Biocides in Plastics: Rapra Review Report 180

Rapra Review Reports

ISSN: 0889-3144

Biocides in Plastics

D. Nichols Thor Overseas Limited

Volume 15, Number 12, 2004

Expert overviews covering the science and technology of rubber and plastics

RAPRA REVIEW REPORTS A Rapra Review Report comprises three sections, as follows: 1. A commissioned expert review, discussing a key topic of current interest, and referring to the References and Abstracts section. Reference numbers in brackets refer to item numbers from the References and Abstracts section. Where it has been necessary for completeness to cite sources outside the scope of the Rapra Abstracts database, these are listed at the end of the review, and cited in the text as a.1, a.2, etc. 2. A comprehensive References and Abstracts section, resulting from a search of the Rapra Polymer Library database. The format of the abstracts is outlined in the sample record below. 3. An index to the References and Abstracts section, derived from the indexing terms which are added to the abstracts records on the database to aid retrieval.

Source of original article Title

Item 1 Macromolecules

33, No.6, 21st March 2000, p.2171-83 EFFECT OF THERMAL HISTORY ON THE RHEOLOGICAL BEHAVIOR OF THERMOPLASTIC POLYURETHANES Pil Joong Yoon; Chang Dae Han Akron,University The effect of thermal history on the rheological behaviour of ester- and etherbased commercial thermoplastic PUs (Estane 5701, 5707 and 5714 from B.F.Goodrich) was investigated. It was found that the injection moulding temp. used for specimen preparation had a marked effect on the variations of dynamic storage and loss moduli of specimens with time observed during isothermal annealing. Analysis of FTIR spectra indicated that variations in hydrogen bonding with time during isothermal annealing very much resembled variations of dynamic storage modulus with time during isothermal annealing. Isochronal dynamic temp. sweep experiments indicated that the thermoplastic PUs exhibited a hysteresis effect in the heating and cooling processes. It was concluded that the microphase separation transition or order-disorder transition in thermoplastic PUs could not be determined from the isochronal dynamic temp. sweep experiment. The plots of log dynamic storage modulus versus log loss modulus varied with temp. over the entire range of temps. (110-190C) investigated. 57 refs. GOODRICH B.F.

Location

USA

Authors and afÀliation

Abstract

Companies or organisations mentioned

Accession no.771897

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Volume 1 Report 35

Polymers in Household Electrical Goods, D.Alvey, Hotpoint Ltd.

Report 36

Developments in Additives to Meet Health and Environmental Concerns, M.J. Forrest, Rapra Technology Ltd.

Report 1

Conductive Polymers, W.J. Feast

Report 2

Medical, Surgical and Pharmaceutical Applications of Polymers, D.F. Williams

Report 3

Advanced Composites, D.K. Thomas, RAE, Farnborough.

Report 4

Liquid Crystal Polymers, M.K. Cox, ICI, Wilton.

Volume 4

Report 5

CAD/CAM in the Polymer Industry, N.W. Sandland and M.J. Sebborn, Cambridge Applied Technology.

Report 37

Report 8

Engineering Thermoplastics, I.T. Barrie, Consultant.

Polymers in Aerospace Applications, W.W. Wright, University of Surrey.

Report 10

Reinforced Reaction Injection Moulding, P.D. Armitage, P.D. Coates and A.F. Johnson

Report 38

Epoxy Resins, K.A. Hodd

Report 39

Polymers in Chemically Resistant Applications, D. Cattell, Cattell Consultancy Services.

Report 40

Internal Mixing of Rubber, J.C. Lupton

Report 41

Failure of Plastics, S. Turner, Queen Mary College.

Report 42

Polycarbonates, R. Pakull, U. Grigo, D. Freitag, Bayer AG.

Report 43

Polymeric Materials from Renewable Resources, J.M. Methven, UMIST.

Report 11

Communications Applications of Polymers, R. Spratling, British Telecom.

Report 12

Process Control in the Plastics Industry, R.F. Evans, Engelmann & Buckham Ancillaries.

Volume 2 Report 13

Injection Moulding of Engineering Thermoplastics, A.F. Whelan, London School of Polymer Technology.

Report 44

Report 14

Polymers and Their Uses in the Sports and Leisure Industries, A.L. Cox and R.P. Brown, Rapra Technology Ltd.

Flammability and Flame Retardants in Plastics, J. Green, FMC Corp.

Report 45

Composites - Tooling and Component Processing, N.G. Brain, Tooltex.

Report 15

Polyurethane, Materials, Processing and Applications, G. Woods, Consultant.

Report 46

Quality Today in Polymer Processing, S.H. Coulson, J.A. Cousans, Exxon Chemical International Marketing.

Report 16

Polyetheretherketone, D.J. Kemmish, ICI, Wilton.

Report 47

Report 17

Extrusion, G.M. Gale, Rapra Technology Ltd.

Chemical Analysis of Polymers, G. Lawson, Leicester Polytechnic.

Report 18

Agricultural and Horticultural Applications of Polymers, J.C. Garnaud, International Committee for Plastics in Agriculture.

Report 48

Plastics in Building, C.M.A. Johansson

Report 19

Recycling and Disposal of Plastics Packaging, R.C. Fox, Plas/Tech Ltd.

Report 20

Pultrusion, L. Hollaway, University of Surrey.

Report 21

Volume 5 Report 49

Blends and Alloys of Engineering Thermoplastics, H.T. van de Grampel, General Electric Plastics BV.

Materials Handling in the Polymer Industry, H. Hardy, Chronos Richardson Ltd.

Report 50

Automotive Applications of Polymers II, A.N.A. Elliott, Consultant.

Report 22

Electronics Applications of Polymers, M.T.Goosey, Plessey Research (Caswell) Ltd.

Report 51

Biomedical Applications of Polymers, C.G. Gebelein, Youngstown State University / Florida Atlantic University.

Report 23

Offshore Applications of Polymers, J.W.Brockbank, Avon Industrial Polymers Ltd.

Report 52

Polymer Supported Chemical Reactions, P. Hodge, University of Manchester.

Report 24

Recent Developments in Materials for Food Packaging, R.A. Roberts, Pira Packaging Division.

Report 53

Weathering of Polymers, S.M. Halliwell, Building Research Establishment.

Report 54

Health and Safety in the Rubber Industry, A.R. Nutt, Arnold Nutt & Co. and J. Wade.

Report 55

Computer Modelling of Polymer Processing, E. Andreassen, Å. Larsen and E.L. Hinrichsen, Senter for Industriforskning, Norway.

Volume 3 Report 25

Foams and Blowing Agents, J.M. Methven, Cellcom Technology Associates.

Report 26

Polymers and Structural Composites in Civil Engineering, L. Hollaway, University of Surrey.

Report 56

Injection Moulding of Rubber, M.A. Wheelans, Consultant.

Plastics in High Temperature Applications, J. Maxwell, Consultant.

Report 57

Joining of Plastics, K.W. Allen, City University. Physical Testing of Rubber, R.P. Brown, Rapra Technology Ltd.

Report 27 Report 28

Adhesives for Structural and Engineering Applications, C. O’Reilly, Loctite (Ireland) Ltd.

Report 58

Report 29

Polymers in Marine Applications, C.F.Britton, Corrosion Monitoring Consultancy.

Report 59

Polyimides - Materials, Processing and Applications, A.J. Kirby, Du Pont (U.K.) Ltd.

Report 30

Non-destructive Testing of Polymers, W.N. Reynolds, National NDT Centre, Harwell.

Report 60

Physical Testing of Thermoplastics, S.W. Hawley, Rapra Technology Ltd.

Report 31

Silicone Rubbers, B.R. Trego and H.W.Winnan, Dow Corning Ltd.

Report 32

Fluoroelastomers - Properties and Applications, D. Cook and M. Lynn, 3M United Kingdom Plc and 3M Belgium SA.

Report 33

Polyamides, R.S. Williams and T. Daniels, T & N Technology Ltd. and BIP Chemicals Ltd.

Report 34

Extrusion of Rubber, J.G.A. Lovegrove, Nova

Volume 6 Report 61

Food Contact Polymeric Materials, J.A. Sidwell, Rapra Technology Ltd.

Report 62

Coextrusion, D. Djordjevic, Klöckner ER-WE-PA GmbH.

Report 63

Conductive Polymers II, R.H. Friend, University of Cambridge, Cavendish Laboratory.

Report 64

Designing with Plastics, P.R. Lewis, The Open University.

Report 65

Decorating and Coating of Plastics, P.J. Robinson, International Automotive Design.

Report 66

Reinforced Thermoplastics - Composition, Processing and Applications, P.G. Kelleher, New Jersey Polymer Extension Center at Stevens Institute of Technology.

Report 67

Plastics in Thermal and Acoustic Building Insulation, V.L. Kefford, MRM Engineering Consultancy.

Report 68

Cure Assessment by Physical and Chemical Techniques, B.G. Willoughby, Rapra Technology Ltd.

Report 69

Toxicity of Plastics and Rubber in Fire, P.J. Fardell, Building Research Establishment, Fire Research Station.

Report 70

Acrylonitrile-Butadiene-Styrene Polymers, M.E. Adams, D.J. Buckley, R.E. Colborn, W.P. England and D.N. Schissel, General Electric Corporate Research and Development Center.

Report 71

Rotational Moulding, R.J. Crawford, The Queen’s University of Belfast.

Report 72

Advances in Injection Moulding, C.A. Maier, Econology Ltd.

