Handbook of Plastic Films

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Handbook of Plastic Films

Editor: Elsayed M. Abdel-Bary

Rapra Technology Limited


Editor: E.M. Abdel-Bary

rapra TECHNOLOGY

Rapra Technology Limited Shawbury, Shrewsbury, Shropshire, SY4 4NR, United Kingdom Telephone: +44 (0)1939 250383 Fax: +44 (0)1939 251118 http://www.rapra.net

First Published in 2003 by

Rapra Technology Limited Shawbury, Shrewsbury, Shropshire, SY4 4NR, UK

©2003, Rapra Technology Limited

All rights reserved. Except as permitted under current legislation no part of this publication may be photocopied, reproduced or distributed in any form or by any means or stored in a database or retrieval system, without the prior permission from the copyright holder. A catalogue record for this book is available from the British Library. Every effort has been made to contact copyright holders of any material reproduced within the text and the authors and publishers apologise if any have been overlooked.

ISBN: 1-85957-338-X

Typeset by Rapra Technology Limited Cover printed by The Printing House, Crewe, UK Printed and bound by Rapra Technology Limited, Shrewsbury, UK

Contents

1. Technology of Polyolefin Film Production ...................................................... 5 1.1

Introduction ........................................................................................... 5

1.2

Structures of the Polyolefins................................................................... 7 1.2.1

Low-Density Polyethylene (LDPE) ............................................. 7

1.2.2

High-Density Polyethylene (HDPE, MDPE, UHMWPE) ........... 8

1.2.3

Linear Low-Density Polyethylene (LLDPE) ................................ 8

1.2.4

Very- and Ultra-Low-Density Polyethylene (VLDPE, ULDPE) ... 8

1.2.5

Polypropylene (PP) ..................................................................... 9

1.2.6

Polypropylene Copolymers ........................................................ 9

1.3

Morphology of Polyolefin Films ............................................................ 9

1.4

Rheological Characterisation of the Polyolefins ................................... 10

1.5

1.6

1.4.1

High-Density Polyethylene ....................................................... 10

1.4.2

Linear Low-Density Polyethylene ............................................ 11

1.4.3

Very- and Ultra-Low-Density Polyethylene .............................. 11

1.4.4

Low-Density Polyethylene, Long Branches .............................. 11

1.4.5

Polypropylene .......................................................................... 12

Blown Film Production (Tubular Extrusion) ........................................ 13 1.5.1

Extruder Characteristics .......................................................... 14

1.5.2

Screw Design ........................................................................... 15

1.5.3

Frost-line and Blow Ratio ........................................................ 15

Cast Film Production ........................................................................... 16 1.6.1

Extrusion Conditions ............................................................... 16

1.6.2

Calendering Finishing .............................................................. 17

1.6.3

Extrusion Coating .................................................................... 17

iii


1.7

1.8

1.9

Orientation of the Film ........................................................................ 18 1.7.1

Orientation During Blowing .................................................... 18

1.7.2

Orientation by Drawing........................................................... 18

1.7.3

Biaxial Orientation (Biaxially Oriented PP, BOPP) .................. 18

Surface Properties ................................................................................ 19 1.8.1

Gloss ........................................................................................ 19

1.8.2

Haze ........................................................................................ 20

1.8.3

Surface Energy ......................................................................... 20

1.8.4

Slip........................................................................................... 21

1.8.5

Blocking ................................................................................... 21

Surface Modification ........................................................................... 21 1.9.1

Corona Discharge .................................................................... 21

1.9.2

Antiblocking ............................................................................ 22

1.9.3

Slip Additives ........................................................................... 23

1.9.4

Lubricants ................................................................................ 24

1.9.5

Antistatic Agents ...................................................................... 24

1.10 Internal Additives ................................................................................ 24 1.10.1 Antioxidants ............................................................................ 24 1.10.2 Ultraviolet Absorbers ............................................................... 24 1.11 Mechanical Properties .......................................................................... 25 1.11.1 Tensile Properties ..................................................................... 26 1.11.2 Impact Properties ..................................................................... 28 1.11.3 Dynamic Mechanical Properties .............................................. 29 1.11.4 Dielectric Properties ................................................................. 30 1.12 Microscopic Examination .................................................................... 31 1.12.1 Optical – Polarised Light Effect with Strain ............................. 31 1.12.2 Scanning Electron Microscopy (SEM) – Etching ...................... 31 1.12.3 Atomic Force Microscopy (AFM) ............................................ 31 1.13 Thermal Analysis ................................................................................. 31 1.13.1 Differential Scanning Calorimetry (DSC) ................................. 31

iv

Contents

1.13.2 Temperature-Modulated DSC (TMDSC) ................................. 32 1.14 Infrared Spectroscopy .......................................................................... 32 1.14.1 Characterisation ...................................................................... 32 1.14.2 Composition Analysis of Blends and Laminates....................... 33 1.14.3 Surface Analysis ....................................................................... 33 1.14.4 Other Properties ...................................................................... 34 1.15 Applications ......................................................................................... 35 1.15.1 Packaging ................................................................................ 35 1.15.2 Laminated Films ...................................................................... 36 1.15.3 Coextruded Films .................................................................... 37 1.15.4 Heat Sealing ............................................................................. 38 1.15.5 Agriculture ............................................................................... 38 1.16 Conclusion ........................................................................................... 38 2. Processing of Polyethylene Films ................................................................... 41 2.1

Introduction ......................................................................................... 41

2.2

Parameters Influencing Resin Basic Properties ..................................... 42

2.3

2.2.1

Molecular Weight (Molar Mass) and Dispersity Index ............ 42

2.2.2

Melt Index (Flow Properties) ................................................... 42

2.2.3

Density .................................................................................... 44

2.2.4

Chain Branching ...................................................................... 45

2.2.5

Intrinsic Viscosity .................................................................... 46

2.2.6

Melting Point and Heat of Fusion ............................................ 47

2.2.7

Melt Properties – Rheology ..................................................... 48

2.2.8

Elongational Viscosity ............................................................. 49

2.2.9

Elasticity .................................................................................. 49

Blown Film Extrusion (Tubular Film) .................................................. 50 2.3.1

Introduction ............................................................................. 50

2.3.2

Description of the Blown Film Process ..................................... 50

2.3.3

Various Ways of Cooling the Film ........................................... 51

v


2.4

2.3.4

Extruder Size ........................................................................... 54

2.3.5

Horsepower ............................................................................. 55

2.3.6

Selection of Extrusion Equipment ............................................ 55

Cast Film Extrusion ............................................................................. 57 2.4.1

Description of the Cast Film Process ........................................ 57

2.4.2

Effects of Extrusion Variables on Film Characteristics ............. 58

2.4.3

Effect of Blow-up Ratio on Film Properties ............................. 61

2.5

Processing Troubleshooting Guidelines ................................................ 62

2.6

Shrink Film .......................................................................................... 62 2.6.1

Shrink Film Types .................................................................... 65

2.6.2

Shrink Film Properties ............................................................. 66

2.6.3

The Manufacture of Shrink Film ............................................. 67

2.6.4

Shrink Tunnels and Ovens ....................................................... 70

3. Processing Conditions and Durability of Polypropylene Films ...................... 73 3.1

Introduction ......................................................................................... 73

3.2

Structures and Synthesis ....................................................................... 78

3.3

Film Processing .................................................................................... 85

3.4

Additives .............................................................................................. 85

3.5

Ultraviolet Degradation of Polypropylene ............................................ 86

3.6

3.7

vi

3.5.1

UV Degradation Mechanisms .................................................. 86

3.5.2

Effect of UV Degradation on Molecular Structure and Properties of PP................................................................. 87

3.5.3

Stabilisation of PP by Additives ............................................... 88

Case Studies ......................................................................................... 90 3.6.1

Materials and Experimental Procedures ................................... 90

3.6.2

Durability-Microstructure Relationship ................................... 91

3.6.3

Durability-Processing Condition Relationship ......................... 94

3.6.4

Durability-Additive Property Relationship ............................... 97

Concluding Remarks ......................................................................... 101

Contents

4. Solubility of Additives in Polymers.............................................................. 109 4.1

Introduction ....................................................................................... 109

4.2

Nonuniform Polymer Structure.......................................................... 109

4.3

Additive Sorption ............................................................................... 110

4.4

Quantitative Data on Additive Solubility in Polymers ....................... 114

4.5

Factors Affecting Additive Solubility ................................................. 118

4.6

4.5.1

Crystallinity and Supermolecular Structure............................ 118

4.5.2

Effect of Polymer Orientation ................................................ 119

4.5.3

Role of Polymer Polar Groups ............................................... 120

4.5.4

Effect of the Second Compound ............................................ 121

4.5.5

Features of Dissolution of High Molecular Weight Additives .. 122

4.5.6

Effect of Polymer Oxidation .................................................. 124

Solubility of Additives and Their Loss ............................................... 125

5. Polyvinyl Chloride: Degradation and Stabilisation ...................................... 131 5.1

Introduction ....................................................................................... 131

5.2

Some Factors Affecting the Low Stability of PVC .............................. 132

5.3

Identification of Carbonylallyl Groups .............................................. 136

5.4

Principal Ways to Stabilise PVC ......................................................... 138

5.5

Light Stabilisation of PVC ................................................................. 144

5.6

Effect of Plasticisers on PVC Degradation in Solution ....................... 145

5.7

‘Echo’ Stabilisation of PVC ................................................................ 151

5.8

Tasks for the Future ........................................................................... 153

6. Ecological Issues of Polymer Flame Retardants ........................................... 159 6.1

Introduction ....................................................................................... 159

6.2

Mechanisms of Action ....................................................................... 160

6.3

Halogenated Diphenyl Ethers – Dioxins ............................................ 162

vii


6.4

Flame Retardant Systems ................................................................... 166

6.5

Intumescent Additives ........................................................................ 168

6.6

Polymer Organic Char-Former ........................................................... 175

6.7

Polymer Nanocomposites .................................................................. 180

7. Interaction of Polymers with the Nitrogen Oxides in Polluted Atmospheres . 187 7.1

Introduction ....................................................................................... 187

7.2

Interaction of Nitrogen Dioxide with Polymers ................................. 188 7.2.1

Vinyl Polymers: PE, PP, PS, PMMA, PAN, PVC and PVF ...... 188

7.2.2

Non-Saturated Polymers ........................................................ 191

7.2.3

Polyamides, Polyurethanes, Polyamidoimides ........................ 196

7.3

Reaction of Nitric Oxide with Polymers ............................................ 202

7.4

Conclusion ......................................................................................... 209

8. Modifications of Plastic Films ..................................................................... 213 8.1

Introduction ....................................................................................... 213

8.2

Modification of Mechanical Properties .............................................. 213

8.3

8.4

viii

8.2.1

Orientation ............................................................................ 214

8.2.2

Crystallisation ........................................................................ 214

8.2.3

Crosslinking ........................................................................... 214

Chemical Modifications ..................................................................... 215 8.3.1

Fluorination ........................................................................... 215

8.3.2

Chlorination .......................................................................... 217

8.3.3

Bromination ........................................................................... 217

8.3.4

Sulfonation ............................................................................ 218

8.3.5

Chemical Etching ................................................................... 218

8.3.6

Grafting ................................................................................. 220

Physical Methods Used for Surface Modification............................... 222 8.4.1

Plasma Treatment .................................................................. 222

8.4.2

Corona Treatment ................................................................. 223

Contents

8.5

8.6

Characterisation ................................................................................ 224 8.5.1

Gravimetric Method .............................................................. 224

8.5.2

Thermal Analyses .................................................................. 225

8.5.3

Scanning Electron Microscopy ............................................... 225

8.5.4

Swelling Measurements .......................................................... 226

8.5.5

Molecular Weight and Molecular Weight Distribution .......... 226

8.5.6

Dielectric Relaxation ............................................................. 226

8.5.7

Surface Properties .................................................................. 227

8.5.8

Spectroscopic Analysis ........................................................... 227

8.5.9

Electron Spectroscopy for Chemical Analysis (ESCA) or X-Ray Photoelectron Spectroscopy (XPS) ......................... 228

Applications ....................................................................................... 228

9. Applications of Plastic Films in Packaging .................................................. 235 9.1

Introduction ....................................................................................... 235

9.2

Packaging Functions .......................................................................... 235

9.3

Flexible Package Forms ...................................................................... 236 9.3.1

Wraps .................................................................................... 237

9.3.2

Bags, Sacks and Pouches ........................................................ 238

9.3.3

Pouch Production .................................................................. 239

9.3.4

Dispensing and Reclosure Features ........................................ 239

9.4

Heat-Sealing ...................................................................................... 240

9.5

Other Uses of Packaging Films........................................................... 241

9.6

Major Packaging Films ...................................................................... 241 9.6.1

Low-Density Polyethylene (LDPE) and Linear Low-Density Polyethylene (LLDPE) ....................................... 242

9.6.2

High-Density Polyethylene (HDPE) ....................................... 243

9.6.3

Polypropylene (PP) ................................................................. 244

9.6.4

Polyvinyl Chloride (PVC)....................................................... 245

9.6.5

Polyethylene Terephthalate (PET) .......................................... 245

ix


9.6.6

Polyvinylidene Chloride (PVDC) ........................................... 246

9.6.7

Polychlorotrifluoroethylene (PCTFE) ..................................... 247

9.6.8

Polyvinyl Alcohol (PVOH) .................................................... 248

9.6.9

Ethylene-Vinyl Alcohol (EVOH) ............................................ 248

9.6.10 Polyamide (Nylon) ................................................................. 249 9.6.11 Ethylene-Vinyl Acetate (EVA) and Acid Copolymer Films ..... 250 9.6.12 Ionomers ................................................................................ 251 9.6.13 Other Plastics ......................................................................... 251 9.7

Multilayer Plastic Films ..................................................................... 252 9.7.1

Coating .................................................................................. 252

9.7.2

Lamination ............................................................................ 253

9.7.3

Coextrusion ........................................................................... 253

9.7.4

Metallisation .......................................................................... 253

9.7.5

Silicon Oxide Coating ............................................................ 254

9.7.6

Other Inorganic Barrier Coatings .......................................... 255

9.8

Surface Treatment .............................................................................. 255

9.9

Static Discharge ................................................................................. 256

9.10 Printing .............................................................................................. 256 9.11 Barriers and Permeation ..................................................................... 257 9.12 Environmental Issues ......................................................................... 261 10. Applications of Plastic Films in Agriculture ................................................ 263 10.1 Introduction ....................................................................................... 263 10.2 Production of Plastic Films ................................................................ 263 10.3 Characteristics of Plastic Films Used in Agriculture ........................... 264 10.4 Stability of Greenhouse Films to Solar Irradiation ............................. 265 10.4.1 Ultraviolet Stabilisers ............................................................. 265 10.4.2 Requirements for Stabiliser Efficiency .................................... 268 10.4.3 Evaluation of Laboratory and Outdoor Photooxidation ........ 271

x

Contents

10.5

10.6

10.7

Other Factors Affecting the Stability of Greenhouse Films .............. 272 10.5.1

Temperature ..................................................................... 272

10.5.2

Humidity .......................................................................... 273

10.5.3

Wind ................................................................................ 273

10.5.4

Fog Formation ................................................................. 273

10.5.5

Environmental Pollution .................................................. 274

10.5.6

Effects of Pesticides .......................................................... 274

Ageing Resistance of Greenhouse Films .......................................... 275 10.6.1

Measurement of Ageing Factors ....................................... 275

10.6.2

Changes in Chemical Structure......................................... 276

Recycling of Plastic Films in Agriculture ......................................... 277 10.7.1

Introduction ..................................................................... 277

10.7.2

Contamination by the Environment ................................. 277

11. Physicochemical Criteria for Estimating the Efficiency of Burn Dressings ... 285 11.1

Introduction .................................................................................... 285

11.2

Modern Surgical Burn Dressings ..................................................... 286

11.3

11.4

11.2.1

Dressings Based on Materials of Animal Origin ............... 286

11.2.2

Dressings Based on Synthetic Materials ............................ 286

11.2.3

Dressings Based on Materials of Vegetable Origin ........... 290

Selection of the Properties of Tested Burn Dressings ....................... 290 11.3.1

Sorption-Diffusion Properties ........................................... 291

11.3.2

Adhesive Properties .......................................................... 292

11.3.3

Mechanical Properties ...................................................... 292

Methods of Investigation of Physicochemical Properties of Burn Dressings ................................................................................ 292 11.4.1

Determination of Material Porosity ................................. 292

11.4.2

Determination of Size and Number of Pores .................... 293

11.4.3

Estimation of Surface Energy at Material-Medium Interface ........................................................................... 294

11.4.4

Determination of Sorptional Ability of Materials ............. 294 xi


11.5

11.6

11.7

11.8

11.4.5

Determination of Air Penetrability of Burn Dressings ...... 295

11.4.6

Determination of Adhesion of Burn Dressings ................. 296

11.4.7

Determination of Vapour Penetrability of Burn Dressings .. 296

Results and Discussion .................................................................... 297 11.5.1

Determination of Sorption Ability of Burn Dressings ....... 297

11.5.2

Kinetics of the Sorption of Liquid Media by Burn Dressings ................................................................. 303

11.5.3

Determination of Vapour Penetrability of Burn Dressings .. 305

11.5.4

Determination of the Air Penetrability of Burn Dressings .. 308

11.5.5

Determination of Adhesion of Burn Dressings ................... 315

The Model of Action of a Burn Dressing ........................................ 318 11.6.1

Evaporation of Water from the Dressing Surface ............. 318

11.6.2

Sorption of Fluid by Burn Dressing from Bulk Containing a Definite Amount of Fluid ............................ 320

11.6.3

Mass Transfer of Water from Wound to Surroundings ..... 321

Criteria for the Efficiency of First-Aid Burn Dressings .................... 322 11.7.1

Requirements of a First-Aid Burn Dressing ...................... 322

11.7.2

Characteristics of First-Aid Burn Dressings ...................... 322

Conclusion ...................................................................................... 324

12. Testing of Plastic Films ................................................................................ 329 12.1

Introduction .................................................................................... 329

12.2

Requirements for Test Methods ...................................................... 330

12.3

12.4

xii

12.2.1

List of Requirements ........................................................ 330

12.2.2

Interpretation of Test Results ........................................... 330

Some Properties of Plastic Films ...................................................... 332 12.3.1

Dimensions ...................................................................... 332

12.3.2

Conditioning the Samples ................................................. 332

Mechanical Tests ............................................................................. 333 12.4.1

Tensile Testing (Static) ...................................................... 333

12.4.2

Impact Resistance ............................................................. 336

Contents

12.5

12.4.3

Tear Resistance ................................................................. 337

12.4.4

Bending Stiffness (Flexural Modulus) ............................... 339

12.4.5

Dynamic Mechanical Properties ....................................... 339

Some Physical, Chemical and Physicochemical Tests ....................... 340 12.5.1

Density of Plastics ............................................................ 340

12.5.2

Indices of Refraction and Yellowness ............................... 340

12.5.3

Transparency .................................................................... 341

12.5.4

Resistance to Chemicals ................................................... 341

12.5.5

Haze and Luminous Transmittance .................................. 341

12.5.6

Ignition, Rate of Burning Characteristics and Oxygen Index (OI) ........................................................... 342

12.5.7

Static and Kinetic Coefficients of Friction ........................ 342

12.5.8

Specular Gloss of Plastic Films and Solid Plastics ............. 343

12.5.9

Wetting Tension of PE and PP Films ................................. 344

12.5.10 Unrestrained Linear Thermal Shrinkage of Plastic Films .. 345 12.5.11 Shrink Tension and Orientation Release Stress ................. 345 12.5.12 Rigidity ............................................................................ 345 12.5.13 Blocking Load by Parallel-Plate Method .......................... 346 12.5.14 Determination of LLDPE Composition by 13C NMR ..... 346 12.5.15 Creep and Creep Rupture ................................................. 346 12.5.16 Outdoor Weathering/Weatherability ................................ 347 12.5.17 Abrasion Resistance ......................................................... 347 12.5.18 Mar Resistance ................................................................. 348 12.5.19 Environmental Stress Cracking......................................... 348 12.5.20 Water Vapour Permeability .............................................. 348 12.5.21 Oxygen Gas Transmission ................................................ 349 12.6

Standard Specifications for Some Plastic Films ................................ 349 12.6.1

Standard Specification for PET Films ............................... 350

12.6.2

Standard Specification for LDPE Films (for General Use and Packaging Applications) ..................................... 350

12.6.3

Standard Specification for MDPE and General Grade PE Films (for General Use and Packaging Applications) ... 350 xiii


12.6.4

Standard Specification for OPP Films ............................... 351

12.6.5

Standard Specification for Crosslinkable Ethylene Plastics . 351

13. Recycling of Plastic Waste ........................................................................... 357 13.1

Introduction .................................................................................... 357

13.2

Main Approaches to Plastic Recycling ............................................ 358

13.3

Primary Recycling ............................................................ 358

13.2.2

Secondary Recycling ......................................................... 358

13.2.3

Tertiary Recycling ............................................................ 359

13.2.4

Quaternary Recycling ....................................................... 360

13.2.5

Conclusion ....................................................................... 362

Collection and Sorting .................................................................... 362 13.3.1

Resin Identification .......................................................... 362

13.3.2

General Aspects of Resin Separation ................................ 363

13.3.3

Resin Separation Based on Density .................................. 364

13.3.4

Resin Separation Based on Colour ................................... 365

13.3.5

Resin Separation Based on Physicochemical Properties .... 365

13.4

Recycling of Separated PET Waste .................................................. 367

13.5

Recycling of Separated PVC Waste ................................................. 368

13.6

13.7

xiv

13.2.1

13.5.1

Chemical Recycling of Mixed Plastic Waste ..................... 369

13.5.2

Chemical Recycling of PVC-Rich Waste ........................... 370

Recycling of Separated PE Waste .................................................... 371 13.6.1

Contamination of PE Waste by Additives ......................... 372

13.6.2

Contamination of PE Waste by Reprocessing ................... 372

Recycling of HDPE ......................................................................... 373 13.7.1

Applications for Recycled HDPE ..................................... 373

13.7.2

Rubber-Modified Products ............................................... 373

13.8

Recycling Using Radiation Technology ........................................... 373

13.9

Biodegradable Polymers .................................................................. 374

Preface

The plastic industry continues to grow very rapidly and plays an important role in many fields such as engineering, medical, agriculture and domestic. It is now very difficult to find the point at which plastic cannot be considered as an essential component. The understanding of the nature of plastic films, their production techniques, applications and their characterisation is essential for producing new types of plastic films. This handbook has been written to discuss the production and main uses of plastic films. Chapter 1 deals with the various types of polyolefins and their suitability for film manufacture. The rheology, structure and properties of the polymers are discussed in relation to the type of film manufacturing processes that are most applicable to the types of polymer. Post-extrusion modifications of the films such as orientation, surface chemistry and additives are discussed. Characterisation methods used to measure film mechanical properties; structure and additives are described, as well as other more specific properties. Finally some particularly important applications that require special structures or modifications are given. In Chapter 2, the main parameters influencing resin basic properties are described. The methods of processing of polyethylene films such as cast film extrusion, blow extrusion of tubular films are discussed. Effects of extrusion variables on film characteristics and effect of blow ratio on film properties are considered. Chapter 3 details the structure, synthesis and film processing of polypropylene. The effects of some additives and UV stabilisers are discussed. The solubility of additives plays an important role in determining the efficiency and the properties of the films as well. For this reason Chapter 4 deals with different aspects of additives solubility in polymers in relation to the polymer degradation and stabilisation. The topic covered in Chapter 5 is the stability of polyvinyl chloride (PVC) films during procesing and service. The possibility of increasing the intrinsic stability of PVC during processing with the minimal contents or in total absence of stabilisers and other additives is discussed.

1

Handbook of Plastic Films Chapter 6 discusses flame retardants, which as special additives have an important role in saving lives. These flame retardant system basically inhibit or even suppress the combustion process by chemical or physical action in the gas or condensed phase. Conventional flame retardants have a number of negative attributes and the ecological issues surrounding their applications are driving the search for new polymer flame retardant systems forward. Chapter 7 covers thermal and photochemical oxidation of polymers under the influence of the aggressive, polluting atmospheric gases. Among pollutants, sulfur dioxide, ozone, nitrogen oxides stand out as the most deleterious impurities of atmosphere. Thus, this chapter is devoted to consideration of the results obtained in studies of interactions of nitrogen oxides with polymers. Chapter 8 discusses the modifications of plastic films to improve their mechanical or physical properties to meet the requirements of certain applications. This can be achieved by subjecting the films to mechanical or chemical treatments. A number of surface modification techniques such as plasma, corona discharge and chemical treatments have been used. Chapter 9 deals with applications of plastic films in packaging. A description of the properties of the most common films used in packaging such as low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), high-density polyethylene (HDPE), polypropylene (PP), PVC, polyvinylidene chloride (PVDC), polyamide (Nylon), and other plastics are given in this chapter. Chapter 10 deals with the application of plastic films in agriculture. The mechanical properties suitable that make these films suitable for use in agriculture are discussed. Stability of these plastic films under the effect of different environmental conditions is reported. Types of UV stabilisers and their compatibility with plastic are given. Also, recycling of plastic films used in agriculture is of great importance and finally, a case study of their reuse as agriculture films is given. Chapter 11 deals with the principal medical treatment of burns using dressings made with a polymeric layer or layers. It is difficult to estimate the effectiveness of the new burn dressings, as their physicochemical properties are not usually presented in literature. Thus, chapter 11 discusses this subject for the first time. The physicochemical criteria for estimating the efficiency of burn dressings and the possibility of using plastic films is given. Chapter 12 covers the most common test methods generally used for plastic films. The requirements necessary for the test methods are summarised.

2

Preface The problem of plastic films recycling is touched on in Chapter 13. The majority of plastic films are made from polyethylene (LDPE, LLDPE or HDPE) which comprise approximately 68% of the total film production. Non-polyethylene resins constitute the remainder of the plastic film. Different types of recycling are given and recycling of some selected types of films are discussed. This handbook represents the efforts of many experts in different aspects of plastic films. Their efforts in preparing contributions to the volume are to be noted and I take the opportunity to express my heartfelt gratitude for their time and effort. My gratitude extends also to many colleagues for their kind comments in many aspects. A special thanks is extended to the staff of Rapra Technology, for the fine production of this Handbook, particularly Claire Griffiths, Editorial Assistant, Steve Barnfield who typeset the book and designed the cover and Frances Powers who commissioned the book and oversaw the whole project. Elsayed M. Abdel-Bary January 2003

3


4

1

Technology of Polyolefin Film Production Robert Shanks

1.1 Introduction A film is a two-dimensional form of a polymer. A film is typified by a large surface area to volume ratio. Films are required to exhibit barrier properties to any contaminating substances that may try to enter, or any desirable substances that may try to leave, across the film. This property is resistance to diffusion. Since a film is very thin, it must have high mechanical properties such as tensile strength, impact resistance and tear strength. The mechanical properties usually depend on molecular structure, molar mass and molar mass distribution. Visibility through a film is often important, so low haze will be required. These are the bulk properties of the film [1]. The film will often be required to improve the appearance of an item contained within it, so surface properties such as gloss and printability are important. The latter property, printability, is related to a relatively high surface energy to achieve wetting and good work of adhesion. Suitable surface energy may be achieved through modification. Protection may also be improved if the friction is low; this property is called slip. When a film is used to enclose and protect items, it may need to provide adhesion to itself or to the contents. The immediate form of adhesion is called tack. Subsequently the polymer must flow to provide complete adhesion. Manufacture of a film will usually be through an extrusion of the melt, so the melt rheology must be suited to the manufacturing process. Rheology is controlled by chemical structure, molar mass and long branches. The way in which the film is extruded, extended and solidified by cooling will control the microstructure and hence many of the properties. A summary of the various polyolefins used in film manufacture is provided in Table 1.1. In this chapter, polyolefin films are reviewed. First, the various types of polyolefins and their suitability for film manufacture are considered. The rheology, structure and properties of the polymers are discussed in relation to the type of film manufacturing processes that are most applicable to the types of polymer. Post-extrusion modifications of the films, such as orientation, surface chemistry and additives, are discussed. Characterisation methods used to measure film mechanical properties, structure and additives are described, as well as other more specific properties. Finally, some important particular applications that require special structures or modifications are described.

5


Table 1.1 Structure, properties and description of the polyolefins used for film production Polyolefin

Comonomer

Density

Preparation method

Mechanical properties

Comments

High-density or linear polyethylene (HDPE, LPE)

No branches

0.94-0.96

Zeigler-Natta

High tensile strength, low impact strength

Brittle film, with good gas barrier properties

Low-density polyethylene (LDPE)

Random short and long branches

0.91

Radical, with autoclave or tubular reactor

Non-Newtonian melt rheology, good impact strength

Good blown extrusion characteristics for flexible films High-clarity, glossy film, difficult to extrude

Linear lowdensity polyethylene (LLDPE)

1-Butene, 1-Hexene, 1-Octene

0.91-0.93

Zeigler-Natta

Intermediate strength with elasticity, melt rheology more Newtonian than LDPE

Very-low-density polyethylene (VLDPE)


0.89-0.91

Single-site metallocene

Tough elastic, moderate strength

High-clarity, very glossy film, very thin films possible

VLDPE with long branches


0.89-0.91

Constrained geometry single site

Tough elastic, moderate strength, nonNewtonian melt rheology

Easy to process, improved melt strength

Ultra-low-density polyethylene (ULDPE), plastomers



Elastic, low tensile strength, low modulus

Thermoplastic elastomer, narrow low temperature melting, good for heat seal

Zeigler-Natta

High tensile strength, brittle, temperature resistance

Transparent, high-strength and temperature-resistant glossy films

Zeigler-Natta

Tough with high melting temperature (block) or softer with lower melting temperature (random)

Tough films, with more milky colour


Narrow molar mass distribution, random comonomer distribution and high isotacticity

Flexible, elastic transparent films

Polypropylene

Polypropylene copolymer with ethylene, block or random

Polypropylene and copolymers with ethylene

6

No branches

Ethylene

Ethylene

0; and in the second, d2[A]p/d[A]m2 < 0. A positive deviation means that the sorption of any molecule of A facilitates the sorption of the next one. Two mechanisms for the positive deviations are possible: (i) an increase in the mobility of the macromolecules caused by a dissolved compound (for example, plasticising the polymer) and (ii) the formation of aggregates (clusters) of several A molecules dissolved in the polymer. Positive deviations may be described by the following equation obtained from the theory of regular solutions:

μ/RT = ln(P/P0) = ln φ1 + φ2 + χφ22

(4.2)

where P is the vapour pressure of the additive in the system, P0 is the pressure of its saturated vapour, and χ is the Flory-Huggins parameter (solvent-solute interaction parameter). Equation (4.2) connects the chemical potential of the solvent (μ) with the volume fractions of the additive (φ1) and the polymer (φ2). The value of χ can be determined in terms of the solubility parameters of the additive and of the polymer, δ1 and δ2, respectively:

χ = V1(δ2 – δ2)2/RT

(4.3)

where V1 is the molar volume of sorbate. Equations (4.2) and (4.3) are widely used in practice and allow one to predict the additive sorption. The values δ1 and δ2 can be obtained from independent experiments or by simulation. In crystalline polymers, it is necessary to take into account the volume accessible for the molecules of additive, which does not always coincidence with the total volume of amorphous phase of the polymer.

110

Solubility of Additives in Polymers A negative deviation from equation (4.1) corresponds to the case when the polymer possesses a limited number (concentration) of centres that can sorb one A molecule each. In this case, with increasing A concentration in the polymer, the number of non-occupied centres decreases. In many cases the negative deviations can be described by a law analogous to the Langmuir equation (4.4) or its combination with Henry’s law (4.5) [2-7, 11]: a[ A] [A]p = 1 + b[Am]

[A]

p

[ ]

=γ A

m

(4.4) m

+

[ ] 1+ b[ A ] aA

m

(4.5) m

where a and b are constants; the ratio a/b corresponds to limit of the A concentration in polymer. The nature of sorption centres may be different. Polymer polar groups interacting with an additive (for example, due to the formation of hydrogen bonds), as well regions with a lower density of a polymer substance (the elements of free volume) in the polymer, may be regarded as such centres. The latter have either a relaxation or a topological nature. Some authors [3-5] consider sorption centres as microvoids and unrelaxed volume in the polymer below the glass transition temperature that disappear at high temperature. In contrast, the centres arising around knots and other chain entanglements are more stable and can also exist in the polymer melt [7, 8, 12]. Suppose that in a polymer a certain concentration of the same centres Zi is present that can interact with compound A. Let us also suppose that the sorption of additive proceeds in two steps. First, the additive forms a true solution, the concentration of A in this solution being related to its concentration around the polymer by Henry’s law, that is, [A] = γ[A]m. Then this truly dissolved additive is reversibly sorbed by centres Zi: K

a A + Zi ←⎯ ⎯ → AZi

(4.6)

The total concentration of A in the polymer is [A]p = [A] + ∑[AZi] = [A] + [AZa], where [A] and [AZa] are the concentrations of true dissolved (mobile) and immobile molecules, respectively. If the additive concentration outside the centres is neglected ([A] 1–CHCl~ Ap

0

~CH2–CHCl–CH2–CHCl~ α0

1

objectively defines the necessity for a new specific approach for studying various aspects of the destruction and stabilisation of PVC. In particular, studies are needed of the new characteristic reactions with unsaturated ketones, confirming the presence of oxovinylene groups in the PVC structure, or the interaction of ~C(O)–CH=CH–CHCl~ groups with organic phosphites P(OR)3 [29-33] and dienes [34, 35].

5.4 Principal Ways to Stabilise PVC Organic phosphites react easily in mild conditions (290-330 K) with oxovinylene groups in the presence of proton donors to yield the stable ketophosphonates:

138

Achievements for Polyvinyl Chloride: Degradation and Stabilisation

~C

CHCl~ O

~C O

C H

C H

CH~

+

P-(OR)3

P(OR)3 O

Cl C

CH2 O

CH

CHCl~

P(OR)3

(5.3)

The reaction kinetics for the interaction of organic phosphites with oxovinylene groups are shown in Figure 5.3. The formation of ketophosphonate structures according to reaction (5.3) results in the disappearance of internal C=C bonds in the PVC structure. As a result, neither ozonolysis of a polymeric product nor especially alkaline hydrolysis leads to degradation of macromolecules and, consequently, decrease of PVC molecular weight.

Figure 5.3 The changes in the ~C(O)–CH=CH~ group content in PVC during interaction with tri(2-ethylhexyl) phosphite (C0 = 10–2 mol/mol PVC): (1) 289 K; (2) 298 K; (3) 448 K 139

Handbook of Plastic Films It is important to note that organic phosphites do not react with β-chloroallyl groups, as has been confirmed by the method of competing reactions of organic phosphites (trialkyl-, arylalkyl- and triarylphosphites) with a mixture (1:1 mol/mol) of methyl vinyl ketone (model of an oxovinylene group) and 4-chloropentene-2 (model of a βchloroallyl group) at 353 K. Practically, the organic phosphite selectively reacts quantitatively (with regarding to proton donor) with methyl vinyl ketone, while 4chloropentene-2 is quantitatively allocated after realisation of the reaction, excluding the small amount (less than 7 wt%) of products of its dehydrochlorination. The main reaction product (up to 75 wt%) is: CH3–C(O)–CH2–CH2–P(OR)2 In this reaction, trialkyl- and alkylarylphosphites are more active than triarylphosphites. Dienophilic oxovinylene groups react with conjugated dienes according to the following Diels-Alder reaction: O O RHC ~C

C H

C H

CHCl~

+

CHR– C H

C H

~C

CH

CH

RHC

CHCl~ CHR–

C H

C H

(5.4)

The reactions of PVC with cyclopentadiene, piperylene, isoprene, 5-methylheptatriene1,3,6, etc., proceed in mild conditions (353 K) and result in destruction of internal unsaturated C=C groups in PVC chains. These reactions are new and have not been reported before, and are similar to the reaction of PVC with organic phosphites, as shown in reaction (5.4). The collection of methods used to increase PVC stability to the action of various factors (such as heat, light, oxygen, etc.), in terms of storage, processing and use is closely connected to the level of theoretical development of PVC degradation. Therefore, it is clear that the significant advances in theoretical developments of the reasons for the thermal instability of PVC (the presence of oxovinylene groups in the backbone), the mechanism of the process (the fundamental influence of adjacent groups of the longrange order) and the kinetics of their disintegration were necessary, and have enabled a new look at the determination of effective methods of PVC stabilisation under thermal and other influences.

140

Achievements for Polyvinyl Chloride: Degradation and Stabilisation According to Scheme 5.2 it is impossible (and unnecessary) to increase the stability of PVC by the reduction of rate VHCl, since this process is rather slow. According to the experimental data, the rate of PVC statistical dehydrochlorination, VHCl, is constant (law of randomness) and does not depend on how the polymer was synthesised or its molecular weight. Hence, it is a fundamental characteristic of PVC, showing that all parts in clusters ~BXBXBX~ participate similarly in the process of HCl elimination under the law of randomness. On the other hand, the rate of formation of the conjugated systems, Vp, differs markedly, since it increases linearly with the content of oxovinylene groups in the initial PVC macromolecules (γ0) (Figure 5.2). Thus, the basis of effective PVC stabilisation, which determines processing properties and the durability of rigid materials and products, is due mainly to the increased selfstability of PVC [17, 36-39]. This can be achieved by chemical stabilisation of the labile oxovinylene groups present in the initial PVC macromolecules, first of all by studying specific polymer analogous reactions with either of the reaction centres 1-3: 2 ~C

1

C H

C H

O

CH~

(5.5)

3 Cl

The conjugation ~C(O)CH=CH~ has to be destroyed and/or the labile chlorine atom has to be replaced with a more stable adjacent group by interaction with the appropriate additives (stabilisers). This principle underlies the stabilisation of PVC in real formulations during manufacture of rigid materials and products, which is called ‘chemical stabilisation’ of PVC [17, 36, 37]. The reactions on centres 1-3 mentioned above are as follows: C O fragments of oxovinylene chloride groups:

(1) Polymer analogous reactions on

~ CH

R3SiH R3GeH O ~C

R3

CH

CH

CHCl~

[5]

Si

O CH

CH

CHCl~

R'

CH

CH

OH

OH

CH

CH

R"

R' R'

R"

CH

~C O

O

CH

CH

C

CHCl~

[40]

(5.6)

R"

141

Handbook of Plastic Films C C

(2) Polymer analogous reactions on

~C

CH2

fragments of oxovinylene groups:

CH

~C

CHCl~

P(OR)3 O

and/or

P(OR)3

CH~ CH

O

CHCl~

P

O

O

OR

[31-35] O

R'

CH

CH

CH

CH

~C

R"

CHCl~ CH

CH

CH

R'

[41] CH

HC

R"

CH

O C HC

O

CH

C

CH C O

O

[21]

CH

O

C O

O

CH

~HC

CHCl~

(5.7)

C H

(3) Polymer analogous reactions on labile

~C(O)

R2Sn(COOR)2

CH

C Cl groups:

CH

CH

CH2–

[16, 17]

Cd(COOR)2

OC(O)R

Zn(COOR)2, etc. O R'

CH

CH

(ZnCl2)

142

R"

~C(O)

CH

CH

CH

[43] O—CHR'—CHClR"

(5.8)

Achievements for Polyvinyl Chloride: Degradation and Stabilisation The concept of oxovinylene activation of the disintegration of PVC has allowed revealing new unexpected possibilities for effective stabilisation – not only thermal, but also light stabilisation – of this polymer. This also allows previously unknown classes of chemical compounds to be used for its stabilisation, in particular, conjugated diene hydrocarbons, Diels-Alder reaction adducts, protonic acids, α,β-dicarbonic compounds, etc. [34, 35, 4046]. It has also enabled new real reactions for PVC stabilisation to be revealed, including the application of known additives that have been used for a long time for PVC stabilisation (for instance, organic phosphites, epoxy compounds, proton-donating compounds, etc.). So, on this basis it is possible to manage the PVC ageing process more effectively (Scheme 5.3). The relation between the chemical structure of additives and their efficiency as stabilisers for

Scheme 5.3

143

Handbook of Plastic Films PVC gives an opportunity for the scientifically based and economically expedient selection of the appropriate stabilisers and their synergistic combinations for producing rigid materials based on PVC.

5.5 Light Stabilisation of PVC Polymer analogous transformations of the oxovinylene groups in PVC macromolecules on chemical stabilisation with the appropriate chemical additives lead not only to increased self-stability of PVC and inhibition of macromolecular crosslinking, but also to a noticeable increase in the colour stability of PVC. The transformation of oxovinylene groups as a result of polymer analogous transformations with chemical additives in the ketophosphonate, cyclohexane, dioxolane, dihydropyran, etc., structural groups in PVC and the ‘curing’ of labile oxovinylene chloride groups result in an increase in the optical density of PVC in the UV region of the spectrum. As a result, these groups act as internal light stabilisers and result in the phenomenon of self-photostabilisation of PVC [47] (Figure 5.4).

Figure 5.4 Dependence of whiteness retention coefficient Kw in PVC films on exposure time: (1) unstabilised PVC; and polymer treated with: (2) 2-tris(2-ethylhexyl) phosphite; (3) 2-ethylhexyl-9,10-epoxy stearate with ZnCl2; (4) piperylene; (5) cyclopentadiene (295 K, λ = 254 nm, 1 to 1.5 × 1015 photon/s cm2) 144

Achievements for Polyvinyl Chloride: Degradation and Stabilisation Thus, the determining factor causing the high rate of PVC disintegration and its need for stabilisation is the presence of abnormal groups, mainly oxovinylene ones, in the structure of its macromolecules.

5.6 Effect of Plasticisers on PVC Degradation in Solution In both plasticised (semi-rigid and flexible) PVC materials as well as PVC in solution, the rates of thermal destruction and effective stabilisation are caused by essentially different fundamental phenomena in comparison to those involved in the ageing of PVC in the absence of solvent. The following aspects of both structure and macromolecular dynamics have a significant influence on the stability of PVC: the chemical nature of the solvent, its basicity, specific and nonspecific solvation, the concentration of PVC in the solution, the segmental mobility of macromolecules, the thermodynamic properties of the solvent, the formation of associates, aggregates, etc. The chemical stabilisation of PVC plays a less significant role. As regards PVC destruction in solution, one of the basic reasons for a change in the kinetic parameters is the nucleophilic activation of the PVC dehydrochlorination reaction. The process is described by an E2 mechanism. Thus, there is a linear dependence between PVC thermal dehydrochlorination rate and the relative basicity of the solvent, B cm–1 (Figure 5.5) [48-50]. The value B cm–1 is evaluated by measuring the shift of a characteristic band (phenolic OH) at λ = 3600 cm–1 in the IR spectrum due to interaction with the solvent [51]. It is very important that, in solvents with relative basicity B > 50 cm–1, the rate of PVC dehydrochlorination is always above the rate of PVC dehydrochlorination without the solvent; while, when B < 50 cm–1, PVC disintegration rate is always less than that without the solvent. The revealed dependence VHCl = f(B) is described by the equation: * VHCl = VHCl + k(B − 50)

(5.9)

Inhibition of PVC disintegration in solvents with basicity B < 50 cm–1 is a very interesting and practically important phenomenon. It has been given the name ‘solvational stabilisation’ of PVC. However, ignoring the fact that, even at low concentration (2 wt%), PVC solutions should be represented not as solutions with isolated macromolecules, but rather as structured systems, results in a number of cases of deviation from a linear dependence of PVC dehydrochlorination rate on the solvent basicity B cm–1. In particular, an abnormal destruction behaviour of PVC is observed in certain ester-type solvents (plasticisers) (Figure 5.5, points 25-28), apparently caused by structural changes of the macromolecules. This has never before been taken into account when working with PVC solutions.

145


Figure 5.5 Influence of the solvent’s basicity on the rate of thermal dehydrochlorination in solution: (1) n-dichlorobenzene, (2) o-dichlorobenzene, (3) naphthalene, (4) nitrobenzene, (5) acetophenone, (6) benzonitrile, (7) di-(n-chlorophenylchloropropyl) phosphate, (8) triphenyl phosphite, (9) phenyl-bis(β-chloroethyl) phosphate, (10) tri-(n-chlorophenyl) phosphate, (11) 2-ethylhexylphenyl phosphate, (12) tricresyl phosphate, (13) cyclohexanone, (14) phenyl-bis(β-chloropropyl) phosphate, (15) tri-β-chloroethyl phosphate, (16) tri-β-chloropropyl phosphate, (17) di-(2-ethylhexyl) phosphate, (18) 2-ethylhexylnonyl phosphate, (19) tri-(2ethylhexyl) phosphate, (20) tributyl phosphate, (21,25) dibutyl phthalate, (22,26) di-(2-ethylhexyl) adipate, (23,27) dioctyl phthalate, (24,28) dibutyl sebacate. Concentration of PVC in solution: (1-24) 0.2 wt%, (25-28) 2 wt%; 423 K, under nitrogen

It was revealed quite unexpectedly that not only ‘polymer-solvent’ interactions, but also ‘polymer-polymer’ interactions, have a significant influence on the rate of PVC disintegration in solution. It is known that the structure and properties of the appropriate structural levels depend on the conformational and configurational nature of the macromolecules, including the supermolecular structure of the polymer, which in turn determines all the basic (both physical and chemical) characteristics of the polymer. ‘Polymer-polymer’ interaction leads to the formation of structures on the supermolecular level. In particular, on going to a more concentrated solution, the PVC-solvent system consistently passes through a number of stages, from isolated PVC macromolecules in

146

Achievements for Polyvinyl Chloride: Degradation and Stabilisation solution (infinitely dilute solution) to associates and aggregates of macromolecules in solution. On further increase of PVC concentration, the formation of a spatial fluctuational net with a structure similar to that of the bulk polymer occurs. When the polymer concentration in solution increases, the rate of the PVC dehydrochlorination reaction changes as well, and various types of effect of the solvent on the PVC disintegration rate in solution are observed depending on the numerical value of the basicity B cm–1 [52-57]. If the relative basicity of the solvents used is B > 50 cm–1, the polymer degradation rate decreases when its concentration increases. If the basicity of the employed solvents is B < 50 cm–1, the polymer degradation rate increases with increasing polymer concentration. In all cases the rate of HCl elimination from the polymer tends to approach the values of PVC dehydrochlorination rate usually observed PVC = 5 x 10–8 (mol HCl/mol PVC)/s (Figure 5.6). in the absence of solvent VHCl

Figure 5.6 The change in PVC dehydrochlorination rate as a function of its concentration in solution: (1) cyclohexanol, (2) cyclohexanone, (3) benzyl alcohol, (4) 1,2,3-trichloropropane, (5) o-dichlorobenzene, (6) no solvent; 423 K, under nitrogen

147

Handbook of Plastic Films Equation (5.9) turns into equation (5.10) if one takes into account that the PVC degradation rate is determined not only by the relative basicity of the solvent, B, but also by its concentration in solution, C (mol PVC/litre). Also, the degree of ‘polymer-polymer’ interaction (degree of macromolecule structurisation in a solution is given by ΔC = C – C0, where C0 is the concentration at the beginning of PVC macromolecule association in solution) is considered: 0

VHCl = VHCl +

A1 (B − 50) ΔC + d1

(5.10)

where A1 = (0.8 ± 0.2) × 10–9 (mol HCl/mol PVC)/s; and d1 is a dimensionless factor reflecting the ‘polymer-solvent’ interaction (d1 = 0.5 ± 0.25). The deviation from the onset of macromolecule association in a solution is taken as an absolute value, since it can be changed in both directions to more concentrated or more dilute polymer solution. Equation (5.10) well describes the change of PVC thermal dehydrochlorination rate as a function of its concentration in a solution of relative solvent basicity B, irrespective of the chosen solvent (Figure 5.7). The observable fundamental effect has significant importance in the production of plasticised materials and products made from PVC, in particular when esters are used. Despite the very high basicity of ester-type plasticisers (B = 150 cm–1) in the range of PVC concentration in solutions above 2%, a noticeable reduction in the degradation rate of PVC is observed (Figure 5.5, points 25-28), and stabilisation of PVC occurs. This effect is caused by the formation of dense globules, associates, etc., in the PVC-plasticiser system. Practically, this allows economic formulations of plasticised materials to be created from PVC with very low content of metal-containing stabilisers, used as HCl acceptors, or without their use at all. Temperature is very important in the formation of heterophase systems. Even at low concentrations of PVC in ester-type plasticisers (for example, in dioctyl phthalate at C > 0.1 mol/l), true solutions are formed only at temperatures above 400 K. The globular structure of PVC suspension and the formation of associates are retained at temperatures up to 430-445 K. In other words, plasticised PVC is able to keep its structural individuality on a supermolecular level, which is formed during polymer synthesis. Specifically, under these conditions an ester-type plasticiser behaves not as a highly basic solvent, but as a stabiliser during PVC thermal degradation due to formation of associates, etc. This leads to a reduction of the amount of stabiliser, extension of useful lifetime of materials and products, etc.

148

Achievements for Polyvinyl Chloride: Degradation and Stabilisation

Figure 5.7 The change in PVC dehydrochlorination rate as a function of its concentration in solution: (1,2) 1,2,3-trichloropropane, (3,4) cyclohexanol; (1,3) experimental data, (2,4) data calculated using equation (5.10) at A1 = 10–9 and d1 = 0.8 and 0.7, respectively, at 423 K, under nitrogen

It is necessary to note that the change in the degradation rate of PVC brought about by association of macromolecules is a general phenomenon and does not depend on how it was achieved. In particular, a change of character of the dehydrochlorination rate of PVC in solution is observed, similar to concentrated PVC solutions (Figures 5.6 and 5.7), if a change of PVC structure in solution occurs upon addition of even chemically inert nonsolvents such as hexane, decane, undecane, polyolefins, polyethylene wax, etc. [53, 56-59] (Figure 5.8). It is interesting to observe that the degree of relative change of PVC disintegration rate under the action of a second inert nonsolvent is much higher than for concentrated PVC solution. This is especially true in the case of using low-basicity solvents (trichloropropane and dichlorobenzene); it is the result of more dense formations on the supermolecular level, corresponding associates and aggregates, and, accordingly, a significant change of PVC destruction rate. The higher the content of nonsolvent (including inert polymer) in a blend and the lower the thermodynamic compatibility of the components in a solution, the more structural

149


Figure 5.8 The change in PVC thermodegradation rate on the content of the second inert polymer in solution of trichloropropane (1,3), dichlorobenzene (2) and cyclohexanol (4-6) for blends of PVC and polyethylene (1,4), polypropylene (2,5) and polyisobutylene (3,6); 423 K, under nitrogen

formation takes place in a solution, including that in the presence of polymer blends (associates, aggregates). Formation of a fluctuational net with participation of macromolecules is the probable explanation. The reason for the change in PVC thermal dehydrochlorination rate in the case of its blends with chemically inert, thermodynamically incompatible polymers is the same. It is due to the fact that in concentrated PVC solution (structural chemical changes of polymer in solution), the parameters determining the rate of PVC disintegration will obviously be similar. Therefore, for PVC thermal destruction, the concentration of the second polymer blended with PVC and its degree of thermodynamic affinity to PVC, in addition to the influence of polymer concentration in the solution, the basicity of the solvent, B cm–1, and ‘polymer-solvent’ interaction forces, have to be taken into account. In view of these factors equation (5.10) turns into: 0 + VHCl = VHCl

150

A1 (B − 50)

ΔC + d1 + C + α n

+

A1α 2α n BC

(5.11)

Achievements for Polyvinyl Chloride: Degradation and Stabilisation where α is the fraction of the second polymer, varying from 0 to 0.99; n is a dimensionless parameter describing the degree of thermodynamic affinity of PVC to the second polymer and varying from 0 (for complete thermodynamic compatibility of the components) up to a value of ~10 (for complete thermodynamic incompatibility of the polymers); and d2 is a dimensionless coefficient reflecting the interaction of the second polymer with the solvent, which equals 2.5 ± 0.1 for the destruction of PVC blended with polyethylene in dichlorobenzene, trichloropropane and cyclohexanol. Observable changes in PVC thermal disintegration rate under the action of a solvent that is thermodynamically incompatible with PVC or for a concentrated solution of PVC are caused by the transformation of the solvent from macromolecular globules of PVC to the structure that existed in the absence of the solvent. This evokes the unexpected effect of ‘solvent action’, either retardation or acceleration of PVC thermal disintegration depending on the solvent basicity, B cm–1. A solvent transformation that accelerates PVC disintegration (B > 50 cm–1) results in a decrease of its interaction with PVC and leads to a delay in the HCl elimination process, i.e., to stabilisation. This occurs in the case of both concentrated PVC solutions as well as the addition of another polymer that is thermodynamically incompatible with PVC. In solvents that slow down PVC disintegration (B < 50 cm–1) by virtue of low nucleophilicity, the effect of solvent transformation and the weakening of its interaction with PVC has the opposite result. In this case an increase of HCl elimination rate from PVC upon increase of its concentration in solution or by using a chemically inert nonsolvent occurs. It is obvious that, irrespective of how the changes to the PVC structure in solution are made, either by increase of its concentration in solution or by addition of another thermodynamically incompatible inert nonsolvent, the varying structuralphysical condition of the polymer results in a noticeable change of its thermal dehydrochlorination rate in solution. These effects are caused by structural-physical changes in the polymer-solvent system, and the previously unknown phenomena can be classified as ‘structural-physical stabilisation’ (in the case of a reduction in the gross rate of PVC disintegration in highly basic solvents at B > 50 cm–1) or ‘structuralphysical antistabilisation’ (in the case of an increase in the gross rate of PVC disintegration in low-basicity solvents with B < 50 cm–1), respectively.

5.7 ‘Echo’ Stabilisation of PVC Finally, it is necessary to describe one more appreciable achievement in the field of ageing and stabilisation of PVC in solution. In real conditions the basic reason for the sharp accelerated ageing of plasticised materials and products is oxidation of the solvent by the oxygen of the air (Figure 5.9, curve 3).

151


Figure 5.9 ‘Echo’ stabilisation of PVC. Elimination of HCl during thermal (argon) (1,2) and thermo-oxidative (air) (3-5) destruction of PVC in solution of dioctyl sebacate: (1-4) unstabilised PVC, (5) PVC stabilised with diphenylpropane (0.02 wt%) – ‘echo’ stabilisation; (2,4) PVC with no solvent; 448 K

•

2 RO2 + RH ⎯ ⎯ ⎯ → ROOH + R •

K

3 ROOH ⎯ ⎯ ⎯ → RO• + HO•

K

•

•

(5.12)

K6

RO2 + RO2 ⎯ ⎯ ⎯→ inactive products

Peroxides, formed by oxidation of ester-type plasticisers, initiate the disintegration of macromolecules. In these conditions the rate of PVC destruction increases by two or more orders of magnitude and is determined by the oxidation stability parameter of the solvent to oxygen Kef = K2 K03.5 K6−0.5 . Thus, a higher oxidation stability of the solvent (in particular, an ester-type plasticiser) lowers the degradation rate of semi-rigid and flexible PVC materials and increases its useful lifetime [60-63]. Inhibition of the oxidation process of the solvents (including plasticisers) due to the incorporation of stabilisers, antioxidants or their synergistic compositions slows down the thermo-oxidative disintegration of PVC in solution (Figure 5.9, curve 5). Effective inhibition of the oxidation of ester-type plasticisers by oxygen of the air causes the rate of PVC thermo-oxidative destruction in concentrated solutions to become closer

152

Achievements for Polyvinyl Chloride: Degradation and Stabilisation to the rate of polymer disintegration. This behaviour is characteristic of the thermal destruction of PVC in the presence of plasticisers acting as solvent. In other words, it becomes slower than PVC disintegration in the absence of solvent. This occurs due to a structural-physical stabilisation. In these cases, inhibition of the solvent oxidation reaction by using ‘echo’-type antioxidant stabilisers improves PVC stabilisation (Figure 5.9, curve 5). This fundamental phenomenon of PVC stabilisation in solution and its thermooxidative destruction has been called ‘echo stabilisation’ of PVC [49, 62, 63].

