Metallic Pigments in Polymers

Metallic Pigments in Polymers

Ian Wheeler

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

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First Published 1999 by

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

©1999, Rapra Technology Limited

The right of Ian Wheeler to be recognised as author of this Work has been asserted by him in accordance with sections 77 and 78 of the Copyright, Designs and Patents Act 1998. 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.

ISBN: 1-85957-166-2

Typeset by Rapra Technology Limited Printed and bound by Polestar Scientifica, Exeter, UK

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Contents

1

2

Introduction and History ................................................................................ 3 1.1

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

1.2

Origins and history ................................................................................ 4

Metal Pigment Types ....................................................................................... 7 2.1

Aluminium ............................................................................................. 7

2.2

Gold bronze ........................................................................................... 8

2.3

Copper ................................................................................................... 8

2.4

Nickel .................................................................................................... 8

2.5

Stainless steel ......................................................................................... 9

2.6

Zinc ....................................................................................................... 9

2.7

Iron ........................................................................................................ 9

2.8

Tin ....................................................................................................... 10

2.9

Silver .................................................................................................... 10

2.10 Gold ..................................................................................................... 10 2.11 Other metals ........................................................................................ 10 3

Manufacture ................................................................................................. 11 3.1

3.2

Dry milling........................................................................................... 11 3.1.1

Aluminium ............................................................................... 12

3.1.2

Gold bronze ............................................................................. 12

Continuous dry milling ........................................................................ 13 3.2.1

Aluminium ............................................................................... 13

3.2.2

Gold bronze ............................................................................. 13

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3.3

Wet milling .......................................................................................... 14 3.3.1

Aluminium ............................................................................... 14

3.3.2

Gold bronze ............................................................................. 19

3.3.3

Silver ........................................................................................ 19

3.3.4

Nickel ...................................................................................... 20

3.3.5

Stainless steel ........................................................................... 21

3.3.6

Zinc ......................................................................................... 22

3.4

Continuous wet milling ........................................................................ 22

3.5

Spherical metal pigments ..................................................................... 22

3.6

Rapidly solidified flakes ....................................................................... 23

3.7

Vacuum deposition .............................................................................. 24

3.8

Cut foil glitters ..................................................................................... 25

3.9

Flakes with coloured surfaces .............................................................. 25 3.9.1

Chemical modification of metal surfaces ................................. 26

3.9.2

Colour formation in situ .......................................................... 27

3.9.3

Attachment of pre-formed colorants ........................................ 28

3.10 Metal coatings on non-metallic substrates ........................................... 29 3.11 Other methods ..................................................................................... 30 References ..................................................................................................... 31 4

Pigment Characteristics ................................................................................. 35 4.1

4.2

Morphology ......................................................................................... 35 4.1.1

Particle size .............................................................................. 35

4.1.2

Particle shape ........................................................................... 38

4.1.3

Aspect ratio ............................................................................. 39

4.1.4

Surface uniformity ................................................................... 41

Physical properties ............................................................................... 41 4.2.1

Specific gravity ......................................................................... 41

4.2.2

Water covering area ................................................................. 41

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4.3

4.4

4.2.3

Specific surface area ................................................................. 42

4.2.4

Heat and lightfastness .............................................................. 42

4.2.5

Chemical resistance .................................................................. 43

4.2.6

Magnetism ............................................................................... 43

Visual properties .................................................................................. 43 4.3.1

Colour and brightness.............................................................. 44

4.3.2

Opacity .................................................................................... 45

4.3.3

Flop ......................................................................................... 45

4.3.4

Leafing and non-leafing ........................................................... 47

4.3.5

Sparkle ..................................................................................... 50

4.3.6

Distinctiveness of image ........................................................... 50

Glitter flakes ........................................................................................ 52

References ..................................................................................................... 53 5

Delivery Forms .............................................................................................. 55 5.1

Dry powder.......................................................................................... 55

5.2

Paste .................................................................................................... 55

5.3

Dispersion in resin and solvent ............................................................ 56

5.4

Plasticiser dispersions ........................................................................... 56

5.5

Granules .............................................................................................. 57

5.6

Dry masterbatch .................................................................................. 59

5.7

Liquid masterbatch .............................................................................. 59

5.8

Compound ........................................................................................... 60

References ..................................................................................................... 61 6

Comparison of Mass Pigmentation and Coating ........................................... 63 6.1

Advantages of coating .......................................................................... 63 6.1.1

Brightness ................................................................................ 63

6.1.2

Colour uniformity .................................................................... 64 iii

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Metallic Pigments in Polymers

6.2

6.1.3

Flop ......................................................................................... 64

6.1.4

Application temperature .......................................................... 65

6.1.5

Vacuum metallisation .............................................................. 65

Mass pigmentation advantages ............................................................ 65 6.2.1

Depth of coloration ................................................................. 65

6.2.2

Single stage versus multistage processing ................................. 66

6.2.3

Environmental and legislative pressures ................................... 66

6.2.4

Cost ......................................................................................... 67

References ..................................................................................................... 67 7

Mass Pigmentation Application Characteristics ............................................ 69 7.1

Colour ................................................................................................. 69

7.2

Dispersibility ........................................................................................ 69

7.3

Opacity and tint strength ..................................................................... 70

7.4

Orientation .......................................................................................... 71

7.5

Mechanical properties .......................................................................... 71

7.6

Cost ..................................................................................................... 75

7.7

Interrelationships ................................................................................. 76

7.8

Compatibility ....................................................................................... 77

7.9

Spherical metal pigments ..................................................................... 78

7.10 Metal flake pigments with coloured surfaces ....................................... 78 7.11 ‘Glitter’ flakes ...................................................................................... 79 8

Flow and Weld Lines in Mass Pigmented Applications ................................. 81 8.1

Description and origins ........................................................................ 81

8.2

Tool design for injection moulding ...................................................... 85

8.3

Orientation in multiphase and glass filled polymers ............................ 87

8.4

Orientation of metal pigments ............................................................. 89

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8.5

Mould tool design for metal pigments ................................................. 92

8.6

Gates, sprues and runners .................................................................... 92

8.7

Tool texturing ...................................................................................... 94

8.8

Additional cavity ................................................................................. 94

8.9

Dynamic melt techniques ..................................................................... 94 8.9.1

SCORTEC ............................................................................... 94

8.9.2

Other techniques ...................................................................... 97

8.10 Localised mould heating ...................................................................... 98 8.11 Other techniques .................................................................................. 99 References ..................................................................................................... 99 9

Formulation of Mass Pigmented Polymers .................................................. 105 9.1

General techniques ............................................................................. 105

9.2

Optimising the formulation ............................................................... 105

9.3

9.2.1

Flake size ............................................................................... 106

9.2.2

Flake concentration ............................................................... 106

9.2.3

Polymer transparency ............................................................ 107

9.2.4

Polymer viscosity ................................................................... 107

9.2.5

Metallic/organic pigment combinations ................................. 108

9.2.6

Deep shades ........................................................................... 108

9.2.7

Spherical pigments ................................................................. 108

Incorporation in polymers ................................................................. 109 9.3.1

Low shear forces .................................................................... 109

9.3.2

Improvement of flake orientation .......................................... 110

9.4

Increasing pigment quality ................................................................. 110

9.5

Summary ............................................................................................ 111

References ................................................................................................... 112 10 Conversion Processes .................................................................................. 113 v

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Metallic Pigments in Polymers

10.1 Injection moulding ............................................................................. 114 10.2 Blow moulding .................................................................................. 115 10.2.1 Blown film ............................................................................. 115 10.2.2 Blown containers ................................................................... 116 10.3 Extrusion ........................................................................................... 117 10.4 Co-extrusion ...................................................................................... 117 10.5 Paint-less film moulding ..................................................................... 117 10.6 In-mould decoration .......................................................................... 118 10.7 Vacuum forming and thermoforming ................................................. 118 10.8 Rotational moulding .......................................................................... 118 10.9 Glass reinforced plastic ...................................................................... 119 10.10 Thermosetting polymers .................................................................... 120 References ................................................................................................... 121 11 Applications of Mass Pigmented Systems .................................................... 123 11.1 Household goods ............................................................................... 123 11.2 Sports goods ...................................................................................... 124 11.3 Agricultural film ................................................................................ 125 11.4 Sacks and bags ................................................................................... 125 11.5 Containers ......................................................................................... 126 11.6 Automotive ........................................................................................ 126 11.7 Pearl simulants ................................................................................... 127 11.8 Mineral simulants .............................................................................. 128 11.9 Fibres and textiles .............................................................................. 128 References ................................................................................................... 129 12 Metal Pigmented Coatings .......................................................................... 131

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12.1 Substrate preparation ......................................................................... 131 12.2 Coating formulation and properties ................................................... 132 12.2.1 Pigment particle size .............................................................. 132 12.2.2 Concentration ........................................................................ 132 12.2.3 Leafing and non-leafing ......................................................... 133 12.3 Dispersion and incorporation ............................................................ 133 12.4 Application to the substrate ............................................................... 134 12.5 Solvent based systems ........................................................................ 134 12.6 Water-based systems .......................................................................... 135 12.7 UV/EB cured coatings ........................................................................ 139 References ................................................................................................... 140 13 Applications of Metal Pigmented Coatings ................................................. 145 13.1 Painting.............................................................................................. 145 13.1.1 Solvent-based paints .............................................................. 146 13.1.2 Water-based paints ................................................................. 148 13.1.3 In-mould coating ................................................................... 152 13.1.4 Miscellaneous paints .............................................................. 153 13.2 Printing .............................................................................................. 153 13.2.1 Solvent-based inks ................................................................. 154 13.2.2 Paste inks ............................................................................... 154 13.2.3 Water-based inks .................................................................... 156 13.2.4 Laminates .............................................................................. 157 13.2.5 Security Inks .......................................................................... 157 13.2.6 Bronze replacement ................................................................ 158 13.3 UV cured coatings .............................................................................. 161 13.4 PVC Plastisols .................................................................................... 162 13.5 Anticorrosive and barrier coatings ..................................................... 162 13.6 Other applications ............................................................................. 163 vii

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14 Powder Coatings ......................................................................................... 167 14.1 Material types and properties ............................................................ 168 14.2 Manufacture ...................................................................................... 168 14.2.1 Dry blend ............................................................................... 169 14.2.2 Bonding ................................................................................. 169 14.2.3 Co-extrusion .......................................................................... 171 14.2.4 Coated flakes ......................................................................... 172 14.2.5 Other technologies ................................................................. 172 14.3 Formulation, application techniques and markets.............................. 173 14.4 Safety and handling ........................................................................... 175 References ................................................................................................... 176 15 Non-colouristic Applications ...................................................................... 179 15.1 Mechanical reinforcement .................................................................. 179 15.2 Microwave heating ............................................................................ 180 15.3 Electrical conductivity ........................................................................ 183 15.3.1 Product forms ........................................................................ 183 15.4 EMI shielding .................................................................................... 184 15.4.1 Origin and measurement ........................................................ 185 15.4.2 Legislative requirements......................................................... 185 15.4.3 Shielding principles and techniques ........................................ 185 15.4.4 Shielding of polymers ............................................................. 186 15.4.5 Coating techniques ................................................................ 187 15.4.6 Mass pigmentation techniques ............................................... 191 15.5 Light exclusion .................................................................................. 192 15.6 Heat and light reflection .................................................................... 193 15.7 Thermal conductivity ......................................................................... 194 15.8 Lubrication and wear reduction ......................................................... 196 15.9 Gas and moisture barrier ................................................................... 196

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15.10 UV protection ................................................................................. 196 15.11 Laser marking ................................................................................. 197 15.12 Magnetic applications ..................................................................... 198 15.13 Corrosion resistance ........................................................................ 199 15.14 Flame retardation ............................................................................ 199 15.15 Radiation absorption ...................................................................... 200 References ................................................................................................... 200 16 Health, Safety and Handling ....................................................................... 203 16.1 Health ................................................................................................ 204 16.1.1 Aluminium ............................................................................. 205 16.1.2 Gold bronze and copper ........................................................ 206 16.1.3 Other metal pigments ............................................................ 207 16.2 Safety ................................................................................................. 207 16.2.1 Aluminium ............................................................................. 208 16.2.2 Gold bronze ........................................................................... 209 16.2.3 Other metal pigments ............................................................ 210 16.3 Health and safety in use ..................................................................... 210 16.3.1 Mass pigmentation ................................................................ 211 16.3.2 Coatings ................................................................................ 211 16.4 Environment ...................................................................................... 212 16.5 Handling, storage and disposal .......................................................... 212 16.5.1 Aluminium ............................................................................. 213 16.5.2 Other metals .......................................................................... 213 16.5.3 UV grades .............................................................................. 213 16.6 Fire fighting ....................................................................................... 214 16.6.1 Aluminium ............................................................................. 214 16.6.2 Other metals .......................................................................... 215 References ................................................................................................... 215

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Metallic Pigments in Polymers

Author Index ..................................................................................................... 221 Company Name Index ....................................................................................... 225 Main Index ........................................................................................................ 227

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Preface and Acknowledgements

The reader expecting this text to commence with the words “Much has been written about…” will be sadly disappointed. In the case of metallic pigments, quite the opposite is the case. The reason lies partly in the narrow nature and therefore limited appeal of the subject matter. More importantly however, relatively few companies, generally using the same raw materials, control the industry. It follows that one product is differentiated from another by the process of manufacture. Much of this is the type of know-how that is not easily patented and effectively policed. In consequence, manufacturers tend not publish their foundation technology. When it comes to elaborating the basic pigment however, there has been an explosion of patent literature, perhaps because such products are now easily identified. From the mid-eighties onwards, surface coloration and adaptation for water-based systems have been the main thrusts. Though there is now a substantial patent literature and a modest amount of reported academic endeavour, there remains a dearth of books on the subject. Indeed the last book devoted specifically to metal pigments, entitled ‘Aluminium and Bronze Flake Powders’ was written by G.W. Wendon and published by Electrochemical Publications as long ago as 1983. Prior to that, one has to go back to 1955 to find a comparable volume, that of Junius Edwards and Robert Wray, entitled ‘Aluminium Paint and Powder’ and published by Reinhold. Both these books were published before the recent upsurge in interest in metal pigments. Increasing interest is not surprising, given the range of visual effects uniquely available from modern metal pigments. This is in addition to their many functional uses. The purpose of this book is to bridge the technology gap since 1983 by providing a comprehensive account of the nature, manufacture, formulation and applications of the diverse metallic pigments commercially available today. Whilst the text concentrates on direct pigmentation of polymers, there are also two chapters on metal pigment coatings for polymer substrates. In addition to the familiar colouristic applications, there is a chapter on the many, often novel functional applications in which colour is either incidental or irrelevant. The tone is practical, rather than theoretical. The intention is to teach the features and benefits of this novel class of pigments, to allow users to achieve the best possible visual result with the most economical cost in use. It is also hoped that those not already familiar with metallic pigments will be persuaded to explore their potential to add value to their products. 1

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Metallic Pigments in Polymers To provide as broad a perspective as possible, all the major manufacturers were invited to contribute material, providing it was not promotional. Their contributions and those of others are gratefully acknowledged as follows: Jim Allan, Irvine Davidson, Sean Earnshaw and David Roach at the University of St. Andrews, electron micrographs, optical microscopy and photographic services. Professor Michael Bevis and Dr Keith Rawson at the University of Brunel and Steve Jordan at Cinpres Inc., SCORTEC. Dr Helge Friesenhan of Carl Schlenk A.G., Copper and gold bronze pigments. Dr Anthony Hart of Hart Coating Technology, nickel, stainless steel and related pigments. Use of photographs in Figures 3.6, 3.7, 15.2, 15.3, 15.4, 15.6 and the graph in Figure 15.5. Dr Margaret Henderson, John Maynard, Dr Geoff Ormerod and Joanne Mitchell of Wolstenholme International, gold bronze pigments, including formulations in Chapter 13. Christine Watters and Colin Hindle at Napier University, Edinburgh, EMI shielding. Chris Williams, Permission to reproduce Figs 15.1 and 15.6 from his book, ‘The Printer’s Ink Handbook’, Maclean Hunter Ltd, Barnet UK, 1992. Many colleagues at Silberline Ltd., UK and Silberline Inc., USA, including David Chapman, Russell Ferguson, Rob Gillan, John Kerr, Steven Kerr, Dr David King, Richard Knowles, Dr Jonathan Knox, Derek Morris, Brian Seath, Dr Malcolm Stock and Dennis Thomson. Special thanks are due to Frances Powers, my technical editor, for her encouragement, enthusiasm and guidance during the writing of this manuscript. Ian Wheeler May 1999

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Introduction and History

1.1 Introduction Colour is one of the key factors influencing a customer buying a retail product. In today’s highly competitive and fast moving markets, all manufacturers are looking for the features that will sell their products rather than those of their competitors. In what are loosely referred to as ‘fashion’ markets, visual appeal is particularly important and tends to come well ahead of functionality. It is in such markets that metal pigments excel. Metal pigments are a small but important and versatile class of colouring agents, composed of fine particles of malleable metals in elemental form. Their significance comes from their ability to provide stunning visual effects that are impossible using traditional organic and metal compound pigments. Their nearest relatives are the pearlescents, composed of flakes of mica, now often coated with titanium dioxide or otherwise surface modified. Such pearl pigments share the same lamellar form, but although potentially whiter than metal pigments, they lack opacity. Thus each finds its place in the market according to the envisaged application. As might be expected, metal pigments retain many of the attributes of the bulk metal, such as solidity (opacity), metallic colour, sheen, malleability, ductility and density. Their use as colouring matter reflects this. Aluminium and gold bronze pigments provide the appearance of silver and gold, respectively, with the aura of quality and prestige that these metals imply. For this reason, metal pigments are often used in prestige applications to add value. The decision to replace solid metal by metal pigmented, coated or mass pigmented polymers can be taken for several other reasons. Design flexibility, ease of fabrication, weight saving, and of course cost reduction are all driving forces. Strictly speaking, metal pigments are a class of inorganic pigment. Nevertheless, the term ‘inorganic pigment’ throughout this book will be reserved for pigments in which the metallic element is chemically combined with at least one other element. A ‘metal pigment’ is therefore defined as a single metallic element or an alloy thereof. In industry the term metal pigment tends to be used somewhat loosely. It encompasses not only a range of morphology from flakes and fibres to spheres, but also applications

3

Metallic Pigments in Polymers going beyond the technical definition of a pigment as an insoluble colouring agent. Thus metal ‘pigments’ find use in many applications in which their colouristic properties are either of limited importance or completely irrelevant. Chapter 15 is devoted to these non-aesthetic applications. The title ‘Metallic Pigments in Polymers’ has been widely interpreted to include not only direct coloration of polymers but also coatings involving combination of metal pigments with polymeric or resinous media. One area not covered in any depth is their use in elastomers. Metal pigments are not widely used in rubbers for several reasons. Most rubbers are inherently coloured, sometimes strongly so and this often makes it difficult to achieve a bright metallic effect. Flexibility and mechanical properties also tend to be adversely affected.

1.2 Origins and history The origins of metal pigments can be traced back to the ancient art of gold beating. Early civilisations, notably the Egyptians, would work the gold into very thin sheets and then overlay wood, bone, or other materials with the precious metal. This art spread to the Far East, India and eventually Europe. As trade developed and demand increased, it became necessary to make thinner and thinner foil. Inevitably, the edges of the very thinnest foils would tend to break off. However, it was soon discovered that by placing these loose particles in a suitable binder, the gold leaf effect could be maintained. Carrying this process a step further, very thin leaf was rubbed through fine screens to generate a gold powder that could be used for ornamental artwork or printing inks. Eventually, because of the extremely high cost of gold, substitutes were sought. Thus gold bronze came into use, which although gold in colour, it is neither metallic gold nor bronze. Gold bronze, the oldest of the gold simulants, is an alloy of copper and zinc, rather than the copper-tin alloy of true bronze. Later, silver and tin were combined to make a silver bronze powder, but the discovery of aluminium smelting was eventually to lead to the development of the largest class of metal pigment. Gold bronze flake pigments were made in Germany as early as 1820. A key advance in the process was the development of the mechanical stamping process by Sir Henry Bessemer in the middle of the 19th century. The process consisted of steel hammers, which fell on steel anvils, thus forming the metal into the flake form recognisable in modern metal pigments. In this way, Bessemer was able to mitigate the very high cost of gold and silver bronze powders. Newly developed smelting processes made aluminium available in quantity. It was quickly introduced to the stamping process as a cheaper

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Introduction and History substitute for silver bronze. To prevent cold welding of the malleable metal, small quantities of oils or fats were introduced as lubricants. Though a technical advance, the stamping process was not without its problems. The fine, dry flake also had a high explosion risk, as well as being a potential contaminant. It was not until the end of the 1920s that a safe, explosion free, wet ball milling process was developed. This manufacturing process has continued largely unchanged in its basic features to this day. An upsurge of interest in metal pigments occurred from the late 1970s onwards, as evidenced by an explosion of patent literature. Products were tailored to the intended application media and increasing account was taken of the need for environmentally friendly product delivery forms. Improvements in flake brightness and advances in flow and weld line minimisation ensured the growth of metal pigments for the direct coloration of polymers. The future is likely to bring further advances in optical brightness through improvements to milling and screening technology. The goal for manufacturers remains attainment of the reflectivity of vacuum deposited metal with the economics of the ball milling process. The apparently insatiable desire for novel colouristic effects will also ensure a place for metal cores with coloured or even multi-coloured surfaces.

5

2

Metal Pigment Types

Six metals and two alloys are used in significant commercial quantities, though not all as colouring agents and not all in polymers or as a coating on polymers. The main factors influencing their effectiveness as pigments are colour, brilliance, corrosion resistance, malleability, specific gravity (SG) and cost. Table 2.1 compares properties for the metals, plus the alloys gold bronze and stainless steel.

Table 2.1 Metal Properties Colour

Brilliance

Corrosion Resistance

Malleability

Aluminium

White

High

Moderate

High

2.70

Moderate

Bronze

Gold

High

Moderate

High

~8.50

Low-moderate

Copper

Reddish

High

8.96

Moderate

Nickel

Off-white

Medium

Good

Moderate

8.90

Moderate-high

Stainless steel

Off-white

Medium

Good

Low

~8.00

Moderate-high

Zinc

Blue-white

Low

Good

Moderate

7.13

Low

Iron

Grey

Low

Poor

Moderate

7.87

Low

Silver

White

Very high

Fair-good

Very high

10.50

Very high

Medium-high Medium-poor

SG

Cost

2.1 Aluminium Aluminium is a silvery, ductile metal. Although by far the most prolific metal pigment world-wide, data on consumption are lacking. Use of aluminium in polymer related applications is estimated at 4,000-5,000 tonnes per annum worldwide. Although mainly used for used for its colouristic properties, aluminium pigments have a wide range of other uses, such as light barriers, light reflectors, moisture barriers and thermal conductors. Chapter 15 is devoted to such non-pigmentary applications. In the plastics market, the full commercially available particle size range is used, ranging from 5-650 µm in ball milled grades, to several millimetres for cut foil types. 7

Metallic Pigments in Polymers The combination of excellent colour, low density and relatively low cost are attractive commercial advantages, but are offset by limited corrosion resistance. Aluminium flake is attacked by water throughout the pH range, but is most stable a few units either side of neutrality. Dry, airborne flakes are explosive, especially the smaller particle sizes. In this respect aluminium flake is no different to many finely divided organic materials.

2.2 Gold bronze The pigment known as gold bronze is actually brass, i.e., a copper-zinc alloy. It remains the prime source of gold effects for all applications except the most technically demanding, such as automotive paints. The shade is controlled by the copper/zinc ratio, which in practice varies between about 70/30, known as ‘rich gold’, through to ‘rich pale’ at 86/14 simulating 22 carat gold, to the redder ‘pale’ at 90/10. Gold bronze flake is susceptible to tarnishing, so for more durable applications it requires a protective coating, usually of silica. Gold bronze flakes are not explosive, but the finer grades are classed as flammable. In recent years the perceived toxicity of the copper component has prompted the replacement of gold bronze with combinations of aluminium pigments and reddish yellow, orange or even brown organic pigments.

2.3 Copper Copper flakes have an attractive reddish sheen, but find limited pigmentary use because they tarnish readily in air, forming coloured salts, such as the green verdigris. For the same reason, the metal’s excellent electrical conductivity can only be exploited if the flakes are incorporated in a reliably impervious matrix. More stable coloured copper flakes, created by controlled surface oxidation, are available.

2.4 Nickel Nickel is used less for its colouristic effects than for its electrical properties. It offers a more expensive, less bright silver than aluminium, but has the advantage of excellent corrosion resistance. Flakes developed for electromagnetic interference (EMI) shielding have a particle diameter of around 12-25 µm and an aspect ratio (see section 4.1.3) of about 20:1. Such flakes may also be heat treated in a controlled atmosphere to further enhance conductivity. For colouristic applications, nickel flake is offered in leafing types (see section 4.3.4) and grades for water-based systems. Both of these can be used in solvent systems. With its

8

Metal Pigment Types slightly yellowish, pewter-like hue and rich lustre, nickel is one of the brighter of the secondary (less common) metals. It therefore finds limited application in protective decorative paints, especially water-based systems, which can take advantage of the metal’s excellent gassing resistance (resistance to generation of hydrogen gas when in contact with water). Water-based grades are non-leafing and can be used to advantage in mixtures with organic and inorganic colorants because they will not obscure the colour underneath as leafing grades would (see section 4.3.4). Applications of nickel are influenced by its health and safety credentials (see Chapter 16).

2.5 Stainless steel Stainless steel is a generic term used to describe a very large range of alloys – in excess of 200 grades in total – all of which are iron based materials containing in excess of 11% chromium. Stainless steels can be classified into four basic types, austenitic, duplex, ferritic and martensitic. The austenitic grades, which normally contain nickel, account for approximately 75% of total production. There is a strong preference as a pigment material for one austenitic grade in particular – UNS-S 31603 (formerly known as AISI Type 316 L) which has a typical composition of 17% chromium, 12% nickel and 2.5% molybdenum. In bulk form it fulfils many demanding roles in chemical plant construction and in food handling applications because of its very high resistance to corrosion in many different aggressive environments.

2.6 Zinc Zinc is a bluish-white lustrous metal. By far its main use is in anticorrosive coatings. The flake form offers a number of technical advantages over the more common zinc powder, such as increased surface coverage and reduced tendency to settling, but zinc flakes have struggled for commercial acceptance due to the additional costs associated with the milling and flake recovery stages. It has no obvious use in mass pigmented polymers.

2.7 Iron Iron is the cheapest of the pigmentary metals. Its use as a pigment is greatly limited by its low brilliance, pewter-like colour, high density and most severely, by rapid tarnishing (rusting) in water-based media. It is however a useful, inexpensive core particle onto which more attractive metals and organic or inorganic colorants can be deposited (see section 3.9). 9

Metallic Pigments in Polymers

2.8 Tin Tin is a durable, low melting, easily malleable metal, silver in colour with a pale gold cast. It makes very attractive flakes by wet ball milling, but these are soft and therefore tend to be poorly shear resistant in applications. The high density and cost of tin combine to render use of these flakes uneconomic, despite their attractive colour and excellent water resistance. Their use is made even less viable because thicker flakes are required to provide adequate degradation resistance. Like zinc, tin is not used for the mass pigmentation of polymers.

2.9 Silver The high cost of silver and its very high density preclude its use as a commodity pigment. It is used in a few critical ink applications, such as printed circuit boards and windscreen de-misting tapes, in which its excellent electrical conductivity and good tarnish resistance are essential. Better use is made of this high priced raw material if it is deposited onto less expensive core particles.

2.10 Gold The ultimate in density and tarnish resistance, but unfortunately also in cost. Real gold has all but disappeared from use as a flake pigment. Its use is not considered further in this text.

2.11 Other metals Titanium is another metal with excellent durability, but it is expensive. It is also difficult to mill, due to its brittleness relative to the other metals considered here. As a result, it has found few if any commercial applications.

10

3

Manufacture

3.1 Dry milling The earliest bronze and copper powders and later aluminium flake powder, were originally produced in Germany by stamping in a two-stage process. Thin metal foil was reduced in size in large circular vessels by pounding in a vertical plane with heavy rods while the vessel and its contents rotated. A lubricant, generally stearic acid, was added to prevent cold welding, i.e., the tendency of adjacent flakes to weld together under the high pressures generated when they are trapped between the milling rods and the body of the vessel. Coarse flakes produced in this step, known as ‘flitter’, were then reduced to powder in small stamping units, with the addition of more stearic acid. The resulting product was rubbed through fine screens to remove excessively large flakes, which would be recycled for further comminution. To increase brightness, a further treatment involved polishing in drums fitted with radially mounted brushes. More fatty lubricant would be added, bringing the concentration up to 4-6%. The stamping process proved slow, cumbersome, very dusty and uneconomic. It is not surprising that dry ball milling superseded it. Economics were further improved by moving to a continuous process (see section 3.2). The batch ball mill consists of a steel cylinder, a metre or more in diameter and perhaps 3-4 metres long, with its long axis horizontal (see Figure 3.1). Solid steel spheres of 3-10 mm diameter occupy around a third of the internal volume. As the mill revolves, these milling media cascade onto the metal powder and lubricant. It is this cascading action that causes flattening of the starting powder to generate flakes. Several narrow steel bars running the length of the mill increase the height from which the media cascade, thereby speeding up the milling process. The considerable mechanical energy expended causes the mill and contents to heat up. For this reason the mill is generally water cooled via an external jacket. Edwards and Wray [1] and also Wendon [2] describe early manufacturing processes.

11

Metallic Pigments in Polymers

Figure 3.1 A typical production ball mill. The mill is configured for draining the milled slurry through the pipe connected to the underside.

3.1.1 Aluminium In the case of aluminium in particular, dry milling is extremely dangerous. Although not pyrophoric, i.e., spontaneously combustible, fine aluminium dust in air only requires an ignition source to cause a very violent explosion. For this reason, inert gas was used in ball mills. Even so, dry milling has declined mainly for safety reasons.

3.1.2 Gold bronze Remarkably little literature exists on milling of gold bronze pigments in general, but there is some information in Wendon [3]. Production begins in the foundry with fusion of molten copper and zinc to form the gold bronze (brass) alloy. A small quantity, usually less than 1%, of aluminium is included as an antioxidant. The molten alloy is then atomised by compressed air to form a fine powder. The stamping type of dry flake production has long since given way to continuous dry milling in conventional ball mills (see section 3.2.2).

12

Manufacture

3.2 Continuous dry milling As might be expected, the main advantages of a continuous process are in throughput and improved economics, but product uniformity is also important.

3.2.1 Aluminium The explosion hazard was reduced by the introduction of what became known as the Hametag process, Hametag being a contraction of Hartstoff-Metall AG [4, 5, 6, 7]. This was a continuous dry ball milling process carried out under a nitrogen purge sufficient to maintain the oxygen content at around 5%. Such a concentration is below the lower explosive limit for aluminium dust, but nevertheless provides sufficient oxygen for controlled re-oxidation of the very reactive nascent aluminium surfaces created by the milling action. Flake separation was initially provided by cyclones into which the flake was carried by the carrier gas. This was found to give an inefficient separation of small flakes from large and was later modified by Mandle [8] and by Carlfors [9]. Booz and Kondis [10] at the Aluminium Company of America (Alcoa) later developed a further variant, involving a vibratory mill. They proposed a continuous feed of aluminium powder, stearic acid lubricant and reduced oxygen gas through a vibrating mill, with removal of product by the carrier gas at an equivalent rate.

3.2.2 Gold bronze Gold bronze flake pigments are manufactured by much the same ball milling processes as aluminium, though allowance must be made for the much higher density. The flake shape is similar. Kramer’s [11] continuous dry milling process has endured to this day, due principally to the lower flammability of gold bronze. For this reason, an air current rather than more expensive inert gas (see section 3.2.1) could be used to remove flakes from the milling chamber. A dry milling process patented by Mandle featured closed loop recycling of oversize flakes back to the mill. In practice, almost all gold bronze flake made by this or any other process is of the leafing type, employing stearic, or occasionally palmitic acids as lubricants. Leafing grades predominate because non-leafing forms of gold bronze tend to have a dull appearance. Until recently, non-leafing forms were not commercially available. Where a non-leafing finish is required in coating applications, leafing flakes are generally deliberately de-leafed by addition of an organic acid, such as citric acid. The terms leafing and non-leafing are defined and further discussed in sections 4.3.4 and section

13

Metallic Pigments in Polymers 12.2.3. It should be noted however that the terms have no real relevance in mass pigmented polymers. The milling action generates fresh gold bronze surfaces. These are re-oxidised at a rate dependent on oxygen availability. This in turn affects the colour of the resulting product, even from a single alloy. In the final polishing stage of production, a slow speed mill is used with a different type of ball charge to induce a very gentle ball milling action. The flakes are smoothed and flattened. Hiding power increases as agglomerates are broken up. Reflectivity increases along with leafing value. It is important that the stearic acid lubricant forms a complete chemically bound coating on the metal surface. In freshly produced powder this reaction is incomplete. Bronze powder manufacturers therefore have to artificially age their products by storage for a period of time until the reaction is complete.

3.3 Wet milling The advent of the wet milling process by Hall [12, 13] was to revolutionise metal flake pigment manufacture. Indeed this profuse inventor also contributed to powder technology by providing an atomising process [14].

3.3.1 Aluminium The Hall milling process (see Figure 3.2) is desirable not only because of its efficiency, but also because the elimination of grinding under dry conditions makes it very much safer. The ball mills used are much the same as those described for dry milling in section 3.1. For aluminium flake pigment manufacture, there are three raw materials to be added to the ball mill. The first is atomised aluminium powder, or in the case of economy grades, foil scrap. This is added to the ball mill, which will already be part filled with the steel grinding media. Then 3-6% (w/w of metal) of a long chain fatty acid, typically oleic acid or stearic acid, is added. The chemical nature of the fatty acid has an effect on the properties of the resulting flakes in coating systems. Stearic acid produces leafing flakes, whilst those derived from oleic acid are non-leafing. Finally, sufficient mineral spirits, otherwise known as white spirit, a high boiling aliphatic hydrocarbon blend, is added to form a mobile slurry.

14

Manufacture

Figure 3.2 The Hall wet milling process for the manufacture of aluminium flake pigment paste

The mill is revolved at a speed that will allow the balls to cascade onto the aluminium metal. As aluminium is one of the more malleable metals, it is readily flattened. Figure 3.3 shows atomised aluminium starting powder. Partway through the milling process, the particles have the appearance shown in Figure 3.4. Eventually the particles become so thin that they begin to break up, reducing the median particle size of the mix and producing the tiny flakes, typically less than one micron thick, that are required (see Figure 3.5). The lubricant prevents cold welding that would otherwise occur when overlapping flakes are trapped between the grinding balls.

