Multifunctional and Nanoreinforced Polymers for Food Packaging (Woodhead Publishing in Materials)

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Multifunctional and nanoreinforced polymers for food packaging

ß Woodhead Publishing Limited, 2011

Related titles: Innovations in food labelling (ISBN 978-1-84569-676-4) Increasingly, consumers desire information about the health, safety, environmental and socioeconomic characteristics of food products. These traits often cannot be detected by sight, smell or taste; therefore, consumers must use food labels to select products that meet their needs and preferences. The growing consumer and industry interest in food labels presents challenges for governments, which must ensure that the product information is accurate, truthful and not misleading to consumers. With the increase in global trade in food, there is also a need to harmonize food labels so that product information is relevant to foreign markets. Innovations in food labelling provides information about the principles and requirements of food labelling and reviews the latest trends in this important area. Development of packaging and products for use in microwave ovens (ISBN 978-1-84569-420-3) Improving the quality and safety of microwavable convenience food products is a priority for manufacturers. Development of packaging and products for use in microwave ovens provides a comprehensive review of this important area. Written by a distinguished team of international contributors, the text discusses the principles, properties of ingredients, materials issues, design, product development and safety of packaging for use in microwaves. Passive and active packaging is explored in detail with an emphasis on practical issues, in addition to the computer simulation of microwave heating of foods in both types of container. Environmentally compatible food packaging (ISBN 978-1-84569-194-3) Food packaging performs an essential function, but packaging materials can have a negative impact on the environment. This collection reviews bio-based, biodegradable and recycled materials and their current and potential applications for food protection and preservation. The first part of the book focuses on environmentally-compatible food packaging materials. The second part discusses drivers for using alternative packaging materials, such as legislation and consumer preference, environmental assessment of food packaging and food packaging eco-design. Chapters on the applications of environmentally-compatible materials for particular functions, such as active packaging, and in particular product sectors then follow. Details of these and other Woodhead Publishing materials books can be obtained by: · visiting our web site at www.woodheadpublishing.com · contacting Customer Services (e-mail: [email protected]; fax: +44 (0) 1223 832819; tel.: +44 (0) 1223 499140 ext. 130; address: Woodhead Publishing Limited, 80 High Street, Sawston, Cambridge CB22 3HJ, UK) If you would like to receive information on forthcoming titles, please send your address details to: Francis Dodds (address, tel. and fax as above; email: francis.dodds@woodhead publishing.com). Please confirm which subject areas you are interested in.


Multifunctional and nanoreinforced polymers for food packaging Edited by JoseÂ-MarõÂa LagaroÂn


Published by Woodhead Publishing Limited, 80 High Street, Sawston, Cambridge CB22 3HJ, UK www.woodheadpublishing.com Woodhead Publishing, 1518 Walnut Street, Suite 1100, Philadelphia, PA 19102-3406, USA Woodhead Publishing India Private Limited, G-2, Vardaan House, 7/28 Ansari Road, Daryaganj, New Delhi ± 110002, India www.woodheadpublishingindia.com First published 2011, Woodhead Publishing Limited ß Woodhead Publishing Limited, 2011 The authors have asserted their moral rights. This book contains information obtained from authentic and highly regarded sources. Reprinted material is quoted with permission, and sources are indicated. Reasonable efforts have been made to publish reliable data and information, but the authors and the publisher cannot assume responsibility for the validity of all materials. Neither the authors nor the publisher, nor anyone else associated with this publication, shall be liable for any loss, damage or liability directly or indirectly caused or alleged to be caused by this book. Neither this book nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, microfilming and recording, or by any information storage or retrieval system, without permission in writing from Woodhead Publishing Limited. The consent of Woodhead Publishing Limited does not extend to copying for general distribution, for promotion, for creating new works, or for resale. Specific permission must be obtained in writing from Woodhead Publishing Limited for such copying. Trademark notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation, without intent to infringe. British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library. ISBN 978-1-84569-738-9 (print) ISBN 978-0-85709-278-6 (online) The publisher's policy is to use permanent paper from mills that operate a sustainable forestry policy, and which has been manufactured from pulp which is processed using acidfree and elemental chlorine-free practices. Furthermore, the publisher ensures that the text paper and cover board used have met acceptable environmental accreditation standards. Typeset by Godiva Publishing Services Limited, Coventry, West Midlands, UK Printed by TJI Digital, Padstow, Cornwall, UK


Contents

1

Contributor contact details

xv

Preface

xix

Multifunctional and nanoreinforced polymers for food packaging LAGAROÂN,

J.-M. Novel Materials and Nanotechnology Group, IATA-CSIC, Spain

1.1 1.2 1.3 1.4 1.5 1.6 1.7

Introduction Structural factors governing barrier properties Novel polymers and blends Nanocomposites Future trends References Appendix: Abbreviations

1

1 7 15 21 25 25 28

Part I Nanofillers for plastics in food packaging 2

Multifunctional nanoclays for food contact applications

J.-M. LAGAROÂN and M.-A. BUSOLO, Novel Materials and Nanotechnology Group, IATA-CSIC, Spain 2.1 2.2 2.3 2.4 2.5

Introduction Antimicrobial nanoclays Oxygen-scavenging nanoclays Future trends References


31

31 33 37 39 39

vi

Contents

3

Hydrotalcites in nanobiocomposites

3.1 3.2 3.3

U. COSTANTINO and M. NOCCHETTI, University of Perugia, Italy and G. GORRASI and L. TAMMARO, University of Salerno, Italy

3.5 3.6

Introduction Hydrotalcite-like compounds (HTlc): basic chemistry Organically modified biocompatible hydrotalcite-like compounds (HTlc) Nanocomposites of biodegradable polymeric matrices and modified hydrotalcites Conclusions and future trends References and further reading

4

Cellulose nanofillers for food packaging

3.4

4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8

5

R. T. OLSSON and L. FOGELSTROÈM, Royal Institute of Technology, Sweden, M. MARTIÂNEZ-SANZ, Novel Materials and Nanotechnology Group, IATA-CSIC, Spain and M. HENRIKSSON, Royal Institute of Technology, Sweden and SP Technical Research Institute of Sweden, Sweden Introduction Morphological and structural characteristics of cellulose nanofillers Extraction and refining of cellulose nanofillers Mechanical properties of cellulose nanofillers Surface modification of cellulose nanofillers Preparation of cellulose-reinforced nanocomposites Future trends and applications of cellulose nanofillers References

Electrospun nanofibers for food packaging applications

S. TORRES-GINER, Novel Materials and Nanotechnology Group, IATA-CSIC, Spain

5.1 5.2 5.3 5.4 5.5 5.6

Electrospinning Functional nanofibers Nanoencapsulation Electrospinning in packaging applications Future trends References

Part II High barrier plastics for food packaging


43

43 45 52 67 75 77

86

86 87 91 95 96 99 101 102

108

108 113 116 119 121 123

Contents

6

Mass transport and high barrier properties of food packaging polymers

F. NILSSON and M. S. HEDENQVIST, Royal Institute of Technology, Sweden 6.1 6.2 6.3 6.4 6.5 6.6

Introduction: the basics of mass transport Diffusivity Solubility What makes a barrier a barrier? Characterisation techniques References

7

Ethylene±norbornene copolymers and advanced single-site polyolefins T. J. DUNN, formerly at Printpack, Inc., USA

7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8

8

Introduction Synthesis and molecular structure: advanced single-site polyolefins Macromolecular structure: advanced single-site polyolefins Macromolecular structure: ethylene±norbornene copolymers Nanocomposite preparation: advanced single-site polyolefins Future trends Sources of further information and advice References

Advances in polymeric materials for modified atmosphere packaging (MAP)

T. K. GOSWAMI, Indian Institute of Technology, India and S. MANGARAJ, CIAE, India

8.1 8.2 8.3 8.4 8.5 8.6 8.7 8.8 8.9 8.10 8.11 8.12

Introduction Modified atmosphere packaging (MAP) Physiological factors affecting shelf-life of fresh produce Post-harvest pathology of fruits and vegetables Response of fresh produce to modified atmosphere packaging Polymeric films for application in modified atmosphere packaging (MAP) Cellulose-based plastics Biodegradable polymers Multilayer plastic films Gas permeation or gas transmission Water vapor permeability Packaging systems in modified atmosphere packaging (MAP)


vii

129

129 130 131 143 146 149

152 152 153 154 155 156 160 161 161

163

163 167 173 188 189 197 204 204 205 208 211 214

viii

Contents

8.13

8.18 8.19

Advanced technology for efficient modified atmosphere packaging (MAP) Package management Design of modified atmosphere packaging (MAP) Mathematical modeling of gaseous exchange in modified atmosphere packaging (MAP) systems Current application of polymeric films for modified atmosphere packaging (MAP) of fruits and vegetables Future trends References and further reading

9

Nylon-MXD6 resins for food packaging

9.1 9.2 9.3 9.4 9.5 9.6 9.7 9.8

Structure and general overview Processing Gas barrier properties Other properties Applications Nylon-MXD6 nanocomposites Future trends References

10

Ethylene±vinyl alcohol (EVOH) copolymers

8.14 8.15 8.16 8.17

10.1 10.2 10.3 10.4 10.5 10.6 10.7 10.8

11

A. AMMALA, CSIRO Materials Science and Engineering, Australia

A. LOÂPEZ-RUBIO, Novel Materials and Nanotechnology Group, IATA-CSIC, Spain Introduction Structure and general properties of ethylene±vinyl alcohol (EVOH) copolymers Ethylene±vinyl alcohol (EVOH) versus aliphatic polyketones Processing in packaging Improving retorting of ethylene±vinyl alcohol (EVOH) Nanocomposites of ethylene±vinyl alcohol (EVOH) and poly(vinyl) alcohol (PVOH) Future trends References

High barrier plastics using nanoscale inorganic films

V. TEIXEIRA, J. CARNEIRO, P. CARVALHO, E. SILVA, S. AZEVEDO and C. BATISTA, University of Minho, Portugal 11.1

Introduction

215 220 221 222 223 226 228

243

243 244 246 250 253 255 258 259

261

261 262 265 266 271 276 280 281

285

285


11.2 11.3 11.4 11.5 11.6 11.7 11.8

12

Contents

ix

Nanotechnologies of thin films for advanced food packaging Thin film technologies for polymer coating using vacuum processes Physical vapour deposition (PVD) processes Inorganic thin film systems Functional properties of diffusion barrier coated polymers Future trends References

287 290 294 299 303 310 311

Functional barriers against migration for food packaging

316

Introduction Food safety issues related to migration Functional barriers Nanostrategies for functional barriers Future trends Sources of further information and advice References and further reading

316 317 319 335 338 339 340

C. JOHANSSON, Karlstad University, Sweden

12.1 12.2 12.3 12.4 12.5 12.6 12.7

Part III Active and bioactive plastics 13

Silver-based antimicrobial polymers for food packaging

A. MARTIÂNEZ-ABAD, Novel Materials and Nanotechnology Group, IATA-CSIC, Spain 13.1 13.2 13.3 13.4 13.5 13.6

Introduction Incorporation of silver into coatings and polymer matrices Antimicrobial silver in food packaging Future trends Sources of further information and advice References and further reading

14

Incorporation of chemical antimicrobial agents into polymeric films for food packaging

BALDEV RAJ, R. S. MATCHE and R. S. JAGADISH, Central Food Technological Research Institute, India

14.1 14.2 14.3 14.4

Introduction Antimicrobial agents Chemical antimicrobial agents Natural antimicrobial agents


347

347 350 356 359 361 362

368

368 371 372 380

x

Contents

14.5 14.6 14.7 14.8 14.9 14.10 14.11

Polymers (synthetic or natural) Nano-antimicrobial agents Antimicrobial films and coatings Antimicrobial activity Future trends References Appendix: Abbreviations

15 15.1 15.2 15.3 15.4 15.5 15.6 15.7 15.8

16

16.1 16.2 16.3 16.4 16.5 16.6

Natural extracts in plastic food packaging

P. SUPPAKUL, Kasetsart University, Thailand

Introduction Natural plant extracts as antimicrobials and antioxidants Designing active plastic packaging systems from natural plant extracts Packaging films based on natural extracts Factors to consider in designing active systems Future trends Sources of further information and advice References and further reading

Bioactive food packaging strategies

A. LOÂPEZ-RUBIO, Novel Materials and Nanotechnology Group, IATA-CSIC, Spain Introduction Definition and technologies Nanotechnologies Controlled release of bioactives Future trends References and further reading

389 390 393 403 404 404 420

421 421 422 430 434 445 448 449 450

460

460 461 470 473 475 476

Part IV Nanotechnology in sustainable plastics for food packaging 17

Polylactic acid (PLA) nanocomposites for food packaging applications

J.-M. LAGAROÂN, Novel Materials and Nanotechnology Group, IATA-CSIC, Spain 17.1 17.2 17.3 17.4

Introduction and properties of polylactic acid (PLA) Nanobiocomposites of polylactic acid (PLA) for monolayer packaging Future trends References and further reading ß Woodhead Publishing Limited, 2011

485

485 486 493 494

Contents

18

18.1 18.2 18.3 18.4 18.5 18.6 18.7 18.8

19

xi

Polyhydroxyalkanoates (PHAs) for food packaging 498

D. PLACKETT and I. SIROÂ, Technical University of Denmark, Denmark

Introduction Commercial developments Polyhydroxyalkanoates (PHAs) and their nanocomposite films Polyhydroxyalkanoate (PHA) foams and paper coatings Conclusions Future trends Sources of further information and advice References

Starch-based polymers for food packaging

R. M. GONZAÂLEZ and M. P. VILLANUEVA, Technological Institute of Plastic (AIMPLAS), Spain

19.1 19.2 19.3 19.4 19.5 19.6 19.7 19.8 19.9

Introduction Market for starch-based materials and potential applications Structure and properties of native and plasticized starch Processing in packaging Mechanical and barrier performance of starch-based systems Nanocomposites Future trends Sources of further information and advice References

20

Chitosan polysaccharide in food packaging applications

P. FERNANDEZ-SAIZ, Novel Materials and Nanotechnology Group, IATA-CSIC, Spain

20.1 20.2 20.3 20.4 20.5 20.6 20.7 20.8

Introduction Structure and properties Processing in packaging Antimicrobial chitosan Barrier performance Nanocomposites Future trends References

21

Carrageenan polysaccharides for food packaging

21.1

Introduction

M. D. SANCHEZ-GARCIA, Novel Materials and Nanotechnology Group, IATA-CSIC, Spain

498 500 502 515 516 517 518 518

527

527 528 531 537 542 546 557 559 560

571

571 572 573 574 582 584 586 587

594

594 ß Woodhead Publishing Limited, 2011

xii

Contents

21.2 21.3 21.4 21.5 21.6

Structure and properties of carrageenan Processing in packaging Barrier performance Nanocomposites References and further reading

595 597 598 601 606

22

Protein-based resins for food packaging

610

Materials (sources, extraction, structure and properties) Structure and properties Packaging materials characterization (barrier performance, mechanical properties) Applications Future trends References

610 618

22.1 22.2 22.3 22.4 22.5 22.6

23

A. A. VICENTE, M. A. CERQUEIRA and L. HILLIOU, University of Minho, Portugal and C. M. R. ROCHA, University of Porto, Portugal

Wheat gluten (WG)-based materials for food packaging H. ANGELLIER-COUSSY, V. GUILLARD, C. GUILLAUME and N. GONTARD, University of Montpellier II, France

23.1 23.2 23.3 23.4 23.5 23.6 23.7

24

Introduction Preparation of wheat gluten-based materials Mechanical and barrier properties of wheat gluten-based materials Wheat gluten-based nanocomposites Example of integrated approach for the packaging of fresh fruits and vegetables Future trends References

Safety and regulatory aspects of plastics as food packaging materials

BALDEV RAJ and R. S. MATCHE, Central Food Technological Research Institute, India 24.1 24.2 24.3 24.4 24.5

Introduction Indirect food additives Nanotechnology in food contact materials Migration of additives Indian Standards for overall migration (IS:9845-1998)


622 634 638 638

649

649 650 652 658 661 664 664

669

669 670 673 674 677

Contents 24.6 24.7 24.8 24.9 24.10 24.11 24.12

xiii

US Food and Drug Administration (US FDA), Code of Federal Regulations (CFR) 681 European Commission Directives on plastic containers for foods 682 Specific migration of toxic additives 684 Recent problems in specific migration 687 Future trends 687 References and further reading 689 Appendix: Abbreviations 691 Index

692



Chapter 4

(* = main contact)

Editor and Chapters 1, 2 and 17 Professor Dr JoseÂ-MarõÂa LagaroÂn Novel Materials and Nanotechnology Group Spanish Council for Scientific Research (CSIC) IATA, Av. Agustin Escardino 7 46980 Paterna Spain E-mail: [email protected]

Chapter 3 Umberto Costantino* and Morena Nocchetti Department of Chemistry University of Perugia 06123 Perugia Italy E-mail: [email protected] Giuliana Gorrasi and Loredana Tammaro Chemical and Food Engineering Department University of Salerno 84084 Fisciano (SA) Italy

Assistant Professor Richard T. Olsson* and Dr Linda FogelstroÈm Department of Fibre and Polymer Technology School of Chemical Science and Technology Royal Institute of Technology Teknikringen 56±58 SE-100 44 Stockholm Sweden E-mail: [email protected] Marta MartõÂnez-Sanz Novel Materials and Nanotechnology Group Spanish Council for Scientific Research (CSIC) IATA, Av. Agustin Escardino 7 46980 Paterna Spain Dr Marielle Henriksson Department of Fibre and Polymer Technology School of Chemical Science and Technology Royal Institute of Technology Teknikringen 56±58 SE-100 44 Stockholm Sweden


xvi


Chapter 8

and SP Technical Research Institute of Sweden P.O. Box 5609 SE-114 86 Stockholm Sweden

Chapter 5 Dr Sergio Torres-Giner Novel Materials and Nanotechnology Group Spanish Council for Scientific Research (CSIC) IATA, Av. Agustin Escardino 7 46980 Paterna Spain E-mail: [email protected]

Chapter 6 Fritjof Nilsson and Professor Michael S. Hedenqvist* School of Chemical Science and Engineering Fiber and Polymer Technology Royal Institute of Technology SE-100 44 Stockholm Sweden E-mail: [email protected]

Tridib Kumar Goswami* Department of Agricultural and Food Engineering Indian Institute of Technology Kharagpur West Bengal 721302 India E-mail: [email protected] Shukadev Mangaraj CIAE Nabibagh Berasia Road Bhopal 462038 (MP) India E-mail: [email protected] [email protected]

Chapter 9 Dr Anne Ammala CSIRO Materials Science and Engineering Private Bag 33 Clayton South MDC Victoria 3169 Australia E-mail: [email protected]

Chapters 10 and 16

Chapter 7 Thomas J. Dunn Flexpacknology LLC 2526B Mt Vernon Road Atlanta GA 30338 USA E-mail: [email protected]

Dr Amparo LoÂpez-Rubio Novel Materials and Nanotechnology Group Spanish Council for Scientific Research (CSIC) IATA, Av. Agustin Escardino 7 46980 Paterna Spain E-mail: [email protected]



Chapter 11 Vasco Teixeira*, Joaquim Carneiro, Pedro Carvalho, Emanuel Silva, Sofia Azevedo and Carlos Batista University of Minho Physics Department GRF-Functional Coatings Group Campus de AzureÂm 4800-058 GuimaraÄes Portugal E-mail: [email protected]

Chapter 12 Associate Professor Caisa Johansson Karlstad University Faculty of Technology and Science Department of Chemical Engineering SE-651 88 Karlstad Sweden E-mail: [email protected]

Chapter 13 Antonio MartõÂnez-Abad Novel Materials and Nanotechnology Group Spanish Council for Scientific Research (CSIC) IATA, Av. Agustin Escardino 7 46980 Paterna Spain E-mail: [email protected]

Chapter 14 Baldev Raj*, Rajeshwar S. Matche and R. S. Jagadish Food Packaging Technology Department Central Food Technological Research Institute Mysore 570020

xvii

India E-mail: [email protected] [email protected] [email protected]

Chapter 15 Assistant Professor Dr Panuwat Suppakul Department of Packaging and Materials Technology Faculty of Agro-Industry Kasetsart University Agro-Industry Building V 50 Phaholyouthin Road Ladyao Chatuchak Bangkok 10900 Thailand E-mail: [email protected]

Chapter 18 David Plackett* and IstvaÂn SiroÂ Solar Energy Programme Risù National Laboratory for Sustainable Energy Technical University of Denmark 4000 Roskilde Denmark E-mail: [email protected]

Chapter 19 R. M. GonzaÂlez* and M. P. Villanueva Extrusion Department Technological Institute of Plastic (AIMPLAS) Calle Gustave Eiffel 4 (Parque TecnoloÂgico) 46980 Paterna Valencia Spain E-mail: [email protected]


xviii


Chapter 20 P. Fernandez-Saiz Novel Materials and Nanotechnology Group Spanish Council for Scientific Research (CSIC) IATA, Av. Agustin Escardino 7 46980 Paterna Spain E-mail: [email protected]

Chapter 21 M. D. Sanchez-Garcia Novel Materials and Nanotechnology Group Spanish Council for Scientific Research (CSIC) IATA, Av. Agustin Escardino 7 46980 Paterna Spain E-mail: [email protected]

Chapter 22 AntoÂnio A. Vicente* and Miguel A. Cerqueira IBB ± Institute for Biotechnology and Bioengineering Centre of Biological Engineering Universidade do Minho Campus de Gualtar 4710-057 Braga Portugal E-mail: [email protected] [email protected] LoõÈc Hilliou Institute for Polymers and Composites/I3N University of Minho Campus de AzureÂm 4800-058 GuimaraÄes

Portugal E-mail: [email protected] Cristina M. R. Rocha REQUIMTE Departamento de Engenharia QuõÂmica Faculdade de Engenharia Universidade do Porto Rua Dr Roberto Frias 4200-465 Porto Portugal

Chapter 23 Dr H. Angellier-Coussy, Dr V. Guillard, Dr C. Guillaume and Pr N. Gontard* UniteÂ Mixte de Recherche IngeÂnierie des AgropolymeÁres et Technologies Emergentes INRA/ENSA.M/UMII/CIRAD UniversiteÂ Montpellier II CC023, pl. E Bataillon 34095 Montpellier Cedex France E-mail: [email protected]

Chapter 24 Baldev Raj* and Rajeshwar S. Matche Food Packaging Technology Department Central Food Technological Research Institute Mysore 570 020 India Email: [email protected] [email protected]


Preface

The current book intends to review the latest developments in the functionalization of high performance plastic materials for food packaging applications. Various polymers, biopolymers and their composites `reinforced' with various organic, inorganic or hybrid engineered nano- or biomaterials, are described which help ensure, or even enhance, the quality and safety of packaged foods. Extending the shelf-life of foods has become of primary interest across the food chain in order to facilitate logistics during production, handling, storage, transportation, presentation by the retailer and even disposal, and to avoid substantial losses due to the deterioration of packaged food quality and safety. An extensive review of the most advanced packaging technologies based on the use of polymers, with special emphasis on polymer-based nanocomposites is presented. In the first chapters of the book several `natural' nanotechnologies of promising value in the food packaging area such as passive and active nanoclays and hydrotalcites, cellulose nanowhiskers and electrospun nanofibres and nanocapsules are presented. These are later discussed in regard to their value in enhancing the physical (chiefly barrier) properties against the transport of low molecular weight components and UV light, their role in modified atmosphere packaging, heat sterilization or retorting, active (antimicrobial, oxygen scavenging, antioxidant, etc.) and bioactive (consumer health promoting) packaging and to provide functional barriers against migration. Finally, an updated chapter on legislation completes the book. JoseÂ-M. LagaroÂn


1

Multifunctional and nanoreinforced polymers for food packaging Â N , Novel Materials and Nanotechnology Group, J.-M. LAGARO IATA-CSIC, Spain

Abstract: The packaging industry has been implementing at a rapidly expanding rate the number of packaging elements made of plastics over recent decades. Plastics, in contrast to more traditional packaging materials like glass and metals, (1) are permeable to the exchange of low molecular weight compounds such as gases and vapours, (2) undergo sorption, so-called scalping, of packaged food constituents, and (3) are amenable to migration into foodstuffs of packaging constituents. Despite these drawbacks, the availability of shapes and forms in which plastics can be conformed, their ease of processing and handling, their low price, their excellent chemical resistance, etc., have made them very attractive in packaging applications. Consequently, a lot of industrial and academic research has been devoted to understanding the mechanisms of mass transport in polymers in order to design new materials and composites with balanced physical properties in general and with improved barrier properties in particular, and to add additional functionalities which may take advantage of their permeability characteristics to positively actuate on the product. This chapter first highlights the factors that make polymers become more impermeable, putting special emphasis on nanotechnology approaches, and then reviews some of the general advances made in the field. Key words: nanotechnology, high barrier polymers/plastics, biopolymers/ bioplastics, packaging, food technology, transport properties.

1.1

Introduction

1.1.1

High barrier concept

High barrier is without doubt a highly desirable property of polymeric materials intended to be used in many packaging applications. The term high barrier usually refers to the low to very low permeability of a material to the transport of low molecular weight chemical species, like gases and vapours. Usually, the lower-limit definition for high barrier typically refers to the performance of PET polymers. However, this property has perhaps never attracted so much attention from industry as over the last decades, when it began to be pursued by some modern food and beverage packaging technologies making use of plastic materials.1±3 In this respect, high barrier has attracted a great deal of recent


2


attention from industry as it has become associated with primary objectives such as commercialization of perishable foods far away from their origin, food shelflife extension and maintaining food quality and safety. Furthermore, it has also become very relevant to a number of other applications including gas separation membranes, packaging of healthcare products, pharmaceuticals and chemicals, and housing of fuels and oxygenated fuels in fuel tanks and lines in the automotive field. The reason for the more recent interest in the development of high barrier polymers and polymer-based structures rests on a widespread trend to implement polymeric materials in an ever-increasing number of applications, in many cases aiming to substitute them for other, more traditional packaging materials. It is common knowledge that the attractiveness of plastics lies in their versatility and ability to offer a broad variety of properties and yet be cheap and easily processed and conformed into a myriad of shapes and sizes. However, polymers do have a number of limitations for certain applications when compared with more traditional materials like metals and alloys or ceramics. Among some of these limitations relevant to the purpose of this chapter are their permeability and comparatively low thermal resistance, and the strong interdependence between these two properties. The permeability of plastics to the exchange of gases and vapours imposes a number of challenges in those applications where high barrier, ideally impermeability, is required. These applications were, for instance, traditionally assumed by tinplate and glass in the food packaging field. However, polymer scientists, engineers and technologists in industry and academia have pulled together a great deal of effort and resources to push the limits of plastics performance towards impermeability, chiefly due to the overwhelming pressure exerted by the numerous other advantages associated with the use of plastics in high barrier applications. Table 1.1 gives typical oxygen permeability and water permeability values for a number of commercial polymers and structures used in food packaging applications.4

1.1.2

Functional packaging

The concept of functional or active/bioactive/intelligent packaging for food applications has been recently exploited, obtaining for the package an active role in the preservation, health-promoting capacity and provision of information concerning the products. Among these, active packaging is perhaps the area that has steered more research and industrial interest. Packages may be termed active when they perform some desired role in food preservation other than providing an inert barrier to external conditions. The opportunity of modifying the inner atmosphere of the package or even the product by simply incorporating certain substances in the package wall has made this group of technologies very attractive, representing an increasingly productive research area. Even though



3

Table 1.1 Water permeability (at 38ëC and 90% RH) and oxygen permeability (at 23ëC) of a number of commercial plastics and multilayer structures Material

PVOH EVOH PAN PAN (70% AN) PVDC PA6 aPA (amorphous) PET PP PC LDPE LCP PET/PVDC PA/PVDC PP/PVDC PET-met. PET/AlOx/PE PET/SiOx/PE PA/SiOx/PE PP/SiOx/PE PLA PLA PHB PHB PHBV PCL PCL PCL

Water permeability 1018 kg m/(m2 s Pa)

Oxygen permeability 1021 m3 m/(m2 s Pa) 0% RH

75% RH

485 000 17 000 2420 8250 30.53 20 600 2420 2300 726 19 400 1200 10 170 160 43 58 21 16 32 13 12 600

0.17 0.77 1.9 10.5 4.5 52 83 135 6750 10 500 21 500 0.42 17.5 18.2 25 3.5 7 4.9 7.7 81 2250

900 91

1689

230

6900 26 600

1590 4380 934 1960

31 225 60

15

2209 1750 5100 3010 7850

Reference of data source

4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 5 6 7 8 7 7 9 10

the first active packaging developments and most of the commercialized technologies consist of sachet technologies, which make use of a small permeable pouch (sachet) containing the active compound that is inserted inside the package, current trends tend towards the incorporation of active ingredients directly into the packaging wall. This strategy is associated with a number of advantages, such as reduction in package size, higher effectiveness of the active principles (which are now completely surrounding the product), and, in many cases, higher throughput in packaging production, since the additional step of incorporating the sachet is eliminated.7 Polymers, and in particular biomassderived polymers, are the preferred materials for active packaging because of their intrinsic properties, constituting an ideal carrier for active principles, with the advantage of being tuneable in terms of controlled release and the possibility


4


of combining several polymers through blending or multilayer extrusion to tailor the application. Active packaging has been used with many products and is under investigation for numerous others. These new food packaging technologies have been developed as a response to trends in consumer preferences towards mildly preserved, fresh, tasty, healthier, and convenient food products with prolonged shelf-life. These novel packaging technologies can also be used to compensate for shortcomings in the packaging design, for instance in order to control the oxygen, water or carbon dioxide levels in the package headspace. In addition, changes in retail practices, such as globalization of markets resulting in longer distribution distances, present major challenges to the food packaging industry, which finally act as driving forces for the development of new and improved packaging concepts that extend shelf-life while maintaining the safety, quality and health aspects of the packaged foods. The combinations of polymers and active substances that can be studied for potential use as active packages are in principle unlimited and it is forecast that the number of applications will increase in the near future. Among the existing active packaging technologies, oxygen scavengers and antimicrobial packaging stand out over the other developments. Both technologies were initially based on the sachet concept, using reducing and inhibitory substances, respectively. Lately, the growth in both areas has been enormous, especially in the case of antimicrobials. Other active packaging applications include systems capable of absorbing carbon dioxide, phase-changing materials, moisture, ethylene and/or flavour/odour taints; releasing carbon dioxide and/or flavour/odour. Traditionally, plastic food packaging has been related to negative food safety issues, due mainly to problems with migration of packaging components. In more recent trends, packaging is being designed more favourably to impact on consumer health by integrating functional ingredients in the packaging structure, through so-called bioactive packaging strategies.8 Novel active and bioactive packaging technologies, combined with bioplastics and nanotechnology, can best help do this. Therefore, proper combination of these technological cornerstones will provide innovation in the food packaging sector over the next few years. Furthermore, due to the shortage of oil resources and waste-management issues, research focus is shifting from synthetic oil-based plastics to biomassderived biodegradable and environmentally friendly polymers. The drawbacks that initially characterized these biopolymers in terms of poor barrier properties and high instability have, in turn, resulted in novel applications, making highly permeable and water-plasticizable biopolymers an ideal partner for active and bioactive packaging where the package is no longer a passive barrier, but actively contributes to the preservation of food by controlled release of the substances. Biopolymers are, thus, the ideal matrix for the incorporation and controlled release of a number of substances to be added to the food. Probably



5

the area that is evolving more quickly is the antimicrobial packaging one, but it is foreseen that biopackages will also serve as reservoirs for vitamins, antioxidants, and pre- and probiotics.

1.1.3

Phenomenology of transport in polymers

According to the above, barrier properties in polymers are necessarily associated with their inherent ability to permit the exchange, to a higher or lower extent, of low molecular weight substances through mass-transport processes like permeation. The phenomenology of permeation of low molecular weight chemical species through a polymeric matrix is generally envisaged down to the molecular level as a combination of two processes, i.e. solution of the solutes and molecular diffusion.11 A permeating gas is first dissolved into the upstream face of the polymer film, and then undergoes a molecular diffusion to the downstream face of the film through typically the polymer amorphous phase, where it evaporates into the external phase again. A solution±diffusion mechanism is thus applied, which can be formally expressed in terms of permeability (P), solubility (S) and diffusion (D) coefficients as follows: P DS

1:1

This permeability coefficient derives from application of Henry's law of solubility to Fick's first law of diffusion as follows: J

q @c Sp p ql ÿD D DS ) P DS At @x l l Atp

1:2

The solubility coefficient S is thermodynamic in nature and is defined as the ratio of the equilibrium concentration of the dissolved penetrant in the polymer to its partial pressure (p) in the gas phase (Henry's law). In polymers, Henry's law is usually obeyed at low penetrant concentrations, i.e. when S is independent of concentration (or of the partial pressure). D characterizes the average ability of the sorbed permeate to move through the polymer chain segments and is typically governed by Fick's first law of diffusion, i.e. the flux of the permeant (J) is proportional to the local gradient of concentration (c) through the thickness of the polymer film (l). During sorption kinetic experiments, if Fickian transport (case I) is assumed, linear behaviour in the penetrant uptake vs. the t1/2 (t being time) curve at small times is usually observed.12 Case II diffusion is defined when linear behaviour is observed in the uptake vs. t curve. This behaviour is observed in a number of systems and is associated with large uptakes and plasticization of the structure by the penetrant. When complex sorption behaviours like sigmoidal shapes are observed it is usually assumed that an ànomalous' or non-Fickian transport occurs. Nevertheless, from recent works a better rationalization of these ànomalous' behaviours has been achieved, in which contributions from the effect of macroscopic elastic constraints arising


6


during the swelling process (geometrical effects) in adsorption experiments have been pointed out.13,14 Concerning the mechanisms of the mass-transport process through polymeric materials, two general approaches can be found, namely (1) molecular models studying the specific penetrant and chain motions in conjunction with the corresponding intermolecular forces, and (2) `free-volume' models which pay attention to the relations between the transport coefficients and the free volume existing in the polymeric matrix, without considering molecular-scale mechanisms. It is also relevant to emphasize here that the mass transport mechanisms, as well as their dependence on permeant partial pressure and testing temperature, are thought to be generally different depending on whether the polymer is in a rubbery or glassy state. Rubbery polymers are above their glass transition temperature (Tg) and, therefore, have very short relaxation times and respond quickly to physical changes. Thus, absorption of small molecules or penetrants causes immediate adjustments to a new equilibrium state and, consequently, there appears to exist a unique mode of penetrant transport for these polymers. Moreover, rubbery polymers are more amenable to show upwardly inflecting permeability responses with increasing penetrant partial pressure due to plasticization. This is typically the case in D-limonene, a common flavour component in fruit juices, in polymers like polyethylene and polypropylene. By comparison, glassy polymers are below their Tg and hence require on average long timescales to fully relax. Gas transport then typically occurs in glassy polymers under nonthermodynamic equilibrium conditions. In this case, penetrant molecules can allocate in holes or irregular cavities with very different diffusional mobility and, consequently, more than one mode of transport may be accessible. A `dual-mode sorption' model satisfactorily describes the dependence of transport properties on penetrant partial pressure in glassy polymers. This model postulates the existence of two different molecular populations dissolved in a glass: one dissolved by an ordinary dissolution process which can typically follow Henry's law (c Sp), and the other dissolved in a limited amount of fixed microcavities which can be described by a Langmuir-like isotherm: c

cH bp 1 bp

1:3

In equation 1.3, cH is the hole saturation constant and b is the hole affinity constant. More complex sorption behaviours have also been postulated for other glassy materials. For instance, a modified dual-mode model requiring Langmuir and Flory±Huggings equations was suggested to explain the sorption of water in an amorphous polyamide.15 In what follows, we first overview some relevant structural factors defining and/or altering high gas barrier properties in polymers, and then comment on recent material developments in the field, i.e. blends, coatings and nanocomposites.



1.2

7

Structural factors governing barrier properties

The structural factors determining inherent high barrier properties in polymers are fundamentally the chemistry, but there are also other relevant factors making a significant impact on barrier properties for a given chemistry, including polymer morphology (crystallinity, thermal history, amorphous density, molecular orientation, etc.), polymer molecular architecture (branches, molecular weight and tacticity), polymer plasticization, temperature, penetrant type and chemistry, and others.

1.2.1

Polymer chemistry

Nowadays, very many chemical combinations and high throughput and selective catalyst technologies are accessible via cutting-edge polymer chemistry, to generate polymeric materials with tailor-made structures and properties. As would be reasonable to expect, then chemistry is the basic and main defining factor determining barrier properties in polymeric materials. Thus, by varying the chemistry of the macromolecule, often by just adjusting the pendant group along the polymer chain, a significantly large variation in barrier properties can be achieved (see Table 1.2). Some commonly employed abbreviations applied to both well-known and new commercial plastics are listed in the Appendix. Behind the significant changes in barrier properties resulting from variations of chemistry are, for instance, the introduction of apolar voluminous groups at the low barrier side of the permeability spectrum, or the incorporation of small and strongly self-interacting chemical groups at the high barrier side of the permeability spectrum. The permeability of a polymer can change by up to six orders of magnitude depending on the grafted chemical groups attached to the polymer backbone. As is well known, most polymeric materials comprise exceedingly long high molecular weight molecules (called polymer chains) which for the case of the most widely used plastics, the thermoplastics family, do not have intermolecular links in the amorphous state other than secondary forces of, for instance, the van der Waals type. Consequently, the presence of Table 1.2 Relative oxygen permeability of polymer materials based on the repetition of CH2±CHX Polymer PVOH PAN PVC PP PS PE

Pending X unit

Relative O2 permeability

±OH ±CN ±Cl ±CH3 ±C6H5 ±H

1 4 800 15 000 42 000 48 000


8


these different pendant groups can either disrupt or enhance the high intermolecular cohesion necessary to maintain high barrier efficiency against the transport of low molecular weight substances. Moreover, chemistry also defines the affinity between a potential permeant and the polymer matrix. As the process of permeation is a bimodal process comprising solution and diffusion, low solubility based on chemical disparity of a permeant and the polymer matrix will also result in low permeability, irrespective of whether the kinetics of diffusion are going to be favourable to the permeant transport. In this chapter, we will rather concentrate, due to their relevance and ease of generalization, on the barrier properties of non-interacting chemicals as is usually the case of the permanent gases. A physical magnitude called the cohesive energy density can be useful in helping to explain, quantify or even predict the behaviour in terms of barrier properties of polymeric materials. The cohesive energy of a substance in a condensed state is defined as the increase in internal energy per mole of substance if all the intermolecular forces are eliminated. For low molecular weight substances this energy can be experimentally calculated from the heat of evaporation. However, for polymers the cohesive energy density (defined as the cohesive energy per unit of volume) can be estimated using additive group contribution models like those devised by, for instance, Van Kreveland for cohesive energy and Traube for molar volume.16 These models propose contribution values for each of the chemical entities building up the polymer chain. Consequently, this parameter tells us about the strength of the interaction between molecules, and how this interaction changes when different chemical groups are added to the polymer chain. The cohesive energy density is often referred to as the square of the solubility parameter. Another important factor strongly associated with barrier properties is the free volume. The free volume comprehends the microcavities present in a polymeric material. Permeants make use of these cavities ± whether permanent or transient ± to diffuse through the polymer matrix. The transport properties of a permeant are therefore dependent on the number and size of these microcavities. This concept is usually expressed through the so-called fractional free volume parameter (Vf) and is indeed strongly related to chemistry (cohesive energy density), but it is also related to a number of other relevant factors having an impact on barrier properties like thermal history, polymer Tg, crystallinity and/or conformational order, etc. The fractional free volume Vf can be easily determined by the following simple equation: V ÿ V0 1:4 V where V is the specific volume of a particular polymer sample determined by density, and V0 is the specific volume at zero solubility (volume exclusively occupied by polymer chains). The latter parameter can be experimentally Vf



9

determined by, for instance, extrapolation of experimental data17 or can be estimated from additive group contributions models. A very useful concept for free volume is that proposed by Cohen and Turnbull18,19 and Fujita20 through a general expression as follows: D / eÿBd =Vf

1:5

In this expression, D is the diffusion coefficient and Bd is a constant that depends only on the size of the penetrant molecule. This model has been shown to adequately describe the transport kinetics of organic vapours and small gas molecules in a number of polymers. More recent efforts have led to the development of an experimental methodology based on a technique called positronium annihilation spectroscopy. This methodology provides an experimental approach to determining free volume, as it enables one to measure hole size on a nanoscale and its fraction.21 Nevertheless, the absolute value of the fractional free volume cannot be directly obtained from only positron lifetime measurements. In spite of that, a study making use of positronium annihilation spectroscopy showed that there exists an excellent correlation between the oxygen permeability and a relative fractional free volume parameter as determined by this technique in a number of EVOH copolymers.22 From the experiments, it was clear that the fractional free volume in these materials does mainly concern the free volume size, as only the free volume size and not the orthopositronium o-Ps lifetime intensity, i.e. the number of holes, varied across composition in these polymers. It is, therefore, relevant to realize that high barrier polymers are the result of a permeable structure (amorphous phase) with a high cohesive energy density and very low fractional free volume. Figure 1.1 plots the oxygen permeability of a number of plastics, superimposed with the performance of bioplastics, vs. the ratio of the cohesive energy density to the fractional free volume. From this figure, it can be seen that EVOH copolymers (with 32 mol% ethylene) are one of the most efficient oxygen barrier materials due to their high intermolecular cohesion and low fractional free volume. Consequently, this material is being increasingly introduced in packaging applications where high barrier properties to gases are required. On the contrary, polymers like HDPE have much lower gas barrier properties due to low intermolecular cohesion and large fractional free volume. High intermolecular cohesion can, however, be distorted by for instance chemical alterations in the material (polymer degradation) due to thermal treatments.23 Polymer chain rigidity or polymer Tg also plays a relevant role in barrier properties since, as explained earlier, penetrant transport mechanisms are greatly altered depending on whether the permeation process occurs above (rubbery state) or below (glassy state) the polymer glass transition temperature. There is a very general trend that indicates that the higher the polymer Tg the lower the gas permeability and the better the permselectivity. However, this does not apply to common polymers like PS or PC which are very rigid glassy materials with


10


1.1 PO2 (cm3 mm/m2 day atm) vs. the fractional free volume/cohesive energy density ratio for a number of polymers typically used in food packaging applications. References to the typical oxygen barrier properties of biopolymers are also included.

values of Tg above 100ëC and very high permeability. This is of course a consequence of the voluminous side groups which indeed reduce chain segment mobility due to steric hindrance but in turn generate large fractional free volumes. On the other hand, polymers like EVOH copolymers, PK copolymers or PVDC have lower values of Tg than for instance PS, PC or other materials like PET and yet have outstanding barrier properties. This is again due to the very high cohesive energy density and low fractional free volume exhibited by the former materials.

1.2.2

Polymer morphology

An important issue that has been implicit in all the previous considerations is the well-known characteristic that polymers are not able to fully crystallize due to metastability, some being in fact totally amorphous. Many polymers used in packaging applications have, therefore, a semicrystalline nature and hence are, from a structural viewpoint, heterogeneous materials. These polymers contain, under the most simplistic two-phase model visualization, both a fraction of chain segments constituting highly packed and conformationally ordered threedimensional structures ± polymer crystalline fraction () ± and another fraction in an amorphous state without conformational regularity and lateral order. As a large body of experimental evidence suggests that polymer crystals are impermeable to the transport of most low molecular weight substances, it is broadly accepted that the amorphous phase is the only phase available for permeation of these substances.



11

It is therefore this particular structural feature, i.e. polymer crystallinity, together with a low intermolecular cohesion between polymer chains in the amorphous phase that best defines many of the most characteristic polymer properties, including permeability. However, polymer crystals not only fill the molecular structure of semicrystalline materials with microscopic impermeable blocks but also affect the surrounding amorphous phase. To begin with, the presence of crystallinity, its morphology (for instance, crystal width-to-thickness ratio) and orientation bring in additional considerations in terms of permeability as the penetrant molecules have to circumvent the crystallites, and thereby travel through a more tortuous diffusive path than in a fully amorphous material. This effect is usually accounted for in the calculations of the transport coefficients (see equation for diffusion below) by the so-called tortuosity or geometrical impedance factor ( ). Thus, the tortuosity factor is in essence the path length that a permeant has to travel across a film thickness divided by its actual thickness. Furthermore and as commented above, the presence of these crystalline blocks also affects the surrounding conformationally disordered amorphous phase. The constraining effects imposed by crystals to the chain segments in the amorphous phase typically depend on factors like crystal surface area and penetrant size. This phenomenon is substantiated from extensive mechanical and transport data, which clearly indicate that the segmental mobility of the non-crystalline fraction is much less than that in the fully amorphous polymer.24,25 This effect is accounted for in the calculations of the transport coefficients (see equation below) by the so-called chain immobilization factor (): Dsemicrystalline

Damorphous 1 ÿ

1:6

As a result of this, being aware of the implications of the crystallinity and its morphology on the barrier properties is, as a matter of fact, a relevant issue, because by adequate processing (thermal history) of polymers these parameters can be optimized to obtain specimens, based on the same chemistry, with enhanced permeability. Polymer molecular orientation due to drawing or processing generally leads to an increase in barrier properties. This is usually attributed to (1) orientationinduced crystallization, (2) fractionation and alignment (perpendicular to the permeant transport) of the crystals in the straining direction (increase in the tortuosity factor), and (3) densification (reduction in free volume) of the amorphous phase due to an increase in conformational order in the non-crystalline chain segments. The oxygen permeability, diffusivity and solubility parameters have been found to decrease with the amount of uniaxial orientation in PET due to conformational transformations of glycol linkages from gauche to trans. However, for a given uniaxial orientation in PET, biaxial drawing results in increased permeability, reducing the barrier performance. Orientation is then generally seen as the process of decreasing excess free volume bringing the nonequilibrium glassy polymer closer to the equilibrium condition.


12


A special case in barrier properties is that of liquid crystal polymers and PVDC. These materials can have gas barrier properties as good as those of high barrier EVOH copolymers. Liquid crystal polymers are often termed `mesomorphic' because they have structures between those of amorphous polymers with no regular order and those with a three-dimensional crystal lattice. The unique packing arrangement of these polymeric systems has raised some fundamental questions about the permeation mechanisms of low molecular weight molecules, i.e. whether they behave more like glasses or conventional crystals. PVDC also shows high barrier properties to gases and water vapour, attributed to high lateral molecular order and hence density. Although the barrier properties of PVDC are somewhat inferior to those of dry EVOH, the former has the advantage that unlike EVOH it is not plasticized by sorption of moisture in medium to high humidity ranges due to its high molecular lateral packing.

1.2.3

Polymer molecular architecture

Some relevant routes to modifying the molecular architecture of polymers, and hence their barrier properties, are copolymerization, i.e. introducing a few side groups or branches along the main chain, and modification of the molecular weight or the stereoisomerism. Linear polyethylene (HDPE) is more crystalline than both branched polyethylenes (e.g. LLDPEs and LDPE) and ultra-high molecular weight polyethylenes and is, therefore, found to be more dense, less permeable and stiffer, albeit less tough. Moreover, the homogeneous or heterogeneous character of the incorporation of the branches along the polymer backbone has a large impact on properties, including barrier properties.26,27 The more recently developed polyolefins obtained by single site catalyst technologies can lead to very low density materials with unprecedented very low barrier properties, which in thin film form can serve as excellent packaging materials for products that have breathing necessities like fruits and vegetables. A significant effect is also the stereoisomerism (tacticity). This is due to the different stereochemical arrangements that can be present along the polymer backbone and that cannot be changed by rotation along the C±C bond. A polymer for which the pendant groups contain the same configuration is said to be isotactic. Polymers for which alternate carbon atoms have the same configuration are called syndiotactic and when the configuration is at random are called atactic. The atactic configuration is in principle more permeable as it usually yields amorphous polymers (e.g. PS or PMMA).

1.2.4

Polymer plasticization

In this context, it is relevant to add here that polymer plasticization (Tg depletion) due to polymer/permeant interactions or due to polymer and surrounding media chemical interactions has very detrimental effects, which



13

usually lead to losses in intermolecular cohesion and decrease in overall barrier performance.28 Relative humidity has a tremendously detrimental impact on the outstanding gas barrier capacity of EVOH polymers, proteins and polysaccharides. This is also the case, albeit to a lesser extent, for other polar polymers like those in the polyamide family. Thus, it is often the case that polymers that are high barrier to gases have very low barrier performance to polar solvents like water, except PVDC. This behaviour is associated with the disruption by moisture of the existing polymer intermolecular self-association promoted by, for instance, hydrogen bonding in EVOH, PVOH and PA.29±31 As opposed to this behaviour, polymers like polyolefins, PE and PP have low barrier properties to gases due to weak self-association but are extremely good barrier materials to water due to their olefinic hydrophobic character. An exceptional case is that of the amorphous polyamide (aPA) and some polyimides, for which oxygen permeability decreases with increasing relative humidity.11 For this aPA, even though the presence of moisture greatly decreases the polymer Tg, the oxygen permeability does not decrease but surprisingly increases (see Table 1.1). Recent spectroscopic work suggests that moisture has a specific interaction with this particular polymer.32 The results indicate that moisture molecules do not disrupt the originally existing hydrogen bonding intermolecular interactions between amide groups, but rather link to the few remaining free amide groups, and most of the sorbed water molecules selfassociate forming clusters, which altogether act as a free volume blocking mechanism to the diffusion of oxygen molecules. This behaviour also occurs in EVOH copolymers but in the low humidity range. For these copolymers, dry EVOH at 0% RH is a lower barrier than EVOH at 30% RH, due to sorbed moisture at low water activities acting as adsorbed blocking elements to the solubility and diffusion of gas molecules.

1.2.5

Temperature

It is well known that temperature affects many of the properties of polymers. Temperature-induced changes in barrier properties are of an exponential nature. In the case of diffusion, the D value increases exponentially with temperature, in agreement with the Arrhenius law (equation 1.7), since activation energies (ED) are always positive. This phenomenon is related to the greater mobility of polymer chains at higher temperatures, which reduces the energy needed by the permeant molecules to jump to the next active site, and with an increase in the free volume of the polymer:33 D D0 eÿED =RT

1:7

In the case of the solubility coefficient, the exponential dependence on T is described by Van't Hoof's Law (equation 1.8). The enthalpy of solution (H S)


14


values is usually positive, although negative values have also been reported.34 In this case, in spite of the larger number of molecules that can be accommodated in the active sites produced by the greater mobility of the polymer chains and the bigger free volume size, the volatility of the sorbates also affects their partition equilibrium between the polymer and the outer medium.35 S S0 eÿAHS =RT

1:8

Finally, as permeability combines sorption and diffusion, its changes with temperature depend on the values of ED and AHS as shown in equation 1.9. Since the values of ED are usually greater than the absolute value of AHS, the permeation equation is considered to be an Arrhenius-type expression, the temperature dependence being described through the activation energy of permeation (EP): P D0 eÿED =RT S0 eÿAHS =RT D0 S0 eÿED ÿAHS =RT P0 eÿEP =RT 1:9 The temperature also affects the state of the polymer, the transport properties of the polymer being affected by it. In the melted polymer, the crystalline regions disappear and transport takes place across the entire matrix, which behaves like a liquid. In this case, all the polymer volume is available for the permeant, which increases its solubility, and the blocking effect of the crystals disappears, which reduces tortuosity and makes diffusion easier. Also, the polymer chains are in constant movement, which facilitates the jumps of the permeant molecules. Changes associated with the glass transition, i.e. with the passage of the polymer from the glassy to the rubbery state, take place as a result of the relaxation or increased mobility of the chain segments in the amorphous phase of the polymer. Above the glass transition temperature (Tg) the amorphous phase of the polymer is in the rubbery state; below this temperature it is in the glassy state. In the rubbery state, relaxation times are shorter and, after the sorption of permeant molecules, a new equilibrium state is reached more quickly. As a result, diffusion is faster when the polymer is in the rubbery state.

1.2.6

The permeant

Characteristics of the permeant like molecular size, shape and chemical nature usually affect its transport properties. Increasing the molecular size in homologous series of permeants (alkanes, esters, aldehydes or alcohols) generally reduces the diffusion and solubility coefficient values of the permeants, mainly for steric reasons. Only when solutes are in the form of vapour do the higher solubilities correspond to the larger molecules, as a consequence of their greater condensabilities.36 The shape of the permeant molecules is also important, as flattened or elongated molecules will diffuse more quickly through the polymer



15

than spherical ones with the same molecular volume.37 The nature of the permeant also affects its transport properties, as described above in the effect of chemistry. If the affinity between the permeant and the polymer is very high it can sometimes cause plasticization of the polymer. In this case, sorption leads to a decrease in the self-association between adjacent macromolecules in the amorphous region. The initial hydrogen bonding and van der Waals forces are replaced by polymer±sorbate interactions, increasing chain mobility and free volume, reducing the Tg and raising the diffusion and solubility coefficients of the solute. Plasticization depends on the penetrant concentration, which has to be above a certain limit for it to take place. However, while outstanding affinity between the sorbate and the polymer and large uptakes are necessary, sometimes they are not sufficient to produce plasticization of the polymer, as described in the case of aPA. When a complex matrix like a foodstuff is placed inside a polymeric package, the polymer will be in contact with a large number of solvents simultaneously and the transport properties of one solute are often affected by the presence of the other co-solvents. Water is the main component of many foodstuffs and also the most frequently reported co-solvent. In hydrophilic polymers like the EVOH copolymers, waterinduced plasticization at high moisture levels has been reported to increase the permeability to hydrophobic and apolar solvents like limonene and oxygen.38 However, as described before in the case of the aPAs, the presence of water can also have a positive effect on the barrier properties of the material. Another co-solute whose effect has been widely described in the literature is limonene, the main component of orange juice flavour. The effect of this terpene on the barrier performance of apolar polyolefins is similar to that of water on polar EVOH copolymers. The presence of high concentrations of limonene has been reported to double the permeability of ethyl-butyrate through HDPE and to increase that of ethyl acetate through biaxially oriented polypropylene by up to 40 times.39 The simultaneous transport of a group of co-solvents with similar transport properties has usually been described as a competition between them for the active sites, resulting in the transport of certain compounds being reduced and that of the rest increased.40 However, positive synergistic effects have also been reported, as in the case of toluene/methanol mixtures.41

1.3

Novel polymers and blends

Novel developments in high barrier plastics mainly come from three sources, namely (1) new polymers including biopolymers, (2) polymer blends including nanocomposites, and (3) inorganic coatings such as aluminium obtained by vacuum deposition technologies and oxides (AlOx or SiOx). Polymeric materials


16


for high barrier applications are challenged today by a broad range of stringent property requirements including ease of processing, higher barrier properties to permanent gases, to moisture and to low molecular weight organic compounds, excellent chemical resistance, permselectivity, low relative humidity dependence for the barrier performance, and ease of recycling and biodegradability. Among the novel high barrier polymers that have been more recently developed are materials like the PK copolymers (aliphatic polyketones).42,43 These semicrystalline materials have an outstanding range of mechanical, thermal and high barrier properties (comparable to some EVOH copolymers, see Fig. 1.1), chemical resistance and reduced relative humidity dependence for barrier properties, which give them significant commercial potential in a broad range of engineering, barrier packaging, fibre and blend application. Another novel, extremely high barrier material that has been recently developed is polyglycolic acid (PGA). This biodegradable polymer is claimed to have very low O2 and CO2 permeabilities, one hundredth that of PET (see Fig. 1.2). Additionally, and as opposed to EVOH and PVOH, the barrier properties of commercial PGA resins are said to be largely insensitive to humidity conditions, making it ideally suited for a variety of beverage and perishable food packaging applications.44 Another family of resins that have been recently developed and are currently making their way into the market are the amorphous vinyl alcohol resins (AVOH).45 Water-soluble but melt-compoundable AVOH is said to have, in addition to excellent gas barrier properties and good chemical resistance compared to PVOH and EVOH, superior extrusion properties, orientability, shrinkability and transparency. This polymer can be used in all extrusion processes such as melt-spinning, oriented film, transparent container and injection, and because it is biodegradable, it lends itself to a variety of applications such as new packaging materials that reduce the burden on the environment.

1.2 OTR/WVTR of some polymers vs. the properties claimed for PGA.



17

Aromatic polyamides such as Ny-MXD6, i.e. polyamide resins produced from meta-xylenediamine and adipic acid, are currently being considered in packaging applications since they provide a transparent high gas barrier at high humidity properties (see Fig. 1.2 and Chapter 9) and can be functionalized to achieve oxygen scavenging properties. Another new range of promising materials that have already been developed and in some cases marketed with success in packaging applications are a number of resins derived from biomass and, therefore, to a higher or lower extent easily biodegradable or compostable.6,46 Among these materials, it is possible to find (1) polymers synthesized from bio-derived monomers such as polylactic acid resins (PLA); (2) polymers produced directly by microorganisms like PHAs, bacterial cellulose, etc.; and (3) polymers extracted directly from biomass such as polysaccharides (plant cellulose, starch, chitosan), proteins (soy protein, gluten, zein) and lipids. These biopolymers can have excellent barrier properties to gases such as for instance plasticized chitosan, although their barrier performance is dramatically reduced in the presence of moisture. However, other polymers like PLA and PHAs have relatively good water barrier properties and their relatively good oxygen barrier, lower than for PET, is largely insensitive to moisture sorption. So in principle, one could devise a bio-based derived high barrier multiplayer system where an inner layer of plasticized chitosan could be sandwiched between high moisture barrier PLA or PHA layers. An interesting property of some of these bio-based polymers, e.g. PLA and starch, is that their permeability to carbon dioxide compared to oxygen (permselectivity) is higher than that of most conventional mineral oil based plastics. This is, for instance, of interest for some food packaging applications where a high barrier to oxygen is required, but CO2 generated by the product should be allowed to exit the package headspace to avoid package swelling. These materials, however, still suffer from high production costs compared to polyolefins but are now competitive with, for instance, PET. An interesting development based on cellulose has been recently published.47 In this study, softwood and hardwood celluloses were oxidized by 2,2,6,6tetramethylpiperidine-1-oxyl radical (TEMPO)-mediated oxidation. The TEMPO-oxidized cellulose fibres were converted to transparent dispersions in water, which consisted of cellulose nanofibres 3±4 nm in width. Films derived from this material were seen to consist of randomly assembled nanofibres, were transparent and flexible, and had extremely low coefficients of thermal expansion caused by the high crystallinity. Moreover, the oxygen permeability of a polylactic acid (PLA) film drastically decreased by a factor of about 750 by forming a thin layer of the cellulose material on the PLA film. Hydrophobization of the originally hydrophilic films was achieved by treatment with alkylketene dimer. Blending polymers is a feasible route to accessing the desired balance of properties by controlling the polymer phase interaction and/or the morphology


18


1.3 Modelling of oxygen permeability for various dispositions of EVOH/aPA blend components facing the transport of oxygen gas and as a function of the volume fraction of EVOH. Experimental data (see arrow) for 80/20 EVOH/PA and EVOH/ionomer melt-mixed blends recently developed in our labs are also provided.

in monolayer barrier systems.48 The most commonly used case is to blend polymers with other polymers that have higher barrier properties. The barrier properties of these blends seem to follow a relationship (see equation 1.10) in good general agreement with that proposed by Maxwell and extended by Roberson (see equation 1.1049) for spheres of a low oxygen barrier phase (aPA in Fig. 1.3), but with higher water resistance, dispersed in a high oxygen barrier (EVOH in Fig. 1.3) continuous matrix which has a lower water resistance.50 This simple model would appear to closely reflect, albeit with a slight positive deviation (due to orientation, see Fig. 1.3), the case of the dispersed morphology found for this EVOH/PA blend. The EVOH/ionomer blend even presents a considerably better barrier than is predicted from equation 1.10 due to the fact that the morphology of the particles is elongated (higher aspect ratio) in the machine direction and normal to the permeation direction. PaPA 2PEVOH ÿ 2VaPA PEVOH ÿ PaPA 1:10 PEVOH=aPA PEVOH PaPA 2PEVOH VaPA PEVOH ÿ PaPA The permeability of blends following the above equation would then approach the permeability of a co-extruded multilayer (see equation 1.11) system comprising two layers, one made of a lower barrier disperse phase and the other of a high barrier matrix; therefore, the overall permeability will be close to the



19

permeability of the neat high barrier matrix for a sufficiently high volume fraction of the matrix (VEVOH). Equation 1.11 presents a very favourable situation in terms of permeability for a non-miscible blend. PEVOH=aPA

PEVOH PaPA VaPA PEVOH VEVOH PaPA

1:11

The circles on the graph in Fig. 1.3 represent the values of permeability obtained by application of a simple additive rule (layers parallel to permeant flow: see equation 1.12). This case would clearly represent a very unfavourable situation in terms of permeability for blends. PEVOH=aPA PEVOH VEVOH PaPA VaPA

1:12

Figure 1.3 shows, as an example, some modelling for the barrier properties of EVOH/aPA blends as a function of blend composition and the orientation of the blend constituents in relation to the direction of oxygen transport. High barrier blends of EVOH with an ionomer and an amorphous polyamide have also been developed.30,31 These blends show excellent barrier properties to gases compared to neat EVOH (see experimental values for EVOH 80/20 blends in Fig. 1.3), and yet much better thermoformability than EVOH alone for the production of thermoformed multilayer rigid food containers. Curiously, the EVOH/aPA blends, that under dry conditions present a lower barrier to oxygen, when submitted to typical packaged food water vapour sterilization (at 120ëC for 20 minutes) processes, have a better oxygen barrier than EVOH due to the decreased water sensitivity of the system. There are also a relatively large number of blends reported in the literature in which a high gas barrier polymer like EVOH was added to improve the barrier properties of a low gas barrier material and, conversely, in which a high water barrier polymer is added to a high gas barrier material to reduce relative humidity dependence in the barrier properties of the latter. In a recent paper, a PVOH-based interpolymer complex stabilized by hydrogen bonding with enhanced gas barrier was reported.51 Thus, hydrogen bonding between poly(methyl vinyl ether-co-maleic acid) (PMVE±MA) and PVOH resulted in films with lower oxygen transmission rates (OTR) than pure PVOH. In the range 20±30% (w/w) PMVE±MA, complexation between the two polymers was maximized. The improved oxygen barrier properties were believed to result from a combination of the relatively intact PVOH crystalline regions and a higher degree of hydrogen bonding in the amorphous regions of the PVOH and PMVE±MA films. This leads to denser amorphous regions that reduce the rate of gases diffusing through the polymer film, hence reducing oxygen permeability. Some other successful blending routes are achieved by blending PET with polyamides. Thus, in a recent study52 PET was blended with an aromatic


20


polyamide, either poly(m-xylylene adipamide) (Ny-MXD6) or a copolyamide based on Ny-MXD6 in which 12 mol% adipamide was replaced with isophthalamide (Ny-MXD6-12I). Incorporating a small amount of sodium 5-sulfoisophthalate into the PET matrix was needed to compatibilize the blends and was seen to reduce the polyamide domain size to 100±300 nm. Blending PET with 10 wt% Ny-MXD6 or Ny-MXD6-12I reduced oxygen permeability of PET by a factor of about 0.8 (P/PPET) when measured at 43% relative humidity (RH), in accordance with the Maxwell model prediction. However, after biaxial orientation, oxygen permeability of blends with 10 wt% Ny-MXD6 was reduced by 0.3 at 43% RH, and permeability of blends with 10 wt% Ny-MXD6-12I was reduced by 0.4. Even at 85% RH, oxygen permeability was reduced by 0.4 and 0.6 for blends with Ny-MXD6 and Ny-MXD6-12I, respectively. The blends were even more effective in reducing carbon dioxide permeability of oriented PET. Transformation of spherical polyamide domains into platelets of high aspect ratio was thought to cause the barrier increase. The platelet aspect ratio predicted by the Nielsen model was confirmed by atomic force microscopy. The higher aspect ratio of Ny-MXD6 domains was ascribed to a lower Tg compared to Ny-MXD6-12I. More interestingly, similar reduction in oxygen permeability was achieved in bottle walls blown from PET blends with Ny-MXD6 or NyMXD6-12I. A very interesting blending technique with high potential is the `layer multiplying co-extrusion' technique, which enables the production of layered films with tens to thousands of alternating layers of two or three different polymers with individual layer thicknesses in the 10 nm to 100 m range and various arrangements.53 Using this technology, polymers with widely dissimilar solid state morphologies and properties can be combined into unique layered and gradient structures. Micro- and nanolayers with up to 4096 layers and individual layer thicknesses less than 20 nm have been successfully produced with the technology. As the layer thickness approaches the micro- and nanometre length scales, useful and interesting changes in gas transport, mechanical and optical properties occur. This technology therefore offers an attractive approach for creating designed architectures from particulate-filled polymers such as alternating filled/unfilled layers with varying thickness and composition. Coupling of carefully chosen inorganic/organic barrier systems with multilayering technology offers the potential for generating tens or hundreds of individual, high aspect ratio barrier domains through which oxygen, carbon dioxide, water vapour or any permeant of interest would have to traverse. Finally, inorganic coatings or nanocoatings such as metallized layers, silicon oxide (SiOx) and aluminium oxide (Al2O3) layers are also being used or developed to reduce permeability in packaging structures. Thus, coating plastics with vacuum-deposited aluminium seeks to increase barrier properties to gases, moisture and organic vapours, and results in better flexibility, greater consumer appeal and lower environmental impact due to reduction in metal consumption



21

and better recyclability than conventional lamination with aluminium foil.54 On the other hand, the metal coating of polymeric films imposes reductions in flexibility, stretchability and thermoformability compared to the performance of the polymer films alone. SiOx coatings possess highly desirable properties, such as transparency, recyclability, retortability and microwave use, and are superior in these regards to the thin metal (generally aluminium-based) coatings currently employed commercially on various polymer substrates. For the SiOx coatings to compete effectively against more established, as well as concurrently emerging barrier technologies, they must demonstrate time and temperature stability and promote substantially reduced oxygen and water vapour permeability. Recent studies of SiOx coatings produced by different processing routes have, in fact, shown that these criteria are usually satisfied. One of the benefits of SiOx coatings lies in the flexibility by which they can be deposited on polymer surfaces. Thus far, sputtering, electronbeam deposition, and plasma-enhanced chemical vapour deposition (PECVD) have all been utilized successfully to produce SiOx barrier coatings on polymer substrates. Of these methods, the last one has become the most popular due to its operational ease and efficacy.55 Thin aluminium oxide (Al2O3) layers have also been considered as high barrier coatings and were trialled on various uncoated papers, polymer-coated papers and boards and plain polymer films using the atomic layer deposition (ALD) technique.56 This study demonstrated that such ALD-grown Al2O3 coatings efficiently enhanced the gas-diffusion barrier performance of the studied porous and non-porous materials against oxygen, water vapour and aromas.

1.4 Nanocomposites Over the last few years there has been a significant increase in the number of research works devoted to enhancing relevant polymer properties, mainly mechanical and barrier properties, but also surface hardness, control released, active and intelligent functionalizations, UV±Vis (ultraviolet±visible light) protection, thermal stability and fire retardancy, in existing polymers by means of nanotechnology. Nanotechnology is by definition the creation and utilization of structures with at least one dimension in the nanometre length scale, typically below 100 nm, that creates novel properties and phenomena otherwise not displayed by either isolated molecules or bulk materials. Among the various existing nanotechnologies available such as metallic antimicrobial and UV light protecting nanoparticles,57 carbon nanotubes and nanofibres,58 the very recently developed grapheme-based materials,59 cellulose nanowhiskers,60 electrospun nanofibres and nanocapsules,61 the one that has attracted more attention in the food packaging field is the use of inorganic layered nanoclays. It has been broadly reported in the scientific literature that the addition of low loadings of nanoclay particles, with thickness in the nanometre scale and with high aspect ratios, to a raw polymer to generate the so-called


22


1.4 Typical modelling examples of permeability reductions in nanocomposites as a result of the application of the Nielsen and Fricke models to layered particles.

nanocomposites can have a profoundly enhancing effect over some material properties, such as mechanical properties, thermal stability, UV±Vis protection,62 active properties, conductivity and gas and vapour barrier properties. Figure 1.4 shows typical modelling examples of permeability reductions in nanocomposites as a result of the application of the Nielsen and Fricke models to layered particles. The model of Nielsen63 (see equation 1.13), and other ulterior refinements such as that of Fredrickson and Bicerano,64 describe systems in which the layered, i.e. thin, flat and squared, particles are perfectly oriented with length and width perpendicular to the permeant transport direction and are homogeneously diluted in the polymer matrix: 1 ÿ Vclay Pnano Pneat 1 L=2W Vclay

1:13

In the above equation, L=W is the aspect ratio of the platelets, Vclay is the volume fraction of the clay filler, Pnano is the permeability of the nanocomposite, and Pneat is the permeability of the pure material. A more realistic system to consider is one in which a discontinuous lowpermeability phase is present in a high-permeability matrix. Maxwell developed a model to describe the conductivity of a two-phase system in which permeable spheres are dispersed in a continuous permeable matrix.50 Fricke extended Maxwell's model to describe the conductivity of a two-phase system in which permeable ellipsoids are dispersed in a more permeable continuous matrix.65



23

This model describes the conductivity of a two-phase system in which lower permeability elongated ellipsoids (Pd) are dispersed in a more permeable continuous matrix (Pm). According to this model, the permeability of a composite system consisting of a blend of the two materials in which the dispersed phase (2 is the volume fraction of the dispersed phase) is distributed as ellipsoids can be expressed as follows:48 P

Pm Pd F 1F

where F

1:14 2

3

6 7 2 1 6 7 4 5 P 1 ÿ 2 d 1 1 ÿ M ÿ1 Pm

1:15

M cos =sin 3 ÿ 12 sin 2 and cos W =L where W is the dimension of the axis of the ellipsoid parallel to, and L the dimension perpendicular to, the direction of transport, and is in radians. In this regard, gas and water vapour permeabilities have been found to decrease, in some cases, to a large extent in the nanocomposites due to, among other factors, increased tortuosity factors.66 For example, an EPDM±clay nanocomposite with a 4 wt% loading was found to decrease N2 permeability by 30% compared to EPDM alone.67 Oxygen permeability decreased by a factor of 3 in polyester±clay nanocomposites at 2.5 wt% loading. A 60% reduction in the water permeability was measured in a 5 wt% loaded poly(vinyl alcohol)/sodium montmorillonite nanocomposite and the material still retained its optical clarity.68 In EVOH nanocomposites, reductions in oxygen permeability of more than 70%, over a range of relative humidity values, have been reported69,70 and reductions in water permeability beyond 90% in some proteins and polysaccharides have also been reported.71 Table 1.3 reports the interesting behaviour of EVOH nanocomposites containing a recently developed kaolinite-based grade complying with food contact legislation,72 in which the oxygen permeability reduction due to the nanoclay is higher with increasing relative humidity with minimum impact on transparency. EVOH resins are known to be strongly sensitive to moisture sorption and hence EVOH nanocomposites are the only efficient technology that can overcome this drawback while retaining transparency and film integrity. Additionally, a higher retorting, i.e. humid heat sterilization resistance is observed in EVOH nanocomposites compared to EVOH alone (see Fig. 1.5). This may have considerable implications in retortable packaged foods, where thick layers of hydrophobic


24


Table 1.3 PO2 of extruded films of EVOH29 and of EVOH29 nanocomposites as a function of relative humidity Material

PO2 (cm3mm)/(m2day)

EVOH29 EVOH29 EVOH29 containing 4 wt% nanoclay EVOH29 containing 4 wt% nanoclay

4.2 (50% RH) 1470.6 (90% RH) 3.0 (50% RH) (28% reduction) 427.8 (90% RH) (71% reduction)

polymers are needed to protect EVOH from significant barrier and structural deterioration. In fact, reducing the water sensitivity of EVOH by blending without significant losses in transparency, with higher barrier properties and with enhanced retorting resistance can only be achieved, to the best of our knowledge, by the nanocomposites technology. Moreover, nanocomposites containing specific nanoclays can also be used as UV-light barrier materials for protection of UV-sensitive packaged products.73 A very recent development is the use of nanoclays as carriers of novel functionalizations such as for the controlled release of antimicrobials, antioxidants and oxygen scavengers of value in, for instance, active food packaging technologies.74,75 Notwithstanding the above, in general, the experimentally measured reductions in permeability have not been in full agreement with the values expected from modelling work for most systems, due to lack of complete exfoliation, insufficient compatibility, morphological alterations, solubility effects and other factors.

1.5 Retorting (humid heat sterilization) resistance experiments at 120ëC for 20 minutes of similar food packaging multilayer systems containing in the intermediate layer (a) pure EVOH and (b) an EVOH nanocomposite with 4 wt% nanoclay.



1.5

25

Future trends

Great efforts have been made by researchers in multidisciplinary fields over the last decades to develop new, high-performance polymeric materials or novel technological solutions for existing materials. The overall objective has been to extend the shelf-life of packaged foodstuffs, retaining or even enhancing their quality and safety attributes. The technological `holy grails' have been both (1) to procure glass-tight barrier performance and to make plastics more functional and versatile while retaining their positive attributes, and (2) to provide property-tailoring solutions for the newly developed and poorly performing renewable and biodegradable first generations of biopolymeric resins. To do so, new materials, but more importantly selected nanotechnology and functionalization tools, have been implemented from simple research ideas into fully functional commercial applications. In the years to come, new nanomaterials and functionalities with property-balancing capacity will continue to make their way from research centres across application fields into the food packaging area to additionally provide more efficiency for innovative food packaging strategies such as emerging preservation, active, bioactive and intelligent technologies. Thus, several cutting-edge nanotechnologies and novel functionalities are currently being trialled by an increasing number of material manufacturers and packaging converters. Nevertheless, for their wide commercial implementation and success they need to comply with current and future legislation and be specifically designed to reach specific targets in materials and properties. It is also clear that there is still a lot of missing information in the food packaging sector regarding their use and potentialities in finished articles and we, the authors and editors, really hope that this book can help steer the mind of the readers towards filling this gap.

1.6

References

1. G. Bureau, J.L. Multon, editors (1995). Food Packaging Technology, Vols 1 and 2, VCH Publishers, New York. 2. A.L. Brody, K.S. Marsh, editors (1997). The Wiley Encyclopedia of Packaging Technology, 2nd edn, John Wiley & Sons, New York. 3. J.F. Hanlon, R.J. Kelsey, H.E. Forcinio (1998). Handbook of Package Engineering, 3rd edn, Technomic Publishing Co., Lancaster, PA. 4. Permeability and Other Film Properties of Plastics and Elastomers (1995). PDL Handbook Series, Plastics Design Library, New York. 5. M.D. Sanchez-Garcia, E. Gimenez, J.M. LagaroÂn (2007). J. Plastic Film and Sheeting, 23, 133±148. 6. K. Petersen, P.V. Nielsen, M.B. Olsen (2001). Starch, 53, 356. 7. A. LoÂpez-Rubio, J.M. LagaroÂn, M.J. Ocio (2008). Active polymer packaging of nonmeat food products. In Smart Packaging Technologies for Fast Moving Consumer Goods, ed. J. Kerry and P. Butler, John Wiley & Sons, Chichester, UK, pp. 19±32. 8. A. LoÂpez-Rubio, R. Gavara, J.M. LagaroÂn (2006). Bioactive packaging: turning


26

9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44.

Multifunctional and nanoreinforced polymers for food packaging foods into healthier foods through biomaterials. Trends Food Sci. Tech., 17, 567± 575. S.K. Young, G.C. Gemeinhardt, J.W. Sherman, R.F. Storey, K.A. Mauritz, D.A. Schiraldi, A. Polyakova, A. Hiltner, E. Baer (2002). Polymer, 43(23), 6101±6114. I. Olabarrieta, D. ForsstroÈm, U.W. Gedde, M.S. Hedenqvist (2001). Polymer, 42, 4401±4408. J. Crank, G.S. Park (1968). Diffusion in Polymers, Academic Press, New York. Z. Zhang, I.J. Britt, M.A. Tung (1999). J. Polym. Sci.: Part B: Polym. Phys., 37, 691. M.A. Samus, G. Rossi (1996). Macromolecules, 29, 2275. G. Rossi (1996). Trends Polym. Sci., 4, 337. R.J. Hernandez, J.R. Giacin, E.A. Grulke (1992). J. Membrane Sci., 65, 187. D.W. Van Krevelen (1990). Properties of Polymers, Elsevier, New York. R.Y.F. Liu, D.A. Schiraldi, A. Hiltner, E. Baer (2002). J. Polym. Sci., Part B: Polym. Phys., 40, 862. M.H. Cohen, D. Turnbull (1959). J. Chem. Phys., 31, 1164. D. Turnbull, M.H. Cohen (1970). J. Chem. Phys., 52, 3038. H. Fujita (1961). Fortschr. Hochpolym. Forsch., 3, 1. K. Tanaka, T. Kawai, H. Kita, K. Okamoto, Y. Ito (2000). Macromolecules, 33, 5513. K. Ito, Y. Saito, T. Yamamoto, Y. Ujihira, K. Nomura (2001). Macromolecules, 34, 6153. J.M. LagaroÂn, E. Gimenez, J.J. Saura (2001). Polym. International, 50, 635. R.H. Boyd (1979). Polym. Eng. Sci., 19, 1010. M. Hedenqvist, A. Angelstok, L. Edsberg, P.T. Larsson, U.W. Gedde (1996). Polymer, 37, 2887. J.M. LagaroÂn, S. Lopez-Quintana, J.C. Rodriguez-Cabello, J.C. Merino, J.M. Pastor (2000). Polymer, 41, 2999. B. Neway, M.S. Hedenqvist, V.B.F. Mathod, U.W. Gedde (2001). Polymer, 42, 5307. S Aucejo, R. Catala, R. Gavara (2000). Food Sci. Technol. Int., 6, 159±164. J.M. LagaroÂn, A.K. Powell, J.G. Bonner (2001). Polymer Testing, 20/5, 569. J.M. LagaroÂn, E. Gimenez, R. Gavara, J.J. Saura (2001). Polymer, 42, 9531. J.M. LagaroÂn, E. Gimenez, J.J. Saura, R. Gavara (2001). Polymer, 42, 7381. J.M. LagaroÂn, E. Gimenez, R. Catala, R. Gavara (2003). Macrom. Chem. Phys., 202(4). L. Nicolas, E. Drioli, H.B. Hopfenbergand, D. Tidone (1977). Polymer, 18, 1137. J. Brandrupand, E.H. Immergut (1989). Polymer Handbook, 3rd edn, John Wiley & Sons, New York. Q. Zhou, K.R. Cadwallader (2004). J. Agric. Food Chem., 52, 6271. G.W. Halek, J.P. Luttmann (1991). In Food Packaging Interactions 2, ed. J.H. Hotchkiss, ACS, Washington, DC. A.R. Berens, H.B. Hopfenberg (1982). J. Membrane Sci., 10(2±3), 283. Z. Zhang, I.J. Britt, M.A. Tung (2001). J. Appl. Polym. Sci., 82, 1886. T.J. Nielsen, J.R. Giacin (1994). Packaging Technol. Sci., 7(5), 247. J. Letinski, G.W. Halek (1992). J. Food Sci., 57(2), 481. C. Gagnard, Y. Germain, P. Keraudren, B. Barriere (2004). J. Appl. Polym. Sci., 92, 676. J.G. Bonner, A.K. Powell (1997). Proc. 213th National American Chemical Society Meeting, ACS Materials Chemistry Publications, Washington, DC. J.M. LagaroÂn, A.K. Powell (2000). Patent WO 0042089. http://www.kureha.com/pdfs/Kureha-KUREDUX.pdf



27

45. http://www.g-polymer.com 46. C.J. Weber, V. Haugaard, R. Festersen, G. Bertelsen (2002). Food Additives and Contaminants, 19, 172. 47. H. Fukuzumi, T. Saito, T. Iwata, Y. Kumamoto, A. Isogai (2009). Biomacromolecules, 10, 162±165. 48. D.R. Paul, C.B. Bucknall, editors (2000). Polymer Blends, Volume 2: Performance, John Wiley & Sons, New York. 49. H.B. Hopfenberg, D.R. Paul (1978). In Polymer Blends, ed. D.R. Paul and S. Newman, Academic Press, New York. 50. J.M. LagaroÂn, E. Gimenez, V. Del-Valle, B. Altava, R. Gavara (2003). Macromolecular Symposia, 198, 473. 51. P.W. Labuschagne, W.A. Germishuizen, S.M.C. Verryn, F.S. Moolman (2008). Eur. Polym. J., 44, 2146±2152. 52. Y.S. Hua, V. Prattipatia, S. Mehtab, D.A. Schiraldia, A. Hiltnera, E. Baera (2005). Polymer, 46, 2685±2698. 53. M. Gupta, Y. Lin, T. Deans, E. Baer, A. Hiltner, D.A. Schiraldi (2010). Macromolecules, 43, 4230±4239. 54. R.S.A. Kelly (1992). I+D Packaging Conference, Sevilla, Spain. 55. A.G. Erlat, R.J. Spontak, R.P. Clarke, T.C. Robinson, P.D. Haaland, Y. Tropsha, N.G. Harvey, E.A. Vogler (1999). J. Phys. Chem. B, 103, 6047±6055. 56. T. Hirvikorpi, M. VaÈhaÈ-Nissi, T. Mustonen, E. Iiskola, M. Karppinen (2010). Thin Solid Films, 518, 2654±2658. 57. A. Travan, C. Pelillo, I. Donati, E. Marsich, M. Benincasa, T. Scarpa, S. Semeraro, G. Turco, R. Gennaro, S. Paoletti (2009). Biomacromolecules, 10(6), 1429±1435. 58. M.D. Sanchez-Garcia, J.M. LagaroÂn, S.V. Hoa (2010). Comp. Sci. Technol., 70(7), 1095±1105. 59. T. Ramanathan, A.A. Abdala, S. Stankovich, D.A. Dikin, M. Herrera-Alonso, R.D. Piner, D.H. Adamson, H.C. Schniepp, X. Chen, R.S. Ruoff, S.T. Nguyen, I.A. Aksay, R.K. Prud'homme, L.C. Brinson (2008). Nature Nanotechnology, 3, 327±331. 60. M.D. Sanchez-Garcia, J.M. LagaroÂn (2010). Cellulose, 17, 987±1004. 61. A. Fernandez, S. Torres-Giner, J.M. LagaroÂn (2009). Food Hydrocolloids, 23(5), 1427±1432. 62. M.D. Sanchez-Garcia, J.M. LagaroÂn (2010). J. Appl. Polym. Sci., 118(1), 188-199. 63. L.E. Nielsen (1967). Models for the permeability of filled polymer systems. J. Macromol. Sci. (Chem.), A1, 929±942. 64. G.H. Fredrickson, J. Bicerano (1999). Barrier properties of oriented disk composites. J. Chem. Phys., 110, 2181±2188. 65. M. Krook, G. Morgan, M.S. Hedenqvist (2005). Barrier and mechanical properties of injection molded montmorillonite/polyesteramide nanocomposites. Polym. Eng. Sci., 45, 136±140. 66. R.K. Bharadwaj, A.R. Hehrabi, C. Hamilton, C. Trujillo, M. Murga, R. Fan, A. Chavira, A.K. Thompson (2002). Structure-property relationships in cross-linked polyester-clay nanocomposites. Polymer, 43, 3699. 67. A. Usuki, A. Tukigase, M. Kato (2002). Polymer, 43, 2185. 68. K.E. Strawhecker, E. Manias (2000). Chem. Mater., 12, 2943. 69. J.M. LagaroÂn, D. Cava, L. Cabedo, R. Gavara, E. Gimenez (2005). Food Additives and Contaminants, 22(10), 994±998. 70. L. Cabedo, E. GimeÂnez, J.M. LagaroÂn, R. Gavara, J.J. Saura (2004). Polymer, 45/15, 5233±5238. 71. J.M. LagaroÂn, E. Gimenez, M.D. SaÂnchez-GarcõÂa, M.J. Ocio, A. Fendler (2007).


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72. 73. 74. 75.

Multifunctional and nanoreinforced polymers for food packaging Food Contact Polymers, Rapra Conference Proceedings, Chapter 19, ISBN 978-184735-012-1. www.nanobiomatters.com J.-M. LagaroÂn-Cabello, M.D. Sanchez-Garcia, E. Gimenez-Torres (2009). Patent WO/2009/065986. M.A. Busolo, P. Fernandez, M.J. Ocio, J.-M. LagaroÂn (2010). Food Additives and Contaminants: Part A, 27(11), 1617±1626. M.A. Busolo, A. Aouad, J.-M. LagaroÂn (2010). Conference Proceedings, ANTEC2010, 2044±2047.

1.7

Appendix: Abbreviations

aPA AVOH EPDM EVOH HDPE LCP LDPE LLDPE Ny-MXD6 PA PA6 PAN PC PCL PE PET PGA PHA PK PLA PMMA PMVE±MA PP PS PVC PVDC PVOH

Amorphous polyamide Amorphous vinyl polymers Ethylene propylene diene monomer Ethylene±vinyl alcohol copolymers High density polyethylene Liquid crystal polymer Low density polyethylene Linear low density polyethylene Aromatic polyamide, poly(m-xylylene adipamide) Polyamide Polyamide 6 (Nylon) Polyacrylonitrile Polycarbonate Polycaprolactone Polyethylene Polyethylene terephthalate Polyglycolic acid Polyhydroxyalkanoates Aliphatic polyketone copolymers Polylactic acid Polymethyl methacrylate Poly(methyl vinyl ether-co-maleic acid) Polypropylene Polystyrene Polyvinyl chloride Polyvinylidene chloride Polyvinyl alcohol


2

Multifunctional nanoclays for food contact applications Â N and M .-A . B U S O L O , Novel Materials and J.-M. L A G A R O Nanotechnology Group, IATA-CSIC, Spain

Abstract: This chapter introduces a novel type of nanomaterials based on nanoclays, which provide in addition to the well-known benefits associated with the reinforcing effect of layered nanoclays, the capacity to deliver active new functionalities to packaging materials. More specifically, it is shown how active metals or their compounds can be nanoscaled and stabilized on the surface of nanoclays to provide antimicrobial and oxygenscavenging capacity while being able to nicely disperse within packaging polymers to deliver both enhanced physical performance and active functionalities. Key words: active packaging, antimicrobials, nanoclays, nanotechnology, oxygen scavengers.

2.1 Introduction There is a current trend to incorporate into packaging materials active agents that will maintain and enhance the quality and safety of packaged goods. These concepts are generally termed active packaging technologies. Thus, active packaging has been defined as a system in which the product, the package and the environment interact in a synergistic manner to extend shelf-life or to achieve some characteristics that cannot be obtained otherwise.1±6 Among these, antimicrobial performance and oxygen scavengers are two of the most desired functionalities in plastic packaging. The main aim of active packaging is thus to respond to changes in the conditions of packaged foods in order to extend packaged product shelf-life. This practice can improve food safety and sensorial properties, while maintaining the quality of packaged foods. Active packaging techniques for preserving or even improving the quality and safety of foods can be divided into three classes: (1) absorbing systems; (2) releasing systems; and (3) other speciality systems for temperature, ultraviolet light and microwave control systems.7 Active packaging materials that can absorb or release active compounds for enhancing the quality and safety of a wide range of foods during extended storage are particularly important.


32


Traditionally, active technologies have been commercially implemented within separate sachet units, but there is now more interest in integrating them within the packaging material to enhance functionality and design.8 For example, the active substances in the polymer permit the absorption of oxygen, control the concentration of carbon dioxide or ethylene, stabilize temperature, control the release of ethanol or antioxidant or antimicrobial substances, and control the humidity and the growth of microorganisms.3,9 Antimicrobial activity can be realized by adding AM agents to a packaging system during manufacture or by using AM polymeric materials.10 The absorption systems remove the essential factors of microbial growth from the food and inhibit the growth of microorganisms. The immobilization systems are not intended to release AM agents and hence limit the biocide action to microorganisms existing at the contact surface. The release systems allow the migration of the AM agent (to the liquid or gas phase) into the food or the headspace inside the package to inhibit the growth of microorganisms. Whereas a gaseous AM agent can penetrate through any space, a solute AM agent cannot migrate through the air space between the food and the packaging material. The release kinetics of packaging systems are typically studied by measuring the release rate of the AM agent into a food simulant or by measuring the effectiveness in inhibiting microbial growth and extending the shelf-life of foods. Controlled-release packaging is thus a new generation of packaging materials that can release active compounds at different controlled rates suitable for enhancing the quality and safety of foods during extended storage. The substances that are being considered for inclusion in release packaging are, among others, nutrients, antimicrobials, antioxidants, enzymes, flavours and nutraceuticals. The antimicrobial substances in the release packaging permit the gradual migration to the food during storage and use. These technologies are very effective in minimizing the superficial contamination of the foods and for that reason the application of this antimicrobial packaging to foods like meat, fruits and vegetables is very attractive. The antimicrobial substances used in food packaging that can migrate to the food should be food additives and need to comply with the new legislation related to active and intelligent packaging.11 As was introduced above, oxygen scavengers also constitute one of the more interesting àctive packaging' technologies as they contribute to keeping the optimal concentration of oxygen inside the packaging in order to preserve the quality (appearance, smell, taste and texture) and prolong the shelf-life of oxygen-sensitive products. An excess of oxygen in packaging can cause undesirable changes in foods such as fat oxidation or growth of bacteria and moulds. Oxygen molecules can remain in the packaging headspace as well as permeate through the packaging film, hence reducing the product shelf-life. Unlike traditional or passive packages, which cannot remove or reduce the oxygen present, the use of active packaging with oxygen scavengers can reduce the oxygen concentration to



33

levels below 0.01%, maintaining those levels during food storage.5 The use of oxygen-free atmospheres in food packaging has to be designed with caution, as anaerobic microorganisms can now break out, leading to potentially serious safety issues. In general, oxygen-scavenging commercial technologies make use of iron powder oxidation; however, a minority of systems are based on ascorbic acid oxidation, catechol oxidation, metallic salts and photosensitive dyes, among others.12 Iron-based scavengers are based on the oxidation of iron into Fe(OH)3: 4Fe + 3O2 + 6H2O ÿ! 4Fe(OH)3 ÿ! 2Fe2O33H2O Iron-based scavenging systems are mostly marketed as sachets (to prevent imparting colour, odour and taste to the food), and more recently some oxygenscavenging laboratory prototype films have been developed by incorporation of commercial iron systems into polymer matrices.5 Considering that the sachets mentioned above have the potential risk of being misused by the consumer and eventually being ingested, as well as the risk of contamination of the product by leakage from the sachet, the use of other types of oxygen-scavenging systems is desirable. The incorporation of active systems into packaging materials allows some advantages such as the potential use with retort packaging, elimination of food product distortion that may occur when a sachet contacts the food, and potential cost savings due to increased production efficiency and convenience. This chapter deals with the introduction of a new nanotechnological toolbox based on the natural dispersability and good properties of nanoclays to impart new active functionalities to plastics and bioplastics of interest in food packaging applications.

2.2

Antimicrobial nanoclays

Nanotechnology in the form of nanocomposites can be designed to control the release of, for instance, antimicrobial natural components from packaging materials. One recent example is the release of natural antimicrobial agents such as thymol and linalool. Thymol is a phenolic monoterpene that has received considerable attention as an antimicrobial agent with very high antifungal activity and very low MIC values13 and as a possible food antioxidant.14 Linalool is another essential oil that has been previously reported to have effective antibacterial15 and antifungal16 properties that would make it suitable for the development of antimicrobial films. The combination of active technologies such as antimicrobials and nanotechnologies such as clay-based nanocomposites can synergistically lead to bioplastic formulations with balanced properties and functionalities for their implementation in packaging applications. As an example of bioactive packaging, the formulation of novel antimicrobial nanocomposites of polycaprolactone (PCL) was presented as a way to control solubility and diffusion of natural biocides such as thymol.17 The


34


2.1 Schematics of the functioning of active nanoclays.

antimicrobial nanocomposites of biodegradable PCL were processed by a solution casting method. The diffusion kinetics of the released biocide were determined by Attenuated Total Reflection Fourier Transformed Infrared (ATRFTIR) spectroscopy. The enhancement of antimicrobial solubility as a result of the presence of the nanoplatelets of mica was possibly due to retention of the apolar biocide agent over the engineered nanofiller surface (see Fig. 2.1). On the other hand, the thymol diffusion coefficient was seen to decrease (from ca. 2.8 10±15 to 1.1 10±15 m2/s) with the addition of the nanoadditive in the biocomposite. This is probably the result of the larger tortuosity effect imposed on the diffusion of the biocide by the dispersed nanoclay. As a result, the incorporation of nanoclays led not only to enhancing the solubility of natural biocides into polymeric matrices but also to controlling the release of natural antimicrobials with interest in the design of novel active antimicrobial film and coating systems. With the exposure of the first commercial active packaging materials, certain concerns were raised by authorities, legislators and consumers with respect to the release of chemical antimicrobial agents such as triclosan or other organic molecules from packaging to, for instance, foods. For this reason, there has been a strong push towards the development of natural antimicrobial technologies derived from mineral, plant or animal sources.18±20 Besides the use of natural extracts, silver is a mineral with very efficient biocide properties known since ancient times. The use of silver-based antimicrobial additives for plastics used in food production and medical equipment is today permitted and regulated.21



35

Thus, silver nanoparticles as well as silver compounds are widely used as efficient biocides. In fact, many commercial antimicrobial products include silver in their formulations as the active ingredient. In this context, many products have been developed for specific applications in quite different areas, i.e. medical devices, liquid disinfectants for large surfaces, personal care products, electronics, food and water storage materials to extend shelf-life, etc. Recent technical innovations and findings facilitate the availability and incorporation of silver products in a wide range of materials at the manufacturing stage, providing novel antimicrobial formulations. Nevertheless, a specific form of efficient silver does not exist for every application, procedure or matrix. In this sense, nanotechnology is becoming a key factor due to the capability of modulating metals, compounds and materials into the nanosize, which often changes their chemical, physical and optical properties, as well as those of the matrices in which they are incorporated. Stable silver nanoparticles can be obtained by using soluble starch as both the reducing and the stabilizing agent22 or by being synthesized via the regular borohydride reduction of Ag+ ions.23,24 Silver nanoparticles were synthesized in the interlamellar space of kaolin by UV radiation or chemical-induced reduction,25,26 in layered laponite suspensions via photoreduction,27 or supported on micro and mesoporous structures after ion exchange followed by in situ reduction.28,29 Silver(I) nitrate adducts with diverse electronic and steric characteristics can be synthesized with N- and P-donor ligands.30 Thus, Ag/SiO2 coating solutions have been prepared to serve for antimicrobial refinement of temperaturesensitive materials like fabrics or wood.31 Moreover, a suspension of silver nitrate in an ammonium salt medium has been reported as a precursor of stable nanoscale AgBr particles.32 In another line of work, many efforts have also been made to develop inorganic materials, such as zeolites, for supporting Ag+ ions due to their ability to incorporate and release ionic species. Coleman et al.33 prepared Ag+- and Zn+-exchanged tobermorites and demonstrated that they have a marked bacteriostatic effect and can be potentially used as antimicrobial materials for in situ bone tissue regeneration. The thermal stability of Ag+-supported È lkuÈ.34 clinoptilolite and possible applications were tested by Akdeniz and U Some reports based on silver-modified clays by a cation exchange method have been published. Oya et al.35 reported the antimicrobial properties of Ag+exchanged montmorillonite in 1991. Keller-Besrest et al.36 prepared a silverloaded montmorillonite for possible topic uses in the treatment of burns. They obtained the coexistence of both silver metal particles and Ag+ ions, and they also observed significant differences in the final silver content in clays using an exchange resin procedure (of up to 10 wt%) with regard to the standard cation exchange capacity (CEC) methodology done in solution (of up to 1 wt%). Quintana et al.37 studied the effects of calcination and mechanical grinding on silver-exchanged Na-MMT and its antimicrobial performance. They reported


36


metallic silver nanoparticles on the clay aggregates, and pointed out that the antibacterial performance is affected by the availability of the ionic silver to be in contact with the bacteria. Praus et al.38 compared the antimicrobial activities of some chemical compounds, silver ions and elemental silver immobilized on montmorillonite. They demonstrated that antibacterial compounds are effective just when they are released from the inorganic carrier, and they concluded that intercalated silver ions are the most effective antibacterial elements while elemental silver does not show any antibacterial effects. In any case, silver species provide colour when incorporated into inorganic carriers and are rather unstable against temperature. However, a recent new patented technology39 that makes use of silver strongly stabilized on nanoclays either in the elemental nanoform or in ionic form (see Fig. 2.2) and that is aimed at dispersion in food contact plastic has been developed, which has been scaled up and is commercially available under the trademark of BactiblockÕ (NanoBioMatters Ltd, Paterna, Spain). This is a white powder material, heat stable and readily dispersable in all kinds of plastics with strong biocide capacity at low dosages (see Table 2.1). Regarding nanobiocomposites, the value of this technology was additionally demonstrated in PLA films.40 From the results, the silver-based nanoclay showed a strong antimicrobial effectiveness against Gram-negative Salmonella spp. with minimum inhibitory concentration and minimum bactericide concentration below 1 mg per 10 ml. PLA nanobiocomposites with different antimicrobial nanofiller loadings were trialled by casting or by melt compounding, showing excellent optical properties. An improved barrier to water was measured for the nanobiocomposites due to the presence of the nanoclay, which also exhibited strong antimicrobial performance (see Table 2.2). In this context, the European Food Safety Authority (EFSA) has recently evaluated the use of several silver-based substances intended to come into contact with foods, and defined a general specific migration limit of 0.05 mg of silver per kg of food (EFSA Journals). The

2.2 Typical TEM pictures of commercial BactiblockÕ nanoclays (a) containing elemental silver nanoparticles and (b) with ionically exchanged silver.



37

Table 2.1 Typical dosages (%) of active BactiblockÕ in nanocomposites of various plastic materials required to overcome the standards JIS Z 2801 or ISO 22196:2007 for antimicrobial performance in surfaces Biocide dosage required to comply with standard JIS Z 2801 or ISO 22196:2007 Thermosets Epoxy based Polyester based

1% 3%

Thermoplastics Polypropylene Polyethylene Polystyrene Polycarbonate

0.5% 0.5% 0.5% 1%

Elastomers EVA

1%

Coatings Solvent based

1%

Source: Unpublished results by the authors.

Table 2.2 Viable cell counts before and after 24 h incubation in antimicrobial activity tests and water permeability of PLA-BactiblockÕ nanocomposite Sample Control without film PLA control film PLA-BactiblockÕ nanocomposite film

Initial CFU/mL

CFU/mL after 24 h incubation

WVTR (g m/m2 s Pa)

2:0 105 2:0 105 2:0 105

4:7 108 6:6 108 3:5 102

± 1:90 10ÿ14 1:28 10ÿ14

study of Busolo et al.40 also proved that those levels of permitted migration can be sufficient to exert strong biocide performance.

2.3

Oxygen-scavenging nanoclays

As mentioned above, supporting scavenging systems on nanoclays is a convenient strategy to develop new materials with multiple functionalities. An example of this technology based on iron is presented below.39 The incorporation of iron into nanoclays has been reported before for several applications such as water treatment and remediation processes41 and for the removal of aqueous Cu2+ and Co2+ ions in waste.42 Iron in organomodified montmorillonite has been previously prepared for the production of flameretardant materials,43 and iron nanoparticles were synthesized in the presence of


38


2.3 Headspace (40 ml) %O2 reduction as a function of time caused by 1.5 g of commercial scavenging technologies.

montmorillonite as an effective protective reagent and support.44 In spite of this, there are many difficulties associated with developing iron-based systems that can lead to efficient oxygen-scavenging materials and that disperse well into packaging plastics with minimum impact on optical and mechanical properties. A feasible proprietary technology that does so, marketed under the trademark of O2BlockÕ (NanoBioMatters Ltd, Paterna, Spain) and based on nanoclays containing iron, results in a highly plastics-dispersable nanomaterial that produces a strong decrease in the headspace oxygen concentration.45 As an example, Fig. 2.3 shows the variation of oxygen content in the headspace of vials containing two commercial scavenging systems as a function of time. Taking into account that oxygen-sensitive products deteriorate relatively quickly, the kinetics of oxygen depletion may become very important, especially in the early stages, but in some other cases it may not be advisable to consume oxygen completely (see later). 46 Regarding this, the O2BlockÕ nanoclay-based grade reported in the study seems to act somewhat more slowly compared to the very efficient commercial sachet material. The reason is that the sachet most likely contains a higher mass fraction of the scavenging principle. Figure 2.4 shows the results for a LDPE containing 5 wt% of an O2BlockÕ grade, indicating that a significant reduction in oxygen content occurs in the nanocomposite. An even higher reduction in the oxygen headspace concentration was also reported in a PLA-FeMMT nanocomposite film.45 In this study, it was seen that a reduction in the oxygen content from 20.9% to 6.8% was seen to occur after six days in solution casting films. In a similar experiment, the commercial sachet system AgelessÕ reduced the oxygen content to 0.5%. Nevertheless, it is relevant to note that the commercial scavenging system AgelessÕ contains ca. 2.8 g of solid inside each sachet, this being mostly elemental iron. Considering that the mass of the nanocomposite film evaluated



39

2.4 Oxygen-scavenging capacity of 1.5 g of LDPE-O2Block nanocomposite film in 40 ml headspace.

in the scavenging tests was 1.7 g, of which only 10 wt% corresponded to the iron-based clay that in turn contained ca. 25 wt% of Fe, the active material equivalent in each vial was only ca. 0.04 g. This means that if equivalent quantities of the active component were to be used the efficiency of the nanocomposite should have been higher compared to the sachet. As a result, these nanocomposites, once they are optimized for the purpose and tailored for specific packaging materials and applications, should provide great interest in the packaging of oxygen-sensitive products.

2.4

Future trends

In summary, the addition of active (antioxidant, antimicrobial, oxygenscavenging, etc.) layered engineered silicates complying with food contact regulations to biodegradable polymers through innovative technology is now available as a formidable tool for improving the properties of polymers and biopolymers and, therefore, to enhance packaged food quality and safety aspects. The fact that these technologies have become commercially available makes them even more interesting for their widespread implementation. Thus, with the advent of this new generation of nanomaterials providing multiple functionalities, i.e. combined physical reinforcement and active performance, to plastics, the plastic packaging field becomes consolidated in its own right as a high-tech area of development.

2.5

References

1. Miltz, J., Passy, N. and Mannheim, C.H. (1995). Trends and applications of active packaging systems. In: Food and Packaging Materials ± Chemical Interaction, Ackerman, P., JaÈgerstad, M. and Ohlsson, P. (eds), The Royal Society of Chemistry, London, pp. 201±210.


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2. Yam, K.L., Takhistov, P.T. and Miltz, J. (2005). Intelligent packaging: concepts and applications. Journal of Food Science, Concise Reviews and Hypotheses, 70, R1± R10. 3. Vermeiren, L., Devlieghere, F., Van Beest, M., de Kruijf, N. and Debevere, J. (1999). Developments in the active packaging of foods. Trends in Food Science and Technology, 10, 77±86. 4. Suppakul, P., Miltz, J., Sonneveld, K. and Bigger, S.W. (2003). Active packaging technologies with emphasis on antimicrobial packaging and its applications. Journal of Food Science, 68, 408±442. 5. LoÂpez-Rubio, A., LagaroÂn, J.M. and Ocio, M.J. (2008). Active polymer packaging of non-meat food products. In: Smart Packaging Technologies for Fast Moving Consumer Goods, Kerry, J. and Butler, P. (eds), John Wiley & Sons, Chichester, UK, pp. 19±32. 6. Ahvenainen, R. (2003). Active and intelligent packaging. In: Ahvenainen, R. (ed.), Novel Food Packaging Techniques, Woodhead Publishing, Cambridge, pp. 5±21. 7. Han, J.H. (2003). Antimicrobial food packaging. In: Ahvenainen, R. (ed.), Novel Food Packaging Techniques, Woodhead Publishing, Cambridge, pp. 50±70. 8. Appendini, P. and Hotchkiss, J.H. (2002). Review of antimicrobial food packaging. Innovative Food Science & Emerging Technologies, 3, 113±126. 9. Gennadios, A., Hanna, M.A. and Kurth, L.B. (1997). Application of edible coatings on meats, poultry and seafoods: a review. Lebensmittel-Wissenschaft und -Technologie, 30, 337±350. 10. Hotchkiss, J.H. (1997). Food packaging interactions influencing quality and safety. Food Additives and Contaminants, 14, 601±607. 11. Commission Directive 2002/72/EC for Food Contact Applications (EFSA), http:// www.efsa.europa.eu/ 12. Ahvenainen, R. (2002). Novel Food Packaging Techniques. CRC Press, Boca Raton, FL, pp. 27±30. 13. Thompson, D.P. (1989). Fungitoxic activity of essential oil components on food storage fungi. Mycologia, 81, 151±153. 14. Youdim, K.A. and Deanes, S.G. (2000). Effect of thyme oil and thymol dietary supplementation on the antioxidant status and fatty acid composition of the ageing rat brain. Journal of Nutrition, 83, 87±93. 15. Onawunmi, G.O., Yisak, W.A. and Ogunlana, E.O. (1984). Antibacterial constituent in essential oil of cymbopogon citratus. Journal of Ethnopharmacology, 12, 279± 286. 16. Reuveni, R., Fleischer, A. and Putievsk, E. (1984). Fungistatic activity of essential oils from Ocimum basilicum. Journal of Essential Oil, 110, 20±22. 17. Sanchez-Garcia, M.D., Ocio, M.J., Gimenez, E. and LagaroÂn, J.M. (2008). Novel polycaprolactone nanocomposites containing thymol of interest in antimicrobial film and coating applications. Journal of Plastic Film and Sheeting, 24(3±4), 239±250. 18. Sun Lee, D. (2005). Packaging containing natural antimicrobial or antioxidative agents. In: Han, J.H. (ed.), Innovations in Food Packaging, Part 2, Elsevier, New York, pp. 108±122. 19. Fernandez-Saiz, P., Ocio, M.J. and LagaroÂn, J.M. (2006). Biopolymers, 83, 577±583. 20. Sanchez-Garcia, M.D., Gimenez, E., Ocio, M.J. and LagaroÂn, J.M. (2008). Technical Papers, Regional Technical Conference, Society of Plastics Engineers, 4, 2084± 2088. 21. Simpson, K. (2003). Plastics, Additives and Compounding, 5, 32. 22. Vigneshwaran, N., Nachane, R.P., Balasubramanya, R.H. and Varadarajan, P.V.



23. 24. 25. 26. 27. 28. 29. 30.

31. 32. 33. 34. 35. 36. 37.

38. 39.

41

(2006). A novel one-pot `green' synthesis of stable silver nanoparticles using soluble starch. Carbohydrate Research, 34, 2012±2018. Lok, C., Ho, C., Chen, R., He, Q., Yu, W., Sun, H., Kwong-Hang Tam, P., Chiu, J. and Che, C. (2007). Silver nanoparticles: partial oxidation and antibacterial activities. Journal of Biology and Inorganic Chemistry, 12, 527±534. Oh, S.G., Lee, G.J., Shin, S.I. and Kim, I.C. (2004). Preparation of silver nanorods through the control of temperature and pH of reaction medium. Materials Chemistry and Physics, 84, 197±204. DeÂkaÂny, I., Patakfalvi, R. and OszkoÂ, A. (2003). Synthesis and characterization of silver nanoparticle/kaolinite composites. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 220, 45±54. DeÂkaÂny, I. and Patakfalvi, R. (2004). Synthesis and intercalation of silver nanoparticles in kaolinite/DMSO complexes. Applied Clay Science, 25, 149±159. Huang, H. and Yang, Y. (2007). Preparation of silver nanoparticles in inorganic clay suspensions. Composite Science and Technology, 68(14), 2948±2953. Yang, X., Yang, L., Wang, X. and Yang, F. (2008). Excellent antimicrobial properties of mesoporous anatase TiO2 and Ag/TiO2 composite films. Microporous and Mesoporous Materials, 114, 431±439. Lv, L., Luo, Y., Ng, W.J. and Zhao, X.S. (2009). Bactericidal activity of silver nanoparticles supported on microporous titanosilicate ETS-10. Microporous and Mesoporous Materials, 120, 304±309. Pettinari, C., Di Nicola, C., Effendy, Marchetti, F., Skelton, B.W. and White, A.H. (2007). Synthesis and structural characterization of adducts of silver(I) nitrate with ER3 (E = P, As, Sb; R = Ph, cy, o-tolyl, mes) and oligodentate aromatic bases derivative of 2,2-bipyridyl, L, AgNO3:ER3:L (1:1:1). Inorganica Chimica Acta, 360, 1433±1450. Mahltig, B., Gutmann, E., Meyer, D.C., Reibold, M., Bund, A. and BoÈttcher, H. (2009). Thermal preparation and stabilization of crystalline silver particles in SiO2based coating solutions. Journal of Sol-Gel Science and Technology, 49, 202±208. Zhang, J., Liu, X., Luo, X., Lu, S., Cao, W. (2007). A novel cetyltrimethyl ammonium silver bromide complex and silver bromide nanoparticles obtained by the surfactant counterion. Journal of Colloid and Interface Science, 307, 94±100. Coleman, N.J., Bishop, A.J., Booth, S.E. and Nicholson, J.W. (2009). Ag+- and Zn2+Ê tobermorites. Journal of the exchange kinetics and antimicrobial properties of 11A European Ceramic Society, 29, 1109±1117. È lkuÈ, S. (2008). Thermal stability of Ag-exchanged clinoptilolite Akdeniz, Y. and U rich mineral. Journal of Thermal Analysis and Calorimetry, 3, 703±710. Oya, A., Banse, T., Ohashi, F. and Otani, S. (1991). An antimicrobial agent derived from montmorillonite. Applied Clay Science, 6, 135±142. Keller-Besrest, F., BeÂnazeth, S. and Souleau, C. (1995). EXAFS structural investigation of a silver-added montmorillonite clay. Materials Letters, 24, 17±21. Quintana, P., MaganÄa, S.M., Aguilar, D.H., Toledo, J.A., Angeles-Chavez, C., CorteÂs, M.A., LeoÂn, L., Freile-PelegrõÂn, Y., LoÂpez, T. and Torres SaÂnchez, R.M. (2008). Antibacterial activity of montmorillonites modified with silver. Journal of Molecular Catalysis A: Chemical, 281, 192±199. Praus, P., MalachovaÂ, K., PavlõÂcÏkovaÂ, Z. and TuricovaÂ, M. (2009). Activity of antibacterial compounds immobilised on montmorillonite. Applied Clay Science, 43, 364±368. LagaroÂn, J.M., Busolo, M. and Fernandez-Saiz, P. (2010). Patent application ES2331640.


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40. Busolo, M.A., Fernandez, P., Ocio, M.J. and LagaroÂn, J.M. (2010). Novel silverbased nanoclay as an antimicrobial in polylactic acid food packaging coatings. Food Additives and Contaminants, 27(11), 1617±1626. 41. Frost, R.L., Xi, Y. and He, H.J. (2009). Colloid Interface Science, doi: 10.1016/ j.jcis.2009.09.027. È zuÈm, C., ErogÏlu, A.E., Hallam, K.R., Scott, T.B. and Lieberwirth, I. 42. Shahwan, T., U (2009). Applied Clay Science, 43, 172±181. 43. Wei, Q., Cai, Y., Wu, N., Zhang, K., Xu, Q., Gao, W., Song, L. and Hu, Y. (2008). Surface and Coating Technologies, 203, 264±270. 44. Yuan, P., Fan, M., Zhu, J., Chen, T., Yuan, A., He, H., Chen, K. and Liu, D. (2009). Journal of Magnetism and Magnetic Materials, 321, 3515±3519. 45. Busolo, M.A. and Lagaron, J.M., (2010). ANTEC 2010 Conference Papers, SPE Publications, Society of Plastics Engineers, Newtown, CT. 46. Miltz, J. and Perry, M. (2005). Packaging Technology and Science, 18, 21±27.