5.8 Tasks for the Future The creation of high-quality and economic semi-rigid and flexible materials and products made from PVC, including those where solvents are employed, requires specific approaches that are essentially different from the principles of manufacture of rigid PVC materials and products. In particular, consideration and use of the following fundamental phenomena should be considered: solvational, structural-physical and ‘echo’ stabilisation of the polymer in solution. As far as paramount tasks of fundamental and applied research in the field of PVC manufacture and processing at the beginning of the 21st century are concerned, they are obviously the following: (1) The manufacture of industrial PVC that does not contain labile groups in its backbone. This will provide a drastic increase in the intrinsic stability of polymeric PVC products, the possibility of processing with the minimal content or total absence of stabilisers and other chemical additives, and the opportunity to create PVC-based materials and products with essentially increased useful service lifetime. (2) Wide use of the latest achievements in the field of destruction and stabilisation of PVC, in both the presence and the absence of solvents. The phenomena of chemical, solvational, structural-physical, self- and ‘echo’ stabilisation of PVC will allow the creation of rigid, semi-rigid and flexible (plasticised) materials and products with minimal content of chemical additives, and will lead to increased useful service lifetime under natural and special conditions. (3) The use of nontoxic and nonflammable products that do not emit toxic and other poisonous gaseous and liquid products at elevated temperature during the manufacture and processing of PVC materials and their products. (4) Complete elimination of all toxic and even low-toxicity (particularly compounds based on barium, cadmium and lead, etc.), chemical additives from all formulations.

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Handbook of Plastic Films (5) The search for nontoxic and highly effective inorganic chemical additives, primarily, stabilisers of zeolite type, modified clays, etc. At the same time, new ‘surprises’ will undoubtedly be presented to us by this outstanding polymer puzzle. Certainly, as we look for a plastic for use as a ‘work-horse’ for many decades, studies on PVC will lead to new stimuli in the development of scientific ideas and practical development, and the opening-up of new pathways. These will result from the essential need to delay PVC ageing in natural and special conditions, and to reduce the amounts of the appropriate chemical additives, down to their complete elimination.

References 1.

V.S. Pudov, Plasticheskie Massy, 1976, 2, 18.

2.

D. Braun and W. Quarg, Angewandte Makromolekulare Chemie, 1973, 29/30, 1, 163.

3.

K.B. Abbas and E.M. Sorvik, Journal of Applied Polymer Science, 1976, 20, 9, 2395.

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L. Valko, I. Tvaroska and P. Kovarik, European Polymer Journal, 1975, 11, 5/6, 411.

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V.P. Myakov, B.B. Troitskii and G.A. Razuvaev, Vysokomolekulyarnye Soedineniya, Series B, 1969, 28, 11, 611.

6.

E.J. Arlman, Journal of Polymer Science, 1954, 12, 547.

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W.C. Geddes, European Polymer Journal, 1967, 3, 2, 267.

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P. Bataille and B.T. Van, Journal of Polymer Science, 1972, A1, 10, 1097.

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K.B. Abbas, M. Erling and E.M. Sorvik, Journal of Applied Polymer Science, 1973, 17, 12, 3577.

10. M. Onoluzka and M. Asahina, Journal of Macromolecular Science, 1969, 3, 2, 235. 11. M. Carrega, C. Bonnebat and G. Zednic, Analytical Chemistry, 1970, 42, 1807. 12. L. Schmidt, Angewandte Makromolekulare Chemie, 1975, 47, 1, 79. 13. V.V. Lisitsky, S.V. Kolesov, R.F. Gataullin and K.S. Minsker, Zhurnal Analiticheskoi Khimii, 1978, 33, 11, 2202.

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Achievements for Polyvinyl Chloride: Degradation and Stabilisation 14. K.S. Minsker, V.V. Lisitsky and G.E. Zaikov, Journal of Vinyl Technology, 1980, 2, 4, 77. 15. K.S. Minsker, Al.Al. Berlin and V.V. Lisitsky, Vysokomolekulyarnye Soedineniya, Series B, 1976, 18, 1, 54. 16. K.S. Minsker and G.T. Fedoseeva, Destruction and Stabilisation of Polyvinyl Chloride, Nauka, Moscow, Russia, 1979, 272. 17. K.S. Minsker, S.V. Kolesov and G.E. Zaikov, Degradation and Stabilisation of Vinyl Chloride-Based Polymers, Pergamon Press, Oxford, UK, 1988. 18. G. Talamini and G. Pezzin, Die Makromolekulare Chemie, 1960, 42, 26. 19. V. Chytry, B. Obereigner and D. Lim, European Polymer Journal, 1969, 5, 4, 379. 20. Z. Mayer, Journal of Macromolecular Science, 1974, 10, 2, 263. 21. Z. Mayer, B. Obereigner and D. Lim, Journal of Polymer Science, 1971, 33, 2, 289. 22. K.S. Minsker, Al.Al. Berlin, V.V. Lisitsky, S.V. Kolesov and R.S. Korneva, Doklady Akademii Nauk SSSR, 1977, 232, 1, 93. 23. K.S. Minsker, V.V. Lisitsky and G.E. Zaikov, Vysokomolekulyarnye Soedineniya, Series A, 1981, 23, 3, 289. 24. K.S. Minsker, S.V. Kolesov, V.M. Yanborisov, Al.Al. Berlin and G.E. Zaikov, Vysokomolekulyarnye Soedineniya, Series A, 1984, 26, 5, 883. 25. M.M. Shemyakin and A.A. Shchukina, Uspekhi Khimii, 1997, 26, 5, 528. 26. K.S. Minsker, Al.Al. Berlin, V.V. Lisitsky and S.V. Kolesov, Vysokomolekulyarnye Soedineniya, Series A, 1977, 19, 1, 32. 27. V.M. Yanborisov, S.V. Kolesov, Al.Al. Berlin and K.S. Minsker, Doklady Akademii Nauk SSSR, 1986, 291, 4, 920. 28. K.S. Minsker, Al.Al. Berlin and V.V. Lisitsky, Vysokomolekulyarnye Soedineniya, Series B, 1976, 18, 1, 54. 29. K.S. Minsker, N.A. Mukmeneva, Al.Al. Berlin, D.V. Kazachenko, M.Ya. Yanberdina, S.I. Agadzhanyan and P.A. Kirpichnikov, Doklady Akademii Nauk SSSR, 1976, 226, 5, 1088.

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Handbook of Plastic Films 30. N.A. Mukmeneva, S.I. Agadzhanyan, P.A. Kirpichnikov and K.S. Minsker, Doklady Akademii Nauk SSSR, 1977, 233, 3, 375. 31. K.S. Minsker, N.A. Mukmeneva, S.V. Kolesov, S.I. Agadzhanyan, V.V. Petrov and P.A. Kirpichnikov, Doklady Akademii Nauk SSSR, 1979, 244, 5, 1134. 32. N.A. Mukmeneva, K.S. Minsker, S.V. Kolesov and P.A. Kirpichnikov, Doklady Akademii Nauk SSSR, 1984, 274, 6, 1393. 33. N.A. Mukmeneva, E.N. Cherezova, L.N. Yamalieva, S.V. Kolesov, K.S. Minsker and P.A. Kirpichnikov, Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, 1985, 5, 1106. 34. K.S. Minsker, S.V. Kolesov and V.V. Petrov, Doklady Akademii Nauk SSSR, 1980, 252, 3, 627. 35. K.S. Minsker, S.V. Kolesov, V.V. Petrov and Al.Al. Berlin, Vysokomolekulyarnye Soedineniya, Series A, 1982, 24, 4, 793. 36. S.V. Kolesov and K.S. Minsker, Vysokomolekulyarnye Soedineniya, Series A, 1983, 25, 8, 1587. 37. K.S. Minsker, Polymer Plastics Technology and Engineering, 1997, 36, 4, 513. 38. K.S. Minsker, S.V. Kolesov and G.E. Zaikov, Journal of Vinyl Technology, 1980, 2, 3, 141. 39. K.S. Minsker, S.V. Kolesov and G.E. Zaikov, Vysokomolekulyarnye Soedineniya, Series A, 1981, 23, 3, 498. 40. S.R. Ivanova, A.G. Zaripova and K.S. Minsker, Vysokomolekulyarnye Soedineniya, Series A, 1978, 20, 4, 936. 41. K.S. Minsker, S.V. Kolesov and V.V. Petrov, Doklady Akademii Nauk SSSR, 1982, 268, 3, 632. 42. S. V. Kolesov, V.V. Petrov, V.M. Yanborisov and K.S. Minsker, Vysokomolekulyarnye Soedineniya, Series A, 1984, 26, 2, 303. 43. K.S. Minsker, S.V. Kolesov and S.R. Ivanova, Vysokomolekulyarnye Soedineniya, Series A, 1982, 24, 11, 2329. 44. K.S. Minsker, S.V. Kolesov, V.M. Yanborisov, M.E. Adler and G.E. Zaikov, Doklady Akademii Nauk SSSR, 1983, 268, 6, 1415.

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Achievements for Polyvinyl Chloride: Degradation and Stabilisation 45. S.V. Kolesov, K.S. Minsker, V.M. Yanborisov, G.E. Zaikov, K. De-Jong and R.M. Akhmetkhanov, Plasticheskie Massy, 1983, 12, 39. 46. S.V. Kolesov, A.M. Steklova, G.E. Zaikov and K.S. Minsker, Vysokomolekulyarnye Soedineniya, Series A, 1986, 28, 9, 1885. 47. K.S. Minsker, G.T. Fedoseeva, L.D. Strelkova, V.V. Petrov and S.V. Kolesov, Vysokomolekulyarnye Soedineniya, Series B, 1983, 25, 3, 165. 48. K.S. Minsker, M.I. Abdullin, V.I. Manushin, L.N. Malyshev and S.A. Arzhakov, Doklady Akademii Nauk SSSR, 1978, 242, 2, 366. 49. K.S. Minskerin in Polymer Yearbook, Volume 11, Ed., R.A. Pethrick, Harwood Academic, Chur, Switzerland, 1994, 229. 50. K.S. Minsker, E.I. Kulish and G.E. Zaikov, Vysokomolekulyarnye Soedineniya, Series B, 1993, 35, 6, 316. 51. V.A. Palm, Osnovy Kolichestvennoi Teorii Organicheskikh Reaktsii (Foundation of Quantitative Theory of Organic Reactions), Khimiya Publishing House, Leningrad, 1977, 114. 52. S.V. Kolesov, E.I. Kulish and K.S. Minsker, Vysokomolekulyarnye Soedineniya, Series B, 1994, 36, 8, 1383. 53. S.V. Kolesov, E.I. Kulish, G.E. Zaikov and K.S. Minsker, Russian Polymer News, 1997, 2, 4, 6. 54. E.I. Kulish, S.V. Kolesov and K.S. Minsker, Bashkirskii Khimicheskii Zhurnal, 1998, 5, 2, 35. 55. E.I. Kulish, S.V. Kolesov, K.S. Minsker and G.E. Zaikov, Vysokomolekulyarnye Soedineniya, Series A, 1998, 40, 8, 1309. 56. S.V. Kolesov, E.I. Kulish, G.E. Zaikov and K.S. Minsker, Journal of Applied Polymer Science, 1999, 73, 1, 85. 57. E.I. Kulish, S.V. Kolesov, K.S. Minsker and G.E. Zaikov, Chemical Physics Reports, 1999, 18, 4, 705. 58. E.I. Kulish, S.V. Kolesov, R.M. Akhmetkhanov and K.S. Minsker, Vysokomolekulyarnye Soedineniya, Series B, 1993, 35, 4, 205. 59. E.I. Kulish, S.V. Kolesov, K.S. Minsker and G.E. Zaikov, International Journal of Polymeric Materials, 1994, 24, 1-4, 123.

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Handbook of Plastic Films 60. V.S. Martemyanov, M.I. Abdullin, T.E. Orlova and K.S. Minsker, Neftekhimiia, 1981, 21, 1, 123. 61. K.A. Minsker, M.I. Abdullin, N.P. Zueva, V.S. Martemyanov and B.F. Teplov, Plasticheskie Massy, 1981, 9, 33. 62. K.S. Minsker and M.I. Abdullin, Doklady Akademii Nauk SSSR, 1982, 263, 1, 140. 63. Chemistry of Chlorine-Containing Polymers: Syntheses, Degradation, Stabilization, Eds., K.S. Minsker and G.E. Zaikov, Nova Science, Huntington, NY, USA, 2000, 198.

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6

Ecological Issues of Polymer Flame Retardants G.E. Zaikov and S.M. Lomakin

6.1 Introduction The use of polymer flame retardants has an important role in saving lives. The main flame retardant systems for polymers currently in use are based on halogenated, phosphorus, nitrogen and inorganic compounds. All of these flame retardant systems basically inhibit or even suppress the combustion process by chemical or physical action in the gas or condensed phase. Conventional flame retardants, such as halogenated, phosphorus or metallic additives, have a number of negative attributes. The ecological issue of their application demands the search for new polymer flame retardant systems. Among the new trends in flame retardancy, the following should be pointed out: intumescent systems, polymer nanocomposites, preceramic additives, low-melting glasses, different types of char-formers and polymer morphology modification processing. Brief explanations of the three major types of flame retardant systems (intumescent systems, polymer nanocomposites and polymer organic char-formers) are the subject of this overview. Our environment has a mostly polymeric nature, and all polymers, whether natural or synthetic, will burn, so the use of polymer flame retardants has an important role in saving lives. There are four main families of flame retardant chemicals: (1) Inorganic flame retardants including aluminium trioxide, magnesium hydroxide, ammonium polyphosphate and red phosphorus. This group represents about 50% by volume of global flame retardant production [1]. (2) Halogenated flame retardants, primarily based on chlorine and bromine. The brominated flame retardants (BFR) are included in this group. This group represents about 25% by volume of global production [1]. (3) Organophosphorus flame retardants are primarily phosphate esters and represent about 20% by volume of global production [1]. Organophosphorus flame retardants may contain bromine or chlorine. (4) Nitrogen-based organic flame retardants are used for a limited number of polymers.

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6.2 Mechanisms of Action Depending on their nature, flame retardants can act chemically and/or physically in the solid, liquid or gas phase. They interfere with combustion during a particular stage of this process, e.g., during heating, decomposition, ignition or flame spread. Substitution of one type of flame retardant by another consequently means a change in the mechanism(s) of flame retardancy. Halogen-containing flame retardants act primarily by a chemical interfering with the radical chain mechanism that takes place in the gas phase during combustion. High-energy OH and H radicals formed during combustion are removed by bromine released from the flame retardant. Although brominated flame retardants are a highly diverse group of compounds, the flame retardancy mechanism is basically the same for all compounds. However, there are differences in the flame retardancy performance of brominated compounds, as the presence of such compounds in the polymer will influence the physical properties of the polymer. In general, aliphatic bromine compounds are easier to break down and hence more effective at lower temperatures, but are also less temperature-resistant than aromatic retardants. Aluminium hydroxide and other hydroxides act in a combination of various processes. When heated, the hydroxides release water vapour, which cools the substrate to a temperature below that required to sustain the combustion processes. The water vapour liberated also has a diluting effect in the gas phase and forms an oxygen-displacing protective layer. Additionally, together with the charring products, the oxide forms an insulating protective layer. Phosphorus compounds mainly influence the reactions taking place in the solid phase. By thermal decomposition, flame retardants are converted to phosphorous acid, which in the condensed phase extracts water from the pyrolysing substrate, causing it to char. However, some phosphorus compounds may, similarly to halogens, act in the gas phase as well by a radical trapping mechanism. Interest in flame retarding polymers goes back to the 19th century with the discovery of highly flammable cellulose nitrate and celluloid. In more recent times a large volume of conventional plastics such as phenolics, rigid polyvinyl chloride (PVC) and melamine resins possess adequate flame retardancy. By the 1970s the major flame retardant polymers were the thermosets, namely, unsaturated polyesters and epoxy resins that utilised reactive halogen compounds and alumina hydrate as an additive. There was also a large market for phosphate esters in plasticised PVC, cellulose acetate film, unsaturated polyesters and modified polyphenylene oxide. Alumina trihydrate (ATH) was the largest-volume flame retardant in unsaturated plastics.

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Ecological Issues of Polymer Flame Retardancy Consumption of halogen-containing flame retardant additives in the 1970s was much less than that of other additives. The term ‘halogenated flame retardants’ covers a large number of different organic substances, all with chlorine or bromine in their molecular structure. Bromine and chlorine have an inhibitory effect on the formation of fire in organic materials. Flame retardants are added to plastics and textiles in order to comply with fire safety requirements. The halogenated flame retardant additives include: (1) Dechlorane Plus, (2) a chlorinated acyclic (for polyolefins), (3) tris(dibromopropyl) phosphate, (4) brominated aromatics, (5) pentabromochlorocyclohexane and (6) hexabromocyclododecane (for polystyrene). A number of chlorinated flame retardant products were produced under the Dechlorane trade name. The products include: (1) two moles of hexachlorocyclopentadiene and contained 78% chlorine, (2) Dechlorane Plus, (3) a Diels-Alder reaction product of cyclooctadiene and hexachlorocyclopentadiene with 65% chlorine, (4) a Diels-Alder product with furan and (5) a product containing both bromine and chlorine with 77% halogen developed for polystyrene and acrylonitrile-butadiene-styrene (ABS) materials [1]. In 1985-1986 a German study detected brominated dioxins and furans from pyrolysis of a brominated diphenyl oxide in the laboratory at 510-630 °C [2]. The relevance of these pyrolysis studies to the real hazard presented by these flame retardants under actual conditions of use has been questioned. Germany and Holland have considered a ban or curtailed the use of brominated diphenyl oxide flame retardants because of the potential formation of highly toxic and potentially carcinogenic brominated furans and dioxins during combustion [1, 2]. The issue has spread to other parts of Europe, where regulations have been proposed to restrict their use.

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Handbook of Plastic Films The chemical stability of the substances – particularly in the cases of polybrominated biphenyls (PBB) and polybrominated diphenyl ethers (PBDE) – is also the reason why brominated flame retardants have been the focus of international environmental debate for many years. PBDE and PBB, which are the most stable of the BFR described, are widespread in the environment, are bioaccumulated and accumulate in sediments, where they are degraded only very slowly.

6.3 Halogenated Diphenyl Ethers – Dioxins Chlorinated dibenzo-p-dioxins and related compounds (commonly known simply as dioxins) are contaminants present in a variety of environmental media. This class of compounds has caused great concern to the general public as well as intense interest in the scientific community. Laboratory studies suggest the probability that exposure to dioxin-like compounds may be associated with other serious health effects, including cancer. Conventional laboratory studies have provided new insights into the mechanisms involved in the impact of dioxins on various cells and tissues and, ultimately, on toxicity [1]. Dioxins have been demonstrated to be potent modulators of cellular growth and differentiation, particularly in epithelial tissues. These data, together with the collective body of information from animal and human studies, when coupled with assumptions and inferences regarding extrapolation from experimental animals to humans, and from high doses to low doses, allow a characterisation of dioxin hazards. Polychlorinated dibenzodioxins (PCDD), polychlorinated dibenzofurans (PCDF) and polychlorinated biphenyls (PCB) are chemically classified as halogenated aromatic hydrocarbons. The chlorinated and brominated dibenzodioxins and dibenzofurans are tricyclic aromatic compounds with similar physical and chemical properties, and the two classes are structurally similar. Certain of the PCB (the so-called coplanar or mono-ortho coplanar congeners) are also structurally and conformationally similar. The most widely studied of these compounds is 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). This compound, often called simply dioxin, represents the reference compound for this class of compounds. The structures of TCDD and several related compounds are shown in Figure 6.1 [3]. These compounds are assigned individual toxicity equivalence factor (TEF) values as defined by the international convention ‘Interim Procedures for Estimating Risks Associated with Exposures to Mixtures of Chlorinated Dibenzo-p-Dioxins and Dibenzofurans’ (US Environmental Protection Agency, USEPA, March 1989). Results of in vitro and in vivo laboratory studies have contributed to the assignment of a relative toxicity value. TEF are estimates of the toxicity of dioxin-like compounds relative to the toxicity of TCDD, which is assigned a TEF of 1.0. All chlorinated dibenzodioxins (CDD) and chlorinated dibenzofurans (CDF) with chlorines substituted in the 2, 3, 7 and 8

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Ecological Issues of Polymer Flame Retardancy

2,3,7,8-Tetrachlorodibenzo-p-dioxin

1,2,3,7,8-Pentachlorodibenzo-p-dioxin

2,3,7,8-Tetrachlorodibenzofuran

2,3,4,7,8-Pentachlorodibenzofuran

3,3′,4,4′,5,5′-Hexachlorobiphenyl

3,3′,4,4′,5′-Pentachlorobiphenyl

Figure 6.1 The structures of dioxin and similar compounds

positions are assigned TEF values [1]. Additionally, the analogous brominated dibenzodioxins (BDD) and brominated dibenzofurans (BDF) and certain polychlorinated biphenyls have recently been identified as having dioxin-like toxicity and thus are also included in the definition of dioxin-like compounds. Generally accepted TEF values for chlorinated dibenzodioxins and dibenzofurans are shown in Table 6.1 [4].

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Table 6.1 Toxicity equivalence factors (TEF) for CDD and CDF [4] Compound*

Toxicity equivalence factors, TEF

Mono-, di- and tri-CDD

0

2,3,7,8-TCDD

1

Other TCDD

0

2,3,7,8-PeCDD

0.5

Other PeCDD

0

2,3,7,8-HxCDD

0.1

Other HxCDD

0

2,3,7,8-HpCDD

0.01

Other HPCDD

0

Mono-, di-, and tri-CDF

0

2,3,7,8-TCDF

0.1

Other TCDF

0

1,2,3,7,8-PeCDF

0.05

2,3,4,7,8-PeCDF

0.5

Other PeCDF

0

2,3,7,8-HxCDF

0.1

Other HxCDF

0

2,3,7,8-HpCDF

0.01

Other HPCDF

0

OCDF

0.001

*CDD, chlorinated dibenzodioxin; CDF, chlorinated dibenzofuran. Prefixes: tetra T, penta Pe, hexa Hx, hepta Hp, octa O.

A World Health Organization/International Program on Chemical Safety meeting held in the Netherlands in December 1993 considered the need to derive internationally acceptable interim TEF for the dioxin-like PCB. Recommendations arising from that meeting of experts suggest that in general only a few of the dioxin-like PCB are likely to be significant contributors to general population exposures to dioxin-like compounds [5]. Dioxin-like PCB may be responsible for approximately one-quarter to one-half of the total toxicity equivalence associated with general population environmental exposures to this class of related compounds.

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Ecological Issues of Polymer Flame Retardancy There are 75 individual compounds comprising the CDD, depending on the positioning of the chlorine(s), and 135 different CDF. These are called individual congeners. Likewise, there are 75 different positional congeners of the BDD and 135 different congeners of the BDF. Only seven of the 75 congeners of the CDD or the BDD are thought to have dioxin-like toxicity; these are ones with chlorine/bromine substitutions in, at least, the 2, 3, 7 and 8 positions. Only 10 of the 135 possible congeners of the CDF or the BDF are thought to have dioxin-like toxicity; these also are ones with substitutions in the 2, 3, 7 and 8 positions. While this suggests 34 individual CDD, CDF, BDD or BDF with dioxin-like toxicity, inclusion of the mixed chloro/bromo congeners substantially increases the number of possible congeners with dioxin-like activity. There are 209 PCB congeners. Only 13 of these 209 congeners are thought to have dioxin-like toxicity; these are PCB with four or more chlorines with just one or no substitution in the ortho position. These compounds are sometimes referred to as coplanar, meaning that they can assume a flat configuration with rings in the same plane. Similarly configured polybrominated biphenyls are likely to have similar properties; however, the database on these compounds with regard to dioxin-like activity has been less extensively evaluated. Mixed chlorinated and brominated congeners also exist, increasing the number of compounds considered dioxin-like. The physical/chemical properties of each congener vary according to the degree and position of chlorine and/or bromine substitution. Very little is known about the occurrence and toxicity of the mixed (chlorinated and brominated) dioxin, furan and biphenyl congeners. In general, these compounds have very low water solubility, high octanol-water partition coefficients and low vapour pressure, and they tend to bioaccumulate. Although these compounds are released from a variety of sources, the congener profiles of CDD and CDF found in sediments have been linked to combustion sources [1]. The Hazards Substance Ordinance in Germany specifies the maximum level of chlorinated dibenzodioxins and furans that can be present in materials marketed in Germany. This has been extended to the brominated compounds. The two largest-volume flame retardants, decabromodiphenyl oxide and tetrabromo-bisphenol A, are said to meet these requirements [2]. The International Program for Chemical Safety (IPCS) of the World Health Organization has made several recommendations. Polybrominated diphenyls production (in France) and use should be limited because of the concern over high persistency, bioaccumulation and potential adverse effects at low levels. There are limited toxicity data on deca- and octabromodiphenyls. Commercial use should cease unless safety is demonstrated. For the polybrominated diphenyl oxides, a Task Group felt that polybrominated dibenzofurans, and to a lesser extent the dioxins, may be formed. For decabromodiphenyl oxide, appropriate industrial hygiene measures need to be taken, and environmental exposure minimised by effluent and emission control. Controlled incineration procedures should be instituted. For octabromodiphenyl oxide, the hexa- and lower isomers should

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Handbook of Plastic Films be minimised. There is considerable concern over persistence in the environment and accumulation in organisms, especially for pentabromodiphenyl oxide. There are no regulations proposed or in effect anywhere around the world banning the use of brominated flame retardants. The proposed EU Directive on the brominated diphenyl oxides has been withdrawn. Deca- and tetrabromo-bisphenol A as well as other brominated flame retardants meet the requirements of the German Ordinance regulating the dioxin and furan content of products sold in Germany [6]. The European search for a replacement for decabromodiphenyl oxide in high-impact polystyrene (HIPS) has led to consideration of other bromo-aromatics, such as Saytex 8010 from Albemarle, and a heat-stable chlorinated paraffin from Atochem. The former product is more costly, and the latter, if sufficiently heat-stable, lowers the heat distortion under load (HDUL) significantly. Neither approach has been fully accepted. In September 1994, the USEPA released a final draft of exposure and risk assessment of dioxins and dioxin-like compounds [5]. This reassessment finds the risks greater than previously thought. Based on this reassessment, a picture emerges that tetrachlorodiphenyl dioxins and related compounds are potent toxicants in animals, with the potential to produce a spectrum of effects. Some of these effects may occur in humans at very low levels, and some may result in adverse impacts on human health. The USEPA also concluded that dioxin should remain classified as a probable human carcinogen [5]. Polymer producers have been seeking non-halogen flame retardants, and the search has been successful in several polymer systems. Non-halogen flame retardant polycarbonate/ABS blends are now commercial. They contain triphenyl phosfate or resorcinol diphosfate (RDP) as the flame retardant. Modified polyphenylene oxide (GE’s Noryl) has used phosfate esters as the flame retardant for the past 15-20 years, and the industry recently switched from the alkylated triphenyl phosphate to RDP. Red phosphorus is used with glass-reinforced Polyamide-6,6 (PA-6,6) in Europe, and melamine cyanurate is used in unfilled PA. Magnesium hydroxide is being used commercially in polyethylene wire and cable. The non-halogen solutions present other problems, such as poor properties (plasticisers lower the heat distortion temperature), difficult processing (high loadings of ATH and magnesium hydroxide), corrosion (red phosphorus) and handling problems (red phosphorus). In this chapter, we have tried to present the basic trends in the flame retardants hierarchy.

6.4 Flame Retardant Systems The main flame retardant systems for polymers currently in use are based on halogenated, phosphorus, nitrogen and inorganic compounds (Figure 6.2). Basically, all these flame retardant systems inhibit or even suppress the combustion process by chemical or physical

166

Ecological Issues of Polymer Flame Retardancy action in the gas or condensed phase. To be effective, the flame retardants must be stable at processing temperatures yet decompose near the decomposition temperature of the polymer in order for the appropriate chemistry to take place as the polymer decomposes. Conventional flame retardants, such as halogenated, phosphorus or metallic additives, have a number of negative attributes. The ecological issue of their application requires that new polymer flame retardant systems are sought. Among the new trends in flame retardancy, the use of intumescent systems, polymer nanocomposites, preceramic additives, low-melting glasses, different types of char-formers and polymer morphology modification should be noted [1]. However, the close interactions between the different flame retardant types should be considered in order to achieve synergistic behaviour. A block scheme of polymer flame retardant systems is given in Figure 6.2.

FLAME RETARDANTS (FR)

HALOGENATED FR

PHOSPHORUS FR

Nitrogen-containing FR

ANTIMONY OXIDE Mg HYDROXIDE, ALUMINA TRIHYDRATE, BORON FR

Ecologically friendly flame retardant systems

INTUMESCENT SYSTEMS

POLYMER NANOCOMPOSITES

POLYMER ORGANIC CHARH FORMERS

Preceramics, Low-melting Glass

POLYMER MORPHOLOGY MODIFICATION

Figure 6.2 A block diagram of polymer flame retardant systems

167

Handbook of Plastic Films Brief discussions of the three major types of flame retardant systems (intumescent systems, polymer nanocomposites and polymer organic char-formers) are presented next.

6.5 Intumescent Additives Intumescent behaviour, resulting from the combination of charring and foaming of the surface of burning polymers, is being widely developed for fire retardancy because it is characterised by a low environmental impact. Among alternative candidates, considerable attention has been paid to intumescent materials because they provide fire protection with the minimum of overall fire hazard [7]. Since the first intumescent coating material was patented in 1938 [8], the mechanism of intumescent flame retardancy has referred to the formation of a foam that acts as an insulating barrier between the fire and the substrate. In particular, such intumescence depends significantly on the ratio of carbon, nitrogen and phosphorus atoms in the compound [7, 9]. Although intumescent coatings are capable of exhibiting good fire protection for the substrate, they have several disadvantages, such as water solubility, brushing problems and relatively high cost [10]. The fire retardation of plastic materials is generally achieved by incorporating fire retardant additives into the plastic during processing [11, 12]. Since the processing requires that additives can withstand temperatures up to about 200 °C or more, intumescent systems with insufficient thermal stability cannot be incorporated into various plastics. The phosphate-pentaerythritol system has been investigated and developed as an intumescent material [7]. For example, a systematic study on a mixture of ammonium polyphosphate and pentaerythritol has shown that intumescence occurs on flaming [13, 14]. Thus, new intumescent materials with appropriate thermal stability have been synthesised for better fire retardancy [15]. The most important inorganic nitrogen-phosphorus compound used as an intumescent flame retardant is ammonium polyphosphate, which is applied in intumescent coatings and in rigid polyurethane foams. The most important organic nitrogen compounds used as flame retardants are melamine and its derivatives, which are added to intumescent varnishes or paints. Melamine is incorporated into flexible polyurethane cellular plastics, and melamine cyanurate is applied to unreinforced PA. Guanidine sulfamate is used as a flame retardant for PVC wall coverings in Japan. Guanidine phosphate is added as a flame retardant to textile fibres, and mixtures based on melamine phosphate are used as flame retardants for polyolefins or glass-reinforced PA. All the above-mentioned compounds – ammonium polyphosphate, melamine, guanidine and their salts – are characterised by an apparently acceptable environmental impact.

168

Ecological Issues of Polymer Flame Retardancy Mechanistic studies in PA-6 with added ammonium polyphosphate (APP), ammonium pentaborate (NH4B5O8; APB), melamine and its salts have been carried out using combustion and thermal decomposition approaches [16, 17]. It was shown that APP interacts with PA6 to produce alkylpolyphosphoric ester, which is a precursor of the intumescent char. On the surface of a burning polymer, APB forms an inorganic glassy layer that protects the char from oxidation and hinders the diffusion of combustible gases. Melamine and its salts induce scission of the H–C–C(O) bonds in PA-6, which leads to increased crosslinking and charring of the polymer [17]. APP added at 10-30 wt% to PA-6 is ineffective in the low molecular weight (low molar mass) polymer since the limiting oxygen index (LOI) remains at the level of 23-24 [18] corresponding to non-fire-retarded PA-6. However, APP becomes very effective at loadings of 40 and 50%, where the LOI increases to 41 and 50, respectively. A condensed-phase fire retardant mechanism is proposed for APP in PA-6 [18]. In fact, an intumescent layer is formed on the surface of burning PA-6/APP formulations, which tends to increase the content of APP. Thermal analysis has shown that APP destabilises PA-6, since thermal decomposition is observed at a temperature 70 °C lower than that of pure PA-6 [18]. However, the intumescent layer effectively protects the underlying polymer from the heat flux. Therefore, in the conditions of the linear pyrolysis experiments, the formulation PA-6/APP (40%) decomposes more slowly than pure polymer [18]. These experiments prove the fire retardant action of the intumescent char. Mechanistic studies of thermal decomposition in the PA-6/APP system show that APP catalyses the degradation of the polymer and interacts with it, forming essentially 5-amidopentyl polyphosphate (Scheme 6.1). On further heating, 5-amidopentyl polyphosphate again liberates polyphosphoric acid and produces the char. The intumescent shielding layer on the surface of the polymer is composed of foamed polyphosphoric acid, which is reinforced with the char [18].

Scheme 6.1 Reaction of APP with PA-6

169

Handbook of Plastic Films The effectiveness of APB in high molecular weight PA-6 (Mn = 35,000) is similar to that of APP as measured by oxygen index [19]. In contrast to APP, APB does not give an intumescent layer. Instead, a brown-black glassy-like compact layer is formed. As thermal analysis has shown, APB destabilises PA-6 since the latter decomposes at 50 °C lower. It is likely that freed boric acid catalyses the thermolysis of the Nylon. In contrast to APP, no other chemical interaction of PA-6 and APB was found. In fact, the residue obtained in thermogravimetry in a nitrogen atmosphere for PA-6/APB formulations corresponds to that calculated on the basis of the individual contributions of PA-6 and APB to the residue [19]. It is likely that a molten glassy layer of boric acid/boric anhydride accumulates on the surface of burning polymer, which protects the char from oxidation. This layer reinforced by the char creates a barrier against diffusion of the volatile fuel from the polymer to the flame, which decreases the combustibility of PA-6 [19]. A systematic mechanistic study of halogen-free fire retardant PA-6, via the combustion performance and thermal decomposition behaviour of non-reinforced PA-6 with added melamine, melamine cyanurate, melamine oxalate, melamine phthalate, melamine pyrophosphate or dimelamine phosphate, has been reported [20]. Melamine, melamine cyanurate, melamine oxalate and melamine phthalate promote melt dripping of PA-6, which increases as the additive concentration increases. These formulations self-extinguish very quickly in air, and their LOI increase with increasing concentration (Table 6.2) [20]. The melt dripping effect is very strong in the case of melamine phthalate, where a small amount of the additive (3-10%) leads to large increases in LOI (from 34 to 53). The combustion behaviour of melamine pyrophosphate and dimelamine phosphate is different from that of melamine itself and the other melamine salts (Table 6.2). The former are ineffective at concentrations below 15% and become effective at a loading of 20-30% because an intumescent char is formed on the surface of burning specimens. The mechanism

Table 6.2 Oxygen indices for high molecular weight PA-6 with added melamine or its salts (for pure PA-6, LOI = 24) [20] Additive

Additive concentration (wt%) 3

5

10

15

20

30

Melamine

-

29

31

33

38

39

Dimelamine phosphate

-

23

24

25

26

30

Melamine pyrophosphate

-

24

25

25

30

32

Melamine oxalate

-

28

29

-

33

-

Melamine cyanurate

-

35

37

39

40

40

Melamine phthalate

34

48

53

-

-

-

170

Ecological Issues of Polymer Flame Retardancy of the fire retardant action of both melamine pyrophosphate and dimelamine phosphate is similar to that of APP, since, analogously with ammonia, melamine volatilises, whereas the remaining phosphoric acids produce esters with PA-6, which are precursors of the char [17]. Some part of the freed melamine condenses, probably forming the derivatives melem and melon [21]. Melamine partially evaporates from the composition PA-6/melamine (30%), whereas the other part condenses, giving 8% solid residue at 450 °C. However, similar behaviour with a more thermostable residue is shown by melamine cyanurate. Melamine pyrophosphate, like dimelamine phosphate [17], gives about 15% of thermostable char. As mentioned before, it is likely that a glassy layer of molten boric acid and boric anhydride accumulates on the surface of the burning polymer and protects the char from oxidation. The glass reinforced by the char creates a barrier against diffusion of the volatile fuel from the polymer to the flame, which decreases the combustibility of PA-6 [19]. As infrared characterisation of solid residue and high-boiling products has shown [17], carbodiimide functionalities are formed on thermal decomposition of PA-6 with melamine and its salts. An unusual mechanism of chain scission of PA-6 through CH2–C(O) bonds [22] is likely to become operative in the presence of melamines (Scheme 6.2). The resultant

Scheme 6.2 Mechanism of thermal decomposition of PA-6 in the presence of melamine [22]

171

Handbook of Plastic Films isocyanurate chain ends undergo dimerisation to carbodiimide or trimerisation to Nalkylisocyanurate. Carbodiimide can also trimerise to N-alkylisotriazine. These secondary reactions increase the thermal stability of the solid residue and increase the yield of the char. In order to understand better the chemical reactions that are responsible for the intumescent behaviour of APP/pentaerythritol (PER) mixtures, as model examples, a study of the thermal degradation of pentaerythritol diphosphate (PEDP) was undertaken [23]. PEDP is a model compound for structures identified in APP/PER mixtures heated below 250 °C. Five major degradation steps between room temperature and 950 °C have been identified using thermogravimetric analysis (TGA), and volatile products are evolved in each step. The formation of the foam reaches a maximum at 325 °C, corresponding to the second step of degradation; foam formation decreases at higher temperatures. There are no differences in the TGA or differential scanning calorimetry (DSC) curves in nitrogen or air up to 500 °C. Above this temperature, thermal oxidation leads to almost complete volatilisation in a single step, which is essentially completed at 750 °C. The elucidation of the chemical reactions that occur upon degradation is easier if each step is studied separately. The separation of the steps is accomplished by heating to a temperature at which one step goes to completion, and the following reaction occurs at a negligible rate [23]. The chemical reactions that occur in the first two steps lead to the initial formation of a char-like structure, which will undergo subsequent graphitisation. The first reaction is the elimination of water, with the condensation of OH groups. This overlaps with the elimination of organics when as little as 28% of the possible water has been evolved. This involves essentially complete scission of the phosphate ester bonds and results in a mixture of polyphosphates and a carbonaceous char. Three mechanisms have been proposed in the literature for this reaction [24, 25]: a freeradical mechanism, a carbonium ion mechanism, and a cyclic cis-elimination mechanism. The free-radical mechanism has been ruled out because of the lack of effect of freeradical inhibitors on the rate of pyrolysis [25]. The carbonium ion mechanism is supported by acid catalysis and kinetic behaviour, and may compete with the ciselimination mechanism [24, 25]. The carbonium ion mechanism should occur exclusively if there is no hydrogen atom on the β-carbon atom, as in PEDP, which is necessary for the cyclic transition state of the elimination mechanism. The olefin is generated from the thermodynamically most stable carbonium ion. Hydride migration or skeletal rearrangement may take place to give a more stable carbonium ion of high reactivity. After ring opening in the ionic ester pyrolysis mechanism, a second ester pyrolysis reaction occurs, which could also take place by the cis-elimination mechanism, as shown in Scheme 6.3. 172


Scheme 6.3 Ester pyrolysis mechanism [24]

The formation of char can occur by either free-radical or acid-catalysed polymerisation reactions from the compounds produced in the pyrolysis. For example, the Diels-Alder reaction followed by ester pyrolysis and sigmatropic (1,5) shifts leads to an aromatised structure as shown in Scheme 6.4 [24]. Repetition of these steps can eventually build up the carbonaceous char, which is observed. The reaction pattern shown in Schemes 6.4 and 6.5 should help to provide the structures observed by spectroscopy in the foamed char [24]. These reactions probably occur in an irregular sequence and in competition with other processes; the final products are obtained by some random combination of polymerisation, Diels-Alder condensation, aromatisation, etc. Ester pyrolysis supplies the chemical structures, which build up the charred material through relatively simple reactions [24]. In summary, intumescent behaviour resulting from a combination of charring and foaming of the surface of burning polymers is being widely developed for fire retardancy because it is characterised by a low environmental impact. However, the fire retardant effectiveness of intumescent systems is difficult to predict because the relationship between the occurrence of the intumescence process and the fire protecting properties of the resulting foamed char is not yet understood. 173


Scheme 6.4 Free-radical char formation [24]

174


Scheme 6.5 Acid-catalysed char formation [24]

6.6 Polymer Organic Char-Former There is a strong correlation between char yield and fire resistance. This follows because char is formed at the expense of combustible gases and because the presence of a char inhibits further flame spread by acting as a thermal barrier around the unburned material. Polymeric additives – poly(vinyl alcohol) (PVOH), systems – that promote the formation of char in the PVOH/PA-6,6 system have been studied [26]. These polymeric additives 175

Handbook of Plastic Films usually produce a highly conjugated system – aromatic structures that char during thermal degradation and/or transform into crosslinking agents at high temperatures:

(

CH

CH2)n

OH

CH

CH2

(

CH

CH2)n

CH

CH2

+ H2O

(6.1)

OH

OH

Scission of several carbon-carbon bonds leads to the formation of carbonyl end-groups. For example, aldehyde end-groups arise from the following reaction:

CH

CH2

OH

CH

CH2

n

CH

CH2

OH

OH

(6.2) CH

CH2

CH

CH2

n

OH

CH + CH3

CH

O

OH

The identification of a low concentration of benzene among the volatile products of PVOH has been taken to indicate the onset of a crosslinking reaction proceeding by a Diels-Alder addition mechanism [27]. Clearly, benzenoid structures are ultimately formed in the solid residue, and the IR spectrum of the residue also indicated the development of aromatic structures:

CH2

CH

(a) CH CH

O CH

+ CH2

CH

CH

CH

OH

CH2

CH OH

OH CH2

CH

CH CH CH CH2

(6.3)

CH CH

(b)

CH C O

176

C

CH2

CH OH

Ecological Issues of Polymer Flame Retardancy Acid-catalysed dehydration promotes the formation of conjugated sequences of double bonds (a), and Diels-Alder addition of conjugated and isolated double bonds in different chains may result in intermolecular crosslinking, producing structures that form graphite or carbonisation (b). In contrast to PVOH, PA-6,6 subjected to temperatures above 300 °C in an inert atmosphere is completely decomposed. The wide range of degradation products, which include several simple hydrocarbons, cyclopentanone, water, CO, CO2 and NH3, suggest that the degradation mechanism is highly complex. Further research has led to the generally accepted degradation mechanism for aliphatic polyamides [28]:

O C

O (CH2)x

C

O NH

(CH2)y

NH

H2O

C

n

O (CH2)x

C

OH + NH2

(CH2)y

NH

n

(a) O C

O

(6.4)

(CH2)x + C + *NH (*CH2)y + *NH n

Hydrocarbons, cyclic ketones, esters, nitriles, carbon char

O C

(*CH2)x + CO2 + NH3 + *(CH2)y + *NH

n

The idea of introducing PVOH into PA-6,6 was based on the possibility of hightemperature acid-catalysed dehydration [29]. This reaction can be provided by the acid products of PA-6,6 degradation hydrolysis, which would promote the formation of intermolecular crosslinking and char. Such a system has been called ‘synergetic carbonisation’ because the char yield and flame suppression parameters of the polymer blend of PVOH and PA-6,6 show significant improvement in comparison with those of pure PVOH and PA-6,6 separately [30]. An additional improvement to the flame resistance properties of the PVOH/PA-6,6 system was suggested by means of substitution of pure PVOH by PVOH-ox [poly(vinyl alcohol) oxidised with potassium permanganate (KMnO4)] [30]. Earlier it was reported that the oxidation of PVOH in alkaline solutions occurs through the formation of two intermediate 177

Handbook of Plastic Films complexes. The final step of this process was attributed to the formation of polyvinylketone as a final product of oxidation of the substrate [31]. The fire retardancy approach was made on the basis of the fire behaviour of PVOH-ox samples. Using cone calorimeter tests, a dramatic decrease in the rate of heat release and a significant increase in the ignition time were shown experimentally for the oxidised PVOH in comparison with the original PVOH (see Table 6.3). One reason for this phenomenon may be the ability of PVOH oxidised by KMnO4 (polyvinylketone structures) to act as a neutral and/or monobasic bidentate ligand [32]. Other experimental results (IR and electronic spectra) provide strong evidence of coordination of the ligand (some metal ions Cd2+, Co2+, Cu2+, Hg2+, Ni2+) through the monobasic bidentate mode [33]. Based on the above, the following structure can be proposed for the polymeric complexes (where M = metal):

H C C

C

O

O M

O

O

C

Polymer complex scheme 6A

C C H

n

Table 6.3 Cone calorimeter data for PA-6,6/PVOH [30] Material PVOH

PVOH-ox*

Heat flux (kW/m2)

Char yield (wt%)

Ignition time (s)

Peak RHR (kW/m2)

Total heat release (MJ/m2)

20

8.8

39

255.5

159.6

35

3. 9

52

540.3

111.3

50

2. 4

41

777.9

115.7

20

30.8

1127

127.6

36.9

35

12.7

774

194.0

103.4

50

9. 1

18

305.3

119.8

*Poly(vinyl alcohol) oxidised with potassium permanganate (KMnO4).

178

Ecological Issues of Polymer Flame Retardancy Cone calorimeter combustion tests for PVOH and PVOH oxidised by KMnO4 (Table 6.3) clearly indicate the substantial improvement of fire resistance characteristics for PVOH-ox in comparison with PVOH. PVOH-ox gives about half the peak rate of heat release (peak RHR, kW/m2), when compared with pure PVOH. Even at 50 kW/m2, the yield of char residue for PVOH oxidised by KMnO4 was 9.1% [30]. The result of elemental analysis of PVOH-ox indicates the presence of 1.5% of manganese remaining in this polymeric structure [30]. It has been suggested that the catalytic amount of chelated manganese structure incorporated in the polymer can provide a rapid hightemperature process of carbonisation followed by formation of char [30]. The sample of PVOH-ox displayed even better flame retardant properties due to the catalytic effect of the manganese-chelate fragments on the formation of char (Table 6.3). However, there is a less satisfactory correlation in the determination of total rate of heat release (Table 6.3) [30]. Although, the cone calorimeter measurements indicated a decrease of total heat release for PA-6,6/PVOH and PA-6,6/PVOH-ox in comparison with pure PVOH, the sample of PA-6,6 with PVOH-ox showed a higher value of total heat release than PA6,6 with PVOH (Table 6.3). This fact has been qualitatively explained by the influence of a catalytic amount of chelated manganese structure incorporated in the polymer on the smouldering of the polymer samples. The flame out time for PA-6,6/PVOH-ox is larger than the flame out times of PA-6,6/PVOH and PA-6,6 alone (Table 6.4). The values of average heat of combustion indicate the exothermal process of smouldering provided by chelated manganese structures (Table 6.4). Approximately equal amounts of char yield for PA-6,6/PVOH and PA-6,6/PVOH-ox have been found [30].

Table 6.4 Cone calorimeter data for the heat of combustion and the flame out time for PA-6,6 compositions at a heat flux of 50 kW/m2 Flame out time (s)

Average heat of combustion (MJ/kg)

PA-6,6

512

31.5

PA-6,6/PVOH (80/20, wt%)

429

25.1

PA-6,6/PVOH-ox (80/20, wt%)

747

29.5

Composition

The polymer organic char-former (PVOH system) incorporated in PA-6,6 reduced the peak rate of heat release from 1124.6 kW/m2 (for PA-6,6) and 777.9 kW/m2 (for PVOH) to 476.7 kW/m2 and increased the char yield from 1.4% (for PA-6,6) to 8.7% due to a ‘synergistic’ carbonisation effect. The cone calorimeter was operated at 50 kW/m2 incident flux.

179

Handbook of Plastic Films Cone calorimeter data of PA-6,6 composition with PVOH oxidised by KMnO4 (manganese chelate complexes) show an improvement in the peak rate of heat release from 476.7 kW/m2 (for PA-6,6/PVOH, 80/20 wt%) to 305.3 kW/m2 (for PA-6,6/PVOH-ox, 80/20 wt%) [30]. On the other hand, the exothermal process of smouldering for PA-6,6/PVOHox compositions has been noted [30]. This reaction is evidently provided by chelated manganese structures, which increases the total heat release of PA-6,6/PVOH-ox blend in comparison with PA-6,6/PVOH blend.

6.7 Polymer Nanocomposites Polymer layered silicate (clay) nanocomposites are materials with unique properties when compared with conventional filled polymers. Polymer nanocomposites, especially polymerlayered silicates, represent a radical alternative to conventionally filled polymers. Solventless, melt intercalation of high molecular weight polymers is a new approach to synthesise polymer-layered silicate nanocomposites. This method is quite general and is broadly applicable to a range of commodity polymers from nonpolar polystyrene to strongly polar Nylon. Polymer nanocomposites are thus processable using current technologies and easily scaled to manufacturing quantities. In general, two types of structures are possible: (1) intercalated and (2) disordered or delaminated with random orientation throughout the polymer matrix. Owing to their nanometre size dispersion, the nanocomposites exhibit improved properties compared to the pure polymers or conventional composites. The improved properties include increased modulus, decreased gas permeability, increased solvent resistance and decreased flammability. For example, the mechanical properties of a PA-6 layered-silicate nanocomposite with a silicate mass fraction of only 5% show excellent improvement over those for pure PA-6 [34]. The nanocomposite exhibits 40% higher tensile strength, 68% greater tensile modulus, 60% higher flexural strength and 126% increased flexural modulus [34]. In the polymer industry there is a need for new, more effective and environmentally friendly flame resistant polymers. Recent data on the combustion of polymer nanocomposites indicate that they could be employed for this purpose [35]. There are several proposed mechanisms as to how the layered silicate affects the flame retardant properties of polymers [35]. The first is increased char layer that forms when nanocomposites are exposed to flame. This layer is thought to inhibit oxygen transport to the flame front, as well as gaseous-fuel transport from the polymer, and therefore reduces the heat release rate of the burning surface. At higher temperatures, the inorganic additive has the ability to act as a radical scavenger due to adsorption on to Lewis acid sites. This may interrupt the burning cycle, as radical species are needed to break polymer chains into fuel fragments. The disordered nanocomposites also inhibit the availability of oxygen as a combustible ‘fuel’ species by increasing the path length of these species to

180

Ecological Issues of Polymer Flame Retardancy the flame front. The path length is dramatically increased due to the surface area of the silicates (approximately 700 m2/g for Na+ montmorillonite). There is also a high possibility of alumina-silicate solid-phase catalysis of polymer decomposition, which can dramatically change the overall scheme of the kinetics of the thermal degradation process. Combustibility of some polymer nanocomposite materials was studied using a cone calorimeter [36, 37] under irradiation of 35 kW/m2, which is equivalent to that typical of a small fire [38]. The RHR, which is one of the most important parameters associated with the flammability and combustion of a material, such as those illustrated in Figure 6.3, can be evaluated during this test [36, 37]. Figures 6.3-6.5 compare the results obtained for PA-6,6 as such and for intercalated PA6,6 hybrid produced by using a Carver press to mix PA-6,6 with 5 wt% of Cloisite 15A (montmorillonite modified by ion exchange with dimethyl-ditallow ammonium, a tetraalkylammonium salt from Southern Clay Products Inc.), in an inert nitrogen atmosphere at 260 °C for 30 minutes. It can be seen that the RHR displays a lower maximum peak in the case of the nanocomposite (Figure 6.3), whereas the quantity of heat released (the area under the RHR curve) is about the same for both products, suggesting that their thermal degradation mechanisms are the same [37]. The release of heat by the nanocomposite over a longer period, however, points to its slower degradation. Figures 6.4 and 6.5 on mass loss and specific extinction area illustrate the advantages of nanocomposite over initial PA-6,6 fire behaviour.

Figure 6.3 Rate of heat release versus time for PA-6,6 and PA-6,6 nanocomposite at a heat flux of 35 kW/m2

181


Figure 6.4 Mass loss rate versus time for PA-6,6 and PA-6,6 nanocomposite at a heat flux of 35 kW/m2

Figure 6.5 Specific extinction area (smoke) versus time for PA-6,6 and PA-6,6 nanocomposite at a heat flux of 35 kW/m2

182

Ecological Issues of Polymer Flame Retardancy During the combustion test of the nanocomposite specimen, the carbon layer that formed on its surface from the start grew over time and resisted the heat. The formation of a carbonised layer on the surface of the polymer is a feature of all the nanocomposites studied so far: the pattern illustrated in Figure 6.6 has been reported for other nanocomposites based on polystyrene, polyethylene and polypropylene [37]. Examinations of this residue by X-ray diffraction and transmission electron microscopy (TEM) have revealed an intercalated nanocomposite structure [37]. The TEM image [37] of the carbon residue obtained by combustion of a PA-6,6 nanocomposite in Figure 6.6 shows the intercalation of silicate layers (dark zones) with ‘carbon’ layers (light zones). It should be emphasised that this intercalated structure was derived from the combustion of a delaminated hybrid. It is clear that the disordered structure collapsed during the combustion and was replaced by a self-assembled, ordered structure.

Figure 6.6 TEM image of carbon residue obtained by combustion of PA-6,6 nanocomposite [37] (Reproduced with permission from J.W. Gilman, T. Kashiwagi, C.L. Jackson, E.P. Giannelis, E. Manias, S. Lomakin, J.D. Lichtenhan and P. Jones in Fire Retardancy of Polymers: the Use of Intumescence, Eds., M. Le Bras, G. Camino, S. Bourbigot and R. Delobel, RSC, Cambridge, UK, 1998. Copyright 1998, RSC.)

References 1.

S.M. Lomakin and G.E. Zaikov, Ecological Aspects of Flame Retardancy, VSP International Science Publishers, Utrecht, The Netherlands, 1999, 170.

2.

H. Beck, A. Dross, M. Ende, R. Wolf and P. Trubiroha, Bundesgesundheitsblatt, 1991, 34, 564.

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R.M.C. Theelen in Biological Basis for Risk Assessment of Dioxin and Related Compounds, Eds., M. Gallo, R. Scheuplein and K. Van der Heijden, Banbury Report No. 35, Cold Spring Harbor Laboratory Press, Plainview, NY, USA, 1991.

4.

U.G. Ahlborg, G.C. Becking, L.S. Birnbaum, A. Brouwer, H.J.G.M. Derks, M. Feeley, G. Golor, A. Hanberg, L.C. Larsen, A.K.D. Liam, S.H. Safe, C. Schlatter, F. Waern, M. Younes and E. Yrjanheikki, Chemosphere, 1994, 28, 6, 1049.

5.

Office of Health and Environmental Assessment Office of Research and Development, Estimating Exposure to Dioxin-Like Compounds, EPA/600/6-88/ 005Ca, Cb, Cc, USEPA, Cinncinnati, OH, USA, 1994.

6.

J. Green, Journal of Fire Sciences, 1996, 14, 426.

7.