15

Metallic Pigments in Polymers

Figure 3.3 Atomised aluminium powder [1 cm = approx. 40 µm]

Figure 3.4 Atomised aluminium powder, partly flattened by wet ball milling [1 cm = approx. 40 µm] 16

Manufacture

Figure 3.5 ‘Cornflake’ type aluminium flake pigment, derived from the Hall process [1 cm = approx. 15 µm]

The length of time the ball mill turns depends on the grade of flake being manufactured and its desired particle size distribution. A milling time of 5 to 30 hours is typical. When the grinding operation has been completed, the slurry is discharged from the ball mill by washing it out with more solvent. This dilute slurry is presented to a vibrating screen of appropriate mesh size. The function of the screen is to recover the required particle size fraction. This will generally be the flakes small enough to pass through the screen. It is this screening process together with the milling time that differentiates one grade from another. Flakes that are too large to pass through the screen are returned to the ball mill for further comminution. The remaining flakes pass to a filter press. Excess solvent is removed in the press, resulting in a filter cake having a volatile content of approximately 20 per cent. The final step in the manufacture of a paste involves homogenising the filter cake in a mixer. Adjustments are made to ensure that the paste meets the specification for that grade in properties such as colour, sparkle and hiding (opacity). Colouristic properties (whiteness, brightness and sparkle) can be influenced by mixing time and paste viscosity. Under suitable conditions a polishing action can be made to take place. This can change

17

Metallic Pigments in Polymers reflectivity and flop characteristics (see section 4.3.3) by reducing surface irregularities and consequent light scattering. In the case of leafing grades, the leafing value is increased (see sections 4.3.3 and 4.3.4). Addition of an aromatic hydrocarbon solvent is often made at the mixing stage, to alter the solvent balance of the paste, to improve compatibility in liquid coating media. A stiff, paste-like consistency is obtained with a volatile content of approximately 2540%, depending on the coarseness of the flakes. The finished paste is finally packed into drums and sealed for dispatch. In practice it is the solids content, known as the non-volatile content (NV or sometimes NVC) that is more generally quoted. NV affects the transport and shelf life of pastes. If the NV is too low, the metal may sediment in transit. Weight based formulae are then under-pigmented. Too high a NV is symptomatic of dry pastes, with reduced shelf life. One grade is distinguished from another by the particle size distribution, shape and surface finish of the flakes. These features are determined by milling parameters such as mill dimensions, rotation speed, milling time and temperature, grinding media loading, size and density, in addition to the amount and nature of metal, solvent and lubricant present. Particle size distribution is further modified at the screening stage. By the time the filter cake is mixed, there are few opportunities for major adjustment of application properties. It follows that the character of the product is formed in the milling and to a diminishing extent, screening and mixing stages of production. In the case of filter cakes intended for plastics applications, the hydrocarbon solvents of manufacture are not suitable carriers for the metal flakes. It is necessary to remove the solvent to give either dry flakes or compositions in which the solvent has been a replaced by a polymer compatible carrier. These and other application-specific delivery forms are described in Chapter 5. The products of the described wet milling process have become known as ‘cornflakes’, due to their resemblance to the well-known breakfast cereal. Developments in milling technology in the 1980s produced thicker, more rounded flakes with smoother surfaces. Due to their shape, these became known as ‘silver dollars’. Silver dollars are an advance in aluminium pigment technology because they are demonstrably whiter and brighter than cornflakes of the same particle size distribution. Their production costs are higher because they require expensive, specialised atomised powders and a more gentle, time consuming, milling regime, involving smaller, more expensive grinding media. Their first and ongoing main application is in automotive

18

Manufacture paints, but they are also used in high quality printing inks. More recently they have begun to appear in plastics, especially in paint replacement applications where maximum brightness comes before cost. A related development is degradation resistant flakes or ‘Tufflakes’ [Registered trademark of Silberline Manufacturing Co. Inc., USA]. These are also thicker flakes, designed to be resistant to shear. A patent to Hieda [15], filed in 1987 defines and claims such degradation resistant aluminium flake pigments and a wet ball milling process for their manufacture. The role of lubricant in the wet milling process was studied by Imasoto [16], using stearic acid. The amount adsorbed increased with milling time up to a limiting concentration of 0.135 mol/kg aluminium. The specific surface area increased up to ~8m2/g. A bimolecular adsorbed layer could be formed depending on the flake size and therefore its surface area. If the surface area is low, the limiting concentration will be as a double layer; if it is high it will be spread out as a monolayer. This has significance in considering the effect of metal flake pigments on the mechanical properties of polymers in which they are incorporated.

3.3.2 Gold bronze As noted previously, milling of gold bronze is not well documented. Nevertheless, Hall [17] provided a wet milling process as part of his 1919 patent, granted in 1925. Milling solvents and lubricants were chosen to allow easy removal of the solvent whilst leaving the flakes coated with lubricant. Stearic acid, aluminium stearate and air blown through the mill combined to aid leafing. The product could be heated to 50 °C, though not appreciably higher, in oxygen to prepare dry flakes with retention of leafing. The vast majority of gold bronze flake manufactured today is dry milled. The tarnish resistance of gold bronze in both coatings and polymers is improved by applying a silica coating from a hot aqueous silicate bath containing a water-soluble salt of an organic acid, followed by drying. Developments of this early process by Atlantic Powdered Metals [18] are still in use today.

3.3.3 Silver A process for the preparation of silver flake powders is disclosed in a patent granted to Du Pont [19]. Precipitated silver powder is bead milled in the presence of water and a small quantity of an unsaturated fatty acid, such as oleic acid or linoleic acid. The flakes are later

19

Metallic Pigments in Polymers separated from the beads and dried. The product obtained is a silver flake substantially covered by a monolayer of fatty acid, believed to be attached to the flake surface via the double bond(s). Whatever the configuration, the flakes can be formulated at over 85% w/w without the high viscosity that reduced the usefulness of uncoated flakes.

3.3.4 Nickel Nickel flakes are manufactured by milling from high purity nickel powders. They are produced in three basic types, a leafing grade – solvent milled with a stearate lubricant, a water compatible non-leafing grade, milled in polyhydric alcohols and a special electrically conductive grade. Filamentary nickel powders are manufactured exclusively by a nickel carbonyl refining process, and their primary application is for nickel alkaline battery manufacture. Their structure, however, also allows them to be used as pigments in electrically conductive materials, particularly paint coatings. These are employed not only for electromagnetic shielding but also in antistatic applications (see Chapter 15).

Figure 3.6 Spherical nickel powder [1 cm = approx. 3 µm] Reproduced with permission of Hart Coating Technology and Novamet

20

Manufacture Discrete spherical nickel powders can be produced in an extremely pure form directly from the nickel refining process (see Figure 3.6). By a combination of screening and air-classification (i.e., size segregation in air, using a hydrocyclone), the size range of a particular grade can be carefully controlled to suit specific applications. The finest particle size material readily available commercially is a sub-10 µm air-classified grade. Such powders are used to produce composites, mostly for electromagnetic shielding applications. To overcome the limited chemical resistance of aluminium flakes, Tundermann and Harrington [20] used nickel, cobalt and related alloy flakes, including stainless steel. They are prepared by a batch wet ball milling process, modified to reduce viscosity, to prevent the fragmentation of these metals that takes place under normal milling conditions.

3.3.5 Stainless steel Stainless steel flakes are manufactured by solvent milling from suitable size powder stock. Like nickel flake pigments, they are also available as stearate milled, leafing grades (see Figure 3.7).

Figure 3.7 Leafing type stainless steel flake [1 cm = approx. 30 µm] Reproduced with permission of Hart Coating Technology and Novamet

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Metallic Pigments in Polymers Unlike nickel, stainless steel particulates are not produced by a basic refining process but by melting a mixture of the necessary constituent elements to form the required alloy, followed by water or gas atomisation. Stainless steel particulates produced in this way are available in a very wide range of sizes from less than 10 µm up to 2,000 µm. Commercially available flakes are virtually all produced from UNS-S 31603 (stainless steel alloy) powder because of its superior corrosion resistance properties. Stainless steel is more time consuming to mill into flake pigment than the softer aluminium and gold bronze. Its colour is also inferior to aluminium. Nevertheless its corrosion and abrasion resistance qualify it for use in both directly pigmented polymers and polymer coatings.

3.3.6 Zinc A batch ball milling process in hydrocarbon solvent was also the process chosen by Marx [21] to prepare leafing zinc flakes of sub-60 µm diameter for anticorrosive applications.

3.4 Continuous wet milling Just as continuous dry milling evolved from the batch dry milling process, so continuous wet milling is a later variant of the Hall process. Alcoa [22] was prominent in this field, patenting continuous feed of metal powder, lubricant and solvent through a tubular ball mill.

3.5 Spherical metal pigments A variation of the wet milling process was patented by McKay, McKay and Ringan of Silberline [23] to produce spherical metal pigments. These are polished aluminium spheres or facetted spheres, from 1 to 300 µm in diameter, derived from substantially spherical atomised aluminium powder. This starting material is created by atomisation under inert gas. Processing consists of gentle polishing in a ball mill or other apparatus in the presence of conventional hydrocarbon solvent and oleic acid lubricant. The energy imparted is insufficient to cause flakes to form, but has the effect of smoothing the metal surface to increase reflectance by at least 10-30% over that of the starting material. The products are used in coatings and mass pigmented plastics.

22

Manufacture In summary, the range of flake particle size obtainable from the Hall process and its developments is very wide, ranging from a D50 (see section 4.1.1) of perhaps 4-5 µm to large, thick, so-called ‘glitter’ flakes (or in the USA, ‘flitter’ flakes) up to 600-700 µm in diameter.

3.6 Rapidly solidified flakes A means of preparing metal flake pigments without the need for ball milling is provided by allowing molten metal to impinge directly on a cooled disc rotating at extremely high speed. The molten droplets cool virtually instantaneously and are dispersed by centrifugal force to be collected in a chamber. Rotor speeds in excess of 20,000 rpm and a cooling rate of over 102 C°/sec are ideally required. The process is variously named spinning disc, centrifugal atomisation, splat cooling or splatomisation. The principle was first described in the 1960s and refined by a number of later patents. Design of the disc and the means of cooling are critical to the quality and consistency of the product. Advantages include compositional uniformity, especially of alloys, leading to improved mechanical and physical properties. Early commercial interest came from the Pratt and Whitney Co., [24, 25] through their Rapid Solidification Rate (RSR) technology. Molten metal flowing through a funnel onto a spinning disc is radially accelerated and dispersed into cooled, almost spherical droplets, in a helium atmosphere. Later, Yeh [26, 27] claimed a similar apparatus. A refinement consisted of a second cooled rotating disc onto which the product of the first was thrown. This configuration, combined with a very high cooling rate of 106 C°/sec produced aluminium and iron elongated flake-like products of 10-600 µm particle diameter and a median particle size of around 200 µm. Boswell [28] at the Batelle Memorial Institute in Switzerland claimed a hybrid of rapid solidification technology and ball milling. In the specification, a molten metal such as tin or an iron alloy is rapidly cooled in contact with rapidly agitated balls. The products are said to have greater angularity but are nevertheless suitable for paints. Considered overall, at the present state of development, rapid solidification processes appear ill suited for the preparation of fine particles with conventional flake geometry.

23

Metallic Pigments in Polymers

3.7 Vacuum deposition Vacuum deposition for flake pigment manufacture is an adaptation of the widely used process for the metallisation of polymer film. It involves coating polymer film, usually polyethylene terephthalate (PET), with a resin deposited from solvent solution to form a release layer (a dry release film that is resoluble in solvent). Under very high vacuum, pure aluminium is deposited on the dried film from the gas phase to a minimum thickness of 40-60 nm. In practice, the 2-3 m wide film is unspooled across the coating head, and subsequently respooled, the whole process taking place in a vacuum chamber. The aluminised film is subsequently run through a solvent bath at ambient pressure to dissolve the resin and release the sheets of ultra reflective aluminium. In the final stage of the process, the sheets are broken down to flakes. The product is offered for sale as a dispersion in solvent containing about 10% metal. Much early work is credited to McAdow [29]. The basic process elements described above were operated to produce aluminium or chromium flakes of 0.075-0.6 µm thickness. A translucent, solvent-based coating containing only 0.028 to 0.15% of these particles, with a surface area of 40,000 to 60,000 cm2/g is claimed. A later modification [30] claimed a larger metal flake, principally aluminium, of greater planarity (flatness) for more efficient light reflection and sparkle. This was made possible by supporting the flake surfaces on both sides with an insoluble film of resin, thin enough to allow coated flakes over 30 µm in diameter to be produced from coated film by the customary mechanical agitation. As the resin coating insulates the flakes from each other, they have no ability to carry electrical charge. They were therefore well-suited to the then emerging technique of electrostatic spraying. A related patent describes the process of preparation [31]. After separating the vacuum deposited metal film from the polymer sheet by dissolving the release layer in solvent, Roberts [32] used ultrasonic energy to disintegrate the film to flakes, as did Levine and co-workers at Revlon [33]. The process proved to be applicable to a wide variety of deposited metals, but magnesium, aluminium, copper, silver and gold are especially favoured. The suggested applications of the product are paints and moulded plastics. A more recent method, disclosed by Gray and co-workers [34], generated flakes by allowing a vacuum deposited film to be disintegrated from the surface of an enclosed glass cylinder following its formation by a gas plasma technique. Miekka at the Avery Dennison Corporation [35] extended the process to create a metal flake pigment with holographic properties. As an example, a polystyrene release coating

24

Manufacture is laid down on a Mylar carrier film with a 200 line rotogravure roll, using a commercial roll coater. The dried film is embossed by heating above the polymer softening point and pressing the outer surface onto a roller engraved with a diffraction pattern. A 30 nm thick aluminium film is then applied by vacuum deposition. The film is separated by dissolving the release layer in a toluene/methyl ethyl ketone (MEK) mixture and mechanically converting it to flakes which exhibit the retained diffraction pattern. Vacuum deposition produces the brightest commercially available aluminium flake. Due to the costs associated with providing a very high vacuum and vapourising aluminium, the product is extremely expensive. This limits its use to the very highest quality coating applications, such as logos, highlighting, spot colour and reflective coatings on lamps, especially coatings on plastic for car headlights. The product appears under the trade names Metalure, Metasheen and Star-Brite.

3.8 Cut foil glitters Cut foil glitter flakes are easily distinguished from ball milled types under an optical microscope. The former have a regular geometry and are available in particle sizes greater than those obtainable from milled flakes (see Figure 4.15). Squares, rectangles, hexagons and diamonds are the most common, ranging from 50 to over 2000 µm in diameter, with a thickness from 15-50 µm. They are produced by stamping or cutting thin foil, usually of aluminium. Particle size is therefore very regular. The cost of the flakes increases with decreasing size, reflecting the increased amount of cutting required. The delivery form is dry flake, since apart from the smallest particle sizes, dusting is not a serious problem. Because of their large particle size, cut foil glitters are too large for inks and all but the most specialised paint applications. They are therefore used most in mass pigmented plastics. Coloured glitters are also available. Their preparation is described in section 3.9.3.

3.9 Flakes with coloured surfaces This section describes a diverse collection of novel approaches used to alter the colour of existing metal flakes. It includes chemical modification of the metal surface, attachment of pre-formed colorants and colour formation in situ from components reacting at the metal surface. A wide range of flake particle size is represented, from fine, high opacity grades to very large glitter flakes.

25

Metallic Pigments in Polymers In the wider context of flake pigments, pearlescent or mica pigments are nowadays also often coated. These will not be considered here, with the exception of mica flakes coated with elemental metals. Interest in surface coloured metal flakes arises because coating metal flakes with organic and inorganic colorants provides visual effects not achievable by physical mixtures of metal flake and colorant. These are considered further in section 7.10. Attempts to colour the surfaces of principally aluminium flakes are recorded in the patent literature as far back as the 1950s. The earliest technology employed was a variant of anodisation in which dyestuffs were absorbed into the porous oxide surface coating of aluminium. The colour was fixed by immersion in boiling water [36]. It was not until the 1980s that chemically coloured metal flakes became commercially significant. An upsurge of research activity provided novel processes for paints, inks and the mass pigmentation of polymers.

3.9.1 Chemical modification of metal surfaces An early development, optionally for water-based applications was patented by Interchemical Corporation [37]. Coloured pigments and a method for their manufacture was the subject of work by Harakawa [38]. A quench solidified (very rapidly cooled) pigment composed mainly of iron alloyed with specific percentages of chromium, phosphorus and carbon is heated to 150-700 °C for 1-5 hours. The surface colour produced is dependent on the treatment temperature and time, ranging from red at low temperatures and times to blue and green at high temperature and times. Knox and Green of Silberline [39] claimed preparation of coloured iron or iron alloy flakes. Wet milled flakes are heated in a furnace at 150-700 °C for various times from 20 seconds to two hours in a reduced oxygen atmosphere. A wide colour spectrum results, ranging from gold through copper-red to blue and eventually black at the highest temperatures. The process is particularly suited to large flakes (~100-600 µm median particle diameter) for the mass coloration of plastics, especially as a component of stoneware and granite effects. The products are offered commercially under the Silcroma trade name. In a very recent development, Fetz [40, 41] of Eckart-Werke described aluminium flakes with a golden hue, prepared by oxidation with a controlled quantity of water in an alcohol medium containing a basic catalyst. Both leafing and non-leafing flakes are suitable, but the latter give better optical properties (brightness, cleanliness).

26

Manufacture The 20-30 nm thick oxide layer controls the colour range, from pale gold to brown. This oxide surface is highly porous, with a surface area around five times that of the starting material. Although UV stable and shear resistant, the products, trade named Aloxal, are not totally water stable. They must be further protected by hydrophobic inhibitors, such as phosphates. An unusual process for creating colours on titanium particles was developed by Greening and Clegg [42]. Fine particle size titanium is agitated in an aqueous electrolyte and subjected to a voltage of 5-100 V, preferably direct current between immersed anode and cathode. The coloration formed on the metal surface is controlled by the applied voltage and treatment time to produce durable products suitable for incorporation in paints.

3.9.2 Colour formation in situ Ostertag, Bittler, Bock, Murphy and Ravella at BASF in Germany [43, 44, 45] pioneered the deposition of metal oxides from the gas phase onto aluminium flakes in a fluidised bed at elevated temperature. Chief amongst these were iron oxide golds formed by decomposition of iron pentacarbonyl to ferric oxide in the presence of oxygen and optionally, water vapour. In later variants, multiple coatings were formed to provide interference or lustre effects, using a wider range of metals, such as titanium, chromium, molybdenum, tin, silicon and zirconium, in addition to iron. Schmid and Mronga [46] disclosed surface coloured aluminium flake pigments comprising a first, colourless or selectively absorbing layer of metal oxide, a second, nonselectively absorbing layer of carbon, metal and/or metal oxide and optionally another layer like the first. The layers are deposited sequentially in a fluidised bed. In a further US Patent, the order of the first and second layers is reversed and higher treatment temperatures introduced [47]. The flakes are also claimed to be less aggregated than those of the earlier patent because the process includes a pre-treatment to remove traces of lubricant from the flake manufacturing stage. Extension of the process to include wet chemistry techniques is the subject of US Patent 5,624,486 [48]. The product group, marking the outcome of the BASF work, named Paliocrome, found its main use in paints for the top end of the automobile market, including coatings on polymeric components. A multi-stage wet chemistry route was used by Nadkarni [49]. Metal flakes, especially aluminium, are dispersed in an aqueous alkaline zirconia sol to form a coating. In a second stage the coated flakes are treated with a solvent solution of a metal salt, such as

27

Metallic Pigments in Polymers cobalt nitrate or iron nitrate. Finally the flakes are heated to fix the coating. Colour intensity can be increased by repeating the treatment cycle. Souma [50] also used wet chemistry to hydrolyse an organic titanium ester in the presence of metal flakes in a suitable organic medium. Claimed metals are aluminium, gold bronze, stainless steel, tin and iron. Attractive iridescent tones are produced, the hue being dependent on the thickness of the titanium oxide coating, which varies between 40 and 155 nm. The products are particularly useful in aqueous coating compositions because the coating is very effective in protecting the underlying aluminium. Interference platelets were disclosed by Philips [51] at Flex Products in the USA. They consist of multi-layer thin films deposited on a flexible web material and subsequently removed and comminuted. They include a metal reflecting layer. The process is akin to vacuum deposition (see section 3.7) producing interesting, though expensive products for prestige coating applications. A related multi-layer technology, developed by BASF [52], generates colour-variable pigments (CVP). In one variant, the core is a silver dollar aluminium flake of 15-20 µm diameter and 300-500 nm thickness. A low refractive index coating of silica is applied by wet chemistry, followed by a very thin layer of iron oxide. The thickness of the coatings controls the visual effect. Automotive paint applications make the best use of the pigments’ goniochromicity, i.e., the variation of colour with angle of viewing, also known as colour flop.

3.9.3 Attachment of pre-formed colorants Coloured metallic glitters are prepared by coating both sides of the starting metal foil with a pigmented resin, generally epoxy or polyester, prior to cutting. Ideally both pigment and coating resin should have a high degree of transparency to maximise metallic brightness. A wide colour range is available. As noted in section 3.8, such metal pigments are too large for most coatings and so are used mainly in plastics. Unfortunately the cheaper grades offer heat stability only up to about 230 °C. Above this temperature the coating resin melts and delaminates, causing the pigment to disperse in the polymer matrix, discolouring it. The problem is particularly acute in polar polymers such as polyamide. This glitter type is therefore only suitable for lower melt point polymers such as PVC, polyolefins and some styrenics. Recently, coloured glitters have been developed which are more durable but more expensive. These have epoxy-based coating resins of increased transparency, providing

28

Manufacture brighter metallic effects and increased heat stability, up to perhaps 250-260 °C in application, depending on the polymer. Applications of these pigments are considered in Chapter 11. Chida [53] at the Showa company in Japan developed coloured metal pigments by attaching finely dispersed organic or inorganic pigments to metal flake surfaces. The process consists of heating a metal pigment such as aluminium to around 90 °C in solvent with a thermally polymerisable fatty acid having multiple carboxylic acid groups. The cooled slurry is concentrated to filter cake and subsequently added to a ball milled predispersion of organic or inorganic pigment in a little solvent. The colorant is fixed to the flake surface by a short kneading step in solvent. Attachment of the colorant is improved by a post-treatment involving free radical polymerisation to form a coating on the flakes. The products are used in both coatings and mass pigmented plastics. Two related processes were disclosed by Hashizume of Toyo Aluminium. The first employed compounds having multiple amino groups rather than multiple carboxylic functions. Aluminium, titanium and silicon compounds could be added to enhance attachment of organic pigment particles to the metal pigment surface [54]. The second process, apparently developed from the first, additionally used metal acids such as molybdic acid, monobasic, aromatic carboxylic acids and polymerisable monomers to further improve bonding of the organic pigment to the metal surface [55]. Stable coloured metal pigments suitable for coatings and the mass coloration of polymers resulted. Keemer [56] prepared a metallic pigment composition by treating a metal pigment, such as aluminium or gold bronze, with a mixture of organic or inorganic pigment particles and wax. An organometallic coupling agent, monomer and initiator were then reacted to form a coloured polymeric shell on the metal surface. No post-treatment was required to fix the colorant in place. Deposition of organic pigments from the gas phase onto metal pigments, especially aluminium, is disclosed by Suzuki [57, 58, 59]. The process is carried out under high vacuum to give a continuous, uniform and homogeneous coloured layer some 0.01– 0.1 µm thick at the metal surface. These products can be used in masterbatches at up to 30% concentration.

3.10 Metal coatings on non-metallic substrates Although stretching the definition of a metal pigment, a group of flake pigments may be obtained by forming a continuous, thin coating of elemental metal on a non-metallic core. Such a product is described by Muller of Du Pont [60]. Nickel is deposited on pre-

29

Metallic Pigments in Polymers sensitised mica or glass flake by a wet chemistry technique. Interference colours are generated by further deposition of a thin layer of nickel oxide or titanium oxide, also from aqueous solution. The products are suitable for use in paints and especially in the mass coloration of plastics. A related technique was used by Yolles [61] to deposit metals onto glass flakes by decomposing an organometallic compound such as dibenzene chromium or a trialkyl aluminium under vacuum at elevated temperature. Neither of these techniques found widespread acceptance, probably due to excessive cost. In addition to the range of pure nickel pigments, two distinct types of coated products are also produced; one in which nickel acts as the coating, the other in which it provides the substrate. Nickel can be applied as a coating on a number of particulate substrates. Two types commercialised by the Novamet Corporation are nickel-coated graphite and nickel-coated carbon fibre. The latter is manufactured using the carbonyl process and is marketed as a resin-encapsulated product. Nickel-coated mica is also known (see section 15.4.5). All are used for EMI shielding. Use of nickel as a core particle is described in section 3.11.

3.11 Other methods The Mond Nickel Company [62] was one of the first to recognise the corrosion resistance advantages of nickel over aluminium. In 1942 the company patented an electrolytic process to deposit nickel in the form of very thin flakes on a cathode immersed in an aqueous solution of a nickel salt, such as the chloride or sulphate. The flakes were dislodged by mechanical action and treated if desired with a leafing agent such as stearic acid for use in paints and inks. Nickel particles can be coated with other metals to provide a surface that gives superior electrically conductive properties. Two forms are commercially available, offering a choice of particle morphology. They are silver-coated nickel spheres and silver-coated nickel flakes. Both forms are employed to produce electrically conductive composite products for use in shielding applications where high performance is vital. The flake form is preferred for coatings and inks whereas the spherical form is used in mass pigmented products such as silicone polymer gaskets.

30

Manufacture

References 1.

J. D. Edwards and R. I. Wray, Aluminium Paint and Powder, 3rd Edn., Reinhold Publishing Corporation, New York, USA, 1955, 1-14.

2.

G. W. Wendon, Aluminium and Bronze Flake Powders, Electrochemical Publications Ltd., UK, 1983, 6-14.

3.

G. W. Wendon, Aluminium and Bronze Flake Powders, Electrochemical Publications Ltd., UK, 1983, 13-14.

4.

E. Podszus, inventor; Hartstoff-Metall AG, assignee, GB Patent 204,055, 1923.

5.

No inventor; Hartstoff-Metall AG, assignee, GB Patent 341,562, 1931.

6.

No inventor; Hartstoff-Metall AG, assignee, GB Patent 360,142, 1931.

7.

No inventor; Hartstoff-Metall AG, assignee, GB Patent 363,604, 1931.

8.

H. H. Mandle, inventor; unassigned, GB Patent 486,845, 1936.

9.

No inventor; Carlfors Aktiebolag, assignee, GB Patent 650,818, 1947.

10. A. D. Booz and T. J. Kondis, inventors; Aluminium Company of America, assignee, US Patent 4,115,107, 1978. 11. E. Kramer, inventor; Metallpulver AG, assignee, US Patent 2,136,445, 1934. 12. E. J. Hall, inventor; Metals Disintegrating Company Inc., assignee, US Patent 1,569,484, 1919. 13. E. J. Hall, inventor; Metals Disintegrating Company Inc., assignee, US Patent 2,002,891, 1935. 14. E. J. Hall, inventor; Metals Disintegrating Company Inc., assignee, US Patent 1,545,253, 1919. 15. T. Hieda, inventor; Toyo Aluminium K. K., assignee, European Patent 305,158 B1 (= T. Hieda, inventor; Toya Aluminium Kabushi Kaisha, assignee, US Patent 4,936,913), 1990. 16. Y. Imasoto, Kotingu Jiho, 1989, 182, 9. (Chemical Abstracts P037287E; in Japanese).

31

Metallic Pigments in Polymers 17. E. J. Hall, inventor; Metals Disintegrating Company Inc., assignee, US Patent 1,569,484, 1919. 18. No inventor; Atlantic Powdered Metals, assignee, GB Patent 1,154,933, 1965. 19. L. W. Tyran, inventor; E.I Du Pont de Nemours and Company, assignee, US Patent 4,273,583, 1981. 20. J. H. Tundermann and J. H. Harrington, inventors; International Nickel Company, Inc., assignee, US Patent 3,941,584, 1976. 21. W. Marx, inventor; Walter Marx & Co. K.G., assignee, British Patent 846, 903, 1956. 22. No inventor; Aluminium Company of America, assignee, GB Patent 1,588,026, 1976. 23. C. F. McKay, A. McKay and E. S. N. Ringan, inventors; Silberline Ltd., assignee, European Patent 0,651,777 B1, 1993. 24. P. R. Holiday and R. J. Patterson, inventors; United Technologies Corporation, assignee, US Patent 4,078,873, 1978. 25. P. R. Holiday and R. J. Patterson, inventors; United Technologies Corporation, assignee, US Patent 4,343,750, 1982. 26. J-W Yeh, K-S Liu, K-Y Shue and Y-C Ho, inventors; National Science Council, Taiwan, assignee, US Patent 5,259,861, 1993. 27. J-W Yeh, K-S Liu, K-Y Shue and Y-C Ho, inventors; National Science Council, Taiwan, assignee, US Patent 5,332,198, 1994. 28. P. Boswell, D. F. Richter and G. Haour, inventors; Batelle Memorial Institute, assignee, US Patent 4,915,729, 1990. 29. W. R. McAdow, inventor; American Marietta Company, US Patent 2,941,894, 1960. 30. W. R. McAdow, inventor; Mobil Oil Corporation, assignee, US Patent 3,692,731, 1972. 31. W. R. McAdow, inventor; Mobil Oil Corporation, assignee, US Patent 3,697,070, 1972.

32

Manufacture 32. C. B. Roberts, inventor; Dow Chemical Company, assignee, US Patent 3,839,012, 1974. 33. S. Levine, M. E. Kamen, A. Defazio and P. Cueli, inventors; Revlon, Inc., assignee, US Patent 4,321,087, 1982. 34. H. R. Gray, R. P. Shimshock and M. E. Krisl, inventors; Deposition Sciences Inc., assignee, European Patent 370,701 B1, 1994. 35. R. G. Miekka, D. R. Benoit, R. M. Thomas, J. P. Rettker and K. Josephy, inventors; Avery Dennison Corporation, assignee, US Patent 5,672,410, 1997. 36. G. W. Wendon, Aluminium and Bronze Flake Powders, Electrochemical Publications Ltd., UK, 1983, 34. 37. R. G. Frieser and P. A. Scardaville, inventors; Interchemical Corporation, assignee, US Patent 3,067,052, 1959. 38. Y. Harakawa, inventor; Teikoku Piston Ring KK, assignee, Japanese Patent Kokai (A) H3-277666, 1990. 39. J. J. W. Knox and W. I. Green, inventors; Silberline Ltd., assignee, GB Patent 2,288,411 B, 1994. 40. A. Fetz, K. Greiwe and H. Birner, Paint & Ink International, 1998, 11, 4, 18. 41. A. Fetz, K. Greiwe and H. Birner, Polymers Paint Colour Journal, 1998, 188, 4409, 42. 42. W. G. Greening, Jr., and N. E. Clegg, inventors; Hi-Shear Corporation, assignee, US Patent 3,199,999, 1961. 43. W. Ostertag, K. Bittler and G. Bock, inventors; BASF AG, assignee, European Patent 033,457, 1981. 44. W. Ostertag, K. Bittler and G. Bock, inventors; BASF AG, assignee, US Patent 4,328,042, 1982. 45. W. J. Murphy and A. Ravella, inventors; BASF AG, assignee, US Patent 5,277,771, 1994. 46. R. Schmid and N. Mronga, inventors; BASF AG, assignee, US Patent 5,364,467, 1994.

33

Metallic Pigments in Polymers 47. R. Schlegal, N. Mronga and R. Rieger, inventors; BASF AG, assignee, US Patent 5,374,306, 1994. 48. R. Schmid and N. Mronga, inventors; BASF AG, assignee, US Patent 5,624,486, 1997. 49. S. K. Nadkarni, inventor; Alcan International Ltd., assignee, US Patent 5,261,955, 1993. 50. T. Souma, M. Ishidoya, T. Nakamichi and N. Takai, inventors; Nippon Oil and Fats Co., Ltd., assignee, European Patent 0,328,906, 1989. 51. R. W. Phillips, S. P. Fisher and P. G. Coombs, inventors; Flex Products Inc., assignee, International Patent Application WO 95/29140, 1995. 52. B. Macdonald, European Chemical & Polymer Engineer, 1998, December, 25. 53. K. Chida, T. Uemura, H. Kitamura and H. Nitta, inventors; Showa Alumi Powder KK, assignee, US Patent 5,037,475, 1991. 54. Y. Hashizume and Kobayashi, inventors; Toyo Aluminium KK, assignee, Japanese Patent 09124973 A, 1955. 55. Y. Hashizume, inventor; Toyo Aluminium KK, assignee, European Patent 810,270, 1997. 56. C. B. Keemer, W. S. Halbach, W. G. Jenkins and R. L. Ferguson, inventors; Silberline Manufacturing Co., Inc., assignee, US Patent 5,558,705, 1996. 57. M. Suzuki, H. Nakaminami and S. Homma, inventors; Japat Ltd., assignee, European Patent 0 769 535 A2, 1997. 58. M. Suzuki, H. Nakaminami and S. Homma, inventors; Ciba Specialty Chemicals Holding, Inc., assignee, US Patent 5,718,753, 1998. 59. Nippon Chibagaigii KK, assignee, Japanese Patent 09132730 A. 60. F. Muller, W. Schmidt and H. Werner, inventors; E.I. du Pont de Nemours Inc., assignee, US Patent 3,536,520, 1971. 61. S. Yolles, inventor; E.I. du Pont de Nemours and Company, assignee, US Patent 3,053,683, 1958. 62. No inventor; The Mond Nickel Company, assignee, UK Patent 545,962, 1942.

34

4

Pigment Characteristics

This chapter describes the morphological, physical and visual properties of metal flake pigments in common use in or on polymers. A description of quality control criteria and test methods is included. There are several important differences between metal pigments and traditional organic and inorganic pigments. The main ones are particle size, specific gravity and particle geometry.

4.1 Morphology 4.1.1 Particle size The term ‘particle size’ is normally interpreted as describing the median of the particle size distribution. This is the size above (and below) which 50% of the total volume of the particles lie. It is also referred to as the D50 or more correctly as D[v, 0.5] to emphasise its volume basis. Measurement of D50 is generally accomplished nowadays by a Low Angle Laser Light Scattering (LALLS) technique, using apparatus from companies such as Malvern or Cilas. Metal flake is dispersed in a clear liquid, such as the white spirits of manufacture, at around 0.03-0.05 g/l of solvent. After a short period of ultrasonic agitation, the dispersion is allowed to relax to remove air bubbles. Laser light is then passed through the sample and the intensity of scattered light measured at a range of scattering angles some 2000 times, as the sample passes through the measuring cell. Computer calculation of results is based on an equivalent sphere that would produce the same scattering intensities as the flake. This is approximately a sphere of equal volume. Results are normally presented as ‘volume % in band’, volume % above’ or ‘volume % below’ graphs, on a logarithmic scale. The Malvern Mastersizer, for example, provides other useful parameters such as D10, D90, span and surface area. The first two represent the 10th and 90th percentiles. Together they provide an indication of the breadth of the distribution and the size of the high and

35

Metallic Pigments in Polymers low particle size tails, so relevant to colouristic properties. They also contribute to a dimensionless quantity called span, defined as D[v,0.9] − D[v,0.1] D[v,0.5]

Span is a useful indication of the tightness of particle size distribution in samples with approximately the same D50. The difference between the broad distribution envelope of a cornflake pigment and the much tighter distribution of a modern silver dollar is illustrated by Figures 4.1 and 4.2, respectively. Surface area measurements have absolute accuracy limitations, but nevertheless provide useful comparisons between samples. For all but glitter flakes, particle sizes tend to be quoted in microns. Products of wet milling range from around 5 to 650 µm. Glitter flakes start at around 100 µm and rise to 2000 µm or more. Organic and inorganic colorants typically have sub-micron dimensions. Metal pigments therefore have particles two or three powers of ten larger. This has implications in printing processes. Metal flakes with median diameters greater than about 20-25 µm are unsuitable for all but screen printing. In injection moulding of metal flake pigmented polymers, flake size may be constrained by the size of the pin gates. Particle size distribution is an important determinant of application properties. A tight distribution, i.e., a low span value, produces a brighter visual effect in a coating than a broader distribution with the same median particle size. This is because there are relatively few large flakes to disrupt smooth orientation or ‘lie down’ of flakes in the coating. There are also fewer small flakes, which although contributing opacity, have a darker appearance. Prior to the introduction of accurate particle size analysers, distribution was measured by washing a sample of paste through a stack of sieves using more of the hydrocarbon solvent of manufacture, according to wet sieving test DIN 53196 [1]. Product specifications were quoted as the percentage of particles passing through a given screen mesh size. This practice endures in modern data sheets. For screening test purposes, a North Gauge can be used to give a comparative indication of the quality of dispersion of the finer grades of metal pigment in a suitable medium. The apparatus, which consists of a recessed wedge down which the dispersion is drawn, is well known in the coatings industry.

36

Pigment Characteristics

Figure 4.1 Particle size distribution curve of a traditional ‘cornflake’ aluminium flake pigment

Figure 4.2 Particle size distribution curve of a modern ‘silver dollar’ aluminium flake pigment

37

Metallic Pigments in Polymers

4.1.2 Particle shape The flake shape of most metal pigments is in contrast to the more spherical appearance of most other colorants. This gives rise to unique visual effects, such as ‘flop’ in coatings (see section 4.3.3). The ‘cornflake’, whose manufacture is described in section 3.3.1, has traditional flake geometry. Its ragged edges are caused by fracture of the flakes during milling (see Figure 4.3). The more modern ‘silver dollars’ (see Figure 4.4) and degradation resistant flakes exhibit much more rounded edges. A comparison of the two illustrations clearly shows these differences. Pearlescent pigments share the same flake shape as metallic pigments. They are distinguished by much lower opacity and in uncolored forms, much increased whiteness. Their visual effect, as their name suggests, is of mother of pearl, rather than a metallic sheen.