3


U . C O S T A N T I N O and M . N O C C H E T T I , University of Perugia, Italy and G . G O R R A S I and L . T A M M A R O , University of Salerno, Italy

Abstract: This chapter deals with the preparative methods, structural aspects and chemical±physical characteristics of hydrotalcite-like compounds (HTlc), an emerging class of layered solids with anion exchange and intercalation properties. Biocompatible HTlc can be modified with molecular anions having pharmaceutical, antimicrobial or antioxidant activity to obtain materials that can release the active anions in different environments with a de-intercalation process. Moreover, the organic±inorganic hybrids can exfoliate when dispersed in polymeric matrices and act as active fillers of biocompatible and biodegradable polymers. The fillers could enhance the mechanical and barrier properties of the polymer and confer on it biological activity for application in food packaging, particularly in active packaging technologies and in biomedical devices. Key words: biocompatible hydrotalcite-like compounds (HTlc), intercalation of biologically active species in HTlc, modified release of drugs and active species, exfoliation of modified HTlc in biocompatible polymers, modified HTlc as active fillers of nanobiocomposites.

3.1

Introduction

Hydrotalcite is the name of a rare mineral discovered in Sweden around 1842. Its chemical formula proposed by Manasse (1915) is magnesium aluminium hydroxycarbonate, Mg6Al2(OH)16CO34H2O, while its layered structure was elucidated independently by Allmann (1968) and Taylor (1969). For a long time hydrotalcite and other isomorphous minerals (i.e. piroaurite, sjogrenite and takovite) were mainly the object of mineralogical studies, but starting from the 1970s it was realized that these rare minerals, called also anionic clays, can be easily and economically prepared on a laboratory scale and have a number of interesting chemical properties (Miyata and Kumura, 1973; Miyata, 1980, 1983). The materials obtained were named hydrotalcite-like compounds (HTlc) or layered double hydroxides (LDH) and are generally represented by the empirical formula [M(II)1±xM(III)x(OH)2]x+[An±x/n]x±mH2O where M(II) and M(III) are bi- and trivalent metal cations with suitable ionic radius, A is the interlayer


44


exchangeable anion with charge ÿn, x is the molar ratio M(III)/[M(III) + M(II)] which ranges between 0.2 and 0.4, and m is the mol of co-intercalated water (Cavani et al., 1991; TrifiroÁ and Vaccari, 1996). It was also realized that a large number of materials with different properties can be obtained by changing the nature of the divalent and trivalent cations, and the type of interlayer molecular anions opening the way for a wide range of applications. At present, HTlc find application as heterogeneous catalysts, support of catalysts (Cavani et al., 1991; Turco et al., 2004; Busca et al., 2006; Costantino et al., 2008a), adsorbents, anion exchangers, anion scavengers (Newman and Jones, 1998; PreÂvot et al., 2001; Khan and O'Hare, 2002), components and/or active principles in pharmaceutical and cosmetic formulations (Costantino and Nocchetti, 2001; Carretero et al., 2007; Choy et al., 2009a) and additives of polymeric blends (Leroux and Taviot-GueÂho, 2005; Evans and Duan, 2006; Costantino et al., 2009a). Recent progress concerns modification of HTlc by intercalation of functional species bearing anionic groups (i.e. carboxylate, phosphonate and sulfonate). Among these species, the following may be mentioned: (1) dyes and chromophors to produce new materials with photochemical and photophysical properties (Ogawa and Kuroda, 1995; Bauer et al., 2003; Latterini et al., 2007); (2) drugs and anions with biological activity and even biomolecules to obtain systems for drug release and for biomedical applications, whenever biocompatible HTlc are used as layered hosts (Choy et al., 2000, 2009b; Hwang et al., 2001; Desigaux et al., 2006; Costantino et al., 2008b); and (3) anions having hydrophobic or hydrophilic tails to render HTlc layers compatible with different polymeric chains and produce novel nanofillers of polymeric nanocomposites (Xu et al., 2004; Costantino et al., 2009a; Xu and Braterman, 2010). This last application is typical of some inorganic layered materials that, when dispersed at low loading (less than 5%) in polymeric blends, are able to exfoliate into single layers each having a thickness of the order of 1 nm, the surface of each layer being functionalized, by ion exchange or grafting reactions, with organic groups that increase the compatibility with the polymers. In addition these layered solids may intercalate polymeric chains into their interlayer regions. In this context, much work has been reported on the use of organically modified smectite clays, in particular montmorillonites, as fillers of polymeric composites, while scarce attention has been paid to anionic clays of hydrotalcite type (Camino et al., 2001; Costantino et al., 2005, 2007; Leroux, 2006; Illaik et al., 2008; Costa et al., 2008; Nyambo et al., 2008; Kovanda et al., 2010). These latter materials compare favourably with natural clays in terms of purity, wellknown stoichiometry, higher ion exchange capacity, and a wider possibility of functionalization with a variety of organic anions generally much more numerous than organic cations, commonly involved in the modification of smectite clays. When biocompatible HTlc modified with organic anions possessing biological activity are exfoliated and homogeneously dispersed in biocompatible



45

and, if possible, biodegradable polymers, an interesting new class of nanobiocomposites is obtained. In these systems the active molecules, fixed by ionic bonds to the inorganic lamellae, can not only improve the compatibility with the polymeric matrix but also carry out the biological activity (i.e. pharmaceutical, antimicrobial or antioxidant) being anchored to the lamellae, or being slowly released in particular environments. The modified HTlc nanofillers thus provide active release systems, simultaneously improving the mechanical and barrier properties of the biopolymer. The present chapter will be concerned mainly with the preparation, characterization, properties and potential use of these new nanobiocomposites. It is divided into three parts: the first part will recall the structural aspect, the preparative methods and the reactivity of HTlc; the second part will deal with the properties of intercalation compounds of biocompatible HTlc with anions having biological activity; and the third part will show the preparation and properties of nanobiocomposites with biodegradable polymers. The chapter will close with a commentary and future trends.

3.2

Hydrotalcite-like compounds (HTlc): basic chemistry

In the last two decades there has been a rapid growth in the number of scientific papers and industrial patents on HTlc, because of their broad possibility of manipulation to obtain materials of interest in many different fields that involve physics and physical chemistry, chemistry and industrial chemistry, medicinal chemistry, pharmaceutical technology and cosmetics. The rich harvest of information obtained has been collected in monographs and reviews to which the reader is referred for a study in depth (Rives, 2001; Braterman et al., 2004; Duan and Evans, 2006; Williams and O'Hare, 2006; Latterini et al., 2007; Perioli et al., 2008; Choy et al., 2009a; Costantino et al., 2009a). However, to make the present contribution self-consistent, in the following sections the fundamental aspects of composition, structure, preparative methods, morphology and thermal behaviour will be recalled.

3.2.1

Composition and structural aspects of HTlc

As already pointed out, this emerging class of compounds, also known as layered double hydroxides or anionic clays, gathers natural and synthetic layered solids commonly represented by the general formula [M(II)1±xM(III)x(OH)2] [Ax/n]mH2O, where M(III) cations are typically Al, Cr, Fe or Ga, M(II) can be Mg, Zn, Ni, Co or Cu, and A is an anion of ionic valence n. The cations have an ionic radius similar to that of Mg2+ (0.065 nm) and prefer the octahedral coordination. Therefore, despite the nature of the cations present, the structure of these compounds is similar to that of hydrotalcite mineral, having composition


46


3.1 (a) View along the ab crystallographic axis of the brucite (Mg(OH)2) sheet; (b) Schematic representation of the packing of four sheets of the MgAl±HTlc in carbonate form.

[Mg0.75Al0.25(OH)2](CO3)0.1250.5H2O. The hydrotalcite structure is clearly described by considering that of brucite, Mg(OH)2, arising from the packing of layers built up of Mg(OH)6 octahedral units with shared edges (see Fig. 3.1a). In the mineral, 25% of Mg(OH)6 units of the brucite layer are substituted by Al(OH)6 octahedral units, the excess of positive charge being balanced by carbonate anions accommodated in the interlayer region (Taylor, 1973). In a similar way, the structure of hydrotalcite-like compounds originates from the packing of brucite layers containing M(II) cations, partially replaced by M(III) cations, surrounded by six OH± ions. Note that the notation M(II) may indicate the presence of more than one type of divalent cation and M(III) of more than one trivalent cation, but the molar ratio x M(III)/[M(III) M(II)] should remain confined between 0.2 and 0.4. Figure 3.1b shows, as an example, the sequence of four layers of a Mg±Al HTlc in which x is equal to 0.33. The presence in the layer of M(III) cations gives rise to positive electrical charges balanced by exchangeable anions (An±) accommodated in the interlayer region, where m mol of water for formula weight are also located. The x value determines the charge density of the layers and hence the ion exchange capacity (IEC) of the materials (Costantino et al., 1998). The IEC is much higher than that of smectite clays and, obviously, depends also on the empirical formula, generally ranging between 2 and 4 mmol of monovalent anion per gram. In natural compounds the brucite-type sheets can stack one to another with two different symmetries, one is rhombohedral (3R) with an ABC ABC . . . stacking sequence, and is typical of pyroaurite mineral (see Fig. 3.1b); the other symmetry is hexagonal (2H) with an AB AB . . . stacking sequence, and is typical of the sjogrenite phase (Taylor, 1973). On the other hand, structural analyses and refinements reported by several authors showed that synthetic HTlc



47

3.2 Rietveld plot for [Mg0.67Al0.33(OH)2](CO3)0.1650.48H2O. Experimental (), calculated (ÿ) and difference (lower) profiles. Inset shows the XRD patterns.

crystallize in the 3R symmetry, although a change in stacking sequence to the 2H polytype has been observed for a Zn±Al HTlc upon dehydration at 150ëC (Hines et al., 2000). Early structural determinations were carried out on natural single crystals. Synthetic HTlc are obtained as a microcrystalline powder (see later) not suitable for single crystal structure analysis, and crystal data have been recently obtained with an X-ray powder diffraction method in which the ab initio crystal structure is refined with the Rietveld procedure. By way of example, Fig. 3.2 and Table 3.1 report the Rietveld refinement of a Mg±Al HTlc Table 3.1 Crystallographic data for [Mg0.67Al0.33(OH)2](CO3)0.1650.48H2O Crystal system Space group ab c

V Z Density Rwp (background subtracted) Rp (background subtracted) RF2

Trigonal* R-3m 0.304535(9) nm 2.2701(1) nm 120ë 0.18232(1) nm3 3 2.12 g/cm3 10.37 7.98 5.56

* Numerals in parentheses represent the standard deviation of the crystallographic data obtained from the retrieved refinement.


48


Table 3.2 Structural parameters of indicated HTlc Sample [Mg0.67Al0.33(OH)2](CO3)0.1650.48H2O [Zn0.67Al0.33(OH)2](CO3)0.1650.51H2O [Co0.68Al0.32(OH)2](CO3)0.160.52H2O [Ni0.68Al0.32(OH)2](CO3)0.160.52H2O [Zn0.52Al0.37Cu0.11(OH)2](CO3)0.1760.47H2O [Ni0.52Zn0.18Al0.30(OH)2](CO3)0.150.55H2O [Ni0.55Mg0.13Al0.32(OH)2](CO3)0.160.52H2O

a (nm)

c (nm)

V (nm3)

0.30454(1) 0.30748(1) 0.30738(1) 0.30749(1) 0.30728(1) 0.30564(1) 0.30622(1)

2.2701(1) 2.2769(1) 2.2840(1) 2.3707(1) 2.2686(1) 2.3148(1) 2.3763(1)

0.18233(1) 0.18642(1) 0.18689(1) 0.19413(1) 0.18551(1) 0.18726(2) 0.19297(1)

* Numerals in parentheses represent the standard deviation of the crystallographic data obtained from the retrieved refinement.

(Costantino et al., 1998), while Table 3.2 reports the structural parameters, obtained with this procedure, of several HTlc having different composition. Recently, structural and thermodynamic parameters have been obtained from molecular modelling (MM) procedures using different force field approaches (Lombardo et al., 2005, 2008).

3.2.2

Methods of preparation of HTlc

Traditional, simple procedures used in gravimetric analysis for the precipitation of insoluble metal hydroxides have been suitably modified to obtain synthetic hydrotalcites in carbonate, chloride or nitrate form. The most common procedures concern, in fact, the co-precipitation of the metal ions (at a given concentration and given molar ratio) and the charge-balancing anions dissolved in a solution maintained at room temperature or at 60±80ëC, under vigorous stirring, with a precipitating alkaline solution. The precipitation may be carried out at almost constant pH value, using as precipitating reagent buffer solutions, i.e. a NaHCO3/Na2CO3 solution, or at variable pH by titrating the metal ion solution with NaOH solution. Furthermore, the precipitation may be carried out at a low or high supersaturation degree according to the solution concentration and the rate of addition of the precipitating reagent. To improve the crystalline degree and the particle size, often the precipitate is aged for some days or hydrothermally treated (Cavani et al., 1991; Rives, 2001). Other preparative routes consider the so-called precipitation from a `homogeneous' solution or the sol gel technique. In the first case, a clear solution containing M(II) and M(III) salts (chloride or nitrate) at a concentration of 0.5±1.0 mol/dm3, and with molar ratio M(II)/M(III) ranging from 2 to 3, has urea added (molar ratio of urea/M(III) about 10). The solution is brought to 90± 100ëC under stirring. The urea hydrolysis generates ammonium carbonate and a pH of about 9 that affords the formation of HTlc in carbonate form. Wellcrystallized powders with a narrow distribution of crystal size are generally



49

obtained (Costantino et al., 1998; Adachi-Pagano et al., 2003). The use of esamethylentetramine, which upon hydrolysis generates ammonia, has also been proposed (Choy et al., 2002; Iyi et al., 2004). In the sol-gel technique the M(II) and M(III) sources are alkoxides or acetylacetonates hydrolysed at a given temperature (Prinetto et al., 2000; Paredes et al., 2006), in some instances also in the presence of microwave irradiation (Rives et al., 2006). Recently, for niche application, methods of obtaining HTlc nanocrystals of dimension 50±250 nm have been proposed. Most are based on the control of the two steps of the precipitation process, that is, nucleation (formation of seeds) and crystal growth (ageing) (Choy et al., 2002; Xu et al., 2006; Duan and Evans, 2006; Rives et al., 2006; Okamoto et al., 2006; Liu et al., 2007; Ma et al., 2007; Gunawan and Xu, 2008). This control has been applied to both the coprecipitation and urea methods. In the former case a fast nucleation process is followed by a hydrothermal treatment at a temperature of 100±120ëC for different times. With the increase of time, particles with increasing crystal size and a sufficiently uniform size distribution are obtained. In the urea method, the addition of ethylene glycol and short refluxing times allows one to obtain particles of nanometric dimensions. It is also worth mentioning methods based on the formation of nanoparticles inside the water pool of reverse micellae (O'Hare and Hu, 2005; Hu et al., 2006; O'Hare et al., 2007; Liu et al., 2008). Colloidal dispersions of Mg±Al, Zn±Al and Ni±Al HTlc in bromide form, having dimensions of 50±100 nm, have been prepared with the double water-inoil microemulsions technique, which consists of mixing two microemulsions, one containing the M(II) and M(III) nitrate salt and the other with ammonia as precipitating reagent. Collisions between the two different micellae allow the formation of HTlc nanocrystals inside the water pool (Bellezza et al., 2009a). For the convenience of the reader, Fig. 3.3 summarizes the synthetic procedure discussed above. It should be clear that the co-precipitation methods are the most appropriate for the preparation of large amounts of HTlc fillers for polymer nanocomposites, and the urea methods for producing materials suited to fundamental studies and for pharmaceutical and cosmetic application; whereas methods for the preparation of nanocrystals produce materials that are used in the formation of thin films or as non-viral transfer vectors in cells and cellular tissue (Choy et al., 2009a).

3.2.3

Physical±chemical characterization of HTlc

Hydrotalcite-like microcrystals, once their chemical composition is known, are characterized by means of the most common techniques used in solid state and material chemistry. The solubility of HTlc in water obviously depends on the composition, though they are generally considered insoluble in the pH interval 4±10 in the absence of complexing agents of the metals. The X-ray powder diffraction (XRPD) patterns furnish various important information: (1) whether


50


3.3 Steps involved in HTlc preparation by the double water-in-oil microemulsions technique.

a single phase or more than one phase is present; (2) the pattern may be indexed to have structural information, the intensity of the XRPD reflections being sensitive to the crystalline degree; and (3) the crystallite size along a given direction can be calculated from the broadening at half-height of the corresponding diffraction peaks by using the Debye±Scherrer equation. Fourier transform-infrared (FT-IR) spectroscopy provides information on the bonded water, the presence of hydrogen bonds, the nature of the intercalated anions, and the presence of impurity charge-balancing anions, such as carbonate and nitrate. The FT-IR spectrum may be considered a fingerprint of a given sample. The thermal properties are commonly studied by performing a coupled thermogravimetric±differential thermal analysis (TG-DTA) (Palmer et al., 2009). In certain cases these techniques are associated with an evolved gas analyser or recording high-temperature XRPD patterns for the identification of the thermally induced phase transitions. In general the thermal decomposition of HTlc can be divided into three endothermic stages, the first stage corresponds to the loss of physisorbed and co-intercalated water and occurs between room temperature and approximately 200ëC; the second stage sees the loss of constitutional water because of the dehydroxylation of brucite layers and occurs in the 250±400ëC range; and the third stage corresponds to the elimination of the charge-balancing anion. If organic anions are present and the TG-DTA analysis is performed in air, their combustion is observed with a strong exothermic effect. Often, the second and third stages overlap and at the end of the decomposition process a mixture of M(II) and M(III) oxides is obtained. At 800±1000ëC spinel



51

3.4 TG±DTA curves of [Mg0.67Al0.33(OH)2](CO3)0.165 0.42H2O (operative conditions: heating rate 10ëC/min, air flow).

phases are formed. By way of example, Fig. 3.4 shows the TG-DTA curves of a Mg±Al HTlc in carbonate form. At 1000ëC a mixture of MgO and MgAl2O4 is obtained. The different preparative methods give rise to materials with the same composition but with different specific surface area and morphology of the microcrystals. The surface area is generally calculated from the N2 absorption isotherms obtained at 79 K, according to the B.E.T. method. It depends on composition and crystalline degree. Materials obtained with co-precipitation methods have a surface area (60±100 m2/g) (Yun and Pinnavaia, 1995) higher than that of materials obtained with urea methods (20±40 m2/g) (Costantino et al., 1998). Scanning electron microscopy (SEM) and sometime transmission electron microscopy (TEM) are used to analyse the morphology of the microcrystals. More or less regular platelets of hexagonal shape and dimension of the order of micrometres are generally found, again depending on the preparative methods. HTlc prepared by co-precipitation and aged and/or subjected to hydrothermal treatment show a rather small crystal size, less than 1 m, which is desirable for catalytic application. The urea method generally affords uniform and well crystallized powders of micron order and well-defined hexagonal shape (see Fig. 3.5a). For use as a filler for polymers, large platelet crystals having, when exfoliated, a high aspect ratio, are looked for. Therefore, studies have been published on the control of the crystal size of HTlc obtained with homogeneous precipitation methods (Choy et al., 2002; O'Hare and Hu, 2005; Xu et al., 2006; Evans and Duan, 2006; Rives et al., 2006; Okamoto et al., 2006; Hu et al., 2006; Liu et al., 2007; Ma et al., 2007; O'Hare et al., 2007; Gunawan and Xu, 2008; Liu et al., 2008).


52


3.5 Micrographs of ZnAl±HTlc obtained by (a) urea method and (b) double water-in-oil microemulsions technique.

In addition, several procedures to exfoliate these microcrystals have been developed with a view to their application in nanotechnology (thin films, layerby-layer stacking) (Adachi-Pagano et al., 2000; Hibino and Jones, 2001; O'Leary et al., 2002; Hibino, 2004; Li et al., 2005; Wu et al., 2005; Hibino and Kobayashi, 2005; Jobbagy and Regazzoni, 2006; Jaubertie et al., 2006; Liu et al., 2006). Nanocrystals, when withdrawn from the colloidal dispersion, tend to aggregate, and very interesting nest-like or globular particles are generally observed (see Fig. 3.5b) (Bellezza et al., 2009a). Many other chemical±physical characterizations performed, for example, with XPS and ESCA (electron spectroscopy for chemical analysis) (Lakshmi Kantam et al., 2006; Fang et al., 2010), solid state nuclear magnetic resonance (MAS-NMR) spectrometry (Sideris et al., 2008), UV-vis spectrophotometry, fluorimetry, confocal fluorescence microscopy (Latterini et al., 2007) and impedance bridges to determine the ionic conductance have been reported to study particular properties and correlated applications of HTlc (Costantino et al., 1997; Mignani et al., 2009).

3.3

Organically modified biocompatible hydrotalcite-like compounds (HTlc)

In the previous section the general characteristics and properties of hydrotalcites have been discussed. The present section will deal with techniques of modification and functionalization of HTlc with different anions and, in line with the present contribution, biocompatible Mg±Al or Zn±Al HTlc and molecular anions with biological activity will be considered. Such association gives rise, in fact, to inorganic±organic hybrid materials in which bioactive species are stored in the interlayer region, often protected from light and oxygen, and potentially being released after a chemical signal. These hybrids have been proposed as systems for modified drug release (Costantino and Nocchetti, 2001; Ambrogi et al., 2001, 2002, 2003; Del Arco et al., 2004, 2009; Dupin et al., 2004; Li et al.,



53

2004; Mohanambe and Vasudevan, 2005; Del Hoyo, 2007; Costantino et al., 2008b, 2009a, 2009b; Ay et al., 2009) and vectors for gene delivery. For example, DNA has been intercalated, protected from nuclease degradation and transfected into nuclei (Choy et al., 2000). Intercalation of molecular anions used in pharmaceutical care (emollients, surfactants, skin nutrients, vitamins and sunscreens) produces new materials to be used in cosmetics (Perioli et al., 2006a, 2006b, 2008). Other interesting products have recently been obtained after intercalation of antimicrobial and antioxidant species (Costantino et al., 2009a, 2009c). Especially these latter hybrids, when homogeneously and efficiently dispersed in polymeric film, may find application in the active packaging of food. In the following, as well as discussion on the procedures to modify the biocompatible HTlc, the composition and properties of the obtained hybrids, divided according to the nature of the intercalated molecular anions, will be reported.

3.3.1

Synthetic routes to obtain biocompatible HTlc intercalated with molecular anions with biological activity

Hydrotalcite-like compounds based on Mg±Al and Zn±Al are biocompatible materials reported in different pharmacopeias and already used in medicine as antacid and antipepsinic agents (Lin et al., 1998; Linares et al., 2004; Konturek et al., 2007) and in many ointments and poultices for the protection of damaged skin. However, the most promising aspect of their development is the use of intercalation compounds with drugs or anions with biological activity to obtain sustained release formulations and active fillers of polymers (Sammartino et al., 2006; Tammaro et al., 2007; Costantino et al., 2009b, 2009c). The conversion of the original HTlc, generally obtained in carbonate form, into intercalation compounds with these species is reached with different procedures; the most used are based on anion exchange reaction, reconstruction of calcinated hydrotalcite and co-precipitation. In designing the anion exchange reaction the nature of the counterion originally present in the HTlc should be considered. The diffusion of bulky anionic species into the interlayer region will be facilitated if the counterions originally present have a low affinity for the matrix and determine a large gallery height. If the known selectivity scale, CO32± > SO42± OH± > F± > Cl± > Br± > NO3± > ClO4±, is taken into account (Miyata, 1983), HTlc containing chloride, or better, nitrate anions are to be considered the most suitable precursors for the uptake of biologically active species. Hence, HTlc containing the strongly held carbonate anions should be converted in chloride form by titration with 0.1M HCl at constant pH of 5; moreover, the HTlc±Cl can be equilibrated with an aqueous solution of 0.5M NaNO3 (molar ratio NO3±/Cl± 10) to obtain the nitrate form of the hydrotalcite (HTlc±NO3). The intercalation


54


3.6 (a) Anion exchange isotherms of MgAl±NO3 towards TIAP. Experimental conditions: concentration 0.1M, temperature 25ëC, reaction time 3 days. (b) Xray powder diffraction patterns of the MgAl±HTlc at different exchange percentages of TIAP: (1) 24.2%, (2) 46.8%, (3) 64.9%, (4) 94.1%.

mechanism and the relative selectivity coefficient can be studied both by determining the anion exchange isotherm and by following the structural changes by taking the XRD patterns of samples at different degrees of exchange (Costantino and Nocchetti, 2001). By way of example, Fig. 3.6a shows the anion exchange isotherm of Mg±Al±HTlc±NO3 towards tiaprofenic anion (TIAP), while Fig. 3.6b shows the X-ray diffraction patterns of the Mg±Al±HTlc at different exchange percentages of TIAP. It may be seen that the drug is exchanged with high selectivity and that the ion exchange process occurs with a first-order phase transition from the NO3 phase to the TIAP phase (Costantino et al., 2008b). The reconstruction procedure, typical of Mg±Al±CO3 and in some instances of Zn±Al±CO3, takes advantage of the so-called `memory effect' of the hydrotalcite heated at 300±500ëC. The calcinated solid, consisting of a mixture of magnesium (or zinc) and aluminium oxides, is able to reconstruct the lamellar structure in water or in aqueous solution of given anions (Rey and Fornes, 1992; Rocha et al., 1999). When the regeneration occurs in CO2-free distilled water, the positive charge of the lamellae will be balanced by OH± ions. The interlayer OH± groups can be replaced by other anions via an acid±base reaction with the corresponding species in acid form. Otherwise, the reconstruction should be carried out in a solution containing the guest in acid form in order to have the direct intercalation of the guest. The direct synthesis by co-precipitation requires the precipitation of the HTlc in the presence of the anionic form of the guests. The chloride or nitrate M(II) and M(III) salts are often used and dissolved in a solution containing the selected guest. Co-precipitation is performed at pH between 9 and 10 by addition of NaOH solution. Well-crystallized samples are formed when the guests have a high selfassembly tendency. In other cases a hydrothermal treatment of the obtained intercalates may improve the crystallinity of the products (Reichle, 1986).



55

3.7 Experimental routes to obtain HTlc intercalation compounds.

Figure 3.7 summarizes the experimental routes described above to obtain intercalation compounds.

3.3.2

HTlc hybrids containing anti-inflammatory and antibiotic drugs

Microcrystals of Mg±Al and Zn±Al hydrotalcites have been used as a reservoir of different non-steroidal anti-inflammatory drugs (NSAID) and of some antibiotics to obtain systems able to release the drugs in different biological fluids (Costantino et al., 2008b, 2009a) The chemical nature and reactivity of hydrotalcites allow one to design drug-intercalated layered materials for sustained release of the drug or for improving solubility and bioavailability of poorly soluble drugs. Drug-intercalated HTlc dispersed in biological fluids with pH around 7 can release the guest species via ion-exchange reactions. The release rate is affected by many factors, such as drug shape and size, arrangement of the drug anions into the interlayer region, selectivity of the HTlc towards the anions present in the release medium, and the dimensions of the HTlc particles (Williams and O'Hare, 2006). Moreover, HTlc are not simply acting as delivery matrices, but can also improve the apparent solubility of the drug; indeed, if the intercalation compounds are in a medium at acid pH (less than 4), the matrix slowly dissolves and the drug is released anion by anion in the medium. Moreover, the hydrotalcite matrix showed barrier properties similar to those of gastric mucus and may provide mucosal protection to the side-effect of the drug (GruÈbel et al., 1997). Thanks to the particular interaction of a drug


56


3.8 Structural formulae and acronyms of NSAID and antibiotics used as guests of HTlc.

with the mucus network, its co-administration with hydrotalcite can not only ensure a protective effect but also improve the drug permeability through gastric mucus (Del Arco et al., 2004; Shaw et al., 2005; Perioli et al., 2010b). Drugs belonging to the NSAID class such as ibuprofen (IBU), diclofenac (DIK), indomethacin (IND), ketoprofen (KET), tiaprofenic acid (TIAP) and



57

Table 3.3 Composition, interlayer distance and drug loading of NSAID-intercalated HTlc NSAID

Intercalation compounda

IND KET TIAP DIK IBU FLU

[MgAl]0.33IND0.20Cl0.130.3H2O [MgAl]0.33KET0.27Cl0.060.4H2O [MgAl]0.33TIAP0.27Cl0.060.4H2O [MgAl]0.33DIK0.331H2O [MgAl]0.33IBU0.330.5H2O [MgAl]0.40FLU0.31Cl0.090.8H2O

d (nm)

Drug loading (%)

2.57 2.27 2.27 2.36 2.17 2.42

50.4 50.0 50.3 55.0 50.0 49.3

a

[MgAl]x indicates [Mg1ÿxAlx(OH)2].

flurbiprofen (FLU) have been chosen as guests of Mg±Al±HTlc. The structural formulae of the selected NSAID are reported in Fig. 3.8. These bioactive species, containing carboxylate groups, have been intercalated both by ionexchange reactions, starting from HTlc±Cl, and by reconstruction of the HTlc structure. The best results, in terms of crystallinity and loading of the intercalation compounds, have been obtained with the former procedure. Composition, drug-loading and interlayer distance of the obtained hybrid materials are reported in Table 3.3. The knowledge of drug anion dimensions and shape as well as of the gallery height of the intercalation compounds allows one to predict the arrangement of the guest species into the interlayer region. In general, the drug anions are packed as a monolayer of partially interdigitated anions, with their principal axis at a slanting angle with respect to the layer plane. The ionogenic groups (±COO±) interact with the positive charges of the sheets, and the organic residues point to the interlayer region. As an example, the computer-generated disposition of the TIAP anions into the interlamellar region of MgAl±HTlc is shown in Fig. 3.9. The high tendency of the guest species to aggregate as a compact monofilm justifies the marked preference of the HTlc for these species (see the isotherm of Fig. 3.6a). Intercalation compounds containing DIK and IBU have been submitted to in vitro drug release studies in simulated intestinal fluid at pH 7.5 and in a solution designed to mimic the ionic conditions of the small intestine (pH 7.0) (Ambrogi et al., 2001, 2002). In the intestinal tract the drug released from intercalated product is due to exchange of drug ions with the phosphates, hydroxides and carbonates present in the intestinal medium. HTlc±DIK and HTlc±IBU have shown a sustained drug release; in particular, at pH 7.5 the dissolution rate of DIK from HTlc±DIK was 38% after 15 min, 60% after 90 min and 90% after 9 h; at pH 7.0, the DIK release from HTlc±DIK was 20% after 15 min, 40% after 2 h, 50% after 4 h, up to a maximum of 70% at the end of the experiment (24 h). In order to study the effect of particle size on the drug release rate, Zn±Al nanosized hydrotalcite, with dimensions of about 350 nm, has been used as host


58


3.9 Computer-generated representation of HTlc±TIAP: TIAP ions are arranged in the interlayer region to form a monolayer partially interdigitated, with their principal axis at a slanting angle with respect to the layer plane.

of DIK and submitted to in vitro drug release studies, and its profile has been compared to that obtained from Zn±Al±HTlc±DIK microparticles (2 m), as shown in Fig. 3.10. The release profiles at the higher pH value (7.5) show different guest release times within the first hours (DIK released from nano- and micro-HTlc: 55% and 38% after 15 min, 80% and 53% after 60 min, respectively). The decrease of particle size determines the increase of the crystal edges and of the amount of intercalated species in the nanocrystal external part. Moreover, the diffusion of the anions through the ZnAl nanoparticles is faster than that through the microparticles due to the decrease of the length of the HTlc galleries. The above considerations affirm that the guest release time from HTlc, within the first hour, depends on the particle size. After the burst effect, the nano- and micro-HTlc±DIK profiles are similar (Perioli et al., 2010a). Some NSAID intercalation compounds have been tested to improve the solubility of poorly water-soluble drugs such as INDO, KET, TIAP and FLU. The solubility measurements of drugs from the intercalate were determined in a gastric juice with pH 1.2 (USP 25 at 37ëC) in which the hydrotalcite quickly dissolves, releasing the drug in molecular form promptly suitable for absorption.



59

3.10 DIK release in phosphate buffer at pH 7.5 from nano- and micro-HTlc± DIK intercalation compounds.

The best results were obtained for indomethacin; indeed, the apparent solubility enhancement was seven times higher than that from the crystalline drug (Ambrogi et al., 2003). Recently, good results have been obtained with FLU and the hypoglycemic gliclazide too: an improvement of the drug dissolution rate in gastric medium and of the permeability through gastric mucus has been observed (Ambrogi et al., 2009; Perioli et al., 2010b). The design of formulations able to maintain pharmacologically active drug levels for long periods, avoiding repeated administrations, and to deliver and release the drug in its pharmaceutical target, and of formulations able to improve the apparent solubility of very insoluble drugs, has been extended to some antibiotic and antibacterial species. Drugs having antibacterial activity belonging to the quinolones (Nalidixic acid), fluoro-quinolones (Ciprofloxacin) and -lactam (Amoxicillin) classes and a bacteriostatic antibiotic (choramphenicol hemisuccinate) have been used as guests of HTlc (Costantino et al., 2009a). The structural formulae and acronyms of the selected drugs are shown in Fig. 3.8. The presence of the carboxylic group makes this species suitable to interact with the positive charges of the hydrotalcite lamellae. Intercalation compounds have been obtained by taking advantage of ion-exchange reactions. Because of its high steric hindrance, hydrotalcite in nitrate form has been used as starting material in order to favour the intercalation of the drugs. In fact the low affinity of nitrate anions for the HTlc and the relatively high interlayer distance of the intercalation compound, 0.87 nm, may promote the diffusion of the big drug anions. Table 3.4 shows the compositions, the interlayer distance and the drug loading (worded as grams of drug per 100 g of hybrid) of the intercalation


60


Table 3.4 Composition, interlayer distance and drug loading of antibioticintercalated HTlc Antibiotic

Intercalation compounda

Cfs Nal Cipro Amox

[MgAl]0.35Cfs0.26(NO3)0.091H2O [MgAl]0.33Nal0.29(NO3)0.040.9H2O [MgAl]0.37Cipro0.35(CO3)0.011.2H2O [MgAl]0.33Amox0.13(CO3)0.10.4H2O

d (nm)

Drug loading (%)

2.47 2.11 1.39 1.87

57.0 46.3 58.6 39.5

a


compounds. On the other hand, the very low-soluble Cipro has been intercalated by reconstructing the HTlc structure. In particular, a stoichiometric amount of solid Cipro, in acidic form, has been added to an aqueous slurry of HTlc in OH± form; an acid±base reaction occurs between the intercalated hydroxyl groups and the Cipro, resulting in intercalation of the drug in anionic form. As an example, Fig. 3.11 illustrates the computer-simulated model, obtained with the Hyperchem program, of the probable arrangement of CFS anions in the HTlc±CFS. The model has been obtained on the basis of the dimensions of the guest, the structural data of the host, and the composition and interlayer distance

3.11 Computer-generated representation of HTlc±CFS.



61

of the intercalation compound. The CFS species are arranged into the interlayer region as a bi-film in which the ± interactions between the benzene rings occur. Using ion exchange, many novel hybrids may be synthesized, and beside anti-inflammatory drugs and antibiotics, tranexamic acid (Trx, trans-4(aminomethyl)cyclohexanecarboxylic acid) has been incorporated into HTlc to obtain nanohybrids that can slowly release these active guest molecules (Costantino et al., 2009b). Tranexamic acid, a synthetic derivative of the amino acid lysine, is an antifibrinolytic molecule also used as a haemostatic agent. It acts against breakdown of clots (by inhibiting or stopping plasminogen activation and fibrinolysis), so it is useful in stopping severe blood loss as it increases clot formation. It is also used in surgical procedures and dental extractions for people with haemophilia.

3.3.3

HTlc hybrids containing amino acids and proteins

Several recent papers have reported the intercalation of amino acids, oligopeptides (Aisawa et al., 2001, 2006; Hibino, 2004; Yasutake et al., 2008; Gao et al., 2009) and proteins into hydrotalcite-like compounds. Aromatic and bicarboxylic amino acids have been incorporated into hydrotalcites via ionexchange reactions starting from Mg±Al±HTlc in nitrate form (Costantino et al., 2009a). Table 3.5 shows the composition and the interlayer distance of intercalation compounds with selected amino acids. Note that incomplete intercalation has been obtained and the amino acid amount has always been insufficient to compensate for the sheet positive charges; anions such as hydroxyl or carbonate have been co-intercalated because of the high pH value of the equilibrating solutions (pH = 9), while in other cases some unexchanged nitrate remained. Studies on the preferential intercalation of pure enantiomers have been carried out. In particular, intercalation reactions have been performed starting both from phenylalanine (Phe) racemic solution and from the L-Phe enantiomeric solution. The specific rotatory power of the DL-phenylalanine solution, Table 3.5 Composition and basal spacing (d) of the indicated intercalation compounds (amino acids) dried at 75% of relative humidity Amino acid

Compositiona

DL-Phe

[MgAl]0.32Phe0.22OH0.10.52H2O [MgAl]0.32Tyr0.15OH0.170.2H2O [MgAl]0.32DOPA0.16OH0.160.32H2O [MgAl]0.37Glu0.24(NO3)0.130.34H2O [MgAl]0.37Asp0.25(NO3)0.120.45H2O

DL-Tyr

L-DOPA L-Glu

DL-Asp a



d (nm) 1.73 1.65 1.32 1.24 1.22

62


3.12 Structural model of the Mg±Al±DL±Phe.

after equilibration with the hydrotalcite, was about zero, showing that the Phe was intercalated as DL dimers. Moreover, the composition and the interlayer distance were not affected by the type of enantiomer present in solution. Figure 3.12 reports a tentative structural model of the compound containing DL-Phe, achieved with the program Hyperchem after having optimized the Phe anions with the MM+ force field. The microcrystals seem to be constituted by assembling two lamellae, one bearing the L enantiomers and the other bearing the D enantiomers, so that the Phe anions make an ordered monolayer of interdigitated moieties into the interlayer region. On the other hand, in the Mg± Al±L±Phe compound the L species can be intercalated with high steric hindrance. Studies on the thermal behaviour of Mg±Al±DL±Phe and Mg±Al±L±Phe, performed by thermogravimetric analysis and in situ high-temperature powder diffraction, have shown that upon water loss the Phe species rearrange their disposition into the interlayer region, reaching an interlayer distance of 2.7 nm at 150ëC. FT-IR spectra of the Mg±Al±DL±Phe and Mg±Al±L±Phe recorded before and after thermal treatment at 150ëC are superimposable, suggesting that the samples do not undergo chemical reactions as polymerizations or grafting. The presence of an OH group in the para position on the Phe phenyl ring, to obtain the aromatic amino acid tyrosine (Tyr), is responsible for the air- and photo-instability of the Tyr. The intercalation of Tyr was performed in order to protect the amino acid from oxygen and light. Similarly to the systems containing Phe, intercalation compounds with DL- or L-Tyr have been prepared (Table 3.5). The compositions and the interlayer distances are independent of the type of enantiomer. However, the OH group in the para position on the phenyl ring causes a contraction of the basal spacing with respect to Mg±Al±DL±Phe which is very probably due to the formation of hydrogen bonds between the guest OH



63

groups and the OH groups of the lamellae through hydration water molecules, as already observed in similar systems. It was of interest to investigate the ability of Mg±Al±HTlc to intercalate L-DOPA (L-3,4-dihydroxyphenylalanine), which is structurally similar to the previously studied amino acids and is used as a drug in Parkinson's disease. The catecholic group of the L-DOPA makes these molecules easily oxidizable to quinone. The intercalation was achieved with success at pH 9 and in the presence of hydrazine as antioxidant. More complex bioactive species such as oligopeptides have also been incorporated into Zn±Al±HTlc by coprecipitation reaction (Aisawa et al., 2006). The interfacial behaviour and the adsorption of biological macromolecules such as proteins on solid inorganic surfaces are two of the major interesting topics in the biotechnology area (Gray, 2004). The adsorption of a protein onto a non-biological solid surface is an important phenomenon, not only because it may affect the biological functioning of the macromolecules (Haynes and Norde, 1995) but also because it is the key to several important applications such as artificial implants, protein purification strategies, biosensors, drug delivery systems, catalysts and catalyst supports (Nakanishi et al., 2001; He et al., 2006; Martinez Martinez et al., 2008). Protein adsorption is a complex process involving many events such as conformational changes, hydrogen bonding and/or hydrophobic and electrostatic interactions. Although surface±protein interaction is not well understood, the chemical nature of the surface has been shown to play a fundamental role in protein adsorption (Bellezza et al., 2002, 2006). Proteins adsorb in different quantities, conformations and orientations, depending on the chemical and physical characteristics of both protein and support surfaces. In the biomaterials field, much research has been devoted to methods that modify the size and textural surface of existing materials in order to achieve more desirable biological integration (Caruso, 2001). HTlc has scarcely been exploited for the adsorption of biological macromolecules such as proteins and enzymes at the solid±liquid interface. Recently, delamination/restacking and co-precipitation methods have been employed to immobilize and adsorb several proteins such as porcine pancreatic lipase (PPL), haemoglobin (Hb), bovine serum albumin (BSA) (An et al., 2009; Charradi et al., 2009), urease (Vial et al., 2008), alkaline phosphatase (AlP) (Mousty et al., 2008) and horseradish peroxidase (HRP) (Chen et al., 2008) into HTlc of micrometric size to develop novel biosensors (Mousty, 2010). Many researchers have indicated that an important factor in determining the biological response of solid materials is the particle size. Attention has been focused on nanomaterials, which offer a new pathway for regulating protein behaviour through surface interactions because they can provide large surface areas for efficient protein binding, and multivalent functionalities can be grafted on their surfaces to meet the structural complexity of biomolecules (Katz and Willner, 2004; Bellezza et al., 2005, 2007).


64


The adsorption of myoglobin (Mb) onto Ni±Al±HTlc nanoparticles has been investigated in terms of structural properties and enzymatic activity. The nanostructured biocomposite is active in the oxidation of 2-methoxyphenol by hydrogen peroxide, and the observed enzyme kinetics follow the Michaelis± Menten mechanism. The catalytic turnover (kcat) and the Michaelis constant (KM) values of adsorbed Mb are lower than those of the native protein, while the catalytic efficiency (kcat/KM) of the adsorbed protein is slightly decreased. In order to explain the decrease of Mb catalytic activity, IR, fluorescence and Raman spectra were collected; the adsorption of protein molecules onto the nanoparticle surface alters the tertiary structure without changing the secondary structure. The absence of catalytic activity for Mb adsorbed onto Ni±Al±HTlc prepared with the urea hydrolysis method, together with the low adsorption capacity of these large HTlc particles, is evidence for the importance of the surface dimensions in the modulation of protein activity (Bellezza et al., 2009b).

3.3.4

HTlc hybrids containing antimicrobial and antioxidant species

Recently, increasing attention has been paid to developing and testing films with antimicrobial properties in order to improve food safety and shelf-life. In this context the preparation of inorganic filler organically modified with antimicrobial species has gained academic interest. The obtained hybrids can be finely dispersed into polymeric matrices and can slowly release the active species. Benzoate derivatives having antimicrobial activity, such as benzoate (Bz), 2,4-dichlorobenzoate (BzDC) and para- and ortho-hydroxybenzoate (pBzOH and o-BzOH), have been chosen as guest model species for HTlc (Costantino et al., 2009c). Benzoate and benzoate derivatives are used as food preservatives and show toxicity at very high levels (maximum acceptable daily intake 5 mg/kg body weight). Intercalation compounds have been obtained by an anion exchange procedure starting from the nitrate form of the hydrotalcite and their chemical compositions and interlayer distances are summarized in Table 3.6. It may be noted that the molecular anions replace almost completely the HTlc nitrate counteranions. FT-IR absorption spectroscopy of the Zn±Al±HTlc±Bz sample suggested the presence of a monodentate carboxylate coordination with the brucite-type layer. This experimental information, together with knowledge of the chemical composition, interlayer distance and van der Waals dimensions of the guests, has allowed structural models of the intercalation compounds to be proposed. Generally, the anions are arranged with the Ph±COO± bond almost perpendicular to the layer and form a partially interdigitated monolayer (see Fig. 3.13). However, the position and nature of the aromatic ring substituents affect the gallery height. Guest species that are ortho-substituted (o-BzOH) are



65

Table 3.6 Composition and basal spacing (d) of the indicated intercalation compounds (antimicrobials) dried at 75% of relative humidity Antimicrobial

Compositiona

ZnAl-Bz ZnAl-o-BzOH ZnAl-p-BzOH ZnAl-BzDC

[ZnAl]0.35Bz0.351H2O [ZnAl]0.35o-BzOH0.27(NO3)0.081H2O [ZnAl]0.35p-BzOH0.33(NO3)0.020.85H2O [ZnAl]0.35BzDC0.32(NO3)0.031H2O

a

d (nm) 1.55 1.55 1.53 1.68

[ZnAl]0.35 indicates [Zn0.65Al0.35(OH)2].

3.13 Computer-generated models showing the most probable arrangement of: (left) Bz and (right) p-BzOH anions between the LDH layers.

arranged into the interlayer region like the unsubstituted species (Bz) because the long axis of the guest anions is unvaried. On the other hand, the presence of a para-substituent (BzDC) causes an increase of the BzDC long axis dimension and then an increase of the interlayer distance of about 0.13 nm with respect to the Zn±Al±Bz. However, the nature of the substituent plays a fundamental role in the interlayer distance value. When the para substituent is the OH group, a network of hydrogen bonds between this group and the OH group of the sheet through the hydration water should occur, bringing the lamellae nearer. Indeed, the interlayer distance of Zn±Al±p-BzOH is smaller (1.53 nm) than that of Zn±Al±Bz (1.55 nm). Another very important topic in food stability and human health is the prevention of lipid oxidation. Many studies have been carried out to search for and develop antioxidants having a natural origin to be used in the food industry to delay the oxidation process. Hydroxycinnamic acid (CA) and its derivatives have drawn attention because they are very diffuse in nature and are potent antioxidants. The aim of a recent work has been to intercalate into the Mg±Al± HTlc the anionic form of CA and ferulic acid; in addition the ascorbate (Asc) has also been considered. Intercalation compounds have been used as active fillers of polycaprolactone (see Section 3.4.4).


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Multifunctional and nanoreinforced polymers for food packaging Table 3.7 Composition and basal spacing (d) of the indicated intercalation compounds (antioxidants) dried at 75% of relative humidity Antioxidant

Compositiona

CA Fer Asc

[MgAl]0.39(CA)0.17(NO3)0.220.46H2O [MgAl]0.36(Fer)0.19(NO3)0.170.89H2O [MgAl]0.36(Asc)0.11Cl0.250.29H2O

d (nm) 1.47 1.73 1.28

a


The anions cinnammate (CA) and ferulate (Fer) have been intercalated via ion exchange starting from Mg±Al in nitrate form, while the Asc has been intercalated in the presence of hydrazine as antioxidant and using the chloride form of the HTlc (Costantino et al., 2009a). Table 3.7 reports the composition and the interlayer distance of the intercalation compounds. CA, Fer and the Asc exchange were about 45%, 53% and 31% respectively. The CA arrangement in the interlayer region is shown in Fig. 3.14; the interlayer distance value is in agreement with the formation of a monofilm partially interdigitate of CA species. Fer anions, very likely, are arranged in the same way; the increasing of the interlayer distance with respect to the Mg±Al±CA is probably due to the presence of the OCH3 group instead of the OH group. The free-radical scavenging activities of the intercalation compounds have been tested and compared with those of the neat antioxidant (see Section 3.4.4).

3.14 Computer-generated models showing the most probable arrangement of CA between the HTlc layers.



3.4

67

Nanocomposites of biodegradable polymeric matrices and modified hydrotalcites

The development of polymer±clay nanocomposites is one of the latest revolutionary steps in polymer technology. Preparations of blends or nanocomposites using inorganic or natural fibres are the route to improving some of the properties of biodegradable polymers. The addition of low percentages of clay to polymers may increase mechanical strength, reduce weight, increase heat resistance, and improve the barrier properties of food packaging materials against moisture, oxygen, carbon dioxide, ultraviolet radiation and volatiles in comparison to the barrier properties of traditional composites. Hence, synthetic polymer nanocomposites have emerged as an area of research in recent years and their development represents a very attractive way to improve and diversify physical and chemical properties of polymers (Messersmith and Giannelis, 1993; Vaia et al., 1994; Giannelis, 1996; Ren et al., 2000; Strawhecker and Manias, 2000). In an ideal nanocomposite structure, all of the inorganic particles must be completely separated into individual layers, forming an exfoliated structure. Therefore, most of the polymer is located at the nanofiller±polymer interface, and the conversion of bulk polymer into interfacial polymer represents the key to impart new and diversified polymer properties. To increase the compatibility between the polymer and the filler, thus favouring the exfoliation, the inorganic compound has to be modified with an organic molecular anion, able to create physical and intermolecular bonds with the polymeric chains. Many methods are used to allow a good dispersion of the inorganic compound into the polymer (Oswald and Asper, 1977; Pinnavaia and Beall, 2000; Alexandre and Dubois, 2000; Kaempfer et al., 2002). In the melt compounding method, polymer nanocomposites can be prepared by conventional compounding techniques (twin-screw extruder or melt compounder). If the compatibility between the polymer chains and the organic modification of the nanoparticles is sufficiently high, polymer chains penetrate into the galleries of the layered materials, and intercalation or exfoliation of the layered clay can occur. The solution-blending method consists of dissolving polymer and organically modified clay in a mutual solvent with subsequent solvent removal. Another interesting possibility is to directly intercalate or exfoliate the clay with a charged polymer that can constitute the counterbalancing ion in the clay galleries. In this case, the organic modification of the clay is not necessary, because the polymer in the charged form can penetrate into the clay galleries by a simple exchange reaction, and intercalate or exfoliate the inorganic solid. The most common nanocomposites investigated so far are composed of polymers and organically modified silicates. Hydrotalcite-like compounds represent a different and interesting class of nanofillers for polymers. As


68


already discussed in Section 3.2, HTlc can be prepared with simple procedures, at a high level of purity. They are cheap and eco-compatible and can be organically modified with a variety of organic anions that may confer to the obtained hybrids special functionalities (see Section 3.3). This latter characteristic will make these layered compounds a very attractive class of lamellar solids because the release of active guest anions from intercalated layered materials is potentially controllable. The new trend of the research is based on the fact that the active molecules, fixed by ionic bonds to the inorganic lamellae, not only can improve the compatibility with the polymer matrix but also can carry out the specific activity being anchored to the lamellae, or being slowly released in a particular environment (Oriakhi et al., 1996; Rives, 2001; Leroux et al., 2003). Recent results obtained using different, mainly biodegradable, polymeric matrices are reported below.

3.4.1

The case of poly(-caprolactone) (PCL)

The need for biodegradable plastics has increased during recent decades, not only due to growing environmental concerns, but also for their biomedical applications. Biodegradable polymers have been extensively investigated for packaging and agricultural products, in order to reduce the environmental pollution caused by plastic wastes (Scott and Gilead, 1995; Mecking, 2004). In the family of synthetic biodegradable polymers, poly(-caprolactone) (PCL), which is a linear, hydrophobic and partially crystalline polyester, is very attractive, not only as a substitute for non-biodegradable polymers for commodity applications, but also for specific applications in medicine and agriculture (Jarrett et al., 1984). The development of new nanohybrid composites obtained by intercalating PCL and active molecular anions into the interlayer region of HTlc is a very promising field for application of PCL in controlled release. The polymeric composite can release the active molecular anion with controlled kinetics, depending on the electronic structure of the active species, the interaction of the species with the matrix component, the concentration of the acceptor medium, and the morphology and polymorphism of the polymeric matrix. At least, the presence of the inorganic compound can improve either mechanical or transport polymer properties. The ability of the composites to release active species makes them useful for many applications as active food packaging films or controlled drug release membranes and scaffolds.

3.4.2

Procedures to obtain films, membranes and fibres of PCL±HTlc composites

PCL±HTlc nanocomposites have been synthesized by in situ ring-opening polymerization of -caprolactone (Tammaro et al., 2005). The polymerization is



69

induced by the alcoholic OH group belonging to 12-hydroxydodecanoate (HD), previously intercalated in Mg±Al±HTlc via an anion-exchange procedure. In particular, 1 g of the intercalation compound of formula [Mg0.65Al0.35(OH)2] (NO3)0.08(HD)0.28 and with interlayer distance of 2.27 nm was dispersed into 22.4 ml of -caprolactone. PCL oligomers were formed in the interlayer region, as evidenced by the increase of the interlayer distance. Finally, nanocomposites containing exfoliated HTlc lamellae were obtained by the solution mixing of high molecular weight PCL with the oligomers of PCL partially intercalated into HTlc±HD. The latter hybrid probably acted as a compatibilizer between the organically modified hydrotalcite and polycaprolactone. The HD-modified hydrotalcite was also used to prepare novel composites based on poly(-caprolactone) with different procedures. Microcomposite systems were obtained by the solution mixing of modified Mg±Al±HTlc with PCL. Other composites of PCL and HTlc±HD have been prepared using meltextrusion processing, a versatile, cheap and environmentally friendly technique (Pucciariello et al., 2007). Although exfoliation has not been achieved and despite the very low content of filler (from 1 to 3% by weight), significant enhancements have been obtained in the physical and mechanical properties of the composites with respect to neat PCL. An alternative and innovative strategy to produce nanocomposites relies on solid-state mixing at near room temperature, which ought to involve an efficient mixing of two or more species by mechanical milling, avoiding high temperatures and solvents. High energy ball milling (HEBM) is an effective unconventional technique currently used in material synthesis and processing. It consists of repeated events of energy transfer, promoted by the milling device, from the milling tools (generally balls) to the milled powder. During the milling the powder particles crack, clean surfaces are produced, and atom diffusion and ìntimate mixing' are promoted. As a consequence of the prolonged milling action, when the energy transferred during the hit is enough to overcome the activation barrier, chemical reactions may occur. It has been proved that HEBM on polymeric materials can help to obtain materials with new characteristics that can be barely achieved through other conventional processes. HEBM of powders constituted of organic polymers and fillers has been proved to be an alternative and efficient technique to produce novel composites. This technique may support the more conventional and more utilized techniques for producing nanocomposites, which are based mainly on in situ polymerization and melt extrusion. Sorrentino et al. (2005) used HEBM to prepare nanocomposites of PCL and an organically modified Mg±Al±HTlc. The molecular weight of PCL decreased and its distribution increased as a consequence of milling. The mechanical parameters derived from the stress±strain curves improved in the composite samples containing up to 2.8 wt% of inorganic filler, with respect to the pure polymer, in spite of the molecular weight decrease. The thermodynamic


70


diffusion coefficient of water vapour in composite samples was lower than in pure PCL, indicating an improvement of the barrier effect. Recently the electrospinning technique, which is able to produce non-woven membranes of micro/nanofibres characterized by high surface area and high porosity, has been demonstrated as a successful method to produce scaffolds having many of the desirable and controllable properties. It is applicable to a wide variety of polymers and composite polymers, both natural and synthetic, already widely used in tissue engineering (Teo and Ramakrishna, 2006; Travis and Horst, 2008; Agarwal et al., 2008). Romeo et al. (2007) reported, for the first time, the successful fabrication of hydrotalcites (Mg±Al±HTlc)-reinforced polycaprolactone (PCL) nanofibres by electrospinning. Either the HTlc in carbonate form or an HTlc organically modified with 12-hydroxydodecanoic acid (HTlc±HA) were incorporated into PCL and electrospin using a voltage of 20 kV. The HTlc±HA was prepared by an ionic exchange reaction from pristine HTlc and encapsulated into PCL from acetone solutions at 15 wt%. The morphological analysis showed pure PCL fibres with an average diameter of 600 50 nm, and this dimension was maintained in the fibres with HTlc, with the inorganic component residing outside the fibres and not exfoliated. At variance, the fibres with the HTlc-HA showed a significantly lower average diameter in the range of 350 50 nm, indicating the improved electrospinnability of PCL. Moreover, the inorganic lamellae were exfoliated, as shown by XRPD, and residing inside the nanofibres, as demonstrated by energy-dispersive X-ray (EDX) spectroscopy analysis. The structural parameters, such as degradation temperature and crystallinity, were investigated for all the samples and correlated with the electrospinning parameters.

3.4.3

PCL nanobiocomposites for modified drug release

The development of controlled-release technology needs materials with more specific drug-delivery properties, and therefore many efforts are being made to develop retarded and tunable drug-release systems. A remarkable innovation in this field is currently coming from nanoscience and nanotechnology ± the aim is to produce polymeric nanobiocomposites for controlled release of a wide variety of pharmaceuticals, or in general more àctive' products. Nanobiocomposites have been prepared by employing as nanoscale reinforcement layered materials functionalized with biologically active molecules that can be successively released by a chemical signal, i.e. exchange reactions. Hydrotalcite containing diclofenac, chloramphenicol hemisuccinate and tranexamic acid has been incorporated into PCL by solvent casting and HEBM procedures (Sammartino et al., 2006; Tammaro et al., 2007; Costantino et al., 2009b). Composites containing different weight percentages of modified hydrotalcites have been processed as films or threads. The composite materials



71

3.15 In vitro release tests of HTlc±Cfs/PCL film composites in a physiological saline solution (0.9% NaCl). The content of HTlc±Cfs in the composites was 5% w/w (l) and 20% w/w (n).

have been analysed by X-ray diffractometry, thermogravimetry and mechanical properties. Studies of the mechanical properties of these composites showed that the presence of the inorganic filler in the polymeric matrix led to an improvement of mechanical parameters except for fracture toughness. Moreover, the composites processed as films were submitted to in vitro release tests in a physiological saline solution (0.9% NaCl). Samples having different HTlc loading show the same qualitative release profile. The typical time-dependent profile of each sample is a fast release in an early period, followed by a reduced release (Fig. 3.15). The drug release consists of two stages: a first stage, very rapid as a burst, in which a small fraction of the drug is released from the surface of the lamellae, and a second stage that is much slower, extending for a longer and longer time, due to the drug de-intercalation from the interlayer region of HTlc inside the polymeric film. The amount of drug released from composite materials depends on both the nature of the incoming counter-anion that will replace the anionic drug, and the counter-diffusion of anions through the polymer. These composites are very promising in the preparation of new hybrid polymeric materials to be used for the controlled molecular delivery of drugs in topical applications, as suture threads or medical scaffolds. In the last few years considerable effort has been made to develop biocompatible scaffolds for tissue engineering. The scaffold should mimic the structure and biological function of native extracellular matrix (ECM) as much as possible, in terms of both chemical composition and physical structure. Native ECM does far more than just provide a physical support for cells. It also provides a substrate with specific ligands for cell adhesion and migration, and


72


regulates cellular proliferation and function by providing various growth factors. In a typical connective tissue, structural protein fibres such as collagen fibres and elastin fibres have diameters ranging from several ten to several hundred nanometres. Polymeric nanofibre non-woven matrix is among the most promising biomaterials for native ECM analogists. Tammaro et al. (2009) reported the incorporation of an Mg±Al hydrotalcitelike compound intercalated with diclofenac anions (HTlc-DIC) into poly(caprolactone) in different concentrations by the electrospinning technique, and the comparison of the obtained non-woven fibres to the pristine pure electrospun PCL. The fibres, characterized by X-ray diffraction, thermogravimetric analysis and differential scanning calorimetry, showed an exfoliated clay structure up to 3 wt%, a good thermal stability of the diclofenac molecules and a crystallinity of PCL comparable to the pure polymer. The scanning electron microscopy revealed electrospun PCL and PCL composite fibres diameters ranging between 500 nm to 3.0 m and a generally uniform thickness along the fibres. As the results suggested, the in vitro drug release from the composite fibres is markedly slower than the release from the corresponding control-spun solutions of PCL and diclofenac sodium salt. Thus, HTlc-DIC/PCL fibrous membranes can be used as an anti-inflammatory scaffold for tissue engineering.

3.4.4

PCL nanobiocomposites for potential food packaging applications

Research and development of nanocomposite materials for food applications such as packaging and other food contact surfaces is expected to grow in the next decade with the advent of new polymeric materials and composites with inorganic nanoparticles. The rationale for incorporating antimicrobials into the packaging is to prevent surface growth in foods where a large proportion of spoilage and contamination occurs (Appendini and Hotchkiss, 2002; LaCoste et al., 2005). This approach can reduce the addition of larger quantities of antimicrobials that are usually incorporated into the bulk of the food. A controlled release from packaging film to the food surface has numerous advantages over dipping and spraying. Hydrotalcite intercalated with benzoate and benzoate derivative anions with antimicrobial activity have been used as fillers of PCL (Costantino et al., 2009c). The composites have been prepared by HEBM and processed, on a laboratory scale, as thin films. According to the nature of the guest, microcomposites and intercalated and/or exfoliated polymeric composites have been obtained and studied. X-ray diffraction analysis and scanning electron microscopy of the composites indicate that the HTlc samples containing BzDC anions are exfoliated into the polymeric matrix, whereas those containing pBzOH anions largely maintain the crystal packing and give rise to microcomposites. Intermediate behaviour was found for HTlc modified with Bz and o-



73

BzOH anions, since exfoliated and partly intercalated nanocomposites have been obtained. When the filler exfoliates into the polymer, the guest anions not only improve the compatibility of the inorganic layer with the polymeric matrix, and hence the mechanical and barrier properties of the composite, but also confer to it their typical biological activity. Preliminary antimicrobial tests indicate that the composites are able to inhibit the growth of Saccharomyces cerevisiae of 40% in comparison with the growth in a pure culture medium. In other words, the growth of the microorganisms in the presence of composites is only 60% of the growth found in the pure culture medium. Such a result gives evidence of the feasibility of the composites as àctive packaging' materials because of the antimicrobial properties of the anions anchored to the HTlc layer. Mechanical and barrier properties of water vapour have been studied for all the nanocomposite films, showing the influence of the morphology on the physical properties. A preliminary study on the release kinetics of the Bz anions bound to HTlc has also been performed, revealing very good perspectives in the field of controlled release of active species (Bugatti et al., 2010). Films with antioxidant activity have been prepared by incorporating hydrotalcite modified with ferulate and ascorbate anions by solvent casting. Microcomposites or exfoliated and partly intercalated nanocomposites have been obtained for HTlc-Asc/PCL and HTlc-Fer/PCL systems respectively (Costantino et al., 2009a). The film antioxidative activity has been evaluated by the scavenging of the stable 2,2-diphenyl-1-picrylhydrazyl (DPPH) free radical. The stable DPPH radical absorbs at 517 nm and the antioxidant activity of the species can be determined by monitoring the decrease in absorbance as a function of time (Brand-Williams et al., 1995). The variation of the absorbance of 10±4M methanol solution of DPPH radical and antioxidant (molar ratio antioxidant/DPPH = 0.5) has been compared with that of the pure DPPH 10±4M methanol solution. In order to investigate the effect of the microenvironment on the antioxidant properties, the radical scavenger ability has been measured for the free species in acid and salt forms, for the anions intercalated into the Mg±Al±HTlc and after dispersing the intercalation compounds into the PCL. In Fig. 3.16 the percentage of the remaining DPPH is plotted against time. The antioxidant activities of the ferulic acid and of its sodium salt are comparable, being attributed to the hydroxyl group of the aromatic ring. The ferulate anions trapped between the sheets preserve the antiradical power, albeit with lower kinetics, probably due to the time of diffusion of the guests from the interlayer region of HTlc to the solution. Within the first hours the antioxidant activity of the composite is higher than that of the Mg±Al±Fer. As observed by X-ray diffraction analysis, a part of the hybrid containing Fer is able to exfoliate into the polymeric matrix, increasing the amount of active


74


3.16 Percentage reduction of DPPH absorbance with time in the presence of Fer and Asc in acid form (only for Fer), as sodium salt, intercalated into the Mg±Al±HTlc and in the PLC±HTlc composites.

anions that can be promptly released. With the increase of time the activity of the above two materials is reversed: the activity of the composite is lower than that of the Mg±Al±Fer since the Fer anions coming from the Mg±Al±Fer, not exfoliated into the PCL have to diffuse through the inorganic sheet and the polymer. Similar results have been obtained for the system containing the ascorbate. Ascorbic acid immediately discoloured the DPPH solution, while the sodium ascorbate reduced by 50% the DPPH concentration. In this case the



75

activity of the PCL composite is always better than that of the intercalation compound. The obtained composites can be considered as promising active food packaging systems due to the presence of antioxidant agents that can control the oxidation of the food.

3.4.5

The case of poly(hydroxyalkanoates) and hydrocolloids

Among biodegradable thermoplastic polyesters, polymers such as poly(3hydroxybutyrate) (PHB) and poly(butylene succinate) (PBS) are promising biomaterials that can be used in packaging, automotive and biomedical fields. However, PHB and PBS present some drawbacks such as low hydrolysis resistance, low barrier to gases and water vapour, low melt stability, and meltviscosity not sufficient for processing for practical end-use applications. In order to improve the thermal and barrier properties, melt blending nanobiocomposites of PHB and different layered phyllosilicates have been prepared. Composites containing kaolinite showed enhanced crystallinity and barrier properties (Sanchez-Garcia et al., 2008). Hydrotalcites grafted on the surface with poly(ethylene glycol) phosphonate have been dispersed into PHB to improve the crystallization kinetics of the nanocomposites (Hsu et al., 2006). Recently, Mg± Al hydrotalcite modified with oleate anion, an ènvironmentally friendly' guest, has been used as filler of PBS. Composites quasi-exfoliated and with improved rheological properties have been obtained for HTlc loading lower than 5% w/w (Zhou et al., 2010). The water vapour permeability and mechanical properties of glycerol plasticized dextrin±alginate films, filled with stearate intercalated hydrotalcite (HTlc-SA), have been investigated (Landman and Focke, 2006). The total filler content, comprising both the stearic acid (SA) and the [Mg4Al2(OH)12CO3 3H2O](HTlc), was fixed at 16.6% w/w of the dried films. The two filler components were allowed to react in the film casting solution for one hour. Sodium alginate acted as a dispersant and facilitated the intercalation of the stearic acid into HTlc that was suspended in the water±alcohol film solution. The resultant cast film properties were not affected when either neat SA or HTlc was the filler. However, superior mechanical and barrier properties were realized at intermediate filler compositions.

3.5

Conclusions and future trends

At present, exfoliated and organically modified smectite clays are the key fillers of different polymeric matrices. However, among other inorganic materials proposed as nanofillers (i.e. carbon nanotubes, perovskites, nanoparticles of silica and modified silica (POSS siloxanes), modified TiO2 nanoparticles), hydrotalcite-like compounds compare favourably with smectite clays for many features. HTlc have a well-known stoichiometry and composition, a higher level


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of purity, may be synthesized with simple and reproducible methods, have a wider possibility of layer modification, have higher ion exchange capacity and show better ability to store and release biologically active species. This last feature allows the design and synthesis of functional HTlc nanofillers for biodegradable and biocompatible polymeric matrices, and the preparation of nanobiocomposites of interest for biomedical and active packaging applications. In this context, the content of the present chapter has been worded to introduce the reader to the chemistry of hydrotalcites, starting from their synthesis and their physical±chemical properties, to their manipulation via anion exchange and intercalation reactions to obtain organic±inorganic hybrids hosting selected drugs, amino acids, proteins or species with antimicrobial or antioxidant properties. It has been shown that these hybrids can release the guest species in a modified way, in different environments, and can be dispersed at nanometric level in biodegradable polymers, conferring to them additional and useful properties. For concerning the potential of hydrotalcites for enhancing barrier properties, it is well known that inorganic fillers may increase the barrier properties of the nanocomposites by creating a more `tortuous path' that retards the progress of the small molecules through the polymeric matrix. The direct benefit of the formation of such a path is clearly observed in all the prepared nanocomposites by dramatically improved barrier properties. There is also evidence that the nanosized platelets restrict the molecular dynamics of the polymer chains surrounding the inorganic, thus retarding the relaxation of polymer chains. The effect of the hydrotalcites on the barrier properties of polymers has been scarcely studied. Film composites constituted by PCL and HTlc functionalized with antimicrobial species have been characterized also for their barrier properties of water vapour (Bugatti et al., 2010). The barrier properties have been investigated by measuring the isotherms of sorption and the diffusion of water vapour for all the composites. The isotherms of the composites follow the same trend as PCL, although showing a higher sorption in all the activity range, due to the higher hydrophilicity of the inorganic lamellae. At variance, the thermodynamic diffusion parameter, at zero vapour concentration, is significantly lower and decreases on increasing the inorganic concentration for all the composites. However, the most effective reduction was found for the exfoliated samples. This chapter will have achieved its aim if the reader acquires the conviction that hydrotalcite-like compounds are an extremely versatile class of materials that can be produced at low cost and can be easily modified with simple procedures, and if the reader envisages the preparation of novel products for unforeseen new applications. This conviction is well established in many academic and industrial laboratories, as documented by the large number of research articles, reviews and patents available. Some chemical firms are producing and selling hydrotalcites as fillers of polymers and this will probably favour an extended and widened interest in these materials in the near future.



3.6

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References and further reading

Adachi-Pagano M, Forano C and Besse J P (2000), `Delamination of layered double hydroxides by use of surfactants', Chem. Commun., 1, 91±92. Adachi-Pagano M, Forano C and Besse J P (2003), `Synthesis of Al-rich hydrotalcite-like compounds by using the urea hydrolysis reaction ± control of size and morphology', J. Mater. Chem., 13, 1988±1993. Agarwal S, Wendorff J H and Greiner A (2008), Ùse of electrospinning technique for biomedical applications', Polymer, 49, 5603±5621. Aisawa S, Takahashi S, Ogasawara W, Umetsu Y and Narita E (2001), `Direct intercalation of amino acids into layered double hydroxides by coprecipitation', J. Solid State Chem., 162, 52±62. Aisawa S, Sasaki S, Takahashi S, Hirahara H, Nakayama H and Narita E (2006), Ìntercalation of amino acids and oligopeptides into Zn±Al layered double hydroxide by coprecipitation reaction', J. Phys. Chem. Solids, 67, 920±925. Alexandre M and Dubois P (2000), `Polymer-layered silicate nanocomposites: preparation, properties and uses of a new class of materials', Mater. Sci. Eng. Rep., 28, 1±63. Allmann R (1968), `The crystal structure of pyroaurite', Acta Crystallogr., B24, 972±977. Ambrogi V, Fardella G, Grandolini G and Perioli L (2001), Ìntercalation compounds of hydrotalcite-like anionic clays with antiinflammatory agents. I. Intercalation and in vitro release of ibuprofen', Int. J. Pharm., 220, 23±32. Ambrogi V, Fardella G, Grandolini G, Perioli L and Tiralti M C (2002), Ìntercalation compounds of hydrotalcite-like anionic clays with antiinflammatory agents. II. Uptake of diclofenac for a controlled release formulation', AAPS PharmSciTech, 3 (article 26). Ambrogi V, Fardella G, Perioli L, Nocchetti M and Grandolini G (2003), Èffect of hydrotalcite-like compounds on dissolution of poorly water soluble drugs', J. Pharm. Sci., 92, 1407±1417. Ambrogi V, Ciarnelli V, Nocchetti M, Perioli L and Rossi C (2009), À novel gliclazide± hydrotalcite nanohybrid for improving drug dissolution rate', Eur. J. Pharm. Sci., 73, 285±291. An Z, Lu S, He J and Wang Y (2009), `Colloidal assembly of proteins with delaminated lamellas of layered metal hydroxide', Langmuir, 25(18), 10704±10710. Appendini P and Hotchkiss J H (2002), `Review of antimicrobial food packaging', Innovative Food Science and Emerging Technologies, 3, 113±126. Ay A N, Zumreoglu-Karan B, Temel A and Rives V (2009), `Bioinorganic magnetic core±shell nanocomposites carrying antiarthritic agents: Intercalation of ibuprofen and glucuronic acid into Mg-Al-layered double hydroxides supported on magnesium ferrite', Inorg. Chem., 48, 8871±877. Bauer J, Behrens P, Speckbacher M and Langhals H (2003), `Composites of perylene chromophores and layered double hydroxides: direct synthesis, characterization, and photo- and chemical stability', Adv. Funct. Mater., 13, 241±248. Bellezza F, Cipiciani A, Costantino U and Negozio M E (2002), `Zirconium phosphate and modified zirconium phosphates as supports of lipase. Preparation of the composites and activity of the supported enzyme', Langmuir, 18, 8737±8742. Bellezza F, Cipiciani A and Quotadamo M A (2005), Ìmmobilization of myoglobin on phosphate and phosphonate grafted-zirconia nanoparticles', Langmuir, 21, 11099± 11104. Bellezza F, Cipiciani A, Costantino U and Marmottini F (2006), Àdsorption of


78


myoglobin onto porous zirconium phosphate and zirconium benzenephosphonate obtained with template synthesis', Langmuir, 22, 5064±5069. Bellezza F, Cipiciani A, Quotadamo M A, Cinelli S, Onori G and Tacchi S (2007), `Structure, stability, and activity of myoglobin adsorbed onto phosphate-grafted zirconia nanoparticles', Langmuir, 23, 13007±13012. Bellezza F, Cipiciani A, Costantino U, Nocchetti M and Posati T (2009a), `Hydrotalcitelike nanocrystals from water-in-oil microemulsions', Eur. J. Inorg. Chem., 2603± 2611. Bellezza F, Cipiciani A, Latterini L, Posati T and Sassi P (2009b), `Structure and catalytic behavior of myoglobin adsorbed onto nanosized hydrotalcites', Langmuir, 25 (18), 10918±10924. Brand-Williams W, Cuvelier M E and Berset C (1995), Ùse of a free radical method to evaluate antioxidant activity', Lebensm. Wiss. u. Technol., 28, 25±30. Braterman P S, Xu Z P and Yarberry F (2004), `Layered double hydroxides (LDHs)', in Handbook of Layered Materials, Auerbach S M, Carrado K A and Dutta P K (eds), Marcel Dekker, New York. Bugatti V, Costantino U, Gorrasi G, Nocchetti M, Tammaro L and Vittoria V (2010), `Nano-hybrid incorporation into polycaprolactone for active packaging applications: mechanical and barrier properties', Eur. Polym. J., 46, 418±427. Busca G, Costantino U, Marmottini F, Montanari T, Patrono P, Pinzari F and Ramis G (2006), `Methanol steam reforming over ex-hydrotalcite Cu±Zn±Al catalysts', Applied Catalysis A: General, 310, 70±78. Camino G, Maffezzoli A, Braglia M, De Lazzaro M and Zammarano M (2001), Èffect of hydroxides and hydroxycarbonate structure on fire retardant effectiveness and mechanical properties in ethylene±vinyl acetate copolymer', Polym. Degrad. Stab., 74, 457±464. Carretero M I, Lagaly G and Showalter (eds) (2007), `Clays and health ± Clays in pharmacy, cosmetics, pelotherapy, and environmental protection', Appl. Clay Sci., 36(1±3), 1±220. Caruso F (2001), `Nanoengineering of particle surfaces', Adv. Mater., 13, 11±22. Cavani F, TrifiroÁ F and Vaccari A (1991), `Hydrotalcite-like anionic clays: preparation, properties and applications', Catal. Today, 11, 173±301. Charradi K, Forano C, Prevot V, Amara A B H and Mousty C (2009), `Direct electron transfer and enhanced electrocatalytic activity of hemoglobin at iron-rich clay modified electrodes', Langmuir, 25(17), 10376±10383. Chen X, Fu C, Wang Y, Yang W and Evans D G (2008), `Direct electrochemistry and electrocatalysis based on a film of horseradish peroxidase intercalated into Ni±Al layered double hydroxide nanosheets', Biosens. Bioelectron., 24, 356±361. Choy J H, Kwak S Y, Jeong Y J and Park J S (2000), Ìnorganic layered double hydroxides as nonviral vectors', Angew. Chem. (Int. Ed.), 39, 4041±4045. Choy J H, Hwang S H and Oh J M (2002), `The effect of synthetic conditions on tailoring the size of hydrotalcite particles', Solid State Ionics, 151, 285±291. Choy J H, Oh J M and Biswick T T (2009a), `Layered nanomaterials for green materials', J. Mater. Chem., 19, 2553±2563. Choy J H, Oh J M, Choi S J, Lee G E and Kim J E (2009b), Ìnorganic metal hydroxide nanoparticles for targeted cellular uptake through clathrin-mediated endocytosis', Chem. Asian J., 4, 67±73. Costa F R, Saphiannikova M, Wagenknecht U and Heinrich G (2008), `Layered double hydroxide based polymer nanocomposites', Adv. Polym. Sci., 210, 101±168. Costantino U and Nocchetti M (2001), `Layered double hydroxides and their intercalation



79

compounds in photochemistry and medicinal chemistry', in: Layered Double Hydroxides: Present and Future, Rives V (ed.), Nova Science Publishers, New York, chapter 12. Costantino U, Casciola M, Massinelli L, Nocchetti M and Vivani R (1997), Ìntercalation and grafting of hydrogen phosphates and phosphonates into synthetic hydrotalcites and AC-conductivity of the compounds thereby obtained', Solid State Ionics, 97, 203±212. Costantino U, Marmottini F, Nocchetti M and Vivani R (1998), `New synthetic routes to hydrotalcite-like compounds ± characterisation and properties of the obtained materials', Eur. J. Inorg. Chem., 1439±1446. Costantino U, Gallipoli A, Nocchetti M, Camino G, Bellucci F and Frache A (2005), `New nanocomposites constituted of polyethylene and organically modified ZnAlhydrotalcites', Polym. Degrad. Stab., 90, 586±590. Costantino U, Montanari F, Nocchetti M, Canepa F and Frache A (2007), `Preparation and characterization of hydrotalcite/carboxyadamantane intercalation compounds as fillers of polymeric nanocomposites', J. Mater. Chem., 17, 1079±1086. Costantino U, Curini M, Montanari F, Nocchetti M and Rosati O (2008a), `Hydrotalcitelike compounds as heterogeneous catalysts in liquid phase organic synthesis. II. Preparation of 4H-chromenes promoted by hydrotalcite doped with hydrous tin(IV) oxide', Micropor. Mesopor. Mater., 107, 16±22. Costantino U, Ambrogi V, Nocchetti M and Perioli L (2008b), `Hydrotalcite-like compounds. Versatile layered hosts of molecular anions with biological activity', Micropor. Mesopor. Mater., 107, 149±160. Costantino U, Nocchetti M, Sisani M and Vivani R (2009a), `Recent progress in the synthesis and application of organically modified hydrotalcites', Z. Kristallogr., 224, 273±281. Costantino U, Nocchetti M, Tammaro L and Vittoria V (2009b), Ìncorporation of active nano-hybrids into poly(-caprolactone) for local controlled release: antifibrinolytic drug', Appl. Clay Sci., 43, 350±356. Costantino U, Bugatti V, Gorrasi G, Montanari F, Nocchetti M, Tammaro L and Vittoria V (2009c), `New polymeric composites based on poly(-caprolactone) and layered double hydroxides containing antimicrobial species', ACS Appl. Mater. Interfaces, 1, 668±677. Del Arco M, GutieÂrrez S, MartõÂn C, Rives V and Rocha J (2004), `Synthesis and characterization of layered double hydroxides (LDH) intercalated with nonsteroidal anti-inflammatory drugs (NSAID)', J. Solid State Chem., 177, 3954±3962. Del Arco M, FernaÂndez A, MartõÂn C and Rives V (2009), `Release studies of different NSAIDs encapsulated in Mg, Al, Fe-hydrotalcites', Appl. Clay Sci., 42, 538±544. Del Hoyo C (2007), `Layered double hydroxides and human health: an overview', Appl. Clay Sci., 36, 103±121. Desigaux L, Belkacem M B, Richard P, Cellier J, LeÂone P, Cario L, Leroux F, TaviotGueÂho C and Pitard B (2006), `Self-assembly and characterization of layered double hydroxide/DNA hybrids', Nano Lett., 6, 199±204. Duan X and Evans D (eds) (2006), Layered Double Hydroxides, Structure and Bonding series, Vol. 119, Springer-Verlag, Berlin. Dupin J C, Martinez H, Guimon C, Dumitriu E and Fechete I (2004), Ìntercalation compounds of Mg±Al layered double hydroxides with dichlophenac: different methods of preparation and physico-chemical characterization', Appl. Clay Sci., 27, 95±106. Evans D G and Duan X (2006), `Preparation of layered double hydroxides and their


80


applications as additives in polymers, as precursors to magnetic materials and in biology and medicine', Chem. Commun., 485±496. Fang W, Zhang Q, Chen J, Deng W and Wang Y (2010), `Gold nanoparticles on hydrotalcites as efficient catalysts for oxidant-free dehydrogenation of alcohols', Chem. Commun., 46, 1547±1549. Gao S P, Lu T H, Li S P and Zhong H (2009), `The mechanism on the pH value influencing the property of glutamic acid/layered double hydroxide compounds', Colloids and Surfaces A: Physicochem. Eng. Aspects, 351, 26±29. Giannelis E P (1996), `Polymer layered silicate nanocomposites', Adv. Mater., 8(1), 29± 35. Gray J J (2004), `The interaction of proteins with solid surfaces', Curr. Opin. Struct. Biol., 14, 110±115. GruÈbel P, Bhaskar K R, Cave D R, Garik P, Stanley H E and Lamont J T (1997), Ìnteraction of an aluminum±magnesium containing antacid and gastric mucus: possible contribution to the cytoprotective function of antacids', Aliment. Pharmacol. Ther., 11, 139±145. Gunawan P and Xu R (2008), `Synthesis of unusual coral-like layered double hydroxide microspheres in a nonaqueous polar solvent/surfactant system', J. Mater. Chem., 18, 2112±2120. Haynes C A and Norde W (1995), `Structures and stabilities of adsorbed proteins', J. Colloid Interface Sci., 169, 313±328. He L, Dexter A F and Middelberg A P J (2006), `Biomolecular engineering at interfaces', Chem. Eng. Sci., 61, 989±1003. Hibino T (2004), `Delamination of layered double hydroxides containing amino acids', Chem. Mater., 16, 5482±5488. Hibino T and Jones W (2001), `New approach to the delamination of layered double hydroxides', J. Mater. Chem., 11, 1321±1323. Hibino T and Kobayashi M (2005), `Delamination of layered double hydroxides in water', J. Mater. Chem., 15, 653±656. Hines D R, Solin S A, Costantino U and Nocchetti M (2000), `Physical properties of fixed-charge layer double hydroxide', Phys. Rev. B, 61, 11348±11358. Hsu S, Wu T and Liao C (2006), Ìsothermal crystallization kinetics of poly(3hydroxybutyrate)/layered double hydroxide nanocomposites', J. Polym. Sci.: Part B: Polym. Phys., 44, 3337±3347. Hu G, Wang N, O'Hare D and Davis J (2006), Òne-step synthesis and AFM imaging of hydrophobic LDH monolayers', Chem. Commun., 287±289. Hwang S H, Han Y S and Choj J H (2001), Ìntercalation of functional organic molecules with pharmaceutical, cosmeceutical and nutraceutical functions into layered double hydroxides and zinc basic salts', Bull. Korea Chem. Soc., 22, 1019±1022. Illaik A, Vuillermoz C, Commereuc S, Taviot-GueÂho C, Verney V and Leroux F (2008), `Reactive and functionalized LDH fillers for polymer', J. Phys. Chem. Solids., 69, 1362±1366. Iyi N, Matsumoto T, Kaneko Y and Kitamura K (2004), À novel synthetic route to layered double hydroxides using hexamethylenetetramine', Chem. Lett., 33, 1122± 1123. Jarrett P, Benedict C, Bell J P, Cameron J A and Huang S J (1984), in Polymers as Biomaterials, Shalaby S W, et al. (eds), Plenum Press, New York, p. 181. Jaubertie C, Holgado M J, San RomaÁn M S and Rives V (2006), `Structural characterization and delamination of lactate-intercalated Zn, Al-layered double hydroxides', Chem. Mater., 18, 3114±3121.



81

Jobbagy M and Regazzoni A E (2006), `Delamination and restacking of hybrid layered double hydroxides assessed by in situ XRD', J. Colloid Interface Sci., 294, 345±348. Kaempfer D, Thomann R and MuÈlhaupt R (2002), `Melt compounding of syndiotactic polypropylene nanocomposites containing organophilic layered silicates and in situ formed core/shell nanoparticles', Polymer, 43, 2909±2916. Katz E and Willner I (2004), Ìntegrated nanoparticle±biomolecule hybrid systems: synthesis, properties, and applications', Angew. Chem. (Int. Ed.), 43, 6042±6108. Khan A I and O'Hare D (2002), Ìntercalation chemistry of layered double hydroxides: recent developments and applications', J. Mater. Chem., 12, 3191±3198. Konturek J W, Beneke M, Koppermann R, Petersen-Braun M and Weingartner U (2007), `The efficacy of hydrotalcite compared with OTC famotidine in the on-demand treatment of gastroesophageal reflux disease: a non-inferiority trial', Med. Sci. Monit., 13, 44±49. Kovanda F, JindovaÂ E, Lang K, KubaÂt P and SedlaÂkovaÂ Z (2010), `Preparation of layered double hydroxides intercalated with organic anions and their application in LDH/ poly(butyl methacrylate) nanocomposites', Appl. Clay Sci., 48, 260±270. LaCoste A, Schaich K M, Zumbrunnen D and Yam K L (2005), Àdvancing controlled release packaging through smart blending', Packaging Technol. Sci., 18, 77±87. Lakshmi Kantam M, Ravindra A, Venkat Reddy C, Sreedhar B and Choudary B M (2006), `Layered double hydroxides-supported diisopropylamide: synthesis, characterization and application in organic reactions', Adv. Synth. Catal., 348, 569±578. Landman E P and Focke W W (2006), `Stearate intercalated layered double hydroxides: effect on the physical properties of dextrin-alginate films', J. Mater. Sci., 41, 2271± 2279. Latterini L, Nocchetti M, Costantino U, Aloisi G G and Elisei F (2007), Òrganized chromophores in layered inorganic matrices', Inorg. Chim. Acta, 360, 728±740. Leroux F (2006), Òrgano-modified anionic clays into polymer compared to smectitetype nanofiller: potential applications of the nanocomposites', J. Nanosci. Nanotechnol., 6, 303±315. Leroux F and Taviot-GueÂho C (2005), `Fine tuning between organic and inorganic host structure: new trends in layered double hydroxide hybrid assemblies', J. Mater. Chem., 15, 3628±3642. Leroux F, Aranda P, Besse J P and Ruiz-Hitzky E (2003), Ìntercalation of poly(ethylene oxide) derivatives into layered double hydroxides', Eur. J. Inorg. Chem., 1242± 1251. Li B, He J, Evans D G and Duan X (2004), Ìnorganic layered double hydroxides as a drug delivery system ± intercalation and in vitro release of fenbufen', Appl. Clay Sci., 27, 199±207. Li L, Ma R, Ebina Y, Iyi N and Sasaki T (2005), `Positively charged nanosheets derived via total delamination of layered double hydroxides', Chem. Mater., 17, 4386± 4391. Lin M S, Sun P and Yu H Y (1998), Èvaluation of buffering capacity and acid neutralizing pH time profile of antacid', J. Formos. Med. Assoc., 97, 704±710. Linares C F, Solano S and Infante G (2004), `The influence of hydrotalcite and cancrinite type zeolita in acidic aspirin solutions', Micropor. Mesopor. Mater., 74, 105±110. Liu S, Chen D, Zhang K, Li J and Zhao N (2008), `Production of hydrogen by ethanol steam reforming over catalysts from reverse microemulsion-derived nanocompounds', Int. J. Hydrogen Energy, 33, 3736±3747. Liu Z, Ma R, Osada M, Iyi N, Ebina Y, Takada K and Sasaki T (2006), `Synthesis, anion


82


exchange, and delamination of Co±Al layered double hydroxide: assembly of the exfoliated nanosheet/polyanion composite films and magneto-optical studies', J. Am. Chem. Soc., 128, 4872±4880. Liu Z, Ma R, Ebina Y, Iyi N, Takada K and Sasaki T (2007), `General synthesis and delamination of highly crystalline transition-metal-bearing layered double hydroxides', Langmuir, 23, 861±867. Lombardo G M, Pappalardo G C, Punzo F, Costantino F, Costantino U and Sisani M (2005), À novel integrated approach X-ray powder diffraction (XRPD) and molecular dynamics (MD) for modelling mixed-metals (Zn, Al) layered double hydroxides (LDH)', Eur. J. Inorg. Chem., 5026±5034. Lombardo G M, Pappalardo G C, Costantino F, Costantino U and Sisani M (2008), `Thermal effects on mixed metal (Zn/Al) layered double hydroxides (LDHs): direct modelling of the X-ray powder diffraction (XRPD) line-shape through molecular dynamics (MD) simulation', Chem. Mater., 20, 5585±5592. Ma R, Liu Z, Takada K, Iyi N, Bando Y and Sasaki T (2007), `Synthesis and exfoliation of Co2+±Fe3+ layered double hydroxides: an innovative topochemical approach', J. Am. Chem. Soc., 129, 5257±5263. Manasse E (1915), `Rocce eritree e di aden della collezione issel', Atti Soc. Toscana Sc. Nat., Proc. Verb., 24, 92. Martinez Martinez V, De Cremer G, J. Roeffaers M B, Sliwa M, Baruah M, De Vos D E, Hofkens J and Sels B F (2008), Èxploration of single molecule events in a haloperoxidase and its biomimic: localization of halogenation activity', J. Am. Chem. Soc., 130, 13192±13193. May Y W and Yu Z Z (2006), Polymer Nanocomposites, Woodhead Publishing, Cambridge, UK. Mecking S (2004), `Nature or petrochemistry? ± Biologically degradable materials', Angew. Chem. (Int. Ed.), 43, 1078±1085. Messersmith P B and Giannelis E P (1993), `Polymer-layered silicate nanocomposites: in situ intercalative polymerization of -caprolactone in layered silicates', Chem. Mater., 5, 1064±1066. Mignani A, Scavetta E, Guadagnini L and Tonelli D (2009), `Comparative study of protective membranes for glucose biosensors based on electrodeposited hydrotalcites', Sensors and Actuators B, 136, 196±202. Miyata S (1980), `Physico-chemical properties of synthetic hydrotalcites in relation to composition', Clays and Clay Minerals, 28, 50±56. Miyata S (1983), Ànion-exchange properties of hydrotalcite-like compounds', Clays and Clay Minerals, 31, 305±311. Miyata S and Kumura T (1973), `Synthesis of new hydrotalcite-like compounds and their physico-chemical properties', Chem. Lett., 2, 843±848. Mohanambe L and Vasudevan S (2005), Ànionic clay containing anti-inflammatory drugs molecules: comparison of molecular dynamics simulations and measurements', J. Phys. Chem. B, 109, 15651±15658. Mousty C (2010), `Biosensing applications of clay-modified electrodes: a review', Anal. Bioanal. Chem., 396, 315±325. Mousty C, Kaftan O, PreÂvot V and Forano C (2008), Àlkaline phosphatase biosensors based on layered double hydroxides matrices: Role of LDH composition', Sensors and Actuators B, 133, 442±448. Nakanishi K, Sakiyama T and Imamura K (2001), Òn the adsorption of proteins on solid surfaces, a common but very complicated phenomenon', J. Biosci. Bioeng., 91, 233±244.



83

Newman S P and Jones W (1998), `Synthesis, characterization and applications of layered double hydroxides containing organic guests', New J. Chem., 22, 105±115. Nyambo C, Songtipya P, Manias E, Jimenez-Gasco M M and Wilkie C A (2008), Èffect of MgAl-layered double hydroxide exchanged with linear alkyl carboxylates on fire-retardancy of PMMA and PS', J. Mater. Chem., 18, 4827±4838. O'Hare D and Hu G (2005), Ùnique layered double hydroxide morphologies using reverse microemulsion synthesis', J. Am. Chem. Soc., 127, 17808±17813. O'Hare D, Hu G, Wang N and Davis J (2007), `Synthesis of magnesium aluminium layered double hydroxides in reverse microemulsions', J. Mater. Chem., 17, 2257± 2266. O'Leary S, O'Hare D and Seeley G (2002), `Delamination of layered double hydroxides in polar monomers: new LDH-acrylate nanocomposites', Chem. Commun., 14, 1506±1507. Ogawa M and Kuroda K (1995), `Photofunctions of intercalation compounds', Chem. Rev., 95, 399±438. Okamoto K, Sasaki T, Fujita T and Iyi N (2006), `Preparation of highly oriented organic± LDH hybrid films by combining the decarbonation, anion-exchange, and delamination processes', J. Mater. Chem., 16, 1608±1616. Oriakhi C O, Farr I V and Lerner M (1996), Ìncorporation of poly(acrylic acid), poly(vinylsulfonate) and poly(styrenesulfonate) within layered double hydroxides', J. Mater. Chem., 6, 103±107. Oswald H R and Asper R (1997), in Physics and Chemistry of Materials with Layered Structures, Vol. 1, Lieth R M A (ed.), D. Reidel Publishing Co., Dordrecht, The Netherlands. Palmer S J, Spratt H J and Frost R L (2009), `Thermal decomposition of hydrotalcites with variable cationic ratios', Journal of Thermal Analysis and Calorimetry, 95, 123±129. Paredes S P, Fetter G, Bosch P and Bulbulian S (2006), `Sol-gel synthesis of hydrotalcitelike compounds', J. Mater. Sci., 41, 3377±3382. Perioli L, Ambrogi V, Bertini B, Ricci M, Giovagnoli S, Nocchetti M, Latterini L and Rossi C (2006a), Ànionic clays for sunscreen agent safe use: photoprotection, photostability and prevention of their skin penetration', Eur. J. Pharm. Biopharm., 62, 185±193. Perioli L, Ambrogi V, Rossi C, Latterini L, Nocchetti M and Costantino U (2006b), Ùse of anionic clays for photoprotection and sunscreen photostability: hydrotalcites and phenylbenzimidazole sulfonic acid', J. Phys. Chem. Solids, 67, 1079±1083. Perioli L, Nocchetti M, Ambrogi V, Latterini L, Rossi C and Costantino U (2008), `Sunscreen immobilization on ZnAl-hydrotalcite for new cosmetic formulations', Micropor. Mesopor. Mater., 107, 180±189. Perioli L, Posati T, Nocchetti M, Bellezza F, Costantino U and Cipiciani A (2010a), Ìntercalation and release of antiinflammatory drug diclofenac into nanosized ZnAl hydrotalcite-like compound', Appl. Clay Sci., in press. Perioli L, Ambrogi V, di Nauta L, Nocchetti M and Rossi C (2010b), `Hydrotalcite as matrix for double modified release of flurbiprofen', Pharm. Res., submitted. Pinnavaia T J and Beall G W (2000), Polymer±Clay Nanocomposites, Wiley Series in Polymer Science, Wiley, New York. PreÂvot V, Forano C and Besse J P (2001), `Hybrid derivatives of layered double hydroxides', Appl. Clay Sci., 18, 3±15. Prinetto F, Ghiotti G, Graffin P and Tichit D (2000), `Synthesis and characterization of sol-gel Mg/Al and Ni/Al layered double hydroxides and comparison with co-


84


precipitated samples', Micropor. Mesopor. Mater., 39, 229±247. Pucciariello R, Tammaro L, Villani V and Vittoria V (2007), `New nanohybrids of poly(-caprolactone) and a modified Mg/Al hydrotalcite: mechanical and thermal properties', J. Polym. Sci. Part B: Polym. Phys., 45, 945±954. Reichle W T (1986), `Synthesis of anionic clay minerals (mixed metal hydroxides, hydrotalcite)', Solid State Ionics, 22, 135±141. Ren J, Silva A S and Krishnamoorti R (2000), `Linear viscoelasticity of disordered polystyrene±polyisoprene block copolymer based layered-silicate nanocomposites', Macromolecules, 33, 3739±3746. Rey F and Fornes V (1992), `Thermal decomposition of hydrotalcites. An infrared and nuclear magnetic resonance spectroscopic study', J. Chem. Soc. Faraday Trans., 88, 2233±2238. Rives V (ed.) (2001), Layered Double Hydroxides: Present and Future, Nova Science Publishers, New York. Rives V, Benito P and Labajos F M (2006), Ùniform fast growth of hydrotalcite-like compounds', Cryst. Growth Des., 6, 1961±1966. Rocha J, del Arco M, Rives V and Ulibarri M A (1999), `Reconstruction of layered double hydroxides from calcined precursors: a powder XRD and 27Al MAS NMR study', J. Mater. Chem., 9, 2499±2503. Romeo V, Gorrasi G, Vittoria V and Chronakis I S (2007), Èncapsulation and exfoliation of inorganic lamellar compounds into polycaprolactone by electrospinning', Biomacromolecules, 8(10), 3147±3152. Sammartino G, Marenzi G, Tammaro L, Bolognese A, Calignano A, Costantino U, Califano L, Mastrangelo F, TeteÁ S and Vittoria V (2006), Ànti-inflammatory drug incorporation into polymeric nano-hybrids for local controlled release', Int. J. Immunopathol. Pharmacol., 18, 55±62. Sanchez-Garcia M, Gimenez E and LagaroÂn J M (2008), `Morphology and barrier properties of nanobiocomposites of poly(3-hydroxybutyrate) and layered silicates', J. Appl. Polym. Sci., 108, 2787±2801. Scott G and Gilead D (1995), in Degradable Polymers. Principles and Applications, Chapman & Hall, London. Shaw L R, Irwin W J, Grattan T J and Conway B R (2005), `The role of gastric mucus as a barrier to the absorption of ibuprofen or paracetamol and the effects of coadministered antacids and modified pH', Int. J. Pharm., 290, 145±154. Sideris P J, Nielsen U G, Gan Z and Grey C P (2008), `Mg/Al ordering in layered double hydroxides revealed by multinuclear NMR spectroscopy', Science, 321, 113±117. Sorrentino A, Gorrasi G, Tortora M, Vittoria V, Costantino U, Marmottini F and Padella F (2005), Ìncorporation of Mg±Al hydrotalcite into a biodegradable poly(3caprolactone) by high energy ball milling', Polymer, 46, 1601±1608. Strawhecker K E and Manias E (2000), `Structure and properties of poly(vinyl alcohol)/ Na+ montmorillonite nanocomposites', Chem. Mater., 12, 2943±2949. Tammaro L, Tortora M, Vittoria V, Costantino U and Marmottini F (2005), `Methods of preparation of novel composites of poly(-caprolactone) and a modified Mg/Al hydrotalcite', J. Polym. Sci.: Part A: Polym. Chem., 43, 2281±2290. Tammaro L, Vittoria V, Costantino U, Bolognese A, Sammartino G, Marenzi G, Califano L, Calignano A and TeteÁ S (2007), `Nanohybrids for controlled antibiotic release in topical applications', Int. J. Antimicrob. Agents, 29, 417±423. Tammaro L, Vittoria V and Russo G (2009), Èncapsulation of diclofenac molecules into poly(-caprolactone) electrospun fibers for delivery protection', J. Nanomater., doi: 10.1155/2009/238206.



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Taylor H F W (1969), `Segregation and cation-ordering in sjoÈgrenite and pyroaurite', Min. Mag., 37, 338±342. Taylor H F W (1973), `Crystal structures of some double hydroxide minerals', Min. Mag., 39, 377±389. Teo W E and Ramakrishna S (2006), À review on electrospinning design and nanofiber assemblies', Nanotechnology, 17, R89±R106. Travis J S and Horst A R (2008), Èlectrospinning: applications in drug delivery and tissue engineering', Biomaterials, 29, 1989±2006. TrifiroÁ F and Vaccari A (1996), `Hydrotalcite-like anionic clays (layered double hydroxides)', in Solid-State Supramolecular Chemistry: Two and Threedimensional Inorganic Networks, of Comprehensive Supramolecular Chemistry, Alberti G and Bein T (eds), Vol. 7, Pergamon Press, Elsevier Science, Oxford, pp. 251±291. Turco M, Bagnasco G, Costantino U, Marmottini F, Montanari T, Ramis G and Busca G (2004), `Production of hydrogen from oxidative steam reforming of methanol. I. Preparation and characterization of Cu/ZnO/Al2O3 catalysts from a hydrotalcitelike LDH precursor', J. Catalysis, 228, 43±55. Vaia R A, Teukolsky R K and Giannelis E P (1994), Ìnterlayer structure and molecular environment of alkylammonium layered silicates', Chem. Mater., 6, 1017±1022. Vial S, PreÂvot V, Leroux F and Forano C (2008), Ìmmobilization of urease in ZnAl layered double hydroxides by soft chemistry routes', Micropor. Mesopor. Mater., 107, 190±201. Williams G R and O'Hare D (2006), `Towards understanding, control and application of layered double hydroxide chemistry', J. Mater. Chem., 16, 3065±3074. Wu Q, Olafsen A, Vistad é B, Roots J and Norby P (2005), `Delamination and restacking of a layered double hydroxide with nitrate as counter anion', J. Mater. Chem., 15, 4695±4700. Xu Z P and Braterman P S (2010), `Synthesis, structure and morphology of organic layered double hydroxide (LDH) hybrids: Comparison between aliphatic anions and their oxygenated analogs', Appl. Clay Sci., 48, 235±242. Xu Z P, Braterman P S, Yu K, Xu H, Wang Y and Brinker C J (2004), Ùnusual hydrocarbon chain packing mode and modification of crystallite growth habit in the self-assembled nanocomposites zinc±aluminum±hydroxide oleate and elaidate (cisand trans-[Zn 2 Al(OH) 6 (CH 3 (CH 2 ) 7 CH=CH(CH 2 ) 7 COO ± )] and magnesium analogues', Chem. Mater., 16, 2750±2756. Xu Z P, Stevenson G S, Lu C Q, Lu G Q, Bartlett P F and Gray P P (2006), `Stable suspension of layered double hydroxide nanoparticles in aqueous solution', J. Am. Chem. Soc., 128, 36±37. Yasutake A, Aisawa S, Takahashi S, Hirahara H and Narita E (2008), `Synthesis of biopolymer intercalated inorganic-layered materials: Intercalation of collagen peptide and soybean peptide into Zn±Al layered double hydroxide and layered zinc hydroxide', J. Phys. Chem. Solids, 69, 1542±1546. Yun S K and Pinnavaia T (1995), `Water content and particle texture of synthetic hydrotalcite-like layered double hydroxides', J. Chem. Mater., 7, 348±354. Zhou Q, Verney V, Commereuc S, Chin I and Leroux F (2010), `Strong interfacial attrition developed by oleate/layered double hydroxide nanoplatelets dispersed into poly(butylene succinate)', J. Colloid Interface Sci., 349, 127±133.


4

Cellulose nanofillers for food packaging È M , Royal Institute of R. T. OLSSON and L. FOGELSTRO Technology, Sweden, M . M A R T IÂ N E Z - S A N Z , Novel Materials and Nanotechnology Group, IATA-CSIC, Spain and M . H E N R I K S S O N , Royal Institute of Technology, Sweden and SP Technical Research Institute of Sweden, Sweden

Abstract: This chapter presents a review of methods for the extraction of cellulose nanofillers, as well as the most important characteristic features related to the exploration of these nanofillers in composite applications. Various methods for the extraction and surface modification of cellulose crystals are presented for the adaption of cellulose crystals in composite applications. A brief review of the different morphological characteristics as well as mechanical properties of different cellulose nanofillers are also presented. Key words: cellulose, microfibrils, extraction, nanocomposite, processing.

4.1

Introduction

Cellulose is the most abundant renewable polymer on earth and is responsible for the structural integrity of wood, plants and algae, as well as some sea animals and microbial cellulose. Ultimately, this structural integrity has been related to rod-like, load-bearing crystal units composed of poly- (1,4)-D-glucopyranoside chains organized parallel in a highly ordered manner.1±3 The crystal units were originally referred to as `cellulose microfibrils' or èlementary fibrils of cellulose', though the terms `nanowhiskers', `protofibrils', `nanofibrils', etc., have also been used to designate cellulose nanofillers (CNFs) as the topic has become the subject of intense research.4,5 Lately, the intrinsic mechanical properties (strength and stiffness) of CNFs have been in focus, and modulus values in excess of 130 GPa have been reported, whereas the mechanical strength may exceed 7±10 GPa.6±13 The mechanical properties are not far from those of some grades of steel, which suggest that CNFs may eventually find use as reinforcement agents in composite formulations with engineering polymers.14 In addition, CNFs also show many other useful characteristics such as high sound attenuation,15 high gas impermeability,16 non-abrasive nature in combination with very high specific surface area17 and low density (ca. 1500 kg/m3),18 which can be considered unique for an inexpensive biodegradable material.



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However, although significant research advances have been reported concerning the specific characteristics of CNFs, a number of hurdles still exist before these materials can be fully exploited in commercial applications. Primarily, the processing of CNFs into polymer composites has proven to be challenging due to the hydrophilic surface of CNFs, often resulting in severe agglomerations caused by CNF surface incompatibility with the polymer host matrix. Another hurdle is related to the procedures used to isolate CNFs from the cellulose source material and the tailoring of their surface properties to improve the CNF surface solubility in different polymers. Extraction procedures need to be cost-efficient and performed with low CNF degradation, whereas efficient surface coatings are necessary for a predictable dispersion of the CNF in polymer matrices.19 Provided that systematic investigations on these topics are performed, CNFs may become important polymer fillers in commercial plastics, and potentially their load-bearing qualities can be taken advantage of in plastics in similar ways as in nature. In this chapter, we will try to survey some of the important features related to CNFs and their potential use in composite applications.

4.2

Morphological and structural characteristics of cellulose nanofillers

Cellulose nanofillers are typically long and slender micron-sized crystal units that show a whisker-like, rectangular cross-sectional area in the nanoscale with dimensions depending on the cellulose source. The cross-sectional dimensions of the plant parenchyma are ca. 2±3 nm, those of CNFs from wood sources 2±4 nm, from bacterial cellulose 4±7 nm, and for cotton linters and ramie 7±9 nm and 10± 15 nm, respectively.20±28 Marine resources yield CNFs with larger diameters: for tunicate marine animals ca. 20 nm, whereas algae contain 10±70 nm wide CNFs.6,29,30 Figure 4.1 shows a selection of CNFs derived from different cellulose sources. It can be approximated that a 3 nm thick and 4 nm wide CNF contains ca. 150 200 poly- (1,4)-D-glucopyranoside chains aligned parallel along the longitudinal direction of the CNF. The chains can be configured slightly differently depending on how the chains are twisted around their axis and interact by intramolecular hydrogen bonding with neighboring chains, thereby creating different allomorphs. It was recently demonstrated that the cellulose crystal unit is a composite of two crystalline phases (allomorphs), I and I , which have been assigned to triclinic and monoclinic unit cells, respectively.35±38 The allomorphs vary in proportion depending on the origin of the CNF. The cellulose I is an unstable phase and tends to transform into the I phase upon thermal heating.39,40 This transformation works best in polar media such as dilute alkali solutions, and it was suggested that medium interaction with the cellulose chains renders the chains more flexible and prone to reconfiguration.41


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4.1 (a) Micrograph of wood pulp cellulose microcrystals, from Battista [31]. (b) Micrograph of freeze-dried microfibrillated cellulose from wood pulp, from Henriksson et al. [32]. (c) Micrograph of microfibrils from Valonia ventricosa (alga), from Horikawa and Sugiyama [33]. (d) Micrograph of rod-like cellulose microcrystals extracted from the mantle of the tunicate Microcosmus fulcatus, from Favier et al. [34].

An explanation for this morphological transformation phenomenon has been proposed as a break-slip model based on molecular dynamics simulation, whereas Wada et al. experimentally related this transformation to heat-induced thermal expansion of the crystal lattice, allowing for rearrangements of the hydrogen bonds between hydroxyl groups.41,42 The morphology of microfibrils extracted by acidic hydrolysis from microbial cellulose and absorbed on a silicon wafer from an aqueous suspension is displayed in Fig. 4.2. Whereas the hydroxyl groups inside the crystal units link the poly- (1,4)-Dglucopyranoside chains by creating hydrogen bonds with oxygen molecules on neighboring chains, the hydroxyl groups are also present on the surface of the CNF and serve to interconnect the CNFs in the formation of bundles (Fig. 4.2a). However, depending on the configuration of the surface-located hydroxyl groups on the poly- (1,4)-D-glucopyranoside chains, the surface reactivity and their capability for inter-nanofiller condensation reactions vary, and have been expressed as an availability ratio between O(2)H:O(3)H:O(6)H groups (Fig. 4.3).43±46 Rowland, Verlac and others43±46 showed that among the three groups, the availability of the O(3)H group is particularly sensitive to the surface perfection of the crystals. If the CNF surface is highly ordered as inside the cellulose



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4.2 (a) FE-SEM micrograph displaying CNF derived from bacterial cellulose by acid hydrolysis (coated with 1±2 nm thick gold/palladium layer). (b) FE-SEM micrograph displaying CNF derived from bacterial cellulose by acid hydrolysis (not coated).

crystals, the O(3)H group is unreactive due to its strong intramolecular bonds with O(50 ), whereas substantial reactivity of the O(3)H groups has been reported for more disorganized cellulose crystals. The CNF crystalline units with the highest perfection have been reported to originate from the green alga Valonia.47 From an interpretive perspective, the crystals from this source may be considered less prone to surface-modification reactions than those from cellulose from cotton, for example, which exhibit less internal chain order and a more disorganized surface, i.e., leaving a larger amount of O(3)H available for modification. The surface reactivity can also be related to induced functional surface molecules remaining from the extraction procedure (Section 4.4). Whereas the cross-sectional dimensions (thickness and width) of CNFs show generic values depending on the source of the crystals, the lengths of CNFs have received less attention. However, it can be presumed that generic length dimensions also can be related to the CNF source material, but due to the inherent difficulty of ascertaining that the extracted CNFs retain their natural length (post extraction) as related to the source, very little systematic information has been reported on this topic. The complications lie in that the CNFs occasionally break at imperfections (possibly less organized crystal regions) along the crystals during extraction and therefore show a distribution of different lengths.48,49



4.3 Molecular structure of the poly- (1,4)-D-glucopyranoside chain with central repeating cellubiose unit.


4.3

91

Extraction and refining of cellulose nanofillers

Cellulose nanofillers (CNFs) can be extracted from the source biomaterial by chemical hydrolysis and/or by applying large mechanical shear forces onto a cellulose suspension.