C.E. Anderson Jr., J. Dziuk Jr., W.A. Mallow and J. Buckmaster, Journal of Fire Sciences, 1985, 3, 151.

8.

H. Tramm, C. Clar, P. Kuhnel and W. Schuff, inventors; Ruhrchemie AG, assignee, US Patent 2,106,938, 1938.

9.

M. Kay, A.F. Price and I. Lavery, Journal of Fire Retardant Chemistry, 1979, 6, 69.

10. D.E. Cagliostro, S.R. Riccitiello, K.J. Clark and A.B. Shimizu, Journal of Fire and Flammability, 1975, 6, 205. 11. R. Delobel, M. Le Bras, N. Ouassou and F. Alistiqsa, Journal of Fire Sciences, 1990, 8, 85. 12. G. Camino, L. Costa and L. Trossarelli, Polymer Degradation and Stability, 1984, 7, 25. 13. G. Camino, G. Martinasso, L. Costa and R. Gobetto, Polymer Degradation and Stability, 1990, 28, 17. 14. S.V. Levchik, L. Costa and G. Camino, Polymer Degradation and Stability, 1992, 36, 31. 15. H. Heinrich, inventor; Chemie Linz (Deutschland) GmbH, assignee, German Patent, DE 4,015,490Al, 1991. 16. S.V. Levchik, G. Camino, L. Costa and G.F. Levchik, Fire and Materials, 1995, 19, 1.

184

Ecological Issues of Polymer Flame Retardancy 17. S.V. Levchik, G.F. Levchik, A.I. Balabanovich, G. Camino and L. Costa, Polymer Degradation and Stability, 1996, 54, 217. 18. S.V. Levchik, L. Costa and G. Camino, Polymer Degradation and Stability, 1992, 36, 229. 19. S.V. Levchik, G.F. Levchik, A.F. Selevich and A.I. Leshnikovich, Vesti Akademii Nauk Belarusi, Seryya Khimichnykh, 1995, 3, 34. 20. S.V. Levchik, G.F. Levchik, G. Camino and L. Costa, Journal of Fire Sciences, 1995, 13, 43. 21. L. Costa, G. Camino and M.P. Luda di Cortemiglia in Fire and Polymers: Hazards Identification and Prevention, ACS Symposium Series No.425, Ed., G.L. Nelson, American Chemical Society, Washington, DC, USA, 1990, 211. 22. S.V. Levchik, L. Costa and G. Camino, Polymer Degradation and Stability, 1992, 43, 43. 23. G. Camino, G. Martinasso and L. Costa, Polymer Degradation and Stability, 1990, 27, 285. 24. G. Camino and S. Lomakin in Fire Retardant Materials, Eds., A.R. Horrocks and D. Price, CRC Press, Boca Raton, FL, 2001, USA. 25. P. Haake and C.E. Diebert, Journal of the American Chemical Society, 1971, 93, 6931. 26. Y. Tsuchiya and K. Sumi, Journal of Polymer Science, 1969, A17, 3151. 27. Polyvinyl Alcohol. Properties and Applications, Ed., C.A. Finch, John Wiley, London, UK, 1973, 622. 28. B.G. Achhammer, F.W. Reinhard and G.M. Kline, Journal of Applied Chemistry, 1951, 1, 301. 29. S.M. Lomakin and G.E. Zaikov, Khimicheskaia Fizika, 1995, 14, 39. 30. G.E. Zaikov and S.M. Lomakin, Plasticheskie Massy, 1996, 39, 211. 31. R.M. Hassan, Polymer International, 1993, 30, 5. 32. R.M. Hassan, S.A. El-Gaiar and A.M. El-Summan, Polymer International, 1993, 32, 39.

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Handbook of Plastic Films 33. R.M. Hassan, M.A. El-Gahami and M.A. Abd-Alla, Journal of Materials Chemistry, 1992, 2, 613. 34. Y. Kojima, A. Usuki, M. Kawasumi, A. Okada, Y. Fukushima, T. Kurauchi and O. Kamigaito, Journal of Materials Research, 1993, 8, 1185. 35. E.P. Giannelis, Advanced Materials, 1996, 8, 1, 29. 36. J.W. Gilman, T. Kashiwagi, M. Nyden, J.E.T. Brown, C.L. Jackson, S. Lomakin, E.P. Giannelis and E. Manias in Chemistry and Technology of Polymer Additives, Eds., S. Al-Malaika, A. Golovoy and C.A. Wilkie, Blackwell Science, Oxford, UK, 1999, 249-265. 37. J.W. Gilman, T. Kashiwagi, C.L. Jackson, E.P. Giannelis, E. Manias, S. Lomakin, J.D. Lichtenhan and P. Jones in Fire Retardancy of Polymers: the Use of

186

7

Interaction of Polymers with the Nitrogen Oxides in Polluted Atmospheres G.B. Pariiskii, I.S. Gaponova and E.Y. Davydov

7.1 Introduction In this chapter, the mechanisms of the reactions of nitrogen oxides with solid polymers are considered. Active participants in reactions with nitrogen oxides are double bonds, the amide groups of macromolecules, alkyl, alkoxy and peroxy radicals, as well as hydroperoxides. The structure of the reaction front during nitration of rubbers has been studied using the electron spin resonance (ESR) imaging technique. The reactions with nitrogen oxides provide a simple way of preparing spin-labelled polymers. The structuralphysical effects on the kinetics and mechanism of reactions of nitrogen dioxide have been demonstrated by the example of filled polyvinylpyrrolidone (PVP). Thermal and photochemical oxidation of polymers have been the subject of detailed and prolonged investigations, because these processes are of major importance for the stabilisation of polymeric materials. However, since the 1960s, the influence of aggressive gases in polluted atmospheres on polymer stability has attracted considerable attention [1]. Among such pollutants in the atmosphere, sulfur dioxide, ozone and the nitrogen oxides stand out as the most deleterious. However, the pursuance of this research has run into a number of problems. The interaction of pollutants with polymers involves the penetration of gases into solids and thus results in a complex kinetic description of the process. Also, as a rule, these reactions are long term for the concentrations of pollutants found in the environment. Consequently, other aging processes occur in the actual conditions of use and storage of polymer materials. To establish the effect of a given aggressive gas on a particular polymer, the reaction is generally studied at pollutant concentrations that are much higher than those actually existing in polluted atmospheres. The results obtained by this means are then linearly extrapolated to the concentrations of reactants found in the atmosphere. This expedient is, a priori, ambiguous in view of the fact that the role of the individual stages of a uniform aging process is changed in conditions of accelerated testing. The problem of non-equivalent kinetics is inherent to polymer reactions in solids [2]. In this case particles existing in different surroundings react with different rate constants. As a result, the most active particles will be removed from the reaction, and the overall rate constant will decrease with time. On the other hand, relaxation processes in polymers restore the initial distribution of particles and so their reactivity. Thus the kinetics will

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Handbook of Plastic Films depend on the relation between the rate of the chemical reaction and the rate of the relaxation processes [3]. This fact also makes it necessary to reconsider critically the validity of extending the results of accelerated tests for polymer ageing. This chapter is devoted to a consideration of the results obtained in studies of the interactions of nitrogen oxides with polymers. There are eight nitrogen oxides, but only NO, NO2 and N2O4 are actually important as pollutants. Nitric oxide (NO) exists as a free radical, but it is reasonably stable in reactions with organic compounds. The paramagnetic nitrogen dioxide (NO2) is more active compared with NO. This gas is universally present in equilibrium with its dimer molecule: 2NO2

N2O4

with Kp = 0.141 atm at 298 K [4]. Nitrogen dioxide absorbs light in the near-UV and visible spectral range. Excited molecules are generated by light with λ > 400 nm. The dissociation of NO2 into an oxygen atom and NO by light with λ < 365 nm takes place with a quantum yield near to unity [5].

7.2 Interaction of Nitrogen Dioxide with Polymers Detailed investigations of the reactions of NO2 with various polymers have been carried out by Jellinek and co-workers [1, 6]. The degradation of polymer films has been studied at different pressures of NO2, in mixtures of NO2 with air, under the combined action of light (λ > 280 nm), O2 and NO2. Based on the data obtained, Jellinek classified all polymers into three groups: (1) vinyl polymers – polyethylene (PE), polypropylene (PP), polystyrene (PS), polymethyl methacrylate (PMMA), polyacrylonitrile (PAN), polyvinyl chloride (PVC) and polyvinyl fluoride (PVF); (2) polymers with non-saturation – primarily rubbers; (3) polyamides, polyurethanes and polyamidoimides. The presentation of the results in this section will be carried out according to this classification.

7.2.1 Vinyl Polymers: PE, PP, PS, PMMA, PAN, PVC and PVF The linear extrapolation of the results of accelerated tests to NO2 concentrations likely to be found in the atmosphere (1-5 ppm) predicts that polymer properties will be essentially constant for a long time.

188

Interaction of Polymers with the Nitrogen Oxides in Polluted Atmospheres The first investigations of the interaction of NO2 with PE and PP were performed by Ogihara and co-workers [7, 8] at 298-383 K and NO2 pressure of 20 kPa. It was established that NO2 reacts at room temperature with the >C=C< double bonds originally contained in PE with the formation of dinitro compounds and nitronitrites by the following reactions: >C=C< + NO2 → >C•−C(NO2)
C(NO2)−C(NO2)
C(ONO)−C(NO2)
373 K. The following mechanism was postulated: RH + NO2 → R• + HNO2

(7.4)

R• + NO2 → RNO2

(7.5)

R• + ONO → RONO

(7.6)

RONO → RO• + NO

(7.7)

The reactions of RO• radicals lead to the formation of macromolecular nitrates, alcohols and carbonyl compounds. The activation energy of the NO2 addition to the double bonds of PE is 8-16 kJ/mol. The activation energies of H atom abstraction are 56-68 kJ/mol in PE and 60 kJ/mol in PP. PE, PP, PAN and PMMA change their characteristics slightly at high concentrations of NO2 (1.3-13 kPa) even under the joint action of pollutant, O2 and UV light [6]. Nitrogen dioxide is capable of abstracting tertiary hydrogen atoms in PS with a low rate (P = 20-80 kPa), with the formation of nitro and nitrite side groups [reactions (7.5) and (7.6)]. This process is accompanied by main-chain scission [9, 10]. The combined action of 0.3 kPa NO2 and light (λ > 280 nm) on PS does not lead to mainchain decomposition in the early stage (10 h), after which the degradation process is developed with a constant rate. PVC and PVF show a minor loss of chlorine and fluorine atoms on exposure to NO2 [1, 6]. An attempt to investigate quantitatively the ageing of PS and poly-tert-butyl methacrylate (P-t-BuMA) has been taken by Huber [11]. The research was performed in a flow system of air containing 60-900 ppm of NO2 and/or 60-900 ppm SO2 at 300 K under the simultaneous action of light with λ > 290 nm. The degradation of P-t-BuMA films was

189

Handbook of Plastic Films expressed in terms of the quantity of ruptures per 10,000 monomer units, α. The kinetic dependence is represented by the equation:

α = (P/Q)[exp(Qt) – 1]

(7.8)

where P and Q are constants. This equation describes an autoaccelerated process. As Q → 0, so α → Pt, that is, the degradation proceeds with a constant rate. The P and Q values decrease as the film thickness increases, and yet the P value diminishes more strongly than Q. Therefore, the accelerated character of the degradation appears more clearly for thin films. PS degradation in the same conditions proceeds much more slowly and has a more pronounced autoacceleration (Table 7.1).

Table 7.1 The P and Q values for P-t-BuMA and PS film degradation under the action of 100 ppm NO2 and light in air Film thickness (mg/cm2)

P × 104 (h–1)

Q × 104 (h–1)

P-t-BuMA

1.4

0.071

0.026

P-t-BuMA

2.6

0.050

-

P-t-BuMA

2.8

0.041

0.017

PS

1.4

0.034

0.036

Polymer type

The autoaccelerated character of P-t-BuMA degradation was linked to the ester group decomposition, with isobutylene formation, which gives free radicals in the reaction with NO2 and thus promotes the degradation process. The IR spectrum of PS shows peaks corresponding to carbonyl (1686 cm–1) and hydroxyl (3400 cm–1) groups after exposure to a mixture of NO2 (100 ppm) and air. No bands connected with the insertion of NO2 into the P-t-BuMA and PS macromolecules were observed. It is believed that the following sequence of reactions occurs in PS [11]: RH + NO2 → R• + HNO2

190

(7.9)

R• + O2 → RO2•

(7.10)

RO2• + RH → ROOH + R•

(7.11)

R• + NO2 → RNO2

(7.12)

R• + NO2 → RONO

(7.13)

Interaction of Polymers with the Nitrogen Oxides in Polluted Atmospheres RONO → RO• + NO

(7.14)

ROOH + NO → RO• + •OH + NO

(7.15)

ROOH + hν → RO• + •OH

(7.16)

RO• → degradation + R•

(7.17)

Hydroperoxide decomposition under the action of NO and light gives rise to accelerated PS degradation.

7.2.2 Non-Saturated Polymers Among these are primarily rubbers. These polymers are far more sensitive to NO2 action that the polyolefins. Appreciable degradation of macromolecules as well as moderate crosslinking were observed for rubbers. Comprehensive kinetic investigations of butyl rubber (a copolymer of isobutylene with 1.75% isoprene) in an NO2 atmosphere (NO2 pressure 1.33-133 kPa), in a mixture of NO2 and air, and under the combined action of NO2, O2 and UV light (λ > 280 nm) have been performed by Jellinek and co-workers [12, 13]. According to the proposed mechanism, the total number of chain ruptures is made up of three parts: (1) ruptures that are due to the NO2 interaction only, (2) ruptures that result only from the action of O2, and (3) ruptures that are caused by the combined action of NO2 and O2. The kinetic dependence of the degree of degradation, α = (1/DPt − 1/DP0), is described by the following equation:

α = kef′ t2 + kef″ [NO2][1 − exp(−k3t)]

(7.18)

where DP0 and DPt are the number-average degrees of polymerisation in the original and degraded macromolecule (at time t). The first term in equation (7.18) is connected with ruptures of macromolecules due to photolysis of the reaction products (hydroperoxides, nitro and nitrite groups). The second term describes the degradation for the (NO2 + O2) system in the absence of light. It should be noted that the assumed mechanism [12, 13] is very complex, involving a wealth of elementary reactions, the rate constants of which are unknown in the solid phase. It is well known that the reaction products can be more active relative to the nitrogen oxides than the original polymer. In connection with this, the application of various physical-chemical techniques is extremely important to investigate the

191

Handbook of Plastic Films degradation process. The development of methods to study the movement of the reaction front across the polymer sample is also required. The use of the ESR technique permits one to draw additional conclusions on the mechanisms of the interaction of polymers with nitrogen oxides from the structure of the resulting free radicals and the kinetics of their formation. The interaction of polyisoprene (PI) with NO2 gives rise to di-tert-alkylnitroxyl radicals [14]. The ESR spectra of these radicals show a characteristic anisotropic triplet signal with a width of 2A||N = 6.2 mT and g|| = 2.0028 ± 0.0005 in the solid polymer, and a triplet with aN = 1.53 ± 0.03 mT and g = 2.0057 ± 0.0005 in dilute solutions. These macroradicals are stable in the absence of NO2 during storage for many months in both inert atmosphere and air. The proposed scheme to explain the formation of these radicals involves three main stages: (1) generation of N-containing alkyl radicals, (2) synthesis of tertiary macromolecular nitroso compounds, and (3) spin-trapping of the tertiary alkyl or allyl radicals: ~CH2-C(CH3)=CH-CH2~ + NO2 ~C•(CH3)-CH(ONO)-CH2~ + RH

~C•(CH3)-CH(ONO)-CH2~ ~C(CH3)(NO)-CH(OH)-CH2~

~C(CH3)(NO)−CH(OH)−CH2∼ + •Rtert → Rtert−N(O•)−Rtert

(7.19) (7.20) (7.21)

The reactions of NO2 with double bonds provide a very simple and rapid method for the synthesis of spin-labelled macromolecules of rubbers. The temperature variation of the rotational mobility of macromolecules in block PI has been studied using spin-labelled samples [14]. The temperature dependence of the rotational correlation time τ is described by τc = τ0 exp(E/RT). The τc values within the fast motion region (τc < 10–9 s) are well described by the parameters E = 34.7 kJ/mol and log τ0 = −14.2. The spatial distribution of these macromolecular nitroxyl radicals allows the estimation of the spatial distribution of the nitration reaction in bulk PI. The possibilities of the ESR imaging technique to determine the form of the reaction front of PI nitration has been considered [15]. The ESR imaging spectra were registered in an inhomogeneous magnetic field on cylindrical samples of 0.4 cm diameter and 1 cm height at NO2 and O2 concentrations of 1 x 10–4 to 2 x10–3 mol/l and 2 x 10–3 to 1.4 x 10–2 mol/l, respectively. The spatial distributions of R2NO• radicals at various reaction times are shown in Figure 7.1. The width of the distribution varies over 2030% for 740 h. The maximum concentration of nitroxyl radicals is observed in the superficial layer, and it progressively decreases towards the centre. The width of this layer is ~1 mm, and radicals are unavailable in the sample centre. The nitroxyl radical yield with respect to absorbed NO2 molecules is 0.01. The shape and variation of the distribution in the presence of O2 are the same as in pure NO2, but the reaction front is narrower. The rate of R2NO• formation in the presence of O2 is much lower than 192

Interaction of Polymers with the Nitrogen Oxides in Polluted Atmospheres

Figure 7.1 ESR-imagination of nitroxyl radicals distribution in cylindrical sample (l = 10 mm, d = 4 mm) of PI in course of interaction with nitrogen dioxide ([NO2]) = 8.8 × 10-4 mole/l; 30 min; 20 °C). The contour lines correspond to vertical sections with equal [R2NO*]. The concentrations are given in arbitrary units ([R2NO*]max = 0.125 au).

in pure NO2 at the cost of a decay of alkyl radicals in the reactions with O2: W(NO2)/ W(NO2+O2) = 102. The distribution at a fixed distance from the surface is likely determined by macrodefects in the sample volume, namely, the availability of cracks and porosity. The front form is determined by the ‘membranous’ regime of the nitration process rather than by structural changes. PMMA, which in itself is stable on exposure to NO2, enters into reactions after previous irradiation by UV light at 293 K [16]. The photolysis of PMMA induces the formation of double bonds as a result of ester group decomposition. The ESR spectrum observed after exposure of samples to NO2 is shown in Figure 7.2. The spectrum represents the superposition of the signals of two nitroxyl radicals at low frequencies of rotational mobility (10–9 s < τc < 10–7 s): •

Dialkylnitroxyl radicals ~C(CH3)(COOCH3)–N(O•)–C(CH3)(CHO)–CH2~ give an anisotropic triplet signal with hyperfine interaction (HFI) constant A||N = 3.2 ± 0.1 mT and g|| = 2.0026 ± 0.0005;

193


Figure 7.2 ESR spectrum of nitroxyl radicals generated by NO2 in PMMA pre-irradiated by UV light at 298 K.

•

Acylalkylnitroxyl radicals ~C(CH3)(COOCH3)−CH(OH)–C(CH3)[N(O•)COOCH3]–CH2~ give a triplet signal that is characterised by A||N = 2.1 ± 0.1 mT and g|| = 2.0027 ± 0.0005.

The free-radical process of NO2 interaction with PMMA containing double bonds is represented by the scheme opposite. The formation of nitroxyl radicals testifies to the fact that main-chain decomposition by reaction (7.24) and side-chain ester group cleavage by reaction (7.26) take place in the polymer. Thus, the interaction of NO2 with double bonds is able to initiate free-radical reactions of polymer degradation when hydrogen atom abstraction reactions from C–H bonds are inefficient.

194


(7.22)

(7.23)

(7.24)

(7.25)

(7.26)

(7.27)

195


7.2.3 Polyamides, Polyurethanes, Polyamidoimides Polymers with amide and urethane groups in the macromolecules represent a special class of materials that are sensitive to NO2. The action of NO2 at pressures of 0.5-2 mm Hg on polyamide-6,6 films with various morphologies has been studied by Jellinek and co-workers [17, 18]. It was shown that a degradation process takes place. The degradation of polyamide is a diffusion-controlled reaction and depends on the degree of crystallinity and the sizes of the crystallites. The process is inhibited by small quantities of benzaldehyde or benzoic acid. Increase of the degradation rate was observed during the combined action of NO2, air and UV light. The assumed mechanism of the process as follows: ~CO–NH~ + NO2 → ~CO–N•~ + HNO2

(7.28)

~CO–N•~ + NO2 → ~CO–N(NO2)~

(7.29)

~CO–N•–CH2~

[~CO–N=CH2 + •CH2~] → chain rupture

(7.30)

There is reason to believe that only a small quantity of amide groups, not linked by the hydrogen bonds, enter into the reaction. These groups can be interlocked by benzoic acid with the formation of the following structure:

HO

O

••••••

NH

••••••

CO

C Ph

Research into the effect of NO2 on polyamide textiles has been described [19]. The exposure of samples in an NO2 atmosphere of low concentration at room temperature for 100 h does not lead to a decrease in the whiteness and tensile strength. However, these characteristics are decreased at higher temperatures. The availability of nitrogen oxides in the air under the action of UV light results in the additional degradation of textiles. The conversion of N−H bonds by nitrogen dioxide is also inherent to polycaproamide (PCA). The UV spectra of PCA films display features of absorption at 390-435 nm during exposure to NO2 at concentrations of 10–4 to 10–3 mol/l [20]. The absorption bands were assigned to nitrosamide groups resulting from N−H group conversion. This

196

Interaction of Polymers with the Nitrogen Oxides in Polluted Atmospheres conclusion is confirmed by IR spectroscopy. The intensity of the band with ν = 3293 cm–1, which is associated with stretching vibrations of the hydrogen-linked N−H groups, decreases sharply. The intensities of the amide I (ν = 1642 cm–1) and amide II (ν = 1563 cm–1) bands, which are characteristic of PCA, also decrease. Instead of these bands, absorptions at ν = 1730 cm–1, which corresponds to the absorption of C=O groups, and at ν = 1504 and 1387 cm–1, which correspond to stretching vibrations of N=O groups of nitrosamides, appear in PCA. Thus, nitrosation through the amide group is the main process of PCA transformation in an NO2 atmosphere, which leads to disintegration of the system of hydrogen bonds. Taking into account the equilibrium: NO+NO3−

N2 O 4

(7.31)

2NO2

the formation of nitrosamides can be represented as follows: ∼CONHCH2∼ + N2O4 → ∼CON(NO)CH2∼ + HNO3

(7.32)

It was found that the initial rate of nitrosamide group accumulation is proportional to [NO2]n, where n ≈ 2. As was shown by ESR, the reaction of NO 2 with N−H bonds also produces acylalkylnitroxyl macroradicals: ∼CONHCH2∼ + NO2

~CON(ONO)CH2~

ΝΟ2

→ ∼CON•CH2∼ → ~CON(ONO)CH2~

(7.33)

~CON(O•)CH2~ + NO

(7.34)

–HNO2

As well as in PCA, the interaction of NO2 with PVP leads to UV bands characteristic of the nitrosamide group [20]. The formation of these groups in PVP is associated with splitting of the side-chain cyclic fragments from the main chain:

PVP + NO2 –HNO2

(R1)

CH2 CHCH2 N

O

(7.35)

197


Types of Hot Runner Systems N

R1

CH2CHCH2

O

+

(7.36)

Thereafter, the reaction of NO 2 with the cyclic double bond gives rise to the nitrosamide product: NO2 N

O

O2 N

N

O R1

+ 2NO2

NO

O NOR O2N

N

O2 N

O OR

N

O

(7.37)

+

The ESR spectra observed when NO2 (10–4 to 10–3 mol/l) reacts with PVP represent the superposition of the signals of acylalkylnitroxyl radicals (A||N = 1.94 mT, g|| = 2.003) and iminoxyl radicals (A||N = 4.33 mT, A⊥N = 2.44 mT, g|| = 2.0029, g⊥ = 2.0053). The formation of these iminoxyl radicals is initiated by the hydrogen atom abstraction reaction from C−H bonds that are in the α-position with respect to the amide group by reaction (7.35) and the following reaction: PVP + NO2 –HNO2

(R2)

CH2 CHCH2 N

O

(7.38)

Nitric acid is thought to be the source of nitrogen oxide in the given system: 2HNO2 → H2O + NO2 + NO

198

(7.39)

Interaction of Polymers with the Nitrogen Oxides in Polluted Atmospheres The recombination of NO and R1• initiates the formation of iminoxyl radicals:

R1 + NO

CH2 CHCH2 N

O

–HNO2 CH2 CHCH2

CH2 CHCH2

(7.40)

+NO2 N

N

O

O

ON

The formation of NO explains the production of acylalkylnitroxyl radicals as follows:

R2

CH2 C CH2 N

CH2 C CH2 O

N

NO O

+ R1 (R2)

CH2 C CH2

(7.41)

R N

N O O

An approach based on the analysis of the composition of nitrogen-containing radicals in PVP depending on the content of filler aerosil has been put forward to elucidate the

199

Handbook of Plastic Films effect of polymer structural-physical organisation [21]. The influence of structural organisation may be manifest in the rates of iminoxyl and acylalkylnitroxyl radical formation. Filling gives the possibility of changing the physical structure of the polymer in interface layers. The decrease in the molecular packing density as a result of filling can accelerate the rate of reaction (7.41) involving breakage of the pyrrolidone cycle. The packing density decrease enhances the reaction rate through the promotion of mutual diffusion of R• macroradicals and nitroso compounds. It is well established that the quantitative relation between iminoxyl and acylalkylnitroxyl radicals is changed with the degree of filling. Formation of a gel fraction has been detected on exposure of polyurethane films to NO2 [21]. Degradation of macromolecules simultaneously takes place in the sol fraction of the samples. The changes in the destruction degree and the gel-fraction yield with time are complex to analyse. The gel fraction at 333 K and P(NO2) = 20 mm Hg initially increases up to 20% and thereafter reduces to nearly zero. The number of scissions in the sol fraction increases at the beginning, subsequently reduces, and then grows again. The exposure of films to NO2 is accompanied by the release of CO2 at all temperatures. The IR spectra in this case show N−H bond (3300 cm–1) consumption. The proposed mechanism includes the reaction of NO2 with the N−H groups of both the main chain and the side branches: ~OCO–NH–CH2~ + NO2 → ~OCO–N•–CH2~ + HNO2

(7.42)

~OCO–N(RH)–CH2~ + NO2 → ~OCO–N(R•)–CH2~ + HNO2

(7.43)

The recombination of ~OCO–N•–CH2~ (R1•) and ~OCO–N(R•)–CH2~ (R2•) results in polymer crosslinking. The conversion of R1• causes macromolecule decomposition and CO2 release. The exposure of polyurethane films to an NO2 atmosphere or a mixture of NO2 with air leads to the progressive reduction of the tensile strength limit [22]. The influence of NO2 on the mechanical properties of polyamidoimide films has been considered at 323 K and P(NO2) = 13 kPa [23]. The temperature dependences of the storage modulus E′ and loss modulus E″ have been obtained for various times of NO2 exposure. A nonmonotonic decrease of E′ was observed at 473 K, but the maximum of the E′ temperature dependence appears at approximately the same temperature. Samples exposed to NO2 for eight days show an increase in E′ at the glass transition temperature (563 K). The phenomenon is associated with chain breakage and the recombination of macroradicals giving rise to crosslinking. Chain breakage is supported by results obtained by the present authors. The ESR spectra of polyamidoimide exposed to an NO2 atmosphere show the formation of iminoxyl radicals with spectral parameters that are close to those of PVP iminoxyl radicals. The possible mechanism of their formation includes the main-chain decomposition step as follows: 200


NH

O

CO

CO

NO2

N

–HNO2

CO

O

N

CO

CO N CO

(7.44) R + O

N

CO

CO N CO

O

N

CO

NO2

O

O

N

CO

N

CO

ON

NO

O

NO

N

CO

RH

O

O

HO

O

N HO

(7.45)

NO

NOH

CO

NO2 –HNO2

O

N

CO

HO

201


7.3 Reaction of Nitric Oxide with Polymers Nitric oxide is a low-activity free radical and can be used as a ‘counter’ of radicals in gas and liquid phases. The reactions of alkyl radicals with NO lead to the formation of nitroso compounds, which are spin traps. Thus, the initiation of free-radical reactions in solid polymers in the presence of nitric oxide provides further information on their mechanism. It is well established that at room temperature NO is not able to remove allylic and tertiary hydrogen atoms and add to isolated double bonds [24-26]. There are discrepant opinions on the capability of NO to react with low molecular weight (low molar mass) dienes and polyenes. Some authors believe that NO is able to add to dienes and polyenes, for example, to substituted o-quinonedimethane, phorone and β-carotene, with the formation of free radicals [27-29]. Another way of looking at these reactions lies in the fact that they can be initiated by NO2 impurities [25, 26]. This section of the review is concerned with radical reactions in polymers, induced by photo- and γ-irradiation, in the presence of nitric oxide. Irradiation of powdered PMMA in an NO atmosphere by the light of a mercury lamp results in the formation of three typesof macromolecular nitroxyl radicals [30]. The radical composition depends on temperature and the wavelength of the light. If the photolysis of PMMA is performed at room temperature using unfiltered light from a high-pressure mercury lamp, acylalkylnitroxyl radicals R1N(O•)C(=O)R2 are formed. The irradiation of samples at 383 K produces, in addition to acylalkylnitroxyl radicals, dialkylnitroxyl macroradicals R1N(O•)R2. Finally, if PMMA irradiation is carried out at room temperature using UV light with 260 nm < λ < 400 nm, the signal of iminoxyl radicals R1C(=NO•)R2 is also observed in the ESR spectrum. Acetyl cellulose (AC) under action of light at room temperature gives rise to dialkyl- and acylalkylnitroxyl radicals [30]. The removal of NO from the samples leads to increasing of components of acylalkylnitroxyl radicals in the ESR spectrum. This phenomenon is probably connected with the formation of diamagnetic complexes of NO with acylalkylnitroxyl radicals. Dialkylnitroxyl radicals do not form complexes of this type at 298 K. The γ-irradiation of PMMA at room temperature, as a photolysis, brings about the formation of acylalkylnitroxyl radicals [30]. Iminoxyl radicals also arise, but their quantity is essentially smaller than in AC under γ-irradiation. The formation of nitroxyl radicals during photolysis as well as in the course of radiolysis of PMMA and AC in the presence of NO is explained by the following scheme: polymer

202

γ, hν

→

R1• (R•2)

(7.46)

R1• + NO → R1NO

(7.47)

R2• + R1NO → R1N(•O)R2

(7.48)

Interaction of Polymers with the Nitrogen Oxides in Polluted Atmospheres The structure of nitroxyl radicals is determined by the nature of the free radicals that are generated by γ- and photo-irradiation of PMMA and AC. Photo-irradiation of PMMA and AC leads to the formation of •C(O)OCH3 radicals, which give in turn acylalkylnitroxyl radicals by reactions (7.46)-(7.48). Dialkylnitroxyl radicals arise when two macroradicals are involved in the reactions with NO. The free-radical reactions in solid polymers in the presence of NO are of particular significance for the preparation of spin-labelled polymers. This method has become particularly important for chemically inert, rigid and insoluble polymers, for instance, polytetrafluoroethylene (PTFE), because of the difficult problem of introducing spin labels by chemical reactions of nitroso compounds, nitrons or nitroxyl biradicals [31]. Oriented PTFE films γ-irradiated at room temperature in air after prolonged NO exposure contain nitroxyl radicals whose ESR spectra are displayed in Figure 7.3 [32].

Figure 7.3 ESR spectra of perfluoronitroxyl radicals in PTFE films stretched to fourfold increase in its length at parallel (a) and perpendicular (b) orientation of magnetic field directions.

203

Handbook of Plastic Films The rotation of the samples leads to changes in angle α between the magnetic field and stretching directions. At 298 K and α = 0°, the ESR spectrum is a triplet consisting of quintets with splitting of AN = 0.46 mT and AF = 1.11 mT, and g|| = 2.0060. At α = 90°, the splittings increase to AN = 1.12 mT and AF = 1.61 mT, and g⊥ = 2.0071. The radicals observed are nitroxyl radicals with the following structure: ~CF2–N(O•)–CF2~. A possible mechanism for nitroxyl macroradical synthesis has been suggested [32]. In an oxygen-containing atmosphere, some of the middle alkyl radicals formed in the course of γ-irradiation are capable of decomposing with rupture of the main chain as a result of the high energy transfer to these radicals: ~CF2–CF2–C•F–CF2~ → ~CF2• + CF2=CF–CF2~

(7.49)

In the presence of oxygen, the terminal alkyl macroradicals can be oxidised to form terminal peroxy radicals: ~C•F2 + O2 → ~CF2OO•

(7.50)

Under the action of NO on samples containing neighbouring terminal double bonds and peroxy radicals, the latter are converted into macromolecular nitrates and nitrites:

~CF2

CF2OO• + NO

~CF2OONO …

(7.51) ~CF2ONO2

~CF2O• + NO2

(7.52) ~CF2O• + NO2

~CF2O• + NO

~CF2ONO

(7.53)

Decomposition of alkoxy radicals in an NO atmosphere causes the synthesis of terminal nitroso compounds: NO

~CF2–CF20• → ~C•F2 + CF2O → ~CF2NO

(7.54)

The adjacent terminal double bonds and terminal nitroso compounds formed can enter into a reaction to synthesise nitroxyl radicals: ~CF2N=O + CF2=CF–CF2~ + NO → ~CF2–N(O•)CF2–CF(NO)–CF2~

204

(7.55)

Interaction of Polymers with the Nitrogen Oxides in Polluted Atmospheres The advantage of the suggested method for the preparation of spin-labelled polymer is that the nitroxyl free-radical fragment is incorporated in the basic macromolecular chain without disturbing its orientation. An analogous investigation of the action of NO on γ-irradiated tetrafluoroethylenehexafluoropropylene copolymer (TFE-HFP) containing 13 mol% of HFP units has been performed [33]. After exposure of powders and films of TFE-HFP to a dose of 105 Gy in air, there are three types of stable peroxide macroradicals: (1) End radicals ~CF2–CF2O2• (denoted ReO2•); (2) Secondary mid-chain radicals ~CF2–CF(OO•)–CF2~ (denoted RcO2•); (3) Tertiary mid-chain radicals ~CF2–C(CF3)(OO•)–CF2~ (denoted RtO2•). Their total concentration is [RO2•] ≈ 3 x 10–3 mol/kg, of which (25 ± 5)% are tertiary peroxy radicals. Under the action of NO on evacuated samples, the radicals decay to form peroxy radical conversion products and tertiary nitroso compounds: ~CF2–C(CF3)(NO)–CF2~ Heating these samples in vacuum up to 473 K leads to the formation of nitroxyl radicals of the type: ~CF2–N(O•)–CF2~ The nitroxyl radicals appear in the temperature range where the tertiary nitroso compounds decay in vacuum with the generation of tertiary alkyl radicals (Rt•). The first step of Rt• formation is β-scission by the reaction: ~CF2−C•(CF3)−CF2−CF2~ → ~CF2−C(CF3)=CF2 + •CF2−CF2~

(7.56)

In the presence of NO formed upon decomposition of the tertiary nitroso compounds, the terminal alkyl radicals can be converted into terminal nitroso compounds, which react with the adjacent double bonds to form nitroxyl macroradicals: NO + •CF2−CF2~ → ON−CF2−CF2~

(7.57)

~CF2–C(CF3)=CF2 + ON−CF2−CF2~ → ~CF2–C•(CF3)–CF2–N(O•)–CF2~ → +X

→ ~CF2–C(CF3)(X)–CF2–N(O•)–CF2~

(7.58)

where X is NO or NO2. Nitrogen dioxide can be formed by the interaction of NO with RO2• in reaction (7.51). 205

Handbook of Plastic Films One more type of nitroxyl macroradical is observed if a powdered TFE-HFP, γ-irradiated in air and exposed to NO with subsequent evacuation, is subjected to light irradiation at λ > 260 nm at 298 K [34]. In this case, a new type of nitroxyl macroradical with the structure ∼CF2–N(O•)–CF3 was registered. The following scheme provides an explanation for the radical formation in TFE-HFP under the action of light: RcO2• + NO → [RcOONO] → RcONO2

(7.59)

ReO2• + NO → [ReOONO] → ReONO2

(7.60)

Rt• + NO → RtNO

(7.61)

RcONO2 + hν → RcO• + NO2

(7.62)

ReONO2 + hν → ReO• + NO2

(7.63)

RtNO + hν → Rt• + NO

(7.64)

RcO• → ~CF2–CFO + •CF2–CF2~

(7.65)

ReO• → ~CF2–CF2• + CF2O

(7.66)

~CF2–CF2• + NO → ~CF2–CF2–NO

(7.67)

~CF2−C•(CF3)−CF2~ + NO2 → ~CF2–C(CF3)(ONO)–CF2~

(7.68)

~CF2–C(CF3)(ONO)–CF2~ + hν → ~CF2–C(CF3)(O•)–CF2~ + NO

(7.69)

~CF2–C(CF3)(O•)–CF2~ → •CF3 + ~CF2–C(=O)–CF2~

(7.70)

•

CF3 + NO → CF3NO

(7.71)

•

CF3 + ON–CF2–CF2~ → CF3–N(O•)–CF2–CF2~

(7.72)

CF3NO + •CF2–CF2~ → CF3–N(O•)–CF2–CF2~

(7.73)

It is obvious that the simultaneous action of light and NO on TFE-HFP results in macromolecular decomposition. Polymer hydroperoxides are active participants in degradation processes. The reactions of nitrogen oxides with these particles are of interest to understand the mechanism of the influence of pollutants on polymer stability in the course of the oxidation process. The phenomenon of hydroperoxide decomposition under the action of NO was discussed

206

Interaction of Polymers with the Nitrogen Oxides in Polluted Atmospheres long ago using both macromolecular peroxides and their low molecular weight analogues [34]. Some authors assumed that the primary stage of peroxide decomposition can be represented by the reaction [35]: ROOH + NO → RO• + HONO

(7.74)

Another mechanism [36] suggests that the reaction proceeds with the formation of peroxide radicals: ROOH + NO → ROO• + HNO

(7.75)

The kinetics of hydroperoxide decomposition in PP at 298 K and various partial pressures of NO has been studied in detail [34]. The decomposition kinetics are shown in Figure 7.4.

Figure 7.4 Kinetics of PP hydroperoxide decomposition in NO at various concentrations (1-3) and NO + NO2 mixture (4): (1) 1.61 × 10-3, (2) 3.22 × 10-3, (3) 4.13 × 10-3, (4) 3.1 × 10-3 NO and 3.0 × 10-6 NO2 mol/l.

As can be seen, the hydroperoxide consumption rate is initially low and then sharply increases. The observed character of the kinetic curves cannot be explained by reactions (7.74) or (7.75). According to the ESR data, the decomposition of PP hydroperoxide in an

207

Handbook of Plastic Films NO atmosphere gives dialkylnitroxyl radicals. It was shown that the induction periods for the hydroperoxide decomposition and nitroxyl radical accumulation are very sensitive to the presence of trace amounts of higher nitrogen oxides. This leads to the conclusion that the interaction of hydroperoxide with NO is more likely to proceed according to the scheme: ROOH + N2O3 → [ROONO] + HNO2

ROONO

(7.76) RONO2

(7.77)

RO• + NO2

(7.78)

RO• + NO2

Alkoxy radicals may decompose or enter into substitution reactions with macromolecules to form chain Rc• and end Re• alkyl macroradicals, and low molecular weight alkyl radicals r•, which with NO give nitroso compounds: RO• → Rc• (Re•, r•)

(7.79)

Rc• (Re•, r•) + NO → RcNO (ReNO, rNO)

(7.80)

The increase in the rate of hydroperoxide decomposition with time can be related to reactions proceeding with participation of such nitroso compounds: r′OOH + r″NO → r′O• + r′–N(O•)–OH

(7.81)

r″–N(O•)–OH → r″• + HNO2

(7.82)

The alkyl radicals formed in the system may stimulate hydroperoxide decomposition [37]: r• (Rc•, Re•) + ROOH → rH (RcH, ReH) + RO2•

RO2• + NO

(7.83)

RO• + NO2

(7.84)

RONO2

(7.85)

ROONO

Another process that can increase the hydroperoxide decomposition rate is the disproportionation of NO to N2 and NO3• with the participation of nitroso compounds [24]:

208


RNO• + 2NO

R–N=N–ONO2

RONO2 + N2

(7.86)

R• + N2 + NO3

(7.87)

R• + N2 + •ONO2

NO + NO3• → 2NO2 Reactions (7.83)-(7.88) may lead to an increasing NO2 concentration in the system and, consequently, result in the acceleration of reaction (7.76).

7.4 Conclusion Nitrogen oxides are capable of influencing the free-radical stages of polymeric material aging in polluted atmospheres. Nitric oxide is a comparatively low-activity free radical, and it cannot abstract even labile hydrogen atoms at ordinary temperatures to initiate the radical degradation process. On the other hand, NO effectively recombines with free radicals. This reaction is apparently controlled in solid polymers by the gas diffusion rate, and NO is capable of terminating the oxidation chain by reaction with peroxy and alkyl macroradicals. The reaction of NO with alkyl radicals gives nitroso compounds, which are spin traps accepting free radicals. This process can slow down polymer degradation in the presence of nitrogen oxides in subsequent conversions, which can break down into alkoxy radicals, effecting the degradation of macromolecules. In addition, nitric oxide initiates the decomposition of hydroperoxides resulting from oxidation of polymers. Nitrogen dioxide is a more active free radical as compared with NO, and is able to break off the labile hydrogen atoms at room temperature as well as to add to the C=C bonds of macromolecules, inducing free-radical degradation of polymers. At the same time, the NO2 radical can inhibit the free-radical reactions giving nitrogen-containing molecules by the reactions with alkyl, alkoxy and peroxy radicals. The thermal and photochemical conversions of these products also affect the aging process of polymeric materials. Nitrogen dioxide is an initiator of the free-radical degradation of polyolefins at elevated temperatures. The low stability of polyamides to the action of NO2 is quite surprising, because the N–H bond of the amide group is rather strong. Therefore, the mechanism of polyamide degradation connected with hydrogen atom abstraction by NO2 from N–H bonds is not fully elucidated.

209


References 1.

H.H.G. Jellinek in Aspects of Degradation and Stabilisation of Polymers, Ed., H.H.G. Jellinek, Elsevier, Amsterdam, The Netherlands, 1978, Chapter 9.

2.

N.M. Emanuel and A.L. Buchachenko, Chemical Physics of Polymer Degradation and Stabilisation, VNU Science Press, Utrecht, The Netherlands, 1987.

3.

O.N. Karpukhin, Usppekhi Khimii, 1978, 47, 6, 1119.

4.

T.C. Hall and F.E. Blacet, Journal of Chemical Physics, 1952, 20, 11, 1745.

5.

J.G. Calvert and J.N. Pitts Jr., Photochemistry, John Wiley, New York, NY, USA, 1966.

6.

H.H.G. Jellinek, F. Flajsman and F.J. Kryman, Journal of Applied Polymer Science, 1969, 13, 1, 107.

7.

T. Ogihara, Bulletin of the Chemical Society of Japan, 1963, 36, 1, 58.

8.

T. Ogihara, S. Tsuchiya and K. Kuratani, Bulletin of the Chemical Society of Japan, 1965, 38, 6, 978.

9.

H.H.G. Jellinek and Y. Toyoshima, Journal of Polymer Science, Part A-1: Polymer Chemistry, 1967, 5, 12, 3214.

10. H.H.G. Jellinek and F. Flajsman, Journal of Polymer Science, Part A-1: Polymer Chemistry, 1969, 7, 4, 1153. 11. A. Huber, Einfluß von Schwefeldioxid und Stickstoffdioxid auf Polymere in Luft unter Belichtung, University of Stuttgart, Germany, 1988, 187. [Ph.D Thesis]. 12. H.H.G. Jellinek and F. Flajsman, Journal of Polymer Science, Part A-1: Polymer Chemistry, 1970, 8, 3, 711. 13. H.H.G. Jellinek and P. Hrdlovic, Journal of Polymer Science, Part A-1: Polymer Chemistry, 1971, 9, 5, 1219. 14. T.V. Pokholok and G.B. Pariiskii, Polymer Science, Series A, 1997, 39, 7, 765. 15. E.N. Degtyarev, T.V. Pokholok, G.B. Pariiskii and O.E. Yakimchenko, Zhurnal Fizicheskoi Khimii, 1994, 68, 3, 461.

210

Interaction of Polymers with the Nitrogen Oxides in Polluted Atmospheres 16. T.V. Pokholok, G.B. Pariiskii and G.O. Bragina, Vysokomolekulyarnye Soedineniya, Seriya A, 1989, 31, 10, 2049. 17. H.H.G. Jellinek and A. Chaudhuri, Journal of Polymer Science, Part A-1: Polymer Chemistry, 1972, 10, 6, 1773. 18. H.H.G. Jellinek, R. Yokota and Y. Itoh, Polymer Journal, 1973, 4, 6, 601. 19. H. Herzlinger, B. Kuster and H. Essig, Textile Praxis International, 1989, 44, 6, 574, 655, 661. 20. I.S. Gaponova, E.Y. Davydov, G.G. Makarov, G.B. Pariiskii and V.P. Pustoshnyi, Polymer Science, Series A, 1998, 40, 4, 309. 21. H.H.G. Jellinek and T.J.Y. Wang, Journal of Polymer Science, Polymer Chemistry Edition, 1973, 11, 12, 3227. 22. H.H.G. Jellinek, F. Martin and J. Wegener, Journal of Applied Polymer Science, 1974, 18, 6, 1773. 23. H. Kambe and R. Yokota, Proceedings of the 2nd International Symposium on Degradation and Stabilisation of Polymers, Dubrovnik, Yugoslavia, 1978, Paper No.39. 24. J.F. Brown, Jr., Journal of the American Chemical Society, 1957, 79, 10, 2480. 25. A. Rockenbauer and L. Korecz, Chemical Communications, 1994, 145. 26. J.S.B. Park and J.C. Walton, Perkin Transactions 2, 1997, 12, 2579. 27. H.-G. Korth, R. Sustmann, P. Lommes, T. Paul, A. Ernst, H. de Groot, L. Hughes and K.U. Ingold, Journal of the American Chemical Society, 1994, 116, 7, 2767. 28. I. Gabr and M.C.R. Symons, Faraday Transactions, 1996, 92, 10, 1767. 29. I. Gabr, R.P. Patel, M.C.R. Symons and M.T. Wilson, Chemical Communications, 1995, 9, 915. 30. I.S. Gaponova, G.B. Pariiskii and D.Ya. Toptygin, Vysokomolekulyarnye Soedineniya, Seriya A, 1988, 30, 2, 262. 31. A.M. Wasserman and A.L. Kovarskii, Spinovye Metki i Zondy v Fizikokhimii Polimerov (Spin Labels and Probes in Physical Chemistry of Polymers), Nauka, Moscow, Russia, 1986.

211

Handbook of Plastic Films 32. I.S. Gaponova and G.B. Pariiskii, Chemical Physics Reports, 1997, 16, 10, 1795 33. I.S. Gaponova and G.B. Pariiskii, Polymer Science, Series B, 1998, 40, 11-12, 394. 34. I.S. Gaponova and G.B. Pariiskii, Polymer Science, Series A, 1995, 37, 11, 1133. 35. J.R. Shelton and R.F. Kopczewski, Journal of Organic Chemistry, 1967, 32, 9, 2908. 36. D.J. Carlsson, R. Brousseau, C. Zhang and D.M. Wiles, Polymer Degradation and Stability, 1987, 17, 4, 303. 37. K. Ingold and B. Roberts, Free-Radical Substitution Reactions, John Wiley, New York, NY, USA, 1972.

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8

Modifications of Plastic Films E.M. Abdel-Bary

8.1 Introduction Modifications of plastic films are generally used to improve mechanical or physical properties so that the films are suitable for certain applications. This can be achieved by subjecting the films to mechanical or chemical treatments. Thus, surface treatments modify the crystalline morphology and surface topography, increase the surface energy and remove contaminants. Removal of contaminants is necessary for good adhesion of the surface to other substrates. Other applications, such as printing, decorating, wetting and lamination, are improved by incorporation of a surfactant to change the surface tension of the adherents. Also, the presence of polar nitrogen-containing monomers on a polymer film surface allows one to obtain ionomers for versatile applications. Thus, such films can be used as anion-exchange membranes in electrodialysis processes, in water desalination [1], as a carrier for immobilisation of medical products [2], as a separator in alkaline batteries [3] and in fuel cells, etc. A number of surface modification techniques, such as plasma, corona discharge and chemical treatments, have been used to modify polymer surfaces, and the chemical methods are of particular interest. In this case, adsorption on and adhesion to polymer surfaces have been modified using many different methods, e.g., oxidation and other chemical reactions, high-energy irradiation and plasma treatment. In the following sections, we shall discuss some of the parameters affecting the mechanical and/or physical or physicochemical characteristics of such films.

8.2 Modification of Mechanical Properties Improved mechanical properties of plastic films can be realised by changing the following parameters: orientation, crystallisation and crosslinking. Regarding the orientation process, the properties of some polymer films can be improved by stretching a film above its glass transition temperature (Tg). Orientation may be in one direction only (uniaxial orientation) or in two directions, i.e., in both machine direction and transverse direction (biaxial orientation). Biaxially oriented film can be further categorised as balanced film, where orientation is roughly equal in both directions, or as unbalanced. This orientation of the molecules in thermoplastics is essentially a stretching process, which tends to align

213

Handbook of Plastic Films the molecules in the direction of the stretching force. Once the molecules have been aligned, the ordered arrangement is frozen in, giving rise to a strained condition.

8.2.1 Orientation Orientation of plastic films improves some of the physical properties such as tensile strength, impact strength, clarity and stiffness. In many instances there is also improvement in gas and water vapour barrier properties. Particularly in the case of polypropylene (PP), the barrier properties are also improved. Films that benefit appreciably from orientation include PP, polyethylene terephthalate (PET) and polyamide (Nylon). Polystyrene (PS) film is brittle material and becomes tough when biaxially oriented. Another aspect of film orientation is that of shrink-wrapping, where films such as low-density polyethylene (LDPE) and polyvinyl chloride (PVC) are stretched at a temperature above their softening points and then cooled to ‘freeze in’ the consequent orientation of the molecules. When these films are reheated, the molecules tend to return to their unstretched dimensions. In contrast, heat-setting ‘annealing’ is used to prevent shrinkage when heating stretched films. If oriented polypropylene, for example, is heated to about 100 °C immediately after being drawn, it shrinks considerably unless it is restrained in some way. This can be prevented by heat-setting. The film is heated, under controlled conditions, and while held under restraint after cooling, the film will not shrink if heated to below the annealing temperature; the film is said to be heat-set. The physical and optical properties of the film remain unchanged.

8.2.2 Crystallisation Crystallisation of polymers occurs as a result of the close approach of molecular chains in ordered, crystalline areas, which leads to the formation of much stronger intermolecular forces than in the amorphous areas. The rate of cooling has a significant effect on the degree of crystallinity and size of crystallites. Thus, rapid quenching of Nylon films during the casting process produces an amorphous film, whereas slow cooling allows the formation of crystals. The properties of the final film are highly dependent on the crystalline state of the polymer. Rapid quenching and consequent inhibition of crystal growth give a transparent film, which is more easily thermoformed.

8.2.3 Crosslinking Crosslinking is used to improve the mechanical properties and to obtain an infusible film. Crosslinked polyethylene film can be achieved by subjecting the film to high-energy

214

Modifications of Plastic Films radiation. When polyethylene film is irradiated, hydrogen and smaller amounts of methane, ethane and propane gases are liberated, and the polymer becomes increasingly insoluble as a result of crosslinking of the molecules via C–C bonds. This process slightly improves the gas and water vapour barrier properties, and the film has good clarity. The tear strength of the film becomes good, and the resistance to tear initiation and to tear propagation becomes high.

8.3 Chemical Modifications Chemical modification of the surface of polymers is an attractive method of improving the barrier characteristics of polymers that are otherwise considered ideal materials for packaging. With the exception of low gas barrier properties, polyolefins are extremely attractive because of their low cost, toughness, processability and excellent water barrier properties. Surface treatment is ideal for such polymers, because they are easily processed and made into better barriers by surface modification, either during processing or afterwards [4, 5]. Chemical reactions of the surface with gases are used to modify the surfaces of existing polymers without changing bulk properties. This modification can be achieved by reacting the polymer surface with gases. Modifications of the surface using fluorine, hydrogen fluoride, sulfur tetrafluoride, chlorine and bromine have been examined.

8.3.1 Fluorination The manufacture of fluoromonomers and their subsequent polymerisations are hazardous and difficult. The fabrication of fluoropolymers is also difficult and expensive. For example, the processing of polytetrafluoroethylene (PTFE) involves costly compaction and a high-temperature (375 °C) sintering process [6]. Hence, the widespread use of fluoropolymers is hindered by these considerations. Fluorination of polymers has been shown to be a successful new route to fluoropolymers. Polymers are fluorinated either directly or indirectly [7]. In direct fluorination, highly active fluorinating agents such as fluorine, hydrogen fluoride, or sulfur tetrafluoride convert the polymeric material completely to a fluorocarbon polymer.

8.3.1.1 Direct Fluorination Fluorine is a highly active fluorinating agent because of its low dissociation energy. It forms extremely stable bonds with carbon [8]. Fluorination of polymers by fluorine may

215

Handbook of Plastic Films be divided into two types: bulk fluorination and surface fluorination [9]. The surface fluorination of polyethylene (PE) film by 10% F2 (diluted with N2) results in a depth of fluorination ranging between 0 and 50 Å [10]. The extent and depth of fluorination during surface fluorination of polycarbonate (PC), PS and polymethyl methacrylate (PMMA) films with F2 diluted with He or N2 increase with reaction time, temperature and F2 gas pressure [11]. The extents of fluorination for PS, PC and PMMA are as high as 64.3%, 55.3% and 20% respectively. The relation between depth of fluorination and reaction time is represented by: d = Kt1/2

(8.1)

where d is the depth of fluorination and t is the reaction time. The proportionality constant K depends on the nature of the polymer; for example, the values for K are 13.2 and 5.6 for PS and PC, respectively. Fluorination may be conducted using hydrogen fluoride [7] and sulfur tetrafluoride [12, 13].

8.3.1.2 Indirect Fluorination In an effort to overcome the disadvantages of conventional fluorinating agents such as F2, HF, or SF4, nontoxic fluorocarbons, chlorofluorocarbons and sulfur hexafluoride are used. These gases cannot be used directly as fluorinating agents. However, when exposed to high-energy environments such as plasma, glow discharge, or γ-radiation, they generate active fluorinating agents [14]. Another approach of considerable interest is to modify the surface of existing polymers without changing the bulk properties. Fluorine attaches to the polymer near the surface and, because of its bulkiness and polar nature, improves gas and nonpolar liquid barrier properties [15]. Bulk fluorinated polymers (by F2 under controlled conditions to reduce crosslinking) can be used for the same purpose in place of fluoropolymers with similar structures prepared from respective monomers [8]. However, to make it cost-effective, surface fluorination rather than complete bulk fluorination of fabricated plastic items may be preferred. Such surface treatments could avoid problems encountered in moulding of fluoropolymers. Large fabricated plastic items can be given a surface coating of fluorinated polymer (0.1 mm thickness) [16]. Fluorinated plastic surfaces are impervious to most solvents and have good chemical, solvent and water resistance [16, 17]. Various plastic bottles, containers and tanks are fluorinated to handle chemicals and solvents safely. Hence, fluorinated plastic containers are found in use as containers for gasoline (petrol), paint, turpentine, motor oil and

216

Modifications of Plastic Films varnish [18, 19]. However, fluorinated materials may not maintain their barrier properties after repeated flexing. Surface fluorination is used to improve the barrier properties of the inner surface of polyethylene during the blow moulding process for formation of bottles. Polyethylene is nonpolar and therefore a poor barrier to nonpolar hydrocarbons. Such treatments with highly polar fluorine significantly improve its barrier properties. Surface-fluorinated containers are commonly used for gasoline (petrol), herbicides, pesticides and other products that normally penetrate polyethylene [20]. Mild surface treatment of polyethylene with low concentrations of fluorine can reduce the permeability of liquid penetrants such as pentane and hexane depending on the solubility and size of the penetrant [21].