Figure 4.3 A typical ‘cornflake’ type aluminium flake pigment [1 cm = approx. 10 µm]

38

Pigment Characteristics

Figure 4.4 A typical ‘silver dollar’ type aluminium flake pigment [1 cm = approx. 5 µm]

4.1.3 Aspect ratio Aspect ratio is defined as the ratio of the largest dimension to the smallest. For commercial spherical pigments, the aspect ratio is therefore close to unity (see Figure 4.5). For metal flake pigments, typical aspect ratios range from 10:1 or 20:1 for degradation resistant or glitter flakes (see Figure 4.6), through 30:1 to 100:1 for silver dollars, to 150:1 or even 200:1 for high opacity cornflake types. The most effective method of estimating aspect ratio is from low magnification electron micrographs. Degradation resistant flakes were developed in response to a need for improved ‘ring line’ stability in car plants. The ring line carries paint in a continuous loop from mixing tank to spray gun. Paint not applied by operating the gun is returned to the mixing tank. By maintaining the paint in motion, settlement of the heavier metal flakes in the lines is avoided. Thicker flakes are necessary to withstand the shear that occurs in the pumps when paint is circulated for lengthy periods. A frequent characteristic of low aspect ratio flakes is their low concentration of flakes of small particle size. Such flakes contribute disproportionately to opacity and appear dark.

39

Metallic Pigments in Polymers

Figure 4.5 Spherical aluminium pigment [1 cm = approx. 15 µm]

Figure 4.6 Degradation resistant aluminium flakes [1 cm = approx. 25 µm]

40

Pigment Characteristics Their elimination helps to maintain brightness. Low aspect ratio flakes also provide cleaner tints and softer tones when combined with pearlescent (mica) pigments [2].

4.1.4 Surface uniformity The last feature of particle geometry, surface uniformity, refers to the degree of surface smoothness. Although not readily measurable, an indication of surface condition is easily gained from a good optical microscope or an electron microscope operating at low magnification. It is a property that can be improved during pigment manufacture. The process used for ‘silver dollar’ flakes (see section 3.3.1) provides a higher degree of surface polish. Brush polishing in the case of dry milled flakes, or controlled mixing of wet milled pastes will increase the degree of surface polish. The visible outcome in application is a brighter and in the case of aluminium, whiter appearance.

4.2 Physical properties 4.2.1 Specific gravity The term specific gravity tends to be used interchangeably with density in describing metal pigments. Values reflect the specific gravities of the metals on which they are based, since any coatings, such as lubricant, tend to be very thin (see Table 2.1). A test method is described in DIN 53217 [3]. In all cases, values are greater than those of organic or organometallic pigments. The ramifications of this are that much higher concentrations of metal pigments are required in applications and the tendency for settlement in liquid coatings increases.

4.2.2 Water covering area The water coverage or water covering area (WCA) of a metal pigment is the value in m2/g of metal, obtained by the method described by Edwards and Wray [4]. The method is based on the fact that if the flakes in a known weight of dry flake powder are spread in a compact monolayer on the surface of water, the area they cover is linked to the flake thickness. The apparatus described by Edwards and Wray consists of a shallow rectangular tray, 75 cm x 15 cm x 1.5 cm deep. Two glass sheets are used to sweep the water filled tray to remove impurities. With one sheet at each end, a known weight of dry flakes is gently and evenly sprinkled onto the water. One glass plate is slowly swept back and forward, sweeping the flakes before it and gradually forming continuous film, free of 41

Metallic Pigments in Polymers visible water. The exercise is repeated with the second plate, until a coherent, monomolecular, wrinkle free film is formed. The distance between the glass plate barriers is measured. The whole sequence is repeated until consecutive measurements are constant. The area covered by one gram of powder is then calculated from the known weight of powder used and the area it occupied. A skilled operator can achieve measurements reproducible to within about 4%. Typical values range from 0.5–6 m2/g, depending on the coarseness of the material. A limitation of the technique is the leafing ability of the flakes. Should they become wetted by the water, the flakes will sink and no measurement will be possible. Accuracy may also be reduced by agglomeration of the particles. Measurement of WCA of metal flake pigment pastes is best achieved by washing out the hydrocarbon solvents with acetone and evaporating the residue to dryness.

4.2.3 Specific surface area Although surface area is a parameter provided by modern laser particle size measuring apparatus (see section 4.1.1), specific surface area (SSA) is a well-established technique in the pigments industry in general. The so-called BET (Brunauer, Emmett and Teller) method of measurement involves degassing the pigment surface and subsequently allowing nitrogen gas to re-coat it. From the volume of gas produced, the SSA can be calculated, expressed as m2/g of metal flake.

4.2.4 Heat and lightfastness A key attribute of metal flakes as pigments is their fastness properties. Whereas organic and inorganic pigments fade with continued exposure to light and may discolour with heat, metal pigments, in some cases with appropriate protective coatings, are immensely resistant. As none of the metal pigments in significant commercial use melts below 420 °C, their physical integrity will comfortably survive the highest temperatures attained in polymer processing equipment. Visual changes that occur are associated with chemical reactions of the metal surfaces with components of the polymers or coatings in which they are incorporated. Gold bronze and copper have some deficiencies in this respect above 200 °C during polymer processing, unless they are of the coated type. Lightfastness of all metal pigments is generally good. In trials, an organic or inorganic pigment will normally fail well before a metal pigment.

42

Pigment Characteristics

4.2.5 Chemical resistance The inherent chemical resistance of the various metal pigments has been mentioned in Chapters 2 and 3, but there are a few instances in which the metals also interact with the polymers in which they are incorporated. Of these, PVC is the most notable. Both aluminium and gold bronze, and indeed many of the other metal pigments, can react with the small quantities of hydrogen chloride released when PVC is processed at too high a temperature. In the case of gold bronze, decomposition of PVC is actually accelerated by the presence of copper and zinc ions. In such instances, silica coated, tarnish resistant grades of metal flake are recommended. Such grades are also specified to avoid colour shifts when gold bronze is used in the higher melt temperature polymers. Even so, it is wise to carry out trials before recycling reground scrap. When processing gold bronze pigments, the dwell time at the processing temperature should not exceed five minutes. Ideally, uncoated grades should not be allowed to exceed 190-200 °C and tarnish resistant grades 260 °C. Polyolefin wax carried granules, formulated with aluminium flake, can be successfully incorporated in polyacetal, providing the recommended moulding temperatures are not exceeded. Polyacetal can be difficult to mould and also decomposes at high temperatures, so care is needed when using it. Any formaldehyde formed by breakdown of the polymer at high temperature will attack the metal. Polymers with appreciable moisture content should not be co-dried with aluminium flake powders. The combination of water vapour and elevated temperature will cause oxidation and possible aggregation of the metal. The carrier component of aluminium flake pigment granules generally provides sufficient protection of the metal to permit simultaneous drying of the metal pigment and the polymer, without problems.

4.2.6 Magnetism Of the major metallic pigments, only iron flakes are magnetic. Stainless steel flakes tend to be of the austenitic type which have no inherent magnetism. It follows that in plastics applications, hopper magnets used to catch ferrous contaminants must be removed or electromagnets switched off when iron based pigments are in use.

4.3 Visual properties Metal pigments provide unique visual effects, derived from their chemical composition and physical characteristics. Depending on particle size and flake orientation, they can display a smooth, lustrous and uniformly metallic finish from fine particle sizes, through to the bright, highly sparkling finish obtained from coarser flakes.

43

Metallic Pigments in Polymers

4.3.1 Colour and brightness With the exception of alloys of variable composition such as gold bronze pigments, use of the word colour in the context of metal flake pigments is not interpreted in the same way as for their organic and inorganic counterparts. There is no property corresponding to colour shift, which would for example distinguish a red-shade yellow from a green-shade yellow. Metal pigment colour is a property of the pure metal itself. Only the cleanliness or brilliance can change, depending on particle size and its distribution, flake shape, surface finish and concentration. A colour shift is possible in the special case of gold bronze. A red-shade bronze results from a high copper to zinc ratio, whilst increasing the zinc content moves the shade towards the green. Even at a fixed ratio, it is possible to obtain a shift towards the green or red, depending on the oxidation conditions during and after milling. Until recently, colour measurement of metallic pigments was subjective and based on visual comparison of coatings or mass pigmented articles by trained colour technicians. The introduction of computerised colour measurement apparatus suitable for such pigments in the early 1990s by such companies as Optronik, Datacolor, X-Rite and Macbeth has allowed numeric description of colour. Figure 4.7 shows a typical printout from such a piece of

Figure 4.7 Colouristic comparison of a batch of aluminium flake pigment against its master standard. Typical commercial tolerance is ±1 ‘L’ unit. 44

Pigment Characteristics equipment. As the technology has advanced, lighter, more portable instruments have been developed to measure colour in situ, for example on an automobile paint spraying line. Although colour matching of solid colours is widely practised, the special properties of metal pigments and the relatively limited size of their market has delayed the introduction of a corresponding system for matching metallics.

4.3.2 Opacity Opacity, also called hiding power or coverage in the specific case of coatings, refers to the ability of a pigmented coating to obliterate a substrate or mass pigmented polymer. After colour, it is arguably the most important property of metal pigments, due to its large influence on formulation cost. The role of opacity in formulating is considered in section 7.3. Here is it sufficient to mention that opacity increases with decreasing flake size, flake thickness and specific gravity. The property is rarely measured in absolute terms or by instrumentation. It is most commonly compared visually with a master standard material incorporated at the same concentration by the same technique. Measurement of the opacity of aluminium flake pigments in printed ink films is described by Kern [5]. The relationship of opacity to flake characteristics and flake concentration in the film is also discussed.

4.3.3 Flop A property not available from conventional organic and inorganic pigments is variously called ‘flop’, ‘flip’ or ‘travel’. It is the apparent change of colour depth with angle of viewing, characteristic of all flake pigments. The effect is readily seen on any metallic pigmented coating applied to a curved surface in sunlight. Surfaces at right angles to the viewer appear bright. Those more parallel appear dark. The article is then said to have a bright face and deep flop. The origin of the effect lies in the almost two-dimensional nature of metal flakes. When incident and reflected light incline towards the normal, the path length of the light is minimised, as a high proportion of flakes are oriented for light reflection. The combination of these factors provides a minimal contribution from the coloured matrix. In contrast, at low angles of viewing, light travels between the flakes in a longer path length. In the case of a coloured formulation, more of the colorant is seen. A deeper shade then results (see Figure 4.8). The presence of an excessive proportion of ‘fines’ (small particle size flakes) in the coating reduces flop by reducing light transmission

45

Metallic Pigments in Polymers

Figure 4.8 The principle of flop

Figure 4.9 The effect of fines on flop Fines curtail the passage of light through the film at low angles (the flop angle) and they inhibit reflection.

through the matrix (see Figure 4.9). Thus the best flop effect is derived from pigments with tight particle size distributions.

46

Pigment Characteristics

4.3.4 Leafing and non-leafing Those metal flakes used mainly for their aesthetic effects, such as aluminium or gold bronze, are available in two forms, called leafing and non-leafing. The terms strictly refer only to coatings and describe the appearance of flakes in the application medium. They are determined by the nature of the lubricant used in the manufacturing process. Leafing pigments, as their name implies, have the ability to cover the surface of a suitable vehicle with layers of overlapping flakes much as leaves cover the ground in autumn (see Figure 4.10). The visual effect is of a particularly bright, continuous and uniform reflectance. There is very little light scatter, because the flakes are almost all oriented parallel to one another. In the flake manufacturing process, the lubricant generally employed to make a leafing pigment is a saturated fatty acid, such as stearic acid. When absorbed on the surface of the flakes, it exhibits both oleophobic and hydrophobic characteristics. The degree of leafing in any vehicle system will depend on the ability of that system to wet the flakes; the greater the wetting, the poorer the leafing. Solvents play a major role in developing the full leafing capability of the pigment, in both manufacture and application. The acid functions of the lubricant react with oxide or hydroxide groups on the metal surfaces to form a monolayer of protruding hydrocarbon chains. The poor wetting of these chains by many application media is responsible for what is observed as leafing. Surface tensions of solvents correlate with leafing ability. Toluene is one of the best solvents for the promotion of leafing. For health and safety reasons its use has given way to alcohols and esters, which are better wetting agents, but

Figure 4.10 Leafing metal flake pigment

47

Metallic Pigments in Polymers poorer leafing agents. Secondary alcohols and esters have lower surface tensions than their primary equivalents and so are less effective. Glycol ethers assist leafing because they have high surface tensions. As evaporation of volatile compounds takes place in a coating, convection currents are set up. These, combined with the oleophobicity of the flake surfaces, cause them to rise to the surface of the film. Surface tension holds the flakes in place. As further evaporation takes place, the viscosity increases, completely immobilising the flakes and leaving a layer of resin beneath the flake layer. Location of the flakes at the film surface makes leafing formulations extremely difficult to colour. The very opaque leafing film will mask any other colorant present in the system. For this reason, leafing finishes are almost always uncolored. On the other hand, the same interlocking orientation protects the vehicle and substrate. This almost impenetrable film greatly inhibits the passage of moisture or oxygen. Because the flakes are metallic, they are completely opaque to visible light and exhibit very high reflectivity for both UV and IR radiation. These properties make leafing films especially suitable for weather resistant and anticorrosion coatings. The degree of leafing is measured on a scale from 0 to 100. A leafing test was developed specifically for metal pigments and is not used elsewhere in the pigment field. Before measuring leafing value, a test medium must be prepared according to ASTM D480-88 [6] or the related DIN 55923 [7]. This is a solution of coumarone-indene resin in mineral spirits. It has very specific properties to ensure good reproducibility. In the leafing test, 1.5 g, or in the case of coarse grades 3.0 g, of metal pigment are dispersed in 25 ml of the vehicle and added to a test tube. A spatula of specified dimensions is inserted and rotated for about ten seconds (see Figure 4.11). Immediately on removal the spatula is suspended in a stoppered cylinder. After three minutes, two measurements are taken. The first is the height of immersion of the spatula. The second is the height of the bright leafing area on the spatula. The leafing value is determined by dividing the second measurement by the first and expressing the result as a percentage. Most commercially available metal flake pastes will have leafing values between 50 and 65%, but exceptionally up to 95%. The effect of the thickness of a stearic acid layer on leafing properties was investigated by Imasoto and Suzuki [8]. They found that as the stearic acid level was increased, a limiting concentration formed at the surface, corresponding to a trimolecular layer. The leafing value of this material was greater than that of either a bimolecular or monomolecular layer.

48

Pigment Characteristics

Figure 4.11 Leafing test apparatus

Non-leafing metal pigments are coated with oleophilic lubricants, the preferred one being oleic acid. Unlike leafing grades, there is good wetting of each flake by the medium in which it is incorporated. The flakes are thus dispersed uniformly throughout the coating (see Figure 4.12). Evaporation of solvent during drying will tend to orient the flakes more parallel to the substrate. The process is generally incomplete, so light tends to be scattered. The visual effect is therefore sparkling rather than uniformly reflective. In mass pigmented polymers, the terms leafing and non-leafing have no meaning. The melt viscosity of polymers is too high to allow true leafing to take place.

Figure 4.12 Non-leafing flake pigment

49

Metallic Pigments in Polymers

4.3.5 Sparkle The sparkle of a metal flake pigment is determined by its particle size and orientation. Applied to both metal flake pigmented coatings and mass pigmented systems, the term describes the ability of flakes to reflect light in a non-uniform manner. Flakes that all lie parallel to one another, as in the case of leafing pigments in a coating, will reflect incident light at the same angle. They therefore appear continuously, uniformly brilliant and exhibit no sparkle. Where flakes make a more random angle with the substrate, as in a non-leafing coating, a proportion will be oriented in such a way as to reflect light falling on their flat surfaces into the eye. Such flakes appear brighter than their neighbours. It is these pinpoints of light that collectively provide sparkle. Particle size is relevant because flakes need to be greater than 30 µm median diameter to be visible to the naked eye. Smaller flakes will show negligible sparkle, irrespective of their orientation, unless subject to high light levels. There is no commercial apparatus for accurately measuring sparkle or providing numerical comparison of samples exhibiting different levels of the property. It is another property determined by comparison with a master standard, a task for the experienced metal pigment technician.

4.3.6 Distinctiveness of image The next property to be considered, called distinctiveness of image (DOI), provides what is currently the closest approach to the measurement of sparkle. DOI is another property specific to metal flake pigmented coatings. Whereas gloss describes how much incident light is reflected from a coating surface, DOI refers to the uniformity of that reflection. Gloss is controlled by the nature of the coating as well as the pigmentation. DOI is influenced by flake orientation and particle size. Very random orientation and large particle size flakes will generate poor DOI. The property is measured by allowing light to shine through text of decreasing point size onto a metal pigmented coating (see Figures 4.13 and 4.14). The text is read by reflection from the surface of the coating until it can no longer be distinguished. A numerical value between 0 and 100 is assigned on an ascending scale corresponding to increasing DOI.

50

Pigment Characteristics

Figure 4.13 Apparatus used to measure distinctiveness of image. Part of the paint panel whose DOI is being measured is visible below the light box

Figure 4.14 Text screen from the DOI apparatus of Figure 4.13. It lies horizontally in the base of the light box, below the lamp

51

Metallic Pigments in Polymers

4.4 Glitter flakes The production process described in section 3.8 provides cut foil glitter flakes of regular geometry (see Figure 4.15). As mentioned previously, the hiding power of a flake decreases as its size increases. Eventually, the flakes are so large that even when incorporated in a polymer at a loading of several percent, there is a negligible contribution to opacity. In such cases, the visual appearance is of sparkling reflection of light from randomly oriented flakes, i.e., a glitter effect. The wet milling process described in section 3.3 can also be used to generate large flakes. Median particle diameters in the range 100 to 600 µm are feasible, but flake shape will never match the geometric regularity of cut foil grades. The process can however generate lower aspect ratios. Thicker flakes are necessary to withstand processing without bending or fracturing. Such a process can be operated more economically than that used for cut foil glitter, especially at the low particle size end of the spectrum. The apparent limitation of lack of geometric regularity is not a constraint in practice, as the eye is not easily able to detect the difference in particle shape up to about 300 µm. It is the light reflecting ability of the flake that appears to be the important factor. For an

Figure 4.15 ‘Glitter’ flake obtained by cutting aluminium foil [1 cm = approx. 50 µm]

52

Pigment Characteristics equivalent visual effect, it is generally necessary to use a wet milled flake of a rather greater diameter than a cut foil square. This is effectively because a disc has a lower surface area from which to reflect light than a square of the same width. The popularity of glitter flakes at low concentrations to provide sparkle in coloured polymers may be due in part to the observation that such formulations are virtually free of the flow line visibility problems discussed in Chapter 8. The flakes have such a low opacity and are so separated from each other that shadows between flakes are eliminated.

References 1.

DIN 53196 Testing of Pigments: Determination of Residue on Sieve Using Organic Solvents as Washing Liquid.

2.

Y. Imasato and M. Suzuki, inventors; Asahi Kasei Metals Ltd., assignee, European Patent 0451785 B1, 1991.

3.

DIN 53217 Determination of Density of Paints and Varnishes and Similar Coating Materials Part 1: Survey of Test Methods Part 2: The Pyknometer Method Part 3: The Displacement Float Method Part 4: The Hydrometer Method Part 5: The Vibration Method

4.

J. D. Edwards and R. I. Wray, Aluminium Paint and Powder, 3rd Edn., Reinhold Publishing Corporation, New York, USA, 1955, 16-22.

5.

G. M. Kern, et al., American Ink Maker, 1991, 69, 10, 60.

6.

ASTM D480-88 Standard Test Methods for Sampling and Testing of Flaked Aluminium Powders and Pastes.

7.

DIN 55923 Pigments; Aluminium Pigments and Aluminium Pigment Pastes for Paints; Technical Delivery Specifications.

8.

Y. Imasoto and M. Suzuki, Kotingu Jiho, 1991, 190, 19, in Japanese.

53

5

Delivery Forms

This chapter considers the forms in which metal pigments are sold by their manufacturers. It also describes the processes by which they are customised by those who purchase them for onward sale to an end user. Metal pigment manufacturers typically add value by supplying the metal in a form compatible with the end use. This specialisation has spawned an enormous number of variants, falling into eight broad categories as follows.

5.1 Dry powder Although dry powder is still sold, its market share is steadily declining, as safer and more environmentally friendly delivery forms evolve. For safety reasons, dry powder is sold in steel drums. Steel drums are less easily damaged than paperboard cartons and without the electrostatic hazard of plastic containers. Aluminium flake powder packaging must in particular retain the powder, even if roughly handled, as escaping powder, apart from being contaminating, is an explosion hazard. A greater percentage of gold bronze than aluminium is still sold in dry form, since bronze is both less combustible and less dusty due to its greater density. Dry powder is used in both coatings and mass pigmented systems. Its popularity in the latter is additionally limited by its poor wetting characteristics. For this reason it is sometimes damped with plasticiser by the manufacturer or end user before incorporation in polymer.

5.2 Paste Pastes have the consistency of butter and contain 60-90% metal, the exact percentage depending on metal density and particle size. The balance of the paste is solvent. Again the product is normally supplied in steel drums. The high volatile content makes the paste form unsuitable for the mass coloration of polymers. Solvent volatilises in the injection moulder or extruder barrel and cannot be adequately removed, even from vented extruders. Mouldings and extrusions are highly vesicular, covered in surface craters, resulting in an unacceptably poor surface finish.

55

Metallic Pigments in Polymers Pastes are however the traditional delivery form of metal pigments destined for surface coatings. Products intended for solvent-based automotive paints are generally sold in the white spirit of manufacture, blended with an aromatic hydrocarbon of similar boiling point to improve solvency in the application system. Variants targeted at the inks market have ‘faster’, i.e., more volatile solvents. These avoid the slow drying and residual odour problems associated with traditional hydrocarbon pastes in this application. Lower esters and ketones, such as ethyl and propyl acetate and MEK are common. These have no active oxygen atoms to react with aluminium. Lower alcohols and ether-alcohols are also used. Aluminium pastes in isopropyl alcohol are commercially available. Contrary to expectation, such pastes are fairly stable, implying that such alcohols have some passivating action. These pastes are also useful in water dilutable systems, since they are water compatible. Recent technical advances have produced aluminium flake pigments passivated against reaction with water. The various chemistries involved are listed in section 12.6. Such products are normally also offered as pastes in water miscible solvents. Water-based aluminium pigment pastes are also available, although there is some unease about their widespread introduction due to the unpredictable nature of the gassing reaction and the risk and financial consequences of any incident. It is more likely that solid, granular forms will become the norm in this market. In handling pastes containing faster solvents, it is important to keep the container well sealed when not in use. Any paste that dries out causes the flakes to aggregate. The paste is then very difficult to reconstitute. Appearance becomes gritty and the opacity is greatly reduced. This is another reason why in the longer term, granular product forms may predominate.

5.3 Dispersion in resin and solvent A dispersion of metal pigment, not in solvent alone, but in a low viscosity resin solution, constitutes a part finished paint or ink. A patent application by Eckart-Werke describes a process for preparing such a product form by milling in the presence of resin and solvent in the ball mill [1]. No fatty acids are employed. The products are directed to printing inks. Resin dispersions are not universally popular with coatings manufacturers as they are perceived as denying them formulating flexibility and an opportunity to add value.

5.4 Plasticiser dispersions Though appropriate for coatings, as noted previously, solvent-containing delivery forms are unacceptable for mass pigmentation of polymers. Even low boiling point solvents cannot

56

Delivery Forms be adequately removed by the heat of polymer processing. Indeed overpressurisation of barrel and screw by volatilised solvent can be potentially dangerous. Replacement of the solvent component of pastes by a non-volatile liquid eliminates this danger. After dry flakes, the next delivery form specifically for metallic pigmentation of polymers is the plasticiser dispersion, introduced in the 1950s. Unlike solvent, the liquid plasticiser is sufficiently high boiling to remain in the processed polymer. Heat stability is nevertheless limited, making them inappropriate dispersions for the highest melting polymers. A further drawback of plasticiser dispersions is their lack of versatility. Thus whereas dry flake pigments can be incorporated in most polymers, albeit with difficulty, plasticiser dispersions are more polymer specific. Typical types in current use include phthalate esters, such as di-octyl phthalate (DOP) and di-isodecyl phthalate (DIDP) for rigid and flexible polyvinyl chloride (PVC), PVC plastisols and styrenics. Mineral oil is a popular choice for the polyolefin group, the more so because grades suitable for direct food contact are available. Polyolefins processed with high concentrations of mineral oil pastes can be prone to screw slip. Concern over the toxicity of phthalates is prompting a search for substitutes (see section 16.1). Adipates and sebacates are contenders despite higher cost.

5.5 Granules The logical development of the plasticiser dispersion is the granule (see Figures 5.1 and 5.2). It consists of a high concentration of metal pigment, generally 70-90%, immobilised by solid carrier resins and/or polymers. It may be regarded as a concentrated masterbatch, but as it is not manufactured by the conventional masterbatch technique, it should more properly be called a colour concentrate. Introduction of metal pigment granules was pioneered by Silberline in the 1980s. Kern [2] claimed a specific composition for the mass coloration of polymers. The products, which took the trade name SILVET®, were combinations of terpene resin, a low density polyethylene and a metal flake pigment, preferably aluminium. The function of the terpene resin was to prevent the agglomeration of aluminium flakes which otherwise took place over time. A more broadly based patent was obtained by Wheeler [3, 4]. Low- or non-dusting, essentially non-volatile metal pigment granules were derived from a process which combined metal pigment, carriers and solvents. Either the mixture was formed into granules or alternative shapes and the solvent removed, or vice versa. Depending on the nature and concentration of the carrier, the products of the invention are used in waterand solvent-based paints and inks and in mass pigmented plastics.

57

Metallic Pigments in Polymers

Figure 5.1 Granular form of metal flake pigment

Figure 5.2 SILVET® aluminium flake pigment granules [13 x actual size]

Typical granules are cylindrical with a length of 3-15 mm and diameters of 3-6 mm. They offer several advantages over all the foregoing product forms. Chief amongst these are colour quality, safety and ease of handling. Unlike conventional masterbatch preparation (see section 5.6), SILVET® is manufactured by a low shear process. This maintains all the brightness and reflectivity of the original flakes. Granules are free flowing and low- or non-dusting. It follows that as well as being safe to handle, they can be easily metered in modern, automatic dosing equipment. This is a particularly useful feature in mass pigmentation of polymers, since it may avoid the need for an intermediate masterbatching or compounding stage. As granules are solvent-free, there is no possibility of them drying out in storage, with consequent loss of dispersibility.

58

Delivery Forms Carrier resins are chosen for compatibility with the intended end use, be it inks, paints or mass pigmented polymers. Those selected for polymer coloration generally have lower melting points than the polymers into which they are incorporated. This ensures that they melt and coat the polymer pellets before the pellets themselves melt. In this way, an excellent dispersion can be achieved without the use of high shear forces. Carriers for solvent-based ink and paint grades are synthetic aldehydes, ketones, acrylics, or less commonly, hydrocarbons or coumarone-indene resins. For water-based coatings, Silberline’s Aquavex ‘S’ and ‘P’ ranges, featuring surfactant and polypropylene glycol carriers, are widely used for inks and paints, respectively.

5.6 Dry masterbatch A masterbatch, otherwise known as a colour concentrate, is intended for dilution with uncolored (‘virgin’ or ‘natural’) polymer prior to conversion. Loadings of 20-50% are common in the industry, the balance being made up of other colorants, universal or polymer-specific carriers and various additives, such as antistatic or flame retarding agents. Typical metal pigment masterbatch concentrations are 1-5% w/w on polymer, the exact level being heavily dependent on the pigment’s particle size, density and concentration in the masterbatch. Masterbatch is divided into dry and liquid types. The latter is small in relation to the former. As entry barriers were low and distribution tended to be localised, there were until recently a large number of masterbatch makers, over 200 in Europe alone. More recently that number has reduced through acquisitions by the major players. PanEuropean converters are increasingly looking for pan-European suppliers who can provide a local supply, technical support and innovation via local subsidiaries. The total Western European masterbatch market in 1996 was estimated at 550,000 tonnes, worth around £1 billion, the majority being white and black [5]. The metallic pigmented sector probably accounts for less than 1% of the total, but is growing rapidly. So also is masterbatch in general, mostly at the expense of compound. Masterbatch coloration of previously difficult polymers such as acrylonitrile-butadiene-styrene (ABS) has become much more reliable.

5.7 Liquid masterbatch As the name implies, liquid masterbatch consists of colorants and optionally additives, incorporated in liquid carriers to give a fairly viscous pre-dispersion. The carrier, which is incorporated in the polymer, is often a plasticiser.

59

Metallic Pigments in Polymers Liquid masterbatch is added to the polymer by dosing pumps. Given suitable equipment, the technique allows very accurate addition levels and a high level of dispersion. Early drawbacks in clean down (the displacement of a (coloured) polymer from the equipment by using another polymer or else allowing it to run out) and colour change have also been largely overcome. Metal pigments present some difficulties, due to the combination of particle size and density. These conspire to induce settling in transit and in delivery lines. Nevertheless, lighter flake pigments such as aluminium are successfully used in smaller particle sizes.

5.8 Compound Compound, in this context, describes the combination of polymer and pigment (and possibly other performance enhancing additives in proportions such that the resulting mass is converted without further dilution into a finished article. As in masterbatch, the manufacturer may add antistatic or flame retarding agents, UV stabilisers and the like. For inexperienced converters, the use of compound gives fewest problems. All the work necessary to control colour, dispersion and other application characteristics has already been done. The main disadvantage is cost, though this is mitigated by the fact that compound producers also tend to be primary polymer producers. They therefore have the advantages of scale. The compound market in Western Europe is around three times that of masterbatch. Coloured compound production in 1996 amounted to over 1.5 m tonnes [6]. Again, metal pigmented compound has a very small, but growing market share. Key players are GE with their Magix range and DSM with Fantasy Colours. A potential disadvantage of using masterbatch or compound is that when the metal flake pigment appears in the final application, it will have been subjected to two potentially severe dispersion regimes. The first is to prepare the compound or masterbatch, the second to incorporate it in the finished article. Direct use of metal pigment granules reduces dispersion energy encountered by the flakes during incorporation, thereby maintaining metallic brightness. If the dispersion energy is too vigorous, flake damage results.

60

Delivery Forms

References 1.

Eckart-Werke, assignee, International Patent Application WO 94/28087, 1994.

2.

G.M. Kern, inventor; Silberline Manufacturing Company Inc., assignee, US Patent 4,544,600, 1982.

3.

I. R. Wheeler, inventor; Silberline, Limited, assignee, European Patent 134676B1, 1984.

4.

I. R. Wheeler, inventor; Silberline, Limited, assignee, US Patent 4725317, 1986.

5.

Plastics and Rubber Weekly, 1998, No.1750, 7.

6.

Thermoplastic Compounding Industry in Western Europe, Applied Market Information Ltd., Bath, 5th Edn., 1997.

61

6

Comparison of Mass Pigmentation and Coating

Mass pigmentation is the incorporation of the pigment in a molten polymer mass during processing into compound or masterbatch, or when an end user or converter prepares a finished article, for example an injection moulding. There is growing interest in replacing coatings by mass pigmentation. There are several reasons for this and in the following sections the advantages and disadvantages of the two techniques are compared.

6.1 Advantages of coating The two key advantages of a metallic coating over a mass pigmented equivalent are reflectivity, i.e., brightness or brilliance, and the potential uniformity of the coating. Use of the word potential is deliberate since poor application techniques can still cause surface blemishes requiring expensive repair. The appearance of metallic pigmented coatings is additionally influenced by the manner in which the flakes lie down in the dry film. If this is not uniform, the overall effect can appear mottled.

6.1.1 Brightness The issue of brightness or brilliance of metallic effect is one of the few in which coating has a very distinct advantage. This arises from the orientation differences between flakes in a coating and those in a polymer mass. A large proportion, sometimes over 80% by weight of a liquid surface coating is solvent. When applied to a substrate, the orientation of metal flakes will be somewhat random. As solvent evaporates, film thickness is reduced. This forces the flakes into an orientation broadly parallel to the substrate surface, because the flake diameter is comparable to the film thickness (see Figure 6.1). The result is a high proportion of flakes with their large surface oriented parallel to the substrate. They are therefore capable of maximising light reflection. In practice, flakes do not lie exactly parallel. An angle up to about 10° with respect to the substrate is common. This is sufficient to scatter light and provide the sparkling effect characteristic of metallic painted automobiles.

63

Metallic Pigments in Polymers

Figure 6.1 The influence of solvent evaporation on flake orientation in a dry coating

In contrast, a metallic flake pigmented injection moulding melt generally has limited flake orientation. Unless there is a mechanism to improve alignment of flakes parallel to the polymer surface, such as the biaxial stretching of polymer film, light reflection from these randomly oriented flakes will be lower.

6.1.2 Colour uniformity Almost irrespective of the method of application to a polymer substrate, a metallic coating exhibits uniformity of visual effect across the coated surface, providing it has been properly applied. Any visible imperfections of the substrate, such as flow or weld lines are obliterated. The best results are obtained by spraying or printing. Dipping is less satisfactory and the only real exception is brushing. Unless care is taken to ensure that brush strokes are all in the same direction, the result can look mottled. This is most acute with self-colours based on fine particle size flakes. It is seldom a problem with deep shades, such as black, hammer finishes, where other pigments provide the majority of the necessary opacity.

6.1.3 Flop Flop is a phenomenon characteristic of all lamellar particles including metal flakes. The observed effect is a change of colour with the angle of viewing. Its origin and effects are discussed in section 4.3.3. Here it is only necessary to note that it is characteristic of coatings which cannot be accurately replicated by a mass pigmented system. Stylists regard flop as a desirable property, that enhances the appeal of coatings.

64

Comparison of Mass Pigmentation and Coating

6.1.4 Application temperature For pigments that will not withstand polymer processing temperatures, coating may be the only suitable technique. Many organic pigments have insufficient heat stability. Those that do, tend to be much more expensive than their coating counterparts, thereby incurring a cost penalty. The temperature stability of metallic pigments is not generally a problem. Unless surface modified, they have the thermal properties of the metals themselves. An exception is gold bronze, which has limited stability in high melting polymers (see section 4.2.4).

6.1.5 Vacuum metallisation Although strictly outside the scope of this book, vacuum metallisation must be mentioned as it is a coating providing the ultimate in metallic reflectiveness. The technique involves allowing vapourised metal to impinge on an article in a high vacuum. The coating thickness is very low, being of the order of tens or a few hundred nanometres. Uniformity of finish is dependent on the surface contours of the article, as metal deposition faithfully follows its contours. Its quality cannot be matched by either a metallic coating or mass pigmentation. This very high reflectivity is harnessed in metal pigments, vacuum deposited onto a release film and subsequently disintegrated to flakes. The technique is described in section 3.7. Vacuum metallisation is expensive, due to the high construction and operating costs of a large, high vacuum chamber. Nevertheless, it is used where maximum reflectivity is essential.

6.2 Mass pigmentation advantages The advantages of using a mass pigmentation route over a coating route divide into colouristic, processing, environmental, legislative and cost issues.

6.2.1 Depth of coloration Coloration throughout a mass pigmented article is preferable to a relatively thin surface coating that can chip or delaminate to expose the often contrasting colour of the polymer substrate. This is a particularly important consideration in the manufacture of automotive lower body parts such as wheel and body trims. Mass pigmentation may also have advantages for complex moulded shapes, for example, where a sprayed coating cannot evenly fill recesses. The reverse is the case if the part

65

Metallic Pigments in Polymers requires tool inserts or sudden changes of thickness. Here the presence of flow and weld lines will disfigure the surface and a more uniform effect can be achieved by coating. Methods of reducing or eliminating flow and weld lines are described in chapter 8.

6.2.2 Single stage versus multi stage processing The main processing issue relates to the number of stages involved in the two techniques. Painting requires careful preparation of polymer substrates that in some cases are difficult to wet. This is particularly true of low energy surfaces such as polyolefins. Flame or chemical treatment may be required to activate the surface sufficiently to give adequate adhesion of the subsequent coating. A considerable technology, involving silane, titanate and zirconate bonding agents has grown up to address this problem. In this area, Kenrich Petrochemicals Inc., [1] is both a prominent supplier and provider of formulation guidance. In the case of metallic pigmented paints, there will usually be two coats; a basecoat of 10-20 µm in thickness, containing the metal pigment and a 30-50 µm thick clearcoat to provide protection. In contrast, mass pigmentation is achieved simultaneously with polymer processing in a single step.