4.3.1

Extraction by chemical hydrolysis

A commonly used extraction methodology of CNFs is acidic hydrolysis of the amorphous regions surrounding the embedded CNFs and cleavage of the bundles, followed by filtration or centrifugation to exclude dissolved noncrystalline elements.50±57 The methodology is beneficial in that it can be performed on very small quantities of cellulose, it requires only the simplest laboratory equipment, and the CNFs can be obtained without any induced imperfections caused by mechanical processing. The conditions typically involve the use of aqueous solutions of sulfuric acid, stirred at 50±60ëC at atmospheric pressure until a homogeneous beige solution is obtained. This procedure results in cellulose nanocrystals having anionic groups on the surfaces (leading to electrostatic stabilization of the nanocrystals in suspension) with the ability to form chiral nematic liquid crystalline phases in concentrated solutions.54,57,58 The obtained form of cellulose was denoted microcrystalline cellulose, MCC, by Battista.31 With chemical hydrolysis the yield of CNF can be high (>80%), provided the original source is highly crystalline.49 It can, however, be expected that the yield is strongly influenced by the conditions used, since excess hydrolysis results in degradation of the CNF. Exaggerated hydrolysis can typically be noted as the solutions turn dark or black in color as the degradation of the CNF occurs. This phenomenon was reported by Roman et al. who assigned the crystal degradation to potential induced thermal degradation related to the sulfate groups introduced as a functional surface on the CNF when sulfuric acid is used for the hydrolysis.59 However, no mechanism for the degradation related to exaggerated hydrolysis has been presented. So far, the literature provides relatively scarce systematic information on optimized extraction conditions as related to different sources of CNF in terms of acidic strength, temperature and pressure, and how these conditions relate to the intrinsic properties of the CNF. It is notable, however, from earlier literature that temperature and acidic strength may provide efficient tools worth considering for successful extraction procedures, and an increase of 10ëC in temperature has a much greater effect on the rate of hydrolysis than doubling the acid concentration.60 Figure 4.4 shows an example of two solutions of extracted CNFs from bacterial cellulose networks, differing only in their respective hydrolysis temperature. In addition to sulfuric acid, hydrochloric acid has also been used for hydrolysis extraction, leading to less stable suspensions due to smaller amounts


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4.4 (a) Suspension of CNFs (white) at optimized acidic hydrolysis conditions from bacterial cellulose networks. (b) Exaggerated acidic hydrolysis conditions for the same bacterial cellulose networks result in a darker solution.

of induced anionic groups on the crystal surfaces.61±63 The smaller amount of surface charges results in the solutions not exhibiting the same gel-like properties and not dispersing in polar solvents as well as cellulose nanocrystals extracted by sulfuric acid solutions. However, cellulose nanocrystals extracted by hydrochloric acid can be dispersed in protic solvents such as formic acid and m-cresol, since these solvents are able to disrupt the hydrogen bonds in aggregated crystals.64 Furthermore, owing to their reported less elongated shape, the hydrochloric acid-hydrolyzed crystals are sometimes easier to disperse and implement as reinforcement in composite materials.65,66

4.3.2

Extraction by mechanical force

The mechanical methods to extract CNFs from wood pulp and parenchyma cells typically involve a high-pressure homogenizer treatment,67±70 a microfluidizer,19,71 a high-pressure refiner, a super-grinder treatment72±75 or ultrasonication.76 The obtained form of cellulose was denoted microfibrillated cellulose, MFC, by Herrick et al.67 and Turbak et al.68 These processing methodologies have in common that they rely on applying large shear forces on cellulose fiber suspensions in order to mechanically liberate the CNFs from the original plant cell wall structure. In a high-pressure homogenizer this is achieved by allowing a cellulose suspension to pass under high pressure through a thin slit where it is subjected to large shear forces. The shear forces serve to disintegrate the microfibrils or microfibril bundles in the plant cell wall, resulting in nanofibers with diameters of about 5±100 nm. During this homogenization, the



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viscosity of the cellulose suspension increases in relation to the increase in the Einstein coefficient, which increases with length per diameter ratio of the suspended particles.77 Practically, this sets a limit on the original cellulose suspension concentration to approximately 2 wt% fibers, as greater concentrations become overly viscous to force through the system due to the limitations on the pump system. However, the character of the original plant cell wall also influences the number of cycles the suspension has to be passed through the slit, and the procedure is usually experimentally evaluated (by microscopy) and optimized to favor the complete disintegration of cellulose nanofibers. The functional part of a slit homogenizer and the general principle of a microfludizer are illustrated in Fig. 4.5. The function of the high-pressure homogenizer is reviewed in detail by Rees.78 Various pretreatment methods have been developed to facilitate the extraction if the flocculation of the cellulose fibers is severe and causes problems during processing, or if the nanofibers are not sufficiently disintegrated to yield individual nanofibers.32,79 These methods include reduction of the pulp fiber length by mechanical cutting,67 acid hydrolysis,80 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-mediated fiber oxidation,70 swelling,79 enzymatic hydrolysis in combination with beating,32,71 and cryocrushing.12,69,81 Herrick et al.67 showed that pre-cut fibers facilitated the disintegration of the nanofibers by increasing the stability of the pulp fiber suspension, preventing it from sedimentation and interfering with the pumping system in the homogenizer. They also suggested that the degree of fibrillation increased with a more significant exposure of the fiber cross-sectional area. In the case of acid hydrolysis, it was suggested that the more facile disintegration stems from a more brittle cell wall resulting from the hydrolysis reaction. 80 Enzymatic treatment with endoglucanase has been suggested to facilitate microfibrillation by swelling of the pulp fibers32 and in combination with processing in a microfluidizer, which results in nanofibers with dimensions of approximately 10±40 nm.32 TEMPOmediated oxidation introduces negative charges on the surface of the microfibrils, resulting in very efficient microfibrillation during light mechanical treatment, and diameters of 3±5 nm have been reported for nanofibers from wood, cotton and tunicates (see Fig. 4.6).70,82 Mechanical extraction has been applied to several types of cellulose sources, such as wood,19,32,67,68,71,79 sugar beet,69,83 potato tuber,84 banana rachis,85 and wheat straw and soy hulls.86 The disintegration of nanofibers from plant sources normally requires less energy and is easier to liberate from the fiber matrix as compared to isolating fine microfibrils from multilayered structures such as wood pulp fibers, especially if hydrogen bonds have formed between the nanofibers, as in dried pulp.87


ß Woodhead Publishing Limited, 2011 4.5 (a) Functional valve causing disruption of agglomerates in a typical homogenizer, from Rees [78]. (b) Principal sketch showing a microfluidizer with interaction chamber for disruption of agglomerates, from Microfluidics Corp.


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4.6 (a) Ca. 20 nm wide TEMPO-mediated oxidized CNF from tunicates, from Habibi et al. [82]; (b) 3±5 nm wide oxidized CNF from wood, from Saito et al. [70]. Both dispersed on TEM grids.

4.4

Mechanical properties of cellulose nanofillers

The true value of the crystal modulus of cellulose is an important property, since it sets an upper limit to what is achievable in terms of reinforcing capacity in polymers. Several values have been suggested in the literature, both theoretical and experimental. Meyer and Lotmar reported the first theoretically modeled value of about 120 GPa in 1936.88 Their structure was found to be incorrect and was later corrected by Treloar, who reported a lower value of 56 GPa.89 The modulus for cellulose I crystals was first determined experimentally by Sakurada et al. to 134 GPa from the observation of the change in the c-spacing measured by X-ray diffraction of deformed fiber bundles.90 This crystal modulus is significantly higher than theoretical values, which may be explained by the fact that most theoretical calculations presume uniform stress in the cellulose crystals. In addition, the orientation and distribution of amorphous and crystalline segments also affect the measurement of the elastic modulus by X-ray diffraction. It was pointed out that experimental and theoretical crystal modulus values differed when a parallel coupling between crystalline and amorphous regions is present in the cellulose structure. The significance of this morphological dependence decreased when the degree of molecular orientation and the crystallinity increased. Therefore, the use of fibers or films with high molecular orientation and crystallinity was recommended for the determination of the crystal modulus by X-ray diffraction.91 This highlights the difficulty of accurately determining the crystal modulus of less crystalline cellulose samples. An alternative method is the determination of the crystal modulus by means of Raman spectroscopy. The shift in the 1095 cm±1 band, which is characteristic for cellulose, as a function of tensile deformation, is measured and related to the modulus of cellulose crystals. This technique was applied to measure the micromechanical properties of microcrystalline cellulose, and an elastic modulus of 25 4 GPa was reported.92 By means of this method, a modulus


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of 143 GPa was determined for tunicate whiskers,93 while a lower value of 114 GPa was determined for bacterial cellulose nanofibers.6 A more recent determination of the cellulose crystal modulus using inelastic X-ray scattering (IXS) reported a value for the elastic modulus of 220 GPa.94 This technique exhibits better selectivity than traditional X-ray diffraction, since it is much less sensitive to the contributions of amorphous regions compared with the crystalline regions. A strong anisotropy was observed, with the elastic modulus of 220 GPa in the fiber direction while the modulus in the perpendicular direction was 15 GPa. Single-fiber measurements of the nanofiber stiffness have been conducted by AFM on bacterial cellulose95 and tunicate whiskers.8 In this case the reported modulus is the macroscopic modulus for the nanofibers in comparison with the previously mentioned measurement on the cellulose crystal stiffness. The modulus of bacterial cellulose was experimentally determined as 78 17 GPa,95 while the modulus for the tunicate whiskers was 145 GPa or 150 GPa depending on the extraction method ± TEMPO-oxidation or acid hydrolysis, respectively.8 The modulus for the tunicate whiskers is comparable with the reported values for the cellulose crystal while the modulus for bacterial cellulose is significantly lower. The reason for this is believed to be due to the differences in crystallinity. The crystallinity for the bacterial cellulose was determined to be about 60%,95 while the tunicate whiskers are highly crystalline. Regardless of the method used for measuring the crystal modulus of cellulose, the obtained values are comparable with those of high-performance synthetic fibers such as aramid (130 GPa).96 The crystal modulus is also well above the modulus for aluminum (70 GPa) and glass fibers (76 GPa).96 The ultimate tensile strength of cellulose is estimated to be 17.8 GPa, which is seven times higher than that of steel per weight.96

4.5

Surface modification of cellulose nanofillers

Cellulose nanofillers have a high tendency for self-association due to their strongly interacting surface hydroxyl groups. These interactions lead to the aggregation of CNFs, which often is undesirable for the preparation of nanocomposites. A uniform dispersion of the CNFs, and adhesion between the nanofillers and the polymer host matrix generally are prerequisites for obtaining improved mechanical properties of the resulting nanocomposites. In fact, the main drawbacks of using cellulose nanofillers as functional elements for nanocomposite preparations is their polar and hydrophilic nature, which causes incompatibility issues with most organic solvents and hydrophobic thermoplastic matrices. However, to achieve a controlled dispersion of cellulose nanofillers within the polymeric matrix, several strategies have been developed. One method to enable dispersion in organic media is to coat the surface of the nanofillers with a surfactant.97±103 The surfactant migrates within the organic



97

medium and adsorbs onto the hydrophilic surface of the CNFs due to its amphiphilic characteristics, leaving its hydrophobic molecule section in the organic phase of the suspension. Stable suspensions of cotton, tunicate and wood crystals coated by surfactants were obtained in tetrahydrofuran (THF), toluene and cyclohexane,97,98,103 and by small angle neutron scattering (SANS) experiments it was revealed that the surfactant layer showed a thickness of Ê covering the crystals.98 The use of a surfactant is a very convenient ca. 15 A method for improving the dispersion in organic solvents. However, successful use of surfactants relies on selecting amphiphilic molecules with a moderately hydrophobic nature adjusted for the intended organic medium. This is due to the fact that a too-optimized solubility match for the organic medium tends to hinder migration so that the surfactant stays in the organic phase. For this reason, sometimes a very high amount of surfactant is required to coat a high specific surface filler such as cellulose crystals (four times the weight of the crystals),97 which occasionally leads to crystals consisting mostly of surfactant (1.6 times surfactant to the weight of the crystals).99 It is also notable that if the cellulose crystals are surfactant-modified within a solvent medium and further transferred by solvent-exchange procedures, or drying, into a composite application, it is difficult to ascertain to which quantity the surfactant remains on the surface of the crystals within the composite. The lack of covalent bonds between the hydroxyl groups on the crystals' surface and hydrocarbon-functional groups on the surfactant may also pose limitations on the use of this technique in composite applications, since covalent bonds in general are more efficient in providing adequate strength to the composites.104±107 As an alternative, covalent modification of the reactive hydroxyl groups on the surface of the CNFs has gained attention and has been widely studied over the last decade. These modifications include silylation, acetylation, esterification and graft-polymerization reactions. Silylation generally relies on the condensation reaction between the hydrolyzed alkoxy, acetoxy or chloro groups of a hydrocarbon-functional silane coupling agent and the hydroxyl groups on the CNFs' surfaces, leading to the formation of a condensed hydrocarbon-functional silsesquioxane cover on the crystals.108 The condensation reaction (the dissociation of the silane molecules) is energy-driven and depends strongly on the amount of water in the solutions.109 The reactions are normally catalyzed by either acids or bases and proceed at a minimum rate at pH 7.110 For example, tunicin whiskers were stabilized in organic solvents of low polarity such as acetone and THF by a partial silylation of their surface, using alkylchloro silanes as silylation agents. Interestingly, the partially modified whiskers appeared to be significantly more flexible than the unmodified crystals. However, strong silylation conditions or excess reaction times resulted in destruction of the whisker morphology; thus, the outcome relied on the compromise between extent of silylation and preservation of the cellulose morphology.111 This phenomenon was later confirmed for the same type of silane


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by Andresen et al. on microfibrillated cellulose from softwood pulp.112 Alkylalkoxy silanes have also been used for the modification of cellulose crystals.113±116 In general, the silylation route can be regarded as very versatile. However, the main drawback with the silylation reactions relates to sensitivity of the reaction conditions, which are strongly influenced by pH, temperature, and ± in particular ± traces of water in the crystal suspensions. Excess access to water affects the dissociation and condensation of the silane molecules, which immediately reacts and starts to produce dimers and larger oligomers also in absence of the cellulose.117 The outcome of the reaction is therefore much related to the competing polymerization reactions in the suspension and the adsorption of condensed smaller and larger complexes on the surface of the crystals.110 For inorganic nanoparticle systems these phenomena have resulted in coatings of agglomerates rather than the individual nanoconstituents.118,119 Surface acetylation or acylation of CNFs is performed by reacting the reactive hydroxyl groups on the surface of the nanofillers with either acid or anhydride groups, leading to the transformation of hydroxyl groups into acetyl groups (acetylation) or more generally into acyl groups (acylation). The gradual conversion of cellulose into cellulose triacetate (CTA) by adding a mixture of acetic anhydride and acetic acid in the presence of a small amount of catalyst was studied in order to elucidate the mechanism of the acetylation. The reaction appears to start within the amorphous, less organized regions and is followed by the acetylation of the cellulose crystals.120 Highly hydrophobic cellulose crystals were obtained by acylation with alkenyl succinic anhydride.121 A single-step process was developed in which surface acetylation, through Fischer esterification, occurred simultaneously with the hydrolysis of the amorphous cellulose, yielding acetylated cellulose nanowhiskers in a one-pot reaction.122 Surface acetylation has also been used to modify the physical properties of bacterial cellulose, while preserving its microfibrillar morphology.123 It is noteworthy that the degree of acetyl substitution has a significant effect on the properties of the obtained material, and that excess acetylation could have detrimental effects on the final properties in end-use applications regarding, for example, optical transmittance and hygroscopicity.124 Cellulose nanocrystals have been esterified by reaction with organic fatty acid chlorides, with varying aliphatic chain lengths (C12 to C18).125 With the applied method, the obtained degree of substitution was high enough so that the long-chain fatty acids (C18) could crystallize on the surface of the cellulose nanocrystals. Another method for surface esterification is the gas-phase process, in which the surface of cellulose nanocrystals can be almost completely reacted with fatty acid chains, while maintaining the original morphology of the crystal, and also leaving the core of the crystal unmodified.126 Polymer grafting can be conducted through two main routes, `grafting-to' and `grafting-from' the cellulose crystals. The `grafting-to' route involves attachment of pre-synthesized polymers to the CNF surface using a coupling agent.



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The possibility of controlling the polymer size and size distribution is a significant advantage with the `grafting-to' approach, which is possible since the polymer is pre-synthesized and can be readily characterized prior to the reaction. However, the approach shows limitations with regard to the grafting density due to steric hindrance because there is a potential for the grafted polymer chains to block the reactive sites on the cellulose surface. The `grafting-from' technique is based on in situ surface-initiated polymerization from immobilized initiators on the substrate. This method may allow access to a higher graft density and better control of the overall structure due to organized growth of the polymer functionality from the surface of the crystals. Grafting of poly(-caprolactone) (PCL) chains onto the surface of cellulose and starch nanocrystals has been performed by previously subjecting the polymeric matrix to reaction with isocyanate functionalities,127 and PCL-grafted nanoparticles were combined with a PCL matrix to obtain films by casting/ evaporation. The grafting of PCL chains on the surface resulted in higher mechanical modulus and ductility of films, thus indicating the formation of a percolating network owing to chain entanglements and co-crystallization. Octadecyl isocyanate has also been used as grafting agent in order to improve the compatibility of MFC with PCL.128 Microfibrillated cellulose was successfully grafted with PCL by means of ring-opening polymerization (ROP) in order to obtain stable suspensions of MFC in non-polar solvents and to improve the compatibility with PCL.129,130 Grafting of cellulose nanocrystals with poly(styrene) (PS) was performed through atom transfer radical polymerization (ATRP). The hydroxyl groups on the cellulose nanocrystals were esterified with 2-bromoisobutyrylbromide to yield 2-bromoisobutyryloxy groups, which were used to initiate the polymerization of styrene.131,132 Similarly, surface-initiated single electron living radical polymerization (SI-SET-LRP) was employed to polymerize N-isopropylacrylamide from the surface of cellulose nanocrystals to produce thermo-responsive substrates.133 It is important to note that aqueous suspensions of CNFs are stable at lower pH values when they have been extracted by means of sulfuric acid treatment, since negatively charged sulfate groups are introduced on the surface, thus inducing electrostatic repulsion between the CNFs. Accordingly, for watersoluble polymers that allow for uniform mixing with aqueous suspensions of CNFs, it may prove unnecessary to surface-modify the nanofillers for obtaining high dispersion of the cellulose component in composite applications.

4.6

Preparation of cellulose-reinforced nanocomposites

The formation of strong hydrogen bonds between cellulose nanocrystals as the water was removed from a suspension of microcrystalline cellulose (CNF) was


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originally demonstrated by Battista.31 The strong hydrogen bonds between adjacent CNFs allowed for the formation of a dry, stiff and strong network of CNFs, which was impossible to disperse in water. The high stiffness and strength of the cellulose CNFs in combination with the vast sources of cellulose promoted further research efforts. However, the ability of CNFs to condense into a hard and dense material can be very useful, as well as completely disastrous in the process of preparing cellulose-based nanocomposites. In the composites the CNF network improves the mechanical and thermal properties of the composite material. Favier et al. prepared composites by mixing tunicate whiskers and a polymer latex, followed by solution casting.33 The stiffness of the composite was increased compared with the unfilled polymer with additions of only a few percent of whiskers. The thermal stability of the composite was also increased. At an addition of 6 wt% of whiskers the storage modulus was stabilized at temperatures well above the glass transition temperature. This improvement was described as being due to the formation of a whisker network within the nanocomposite. The mechanical potential of CNF networks can also be demonstrated by cellulose nanopapers, i.e., cellulose nanofiber networks. Moduli for these cellulose nanopapers are reported to be in the range of 1±16 GPa19,69,75,134±137 and as high as 30 GPa for bacterial cellulose-based nanopapers.138 There are different approaches reported in the literature on how to prepare cellulose-reinforced nanocomposites. Due to the strong hydrogen bonds formed between adjacent CNFs during drying procedures, the most successful practice in nanocomposite preparation methodologies relies on maintaining the CNFs in the never-dried dispersed state (with or without surface modification) prior to the incorporation in the polymer matrix. Some methods are based on casting where a water-soluble polymer is mixed with the CNF water suspensions and cast. The composite is then formed after water evaporation.19,139,140 Nonsoluble polymers can be used as water-borne latex and directly mixed with the nanofiller suspension in a similar manner as water-soluble polymers. During drying, the polymer particles coalesce and a reinforced polymer composite is formed.33 Solvent-exchange procedures allow for maintaining the CNFs in their non-agglomerated wet state in some organic solvents, thereby omitting drying of the CNFs post extraction. Unmodified CNFs have been reported to successfully disperse in different aprotic solvents,141,142 whereas dispersion in other solvents can be facilitated by the use of surfactants, by chemical modifications, and by polymer grafting. However, in many systems the CNFs still show limited surface solubility regardless of surface modifications, and additional energy is required to maintain the dispersion at a reasonably high level. This energy could be supplied in the form of ultrasound, high-shear mixing or equivalent, which potentially will provide sufficient energy for instant disruption of the aggregated nanoclusters, and thereby open a processing window.



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A different approach to nanocomposite preparation is to impregnate the porous nanopaper structures with a monomer or polymer, which is then cured. The nanopapers are prepared by filtration of fiber suspensions. The wet nanofiber networks can be directly impregnated with the matrix solution143,144 or dried, after which the dry nanopaper is immersed in the solution.13,18,145 Attempts have also been made to process CNFs by melt extrusion.146,147 Provided that sufficient dispersion can be achieved and that the nanocomposite material is formed with a uniform dispersion on a macroscopic as well as a nanoscopic level, there are a number of parameters that can be considered important in terms of evaluating the enhancement in mechanical properties of cellulose nanocomposites: · Matrix/filler and filler/filler interactions. The predominance of one of these interactions over the other depends on the matrix structure and its affinity for the CNF. In the case of cellulose whisker composite materials, it has been observed that too high an affinity between the cellulose whiskers and the polymeric matrix may not always favor the mechanical properties.148,149 Accordingly, matrix±CNF as well as CNF±CNF interactions play a crucial role in the reinforcement effect, since hydrogen bonding among the whiskers leads to the formation of rigid whisker networks, which facilitate the stress transfer from the matrix. · The geometrical aspect ratio (L/d) of the filler, i.e., the ratio between the length L and the diameter d of the nanofillers. This parameter is directly influenced by the source of cellulose and the preparation conditions of the CNFs. It is advantageous to have an aspect ratio larger than 50 in order to have a considerable reinforcement effect when compared with micron-sized filaments.150 Nevertheless, for aspect ratios larger than 100, Young's modulus reaches a plateau which corresponds to the maximum point of reinforcement.150 · The processing method. The solvent-casting technique was found to give higher mechanical performance nanocomposites than those obtained by freeze-drying/molding.12,151 This behavior was ascribed to the sedimentation of the filler during evaporation of the solvent and to the whisker/whisker interactions. A decrease in the apparent aspect ratio of whiskers was thought to take place when hot pressing or extrusion is used, due to a gradual breakage and/or orientation of the whiskers.152

4.7

Future trends and applications of cellulose nanofillers

CNFs are likely to be of high relevance in future polymer composite formulations provided that means for their extraction, isolation and refinement, as well as surface modifications, can be systematically developed. It can be


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presumed that exploitations of the CNFs initially will find use in high-end applications, where their use presents significant advantages in limited quantities. Possibly, these applications include membranes/filters and wound dressings, dental implants, advanced glue systems, strong adhesive tapes and products requiring high transparency in combination with improved mechanical properties. It was recently demonstrated that crystals from cellulose not only provide improved mechanical properties but can also be associated with a high level of optical transparency, provided that polymer matrix host material is selected with care.143,153 As inexpensive large-scale production of CNFs are continuously being developed, the main hurdle for their implementation on an industrial level will undoubtedly be related to finding the extraction procedures and simple methodologies to surface-modify CNFs as integrated in a system. It is noteworthy that some recent literature shows improved barrier properties as related to the cellulose crystal contents, which implies that CNFs may eventually find use in more environmentally friendly packaging materials.154±159 Implementation of CNFs as barrier agents in packaging films would present a significant leap forward in the direction of creating a more sustainable environment. Currently, layered silicates are the most commonly used barrier modifiers in the plastics industry; however, the natural layered silicates are not from a renewable resource or biodegradable, whereas CNFs possess these characteristics.160±165 A promising area relates to the implementation of CNFs in current industry-emerging biopolymers, such as poly(lactic acid) (PLA), polyhydroxylalkanoates (PHA) and polycaprolactones (PCL), of which PLA and PHA are fully renewable and biodegradable plastics.

4.8 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

References Mutwil, M.; Debolt, S.; Persson, S. Curr. Opin. Plant Biol., 2008, 11, 252±257. Delmer, D. P.; Amor, Y. Plant Cell, 1995, 7, 987±1000. Neville, A. C.; Levy, S. Planta, 1984, 162, 370±384. Ohad, I.; Danon, I. O.; Hestrin, S. J. Cell. Biol., 1962, 12, 31±46. Ohad, I.; Danon, D. J. Cell. Biol., 1964, 22, 302±305. Hsieh, Y.-C.; Yano, H.; Nogi, M.; Eichhorn, S. J. Cellulose, 2008, 15, 507±513. Nishiyama, Y. J. Wood Sci., 2009, 55, 241±249. Iwamoto, S.; Kai, W.; Isogai, A.; Iwata, T. Biomacromolecules, 2009, 10, 2571± 2576. Hepworth, D. G.; Bruce, D. M. J. Mater. Sci., 2000, 35, 5861±5865. Mark, R. E. Cell Wall Mechanics of Tracheids. Yale University Press, New Haven, CT, 1967. Kroon-Batenburg, L. M. J.; Kroon, J.; Northolt, M. G. Polym. Commun., 1986, 27, 290±292. Helbert, W; CavailleÂ, J.-Y.; Dufresne, A. Polym. Compos., 1996, 17, 604±611. Nakagaito, A. N.; Yano, H. Appl. Phys. A ± Mater., 2005, 80, 155±159. Hubbe, M. A.; Rojas, O. J. BioResources, 2008, 3, 1419±1491. George, J.; Sreekala, M. S.; Thomas, S. Polym. Eng. Sci., 2001, 41, 1471±1485.



103

16. Fukuzumi, H.; Saito, T.; Iwata, T.; Kumamoto, Y.; Isogai, A. Biomacromolecules, 2009, 10, 162±165. 17. Olsson, R. T.; Azizi Samir, M. A. S.; Salazar-Alvarez, G.; Belova, L.; StroÈm, V.; Berglund, L. A.; Ikkala, O.; NogueÂs, J.; Gedde, U. W. Nat. Nanotechnol., 2010, 5, 584±588. 18. Henriksson, M.; Berglund, L. A. J. Appl. Polym. Sci., 2007, 106, 2817±2824. 19. Zimmermann, T.; PoÈhler, E.; Geiger, T. Adv. Eng. Mater., 2004, 6, 754±761. 20. Thimm, J. C.; Burritt, D. J.; Sims, I. M.; Newman, R. H.; Ducker, W. A.; Melton, L. D. Physiol. Plantarum, 2002, 116, 164±171. 21. Frey-Wyssling, A.; MuÈhlethaler, K. Makromol. Chem., 1963, 62, 25±30. 22. Lai-Kee-Him, J.; Chanzy, H.; MuÈller, M.; Putaux, J.-L.; Imai, T.; Bulone, V. J. Biol. Chem., 2002, 277, 36931±36939. 23. Andersson, S.; Serimaa, R.; Paakkari, T.; SaranpaÈaÈ, P.; Pesonen, E. J. Wood Sci. 2003, 49, 531±537. 24. Jakob H. F.; Tschegg, S. E.; Fratzl, P. Macromolecules, 1996, 29, 8435±8440. 25. Peura, M.; Andersson, S.; Salmi, A.; Karppinen, T.; Torkkeli, M.; HñggstroÈm, E.; Serimaa, R. Mater. Sci. Forum, 2009, 599, 126±136. 26. Brown, R. M.; Willison, J. H. M.; Richardson, C. L. Proc. Natl. Acad. Sci. USA, 1976, 73, 4565±4569. 27. Klemm, D.; Schmauder, H. P.; Heinze, T. Biopolymers: Cellulose, eds Vandamme, E. J.; de Baets, S.; SteinbuÈchel, A. Vol. 6, Wiley-VCH Verlag, Weinheim, Germany, 2003. 28. Fink, H.-P.; Hofman, D.; Philipp, B. Cellulose, 1995, 2, 51±70. 29. Nieduszynski, I.; Preston, R. D. Nature, 1970, 225, 273±274. 30. Tsekos, I.; Orologas, N.; Herth, W. Phycologia, 1999, 38, 217±224. 31. Battista, O. A. Microcrystal Polymer Science. McGraw Hill, New York, 1975. 32. Henriksson, M.; Henriksson, G.; Berglund, L.A.; LindstroÈm, T. Eur. Polym. J., 2007, 43, 3434±3441. 33. Horikawa, Y.; Sugiyama, J. Cellulose, 2008, 15, 419±424. 34. Favier, V.; Chanzy, H.; CavailleÂ, J.-Y. Macromolecules, 1995, 28, 6365±6367. 35. Atalla, R. H.; VanderHart, D. L. Science, 1984, 223, 283±285. 36. VanderHart, D. L.; Atalla, R. H. Macromolecules, 1984, 17, 1465±1472. 37. Horii, F.; Hirai, A.; Kitamaru, R. Macromolecules, 1987, 20, 2117±2120. 38. Sugiyama, J.; Vuong, R.; Chanzy, H. Macromolecules, 1991, 24, 4168±4175. 39. Horii, F.; Yamamoto, H.; Kitamaru, R.; Tanahashi, M; Higuchi, T. Macromolecules, 1987, 20, 2946±2949. 40. Yamamoto, H.; Horii, F. Macromolecules, 1993, 26, 1313±1317. 41. Wada, M.; Kondo, T.; Okano, T. Polym. J., 2003, 35, 155±159. 42. Hardy, B. J.; Sarko, A. Polymer, 1996, 37, 1833±1839. 43. Rowland, S. P.; Roberts, E. J.; Wade, C. P. Text. Res. J., 1969, 39, 530±542. 44. Rowland, S. P.; Roberts, E. J. J. Polym. Sci. A1, 1972, 10, 2447±2461. 45. Rowland, S. P.; Howley, P. S. Text. Res. J., 1988, 58, 96±101. 46. Verlhac, C.; Dedier, J. J. Polym. Sci. Polym. Chem., 1990, 28, 1171±1177. 47. Sugiyama, J.; Okano, T.; Yamamoto, H.; Horii, F. Macromolecules, 1990, 23, 3196±3198. 48. LeppaÈnen, K.; Andersson, S.; Torkkeli, M.; Knaapila, M.; Kotelnikova, N.; Serimaa, R. Cellulose, 2009, 16, 999±1015. 49. Olsson, R. T.; KraÈmer, R. H.; LoÂpez-Rubio, A.; Torres-Giner, S.; Ocio, M.-J.; LagaroÂn, J.-M. Macromolecules, 2010, 43, 4201±4209. 50. RaÊnby, B. G. Acta Chem. Scand. 1949, 3, 649±650.


104 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71. 72. 73. 74. 75. 76. 77. 78. 79. 80. 81. 82. 83. 84. 85.

Multifunctional and nanoreinforced polymers for food packaging Battista, O. A. Ind. Eng. Chem., 1950, 42, 502±507. Mukherjee, S. M.; Woods, H. J. Biochim. Biophys. Acta, 1953, 10, 499±511. Marchessault, R. H.; Morehead, F. F.; Walter, N. M. Nature, 1959, 184, 632±633. Revol, J.-F.; Bradford, H.; Giasson, J.; Marchessault, R. H.; Gray, D. G. Int. J. Biol. Macromol., 1992, 14, 170±172. Revol, J.-F.; Godbout, L.; Dong, X.-M.; Gray, D. G.; Chanzy, H.; Maret, G. Liq. Cryst., 1994, 16, 127±134. Dong, X. M.; Kimura, T.; Revol, J.-F.; Gray, D. G. Langmuir, 1996, 12, 2076±2082. de Souza Lima, M. M.; Borsali, R. Langmuir, 2002, 18, 992±996. Fleming, K.; Gray, D., Prasannan, S.; Matthews, S. J. Am. Chem. Soc., 2000, 122, 5224±5225. Roman, M.; Winter, W. T. Biomacromolecules, 2004, 5, 1671±1677. Harris, E. E.; Kline, A. A. J. Phys. Chem., 1949, 53, 344±351. Araki, J.; Wada, M.; Kuga, S.; Okano, T. Colloid Surface A, 1998, 142, 75±82. Araki, J.; Wada, M.; Kuga, S.; Okano, T. J. Wood Sci., 1999, 45, 258±261. Araki, J.; Wada, M.; Kuga, S.; Okano, T. Langmuir, 2000, 16, 2413±2415. van den Berg, O.; Capadona, J. R.; Weder, C. Biomacromolecules, 2007, 8, 1353± 1357. Berglund, L. Natural Fibers, Biopolymers, and Biocomposites: Cellulose-based Nanocomposites, eds Mohanty, A. K.; Misra, M.; Drzal, L. Taylor & Francis, Boca Raton, FL, 2000. Hubbe, M. A.; Rojas, O. J.; Lucia, L. A.; Sain, M. BioResources, 2008, 3, 929±980. Herrick, F. W.; Casebier, R. L.; Hamilton, J. K.; Sandberg, K. R. J. Appl. Polym. Sci: Appl. Polym. Symp., 1983, 37, 797±813. Turbak, A. F.; Snyder, F. W.; Sandberg, K. R. J. Appl. Polym. Sci.: Appl. Polym. Symp., 1983, 37, 815±827. Dufresne, A.; CavailleÂ, J.-Y.; Vignon, M. R. J. Appl. Polym. Sci., 1997, 64, 1185± 1194. Saito, T.; Nishiyama, Y.; Putaux, J.-L.; Vignon, M.; Isogai, A. Biomacromolecules, 2006, 7, 1687-1691. È sterberg, M.; PaÈaÈkkoÈ, M.; Ankerfors, M.; Kosonen, H.; NykaÈnen, A.; Ahola, S.; O Ruokolainen, J.; Laine, J.; Larsson, P. T.; Ikkala, O.; LindstroÈm, T. Biomacromolecules, 2007, 8, 1934±1941. Chakraborty, A.; Sain, M.; Kortschot, M. Holzforschung, 2005, 59, 102±107. Taniguchi, T. Zairyo, 1996, 45, 472±473. Taniguchi, T.; Okamura, K. Polym. Int., 1998, 47, 291±294. Iwamoto, S.; Nakagaito, A. N.; Yano, H. Appl. Phys. A, 2007, 89, 461±466. Zhao, H.-P.; Feng, X.-Q.; Gao, H. Appl. Phys. Lett., 2007, 90, 073112. Nielsen, L. E.; Landel, R. F., Mechanical Properties of Polymers and Composites, 2nd edn, Marcel Dekker, New York, 1994. Rees, L. H., Chemical Engineering (New York), 1974, 81, 86±92. Abe, K.; Iwamoto, S.; Yano, H. Biomacromolecules, 2007, 8, 3276±3278. Boldizar, A.; Klason, C.; KubaÂt, J.; NaÈslund, P.; SaÂha, P. Int. J. Polym. Mater., 1987, 11, 229±262. Bhatnagar, A.; Sain, M. J. Reinf. Plast. Compos., 2005, 24, 1259±1268. Habibi, Y.; Chanzy, H.; Vignon, M. R. Cellulose, 2006, 13, 679±687. Dinand, E.; Chanzy, H.; Vignon, M. R. Food Hydrocolloid, 1999, 13, 275±283. Dufresne, A.; Vignon, M. R. Macromolecules, 1998, 31, 2693±2696. Zuluaga, R.; Putaux, J.-L.; Restrepo, A.; Mondragon, I.; Ganan, P. Cellulose, 2007, 14, 585±592.


Cellulose nanofillers for food packaging 86. 87. 88. 89. 90. 91. 92. 93. 94. 95. 96. 97. 98. 99. 100. 101. 102. 103. 104. 105. 106. 107. 108. 109. 110. 111. 112. 113. 114. 115. 116. 117. 118. 119. 120. 121. 122.

105

Alemdar, A.; Sain, M. Bioresource Technol., 2008, 99, 1664±1671. Iwamoto, S.; Abe, K.; Yano, H. Biomacromolecules, 2008, 9, 1022±1026. Meyer, K.H.; Lotmar, W. Helv. Chim. Acta, 1936, 19, 68±86. Treloar, L. R. G. Polymer, 1960, 1, 290±303. Sakurada, I.; Nukushina, Y.; Ito, T. J. Polym. Sci., 1962, 57, 651±660. Matsuo, M.; Sawatari, C.; Iwai, Y.; Ozaki, F. Macromolecules, 1990, 23, 3266± 3275. Eichhorn, S. J.; Young, R. J. Cellulose, 2001, 8, 197±207. Sturcova, A.; Davies, G. R.; Eichhorn, S. J. Biomacromolecules, 2005, 6, 1055±1061. Diddens, I.; Murphy, B.; Krisch, M.; Muller, M. Macromolecules, 2008, 41, 9755± 9759. Guhados, G.; Wan, W.; Hutter, J. L. Langmuir, 2005, 21, 6642-6646. Kamel, S. Express Polym. Lett., 2007, 1, 546±575. Heux, L.; Chauve, G.; Bonini, C. Langmuir, 2000, 16, 8210±8212. Bonini, C.; Heux, L.; CavailleÂ, J.-Y.; Lindner, P.; Dewhurst, C.; Terech, P. Langmuir, 2002, 18, 3311±3314. Ljungberg, N.; Bonini, C.; Bortolussi, F.; Boisson, C.; Heux, L.; CavailleÂ, J.-Y. Biomacromolecules, 2005, 6, 2732±2739. Kvien, I.; Tanem, B. S.; Oksman, K. Biomacromolecules, 2005, 6, 3160±3165. Ljungberg, N.; CavailleÂ, J.-Y.; Heux, L. Polymer, 2006, 47, 6285±6292. Rojas, O. J.; Montero, G. A.; Habibi, Y. J. Appl. Polym. Sci., 2009, 113, 927±935. Kim, J.; Montero, G.; Habibi, Y.; Hinestroza, J. P.; Genzer, J.; Argyropoulos, D. S.; Rojas, O. J. Polym. Eng. Sci., 2009, 49, 2054±2061. Judeinstein, P.; Sanchez, C. J. Mater. Chem., 1996, 6, 511±525. Guo, Z.; Pereira, T.; Choi, O.; Wang, Y.; Hahn, H. T. J. Mater. Chem., 2006, 16, 2800±2808. Keusch, S.; Haessler, R. Compos. Part A ± Appl. S., 1999, 30, 997±1002. Kessler, A.; Bledzki, A. Compos. Sci. Technol., 2000, 60, 125±130. Arkles, B. Chem. Tech., 1977, 7, 766±776. Nass, R.; Arpac, E.; Glaubitt, W.; Schmidt., H. J. Non-Cryst. Solids, 1990, 121, 370±374. Daniels, M. W.; Sefcik, J.; Francis, L. F.; McCormick, A. V. J. Colloid Interf. Sci., 1999, 219, 351±356. GousseÂ, C.; Chanzy, H.; Excoffier, G.; Soubeyrand, L.; Fleury, E. Polymer, 2002, 43, 2645±2651. Andresen, M.; Johansson, L.-S.; Tanem, B. S.; Stenius, P. Cellulose, 2006, 13, 665± 677. Lu, J.; Askeland, P.; Drzal, L. T. Polymer, 2008, 49, 1285±1296. Grunert, M.; Winter, W. T. J. Polym. Environ., 2002, 10, 27±30. Grunert, M.; Winter, W. T. Polym. Mater. Sci. Eng., 2000, 82, 232±238. Roman, M. and Winter, W. T. Cellulose Nanocomposites: Processing, Characterization, and Properties, eds Oksman, K.; Sain, M.; ACS Symposium Series 938, American Chemical Society, Washington, DC, 2006. Piana, K.; Schubert, U. Chem. Mater., 1994, 6, 1504±1508. Prozorov, T.; Prozorov, R.; Gedanken, A. Adv. Mater., 1998, 10, 1529±1532. Olsson, R. T.; Hedenqvist, M. S.; StroÈm, V.; Deng, J.; Savage, S. J.; Gedde, U. W. Polym. Eng. Sci., 2011, doi: 10.1002/pen.21892 Sassi, J.-F.; Chanzy, H. Cellulose, 1995, 2, 111±127. Yuan, H.; Nishiyama, Y.; Wada, M.; Kuga, S. Biomacromolecules, 2006, 7, 696±700. Braun, B.; Dorgan, J. R. Biomacromolecules, 2009, 10, 334±341.


106


123. Kim, D.-Y.; Nishiyama, Y.; Kuga, S. Cellulose, 2002, 9, 361±367. 124. Ifuku, S.; Nogi, M.; Abe, K.; Handa, K.; Nakatsubo, F.; Yano, H. Biomacromolecules, 2007, 8, 1973±1978. 125. Berlioz, S.; Molina-Boisseau, S.; Nishiyama, Y.; Heux, L. Biomacromolecules, 2009, 10, 2144±2151. 126. de Menezes, A. J.; Siqueira, G.; Curvelo, A. A. S.; Dufresne, A. Polymer, 2009, 50, 4552±4563. 127. Habibi, Y.; Dufresne, A. Biomacromolecules, 2008, 9, 1974±1980. 128. Siqueira, G.; Bras, J.; Dufresne, A. Biomacromolecules, 2009, 10, 425±432. 129. LoÈnnberg, H.; FogelstroÈm, L.; Samir, M. A. S. A.; Berglund, L.; MalmstroÈm, E.; Hult, A. Eur. Polym. J., 2008, 44, 2991±2997. 130. Habibi, Y.; Goffin, A.-L.; Schiltz, N.; Duquesne, E.; Dubois, P.; Dufresne, A. J. Mater. Chem., 2008, 18, 5002±5010. 131. Yi, J.; Xu, Q. X.; Zhang, X.; Zhang, H. Polymer, 2008, 49, 4406±4412. 132. Morandi, G.; Heath, L.; Thielemans, W. Langmuir, 2009, 25, 8280±8286. 133. Zoppe, J. O.; Habibi, Y.; Rojas, O. J.; Venditti, R. A.; Johansson, L.-S.; Efimenko, È sterberg, M.; Laine, J. Biomacromolecules, 2010, 11, 2683±2691. K.; O 134. Leitner, J.; Hinterstoisser, B.; Wastyn, M.; Keckes, J.; Gindl, W. Cellulose, 2007, 14, 419±425. 135. Bruce, D. M.; Hobson, R. N.; Farrent, J. W.; Hepworth, D. G. Compos. Part A ± Appl. S., 2005, 36, 1486±1493. 136. Yano, H.; Nakahara, S. J. Mater. Sci., 2004, 39, 1635±1638. 137. Henriksson, M.; Berglund, L. A.; Isaksson, P.; LindstroÈm, T.; Nishino, T. Biomacromolecules, 2008, 9, 1579±1585. 138. Nishi, Y.; Uryu, M.; Yamanaka, S.; Watanabe, K.; Kitamura, N.; Iguchi, M.; Mitsuhashi, S. J. Mater. Sci., 1990, 25, 2997±3001. 139. Mathew, A. P.; Dufresne, A. Biomacromolecules, 2002, 3, 609±617. 140. Zimmermann, T.; Pohler, E.; Schwaller, P. Adv. Eng. Mater., 2005, 7, 1156±1161. 141. Samir, M. A. S. A.; Alloin, F.; Sanchez, J.-Y.; El Kissi, N.; Dufresne, A. Macromolecules, 2004, 37, 1386±1393. 142. van den Berg, O.; Capadona, J. R.; Weder, C. Biomacromolecules, 2007, 8, 1353± 1357. 143. Nogi, M.; Ifuku, S.; Abe, K.; Handa, K.; Nakagaito, A. N.; Yano, H. Appl. Phys. Letter, 2006, 88, 133124. 144. Henriksson, M.; FogelstroÈm, L.; Berglund, L. A.; Johansson, M.; Hult, A. Compos. Sci. Technol., 2011, 71, 13±17. 145. Nakagaito, A. N.; Iwamoto, S.; Yano, H. Appl. Phys. A ± Mater., 2005, 80, 93±97. 146. Oksman, K.; Mathew, A. P.; Bondeson, D.; Kvien, I. Compos. Sci. Technol., 2006, 66, 2776±2784. 147. Bondeson, D.; Oksman, K. Compos. Part A ± Appl. S., 2007, 38, 2486±2492. 148. AngleÁs, M. N.; Dufresne, A. Macromolecules, 2001, 34, 2921±2931. 149. Dufresne, A.; Kellerhals, M. B.; Witholt, B. Macromolecules, 1999, 32, 7396±7401. 150. Eichhorn, S. J.; Dufresne, A.; Aranguren, M.; Marcovich, N. E.; Capadona, J. R.; Rowan, S. J.; Weder, C.; Thielemans, W.; Roman, M.; Renneckar, S.; Gindl, W.; Veigel, S.; Keckes, J.; Yano, H.; Abe, K.; Nogi, M.; Nakagaito, A. N.; Mangalam, A.; Simonsen, J.; Benight, A. S.; Bismarck, A.; Berglund, L. A.; Peijs, T. J. Mater. Sci., 2010, 45, 1±33. 151. Dufresne, A.; CavailleÂ, J.-Y.; Helbert, W. Polym. Composite., 1997, 18, 198±210. 152. Hajji, P.; CavailleÂ, J.-Y.; Favier, V.; Gauthier, C.; Vigier, G. Polym. Composite, 1996, 17, 612±619.



107

153. Nogi, M.; Iwamoto, S.; Nakagaito, A. N.; Yano, H. Adv. Mater., 2009, 21, 1595± 1598. 154. Sanchez-Garcia, M. D.; Lopez-Rubio, A.; LagaroÂn, J. M. Trends Food Sci. Tech., 2010, 21, 528±536. 155. Fendler, A.; Villanueva, M. P.; Gimenez, E.; LagaroÂn, J. M. Cellulose, 2007, 14, 427±438. 156. Hult, E.-L.; Iotti, M.; Lenes, M. Cellulose, 2010, 17, 575±586. 157. Bilbao-SaÂinz, C.; Avena-Bustillos, R. J.; Wood, D. F.; Williams, T. G.; McHugh, T. H. J. Agric. Food Chem., 2010, 58, 3753±3760. 158. Spence, K. L.; Venditti, R. A.; Rojas, O. J.; Habibi, Y.; Pawlak, J. J. Cellulose, 2010, 17, 835±848. 159. Petersson, L.; Oksman, K. Compos. Sci. Technol., 2006, 66, 2187±2196. 160. Ray, S. S.; Maiti, P.; Okamoto, M.; Yamada, K.; Ueda, K. Macromolecules, 2002, 35, 3104±3110. 161. Maiti, P.; Yamada, K.; Okamoto, M.; Ueda, K.; Okamoto, K. Chem. Mater., 2002, 14, 4654±4661. 162. Di, Y.; Iannac, S.; Sanguigno, L.; Nicolais, L. Macromol. Symp., 2005, 228, 115± 124. 163. Sanchez-Garcia, M. D.; Gimenez, E.; LagaroÂn, J. M. J. Plast. Film Sheet., 2007, 23, 133±148. 164. Chowdhury, S. R. Polym. Int., 2008, 57, 1326±1332. 165. Hamad, W. Cellulosic Materials; Fibers, Networks, and Composites. Kluwer Academic Publishers, Dordrecht, The Netherlands, 2002.


5

Electrospun nanofibers for food packaging applications S . T O R R E S - G I N E R , Novel Materials and Nanotechnology Group, IATA-CSIC, Spain

Abstract: Electrospinning is a novel fabrication technology based on high electric fields that can be used to produce polymer- and biopolymer-based mats composed of nanofibers or other nanostructures. Electrospun mats have been shown to possess unique functionalities originating from the very large surface-to-volume ratios of the nanofibers, the use of functional and/or renewable polymers or the encapsulation of bioactive non-polymer substances. The functional electrospun mats can be used as tools for the development of nanocomposite fabrics from a wide variety of plastics with improved performance for packaging applications. They could serve, for example, as a reinforcement to enhance the physical properties of plastics and bioplastics, as transparent layers to a gas barrier, and even as an emerging technology to design bioactive packaging with antimicrobial protection or delivery of nutraceuticals to foods. Key words: electrospinning, ultrathin fibers, nanocomposites, active and bioactive packaging.

5.1

Electrospinning

Looking genuinely at nature, nanofibers often serve as a basic platform on which either organic or inorganic components are built. For instance, cellulose nanofibers would represent the building blocks in plants while collagen nanofibers would do so in the animal body. The fiber structure exhibits, from a structural point of view, a certain ability to transmit forces along its length, thus reducing the amount of materials required. While strong enough for their designed purpose, nanofibers have the added advantage of giving high porosity to the natural supports, which allows faster diffusion of chemicals to the inner structure. To follow this extraordinary natural design, a process that is able to fabricate fiber nanostructures from a variety of materials and mixtures is an indispensable pre-requisite. Control of the nanofiber arrangement is also necessary to optimize such structural requirements. Electrospinning is a physical process used for the formation of ultrathin fibers by subjecting a polymer solution to high electric fields. A schematic representation of a typical laboratory electrospinning setup is shown in Fig. 5.1. In this configuration, a polymer solution is placed inside a syringe lying



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5.1 Typical electrospinning setup where a non-woven nanofiber mat is collected.

horizontally on a digitally controlled pump which permits adjustment and precise control of the solution flow-rate. The polymer solution is then pumped to a metallic millimeter-size needle which is connected to a high voltage power supply operated in positive DC mode and with low current intensity. At a critical high voltage (5±25 kV), the polymer solution droplet at the tip of the needle distorts and forms a Taylor cone to be ejected as a charged polymer jet. This stretches and is accelerated by the electrical field towards a grounded and oppositely charged collector. As the electrospun jet travels through the electrical field, the solvent completely evaporates while the entanglements of the polymer chains prevent it from breaking up. This results in the deposition of ultrathin polymer fibers on a metallic collector to habitually assemble the fibers as nonwoven mats. As elongation is accomplished via a contactless scheme, electrospun fibers are considerably thinner in diameter and thus higher in surface-to-volume ratio than fibers fabricated using conventional mechanical extrusion or a classical spinning process. For instance, while electrospun fiber diameters are habitually under the micron and also within the nanometric range, synthetic fibers produced via extrusion and fibers of biological origin such as cotton, wool, or silk are characterized by diameters in the range of various micrometers and above. Moreover, since the electrospinning is a continuous process, fibers when wound can be as long as several meters or even kilometers. The formed fibers are not only ultrathin and relatively long but also fully interconnected to form a three-dimensional network. Such small dimensions generally lead to very high ratios of specific surface area to mass and provide the electrospun products with extraordinary functionalities which are not found in similar materials of larger sizes. Fiber formation via electrospinning basically requires the materials to be processed to display specific viscoelastic properties, electrical conductivities in


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a limited range of values, and specific surface energies. Such requirements can only be met by polymers provided that molecular weights are chosen in an appropriate range. In order to carry out the process, the polymer must first be in a liquid form, either as molten polymer or as polymer solution. Melting electrospinning is limited by the fact that fibers are generally above the micron size and electrospinning from polymer solutions is then habitually preferred. In this last case, solvent properties such as boiling point and conductivity play a significant part in the electrospinning process and in the resultant electrospun morphology (Torres-Giner et al., 2008a). In particular, round-like fibers from less than 40 nm to a few microns can be fabricated by electrospinning of more than 50 different types of polymers (Li and Xia, 2004). In addition to observational changes in the fiber diameter, through the modification of the electrospinning setup other fiber and non-fiber assemblies with novel remarkable features can be produced, such as uniform or variable flat and round-like fibers, bead-like or round particles, core±sheath or multilayer coaxial structures, hollow tubes or porous fibers, aligned fibers, crosslinked fibers, and multi-jet fibers. This adjustability certainly enhances the performance of the electrospun materials, allowing application-specific modifications. Electrospun shapes are particularly developed by changing the polymer solution and process conditions: polymer concentration, solvent nature, tip-tocollector distance, voltage and flow-rate. Among all process parameters, the polymer concentration of the solution for electrospinning is the most relevant factor determining the fiber diameter (Torres-Giner et al., 2008b). Thus, higher concentrations generally result in the formation of fibers with larger diameters. Reduction of the polymer concentration below a threshold value would result in beads or in extremely thin fibers with beads along their length. This variation within the electrospinning technology is also called èlectrospraying' because the process acts like an atomization procedure to form nearly mono-dispersed and non-fibrillar ultrathin particles (Torres-Giner et al., 2010). Figure 5.2 shows the most common structures that may be fabricated by the electrospinning technology. Fibers composed of blends of different polymers can be prepared by electrospinning from solutions containing different polymer species in a common solvent. The resulting morphology can be either of a matrix-dispersed phase type or co-continuous, depending on the thermodynamic and kinetic properties of the electrospinning solutions. Another interesting characteristic of the electrospinning procedure is the ability to form porous nanostructures, which may imply a tremendous increase of the fiber surface area (see Fig. 5.3). These pores can be produced when, in the electrospinning of a blend consisting of two polymers, one of the polymers is partially removed after the fiber formation by dissolving it in a solvent in which the other polymer is insoluble (Torres-Giner et al., 2008a). It is even possible to vary the pore size and density by further controlling the processing parameters (Ramakrishna et al., 2006). Pore



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5.2 Selected SEM images of electrospun zein networks for: (a) thick tubular fibers obtained from concentrated solutions; (b) thin tubular nanofibers obtained using long tip-to-collector distances; (c) nanobeads obtained from diluted solutions; (d) ribbon-like ultrathin fibers obtained from the acidified solution. Scale markers of 5 m in all cases (Torres-Giner et al., 2008b).

formation can also be induced in the presence of high humidity, where condensation processes lead to the formation of water islands within the fibers, subsequently causing pore formation. Electrospinning onto very cold substrates such as liquid nitrogen can also result in highly nanoporous fibers. On the other hand, similar to this, fiber surfaces may not always be smooth, depending mainly on the solvent volatility used in the electrospinning. For instance, fibers may fuse together if the solvents are not completely evaporated, to yield threedimensional networks that have foam or sponge-like structures (Gomes et al., 2007). Coaxial electrospinning, also habitually called co-electrospinning, can be further applied for the preparation of polymer core±shell fibers and hollow


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5.3 Chitosan porous nanofibers resulting from specific solvent post-treatment. Scale marker of 2.5 m (Torres-Giner et al., 2008a).

fibers. This is a frequent solution for those cases in which the material cannot be electrospun to produce fibers, such as oils and low molecular weight polymers. It consists of the same electrospinning setup as for polymer blends with the exception of the use of two needles which are arranged in a concentric configuration and connected to two different reservoir solutions. If the two solutions are immiscible and contain two different polymers, a core±shell nanofiber is formed, while if the inner solution is free of polymer a hollow fiber is produced from the outer polymer solution. In the latter case, the inner solution usually consists of a non-polymer fluid or an immiscible solvent with the solvent used for the outer solution. It is frequently preferable to apply the shell from the immiscible solvent which can be easily removed by a vapor phase separation rather than from a fluid phase, which may give rise to mechanical forces causing a disruption or a swelling of the fibers. Furthermore, typical electrospinning results in planar random non-wovens, that is, with an isotropic orientation of the fibers, if planar electrodes are used. However, a parallel arrangement of the fibers can be obtained by employing cylindrically shaped electrodes which rotate and collect the fibers during the process (Xu et al., 2004). Tubular arrangements become accessible in this way, with the fibers being either parallel or random depending, among other parameters, on the rotation speed. A further approach consists in the use of the so-called split electrodes which are composed of, for instance, a set of two parallel electrodes or a set of four electrodes arranged in a rectangular pattern.



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The expansion of the technology has also brought alternative methods to produce the nanofibers in a more controlled and efficient manner. For instance, similar nanofibers can be made from solution blow spinning, which does not require high voltage equipment or any electrically conductive collector and, more importantly, avoids the use of aggressive solvents. This setup consists of a source of compressed gas such as nitrogen, argon and air, equipped with a pressure regulator system of concentric nozzles: an inner nozzle through which the polymer solution is pumped, and an outer nozzle through which a high pressure stream of the gas passes (Medeiros et al., 2009). This particular nozzle geometry creates a region of low pressure around the inner nozzle that helps draw the polymer solution into a cone, which then elongates into a fiber. The certain ability to fabricate polymer-based nanofibers with controllable size and porous structure in the form of non-woven mats or three-dimensional porous structures could provide virtually unlimited novel sources for the development of natural polymer-based applications (Torres-Giner et al., 2008a, 2008b). The highest potential role that the electrospinning process can play is possibly the construction and reproduction of multi-level materials. Unlike the bottom-up methods, electrospun fibers are produced through a top-down process, which results in continuous and low-cost fibers that are also relatively easy to align, assemble, and process into applications. Also, as previously mentioned, deposited structures can also be particulates or mixtures of fibers and particles. The use of such ultrathin structures as components in subsequent largescale derived products is attracting considerable interest in the industry of functionalized materials for their utilization as tissue engineering scaffolds, pharmaceutical drug release dosages, highly functional food and ingredients, and active and bioactive packaging materials, and in general for the manufacture of advanced functional materials.

5.2

Functional nanofibers

The electrospinning technique has mainly dealt with synthetic polymers due to their low cost, high availability, and well-defined chemical properties that allow for more uniform behavior during the electrospinning process. However, instead, the technology also opens up enormous possibilities for the implementation of bio-based materials and food hydrocolloids such as proteins and polysaccharides, to make novel biodegradable and renewable structures of interest in various functional applications. In the context of the literature review, there are numerous studies that focus on the use and production of electrospun biopolymer-based ultrathin fibers. The main advantage of biopolymers is that in a natural way they usually compromise bioactive properties without including for this function any other kind of compounds. The reason for this is that biopolymers are macromolecules produced in nature by living organisms and plants, and because of their participation in the natural biocycles, they can


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compromise certain functionalities. Biopolymers are of great interest in various industries, chief amongst them being the food industry, because they may be (a) non-toxic, edible, and digestible, (b) biocompatible and biodegradable, and (c) renewable and sustainable, giving rise to a broader utilization, especially in fields such as biomedical sciences, pharmaceuticals, cosmetics, and other related fields. Further, biopolymer uses could allow for the creation of value-added products if currently under-utilized but abundantly available biomass were to be used as raw material for electrospinning. Naturally occurring polysaccharides and proteins have been known to be biocompatible and safe for many different applications. A wide range of them can be electrospun into ultrathin fiber mats with a specific fiber arrangement, structural integrity, and full biocompatibility. Many studies have been conducted using polysaccharides and proteins for an electrospun fabrication that could be potentially useful for functionalizing materials. Table 5.1 gathers the most widespread researches on electrospun biopolymers with their expected applications. Till now, polysaccharides including, but not limited to, alginates (Nie et al., 2008), cellulose (Zhang et al., 2008; Ma and Ramakrishna, 2008), and chitosan (Torres-Giner et al., 2008a; Neamnark et al., 2008) have been electrospun as good examples of novel functional materials. For instance, alginate nanofibers have excellent biocompatibility, low toxicity, non-immunogenicity, relatively low cost and simple gelation behavior with divalent cations which have been studied for many biomedical applications. On the other hand, cellulose-based electrospun nanofibers have been largely proposed in the pharmaceutical and biomedical fields, including for applications as adsorbent beads, filters and barrier membranes, artificial tissue/skin, antimicrobial membranes and protective clothing. Furthermore, studies on antibacterial activities of Table 5.1 Summary of promising properties related to the electrospinning of functional polymers Polymer nanofibers

Functional properties

Alginates Celluloses

Wound dressings for tissue engineering Ion-exchange medium for protein separations in biomembranes Cell support, antimicrobial performance Enlarges cell proliferation and osteoblastic activity Proliferation of muscle, bone and skin cells Improved frictional surface and cell adhesion Biofillers of mechanically improved packaging Water entrapper, thermally resistant, and biologically valuable Increases cell biocompatibility and antibacterial activity Enhances Cu2+ adsorption capacity in water purification filters

Chitosan Starch Collagen/gelatin Silk fibroin Gluten Zein Collagen and chitosan Wool keratose and silk



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chitosan nanofibers have shown that the salts of this biopolymer inhibit the growth of S. aureus bacteria in the presence of very low fiber amounts (TorresGiner et al., 2008a, 2009a). The polycationic nature of migrated glucosamine fractions of chitosan is considered to interfere with the negatively charged residues of macromolecules at the cell membrane surface, which results in the death of the microorganisms. In this way, composite materials which contain the chitosan fibers can be used in far more applications with respect to their biodegradability and unusual properties that could help control the microorganisms. Other polysaccharides, such as hyaluronic acid, starch, dextran, and heparin have also shown potential in the electrospinning process, with or without polymer additives (Lee et al., 2009). Some proteins are original counterparts of the animal body and their electrospun mats can produce specific biological responses. In the tissue engineering field, electrospun ultrathin fiber interfaces made of animal proteins can easily approximate the nanostructural morphology of natural tissues by assessing appropriate levels and sizes of porosity to allow cell migration, sufficient surface area and a variety of surface chemistries to encourage cell adhesion, growth, migration, and differentiation, and adjust the degradation rate to match tissue regeneration (Lannutti et al., 2007). In this context, collagen can be considered as the most promising natural polymer for tissue restoration processes and, therefore, the electrospinning of collagen has been widely reported (TorresGiner et al., 2009b). Another interesting protein, which shows not only cellular responsive features but also potential uses in functional textiles and clothing design, is silk fibroin. The use of silk for active surfaces as electrospun nanofiber assemblies with other polymers is confirming very encouraging results due to improved frictional properties (Akada et al., 2007). Electrospun ultrathin structures of zein prolamine from corn also have good potential in the food technology area, not only as a reinforcing fiber in, for instance, plastic food packaging applications, but also as an edible carrier for encapsulation of food additives (Torres-Giner et al., 2010) or to modify food properties and in the design of novel active and bioactive packaging technologies (Torres-Giner et al., 2008b). As a most recent and natural approach to increase the bioactivity of the electrospun polymers, the latest research is being focused on blending two biological polymers. Probably the clearest example that shows benefits of this biological combination in the electrospinning is the collagen±chitosan complex, which has been proposed as optimal wound dressings because it combines the enhanced cell biocompatibility of collagen with the antibacterial activity of chitosan (Chen et al., 2008). Apart from the in tissue engineering field, other electrospun bioblends are promising candidates for different functional applications: wool keratose and silk fibroin as metallic particle filters in water treatment (Ki et al., 2007) or zein and chitosan for antibacterial and functional packaging (Torres-Giner et al., 2009a).


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5.3


Nanoencapsulation

Electrospun fibers may be given additional functionality by incorporating functional nanoscalar compounds, e.g. drugs, into the electrospinning solution. Further investigations on this approach permit one to functionalize or add extra functionalities to electrospun materials to display specific properties. The process generally consists of the introduction of very diverse non-polymeric agents into the polymer solution to electrospin. This results in hybrid ultrathin fibers in which the incorporated agents remain packed into the polymer matrix as individual nanodroplets of liquid or solid nanoparticles. Such nanostructural distribution can overcome the issue of low stability of encapsulated ingredients as well as improve the distribution of the functional components. This approach also becomes crucial when synthetic polymers, such as poly(lactic acid) (PLA), poly(vinyl alcohol) (PVA), poly(vinyl pyrrolidone) (PVP), poly(ethylene oxide) (PEO), poly(-caprolactone) (PCL), poly(styrene) (PS), and their copolymers, are employed because this constitutes their unique current way to functionalization. In particular, the electrospun morphology can provide high efficiency in drug-loading and, therefore, has been proposed to provide novel functional carriers in pharmaceutical compositions by nanoencapsulating specific therapeutic compounds (Sawicka and Gouma, 2006). The rate profiles can also be controlled, obtaining desired adjustable releases, such as rapid, immediate, or delayed dissolution, or a modified release profile, for instance the sustained and/ or pulsatile release characteristic. This is specifically modulated by changing the morphology, porosity, and hydrophilic/hydrophobic composition of the electrospun fiber membrane. For instance, the application of electrospraying to substance-loading polymer solutions results in the formation of nanocapsules of high substance encapsulation efficiency and performance because of the ultrathin particle sizes which can reduce the losses of the entrapped substances and enhance the bioavailability (Torres-Giner et al., 2010). Several examples can be found in the literature regarding encapsulation of substances using electrospinning with practical application in the packaging field. Table 5.2 includes different electrospun encapsulation works that report uses in the design of functional nanostructures of additives, coatings or interlayers. Improving the surface functionality of electrospun fibers with bioactive molecules could be very important for specific bioactive applications. For example, clays and other minerals have been proposed to be introduced in electrospun nanofibers of zein, such as organo-modified and unmodified mica, kaolinite, montmorillonite (MMT) and zeolite (Torres-Giner and LagaroÂn, 2010). These zein±clay nanofibers are proposed to reinforce the mechanical, thermal, barrier and control release properties of both plastic and bioplastic matrices without implying losses in biodegradability and optical properties. For active food packaging, incorporation of enzymes and bioactive molecules into a matrix is usually challenging, as they are generally sensitive to heat and may



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Table 5.2 Summary of promising properties related to nanoencapsulation using electrospinning Polymer nanofibers


Zein±clays

Improved mechanical and thermal properties, controlled release Enhancement and control of water resistance

PVA±polyhedral oligosilsesquioxane PLA±lysozyme PVA±lipase PLA±paracetamol Chitosan±ibuprofen PVA±acetylsalicylic acid PCL±resveratrol and gentamicin sulfate PLA±tetracycline Cellulose±vitamin A, E PLA±biotin Cellulose±erythromycin Zein± -carotene PVA±Ag PVP±Iron particles PEO±DNA PVA±bacteria

Antibacterial, controlled release Fast transesterification activity Analgesic sustained release Sustained release of an anti-inflammatory Controlled release of analgesic Sustained release of antioxidant and antibiotic substances Controlled release of antibiotic Controlled release of vitamins as transdermal and dermal therapeutic agents Release of vitamin H Artificial gastric juice of acid protection Antioxidant activity Films of broad-spectrum antibacterial and antifungal functionability Offers enhanced protection to prevent from oxidation Fluorescently labeled proteins Protection and release of living organisms

lose their activity when in contact with certain chemicals. Electrospinning can be carried out in room temperature conditions and this allows such molecules to be satisfactorily incorporated. Bioactive molecules can include enzymes such as lysozyme and lipase, pharmaceuticals such as paracetamol, ibuprofen, and acetylsalicylic acid, or antibiotics such as gentamicin sulfate or tetracycline. Nutraceuticals can also be nanoencapsulated as functional food, which promotes health beyond providing basic nutrition and can be included in bioactive food packaging applications. For example, electrospinning technology has been presented as novel route to stabilization of vitamins or antioxidants such as carotene (Fernandez et al., 2009) and omega-3 fatty acids (Torres-Giner et al., 2010). They can bioact on the containing food from the incorporated package or be released from it when it is open. Figure 5.4 shows zein/ -carotene ultrathin fibers which have been proved to present a remarkably good protection against oxidation when exposed to UV±vis irradiation. The technology is therefore reported here as capable of producing added-value biopolymer nanofibers that can have good potential in food and nutraceutical formulations and coatings, bioactive food packaging and food processing industries. In other more complex cases such as with some metals, semiconductors or inorganic compounds, it can be difficult to be able to fulfill the physical


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5.4 Fluorescence image of electrospun zein/ -carotene nanofibers (Fernandez et al., 2009).

electrospinning requirements. The answer to this problem is to add their low molar mass precursor materials such as metal salts, for instance, in high concentrations to the polymer solution. This, in turn, is electrospun to yield fibers in which the precursor molecules are dispersed within the polymer carrier. The precursor species are subsequently subjected to further chemical modifications such as, for instance, a reduction process. A clear example for this case is the inclusion of silver salts in natural and synthetic polymer solutions. Silvercontaining electrospun fibers are then thermally activated and the resulting materials are highly antimicrobial because they can function either as release systems for Ag+ ions or as contact-active materials (Dong et al., 2009). Other cases include the dispersion of ferrofluids into the fibers to produce materials which display ferromagnetism and supermagnetism functionalities. Fe3O4, for example, is a well-known ferromagnetic material which, however, becomes superparamagnetic as the size of the particles is reduced down to the nanometer scale (Graeser et al., 2007). The coaxial electrospinning method has a number of additional advantages that make it more attractive for the production of functional nanofibers. From the point of view of the nanoencapsulated objects, first of all this provides a more natural environment, and secondly the electric charges are located practically only at the outer surface, so that the objects dispersed in the inner parts are not charged at all. It may also reduce the contact between the nanoencapsulated components and the organic solvents, which is expected to improve the bioactivity of the resultant nanofibers. They would only be affected mechanically by the predominantly viscous stresses in the droplet during electrospinning. Apart from requiring milder conditions, the coaxial method also



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has the advantage of exerting greater control over the release behavior, which increases the loading efficiency. This method effectively protected fragile biological agents, enabling the incorporation of proteins, enzymes, and DNA (Yarin et al., 2007). This can be applied even to obtaining `living' membranes of non-wovens made from fibers containing biological objects, for instance viruses, bacteria and cells, in such a way that these objects keep their specific functions to adapt to their new environment (Lopez-Rubio et al., 2009).

5.4

Electrospinning in packaging applications

The ease and versatility of the electrospinning technique has offered new opportunities for researchers to investigate the effectiveness of nanofibers as a reinforcement to enhance the properties of a matrix material (Huang et al., 2003). Composites are combinations of two distinct material phases, the bulk phase or matrix, and the reinforcing phase. From a mechanical point of view, it is the combination of the strength of the reinforcement and the toughness of the matrix that gives composites their superior properties that are not present in single conventional materials. When fiber reinforcements close to 100 nm in cross-section are used, the final materials are called nanocomposites, and they generally maintain their original optical properties. Also as anticipated, nanofibers not only can have better mechanical properties than microfibers of the same materials but also may possess some additional merits which cannot be shared by traditional composites. Additionally, their application to renewable biopolymers represents an emerging new class of nanocomposites, also referred to as nanobiocomposites, with reduced environmental impact and high functionality, which are the wave of the future and are considered as the materials of the next generation (Pandey et al., 2005). In the fiber reinforcement context, depending on the type of application, the arrangement of nanofibers can basically take two different forms: aligned or randomly distributed. For load-bearing applications, the most effective arrangement is to have the nanofibers aligned in the stress direction to form a laminated nanocomposite (see Fig. 5.5a). Such a material is strong in the direction coinciding with the aligned nanofibers but weak in the perpendicular direction. Randomly distributed nanofibers, in the form of non-woven nanocomposites (Fig. 5.5b), can also be effective in other areas, for instance as gas barrier layers. The introduction into the matrix of nanometer-sized spaces produced by the nanofiber mesh can lower the gas permeability because of a tortuosity effect in the gas diffusion. Recently, electrospinning has been presented as a potential technology for use as a platform for multifunctional, hierarchically organized nanocomposites (Teo and Ramakrishna, 2009). Electrospun nanofibers can be incorporated as reinforcements to plastic matrices by different methods, such as melt mixing or solvent casting, to generate novel nanocomposites consisting of nanofiber layers intercalated into a


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5.5 Basic nanocomposite reinforcements based on electrospun fibers: (a) laminated nanocomposites; (b) non-woven nanocomposites.

plastic matrix. In other cases, with a greater emphasis on functional capability, bioactive nanofibers can be exposed on the matrix surface to the environment to modify external stimuli or to the food as drug vehicle. Figure 5.6 presents these different fiber dispositions in the electrospun nanocomposites according to the desired functionality for food packaging applications. Nevertheless, to date, little or no research has been done on this topic and only a few researchers have tried to make nanocomposites reinforced with electrospun polymer nanofibers. Some works have proved that the mechanical performance of certain epoxy composites can be expanded when impregnation with electrospun nanofibers is realized (Kim and Reneker, 1999; Fong, 2004). Nanofibers can thus penetrate through the resin matrix to form a stronger and stiffer network, in which their very high surface-to-volume ratio may improve the resultant material toughness. Such results in long-term materials have triggered research on the exploitation of nanofibers for biodegradable plastics. As an example, it is reported that electrospun PVA fibers can be coffined into a PS matrix to produce a reinforced film with enhanced mechanical properties (Tsutsumi and Hara, 2008). In other work (Chen and Liu, 2008), electrospun cellulose mats composed of fibers 200±

5.6 Electrospun nanocomposites in the design of novel food packaging materials.



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800 nm long have been incorporated in soybean protein isolate to improve the mechanical stress at yield and Young's modulus by 13 and 6 times, respectively. More interestingly, this composite exhibited high visible light transmittance of ca. 75%. Regarding this optical characteristic, a nanocomposite using electrospun Nylon-4,6 nanofibers as reinforcement and phenolic epoxy resin as matrix was shown to be also transparent (Bergshoef and Vancso, 1999). This is because light was able to transmit over the entire range of wavelength of the visible spectrum (400±700 nm). Since light neither reflects nor refracts at air± nanomaterial interfaces if the material size is less than one-tenth of the wavelength of visible light, the reinforcing nanofibers serve greatly in the manufacture of transparent reinforced materials. Using nanofibers as composite reinforcements has shown a few interesting optical and mechanical results, but they have not been so well studied in regard to the simultaneous reinforcement of the barrier properties of plastics and bioplastics in packaging and membrane applications. The gas and vapor permeability of biopolymers can be enhanced dramatically by incorporating nanofiber layers of fairly low thickness as interlayers or coatings. For instance, the incorporation of a thin interlayer of electrospun zein ultrathin fibers into PLA-based films has shown to reduce up to 71% the oxygen permeability in comparison to the same unreinforced matrix (Busolo et al., 2009). As can be seen by optical microscopy, in the interior of the PLA film of Fig. 5.7 (top image) the thin reinforcement results in completely transparent and colorless sheets similar to the original one (bottom images).

5.5

Future trends

To conclude, electrospinning allows extensive tunability in material properties and functions through specific selection of the solution composition. Although electrospun materials are predominantly polymer-based, ceramic, metallic and other bioactive particles can also be introduced into the fibers and subsequently be part of the final nanocomposites. At first, non-polymer particles or a second polymer can be mixed into the primary polymer solution and electrospun to form hybrid ultrathin fibers. As an example, results of the nanodispersion of commercial minerals into electrospun ultrathin zein fibers have shown a considerable increase in thermal resistance at mineral contents below 10 wt% (Torres-Giner and LagaroÂn, 2010). Further modifications of the electrospinning technique can be performed to increase the number of functional materials and to broaden the range of potential applications. This can be represented in the modification of the morphology or surface of nanofibers, the use of coaxial electrospinning technology to produce a second layer of polymer material, and the orientation and organization of the nanofibers by modification of the collector to optimize its performance. The development of new electrospinning configurations such as solution blow spinning can provide novel nanofibers with greater potential for


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5.7 Top: PLA±zein nanocomposite micrographs in top view by optical microscopy (scale marker is 100 m); bottom: images of the original film (left) and resultant composite (right) with electrospun nanofibers (Busolo et al., 2009).

commercial scale-up. Such additional adaptations will allow creating advanced multi-functional nanocomposites, in which various functions are incorporated for plastics in multi-sectorial applications. In this sense, future hybrid nanostructures will be applied as functional reinforcing fillers in uses such as coatings, packaging, and other applications. In the future it will be important to focus research on gaining a better fundamental understanding of the electrospinning process, but even more importantly on how this technique can be used as a tool in developing new materials. The studies described above indicate that the production of nanocomposites from electrospun fibers is feasible. However, some more essential



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studies are still required and many challenges remain to be faced. In particular, the ability to integrate the nanofibers into packaging materials in an efficient and reproducible manner remains a major challenge. Although many works have reported on the control, orientation, size, and other target characteristics, further advances concerning the reproducibility of locating the nanofibers in specific positions and orientations will be necessary. The encapsulation and posterior release of bioactives such as nutraceuticals or antimicrobials will also require further studies to prove the expected bioactive properties in the resultant material. Regarding fiber productivity, scale-up and commercial production are other general challenges which need to be addressed. The design and construction of process equipment for controllable and reproducible electrospinning will act as a stimulus to provide novel products based on electrospinning technology.

5.6

References

Akada M, Kotaki M, Sato M, Sukigara S (2007) `Surface frictional properties of silk/ nylon blended nanofiber assemblies' J. Textile Eng. 53 245±248. Bergshoef M, Vancso G (1999) `Transparent nanocomposites with ultrathin, electrospun nylon-4,6 fiber reinforcement' Adv. Mater. 11 1362±1365. Busolo MA, Torres-Giner S, LagaroÂn JM (2009) Ènhancing the gas barrier properties of polylactic acid by means of electrospun ultrathin zein fibers' Annual Technical Conference ± ANTEC, Conference Proceedings 5 2763±2767. Chen G, Liu H (2008) Èlectrospun cellulose nanofiber reinforced soybean protein isolate composite film' J. Appl. Polym. Sci. 110 641±646. Chen Z, Mo X, He C, Wang H (2008) Ìntermolecular interactions in electrospun collagen±chitosan complex nanofibers' Carbohyd. Polym. 72 410±418. Dong G, Xiao X, Liu X, Qian B, Liao Y, Wang C, Chen D, Qiu J (2009) `Functional Ag porous films prepared by electrospinning' Appl. Surf. Sci. 255 7623±7626. Fernandez A, Torres-Giner S, LagaroÂn JM (2009) `Novel route to stabilization of bioactive antioxidants by encapsulation in electrospun fibers of zein prolamine' Food Hydroc. 23 1427±1432. Fong H (2004) Èlectrospun nylon 6 nanofiber reinforced BIS-GMA/TEGDMA dental restorative composite resins' Polymer 45 2427±2432. Gomes DS, da Silva ANR, Morimoto NI, Mendes LTF, Furlan R, Ramos I (2007) `Characterization of an electrospinning process using different PAN/DMF concentrations' PolõÂmeros: CieÃncia e Tecnologia 17 206±211. Graeser M, Bognitzki M, Massa W, Pietzonka C, Greiner A, Wendorff JH (2007) `Magnetically anisotropic cobalt and iron nanofibers via electrospinning' Adv. Mater. 19 4244±4247. Huang Z, Zhang Y, Kotaki M, Ramakrishna S (2003) À review on polymer nanofibers by electrospinning and their applications in nanocomposites' Comp. Sci. Tech. 63 2223±2253. Ki CS, Gang EH, Um IC, Park YH (2007) `Nanofibrous membrane of wool keratose/silk fibroin blend for heavy metal ion adsorption' J. Memb. Sci. 302 20±26. Kim JS, Reneker DH (1999) `Mechanical properties of composites using ultrafine electrospun fibers' Polym. Comp. 20 124±131.


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Lannutti J, Reneker D, Ma T, Tomasko D, Farson D (2007) Èlectrospinning for tissue engineering scaffolds' Mater. Sci. Eng. C27 504±509. Lee KY, Jeong L, Kang YO, Lee SJ, Park WH (2009) Èlectrospinning of polysaccharides for regenerative medicine' Adv. Drug Deliv. Reviews 61 1020± 1032. Li D, Xia Y (2004) Èlectrospinning of nanofibers: Reinventing the wheel?' Adv. Mater. 16 1151±1170. Lopez-Rubio A, Sanchez E, Sanz Y, LagaroÂn JM (2009) Èncapsulation of living bifidobacteria in ultrathin PVOH electrospun fibers' Biomacromolecules 10 2823± 2829. Ma Z, Ramakrishna S (2008) Èlectrospun regenerated cellulose nanofiber affinity membrane functionalized with protein A/G for IgG purification' J. Membr. Sci. 319 23±28. Medeiros ES, Glenn GM, Klamczynski AP, Orts WJ, Mattoso LHC (2009) `Solution blow spinning: A new method to produce micro- and nanofibers from polymer solutions' J. Appl. Polym. Sci. 113 2322±2330. Neamnark A, Sanchavanakit N, Pavasant P, Rujiravanit R, Supaphol P (2008) Ìn vitro biocompatibility of electrospun hexanoyl chitosan fibrous scaffolds towards human keratinocytes and fibroblasts' Eur. Polymer J. 44 2060±2067. Nie H, He A, Zheng J, Xu S, Li J, Han CC (2008) Èffects of chain conformation and entanglement on the electrospinning of pure alginate' Biomacromolecules 9 1362± 1365. Pandey JK, Reddy KR, Kumar AP, Singh RP (2005) Àn overview on the degradability of polymer nanocomposites' Polym. Degrad. Stab. 88 234±250. Ramakrishna S, Fujihara K, Teo W-E, Yong T, Ma Z, Ramakrishna R (2006) Èlectrospun nanofibers: Solving global issues' Mater. Today 9 40±50. Sawicka K, Gouma P (2006) Èlectrospun composite nanofibers for functional applications' J. Nanopart. Res. 8 769±781. Teo W-E, Ramakrishna S (2009) Èlectrospun nanofibers as a platform for multifunctional, hierarchically organized nanocomposite' Comp. Sci. Tech. 69 1804±1817. Torres-Giner S, LagaroÂn JM (2010) `Zein-based ultrathin fibers containing ceramic nanofillers obtained by electrospinning. I. Morphology and thermal properties' J. Appl. Polym. Sci. 118 778±789. Torres-Giner S, Ocio MJ, LagaroÂn JM (2008a) `Development of active antimicrobial fiber based chitosan polysaccharide nanostructures using electrospinning' Eng. Life Sci. 8 303±314. Torres-Giner S, Gimenez E, LagaroÂn JM (2008b) `Characterization of the morphology and thermal properties of zein prolamine nanostructures obtained by electrospinning' Food Hydroc. 22 601±614. Torres-Giner S, Ocio MJ, LagaroÂn JM (2009a) `Novel antimicrobial ultrathin structures of zein-chitosan blends obtained by electrospinning' Carbohyd. Polym. 77 261± 266. Torres-Giner S, Gimeno-AlcanÄiz JV, Ocio MJ, LagaroÂn JM (2009b) `Comparative performance of electrospun collagen cross-linked by means of different methods' Appl. Mat. Inter. 1 218±223. Torres-Giner S, Martinez-Abad A, Ocio MJ, LagaroÂn JM (2010) `Stabilization of a nutraceutical omega-3 fatty acid by encapsulation in ultrathin electrosprayed zein prolamine' J. Food Sci. doi: 10.1111/j.1750-3841.2010.01678.x. Tsutsumi H, Hara C (2008) `Characterization of new type polymer composites prepared



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by in situ coffining electrospun fibers into polymer matrixes' Technical Proceedings of the 2008 NSTI Nanotechnology Conference and Trade Show, NSTI-Nanotech, Nanotechnology 2 733±736. Xu CY, Inai R, Kotaki M, Ramakrishna S (2004) Àligned biodegradable nanofibrous structure: A potential scaffold for blood vessel engineering' Biomaterials 25 877± 886. Yarin AL, Zussman E, Wendorff JH, Greiner A (2007) `Material encapsulation and transport in core±shell micro/nanofibers, polymer and carbon nanotubes and micro/ nanochannels' J. Mater. Chem. 17 2585±2599. Zhang L, Menkhaus TJ, Fong H (2008) `Fabrication and bioseparation studies of adsorptive membranes/felts made from electrospun cellulose acetate nanofibers' J. Membr. Sci. 319 176±184.


5

Electrospun nanofibers for food packaging applications S . T O R R E S - G I N E R , Novel Materials and Nanotechnology Group, IATA-CSIC, Spain

Abstract: Electrospinning is a novel fabrication technology based on high electric fields that can be used to produce polymer- and biopolymer-based mats composed of nanofibers or other nanostructures. Electrospun mats have been shown to possess unique functionalities originating from the very large surface-to-volume ratios of the nanofibers, the use of functional and/or renewable polymers or the encapsulation of bioactive non-polymer substances. The functional electrospun mats can be used as tools for the development of nanocomposite fabrics from a wide variety of plastics with improved performance for packaging applications. They could serve, for example, as a reinforcement to enhance the physical properties of plastics and bioplastics, as transparent layers to a gas barrier, and even as an emerging technology to design bioactive packaging with antimicrobial protection or delivery of nutraceuticals to foods. Key words: electrospinning, ultrathin fibers, nanocomposites, active and bioactive packaging.

5.1

Electrospinning

Looking genuinely at nature, nanofibers often serve as a basic platform on which either organic or inorganic components are built. For instance, cellulose nanofibers would represent the building blocks in plants while collagen nanofibers would do so in the animal body. The fiber structure exhibits, from a structural point of view, a certain ability to transmit forces along its length, thus reducing the amount of materials required. While strong enough for their designed purpose, nanofibers have the added advantage of giving high porosity to the natural supports, which allows faster diffusion of chemicals to the inner structure. To follow this extraordinary natural design, a process that is able to fabricate fiber nanostructures from a variety of materials and mixtures is an indispensable pre-requisite. Control of the nanofiber arrangement is also necessary to optimize such structural requirements. Electrospinning is a physical process used for the formation of ultrathin fibers by subjecting a polymer solution to high electric fields. A schematic representation of a typical laboratory electrospinning setup is shown in Fig. 5.1. In this configuration, a polymer solution is placed inside a syringe lying



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5.1 Typical electrospinning setup where a non-woven nanofiber mat is collected.

horizontally on a digitally controlled pump which permits adjustment and precise control of the solution flow-rate. The polymer solution is then pumped to a metallic millimeter-size needle which is connected to a high voltage power supply operated in positive DC mode and with low current intensity. At a critical high voltage (5±25 kV), the polymer solution droplet at the tip of the needle distorts and forms a Taylor cone to be ejected as a charged polymer jet. This stretches and is accelerated by the electrical field towards a grounded and oppositely charged collector. As the electrospun jet travels through the electrical field, the solvent completely evaporates while the entanglements of the polymer chains prevent it from breaking up. This results in the deposition of ultrathin polymer fibers on a metallic collector to habitually assemble the fibers as nonwoven mats. As elongation is accomplished via a contactless scheme, electrospun fibers are considerably thinner in diameter and thus higher in surface-to-volume ratio than fibers fabricated using conventional mechanical extrusion or a classical spinning process. For instance, while electrospun fiber diameters are habitually under the micron and also within the nanometric range, synthetic fibers produced via extrusion and fibers of biological origin such as cotton, wool, or silk are characterized by diameters in the range of various micrometers and above. Moreover, since the electrospinning is a continuous process, fibers when wound can be as long as several meters or even kilometers. The formed fibers are not only ultrathin and relatively long but also fully interconnected to form a three-dimensional network. Such small dimensions generally lead to very high ratios of specific surface area to mass and provide the electrospun products with extraordinary functionalities which are not found in similar materials of larger sizes. Fiber formation via electrospinning basically requires the materials to be processed to display specific viscoelastic properties, electrical conductivities in


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a limited range of values, and specific surface energies. Such requirements can only be met by polymers provided that molecular weights are chosen in an appropriate range. In order to carry out the process, the polymer must first be in a liquid form, either as molten polymer or as polymer solution. Melting electrospinning is limited by the fact that fibers are generally above the micron size and electrospinning from polymer solutions is then habitually preferred. In this last case, solvent properties such as boiling point and conductivity play a significant part in the electrospinning process and in the resultant electrospun morphology (Torres-Giner et al., 2008a). In particular, round-like fibers from less than 40 nm to a few microns can be fabricated by electrospinning of more than 50 different types of polymers (Li and Xia, 2004). In addition to observational changes in the fiber diameter, through the modification of the electrospinning setup other fiber and non-fiber assemblies with novel remarkable features can be produced, such as uniform or variable flat and round-like fibers, bead-like or round particles, core±sheath or multilayer coaxial structures, hollow tubes or porous fibers, aligned fibers, crosslinked fibers, and multi-jet fibers. This adjustability certainly enhances the performance of the electrospun materials, allowing application-specific modifications. Electrospun shapes are particularly developed by changing the polymer solution and process conditions: polymer concentration, solvent nature, tip-tocollector distance, voltage and flow-rate. Among all process parameters, the polymer concentration of the solution for electrospinning is the most relevant factor determining the fiber diameter (Torres-Giner et al., 2008b). Thus, higher concentrations generally result in the formation of fibers with larger diameters. Reduction of the polymer concentration below a threshold value would result in beads or in extremely thin fibers with beads along their length. This variation within the electrospinning technology is also called èlectrospraying' because the process acts like an atomization procedure to form nearly mono-dispersed and non-fibrillar ultrathin particles (Torres-Giner et al., 2010). Figure 5.2 shows the most common structures that may be fabricated by the electrospinning technology. Fibers composed of blends of different polymers can be prepared by electrospinning from solutions containing different polymer species in a common solvent. The resulting morphology can be either of a matrix-dispersed phase type or co-continuous, depending on the thermodynamic and kinetic properties of the electrospinning solutions. Another interesting characteristic of the electrospinning procedure is the ability to form porous nanostructures, which may imply a tremendous increase of the fiber surface area (see Fig. 5.3). These pores can be produced when, in the electrospinning of a blend consisting of two polymers, one of the polymers is partially removed after the fiber formation by dissolving it in a solvent in which the other polymer is insoluble (Torres-Giner et al., 2008a). It is even possible to vary the pore size and density by further controlling the processing parameters (Ramakrishna et al., 2006). Pore



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5.2 Selected SEM images of electrospun zein networks for: (a) thick tubular fibers obtained from concentrated solutions; (b) thin tubular nanofibers obtained using long tip-to-collector distances; (c) nanobeads obtained from diluted solutions; (d) ribbon-like ultrathin fibers obtained from the acidified solution. Scale markers of 5 m in all cases (Torres-Giner et al., 2008b).

formation can also be induced in the presence of high humidity, where condensation processes lead to the formation of water islands within the fibers, subsequently causing pore formation. Electrospinning onto very cold substrates such as liquid nitrogen can also result in highly nanoporous fibers. On the other hand, similar to this, fiber surfaces may not always be smooth, depending mainly on the solvent volatility used in the electrospinning. For instance, fibers may fuse together if the solvents are not completely evaporated, to yield threedimensional networks that have foam or sponge-like structures (Gomes et al., 2007). Coaxial electrospinning, also habitually called co-electrospinning, can be further applied for the preparation of polymer core±shell fibers and hollow


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5.3 Chitosan porous nanofibers resulting from specific solvent post-treatment. Scale marker of 2.5 m (Torres-Giner et al., 2008a).

fibers. This is a frequent solution for those cases in which the material cannot be electrospun to produce fibers, such as oils and low molecular weight polymers. It consists of the same electrospinning setup as for polymer blends with the exception of the use of two needles which are arranged in a concentric configuration and connected to two different reservoir solutions. If the two solutions are immiscible and contain two different polymers, a core±shell nanofiber is formed, while if the inner solution is free of polymer a hollow fiber is produced from the outer polymer solution. In the latter case, the inner solution usually consists of a non-polymer fluid or an immiscible solvent with the solvent used for the outer solution. It is frequently preferable to apply the shell from the immiscible solvent which can be easily removed by a vapor phase separation rather than from a fluid phase, which may give rise to mechanical forces causing a disruption or a swelling of the fibers. Furthermore, typical electrospinning results in planar random non-wovens, that is, with an isotropic orientation of the fibers, if planar electrodes are used. However, a parallel arrangement of the fibers can be obtained by employing cylindrically shaped electrodes which rotate and collect the fibers during the process (Xu et al., 2004). Tubular arrangements become accessible in this way, with the fibers being either parallel or random depending, among other parameters, on the rotation speed. A further approach consists in the use of the so-called split electrodes which are composed of, for instance, a set of two parallel electrodes or a set of four electrodes arranged in a rectangular pattern.



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The expansion of the technology has also brought alternative methods to produce the nanofibers in a more controlled and efficient manner. For instance, similar nanofibers can be made from solution blow spinning, which does not require high voltage equipment or any electrically conductive collector and, more importantly, avoids the use of aggressive solvents. This setup consists of a source of compressed gas such as nitrogen, argon and air, equipped with a pressure regulator system of concentric nozzles: an inner nozzle through which the polymer solution is pumped, and an outer nozzle through which a high pressure stream of the gas passes (Medeiros et al., 2009). This particular nozzle geometry creates a region of low pressure around the inner nozzle that helps draw the polymer solution into a cone, which then elongates into a fiber. The certain ability to fabricate polymer-based nanofibers with controllable size and porous structure in the form of non-woven mats or three-dimensional porous structures could provide virtually unlimited novel sources for the development of natural polymer-based applications (Torres-Giner et al., 2008a, 2008b). The highest potential role that the electrospinning process can play is possibly the construction and reproduction of multi-level materials. Unlike the bottom-up methods, electrospun fibers are produced through a top-down process, which results in continuous and low-cost fibers that are also relatively easy to align, assemble, and process into applications. Also, as previously mentioned, deposited structures can also be particulates or mixtures of fibers and particles. The use of such ultrathin structures as components in subsequent largescale derived products is attracting considerable interest in the industry of functionalized materials for their utilization as tissue engineering scaffolds, pharmaceutical drug release dosages, highly functional food and ingredients, and active and bioactive packaging materials, and in general for the manufacture of advanced functional materials.

5.2

Functional nanofibers

The electrospinning technique has mainly dealt with synthetic polymers due to their low cost, high availability, and well-defined chemical properties that allow for more uniform behavior during the electrospinning process. However, instead, the technology also opens up enormous possibilities for the implementation of bio-based materials and food hydrocolloids such as proteins and polysaccharides, to make novel biodegradable and renewable structures of interest in various functional applications. In the context of the literature review, there are numerous studies that focus on the use and production of electrospun biopolymer-based ultrathin fibers. The main advantage of biopolymers is that in a natural way they usually compromise bioactive properties without including for this function any other kind of compounds. The reason for this is that biopolymers are macromolecules produced in nature by living organisms and plants, and because of their participation in the natural biocycles, they can


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compromise certain functionalities. Biopolymers are of great interest in various industries, chief amongst them being the food industry, because they may be (a) non-toxic, edible, and digestible, (b) biocompatible and biodegradable, and (c) renewable and sustainable, giving rise to a broader utilization, especially in fields such as biomedical sciences, pharmaceuticals, cosmetics, and other related fields. Further, biopolymer uses could allow for the creation of value-added products if currently under-utilized but abundantly available biomass were to be used as raw material for electrospinning. Naturally occurring polysaccharides and proteins have been known to be biocompatible and safe for many different applications. A wide range of them can be electrospun into ultrathin fiber mats with a specific fiber arrangement, structural integrity, and full biocompatibility. Many studies have been conducted using polysaccharides and proteins for an electrospun fabrication that could be potentially useful for functionalizing materials. Table 5.1 gathers the most widespread researches on electrospun biopolymers with their expected applications. Till now, polysaccharides including, but not limited to, alginates (Nie et al., 2008), cellulose (Zhang et al., 2008; Ma and Ramakrishna, 2008), and chitosan (Torres-Giner et al., 2008a; Neamnark et al., 2008) have been electrospun as good examples of novel functional materials. For instance, alginate nanofibers have excellent biocompatibility, low toxicity, non-immunogenicity, relatively low cost and simple gelation behavior with divalent cations which have been studied for many biomedical applications. On the other hand, cellulose-based electrospun nanofibers have been largely proposed in the pharmaceutical and biomedical fields, including for applications as adsorbent beads, filters and barrier membranes, artificial tissue/skin, antimicrobial membranes and protective clothing. Furthermore, studies on antibacterial activities of Table 5.1 Summary of promising properties related to the electrospinning of functional polymers Polymer nanofibers


Alginates Celluloses

Wound dressings for tissue engineering Ion-exchange medium for protein separations in biomembranes Cell support, antimicrobial performance Enlarges cell proliferation and osteoblastic activity Proliferation of muscle, bone and skin cells Improved frictional surface and cell adhesion Biofillers of mechanically improved packaging Water entrapper, thermally resistant, and biologically valuable Increases cell biocompatibility and antibacterial activity Enhances Cu2+ adsorption capacity in water purification filters

Chitosan Starch Collagen/gelatin Silk fibroin Gluten Zein Collagen and chitosan Wool keratose and silk



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chitosan nanofibers have shown that the salts of this biopolymer inhibit the growth of S. aureus bacteria in the presence of very low fiber amounts (TorresGiner et al., 2008a, 2009a). The polycationic nature of migrated glucosamine fractions of chitosan is considered to interfere with the negatively charged residues of macromolecules at the cell membrane surface, which results in the death of the microorganisms. In this way, composite materials which contain the chitosan fibers can be used in far more applications with respect to their biodegradability and unusual properties that could help control the microorganisms. Other polysaccharides, such as hyaluronic acid, starch, dextran, and heparin have also shown potential in the electrospinning process, with or without polymer additives (Lee et al., 2009). Some proteins are original counterparts of the animal body and their electrospun mats can produce specific biological responses. In the tissue engineering field, electrospun ultrathin fiber interfaces made of animal proteins can easily approximate the nanostructural morphology of natural tissues by assessing appropriate levels and sizes of porosity to allow cell migration, sufficient surface area and a variety of surface chemistries to encourage cell adhesion, growth, migration, and differentiation, and adjust the degradation rate to match tissue regeneration (Lannutti et al., 2007). In this context, collagen can be considered as the most promising natural polymer for tissue restoration processes and, therefore, the electrospinning of collagen has been widely reported (TorresGiner et al., 2009b). Another interesting protein, which shows not only cellular responsive features but also potential uses in functional textiles and clothing design, is silk fibroin. The use of silk for active surfaces as electrospun nanofiber assemblies with other polymers is confirming very encouraging results due to improved frictional properties (Akada et al., 2007). Electrospun ultrathin structures of zein prolamine from corn also have good potential in the food technology area, not only as a reinforcing fiber in, for instance, plastic food packaging applications, but also as an edible carrier for encapsulation of food additives (Torres-Giner et al., 2010) or to modify food properties and in the design of novel active and bioactive packaging technologies (Torres-Giner et al., 2008b). As a most recent and natural approach to increase the bioactivity of the electrospun polymers, the latest research is being focused on blending two biological polymers. Probably the clearest example that shows benefits of this biological combination in the electrospinning is the collagen±chitosan complex, which has been proposed as optimal wound dressings because it combines the enhanced cell biocompatibility of collagen with the antibacterial activity of chitosan (Chen et al., 2008). Apart from the in tissue engineering field, other electrospun bioblends are promising candidates for different functional applications: wool keratose and silk fibroin as metallic particle filters in water treatment (Ki et al., 2007) or zein and chitosan for antibacterial and functional packaging (Torres-Giner et al., 2009a).


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5.3


Nanoencapsulation

Electrospun fibers may be given additional functionality by incorporating functional nanoscalar compounds, e.g. drugs, into the electrospinning solution. Further investigations on this approach permit one to functionalize or add extra functionalities to electrospun materials to display specific properties. The process generally consists of the introduction of very diverse non-polymeric agents into the polymer solution to electrospin. This results in hybrid ultrathin fibers in which the incorporated agents remain packed into the polymer matrix as individual nanodroplets of liquid or solid nanoparticles. Such nanostructural distribution can overcome the issue of low stability of encapsulated ingredients as well as improve the distribution of the functional components. This approach also becomes crucial when synthetic polymers, such as poly(lactic acid) (PLA), poly(vinyl alcohol) (PVA), poly(vinyl pyrrolidone) (PVP), poly(ethylene oxide) (PEO), poly(-caprolactone) (PCL), poly(styrene) (PS), and their copolymers, are employed because this constitutes their unique current way to functionalization. In particular, the electrospun morphology can provide high efficiency in drug-loading and, therefore, has been proposed to provide novel functional carriers in pharmaceutical compositions by nanoencapsulating specific therapeutic compounds (Sawicka and Gouma, 2006). The rate profiles can also be controlled, obtaining desired adjustable releases, such as rapid, immediate, or delayed dissolution, or a modified release profile, for instance the sustained and/ or pulsatile release characteristic. This is specifically modulated by changing the morphology, porosity, and hydrophilic/hydrophobic composition of the electrospun fiber membrane. For instance, the application of electrospraying to substance-loading polymer solutions results in the formation of nanocapsules of high substance encapsulation efficiency and performance because of the ultrathin particle sizes which can reduce the losses of the entrapped substances and enhance the bioavailability (Torres-Giner et al., 2010). Several examples can be found in the literature regarding encapsulation of substances using electrospinning with practical application in the packaging field. Table 5.2 includes different electrospun encapsulation works that report uses in the design of functional nanostructures of additives, coatings or interlayers. Improving the surface functionality of electrospun fibers with bioactive molecules could be very important for specific bioactive applications. For example, clays and other minerals have been proposed to be introduced in electrospun nanofibers of zein, such as organo-modified and unmodified mica, kaolinite, montmorillonite (MMT) and zeolite (Torres-Giner and LagaroÂn, 2010). These zein±clay nanofibers are proposed to reinforce the mechanical, thermal, barrier and control release properties of both plastic and bioplastic matrices without implying losses in biodegradability and optical properties. For active food packaging, incorporation of enzymes and bioactive molecules into a matrix is usually challenging, as they are generally sensitive to heat and may



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Table 5.2 Summary of promising properties related to nanoencapsulation using electrospinning Polymer nanofibers


Zein±clays

Improved mechanical and thermal properties, controlled release Enhancement and control of water resistance

PVA±polyhedral oligosilsesquioxane PLA±lysozyme PVA±lipase PLA±paracetamol Chitosan±ibuprofen PVA±acetylsalicylic acid PCL±resveratrol and gentamicin sulfate PLA±tetracycline Cellulose±vitamin A, E PLA±biotin Cellulose±erythromycin Zein± -carotene PVA±Ag PVP±Iron particles PEO±DNA PVA±bacteria

Antibacterial, controlled release Fast transesterification activity Analgesic sustained release Sustained release of an anti-inflammatory Controlled release of analgesic Sustained release of antioxidant and antibiotic substances Controlled release of antibiotic Controlled release of vitamins as transdermal and dermal therapeutic agents Release of vitamin H Artificial gastric juice of acid protection Antioxidant activity Films of broad-spectrum antibacterial and antifungal functionability Offers enhanced protection to prevent from oxidation Fluorescently labeled proteins Protection and release of living organisms

lose their activity when in contact with certain chemicals. Electrospinning can be carried out in room temperature conditions and this allows such molecules to be satisfactorily incorporated. Bioactive molecules can include enzymes such as lysozyme and lipase, pharmaceuticals such as paracetamol, ibuprofen, and acetylsalicylic acid, or antibiotics such as gentamicin sulfate or tetracycline. Nutraceuticals can also be nanoencapsulated as functional food, which promotes health beyond providing basic nutrition and can be included in bioactive food packaging applications. For example, electrospinning technology has been presented as novel route to stabilization of vitamins or antioxidants such as carotene (Fernandez et al., 2009) and omega-3 fatty acids (Torres-Giner et al., 2010). They can bioact on the containing food from the incorporated package or be released from it when it is open. Figure 5.4 shows zein/ -carotene ultrathin fibers which have been proved to present a remarkably good protection against oxidation when exposed to UV±vis irradiation. The technology is therefore reported here as capable of producing added-value biopolymer nanofibers that can have good potential in food and nutraceutical formulations and coatings, bioactive food packaging and food processing industries. In other more complex cases such as with some metals, semiconductors or inorganic compounds, it can be difficult to be able to fulfill the physical


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5.4 Fluorescence image of electrospun zein/ -carotene nanofibers (Fernandez et al., 2009).

electrospinning requirements. The answer to this problem is to add their low molar mass precursor materials such as metal salts, for instance, in high concentrations to the polymer solution. This, in turn, is electrospun to yield fibers in which the precursor molecules are dispersed within the polymer carrier. The precursor species are subsequently subjected to further chemical modifications such as, for instance, a reduction process. A clear example for this case is the inclusion of silver salts in natural and synthetic polymer solutions. Silvercontaining electrospun fibers are then thermally activated and the resulting materials are highly antimicrobial because they can function either as release systems for Ag+ ions or as contact-active materials (Dong et al., 2009). Other cases include the dispersion of ferrofluids into the fibers to produce materials which display ferromagnetism and supermagnetism functionalities. Fe3O4, for example, is a well-known ferromagnetic material which, however, becomes superparamagnetic as the size of the particles is reduced down to the nanometer scale (Graeser et al., 2007). The coaxial electrospinning method has a number of additional advantages that make it more attractive for the production of functional nanofibers. From the point of view of the nanoencapsulated objects, first of all this provides a more natural environment, and secondly the electric charges are located practically only at the outer surface, so that the objects dispersed in the inner parts are not charged at all. It may also reduce the contact between the nanoencapsulated components and the organic solvents, which is expected to improve the bioactivity of the resultant nanofibers. They would only be affected mechanically by the predominantly viscous stresses in the droplet during electrospinning. Apart from requiring milder conditions, the coaxial method also



119

has the advantage of exerting greater control over the release behavior, which increases the loading efficiency. This method effectively protected fragile biological agents, enabling the incorporation of proteins, enzymes, and DNA (Yarin et al., 2007). This can be applied even to obtaining `living' membranes of non-wovens made from fibers containing biological objects, for instance viruses, bacteria and cells, in such a way that these objects keep their specific functions to adapt to their new environment (Lopez-Rubio et al., 2009).

5.4

Electrospinning in packaging applications

The ease and versatility of the electrospinning technique has offered new opportunities for researchers to investigate the effectiveness of nanofibers as a reinforcement to enhance the properties of a matrix material (Huang et al., 2003). Composites are combinations of two distinct material phases, the bulk phase or matrix, and the reinforcing phase. From a mechanical point of view, it is the combination of the strength of the reinforcement and the toughness of the matrix that gives composites their superior properties that are not present in single conventional materials. When fiber reinforcements close to 100 nm in cross-section are used, the final materials are called nanocomposites, and they generally maintain their original optical properties. Also as anticipated, nanofibers not only can have better mechanical properties than microfibers of the same materials but also may possess some additional merits which cannot be shared by traditional composites. Additionally, their application to renewable biopolymers represents an emerging new class of nanocomposites, also referred to as nanobiocomposites, with reduced environmental impact and high functionality, which are the wave of the future and are considered as the materials of the next generation (Pandey et al., 2005). In the fiber reinforcement context, depending on the type of application, the arrangement of nanofibers can basically take two different forms: aligned or randomly distributed. For load-bearing applications, the most effective arrangement is to have the nanofibers aligned in the stress direction to form a laminated nanocomposite (see Fig. 5.5a). Such a material is strong in the direction coinciding with the aligned nanofibers but weak in the perpendicular direction. Randomly distributed nanofibers, in the form of non-woven nanocomposites (Fig. 5.5b), can also be effective in other areas, for instance as gas barrier layers. The introduction into the matrix of nanometer-sized spaces produced by the nanofiber mesh can lower the gas permeability because of a tortuosity effect in the gas diffusion. Recently, electrospinning has been presented as a potential technology for use as a platform for multifunctional, hierarchically organized nanocomposites (Teo and Ramakrishna, 2009). Electrospun nanofibers can be incorporated as reinforcements to plastic matrices by different methods, such as melt mixing or solvent casting, to generate novel nanocomposites consisting of nanofiber layers intercalated into a


120


5.5 Basic nanocomposite reinforcements based on electrospun fibers: (a) laminated nanocomposites; (b) non-woven nanocomposites.

plastic matrix. In other cases, with a greater emphasis on functional capability, bioactive nanofibers can be exposed on the matrix surface to the environment to modify external stimuli or to the food as drug vehicle. Figure 5.6 presents these different fiber dispositions in the electrospun nanocomposites according to the desired functionality for food packaging applications. Nevertheless, to date, little or no research has been done on this topic and only a few researchers have tried to make nanocomposites reinforced with electrospun polymer nanofibers. Some works have proved that the mechanical performance of certain epoxy composites can be expanded when impregnation with electrospun nanofibers is realized (Kim and Reneker, 1999; Fong, 2004). Nanofibers can thus penetrate through the resin matrix to form a stronger and stiffer network, in which their very high surface-to-volume ratio may improve the resultant material toughness. Such results in long-term materials have triggered research on the exploitation of nanofibers for biodegradable plastics. As an example, it is reported that electrospun PVA fibers can be coffined into a PS matrix to produce a reinforced film with enhanced mechanical properties (Tsutsumi and Hara, 2008). In other work (Chen and Liu, 2008), electrospun cellulose mats composed of fibers 200±

5.6 Electrospun nanocomposites in the design of novel food packaging materials.



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800 nm long have been incorporated in soybean protein isolate to improve the mechanical stress at yield and Young's modulus by 13 and 6 times, respectively. More interestingly, this composite exhibited high visible light transmittance of ca. 75%. Regarding this optical characteristic, a nanocomposite using electrospun Nylon-4,6 nanofibers as reinforcement and phenolic epoxy resin as matrix was shown to be also transparent (Bergshoef and Vancso, 1999). This is because light was able to transmit over the entire range of wavelength of the visible spectrum (400±700 nm). Since light neither reflects nor refracts at air± nanomaterial interfaces if the material size is less than one-tenth of the wavelength of visible light, the reinforcing nanofibers serve greatly in the manufacture of transparent reinforced materials. Using nanofibers as composite reinforcements has shown a few interesting optical and mechanical results, but they have not been so well studied in regard to the simultaneous reinforcement of the barrier properties of plastics and bioplastics in packaging and membrane applications. The gas and vapor permeability of biopolymers can be enhanced dramatically by incorporating nanofiber layers of fairly low thickness as interlayers or coatings. For instance, the incorporation of a thin interlayer of electrospun zein ultrathin fibers into PLA-based films has shown to reduce up to 71% the oxygen permeability in comparison to the same unreinforced matrix (Busolo et al., 2009). As can be seen by optical microscopy, in the interior of the PLA film of Fig. 5.7 (top image) the thin reinforcement results in completely transparent and colorless sheets similar to the original one (bottom images).

5.5

Future trends

To conclude, electrospinning allows extensive tunability in material properties and functions through specific selection of the solution composition. Although electrospun materials are predominantly polymer-based, ceramic, metallic and other bioactive particles can also be introduced into the fibers and subsequently be part of the final nanocomposites. At first, non-polymer particles or a second polymer can be mixed into the primary polymer solution and electrospun to form hybrid ultrathin fibers. As an example, results of the nanodispersion of commercial minerals into electrospun ultrathin zein fibers have shown a considerable increase in thermal resistance at mineral contents below 10 wt% (Torres-Giner and LagaroÂn, 2010). Further modifications of the electrospinning technique can be performed to increase the number of functional materials and to broaden the range of potential applications. This can be represented in the modification of the morphology or surface of nanofibers, the use of coaxial electrospinning technology to produce a second layer of polymer material, and the orientation and organization of the nanofibers by modification of the collector to optimize its performance. The development of new electrospinning configurations such as solution blow spinning can provide novel nanofibers with greater potential for


122


5.7 Top: PLA±zein nanocomposite micrographs in top view by optical microscopy (scale marker is 100 m); bottom: images of the original film (left) and resultant composite (right) with electrospun nanofibers (Busolo et al., 2009).

commercial scale-up. Such additional adaptations will allow creating advanced multi-functional nanocomposites, in which various functions are incorporated for plastics in multi-sectorial applications. In this sense, future hybrid nanostructures will be applied as functional reinforcing fillers in uses such as coatings, packaging, and other applications. In the future it will be important to focus research on gaining a better fundamental understanding of the electrospinning process, but even more importantly on how this technique can be used as a tool in developing new materials. The studies described above indicate that the production of nanocomposites from electrospun fibers is feasible. However, some more essential



123

studies are still required and many challenges remain to be faced. In particular, the ability to integrate the nanofibers into packaging materials in an efficient and reproducible manner remains a major challenge. Although many works have reported on the control, orientation, size, and other target characteristics, further advances concerning the reproducibility of locating the nanofibers in specific positions and orientations will be necessary. The encapsulation and posterior release of bioactives such as nutraceuticals or antimicrobials will also require further studies to prove the expected bioactive properties in the resultant material. Regarding fiber productivity, scale-up and commercial production are other general challenges which need to be addressed. The design and construction of process equipment for controllable and reproducible electrospinning will act as a stimulus to provide novel products based on electrospinning technology.