8.3.2 Chlorination The chlorination reaction is too slow and not practicable, but it results in good barrier properties with more resistance to flexing. The gas-phase chlorination of the surface of LDPE has been studied under ambient light [22, 23] as well as in the presence of ultraviolet (UV) radiation [24]. The resultant surface was reported to consist of C–Cl and C–Cl2 moieties [22, 23]. However, chlorination of the surface of PE leads also to the formation of allyl chloride and vinyl chloride moieties [24].

8.3.3 Bromination The introduction of Br moieties on the polyolefin surface opens up a synthetic pathway to introduce a wide range of specific functional groups on the surface under mild conditions via nucleophilic substitution of Br moieties by different nucleophiles [25]. The gas-phase bromination of PE, PP and PS film surfaces by a free-radical photochemical pathway occurs with high regioselectivity. The surface bromination was accompanied by simultaneous dehydrobromination resulting in the formation of long sequences of conjugated double bonds. Thus, the brominated polyolefin surface contains bromide (Br) moieties in different chemical environments. As an example, we consider the free-radical mechanism for the bromination of the surface of PE film. The first step in this reaction is the homolytic bond cleavage of the bromine molecule into two bromine radicals upon exposure to radiation [26]: UV

Br2 → 2Br•

(8.2)

217

Handbook of Plastic Films In the second step, the bromine radical abstracts a hydrogen atom from the methylene unit of LDPE, which results in the formation of a radical centre on the LDPE chain: –CH2–CH2– + Br• → –•CH–CH2– + HBr

(8.3)

This radical centre further reacts with a bromine molecule to form the C–Br moiety and a bromine radical: –•CH–CH2– + Br2 → –CHBr–CH2– + Br•

(8.4)

This bromine radical then reacts with another –CH2– unit [equation (8.3)] and this chain reaction continues: –CHBr–CH2– + Br• → –CHBr–•CH– + HBr

(8.5)

The effects of the structure of the polymer on the mechanism of the bromination have been studied. Since the PS backbone contains 50% benzyl carbon atoms and 50% secondary carbon atoms, an increased rate of bromination compared to that of PE is expected.

8.3.4 Sulfonation Sulfonation involves exposure of the polymer surface to SO3/air followed by neutralisation with NH4OH, NaOH, or LiOH. Chemical reduction of copper, tin, or silver counterions present from the neutralisation process following sulfonation is called ‘reductive metallisation’. When combined with a thin protective overcoating of compatible barrier copolymer, dramatic permeation flux reductions of nearly 200-fold have been reported [5]. Sulfonation of polystyrene and aromatic polymers can be used to obtain protonconducting polymer electrolytes for use in fuel cells [27]. The aromatic polymers are easily sulfonated by concentrated sulfuric acid, by chlorosulfonic acid, by pure or complexed sulfur trioxide, or by acetyl sulfate. Sulfonation with chlorosulfonic acid or fuming sulfuric acid sometimes causes chemical degradation in these polymers [1]. Surface sulfonation yields excellent gas barrier properties under dry conditions, is relatively simple and does not affect the mechanical stability of the polymer [5].

8.3.5 Chemical Etching Chemical treatment is usually used for irregular and, in particular, large articles when other treatment methods are not applicable. It involves immersion of the article [LDPE and high-density PE (HDPE)] in an etchant solution such as chromic acid [28],

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Modifications of Plastic Films permanganate, sulfuric acid [29,30] or chlorosulfonic acid. Reflection infrared (IR) studies reveal extensive chemical changes on the surface in the case of LDPE but not HDPE or PP. New bands corresponding to the introduction of –OH, >C=O and SO3H groups were detected. Oxidation of PE by sulfuric acid and potassium chlorate [29, 30] has been carried out. In this case, the free energy of adhesion of the polymer is found to increase linearly with the surface density of the hydrophilic sites created by oxidation. The surface tension, polarity, wettability and bondability of fluoropolymer are improved by sodium etching [31, 32]. The etching solution is the equimolar complex of sodium and naphthalene dissolved in tetrahydrofuran. X-ray photoelectron spectroscopy (XPS) shows the complete disappearance of the fluorine peak, the appearance of an intense oxygen peak, and broadening and shifting of the C 1s peak to lower binding energy. A significant number of functional groups, such as carbonyl, carboxyl and C=C unsaturation, are introduced. The oxidation methods described up to now are heterogeneous in nature, since they involve chemical reactions between substances located partly in an organic phase and partly in an aqueous phase. Recently, a technique that is commonly referred to as phase transfer has come into prominence. This technique involves the use of phasetransferred permanganate (purple hydrocarbon) as an oxidant in a polar medium. Konar and co-workers [33, 34] have oxidised several polyolefins with the help of tetrabutylammonium permanganate in a hydrocarbon medium. Characterisation of the oxidised polyolefins confirmed the introduction of polar functional groups on the polar surface [35, 36]. Other phase-transfer catalysts, such as tetrabutylammonium bromide, tetrapentylammonium iodide, dicyclohexyl-18-crown-6 (DC-18-C6) and benzyl triphenyl phosphonium chloride (BTPC), have been investigated [37]. The results obtained show that LDPE oxidised using DC-18-C6 and BTPC catalysts has a relatively greater polar contribution to the total surface free energies than when using other catalysts. The carboxyl percentage attains 15.0% and 20.0%, respectively [38] while hydroperoxide attains 22.2% and 15.2%, respectively [36]. When a polymer is soaked in a heavily oxidative chemical liquid, such as chromic anhydride/ tetrachloroethane, chromic acid/acetic acid or chromic acid/sulfuric acid, and treated under suitable conditions, polar groups are introduced on the polymer surface [39, 40]. The surface of the polymer is heavily oxidised by nascent oxygen generated during the reaction as follows: K2Cr2O7 + 4H2SO4 → Cr2(SO4)3 + K2SO4 + 4H2O + 3[O]

(8.6)

219


8.3.6 Grafting Graft copolymerisation of vinyl monomers on to polymeric materials has been the subject of extensive studies for about four decades. In spite of the huge number of published papers and patents, and the interesting results obtained, there has been comparatively little commercialisation of the grafting process. The reasons for the lack of industrialisation on a large scale have been partly economic. Among the technical problems, which still remain to a considerable extent, are the concurrent formation of homopolymer in most cases and the lack of reproducibility in these largely heterogeneous reactions. In addition, there is the difficulty of controlling the grafted side chains in the molecular weight (molar mass) distribution. There are now a considerable number of methods available for effecting graft copolymerisation on to preformed polymers, each with its own particular advantages and disadvantages. Graft copolymerisation is effected, generally, through an initiation reaction involving attack by a macroradical on the monomer to be grafted. The generation of the macroradical is accomplished by different means such as: (1) Decomposition of a weak bond or liberation of an unstable group present in side groups in the chemical structure of the polymer; (2) Chain transfer reactions; (3) Redox reaction; (4) Photochemical initiation; and (5) Gamma-radiation-induced copolymerisation. Grafting using γ-radiation is concentrated on polyolefins and some vinyl polymers and elastomers, which are usually difficult to graft by chemical means without prior chemical modification of the substrate.

8.3.6.1 Grafting Using High-Energy Radiation The surface properties of commercial polymer thin films can be tailored under appropriate experimental conditions of radiation-induced grafting. The growth in popularity of radiation as the initiating system for grafting arises from the availability and cost of ionising radiation. This is due to the introduction of more powerful nuclear reactors. Apart from its cheapness, radiation is a very convenient method for graft initiation because it allows a considerable degree of control to be exercised over such structural factors as

220

Modifications of Plastic Films the number and length of the grafted chains by careful selection of the dose and dose rate. Thus, the advantages of radiation-chemical methods are: (1) Ease of preparation as compared to the conventional chemical method; (2) General applicability to a wide range of polymer combinations (due to the relatively nonselective absorption of radiation in matter); and (3) More efficient (and thus more economical) energy transfer provided by radiation compared to chemical methods requiring heat. The theory of radiation-induced grafting has received extensive treatment. The direct effect of ionising radiation in material is to produce active radical sites. The typical steps involved in free-radical polymerisation are also applicable to graft copolymerisation, including initiation, propagation and chain transfer. However, the complicating role of diffusion prevents any simple correlation of individual rate constants to the overall reaction rate. Among the various methods of radiation grafting, four have received special attention: (1) Direct radiation grafting of a vinyl monomer on to a polymer; (2) Grafting on radiation-peroxidised polymer; (3) Grafting initiated by trapped radicals; and (4) The intercrosslinking of two different polymers. Acrylic acid (AA) has been grafted on to PE films using γ-radiation [41, 42]. Gammaradiation grafting of styrene on to PE films has been carried out [43]. The styrene-grafted films were then sulfonated to form cationic exchange membranes. Rieke and co-workers described the properties obtained from grafting AA on to HDPE [44]. Their study pursued the concept of producing thermally sensitive crosslinks that could improve the properties of PE, (i.e., increase chemical reactivity). In 1977, Toi and co-workers determined the thermal properties for styrene-grafted HDPE by using γ-radiation [45]. No effect was observed on the crystallite size and the glass transition temperature after grafting. Ishigaki and co-workers reported the graft polymerisation of AA on to PE film by the preirradiation method [46, 47]. LDPE and HDPE were irradiated by electron beams of 2-50 Mrad and then immersed in an AA aqueous solution. These products were tested as semipermeable membranes for water desalination under reverse osmosis conditions [48]. Hydrophilic monomers such as AA or vinylpyridine were grafted on to PE via 60Co γradiation. The hydrophilic monomer-grafted PE could be treated further for functionalisation, leading to the investigation of a few applications such as separation membranes, polymeric catalysts and biosensors [49-53].

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8.3.6.2 Photografting The surface photografting process is based on surface grafting reactions initiated by ultraviolet radiation. These reactions are efficient and fast. They are limited to the surface of the polymer without affecting bulk properties, and they give very thin layers (less than 10 nm) of grafted polymer [54]. The grafting of sheets of low-density and high-density polyethylene with acrylic acid by UV irradiation from a high-pressure mercury lamp using a batch process has been reported [55]. The surface of HDPE is more difficult to graft than that of LDPE because of the linear chain structure of HDPE, and consequently its higher degree of crystallinity, which gives it a rougher surface structure than LDPE. Surface grafting with acrylic acid, as expected, decreases the contact angle of water, approaching complete wetting for LDPE. The molecular mechanism of bulk surface photografting has been given [56]. The primary grafting given in this mechanism using benzophenone involves initiation and propagation of short linear chains which is terminated by the addition of ketyl radical. Benzophenone acts as both initiator and terminator. The main effects that are important for applications are increased wettability, as mentioned before, increased adhesion of inks and other substrates, and increased adsorption of dyes. By grafting of reactive monomers like glycidyl acrylate, the polymer surface is made reactive to stabilisers, hydrophilic polymers, heparin and other bioactive agents, which gives functional properties of great interest [57-59]. Biomedical applications are of particular interest [60]. Examples of other recent publications on surface photografting include the preparation of polymeric catalyst [61, 62], polyethylene films for studying electrostatic interactions [63], and films for immobilisation of enzymes [64].

8.4 Physical Methods Used for Surface Modification Modification techniques using physical methods have been designed to achieve increased hydrophilicity, chemical modification and attachment of pharmacologically active agents. These physical methods include plasma treatment, corona treatment, ultraviolet and gamma radiation.

8.4.1 Plasma Treatment The implantation process that occurs in plasma treatment is one of the most effective methods of surface modification of polymeric materials. The plasma activates gas molecules, such as oxygen and nitrogen. The activated species interact with the polymer’s surfaces, and then special functional groups, such as hydroxyl, carbonyl, carboxyl, amino

222

Modifications of Plastic Films and amido groups, are formed at the surface of the polymers. As a result, the implantation reactions lead to large changes in the surface properties of the polymer; for example, the polymers change from hydrophobic to hydrophilic. ‘Plasma treatment’ is frequently used for the improvement of the adhesion and wettability of polymeric materials. A polyethylene film was treated with a nitrogen plasma, and its surface was inspected by XPS (C 1s and N 1s core levels) [65]. The original polyethylene film provides a sharp and symmetrical C 1s core-level spectrum whose peak appears at 285 eV with no N 1s core-level spectrum. However, the plasma-treated film gives an asymmetrical C 1s spectrum with a tail at more than 285 eV, and a strong N 1s core-level spectrum. This comparison indicates that some nitrogen functionalities were generated at the polyethylene film surface through nitrogen plasma treatment. Similarly, oxygen plasma treatment leads to the formation of some oxygen functionalities at the surface of polyethylene film [66]. It is clear that plasma treatment implants atomic residues at the surface of polymeric materials. Carbon monoxide, carbon dioxide, nitrogen monoxide, nitrogen dioxide and ammonia are used as plasma gases for hydrophilic surface modification. Polypropylene, polyester, polystyrene, rubber and polytetrafluoroethylene, among others, but not polyethylene, have been successfully modified by plasma treatment. The details of the implantation process are reviewed in the literature [67].

8.4.2 Corona Treatment In this technique, a sufficiently high-voltage electrical discharge is applied to the surface of a moving substrate (sheet or film). Pretreatment of films is usually carried out at the same time as film extrusion, which is an advantage where antistatic and other additives are present in the film. When film was extruded and stored prior to treatment, it was found that the additives had bloomed to the surface, and this made it difficult to achieve an even treatment. In one method, the film is passed between two electrodes, one of which is a metal blade connected to a high-voltage, high-frequency generator. The other is an earthed roller, which is separated from the high-voltage electrode by a narrow gap. The metal electrode should be slightly narrower in width than the film that is to be treated in order to prevent direct discharges to the roller. The electrical discharge is accompanied by the formation of ozone. This oxidises the film surface, rendering it polar. The level of treatment is governed by the generator output and the speed of throughput. Both under- and overtreatment should be avoided – the latter causing surface powdering, brittleness and sealing difficulties. The effect of treatment diminishes with time, and the treated surface is sensitive to handling and dust pickup. The corona treatment functions at atmospheric pressure and relatively high temperature. In this case, very significant surface oxidation occurs [6].

223

Handbook of Plastic Films One simple test for determining whether or not a film has been treated is to run water over the surface. If the film is untreated, the water will be repelled, whereas a treated film will retain the liquid for several minutes. Between these two extremes, a partially treated film will tend to show areas of both good and bad adhesion, so that the test is only satisfactory for seeing whether or not the surface of the film has been treated but not for detecting overtreatment. An improvement on this primitive test method is the peel adhesion test. This is carried out by applying a specified pressure-sensitive tape to the film surface, using a roller. The peel strength is then measured with the aid of a tensiometer. The higher the level of treatment, the higher the peel strength. The chemical changes occurring on the surface can be detected by using the XPS technique. This technique enables one to identify the presence of hydroxyl, ether, ester, hydroperoxide, aldehyde, carbonyl or carboxylic groups in corona dischargetreated polyolefins.

8.5 Characterisation Characterisation of modified films depends on the method of modification. For instance, the change in mechanical properties due to stretching can be evaluated by measuring the changes in mechanical properties using tensile testing machines according to standard methods. Characterisation of grafted films also differs somewhat from that of physically treated films. However, the selection of one or other measuring technique depends generally on the extent of modification.

8.5.1 Gravimetric Method Graft products are usually characterised by different methods. The first method is the calculation of graft parameters known as the grafting percentage (GP), grafting efficiency (GE) and weight conversion percentage (WC). These parameters can be calculated according to the following equations: grafting percentage (GP) =

grafting efficiency (GE) =

224

A−B × 100 B A−B × 100 C

(8.7)

(8.8)

Modifications of Plastic Films

weight conversion percentage (WC) =

A × 100 B

(8.9)

where A, B and C are the weights of the extracted graft product, substrate and monomer, respectively. This gravimetric method gives direct and rapid information about the graft reaction. Other characterisation methods are usually used to detect the changes in physical properties, which usually result from the changes in the morphology and structures of the substrates due to grafting.

8.5.2 Thermal Analyses In polymers having a certain degree of crystallinity, differential scanning calorimetry (DSC) is used to determine the heat of fusion and, consequently, the changes in the degree of crystallinity in grafted and ungrafted samples. The changes in the crystallinity of PE found after grafting include a small 2.5 °C drop in the location of the maximum in the melting curve and a significant decrease in the area under the melting peak [69]. Similar results were observed in the case of grafting PP and PE/ethylene-vinyl acetate (EVA) blends [70]. While the decrease in the melting temperature, represented by the shift in the melting curve, indicates that there is some change in the crystallinity caused by grafting, comparison of the areas before and after grafting indicates that this may be a small effect. By assuming that the difference in areas is due only to a difference in the amount of PE or PP present (in other words, no difference in the degree of crystallinity), the per cent graft can be calculated from: %G =

A1 − A2 ρPAN × × 100 A2 ρPE

(8.10)

where A1 is the area before grafting, A2 is the area after grafting, and ρ is the density.

8.5.3 Scanning Electron Microscopy Scanning electron microscopy (SEM) is generally used to detect the topography of a grafted surface, which usually changes due to monomers grafted on to the surface. In addition, this method can also be used to detect the depth of grafting into the matrix. If a binary monomer mixture was used for grafting, scanning electron micrographs help to detect the grafted monomer distribution by comparison with micrographs of each grafted monomer separately.

225


8.5.4 Swelling Measurements Equilibrium swelling of grafted samples in a proper solvent helps to detect the presence of grafted monomer. For instance, polyethylene does not swell to any noticeable extent in water. However, if polyethylene is grafted with water-soluble polymers such as polyacrylic acid or polyacrylamide, the equilibrium swelling of the product obtained increases markedly. Accordingly, the increase in swelling is evidence of grafting. In contrast, the swellability of natural rubber or styrene-butadiene-rubber vulcanisates in gasoline or benzene decreases markedly due to grafting with polyacrylonitrile (PAN). This decrease in swelling, again, is evidence of grafting.

8.5.5 Molecular Weight and Molecular Weight Distribution It is essential to know the molecular weight (molar mass) distribution of a graft in order to design functional polymeric membranes precisely by application of radiation-induced graft polymerisation and to control the grafting process. For example, the length and density of the polymer chains grafted on to a cellulose triacetate microfiltration membrane will determine the permeability of liquid through and the adsorptivity of molecules on the functionalised microfiltration membrane. Thus, the molecular weight distribution of methyl methacrylate grafted on to cellulose triacetate has been determined by acid hydrolysis of the substrate. From the gel-permeation chromatogram, the molecular weight distribution was determined [71]. This method is valid only when it is possible to degrade the substrate. In the case of grafted natural rubber, for example, ozonolysis is a very convenient process to use to destroy the natural rubber segments, leaving the plastomer chains intact [72]. Alternatively, oxidation with perbenzoic acid can be used [73]. Osmometry or solution viscosity may then be used to determine the molecular weight of the isolated non-rubber fraction.

8.5.6 Dielectric Relaxation Dielectric relaxation measurements of polyethylene grafted with AA, 2-hydroxyethyl methacrylate (HEMA) and their binary mixture were carried out in a trial to explore the molecular dynamics of the grafted samples [74]. Such measurements enable information to be obtained about their molecular packing and interaction. It was possible to predict that the binary mixture used yields a random copolymer PE-g-P(AA/HEMA) that is greatly enriched with HEMA. This method of characterisation is very interesting and is likely to be developed in different polymer/monomer systems.

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Modifications of Plastic Films

8.5.7 Surface Properties The surface properties of modified plastic films are very important in industrial applications. A number of techniques are available for determining the composition of a solid surface. This is very important in many processes, such as oxidation discoloration, wear and adhesion. The technique used depends upon such important considerations as sampling depth, surface information, analysis environment and surface suitability. The most widely used techniques for surface analysis are Auger electron spectroscopy (AES), XPS, secondary ion - mass spectroscopy (SIMS), Raman and IR spectroscopy, and contact angle measurements.

8.5.8 Spectroscopic Analysis 8.5.8.1 Infrared (IR) Spectroscopy Proof of chemical modification or changes in chemical structure due to physical treatments such as corona discharge can be followed up by spectroscopic analysis using IR. Thus, the amount of acrylonitrile grafting on to PE using an electron beam was determined from the absorbance of the nitrile group at 2240 cm–1 after extraction of homopolymer [69]. In order to minimise the effects of weighing error, an internal reference method utilising the methylene absorbance of PE at 730 cm–1 was adopted. Thus, the mass of PAN in a sample was correlated to the ratio of the absorbance A2240/A730, and the weight per cent graft defined before was computed from the mass of PAN.

8.5.8.2 X-Ray Fluorescence Spectroscopy (XFS) This method can be used to detect and characterise the first several hundred nanometres of depth of a solid. It can be attached to a scanning electron microscope. The main principle is that energetic electrons bombard the sample, where ionisation takes place. Ions with an electron vacancy in their atomic core rearrange to a lower energy state, resulting in the release of electromagnetic energy of a specific wavelength. Analysis of the wavelengths of the X-radiation emitted identifies the atomic species present.

8.5.8.3 Auger Electron Spectroscopy (AES) This technique is used to characterise the chemical bonding state of the elements on the surface. The maximal depth from which Auger electrons can escape is only about 0.30.6 μm. For most materials, AES uses a low-energy, 1-5 keV, electron beam gun for surface bombardment to minimise surface heating.

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8.5.9 Electron Spectroscopy for Chemical Analysis (ESCA) or X-Ray Photoelectron Spectroscopy (XPS) In this method the surface is bombarded with low-energy X-rays, which is less disruptive than an electron beam. The energy is absorbed by ionisation, resulting in the direct ejection of a core-level electron, i.e., a photoelectron. Hence ESCA is also known as Xray photoelectron spectroscopy. These electrons have an escape depth of less than a nanometre. Although XPS is less sensitive than AES, it provides a direct measure of the binding energy of core-level electrons through the relation: binding energy of ionised core-level electron = energy of emitted photoelectron – incident X-ray energy and it gives simpler spectral line shapes than AES. This technique can be used to distinguish between different elements and different chemical bonding configurations. It is the most popular surface analytical technique for providing structural, chemical bonding and composition data for polymeric systems. All elements, except hydrogen, are readily identified by XPS, since the different core-level binding energies are highly characteristic. By measuring the relative peak intensities and dividing them by the appropriate sensitivity factors, one may find the concentration of different elements on a surface. Moreover, small shifts in the binding energy of a core level are corroborated by considering the presence of different functional groups. For example, when a carbon atom is bonded to different groups of atoms of increasing electronegativity, a systematic shift in the binding energy of the C 1s peak is observed. The higher the electronegativity of the group, the higher the binding energy of the C 1s peak.

8.6 Applications Since bringing about changes in physical properties is often the impetus for grafting, it is necessary to touch upon this briefly in this section. A number of general reviews on grafting have also included some discussion on the changes in physical properties that usually determine the field of applications. Grafting has often been employed to change the moisture absorption and transport properties of plastic films when hydrophilic monomers such as acrylamide, acrylic acid and methacrylic acid are grafted. Radiation grafting of anionic and cationic monomers to impart ion-exchange properties to polymer films and other structures is rather promising. Thus, grafting of acrylamide and acrylic acid on to polyethylene and polyethylene/ethylene-vinyl acetate copolymer blend [70] allows a new product to be obtained with reasonable ion-exchange capacity.

228

Modifications of Plastic Films A number of possible uses of radiation grafting are being explored for microlithography, diazo printing, and various copying and printing processes. Radiation grafting for various biomedical applications remains an extremely active field of development. The grafted side chains can contain functional groups to which bioactive materials can be attached. These include amine, carboxylic and hydroxyl groups, which can be considered as centres for further modifications. Photodegradation of polyethylene waste can be markedly accelerated via its grafting with acrylamide [70]. In contrast, photostabilisation of polyethylene and polypropylene can be achieved as a result of the grafting of 2-hydroxy-4-(3-methacryloxy-2hydroxypropoxy)benzophenone using γ-radiation [75]. In this case, the grafted compound, acting as a UV stabiliser, is chemically bound to the backbone chain of the polymer, and its evaporation from the surface can be avoided.

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Handbook of Plastic Films 45. K. Toi, M. Kikuchi and T. Tokuda, Journal of Applied Polymer Science, 1977, 21, 535. 46. I. Ishigaki, T. Sugo and T. Senoo, Radiation Physics and Chemistry, 1981, 18, 899. 47. I. Ishigaki, T. Sugo K. Senco, T. Okada, J. Okamoto and S. Machi, Journal of Applied Polymer Science, 1982, 27, 1033. 48. M.I. Aly, K. Singer, N.A. Ghanem and M.A. El-Azmirly, European Polymer Journal, 1978, 14, 545. 49. G-H. Hsiue and W-K. Huang, Journal of Applied Polymer Science, 1985, 30, 1023. 50. G-H. Hsiue and W-K. Huang, Journal of the Chinese Institute of Chemical Engineers, 1987, 16, 257. 51. G-H. Hsiue and J-S. Yang, Journal of Membrane Science, 1993, 82, 117. 52. G-H. Hsiue, W-K. Huang and H-L. Chu, Journal of Polymer Science, Part A: Polymer Chemistry, 1989, 27, 4397. 53. G-H. Hsiue, T.L Perng and J.M. Yang, Journal of Applied Polymer Science, 1991, 42, 1899. 54. B. Ranby, Die Makromolekulare Chemie - Macromolecular Symposia, 1992, 63, 55. 55. K. Allmer, A. Hult and B. Ranby, Journal of Polymer Science: Polymer Chemistry, 1988, 26, 2099. 56. B. Ranby, International Journal of Adhesion and Adhesives, 1999, 19, 5, 337. 57. B. Zhang and B. Ranby, Journal of Applied Polymer Science, 1991, 43, 621. 58. K. Allmer, A. Hult and B. Ranby, Journal of Polymer Science, Part A: Polymer Chemistry, 1989, 27, 1641. 59. K. Allmer, A. Hult and B. Ranby, Journal of Polymer Science, Part A: Polymer Chemistry, 1989, 27, 3405. 60. K. Allmer, J. Hilborn, P.H. Larsson, A. Hult and B. Ranby, Journal of Polymer Science, Part A: Polymer Chemistry, 1990, 28, 173. 61. H. Kubota, European Polymer Journal, 1992, 28, 3, 267.

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Modifications of Plastic Films 62. H. Kubota, European Polymer Journal, 1993, 29, 4, 551. 63. J. Zhang, E. Uchida, Y. Uyama and Y. Ikada, Journal of Colloid and Interface Science, 1997, 188, 431. 64. J.R. Bellobono, E. Selli, A. Polissi and F. Mufatto, Biotechnology and Bioengineering, 1990, 35, 6, 646. 65. H. Yasuda, H.C. Marsh, S. Brandt and C.N. Reilly, Journal of Polymer Science, Polymer Chemistry Edition, 1977, 15, 991. 66. H. Yasuda, Journal of Polymer Science: Macromolecular Reviews, 1980, 16, 199. 67. N. Inagaki, Plasma Surface Modification and Plasma Polymerisation, Technomic, Lancaster, PA, USA, 1996. 68. C.Y. Kim and D.A.I. Goring, Journal of Applied Polymer Science, 1971, 15, 1357. 69. P.W. Morgan and J.C. Corelli, Journal of Applied Polymer Science, 1983, 28, 1879. 70. E.M. Abdel Bary and E.M. El-Nesr, Radiation Physics and Chemistry, 1996, 48, 5, 689. 71. H. Yamagishi, K. Saito, S. Furusaki, T. Sugo, F. Hoson and J. Okamoto, Journal of Membrane Science, 1993, 85, 71. 72. P.W. Allen, G. Ayrey, C.G. Moore and J. Scanlan, Journal of Polymer Science, 1959, 36, 55. 73. J.A. Blanchette and L.E. Nielson, Journal of Polymer Science, 1956, 22, 317. 74. A.A. Mansour, E.M. Abdel-Bary and E.M. El-Nesr, Journal of Elastomers and Plastics, 1994, 26, 355. 75. F. Ranogajec, M. Mlinac and I. Dvornik, Radiation Physics and Chemistry, 1981, 18, 511.

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9

Applications of Plastic Films in Packaging Susan E. Selke

9.1 Introduction Nearly all products are packaged at some point in their life-cycle. Plastic films are widely used in packaging, and continue to grow in use as more and more applications switch from rigid to flexible packages. Flexible packages generally take up much less space than the rigid structures they replace, especially before they are filled with product. They commonly require less material, as well. Therefore, switching from rigid to flexible packaging can provide significant economic savings in warehouse space and transportation, as well as in package cost. On the other hand, because flexible packaging does not usually have as much strength as rigid packaging, stronger distribution packaging may be required. Opening and reclosing of flexible packaging may also be less userfriendly, and consumers may perceive some types of products in flexible packaging as being lower in quality than equivalent products in rigid or semi-rigid packages. Common flexible packaging forms include wraps, bags and pouches. In these packages, plastic films may be used alone or combined with paper and/or metal to serve the basic packaging functions of containment, protection, communication and utility in the delivery of quality products to the consumer. While plastic films are most often found in flexible package structures, they may also be used as a component in rigid or semi-rigid package structures, for example, as a liner inside a carton, or as lidding on a cup or tray. The most common film used in packaging is low-density polyethylene (LDPE), defined broadly to include linear low-density polyethylene (LLDPE). Appreciable amounts of high-density polyethylene (HDPE), polypropylene (PP), polyvinyl chloride (PVC), polyvinylidene chloride (PVDC), polyamide (Nylon) and other plastics are also used

9.2 Packaging Functions Before examining applications of plastic films in packaging, it is useful to take a moment to consider why we use packages at all, since that will help in evaluation of the advantages and disadvantages of plastic films as packaging materials. The functions of packaging can be described in many ways. One simple way of organising them is to consider the basic packaging functions as containment, protection, communication and utility.

235

Handbook of Plastic Films The containment function of packaging is the most basic. Many types of goods cannot be readily moved from one place to another unless they are contained in some manner. This is obvious for liquids and gases, but is also true for many small solid items, for example, marbles, nails, laundry detergent powder and potato chips (crisps). The package confines these items in a way that makes it feasible to transport them. Sometimes the containment function is considered part of the more inclusive protection function of packaging. An important attribute of many forms of packaging is the ability of the package to protect the product from some type of damage associated with its interaction with the environment. In the example above, the marbles and nails must be protected against exposure to dust and dirt to remain in a condition that will permit their sale. The laundry detergent needs to be protected from exposure to excess moisture that could cause caking. The potato chips must be protected against light and oxygen, which can cause rancidity. In some cases, protection of the environment from the product is provided by the package. A water-soluble pouch for agricultural chemicals has, as its prime function, protection of the user from exposure to the hazardous undiluted chemical. Packages also serve as a vehicle for communication. In most cases, the package must in some way communicate what it contains. Sometimes this is as simple as being transparent, so that the user can see what is inside. Since the package is often the primary sales tool, however, communication needs are usually much more extensive. The package must not only communicate what is inside, but also act to convince the potential consumer to purchase the product. Often, there are a number of legally required communications, such as the amount of product, where and by whom the product is made, required warnings, etc. Packages also provide utility, either for the end-user or for others who interact with the package along the supply chain. Utility includes attributes such as a tear strip for opening and tamper evidence, a zipper closure for resealing, and a hole for use in hanging the packages on a display. Individual packages or package elements often provide more than one function, simultaneously. For example, a stand-up pouch for snack foods provides: containment; protection of the product against oxygen and moisture; communication of identification, legally required, and sales messages; opening and reclosure features for consumer utility; and the ability to stand conveniently on the retailer’s shelf and present a reasonably flat front panel to catch the consumer’s eye.

9.3 Flexible Package Forms Flexible packages come in two basic forms: wraps, and bags or pouches. A wrap consists of plastic film that has not been formed into a package shape. The film is simply wound around

236

Applications of Plastic Films in Packaging the product or products to be contained, and held in place in some fashion. In a bag or pouch, some shaping of the plastic is done, either before or at the same time as the product is added. Most often, this shaping is done by heat-sealing the edges of the plastic together.

9.3.1 Wraps 9.3.1.1 Stretch-Wrap One of the largest uses of plastic films in packaging is in stretch-wrap used for bundling pallet-loads of products together, in order to unitise them for distribution. The plastic film, most often linear low-density polyethylene, is stretched as it is wound around the products and pallet, usually in a spiral fashion. When enough has been applied, the film is cut, and the tail of the film is adhered to the load, usually by self-cling. When the stretching force is released, the film’s tendency to return to its unstretched dimensions causes a restraining force to be exerted on the load, thus unitising it and keeping it from shifting when the load is moved during distribution. In addition to its unitising function, stretch-wrap also protects the load against moisture, dust and abrasion. Stretch-wrap can also be used to provide this protection to single items, or to unitise smaller than pallet-load quantities of goods. While stretch-wrap is simple in conception, it may have a fairly complex structure. It is desirable for each layer of stretch-wrap to stick to the layers below, but it is undesirable for adjacent shrink-wrapped loads to stick to each other, or to other things with which they come in contact. Therefore, the stretch-wrap may have a multilayer structure, with tackifying agents added to the inside layer to enhance cling. Low-density polyethylene, polyvinyl chloride, ethylene-vinyl acetate and other polymers are used as stretch films, in addition to LLDPE.

9.3.1.2 Shrink-Wrap Shrink-wrap is an alternative to stretch-wrap for unitisation. When shrink-wrap is exposed to a source of heat, the previously aligned (oriented) molecules try to return to the lowerenergy, unoriented, random-coil conformation. The product prevents the film from returning to its unstretched dimensions, and the force exerted by the material on the product acts to unitise the load. For unitising pallet-loads of goods, stretch-wrap is much more common than shrinkwrap, since it generally requires less energy and is more economical. Shrink-wrap is

237

Handbook of Plastic Films more commonly used as a bundling wrap, unitising two or several products (either the same or different), rather than for full pallet-loads of goods. Often, shrink-wrap is used for product protection rather than unitisation, in applications ranging from meat to toys. It can be designed to form a tight enclosure around the entire product, providing excellent protection against dirt, moisture and abrasion. Usually, the wrap is formed into a loose pouch before it is shrunk tightly around the product in a shrink tunnel, where the packaged product is exposed to hot air. LDPE and LLDPE are common materials for shrink films. PVC and PP are used in lesser quantities, as are some specialty films.

9.3.2 Bags, Sacks and Pouches To make a bag, sack or pouch, two or more edges of a plastic film are sealed together, forming a cavity in which the product can be placed. In most applications, the opening is then closed so that the product is completely enclosed by the package. In some cases, such as merchandise sacks, one side remains open. The terms ‘bag’, ‘sack’ and ‘pouch’ can be confusing. According to some authorities, sacks are larger than bags, and both refer to packages in which the top is open, while pouches are smaller, and refer to packages that are totally sealed. However, these definitions do not conform to common use of these terms, which, in practice, are often used interchangeably. Common styles of pouches include pillow pouches, three-side-seal pouches and fourside-seal pouches. Pillow pouches are produced by forming the plastic film into a cylinder and sealing the edges together in what will become the back seam in the finished package. The bottom of the cylinder is collapsed and sealed, the product introduced, and then the top seam added. The shape of the filled package resembles a pillow – hence its name. Three-side-seal pouches are formed, as the name indicates, by folding the film into a rectangle and sealing the three non-fold sides. In some cases, the fourth side is sealed as well, for additional strength. Four-side-seal pouches are formed from two pieces of material that are sealed together on all sides. Therefore, four-side-seal pouches need not be rectangular in shape. In contrast to pillow and three-side-seal pouches, the front and back of a four-side-seal pouch may be made from different types of plastic film. In any of these pouch styles, gussets may be added to expand the capacity of the pouch without increasing its width or height. Pouches may be used alone, or may be combined with another package for product distribution and/or sale. One very common package structure is bag-in-box packages, which consist of a pouch inside a folding carton or corrugated box.

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Applications of Plastic Films in Packaging The pouch material may be plastic film alone, or a multilayer material containing paper and/or aluminium foil. Paper may be used to add strength, rigidity, printability or bulk to the flexible package. Foil may be incorporated to improve the barrier to permeants such as oxygen, water vapour, odours or flavours. In the past several years, stand-up pouches have increasingly been used as substitutes for cartons or bottles. Stand-up pouches are designed to stand upright on the retail shelf. Their design involves gussets and special shaping of the bottom panel.

9.3.3 Pouch Production There are two primary ways of using bags, sacks and pouches for packaging: as preformed pouches, or in form-fill-seal operations. In a form-fill-seal (FFS) operation, the web stock (usually preprinted, if applicable) is fed into either a horizontal or vertical FFS machine, in which it is formed into a pouch, the product added and the final seal formed. If preformed pouches are used, the packages are formed and an opening left for product introduction. The product is added to the package in a separate operation, and then the package is sealed. Form-fill-seal operations are usually economically advantageous for large-scale production. Buying of preformed pouches is generally more economical if production quantities are small, or in cases where the material is difficult to seal and poses quality control problems.

9.3.4 Dispensing and Reclosure Features One of the long-standing drawbacks of flexible packaging has been the difficulty of providing easy-to-use and effective dispensing and reclosure. In the past few years, several innovations have provided significant improvements in these package attributes. The most common way to dispense products from flexible packages is to cut or tear the package open, or to peel open one of the seams. For some products, such as breakfast cereal in bag-in-box packages, this is a significant source of consumer complaints. The seals often do not peel easily, and all too often the result is a bag with a split down the side, spilling cereal into the carton and making it nearly impossible to reclose the pouch to protect product freshness. Some flexible packages now incorporate zipper closures, often accompanied by a tear strip for initial opening. Other packages have resealable flaps, usually located along a seam. For liquid products, some packages incorporate a threaded spout with a standard threaded cap. This may be located on the top of the pouch, or on the bottom, depending on the

239

Handbook of Plastic Films product and the package size. In bag-in-box packages, the outer carton may include a flap through which the spout can be extended for dispensing. For single-serve drinks, it is common to provide an attached straw (protected from dirt in its own pouch, which is glued to the side of the drink pouch), along with a designated spot on the package that has been modified for easy puncturing.

9.4 Heat-Sealing Heat-sealing is the usual method for producing seals and seams in flexible packaging. Occasionally, adhesive systems are used. There are a wide variety of types of heat-sealing systems, but the most common, especially for films, are thermal or bar sealing, and impulse sealing. Thermal or bar sealing uses two heated bars that exert pressure on the materials to be sealed and at the same time conduct heat to the interface, melting the materials. The pressure ensures good contact between the materials, and assists in interpenetration of the melted viscous materials at the interface. When sufficient time has elapsed to produce an initial seal, the materials are released. Therefore, the hot tack of the material is crucial in forming an adequate seal. The full strength of the seal forms as the material cools, but the initial strength must be sufficient to maintain the seal integrity while cooling proceeds. The sealing bars usually have rounded edges to avoid puncturing the material, and often one bar is fitted with a resilient surface to aid in achieving uniform pressure during sealing. Usually, the heat-seal jaws are serrated rather than flat, and produce a patterned seal. In variants of thermal sealing, only one bar is heated and the other is not. Especially for sealing lidding on containers, the bars may be shaped rather than rectangular, producing shaped seals. Another variant uses heated rollers rather than bars; the pouch is sealed as it travels through the rollers. Impulse sealing also uses two jaws to produce the seal, but heat is generated by flow of an impulse of electric current through a nichrome wire. The jaws do not remain hot, but cool down after each electrical impulse. The material being sealed is captured between the jaws, the current flows to produce heating, and the material remains between the jaws for a cool-down period before it is released. Cooling may be aided by circulation of cooling water through the jaws. With impulse sealing, materials do not require as good hot tack as with thermal sealing. The seal will increase in strength during the cooling phase, before it is released from the heat-sealer, so it is not as subject to immediate failure or distortion. On the other hand, the impulse seal is typically much narrower than the bar seal, and therefore is often not as strong. Impulse sealing is particularly advantageous for oriented materials, which have a tendency to wrinkle during sealing. As with bar sealing, the jaws can be shaped to produce shaped seals.

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Applications of Plastic Films in Packaging Hot wire or knife sealing uses a heated wire or knife to cut and seal films simultaneously. It is often used to produce thin polyethylene bags for applications such as produce packaging. The seals are very narrow, often nearly invisible, and relatively weak. In band sealing, often used for the final seal on filled preformed pouches, the materials to be sealed are moved through heated bands. Both heating and cooling phases can be provided. Other types of heat-sealing are used less frequently in production of flexible packaging.

9.5 Other Uses of Packaging Films Plastics films are sometimes used as components in rigid or semi-rigid packaging structures. They can serve as liners inside closures for bottles and jars, as lidding on trays or cups, or can be laminated on paperboard or other materials. While the plastic resins used as coatings are not produced as stand-alone films, they are deposited on or in packages as films. Two common packaging applications of plastic films outside the flexible packaging category are skin packaging and bubble-wrap. In skin packaging, a product is held tightly to a backing material by a plastic film. Usually, the backing material consists of a heat-seal coated paperboard. The product is placed on the board, and then the heated plastic film lowered on to it. A vacuum is drawn through the backing material, causing the film to form tightly around the product and seal to the board. Usually, the product is displayed in the retail environment by hanging the backing from a peg. Obviously, the product must be able to withstand momentary contact with the hot plastic without damage, and the plastic must not adhere to the product. The coated backing often requires perforations to permit adequate evacuation of trapped air. In some cases, films are used that permit sealing to uncoated board. Heavy-duty films sealed to corrugated board can be used to provide protection to products during distribution, by physically isolating the skin-packaged product from impacts to the outer container. Bubble-wrap is a cushioning material produced by forming bubbles of air, of a defined size, between two plastic films. The bubbles can be various sizes, depending on the enduse of the material. Generally, smaller bubbles are used to protect lighter-weight products, and larger bubbles are used for heavier products. Bubble-wrap does not provide suitable protection for products that are very heavy, however.

9.6 Major Packaging Films A variety of plastic resins are used to make packaging films. Sometimes they are used alone, and often they are used in combinations that provide the benefits of multiple materials. The most commonly used packaging resins will be described in this section.

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9.6.1 Low-Density Polyethylene (LDPE) and Linear Low-Density Polyethylene (LLDPE) Low-density and linear low-density polyethylene are the most commonly used packaging films. Low-density polyethylene is produced by a high-temperature, high-pressure process that results in considerable short- and long-chain branching of the molecules. Linear low-density polyethylene is produced at temperatures similar to those used for highdensity polyethylene, resulting in linear molecules. The reduction of density comes about through the use of comonomers that put side groups on the main chain that act like branching in decreasing crystallinity. In traditional Ziegler-Natta catalyst polymerisations, these comonomers are butene, hexene or octene. Some of the new family of polyethylenes using metallocene catalysts incorporate higher alpha-olefins into the polymer structure, producing longer side groups, which act much like the long-chain branching in highpressure LDPE. LDPE and LLDPE are soft, flexible materials, with a hazy appearance. At equal density and thickness, LLDPE has higher impact strength, tensile strength, puncture resistance and elongation than LDPE. LLDPE based on octene generally has the highest strength, followed by hexene- and butene-based polymers, in that order. The cost per unit mass of the materials generally also follows the order octene > hexene > butene. LDPE has better heat-seal properties than LLDPE. It seals at lower temperatures, seals over a wider temperature range, and has better hot tack, all of which result, to a great extent, from its long-chain branching. Metallocene LLDPE containing higher alpha-olefins was designed, in part, to remedy this disadvantage of LLDPE. Another approach that has commonly been taken to producing the best mix of properties for a given application is to blend LLDPE and LDPE. LDPE and LLDPE are good barriers to water vapour, but are poor barriers to oxygen, carbon dioxide and many odour and flavour compounds. They have good grease resistance, and are quite inert. Low-temperature performance is good, as these materials retain their flexibility at very low temperatures. They soften and melt at moderately elevated temperatures, so they are not suitable for applications involving significant exposure to heat. Some characteristic LDPE and LLDPE properties are presented in Table 9.1. LDPE is generally the cheapest plastic film, on a per-unit-mass basis. Since LLDPE often permits considerable down-gauging, it can be the lowest cost alternative on a per-use basis. Very low-density polyethylene (VLDPE) is LLDPE with a higher concentration of comonomer, which reduces crystallinity, and consequently density, below the traditional range for LLDPE, to 0.905-0.915 g/cm3. These materials are very soft films, with excellent cling but reduced strength.

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Applications of Plastic Films in Packaging

Table 9.1 Typical properties of polyethylene films [1, 2] Property

Polymer LDPE

LLDPE

HDPE

–120

–120

–120

105-115

122-124

128-138

Glass transition temperature (Tg; °C) Melting temperature (Tm; °C) Heat distortion temperature, at 455 kPa (°C) Density (g/cm3)

40-44 0.915-0.940

62-91 0.915-0.935

0.94-0.97

Tensile modulus (GPa)

0.2-0.5

0.6-1.1

Tensile strength (MPa)

8-31

20-45

17-45

Elongation (%)

100-965

350-850

10-1200

WVTR* at 37.8 °C and 90% RH (g μm/m2 d)

375-500

125

O2 permeability, at 25 °C (103 cm3 μm/m2 d atm)

160-210

40-73

*WVTR: Water vapour transmission rate (d = day, 24 h) RH: relative humidity

9.6.2 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 vapour barrier is better, as is their gas barrier. However, permeability to oxygen and carbon dioxide is still much too high for HDPE to be suitable as a barrier for these permeants. As is the case for LDPE, HDPE is very inert, and has good oil and grease resistance. Using high molecular weight (high molar mass) resin, HMW-HDPE, which permits considerable down-gauging, can reduce the cost of HDPE films on a per-use basis. This material is higher in cost per unit mass, and is also somewhat more difficult to process than lower molecular weight materials, due to its high viscosity. Another alternative for reducing the cost of HDPE film is the use of recycled material, often originating in milk cartons. 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 9.1.

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9.6.3 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, 0.890.91 g/cm3. PP films are suitable for high-speed packaging applications that demand a relatively stiff material, since they are considerably stiffer than HDPE, and also have much improved clarity. Clarity can be further improved by using copolymer resins containing some ethylene units, to reduce crystallinity. Another approach to improving transparency is the use of nucleating agents to reduce average crystallite size. Barrier properties of PP are comparable to those of HDPE. Unoriented PP film tends to be somewhat brittle, especially at low temperatures. In many applications, biaxially oriented film (BOPP) is preferred. Orientation also increases the stiffness of the film. PP, especially BOPP, does not heat-seal well. Therefore, it is commonly coated or coextruded with sealants to make heat-sealable films. Typical PP properties are shown in Table 9.2.

Table 9.2 Typical properties of polypropylene (PP), biaxially oriented polypropylene (BOPP) and polyvinyl chloride (PVC) films [1-4] Property

Polymer PP

BOPP

PVC

Tg (°C)

–10

–10

75-105

Tm (°C)

160-175

160-175

212

Heat distortion temperature, at 455 kPa (°C)

107-121

Density (g/cm3)

0.89-0.91

0.89-0.91

1.35-1.41


1.1-1.5

1.7-2.4

to 4.1


31-43

120-240

10-55

Elongation (%)

500-650

30-150

14-450

WVTR, at 37.8 °C and 90% RH (g μm/m2 d)

100-300

100-125

750-15,700

50-94

37-58

3.7-240


244

57-82


9.6.4 Polyvinyl Chloride (PVC) Polyvinyl chloride films are formed by combining PVC resin, produced by addition polymerisation of vinyl chloride, with plasticisers and other additives to produce a flexible film. Unmodified PVC is quite brittle and difficult to process because of its heat sensitivity. However, because of its polar nature, PVC has a high affinity for plasticisers, and hence can be substantially modified. Plasticisers generally consist of high-boiling-point organic liquids, which serve a lubricating function in the resin. Some soft and flexible PVC films are approximately 50% plasticiser by weight. For food packaging uses, plasticisers and other ingredients must be suitable for direct food contact. The major plasticisers used in such applications are adipates. Often, epoxidised soybean oil is added as a secondary plasticiser. For non-food use, a wider range of plasticisers is available. Adipates and phthalates are most common. In addition to plasticisers, PVC films contain stabilisers, as the resin is heat-sensitive. Oil epoxides have some stabilising functionality, and in food packaging uses supplement the activity of calcium, magnesium or zinc stearates. Phosphites may also be used. In non-food applications, organometallic salts of barium and zinc are commonly used. The properties of PVC films are strongly influenced by the type and level of modifying ingredients, especially plasticisers, that have been added. In general, the films are quite soft and flexible, easy to heat-seal, and have excellent self-cling, toughness, resilience and clarity. Permeability is relatively high. Both oriented and unoriented films are available. Properties of PVC film are listed in Table 9.2. Heavier gauge PVC, sheet rather than film, is often used in thermoformed packaging, such as in blister packaging.

9.6.5 Polyethylene Terephthalate (PET) Polyethylene terephthalate is formed by condensation polymerisation of ethylene glycol and either terephthalic acid or dimethyl terephthalate. It is commonly used in biaxially oriented form, and has excellent transparency and mechanical properties. Heat-setting enables the film to be used for extended periods at temperatures ranging from –70 to +150 °C. It can tolerate considerably higher temperatures for short periods, such as in dual ovenable packaging for frozen foods. PET has good barrier properties, especially for odours and flavours. The barrier properties can be enhanced by coating with PVDC, or by metallising, as will be discussed in subsequent sections. Coating or coextrusion is often used to provide good heat-seal properties. Typical PET properties are listed in Table 9.3.

245


Table 9.3 Typical properties of polyethylene terephthalate (PET) films [1, 2, 5] Property

PET polymer Unoriented

Oriented

Tg (°C)

73-80

73-80

Tm (°C)

245-265

245-265

Heat distortion temperature, at 455 kPa (°C) 3

Density (g/cm )

38-129 1.29-1.40

1.40


2.8-4.1


48-72

220-270

Elongation (%)

30-3,000

70-110

WVTR, at 37.8 °C and 90% RH (g μm/m2 d)

390-510

440


1.2-2.4

1.1

9.6.6 Polyvinylidene Chloride (PVDC) Polyvinylidene chloride is an addition polymer of vinylidene chloride. It is an excellent barrier to oxygen, water vapour, odours and flavours. However, its high crystallinity and sensitivity to heat-induced degradation make it extremely difficult to process. Therefore, homopolymer PVDC is not used commercially. Copolymerisation of vinylidene chloride with various amounts and types of comonomers, usually vinyl chloride, acrylonitrile, methacrylonitrile, methacrylates or alkyl acrylates, produces a family of PVDC copolymer resins with improved processability, while maintaining desired barrier properties. Vinylidene chloride content typically ranges from 72 to 94 wt%; molecular weights range from about 65,000 to 150,000 [6]. In general, the highest barrier resins are not melt-processable, but instead are applied by solvent or latex coating. Extrudable resins have undergone more modification, so consequently have somewhat decreased barrier properties. PVDC films produced for household use are plasticised copolymers, and have even poorer barrier performance. However, they remain much better barriers than competitive polyethylene films. Representative properties are shown in Table 9.4. PVDC copolymer films can be heat-sealed. Therefore, in PVDC copolymer coatings or coextrusions, the PVDC can serve as a combination barrier and heat-seal layer. However, the best barrier films generally do not provide the best heat-seal capability, and vice versa, so when both heat-sealability and barrier are desired, sometimes two differently formulated PVDC copolymer coatings are applied.

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Table 9.4 Typical properties of polyvinylidene chloride (PVDC) films [1, 2, 6] PVDC polymer Property

Generalpurpose

Highbarrier

Tg (°C)

–15 to +2

–15 to +2

Tm (°C)

160-172

160-172

1.60-1.71

1.73


0.3-0.7

0.9-1.1


48-100

83-148

Elongation (%)

40-100

50-100

79

20

0.31-0.43

0.031

Heat distortion temperature, at 455 kPa (°C) Density (g/cm3)

WVTR, at 37.8 °C and 90% RH (g μm/m2 d) O2 permeability, at 25 °C (103 cm3 μm/m2 d atm)

Nearly all cellophane produced in North America is solvent-coated with PVDC copolymers. Solvent and latex coatings are also used on plastic sheet for thermoformed containers, and on blow-moulded plastic bottles. Common substrates include polyolefins, polyesters, polyamides and styrenics. Coextrusions of PVDC copolymer with polyethylene or polypropylene are used in shrinkable films for meat, cheese and other moisture- or oxygen-sensitive foods. Latex coatings of PVDC copolymers are used to provide moisture resistance, grease resistance and barrier to paper and paperboard packages.

9.6.7 Polychlorotrifluoroethylene (PCTFE) Polychlorotrifluoroethylene (PCTFE) is another polymer with good barrier characteristics, especially for water vapour. The homopolymer is very difficult to process because of its extremely high melt viscosity. A small amount of modification by copolymerisation yields AlliedSignal Corporation’s trademarked Aclar films, which contain greater than 95% chlorotrifluoroethylene by weight. These films are considered the best available transparent moisture barriers for flexible packaging; however, they are rather expensive. Aclar films can be used alone, or can be laminated to paper, polyethylene, aluminium 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.

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9.6.8 Polyvinyl Alcohol (PVOH) Polyvinyl alcohol films are unique in several respects. Polyvinyl alcohol polymers are produced by hydrolysis (more correctly, alcoholysis) of polyvinyl acetate. If fully hydrolysed, the polymer is readily soluble in water. Controlling the degree of hydrolysis can produce films that are soluble in hot water but not in cold water. Because PVOH degrades at temperatures well below melt, it cannot be processed by extrusion. Therefore, casting from a water solution is used to make film. As produced, the film is amorphous, but orientation induces some crystallinity. The water-solubility of PVOH is the major reason for its use in niche markets where this is a desired attribute. One application is as an inner pouch in packaging of agricultural or other chemicals, to limit human exposure. The pouch with its contents can be placed into the dilution and dispensing apparatus, without direct contact between the user and the chemical. In the water, the pouch dissolves, releasing the chemical. The dissolved polymer does not clog spray nozzles, and is biodegradable. Another application is in hospital laundry bags. Here, the hot-water-soluble variety is used. Soiled laundry is placed in the bags, and then bag and all can be placed into the washer, so that no contact between the launderer and the potentially infectious linen is required. Since the polymer does not dissolve in cold water, it will not be affected by residual liquid in the linens, but will dissolve readily in the hot wash water.