6.2.3 Environmental and legislative pressures Environmental constraints are increasingly being attached to solvent emissions and the laundering and disposal of paint residues. The most significant recent legislation is the proposed EC Solvents Directive [2]. This suggests that the contribution of solvent use to the 12 million tonnes per annum of volatile organic compound (VOC) emissions in the EC is around 30%. VOCs would have to be reduced by 70-80% from 1990 levels to bring atmospheric ozone levels down to acceptable levels for the long term. Disposal of solid paint residues by landfill now incurs an additional tax, calculated by weight. The tax rate is likely to increase in real terms to encourage recycling schemes. Legislation is likely to tighten further in the future. Though there are moves towards water-based coatings, the capital costs of corrosion resistant plant are high. In contrast, mass pigmentation has negligible emissions. In most cases there is also no waste disposal. Sprues, runners and flawed mouldings or extrusions can be ground up and recycled, in either the same or a different product.

66

Comparison of Mass Pigmentation and Coating

6.2.4 Cost This is arguably the most important factor in today’s highly competitive marketplace. The capital and operational costs of paint application lines and drying tunnels are considerable. Repair of blemishes in the paint film, by rubbing down and re-coating, is labour intensive and therefore also expensive. Anecdotal evidence suggests that up to 40% of the cost of a moulded item such as a television cabinet or car wheel trim is associated with the painting operation. Against this background it is not difficult to see why mass pigmentation is gaining ground. This trend would have been more rapid but for the difficulty of matching the appearance of a painted part by mass pigmentation. The painted part always looks brighter for the reasons given in section 6.1.1. The extent to which the gap can be closed by optimising the formulation and incorporation of metallic, mass pigmented polymers is covered in Chapter 9. Where a coloured metallic effect is required, the cost advantage of mass pigmentation is reduced by the use of generally higher cost organic pigments with sufficient heat stability to survive processing.

References 1.

Ken-React Reference Manual, Kenrich Petrochemicals Inc., Bayonne, New Jersey, USA, 2nd Revised Edition, 1993.

2.

Proposed Council Directive, Commission of the European Communities, COM(96) 538 Final; 96/0276 (SYN), November 1996.

67

7

Mass Pigmentation Application Characteristics

This chapter looks at the effect of the inherent properties of metal flake pigments, described in Chapter 4, on mass pigmented systems. The term ‘mass pigmentation’ encompasses all the processes by which metal flakes are combined with polymer to ultimately form a finished article. Of these techniques, injection moulding is the most technically challenging, because it has the most variables requiring control.

7.1 Colour As discussed in section 4.3.1, colour in the context of metal pigments generally means brilliance, otherwise referred to as brightness. It broadly corresponds to cleanliness in organic pigments. Metallic colour is influenced by the surface finish of the pigment, its tint strength (see section 7.3) and its concentration. In the specific case of aluminium pigments, improved colour equates to increased whiteness. These properties are in turn dependent on the particle size distribution and the surface smoothness of the flake. There is a complex relationship between colour, particle size, opacity, loading and cost. This is summarised in section 7.7, after these factors have been considered individually.

7.2 Dispersibility The process of dispersion starts with a combination of metal flake and polymer in the dry state. The hazards of blending dry aluminium flake with polymer are sufficiently well known for this practice to have given way to masterbatch pre-blending. Masterbatch in this context covers metal flake pigment damped or carried by any suitable organic material, such as a plasticiser or a polymer. A short period of dry tumbling, for example in a double cone blender, should be sufficient for most commonly available aluminium pigment masterbatches. High speed powder blenders are not recommended for two reasons. Firstly, there is a danger that the high shear will break down the masterbatch, releasing single flakes with a consequent danger of explosion. Secondly, the same high shear can bend or break the flakes, leading to a loss of metallic brightness.

69

Metallic Pigments in Polymers For most applications, it is not necessary to pre-dry aluminium flake masterbatches. Where polymers are routinely dried, the masterbatch-polymer premix may be dried together, unless the masterbatch carrier becomes molten at the drying temperature. Most metal flake pigments are very resistant to heat. The flakes themselves remain stable well above the highest polymer processing temperatures. In practice, the maximum processing temperature is often dictated by either the masterbatch carrier or the polymer.

7.3 Opacity and tint strength Opacity or hiding power is a function of flake diameter, thickness and the density of the metal. At constant thickness and density, opacity is roughly proportional to the inverse square of the diameter. The smaller the diameter and/or thickness, the greater the number of flakes per unit weight and the higher the total surface area. With more surface area available to obliterate the substrate, opacity is higher. This relationship is explored further in section 7.7. Tint strength refers to the ability of a metal pigment to modify the colour depth of a colorant with which it is incorporated. Thus, a deep blue metallic effect will become paler if further metal flake pigment is added to the formulation. ‘White’ pigments such as aluminium will cause little change in hue. Gold bronze moves the effect towards the green. A fine particle size flake will create a greater colour shift than the same weight of a coarser flake. Increasing the flake thickness, i.e., reducing the aspect ratio, is practised to improve degradation resistance. It is inevitably at the expense of opacity. Flake damage can have an ambivalent effect on opacity. On the one hand, folded and bent flakes have reduced effective surface area. On the other hand, if the flakes become thinned and break, effective surface area is increased. This is however accompanied by loss of brightness, due to both the increased concentration of fine particles and the disruption of uniform orientation caused by bent and folded flakes. Loss of opacity becomes very marked at large particle diameters. For this reason, very large aluminium flakes are generally used to provide a random sparkle effect in combination with other colorants. The large density range encountered with metal flakes greatly influences opacity. About two to three times as much gold bronze as aluminium flake of comparable geometry would be required for equal opacity. The relatively low density of aluminium is undoubtedly a contributing factor to its popularity.

70

Mass Pigmentation Application Characteristics Pigment concentration or loading is determined by the visual requirements of the pigmented article. It is also likely to be linked to whether opacity is essential, desirable or irrelevant. Commercial concentrations of metal flake pigments are much higher than organic or inorganic colorants. Even the finest metal flake can require 0.2% or more to make a 1 mm thick section of transparent polymer opaque. Larger flakes need considerably more, so that in practice, concentrations of 1-4% are common. Organic and inorganic pigments tend to be used at around a tenth of these levels, due to their much smaller particle diameters, as well as higher inherent tinctorial strength.

7.4 Orientation For a metal flake pigment, orientation influences brightness and to some extent opacity. The terms ‘leafing’ and ‘non-leafing’, so relevant to coatings (see section 4.3.4), have no relevance to mass pigmented polymers because the melt viscosity is too high for leafing to take place. Flakes that reflect light from the full area of their faces appear very much brighter than flakes presenting their edges. In mass pigmented articles, orientation is determined by polymer flow characteristics. Flakes tend to align themselves to reduce their resistance to flow.

7.5 Mechanical properties Metal flakes in polymer melts tend to cause an increase in viscosity if added as dry flake. This is attributed to poor wetting of the flake surface. In elaborated products, such as plasticiser dispersions and granules, the carrier can have a more marked effect than the metal flake itself. Many carriers have a lubricating action, which serves to decrease the melt viscosity and therefore increase the melt flow index. Low loadings of metal flake pigments in polymers generally cause a very little change in tensile and impact strength. Indeed in certain polymers, particularly polyolefins, these properties may be enhanced. Tables 7.1 to 7.7 show the effects of including granular aluminium flake pigments in a range of common polymers at loadings between 0.5 and 6% by weight of polymer. A number of conclusions can be drawn. For example, the degree of mechanical strength loss is broadly proportional to metal flake loading. Larger flake particle sizes are less prone to strength loss, whilst the more polar polymers, such as ABS and polyamide, show the greatest percentage loss at higher loadings.

71

Metallic Pigments in Polymers Polyolefins exhibit the best mechanical property retention. This may be because the thin fatty acid lubricant coating on the flakes has a largely hydrocarbon structure, similar to that of the polymer. This would be expected to enhance chain entanglement in the melt. However, even in polyolefins, mechanical properties are usually reduced at higher loadings.

Table 7.1 Percentage change in mechanical properties as a result of pigmentation of PP by aluminium flake pigment granules Metal particle size

Metal %

Tensile break

Elongation

10 µm

0.7 1.4 2.8 5.6

-4 -5 -5 -6

-42 -47 -63 -86

30 µm

0.7 1.4 2.8 5.6

-1 -2 -3 -4

-38 -65 -67 -79

Table 7.2 Percentage change in mechanical properties as a result of pigmentation of elastomer modified PP by aluminium flake pigment granules (NC = no change) Metal particle size

Metal %

Impact strength

Tensile break

Elongation

1 0 µm

1.0 1.5 2.0

+15 +24 +29

NC NC +4

-23 -64 -76

3 0 µm

0.5 1.0 1.5 2.0

-5 -4 -5 NC

+4 +1 -1 -2

-63 -51 -61 -59

7 5 µm

1.0 1.5 2.0 3.0 4.0

-20 -26 -34 -36 -30

+1 +3 +4 +1 +4

-59 -69 -74 -75 -74

72

Mass Pigmentation Application Characteristics

Table 7.3 Percentage change in mechanical properties as a result of pigmentation of high impact PS by aluminium flake pigment granules (NC = no change) Metal particle size 36 µm

Metal %

Impact strength

Tensile break

Elongation

1.0 5.0

NC NC

-8 -11

-25 -30

Table 7.4 Percentage change in mechanical properties as a result of pigmentation of crystal PS by aluminium flake pigment granules Metal particle size

Metal %

Impact strength

Tensile break

Elongation

10 µm

0.7 1.4 2.8

-38 -13 -21

-13 -21 -20

-64 +62 +136

3 0 µm

0.7 1.4 2.8 5.6

-17 -18 -3 -14

-16 -18 -20 -22

-4 +22 +64 +118

75 µm

0.7 1.4 2.8 5.6

-18 -24 -5 -2

-12 -12 -14 -23

-18 -16 -6 +12

Table 7.5 Percentage change in mechanical properties as a result of pigmentation of ABS by aluminium flake pigment granules Metal particle size 36 µm

Metal %

Impact strength

Tensile break

Elongation

0.7 3.5

-20 -60

-2 -4

-52 -56

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Table 7.6 Percentage change in mechanical properties as a result of pigmentation of polyamide by aluminium flake pigment granules Metal particle size 3 0 µm

Metal %

Impact strength

Tensile break

Elongation

0.7 2.8

-27 -34

-2 -5

+4 +2

Table 7.7 Percentage change in mechanical properties as a result of pigmentation of plasticised PVC by aluminium flake pigment granules Metal particle size 10 µm

Metal %

Tensile break

Elongation

0.7 1.4 2.8 5.6

-8 -7 -7 -7

-3 -4 -5 +3

Figure 7.1 Electron micrograph showing capillary channels at a fracture surface [1 cm = approx. 35 µm]

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Mass Pigmentation Application Characteristics Microscopic examination reveals the reasons for loss of mechanical properties. Figure 7.1 is an electron micrograph of the fractured surface of an impact test specimen pigmented by metal flake. The lack of bonding between the metal flake and the polymer matrix is clearly visible. At least part of the reason is believed to be the difference in the coefficients of thermal expansion of metal and polymer matrix. Thus, as a pigmented polymer cools from the melt phase, differential shrinkage occurs. Unless there is efficient bonding between metal and matrix, capillary channels are formed as in Figure 7.1. These interfaces are sites of mechanical weakness, allowing crack propagation to take place more readily. The result, at a sufficient flake concentration, is the observed reduction in mechanical properties, particularly impact strength.

7.6 Cost The cost of a metal pigment in a formulation depends upon the metal concentration in the pigment, the loading and the unit price. Unit prices for both aluminium and gold bronze pigments are comparable with the least expensive classical organic pigments. Although comparably priced at the commodity end of the market, the range of aluminium pigment prices reaches a ceiling well above that for gold bronze. Cost in use has to take account of the greatly reduced tinting strength of metal pigments, controlled by their density and particle size. As described in section 7.3, loading has an inverse relationship with particle size. The unit cost of the pigment is less influenced by particle size than by the tightness of the particle size distribution and by its colour. Particle size only influences cost in the respect that from any given starting atomised powder, finer particle size grades tend to be derived from a longer and therefore more costly milling regime. A tight particle size distribution implies that more of the milled product is rejected at the screening stage. Thus the process yield is low. Pigment colour is improved by a slower speed, more gentle milling action and a reduced metal charge in the mill. If taken with a low process yield, the result is an increase in unit production costs that have to be recovered by a higher selling price. Dry granular forms attract a further price premium for their handling advantages. This helps to offset their production costs, which are inevitably higher since granules are derived from pastes. Compared at equal metal content, the granule must bear the higher unit cost of carrier compared to solvent, plus the extra processing costs associated with granulation and drying. In coating systems, the carrier is generally also a film former. Its use reduces the amount of solid binder required, which in turn offsets the higher cost of the granule.

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Metallic Pigments in Polymers

7.7 Interrelationships The complex relationship between colour, particle size, opacity, loading and cost referred to in section 7.1 is illustrated for aluminium flake pigments in Figure 7.2. The y axis displays some measure of colour (brilliance). Pigment concentration (loading) is plotted on the x axis. Small, medium and large median particle size grades are displayed.

Figure 7.2 Relationship between colour, particle size, opacity (decreases with increasing particle size), loading and cost

The smallest particle size material with its high opacity requires a low concentration to reach colour saturation, defined as the point at which even with increased pigment loading, no further increase in brightness is obtained. A medium particle size grade requires a higher loading to attain saturation, but the colour at that point is brighter. The same applies to the coarsest grade. Cost also increases with loading, so in practice, cost-effective formulating demands that a compromise be reached. Although generally less expensive per unit weight than organic or inorganic colorants sold for mass pigmentation of polymers, metal pigment loadings can be five or ten times higher. Coloured metallic formulations may contain large flake grades which are more cost-effective. The colorant can provide the opacity that the large flakes lack.

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Mass Pigmentation Application Characteristics The foregoing relationships may be summarised as follows: Fine particle size

Coarse particle size

Dark colour and less bright

White colour and increased brightness

High opacity

Low opacity

Low sparkle

High sparkle

0.25%-1.5% loading

1.5%-5% loading

Low cost in use

High cost in use

7.8 Compatibility The metals used as metallic pigments are compatible with most polymers, however there are a few combinations to avoid. No pigments except tin have any problem with heat stability at normal thermoplastic moulding temperatures. Aluminium pigments can react with PVC at high temperatures. The metal does not appear to promote polymer decomposition but will react with the hydrogen chloride breakdown product. In the case of polyacetal, generation of formaldehyde at high temperatures does appear to be exacerbated by the presence of the metal. In both cases, processing is successful if normal operating temperatures are not exceeded. Lead stabilisers should be avoided, as they reduce the metallic effect. Tin stabilisers are satisfactory. When incorporating high loadings of aluminium or gold bronze flake into polyolefins, for example when preparing masterbatch, an unpleasant smell can be caused by decomposition of the fatty acid lubricant on the flakes. This can be eliminated by addition of antioxidants. Only 0.1-0.2% is generally necessary. A 1:1 combination of Irganox 1010 (Ciba) and MD 1024 (Ciba) is a popular choice. Gold bronze also presents some challenges when used with polyolefins (generally processed at lower temperature). At high temperatures the polymers become oxidised and brittle if processed with copper, either as copper flake itself or from that present in gold bronze. Surface coated grades, such as the Tarnish Resistant range from Wolstenholme International, provide improved stability. Ideally, however, articles in which strength is important and/or which are intended for extended service should not be pigmented with copper-containing pigments. Alternatively, an aluminium flake with a red shade, yellow colour or an orange organic pigment could be used to achieve gold shades. Other instances of incompatibility are a consequence of the nature of the carrier. It is one of the few merits of dry metal flakes that such compatibility issues do not exist. In all

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Metallic Pigments in Polymers other product forms, such as plasticiser dispersions and granules, some constraints apply. The historically popular phthalate ester carriers are most compatible with PVC and styrenics. Mineral oil types are best restricted to polyolefins, not so much because of chemical compatibility as limited heat stability. Processing with mineral oils above 250 °C is not recommended. Similarly, phthalate esters begin to degrade above about 260 °C. This precludes the use of such materials in the newer engineering polymers. Commercially available granules generally have modified polyolefin carriers, stable to around 300 °C. They do not have truly universal compatibility, but do find use in all the common polymers, including styrenics, polyolefins, polyesters, polyurethanes, polyamides, polycarbonate, acrylics and polyacetal. Of the common polymers only PVC and ethylenevinyl acetate copolymer (EVA) occasionally give problems. In the former case, this is more likely to be a result of incompatibility with additives in the polymer than with the polymer itself.

7.9 Spherical metal pigments Being almost true spheres, these pigments have no orientation in plastics and therefore do not accentuate weld and flow lines by conventional orientation mechanisms. Spherical metal pigments offer pinpoint light reflection (sharp points of light, reflected from a small area of a sphere’s surface). The remainder of the surface area is non-contributory. If used alone, such pigments have very low opacity and appear less bright than flakes of comparable diameter. A combination of flakes and spheres can capitalise on the beneficial properties of each. The flakes contribute the brightness and opacity that spheres lack. The ratio of flake to spherical pigment is critical. As flake pigment is introduced into the formulation, it contributes brightness. Due to the mitigating effect of the spherical pigment, a higher flake loading can be achieved before flow and weld lines become visible. The overall effect is brighter and more uniform. A useful advantage of spherical pigments in processing terms is their high degree of shear resistance.

7.10 Metal flake pigments with coloured surfaces Pigments in this category cover a wide range of visual effect and particle size. Together with coloured glitter flakes considered in the next section, they have become known as

78

Mass Pigmentation Application Characteristics ‘effect’ pigments. Many are supplied in dry powder form, especially in the larger particle size ranges. As a result, wetting into a polymer can present challenges. Extended mixing or compounding promotes detachment of the coloured surface, as can excessively high processing temperatures. In the latter case, the coloured coating can melt and disperse in the polymer matrix. It is therefore important to incorporate these products with the absolute minimum of shear and at the lowest possible temperature. When used in coatings, bleed of the colorant into the application solvent is likely to be a greater problem than shear damage. More modern products have improved properties in this respect.

7.11 ‘Glitter’ flakes The most influential characteristics of glitter flakes are their high sparkle and lack of opacity. Adequate wetting can be a problem with large, dry flakes in low surface energy polymers. For this reason they are best added as granules or plasticiser dispersions. Glitter flakes have very low surface areas, so elaborated forms require very little carrier to meet dispersibility requirements. They therefore provide greater formulation flexibility and have wider polymer compatibility than conventional flakes.

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8

Flow and Weld Lines in Mass Pigmented Applications

8.1 Description and origins Together with poor colour, the prominence of flow and weld lines in metal flake pigmented polymers is cited by plastics converters as one of their main problems. Indeed flow and weld lines arguably present the greatest challenge to the wider introduction of flake pigments in general. The assignment of a whole chapter to this topic demonstrates the importance of the subject. This chapter describes the origins of flow and weld lines and explains why they are more visible with metallic flake pigmentation. Overcoming flow and weld lines can be split into three distinct areas. Methods are described that reduce or even eliminate them by optimising equipment design and operation. Weld lines can also be mitigated by skilful formulation, whilst the third technique is improved pigment design. Techniques in these last two categories are described in Chapter 9. In practice, the best results are obtained by combinations of all three techniques, rather than using any of them in isolation. Flow and weld lines are principally a phenomenon of injection mouldings, but can also be seen in extrusions, blow mouldings and other less common polymer processing techniques. Flow lines may occur where a localised discontinuity in polymer flow alters polymer chain orientation. They may also be caused by moulding defects which allow frozen off (solidified) polymer, perhaps from the barrel tip, to be carried into the cavity with the next shot. Flow lines may be present, though not necessarily visible, in mouldings from natural or virgin polymer. However, pigmentation can increase visibility markedly, especially with high aspect ratio pigments, such as fibres or flakes. Common locations for flow lines are on the cavity side of pin gates, in thinned sections of an injection moulding or where the melt is required to undergo sharp changes of direction (see Figure 8.1). Heuzey and co-workers [1] studied flow marks in injection moulded linear polyethylene (PE). They concluded that injection speed was the main controlling factor. A previously suggested cause, wall slip, was found to have no influence on the generation of flow marks. Microscopic observation instead suggested that flow lines result from filamentation and stretching of semi-solidified material.

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Metallic Pigments in Polymers

Figure 8.1 Flow lines in an article pigmented by metal flakes

Weld lines are also known as meld lines, knit lines, flow defects or flow aberrations. A comprehensive review of the subject is presented by Malguarnera [2]. Weld lines are described as complex, three-dimensional areas of finite dimensions, occurring whenever two or more molten polymer interfaces are brought into contact. They have aesthetic, morphological and mechanical properties which are generally inferior to those of the polymer making up the rest of the component, even though processing conditions are the same. If appreciable melt cooling has already occurred in the cavity, the melt fronts may not even fuse completely on contact, thereby exacerbating mechanical weakness. The term ‘meld line’ merits further comment. It is used interchangeably with weld line, but strictly refers to the type of weld line in which broadly parallel melts fuse laterally. Thus ‘weld line’ describes the collision of opposing polymer melt fronts. The distinction is somewhat arbitrary, but a meeting angle between two melts of 130°-140° is generally accepted as the boundary (see Figure 8.2) [3].

Figure 8.2 Weld and meld line formation in metal flake pigmented polymer

82

Flow and Weld Lines in Mass Pigmented Applications The weld line phenomenon has been increasingly studied from the early 1980s. Three types of weld line are identified. Multiple-gated injection mould tools create melt fronts that have to impinge at some point. This is the true weld line, occurring by impact of melt fronts coming from opposite directions (see Figure 8.3). The second arises from recombination of two melt fronts derived from a single melt front that has divided around an obstacle in the tool. The fronts are moving parallel to each other in the same direction, eventually coalescing laterally to form a meld line (see Figure 8.2). The mould filling sequence that generates this type of weld line is shown in Figure 8.4.

Figure 8.3 Weld line arising from the impact of opposing melt fronts

Figure 8.4 Mould filling sequence showing the generation of weld and meld lines by division and recombination of melts

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Metallic Pigments in Polymers

Figure 8.5 Weld lines caused by jetting

The third type of weld line is less common, since it results from a moulding defect. Jetting or squirting of melt into the tool cavity causes a snaking appearance in which the melt fronts again coalesce from a parallel orientation (see Figure 8.5). In this case, however, the directions of flow are opposed. In extrusion, weld lines result whenever there is an internal mandrel in the die head. Such a configuration typically occurs in producing hollow objects with a constant cross section, such as pipes. Weld lines of the parallel flow type occur where the melt flows divide around the die supports to recombine downstream. The number of weld lines is defined by the number of die supports or ‘spider’s legs’ used. In blown film, weld lines are deemed satisfactory if the appearance and weld strength are adequate for the packaging applications for which they are mainly used. In blow moulded containers, welds form by a similar mechanism to extrusion, since the first stage of blow moulding is extrusion of a slug of polymer known as the parison. Where this melt is split by the mandrel support or the mandrel itself, recombination generates a weld line which may incompletely fuse during the inflation stage. A fault known as parison pleating, in which polymer emerging from the extruder has the appearance of a frilled skirt, is similar to jetting in injection moulding. Weld lines also form when the parison is pinched off by the closing mould halves, immediately prior to inflation. Despite the many different polymer conversion techniques, Malguarnera contended that the physical nature of weld lines that exist in almost every manufactured plastic item is essentially the same.

84

Flow and Weld Lines in Mass Pigmented Applications

8.2 Tool design for injection moulding The performance of flow and weld regions can be changed by modifying tool design and moulding parameters. The essential requirement is to employ any mechanism that encourages molecular entanglement of impacting polymer chains to re-establish the morphology characteristic of areas remote from the weld region. Hobbs [4] studied isotactic PP using a mould containing a pin in the cavity around which the melt was forced to divide. The mould temperature was varied from 25 °C to 220 °C whilst holding the melt temperature constant. As the mould temperature was raised, the surface appearance of the weld line formed beyond the pin improved until at 180 °C it was invisible. Hobbs was also able to show that air entrapment in the weld region should be avoided as it can cause a surface notch coincident with the weld line. Hagerman [5] concurs, based on his study of four grades of ABS by electron microscopy. Sufficiently high melt and tool temperatures assist recovery of mechanical properties at the weld line. Weld lines produced by melt flowing past inserts in an injection moulding tool were also studied by Barrick [6]. He observed that the relative tensile strength of weld lines in high melt flow index materials was greater than that for low melt flow index polymers. Weld strength decreased as the distance from the gate increased. This was interpreted as being due to melt cooling which progressively occurred as the melt moved away from the gate region. The study concluded that melt index and density were the most important material properties affecting weld line strength. Malguarnera [7] investigated the effect of melt temperature, mould temperature, injection speed and cooling time on the weld strength of freshly moulded and annealed specimens of a high flow and a low flow rate PP. It was found that annealing and higher melt and mould temperatures strengthened weld lines in both materials. Higher injection speeds had only a slight positive affect on the low flow rate material at low melt temperatures. Cooling time had little effect. A later study of four commercially available grades of PP, also involving Malguarnera [8], concluded that melt and mould temperatures and annealing had the greatest influence on the weld region’s tensile properties. Confirmation that weld line impact strength is directly related to the extent of diffusion across the weld line during processing came from Pecorini and Seo [9, 10]. They developed a model describing polymer self-diffusion which correctly predicted the relation between weld line healing and injection moulding temperature for a plasticised cellulose acetate propionate (CAP) resin. The 42nd SPE Annual Technical Conference in 1984 provided two contributions to weld line knowledge. Kim [11] was able to predict and experimentally correlate the strength 85

Metallic Pigments in Polymers of weld lines from PS processing parameters by determining the effect of molecular orientation and bonding. In a study of tensile properties of polymethyl methacrylate (PMMA), styrene acrylonitrile copolymer (SAN), polystyrene (PS) and polycarbonate (PC), Mosle [12] suggested that the influences of weld lines could be minimised by lowering the viscosity of the melt during injection moulding. This theme was continued by Piccarolo [13] in a comparison of the weld line performance of PS and Nylon 6 under a wide range of processing parameters. The width of grooves forming at weld lines was mainly related to the operating conditions influencing flow front temperature. The mechanism of weld line formation was determined by the viscosity of the layers, which squeezed towards the mould wall after the melt fronts collided. Janicki and Peters [14] of Dow Chemical Co., claimed that mould temperature had no effect on amorphous, general purpose PS. The average molecular weight however was significant, a higher molecular weight leading to increased weld line strength. The relationship between weld line properties and the relaxation behaviour of the polymer melt was studied by Hamada and co-workers [15]. A polymer melt with a relatively short viscoelastic relaxation time was found to have superior weld line strength. This means that such polymers can relax, and entanglement in the weld line region increases smoothly before the melt is frozen off. The relaxation behaviour was also demonstrated by optical data that corresponded to molecular orientation. Two studies by Merhar [16, 17] looked at the influence of processing parameters on weld line formation in PVC. Taguchi design of experiments was used to show that mould temperature was the most important factor affecting weld line strength. Reductions in strength were observed at the highest temperature and injection rate, but this was ascribed to thermal degradation or the complex morphology of the material. Higher melt and mould temperatures produced more brittle parts, while higher injection rates increased the ductility, thought to be due to variations in crystallinity at the weld line. Overall, the grade of PVC studied was insensitive to both strength loss at the weld line and to processing conditions. As early as 1951 an ingenious glass faced injection mould allowed Gilmore and Spencer [18] to observe the behaviour of molten polymer flow inside the mould. They were able to photograph severe retardation of the melt as it passed a cavity insert. More significant was the pocket of air entrapped when the flow fronts recombined. Air entrapment at the weld may be at or below the surface, the latter as voids. Both affect the appearance and strength of the weld region. Yokoi [19] later used a similar glass faced mould to analyse weld line formation. A method was developed which clearly determined a specific meeting angle between two melt fronts and where a weld line vanished. The flow-front meeting angle could be used to predict when 86

Flow and Weld Lines in Mass Pigmented Applications a weld line would disappear, almost regardless of moulding conditions or cavity shape. The vanishing angle ranged from 118° to 148°, depending on the nature of the polymer. In the early 1990s, the rapidly increasing power of computers allowed ever more sophisticated models to be constructed to predict flow behaviour in injection mould tools. It was not long before companies such as C-Mold, and Moldflow turned their attention to developing an automatic weld line interpretation algorithm. Lautenbach [20], described such a programme in 1991. It accurately predicts the correct number and location of actual weld lines on two production moulded parts. A slight discrepancy in predicted weld line length versus actual visible length was noted, but satisfactory refinement of the model is anticipated. The following year, Kuvshinikov [21] reported the application of a Moldflow programme to successfully predict the appearance of a weld line in a low gloss ABS automotive dashboard panel. A theory to predict the tensile strength of weld lines, based on the effects of molecular orientation and bonding at the melt front interface, was presented by Jong, Chan and Wu [22]. The theory is integrated with computer aided engineering results and the interpretation of weld line locations to predict weld line strength. The results obtained showed good agreement with experimental data. Mekhilef [23] developed a model for the prediction of weld line strengths of amorphous polymers based on diffusion and free energy concepts. The concept was extended to a study of weld line strength of injection moulded polycarbonate/high density polyethylene (PC/HDPE) blends. Specimens were tested for tensile strength and phase morphology studied by scanning electron microscopy (SEM). These showed the importance of blend morphology in characterising the structure and adhesion at the melt interface. Acceptable agreement between prediction and experimental results is claimed. Lubricants may have a modifying influence on weld line strength. A study by Herten [24] proposed that lubricants reduce the viscoelastic stresses at the plastic/mould interface. The flow behaviour of lubricated ABS was correlated with weld line strength measurements of injection mouldings.

8.3 Orientation in multiphase and glass filled polymers There is very little literature describing the flow and weld line performance of metal flakes. Much work has however been carried out on traditional filled polymers, including short cut glass fibres. Some pointers to metal pigment performance can be derived from these.

87

Metallic Pigments in Polymers In 1990, Miller [25] cited injection speed, barrel and mould temperatures, back pressure and screw speed as the critical parameters to note, to avoid the pitfalls in moulding reinforced thermoplastics. The following year, Lalande [26] reported on a study of PP reinforced by 30%, 40% and 50% of long glass fibre. Fibre orientation in the matrix governed mechanical properties. Morphology at the weld line gave a parallel orientation of the fibres to the flow front and to the weld line. This orientation gave a low tensile strength because the stress at the weld line must be borne by the matrix alone. Thamm [27] studied the phase morphology of PP/EPDM and PP/HDPE/EPDM blends, which are expected to have high impact strength. The PP has a well defined skin and core structure. Within the core region, the dispersed EPDM has a spherical or globular shape. In the skin region, however, the dispersed discontinuous phase is sheet or disc like, oriented parallel to the surface of the moulding. Where weld lines occur, these discs orient vertically to the flow direction, parallel to the weld line. In extreme cases, they prevent the tangling of molecular chains of the matrix necessary to achieve high weld line strength. Indeed they may act as stress concentrators, leading to the observed loss of weld line strength. Thamm found that this behaviour could be minimised by using high viscosity, less easily dispersed EPDM compositions. A key observation relevant to metal flake pigmented mouldings is that no elastomeric particles were observed at the centre of the weld line. Later work by Savadori [28] also tested samples of PP/EPDM, this time filled with short glass fibres or glass spheres. It was shown that the weld line had poorer mechanical properties when it was due to opposing flows. The tensile strength decreased with an increase in rubber or filler content and with a decrease in rubber molecular weight. However Peacock [29] claimed that addition of low concentrations of low molecular weight rubber to rubber modified PP reduced the severity of flow marks. Further pointers to the behaviour of metal flakes in polymers come from work by Lim [30] on polycarbonate (PC) and polyphenylene sulphide (PPS), reinforced by short glass fibres. Microscopy was used to study microstructure. The weld region showed flowerand volcano-like patterns. Crack growth rate was shown to be dependent on fibre orientation. Akay and Barkley [31] found similar effects in short fibre reinforced PP and polyamide. At the weld line there is a discontinuity in fibre-orientation profile, the fibres aligning themselves along the weld line, preventing reintegration of the flows. Support for this conclusion has come more recently from Kim and co-workers [32]. They used a mould cavity with a rectangular insert near the gate. Tensile strength and elongation at break in glass fibre filled polybutylene terephthalate (PBT) with a weld line, were about half those of specimens without a weld. This was attributed to the fibres near the weld lines being oriented parallel to the weld line direction, i.e., perpendicular to the tensile force direction, due to stretching flow.

88

Flow and Weld Lines in Mass Pigmented Applications Hashemi [33] claimed that short glass fibre filler had a more adverse effect on weld line strength than talc in the same PP polymer. The effect was also more pronounced for weld lines produced by impinging flows than those produced by joining of two parallel melt fronts.

8.4 Orientation of metal pigments Although flow and weld lines can be visible in mouldings coloured by organic and in organic pigments, metal flake pigments do tend to make the effect more prominent. Much of the reason lies in the flake shape. Non-flake pigment particles have three broadly similar dimensions. Therefore their orientation in the polymer has relatively little effect on their perceived colour. The same is not true for flake pigments. Other relevant factors are the relative inflexibility of metal flakes and the considerable differences in the coefficient of thermal expansion compared to polymer matrices. Flow lines in metal flake pigmented mouldings also occur where there is a constriction of flow. A typical example is where the gate causes the flakes to orient to minimise their resistance to flow. This is particularly pronounced where the gate dimensions are of the same order of magnitude as the flake particle size. If the flake orientation happens to be at right angles to the surface of the moulding, the result is a dark line created by the relatively low reflectivity of the flake edges, coupled with the absence of reflected light from the spaces between such flakes. The line may well tail off on the other side of the gate as the pressure drop imposes a more random flake orientation. Any localised differences in the concentration of metal flakes may also contribute to flow lines, the areas of lower concentration appearing darker. This effect is often seen at pin gates, where larger flakes can pack like a log jam behind the gate. When they are released, for example by an injection pressure increase, a localised high flake concentration is created in the cavity. As described in section 7.7, the higher the flake concentration below colour saturation, the brighter it appears. This area will therefore appear brighter than its surroundings and will be interpreted by the eye as a flow line. The relationship between flow and weld lines and flake orientation is illustrated by Figure 8.6. Flakes which are parallel to the plane of the photograph are bright. Those at right angles to the plane of the photograph appear dark. The dark clusters of particles, similarly oriented vertically with respect to the surface can be clearly seen. This illustration is not metal flakes but pieces of slate on a beach. Each of the ‘particles’ is many centimetres in diameter. Whether on a large or small scale however, the optical effect is the same. Flake edges appear dark.

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Metallic Pigments in Polymers

Figure 8.6 Pieces of slate on a beach, illustrating the appearance of flow and weld lines in flake pigmented polymers [1cm = approx. 12 cm]

Sanschagrin [34, 35] was amongst the first to seriously study flake pigments. In a study involving many different particle shapes, he found that high aspect ratio flake and fibre reinforced polymers exhibited the greatest loss of strength. The study covered weld lines formed both as a result of head-on collision and those resulting from division of a melt front around an insert in the tool. In the case of the former, a V-notch, absent from moulding of unfilled PP, was formed at the weld for all filled mouldings, despite good venting in the immediate area. The notch became wider and deeper as the particles went from spheres to fibres to flakes. In flake filled polymers, Sanschagrin observed a surface layer in which flakes were broadly oriented parallel to the surface. At the core, the orientation was more random. After melt recombination, the distance required to reimpose uniform flow was long. This was explained not by rotation hindrance due to neighbouring flakes, but to the consequences of the ‘fountain flow’ mechanism of injection mould filling [36]. In terms of strength, Sanschagrin concluded that a weld line will always be much weaker than other areas of the part as long as differences in filler orientation remain. It appears from optical and electron microscopy of injection moulded parts exhibiting weld lines that there is orientation of the long axes of the flakes parallel to the weld and a relative absence of flakes at the tip of the melt front (see Figures 8.7 and 8.8) [27]. This

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Flow and Weld Lines in Mass Pigmented Applications

Figure 8.7 Electron micrograph showing the relative absence of flakes in the weld region, between the parallel lines. The orientation of flakes on each side of the weld is shown by arrows. [1 cm = approx. 60 µm]

Figure 8.8 Schematic diagram of the melt front region of which Figure 8.7 is a part.

complements Thamm’s observations above. The concept of flake inertia, consequent upon their very much higher density relative to the polymer melt, is proposed to explain the relative absence of flakes at the tip of the advancing melt front.