Volume 7

Separation Performance, T. deV. Naylor, The Smart Chemical Company. Report 90

Rubber Mixing, P.R. Wood.

Report 91

Recent Developments in Epoxy Resins, I. Hamerton, University of Surrey.

Report 92

Continuous Vulcanisation of Elastomer ProÀles, A. Hill, Meteor Gummiwerke.

Report 93

Advances in Thermoforming, J.L. Throne, Sherwood Technologies Inc.

Report 94

Compressive Behaviour of Composites, C. Soutis, Imperial College of Science, Technology and Medicine.

Report 95

Thermal Analysis of Polymers, M. P. Sepe, Dickten & Masch Manufacturing Co.

Report 96

Polymeric Seals and Sealing Technology, J.A. Hickman, St Clair (Polymers) Ltd.

Volume 9 Report 97

Rubber Compounding Ingredients - Need, Theory and Innovation, Part II: Processing, Bonding, Fire Retardants, C. Hepburn, University of Ulster.

Report 98

Advances in Biodegradable Polymers, G.F. Moore & S.M. Saunders, Rapra Technology Ltd.

Report 99

Recycling of Rubber, H.J. Manuel and W. Dierkes, Vredestein Rubber Recycling B.V.

Report 100

Photoinitiated Polymerisation - Theory and Applications, J.P. Fouassier, Ecole Nationale Supérieure de Chimie, Mulhouse.

Report 73

Reactive Processing of Polymers, M.W.R. Brown, P.D. Coates and A.F. Johnson, IRC in Polymer Science and Technology, University of Bradford.

Report 74

Speciality Rubbers, J.A. Brydson.

Report 75

Plastics and the Environment, I. Boustead, Boustead Consulting Ltd.

Report 101

Report 76

Polymeric Precursors for Ceramic Materials, R.C.P. Cubbon.

Solvent-Free Adhesives, T.E. Rolando, H.B. Fuller Company.

Report 102

Advances in Tyre Mechanics, R.A. Ridha, M. Theves, Goodyear Technical Center.

Plastics in Pressure Pipes, T. Stafford, Rapra Technology Ltd.

Report 103

Gas Assisted Moulding, T.C. Pearson, Gas Injection Ltd.

Report 104

Plastics ProÀle Extrusion, R.J. Kent, Tangram Technology Ltd.

Report 105

Rubber Extrusion Theory and Development, B.G. Crowther.

Report 106

Properties and Applications of Elastomeric PolysulÀdes, T.C.P. Lee, Oxford Brookes University.

Report 77 Report 78

PVC - Compounds, Processing and Applications, J.Leadbitter, J.A. Day, J.L. Ryan, Hydro Polymers Ltd.

Report 79

Rubber Compounding Ingredients - Need, Theory and Innovation, Part I: Vulcanising Systems, Antidegradants and Particulate Fillers for General Purpose Rubbers, C. Hepburn, University of Ulster.

Report 80

Anti-Corrosion Polymers: PEEK, PEKK and Other Polyaryls, G. Pritchard, Kingston University.

Report 107

High Performance Polymer Fibres, P.R. Lewis, The Open University.

Report 81

Thermoplastic Elastomers - Properties and Applications, J.A. Brydson.

Report 108

Chemical Characterisation of Polyurethanes, M.J. Forrest, Rapra Technology Ltd.

Report 82

Advances in Blow Moulding Process Optimization, Andres Garcia-Rejon,Industrial Materials Institute, National Research Council Canada.

Report 83

Report 84

Molecular Weight Characterisation of Synthetic Polymers, S.R. Holding and E. Meehan, Rapra Technology Ltd. and Polymer Laboratories Ltd.

Volume 10 Report 109

Rubber Injection Moulding - A Practical Guide, J.A. Lindsay.

Report 110

Long-Term and Accelerated Ageing Tests on Rubbers, R.P. Brown, M.J. Forrest and G. Soulagnet, Rapra Technology Ltd.

Report 111

Polymer Product Failure, P.R. Lewis, The Open University.

Report 112

Polystyrene - Synthesis, Production and Applications, J.R. Wünsch, BASF AG.

Rheology and its Role in Plastics Processing, P. Prentice, The Nottingham Trent University.

Volume 8 Report 85

Ring Opening Polymerisation, N. Spassky, Université Pierre et Marie Curie.

Report 113

Report 86

High Performance Engineering Plastics, D.J. Kemmish, Victrex Ltd.

Rubber-ModiÀed Thermoplastics, H. Keskkula, University of Texas at Austin.

Report 114

Developments in Polyacetylene - Nanopolyacetylene, V.M. Kobryanskii, Russian Academy of Sciences.

Report 87

Rubber to Metal Bonding, B.G. Crowther, Rapra Technology Ltd.

Report 115

Metallocene-Catalysed Polymerisation, W. Kaminsky, University of Hamburg.

Report 88

Plasticisers - Selection, Applications and Implications, A.S. Wilson.

Report 116

Compounding in Co-rotating Twin-Screw Extruders, Y. Wang, Tunghai University.

Report 89

Polymer Membranes - Materials, Structures and

Report 117

Rapid Prototyping, Tooling and Manufacturing, R.J.M.

Report 118

Liquid Crystal Polymers - Synthesis, Properties and Applications, D. Coates, CRL Ltd.

Volume 13

Report 119

Rubbers in Contact with Food, M.J. Forrest and J.A. Sidwell, Rapra Technology Ltd.

Report 145

Multi-Material Injection Moulding, V. Goodship and J.C. Love, The University of Warwick.

Report 120

Electronics Applications of Polymers II, M.T. Goosey, Shipley Ronal.

Report 146

In-Mould Decoration of Plastics, J.C. Love and V. Goodship, The University of Warwick.

Report 147

Rubber Product Failure, Roger P. Brown.

Volume 11

Report 148

Plastics Waste – Feedstock Recycling, Chemical Recycling and Incineration, A. Tukker, TNO.

Report 121

Polyamides as Engineering Thermoplastic Materials, I.B. Page, BIP Ltd.

Report 149

Analysis of Plastics, Martin J. Forrest, Rapra Technology Ltd.

Report 122

Flexible Packaging - Adhesives, Coatings and Processes, T.E. Rolando, H.B. Fuller Company.

Report 150

Mould Sticking, Fouling and Cleaning, D.E. Packham, Materials Research Centre, University of Bath.

Report 123

Polymer Blends, L.A. Utracki, National Research Council Canada.

Report 151

Rigid Plastics Packaging - Materials, Processes and Applications, F. Hannay, Nampak Group Research & Development.

Report 124

Sorting of Waste Plastics for Recycling, R.D. Pascoe, University of Exeter.

Report 152

Report 125

Structural Studies of Polymers by Solution NMR, H.N. Cheng, Hercules Incorporated.

Natural and Wood Fibre Reinforcement in Polymers, A.K. Bledzki, V.E. Sperber and O. Faruk, University of Kassel.

Report 153

Report 126

Composites for Automotive Applications, C.D. Rudd, University of Nottingham.

Polymers in Telecommunication Devices, G.H. Cross, University of Durham.

Report 154

Polymers in Building and Construction, S.M. Halliwell, BRE.

Report 127

Polymers in Medical Applications, B.J. Lambert and F.-W. Tang, Guidant Corp., and W.J. Rogers, Consultant.

Report 155

Styrenic Copolymers, Andreas Chrisochoou and Daniel Dufour, Bayer AG.

Report 128

Solid State NMR of Polymers, P.A. Mirau, Lucent Technologies.

Report 156

Life Cycle Assessment and Environmental Impact of Polymeric Products, T.J. O’Neill, Polymeron Consultancy Network.

Report 129

Failure of Polymer Products Due to Photo-oxidation, D.C. Wright.

Report 130

Failure of Polymer Products Due to Chemical Attack, D.C. Wright.

Report 131

Failure of Polymer Products Due to Thermo-oxidation, D.C. Wright.

Report 132

Stabilisers for PolyoleÀns, C. Kröhnke and F. Werner, Clariant Huningue SA.

Volume 12 Report 133 Report 134

Volume 14 Report 157

Developments in Colorants for Plastics, Ian N. Christensen.

Report 158

Geosynthetics, David I. Cook.

Report 159

Biopolymers, R.M. Johnson, L.Y. Mwaikambo and N. Tucker, Warwick Manufacturing Group.

Report 160

Emulsion Polymerisation and Applications of Latex, Christopher D. Anderson and Eric S. Daniels, Emulsion Polymers Institute.

Report 161

Advances in Automation for Plastics Injection Moulding, J. Mallon, Yushin Inc.

Emissions from Plastics, C. Henneuse-Boxus and T. Pacary, Certech.

Report 162

Infrared and Raman Spectroscopy of Polymers, J.L. Koenig, Case Western Reserve University.

Analysis of Thermoset Materials, Precursors and Products, Martin J. Forrest, Rapra Technology Ltd.

Report 163

Polymer/Layered Silicate Nanocomposites, Masami Okamoto, Toyota Technological Institute.

Report 164

Cure Monitoring for Composites and Adhesives, David R. Mulligan, NPL.

Report 165

Polymer Enhancement of Technical Textiles, Roy W. Buckley.

Report 135

Polymers in Sport and Leisure, R.P. Brown.

Report 136

Radiation Curing, R.S. Davidson, DavRad Services.

Report 137

Silicone Elastomers, P. Jerschow, Wacker-Chemie GmbH.

Report 138

Health and Safety in the Rubber Industry, N. Chaiear, Khon Kaen University.