5.6

References

Akada M, Kotaki M, Sato M, Sukigara S (2007) `Surface frictional properties of silk/ nylon blended nanofiber assemblies' J. Textile Eng. 53 245±248. Bergshoef M, Vancso G (1999) `Transparent nanocomposites with ultrathin, electrospun nylon-4,6 fiber reinforcement' Adv. Mater. 11 1362±1365. Busolo MA, Torres-Giner S, LagaroÂn JM (2009) Ènhancing the gas barrier properties of polylactic acid by means of electrospun ultrathin zein fibers' Annual Technical Conference ± ANTEC, Conference Proceedings 5 2763±2767. Chen G, Liu H (2008) Èlectrospun cellulose nanofiber reinforced soybean protein isolate composite film' J. Appl. Polym. Sci. 110 641±646. Chen Z, Mo X, He C, Wang H (2008) Ìntermolecular interactions in electrospun collagen±chitosan complex nanofibers' Carbohyd. Polym. 72 410±418. Dong G, Xiao X, Liu X, Qian B, Liao Y, Wang C, Chen D, Qiu J (2009) `Functional Ag porous films prepared by electrospinning' Appl. Surf. Sci. 255 7623±7626. Fernandez A, Torres-Giner S, LagaroÂn JM (2009) `Novel route to stabilization of bioactive antioxidants by encapsulation in electrospun fibers of zein prolamine' Food Hydroc. 23 1427±1432. Fong H (2004) Èlectrospun nylon 6 nanofiber reinforced BIS-GMA/TEGDMA dental restorative composite resins' Polymer 45 2427±2432. Gomes DS, da Silva ANR, Morimoto NI, Mendes LTF, Furlan R, Ramos I (2007) `Characterization of an electrospinning process using different PAN/DMF concentrations' PolõÂmeros: CieÃncia e Tecnologia 17 206±211. Graeser M, Bognitzki M, Massa W, Pietzonka C, Greiner A, Wendorff JH (2007) `Magnetically anisotropic cobalt and iron nanofibers via electrospinning' Adv. Mater. 19 4244±4247. Huang Z, Zhang Y, Kotaki M, Ramakrishna S (2003) À review on polymer nanofibers by electrospinning and their applications in nanocomposites' Comp. Sci. Tech. 63 2223±2253. Ki CS, Gang EH, Um IC, Park YH (2007) `Nanofibrous membrane of wool keratose/silk fibroin blend for heavy metal ion adsorption' J. Memb. Sci. 302 20±26. Kim JS, Reneker DH (1999) `Mechanical properties of composites using ultrafine electrospun fibers' Polym. Comp. 20 124±131.


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Lannutti J, Reneker D, Ma T, Tomasko D, Farson D (2007) Èlectrospinning for tissue engineering scaffolds' Mater. Sci. Eng. C27 504±509. Lee KY, Jeong L, Kang YO, Lee SJ, Park WH (2009) Èlectrospinning of polysaccharides for regenerative medicine' Adv. Drug Deliv. Reviews 61 1020± 1032. Li D, Xia Y (2004) Èlectrospinning of nanofibers: Reinventing the wheel?' Adv. Mater. 16 1151±1170. Lopez-Rubio A, Sanchez E, Sanz Y, LagaroÂn JM (2009) Èncapsulation of living bifidobacteria in ultrathin PVOH electrospun fibers' Biomacromolecules 10 2823± 2829. Ma Z, Ramakrishna S (2008) Èlectrospun regenerated cellulose nanofiber affinity membrane functionalized with protein A/G for IgG purification' J. Membr. Sci. 319 23±28. Medeiros ES, Glenn GM, Klamczynski AP, Orts WJ, Mattoso LHC (2009) `Solution blow spinning: A new method to produce micro- and nanofibers from polymer solutions' J. Appl. Polym. Sci. 113 2322±2330. Neamnark A, Sanchavanakit N, Pavasant P, Rujiravanit R, Supaphol P (2008) Ìn vitro biocompatibility of electrospun hexanoyl chitosan fibrous scaffolds towards human keratinocytes and fibroblasts' Eur. Polymer J. 44 2060±2067. Nie H, He A, Zheng J, Xu S, Li J, Han CC (2008) Èffects of chain conformation and entanglement on the electrospinning of pure alginate' Biomacromolecules 9 1362± 1365. Pandey JK, Reddy KR, Kumar AP, Singh RP (2005) Àn overview on the degradability of polymer nanocomposites' Polym. Degrad. Stab. 88 234±250. Ramakrishna S, Fujihara K, Teo W-E, Yong T, Ma Z, Ramakrishna R (2006) Èlectrospun nanofibers: Solving global issues' Mater. Today 9 40±50. Sawicka K, Gouma P (2006) Èlectrospun composite nanofibers for functional applications' J. Nanopart. Res. 8 769±781. Teo W-E, Ramakrishna S (2009) Èlectrospun nanofibers as a platform for multifunctional, hierarchically organized nanocomposite' Comp. Sci. Tech. 69 1804±1817. Torres-Giner S, LagaroÂn JM (2010) `Zein-based ultrathin fibers containing ceramic nanofillers obtained by electrospinning. I. Morphology and thermal properties' J. Appl. Polym. Sci. 118 778±789. Torres-Giner S, Ocio MJ, LagaroÂn JM (2008a) `Development of active antimicrobial fiber based chitosan polysaccharide nanostructures using electrospinning' Eng. Life Sci. 8 303±314. Torres-Giner S, Gimenez E, LagaroÂn JM (2008b) `Characterization of the morphology and thermal properties of zein prolamine nanostructures obtained by electrospinning' Food Hydroc. 22 601±614. Torres-Giner S, Ocio MJ, LagaroÂn JM (2009a) `Novel antimicrobial ultrathin structures of zein-chitosan blends obtained by electrospinning' Carbohyd. Polym. 77 261± 266. Torres-Giner S, Gimeno-AlcanÄiz JV, Ocio MJ, LagaroÂn JM (2009b) `Comparative performance of electrospun collagen cross-linked by means of different methods' Appl. Mat. Inter. 1 218±223. Torres-Giner S, Martinez-Abad A, Ocio MJ, LagaroÂn JM (2010) `Stabilization of a nutraceutical omega-3 fatty acid by encapsulation in ultrathin electrosprayed zein prolamine' J. Food Sci. doi: 10.1111/j.1750-3841.2010.01678.x. Tsutsumi H, Hara C (2008) `Characterization of new type polymer composites prepared



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by in situ coffining electrospun fibers into polymer matrixes' Technical Proceedings of the 2008 NSTI Nanotechnology Conference and Trade Show, NSTI-Nanotech, Nanotechnology 2 733±736. Xu CY, Inai R, Kotaki M, Ramakrishna S (2004) Àligned biodegradable nanofibrous structure: A potential scaffold for blood vessel engineering' Biomaterials 25 877± 886. Yarin AL, Zussman E, Wendorff JH, Greiner A (2007) `Material encapsulation and transport in core±shell micro/nanofibers, polymer and carbon nanotubes and micro/ nanochannels' J. Mater. Chem. 17 2585±2599. Zhang L, Menkhaus TJ, Fong H (2008) `Fabrication and bioseparation studies of adsorptive membranes/felts made from electrospun cellulose acetate nanofibers' J. Membr. Sci. 319 176±184.


6

Mass transport and high barrier properties of food packaging polymers F . N I L S S O N and M . S . H E D E N Q V I S T , Royal Institute of Technology, Sweden

Abstract: Polymers are to various extents permeable towards gases and liquids. For proper material selection it is therefore important to be able to predict and assess their permeation properties in actual/realistic environments. This chapter deals with the basics of the transport properties of polymers and their barrier properties, with the fundamental equations of mass transport. The second and third parts describe the physics behind the two parameters governing the transport: solute diffusivity and solubility. Since the focus of this chapter is on the prediction of solubility, the diffusivity description is very brief. The fourth part shows ways of obtaining high barrier properties of polymers by limiting the diffusivity and/or the solubility. Finally, the fifth part exemplifies ways of measuring the mass transport properties of polymers. Key words: transport properties, diffusivity, solubility, barrier, prediction.

6.1

Introduction: the basics of mass transport

The amount of solute transferred through unit cross-section per unit time is called the flux (F). According to Fick's first law, the flux depends only on the diffusion rate (D) and the concentration gradient (c=x) (Crank, 1986). Consider the plate in Fig. 6.1 which is subjected to an unlimited amount of nitrogen gas on the left side. At steady state, it is possible to calculate the flux as: @c c2 ÿ c1 6:1 F ÿD ÿD x2 ÿ x1 @x where c1 and c2 are the solute concentrations in the plate at the two boundaries. Henry's law (c Sp) gives a relationship between the solute vapour pressure (p) and the solute equilibrium concentration (c) through the solubility coefficient (S). The law is, at least at low pressures, valid for most non-glassy gas/polymer combinations. Assuming that Henrys law holds, eq. 6.1 can be rewritten as: p 1 ÿ p2 6:2 F DS x2 ÿ x1 This equation can be further simplified by introducing the permeability coefficient (P), defined as:


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6.1 A plate subjected to a steady-state gas transport from left to right.

P DS

6:3

In essence this means that the solute permeation rate depends on two factors, the diffusivity (D) and the solubility (S). By controlling these factors it is possible to steer towards high barrier properties or specific membrane characteristics. A useful expression is Fick's second law, which describes the solute concentration (uptake or loss) with time: @c @ @c D 6:4 @t @x @x This equation is easily obtained for a plate geometry by a one-dimensional mass balance and the use of eq. 6.1.

6.2

Diffusivity

The diffusivity depends on the size and shape of the solute and on the mobility and structure of the polymer chain network. The diffusivity will be reduced in the presence of polymer crystals and will increase with solute concentration if the solute plasticises the polymer. Thornton et al. (2009) were recently able to model a large set of permeability and diffusivity data over a broad range of free volumes. Data included both conventional polymers and those with extra large free volume where diffusion also occurred in percolated channels. The diffusivity could be expressed as an exponential function depending on the fractional free volume ( f ) and two empirical constants ( and ): D exp f A corresponding expression for the solubility was also derived.


6:5

Mass transport and high barrier properties

131

6.2 as a function of the square of the kinetic diameter obtained from permeability ( ) and diffusivity (l) data. Lines are best fits using a linear relationship. Drawn after Thornton et al. (2009).

The fractional free volume ( f ) for a specific polymer can be obtained directly by Bondi's group contribution method (Bondi, 1964) without considering that the accessible free volume may be different for different gases. An alternative approach for obtaining f (Park and Paul, 1997; Greenfield and Theodorou, 1993) is to consider that the accessible fractional free volume is different for different gases. For a polymer with a total specific volume v, the fractional free volume f of gas n is dependent on the specific free gas volume (v0 ): fn v ÿ v0 n =v 6:6 For a polymer with K repeating units, where each segment has a van der Waals volume vw and an empirically determined gas±polymer interaction parameter

nk , the variable v0 can be calculated as a summation over all repeating units: v 0 n

K X k1

nk nw n

6:7

The constants and in eq. 6.5 can both be experimentally determined. For example, is an approximately linear function of the square of the kinetic gas diameter, which is obtained from diffusivity or permeability experiments: see Fig. 6.2.

6.3

Solubility

The solute (gas/vapour) solubility depends on a number of factors, including the size and shape of the solute and polymer molecules and their polarity and


132


hydrogen-bonding capacities. In addition, molecular crosslinking, orientation and crystallisation are important factors. Several models, both semi-empirical and theoretical, have been developed to predict gas, vapour and even liquid solubilities in polymers. Some examples of models based on statistical thermodynamics will be given here. In these methods, relationships between pressure, temperature and volume of the pure components are usually developed first and a rule of mixing is then used to determine the properties of the mixture. Note that this chapter should not be considered as a complete survey of existing models, but rather as a presentation of interesting examples for systems above and below the glass transition temperature Tg. For a comprehensive review on the topic (equation-of-state models before 2000) please consult Wei and Sadus (2000). Examples of models that, for sake of space, we have not considered include the UNIFAC and/or free-volume-based models (Rolker et al., 2007; Radfarnia et al., 2005; Wibawa and Widyastuti, 2009; Wang 2007; Serna et al., 2008).

6.3.1

The Sanchez±Lacombe equation-of-state model (SL-EOS)

Sanchez and Lacombe presented in an early pioneering work a lattice-fluid equation of state model for polymers (Sanchez and Lacombe, 1976; Lacombe and Sanchez, 1976; Challa and Visco, 2005). The starting point in the development of the theory was the relationship between the free energy (G) and the configurational partition function (Z) in the pressure ensemble: G ÿkT ln ZT; p

6:8

where k, p and T are respectively the Boltzmann's constant, temperature and pressure. Z is, in turn, a function of the number of configurations E; V ; N available in a system of N molecules having configurational/potential energy E and volume V: XX ZT; p

E; V ; N exp ÿ 1 E pV 6:9 V

E

In the ensemble studied, T and p were constant and 1 1=kT. The main problem here was to determine , and the approach was to use the Guggenheim solution with a mean field approximation. Consider first a binary mixture of N0 empty sites and N linear r-mers (molecule-chains) giving the total number of sites as Nr N0 rN . The interior r-mer is surrounded by z ÿ 2 nearest nonbonded and two bonded neighbours, where z is the coordination number. Consider an orthogonal lattice with z 6 (Fig. 6.3). One of the middle mers is surrounded by two bonded neighbours and four non-bonded neighbours/ vacancies. The corresponding numbers for the end-mers are 1 and z ÿ 1 5. In the general case, an r-mer is surrounded by qz nearest neighbours:



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6.3 The fluid lattice.

qz r ÿ 2z ÿ 2 2z ÿ 1 rz ÿ 2 2

6:10

The total number of nearest neighbour pairs is z=2Nr and the number of nonbonded pairs is z=2Nq , where Nq N0 qN . In the derivation, a symmetry number and a flexibility parameter were introduced. The first parameter was equal to 2 if the two chain ends were indistinguishable and was equal to 1 if they were different. The variable described the internal degrees of freedom of the rmer. Its maximum value is zz ÿ 1rÿ2 for a flexible linear chain. Hence, for a 2-mer component, d z holds. In obtaining a useful solution for , Sanchez and Lacombe made use of, e.g., Guggenheim's findings/derivations which at large z yielded: N0 N 1 w 6:11 lim z!1 f0 f where w =erÿ1 and the fractions of empty and occupied sites are f0 N0 =Nr and f rN =Nr . In the following, it is assumed that d and the closepacked volume (rv ) are independent of pressure and temperature. The closepacked volume of a mer is the same as that of an (empty) lattice site (Fig. 6.3) and can be obtained from the close-packed mass density (r ) through knowledge of the molar mass M and the close-packed mass density : rv M= , where the energy of the system depends only on the nearest neighbour interactions. It can be written: XX E ÿz=2Nr pi; jij 6:12 i

j


134


where pi; j is the joint-pair probability between components i and j and ij is the corresponding interaction energy. As stated before, the only non-zero contributions to the energy are the mer-to-mer interactions. Assuming random mixing of r-mers and holes, the probability term becomes, for large z, pmer, mer rN =Nr 2 f 2 which yields an energy equal to: E ÿNr z=2 f

2

6:13

or using the fact that the close-packed volume of an r-mer system is V N rv and that the total volume is V N0 rN v Nr v V =f

6:14

the equation: E=rN ÿz=2V =V ÿ V =V f

6:15

The variable can be considered as the energy to make a hole and r is the molecular energy in the absence of holes. Both energy and volume are solely functions of the number of holes, and this leads to a simplified form of the configurational partition function: Zp; T

1 X

exp 1 E pV

6:16

N0 0

This can be solved by approximating the above sum by its maximum term. This is the same as inserting the generic term of the partition function in eq. 6.7 and finding the minimum of the Gibbs free energy: G E pV ÿ kT ln

6:17

Using eqs 6.11, 6.14 and 6.15, eq. 6.17 can, be expressed in dimensionless variables: ~ 1 1 p ~ ÿ~ T~ G=Nr G ÿ 1 ln1 ÿ ~ ln~ =w 6:18 ~ r ~ where ~p p=p , ~ = and T~ T=T are, respectively, the reduced pressure, mass density and temperature. Further, p =v , T =k and ~ 1=~v V =V . In other words, p and are the `hypothetical' pure component cohesive energy density and mass density. T is proportional to the depth of the potential energy well. The occupied fraction can be written f = ~ 1=~v. By obtaining the minimum of the Gibbs free energy with respect to the volume ~v, the following expression is finally obtained: 1 2 ~ p T ln1 ÿ ~ 1 ÿ ~ 0 ~ ~ 6:19 r The characteristic fluid length is r p M=RT , where M is the pure component molar mass. For n 1 mol solute, the Sanchez-Lacombe equation



135

can be written in the standard form Z Fp; V ; T by expanding eq. 6.19 in the virial form: pV 1 1 r r Z 6:20 1r ÿ ~ ~2 ~3 . . . RT 2 T~ 3 4 Challa and Visco (2005) used the SL-EOS model to predict the solubilities of blowing agent in polyols. First, the three parameters p , T and of the pure systems were determined through a minimisation of an objective function including experimental p, T and data. The binary system (i, j) with blowing agent and polyol were subsequently predicted using the following combining rules, which differed slightly from the mixing formulae used by Sanchez-Lacombe: XX p i j pij 6:21 i

where

j

0:5 ÿ pij 1 ÿ fij Pi Pj

6:22

and fij is the SL-EOS binary interaction parameter. Further, T p

X 0 T 0 i

i

and

pi

i

6:23

1 X 0i r ri0 i

6:24

with 0i

i pi =Ti j pj =Tj

and

!i i X i !j j

6:25

j

The zero subscript refers to the pure state and i is the segment fraction of component i. For a binary solute±polymer mixture, j is the volume fraction of solute. !i is the mass fraction of component i. The solubility was defined as the mole-fraction (x1 ) of the blowing agent divided by the pressure when the pressure approaches zero, and the mole-fraction x1 is related to the volume fraction 1 through 1 r1 x1 =r: x1 6:26 S lim p!0 p The solubilities estimated with the SL-EOS model and with a `variable-range statistical associating fluid theory' (VR-SAFT) by Challa and Visco (2005) are compared with experimental data in Table 6.1. In the analysis, it was assumed


136

Multifunctional and nanoreinforced polymers for food packaging Table 6.1 Henry's law solubility constants for blowing agents in PluracolÕ 975* Blowing agent HFC 32 HFC 134a HFC 143a HFC 125 HFC 152a

SSL (bar/mol)

SVR-SAFT (bar/mol)

Sexp (bar/mol)

0.1267 0.2364 0.1558 0.1170 0.3519

0.0605 2.1853 2.0899 2.1263 2.1340

0.1220 0.1761 0.0649 0.0976 0.1647

*Data obtained from Challa and Visco (2005).

that the vapour temperature and pressure were the same as the liquid temperature and pressure, that the chemical potential of the blowing agent was the same in the liquid and vapour phases, and that the vapour pressure of the polyol was zero. It was noticed that the SL-EOS model gave good results compared to experimental data, and clearly better than the VR-SAFT model (which will be described later in this chapter).

6.3.2

Statistical associated fluid theory (SAFT) models

An important class of statistical thermodynamic methods are the statistical associated fluid (SAFT) models. An early development of the original SAFT model was the `variable range statistical associating fluid theory' (VR-SAFT), which was mentioned briefly in Section 6.3.1 (Gil-Villegas et al., 1997; Galindo et al., 1998; McCabe et al., 2001). Here it is considered that the Helmholtz free energy (A) consists of the ideal-gas free energy (AIDEAL), the monomer free energy (AMONO), the free energy due to the formation of chains of monomers (ACHAIN) and the free energy due to the formation of association complexes (AASSOC): A AIDEAL AMONO ACHAIN AASSOC

6:27

When a square well is used to describe the interactions between the segments, the compressibility becomes: pV @A=kB T A 6:28 Z ÿ NkB T @N Nk BT T;V where kB is the Boltzmann constant. A more complete description of the model can be found in Gil-Villegas et al. (1997), Galindo et al. (1998) and McCabe et al. (2001). The most well-known of the successfully improved SAFT-type models is the perturbed chain SAFT model (PC-SAFT), presented by Gross and Sadowski (2001). This model involves a perturbation theory with a hard chain reference fluid rather than the spherical molecules used in earlier SAFT work. The compressibility was considered to consist of a hard chain and a dispersion component:


Mass transport and high barrier properties Z 1 Z hc Z disp

137 6:29

The hard chain part can be expressed as Z

hc

! 3 31 2 323 ÿ 3 23 m ÿ 1 ÿ 3 0 1 ÿ 3 2 0 1 ÿ 3 3 X i

xi mi ÿ 1giihs ÿ1

@giihs @

6:30

where giihs is the radial distribution function of the hard sphere fluid, xi is the mole fraction and mi is the number of segments per chain of component i, is P the total number density of molecules, and n =6 i xi mi din , where di is the segment diameter of component i. The dispersion term is @I1 @I2 disp C1 Z ÿ2 6:31 C3 ÿ m C2 I2 C4 @ @ where is the packing fraction and I1 and I2 are integrals of density, segment number and temperature. For a complete description of how to calculate these terms, and the coefficients Ci , please refer to Gross and Sadowski (2001). Not surprisingly, since the chain feature was implemented in the model, the fit to PVT data of non-spherical molecules was superior to the original SAFT, as illustrated for toluene in Fig. 6.4. Interestingly, the prediction of the pressuredependent solubility of n-pentane in polyethylene was also good. The fitted binary interaction parameter was small (ÿ0:0195) using polyethylene parameters from the extrapolation of n-alkane parameters to high molar mass. It

6.4 Experimental saturated liquid and vapour densities for toluene ( ) and the corresponding fits using PC-SAFT (solid line) and SAFT (broken line). Drawn after Gross and Sadowski (2001).


138


6.5 n-Pentane-polyethylene (l, left y-axis) and CO2-polyamide 11 ( , right y-axis) experimental data and corresponding fits using the simplified SAFT model (lines). The binary interaction parameters used were 0 for the polyethylene case and ÿ0.05 for the PA11 case. Drawn after von Solms et al. (2005).

should be noted that the assessment of the quality of the model was based on the fit and predictions of non-associating or only weakly polar substances. In improved versions of the PC-SAFT model, the same ideal and dispersion terms were used, but simplified terms for the hard-chain contribution were used (simplified PC-SAFT) and a term for associating energies was added (associating complexes) (von Solms et al., 2005). Figure 6.5 shows high-pressure data predicted with the simplified PC-SAFT method together with experimental data for concentration versus pressure for two very different systems: n-pentane±polyethylene and CO2±polyamide 11. In the polyethylene case, a finite binary interaction parameter was in fact unnecessary; the temperature dependence of the gas solubility was still correctly predicted. However, in order to obtain a good fit in the PA11 case, a small but finite kij was necessary. In fact, a temperature-dependent kij had to be included in some cases. In these cases (CH4/HDPE and CH4/PVDF), the kij was considered to increase linearly with increasing temperature. A group-contribution simplified PC-SAFT for the prediction of polymer systems was presented by Tihic et al. (2008) (GC-PC-SAFT). The methodology is similar to that described above, except that the mixing rules are different. Both first- and second-order interactions (for molecular length m, segment diameter and the energy term ) are considered. For details refer to Tihic et al. (2008). Pedrosa et al. (2006) compared the PC-SAFT model with the soft-SAFT model, in which the reference term involves spherical Lennard±Jones species. The chain feature was, however, included; a perturbation to the reference term (using Wertheim's theory) was applied, followed by the use of a radial



139

6.6 Experimental ( ) n-pentane mass concentration in polyethylene at 423.65 K and predicted values using the SOFT-SAFT (solid line) and the PCSAFT (dashed line). Drawn after Pedrosa et al. (2006).

distribution function for the Lennard±Jones fluid. Figure 6.6 illustrates good agreement between the experimental n-pentane solubility in polyethylene and predictions based on the two models.

6.3.3

The non-equilibrium lattice fluid (NELF) model

Doghieri, Baschetti, Sarti and coworkers in Bologna, Italy, have modelled the solubility of gases in glassy polymers using a non-equilibrium lattice fluid (NELF) model. The approach was to use the polymer density as an internal state variable describing the departure from equilibrium. In essence, only the PVT data of the pure components and the density of the solid mixture were needed for the calculations. Through a pseudo-equilibrium between the chemical potentials of the gas (G ) and of the solid (S ), it is possible to calculate the solute volume fraction (1 ) as a function of gas pressure (p) and temperature (T): S T; ~S ; 1 G T; p The left-hand side (S ) of eq. 6.32 is calculated from S r1 ÿ r10 S 0 ln1 ÿ ~S ÿ r1 ÿ ln~ 1 ÿ ri RT ~S ÿ 0 2 S r1 v1 p1 p ÿ 2 p ~ RT


6:32

6:33

140


where p1 and p2 are tabulated gas normalisation constants for the gas and the polymer, respectively. The density constants 1 and 2 of the pure components, as well as the constants v1 and v2 , are tabulated.ÿp p 2 The binary pressure parameter is p p1 ÿ p2 , a first-order approximation where fij 1, the site molar volume is v v1 v2 =v1 2 v2 1 , the site occupation number is r10 M1 =1 v1 and the mixture occupation number is r1 r10 v1 =v . The reduced solid density ~S can be approximated by 02 w1 1 ÿ w1 6:34 ~S 1 ÿ w1 1 2 where 02 is the density of the pure polymer and w1 is the weight fraction of gas. The relation between weight fraction and volume fraction is: 1

w1 =1 w1 =1 w2 =2

6:35

The right-hand side of eq. 6.32 can be derived from the Sanchez-Lacombe equation of state, resulting in: G ~Eq r10 v1 p1 ln~ Eq ÿ r10 ln1 ÿ ~Eq ÿ r10 ÿ RT RT

6:36

Finally, the equilibrium density Eq of the gas must be pre-calculated by minimising the chemical potential of the Sanchez±Lacombe equation of state, resulting in (cf. eq. 6.18): ÿ Eq 2 ÿ 1 Eq Eq ~ 0 6:37 ~ ~ p T ln 1 ÿ ~ 1 ÿ ~ r Examples included the prediction of the CO2 dual sorption isotherm of PC at 35ëC as well as the reduced dual character for the annealed material (Doghieri and Sarti, 1996). Different desorption isotherms for the same system but from different sorption pressures were also predicted (hysteresis). Later, the model was successfully tested on low-pressure sorption isotherms using a constant density equal to the pure unpenetrated polymer, i.e. without the need for dilation data. Wherever significant swelling was absent, the model yielded in general a satisfactory description of the sorption behaviour. Examples studied included vinyl chloride in PVC and CO2 in PMMA (Sarti and Doghieri, 1998; Doghieri and Sarti, 1998). If dilation data are missing, and eq. 6.34 is not assumed to be valid, it is still possible to obtain the polymer pseudo-equilibrium density by considering that it decreases linearly with increasing penetrant pressure (Giacinti Baschetti et al., 2001): S p 02 1 ÿ kp


6:38


141

6.7 Solute (CO2)±polymer mass ratio as a function of solute pressure. Lines correspond to NELF one-parameter fitting to PMMA (33ëC, ) and PS (35ëC, l) data. Drawn after data of Giacinti Baschetti et al. (2001).

where k, the swelling coefficient, and the pure polymer density are nonequilibrium parameters. At least one of the two parameters k and 02 is usually unknown; the authors, however, presented convenient ways of obtaining these from a small amount of data. Figure 6.7 shows a fit where the polymer density is known and the swelling coefficient is determined from a single high-pressure solubility datum (one-parameter correlation). The estimated swelling coefficient values of 0.0097 (PS) and 0.0218 (PMMA) were indeed close to the experimental values: 0.0121 (PS) and 0.0243 (PMMA). It is useful to know that, at the limit of vanishing pressure, the solubility coefficient can be obtained from (De Angelis et al., 2007): TSTP M1 p1 T1 p2 2 02 exp 1 ÿ 1 0 ln 1 ÿ S0 TpSTP 1 RT1 T2 p1 2 2 T1 p2 02 T1 6:39 ÿ 1 p1 p2 ÿ p12 T2 p1 2 p1 T

6.3.4

The non-equilibrium perturbed hard-sphere chain (NE-PHSC) model

A non-equilibrium perturbed hard-sphere-chain (NE-PHSC) model has been used to predict the solubility of gases and vapours in glassy polymers (Doghieri et al., 2006). The non-equilibrium thermodynamics for glassy polymers (NETGP), previously used with lattice-fluid models, was here combined with the


142


perturbed hard-sphere-chain (PHSC) theory. In the latter, the residual Helmholtz free energy is obtained as the sum of two contributions: ares aref aper

6:40

ref

where a accounts for the chain connectivity and a hard-sphere interaction and aper accounts for mean field forces. Two different approaches were used to describe the perturbation, either a van der Waals approach or a square well potential with a variable width. The NE-PHSC was solved, in the limit of low penetrant concentrations, considering the following pseudo-equilibrium condition: " # M M NES 0 sol T; p; PE ; s ; p ; ; p ; ksp sol ; pol ; s ; r s r p M EQG T; p; s ; ; s sol 6:41 r s 0 where the suffixes s and p refer to the solute and polymer, respectively. PE sol , pol and ksp are the pseudo-equilibrium solute mass per polymer mass, the pure polymer density and the binary interaction parameter. The chemical potential in the solid (left-hand side of eq. 6.39) is obtained from NET-GP calculations and the solute chemical potential in the gas phase (right-hand side) is obtained from equilibrium EOS. The methodology was tested on different solute±polymer pairs. Figure 6.8 gives an example where experimental CO2 solubilities in PC were fitted with the square well potential type of EOS. It is interesting to note the large improvement in the fitting of the glassy data when PHSC was combined with NET-GP.

6.8 CO2 solubility ( ) as a function of the inverse temperature predicted by the PHSC model with the square well potential (constant relative width of 1.455) with (solid curve) and without (dashed curve) the NET-GP approach. The binary interaction parameter was 0.075. Drawn after data of Doghieri et al. (2006).



6.4

143

What makes a barrier a barrier?

The starting point when aiming at developing materials with excellent barrier properties is to consider eq. 6.3. A low permeation is obtained by targeting a low diffusivity and/or a low solubility. There are several ways to achieve this and some of them will be briefly discussed in the following. For a more complete description, please consult Hedenqvist (2005). Crystals are impermeable to most solutes and the presence of crystals therefore lowers the solubility. The fact that the solutes have to circumvent the crystals also leads to a decrease in the diffusivity due to a tortuosity effect. The spherulitic morphology of polyethylene is complex and the prediction of the tortuosity is not straightforward. The radially growing, splining, splaying crystal lamellae form a network of crystals that the solute molecule has to pass during its journey through the material (Fig. 6.9). A Monte-Carlo-generated random walk yielded the tortuosity effect given in Fig. 6.10, which is compared with experimental data for four different solutes. Note that the tortuosity effect in the present cases leads to a reduction of the diffusivity by, at most, a factor of ca. 10 when going from 0 to ca. 80 vol% crystallinity.

6.9 Generated spherulite based on 100000 `crystalline' bricks having an aspect ratio of 10. The volume crystallinity is around 25%.


144


6.10 The reciprocal of the tortuosity () as a function of volume crystallinity for different polyethylenes with the following penetrants: n-hexane at a specific penetrant concentration in the amorphous component (15 vol%), n CH4, N2, s Ar. The latter three sets of data were obtained from permeation data taken at 80ëC and 10 MPa. The line predicted by simulation is displayed as a solid line ending with l. Drawn after Nilsson et al. (2009).

In Fig. 6.10, three of the solutes are small and in the case of n-hexane the polymer is swollen with a mobile amorphous interphase. Thus the solute mobility reduction near the crystal faces is expected to be reasonably small even though the self-diffusivity of the amorphous chains near the crystals is low. However, in the case of larger solutes or unswollen polymers, the constraint effect will most probably be more pronounced. The reduction in the diffusivity due to the constraint effect is larger for materials of greater crystallinity. The constraint of the amorphous component and hence the permeation can also be decreased by chemical crosslinking. Figure 6.11 shows the permeability of methanol through elastomeric crosslinkable polysilicon olefins. These contain unsaturations that oxidise during a high temperature (120ëC) treatment and produce intermolecular crosslinks. Interestingly, the crosslinking effect is greater for the larger solutes. It should, however, be noted that increasing the degree of crosslinking does not always lead to a lower permeability. It has been shown for poly(ethylene glycol) diacrylate (Lin et al., 2005) and for photocrosslinked polyethylene (Chen and RaÊnby, 1989) that the constraining effect may be small, absent or of less importance than other effects leading to opposite trends. The presence of strong secondary intermolecular bonds increases the barrier for non-polar solutes. For this reason, the hydrogen-bonded EVOH has a significantly lower oxygen permeability than polyethylene. The weak point is that moisture interacts with the hydrogen bonds in the material, and in the worst



145

6.11 Methanol permeability through polyorganosilicone-containing elastomers. The treatment time corresponds to the extent of crosslinking. The data correspond to (from bottom to top) the permeation of CH4, C2H6, C3H8 and C4H10. Drawn after data of Kim et al. (2005).

case this effectively leads to the disappearance of the barrier. The high gas barrier component, in this case, has to be protected from the moist environment with a `waterproof' layer. In addition, the layers need good bonding that then requires a tie layer on both sides of the barrier. The resulting films are therefore often relatively complex and advanced. Numerous examples of multilayer films and their barrier properties are given in Massey (2003). Biaxial stretching in a sequential or simultaneous mode reduces the permeability. In a study on polypropylene, Lin et al. (2008) showed that, even though the amorphous content increased with drawing, the oxygen permeability decreased. They suggested that this was due to the reduced mobility of stretched tie chains, which reduced the frequency by which connecting channels form between neighbouring free-volume holes. Pinhole-free metals and defect-free inorganic glasses are considered to be àbsolute' barriers. Consequently, when extra high barriers are needed, polymer layers may be combined with one or both of these. BarixTM is an interesting barrier solution (www.vitexsys.com). It consists of several layer-pairs of a thin polymer film and an AlOx layer. The thin film is produced from a liquid that is cured with UV and the ceramic is applied by physical vapour deposition. The polymer films are generally between 0.25 and 1 m thick and the ceramic films are significantly thinner. It has been shown that a 20 nm thick ceramic layer can give good barrier properties. The resulting multilayer BarixTM film is a very thin and thus transparent and flexible material that can, for instance, be used as a coating on thicker substrates. The key to success here is that the polymer film is smooth, continuous and clean, which gives a uniform nucleation and a more


146


defect-free AlOx film during the deposition. In addition, a possible defect or crack in the ceramic layer cannot easily grow past the adjacent polymer film layers. The resulting multilayer film is claimed to be a million times tighter to water vapour and oxygen than conventional food packaging. The 3M ScotchpakTM is another example of a high barrier film obtained with an AlOx layer. There are also materials using SiOx vacuum-coated layers (see, e.g., the `TechbarrierÕ' material at www.techbarrier.com). With biaxially oriented PET as the base, the water vapour transmission rate is 0.3 g/(m2 24 h) and the oxygen transmission rate is 0.3 cm3/(m2 24 h atm). With SiOx-coated oriented polyvinylalcohol (OPVA), the values of the Techbarrier go down to, respectively, 100

Commodities

Artichoke, asparagus, cauliflower, cherry, citrus, grape, jujube, strawberry, pomegranate, leafy vegetables, root vegetables, potato, most cut flowers Blueberry, cranberry, cucumber, eggplant, okra, olive, pepper, persimmon, pineapple, pumpkin, raspberry, tamarillo, watermelon Banana, fig, guava, honeydew melon, mango, plantain, tomato

Apple, apricot, avocado, cantaloupe, feijoa, kiwifruit (ripe), nectarine, papaya, peach, pear, plum Cherimoya, mammee apple, passion fruit, sapota

Table 8.2 Summary of respiration and ethylene production rates of some fruits at different temperatures ß Woodhead Publishing Limited, 2011

Commodity

Apple: Fall Summer Apricot Artichoke Asian pear Avocado Banana (ripe) Beets Blackberry Blueberry Cherry Grape, American Grape, Muscadine Grape, Table Grapefruit Guava

Respiration rate (mg CO2/kg-h) at a temperature of 0ëC

5ëC

10ëC

15ëC

20ëC

25ëC

3 5

6 8

9 17

15 25

20 31

na na

6 30 5 na na 5 19 6 8 3 10 3 na na

na 43 na 35 na 11 36 11 22 5 13 7 na na

16 71 na 105 80 18 62 29 28 8 na 13 na 34

na 110 na na 140 31 75 48 46 16 na na 1%) also retard fruit ripening and their effects are additive to those of reduced O2 atmosphere (Daniels et al. 1985; Dixon and Kell 1989; Kader et al. 1989). The effects of MA/CA on delay or inhibition of ripening are greater at higher temperature (Saltveit 2005). Thus use of MA may allow handling of ripening (climacteric-type) fruits at temperatures higher than their optimum temperature. This is especially beneficial for chill-sensitive fruits such as tomatoes, melons, avocados, bananas and mangoes to avoid their exposure to chilling temperature (Zagory and Kader 1988). MA conditions reduce respiration rates as long as the levels of O2 and CO2 are within those tolerated by the commodities. These, combined with the decreased C2H4 production and reduced sensitivity to C2H4 action, result in delayed senescence and extending shelf-life as indicated by retention of chlorophyll (green color), textural quality


Table 8.9 Optimum conditions of MA/CA for some fruits and their shelf-life Commodity

Storage temperature (ëC)

Optimum MA/CA

Injurious atmosphere

Marketable life (days)


% O2

% CO2

% O2

% CO2

RA storage

CA storage

0±3

3

3

2

10

200

300

Avocado Banana

7 12±15

2±5 2

3±10 5

1 1

15 8

12 21

56 60

Grape Guava

0±2 12±15

3±5 2±5

1±3 2±5

1 2

10 12

40 15±20

90±100 45

Lemon Litchi Mango Orange Papaya Pear

15 0±5 13 5±10 13 0±1

3±5 3±5 3±5 10 3±5 2±3

0±5 3±5 5±8 5 5±8 0±1

1 2 2 5 2 1

6 14 8 5 8 2

130 20±30 14±28 42 14±28 200

220 2230 21±45 84 21±35 300

Pineapple Strawberry

10±15 0

2±5 4±10

10 15±20

2 1

10 12

12 7

10±15 7±15

Apple

Sources: Saltveit 1993, 1997; Kader 1997; Mahajan 2001, Irtwange 2006.

Major benefit under MA/CA storage

Commercial potential

Maintains firmness and acidity Delays softening Suppresses climacteric pattern Controls disease Delays ripening and chilling injury Retains green color Delays ripening Delays ripening Maintains firmness Less decay Delays flesh and core browning Reduces chilling injury Less decay

Excellent Good Excellent Fair Good Good Good Fair Fair Excellent Fair Excellent

194


Table 8.10 O2% limits below which injury can occur for some commodities Minimum O2 concentration tolerated (%) 0.5 or less 1

1.5 2

2.5 3 4 5 10 14

Commodities

Chopped greenleaf, redleaf, Romaine and iceberg lettuce, spinach, sliced pear, broccoli, mushroom Broccoli florets, chopped butterhead lettuce, sliced apple, Brussels sprouts, cantaloupe, cucumber, crisphead lettuce, onion bulbs, apricot, avocado, banana, cherimoya, atemoya, sweet cherry, cranberry, grape, kiwifruit, litchi, nectarine, peach, plum, rambutan, sweetsop Most apples, most pears Shredded and cut carrots, artichoke, cabbage, cauliflower, celery, bell and chilli pepper, sweet corn, tomato, blackberry, durian, fig, mango, olive, papaya, pineapple, pomegranate, raspberry, strawberry Shredded cabbage, blueberry Cubed or sliced cantaloupe, low permeability apples and pears, grapefruit, persimmon Sliced mushrooms Green snap beans, lemon, lime, orange Asparagus Orange sections

Sources: Bohling and Hansen 1984; Kader et al. 1989; Kays 1991, 1997; Gorny 1997; Kader 1997; Kupferman 1995; Richardson and Kupferman 1997; Saltveit 1997; Beaudry 2000.

(decreased lignification), and sensory quality of fruits and vegetables (Makhlouf et al. 1989; Pal and Buescher 1993; Saltveit 2005; Menon and Goswami 2008; Lu Shengmin 2009). Exposure of fresh fruits and vegetables to O2 levels below their tolerance limits or to CO2 levels above their tolerance limits (Tables 8.10 and 8.11) may increase anaerobic respiration and the consequent accumulation of ethanol and acetaldehyde, causing off-flavor (Bohling and Hansen 1984; Kader et al. 1989; Kays 1991, 1997; Gorny 1997; Kader 1997; Saltveit 1997; Beaudry 2000). Low O2 and/or high CO2 concentrations can reduce the incidence and severity of certain physiological disorders such as those induced by C2H4 (scald of apple and pear) and chilling injury of some commodities (Kader 1997; Saltveit 1997; Irtwange 2006). Besides, O2 and CO2 levels beyond those tolerated by the commodity can induce physiological disorder such as brown stain on lettuce, internal browning and surface pitting of pome fruits, and black heart of potato. CA/MA combinations have direct and indirect effects on post-harvest pathogens. El-Goorani and Sommer (1981) pointed out that delaying senescence, including fruit ripening, by MA/CA reduced the susceptibility of fruits and vegetables to pathogens. On the other hand, MA conditions unfavorable to a given commodity can induce its physiological breakdown and render it more


Advances in polymeric materials for MAP

195

Table 8.11 CO2% limits above which injury can occur for some commodities Maximum CO2 concentration tolerated (%) 2 3 5 7 8 10 15 20 25 30

Commodities

Lettuce (crisphead), pear Artichoke, tomato Apple (most cultivars), apricot, cauliflower, cucumber, grape, nashi, olive, orange, peach (clingstone), potato, pepper (bell) Banana, bean (green snap), kiwifruit Papaya Asparagus, Brussels sprouts, cabbage, celery, grapefruit, lemon, lime, mango, nectarine, peach (freestone), persimmon, pineapple, sweet corn Avocado, broccoli, litchi, plum, pomegranate, sweetsop Cantaloupe (muskmelon), durian, mushroom, rambutan Blackberry, blueberry, fig, raspberry, strawberry Cherimoya

Sources: Herner 1987; Gorny 1997; Kader 1997; Kays 1997; Kupferman 1995; Richardson and Kupferman 1997; Saltveit 1997.

susceptible to pathogens. Elevated CO2 concentrations inhibit the growth of some types of microorganisms, bacteria and fungi during storage. Oxygen levels below 1% and/or CO2 levels above 10% are needed to significantly suppress fungal growth (Farber 1991; Kader et al. 1989). Elevated CO2 levels (10±15%) can be used to provide fungistatic effects on commodities that tolerate such CO2 levels (Daniels et al. 1985; Dixon and Kell 1989).

8.5.2

Tolerance limit of commodities to modified atmosphere

The extent of benefits from the use of MA depends upon the commodity, cultivar, physiological age (maturity stage), initial quality, concentration of O2 and CO2, temperature and duration of exposure to such conditions. Subjecting a cultivar of a given commodity to an O2 level below and/or a CO2 level above its tolerance limit at a specific temperature±time combination will result in stress to the living plant tissue, which is manifested as various symptoms, such as irregular ripening, initiation and/or aggravation of certain physiological disorders, development of off-flavors and increased susceptibility to decay and fungal growth (Marcellin 1974; Lipton 1975; Isenberg 1979; Smock 1979; Kader et al. 1989; Ke and Saltveit 1989; Varoquaux et al. 1996; Beaudry 2000; Watkins 2000). Fruits and vegetables are classified according to their relative tolerance to low O2 or elevated CO2 concentration (Kader et al. 1989; Kader 1997; Saltveit 1997; Beaudry 2000; Watkins 2000) when kept at their optimum


196


storage temperature and relative humidity ± see Tables 8.10 and 8.11. The limits of tolerance to low O2 levels would be higher than those indicated in Table 8.10 to maintain aerobic respiration if the storage temperature and/or duration are increased. For some commodities, susceptibility to low O2 and/or high CO2 stress is influenced by maturity stage. For example ripe fruits often tolerate higher levels of CO2 than mature green fruits. Minimally processed fruits and vegetables have fewer barriers to gas diffusion, and consequently they tolerate higher concentrations of CO2 and lower O2 levels than intact commodities. The effects of stress resulting from exposure to undesirable MA/ CA conditions (i.e. level of O2 and/or CO2) can be additive to other stresses (such as chilling injury, wounding, or ionizing radiation) in accelerating the deterioration of fresh produce. Successful MAP must maintain near-optimum O2 and CO2 levels to attain the beneficial effects of MA without exceeding the limits of tolerance which may increase the risk of physiological disorders and other detrimental effects (Watkins et al. 1998; Kader et al. 1989; Beaudry 2000; Watkins 2000).

8.5.3

Physiological and biochemical effects of modified atmosphere packaging

The respiration rate is considerably reduced by the low O2 and high CO2 atmosphere in MAP. The low respiration rate reduces the overall metabolic and biochemical activities (ethylene production and sensitivity to ethylene, rapid acid catabolism, changes of pectic substances in the cell wall leading to softening, etc.) in the cell, thereby reducing the rate of utilization of food reserves (Kader 1986; Kader et al. 1989). By reducing the respiration rates, MA also lowers the production of heat due to respiration. Carbon dioxide has an antagonistic effect on enzymes involved in ethylene biosynthesis. As O2 is required in the production of ethylene, low O2 concentrations suppress ethylene production. During MAP, a compound precursor to ethylene is accumulated in the products. Therefore, when the products are transferred to air, ethylene is rapidly produced and the products ripen faster (Wang 1990). Modified atmospheres delay the onset of the ripening process and increase firmness in fruits. By inhibiting the enzyme polyphenol oxidase in litchi, strawberries, lettuces and mushrooms, MA prevents browning of their tissues (Wang 1990; Renault et al. 1994b; Stewart et al. 2003; Sivakumar and Korsten 2006; Del Nobile et al. 2008). When O2 is not available, fruits and vegetables degrade glucose anaerobically by glycolysis to generate energy. In the glycolysis pathway, aldehydes, alcohols and lactates are produced (Kader 1986). Accumulation of these anaerobic by-products produces off-flavors associated with physiological disorders, leading to an unacceptable eating quality (Meyer et al. 1973; Herner 1987; Kader et al. 1989; Ke and Saltveit 1989; Varoquaux et al. 1996). Therefore, a minimum of 2±3% levels of O2 must be maintained



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in the MAP to prevent anaerobic respiration (Wang 1990; Jayas and Jeyamkondan 2002). Hence, the objective of MAP design is to define the conditions so as to achieve the optimum concentration of O2 and CO2 inside the package with the shortest possible time to preserve the quality and extend shelf-life.

8.5.4

Required characteristics of plastic films for MAP

The desirable characteristics of a polymeric film for modified atmosphere packaging depend on the respiration rate of the produce at the transit and storage temperature to be used and on the known optimum O2 and CO2 concentrations for the produce that will result in optimum MA conditions within a definite time period. For most produce, a suitable film must be much more permeable to CO2 than to O2 (Sacharow and Griffin 1980; Ben-Yehoshua 1985; Kader et al. 1989; Exama et al. 1993). The major factors to be taken into account when selecting the packaging materials are: · The type of package (i.e. flexible pouch or rigid or semi-rigid lidded tray) · The barrier properties needed (i.e. permeabilities of individual gases and gas ratios when more than one gas is used) · The physical properties of machinability, strength, clarity and durability · Integrity of closure (heat sealing), fogging of the film as a result of product respiration · Sealing reliability · Water vapor transmission rate · Resistance to chemical degradation · Non-toxic and chemically inert · Printability · Commercial suitability with economic feasibility.

8.6

Polymeric films for application in modified atmosphere packaging (MAP)

Flexible plastic packaging materials comprise nearly 90% of the materials used in MAP, with paper, paperboard, aluminum foil, metal and glass containers accounting for the remainder. This is largely due to changing consumer demand in which convenience, quality, safety and impact on the environment are of prime consideration. These materials provide a range of permeability to gases and water vapor together with the necessary package integrity needed for MAP (Tables 8.12 and 8.13). Sometimes the films are used alone, but often they are used in combinations that provide the benefits of multiple materials. The most commonly used polymeric films for modified atmosphere packaging are as follows.


Table 8.12 Absorbers used for active MAP for extending shelf-life


Packaging system

Example of working principle/mechanism/reagents

Purpose

Oxygen absorbers (sachet, labels, films, corks)

Ferro-compound (iron powders), ascorbic acid, metal salt, glucose oxidase, alcohol oxidase

Carbon dioxide absorbers (sachet)

Calcium hydroxide and sodium hydroxide or potassium hydroxide, calcium oxide, magnesium oxide, activated charcoal and silica gel Aluminum oxide and potassium permanganate (sachets), activated hydrocarbon (squalane, apiezon) + metal catalyst (sachets), builder-clay powders (films), zeolite (films), Japanese oya stone (films) and other compounds like silicones (phenylmethyl silicone) Polyacrylates (sheet), polypropylene glycol (film), silica gel (sachet), clays (sachet)

Reducing/preventing respiration rate, mold, yeast and aerobic bacteria growth; prevention of oxidation of fats, oils, vitamins, colors; prevention of damage by worms, insects and insect eggs Removing excess carbon oxide formed during storage to prevent fruit damage and bursting of package Prevention of too fast ripening and softening

Ethylene absorbers (sachets, films)

Humidity absorbers (drip absorbent sheets, films, sachets) Absorbers of off-flavors, amines and aldehydes (films, sachets) UV-light absorbers Reagents Preservative films

Cellulose acetate film containing narinaginase enzyme, ferrous salt and citric or ascorbic acid (sachet), specially treated polymer Polyolefins like polyethylene and propylene doped in the material with a UV-absorbent agent; crystallinity modification of nylon 6 Ferrous carbonate: 4FeCO3 + O2 + 6H2O ! 4Fe (OH)3 + 4CO2 Slowly diffuse preservatives such as nisin, sorbate, glycol, antioxidants, antibiotics, ethanol or ethylene into the package

Sources: Kader et al.1989; Labuza and Breene 1989; Ahvenainen 2003.

Control of excess moisture in packed produce, reduction of water activity on surface of food in order to prevent growth of molds, yeast and spoilage bacteria Reduction of bitterness in fruit, improving flavor, and oil-containing foods Restricting light induction oxidation For quickly developing an MA within a package. The reaction quickly builds up the CO2 content of the package while reducing the O2 content somewhat Control microbial growth; suppress unwanted biochemical reactions

Table 8.13 Permeability of polymeric films available for MAP Name of film


Polyvinyl fluoride (PVF) Polyvinylidene fluoride (PVDF) Polyamide (nylon-6) Polycarbonate Polyethylene terephthalate (PET) Polyamide Low density polyethylene (LDPE) Linear low density polyethylene (LLDPE) Medium density polyethylene (MDPE) Linear medium density polyethylene (LMDPE) High density polyethylene (HDPE) Ethylene vinyl acetate copolymer (EVA) Ethylene vinyl alcohol copolymer (EVOH) Polypropylene (cast film) Polypropylene (BOPP) Polybutylene Polyvinyl alcohol (PVOH) Polystyrene (PS) Mylar (polyester) Oriented polystyrene (OPS) Polyvinyl chloride (PVC) ± plasticized Polyvinylidene chloride (PVDC) Cellulose acetate Rubber hydrochloride Ethylcellulose Methylcellulose Cellulose triacetate Vinylchloride acetate Natural rubber Silicone rubber

Permeability (cm3.m/m2.h. atm) O2

CO2

50 81.66 105.83 2829.17 50±100 416.66 11416.68 2916.66±8333.34 4083.33±8791.67 3666.66 1640.41±3280.83 7500 0.1 2458.33±2675 2675 6316.66 3.75 4875±6316.67 54.16±137.5 4100 422.5±32666.67 16.25 1919.58 623.33 32808.33 1312.08 2460.41 246.25 63500 1058333

179.16 408.33 423.33 18166.67 245.83±408.64 708.33 39958.33 ± 1625±41000 ± 9841.67±11482.92 45833.33 3.33 8166.67±13041.67 8833.33 23375 1.66 16375±23375 190.41±412.5 11500±21958.33 1633.33±49000 62 14107.50 4724.16 82020.83 6561.66 14435.66 902.5 370416.66 6350000

Sources: Karel et al.1975; Kader et al.1989; Abdel-Bary 2003; Massey 2003.

Permeability (g.m/m2.h) to water vapor

CO2/O2 ratio

54.16 8.16 640 62.5 16.25±21.25 20.83±56.25 18.75 12.5 11.66 9.16 6.66 187.5 33.33±100 ± 6.25±11.25 19.58±30.83 ± 32.5±162.5 ± 145.83 147.91±180.83 3.33 1230.84 8.25 328 3280.83 78.31 65.61 ± 72

3.58 5.00 4.00 6.42 4.91 1.70 3.49 ± 4.66 ± 5.99 6.11 33.3 3.32 3.30 3.70 2.00 3.35 3.51 2.80 3.86 3.81 7.34 7.57 2.5 5.00 5.86 3.66 5.83 6.00

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8.6.1


Polyolefins

Polyolefin is a collective term for polyethylene and polypropylene, the two most widely used plastics in food packaging industries. Polyethylene and polypropylene both possess a successful combination of properties, including flexibility, strength, lightness, stability, moisture and chemical resistance, and easy processability, and are well suited for recycling and reuse (Karel et al. 1975; Abdel-Bary 2003; Marsh and Bugusu 2007).

8.6.2

Low-density polyethylene (LDPE)

The simplest and most inexpensive plastic made by addition polymerization of ethylene is polyethylene. Low-density polyethylene is the most commonly used packaging film. LDPE seals at a lower temperature and over a wider temperature range, and has better hot tack, all of which result, to a great extent, from its longchain branching (Prasad 1995; Moyls et al. 1998; Abdel-Bary 2003). LDPE forms a good barrier to water vapor but a poor barrier to oxygen, carbon dioxide and many odor and flavor compounds. Because LDPE is relatively transparent, it is predominantly used in film applications and in applications where heat sealing is necessary. Some properties and characteristics of LDPE are presented in Table 8.13. LDPE is generally the cheapest plastic film on a per-unit-mass basis.

8.6.3

Linear low-density polyethylene (LLDPE)

Linear low-density polyethylene is also one of the most commonly used packaging films in the packaging industry. The reduction of density comes about through the use of comonomers that put side groups on the main chain that act like branches in decreasing crystallinity. LLDPE is also a soft, flexible material with a hazy appearance. At equal density and thickness, LLDPE has higher impact strength, tensile strength, puncture resistance and elongation than LDPE. Like LDPE, LLDPE has good water vapor barrier properties but is a poor barrier to oxygen, carbon dioxide and many odor and flavor compounds (Abdel-Bary 2003; Massey 2003). Since LLDPE often permits considerable downgaging, it can be the lowest cost alternative on a per-use basis.

8.6.4

High-density polyethylene (HDPE)

High-density polyethylene is a linear addition polymer of ethylene, produced at temperatures and pressures similar to those used for LLDPE, and with only very slight branching. HDPE films are stiffer than LDPE films, though still flexible, and have poorer transparency. Their water vapor barrier is better, as is their gas barrier. However, permeability to oxygen and carbon dioxide is still much too



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high for HDPE to be suitable as a barrier for these permeants (Burton et al. 1987; Marsh and Bugusu 2007). Because of the distinctly cloudy appearance of HDPE film, a small amount of white pigment is commonly added to provide an attractive opaque white film. Typical HDPE properties are shown in Table 8.13.

8.6.5

Polypropylene (PP)

Polypropylene is a linear addition polymer of propylene; resins used in packaging are predominantly isotactic. PP has the lowest density of the commodity plastics, 0.89±0.91 g/cm3. Harder and more transparent than polyethylene, PP has good resistance to chemicals and is effective at barring water vapor. Its high melting point makes it suitable for application where thermal resistance is required. Barrier properties of PP are comparable to those of HDPE (Crosby 1981; Exama et al. 1993; Abdel-Bary 2003). In many applications, biaxially oriented film (BOPP) is preferred. BOPP film is explicitly used in modified atmosphere packaging of food commodities.

8.6.6

Polyvinyl chloride (PVC)

Polyvinyl chloride films are formed by combining PVC resin, produced by addition polymerization of vinyl chloride, with plasticizers and other additives to produce a flexible film. In general, the films are quite soft and flexible, easy to heat-seal, and have excellent self-cling, toughness, medium strength, excellent resistance to chemicals, resilience and clarity. Permeability is relatively high (Kader et al. 1989; Exama et al. 1993; Ahvenainen 2003; Massey 2003). Both oriented and unoriented films are available. The properties of PVC films are listed in Table 8.13.

8.6.7

Polyesters

Polyethylene terephthalate (PET), polycarbonate and polyethylene naphthalate (PEN) are polyesters, which are condensation polymers formed from ester monomers that result from the reaction between carboxylic acid and alcohol. The most commonly used polyester in food packaging is polyethylene terephthalate (Kader et al. 1989; Abdel-Bary 2003).

8.6.8

Polyethylene terephthalate (PET)

PET is commonly used in biaxially oriented form, and has excellent transparency and mechanical properties. PET provides a good barrier to gases (O2 and CO2), to moisture and especially to odors and flavors. The barrier properties can be enhanced by coating with PVDC. Coating or coextrusion is often used to provide good heat-seal properties. It can tolerate considerably higher


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temperatures for short periods, such as in dual ovenable packaging for frozen foods. The main reasons for its popularity in the food packaging industry are its glass-like transparency, adequate gas barrier properties, light weight and shatter resistance (Abdel-Bary 2003; Kirkland et al. 2008). Typical PET properties are listed in Table 8.13.

8.6.9

Polyvinylidene chloride (PVDC)

Polyvinylidene chloride is an addition polymer of vinylidene chloride. It is heat-sealable and serves as an excellent barrier to oxygen, water vapor, odors and flavors (Kader et al. 1989). The PVDC copolymer can be heat-sealed and serves as an excellent barrier to gases. However, the best barrier films generally do not provide the best heat-seal capability, and vice versa, so when both heat-sealability and barrier properties are desired, sometimes two differently formulated PVDC copolymer coatings are applied. The major applications of PVDC include packaging of poultry, cured meats, cheese, snack foods, tea, coffee and confectionery and modified atmosphere packaging of food products.

8.6.10 Ethylene±vinyl alcohol (EVOH) Ethylene±vinyl alcohol is a copolymer of ethylene and vinyl alcohol. The presence of ±OH groups in the structure results in strong intermolecular hydrogen bonding. EVOH gives an excellent barrier to gases (especially O2), odors and flavors. However, the hydrogen bonds also make it a moisturesensitive material, and high humidity decreases its barrier capability (Marsh and Bugusu 2007). EVOH is most often used as an oxygen barrier. Typical EVOH properties are listed in Table 8.13.

8.6.11 Polyamide (nylon) Nylon films are used for specialty applications in packaging, where performance requirements justify their relatively high cost. Nylons have mechanical (excellent strength) and thermal properties (high-temperature performance) similar to those of PET and have similar usefulness. Nylons also provide an excellent odor and flavor barrier, and a reasonably good oxygen barrier (Crosby 1981; Kader et al. 1989). They are very poor water vapor barriers, and generally have a tendency to lose some barrier performance when exposed to large amounts of moisture. However, their performance is not as water-sensitive as that of EVOH. Owing to their relatively high cost, they are often coextruded with other plastics. Typical properties of some nylon films are given in Table 8.13. Nylon-6 tends to be the most-used nylon packaging film in industry.



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8.6.12 Polychlorotrifluoroethylene (PCTFE) These films are considered the best available transparent moisture barriers for flexible packaging; however, they are rather expensive. Aclar films can be laminated to paper, polyethylene, aluminum foil or other substrates. The film is heat-sealable and can be thermoformed. Aclar blister packages are often used for unit packages for highly moisture-sensitive pharmaceuticals.

8.6.13 Polyvinyl alcohol (PVOH) Polyvinyl alcohol polymers are produced by hydrolysis of polyvinyl acetate. Because PVOH degrades at temperatures well below melt, it cannot be processed by extrusion. Therefore, casting from a water solution is used to make the film. As produced, the film is amorphous, but orientation induces some crystallinity.

8.6.14 Ethylene±vinyl acetate (EVA) Ethylene±vinyl acetate is produced by addition copolymerization of ethylene and vinyl acetate. EVA has higher permeability to water vapor and gases than LDPE. These films have excellent transparency, and provide very good heat-seal and adhesive properties, with excellent toughness at low temperatures. In both lidding and base films, EVA is mainly used as a component of the sealant layer.

8.6.15 Ionomers The heat-seal performance of ionomers is outstanding. Ionomer films have excellent clarity, flexibility, strength and toughness, which make them suitable for modified atmosphere packaging of commodities. They can be used to package sharp objects, which break through many alternative materials when subject to vibration during distribution. Ionomers have relatively poor gas barrier properties, and tend to absorb water readily. They are also relatively costly compared to films such as ethylene±vinyl acetate (Massey 2003).

8.6.16 Polycarbonate films Polycarbonate films have excellent transparency, toughness and heat resistance, but high cost. They have some use in skin packaging, food packaging where exposure to high temperatures for in-bag preparation is required, and medical packaging.

8.6.17 Polystyrene Polystyrene is another thermoplastic film with excellent transparency, with a high tensile strength but giving a poor barrier to moisture vapor and gases


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(Benning 1983; Kader et al. 1989; Abdel-Bary 2003). It is often used in window envelopes and window cartons. Because of its low gas barrier, it can be used for produce where a `breathable' film is required. Polystyrene alone is brittle, but it can be blended or generally biaxially oriented to get the required properties. In heavier gages, polystyrene is widely used for transparent thermoformed trays.

8.7

Cellulose-based plastics

Cellulose-based plastics such as cellulose acetate, cellulose butyrate, cellulose propionate and copolymers are also used to a relatively small extent, most often as sheet rather than film. Their high price and water sensitivity limit their usefulness.

8.8

Biodegradable polymers

Biodegradable polymers are derived from replenishable agricultural feedstocks, animal sources, marine food processing industry wastes, or microbial sources. Biodegradable polymers are made from cellulose and starches. Cellophane is the most common cellulose-based biopolymer. Starch-based polymers include amylose, hydroxylpropylated starch and dextrin. Other starch-based polymers are polylactides (PLA), polyhydroxyalkanoate (PHA), polyhydroxybutyrate (PHB), and a copolymer of PHB and valeric acids (PHB/V). Made from lactic acid formed from microbial fermentation of starch derivatives, polylactide does not degrade when exposed to moisture (Marsh and Bugusu 2007; Siracusa et al. 2008). In addition, biodegradable films can also be formed from chitosan, which is derived from the chitin of crustacean and insect exoskeletons. Chitin is a biopolymer with a chemical structure similar to that of cellulose. Edible films, in the form of a thin layer of edible materials applied to food as a coating or placed on or between food components, are another form of biodegradable polymer. They serve several purposes, including inhibiting the migration of moisture, gases and aromas and improving the food's mechanical integrity or handling characteristics, aiming to achieve modified atmosphere packaging conditions (Ben-Yehoshua et al. 1994; Marsh and Bugusu 2007). At present, bioplastics are more expensive than petroleum-based polymers, so substitution would likely result in increased packaging cost. Commercialization of bioplastics is underway. Polylactides are commercially produced from natural products (corn sugar). After the original use, the polymer can be hydrolyzed to recover lactic acid, thereby approaching the cradle-to-cradle objective (that is, imposing zero impact on future generations). In addition, Wal-Mart uses biopolymers by employing polylactides to package fresh and cut produce (Marsh and Bugusu 2007).



8.9

205

Multilayer plastic films

In many cases, the best combination of packaging attributes at the lowest cost is achieved by using a combination of materials. Therefore, plastic packaging films are often combined with one another or with other materials such as paper or aluminum through processes such as coating, lamination, coextrusion and metalization.

8.9.1

Coating

Coating is commonly used to add a thin layer of a plastic on the surface of another plastic film or more commonly on a non-plastic substrate such as paper, cellophane or foil. The coating may be applied as a solution, a suspension, or a melt. Common reasons for using coating in flexible packaging are to impart heat-sealability for plastics that are not heat-sealed easily; to provide moisture protection for paper or cellophane; to improve barrier properties; and to provide protection from direct contact of the base material with the product (Banks 1984, 1985; Smith et al. 1987a; Ben-Yehoshua et al. 1994). PVDC copolymer coatings are often used to improve barrier properties and heat-sealability.

8.9.2

Lamination

Lamination is the process of combining two webs of film together (Prasad 1995; Marsh and Bugusu 2007). In flexible packaging applications, lamination is often used to combine a plastic film with another film, paper or foil. A variety of lamination methods are used. When plastic films are involved, either as a substrate or as an element in the finished structure, the laminating adhesive is often a low-density polyethylene, applied by extrusion, and the process is known as extrusion laminating. When paper is contained in a flexible package, it is most often being used for its excellent printability, along with its ability to impart substance and strength. Another significant use of lamination is to produce a web with buried printing.

8.9.3

Coextrusion

Coextrusion results in the production of a multilayer web without requiring initial production of individual webs and a separate combining step. The melted polymers are fed together carefully to produce a layered melt, which is then processed in conventional ways to produce a plastic film or sheet. When only plastics are being used in a flexible packaging structure, coextrusion is generally preferred to lamination, unless buried printing is involved. A major advantage of coextrusion over lamination is its ability to incorporate very thin layers of a material, much thinner than those that can be produced as a single web. This is


206


particularly important for expensive substrates, such as those often used to impart barrier properties. The amount of the expensive barrier resin used need only be enough to provide the desired performance (Abdel-Bary 2003). The thinness of the layer is not limited by the need to produce an unsupported film and handle it in a subsequent lamination step.

8.9.4

Metalization

Metalization is a way of applying a thin metal layer on a plastic film. In commercial packaging practice, the metal being deposited is almost always aluminum. Metalized films have significantly enhanced barrier characteristics, and are usually chosen for this reason. In addition to a gas barrier, metalized film provides an essentially total light barrier (Hernandez et al. 2000).

8.9.5

Barriers and permeation

The mechanism by which substances travel through an intact plastic film is known as permeation. It involves dissolution of the penetrating substance, the permeant, in the plastic, followed by diffusion of the permeant through the film, and finally by evaporation of the permeant on the other side of the film, all driven by a partial pressure differential for the permeant between the two sides of the film (Karel et al. 1975; Nemphos et al. 1976; Koros 1989; Prasad 1995; Massey 2003). The barrier performance of the film is generally expressed in terms of its permeability coefficient or permeability. For one-dimensional steady-state mass transfer, the permeability coefficient is related to the quantity of permeant, which is transferred through the film as represented by the equation: P

Qx Atp

8:1

where P is the permeability coefficient, Q is the amount of permeant passing through the material, x is the thickness of the plastic film, A is the surface area available for mass transfer, t is time, and p is the change in permeant partial pressure across the film. Hence the permeability coefficient (P) is the proportionality constant between the flow of the penetrant gas per unit film area per unit time and the driving force (partial pressure difference) per unit film thickness. The amount of gas penetrating through the film is expressed in terms of either moles per unit time (flux) or weight or volume of the gas at STP. Commonly, it is expressed in terms of volume. It can be shown that the permeability coefficient P, as defined by equation 8.1, is equal to the product of the Fick's law diffusion coefficient, D, and the Henry's law solubility coefficient, S (P DS) in situations where these laws



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adequately represent mass transfer (ideally dilute solutions, with diffusion independent of concentration). The permeability coefficient under these circumstances is a function of temperature but is not a function of film thickness or permeant concentration.

8.9.6

Concept and theoretical approach

Gases and vapors can permeate through materials by macroscopic or microscopic pores and pinholes or they may diffuse by a molecular mechanism, known as activated diffusion. In activated diffusion the gas is considered to dissolve in the film at one surface, to diffuse through the film by virtue of concentration gradient, and to reevaporate at the other surface of the packaging film. The equilibrium and kinetics considerations governing mass transfer have been applicable here as well. The gas transport properties through polymers can be described by three parameters: the diffusion coefficient, the permeability coefficient, and the solubility. These terms are interrelated, although the precise nature of the correlation is dependent on the type of diffusion that occurs. Generally the Fickian diffusion process is considered for gas transport in polymers. The rate of diffusion is the speed with which a gas molecule penetrates through the polymer. The diffusion coefficient D is based on Fick's first law of diffusion. It states that the flux J in the x direction is proportional to the concentration gradient @c=@x: @c 8:2 J ÿD @x The flux, J, is the volume of substance diffusing across unit area in unit time, independent of the state of aggregation of the polymer. This first law is applicable to diffusion in the steady state, that is, where concentration is not varying with time. The change in concentration with time at a distance x into a thin film sheet, where the flux is in the x-direction only, is given by @c @Jx ÿ @x @x Substituting the values of equation 8.2 in equation 8.3, we have @c @ @c D @x @x @x If D is independent of concentration, then equation 8.4 can be written as 2 @c @ c D @x @x2


8:3

8:4

8:5

208


The permeability coefficient, P, concerns the steady-state flux, J, of gas passing through the polymer and the pressure difference across it, which gives the driving force: p ÿ p 1 2 J P 8:6 x where p1 and p2 are the partial pressures on opposite sides of a film of thickness x. P is expressed in cm3 of gas at STP per cm2 of film, unit cm of film thickness per second for a pressure difference of 1 atm. The solubility, S, is defined as the amount of dissolved gas in the polymer divided by the volume of the sample for 1 atm of gas on the sample surface: c1 ÿ c2 Sp1 ÿ p2 When p2 0 and c2 0, the above equation can be written as c1 Sp1

8:7

and when p1 1 atm S c1 where c1 is the concentration in the sample when equilibrium is reached. Equation 8.6 obeys Henry's law when S is independent of p, and hence equations 8.2, 8.6 and 8.7 can be combined to be written as P DS

8:8

The solubility S is expressed in cm3 of gas at STP per cm3 of the solid at a pressure of 1 atm (cm3 STP/cm3.atm), The diffusivity or diffusion coefficient D is expressed as the diffusion of penetrants in cm2 across the film at STP per second (cm2/s). P is expressed in cm3 of gas at STP per cm2 of film, unit cm of film thickness per second for a pressure difference of 1 atm (cm3 STP-cm/ cm2.s.atm).

8.10

Gas permeation or gas transmission

Conceptually, the gas permeability coefficient is the same as the gas transmission rate (GTR). The GTR is defined as the volume of gas that passes through a sample of unit area under unit pressure differential, at a given temperature and film thickness, with the rate being determined after the gradient of the recorded volume±time curve has become constant. The gas transmission rate is usually expressed for the total thickness of the film, while gas permeability is expressed on the basis of per unit film thickness. For composite films, it is more appropriate to use gas transmission rate values since permeation in composite films does not usually vary linearly with film thickness. For some single material (polymer) films also, the relationship is not linear either. In such cases extrapolation may be erroneous (Nemphos et al. 1976; Laffin et al. 2009).



209

8.10.1 Measurement of gas permeability There are many methods for measuring permeability of gases and it is not possible to review them in detail here. Two major types of methods used for the measurement of gas transmission rates are the pressure-increase method (or differential pressure principle) and the concentration-increase method (or equal pressure principle) (Karel et al. 1975; Prasad 1995). Pressure-increase method/differential pressure principle The test specimen is placed between the upper and lower chambers and clamped tightly. First the lower-pressure chamber (lower chamber) is vacuumized and then the whole system. When the specified degree of vacuum is reached, the lower test chamber is shut off and test gas of a certain pressure is fed to the upper test chamber (high-pressure chamber). It is ensured that a constant differential pressure (adjusted) is maintained across the specimen. Hence under the gradient of differential pressure the test gas permeates from the high-pressure side to the low-pressure side. By monitoring and measuring the pressure in the low-pressure side, the various barrier parameters (permeability coefficient) of the tested specimen are calculated (Labthink 2008). Karel et al. (1975) reported that in the pressure-increase method a membrane is mounted between the high-pressure and the low-pressure sides of a permeability cell. In this method, both sides are evacuated and the membrane is degassed. Then, at zero time a known constant pressure PH of the test gas is introduced on the high side, and PL (low-side pressure) is measured as a function of time. If the measurement is continued only as long as PH (high-side pressure) remains much larger than PL (low-side pressure), P remains essentially constant and the permeability coefficient can be calculated as follows: pL VL 273 x P 8:9 t 760 T A where P is the permeability coefficient (cm3-mm/cm2.s.cm Hg), PH is the pressure introduced at the high side, PL is the low-side pressure, pL =t is the steady gas pressure increment in the low-side pressure and is obtained from the slope of the increments of low-side pressure vs. time plot, VL is the calibrated volume of the low-side pressure of the cell, x is the thickness of the film, and A is the effective permeation area. Banerjee et al. (2004) measured the gas permeability of films using a laboratory-made high-vacuum apparatus with static permeation cell at 1 atm for different temperatures. The polymer film was degassed for 24 h within the permeation cell prior to the experiment. To start the measurement, desired gas pressure (Pi 1 bar) was applied instantaneously to the pressure side of the film. On the downstream side a reservoir of constant volume was connected with a pressure transducer, so that the total amount of gas that passed the polymer


210


film could be monitored. The time-lag method was employed for the gas transport measurements. This technique allows the determination of the mean permeability coefficient P from the steady-state gas pressure increment (dp/dt)s in the calibrated volume V of the product side of the cell. The permeability coefficient is reported in barrer and was calculated from equation 8.10: dp V T0 x P 8:10 dt S P0 P i T A where P is the permeability coefficient in barrer (cm3-cm/cm2.s.cm Hg), dp/dt is the steady-state gas pressure increment in the calibrated volume V of the of the cell and is obtained from the slope of the increments of downstream pressure vs. time plot, V is the calibrated volume of the product side of the cell in cm3, T0 is the standard temperature 273.15 K, P0 is the standard pressure 1.013 bar, Pi is the upstream side pressure (desired gas pressure to be applied at the highpressure side of the film, i.e. 1 bar), T is the temperature of measurement in K, x is the thickness of the film in cm, and A is the effective permeation area in cm2. Laffin et al. (2009) measured gas permeability of LDPE/LLDPE films under controlled conditions of pressure, temperature and relative humidity. The test consisted of placing the film sample between a partition test cell and an evacuated manometer, with the pressure across the film at 1 atm. As the gas passes through the film sample, the mercury in the capillary of the manometer is depressed. After a constant transmission rate was achieved, a plot of mercury height against time gave a constant gradient. The slope of the gradient was then used to calculate the gas transmission rates. The quantity of gas passing through the film is directly proportional to the difference in gas pressure on either side of the film, and inversely proportional to the thickness of the film. In addition, it is directly proportional to the time during which permeation has occurred, and to the exposed area of the sample (Karel et al. 1975). Hence, Q/

Atp1 ÿ p2 x

8:11

PAtp1 ÿ p2 8:12 x where Q is the quantity of gas which passes through the film, A is the surface area in contact with the gas, t is the time, p1 ÿ p2 is the partial pressure differential, and x is the thickness of film. Q

Concentration-increase method/equal pressure principle The test principle is that high oxygen flows on one side of the film and high pure nitrogen flows on the other side of the film. Oxygen molecules pass through the film into the nitrogen on the other side, and are taken to the sensor by the flowing nitrogen. The transmission of oxygen is tested by analyzing the



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concentration of oxygen detected by the sensor. As for the packaging container, nitrogen flows in the container, and air or high pure oxygen covers the outside of the container. In most cases, gas transmission rates have been measured by employing the pressure gradient method (p 1 atm). Normally, in MA packages a pressure gradient of 1 atm between the internal atmosphere of the package and the external atmosphere is a rare possibility. A certain degree of flexibility in the package free volume and the presence of a pressure-balancing gas, namely, N2, help in maintaining a low pressure gradient without causing considerable variation in the concentration gradient. Hence the concentration-increase or concentration gradient method facilitates close simulation of the conditions under which gas transmission takes place in MAP (Taylor et al. 1960; Yasuda et al. 1968; Prasad 1995).

8.11

Water vapor permeability

It is also important to calculate the water vapor transmission rate of the packaging system. In this case, the partial pressure difference for water vapor between the inside and the outside of the package is almost never constant. Simplifying equation 8.1, the rate of moisture gain or loss in the product is given by the resulting differential equation as follows (Abdel-Bary 2003): dQ 1 Pwv Ap2 ÿ p1 8:13 dt x where Q is the mass of permeant (water vapor) passing through the material in g, Pwv is the water vapor permeability of the packaging filom (g-mm/m2.day.Pa), p1 and p2 are the partial pressures of water vapor outside and inside the package respectively, in Pa, and p2 is function of Q. The principle involved is that saturated water vapor is transmitted through the test specimen in unit time under specified conditions of temperature and humidity. The transmitted mass is determined by testing the decreasing weight of distilled water as time passes. In a desiccant system of measurement, silica gel is used as the desiccant and is placed directly inside the film pouch whose Pwv is to be measured under controlled conditions of 38ëC and 90% relative humidity. Water vapor permeability is computed from the measured values of the change in weight of the packages with time, employing the following equation (Jaya 2005): Pwv dw 1 8:14 x dt Ap where Pwv is the water vapor permeability of the packaging film (g-mm/ m2.day.Pa), dw/dt is the weight gain by the desiccant with time and is obtained from the slope of the increments of weight vs. time plot, t is the time in days, w


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is the weight gain by the desiccant in g, x is the thickness of the film in mm, A is the area of the package in m2, and p is the water vapor pressure at 38ëC in Pa.

8.11.1 Effect of temperature on permeability The permeability of O2 and CO2 in polymeric films is temperature dependent and this dependence is commonly described by an exponential equation (Arrhenius-type equation) (Yam and Lee 1995; Exama et al. 1993). The relationship of O2 permeability and CO2 permeability with temperature can be depicted by this model. The generalized form and the specific form (permeability to O2 and CO2) of the Arrhenius equations are as follows: P ÿEa P 8:15 P P exp RT EaP HS ED HS HC Hm where HS is the apparent heat of solution, ED is the activation energy for diffusion, HC is the heat of condensation, Hm is the heat of mixing, P is the permeability of gas at temperature T, PP is the permeability pre-exponential factor for gas, EaP is the activation energy of permeation for gas, R is the universal gas constant, and T is the absolute temperature When permeability coefficients are not available at the temperature of interest, an Arrhenius relationship can be used to determine the required value from the permeability coefficient at a nearby temperature and the activation energy. The equation used is the following: Ea 1 1 P2 P1 exp 8:16 ÿ R T1 T 2 where T1 is the temperature at which P1 is known, T2 is the temperature at which P2 is to be calculated, Ea is the activation energy, and R is the gas constant. The permeability coefficient, as indicated, is a product of the diffusion coefficient and the Henry's law solubility constant. Since these vary in different ways with temperature, equations 8.15 and 8.16 are valid only over reasonably small temperature ranges. A particular concern is that permeation rates are much higher above the glass transition temperature (Tg) than below this temperature, and the rate of change with temperature differs. Generally, the above equations would accurately characterize a polymer's gas diffusivity/temperature behavior, except where there are strong interactions between the polymer and the gas molecules (e.g., water vapor and hydrophilic polymers). In addition, the above equations would only predict the effect of temperature above Tg, since most films show a discontinuity of diffusion at the transition. At or below Tg, the polymer conformation is set and rotational movements responsible for diffu-



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sional properties are blocked (Cowie 1973). Therefore, equation 8.15 should never be used to calculate the permeability coefficient across a temperature range that spans Tg of the plastic.

8.11.2 Temperature quotient for permeability The influence of temperature on permeability of polymeric films was quantified in Section 8.3.3 with the QP10 value, which is the permeability increase for a 10ëC rise in temperature and is given by the following equation: 10=T2 ÿT1 P2 P Q10 8:17 P1 where QP10 is the temperature quotient for permeability, and P1 and P2 are the permeabilities at temperatures T1 and T2 .

8.11.3 Permeability coefficient of multiplayer films Permeability coefficients for multilayer plastic film or sheet, either laminations or coextrusions, can be calculated from the thickness and permeability coefficients of the individual layers, as follows (Abdel-Bary 2003): xt 8:18 Pt X n xi =Pi i1

where the subscript t indicates the value for the total structure, i indicates the value for an individual layer, and there are n layers in the structure. When two films are combined to form a film laminate, equation 8.18 can be expressed as follows (Karel et al. 1975): 1 x1 x2 Pla xP1 xP2

8:19

where Pla is the permeability of the film laminate (cm3/m2.h.atm), P1 and P2 are the permeabilities of the individual films, i.e. of film 1 and film 2 respectively, x1 and x2 are the thicknesses of the individual films, and x is the thickness of the film laminate.

8.11.4 Effect of sub-zero temperature on permeability Lambden et al. (1985) investigated the effect of sub-zero temperatures on the oxygen transmission rate (OTR) of packaging films. They inferred that around 0ëC, a small variation in temperature greatly alters the permeabilities and thus their prediction is not possible with an Arrhenius relationship.


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8.11.5 Influence of polymer structure and morphology on permeability Salame (1986) correlated polymer structure and morphology with gas permeability. Based on cohesive energy density and fractional free-volume of the polymer, he derived a numerical scale of `permachor' values () to predict gas permeability for non-interacting polymer-penetrant systems as given below: P A=T0 eÿS

8:20

where A and S are constant and T0 is the tortuosity (oriented crystalline polymers).

8.12

Packaging systems in modified atmosphere packaging (MAP)

The polymeric films used for MAP are of three types: (1) polymeric films without perforations or microperforated, (2) macroperforated polymeric films, and (3) perforation-mediated packaging systems. Microperforated or non-perforated polymeric films yield low O2 and low CO2 concentrations because the CO2 permeability of these materials is generally three to six times that of O2 permeability (Yam and Lee 1995; Exama et al. 1993). These materials are suitable for less CO2-tolerant commodities such as mangoes, bananas, grapes and apples. The gas permeability in microperforated polymeric films is temperature dependent and this dependence is commonly described by Arrhenius-type equations (Exama et al. 1993; Mahajan et al. 2007) as follows: P ÿEa 1 1 8:21 ÿ P Pref exp R T Tref where P is the permeability at temperature T, EaP is the activation energy for permeation, R is the ideal gas constant, T is the absolute temperature, Tref is the reference temperature, and Pref is the permeability at the reference temperature. Perforated films have higher permeability rates but the ratio of CO2 to O2 permeability is much lower, approaching unity. Such films are, therefore, of great interest for commodities tolerating simultaneously low O2 and high CO2 levels such as fresh-cut products, strawberries and mushrooms ± commodities having a high respiration rate (Fonseca et al. 2000; Montero-CalderoÂn et al. 2008; Rediers et al. 2009). Macroscopic perforations in a polymeric film represent a parallel route for gas transport. An apparent permeability term is used for these films, which is a function of the film permeability and of the number and size of holes. For holes of equal size, the governing equation which describes the effect of temperature



215

on permeability of macroscopic films is as follows (Fishman and Ben-Yehoshua 1996; Mahajan et al. 2007; Techavises and Hikida 2008): Pa P

R2h 16:4 10ÿ6 Nh x Rh

8:22

where Pa is the apparent permeability of the macroscopic perforated film, Rh is the radius of the holes/macroperforations, Nh is the number of holes, x is the film thickness, and P is the permeability of the non-perforated films. In the perforation-mediated packaging, tubes are inserted in an airtight package (Fonseca et al. 2000, Mahajan et al. 2007). This system is also adequate for products requiring high CO2/low O2 concentrations and minimizes water accumulation inside the package. The perforation-mediated packaging system is a rigid one, which is suitable for bulk products and for products sensitive to mechanical damage. The gas exchange in perforation-mediated packages has been found to be independent of temperature within the biological range of temperature (0±25ëC). However, the permeability depends on dimensions, numbers and porosity of the tubes. The permeability of perforation-mediated packages can be represented by PAp/x as described in the following equation (Fonseca et al. 2000; Mahajan et al. 2007): PAP D1:45 4:80 10ÿ6 NP 0:598 x LP

8:23

where NP is the number of perforations (tubes), is the porosity ( 1 when the tubes have no packing), AP is the surface area of the package through which O2 and CO2 permeate, D is the diameter of perforation (tube) and LP is the length of perforation (tube). All variables in the above equation are in SI units.

8.13

Advanced technology for efficient modified atmosphere packaging (MAP)

The goal of MAP of fresh commodities is to create an equilibrium package atmosphere with %O2 low enough and %CO2 high enough to be beneficial to the produce and not injurious. This is accomplished through the proper balance of several variables that affect package atmosphere (Das 2005; Kader et al. 1989; Mahajan et al. 2007; Del-Valle et al. 2009). MAP has progressed in the past several years. Appropriate packaging materials have been developed for most commodities. Recent advances in polymer science and technology have made it possible to manufacture films with desired and well-designed gas transmission rates, especially for O2. There is consensus as to which films are appropriate for standard-sized packages of various food commodities. Knowledge of how to effectively seal packages and reduce incidence of leakages has developed and printing capabilities have provided ever more attractive packages. Technical


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challenges still exist in produce packaging. Some of the technologies currently available to meet those challenges are as follows.

8.13.1 Continuous films The movement of O2 and CO2 is usually directly proportional to the differences in gas concentration across the film. Steady-state (constant) O2 and CO2 levels are achieved in the package when the O2 uptake and CO2 production by the product are equal to that permeating through the film (Kader et al. 1989; Exama et al. 1993; Del-Valle et al. 2009).

8.13.2 Tailoring of film laminates Plastic films can be manufactured either as a single film or as a combination of more than one plastic. There are two ways of combining plastics: lamination and coextrusion. The tailoring of the film laminates is done when the gas permeability characteristics of any of the selected films do not match the gas permeability requirements of the MAP system satisfactorily. Thus various combinations of different films are taken and film laminates are prepared in order to bring the gas permeability characteristics as close as possible to the gas permeability requirements of the MAP (Burton et al. 1987; Prasad 1995; Jacomino et al. 2001). Based on the gas permeability characteristics of the individual films, two different films are combined to form the laminates. Lamination involves bonding together two or more plastics or bonding plastics to another material such as paper or aluminum. Bonding is commonly achieved by use of water-, solvent-, or solid-based adhesive. After the adhesive has been applied to one film, the two films are passed between rollers to pressure-bond them together. Lamination using lasers rather than adhesives has also been used for thermoplastics (Kirwan and Strawbridge 2003). Lamination enables reverse printing, in which the printing is buried between layers and thus is not subjected to abrasion and can add or enhance heat-sealability. Curwood has introduced laminations of 35±50 m polyester and linear LDPE film, which has been microcut. These microcuts permit better flow of oxygen and carbon dioxide, and thus minimize the probability of respiratory anaerobiosis.

8.13.3 Coextrusion In coextrusion, two or more layers of molten plastics are combined during film manufacture. This process is more rapid but requires materials that have thermal characteristics that allow coextrusion. Because coextrusion and lamination combine multiple materials, recycling is complicated. However, combining materials results in the additive advantage of properties from each individual material and often reduces the total amount of packaging material required.



217

Therefore, coextrusion and lamination can be sources of packaging reduction (Marsh and Bugusu 2007).

8.13.4 Tailored oxygen transmission rate (OTR) The flexible packaging industry has become increasingly responsive to the specific gas requirements of fresh produce and is now providing films specifically designed for given produce items. Films for low, medium and high respiration rate commodities are now available from many package vendors and the process of matching OTR to product is being constantly refined. This has also allowed fresh-cut processors to begin to provide a much greater diversity of products, which now includes artichoke hearts, baby salad greens, sliced strawberries and many others. Very high respiration rate commodities such as litchi, strawberries, broccoli, asparagus and mushrooms have always presented a challenge to packagers. New technologies are now allowing the manufacture of very high OTR (>15,000 cm3/m2day) films for these applications (Zagory 1998).

8.13.5 Metallocene technology Technology developed independently by Dow Chemical Co. and Exxon Chemical Co. uses new single-site catalysts to produce desired polymer resins. These catalysts, when applied to the manufacture of polyethylene and other polymers, can provide a much narrower distribution of polymer chain length, molecular weight and density. This results in flexible plastic films with very high OTR, low moisture vapor transmission rate, enhanced clarity, superior strength, low seal initiation temperature and very rapid bonding of the seal. These stronger films with stronger seals are finding wide application in produce packaging (Zagory and Davis 1997).

8.13.6 Perforated films The rate of gas movement through a perforated film is a sum of gas diffusion through the perforation and gas permeation through the polymeric film. Generally, total gas flow through the perforations is much greater than gas movement through the film. Gas transmission through microperforations has been modeled (Fishman and Ben-Yehoshua 1996). The rate of gas exchange through perforations in a film is so much greater than through continuous films that a 1 mm perforation in a 0.0025 mm (1 mil) thick LDPE film has nearly the same gas flux as a half square meter area of the film. As might be surmised, perforated packages are more suitable for produce having a high O2 demand (Fonseca et al. 2002b).


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8.13.7 Microperforated films The concept of a packaging material with CO2/O2 transmissions compatible with the needs of the contained produce has been advanced. Two basic types of film materials have been proposed, tested and, to some extent, introduced on a commercial scale: microperforated and mineral filled. Alternative approaches to providing a high OTR, especially in applications where there is limited package surface area for gas exchange, have included films with holes or pores. P-Plus microperforated technology owned by Print Pak, and proprietary microperforation technologies owned by Respire Films in America and Sidlaw in England, are finding applications in the rapidly emerging fresh, lightly processed and cut fruit market (Cameron et al. 1993). The P-Plus film is manufactured by perforating a polyolefin film with very tiny orifices using laser beams. The gas permeabilities are designed to balance the respiration rate of the produce being packed. P-Plus films represent a range of base film substrates displaying permeabilities precisely matching the demands of the produce (Ben-Yehoshua et al. 1994). Most cut fruit is packaged in rigid, gas-impermeable trays with a permeable film lidstock sealed to the tray. Because the tray is impermeable to gases, there is reduced surface area for gas exchange. All the gas exchange must occur through the lidstock. Until recently, few films had a high enough OTR to be useful in these applications. Those films that had a high OTR often would not seal to the trays. However, these microperforated films display the properties required for MAP of highly respiring produce. Microporous and microperforated films allow much more rapid gas exchange than would normally be possible through plastic films (Geeson et al. 1994; Artes-Hernandez et al. 2006). Perforation retains many of the good results of sealing in reduction of water loss and alleviation of water stress without the possible deleterious effects of anaerobiosis such as off-flavors, fermentation or CO2 damage. Furthermore, perforation of polyolefin films enables the attainment of some of the advantages of seal packaging (Ben-Yehoshua et al. 1994). Microperforated packaging techniques have also proven effective in retarding deterioration in several other commodities (Geeson et al. 1985; Geeson and Smith 1989). Additionally, perforation enables MAP for highly respiring produce such as litchi, capsicums and mushrooms (Burton et al. 1987; Huang et al. 1990). Perforation may also enable MAP for produce that is sensitive to even small changes in concentrations of O2, CO2 and C2H4 (Ben-Yehoshua et al. 1994).

8.13.8 Microporous films Microporous films, which are engineered to pass low molecular weight gases such as O2, CO2, water vapor, nitrogen etc., specifically for the purpose of adjusting the gaseous concentration within the package, generally fall into two



219

categories: those that are intentionally perforated with very small orifices which pass gases at a very slow rate relative to the total area; and those that contain intentional additives that interfere with the continuity of the plastic materials and thus alter their gas transmission rates (Ben-Yehoshua et al. 1994; Techavises and Hikida 2008). The two most popular microporous film technologies are those of van Leer (Belgium) and FreshHold, owned by Albert Fisher, plc. (USA), with the latter receiving major attention. In this type of material, the plastic polymer is admixed with an inert inorganic mineral such as crushed calcium carbonate or talc. The mineral fill is encapsulated in discrete particulates by the polymer and imparts a variety of properties such as stiffness. Those films exhibiting high gas permeability by virtue of their nature or by reason of being polymeric blends are technically not microporous. Among these are high (10±20%) EVA content polyethylene films such as Shields Bag or Cryovac, or polycyclic terepene film, Phillips K-resin block copolymer styrene film and Dow Chemical's Attane ultra-low-density ethylene octane copolymer films produced in the past by Bunzl in the UK. These films are being suggested as high gas permeability packaging materials for MAP of respiring produce (Abdel-Bary 2003).

8.13.9 Interactive package MAP application may require packaging materials capable of passing controlled quantities of water, oxygen, carbon dioxide and ethylene in order to control the concentrations of these gases in the internal package environment and to avoid anaerobiosis (Martinez-Romero et al. 2009). Thus was born the term, `smart' packaging, or packaging that could somehow sense the changing internal packaging environment and admit O2 from the outer atmosphere, allow excess CO2 to escape, or both. This terminology then translated into active packaging, which encompasses a broad spectrum of materials sensitive to the packaged produce requirements and its surrounding environment. The latter group includes families of package supplements such as in-package sachets of chemicals to absorb O2, CO2 or C2H4 and even to provide O2 and CO2 when the package environment has been depleted of desired gases (Ben-Yehoshua 1985; Ahvenainen 2003).

8.13.10 Tray/lidstock compatibility The high OTR requirements for lidstocks sealed to impermeable trays have often conflicted with poor sealing properties. Advances in coextrusion technology, coupled with single-site catalyst-based plastic resins, have provided better breathing, better sealing films just in time to meet the needs of the fresh-cut fruit industry (Zagory and Hurst 1996).


220


8.13.11 Customizable packaging materials Because each produce item has differing, often unique, packaging requirements, the ability to customize the package to the product has been the aim of produce package development efforts. FreshHold labels can be customized to provide almost any desired OTR, as is true of microperforated films. Some film vendors provide an array of OTRs by varying the thickness of a given film. Thinner films have higher OTRs. Very thin films do not run well on modern automated packaging machinery and so this approach is limited. Landec Corporation, Inc. of California, USA, has developed side-chain polymer technology that allows the film OTR to increase rapidly as temperature increases, thereby avoiding anaerobic conditions subsequent to loss of temperature control. In addition, these polymers can provide very high OTRs, an adjustable CO2/O2 permeability ratio, and a range of moisture vapor transmission rates. These polymers are available as attachable patches that can go on bags or overwraps and represent the first truly customizable packaging system (Zagory and Davis 1997; Zagory 1998).

8.13.12 Antifog properties Potential buyers like to see fresh produce before they buy it. Therefore plastic packages need to be clear and the product visible. Condensation of water inside the package can often occlude the view of the product. Antifog compounds have been developed that, when included in coextruded films, migrate to the inner surface of the film and prevent large water drops from forming. This results in a more attractive package and a better view of the product. However, antifog coatings can interfere with seal integrity and so newer technology relies on register coatings that apply the antifog material only on selected areas of the film away from the seal.

8.14

Package management

There has emerged an increased appreciation that packaging can deliver its promised benefits to fresh produce only within a specific temperature range. In addition, an emphasis on shelf-life extension has shifted to an emphasis on quality preservation. As the marketplace for fresh and cut products becomes more competitive, it is the quality that sells, not shelf-life. This has resulted in increased attention to maintaining low temperatures and rapid distribution. Such changes in perspective have helped realize the benefits that modern packaging films can provide (Kablan et al. 2007).



8.15

221

Design of modified atmosphere packaging (MAP)

The basis of MAP is that a reduced O2 environment suppresses respiration by the commodity, thereby slowing vital processes and prolonging the maintenance of post-harvest quality. A secondary but potentially important factor is a concomitant decrease in respiration in response to elevated CO2. So this modified atmosphere can potentially reduce respiration rate, ethylene biosynthesis and sensitivity to ethylene, decay and physiological changes, namely oxidation (Kader et al. 1989; Saltveit 1993; Mahajan et al. 2007). The objective of MAP design is to define conditions that will create the atmosphere best suited for the extended storage of a given produce while minimizing the time required to achieve this atmosphere. This can be done by matching the film permeation rate for O2 and CO2 with the respiration rate of the packaged produce. As different products vary in their behavior and as MA packages will be exposed to a dynamic environment, each package has to be optimized for specific demands (Chau and Talasila 1994; Jacxsens et al. 2000; Mahajan et al. 2007). MAP is a dynamic system during which respiration and permeation occur simultaneously. Factors affecting both respiration and permeation must be considered when designing a package (Cameron et al. 1989; Mannapperuma et al. 1989; Yam and Lee 1995; Jacxsens et al. 2000). The commodity mass kept inside the package, storage temperature, oxygen, carbon dioxide and ethylene partial pressures, and stage of maturity are known to influence respiration in a package (Kader et al. 1989; Beaudry et al. 1992; Ben-Yehoshua et al. 1994; Das 2005). Type, thickness, unintended holes, and surface area of the packaging film that is exposed to atmosphere and across which permeation of O2 and CO2 takes place, volume of void space present inside the package, as well as temperature, relative humidity, and gradient of oxygen and carbon dioxide partial pressures across the film, are known determinants of permeation (Ashley 1985; Beaudry et al. 1992; Kader 1997; Renault et al. 1994a; Das 2005). In a MAP packaging system, fresh fruits are sealed in perm selective polymeric film packages. Due to respiration of the packaged fruits, O2 starts depleting and CO2 starts accumulating within the package because of the consumption of O2 and the production of CO2 in the respiration process. Consequently, respiration begins to decrease while O2 and CO2 concentration gradients between package and ambient atmosphere begin to develop. The development of concentration gradients induced ingress of O2 and egress of CO2 through the packaging films (Cameron et al. 1989; Merts et al. 1993; Chau and Talasila 1994; Renault et al. 1994a; Mahajan et al. 2007). In a properly designed MAP, after a period of transient state an equilibrium state is established. At equilibrium, the amount of O2 entering the package and that of CO2 permeating out of the package become equal to the amount of O2 consumed and that of CO2 evolved by the packaged fruit, respectively (Jacxsens et al. 2000; Del Nobile et


222


al. 2007; Techavises and Hikida 2008). The package atmosphere is then considered to be in dynamic equilibrium with the external atmosphere. An ideal package system will equilibrate and maintain the levels of O2 and CO2 that are known to be optimal for storage, transport and handling throughout the market chain for a specific commodity (Jacxsens et al. 1999; Van der Steen et al. 2001; Fonseca et al. 2000; Paul and Clarke 2002; Mahajan et al. 2007).

8.15.1 Design methodology MAP design requires the determination of intrinsic properties of the produce, i.e. respiration rate, optimum O2 and CO2 gas concentrations, and film permeability characteristics (Cameron et al. 1989; Talasila and Cameron 1997). The ultimate aim of this design process is to select suitable films for a given product, its area and thickness, filling weight, equilibrium time, and the equilibrium gas composition at isothermal and non-isothermal conditions. The design involves the mathematical modeling of the gaseous exchange in MAP, the respiration rate of the commodity, the permeability of the films and the optimization of package parameters (Exama et al. 1993; Mahajan et al. 2007; Das 2005; Kader et al. 1989).

8.16

Mathematical modeling of gaseous exchange in modified atmosphere packaging (MAP) systems

When fresh fruits and vegetables are sealed in a selected polymeric film package, it constitutes a dynamic system in which respiration of the product and gas permeation through the film take place simultaneously. In the respiration process O2 is consumed and the produce evolves CO2. The simplest concept is that the plastic film serves as the regulator of O2 flow into the package and of flow of CO2 out of the package. For a small length of transient period and at a given temperature, the rates of O2 consumption and CO2 evolution of the packaged commodity depend greatly on the O2 and CO2 concentrations. The differential mass balance equations that describe the O2 and CO2 concentration changes in a package containing respiring product are given in equations 8.24 and 8.25, respectively: Wp Ap PO2 dYO2 a RO2 YO2 ÿ ÿ YO2 8:24 dt Vfp Vfp Wp Ap PCO2 dZCO2 a RCO2 ÿ ZCO2 ÿ ZCO2 ÿ 8:25 dt Vfp Vfp where Ap is the area of the package through which the O2 and CO2 permeate a a and ZCO2 are the O2 and CO2 concentrations in the atmospheric air (m2), YO2 3 3 (cm per cm of air) respectively, YO2 and ZCO2 are the O2 and CO2



223

concentrations inside the package (cm3 per cm3 of air) respectively, PO2 and PCO2 are the O2 and CO2 permeabilities of the packaging material (cm3.m±2.h±1. [conc. diff. of O2 in volume fraction]±1) respectively, Wp is the weight of the fruit kept inside the package (kg), RO2 and RCO2 are the respiration rates for O2 consumption and CO2 evolution by the fruits (cm3.kg±1.h±1) respectively, Vfp is the free volume in the package (cm3), t is the storage time (h) and dYO2/dt and dZCO2/dt are the rates of change of O2 concentration (YO2) and CO2 concentration (ZCO2) within the package at time t of storage (cm3 per cm3 of air.h±1), respectively. Equations 8.24 and 8.25 coupled to the model that describes the dependence of respiration rate on gas composition, temperature (and eventually time) and models that describe the dependence of packaging material on temperature, constitute the basis of MAP design (Exama et al. 1993; Chau and Talasila 1994; Talasila et al. 1994; Talasila and Cameron 1997; Makino and Iwasaki 1997; Maneerat et al. 1997; Jacxsens et al. 2000; Das 2005; Del Nobile et al. 2007; Mahajan et al. 2007; Torrieri et al. 2007). The effect of gas composition on respiration rate is often described by the Michaelis±Menten equation with different types of inhibitions, while the effect of temperature is quantified by an Arrhenius-type equation (Lee et al. 1991; Peppelenbos and Leven 1996; Mangaraj and Goswami 2008a). The permeability of O2 and CO2 in polymeric films is temperature dependent and this dependence is commonly described by an exponential equation (Arrhenius-type equation) (Exama et al. 1993; Mahajan et al. 2007; Mangaraj and Goswami 2008b).

8.17

Current application of polymeric films for modified atmosphere packaging (MAP) of fruits and vegetables

Many plastic films have been used for MAP of varieties of produce (Table 8.3). The packaging of apples using polyethylene, PVC, PET, etc., films was found to be successful in increasing shelf-life and maintaining quality (Veeraju and Karel 1966; Anzueto and Rizvi 1985; Smith et al. 1987b, 1988; Prasad 1995; Kader 1997; Guan Wenqiang et al. 2004). Rocha et al. (2004) packed apple using polypropylene of 100 m during 6.5 months at 4ëC and 85% R.H. and found that apples packed in MA lost less weight, presented better color, and preserved better firmness than fruits stored in air. Prasad (1995) developed MA packages for apple, combining BOPP and PVC films by tailoring of film laminates. The MA packed apples were reported to have retained orchard freshness and increased shelf-life considerably. The incidence of bitter pit that developed during storage of apple was progressively reduced from 50% to less than 5% using LDPE packages (Hewett 1984). Packaging citrus fruits in polyethylene films maintains high R.H. inside the package and hence reduction of shrinkage (Oswin 1975; Barmore et al. 1983;


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Ben-Yehoshua 1985; Smith et al. 1987a). Guava packed with PVC, LDPE or PET and stored for two and three weeks at 5 and 8ëC hindered the development of peel color and the loss of firmness (Gaspar et al. 1997; Mohamed et al. 1994; Pal et al. 2002; Pereira et al. 2004). Sanjuka et al. (2003) observed that guava stored with the silicon membrane had good overall quality after storage and after ripening. Combrink et al. (2004) reported that unperforated polyethylene bags maintained guava fruit quality better than perforated bags. Most workers have used polyethylene and PVC films (Saguy and Mannheim 1975; Nakashi et al. 1991) to extend tomato shelf-life up to 21 days. Banana has greatly benefited from MAP using LDPE films due to reduced C2H4 sensitivity associated with high CO2 and low O2 (Banks 1984; Aradhya et al. 1993; Bhande 2007). MAP has been considered to be beneficial to maintain high humidity, essential for prevention of water loss and browning of litchi pericarp (Scott et al. 1982; Nip 1988; Underhill and Critchley 1992; Kader 1995; Ray 1998; Pesis et al. 2002; Duan et al. 2004; Tian et al. 2005; Sivakumar and Korsten 2006). Litchi treated with 1% HCl, packed with polyethylene films (Jiang Yueming et al. 2004) and with or without SO2 treatment, sealed with polyethylene and PVC films (Paull and Chen 1987; Paull et al. 1998; Chaiprasart 2003), prevented dehydration and pericarp browning. Sivakumar and Korsten (2006) reported that MAP of litchi fruits using BOPP film after post-harvest treatment minimized the rate of transpiration, preventing weight loss and deterioration of fruit quality. Microperforated LDPE (30 m) films provided effective, favorable atmospheres for Bramley apples during a simulated four-week marketing period under ambient conditions and the shelf-life benefit was observed (Geeson 1989). Artes-Hernandez et al. (2006) studied the quality of superior seedless table grapes under MAP using microperforated and oriented polypropylene films and reported that SO2-free MAP kept the overall quality of clusters close to that at harvest. Packaging of strawberries using LDPE, PVC and polypropylene films with or without perforations showed a considerable improvement in quality in terms of fruit firmness, weight loss, desiccation and decay (Pinatauro 1978; Aharoni and Barkai-Golan 1987; El-Ghaouth et al. 1992; Sanz et al. 2000; Van der Steen et al. 2001; Wu Ying et al. 2007; Zheng Yonghua 2008). A number of researchers have worked on MAP of cherries and other similarly perishable fruits including blueberries and raspberries (Beaudry and Lakakul 1992; Crisosto et al. 1993; Cameron et al. 1995; Reed 1995; Moyls et al. 1998; Van der Steen et al. 2001; Petracek et al. 2002) using PVC, polypropylene, LDPE and microperforated films or films that are unperforated but have a selective permeability to oxygen and carbon dioxide. Valero et al. (2006) developed active packaging by adding eugenol or thymol to table grapes stored for 56 days under MA condition. The sensory, nutritional and functional property losses were significantly reduced in packages with added eugenol or thymol. In addition, lower microbial spoilage counts were achieved with the active packaging (Labuza 1990).



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Jacomino et al. (2001) observed that multilayer coextruded polyolefinic film with selective permeability (PSP) can prolong storage of guava up to three weeks, while low-density polyethylene film with incorporated minerals (LDPEm) is suitable for guava storage at 10ëC with 85±90% relative humidity. The PSP film and LDPE film with mineral incorporation provided an atmosphere that kept the fruit with good sensorial characteristics for 28 days and 14 days, respectively. MAP was proven to extend the shelf-life of several vegetables. Both high CO2 and low O2 concentrations retard respiration and senescence of broccoli heads (Serrano et al. 2006). Broccoli maintains its quality longer in both perforated and sealed polyethylene, BOPP and PVC film packages (Aharoni et al. 1987; Rij and Ross 1987; Granado-Lorencio et al. 2008). Christie et al. (1995) successfully developed MA packages for broccoli using LDPE films impregnated with inorganic particles. It was observed that the overall quality of broccoli packaged in LDPE which contained an ethylene absorber was perceived to be the sample most similar to fresh broccoli (Martinez-Romero et al. 2007). A biodegradable laminate of a chitosan±cellulose was found to be suitable as a packaging material for MAP and storage of broccoli (Yoshio and Takashi 1997). Serrano et al. (2006) stored broccoli using macroperforated, microperforated and unperforated polypropylene films and observed that all changes related to loss of quality were significantly reduced and delayed with time, especially with perforated and unperforated films. Packaging of peppers with plastic films extended the shelf-life but the major benefit appeared to be mediated by the maintenance of high relative humidity inside the package which reduced the rates of transpiration (Maxie et al. 1974; Ben-Yehoshua et al. 1983; Miller et al. 1986b; Watada et al. 1987; Irtwange 2006). MAP is an economical and effective way of extending shelf-life of fresh mushrooms. Packaging of mushrooms using PVC wrap and polyolefin films increased shelf-life by retarding cap opening, reducing respiration, reducing internal browning and reducing weight loss, consequently resulting in higher quality (Nichols and Hammond 1975; Kim et al. 2006). In the perforated film packs the quality of mushroom varied according to film permeability and number of perforation holes (Nichols and Hammond 1975). Koide and Shi (2007) compared the storage quality of green peppers using PLA-based biodegradable film packaging with LDPE, and perforated LDPE film packaging. It is suggested that the biodegradable film with higher water vapor permeability can be used to maintain the quality and sanitary conditions of freshly harvested green peppers in modified atmosphere packaging. Almenar et al. (2008) packed highbush blueberries in polylactide (PLA) containers and stored them at 10ëC for 18 days and at 23ëC for nine days. Physicochemical and microbiological studies were carried out in order to know the efficacy of PLA packages. Results showed that the PLA containers prolonged blueberry shelf-life at different storage temperatures. Del Nobile et al.


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(2008) used four different films ± two polyester-based biodegradable films, a multilayer film made by laminating an aluminum foil with a polyethylene film, and an oriented polypropylene film ± to study the ability of biodegradable films to prolong the shelf-life of minimally processed lettuce stored at 4ëC. Results suggest that the gas permeability of the investigated films plays a major role in determining the quality of the packed produce. Moreover, it was observed that biodegradable films guarantee a shelf-life longer than with an oriented polypropylene film package. Jacxsens et al. (2000) designed equilibrium MA packages for fresh-cut vegetables such as bell peppers, broccoli, carrots, chicory, cucumbers, French beans, iceberg lettuce, mixed lettuce and mungbean sprouts subjected to changes in temperature that are similar to those encountered in the distribution chain. Higher respiration rates and temperature dependence of cut/shredded produce was observed compared to unprocessed vegetables. Hence, they developed new unperforated packaging films with high O2 and CO2 permeability. The EMA packages were designed by combining mathematical models that describe the effect of temperature, O2 and CO2 levels on produce respiration. The influence of temperature on respiration was described by an Arrhenius type of equation, while the influence of O2 and CO2 on respiration was modeled by Michaelis± Menten kinetics (Gonzalez-Aguilar et al. 2004; Mahajan et al. 2007; Del Nobile et al. 2007). Fresh-cut produce seemed to be more temperature sensitive than unprocessed fresh vegetables. The proposed packaging systems with the new polymer films of higher permeabilities provided a sufficient low headspace O2 level in a temperature range between 2 and 10ëC. Ultrathin SiO2 films in the range of 2±50 nm thickness were readily fabricated from inexpensive sodium silicate as starting material by its alternate adsorption with cationic polymer and subsequent treatment with O2 plasma and calcination. Film thickness can be controlled by adjusting the number of adsorption cycles and the pH value of silicate solution. The film surface is generally smooth (small roughness) and remains unchanged after O2 plasma treatment or calcination. Whereas a nanoporous thin film is obtained by O2 plasma treatment, a dense silica film is produced through calcination at 450ëC. These preparative methods prove that inexpensive sodium silicates are converted to advanced silica-based materials, such as functional ultrathin films, coatings, capsules, and catalyst, by a simple procedure.

8.18

Future trends

The use of MAP for fresh produce is quite restricted for a number of reasons. No single polymer offers all the properties required for MAP. In addition to barrier properties, properties like machinability/sealability must also be taken into account. One inherent requirement for all MAP packs is the ability to retain the desired atmosphere for as long as possible. This is achieved first by choosing a



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film or films to provide the required gas and moisture vapor permeability characteristics and second by ensuring seal integrity of the packs. To achieve the above film characteristics the different plastic films are either laminated or coextruded. The unavailability of appropriate films that provide a safe modified atmosphere, especially under the abusive temperature conditions that can occur in the handling chain, can be a significant problem. Packages that provide a safe atmosphere at one temperature may result in anaerobic conditions at higher temperature. As of today, the application of MAP is limited to certain produce and/or specific purposes that subsequently provide reduced profit margin. If MAP was applied commercially to a large number of crops, the profit margin would increase substantially. Some problems are associated with maintaining package integrity during storage and transportation. The plastic films used for MAP must be flexible and easy to use, but strong enough to survive normal handling operations. Based on the above points, some of the future research works that can be suggested are as follows. The conditions for MAP of all commodities should be standardized based on appropriate design steps which include matching the permeability requirements of films. This may lead to the commercial production of polymeric films of desirable properties for MAP and their availability in the market for sustainable growth and development. The polymeric films providing recommended gas transmission rates and other characteristics (strong, flexible, transparent, durable and food grade) required for MAP for all commodities should be produced commercially, as either single, coextruded or laminated polymers, for the success and popularization of MAP technology. When selecting polymer films for particular packaging applications, it is important that the film permeabilities be measured under the envisaged storage conditions and using a mixed gas technique to give realistic predictions in the MAP system. New trends of research are needed in the development of interactive or smart films. These new films may somehow sense the changing internal packaging atmosphere and admit O2 from the outer atmosphere or allow excess CO2 to escape. Package modeling can improve understanding of how package, plant and environmental factors interact and can be useful in package design. The creation and maintenance of an optimal atmosphere inside an MA package depends on the respiration rate of the product and the permeability of the films to O2 and CO2, both of which are affected by temperature. However, an increase in temperature has different effects on these two parameters: the increase in the respiration rate as a function of temperature described by QR10 is generally substantially greater than the increase in the permeability of the packaging material. Hence films should be produced to match the QR10 and QP10 to counter the effects of temperature fluctuations encountered during the transport, storage and distribution chain.