9.6.9 Ethylene-Vinyl Alcohol (EVOH) Ethylene-vinyl alcohol resins are produced by hydrolysis (alcoholysis) of ethylene-vinyl acetate random copolymer, analogous to the route for production of polyvinyl alcohol from polyvinyl acetate. Commercially available materials contain a substantial percentage of ethylene, typically 27 to 48 mol%. The presence of ethylene renders the resins melt-processable. The presence of –OH groups in the structure results in strong intermolecular hydrogen bonding. While EVOH is a random copolymer, CH2 and CHOH groups are isomorphous; they fit into the same crystalline structure. Therefore, the polymer crystallises readily. The combination of strong intermolecular forces and crystallinity makes it an excellent barrier to gases, odours and flavours. However, the hydrogen bonds also make it a moisture-sensitive material, and high humidity decreases its barrier capability. EVOH is most often used as an oxygen barrier. Since, in most applications, it is likely to be exposed to moisture either from the environment or in the product, it is usually used as a buried inner layer in a coextruded structure, where a good moisture barrier, often a

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Table 9.5 Typical properties of ethylene-vinyl alcohol (EVOH) films [1, 2, 7] EVOH polymer Property

32 mol% ethylene

44 mol% ethylene

Tg (°C)

69

55

Tm (°C)

181

164

Density (g/cm3)

1.19

1.14


2.6

2.1


77

59

Elongation (%)

230

380

WVTR, at 40 °C and 90% RH (g μm/m2 d)

1535

724

0.0078

0.030



polyolefin, protects it. Monolayer EVOH films, oriented or unoriented, are also available, which can be used alone but are usually combined with other materials by laminating, or coating. Typical EVOH properties are listed in Table 9.5.

9.6.10 Polyamide (Nylon) Polyamides, or Nylons, are a family of plastics containing characteristic amide functionality. They are commonly formed by condensation polymerisation of amino acids, or of carboxylic acids and amines. Nylon films are used for specialty applications in packaging, where performance requirements justify their relatively high cost. Nylons have excellent high-temperature performance, so can be used, for example, in boil-in-bag packages. Nylons also provide excellent odour and flavour barrier, and reasonably good oxygen barrier. They are very poor water vapour 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 EVOH. Most Nylons used in packaging have some crystallinity; the amount is heavily dependent on processing conditions, since Nylons have a narrow window for crystallisation. Films generally retain good flexibility at low temperatures, and have excellent strength properties. Owing to their relatively high cost, they are often coextruded with other plastics.

249


Table 9.6 Typical properties of polyamide (Nylon) films [1, 2] Polymer Property

Nylon-6

Nylon-11

Nylon MXD6

Tg (°C)

60

64

Tm (°C)

210-220

180-190

243

Density (g/cm3)

1.13-1.16

1.03-1.05

1.20-1.25


0.69-1.7

1.3

3.8-4.1


41-165

55-65

220-230

300

300-400

72-76

3,900-4,300

1,000-2,000

630

0.47-1.02

12.5

0.06-0.26


Elongation (%) WVTR, at 40 °C and 90% RH (g μm/m2 d) O2 permeability, at 25 °C (103 cm3 μm/m2 d atm)

Nylon MXD-6: Mitsubishi Gas Chemicals America, Inc., New York, NY, USA

Polyamides manufactured from straight-chain amines and carboxylic acids are typically named with numbers representing the number of carbons in each of the starting monomers. For example, Nylon-6,10 is made from a six-carbon amine and a ten-carbon carboxylic acid. Similarly, polyamides made from amino acids have a number designating the number of carbons in the acid. When the carbons are not in a straight chain, more complex names are necessary. Typical properties of some Nylon films are given in Table 9.6. Nylon-6 tends to be the most-used Nylon packaging film in the USA, and Nylon-11 in Europe.

9.6.11 Ethylene-Vinyl Acetate (EVA) and Acid Copolymer Films Ethylene-vinyl acetate is produced by addition copolymerisation of ethylene and vinyl acetate. The acetate groups provide polar functionality that increases intermolecular forces in the film, and, because of the structural irregularity thus introduced, interfere with crystallisation. These films have excellent transparency, and provide very good heatseal and adhesive properties, with excellent toughness at low temperatures. Typical filmgrade EVA resins contain between 5 and 18% vinyl acetate. Resins designed for use as an adhesive layer in a multilayer structure are typically at the higher end, and standalone films at the lower end, of this concentration range.

250

Applications of Plastic Films in Packaging Common markets for EVA are poultry and meat wrap, stretch film and ice bags. The films tend to be sticky, so may require the use of slip and antiblock additives. Copolymers of ethylene with acrylic acid and with methacrylic acid are also available, and are commonly called acid copolymer resins. They are characterised by good clarity, strong adhesion to polar substances such as paper, and also to foil, and low melt and heat-seal temperatures.

9.6.12 Ionomers Ionomers are formed by neutralisation of ethylene-acrylic acid or ethylene-methacrylic acid copolymers containing 7 to 30 wt% acid, to yield sodium or zinc salts. The resulting ionic bonds function as reversible crosslinks in the polymer, readily disrupted by heat, but reforming on cooling. Therefore, these materials provide very strong bonding to numerous substrates. Ionomers can be used for skin packaging to uncoated corrugated board, for example. The heat-seal performance of ionomers is outstanding, even permitting sealing through grease contamination, which makes them ideal for packaging of processed meat. They have superior hot tack, and excellent melt strength. Ionomer films have excellent clarity, flexibility, strength and toughness. 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, and tend to absorb water readily. They also are relatively high cost compared to films such as ethylene-vinyl acetate.

9.6.13 Other Plastics Several other types of plastics are used in packaging films to some extent. 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. Polystyrene is another film with excellent transparency, often used in window envelopes and window cartons. It has low gas barrier, so can be used for produce where a ‘breathable’ film is required. In heavier gauges, polystyrene is widely used for transparent thermoformed trays. Expanded polystyrene is used for trays, egg cartons and other applications where its cushioning properties are desired. In general, these materials are

251

Handbook of Plastic Films classified as sheet, rather than film. Polystyrene film is generally biaxially oriented to improve its properties, since the unmodified material is too brittle for most applications. Impact-modified polystyrene sheet that incorporates polybutadiene is often used where transparency can be sacrificed for impact resistance. 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 limits their usefulness. A wide variety of copolymers are available. Some of these have been discussed already. It is quite common to modify the chemical structure of a polymer to obtain a more desirable mix of properties. Another way to combine properties is to use blends of polymers. High-impact polystyrene (HIPS) is actually partially a copolymer and partially a blend of polybutadiene and polystyrene.

9.7 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, aluminium or even glass, through processes such as coating, lamination, coextrusion and metallisation.

9.7.1 Coating Coating is commonly used to add a thin layer of a plastic on the surface of another plastic film or sheet, 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 paper, cellophane, foil or plastics that are not themselves easily heat-sealed; 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. Coating with low-density polyethylene is often used on paper to give heat-sealability and moisture protection, as well as to protect printing from abrasion. It is often used on aluminium foil to provide heat-sealability and abrasion resistance, and to prevent interaction between the foil and the product. PVDC copolymer coatings are often used to improve barrier and heat-sealability.

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9.7.2 Lamination Lamination is the process of combining two webs of film together. In flexible packaging applications, lamination is often used to combine a plastic film with paper or foil, or to join paper and foil together. 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 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. When aluminium is used, it is most often employed for its excellent barrier to light and to permeation. Occasionally, it is used primarily for its desirable appearance. Another significant use of lamination is to produce a web with buried printing. In these materials, one web is reverse-printed, and is then laminated to a second web, either made from the same or a different polymer. The printing can be seen through the transparent plastic, and is protected against abrasion so it maintains a fresh attractive appearance much better than surface-printed materials.

9.7.3 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. Obviously, coextrusion cannot be used to incorporate nonthermoplastic materials. A major advantage of coextrusion over lamination is its ability to incorporate very thin layers of a material, much thinner than those which can be produced as a single web. This is 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. The thinness of the layer is not limited by the need to produce an unsupported film and handle it in a subsequent lamination step.

9.7.4 Metallisation Metallisation is a way of applying a thin metal layer on a plastic film (or on paper), as an alternative to using a lamination with aluminium foil. In commercial packaging practice, the metal being deposited is almost always aluminium. The process, known as vacuum

253

Handbook of Plastic Films metallising, involves evaporation of aluminium inside a vacuum chamber, and deposition of the aluminium vapour on a plastic film. The operation is usually done in a batch mode, with the substrate being metallised and an aluminium wire placed inside the vacuum chamber. The film is rolled past a chill roll, which removes the heat from the condensing aluminium, preventing melting of the film. Very high vacuums are needed, which has retarded the development of continuous metallisation processes, although some are available. Vaporisation of the aluminium is most often achieved by resistance heating. Induction and electron-beam heating are used to a lesser extent. Metallised films have significantly enhanced barrier characteristics, and are usually chosen for this reason. Cost of metallised film is generally less than that of foil-containing laminated materials. In snack packaging, for example, metallised film has almost totally replaced foil laminations. The barrier performance of metallised film, as initially produced, is somewhat inferior to foil, and is dependent on the thickness of the deposited metal layer. However, stress during product distribution can lead to the development of flex cracks in foil, which then provide a route for gas transfer. Metallised foil, since it retains the flexibility and other mechanical characteristics of the film substrate, is not usually subject to flex cracking. Therefore, the barrier characteristics of metallised foil are sometimes superior to those of foil laminations, at later points in distribution. Also, many oxygen-sensitive products require better barrier than can be attained with plastic alone, but can be successfully protected with metallised film. In addition to gas barrier, metallised film provides an essentially total light barrier. Occasionally, metallised foil is used for its appearance, rather than for its barrier characteristics. This is particularly the case when it is used for labels. In many such applications, however, paper, rather than film, is the metallisation substrate.

9.7.5 Silicon Oxide Coating One of the disadvantages of metallised film is that the resultant material is opaque, and is not suitable for use in microwave ovens. The desire for transparent high-barrier coatings led to the development of glass-like coatings based on silicon oxide, SiO2. Silicon oxide coatings on film are usually applied in a manner analogous to vacuum metallising. The silicon oxide is evaporated, using electron-beam heating, and condensed on the film substrate in a vacuum chamber. The film is very thin, 400 to 1000 Å, and does not affect the mechanical properties of the material to any significant degree. The chemical composition of the deposited film depends somewhat on conditions, and is

254

Applications of Plastic Films in Packaging characterised as SiOx, where the value of x is between 1.0 and 2.0. At values close to 1, the layer imparts a distinct yellowish colour to the film. At values close to 2, it is nearly colourless. The yellowing is a significant concern in some applications. The SiOx layer greatly increases barrier of the film, and it is transparent to microwave radiation. Therefore, it can be used in packages that will be heated in microwave ovens. The most common substrate is PET, in thickness of 12.5-25 μm, although polypropylene, polystyrene and polyamides can also be used. An alternative to evaporative deposition is chemical plasma deposition, in which a siliconcontaining gas such as tetramethyldisiloxane or hexamethyldisiloxane is used as the silica source. Little heat is required, and the degree of vacuum needed is lower. Therefore, plasma deposition can be used on heat-sensitive materials such as LDPE and oriented PP. The coating produced is thinner, and less yellow. Plasma deposition is the method of choice for SiOx coating of containers, and can also be used for film.

9.7.6 Other Inorganic Barrier Coatings Processes have also been developed that deposit aluminium oxide coatings on plastic films, to increase barrier properties. Combinations of SiO and MgO have also been used. Another type of inorganic barrier coating uses clay nanocomposites, which are deposited on the film from a solution of PVOH/EVOH copolymer, in a mix of water and isopropyl alcohol, with nanodispersed 7 nm diameter silica and titanium dioxide particles. Microgravure equipment is used to coat the solution on to the film substrate. Barrier is reportedly comparable to that of films metallised with aluminium, but the coatings are transparent. These materials are all still in relatively early phases of development.

9.8 Surface Treatment In many packaging applications, it is necessary for something to stick to a plastic film. This may involve placing a label on a pouch or on a stretch-wrapped pallet, adhering two films together in a lamination, or, as is often the case, printing the film. Adequate adhesion requires that secondary bonding forces between the film and the object, such as the ink, which is to be adhered, be sufficient to retain the material. Historically, this has been a significant problem for plastic films, since the surface energy of the films is often low, causing poor adhesion. Several techniques are commonly used to increase the surface energy of polymers, hence improving adhesion. For films, the most common treatment is corona discharge.

255

Handbook of Plastic Films In corona discharge treatment, the film surface is exposed to a discharge between grounded and powered electrodes, at high voltage. The discharge of the electric current ionises the air in the gap between the electrodes. The ions produced initiate free-radical reactions with the film surface, causing bond cleavage followed by oxidation. Oxidation of the surface increases its ability to adhere to substances such as inks and adhesives. The effectiveness of corona discharge treatment dissipates with time, so ideally it should be applied within a short time of the subsequent printing. In a roll of corona dischargetreated material that has been stored, the effectiveness of the treatment is likely to be significantly higher on the inside layers than on the first few outside layers of film. Other surface treatments that are sometimes applied to film also exist. However, corona discharge is by far the most frequently encountered.

9.9 Static Discharge Plastics, because they are nonconductive, are subject to build-up of electrostatic charges. When such charges build up, the result can range from the attraction of dust and lint to material handling problems, shocks and sparks. Methods for controlling the build-up of static charges include charge neutralisation through ionisation of the surrounding air and incorporation of conductive materials to dissipate the charge. Antistatic agents can be incorporated into the film as additives, or can be used as a surface treatment. The agents commonly used include non-ionic ethoxylated alkylamines, anionic aliphatic sulfonates and phosphates, and cationic quaternary ammonium compounds. In some cases, humidifying the area can control static, so that a thin layer of water is absorbed on the film surface, which conducts the charge to ground. Control of static discharge is especially important for packaging for sensitive electronic devices. Film designed for such applications, usually polyethylene, is generally pigmented pink to denote that it contains antistatic agents.

9.10 Printing In many packaging applications, plastic films are printed to convey information to the user. When printing is desired, it is usually done on roll stock before packages, such as pouches, are formed. Printing on formed flexible packages is usually limited to date or lot coding. Flexography is the printing method used most often for flexible packaging materials. In this process, a subcategory of relief printing, the printing plates are flexible elastomers,

256

Applications of Plastic Films in Packaging with the images, or printing areas, raised above the nonprinting surrounding areas. Thin, highly fluid, rapid-drying inks are used. The ink is transferred by a system of rollers to the top surface of the printing plates, which in turn transfer the ink to the film. In lithography, the printing image and the background are on the same plane of the thin metal printing plate. The plate is treated to attract water and repel ink in the non-image areas, and the reverse in the image areas. A system of rollers is used to transfer both ink and water to the plate. The image on the plate is then transferred (offset) to a rubberblanket-covered cylinder, and then to the film. Rotogravure uses copper-plated printing cylinders, which have the image engraved into the cylinder in the form of tiny cells. The cylinder rotates in an ink bath, filling the cells with ink. Excess ink is wiped off by a doctor blade, and then the image is transferred to the film as it is pressed against the printing cylinder by an elastomer-covered impression cylinder. For printing date and lot codes on formed packages, ink-jet printing is commonly used. In this process, electrically charged drops of ink are sprayed out of jets, and electrostatically directed to the desired printing location. This is an impactless form of printing, and is ideal for printing rapidly changing information such as these codes. Other types of printing, such as screen printing, as well as variations of the basic processes described above, are used less frequently for plastic packaging films.

9.11 Barriers and Permeation As has been discussed, in many packaging applications, protection of the product from gain or loss of gases or vapours is important. 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. The barrier performance of the film is generally expressed in terms of its permeability coefficient. 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=

Ql AtΔp

(9.1)

257

Handbook of Plastic Films where P is the permeability coefficient, Q is the mass of permeant passing through the material, l is the thickness of the plastic film, A is the surface area available for mass transfer, t is time, and Δp is the change in permeant partial pressure across the film. It can be shown that the permeability coefficient, as defined by equation (9.1), is equal to the product of the Fick’s law diffusion coefficient, D, and the Henry’s law solubility coefficient, S, in situations where these laws adequately represent mass transfer (ideally dilute solutions, diffusion independent of concentration): P = DS

(9.2)

The permeability coefficient, under these circumstances, is a function of temperature, but is not a function of film thickness or permeant concentration. While this is a very simplified approach to mass transfer, it is adequate for many packaging situations. For example, with oxygen-sensitive products, reaction with oxygen is commonly rapid compared to the rate of transfer, so the oxygen concentration within the package is relatively constant at nearly zero. Oxygen concentration in the surrounding air, measured as partial pressure, is constant at approximately 21 kPa. Regardless of the shape of the flexible package, mass transfer is essentially one-dimensional, through the thickness of the film. If temperature is constant and P is known, the amount of oxygen transported through the film in a given period can be easily calculated using equation (9.1). Conversely, if the sensitivity of the product is known in terms of the maximum amount of oxygen that can be taken up without resulting in unacceptable product quality, the time required for that amount of transfer (the product shelf-life) can be calculated. A similar approach can often be taken for transfer of odour or flavour compounds. While the diffusivity, and hence the permeability coefficient, of such organic substances is likely to be concentration-dependent, at the low levels associated with most packaging situations, the dependence is slight. Calculating shelf-life when water vapour transmission is involved is more problematic. In such cases, the partial pressure difference for water vapour between the inside and the outside of the package is almost never constant. Simplifying assumptions generally used consider the time for moisture in the product itself and in the product headspace to reach equilibrium to be small compared to the time required for permeation, and ignore moisture change in the headspace itself, calculating only moisture gain or loss in the product. The resulting differential equation is: dQ 1 = PA( p2 − p1 ) dt l

258

(9.3)

Applications of Plastic Films in Packaging where p1 is the partial pressure of water vapour outside the package, p2 is the partial pressure of water vapour inside the package, and p2 is a function of Q. Solution of this equation requires knowledge of the moisture sorption isotherm for the product, which relates the moisture content of the product to the equilibrium relative humidity of the air in contact with the product, and thus to p2. In the case where the sorption isotherm at the storage temperature can be approximated as linear over the range of moisture contents of interest, it can be written as: W = a + bM

(9.4)

where W is the water activity of the air in equilibrium with the product with moisture content M (dry weight basis), and a and b are the best-fitting straight-line constants. Rewriting the basic permeability equation [equation (9.3)] in terms of water activity, and substituting: Q = (M – Mi)w

(9.5)

where Mi is the initial moisture content and w is the dry weight of the product gives: w

dM PAps = (W2 − W1 ) dt l

(9.6)

where W1 and W2 are the water activities at times 1 and 2, and ps is the saturation water vapour pressure at the storage temperature. This equation can be integrated, giving the following relationship for moisture gain or loss:

[ [

] ]

1 ⎛ W2 − W1 t ⎞ PApst - ln⎜ ⎟= b ⎝ W2 − W1 ⎠ lw 0

(9.7)

Mass transfer characteristics for plastics are often expressed in terms of water vapour transmission rates (WVTR), rather than permeability coefficients. WVTR reflect the rate of water vapour transfer under specific conditions, and must be translated to permeability coefficients for application at different conditions. The relationship between Pwater and WVTR is the following: Pwater =

WVTR Δp

(9.8)

259

Handbook of Plastic Films where Δp is the difference in water vapour partial pressure under the conditions at which the WVTR was measured. In many cases, this was the ASTM standard condition of 32.2 °C (90 °F) and 90% relative humidity (RH). 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: 1 ⎤ ⎪⎫ ⎪⎧⎛ E ⎞ ⎡ 1 P2 = P1 exp⎨⎜ a ⎟ ⎢ − ⎥ ⎬ ⎪⎩⎝ R ⎠ ⎣ T1 T2 ⎦ ⎪⎭

(9.9)

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. Care must be taken in applying equation (9.9). 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, equation (9.9) is valid only over reasonably small temperature ranges. A particular concern is that permeation rates are much higher above the Tg than below this temperature, and the rate of change with temperature differs. Therefore, equation (9.9) should never be used to calculate the permeability coefficient across a temperature range that spans Tg of the plastic. Permeability coefficients for multilayer plastic film or sheet, either coextrusions or laminations, can be calculated from the thickness and permeability coefficients of the individual layers, as follows: Pt =

lt

∑ (l / P ) i=n

i=1 i

i

(9.10)

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. Special care must be taken when the barrier characteristics of a polymer are affected by the presence of the permeant or of some other substance that may also be permeating. This situation is most often encountered with water-sensitive plastics, such as ethylenevinyl alcohol, since co-permeation of water vapour and other components of interest, such as oxygen, may well occur during processing and storage. It may also arise in other situations, such as co-permeation of organics involved in odour and taste.

260


9.12 Environmental Issues In recent years, consideration of the environmental effects of packaging decisions has become more common. While thorough discussion of such issues is beyond the scope of this chapter, some general observations and conclusions will be made. It is generally agreed that evaluation of the environmental impacts of a product or package requires consideration of the total life-cycle of the object. Such ‘cradle to grave’ analysis is commonly referred to as life-cycle assessment. Usually, when such analyses are carried out, the most influential life-cycle stage is that of production of the raw materials and packages, rather than transportation or disposal. Packages that minimise material use are therefore likely to have reduced environmental impact. Since flexible packaging systems usually (although not always, since distribution packaging must be included) use less overall packaging material, they often have reduced environmental impact, compared to the rigid packaging systems they replace. In examining the impacts of waste disposal, two general conclusions can be drawn. In most cases, flexible packaging is less likely to be recovered for recycling than rigid or semi-rigid packaging. Therefore, a higher proportion of flexible packaging is likely to require disposal. On the other hand, flexible packaging, as discussed above, usually means less total material requires handling. Unless recycling rates for the alternatives to the flexible packages are very high, use of flexible packaging is likely to mean less material requiring disposal. Also, flexible packages containing plastics are sources of recoverable energy in appropriate systems.

References 1.

R.J. Hernandez, S.E.M. Selke and J.D. Culter, Plastics Packaging: Properties, Processing, Applications, and Regulations, Hanser, Munich, Germany, 2000.

2.

S.E.M. Selke, Understanding Plastics Packaging Technology, Hanser, Munich, Germany, 1997.

3.

D. Kong in The Wiley Encyclopedia of Packaging Technology, Eds., A.L. Brody and K.S Marsh, Wiley, New York, NY, USA, 1997, 407.

4.

E. Mount and J. Wagner in The Wiley Encyclopedia of Packaging Technology, Eds., A.L. Brody and K.S. Marsh, Wiley, New York, NY, USA, 1997, 415.

5.

J. Newton in The Wiley Encyclopedia of Packaging Technology, Eds., A.L. Brody and K.S. Marsh, Wiley, New York, NY, USA, 1997, 408.

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P. DeLassus, W. Brown and B. Howell in The Wiley Encyclopedia of Packaging Technology, Eds., A.L. Brody and K.S. Marsh, Wiley, New York, NY, USA, 1997, 958.

7.

R. Foster Newton in The Wiley Encyclopedia of Packaging Technology, Eds., A.L. Brody and K.S. Marsh, Wiley, New York, NY, USA, 1997, 355.

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10

Applications of Plastic Films in Agriculture E.M. Abdel-Bary, A.A. Yehia and A.A. Mansour

10.1 Introduction The quantity of plastic materials used annually in the world in the agricultural sector amounts to 2 million tons. About 50% of this is used in protected cultivation greenhouses, as mulching, for low tunnels, as temporary coverings of structures for fruit trees, etc. [1]. Thin plastic film produced with low investment is economically and technically feasible, and provides the best cost/benefit ratio for use in greenhouses and low tunnels. The area covered by both greenhouses and tunnels has been experiencing continual growth. This growth is expected to appear in many countries where protected cultivation replaces the traditionally used more expensive glass-clad greenhouses. Low-density polyethylene (LDPE), ethylene-vinyl acetate (EVA) and linear low-density polyethylene (LLDPE) films are the most common greenhouse covering films in agriculture. This chapter looks at the production of polyethylene-based plastic films for protected cultivation. The mechanical properties that make these films suitable for the use in agriculture are discussed. The stability of these plastic films under the effects of different environmental conditions is reported. These include solar irradiation, temperature, humidity, wind, fog formation and pesticides. Types of ultraviolet (UV) stabilisers and a determination of their compatibility are given. Also, the recycling of plastic films used in agriculture is of great importance, and a case study of their recycling as agricultural films is given.

10.2 Production of Plastic Films LDPE films dominate the market for protected cultivation in the countries of both the Mediterranean region and worldwide. Most of these contain special additives, which are used either to enhance the performance of the film in the special conditions met in a greenhouse, or to prolong its lifetime by minimising the effects of the environment on the structure of the film. Advances in the formulation of the LDPE films in use today have led to an expected lifetime of between one and five cultivating seasons [2]. The expected lifetime is, in fact, significantly affected by the environmental conditions that the film will face during its use. The climate of the region, the greenhouse design, the microclimate developing inside

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Handbook of Plastic Films the greenhouse, the use of agrochemicals and the environmental pollution of the area can all severely affect the lifetime of the material by inducing ageing of the plastic film to various degrees. Thus, a film whose lifetime is estimated to be four seasons in NorthCentral Europe will only last two or three seasons in the Mediterranean [2]. The varying requirements for greenhouse systems between different regions because of different climatic conditions and differences in production methods has led, until very recently, to a significant variation in approaches, standards and practices adopted or implemented by the interested National Research Institutions, Commercial Agencies and relevant industries [3]. Some of the consequences of this differentiation and variability are reflected in the lack of standardisation concerning the testing methods for covering materials of greenhouses. Usually, the testing methods used for plastics in general are also applied to greenhouse covers, despite important functional differences. As a consequence, quality control data provided by the producers of the covering materials of greenhouses are usually limited to only a few properties of these materials. In most cases, it is not possible to reproduce the relevant technical information provided, as this is not obtained systematically and is available in a somewhat confusing way [3]. The manufacturing of plastic films used in agriculture is usually carried out by blown film extrusion (tubular extrusion). Readers are asked to consult Chapters 1 and 2, where the manufacturing process is given in detail.

10.3 Characteristics of Plastic Films Used in Agriculture The film products of interest here have been evaluated for their applied efficiency on the basis of their characteristics and requirements related to mechanical resistance, total percentage light transmittance (T %) to visible solar and long-wavelength ultraviolet (UV-A) radiation, useful lifetime and energy-saving potential (greenhouse effect). Visible solar radiation regulates the nutrition of plants through the ‘chlorophyll function’. Longwavelength ultraviolet radiation favours the formation of pigments and vitamins, which is advantageous for the quality characteristics of the crop involved, with regard to flavour, intensity of colour, perfume or smell and good keeping of fruit or vegetables. With reference to energy saving, values of the total thermal transmittance measured in W/m2 °C for the different covering materials and their possible combinations have been estimated. The total thermal transmittance of a covering material indicates the general heat loss (management, convection, radiation), estimated in watts, through a 1 m2 surface referred to a difference of 1 °C between the internal and external environmental temperatures of the prepared covering. These values enable estimation of the theoretical thermal yield of a manufactured film related to the heating needs for a thermal difference of 1 °C across the film.

Applications of Plastic Films in Agriculture The incident heat calculation enables the agricultural operator to choose the proper covering material with respect to any thermal (°C) and luminous (flux) needs of the species to be established in the agricultural crop rotation desired. This can be done by using the results showing the total thermal transmittance (W/m2 °C) and total light transmittance (T %) of some materials for greenhouse covering [4].

10.4 Stability of Greenhouse Films to Solar Irradiation The performance and lifetime of the plastic films used as covering materials in protected cultivation depend strongly on: (a) the original chemical structure of the materials, (b) the change in the properties of the material brought about by induced ageing, (c) the type of physical structure used, (d) the climatic conditions of the area where the structure is installed, and (e) the use of agrochemicals, among other things. A brief description of the factors affecting the stability of polyethylene (PE) as a greenhouse covering under the effect of different environmental conditions is given below. It is well known that photodegradation of many plastic materials occurs on subjecting these materials to solar radiation with wavelengths of 290-1400 nm [5, 6], the most energetic part of the solar spectrum. UV radiation in the range 290-400 nm can be absorbed by the plastic, and this is followed by bond cleavage and depolymerisation, causing photodegradation. The photodegradation process of the covering materials of a greenhouse is further complicated by various interacting factors. The effect of UV radiation combined with varying temperature, humidity, critical mechanical loads, friction, abrasion, exposure to agrochemicals, etc., accelerate ageing at various rates. Accordingly, it is difficult to predict the lifetime of plastic films by laboratory testing of the photostability of films. For instance, high abrasion of the film by sand or soil particles carried by the wind leads to the formation of high concentrations of active centres giving rise to an increase in photodegradation.

10.4.1 Ultraviolet Stabilisers Theoretically, LDPE should be stable under the effect of UV due to its stable structure and the absence of chromophores. However, during processing, it suffers partial oxidation, in which carbonyl and hydroxyl groups are formed. Also, it contains some impurities (photo-absorbing chromophores). Both impart photosensitivity to LDPE films [7, 8]. Special measures are therefore needed in order to protect greenhouse films against solar radiation and especially its most energetic and therefore harmful portion, namely, UV

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Handbook of Plastic Films radiation. Inhibition or at least retardation of the reaction responsible for degradation is, of course, a necessity for successful UV stabilisation. Retardation or protection against photodegradation can take place by using additives. These additives may retard the photodegradation of the polymer in three ways, namely, ultraviolet screening, ultraviolet absorption and excited-state quenching. Thus, stabilisers are often included in the polymer to provide stability against photooxidation to protect the material from UV light damage. The effectiveness of a light stabiliser depends on many factors, including its solubility and concentration in the polymer matrix [9].

10.4.1.1 Ultraviolet Screening Ultraviolet screening compounds are based on inorganic or organic additives. In this type of protection, the ultraviolet light is blocked before it can reach the polymer. Screening is provided by pigments or by reflective coatings. Carbon black is also very effective and is used to stabilise many outdoor grades of polymers. In this case of UV screeners, any damage is confined to surface regions because UV penetration is restricted to very short distances. However, many of the pigments, like chalk, talc, short glass fibres and carbon black, impart an unattractive appearance, the grey, brown and black colours generally being unappealing. TiO2 is another common additive, which may act as a screener, but it may occur in different forms, some of which are chemically active and can promote photodegradation. The first class of organic additives for improving the resistance to UV radiation is the UV absorbers. They act by absorbing the harmful UV radiation above 290 nm, and thus do not allow it to reach the chromophores present in the chemical structure of LDPE as a result of processing or as impurities. Many organic compounds absorb light in the desired region but few act as stabilisers. Some have little or no effect when added to polymers and may actually increase the rate of degradation. For a UV absorber to be effective, it must be able to dispose of its excitation energy without interacting with the polymer in harmful ways and without undergoing any photochemical reaction that would destroy its effectiveness. Accordingly, a stabiliser must have a structure that provides a rapid cascade back to the ground state through thermally excited levels with a quantitative efficiency for return to the ground state not less than 0.999%, i.e., less than one molecule can be destroyed for every 100,000 molecules that are excited. Derivatives of o-hydroxybenzophenone or benzotriazole are examples of UV absorbers. However, this class of stabilisers seems to perform better in thicker materials and not well in the thin LDPE greenhouse films [8].

Applications of Plastic Films in Agriculture

10.4.1.2 Excited-State Quenchers The second class of UV stabilisers is the nickel excited-state quenchers. These quenchers act by deactivating the excited states of the chromophoric groups responsible for the photo-initiation by energy transfer, instead of relying on direct absorption of the UV radiation [8]. With proper selection of the Ni quenchers, the results of the UV stabilisation are satisfactory. A typical example of a nickel excited-state quencher is nickel dibutyldithiocarbamate. However, formulations containing such Ni quenchers are prohibited because of the environmental impact of nickel compounds.

10.4.1.3 Hindered-Amine Light Stabilisers (HALS) HALS, based on bis(2,2,6,6-tetramethyl-4-piperidyl) sebacate, are the most recent and innovative class of light stabilisers. HALS do not absorb any light above 250 nm and so cannot be regarded as UV absorbers or as excited-state quenchers. Through oxidation of the piperidyl group to the nitrosyl group, a radical becomes available that starts a very efficient cycle of radical scavenging and peroxide decomposition. Thus, HALS are converted to the corresponding nitroxyl radicals, which are the real species responsible for polymer stabilisation. Hindered nitroxyl radicals are effective chain breaking antioxidants that act by trapping alkyl radicals to give hydroxylamines and/or alkylhydroxylamines – the former regenerates nitroxyl. The overall high efficiency of HALS as UV stabilisers in polyolefins is attributed to the regeneration of the nitroxyl radical. The complementary nature of the chain breaking antioxidant mechanisms involved [10] are shown in Scheme 10.1. From these reactions, nitroxyl and alkoxyl radicals are formed according to equations (10.1) and (10.3)-(10.5). These radicals act as scavengers for any radicals formed during UV irradiation [equation (10.2)]. This means that HALS operate as excellent antioxidants. Some of the HALS contain further antioxidant groups; others are polymeric and less extractable. The main difference between UV absorbers and HALS is that the former absorb UV radiation and in turn are destroyed by it, while the latter do not absorb UV radiation and are much more slowly altered by secondary side reactions. Thus HALS act as radical traps for radicals produced from photochemical oxidation [8, 9]. They offer an excellent approach to ultraviolet stabilisation and have replaced nickel quenchers and ultraviolet absorbers in many applications. Highly efficient chemically resistant light stabiliser systems have been developed. Market demands for extended-life greenhouses and thinner mulch films require even more powerful stabilisers. New noninteracting chemistries based on alkoxylamine HALS will offer a new generation of stabilisers for agricultural polyethylene films.

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hν, [O]

N

H

N

O• + P•

N

O

N

ΔH

O•

N

P + POO•

NH + POOH

N• + POO•

(10.1)

O

P

N

(10.2)

O• + POOP

(10.3)

N• + H2O + PO•

(10.4)

NO• + PO•

(10.5)

Scheme 10.1

10.4.2 Requirements for Stabiliser Efficiency The effectiveness of long-time stabilisation depends not only on the chemical nature but also on the rate of additive loss, which in turn depends on the compatibility of the additive with the polymer and is controlled by its volatility, solubility and diffusion coefficient.

10.4.2.1 Compatibility of the Additive Compatibility is the main problem for light stabilisers, because light stabilisers are generally used in concentrations up to 2%. Ideally, stabiliser molecules should be disposed singly throughout the polymer matrix. This will not generally happen, but compatibility with the polymer should be sufficient to prevent gross phase separation.

Applications of Plastic Films in Agriculture Stabilisers are normally dissolved in the polymer melt at the processing temperature. However, their solubility limit may be exceeded on cooling and this may lead to visually observable blooming. These processes depend on the nature of the chosen substrate as well as on its morphology. Additive diffusion is primarily a consequence of the thermal motion of the polymer chains above the glass transition temperature and of the related formation and disappearance of free volumes. If the chains are flexible and move easily, only small amounts of energy are necessary to move the polymer segments. With increasing orientation of the polymer chains or on crosslinking and with increasing crystallinity, the diffusion constant decreases. For this reason, the diffusion of additives is faster in LDPE than in high-density polyethylene (HDPE). Numerous studies [11-14] confirm that the diffusion behaviour of UV absorbers depends mainly on polymer structure and morphology and to a minor extent on additive structure. The solubility and compatibility of light stabilisers are particularly a problem when highly polar light stabilisers are used for non-polar plastics such as polyolefin. However, even in polyurethane, the compatibility of light stabilisers may become a problem.

10.4.2.2 Determination of Compatibility Stabilisers such as antioxidants, metal deactivators and UV absorbers are added to polymers to reduce degradation during the manufacturing process and throughout the lifetime of the polymer products. In order to study the degradation of polymers or the compatibility between additives and polymers, it is essential to have an analytical method that can provide both identification and a quantitative measure of additives in the polymers. Fourier transform infrared (FTIR) spectroscopy [15, 16], UV spectroscopy [17], near-infrared reflectance gas chromatography, high-performance liquid chromatography (HPLC) and differential scanning calorimetry (DSC) can all be used as analytical tools for identification and determination of the concentration of dissolved stabilisers and their homogeneous distribution. FTIR and UV spectroscopy are the most important techniques used, as they can be applied directly to the sample without disturbing the morphology in the solid state. In addition, it is possible to detect any degradation or changes taking place at earlier stages due to the sensitivity of these tools. Furthermore, the diffusion coefficient of additives can be estimated by using the disc-stacking technique [18], where a disc doped with the additives is placed in the middle of a stack of undoped discs. Diffusion is then allowed to take place at an appropriate temperature for an appropriate time. Then spectroscopic measurements can be done on different discs to evaluate the concentration of the additives in each disc. Accordingly, the diffusion

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Handbook of Plastic Films coefficient can be determined by knowing both the thickness of the discs and the concentration of the additive by using a characteristic absorption band. The principle behind the use of near-infrared reflectance analysis is the measurement of the light reflected by a sample when exposed to light in this region. The logarithm of the inverse of this reflected light can be related to the concentration of a particular component found in the sample. The obtained concentration value for different discs can also be used for determination of either the diffusion coefficient or the solubility of the additive. The same can be said for gas chromatography or HPLC. On the other hand, differential scanning calorimetry can be used to determine the exothermic peak of the oxidation of antioxidants, which allows direct determination of the concentration of the dissolved antioxidants. However, the commonly used methods for determination of compatibility do not give adequate information about the molecular association of the stabilisers, but only information about the volume concentration of the stabilisers in the polymeric matrix. This means that it makes no difference whether the stabilisers are present as stacks of molecules or as single molecules. Such aggregation will lead to lower efficiency of stabilisers. Of course, FTIR can offer some information from the change in the perturbation potential that results from polymer-stabiliser and stabiliser-stabiliser molecular interactions. However, this needs a very careful study of the sample, and to have references for molecularly dispersed stabilisers as well as sophisticated calculation for the obtained spectra. On the other hand, broad-band dielectric spectroscopy [19-22] can be applied to investigate solubility and compatibility, as it offers an excellent possibility of detecting the molecular reorientation of stabiliser molecules and segments simultaneously at the same temperature. Accordingly, the degree of compatibility with most polymeric segments can be evaluated, where detailed investigation of the molecular dynamics have been carried out [19-22] for various additives having different shapes, sizes and polarities in different polymeric matrices. An empirical relation that determines the dependence of the relaxation frequency differences between the cooperative process of the additive and the glass process of the matrix (macro-Brownian cooperative reorientation of the segment associated with the glass temperature) and the additive length has been given [21]: Δ log fm = 4 log[(L/d) – 1]

(10.6)

where Δ log fm is the difference between log fm of the cooperative process of the additive and log fm of the glass process of the matrix; L is the length of the additive; and d is the

Applications of Plastic Films in Agriculture polymer inter-chain distance. This equation was found to be valid not only for the relaxation process of an additive in a polymer but also for the δ relaxation process of side-chain liquid-crystalline polymers and their additives. This equation implies that, if the molecules are molecularly dispersed, and have a length not longer than 1.8 nm, they must relax cooperatively with the cooperatively reorienting segments of the glass process at the same relaxation frequency. However, in the case of Tinuvin P, a commercially available UV stabiliser (a benzotriazole derivative), the difference in the relaxation frequency maxima of the stabiliser peak and the glass process of the matrix, Δ log fm, is greater than three decades of frequency. These results indicate that the reorientations of the stabiliser are not coupled with the glass process of polystyrene segments, where the stabiliser molecules can relax locally at higher frequencies, (i.e., faster by a factor of 1000). Furthermore, short additives can relax either cooperatively with the polymeric segments at the same relaxation frequency as the segments, or locally at higher frequencies. The ratio of the local contributions to the total relaxation strength (cooperative plus local) of the additives depends on the size of the stabiliser. The biodegradation of representative samples of available commercial photo(bio)degradable polyethylene films was examined with respect to the rate and extent of degradation, oxidation products and changes in molecular weight both during outdoor exposure and in laboratory photo-ageing devices with different accelerating factors [23]. Although the rate of photooxidation was found to depend on the type of degradation system used, all the samples showed a rapid rate of carbonyl formation, with concomitant reduction in molecular weight and mechanical properties on exposure to UV light. The photo-fragmented polymers were shown to be much more hydrophilic in nature compared to the unoxidised analogues, and photo-fragments of all samples were found to contain high levels of low molecular weight (low molar mass) bioassimble carboxylic acids and esters. The recycling behaviour of virgin polyolefins, both as homopolymers and as heterogeneous polymer blends, which contained 10% of non-oxidised and photooxidised photo(bio)degradable plastics, has been examined. It was found that the initial mechanical performance of homogeneous blends was not greatly affected by the presence of nonoxidised degradable materials. However, blends containing degradable films that were initially partially photooxidised had a much more detrimental effect on the properties of the recycled blends during processing and weathering; the effect was minimal for degradable polymers containing the iron-nickel dithiocarbamate system.

10.4.3 Evaluation of Laboratory and Outdoor Photooxidation Laboratory and outdoor photooxidation of plastic films were evaluated using different techniques [24]. Thus melt blown, biaxially oriented, unstabilised and stabilised LDPE films with various thicknesses were exposed in two accelerated artificial weathering devices

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Handbook of Plastic Films with xenon (Xenotest) and UV-B fluorescent tube (QUV Weatherometer) sources under controlled temperature and humidity conditions. The structural changes during combined photo- and thermal degradation have been studied using tensile tensiometric, IR spectrophotometric and DSC methods. The effects of HALS and film thickness on the time-dependent changes in elongation, carbonyl group concentration, crystallinity and the onset temperature (Ton) of the post-fusion DSC oxidation exotherm have been observed. Photooxidation is accompanied by increased crystallinity, which maximises as mechanical properties start to deteriorate significantly and the rate of carbonyl group formation increases. While there is a poor correlation between the reduction in mechanical properties and increased carbonyl index values, the former correlates well with the DSCderived Ton values for unstabilised and stabilised films. This suggests that thermal analysis may be used to detect the physicochemical changes occurring in exposed films more effectively than other techniques such as IR. However, many problems of premature film failure can occur during their use in greenhouses, due to their interaction with the agrochemicals used. Both sulfur- and chlorine-containing agrochemicals inhibit the functioning of HALS, and can have a very detrimental effect on the life of greenhouse films [25, 26]. The concentration of HALS in LDPE covering films before and after exposure to natural weathering and accelerated photooxidation conditions has been determined [27]. It has been found that photostabiliser disappearance above 0.4% up to 600 days is mostly probably the result of its physical loss during long photooxidation times under both photooxidative conditions. On the contrary, photostabiliser disappearance in the initial stage is due to chain scission and the consequent volatilisation and diffusion of these fragments on the surface.

10.5 Other Factors Affecting the Stability of Greenhouse Films 10.5.1 Temperature Cyclic temperature changes and the high temperatures developed at the metallic parts of greenhouse constructions during hot and sunny days lead to increased degradation. One can observe a lot of damage in the places where the plastic greenhouse films come into contact with the metallic structural elements, especially when they are not painted. The temperature at these contact points may reach up to 70 °C and more depending on climatic conditions. In this case the diffusion of metal ions enhances the degradation process. Metal particles, especially iron, may catalyse the decomposition of hydroperoxides formed as a result of oxidation, leading to unnecessarily high rates of degradation. The degradation

Applications of Plastic Films in Agriculture mechanism of PE films containing additives with metal ions at a simulated composting temperature has been studied. The hydroperoxide concentration [POOH] in the films was traced quantitatively by using iodometric potentiometric titration, and compared with Fourier transform infrared spectrometry (FTIR). The results show that [POOH] increases during the early stage of degradation, followed by a more or less flat maximum, before it starts to decrease. At the same time, similar results are obtained by FTIR analysis. It is also found that the rate laws for the carbonyl index and [POOH] increases seem more complicated than an exponential-type increase in the early stage of oxidation [28]. Moreover, high temperatures lead to an increase in the rate of reaction for both photooxidation and chemical oxidation by agrochemicals, and thus to higher degradation rates. As mentioned before, HALS compounds act as radical traps, and consequently they also act as heat stabilisers and minimise the effects of high temperature [29].

10.5.2 Humidity Lower resistance to oxidation and enhancement of degradation occur as a result of increased humidity as well as rainfall. This is due to the gradual washout of additives that may bloom on to the surface of plastic films. Besides, the degradation of plastic films may occur due to hydroxyl radicals or other reactive species generated as a result of photolysis [30].

10.5.3 Wind It has been suggested that tearing due to high winds can be a major problem in greenhouses. Another problem connected with windy areas is the wind load. This load can impose increased stress on plastic films and lead to premature failure of the film. Abrasion caused by soil and other particles, which are carried by the wind and impinge on the surface of the greenhouse film, may also be another problem.

10.5.4 Fog Formation The term ‘fog’ is used to describe the condensation of water vapour in the form of small discrete droplets on the surface of transparent plastic films. The physical conditions that lead to this formation may be summarised as follows [31]: (1) A fall in temperature of the inside surface of the film to below the dew point of the enclosed air/water vapour mixture;

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Handbook of Plastic Films (2) Cooling of the air near the film to a temperature at which it can no longer retain all the water vapour, so that excess water condenses upon the film; (3) The difference between the surface tension of condensed water and the critical wetting tension of the film surface, which causes the water to condense as discrete droplets, rather than as a continuous film. A number of undesirable effects may result from fog formation in greenhouse films and leads to the following: (1) Light transmission will be reduced where the total internal reflection of incident light occurs. Consequently, the rate of plant growth will reduce, crop maturity is delayed and the crop yield decreases; (2) Light and heat transmission may be focused on delicate plant tissues owing to water droplets acting as lenses. This causes burning of the plants and crop spoilage. To prevent fog formation, surface-active agents are usually added to PE during film production. These compounds are incompatible with the polymer and subsequently migrate to the film surface where they increase the critical wetting tension. The result is reduction in contact angle between water and polymer surface, permitting the water to spread into a more uniform layer [31].

10.5.5 Environmental Pollution Atmospheric pollution, such as nitrogen oxides, sulfur dioxides, hydrocarbons and particulates, can enhance the degradation of polymers [32] and must also be taken into consideration. For instance, infrared studies have revealed that polyethylene reacts with NO2 at elevated temperature and that chemical attack is observed even at 25 °C. Similarly, SO2 is rather reactive, especially in the presence of UV radiation, which it readily absorbs and forms triplet excited sulfur dioxide. This species is capable of abstracting hydrogen from polymer chains, leading to the formation of macroradicals in the polymer structure, which in turn can undergo further depolymerisation [33].

10.5.6 Effects of Pesticides The use of agrochemicals in greenhouses severely affects polymer films [29]. The pesticides used for the protection of the crop influence the degradation and lifetime of the films. Usually pesticides have complicated formulations, and contain a number of compounds besides the active component. They contain sulfur and halogen in their chemical structure. It is a well-known fact that films are destroyed under the effect of pesticides.

Applications of Plastic Films in Agriculture Pesticides react with the stabilisers present in the film, decreasing its effect or completely destroying it. Experimental results clearly show that pesticides with sulfur-containing active compounds enter into antagonistic interaction with the stabilisers. One simple explanation [34] is that the interaction of the pesticide and the HALS compound prohibits the latter in executing its effect. Some sulfur-containing compounds and organic halogenides initiate the oxidation of PE and bring about rapid deterioration of its mechanical properties. The extent of this negative effect depends on the molecular weight, dispersion, allotropic modification, etc., of the elementary sulfur. The introduction of a UV absorber into film considerably improves the lifetime and light stability of the film. Also, HALS-stabilised greenhouse films were shown to last 33% longer than Ni-stabilised films in testing under real conditions. However, the polymer materials used for greenhouse films are changing, and, in particular, the use of blends is continuously increasing, like the use of additives. These additives are used in relatively large amounts for different aims, like photooxidation resistance, antifogging, etc. Moreover, the films can absorb fertilisers and pesticides, which can compromise the use of secondary materials coming from greenhouse covering films in many applications. UV exposure gives rise to major modifications of the macromolecular chains, with chain breaking, formation of oxygenated groups, possible formation of branching and crosslinking, and so on [35-38]. Finally, the reprocessing operations can induce further degradation due to the thermomechanical treatment in the melt [39-42].

10.6 Ageing Resistance of Greenhouse Films 10.6.1 Measurement of Ageing Factors Evaluation of the stability and durability of greenhouse films is usually carried out using laboratory equipment. There is no standard testing scheme for evaluating the degradation of these properties when the plastic film is used as a greenhouse covering material. This is due to the fact that there are several interconnected factors that can lead to the degradation of the mechanical properties. These factors are usually difficult to realise in the laboratory. Following up the changes that occur in the mechanical properties of plastic films as a result of ageing is very important in order to throw some light on the problem and to identify the conditions of the film. However, other properties of the film, such as physical and chemical properties, are also affected by the degradation, e.g., abrasion directly affects light transmittance and also other mechanical properties. Many research groups have paid attention to this problem and concentrated their efforts on measuring the effects of various ageing factors on the degradation of plastic materials [6, 43, 44]. Some of them are concerned with the very specific problem of ageing of agricultural plastic film [24, 45].

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Handbook of Plastic Films In principle, all parameters should be covered in order to be able to predict accurately the performance and lifetime of the material under the particular circumstances that would occur when the material is used in a greenhouse. The accelerated ageing process consists of simulating in an intensified manner the most critical parameters that lead to the degradation of plastics. Several accelerated ageing tests for plastic films have become commercially available during the past decade. However, their relationship to real outdoor ageing is extremely questionable. These tests are based on inducing artificial ageing in the material through an intense UV source coupled with a day-night cycle and a water spray cycle. Only a few studies have considered the use of pesticides or stress due to wind loading combined with the effect of UV-induced ageing [25]. An empirical correlation between the lifetime of films under artificial weathering and in the greenhouse situation has been given [46]. This standard defines three climatic zones depending on the level of solar radiation energy that they receive: 70-100, 100-130 or 130-160 kLy/yr, where the kilolangley is given by 1 kLy = 4.184 kJ/cm2. However, no special conditions – such as the contact between the film and the metal parts of the greenhouse structure, and the application of pesticides that can strongly influence the actual weathering of the plastic films – have been considered. Thus, the artificial ageing tests can only provide a rough estimate of the actual behaviour of plastic films when exposed to the real and complicated environmental factors that affect the plastic during its use [47]. For this reason several researchers have studied the degradation of plastic films under natural weathering conditions (outdoor tests) [48]. Only a limited number of tests were performed in greenhouses by this group [48].

10.6.2 Changes in Chemical Structure Changes in chemical structure resulting from plastic film ageing have been followed using spectroscopic methods. FTIR is the most frequently used technique [47, 49]. It provides information about the chemical structure of the macromolecules. For instance, when the chains are oxidised, carbonyl and OH groups are formed. The additive concentrations and their changes can also be detected by the same technique. The presence of parts of the agrochemicals in the film can also be detected spectroscopically. Electron spin resonance (ESR) is also used to detect the creation of free radicals during degradation [49]. Thermal analysis, such as DSC, is used to study the oxidation process as well as the changes in the crystallinity of plastic films due to ageing [50]. Gel permeation chromatography (GPC) may be used to evaluate the changes in the molecular weight and molecular weight distribution of the films [8]. The integrated area between 1770 and 1690 cm–1 of the absorption band at 1734 cm–1 was used to determine the concentration of Tinuvin 622 [poly(N-β-hydroxyethyl-2,2,6,6-tetramethyl-4-

Applications of Plastic Films in Agriculture hydroxypiperidyl succinate)] before and after film exposure. The oxidation degree, i.e., carbonyl index (CI), under different oxidation conditions was obtained by calculating the carbonyl absorption at 1713 cm–1 from the FTIR spectra at various oxidation times using the spectrum of the unoxidised starting material as a reference. All measured absorbances should be normalised by the film thickness using the equation: CI = (A1713/d) x 100

(10.7)

where A1713 is the measured absorbance at 1713 cm–1 at a certain exposure time, and d is the film thickness in micrometres.

10.7 Recycling of Plastic Films in Agriculture 10.7.1 Introduction The amount of plastic materials used in agriculture has been continually increasing. Plastic materials are used for greenhouse covers, mulching, piping, packaging and other applications. Films used for greenhouses can be considered as an easy source of materials for recycling. Indeed, large amounts of film can be easily collected and, because of the homogeneity of the polymers used for this application, the recycling operations can be relatively easy. However, UV exposure gives rise to major modifications of the macromolecular chains, with chain breaking, formation of oxygenated groups, possible formation of branching and crosslinking, and so on [35, 36, 51, 52]. The recycling of post-consumer films for greenhouses is strongly dependent on the initial structure of the plastic materials and on the processing conditions. Such films contain small amounts of low molecular weight compounds probably coming from the photooxidation of the PE molecules and from the absorption of fertiliser and pesticide residues. The amount of these compounds is small, however, and does not prevent the use of the recycled materials in many applications. The properties of the secondary materials deteriorate with the number of extrusion steps, but especially with the increasing extent of photooxidative degradation. However, the mechanical properties of the recycled post-consumer film remain relatively good even after many extrusion passes, and such film is useful for many applications [53].

10.7.2 Contamination by the Environment Dow Chemicals have actively investigated the recycling of mulch film because the normal practice of disposal by burning it on the fields is environmentally undesirable. The

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Handbook of Plastic Films contamination levels in mulch film make its recycling particularly challenging. For instance, soil contamination can be as high as 30-40%. Furthermore, the soil can contain up to 3% iron, which is a polyethylene prodegradant [54]. In addition, it was found that vegetable matter derived from harvested plants could not be removed during the washing operations [54]. Other contaminants are fumigants, (e.g., methyl bromide), and the oxidised fractions of LDPE resulting from photodegradation of the mulch film. Recently effort has been focused on the recycling of LDPE mulch film and greenhouse film, both of which contain pesticide residues. This area poses special problems that are difficult to overcome. It has been discovered that organochlorine and organosulfur pesticide residues can deactivate HALS. This deactivation is believed to occur as a result of hydrolysis of the pesticides to acidic species that then react with the HALS. This has implications with respect to the long-term outdoor stability of the recycled product. During in-service use, PE can become badly degraded and can form low molecular weight oxygenated products, (e.g., aldehydes, acids, ketones, waxes, etc.). These impurities can lead to embrittlement of the recycled polymer because low molecular weight oxidised fractions are segregated from the melt during crystallisation and concentrate at the spherulite boundaries [55]. The resulting zone, rich in oxidised material, has very low fracture toughness. Moreover, oxygenated degradation products of PE, such as carbonyl groups, are active chromophores and can sensitise the reprocessed polymer to photodegradation. Plastic waste management, in general, is a global environmental problem. The management of such waste may be through the famous 4R approach: •

Reduction (of source material);

•

Reuse;

•

Recycling;

•

Recovery.

The recycling and reuse of plastic waste films generated from greenhouses can share in solving the problem. The disposal of municipal solid waste has become an environmental issue of growing concern [56]. It was determined that discarded plastics represent close to 20% of municipal solid waste on a volume basis [57, 58]. This is due to the high volume-to-weight ratio of polymeric materials. The management of plastic waste follows the scheme: •

Source reduction;

•

Recycling;

Applications of Plastic Films in Agriculture •

Thermal reduction by incineration;

•

Land-filling.