Report 166

Developments in Thermoplastic Elastomers, K.E. Kear

Report 139

Rubber Analysis - Polymers, Compounds and Products, M.J. Forrest, Rapra Technology Ltd.

Report 167

PolyoleÀn Foams, N.J. Mills, Metallurgy and Materials, University of Birmingham.

Report 140

Tyre Compounding for Improved Performance, M.S. Evans, Kumho European Technical Centre.

Report 168

Plastic Flame Retardants: Technology and Current Developments, J. Innes and A. Innes, Flame Retardants Associates Inc.

Report 141

Particulate Fillers for Polymers, Professor R.N. Rothon, Rothon Consultants and Manchester Metropolitan University.

Volume 15

Report 142

Blowing Agents for Polyurethane Foams, S.N. Singh, Huntsman Polyurethanes.

Report 169

Engineering and Structural Adhesives, David J. Dunn, FLD Enterprises Inc.

Report 143

Adhesion and Bonding to PolyoleÀns, D.M. Brewis and I. Mathieson, Institute of Surface Science & Technology, Loughborough University.

Report 170

Polymers in Agriculture and Horticulture, Roger P. Brown.

Report 171

PVC Compounds and Processing, Stuart Patrick.

Rubber Curing Systems, R.N. Datta, Flexsys BV.

Report 172

Troubleshooting Injection Moulding, Vanessa Goodship, Warwick Manufacturing Group.

Report 144

Report 173

Regulation of Food Packaging in Europe and the USA, Derek J. Knight and Lesley A. Creighton, Safepharm Laboratories Ltd.

Report 174

Pharmaceutical Applications of Polymers for Drug Delivery, David Jones, Queen's University, Belfast.

Report 175

Tyre Recycling, Valerie L. Shulman, European Tyre Recycling Association (ETRA).

Report 176

Polymer Processing with Supercritical Fluids, V. Goodship and E.O. Ogur.

Report 177

Bonding Elastomers: A Review of Adhesives & Processes, G. Polaski, J. Means, B. Stull, P. Warren, K. Allen, D. Mowrey and B. Carney.

Report 178

Mixing of Vulcanisable Rubbers and Thermoplastic Elastomers, P.R. Wood.

Report 179

Polymers in Asphalt, H.L. Robinson, Tarmac Ltd, UK.

Biocides in Plastics

Dean Nichols (Thor Overseas Limited)

ISBN 1-85957-512-9

Biocides in Plastics

Contents 1

Introduction .................................................................................................................................................3

2

The Need for Biocides in Plastics and Basic Microbiology .......................................................................5

3

2.1

Bacteria .............................................................................................................................................5

2.2

Fungi.................................................................................................................................................5

2.3

Algae ................................................................................................................................................6

Plastic Materials Requiring Biocides ..........................................................................................................6 3.1

3.2

3.3 4

Test Methods .............................................................................................................................................12 4.1

4.2

4.3 5

Biostabiliser Effects .........................................................................................................................6 3.1.1 Nutrient Sources for Fungi and Bacteria .............................................................................7 3.1.2 Microbiological Effects........................................................................................................7 3.1.3 Organisms of Importance .....................................................................................................8 Hygienic Applications ......................................................................................................................8 3.2.1 Organisms of Interest ...........................................................................................................8 3.2.2 Merits of Such Biocides .......................................................................................................9 3.2.3 The Bacterial 'Problem' ......................................................................................................10 3.2.4 False Claims .......................................................................................................................11 3.2.5 Conclusions Regarding Hygienic Applications .................................................................11 Active Packaging ............................................................................................................................11

Fungal Test Methods ......................................................................................................................13 4.1.1 Fungicidal Procedures ........................................................................................................13 4.1.2 Fungistatic Procedures .......................................................................................................13 4.1.3 Soil Burial ..........................................................................................................................14 4.1.4 Humidity Chamber or Vermiculite Bed .............................................................................14 Bacterial Test Methods ...................................................................................................................15 4.2.1 Resistance of Plastic to Bacteria .......................................................................................15 4.2.2 Antimicrobial Plastic .........................................................................................................17 4.2.3 Pink Stain Test ...................................................................................................................18 Laboratory Tests versus use Conditions .........................................................................................19

Available Active Ingredients .....................................................................................................................19 5.1

5.2

Migratory Biocides .........................................................................................................................19 5.1.1 OBPA .................................................................................................................................19 5.1.2 OIT .....................................................................................................................................20 5.1.3 Butyl BIT ..........................................................................................................................21 5.1.4 Zinc Pyrithione...................................................................................................................21 5.1.5 Iodo-Propylbutyl Carbamate (IPBC) .................................................................................22 5.1.6 N-Haloalkylthio Compounds .............................................................................................23 5.1.7 Carbendazim (N-benzimidazol-2-ylcarbamic acid methylester) .......................................24 5.1.8 Bethoxazin (3-Benzo(b)thien-2-yl-5,6-dihydro-1,4,2-oxathiazine 4-oxide) .....................24 Non or Low Migratory Biocides ...................................................................................................25 5.2.1 Triclosan (2,2,4-dicholoro-2-hydroxydiphenyl ether) .......................................................25 5.2.2 DCOIT ..............................................................................................................................25 5.2.3 Silver ..................................................................................................................................26 5.2.4 Sustainable Antimicrobial Polymers (Degussa).................................................................27

1

Biocides in Plastics

5.3 6

7

5.2.5 Titanium Dioxide Nanoparticles ........................................................................................28 Other Ingredients ............................................................................................................................28

Legislation Regarding Biocides ................................................................................................................28 6.1

Limitations of Use ..........................................................................................................................28

6.2

Future Requirements ......................................................................................................................29

6.3

BPD Exemptions ............................................................................................................................30

Summary ...................................................................................................................................................30

Additional References .......................................................................................................................................31 Unpublished References ...................................................................................................................................31 Bibliography .....................................................................................................................................................31 Acknowledgements ...........................................................................................................................................31 Abbreviations ....................................................................................................................................................32 Subject Index ....................................................................................................................................................99 Company Index ...............................................................................................................................................113

The views and opinions expressed by authors in Rapra Review Reports do not necessarily reÁect those of Rapra Technology Limited or the editor. The series is published on the basis that no responsibility or liability of any nature shall attach to Rapra Technology Limited arising out of or in connection with any utilisation in any form of any material contained therein.

2

Biocides in Plastics

The use of biocides in plastics is commonplace. They are added to protect the plastic itself from degradation by microbes or alternatively to provide an external antimicrobial hygienic surface. The choice of suitable test method and the appropriate biocide can be difÀcult, as the different ways in which biocides work will affect how they perform under certain test protocols. A list of frequently used test methods and commonly available biocides is provided with details of their strengths and weaknesses. In an ever-changing regulatory environment, an examination is also made of the inÁuence of legislation on the current and future use of such biocides.

1 Introduction Biocides are additives used in a variety of articles including plastics. When used in plastics the annual biocide sales volumes are in excess of 2,500 tons with a value of over $175 million (147). They are used traditionally to prevent degradation of the plastic itself or more recently to impart an external anti-microbial or hygienic effect. They achieve this by killing or

preventing the growth of microbes that may otherwise consume nutrients within the plastic or settle and grow on its surface. Microorganisms grow by adopting some of the raw materials within the plastic as their nutrient carbon source (food) but in addition need the correct pH, trace elements and crucially, water. When these organisms grow (and use the plastic's ingredients) it may cause the plastic to relinquish its mechanical properties. It could become brittle and lose its conductivity or Áexibility. Alternatively organisms may colonise the surface of the plastic making it unsightly, create odours, trap water or create unwanted unhygienic conditions. Biocides can be selected on the basis of their function and the application for which they are intended but choosing the right biocide is often not so simple. By collecting data on a variety of different active ingredients (actives), the minimum inhibitory concentration (MIC) may be used to provide a rough comparison between active ingredients (see Table 1). This can be used to make a preliminary choice of the most suitable active ingredient for a given application or against a speciÀc organism. However, this cannot be used in isolation, as not only is biocidal performance a criterion, the inprocess stability, migration, leachability, UV and heat stability may all be important factors.

Biocide active/ microorganism

Alternaria alternata

Aspergillus niger

Trichoderma viride

Aureobasidium pullulans

Chaetomium globosum

Cladosporium cladosporoides

Sclerophoma pityphila

Penicillium glaucum

Pseudomonas aeruginosa

Staphylococcus aureus

Table 1 Minimum inhibitory concentration values of some biocide actives in ppm

OBPA

10

10

10

10

10

10

10

10

10

10

OIT

1.5

5

-

0.5

10

-

-

2.5

500

10

DCOIT

10

5

100

50

5

5

100

15

13

5

BBIT

2

31

32

4

0.5

0.5

-

5

500

2

IPBC

2

2

100

1

5

2

1

1

-

200

7.5

100

50

15

20

5

5

50

400

100

0.01

Silver ion

-

0.003

-

-

-

-

-

-

0.008

0.008

Silver *

-

500

-

-

500

-

-

500

62.5

250

Zinc pyrithione

*Typical zeolite or similar (this example ion exchange resin) OBPA: 10,10 Oxybisphenoxarsine OIT: N-Octyl-isothiazolinone DCOIT: Dichloro-n-octyl-isothiazolinone BBIT: Butyl-benzisothiazolinone IPBC: Iodo-propylbutyl carbamate

3

Biocides in Plastics

The MIC values also take no account of the availability of the active at the surface of the plastic. Those that readily migrate from the plastic article will yield a high concentration of biocide in the vicinity for a period until the biocide is used up. The rate of migration will depend on the active ingredient. Migration occurs either with the plasticiser and/or by a process of leaching, the latter being usually dependent on available moisture and therefore the water solubility of the active concerned. For plastics that may be in contact with fats or oils (for example with packed meats) the solubility in fats may also be a factor. Those plastics with low or non-migratory biocides may not have sufÀcient active available on the surface to kill the microbes that may settle there. Water solubility and rate of migration should also inÁuence the choice of biocide. This can be related to the MIC value to give an index of solubility (MIC/solubility in water) (Table 2). Values of less than one for the index of solubility could indicate that theoretically, insufÀcient biocide would be present on the surface to kill the microorganisms. High solubility values could indicate a tendency for excess leaching in high moisture environments.