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Metallocene technology, ultrathin SiO2 films, O2 and CO2 tailored film, oxygen transmission rate, microperforated films, interactive packaging film, customizable packaging materials, mineral incorporated film, and various additives to preserve the quality of fresh produce are the future needs for the application of MAP. Modified atmosphere packaging is used as a supplement to low-temperature preservation of fruits and vegetables. It is mainly used to reduce respiration rate and retard the ripening process, thereby increasing resistance to diseases for the host. Commercially, MA is successful for storage of apples, pears, fresh cut (minimally processed) fruits and vegetables and for highly perishable and highvalue commodities, such as cherries, figs, raspberries, strawberries, litchi, capsicums, mushrooms, etc. (Gran and Beaudry 1992; Meheriuk et al. 1995). A marginal increase in storage life and quality by MA storage is not enough for the added cost of implementing MA technology commercially for most fruits and vegetables. An important problem in the commercial application of MA in fruits and vegetables is that the effect of MA is different for the same cultivar grown in different locations, under different cultural practices or in different seasons. Therefore, trial-and-error studies have to be conducted to determine the optimum atmosphere for each cultivar in a given place and season. Film packaging offering any desired values of permeability to O2, CO2 and water vapor is available in developed nations, but not in developing countries. Models describing the respiration rates of fresh fruits and vegetables and gas permeability need to be developed. Based on such models, and critical design of MAP systems, it is possible to maintain an optimum atmosphere recommended for safe storage and extended shelf-life of commodities. As fruits and vegetables are more sensitive to environmental conditions, accurate design of the MAP is essential to achieve superior product quality, and the development of models for different fruits and vegetables is a pre-requisite. Research is also needed in integrating active packaging with MAP to make this technology economically viable. Current ethylene-removing techniques (catalytic or chemical oxidation) are not commercially successful. Active packaging involving ethylene-absorbing substances should be studied. MAP and related technology can be selectively used in post-harvest handling of fresh fruits and vegetables with good results. There is a need for commercial application of the technology by processors and fresh produce retailers. There is also a need for research into MAP and related technologies for local crops under local conditions in developing countries.

8.19

References and further reading

Abeles FB, Morgan PW, Saltveit ME (1992) Ethylene in Plant Biology, 2nd edition, Academic Press, New York Abdel-Bary EM (2003) Handbook of Plastic Films, Rapra Technology Ltd, Shawbury, Shrewsbury, UK



229

Adsule RN, Kadam SS (1995) Guava. In DK Salunkhe and SS Kadam (eds), Handbook of Fruit Science and Technology, Marcel Dekker, New York, pp. 419±434 Aharoni N, Philosoph-Hadas S, Barkai-Golan R (1987) Modified atmosphere to delay senescence and decay of broccoli. Proc. 4th National Controlled Atmospheres Research Conf., July 1985, North Carolina, pp. 169±177 Aharoni Y, Barkai-Golan R (1987) Pre-harvest fungicide sprays and polyvinyl wraps to control botrytis rot and prolong post harvest storage life of strawberries. J. Hortic. Sci. 62, 175±180 Ahvenainen R (2003) Novel Food Packaging Technology, CRC Press, Boca Raton, FL, and Woodhead Publishing, Cambridge, UK Almenar E, Samsudin H, Auras R, Harte B, Rubino M (2008) Postharvest shelf life extension of blueberries using a biodegradable package. Food Chem. 110, 120±127 Andres C, Bert EV, Francisco AH, Bart N, Francisco A (2007) Respiration rates of freshcut bell peppers under superatmospheric and low oxygen with or without high carbon dioxide. Postharvest Biol. Technol. 45, 81±88 Andrich G, Zinnai A, Balzini S, Silvestri S, Fiorentini R (1998) Aerobic respiration rate of golden delicious apples as a function of temperature and pO2. Postharvest Biol. Technol. 14, 1±9 Anzueto CR, Rizvi SSH (1985) Individual packaging of apples for shelf life extension. J. Food Sci. 50, 891±899 Aradhya SM, Habbibunnisa B, Prasad A, Vasantha MS, Ramana KVR, Ramachandra BS (1993) Extension of storage life of Rasthale banana under modified atmosphere at low temperature, paper no. FVP-39, presented at the 3rd International Food Convention, 7±12 September, Mysore, India. Artes-Hernandez F, Tomas-Barberan FA, Artes F (2006) Modified atmosphere packaging preserves quality of SO2-free `Superior seedless' table grapes. Postharvest Biol. Technol. 39, 146±154 Ashley RJ (1985) Permeability and plastics packaging. In J Comyn (ed.), Polymer Permeability, Elsevier, New York, pp. 269±308 Ayers JC, Pierce LC (1960) Effects of packaging films and storage temperatures on the ripening of mature green tomatoes. Food Technol. 14, 644±650 Banerjee S, Maier G, Dannenberg C, Spinger J (2004) Gas permeabilities of novel poly (arylene ether)s with terphenyl unit in the main chain. J. Membrane Sci. 229, 63±71 Banks NH (1984) Some effects of TAL Pro-long coating on ripening bananas. J. Exp. Botany 35, 127±137 Banks NH (1985) Responses of banana fruit to prolong coating at different times relative to the initiation of ripening. Sci. Hortic. 26, 146±151 Barkai-Golan R (1990) Post harvest disease suppression by atmospheric modification. In M CalderoÂn and R Barkai-Golan (eds) Food Preservation by Modified Atmospheres, CRC Press, Boca Raton, FL, pp. 237±264 Barmore CR, Purvis AC, Fellers PJ (1983) Polyethylene film packaging of citrus fruit: Containment of decaying fruit. J. Food Sci. 48, 1554±1559 Beaudry R (2000) Responses of horticultural commodities to low O2: Limits to the expanded use of modified atmosphere packaging. Hort. Technol. 10, 491±500 Beaudry R, Lakakul R (1992) Basic principles of modified atmosphere packaging. Tree Fruit Postharvest J. 6(1), 7±13 Beaudry RM (1993) Effect of carbon dioxide partial pressure on blueberry fruit respiration and respiratory quotient. Postharvest Biol. Technol. 3, 249±258 Beaudry RM, Cameron AC, Shirazi A, Dostal-Lange DL (1992) Modified-atmosphere packaging of blueberry fruit: effect of temperature on package O2 and CO2. J. Am.


230


Soc. Hortic. Sci. 117, 436±441 Beit-Halachmy I, Mannheim CH (1992) Is modified atmosphere packaging beneficial for fresh mushrooms? Lebensm. Wiss. u. -Technol. 25, 426±432 Ben-Arie R, Sonego L (1985) Modified atmosphere storage of kiwifruit (Aclinidia chinensis Planch) with ethylene removal. Sci. Hortic. 27, 263±273 Benning CJ (1983) In Plastic Films for Packaging, Technomic Publishing, Lancaster, PA, pp. 181±196 Ben-Yehoshua S (1985) Individual seal-packaging of fruits and vegetables in plastic film ± new postharvest technique. HortScience 20, 32±37 Ben-Yehoshua S, Shapiro S, Even-Chen Z, Lurie S (1983) Mode of action of plastic film in extending life of lemon and bell pepper fruits by alleviation of water stress. Plant Physiol. 73, 87±93 Ben-Yehoshua S, Burg SP, Young R (1985) Resistance of citrus fruit to mass transport of water vapor and other gases. Plant Physiol. 79, 1048±1053 Ben-Yehoshua S, Fishman S, Fang D, Rodov V (1994) New development in modified atmosphere packaging and surface coatings for fruits. ICIAR Proceedings, aphnet.org, http://www.aphnet.org/workshop/postharvest Bhande SD (2007) Modelling of respiration kinetics of banana fruits for controlled atmosphere storage and modified atmosphere storage. Unpublished M. Tech. thesis, Department of Agriculture and Food Engineering, Indian Institute of Technology, Kharagpur, India Bhande SD, Ravindran MR, Goswami TK (2008) Respiration rates of banana fruits under aerobic conditions at different storage temperatures. J. Food Eng. 87(1), 116±123 Biale JB, Young RE (1981) Respiration and ripening in fruits ± retrospect and prospect. In J Friend and MJC Rhodes (eds), Recent Advances in the Biochemistry of Fruits and Vegetables, Academic Press, New York Blanke MM (1991) Respiration of apple and avocado fruits. Postharvest News and Information 2, 429±436 Bohling H, Hansen H (1984) Influence of extremely low O2 storage atmospheres on the respiration behaviors of apples. Acta Hort. 157, 283±294 Brecht JK (1995) Physiology of lightly processed fruits and vegetables. HortScience 30(1), 18±21 Bureau S, Ruiz D, Reich M, Gouble B, Bertrand D, Audergon J-M, Renard C (2009) Rapid and non-destructive analysis of apricot fruit quality using FT-near-infrared spectroscopy. Food Chem. 113, 1323±1328 Burg SP, Burg EA (1965) Gas exchange in fruits. Physiol. Plantarium 18, 870±884 Burton KS, Frost CE, Nichols R (1987) A combination of plastic permeable film system for controlling post-harvest mushroom quality. Biotechnol. Lett. 9, 529±534 Burton WC (1975) Some biophysical principles underlying the controlled atmosphere storage of plant materials. Ann. Appl. Biol. 78, 149±168 Burzo I (1980) Influence of temperature level on respiratory intensity in the main vegetable varieties. Acta Hort. 116, 61±64 CalderoÂn M, Barkai-Golan R (eds) (1990) Food Preservation by Modified Atmospheres, CRC Press, Boca Raton, FL, pp. 237±264 Cameron AC (1989) Modified atmosphere packaging: a novel approach for optimizing package oxygen and carbon dioxide. In 5th International Controlled Atmosphere Conf, Washington, DC, 2, 197±209 Cameron AC, Boylan-Pett W, Lee J (1989) Design of modified atmosphere packaging systems: modeling oxygen concentrations within sealed packages of tomato fruits. J. Food Sci. 54 (6), 1413±1421



231

Cameron AC, Patterson BD, Talasila PC, Joles DW (1993) Modeling the risk in modified-atmosphere packaging: a case for sense-and-respond packaging. In GD Blanpied, JA Bartsch and JR Hicks (eds), Proc. 6th Int. Controlled Atmosphere Research Conf., Vol. I, Ithaca, NY, pp. 95±102 Cameron AC, Beaudry RM, Banks NH, Yelanich MV (1994) Modified atmosphere packaging of blueberry fruit: modeling respiration and package oxygen partial pressures as a function of temperature. J. Am. Soc. Hortic. Sci. 119, 534±539 Cameron AC, Talasila PC, Joles DW (1995) Predicting film permeability needs for modified-atmosphere packaging of lightly processed fruits and vegetables. HortScience 30(1), 25±34 Carlin F, Nguyen-the C, Hilbert G, Chambroy Y (1990) Modified atmosphere packaging of fresh, `ready-to-use' grated carrots in polymeric films. J. Food Sci. 55, 1033± 1038 Chaiprasart P (2003) Effect of modified atmosphere packaging by PE and PVC on quality changes of litchi fruits. ISHS Acta Horticulturae 665: II International Symposium on Lychee, Longan, Rambutan and Other Sapindaceae Plants Chau KV, Talasila PC (1994) Design of modified atmosphere packages for fresh fruits and vegetables. In RP Singh and FAR Oliveira (eds), Minimal Processing of Foods and Process Optimisation, CRC Press, Boca Raton, FL, pp. 407±416 Chen PM, Mellenthin WM, Kelly SB, Facteau TJ (1981) Effects of low oxygen and temperature on quality retention of `Bing' cherries during prolonged storage. J. Am. Soc. Hortic. Sci. 106, 533±535 Chen YZ, Wang YR (1989) Study on peroxidases in litchi pericarp. Acta Botanica Austro Sinica 5, 47±52 Christie GBY, Macdiarmid JI, Schliephake K, Tomkins RB (1995) Determination of film requirements and respiratory behaviour of fresh produce in modified atmosphere packaging. Postharvest Biol. Technol. 6, 41±54 Church IJ, Parsons AL (1995) Modified atmosphere packaging technology: a review. J. Sci. Food Agric. 67, 143±152 Church N (1994) Development in modified-atmosphere packaging and related technologies. Trends Food Sci. Technol. 5, 345±352 Combrink JC, De-Kock SL, Van-Ecden CJ (2004) Effect of postharvest treatment and packaging on the keeping quality of fresh guava fruit. Acta Hort. 275, 539±645 Cowie JMJ (1973) In K Stead (ed.), Polymers: Chemistry and Physics of Modern Materials, Intertext, London, pp. 202±222 Crisosto CH, Garner D, Doyle J, Day KR (1993) Relationship between fruit respiration, bruising susceptibility, and temperature in sweet cherries. HortScience 28(2), 132± 135 Crosby NT (1981) Food Packaging Materials ± Aspect Analysis and Migration of Contaminates, Applied Science Publishers, London Cussler EL (1984) Diffusion: Mass Transfer in Fluid Systems, Cambridge University Press, Cambridge, UK, pp. 105±145 Daniels JA, Krishnamurti R, Rizvi SSH (1985) A review of effects of CO2 on microbial growth and food quality. J. Food Prot. 48, 532±537 Das H (2005) Food Processing Operations Analysis, Asian Books Private Ltd, New Delhi, India, p. 406 Daun H, Gilbert SG, Ashkenazi Y, Henig Y (1973) Storage quality of bananas packaged in selected permeability films. J. Food Sci. 38, 1245±1250 Davies AR (1995) In GW Gould (ed.), Advances in Modified Atmosphere Packaging, New Method of Food Preservation, Blackie, Glasgow, UK, pp. 304±320


232


Day BPE (1996) High oxygen modified atmosphere packaging for fresh prepared produce. Postharvest News and Information 7(3), 31±34 Deily KR, Rizvi SS (1981) Optimization of parameters for packaging of fresh peaches in polymeric films. J. Food Process Eng. 5, 20±26 Del Nobile MA, Licciardello F, Scrocco C, Muratore G, Zappa M (2007) Design of plastic packages for minimally processed fruits. J. Food Eng. 79, 217±224. Del Nobile MA, Conte A, Cannarsi M, Sinigaglia M (2008) Use of biodegradable films for prolonging the shelf life of minimally processed lettuce. J. Food Eng. 85(3), 317±325 Del-Valle V, Hernandez-Munoz P, Catala R, Gavara R (2009) Optimization of an equilibrium modified atmosphere packaging (EMAP) for minimally processed mandarin segments. J. Food Eng. 91, 474±481 Dilley DR (1990) Historical aspects and perspectives of controlled atmosphere storage. In M CalderoÂn and R Barkai-Golan (eds), Food Preservation by Modified Atmospheres, CRC Press, Boca Raton, FL, pp. 187±196 Dixon NM, Kell DB (1989) The inhibition by CO2 of the growth and metabolism of micro-organisms. J. Appl. Bacteriol. 67, 109±136 Duan Xuewu, Jiang Yueming, Su Xinguo, Liu Hai, Li Yuebiao, Zhang Zhaoqi, Zheng Yonghua, Jiang Weibo (2004) Role of pure oxygen treatment in browning of litchi fruit after harvest. Plant Science 167, 665±668 El-Ghaouth A, Arul J, Grenier J, Asselin A (1992) Antifungal activity of chitosan on two postharvest pathogens of strawberry fruits. Phytopathology 82, 398±402 El-Goorani MA, Sommer NF (1981) Effects of modified atmospheres on postharvest pathogen of fruits and vegetables. Hortic. Review 3, 412±468 Emond JP, Chau KV, Brecht JK (1993) Modeling respiration rates of blueberry in a perforation-generated modified atmosphere package. In GD Blanpied, JA Bartsch and JR Hicks (eds), Proc. 6th Int. Controlled Atmosphere Research Conf., Ithaca, NY, pp. 134±144 Exama A, Arul J, Lencki RW, Lee LZ, Toupin C (1993) Suitability of plastic films for modified atmosphere packaging of fruits and vegetables. J. Food Sci. 58(6), 1365± 1370 Fallik E, Chalupowicz D, Aharoni Z, Aharoni N (2002) Modified atmosphere in a water vapour-permeable film maintains snap bean quality after harvest. Folia Hortic. 14, 85±94. Farber JM (1991) Microbiological aspects of modified-atmosphere packaging technology ± a review. J. Food Prot. 54(1), 58±70 Fernandez-Trujillo PJ, Obando-Ulloa JM, MartõÂnez JA, Moreno E, Garcia-Mas J, Monforte AJ (2008) Climacteric and non-climacteric behavior in melon fruit. 2. Linking climacteric pattern and main postharvest disorders and decay in a set of near-isogenic lines. Postharvest Biol. Technol. 50, 125±134 Fidler JC, North CJ (1967) The effect of conditions of storage on the respiration of apples, I. The effects of temperature and concentrations of carbon dioxide and oxygen on the production of carbon dioxide and uptake of oxygen. J. Hortic. Sci. 42, 189±206 Fishman S, Ben-Yehoshua R (1996) Mathematical model for perforation effect of oxygen and water vapor dynamics in modified atmosphere packages. J. Food Sci. 61(5), 956±961 Fonseca SC, Oliveira FAR, Lino IBM, Brecht JK, Chau KV (2000) Modeling O2 and CO2 exchange for development of perforation mediated modified atmosphere packaging. J. Food Eng. 43(1), 9±15



233

Fonseca, SC, Oliveira FA, Brecht JK (2002a) Modelling of respiration rate of fresh fruits and vegetables for modified atmosphere packages: a review. J. Food Eng. 52, 99± 119 Fonseca SC, Oliveira FAR, Frias JM, Brecht JK, Chau KV (2002b) Modelling respiration rate of shredded Galega kale for development of modified atmosphere packaging. J. Food Eng. 54(4), 299±307 Forward DF (1960) Effect of temperature on respiration. Encyclopedia of Plant Physiology, 12(2), 234±258 Gaspar JW, Couto FAA, Salomao LC (1997) Effect of low temperature and plastic films on postharvest life of guava (Psidium guajava L.). Acta Hort. no. 452 Geeson JD (1989) Modified atmosphere packaging of fruits and vegetables. Acta Hort. 258, 143±150 Geeson JD, Smith SM (1989) Retardation of apple ripening during distribution by the use of modified atmospheres. Acta Hort. 258, 245±253 Geeson JD, Browne KM, Maddison K, Sheperd J, Guaraldi F (1985) Modified atmosphere packaging to extend the shelf life of tomatoes. J. Food Technol. 20, 336±341 Geeson JD, Genge PM, Sharpies RO (1994) The application of polymeric film lining systems for modified atmosphere box packaging of English apples. Postharvest Biol. Technol. 4: 35±48 Genigeorgis C (1985) Microbial and safety implication of the use of modified atmosphere to extend the storage life of fresh fish and meat. Int. J. Food Microbiol. 1, 237±251 Giusti AM, Bignetti E, Cannella C (2008) Exploring new frontiers in total food quality definition and assessment: from chemical to neurochemical properties. Food Bioprocess Technol. 1, 130±142 Gong S, Corey KA (1994) Predicting steady state oxygen concentrations in modified atmosphere packages of tomatoes. J. Am. Soc. Hortic. Sci. 119, 546±550 Gonzalez-Aguilar G, Ayala-Zavala JF, Ruiz-Cruz S, Acedo-Felix E, Diaz-Cinco ME (2004) Effect of temperature and modified atmosphere packaging on overall quality of fresh-cut bell peppers. Lebensm. Wiss. u. Technol. 37, 817±826 Gorny J (1997) CA'97 Proceedings, vol. 5: Fresh-cut fruits and vegetables and MAP. Postharvest Horticulture Series no. 19, Davis, University of California Postharvest Outreach Program, Davis, CA, 168 pp. Gorris L, Tauscher B (1999) Quality and safety aspects of novel minimal processing technology. In FAR Oliveira and JC Oliveira (eds), Processing of Foods: Quality Optimization and Process Assessment, CRC Press, Boca Raton, FL, pp. 325±339 Gorris LGM, Peppelenbos HW (1992) Modified atmosphere and vacuum packaging to extend the shelf life of respiring food products. Hort. Technol. 2(3), 303±309 Gran CD, Beaudry RM (1992) Determination of the low oxygen limit for several commercial apple cultivars by respiratory quotient breakpoint. Postharvest Biol. Technol. 3, 259±267. Granado-Lorencio F, Olmedilla-Alonso B, Herrero-Barbudo C, Sanchez-Moreno C, de Ancos B, Martinez JA, Perez-Sacristan B, Blanco-Navarro I (2008) Modifiedatmosphere packaging (MAP) does not affect the bioavailability of tocopherols and carotenoids from broccoli in humans: a cross-over study. Food Chem. 106, 1070± 1076 Guan Wenqiang, Chen Li, Hong-Li Xi, Hu-Yun Feng (2004) Effect of modified atmosphere packaging on the quality of Fuji apple. Trans. Chinese Soc. Agric. Eng. 20(5), 218±221 Hagger PE, Lee DS, Yam KL (1992) Application of an enzyme kinetic based respiration


234


model to closed system experiments for fresh produce. J. Food Process Eng. 15, 143±157 Hardenburg RE (1971) Effect of packaging environment on keeping quality of fruits and vegetables. HortScience 6, 194±202 Henig YS, Gilbert SG (1975) Computer analysis of the variables affecting respiration and quality of produce packaged in polymeric films. J. Food Sci. 40, 1033±1035 Hernandez RJ, Selke SEM, Culture JD (2000) Plastics Packaging: Properties, Processing, Applications, and Regulations, Hanser, Munich, Germany Herner RC (1987) High CO2 effects on plant organs. In J Weichmann (ed.), Postharvest Physiology of Vegetables, Marcel Dekker, New York, pp. 239±253 Herr B (1991) Overwrap films: More than just a dust cover. Mushroom News 39(7), 24±26 Hewett EW (1984) Bitter pit reduction in Cox's Orange pippin apple by controlled and modified atmosphere storage. Sci. Hort. 23, 59±66 Hintlian CB, Hotchkiss JH (1986) The safety of modified atmosphere packaging: a review. Food Technol. 40, 70±76 Hotchkiss JH (1989) Microbiological hazards of control/modified atmosphere food packaging. J. Assoc. Food Drug Offic. 53(3), 41±49 Huang S, Hart H, Lee H, Wicker L (1990) Enzymatic and color changes during postharvest storage of lychee fruit. J. Food Sci. 55, 1762±1763 Hulme AC (1971) The Biochemistry of Foods and their Products, Academic Press, London, pp. 537±554 Iqbal T, Rodrigues FAS, Mahajan PV, Kerry JP (2009) Mathematical modeling of the influence of temperature and gas composition on the respiration rate of shredded carrots. J. Food Eng. 91, 325±332 Irtwange SV (2006) Application of modified atmosphere packaging and related technology in postharvest handling of fresh fruits and vegetables. Agricultural Engineering International: the CIGR Ejournal. Invited overview no. 4, Vol. VIII, February 2006 Isenberg FMR (1979) Controlled atmosphere storage of vegetables. In J Janick (ed.), Horticultural Reviews, AVI Publishing, Westport, CT, Vol. 1, pp. 337±394 Jacomino AP, Kluge RA, Sarantopoulos CIGL, Sigrist JMM (2001) Evaluation of plastic packages for guava refrigerated preservation. Packaging Technol. Sci. 14, 11±19. Jacxsens L, Devlieghhere F, Debevere J (1999) Validation of a systematic approach to design equilibrium modified atmosphere packages for fresh-cut produce. Lebensm. Wiss. u. Technol. 32, 425±432 Jacxsens L, Devlieghere F, Rudder TD, Debevere J (2000) Designing equilibrium modified atmosphere packages for fresh-cut vegetables subjected to changes in temperature. Lebensm. Wiss. u. Technol. 33, 178±187 Jaya L (2005) Technology development for production of vacuum dried coconut milk powder. Unpublished Ph.D. thesis, Department of Agriculture and Food Engineering, Indian Institute of Technology, Kharagpur, India Jayas DS, Jeyamkondan S (2002) Modified atmosphere storage of grain, meat, fruit and vegetables. Biosystem Engineering 82(3), 235±251 Jiang Yueming, Li Yuebiao, Li Jianrong (2004) Browning control, shelf life extension and quality maintenance of frozen litchi fruit by hydrochloric acid. J. Food Eng. 63, 147±151 Joles DW, Cameron AC, Shirazi A, Petracek PD, Beaudry RM (1994) Modified atmosphere packaging of `Heritage' red raspberry fruit: respiratory response to reduced oxygen, enhanced carbon dioxide and temperature. J. Am. Soc. Hortic. Sci. 119, 540±545



235

Jurin V, Karel M (1963) Studies on control of respiration of McIntosh apples by packaging methods. Food Technol. 17(6), 104±108 Kablan T, Mathias KO, Gilles D, Robert WL, Joseph A (2007) Comparative evaluation of the effect of storage temperature fluctuation on modified atmosphere packages of selected fruit and vegetables. Postharvest Biol. Technol. 46, 212±221 Kader AA (1980) Prevention of ripening in fruits by use of controlled atmosphere. Food Technol. 34(3), 51 Kader AA (1986) Biochemical and physiological basis for effects of controlled and modified atmospheres on fruits and vegetables. Food Technol. 40(5), 99±104 Kader AA (1987) Respiration and gas exchange of vegetables. In J Weichmann (ed.), Postharvest Physiology of Vegetables, Marcel Dekker, New York, pp. 25±43 Kader AA (1992) Quality assurance of harvested horticultural perishables, ISHS Acta Horticulturae 553: IV International Conference on Postharvest Science Kader AA (1995) Regulation of fruits physiology by controlled and modified atmosphere. Acta Hort. 398, 59±70 Kader AA (1997) A summary of CA requirements and recommendations for fruits other than apples and pears. In AA Kader (ed.), Proc. 7th Int. Controlled Atmosphere Research Conf., Davis, CA, Vol. 3, pp. 1±34; also in A Kader (ed.), Fruits Other than Apples and Pears. Postharvest Hort. Series No. 17, University of California, Davis, CA, CA'97 Proc. 2, pp. 1±36 Kader AA, Zagory D, Kerbel EL (1989) Modified atmosphere packaging of fruits and vegetables. CRC Crit. Rev. Food Sci. Nutr. 28, 1±30 Karel M, Fennema OW, Lund DB (1975) In OW Fennema (ed.), Principles of Food Science, Marcel Dekker, New York, p. 474 Kawada K, Kitagawa H (1988) Plastic film packaged storage of citrus and some other fruits in Japan. In R Goren and K Mendel (eds), Proc. 6th Int. Citrus Congr. Tel Aviv, Israel, pp. 1555±1565 Kays SJ (1991) Postharvest Physiology of Perishable Plant Products, Van Nostrand, New York, p. 532 Kays SJ (1997) Metabolic processes in harvested products respiration. In SJ Kays (ed.), Post Harvest Physiology of Perishable Plant Products, Van Nostrand Reinhold, New York Ke D, Saltveit ME Jr. (1989) Carbon dioxide-induced brown stain development as related to phenolic metabolism in iceberg lettuce. J. Am. Soc. Hortic. Sci. 114, 789±794 Kim KM, Jung AK, Jin SL, Hyun JP, Hanna MA (2006) Effect of modified atmosphere packaging on the shelf-life of coated, whole and sliced mushrooms. LWT 39, 364± 371 Kirkland BS, Clarke R, Paul DR (2008) A versatile membrane system for bulk storage and shipping of produce in a modified atmosphere. J. Membrane Sci. 324, 119±127 Kirwan MJ, Strawbridge JW (2003) Plastics in food packaging. In R Coles, D McDowells and MJ Kirwan (eds), Food Packaging Technology, Blackwell Publishing, London and CRC Press, Boca Raton, FL, pp. 4±8 Koide S and Shi J (2007) Microbial and quality evaluation of green peppers stored in biodegradable film packaging. Food Control 18, 1121±1125 Konze JR, Jones JF, Boller T, Kende H (1980) Effect of 1-aminocyclopropane-1carboxylic acid on the production of ethylene in senescing flowers of Ipomoea tricolor Cav. Plant Physiol. 66, 566±571 Koros WJ (1989) Barriers Polymers and Structures. ACS Symposium Series, 197th National Meeting of the American Chemical Society, 9±14 April, Dallas, TX, pp. 1±8


236


Kubo Y, Inaba A, Nakamura R (1989) Effects of high CO2 on respiration in various horticultural crops. J. Jap. Soc. Hortic. Sci. 58, 731±736 Kupferman E (1995) Cherry temperature management. Tree Fruit Postharvest J., 6(1), 3±6 Labthink (2008) Labthink Instruments Co. Ltd, China; www.labthink.on/www/ labthink.net Labuza TP (1990) Active food packaging technologies. Food Sci. Technol. Today 4(1), 53±54 Labuza TP, Breene WM (1989) Applications of àctive packaging' for improvement of shelf-life and nutritional quality of fresh and extended shelf-life foods. J. Food Proc. Pres. 13, 1±69 Laffin C, Forristal PD, O'Kiely P (2009) Evolution of CO2 permeation properties of LDPE/LLDPE films upon uni-axial stretching, Packaging Technol. Sci. 22, 9±29 Lakakul R, Beaudry RM, Hernandez RJ (1999) Modelling respiration of apple slices in modified atmosphere packages. J. Food Sci. 64(1), 105±110 Lambden AE, Chadwick D, Gill CO (1985) Oxygen permeability at subzero temperature of plastic film used for vacuum packaging of meat. J. Food Sci. Technol. 20(6), 781±783 Lee DS, Hagger PE, Lee J, Yam KL (1991) Model for fresh produce respiration in modified atmosphere based on principles of enzyme kinetics. J. Food Sci. 56(6), 1580±1585 Lee DS, Song Y, Yam KL (1996) Application of an enzyme kinetics based respiration model to permeable system experiment of fresh produce. J. Food Eng. 27, 297±310 Lioutas TS (1988) Challenges of controlled and modified atmosphere packaging: a food company's perspective. Food Technol. 49, 78±86 Lipton WJ (1975) Control atmosphere for fresh vegetables and fruits ± why and when. In NF Haard and DK Salunke (eds), Postharest Biology and Handling of Fruits and Vegetables, AVI Publishing, Westport, CT, p. 130 Lopez-Galvez G, Saltveit ME, Cantwell M (1996) Wound-induced phenylalanine ammonia lyase activity: factors affecting its induction and correlation with the quality of minimally processed lettuce. Postharv. Biol. Technol. 9, 223±233 Lopez-Galvez G, El-Bassuoni R, Nie X, Cantwell M (1997) Quality of red and green fresh-cut peppers stored in controlled atmospheres. In JR Gorny (ed.), Proc. 7th Int. Controlled Atmosphere Research Conf., Davis, CA, Vol. 5, pp. 152±157 Lu Shengmin (2009) Effects of bactericides and modified atmosphere packaging on shelflife of Chinese shrimp (Fenneropenaeus chinensis). LWT ± Food Sci. Technol. 42, 286±291 Mahajan PV (2001) Studies on Control atmosphere storage for apple and litchi using liquid nitrogen. Unpublished Ph.D. thesis, Department of Agriculture and Food Engineering, Indian Institute of Technology, Kharagpur, India Mahajan PV, Goswami TK (2001) Enzyme kinetics based modeling of respiration rate for apple. J. Agric. Eng. Res. 79(4), 399±406 Mahajan PV, Oliveira FAR, Montanez JC, Frias J (2007) Development of user-friendly software for design of modified atmosphere packaging for fresh and fresh-cut produce. Innovative Food Sci. Emerging Technol. 8, 84±92 Makhlouf J, Wellemot C, Arul J, Castaigne F, Emond JP (1989) Regulation of ethylene biosynthesis in broccoli flower buds in controlled atmospheres. J. Am. Soc. Hortic. Sci. 114(6), 955±958 Makino YK, Iwasaki TH (1997) Application of transition state theory in model development for temperature dependence of fresh produce. J. Agric. Eng. Res. 67, 43±52



237

Manay N, Shadaksharaswamy M (2006) Foods: Facts and Principles, New Age International Publishers, New Delhi, India, p. 540 Maneerat C, Tongta A, Kanlayanarat S, Wongs-Aree C (1997) A transient model to predict O2 and CO2 concentrations in modified atmosphere packaging of bananas at various temperatures. In JR Gorny (ed.), Proc. 7th Int. Controlled Atmosphere Research Conf., Davis, CA, Vol. 5, pp. 191±197 Mangaraj S, Goswami TK (2008a) Respiration rate modelling of royal delicious apple at different temperature. Fresh Produce, 2(2), 72±80, published by Global Science Books, UK Mangaraj S, Goswami TK (2008b) Modelling of respiration rates of litchi fruit under aerobic condition. Food Bioprocess. Technol., in press, doi: 10.1007/s11947-0080145-z Mannapperuma JD, Zagory D, Singh RP, Kader AA (1989) Design of polymeric packages for modified atmosphere storage of fresh produce. In JK Fellman (ed.), Proc. 5th Int. Controlled Atmosphere Research Conf., Wenatchee, WA, Vol. 1, pp. 225±233 Marcellin P (1974) Conservation de leÂgumes en atmospheÁre controlleÂe dans des sacs en polyeÂthyleÁne avec feneÃtre de elastomeÁre de silicone. Acta Hortic. 38, 30±37 Marsh K, Bugusu B (2007) Food packaging ± roles, materials, and environmental issues. J. Food Sci. 72(3), R39±R54 Martinez-Romero D, Serrano M, Guillen F, Castillo S, Valero D (2007) Tools to maintain postharvest fruit and vegetable quality through the inhibition of ethylene action: a review. Crit. Rev. Food Sci. Nutr. 47, 543±560 Martinez-Romero D, Guillen F, Castillo S, Zapata PJ, Serrano M, Valero D (2009) Development of a carbon-heat hybrid ethylene scrubber for fresh horticultural produce storage purposes. Postharvest Biol. Technol. 51, 200±205 Martins RC, Lopes VV, Vicente AA, Teixeira JA (2008) Computational shelf-life dating: complex systems approaches to food quality and safety. Food Bioprocess Technol. 1, 207±222 Massey LK (2003) Permeability Properties of Plastics and Elastomers. A Guide to Packaging and Barrier Materials. Plastic Design Laboratory/William Andrew Publishing, Norwich, NY, p. 601 Maxie EC, Mitchell FG, Sommer NF, Synder RG, Rae HL (1974) Effect of elevated temperature on ripening of Bartlett pear, Pyrus communis L. J. Am. Soc. Hortic. Sci. 99, 344±149 McLaughlin CP, O'Beirne D (1999) Respiration rate of a dry coleslaw mix as affected by storage temperature and respiratory gas concentrations. J. Food Sci. 64(1), 116±119 Meheriuk M, Girard B, Moyls L, Beveridge HJT, McKenzie DL, Harrison J, Weintraub S, Hocking R (1995) Modified atmosphere packaging of `Lapins' sweet cherry. Food Res. Int. 28, 239±244 Menon RR, Goswami TK (2008) Modelling of respiration rates of green mature mango under aerobic conditions. Biosystems Engineering 99(2), 239±248 Merts I, Cleland DJ, Banks NH, Cleland AC (1993) Mathematical model of a modified atmosphere packaging system for horticultural produce. Sci. Technol. Froid. 3, 440±447 Meyer BS, Anderson DB, Bohling RH, Fratianne DG (1973) Introduction to Plant Physiology (2nd edn), Van Nostrand, Princeton, NJ Miller WR, Risee LA, McDonald RE (1986a) Deterioration of individually wrapped and non wrapped bell peppers during long-term storage. Trop. Sci. 26, 1±5 Miller WR, Spalding DH, Hale PW (1986b) Film wrapping mangoes at advancing stages


238


of post harvest ripening. Trop. Sci. 26, 6±11 Mohamed S, Kyi KMM, Yusof S (1994) Effects of various surface treatments on the storage life of guava (Psidium guajava L.) at 10ëC. J. Sci. Food Agric. 66, 9±11 Montero-CalderoÂn M, Rojas-GrauÈ MA, MartõÂn-Belloso O (2008) Effect of packaging conditions on quality and shelf-life of fresh-cut pineapple (Ananas comosus). Postharvest Biol. Technol. 50, 82±89 Moyls AL, McKenzie DL, Hocking RP, Toivonen PMA, Delaquis P, Girard B, Mazza G (1998) Variability in O2, CO2, and H2O transmission rates among commercial polyethylene films for modified atmosphere packaging. Trans. Am. Soc. Agric. Eng. 41(5), 1441±1446 Nakashi S, Schlimme D, Solomos T (1991) Storage potential of tomatoes harvested at the breaker stage using modified atmosphere packaging. J. Food Sci. 56(1), 55±59 Nemphos SP, Salame M, Steingiser S (1976) Barrier polymers. In Encyclopedia of Polymer Science and Technology 1(Suppl.), 65±95 Nichols R, Hammond JBW (1975) The relationship between respiration, atmosphere and quality in intact and perforated mushroom prepacks. J. Food Technol. 10, 424±429 Nip WK (1988) Handling and perservation of lychee (Litchi chinensis Sonn.) with emphasis on color relation. Trop. Sci. 28, 5±11 Opara LU, Al-Ani MR, Al-Shuaibi YS (2008) Physico-chemical properties, vitamin C vontent, and antimicrobial properties of pomegranate fruit (Punica granatum L.). Food Bioprocess. Technol. doi 10.1007/s11947-008-0095-5 Oswin CR (1975) Plastic Films and Packaging. Applied Science Publishers, London Pal RK, Buescher RW (1993) Respiration and ethylene evolution of certain fruits and vegetables in response to carbon dioxide in controlled atmosphere storage. J. Food Sci. Technol. 30, 29±32 Pal RK, Singh SP, Singh CP, Asre R (2002) Response of guava fruits (Psidium guava L. cv. Lucknow-49) to control atmosphere storage. ISHS Acta Horticulturae 735: I, International Guava Symposium Parry RT (1993) In RT Parry (ed.), Principles and Application of Modified Atmosphere Packaging of Food, Blackie, Glasgow, UK, pp. 1±18 Paul DR, Clarke R (2002) Modeling of modified atmosphere packaging based on designs with a membrane and perforations. J. Membrane Sci. 208, 269±283 Paull RE, Chen JN (1987) Effect of storage temperature and wrapping on quality characteristics of litchi fruit. Sci. Hort. 33, 223±236 Paull RE, Reyes MEQ, Reyes MU (1998) Sulfite residues on litchi fruit treated with sulfur dioxide. Postharvest Biol. Technol. 14, 229±233 Peleg K (1985) In Produce Handling, Packaging and Distribution. AVI Publishing, Westport, CT, p. 625 Peppelenbos HW, Leven J (1996) Evaluation of four types of inhibition for modelling the influence of carbon dioxide on oxygen consumption of fruits and vegetables. Postharvest Biol. Technol. 7, 27±40 Pereira LM, Rodrigues ACC, Sarantopoulos CIGL, Junqueira VCA, Cunha RL, Hubinger MD (2004) Influence of modified atmosphere packaging and osmotic dehydration on the quality maintenance of minimally processed guavas. J. Food Sci. 69(4), 1107±1111 Pesis E, Dvir O, Feygenberg O, Arie RB, Ackerman M, Lichter A (2002) Production of acetaldehyde and ethanol during maturation and modified atmosphere storage of litchi fruit. Postharvest Biol. Technol. 26, 157±165 Petracek PD, Joles DW, Shirazi A, Cameron AC (2002) Modified atmosphere packaging of sweet cherry (Prunus avium L., cv. `Sams') fruit: metabolic responses to oxygen,



239

carbon dioxide, and temperature. Postharvest Biol. Technol. 24, 259±270 Phillips CA (1996) Review: Modified atmosphere packaging and its effects on the microbiologcal quality and safety of produce. Int. J. Food Sci. Tech. 31, 463±479 Pinatauro ND (1978) Food Packaging. Noyes Data Corp., Park Ridge, NJ, pp. 379±387 Prasad M (1995) Development of modified atmosphere packaging system with permselective films for storage of red delicious apples. Unpublished Ph.D. thesis, Department of Agriculture and Food Engineering, Indian Institute of Technology, Kharagpur, India Prasad M, Singh RP (1994) Development of modified atmosphere packaging system for fruit storage through gas barrier polymeric films. In IS Bhardwaj (Ed.), Polymer Science ± Recent Advances, Vol. 2, Allied Publishers, New Delhi, India, pp. 971± 987 Prince TA, Herner RC, Lee J (1986) Bulb organ changes and influence of temperature on gaseous levels in a modified atmosphere package of precooled tulip bulbs. J. Am. Soc. Hortic. Sci. 111, 898±904 Quevedo RA, Aguilera JM, Pedreschi F (2008) Color of salmon fillets by computer vision and sensory panel. Food Bioprocess Technol. doi 10.1007/s11947-008-0106-6 Raghavan GSV, Gariepy Y (1985) Structure and instrumentation aspects of storage systems. Acta Hort. 157, 5±30 Ratti C, Raghavan GSV, Gariepy Y (1996) Respiration rate model and modified atmosphere packaging of fresh cauliflower. J. Food Eng. 28, 297±306 Ray PK (1998) Post-harvest handling of litchi fruits in relation to colour retention ± a critical appraisal. J. Food Sci. Technol. 35, 103±116 Rediers H, Claes M, Peeters L, Willems KA (2009) Evaluation of the cold chain of freshcut endive from farmer to plate. Postharvest Biol. Technol. 51, 257±262 Reed AN (1995) Commercial considerations for modified atmospheric packaging of cherries. Tree Fruit Postharvest J. 6(4), 11±14 Renault P, Souty M, Chambroy Y (1994a) Gas exchange in modified atmosphere packaging. 1. A new theoretical approach for micro-perforated packs. Int. J. Food Sci. Technol. 29, 365±378 Renault P, Houal L, Jackuemin G, Chambroy Y (1994b) Gas exchange in modified atmosphere packaging. 2. Experimental results with strawberries. Int. J. Food Sci. Technol. 29(4), 379±394 Rhodes MJC (1980) The physiological basis for the conservation of food crops. Prog. Food Nutr. Sci. 4, 11±20 Richardson DG, Kupferman E (1997) Controlled atmosphere storage of pears. In EJ Mitcham (ed.), Apples and Pears. Postharvest Hort. Series No. 16, University of California, Davis, CA, CA'97 Proc., Vol. 2, pp. 31±35 Rij RE, Ross SR (1987) Quality retention of fresh broccoli packaged in plastic films of defined CO2 transmission rates. Packag. Technol. 22, 14±18 Rocha AMCN, Barreiro MG, Morais AM MB (2004) Modified atmosphere package for apple `Bravo de Esmolfe'. J. Food Control 15(1), 61±64 Roy S, Anantheswaran RC, Beelman RB (1995) Fresh mushroom quality as affected by modified atmosphere packaging. J. Food Sci. 60(2), 334±340 Ryall AL, Lipton WJ (1979) Handling, Transportation and Storage of Fruits and Vegetables, Vol. 1, Vegetables and Melon, 2nd edition, AVI Publishing, Westport, CT, pp. 1±13 Ryall AL, Pentzer WT (1974) Handling, Transportation and Storage of Fruits and Vegetables, Vol. 2, Fruits, AVI Publishing, Westport, CT, pp. 4±12 Sacharow S, Griffin RC (1980) Principles of Food Packaging, 2nd edition, AVI


240


Publishing, Westport, CT Saguy I, Mannheim CH (1975) The effect of selected plastic films and chemical dips on the shelf life of marmande tomatoes. J. Food Technol. 10, 544±549 Salame M (1986) Prediction of gas barrier properties of high polymers. Polym. Eng. Sci. 26(22), 1543±1546 Saltveit ME (1993) A summary of CA and MA requirements and recommendations for the storage of harvested vegetables. In GD Blanpied, JA Bartsch and JR Hicks (eds), Proc. 6th Int. Controlled Atmosphere Research Conf., Ithaca, NY, Vol. II, pp. 800±818 Saltveit ME (1996) Physical and physiological changes in minimally processed fruits and vegetables. In FA Tomas-Barberan (ed.), Psytochemistry of Fruits and Vegetables, Oxford University Press, Oxford, pp. 205±220. Saltveit ME Jr (1997) A summary of CA and MA requirements and recommendations for harvested vegetables. In ME Saltveit (ed.), Proc. 7th Int. Controlled Atmosphere Research Conf., Davis, CA, Vol. 4, pp. 98±117 Saltveit ME (2005) Commercial storage of fruits, vegetables and florist and nursery crops. Postharvest Technology Centre RIC, Department of Plant Science, 104 Mann Laboratories, University of California, USA Salunkhe DK, Kadam SS (1995) Handbook of Fruit Science and Technology, Marcel Dekker, New York Sanjuka PS, Nieuwenhof F, Raghavan GSV (2003) Extension of storage life of guava using silicon membrane system. Written for presentation at the CSAE/SCGR 2003 Meeting, Montreal, QueÂbec, 6±9 July, 2003 Sanz C, Perez AG, Olias R, Olias JM (2000) Modified atmosphere packaging of strawberry fruit: Effect of package perforation on oxygen and carbon dioxide. Food Sci. Technol. Int. 6(1), 33±38 Scott KJ, Brown BI, Chaplin GR, Wilcox ME, Bain JM (1982) The control of rotting and browning of litchi fruit by hot benomyl and plastic film. Sci. Hort. 16, 253±262 Serrano M, Martinez-Romero D, Guillen F, Castillo S, Valero D (2006) Maintenance of broccoli quality and functional properties during cold storage as affected by modified atmosphere packaging. Postharvest Biol. Technol. 39, 61±68 Shirazi A, Cameron AC (1992) Controlling relative humidity in modified atmosphere packages of tomato fruit. HortScience 27, 336±339 Siracusa V, Rocculi P, Romani S, Dalla Rosa M (2008) Biodegradable polymers for food packaging: a review. Trends Food Sci. Technol. 19, 634±643 Siripanich J, Kader AA (1985) Effect of CO2 on total phenolics, phenylalanine ammonia lyase and polyphenol oxidase in lettuce tissue. J. Am. Soc. Hortic. Sci. 110, 249± 253 Sivakumar D, Korsten L (2006) Influence of modified atmosphere packaging and post harvest treatments on quality retention of litchi cv. Mauritius. Postharvest Biol. Technol. 41, 135±142 Sivakumar D, Korsten L, Zeeman K (2007) Postharvest management on quality retention of litchi during storage. Fresh Produce, 1(1), 66±75 Sivakumar D, Arrebola E, Korsten L (2008) Postharvest decay control and quality retention in litchi (cv. McLean's Red) by combined application of modified atmosphere packaging and antimicrobial agents. Crop Protection 27, 1208±1214 Smith S, Geeson J, Stow J (1987a) Production of modified atmosphere in deciduous fruits by the use of films and coatings. HortScience 22, 772±776 Smith SM, Geeson JD, Browne KM, Genge PM, Everson HP (1987b) Modified atmosphere retail packaging of discovery apples. J. Sci. Food Agric. 40, 161±167



241

Smith SM, Geeson JD, Genge PM (1988) Effects of harvest date on the responses of discovery apples to modified atmosphere retail packaging. Int. J. Food Sci. Technol. 23, 78±86 Smock RM (1979) Controlled atmosphere storage of fruits. In J Janick (ed.), Horticultural Reviews, AVI Publishing, Westport, CT, Vol. 1, pp. 301±336 Solomos T, Kanellis A (1989) Low oxygen and fruit ripening. Acta Hort. 258, 151±160 Song Y, Kim HK, Yam KL (1992) Respiration rate of blueberry in modified atmosphere at various temperatures. J. Am. Soc. Hortic. Sci. 117, 925±929 Stewart D, Oparka J, Johnstone C, Iannetta PPM, Davies HV (2003) Effect of modified atmosphere packaging (MAP) on soft fruit quality. Plant Biochem. Phytochem. 119±124 Stewart JK, Uota M (1971) Carbon dioxide injury and market quality of lettuce held in controlled atmosphere. J. Am. Soc. Hortic. Sci. 96, 27±30 Talasila PC (1992) Modeling of heat and mass transfer in a modified atmosphere package. Ph.D. dissertation, University of Florida, Gainesville, FL Talasila PC, Cameron AC (1997) Prediction equations for gases in flexible modifiedatmosphere packages of respiring produce are different than those for rigid packages. J. Food Sci. 62, 923±934 Talasila PC, Chau KV, Brecht JK (1992) Effects of gas concentrations and temperature on O2 consumption of strawberries. Trans. ASAE 35(1), 221±224 Talasila PC, Cameron AC, Joles DW (1994) Frequency distribution of steady-state oxygen partial pressures in modified atmosphere packages of cut broccoli. J. Am. Soc. Hortic. Sci. 119, 556±562 Taylor AA, Karel M, Proctor BE (1960) Measurement of O2 permeability. Mod. Package, 33(1), 131±136 Techavises N, Hikida Y (2008) Development of a mathematical model for simulating gas and water vapor exchanges in modified atmosphere packaging with macroscopic perforations. J. Food Eng. 85, 94±104 Tian SP, Li BQ, Xu Y (2005) Effect of O2 and CO2 concentrations on physiology and quality of litchi fruits in storage. Food Chem. 91, 659±663 Tolle WE (1962) Variables affecting film permeability requirements for modifiedatmosphere storage of apple. USDA Tech. Bull. 1418±1429 Tomas-Barberan FA, Loaiza-Velarde J, Bonfanti A, Saltveit ME (1997) Early woundand ethylene-induced changes in phenylpropanoid metabolism in harvested lettuce. J. Am. Soc. Hortic. Sci. 122(3), 399±404 Torrieri E, Cavella S, Masi P (2007) Modelling the respiration rate of fresh-cut Annurca apples to develop modified atmosphere packaging. Int. J. Food Sci. Technol. doi: 10.1111/j.1365-2621.2007.01615.x Underhill SJR, Critchley C (1992) Anthocyanin decolorisation and its role in lychee pericarp browning. Aust. J. Exp. Agric. 34, 115±122 Valero D, Valverde JM, MartõÂnez-Romero D, Guillen F, Castillo S, Serrano M (2006) The combination of modified atmosphere packaging with eugenol or thymol to maintain quality, safety and functional properties of table grapes. Postharvest Biol. Technol. 41, 317±327 Van der Steen C, Jacxsens L, Devlieghere F, Debevere J (2001) A combination of high oxygen atmosphere and equilibrium modified atmosphere packaging to improve the keeping quality of red fruits. Information Hyperlinked Over Protein (IHOP) Varoquaux P, Mazollier J, Albagnac G (1996) The influence of raw material characteristics on the storage life of fresh-cut butterhead lettuce. Postharvest Biol. Technol. 9, 127±139


242


Veeraju P, Karel M (1966) Controlling atmosphere in a fresh-fruit package. Mod. Package 402, 164±172 Wang CY (1990) Physiological and biochemical effects of controlled atmosphere on fruits and vegetables. In M CalderoÂn M and R Barkai-Golan (eds), Food Preservation by Modified Atmospheres, CRC Press, Boca Raton, FL, pp. 197±224 Watada AE, Kim SD, Kim KS, Harris TC (1987) Quality of green beans, bell peppers and spinach stored in polyethylene bags. J. Food Sci. 526, 1635±1369 Watkins C (2000) Responses of horticultural commodities to high CO2 as related to modified atmosphere packaging. Hort. Technol. 10, 501±506 Watkins CB, Brookfield PL, Elgar HJ, McLeod SP (1998) Development of a modified atmosphere package for export of apple fruit. In S Ben-Yehoshua (ed.), Proc. 1997 Int. Congr. Plastics Agric., Laser Pages Pub. Ltd, Jerusalem, pp. 586±592 Wills RBH, Lee TH, Graham D, McGlasson WB, Hall EG (1981) Postharvest ± An Introduction to the Physiology and Handling of Fruits and Vegetables. AVI Publishing, Westport, CT, p. 163 Wills RBH, McGlasson WB, Graham D, Lee TH, Hall EG (1989) Postharvest: An Introduction to the Physiology and Handling of Fruits and Vegetables. Chapman & Hall, New York Wolfe SK (1980) Use of CO- and CO2-enriched atmospheres for meats, fish and produce. Food Technol. 34(3), 55±63 Wu Ying, Deng Yun, Li Yunfei (2007) Changes in enzyme activities in abscission zone and berry drop of `Kyoho' grapes under high O2 or CO2 atmospheric storage. LWT ± Food Sci. Technol. doi: 10.1016/j.lwt.2007.01.015 Yam KL, Lee DS (1995) Design of modified atmosphere packaging for fresh produce. In ML Rooney (ed.), Active Food Packaging, Blackie Academic and Professional, London, p. 55 Yang CC, Chinnan MS (1988) Modeling the effect of O2 and CO2 on respiration and quality of stored tomatoes. Trans. ASAE 30(3), 920±925 Yang SF (1985) Biosynthesis and mechanism of ethylene action. HortScience 20, 41±45 Yasuda H, Clark HG, Stannett V (1968) Permeability. In Encyclopedia of Polymer Science and Technology, 9: 794±807 Yoshio M, Takashi H (1997) Modified atmosphere packaging of fresh produce with a biodegradable laminate of chitosan-cellulose and polycaprolactone. Postharvest Biol. Technol. 10, 247±254 Young RE, Romani RJ, Biale JB (1962) Carbon dioxide effects on fruit respiration. II. Response of avocados, bananas and lemons. Plant Physiol. 37, 416±422 Zagory D (1998) An update on modified atmosphere packaging of fresh produce. Packaging International, http://www.davisfreshtech.com Zagory D, Davis CA (1997) Advances in modified atmosphere packaging (MAP) of fresh produce. Perishables Handling Newsletter, no. 90, 2±4 Zagory D, Hurst WC (eds) (1996) Food Safety Guidelines For The Fresh-cut Produce Industry, 3rd edition, International Fresh-cut Produce Association, p. 125 Zagory D, Kader AA (1988) Modified atmosphere packaging of fresh produce. Food Technol. 42(9), 70±77 Zheng Yonghua, Yang Zhenfeng, Chen Xuehong (2008) Effect of high oxygen atmospheres on fruit decay and quality in Chinese bayberries, strawberries and blueberries. Food Control 19, 470±474


9

Nylon-MXD6 resins for food packaging A . A M M A L A , CSIRO Materials Science and Engineering, Australia

Abstract: This chapter reviews the properties and uses of Nylon-MXD6 as a high barrier polymer in food packaging applications. Nylon-MXD6 is a generic name given to a range of crystalline polyamides produced from meta-xylenediamine and adipic acid. Nylon-MXD6 has found widespread use in polymer blends and multilayer food packaging applications. Numerous examples of gas barrier performance are given, together with other properties such as aroma-retaining properties, mechanical properties and retortability. The chapter also discusses examples of commercially available systems employing Nylon-MXD6, including the use of oxygen scavenging systems and Nylon-MXD6 nanocomposites. Key words: Nylon-MXD6, poly(m-xylene adipamide), barrier packaging, gas barrier, nylon nanocomposites.

9.1

Structure and general overview

Nylon-MXD6 is a generic name given to a range of crystalline polyamides produced from meta-xylenediamine and adipic acid. Alternative names for Nylon-MXD6 include poly(m-xylene adipamide) and poly(m-xylylene adipamide). Compared to Nylon-6 and Nylon-6,6, Nylon-MXD6 contains an aromatic ring in its structure as shown in Fig. 9.1. The synthesis of Nylon-MXD6 as the meta polymer configuration was first reported in the 1950s.1,2 At the time, the application area of interest was for the textile industry and the production of superior synthetic fibres. The fact that the unsymmetrical meta isomer possessed a crystalline structure was of great interest, since it was previously thought this would result in an amorphous polymer. In addition, Carlston and Lum also reported that the use of aliphatic

9.1 Chemical structure of Nylon-MXD6.


244


acids containing an even number of six to ten carbon atoms resulted in crystalline polymers, while an odd number of carbon atoms gave amorphous polymers. The polymer made from adipic acid resulted in the highest melting temperature of 243ëC. It was not until the 1980s that the Mitsubishi Gas Chemical Company (MGC), based in Japan, commercialized the production of Nylon-MXD6.3 This was made possible by previous work performed by MGC on purifying the xylene isomer and developing a patented process4,5 for preparing Nylon-MXD6 using direct polycondensation of the diamine and dicarboxylic acid at atmospheric pressure without the need for water, as was previously described by Lum and Carlston. The growing use of Nylon-MXD6 in food packaging applications stems largely from its excellent gas barrier properties as well as a number of other favourable characteristics. These properties are described in Sections 9.3 and 9.4. Due to its processability and similar moulding criteria to other polymers used in food packaging, Nylon-MXD6 has found widespread use in blends and multilayer applications. It is easy to combine Nylon-MXD6 with polymers such as polyethylene terephthalate (PET), polypropylene (PP) or polyethylene (PE) for co-extrusion and co-injection moulding to produce laminated films, sheets and bottles. Application examples are described in further detail in Section 9.5. The incorporation of nanoparticles into Nylon-MXD6 has been shown to further enhance the barrier properties while preserving the processing characteristics. Nylon-MXD6 nanocomposites are reviewed in Section 9.6.

9.2

Processing

9.2.1

Drying and handling

As a general class of polymers, nylons are susceptible to moisture absorption, which can potentially lead to hydrolysis during processing and alter the material properties. The aromatic structure and crystallinity of Nylon-MXD6 results in comparatively less moisture absorption than that observed for amorphous polyamides. Nylon-MXD6 has a water absorption value of 0.31% (24 h, ASTM D570) compared to 1.2% for Nylon-6,6 and 1.6% for Nylon-6.6 Nylon-MXD6 is usually packaged by the manufacturer in moisture-proof packaging as pellets with a moisture content of less than 0.1%. If the product is used immediately after opening then no further drying is recommended; however, Nylon-MXD6 will absorb moisture from the atmosphere, so it is important to process the polymer under dry conditions. The manufacturer recommends drying at 120±140ëC for 4±5 hours under reduced pressure (0.5±2.0 mm Hg). If a dehumidifier hopper dryer is used then it should be operated at 80ëC with dried air circulation (dewpoint ÿ20ëC) to prevent moisture absorption prior to processing.



9.2.2

245

Extrusion

In addition to processing Nylon-MXD6 under dry conditions, it is also necessary to minimize residence time and to avoid any prolonged exposure of molten and hot nylon to air in order to prevent discolouration and yellowing. Typically, nylons are extruded under a nitrogen purge to minimize oxidation. A vacuum line attached to a vent in the barrel can also be used to remove volatiles and moisture. Processing temperatures of Nylon-MXD6 are usually in the range 250± 290ëC. Temperatures above this will start to degrade the polymer and should be avoided. The thermal stability of Nylon-MXD6 is discussed in more detail in Section 9.4.1.

9.2.3

Injection moulding

One of the more favourable characteristics of Nylon-MXD6 compared to other nylons is its ease of moulding. Its crystallization behaviour is similar to that of PET and it crystallizes most quickly at temperatures of 150±170ëC. Its favourable crystallization speed over other polymer resins allows thermal forming and drawing over a wide range of conditions. Nylon-MXD6 is available in several grades of varying molecular weight and melt flow index to match the desired application (Table 9.1).7 With melt indices varying from 0.5 to 7.0 g/10 min it is possible to select a grade to produce films, sheets, trays, bottles or cups. Application examples are presented in Section 9.5.

9.2.4

Biaxially drawn films

In the production of biaxially drawn Nylon-MXD6 film the polymer is drawn in both the extruded (longitudinal) and transverse directions. This process orientates the polymer chains and will generally result in improved physical properties compared to undrawn film. One of the properties that can be significantly improved due to orientation of the polymer through drawing or processing is gas barrier performance. LagaroÂn Table 9.1 Grades of Nylon-MXD6 Property Melting point (ëC) Relative viscosity Melt index (g/10min) Average molecular weight

6001

6007

6121

240 5 2.1 7.0 16,000

240 5 2.7 2.0 25,000

240 5 3.5 0.5 39,000

Sources: adapted from http://www.mgc.co.jp/eng/products/nop/nmxd6/grade.html and Mitsubishi Gas Chemical Catalogue ± Polyamide MXD6 (2003).


246


et al.8 have noted that this is usually attributed to (1) orientation-induced crystallization, (2) fractionation and alignment (perpendicular to the permeant transport) of the crystals in the straining direction (increase in tortuosity), and (3) densification (reduction in free volume) of the amorphous phase owing to an increase in conformational order in the non-crystalline chain segments.

9.3

Gas barrier properties

9.3.1

Oxygen transmission rate

One of the most notable properties of Nylon-MXD6 is its excellent oxygen barrier properties. Table 9.2 compares the oxygen transmission rates to those of various other polymer films at different relative humidity. The oxygen barrier properties of Nylon-MXD6 exceed that of many other polymers used in packaging. It is particularly useful that Nylon-MXD6 can retain its excellent gas barrier properties at high humidity. In contrast, ethylene vinylalcohol copolymer (EVOH) films do not perform as well under a high humidity environment. Figure 9.2 shows the oxygen permeability dependence on relative humidity for Nylon-MXD6. This behaviour at both low and high relative humidity makes Nylon-MXD6 a very practical polymer for both dry and wet food packaging applications. Oxygen scavenging systems In the late 1980s the UK packaging company Carnaud MetalBox (CMB) found that the incorporation of small amounts of cobalt produced an oxygen scavenging system that enhanced the oxygen barrier performance of NylonTable 9.2 Oxygen transmission rates of various polymer films Film

Oxygen transmission rate (cm3/m2.day.atm), 20 m, 23ëC 60% RH

Nylon-MXD6 (oriented: 4 4) 2.8 Nylon-MXD6 (non-oriented) 4.3 EVOH (32 mol% ethylene) 0.5 EVOH (44 mol% ethylene) 2.0 PAN copolymer 17 Nylon-6 (oriented) 40 Nylon-6 (oriented, PVDC coated) 10 PET (oriented) 80 PP (oriented) 2500 PP (PVDC coated) 14

80% RH

90% RH

3.5 7.5 4.5 8.5 19 52 10 80 2500 14

5.5 20 50 42.5 22 90 10 80 2500 14

Source: http://www.mgc.co.jp/eng/products/nop/nmxd6/barrier.html



247

9.2 Humidity dependence of oxygen permeability at 23ëC. Source: http:// www.mgc.co.jp/eng/products/nop/nmxd6/barrier.html

MXD6/PET systems.9 The cobalt salt catalyses the reaction of Nylon-MXD6 with oxygen passing through the package wall and prevents oxygen reacting with the food product. Bottles made using this technology were successful in demonstrating the inhibition of oxygen reacting with oxygen-sensitive products like orange juice, beer and wine. In today's market, there are two major producers of Nylon-MXD6/PET bottles that use the oxygen scavenging technology. Constar International market a product under the name OxbarTM for multilayer bottles and in addition they have monolayer systems, MonoxbarTM and the high clarity DiamondClearTM. Amcor market the system called Bind-OxTM, where the oxygen scavenger is incorporated in the middle layer of a three-layer multilayer bottle. In this system, the rate at which oxygen is bound is faster than the rate of oxygen permeation, so the Bind-OxTM layer acts like a sponge, capturing oxygen which has virtually no chance of reaching the product. Although oxygen scavenging systems are highly effective, they do have a limited lifetime and will only stay active until there is sufficient catalyst present to sustain the oxidation reaction. The barrier properties are usually a function of scavenging capacity and rate of consumption.10 The composition (MXD6 and cobalt content) as well as the container wall thickness are factors that can affect the scavenging capacity. Similarly, the barrier performance is dependent on


248


package surface area/volume ratio.11 Large surface/volume ratios limit costeffective performance and for this reason oxygen scavengers have primarily been used for bottled products. An example of the oxygen barrier improvement observed with the use of OxbarTM has been reported by Brody et al. (2001).10 A 1-litre PET-only bottle showed an oxygen transmission rate of 3.5 cm3/m2/day (22ëC), while the equivalent OxbarTM bottle (PET/MXD6/Co) had a transmission value of less than 0.04 cm3/m2/day. This low level persisted for a period approaching two years.

9.3.2

Carbon dioxide transmission rate

Nylon-MXD6 exhibits excellent carbon dioxide (CO2) gas barrier properties. One of the main application areas of Nylon-MXD6 is for carbonated beverage containers. For carbonated soft drinks and beer, one of the primary concerns is the rate at which CO2 escapes from the bottle. If the rate of diffusion is too fast then the product will go flat on the shelf before it is consumed, so it is critical to have a high CO2 retention. The technology involving Nylon-MXD6 for improving CO2 retention is centred around the use of multilayer bottles and polymer blends. Figure 9.3a shows the change in retention of CO2 in a Nylon-MXD6/PET multilayer bottle. It can be seen that there is significant improvement in CO2 retention over a longer time with the use of MXD6. Similarly Fig. 9.3b shows how increasing MXD6 content in a MXD6/PET blend bottle can extend the shelf-life of a carbonated drink.

9.3.3

Aroma-retaining and odour-blocking properties

Aroma compound retention and the blocking of foreign odours is an important aspect of food packaging. Aroma compounds can be lost by chemical reactions within the food product or by sorption or permeation through the packaging. The loss of aroma compounds can be minimized through the use of a high barrier packaging material. Table 9.3 shows the aroma-retaining and odour-blocking properties of Nylon-MXD6 in comparison to various other gas barrier films. It can be seen that Nylon-MXD6 films perform very well at retaining aroma and flavour. The heat treatment of beverages such as extended shelf-life milk which undergoes ultra-high temperature (UHT) processing can cause aldehyde and ketone off-flavours. Suloff et al.12 have reported on the use of PET blends with Nylon-MXD6, D-sorbitol and -cyclodextrin as selective scalping agents which can be used to improve the flavour profile and remove these carbonyl compounds that create the stale flavours during storage. In the case of Nylon-MXD6, the free amino groups react with the carbonyls to form imines.



249

9.3 (a) Carbonation retention of PET/Nylon-MXD6 multilayer bottle (500 cm3 bottle, average wall thickness 350 m, storage conditions 20ëC, inside 100% RH, outside 65% RH). Source: http://www.mgc.co.jp/eng/products/nop/ nmxd6/bottle.html; (b) Nylon-MXD6 content in Nylon-MXD6/PET blended bottles and the relationship between the carbon dioxide transmission coefficient and the shelf-life of a carbonated drink. Source: http:// www.mgc.co.jp/eng/products/nop/nmxd6/bottle.html


250


Table 9.3 Aroma-retaining and odour-blocking properties of various barrier films Film (thickness 15 m)

Evaluated food

Nylon-MXD6 (oriented) Nylon-MXD6/Nylon-6 blend (oriented) Nylon-6 (oriented) PVDC-coated drawn Nylon-6 PET (oriented) PE

Soy sauce

Vinegar

Worcester sauce

l (one month) l (one month)

n n

l (three months) n

n l (one month) l (one month) u

u n s u

n n s u

Film (thickness 15 m)

Evaluated flavour

Nylon-MXD6 (oriented) Nylon-MXD6/Nylon-6 blend (oriented) Nylon-6 (oriented) EVOH PVDC-coated rolled Nylon-6 PET (oriented) PP (oriented) PE

D-limonene

Vanilla essence

L-menthol

l n

l s

l l

n n l n u u

u s l l u u

l l l l n u

Conditions: 23ëC, 50% RH, shaded from light; evaluation method: sensory analysis. Key: l no less than 2 weeks; n 1to 2 weeks; s 3 days to 1week; u within 3 days. Source: http://www.mgc.co.jp/eng/products/nop/nmxd6/blendfilm.html

9.4

Other properties

9.4.1

Thermal properties

Nylon-MXD6 is more thermally stable than other commonly used barrier packaging plastics. Table 9.4 summarizes some of the thermal properties. The glass transition temperature and heat distortion temperature are significantly higher than those of Nylon-6 and Nylon-6,6. Because of its excellent thermal stability, Nylon-MXD6 also exhibits superior recyclability. It is easy to recycle since it does not show any gel formation or decomposition.

9.4.2

Retortability

Retort packaging involves the use of steam or boiling water to cook food in its own package and extend shelf-life. Because plastic packaging is less bulky than traditional cans and glass jars, foods cook faster, providing a better-tasting product for the consumer. Nylon-MXD6 has excellent retortability due to its



251

Table 9.4 Thermal properties of Nylon-MXD6 (injection-moulded specimens) Property Melting point (ëC) Heat distortion temperature (ëC) Glass transition temperature (ëC) Coefficient of thermal expansion (cm/cmëC)

NylonMXD6

Nylon-6,6

Nylon-6

PET

237 96 85 5 10ÿ5

260 75 50 10 10ÿ5

220 65 48 8 10ÿ5

255 85 77 7 10ÿ5

Source: adapted from http://www.mgc.co.jp/eng/products/nop/nmxd6/nature.html

resistance to boiling treatment and quick performance recovery. Its barrier and strength are also features that make Nylon-MXD6 an excellent candidate for retort packaging. An example of the retort properties of Nylon-MXD6 is illustrated in Table 9.5 which shows the change in oxygen transmission rate of a laminated multilayer PP/Nylon-MXD6/PP container. In comparison to the same laminated container made with EVOH instead of Nylon-MXD6, it is clear that the use of Nylon-MXD6 gives superior results. This is further illustrated in Fig. 9.4 showing the cumulative oxygen transmission rate of the containers. The patent JP7276582 (assigned to Sumitomo Bakelite Co. Ltd) refers to a multilayer film for retort pasteurization applications.13 The reported advantages of using Nylon-MXD6 include improved heat resistance, improved mechanical strength and no curling as well as enhanced gas barrier performance.

9.4.3

Mechanical properties

Compared to other polyamides, Nylon-MXD6 exhibits greater strength and stiffness. Some of the physical properties are compared in Table 9.6. Blended Table 9.5 Change in oxygen transmission rate of laminated containers after retorting Structure of laminated container

Thickness (m)

Oxygen transmission rate (cm3/m2.day.atm), 23ëC Initial

PP/Nylon-MXD6/PP PP/EVOH/PP (ethylene 32 mol%)

140/40/180 140/40/180

0.6 0.25

Time after retorting (days) 1

7

14

30

12.0 22.0

0.9 16.0

0.64 8.0

0.62 3.5

Retorting conditions: 121ëC, 30 min. Measuring conditions: 23ëC, 100% RH (inside), 50% RH (outside). Source: http://www.mgc.co.jp/eng/products/nop/nmxd6/container.html


252


9.4 Cumulative oxygen transmission coefficient of Nylon-MXD6/polypropylene laminated containers after retorting. EVOH: ethylene 32 mol%. Retorting conditions: 121ëC, 30 min. Container specifications: volume 350 cm3, surface area 310 cm2, (inside)PP / Nylon-MXD6 / PP(outside) 140/ 10/40/10/180 m (average). Measuring conditions: 23ëC, 100% RH (inside), 50% RH (outside). Source: http://www.mgc.co.jp/eng/products/nop/ nmxd6/container.html

Table 9.6 Physical properties of Nylon-MXD6 (injection-moulded specimens) Property

Tensile strength

ASTM test method (units)

NylonMXD6

D638 1010 (kg/cm2) Tensile elongation D638 2.3 (%) Tensile modulus D638 48 103 (kg/cm2) Flexural strength D790 1600 (kg/cm2) Flexural modulus D790 45 103 (kg/cm2) Izod impact (notched) D256 2 (kg-cm/cm) Rockwell hardness D785 108 (M scale)

Nylon-6,6

Nylon-6

PET

780

630

800

60

200

5.8

32 103

26 103

31 103

1300

1250

1250

30 103

24 103

35 103

4

6

4

89

85

106

Source: adapted from http://www.mgc.co.jp/eng/products/nop/nmxd6/nature.html



253

and oriented films of Nylon-MXD6 also exhibit good pin-hole resistance and easy tearing properties that further enhance the use of polymers in easy to open retort pouches.14

9.5

Applications

As illustrated in the examples in previous sections of this chapter, Nylon-MXD6 alone exhibits many useful properties for food packaging; however, its presence in a blend or multilayer structure can satisfy multiple performance criteria for bottle, film and sheet applications. This section will focus on further examples where the use of Nylon-MXD6 together with other polymers can achieve optimal properties for food packaging.

9.5.1

Polymer blends

The blending of polymers can result in improved physical properties and this is often done at a lower cost than developing a new polymer. In food packaging applications Nylon-MXD6 can be blended with other polymers such as Nylon-6, PET or PP to achieve higher gas barrier properties and greater thermal resistance. Recently, blends of Nylon-MXD6 with Nylon-6 and Nylon-6,6 have been reported as shrinkage films for food packaging.15 The Nylon-MXD6 blends achieved effective gas barrier properties and good controlled shrinkage, offering a halogen-free alternative to polyvinylidene chloride (PVDC) films, which have raised some environmental concerns. In addition, the use of Nylon-MXD6 blends is reported to give superior results compared with that of PET blends which can often have problems with rippling and appearance after boiling sterilization.15 As mentioned previously in an example of a blended product, Fig. 9.5 shows the relationship between carbon dioxide transmission rate and the percentage of Nylon-MXD6 in a Nylon-MXD6/PET blended bottle, as plotted against shelflife. It can be seen that as the percentage of Nylon-MXD6 in the blend increases then the carbon dioxide transmission decreases, resulting in a longer product shelf-life of a carbonated beverage. The compatibilization of Nylon-MXD6 blended with other polymers has been the subject of several studies. For instance, in PET/Nylon-MXD6 blends haze was found to increase as the level of Nylon-MXD6 increased.16 Stretching was also found to increase haze in the immiscible blends. Refractive index mismatch and particle size effects were identified as the contributing factors. In order to overcome the incompatibility, the use of a modified PET containing isophthalate to partially replace terephthalate has been reported to improve transparency after biaxial stretching while maintaining good barrier properties.17


254


9.5 Image of a multilayer bottle preform showing cross-section of five layers (PET/Nylon-MXD6/PET/Nylon-MXD6/PET).

9.5.2

Multilayer products

Multilayer food packaging structures have been used for many decades to deliver multiple performance criteria in a single structure. Nylon-MXD6 can be incorporated into multilayer packaging with other polymers to optimize gas barrier properties and to improve retortability. An example is the patented process, known as SurshotTM, developed by Owens Illinois for co-injection moulding of a five-layer plastic bottle.18 The technology uses outer, middle and inner layers of PET. Sandwiched between them are two layers of proprietary SurshieldTM barrier which consists of Nylon-MXD6 and an oxygen scavenger. It is reported to improve CO2 gas barrier by 40%.18 The technology is now owned by Graham Packaging Company19 and is suitable for a range of products from beer to pasta sauces. A similar example of a five-layer bottle preform containing Nylon-MXD6 is shown in Fig. 9.5.20 Similarly, the OxbarTM technology described earlier uses a three-layer construction of PET and Nylon-MXD6 with oxygen scavenger to achieve improved barrier properties as presented in Fig. 9.6.



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9.6 Multilayer bottle showing three layers (PET/Nylon-MXD6/PET) with OxbarTM oxygen scavenging additive incorporated into centre layer. Source: adapted from http://www.constar.net/tech-barrier-oxbar.php

Apart from multilayer bottle applications, Nylon-MXD6 has also found applications in multilayer films and sheets suitable for food packaging. The patent WO 2001 092011 (assigned to Cryovac Inc.) describes the use of Nylon-MXD6 for co-extruded multilayer films that are able to maintain dimensional stability at high temperatures as well as having low oxygen transmission rates and good interlaminar bond strength.21 These multilayer films are suitable for both `hot fill' packaging products such as soups, sauces, beverages and other liquified foods as well as products that are not `hot filled' but still require heat sealing such as lidding films for lunch meats or dairy products, where, without dimensional stability, distortion of the package would otherwise occur. The patent EP 1529635 (US 2005 0100729)22 also describes the use of Nylon-MXD6 in a multilayer film. In this application Nylon-MXD6 is used in a multilayer film as a cover film for oven-ready meal trays. The heat-sealable product exhibits good oxygen barrier properties, interlayer adhesion and mechanical properties as well as easy-peel properties. Nylon-MXD6 has also found applications in multilayer laminated paper packaging materials. The patent WO 2001 19611 (assigned to Nippon Tetra Pak and Tetra Laval)23 describes the use of Nylon-MXD6 in such packaging for food and beverages including milk, soup, fruit juice, tea, wine and seasoning sauce. The package exhibits strength, barrier properties and taste retention.

9.6

Nylon-MXD6 nanocomposites

While Nylon-MXD6 alone is a proven gas barrier polymer, the addition of nanoclays can form nanocomposites with significantly enhanced barrier properties. The combination of Nylon-MXD6 and nanotechnology is ideal for food


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Table 9.7 Properties of ImpermTM (non-oriented, grade 105) Property OTR, 23ëC, 60% RH (cm3.mm/m2.day.atm) CO2TR, 23ëC, 60% RH (cm3.mm/m2.day.atm) WVTR, 40ëC, 90%RH (g.mm/m2.day) Haze (%) Tensile strength (MPa) Tensile modulus (GPa) Tensile elongation (%)

ImpermTM 105

Nylon-MXD6

0.02 0.11 0.43 1.5 89 4.4 2.6

0.09 0.30 1.36 1.4 85 3.1 3.3

Source: adapted from http://www.nanocor.com/tech_sheets/I105.pdf

packaging as shelf-life can be enhanced while transparency remains high and processing characteristics remain similar to those of Nylon-MXD6 itself. Currently, a commercially available nanocomposite Nylon-MXD6 resin is manufactured by Mitsubishi Gas Chemical Co. and Nanocor Inc. with the trademark ImpermTM. The uniform dispersion of clay platelets in ImpermTM is believed to enhance barrier by creating a `tortuous path' for gas molecule permeation.24 There are grades of ImpermTM suitable for multilayer bottles and for film and sheet applications. Table 9.7 shows some properties of non-oriented films of ImpermTM Grade 105. Oxygen barrier is improved by a factor of 4.5 times compared to the base resin. Similarly, the carbon dioxide barrier and water vapour barrier improve by a factor of 3. Figure 9.7 also shows the shelf-life improvement that can be achieved for carbonated beverages with the use of ImpermTM grade 103 (previous grade name M9). While a 5 wt% barrier layer of Nylon-MXD6 can extend

9.7 Carbon dioxide retention comparison of PET and multilayer PET bottles containing Nylon-MXD6 and nanocomposite Nylon-MXD6 (500 ml bottle, 28 g weight, 390 m thickness, 5 wt% barrier layer, test conditions 23ëC, inside 100% RH, outside 50% RH). Source: http://www.nanocor.com/tech_papers/ NOVAPACK03.pdf



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9.8 TEM micrographs of (a) MXD6±kaolinite nanocomposite, and (b) MXD6± montmorillonite nanocomposite.

the shelf-life of a monolayer PET bottle from 7 weeks to 14 weeks, ImpermTM can further extend the container shelf life to 21 weeks (using 90% CO2 retention as the cut-off). While the tortuous path theory is the most widely accepted model for gas barrier improvement of nanocomposites, other variables have also been shown to contribute to improvements in gas barrier. An example is the increase in crystallinity that results from nanoparticles acting as nucleating agents.25 Kaolinite and montmorillonite clays were dispersed in Nylon-MXD6 and then blow-moulded to form multilayer bottles. Both clays did not cause any haze in the final products, and despite the fact that kaolinite was only partially exfoliated compared to the montmorillonite clay (Fig. 9.8), both samples showed improvement in CO2 gas barrier over the neat Nylon-MXD6 resin alone. The nanoclay additives were shown to increase the crystallinity of the Nylon-MXD6, and because the crystalline regions are more impermeable to the transport of gases compared to amorphous regions, this can explain the improvement in gas barrier. Recently, it was also demonstrated that the choice of organic modifier used on montmorillonite clays can have a significant effect on the gas barrier properties of resulting Nylon-MXD6 nanocomposites.26 Positron annihilation lifetime spectroscopy (PALS) was used to study the free-volume changes occurring when different surface-modified montmorillonite clays were incorporated into Nylon-MXD6. The molecular transport of gases through a polymer depends on the amount of free volume present due to chain packing and chain segment rearrangement. The interactions between modified clay nanoparticles


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and polymer that result in lower free volumes are favourable and this information can be used to tailor lower gas permeability packaging materials. In the study described above, Cloisite 10A, a montmorillonite clay modified with a quaternary ammonium salt containing an aromatic functional group, was found to reduce free volume compared to other modified clays and neat NylonMXD6 resin. It also gave a 66% reduction in oxygen transmission rate over neat Nylon-MXD6. These results suggest that there are favourable interactions between the aromatic groups on the modified clay and the aromatic groups on the nylon-MXD6 chain. In addition to improving gas barrier properties, the use of nanoclays in Nylon-MXD6 can also improve mechanical properties of the polymer. Recently, it was reported that co-extruded multilayer films containing a montmorillonite clay in Nylon-MXD6 significantly enhanced oxygen barrier performance as well as decreasing film elongation while improving tear resistance of the films.27 The films are currently being investigated as potential replacements for foil-based packaging in the military food supply chain. Nanocor's ImpermTM products24 are also reported to enhance mechanical reinforcement, with the nanoclay additives acting to restrict the Brownian motion of the Nylon-MXD6 chains.24 In summary, the use of Nylon-MXD6 clay nanocomposites has already created a significant impact on food packaging technology and will most likely continue to expand commercially in the decades to come.

9.7

Future trends

Since its introduction into commercial markets, the demand for Nylon-MXD6 as a packaging material has expanded steadily. With increasing safetyconsciousness about foods, it is expected that the need for effective gas barrier packaging that enables long-term storage without damaging the freshness of foods will become higher in the future. Convenience continues to be a major driver in food packaging with consumers wanting products that are easy to handle and quick to prepare. The retortability of Nylon-MXD6 coupled with its high gas barrier properties makes it a polymer of choice for convenience food packaging. The further commercial development of nanotechnology with Nylon-MXD6 will be strongly dependent on the public perception of nanotechnology in food contact materials. While existing Nylon-MXD6 nanocomposite products like ImpermTM are fully approved for use as internal barrier layers in multilayer structures, they are still only approved for non-direct food contact applications. While clay nanoparticles are the most commonly used commercial application of nanoparticles in food packaging, the use of other innovative nanoparticle technologies with Nylon-MXD6 has yet to be fully explored. Possibilities could include nanoparticles for a wide range of applications, including antimicrobial functionality and UV protection which when combined



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with the previously discussed properties of Nylon-MXD6 have the potential to greatly influence food packaging markets in the future.

9.8

References

1. F. G. Lum and E. F. Carlston, Synthetic fiber-forming polymers from meta-xylylene diamine and adipic acid, US Patent 2,766,221 (1956). 2. E. F. Carlston and F. G. Lum, Ind Eng Chem 49: 1239±1240 (1957). 3. Mitsubishi Gas Chemical Co., Plant and Production History. Available from: http:// www.mgc.co.jp/eng/about/history/plant.html [accessed 25 February 2010]. 4. A. Miyamoto, S. Shimizu, M. Harada, T. Ajiro and H. Hara, Preparation of polyamide from meta-xylylene diamine and adipic acid by adding diamine to acid in two stages at atmospheric pressure, European Patent EP 84661-A1 (1982). 5. A. Miyamoto, S. Shimizu, K. Yamamiya and M. Harada, Polyamide preparation from m-xylylene di:amine and adipic acid by maintaining polycondensing mixture in uniformly fluidized state, US Patent 4,433,136-A (1984). 6. R. J. Palmer, `Polyamides, Plastics', in Kirk Othmer, Encylopedia of Chemical Technology (2005) John Wiley & Sons. Available from http://www.mrw. interscience.wiley.com/emrw/9780471238966/kirk/article/plaspalm.a01/current/html [accessed 25 February 2010]. 7. Mitsubishi Gas Chemical Co., Various Grades and Uses. Available from http:// www.mgc.co.jp/eng/products/nop/nmxd6/grade.html [accessed 25 February 2010]. 8. J. M. LagaroÂn, R. CatalaÂ and R. Gavara, Mater Sci Technol 20: 1±7 (2004). 9. A. Cochran, R. Folland, J. W. Nicholas, M. E. R. Robinson, M. A. Cochran and M. E. Riddell, Packaging material with oxygen-scavenging properties containing a polymer, metal oxidation catalyst and oxidisable polymer, European Patent EP 301719-A1 (1988). 10. A. L. Brody, E. R. Strupinksy, L. R. Kline, Active Packaging for Food Applications (2001), CRC Press. Available from http://books.google.com.au/books?id= 6z1QbQTmcukC&pg=PA56&lpg=PA56&dq=cobalt+mxd6&source=bl&ots=nGnEGrNG3b&sig=NTJo-GljE7LzKdkQzh7gSVEJA-0&hl=en&ei= XMN3SqzuA9eBkQXyo5mzBg&sa=X&oi=book_result&ct=result&resnum=1#v=onepage&q=cobalt%20mxd6&f=false2001] [accessed 25 February 2010]. 11. P. Maul, `Barrier enhancement using additives', Pira International Conference, Brussels, Belgium, 5±6 December 2005. Available from http://www.nanocor.com/ tech_papers/BARRIER%20ENHANCEMENT%20USING%20 ADDITIVES%20110605.pdf [accessed 25 February 2010]. 12. E. C. Suloff, J. E. Marcy, B. A. Blakistone, S. E. Duncan, T. E. Long and S. F. O'Keefe, J Food Sci 68: 2028±2033 (2003). 13. K. Susumu and O. Yoshiyuki, Multilayer film for packaging food to be retortpasteurised ± has outer and intermediate layers of a polycondensation product prepared from metaxylenediamine adipic acid and an aliphatic acid nylon, Japanese Patent JP 7276582-A (1995). 14. Mitsubishi Gas Chemical Co. brochure, Nylon MXD6 superior performance in barrier packaging. Available from http://www.idspackaging.com/Common/ exhib_349/Mxd6bro.1.pdf [accessed 25 February 2010]. 15. M. Takashige and T. Kanai, J Polym Eng 28: 179±201 (2008). 16. Y. Maruhashi and S. Iida, Polym Eng Sci 41: 1987±1995 (2001). 17. Y. S. Hu, V. Prattipati, A. Hiltner, E. Baer and S. Mehta, Polymer 46: 5202±5210


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(2005). 18. Handbook of Food Science, Technology and Engineering. Available from http:// books.google.com.au/books?id=Wt9QVCCLKOQC&pg=PT429&lpg=PT429&dq= mxd6+future+packaging&source=bl&ots=fLYSktweo3&sig=r-b2lb9 WIsod9A61_bgd6rBmJdM&hl=en&ei=uYZRSsqGMoamNoDwmFA&sa=X&oi= book_result&ct=result&resnum=3#v=onepage&q=mxd6%20future%20packaging& f=false2006 [accesssed 25 February 2010]. 19. Graham Packaging Company, New Technology. Available from http:// www.grahampackaging.com/technology/new-technology.asp [accessed 25 February 2010]. 20. A. Ammala, A. J. Hill, K. Lawrence and T. Tran, Nanocomposite Barrier Improvement for Plastics Packaging. Confidential Technical Report 169. CSIRO CMIT Australia (2004). 21. F. M. Hofmeister, P. J. Satterwhite, T. D. Kennedy, M. Hofmeister, J. Satterwhite and D. Kennedy, Multilayer film for packaging applications, has first layer having amorphous polyamide, second layer, and third layer having ethylene/vinyl alcohol copolymer, polyamide, polyvinylidene chloride and/or polyacrylonitrile, PCT International Patent WO 2001 92011-A (2001). 22. H. Pfeiffer, B. Janssen, G. Hilkert, M. Konrad, H. Peiffer and B. Janssens, Coextruded, biaxially-oriented polyester film for use e.g. as cover film for ovenready meal trays, has a base layer containing poly-m-xylylene-adipamide and a heatsealable outer layer of aromatic±aliphatic polyester, US Patent US 2005 0100729 (2005). 23. P. Frisk, N. Kobayashi, H. Ogita, K. Norio and O. Hiroaki, Laminated material for packaging foods and beverages as well as other liquid products, with strength, barrier properties and taste retention, as well as improved productivity and cost performance ratio, PCT International Patent WO200119611-A (2001) 24. Imperm Technical data sheet. Available from http://www.nanocor.com/tech_sheets/ I105.pdf [accessed 25 February 2010]. 25. A. Ammala, A. J. Hill, K. A. Lawrence and T. Tran, J Appl Polym Sci 104: 1377± 1381 (2007). 26. A. Ammala, S. J. Pas, K. A. Lawrence, R. Stark, R. I. Webb and A. J. Hill, J Mater Chem 18: 911±916 (2008). 27. C. Thellen, S. Schirmer, J. A. Ratto, B. Finnigan and D. Schmidt, J Membr Sci 340: 45±51 (2009).


10


Â P E Z - R U B I O , Novel Materials and Nanotechnology A. LO Group, IATA-CSIC, Spain

Abstract: Ethylene±vinyl alcohol (EVOH) copolymers are excellent gasbarrier semicrystalline materials with very good chemical resistance and, as such, they are widely used in a number of packaging applications. One of the most widely implemented applications is that of an intermediate barrier layer in multilayer structures, to be used in various packaging designs for foodstuffs. The presence of EVOH in the packaging structure is key to food quality and safety because, for instance, it delays the ingress of oxygen, the agent responsible for a number of food deterioration processes. The effects of industrial processing on the structure and properties of these polymers are compiled, together with solutions to overcome the deleterious effects of industrial retorting processes commonly used in the food industry. Finally, the property improvements attained upon addition of different nanoclays to EVOH matrices and to the homopolymer poly(vinyl alcohol) (PVOH) will be described. Keywords: ethylene±vinyl alcohol (EVOH), nanocomposites, poly(vinyl alcohol) (PVOH), retorting, barrier properties.

10.1

Introduction

The use of polymers in the food packaging area has been steadily increasing over the last decades due to the numerous advantages, such as lightness, cost and versatility, which these materials present over the traditionally employed glass or tinplate. One of the main disadvantages arising from the use of polymers is probably the fact that they are not impermeable materials, thus allowing the passage of gases and aromas that could compromise food quality and, more importantly, food safety. In the specific case of oxygen-sensitive products, the use of high-barrier packaging materials is a requirement. A high-barrier material in food packaging refers to a material that has low oxygen permeability and, more specifically, an oxygen permeability lower than 1 cm3/m2.day.atm. Ethylene±vinyl alcohol (EVOH) copolymers are widely used as high barrier layers in multilayer food packaging structures due to their outstanding properties and very low permeability to oxygen and food aromas. The use of high barrier packaging concepts containing ethylene±vinyl alcohol copolymer resins actually began in the mid-1970s, although it was not until 1983 when their use in high barrier applications rapidly expanded (Foster, 1991).


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Today EVOH packages are used in low acid retortable applications, high acid hot fill applications, aseptic packages, and packages to prevent flavour and aroma scalping, and the resins have evolved into four different packaging segments ± flexible, formed, bottles and co-extrusion coating (Foster, 1991). In this chapter, the main characteristics and drawbacks of EVOH copolymers for food packaging applications are described together with some of the existing solutions, which include blending with other materials and addition of nanoclays to generate EVOH nanocomposites.

10.2

Structure and general properties of ethylene± vinyl alcohol (EVOH) copolymers

Ethylene±vinyl alcohol copolymers are a family of semicrystalline materials with excellent barrier properties to gases and hydrocarbons. The presence of hydroxyl groups from the vinyl alcohol fraction provides the materials with very high inter- and intramolecular cohesivity, reducing the free volume between the polymer chains available for the exchange of low molecular weight compounds such as gases and aromas. Figure 10.1 shows the chemical structure of the copolymers having a random distribution of the hydroxyl groups along their chains. EVOH copolymers are commonly produced via a saponification reaction of a parent ethylene-co-vinyl acetate copolymer, whereby the acetoxy group is converted into a secondary alcohol. These materials have been increasingly implemented in many pipe and packaging applications where stringent criteria in terms of chemical resistance and in gas, water, aroma, and hydrocarbon permeation are to be met. The composition of the copolymers can be varied by changing the ratio of the ethylene/vinyl alcohol fractions, which also causes a change in the physicochemical properties. In particular, the copolymers with low contents of ethylene (below 38 mol% ethylene) have outstanding barrier properties, under dry conditions, compared to other polymeric materials. The crystalline morphology of these copolymers is relatively well known across composition and has been the subject of several studies (Cerrada et al.,

10.1 Chemical structure of EVOH copolymers.



263

1998; Takahashi et al., 1999). Both the melting point and the density of the materials vary as a function of the ethylene/vinyl alcohol ratio, indicating that the crystal structure itself changes continuously with the change in the copolymer composition. While the unit cell of EVOH samples with vinyl alcohol contents greater than 26 mol% has been observed to be monoclinic (like the polyvinyl alcohol ±PVOH± unit cell structure), for lower vinyl alcohol contents the crystal unit cell has been described as orthorhombic and, thus, more similar to the one from polyethylene (PE). A boundary of crystal structure transformation between the PVOH type and the PE type is considered to be located at vinyl alcohol contents between 27 and 14 mol% because the X-ray pattern can be interpreted almost equally on the basis of both the PVOH- and PE-type structure models (Takahashi et al., 1999). However, the crystal structure also depends on the thermal history of the polymeric samples, in a way that quenching EVOH copolymers with high vinyl alcohol contents leads to an orthorhombic crystal morphology (Cerrada et al., 1998). The changes in crystal structure as a function of the copolymer composition also result in changes in the oxygen permeability of the materials. Therefore, depending on the application, the copolymer grade can be chosen for the expected oxygen performance. The oxygen permeability of dry EVOH measured at 45ëC varies from 0.45 to 32 cm3/m2.day.atm for copolymers having an ethylene content of 26 and 48 mol%, respectively (Lopez-Rubio et al., 2005a). One of the main drawbacks of these materials is their high water sensitivity and, thus, the same hydroxyl groups that provide the high cohesivity between the polymer chains also make the copolymers very hydrophilic, so that in the presence of water or in high humidity environments, the structure of the material becomes plasticized and the barrier properties are greatly deteriorated. While under dry conditions, the glass transition temperature (Tg) is unaffected by the copolymer ethylene content, when increasing the relative humidity (RH) a drop in the Tg has been observed. Tg varies from around 50ëC (dry) to below room temperature in the presence of water vapour and, thus, EVOH copolymers are glassy polymers when dry and rubbery polymers at high RHs (Aucejo et al., 1999). This plasticization when increasing the RH, and especially above 75% RH, is reflected in significant increments of the oxygen permeability values. For instance, a ten-fold increase in oxygen permeability was observed for EVOH with 32 mol% of ethylene at 80% RH (Zhang et al., 2001). Zhang and coworkers (2001) also observed that at low relative humidity (up to 35%) an improvement in the oxygen permeability of the copolymers occurred, which was explained by strong adsorption of water molecules by the polymer, therefore occupying the free volume that would otherwise be available for oxygen. Exclusion of oxygen by water from the free volume of the polymer matrix reduced the available diffusive pathways for the oxygen. As a result, oxygen permeation decreased as the RH increased up to an intermediate RH. Concurrently, however, adsorbed water molecules interacted with polar hydroxyl


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10.2 Oxygen transmission rate versus relative humidity at 20ëC of three EVOH copolymer grades, in comparison with other commercial synthetic polymers. Graph extracted from EVAL Europe (www.eval.be).

groups of the polymer and weakened the intermolecular and intramolecular hydrogen bonding, thereby facilitating segmental motion and oxygen diffusion. At high RH, this bonding effect was more pronounced than the reduction of absorption sites because the polymer was increasingly plasticized by the sorbed water, resulting in the large increase in oxygen transmission rate observed (Zhang et al., 2001). Figure 10.2 shows the oxygen permeability of three grades of EVOH, in comparison with that of other commercial synthetic polymers, as a function of relative humidity (data extracted from EVAL Europe, Kuraray Co. Ltd). Mechanical performance of EVOH copolymers is also affected by humidity. Fourier transform infrared spectroscopy (FT-IR) was used to analyse both the transport properties of water through food packaging films made of EVOH with various ethylene contents, i.e. ranging from 26 to 48 mol% of ethylene, and the water±polymer interactions (Cava et al., 2006). From the results, an unreported Langmuir contribution was found at low relative humidity conditions for the copolymers, which was thought to be responsible for the unusual trend in oxygen permeability reported earlier for these materials. Furthermore, a distribution of water molecules with different hydrogen bonding strengths and different diffusion rates was encountered, which indicated that the interaction and transport properties of moisture in these polymers is far from being a simple process. A proper understanding of the above moisture sorption effect on the mechanical performance of EVOH copolymers is also of great importance to predict the effect of moisture uptake on the EVOH based packaging structures in service applications. In agreement with the mass transport results, a surprising



265

anomalous antiplasticization regime for some mechanical properties below 30% RH was observed in EVOH materials, reflected in an increase of the modulus for all the copolymers analysed (Cabedo et al., 2006). Beyond this point, further moisture sorption acts as the well-reported effective plasticization agent for the mechanical properties (Cabedo et al., 2006). These results have a direct impact on food packaging, as water present in food products could affect EVOH behaviour in terms of barrier properties, resulting in reduced packaged food product shelf-life. For this reason, in most applications EVOH is used as an intermediate layer in multilayer packaging structures, protected by layers of hydrophobic materials such as polyethylene (PE) or polypropylene (PP). Other important characteristics of EVOH which make them very suitable for a variety of applications are their high chemical and thermal resistance, good optical characteristics and high crystallization velocity.

10.3

Ethylene±vinyl alcohol (EVOH) versus aliphatic polyketones

Aliphatic polyketones (PKs) are a family of semicrystalline thermoplastics prepared by the polymerization of -olefins and carbon monoxide in a perfectly 1:1 alternating sequence using palladium catalysts (Drent and Budzelaar, 1996; Sommazzi and Garbassi, 1997). The simplest member of the family of perfectly alternating polyketones is the copolymer of ethylene and carbon monoxide. This copolymer is a white powder of relatively high crystallinity (35±50% as determined by X-ray diffraction) and a melting temperature of 260ëC. Its alternating structure has been confirmed by elemental analysis, infrared and nuclear magnetic resonance spectra (Lai and Sen, 1984; Zhao and Chien, 1992; Drent et al., 1991). Lower melting point polymers can be produced by incorporation of propylene in addition to ethylene in order to avoid degradation phenomena induced by processing at such high temperatures. The lowering of the melting point is proportional to the number of propylene units present in the polymer chain (Sommazzi and Garbassi, 1997). When both ethylene and propylene are used as co-monomers, their distribution is statistically random. The general structure of this family of polymers is represented in Fig. 10.3.

10.3 Chemical structure of polyketone terpolymers.


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These polymers are reported to have a useful combination of mechanical, high-temperature, chemical resistance, wear resistance and barrier properties, giving them significant commercial potential in a broad range of engineering, barrier packaging, fibre and blend applications (Bonner and Powell, 1997). Their properties are at the border between commodity polymers (like polyethylene and polyvinyl chloride) and engineering polymers of medium performance like polyamides and polyesters. Ethylene/propylene/CO terpolymers show good chemical resistance. They are resistant to a wide range of chemicals including automotive fluids, solvents and other industrial chemicals. In contrast with EVOH copolymers, polyketones are particularly resistant to aqueous media, displaying minor swelling phenomena. Water absorption at 100ëC is not negligible, rising to a level of about 3.5%. However, the polymer exhibits good stability and water absorption does not produce hydrolytic degradation (Sommazzi and Garbassi, 1997). Furthermore, mechanical properties are only slightly affected by moisture sorption. The tensile strength at yield decreases from 60 MPa to 56 MPa, the elongation at yield increases from 25% to 28% and the elongation at break does not change at high RH. Polyketones are attractive polymers to be used in food-packaging applications. This class of polymers is reported to fulfil both processing and food preservation requirements. In fact, they are one-step processable, have good impact properties, are dimensionally heat-stable, are easily compoundable with other polymers used for food packaging (nylon, polycarbonate, and ethylene± vinyl alcohol copolymer), and have good barrier properties competitive with those of nylon and poly(ethylene terephthalate) (PET) (Del Nobile et al., 1993).

10.4

Processing in packaging

10.4.1 Retorting of EVOH and consequences on structure and barrier properties As mentioned in the first section, EVOH copolymers are widely used as a highbarrier layer in multilayer food packaging structures due to their outstanding properties and very low permeability to oxygen and food aromas. The interest for these materials in the food packaging area is based on the fact that many food products are to be packaged with high-barrier polymeric materials because oxygen is a ubiquitous element involved in many food deterioration reactions, such as fat oxidation, vitamin loss, etc. But furthermore, several food products are thermally treated within the food packages and, therefore, apart from the already mentioned high-barrier conditions, plastic packages must withstand such kinds of processes without suffering undesirable changes. Specifically, precooked foods (ready-to-eat products) for which there is a continuously increasing demand require a retorting treatment inside the package before



267

being commercialized (typically 121ëC during 20 min in an industrial autoclave, i.e., in the presence of pressurized water vapour) (Ramesh, 1999). Ethylene±vinyl alcohol (EVOH) copolymers are commonly used in retortable packaging structures, but because of the above-mentioned high water sensitivity, these materials are used as intermediate high-barrier layer in multilayer structures protected from the external relative humidity by at least two layers of a hydrophobic material such as polypropylene (PP). However, it is common knowledge that even protected between these hydrophobic materials, retorting processes have a tremendous impact on the gas barrier performance of the copolymers partly due to extensive plasticization of the EVOH layer. The plasticizing effect of water on the barrier properties of EVOH is time-dependent, especially if the hydrophilic layer is protected by a water barrier such as polypropylene as in the case of packages for food retorting. When such retortable packages (containing aqueous foodstuffs) are subjected to steam retorting, water passing through the protective hydrophobic layer is thought to be sorbed on the EVOH layer in such quantities that the barrier layer becomes quite permeable to oxygen. The rate of water release through the outer polypropylene layer becomes very slow on cooling, so the oxygen permeability can remain elevated for many weeks (Tsai and Wachtel, 1990). Tsai and Jenkins (1988) reported that the oxygen barrier of retortable packages containing an EVOH barrier layer was initially reduced by two orders of magnitude when these containers were subjected to steam or pressurized water during thermal processing, and during long-term storage (>200 days) the barrier was partially recovered (by a factor of 10). In a more recent work (Lopez-Rubio et al., 2003) it was demonstrated that this huge increase in permeability was caused not only by the plasticization of the EVOH structure, as had been previously reported (Tsai and Jenkins, 1988; Zhang et al., 2001), but also by a deterioration of the copolymer crystallinity that takes place even in multilayer structures during the retorting process (LopezRubio et al., 2005a). It is worth noting that the retorting treatment is carried out at temperatures well below the copolymer's maximum of melting, albeit above its glass transition temperature. Nevertheless, polymer films, typically utilized in foodpackaging applications, did not withstand the treatment and irreversibly lost dimensional stability. Therefore, in order to evaluate the effects of retorting on pure EVOH matrices, thicker plates were prepared and submitted to this food preservation method (Lopez-Rubio et al., 2003). As can be observed in Fig. 10.4, after retorting, the samples appeared, by visual inspection, to be dramatically damaged, with the presence of voids and loss of transparency, i.e., intense whitening or hazing. These effects are attributed to pressurized water vapour having penetrated the amorphous phase but also diffusing through the crystal edges and inducing partial melting and fractionation of the crystalline morphology. Voiding and loss of transparency could arise from bubbles formed by water evaporation as


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10.4 EVOH32 (a) before and (b) after retorting in autoclave (121ëC, 20 min).

the pressure is released rapidly after retorting in the autoclave. These damaging effects do not occur upon water exposure or uptake at room temperature and evidently neither due to annealing. High temperature and humidity thus appear to be a very severe and aggressive combination of factors for these materials. Figure 10.5 shows the X-ray patterns as a function of temperature obtained after real-time synchrotron experiments of EVOH monolayers. Comparing the patterns of the dry and water-saturated specimens (the latter one simulating an in situ retorting process), it can be seen that while the dry sample melts around 183ëC (in accordance with DSC data), the X-ray patterns of the water-saturated EVOH specimen disappear around 100ëC, which implies that the polymer melts 83ëC earlier than expected in the presence of heated water vapour (Lopez-Rubio et al., 2003). The morphology and thermal characteristics of retorted specimens typically used in food-packaging structures (i.e. protected by polypropylene layers) were also seen to be greatly affected by the combination of temperature and humidity. An FTIR methodology was developed to ascertain the changes in crystallinity of EVOH films and a significant reduction in crystallinity was observed after retorting of the multilayer packaging structures (Lopez-Rubio et al., 2003). FTIR spectroscopy technique is an appropriate tool to study morphological alterations in these materials because of its high sensitivity to detect both crystallinity alterations through the use of the 1140 cmÿ1 band and the presence of humidity in the sample through observation of the OH in-plane bending band at 1650 cmÿ1. The 1140 cmÿ1 band is likely attributable to C±O±C stretching or to C±C stretching coupled with a C±O stretching mode. The absorbance of this band (divided by that of the internal standard at 1333 cmÿ1) can thus give us an indication of potential alterations in crystallinity after the various treatments, irrespective of differences in optical path and of minor thickness variations between different specimens (Lopez-Rubio et al., 2005a).



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10.5 WAXS patterns as a function of temperature of (a) dry and (b) watersaturated EVOH32 film specimens (from Lopez-Rubio et al., 2003).

10.4.2 Effects of other novel food preservation technologies on EVOH packaging structures Consumer preferences towards mildly preserved, high quality and more freshlike products are leading to the substitution of these traditional thermal treatments by other emerging preservation technologies based on the application of irradiation, microwave pasteurization, electric fields, high hydrostatic pressure and their combination with mild thermal treatments. Among these emerging technologies, high pressure processing (HPP) is receiving a great deal of attention due to its unique advantages over conventional thermal treatments, including application at low temperatures, which improves the retention of food quality. High pressure treatments are independent of product size and geometry and their effect is uniform and instantaneous (Palou et al., 1999). Most HPP equipment works in batch processes. Foodstuffs are packaged at the end of the production line and then pressurized at hundreds of MPa to produce a high


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nutritional and sensorial quality product, with more desirable texture and longer shelf-life (Ledward, 1995). It is then crucial to understand the effects of high pressure on relevant properties of plastic materials to assure the safety of the foodstuffs throughout their shelf-life. Irradiation of pre-packaged foodstuffs using gamma and electron beam radiation is also gaining ground as a method of food preservation. To enhance the protection offered by irradiation, foodstuffs are usually pre-packaged in flexible packaging materials prior to the treatment and, in that way, subsequent recontamination by microorganisms is prevented. Moreover, this technique is also being used for the sterilization of flexible packages utilized later on in aseptic packaging technology (Azuma et al., 1983). Plastics are affected in various ways when exposed to high-energy radiation and, thus, it is also important to characterize the effects of this preservation treatment on the structure and properties of polymers in order to ensure the adequate final performance, especially if they are going to be used for food packaging. The effects of these novel food preservation technologies on EVOH materials have also been studied. In contrast with the damaging effects of retorting, HPP and irradiation technologies do not alter significantly the morphology and physico-chemical properties of the packaging structures. Specifically, high hydrostatic pressures even lead to a slight improvement in the oxygen barrier properties of EVOH, which can be explained not only by a pressure-induced reduction of the free volume but also by a slight increase in the crystallinity of the materials (Lopez-Rubio et al., 2005b). Table 10.1 shows the oxygen transmission rate values for two different grades of EVOH that had been submitted to various high pressure treatments in comparison with the values obtained for the retorted specimens. In the case of irradiation, formation of radiolysis compounds was detected, which was observed to be dose-dependent (Riganakos et al., 1999). However, it Table 10.1 Oxygen transmission rate (cm3/m2.day) of multilayer structures PP// EVOH//PP high pressure processed, retorted and untreated

Untreated 400 MPa, 40ëC, 5 min 400 MPa, 75ëC, 5 min 800 MPa, 40ëC, 5 min 800 MPa, 75ëC, 5 min 400 MPa, 40ëC, 10 min 400 MPa, 75ëC, 10 min 800 MPa, 40ëC, 10 min 800 MPa, 75ëC, 10 min Retorted Retorted (after 150 h)

EVOH26

EVOH48

0.60 0.44 0.36 0.50 0.62 0.50 0.48 0.58 0.59 392.00 1.63

40.88 39.24 41.00 39.63 37.50 37.00 38.25 39.38 38.13 645.00 28.55



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was demonstrated that all the radiolysis and non-radiolysis products of ethylene± vinyl alcohol (EVOH) copolymers were within the threshold of regulation (Kothapalli and Sadler, 2003), so in principle, this confirms the safety of irradiated EVOH as a food contact material. Moreover, electron beam irradiation at doses of 30 and 90 kGy was seen to impart some oxygen scavenging capacity in an ethylene±vinyl alcohol copolymer (EVOH29, i.e., 29 mol% of ethylene). This oxygen-blocking activity is thought to arise from the reaction of oxygen with the free radicals formed during the irradiation process and was observed to be dependent on the irradiation dose, i.e., the higher the dose, the longer the time the polymer was able to react with oxygen (Lopez-Rubio et al., 2007). After exhaustion of this capacity the irradiated films are slightly more permeable to oxygen as a consequence of faster oxygen diffusion. From a food packaging application point of view, the e-beam irradiation of EVOH-containing structures can produce the desired reduction of microbial contamination, and an oxygenscavenging activity which might be of great interest for the packaging of oxygen-sensitive products.

10.5

Improving retorting of ethylene±vinyl alcohol (EVOH)

10.5.1 Strategies to improve EVOH resistance to the retorting treatment It is well known that the application of a thermal treatment below the melting point of semicrystalline polymeric materials ± particularly above the glass transition temperature of the material ± favours the mobility of chain segments at the crystals' interphase and within the crystals towards the development of a more stable and thicker crystalline morphology, a phenomenon known as annealing. Therefore, this phenomenon leads to the elimination of defects through partial melting and recrystallization of the most ill-defined (less metastable) crystals, generating a more regular stacking of the lamellae and higher crystallinity. Therefore, two of the strategies advocated for reducing the detrimental effects of retorting over EVOH structures were thermally treating the multilayer structures of PP/EVOH/PP either before or after the retorting treatment (Lopez-Rubio et al., 2005a). Different multilayer structures of various compositions of EVOH protected by PP layers were retorted and then dried at 70ëC for one week in a vacuum oven. Surprisingly, it was found that the oxygen transmission rate values of the retorted and then dried samples were found to be not only better than the values from the just retorted specimens, but also superior to those of the untreated ones, where this improvement was more pronounced for those copolymers with higher ethylene content. Analysing the FTIR spectra of the specimens, it was also found that drying is an effective process in reducing sorption-induced polymer plasticization, as the water band at 1650 cmÿ1 was not so clearly seen in the retorted and dried samples.


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10.6 FTIR spectra of EVOH32 specimens: from top to bottom, untreated, retorted, and retorted and then dried, delaminated from PP/EVOH/PP structures (from Lopez-Rubio et al., 2005a).