The most feasible methods for developing countries are source reduction and recycling. Source reduction is any measure that reduces the volume of plastic waste produced. This is accomplished through material efficiency, i.e., reducing the quantity of plastic material used to produce a particular item. Recycling generally involves the collection of waste plastic materials for reprocessing [59, 60]. Polyolefin blend technology is of critical importance to various applications, including greenhouse films. For instance, the LLDPE/LDPE blend is characterised by reduced haze and better bubble stability. One of the most common blends is LDPE/ ethylene-propylene-diene terpolymer (EPDM) with improved low-temperature flexibility, rubbery properties, weathering resistance and high-temperature mechanical properties. The addition of EVA to LDPE has been commercially utilised to improve environmental stress cracking resistance, toughness, film tearing resistance, flexibility and optical properties. Both blending and coextrusion have been employed to deal with the problem of agricultural plastic film waste. The main goal is to find a solution to the problem of agricultural plastic waste from greenhouses by recycling and converting the waste into products usable in the mulch and greenhouse film applications. The proposed solution is based on the development of multilayer films consisting essentially of a top layer made from virgin resin and a bottom layer consisting of a blend of recycled PE waste film material in combination with virgin resin and other ingredients. Evaluation of greenhouse plastic wastes revealed that it is possible to obtain useful transparent plastic films to be reused with reduced cost [61]. Multilayer films for greenhouses are a current trend in the industry. LDPE films for the top layer, stabilised with different concentrations (0.1, 1.0 and 2.5%) of UV quencher, have been produced in the laboratory by blow extrusion. The effect of natural weathering on the film properties was investigated over a period of 12 months [62]. Significant changes in the mechanical properties were observed in the later stages of degradation. Films stabilised with 0.1% stabiliser crumbled after 12 months of natural weathering, whereas films with higher concentrations retained their mechanical properties. It is believed that the inclusion of a UV stabiliser interferes with the crystallisation process and that the stabiliser particles accumulate in the amorphous matrix. Degradation of the imperfect crystalline region with its low oxygen permeability proceeds via crosslinking, whereas chain scission predominates in the amorphous region with excess of oxygen. Films stabilised with 2.5% UV quencher form a barrier against the transmission of UV radiation and the bottom layers are less affected by UV radiation. Recycled material can, therefore, be incorporated at high concentrations into these layers.

279

Handbook of Plastic Films Optimisation of the top layer was based on a fixed concentration of three thermoplastics, i.e., 80% LDPE, 10% LLDPE and 5% EVA, the remaining 5% being a specially prepared master batch of LDPE containing 25% UV and heat stabilisers. Different types of UV stabilisers were taken in different concentrations. These are: Cyasorb 1084 [n-butylaminenickel-2,2′-thio-bis(4-tert-octyl phenolate)] and Chimassorb 81 [2-hydroxy-4-noctoxybenzophenone], acting as UV light absorbers; Chimassorb 944 LD [poly{6-(1,1,3,3tetramethylbutyl)imino-1,3,5-triazine-2,4-diyl amino-hexamethylene-4-(2,2,2,6tetramethylpiperidyl)imine}] and Tinuvin 622 LD, acting as radical scavengers; an energy transfer agent; and a peroxide decomposer. The data obtained show that the haziness of all plastics films are within the range required for agricultural films. It has also been found in the case study [62] that the utilisation of a single UV stabiliser is less efficient than the utilisation of a two- or three-component UV stabiliser. Thus, films containing three-component UV stabilisers in addition to a thermal stabiliser (Irganox 1076) can retain at least 94% and 81% of tensile strength and elongation at break, respectively, after exposure to UV radiation for 600 h. The good resistance of these plastic films can be attributed to the different mechanisms of action of the utilised stabilisers. In other words, if the UV absorber Chimassorb 81 is added alone to the plastic blend, the films retain about 63% of the original elongation; whereas in combination with Cyasorb 1084, the retained elongation is increased to about 80%. Consequently, these results indicate the necessity of using a combination of UV absorbers and radical scavengers [62]. Furthermore, some plastic films of various compositions were subjected to outdoor weathering tests in two different locations, Cairo and Upper Egypt. The results obtained indicate that the unprotected films deteriorate completely within three months, whereas the protected films can withstand almost one year without a drastic decrease in mechanical properties.

References 1.

J.C. Garnaud in Proceedings of the 13th International Congress of CIPA, Verona, Italy, 1994.

2.

P.A. Dilara and D. Briassoulis, Journal of Agricultural Engineering Research, 2000, 76, 309.

3.

D. Briassoulis, D. Waaijenberg, J. Gratraud and B. von Elsner, Journal of Agricultural Engineering Research, 1997, 67, 1.

4.

L. Pacini, Plasticulture, 1999, 117, 25.

5.

M.B. Amin, S.H. Hamid and J.H. Khan, Journal of Polymer Engineering, 1995, 14, 253.

Applications of Plastic Films in Agriculture 6.

S.H. Hamid, A.G. Maadhah and M.B. Amin in Handbook of Polymer Degradation, Eds., S.H. Hamid, A.G. Maadhah and M.B. Amin, Marcel Dekker, New York, NY, USA, 1992, 219.

7.

J.F. Rabek, Polymer Photodegradation. Mechanisms and Experimental Methods, Kluwer Academic Publishers, Dordrecht, The Netherlands, 1994, 73.

8.

F. Gugumus in Developments in Polymer Stabilisation – I, Ed., G. Scott, Applied Science, London, UK, 1979.

9.

P.P. Klemchuk in Polymer Stabilisation and Degradation, Ed., P.P. Klemchuk, American Chemical Society, Washington, DC, USA, 1985, 1.

10. S. Al-Malaika, E.O. Omikorede and G. Scott, Journal of Applied Polymer Science, 1987, 33, 703. 11. R.G. Hauserman and M. Johnson, Journal of Applied Polymer Science, 1976, 20, 2533. 12. M. Johnson and R.G. Hausermann, Journal of Applied Polymer Science, 1977, 21, 3457. 13. F. Gugumus, Proceedings of the 3rd International Conference on Polypropylene Fibres and Textiles, York, UK, 1983, Paper No.18. 14. F. Gugumus, Kunststoffe, 1987, 77, 1065. 15. K. Moeller, T.O. Gevert and I. Jakubowicz, Proceedings of the International Conference on Environmental Science, Mount Prospect, IL, USA, 1990, 635-640. 16. D.R. Bauer, J.L. Gerlock, D.F. Mielewski, M.C.P. Peck and R.O. Carter, Polymer Degradation and Stability, 1990, 28, 1, 39. 17. B. Bell, D.E. Beyer, N.L. Maeker, R.R. Papenfus and D.B. Priddy, Journal of Applied Polymer Science, 1994, 54, 1605. 18. J.Y. Moisan, European Polymer Journal, 1980, 16, 979. 19. A.A. Mansour, B. Stoll and W. Pechhold, Colloid & Polymer Science, 1992, 270, 219. 20. A.A. Mansour and B. Stoll, Colloid & Polymer Science, 1994, 272, 25. 21. A.A. Mansour and B. Stoll, Colloid & Polymer Science, 1994, 272, 17.

281

Handbook of Plastic Films 22. A.A. Mansour, Ph.D. Thesis, University of Ulm, Germany, 1992. 23. S. Al-Malaika, S. Chohan, M. Coker, G. Scott, R. Arnaud, P. Dabin, A. Fauve and J. LeMarie, Journal of Macromolecular Science A, Applied Chemistry, 1995, 32, 4, 709. 24. M.G. Liu, A.R. Horrocks and M.E. Hall, Polymer Degradation and Stability, 1995, 49, 1, 151. 25. P.C. Powell, Engineering Design Guides, 1979, 19, 1. 26. P. Desriac, Plasticulture, 1991, 89, 1, 9. 27. M. Scoponi, S. Cimmino and M. Kaci, Polymer, 2000, 41, 2, 7969. 28. J.G. Yu, H.S. Li, M.Q. Zhang and M.L. Zhang, Journal of Applied Polymer Science, 2000, 75, 4, 523. 29. F. Henninger in Handbook of Polymer Degradation, Ed., S.H. Hamid, A.G. Maadhah and M.B. Amin, Marcel Dekker, New York, NY, USA, 1992, 411. 30. J.E. Bonekamp and N.L. Maecker, Journal of Applied Polymer Science, 1994, 54, 1593. 31. ICI Europe, Surfactants, Report, Ciba Speciality, Everberg, Belgium, 1998. 32. B. Ranby and J.F. Rabek in The Effects of Hostile Environments on Coatings and Plastics, Eds., D.P. Garner and G.A. Stahl, American Chemical Society, Washington, DC, USA, 1983, 291-307. 33. W. Schnabel in Polymer Degradation: Principles and Practical Applications, Hanser International, New York, NY, USA, 1981. 34. E. Epacher and B. Pukanszky, Proceedings of Antec ’99, New York, NY, USA, 1999, Volume III, 3785. 35. F.P. La Mantia, Radiation Physics and Chemistry, 1984, 23, 699. 36. A. Tidjani, R. Arnaud and A. Dasilva, Journal of Applied Polymer Science, 1993, 47, 211. 37. M. Sebaa, C. Servens and J. Pouyet, Journal of Applied Polymer Science, 1993, 47, 1897.

Applications of Plastic Films in Agriculture 38. J.L. Angùlo-Sanchez, H. Ortega-Ortiz and S. Sànchez-Valdes, Journal of Applied Polymer Science, 1994, 53, 847. 39. M.K. Loultcheva, M. Proietto, N. Jilov and F.P. La Mantia, Polymer Degradation and Stability, 1997, 57, 77. 40. A.T.P. Zahavich, B. Latto, E. Takacs and J. Vlachopoulos, Advances in Polymer Technology, 1997, 16, 11. 41. M. Marrone and F.P. La Mantia, Polymer Recycling, 1996, 2, 17. 42. J.I. Eguiazàbal and J. Nazàbal, Polymer Engineering Science, 1990, 30, 527. 43. F.S. Qureshi, M.B. Amin, A.G. Maadhah and S.H. Hamid, Polymer Plastics Technology and Engineering, 1989, 28, 649. 44. G. Yanai, A. Ram and J. Miltz, Journal of Applied Polymer Science, 1995, 57, 303. 45. A. Ram, T. Meir and J. Miltz, International Journal of Polymeric Materials, 1980, 8, 323. 46. G. Grünwald, Plastics: How Structure Determines Properties, Hanser, Germany, Munich, 1992. 47. M.R. Kamal and B. Huang in Handbook of Polymer Degradation, Eds., S.H. Hamid, M.B. Amin and A.G. Maadhah, Marcel Dekker, New York, NY, USA, 1992, 127. 48. J.H. Khan and S.H. Hamid, Polymer Degradation and Stability, 1995, 48, 137. 49. A.A. Popov, N.N. Blinov, B.E. Krisyuk, S.G. Karpov, L. Privalova and G.E. Zoukov, Journal of Polymer Science, 1983, 21, 1017. 50. L. Peeva and S. Evtimova, European Polymer Journal, 1984, 20, 1049. 51. A. Tidjani, R. Arnaud and A. Dasilva, Journal of Applied Polymer Science, 1993, 47, 211. 52. L. Angùlo, H. Ortega and S. Sànchez, Journal of Applied Polymer Science, 1994, 53, 847. 53. N.T. Dintcheva, F.P. La Mantia, D. Acierno, L. Di Maio, G. Camino, F. Trotta, M.P. Luda and M. Paci, Polymer Degradation and Stability, 2001, 72, 1, 141.

283

Handbook of Plastic Films 54. C. Llop and A. Perez, Die Makromolekulare Chemie - Macromolecular Symposia, 1992, 57, 115. 55. D.T. Wark, Proceedings of the ECM International Conference, Advances in High Performance Polymer Blends and Alloys, 1991. 56. D.R. Paul in Multicomponent Polymer Materials, Eds., D.R. Paul and L.R. Sperling, American Chemical Society, Washington, DC, USA, 1986, 2-19. 57. A.P. Plochocki, Polymer Engineering and Science, 1983, 23, 618. 58. Plastic Recycling, Ed., R.J. Ehrig, Hanser, Munich, Germany, 1989. 59. A. Hansen in Plastic Extrusion Technology, 2nd Edition, Ed., F Hensen, Hanser, Munich, Germany, 1997. 60. A. Yehia, E.M. Abdel-Bary, A.A. Abdel-Hakim and M.N. Ismail, Proceedings of the 1st Egyptian-Syrian Conference on Chemical Engineering, Suez, Egypt, 1995. 61. N. Khraishi and A. Al-Robaidi, Polymer Degradation and Stability, 1991, 32, 1, 105. 62. E.M. Abdel-Bary, M.N. Ismail, A.A. Yehia and A.A. Abdel-Hakim, Polymer Degradation and Stability, 1998, 62, 1, 111.

11

Physicochemical Criteria for Estimating the Efficiency of Burn Dressings Klara Z. Gumargalieva and Gennady E. Zaikov

11.1 Introduction The principal medical treatment of burns is the use of dressings, which often worsen the effects of the injury. It is difficult to estimate the effectiveness of new burn dressings, as their physicochemical properties are not usually presented in the literature. This chapter is devoted to a discussion of this subject for the first time. The authors address the complexity of physicochemical methods of analysis in order to create criteria for efficient dressings for a burn wound surface. The characteristics typical of burn wounds for which dressings are required are shown in Table 11.1.

Table 11.1 Characteristics of burn wounds [3] Burn degree Image of damage

Physiological process

Burn depth (mm)

I

Redness and oedema (medium oedema)

Aseptic inflammatory process

0

II

Sac formation

Aseptic inflammatory process

0

III

Damage of skin cover, exuding wound surface

Skin necrosis, tissue necrosis

1-2

IV

Exuding wound surface

Full necrosis of tissues, carbonisation of tissues

2-5

Based on theoretical and experimental data, it was found that the maximal sorptional ability of a burn dressing is determined by the free volume of the dressing material calculated from the value of the material density. Kinetic parameters were determined from the sorption curves. These parameters help in predicting the behaviour of burn dressings. Criteria for estimating the efficiency of first-aid burn dressings are then formulated.

285


11.2 Modern Surgical Burn Dressings Dressings for wounds and burns must primarily be protective, sorptional and atraumatic. In currently used dressings, these properties are provided by a multilayer structure or structural modifications. Different classifications of dressings can be found in the literature: by material, by construction or by function [1-3]. The dressings applied in the modern treatment of wounds and burns are subdivided into three groups according to the material of the layer sorbing the wound exudate. The material may be of animal origin, synthetic foamed polyurethane or of vegetable origin (Table 11.2).

11.2.1 Dressings Based on Materials of Animal Origin Typical dressings in this group are collagen sponges. Besides hydrophilic properties, collagen sponges provide higher sorption of liquid (in the range of 40-90 g/g) [1, 4-9]. The patent literature describes in detail the methods of obtaining collagen dressings for wounds and burns in the form of sponges and felt [10-13] based on materials of animal origin. Also, the materials used include that made from biological artificial leathers based on lyophilised bodies and swine cutis, produced as plates 0.5-0.7 mm thick. However, these materials possess lower sorptional capacity than collagen dressings. Dressings called ‘cultivated cutis’ are also obtained from the epithelia of cells of the patient himself [13]. The shortcoming of biological artificial leathers or bio-dressings is their expense and, as a rule, their inability to retain their properties on storage.

11.2.2 Dressings Based on Synthetic Materials The demands for inexpensive raw materials for the production of wound and burn dressings has led to the production of materials based on synthetic polymers, particularly cellular polyurethane [10, 14-18]. Cellular polyurethane intended for medical purposes is synthesised using toluene diisocyanate and polyoxypropyleneglycol [19]. Dressings based on polyurethanes have a pore distribution of about 200-300 pores/cm2, and allow the regulation of the number and size of pores in layers [20]. Dressings from this group are prepared as a double layer; the density of the outside layer is high in order to prevent liquid evaporation and penetration of microorganisms. In rare cases, these dressings are homogeneous through their thickness. The influence of the pore size on the sorption properties of polyurethane sponges has been reported [21], where macroporous sponge with a pore size from 200 up to 2000 μm is completely nourished by exudate under pressure only. In this case, the size of the pores should be of the order of several micrometres.

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Physicochemical Criteria for Estimating the Efficiency of Burn Dressings

Table 11.2 Characteristics of dressings in brief No. Name

Company

Country Structure

Composition

Group 1 1.

Collagen burn dressing

‘Helitrex’

USA

Dressings uniform by thickness, dense, pores of 0.01 mm size. It has gauze cover

Collagen

2.

Collagen sponge

‘Helitrex’

US A

Similar to No. 1, differs by big radius of pores, formed by fibril weaving of cylindrical form preferably

Collagen

3.

Collagen dressing

‘Bayer’

Germany Friable dressing, possesses rough porous structure with pore/hole sizes from 1.5 to 0.1 mm

Collagen

4.

Burn curative dressing

‘Combutec-2’

USA

Dressing of large-porous structure with pore size from 1 to 0.05 mm. Pores are of cylindrical form preferably formed by fibril weaving of collagen

Collagen

5.

Biological dressing

‘Corretium-2’

US A

Dense, pressed plate. Fibrillar structure is observed in dense layers

Collagen

6.

Biological dressing

‘Corretium-3’

USA

The same as No. 5

Collagen

Group 2 7.

Compositional burn dressing

‘Biobrant’

USA

Double-layered elastic, porous dressing, consists of the upper layer of 0.010.005 mm and flexible fabric Nylon base. It represents combination of hydrophilic components with elastic silicon films

Silicon, the main layer, base made from polyamide

8.

Synthetic dressing

‘Epigard’

USA

Double-layered elastic porous dressing. Upper layer is dense, nonporous 0.2 mm thick

The main layer made from polyurethane, the upper one made from polypropylene

287


Table 11.2 Characteristics of dressings in brief continued No. Name Company Country Structure Composition 9.

Synthetic burn dressing

10.

‘Syncrite’

ChSSR

Single-layered dressing on gauze base with through large pores, medium flexible

Polyurethane

Synthetic ‘Syspurderm’ wound dressing

Germany

Dressings of homogeneous Polyurethane composition, with different pore distribution: upper layer is 0.1 mm thick, possesses small porous structure with pores of 0.01 mm size; lower layer, adjoining wound possesses large pores of 0.05 mm size. The dressing is ‘elastic’, accepts a form badly

11.

Synthetic ‘Farmexplant’ wound dressing

PB R

Antiseptic double-layered dressing. Main polyurethane layer possesses pores of 0.1-1.5 mm. Upper layer is 0.1 mm thick, more dense, non-porous

Polyurethane

12.

Atraumatic caproic dressing

USSR

Large-cellular dressing on basis of woven Nylon

Polyamide

USSR

Wound large-cellular dressing, homogeneous by its composition

Alginic acid salts

USSR

Porous cotton balling dressing with atraumatic layer

Cellulose

Group 3 13.

Cover for wound, burns

14.

Needle-pierced fabric

15.

Wound nonadhering dressing

288

VNII medpolymer ‘Algipor’

‘Bayersdorf’

Germany Three-layered dressing of plaster type with tricot lower layer. Dressing is of the sandwich type: upper layer is crepe paper, main part is cotton balling, lower layer is tricot network. Atraumatic action is provided by the effect of dressing ‘bending’ (tunnelling effect)

Main and upper layers made from cellulose, lower one is a film of Dacron or Nylon


Table 11.2 Characteristics of dressings in brief continued No. Name

Company

Country Structure

Composition

16.

Wound absorbing dressing

‘Johnson-Joh- USA nson’

17.

Haemostatic

18.

Wound dressing

19.

Surgical dressing

20.

Dressing with perforated metallised layer

Germany First aid dressing with hydrophobic layer and lower metallised layer. Internal layers represent non-fabric pressed layer of crepe paper

Main layer made from cellulose, lower layer is aluminium spray-coated

21.

Dressing lower layer is not metallised

Germany Similar to No. 20

Cellulose with spray-coated lower layer

22.

Non-adhesive dressing

ChSSR

Dense cotton balling dressing, lower and upper layers are nonfixed Nylon networks

Cellulose and polyamide

23.

Series of experimental dressings with various quantitative viscose-cotton composition

USSR

Cellulose or viscose dressings with atraumatic layer

Three-layered dressing with perforated lower and upper layers 0.01 mm thick, main part is cotton balling, porous

Main layer made from cellulose, external layers made from polypropylene

Sweden

Double-layered dressing with perforated lower layer, sewn to the main layer

Viscose main layer, atraumatic one made from polyethylene

‘Mesorb’

France

Cotton balling or viscose dressing, crepe paper lower and upper layers

Cellulose

‘Kendall’

US A

Similar to No. 16 with cellulose base and atraumatic synthetic lower layer

Cellulose

‘Switin’

289

Handbook of Plastic Films Apart from polyurethanes, other polymeric materials (polyvinyl chloride, Nylon, etc.) have been used as the sorbing layer [22-25]. This group includes a compositional burn dressing based on a silicon film, polyamide network and hydrophilic admixture, produced by Hall Woodroof Co., (USA) [13]. Polyurethane coverings with an atraumatic lower layer made from polyglycolic acid may be considered as a variety of compositional dressings [26]. It is characteristic of dressings from this group that they preserve their high strength properties even after absorption of wound exudate. A two-component protective dressing ‘Hydron’ was recently applied in the treatment of burns. It is a film formed on the wound, and consists of a powder of poly(2-hydroxyethyl methacrylate) dissolved in polyethyleneglycol 400 [2, 27]. Although they possess good protective properties, ‘Hydron’ dressings have low strength and sorptional capacity.

11.2.3 Dressings Based on Materials of Vegetable Origin A large number of burn dressings, the so-called ‘cotton balling’, are based on cellulose, viscose or a combination of the two [28-32]. These dressings differ from each other by structure and composition of the upper and lower layers. Most often, a sorption layer based on cellulose is used in complex dressings. Such dressings are usually layered, with the separate layers being produced from either the same or different materials; the layers may be fixed mechanically or by using thermoplastic material. To decrease their adhesion to the wound surface, the lower layer is produced from various fabric and non-fabric materials (perforated Dacron, polypropylene, pressed paper, metallised fabric material, etc.). The total sorptional ability of these dressings is defined by the hydrophilicity and porosity of the basic material and is usually equal to 15-25 g/g. Data on the action of wound and burn dressings based on another vegetable material – derivatives of alginic acid – have been reported [1, 33, 34]. Typical ‘Algipor’ specimens used are based on the mixed sodium-calcium salts of alginic acid as spongy plates of about 10 mm thickness with high absorption ability.

11.3 Selection of the Properties of Tested Burn Dressings The data from the literature showed that burn dressings, particularly the first-aid ones, must perform three main functions [1, 2, 35, 36]: (1) Absorb the wound exudate, which contains metabolic products and toxins; (2) Provide optimum water, air and heat exchange between the wound and the atmosphere; (3) Protect the wound from the penetration of microorganisms from the air.

290

Physicochemical Criteria for Estimating the Efficiency of Burn Dressings Moreover, the burn dressing must be removable from the wound without further injury to the patient. Therefore, the following properties of burn dressings have been studied to determine their efficiency.

11.3.1 Sorption-Diffusion Properties The sorption-diffusion properties of dressings are extremely important, because they determine the performance of the three main functions of dressings just mentioned.

11.3.1.1 Water absorption Water is the main component of the exudate from wounds. At present, there is no opinion on how fast and to what degree the dressing must absorb the exudate in order to clean the wound from toxins and metabolic products while at the same time keeping the wound wet enough to prevent the removal of water from healthy tissue [1, 2, 35, 36].

11.3.1.2 Air penetrability Sufficient air must be allowed to penetrate the dressing, since an increase of oxygen concentration helps the healing process.

11.3.1.3 Vapour penetrability Vapour penetrability of the skin of a healthy man may reach 0.5 mg cm–2 h–1 [37]. Water loss by evaporation from burns is even higher (Table 11.3). In the absence of technical data, it may be concluded that high vapour penetrability will lead to ‘drying’ of the dressing, with a corresponding change in the surface energy of the dressing-wound

Table 11.3 Water losses by evaporation from different types of burns Surface type

Evaporation (cm3/cm2-h)

Natural skin

1-2

First degree burn

1-2.5

Second degree burn with blisters intact

2.8

Second degree burn with no damage of fermentative layer

37

Third and fourth degree burns

20-31

291

Handbook of Plastic Films interface. This will promote undesirable removal of water from the tissues, and may cause the dressing to come off the wound. Low vapour penetrability of the dressing will lead to the accumulation of liquid under the dressing, which may cause oedema.

11.3.1.4 Microorganism Penetrability Penetration of microorganisms through the dressing must be blocked to prevent infection.

11.3.2 Adhesive Properties The adhesive properties of dressings determine their ability to stay attached to the wound. Thus, the surface energy of the dressing surface facing the wound must always be lower than that of the wound surface.

11.3.3 Mechanical Properties Two mechanical properties are important for dressings: (a) flexural rigidity and (b) strength at break. The former defines the ability of the dressing to mould to the wound profile; the latter is important since it allows the dressing to be removed from the wound completely without breaking.

11.4 Methods of Investigation of Physicochemical Properties of Burn Dressings 11.4.1 Determination of Material Porosity The porosity of materials (the relation of pore space volume to total volume) is determined by the following two methods. (1) By measuring the density, and then using: l pores 1 l = + mat P∑ l ∑ Ppores l ∑ Pmat

(11.1)

where Q is the material porosity, ρ is the observable density, and ρ0 is the density of the material forming the porous medium. The value of ρ is determined by weighing

292

Physicochemical Criteria for Estimating the Efficiency of Burn Dressings a sample of known geometrical size. The value of ρ0 is determined similarly for samples pressed at 500 GPa. (2) From photos obtained by a light microscope (MIN-10) we get: ⎡ S pores ⎤ Q=⎢ ⎥ ⎢⎣ S0 ⎥⎦

3/ 2

(11.2)

where Spores and S0 are total surface area of pores and general surface area of the material in the field of vision of the microscope, respectively.

11.4.2 Determination of Size and Number of Pores The number and size of the pores are determined with the help of the MIN-10 microscope in reflected light. The pore distribution curve (number of pores as a function of radius) is calculated; typical results are given in Figure 11.1.

Figure 11.1 Typical curves of pore size distribution for various burn dressing materials. 1: Farmexplant; 2: Syncrite; 3: Bayer brown collagen dressing; 4: Syspurderm

293


11.4.3 Estimation of Surface Energy at Material-Medium Interface The surface energy of a material-medium interface is estimated using the wetting angle of the material surface by the medium. A drop of liquid is applied to the surface of the material, and the angle is measured between the tangent at the base of the drop and the material surface. The wetting angle is determined using a horizontal microscope. The accuracy of angle measurement does not exceed ±1°.

11.4.4 Determination of Sorptional Ability of Materials The total amount of liquid sorbed by a ‘tiled’ material includes the liquid in macropores with size over 0.1 μm, that is micropores with size smaller than 0.1 μm, and that in the material matrix itself (dissolved liquid). The amount of dissolved liquid, and of liquid filling the micropores, is calculated from the vapour pressure of the sorbed liquid over the sample (sorption isotherms). The sorption isotherm for a material with micropores possesses an S-type form (Figure 11.2). The first part of the curve is connected with the real dissolved liquid, and the second part with the condensed liquid in micropores.

Figure 11.2 A typical sorption isotherm of a low molecular weight liquid by a microporous material: part ➀ of the curve represents real dissolving of the liquid by the material, and ➁ represents condensation of the liquid in the micropores within the material. Here Δm is the (change in) sorbed liquid mass; and P/P0 is the relative pressure of liquid in the thermostated vessel (where P0 is the saturated vapour pressure of the liquid under the conditions used)

294


Figure 11.3 A schematic diagram of the device for determining the absorbtion ability of porous materials. 1: sample; 2: perforated plate; 3: thermostated bath; 4: float

The maximal sorption (the amount of liquid, really dissolved and filling micro- and macropores) is determined using the device shown in Figure 11.3. The device represents a vessel with liquid medium, in which a float of special perforated square construction is placed. The float construction is calculated to prevent its sinking. This requires that the liquid medium does not penetrate through the perforations of the square, but instead forms a meniscus on the side of the square facing the porous interlayer. The change of the mass of the porous material is determined from the immersion of the float with the sample. It is measured using a horizontal microscope.

11.4.5 Determination of Air Penetrability of Burn Dressings Air penetrability (the volume of air that passes through a specific surface area during a specific time) was determined using a device specially designed for this purpose. The device is a cylindrical cell with perforated plate supporting the sample (Figure 11.4). The air was passed through the cell with the help of an air compressor, equipped with a manometer and pressure controller. The time required to fill a polyethylene sack (45 litres in volume) with air was measured. A round form sample was prepared. The sample was then placed on the perforated plate of the cell. The compressed air passed through the cell pressed on the sample. The time taken for the polyethylene sack to fill was measured. The method allows determination of the air penetrability of dry or wet materials.

295


Figure 11.4 A schematic diagram of the device for determining the air penetrability of porous materials. 1: sample; 2: perforated plate; 3: polyethylene sack; 4: manometer; 5: pressure controller

11.4.6 Determination of Adhesion of Burn Dressings Adhesion of burn dressings was investigated on a modified form of a device previously described [38]. Thus, a 1 mm thick fibreglass plate, covered with three layers of medical gauze, was placed into a fibreglass cell having a working surface of 3 × 10 mm2. The cell was filled with 5 ml of whole blood and 1 ml of 2% thrombin. The dressing to be tested was then placed on the plate surface for 1 min. The cell containing the sample was placed into a thermostat at 37 °C for 24 hours. Sample removal was performed at a 90° angle to the surface of the tested material.

11.4.7 Determination of Vapour Penetrability of Burn Dressings Vapour penetrability (the mass of water that passes through a specific surface area during a specific time) was determined using a device described elsewhere [39]. A glass vessel was filled with a known amount of liquid, e.g., water or aqueous solution of sulfuric acid. This amount provided a known relative humidity. The investigated sample was placed on the vessel surface; and a metal ring was set and pressed to the vessel by a special clamp. The vessel with the contents was weighed and placed into desiccator with

296

Physicochemical Criteria for Estimating the Efficiency of Burn Dressings dryer at 37 °C. After measured time periods the vessel was taken out from the desiccator, weighed and then put back in the desiccator. The amount of water that has passed through the sample was determined by the mass loss of the vessel contents. The vessel dimensions used in the experiments were 40 mm diameter and 20 mm height.

11.5 Results and Discussion 11.5.1 Determination of Sorption Ability of Burn Dressings On applying a dressing to a burn wound, first wetting of the surface layer of the material occurs, followed by sorption of the wound exudate into the dressing volume. Thus it is necessary: (1) to know the components of the burn wound exudate, which need to be sorbed by the material, and the way in which sorption occurs; and (2) to determine the maximum sorption of the separate components of the exudate by the dressing material. With respect to the second item, the maximum water sorption of different materials has been determined previously [40]. For this purpose, the sample was immersed in water, dried rapidly using filter paper and then weighed. However, this method did not allow the sorption kinetics to be measured, and the accuracy of the maximum sorption was low. That is why we have developed the device for continuous measurement of sorption. The first item mentioned above has not yet been addressed in the published literature. The exudate from wounds contains water, salts, proteins, damaged cells and various low and high molecular weight (low and high molar mass) substances in relatively lower amounts. Table 11.4 shows the approximate composition of oedema liquid in a burn wound. The composition of oedema liquid changes depending on the burn degree: the worse the burn, the higher the content of protein and the lower the albumin/globulin ratio [3]. Similar data for blood plasma are also shown for comparison in Table 11.4.

Table 11.4 Composition of oedema liquid and blood plasma (g/cm3) Components

Oedema liquid

Blood plasma

Urea

5.1 × 10

5.5 × 10-4

Sugar

5.8 × 10-6

11.0 × 10-6

Protein

3.4 × 10-2

7.2 × 10-2

Salts

1.0 × 10-2

1.0 × 10-2

3. 9

1.5

Albumin/Globulin

-4

297

Handbook of Plastic Films Sorption of wound exudate may proceed via filling of micro- and macropores, or dissolving in the material matrix. Let us consider the sorption of the various components of the wound exudate by the dressing material. (1) Water fills pores and dissolves in the material matrix. Water solubility is defined by the material hydrophilicity. The solubility of water, salts and other low molecular weight substances in polymers is subject to the following rules: • In hydrophilic polymers, solubility is defined by the size and charge of the low molecular weight substance; • In hydrophobic polymers, solubility is defined by vapour pressure (the higher the vapour pressure, the higher the solubility) [41]. (2) Protein fills pores up to 10–2 m in size and may dissolve only in hydrogels of ‘Hydron’ type with water content over 30% by mass. (3) Cells fill only open pores over 0.1-0.2 μm in size.

11.5.1.1 Solubility of water in polymers As mentioned previously, modern burn dressings are heterogeneous materials, usually consisting of several layers. The upper one exposed to the air is usually more hydrophobic and less porous than the others. The solubility of water in this layer will define its evaporation from the dressing surface and the heat exchange between the wound and the surroundings. Information about solubility of water in various polymers is reported in Table 11.5 [42]. The solubility of water was determined by the sorption method. Extreme values of sorption at known water vapour pressures were calculated from the sorption curves, and then the sorption isotherms were constructed using the method described elsewhere [43]. ∞ Extreme values of solution φH at the saturation pressure were determined by extrapolation 2O ∞ of φH 2O to P/Ps = 1. The value φH of equals the solubility of water in the polymer. 2O

11.5.1.2 Maximum sorption ability of burn dressings Modern burn dressings are heterogeneous materials that either have large pores or are fibrillar, and they possess a high free volume. In contact with a wound, the exudate will fill the free volume of the dressing. The degree of filling is defined by the hydrophilicity of the material, and the size and geometry of the free volume fraction.

298


Table 11.5 Solubility of water in various polymers Solubility (102 g/g)

T (K)

Cellophane

40

303

Viscose fibre

46

303

Cotton

23

303

Cellulose diacetate

18

303

Cellulose triacetate

11.5

303

Polycaproamide

8.5

303

Polyethyleneterephthalate

0.3

303

Polydimethylsiloxane

0.07

308

Poly(2-oxyethylmethacrylate)

40*

310

Polypropylene

0.007

298

Polytetrafluoroethylene

0.01

293

Polyethylene (ρ = 0.923)

0.006

298

Polyurethane

1*

298

Polyvinyl chloride

1.5

307

Polymer

*Measured by the authors.

11.5.1.3 Maximum sorption of water by burn dressings Sorption of water by burn dressings is measured using a device developed for this purpose by the present authors. Experiments were performed in the following way. First, different masses were placed on the perforated plate of the device, and the relative immersion of the device into the water was measured, in units of the eyepiece graticule of the horizontal microscope. A calibration curve was then drawn using the coordinates ‘mass’ versus ‘depth of immersion’. The slope coefficient of this calibration curve equals 0.70 ± 0.02 g/ unit. Then, a sample of a dressing was placed into the device, and the depth of immersion during time h was measured. The mass of the medium sorbed by the material was calculated from the correlation: mc – 0.70h

(11.3)

The extreme value of the mass of sorbed medium was determined at t → ∞.

299

Handbook of Plastic Films Table 11.6 shows the experimental and theoretical [calculated from equation (11.3)] values of CH∞ 2O , and values of ρ0 determined experimentally and used for the theoretical calculations. A good correlation was observed between the experimental and theoretical values of CH∞ 2O for the majority of dressings. This shows that practically the entire free volume is filled by liquid medium for the contact of dressings with water.

Table 11.6 Experimental and theoretical data of the maximum sorption of water by burn dressings ∞ CH 2O

Covering name (material)

ρ0 (g/cm3)

Experimental

Theoretical

Helitrex (collagen)

32 ± 2

0.030 ± 0.007

33 ± 2

Helitrex (collagen sponge)

58 ± 3

0.018 ± 0.005

55 ± 3

Collagen dressing

1.8 ± 0.1

0.350 ± 0.07

2.8 ± 0.3

Corretium-2 (collagen)

3.5 ± 0.3

0.300 ± 0.07

3.3 ± 0.3

Corretium-3 (collagen)

2.1 ± 0.2

0.330 ± 0.05

3.0 ± 0.1

Combutec-2 (collagen)

77.0 ± 5.0

0.015 ± 0.005

66.0 ± 3.0

Epigard (foamy polyurethane

10.0 ± 0.3

0.067 ± 0.005

15.0 ± 1.0

Silicon-Nylon composite

7.5 ± 0.2

0.130 ± 0.03

7.7 ± 0.5

Syspurderm (foamy polyurethane)

6.2 ± 0.2

0.140 ± 0.03

7.1 ± 0.5

Syncrite (foamy polyurethane)

20.0 ± 2.0

0.050 ± 0.01

22.0 ± 1.5

Farmexplant (foamy polyurethane)

12.0 ± 0.5

0.064 ± 0.007

15.6 ± 3.0

Johnson & Johnson (cellulose)

11.4 ± 0.5

0.100 ± 0.03

10.0 ± 1.5

Blood-stopping (cellulose)

15.7 ± 0.9

0.080 ± 0.006

12.5 ± 0.7

Tunnelling (cellulose)

4.3 ± 0.2

0.200 ± 0.005

5.0 ± 0.4

Switin (cellulose)

18.0 ± 2.0

0.050 ± 0.005

20.0 ± 1.0

Metallised (cellulose-paper)

12.4 ± 0.7

0.100 ± 0.04

10.0 ± 0.5

Needle-perforated (cellulose-viscose)

28.0 ± 2.5

0.033 ± 0.007

30.0 ± 2.0

100% Viscose

25.0 ± 2.0

0.033 ± 0.007

30.0 ± 2.0

70% cotton + 30% viscose

31.0 ± 3.0

0.030 ± 0.007

33.3 ± 3.0


25.0 ± 2.0

0.035 ± 0.007

28.5 ± 2.0


28.0 ± 2.0

0.036 ± 0.007

27.7 ± 2.0

Algipor (vegetable)

30.0 ± 3.0

0.011 ±0.0002

90.0 ± 5.0

300

Physicochemical Criteria for Estimating the Efficiency of Burn Dressings The exception is the ‘Algipor’ dressing, the large pores of which become denser on filling with water because of the collapse of the pore walls. At the end this leads to the decrease of the total volume of the dressing. The liquid medium may not fill the whole volume of the dressing if the material is sufficiently hydrophobic and poorly wetted with water. To test this assumption, seven collagen materials that differ in production method were investigated for: density, maximal water sorption, wetting angle and heat of sorption of water by the material. The latter was determined using a microcalorimeter (LKB 2107) as follows: A sample of known mass was exposed to vacuum in a thermostated Butch-type cell, and then an excess amount of water was introduced into the cell, causing the forced filling of the material volume. The results obtained are presented in Table 11.7 and Figure 11.5.

Table 11.7 Density, maximal water sorption, wetting angle and heat of sorption of water by different collagens ∞ CH (g/g) 2O

ρ0 (g/cm3)

Experimental

0.011

Theoretical

ø (deg)

ΔH (cal/g)

74

91

170

34.6

0.016

53

62.5

70

25.4

0.013

49

77

90

30.2

0.013

47

77

110

31.9

0.013

8

77

120

31.2

0.014

4

71.4

110

29.8

0.014

30

71.4

50

27.2

Figure 11.5 The dependence of the maximum sorption of water on the heat of sorption for various collagen materials

301

Handbook of Plastic Films The following conclusions can be derived from the data presented in Table 11.7: (1) The experimental value of CH∞ 2O is lower than the ‘theoretical’ one. This may be explained by two reasons: the decrease of the total volume (as in the case of ‘Algipor’), and the non-filling of a part of the material free volume by water. (2) A satisfactory correlation exists between the theoretical values of CH∞ 2O and ΔH. Thus, the main reason for the difference between the experimental and theoretical values of ∞ CH is evidently the non-filling of a part of the material free volume by water. 2O (3) The absence of a correlation between maximal water sorption and wetting angle, defined on the external surface of the material, shows that the values obtained as mentioned above do not reflect the real interaction of water with the internal surface of the collagen material.

∞ Figure 11.6 The dependence of C H on the free volume of various burn dressing 2O materials: 1, Helitrex collagen dressing; 2, Neutron collagen sponge; 3, Bayer brown collagen dressing; 4 and 5, Corretium-2 and -3 artificial leathers; 6, Combutec-2; 7, Epigard synthetic dressing; 8, Syspurderm foamy polyurethane dressing; 9, Syncrite synthetic dressing; 10, Farmexplant foamy polyurethane dressing; 11, burn face mask; 12, Biobrant compositional dressing; 13, Johnson & Johnson cellulose dressing; 14, Kendall cellulose dressing; 15, Torcatee non-adhesive cellulose dressing; 16, bloodstopping cellulose dressing; 17, Mesorb cotton balling dressing; 18, dressing with tunnelling effect; 19, cellulose dressing with non-adhesive synthetic layer; 20, Switin cotton balling dressing; 21 and 22, metallised dressings; 23 and 24, needle-perforated fabric with atraumatic layer; 25, 100% viscose; 26 to 29, viscose-cotton balling

302

Physicochemical Criteria for Estimating the Efficiency of Burn Dressings Thus, it may be concluded that, for the majority of hydrophilic burn dressings, the maximum sorption capacity with reference to water may be predicted satisfactorily. For example, the experimental values of CH∞ 2O correlate well with the free volume part of the materials (Figure 11.6), the correlation coefficient being 0.96.

11.5.1.4 Maximum sorption of plasma by burn dressings Sorption of blood plasma by burn dressings was determined by a similar method. Plasma was obtained by centrifugation of conserved blood. The treatment of the experimental results was carried out similarly to the case of the investigation of the maximum sorption ∞ of water. The value of C plasma differs from CH∞ 2O . The difference is not higher than 10%, ∞ are not presented in Table 11.7. which is why data for C plasma

11.5.2 Kinetics of the Sorption of Liquid Media by Burn Dressings The study of the kinetics of the sorption of wound exudate by burn dressings is of great importance for the estimation of their efficiency. There are difficulties in the mathematical description of the kinetics of the sorption process connected with the absence of a strictly quantitative description of dressing structure.

11.5.2.1 Structure of burn dressings Burn dressings are heterogeneous systems, consisting of several component phases. As general attention in dressings must be paid to the material possessing the maximum penetrability with reference to the liquid medium, it is necessary to classify the types of heterogeneous systems. For example, the penetrable parts of the material are placed under a layer of another weakly penetrable material in such a way that diffusing flow is perpendicular to the surface layer. This is the case for double-layered dressings with a dense external layer. The penetrable parts of the material can be dispersed in a continuous weakly penetrable phase. Dressings based on collagen and cellulose possess fibrillar structure and the fibres are randomly placed. In some cases, spatial orientation of fibres is present. The number of open pores in dressings of this type is large but the open pores possess irregular form and great tortuosity in the direction of mass transfer. Modern burn dressings are multilayer with a denser external layer. Table 11.8 shows the mean radius of macropores and their number per unit area for dressings based on polyurethane.

303


Table 11.8 Mean radius R of macropores and their number N per unit area for dressings based on polyurethane R (10–2 cm)

N (cm–2)

Epigard

2.2 ± 0.2

370 ± 10

Syspurderm

1.8 ± 0.2

266 ± 5

Syncrite

2.8 ± 0.2

275 ± 5

Farmexplant

2.2 ± 0.2

300 ± 10

Dressing name

Detailed analysis of a number of mathematical models and results of experimental investigations of heterogeneous systems has been performed by Zaikov [44]. It is known that the calculation of diffusion coefficients in heterogeneous systems is very difficult. According to ideas accepted at the present time, the penetration of liquid into a porous body is ruled by the laws of capillarity. These ideas have been successfully applied to interpret the penetration of water into paper, leather, fabrics, etc. [45, 46]. An equation that takes into account the real structure of porous bodies was obtained by Deriagin [47].

11.5.2.2 Kinetics of sorption The kinetics of sorption of water and blood plasma was investigated using the device for the maximal sorption of water. Figure 11.7 shows typical kinetic curves of sorption of water and plasma by various dressings. All curves are satisfactorily described by the equation: 1/ 2

⎡ Dt ⎤ mt = 2⎢ 2 ⎥ m∞ ⎣ πl ⎦

(11.4)

The following conclusions can be made from the data obtained: (1) Burn dressings differ significantly in their rates of sorption of liquid media; (2) The rate of sorption is determined by the pore size and the material hydrophilicity.

304


Figure 11.7 Curves for the sorption of water and blood plasma by various burn dressings. 1: water, and 2: plasma, by needle-perforated material; 3: water, and 4: plasma, by Syspurderm polyurethane dressing

11.5.3 Determination of Vapour Penetrability of Burn Dressings With multilayer dressings, the external layer, which regulates the mass transfer of water from the wound into the surroundings, is denser than the inner ones. The process of mass transfer of water through the material layers is often called aqua-, water or vapour penetrability. Penetrability and diffusion of water in polymers has been the subject of numerous investigations. The results given in some reviews and monographs [47, 48] are presented in Table 11.9. The mass transfer of water molecules in polymers possesses a list of features. In hydrophobic matrices, the interaction between water molecules and the material matrix is weak (low solubility). Nevertheless, the interaction of water molecules with each other stipulates a specific transfer mechanism. In hydrophilic materials, the interaction between water molecules and the hydrophilic groups of the material matrix stipulates high solubility of water in the matrix and increased aqua-penetrability. Consequently, high aqua-penetrability may be a property

305


Table 11.9 Penetrability and diffusion of water vapours in polymers [48] Polymer

T (K)

p p0

P × 1015 (mol m/m2 s Pa)

D × 1012 (m2/s)

Cellulose

298

1.0

8500

–

Regenerated cellulose

298

0.2

5700

0.1

Cellulose acetate

303

0.5 - 1.0

2000

1.7

Cellulose diacetate

298

1.0

15.7

–

Cellulose triacetate

298

1.0

5.5

–

Ethylcellulose

298

0.84

7950

18

Polydimethylorganosiloxane

308

0.2

14400

7000

Polyethylene (ρ= 0.922)

298

0 - 0.1

30

23


298

0 - 0.1

58.6

0.39

Polypropylene

298

0 - 0.1

17

24

Polyvinyl chloride

303

–

–

2.3

Polycaproamide

298

0.5

134

0.097

of hydrophobic as well as of hydrophilic materials; however, the causes will be different. For example, in hydrophilic polydimethylorganosiloxane, the high mobility of water molecules is stipulated by the high mobility of the chain units in this polymer. That is why, despite the low solubility of water in polydimethylorganosiloxane, the coefficient of aqua-penetrability is significant. In contrast, in regenerated cellulose, the diffusion coefficient is low because only the dissolved water molecules, which are not connected with the matrix of this polymer, participate in the mass transfer. In this case the high value of aqua-penetrability is stipulated by increasing the dissolved water content in the regenerated cellulose, which increases the fraction of the water molecules participating in mass transfer. This in turn leads to the increase of both the diffusion coefficient and the penetrability coefficient. The mass transfer of water through a porous body is practically equal to that of gases in a polymer, provided there is no interaction between water molecules and the matrix of the polymeric material. Since hydrophilic materials, which actively interact with water molecules, are commonly used for the production of dressings, diffusion should be considered simultaneously with absorption.

306

Physicochemical Criteria for Estimating the Efficiency of Burn Dressings As a rule, the rate of the absorption process is significantly higher than the diffusion rate. Therefore, it can be assumed that the absorption equilibrium is immediately reached, and the concentration of water in the material CH 2O is obtained from the equation: ∂C H 2 O ∂t

= DH 2O

∂2CH 2O ∂x 2

−

a ∂CH 2O

(11.5)

∂t

where DH 2O is the coefficient of water diffusion in the material, x is the diffusion a is the concentration of absorbed water. coordinate, and CH 2O The concentration of absorbed water can be calculated for particular cases. For example, if the concentration of functional groups able to link water molecules irreversibly is limited and equals Cf, we can assume that the bonded water molecules no longer participate in the diffusion process, but form domains in which fast absorption occurs. For the case when the concentration of water at one of the surfaces (x = 0) is constant 0 and equals CH , the reaction zone reaches the second surface of the membrane, which 2O has thickness l, during the time t [49]. Thus, during time t there will be no water flow through the surface x = l on the membrane exterior, and then steady-state flow will be set up immediately. The amount of water passing through the membrane is given by: mH 2O = DH 2O

ΔCH 2O l

(11.6)

St

where S is the area of the membrane and

ΔCH 2O l

is the concentration gradient.

If the solubility of water in the material is ruled by Henry’s law: CH 2O = σP

(11.7)

where P is the water vapour pressure over the material, then substituting equation (11.7) into equation (11.6) gives: mH 2O = DH 2O σ H 2O

ΔP St l

(11.8)

Considering the diffusion coefficient DH2O being given by: PH 2O = DH 2O σ H 2O

(11.9)

307

Handbook of Plastic Films we obtain: PH 2O =

mH 2O

(11.10)

ΔP S t

The aqua-penetrability of burn dressings has been determined on the device described in Section 11.3. The values of the penetrability coefficients were calculated by equation (11.10). Table 11.10 shows the values of the coefficients of aqua-penetrability PH 2O for various burn dressings.

Table 11.10 Values of aqua-penetrability coefficients of burn dressings at 37 °C Dressing name (material)

PH 2 O × 10 9

(mol m/m2 s Pa)

Helitrex (collagen)

1.6 ± 0.1

Helitrex sponge (collagen)

11.0 ± 1.0

Brown dressing (collagen)

6.6 ± 0.6


0.8 ± 0.2


1.2 ± 0.2

Epigard (foamy polyurethane)

4.3 ± 0.4


3.3 ± 0.3

Biobrant (silicon-polyamide)

1.6 ± 0.16


2.0 ± 0.2

Perforated metallised dressing (cellulose)

9.0 ± 0.7

Face mask dressing (cellulose)

2.5 ± 0.2

Burn towel (cellulose)

5.4 ± 0.5


8.0 ± 0.7


8.0 ± 0.7

100% viscose

7.0 ± 0.7

11.5.4 Determination of the Air Penetrability of Burn Dressings As mentioned in Section 11.5.1, active sorption of wound exudate occurs for several minutes after putting a dressing on a burn wound. Later, the evaporation of water from the external

308

Physicochemical Criteria for Estimating the Efficiency of Burn Dressings side of the dressing proceeds. This leads to a change in the state of the exudate in the material mass. On the whole, this changes the penetrability of the dressing with respect to air. In this case, in order for anaerobic conditions not to be created in the wound, it is necessary to provide optimal air penetrability during the entire period of application. Data on the penetrability of dressings to dry air are known in the literature. Thus, for example, it is recommended [50] to determine air penetrability with the help of the industrially produced VPTM-2 device. This device records automatically the amount of air passing through a dressing of known area during time t under pressure oscillations of about 5 mm H2O. However, the application of such a device does not allow investigation of the air penetrability of dense materials such as foamy polyurethane compositions and, most importantly, of dressings in the wet state. The construction and principle of action of a device, developed by the present authors, that allows thse determination of the air penetrability of any material in any state and under any conditions were described in Section 11.3.

11.5.4.1 Penetrability of various materials to oxygen and nitrogen The coefficient of gas penetrability (as well as the coefficient of vapour penetrability) is calculated according to equation (11.10). Literature data on the penetrability of various polymers to oxygen and nitrogen are given in Table 11.11. As the data in this table show,

Table 11.11 Penetrability and separation coefficients of gases in polymers. Polymer

Penetrability coefficient ×1015 (mol m/m2 s Pa)

Separation coefficient

O2

N2

O2 / N2

Polycaproamide

0.013

0.0033

3.8

Polyvinyl chloride

0.022

0.008

2.8

Polyurethane elastomer

0.032

0.10

3.2

Polyethylene (ρ = 0.922)

0.35

0.13

2.7

Polystyrene

3.13

0.73

2.9

Teflon

2.07

0.67

3.1

Ethylcellulose

3.2

0.93

3.4

Polydimethylsiloxane

168

83.0

2.0

Silicon rubber

200

87.0

2.3

309


Table 11.12 Values of penetrability coefficients P (mol m/m2 s Pa), diffusion D ( m 2/ s ) a n d s o l u b i l i t y σ ( m o l / m 3 P a ) o f g a s e s i n t o p o l y d i m e t h y l s i l o x a n e at 20 °C [51]. P × 1015

D × 1010

σ × 106

O2

83

23.3

36

N2

164

30

55.6

CO2

720

–

–

Gases

the penetrability of polymers may differ by four orders of magnitude. Special attention should be paid to the high gas penetrability of polydimethylsiloxane and compositions based on it, which is the result of the increased solubility of gases in them at high rates of diffusion (Table 11.12) [51].

11.5.4.2 Penetrability of porous materials filled by a liquid medium A short list of studies considering the investigation of the gas penetrability of polymeric membranes in contact with a liquid is given elsewhere [52]. It is observed that the sorption of liquid by a polymer leads to a decrease in the gas penetrability coefficient in comparison with that of the liquid-free polymer.

11.5.4.3 Air permeability Let us consider the mass transfer of air through a porous body in two cases: one in which the free volume of all the pores is filled by air, and the other with the free volume filled by a liquid medium. The porous body may be represented as consisting of two phases: the material forming the body’s matrix, and the free space. We also assume that pores have cubic form and are disposed within the volume of the body in such a way that they do not join up with each other. Such a model is sufficient for porous burn dressings. Let us determine the total thickness of the body in the direction of mass transfer, the total thickness of free space occupied by pores, and the total thickness of the layer occupied by the material.

310

Physicochemical Criteria for Estimating the Efficiency of Burn Dressings •

The total thickness of the body in the direction of mass transfer is given by: lΣ = V/S

(11.11)

where V and S are the volume and surface area, respectively. •

The total thickness of the free space occupied by pores is given by: Qpores = lΣQ1/3 = (V/S)Q1/3

(11.12)

where Qpores = Vpores/V is the porosity. •

The total thickness of the layer occupied by the material is given by: lmat = lΣ – lpores = (V/S)(1 – Q1/3)

(11.13)

Thus, air passing through a porous body will overcome the resistance of two layers, each possessing its own penetrability coefficient with respect to air. The total penetrability coefficient PΣ of a porous body is thus given by: l pores 1 l = + mat P∑ l ∑ Ppores l ∑ Pmat

(11.14)

where Ppores and Pmat are the penetrability coefficients of the porous medium and the material forming the body’s matrix, respectively. The following equation can be used to determine the ratio of the penetrability coefficients of air for the porous body when its pores are filled with liquid and air: P∑( liq) P∑(air )

=

( Pmat / Pair ) + 1 ξ( Pmat / Pliq )

(11.15)

where: ξ = Q1/3/(1 – Q1/3)

(11.16)

Values of Pmat are shown in Table 11.10. Values of Pair and Pliq can be estimated from the coefficients of diffusion and solubility of oxygen in air, water, plasma and blood at 37 °C (Table 11.13).