In reality, the plasticiser and other components in the PVC affect migration of actives, but most bacteria and fungi will only grow and reproduce where water (liquid or vapour) is present. Some microorganisms will survive in the absence of water but will not grow, proliferate or cause deterioration. Laboratory microbiological testing can further assist in selecting the most appropriate active for the plastic substrate but this must be chosen on the basis of suitability for the application rather than suitability of the active ingredient being tested. Thermal stability has long been an issue promoted by the inorganic biocide companies as a main advantage for these over organic biocides. However, simple processing temperature is a very one-dimensional assessment of organic biocide stability. It is also a question of how long certain organic biocides are heated at these temperatures, what loss of activity is acceptable and whether any discoloration of the plastic is tolerable. A temperature pyramid (Figure 1) can provide some information of the suitability of biocides and commodity plastics. Engineering plastics can often use organic biocides as well as inorganic biocides.

Table 2 Other properties of active ingredients Biocide

Water Solubility mg/l (ppm)

Heat stable in higher temperature polyoleÀns

Heat stable in most PVC, PU and lower temperature plastics

UV stability (discoloration)

Index of solubility (MIC/solubility in water)

OBPA

5

No

Yes

Moderate

0.5

480

No

Yes

Good

1-560

2

No

Yes

Good

0.01- 0.5

BBIT

480

No

Yes

Good

15-560

IPBC

156

No

Limited

Very poor

0.8-156

Zinc Pyrithione

20

No

Yes

Poor

0.05-4

Triclosan

10

No

Yes

Not known

0.1-10

0.004

Yes

Yes

Poor

0.5-1.0

OIT DCOIT

Silver

PVC: Polyvinyl chloride PU: Polyurethane

4

Biocides in Plastics

use temperature

300 °C HIGH TEMPERATURE PLASTICS PB - Polybutylene PEEK - Polyetheretherketone LCP - Liquid Crystal Polymer PTFE - PolytetraÁuorethylene PES - Polyether Sulfone PVDF - Polyvinyldene Áuoride

150 °C PC - Polycarbonate ENGINEERING PLASTICS

PET - Polyethylene Terephthalate PMP - Polymethyl pentene POM - Polyoxymethylene PA - Polyamine (nylon)

100 °C PPE - Polyphenylene ether PMMA - Polymethylacrylate ABS - Acrylonitrile Butadiene Styrene SAN - Styrene acrylonitrile

PP - Polypropylene COMMODITY PE - Polyethylene

PLASTICS

Figure 1 Temperature pyramid

2 The Need for Biocides in Plastics and Basic Microbiology The microorganisms of importance fall into three broad groups (Figure 2). Viruses may also be mentioned although these are less well documented as an issue for growth in or on plastics.

2.1 Bacteria Can degrade plastics by utilising raw materials as a food source, potentially causing surface staining (Figure 3), pitting and malodours. Plastic material can also provide

a surface for the growth and proliferation of pathogenic organisms.

2.2 Fungi Moulds can degrade plastics and grow within them (Figure 4), reducing structural strength, conductivity or other physical properties. They can cause unsightly and aesthetically unpleasant growth on the surface and potentially enable the growth of some fungi that produce harmful mycotoxins. Yeast can also stain and cause malodours on plastics.

Figure 2

Figure 3

The three different types of microrganisms

Staining of PVC Àlm

5

Biocides in Plastics

Examples of applications where biocides are used include:

Figure 4

•

PVC – Tarpaulin, truck canopies, pool and pond liners, silage pit liners, artiÀcial leather, sheeting, awnings, textile coatings, flooring, wallpaper, rooÀng materials, garden furniture, refrigerator gaskets, automobile components.

•

PU – Shoe covering, foams, textile coatings, shower curtains, electrical components.

•

Polypropylene/polyethylene (PE) – Typically hygienic uses including textiles, pillows, quilts, carpets.

•

Wood plastics composites (WPC) – Anti rot, surface growth and anti-algal products for wood plastics composites (the growth of wood rot fungi on WPC is seen in Figure 6).

Microscopic view of Penicillium sp. growing within a matrix

2.3 Algae Although they can be unsightly with green, brown or black discoloration of surfaces, they themselves cause no damage to plastics, as they do not use them as a nutrient source. They can however trap water, encouraging fungal growth (Figure 5) and physical ‘freeze and thaw’ effects.

3.1 Biostabiliser Effects This is achieved by addition of a biocide whose function is to maintain the properties of the plastic article. These biocides are not intended to offer any external or hygienic effect but merely prevent the effects of microbial growth on or in the plastic itself.

Figure 5 Fungal colonies growing on roof felt

3 Plastic Materials Requiring Biocides Many plastics articles may contain a biocide, although even in the same type of product the manufacturer may decide to add or omit a biocide dependent on the quality required of the Ànal article. This is because the use of a biocide will impact on the cost of the Ànal article and therefore its use must provide some essential or value added component.

6

Figure 6 Wood rot fungus Gloeophyllum trabeum growing over a WPC block (sample of decking) with a wood block control above

Biocides in Plastics

3.1.1 Nutrient Sources for Fungi and Bacteria The sources of food for microorganism growth within plastics are the raw materials used in their manufacture. The obvious example is the plasticiser used in PVC. The type of plasticiser will vary the susceptibility of the PVC to microbial attack. This is because the type and propensity of organisms will depend on their ability to utilise the plasticiser as a food source. As such, plasticisers can be ranked in terms of their susceptibility to attack (Table 3). Of course, PVC is only one example of a plastic but other plastics based on PU will also be susceptible. The ingredients in PU that are susceptible may include esters, ureas, urethanes, amides, biurets and allophanates. Indeed most polymers are receptive to attack including cellulose nitrate, cellulose acetate, polycaprolactone, polyethylene succinate, polyethylene adipate, polyvinyl alcohol, polybutadiene, styrene butadiene, butyl acrylonitrile, butadiene acrylonitrile, polyester polyurethanes, polyacetate, polyglycollate, polydioxanone and Nylon 2,6.

3.1.2 Microbiological Effects Fungal and algal growth is very common on surfaces remaining damp and/or becoming soiled, especially in conditions of high humidity and for algal growth, the presence of light. Where there is a nutrient contribution from the product, e.g., plasticiser migration, growth can be profuse. On exterior surfaces, microbial growth can result in wide ranging problems. The most serious effect can be physical – causing cracking, pitting (Figure 7), weight loss, structural

Figure 7 Pitting of a plastic surface due to fungal attack

integrity loss, brittleness or corrosion of an underlying substrate. Safety issues of slipping are also relevant for plastics intended for human trafÀc such as decking. Fungal growth on interior plastics is important too and there are suspicions that such growth may play a part in the so-called 'Sick Building Syndrome' that is more correctly called 'building related illness'. Toxins produced by fungi could cause persons to become ill but only when working or living in a contaminated building. Once away from the building the symptoms cease. Building related illness is rather more complicated than is suggested in the literature and other factors may play a more important role in this phenomenon. Other effects are aesthetic. Staining, discoloration or visible surface growth at point of sale or in service is undesirable. Fungal growth on some surfaces can cause contamination of other products. Fungal growth on plastic containers for food or industrial products can secondarily contaminate these products (Figure 8).

Table 3 Plasticiser susceptibility (decreasing down each column) Tricresyl phosphate

Diisooctyl adipate

Dibenzyl sebacate

Diisooctyl phthalate

Diisooctyl sebacate

Polypropylene sebacate

Dioctyl phthalate

Butyl stearate

Methyl ricinoleate

Dibutyl phthalate

Dioctyl sebacate

Butyl ricinoleate

Dinonyl phthalate

Dihexyl adipate

Butoxyethyl stearate

Dioctyl adipate

Dibutyl sebacate

Zinc ricinoleate

Dimethyl sebacate

Dicapryl adipate

7

Biocides in Plastics

Figure 8 Fine surface fungal growth on the lid of a plastic paint container. Water condensing on the lid can provide contaminant organisms that may infect the paint

3.1.3 Organisms of Importance The following lists some of the fungal organisms that can potentially grow on plastics or their raw materials: Alternaria alternata Aspergillus fumigatus Aspergillus niger Aureobasidium pullulans Botryotrichum sp. Cephalosporium sp. Chaetomium globosum Cladosporium resinae Cladosporium sp. Corollospora maritima Curvularia sp. Epicoccum purpurascens Fusarium semitectum Gliocladium roseum Haligena unicordata Helminthosporium sp. Lulworthia purpurea Mesabotrys sp. Monilia sp. Nigrospora oryzae Nigrospora spherica Paecilomyces variotii Penicillium citrinum Pestalotia neglecta Phoma sp. Phytophthora sp. Pythium sp. Rhizopus sp. Rhodotorula sp. Scopulariopsis sp. Spicaria Sporobolomyces roseus Stemphylium sp. Tetracoccosporium sp. Trichoderma harzianum Ulocladium chartarum Zalerion maritima

8

Figure 9 Fungal staining of cladding

3.2 Hygienic Applications Hygienic applications do not necessarily provide any protection to the plastic article itself, although by their nature could do so. They provide protection against organisms that may come into contact or settle on them, thereby limiting transmission of disease. The majority of biocides are targeted towards antibacterial applications. They are designed to enhance hygiene and although they should not replace traditional disinfection practices, are sometimes promoted as an alternative or in addition to good housekeeping. Examples include toilet seats, all manner of hospital applications including medical devices, ward furniture, Áooring and coated textiles as well as perceived added value products such as antibacterial socks, bedding, chopping boards, worktops and other items in food preparation.