Figure 10.6 compares the FTIR spectra of EVOH32 specimens untreated, retorted, and retorted and then dried, delaminated from PP/EVOH/PP structures, with the 1650 cmÿ1 point arrowed. From this figure it can also be observed that the crystallinity band at 1140 cmÿ1 shows the highest absorbance, even higher than that of the unmodified sample, an observation that confirms that the crystallinity is the highest for this specimen. Thus, pressurized water vapour, which penetrated the multilayer structure during retorting, is thought to melt and disrupt part of the EVOH crystalline morphology. The subsequent annealing process in the vacuum oven at 70ëC for one week allowed the polymer chains to reorganize and anneal, giving rise to a significantly improved crystalline structure (Lopez-Rubio et al., 2005a). From the above results and from an applied problem-solving perspective, it is apparent that a drying step after sterilization of the package can restore or even improve the barrier performance of the materials by removing sorbed moisture and by rebuilding a more favourable morphology. Being aware of the implications of crystallinity and its morphology on barrier properties and of the ability of water sorption to modify the polymer morphology, the second strategy considered was to improve the initial crystallinity of the samples before retorting through the application of an annealing process. The different multilayer structures were first annealed for 20 min in an oven at a selected optimum temperature (beyond which crystallinity was seen to decrease through annealing) that had previously been determined through FTIR. Then, the specimens were retorted in the autoclave, delaminated, and the EVOH layer FTIR recorded. The rationale behind this annealing experiment was to provide the most adequate polymer morphology in terms of crystallinity content and robustness (i.e., higher crystalline density) before retorting.



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Table 10.2 FTIR absorbance of the 1440 cmÿ1 peak divided by the absorbance of the 1333 cmÿ1 reference peak Material

Not treated

Annealed

Retorted

Annealed and retorted

Retorted and dry

EVOH26 EVOH29 EVOH32 EVOH38 EVOH44

1.26 1.25 1.24 1.21 1.21

1.45 1.43 1.40 1.38 1.34

1.13 1.12 1.14 1.19 1.19

1.25 1.20 1.16 1.19 1.19

1.28 1.28 1.26 1.25 1.24

Variations in crystallinity were estimated through the ratio of the absorbance of the crystallinity band at 1140 cmÿ1 to that of the band at 1333 cmÿ1. The values of this ratio for the various copolymer grades and thermal histories are displayed in Table 10.2. From these results, it can be seen that pre-annealed EVOH26, EVOH29 and EVOH32 specimens can attain a higher level of crystallinity after retorting than that of retorted-only specimens. In fact, preannealed sample EVOH26 even shows a degree of crystallinity after retorting similar to that of the untreated specimen (dried under vacuum at 70ëC for one week). For EVOH38, EVOH44 and EVOH48, prior annealing of the specimens did not lead to improved morphology compared to that of untreated specimens (Lopez-Rubio et al., 2005a). The third strategy discovered to protect EVOH from the effects of retorting was by appropriate shielding of the EVOH layer between polypropylene layers of a critical thickness (40 m instead of the 10 m normally used in foodpackaging structures). From synchrotron radiation studies, it was observed that the integrity of the EVOH layer was largely maintained during a typical retorting process when using these sufficiently thick PP layers. Figure 10.7 shows the WAXS patterns of the PP, EVOH32 and PP/EVOH32/PP structures during different stages of the retorting process. In Fig. 10.7, the arrow indicates the presence of the (110) crystalline peak of EVOH in the structure during the whole retorting experiment, suggesting that the barrier layer does not melt in a multilayer structure (Lopez-Rubio et al., 2005c). Therefore, even though a plasticization of the EVOH amorphous structure will still take place, as the crystalline structure is not melted during the retorting process, the barrier effects are expected to be much lower in multilayer structures containing thicker layers of PP.

10.5.2 Blending EVOH with other materials Another strategy that showed promising results for palliating the deleterious effects of retorting treatments over EVOH structure was the use of blends. Specifically, blends of EVOH with amorphous polyamide (aPA) and nylon-


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10.7 WAXS patterns of PP, EVOH32 and PP/EVOH/PP structures during different stages of the retorting process (from Lopez-Rubio et al., 2005c).

containing ionomer were originally developed to diminish the barrier deterioration of EVOH when exposed to high-humidity environments (LagaroÂn et al., 2001, 2003a). The main drawback for blending EVOH with other polymers is the strong polymer self-association promoted by the hydroxyl groups, which leads to poor compatibility. Binary and ternary blends of amorphous polyamide (aPA) and nylon-containing ionomer with EVOH have proven to have beneficial effects such as improved processability during thermoforming. Even when these blends were found to be immiscible, good phase dispersion and adhesion at the interphase were generally found (LagaroÂn et al., 2001, 2003a). From a barrier perspective, the materials were found to provide a positive deviation from the Maxwell model in oxygen permeability, which led to better barrier properties than expected (LagaroÂn et al., 2003a). Furthermore, both the aPA and the nyloncontaining ionomer exhibited lower water sorption than the hydrophilic EVOH. This lower water sorption was probably the reason why, when these blends were exposed to a combination of temperature and pressurized water vapour (i.e. retorting), they did not experience detrimental structural changes as was the case in pure EVOH (Lopez-Rubio and LagaroÂn, 2008). In fact, improvements in the thermal properties, crystalline structure and water resistance of the blends were found upon retorting compared with neat EVOH. However, only the binary blend with aPA showed a real enhancement in the oxygen barrier properties immediately after retorting compared with neat EVOH. This effect was ascribed to the retorting-induced compatibilization between EVOH and aPA components of the blend as determined by SEM. In the blends with ionomer, increased oxygen permeability values were observed, probably due to the melting of the



275

ionomer at the temperatures applied and, thus, upon cooling after the treatment, voids were left in the films, causing the drop in barrier properties (Lopez-Rubio and LagaroÂn, 2008).

10.5.3 Alternatives to EVOH for retortable packages As a consequence of the detrimental effects of common retorting processes over the structure and permeability of the EVOH copolymers, alternative high-barrier materials are being studied as potential substitutes in retortable food-packaging structures. Aliphatic polyketones and amorphous PA are high and medium-high barrier materials with potential in retortable applications. A very recent study has already proven that the aliphatic polyketones are adequate materials to withstand packaged food retorting conditions, as the deterioration in oxygen barrier suffered by these polymers even in monolayer structures is very small compared with the deterioration suffered by EVOH-based multilayer structures (Lopez-Rubio et al., 2006a). Moreover, the crystalline structure of polyketones withstands the retorting process as observed in the WAXS patterns during timeresolved in situ retorting of the specimens (cf. Fig. 10.8). Amorphous polyamides (aPAs), on the other hand, offer favourable properties such as dimensional stability, good dielectric and barrier properties, and low mould shrinkage (Granado et al., 2004) and, furthermore, exhibit an antiplasticization behaviour at high relative humidity conditions. Thus, it is known that the oxygen barrier performance of this material increases at high relative humidity conditions in contrast with the behaviour of most polar polymers, including most polyamides, aliphatic polyketones and EVOH (LagaroÂn et al., 2003b). When aPA was submitted to a retorting process, it was fortuitously found that the combination of heat and moisture was capable of inducing some crystallization in the otherwise amorphous polymer. The crystallization of the polymer began as characterized by wide angle X-ray scattering and differential scanning calorimetry in the presence of humidity at 90ëC and extended up to 120ëC under autoclave conditions, and is thought to be the result of heated

10.8 Synchrotron WAXS traces versus temperature (ëC) taken during a typical retorting run of PK specimens (from Lopez-Rubio et al., 2006a).


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moisture being able to disrupt the intense amide groups' self-association. Thus, the thermally activated molecular structure is thought to become plasticized by the combined presence of heat and water which, in turn, provoke sufficient segmental molecular mobility in the system to promote some degree of lateral order. Property-wise, the resulting consequences of this behaviour are an increase in the barrier properties to oxygen and a reduction in water sorption. From an applied viewpoint, it is suggested that this unexpected behaviour could make this polymer of significant interest in retortable food packaging applications (Lopez-Rubio et al., 2006b).

10.6

Nanocomposites of ethylene±vinyl alcohol (EVOH) and poly(vinyl) alcohol (PVOH)

A great deal of research has been put forward to minimize mass transport processes in polymeric materials for the packaging of, for instance, oxygensensitive foodstuffs and, thus, to guarantee food quality and safety during the extended shelf-life of these products (LagaroÂn et al., 2004). One of the most promising routes to accomplish this is through the development of ùltra-high' barrier materials by means of nanotechnology. Specifically, the generation of nanocomposites through the addition of low loadings of nanoparticles (generally nanoclays) to a raw polymer has been reported to have enhancing effects over some material properties, such as mechanical properties, thermal stability (Kotsilkova et al., 2001) and gas barrier properties, without significant reduction in other relevant characteristics such as toughness (Alexandre and Dubois, 2000) and transparency below a critical loading level (Wan et al., 2003). The addition of nanoclays to EVOH matrices is thought to result in ultra-high barrier properties mainly due to a tortuosity-driven decrease in molecular diffusion of gases and vapours and to increased thermal resistance. EVOH nanocomposites have been developed mainly using two types of layered silicates: commercial modified montmorillonites (MMT) and natural and modified kaolinites. The montmorillonites used in EVOH nanocomposites are organically modified by exchanging the interlayer cations, normally with alkylammonium ions. The kaolinite is a very common material in earth, very cheap and, hence, widely used as raw material in many industrial sectors (LagaroÂn et al., 2005). Surface chemical treatment of kaolinite platelets can also be applied to facilitate intercalation of the polymer chains and further exfoliation of the clay. The treatments of the platelets with chemical agents, such as dimethyl-sulfoxide, methanol and octadecylamine, lead to increases in the basal spacing of the clay as well as to increased compatibility with the polymer, thus facilitating mixing, exfoliation and dispersion (Cabedo et al., 2004). X-ray diffraction is a conventional method to characterize the gallery height in clay particles, which is indicative of the extent of intercalation. The (001) basal crystalline plane observed in the X-ray diffractogram is usually shifted



277

towards lower angles upon chemical treatments (Cabedo et al., 2004; Villanueva et al., 2010). This displacement indicates that the clay interlayer distance increases as a result of the treatment. The shift in the basal crystalline plane is generally also observed after mixing with the polymer, indicating that intercalation takes place (Artzi et al., 2001; Lee et al., 2006). Further mixing can lead to the disappearance of the basal peak, which has been ascribed to exfoliated structure (Artzi et al., 2005). Because the silicate layers have hydroxyl groups, it is generally recognized that polymers with polar groups should be used to obtain a highly intercalated structure or exfoliation state. The presence of hydroxyl groups in EVOH thus makes these copolymers suitable for the interaction with the silicate layers (Jeong et al., 2005). However, some of the recent theoretical and experimental research results demonstrated that the adhesive role of a polar polymer between hydrophilic clay layers, the so-called glue effect, tends to strongly prohibit complete dissociation of the layered structure of clay, resulting in only an ordered intercalated state (Lyatskaya and Balazs, 1998; Lee et al., 2001). This has been confirmed by transmission electron microscopy (TEM) analysis of EVOH nanocomposites with 5 wt% montmorillonite organoclays, prepared by dynamic melt intercalation, where the silica platelets were observed to be highly associated, while the same nanocomposites prepared with a polymer with poor interfacial affinity with the clay showed much higher disintegration of the layered structure (Lee et al., 2006). The latter authors also demonstrated that even though there was lower dissociation in EVOH matrices, a dramatic increase in tensile strength and modulus was observed for these nanocomposites. Specifically, increases of 10% and 22% were observed for the tensile strength and modulus respectively (Lee et al., 2006). Based on these findings, it was concluded that the glue effect is important in determining the degree of intercalation, and the optimum value of interaction between the polymer and the clay surface is of critical consideration in designing high-performance clay nanocomposites with an intercalated structure (Lee et al., 2006). This has been further substantiated by several studies where through SEM and TEM a strong adhesion between EVOH and clay and intercalation (but not total exfoliation) was observed, respectively, which were correlated with 3 to 6ëC higher Tg values for the nanocomposites and, thus, higher rigidity (Cabedo et al., 2004; LagaroÂn et al., 2005). As an example, a TEM image of an EVOH±kaolinite composite is shown in Fig. 10.9. However, it is important to note that the amount of exfoliation appears to be strongly affected by the processing conditions. Generally, the level and mode of dispersion are governed by parameters such as matrix viscosity, level of shear field, and residence time in the process (Cho and Paul, 2001). Artzi and coworkers (2005) claimed to have both intercalation and delamination of organoclays in EVOH nanocomposites processed by melt mixing. The montmorillonite clays used by the previous authors had been modified using


278


10.9 TEM image of an EVOH nanocomposite containing 2% of a modified kaolinite clay (from Cabedo et al., 2004).

octadecylamine instead of alkylammonium ions, the fact that could also influence the clay behaviour during processing of the nanocomposites. The fracturing process of the organoclay particles into smaller tactoids and posterior delamination during the mixing process with EVOH can be followed using a plastograph mixing cell and is indicated by the increase in processing torque. Residence mixing time has been found to strongly affect the structuring process of the composites, the level of exfoliation, the degree of EVOH crystallinity, and the resulting properties (Artzi et al., 2001). Subsequent processing of the nanocomposites by, for instance, extrusion results in higher delamination and platelet dispersion. Another factor contributing to the higher delamination level is the higher stress developed at the elevated rotational speed. Extrusion residence time, successive extrusion passes, screw rotational speed and processing temperature were all found to affect the morphology and the associated thermal and mechanical properties (Artzi et al., 2005). EVOH±clay nanocomposite extrusions processed at 200ëC and 40 rpm presented an exfoliated structure and, regardless of clay type or content, the tensile modulus and strength were observed to increase up to 40% and 30% respectively relative to the neat EVOH with only 0.5% clay addition, again highlighting the high level of interaction of the clays with the EVOH matrix (Artzi et al., 2004).



279

The high interaction level in EVOH±clay systems has also been supported by DSC studies. Upon addition of montmorillonite clays, some authors observed a decrease in the melting temperature and enthalpy values, which could be explained by the interactions of the platelets with the polymeric chains, partially hindering segmental mobility and, thus, the crystallization process. The EVOH crystallization process is quite sensitive to crystallization conditions. The crystallization process in the presence of clay particles, especially clay nanoplatelets, can generate `smaller' EVOH crystals having a lower melting temperature. In this case, the clay is probably playing a role of hindering the crystallization process, owing to the high interaction level with EVOH (Artzi et al., 2004). However, these previous results contrast with the increase in both the melting temperature and enthalpy of fusion observed for the various EVOH nanocomposites based on kaolinite clays. In this case, the increase in the melting enthalpy was ascribed to the nucleating role of clay nanoplatelets on cooling from the melt (Cabedo et al., 2004). The latter observation is very relevant from a barrier perspective, as crystals are generally impermeable to the transport of low molecular weight substances and, thus, the combination of higher crystallinity and clay impermeable elements was seen to result in oxygen permeation rates for the nanocomposites below the experimental error of the instrument, i.e. below 10ÿ5 (cm3 m)/(m2 day atm) (Cabedo et al., 2004). Incorporation of kaolinite nanoclays to EVOH has been observed to result in significantly improved oxygen barrier properties, both under dry conditions and at high relative humidity. In this latter case, i.e. at high RH, reductions in the oxygen permeability of 70% were observed in the copolymer with 32 mol% of ethylene (LagaroÂn et al., 2005). Apart from the already mentioned improvement in mechanical and oxygen barrier properties of EVOH nanocomposites, addition of dispersed, intercalated nanoclays also results in better thermal stability. An increase in the temperature of the maximum weight loss rate of EVOH±kaolinite nanocomposites has been observed, with increases in the maximum rate of degradation temperature of more than 21ëC (Cabedo et al., 2004). The thermal stability increase could be explained by the fact that the clay, together with the solid degradation products, led to a dense coating that hinders the development of further degradation by opposing a strong mass transport resistance to the agents involved in the reaction, resulting in a decrease of the degradation kinetics (Zanetti et al., 2001). Nanocomposites of clays and the homopolymer poly(vinyl) alcohol (PVOH) have also been developed. PVOH has excellent film-forming characteristics and outstanding oxygen barrier properties in the dry state. As PVOH is even more hydrophilic than EVOH, its nanocomposites exhibit enhanced interfacial interactions through hydrogen bonding, due to adsorption of the polymer molecules on the clay surface (Grunlan et al., 2004; Ogata et al., 1997). The morphologies observed for these composites show both exfoliated and intercalated structures that are well known for a number of other polymer±clay systems. The macro-


280


scopic composite properties are greatly affected by the size and morphology of the dispersed clay and, for instance, the thermomechanical properties of PVOH have been observed to increase or decrease with clay loadings, depending on the preparation conditions employed (Grunlan et al., 2004; Ogata et al., 1997; Strawhecker and Manias, 2000). It has been observed that sodium-exchanged montmorillonite clays are more easily dispersed in PVOH matrices than alkyl ammonium ion-exchanged clays (Chang et al., 2003), leading to significant increases in the Tg of the materials, and thus to increased rigidity, with just 1 wt% clay additions (Bandi and Schiraldi, 2006). The previous authors observed that increasing the amount of clay led to decreases in the glass transition temperature and explained the relative changes in Tg as being the result of two competing effects: on one hand the surface interaction between the polymer and the clay which strengthens the interface (decreasing chain mobility), and on the other hand, enhanced interfacial free volume due to the lower bulk crystallinity of polymer chains (increasing chain mobility) (Bandi and Schiraldi, 2006). A similar trend in Tg of PVOH nanocomposites has been reported by other authors (Ogata et al., 1997). However, as explained previously, the method of preparation is crucial in the final characteristics of the materials and, thus, some studies show improved properties of the hybrid materials at higher clay loadings (Chang et al., 2003; Grunlan et al., 2004). For instance, Grunlan and co-workers (2004) observed a significant reduction in oxygen permeability at 55% RH in PVOH with sodium montmorillonite with 10 wt% clay loadings. This low permeability was ascribed to the great interaction between the polymer and the clay as evidenced by an increase of more than 10ëC in the glass transition temperature at this clay concentration. Strawhecker and Manias (2000) observed significant enhancements in thermal, mechanical and barrier properties of PVOH with 5 wt% sodium montmorillonite clay addition. Interestingly, a 60% water permeability reduction was seen for these hybrid materials (Strawhecker and Manias, 2000). The ability to reduce the oxygen and water vapour permeability of PVOH-based systems at elevated humidity may prove advantageous for applications in food packaging, where moisture sensitivity currently prevents them from being used.

10.7

Future trends

The use of EVOH-based resins is expected to further expand in the food packaging sector, due both to the inherently excellent properties of these copolymers and to the newly developed solutions (including blending with other materials and generation of nanocomposites) to counteract the drawbacks that these materials present. Due to the generally poor miscibility of EVOH with other polymers, it is foreseen that the nanocomposite route would be the preferred one, which can also have an impact on the water barrier properties of the copolymers.



281

On the other hand, the plasticization of EVOH in the presence of humidity can be exploited for the development of novel active packaging structures which contain active substances incorporated in the polymeric structure. Active packaging is certainly one of the most important innovations in the packaging field for food preservation and is covered in other corresponding chapters. Active packages are designed to perform a role other than to provide an inert barrier between the product and the outside environment, using the possible interactions between food and the packaging in a positive way to improve product quality and acceptability (Fernandez Alvarez, 2000). Contact of the EVOH with water or plasticization of the structure at elevated relative humidity can serve to trigger the active mechanism, either by favouring the release of the active agents towards the food product, or by facilitating the interaction of gases that need to be removed from package headspace with the scavenging elements incorporated into the packaging structure. Research into this area is steadily increasing and EVOH matrices have a great potential for the development of novel active packaging concepts. Therefore, in the food packaging area, EVOH materials have a very promising future for a wide variety of food products.

10.8

References

Alexandre, M., Dubois, P. (2000). Polymer-layered silicate nanocomposites: preparation, properties and uses of a new class of materials. Materials Science and Engineering 28, 1±63. Artzi, N., Nir, Y., Wang, D., Narkis, M. (2001). EVOH/clay nanocomposites produced by melt processing. Polymer Composites 22, 710±720. Artzi, N., Narkis, M., Siegmann, A. (2004). EVOH/clay nanocomposites produced by dynamic melt mixing. Polymer Engineering and Science 44, 1019±1026. Artzi, N., Tzur, A., Narkis, M., Siegmann, A. (2005). The effect of extrusion processing conditions on EVOH/clay nanocomposites at low organo-clay contents. Polymer Composites 26, 343±351. Aucejo, S., Marco, C., Gavara, R. (1999). Water effect on the morphology of EVOH copolymers. Journal of Applied Polymer Science 74, 1201±1206. Azuma, K., Hirata, T., Tsunoda, H., Ishitani, T., Tanaka, Y. (1983). Identification of the volatiles from low-density polyethylene film irradiated with an electron-beam. Agricultural and Biological Chemistry 47, 855±860. Bandi, S., Schiraldi, D.A. (2006). Glass transition behavior of clay aerogel/poly(vinyl alcohol) composites. Macromolecules 39, 6537±6545. Bonner, J.G., Powell, A.K. (1997). In: Proceedings of 213th National ACS Meeting, ACS Materials Chemistry Publications, Washington, DC and San Francisco, CA. Cabedo, L., GimeÂnez, E., LagaroÂn, J.M., Gavara, R., Saura, J.J. (2004). Development of EVOH±kaolinite nanocomposites. Polymer 45, 5233±5238. Cabedo, L., LagaroÂn, J.M., Cava, D., Saura, J.J., GimeÂnez, E. (2006). The effect of ethylene content on the interaction between ethylene-vinyl alcohol copolymers and water ± II: Influence of water sorption on the mechanical properties of EVOH copolymers. Polymer Testing 25, 860±867.


282


Cava, D., Cabedo, L., GimeÂnez, E., Gavara, R., LagaroÂn, J.M. (2006). The effect of ethylene content on the interaction between ethylene±vinyl alcohol copolymers and water: (I) Application of FT-IR spectroscopy to determine transport properties and interactions in food packaging films. Polymer Testing 25, 254±261. Cerrada, M.L., Perez, E., PerenÄa, J.M., Benavente, R. (1998). Wide-angle X-ray diffraction study of the phase behavior of vinyl alcohol ethylene copolymers. Macromolecules 31, 2559±2564. Chang, J.-H., Jang, T.-G., Ihn, K.J., Lee, W.-K., Sur, G.S. (2003). Poly(vinyl alcohol) nanocomposites with different clays: pristine clays and organoclays. Journal of Applied Polymer Science 90, 3208±3214. Cho, J.W., Paul, D.R. (2001). Nylon 6 nanocomposites by melt compounding. Polymer 42, 1083±1094. Del Nobile, M.A., Mensitieri, G., Nicolais, L., Sommazzi, A., Garbassi, F. (1993). Gastransport properties of ethylene/propylene/carbon monoxide polyketone terpolymer. Journal of Applied Polymer Science 50, 1261±1268. Drent, E., Budzelaar, P.H.M. (1996). Palladium-catalyzed alternating copolymerization of alkenes and carbon monoxide. Chemical Reviews 96, 663±681. Drent, E., van Broekhoven, J.A.M., Doyle, M.J. (1991). Efficient palladium catalysts for the copolymerization of carbon monoxide with olefins to produce perfectly alternating polyketones. Journal of Organometallic Chemistry 417, 235±351. Fernandez Alvarez, M. (2000). Review: Active food packaging. Food Science and Technology International 6, 97±108. Foster, R.H. (1991). Improved ethylene vinyl alcohol copolymer (EVOH) resins for high barrier plastics packaging. Conference Proceedings of ANTEC 91, Montreal. Granado, A., Eguiazabal, J.I., Nazabal, J. (2004). Solid-state structure and mechanical properties of blends of an amorphous polyamide and a poly(amino-ether) resin. Macromolecular Materials and Engineering 289, 281±287. Grunlan, J.C., Grigorian, A., Hamilton, C.B., Mehrabi, A.R. (2004). Effect of clay concentration on the oxygen permeability and optical properties of a modified poly (vinyl alcohol). Journal of Applied Polymer Science 93, 1102±1109. Jeong, H.M., Kim, B.C., Kim, E.H. (2005). Structure and properties of EVOH/ organoclay nanocomposites. Journal of Materials Science 40, 3783±3787. Kothapalli, A., Sadler, G. (2003). Determination of non-volatile radiolytic compounds in ethylene co-vinyl alcohol. Nuclear Instruments and Methods in Physics Research B 208, 340±344. Kotsilkova, R., Petkova, V., Pelovski, Y. (2001). Thermal analysis of polymer-silicate nanocomposites. Journal of Thermal Analysis and Calorimetry 64, 591±598. LagaroÂn, J.M., Gimenez, E., Saura, J.J., Gavara, R. (2001). Phase morphology, crystallinity and mechanical properties of binary blends of high barrier ethylenevinyl alcohol copolimer and amorphous polyamide and a polyamide-containing ionomer. Polymer 42, 7381±7394. LagaroÂn, J.M., Gimenez, E., Altava, B., Del-Valle, V., Gavara, R. (2003a). Characterization of extruded ethylene-vinyl alcohol copolimer based barrier blends with interest in food packaging applications. Macromolecular Symposia 198, 473± 482. LagaroÂn, J.M., Gimenez, E., Catala, R., Gavara, R. (2003b). Mechanisms of moisture sorption in barrier polymers used in food packaging: Amorphous polyamide vs. high-barrier ethylene±vinyl alcohol copolymer studied by vibrational spectroscopy. Macromolecular Chemistry and Physics 204, 704±713. LagaroÂn, J.M., Catala, R., Gavara, R. (2004). Structural characteristics defining high



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barrier properties in polymeric materials. Materials Science and Technology 20, 1±7. LagaroÂn, J.M., Cabedo, L., Cava, D., Feijoo, J.L., Gavara, R., Gimenez, E. (2005). Improving packaged food quality and safety. Part 2: Nanocomposites. Food Additives and Contaminants 22, 994±998. Lai, T.W., Sen, A. (1984). Transition-metal catalyzed copolymerization of carbonmonoxide with olefins. 2. Palladium(II)-catalyzed copolymerization of carbon monoxide with ethylene ± direct evidence for a single mode of chain growth. Organometallics 3, 866±870. Ledward, A.D. (1995). High pressure processing ± the potential. In A.D. Ledward, D.E. Johnston, R.G. Earnshaw and A.P.M. Hasting (eds), High Pressure Processing of Foods. Nottingham, UK: Nottingham University Press, pp. 1±5. Lee, S.-S., Lee, C.S., Kim, M.-H., Kwak, S.Y., Park, M., Lim, S.H., Choe, C.R., Kim, J. (2001). Specific interaction governing the melt intercalation of clay with poly(styrene-co-acrylonitrile) copolymers. Journal of Polymer Science Part B ± Polymer Physics 39, 2430±2435. Lee, S.-S., Hur, M.H., Yang, H., Lim, S., Kim, J. (2006). Effects of interfacial attraction on intercalation in polymer/clay nanocomposites. Journal of Applied Polymer Science 101, 2749±2753. Lopez-Rubio, A., LagaroÂn, J.M. (2008). Improving the resistance to humid heat sterilization of EVOH copolymers through blending. Journal of Applied Polymer Science 109, 174±181. Lopez-Rubio, A., LagaroÂn, J.M., GimeÂnez, E., Cava, D., Hernandez-MunÄoz, P., Yamamoto, T., Gavara, R. (2003). Morphological alterations induced by temperature and humidity in ethylene±vinyl alcohol copolymers. Macromolecules 36, 9467±9476. Lopez-Rubio, A., Hernandez-MunÄoz, P., GimeÂnez, E., Yamamoto, T., Gavara, R., LagaroÂn, J.M. (2005a). Gas barrier changes and morphological alterations induced by retorting in ethylene vinyl alcohol-based food packaging structures. Journal of Applied Polymer Science 96, 2192±2202. Lopez-Rubio, A., LagaroÂn, J.M., HernaÂndez-MunÄoz, P., Almenar, E., Catala, R., Gavara, R., Pascall, M.A. (2005b). Effect of high pressure treatments on the properties of EVOH-based food packaging materials. Innovative Food Science and Emerging Technologies 6, 51±58. Lopez-Rubio, A., Hernandez-MunÄoz, P., Catala, R., Gavara, R., LagaroÂn, J.M. (2005c). Improving packaged food quality and safety. Part 1: Synchrotron X-ray analysis. Food Additives and Contaminants 22, 988±993. Lopez-Rubio, A., Gimenez, E., Gavara, R., LagaroÂn, J.M. (2006a). Gas barrier changes and structural alterations induced by retorting in a high barrier aliphatic polyketone terpolymer. Journal of Applied Polymer Science 101, 3348±3356. Lopez-Rubio, A., Gavara, R., LagaroÂn, J.M. (2006b). Unexpected partial crystallization of an amorphous polyamide as induced by combined temperature and humidity. Journal of Applied Polymer Science 102, 1516±1523. Lopez-Rubio, A., LagaroÂn, J.M., Yamamoto, T., Gavara, R. (2007). Radiation-induced oxygen scavenging activity in EVOH copolymers. Journal of Applied Polymer Science 105, 2676±2682. Lyatskaya, Y., Balazs, A.C. (1998). Modeling the phase behavior of polymer-clay composites. Macromolecules 31, 6676±6680. Ogata, N., Kawakage, S., Ogihara, T. (1997). Poly (vinyl alcohol)±clay and poly (ethylene oxide)±clay blends prepared using water as solvent. Journal of Applied


284


Polymer Science 66, 573±581. Palou, E., LoÂpez-Malo, A., Barbosa-CaÂnovas, G.V., Swanson, B.G. (1999). High-pressure treatment in food preservation. In M.S. Rahman (ed.), Handbook of Food Preservation. New York: Marcel Dekker, pp. 533±576. Ramesh, M.N. (1999). In M.S. Rahman (ed.), Handbook of Food Preservation. New York: Marcel Dekker, p. 95. Riganakos, K.A., Koller, W.D., Ehlermann, D.A.E., Bauer, B., Kontominas, M.G. (1999). Effects of ionizing radiation on properties of monolayer and multilayer flexible food packaging materials. Radiation Physics and Chemistry 54, 527±540. Sommazzi, A., Garbassi, F. (1997). Olefin carbon monoxide copolymers. Progress in Polymer Science 22, 1547±1605. Strawhecker, K.E., Manias, E. (2000). Structure and properties of poly(vinyl alcohol)/ Na+ montmorillonite nanocomposites. Chemical Materials 12, 2943±2949. Takahashi, M., Tashiro, K., Amiya, S. (1999). Crystal structure of ethylene±vinyl alcohol copolymers. Macromolecules 32, 5860±5871. Tsai, B.C., Jenkins, B.J. (1988). Effect of retorting on the barrier properties of EVOH. Journal of Plastic Film and Sheeting 4, 63±71. Tsai, B.C., Wachtel, J.A. (1990). In: W.J. Koros (ed.), Barrier Polymers and Structures, American Chemical Society, Washington, DC, pp. 192±202. Villanueva, M.P., Cabedo, L., LagaroÂn, J.M., Gimenez, E. (2010). Comparative study of nanocomposites of polyolefin compatibilizers containing kaolinite and montmorillonite organoclays. Journal of Applied Polymer Science 115, 1325±1335. Wan, C., Qiao, X., Zhang, Y., Zhang, Y. (2003). Effect of different clay treatment on morphology and mechanical properties of PVC±clay nanocomposites. Polymer Testing 22, 453±461. Zanetti, M., Camino, G., Thomann, R., MuÈlhaupt, R. (2001). Synthesis and thermal behavior of layered silicate±EVA nanocomposites. Polymer 42, 4201±4207. Zhang, Z., Britt, I.J., Tung, M.A. (2001). Permeation of oxygen and water vapour through EVOH films as influenced by relative humidity. Journal of Applied Polymer Science 82, 1866±1872. Zhao, A.X., Chien, J.C.W. (1992). Palladium catalyzed ethylene±carbon monoxide alternating copolymerization. Journal of Polymer Science Part A ± Polymer Chemistry 30, 2735±2747.


11

High barrier plastics using nanoscale inorganic films V. TEIXEIRA, J. CARNEIRO, P. CARVALHO, E . S I L V A , S . A Z E V E D O and C . B A T I S T A , University of Minho, Portugal

Abstract: Packaging materials for food containers need to fulfil extremely tight standards towards the fresh-like quality maintenance of packed products. Even though polymers are normally preferred for the production of packaging systems, their permeability to gases, water vapour and odours remains a concern. In this sense, new approaches using nanoscale effects are under development to design, create or model gas-barrier nanocoatings with significantly optimized properties. It is generally agreed that the final barrier performances of the deposited inorganic materials are strongly coupled with mechanical and morphological properties. Basically, a good barrier system should have a dense morphology without cracks, good adhesion to the substrate, low stress, uniform thickness and reproducibility. At the moment, researchers aim for development of cost effective mass production techniques. Key words: food industry, shelf-life, flexible polymers, nanotechnology, nanopackaging systems, gas-barrier nanocoatings, thin films, inorganic materials, deposition techniques, mechanical properties, diffusion mechanisms, WVTR, OTR.

11.1

Introduction

Food packaging is a constant presence in the daily life of all individuals who make up so-called modern societies in developed countries. In recent years, food preparation and consumption habits have changed with people's lifestyles, leading to a considerable increase in the market supply of pre-prepared and packaged food. Consequently, high quality packaging is an important tool for the safety and well-being of people as well as for the successful marketing of food products. Basically, the protective materials used in the food industry aim to extend shelf-life, preserving products from air (and oxygen), light (and UV radiation), microbial contamination, loss of gas (e.g. carbonated beverages), storage temperature variation, foreign aroma compounds, moisture loss/incorporation and mechanical influences [1]. As a conclusion, the packaging must ensure that the interaction between the environment and the packaged food is minimal.


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To get together all these requirements, different packaging concepts have been introduced into the food market. In order to understand the real importance of food packaging, it must be seen not only from the consumer's point of view but also from the market one. As stated, the market needs have changed: consumers are more selective, critical and rigorous in their concern for fresh-like qualities and, at the same time, there have been considerable changes in retail and distribution practices. The centralization of activities, new trends like shopping via the Internet and internationalization of markets have led to a considerable increase in distribution distances as well as to longer storage times of different products with different temperature requirements, which pushes the food packaging industry to higher limits in terms of product quality. For this reason novel nanomaterials for flexible packaging systems are being developed to increase the safety of foods and keep their natural characteristics, among them bioactive packaging, inorganic nanocoatings and smart labels based on nanotechnology concepts. Polymers are a common choice as protective materials since they combine flexibility, variable sizes and shapes, relative light weight, stability, resistance to breaking, barrier properties and perceived high-quality image with costeffectiveness. But still, traditional packaging concepts are limited in their ability to prolong the shelf-life of food products. Nanobiocomposites of biopolymers containing low additions (below 8%) of modified food-compliant nanolayered clays and bionanofibres can have a tremendous positive impact on a number of physical properties such as barrier properties to gases and vapours, UV protection and mechanical and thermal properties without significant losses in transparency and toughness, that can lead to enhance quality and safety of packaged foods. Moreover, these inorganic structures can also be surface modified to become active and bioactive nanoadditives, which by integration in biomaterials, can lead to novel packaging materials, coatings and encapsulates with active (antimicrobial, antioxidant or oxygen scavenging capacity) and bioactive properties (protection of functional ingredients such as body antioxidants, prebiotics and probiotics) which can more effectively enhance the quality, safety and health aspects of foods and packaged foods. In the field of nanotechnology-based thin films and nanostructured coating materials, new approaches using nanoscale effects will be used to design and create advanced nanocoating systems with significantly optimized or enhanced properties of high interest to the food, health and even biomedical industries. With the development of nanotechnology in various areas of materials science, there is great potential in the use of novel functional surfaces and more reliable nanomaterials by employing nanocomposite and nanostructured thin films in food packaging, security pharmaceutical labels and novel polymeric containers for food contact, as well as in applying these concepts to medical surface instruments, bio-implants, and even coated nanoparticles for bionanotechnology. During recent years, the food industry has been investing millions of dollars in



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11.1 Chart presenting some nanostructured materials used in food packaging.

R&D towards the application of nanoscience and nanotechnology. Some names like NestleÂ, Altria, H.J. Heinz and Unilever are at the top of the list, followed by hundreds of smaller companies. Currently, various technologies are the subject of intense investigation in both the food packaging and food security sectors. The incorporation of nanostructured materials can, for example, enhance the barrier properties of polymers while reducing the use of raw materials and, therefore, waste generation. Different applications of nanomaterials can be listed: (1) improved packaging (gas and moisture barriers, tensile strength); (2) shelf-life extension (via active packaging); (3) nanoadditives and nanofilms; (4) delivery and controlled release of nutraceuticals; (5) antibacterial (or self-cleaning) packaging and (6) monitoring product conditions during transportation using different nanosensing devices [2]. Figure 11.1 illustrates some of the nanotechnologies under intense research.

11.2

Nanotechnologies of thin films for advanced food packaging

Generally, untreated materials used in the design of flexible food packages are permeable to gases (oxygen, carbon dioxide, etc.), water vapour and odours [3, 4]. Packaging materials should present good barrier performance, highquality mechanical properties, sealability and cost-efficiency [5]. Microwave compatibility and package transparency are further important parameters pointed out by consumers.


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In vacuum coating of plastic films, which form the majority of roll-to-roll vacuum web coating, about 15 billion square metres per year are presently coated worldwide, with a continuous, long-term growth over the last decades. In this area, packaging films today represent about two-thirds of the overall volume. The rest is covered by technical and decorative applications. In packaging, the most important material used is still aluminium in the form of sheet or as thin film material thermally evaporated. However, it is not transparent and has a negative environmental impact. In addition, a new market perspective is seen in transparent and flexible barrier layers on the basis of oxides based on silicon and/or aluminium. In order to meet all the requirements listed, nowadays the package has a combination of different layers (each with a certain specification), the most critical barrier being the one that represents the highest fraction of the total cost of the laminate. In this way, R&D strategies have moved towards the development of coated polymers with thin films of transparent metal oxides to be used in advanced food package assembly. Silicon oxide (SiOx) coated polymers, e.g. poly(ethylene terephthalate), have been studied as particularly useful diffusion barrier films due to their low oxygen transmission rate and high transparency. A good barrier system should present certain requirements, such as dense morphology without cracks, pinholes or other defects, good adhesion to the substrate, low stress, uniform thickness and reproducibility [6]. Gas-barrier systems for food packaging are developed to prevent oxidative processes in order to assure long-term high-quality products. Typical permeation rates of uncoated to coated polymer of the order of 100 [7] are achieved with 15 nm thick aluminium coatings formed by resistive evaporation [8]. In food packaging, transparent inorganic layers several dozen nanometres thick deposited onto polymers can produce excellent results in terms of transparency and barrier properties to oxygen, water vapour, carbon dioxide and aromas, increasing the shelf-life of the packed product. These transparent oxide-type coatings present a commercial interest for those packaging applications that require microwaveability or product visibility. At present, silicon oxide thin films are competitive with Al-coated polymer films which, though they present adequate barrier characteristics, are not transparent or suitable for microwave ovens [9]. Silicon oxide thin films have been produced since the early 1980s by means of plasma enhanced chemical vapour deposition (PECVD) and physical vapour deposition (PVD) [10]. Although SiOx nanocoatings suffer from problems that include cracking and poor adhesion to polymer substrates [11], different studies are still being carried out in order to improve the quality of SiOx based thin films as well as to change the substrate used. For industry, polypropylene (PP) can be an interesting substitute for PET as packaging polymer since it has an inherent water vapour barrier, low density and extremely low cost.



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At the same time, SiOx substitutes are now being considered. The most common coatings for this specific application are silicon, aluminium and titanium based thin films in the form of nitride, oxide, oxynitride and carbonitride [12±20]. It is believed that by changing the chemical structure of the transparent oxide film through the introduction of different chemical species, the intrinsic defects of the nanocoating can be modified, resulting in better barrier films. It has already been proved that silicon nitride (SiN) presents better barrier quality at lower thicknesses relative to the widely used SiOx [21, 22]. Erlat and co-workers produced aluminium oxynitride (AlOxNy) films by reactive magnetron sputtering on PET substrates. They attained nanocoatings with good oxygen barrier properties as well as excellent water vapour resistance (comparable to high-quality SiOx and AlOx layers). The authors also observed that the produced AlOxNy films have approximately the same barrier performance as the sputtered AlOx. On this basis, they concluded that AlOxNy can be a promising barrier layer material [14]. Gas-barrier properties are also very important for beverage containers produced in PET. The carbon dioxide gas in beer can easily escape through PET bottles and the ingredients of orange juice can easily be oxidized by incoming oxygen through the bottle [23]. Although silicon, aluminium and titanium alloy thin films were reported to be good gas-barrier materials, amorphous hydrogenated-carbon (a-C:H) films or diamond-like carbon (DLC) [24] were also considered good materials for food packaging purposes, due to their flexibility, recyclability and biocompatibility [25]. DLC is known to be a very hard, very low friction material that possesses low wear rate, excellent tribological properties and excellent corrosion resistance. It consists of dense amorphous carbon or hydrocarbon with high electrical resistivity, high refractive index and chemical inertness. The mechanical properties of DLC thin films are between those of graphite and diamond [26]. Generally, DLC films are produced by PECVD methods [27]. Finch and co-workers [28] coated the interior of PET bottles with 10±100 nm thick a-C:H films, improving their gas barrier properties by 20±30 times compared with uncoated bottles. Enhancing gas barrier performance extends the shelf-life of bottles since they can preserve the taste and quality of stored food/ drink in better conditions for a longer time [29]. These coated bottles are already on the market as beer containers in North America, Europe and Korea. Recently, under the Portugal±Spain International Nanotechnology Laboratory Nanotechnology Projects Call (Subject: Nanotechnologies; Topics: Security and Food Quality Control), a project in which University of Minho participation was approved. Entitled NanoPackSafer: NANO-engineered PACKaging systems for improving quality, SAFEty and health characteristics of foods (www.nanopacksafer.com) has been launched. Within the framework of the NanoPackSafer project, novel nanomaterials for flexible packaging systems are being developed to increase the safety of foods while keeping their


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natural characteristics, among them active packaging, inorganic coatings and edible coatings. Edible coatings and films are a thin layer of a biopolymer that is deposited on the surface of a food and is co-consumed. Until now it has been used to improve handling properties (`M&M'sÕ melt in your mouth, not in your hand'), to prevent moisture loss (wax coatings on fruits and vegetables), for seasonings on snack foods (e.g. as adhesive of salt on dry roasted peanuts), as a glaze on baked goods (instead of egg-based coatings), etc. Besides edible films, which are outside the scope of this chapter, the University of Minho is responsible for applying nanotechnology approaches to the development of non-edible films, such as novel thin films, nanoreinforced biopolymers and nanosensors. The main task of this R&D project involves the production and characterization of advanced nanocomposite oxide-based sputtered thin coatings on plastic packaging materials. The coating deposition systems available at the University of Minho are being used to produce novel nanocomposite metal oxides for high-performance barriers, for catalysis, and with antimicrobial capabilities that can minimize biological attachment and biofilm formation in the functional food pack. Another research task is concerned with the incorporation of sensorial functions (e.g. smart labels for `smart packaging'), such as the development of nanoparticle-based sensors made by soft templating methods and sensorial thin film systems. The active surface of the resulting nanoparticles will be functionalized with appropriate molecules in order to give the desired response to O2, CO2, pH or temperature [30±32].

11.3

Thin film technologies for polymer coating using vacuum processes

11.3.1 Basics of thin film preparation Thin film deposition technologies using vacuum processes are usually classified into two main classes due to the nature of the processes themselves: physical processes and chemical processes. Several different combinations of any of these processes have been tried because in many cases a single process is not able to give rise to films with the required properties. The number of these socalled hybrid processes has increased significantly during the last decade and is already virtually countless. Many of the processes have more than one name designation and sometimes there is an overlap in process mechanisms. For instance, for physical vapour deposition (PVD) processes, although there have been several attempts to classify deposition processes, the best is thought to be by Bunshaw and Mattox [33], which dates from 1970, with addition of advanced techniques [34]. In this section we will focus on and describe solely the processes used for depositing transparent oxide barrier films applied to flexible food packaging



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applications, although they are also applied in encapsulation of flexible devices such as organic light emitting diodes (OLEDS) or solar cells. The process principles and materials used in the different film technologies are discussed. The different vacuum processes are described and the advantages and disadvantages of each process are summarized. Thin film materials exhibit unique material properties resulting from the atomistic growth process which is clearly dependent on the deposition process and chosen parameters. The synthesis of a film prepared using vacuum processes comprises three main steps: (1) creation of the appropriate atomic, molecular, or ionic depositing species; (2) transport of the species from the source to the substrate; and (3) condensation of the depositing species on the substrate directly or via chemical reaction with reactive constituents, forming a solid deposit. In atomistic processes, the solid film is formed by condensation of the atoms in the vapour phase onto a substrate and migration to nucleation and growth sites. The adsorbed atoms, so-called adatoms, require energy enough to occupy their lowest possible energy configurations avoiding structural imperfections. The microstructure and morphology of the growing film is also a result of the energy of the atoms which in turn is dependent on the deposition process and respective parameters. The deposition processes that are presently being utilized for the deposition of transparent inorganic barrier film will be hereinafter described, though the influence of the parameters in each process on the final properties of the films will not be addressed.

11.3.2 Chemical vapour deposition (CVD) processes In a CVD process the solid film is deposited from a vapour by a chemical reaction occurring on or in the vicinity of a normally heated substrate surface. The experimental parameters such as power, precursor compositions and flows, working pressure, temperature, substrate material, etc. will dictate the final properties of the film. CVD processes are generally classified and categorized according to the characteristics of the processing parameters such as temperature, pressure, wall/substrate temperature, precursor nature, depositing time, gas flow state and activation manner [35]. Herein, we will explore solely the most common CVD processes found in the literature concerning the production of thin films for gas-barrier application applied to food packaging. The most common CVD processes are plasma-enhanced chemical vapour deposition (PECVD), atmospheric-pressure chemical vapour deposition (APCVD) and catalytic chemical vapour deposition (Cat-CVD) also designated hot-wire chemical vapour deposition (HW-CVD). In PECVD the chemical process is activated by plasma generation, in APCVD the process, as stated by its designation, is conducted at atmospheric pressure, and in Cat-CVD source gases are decomposed by the catalytic cracking reaction with heated catalyser.


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Plasma enhanced CVD (PECVD) PECVD is a widely used technique to obtain device quality thin films at low substrate temperatures [36]. In PECVD, source gases are decomposed in plasma by the collisions between energetic electrons and gas molecules. For the deposition of SiOx films, the plasma promotes the decomposition of silicon source gases (silane (SiH4), tetramethoxysilane (TMOS) or hexamethyldisiloxane (HMDS)) to silicon radicals and allows them to react with oxygen radicals that issue from oxygen or nitrous oxide (N2O) source gases [37]. PECVD processes could be operated using microwave (2.45 GHz), AC (50 Hz), MF (kHz range) or RF (13.56 MHz) electrical power supplies, the latter being the most common. A schematic diagram of a RF PECVD system used to prepare SiOx films onto PET below 100ëC for food packaging products [18] is illustrated in Fig. 11.2. The system comprises a vacuum chamber where the deposition process takes place, and an evacuating system composed of a rotary mechanical pump for the primary vacuum level and to backup a turbomolecular pump which then takes the pressure down to 10ÿ4 Pa. The feeding of gases (tetramethoxysilane (TMOS) and oxygen) is controlled by a mass flow controller which dictates the partial pressures of the source gases. The initiation of the plasma and decomposition of the source gases are accomplished with a power supply operating at the RF (13.56 MHz) and require an impedance matching network. A low-pressure microwave plasma reactor system based on a plasmaline antenna, shown in Fig. 11.3, has been used for plasma deposition of SiOx

11.2 Schematic diagram of a RF PECVD system, from Ref. [18].



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11.3 Schematic illustration of the plasmaline antenna reactor system used for SiOx coating of PET bottles, from Ref. [39].

coatings in PET bottles [38, 39]. In this approach, liquid HMDS is evaporated as process gas for deposition of barrier coatings and is fed into the chamber as a mixture with oxygen. Microwave power is applied to the system by means of a modified plasmaline antenna operating at 2.45 GHz. This antenna consists of a copper tube, which is the inner conductor of the microwave antenna, with surrounding inner (also used to deliver the feeding gases) and outer quartz tube. The bottles are inserted upside-down, allowing the copper tube inside the bottle. The plasma is generated inside the PET bottle along with the source gases, which makes the deposition restricted to the bottle interior. Various kinds of inorganic films with gas barrier properties can be prepared by PECVD by simply varying the gas sources and their ratios. PECVD has been successfully used to prepare silicon suboxide (SiOx) films [18], silicon nitride (SiNx) films [40], silicon oxynitride SiOxNy coatings [41], and more recently ternary SiCxNy coatings [13] with improved barrier properties. SiOxCyHz films [42] have also been prepared by this method, although they revealed poor barrier performances. The nature of the power source generated plasma may also be an important factor in influencing the final properties of the films. As reported by Zhang et al. [43], the use of medium-frequency (MF) power generated plasma resulted in SiO2 coatings instead of the low purity polymer-like SiOxCyHz obtained in RF plasma. PECVD is a cold plasma deposition technique and is moderately useful for coating heat-sensitive materials such as polyimide (PI) [40], poly(ethylene 2,6-naphthalate) (PEN) [13], polyethylene terephthalate (PET) [44], and polyethersulfone (PES) [45]. However, it seems to be limited until now to silicon-based coatings and DLCs. Furthermore, the speed of deposition is still far below that of thermal evaporation.


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Atmospheric pressure CVD (APCVD) APCVD is a novel technique that has the great advantage of not needing vacuum for the deposition of films, therefore no vacuum chamber and vacuum equipment are required for the process. Operating at a power source frequency over 1 kHz, inserting dielectric plates between metal electrodes, and using helium as the dilution gas, it is possible to obtain an atmospheric-pressure glow (APG) plasma stabilized in air, argon, oxygen and nitrogen [46]. The APG plasma technique enabled development of a low-cost line-type production system. This technique has been used to deposit SiOx films [47]. These films, prepared from tetraethoxysilane (TEOS)/air, showed low porosity and high hardness and transparency [48]. APCVD has also been used to deposit DLC films with gasbarrier properties inside PET bottles [29]. The gas-barrier properties of AP-DLC films ~1 m thick were 5±10 times those of uncoated PET substrates [48]. Catalytic CVD (Cat-CVD) Cat-CVD, often called hot-wire CVD [36], is a recent low-temperature deposition technique working without any help from plasma. In Cat-CVD, source gases are decomposed by the catalytic cracking reaction with heated catalyser, usually a heated tungsten wire placed near substrates [36]. The decomposition mechanism, being different from that of a PECVD method, results in films with also different properties. SiNx and SiOxNy have been obtained by this technique for barrier purposes [17, 49]. SiNx films on polymeric substrates revealed high gas-barrier ability as well as high transparency and low stress. SiOxNy is a very recent achievement with this technique and along with SiNx in a multilayered architecture has low water vapour transmission rates [17]. The preparation of such films was limited to the use of SiH4 but now HMDS, which is safe and inexpensive, is found to be an effective alternative.

11.4

Physical vapour deposition (PVD) processes

The use of PVD processes has been increasing rapidly since current technology requires engineered materials with several distinctive properties, often incompatible, combined in the same product. Such processes are very flexible, allowing the deposition of practically every type of inorganic materials: metals, alloys, compounds and also some organic materials. This is perhaps the major advantage over any other deposition process. Moreover, these processes are appropriate to form multilayer coatings, graded composition deposits, very thick deposits and freestanding structures. In CVD processes, the three steps for the preparation of a film referred to in the previous section ± creation of the vapour phase, transport and condensation ± occur simultaneously at the substrate and cannot be independently controlled. Unlike in PVD processes, these steps can be independently controlled and



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therefore have a much greater degree of flexibility in controlling the structure and properties, and the deposition rate. There are several other advantages of PVD processes over CVD: versatility in composition of film; aptitude for the preparation of unusual microstructures and crystallographic modifications; films can have a very low contamination level; films have excellent adhesion to the substrate and excellent surface finish; the substrate temperature can go from subzero to high temperatures; and absence of pollutants in the process, which is a very important ecological factor. However, there are also some limitations, including the lack of ability to deposit most of the polymeric materials, the difficulty in coating complex substrate shapes, and the high initial cost of the processing equipment.

11.4.1 Evaporation In the evaporation process, the vapour phase is obtained from the source material being heated by direct resistance, radiation, eddy currents, e-beam, laser beam, or an arc discharge [34]. The vaporized material is transported in vacuum, typically at 10ÿ5±10ÿ6 mbar, in a straight line for a given distance (which depends on the gas pressure) prior to condensation on the substrate. As reported in a review paper on inorganic coatings on polymers, this deposition method was the first used for barrier purposes [37].

11.4.2 Electron beam evaporation (EBE) This process is particularly appropriate for the evaporation of materials with very high melting temperatures. Commercially available sources typically use magnetic deflection to direct an intense electron beam into a water-cooled crucible which contains the material to be evaporated. Scanning, if available, allows the beam to sweep over the charged surface, improving material usage and extending source lifetime [50]. The common problems associated with electron beam deposition are short filament (source of electrons) lifetime, deposition non-uniformity, and spatter. EBE systems involve similar costs to sputtering but the deposition rates are higher; however, the thickness control of the films is not as precise as in a sputtering system. A ten-fold reduction in the oxygen permeation rate has been observed in SiO films prepared by electron beam reactive evaporation in the presence of oxygen. In other studies, reductions of 60-fold in the oxygen permeation rate through 12 m PET were obtained with SiOx coatings deposited by electron-beam reactive evaporation of silicon monoxide in the presence of oxygen [7]. SiOx films can be also prepared directly by EBE from raw Si±O material without the presence of an oxygen atmosphere [19, 51]. It is also possible to prepare Al2O3 films by EBE from an Al2O3 source. However, the deposition rates obtained are low: 0.3±0.5 nm/min at ~6 kV.


296


11.4.3 Sputter deposition Sputter deposition is a non-thermal vaporization process which uses a physical phenomenon to produce the microscopic spray effect [34]. When a fast ion strikes the surface of a material (target), atoms of that material are ejected by a momentum transfer process, as illustrated in Fig. 11.4. As with evaporation, the ejected atoms or molecules can be condensed on a substrate to form a surface coating. When the process involves a partial pressure of a reactive gas which reacts with the sputtered material to form a compound surface coating, the process is called reactive sputter deposition. This is, in fact, the case when depositing gas barrier layers such as SiOx or SiNxOy from a Si target, using O2 (or O2 + N2) as the reactive gas. Reactive magnetron sputtering In this process Ar gas is introduced as sputtering working gas which will be ionized and accelerated towards the target material. The planar sputtering source with no magnetic enhancement (diode sputtering) has a high loss of electrons and hence a poor ionization efficiency. The result of this is that the power supply to drive the source needs to deliver the required current at around 2 kV [52]. The use of a magnetron allows trapping of the electrons by the magnetic field lines close to the sputtering target in a well-defined region: see Fig. 11.5. Therefore, electrons stay within the plasma for a considerably longer time, increasing the probability of ionizing the working gas. This increase in ionizing efficiency results in a denser plasma capable of carrying a much higher current at a significantly low voltage. Magnetrons can be configured in several different forms such as planar in a variety of shapes or cylindrical. Moreover, they can operate in conjunction with other magnetrons or other vacuum deposition sources, simultaneously or alternately, for instance for deposition of composite or multilayer films from different material sources, respectively. Magnetron sputtering cathodes can be run using AC or DC power supplies. The most common AC power supplies

11.4 Schematics of the physical sputtering process: a collision of an accelerated ion with a target surface.



297

11.5 Configuration of a magnetron cathode.

usually operate in the RF range at 13.56 MHz although medium frequency (MF) range is also used. DC power can be supplied to the cathode with a negative constant voltage, which is the conventional method, or pulsed in the medium frequency (kHz) range. The power pulsing can be unipolar, which allows only negative voltage pulses to the target, or bipolar, which alternately switches from negative to positive voltage during the process. This way the target is frequently alternating between sputtering and positive charge dissipation. This is of particular interest when dealing with conductive targets which form insulator films in a reactive atmosphere. RF power supplies are more expensive than DC ones and require a complex impedance matching system [50, 53]. Moreover, the deposition rate with RF power is very low, about half the rate for DC power, for an equivalent amount of power [53]. Figure 11.6 shows a typical planar magnetron system operated by DC power. A technique known as successive pulsed plasma anodization (SPPA) has been used to deposit AlOx gas barrier films on PET substrates [20]. This technique makes use of a coaxial dual magnetron system in which a thin metal layer is firstly deposited, in this case Al, and subsequently anodized through self-biased oxygen plasma. The cycle is repeated until the desired oxide thickness is achieved. SiOx thin films have been deposited on PET films by reactive sputtering using a DC magnetron sputtering from a Si target and in an atmosphere of Ar + O2 [54]. Results showed that low oxygen concentrations are preferable since less micro-defects are observed in the films and consequently the oxygen transmission rates are lower. SiON films on PET prepared in N2 + Ar atmosphere from Si target showed oxygen transmission rates even lower than those of SiOx films [15]. The films revealed a fine and amorphous structure without pinholes or cracks. Other studies [55] on SiOx gas barrier layers deposited on PC film by a roll-to-roll DC magnetron reactive sputtering method revealed an extremely smooth surface and excellent gas barrier performance against moisture and oxygen with a 40 nm thick film. Reactive pulsed DC sputtering in double-ring magnetron is also an effective method for SiO2, Si3N4 and SiOxNy single barrier layers [56]. Al2O3 films were reactively deposited


298


11.6 Schematic illustration of a planar magnetron sputtering system using DC discharge current.

from Al targets onto PET by AC magnetron sputtering in a dual-magnetron rollto-roll process, for OLED devices [16]. With a layer thickness of 200 nm, the permeation barrier can be improved by nearly three orders of magnitude. AlOxNy are also a possibility as gas barrier films and, when produced by medium frequency (40 kHz) AC magnetron sputtering, utilizing a coaxial dual magnetron source with Al targets, seem to be a promising barrier layer material, demonstrating highly competitive gas permeation values for oxygen and water vapour [14]. TiNxOy films were deposited on PET substrates from pure Ti and N2 + Ar atmosphere by means of RF magnetron sputtering and applying substrate bias [57]. The deposited films exhibited an amorphous or a columnar structure with fine crystallites dependent on power density. The residual O2 plus the eventual O2 coming from the PET substrate are enough to form the oxynitride. Ion beam sputter deposition (IBD/IBAD) Ion beam sputtering uses an ion source to generate a relatively focused ion beam direct at the target to be sputtered. The ion source comprises both the cathode and the anode which are concentrically aligned. With application of a high



299

voltage field of 2±10 kV an electrostatic field is created inside the ion source, confining electrons in the centre of the source. When argon gas is injected into the ion gun, the high electric field causes the gas to ionize, creating a plasma inside the source region. The ions are then accelerated from the anode region to the exit aperture (cathode), creating a collimated ion beam. The resulting ion beam impinges upon a target material and, via momentum transfer between the ion and the target, sputters the target material towards the substrate. A second ion gun may be also used to assist the deposition (ion beam assisted deposition ± IBAD) by bombarding the growing film. Dual ion beam sputtering utilizing ion beam assisted deposition conditions can improve the adhesion, density, control of the stoichiometry, and low optical absorption (at short wavelengths) in thin films [58]. Low-energy reactive ion beam bombardment can increase the rate of compound formation, control the stoichiometry and improve adhesion of the deposited thin film. SiOx films prepared by sputtering by 1 keV Ar ion beam from a Si target and assisted by a second Ar + O2 ion beam [58] revealed a dense amorphous nature when oxygen content is 20% and presented an oxygen transmission rate lower than that obtained by CVD and e-beam processes.

11.5

Inorganic thin film systems

Nowadays, high food quality is a requirement in most developed societies. Consumers acquire packaged goods that, generally, are produced thousands of kilometres away. This has been the most important driving force for research and improvement of packages characteristics. Polymeric materials play an important role in the food packaging industry and can be found in several forms such as films, bottles and other kinds of packages. The most important reasons for this fact are their low cost, high flexibility and relatively good chemical stability. Table 11.1 summarizes the most common polymers used for food packaging. From this table it is clear that these polymers present a poor barrier performance to oxygen. Table 11.1 Multilayered and composites systems and O2 permeability Polymer

Permeability, PO2 (1016 cm3 (STP).cm/cm2/s/Pa)

LDPE/EVOH blends PET/EVOH blends Polypropylene/polyamide 6 blends PET copolymer/talc 32 wt% composite Polyethylene/mica 10 wt% composite Polyamide 6/layered silicate nanocomposite Source: ref. [59].


3.8 5.1 1.7 14 2000 10

300


To improve the barrier properties of polymers to oxygen and water vapour, several technologies have been developed, the most common being the deposition of inorganic thin films. In the last 20 years some prominent research groups have been working in this field, with the specific objective of developing materials and techniques for depositing metals or oxides in polymeric substrates [37]. Inorganic oxide coatings present some advantages over aluminium, such as optical transparency and microwaveability. In 1996 Chatham [7] presented a review of several inorganic materials deposited by different techniques. Table 11.2 summarizes probably the best reported results since 1996 for different inorganic systems, deposition techTable 11.2 Substrate/thin film systems and respective deposition techniques, oxygen transmission rate (OTR) and water vapour transmission rate (WVTR) in works reported between 1996 and 2003 Ref.

51 20 60 14 9 13 15 54 61 62 63 45 29 58 64 65 49 42 57 11 66 67 68

Coating material

Deposition technique

EB evap. SiOx Al MS a-C:H CVD Al MS MS AlOxNy MS SiNx PECVD SiOx PECVD SiCxNy SiON MS MS SiOx DLC PECVD DLC PSII DLC (Si) PECVD PECVD SiOxNy a-C:H CVD DIBAD SiOx PECVD SiOx PECVD SiOx/ parylene CatCVD SiNx PECVD SiOxCyHz MS TiNxOy MMT clay LbL and CPAM EB evap. SiOx ALD Al2O3 Al2O3

MS

OTR WVTR (cm3.mÿ2. (g.mÿ2.dayÿ1) dayÿ1.atmÿ1) 5

Application field FP FP Packaging FP FP FP FP EE FP FP Packaging FP FP EE FP ± FP EE

± 0.012 0.02 1 0.6 0.6 ± ± 0.8 0.4 0.17 ± ± 5 2 0.27 ±

0.05 0.1 1.06 ± 0.2 0.4 ± ± ± 0.032 0.0235 ± ± ± 0.005

PET PET PET PET PET PET PET PEN PET PET PET PET PET PES PET PC PET PES

± ± 0.6 0.05

0.0045 0.6 0.98 ±

PET PET PET PET

EE Packaging EE Packaging

PET PEN PP PET PLA PET PES

EE FP

1.5 10 ±

± 0.17

Subs

± 0.6 10ÿ4


EE


301

niques and substrates. The authors related in their works important features like defects in mechanical properties, aspects of nucleation and growth of thin films and their influence in the final barrier properties. The majority of these authors focused their work on systems for food packaging (FP) applications as well as for electronic device encapsulation (EE). From a general overview of this table it is possible to conclude that silicon based materials are the most studied system. However, aluminium-based and a-C:H (DLC) compounds are becoming very promising materials for providing a gas barrier in food packaging applications. Regarding the specific values of OTR it is possible to observe that the best results were achieved with CVD by Vasquez-Borucki et al. [60]. In this work the authors successfully synthesized a-C:H in PET substrates. GruÈniger et al. [64] claimed 0.27 cm3.mÿ2.dayÿ1.atmÿ1 for OTR for silicon oxide thin films, deposited by PECVD. Promising results were reported by Erlat et al. [14] using magnetron sputtering, where the OTR for an aluminium metallization was 0.02 cm3.mÿ2.dayÿ1.atmÿ1. Concerning WVTR, the best results were obtained with Al2O3 thin film deposited by magnetron sputtering. The authors reported an OTR value of 0.001 g.mÿ2.dayÿ1 [68]. Different authors [40, 69, 70] claim that the barrier performance of the systems and consequently their success for food packaging applications are greatly influenced by the presence of defects on deposited films, their thickness and mechanical properties. It is also worth noting the increasing number of studies of barrier systems for electronic device encapsulation in comparison to food packaging. Considerable attention has been given in the last years to carbon compounds for applications as gas barrier coatings. Inhibition of CO2 loss in drink bottles is an attractive area for implementation of carbon compounds, where the transparency of the final product is less important than for other types of food packs [46]. The increasing market for flexible electronic devices has led to new needs for flexible packaging, and gas barrier thin films can be an important technology. Figure 11.7 shows the permeability requirements for the different application fields. From this figure it is possible to observe that the requirements for encapsulation of OLEDs and organic solar cells are much more exacting than for food packaging. For this reason, any technological progress in gas barrier properties for electronic encapsulation can also be applied to food packaging systems. Industrial demand for packaging is the key to a new challenge in the research community. The development of technologies for mass and continuous production is now one of the most important research fields. The development of, for example, roll-to-roll equipment [16] and hardware for thin film deposition in the internal walls of bottles [71] is now a big research and industrial challenge. Moreover, the increased interest in high flexible gas barrier solutions for packaging (for both electronic encapsulation and food containers) has led in


302


11.7 Gas barrier requirements for different technological areas, adapted from Refs [16] and [52].

recent years to an expansion of the research in many different areas. In Fig. 11.8 it is possible to observe how the scientific community drives the research in this area. The present data were compiled from all the publications cited in this section and from the specific work area for each one. In this way it is possible to observe the preferred paths being followed by the different authors with relevant research work in this area. The development of flexible organic electronic devices represents a new challenge in terms of encapsulation. PECVD appears to have a preferential deposition technique. The lower concentration of defects in the films deposited by this technique can explain the preference, but no less important are the base materials used, silicon and carbon. However, sputtering techniques are always very good solutions because of the high quality of the films they produce, their simplicity, their upscaling facility and their high flexibility in terms of industrial setup. The recent developments of a new generation of pulsed power supplies and rotating targets for the case of magnetron sputtering make this technique a valid solution. Table 11.3 presents a good example of non-conventional sputtered materials for this type of applications, with promising results achieved by Fahlteich et al. [72]. Understanding these organic/inorganic systems for packaging is the key factor for their successes in this field. Barrier properties of inorganic systems have high importance in the final system behaviour. For this reason, great efforts are spent in achieving new materials with superior properties. Nevertheless, the need to apply the knowledge obtained from already existing commercial



303

11.8 Percentage of total publications cited in this section for the different deposition techniques, materials systems, work objectives and application fields.

Table 11.3 Lowest reported oxygen transmission rates (OTR) for different sputtered barrier layers Material Thickness (nm) OTR (cm3/m2dbar)

ZTO

ZnO

SiO2

TiO2

Al2O3

>120 PCL > PLLA > PHB > PGA, which was partly attributed to differences in the crystallinity of these polymers. The CO2 permeability of PHB is low and is comparable with that of polyvinylidene chloride (PVDC). A CO2 diffusion coefficient value of 1 10ÿ9 cm2 sÿ1 at 25ëC was reported for PHB by Poley et al. (2005), which is slightly higher than that measured earlier by Miguel et al. (1997) (4.4±4:7 10ÿ10 cm2 sÿ1 at 30ëC). The oxygen diffusion coefficient was 0:4 10ÿ9 cm2 sÿ1 at 25ëC for PHB and this increased slightly with increasing HV content (8±22 wt%), which was in this case attributed to decreasing crystallinity. The barrier properties of polymers can be enhanced by the addition of inorganic laminar nanofillers (e.g., clays). This well-known effect is associated with an increase in the tortuosity of the diffusion path as a result of introducing impermeable nanoplatelets (Sanchez-Garcia et al., 2008; de Azeredo, 2009). Articles in the literature report a decrease in the oxygen permeability of PLA (Ray et al., 2003; Chowdhury, 2008; Zenkiewicz and Richert, 2008; Sabet and Katbab, 2009), PCL (Sanchez-Garcia et al., 2007, 2008), PET (Frounchi and Dourbash, 2009) and PP (Mirzadeh and Kokabi, 2007; Villaluenga et al., 2007) when nanoclays are incorporated. However, the number of studies on the improvement of PHA barrier properties through addition of nanofillers is limited. As one of the few examples, Sanchez-Garcia et al. (2008) compared the thermal and barrier properties of organically modified kaolinite and OMMT in PHB-based nanocomposites prepared by melt blending with PCL added as a plasticiser. The result was a non-miscible but compatible interphase blend. Overall, the nanocomposites exhibited increased gas, aroma and water vapour


508


barrier performance. For example, the oxygen permeability of PHB- and PHB/ PCL-based nanocomposites containing 4 wt% nanoclay measured at 24ëC and 0% RH decreased by up to 43%. The same researchers reported ~20 and 27% reduction in oxygen permeability of PHB and PHBV films, respectively when 5 wt% OMMT was added (Sanchez-Garcia et al., 2007). Although a decrease in permeability is generally expected as a result of nanocomposite formation using layered clay silicates, the coexistence of phases with different permeabilities can result in complex transport phenomena. On the one hand, an organophilic clay can give rise to superficial adsorption and specific interactions with the penetrants while, on the other hand, the polymer phase can be considered as a two-phase crystalline-amorphous system in which the crystalline regions are impermeable to penetrant molecules. At the same time, changes in matrix crystallinity and chain mobility, induced by the presence of the nanofiller, need to be considered (Osman and Atallah, 2004; Osman et al., 2004; Pavlidou and Papaspyrides, 2008). Table 18.3 presents a summary of representative permeability data derived from the literature in which PHB is compared with PHBV, PLA, PCL and a number of conventional synthetic plastics. As shown, this is not an easy task since the values reported for the different polymer types can cover a wide range. This observation is probably due to the use of different measuring techniques and equipment as well as variations in the specific properties of the tested polymers (e.g., crystallinity and molecular weight). Broadly speaking, the barrier properties of PHB and PHBV appear to be slightly better than those of PLA and potentially competitive with those of various synthetic plastics; Table 18.3 Permeability properties of PHAs and some other polymers of interest for food packaging. Presented values are indicative only Polymer*

PHB PHBV PLA PCL LDPE PET PP PS PVC

O2 permeability at 23ëC, 0±50% RH (ml mm mÿ2 dayÿ1 atmÿ1)

Water vapour permeability at 23ëC±38ëC, 50±90% RH (g mm mÿ2 dayÿ1)

CO2 permeability at 23ëC, 0±50% RH (ml mm mÿ2 dayÿ1 atmÿ1)

2±10 5±14 15±25 20±200 50±200 1±5 50±100 100±150 2±8

1±5 1±3 5±7 300 0.5±2 0.5±2 0.2±0.4 1±4 1±2

3 ± 35±70 ± 800±1000 15±20 200±400 250±500 10±15

* Values for individual films will depend upon a number of factors including polymer molecular weight and crystallinity as well as the permeability testing conditions. Source: adapted from Lange and Wyser, 2003; Miguel et al. 1997; Thellen et al. 2008; SanchezGarcia et al. 2007, 2008.


Polyhydroxyalkanoates (PHAs) for food packaging

509

however, given the spread of data reported in the literature and the various conditions under which materials were measured, caution is suggested and the evaluation of individual PHAs in terms of the barrier requirements for packaging of particular food types would be highly recommended.

18.3.4 Thermal stability The thermal instability of PHAs has been a limiting factor in the processing and application of these polymers (Chen et al., 2004; Erceg et al., 2009). Thermal degradation of PHB and PHBV and various approaches to improving thermal stability have been widely studied using techniques including TGA, DSC, timeresolved pyrolysis MS and pyrolysis GC/MS (Galego and Rozsa, 1999; He et al., 2001; Aoyagi et al., 2002; Carrasco et al., 2006). The thermal degradation of PHAs near the melting point occurs almost exclusively by a non-radical random chain-scission reaction and the depolymerisation of the macromolecular chains is the controlling step (Grassie et al., 1984; Spyros et al., 1997; Carrasco et al., 2006). Thermal degradation becomes particularly significant at temperatures above 200ëC (Galego and Rozsa, 1999). In the literature there is a consensus that increasing the HV content in a PHBV copolymer leads to a reduction in melting point. As a result the processing temperature window is increased and degradation rates are maintained within acceptable limits (Poirier et al., 1995; Kuusipalo, 2000a; He et al., 2001; Thellen et al., 2008; Bordes et al., 2009a, 2009b). Da Silva et al. (2005) and Poley et al. (2005) reported that the melting temperature (TM) of PHB decreased from 176ëC to 158ëC with 22 wt% HV content, while Bordes et al. (2009b) described the same reduction in TM for PHBV with only 8 wt% HV content. He et al. (2001) observed a 70ëC decrease in TM for PHBV containing 30 wt% HV. However, as Bordes et al. (2009b) pointed out, the initial molecular weight can have a more significant influence on thermal degradation than the HV content. The lower the Mw, the greater the degrading effect due to random chain-scission. The characteristic thermal properties of some biodegradable polymers of relevance to food packaging are presented in Table 18.4. Blending with other biopolymers as a method for increasing the thermal stability of PHAs has been the subject of many research studies (Chun and Kim, 2000; El-Hadi et al., 2002a; Godbole et al., 2003; Erceg et al., 2005; Ohashi et al., 2009; Zhang and Thomas, 2010). The thermal stability of PHAs can be improved by addition of inorganic nanofillers including MMTs and LDHs (Choi et al., 2003; Lim et al., 2003; Bruzaud and Bourmaud, 2007; Wu et al., 2008; Bordes et al., 2009a, 2009b; Dagnon et al., 2009a, 2009b; Erceg et al., 2009, 2010; Botana et al., 2010). Such improvements are usually attributed to the dispersed silicate layers acting as a barrier to oxygen and to the volatiles generated during thermal decomposition (Cho and Paul, 2001; Choi et al., 2003; Lim et al., 2003; Bruzaud and Bourmaud


510

Multifunctional and nanoreinforced polymers for food packaging Table 18.4 Thermal properties of PHAs and polyolefins used in food packaging Polymer PHB PHBV LDPE PP

Tg (ëC)

Tm (ëC)

15 ÿ1 ÿ81 ÿ7 to ÿ35

175 136±162 105±110 160±168

Source: adapted from Kuusipalo, 2000a.

2007). For example, addition of 1±3 wt% CloisiteÕ 30B to PHBV increased the decomposition onset temperature from 252 to 263ëC (Choi et al., 2003). The temperature corresponding to 50% degradation of neat PHBV was found to increase by 30ëC with 5 wt% CloisiteÕ 15A nanoclay addition (Bruzaud and Bourmaud, 2007). The influence of nanofillers on the thermal stability of PHAs is complex and reports in the literature sometimes conflict. The degree of dispersion, in particular, significantly affects the thermal stability of nanocomposites, as agglomerates can cause local accumulation of heat and trigger more rapid thermal decomposition (Lim et al., 2003; Erceg et al., 2010). Dispersion of nanoclays within a polymer matrix depends on factors such as the amount and the nature of clay, the type and quantity of organomodifier, and processing conditions. The nanofiller content can also be crucial as agglomerates can form above a certain loading. As an example, Lim et al. (2003) demonstrated that although the decomposition onset temperature for PHB with 3 wt% OMMT was higher than that for unreinforced PHB, the thermal stability of the nanocomposites was reduced by further nanofiller addition. Similarly, Erceg et al. (2009) reported 5 wt% as an OMMT load limit for increasing the thermal stability of PHB. In contrast, Wang et al. (2005) found improved thermal stability at up to 10 wt% OMMT concentration in PHBV, despite the presence of agglomerates at the highest loading. It is suggested elsewhere that the presence of aluminium Lewis acid sites in the silicate layers enhances the thermal degradation of PHB by catalysing the hydrolysis of ester linkages and this phenomenon is more pronounced at higher loading levels (Erceg et al., 2009). The effect of nanofiller type on PHA thermal stability is exemplified in the research reported by Maiti et al. (2007). The addition of OMMT (1.2±3.6 wt%) increased the decomposition temperature but this was lowered when hydrophilic unmodified MMT (2.2 wt%) was used, probably as a result of poor dispersion. Other studies have shown that clay organomodifiers (e.g., quaternary ammonium salts) can have a strong catalytic influence on thermal degradation of PHB and PHBV (Xie et al., 2001; Hablot et al., 2008; Cabedo et al., 2009). The proposed degradation mechanism involves conversion of the quaternary ammonium surfactant into an amine through nucleophilic attack or Hofmann elimi-



511

18.3 PHB random chain scission reaction.

nation (Bordes, 2009b). The released acidic proton and/or nucleophilic amine can act as catalyst for PHB random chain scission (Fig. 18.3). It has also been suggested that these surfactants can act in synergy with fermentation residues to degrade PHB; however, the corresponding mechanism is still unexplained (Hablot et al., 2008). The release of tightly bound water from nanoclay surfaces at elevated temperature may contribute to PHBV degradation during processing. Interestingly, in support of this suggestion, it has be shown that kaolinite-based nanofillers, which release water at much higher temperatures, do not catalyse the degradation of PHBV (Cabedo et al., 2009). Erceg et al. (2009, 2010) investigated the influence of two OMMTs, namely CloisiteÕ 30B and CloisiteÕ 25A, on the thermal stability of PHB and analysed the degradation kinetics. These authors concluded that the isothermal degradation of pure PHB and PHB/ CloisiteÕ 30B nanocomposites occurs in two distinct regions ± a first in which relatively low mass loss takes place, and a second, the main degradation mechanism, in which the greater mass loss takes place. Other reports indicate that a simple first-order kinetic model cannot be applied to describe the isothermal degradation behaviour of PHB and PHB-based nanocomposites due to the contribution of different mechanisms, including autocatalytic or chain reactions (Kopinke et al., 1996; Wu et al., 2008). Although some researchers have studied solution-cast PHA/clay nanocomposites in order to circumvent thermal degradation issues (Bruzaud and Bourmaud, 2007; Wu et al., 2008; Cabedo et al., 2009), melt processing remains a more industrially practicable method for fabrication of polymer nanocomposites and in this case high shear rates during extrusion, which are usually needed in order to achieve nanoclay exfoliation, can contribute to PHA degradation (Bordes et al., 2009a). The decrease in PHA molecular weight at high screw speeds in an extruder can lead to stickiness on the metal surface of the chill rolls or injection moulding tools and increased crystallisation times (ElHadi et al., 2002b). The extent of degradation in melt-processed PHBV is highly


512


dependent on the type of mixing apparatus, the processing time and the processing atmosphere. For example, a reduction in PHBV degradation during nanocomposite compounding under nitrogen has been reported (Cabedo et al., 2009). There have been relatively few reports on LDH-based PHA nanocomposites, and investigations on thermal stability are also limited in number. In one of the few examples, Wu et al. (2008) explored the thermal degradation mechanism of PHB containing 2% and 5% poly(ethylene glycol) phosphonate (PEOPA)modified LDH (PMLDH). These authors found that the incorporation of organically modified LDH did not improve the thermal stability of the nanocomposites, with the decomposition onset decreasing from 263.6ëC in neat PHB to 240.2ëC in samples containing 5% PMLDH, suggesting that the organic modifier may catalyse the thermal degradation of PHB. To conclude, although improvement of the thermal stability of PHAs by addition of nanofillers can in principle be a viable option, the filler type and content, the organomodifier and the processing conditions have to be selected carefully in order to achieve the desired effect. In addition to thermal stability during melt processing, thermal stability of finished products at lower temperatures during storing or transportation of PHAbased packaging would clearly be important. In the case of PLA, it is known that stacked packaging trays can lose mechanical stability and collapse at temperatures above the Tg, which is typically in the range 50±59ëC (Huda et al., 2006, 2007). However, as noted (http://www.biomer.de), processed PHB can be highly crystalline and exhibit no such softening at temperatures likely to be encountered in storage and transport.

18.3.5 Migration For food packaging purposes, migration is a key issue, since monomers or additives used in PHA manufacturing processes may not be common in conventional food contact materials and might conceivably migrate into packaged food. However, to the authors' knowledge there has been no study yet which has monitored the migration of specific components from PHA packaging. The total migration from PHB films into different food simulants, including distilled water, 3% acetic acid, 15% ethanol and n-heptane has been investigated (Bucci et al., 2007). Tests were run at 40ëC for 10 days, with the exception of n-heptane where the tests were performed at 20ëC for 30 minutes. For all the simulants, total migration was below the recommended limit of 8.0 mg/dm2 or 50 mg/kg, suggesting that PHAs should be safe for packaging of various food products. Although in the European legislation both conventional and bio-based food contact materials are regulated in the same way, some special issues have to be considered in the latter case (Chowdhury, 2008). A challenge which the food packaging industry has to face in relation to the use of biodegradable polymers



513

and which also complicates the migration issue is the durability of the packaging in relation to the product shelf-life. The environmental conditions that lead to degradation of packaging must clearly be avoided during the storage of the food product and should only exist after the packaging has been discarded (Petersen et al., 1999; Haugaard et al., 2001). While it is known that pure PHB and PHBV are non-toxic, more information is needed regarding the potential toxicity and migration behaviour of degradation products produced during either processing or biodegradation (HaÈnggi, 1995). Another issue to be considered in respect to future packaging applications is the potential migration of nanoparticles from PHA nanocomposite films into food products. Concerns may arise because nanoparticles are generally much more reactive than corresponding macroparticles. The large surface area of nanoparticles allows a greater contact with cellular membranes as well as a greater capacity for absorption and migration (Li and Huang, 2008). There is, however, limited scientific data about the migration of nanoparticles from packaging material into food or the eventual toxicological effects (de Azeredo, 2009). SÏimon et al. (2008) discussed the theory of particle migration from nanocomposites and concluded that only very small particles with a diameter of ~1 nm should migrate. Avella et al. (2005) determined the migration of certain minerals (Fe, Mg, Si) from biodegradable starch/nanoclay nanocomposite films; the results of this study showed an insignificant trend in the levels of Fe and Mg in packaged vegetables but a consistent increase in the amount of Si, one of the main elements present in MMT nanoclays. In a recent study (Schmidt et al., 2009), isotopes of Zr and Mg were selected for on-line detection of CloisiteÕ 30B in order to follow the potential migration of this nanoclay from PLA nanocomposite films. The technique involved particle separation using field flow fractionation and then multi-angle light scattering to determine particle sizes in combination with ICP±MS for chemical characterisation. Although nanoparticles in the range of 50±800 nm were detected, ICP±MS signals corresponding to clay minerals were absent. A more recent study (MauricioIglesias et al., 2010) suggests that the specific migration properties of nanoparticles should be monitored, rather than the migration of their constituent elements, although this can be challenging from an analytical viewpoint (Tiede et al., 2009). In summary, there are presently no data available concerning the migration of specific components including possible degradation products from PHA packaging materials, which is a critical issue in terms of food safety. Similarly, there are no studies reporting the migration of nanoparticles from PHA-based nanocomposite films, although it is reasonable to assume that migration may occur and hence, if these materials are to be developed in the future, there will be a continuing need for risk evaluation.


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18.3.6 Degradability The biodegradation of PHAs in both aerobic and anaerobic environments has been extensively studied (Abou-Zeid et al., 2001, 2004; Bucci et al., 2007). As a rule, PHAs are considered more readily biodegradable than PLA (Sudesh et al., 2000; Sudesh and Iwata, 2008). The biodegradation of PHAs involves biotic or abiotic hydrolysis followed by bioassimilation (CorreÃa et al., 2008). Various microorganisms can excrete extracellular PHA depolymerases which hydrolyse high molecular weight PHAs into water-soluble oligomers and monomers and subsequently utilise these products as nutrients (Khanna and Srivastava, 2005). The eventual metabolic products are water and carbon dioxide (Renard et al., 2004). Extracellular PHA depolymerase has been isolated and purified from several bacteria and fungi that are known to degrade PHAs. In this respect, the dominant genera among bacteria are Pseudomonas, Azotobacter, Bacillus and Streptomyces, and among fungi they are Penicillium, Cephalosporum, Paecilomyces and Trichoderma (Savenkova et al., 2000). The PHA depolymerase mechanism has been widely studied (Timmins et al., 1997; Iwata et al., 2002; Abe et al., 2005; Li et al., 2007). The rate of PHA biodegradation is influenced by (1) molar mass, copolymer composition, crystallinity, stereochemistry, hydrophilic/hydrophobic balance, and chain mobility; and (2) environmental factors including the microbial population, temperature, moisture, pH and nutrient supply (Khanna and Srivastava, 2005). Numerous studies have explored the factors determining the biodegradability of PHA materials in soil (Savenkova et al., 2000; Tsuji et al., 2003; dos Santos Rosa et al., 2004; CorreÃa et al., 2008), fresh water (Kasuya et al., 1998; Kusaka et al., 1999), marine environments (Tsuji and Suzuyoshi, 2002a, 2002b, 2003; Thellen et al., 2008), sewage environments (Briese et al., 1994; Bucci et al., 2007) and compost media (Yue et al., 1996; Maiti et al., 2007). In general, the higher the polymer crystallinity and melting point, the lower the degradation rate. In PHBV, an increased HV content is associated with faster degradation (Renard et al., 2004). Degradation mechanisms under aerobic conditions are different from those in anaerobic situations and reports indicate that PHBV degrades more rapidly than PHB under aerobic conditions (Mergaert et al., 1993; Yue et al., 1996; dos Santos Rosa et al., 2004; Li et al., 2007); however, the opposite effect has been reported by Abou-Zeid et al. (2001, 2004). Biodegradation of PHA-based nanocomposites has been investigated and a decrease in the rate of PHB or PHBV biodegradation with increasing nanoparticle content is often reported. For example, Wang et al. (2005) showed that the biodegradability of PHBV/OMMT in soil suspension decreased with increasing OMMT content. The reduced rate of biodegradation when high aspect ratio nanoclays are present in the matrix has been attributed to the formation of a tortuous path which can hinder penetration of microorganisms into the bulk of the material (Wang et al., 2005; Maiti et al., 2007). Reduced water permeability



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and an antimicrobial effect in some OMMTs may also play a role in lowering the rate of biodegradability (Bordes et al., 2009a). However, conflicting reports about the effect of nanoclays on polymer biodegradability can be found in the literature. As an example, titanate-modified MMT enhanced the biodegradation of toughened PHB several-fold. In the proposed mechanism, the terminal hydroxylated edge groups of the silicate clay layers can absorb moisture from compost and act as initiation sites for polyester hydrolysis (Ray et al., 2003). Typically, any factor which increases the hydrolytic tendency of PHAs will ultimately control the degradation (Pavlidou and Papaspyrides, 2008). In addition, there is evidence that well-dispersed clay particles cause the polymer chains to fragment more rapidly, resulting in increased degradation (Parulekar et al., 2007). In one study the biodegradation rate of PHB was enhanced significantly in the presence of 2 wt% organo-modified fluoromica with nearcomplete degradation observed in about seven weeks (Maiti et al., 2007). The authors also reported that at higher temperatures the rate of biodegradation drastically decreased for both neat PHB and PHB nanocomposites. This reduced biodegradation rate may be due to the suppression of microorganisms at and above 60ëC or to an increase in polymer crystallinity in these samples. The latter is considered important since the amorphous interspherulitic regions are prone to hydrolysis followed by microorganism attack. In summary, as indicated, the factors which influence the rate of PHA nanocomposite degradation remain under discussion and are the subject of continuing research. The use of PHAs for food packaging materials would ultimately require advance knowledge of performance in service under a variety of conditions and to date there have been few investigations in this direction. Kantola and HeleÂn (2001) analysed the performance of BiopolÕ-coated paperboard trays overwrapped with a Mater-BiÕ (Novamont) starch-based film when used to package organic tomatoes. The key finding was that the tomatoes stayed as fresh in the PHB-coated paperboard trays as those wrapped in perforated LDPE bags. Haugaard et al. (2003) explored packaging of an orange juice simulant and a dressing in PHB cups and concluded that the performance was as good as that of HDPE and superior when samples were stored under light. Hermida et al. (2008) discovered that there was no significant reduction in PHB properties when exposed to the levels of gamma radiation needed to sterilise food or packaging materials.

18.4

Polyhydroxyalkanoate (PHA) foams and paper coatings

As with other applications, cost of production is a significant problem in terms of introducing PHA to the price-sensitive foam packaging market. Kaneka Corporation, also known as Kanegafuchi, has been active in foaming of biodegradable polymers and in recent years has filed two patents on expandable


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PHA. In the first of these (Hirose et al., 2007), solid beads of a poly(3hydroxyalkanoate) are formed, suspended in a blowing agent in a sealed vessel, and then pressurised and heated to start the expansion. The blowing agent is preferably dimethyl ether, diethyl ether or methyl ethyl ether, all of which have low boiling points and will impregnate the polymer beads. Once the beads are sufficiently saturated with ether and heated to a temperature not far below their melting point, the vessel is opened and the beads complete their expansion. In a second patent (Miyagawa et al., 2007), benefits are claimed for the addition of an isocyanate chain extender to the polymer and a wider range of blowing agents is claimed. Current industrial applications of foamed PHAs have yet to develop because of manufacturing problems, largely as a result of thermal instability. Thermal degradation makes foaming difficult because of the low viscosity after addition of a foaming agent and the resulting cell collapse. In one of the few examples cited in the literature, a mixture of PHB, PVOH and starch with optimum viscosity was produced and azodicarbonamide used as a foaming agent. Due to a greater surface area, the resulting foam was found to have a faster rate of biodegradation than bulk materials of the same composition (Grosu et al., 2007). Although there have been studies reported and patents issued on the use of PHA foams for medical applications (e.g., tissue engineering), development for packaging uses has to the authors' knowledge not yet been reported. As reported elsewhere in this chapter, there have been a few studies on PHAs as coatings for paper and paperboard which might be suitable for food packaging (Cyras et al., 2007, 2009). Bourbonnais and Marchessault (2010) recently reported the effect of natural or artificially produced PHB and PHBV granules as paper sizing agents. Differences in the sizing effect were noticed when papers were impregnated and dried at ~110ëC and this was associated with differences in PHA particle morphology. Much improved sizing was noted when impregnated papers were pressed and heated at ~160ëC, under which conditions melted granules formed a thin film at the paper surface. The authors noted that the preparation technique might also open the way to new fibre-reinforced PHA films. As examples, PHB or PHBV coatings on paper or cardboard were successfully applied to reduce the moisture absorption and the water vapour permeability of these materials (Kuusipalo 2000a, 2000b; Cyras et al., 2007, 2009).

18.5

Conclusions

PHAs have been widely investigated in terms of their chemistry, biosynthesis, properties and potential applications, and the means of tailoring the specific polymers, as well as their direct extraction from bacterial cultures, have attracted commercial interest for some 50 years. As discussed in this chapter, commercial interest is now on the rise again as a result of industrial interest in `green'



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materials and concerns about the environment. Although a number of companies have developed and can supply PHB or PHBV raw materials, their conversion into readily processable bioplastics including, for example, nucleating agents and plasticisers has been the focus of fewer manufacturers. Considering the question of food packaging, wide uptake of PHAs is presently limited by cost factors and this appears unlikely to change in the short term. Putting the cost issue aside, by examining past literature, this chapter has addressed whether the properties of PHAs or PHA nanocomposites might generally match the needs for food packaging. Clearly, this is a complex question, since requirements vary according to food product and type of packaging; however, some overall indications can be gained. In terms of mechanical properties, increased flexibility appears to be a key issue and, although blending with other polymers offers this possibility, the introduction of nanofillers may in this respect have an undesirable effect. The literature on permeability of PHA films, although not always easy to interpret, suggests that this should not be a major limiting factor. Control of the thermal stability of PHAs during melt processing is a challenge but, as discussed and as also demonstrated industrially, is not an insurmountable problem. Also on the positive side, it seems that PHAs can be processed so as to be thermally stable at storage and transportation temperatures, the lack of which continues to be an issue in regards to wider adoption of PLA-based food packaging trays. The migration properties of PHAbased films for food packaging and their likely performance in service have received relatively little attention so far and will need further study if PHAbased materials are to enter this sector in the future.

18.6

Future trends

As indicated, a principal reason for the lack of PHA-based products in food packaging is the relatively high cost of PHAs. Typically, PHAs are produced using pure cultures of defined bacterial isolates and highly purified carbon sources such as glucose. This type of production yields an average cost of around ¨10/kg, which is considerably more than that of petroleum-based plastics such as PE (Mooney, 2009). However, production of PHA from mixed cultures using various waste streams has been proposed as an effective way to reduce these costs (Lemos et al., 2003; Reis et al., 2003; Khardenavis et al., 2005, 2007). Although PHAs are still expensive relative to petroleum-derived plastics, future increases in production volume combined with the possibility of higher oil prices could help to reduce the price gap. From a technical perspective, amongst other issues, processing technologies are needed to deal with low thermal stability, and optimised methods to increase the toughness of PHB are required. The use of PHAs in high-value medical applications is quite feasible but volumes are not sufficient to justify economical production, estimated to be at least 20,000 tons per annum.


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Considering biodegradable polymers as a whole in food packaging uses, market forces and government regulations are driving manufacturers in this direction and, if suitable processing parameters, properties and viable prices can be achieved, PHAs could play a significant role in future food packaging. As an example, partial substitution of PET bottles, where the price gap is not so significant, may be a future target for PHA products, providing that technological difficulties can be solved.

18.7

Sources of further information and advice

For further information on the subject of biodegradable polymers, including PHAs, a good starting point is http://www.biopolymer.net. This website provides a listing of commercial bioplastics by trade name, material type and application as well as links to relevant institutions and organisations such as the European Bioplastics Association (http://www.european-bioplastics.org). Excellent sources of information about biopolymers, including PHAs, are the textbooks edited by Doi and SteinbuÈchel (2002) and by Belgacem and Gandini (2008). The latter also covers the topic of composite materials based on renewable polymers. With regard to reviews on the topic of PHAs, mention should be made of the publication by Lenz and Marchessault (2005) and a more recent one on PHA origins, properties and applications by Chodak (2008). Philip et al. (2007) also produced a useful review on PHAs. A recent book by Sudesh and Abe (2010) entitled Practical Guide to Microbial Polyhydroxyalkanoates provides another general introduction. The websites of the key PHA manufacturers provide good introductions to PHAs (e.g., Metabolix, Biomer). Features available on the website http://www.makeitfrom.com provide basic property information about PHAs and also allow a quick and general comparison to be made between PHAs and other materials. For a good recent reference to detailed information on biogenesis and structure of PHAs, albeit largely with reference to nano-/micro-bead applications in biotechnology and medicine, the reader is referred to Grage et al. (2009).

18.8

References

Abe T, Kobayashi T and Saito T (2005), `Properties of a novel intracellular poly(3hydroxybutyrate) depolymerase with high specific activity (PhaZd) in Wautersia eutropha H16', J. Bacteriol., 187, 6982±6990. Abou-Zeid D, MuÈller R and Deckwer W (2001), `Degradation of natural and synthetic polyesters under anaerobic conditions', J. Biotechnol., 86, 113±126. Abou-Zeid D, MuÈller R and Deckwer W (2004), `Biodegradation of aliphatic homopolyesters and aliphatic±aromatic copolyesters by anaerobic microorganisms', Biomacromolecules, 5, 1687±1697. Anderson A J and Dawes E A (1990), Òccurrence, metabolism, metabolic role, and industrial uses of bacterial polyhydroxyalkanoates', Microbiol. Rev., 54, 450±472.



519

Aoyagi Y, Yamashita K and Doi Y (2002), `Thermal degradation of poly[(R)-3hydroxybutyrate], poly[-caprolactone], and poly[(S)-lactide]', Polym. Degrad. Stab., 76, 53±59. Arora A and Padua G W (2010), `Review: Nanocomposites in food packaging', J. Food Sci., 75, R43±R49. Avella M, De Vlieger J J, Errico M E, Fischer S, Vacca P and Volpe M G (2005), `Biodegradable starch/clay nanocomposite films for food packaging applications', Food Chem., 93, 467±474. Barham P J and Keller A (1986), `Relationship between microstructure and mode of fracture in polyhydroxybutyrate', J. Polym. Sci. A-2, 24, 69±77. Barham P J, Barker P and Organ S J (1992), `Physical properties of poly(hydroxybutyrate) and copolymers of hydroxybutyrate and hydroxyvalerate', FEMS Microbiol. Rev., 103, 289±298. Belgacem M N and Gandini A (2008), Monomers, Polymers and Composites from Renewable Resources, Amsterdam, Elsevier. Bordes P, Pollet E and AveÂrous L (2009a), `Nano-biocomposites: Biodegradable polyester/nanoclay systems', Prog. Polym. Sci. (Oxford), 34, 125±155. Bordes P, Hablot E, Pollet E and AveÂrous L (2009b), Èffect of clay organomodifiers on degradation of polyhydroxyalkanoates', Polym. Degrad. Stab., 94, 789±796. Botana A, Mollo M, Eisenberg P and Torres Sanchez R M (2010), Èffect of modified montmorillonite on biodegradable PHB nanocomposites', Appl. Clay. Sci., 47, 263± 270. Bourbonnais R and Marchessault R H (2010), Àpplication of polyhydroxyalkanoate granules for sizing of paper', Biomacromolecules, 11, 989±993. Briese B H, Jendrossek D and Schlegel H G (1994), `Degradation of poly(3hydroxybutyrate-co-3-hydroxyvalerate) by aerobic sewage sludge', FEMS Microbiol. Lett., 117, 107±112. Bruno M, Tavares M I B, Motta L M, Miguez E, Preto M and Fernandez A O R (2008), Èvaluation of PHB/clay nanocomposite by spin-lattice relaxation time', Mater. Res., 11, 483±485. Bruzaud S and Bourmaud A (2007), `Thermal degradation and (nano)mechanical behavior of layered silicate reinforced poly(3-hydroxybutyrate-co-3hydroxyvalerate) nanocomposites', Polym. Test., 26, 652±659. Bucci D Z, Tavares L B B and Sell I (2007), `Biodegradation and physical evaluation of PHB packaging', Polym. Test., 26, 908±915. Cabedo L, Plackett D, GimeÂnez E and LagaroÂn J M (2009), `Studying the degradation of polyhydroxybutyrate-co-valerate during processing with clay-based nanofillers', J. Appl. Polym. Sci., 112, 3669±3676. Carrasco F, Dionisi D, Martinelli A and Majone M (2006), `Thermal stability of polyhydroxyalkanoates', J. Appl. Polym. Sci., 100, 2111±2121. Chen G X, Hao G J, Guo T Y, Song M D and Zhang B H (2002), `Structure and mechanical properties of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV)/ clay nanocomposites', J. Mater. Sci. Lett., 21, 1587±1589. Chen G X, Hao G J, Guo T Y, Song M D and Zhang B H (2004), `Crystallization kinetics of poly(3-hydroxybutyrate-co-3-hydroxyvalerate)/clay nanocomposites', J. Appl. Polym. Sci., 93, 655±661. Cho J W and Paul D R (2001), `Nylon 6 nanocomposites by melt compounding', Polymer, 42, 1083±1094. Chodak I (2008), `Polyhydroxyalkanoates. Origins, properties and applications', in Belgacem M N and Gandini A, Monomers, Polymers and Composites from


520


Renewable Resources, Amsterdam, Elsevier, 451±478. Choi W M, Kim T W, Park O O, Chang Y K and Lee J W (2003), `Preparation and characterization of poly(hydroxybutyrate-co-hydroxyvalerate)-organoclay nanocomposites', J. Appl. Polym. Sci., 90, 525±529. Chowdhury A A (1963), `Poly- -hydroxybuttersaÈure abbauende Bakterien und Exoenzym', Archiv fuÈr Mikrobiologie, 47, 167±200. Chowdhury S R (2008), `Some important aspects in designing high molecular weight poly(L-lactic acid)-clay nanocomposites with desired properties', Polym. Int., 57, 1326±1332. Chun Y S and Kim W N (2000), `Thermal properties of poly(hydroxybutyrate-cohydroxyvalerate) and poly(-caprolactone) blends', Polymer, 41, 2305±2308. CorreÃa M C S, Rezende M L, Rosa D S, Agnelli J A M and Nascente P A P (2008), `Surface composition and morphology of poly(3-hydroxybutyrate) exposed to biodegradation', Polym. Test., 27, 447±452. Cyras V P, Commisso M S, Mauri A N and VaÂzquez A (2007), `Biodegradable doublelayer films based on biological resources: Polyhydroxybutyrate and cellulose', J. Appl. Polym. Sci., 106, 749±756. Cyras V P, Commisso M S and VaÂzquez A (2009), `Biocomposites based on renewable resource: Acetylated and non-acetylated cellulose cardboard coated with polyhydroxybutyrate', Polymer, 50, 6274±6280. Da Silva M G, Vargas H, Poley L H, Rodriguez R S and Baptista G B (2005), `Structural impact of hydroxyvalerate in polyhydroxyalkanoates (PHA scl) dense film monitored by XPS and photothermal methods', J. Brazil. Chem. Soc., 16, 790±795. Dagnon K L, Chen H H, Innocentini-Mei L H and D'Souza N A (2009a), `Poly[(3hydroxybutyrate)-co-(3-hydroxyvalerate)]/layered double hydroxide nanocomposites', Polym. Int., 58, 133±141. Dagnon K L, Robinson C, Chen H H and D'Souza N A (2009b), À study of crystallization kinetics of poly(3-hydroxybutyrate-co-3-hydroxyvalerate)/Zn±Al layered double hydroxide (LDH) stearate nanocomposites', Annual Technical Conference ± ANTEC, Conference Proceedings, 2757. de Azeredo H M C (2009), `Nanocomposites for food packaging applications', Food Res. Int., 42, 1240±1253. De Smet M J, Eggink G and Witholt B (1983), `Characterization of intracellular inclusions formed by Pseudomonas oleovorans during growth on octane', J. Bacteriol., 154, 870±878. Doi Y and SteinbuÈchel A (2002), Biopolymers. Vol. 3b, Polyesters. II: Properties and Chemical Synthesis, Weinheim, Wiley-VCH. dos Santos Rosa D, Calil M R, Fassina Guedes C d G and Rodrigues T C (2004), `Biodegradability of thermally aged PHB, PHB-V, and PCL in soil compostage', J. Polym. Environ., 12, 239±245. Du G and Yu J (2002), `Green technology for conversion of food scraps to biodegradable thermoplastic polyhydroxyalkanoates', Environ. Sci. Technol., 36, 5511±5516. Dufresne A, Kellerhals M B and Witholt B (1999), `Transcrystallization in Mcl-PHAs/ cellulose whiskers composites', Macromolecules, 32, 7396±7401. El-Hadi A, Schnabel R, Straube E, MuÈller G and Henning S (2002a), `Correlation between degree of crystallinity, morphology, glass temperature, mechanical properties and biodegradation of poly(3-hydroxyalkanoate) PHAs and their blends', Polym. Test., 21, 665±674. El-Hadi A, Schnabel R, Straube E, MuÈller G and Riemschneider M (2002b), Èffect of melt processing on crystallization behavior and rheology of poly(3-



521

hydroxybutyrate) (PHB) and its blends', Macromol. Mater. Eng., 287, 363±372. Erceg M, KovacÏicÂ T and KlaricÂ I (2005), `Dynamic thermogravimetric degradation of poly(3-hydroxybutyrate)/aliphatic±aromatic copolyester blends', Polym. Degrad. Stab., 90, 86±94. Erceg M, KovacÏicÂ T and KlaricÂ I (2009), `Poly(3-hydroxybutyrate) nanocomposites: Isothermal degradation and kinetic analysis', Thermochim. Acta, 485, 26±32. Erceg M, KovacÏicÂ T and Sanja P (2010), Ìsothermal degradation of poly(3hydroxybutyrate)/organically modified montmorillonite nanocomposites', Polym. Compos., 31, 272±278. Frounchi M and Dourbash A (2009), Òxygen barrier properties of poly(ethylene terephthalate) nanocomposite films', Macromol. Mater. Eng., 294, 68±74. Furukawa T, Sato H, Murakami R, Zhang J, Noda I, Ochiai S and Ozaki Y (2006), `Raman microspectroscopy study of structure, dispersibility, and crystallinity of poly(hydroxybutyrate)/poly(L-lactic acid) blends', Polymer, 47, 3132±3140. Galego N and Rozsa C (1999), `Thermal decomposition of some poly( hydroxyalkanoates)', Polym. Int., 48, 1202±1204. Godbole S, Gote S, Latkar M and Chakrabarti T (2003), `Preparation and characterization of biodegradable poly-3-hydroxybutyrate±starch blend films', Bioresour. Technol., 86, 33±37. Grage K, Jahns A C, Parlane N, Palanisamy R, Rasiah I A, Atwood J A and Rehm B H A (2009), `Bacterial polyhydroxyalkanoate granules: Biogenesis, structure, and potential use as nano-/micro-beads in biotechnological and biomedical applications', Biomacromolecules, 10, 660±669. Grassie N, Murray E J and Holmes P A (1984), `The thermal degradation of poly(-(D)- hydroxybutyric acid): Part 2 ± Changes in molecular weight', Polym. Degrad. Stab., 6, 95±103. Gregorova A, Wimmer R, Hrabalova M, Koller M, Ters T and Mundigler N (2009), Èffect of surface modification of beech wood flour on mechanical and thermal properties of poly(3-hydroxybutyrate)/wood flour composites', Holzforschung, 63, 565±570. Grosu E, Nemes E, Rapa M, Scheau A, Cornea P C and Lupescu I (2007), Proc. Int. Conf. on Biofoams, Capri, September 2007. Hablot E, Bordes P, Pollet E and AveÂrous L (2008), `Thermal and thermo-mechanical degradation of poly(3-hydroxybutyrate)-based multiphase systems', Polym. Degrad. Stab., 93, 413±421. HaÈnggi U J (1995), `Requirements on bacterial polyesters as future substitute for conventional plastics for consumer goods', FEMS Microbiol. Rev., 16, 213±220. Haugaard V K, Udsen A M, Mortensen G, Hoegh L, Petersen K and Monahan F (2001), `Potential food applications of biobased materials. An EU-concerted action project', Starch-StaÈrke, 53, 189±200. Haugaard V K, Danielsen B and Bertelsen G (2003), Ìmpact of polylactate and poly(hydroxybutyrate) on food quality', Eur. Food Res. Technol., 216, 233±240. He J, Cheung M K, Yu P H and Chen G (2001), `Thermal analyses of poly(3hydroxybutyrate), poly(3-hydroxybutyrate-co-3-hydroxyvalerate), and poly(3hydroxybutyrate-co-3-hydroxyhexanoate)', J. Appl. Polym. Sci., 82, 90±98. Hermida E B, Mega V I, Yashchuk O, FernaÂndez V, Eisenberg P and Miyazaki S S (2008), `Gamma irradiation effects on mechanical and thermal properties and biodegradation of poly(3-hydroxybutyrate) based films', Macromol. Symp., 263, 102±113. Hirose F, Miyagawa T, Senda K and Fukugawa S (2007), Èxtruded polyhydroxy-


522


alkanoate foams with good appearance and biodegradability and high open cell ratio, and their stable manufacture', Japanese patent JP 2007130763. Holmes P A (1988), `Biologically produced PHA polymers and copolymers', in Bassett D C, Developments in Crystalline Polymers, London, Elsevier, 1±65. Hsu S, Wu T and Liao C (2007), `Nonisothermal crystallization behavior and crystalline structure of poly(3-hydroxybutyrate)/layered double hydroxide nanocomposites', J. Polym. Sci. Part B, 45, 995±1002. Huda M S, Drzal L T, Mohanty A K and Misra M (2006), `Chopped glass and recycled newspaper as reinforcement fibers in injection molded poly(lactic acid) (PLA) composites: A comparative study', Comp. Sci. Technol., 66, 1813±1824. Huda M S, Drzal L T, Mohanty A K and Misra M (2007), `The effect of silane treatedand untreated-talc on the mechanical and physico-mechanical properties of poly(lactic acid)/newspaper fibers/talc hybrid composites', Composites Part B: Engineering, 38, 367±379. Iordanskii A L, Razumovskii L P, Krivandin A V and Lebedeva T L (1996), `Diffusion and sorption of water in moderately hydrophilic polymers: From segmented polyetherurethanes to poly-3-hydroxybutyrate', Desalination, 104, 27±35. Iordanskii A L, Kamaev P P and Zaikov G E (1998), `Water sorption and diffusion in poly(3-hydroxybutyrate) films', Int. J. Polym. Mater., 41, 55±63. Iordanskii A L, Kamaev P P and Zaikov G E (1999), `Water sorption and diffusion in poly-(3-hydroxybutyrate) films', Polym. Plast. Technol. Eng., 38, 729±738. Iwata T, Shiromo M and Doi Y (2002), `Surface structures of poly[(R)-3hydroxybutyrate] and its copolymer single crystals before and after enzymatic degradation with an extracellular PHB depolymerase', Macromol. Chem. Phys., 203, 1309±1316. Jiang L, Morelius E, Zhang J, Wolcott M and Holbery J (2008), `Study of the poly(3hydroxybutyrate-co-3-hydroxyvalerate)/cellulose nanowhisker composites prepared by solution casting and melt processing', J. Composite Mater., 42, 2629±2645. Kantola M and HeleÂn H (2001), `Quality changes in organic tomatoes packaged in biodegradable plastic films', J. Food Quality, 24, 167±176. Kasuya K, Takagi K, Ishiwatari S, Yoshida Y and Doi Y (1998), `Biodegradabilities of various aliphatic polyesters in natural waters', Polym. Degrad. Stab., 59, 327±332. Khanna S and Srivastava A K (2005), `Recent advances in microbial polyhydroxyalkanoates', Process Biochem., 40, 607±619. Khardenavis A, Guha P K, Kumar M S, Mudliar S N and Chakrabarti T (2005), Àctivated sludge is a potential source for production of biodegradable plastics from wastewater', Environ. Technol., 26, 545±552. Khardenavis A A, Suresh Kumar M, Mudliar S N and Chakrabarti T (2007), `Biotechnological conversion of agro-industrial wastewaters into biodegradable plastic, poly -hydroxybutyrate', Bioresour. Technol., 98, 3579±3584. Kopinke F, Remmler M and Mackenzie K (1996), `Thermal decomposition of biodegradable polyesters ± I: Poly( -hydroxybutyric acid)', Polym. Degrad. Stab., 52, 25±38. Kusaka S, Iwata T and Doi Y (1999), `Properties and biodegradability of ultra-highmolecular-weight poly[(R)-3-hydroxybutyrate] produced by a recombinant Escherichia coli', Int. J. Biol. Macromol., 25, 87±94. Kuusipalo J (2000a), `PHB/V in extrusion coating of paper and paperboard: Part I: Study of functional properties', J. Polym. Environ., 8, 39±47. Kuusipalo J (2000b), `PHB/V in extrusion coating of paper and paperboard ± Study of functional properties. Part II', J. Polym. Environ., 8, 49±58.