311


Table 11.13 Values of the coefficients of penetrability, diffusion and solubility of oxygen in air, water, plasma and blood at 37 °C (dimensions as in Table 11.9) P

D

σ

Air

2.5 × 10-9

2.7 × 10-5

9.4 × 10-5

Water

7.4 × 10-14

3.0 × 10-9*

2.5 × 10-5*

Plasma

–

2.0 × 10-9*

–

Blood

1.4 × 10-14

1.4 × 10-9*

1.0 × 10-5*

Medium

* Values taken from [53]

Values of the penetrability coefficient of oxygen in various media may be calculated according to the following expression: P = Dσ

(11.17)

For any material Pair >> Pmat, so we obtain the simpler expression: P∑( air ) P∑( liq )

≈ξ

Pmat +1 Pliq

(11.18)

As, for the majority of dressings, ξ >> 1, and Pmat and Pliq are of the same order of magnitude, the decrease in air penetrability of a dressing when the pores fill with liquid must be significant. It has been shown by special experiments that air humidity (from 40 to 100%) does not practically influence the rate of penetration. The experiments were performed according to the following scheme. First we determined the time t0 to fill a polyethylene sack, of 45 litre volume, with air in conditions when the sample was not in the cell. This time t0 (a constant of the device) depended on the pressure in the system (p): log(1/t0) = –2.00 + 0.44 log p

(11.19)

The time for polyethylene sack filling at p = 100 Pa was selected as the standard. At T = 21 ± 1 °C, it is found that t0 = 16.0 ± 0.1 min. Subsequently, the time tx to fill the polyethylene sack when the sample was placed into the cell was similarly determined. It was observed (Figure 11.8) that the dependence of tx on p

312


Figure 11.8 The dependence of 1/log t on the pressure in the system for dry air. 1: needle-perforated material; 2: collagen sponge; 3: Syspurderm; 4: Syncrite; 5: Switin cellulose dressing; 6: Farmexplant; 7: Epigard

has the same slope as in equation (11.19) for all investigated dressings in conditions of dry air penetration: lg

1 = − Ax + 0.44 lg p tx

(11.20)

where Ax is a constant depending on the structure and properties of the dressing material. By bubbling humid air through a dressing saturated by water, the slope increased significantly. That is why it is necessary to perform several experiments for each dressing at different pressures in order to extrapolate tx to the pressure of 100 Pa with the required accuracy (Figure 11.9). The increase in the slope of log(1/tx) versus log p on bubbling air through a dressing saturated with water was attributed to the change in the material structure of the dressing resulting from the changes of form and size of the macropores. This is often accompanied by a decrease of the total volume of the dressing. The coefficient of air penetrability of the dressing (Px) was calculated according to: Px =

ml x S t (t x − t0 )

(11.21)

where m is the polyethylene sack bulk (equal to 2 mol of air at 21 ± 1 °C), and S is the surface area in contact with the bubbling air (equal to 1.8 × 10–3 m2); and p = 100 Pa. Thus,

313


Figure 11.9 The dependence of 1/log t on the pressure in the system for humid air. 1: needle-perforated material; 2: Farmexplant; 3: Combutec-2; 4: Syspurderm; 5: Biobrant (silicon-polyamide) compositional dressing; 6: Epigard

Px = 11

lx

(t x − t0 )

(11.22)

The values of air penetrability coefficients for dry dressings and dressings saturated with water are shown in Table 11.14. From this table it can be seen that a significant decrease of air penetrability takes place on saturation with water for all dressings except for ‘Biobrant’. The penetrability coefficient for dry dressings can be calculated according to the equation:

(

13 Q1 3 1 + Q = + P∑(air ) Pair Pmat

1

)

(11.23)

Pair is obtained from equation (11.17) using D = 2.7 × 10–5 m2/s and solubility at atmospheric pressure equal to 45 mol/m3. The value of Pair is 1.2 × 10–3 mol m/m2 s Pa. The values of PΣ(air) were taken from Table 11.11.Values of Pmat were calculated from equation (11.18). The values of PΣ(H2O) can be obtained from the equation:

(

1 + Q1 3 Q1 3 = + P∑(H 2O) PH 2O Pmat 1

314

)

(11.24)


Table 11.14 Coefficients of air penetrability for dry and water-saturated burn dressings at temperature of 21 ± l °C. Dressing name (material)

Coefficient of air penetrability (mol m/m2 s Pa) Dry

Wet

Helitrex (collagen)

2.7 × 10

Combutec-2 (collagen)

1.1 × 10

0


-4

1.3 × 10

1.3 × 10-5


1.3 × 10-4

1.0 × 10-6


1.1 × 10-3

4.0 × 10-5


4.5 × 10-5

0

Biobrant (polyamide + silicon)

-4

1.8 × 10

7.0 × 10-5


1.6 × 10-4

3.0 × 10-6

Needle-perforated material (cellulose)

1.1 × 10-3

–

-5

0

-3

The calculated values of PΣ(H 2O) fall close to 10–8 mol m/m2 s Pa for the majority of dressings. This result reveals the extremely low air penetrability for the listed dressings. For some dressings, the value of PΣ(H 2O) is significantly higher than 10–8 mol m/m2 s Pa. This can be explained by two effects: (1) The presence of air flow along the surface of pores (surface flow) [49]; (2) The pressure of channels in the materials that are free of water. To test these suppositions, additional investigations are required.

11.5.5 Determination of Adhesion of Burn Dressings Adhesion properties play a key role in dressing performance. The lower layer of a dressing must be easily wetted, providing good adhesion of the dressing to the wound. Besides, the surface energy at the dressing-wound interface must be minimal to provide the smallest trauma on its removal from the wound.

315


11.5.5.1 Adhesive strength: theory Adhesive strength characterises the ability of an adhesive structure to preserve its integrity. Adhesive strength as well as the strength of homogeneous solids is of kinetic nature. That is why the rates of surface tension and temperature increase affect the adhesive strength, and why the scale factors, (i.e., sample dimensions), are also of great importance. Different theories of adhesion of polymers have previously been suggested [53, 54] as follows: (1) Mechanical theory (MacBain), according to which the main role is devoted to mechanical filling of defects and pores of the surface (dressing) by the adhesive (blood); (2) Adsorption theory (Mac-Loren), considering adhesion as a result of the performance of molecular interaction forces between contacting phases – according to this theory, low adhesion, for example, may be reached between a substrate (dressing) with nonpolar groups and polar adhesive (blood); (3) Electrical theory (Deriagin), based on the idea that the main factor controlling the strength of adhesive compounds rests in the double electrical layer that is formed on the adhesive-substrate interface; (4) Diffusion theory (Vojytzky), considering the adhesion to be a result of interweaving of the polymer chains; (5) Molecular-kinetic theory (Lavrentiev), which assumes that a continuous process of restoration and breakage of bonds proceeds in the zone of adhesive-substrate contact – thus, adhesive strength is defined by the difference between the activation energies for breakage and formation of bonds, and also depends on the correlation between the amount of segments participating in the formation of bonds and the average number of molecular bonds per unit contact area. In recent years, the thermodynamic concept has received the most attention. Thus, the main role is devoted to the correlation of the surface energies of adhesive and substrate. The thermodynamic work of adhesion of a liquid to a solid (Wa) is described by the Dupret-Jung equation: Wa = γl(1 – cos θ)

(11.25)

where gl is the surface tension of the liquid, and θ is the wetting angle. Substituting Jung’s equation:

γs-l = γs – γs-l cos θ

316

(11.26)

Physicochemical Criteria for Estimating the Efficiency of Burn Dressings into equation (11.25), we obtain the correlation: Wa = γs + γl – γs-l

(11.27)

where γs and γs-l are the surface tension of the solid and of the solid-liquid interface, respectively. It follows from equation (11.27) that, the higher Wa, the larger are the values of γs and γl while γs-l are smaller. However, according to equation (11.27), the increase of γs must lead to the growth of Wa and to an increase of γs-l. That is why the increase of the surface tension of the substrate is accompanied by the action of two effects. The necessary condition for adhesive strength is γl >> γs. Values of γl and Ws −H 2O for different materials are shown in Table 11.15.

Table 11.15 Values of the surface tension and thermodynamic work of adhesion of various materials [27] Material

γs (mN/m)

Ws −H 2O

(mN/m)

Polytetrafluoroethylene

18.5

83

Silicon rubber

21.0

78

Polyethylene

31.0

99

Polystyrene

33.0

105

Polymethylmethacrylate

39.0

103

Polyvinyl chloride

39.0

101


43.0

104

Polycaproamide

46.0

107

Glass

170.0

222

11.5.5.2 Adhesive strength of dressings The adhesive strength of burn dressings was determined according to the method described in Section 11.3. Table 11.16 shows the adhesive strength of various burn dressings and the angle of wetting by water.

317


Table 11.16 Adhesive strength (A) and the angle of wetting by water (θ) of various burn dressings A (mN/m)

θ (deg)

Corretium (collagen)

220 ± 20

75 ± 2


210 ± 20

–


350 ± 50

125 ± 3


200 ± 20

130 ± 2

Bern-pack (cellulose)

170 ± 20

–

Biobrant (silicon-polyamide)

70 ± 10

–


20

–

Blood-stopping non-adhesive dressing (cellulose)

20

–

170 ± 50

–

Dressing name (material)

Dressing with metallised lower layer (cellulose)

11.6 The Model of Action of a Burn Dressing Three main processes proceed after the application of a dressing to a wound: (1) Sorption of the wound exudate by the dressing; (2) Water evaporation from the dressing surface; (3) Mass transfer of gases through the dressing under conditions of ongoing sorption and evaporation. Processes (1) and (3) were analysed in detail in Section 11.4. It was found that sorption of liquid media (water, plasma) proceeds rapidly and reaches a limiting value (maximal sorption ability) after several minutes for most dressings, i.e., a time that is significantly shorter than the time for which the dressing acts (2-3 days). The mass transfer of gases (oxygen and nitrogen) through the dressing is 2-4 orders of magnitude slower with wet samples than with the dry ones in similar conditions. Next, we consider water evaporation from the dressing surface.

11.6.1 Evaporation of Water from the Dressing Surface Suppose that a dressing is saturated with water in air at 20 °C and 50% humidity. The temperature of the dressing surface is 32 °C. These conditions are chosen to take into account the temperature gradient in the matrix of the dressing.

318

Physicochemical Criteria for Estimating the Efficiency of Burn Dressings Let us determine the amount of water that evaporates from the surface of the dressing during a given time period under stationery atmospheric pressure, and when the dressing surface is completely saturated with water. The partial pressure of air at 20 °C and 50% relative humidity equals: PH 2O = 1.26 × 10–3 kg/cm2, Pair = 1.02 kg/cm2

For air at 32 °C in the saturated state: PH 2O = 4.85 × 10–2 kg/cm2, Pair = 0.98 kg/cm2

The values of density, viscosity, heat conductivity and heat capacity of air at 26 °C equal: ρ = 1.185 kg/cm3 μ = 1.861 × 10–6 g/m s λ = 6.1 × 10–6 kcal/m s °C Cp = 0.24 kcal/°C After mathematical transformations using the method described elsewhere [55-58], the following equation for the mass transfer of water in a dressing can be obtained: W = am

P ( p1 − p2 ) RT ρ av

(11.28)

where am is the coefficient of heat conductivity, ρav is the average value of the mixture density over and near the surface of the dressing, p1 and p2 are the partial pressure PH 2O at 37 °C and 20 °C, respectively, R is the universal gas constant, and P is the normal pressure. Substituting numerical values for a dressing of 1 m × 1 m size, we obtain: W = 1.2 × 10–1 g/m2 s If the dressing surface is not completely occupied by water, we should apply the equation: W=

Csurf (H 2O) 0 Csurf (H 2 O )

× 1.2 × 10 −1 g/m 2 s

(11.29)

0 where Csurf (H 2O) and Csurf (H 2O) are the surface concentrations of water on the external side of the dressing and on the free water surface, respectively.

319


11.6.2 Sorption of Fluid by Burn Dressing from Bulk Containing a Definite Amount of Fluid Let us consider the case where a burn dressing is applied to a wound containing a definite amount of liquid. Assume that a dressing membrane of given size (thickness and surface area S) is in contact with the solution of restricted bulk volume V, which contains a concentration C0(s-s) of diffusive substance. As the dressing becomes saturated by this substance, the concentration of the latter in the bulk will decrease. The solution of the diffusion equation has the following form [55-58]: m m∞

= 1−

2a(1 − a) 1 + a + a2 q 2

⎡ 4Dq 2t ⎤ exp⎢ 2 ⎥ ⎢⎣ l ⎥⎦

(11.30)

where q is the positive solution of the characteristic equation: tgq = − aq ; a =

V (σSl )

where σ is the distribution coefficient of the substance between the membrane and the solution. When a sufficient part of the substance in solution is sorbed by the membrane, the value of a is small and a simpler expression can be used: ⎡

⎤ ⎥ mt ≈ m ⎢1 − 12⎥ ⎢ ⎥ 4πDt / l 2 ⎣ ⎦ ∞⎢

a

(

)

(11.31)

From equations (11.30) and (11.31), two important correlations can be obtained. The sorption ability of the dressing, i.e., the part of the substance sorbed from the solution under equilibrium conditions, equals: m∞/m0 = 1/(1 + a)

(11.32)

Thus, for the efficient action of the dressing, it is necessary that the concentration C be as high as possible in relation to the products of metabolism and toxins. Relating to water, CH2O ~ 1, it is desirable that the dressing volume (lS) should be close to the volume of wound exudate (V).

320

Physicochemical Criteria for Estimating the Efficiency of Burn Dressings The time to reach the degree 0.85 of maximum sorption of liquid media by the dressing equals: t0.85 = 12

a2l 2 V2 = 12 πD πDσ 2 S 2

(11.33)

It depends on many parameters, each being able to affect the time of completion of the sorption process.

11.6.3 Mass Transfer of Water from Wound to Surroundings Generally, the change in the amount of water under the dressing in the wound ( mH 2O ) is determined from the correlation derived from equations (11.29) and (11.31):

0 mH 2O = V CH 2O

⎡ ⎢ a − mH O dressing ⎢1 − ) 2 ( ⎢ 4πDt / l 2 ⎣

(

⎤ ⎥ Csurf (H 2O) − 0 × 1.2 × 10 −1 S t 12⎥ C surf (H 2 O ) ⎥ ⎦

)

(11.34)

Let us consider the application of the correlation (11.34) for the following case. The wound characteristics are: 0 0 S = 10–2 m–2, CH = 106 g/m3, mH = 50 g 2O 2O

V = 5 × 10–5 m3, l = 10–3 m, σ H 2O = 1 Csurf (H 2O) 0 Csurf (H 2 O )

= 0.5

Under these conditions: t0.85 = 12

m∞ m

0

=

25 × 10 −10 π × 10 −9 × 10 −4

(

1 + 5 × 10

= 9.5 × 102 s# (or ~15 min)

1 −5

/ 10 −2 × 10 −3

)

= 0.17#

(or 8.5 g)

321

Handbook of Plastic Films During the same time the following amount of water will evaporate from the dressing surface: mevap ( H2O) < 0.5 × 1.2 × 10 −1 × 950 × 10 −2 = 0.6 g

i.e., the rate of evaporation is significantly (14 times) lower than that of water sorption by the dressing. All the amount of water from the wound (wound exudate) will evaporate during the time: t=

50 0.5 × 1.2 × 10 −1 × 10 −2

= 8.3 × 10 4 s# (or ~23 h)

11.7 Criteria for the Efficiency of First-Aid Burn Dressings 11.7.1 Requirements of a First-Aid Burn Dressing A first-aid burn dressing must meet the following criteria: (1) Sorption of the wound exudate, containing products of metabolism and toxic substances, during the period of dressing action (24-48 h); (2) Wound isolation from infection of the external medium; (3) Optimum air and water transfer between wound and surroundings; (4) Easy removal from the wound, causing no damage to the wound surface. The characteristics of burn wound dressings, based on the approximate estimations discussed previously, are listed next. Note that no quantitative data have been reported in the literature.

11.7.2 Characteristics of First-Aid Burn Dressings 11.7.2.1 Sorption ability of dressings A second- or third-degree burn wound releases on average 5 × 103 g/m2 of exudate. As may be seen from Table 11.11, the water amount is about 90%. The sorption of different components of the exudate proceeds at different rates. In this case, the free volume of the dressing material will be first filled with water. The diffusion of proteins and cells takes place in space occupied by water.

322

Physicochemical Criteria for Estimating the Efficiency of Burn Dressings Modern burn dressings possess a porosity of 0.9 and almost the entire free volume can be filled with water (Figure 11.5). The maximum sorption ability for such dressings equals: CH 2O ≈

ρH 2O ρ

and the amount of the liquid sorbed per unit area is: ∞ mρ ≈ C H ρl = 2O

ρH 2O ρ

ρ l ≈ 106 l g/m 2

because ρH 2O = 106 g/m3. As a first-aid burn dressing must sorb 5 × 103 g/m3, it follows that: 5 × 103 ≈ 106l

(11.35)

and therefore the thickness of a first-aid burn dressing equals: l ≈ 5 × 103/106 ≈ 5 × 10–3 m (or 0.5 cm) Thus, the first criterion for the efficiency of a first-aid burn dressing can be formulated as follows: A first-aid burn dressing must use its entire free volume for sorption. This volume must be 0.9 or more of the total volume of the dressing. Dressing thickness must be 0.5 cm or more. The majority of foreign, (i.e., non-Russian), first-aid dressings fulfil this criterion.

11.7.2.2 Air penetrability of dressings The air penetrability of most of the dry dressings ranges between 10–4 and 10–5 mol m/m2 s Pa (Table 11.14). The air penetrability of the dressings saturated with water is much lower and decreases to values between 10–6 and 10–5 mol m/m2 s Pa, that is, 0.2-2 dm3/ m2 s. Thus, the second criterion for the efficiency of first-aid burn dressings can be formulated as follows: A first-aid burn dressing must possess an air penetrability of 10–5 mol m/m2 s Pa or higher after the sorption of water. For example, the Biobrant burn dressing fulfils this criterion. 323


11.7.2.3 Adhesion of dressing to wound The adhesion strength of dressings with respect to coagulated blood (Table 11.16) varies in a wide range, but it has the minimum value of ~20 N/m. This value should be accepted as the optimal one, because it corresponds to the minimal pain and damage on removal from the surface of natural skin. Thus, the third criterion for the efficiency of first-aid burn dressing can be formulated as follows: A first-aid burn dressing must possess an adhesive strength to the wound of 20 N/m or less after the end of its action. The following burn dressings, for example, fulfil this criterion: Biobrant, blood-stopping remedy, Johnson & Johnson.

11.7.2.4 Isolation of wound from infection from external medium It is known that microorganisms causing wound infection do not penetrate through filters possessing average pores size ~0.5 μm. So the fourth criterion for the efficiency of first-aid burn dressings is as follows: A first-aid burn dressing must possess no open pores with average diameter larger than 5 × 10–7 m (0.5 μm). Moreover, it is implied that first-aid burn dressings possess sufficient mechanical strength and elasticity in both dry and humid conditions.

11.8 Conclusion Experimental methods to estimate the main physicochemical properties of burn dressings were worked out. Based on theoretical and experimental data we found the following: (1) The maximal sorption ability of a burn dressing equals the free volume of the dressing material, calculated from the value of the material density. (2) Water can be used as a model liquid in the study of sorption ability instead of blood plasma. (3) Kinetic parameters were determined from the sorption curves. These parameters showed that first-aid burn dressings markedly differ in the value of the rate of liquid media sorption at stages close to the sorption limits. (4) The air penetrability parameter in the wet state decreases abruptly by 2-3 orders of magnitude for the majority of tested dressings. This is due to the filling of pore space by the liquid medium.

324

Physicochemical Criteria for Estimating the Efficiency of Burn Dressings (5) Accordingly, it is recommended that the air penetrability parameter should be determined in the wet state, which represents the common condition of action for first-aid burn dressings. (6) The value of the adhesive strength after the end of its action on the wound should not exceed 20 N/m. From the data obtained in this study, we formulated the following criteria to estimate the efficiency of first-aid burn dressings: •

Maximum sorption ability for water must be at least 10 g/g;

•

Optimal thickness of dressings, fulfilling this value of sorptional capacity, must be about 5 × 10–3 m (0.5 cm);

•

Adhesive strength must not exceed 20 N/m;

•

Average diameter of open (connected) pores must not exceed 5 × 10–7 m.

References 1.

M.I. Fel’dshtein, V.S. Yakubovich, L.V. Raskina and T.T. Daurova, Polymer Coatings for Wound and Burn Treatment, Institute of Information, Moscow, Russia, 1981, 299 (in Russian).

2.

G.B. Park, Biomaterials, Medical Devices and Artificial Organs, 1978, 6, 1.

3.

V. Rudkovsky, V. Nezelovsky, V. Zitkevich and N. Zinkevich, Theory and Practice of Burn Treatment, Meditsina, Moscow, Russia, 1988, 200 (in Russian).

4.

A. Robin and K.H. Stenzel in Biomaterials, Eds., L. Stark and G. Agarwal, Plenum Press, New York, NY, USA, 1969, 157.

5.

A. Robin, R.R. Riggio and R.L. Nachman, Transactions of the American Society of Artificial Internal Organs, 1968, 14, 1669.

6.

H.C. Grillo and I. Gross, Surgical Research, 1962, 2, 69.

7.

J. Oluwasanmi and M. Chapil, Journal of Trauma, 1976, 16, 348.

8.

G.E. Zaikov, International Journal of Polymeric Materials, 1994, 24, 1.

325


J.I. Abbendhaus, R.A. McMahon, J.G. Rosenkranz and I.C. McNeil, Surgical Forum, 1965, 16, 477.

10. F.J. Richter and C.T. Riall, inventors; American Cyanamid Company, assignee; US Patent 3,566,871, 1971. 11. Polymers in Medicine, Ed., G.E. Zaikov, Nova Science Publishers, New York, NY, USA, 1997. 12. G.E. Zaikov, Degradation and Stabilisation of Polymers, Nova Science Publishers, New York, NY, USA, 1998. 13. Polymers in Medicine, Ed., G.E. Zaikov, Nova Science Publishers, New York, NY, USA, 1995, 286. 14. J.H. Gardner and D.T. Rovee, inventors; Johnson & Johnson, assignee; US Patent 3,521,631, 1970. 15. L.M. Wheeler, inventor; Parke Davis and Company, assignee; US Patent 3,648,692, 1972. 16. No inventors; Johnson & Johnson, assignee; UK Patent 1,309,768, 1973. 17. G.L. Wilks and L.L.J. Samuels, Biomedical Materials Research, 1973, 7, 541. 18. I.A. Agureev, Voenno-Meditsinskii Zhurnal, 1963, 6, 74 (in Russian). 19. P. Lock, inventor; no assignee; French Patent 2,156,068A1, 1973. 20. K. Gorkisch, E. Vaubel and K. Hopf, Proceedings of the 2nd International Congress on Plastics in Medicine, Amsterdam, The Netherlands, 1973, Paper No.16. 21. A.L. Iordanskii, G.E. Zaikov and T.E. Rudakova in Transport, Kinetics, Mechanism, VSP Science Press, Utrecht, The Netherlands, 1993, 288. 22. USSR Certificate No. 245,281, 1969, Bulletin of Certificates, No. 19. 23. W.M. Chardack, M.M. Martin, T.C. Jewett and E.M. Pearce, Plastic and Reconstructive Surgery, 1962, 30, 554. 24. C.W. Hall, D. Liotta, J.J. Chidoni, V.M.M. Lobo and A. Valente, Journal of Biomedical Materials Research, 1972, 6, 571. 25. J.J. Guldarian. C. Jelenko, D. Calloway, L. Kalle and M.Lewin, Journal of Trauma, 1973, 13, 32.

326

Physicochemical Criteria for Estimating the Efficiency of Burn Dressings 26. E.E. Schmitt and R.A. Polistina, inventors; American Cyanamid Company, assignee; US Patent 3,875,937, 1975. 27. S. Madou, Ed., Polymers for Medicine, Meditsina, Moscow, Russia, 1981, 350 (in Russian). 28. USSR Certificate No. 267,010, Bulletin of Certificates, 1970, No.12. 29. G.E. Zaikov, A.L. Buchachenko and V.B. Ivanov, Ageing of Polymers, Polymer Composites and Polymer Blends, Nova Science Publishers, New York, NY, USA, 2002. 30. F.C. Moore and L.A. Perkinson, inventors; Moore-Perk Corporation, assignee; US Patent 3,678,933, 1972. 31. M.G.M. Nilsson, R.G.A.B. Udden, P.E.C. Udden and B.A. Wennerblom, inventors; Svenska cellulos Aktiebolaget, assignee, US Patent 3,654,929, 1972. 32. H. Kinkel and S. Holzman, Chirurgie, 1965, 36, 535. 33. USSR Certificate No. 6,658,148, Bulletin of Certificates, 1979, No.15. 34. M.I. Kuzin, V.K. Sologub, V.V. Yudenich, Y.B. Monakov, K.S.Minsker and A.A. Berlin, Khirurgiya, 1979, 8, 86 (in Russian). 35. I.V. Yannas and J.F. Burke, Journal of Biomedical Materials Research, 1980, 14, 65. 36. S. Jacobson and U. Rothenaw, Journal of Plastic and Reconstructive Surgery, 1976, 10, 65. 37. D. Spruit and K.E. Malten, Dermatology, 1966, 132, 115. 38. USSR Certificate No. 685,292, Bulletin of Certificates, 1979, No.34. 39. Textbook on Polymer Materials, Ed., N.A. Plate, Khimiya Publishers, Moscow, Russia, 1980, 255 (in Russian). 40. G.E. Zaikov, A.L. Iordanskii and V.S. Markin, Diffusion of Electrolytes in Polymers, VSP Science Press, Utrecht, The Netherlands, 1988, 328. 41. Y.V. Moiseev and G.E. Zaikov, Chemical Resistance of Polymers in Reactive Media, Plenum Press, New York, NY, USA, 1986, 586. 42. I.A. Barrie in Diffusion in Polymers, Eds., J. Crack and G.S. Park, Academic Press, London, UK, 1968, 452.

327

Handbook of Plastic Films 43. S.I. Papkov and E.Z. Fainberg, Interaction of Cellulose and Cellulose Materials with Water, Khimiya, Moscow, Russia, 1976, 231 (in Russian). 44. G.E. Zaikov, Chemical and Biochemical Kinetics, Nova Science Publishers, New York, NY, USA, 2002. 45. S.S. Voyutskii, Physico-Chemical Principles of Fiber Materials – Sorption by Polymer Dispersions, Khimiya, Leningrad, 1969, 336 (in Russian). 46. D.A. Fridrikhsberg, Course of Colloid Chemistry, Khimiya, Leningrad, 1974, 351 (in Russian). 47. M.I. Al’tshuller and B.V. Deryagin, in Investigations in the Field of Surface Force, Nauka, Moscow, Russia, 1967, 235 (in Russian). 48. M.M. Mikhailov, Moisture Permeability of Organic Dielectrics, Gosenergoizdat, Moscow, Russia, 1960, 162 (in Russian). 49. N.I. Nikolaev, Diffusion in Membranes, Khimiya, Moscow, Russia, 1980, 232 (in Russian). 50. Textbook on Textile Materials, Legkaya Industriya, Moscow, Russia, 1974, 342 (in Russian). 51. I.M. Raigorodskii and V.A. Savin, Plasticheskie Massy, 1976, 1, 65 (in Russian). 52. V.N. Manin and A.N. Gromov, Physico-Chemical Resistance of Polymer Materials During Exploitation, Khimiya, Moscow, Russia, 1980, 247 (in Russian). 53. E. Laifut, Transfer Phenomena in Living Systems, Mir, Moscow, Russia, 1977, 520 (in Russian). 54. V.E. Basin, Adhesion Durability, Khimiya, Moscow, Russia, 1981, 208 (in Russian). 55. L.M. Batuner and M.E. Pozin, Mathematical Methods in Chemical Technology, Khimiya, Leningrad, 1971, 822 (in Russian). 56. Polymer Analysis and Degradation, Eds., A. Jimenez and G. Zaikov, Nova Science Publishers, Huntington, NY, USA, 2000, 287. 57. Polymers in Medicine, Ed., G.E. Zaikov, Nova Science Publishers, New York, NY, USA, 1998, 245. 58. A.Y. Polishchuk and G.E. Zaikov, Multicomponent Transport in Polymer Systems for Controlled Release, Gordon and Breach, New York, NY, USA, 1996, 231.

328

12

Testing of Plastic Films E.M. Abdel-Bary and G. Akovali

12.1 Introduction Plastics are a very important group of materials. They differ from most of the ‘natural’ materials – such as metals, papers, ceramics, natural fibres – mainly as a result of their ‘viscoelastic’ behaviour. The word ‘viscoelastic’ is used to describe behaviour that shows both viscous and elastic characteristics even at ambient conditions, when stressed. This behaviour is a direct result of the long-chain nature of the polymeric molecules that constitute the plastic material. Whereas the gross mechanical behaviour of most ‘natural’ materials under stress could be considered as elastic or deformation flow, the response of all plastics to stress is a combination of the two. The ratio of viscous and elastic components, termed ‘damping’, can vary greatly over quite a narrow temperature range for plastics and it also depends markedly on the rate of stressing. One of the most common forms of plastic material is the ‘film’. Test methods for plastic films have evolved not only from the techniques of the preceding technologies. The bigger manufacturers and users have also devised their own laboratory procedures to enable them to control film properties or determine the suitability of a film for a particular process or application. In addition, research scientists have published the methods that they have used to study the theoretically interesting properties of polymers. Standards organisations have attempted to devise standard test methods acceptable to all branches of the industry. This chapter reports briefly on the most common test methods generally used for plastic films, according to the field of applications. Although most countries have their own standards and standards organisation(s), consideration here will be restricted to tests published by the American Society for Testing and Materials (ASTM). Anyone interested in the details of the ASTM tests can find them in ASTM D883 [1], which is one of the many parts of the ASTM Standards and is available in libraries or directly from the ASTM or the US Government Printing Office.

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12.2 Requirements for Test Methods 12.2.1 List of Requirements There are several requirements necessary for a test method to be developed, some of which are summarised below: (1) The test method should be rapid, so that results can be used in quality control on high-output machinery without delaying producing or dispatch. (2) Results must be reproducible and consistent between different testing stations and machines. This means that the test should be insensitive to minor variations in specimen preparation, to wear and to other small differences in test apparatus. (3) The precision of the results should be no more than is required. The cost of extreme accuracy is rarely justified in industry, and often a value that is accurate to within a few per cent will give all the information that is wanted. (4) It is preferable that the results are scientifically significant. It is imperative that they are of technological significance and give a meaningful indication of the real-life performance of the film. The main advantage of a standard method is that results obtained by its use in different laboratories can be compared.

12.2.2 Interpretation of Test Results The main difficulties encountered both in deriving significant tests for polymers and in interpreting the results are the (relatively rapid) changes in properties with rate of deformation and, particularly, with temperature. The mechanical behaviour of conventional materials is fairly insensitive to temperature in the normal range of ambient and packaging-processing temperature for the films used in the packaging industry. However, a polymer, being viscoelastic, may change from a glassy solid through a leathery and then a rubbery stage to a sticky liquid in a temperature range of less than 100 °C. This variation can be of practical importance not only for the manufacturer, who is prevented from using the high temperature sometimes demanded (for example, in print drying), but also for the designer wishing to provide packages that can be used in

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Testing of Plastic Films environments ranging from cold storage at –30 °C to a window display in hot sunshine, where the temperature can exceed 60 °C. Viscoelasticity is a complex subject, and all polymers exhibit a similar pattern of behaviour, the details of which are determined by the chemical nature of the polymer, its molecular weight (molar mass) and molecular weight distribution, degree of crystallinity and so on. Taking polystyrene (PS) as an example of a simple amorphous polymer, one finds that the elastic modulus is constant in the temperature range up to about 100 °C, which is the glassy region. Increasing the temperature above 100 °C leads to a drastic decrease in the elastic modulus, as it exists in the leathery region. Further increase in temperature has no effect on the elastic modulus as PS falls in the rubbery region. In all these three regions – glassy, leathery and rubbery – the moduli of commercially useful polymers are independent of molecular chain length. In the last region, at temperatures exceeding about 170 °C, the polymer falls in the flow region. The basic molecular phenomena causing these different types of behaviour are reasonably well understood. In the glassy region, the long polymer molecule is frozen, with the atoms vibrating about fixed positions as in any rigid solid. In the leathery (transition) region, where the modulus changes rapidly with temperature, short-range diffusion of segments of the polymer chains takes place, but any movement is restricted to individual atoms of two or three adjacent segments, and the molecule as a whole does not move. In the rubbery region, the modulus is fairly constant; here the shortrange motions of polymer segments are very fast, and the cooperative movement of adjacent segments takes place. Entanglements restrict the length of chain that can move. In the rubbery flow region, the motion of molecules as a whole becomes important as a result of slippage of the entanglements; while in the region of flow, changes in the entire molecule take place quicker than the rate of testing, and there is little elastic recovery at this time-scale. For the last two regions, the modulus depends on the chain length and its distribution. The modulus versus temperature curve is also rate- (of testing or stressing) dependent, since the major changes in the modulus take place when a particular molecular activity is occurring at large magnitudes at rates faster than the test. This behaviour for the modulus holds true for any usual mechanical property such as yield strength, breaking strength, breaking elongation, impact strength (or total breaking energy), etc. Meanwhile, it is important to consider the parameters in the test, which are important for the proposed application, e.g., temperature, rate, humidity and geometry, so that they cover the range met in use. If not, the data of some other standard test should be able to provide the necessary, and probably the most important, information.

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12.3 Some Properties of Plastic Films Several standard methods can be used to determine the properties of plastic films. Properties can be purely physical, physico-chemical, chemical or mechanical. For the first three, usually the form of the sample does not matter, and the same methods are used for film samples as well as samples with bar shape. Most of the mechanical tests are also the same methods that are employed in testing plastics in any form, while some are specific for the plastic films. Following the procedure described in a relevant standard for tensile properties, it was shown that it is very important to define appropriately the whole set of parameters involved in the test. In addition, special adaptation of the equipment used is required. Harmonisation of the testing methods for impact and initial tear resistance proved to be more readily obtained. However, some parameters entering the corresponding measuring procedures had to be adapted. In general, harmonisation has been achieved regarding the measurement of the specific mechanical properties [2]. Some of the characteristics of these films, usually taken into consideration, are first given below, followed by the mechanical tests and then other tests.

12.3.1 Dimensions Measurement of the average thickness of a film is straightforward, and no special problems should be encountered in their measurement. The accurate measurement of film thickness is important because the values of some of the other properties – such as tensile strength, elongation at break, impact resistance, resistance to tear propagation – depend strongly on the thickness of the material. In general, the thicknesses of plastic films are several tens of micrometres, (e.g., low-density polyethylene (LDPE) agricultural films usually range from 50 μm to more than 200 μm, the latter for greenhouse films). The trend is to reduce thickness to avoid the huge amount of waste at the end of their lifetime.

12.3.2 Conditioning the Samples In general, the physical and electrical properties of plastics and electrical insulating materials are strongly influenced by the temperature and stress history of the samples (used during their preparation) as well as the humidity. In order to make reliable comparisons, it is necessary to standardise the temperature and humidity conditions to which plastics are subjected prior to and during testing. Unless otherwise specified for special polymers, the standard procedure recommended for conditioning samples prior to testing is described by ASTM D618-61/90 Procedure A [3]. In this method, for specimens thinner or thicker than 7 mm, condition the specimens for a minimum 40 h immediately prior to testing (or 88 h for the latter, over 7 mm thickness) in the

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Testing of Plastic Films standard laboratory atmosphere at 23 °C and 50% relative humidity (RH), whilst providing adequate air circulation on all sides. This can be achieved by placing the samples in suitable racks, hanging them from metal clips or laying them on widemesh, wire screen frames with at least 25 mm between the screen and the surface of the bench.

12.4 Mechanical Tests 12.4.1 Tensile Testing (Static) Tensile tests are mainly used to determine the tensile strength of a material. Such testing provides data for research, development and engineering design as well as for quality control and specification. In tensile testing there are certain difficulties with thin films. It is essential that the cut edges of the tensile specimen are free from nicks or flaws from which premature failure could start. For thinner films, grip surfaces are a problem. Both slippage in the grip and fracture of the sample at the grips must be avoided. Any technique, such as the use of a thin coating of rubber on the faces or the use of emery cloth, that prevents slipping in the grips, prevents grip fractures and does not interfere with the portion of the sample under test, is acceptable. From tensile tests, some material characteristics – such as the (tensile) modulus, percent elongation at break, yield stress and strain, tensile strength and tensile energy to break values – can be obtained. Tensile properties (static) of plastics are covered in ASTM D638 (general) [4] and ASTM D882 (films) [5].

12.4.1.1 Tensile Strength Tensile strength is calculated by dividing the maximum load by the initial crosssectional area of the specimen, and is expressed as force per unit area (usually in megapascals, MPa).

12.4.1.2 Yield Strength Yield strength is the load at the yield point divided by the initial cross-sectional area, and is expressed as force per unit area (MPa), usually to three significant figures.

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12.4.1.3 Tensile Modulus of Elasticity The tensile modulus of elasticity (or simply the elastic modulus, E) is an index of the stiffness, while the tensile energy to break (TEB, or toughness) is the total energy absorbed per unit volume of the specimen up to the point of rupture. The tensile modulus of elasticity is calculated by drawing a tangent to the initial linear portion of the load versus extension curve, selecting any point on this tangent, and dividing the tensile force by the corresponding strain. The results are expressed in MPa, and are usually reported to three significant figures. Secant modulus (used for cases where no initial linear proportionality exists between stress and strain) is defined at a designated strain. TEB is calculated by integrating the energy per unit volume under the stress-strain curve, or by integrating the total energy absorbed divided by the volume of the original gauge region of the specimen. TEB is expressed as energy per unit volume (in megajoules per cubic metre, MJ/m3), usually to two significant figures.

12.4.1.4 Tensile Strength at Break Tensile strength at break is calculated in the same way as the tensile strength, except that the load at break is used in place of the maximum load. It should be noted that, in most cases, tensile strength and tensile strength at break values are identical.

12.4.1.5 Percent Elongation at Break Percent elongation at break is the extension at the point of rupture divided by the initial gauge length. It is usually reported to two significant figures.

12.4.1.6 Percent Elongation at Yield Percent elongation at yield is the extension at the yield point divided by the initial gauge length of the specimen, usually given to two significant figures.

12.4.1.7 Package Yield of a Plastic Film A specific ASTM test method (ASTM D4321; [6]) exists for the determination of the ‘package yield’ of plastic films, in terms of area per unit mass of the sample. In this test, values such as the nominal yield (the target value of the yield as agreed between the user and supplier), package yield (yield calculated by the standard), nominal thickness (the

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Testing of Plastic Films target value of the film thickness as agreed between the user and supplier), nominal density and measured density are defined and obtained. The value of package yield is important for the manufacturer, because it determines the actual number of units or packages that can be derived from a given mass of film in a particular application.

12.4.1.8 ASTM D882 Test for Thin Films In tensile measurements, discrepancies can and usually do occur in the results, either because of the use of different specimen types with different geometries and/or because different test speeds are employed in the testing procedure. However, the data from such tests cannot be considered appropriate for applications whose load time-scales differ widely from those actually used in the test employed. In fact, the shape of the specimens suggested may be different depending on the film thickness. They are specified in different standards (such as ISO 527 for thick films [7-9], and ISO 1184 [9] and ASTM D882 for films less than 0.25 mm; [5]). A brief description of D882-95a is given next. A load range is selected such that specimen failure occurs within its upper two-thirds, for which a few trial runs are recommended. The cross-sectional area, width (to an accuracy of 0.25 mm) and thickness (to an accuracy of 0.025 mm for thin films with thicknesses less than 0.25 mm, and for thicker films to an accuracy of 1%) of the sample are measured at several points. The grip separation rate is set and the test specimen is placed in the grips and tightened evenly. The machine is started, and load versus extension values are recorded. Some characteristic tensile values of different plastic films are presented in the table given in ASTM D882-95a. LDPE is one of the weakest films used as the covering of greenhouses, in terms of tensile strength (11-37.9 MPa) [10]. As the density of polyethylene (PE) increases from LDPE to high-density polyethylene (HDPE), tensile strength at yield and stiffness values are seen to increase, while elongation and flexibilities decrease [11]. This is because the crystalline regions significantly increase the modulus of elasticity and hence the ability of the plastics to support loads at elevated temperature [12]. Another effect observed from the table in ASTM D882-95a is that of strengthening due to the molecular orientation imparted during film blowing. This is because, on a molecular level, tensile properties are higher in the direction of the covalent C–C bond in the chain than in the transverse direction, which is dominated by the much weaker van der Waals’ bonds. Since the crystals of LDPE films are preferentially oriented parallel to the machine direction, load applied in the machine direction may yield higher values of tensile strength than load

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Handbook of Plastic Films applied perpendicular to that direction. In fact, not only the direction of the film, but also the process parameters – such as melt temperature, die parameters, blow-up ratio, draw ratio, frost-line height and cooling conditions – can lead to different mechanical properties between two films with the same composition [13] (details are given in Chapter 2).

12.4.2 Impact Resistance Impact values represent the total ability of the material to absorb impact energy, which is composed of two parts: (a) the energy required to break the bonds, and (b) the work consumed in deforming a certain volume of the material. The impact resistance of plastics in general is specified by ASTM D256 [14] as the energy extracted from standardised pendulum-type hammers with one pendulum swing done either with milled notched (Izod and Charpy tests) or unnotched samples, for relatively brittle samples. The results are reported in terms of energy absorbed per unit specimen width. For tough plastic films, on the other hand, the free-falling dart method is recommended. There is one specific ASTM standard given for the impact resistance of LDPE measured by the free-falling dart method (ASTM D1709 [15] or ISO 7765-1 [16] and ISO 7765-2 [17]), which is reported in two different cases, for 260 g and 881 g (for 0.20 mm thick) film. LDPE has good toughness, which decreases with the density of the material. ASTM D1790 [17a] and D746 [18] are test methods for the routine determination of the specific ‘brittleness’ temperature at which plastics exhibit brittle failure under specified impact conditions. The first method is given for a thin (0.25 mm or less) plastic film, and the second is for real loading conditions. Thus ways to predict the behaviour of the material at low temperatures can be made, which is important for plastic films that are used in variable temperature conditions. The test applies for similar conditions of deformation, and the brittleness temperature is estimated statistically in the test as that at which 50% of the specimens would fail.

12.4.2.1 Impact Resistance by Free-Falling Dart Method The test method ASTM D1709-91 [15] covers the determination of the energy that causes a plastic film to fail under specified conditions of impact of a free-falling dart. This energy is expressed in terms of the weight (mass of the missile), falling from a specified height, that would result in 50% failure of the specimens tested. The impact resistance of a plastic film, while partly dependent on its thickness, has no simple correlation with sample thickness.

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Testing of Plastic Films The specimen for the test should be large enough to extend outside the specimen clamp gaskets at all points. The specimens will be representative of the film under study, and should be free from pinholes, wrinkles, folds or other obvious imperfections, unless such imperfections constitute variables under study.

12.4.2.2 Pendulum Impact Resistance Like other techniques to measure toughness, this test method (ASTM D256 [14]) provides a means to determine the parameters of a material at strain rates close to those applicable in some enduse applications, and the results are more valid than those provided by low-speed uniaxial tensile tests. The dynamic tensile behaviour of a film is important, particularly when the film is used as a packaging material. The same uncertainties about correlations with thickness that apply to other impact tests (such as ASTM D1709 [15]) also apply to this test. Several impact test methods are used for film samples. It is sometimes desirable to know the relationships among the test results derived by different methods. A study was conducted in which films made from two resins [polypropylene (PP) and linear low-density polyethylene (LLDPE)], with two film thicknesses for each resin, were impacted using ASTM test methods D1709 [15], D3420 [19] and D4272 [20]. Differences in results between test methods D1709 and D4272 may be expected, since test method D1709 represents failure-initiated energy while test method D4272 represents initiation plus completion energy.

12.4.2.3 Hail Resistance Although impact resistance is a valuable property to measure, the complexity and multiplicity of events occurring during impact make the value obtained applicable only under narrow conditions and not suitable for general design purposes. Thus, servicerelated impact tests have been devised for large-volume applications as greenhouse coverings. According to this method, a complete half of a greenhouse roof is built horizontally and is randomly shot with Nylon balls. The impact damage is registered with a camera. Single glass, 4 mm thick, is considered to be the reference material, and all other materials are compared to that.

12.4.3 Tear Resistance The tear resistance of a plastic film is a complex function of its ultimate resistance to rupture. There are different ASTM standards available for the tear resistance of films: ASTM D1004 [21] is designed to measure the force necessary to initiate tearing at very

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Handbook of Plastic Films low rates of loading, while ASTM D1938 [22]covers the force necessary to propagate a tear by a single tear. ASTM D1922 [23] is the determination of the average force to propagate tearing through a specified length of plastic film by use of an Elmendorf-type tearing tester. In ASTM D2582 [24], the puncture-propagation tear resistance of films is of interest. In these tests, two different values are of interest and are measured: (1) the force required to initiate the tear (ASTM D1004 and ISO 344 [25]); (2) the force needed to propagate the tear (ASTM D1938, D1922 and ISO 6383-1; [26]). ISO standards are specific to applications in greenhouses. The second value (the force needed to propagate the tear) can be considered to be of most interest, because, while it might occasionally be impossible to prevent a film from tearing in greenhouse applications, (e.g., when the film is not fastened securely, flaps in the wind, and hits a protruding part of the structure), it is highly beneficial if the tear propagates with great difficulty. Resistance to initiation of tear is also important and cannot be neglected in general. The tear resistance of plastic films is very important with regard to their overall mechanical behaviour and common failure mechanisms, i.e., for agricultural plastic films. The resistance to tear propagation for LDPE film is found to vary significantly. The reported value of resistance to tear propagation is 5-20 N [27]. Possible sources of this variation are the anisotropy, elongation effects and variable thickness of the tested films, as well as the use of different speeds during tearing.

12.4.3.1 Propagation Tear Resistance of Plastic Film and Thin Sheeting by Pendulum Method This test (ASTM D1922-94a; [23]) covers the determination of the average force to propagate tearing through a specified length of plastic film. It is widely used in packaging applications. While it may not always be possible to correlate film tearing data with other mechanical or roughness properties, the apparatus for this test method provides a controlled means for tearing specimens at straining rates approximating some of those found in actual packaging service. Owing to orientation during manufacture, plastic films and sheeting frequently show marked anisotropy in their resistance to tearing. This is further complicated by the fact that some films elongate greatly during tearing, even at the relatively rapid rates of loading encountered in this test method. The degree of this elongation is dependent in turn on film orientation and the inherent mechanical properties of the polymer from which it is made. There is no direct relationship between tearing force and specimen thickness. The tearing force is usually expressed in milli newtons (mN) or gram-force.

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Testing of Plastic Films A comparison of propagation tear resistance (Elmendorf tear) in machine direction and transverse direction of different types of plastic film is given in a table in ASTM D192294a. From the data given in this table,, it can be observed that LLDPE possesses the highest value of tear resistance in both machine and transverse directions. PP, which shows a low value of tear resistance in the machine direction, has a higher value in the transverse direction. The difference between the two directions reflects the degree of orientation and anisotropy of the material. PS orientation during processing is not remarkable, and consequently its tear resistance does not differ in the two directions.

12.4.3.2 Puncture-Propagation Tear Resistance This test method (ASTM D2582-93; [24]) covers the determination of the dynamic tear resistance of plastic film and film sheeting subjected to enduse snagging-type hazards. The puncture-propagation tear test measures the resistance of a material to snagging, or, more precisely, to dynamic puncture and propagation of that puncture resulting in a tear. Failure due to snagging hazard occurs in a variety of enduses, including industrial bags, liners and tarpaulins. The tear resistance measured by the instrument in this test is in newtons (N). Tear resistance can be measured using a standard drop height of 508 ± 2 mm or a nonstandard drop height (or carriage weight).

12.4.4 Bending Stiffness (Flexural Modulus) Test methods ASTM D747 [28] and D790 [29] cover the determination of the bending stiffness of plastic sheets and films. In the test, specimens are subjected to three- or fourpoint bending loads, such as a cantilever beam, and the force and angle of bending are used to determine the apparent flexural modulus (or bending stiffness) and yield strength.

12.4.5 Dynamic Mechanical Properties Tests by dynamic mechanical analysis (DMA) provide the elastic and loss moduli as well as the loss tangent (damping) as functions of temperature, frequency and/or time. These plots are indicative of the viscoelastic characteristics of the plastic. As the modes of molecular motion in the specimen change with temperature (or frequency), a corresponding transition temperature occurs. The most significant transition temperatures are the glass transition temperature (Tg) and the melting temperature (Tm). In addition,

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Handbook of Plastic Films there may be a number of sub-glass transition temperatures, which can be very important in determining the toughness of the material. In the temperature ranges where significant changes are observed in the modes of molecular motion, a number of mechanical properties, e.g., elastic modulus, decrease rapidly with increasing temperature (at constant or near-constant frequency) or increase with increasing frequency (at constant temperature). Hence DMA tests (provided by ASTM D4065; [30]) provide determination of transition temperatures, elastic modulus and loss modulus over a range of temperatures (from –160 °C to degradation), frequencies (0.01 to 1000 Hz) and times, by free vibration and resonant or nonresonant forced vibration techniques. DMA is usually applied for materials with elastic modulus from 0.5 MPa to 100 GPa [31]. DMA tests have been shown to be useful to evaluate a number of properties, for example, (1) degree of phase separation (in multicomponent systems), (2) effects of a certain processing treatment, and (3) filler type and amount, among others. DMA is very useful for quality control in general, for specification acceptance and in research, and it can also be used to determine, e.g., (1) stiffness and its change with temperature, (2) degree of crystallinity, (3) magnitude of triaxial stress state in rubber phase for rubber-modified plastics, etc. DMA tests incorporate laboratory practice for determining the dynamic mechanical properties of plastic films subjected to various oscillatory deformations on a variety of instruments (generally called dynamic mechanical analysers, thermomechanical analysers, mechanical spectrometers or even viscoelastometers).

12.5 Some Physical, Chemical and Physicochemical Tests 12.5.1 Density of Plastics The density of solid plastics is a conveniently measurable property, which is useful to follow the occurrence of physical changes, as well as to indicate uniformity among samples. ASTM D1505 [32] covers the method for density determination through observation of the level to which a test specimen sinks in a liquid column exhibiting a density gradient, in comparison with standards of known density.

12.5.2 Indices of Refraction and Yellowness The refractive index test is useful for controlling the purity and composition of films of transparent plastics for simple identification purposes, and it is done by use of a refractometer (ASTM D542; [33]), usually to four significant figures.

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Testing of Plastic Films For homogeneous, non-fluorescent, nearly colourless transparent and/or nearly white translucent-opaque plastic films, the yellowness index test is recommended to determine the degree of yellowness or degree of its change. Yellowness is defined as the deviation in chroma from whiteness in the dominant wavelength range from 570 to 580 nm relative to magnesium oxide for CIE Source C. In the test, data are collected using a Hardy GE type spectrophotometer or an equivalent system. A change in the yellowness index is taken as a measure of degradation (under exposure to heat, light or other environment) and has proved to be a very useful parameter for plastic films.

12.5.3 Transparency The clarity of a film is measured by its ability to transmit light in the visible region. The regular transmittance of film and sheet materials (defined as the ratio of undiffused transmitted flux to the incident flux) can be obtained by following ASTM D1746 [34].

12.5.4 Resistance to Chemicals Plastic films can be subjected to various chemicals and corrosive conditions, and their resistance to these should be tested. ASTM D543 [35] covers a general test method for all type of plastic materials. The test follows the changes in weight, dimensions, appearance and strength properties. As indicated in the test, the choice of type and concentration of reagent, duration of immersion and temperature are all arbitrary, and this poses the main limitation of the method.

12.5.5 Haze and Luminous Transmittance Light scattered from a film can produce a hazy or smoky field when viewed through the material. Haze is the cloudy or turbid appearance of an otherwise transparent material as a result of light scattered within or from the surface of the specimen. ASTM D1003 [36] provides a test method for the evaluation of specific light-transmitting and lightscattering properties of transparent plastic films. A hazemeter or a spectrometer [37] is used, which can provide very useful diagnostic data for the reason for the haze. In the test, the intensity of the incident light (I1), the total light transmitted by the specimen (I2), the light scattered by the instrument (I3) and the light scattered by the instrument and specimen (I4) are all measured. From these, the total transmittance (Tt) is calculated as Tt = I2/I1; and the diffuse transmittance (Td) is calculated from: Td =

I 4 − I 3( I2 / I1 ) I1

(12.1)

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Handbook of Plastic Films From these, the per cent haze is calculated as: haze =

Td × 100 Tt

(12.2)

Materials having a haze value greater than 30% are considered diffusing and should be tested.

12.5.6 Ignition, Rate of Burning Characteristics and Oxygen Index (OI) Most plastic films are flammable. There are three different ASTM methods available to test the ignition and rate of burning characteristics and to evaluate the per cent oxygen necessary to initiate burning, namely the oxygen index. ASTM D635 [38] covers a smallscale laboratory screening procedure to compare the relative rates of burning of selfsupporting plastic films tested in the horizontal position using a burner. ASTM D1929 [39] is for determination of the self-ignition temperature (the lowest initial temperature of air passing around the specimen at which, in the absence of an ignition source, the selfheating of the specimen leads to ignition) and flash ignition temperature (the lowest initial temperature of air passing around the specimen at which a sufficient amount of combustible gas is evolved to be ignited by a small external flame) of plastics by using a hot-air ignition furnace. The oxygen index test (ASTM D2863; [40]) covers tests that find the minimum oxygen concentration to support candle-like combustion of plastic film.

12.5.7 Static and Kinetic Coefficients of Friction The frictional properties of film surfaces may contribute markedly to film behaviour in packaging machinery and to the stacking properties of sacks. Slip agents are frequently added to film to improve its frictional behaviour. However, films containing additives often take considerable time to develop their full properties while the additives diffuse to the surface, and care must be taken in choosing the time after manufacture to carry out the test. The ASTM D1894-95 test method [41] covers determination of the coefficients of starting and sliding friction of plastic film and sheeting, when relative sliding occurs between the film and other substances under specified test conditions. The procedure permits the use of a stationary sled with a moving plane film, or the use of a moving sled with a stationary plane film. The static or starting coefficient of friction (μs) is related to the force measured to begin movement of the surfaces relative to each other. The kinetic or sliding coefficient of friction (μk) is related to the force required to sustain this movement.

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Testing of Plastic Films Measurements of frictional properties may also be made on a film or sheet specimen when sliding over itself or over another substance. The coefficient of friction is related to the slip properties of plastic films, which are of wide interest in packaging applications. These methods yield empirical data for control purposes in film production. For instance, slip properties are generated by additives in some plastic films, for example, polyethylene. These additives have varying degrees of compatibility with the film matrix. Some of them bloom, or extrude to the surface, lubricating it and making it more slippery. Because this blooming action may not always be uniform in all areas of the film surface, values from these tests may be limited in reproducibility. Besides, this blooming action of many slip additives is time-dependent. For this reason, it is sometimes meaningless to compare the slip and friction properties of films or sheets produced at different times, unless the method is designed to study this effect. Plastic films (not greater than 0.245 mm thick) and sheeting (greater than 0.245 mm thick) may exhibit different frictional properties in their respective principal directions due to anisotropy or extrusion effects. Specimens may be tested with their long dimensions in either the machine direction or transverse direction of the sample, but it is more common to test the specimen with its long direction parallel to the machine direction. The test surface must be kept free of dust, lint, fingerprints, or any foreign matter that might change the surface characteristics of the specimen. The static and kinetic coefficients of friction (μs and μk, respectively), are calculated from:

μs = As/B

(12.3)

μk = Ak/B

(12.4)

where As is the initial scale reading (g) at which motion just begins, Ak is the average scale reading (g) obtained during uniform sliding of the film surface and B = sled weight (g).

12.5.8 Specular Gloss of Plastic Films and Solid Plastics This test (ASTM D2457-90; [42]) covers the measurement of gloss of plastic films, both opaque and transparent. Specular gloss is defined as the relative luminous reflectance factor of a specimen in the mirror direction. Specular gloss is used primarily as a measure of the shiny appearance of film and surfaces. Precise comparisons of gloss values are meaningful only when they refer to the same measurement procedure and the same general type of material. In particular, gloss values for transparent films should not be compared with those of opaque films, and vice versa. Gloss is a complex attribute of a surface, which cannot be completely measured by any single number. Specular gloss usually varies with surface smoothness and flatness. The instrument

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Handbook of Plastic Films used consists of an incandescent light source to produce the incident beam, a means to locate the surface of the specimen, and a receptor located to receive the required pyramid of rays reflected by the specimen. The receptor is a photosensitive device that is responsive to visible radiation. The receptor measurement mechanism should give a numerical indication that is proportional to the light flux passing the receptor field stops within ±1% of full-scale readings. Specimen surfaces should have good planarity, since surface warpage, waviness or curvature may seriously affect test results. The direction of machine marks, or similar texture effects, should be parallel to the plane of the axes of the two beams. Surface test areas must be kept free of soiling and abrasion. Gloss is due chiefly to reflection at the surface; therefore, anything that changes the surface physically or chemically is likely to affect gloss.