3.2.1 Organisms of Interest Since the target for the use of these products is typically hospital or food preparation areas, the main organisms of concern are pathogenic bacteria such as methycillin resistant Staphylococcus aureus (MRSA), Escherichia coli and other enteric organisms. Other organisms may include dust mites and anti-viral applications such as severe acute respiratory syndrome (SARS). MRSA, where UK deaths have doubled in the period from 1999 to 2003 up to nearly 1000 per year (Daily Mail, February 25, 2005 quoting from the Office of National Statistics), is a bacterium of signiÀcant interest (Figure 10). The danger of producing plastic

Biocides in Plastics

articles that show in laboratory tests to be impervious to the growth of MRSA may not be representative of what happens in practice. In an increasingly litigious society, it is a high risk to make claims regarding such organisms, should the article fail in service. However, some producers may feel that the marketing advantage currently outweighs the concerns in these cases. New legislation regarding the use of biocides will cause companies to think very hard before joining the growing list of producers making such claims.

person touches a surface or object contaminated with infectious droplets and then touches his or her mouth, nose, or eye(s).

SARS is also a disease in the news over recent years and a plastic resistant to SARS has been mentioned in some journals. From the Centre for Disease Control (www.cdc.gov) fact sheet on SARS dated 13 January 2004, SARS is a viral respiratory illness caused by a coronavirus, called SARS-associated coronavirus.

These biocides are usually provided to offer a value added component to existing applications. An example would be an anti-bacterial chopping board promoted as a safer alternative to a board that does not claim to have any anti-bacterial additive. However, there are no internationally recognised test methods available to prove the performance of such systems and so most applications are based either on certain laboratory test protocols, anecdotal evidence of performance or by marketing claims alone.

The main way that SARS seems to spread is by close person-to-person contact. The virus that causes SARS is thought to be transmitted most readily by respiratory droplets (droplet spread) produced when an infected person coughs or sneezes. Droplet spread can happen when droplets from the cough or sneeze of an infected person are propelled a short distance (generally up to three feet) through the air and deposited on the mucous membranes of the mouth, nose, or eyes of persons who are nearby. The virus also can spread when a

A plastic article that may have a biocide or is itself resistant to SARS will therefore be likely to have little or no effect on the spread of the disease.

3.2.2 Merits of Such Biocides

Of course, bacteria can be isolated from interior and exterior surfaces and the organisms recovered will frequently reÁect the activity taking place close by. In food processing factories these can be organisms associated with foodstuffs, in hospitals it is no doubt possible to Ànd bacteria associated with human illness.

1000 900 800 700 600 500 400 300 200 100 0

98

19

99

19

00

20

01

20

02

20

03

20

Figure 10 Number of death certiÀcates mentioning Staphylococcus aureus by methycillin resistance, England & Wales (number of deaths)

9

Biocides in Plastics

Indeed, it can be assumed that use of biocides in this application Àeld would only be of beneÀt but this may not be the case in some instances. It is a commonly held view that 'all bacteria are harmful' but it is just not the case. Of course, pathogenic organisms such as methycillin resistant Staphylococcus aureus (MRSA), E. coli 0157 and many others are harmful to humans but the vast majority, e.g., intestinal tract and waste breakdown organisms, animal rumen bacteria, are essential to us. Additionally, there is strong evidence that children acquire resistance to disease and illnesses such as asthma by exposure to microorganisms when they are very young. We would not survive in a sterile environment.

to be effective. There are several ways to make surfaces more resistant to the survival of bacteria: •

Make the surface resistant to heat, abrasion, cleaning and disinfecting agents, physically 'strong' and able to be very vigorously cleaned.

•

Make a very smooth surface that is easy to clean conventionally and to which it is more difÀcult for soiling and soiling organisms to attach.

•

Include an antibacterial agent, whilst remembering that such a product could only work if bacteria are directly on the surface and it is wet enough to allow migration of the biocide (Figure 12).

3.2.3 The Bacterial 'Problem' Bacteria need water to survive and will therefore only do so in very damp environments - in fact, they simply cannot grow on dry plastic surfaces. Most are easily killed by even diffuse UV light, although the spores of bacteria such as Bacillus and Clostridium sp. are resistant. It should be remembered that the surface of a plastic article is rarely nutritious, so bacteria need extraneous nutrients – splashed food in processing factories, body Áuids in hospitals and institutions, mud on Áooring and so on. When examining growth on a plastic Àlm (for example PVC Áooring), it can be seen, when magniÀed, that bacteria will grow on the soiling rather than on the plastic (Figure 11). A barrier is then formed, through which an antimicrobial would have to migrate in order

Dust particle coated with bacteria

Bacteria grow

Bacteria grow

'Soiling'

'Soiling'

Plastic

Plastic

Figure 11 Schematic of surface soiling and potential for bacterial growth

Bacteria in contact with surface die Others remain viable

'Antibacterial' surface

Figure 12 Schematic showing effect of antibacterial agent on a bacterium

10

Biocides in Plastics

Antibacterial and hygienic surfaces are possible but there are many theoretical and not fully substantiated claims, e.g.,

3.2.5 Conclusions Regarding Hygienic Applications

•

Nanoparticulate TiO2 giving off O-

There are many reasons why the development of biocide-based antibacterial products should be carefully considered and evaluated:

•

Poly-(4-vinyl-N-alkylpyridinium) bromide (hexylPVP) destroying bacteria by modifying the electrostatic charge on their cell walls

•

They can give users a false sense of security.

•

Their use may result in less frequent/less thorough cleaning, the most effective method of dealing with bacterial contamination.

•

They might lead to the development of tolerant/ resistant organisms.

•

They may lead to the development of lower childhood antibody production and greater incidence of certain conditions, e.g., asthma.

•

They could result in claims against the product manufacturers in cases of failure, leading to claims against the biocide supplier.

•

This application could be regarded as an ‘unethical’ use of our industry’s products.

•

In the EU, antibacterial plastics could be classiÀed as ‘Biocidal Products’ under the Biocidal Products Directive (BPD) and the cost of registration could be as much as €200,000.

•

Polymers containing nanoparticulate silver

More common is the use of antibacterial chemical additives, e.g., Triclosan, silver compounds and complexes, and zinc pyrithione

3.2.4 False Claims Many misleading claims are made based on spurious test methods because there are no internationally recognised standard methods for determining the efÀcacy of so-called antibacterial plastics. Methods used to substantiate claims include: •

Zone of inhibition or 'halo' tests using liquid or solid additives

•

Zone of inhibition tests using a test piece

•

Direct inoculation of the surface under evaluation

This zone of inhibition test shows only that the plastic contains an antibacterial agent that is able to migrate from the product being examined. An alternative test, becoming more widely used, is based on the Japanese standard JIS Z2801:2000 (a.1), a test for determining the antibacterial activity of plastic surfaces and is commonly referred to as the ‘Film Adherence Method’. Whereas zone of inhibition tests are only able to determine whether or not a product contains an antibacterial agent, the Àlm adherence method can give a measure of the ability of the surface to kill applied microorganisms. However, it does not take into consideration the soiling which most commonly accompanies bacterial contamination. Such methods are explained in further detail later in this review in Section 4.2.2.

3.3 Active Packaging This is a relatively new development whereby a biocide is incorporated into a plastic packaging article to prolong or improve the shelf life of the food in the packaging. Various studies have looked at biocide products like Triclosan and silver zeolites (114) but with limited success. Additionally these actives have some issues related to regulatory approval. This may be because release of biocide from the plastic is limited and although inherently valuable to reduce contamination of the food itself, more is needed in some instances. Intimate contact between the plastic Àlm or a carton with the food would also be required. Triclosan in the main, shows that in vitro testing methods used to simulate in-use conditions did not reÁect real life use. This is due to the additional factors

11

Biocides in Plastics

that may be required for the activity of some biocide actives including oxygen and light that are not available in some vacuum packed foodstuffs.

4 Test Methods

growth and to check and demonstrate the effectiveness of the biocide to prevent colonisation and growth of microorganisms. A distinction should be made between those added as a biostabiliser (to protect the plastic itself) or those intended to provide an anti-microbial (hygienic) effect. This must also be disassociated from a disinfectant which has quick cleansing but relatively short-term effect (see Figure 13).