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Metallic Pigments in Polymers The effect of polymer viscosity on flow and weld line visibility was studied by Watters, Kerr and Ringan at Silberline. Their unpublished work is described in section 9.2.4. Mechanical properties reduction is also observed in polymers containing other flake pigments. Mica filled polymers exhibit very low weld line strength, probably due to delamination of the mica flakes themselves [37]. Whilst flow and weld lines are undeniably a challenge, there several ways of mitigating or even eliminating them. The first of these is improving the tool design.

8.5 Mould tool design for metal pigments Very often a plastics converter without previous experience of metal flake pigments will attempt to use them in a mould tool designed for non-flake organic or inorganic pigments and be disappointed by the results. The best cure for flow and weld lines lies in imaginative tool design. The aim should be to avoid as far as possible, any constriction of the flow of molten polymer, particularly any sharp changes of direction at any point in the melt’s journey from barrel to the furthest recesses of the mould cavity. Designing moulds where the melt front expands laterally after the weld line is formed will help to reduce its visibility [37]. One possible remedy is to move the flow and weld lines to positions in which they are not seen. Modern, computer aided design packages, such as those offered by Moldflow, C-Mold and others go a long way to achieve this, but are only generally applicable to new tools. Where an existing tool has to be used, options for modification may be very limited. Rosato [38] has published an introduction to the subject which contains a large section on all aspects of injection mould tool design, with 25 references.

8.6 Gates, sprues and runners Wilkinson at Du Pont [39] produced useful guidance on optimising the gate position in injection mouldings. The flow front profile and the consequent effectiveness of the holding pressure determine the strength and other properties of the moulding. More recently, Bradley [40] also discussed the importance of gate location and numbers. Multiple gates are claimed to offer a number of processing advantages. Catoen of Husky Injection Moulding Systems [41] cited faster injection times, lower melt temperatures, less

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Flow and Weld Lines in Mass Pigmented Applications solid layer build up, thinner walls and the possibility of being able to process higher viscosity resins. Attractive as this may be, multiple gates mean multiple head-on weld lines. Bryce at Texas Plastic Technologies [42] offers guidelines for the design of gates, runners and vents for injection moulding. In considering formulations containing metal flake pigments, the size of the gate is particularly important. Pin gates may be suitable for formulations incorporating nonmetallic colorants, but fail to allow the much larger metal flakes to pass unhindered into the mould cavity. If the gate is too small, irregular cavity filling can occur, as the larger flakes tend to pack in front of the gate. In general, the largest possible gate size, consistent with the other constraints, is preferred. This will create the least disruption to smooth polymer flow. An absolute minimum dimension at least three times that of the largest flake diameter likely to be used is recommended. Strip gates are often advantageous where other considerations permit their use. Hatch [43] describes flow and sink marks in injection moulded ABS parts. Undersized sprue and gates and almost non-existent venting were found to be the main causes. On rectifying these, cycle time was slightly extended, but no sink marks were produced. The size and location of runners is critically important in metal flake pigmented mouldings. They should be as large a diameter as possible and preferably free of sudden changes of direction, especially close to the gate. If a sharp change of direction is unavoidable, the runner should have a generous radius. Hot runners are very useful in preventing the melt from freezing off before weld lines have fused. They also help to avoid the need to turn up barrel heat, with the consequent possibility of burn off of the additives package and plate out of degradation products on the tool surfaces. Two ways to mitigate weld lines using hot runners are described in Injection Moulding International [44]. The article notes the trend to decreasing wall thickness in moulded parts, which tends to increase the visibility of flow and weld lines. Actively influencing mould filling improves control of mould front formation. One of the advantages of metal pigmentation is that the excellent thermal conductivity of metals allows more efficient and uniform heat conduction throughout the melt. In some cases, introduction of metal pigment can allow the barrel temperature to be reduced by several degrees. When granular metal pigments are used, the contribution from thermal conductivity is enhanced by the effect due to the lower melt viscosity of the carrier component. Advances in runnerless moulding technology are reviewed by Bernhardt and Bertacchi [45]. Underlying principles are discussed with practical examples. Aspects examined in detail include use of insulated runners, internally heated insulated runners, purely hot

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Metallic Pigments in Polymers runners, valve gates, sequential filling, gas assisted injection moulding with hot runners and displacement and strengthening of weld lines.

8.7 Tool texturing Another technique for reduction of flow and weld line visibility is texturing the article. This is most readily achieved in injection mouldings by treatment of the mould tool surface. Stippling and crosshatching are usually the most effective.

8.8 Additional cavity Additional cavities, into which the melt can be displaced, can relocate and reduce the visibility of weld lines. Hamada [46] suggests an additional cavity as a means of linearising the flow of fibre reinforced thermoplastics (FRTP). These otherwise displayed very poor visual quality, coupled with reduced mechanical strength. Strength increments were significant in a multicavity tool, due to realignment of fibres to the flow, caused by ‘back flow’ of the material during the holding stage.

8.9 Dynamic melt techniques Many large injection mouldings are conspicuous components with high specifications for surface quality. In recent years the drive for ever lower unit costs has seen typical wall thickness reduced from 4-6 mm to only 2-3 mm. This places critical constraints on both the tool maker and the moulder. To fill large components of such thin section, multiple gating has become more common. As already noted however, the resulting proliferation of melt fronts induces multiple weld lines. This section describes several relatively recent techniques for weld line suppression, all of which rely on keeping the polymer melt in motion to minimise the opportunity for flow and weld line formation in these injection moulded parts.

8.9.1 SCORTEC One of the earliest means of keeping melt in motion during the cooling phase used the injection moulding machine screw to produce an oscillating pressure [47]. This overcame mould filling difficulties such as poor mould packing and dimensional instability of the moulded part.

94

Flow and Weld Lines in Mass Pigmented Applications A more refined development by Bevis and Allan at Brunel University, was originally known as Multiple Live Feed Moulding [48]. Later SCORTEC, or Shear Controlled ORientation TEChnology was coined as a generic name for a family of technologies, all capable of improving the appearance and strength of the moulded part [49, 50, 51, 52]. Grossman of Scortec Inc., has described the SCORIM (Shear Controlled ORientation in Injection Moulding) process as offering an economical answer to flow and weld lines in both thick and thin sectioned parts [53]. Mechanical properties are improved and sink lines, voids and dimensional instability in filled and unfilled polymers eliminated. The technique requires that a processing head is inserted between the nozzle of an injection moulding machine and the rear of the tool. The head, shown in Figure 8.9, directs a split melt to opposite ends of the tool cavity. Hydraulic pistons act on the melt to keep it in motion during the cooling phase of the cycle. A controller linked to the injection moulding machine’s own electronics, synchronises the operation of the pistons. The most usual initial configuration is out of phase, i.e., one pushing as the other pulls. In this way an oscillation of the melt takes place. Screw pressure is maintained, depending on the part shape and thickness. As the melt cools, in-phase compression may be applied to fill any sink marks or voids.

Mould tool

Component

Hyraulic pistons

Runner system

SCORIM head

Barrel of injection moulding machine

Figure 8.9 The SCORIM process

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Metallic Pigments in Polymers

Figure 8.10 Comparison of a conventional moulding (bottom) and a moulding prepared with SCORIM. The weld line, visible in the lower moulding, is absent in the upper.

Figure 8.11 Styling possibilities with SCORIM. The designs are produced by varying the piston action, using melts of contrasting colours. 96

Flow and Weld Lines in Mass Pigmented Applications Figure 8.10 shows the effect of SCORIM versus conventional moulding of a tensile test specimen simultaneously injected from opposite ends. Experiments with fibres confirm that they too orient to minimise their cross section in the direction of movement. Another commercially useful spin-off from this technology is its potential for moulding multi-coloured parts in a controlled and reproducible manner. Two injection barrels are required, each providing a differently coloured melt. Use of SCORIM then creates mouldings comprising the original two colours and their mixture. The degree of mixing and the shapes obtained are controlled by the SCORIM processor (see Figure 8.11). SCOREX (Shear Controlled ORientation in EXtrusion) is the application of the technique to extruded tubes and other profiles that inevitably involve recombination of flows on the downside of the die supports.

8.9.2 Other techniques Sequential injection moulding, proposed by Gazonnet, is suitable for large, twodimensional parts. It operates by opening and closing the needle valve nozzles of a hot runner system to displace weld lines into non-critical areas of the moulding. Nozzle action is controlled pneumatically or hydraulically. Effective venting is essential. Gazonnet [54, 55] describes the principles and advantages of this technique, together with some applications covering both elimination and relocation of weld lines. Software developed by Pole Européen de Plasturgie and SISE to control the process is also discussed. Through the related technique of cascade injection, mouldings free from weld lines are produced by arranging injection nozzles in series. Injection commences at the central nozzle, adjacent nozzles then being opened stepwise within the series as they are reached by the melt front. Relocation of weld lines through displacement to less critical areas of the moulding can again be achieved [56]. Gardner and Malloy [57] describe a moving boundary method, providing melt mixing in the weld region. The process uses a cam operated reciprocating pin, or two such pins, to displace the melt during mould filling. The technique is claimed to be very effective in strengthening weld lines in glass fibre reinforced polymers. Kazmer and Roe [58] successfully used a hot manifold with timed valve gate control of flow into the cavity to improve weld line strength. A mould with flow diverters was used by Walker [59] to create reversible transient flow in injection moulded PP and PS. This shifted the flow under the frozen surface layer of the moulding and increased both the tensile strength and flexural modulus. Flow analysis software determined the effects of subsurface orientation.

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Metallic Pigments in Polymers

8.10 Localised mould heating As noted in section 8.2, any mechanism that encourages molecular entanglement of impacting polymer chains will relieve weld lines. Extending the period of time during which the polymer is molten contributes to this aim. Wada [60] of the Asahi Chemical Industry Company built on this concept to patent a novel injection moulding technology called Bright Surface Moulding (BSM). The technique involves robot insertion of a high frequency induction heater between the tool cavity surfaces after ejection of the moulded part. The cavity surface temperature rises very rapidly, whereupon the heater is retracted and the moulding cycle continues in the conventional way. As injected polymer touches the heated part of the cavity, its temperature is temporarily raised. If the heated section coincides with a weld line, the opportunity for knitting of molecular strands is much increased, with a corresponding reduction in weld line visibility and an increase in weld strength. Simultaneous use of this technique with SCORIM has been conducted with aluminium flake pigments by Rawson, Allan and Bevis [61, 62]. The combination gives a less visible weld line than either technique used alone (see Figure 8.12). The technique is effective

Stylac AT-30 ABS, with SILVET® 764-30-E1 aluminium flake pigment. (90 µm median diameter)

Figure 8.12 Weld line visibility as a function of moulding technique. A combination of SCORIM and BSM renders the weld invisible.

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Flow and Weld Lines in Mass Pigmented Applications and reproducible. An optimum heating time, corresponding to an optimum mould surface temperature, facilitates controlled re-alignment of the aluminium flakes at the moulding’s surface. Such melt manipulation, as well as removing weld lines, imparts uniform surface reflectivity across the weld region, thereby removing any tendency for flop characteristics. Weld strength is also increased.

8.11 Other techniques Two other ingenious solutions to the flow and weld line problem can be discussed here. An injection moulding with continuous thin sections promotes smooth flow of metal flake pigmented polymer. The unwanted thin sections are subsequently punched out. This approach has been used on car wheel trims as an alternative to spray painting an unpigmented moulding. Though giving a blemish-free surface, the disadvantage is the cost of the additional process step. The technique has similarities to thermoforming (see section 10.7). Robot placing or spooling of uniformly pigmented extruded film or sheet in an injection moulding tool, followed by injection of natural polymer can also provide a solution. This technique, sometimes known as in-mould decoration, has also been applied to wheel trims. It is appropriate where only one side of the moulded part is visible in the final application. The technique is considered further in section 10.6.

References 1.

M-C. Heuzey, J. M. Dealy, D. M. Gao and A. Garcia-Rejon, Proceedings of the Antec 97 Conference, Toronto, Canada, 1997, Vol. 1, 532.

2.

S. C. Malguarnera, Polymer Plastics Technology and Engineering, 1982, 18, 1, 1.

3.

C-Mold Design Guide, Advanced CAE Technology Inc., Ithaca, New York, 1997.

4.

S. Y. Hobbs, Polymer Engineering and Science, 1974, 14, 9, 621.

5.

E. M. Hagerman, Proceedings of the SPE Antec 73 Conference, Montreal, Canada, 1973, Paper No.12.

6.

P. L. Barrick, R. H. Crawford, B. L. Espy, W. F. Robb and E. E. Swain, SPE Journal, 1964, 20, 1, 69.

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Metallic Pigments in Polymers 7.

S. C. Malguarnera, Plastics Engineering, 1981, 37, 5, 35.

8.

S. C. Malguarnera, A. I. Manisali and D. C. Riggs, Polymer Engineering and Science, 1981, 21, 17, 1149.

9.

T. J. Pecorini and K. S. Seo, Proceedings of the SPE Antec 95 Conference, Boston, USA, 1995, 1794.

10. T. J. Pecorini and K. S. Seo, Plastics Engineering, 1996, 52, 6, 31. 11. S. G. Kim and N. P. Suh, Proceedings of the SPE Antec 84 Conference, New Orleans, USA, 1984, 777. 12. H. G. Mosle, R. M. Criens and H. Dick, Proceedings of the SPE Antec 84 Conference, New Orleans, USA, 1984, 772. 13. S. Piccarolo and M. Saiu, Plastics and Rubber Processing and Applications, 1988, 10, 1, 11. 14. S. L. Janicki and R. B. Peters, Proceedings of the Antec 91 Conference, Montreal, Canada, 1991, Vol. I, 391. 15. H. Hamada, K. Tomari, H. Yamane, T. Senba and M. Hiragushi, Proceedings of the Antec 97 Conference, Toronto, Canada, 1997, Vol.I, 1071. 16. C. F. Merhar, K. A. Beiter and K. Ishii, Engineering Plastics, 1994, 7, 2, 81. 17. C. F. Merhar, K. A. Beiter and K. Ishii, Proceedings of the Antec 94 Conference, San Francisco, USA, 1994, Vol.III, 3450. 18. G. D. Gilmore and R. S. Spencer, Modern Plastics, 1951, 28, 8, 117. 19. H. Yokoi, Y. Murata, K. Oka and H. Watanabe, Proceedings of the Antec 91 Conference, Montreal, Canada, 1991, 367. 20. S. Lautenbach, K. K. Wang, H. H. Chiang and W. R. Jong, Proceedings of the Antec 91 Conference, Montreal, Canada, 1991, 372. 21. P. J. Kuvshinikov, Proceedings of the Antec 92 Conference, Detroit, USA, 1992, Vol.II, 2511. 22. W. R. Jong, C. H. Chan and C. C. Wu, Proceedings of the Antec 96 Conference, Indianapolis, USA, 1996, Vol.I, 783.

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Flow and Weld Lines in Mass Pigmented Applications 23. N. Mekhilef, A. Ajji and A. Ait-Kadi, Proceedings of the Antec 93 Conference, New Orleans, USA, 1993, Vol.I, 516. 24. J. F. Herten and B. Louies, Kunststoffe, 1985, 75, 10, 743. 25. B. Miller, Plastics World, 1990, 48, 12, 28. 26. F. Lalande, Proceedings of the Antec 91 Conference, Montreal, Canada, 1991, 404. 27. R. C. Thamm, Proceedings of the ACS Rubber Division, Fall Meeting, San Francisco, USA, 1976, Paper No.29. 28. A. Savadori, A. Pelliconi and D. Romanini, Plastics and Rubber Processing and Applications, 1982, 3, 3, 215. 29. A. J. Peacock, inventor; Exxon Chemical Patents Inc., assignee, US Patent 5,468,808-A, 1995. 30. J. K. Lim, M. Nakajima and T. Shoji, Proceedings of ICCM/9, Ceramic Matrix Composites and other Systems, Madrid, Spain, 1993, Vol.2, 205. 31. M. Akay and D. Barkley, Plastics & Rubber Composites Processing & Applications, 1993, 20, 3, 137. 32. J. K. Kim, J. H. Song and T. H. Kwon, Polymer Engineering & Science, 1997, 37, 1, 228. 33. S. Hashemi, G. Gara and B. Stanworth, Plastics & Rubber Composites Processing & Applications, 1994, 22, 2, 105. 34. B. Sanschagrin, R. Gauvin, B. Fisa and T. V. Khank, Plastics Compounding, 1987, 10, 3, 37. 35. B. Sanschagrin, R. Gauvin, B. Fisa and T. Vu-Khanh, Proceedings of the 42nd SPI Annual Conference & Expo ’87, Cincinnati, USA, 1987, Paper No.13-A. 36. Z. Tadmor, Journal of Applied Polymer Science, 1973, 18, 6, 1753. 37. B. Sanschagrin, R. Gauvin, B. Fias and T. V. Khank, Plastics Compounding, 1987, 10, 3, 37. 38. D. V. Rosato, Injection Mould Design, Injection Moulding Handbook. The Complete Molding Operation Technology, Performance, Economics, Van Nostrand Reinhold, Co., Inc., New York, 1986, 160-234.

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Metallic Pigments in Polymers 39. R. Wilkinson, E. E. Pope, K. Leidig and K. Schirmer, Plastics and Rubber Weekly, 1997, No.1708, 12. 40. D. Bradley, Plastics News International, 1998, April, 28. 41. B. Catoen, Proceedings of the Antec 93 Conference, New Orleans, USA, 1993, Vol.I, 508. 42. D. M. Bryce, Proceedings of the Antec 94 Conference, San Francisco, USA, 1994, Vol.I, 740. 43. B. Hatch, Injection Moulding, 1997, 5, 8, 92. 45. A. Bernhardt and G. Bertacchi, Proceedings of the Antec 94 Conference, San Francisco, USA, 1994, Vol.I, 1164. 46. H. Hamada, Z. Maekawa, T. Horino, K. Lee and K. Tomari, International Polymer Processing, 1988, 2, 3/4, 131. 47. H. A. Hengesbach and K. Schramm, Plastverarbeiter, 1976, 27, 12, 667. 48. Brunel University of West London, assignee, British Patent 2,170,142 B, 49. R. Malloy, G. Gardner and E. Grossman, Proceedings of the Antec 93 Conference, New Orleans, USA, 1993, Vol.I, 521. 50. E. M. Grossman, Proceedings of Injection Moulding Outlook RETEC, Dallas, USA, 1993, Paper No.Y. 51. A. McDonald, Engineer, 1994, 279, 7216, 21. 52. E. M. Grossman, Proceedings of the World Class Injection Moulding RETEC, Charlotte, USA, 1994, 35. 53. E. M. Grossman, Proceedings of the Emerging Technologies RETEC, Erie, USA, 1995, Paper No.11. 54. J. P. Gazonnet, Revue Generale des Caoutchoucs & Plastiques, 1994, 71, 736, 44. 55. J. P. Gazonnet, Plastiques Modernes et Elastomers, 1994, 46, 9, 41. 56. W. Homes, Kunststoffe Plast Europe, 1996, 86, 9, 13.

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Flow and Weld Lines in Mass Pigmented Applications 57. G. Gardner and R. Malloy, Proceedings of the Antec 94 Conference, San Francisco, USA, 1994, Vol.I, 626. 58. D. O. Kazmer and D. S. Roe, Proceedings of the Antec 94 Conference, San Francisco, USA, 1994, Vol.I, 631. 59. W. H. Walker, Proceedings of the SPE Antec 94 Conference, 1994, San Francisco, USA, Vol.II, 1880. 60. K. Tazaki, T. Tahara, A. Wada, H. Suzuki and Y. Mizutani, inventors; Asahi Dow Ltd., assignee, GB Patent 2,081,171B, 1980. 61. P. S. Allan, M. J. Bevis, K. Yasuda, inventors; Brunel University of West London and Asahi Kasei Kogyo, assignees, GB Patent 2,299,780 B, 1996. 62. K. W. Rawson, P. S. Allan and M. J. Bevis, Polymer Engineering and Science, 1999, 39, 1, 177.

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9

Formulation of Mass Pigmented Polymers

This chapter concentrates on formulation skills and metal pigment developments. It also brings together elements of equipment design and pigment incorporation introduced in previous chapters.

9.1 General techniques There are many ways to improve the visual effect of metallic, mass pigmented polymers. It is important to reiterate that in most instances, the best results will be achieved from combinations of the materials and techniques described here, rather than from any single factor. The full arsenal of methods employed comprises: optimising the formulation, improving pigment incorporation and the equipment used to achieve this and finally improving the quality of the metal pigments themselves. A few principles are common to all metal flake pigments and incorporation techniques. These are examined first. Techniques for specific processes, such as extrusion or injection moulding, are described later. A recurring theme in the latter is reducing the visibility of flow and weld lines.

9.2 Optimising the formulation A cornerstone of formulation with metal pigments is a sound understanding of the relationships between particle size, colour and colour saturation, opacity, sparkle, pigment concentration and cost. These characteristics are defined in Chapter 4 and their interrelationship described in section 7.7. Competitive pressures in the plastics market demand ever increasing cost-effectiveness. It therefore becomes even more important to choose the correct pigment for the envisaged application. Cost is linked to concentration, so it follows that concentration must be minimised, consistent with other factors.

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Metallic Pigments in Polymers The ideal metal pigment will have the following attributes: Lowest density Thinnest flakes Smallest flakes

=

more opacity (more flakes per unit weight)

Smooth surfaces Rounded flakes Largest flakes Good orientation

=

best colour

Thickest flakes

=

best colour retention (degradation resistance)

Rounded flakes Largest flakes

=

easiest dispersion

It is immediately apparent that some of the requirements are contradictory, the more so when cost considerations are added. The lowest density is provided by aluminium at 2.7 g/cm3. The thinner cornflakes offer the greatest opacity, providing dispersion forces are minimised to prevent flake damage. For commodity applications, the constraints of small flakes may be acceptable because their high opacity reduces the concentration required. This in turn reduces the cost per unit weight of pigmented polymer. The better colour demanded of prestige applications requires the more expensive round-edged, smooth surfaced silver dollar flakes. If a degree of sparkle is specified, this can only be achieved from larger flakes. Higher concentrations and therefore cost will be the penalty to achieve equivalent opacity.

9.2.1 Flake size As a general principle, the largest possible flake should be used, consistent with cost and appearance. In mass pigmented polymers, the unaided eye is barely able to distinguish flakes below about 20 µm median diameter, no matter how bright they may be. As median flake size increases above 30 µm, a random orientation of the flakes will begin to appear sparkling.

9.2.2 Flake concentration The pigment loading should be adjusted so that the moulded part appears just opaque in its thinnest section. This does not necessarily mean that the part is actually opaque, merely that it appears so. A bottle for example, can appear opaque when viewed from 106

Formulation of Mass Pigmented Polymers the side, but appears remarkably translucent when viewed through the aperture. Such an approach allows the maximum metallic reflection from deep within the polymer. This adds to the richness of the effect in cases where solidity is also important. It also has the added benefit of being very cost-effective.

9.2.3 Polymer transparency Where possible, an optically clear grade of polymer should always be used. Inherently coloured polymers, such as ABS will require 10-40% more pigment for an equivalent visual effect. In extreme cases, they may never reach the brightness and gloss of the same pigment in a transparent polymer.

9.2.4 Polymer viscosity The viscosity of a polymer influences its ability to carry the metal pigment at the tip of the advancing melt in an injection moulding. This phenomenon was investigated by Watters, Kerr and Ringan [1] who moulded iron flakes in three grades of PP of widely differing melt flow. Iron flakes were selected because of their particularly high density, which it was hoped would make any effect more easily visible. Figure 9.1 shows the weld line formed by division of melt round a pin in the tool. The two mouldings are distinguished only by the viscosity of the polymer grade used.

Figure 9.1 The effect of polymer viscosity on weld line appearance. Low melt flow polymer (A), left, exhibits a sharper weld line than a high melt flow polymer (B) when pigmented by 2% iron flake of 110 µm median particle diameter 107

Metallic Pigments in Polymers The higher viscosity PE (A), with a low melt flow index of 20 g/10 min at 190 °C, carries the metal flakes well, producing a narrow, but very prominent weld line. The flake-free band at melt contact is very narrow. A less viscous grade (B), of 200 g MFI has a greater ability to flow past the high inertia flakes. The resulting weld line is much more diffuse. It is also very wide, as is the area free of flakes where the melt fronts combine. These results confirm Mosle’s [2] findings for fibres. The effect is much less marked with the lower density aluminium pigment and the normal MFI range of commercial polymers.

9.2.5 Metallic/organic pigment combinations The most severe instances of flow and weld lines come from fine particle size flakes at concentrations high enough to confer opacity on the moulded article. If low concentrations of larger flakes can be used, in combination with organic and inorganic colours to provide opacity, unsightly lines can normally be avoided. Considerable skill is required from formulators to achieve optimum results. In certain polymers, dyestuffs may provide a richer effect, if their relative transparency can be tolerated. Large glitter flake pigments can be used to advantage in combination with organic and inorganic colorants. The latter provide the opacity that the glitters lack.

9.2.6 Deep shades In coloured metallic formulations, for a given metal pigment and loading, deeper shades mask weld and flow lines better than pale shades. Though not always an option, it is a mitigating factor that can be combined with others in this section.

9.2.7 Spherical pigments Manufacture of these pigments is described in section 3.5. Being almost true spheres, spherical metal pigments have no orientation in plastics and therefore cannot generate weld and flow lines by conventional orientation mechanisms. It has been commercially confirmed that their introduction into metal flake pigmented formulations reduces the visibility of flow and weld lines. This is believed to be both because

108

Formulation of Mass Pigmented Polymers the spherical pigments disrupt the orientation of flakes in the region of flow and weld lines, and because a lower concentration of flake pigment is required. This is another area in which skilled formulators can significantly improve the appearance of the moulded part. The ratio of flake to spherical pigment is critical. Spherical metal pigments offer pinpoint light reflection. The remainder of the surface area is non-contributory. If used alone, such pigments have very low opacity and appear less bright than flakes. For this reason, they are best suited to deeper shades. As flake pigment is introduced into the formulation, it contributes brightness. Due to the mitigating effect of the spherical pigment, a higher flake loading can be achieved before flow and weld lines become visible. The overall effect is brighter and more uniform. The combination of a 30 µm diameter spherical aluminium pigment with a 50 µm average particle diameter aluminium flake pigment gives particularly good results. A useful attribute of spherical metal pigments in processing terms is their high degree of shear resistance.

9.3 Incorporation in polymers 9.3.1 Low shear forces This is the most important processing factor. In order to manufacture flakes, all the metals used as pigments must be malleable. Aluminium, for example, is a soft and ductile metal. Derived flakes are easily deformed by the high shear forces commonly encountered during plastics processing. Bent or broken flakes have reduced surface area from which to reflect light. The visual result is then dull, grey and non-metallic. Figure 9.2 shows aluminium flakes carefully isolated from a commercial masterbatch. Broken and folded flakes, with their characteristic straight edges, can be clearly seen. Where it is necessary to disperse other colorants using high shear, as in a twin screw extruder, the metal pigment should be introduced close to the die, perhaps via a degassing port. Though a batch apparatus, the Banbury type of mixer is ideal for metallics, as shear can be controlled throughout the dispersion process. Modern metal pigment delivery forms, such as Silberline’s widely patented SILVET® aluminium flake pigment granules, are designed for low shear incorporation. The metal flakes are immobilised in 20-30% of a widely compatible carrier resin, which melts below the melt point of the polymer granules. The pigment is thus distributed as a predispersion over the polymer granule surfaces. Thereafter there is only a requirement for

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Metallic Pigments in Polymers

Figure 9.2 Electron micrograph of aluminium flake, damaged by high shear processing [1 cm = approx. 4 µm]

mixing, rather than shearing, to fully disperse the metal flakes throughout the polymer mass. The granule dimensions have been carefully chosen to minimise the stratification in the hopper that would otherwise occur due to the substantial density difference between metal pigment and polymer.

9.3.2 Improvement of flake orientation Advantage should be taken of any mechanism that helps to increase the proportion of flakes orienting with their faces parallel to the polymer surface. This includes mono and biaxial stretching, as, for example, during sheet extrusion or blown film manufacture. Modern injection-stretch-blow moulding machines give particularly good results.

9.4 Increasing pigment quality A key method of achieving mass pigmented polymers approximating a painted appearance is to use new, highly polished, extremely bright flakes specially designed for the purpose.

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Formulation of Mass Pigmented Polymers In practice, such pigments tend to be derived from aluminium flakes originally developed to meet the very exacting requirements of the automotive paint market. These are the silver dollar flakes whose production is described in section 3.3.1. It is ironic that pigments originally developed for automotive paints should now be instrumental in forcing a move to mass pigmentation. For this application it has been found that flake diameters almost twice those of the corresponding paint grades are required to compensate for the lack of orientation in the moulding. Typical automotive and bicycle paint pigments have median particle sizes of 14-20 µm and 25-35 µm, respectively. Corresponding grades for mass pigmentation require diameters of 25-35 µm and 45-60 µm. Loadings of ~2.5% and ~3.5% w/w, respectively, are then required for colour saturation in clear polymers. Work by Sanschagrin [3] showed that as the aspect ratio of a filler in injection moulded polypropylene decreased, strength retention increased. It follows that spherical metal pigments and thick, degradation resistant metal flakes should offer better mechanical strength retention than conventional, high aspect ratio flakes.

9.5 Summary The following points are provided as a quick reference summary. They collect together all the techniques from this and Chapter 8 which provide the brightest, most cost-effective visual effects, with minimised weld line visibility and increased weld strength in metallic pigmented mouldings.

Tool design • Move weld lines out of sight at the design stage where feasible. • Locate unavoidable welds as close to the gate as possible. • Ensure good venting in the weld region. • Texture the tool surface. • Investigate dynamic melt techniques.

Formulation Use: • the largest, thickest, preferably silver dollar flake, consistent with cost and appearance. • the minimum pigment concentration for opacity in the thinnest section of the moulding. • deep organic or inorganic colorants. 111

Metallic Pigments in Polymers • spherical pigments, where the effect required is a deep shade. • the most transparent polymer possible. • the polymer with the highest possible melt flow. • the highest molecular weight polymer, consistent with other constraints. • a polymer with a relatively short viscoelastic relaxation time.

Processing • Minimise shear. • Increase mould temperature. • Increase injection speed, but beware of jetting. • Increase melt temperature within the heat stability limits of melt components.

References 1.

C. Watters, S. Kerr and E. Ringan, unpublished work, Silberline Ltd.

2.

H. G. Mosle, R. M. Criens and H. Dick, Proceedings of the Antec 84 Conference, New Orleans, USA, 1984, 772.

3.

B. Sanschagrin, R. Gauvin, B. Fisa and T. V. Khank, Plastic Compounding, 1987, 10, 3, 37.

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10

Conversion Processes

In this context, the term conversion describes the process of transforming uncoloured, i.e., natural or virgin polymer powder or granules into an optionally coloured, finished article by melting and fusing the polymer. In its widest sense conversion includes the manufacture of masterbatch or compound precursors to the finished article. In this chapter, a number of the most important conversion processes will be covered with particular reference to the role of metal pigments in their coloration. Techniques for combining metal pigment with the polymer are common to most of the conversion processes described here. The key requirement is a homogeneous dispersion achieved without the use of high shear forces. The quality of the pre-dispersion must be good enough to accommodate any deficiencies in the mixing action of the conversion equipment. The physical form of the raw materials is the determinant of the mixing process. Polymer may be supplied in powder, spherical, flake or granule forms, with metal pigment present as dry flake, plasticiser dispersion or granule. Dry flake is the least technically satisfactory product form. Incorporation is by tumbling or gentle mixing by a blade capable of moving the whole mass. Dry flake should not be mixed in any form of high speed powder blender for two reasons. Firstly, the shear is too great, causing darkening and loss of metallic effect. Secondly, it is extremely hazardous in the case of some metals, especially aluminium, even if carried out under a blanket of inert gas. Dry flake will mix well with polymer powder, but it is difficult to wet it adequately into the polymer melt. Plasticiser dispersions have better wetting characteristics, but have a tendency to form into balls during mixing with polymer. If double cone blenders are used, the polymer should be charged first. A small further addition of plasticiser, allowed to coat the polymer surfaces, is sometimes beneficial before addition of the metal pigment. Polymer powder is the least preferred physical form here as it tends to form lumps itself under these conditions. Metal pigment granules offer a means of avoiding premixing altogether because they are easy to handle and incorporate. In mass pigmented plastics they overcome the main deficiencies of plasticiser dispersions, offering easier handling and meterability in modern

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Metallic Pigments in Polymers automatic dosing systems. For most applications, dry tumbling with polymer granules followed by extrusion or moulding will provide a satisfactory degree of dispersion. For the most demanding applications, where a high degree of dispersion is required, for example, in blown film applications, low metal content granules are available. Commercially available pigment granules tend to have a low melting carrier system. It melts first in the processing equipment, coating the surface of polymer pellets, granules or spheres before the polymer itself melts. In this way, good pigment wetting and efficient pre-dispersion is obtained. This in turn allows the essential low shear incorporation equipment to be used for a full dispersion. Because of the high metal content, granules can, if desired, be co-dried with the polymer without premature break up. With care, even temperatures above the carrier softening point can be used. Organic and inorganic pigments, unless pre-dispersed in a polymer or carrier resin, are usually aggregates of primary pigment particles. To achieve the maximum colour strength from such pigments, shear may be required to break down the aggregates and disperse the primary pigment particles. It is therefore tempting for the converter to use the same equipment to disperse metal flake pigments as for dispersing such colorants, in the belief that this too will give rapid and efficient dispersion. This is wrong, as the disappointingly dark, dull and non-metallic results will testify. High shear processing has bent, folded or fractured the very thin metal flakes, dramatically reducing their colour, brightness and metallic lustre. If high shear is required to disperse other components of a plastics formulation, that step should be complete if possible, before the granule is added to the system. In twin screw extruders it may be possible to meter granules into a downstream de-gassing port, or via a side feeder near the die, as is current practice for other shear sensitive fillers such as glass fibres. When using metal pigments in general, it has been found that the thermal conductivity through the molten polymer mass improves. This can allow a barrel temperature reduction of 5 °C or more, with consequent energy savings and reduced tendency for thermal degradation of the polymer.

10.1 Injection moulding Injection moulding accounts for approximately one-third of all polymers processed. The technique is versatile and has relatively low set up costs. As a result, injection moulding companies tend to be small and numerous. A very comprehensive introduction to the theory and practice of injection moulding is provided by Rosato and Rosato [1]. It includes information on the design of the moulded 114

Conversion Processes part for optimum performance, ease of manufacture and lowest cost. Choice of polymer is considered as well as moulding techniques themselves. Points to consider in injection moulding with metal pigments are mainly related to the avoidance of flow and weld lines. These issues are considered in detail in Chapter 8. Colour retention is less of a problem in injection moulding than it is in extrusion, as the barrel and screw are generally configured for mixing rather than applying high shear. Nevertheless, screw return times (the time taken for the injection moulder’s screw to return with plasticised material, ready for the next shot) should be minimised, consistent with a satisfactory dispersion.

10.2 Blow moulding The term blow moulding refers to both container and continuous film production. The production processes are different, but they share the use of air to expand the polymer melt, hence the name.

10.2.1 Blown film Blown film extrusion is a continuous process in which a tube of extruded polymer melt is drawn up into nip rollers. Air is injected to create a bubble whilst maintaining tube integrity as the polymer rises and cools. A detailed description of the apparatus and its operation is provided by Knittel [2]. Polyolefins are the most commonly used polymers, producing film thicknesses around 0.1-0.5 mm. The main markets are rubbish and shopping bags and agricultural film for silage, weed suppression, etc. Co-extruded films have food packaging uses, by virtue of internal barrier layers. The most commonly used pigment is carbon black, since it is the most cost-effective source of opacity. Metal pigments, principally aluminium, but also gold bronze, are used in applications in which appearance takes precedence over function, for example, plastic shopping bags. Due to the extreme thinness of blown films, only fine particle size grades of metal flake pigments can be used. They must be free of grossly oversize flakes that could otherwise cause the film bubble to pinhole. Dispersion is also very important, as any undispersed flake aggregates will have the same effect. In practice therefore, grades of 6-12 µm diameter

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Figure 10.1 Uniform orientation of aluminium flakes in biaxially stretched polymer film [1 cm = approx. 15 µm]

with fairly tight particle size distributions are specified at loadings of approximately 0.51%. They may be offered as compound or as a low metal content masterbatch. The biaxial stretching that occurs automatically in this process orients metal flakes parallel to the film surface, thereby increasing brightness (see Figure 10.1).