12.5.9 Wetting Tension of PE and PP Films In this test method (ASTM D2578-94 [43]) drops of a series of mixtures of formamide and cellosolve (ethyleneglycol monoethyl ether) of gradually increasing surface tension are applied to the surface of the polyethylene or polypropylene film until a mixture is found that just wets the film surface. The wetting tension of the PE or PP film surface will be approximated by the surface tension of this particular mixture. The ability of PE and PP films to retain inks, coating, adhesives, etc., is primarily dependent upon the character of their surfaces, and can be improved by one of several surface-treatment techniques mentioned in Chapter 8. The same treatment techniques have been found to increase the wetting tension of PE or PP film surfaces in contact with a mixture of formamide and ethyl cellosolve in the presence of air. It is therefore possible to relate the wetting tension of a PE or PP film surface to its ability to accept and retain inks, coating, adhesives, etc. The measured wetting tension of a specific film surface can only be related to acceptable ink, coating, or adhesive retention through experience. Wetting tension in itself is not a completely reliable measure of ink or coating retention, or adhesion. A wetting tension of 3.5 × 10-2 N/m or higher has generally been found to reveal a degree of treatment normally regarded as acceptable for tubular film made from PE and intended for commercial flexographic printing. A table showing the measured wetting tension of PE and PP film as a function of the concentration of a mixture of ethyl cellosolve and formamide is given in ASTM D257894 [43]. Note that a solution is considered to wet a test specimen when it remains intact as a continuous film of liquid for at least 2 seconds. The reading of the liquid film behaviour should be made in the centre of the liquid film. Shrinking of the liquid film about its periphery does not indicate lack of wetting. Breaking of the liquid film into droplets

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Testing of Plastic Films within 2 seconds does indicate lack of wetting. Too much liquid being placed upon the film surface may cause severe peripheral shrinkage.

12.5.10 Unrestrained Linear Thermal Shrinkage of Plastic Films Unrestrained linear thermal shrinkage, expressed as the percentage of the original dimension, is defined as ‘the irreversible reduction in linear dimension at elevated temperatures where no restraint to inhibit shrinkage is present’. During the manufacturing processes, internal stresses that occur might be locked into the film, which can be released afterwards by proper heating. The temperature at which shrinkage occurs is mainly related to the processing techniques employed and may also be related to the phase transition in the base resin. The magnitude of the shrinkage varies with the temperature. Shrinkage of a particular material produced by a process may be characterised by the ASTM D2732 test method [44], by making measurements at several temperatures through the shrinkage range of the material. The experiment is usually carried out in a constant-temperature liquid bath accurate to ±0.5 °C. It is a prerequisite that the liquid of the bath should not plasticise or react with the specimen. Polyethyleneglycol, glycerine and water have been found to have wide applicability for this purpose. Immersion of the sample (100 × 100 mm2) for 10 s has been determined to be generally adequate for most thermoplastics of up to 50 μm thickness. Unrestrained linear shrinkage is calculated using: unrestrained linear shrinkage (%) =

L0 − Lf × 100 L0

(12.5)

where L0 is the initial length of side (100 mm) and Lf is the length of side after shrinkage.

12.5.11 Shrink Tension and Orientation Release Stress The ASTM D2838 test [45] measures the maximum force of a totally restrained specimen and the maximum force of a specimen permitted to shrink a predetermined amount prior to restraint in a liquid bath at selected temperatures. The results obtained are especially important and useful for shrink-wrap films and shrink-wrap packaging design.

12.5.12 Rigidity Rigidity affects the machinability of plastics. It depends mainly on the stiffness of the material, on its thickness, as well as on a number of other factors such as static electricity, frictional

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Handbook of Plastic Films properties, etc. The standard test method ASTM D2923 [46] is specific for the rigidity of polyolefin films and sheeting. In the test, the resistance of the sample to flexure is measured (by a strain gauge affixed to the end of the sample) and by use of a microammeter connected to the gauge and calibrated; rigidity is read directly as grams per centimetre of sample width.

12.5.13 Blocking Load by Parallel-Plate Method Blocking (unwanted adhesion) is a problem with plastic films, which develops during processing and/or storage, and happens when touching layers of films are in intimate contact with almost complete exclusion of air between them. Blocking is induced by increase of temperature and/or pressure. The standard test method provided by ASTM D3354 [47] simulates the operation of separating blocked films in some enduse applications. The load (in grams) needed to separate blocked samples [five groups of specimens each cut to 100 × 180 mm2] is measured by a beam-balance system (similar to an analytical balance). The test, in summary, is as follows: One sheet of the blocked specimen is secured to an aluminium block suspended from the end of the balance beam, while the other end is fixed to another aluminium block fastened to the balance base. Weight is then added equivalent to 90 ± 10 g/m to the other side of the beam until the films totally separate (or until they reach 1.905 cm separation). The film-to-film adhesion is expressed as grams, and the test is limited to maximum 200 g of load.

12.5.14 Determination of LLDPE Composition by 13C NMR The performance properties of ethylene copolymer plastic films depend on the number and type of short-chain branches. The ASTM D5017 method [48] allows one to measure them for ethylene copolymers with propylene, 1-butene, 1-octene and 4-methyl-1-pentene. For this, the polymer sample (about 1.2 g) is dispersed in a solvent (1.5 ml) and a deuterated solvent (1.3 ml), put into a 10 mm nuclear magnetic resonance (NMR) tube, and analysed at high temperatures by using 13C NMR spectroscopy, usually a 13C pulsed Fourier transform with a field strength of at least 2.35 T. Spectra are recorded under conditions such that the responses of each chemically different carbon are identical. The integrated responses for carbons originating from different comonomers are used for calculation of the copolymer composition. Results are presented as mole per cent alkene and/or branches per 1000 carbon atoms.

12.5.15 Creep and Creep Rupture Creep is defined as the increasing strain over time in the presence of a constant stress, and is expressed as the per cent extension (creep strain per cent). The practical importance

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Testing of Plastic Films of creep is due to the need (a) to determine the limits of excessive deformation and (b) to understand creep rupture. The two mechanisms are especially important for LDPE greenhouse covering materials, because LDPE, with 8.23% creep, has the second highest creep value of all greenhouse covering films [11]. There is a significant variation in the creep values of LDPE, which is attributed to the fact that the creep resistance of LDPE increases with density and with the content of ethylene-vinyl acetate (EVA) in the material’s composition. Creep is also strongly dependent on the service temperature of the covering material. ASTM D2990 [49] is a general test method for plastics to characterise creep and creep rupture. The test is applicable to different loading conditions, (e.g., tensile, flexural, compressive, etc.), and helps to determine the creep strength and modulus of standard specimens for use in comparing materials and in design.

12.5.16 Outdoor Weathering/Weatherability The ASTM D1435 test [50] is used to evaluate the stability of plastic films when exposed outdoors to the varied influences of the atmosphere and weather. The general climate, the season, the time of day, the presence or absence of industrial pollutants in the atmosphere, and annual variations in the weather are the most important factors, and the results are taken as indicative only. Short-term accelerated exposure tests are also available by use of a special chamber equipped with a carbon-arc light (ASTM G152 [51] and ASTM G153 [52]), which can indicate the relative outdoor performance, but cannot be used to predict the absolute long-term performance.

12.5.17 Abrasion Resistance Abrasion is a surface phenomenon that occurs mechanically, and it is important in the sense that it can significantly degrade certain physical properties (light transmission, thermal effect through loss of thickness, etc.), as well as some mechanical properties, (e.g., impact resistance, tear resistance). As a result it has a direct impact on the functional characteristics of covering materials. Abrasive damage to transparent plastic films is judged by following the change of the optical properties (ASTM D1044; [53]) as well as by volume loss in general (by using abrasion testing machines, ASTM D1242; [54]). The abrasion resistance of plastic films used in greenhouses is of utmost importance. Abrasion, in this case, occurs due to the effect of particles carried by the wind, which can be significant in some areas where greenhouses are built. In this case, abrasion can lead to the loss of transparency and reduction in mechanical properties much earlier than expected. Abrasion in general is affected by the exact formulation of the film, and by the incorporation of (amount and type of) filler, additives and pigments, which can lead to varying results. Another important factor is that rapid chemical oxidation of the surface layer may occur due to the buildup of localised high temperatures during abrasion [55].

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Handbook of Plastic Films It is worth mentioning that abrasion can ultimately lead to increased degradation of the film, since more active centres for photooxidation are in general created by this procedure. There is a direct relationship between the density of a PE film and its abrasion resistance: increase of density increases the abrasion resistance.

12.5.18 Mar Resistance Test ASTM D673 [56] covers the determination of the extent of resistance of plastic film surfaces to surface marring, mainly caused by falling abrasive particles. The test simulates the relatively mild airborne abrasive action that occurs in actual use, and different materials are ranked according to their relative mar resistances.

12.5.19 Environmental Stress Cracking The ability of a polymer surface to withstand an aggressive medium under load is known as environmental stress-cracking resistance (ESCR). Environmental stress cracking is a characteristic that depends on the nature and level of stresses applied as well as on the thermal history of the sample and the environment, and is also called stress corrosion [57]. Under certain conditions of stress and in the presence of certain environments, environmental stress cracking occurs. For example, in the presence of soaps, wetting agents and detergents, ethylene plastics may exhibit mechanical failure by cracking. Typically, increased ESCR is obtained with increased polymer molecular weight. ASTM D1693 [58] is specific for the environmental stress cracking of ethylene plastics. A stress crack is an external or internal rupture in the film caused by tensile stresses less than its short-time mechanical strength. The environment accelerates the development of stress cracks. The appearance of what seem to be cracks on the surfaces of transparent polymers develops under tensile stress, with the plane of the craze being normal to the stress direction. Crazes usually initiate at surfaces but can develop internally under special circumstances as well. They reflect light in a manner similar to cracks, and indeed often precede early fracture of the film. In the test, bent specimens, each having a controlled imperfection on one surface, are exposed to the action of a surface-active agent, and the proportion of the total number of specimens that crack in a given time is reported.

12.5.20 Water Vapour Permeability In the packaging of hygroscopic materials, and particularly in packaging of food, the permeability properties of the film to water vapour and other gases is very important.

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Testing of Plastic Films ASTM D3079 [59] and E96 [60]/F372 [61]/F1249 [62] cover standard test methods for plastic packaging films and plastics for water vapour permeability. These tests are especially important and are used for packaging plastic films. In the first of these, desiccant- or product-filled packages are exposed to a normal atmosphere of 90 ± 2% RH at constant temperature, and weighings are repeated to constant rate of moisture gain. Water vapour permeabilities are reported in grams per 30 days. In the second test method, desiccant- or product-filled packages are again exposed to a normal atmosphere of 90 ± 2% RH at two different temperatures for 24 hours and 6 days, respectively. Hence cycling between cold and hot/moist atmospheres is achieved. In this test, weighings are repeated to constant rate of moisture gain, and water vapour permeability is reported in grams per cycle. In the third method mentioned, desiccant- or product-filled packages are exposed to a normal atmosphere of 90 ± 2% RH at constant temperature for at least 1 month; average rate of water gain is reported. In the last two methods, infrared (F372) and modulated infrared (F1249) detection of water vapour transmitted from a moist atmosphere to a dry air stream is made, which provides a measure of water vapour transmission rates.

12.5.21 Oxygen Gas Transmission ASTM D1434 [63] and D3985 [64] cover standard test methods for packaging plastic films and sheeting materials for their oxygen gas transmission. Basically the methods used can be divided into three types, varying either pressure, volume or concentration. In variablevolume methods, gas is introduced at a high pressure on one side of the film, the chamber on the other side normally being at atmospheric pressure. The change in volume is followed as a function of time. A manometer is used to measure the pressure of oxygen transmitted, from which the rate of transmission at steady state can be calculated. In another method, a coulometric sensor is used, which measures the rate of oxygen transmitted through a specimen exposed on one surface to oxygen and on the other to nitrogen. Considerable experimental difficulties are normally encountered in achieving airtight seals and in the initial calibration of the instrument to allow for the deadspace in the filter paper and discs used to support the film

12.6 Standard Specifications for Some Plastic Films There are several standards available to specify plastic films, such as: •

ASTM D5047 [65] for polyethylene terephthalate (PET) films;

•

ASTM D4635 [66] for LDPE films;

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Handbook of Plastic Films •

ASTM D3981 [67] for medium-density polyethylene (MDPE) films;

•

ASTM D2103 [68] for PE films;

•

ASTM D2673 [69] for oriented polypropylene (OPP) films;

•

ASTM D2647 [70] for crosslinkable ethylene plastics.

12.6.1 Standard Specification for PET Films Specification ASTM D5047 [65] covers biaxially oriented PET films in the range of 1.5-35.5 μm, containing at least 90% PET homopolymer. The thicknesses should be within ±18% to ±14% of nominal for film tested in accordance with ASTM D374 [71]; while the requirements for the width (within ±1.6 mm and ±3.2 mm of nominal for rolls up to 1 m or larger, respectively), and weight (within ±10% and ±5% for orders up to 110 kg or over, respectively), are also given. The film will be tested appropriately to establish conformance to the critical requirements as agreed by the purchaser and seller.

12.6.2 Standard Specification for LDPE Films (for General Use and Packaging Applications) Specification ASTM D4635 [66] covers unpigmented, unsupported, tubular LDPE films with densities between 910 and 925 kg/m3 (0.910-0.925 gm/cm3), for general use and for packaging applications. It is also applicable to polyethylene copolymer (low-pressure PE and LLDPE) as well as for blends of homopolymers and copolymers, including ethylenevinyl acetate copolymers. The thicknesses are 100 μm or less and the maximum widths are 3 m. The specification covers dimensional tolerances (including thickness, width, length and yield), intrinsic quality requirements (density, workmanship, tensile strength, heat sealability, odour, impact strength, coefficient of friction, optical properties, surface treatment, etc.), and test methods.

12.6.3 Standard Specification for MDPE and General Grade PE Films (for General Use and Packaging Applications) Specification ASTM D3981 [67] is for unpigmented, unsupported, sheet or tubular MDPE films with densities between 926 and 938 kg/m3 (0.926-0.938 g/cm3), for general use and for packaging applications. It is also applicable to polyethylene copolymer

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Testing of Plastic Films (low-pressure PE and LLDPE) as well as for blends of homopolymers and copolymers, including ethylene-vinyl acetate copolymers. The thicknesses are 25-100 μm and the maximum widths are 3.05 m. The standard excludes heat-shrinkable films. The specification covers dimensional tolerances (including thickness, width, length and yield), intrinsic quality requirements (density, workmanship, tensile strength, heat sealability, odour, impact strength, coefficient of friction, optical properties, surface treatment, etc.) and test methods. Specification ASTM D2103 [68] covers general specifications for polyethylene films.

12.6.4 Standard Specification for OPP Films Specification ASTM D2673 [69] covers OPP films of 10-50 μm thickness with ±10% of the nominal value, composed of Group 1 or 2 propylene (ASTM D4101 [72]), or a blend of such Group 1 and/or Group 2 polypropylene with one or more other types of polymers where the polypropylene fraction is the main component. It must have normal appearance (be free of gel, streaks, pinholes, particulates, etc., as well as undispersed raw materials) and it should not block excessively. The average width will be within –3 to +19 mm of nominal. If the film yields a minimum tensile strength of 103 MPa in at least one principal (machine or transverse) direction, it is termed oriented polypropylene (OPP). If the film is oriented in one (machine or transverse) direction and yields a minimum tensile strength of 103 MPa in the orientation direction, it is called as uniaxially oriented PP film. If the film tensile strengths in both the machine and transverse directions exceed 103 MPa, it is biaxially oriented PP. If the film tensile strengths in both the machine and transverse directions exceed 103 MPa, but do not differ by more than 55 MPa, and the machine and transverse elongations do not differ by more than 60%, it is balanced oriented PP.

12.6.5 Standard Specification for Crosslinkable Ethylene Plastics Specification ASTM D2647 [70] covers crosslinkable ethylene plastics and compounds. There are mainly two different types: mechanical types (type I) and electrical types (type II). In the former, mechanical properties (strength, ultimate elongation, elongation retention after ageing, apparent modulus of rigidity, brittleness temperature) are the most important in applications.

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References 1.

D833, Terminology Relating to Plastics, 2000.

2.

D. Briassoulis and A. Aristopoulou, Polymer Testing, 2001, 20, 615.

3.

ASTM D618-00, Standard Practice for Conditioning Plastics for Testing, 2000.

4.

ASTM D638-02, Standard Test Method for Tensile Properties of Plastics, 2002.

5.

ASTM D882-02, Standard Test Method for Tensile Properties of Thin Plastic Sheeting, 2002.

6.

ASTM D4321-99, Standard Test Method for Package Yield of Plastic Film, 1999.

7.

ISO 527-1, Plastics - Determination of Tensile Properties - General Principles, 1994.

8.

ISO 527-2, Plastics - Determination of Tensile Properties - Test Conditions for Moulding and Extrusion Plastics, 1994.

9.

ISO 527-3, Plastics - Determination of Tensile Properties - Part 3: Test Conditions for Films and Sheets, 2001.

10. D. Briassoulis, D. Waayenberg, J. Gratraud and B.J. Von Elsner, Journal of Agricultural Engineering Research, 1997, 67, 81. 11. P.C. Powell, Engineering Design Guides, 1979, 19, 1. 12. G. Gruenwald, Plastics: How Structure Determines Properties, Hanser, Munich, Germany, 1992. 13. R.M. Patel, Polymer Engineering Science, 1994, 34, 1506. 14. ASTM D256-00e1, Standard Test Methods for Determining the Izod Pendulum Impact Resistance of Plastics, 2000. 15. ASTM D1709-01, Standard Test Methods for Impact Resistance of Plastic Film by the Free-Falling Dart Method, 2001. 16. ISO 7765-1, Plastics Film and Sheeting - Determination of Impact Resistance by the Free-Falling Dart Method - Part 1: Staircase Methods, 1999. 17. ISO 7765-2, Plastics Film and Sheeting — Determination of Impact Resistance by the Free-Falling Dart Method — Part 2: Instrumented Puncture Test, 1999.

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Testing of Plastic Films 17a. ASTM D1790-02, Standard Test Method for Brittleness Temperature of Plastic Sheeting by Impact, 2002. 18. ASTM D746-98e1, Standard Test Method for Brittleness Temperature of Plastics and Elastomers by Impact, 1998. 19. ASTM D3420-95, Standard Test Method for Pendulum Impact Resistance of Plastic Film, 1995. 20. ASTM D4272-99, Standard Test Method for Total Energy Impact of Plastic Films By Dart Drop, 1999. 21. ASTM D1004-94a, Standard Test Method for Initial Tear Resistance of Plastic Film and Sheeting, 1994. 22. ASTM D1938-02, Standard Test Method for Tear-Propagation Resistance (Trouser Tear) of Plastic Film and Thin Sheeting by a Single-Tear Method, 2002. 23. ASTM D1922-00a, Standard Test Method for Propagation Tear Resistance of Plastic Film and Thin Sheeting by Pendulum Method, 2000. 24. ASTM D2582-00, Standard Test Method for Puncture-Propagation Tear Resistance of Plastic Film and Thin Sheeting, 2000. 25. ISO 344, Textile Machinery and Accessories - Spinning Machines - Flyer Bobbins, 1981. 26. ISO 6383-1, Plastics - Film and Sheeting - Determination of Tear Resistance Trouser Tear Method, 1983. 27. F. Henninger in Handbook of Polymer Degradation, Eds., S.H. Hamid, M.B. Amin and A.G. Maadhah, Marcel Dekker, New York, NY, USA, 1992, 411. 28. ASTM D747-02, Standard Test Method for Apparent Bending Modulus of Plastics by Means of a Cantilever Beam, 2002. 29. ASTM D790-02, Standard Test Methods for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials, 2002. 30. ASTM D4065-01, Standard Practice for Plastics: Dynamic Mechanical Properties: Determination and Report of Procedures, 2001. 31. J.D. Ferry, Viscoelastic Properties of Polymers, 2nd Edition, Wiley, New York, NY, USA, 1961.

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Handbook of Plastic Films 32. ASTM D1505-98e1, Standard Test Method for Density of Plastics by the Density-Gradient Technique, 1998 33. ASTM D542-00, Standard Test Method for Index of Refraction of Transparent Organic Plastics, 2000. 34. ASTM D1746-97, Standard Test Method for Transparency of Plastic Sheeting, 1997. 35. ASTM D543-95 (2001), Standard Practices for Evaluating the Resistance of Plastics to Chemical Reagents, 2001. 36. ASTM D1003-00, Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics, 2000. 37. F.W. Billmeyer, Jr. and Y. Chen, Color Research and Application, 1985, 10, 219. 38. ASTM D635-98, Standard Test Method for Rate of Burning and/or Extent and Time of Burning of Plastics in a Horizontal Position, 1998. 39. ASTM D1929-96(2001)e1, Standard Test Method for Determining Ignition Temperature of Plastics, 2001. 40. ASTM D2863-00, Standard Test Method for Measuring the Minimum Oxygen Concentration to Support Candle-Like Combustion of Plastics (Oxygen Index), 2000. 41. ASTM D1894-01, Standard Test Method for Static and Kinetic Coefficients of Friction of Plastic Film and Sheeting, 2001. 42. ASTM D2457-97, Standard Test Method for Specular Gloss of Plastic Films and Solid Plastics, 1997. 43. ASTM D2578-99a, Standard Test Method for Wetting Tension of Polyethylene and Polypropylene Films, 1999. 44. ASTM D2732-01, Standard Specification for Polyethylene (PE) Plastic Tubing, 2001. 45. ASTM D2838-02, Standard Test Method for Shrink Tension and Orientation Release Stress of Plastic Film and Thin Sheeting, 2002. 46. ASTM D2923-01, Standard Test Method for Rigidity of Polyolefin Film and Sheeting, 2001. 47. ASTM D3354-96, Standard Test Method for Blocking Load of Plastic Film by the Parallel Plate Method, 1996.

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Testing of Plastic Films 48. ASTM D5017-96, Standard Test Method for Determination of Linear Low Density Polyethylene (LLDPE) Composition by Carbon-13 Nuclear Magnetic Resonance, 1996. 49. ASTM D2990-01, Standard Test Methods for Tensile, Compressive, and Flexural Creep and Creep-Rupture of Plastics, 2001. 50. ASTM D1435-99, Standard Practice for Outdoor Weathering of Plastics, 1999. 51. ASTM G152-00ae1, Standard Practice for Operating Open Flame Carbon Arc Light Apparatus for Exposure of Nonmetallic Materials, 2000. 52. ASTM G153-00ae1, Standard Practice for Operating Enclosed Carbon Arc Light Apparatus for Exposure of Nonmetallic Materials, 2000. 53. ASTM D1044-99, Standard Test Method for Resistance of Transparent Plastics to Surface Abrasion, 1999. 54. ASTM D1242-95a, Standard Test Methods for Resistance of Plastic Materials to Abrasion, 1995. 55. V. Shah, Handbook of Plastic Testing Technology, Wiley, New York, NY, USA, 1984. 56. ASTM D673 discontinued not replaced 57. R.P. Kambour and A.S. Holik, Journal of Polymer Science, A-2: Polymer Physics, 1969, 7, 1393. 58. ASTM D1693-01, Standard Test Method for Environmental Stress-Cracking of Ethylene Plastics, 2001. 59. ASTM D3079-94 (1999), Standard Test Method for Water Vapor Transmission of Flexible Heat-Sealed Packages for Dry Products, 1999. 60. ASTM E96-00e1, Standard Test Methods for Water Vapor Transmission of Materials, 2000. 61. ASTM F372-99, Standard Test Method for Water Vapor Transmission Rate of Flexible Barrier Materials Using an Infrared Detection Technique, 1999. 62. ASTM F-1249-01, Standard Test Method for Water Vapor Transmission Rate Through Plastic Film and Sheeting Using a Modulated Infrared Sensor, 2001. 63. ASTM D1434-82(1998), Standard Test Method for Determining Gas Permeability Characteristics of Plastic Film and Sheeting, 1998.

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Handbook of Plastic Films 64. ASTM D3985-02, Standard Test Method for Oxygen Gas Transmission Rate Through Plastic Film and Sheeting Using a Coulometric Sensor, 2002. 65. ASTM D5047-95, Standard Specification for Polyethylene Terephthalate Film and Sheeting, 1995. 66. ASTM D4635-01, Standard Specification for Polyethylene Films Made from Low-Density Polyethylene for General Use and Packaging Applications, 2001. 67. ASTM D3981-95, Standard Specification for Polyethylene Films Made from Medium-Density Polyethylene for General Use and Packaging Applications, 1995. 68. ASTM D2103-97 Standard Specification for Polyethylene Film and Sheeting, 1997. 69. ASTM D2673-99, Standard Specification for Oriented Polypropylene Film, 1999. 70. ASTM D2647-94 (2000) e1, Standard Specification for Crosslinkable Ethylene Plastics, 2000. 71. ASTM D374, Standard Test Methods for Thickness of Solid Electrical Insulation, 1999. 72. ASTM D4101, Standard Specification for Polypropylene Injection and Extrusion Materials, 2002.

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13

Recycling of Plastic Waste E.M. Abdel-Bary

13.1 Introduction Polymeric materials (plastics and rubbers) comprise a steadily increasing proportion of the municipal and industrial waste going into landfill. Owing to the huge amount of plastic wastes and environmental pressures, recycling of plastics has become a predominant subject in today’s plastics industry. Development of technologies for reducing polymeric waste, which are acceptable from the environmental standpoint and are cost-effective, has proven to be a difficult challenge, because of the complexities inherent in the reuse of polymers. Establishing optimal processes for the reuse/recycling of polymeric materials thus remains a worldwide challenge in the new century. Compared with other countries, there is a huge amount of plastic waste in the USA (taken as a reference), where five main types of polymers dominate in the plastics waste stream. The highest polymer waste results from low-density polyethylene (LDPE), at 5 million tons per year; high-density polyethylene (HDPE) is second, at 4.1 million tons; then come polypropylene (PP) at 2.6 million tons, followed by polystyrene (PS) at 2 million tons and polyethylene terephthalate (PET) at 1.7 million tons [1]. These five polymer types, together with polyvinyl chloride (PVC), also dominate the plastics waste stream in the European Community [2]. Plastic films find applications in agriculture as well as in plastic packaging, which is a high-volume market owing to the many advantages of plastics over other traditional materials. However, such material is also the most visible in the waste stream, and has received a great deal of public criticism as films have comparatively short life-cycles and usually are non-degradable [3]. The majority of plastic films are made from LDPE or linear low-density polyethylene (LLDPE), comprising approximately 68% of the total film production. In addition, HDPE resins are commonly used in film plastics. Non-polyethylene resins constitute the remainder of the film plastic types found in the market place. PP, PVC and Nylon resins comprise the bulk of these other film types. Increasingly, certain multilayer or coextruded films are used in special applications that seek to combine the performance attributes of two or more resins for such applications.

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Handbook of Plastic Films Businesses can save money by reducing their disposal expenses, in the form of both tonnagebased tipping fees and container hauling fees. This is especially evident with plastic films, where a high volume-to-weight ratio can mean more container pulls per ton hauled.

13.2 Main Approaches to Plastic Recycling There are four main approaches to recycling plastics (excluding, as not acceptable, dumping on land or at sea with or without prior treatment) [4]. These are primary, secondary, tertiary and quaternary recycling.

13.2.1 Primary Recycling This is the recycling of clean, uncontaminated, single-type waste, and it remains the most popular as it ensures simplicity and low cost, especially when done ‘in-plant’ and fed with scrap of controlled history [5]. The recycled scrap or waste is either mixed with virgin material to assure product quality or used as second-grade material [6]. Primary recycling is very simple without any precautions except the proper and clean collection of the waste in the plant.

13.2.2 Secondary Recycling 13.2.2.1 Approaches to Secondary Recycling There are two main approaches to secondary recycling. One approach is to separate the plastics from their contaminants and then segregate the plastics into generic types, one or more of which is then recycled into products produced from virgin or primary recycled material. The other approach is to separate the plastics from their associated contaminants and remelt them as a mixture without segregation. The treatment of the plastics-containing waste streams may include: size reduction by granulators, shredders or crumblers; separation of plastics from other waste materials and from one another; cleaning; drying; and compounding [7, 8]. The actual order and number of operations in a particular treatment system depends on the waste being processed and the desired quality of the final material [9].

13.2.2.2 Mechanical Recycling Mechanical recycling is mainly related to secondary recycling. The main steps include separating, sorting and washing to get rid of contamination, especially for plastic films,

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Recycling of Plastic Waste which possess a large surface area and consequently have a large degree of contamination. In a chemical recycling plant, one should have shredders, metal and mineral separators, prewashing and granulation, second washing stage, mechanical grinding and dirt removal, hydrocyclone separation, dewatering and melt processing. The separation of plastic waste is one of the main factors restricting high performance in plastic recycling. The separation of plastics into desired categories as well as the elimination of contaminants is an ongoing technological development process. The aim is to develop automatic and continuous separation technology to minimise the handling of waste and to achieve a more efficient recycling process. Probably the best alternative for pure plastic streams is not to allow them to mix in the first place, neither among themselves nor with contaminants. If separation starts at the consumer level and at the source point of collection, there will be fewer difficulties during the recycling.

13.2.3 Tertiary Recycling Tertiary recycling includes chemical recycling. The terms ‘chemical recycling’ and ‘feedstock recycling’ of plastics are sometimes collectively referred to as ‘advanced recycling technologies’. In these processes, solid plastic materials are converted into smaller molecules as chemical intermediates through the use of heat. These chemical intermediates, usually liquids or gases, but sometimes solids or waxes, are suitable for use as feedstocks for the production of new petrochemicals and plastics. The technical and economic feasibility and overall commercial viability of advanced recycling methods must be considered in each step of the recycling chain, consisting of collection, processing and marketing. All of them are critical to the success of chemical and feedstock recycling. Today, most of these technologies remain developmental and have not yet proven themselves sustainable in a competitive market. Nevertheless, they remain of considerable interest in their longer-term potential. The term ‘feedstock recycling’ encompasses chemical recycling but is often applied to the thermal depolymerisation of polyolefins and substituted polyolefins into a variety of smaller hydrocarbon intermediates. Fluidised bed pyrolysis investigations of LDPE have provided data on the suitability of the process and on the influence of the process conditions on the compatibility of the feedstock produced with the conventional petroleum feedstock [10]. The gases produced from the pyrolysis of LDPE are mainly hydrogen, methane, ethane, ethylene, propane, propene, butane and butene. Also, it has been reported that the thermolysis products of HDPE consist of 80-90 wt% straight-chain alkanes and 1-alkenes. Subsequent hydrogenation of the PE oil resulted in a diesel fuel with high cetane index and low sulfur and aromatic contents [11].

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Handbook of Plastic Films In some cases, addition polymers such as polystyrene and polymethyl methacrylate can be thermally depolymerised back to their corresponding monomers in reasonably high yield. The term ‘chemical recycling’ is often applied also to the depolymerisation of certain condensation polymers back to monomers. Examples of these types of plastics are polyesters, polyamides and polyurethanes. Chemical recycling thus mainly includes pyrolysis, gasification, hydrogenation, hydrolysis, glycolysis and depolymerisation. A new reactor system was developed for the recovery of fuels from waste plastic mixtures in a steam atmosphere. The degradation mechanisms of two polyolefins (PE and PP), two polyamides (Nylon-6 and Nylon-6,6), polystyrene and three polyesters (polycarbonate, polybutylene terephthalate and polyethylene terephthalate) in both nitrogen and steam as the carrier gas have been investigated [12]. The oil produced from the proposed reactor system was continuously upgraded to produce gasoline and kerosene over a Raney nickel catalyst in a steam atmosphere.

13.2.4 Quaternary Recycling Quaternary recycling includes the recovery of the energy content of plastic wastes. Owing to a lack of other recycling possibilities, incineration (combustion) aimed at the recovery of energy is currently the most effective way to reduce the volume of organic material. This may then be disposed of to landfill. Plastics, either thermoplastics or thermosets, are actually high-yielding energy sources. For example, one litre of heating oil has a net calorific value of 10,200 kcal, whereas 1 kg of plastics releases 11,000 kcal worth of energy; for comparison, it should be added that 1 kg of briquettes (blocks of pressed coal dust) has a net calorific value of 4,800 kcal. It has been estimated that, by burning 1 ton of organic waste, approximately 250 litres of heating oil could be saved [13]. Clean incineration of municipal solid waste (MSW) is widely accepted in countries like Sweden and Germany (50% of total MSW), Denmark (65%), Switzerland (80%) and Japan (70%) [14]. Although there are very stringent emissions regulations, more than 50 refuse incineration units are working in Germany. The energy that can be recovered from the incineration of plastics depends on the type of plastic. It has been estimated (in kcal/kg) as: 18,720 for PE; 18,343 for PP; 16,082 for PS; 13,179 for phenol-formaldehyde; 11,362 for foamed polyurethane (PU); 10,138 for Nylon; 8,565 for polyvinyl acetate (PVAc); 7,516 for PVC; and 7,014 for PU. This energy is on average 10,000 kcal/kg. Each ton will release about 107 kcal. However, plastics emit some objectionable gases and form some hazardous compounds. Thus, recovering energy from plastic waste is not cheap. The main goal must be to avoid the formation of these hazardous compounds by the correct construction of incinerators

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Recycling of Plastic Waste and by considering all proper means to avoid pollution. The costs of operating, maintaining and monitoring an incinerator are quite substantial compared to those for a conventional power plant. Energy from waste (EFW) is a reliable and renewable source of energy, especially if the MSW is rich in organic matter. Furthermore, it reduces the amount of waste to be landfilled at the final stage. Costs involved in developing new landfills can partly offset the high costs of energy recovery from an EFW facility. Incinerators with EFW installations are not considered just as power plants, as their main purpose is to reduce the amount of garbage being landfilled within the purpose of an integrated waste management system. Incineration plants should be designed and operated to produce the least amount of pollution. The use of incineration plants is mandatory for plastic wastes from hospitals and similar institutions, which is considered as a potential source of disease. Incinerators do not emit ethane gas, as this gas is completely combusted into CO2 and water, even at low temperature. However, incinerators have often been associated with dioxin and furan emissions, which are avoided in modern ones by working at temperatures that are high enough to decompose such chemicals and prevent them from reaching the ecosystem. Although dioxin and furan are often perceived as two individual chemical products, there are in fact 75 congeners of polychlorinated dibenzodioxins (PCDD) and 135 congeners of polychlorinated dibenzofurans (PCDF), each differing in its chemical configuration and degree of toxicity. The most toxic of the dioxins is 2,3,7,8tetrachlorodibenzo-p-dioxin (TCDD). When assessing health risks, the total congeners are converted to the equivalent TCDD. Dioxins and furans are present in Nature and are generated by many sources, such as forest fires and as a by-product of a certain chemical processes and the burning of wood in stoves and fireplaces, barbecues, diesel engines, power plants, ponds and so on. Scientists can account for about 60% of the dioxins found in Nature (referred to as ‘background level’), while the source of the remainder is still unknown. Dioxins enter the human body through the food chain, inhalation and skin contact. As long as the quantities absorbed are very minute, however, they do not represent a health hazard. The complete combustion of organic matter removes all the dioxins present in the garbage However, during cooling of the flue gases, traces of dioxins are formed. An energy-fromwaste facility acts as a reliable and renewable source of energy. It is a safe method of reducing the volume of waste dumped in landfills. A considerable reduction in the emission of greenhouse gases compared to landfills can be achieved. However, further research is needed to avoid completely this emission. Recovery of energy from solid waste constitutes the fourth ‘R’ after reducing, reusing and recycling. Research as the fifth ‘R’ is the key element. Scientists and environmental scientists have to work together to develop new methods for recycling more products.

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13.2.5 Conclusion In conclusion, primary recycling is ideal for clean, uncontaminated single-type scrap, but degradation during service life or reprocessing should be taken under serious consideration. Secondary recycling by type can be accomplished by various methods, but the cost associated with the separation and decontamination of the wastes undoubtedly poses an inherent obstacle. Dissolution-based techniques seem worth developing, but cannot yet be considered to be the complete answer. Secondary recycling of plastics mixtures by remelting is intended to produce downgraded products as a result of incompatibility problems. Compatibilisation is effective only in specific cases of plastics mixtures. Tertiary or ‘chemical’ recycling processes involve high levels of investment and succeed in recovering the chemical products, but negate the value added during the polymerisation. The latter comment is valid also for the last resort, quaternary recycling (energy recovery of plastics waste), which can substitute other energy sources and solve disposal problems. However, it stands strongly accused of undesirable emissions. The first and most important steps in plastic recycling are collection and sorting, after which the recycling process depends on the type of plastic and the field of application. These issues will now be addressed.

13.3 Collection and Sorting Collection involves gathering lightweight packaging films and other materials. Plastic packaging from separate collection streams is separated from other lightweight packaging material and sorted into fractions comprising film containers, mixed plastics and residues. Identification of the plastic type is one of the most important elements in recycling because most recycling processes prohibit certain types of plastics. For example, severe problems appear during processing of recycled resin of unknown origin. Thus, extrusion and injection moulding require accurate identifications of plastic waste, otherwise a product with bad appearance and impaired quality, especially poor mechanical properties, is obtained.

13.3.1 Resin Identification Identification has become more complicated, not only due to the presence of plastic materials compounded with additives such as plasticisers, stabilisers, flame retardants, fillers and others, but also due to the presence of polymer blends in the waste. Some time ago, the Society of the Plastic Industry (SPI) introduced a labelling system for recyclable

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Recycling of Plastic Waste plastic material. It is now common for manufacturers to use this code system printed or moulded on the product surface for easy plastic identification. The code is a three-sided triangular arrow with a number in the centre and letters underneath, indicating the type of resin: 1 for PET; 2 for HDPE; 3 for vinyl polymers, especially PVC; 4 for LDPE; 5 for PP; 6 for PS; and 7 for others. It is thus easy to develop automated scanning systems that can read the SPI or other codes. This will help to identify the resin used [15]. Furthermore, separation of plastic containers has been proposed by printing bar codes on them [16]. Plastic films are more difficult to identify than plastic containers, because most films do not carry a code, and producers and recyclers need training on how to distinguish between film types. Sorting generally must occur early in the recovery process, near the initial point of generation, to be successful. Optical systems for identification of mixed plastics have been used. A few technologies originally designed and used in the film and packaging industry were considered earlier. Electromagnetic scanning equipment was used to recognise chlorine molecules and so to sort PVC from PET [17]. An X-ray fluorescence (XRF) analyser as a photoelectric sensor was also used to identify transparent PET, green PET, translucent or natural HDPE, pigmented HDPE and PVC. The sensor system is connected to an automatic sorting line. Automation of the process reduces costs and improves the resale value of the separated plastics. Although dirt does not significantly influence the fluorescence intensity from bottles, paper labels do reduce the intensity but do not pose an obstacle in detecting vinyl bottles [17]. Paper labels are virtually blind to X-rays. Infrared and other spectral separation devices have been reported for the continuous examination of waste products [18]. However, a satisfactory process for the identification of plastic products for commercial purposes has yet to be developed.

13.3.2 General Aspects of Resin Separation Resin separation from contaminants or from undesired materials to obtain the desired stream can be achieved by a number of processes. These are: magnetic separation for the removal of ferrous materials; an electrostatic method for nonferrous, mainly aluminium, separation; air separation via cyclones to separate paper; and flotation tanks or hydrocyclones used to separate various resins based upon specific gravity. After that the processed materials are shredded. Automatic separation of shredded plastic waste is very difficult if the resins have similar specific gravity. Fortunately, 85% by volume of world plastic consumption is of four main thermoplastic resins: PE, PP, PVC and PS [19]. In the next sections, some separation techniques based on different properties will be discussed.

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13.3.3 Resin Separation Based on Density Air classification can be used to separate plastics on the basis of their bulk densities; thus film and foamed plastics may be separated from heavier forms of plastic material or paper [20-22]. The densities of the major thermoplastics give the potential to separate them into types by a series of float-sink operations [23, 24]. Water may be used to separate PP, LDPE and HDPE from PS, PVC and PET. A liquid having a density of about 930 kg/m3 may be used to separate PP and LDPE from HDPE; the PP and LDPE may then be separated using a liquid having a density of about 910 kg/m3. The PS and PVC may be separated using a liquid with a density of about 1150 kg/m3 [20]. Blending or filling a plastic may change its density to the point where it could cause difficulties in the float-sink operation processes. Labels, residual adhesives, metals and metallic-plastic composites cause similar difficulties, and therefore processes have been developed to remove these contaminants before the mixed plastic materials enter a separation system [25]. As an example, a solvent washing stage, using either tetrachloroethylene- or hexane-related solvents, was added to the classic water-washing treatment. These solvents were believed to remove not only the glues but also any toxic organic chemicals that have been stored in beverage bottles by consumers or are present as additives in plastics and that inadvertently will be present in the end-products [26]. A different approach for separating mixed plastic wastes by density has been reported [27, 28]. The process uses the properties of a fluid near its critical point to allow fine separations at mild temperatures and pressures. The density of the medium can be varied over a wide range and controlled to a sensitivity of ±0.01 g/cm3. Carbon dioxide is the most commonly used supercritical fluid and can be compressed to densities in the range of 1000 kg/m3. Since the separation of non-olefin thermoplastics will require fluid densities up to approximately 1400 kg/m3, mixtures of carbon dioxide and sulfur hexafluoride, a very dense supercritical fluid, may be used. By effecting small incremental changes of pressure, pure CO2 efficiently separated LDPE, HDPE and PP. Separation of green PET, clear PET and PVC has also been demonstrated, and separation of light- and dark-coloured HDPE is possible. The different densities exhibited by PET in the neck and the body of PET bottles can be separated by CO2/SF6. The possibility of separating various components of wire and cable scrap also exists. The centrifugal field produced in a hydrocyclone has been extensively used for the separation of plastics. In a hydrocyclone, the flow rate referred to the separation area is 100 times higher than in a static float-sink separator. The contamination of plastics is of only minor relevance in this process compared to flotation. For the separation of an n-component mixture, n – 1 separation stages (cyclone plants) are necessary.

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Recycling of Plastic Waste Furthermore, continuously operated plants must be equipped with a feeding device and screens for dehydration purposes [29]. Using hydrocyclones, PS, PET and PVC can be separated from one another or from polyvinyl alcohol; polyolefins from municipal solid wastes; PET from PE, PP and paper; and PP car bumpers from metals and other contaminants [30-32].

13.3.4 Resin Separation Based on Colour Photoelectric sensors are used for the separation of mixed, whole, or baled plastic containers. One of the systems uses mechanical means to reduce the baled plastic into individual bottles and to screen contaminants. After deballing and screening, the containers are manipulated into a single-line presentation to an optical sensor that performs a threeclass identification: Class 1, dairy HDPE and PP; Class 2, PET and PVC; and Class 3, mixed colour HDPE containers. Another optical sensor can be used to discriminate green and amber PET from clear PET containers, PP from dairy HDPE containers, and mixed colour HDPE according to seven colour classifications. However, reliable identification of post-consumer containers requires that measurements from much of the container surface should be ignored. These areas include closures or necks, labels, edges, bottoms and areas with residue of dirt [33, 34].

13.3.5 Resin Separation Based on Physicochemical Properties 13.3.5.1 Electrification The separation of mixed plastic wastes can be achieved using high-voltage drums, taking advantage of their different relative positions in the charging sequence. The process involves first tribo-electrification of the shredded plastic particles of the mixture by fluidisation. Subsequently, the electrified mixture is conveyed through an electrostatic field that separates the individual particles according to the magnitude and the polarity of the electric charges acquired during the tribo-electrification. When fluidising a mixture of two shredded plastics, the particles of the plastic with the lower work function transfer electronic charges to those with the higher work function. For example, the tribo-electrical contact between PVC and PET results in PVC having a negative charge and PET a positive charge. In the case of PET/PS mixtures, PET has a negative charge and PS a positive charge [35]. Mixtures containing more than two plastic species pose a substantial problem with regard to their charging behaviour. Also, owing to the various additives contained in different types of resin, the respective positions of the plastic species are prone to change.

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13.3.5.2 Spectroscopy Spectroscopy in the near-infrared (NIR) region of the spectrum could be a key to the rapid identification of various plastics and their subsequent recovery. By illuminating the sample with light in the near-infrared and measuring the reflected light, a so-called NIR spectrum of the material is obtained, which contains information about the molecular vibrations excited by the light energy. For example, the IR vibrational spectra of plastics show characteristic absorption bands at wavenumbers ν of 1200, 1400, 1700 and 22002500 cm–1 for CH, and at 1300-1500 and 1900-2100 cm–1 for OH. The NH (1500 and 2050 cm–1) and the CO (1730-1740 cm–1) vibrations contain the relevant spectral information for plastics. Plastic objects, such as beverage bottles, can be dropped through a vertical tube, and are identified and separated while falling. This also simulates other transportation possibilities like conveyor belts. From the performance data, it was found that identification can be achieved within 0.2 s, although several measurements were needed to avoid mistakes due to dirt or labels. The problem of transparency for satisfactory measurements could be overcome by reflectance measurements [36]. Other spectroscopic methods are also possible. The identification of several thermoplastics – such as polycarbonate (PC), acrylonitrile-butadiene-styrene (ABS), PP, PS, high-impact PS (HIPS) and PVC – can be achieved by Fourier transform IR (FTIR), based on similar principles [37]. When exposed to γ-radiation, the high molecular weight (high molar mass) molecules of PVC containing chlorine atoms emit an X-ray return signature easily visible by an XRF analyser. Polyolefins, which have much lower molecular weights, emit a lower backscattering signal that barely shows up on the XRF analyser, and so is easily identified and separated [16, 38]. Bayer have proposed a process for automatic identification and sorting of post-consumer plastics, in which fluorescent dyes were added to resins during compounding, a different one for each resin type. These dyes, having high detection sensitivity, can be added in minute quantities, and so 5 g of dye per ton of polymer were sufficient for identification by a diode device [39].

13.3.5.3 Selective Dissolution of Polymer Mixtures Finally, it must be emphasised that solvent recycling of a single-type plastic scrap serves as a model process providing fundamental knowledge for the development of a selective dissolution process. The principle of the selective dissolution of a single polymer in a polymer mixture can be used to separate the polymers. According to the concept of selective dissolution, one polymer could be dissolved at a time, and thus dissolution-based processes can deal with mixtures of polymers. This has an evident impact on the recycling of plastics in municipal solid

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Recycling of Plastic Waste waste. Mixtures of polyolefins, PS, PVC, thermosetting plastics and natural polymers (rubbers, fibres, paper, etc.) can be separated using certain solvent systems [40]. The mixture is first treated with xylene at 5-50 °C to dissolve PS, and after separation the mixture is further heated at 90-150 °C to dissolve PE, leaving PP insoluble. The three main thermoplastics – polyolefins, PS and PVC – may be separated by dissolving them in a mixed solvent of xylene (85%) and cyclohexane (15%). From the dissolution, three separate phases could be obtained containing 99% each of the pure plastics, indicating that excellent separation can be achieved. [41]. In relevant studies, toluene, xylene and kerosene have been proposed as suitable solvents for the selective dissolution of LDPE, but the information given is very limited [42, 43]. Such selective dissolution is accomplished for each of the polymers of the mixture by heating the waste dispersion at various temperatures.

13.4 Recycling of Separated PET Waste The worldwide production of PET is above 1 × 106 tonnes per year. With such large consumption, the effective utilisation of PET waste is of considerable commercial and technological significance. PET waste may be converted into extruded or moulded articles after repelletising it. Recently, waste PET films or sheets have been cleaned, crushed, dried and mixed with LDPE waste [44]. The obtained mix was pelletised and blowextruded into films. The maximum concentration of PET does not exceed 20%. The films obtained were found to possess very good mechanical properties compared with LDPE only. Also, the films are expected to possess good printability due to the polar nature of PET. PET may be depolymerised to yield raw materials for resin synthesis. Recycling of segregated waste may be possible by blending in small quantities with virgin monomer, bis(hydroxyethyl) terephthalate. However, it often lowers the quality of the final product [45]. It is therefore desirable to break down the polymer into smaller fragments or oligomers [46]. PET can also be fully depolymerised into dimethyl terephthalate (DMT). However, the regenerated DMT exhibits a significantly higher carboxyl content, adversely affecting product quality. It is more economic to convert PET into low molecular weight oligomers by glycolysis in the presence of a transesterification catalyst [47-49]. When glycolysis is carried out using ethyleneglycol, the oligomers may be directly recycled into the polycondensation stage in PET manufacture, but this also lowers the product quality. Glycolysis can be carried out using different glycols, and the oligomers can be used in the synthesis of unsaturated polyester by reaction with an unsaturated anhydride [50] or used to synthesise other polymers [51].

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Handbook of Plastic Films There are two distinct advantages of the process: (1) the PET waste is converted into a commercial value-added product; and (2) unsaturated polyester resins based on terephthalic acid (TPA) are obtained without the processing difficulties encountered with the use of plain TPA. Yang and Tsai [52] degraded PET fabric waste to glycolysed product by treating the PET waste with sodium hydroxide using ethyleneglycol or glycerol as the solvent. Compared with the conventional aqueous alkaline hydrolysis, they found that the degradation rate in ethyleneglycol increases tenfold. They reported that the kinetics of the alkaline ethyleneglycol treatment show that the weight loss is linear with respect to time. They concluded that using ethyleneglycol can greatly shorten the treatment time to achieve results similar to those with the conventional aqueous system. A new chemical recycling process for PET using supercritical water has been developed by Yoshiyuki and co-workers [53]. In this method, the monomers obtained from supercritical water hydrolysis are the raw materials of each polymer. The purity of the terephthalic acid obtained from PET is about 99 wt%. It was confirmed that this process has the advantage of reducing the reaction time and simplicity of the process when compared with conventional methods such as methanolysis and glycolysis.

13.5 Recycling of Separated PVC Waste As mentioned before, most of the technologies for the recycling of plastic wastes include degradative extrusion, pyrolysis, hydrogenation, gasification, glycolysis, hydrolysis, methanolysis, incineration with HCl recovery, or input as a reducing agent into blast furnaces. Most of these technologies are still in the research phase, or simply are not suitable for PVCcontaining waste. The latter is particularly true for technologies such as glycolysis and hydrolysis, which play a role only for well-defined single-waste streams such as PET. Some of these technologies are currently generally regarded as the most feasible ones for realisation on a practical scale. However, one group of these technologies is not designed specifically for PVC waste, but deals with mixed plastic waste (MPW) in general. These technologies mainly concentrate on recovering the organic part of the MPW. They often have restrictions with regard to the maximum permissible chlorine (or PVC) input. Other technologies are designed to deal specifically with PVC waste (chlorine concentrations of well over 10%). They emphasise recovery of the chlorine fraction in a useful form. Hence, together with the competing technologies for chemical recycling, three types of technologies have been discussed [54]: (1) Technologies for chemical recycling of mixed plastic waste; (2) Technologies for chemical recycling of PVC-rich waste; (3) Alternatives to chemical recycling (incineration, mechanical recycling).

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Recycling of Plastic Waste

13.5.1 Chemical Recycling of Mixed Plastic Waste Regarding the chemical recycling of MPW with a PVC content of up to several per cent, the process consists of two parts, a liquefaction step and an entrained bed gasifier. In the liquefaction step, the plastic waste is mildly thermally cracked (depolymerised) into synthetic heavy oil and some condensable and non-condensable gas fractions. The non-condensable gases are reused in the liquefaction as fuel together with natural gas. The heavy oil is filtered to remove large inorganic particles. The oil and condensed gas are then injected into the entrained gasifier. Also, chlorine-containing gases from the plastic waste are fed to the gasifier. The gasification is carried out with oxygen and steam at a temperature of 1200-1500 °C [55]. The products of the process are synthesis gas (predominantly H2/CO), pure sulfur and NH4Cl.

13.5.1.1 Polymer Cracking Process In the polymer cracking process, some elementary preparation of the waste plastics feed is required, including size reduction and removal of most nonplastics. The reactor operates at approximately 500 °C in the absence of air. The plastics crack thermally under these conditions to hydrocarbons, which vaporise and leave the bed with the fluidising gas. The gas has a high content of monomers (ethylene and propylene) and other useful hydrocarbons, with only some 15% being methane.

13.5.1.2 Conversion Process The feedstock recycling process was designed to handle the recycling of mixed plastic waste supplied by the collection system. The conversion of the pretreated mixed plastic into petrochemical raw materials takes place in a multistage melting and reduction process. In the first stage the plastic is melted and dehalogenised to preserve the subsequent plant segments from corrosion. The hydrogen chloride separated out in this process is absorbed and processed in the hydrochloric acid production plant. Hence, the major part of the chlorine present in the input, (e.g., from PVC), is converted into saleable HCl. Minor amounts become available as NaCl or CaCl2 effluent [56]. Gaseous organic products are compressed and can be used as feedstock in a cracker. In the subsequent stages the liquefied plastic waste is heated to over 400 °C and cracked into components of different chain lengths. About 20-30% of gases and 60-70% of oils are produced and subsequently separated in a distillation column.

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Handbook of Plastic Films Naphtha produced by the feedstock process is treated in a steam cracker, and the monomers, (e.g., ethylene, propylene), are recovered. These raw materials are used for the production of virgin plastic materials. The process is carried out under atmospheric pressure in a closed system and, therefore, no other residues or emissions are produced. In sum, the products of the process are: (1) HCl, which is neutralised or processed in a hydrochloric acid production plant; (2) Naphtha, to be treated in a steam cracker; (3) Monomers, (e.g., ethylene, propylene), which can be used for the production of virgin plastic materials; (4) High-boiling oils, which can be processed into synthesis gas or conversion coke and then transferred for further use; (5) Residues.

13.5.2 Chemical Recycling of PVC-Rich Waste These processes aim to recover as much as possible of the chlorine present in PVC in a usable form (HCl or a saleable chloride salt). The two processes in question, which are discussed below, are: •

Incineration process;

•

Pyrolysis process.

13.5.2.1 Incineration Process A plant for the processing of chlorine-containing fluid and solid waste streams is used. The goal is to process the waste by thermal treatment and to produce HCl using the energy from the process itself. The plant is based on a rotary kiln and has a capacity of 45 kilotonnes per year, (i.e., not only PVC waste), with a heat production capacity of 25 MW at ca. 7500 production hours per year. The waste is incinerated in the rotary kiln and a post-combustion chamber, directly after the rotary kiln, at temperatures of 900-1200 °C. During this treatment HCl is released and recovered. In this way a continuous production of high-quality HCl can be assured. Also, the formation of dioxins and furans can be diminished in this way, as the goal of the process is to oxidise the waste fully, so that no toxic chemicals (dioxins and furans) are formed.

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Recycling of Plastic Waste

13.5.2.2 Pyrolysis Process In this process the chemical and thermal degradation of the PVC waste takes place in a reactor at low pressures (200-300 kPa) and moderate temperatures (maximum 375 °C). Chlorine generated from the PVC reacts with fillers, forming calcium chloride. Simultaneously, the metal stabilisers that may be present in PVC waste (lead, cadmium, zinc and/or barium) are converted to metal chlorides. This product consists of over 60% lead and may be purified and reused. After completion of the reactions, three main intermediate products are formed: a solid-phase product, a liquid product and a gas-phase product. In sum the products of the process are: (1) Calcium chloride (

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