A variety of international standard methods are available to test the susceptibility of various plastics to microbial

Biostabiliser Incorporated into plastics Anti-fungi, algae and bacteria Prevent discoloration Stop cracking Prevent pitting Prevent brittleness Improve product life span Maintain aesthetic appeal Medium to long life

Antimicrobial Within or coated on Mostly antibacterial Prevent discoloration Prevent odours Impart ‘feel good factor’ Improve product hygiene Reduce contamination Intended long life

Disinfectant Surface applied Mostly antibacterial Prevent discoloration Prevent odours Sterilising effect Eliminate contamination Short life

Figure 13 Differences between biostabilisers, antimicrobials and disinfectants

12

Biocides in Plastics

Many plastics are non-polar and hydrophobic in character, causing water droplets to run away or pool. ‘Wetting’ of the plastic surface may be enhanced by the presence of soiling or a surfactant. Agar plate tests attempt to mitigate the problem of surface wetting by creating a high humidity environment in a petri dish. It is appropriate for plastics that are used outdoors or in wet environments to be pre-treated or conditioned by artiÀcial weathering (exposure to UV light, condensation and water spray) or leaching before testing. This also alleviates the hydrophobic effect. Most laboratory tests are used routinely to provide relatively rapid results. They serve as a model to mimic conditions found in practice. However many procedures are not reliable in this respect, since they do not take account of factors such as: the degree of soiling, the potential level of contamination, presence of cleaning residues, increased surface area caused by abrasion, the age, exposure to weather and so on. Agar plate techniques evaluate the growth of microorganisms on the test plastic and also in the surrounding agar. A zone of no growth (zone of inhibition) can either indicate an effective biostabiliser, a high concentration or a high rate of migration or leaching of the active from the plastic into the surrounding agar. However large zones of inhibition indicate high water solubility of the active and therefore low retention and vulnerability to leaching. It is even possible for the plastic to support microbial growth and yet still have a large zone of inhibition. Thus care must be taken when interpreting such results. Most methods can be modiÀed to suit applications, but for speciÀc applications and to account for long-term efÀciency, near faithful conditions may be afforded by humidity chamber techniques, such as soil burial or vermiculite bed. Often a combination of tests is worthwhile in order to reÁect performance under actual climatic and environmental conditions.

4.1 Fungal Test Methods There are many methods available to the microbiological test laboratory that determine fungicidal and fungistatic properties of plastics (a.2-a.4).

4.1.1 Fungicidal Procedures Fungicidal procedures demonstrate the ability of a biostabiliser to confer resistance to the plastic and kill

Figure 14 ISO 846A samples of PVC

fungi. Tests such as ISO 846 Part A (a.5) and ASTM G21 (a.6) employ a buffered mineral salt agar without any organic carbon source as a nutrient. Fungi can only grow by utilising components from the plastic formulation or from stored nutrients (a.7) causing deterioration of the plastic (see Figure 14). Typical fungi used in these tests include: Aspergillus niger, Chaetomium globosum, Paecilomyces variotii, Penicillium funiculosum and either Trichoderma longibrachiatum or Gliocladium virens (which is also known as Trichoderma viride). Plastic test discs are placed on the mineral salt agar surface and spray inoculated with a mixed fungal spore suspension. The plates are incubated at 20-25 ºC for four weeks or more. The degree of growth on the plastic and diameter of any zone of inhibition are recorded. A similar procedure, ISO 16869 (a.8), overlays the test plastic with inoculated agar instead of inoculation by spraying. These methods ought not to be carried out in isolation, since they do not account for ‘soiling’ of the plastic surface, which is likely to be present in normal use.

4.1.2 Fungistatic Procedures Fungistatic procedures demonstrate the ability of a biostabiliser to inhibit fungal germination (a.9). Tests such as ISO 846 part B (a.5) and JIS Z2911 (a.10) use a buffered mineral salts agar but supplemented with a readily available carbon source in the form of glucose.

13

Biocides in Plastics

their microbiological properties. They require regular monitoring and incubation is over a longer period than agar plate tests. Also short-term weight loss over the Àrst few weeks may not necessarily reÁect longerterm migration effects. Where garden or forest soil is used, reproducibility between tests is near impossible - conversely when a standardised soil, such as John Innes, is used the ‘seed inoculation’ of a Àxed variety of laboratory organisms might not reÁect the natural soil microÁora.

Figure 15 ISO 846B Typical zone of inhibition (OIT coated textile)

Another variation on a theme includes AATCC 30 Soil Burial Test (a.11). A temperature of 28 ºC, relative humidity of 85% and water content of 20-30% for incubation over 16 weeks is the change in essential criteria.

Test methodology and assessment is the same as the fungicidal procedures, except that the incubation period is much shorter since fungi grow more quickly in the presence of a rich nutrient source. Even where the test substrate is not used as a source of nutrient, metabolic products from growth on the agar may cause deterioration of the test plastic. Fungistatic activity is shown by any inhibition of growth either on the plastic or in the agar (Figure 15). However, some of these types of test introduce a high level of nutrient that may not necessarily be found in practice.

4.1.3 Soil Burial

Figure 16 Typical soil burial test, here on a PVC ground sheet

For plastics that are in contact with soil or are exposed to high humidity, for example silage liners, the test method ISO 846 part D (a.5) is the appropriate standard test to use. Test samples are weighed, completely buried in damp microbiologically active soil, and incubated from one month up to 48 months at 29 ºC. Changes in physical characteristics, such as cracking or discoloration, are monitored, recorded and Ànal weights compared with original weights (Figure 16). This test method assumes weight loss is due to metabolism of the plasticiser and also loss of other components such as lubricants, stabilisers, stearates or PU. It is possible that the measured loss is lower than the actual loss since metabolic by-products may remain in the plastic. One of the disadvantages of these test methods is that they monitor differences in the migration of the plasticiser and components of biostabilisers rather than

14

4.1.4 Humidity Chamber or Vermiculite Bed A few test laboratories use simple in-house vermiculite bed or humidity chamber techniques to determine the susceptibility of plastic to contamination by fungi. These have been adapted from methods such as BS 3900-G6 (a.12) used for industrial coatings. As reported (249) and used widely within the International Biodeterioration Research Group (IBRG) this method involves supporting plastic samples over an inert medium such as vermiculite. This is saturated with sterile water to provide high relative humidity. Samples can also be suspended on clips in a humidity cabinet. The plastic surface is Àrst sprayed with a mixed fungal spore suspension and the closed humidity chamber (cabinet or vermiculite bed) incubated at 25 ºC for up to three months. Fungal growth is periodically assessed,

Biocides in Plastics

using a stereoscopic microscope, as a percentage of the surface covered. A rating of 0-5 is given but the pass criteria are perhaps subjective. Often 0-2 rating (i.e., growth of less than 10% of the area of the plastic surface) is used as a benchmark (Figure 17). To aid the visual assessment a dye (LoefÁer’s methylene blue) may be used to stain the fungal structures at the end of the test (a.7). This method relies on the plastic itself as the sole carbon source for fungal growth. It is often difÀcult to achieve good fungal growth, even on biostabiliser-free samples, however the procedure perhaps emulates actual conditions more closely than agar plate tests. Another advantage of this method is that weathering, either naturally or by artiÀcial methods, of sample prior to the test can give an indication of exterior performance.

of very hydrophobic surfaces. Many methods suggest the use of only one or two microbial species that can only give a vague indication of the variety of potential contaminants that might be found in practice. Common tests cite MRSA or E. coli in a ‘feel good factor’ but in reality other organisms may be more applicable to certain applications. For every different species each biocide has a different MIC value, thus some active suppliers can choose a speciÀc bacteria or fungus that will give results that favour one active over another. It is important that plastic manufacturers do not make extravagant or unsubstantiated claims of hygienic properties or anti-microbial resistance. They must be aware that claims of antimicrobial performance may, in future, require registration of the plastic article as a biocidal product (98). This registration process will incur substantial costs. Many experts are debating the merits of testing these claims, since all existing standard test protocols fall outside of this new scope. Issues currently under discussion include: deÀnition of a hygienic surface, information required for registration, testing of the products, permitted biocide actives, labelling of products and guidelines and advice for consumers. Several draft methods are available to organisations that are collaborating to develop meaningful tests but experts have yet to agree on test pass and fail criteria, test temperature, test organisms and presence of nutrient. There are several test protocols being promoted as follows:

Figure 17

4.2.1 Resistance of Plastic to Bacteria

Humidity chamber test method (PVC roof sheet)

4.2.1.1 ISO 846 C (a.5) The humidity chamber technique can also be used to determine the susceptibility of plastic to contamination by algae, the main difference being incubation under intermittent daylight type lights for a longer period to allow the algae to grow. The use of selective fungicides can prevent overgrowth of the algae by fungi where testing is required to be algal speciÀc. An alternate method for algal testing, ASTM G29-96 (a.13), is slightly more complicated and requires propagation tanks for culturing the algae and large test chambers.

4.2 Bacterial Test Methods It is actually difÀcult to achieve bacterial colonisation on the surface of most plastics without soiling, especially

Is often determined using the zone of inhibition test ISO 846 part C. Plastic test discs are sandwiched between two layers of a non nutrient mineral salts agar which has been inoculated with a cell suspension of Pseudomonas aeruginosa. The plates are incubated at 29 ºC for four weeks or more. The degree of growth on or over the plastic and diameter of any zone of inhibition is recorded, although bacterial colonies are often difÀcult to see with the naked eye. Addition of tetrazolium chloride dye to stain bacterial colonies pink and use of a microscope aids evaluation. A quicker method employs nutrient agar. If there is no bacterial growth in the agar around the plastic of an untreated control sample, then it does not contain nutrient components to support growth. The detection of colonies growing in a non-nutrient agar (incomplete mineral salt) means that the nutrients for

15

Biocides in Plastics

E. Coli

Staph. Aureus BACTERIA

Ps. Aeruginosa

Candida albicans YEAST

Rhodotorula rubra

Figure 18 Plastic Àlm with test method ISO 846 C (a.5) with a variety of different organisms, including some yeast

growth also have to migrate from the plastic. Such a test is therefore highly dependent on the food sources available to the bacteria and is therefore only suitable for testing a speciÀc PVC formulation against speciÀc bacteria (Figure 18).