10.2.2 Blown containers Blow moulding is extensively used for a wide range of shapes and sizes of containers, the most notable being cosmetic and toiletry containers, beverage and oil bottles. Its usefulness lies in the wide variety of shapes of container that can be accommodated, many impossible to produce by the other common moulding techniques. A comprehensive review of the subject is provided by Fritz [3]. The process involves extruding a slug of molten polymer into a mould, constructed somewhat like an injection mould. The polymer is expanded against the walls of the closed mould by air pressure. A larger wall thickness than blown film permits a wider pigment range. Metal pigment grades with median diameters over 200 µm have been successfully moulded by this technique.

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Conversion Processes Because blow moulders are not primarily designed for dispersion, compound, or less commonly, masterbatch is the preferred form of feed. Occasionally, metal flakes, especially coarse particle sizes, rub off the surface of blow moulded containers. Weld strength of the seam or parison line may also be reduced. In both cases, this is generally a symptom of insufficient wetting, which in turn points to insufficient energy input at the compounding or masterbatch stage. It can sometimes be overcome by raising the melt temperature of the blow moulder, but the real remedy is the responsibility of the compound or masterbatch manufacturer. Metal flake concentrations can be quite low, as only translucency, rather than opacity, is required to hide the eventual contents of the container. The technique is particularly suited to the creation of coloured pearlescent effects (see section 11.7).

10.3 Extrusion Comparable with injection moulding in volume, extrusion is the other main polymer processing method. It encompasses sheet, film and profile production and extrusion coating. This process is well suited to metal pigments. A uniform extrudate assists flake alignment. This provides one of the main attributes of this technique, a uniform metallic effect. Brightness is assisted by any mono or biaxial stretching that takes place. Flake particle size distribution is less critical, as the extrusion process helps to align the particles parallel to the surface. Very large flakes should still be avoided in the thinner films, as they can still cause pinholing.

10.4 Co-extrusion Co-extrusion allows the core layers to be composed of different materials to the skins. High opacity metal pigments with median particle sizes below about 10 µm can be used in outer layers, where they will obliterate differently coloured core layers. Scrap or regrind can then be more readily recycled in the core.Without special treatment, metal flakes have limited barrier properties. Their use in co-extrusions therefore tends to be cosmetic rather than functional.

10.5 Paint-less film moulding Paint-less Film Moulding (PFM) is a new concept for the production of mass coloured automotive body components without the costs associated with painting [4]. It was

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Metallic Pigments in Polymers developed jointly by four industrial partners, BASF, Senoplast, Rohm and Engel, by bringing together injection moulding, co-extrusion and thermoforming techniques. An outer layer of abrasion and weather resistant PMMA acrylic is coextruded with acrylonitrile/styrene/acrylate (ASA) copolymer or an ASA/PC blend. After thermoforming, a backing polymer, such as ABS, polybutylene terephthalate (PBT)/PC or ASA/PC is applied by injection moulding, using the thermoformed part as a mould insert.

10.6 In-mould decoration There are several variants under this name involving both coating and mass pigmentation. The principle governing both is that a thin, pigmented layer is inserted into an injection moulding tool. Uncoloured virgin or reground polymer is then injected to bond to the pigmented layer. The technique is only applicable to articles such as automobile wheel covers that have a limited number of visible surfaces. In-mould decoration by mass pigmentation involves insertion of a thin sheet or film into the cavity of an injection mould tool. In the case of sheet, placement may be by robot. Film is generally spooled through the tool. The mould faces are closed and molten polymer injected. Under heat and pressure, the insert assumes the shape of the mould tool. The advantage of this technique for metal flake pigments is that flakes can be oriented by biaxial stretching during extrusion of the sheet or film. In this way a more uniform, brighter visual effect, closer to that of a paint and free from flow and weld lines is obtained. The thickness of the pigmented layer can be controlled through a wide range, for example to provide stone chip resistance in wheel covers. These advantages help to offset the higher cost of this technique over direct mass pigmentation.

10.7 Vacuum forming and thermoforming These two related techniques are useful for metal flake pigments because they too eliminate weld and flow lines. The extruded sheet starting material, with its oriented metal flakes, offers increased brightness and uniform appearance. Very deep draws should be treated with caution because non-uniform stretching of the sheet can alter metallic appearance.

10.8 Rotational moulding Rotational moulding or rotomoulding is one of the fastest growing polymer processing methods. It is a cost-effective alternative to blow moulding, especially for large mouldings.

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Conversion Processes Very short production runs are economically possible. PE is by far the major polymer used for this technique. The process is particularly simple. A weighed quantity of plastic powder is heated inside a closed mould that slowly rotates in two planes. The melting polymer forms a skin over the mould’s inside surface. There follows a cooling phase during which the polymer solidifies. The tool is then opened and the moulding removed. Tooling costs are low since high pressures are not required. This also means stress free mouldings. Versatile design options allow hollow shapes to be moulded without seams, sprues, runners or ejector pin marks. Wastage is therefore also low. Low pressures are an advantage where metal flakes are concerned, since flake damage is minimised. The flake orientation is likely to be fairly uniform, but with little orientation. Nevertheless, flow lines are minimised and with no converging melts, weld lines are completely eliminated. A potential disadvantage of the technique is that powdered polymer is used. Ideally the metal flake is incorporated in the polymer powder, as a masterbatch route tends to give poor uniformity. It is difficult to avoid flake damage as the compounded polymer is powdered. In practice, all sizes of metal flake are used. Large glitter flakes in coloured formulations often give the best results in the large mouldings for which rotomoulding is most often used. Concentrations are comparable to those of injection moulding.

10.9 Glass reinforced plastic Bright metallic silver effects can be achieved in glass reinforced plastic (GRP) using nonleafing aluminium flake pigment grades. A wide range of particle sizes and therefore of visual effects is possible, from fine, smooth finishes with high opacity, to an attractive sparkle. The technique tends to demand good pigment durability, both in service and also because some of the components are acidic or otherwise aggressive to metals. Thus higher purity, acid resistant metal pigments are generally specified. Small quantities of organic or inorganic colorants may be added, providing their lightfastness and bleed resistance are adequate. Dry flake, plasticiser dispersions and granules formulated with solvent soluble carriers can all be used. In the last category, aldehyde, ketone and hydrocarbon resin carriers are all suitable. Formulation involves dispersion of any non-metallic pigments in the gel coat

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Metallic Pigments in Polymers resin, which is usually styrene-based. Unless a predispersed colorant is used, this step should be completed first, as high shear may be required for complete dispersion and colour strength development. For the best results, the metal pigment should be allowed to soak in the liquid GRP medium for several hours with occasional gentle stirring. This allows sufficient time to release and wet individual metal flakes. Dispersion is thereafter best achieved by low energy stirring, using a slow speed blade capable of moving the whole mass. High shear should be avoided as it bends and breaks the flakes, reducing their brightness. The pigmented mix should be gently agitated before and preferably also during use to avoid settlement of the metal flakes. To produce the brightest effects, the metallic pigmented GRP resin should be applied to the mould by spraying. This has the advantage of allowing the pigments to flow naturally and has the additional benefit of ease of use. Application by brush will also give a metallic effect, but may cause local differences in flake orientation and consequently an uneven appearance. The suggested sequence is an initial coating of the mould by a clear resin coat, followed by application of the metallic pigmented resin layer and finally laying up with the fibreglass reinforcing material. On ejection from the mould, the component should display a clean metallic finish, comparable to a painted article. The quality of the visual effect will depend on the thickness of the initial clear layer, the smoothness of application and the metal pigment particle size and concentration. Brightness increases with loading up to colour saturation. This could be 0.5-2% for a fine grade but 5-7% or more for a coarse type. An alternative dispersion method involves the addition of acetone directly to the metal pigment. This accelerates the dispersion process. When fully dispersed and homogeneous, the premix can be added directly to the GRP resin. Like rotational moulding, GRP is useful for manufacturing large structures incapable of production by any of the other thermoplastic processes in this section. Use of metal flake in GRP boat hulls has the incidental advantage of improving their visibility on radar.

10.10 Thermosetting polymers Unlike thermoplastics, which can undergo any number of melting and re-solidification cycles, thermosetting polymers, once reacted, cannot be re-melted.

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Conversion Processes Metal pigments are not widely used in thermosets. One reason is that many such polymers are inherently coloured, sometimes strongly so. As in similarly opaque elastomers, the metallic effect is then diluted or even lost altogether. For thermosets prepared from liquid components, dry flakes and plasticiser dispersions can be used. Granules with resin rather than polymeric carriers are also suitable. Powder precursors can be combined with dry flake or plasticiser dispersions.

References 1.

Injection Moulding Handbook, 2nd Edition, Eds., Donald V. Rosato and Dominik V. Rosato, Chapman and Hall, New York, 1995.

2.

R. Knittel, Proceedings of the SPE Antec 96 Conference, Indianapolis, USA, 1996, Vol.I, 92.

3.

H. G. Fritz, in Plastics Extrusion Technology, Ed., F. Hensen, Hanser Publishers, Munich, 1988, 363.

4.

Plastics and Rubber Weekly, 1998, No. 1750, 1.

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11

Applications of Mass Pigmented Systems

This chapter describes at the most common colouristic applications of metal pigments in mass pigmented polymers. Non-colouristic applications are described in Chapter 15.

11.1 Household goods Metal pigments, principally aluminium, are very effective in breaking up the uniform appearance of large areas of solid colour, especially dark colours. There is increasing interest in this technique in the teletronics market. Traditional black TV and video cabinets are being subtly pigmented by metal pigments to give renewed customer appeal. Most recently, the technique has spread to shavers and mobile telephones. Aluminium flakes in the 20-60 µm range predominate. Many of these appliances require cut-outs for switch and cable attachment, which are a source of flow and weld lines. These challenges are being met by combinations of flake and spherical pigments and by SCORIM (see Chapter 8). Larger flakes are used in footwear, especially in thermoplastic elastomers for soles and in trims for trainer shoes. Glitter flakes appear in transparent or tinted children’s sandals. Their sparkling appearance increases the appeal of toys. Pigmentation by coarser aluminium flake is also feature of larger articles, such as vacuum cleaners, coffee makers, toasters, suitcases and the like. To mitigate the reduced opacity of coarser pigments, they are generally formulated with organic and inorganic colours. A selection of metal flake pigmented articles is shown in Figures 11.1. Although more often surface printed, floor coverings are also pigmented by extrusion coating over a foamed polymer substrate. The main advantage is uniform coloration throughout a layer of increased thickness and therefore increased wear resistance [1]. For the garden, lawnmower and hedge trimmer housings, patio sets and hand tools are produced with metallic finishes. Some are painted, the others pigmented. Many traditional white patio sets are now coloured with low concentrations of black glitter flakes to give a good contrast with the white.

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Figure 11.1 Metal flake pigmented mouldings

11.2 Sports goods This is very definitely a fashion driven market. Styles therefore come and go in cycles. A particularly prominent application for aluminium flake pigments in the mid 1990s was ski boots. Coarse flakes in the 70-230 µm range were used with solid colours to provide sparkle and avoid weld lines. Bicycle parts and accessories, such as chain guards, mud guards, brake mechanisms and cycle helmets are also mass pigmented by metal pigments. A key consideration for functional parts is the effect of such pigmentation on mechanical properties. Chip resistance is also important. Bicycle frames continue to be made from steel or lightweight alloys and are increasingly painted with effect pigments.

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11.3 Agricultural film The light reflecting and opacifying properties of aluminium flake pigments offer several potential advantages in agricultural film applications. These include crop ripening, silage wrapping and weed suppression. Black, grey and white and pigmented plastic sheets were some of the first to be patented for their plant mulching properties [2, 3]. Mica flakes for this application are claimed in US Patent 3,099,897 [4]. An early use of aluminium flakes is disclosed in US Patent 3,382,610 [5], in which the flakes are combined with an asphalt binder. PE film, mass pigmented by around 1% of a particularly fine particle size aluminium flake pigment, has been spread on the ground under soft fruits. The reflection of sunlight and heat by the metal flakes is believed to retard the rate of plant root development. This has the effect of delaying the onset of ripening by a few weeks, providing a significant commercial advantage. The more diffuse reflective properties of this type of film are said to be an advantage over vacuum metallised film, which can concentrate the sun’s rays, causing scorching of the plants. Although metal pigment offers some inherent UV protection, UV inhibited polymer should be used to ensure maximum film life. In a modification of this process, Fawcett [6] of Transmet Corporation patented a mulch sheet to be placed on the soil around the stalk of a plant. The mulch comprises a woven mesh with aluminium flakes which will reflect the sun’s rays up onto the underside of the leaves. The mesh retains moisture that has seeped into the ground through the porous mat. The aluminium flakes are applied to the upper tacky surface of thermoplastic woven mat and pressed into position. They are fixed there when the polymer cools. A pod containing a seed may be secured to the underside of the matting in the open hole. A layer on the underside of the mat could include a layer of fertiliser. In weed suppression film, fine grades of aluminium flake exclude light, but unlike black pigmented films, which absorb heat and stimulate growth, aluminium flake pigmented films appear to discourage weeds more effectively by reducing soil temperature. The use of such film for wrapping silage for temperature modification to control storage properties is a related potential application. The main parameters affecting the service life of PE agricultural films are discussed by Henninger [7].

11.4 Sacks and bags This application area comprises rubbish bags, carrier bags and general flexible packaging. Cost is the main factor in determining the products used. Both aluminium and gold

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Metallic Pigments in Polymers bronze are employed, generally relatively low quality, wide particle size distribution ‘cornflake’ grades, in particle size in the range 5-12 µm diameter for maximum opacity. Even the finest grades of metal flake do not compete on price with carbon black for garbage bags. Applications tend to be restricted to the relatively up-market areas of carrier bags, or where the non-pigmentary attributes of metal pigments are needed, i.e., light exclusion to hide the contents or light reflection to keep the contents cool.

11.5 Containers Containers of all types are a major target for ‘effect’ pigments, particularly where their use has a fashion component. Thus injection and blow moulded cosmetic and toiletry containers, extruded toothpaste tubes and the like are frequently styled with metal pigments. Containers for car engine oils are another major, though less fashion driven outlet in this sector. Even PET beverage bottles have been launched which contain a low concentration of a coarse, food contact grade aluminium flake. Toiletry bottles are a popular application. This is at least partly due to reduced opacity requirements. A rich, deep metallic effect can be obtained from a relatively low pigment concentration, yet there is sufficient opacity to obscure the bottle’s contents. Plant pots, tubs and containers are a consumer of ‘effect’ pigments, especially coloured glitter flakes. These are formulated with white, cream, beige, pink and grey to provide simulations of stoneware and marble (see section 11.8).

11.6 Automotive It has been estimated that up to a quarter of the roughly 1,150 kg weight of a modern mid-range car is made from synthetic polymers [8]. The largest element of this is the 125 kg comprising body parts, interior trim, instrument panels and headlights; all applications in which visual quality is important. Metal automotive components are increasingly being replaced by polymer, especially where mechanical properties are not critical. Corrosion resistance and weight saving are the two main driving forces for the use of polymers in general. In addition, the need for a prestige appearance and the perceived strength attributes of metals demand that polymers be made to look like metal. This applies not just to internal and exterior trim components, but also to engine compartment parts such as air filter housings and battery mountings. Thus, for example, Fritzsche describes metal pigments for polyamide engine components, without surface marks, stable to 300 °C and required to look like diecast aluminium [9]. 126

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Figure 11.2 Accelerator pedal; Nylon 66, pigmented by 2% of a 33 µm median diameter aluminium flake pigment

A recent example of the replacement of a cast metal accelerator pedal by a metallic pigmented polymer equivalent is shown in Figure 11.2. Skilful design allowed the necessary mechanical properties to be retained with a considerable weight reduction. Extension of metallic pigmentation to larger and more visible components is frequently hampered by flow and weld lines and by the difficulty of colour matching to painted body panels. One solution is to move to mass coloured body panels. This has already occurred with the launch of a mass market automobile in China [10]. The so-called ‘Smart’ car, a joint venture between Mercedes Benz AG and the Swiss SMH AG company, will feature easily replaceable, mass coloured body panels and bumpers [11]. The very new technique of paint-less film moulding, described in section 10.5, provides the high quality of finish demanded by the automotive industry. The all-polymer panels have high surface uniformity and brilliance, toughness, good long-term durability and an attractive price/performance ratio to add to the obvious weight-saving advantage.

11.7 Pearl simulants By reducing the concentration of very high quality, fine particle size, surface polished aluminium flakes to very low levels in optically transparent polymers, it is possible to achieve light reflection from deep within the polymer. This is a characteristic of pearlescent (mica) pigments, which have much less inherent opacity than metal pigments.

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Metallic Pigments in Polymers Thus the visual effect of using 2-6% of pearl pigment may be reproduced from as little as 0.01 to 0.2% w/w aluminium flake. This ensures extremely cost-effective formulations. It must be emphasised however, that because of the inherent colour of aluminium metal, this approach is limited to silver and deeper coloured pearl effects. Aluminium flake pigments are available that have been specially designed for this application. They will typically have much lower metal contents than general purpose metallic pigment masterbatch. This is to ensure the high degree of dispersion required at the very low concentrations required. Toiletry and cosmetics containers are typical applications of this approach.

11.8 Mineral simulants The aim in this group of effects is to simulate in a mass pigmented polymer, the visual effect of such minerals as granite and marble. These formulations generally consist of two or more coarse metal flake pigments of contrasting colour in a white or coloured background. Thus the black and white particulate appearance of granite is simulated by a 200-600 µm diameter aluminium flake, mixed with a black surfaced flake of similar dimensions. Chopped foil glitters can be used, but the visual effect can look overly uniform. Milled flakes, with their irregular geometry can often provide a more natural appearance. Lightly pigmented white, cream, ochre and pink polymers with good surface gloss give the best results as hosts for the metals. PMMA and PP are the popular choices. Because the hiding power of large flakes is so low, a 1-2% loading of each may be required. The technique can also be extended to the simulation of man-made products such as stoneware. Together with the mineral simulations, these effects are found in resin bonded laminate sheets for kitchen and bathroom work surfaces and in plastic flower pots, tubs and other garden applications [12, 13].

11.9 Fibres and textiles Due to their very much larger size compared to organic and inorganic pigments, metal pigments have limited application in fibres. Indeed the diameter of a fibre strand can be less than that of many flake pigments. At the very least, mechanical strength is lost. Brightness and metallic effect are also compromised because only the smallest and therefore darkest flakes can be used. Nevertheless, Blechschmidt [14] has described the preparation and properties of polyolefin tape yarns, coloured by metallic coloured masterbatches during extrusion.

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Applications of Mass Pigmented Systems A more common application involving textiles is coating onto a textile substrate for the manufacture of synthetic leather, or leathercloth. The coating is generally a PVC plastisol, applied by knife coating and thermally cured.

References 1.

C. Cussot and D. Delage, inventors; Societe Francaise Bitumastic SA, assignee, European Patent 296,976 A1, 1988.

2.

H. A. Lemaire and C. d’Azergues, inventors; US Patent 3,252,250, 1964.

3.

N. J. Smith, inventor; US Patent 3,955,319, 1976.

4.

H. A. Letteron, inventor; General Electric Company, assignee, US Patent 3,099,897, 1961.

5.

R. L. Ferm, inventor; Chevron Research, assignee, US Patent 3,382,610, 1968.

6.

S. L. Fawcett, M. S. Fawcett and D. L. Cullen, inventors; Transmet Corporation, assignee, US Patent 4,794,726, 1989.

7.

F. Henninger and E. Pedrazzetti, The Arabian Journal for Science and Engineering, 1988, 13, 4, 473.

8.

K. Grace, British Plastics & Rubber, 1996, November, 26.

9.

T. Fritzsche, W. Pankewitz and P. Wolf, Kunststoffe, 1997, 87, 3, 297.

10. M. C. Gabriele, Modern Plastics International, 1998, 28, 8, 28. 11. Financial Times, 1997, 1st July, No. 33331, 16. 12. M. Yoshioka and H. Fukuda, inventors; Tochu Plastic Kogyo KK, assignee, Japanese patent 01/14138, 1987. 13. Kawanami and Yasutaro, inventors; assignee unknown, JP 02170847, 1988. 14. D. Blechschmidt, W. Kittelmann and H. Halke, Chemical Fibers International, 1996, 46, 5, 352.

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12

Metal Pigmented Coatings

The volume of coatings for the decoration and protection of plastic substrates (excluding automotive OEMs) has been estimated at 52 million litres in Europe in 1997, with a growth rate of 5-10% per annum [1]. Automotive plastics OEMs add perhaps another 30-50 million litres. The major players are Akzo Nobel, BASF, Du Pont, Herberts, NPA, PPG Industries, Sonneborn & Rieck, Verilac and Weilburger, with applications in automotive, teletronics and building products. This chapter considers metal pigmented coatings incorporating polymeric components as well as coatings specifically on polymer substrates. West [2] has produced a useful summary of the problems involved in painting on various polymers for automotive applications, especially those connected with on-line painting. Factors affecting adhesion to elastomer modified PP are discussed. Adhesion of paints to plastics was also the theme of a workshop in the USA [3]. A successful coating is heavily dependent on the formulation of the coating vehicle, be it ink or paint, as well as its application to the substrate. Although metal flake pigmentation influences film properties, formulations developed for organic and inorganic pigmented coatings can usually be readily modified to accommodate the unique physical and chemical characteristics of metal pigments.

12.1 Substrate preparation Another key requirement for a durable coating is good surface preparation to provide satisfactory adhesion of the coating when dry. Adhesion is particularly problematic on low energy polymer substrates, such as polyolefins, but several techniques exist to activate them to receive and bind the coating. These include flame treatment which is popular in Europe, corona discharge and also chemical treatments involving permanganates and chromates. However, these latter materials, widely used in the USA, are coming under environmental pressure. Adhesion to metal substrates can be improved by degreasing with solvent or by laying down a primer coat, as in the case of automotive coatings.

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12.2 Coating formulation and properties 12.2.1 Pigment particle size The relationship between pigment particle size and film thickness has an influence on surface texture and gloss of the coating. Flakes whose diameters are larger than the dry film thickness orient poorly, especially if their concentration is too high. In the extreme they can protrude through the film surface, causing a noticeable reduction of gloss and DOI. The problem is exacerbated by ‘seeds’, i.e., three-dimensional aggregates of flakes, a phenomenon described by Knowles [4] and shown in Figure 12.1.

Figure 12.1 The origin of metal flake pigment seeds in a coating In some ink processes, the very much larger dimensions of metal flake pigments restrict the range that can be accommodated. In gravure for example, particles with any dimension larger than about 25 µm will block the cells of the print cylinder, thereby reducing definition. Metal flake pigments offered for ink applications, with the exception of the screen process, tend to have fairly tight particle size distributions, with median diameters around 8-15 µm and few coarse flakes. Grades up to 100 µm can be used in screen printing, given an appropriate choice of screen mesh size.

12.2.2 Concentration The much greater density and particle size of metal pigments compared to organic and inorganic pigments dictate much higher loadings to achieve sufficient opacity. Whilst automotive paints can have aluminium flake concentrations of 4-6% of formulation, typical aluminium pigment loadings in inks are around 15-20%. Gold bronze inks can require loadings of 40% or more and pure silver, at the high end of the density spectrum, even higher loadings.

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12.2.3 Leafing and non-leafing Of the several properties of metal pigments which only assume relevance in coating applications, leafing and non-leafing are arguably the most important. The terms describe the orientation of metal flake pigment particles in a liquid coating system. This in turn determines their appearance and applications. Information about manufacturing processes for these pigment types can be found in Chapter 3 and a description of their properties in section 4.3.4. An integral part of formulations incorporating metal flakes is careful selection of the vehicle. This is generally dictated by the end use of the coating system. Exterior versus interior use, chemical and moisture resistance and drying speed are a few of the factors to be considered. Formulation with leafing flake pigments demands that components are used that will not adversely affect the ability of the flakes to leaf. In practice this means relatively non-polar, low acid number species. Use of leafing pigments somewhat limits solvent choice. Aliphatic and aromatic hydrocarbons are preferred. It is important to avoid low molecular weight, polar types if leafing stability is to be maintained. Polar solvents, such as lower acetates, ketones and to a lesser extent alcohols will remove the thin fatty acid coating from the flakes and with it the ability of the flake to leaf. If toluene or xylene is permitted in the formulation, leafing will be improved. The same result is achieved by increasing the surface tension of the solvent. Non-leafing flake pastes are rather less stringent in their compatibilities. As leafing ability is not an issue, the choice of solvent is more likely to be limited by compatibility with the resinous or polymeric film forming component.

12.3 Dispersion and incorporation A clear understanding of dispersion and incorporation techniques is fundamental to achieving optimum visual quality from a metal pigmented coating. Any attempt to apply the principles used for organic and inorganic pigments will be disastrous. Non-metallic pigment particles for example are sub-micron in size and often difficult to deform during dispersion. Indeed some high energy dispersion equipment such as ball mills, sand mills or high-speed dispersers are necessary to separate aggregates into primary pigment particles and develop full colour strength. Because metal pigments are fragile, the use of high energy dispersion techniques should be avoided. Ball milling or sand milling completely destroys the integrity of the flake, resulting in a much finer particle size, darker, non-metallic finish. High-speed dispersion 133

Metallic Pigments in Polymers equipment, such as sawtooth bladed or rotor/stator dissolvers should also be avoided. When running at high speed they will deform the flakes by fracturing, folding and bending. When this occurs, the resulting coating will be more grey, exhibit less hiding and may also generate seeds by cold welding. The colour loss is due to a change of orientation of flake in the film. Loss of hiding and grittiness are related to flake deformation and the resulting change in the flake surface area. The recommended procedure for dispersion of a metal flake paste is to add it to a mixing vessel fitted with a slow speed, paddle bladed agitator capable of moving the whole mass. To the paste is added one to two times its weight of vehicle or preferably solvent. Where possible a soaking period of a few hours is beneficial to the dispersion process. This is especially true for leafing pastes and smaller flake sizes. Slow speed mixing will then reduce this mixture to a thick creamy consistency. At this point most of the flakes will be separated from one another by a layer of solvent or vehicle. When this consistency has been achieved with no soft agglomerates present, further reduction with solvent or vehicle can take place. Using this technique will greatly reduce dispersion difficulties and minimise related problems.

12.4 Application to the substrate Though some modification of conditions may be required, metal pigmented coatings can be applied by almost all the techniques employed for metal free compositions. Sprayed coatings may require a gun with a larger nozzle diameter and printing equipment needs to take account of large flakes as noted in section 12.2.1.

12.5 Solvent-based systems As metal pigments are routinely manufactured by a solvent process, their incorporation in solvent-based coating systems is fairly straightforward. The main issue to consider is whether the non-metal components are compatible with the constituents of the coating system. For traditional solvent-based pastes, this means aliphatic, aromatic or mixed aliphatic/aromatic hydrocarbons. To reduce the odour in printed films and speed up drying, metal pastes are widely supplied in faster, i.e., more volatile solvents. The penalty is the tendency of such pastes to dry out and aggregate more rapidly than conventional types. The granular form overcomes these difficulties, as well as offering easy handling. Aluminium and gold bronze granules are available with aldehyde, ketone or acrylic

134

Metal Pigmented Coatings carriers (see also sections 5.5 and 13.2.1). These versatile products are suitable for a wide range of solvent-based paints and inks. In the case of gold bronze, stability is enhanced by using resins of low acid number. This restricts carrier choice to the aldehyde and ketone types. Ferguson [5] has produced an account of the manufacture and use of metal pigments in paints, including details of test methods.

12.6 Water-based systems Water-based media present considerable challenges to the application of both aluminium and to a lesser extent, gold bronze and zinc pigments. The main problem to be overcome is the tendency for chemical reaction. Aluminium in contact with water is afforded some limited protection from the inert oxide coating and traces of fatty acid lubricants left from the manufacturing process. Also, in general, the purer the metal, the more resistant it is. After a few hours at ambient temperature however, hydrogen gas is generated according to the equations: 2Al + 3H2O = Al2O3 + 3H2 and

2Al + 6H2O = 2Al(OH)3 + 3H2

Apart from being highly flammable, hydrogen causes a build up of pressure in the container of paint or ink, creating a secondary hazard. Aluminium pigment manufacturers have developed a simple test for gas generation. The apparatus is shown in Figure 12.2. A 200 g sample of the coating containing the aluminium flake pigment is placed in an Erlenmeyer flask, ensuring that none coats the neck. The flask, with the stopper securely inserted, is placed in a bath maintained at 50 °C. The water level in the burette is adjusted to the 100 ml mark and the temperature allowed to equilibrate for at least an hour. At the end of this time, the water level in the burette is noted. This is the zero point from which subsequent measurements are made. The test is normally conducted for at least one week, recording the water level daily. It is important to control the ambient air temperature to avoid volume measurement errors. Gas generation of less than 10 ml in this period is generally considered a satisfactory result for an automotive coating. Industrial finishes can tolerate higher levels. For each test material, it is good practice to simultaneously run a control sample of known gassing characteristics.

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Metallic Pigments in Polymers

Figure 12.2 Gas generation testing apparatus

A considerable effort has been made by all the major metal pigment manufacturers to find means of inhibiting or passivating metal pigments, especially aluminium, against aqueous attack. A full survey of the dozens of literature references and patents in this field is outside the scope of this book. Instead a brief summary is presented of all the major techniques. These can be divided into organic inhibitors, inorganic or organometallic treatments and encapsulation by resins or polymers. Brown and Rolles [6, 7] discovered that merely mixing a few percent of nitro derivatives of aliphatic or aromatic hydrocarbons or 3- or 5-nitro salicylic acid into an aluminium paste increased its water resistance. Toyo Aluminium [8] claimed enhanced aqueous performance by replacing the conventional lubricant in the wet milling process by dimer acids, derived for example from oleic or linoleic acids. Inorganic treatments create a deposit of one or more inorganic salts by chemical reaction from solution. Oxides of chromium, precipitated from ammonium dichromate, are particularly effective [9, 10, 11, 12]. Such treatments are coming under increasing environmental scrutiny, due to the difficulty in removing traces of Cr6+. The EC Waste Packaging Directive, 94/62/EC [13], requires that hexavalent chromium levels in packaging do not exceed 100 ppm from 2001. Eckart-Werke [14] in Germany continues to offer chromium oxide inhibited aluminium flake pigments under the Hydrolux trade name. 136

Metal Pigmented Coatings Other inorganic or organometallic passivation processes involving molybdenum [15, 16], phosphorus [17, 18, 19, 20, 21, 22, 23, 24], silicon [25], titanium [26], cerium [27, 28] and vanadium [29, 30] have all been the subject of patent applications. Of these, silicon and phosphorus are most used commercially. A passivation mechanism involving metal pigment treatment with chromium, vanadium or phosphorus compounds in the gas phase is the subject of a patent to Schmid of BASF [31]. Indeed BASF was a pioneer of gas phase reactions on metal flakes maintained in motion by means of a fluidised bed. Encapsulation processes involve numerous compounds, oligomers and polymers, precipitated from solution or created at the flake surface, for example by polymerisation. Banba [32] describes an encapsulant for aluminium flake containing vinyl and epoxy functionality. EckartWerke [33] claimed a siloxane, covalently bonded to the surface of aluminium flake, and a three-dimensionally cross-linked synthetic resin coating covalently bonded to the siloxane. A nitro substituted polymeric coating for aluminium flake is employed by Carpenter [34], whilst Iri [35] disclosed a reaction of isocyanate and phosphorus compounds for the protection of aluminium and gold bronze. A stable, high solids metal flake pigment dispersion is provided by Chang [36] by neutralising a phosphated acrylic polymer. In addition to the above, there are a few techniques for surface modification of the aluminium flake which attempt to capitalise on the limited degree of protection afforded by the natural oxide surface and traces of lubricant remaining from the production process. Preparing an alloy of aluminium with other metals as a means of improving water resistance has also been examined. Uchimura at Toyo [37] applied for a patent for an alloy of aluminium, zinc and silicon, with minor amounts of indium or tin. Much recent work on the mechanisms of aluminium flake stabilisation has been published by Müller in Germany. The rate of the corrosion reaction was followed by measuring the volume of hydrogen evolved. In studies of mono, di, and polysaccharides and related hydroxycarboxylic acid compounds, strongly reducing enediols, such as ascorbic acid or glycolaldehyde were found to be the most effective gassing inhibitors [38]. They also retained good colour in coatings, unlike phenolic based inhibitors. Ortho substituted phenol derivatives, especially salicyl alcohol, are nevertheless effective in water-butyl glycol mixtures at pH 10. In contrast, ortho substituted anilines offered no passivation [39]. Esters of gallic acid increased in passivation activity with increasing chain length [40]. Hydrogen evolution of styrene-maleic acid copolymer inhibitors could be correlated with the acid number, lower acid numbers and higher molecular weight being most effective

137

Metallic Pigments in Polymers [41]. Such copolymers were very much more effective than high molecular weight polyacrylic acids. As expected from other studies, analysis revealed some formation of partially soluble aluminium salts of the polymers [42]. Inhibition is enhanced in alkaline media but overall the effect of the pH of the system is variable [43]. For low molecular weight (<60,000) styrene maleic acid copolymers, stability is greater at pH 8 than at pH 10. At higher molecular weights, (>100,000) the effect is reversed [44]. The isoelectric point (IEP) of aluminium oxide (~pH 9) is significant in controlling the corrosion inhibiting effect of polyacrylic acid in alkaline media. Above the IEP (pH 10) high and low molecular weight polyacrylic acids inhibited corrosion, the effectiveness decreasing with increasing molecular weight. Below the IEP (pH 8), low molecular weight material showed inhibition, but less effectively than at pH 10. Again effectiveness decreased with increasing molecular weight. High molecular weight polymer at pH 8 actually stimulated corrosion [45]. Some other classes of resin also show some corrosion inhibiting ability. Alkyd resins showed good inhibition at pH 10. There is some effect from epoxy resins, and oil free saturated polyesters were found to be the poorest of the resin classes examined [46]. The solvency of the inhibitor is important. Water soluble and ionomer dispersions tend to inhibit corrosion better than emulsions. Certain functional groups, notably carboxy, phenolic hydroxy and longer chain unsaturated alkyl groups, are also more effective [47]. The passivation activity of phenolic resins is rationalised by improved adhesion to the metal surface through formation of a surface-chelate complex [48]. A common feature of all inhibiting agents for aluminium is specificity. The inhibited metal flake product which is highly resistant to gassing in one commercial paint or ink medium may fail rapidly in another. At the time of writing, there is no 100% reliable inhibited aluminium pigment on the market. This reflects the difficulty of entirely inhibiting every flake surface. Although the evidence is circumstantial, the gassing reaction appears to be capable of being initiated by very few partially coated or uncoated flakes. The oxidation reaction is so exothermic that adjacent flakes are attacked. If the heat cannot be easily dissipated, as for example when product is tightly packed in a sealed drum, catastrophic failure can occur. Unless fitted with a vent plug, the drum will then bulge or even rupture due to the pressure of the generated hydrogen gas. For this reason, aluminium flakes intended for water-based applications are best not carried in water. The recent introduction of dry granular product forms to the market overcomes this problem. Carriers include non-ionic surfactants, intended for inks, and polypropylene glycol, used for paints. Of other commercially available product forms, pastes in water compatible solvents, such as alcohols, are the most prominent (see also Chapter 5). The popular isopropyl alcohol appears to offer some inhibition of gassing, but for total reliability in aluminium pastes, some extra passivation of the metal is desirable. 138

Metal Pigmented Coatings Passivation of gold bronze is a less important issue due to the inherently lower chemical reactivity of its copper and zinc constituents. This is reflected in the lack of both patents and literature on the subject. Coating with silica is widely practised as a means of improving tarnish resistance in general. Combining metal powder, particularly zinc, with a polymerisable monomer and spray drying gives a pigment with greater resistance in aqueous lacquers [49]. Inhibition of hydrogen evolution in an aqueous alkaline, zinc pigmented paint medium was also investigated by Müller. Propyl, octyl and dodecyl gallate were shown to be good inhibitors, as were oligomeric phenolic resins, the latter through chelation to zinc(II) [50]. In contrast, citric acid accelerated hydrogen generation in alkaline media. Some metal salts are effective as inhibitors, the most notable being cerium(III) chelates of citric acid. In this case, however, the pronounced inhibiting effect could be ascribed to the cerium rather than the charge of the chelates [51].