523

Lange J and Wyser Y (2003), `Recent innovations in barrier technologies for plastic packaging ± a review', Pack. Techn. Sci., 16, 149±158. Lemoigne M (1925), ÈÂtudes sur l'autolyse microbienne acidification par formation d'acide', Ann. Inst. Pasteur Paris, 39, 144. Lemoigne M (1926), Produit de deÂshydratation et de polymeÂrisation de l'acide oxybutyrique', Bull. Soc. Chim. Biol., 8, 770±782. Lemos P C, Serafim L S, Santos M M, Reis M A M and Santos H (2003), `Metabolic pathway for propionate utilization by phosphorus-accumulating organisms in activated sludge: 13C labeling and in vivo nuclear magnetic resonance', Appl. Environ. Microbiol., 69, 241±251. Lenz R W and Marchessault R H (2005), `Bacterial polyesters: Biosynthesis, biodegradable plastics and biotechnology', Biomacromolecules, 6, 1±8. Li S and Huang L (2008), `Pharmacokinetics and biodistribution of nanoparticles', Mol. Pharmaceutics, 5, 496±504. Li Z, Lin H, Ishii N, Chen G and Inoue Y (2007), `Study of enzymatic degradation of microbial copolyesters consisting of 3-hydroxybutyrate and medium-chain-length 3-hydroxyalkanoates', Polym. Degrad. Stab., 92, 1708±1714. Lim S T, Hyun Y H, Lee C H and Choi H J (2003), `Preparation and characterization of microbial biodegradable poly(3-hydroxybutyrate)/organoclay nanocomposite', J. Mater. Sci. Lett., 22, 299±302. Lovera D, MaÂrquez L, Balsamo V, Taddei A, Castelli C and MuÈller A J (2007), `Crystallization, morphology, and enzymatic degradation of polyhydroxybutyrate/ polycaprolactone (PHB/PCL) blends', Macromol. Chem. Phys., 208, 924±937. Macrae R M and Wilkinson J F (1958), `Poly- -hyroxybutyrate metabolism in washed suspensions of Bacillus cereus and Bacillus megaterium', J. Gen. Microbiol., 19, 210±222. Maiti P, Batt C A and Giannelis E P (2007), `New biodegradable polyhydroxybutyrate/ layered silicate nanocomposites', Biomacromolecules, 8, 3393±3400. Mauricio-Iglesias M, Peyron S, Guillard V and Gontard N (2010), `Wheat gluten nanocomposite films as food-contact materials: Migration tests and impact of a novel food stabilization technology (high pressure)', J. Appl. Polym. Sci., 116, 2526±2535. McChalicher C W J and Srienc F (2007), Ìnvestigating the structure±property relationship of bacterial PHA block copolymers', J. Biotechnol., 132, 296±302. Mergaert J, Webb A, Anderson C, Wouters A and Swings J (1993), `Microbial degradation of poly(3-hydroxybutyrate) and poly(3-hydroxybutyrate-co-3hydroxyvalerate) in soils', Appl. Environ. Microbiol., 59, 3233±3238. Miguel O and Iruin J J (1999a), Èvaluation of the transport properties of poly(3hydroxybutyrate) and its 3-hydroxyvalerate copolymers for packaging applications', Macromolecular Symposia, 144, 427±438. Miguel O and Iruin J J (1999b), `Water transport properties in poly(3-hydroxybutyrate) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) biopolymers', J. Appl. Polym. Sci., 73, 455±468. Miguel O, Fernandez-Berridi M J and Iruin J J (1997), `Survey on transport properties of liquids, vapors, and gases in biodegradable poly(3-hydroxybutyrate) (PHB)', J. Appl. Polym. Sci., 64, 1849±1859. Miguel O, Barbari T A and Iruin J J (1999), `Carbon dioxide sorption and diffusion in poly(3-hydroxybutyrate) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate)', J. Appl. Polym. Sci., 71, 2391±2399. Mirzadeh A and Kokabi M (2007), `The effect of composition and draw-down ratio on


524


morphology and oxygen permeability of polypropylene nanocomposite blown films', Eur. Polym. J., 43, 3757±3765. Miyagawa T, Okuma K, Hirose F and Fukunaga S (2007), `Lightweight poly(3hydroxyalkanoate) foams and their manufacture', Japanese patent JP 2007291159. Mooney B P (2009), `The second green revolution? Production of plant-based biodegradable plastics', Biochem. J., 418, 219±232. Ohashi E, Drumond W S, Zane N P, Barros P W D F, Lachtermacher M G, Wiebeck H and Wang S H (2009), `Biodegradable poly(3-hydroxybutyrate) nanocomposite', Macromolecular Symposia, 279, 138±144. Olkhov A A, Vlasov S V, Iordanskii A L, Zaikov G E and Lobo V M M (2003), `Water transport, structure features and mechanical behavior of biodegradable PHB/PVA blends', J. Appl. Polym. Sci., 90, 1471±1476. Osman M A and Atallah A (2004), `High-density polyethylene micro- and nanocomposites: Effect of particle shape, size and surface treatment on polymer crystallinity and gas permeability', Macromol. Rapid Commun., 25, 1540±1544. Osman M A, Mittal V and Lusti H R (2004), `The aspect ratio and gas permeation in polymer-layered silicate nanocomposites', Macromol. Rapid Commun., 25, 1145± 1149. Parulekar Y and Mohanty A K (2007), Èxtruded biodegradable cast films from polyhydroxyalkanoate and thermoplastic starch blends: Fabrication and characterization', Macromol. Mater. Eng., 292, 1218±1228. Parulekar Y, Mohanty A K and Imam S H (2007), `Biodegradable nanocomposites from toughened polyhydroxybutyrate and titanate-modified montmorillonite clay', J. Nanosci. Nanotechnol., 7, 3580±3589. Pavlidou S and Papaspyrides C D (2008), À review on polymer-layered silicate nanocomposites', Prog. Polym. Sci., 33, 1119±1198. Petersen K, Vñggemose Nielsen P, Bertelsen G, Lawther M, Olsen M B, Nilsson N H and Mortensen G (1999), `Potential of biobased materials for food packaging', Trends Food Sci. Technol., 10, 52±68. Philip S, Keshavarz T and Roy I (2007), `Polyhydroxyalkanoates: biodegradable polymers with a range of applications', J. Chem. Technol. Biotechnol., 82, 233±247. Poirier Y, Nawrath C and Somerville C (1995), `Production of polyhydroxyalkanoates, a family of biodegradable plastics and elastomers, in bacteria and plants', Bio/ Technology, 13, 142±150. Poley L H, Siqueira A P L, Da Silva M G, Sanchez R, Prioli R, Mansanares A M and Vargas H (2005), `Photothermal methods and atomic force microscopy images applied to the study of poly(3-hydroxybutyrate) and poly(3-hydroxybutyrate-co-3hydroxyvalerate) dense membranes', J. Appl. Polym. Sci., 97, 1491±1497. Ray S S and Okamoto M (2003), `Polymer/layered silicate nanocomposites: A review from preparation to processing', Prog. Polym. Sci., 28, 1539±1641. Ray S S, Yamada K, Okamoto M, Fujimoto Y, Ogami A and Ueda K (2003), `New polylactide/layered silicate nanocomposites. 5. Designing of materials with desired properties', Polymer, 44, 6633±6646. Reis K C, Pereira J, Smith A C, Carvalho C W P, Wellner N and Yakimets I (2008), `Characterization of polyhydroxybutyrate±hydroxyvalerate (PHB±HV)/maize starch blend films', J. Food Eng., 89, 361±369. Reis M A M, Serafim L S, Lemos P C, Ramos A M, Aguiar F R and Van Loosdrecht M C M (2003), `Production of polyhydroxyalkanoates by mixed microbial cultures', Bioprocess. Biosyst. Eng., 25, 377±385. Renard E, Walls M, GueÂrin P and Langlois V (2004), `Hydrolytic degradation of blends



525

of polyhydroxyalkanoates and functionalized polyhydroxyalkanoates', Polym. Degrad. Stab., 85, 779±787. Rychter P, Biczak R, Herman B, Smytta A, Kurcok P, Adamus G and Kowalczuk M (2006), Ènvironmental degradation of polyester blends containing atactic poly(3hydroxybutyrate). Biodegradation in soil and ecotoxicological impact', Biomacromolecules, 7, 3125±3131. Sabet S S and Katbab A A (2009), Ìnterfacially compatibilized poly(lactic acid) and poly(lactic acid)/polycaprolactone/organoclay nanocomposites with improved biodegradability and barrier properties: Effects of the compatibilizer structural parameters and feeding route', J. Appl. Polym. Sci., 111, 1954±1963. Sanchez-Garcia M D, Gimenez E and LagaroÂn J M (2007), `Novel PET nanocomposites of interest in food packaging applications and comparative barrier performance with biopolyester nanocomposites', J. Plastic Film Sheeting, 23, 133±148. Sanchez-Garcia M D, Gimenez E and LagaroÂn J M (2008), `Morphology and barrier properties of nanobiocomposites of poly(3-hydroxybutyrate) and layered silicates', J. Appl. Polym. Sci., 108, 2787±2801. Savenkova L, Gercberga Z, Nikolaeva V, Dzene A, Bibers I and Kalnin M (2000), `Mechanical properties and biodegradation characteristics of PHB-based films', Process Biochem., 35, 573±579. Schmidt B, Petersen J H, Bender Koch C, Plackett D, Johansen N R, Katiyar V and Larsen E H (2009), `Combining asymmetrical flow field-flow fractionation with light-scattering and inductively coupled plasma mass spectrometric detection for characterization of nanoclay used in biopolymer nanocomposites', Food Addit. Contam., 26, 1619±1627. SÏimon P, Chaudhry Q and BakosÏ D (2008), `Migration of engineered nanoparticles from polymer packaging to food ± A physicochemical view', J. Food Nutr. Res., 47, 105±113. Spyros A, Argyropoulos D S and Marchessault R H (1997), `Study of poly(hydroxyalkanoate)s by quantitative 31P NMR spectroscopy: Molecular weight and chain cleavage', Macromolecules, 30, 327±329. Sudesh K and Abe H (2010), Practical Guide to Microbial Polyhydroxyalkanoates, Shawbury, Shrewsbury, UK, Smithers Rapra Publishing. Sudesh K and Doi Y (2000), `Molecular design and biosynthesis of biodegradable polyesters', Polym. Adv. Technol., 11, 865±872. Sudesh K and Iwata T (2008), `Sustainability of biobased and biodegradable plastics', Clean ± Soil, Air, Water, 36, 433±442. Sudesh K, Abe H and Doi Y (2000), `Synthesis, structure and properties of polyhydroxyalkanoates: Biological polyesters', Prog. Polym. Sci., 25, 1503±1555. Thellen C, Coyne M, Froio D, Auerbach M, Wirsen C and Ratto J A (2008), À processing, characterization and marine biodegradation study of melt-extruded polyhydroxyalkanoate (PHA) films', J. Polym. Environ., 16, 1±11. Tiede K, HasselloÈv M, Breitbarth E, Chaudhry Q and Boxall A B A (2009), `Considerations for environmental fate and ecotoxicity testing to support environmental risk assessments for engineered nanoparticles', J. Chromatogr. A, 1216, 503±509. Timmins M R, Lenz R W and Fuller R C (1997), `Heterogeneous kinetics of the enzymatic degradation of poly( -hydroxyalkanoates)', Polymer, 38, 551±562. Tsuji H and Suzuyoshi K (2002a), Ènvironmental degradation of biodegradable polyesters 1. Poly(-caprolactone), poly[(R)-3-hydroxybutyrate], and poly(Llactide) films in controlled static seawater', Polym. Degrad. Stab., 75, 347±355.


526


Tsuji H and Suzuyoshi K (2002b), Ènvironmental degradation of biodegradable polyesters 2. Poly(-caprolactone), poly[(R)-3-hydroxybutyrate], and poly(Llactide) films in natural dynamic seawater', Polym. Degrad. Stab., 75, 357±365. Tsuji H and Suzuyoshi K (2003), Ènvironmental degradation of biodegradable polyesters. IV. The effects of pores and surface hydrophilicity on the biodegradation of poly(-caprolactone) and poly[(R)-3-hydroxybutyrate] films in controlled seawater', J. Appl. Polym. Sci., 90, 587±593. Tsuji H, Suzuyoshi K, Tezuka Y and Ishida T (2003), Ènvironmental degradation of biodegradable polyesters: 3. Effects of alkali treatment on biodegradation of poly(caprolactone) and poly[(R)-3-hydroxybutyrate) films in controlled soil', J. Polym. Environ., 11, 57±65. Villaluenga J P G, Khayet M, Lopez-Manchado M A, Valentin J L, Seoane B and Mengual J I (2007), `Gas transport properties of polypropylene/clay composite membranes', Eur. Polym. J., 43, 1132±1143. Wang S, Song C, Chen G, Guo T, Liu J, Zhang B and Takeuchi S (2005), `Characteristics and biodegradation properties of poly(3-hydroxybutyrate-co-3-hydroxyvalerate)/ organophilic montmorillonite (PHBV/OMMT) nanocomposite', Polym. Degrad. Stab., 87, 69±76. Wu T, Hs S, Shih Y and Liao C (2008), `Thermal degradation kinetics of biodegradable poly(3-hydroxybutyrate)/layered double hydroxide nanocomposites', J. Polym. Sci. Part B, 46, 1207±1213. Xie W, Gao Z, Pan W, Hunter D, Singh A and Vaia R (2001), `Thermal degradation chemistry of alkyl quaternary ammonium montmorillonite', Chem. Mater., 13, 2979±2990. Xu C and Qiu Z (2009), Ìsothermal melt crystallization kinetics study of biodegradable poly(3-hydroxybutyrate)/multiwalled carbon nanotubes nanocomposites', Polym. J., 41, 888±892. Yoon J, Jung H, Kim M and Park E (2000), `Diffusion coefficient and equilibrium solubility of water molecules in biodegradable polymers', J. Appl. Polym. Sci., 77, 1716±1722. Yue C L, Gross R A and McCarthy S P (1996), `Composting studies of poly( hydroxybutyrate-co- -hydroxyvalerate)', Polym. Degrad. Stab., 51, 205±210. Yun S I, Gadd G E, Latella B A, Lo V, Russell R A and Holden P J (2008), `Mechanical properties of biodegradable polyhydroxyalkanoates/single wall carbon nanotube nanocomposite films', Polym. Bull., 61, 267±275. Zenkiewicz M and Richert J (2008), `Permeability of polylactide nanocomposite films for water vapour, oxygen and carbon dioxide', Polym. Test., 27, 835±840. Zhang M and Thomas N L (2010), `Preparation and properties of polyhydroxybutyrate blended with different types of starch', J. Appl. Polym. Sci., 116, 688±694.


19


Â L E Z and M . P . V I L L A N U E V A , R. M. GONZA Technological Institute of Plastic (AIMPLAS), Spain

Abstract: This chapter presents a review of the use of starch in food packaging. The chapter first reviews the use of starch during recent years and its particular properties. Then, it is focused on the latest developments in starch processing techniques for food packaging production. Finally, specific sections about the mechanical and barrier performance and the use of starchbased nanocomposites are included, due to their relevant importance in order to meet the often stringent food packaging requirements. Key words: starch structure, starch properties, processing, mechanical and barrier performance, starch nanocomposites.

19.1

Introduction

The use of natural polymers has received increased attention in recent years, having great potential as substitutes for conventional polymers in a broad range of applications. The development of more environmentally friendly thermoplastic materials has been the subject of a large number of studies and investigations. One of the most interesting applications for these materials is food packaging in which a short shelf-life of packages is required and there is an increasing demand for food products to be packed. The use of these new materials would allow new management of plastic residues and reduction of the dependence on petroleum. Among the natural polymers, there has been particular interest in the use of starch. Due to its nature, starch is inherently biodegradable. Only carbon dioxide and water are needed by plants to synthesize it by photosynthesis (Teramoto et al., 2003). On the other hand, its labile bonds can be hydrolysed into glucose by microorganisms or enzymes, and then metabolized into carbon dioxide and water (Primarini and Ohta, 2000). In addition, starch is a cheap material, abundant in the nature and renewable. In food packaging, starch films have been very interesting as they are excellent oxygen barriers due to their particular chemical structure (McHugh and Krochta, 1994; Nisperos-Carriedo, 1994). In this chapter, a review of the use of starch in food packaging is presented, starting with the evolution of the use of starch during recent years (Section 19.2) and its particular properties (Section 19.3). In the second stage, a summary of


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the latest developments in starch processing techniques for food packaging production is provided (Section 19.4). Finally, specific sections concerning the mechanical and barrier performance (Section 19.5) and the use of starch-based nanocomposites (Section 19.6) are included, due to their relevant importance in meeting food packaging requirements. To complete the chapter, a section about future trends in starch developments is included (Section 19.7).

19.2

Market for starch-based materials and potential applications

Starch production and its applications have undergone considerable evolution through the last few years. The following paragraphs explain the evolution of starch in the market, focused on the main manufacturers.

19.2.1 Evolution of starch in the plastic industry The introduction of starch in the plastic sector was motivated by its low cost and biodegradability. Starch was first used as a filler in commodity plastics to reduce the price and to increase the rate of biodegradation of synthetic polymers. Afterwards, the developments in starch plasticization and starch processing led to the use of different starch-based systems for different purposes. Currently, the tendency in the market is the production of starch-based materials with a high content of starch together with other biodegradable plastics (30±80%). These can be co-polymers derived from natural sources such as poly(lactic acid) (PLA), polyhydroxyalkanoate (PHA) and polyhydroxybutyrate (PHB), or derived from fossil fuels such as poly(butylene succinate) (PBS), poly(butylene succinate-co-adipate) (PBSA), poly(butylene adipate-coterephthalate (PBAT), polyvinyl alcohol (PVOH) and polycaprolactone (PCL) (Schwach and AveÂrous, 2009; Vroman and Tighzert, 2009). Some of them (PBS, PBSA, PBAT) can be potentially produced from bio-based succinic acid by fermentation. However, the fully bio-based blends are not yet commercially available. In 2003, the market for starch-based bioplastics accounted for about 25,000 tons/year (Shen et al., 2009). The market share of these products accounted for about 70% of the global market for bioplastics (Bastioli, 2005). The global consumption of starch-based biodegradable polymers increased up to 114,000 tons in 2007. However, the production capacity according to the latest data reported by the University of Utrecht shows a projected increase to 810,000 tons for 2020 (Fig. 19.1). The estimated production volumes of the main producers for 2009 and 2013 are given in Table 19.1. Figure 19.2 shows the global consumption of starch-based biodegradable polymers in the main sectors. As can be seen, loose-fill packaging represents the greatest consumption of starch (52%), followed by bags and sacks with 28% and



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19.1 Current and potential volume production of starch-based materials in Europe (obtained from Shen et al., 2009).

packaging with 14%. Other sectors (6%) include agricultural films, hygiene products and injected parts. The price of starch-based materials has been decreasing over recent years, allowing them to compete with traditional plastics in some limited areas. According to the composition of the blend the price may vary. An example of this is the price of Mater-BiTM which ranges at present from 1.5 to 4 ¨/kg, compared to prices of between 3 and 5 ¨/kg in 2003 (Bastioli, 2000). However, this is still a high price compared to commodity plastics such as polyethylene (PE), polypropylene (PP), polystyrene (PS), polyethylene terephthalate (PET) and polyvinyl chloride (PVC) (1±1.4 ¨/kg), which limits the use of starch plastics in some applications.

Table 19.1 Production capacity of starch-based polymers by the main producers in Europe Producer Novamont (MaterBiÕ) Biotec (BioplastÕ) Rodenburg (SolanylÕ) BIOP (BioParÕ)

Year 2009 (tons)

Year 2013 (tons)

60,000 60,000 40,000 4,000

100,000 60,000 40,000 24,000

Source: adapted from Shen et al., 2009.


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19.2 Global consumption of starch-based biodegradable polymers by end use sector, year 2005 (includes Western Europe, North America and Asia Pacific) (obtained from Platt, 2006).

19.2.2 Main applications and manufacturers One of the first applications of starch-based biodegradable materials in the packaging sector was introduced in the market with the production of foamed starch loose-fill packaging. The company National Starch Co. introduced two technologies for the production of this product: one from hydroxypropylated high-amylose starch and the second from unmodified starch. Currently, among the applications of starch-based blends, packaging is the dominant area. In 2003, Novamont devoted 75% of its total production to packaging, while BIOP devoted 80% in 2007. Starch-based plastics available on a large scale in the market are mainly to produce foams, films and mouldable products (De BragancËa and Fowler, 2004). · Foams: Starch has increased the production of loose-fill foam packaging in the last 10 years, mainly replacing polystyrene foams. The rate is increasing as the quality of starch-based materials is improving. For example in the US, around 25% of expanded polystyrene use has been replaced by starch-based foams. · Films and nets: Starch-based films may be applied to agriculture, e.g. mulch films, plastic shopping bags, the composting sector, e.g. bags and sacks, laminated paper and food containers. Such markets are developed in Italy, Germany and the Scandinavian countries. Applications as nets for fresh fruit, vegetables or seafood are also possible. · Moulded products: Compounded thermoplastic starch is mouldable in a similar way to traditional mouldable plastics like acrylonitrile butadiene styrene (ABS), polystyrene (PS) and low density polyethylene (LDPE).



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Commercially, the most important sector is the production of thermoformed products for fast-food packaging, for example trays for fresh food, plates, bowls and cups. In most of these products, starch is foamed. Injected parts are also possible to obtain hygiene products, sanitary products, etc. Nowadays, the two big worldwide producers of starch-based materials are Novamont and Biotec, but there are other relevant starch-based polymer producers as shown in Table 19.2.

19.3

Structure and properties of native and plasticized starch

Starch is a natural carbohydrate which acts as the main means of energy storage in a great variety of plants such as wheat, potato, corn, cassava, tapioca, rice or pea among others, being located in the roots, seeds and stems in granule form. The shape and size of these granules depend on the starch source, typical dimensions ranging between 0.5 and 175 microns (Donald, 2004). Chemically, starch is a polysaccharide consisting of a mixture of amylose, a linear polymer, and amylopectin, a highly branched polymer having the same backbone structure as amylose but with branched points. The polymer building block of both components is the monomer glucose. In the case of amylose, -1,4linkages take place to form a linear structure with a molecular weight of 105±106, whereas in the case of amylopectin, the linear chain based on -1,4 bonds has also -1,6-linkages forming the branches, the molecular weight being 107±109. Figure 19.3 shows schemes of (a) amylose and (b) amylopectin structures. The amylose/amylopectin ratio in starch granules varies with the source of the starch. The level of amylose in starch is usually between 20% and 30% in weight (Oxford et al., 1987; Parker and Ring, 1996; Ramesh et al., 1999), although in some cases it can be higher. AveÂrous and Halley (2009) reported that some mutant plant species present singular compositions, such as the case of amylose-rich starches, like amylomaize, where the amylose level is up to 80%, and some amylopectin-rich starches, like the waxy maize, with an amylopectin level of 99%. There is evidence of the great influence of the amylose/ amylopectin ratio in the physical and chemical properties of a particular starch, as well as subsequent mechanical processing (Fang and Fowler, 2003). Starch granules also contain small amounts of lipids and proteins. Physically, most native starches are semi-crystalline. Their crystallinity has been reported to be between 20% and 45% (Whistler et al., 1984). Amylose and amylopectin are arranged in the granules in complex structures consisting of crystalline and amorphous areas (French, 1984; Blanshard, 1987). The crystalline areas are formed as a consequence of the specific arrangement of the branches in the amylopectin chains (Manners, 1989). The short branches are believed to form double helices, which to a great extent are organized into


532


Table 19.2 Summary of commercial biodegradable starch-based materials with applications in the packaging sector Commercial starch

Processing method

MaterBiÕ (Novamont)

Injection moulding, extruded articles, film, thermoforming

Applications in packaging

Bags, film for packaging, trays for fresh food, sheets, nets for fruit and vegetables, expanded trays, loose-fill packaging Injection moulding, sheet Packaging, film, trays, carrier BioplastÕ (Biotec GmbH & film extrusion, blown film, and refuse bags, net bags, Co.) thermoforming thermoformed products, single-use disposable fast food packaging BioparÕ Mono and co-extruded film Barrier packaging, food (BIOP Biopolymer blowing, bottle blowing, packaging, fruit and vegetable Technologies AG) cast film, injection packaging, shopping bags, moulding, thermoforming refuse and waste bags Injection moulding, Thermoformed trays and CereplastÕ thermoforming, extrusion packages, mugs (Cereplast, Inc.) coating, blow moulding, profile extrusion Injection moulding, film Foamed products: trays, EverCornTM (Japan Corn Starch blowing, sheet extrusion, packaging; multifilm, food Co. Ltd) thermoforming wrapping film, mouldings such as knife, fork, cup, etc. PlanticÕ Injection moulding, Packaging for chocolates, (Plantic Technologies thermoforming, sheet and flexible and rigid packaging GmbH) casting extrusion Injection moulding Protection corner for SolanylÕ (Rodenburg packaging Biopolymers B.V.) TerratekÕ Injection moulding Tableware (MGP Ingredients, Inc.) GraceBioÕ Film blowing Shopping bags, net bags (Grace Biotech Corporation) PSM Film blowing, injection, Flexible and rigid packaging (PSM North extrusion, foaming America) Film blowing Film for packaging fresh food, BiostarchÕ (Biostarch shopping bags Technology Pte. Ltd.) Film blowing, sheet Films, packaging, cosmetics, TerraloyTM (Teknor Apex extrusion, injection catering/housewares, Company) consumer products, food contact products Note: More information about these commercial grades can be found in the web pages of the companies specified in Section 19.8. Source: information obtained from the web pages of the manufacturers.



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19.3 Molecular structure of (a) amylose and (b) amylopectin.

crystallites. On the other hand, amylose is believed to form part of the amorphous regions, together with the long chains of amylopectin (AveÂrous and Halley, 2009). The amorphous and crystalline parts are arranged in alternate layers within the starch granules (AveÂrous and Halley, 2009; Perez and Imberty, 1996). As some authors reported (Wang et al., 1998; Hedley, 2001), according to the arrangement of the amylopectin double helices, their packing density and the amount of bound water within the crystal structure, two types of crystallites or polymorphs can be found in the starch granules, called A and B. The Apolymorph is a more dense packed structure and contains less water molecules than the B-polymorph (Sarko and Wu, 1978; Imberty and Perez, 1988). Starches can contain either A or B or both polymorph forms, classifying starches as A, B or C respectively. Whereas A-starches are present in cereals, B-starches can be found in tubers and C-starches in legumes (Manners, 1989; Oxford et al., 1987). Table 19.3 shows starch composition, granule diameter and degree of crystallinity of starches from different sources (AveÂrous, 2004). As can be seen, starches containing the less amount of amylopectin, in this case the amylomaize, show lower percentages of crystallinity. Among the other starches, all those with higher amounts of amylopectin, A-type (wheat, maize and waxy starch), showed a percentage of crystallinity between 36% and 39%, whereas in the case of Btype (potato), this is 25%. This difference may be associated with the differences in the packing density between the A- and B-polymorphs. Native starch is a powder insoluble in cold water or organic solvents (Radley, 1953). The melting temperature of pure dry starch has been reported to be


534


Table 19.3 Composition and characteristics of different starches determined on a dry basis Starch source Wheat Maize Waxy starch Amylomaize Potato

Amylose Amylopectin Lipid content content content (%) (%) (%) 26±27 26±28 5 mm) and fibre weight content (>10%) led to an increase of TS and Young's modulus and a decrease of EB (Lodha and Netravali, 2002). The same type of protein was reinforced by the incorporation of different volume fractions of alkali-treated and untreated banana fibres, with different amounts of glycerol as plasticizer. Results showed that TS and Young's modulus of SPI reinforced with the alkali-treated fibre increased by 82% and 963%, respectively, compared to soy protein film without fibres (Kumar et al., 2008). Caseinate films were reinforced with two lignocellulosic fibres: wood pulp fibre and flax bast fibre. Young's modulus increased from 200 MPa to 1200 MPa when 20% of fibre was added to the film. Simultaneously, TS increased from 6 to 30 MPa, for films with 0 and 20% of fibre, respectively (Fossen et al., 2000). Pressure, temperature and holding time used for thermoforming whey protein sheets do not significantly affect the mechanical properties (Sothernvit et al., 2007). However, Table 22.2 shows that for nearly similar formulations, the process used to produce films dramatically changes the mechanical properties. A systematic comparison between solution casting and compression moulding suggested that improved mechanical properties are obtained in terms of enhanced tensile strength and elongation at break (Sothernvit et al., 2007). Extrusion seems to perform even better if elongation at break is the film's mechanical property to optimize. Extruded whey protein isolate films plasticized with glycerol (Hernandez-Izquierdo et al., 2008) show doubled elongation at break when compared to film obtained from cast solution (Sothornvit et al., 2009). Film blowing is the most popular cost-effective process to produce thin films and pouches, and low density polyethylene grades specifically designed for this


Table 22.2 Comparison of the tensile strength (TS), elongation-at-break (EB) and Young's modulus (YM) of protein films Film type


Test conditions

TS (MPa)

EB (%)

Soy protein concentrate Soy protein concentrate:Phytagel (40%) Soy protein concentrate:Phytagel (40%):clay(7%) Soy protein isolate (5%):glycerol (2.5%) Soy protein isolate:glycerol (10%) (extrusion) Soy protein isolate:glycerol (40%) (extrusion) Soy protein isolate:glycerol (40%) (compression moulding) Caseinate:glycerol (17%) Caseinate:glycerol (17%):wood pulp fibre (20%) Caseinate:glycerol (17%):flax bast fibre (20%) Soy protein isolate: cellulose (9:1) Soy protein isolate: cellulose (1:9) Whey protein isolate: glycerol (2:1) Whey protein isolate: glycerol (2:1) (extrusion)

21ëC, 65% RH 21ëC, 65% RH 21ëC, 65% RH 25ëC 25ëC, 50% RH 25ëC, 50% RH 25ëC

14.7 50.1 74.5 1 41 9 2.6

20ëC, 60% RH 20ëC, 60% RH 20ëC, 60% RH 20ëC, 70% RH 20ëC, 70% RH 25ëC, 50% RH

5 30 33 10.7 38.1 3.4 3.5

48.6 108.1 50.9 121

Whey protein isolate: glycerol (2:1) (compression moulding) Whey protein isolate: glycerol:Cloisite 20A (2:1.5%) Whey protein isolate: glycerol:Cloisite Na+ (2:1.5%) Soy flour/pectin Soy flour/pectin/TGase Wheat gluten:glycerol (15:6) Wheat gluten:glycerol: sucrose (15:3:3) Zein:polyethylene-glycol (75:25) (film blowing)

23ëC, 50% RH

4

94

25ëC, 50% RH

1.55

29.1

115.5

Sothornvit et al., 2009

25ëC, 50% RH

2.98

42.4

109.3

Sothornvit et al., 2009

23ëC, 50% RH 23ëC, 50% RH 25ëC, 50% RH 25ëC, 50% RH

6.8 12.4 4.2 3.8 0.04

25.7 14.8 9.5 275 3 159 74.5

89 11 270

YM (MPa) 201 717 30 1226 176 190 1050 600 171.8 37 60

11.61 7.21 5.7

Reference Huang and Netravali, 2006 Huang and Netravali, 2006 Huang and Netravali, 2006 Chen and Liu, 2008 Zhang et al., 2001 Zhang et al., 2001 Cunningham et al., 2000 Fossen et al., 2000 Fossen et al., 2000 Fossen et al., 2000 Wu et al., 2009 Wu et al., 2009 Sothornvit et al., 2009 Hernandez-Izquierdo et al., 2008 Sothornvit et al., 2007

Mariniello et al., 2003 Mariniello et al., 2003 Cherian et al., 1995 Cherian et al., 1995 Oliviero et al., 2010


633

plastic transformation are marketed. Common strains at break are of the order of 500±700% and are achieved through a complex tailoring of process parameters and macromolecular architecture (namely, long-chain branching which promotes enhanced strain hardening of the melt). Recently, thermoplastic zein was successfully processed into extruded tubes to be further blown into films. The mechanical properties of the film are still below those of conventional plastics (see Table 22.2), but the film-blowing ability of such zein-based materials plasticized with polyethylene glycol, directly in the extruder, will turn this biodegradable protein-based material into a commercially attractive alternative to petroleum-based packages. Furthermore, Oliviero et al. (2010) studied the structural properties of zein batches which showed best performance in film blowing. They concluded that the optimum strain-hardening behaviour of the plasticized paste was correlated with a large content of -helices. This means that besides the enhanced mechanical properties achieved though the addition of plasticizers, the secondary structural properties of proteins and process-induced crystalline structures play an important role in the toughness and stretching of the final film. The storage time is one of the problems of protein films. Some works showed that the ageing of films leads to deterioration and breakdown of their properties due to the loss of plasticizers. Wheat gluten and corn-zein films with glycerol and poly(ethylene glycol) as plasticizers after 20 days of storage presented changes in mechanical properties (Park et al., 1994a). Also the mechanical properties of whey protein films plasticized with glycerol were changed after four months of storage; however, when glycerol was replaced by sorbitol they remained stable (Anker et al., 2001). Table 22.2 shows values for mechanical parameters of protein films. Despite the problems still encountered in protein-based films when mechanical properties are considered, if biodegradable protein films with satisfactory mechanical properties and good appearance are obtained they can become potential and ecological alternatives for replacing synthetic packaging materials in food and pharmaceutical applications.

22.3.4 Opacity and colour parameters Opacity provides an indication of how much light passes through a film; this may be crucial in cases where it is important to control the incidence of light on the food products. Also, the film colour can be an important factor in terms of consumer acceptance. In the L* a* b* colour system, L* represents the lightness, and a* and b* are colour coordinates, where a* is in the red direction, ÿa* is in the green direction, +b* is in the yellow direction, ÿb* is in the blue direction, low L* is dark, and high L* is light. Also the whiteness index, the yellowness index and the total colour difference (E) are often used to characterize the colour of films.


634


The addition of oil decreased the transparency of caseinate films. Oleic acid presented higher values of transparency than fatty acids and beeswax, possibly due to the heterogeneity of the latter two components when present in the film's structure (Fabra et al., 2009). The same behaviour was observed for gelatine films when sunflower oil was incorporated: the films without oil were transparent, but when oil was added the transparency decreased by approximately 10% (PeÂrez-Mateos et al., 2009). The colour evaluation of WPI films showed that the incorporation of mesquite gum decreased lightness (L*) and increased a* and b*, leading to more reddish and yellowish films (OseÂs et al., 2009). SPI films reinforced with cellulose nanofibrous material showed high visible light transmittance with values over 75% at 700 nm (Chen and Liu, 2008). The incorporation of clay composite appears to influence colour film properties. WPI/Cloisite 30B organoclay films with different amounts of clay resulted in films with an opaque appearance, which depended on the amount of clay added (Sothornvit et al., 2010). The drying temperature and pH of film-forming solutions can also influence the colour parameters of protein films. Peanut protein films were obtained at different pH values and dried under different temperatures. Results showed that colour parameters L* and a* decreased with increasing pH, thus influencing the darkness of the film, while b* increased with increasing temperature, leading to more yellowish films (Jangchud and Chinnan, 1999).

22.4

Applications

Edible films can be useful as barriers to gases (water vapour, oxygen, carbon dioxide, aromas). Nevertheless, when possessing the appropriate mechanical properties, they can also be useful for food protection, reducing bruising and breakage, for example, and thus improving food integrity. Recently, edible films have also been used as a vehicle for antioxidants and/or antimicrobials, enhancing their functional properties (Han and Gennadios, 2005; GoÂmez-Estaca et al., 2009; Guillard et al., 2009; Lu et al., 2009).

22.4.1 Incorporation of functional compounds Antimicrobial compounds Antimicrobial packaging materials may be classified in two types: those that contain antimicrobial agents that migrate to the surface of the packaging material and thus can contact the food; and those that are effective against food surface microbiological growth without the migration of the active agent to the food surface (Brody et al., 2001). This means that different packaging strategies can be adopted. The bioactive compound may be (1) incorporated directly into the polymer; (2) adsorbed onto polymer surfaces; (3) immobilized in the



635

polymers through ion or covalent linkages; or (4) not used, when the polymers making part of the packaging material are already inherently antimicrobial (Appendini and Hotchkiss, 2002). Protein-based films can be used as a solid thin layer that is subsequently applied on the food. Their utilization as coatings presents great advantages. They are applied in the liquid form on a food by dipping, spraying or brushing (Krochta, 2002; Zhao and McDaniel, 2005). They can also serve as carriers for a wide range of food additives, including colourants, flavours, antibrowning agents, spices, nutrients and various antimicrobials that can extend product shelf-life and reduce the risk of pathogen growth on food surfaces (Baldwin et al., 1996; Han, 2000; Lee, 2005). Incorporating antimicrobial compounds into edible films or coatings provides a way to improve the safety and shelf-life of ready-to-eat foods (Cagri et al., 2004). Various antimicrobial agents may be incorporated in the packaging system, such as chemical antimicrobials, natural antimicrobials, antioxidants and antimicrobial polymers (Appendini and Hotchkiss, 2002), among others. Several works have tested these functional compounds against some of the most problematic microorganisms in the food industry, often showing good results. WPI films with particles of Cloisite 30B showed a bacteriostatic effect against Listeria monocytogenes (Sothornvit et al., 2009). SPI films combined with grape seed extract, nisin and EDTA showed the greatest inhibitory activity against L. monocytogenes, Escherichia coli O157:H7 and Salmonella typhimurium, with reductions of 2.9, 1.8 and 0.6 log CFU/mL, respectively (Sivarooban et al., 2008). The antimicrobial properties of WPI films containing oregano, rosemary and garlic essential oils were tested against E. coli O157:H7, Staphylococcus aureus, S. enteritidis, L. monocytogenes and Lactobacillus plantarum. All of the essential oils are shown to be effective against those bacteria; however, oregano essential oil was the most effective when used at a 2% level (Seydim and Sarikus, 2006). Film blends of casein and starch were used to incorporate neem (Melia azadirachta) extract, which was shown to be effective in inhibiting the growth of E. coli, S. aureus, B. cereus, L. monocytogenes and Pseudomonas spp. (Jagannath et al., 2006). Antioxidants Oxidation can seriously limit food preservation, and is involved in one of the most important degradation reactions in foodstuffs (NerõÂn et al., 2008). The use of antioxidants is therefore very important in the food industry in order to decrease food oxidation and thus increase shelf-life. Protein-based films can provide a good vehicle for antioxidant application, incorporated in a film or in a coating (Zhao and McDaniel, 2005). Gelatine (tuna-fish) films were successfully enriched with murta extracts and showed ability to act as antioxidant carriers (GoÂmez-Estaca et al., 2009). Films


636


of SPI with ferulic acid showed antioxidant activity, presenting an optimal concentration of 100 mg of ferulic acid per 100 g of SPI in the film-forming solution (Ou et al., 2005). Calcium caseinate and WPI films were prepared with a mixture of calcium lactate and gluconate and -tocopherol acetate, showing capabilities of carrying high concentrations of these antioxidant compounds (Mei and Zhao, 2003). Other compounds Application to agriculture can also be foreseen for biodegradable plastics as long as water absorbance and controlled release of fertilizers are inherent properties of the covering layers. Wheat gluten was mixed with glycerol and water in order to obtain a dough-like product to be compressed and moulded at high temperature (GoÂmez-MartõÂnez et al., 2009). KCl and citric acid were also added to the mixture during compounding in order to play the role of the released agent and plasticizer, respectively. Compression moulding of the optimal formulations in terms of rheological properties led to the successful production of sheets that were screened for their water absorption and KCl release properties. It was found that citric acid slowed down the release of KCl and significantly increased the water absorption of the sheets. From the mechanical testing of sheets, the authors hypothesized that the addition of citric acid actually modified the microstructure of the bioplastic. However, the exact mechanism of this additive on the slow release and improved water uptake still needs to be identified.

22.4.2 Applications to foods Protein-based films and coatings were tested in different food products in order to extend their shelf-life. The following is a summary. · Casein protein coatings were successfully used to reduce water loss in zucchini (Avena-Bustillos et al., 1994). · WPI was used to coat dry roasted peanuts and was shown to decrease their oxidative deterioration (Mate et al., 1996). Also WPI used with acetylated monoglyceride was shown to reduce peroxide and moisture loss values in stored frozen king salmon (Stuche and Krochta, 1995). · Corn-zein films delayed ripening and colour change in tomatoes during storage (Park et al., 1994b). · Edible films composed of soybean protein, stearic acid and pullulan were used to preserve kiwifruit; results showed that the use of edible film retarded the senescence process, the softening rates of coated and uncoated kiwifruit being 29% and 100%, respectively, after 37 days' storage (corresponding to a three-fold extension of the shelf-life of the product) (Xu et al., 2001). · Films of wheat gluten and a bilayer of lipids have shown a significant effect



· ·

·

·

·

637

on the retention of firmness, and reduced the weight loss of strawberries. However, the appearance and taste of bilayer-coated fruits were unacceptable (Tanada-Palmu and Grosso, 2005). Films of collagen and galactomannan have been used to decrease the gas transfer rate of fruits (apple and mango), giving good indications for fruit preservation (Lima et al., 2010). Edible coatings of WPC decreased the initial respiration rate of minimally processed apples at 25ëC. Later, WPI coatings with an antibrowning (CaCl2) agent were shown to effectively increase the shelf-life of minimally processed apples by 2 weeks at 3ëC (Lee et al., 2003). Milk protein-based edible films containing oregano, paprika or mixed oregano±paprika essential oils were applied on beef muscle slices to control the growth of pathogenic bacteria and increase the shelf-life during storage at 4ëC. The film containing oregano was the most effective against bacteria (Pseudomonas spp. and E. coli O157:H7), whereas the film containing paprika oil seemed to be the least effective against those two bacteria. Films containing oregano extract showed 0.95 and 1.12 log reductions against Pseudomonas spp. and E. coli O157:H7, respectively, when compared to samples without film (Oussalah et al., 2004). Muscle protein films with a combination of palm oil and chitosan were used to cover dried fish powder. Samples showed lower thiobarbituric acid reactive substances and lower yellowness than other samples during extended storage up to 21 days (Artharn et al., 2009). WPI films incorporating different levels of oregano oil were used to increase the shelf-life of fresh beef (5ëC, 12 days). Wrapping of beef cuts with the antimicrobial films resulted in a decrease of colour changes, and while the maximum growth rate and total flora of Pseudomonas spp. were reduced, the growth of lactic acid bacteria was completely inhibited (Zinoviadou et al., 2009). The same authors used sodium lactate and 3-polylysine in WPI films against the spoilage in fresh beef. They showed that the total flora was reduced with antimicrobial films containing 3-polylysine, while for films with sodium lactate a decrease of growth of the total flora and Pseudomonas spp. was observed (Zinoviadou et al., 2010).

Protein-based films and coatings can be successfully used in enhancing and maintaining the quality of food products through increasing shelf-life and improving safety. The main problems associated with these materials are their mechanical and transport properties, together with their water solubility: though significant progress has been made, those properties are still not all at a satisfactory level when compared to, e.g., petroleum-based materials. It is expected, however, that the advantages mentioned above combined with the environmental friendliness of using biodegradable materials will certainly foster further research and application of protein-based resins for packaging.


638

22.5


Future trends

In recent years great progress has been made in developing edible films from protein sources. Protein-based films present a good barrier to gases (oxygen and carbon dioxide), are promising vehicles for a great number of functional compounds, and have been shown to be usable in different food products in order to extend their shelf-life. Protein-based film production through solution casting is the method most often used; however, the `dry process' using equipment already used in commercial plastics has been tested and shown to be a promising method (within limits) for protein-based film production. The main drawbacks of protein-based films are their poor water vapour barrier, mechanical resistance and thermal properties. As already reported above, nanotechnology can be a feasible way to improve these properties. Particles on the nanoscale can affect film properties: they can be used to improve mechanical strength, increase heat resistance and decrease permeability to gases. Also the utilization of nanofilms and multi-nanolayer films and coatings holds promise for food applications, mainly as part of food preservation and safety strategies. The use of this technology is envisaged, e.g., in systems aimed at integrating the sensing, localization, reporting and remote control of food products and for the encapsulation of functional food ingredients (e.g. multi-nanolayer films could include various functional agents such as antioxidants, antimicrobials, flavours, enzymes, etc.). A more significant research effort should be undertaken in order to understand how the utilization of materials on the nanoscale can change the properties of protein-based film and show how such materials can affect, e.g., thermoplastic processing steps in order to make this an alternative to their production by conventional casting. In conclusion, the utilization of multinanolayered protein-based films/coatings is essentially unexplored and is one of the future trends in applying nanotechnology to protein-based packaging materials.

22.6

References

Anker, M., Stading, M. and Hermansson, A. M. (2001). Àging of whey protein films and the effect on mechanical and barrier properties'. Journal of Agricultural and Food Chemistry 49: 989±995. Appendini, P. and Hotchkiss, J. H. (2002). `Review of antimicrobial food packaging'. Innovative Food Science & Emerging Technologies 3: 113±126. Artharn, A., Prodpran, T. and Benjakul, S. (2009). `Round scad protein-based film: Storage stability and its effectiveness for shelf-life extension of dried fish powder'. LWT ± Food Science and Technology 42: 1238±1244. Arvanitoyannis, I., Nakayama, A. and Aiba, S. (1998). Èdible films made from hydroxypropyl starch and gelatin and plasticized by polyols and water'. Carbohydrate Polymers 37: 371±382. Athamneh, A. I. and Barone, J. R. (2009). Ènzyme-mediated self-assembly of highly



639

ordered structures from disordered proteins'. Smart Materials and Structures 18: 104024. Avena-Bustillos, R. J. and Krochta, J. M. (1993). `Water vapor permeability of caseinatebased edible films as affected by pH, calcium crosslinking and lipid content'. Journal of Food Science 58(4): 904±907. Avena-Bustillos, R. K., Saltveit, M., Rojas-Villegas, R. and Sauceda-Perez, J. (1994). Òptimization of edible coating formulations on zucchini to reduce water loss'. Journal of Food Engineering 21: 197±214. Ayranci, E. and Tunc, S. (2003). À method for the measurement of the oxygen permeability and the development of edible films to reduce the rate of oxidative reactions in fresh foods'. Food Chemistry 80(3): 423±431. Azeredo, H. M. C. (2009). `Nanocomposites for food packaging applications'. Food Research International 42: 1240±1253. Baldwin, E. A., Nisperos, M. O., Chen, X. and Hagenmaier, R. D. (1996). Ìmproving storage life of cut apple and potato with edible coating'. Postharvest Biology and Technology 9: 151±163. Barone, J. R., Dangaran, K. and Schmidt, W. F. (2006a). `Blends of cysteine-containing proteins'. Journal of Agricultural and Food Chemistry 54: 5393±5399. Barone, J. R., Schmidt, W. F. and Gregoire, N. T. (2006b). Èxtrusion of feather keratin'. Journal of Applied Polymer Science 100: 1432±1442. Bergo, P. and Sobral, P. J. A. (2007). Èffects of plasticizer on physical properties of pigskin gelatin films'. Food Hydrocolloids 21: 1285±1289. Boundy, J. A., Turner, J. E., Wall, J. S. and Dimler, R. J. (1967). Ìnfluence of commercial processing on composition and properties of corn zein'. Cereal Chemistry 44: 281±287. Bourtoom, T. (2008). Èdible films and coatings: characteristics and properties'. International Food Research Journal 15(3): 237±248. Bourtoom, T. (2009). Èdible protein films: properties enhancement'. International Food Research Journal 16(1): 1±9. Brandenburg, A. H., Weller, C. L. and Testin, R. F. (1993). Èdible films and coatings from soy protein'. Journal of Food Science 58(5): 1086±1089. Brault, D., D'Aprano, G. and Lacroix, M. (1997). `Formation of free-standing sterilized edible films from irradiated caseinates'. Journal of Agricultural and Food Chemistry 45: 2964±2969. Brody, E.P. (2000). `Biological activities of bovine glycomacropeptide'. British Journal of Nutrition, 84: S39±S46. Brody, A. L., Strupinsky, E. R. and Kline, L. R. (2001). Active Packaging for Food Applications. Lancaster, PA, Technomic Publishing. Caessens, P. W. J. R., Visser, S. and Gruppen, H. (1997). `Method for the isolation of bovine beta-lactoglobulin from a cheese whey protein fraction and physicochemical characterization of the purified product'. International Dairy Journal 7(4): 229± 235. Cagri, A., Uspunol, Z. and Ryser, E. (2004). Àntimicrobial edible films and coating'. Journal of Food Protection 67(4): 833±848. Chen, G. and Liu, H. (2008). Èlectrospun cellulose nanofiber reinforced soybean protein isolate composite film'. Journal of Applied Polymer Science 110: 641±646. Chen, H. (1995). `Functional properties and applications of edible films made of milk proteins'. Journal of Dairy Science 78: 2563±2583. Chen, H. (2002). `Formation and properties of casein films and coatings'. Protein-based Films and Coatings. Gennadios, A., Boca Raton, FL, CRC Press.


640


Cherian, G., Gennadios, A., Weller, C. and Chinachoti, P. (1995). `Thermomechanical behaviour of wheat gluten films: Effect of sucrose, glycerin, and sorbitol'. Cereal Chemistry 72(1): 1±6. Chiellini, E., Cinelli, P., Fernandes, E. G., Kenawy, el-R. S. and Lazzeri, A. (2001). `Gelatin-based blends and composites. Morphological and thermal mechanical characterization'. Biomacromolecules 2: 806±811. Ciesla, K., Salmieri, S. and Lacroix, M. (2006). `Modification of the properties of milk protein films by gamma radiation and polysaccharide addition'. Journal of the Science of Food and Agriculture 86: 908±914. Clark, A.H., Kavanagh, G.M. and Ross-Murphy, S. B. (2001). `Globular protein gelation ± theory and experiment'. Food Hydrocolloids 15(4±6): 383±400. Considine, T., Patel, H. A., Anema, S. G., Singh, H. and Creamer, L. K. (2007). Ìnteractions of milk proteins during heat and high hydrostatic pressure treatments ± a review'. Innovative Food Science & Emerging Technologies 8(1): 1±23. Covas, J. A. and Machado, A. V. (2004). `Monitoring reactive processes along the extruder'. International Polymer Processing 91, 2711±2720. Cunningham, P., Ogale, A. A., Dawson, P. L. and Acton, J. C. (2000). `Tensile properties of soy protein isolate films produced by a thermal compaction technique'. Journal of Food Science 65(4): 668±671. Cuq, B., Gontard, N., Cuq, J.-L. and Guilbert, S. (1997). `Selected functional properties of fish myofibrillar protein-based films as affected by hydrophilic plasticizers'. Journal of Agricultural and Food Chemistry 45: 622±626. Cuq, B., Gontard, N., Cuq, J.-L. and Guilbert, S. (1998). `Packaging films based on myofibrillar proteins: Fabrication, properties and applications'. Nahrung 42 (3/4), 260±263. Denavi, G. A., PeÂrez-Mateos, M., AnÄoÂn, M. C., Montero, P., Mauri, A. N. and GoÂmezGuilleÂn, M. C. (2009). `Structural and functional properties of soy protein isolate and cod gelatin blend films'. Food Hydrocolloids 23: 2094±2101. de Wit, J. N. (1998). `Nutritional and functional characteristics of whey proteins in food products'. Journal of Dairy Science 81(3): 597±608. de Wit, J. N. (2001). `Whey protein concentrates: manufacture, composition and applications'. Industrial Proteins 9(3): 3±5. de Wit, J. N. and Moulin, J. (2001). `Whey protein isolates: manufacture, composition and applications'. Industrial Proteins 9(3): 6±8. Dickey, L. C., Parris, N., Craig, J. C. and Kurantz, M. J. (2001). Èthanolic extraction of zein from maize'. Industrial Crops and Products 13: 67±76. Dickey, L. C., Parris, N., Craig, J. C. and Kurantz, M. J. (2002). `Serial batch extraction of zein from milled maize'. Industrial Crops and Products 15: 33±42. Durham, R. J., Hourigan, J. A., Sleigh, R. W. and Johnson, R. L. (1997). `Whey fractionation: wheying up the consequences'. Food Australia 49(10): 460±465. Elsalam, M. H. A., El Shibiny, S. and Buchheim, W. (1996). `Characteristics and potential uses of the casein macropeptide'. International Dairy Journal 6(4): 327± 341. Esen, A. (1986). `Separation of alcohol-soluble proteins (zeins) from maize into three fractions by differential solubility'. Plant Physiology 80: 623±627. Esen, A. (1987). À proposed nomenclature for the alcohol soluble proteins (zeins) of maize. Journal of Cereal Science 5: 117±128. Fabra, M. J., Talens, P. and Chiralt, A. (2008). `Tensile properties and water vapor permeability of sodium caseinate films containing oleic acid±beeswax mixtures'. Journal of Food Engineering 85: 393±400.



641

Fabra, M. J., JimeÂnez, A., AtareÂs, P., Talens, P. and Chiralt, A. (2009). Èffect of fatty acids and beeswax addition on properties of sodium caseinate dispersions and films'. Biomacromolecules 10: 1500±1507. Fabra, M. J., Talens, P. and Chiralt, A. (2010). `Water sorption isotherms and phase transitions of sodium caseinate-lipid films as affected by lipid interactions'. Food Hydrocolloids 24: 384±391. Foegeding, E. A. (2006). `Food biophysics of protein gels: A challenge of nano and macroscopic proportions'. Food Biophysics 1(1): 41±50. Fossen, M., Ormel, I., Van Vilsteren, G. E. T. and Jongsma, T. J. (2000). `Lignocellulosic fibre reinforced caseinate plastics'. Applied Composite Materials 7: 433±437. Fox, P. F. (2001). `Milk proteins as food ingredients'. International Journal of Dairy Technology 54 (2):41±55. Fox, P. F. and McSweeney, P. L. H. (1998). Dairy Chemistry and Biochemistry. London, Blackie Academic & Professional. Gemili, S., Yemenicioglu, A. and Altnkaya, S. A. (2010). `Development of antioxidant food packaging materials with controlled release properties'. Journal of Food Engineering 96: 325±332. Gennadios, A., Weller, C. L. and Testin, R. F. (1993). `Temperature effect on oxygen permeability of edible protein-based films'. Journal of Food Science 58(1): 212± 214. Gennadios, A., Ghoparde, V. M., Weller, C. L. and Hanna, M. A. (1996). `Heat-curing of soy protein films'. Transactions of the ASAE 39(2): 575±579. Ghanbarzadeh, B. and Oromiehie, A. R. (2008). `Studies on glass transition temperature of mono and bilayer protein films plasticized by glycerol and olive oil'. Journal of Applied Polymer Science 109: 2848±2854. Ghoparde, V. M., Gennadios, A. and Hanna, M. A. (1995). `Chemically modified soy protein films'. Transactions of the ASAE 39(2): 1805±1808. Gillgren, T., Barker, S. A., Belton, P. S., Georget, D. M. R. and Stading, M. (2009). `Plasticization of zein: A thermomechanical, FTIR, and dielectric study'. Biomacromolecules 10: 1135±1139. Gnanasambadam, R., Hettiarachchy, N. S. and Coleman, M. (1997). `Mechanical and barrier properties of rice bran films'. Journal of Food Science 62(2): 395±398. GoÂmez-Estaca, J., Bravo, L., GoÂmez-GuilleÂn, M. C., AlemaÂn, A. and Montero, P. (2009). Àntioxidant properties of tuna-skin and bovine-hide gelatin films induced by the addition of oregano and rosemary extracts'. Food Chemistry 112: 18±25. GoÂmez-MartõÂnez, D., Partal, P., Martinez, I. and Gallegos, C. (2009). `Rheological behavior and physical properties of controlled-release gluten-based bioplastics'. Bioresource Technology 100: 1828±1832. Gontard, N., Marchesseau, S., Cuq, J. L. and Guilbert, S. (1995). `Water vapour permeability of edible bilayer films of wheat gluten and lipids'. International Journal of Food Science & Technology 30: 49±56. Gounga, M. E., Xu, S.-Y. and Wang, Z. (2007). `Whey protein isolate-based edible films as affected by protein concentration, glycerol ratio and pullulan addition in film formation'. Journal of Food Engineering 83: 521±530. Guilbert, S. and Gontard, N. (1997). `Recent innovations in edible and/or biodegradable packaging materials'. Food Additives and Contaminants 14(6±7): 741±751. Guilbert, S. and Gontard, N. (2005). Àgro-polymers for edible and biodegradable films: Review of agricultural polymeric materials, physical and mechanical characteristics'. In: Innovations in Food Packaging, J. Han (ed.), Oxford, UK, Elsevier Science & Technology, pp. 263±276.


642


Guilbert, S., Gontard, N., Morel, M.H., Chalier, P., Micard, V. and Redl, A. (2002). `Formation and properties of gluten films and coatings'. In: Protein-based Films and Coatings, A. Gennadios (ed.), Boca Raton, FL, CRC Press. Guilherme, M. R., Mattoso, L. H. C., Gontard, N., Guilbert, S. and Gastaldi, E. (2010). `Synthesis of nanocomposite films from wheat gluten matrix and MMT intercalated with different quaternary ammonium salts by way of hydroalcoholic solvent casting'. Composites: Part A 41: 375±382. Guillard, V., Issoupov, V., Redl, A. and Gontard, N. (2009). `Food preservative content reduction by controlling sorbic acid release from superficial coating'. Innovative Food Science & Emerging Technologies 10: 108±115. Hamaguchi, P. Y., WuYin, W. and Tanaka, M. (2007). Èffect of pH on the formation of edible films made from the muscle proteins of blue marlin (Makaira mazara)'. Food Chemistry 100: 914±920. Han, J. (2000). Àntimicrobial food packaging'. Food Technology 54(3): 56±65. Han, J. H. and Gennadios, A. (2005). Èdible films and coatings: a review'. In: Innovations in Food Packaging, J. H. Han (ed.), New York, Academic Press, pp. 239±259. Hermansson, A.-M. (1986). `Soy protein gelation'. Journal of the American Oil Chemists' Society 63(5): 658±666. Hernandez-Izquierdo, V. M. and Krochta, J. M. (2008). `Thermoplastic processing of proteins for film formation ± a review'. Journal of Food Science 73: R30±R39. Hernandez-Izquierdo, V. M., Reid, D. S., McHugh, T. H., Berrios, J. D. J. and Krochta, J. M. (2008). `Thermal transitions and extrusion of glycerol-plasticized whey protein mixtures'. Journal of Food Science 73: E169±E175. Hernandez-MunÄoz, P., LoÂpez-Rubio, A., Del-Valle, V., Almenar, E., and Gavara, R. (2004). `Mechanical and water barrier properties of glutenin films influenced by storage time'. Journal of Agricultural and Food Chemistry 52: 79±83. Huang, X. and Netravali, A. N. (2006). `Characterization of nano-clay reinforced phytagel-modified soy protein concentrate resin'. Biomacromolecules 7: 2783± 2789. Huang, X. and Netravali, A. (2007). `Characterization of flax fiber reinforced soy protein resin based green composites modified with nano-clay particles'. Composites Science and Technology 67: 2005±2014. Huffman, L. M. and Harper, W. J. (1999). `Maximizing the value of milk through separation technologies'. Journal of Dairy Science 82(10): 2238±2244. Ikeda, S. and Li-Chan, E. C. Y. (2004). `Raman spectroscopy of heat-induced fine-stranded and particulate beta-lactoglobulin gels'. Food Hydrocolloids 18(3): 489±498. Jagannath, J. H., Radhika, M., Nanjappa, C., Murali, H. S. and Bawa, A. S. (2006). Àntimicrobial, mechanical, barrier, and thermal properties of starch±casein based, neem (Melia azardirachta) extract containing film'. Journal of Applied Polymer Science 101: 3948±3954. Jangchud, A. and Chinnan, M. S. (1999). `Peanut protein film as affected by drying temperature and pH of film forming solution'. Journal of Food Science 64(1): 153±157. Johnston-Banks, F. A. (1990). `Gelatine'. In Food Gels, P. Harris (ed.), London and New York, Elsevier Applied Science, pp. 233±289. Kester, J. and Fennema, O. (1986). Èdible films and coatings: a review'. Food Technology 40(12): 47±59. Khwaldia, K., Perez, C., Banon, S., Desobry, S. and Hardy, J. (2004). `Milk proteins for edible films and coatings'. Critical Reviews in Food Science and Nutrition 44: 239± 251.



643

Kim, K. M., Weller, C. L., Hanna, M. A. and Gennadios, A. (2002). `Heat curing of soy protein films at selected temperatures and pressures'. LWT ± Food Science and Technology 35(2): 140±145. Kokoszka, S., Debeaufort, F., Hambleton, A., Lenart, A. and Voilley, A. (2010a). `Protein and glycerol contents affect physico-chemical properties of soy protein isolatebased edible films'. Innovative Food Science and Emerging Technologies 11(3): 503±510. Kokoszka, S., Debeaufort, F., Lenart, A. and Voilley, A. (2010b). `Water vapour permeability, thermal and wetting properties of whey protein isolate based edible films'. International Dairy Journal 20: 53±60. Kristo, E., Koutsoumanis, K. P. and Biliaderis, C. G. (2008). `Thermal, mechanical and water vapor barrier properties of sodium caseinate films containing antimicrobials and their inhibitory action on Listeria monocytogenes'. Food Hydrocolloids 22: 373±386. Krochta, J. M. (2002). `Proteins as raw materials for films and coatings: Definitions, current status, and opportunities'. In: Protein-Based Films and Coatings, A. Gennadios (ed.), Boca Raton, FL, CRC Press. Krochta, J. M. and de Mulder-Johnson, C. (1997). Èdible and biodegradable polymer films: Challenges and opportunities'. Food Technology 51(2): 61±74. Kumar, R., Choudhary, V., Mishra, S. and Varma, I. (2008). `Banana fiber-reinforced biodegradable soy protein composites'. Frontiers of Chemistry in China 3(3): 243± 250. Kuwata, K., Hoshino, M., Forge, V., Era, S., Batt, C. A. and Goto, Y. (1999). `Solution structure and dynamics of bovine beta-lactoglobulin A'. Protein Science 8(11): 2541±2545. Lacroix, M. and Cooksey, K. (2005). Èdible films and coatings from animal-orlgln proteins'. In: Innovations in Food Packaging, J. H. Han (ed.), New York, Academic Press, pp. 301±312. Lee, D. S. (2005). `Packaging containing natural antimicrobial or antioxidative agents'. In: Innovations in Food Packaging, J. Han (ed.), Oxford, UK, Elsevier Science & Technology, pp. 108±119. Lee, J. Y., Park, H. J., Lee, C. Y. and Choi, W. Y. (2003). Èxtending shelf-life of minimally processed apples with edible coatings and antibrowning agents', LWT ± Food Science and Technology 36: 323±329. Lim, L., Mine, Y., Britt, I. J. and Tung, M. A. (2002). `Formation and properties of egg white films and coatings'. In Protein-based Films and Coatings, A. Gennadios (ed.), Boca Raton, FL, CRC Press. Lima, A. M., Cerqueira, M. A., Souza, B. W. S., Santos, E. C. M., Teixeira, J. A., Moreira, R. A. and Vicente, A. A. (2010). `New edible coatings composed of galactomannans and collagen blends to improve the postharvest quality of fruits ± Influence on fruits gas transfer rate'. Journal of Food Engineering 97: 101±109. Lodha, P. and Netravali, A. N. (2002). `Characterization of interfacial and mechanical properties of ``green'' composites with soy protein isolate and ramie fiber'. Journal of Materials Science 37: 3657±3665. Lu, S., Wang, X., Lu, Q., Hu, X., Uppal, N., Omenetto, F. G. and Kaplan, D. L. (2009). `Stabilization of enzymes in silk films'. Biomacromolecules 10: 1032±1042. Mariniello, L., Di Pierro, P., Esposito, C., Sorrentin, A., Masi, P. and Porta, R. (2003). `Preparation and mechanical properties of edible pectin-soy flour films obtained in the absence or presence of transglutaminase'. Journal of Biotechnology 102: 191± 198.


644


MarquieÂ, C. and Guilbert, S. (2002). `Formation and properties of cottonseed protein films and coatings'. In: Protein-based Films and Coatings, A. Gennadios (ed.), Boca Raton, FL, CRC Press. Mate, J. I., Frankel, E. N. and Krochta, J. M. (1996). `Whey protein isolate edible coatings: Effect on the rancidity process of dry roasted peanuts'. Journal of Agricultural and Food Chemistry 44: 1736±1740. McHugh, T. H. and Krochta, J. M. (1994a). `Milk-protein-based edible films and coatings'. Food Technology 48(1): 97±103. McHugh, T. H. and Krochta, J. M. (1994b). `Sorbitol- vs glycerol-plasticized whey protein edible films: Integrated oxygen permeability and tensile property evaluation'. Journal of Agricultural and Food Chemistry 42(4): 841±845. McKinney, L. L. (1958). `Zein'. In: The Encyclopedia of Chemistry (Supplement), G. L. Clark (ed.), New York, Reinhold, pp. 319±320. Mei, Y. and Zhao, Y. (2003). `Barrier and mechanical properties of milk protein-based edible films containing nutraceuticals'. Journal of Agricultural and Food Chemistry 51: 1914±1918. Miller, K. S. and Krochta, J. M. (1997). Òxygen and aroma barrier properties of edible films: a review'. Trends in Food Science and Technology 8: 228±237. Miller, K. S., Chiang, M. T. and Krochta, J. M. (1997). `Heat curing of whey protein films'. Journal of Food Science 62(6): 1189±1193. Mo, X. and Sun, X. (2003). Èffects of storage time on properties of soybean proteinbased plastics'. Journal of Polymers and the Environment 11: 15±22. Monedero, F. M., Fabra, M. J., Talens, P. and Chiralt, A. (2009). Èffect of oleic acid± beeswax mixtures on mechanical, optical and water barrier properties of soy protein isolate based films'. Journal of Food Engineering 91: 509±515. Morillon, V., Debeaufort, F., Blond, G., Martine, C. and Voilley, A. (2002). `Factors affecting the moisture permeability of lipid-based edible films: a review'. Critical Reviews in Food Science and Nutrition 42(1): 67±89. Mujica-Paz, H. and Gontard, N. (1997). Òxygen and carbon dioxide permeability of wheat gluten film: Effect of relative humidity and temperature'. Journal of Agricultural and Food Chemistry 47: 4101±4105. NerõÂn, C., Tovar, L. and Salafranca, J. (2008). `Behaviour of a new antioxidant active film versus oxidizable model compounds'. Journal of Food Engineering 84: 313± 320. Olabarrieta, I., GaÈllstedt, M., Ispizua, I., Sarasua, J. R. and Hedenqvist, M. S. (2006). `Properties of aged montmorillonite±wheat gluten composite films'. Journal of Agricultural and Food Chemistry 54(4): 1283±1288. Oliviero, M., Di Maio, E. and Iannace, S. (2010). Èffect of molecular structure on film blowing ability of thermoplastic zein'. Journal of Applied Polymer Science 115: 277±287. OseÂs, J., Fabregat-VaÂzquez, M., Pedroza-Islas, R., TomaÂs, S. A., Cruz-Orea, A. and MateÂ, J. I. (2009). `Development and characterization of composite edible films based on whey protein isolate and mesquite gum'. Journal of Food Engineering 92: 56±62. Ou, S., Wang, Y., Tang, S., Huang, C. and Jackson, M. G. (2005). `Role of ferulic acid in preparing edible films from soy protein isolate'. Journal of Food Engineering 70: 205±210. Oussalah, M., Caillet, S., SalmieÂri, S., Saucier, L. and Lacroix, M. (2004). Àntimicrobial and antioxidant effects of milk protein-based film containing essential oils for the preservation of whole beef muscle'. Journal of Agricultural and Food Chemistry 52: 5598±5605.



645

Ozdemir, M. and Floros, J. D. (2008). Òptimization of edible whey protein films containing preservatives for water vapor permeability, water solubility and sensory characteristics'. Journal of Food Engineering 86: 215±224. Papiz, M. Z., Sawyer, L., Eliopoulos, E. E., North, A. C. T., Findlay, J. B. C., Sivaprasadarao, R., Jones, T. A., Newcomer, M. E. and Kraulis, P. J. (1986). `The structure of beta-lactoglobulin and its similarity to plasma retinol-binding protein'. Nature 324(6095): 383±385. Park, H. J., Bunn, J. M., Weller, P. J., Vergano, P. J. and Testin, R. F. (1994a). `Water vapor permeability and mechanical properties of grain protein-based films as affected by mixtures of polyethylene glycol and glycerin plasticizers'. Transactions of the ASAE 37: 1281±1285. Park, H. J., Chinnan, M. S. and Shewfelt, R. L. (1994b). Èdible corn-zein film coatings to extend storage life of tomatoes'. Journal of Food Processing and Preservation 18: 317±331. Park, S. K., Hettiarachchy, N. S., Ju, Z. Y. and Gennadios, A. (2002). `Formation and properties of soy protein films and coatings'. In: Protein-based Films and Coatings, A. Gennadios (ed.), Boca Raton, FL, CRC Press. Pavlath, A. E. and G. H. Robertson (1999). `Biodegradable polymers vs. recycling: what are the possibilities'. Critical Reviews in Analytical Chemistry 29(3): 231±241. Pereda, M., Aranguren, M. I. and Marcovich, N. E. (2008). `Characterization of chitosan/ caseinate films'. Journal of Applied Polymer Science 107: 1080±1090. Pereda, M., Aranguren, M. I. and Marcovich, N. E. (2009). `Water vapor absorption and permeability of films based on chitosan and sodium caseinate'. Journal of Applied Polymer Science 111: 2777±2784. PeÂrez-Gago, M. B. and Krochta, J. M. (2002). `Formation and properties of whey protein films and coatings'. In: Protein-based Films and Coatings, A. Gennadios (ed.), Boca Raton, FL, CRC Press. PeÂrez-Gago, M. B. and Krochta, J. M. (2005). Èmulsion and bi-layer edible films'. In: Innovations in Food Packaging, J. Han (ed.), Oxford, UK, Elsevier Science & Technology, pp. 384±399. PeÂrez-Mateos, M., Montero, P. and GoÂmez-GuilleÂn, M. C. (2009). `Formulation and stability of biodegradable films made from cod gelatin and sunflower oil blends'. Food Hydrocolloids 23: 53±61. Permyakov, E. A. and Berliner, L. J. (2000). àlpha-Lactalbumin: structure and function'. FEBS Letters 473(3): 269±274. Pol, H., Dawson, P., Acton, J. and Ogale, A. (2002). `Soy protein isolate/corn-zein laminated films: transport and mechanical properties'. Journal of Food Science 67: 212±217. Pommet, M., Redl, A., Guilbert, S. and Morel, M-H (2005). Ìntrinsic influence of various plasticizers on functional properties and reactivity of wheat gluten thermoplastic materials'. Journal of Cereal Science 42: 81±91. Prudencio-Ferreira, S. H. and Areas, J. A. G. (1993). `Protein-protein interactions in the extrusion of soya at various temperatures and moisture contents'. Journal of Food Science 58: 378±381. Rayas, L. M., Hernandez, R. J. and Ng, P. K. W. (1997). `Development and characterization of biodegradable/edible wheat protein films'. Journal of Food Science 62: 160±162, 189. Redl, A., Morel, M. H., Bonicel, J., Vergnes, B. and Guilbert, S. (1999). Èxtrusion of wheat gluten plasticized with glycerol: Influence of process conditions on flow behavior, rheological properties and molecular size distribution'. Cereal Chemistry 76: 361.


646


Riande, E., DõÂaz-Calleja, R., Prolongo, M. G., Masegosa, R. M. and Salom, C. (2000). `Reinforced polymers'. In: Polymer Viscoelasticity: Stress and Strain in Practice, E. Riande, R. DõÂaz-Calleja, M. G. Prolongo, R. M. Masegosa and C. Salom (ed), New York, Marcel Dekker. Rivero, S., GarcõÂa, M. A. and Pinnoti, A. (2010). `Correlations between structural, barrier, thermal and mechanical properties of plasticized gelatin films'. Innovative Food Science & Emerging Technologies 11(2): 369±375. Ross-Murphy, S. B. (1992). `Structure and rheology of gelatin gels: Recent progress'. Polymer 33(12): 2622±2627. Rossman, J. M. (2009). `Commercial manufacture of edible films'. In: Edible Films and Coatings for Food Applications, K. C. Huber and M. E. Embuscado (eds), New York, Springer, pp. 367±391. Sabato, S. F., Ouattara, B., Yu, H., D'Aprano, G. and Lacroix, M. (2001). `Mechanical and barrier properties of cross-linked soy and whey protein based films'. Journal of Agricultural and Food Chemistry 49: 1397±1403. Santosa, F. X. B. and Padua, G. W. (1999). `Tensile properties and water absorption of zein sheets plasticized with oleic and linoleic acids'. Journal of Agricultural and Food Chemistry 47: 2070±2074. Sawyer, L., Barlowa, P. N., Boland, M. J., Creamer, L. K., Dentona, H., Edwards, P. J. B., Holte, C., Jamesond, G. B., Kontopidisa, G., Norris, G. E., Uhrnova, S. and Wu, S. (2002). `Milk protein structure ± what can it tell the dairy industry?'. International Dairy Journal 12: 299±310. Seydim, A. C. and Sarikus, G. (2006). Àntimicrobial activity of whey protein based edible films incorporated with oregano, rosemary and garlic essential oils'. Food Research International 39: 639±644. Shih, F.F. (1998). `Film-forming properties and edible films of plant proteins'. Nahrung 42 (3/4): 254±256. Shukla, R. and Cheryan, M. (2001). `Zein: the industrial protein from corn'. Industrial Crops and Products 13: 171±192. Sivarooban, T., Hettiarachchy, N. S. and Johnson, M. G. (2008). `Physical and antimicrobial properties of grape seed extract, nisin, and EDTA incorporated soy protein edible films'. Food Research International 41: 781±785. Sobral, P. J. A., Monterrey-Quintero, E. S., and Habitante, A. M. Q. B. (2002). `Glass transition study of Nile Tilapia myofibrillar protein films plasticized by glycerol and water'. Journal of Thermal Analysis and Calorimetry 67: 499±504. Sorrentino, A., Gorrasi, G. and Vittoria, V. (2007). `Potential perspectives of bionanocomposites for food packaging applications'. Trends in Food Science and Technology 18: 84±95. Sothornvit, R. and Krochta, J. M. (2001). `Plasticizer effect on mechanical properties of lactoglobulin films'. Journal of Food Engineering 50: 149±155. Sothornvit, R., Olsen, C. W., McHugh, T.H. and Krochta, J. M. (2003). `Formation conditions, water vapor permeability, and solubility of compression-molded whey protein films'. Journal of Food Science 68: 1985±1989. Sothornvit, R., Olsen, C. W., McHugh, T.H. and Krochta, J. M. (2007). `Tensile properties of compression-molded whey protein sheets: determination of molding condition and glycerol-content effects and comparison with solution-cast films'. Journal of Food Engineering 78: 855±860. Sothornvit, R., J.-W. Rhim and Hong, S. I. (2009). Èffect of nano-clay type on the physical and antimicrobial properties of whey protein isolate/clay composite films'. Journal of Food Engineering 91: 468±473.



647

Sothornvit, R., Hong, S.-I., An, D. J. and Rhim, J.-W. (2010). Èffect of clay content on the physical and antimicrobial properties of whey protein isolate/organo-clay composite films'. LWT ± Food Science and Technology 43: 279±284. Sperling, L. H. (2006). Introduction to Physical Polymer Science. Hoboken, NJ, John Wiley & Sons. Stuche, Y. M. and Krochta, J. M. (1995). Èdible coatings on frozen king salmon: Effect of whey protein isolate and acetylated monoglycerides on moisture loss and lipid oxidation'. Journal of Food Science 60(1): 28±31. Tanada-Palmu, P. S. and Grosso, C. R. (2005). Èffect of edible wheat gluten-based films and coatings on refrigerated strawberry (Fragaria ananassa) quality'. Postharvest Biology and Technology 66: 199±208. Tang, C.-H., Jiang, Y., Wen, Q.-J. and Yang, X.-Q. (2005). Èffect of transglutaminase treatment on the properties of cast films of soy protein isolates'. Journal of Biotechnology 120: 296±307. Thoma-Worringer, C., Sorensen, J. and Lopez-Findino, R. (2006). `Health effects and technological features of caseinomacropeptide'. International Dairy Journal 16(11): 1324±1333. Tobitani, A. and Ross-Murphy, S. B. (1997). `Heat-induced gelation of globular proteins.1. Model for the effects of time and temperature on the gelation time of BSA gels'. Macromolecules 30(17): 4845±4854. Tunc, S., Angellier, H., Cahyana, Y., Chalier, P., Gontard, N. and Gastaldi, E. (2007). `Functional properties of wheat gluten/montmorillonite nanocomposite films processed by casting'. Journal of Membrane Science 289: 159±168. Turner, J. E., Boundy, J. A. and Dimler, R. J. (1965). `Zein: a heterogeneous protein containing disulfide-linked aggregates'. Cereal Chemistry 42: 452±461. Verbeek, C. J. R. and van den Berg, L. E. (2010). Èxtrusion processing and properties of protein-based thermoplastics'. Macromolecular Materials and Engineering 295: 10±21. Weber, C. J. (2000). Biobased Packaging Materials for the Food Industry: Status and Perspectives. Report of the EU Food Biopack Project, Frederiksberg, Denmark. Weller, C. L., Gennadios, A. and Saraiva, R. A. (1998). Èdible bilayer films from zein and grain sorghum wax or carnauba wax'. LWT ± Food Science and Technology 31: 279±285. Wolf, W. J. (1970). `Soybean proteins: Their functional, chemical, and physical properties'. Journal of Agriculture and Food Chemistry 18(6): 969±976. Wu, R.-L., Wang, X.-L., Wang, Y.-Z., Bian, X.-C. and Li, F. (2009). `Cellulose/soy protein isolate blend films prepared via room-temperature ionic liquid'. Industrial & Engineering Chemistry Research 48(15): 7132±7136. Xu, S., Chen, X. and Sun, D.-W. (2001). `Preservation of kiwifruit coated with an edible film at ambient temperature'. Journal of Food Engineering 50: 211±216. Yakimets, I., Wellner, N., Smith, A. C., Wilson, R. H., Farhat, I. and Mitchell, J. (2005). `Mechanical properties with respect to water content of gelatin films in glassy state'. Polymer 46: 12577±12585. Zhang, J., Mungara, P. and Jane, J. (2001). `Mechanical and thermal properties of extruded soy protein sheets'. Polymer 42: 2569±2578. Zhao, Y. and McDaniel, M. (2005). `Sensory quality of foods associated with edible film and coating systems and shelf-life extension'. In: Innovations in Food Packaging, J. H. Han (ed.), New York, Academic Press, pp. 434±449. Zhou, J. J., Wang, S. Y. and Gunasekaran, S. (2009). `Preparation and characterization of whey protein film incorporated with TiO2 nanoparticles'. Journal of Food Science


648


74(7): N50±N56. Zinoviadou, K. G., Koutsoumanis, K. P. and Biliaderis, C. G. (2009). `Physico-chemical properties of whey protein isolate films containing oregano oil and their antimicrobial action against spoilage flora of fresh beef'. Meat Science 82: 338± 345. Zinoviadou, K. G., Koutsoumanis, K. P. and Biliaderis, C. G. (2010). `Physical and thermo-mechanical properties of whey protein isolate films containing antimicrobials, and their effect against spoilage flora of fresh beef'. Food Hydrocolloids 24: 49±59. Zoccola, M., Aluigi, A. and Tonin, C. (2009). `Characterisation of keratin biomass from butchery and wool industry wastes'. Journal of Molecular Structure 938: 35±40.


23

Wheat gluten (WG)-based materials for food packaging H. ANGELLIER-COUSSY, V. GUILLARD, C . G U I L L A U M E and N . G O N T A R D , University of Montpellier II, France

Abstract: This chapter discusses the use of wheat gluten (WG) based materials for food packaging. It presents the two technological processes to prepare WG-based materials and reviews the ways to modulate mechanical and mass transfer properties, with a specific section devoted to WG-based nanocomposites. The chapter also includes a case study of using WG-based materials (paper coated by wheat gluten) as modified atmosphere packaging for fruits and vegetables. Key words: wheat gluten, food packaging, nanocomposites, barrier properties.

23.1

Introduction

Wheat gluten (WG) is a by-product of the wheat starch industry which is extensively used for both food and non-food applications. The use of wheat gluten for non-food applications is part of a trend to produce biodegradable materials with a large range of functional properties. WG has been widely investigated as a protein source because it is annually renewable and readily available at a reasonable cost (between 1 and 1.3 ¨/kg). With respect to its unique viscoelastic and film-forming properties, WG is an interesting raw material that can be used as a food packaging material. It is fully biodegradable without releasing toxic products (Domenek et al., 2004a). WG based-films present an attractive combination of strength and flexibility (Angellier-Coussy et al., 2008a; Cuq et al., 2000), a high gas (oxygen and carbon dioxide) permeability in dry condition and a significant gas perm-selectivity at high relative humidity (Gontard et al., 1996; MujicaPaz and Gontard, 1997; Guillaume et al., 2010), and good grease (Guillaume et al., 2010) and aroma barrier properties (Chalier et al., 2007a) which are key functional properties for food quality preservation. They also are translucent (Angellier-Coussy et al., 2008a) and can be heat-sealed (Cho et al., 2007). A downside to the use of WGbased materials is their per se reactivity and thus lower inertia than most


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conventional petrochemical-based plastics. WG-based films are sensitive towards water due to the hydrophilic nature of many amino acids constituting the protein chains and to the substantial and necessary amount of added hydrophilic plasticizer (glycerol). As a consequence, their mechanical properties and water vapour barrier properties in high moisture conditions are relatively poor as compared to synthetic films such as low-density polyethylene (PDL Handbook Series, 1995). Improving moisture resistance and mechanical properties are two of the critical issues in the development of WG-based materials for sustainable food packaging applications. Many studies have already been devoted to exploring the ways to improve the mechanical properties of WG-based materials, but only a few papers deal with the modulation of mass transfer properties which are key properties for food packaging purposes. This chapter will first give an overview on the main technological approaches in preparing WG-based materials. The mechanical and mass transfer properties of WG-based materials are discussed. The case of WG-based nanocomposites is considered in the following section. Finally, an example of integrated approach for the development of adequate food packaging will be presented. It will deal with the use of paper coated by wheat gluten as material for modified atmopshere packaging of fruits and vegetables like parsley having a high respiration activity.

23.2

Preparation of wheat gluten-based materials

Wheat gluten proteins consist of monomeric gliadins with a molecular weight ranging from 15,000 to 85,000 and a mixture of glutenin polymers with a molecular weight between about 80,000 to several million. Although proteins themselves are already heteropolymers, with -amino acids being their monomer units, the terms monomeric and polymeric refer in this case to the quaternary structure of the proteins. Gliadins represent a heterogeneous mixture of single-chained or monomeric gluten proteins, while glutenins consist of peptide chains associated through interchain disulfide bonds. While gliadins are readily extractable in aqueous alcohols, glutenins are partly insoluble in most common solvents due to their large size. However, their subunit building blocks have solubilities comparable to those of gliadins. The glutenin subunits can be obtained by treatment of glutenin with a disulfide reactive agent. WG proteins can undergo disulfide interchange upon heating, which leads to the formation of a covalent three-dimensional macromolecular network. For glutenins, crosslinking reactions occur above 60±70ëC, whereas for gliadins the reactive zone is evidenced around 90ëC (Domenek et al., 2002; Schofield et al., 1983). The reactivity of the WG proteins towards chemicals and temperatures permits the preparation of WG-based materials by using two technological approaches: either a solvent-based process, also called casting, or a common thermo-


Wheat gluten (WG)-based materials for food packaging

651

23.1 Schematic representation of the two technological processes used to prepare wheat gluten-based materials.

mechanical process based on the thermoplastic properties of WG. In both processes, the formation of a macromolecular network from proteins requires three steps (Fig. 23.1). The first one consists in disrupting intermolecular bonds that stabilize polymers in the native state. This first step enables the second one, which consists in rearranging and reorienting the polymer chains (shaping), leading to the formation of a three-dimensional network stabilized by new interactions and bonds after the agent that ruptures intermolecular bonds is removed (Cuq et al., 1998).

23.2.1 Solvent-based process The solvents used to prepare protein film-forming solutions are generally based on water and ethanol (Gontard et al., 1992, 1993). Dispersing proteins in solvents may require the addition of disruptive agents, pH adjustment by the addition of acids or bases, or ionic strength control by electrolyte addition. The functional properties of protein-based films prepared by casting depend on protein concentration in solution, pH, additives, solvent polarity, drying rate and temperature (Cuq et al., 1998). This process involves the use and elimination of large amounts of solvent, which could not be in line with the promotion of low cost and low environmental impact process, except for specific applications such as coatings.


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23.2.2 Thermo-mechanical process Thermal processing of wheat proteins has become a challenge for polymer material scientists as this technique can avoid the usage of solvents. Furthermore, as this technique enables the use of common processes like extrusion, it ensures subsequent take-up in existing converting thermo-mechanical processes at plant scale. Wheat proteins could be considered as partially thermoplastic polymers that could be changed in a reversible way from a rigid state to a soft state through a temperature increase and the plasticization through the addition of small polar molecules. In that regard, WG-based materials can be shaped by existing plastics processing machinery including extrusion (Hochstetter et al., 2006; Redl et al., 1999), lamination, roller milling or thermomolding (AngellierCoussy et al., 2008a; Gallstedt et al., 2004; Sun et al., 2008), which are all available at the industrial scale. The difficulty in extruding these materials is due to the strong self-association among protein chains through inter/intramolecular interactions and cross-linking via disulfide bonds. In addition the properties of wheat proteins are significantly modified on heating before melting. Consequently, a large amount of plasticizer is always required in thermal processing to reduce the strong intra/intermolecular interactions among wheat protein chains. Water is the most ubiquitous plasticizer of WG because of its high capability to modify the mobility of proteins, whereas glycerol is the other common residual plasticizing agent having a high retention in the WG-based material due to its high boiling point and strong hydrogen bonding with proteins (Zhang et al., 2005).

23.3

Mechanical and barrier properties of wheat gluten-based materials

23.3.1 Barrier properties of WG-based materials The particular interest of WG-based films compared to usual plastic films is their gas permeability and selectivity which are sensitive to temperature but more particularly to relative humidity (MujicaPaz and Gontard, 1997). At low relative humidity (RH), WG-based films display impressive gas barrier properties, especially towards O2. The increasing RH effect was attributed to a modification of the wheat gluten network structure and polymeric chain mobility, related to a change from a glassy to a rubbery state (Gontard and Ring, 1996). The increase of CO2 permeability with RH was much more pronounced than that of O2 permeability (Fig. 23.2). This phenomenon was explained by a selective sorption of CO2 due to the specific interactions setting up between carbon dioxide and the water-plasticized protein matrix, especially high content amide groups of wheat gluten protein (Pochat-Bohatier et al., 2006). At high RH value, adsorption of water should provide a better accessibility to active sites of CO2 sorption located on the mobile polymeric protein chains. Consequently,


ß Woodhead Publishing Limited, 2011 23.2 Effect of temperature and relative humidity on carbon dioxide permeability, oxygen permeability, selectivity (CO2/O2) and ethylene permeability of a wheat gluten film (adapted from MujicaPaz and Gontard, 1997, and Paz et al., 2005).

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WG-based films exhibit a large range of selectivity values (from 3 to 28) as a function of moisture content, contrary to the selectivity of most synthetic films which is usually varying between 4 and 6 (MujicaPaz and Gontard, 1997). The high selectivity value of WG-based films (28 at 24ëC and 100% RH) and moreover a high ethylene permeability (Paz et al., 2005) could be very interesting for fresh or minimally processed fruits and vegetables preservation under modified atmosphere (Fig. 23.2). However, because of their hydrophilic nature, WG-based materials display a poor water resistance, which is revealed by an important swelling when they are immersed in liquid water (Domenek et al., 2004b; Tunc et al., 2007) and a high water adsorption in high moisture conditions. Their water vapour barrier properties (water vapour permeability (WVP) for a 0±100% RH difference ranging from 5 10ÿ12 mol.mÿ1.sÿ1.Paÿ1 to 6:2 10ÿ11 mol.mÿ1.sÿ1.Paÿ1 depending on the film preparation process; Gontard et al., 1993; Pommet et al., 2003) are also relatively poor compared to those of synthetic films such as lowdensity polyethylene (WVP 0:05 10ÿ12 mol.mÿ1.sÿ1.Paÿ1; PDL Handbook Series, 1995). Furthermore, their mechanical properties and water barrier properties are strongly affected by the presence of water or other plasticizers (Gontard et al., 1993), which restrict their utilization to a targeted range of applications. Decreasing water vapour permeability is one of the main critical issues in the development of biopolymers for extending potential food packaging applications.

23.3.2 How to modulate the functional properties of WGbased materials To widen the end-uses of wheat gluten based-films, their functional properties (especially mechanical and water barrier properties) must be improved. It is known that mechanical properties of WG-based materials mainly depend on the type and density of intra- and intermolecular interactions, but also from interactions with other constituents (Guilbert et al., 2002). Globally, when covalent bonds, such as disulfur bonds, stabilize the network or when the density of bond energy is high, WG-based films are resistant and relatively elastic (Guilbert et al., 2002). Thus, the most common studied way to alter gluten functionality, especially mechanical properties, is to modulate the degree of protein crosslinking. Crosslinking reactions can be induced by chemical (vapours of formaldehyde) (Micard et al., 2000), thermal (Ali et al., 1997; Angellier-Coussy et al., 2008a; Cuq et al., 2000; Micard et al., 2000; Song et al., 2008), enzymatic (Lai and Chiang, 2006; Larre et al., 2000; Wang et al., 2005) and radiation (Micard et al., 2000) treatments. These treatments were applied either as `pre-treatment' (Ali et al., 1997; Lai and Chiang, 2006), meaning that changes occurred in the film-forming solution, as `posttreatments', i.e. applied on the final films (Micard et al., 2000), or during the



655

treatment itself as in the case of the thermo-mechanical process (AngellierCoussy et al., 2008a). Heat treatment, which is often combined with pressure and shear, is the most efficient and common way to modify the structure of WG-based materials as it avoids the use of solvents and chemical compounds. For example, increasing thermoforming temperature (from 60 to 120ëC) led to a rise in both the resistance and the deformability of WG-based materials due to the establishment of a covalent network. The increase in deformability was attributed to the thermo-mechanical treatment in a two-blade counter-rotating batch mixer inducing shearing that may favour the parallel orientation of protein chains, thus leading to an increase in the mean length of the polymer (Angellier-Coussy et al., 2008a). This result was also observed by Sun et al. (2008). Concerning the casting process, some authors have also tried to optimize the conditions. Gontard et al. (1992) showed that WG films made at low pH (pH 4 with acetic acid) from an ethanol solution were stronger than films obtained from alkaline conditions. Under those high pH conditions, the reduction of disulfide bonds to sulfydryl groups is favoured, thus allowing the film to stretch further. Later, Kayserilioglu et al. (2001) and Zhang et al. (2006) confirmed this result. They showed that alkaline conditions (pH 11 with NaOH) caused some level of deamidation of WG that modified the chemical and aggregation structure, enhancing intermolecular interactions between water and all components in WG (proteins, starch, lipids), and thus resulting in a more stable crosslinked WG network with strong intermolecular interactions. Figure 23.3 highlights that all the possible treatments previously listed, which influence the crosslinking degree of proteins, allow one to achieve a quite large window of mechanical properties and an attractive combination of strength and deformability as compared to polystyrene (PS) and biodegradable polyesters such as polylactic acid (PLA) and polycaprolactone (PCL). However, the mechanical properties of WG-based materials remain low compared to those of conventional plastics like polyolefins (Fig. 23.3). If the mechanical properties of WG-based films can be greatly improved by increasing the crosslinking degree of proteins, their permeability to water vapour, gases and organic volatile compounds as well as their solubility in water are usually less affected. Correlated to the mechanical changes observed, the wheat gluten solubility in water decreases with the increase of temperature (Angellier-Coussy et al., 2008a). Water vapour permeability is also reduced, but in a limited range, with more severe heat treatment (Ali et al., 1997) or thermoforming temperature (Angellier-Coussy et al., in press). Changes in water vapour transport properties (permeability, sorption and diffusion) were directly related to changes in the structure of the films. It was shown that an increase in temperature from 80ëC to 120ëC led to a decrease in both swelling and WVP values. This was related to an increase in the crosslinking degree of WG-based films resulting in a more locked structure less prone to chain mobility and


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23.3 Mechanical properties of wheat gluten-based materials (u) as compared to conventional plastics (ú) and biodegradable polyesters (s) (adapted from Angellier-Coussy et al., 2008b).

rearrangement in contact with water. The decrease in WVP was only related to a decrease in the apparent diffusivity of water, since the thermoforming temperature had no significant effect on the moisture sorption isotherms (AngellierCoussy et al., in press). The development of bilayer or multilayer films associating wheat gluten with lipids or another polymeric layer (synthetic polymer or biopolymer) which affords water resistance and/or mechanical resistance is a third strategy to extend the functional properties of wheat gluten films. For example, in order to tentatively increase the functional properties and notably the moisture resistance of WG-based films, edible composite films comprising wheat gluten as the structural matrix and various concentrations of different lipids as the moisture barrier component were tested (Gontard et al., 1994). Among the 10 lipids tested, beeswax was the most effective lipid for improving moisture barrier properties of the films (WVP was reduced by 74% as the beeswax concentration was increased from 0 to 36.8 g.100 gÿ1 dry matter). But these films were opaque, weak and disintegrated easily in water. Combining wheat gluten proteins with diacetyl tartaric ester of monoglycerides (20 g.100 gÿ1 dry matter) reduced WVP about 50%, increased strength and maintained transparency. Solid lipids such as beeswax or paraffin wax could be also deposited in a molten state onto the base film in a thin layer to form bilayer films. These films were proved to be more efficient than composite film of the same formulation (Table 23.2). For instance, a film consisting of wheat gluten, glycerol and diacetyl tartaric ester of monoglyceride as one layer, and beeswax as the other, yielded a water vapour permeability of 2:32 10ÿ14 mol.mÿ1.sÿ1.Paÿ1, which was less than that obtained with low density polyethylene (Gontard et al., 1995). Another example of such association is the realization of a bilayer film made of a layer of functionalized polyethylene (for example, ethylene/acrylic ester/



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maleic anhydride terpolymer or ethylene/glycidyl methacrylate copolymer) and a layer of pure wheat gluten with the aim of obtaining a bilayer film combining the low water vapour permeability of the polyethylene layer with the high gas permeability of the wheat gluten layer (Irissin-Mangata et al., 1999). The use of functionalized polyethylene had no influence on film opacity, and was effective in reducing dispersion in water and water vapour permeability of pure wheat gluten films. However, even if the high selectivity ratio of gluten films was preserved, the O2 and CO2 permeability values of the gluten/terpolymer bilayer were lower than those of pure gluten films, which reduce the window of application of the bilayer. Tables 23.1 and 23.2 show the gas and water vapour permeabilities of various WG-based films. A last example of association with the aim of improving mechanical properties of WG consists in reinforcing the protein matrix by associating WG to paper, exploiting their naturally occurring compatibility due to the hydrophilic nature of both constituents. WG proteins can be used as functional coatings for paper, the latter acting as a mechanical support packaging (Chalier et al., 2007a; Gastaldi et al., 2007; Guillaume et al., 2010). Moreover, due to the biodegradable feature of proteins, both the recyclability and biodegradability attributes of the paper would be maintained, unlike most of the synthetic coatings currently used for cellulosic substrates. Using wheat gluten as coating for paper (considered as porous) by using either a thermomoulding process (Gallstedt et al., 2005) or casting (Guillaume et al., 2010) underlined the necessity to have a continuous and thick enough coating to significantly improve Table 23.1 Oxygen and carbon dioxide permeabilities (10ÿ18 mol.mÿ1.sÿ1.Paÿ1) of wheat gluten-based films O2 perme- CO2 perme- T RH ability ability (ëC) (%) Paper/wheat gluten

8328

16927

25

Wheat gluten (casting)

1970

55580

24 100

Low density polyethylene 1078 Wheat gluten/DATEM* 790 Wheat gluten/beeswax 687 Wheat gluten (casting) 152

4134 11142 6614 545

20 100 25 93 25 91 24 50

Wheat gluten/DATEM* and beeswax (bilayer film) Wheat gluten/beeswax and beeswax (bilayer film) Polyamide 6 EVOH

11

76

25

56

Guillaume et al., 2010 MujicaPaz and Gontard, 1997 Charles et al., 2003 Gontard et al., 1996 Gontard et al., 1996 MujicaPaz and Gontard, 1997 Gontard et al., 1996

10

61

25

56

Gontard et al., 1996

10 6

(±) (±)

80

Reference

23 100 23 95

*DATEM is diacetyl tartaric ester of monoglyceride.


Ashley, 1985 Salame, 1986

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Table 23.2 Water vapour permeability (WVP, 10ÿ12 mol.mÿ1.sÿ1.Paÿ1) of wheat gluten-based films WVP

RH (%)

T (ëC)

Wheat gluten (casting) Wheat gluten/refined paraffin Wheat gluten/carnauba wax Wheat gluten (casting) Wheat gluten/soy protein Wheat gluten/DATEM Wheat gluten/mineral oil Wheat gluten/beeswax Low density polyethylene

5.31 4.10 3.86 3.11 2.84 2.32 2.28 1.83 0.048

100±0 100±0 100±0 11±0 11±0 100±0 11±0 100±0 95±0

30 30 30 23 23 30 23 30 38

Wheat gluten and beeswax (bilayer film)

0.023

100±0

30

Reference Gontard et al., 1994 Gontard et al., 1994 Gontard et al., 1994 Gennadios et al., 1993a Gennadios et al., 1993b Gontard et al., 1994 Gennadios et al., 1993a Gontard et al., 1994 PDL Handbook Series, 1995 Gontard et al., 1995

the gas barrier properties of the paper and to manage to come near the oxygen permeability of the pure wheat gluten film. On the contrary, when the wheat gluten coating (deposited by casting) penetrated deeply into the paper, the resulting composite structure displayed transfer properties (water vapour, O2 and CO2) equivalent to those of pure wheat gluten film (Guillaume et al., 2010). The composite paper/wheat gluten structure thus appears more efficient than a strictly bilayer structure.

23.4

Wheat gluten-based nanocomposites

The development of food packaging materials with tailored barrier properties has necessitated study of the structure of such materials at the nanometric scale. In this context, many studies have been devoted to the use of hybrid organic± inorganic systems and, in particular, to those in which layered silicates are dispersed at a nanometric level in a polymeric matrix (Giannelis, 1996). The nanoscale plate morphology of layered silicates and other fillers often promotes improved physical properties of biopolymers, including improved mechanical properties, thermal stability and gas barrier properties (Alexandre and Dubois, 2000). This strategy has been recently applied to WG-based materials to modulate their mechanical and barrier properties (Angellier-Coussy et al., 2008a; Guilherme et al., 2010; Olabarrieta et al., 2006; Tunc et al., 2007; Zhang et al., 2007) as well as to create antimicrobial delivery systems (Mascheroni et al., 2010). All the studies devoted to WG-based nanocomposites involve the use of non-modified or organo-modified montmorillonites. The properties of these nanocomposite materials are directly related to their structure and, in this respect, the state of dispersion (intercalation/exfoliation) of the inorganic phase seems to be a key parameter (Ray and Bousmina, 2005).



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23.4.1 Preparation and structure of WG-based nanocomposites Several methods have been considered to prepare polymer/layered silicate nanocomposites with an optimal dispersion state of nanofillers (Alexandre and Dubois, 2000). Two of them are compatible with the production of WG-based materials: (1) the melt-intercalation technique, which is compatible with the preparation of protein-based materials via a thermo-mechanical process based on the thermoplastic properties of wheat gluten, and (2) the intercalation of proteins from solution, which is compatible with the preparation of WG-based materials via casting. So far the latter technique has been preferred due to a higher facility to exfoliate MMT nanoparticles due to the capacity of layered silicates to swell in an appropriate solvent. In both cases, layered silicates are mixed to wheat gluten in the first step of the WG-based materials preparation (Fig. 23.1). The state of dispersion of nanoparticles within the WG matrix is evaluated by wide-angle X-ray diffraction analysis (XRD) and transmission electron microscopy (TEM), which are the two tools most frequently used. Other techniques such as FTIR are not used because of the difficulty of interpreting data related to the chemical complexity of the WG. It is shown that intercalated/ exfoliated systems could be achieved using either the solvent or the meltintercalation technique. However, the level of exfoliation depends greatly on the nature of the montmorillonites. Almost fully exfoliated structures were obtained in the case of non-modified sodium MMT (Nanofil EXM 757 from SuÈd-Chemie (from Tunc et al., 2007) or Na+ Cloisite from Southern Clay Products, Inc. (from Olabarrieta et al., 2006)) whereas the presence of both stacks on TEM pictures and a d001 peak characteristic of an intercalated structure on XRD patterns evidenced that organo-modified montmorillonites (Cloisite 10A (Olabarrieta et al., 2006) and Cloisite 30B (Zhang et al., 2007) from Southern Clay Products, Inc.) were not completely exfoliated. This highlighted the fact that the affinity between wheat proteins and nanoclays is a key parameter governing the state of dispersion of the nanoclays. The use of non-modified MMTs which are hydrophilic seems to be more appropriate.

23.4.2 Properties of WG-based nanocomposites All studies demonstrated that the introduction of MMT nanoparticles resulted in a significant improvement in mechanical strength and resistance accompanied by a decrease in deformability. For example, the addition of 5 wt% of nonmodified MMT in WG led to an almost threefold increase of the Young's modulus and the stress at break (Tunc et al., 2007). It appeared that an optimal filler content of 5 wt% was required to achieve the greatest improvement of mechanical properties to allow layered silicates to form a connected threedimensional network. In the case of organo-modified MMT, the quaternary


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alkylammonium could act as a plasticizer in the WG matrix, which might play a negative role in enhancing the strength of the nanocomposites (Zhang et al., 2007). Combinations of wheat gluten proteins with non-modified MMT have been demonstrated to efficiently reduce the moisture sensitivity of WG (Olabarrieta et al., 2006; Tunc et al., 2007). For example, the liquid water uptake in cast WGbased nanocomposite films was halved when 10 wt% of MMT was added in the matrix (Tunc et al., 2007). A decrease in WVP from 1:6 10ÿ11 to 0:6 10ÿ11 mol.mÿ1.sÿ1.Paÿ1 between the neat matrix and the nanocomposite (from 5 to 10 wt% of MMT) may be related to the establishment of hydrophilic interactions between gluten proteins and nanoclays, which resulted in a lower availability of the hydrophilic sites for water vapour. As regards O2 and CO2 permeability (evaluated at 25ëC and 80 and 90% RH), the addition of MMT into WG had no significant effect on the permeability of the films towards gases (O2 and CO2) over the range of studied MMT contents (up to 10 wt%) (Tunc et al., 2007). It was pointed out that O2 and CO2 permeabilities of nanocomposite materials increased with increasing RH. This phenomenon was also observed in pure wheat gluten films (Gontard and Ring, 1996) suggesting that the neat matrix of wheat gluten was the key element in the gas barrier properties of the studied materials. Increased barrier properties towards 2-heptanone were also observed for MMT contents 5 wt%. This decrease in permeability was attributed to a tortuosity effect liable for a significant decrease in diffusivity with increasing MMT contents. WG-based nanocomposite systems were also used for the controlled release of active agents. It has been shown that the release of a volatile compound, carvacrol, from paper coated with wheat gluten is RH-dependent (Mascheroni et al., 2010). Such behaviour is very interesting both for limiting volatile active agent losses before using the material as food packaging, and for triggering the active agent release in the presence of the food. In this study (Mascheroni et al., 2010), the release of carvacrol was assessed at 25ëC as a function of time and using a two-step gradient of relative humidity. It was shown that WG/paper materials lost more than 70% of carvacrol within 20 days of storage at 60% RH. This means that only 30% of the active agent would be available for release towards the food during its storage within the packaging. Once placed at 100% RH, this 30% was entirely released within 8 days. When MMT nanoparticles were introduced in the WG coating layer, only 20% of the carvacrol was released during the 20 days of storage at 60% RH. Consequently 80% of the volatile active agent remained available for release during the period in which food is packaged. Once placed at 100% RH, this 80% was entirely released within 13 days (from day 22 to day 35) (Mascheroni et al., 2010). However, if these novel materials are used in commercial applications, how should such materials be proven to comply with regulations? An important issue is the potential release of engineered nanomaterials (ENM) from nanocomposite



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materials. In Mauricio-Iglesias et al. (2010), it is found that the content of silicon increased for the three aqueous food simulating liquids (FSL) in contact with WG/MMT. In the case of 3% acetic acid, the levels of aluminium also increased. In this study as in previous ones (Avella et al., 2005), the results were obtained by elementary analysis. Therefore, it is impossible to know whether the `whole ENM' has migrated, or only a part, or how the ENM migrated. The toxicity of ENM depends on a large number of factors including their structure, surface area, particle number, charge, chemistry, size and size distribution, state of aggregation, shape and elemental composition (Oberdorster et al., 2005). Such a characterization of ENM in food or FSL represents a real challenge for the current analytical methodologies (although recently Tiede et al. (2009) reported that wet scanning electron microscopy was suitable for characterizing ENM in liquid environment).

23.5

Example of integrated approach for the packaging of fresh fruits and vegetables

Preservation of fresh or minimally processed fruits and vegetables still constitutes one of the most challenging applications for food packaging materials: this living produce is sensitive to over-maturation and physiological disorders due to the surrounding atmospheric composition. The use of modified atmosphere packaging (MAP) can reduce these losses by creating a favourable surrounding atmosphere able to slow down the respiration metabolism. Atmospheric composition within the package is the result of both respiration of the commodity and diffusion/permeation of gases through the film until a steady modified atmosphere is reached (Floros and Matsos, 2005). This steady atmosphere should be close to an optimal gas concentration specific to each produce. Hence, gas transfer properties of the packaging must correctly match physiological requirements of the commodity and could thus be called `passive MAP'. A favourable MAP can also be created actively by using packaging material able to absorb, for example, oxygen (Charles et al., 2003) or to release volatile active agent, e.g. antimicrobial agents (Matan et al., 2006; Valero et al., 2006; Valverde et al., 2005), for creating a so-called active MAP. In passive MAP, most conventional plastic films exhibit too low gas permeability, leading to a rapid and sharp drop in O2 followed by detrimental anaerobic conditions (Zagory and Kader, 1988). Therefore, they need to be perforated (with macro- or micro-perforations) to allow sufficient gas exchange. In that case, CO2 diffuses through the film at a similar rate to O2 and therefore the gas permselectivity value (ratio of CO2 permeability to O2 permeability) of perforated materials is close to 1. As an alternative to conventional plastics, the development of WG-based materials appears to be of great interest because of their high gas permeability and permselectivity values that are able to create a unique low oxygen and low carbon dioxide atmosphere, adapted to the


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preservation of fresh fruits and vegetables, especially CO2-sensitive commodities (Barron et al., 2002; Gontard et al., 1996). In active MAP, WG-based materials have also been demonstrated to make efficient antimicrobial packaging for their ability to control, in a relevant way, the release of volatile extracts of various essential oils (allylisothiocyanate, carvacrol, cinnamaldehyde or eugenol) (Ben Arfa et al., 2006, 2007; Chalier et al., 2007b). We present below an illustration of combining wheat gluten proteins with paper and MMT. Nanocomposite and composite materials were manufactured by coating a support paper with a wheat gluten layer containing MMT (nanocomposite) or not (composite). Characteristics and permeability of these materials were studied and passive MAP experiments were performed for assessing the effect on the quality of a real food: parsley (Gontard et al., 2008). O2 and CO2 permeabilities of WG/paper-based materials were evaluated at 25ëC and at 80% and 90% RH, in comparison with control uncoated paper, and results are presented in Table 23.3. The continuous layer formed by WG proteins onto the support paper greatly reduced the gas permeability of the paper. The coated paper could thus no longer be considered porous. O2 and CO2 permeability values were not significantly affected by the presence of MMT in the wheat gluten network, whatever the RH. Since permeability is known to be governed by two mechanisms, diffusion and sorption, it was assumed that introduction of MMT did not change solubility nor diffusivity of O2 and CO2, as observed in a previous study (Tunc et al., 2007). It should be pointed out that, in Table 23.3, O2 and CO2 permeability of both composite and nanocomposite materials increased with increasing RH. This phenomenon was also observed on pure wheat gluten films (Gontard et al., 1996), suggesting that the wheat gluten-based coating layer is the key element in the gas barrier properties of the studied materials. The increasing RH effect was attributed to a modification of the wheat gluten network structure and polymeric chain mobility, related to a change from Table 23.3 Gas permeability and permselectivity ( ) of control paper, composite and nanocomposite paper-based materials at 25ëC as a function of relative humidity (RH %) RH (%)

PO2*

PCO2*

Control

80 90

> 5 109 > 5 109

> 5 109 > 5 109

Composite

80 90

5210 8512

10130 67790

1.9 7.9

Nanocomposite

80 90

6400 9000

9870 68050

1.5 7.5

1 1

* O2 and CO2 permeability (PO2 and PCO2) expressed in amol.mÿ1.sÿ1.Paÿ1. Mean standard deviation is 300 and 400 amol.mÿ1.sÿ1.Paÿ1 for PO2 and PCO2 respectively.



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a glassy to a rubbery state. The increase of CO2 permeability was more pronounced than that of O2 permeability. This phenomenon was explained by a selective sorption of CO2 due to the specific interactions setting up between carbon dioxide and the water-plasticized protein matrix, especially high content amide groups of wheat gluten protein (Pochat-Bohatier et al., 2006). At high RH value, adsorption of water should provide better accessibility to active sites of CO2 sorption located on the mobile polymeric protein chains. As a consequence, gas permselectivity was greatly affected by RH and rose from 1.9 to 7.9 and from 1.5 to 7.5 for composite and nanocomposite materials, respectively. These results show that the unique gas permselectivity properties are preserved when combined with paper or MMT. Passive MAP experiments were conducted on parsley with the uncoated control paper and the nanocomposite material at 20ëC. As expected for a highly porous material, O2 and CO2 partial pressures at the steady state obtained when using control paper were close to those in the composition of air (21 and 0 kPa respectively). Such an atmosphere was detrimental to the quality of the product. After only four days of storage, more than 50% of ascorbic acid and chlorophyll were lost, and leaves were fully yellow (data not shown). In comparison, nanocomposite material generated a headspace atmosphere containing lower O2 (11 kPa) and higher CO2 (4 kPa) content. This steady atmosphere was in agreement with the atmosphere recommended by UC Davis1 and clearly improved the quality attributes of parsley during storage, by maintaining a high chlorophyll content (directly linked to the green colour of the herb) and ascorbic acid during eight days of storage. For consumers, the shelf-life of parsley is mainly determined through its discoloration. Sensory analysis results were perfectly correlated to chlorophyll content. A critical level of 1.5 mg/g of fresh parsley (corresponding to 70% of the initial content) was reached after only two days of storage when it was packed at 20ëC using uncoated paper. If a nanocomposite material was used, discoloration was delayed to more than eight days. As regards ascorbic acid, one of the major components of parsley, preserving at least 60% of initial vitamin C content could be considered as a critical level. It was reached after only three days of storage with uncoated paper against more than eight days for composite material. The use of nanocomposite materials for MAP of parsley led to an equilibrium atmosphere favourable for maintaining the quality of the parsley, by slowing down the oxidation reactions and physiological reactions that are responsible for product degradation.

1. University of California Davis, Postharvest Technology Research and Information Center, http:// [email protected].


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23.6


Future trends

It can be concluded from this chapter on wheat gluten-based materials for food packaging that there is still an important need for improved knowledge on how `food and agropolymer sciences' can facilitate the development of efficient WGbased packaging for food. To make certain that the developed packaging materials will optimally fulfil the requirements of the food industry, compounds producers, packaging converters, food retailers, waste management and legislative bodies and consumers, future projects should adopt a holistic approach to preparing the ground for addressing the long-term development of WG-based materials for food packaging. It can be reasonably expected that the next studies will include the identification of targeted food products and the quantification of their needs in terms of packaging properties (mechanical and gas transfer properties, moisture resistance, stability towards pH, temperature, light, etc.) to increase their shelflife. An essential step in the application of WG-based packaging by the food industry is thus to develop integrated studies of the process±structure±properties relationships of WG-based materials based on the latest innovative developments for the characterization of complex polymeric materials. The next projects will also be focused on developing mathematical modelling tools to calculate how the structure±function relations at different scales will determine the end properties.