4.2.1.2 AATTC 147:2004 Antibacterial Activity Assessment of Textile Materials: Parallel Streak Method (a.14) This is a method originating from the textile industry but is sometimes also used as a base for testing textiles or other substrates coated with PVC or PU. A swab of pure bacteria, usually Staphyloccocus aureus or Klebsiella sp., but others can also be used, is drawn across nutrient agar in Àve parallel lines. The sample for testing is then intimately placed across the Àve streaks. If the biocide migrates from the sample, it will prevent growth of the bacterium near the edge and under the sample itself. This method relies on migration of the biocide to provide a positive result but also is sometimes problematic as bacteria may not grow in the anaerobic conditions that may be present under the sample itself. This could lead to the incorrect conclusion that a blank sample (without a biocide) may inhibit bacterial growth when this may not be the case. Low migratory or non-migratory biocides will therefore likely fail such a test. In addition, initial zones, caused by temporary inhibition or extended lag phase may eventually cause the plates to be covered by bacteria over an extended period (Figure 19).

16

Figure 19 AATTC 147:2004 plate showing small zone of inhibition using ACTICIDE® PLN 9 against the organism, Staphylococcus aureus after a few days incubation

4.2.1.3 Kirby-Bauer Disk-Diffusion Method and ModiÀed Kirby-Bauer Susceptibility Test (a.15) This method originates from work carried out by Kirby and Bauer in the 1950s looking at single disc antibiotic sensitivity of Staphylococci. Some laboratories and organisations have adapted this in vitro method used as a guide for in vivo infections to utilise it in the plastics sector (Figure 20). It is essentially very similar to the ISO 846 C test method (a.5), whereby different biocides or concentrations of biocides within a plastic are examined. Plastic samples

Biocides in Plastics

stipulate elimination of viable bacteria over a set time period, as the former can allow for a bacterial growth lag. There are fears that this 2 log reduction could permit microbial adaptation to the antimicrobial and eventually could lead to the development of tolerant strains of microorganisms. Staphylococcus aureus, Klebsiella pneumoniae, E. coli are often used in these tests. One such test, the Japanese standard JIS Z2801 (a.1) or Àlm adherence method, determines the ‘antimicrobial effect’ of plastic/textile materials by measuring a 2 log reduction (99%) in number of bacteria in contact with a treated plastic over 24 hours compared with a biostabiliser-free plastic. A bacterial cell suspension is sandwiched between a PVC test piece and an inert material such as a glass cover slip. A humid environment is maintained by incubation in a petri dish. After typical contact times of 1-24 hours at 35 ºC the test surfaces are rinsed in saline and the viable bacteria enumerated (Figures 21 and 22).

Figure 20 Disk diffusion in agar

are cut into small discs and dropped on agar containing an inoculum of a speciÀc bacterial species. It is then incubated for a number of hours and a zone of inhibition against the growth of the bacterium demonstrates the migration of the biocide from the plastic into the agar.

4.2.2 Antimicrobial Plastic Some methods, based on standard disinfectant protocols, state a requirement for reduction of bacterial cells in a one hour period (e.g., a 2 log reduction - a 99% decrease in the number of viable bacteria). Others

It is possible, however, that the cover slip could cause the cell suspension to be artiÀcially depleted of oxygen, which will inÁuence the survival or growth of the bacteria. To avoid anaerobic conditions, a porous acetate membrane Àlter can be used to cover the inoculum. This modiÀed technique is known as the membrane Àlter method. Recent work by the IBRG has, however, shown little inÁuence on bacterial recovery rates when varying the type of cover used. As with all ‘antimicrobial effect’ test methods, the pass and fail criteria are debatable. Log 2 (99%) reduction

Place test piece in a petri-dish and inoculate Incubate for Prepare bacterial cell 24 hrs at 20 °C 6 -1 suspension - 10 /ml

Membrane Àlter Cell suspension Test surface

Cover with sterile membrane Àlter Carry out Total Viable Count (TVC) Transfer Àlter and swab to neutraliser

Swab surface

Figure 21 Scheme of method used in JIS Z2801

17

Biocides in Plastics

Escherichia coli Log reduction (20 mins extrusion samples) Control

2.00E+03 1.80E+03 1.60E+03 1.40E+03 1.20E+03 1.00E+03 8.00E+03 6.00E+03 4.00E+03 2.00E+03 0.00E+03

1 OPBA 2 ZP + OIT 3 OIT 4 Butyl parabens 5 Silver 6 Butyl BIT

Figure 22 Typical log reduction data for JIS Z 2801 test method for a variety of actives

is often used as a measure of acceptance, but this is clearly not desirable where slower acting, long-term biostabilisers are employed.

plastic, not on the growth of the organism or the zone of inhibition that might be produced. The organism is not thought to cause deterioration of plastic but it produces a soluble pink pigment that diffuses through the agar to stain the plastic (a.7) (Figure 23).

4.2.3 Pink Stain Test Perhaps ASTM E1428 (a.16), commonly referred to as the ‘Pink Stain’ test, is one of the most recognised plastic test methods. The actinomycete Streptoverticillium reticulum is used as an indicator organism to inoculate yeast malt extract agar. Test discs of plastic are placed on the surface and incubated at 29 ºC for three weeks. Assessment focuses on the extent of staining of the

This pigment is a prodigiosin-like pigment, actually a mixture of two prodiginines (a.17) but is not exclusive to Streptoverticillium sp. Absence of staining indicates the effectiveness of the biostabiliser when compared to a control plastic. Streptoverticillium waksmanii produces a similar effect.

Figure 23 ASTM E1428 ‘pink stain’ (178) test of unprotected PVC samples

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Biocides in Plastics

4.3 Laboratory Tests versus use Conditions Laboratory tests can only provide an idea of actual performance in the Àeld. They cannot usually account for long-term performance of products because accelerated weathering tests such as QUV or leaching methods cannot truly mimic the conditions in practice, partly because of the huge variability in geographical weather conditions. Although they may give an indication of performance, such tests show excessive effects of strong UV and the hydroscopic effect of the weathering process. This may not be so pronounced or so intense in ‘Àeld’ use because a ‘depot’ of biocide may be present within a plastic matrix. The migration within the plastic itself may be the limiting factor rather than the hydroscopic effect of artiÀcial weathering (Figure 24) . Of course not all plastics are used in an exterior environment and therefore do not need weathering. In many other cases alternate wear and in-use characteristics may be relevant such as soiling, scufÀng or abrasion caused by trafÀc or cleaning and general wear and tear. These cannot really be adequately accounted for by laboratory testing. The most realistic way to determine true antimicrobial performance of a biocide containing plastic is to test it in the Àeld against a sample that does not contain a biocide (as well as against other biocides). Such comparative testing is however sometimes difÀcult, especially for companies that may not produce or have control over the Ànal product such as masterbatch manufacturers and the biocide suppliers themselves. Often end producers may not have sufÀcient microbiological expertise to

assess biocide performance adequately and so a good partnership between biocide company, masterbatch producer and Ànal article manufacturers is important.

5 Available Active Ingredients There are many organic and inorganic actives available for use in plasticised PVC and other plastics. The major actives are summarised next and are split into two categories according to whether they migrate readily or not.

5.1 Migratory Biocides

5.1.1 OBPA Known widely by its trade name Vinyzene (Morton now part of Rohm and Haas) this is typically available in 2% or 5% concentration in plasticiser or PVC pellets, 10,10 oxybisphenoxarsine (OBPA; Figure 25) was initially registered in the United States in 1965. It is used at a dose rate of 0.6-5% based on the total weight of the formulation. OBPA currently accounts for about half of the market for plastic biostabilisers worldwide but this level is slowly diminishing due to environmental and health concerns. These considerations are highlighted by the potential leaching of arsenic from plastic articles

hydroscopic effect removes active

Leaching

1 day

surface depleted and fails in subsequent tests

2 days

5 days

depot of biocide remains in plastic matrix

less aggressive hydroscopic effect removes active more slowly surface active removed and continually replaced depot of biocide can migrate to upper layers to replace that lost

actual Àeld use time in months/years

Figure 24 Available surface active may be limited by external leaching or by internal migration within the plastic

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Biocides in Plastics

leachability. Additionally results of Redlich highlight poor UV stability. A relatively low water solubility of 5 mg/l causes confusion since OBPA has a tendency to migrate with some plasticisers used in PVC thus creating large zones of inhibition in some agar plate test methods, as discussed in section 4.1.2.

Figure 25 Structure of 10,10 oxybisphenoxarsine

disposed of in landÀll sites and it has been hypothesised that toxic trimethylarsenate can be released following fungal degradation by Scopulariopsis brevicaulis. This latter theory has perhaps erroneously been linked to cases of Sudden Infant Death Syndrome, along with a number of other possible contributory chemicals such as antimony Áame-retardants, in PVC covered cot mattresses (a.18). The levels of arsenic are however fairly low which is often sufÀcient to achieve the MIC against the majority of fungi and bacteria of 10 mg/l. Upsher & Roseblade (a.19) have reported in 1984 that long-term activity of OPBA exposed at a jungle site is limited due to its

Large zones of inhibition created by OBPA have for some while been used as a marketing tool for OBPA suppliers to extol the advantages over other actives that may not produce such large zones in typical ASTM G21 (a.6) or ISO 846 (a.5) test methods. However, potentially a large zone does mean the product migrates readily from the plastic in which it is added which means that any active ingredients are lost fairly readily in comparison to actives which exhibit smaller zones in such tests (Figure 26).