12.7 UV/EB cured coatings UV curing has emerged as a coating technique in the last 20-30 years. It currently represents only 1% of the total European coatings market and 3% of the inks market [52]. Electron beam (EB) curing is a higher energy technique, still in its commercial infancy. A comprehensive introduction to radiation curing technology in general is provided by Holman and Oldring [53]. The key advantage of both techniques is that they are VOC free, i.e., all the coating applied to the substrate is retained after cure. No heat is required, making them useful techniques for plastic substrates. Curing, which is virtually instantaneous, is effected by a polymerisation of liquid monomers, initiated by an artificial UV light source. Such radiation curing systems are environmentally friendly, but the monomers are expensive and there are concerns over the perceived toxicity of some of the active components. It is a popular misconception that metal pigments cannot be used in UV systems due to inhibition of through-curing. What is true is that unprotected metal flake surfaces, especially aluminium, catalyse cure in the absence of any source of UV energy. For this reason, metal pigmented coatings are offered as either ‘one pack’ or ‘two pack’ systems. For the former, the metal flakes are prevented from causing premature cure by resin or inorganic compound encapsulation. For the latter, unprotected flakes are used, but they are supplied separately from the monomer and photoinitiator system. The two components are brought together immediately before use. At ambient temperatures, the ready-to-use coating will remain usable for 6-24 hours, depending on metal purity, concentration and

139

Metallic Pigments in Polymers flake size. High loadings of more active aluminium metal, in fine particle sizes, corresponding to high surface area for reaction, have the shortest pot life. In contrast, inhibited one pack types can provide usable coatings that can be stored for many months, even years, providing light is excluded from the pack. Indeed to prevent premature curing, all active UV formulations must be stored in opaque, preferably plastic, containers. Metal pigment product forms available in the market and appropriate for UV/EB cured coatings, are dry flakes and also stiff pastes in which the carrier is an inert diluent component of the formulation. A fatty alcohol is used by at least one manufacturer. Granular forms may also be used, providing the carrier will dissolve in the other components and does not interfere with application properties. In practice it is difficult to attain the visual quality of conventional liquid coatings from a UV curing system. The reasons for this are not fully understood, but are at least partly due to higher application viscosity and lack of flake orientation. As there is no loss of volatiles, there is no mechanism to align flakes parallel to the surface to increase reflectivity as would occur in a conventional liquid coating. For these reasons, high quality, particularly reflective flake pigments must be used. This further reduces the competitiveness of UV systems versus their liquid counterparts. Overall, however, the attractions of the technology outweigh the disadvantages, as evidenced by the continuing growth of the metallics UV market.

References 1.

The Coatings Agenda Europe, 1997, 81.

2.

E. J. West, Proceedings of a Symposium on Coatings for Plastics, Harrogate, UK, 1986, Paper No.9.

3.

Adhesion of Paints to Plastics Workshop, Presented at Adhesion and Coupling Agent Technology 97, Boston, USA, 1997, Intertech Conferences.

4.

R. Knowles, Polymers Paint Colour Journal, 1991, 181, 4297, 714.

5.

R. L. Ferguson, Paint and Coating Test Manual (Gardner-Sward Handbook 14th Edn.), Ed., J. V. Koleske, ASTM, 1995, 223.

6.

M. H. Brown, inventor; Aluminium Company of America, assignee, US Patent 2,848,344, 1953.

7.

M. H. Brown and R. Rolles, inventors; Aluminium Company of America, assignee, US Patent 3,244,542, 1962.

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Metal Pigmented Coatings 8.

E. Uchimura and Y. Hashizume, inventors; Toyo Aluminium KK, assignee, GB Patent 2,043,092, 1980.

9.

R. L. Hawkins, Jr., G. Mills and E. G. Bobalek, inventors; The Empire Varnish Company, assignee, US Patent 2,904,523, 1955.

10. J. A. DeRidder, inventor; Diamond Shamrock Corporation, assignee, US Patent 3,940,280, 1976. 11. T. Higashiyama and T. Nishikawa, inventors; Diamond Shamrock Corporation, assignee, US Patent 4,266,975, 1981. 12. T. Kondis, inventor; Silberline Manufacturing Co., Inc., assignee, US Patent 4,693,754, 1987. 13. Coatings COMET, 1998, 6, 1, 63. 14. R. Besold, W. Reisser and E. Roth, Pitture e Vernici, 1991, 67, 10, 9. 15. S. Setoguchi, H. Katoh and T. Matsufuji, inventors; Toyo Aluminium KK, assignee, US Patent 5,480,481, 1996. 16. T. Yamamoto, M. Uenishi, H. Katoh and S. Setoguchi, inventors; Toyo Aluminium KK, assignee, US Patent 5,494,512, 1996. 17. S. Ishijima, Y. Hayashi and T. Kiritani, inventors; Asahi Kasei Kogyo KK, assignee, GB Patent 2,053,258 B, 1979. 18. T. Kondis, inventor; Silberline Manufacturing Co., Inc., assignee, US Patent 4,808,231, 1989. 19. T. Kawabe, T. Bamba, T. Matsufuji, H. Ueshimo, Y. Hashizume, E. Uchimura, M. Harasada, M. Aoki and T. Kimura, inventors; Toyo Aluminium KK, assignee, US Patent 4,869,754, 1989. 20. R. Schmid, N. Mronga, H. Keller and J. A. G. Gomez, inventors; BASF AG, assignee, US Patent 5,474,605, 1995. 21. C. Keemer, W. G. Jenkins, H. T. Lamboin and J. B. Scheller, inventors; Silberline Manufacturing Co., Inc., assignee, US Patent 5,215,579, 1993. 22. W. G. Jenkins, C. Keemer, H. T. Lambourn and M. Curcio, inventors; Silberline Manufacturing Co., Inc., assignee, US Patent 5,296,032, 1994.

141

Metallic Pigments in Polymers 23. W. G. Jenkins, C. Keemer, H. T. Lambourn and M. Gurcio, inventors; Silberline Manufacturing Co., Inc., assignee, US Patent 5,356,469, 1994. 24. C. B. Keemer, W. G. Jenkins, H. T. Lambourn and J. B. Scheller, inventors; Silberline Manufacturing Co., Inc., assignee, US Patent 5,470,385, 1995. 25. R. Schmid, N. Mronga, H. Keller and J. A. G. Gomez, inventors; BASF AG, assignee, US Patent 5,474,605, 1995. 26. W. Ostertag and N. Mronga, inventors; BASF AG, assignee, US Patent 4,978,394, 1990. 27. J. S. DePue, C. W. Carpenter and L. G. Bemer, inventors; BASF Corporation, assignee, US Patent 5,372,638, 1994. 28. J. S. DePue, C. W. Carpenter and L. G. Bemer, inventors; BASF Corporation, assignee, US Patent 5,322,560, 1994. 29. C. B. Keemer, W. G. Jenkins, H. T. Lambourn and J. B. Scheller, inventors; Silberline Manufacturing Co., Inc., assignee, US Patent 5,470,385, 1995. 30. T. Kondis, inventor; Tom Kondis, assignee, US Patent 4,693,754, 1987. 31. R. Schmid, N. Mronga and J. A. G. Gomez, inventors; BASF AG, assignee, US Patent 5,352,286, 1994. 32. T. Banba, inventor; Toyo Aluminium KK, assignee, US Patent 4,434,009, 1984. 33. W. Reisser and G. Sommer, inventors; Eckart-Werke, assignee, US Patent 5,332,767, 1994. 34. C. W. Carpenter and J. M. De Haan, inventors; BASF Corporation, assignee, US Patent 5,389,139, 1995. 35. K. Iri and M. Suzuki, inventors; Asahi Kasei Metals Ltd., assignee, US Patent 5,272,223, 1993. 36. D. C. K. Chang, inventor; E. I. Du Pont de Nemours & Co., assignee, US Patent 5,104,922, 1992. 37. E. Uchimura, inventor; Toyo Aluminium KK, assignee, British Patent 2,147,310, 1984. 38. B. Müller and M. Kurfess, Werkstoffe und Korrosion, 1993, 44, 9, 373.

142

Metal Pigmented Coatings 39. B. Müller, British Corrosion Journal, 1996, 31, 4, 315. 40. B. Müller, M. Müller and I. Lohrke, Farbe Lack, 1994, 100, 7, 528. 41. B. Müller, Journal of Coatings Technology, 1995, 67, 846, 59. 42. B. Müller and T. Schmelich, Corrosion Science, 1995, 37, 6, 877. 43. B. Müller and U. Davidowski, Coating, 1995, 28, 9, 345. 44. B. Müller and A. Holland, Materials and Corrosion, 1997, 48, 2, 95. 45. B. Müller and U. Dawidowski, Materials and Corrosion, 1996, 47, 3, 154. 46. B. Müller and A. Holland, Surface Coatings International, 1997, 80, 7, 321. 47. B. Müller, T. Schmelich and M. Gampper, Farbe Lack, 1995, 101, 2, 101. 48. B. Müller, Adhasion Kleben & Dichten, 1996, 40, 12, 32. 49. E. Weiderhold, inventor; no assignee, GB Patent 1,115,338, 1965. 50. B. Müller and I. Förster, Corrosion, 1996, 52, 10, 786. 51. B. Müller, W. Kläger and G. Kubitzki, Corrosion Science, 1997, 39, 8, 1481. 52. J. Kerr and B. Seath, Proceedings of Radcure Coatings and Inks: Application and Performance, Harogate, UK, 1996, Paper No.5. 53. UV and EB Curing Formulation for Printing Inks, Coatings and Paints, Eds., R. Holman and P. Oldring, SITA Technology, London, 1988.

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13

Applications of Metal Pigmented Coatings

The main application areas for metal pigmented coatings are described here. Representative starting-point formulations are provided for the most common coating types. They are intended as general guidance and are not optimised for any given application. Polymers feature as substrates, binder components, or sometimes both.

13.1 Painting Coating polymer substrates is the alternative to the mass pigmentation options described in Chapter 11. The strengths and weaknesses of the two techniques are covered in Chapter 6. World Paint File 1998-2002 [1] provides an overview of production and consumption trends and applications, plus market forecasts from 30 leading global paint makers. The market penetration of conventional solvent-based, water-based, powder coat and radiation cured coatings in Europe is described in Coatings Agenda [2]. Water-based coatings are estimated to have 11.6% of the market share in Europe, almost the same as in the USA and Japan. Conventional and high solids solvent-based paints together account for almost three-quarters of consumption. Thus, penetration of the aqueous coatings market has not been as rapid as anticipated in the early 1990s. Paint application areas for metal pigments, specifically on polymer substrates, include automotive trim components, teletronics and domestic appliance housings; indeed many of the same applications being explored by mass pigmentation. Both solvent-based and water-based formulations are used, though the latter must take account of the water sensitivity of some metal pigments, as discussed in section 12.6. Other coating applications make use of the protective and anticorrosion properties of some of the metals. Efficient pre-dispersion of the metal flake, described in section 12.3, is a prerequisite for a high quality finish. This is irrespective of the application or the metal pigment delivery form. Ferguson [3] gives formulation guidelines for both leafing and non-leafing aluminium pigments in paints and inks. In considering the application of coatings to polymer substrates, adhesion is a key concern. PP is the polymer of choice for many high volume applications involving metallic paints. 145

Metallic Pigments in Polymers To achieve satisfactory adhesion onto such low energy surfaces, the surface must be activated, for example by corona discharge. In this respect, the subsequent application of a metallic coating is no different to that of a coating free of metal pigment. Silanes, titanates and more recently zirconates have been employed to aid adhesion. Du Pont [4] have patented an alkyl titanate for the purpose.

13.1.1 Solvent-based paints Metal flake pigment in the traditional hydrocarbon solvent paste form is generally suitable for solvent-based paints, especially where aromatic solvent has enhanced the solvency of the paste. It is this form that is widely used in automotive formulations. A basecoat formulation suitable for both OEM and refinish is shown in Table 13.1. It is applied to an approximately 15 µm thickness and overcoated with a clear film some 40 µm thick. For general industrial coatings where a single coat suffices, Table 13.2 shows a typical formulation, again using a medium-fine aluminium paste. Hammer finishes are an other important outlet for aluminium flakes. Their function is protective as well as decorative. A silicone additive generally provides the distinctive mottled effect (see Table 13.3). Applications include metal fences and railings and to provide an antique gold effect, for example on gas and electric fires. Fine and medium-fine grades of both aluminium and gold bronze are used in aerosol paints. A starting point formulation for the latter is reproduced in Table 13.4. A simple formulation to provide paint suitable for application by brush, for example in modelling paints and general gilding simulation work, is provided in Table 13.5. The advantages of a dry, granular form of metal pigment, so familiar to ink makers, are now being recognised by paint makers. Formulations are adjusted for the generally higher metal content of granules and for the presence of carrier that will replace an equal weight of resin in the formulation.

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Applications of Metal Pigmented Coatings

Table 13.1 Silver metallic basecoat formulation for OEM or refinish Polyester resin

17.4%

Melamine resin

13.7%

CAB (cellulose acetate butyrate) resin, 20% solution in n-butyl acetate

31.5%

Polyethylene wax, 10% dispersion in 5:4:1 xylene:n-butyl acetate:isobutanol

25.2%

Aluminium flake pigment paste (NV (non-volatile content) = 65%; D50 = 15 µm), dispersed 1:1 in 2:1 butyl acetate:xylene

12.2%

The resin solutions and wax dispersion are combined in a high-speed disperser and filtered to remove any gelled particles. The metal dispersion is mixed in with gentle agitation and the system diluted to a spray viscosity of 15 s, DIN 4, using 2:1 butyl acetate:xylene. The paint should be filtered again before use.

Table 13.2 One coat silver industrial stoving paint Short-oil, non-drying alkyd (70% in xylene)

50.0%

Melamine formaldehyde resin (65% in n-butanol)

15.0%

Xylene

13.0%

n-Butanol

3.0%

2-Butoxyethanol

5.0%

Aluminium flake pigment paste (NV = 64%; D50 = 16 µm), dispersed 1:1 in xylene

14.0%

Resins and solvents are combined to form a smooth dispersion, to which the aluminium flake dispersion is added with gentle paddle agitation. The paint is thinned to 20 s in a Ford No.4 cup with a blend of 4:1 xylene:n-butanol before use.

Table 13.3 Silver industrial hammer finish paint Aluminium flake pigment paste (NV = 65%; D50 = 30 µm), dispersed 1:1 in xylene

20.0%

Styrenated alkyd resin (60% in xylene)

79.7%

Silicone oil

0.2%

Cobalt drier

0.1%

The aluminium flake dispersion is added to the resin solution with gentle paddle agitation. Silicone oil and cobalt drier are stirred in and the paint thinned with ethyl acetate, butyl acetate, xylene, or a mixture of these, to 20-30 s spraying viscosity in a Ford No.4 cup.

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Metallic Pigments in Polymers

Table 13.4 Gold bronze aerosol paint Acrylic resin (65% in xylene)

6.5%

Toluene

10.5%

Xylene

18.0%

Acetone

50.0%

Rich pale, extra fine lining paste (NV = 85% in white spirit; D50 = 6 µm), dispersed at 2:1 in xylene

15.0%

The bronze pigment premix is gently stirred into a dispersion of the remaining components.

Table 13.5 Gold bronze brushing paint Polyurethane alkyd varnish (65% in xylene)

70.0%

Rich, fine lining paste (NV = 85% in white spirit; D50 = 11 µm)

30.0%

The gold bronze pigment paste is gently combined with the varnish and white spirit added to application viscosity, whilst under agitation.

13.1.2 Water-based paints Paint manufacturers have been active in the passivation of metal flake pigments. A patent to Nippon Paint [5] discloses surface treatment of aluminium, copper or zinc by aqueous phosphorus or vanadium ions. A related patent [6] claims a phosphated aluminium flake pigment and derived water-based paint for automotive finishes. Frangou and Backhouse [7, 8, 9, 10] at ICI researched vinyl and addition polymer resins containing phosphorus, optionally as a metal salt. These are used to passivate aluminium flake pigment in aqueous automotive basecoats. Resin formulations are tailored to improve adhesion between basecoat and clearcoat. The Eastman Kodak Company [11] patented an aqueous metallic coating containing a cellulose mixed ester and a compatible amine neutralised acrylic resin. Cellulose esters impart improved metallic orientation, solvent release and high gloss. The acrylic resin contributes strength and hardness in the final film. Anderson at BASF [12, 13] described a two pack system in which unpassivated aluminium flake was retained in solvent and added to the water-based resin system immediately before use. The vehicle is a water reducible acrylic latex, with non-ionic, acid or urethane functionality, plus a rheology control agent. The resulting paint is intended for automotive use.

148

Applications of Metal Pigmented Coatings Anionic PU and acrylic resins, formulated with aluminium pigment inhibited by a phosphate ester, have also been patented by BASF [14] for automotive use. A stable, water-borne, high solids, metal flake pigment paint, incorporating a neutralised phosphated acrylic film former, was patented by Chang of Du Pont [15]. An alkylated melamine formaldehyde polymer provides cross-linking. Outside the patent literature, the large paint companies tend to guard their formulations, making it difficult to provide specific guidance. Nevertheless, there are a few ground rules. A pH range from 5-9 is desirable, with 7.5-8.5 preferred. Gassing can be expected to become more of a problem in aluminium flake pigmented systems the further the pH moves away from this range. Wherever possible, pH should be adjusted prior to the addition of the metal pigment. Unlike solvent-based coatings, water-based formulations tend to be very system specific. There is no substitute for thorough compatibility and stability testing on a small scale. Many of the resin systems in use require neutralisation in situ. It has been found that complex amines which are used to control the pH and also to take the resins into solution, such as dimethylethanolamine (DMEA), trimethylethanolamine (TMEA), 2dimethylamino-2-methyl-1-propanol and the well known 2-amino-2-methyl-1-propanol (AMP-95) are less aggressive to the metals than triethylamine or the cheaper ammonia solution. Chlorinated polyolefins are reputed to improve the orientation of flakes in basecoat-clear systems [16]. This allows thinner basecoats to be used, yet with greater opacity. Film forming resins include acrylics, polyesters, and polyurethanes. A useful water-based automotive paint test system, based on an inhibited grade of aluminium flake pigment, is provided in Table 13.6. Acceptable stability is typically a release of <10 ml of hydrogen in the gas test apparatus of Figure 12.2 after 7 days at 50 °C or sometimes 4 weeks at 40 °C.

149

Metallic Pigments in Polymers

Table 13.6 Water-based silver automotive paint test system Premix Inhibited aluminium flake pigment paste (NV = 65%; D50 = 12 µm)

6.9%

Ethylene glycol monobutyl ether

6.0%

Trimethyl pentane diol mono propionate

0.3%

Vehicle Acrylic resin (50% NV in water)

66.7%

Dipropylene glycol methyl ether

3.3%

Deionised water Rheology modifier

16.5% 0.3%

Aluminium and ethylene glycol ether are gently blended to a creamy paste, to which the surfactant is added. In a separate vessel, resin, water and dipropylene glycol ether are combined and the aluminium premix added with low shear agitation. Finally, the rheology modifier is added, again under slow agitation.

As an example of a general purpose, water dilutable coating, Table 13.7 shows the use of an inhibited aluminium flake pigment paste, incorporated in a water reducible alkyd resin system.

Table 13.7 Water reducible, general purpose silver industrial paint Premix Inhibited aluminium flake pigment paste (NV = 66%; D50 = 18 µm)

7.0%

2-Butoxyethanol

8.4%

Vehicle Water reducible alkyd

27.6%

Manganese aqueous curing agent, 8%

0.4%

Cobalt aqueous curing agent, 5%

0.7%

Water 25% aqueous ammonia solution

54.5% 1.4%

The vehicle components are blended together in turn, finally adjusting the pH to 8.3 with the ammonia solution. The aluminium flake, soaked and well dispersed in the 2-butoxyethanol, is gently stirred into the prepared vehicle.

150

Applications of Metal Pigmented Coatings A silver, water-based acrylic formulation, equally suitable for industrial coatings on metal and plastic, is described in Table 13.8. When spray applied, this coating has a fast touchdry time.

Table 13.8 Fast drying, water-based acrylic, general industrial paint Premix Inhibited aluminium flake pigment paste (NV = 65%; D50 = 30 µm)

3.8%

2-Butoxyethanol

2.4%

Diethylene glycol monobutyl ether

3.2%

Vehicle Aqueous acrylic resin solution (NV = 55%) Manganese aqueous curing agent, 8% 2-Butoxyethanol

70.0% 0.4% 14.6%

Water

3.8%

Dibutyl phthalate

1.4%

The vehicle components are blended together in turn, with a final adjustment of the pH to 8.3 with the ammonia solution. A well dispersed aluminium flake premix in 2-butoxyethanol alone is diluted with the diethylene glycol monobutyl ether under gentle agitation. The completed premix is gently stirred into the prepared vehicle and the following components added under slow agitation. Rheology modifier

0.1%

Modified polydimethylsiloxane (50% in 2-butoxyethanol)

0.3%

Heat resistant, water dilutable aluminium pigment formulations are used in coatings on automobile exhaust manifolds, exhaust pipes and the like. A simple formulation is given in Table 13.9. The coating can be applied by brush, or thinned if necessary for spray application. Curing is typically carried out at around 210 °C. The cured coating can withstand temperatures in excess of 600 °C without losing adhesion.

Table 13.9 Heat resistant, water dilutable silver paint Silicone resin emulsion (NV = 55%)

75.0%

Inhibited aluminium pigment paste (NV = 64% in water; D50 = 19 µm)

25.0%

The aluminium paste is gently stirred directly into the vehicle.

151

Metallic Pigments in Polymers Table 13.10 shows a gold metallic water-based paint, suitable for spraying or use in an aerosol. In the latter case, a propellant, such as dimethyl ether, is required in a ratio of 3 parts paint to 2 parts ether. The formulation shows some stability under ambient conditions. If the paint is to be used in an aerosol, tests should be carried out to verify its stability in the presence of the propellant.

Table 13.10 Water-based gold-bronze aerosol paint Acrylic resin emulsion (NV = 50%)

33.4%

Deionised water

23.0%

Isopropyl alcohol

16.2%

Propylene glycol monomethyl ether

8.3%

Antifoam

0.8%

2-amino-2-methyl-1-propanol (95% in water)

4.7%

Wetting agent solution

3.3%

Extra fine lining gold bronze powder (D50 = 9 µm)

10.3%

The resin is diluted with the water and the amine added. The remaining components are mixed in with slow agitation, leaving the metal flake addition to last. The final pH should be adjusted to between 7.5 and 8.5 if necessary.

13.1.3 In-mould coating This technique is the coating equivalent of the mass pigmentation in-mould decoration method described in section 10.6. In this case, a pigmented system is applied to one face of the mould tool by spraying, optionally by robot. A liquid, thermally cross-linkable polymer-based coating system is normally chosen to eliminate any problems from solvent release [17]. Polymer is then injected to bond to the pigmented layer. Thereafter the completed part is ejected. Although a more expensive technique than direct mass pigmentation, it provides the visual quality and uniformity of a painted finish without any of the application and drying problems of a separate painting operation. It is particularly useful for metal flake pigments as troublesome flow and weld lines are eliminated.

152

Applications of Metal Pigmented Coatings

13.1.4 Miscellaneous paints Both leafing and non-leafing grades of nickel flake are used in decorative coating systems. They produce an attractive, bright finish with a warm hue, somewhat reminiscent of metallic silver or pewter. They can be incorporated into solvent-based paint and powder coatings systems and also into water-based coatings, since nickel shows excellent resistance to attack in aqueous media. The largest application of nickel flake pigment is in the manufacture of electrically conductive coatings. These have become very important over the past twenty years in the electronics industry, for shielding purposes where they are used to achieve compliance to electromagnetic compatibility legislation. A special grade of nickel flake that is selectively treated to give a surface that maximises its electrical properties is produced specifically for this purpose and described as a conductive grade (see sections 15.3 and 15.4). Stainless steel flakes are widely employed in decorative and functional coating applications. They are somewhat darker than aluminium, with a bluish overtone. A wide size range is available, larger flake sizes giving a brighter appearance than the fine. Finer grades are used to produce coatings that have an appearance similar to that of pewter. The major advantage of these flakes is undoubtedly their resistance to corrosive attack. The UNS-S 31603 alloy composition confers excellent properties in a whole range of corrosive environments. This enables finishes containing stainless steel flakes to be used, for example, in external atmospheric exposure applications without any further protective top coat system. Other significant applications include the production of aerosol coatings.

13.2 Printing In general, flexible packaging is the largest outlet for inks containing metal pigments. High quality, usually silver dollar type aluminium and some coated gold bronze pigments are used for spot colour, highlights and borders in high quality packaging, gift wrap and in prestige publications such as company reports. State of the art aluminium pigments are also beginning to compete with aluminium foil and golds prepared by coating aluminium foil with yellow lacquer. The ultimate in colouristic quality is found by using aluminium flake prepared by vacuum deposition, whose preparation is described in section 3.7. Properly applied, their uniformity of reflection approaches that of vacuum deposited film. Lower price, cornflake grades of aluminium flake are extensively used in commodity packaging, especially foodstuff wrappers. For inks applied to polymeric substrates, the same concerns about adhesion that are expressed for paints, continue to apply.

153

Metallic Pigments in Polymers

13.2.1 Solvent-based inks Metal pigments of choice for this application are pastes and granules. Dry flakes are still used, especially gold bronze, but the same drawbacks of poor wetting and contamination of the environment apply. Traditional solvent pastes, i.e., those containing high boiling hydrocarbon solvents are unsuitable for solvent-based liquid inks. Their drying rate is too slow for modern, highspeed presses and they tend to leave residual odour in the printed stock. The response of pigment manufacturers has been to offer paste forms in the faster, i.e., more volatile solvents, widely used in the industry. Thus aluminium and gold bronze flakes are commercially available in esters such as ethyl or isopropyl acetate, or alcohols such as isopropanol. As it is difficult to get adequate brightness from non-leafing bronze pigments made by milling, leafing grades are de-leafed as required. The greatest disadvantage of pastes supplied in fast solvents is that the solvents tend to evaporate in the can. This leads to flake aggregation (seeds) and loss of hiding and surface smoothness in application. For this reason, containers must be securely lidded when not in use. The disadvantages of pastes are overcome by granular forms. The most popular granular grades for solvent-based inks use a synthetic aldehyde or sometimes a ketone resin. These resins have very wide solvent and binder compatibility. They are also film formers in their own right and so form a part of the dried ink film. Granules are more expensive to produce since they are derived from pastes. The inclusion of carrier resin that would otherwise have to be added by the ink maker is a cost-mitigating factor that is often overlooked. Tables 13.11 and 13.12 show solvent ink formulations for gravure and flexographic printing. For the former, pigments with median particle sizes below 14 µm are preferred. To print such inks satisfactorily onto polyolefin film, the film surface must be activated, for example by corona discharge, as described for paints.

13.2.2 Paste inks Metal pigments intended for litho printing are supplied in paste form, often in a high boiling petroleum distillate, known as High Boiling Petroleum Fraction (HBPF), with a typical boiling point of 160-180 °C. This is the same solvent that is commonly used in such inks, thereby guaranteeing compatibility. Table 13.13 shows one such formulation.

154

Applications of Metal Pigmented Coatings

Table 13.11 Solvent-based aluminium ink for gravure printing Premix Aluminium flake pigment granule (D50 = 12 µm; 80% in aldehyde resin)

18.0%

Ethyl acetate

18.0%

Vehicle Acrylic resin (100%)

19.0%

Isopropyl acetate

40.0%

Methoxy propanol

5.0%

The vehicle components are homogenised by high-speed stirring and the premix thereafter added under slow speed mixing.

Table 13.12 Solvent-based gold bronze ink for flexographic printing Gold bronze flake powder (D50 = 12 µm)

35.0%

Polyvinylbutyral resin

12.0%

Isopropanol

52.8%

Adhesion promoter

0.2%

The bronze powder is gently added to the remaining components which have been pre-dispersed by high speed stirring. Isopropanol is used to reduce to a printing viscosity of around 25-30 seconds (Zahn No. 2. cup)

Table 13.13 Silver ink for lithographic printing Aluminium flake, (D50 = 10 µm), supplied at 80% in HBPF

25%

Hydrocarbon resin solution

72%

Wax dispersion

3%

The components are combined by low energy agitation to a homogeneous, stiff mix.

155

Metallic Pigments in Polymers

13.2.3 Water-based inks The same general product forms, powder, paste and granules, can be used in aqueous inks. In addition, ink concentrates are offered. These consist of a high loading of metal flake, with sufficient resin and solvent or water to prevent aggregation. The wetting problems of dry powders are exacerbated by the high surface tension of water. Use of surfactants to wet out metal flake powders must be carried out sparingly, as excessive amounts can later lead to foaming of the inks and poor adhesion of the subsequent film to the substrate. Isopropyl alcohol carried pastes and the more modern granular forms with low foaming, non-ionic surfactant carriers are widely used. Metal pigment/resin dispersions are often acrylic, for compatibility with the acrylic resins widely used in metallic water-based inks. Passivated aluminiums may be required for derived inks to be adequately gas stable in some applications. In practice the stability of gold bronze limits the range of acceptable binder systems. Cartons, labels, gift wrap and cigarette, cosmetic and confectionery packaging are the main applications. Table 13.14 illustrates a general purpose ink, based on a leafing aluminium flake granule. A corresponding granule containing non-leafing flake can be substituted, as can a paste of non-leafing flake in isopropanol. Alcohol pastes are best wetted and dispersed in part of the organic solvent component, as loss of leafing is not an issue. The presence of some solvent in the formulation improves print quality.

Table 13.14 General purpose, water-based, leafing aluminium ink Premix Inhibited aluminium flake granule (D50 = 15 µm; 80% in non-ionic surfactant)

18.5%

Deionised water

18.5%

Vehicle Pure acrylic solution (35% in water)

50.0%

Isopropanol

2.0%

Propylene glycol monomethyl ether

7.5%

Deionised water

3.5%

The granule is allowed to soak in the water and subsequently mixed to a smooth, creamy paste by low energy agitation. The components of the vehicle are combined and the premix gently stirred in. Let down by 25-30% with water is required for press readiness.

156

Applications of Metal Pigmented Coatings

Table 13.15 Non-leafing aluminium-based silver gravure ink Inhibited aluminium flake (D50 = 14 µm; 38% with 17% acrylic resin in water)

35.0%

Acrylic resin emulsion (NV = 55%)

64.7%

Antifoam

0.1%

Wax dispersion

0.2%

The components are combined with gentle agitation. The desirable final pH is within the range 7.5-8.5.

Table 13.16 Rich gold bronze gravure ink Stabilised rich gold bronze (D50 = 7 µm; 73% in water)

54.0%

Acrylic resin emulsion (NV = 55%)

45.8%

Antifoam

0.2%

Preparation is as in Table 13.15.

Silver and bronze rotogravure formulations are given in Tables 13.15 and 13.16, respectively. The silver uses an acrylic dispersion, the gold a stabilised bronze flake in water. The difference in pigment content is an illustration of the density difference between gold bronze and aluminium. Aluminium is present as 13.3% of the formulation, whilst gold bronze comprises 39.4%. The difference is further emphasised by the fact that the coarser aluminium flake has less hiding power.

13.2.4 Laminates The preparation of decorative surfaces, such as kitchen worktops, is achieved by printing down a design that is subsequently laminated to layers of thermoplastic or thermosetting sheet. Alternatively, the printed substrate is overcoated by thermosetting polymer, applied by knife or curtain coating and cured. Only the printing process selected limits the range of metal pigments that can be used in this application.

13.2.5 Security Inks Metal pigments, especially novel effect types, play a valuable role in security printing. Here cost is subsidiary to protection from forgery. Metal pigments are difficult to forge 157

Metallic Pigments in Polymers without the production expertise of the few specialist manufacturers. Even under a low resolution microscope, it is easy to compare flake particle size, surface finish, flake roundness and the smoothness of edges. An example of the novelty required is provided by Thomas de la Rue & Company [18]. A coating containing combinations of luminescent pigments and 10-20 µm diameter aluminium or nickel flakes is disclosed for application to credit cards, cheques and the like. Although silver in daylight, such coatings appear coloured under UV light.

13.2.6 Bronze replacement For coatings in general, and inks in particular, there is interest in replacing gold bronze because of increasing health and environmental concerns over its copper component. The growing interest in water-based formulations for environmental reasons also creates difficulties. Unless surface treated, which increases their cost, gold bronze pigments tend to tarnish in water-based media. It has long been known that gold effects could be produced from mixtures of aluminium flake pigments and organic pigments or dyes. Transparent yellow, orange and red colorants are mixed for green and red-shade golds, with brown giving ‘antique’ golds. Because of their ‘flop’ effect, such mixtures are not able to accurately reproduce the visual effect of gold bronze. The origin of flop, defined as the change of colour depth with angle of viewing, is described in section 4.3.3. In practice, due to the flop effect, gold bronze formulations can only be accurately matched at a given angle of viewing. This is not necessarily a disadvantage in new formulations as it may not be necessary to provide an exact match. Aluminium based formulations can compete on cost because of the much lower density of aluminium compared to gold bronze. The lower cost associated with the lower concentration required to provide a given opacity per square metre helps to offset the usually higher cost of the organic pigment component of the formulation. Table 13.17 shows the use of mixtures of organic red and yellow tinted, solvent-based aluminium inks to provide a wide range of gold shades. A corresponding water-based system is illustrated in Table 13.18. The formulation shown provides a mid-shade gold, but the ratio of yellow to red can be manipulated to give a wide shade range.

158

Applications of Metal Pigmented Coatings

Table 13.17 Solvent-based gold inks prepared from aluminium pigment Ink bases

Ink A

C.I. Yellow 13 pigment powder

12.0%

C.I. Red 48.2 pigment powder

Ink B 12.0%

Nitrocellulose 400

5.0%

5.0%

Ketone resin

8.0%

8.0%

Ethoxypropanol

10.0%

10.0%

Ethyl acetate

13.0%

13.0%

Ethanol

52.0%

52.0%

The components are ball milled or bead milled to provide a well dispersed coloured concentrate. Vehicle Nitrocellulose 400

9.0%

Ketone resin

8.0%

Ethoxypropanol

10.0%

Ethyl acetate

15.0%

Ethanol

55.0%

Silicone oil

2.0%

PE wax dispersion

1.0%

Gold ink Aluminium pigment granule (D50 = 12 µm; 80% in aldehyde resin)

35.0%

Ink base A + B

30.0%

Vehicle

35.0%

The granule is soaked directly in the vehicle and later gently dispersed by low shear agitation. A blend of ink bases is mixed in to give the desired shade.

159

Metallic Pigments in Polymers

Table 13.18 Water-based gold inks prepared from aluminium pigment Inhibited aluminium pigment paste (D50 = 16 µm; 65% in isopropanol)

24.6%

Pure acrylic solution (35% in water)

53.7%

Deionised water

12.4%

C.I. Yellow 13 (40% water-based, surfactant dispersion)

8.0%

C.I. Red 53.1 (40% water-based, surfactant dispersion)

1.0%

Antifoam

0.3%

The aluminium pigment is mixed to a paste-like consistency with the water and let down into a prepared combination of the remaining components.

A key disadvantage of aluminium-based gold formulations is the potential for pigment separation. This is best illustrated on corrugated board packaging (see Figure 13.1). Although homogeneous when applied, the organic pigment or dyestuff component of the ink is carried to the more absorbent areas between the corrugations. Little is left at the paper junctions where the glue has reduced the absorbency. The less mobile aluminium flake tends to remain where it is deposited. In an extreme case, the visual result is stripes, alternately silver and excessively pigmented metallic gold. To overcome this deficiency and also to accurately reproduce the visual effect of gold bronze with increased durability, industrial researchers looked for ways to surface colour aluminium flakes. The discovery that many surface coloration techniques increased the resistance of the underlying aluminium to aqueous attack proved a useful bonus in waterbased applications. Cost, however, is a deterrent to commercialisation.

Figure 13.1 Organic pigment migration in corrugated packaging.

160

Applications of Metal Pigmented Coatings

13.3 UV cured coatings Coatings cured by UV light are an alternative to water-based systems where low or zero VOC are required. Most UV cured metallic formulations are inks. Applications are mainly in packaging, where freedom from solvent taint is required. Advantages of UV curing, which help to offset its generally higher cost, include dry handling direct from the press, plus the absence of set off and odour or taint. Superior abrasion resistance is an advantage in the printing of vinyl floorcoverings and leathercloth, because tougher, thicker films are produced. Starting point formulations for UV cured inks are provided by Holman and Oldring [19] and by Kerr [20]. Aluminium flake pigment concentrations are 10-20% by weight of formulation, for flexographic and screen inks. A higher concentration, some 15-25%, is required for lithographic ink. The exact percentage is dependent on particle size, lower concentrations being appropriate for finer grades. Table 13.19 contains a typical starting point formulation for a stable, one pack aluminised UV curing ink for flexographic application to polyolefin surfaces. The aluminium must be inhibited by surface treatment to prevent premature cure.