23.7

References

Alexandre M and Dubois P (2000), `Polymer-layered silicate nanocomposites: preparation, properties and uses of a new class of materials', Materials Science & Engineering R ± Reports, 28, 1±63. Ali Y, Ghorpade V M and Hanna M A (1997), `Properties of thermally-treated wheat gluten films', Industrial Crops and Products, 6, 177±184. Angellier-Coussy H, Torres-Giner S, Morel M H, Gontard N and Gastaldi E (2008a), `Functional properties of thermoformed wheat gluten/montmorillonite materials with respect to formulation and processing conditions', Journal of Applied Polymer Science, 107, 487±496. Angellier-Coussy H, Morel M H, Gastaldi E, Chalier P, Guillaume C, Gontard N and Guilbert S (2008b), `Functional and tailormade wheat gluten based materials', 59th Starch Convention, Detmold, Germany, 16±17 April. Angellier-Coussy H, Gastaldi E, Gontard N and Guillard V (in press), Ìnfluence of processing temperature on the water vapour transport properties of wheat gluten based agromaterials', Industrial Crops and Products. Ashley R J (1985), `Permeability and plastics packaging', in Polymer Permeability, Comyn J (ed.), Elsevier, London, 269±307. Avella M, De Vlieger J J, Errico M E, Fischer S, Vacca P and Volpe M G (2005), `Biodegradable starch/clay nanocomposite films for food packaging applications', Food Chemistry, 93, 467±474. Barron C, Varoquaux P, Guilbert S, Gontard N and Gouble B (2002), `Modified atmosphere packaging of cultivated mushroom (Agaricus bisporus L.) with hydrophilic films', Journal of Food Science, 67, 251±255.



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Ben Arfa A, Combes S, Preziosi-Belloy L, Gontard N and Chalier P (2006), Àntimicrobial activity of carvacrol related to its chemical structure', Letters in Applied Microbiology, 43, 149±154. Ben Arfa A, Preziosi-Belloy L, Chalier P and Gontard N (2007), Àntimicrobial paper based on a soy protein isolate or modified starch coating including carvacrol and cinnamaldehyde', Journal of Agricultural and Food Chemistry, 55, 2155±2162. Chalier P, Peyches-Bach A, Gastaldi E and Gontard N (2007a), Èffect of concentration and relative humidity on the transfer of alkan-2-ones through paper coated with wheat gluten', Journal of Agricultural and Food Chemistry, 55, 867±875. Chalier P, Ben Arfa A, Preziosi-Belloy L and Gontard N (2007b), `Carvacrol losses from soy protein coated papers as a function of drying conditions', Journal of Applied Polymer Science, 106, 611±620. Charles F, Sanchez J and Gontard N (2003), Àctive modified atmosphere packaging of fresh fruits and vegetables: Modeling with tomatoes and oxygen absorber', Journal of Food Science, 68, 1736±1742. Cho S W, Ullsten H, Gallstedt M and Hedenqvist M S (2007), `Heat-sealing properties of compression-molded wheat gluten films', Journal of Biobased Materials and Bioenergy, 1, 56±63. Cuq B, Gontard N and Guilbert S (1998), `Proteins as agricultural polymers for packaging production', Cereal Chemistry, 75, 1±9. Cuq B, Boutrot F, Redl A and Lullien-Pellerin V (2000), `Study of the temperature effect on the formation of wheat gluten network: Influence on mechanical properties and protein solubility', Journal of Agricultural and Food Chemistry, 48, 2954±2959. Domenek S, Morel M H, Bonicel J and Guilbert S (2002), `Polymerization kinetics of wheat gluten upon thermosetting. A mechanistic model', Journal of Agricultural and Food Chemistry, 50, 5947±5954. Domenek S, Feuilloley P, Gratraud J, Morel M H and Guilbert S (2004a), `Biodegradability of wheat gluten based bioplastics', Chemosphere, 54, 551±559. Domenek S, Brendel L, Morel M H and Guilbert S (2004b), `Swelling behavior and structural characteristics of wheat gluten polypeptide films', Biomacromolecules, 5, 1002±1008. Floros J D and Matsos K I (2005), Ìntroduction to modified atmosphere packaging', in Innovations in Food Packaging, Han J (ed.), Elsevier Academic Press, New York, 159±172. Gallstedt M, Mattozzi A, Johansson E and Hedenqvist M S (2004), `Transport and tensile properties of compression-molded wheat gluten films', Biomacromolecules, 5, 2020±2028. Gallstedt M, Brottman A and Hedenqvist M S (2005), `Packaging-related properties of protein- and chitosan-coated paper', Packaging Technology and Science, 18, 161± 170. Gastaldi E, Chalier P, Guillemin A and Gontard N (2007), `Microstructure of proteincoated paper as affected by physico-chemical properties of coating solutions', Colloids and Surfaces A ± Physicochemical and Engineering Aspects, 301, 301± 310. Gennadios A, Weller C L and Testin R F (1993a), `Modification of physical and barrier properties of edible wheat gluten based films', Cereal Chemistry, 70, 426±429. Gennadios A, Weller C L and Testin R F (1993b), `Property modification of edible wheat gluten based-films', Transactions of the ASAE, 36, 465±470. Giannelis E P (1996), `Polymer layered silicate nanocomposites', Advanced Materials, 8, 29±35.


666


Gontard N and Ring S (1996), Èdible wheat gluten film: Influence of water content on glass transition temperature', Journal of Agricultural and Food Chemistry, 44, 3474±3478. Gontard N, Guilbert S and Cuq J L (1992), Èdible wheat gluten films ± influence of the main process variables on film properties using response surface methodology', Journal of Food Science, 57, 190±195. Gontard N, Guilbert S and Cuq J L (1993), `Water and glycerol as plasticizers affect mechanical and water vapor barrier properties of an edbile wheat gluten film', Journal of Food Science, 58, 206±211. Gontard N, Duchez C, Cuq J L and Guilbert S (1994), Èdible composite films of wheat gluten and lipids ± Water vapor permeability and other physical properties', International Journal of Food Science and Technology, 29, 39±50. Gontard N, Marchesseau S, Cuq J L and Guilbert S (1995), `Water vapor permeability of edible bilayer films of wheat gluten and lipids', International Journal of Food Science and Technology, 30, 49±56. Gontard N, Thibault R, Cuq B and Guilbert S (1996), Ìnfluence of relative humidity and film composition on oxygen and carbon dioxide permeabilities of edible films', Journal of Agricultural and Food Chemistry, 44, 1064±1069. Gontard N, Guillaume C, Gastaldi E and Chalier P (2008), Ìntegrated approach to design active and biodegredable tailor made food packaging', Society of Plastics Engineers, Annual Technical Conference, ANTEC, Milwaukee, WI, 4±8 May. Guilbert S, Gontard N, Morel M H, Chalier P, Micard V and Redl A (2002), `Formation and properties of wheat gluten films and coatings', in Protein-based Films and Coatings, Gennadios A (ed.), CRC Press, Boca Raton, FL, 69±122. Guilherme M R, Mattoso L H C, Gontard N, Guilbert S and Gastaldi E (2010), `Synthesis of nanocomposite films from wheat gluten matrix and MMT intercalated with different quaternary ammonium salts by way of hydroalcoholic solvent casting', Composites Part A ± Applied Science and Manufacturing, 41, 375±382. Guillaume C, Pinte J, Gontard N and Gastaldi E (2010), `Wheat gluten-coated papers for bio-based food packaging: Structure, surface and transfer properties', Food Research International, doi10.1016/j.foodres.2010.1004.1014. Hochstetter A, Talja R A, Helen H J, Hyvonen L and Jouppila K (2006), `Properties of gluten-based sheet produced by twin-screw extruder', LWT ± Food Science and Technology, 39, 893±901. Irissin-Mangata J, Boutevin B and Bauduin G (1999), `Bilayer films composed of wheat gluten and functionalized polyethylene: Permeability and other physical properties', Polymer Bulletin, 43, 441±448. Kayserilioglu B S, Stevels W M, Mulder W J and Akkas N (2001), `Mechanical and biochemical characterisation of wheat gluten films as a function of pH and cosolvent', Starch±StaÈrke, 53, 381±386. Lai H M and Chiang I C (2006), `Properties of MTGase treated gluten film', European Food Research and Technology, 222, 291±297. Larre C, Desserme C, Barbot J and Gueguen J (2000), `Properties of deamidated gluten films enzymatically cross-linked', Journal of Agricultural and Food Chemistry, 48, 5444±5449. Mascheroni E, Chalier P, Gontard N and Gastaldi E (2010), `Designing of a wheat gluten/ montmorillonite based system as carvacrol carrier: Rheological and structural properties', Food Hydrocolloids, 24, 406±413. Matan N, Rimkeeree H, Mawson A J, Chompreeda P, Haruthaithanasan V and Parker M (2006), Àntimicrobial activity of cinnamon and clove oils under modified



667

atmosphere conditions', International Journal of Food Microbiology, 107, 180± 185. Mauricio-Iglesias M, Peyron S, Guillard V and Gontard N (2010), `Wheat gluten nanocomposite films as food-contact materials: Migration tests and impact of a novel food stabilization technology (high pressure)', Journal of Applied Polymer Science, 116, 2526±2535. Micard V, Belamri R, Morel M H and Guilbert S (2000), `Properties of chemically and physically treated wheat gluten films', Journal of Agricultural and Food Chemistry, 48, 2948±2953. MujicaPaz H and Gontard N (1997), Òxygen and carbon dioxide permeability of wheat gluten film: Effect of relative humidity and temperature', Journal of Agricultural and Food Chemistry, 45, 4101±4105. Oberdorster G, Maynard A, Donaldson K, Castranova V, Fitzpatrick J, Ausman K, Carter J, Karn B, Kreyling W, Lai D, Olin S, Monteiro-Riviere N, Warheit D, Yang H et al. (2005), `Principles for characterizing the potential human health effects from exposure to nanomaterials: Elements of a screening strategy', Particle Fibre Toxicology, 2, 8. Olabarrieta I, Gallstedt M, Ispizua I, Sarasua J R and Hedenqvist M S (2006), `Properties of aged montmorillonite±wheat gluten composite films', Journal of Agricultural and Food Chemistry, 54, 1283±1288. Paz H M, Guillard V, Reynes M and Gontard N (2005), Èthylene permeability of wheat gluten film as a function of temperature and relative humidity', Journal of Membrane Science, 256, 108±115. PDL Handbook Series (1995), Permeability and Other Film Properties of Plastics and Elastomers, Plastics Design Library (PLD), Norwich, NY. Pochat-Bohatier C, Sanchez J and Gontard N (2006), Ìnfluence of relative humidity on carbon dioxide sorption in wheat gluten films', Journal of Food Engineering, 77, 983±991. Pommet M, Redl A, Morel M H and Guilbert S (2003), `Study of wheat gluten plasticization with fatty acids', Polymer, 44, 115±122. Ray S S and Bousmina M (2005), `Biodegradable polymers and their layered silicate nano composites: In greening the 21st century materials world', Progress in Materials Science, 50, 962±1079. Redl A, Morel M H, Bonicel J, Vergnes B and Guilbert S (1999), Èxtrusion of wheat gluten plasticized with glycerol: Influence of process conditions on flow behavior, rheological properties, and molecular size distribution', Cereal Chemistry, 76, 361± 370. Salame M (1986), `Barrier polymers', in The Wiley Encyclopedia of Packaging Technology, Bakker M (ed.), Wiley, New York, 48±54. Schofield J D, Bottomley R C, Timms M F and Booth M R (1983), `The effect of heat on wheat gluten and the involvement of sulfhydryl-disulfide interchange reactions', Journal of Cereal Science, 1, 241±253. Song Y H, Zheng Q and Lai Z Z (2008), `Properties of thermo-molded gluten/glycerol/ silica composites', Chinese Journal of Polymer Science, 26, 631±638. Sun S M, Song Y H and Zheng Q (2008), `Thermo-molded wheat gluten plastics plasticized with glycerol: Effect of molding temperature', Food Hydrocolloids, 22, 1006±1013. Tiede K, Tear S P, David H and Boxall A B A (2009), Ìmaging of engineered nanoparticles and their aggregates under fully liquid conditions in environmental matrices', Water Research, 43, 3335±3343.


668


Tunc S, Angellier H, Cahyana Y, Chalier P, Gontard N and Gastaldi E (2007), `Functional properties of wheat gluten/montmorillonite nanocomposite films processed by casting', Journal of Membrane Science, 289, 159±168. Valero D, Valverde J M, Martinez-Romero D, Guillen F, Castillo S and Serrano M (2006), `The combination of modified atmosphere packaging with eugenol or thymol to maintain quality, safety and functional properties of table grapes', Postharvest Biology and Technology, 41, 317±327. Valverde J M, Guillen F, Martinez-Romero D, Castillo S, Serrano M and Valero D (2005), Ìmprovement of table grapes quality and safety by the combination of modified atmosphere packaging (MAP) and eugenol, menthol or thymol', Journal of Agricultural and Food Chemistry, 53, 7458±7464. Wang J S, Zeng Y W and Zhao M M (2005), `Development and physical properties of film of wheat gluten cross-linked by transglutaminase', Journal of Wuhan University of Technology ± Materials Science Edition, 20, 78±82. Zagory D and Kader A A (1988), `Modified atmosphere packaging of fresh produce', Food Technology, 42, 70±77. Zhang X Q, Burgar I, Do M D and Lourbakos E (2005), Ìntermolecular interactions and phase structures of plasticized wheat proteins materials', Biomacromolecules, 6, 1661±1671. Zhang X Q, Hoobin P, Burgar I and Do M D (2006), `pH effect on the mechanical performance and phase mobility of thermally processed wheat gluten-based natural polymer materials', Biomacromolecules, 7, 3466±3473. Zhang X Q, Do M D, Dean K, Hoobin P and Burgar I M (2007), `Wheat-gluten-based natural polymer nanoparticle composites', Biomacromolecules, 8, 345±353.


24

Safety and regulatory aspects of plastics as food packaging materials B A L D E V R A J and R . S . M A T C H E , Central Food Technological Research Institute, India

Abstract: Polymeric materials are used extensively in food packaging. In addition to the basic polymers, plastics also contain additives added in small quantities to alter the properties of the polymers in the desired way and simplify their processing. These additives along with low-molecular-weight non-polymeric components, which may remain in plastic packaging materials, possess high mobility. It is likely that some transfer of these lowmolecular-weight non-polymeric components will occur from the plastic packaging material into the packaged content, thereby contaminating the product with the risk of toxic hazard to the consumer. This chapter reviews guidelines for proper use of plastics for food packaging applications and discusses the specific migration of some of the toxic additives like acetaldehyde, terephthalic acid, methyl ethyl glycol and bisphenol-A. Nanocomposites are also used in food packaging materials. There are many safety concerns about nanomaterials, as their size may allow them to penetrate into cells and eventually remain in the system. Manufacturers have to follow good manufacturing practice using only the additives listed in the positive list. Prior to categorizing such plastics as toxic, evidence regarding degree of migration of their constituents has to be ascertained. In general, migration and extraction studies need to be simultaneously conducted on actual foodstuffs under conditions that are slightly more stringent than those encountered in normal usage. Hence, for good measure, the overall migration of all the migrants put together is considered for safe use, unless they are especially toxic and their specific limits are fixed by the regulatory authorities such as: Bureau of Indian Standards, the European Commission Directives, and the Code of Federal Regulations of the US Food and Drug Administration. Key words: food contact materials (FCMs), indirect additives, antimicrobial agent, migration, safety nanocomposites, legislation, food stimulants, toxic additives, GRAS.

24.1

Introduction

The global retail market is flourishing day by day with different innovative and designed polymeric materials and items. Today, plastic has almost replaced metal, wood, glass and paper in the field of packaging but there is no substitute for plastics. Plastic is one of the greatest inventions of the last millennium. There has been enormous development in the field of food packaging with plastics.


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Food requires protection against various environmental factors from the time of its production till it is consumed. Hence, packaging is required to protect the food. The shelf-life of packaged foods may vary from a few days to more than a year. Thus, the properties of packaging material must have sufficient permanence to assure that shelf-life is not compromised (Matche and Baldev, 2005/06; Vijayalakshmi and Baldev, 2010). In addition to the basic polymers, plastics also contain additional chemical components, called additives, which are added in small amounts to alter the properties of the polymers in the desired way and/or to simplify their processing. Only fillers and softeners (plasticizers) are used at high concentration to increase volume and/or weight to improve softening flexibility, elasticity, malleability and processability. Other additives are mostly low-molecular-weight components like stabilizers, antioxidants, antistatic agents, light stabilizers (UV absorbers), lubricants (slip agents), optical brighteners, etc. Polymer packaging materials may also contain small quantities of monomers, oligomers as well as polymerization catalysts and regulators, crosslinking agents, emulsifying agents, etc. These additives along with lowmolecular-weight non-polymeric components, which may remain in plastic packaging materials, possess high mobility. It is likely that some transfer of these low-molecular-weight non-polymeric components will occur from the plastic packaging material into the packaged content, thereby contaminating the product with the risk of toxic hazard to the consumer. However, it is to be remembered that useful properties of the plastics are not manifested without the addition of these additives. Therefore, guidelines for proper use of plastics for food packaging applications have been realized all over the world, which are necessary to safeguard the health of consumers (Baldev, 2001).

24.2

Indirect food additives

Concern over the safety-in-use of plastics as food packaging materials arises principally from the possible toxicity of other low-molecular-weight constituents that may be present in the plastics and hence may be leached into the foodstuff during storage. As stated above, such constituents arise from two sources. Polymerization residues include monomers, oligomers (with a molecular weight less than 200), catalysts (mainly metallic salts and organic peroxides), solvents, emulsifiers, wetting agents, raw material impurities, plant contaminants, inhibitors, decomposition and side-reaction products. The more volatile gaseous monomers, e.g. ethylene, propylene and vinyl chloride, usually fall in concentration with time, but very low levels may persist in the finished product almost indefinitely. Styrene and acrylonitrile residues are more difficult to remove. Processing aids such as antioxidants, antiblock agents, antistatic agents, heat and light stabilizers, plasticizers, lubricants and slip agents, pigments, fillers, mould release agents and fungicides are added to assist production processes or to enhance the properties and stability of the final product. They may be present in


Safety and regulatory aspects of plastics

671

amounts varying from a few parts per million up to several percent (Crosby, 1981, Robertson, 2005; Jenkins and Harrington, 1991). Since compounds of the first group are present inadvertently, there is not a lot that can be done to remove them. However, the efforts made by the industries to reduce vinyl chloride monomer levels, in particular, illustrate the advantages of optimum manufacturing processes on the purity of the final product. Chemicals added deliberately during formulation to alter the processing, mechanical or other properties of the polymer are likely to be present in greater amounts than polymerization residues and should be subjected to strict quality control. They are normally restricted to compounds appearing on an approved list for food contact use. A brief mention of the function of some major additives is presented below.

24.2.1 Antiblock agents These agents are added to roughen the surface of thin films and, hence, prevent them sticking together during machine processing. Silica is most commonly used because its poor solubility in most polymers helps to increase the surface concentration and so introduces irregularity. Similarly, slip additives such as fatty acid amides are used to reduce mobility.

24.2.2 Antioxidants These additives prevent degradation of the polymer by reacting with atmospheric oxygen during moulding operations at high temperatures or when used in contact with hot foods, and to prevent deterioration during storage. Derivatives of phenols and organic sulphides are most frequently used as antioxidants. Some of these compounds are classified as heat stabilizers.

24.2.3 Antistatic agents Since all plastics are good electrical insulators and are in fact used on a large scale for this purpose, they will retain electrostatic charges produced by friction from contact with processing machinery. Accumulation of static electricity can cause problems through the pick-up of dust, adhesion between layers or particles of plastics, sparking, electrical shock and possibly fire hazards. Most antistatic agents are glycol derivatives or quaternary ammonium compounds, which increase the electrical conductivity and plate-out onto the surface of plastic.

24.2.4 Lubricants These are added to reduce frictional forces and are usually low to medium molecular weight hydrocarbons. They should possess good solubility in the plastic, low volatility and be relatively stable compounds.


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24.2.5 Plasticizers Plasticizers are added to make the product more flexible and less brittle. They are usually high molecular weight esters. The plasticizer also gives the material the limp and tacky qualities found in `cling' films. PVC containts about 20±30% of plasticizers. Typically phthalic esters such as dioctyl phthalate (DOP), also known as di-2-ethylhexyladipate (DEHA), are used as plasticizers.

24.2.6 Ultraviolet stabilizers UV stabilizers are needed to protect the product from deterioration by sunlight or even supermarket lighting. It is not only the finished packaging material but food products containing nutrients such as vitamin C that are also susceptible to this form of deterioration. Different UV stabilizers are utilized depending upon the substrate, intended functional life, and sensitivity to UV degradation. UV stabilizers, such as benzophenones, work by absorbing the UV radiation and preventing the formation of free radicals. Depending upon substitution, the UV absorption spectrum is changed to match the application. Concentrations normally range from 0.05% to 2%, with some applications going up to 5%.

24.2.7 Optical property modifiers The optical properties of a material from a technological aspect are normally described in terms of their ability to transmit light, to exhibit colour and reflect light from the surface (i.e. gloss). The majority of virgin food packaging films are unpigmented but some are coloured by the addition of colourants. The principal pigments for use as colourants in packaging materials are carbon black, white titanium dioxide, red iron oxide, yellow cadmium sulfide, molybdate orange, ultramarine blue, blue ferric ammonium ferrocyanide, chrome green, and blue and green copper phthalocyanins.

24.2.8 Foaming agents There are two types of foaming or blowing agents: physical and chemical, which are used for the production of cellular products. In the physical process gas is generated to produce the cells; this takes place through a physical transition, i.e. evaporation or sublimation. In a chemical process, decomposition reactions take place which result in evolution of gases. In food packaging applications, physical blowing agents are normally used. In expanded and extruded polystyrene foams fluorocarbon or light aliphatic hydrocarbon such as pentane is used as a blowing agent.



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24.2.9 Antimicrobial agents Antimicrobial packaging is playing an important role in the inhibition of pathogenic contamination in foods, thereby extending the shelf-life of foods. With the potential in providing food quality and safety, antimicrobial packaging is gaining a lot of interest in research and development. The major potential food applications of antimicrobial films include some for sensitive foods like bakery products, dairy products (cheese), fresh produce such as fruits and vegetables, and meat, fish and poultry products. Antimicrobials such as algicides, bactericides and fungicides can be added to polymers to prevent the growth of microorganisms inside the package. However, their use in food packaging is rare because of the possibility of migration into the food itself. Regulations might require some amendments related to toxicology and testing of antimicrobial compounds for the newer materials, as they might not be covered under the regulations.

24.3

Nanotechnology in food contact materials

Food contact materials (FCMs) and articles including packaging materials, cutlery, dishes, processing machines, containers, etc., intended to come into contact with foodstuffs, are based on metal/metal oxide nanoparticles and nanoclays. Nanotechnology in food contact materials is a newer area to be developed for the future. At present there is some hesitation to incorporate nanomaterials because of the uncertainty of future regulations and standards and for fear of negative consumer reactions (Lyndhurst, 2009). The report also indicates that attitudes to novel food technologies in the USA and Asia seem to be generally more positive than in Europe. Nevertheless, there is a possibility that the general public's attitude to nanotechnologies in food packaging might be less negative than to nanotechnologies incorporated into food itself. The use of nanotechnologies in food packaging in Europe is in principle sufficiently regulated by Commission Directive 2004/1935/EC which covers all materials coming in contact with foodstuffs. According to this Commission Directive, individual Member States may ask the European Food Safety Authority (EFSA) to conduct a safety evaluation of food contact materials. Food contact plastics are subject to additional measures regulated by Commission Directive 2008/282/ EC on recycled plastic materials and articles, and by Commission Directive 2009/450/EC which sets down additional requirements to Commission Directive 2004/1935/EC for active and intelligent materials and articles. Finely dispersed nanosilver particles permanently embedded in plastic containers significantly reduce bacterial growth by 99% and ensure safer, fresher and tastier food. Nanotitanium particles are used as antibacterial agents in ultrafine filters which can capture and eliminate bacteria and odours from up to 99% of the particles and ensure that fresh and purified air is circulated through the fridge compartments (for instance, Hitachi's Advanced Multi Flow system).


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Aluminium foil, widely used in flexible packaging for food with surface properties improved by anti-adhesive coating or black coating by nanotechnology, is used as baking foil which does not reflect heat in an oven. Moreover, ZnO nanoparticles do not discolour, nor do they require ultraviolet to get activated. These properties make nano-ZnO a superior non-organic antibacterial agent (Observatory Nano, 2009). With respect to the use of nanoparticles of the additive in the polymer matrix, there is no reason to believe that àdequately' modified nanocomposites making use of substances in positive lists can impose any immediate risk for food-contact applications; however, studies concerning potential migration issues and life-cycle analysis have to be undertaken to corroborate the fact (LagaroÂn et al., 2005). In silver-based nanoclay polylactic acid film, migration levels of silver, within the specific migration levels referenced by the European Food Safety Authority (EFSA), exhibit antimicrobial activity, supporting the potential application of this biocidal additive in active foodpackaging applications to improve food quality and safety (Busolo et al., 2010).

24.4

Migration of additives

The ingredients in plastic packaging materials may cause toxicity as a result of their migration to the foodstuffs that are packed in them. Therefore positive lists of constituents (additives) permitted in the respective plastics used in contact with foodstuffs, pharmaceuticals and drinking water have been specified by the Bureau of Indian Standards (BIS) (Table 24.1). Manufacturers have to follow good manufacturing practice (GMP), using only such additives as are listed in the positive list. Prior to categorizing such plastics as toxic, evidence regarding the degree of migration of their constituents has to be ascertained. In general, migration and extraction studies need to be simultaneously conducted on actual foodstuffs under conditions that are slightly more stringent than those encountered in normal usage. It is, however, not always possible to analyse actual foodstuffs for the nature and quantity of migrants from the plastics. In order to simplify such assessment, food simulants/extractants have to be substituted for the actual foodstuffs. Further, it is also very difficult to estimate all the migrants individually. Hence, for good measure, the overall migration of all the migrants put together is considered for safe use, unless they are especially toxic and their specific limits are fixed (Bureau of Indian Standards (BIS) IS:9845-1998; Commission Directive 2002/72/EC; US Food and Drug Administration (US FDA) Code of Federal Regulations (CFR) 21,176.170).

24.4.1 Migration model The extent of migration of a substance depends on its concentration in the material, the degree to which it is bound or mobile within the matrix of the material, the thickness of the packaging material, the nature of the food with



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Table 24.1 Indian standards for plastics in contact with foodstuffs, pharmaceuticals and drinking water IS no.

Title

10171:1999

Guide on suitability of plastics for food packaging (second revision) List of pigments and colorants for use in plastics in contact with foodstuffs, pharmaceuticals and drinking water Determination of overall migration of constituents of plastics materials and articles intended to come in contact with foodstuffs ± method of analysis (second revision) Polyethylene for its safe use in contact with foodstuffs, pharmaceuticals and drinking water Positive list of constituents of polyethylene in contact with foodstuffs, pharmaceuticals and drinking water (first revision) Positive list of constituents of polypropylene and its copolymers for its safe use in contact with foodstuffs, pharmaceuticals and drinking water (first revision) Polypropylene and its copolymers for its safe use in contact with foodstuffs, pharmaceuticals and drinking water Positive list of constituents of polystyrene (crystal and high impact) in contact with foodstuffs, pharmaceuticals and drinking water Polystyrene (crystal and high impact) for its safe use in contact with foodstuffs, pharmaceuticals and drinking water (first revision) Positive list of constituents of polyvinyl chloride and its copolymers for safe use in contact with foodstuffs, pharmaceuticals and drinking water Polyvinyl chloride (PVC) and its copolymers for its safe use in contact with foodstuffs, pharmaceuticals and drinking water Positive list of constituents of ionomer resins for its safe use in contact with foodstuffs, pharmaceuticals and drinking water Ionomer resins for its safe use in contact with foodstuffs, pharmaceuticals and drinking water Positive list of constituents of ethylene/acrylic acid (EAA) copolymers for their safe use in contact with foodstuffs, pharmaceuticals and drinking water Ethylene/acrylic acid (EAA) copolymers for its safe use in contact with foodstuffs, pharmaceuticals and drinking water Positive list of constituents of polyalkylene terephthalates (PET & PBT) for their safe use in contact with foodstuffs, pharmaceuticals and drinking water Polyalkylene terephthalates (PET & PBT) for its safe use in contact with foodstuffs, pharmaceuticals and drinking water

9833:1981 9845:1998 10146:1982 10141:1982 10909:1984 10910:1984 10149:1982 10142:1999 10148:1982 10151:1982 11435:1985 11434:1985 11705:1986 11704:1986 12229:1987 12252:1987


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Table 24.1 Continued IS no.

Title

12248:1998

Positive list of constituents of Nylon-6 polymer for its safe use in contact with foodstuffs, pharmaceuticals and drinking water Nylon-6 polymer for its safe use in contact with foodstuffs, pharmaceuticals and drinking water Positive list of constituents of ethylene vinyl acetate (EVA) for its safe use in contact with foodstuffs, pharmaceuticals and drinking water Ethylene vinyl acetate (EVA) copolymers for its safe use in contact with foodstuffs, pharmaceuticals and drinking water Positive list of constituents of ethylene methacrylic (EMMA) copolymer and terpolymers in contact with foodstuffs, pharmaceuticals and drinking water Ethylene methacrylic and (EMMA) copolymer and terpolymers for their safe use in contact with foodstuffs, pharmaceuticals and drinking water Positive list of constituents of polycarbonate resins in contact with foodstuffs, pharmaceuticals and drinking water Polycarbonate resins for its safe use in contact with foodstuffs, pharmaceuticals and drinking water Positive list of constituents of melamine-formaldehyde resins in moulded articles in contact with foodstuffs, pharmaceuticals and drinking water Melamine-formaldehyde resins in moulded articles in contact with foodstuffs, pharmaceuticals and drinking water Positive list of constituents of modified poly(phenylene oxide) (PPO) in contact with foodstuffs, pharmaceuticals and drinking water Modified poly(phenylene oxide) (PPO) resins in contact with foodstuffs, pharmaceuticals and drinking water Positive list of constituents of unsaturated polyester resins in contact with foodstuffs, pharmaceuticals and drinking water

12247:1998 13449:1992 13601:1993 13557:1992 13576:1992 Doc: PCD 12(1328) Doc: PCD12 (1329) Doc: PCD 12(1331) Doc: PCD 12(1332) Doc: PCD 12(1375) Doc: PCD 12(1375) Doc: PCD 12(1516)

which the material is in contact (dry, aqueous, fatty, acidic or/and alcoholic in nature), the solubility of the substance in the food, the duration of contact, and the temperature. In a polymer/food system as presented in Fig. 24.1, there is food on the left that can migrate into the polymer layers on the right side, along with an intermediate layer of swollen polymer with a profile of the migrating food component. On the other hand, we have a concentration gradient of the considered additives, where certain diffusion in the undisturbed polymer layer and a much improved mobility of the additive in the swollen layer and concentration jump at the interfaces are assumed.



677

24.1 Migration model for the polymer/food system (adapted from Baldev Raj, 2001).

The following general formula relates the migration of an additive. In a system where a cut piece of the plastic P into a food F at a certain time t is kept at constant temperature, the model predicts direct proportionality of migration of the concentration CAP of the considered additive in the polymer and to the square root of time (t): p MAF T CAP t where MAF T is the migration of additive A into test food F at a temperature T (Crosby, 1981).

24.5

Indian Standards for overall migration (IS:9845-1998)

Central Food Technological Research Institute, Mysore, India, has drafted IS:9845-1998 for `Determination of overall migration of constituents of plastics materials and articles intended to come in contact with foodstuffs ± method of analysis (second revision)' which is now implemented to be followed for overall migration of plastics constituents for their food grade quality, in the country. This standard is the result of R&D work in the laboratory on the study of various factors affecting the migration of additives in food simulants, and is at par with other international standards like US FDA, European Commission Directives, etc. A collection of data regarding the main composition and overall extractable amount of plastic constituents can help with the estimation of migration. This


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Multifunctional and nanoreinforced polymers for food packaging Table 24.2 Migration tests of plastic materials and articles for certain types of food Sl no.

Food type

Simulant

1 2 3 4 5 6 7

Aqueous foods All aqueous and acidic foods Alcoholic foods Fats/oils and fatty foods Alcoholic and acidic foods Fatty and aqueous foods All fatty and acidic foods

A B C D C and D D and A D and B

A: Distilled water B: 3% acetic acid C: 8%, 10%, 50% ethanol D: n-heptane or substitute of olive oil (isooctane and 95% ethanol).

can be a considerable asset both to the producers of such articles and for quality control laboratories. Much time and money may also be saved if studies are made in the evaluation of laminates containing layers of recycling material with unknown impurities which can migrate through the virgin plastic layer (functional barrier) in contact with food. A BIS list of all the specifications on different polymeric materials coming in contact with food application is given in Table 24.1. The choice of simulating solvents and test conditions (time±temperature) depends on the type of foods and conditions of use of food products. Food products have now been classified into seven major groups as shown in Table 24.2. This table has been prepared on the lines of the accepted classification of foodstuffs for such a purpose. The table also gives suitable simulants to be used for different types of foods.

24.5.1 Selection of samples Test samples representing the lot/batch have to be conducted in triplicate. Samples in each replicate shall consist of a number of containers (preformed or converted products) with nearest exposed area of 1000 cm2. In the case of heatsealable films a representative sample shall be of sufficient size to convert into two pouches with an exposed surface area of 1000 cm2 (size of each pouch 12.5 cm in width and 20 cm in length) and non-heat-sealable homogeneous films of size 50 cm 10 cm to be exposed over both sides with 1000 cm2 surface area coming in contact. In the case of lids/wads, 10 pieces are to be sealed to glass bottles only in the smallest size in actual use, to be placed reverted in position with simulant inside during the test period. The samples in the form of containers/pouches/film/lids used shall be carefully rinsed with water (25±30ëC) to remove extraneous materials prior to the actual migration test.



679

24.5.2 Procedure Sample containers/pouches in each replicate are filled to their capacity with preheated simulant and closed/sealed. Non-heat-sealable film samples are exposed on both sides with preheated simulant at the test temperature (at least 1 ml/cm2 of contact area). The test samples exposed to the simulant are maintained at a specified temperature in an oven/water bath/autoclave for the specified duration. After completion of exposure time the extracted simulant is transferred into a clean Pyrex glass beaker/container along with three washings of the specimen with a small quantity of the fresh simulant.

24.5.3 Determination of amount of extractive The extracted simulant is evaporated/distilled in a Pyrex beaker/round-bottom flask to about 50±60 ml and transferred into a clean tared stainless steel dish along with three washings with a small quantity of fresh simulant. Further, the concentrate is evaporated in the dish to dryness in an oven at 100 5ëC. The dish with extractive is cooled in a desiccators for 30 minutes and weighed to the nearest 0.1 mg till a constant weight of residue is obtained. The extractives are calculated as mg/dm2 and mg/kg or ml/l or ppm of the foodstuff with respect to the capacity of the container/pouch to be used. A blank shall also be carried out without the sample for adjustment, if necessary. Then: Amount of extractive (Ex)

M 100 mg/dm2 A M 1000 mg/kg or mg/l or ppm V

where M mass of residue in mg minus blank value, A total surface area in cm2 exposed in each replicate, and V total volume in ml of simulant used in each replicate or filled capacity of containers. The simulants and test conditions (time±temperature) for extractability studies to be carried out as per different national and international standards depending on the type of food and conditions of use are given in Table 24.3.

24.5.4 Migration limits The test material shall comply with the overall migration limit when tested by the method prescribed in IS:9845-1998. In the case of liquid foodstuffs or of simulants, the upper limit shall be 60 mg/l or ppm. However, for the value of the overall migration the upper limit shall be 10 mg/dm2 of the surface of the material or article. In case of lids/wads the results can be expressed only as mg/kg, with 60 mg/l or ppm as the upper threshold limit.


Table 24.3 Time±temperature test conditions using food simulants for overall migration in plastics SI no.

Condition of contact

H2O

3% acetic acid

Ethanol

(gen)

(BIS, EEC)

A

B

8% (US FDA) 10% (BIS, EEC) 50% (gen) C

n-Heptane* (BIS, US FDA)

Fat simulants D Substitute for olive oil (EEC) Isooctane 95% ethanol


1

High temperature heat sterilized (retorting) (BIS + EEC + US FDA ÿ gen)

121ëC, 2 h

121ëC, 2 h

±

66ëC, 2 h

60ëC, 2.5 h

60ëC, 4.5 h

2

Hot filled or pasteurized above 66ëC, below 100ëC (gen)

100ëC, 2 h 100ëC, 0.5 h

100ëC, 2 h

±

49ëC, 0.5 h

60ëC, 1.5 h

60ëC, 3.5 h

3

Hot filled or pasteurized below 66ëC (gen)

70ëC, 2 h 66ëC, 2 h

70ëC, 2 h

70ëC, 2 h 66ëC, 2 h

38ëC, 0.5 h

40ëC, 0.5 h

60ëC, 2 h

4

Room temperature filled and stored and also in refrigerated and frozen condition (no thermal treatment in container) (gen)

40ëC, 10 d 49ëC, 1 d

40ëC, 10 d

40ëC, 10 d

38ëC, 0.5 h

20ëC, 2 d

40ëC, 10 d

5

Refrigerated storage (no thermal treatment in container) (US FDA)

21ëC, 2 d

±

21ëC, 2 d

21ëC, 0.5 h

±

±

6

Frozen storage (no thermal treatment in container) US FDA

21ëC, 1 d

±

21ëC, 0.5 h

±

±


24.6

681

US Food and Drug Administration (US FDA) Code of Federal Regulations (CFR)

In United States of America all the packaging materials are evaluated for food contact application as per the US FDA, CFR 21, Parts 170 to 199, revised as of 1 April 2009.

24.6.1 Indirect food additives: general Regulations prescribing conditions under which food additive substances may be safely used predicate usage under conditions of good manufacturing practice. The quantity of any food additive substances that may be added to food as a result of use in articles that contact food shall not exceed, where no limits are specified, that which results from use of the substance in an amount not more than reasonably required to accomplish the intended physical or technical effect in the food-contact article; shall not exceed any prescribed limitations; and shall not be intended to accomplish any physical or technical effect in the food itself, except as such may be permitted by the regulations. Any substance used as a component of articles that contact food shall be of purity suitable for its intended use. The existence of a regulation prescribing safe conditions for the use of a substance as an article or component of articles that contact food shall not be constructed as implying that such substance may be safely used as a direct additive in food. Substances that under conditions of good manufacturing practice may be safely used as components of articles that contact food include the following subjects to any prescribed limitations: · Substances generally recognized as safe in or on food · Substances generally recognized as safe for their intended use in food packaging · Substances used in accordance with a prior sanction or approval · Substances permitted for use by regulations as such and parts.

24.6.2 Threshold of the regulations and migration limits Substances used in food-contact articles (e.g., food-packaging or foodprocessing equipment) that migrate, or may be expected to migrate, into food at negligible levels may be reviewed under the regulation US FDA, CFR 21, Parts 170 to 199. In the finished form in which it is to contact food, when extracted with the solvent or solvents characterizing the type of food, and under conditions of time and temperature characterizing the conditions of its intended use as determined from Tables 24.2 and 24.3, the extractives shall not exceed 0.5 mg per square inch (7.75 mg/dm2) of food-contact surface, nor exceed 50 parts per million of the water capacity of the container in general or


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other limits specified for a specific material when tested as per the prescribed method.

24.7

European Commission Directives on plastic containers for foods

At the European level Framework Directive 1989/109/EC defines comparable general requirements for plastic containers. In the early 1980s corresponding separate directives in the field of plastic utensils were adopted at the European level, which also included procedures for carrying out such migration tests. European regulations have been harmonized to a large extent, at least with regard to admissible monomers and starting substances (positive lists) as well as to maximum admissible migration of ingredients of plastics utensils: this also applies to overall migration limitations, maximum admissible residual content of certain monomers and starting substances in plastic containers (so-called QM(A) limits), and maximum admissible migration limits of defined specific substances (so-called SML(T)) (Commission Directive 2004/19/EC). Commission Directives (Table 24.4) have laid down procedures for selecting food simulants and also requirements for testing migration based on actual conditions of use (time/temperature combinations). On the other hand, the existing European Directives mentioned above partly cover the use of plastic additives and at present provide no regulations at all with regard to aids to polymerization and colouring materials in plastics. In practice, the evaluation of plastic containers Table 24.4 European Framework Directives on separate materials in contact with food Directive no.

Subject

Directive 2002/72/EEC Directive 90/128/EEC Directive 82/711/EEC Directive 85/572/EEC Directive 80/766/EEC Directive 81/432/EEC

Plastic materials and articles Plastic monomers Basic rules for migration tests List of simulants/foodstuffs VC in PVC Method of analysis for vinyl chloride released into foodstuffs Limits of vinyl chloride monomer Determining symbols Regenerated cellulose film (RCF) Ceramic articles First amendment to 83/229/ECC Amendment to 83/229/ECC First amendment to 82/711/ECC Nitrosamines in elastomers and rubber Second amendment to 82/711/ECC Epoxy derivatives

Directive 78/142/EEC Directive 80/590/EEC Directive 83/329/EEC Directive 84/500/EEC Directive 86/388/EEC Directive 92/15/EEC Directive 93/8/EEC Directive 93/11/EEC Directive 97/48/EEC Directive 2001/61/EEC



683

(in particular packages) with regard to compliance with food regulation is a twostep procedure in most cases: first the ingredients of the recipe are examined so as to ensure that the materials used are admissible in principle. This examination is based on existing European Directives. If, in this first step, all components of the recipe turn out to be admissible in principle, migration tests are carried out. In the next step, individual components of the plastic container in question (e.g., additives, colouring materials, monomers, etc.) are not transmitted to the filling material (foodstuff) to an inadmissibly great extent. The corresponding tests are preferably carried out directly on the respective containers or on a test specimen taken from it, with specific attention paid to the requirement that the overall migration limit and any specific migration limits be met (Franz et al., 1992; Till et al., 1987).

24.7.1 Active and intelligent food contact materials As per Commission Directive 2004/1935/EC, active and intelligent food contact materials and articles designed to actively maintain or improve and monitor the condition of the food are not inert by their nature. It is therefore necessary, for reasons of clarity and legal certainty, to be included in the scope of the Regulation. Further requirements should be stated in specific measures, to include positive lists of authorized substances and/or materials and articles, which should be adopted as soon as possible. Active food contact materials and articles are designed to deliberately incorporate àctive' components intended to be released into the food or to absorb substances from the food. They should be distinguished from materials and articles which are traditionally used to release their natural ingredients into specific types of food during the process of their manufacture. Active food contact materials and articles may change the composition or the organoleptic properties of the food only if the changes comply with the Community provisions applicable to food, such as the provisions of Commission Directive 1989/ 107/EC(4) on food additives. In particular, substances such as food additives deliberately incorporated into certain active food contact materials and articles for release into packaged foods or the environment surrounding such foods, should be authorized under the relevant Community provisions applicable to food and also be subject to other rules which will be established in a specific measure. As per amendments of Regulation (EC) No. 2004/1935/EC described in Regulation (EC) No. 2009/596/EC, active and intelligent food contact materials and articles should not change the composition or the organoleptic properties of a food or give information about the condition of the food that could mislead consumers. For example, active food contact materials and articles should not release or absorb substances such as aldehydes or amines in order to mask an incipient spoilage of the food. Such changes, which could manipulate signs of


684


spoilage, could mislead the consumer and should therefore not be allowed. Similarly, active food contact materials and articles which produce colour changes to the food, giving the wrong information concerning the condition of the food, could mislead the consumer and therefore should also not be allowed. In addition, adequate labelling or information should support users in the safe and correct use of active materials and articles in compliance with the food legislation, including the provisions on food labelling. On grounds of efficiency, the normal time limits for the regulatory procedure with scrutiny should be curtailed for the adoption of a list of substances authorized for use in the manufacture of active or intelligent food contact materials and articles. When necessary, special conditions of use, purity standards and specific limits on the migration into or on to food are to be used. For substances exempt from specific migration limits or other restrictions, a generic specific migration limit of 60 mg/ kg or 10 mg/dm2, according to the case, is applied. However, the sum of all the specific migrations should not exceed the overall migration limits.

24.8

Specific migration of toxic additives

In addition to creating safety and health problems during production, many chemical additives that give plastic products desirable packaging qualities also have negative environmental and human effects. These effects include direct toxicity as in the case of lead, cadmium and mercury. Most of the colourful plastic containers, which are manufactured by recycling, would have these toxic additives. Plastic containers can contaminate food because some chemicals diffuse from the packaging polymer of which they are made to the foods they contain. Migration potential exists for traces of monomers, oligomers, additives, stabilizers, plasticizers and lubricants. Such substances may be toxic. A report of the Berkeley (US) Plastics Task Force published in 1996 found that styrene from polystyrene, plasticizers from polyvinyl chloride (PVC), antioxidants from polyethylene and acetaldehyde from polyethylene terephthalate (PET) have the potential to contaminate food (Stover et al., 1996/Berkeley Report).

24.8.1 Vinyl chloride As per mutagenicity and metabolism of vinylchloride monomer (VCM), a wide range of toxic effects has been reported in human case studies. The principal effects observed include lesions of the bones in the terminal joints of the fingers and toes (acro-osteolysis) as well as changes in the liver and spleen. Long-term exposure gives rise to a rare form of liver cancer (angiosarcoma) and the association with exposure to VCM has been reported amongst plant operatives in several countries. In recent years, however, exposure to VCM at production and polymerization plants has been markedly reduced. It is well known that vinyl chloride causes angiosarcomas for the liver as well as tumours of the brain,



685

lung and haematolymphopoietic systems in humans. As per European Commission Directive, the limits for the level of vinyl chloride in materials and articles and for the level of vinyl chloride released by materials and articles to foodstuffs shall be 1 mg/kg of PVC material and 0.01 mg/kg of food (Commission Directive 1978/142/EC). As per Indian Standard, the vinyl chloride monomer content of PVC suspension resin used for manufacture shall not exceed 5 ppm, and in PVC containers/ film used for food packaging shall not exceed 1 ppm. The residual migration of VCM into foodstuffs being packed shall not exceed 10 ppb. The method developed at Central Food Technological Research Institute (CFTRI), Mysore, is suitable for estimation of residual VCM content in PVC material and foods packed in them up to 0.01 ppm levels (Ravi et al., 2000).

24.8.2 Vinylidene chloride (VDC) Less is known of the toxicology of VDC, both in animals and in humans. The LD value for rats is around 1500 mg/kg body weight, while in mice the value is 200 mg/kg body weight. VDC affects the activity of several rat liver enzymes and decreases the store of glutathione. Some tumours have been observed after prolonged exposure but no teratogenic effects were seen in rats or rabbits. The main pathway of excretion is via the lungs, with other metabolites being discharged by the kidneys.

24.8.3 Acrylonitrile (AN) Acrylonitrile is considerably more toxic than the chlorinated monomers and has a lethal dose (LD) value of 80±90 mg/kg body weight in rats and 27 mg/kg body weight in mice. It has also been shown to be mutagenic after metabolic activation with liver enzymes. In animals AN is metabolized to cyanide, which is converted to thiocyanate and excreted in the urine. There is also some evidence of carcinogenicity in animals and possibly humans too. As per US FDA, styrene±maleic anhydride copolymers shall not contain residual acrylonitrile monomer more than 0.1 wt%. In nitrile rubber modified acrylonitrile±methyl acrylate copolymers, the residual acrylonitrile monomer content is not more than 11 parts per million (US FDA, CFR 21, 177-1020).

24.8.4 Styrene The LD value of styrene for rats is 5 g/kg body weight. It is metabolized to styrene and its oxide, which is a potent mutagen in a number of test systems. Both styrene and its oxide have been found to produce chromosomal aberrations under certain conditions. Toxic effects of styrene in humans have been reviewed by the International Agency for Research on Cancer (IARC). The most fre-


686


quently observed changes were of a neurological and psychological nature. The total residual monomers, when present, shall not exceed 0.2% by mass of the polymer, as per Indian Standard. As per US FDA styrene±maleic anhydride copolymers shall not contain residual styrene monomer more than 0.3 wt%. Polystyrene basic polymers shall not contain more than 1 wt% of total residual styrene monomer (US FDA, CFR 21, 177-1020).

24.8.5 Colourants in plastics Plastics are increasingly coloured to enhance the attractiveness of packaging, to protect the contents from the adverse effects of light or to differentiate between products. Depending on the type of packaging, the contents and the storage conditions, it is possible that components in the packaging, including colourants, could migrate to the food. It must therefore be ensured that the packaging components, including colourants, do not pose a health hazard for the consumer. This is also the aim of the relevant Directives, laws and regulations. The colouration of plastics that come into contact with food is an important application for the colourants industry. The following basic criteria are decisive for the safe use of colourants for the colouration of food contact articles and packaging: · Purity criteria of the colourants · Its fastness to migration · The tested toxicological properties. Colourant manufacturers guarantee that the colourants have been toxicologically tested and that the purity criteria are met. The manufacturers of the food contact article or packaging material are responsible for the migration testing. Consumer safety is the joint responsibility of manufacturers, processors and authorities. For the consumer the safe use of coloured plastic food contact articles is already provided for by the current regulations in combination with a responsible approach by the pigment manufacturers and processors. Nevertheless, new knowledge must always be taken into account. Basically the problem of colour migration is found in vegetable oils, when they are packed into coloured polythene containers. CFTRI has developed simple methods to detect the colour migration qualitatively from plastics. Migration of the colour can be observed by exposing coloured plastic pieces in decolourized coconut oil when compared with blank. However, quantitative estimation can also be done using spectroscopic analysis (Baldev et al., 2007). As per US FDA, CFR 21, 178.3297 Colourants and polymers, the substances may be safely used as colourants in the manufacture of articles or components of articles intended for use in producing, manufacturing, processing, preparing, treating, packaging, transporting or holding food. The term colourant means a



687

dye pigment, or other substance that is used to impart colour to, or to alter the colour of, a food-contact material, but that does not migrate to food in amounts that will contribute to that food any colour apparent to the naked eye. As per Indian Standards, colour migrated to food simulant or decolourised coconut oil or packed food shall not be apparent to the naked eye. If the colour migrated is clearly visible, such materials are not suitable for food contact applications, even though the extractive value is within the limit (IS: 98331981).

24.9

Recent problems in specific migration

In recent years, there has been considerable demand by the food industries for information concerning the specific migration of some additives and their estimation ± acetaldehyde, terephthalic acid and methyl ethyl glycol in PET containers (Ewender et al., 2003), bisphenol-A (BPA) content in epoxy coatings and polycarbonate (Yoshiki et al., 2005). European Directives used for plastic materials and articles in contact with food regulations have fixed upper limits for the specific migration of hundreds of additives. The methodology of such specific migration of additives is not available. Work has been reported only on the few additives in plastics. There is a need to set up a facility to standardize the methodology for estimating such specific additives to help the industries to evaluate their packaging materials for safety and to prevent any health hazard to the consumer. Due to the estrogenic activity of BPA used as monomer in polycarbonate feeding bottles and epoxy-coated cans, there is a need for newer methods in order to have reliable tools for risk assessment and control of human exposure to BPA (Ballesteros-GoÂmez et al., 2009). Chemical degradation of epoxy resin into monomer using solvent has been reported (Sato et al., 2001; Braun et al., 2001). Formation of monomer (BPA) by recycling of polycarbonate resin was reported by Oku and co-workers (Oku et al., 2000; Hata et al., 2002; Kawai et al., 2005).

24.10 Future trends In recent years nanotechnology has entered the field of food packaging technology. Nanocomposites are used in food contact materials (FCMs), since the addition of nanoreinforcements can not only passively protect the food against environmental factors, but also incorporate properties to the packaging material related to improvements in overall performance by enhancing their mechanical, thermal and barrier properties, usually even at very low contents. Moreover, several nanoparticles can provide active and/or `smart' properties to food packaging materials, such as antimicrobial properties, oxygen scavenging ability, enzyme immobilization, or indication of the degree of exposure to some degradation-related factor.


688


Current legislation pertaining to food ingredients, food additives and FCMs does not differentiate between substances produced routinely by `standard' manufacturing methods and those developed by nanotechnology. There is currently no size limitation on particle size for food additives except for E460 cellulose (microcrystalline). There are many safety concerns about nanomaterials, as their size may allow them to penetrate into cells and eventually remain in the system. There is no consensus about categorizing nanomaterials as new (or unnatural) materials. On the one hand, the properties and safety of materials in their bulk form are usually well known, but their nano-sized counterparts frequently exhibit different properties from those found at the macro-scale. There is limited scientific data about migration of most types of nanoparticles (NPs) from the packaging material into food, as well as their eventual toxicological effects. It is reasonable to assume that their migration may occur into foods, hence the need for accurate information on the effects of NPs on human health following chronic exposure is imperative (de Azeredo, 2009). There may not be the need to develop a new approach to risk assessment of nanomaterials, but there is a clear need to provide hazard identification data on the widest possible range of nanomaterials. In the absence of such data, it is not possible to derive conclusions about the spectrum of toxicological effects that might be associated with nanomaterials. There is a need for rules on substances and materials that are problematic and not dealt with elsewhere in the legislation. If and until such legislation is completed and adopted, the products of nanotechnology will continue to be dealt with by a combination of general food law and more specific controls on particular materials and articles. Specific legislation dealing with nanocomponents in food and FCMs is only likely to be made if there is sound scientific evidence to show that such materials present a higher risk than their macro equivalents. In the absence of detailed toxicological data but in view of the potential of some nanoparticles to cause harm, it may also be appropriate to consider application of the precautionary principle (PP) for certain applications of nanotechnology in the food sector. The PP is a wellaccepted tenet of international law and is an attempt to legally codify the maxim `better safe than sorry'. Although evidence is emerging to suggest that certain engineered nanoparticles have the potential to cause harm to human health, it is not clear at present whether there is enough scientific basis to invoke the PP in all applications of nanotechnology for food contact materials. There is a need for research on any significant risk of indirect contamination of food through migration of nanoparticles from food packaging or active surfaces used in food processing. Interdisciplinary research is vital to address the current uncertainties and much can be learnt from parallel areas. Like any other new technology, public confidence, trust and acceptance are likely to be the key factors determining the success or failure of nanotechnology applications for FCMs (Chaudry et al., 2008).



689

24.11 References and further reading Baldev Raj (2001), `Food and packaging interaction ± Migration concepts and regulations', Indian Food Industries, 20, 67±74. Baldev Raj, Vijayalakshmi N S, Ravi P and Srinivas P (2007), `Migration behaviour and estimation of colourants from coloured plastics to edible oils', Deutsche Lebensmittel-Rundschau, 1, 15±20. Ballesteros-GoÂmez A, Rubio S and PeÂrez-Bendito D (2009), Ànalytical methods for the determination of bisphenol A in food', Journal of Chromatography: A, 1216(3), 449±469. BIS, IS:9833-1981, List of pigments and colourants for use in plastics in contact with foodstuffs, pharmaceuticals and drinking water (reaffirmed 2003). BIS, IS:9845-1998, Determination of overall migration of constituents of plastics materials and articles intended to come in contact with foodstuffs ± method of analysis. BIS, IS:10146-1982, Specification for polyethylene for its safe use in contact with foodstuffs, pharmaceuticals and drinking water. Braun D, von Gentzkow W and Rudolf A P (2001), `Hydrogenolytic degradation of thermosets', Polymer Degradation and Stability, 74, 25±32. Busolo M A, Fernandez P, Ocio M J and LagaroÂn J M (2010), `Novel silver-based nanoclay as an antimicrobial in polylactic acid food packaging coatings', Food Additives and Contaminants: Part A, DOI: 10.1080/19440049.2010.506601. Chaudry Q, Scotter M, Blackburn J, Ross B, Boxall A, Castle L, Aitken R and Watkins R (2008), `Review: Applications and implications of nanotechnologies for the food sector', Food Additives and Contaminants, 25, 241±258. Commission Directive 1978/142/EC relating to limits of vinyl chloride monomer. Commission Directive 1989/107/EC of 21 December 1988 relating to food additives authorized for use in foodstuffs intended for human consumption. Commission Directive 1989/109/EC of 21 December 1988 relating to materials and articles intended to come into contact with foodstuffs. Commission Directive 2002/72/EC of 6 August 2002 relating to plastic materials and articles intended to come into contact with foodstuffs. Commission Directive 2004/19/EC amending Directive 2002/72/EC relating to plastic materials and articles intended to come into contact with foodstuffs. Commission Directive 2004/1935/EC of 27 October 2004 relating to materials and articles intended to come into contact with food and repealing Directives 80/590/ EEC and 89/109/EEC. Commission Directive 2008/282/EC of 17 March 2008 relating to recycled plastic materials and articles intended to come into contact with foods and amending Decision 2007/76/EC. Commission Directive 2009/450/EC of 29 May 2009 relating to active and intelligent materials and articles intended to come into contact with food. Commission Directive 2009/596/EC of 18 June 2009 relating to a number of instruments subject to the procedure referred to in Article 251 of the Treaty to Council Decision 1999/468/EC with regard to the regulatory procedure with scrutiny. Crosby N T (1981), `Food packaging materials: Aspects of analysis and migration of contaminants', in Food Packaging Materials, London, Applied Science Publishers. de Azeredo H M C (2009), `Review ± Nanocomposites for food packaging applications', Food Research International, 42, 1240±1253. Ewender J F R, Mauer A and Welle F (2003), `Determination of the migration of


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acetaldehyde from PET bottles into non-carbonated and carbonated mineral water', Deutsche Lebensmittel-Rundschau, 99, 215±221. Franz R, Lee K T, Knezevic G, Wolff E and Piringer O (1992), `Measuring and evaluation of the global migration from food contact materials into food: a comparison between official EC-techniques and alternative methods', Internationale Zeitschrift fuÈr Lebensmittel-Technik, Marketing, Verpackung und Analytik, 43, 291±296. Hata S, Goto H, Yamada E and Oku A (2002), `Chemical conversion of polycarbonate to 1,3-dimethyl-2-imidazolidinone (DMI) and bisphenol A', Polymer, 43, 2109±2116. Jenkins W A and Harrington J P (1991), Packaging Foods with Plastics, Lancaster, PA, Technomic Publishing Co. Kawai N, Tsujita K, Kamo T and Sato Y (2005), `Chemical recovery of bisphenol-A from polycarbonate resin and waste CDs', Polymer Degradation and Stability, 89, 317± 326. LagaroÂn J M, Cabedo L, Cava D, Feijoo J L, Gavara R and Gimenez E (2005), Ìmproving packaged food quality and safety. Part 2: Nanocomposites', Food Additives and Contaminants: Part A, 22, 994±998. Lyndhurst B (2009), An Evidence Review of Public Attitudes to Emerging Food Technologies, Social Science Research Unit, Food Standards Agency, March 2009. Matche R S and Baldev Raj (2005/06), Àpplications of plastics in food packaging', Packaging India, Dec.±Jan., 38, 33±48. Observatory Nano (2009), 2 Agrifood market report ± content, 2.5.4 Food contact materials (FCMs) based on metal/metal oxide nanoparticles. Oku A, Tanaka S and Hata S (2000), `Chemical conversion of polycarbonate to bis(hydroxyethyl) ether of bisphenol A ± An approach to the chemical recycling of plastic wastes as monomers', Polymer, 41, 6749±6753. Paine F A and Paine H Y (1983), A Handbook of Food Packaging, Council of The Institute of Packaging, London, Leonard Hill. Proceedings of the Second International Symposium on Feedstock Recycling of Plastics and Other Innovative Recycling Technology, 27. Ravi P, Baldev Raj, Vijayalakshmi N S and Srinivas P (2000), Èstimation of vinyl chloride monomer in PVC and food materials under publication', Packaging India, 32, 33±37. Robertson G L (2005), Food Packaging: Principles and Practice, New York, Marcel Dekker. Sato Y, Tsujita K and Kawai N (2001), `Recovery of bisphenol-A from polycarbonate and epoxy resins by liquid-phase chemical recycling', Proceedings of the Polymer Degradation Discussion Group, 24th Meeting, C-8. Schwope A D and Reid R C (1988), `Migration to dry foods', Food Additives and Contaminants, 5, 445±454. Stover R L, Evans K and Pickett K (1996), Report of the Berkeley Plastics Task Force, 1± 48. Till D, Schwope A D, Ehntholt D J, Sidman K R, Whelan R H, Schwartz P S and Reid R C (1987), Ìndirect food additive migration from polymeric food packaging', CRC Critical Reviews in Toxicology, 18, 215±243. US FDA (2009), CFR 21, 177.1020, Acrylonitrile/butadiene/styrene copolymer, revised as of 1 April 2009. US FDA (2009), CFR 21, Parts 170 to 199, revised as of 1 April 2009. US FDA (2009), CFR 21, 178.3297, Colorants for polymer. US FDA (2009), CFR 21, 176.170, Components of paper and paperboard in contact with



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aqueous and fatty foods. Vijayalakshmi N S and Baldev Raj (2010), `Suitability of plastic containers for drinking/ potable water and regulations', Indian Food Packer, 64, 66±73. Yoshiki S, Yasuhiko K, Koji T and Noboru K (2005), `Degradation behaviour and recovery of bisphenol-A from epoxy resin and polycarbonate resin by liquid-phase chemical recycling', Polymer Degradation and Stability, 89, 317±326.

24.12 Appendix: Abbreviations AN BIS BPA CFR CFTRI DOP DEHA EC EFSA FCMs GMP IARC IS LD NPs PET PP PVC QM(A) SML(T) US FDA VCM VDC

acrylonitrile Bureau of Indian Standards bisphenol-A Code of Federal Regulations Central Food Technological Research Institute dioctyl phthalate di-2-ethylhexyladipate European Commission European Food Safety Authority food contact materials good manufacturing practice International Agency for Research on Cancer Indian Standards lethal dose nanoparticles polyethylene terephthalate precautionary principle polyvinyl chloride quantity in material or article specific migration limit (test) US Food and Drug Administration vinylchloride monomer vinylidene chloride


Index

-galactosidase, 470 -lactalbumin, 613 -tocopherol, 465 -zein, 616 Acetyl-CoA, 499 acrylonitrile, 685 Acticoat, 353 Actisorb, 353 active packaging, 3, 4, 31, 356±7, 460±1, 462 additives, 670 additives migration, 674±7 Indian standards for plastics in contact with foodstuffs, pharmaceuticals and drinking water, 675±6 migration model, 674, 676±7 polymer/food system, 677 A-DO Korea, 392 adsorption, 319 advanced single-site polyolefins and ethylene±norbornene copolymers, 152±61 macromolecular structure, 155±6 future trends, 160±1 macromolecular structure, 154±5 crystallinity and composition continuum for ethylene±propylene polymers, 155 single-site polyethylenes mechanical properties, 154 nanocomposite preparation, 156±60 blend preparation, 157 BUR film properties, 158 material characteristics, 157 non-polypropylene blends with compatibiliser, 160 synthesis and molecular structure, 153±4 isotactic ethylene/butylene copolymer, 154 metallocene structure, 153 agar diffusion method, 579 Ageless, 38

AgIon, 358 AIT see allyl isothiocyanate ALD see atomic layer deposition alginates, 473 alipathic polyketones, 265±6 allerginicity, 611 allyl isothiocyanate, 380, 432, 446, 447 alpha-olefins, 153 Alphasan, 358 aluminium silicate, 377 amino acids, 61±4 indicated intercalation compounds composition and basal spacing, 61 MgAl-DL-Phe structural model, 62 amorphous polyamides, 275±6 amorphous vinyl alcohol resins, 16 amylomaize, 533 amylopectin, 531, 533 amylose, 531, 533 anionic clays, 43, 45 antibiotic drugs, 55±61 HTIc±CFS computer-generated representation, 60 intercalated HTIc composition, interlayer distance and drug loading, 60 structural formulae and acronyms, 56 antiblock agents, 671 antimicrobial activity, 403±4, 574±82 antimicrobial agents, 64±6, 371±2, 373±6, 673 antimicrobial activity, 403±4 evaluation methods, 403 Bz and p-Bz-OH anions computergenerated models, 65 chemical antimicrobial agents, 372, 377±80 antioxidants, 377 fungicides, 378 gases, 378±80 inorganic materials, 377±8 organic acids and salts, 372, 377 triclosan, 377


Index classification, 372 films and coatings, 393±402 antimicrobial packaging system, 394±5 biopolymer-based packaging, 397±400 commercial antimicrobial systems, 400±2 requirements, 394 synthetic polymeric packaging materials, 395±7 future trends, 404 incorporation into polymeric films for food packaging, 368±420 indicated intercalation compounds composition and basal spacing, 65 nano-antimicrobial agents, 390±3 packaging materials with nanoantibacterial agents, 393 natural antimicrobial agents, 380±9 bacteriocins, 380±7 enzymes, 387±8 plant origin, 388±9 polymers, 389±90 release through multilayer film and through single-layer film, 370 antimicrobial compounds, 634±5 antimicrobial films and coatings, 393±402 antimicrobial packaging system, 394±5 biopolymer-based packaging, 397±400 alginates, 398±9 chitosan films, 399±400 paper, 398 polysaccharide-based, 397±8 protein-based films and coatings, 400 commercial antimicrobial systems, 400±2 materials, 401±2 requirements, 394 synthetic polymeric packaging materials, 395±7 coatings, 396±7 ethylene acrylic acid, 396 linear low density polyethylene, 396 low density polyethylene, 395±6 multilayer structures, 397 antimicrobial nanoclays, 33±7 active nanoclays functioning, 34 antimicrobial packaging, 369, 586±7 applications, 373±6 future trends, 404 primary goals, 369 antimicrobial packaging films, 434±42 antioxidant packaging films, 442±5 antioxidants, 64±6, 377, 635±6, 671 hydroxycinnamic acid arrangement computer-generated models, 66 indicated intercalation compounds composition and basal spacing, 66 antistatic agents, 671 aPA see amorphous polyamides

693

Apacider, 358 Aquacel-Ag, 353 Archer Daniels Midland Company, 501 Arrhenius equation, 212, 214 Arrhenius law, 13 ascorbic acid, 377 atmospheric pressure chemical vapour deposition, 294 atomic layer deposition, 21 atom transfer radical polymerisation, 99 ATR see attenuated total reflection ATR-FTIR spectroscopy see attenuated total reflection Fourier transformed infrared spectroscopy ATRP see atom transfer radical polymerisation attenuated total reflection, 475 attenuated total reflection Fourier transformed infrared spectroscopy, 34, 574 AVOH see amorphous vinyl alcohol resins -lactoglobulin, 612±13 -zein, 616 Baby Dream Co. Ltd, 392 Bactekiller, 358 bacteriocins, 380±7 combination of agents, 380±2 combination with other antimicrobial agents, 383±6 Bactiblock, 36, 358, 400 dosages in plastic materials, 37 PLA-Bactiblock viable cell counts before and after incubation, 37 TEM pictures, 36 Bardex, 353 Barix, 145 benomyl, 378 benzophenone, 330 betel oil, 423±4 BHA see butylate hydroxy anisole BHT see butylate hydroxy toluene Bind-Ox, 247 bioactive food packaging, 460±76 controlled release of bioactives, 473±5 characterisation methods, 474±5 developed methods, 473±4 definition, 461±2 existing technologies to improve shelf-life or food functionality, 462±70 existing and potential bioactive packaging developments, 465±70 recent developments in active packaging technologies, 463±5 future trends, 475±6 nanotechnologies, 470±3 bioactive packaging, 461, 462 strategies, 4


694

Index

biocide effects, 576±81 Biocote, 358 Biocycle, 501 biodegradable polymers, 204 biological reduction, 352 Biomaster, 358 Biomer, 501 Biomer P-series, 501 Bio-On, 501 BIOP, 530 bioplastic films, 338 bioplastics, 204 Biopol, 500, 501, 515 biopolymers, 4, 113, 114, 360, 470±1 bioriented polypropylene matrix, 573, 575 Biotec, 531 Biotec Company, 543 Boltzmann's constant, 132 Bondi's group contribution method, 131 bottom-up technique, 335, 350 bovine serum albumin, 613 brittleness, 574 butylate hydroxy anisole, 377 butylate hydroxy toluene, 377 CA see controlled atmosphere canola oil, 445 carbon dioxide, 311 carbon dioxide permeability, 626±7 carbon dioxide transmission rate, 248 carbon nanofibres, 493 carbon nanotubes, 391, 493 carboxymethyl cellulose, 545 carrageenan polysaccharides barrier performance, 598±601 biodegradable vs synthetic films WVP values, 598 glycerol effect on water permeability and water % uptake, 601 oxygen permeability coefficients of biodegradable vs synthetic films, 600 food packaging, 594±606 nanocomposites, 601±6 TEM of casting carrageenan, 603 UV±vis spectra of the castings, 605 water permeability, 603 water uptake at 11%, 4% and 75% RH, 604 processing in packaging, 597 structure and properties, 595±6 flow diagram for extraction, 596 monomers molecular structure, 596 carvacrol, 465 casein, 611 caseinmacropeptide see glycomacropeptide casting, 539, 573, 651 catalytic chemical vapour deposition, 294 cation exchange capacity, 35

CEC see cation exchange capacity cell age, 579 cellophane, 204 cellulose, 360, 630±1 cellulose-based plastics, 204 cellulose nanocrystals, 556 cellulose nanofiller cellulose-reinforced nanocomposites preparation, 99±101 extraction and refining, 91±5 CNFs (white) suspension, 92 extraction by chemical analysis, 91±2 extraction by mechanical force, 92±5 functional valve and microfluidiser with interaction chamber, 94 oxidised CNF from tunicates and wood, 95 food packaging, 86±102 future trends and applications, 101±2 mechanical properties, 95±6 morphological and structural characteristics, 87±90 FE-SEM micrograph displaying CNF from bacterial cellulose, 89 poly- (1,4)-D-glucopyranoside chain molecular structure, 90 wood pulp micrograph, 88 surface modification, 96±9 cellulose nanowhiskers, 492±3, 553, 602 chain immobilisation factor, 11 chelating agents, 381±2 chemical antimicrobial agents, 372, 377±80 antioxidants, 377 fungicides, 378 gases, 378±80 alcohols, 378±9 chlorine dioxide, 379 other gases, 380 inorganic materials, 377±8 organic acids and salts, 372, 377 triclosan, 377 chemical reduction, 351±2 chemical vapour deposition, 291±4 Chemie Linz, 500 chitin, 389, 572 chitin nanoparticles, 556 chito-oligosaccharides, 576 chitosan, 115, 389, 391, 464±5, 473, 544 chitosan films, 581±2 chitosan nanoparticles, 556 chitosan polysaccharide antimicrobial activity, 574±82 ATR-FTIR spectra, 575 biocide properties optimisation, 576±81 film-forming and storage condition optimisation, 581±2 barrier performance, 582±4 water permeability, 584


Index food packaging applications, 571±87 processing in packaging, 573±4 structure and properties, 572±3 chitin and chitosan chemical structure, 572 chlorine dioxide, 379 cholesterol reductase, 470 cinnamon oil, 424±5, 426 antioxidant activity against oxidative bleaching of -carotene, 426 DPPH radical scavenging activity, 426 Citrex, 400 clays, 470±1 Cloisite 6A, 550, 552 Cloisite 10A, 258, 550, 552, 659 Cloisite 15 A, 504±5 Cloisite 30A, 550, 552 Cloisite 30B, 504±5, 513, 659 Cloisite Na+, 504 clove oil, 425±6 CMC see carboxymethyl cellulose CNF see carbon nanofibres; cellulose nanofiller CNT see carbon nanotubes CNW see cellulose nanowhiskers coating, 205, 619 coating fragmentation, 308 coaxial electrospinning, 111±12, 118±19 co-electrospinning see coaxial electrospinning coextrusion, 205±6, 216±17 cohesive energy density, 8 collagen, 617 collagen-chitosan complex, 115 colour parameters, 633±4 Commission Directive 2002/72/CE, 491 Commission Directive 2007/19/CE, 491 Commission Directive 1978/142/EC, 685 Commission Directive 1989/107/EC, 683 Commission Directive 2002/72/EC, 674 Commission Directive 2004/19/EC, 682 Commission Directive 2004/1935/EC, 683 Commission Directive 2009/450/EC, 673 composites, 119 conglycinin, 614±15 Contreet, 353 controlled atmosphere, 165 controlled-release packaging, 32, 433 Coomassie Plus total protein assay, 474 corn zein, 616±17 co-rotating extrusion, 620 cotton, 360 cottonseed proteins, 618 crosslinking, 474 CRP see controlled-release packaging crystals, 143 cup method, 147 cyclodextrins, 431±2

695

deacetylation, 577 delamination, 308 di(2-ethylhexyl) maleate, 330±1 differential scanning calorimetry, 534, 555 2,2-diphenyl-1-picryhydrazyl assay, 426 Directive 2002/72/CE, 359 Directive 1994/36/EC, 359 DMTA see dynamic mechanical thermal analysis DPPH assay see 2,2-diphenyl-1picryhydrazyl assay dual-mode sorption model, 6 DuPont, 392 dynamic mechanical thermal analysis, 555 EAA see ethylene acrylic acid ECM see extracellular matrix egg white, 618 electromagnet, 147±8 electron beam evaporation, 295 electrospinning, 108±13, 119±21, 122, 467, 491 chitosan porous nanofibres, 112 electrospun zein networks SEM images, 111 packaging applications, 119±21, 122 electrospun nanocomposite in novel food packaging materials design, 120 nanocomposite reinforcements based on electrospun fibres, 120 PLA-zein nanocomposite micrographs, 122 set-up, 109 electrospraying, 110 electrospun biopolymer-based ultrathin fibres, 113 electrospun nanofibres electrospinning, 108±13, 119±21, 122 chitosan porous nanofibres, 112 electrospun nanocomposite in novel food packaging materials design, 120 electrospun zein networks SEM images, 111 nanocomposite reinforcements based on electrospun fibres, 120 packaging applications, 119±21, 122 PLA±zein nanocomposite micrographs, 122 set-up, 109 functional nanofibres, 113±15 functional polymers electrospinning properties, 114 future trends, 121±3 nanoencapsulation, 116±19 electrospun zein/ -carotene nanofibres fluorescence image, 118 promising properties using electrospinning, 117


696

Index

packaging applications, 108±23 encapsulation, 467±8 engineered nanomaterials, 660±1 Enmat, 501 enterocin, 381 enzymes, 387±8 EP 1529635, 255 essential oils, 382, 388±9 ethanol, 378±9 ethylene, 187 ethylene acrylic acid, 396 ethylene±norbornene copolymers and advanced single-site polyolefins, 152±61 macromolecular structure, 154±5 nanocomposite preparation, 156±60 synthesis and molecular structure, 153±4 future trends, 160±1 macromolecular structure, 155±6 norbornene and polymerisation modes, 156 ethylene/propylene/CO terpolymers, 266 ethylene±vinyl acetate, 203 ethylene±vinyl alcohol copolymers, 9, 202, 261±81, 584 future trends, 280±1 improving retorting, 271±6 alternatives for retortable packages, 275±6 blending with other materials, 273±4 FTIR absorbance, 273 FTIR spectra, 272 resistance to treatment, 271±3 synchrotron WAXS traces vs temperature, 275 WAXS patterns of retorting stages, 274 and poly(vinyl) alcohol nanocomposites, 276±80 EVOH nanocomposite, 278 processing in packaging, 266±71 EVOH32 before and after retorting, 268 novel food preservation technologies, 269±71 oxygen transmission rate of multilayer structures, 270 retorting, 266±9 WAXS patterns, 269 structure and general properties, 262±5 chemical structure, 262 oxygen transmission rate vs relative humidity, 264 vs alipathic polyketones, 265±6 polyketone terpolymers chemical structure, 265 eugenol, 425

European Commission Directive 2002/72/ EC, 321 European Commission Directives, 682±4 active and intelligent food contact materials, 683±4 European Framework Directives, 682 European Commission Regulation No. 450/ 2009, 337 European Council Directive 97/48/EC, 326, 327 European Council Directive 82/711/EEC, 326 European Council Directive 85/572/EEC, 326 European Council Directive 89/109/EEC, 317±18 EVAflex150/LDPE, 441 extracellular matrix, 71 extraction, 319 fibrous protein, 610±11 Fick's law, 5, 129, 130, 304 Fick's law diffusion coefficient, 206 Fick's law of diffusion, 207 film blowing, 539±40 fingerroot oil, 427 foaming agents, 672 food contact multifunctional nanoclays, 31±9 antimicrobial nanoclays, 33±7 future trends, 39 oxygen-scavenging nanoclays, 37±9 food contact materials, 673±4 food packages, 460 food packaging carrageenan polysaccharides, 594±606 barrier performance, 598±601 nanocomposites, 601±6 processing in packaging, 597 structure and properties, 595±6 cellulose nanofiller, 86±102 cellulose-reinforced nanocomposites preparation, 99±101 extraction and refining, 91±5 future trends and applications, 101±2 mechanical properties, 95±6 morphological and structural characteristics, 87±90 surface modification, 96±9 chemical antimicrobial agents incorporation into polymeric films, 368±420 antimicrobial activity, 403±4 antimicrobial agents, 371±2, 373±6 antimicrobial films and coatings, 393±402 chemical antimicrobial agents, 372, 377±80


Index future trends, 404 nano-antimicrobial agents, 390±3 natural antimicrobial agents, 380±9 polymers, 389±90 chemical vapour deposition processes, 291±4 atmospheric pressure CVD, 294 catalytic CVD, 294 plasma enhanced CVD, 292±3 plasmaline antenna reactor system, 293 RF PEVCD system schematic diagram, 292 chitosan polysaccharide, 571±87 antimicrobial activity, 574±82 barrier performance, 582±4 future trends, 586±7 nanocomposites, 584±6 processing in packaging, 573±4 structure and properties, 572±3 diffusion barrier coated polymers functional properties, 303±10 coating fragmentation, 308 gas transport through a film, 304 micron-scale and nanoscale defects, 305 system coating/substrate nominal stress-strain derivative, 309 electrospun nanofibres, 108±23 electrospinning, 108±13 electrospinning in packaging applications, 119±21, 122 functional nanofibres, 113±15 future trends, 121±3 nanoencapsulation, 116±19 functional barriers against migration, 316±40 food safety issues related to migration, 317±19 functional barriers, 319±34 future trends, 338±9 nanostrategies for functional barriers, 335±7 functional packaging, 2±5 high barrier concept, 1±2, 3 plastics water and oxygen permeability, 3 high barrier plastics using nanoscale inorganic films, 285±311 future trends, 310±11 inorganic thin film systems, 299±303 deposition techniques and substrates, 300 deposition techniques publications, 303 gas barrier requirements, 302 lowest reported oxygen transmission rates, 303 multilayered and composites systems and O2 permeability, 299

697

mass transport and high barrier properties of polymers, 129±49 barrier, 143±6 characterisation techniques, 146±9 diffusivity, 130±1 mass transport basics, 129±30 solubility, 131±42 multifunctional and nanoreinforced polymers, 1±25 future trends, 25 nanocomposites, 21±4 novel polymers and blends, 15±21 structural factors governing barrier properties, 7±15 transport phenomenology in polymer, 5±6 nanostructured materials, 287 nanotechnologies of thin films, 287±90 natural extracts in plastic food packaging, 421±49 designing active plastic packaging systems, 430±4 factors to consider in designing active systems, 445±8 future trends, 448±9 packaging films, 434±45 plant extracts as antimicrobials and antioxidants, 422±30 Nylon-MXD6 resins, 243±59 applications, 253±5 future trends, 258±9 gas barrier properties, 246±50 nanocomposites, 255±8 other properties, 250±3 processing, 244±6 structure and general overview, 243±4 physical vapour deposition processes, 294±9 electron beam evaporation, 295 evaporation, 295 polyhydroxyalkanoates, 498±518 commercial developments, 500±1 foams and paper coatings, 515±16 future trends, 517±18 PHAs and their nanocomposite films, 502±15 polylactic acid nanocomposites, 485±94 future trends, 493±4 nanobiocomposites for monolayer packaging, 486±93 properties of polylactic acid, 485±6 protein-based resins, 610±38 applications, 634±7 future trends, 638 packaging materials characterisation, 622±34 sources, extraction, structure and properties, 610±18


698

Index

structure and properties, 618±22 safety and regulatory aspects of plastics, 669±91 additives migration, 674±7 European Commission Directives on plastic containers for foods, 682±4 future trends, 687±8 Indian Standards for overall migration, 677±80 indirect food additives, 670±3 nanotechnology in food contact materials, 673±4 problems in specific migration, 687 specific migration of toxic additives, 684±7 US Food and Drug Administration Code of Federal Regulations, 681±2 silver-based antimicrobial polymers, 347±62 antimicrobial silver, 356±9 effects on human health, 349±50 future trends, 359±61 historical use of silver as antimicrobial agent, 347 incorporation of silver into coatings and polymer matrices, 350±6 nanosilver antimicrobial mechanism, 348±9 nanosilver for limitless applications, 348 sputter deposition, 296±9 ion beam sputter deposition, 298±9 magnetron cathode configuration, 297 planar magnetron sputtering system, 298 reactive magnetron sputtering, 296±8 schematics, 296 starch-based polymers, 527±60 future trends, 557±9 market of starch-based materials and potential applications, 528±31 mechanical and barrier performance of starch-based systems, 542±6 nanocomposites, 546±57 processing in packaging, 537±41 structure and properties of native and plasticised starch, 531±7 thin film technologies for polymer coating, 290±4 preparation, 290±1 wheat gluten-based materials, 649±64 future trends, 664 integrated approach for fresh fruit and vegetable packaging, 661±3 material preparation, 650±2 mechanical and barrier properties, 652±8 nanocomposites, 658±61

food simulating liquid, 661 Fosfargol, 358 Fourier transform infrared spectroscopy, 50, 264, 475 fractional free volume, 8, 9 FresherLonger Miracle Food Storage Containers, 392 FTIR see Fourier transform infrared spectroscopy functional barriers, 319±34 additives in plastic packaging, 324 impurities in dispersion coatings, 325 mass transfer of molecules, 319 mechanisms behind migration, 320 migrating substances sources and identity, 320±6 against migration, 316±40 food safety issues related to migration, 317±19 future trends, 338±9 against migration for food packaging, 316±40 migration modelling, 333±4 migration of substances from packaging to food, 319±20 migration testing, 326±8 nanostrategies, 335±7 related food safety issues, 337 plastic materials, typical additives and applications, 322±3 in practice, 328±33 novel approaches, 331±3 paper constituents and adhesives, 331 plastic packaging components, 328±9 printing ink components, 329±31 functional packaging, 2±5 fungicides, 378 galangal oil, 427 gas chromatography, 475 gases, 378±80 gas permeation, 208±11 gas-phase process, 98 gas transmission, 208±11 gas transmission rate, 208 gelatine, 474, 617 gellan gum see Phytagel GFSE see grapefruit seed extract Gibbs free energy, 132, 134 gliadin, 615, 650 globular protein, 610±11 glue effect, 277 glutenin, 615, 650 glycerol, 536, 548, 650 glycinin, 615 glycomacropeptide, 613 good manufacturing practice, 674 grapefruit seed extract, 434


Index `green' synthesis, 351 GTR see gas transmission rate Guggenheim solution, 132 Hawaii Natural Energy Institute, 501 HCIc see hydrotalcite-like compounds HDPE see high-density polyethylene heat treatment, 655 HEBM see high energy ball milling Helmholtz free energy, 136 Henry's law, 5, 129 Henry's law solubility coefficient, 206±7 high barrier, 1 high barrier plastics, 285±311 high-density polyethylene, 200±1, 599 high energy ball milling, 69 high pressure processing, 269 hinokithiol, 380 HPP see high pressure processing hydrocolloids, 75, 595 hydrotalcite-like compounds, 45±52 hydrotalcites composition, interlayer distance and drug loading antibiotic-intercalated HTIc, 60 NSAID-intercalated HTIc, 57 computer-generated models Bz and p-Bz-OH anions, 65 HTIc±CFS, 60 HTIc±TIAP, 58 hydroxycinnamic acid arrangement, 66 future trends, 75±6 HTIc hybrids, 55±66 amino acids and proteins, 61±4 anti-inflammatory and antibiotic drugs, 55±61 antimicrobial and antioxidant species, 64±6 diclofenac release in phosphate buffer, 59 MgAl±DL±Phe structural model, 62 NSAID and antibiotics structural formulae and acronyms, 56 hydrotalcite-like compounds basic chemistry, 45±52 brucite sheet and MgAl±HTIc representation, 46 composition and structural aspects, 45±8 [Mg0.67Al0.33(OH)2] (CO3)0.1650.42H2O TG-DTA curves, 51 physical±chemical characterisation, 49±52 preparation methods, 48±9 steps in preparation by double waterin-oil microemulsions technique, 50 structural parameters, 48 ZnAl±HTIc micrographs, 52

699

indicated intercalation compounds composition and basal spacing amino acids, 61 antimicrobials, 65 antioxidants, 66 [Mg0.67Al0.33(OH)2] (CO3)0.1650.48H2O crystallographic data, 47 rietveld plot, 47 modified and biodegradable polymeric matrices nanocomposites, 67±75 poly(-caprolactone) case, 68 poly(hydroxyalkanoates) and hydrocolloids case, 75 procedures to obtain films, membranes and PCL-HTIc composites fibres, 68±70 nanobiocomposites, 43±76 organically modified biocompatible HTIc, 52±66 experimental routes to obtain HTIc intercalation compounds, 55 MgAl±NO3 anion exchange isotherms and MgAl±HTIc X-ray diffraction, 54 synthetic routes with molecular anions with biological activity, 53±5 PCL nanobiocomposites, 70±5 DPPH absorbance percentage reduction, 74 HTIc-Cfs/PCL film composites in vitro release tests, 71 modified drug release, 70±2 potential food packaging application, 72±5 hydroxycinnamic acid, 65 imazalil, 378 Imperm, 256 incubation temperature, 580 Indian Standards for overall migration, 677±80 extractive amount determination, 679 migration limits, 679 procedure, 679 selection of samples, 678 time-temperature test conditions, 680 indirect food additives, 670±3, 681 antiblock agents, 671 antimicrobial agents, 673 antioxidants, 671 antistatic agents, 671 foaming agents, 672 lubricants, 671 optical property modifiers, 672 plasticizers, 672 ultraviolet stabilisers, 672 inelastic X-ray scattering, 96 injection moulding, 541, 546


700

Index

inorganic materials, 377±8 International Agency for Research on Cancer, 685±6 ion beam sputter deposition, 298±9 ionomers, 203 Irgaguard, 358 iron, 37±8 iron-based scavenging systems, 33 IXS see inelastic X-ray scattering JP7276582, 251 Kanegafuchi, 515 Kaneka Corporation, 515 kaolinite, 257, 276 keratins, 618 lacticin, 381 lactoferrin, 387 lamination, 205 laser ablation, 350 layered double hydroxides, 45 layer multiplying coextrusion technique, 20 LDPE see low-density polyethylene linalool, 33, 432 linear low-density polyethylene, 200, 396 liquid crystal polymers, 12 LLDPE see linear low-density polyethylene low-density polyethylene, 200, 395±6, 598 lubricants, 671 lysozyme, 387, 464 MA see modified atmosphere `macromolecules', 498 magnesium aluminium hydroxycarbonate, 43 magnesium oxide, 377 MAP see modified atmosphere packaging mass spectrometry, 475 mass transport barrier, 143±6 generated spherulite, 143 methanol permeability, 145 reciprocal of tortuosity, 144 basics, 129±30 plate subjected to steady-state gas transport, 130 characterisation techniques, 146±9 cup measurement with liquid water in the cup, 148 diffusivity, 130±1 from permeability and diffusivity data, 131 and high barrier properties of food packaging polymers, 129±49 non-equilibrium lattice fluid model, 139±41 solute (CO2)±polymer mass ratio, 141

non-equilibrium perturbed hard-sphere chain model, 141±2 CO2 solubility, 142 Sanchez±Lacombe equation-of-state model, 132±6 fluid lattice, 133 Henry's law solubility constants in Pluracol, 136 solubility, 131±42 statistical associated fluid theory models, 136±9 experimental n-pentane mass concentration in polyethylene, 139 experimental saturated liquid and vapour densities for toluene, 137 n-Pentane-polyetylene and CO2polyamide 11 data, 138 Mater-Bi, 515 medium chain length triglyceride oil, 625 melt-compounding method, 67 melt-intercalation technique, 659 melt mixing, 486 Mesosilver, 359 Metabolix Inc, 501 metalisation, 206 metallocene, 153±4 general structure, 153 metallocene technology, 217 Microban, 400 microcrystalline cellulose, 336±7, 545 microencapsulation, 467, 471 MicroFree, 400 MicroGard, 400 microperforated films, 218 microperforated polymeric films, 214 microporous films, 218±19 micro-winceyette fibres, 546 midpoint cracking, 308 migration, 319 functional barriers for food packaging, 316±40 functional barriers, 319±34 future trends, 338±9 nanostrategies for functional barriers, 335±7 mechanisms, 320 modelling, 333±4 related food safety issues, 317±19 substances from packaging to food, 319±20 testing, 326±8 milk proteins, 611±14 see also casein; whey MINERV-PHA, 501 minimal inhibitory concentration, 578±9 Mirel, 501 MMT see montmorillonites modified atmosphere, 165, 190


Index modified atmosphere packaging, 165±7, 661±3 advanced technology, 215±20 antifog properties, 220 coextrusion, 216±17 continuous films, 216 customisable packaging materials, 220 film laminates tailoring, 216 interactive package, 219 metallocene technology, 217 microperforated films, 218 microporous films, 218±19 perforated films, 217 tailored oxygen transmission rate, 217 tray/lidstock compatibility, 219 advances in polymeric materials, 163±228 biodegradable polymers, 204 cellulose-based plastics, 204 future trends, 226±8 gas permeation or gas transmission, 208±11 mathematical modelling of gaseous exchange, 222±3 package management, 220 packaging systems, 214±15 polymeric films application for fruits and vegetables, 223±6 post-harvest pathology of fruits and vegetables, 188±9 advantages and disadvantages, 172±3 applications, 171±2 definition, 168 design, 221±2 methodology, 222 effect, 170 factors affecting respiration rate, 181±6 atmospheric composition, 183±4 climacteric pattern of respiration in ripening fruit, 185 fruits according to respiratory behaviour during ripening, 186 physical stress, 184 stage of development/maturity stage of the commodity, 184±6 temperature, 181±3 temperature on rate of deterioration, 182 variation of temperature quotient for respiration, 182 fresh produce response, 189±97 CO2% limits above occurrence of injury, 195 favourable and injurious effects, 191±5 MA/CA benefit for fresh fruits, 191 MA/CA benefit for fresh vegetables, 192 MA/CA optimum conditions, 193

701

O2 % limits below occurrence of injury, 194 physiological and biochemical effects, 196±7 required characteristics of plastic films, 197 tolerance limits, 195±6 gases, 172 history, 167±8 measurement of gas permeability, 209±11 concentration-increase method/equal pressure principle, 210±11 pressure-increase method/differential pressure principle, 209±10 multilayer plastic films, 205±8 barriers and permeation, 206±7 coating, 205 coextrusion, 205±6 concept and theoretical approach, 207±8 lamination, 205 metalisation, 206 objective/goal, 169 physiological factors affecting shelf-life of fresh produce, 173±88 biological structure, 186 compositional changes, 187±8 developmental processes, 188 ethylene production and sensitivity, 187 physiological breakdown, 188 respiration rate, 174±9 respiratory quotient, 180±1 transpiration, 187 polymeric films for application, 197±204 absorbers for extending shelf-life, 198 ethylene±vinyl acetate, 203 ethylene±vinyl alcohol, 202 high-density polyethylene, 200±1 ionomers, 203 linear low-density polyethylene, 200 low-density polyethylene, 200 permeability, 199 polyamide, 202 polycarbonate films, 203 polychlorotrifluoroethylene, 203 polyesters, 201 polyethylene terephthalate, 201±2 polyolefins, 200 polypropylene, 201 polystyrene, 203±4 polyvinyl alcohol, 203 polyvinyl chloride, 201 polyvinylidene chloride, 202 principles, 168±9 respiration and ethylene production rates fruits, 176±7 horticultural commodities, 175


702

Index

vegetables, 178±9 utility, 170±1 water vapour permeability, 211±14 permeability coefficient of multiplayer films, 213 polymer structure and morphology on permeability, 214 subzero temperature on permeability, 213 temperature on permeability, 212±13 temperature quotient for permeability, 213 Monoxbar, 247 montmorillonites, 257, 276, 472, 548, 550, 585, 628±9 3M Scotchpak, 146 multifunctional nanoclays antimicrobial nanoclays, 33±7 active nanoclays functioning, 34 Bactiblock dosages in plastic materials, 37 PLA-Bactiblock viable cell counts before and after incubation, 37 TEM pictures, 36 food contact applications, 31±9 future trends, 39 oxygen-scavenging nanoclays, 37±9 headspace reduction, 38 LDPE-O2Block oxygen-scavenging capacity, 39 multilayer plastic films, 205±8 myofibrillar proteins, 617±18 Na+ Cloisite, 659 nano-antimicrobial agents, 390±3 nanobiocomposites, 119, 286 hydrotalcites, 43±76 future trends, 75±6 hydrotalcite-like compounds basic chemistry, 45±52 modified and biodegradable polymeric matrices nanocomposites, 67±75 organically modified biocompatible HTIc, 52±66 Nanobiomatters, 159 Nanobioter 202 A1.41, 159 Nanobioter 202 A1.49, 159 Nanobioter 404 C1.33, 159 Nanobioter 434 C1.33, 159 Nano Care Technology Ltd, China, 392 nanoclays, 471, 556 see also specific nanoclays nanocomposites, 119 carrageenan polysaccharides, 601±6 chitosan polysaccharide, 584±6 wheat gluten-based materials, 658±61 preparation and structure, 659 properties, 659±61

nanoencapsulation, 116±19 Nanofil EXM 757, 659 nanolithography, 350 nanopackaging systems high barrier plastics using nanoscale inorganic films, 285±311 diffusion barrier coated polymers, 303±10 inorganic thin film systems, 299±303 nanotechnologies of thin films, 287±90 physical vapour deposition, 294±9 thin films technologies, 290±4 nanoparticles, 471±2 nanosilver, 392 antimicrobial mechanism, 348±9 applications, 348 Nano Silver Baby Milk Bottle, 392 Nano Silver Food Containers, 392 nanotechnology, 21, 335±7 food contact materials, 673±4 nano titanium dioxide, 390±1, 392 naringinase, 469 National Committee for Clinical Laboratory Standards, 578 National Starch Co., 530 native starch, 533±4, 535 natural antimicrobial agents, 380±9 bacteriocins, 380±7 enzymes, 387±8 plant origin, 388±9 essential oils, 388±9 plant extracts, 388 natural extracts antimicrobials and antioxidant additives for food packaging materials, 424 designing active plastic packaging systems, 430±4 coating of active additives, 430 incorporation of active additives, 431±2 smart blending of active additives, 433 systematic approach for smart blending of active additives, 434 packaging films based on natural extracts, 434±45 plant extracts as antimicrobials and antioxidants, 422±30 betel oil, 423±4 cinnamon oil, 424±5, 426 clove oil, 425±6 fingerroot oil, 427 galangal oil, 427 oregano oil, 427±8 rosemary oil, 428 sweet basil oil, 428±30 plastic food packaging, 421±49 neutralisation, 577±8


Index Ningbo Tianan Biologic Material Co., Ltd, 501 Nisaplin, 380, 400 nisin, 380, 464 nitrate, 350 nitrite, 381 Nodax, 501 non-equilibrium lattice fluid model, 139±41 solute (CO2)±polymer mass ratio, 141 non-equilibrium pertubed hard-sphere chain model, 141±2 CO2 solubility, 142 non-gravimetric methods, 146 non-steroidal anti-inflammatory drugs, 55±61 diclofenac release in phosphate buffer, 59 HTIc-TIAP computer-generated representation, 58 intercalated HTIc composition, interlayer distance and drug loading, 57 structural formulae and acronyms, 56 norbornene, 155 polymerisation modes, 156 Novamont, 530, 531 Novaron, 358, 400 NSAIDs see non-steroidal anti-inflammatory drugs nylon, 202 Nylon-MXD6 applications, 253±5 polymer blends, 253 aroma-staining and odour-blocking properties, 248, 250 barrier films, 250 food packaging, 243±59 future trends, 258±9 gas barrier properties, 246±50 carbonation retention, 249 carbon dioxide transmission rate, 248, 249 mechanical properties, 251±3 physical properties, 252 multilayer products, 254±5 multilayer bottle preform, 254 multilayer bottle with Oxbar oxygen scavenging additive, 255 nanocomposites, 255±8 carbon dioxide retention, 256 Imperm, 256 MXD6-kaolinite and MXD6montmorillonite, 257 other properties, 250±3 oxygen transmission rate, 246±8 humidity dependence of oxygen permeability, 247 oxygen scavenging systems, 246±8 polymer films, 246 processing, 244±6

703

biaxially drawn films, 245±6 drying and handling, 244 extrusion, 245 grades, 245 injection moulding, 245 retortability, 250±1 laminated containers, 251 structure and general overview, 243±4 chemical structure, 243 thermal properties, 250, 252 cumulative oxygen transmission coefficient, 252 injection-moulded specimens, 251 O2Block, 38 OMMT see organo-modified montmorillonite opacity, 633±4 optical density, 577, 579 optical property modifiers, 672 order±disorder transition, 534 oregano oil, 427±8 organic acids, 381 organoclays, 556 organo-modified montmorillonite, 504 OTR see oxygen transmission rate overall migration, 327 Oxbar, 247, 248, 254±5 oxygen permeability, 598±9, 626±7 oxygen-scavenging nanoclays, 37±9 headspace reduction, 38 LDPE-O2Block oxygen-scavenging capacity, 39 oxygen transmission rate, 213, 217, 246±8 ozone, 380 packaging films see antioxidant packaging films PCL see poly(-caprolactone) PC-SAFT see perturbed chain SAFT model PCTFE see polychlorotrifluoroethylene pediocin, 380, 381 PEN see polyethylene naphthalate PEO see polyethylene oxide peptides, 387±8 perforated polymeric films, 214 perforation-mediated packaging, 215 permachor values, 214 permeability, 623 permeability coefficients, 206±7, 212, 213 permeants, 8, 14±15 permeation, 206±7 pertubed hard-sphere-chain theory, 141 perturbed chain SAFT model, 136 PET see polyethylene terephthalate pH, 580 PHA see polyhydroxyalkanoates PHA synthases, 499


704

Index

PHB see polyhydroxybutyrate PH3B see poly-3-hydroxybutyrate PHB Industrial S.A., 501 PHBV see polyhydroxybutyrate-co-valerate PHV see polyhydroxyvalerate physical reduction, 350±1 physical vapour deposition, 294±9, 353 Phytagel, 630 phytochemicals, 468 plant extracts, 388 plasma enhanced chemical vapour deposition, 292±3 plastic colourants, 686±7 plastic food packaging designing from natural plant extracts, 430±4 coating of active additives, 430 incorporation of active additives, 431±2 smart blending of active additives, 433 systematic approach for smart blending of active additives, 434 factors to consider in designing active systems, 445±9 characteristics of active additives and foods, 445 chemical interaction of active additives with film matrix, 445±6 cost, 447 food contact approval, 447±8 mass transfer coefficients and modelling, 446 packaging materials properties, 446±7 process conditions and residual active activity, 445 storage temperature, 446 future trends, 448±9 natural extracts, 421±49 packaging films based on natural extracts, 434±45 antimicrobial food packaging materials based on natural plant extracts, 435±7 antimicrobial packaging films, 434±42 antioxidant food packaging materials based on natural plant extracts, 443 antioxidant packaging films, 442±5 DDPH radical scavenging activity of cellulose-ether films, 444 growth inhibition of selected microorganisms by cellulose-ether coated LDPE film, 440, 441 plasticisers, 574, 599±600, 620±1, 672 plastics safety and regulatory aspects of food packaging materials, 669±91 additives migration, 674±7

European Commission Directives on plastic containers for foods, 682±4 future trends, 687±8 Indian Standards for overall migration, 677±80 indirect food additives, 670±3 nanotechnology in food contact materials, 673±4 problems in specific migration, 687 specific migration of toxic additives, 684±7 US Food and Drug Administration Code of Federal Regulations, 681±2 Plastic Storage Bags, 392 polyacrylamides, 360 polyacrylates, 360 polyamide, 202 poly(butylene succinate), 75 polycaprolactone, 472, 544, 598, 655 polycarbonate films, 203 polychlorotrifluoroethylene, 203 poly(-caprolactone), 68, 99 for modified drug release, 70±2 HTIc-Cfs/PCL film composites in vitro release tests, 71 for potential food packaging application, 72±5 DPPH absorbance percentage reduction, 74 polyesters, 201 polyethylene naphthalate, 201 polyethylene oxide, 336 polyethylene terephthalate, 201±2 polyhydroxyalkanoates, 498±518, 598 commercial developments, 500±1 foams and paper coatings, 515±16 future trends, 517±18 main suppliers world-wide, 501 PHAs and their nanocomposite films, 502±15 commercial plastics vs PHB and PHBV mechanical properties, 504 degradability, 514±15 mechanical properties, 503±6 migration, 512±13 permeability, 506±9 permeability data of PHB vs PLA and conventional synthetic plastics, 508 PHAs and polyolefins thermal properties used in food packaging, 510 PHB random chain scission reaction, 511 thermal stability, 509±12 PH3B, PHV and PHBV structures, 499 synthesis of polyhydroxybutyrate, 500 polyhydroxybutyrate, 498 poly-3-hydroxybutyrate, 75, 499


Index polyhydroxybutyrate-co-valerate, 499, 500 polyhydroxyvalerate, 499 polyketones, 266 polyketone terpolymers, 265 polylactic acid, 598, 655 future trends, 493±4 general properties of commercial PLA grade, 487 melt compounded films, 490 nanobiocomposites for monolayer packaging, 486±93 nanoclays, 486±91 other fillers, 491±3 nanocomposite containing a food contact compliant nanoclay UV blocking, 489 nanocomposites for food packaging applications, 485±94 properties, 485±6 reductions in oxygen and water vapour permeability, 488 polylactides, 204 polymer blending, 573±4 polymer blends, 253 polymer chain rigidity, 9 polymer chains, 7 polymer grafting, 98±9 grafting-from technique, 99 grafting-to technique, 98 polymers, 389±90 chemistry, 7±10 polymer materials relative oxygen permeability, 7 PO2 vs fractional free volume/cohesive energy density ratio, 10 functional packaging, 2±5 future trends, 25 high barrier concept, 1±2, 3 plastics water and oxygen permeability, 3 molecular architecture, 12 morphology, 10±12 multifunctional and nanoreinforced for food packaging, 1±25 nanocomposites, 21±4 extruded films PO2 of EVOH29, 24 food retorting resistance experiments, 24 permeability reductions, 22 novel polymers and blends, 15±21 oxygen permeability modelling for EVOH/aPA blend components, 18 polymers OTR/WVTR vs properties claimed for PGA, 16 plasticisation, 12±13 structural factors governing barrier properties, 7±15 permeant, 14±15

705

temperature, 13±14 transport phenomenology, 5±6 see also specific polymers polymer-sorbate interactions, 15 polymer surface modification, 583 polyolefins, 200 polypropylene, 201 polysaccharides, 114, 115 polystyrene, 203±4, 655 polyvinyl alcohol, 13, 203, 474, 543±4 polyvinyl alcohol nanocomposites, 276±80 polyvinyl chloride, 201 polyvinylidene chloride, 202 positronium annihilation spectroscopy, 9 post-harvest pathology, 188±9 post-processing ageing, 537 potassium sorbate, 377 pressure decay method, 148 Preventol, 398 Printpack, 159 probiotics, 467 Procter & Gamble, 501 prolamin see gliadin protein-based resins applications, 634±7 applications to foods, 636±7 incorporation of functional compounds, 634±6 food packaging, 610±38 future trends, 638 packaging materials characterisation, 622±34 opacity and colour parameters, 633±4 tensile strength, elongation-at-break and Young's modulus, 632 thermal and mechanical properties, 629±33 transport properties, 623±7 water sensitivity, 627±9 sources, extraction, structure and properties, 610±18 collagen and gelatine, 617 corn zein, 616±17 milk proteins, 611±14 other proteins, 617±18 soy protein, 614±15 wheat gluten protein, 615 structure and properties, 618±22 processing aids, 620±1 protein modification, 621±2 solution casting, 618±19 thermoplastic processing, 619±20 protein modification, 621±2 proteins, 114 quartz crystal microbalance, 146 random cracking, 308


706

Index

reactive magnetron sputtering, 296±8 relative humidity, 627, 652 respiration rate, 174±9, 181±6 respiratory quotient, 180±1 retortability, 250±1 retorting, 271±6 retort packaging, 250±1 Rhetech, 159 ring-opening polymerisation, 99 ROP see ring-opening polymerisation rosemary oil, 428 rubbery polymers, 6 Sanchez±Lacombe equation-of-state model, 132±6, 140 fluid lattice, 133 Henry's law solubility constants in Pluracol, 136 Sanitized, 400 SANS see small angle neutron scattering scanning electron microscopy, 51, 72 seaweeds, 595±6 self-assembly technique, 350 Sharper Image, 392 shelf-life, 173 silk fibroin, 115 silver, 34, 35, 36, 356±7, 377 antimicrobial effectiveness, 354±6 related issues and inactivation, 355±6 size and shape, 355 antimicrobial polymers for food packaging, 347±62 future trends, 359±61 scientific articles dealing with silverbased nanocomposites, 360 antimicrobial silver for food packaging, 356±9 active packaging and silver, 356±7 ion-exchange from minerals, 357±9 regulatory issues, 359 silver ion exchange mechanism, 358 effects on human health, 349±50 future trends, 359±61 historical use as antimicrobial agent, 347 incorporation into coatings and polymer matrices, 350±6 nanoparticles preparation, 350±2 biological reduction, 352 chemical reduction, 351±2 physical reduction, 350±1 nanosilver antimicrobial mechanism, 348±9 nanosilver for limitless applications, 348 silver-based nanocomposites, 352±4 medical field, 352±3 techniques and materials, 353±4 Silvercell, 353 Silverex, 353

silver hydrogels, 360 SilverIon, 353 silver ion-exchange, 357±9 silver nanoparticles, 391±2 silver sulfadiazine, 353 silylation, 97, 98 single-screw extruder, 550, 553 single-site polyolefins, 153 sisal fibres, 546 Skygreen, 507 small angle neutron scattering, 97 SME see specific mechanical energy sodium chloride, 381 sol-gel technique, 49 solubility coefficient, 604 solution blending method, 67 solution casting, 486, 618±19 solvent-based process, 651 solvent-casting technique, 101 solvent process see solution casting sorbitol, 536 soy flour, 615 soy protein, 614±15 soy protein concentrate, 615, 630 soy protein isolate, 615, 628, 630±1 specific mechanical energy, 535 specific migration, 327 split electrodes, 112 SPPA see successive pulsed plasma anodisation sputter deposition, 296±9 SSE see single-screw extruder starch-based polymers, 527±60 future trends, 557±9 technical substitution of synthetic plastics by starch plastics, 558 main applications and manufacturers, 530±1 films and nets, 530 foams, 530 moulded products, 530±1 market of starch-based materials and potential applications, 528±31, 532 current and potential volume production of starch-based materials in Europe, 529 global consumption of starch-based biodegradable polymers, 530 production capacity of starch-based polymers, 529 starch-based polymer producers, 532 starch evolution in plastic industry, 528±30 mechanical and barrier performance, 542±6 plasticised cassava starch films, 536 nanocomposites, 546±57 barrier properties, 555±7


Index cellulose nanocrystals content effect on pea starch modulus and tensile strength, 554 evolution of tensile strength and elongation break with montmorillonite content, 552 influence of interlayer cation on the morphology, 549 mechanical improvements in starch± montmorillonite nanocomposites, 551 mechanical properties, 550±4 mechanical properties of starch with cellulose nanowhiskers, 553 methods of preparation, 547±8 montmorillonite effect on WVP of starch/montmorillonite nanocomposites, 557 morphology, 548±50 TEM picture showing partial exfoliation, 550 thermal properties, 555 processing in packaging, 537±41 structure and properties of native and plasticised starch, 531, 533±7 amylose and amylopectin molecular structures, 533 composition and characteristics of different starches determined on dry basis, 534 starch destructurisation, 534 starch gelatinisation, 534 statistical associated fluid theory models, 136±9 experimental n-pentane mass concentration in polyethylene, 139 experimental saturated liquid and vapour densities for toluene, 137 n-Pentane-polyetylene and CO2polyamide 11 data, 138 stereoisomerism, 12 styrene, 685±6 successive pulsed plasma anodisation, 297 surface acetylation, 98 surface-initiated single electron living radical polymerisation, 99 surface plasmon resonance, 355 surfactant, 96±7 Surshield, 254 Surshot, 254 sweet basil oil, 428±30 synthetic peptides, 389±90 TBHQ see tert-butyl hydroquinone Techbarrier, 146 teichoic acids, 579 TEMPO see tetramethypiperidine-1-oxyl radical Tenax, 327, 328

707

tert-butyl hydroquinone, 377 tetramethypiperidine-1-oxyl radical, 17 TG-DTA see thermogravimetric±differential thermal analysis thermal processing, 652 thermogravimetric±differential thermal analysis, 50 thermoplastic processing, 619±20 thermoplastic starch, 534, 535, 537, 541, 545 thymol, 33, 472 titanium dioxide, 354 tocopherols, 377 Topas, 156, 159 top-down technique, 335, 350 toxic additives, 684±7 acrylonitrile, 685 plastic colourants, 686±7 styrene, 685±6 vinyl chloride, 684±5 vinylidene chloride, 685 TPS see thermoplastic starch tranexamic acid, 61 transglutaminase, 630 transmission electron microscopy, 51, 602±3, 659 transpiration, 187 triclosan, 377 TSE see twin-screw extruder twin-screw extruder, 550, 553 Ultra-Fresh, 400 ultraviolet stabilisers, 672 Urgotul, 353 US Food and Drug Administration Code of Federal Regulations, 681±2 indirect food additives, 681 regulation and migration limits threshold, 681±2 UV light, 605±6 van der Waals, 15, 131, 142 Van't Hoof's law, 13 variable-range statistical associating fluid theory, 135, 136 vinyl chloride, 684±5 vinylidene chloride, 685 Vitamin E, 329 VR-SAFT see variable-range statistical associating fluid theory WasaOuro, 400 water barrier, 582±4 decreased permeability, 584 water clustering, 601 water sensitivity, 627±9 water vapour permeability, 211±14, 542, 544, 599, 601, 623±6


708

Index

edible protein films, 626 water vapour transmission rate, 158 wheat gluten-based materials food packaging, 649±64 future trends, 664 integrated approach for fresh fruit and vegetable packaging, 661±3 material preparation, 650±2 solvent-based process, 651 technological processes, 651 thermo-mechanical process, 652 mechanical and barrier properties, 652±8 barrier properties, 652±4 functional properties modulation, 654±8 oxygen and carbon dioxide permeabilities, 657 temperature and relative humidity effect on CO2, 653 vs conventional plastics and biodegradable polyesters, 656 water vapour permeability, 658

nanocomposites, 658±61 preparation and structure, 659 properties, 659±61 wheat gluten protein, 615 whey, 611, 613±14 whey protein concentrate, 612 whey protein isolate, 612 wide-angle X-ray diffraction analysis, 659 wool keratose, 115 WVP see water vapour permeability WVTR see water vapour transmission rate XPRD see X-ray powder diffraction X-ray diffraction, 72, 73±4, 95, 96, 156 X-ray powder diffraction, 49±50 zein, 115, 633 Zeneca BioProducts, 501 zeolite, 378 Zeomic, 358 Ziegler±Natta catalysts, 153 zinc oxide, 377


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