5.1.2 OIT N-Octyl-isothiazolinone (OIT; Figure 27) has been widely available in many applications for more than 25 years and more recently is being used in the plastic

Figure 27 Structure of N-octyl-isothiazolinone

Figure 28 Figure 26 ISO 846 A (top) and ISO 846 B (bottom) tests for PVC containing OBPA (5% pellet type) at extrusion time of 20 minutes and 120 minutes at 180 ºC

20

ISO 846 A (top) and ISO 846 B (bottom) tests of a PVC with ACTICIDE® PLN 9 (9% OIT in diisononyl phthalate (DINP)) at extrusion time of 20 minutes and 120 minutes at 180 °C

Biocides in Plastics

industry. The major producers include Thor as well as Rohm and Haas with tradenames of ACTICIDE® PL and Vinyzene IT, respectively. It is available from 5% to 45% concentrations in a variety of plasticisers, carriers and PVC pellets. Its relatively high water solubility of 480 g/l at 25 ºC confers some advantages: providing good anti-fungal effects in interior applications (Figure 28). Conversely, it can be subject to excessive leaching where precipitation is high in exterior locations. OIT is inactivated by reducing agents and is classed as a skin sensitiser in its liquid form, although once bound in PVC poses no hazard in most applications. It provides good performance in many applications against most fungal species (MIC from 0.5-10 ppm for the majority of fungi), although is only an effective bactericide against a few bacterial species. It is one of the main replacements for heavy metal free alternatives to OPBA.

5.1.3 Butyl BIT Available from Arch chemicals (previously Avecia biocides), BBIT (Figure 29) has a similar solubility to the more commonly used octyl isothiazolinone, this patented and relatively new (to PVC) active is promoted as giving similar performance to OIT.

Figure 30 ISO 846 A (top) and ISO 846 B (bottom), tests of PVC incorporating BBIT (100%) at 0.2 phr and extruded at 20 minutes and 106 minutes at 180 ºC

5.1.4 Zinc Pyrithione

It also has good activity against some bacteria but is weak against Pseudomonas aeruginosa, as well as some Enterobacter and Klebsiella species. The toxicological profile of BBIT is good (except in vitro mutagen positive, in vitro cytotoxicity (IVC) tests) and like all isothiazolinones, it is a skin sensitiser. It is probably the most costly isothiazolinone active currently available for PVC applications. In comparable tests at extremes of temperature the stability of BBIT in PVC formulations is shown to be poorer than OIT (Figure 30).

Well known for its use as an anti-dandruff agent, zinc pyrithione (Figure 31) is typically supplied as a powder that has solubility in water of 20 mg/l. It is produced by a number of companies like Arch chemicals, Rutgers Organics as well as many Chinese producers and is therefore available from many suppliers. It is often promoted as an anti-bacterial additive although its activity is mainly anti-fungal. MIC against fungi range from only 7.5 ppm against Alternaria alternata to 50 ppm against species such as Trichoderma and up to 100 ppm for Aspergillus niger. The range is similar for bacteria, with only 10 ppm against Staphylococcus sp., but up to 400 ppm for Pseudomonas aeruginosa. Dose rates required to achieve good efÀcacy vary from 1000 ppm to more than 4000 ppm active ingredient depending on product and processing conditions.

Figure 29

Figure 31

Structure of butyl BIT

Structure of zinc pyrithione

It is a good fungicide conferring a similar degree of protection as OIT in MIC tests, possessing activity against most fungal species with a few exceptions (MIC for Penicillium sp. < 5 ppm).

21

Biocides in Plastics

Patented systems (51) try to overcome this problem by inclusion of an hydrotalcite. Hydrotalcite is a naturally found layered hexagonal structure of carbonates sandwiched between layers of magnesium and aluminium hydroxides that can also be produced synthetically. It can be used with calcium/zinc heat stabilisers and acts by scavenging chloride ions (acid scavenger) effectively swapping them for the carbonate ions within the layered lattice. These solutions do add an extra cost but are an option to reduce yellowing. Where other plastics, such as PE or other polyoleÀns are concerned (where it is used as an antibacterial additive) high dose levels (in excess of 1000 ppm) may be required but the yellowing issue is not so signiÀcant. This effect is demonstrated by zone of inhibition testing and therefore the active migrates from the plastic.

Figure 32 ISO 846 A (top) and ISO 846 B (bottom) tests of PVC samples containing zinc pyrithione at extrusion times of 20 minutes and 60 minutes at 180 ºC

Zinc is a crucial element in the formulation of many heat stabilisers used in PVC, however excessive use can have a negative impact on the degradation of the PVC (30). This is because zinc pyrithione typically contains 20% zinc. Some PVC formulations, particularly those calendered or extruded under higher sheer, are zinc sensitive as they react with hydrogen chloride and form zinc chloride, which catalyses further degradation of the PVC. The outcome is yellowing and poor PVC stability (Figure 32).

5.1.5 Iodo-Propylbutyl Carbamate (IPBC) IPBC is an iodine containing fungicide used mainly in coatings but sometimes used in plastics applications (Figure 33). Various suppliers offer IPBC powder and IPBC within a plasticiser carrier but its use in plastics at present is not widespread. It is not particularly stable at elevated temperatures and can be prone to strong discoloration due to the iodine component (Figure 34). Degradation products

Figure 33 Structure of IPBC

Figure 34 Oxime sealant with no biocide, ACTICIDE® PIN 8 (8% IPBC in diisononyl phthalate carrier), 20% IPBC (in glycol ether) and 30% IPBC (in glycol), respectively, before and after heating to 50 ºC

22

Biocides in Plastics

can, however, still be microbiologically active and so if discoloration is not an issue, as with backing layers and heavily pigmented plastics, IPBC can be used.

5.1.6 N-Haloalkylthio Compounds

Figure 36 Structure of captan

5.1.6.1 Folpet (N-Trichloromethylthiophthalimide) Folpet is an N-haloalkylthio compound available from Bayer under the Preventol trademark (Figure 35). These are typically white powders and have been used in some plastics applications. They are however reactive with sulfides and mercapto compounds and have various weaknesses against some fungal species such as Trichoderma sp., and Alternaria sp., which means they are rarely used in isolation.

Figure 35 Structure of Folpet Technical Folpet is usually 90% pure. The main impurities are phthalimide (up to 4.0%) and sodium chloride (up to 5%). Pure Folpet is a white crystalline solid with a reported melting point of 177 °C. Solubility in water is only 1 ppm at room temperature. In the dry state, it is stable at room temperature, but it is hydrolysed in an aqueous solution at a rate that depends on the pH. Degradation products and hydrolysis in water can be carbon dioxide, hydrochloric acid, hydrogen sulÀde, phthalamic acid, and phthalic acid.

is not so commonly used in plastics but is structurally very similar to Folpet. This is perhaps because it was in the past classiÀed as a mutagen and carcinogen. It has, however, good microbiological activity with MIC values in the range 25-100 ppm. The Environment Protection Agency (EPA) has released an amendment to the 1999 Reregistration Eligibility Document (RED) for captan - the comment period closed January 24, 2005. The amendment makes minor changes to the original RED, and the EPA changed the cancer classiÀcation for captan. Captan has been registered for more than 40 years. The fungicide is used to control diseases in orchard crops, ornamentals, and turf. It is also used as a seed treatment and as a preservative in paints and adhesives. Captan is a serious eye irritant and until recently, EPA classiÀed captan as a probable human carcinogen. The Agency concluded that captan is only a potential human carcinogen at an exposure threshold signiÀcantly greater than likely dietary or non-dietary exposure. In other words, the EPA concluded that labelled uses of captan are unlikely to cause human cancers (EPA Pesticide Program Update, 12-9-04).

5.1.6.3 Fluorfolpet (N-DichloroÁouromethylthiophthalimide) Again, this compound (Figure 37) works in a similar manner to the actives described previously. Mura and co-workers (178) have shown by microscopy that this active is readily leached from PVC products,

5.1.6.2 Captan (N-(trichloromethyl-thio-4cyclohexene-1,2-dicarboximide)) Another N-haloalkylthio compound, captan (Figure 36), was Àrst introduced in 1949 and is a white crystalline powder of very low water solubility of 2 ppm. Since captan decomposes slowly when heated to its melting point, a melting point range of 158-170 °C has been reported for the technical product. This active ingredient

Figure 37 Structure of Áuorfolpet

23

Biocides in Plastics

despite its relatively low water solubility of 0.015 g/l. In water it hydrolyses rapidly and is degraded by alkaline products, amines and sulÀdes. All the N-haloalkylthio compounds have good MIC against a variety of fungal species and against algae but have a weakness against the fungi Trichoderma viride.

It is thermally stable up to 170 ºC and has solubility in water of about 33 ppm. Its primary promotion has been in the wood preservation industry as it is effective against a range of wood decay organisms. Its use in the plastics industry is only so far promotional. It is a white to yellow solid that is both metal and halogen free (Figure 40).

5.1.7 Carbendazim (N-benzimidazol2-ylcarbamic acid methylester)

SigniÀcantly reduced performance shown following exposure to QUVTM means it is not likely to be as effective as DCOIT in exterior applications.

This pale grey powder has been widely used as a fungicide in a variety of different applications for many years (Figure 38). Its very low water solubility of