Table 13.19 One-pack UV curing flexographic ink for polyolefin substrates Inhibited aluminium pigment (D50 = 12 µm; 80% in long chain aliphatic alcohol)

20.0%

Polyurethane acrylate prepolymer

60.0%

Acrylic monomers

10.0%

Photoinitiators and synergists

6.0%

Silane adhesion promoter

1.5%

Additives (e.g., waxes, flow modifiers)

2.5%

The pigment is wetted in the monomers and gently mixed into a blend of the remaining components.

For flexible PVC sheet and leathercloth applications, urethane acrylates with flexible backbones are used in conjunction with mono and difunctional monomers. Medium particle size grades of aluminium pigment give the greater brightness and degree of sparkle required (see Table 13.20).

161

Metallic Pigments in Polymers

Table 13.20 One-pack UV curing coating for PVC and leathercloth applications Inhibited aluminium pigment (D50 = 22 µm; 80% in long chain aliphatic alcohol)

20.0%

Urethane acrylate prepolymer

46.0%

n-vinyl pyrrolidone

15.5%

Alkyl diglycol acrylate

12.0%

Alkyl diol diacrylate

4.0%

Photoinitiators

2.5%

The pigment is wetted out in the pyrrolidone and added with low shear agitation to a blend of the remaining components.

13.4 PVC Plastisols Leathercloth and other fabric coatings, along with T-shirt and sweatshirt decoration and the like, are market areas for metal pigmented PVC plastisols. These mixtures of PVC polymer, finely dispersed in plasticiser, can be printed or spread by knife and cured to tough, flexible films by exposure to around 160 °C for a few minutes. They are of virtually 100% NV, with high viscosity for a high film build. Application of a printed design is usually by the silk screen process, which accommodates the larger metal flakes well. A basic formulation appears in Table 13.21. The viscosity reducer increases ink penetration, especially into fibrous substrates, with a corresponding improvement in bond strength.

Table 13.21 Aluminium pigmented plastisol coating Aluminium flake pigment (D50 = 30 µm; 80% in dodecyl phthalate)

24.0%

PVC plastisol base

72.0%

Viscosity reducer

4.0%

The aluminium pigment is gently blended into the plastisol and the viscosity reducer added last. The amount shown may not always be required.

13.5 Anticorrosive and barrier coatings Aluminium and stainless steel flakes and zinc dust (see section 2.6) are used extensively in so-called barrier coatings for anticorrosion applications. Applications are often in 162

Applications of Metal Pigmented Coatings harsh environments, such as chemical plant, external storage tanks, bridges and roof coatings. Such solvent-based paints are mainly applied to steel, rather than polymer substrates, but are often formulated with polymeric binders. Much work on the effectiveness of both metal and non-metal flake pigments has been carried out by Hare (see section 15.13). A typical starting point formulation for a general purpose barrier coating is shown in Table 13.22.

Table 13.22 Non-toxic, aluminised, silicone-alkyd barrier paint Calcium borosilicate Leafing aluminium pigment paste (D50 = 9 µm; 68% in white spirit) Organo-montmorillonite suspending agent

5.0% 14.6% 0.1%

Silicone resin modified long soya oil alkyd, 60% NV in white spirit.

31.3%

White spirit

48.0%

Zirconium/cobalt drier

1.0%

The aluminium paste is soaked in its own weight of the white spirit. After gentle agitation to a smooth dispersion, it is stirred into a dispersion of the remaining components.

The Aluminum Association [21] in the USA has produced a short guide to aluminium pigmented maintenance coatings, including formulation guidelines and seven representative formulations.

13.6 Other applications The application of metal pigments in wax crayons is not so much a coating as a means of making a coating. Thus glitter flakes are incorporated with a colorant, an emollient and a waxy material such as a plasticiser or fatty acid in a patent to Binney and Smith [22]. There is a small market for filler pastes with a metallic appearance. Aluminium and stainless steel flakes are the pigments of choice, incorporated in polyester or epoxy resins. The main application is repair of accident damaged vehicles. High purity aluminium powder is combined with free radical initiated acrylic monomer to provide an adhesive in a patent to Loctite [23].

163

Metallic Pigments in Polymers

References 1.

Market Trade International, World Paint File 1998-2002, DMG Business Media, Redhill, UK, 1998.

2.

The Coatings Agenda Europe, Campden Publishing Limited, London, UK, 1998, 49.

3.

R. L. Ferguson, Pigment Handbook, Vol.1, Ed., P. A. Lewis, Wiley-Interscience, New York, 1988, 785-801.

4.

E.I. Du Pont de Nemours Inc., assignee, US Patent 2,943,955,

5.

T. Okai, Y. Okamura, M. Oda, T. Yamamoto and T. Kuwajima, inventors; Nippon Paint Co., Ltd., assignee, US Patent 4,885,032, 1989.

6.

T. Kuwajima, S. Nagahata and S. Konishi, inventors; Nippon Paint Co., Ltd., assignee, US Patent 5,057,156, 1991.

7.

A. Frangai, inventor; ICI plc, assignee, British Patent 2,182,939, 1986.

8.

Z. Vachlas and S. J. Thorne, inventors; Imperical Chemical Industries plc, assignee, European Patent 0,238,222, 1987.

9.

A. J. Backhouse, A. Frangou and S. J. Thorne, inventors; Imperical Chemical Industries plc, assignee, European Patent 0,170,474, 1985.

10. A. Frangou, inventor; Imperial Chemical Industries plc, assignee, US Patent 4,675,358, 1987. 11. K. R. Walker, inventor; Eastman Kodak Company, assignee, US Patent 5,286,768, 1994. 12. J. L. Anderson, Jr., H. J. Finkenauer, D. L. Newton and J. P. Jones, inventors; BASF Corporation, assignee, US Patent 5,168,105, 1992. 13. J. L. Anderson, Jr., H. J. Finkenauer, D. L. Newton and J. P. Jones, inventors; BASF Corporation, assignee, US Patent 5,204,401, 1993. 14. C. W. Fowler, M. C. Knight and A. J. Nichols, inventors; BASF Corporation, assignee, European Patent 0,394,737 B2, 1990. 15. D. C. K. Chang, inventor; E.I. Du Pont de Nemours and Company, assignee, US Patent 5,104,922, 1992.

164

Applications of Metal Pigmented Coatings 16. R. A. Cowles, inventor; Inmont Corporation, assignee, US Patent 4,539,360, 1983. 17. Matsushita Electrical Works, assignee, Japanese Patent 63/239052. 18. J. Beck and A. Nutton, inventors; Thomas de la Rue & Company Ltd., assignee, European Patent 0,253,543, 1987. 19. R. Holman and P. Oldring, UV and EB Curing formulations for Printing Inks, Coatings and Paints, SITA technology, London, 1988, 182-187. 20. S. Kerr, European Chemical & Polymer Engineer, 1998, December, 34. 21. Aluminium Pigmented Coatings for Industrial Maintenance Applications, The Aluminum Association, Washington, D.C., USA, 1996. 22. M. S. Craig, inventor; Binney & Smith Inc., assignee, US Patent 5,261,952, 1993. 23. D. J. Dunn, P. P. Vano, J. P. Moran, Jr., M. Holmes and E. Frauenglas, inventors; Loctite Corporation, assignee, US Patent 4,722,960, 1988.

165

14

Powder Coatings

Powder coating, sometimes known as dry paint, is both an environmentally friendly or ‘green’ technology and one of the least expensive finishing techniques on the market. Globally it has grown eleven-fold since the early 1970s and growth continues at around 11% p.a. By 1995, world production was estimated at around 400,000 tonnes, or about 15% of the industrial coatings market [1]. It has been projected to reach 836,000 tonnes by the year 2000 [2]. Within this figure, the part of the market using metallic, sparkle or hammer finishes is estimated at a minimum of 40,000 tonnes, offering a total market potential of at least 1000 tonnes of aluminium flake pigment, of which about half is in Europe. The Powder Coating Institute in the USA has provided a very comprehensive overview of powder coatings and their manufacture and application [3]. World markets and coating types favoured in each geographical area are described by Dreher [4]. In its most basic form, the manufacturing process consists of compounding polymer resins with a curing agent, other additives and colorants and reducing the resultant mass to fine powder particles. Application is by coating a substrate with the powder, by dipping or more commonly, electrostatic spraying. The coating is then heated above its melt point to cause flow out to a continuous film. The curing agent acts by a chemical mechanism that is irreversible, thereby providing a very durable finish. Powder coating is energy efficient because it is VOC free. Equally there are no aqueous coating residues requiring increasingly expensive disposal. Curing can be carried out in static air, so the considerable cost of heating high airflow drying tunnels, characteristic of liquid coatings, is avoided. Labour costs can be lower because the process lends itself better to automation and can be operated by a less skilled work force. Application skill is not so critical because the coatings do not sag or run, resulting in fewer rejects. Finally, the absence of flammable solvents helps to reduce fire insurance premiums. The typical payback period for conversion of a ‘wet paint’ line to a powder coating system is 6-18 months. Since overspray powder, i.e., the proportion that misses the target, can be recycled and reused, material utilisation can exceed 97% of input. This is far greater than for wet paint. Powder coating can be expected to become more cost competitive as legislative and cost constraints on emission of volatiles tighten.

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Metallic Pigments in Polymers In film performance terms, powder coating provides a thicker film that also has a higher cross-link density than its liquid counterparts. This provides better impact, abrasion, corrosion and humidity resistance. The last of these is particularly relevant to aluminium flake pigmented systems. The disadvantages of the technique are mostly related to the quality of surface finish. Metal flake pigments have a major influence on this. In practice it is difficult to approach the brightness achievable from the same grade of metal pigment in a liquid coating for reasons covered in section 14.3.

14.1 Material types and properties Powder coating is by no means a new technology. Indeed its origins can be traced back to the fluidised bed applied, thermoplastic dip coatings of the early 1950s. The use of thermoplastic polymers such as vinyl, polyamide and especially polyolefins has declined in favour of thermosets, initially epoxy types. Poor UV resistance and chalking caused these in turn to be overtaken by epoxy-polyesters around the mid 1970s. This type remains the mainstay of the European market today. Polyester-triglycidyl isocyanurate (TGIC), is currently in second place, but will lose ground due to health concerns. A UK Health and Safety Executive guidance note [5] describes the known health effects, maximum exposure limits and sampling and analysis methods. On 31st May 1998 the European Union assigned TGIC a Category 2 mutagenic classification, which requires the ‘T’ (toxic) symbol and corresponding risk phrases [6]. Implications of the legislation and alternative crosslinkers are considered by Goemans [7]. The main contender as a TGIC replacement is hydroxy alkyl amide (HAA). Other candidates are described by Osmond and Steele [8]. Polyester-urethane has a very low European market share, and acrylic types are under 1%, mainly due to their higher cost. Some work had been done on acrylic and melamine compounds in the early seventies with no success. A revival in acrylic technology has recently taken place for niche applications where a higher cost can be tolerated. Metal flake pigments can be made compatible with all these polymer types. Preferences outside Europe are rather different. Polyester and polyester/urethanes hold the majority of the North America market, with the latter even more popular in Japan where it has a claimed market share of 34%.

14.2 Manufacture There are four routes to the manufacture of metal pigmented powder coatings. The oldest of these is the physical blending of dry metal flakes with micronised powder resin

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Powder Coatings particles prior to spraying. The hazards and technical limitations of this method prompted development of the so-called ‘bonded’ route, pioneered by Alcoa [9], in which metal flakes are attached to the resin particles. The high costs and limited palette associated with bonded powders were in turn a driving force for the challenges of the extrusion route, also referred to as co-extrusion. Here resin, metal flake and other colorants and additives are mixed together, extruded and subsequently micronised. Most recently, individual flakes have been encapsulated with resins capable of simulating the density and charging characteristics of the micronised resin powders into which they are subsequently mixed. This brings the technology full circle, since it is merely a more sophisticated form of dry blending. A useful review of all aspects of metal pigments in powder coatings has been provided by Besold [10].

14.2.1 Dry blend Dry blend describes the mixing of pre-compounded micronised powder coating resin with dry metal flake. Its main advantages are low cost and the retention of flake integrity due to the low energy involved in blending the pigment with the resin. The method was fraught with problems in its early days as the flake was uncoated. In the case of aluminium there was a very high risk of fire or explosion. The flake also has completely different charging characteristics to those of the powder, making the coating uneven. Particularly prevalent is a fault known as ‘picture framing’, which describes the much brighter edges of a coated article compared to the remaining area. This is caused by the preferential deposition of loosely adherent flakes. The technique is unsuitable for application by tribo (see section 14.3) and there is poor penetration into recessed areas. This has been attributed to the Faraday Cage effect [11]. As the densities of the two components are also widely different, the overspray has a different composition to that of the starting material, so cannot be readily recycled. Despite these disadvantages this method of coloration is inexpensive, so it is still widely used, although now declining.

14.2.2 Bonding The bonded powder process was developed as a response to the safety and application drawbacks of the dry blending of uncoated metal flakes. As the name implies, dry metal flake powder is physically attached to powder coating resin that has already been micronised [12]. The technique is preferred in Europe due to the powder

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Metallic Pigments in Polymers producers’ awareness of the explosion hazards of dry flakes. Colouristic properties are good and both leafing and non-leafing finishes are possible. Another advantage is that overspray will have a composition that is not significantly different from that of the starting material. The main disadvantage is high cost. This is derived at least in part from an historic lack of competition, since the technology has limited licences. Also, in order to make a homogeneous product, all the powder must be bonded. This is akin to using coloured compound compared to masterbatch. For this reason too, the bonding route is less versatile in the available range of visual effects. Care in preparation and application is needed if abrasion is not to detach flakes from the resin surfaces. Should this occur to any extent, the application characteristics will move towards those of dry blend, described above. An early patent for this technique was filed by Rolles [13]. A brush polishing device for combining the metal flakes with thermoplastic or thermosetting polymer particles at an elevated temperature is described. The polymer either forms a continuous film on the flake surfaces or the flakes coat the polymer particles. In either case the integrity of the fused mass is maintained by cooling. Screens can be used to remove any aggregates formed. Examples show the utility of the process with various metals such as aluminium, zinc and gold bronze. Richart and Daly [14] patented a flake pigmented coating powder prepared by mixing a thermosetting resin, such as a polyester based system, with a metallic flake such as leafing or non-leafing aluminium. The temperature is maintained between the softening point and the melting point of the resin, with sufficient mechanical shear to prevent agglomerates from forming. The mixing time is adjusted until at least 75% of the flakes are embedded in the resin particles. A similar process, using leafing aluminium flake pastes, ball milled with resin powder, was patented by the H.B. Fuller company [15]. The milled material is screened to remove aggregates and post-treated with a fluidising agent such as fumed silica, to maintain flow properties. Spray drying also achieves the objectives of the bonding process. Thus in 1976, Camelon and Gibeau [16] at Ford Motor Company patented powder paint consisting of aluminium flakes individually coated with a continuous film of thermoplastic polymer in a spray dryer. A useful feature is the possibility of using the same polymer as flake encapsulant as that used as the principal film former in the final powder coating formulation.

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14.2.3 Co-extrusion For metal-free powder coatings, the process of choice remains extrusion, introduced in commercial volume in the late 1960s. Harris [17] provides a review of production machinery. Manufacture routinely consists of four stages. In the first, resin, colorants, and any additives such as levelling agents, are intimately mixed. The particle size of the resin is influential in the efficiency of dispersion, coarse particles some millimetres in size giving a scouring action that prevents pigment and additive agglomeration. An enclosed, high speed, vertical axis mixer is normally used, often with second impeller mounted at right angles to act as a vortex breaker. The homogeneous premix is continuously fed into an extruder. Extruded melt is cooled from around 120 °C to ambient temperature using a mill with chilled nip rolls to produce a band of material around a metre wide and 3 mm thick. This friable material is flaked off the rolls by doctor blades and broken down into pieces around 1 cm square. In the third stage of production, these are subjected to micronising by milling before final screening to produce the desired particle size, generally in the range 10-80 µm. Metal flake pigments incorporated by the co-extrusion route must be specially designed to withstand the high degree of attrition inherent in each stage of production. Traditional extrusion techniques have also had to be modified to avoid a detrimental effect on flake aesthetics. Metal flake pigment may be added to the premix with the other ingredients, but flake damage can be minimised by homogenising the other components first. Modern granular forms of metal pigment are predispersed and therefore require only brief further compounding. Their carriers are compatible with all the commonly used resin systems and the carrier itself becomes an integral part of the coating. Those metal flake pigment grades developed specifically for the extrusion route are thicker, in order to be more degradation resistant. They also contain fewer fine particles that can contribute to a dark appearance. Figure 14.1 shows a typical low aspect ratio flake for this application. If conventional flakes must be used, it is customary to use a rather larger flake size than is required in the finished product. In this way, after incorporation, the flake size is brought into the desired range. One advantage of this route is that, like bonded material, the powder is homogeneously pigmented. The overspray will therefore have virtually the same composition as the starting powder and so can be readily recycled. Another benefit is that such powder coating formulations are suitable for both electrostatic and tribo application. Penetration into cavities is excellent and finishes are free of ‘picture frame’ edge effects.

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Figure 14.1 Typical low aspect ratio aluminium flake pigment for powder coatings prepared by the extrusion route. [1 cm = approx. 60 µm]

14.2.4 Coated flakes The original dry blend method was recognised to have the advantage of simplicity, versatility and low cost, all very attractive attributes if the hazards and limited aesthetics could be overcome. The modern response of metal pigment manufacturers has been to coat or encapsulate each metal flake with a resinous or inorganic coating. Several examples can be found in the patent literature [18, 19, 20, 21, 22, 23] and both types are commercially available. In practice it is difficult to ensure uniform coating of single flakes, particularly with resinous polymers. As a result, encapsulated grades tend to be more aggregated and provide less opacity than the powders or pastes on which they are based. Comparisons of dry blending with the bonding and extrusion processes are provided by Kerr [1] and by Birch [24]. This comprehensive review covers powder recovery, metallic effect, curing gun blockages and reduction of picture framing effects. Some improvements in bonded coatings, including resin and pigment development are also described.

14.2.5 Other technologies A hybrid of dry powder and bonding is provided by so-called dedusted flakes, such as the DF range of Silberline Inc. Here, dry aluminium flakes are restrained by entanglement 172

Powder Coatings with PTFE microfibres. Environmental contamination and explosion hazards are reduced. Very low concentrations of PTFE, typically 0.1%, are required. The treatment is sufficient to prevent significant dusting in transit, but the free flake form is regenerated during mixing with powder resin prior to spraying. As the resulting mix retains the limitations of dry flakes, the technique has been developed to allow dedusting of resin coated flakes, now marketed as the LE range. Resin coating alone does not prevent dusting but improves the compatibility on application, i.e., reducing picture framing, etc. Another variant and one of the most promising for the future is the use of water to carry the powder. Thus Williams and Gessner at BASF [25] applied for patent protection for a process in which conventionally produced powder is combined with water, surfactants and rheology control agents. Such powder slurries are suggested for formulating primers and coloured basecoats with metallic effects, applied by spraying and subsequently cured at elevated temperature. A wide range of powdered polymers can be used, but acrylic resins are preferred. Like water-based coatings, formulations involving aluminium flake pigments will require the metal to be passivated against aqueous attack if long-term storage stability is a prerequisite.

14.3 Formulation, application techniques and markets Aluminium, gold bronze, zinc and stainless steel flake are all offered for powder coating, the first two for their aesthetic effects, zinc and stainless steel in small quantities for their anticorrosion properties. The thick film build of powder coating compared to liquid coatings allows obliteration of the substrate by as little as 2% w/w on formulation of a fine particle size metal pigment. Coarser grades may require up to 5%. Another useful advantage of high film build is that coarser grades of flake, which would protrude through a conventional liquid coating, can be used. Disadvantages include cost. The main application methods are fluidised bed or electrostatic spraying. In the former, the article to be coated is heated above the softening point of the powder resin, then dipped in a bath of the powder, held in an agitated state by air injected from below. The coating that forms on the part by this dip coating method is subsequently hardened by curing in an oven. The coating is prevented from running off the part by adhesion and controlled viscosity. The technique is useful for limited production runs, but is not as easy to automate as spraying. Electrostatic spraying requires that the article to be coated is well earthed. Coating is achieved by applying an electrostatic charge to the pigmented powder, which is then attracted to the part. When a sufficient thickness of coating powder has been attached, the part is cured in an oven. In practice, the operation is generally continuous, the parts

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Metallic Pigments in Polymers being carried on a moving coating line past the spray heads and into a curing tunnel. Electrostatic charging may be by an induced or corona mechanism, or by tribo, in which the charge is created by frictional contact of powder particles in the gun. Metal flake pigments that are neither coated nor physically attached to the powder, tend to cause electrical short circuiting of the spray gun. The charging characteristics of metal flakes are in any case very different to powder, so the quality of finish tends to be at the margins of acceptance. For these reasons, co-extrusion and bonding are the currently preferred techniques. In comparison with the metallic effect obtained from liquid coatings, powder coatings have some deficiencies. In particular, orientation and therefore brightness are adversely affected by the high melt viscosity and the short residence time in the liquid state. There is also no appreciable film shrinkage. Consequently the solvent loss from liquid coatings, responsible for forcing an alignment of flakes parallel to the substrate thus increasing brightness, is absent. Lower melt viscosity binders and longer cure times will help, but there are then limitations of sag and low productivity. On the positive side, powder coatings can provide thicker coatings capable of accommodating larger flakes. They also offer better chemical, weather and abrasion resistance than the corresponding liquid coatings. For outdoor applications, especially for leafing effects, a clear powder topcoat is often applied to give further protection. The main markets for metal pigmented powder coatings are domestic appliances, metal furniture, tools, automotive components and general exterior and architectural extrusions, especially window frames. All the metallic effects available from liquid coatings are also available from powder coatings, including sparkle and hammer finishes, metallic, coloured metallic, tinted lacquers, fluorescent and pearlescent effects. For technical and commercial reasons the last three of these have made little impact. A sizeable fashion driven market exists for sparkle finish in bicycles, metal furniture, garden implements and to a lesser extent, automotive components. Hammer finishes appear in the domestic sector, particularly on gas and electric fires, to give an antique appearance. Straightforward metallic effects are prominent in vehicle wheel trims where the high film thickness gives enhanced stone chip resistance. Clear powder coats are increasingly applied over conventional metallic pigmented basecoats in automotive applications. Gold bronze flakes must be protected to provide sufficient colour stability in powder coating applications. Silica coated grades such as the Resist range of Eckart-Werke, Schlenk’s ‘Constant’ or Wolstenholme International’s Tarnish Resistant have been commercially available for many years. 174

Powder Coatings Zinc flake is used for its protective properties. It enhances the already good barrier properties of powder coating films, even at relatively low concentrations. In this respect the flake is superior to the more common powder form and delivers reasonable cathodic protection [26]. Further advantages are film flexibility, resistance to blistering and easy overpainting. Against this must be set the necessity for rather high loadings, around 1015%, partly due to the metal’s higher density. One substantial application of stainless steel flakes is in the manufacture of decorative organic powder coating systems, particularly those intended for use in demanding environments. They are used extensively in both epoxy and polyurethane resin systems as coatings for structural steelwork in food processing plants. Their resistance to chemical attack in aqueous media also makes them eminently suited for use in water-based resin systems, including water carried powder coat (see section 14.2.5).

14.4 Safety and handling Powder coatings pose a lower safety risk than liquid coatings. A resin dust in the atmosphere requires 50-100 times more ignition energy than a solvent/air mixture [27]. Nevertheless, the high surface area of fine resin particles requires precautions to prevent dust explosions. Dusts can also be a mild respiratory irritant. Electrical grounding and avoidance of dust clouds are the main considerations. Safety aspects are fortunately well understood in the industry. Reference has already been made to the hazards of dry metal flake in the electrically charged environment of a powder spray gun. The energy for ignition is known as the MIE or minimum explosive energy, measured in millijoules (mJ). Data quoted by Besold [26] show that aluminium flake poses the greatest hazard, having an MIE comparable to that of the resin powder itself. The value is claimed to be independent of whether the pigment is dry blended, extruded or bonded. The MIE value for zinc is some 20 times higher than that for aluminium. Gold bronze, whilst flammable in fine particle sizes, is not considered an explosion hazard. The Paintmakers Association of Great Britain publishes a code of safe practice for the application of powder coatings by electrostatic spraying [28]. Current health and safety concerns relating to powder coatings in general are described by Cooke [29].

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References 1.

J. D. Kerr, Product Finishing, 1995, September.

2.

Coatings COMET, World Outlook, 1997, 7, 2, iii-xiv.

3.

Powder Coating, The Complete Finisher’s Handbook, Ed., N. P. Liberto, The Powder Coating Institute, Alexandria, VA, USA, 1994.

4.

B. Dreher, Polymers Paint Colour Journal, 1993, 183, 4326, 166.

5.

EIS 15, Control of Exposure to TGIC in Coating Powders, UK Health and Safety Executive.

6.

Coatings COMET, 1996, 4, 7, 66.

7.

C. Goemans et al., Polymers Paint Colour Journal, 1998, 188, 4405, 19.

8.

M. F. Osmond and G. D. Steele, Polymers Paint Colour Journal, 1992, 182, 4303, 182.

9.

R. Rolles, J. E. Williams, Jr., and T. J. Kondis, inventors; Aluminium Company of America, assignees, US Patent 4,138,511, 1976.

10. R. Besold, et al., 13th International Conference, Paint Research Association, UK, 1993, Paper No.18. 11. Powder Coating, The Complete Finisher’s Handbook, Ed., N. P. Liberto, The Powder Coating Institute, Alexandria, VA, USA, 1994, 99. 12. H. Groebl, Farbe und Lack, 1974, 80, 10, 930. 13. R. Rolles, J. E. Williams, Jr., and T. J. Kondis, inventors; Aluminium Company of America, assignee, US Patent 4,138,511, 1976. 14. D. S. Richart and A. T. Daly, inventors; Morton International Inc., assignee, US Patent 5,187,220, 1993. 15. S. V. Bigalk and S. C. Hart, inventors; H.B. Fuller Licensing & Financing, Inc., assignee, US Patent 5,045,114, 1991. 16. M. J. Camelon and R. C. Gibeau, inventors; Ford Motor Company, assignee, US Patent 3,932,320, 1976.

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Powder Coatings 17. S. T. Harris, Polymers Paint Colour Journal, 1992, 182, 4303, 186. 18. C. A. Ponyik, Jr., inventor; Mobil Oil Corporation, assignee, US Patent 3,575,900, 1969. 19. T. Banba, inventor; Toyo, Aluminium Kabushiki Kaisha, assignee, US Patent 4,434,009, 1984. 20. K. Higashi, Y. Imasato and K. Iri, inventors; Asahi Kasei Metals Ltd., assignee, European Patent 0,280,749 B1, 1987. 21. W. Reisser and G. Sommers, inventors; Eckart-Werke Standard BronzpulverWerke Carl Eckart GmbH & Co., assignee, US Patent 5,332,767, 1994. 22. R. Schmid, N. Mronga, J. A. G. Gomez, R. Rieger and R. Schlegal, inventors; BASF AG, assignee US Patent 5,505,991, 1996. 23. Asahi Kasei Metals, assignee, PCT 9,638,506, 24. J. Birch, European Coatings Journal, 1997, No.7-8, 709. 25. C. F. Williams and M. A. Gessner, inventors; BASF Corporation, assignee, European Patent 0,652,264 A2, 1994. 26. R. Besold, Farbe und Lack, 1983, 89, 3, 166. 27. Safe Powder Coating, 4th Edn., CEP, 1990. 28. Application of Powder Coatings by Electrostatic Spraying, Paintmakers Association of Great Britain Ltd, London, 1983. 29. M. Cooke, Surface Coatings International, 1998, 81, 2, 86.

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15

Non-colouristic Applications

Though metal flakes are best known for their role as pigments in paints, ink and plastics, there are many existing and potential applications in which these pigmentary properties are of lesser or even no relevance. In such applications, the properties of the metal itself are often more important than the shape or size of the flakes. The non-pigmentary applications relevant to polymers are discussed in the following sections. Some have already yielded commercially viable products. Others can be regarded as subjects for research and development. In total however, the contents of the section should dispel the impression that metal flakes are only useful for their pigmentary properties.

15.1 Mechanical reinforcement The influence of metal pigments on the mechanical properties of polymers is discussed in section 7.5. Developments in mechanical reinforcement are concentrated on modifying the surface of the metal flake to improve the bonding between the flake surface and the polymer matrix. Lack of bonding in the gaps between flakes and matrix at a fracture surface was shown in Figure 7.1. Suitable bonding or crosslinking agents (used to create a bond between the metal surface and the matrix) include multi-functional organic titanates, zirconates and silanes. Applications are those in which a metallic appearance is required, combined with mechanical properties closer to those of the pure metals, i.e., ‘metal replacement’ applications. Weight saving, as in automotive parts, is a secondary attraction. There is a compelling commercial justification for surface treated pigments in mass pigmented polymers. Polyolefins have attractive properties such as good chemical resistance and low cost. If their relatively poor mechanical properties can be raised towards the level of the engineering polymers such as ABS, PC and the like, the pigment cost is offset against the more expensive polymer that would otherwise have to be used.

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15.2 Microwave heating Interest in metal flakes for microwave heating comes from the observation that they heat up in a microwave beam when incorporated in high melting polymers such as polyethylene terephthalate (PET) [1]. PET is temperature stable and used in food contact applications. Microwave packages can be divided into two groups, passive and active containers. Passive containers simply hold the food and cannot provide any additional heating. These containers are made from microwave transparent materials that allow energy to pass through with little or no resistance, thus allowing energy absorption by the food mass. Passive containers are commonly made from glass, polymer compounds, or coated fibreboard. Food containers formed from high temperature polymers such as PET are unable to heat the outside surfaces of foods contained in them. As a result, the browning and crisping of bread, pastry, pizzas, french fries, batter coated and related foodstuffs, that would occur naturally in a conventional oven, does not take place. Worse, such foods can become soggy and unappetising. Active containers not only hold the food for heating purposes but are also designed to interact with microwave energy. This implies that part or all of the container absorbs or reflects microwave radiation, thereby heating that part of the food in immediate contact. The shape, dimensions and construction materials contribute to the efficiency of cooking. To achieve significant browning and crisping, the food surface must be heated above 190 °C. This can be achieved by the use of susceptors. The development of the microwave susceptor or receptor, was to overcome this deficiency and revolutionise this part of the fast food market. Rapid developments in the early 1980s led to inexpensive, one trip ‘pad susceptors’, formed from aluminium, vacuum deposited onto polymer film, itself usually laminated to a paperboard backing. Seiferth [2] describes ‘a laminate for use in a disposable container, adapted to heat the surface of a quantity of food when exposed to microwave radiation’. The laminate consists of a continuous vacuum deposited layer of aluminium less than 70 nm thick, a heat resistant polymer film and a structural support of paper stock. The extreme thinness of the metal film confers the microwave activity, since it was observed that thicker films have no heating potential. The logical development was to move to a flake metal pigment. If the coating is applied by a printing process, the position and degree of heating within a single sheet of composite material can be controlled to a greater degree than is possible with a pad susceptor. This is of potential interest to manufacturers of lidding films, the flexible 180

Non-colouristic Applications polymer film used to cover and seal the tray. In such cases the film acts to reduce or prevent direct microwave irradiation of the food, thus giving additional control of heating rate. Huang and Plorde [3] at Du Pont patented a composite consisting of a polymeric substrate such as PET film, coated by a mixture of an electrically conductive metal or metal alloy in flake form in a thermoplastic dielectric matrix. Circular flakes with flat surfaces and smooth edges were found to give the most efficient heating performance. A particle size range of 10-35 µm is generally preferred, incorporated in a coating system to give a 50-70% concentration in a dry film weight of 40-70 g/m2. The coating system itself is advantageously formulated with a polyester resin for good compatibility with a PET substrate. Formulations of this type are capable of attaining surface temperatures of over 200 °C in a 2450 MHz, 650 watt microwave oven in periods as short as one to two minutes. Paleari [4] developed a multilayer sheet having a sealing layer, an inner layer and an outer, abuse resistant layer. The film included 0.05-2.5% by weight, based on total film weight, of fine aluminium powder, dispersed between two of the layer components. The composition, which has a metallic appearance, is thermoformable into complex shapes and has some gas barrier properties. A greater measure of temperature control could be obtained because a pigmented system could be printed down in grids or other patterns in which temperature was controlled by flake concentration per unit area. Carbon black will absorb microwave radiation in a similar fashion, but is difficult to control. Mixtures of aluminium and carbon black with metal flakes have proved effective [1]. A microwave interactive double bag, especially suitable for popcorn was patented by Hartman [5]. The active component is a printed coating of aluminium flake, clay and a binder. The two main drawbacks of continuous film pad susceptors are that they are limited to flat sheets and only function in one heating cycle. The heat generated destroys the continuity of the metal film. Once crazed in this way, they cease to operate. Thus they are regarded as ‘one trip’ disposable packaging, suitable for pies, pizzas, french fries and popcorn. Incorporating metal flake by mass pigmentation provided a route to the reusable or ‘rotable’ container. A low cost alternative to the present ceramic browning dishes also became possible, making the microwave frying pan an economically attainable goal.

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Metallic Pigments in Polymers Substantial R&D effort has been expended to establish the optimum metal type, flake size and loading [6, 7]. Metal concentration per unit area of a tray controls the temperature. At all metal concentrations, a steady state is reached at which heat is radiated from the surface at the same rate as incident microwave radiation is absorbed and converted into heat energy. Matching the metal content of a tray to the anticipated food load is required for generation of the required 190-230 °C browning temperature in an average microwave oven. Using aluminium flakes, a loading between about 10% and 22% w/w with the polymer is required in a 0.5-0.7 mm thick PET tray, used in a 650-700 watt oven operating at 2450 MHz. The heating rate is relatively insensitive to flake size. Large flakes appear to function slightly better than smaller particle sizes. A useful advantage is the ability to generate a different temperature in different areas of a container by varying the thickness of the moulding and with it, the flake concentration per unit area. This is particularly useful for the so-called TV dinner. This is a multi compartment meal, usually composed of meat, a vegetable and potato or rice. Injection moulding allows areas of differing thickness within a single tray. When aluminium pigmented, such trays offer differential heating rates to accommodate the differing heat capacities of various foods. All the components of the TV dinner can then be brought to the same temperature at the same time. In trials of an empty container, the hottest areas were surprisingly not at the corners, which are reputed to concentrate microwave energy, but at the centre of the panels. Even at loadings of over 20% w/w aluminium in PET trays, no arcing occurred. The tray was also little affected by its position in the oven and retained its structural integrity well throughout trials, including multiple dishwasher cycles. A practical example of microwave heating using an aluminium flake pigmented PET tray is to poach or fry eggs since they are notoriously difficult to cook in a microwave oven. The technique used is to exclude microwave energy from the egg itself and to cook it by the thermal energy emitted by the tray and its similarly pigmented cover. Cooking time is around 1.5 minutes at a medium-high power setting. The result is comparable to frying an egg in a conventional frying pan. Products are commercially available in which aluminium flake is formulated with a carrier resin, approved for direct food contact. The composition is offered in granular form at the highest metal/carrier ratio that can be sustained to retain adequate dispersibility in polyesters such as PET at the high loadings required. It is possible that the demonstrated microwave heating ability of metal flake filled polymers may have other less obvious uses in the area of selective heating. For example, a metal flake pigmented plastic sleeve could be softened using microwave radiation to bond two plastic pipes together without heating or deforming the pipes themselves.

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15.3 Electrical conductivity Although advances have been made in inherently electrically conductive polymers, a review is outside the scope of this book. In a non-conductive polymer matrix, the filler provides conductivity, whether it be conductive carbon black or a metal. For metal pigments, the key requirement apart from high conductivity in the bulk metal is a high aspect ratio. In this respect, metal fibres can be more effective than flakes.

15.3.1 Product forms It is widely assumed that since bulk metals conduct electricity, so also must derived flake pigments. Whilst true for most metals, it is not so for aluminium. Given its widespread use and apparent lack of chemical activity, it is surprising to find aluminium so high in the electrochemical series. Indeed it lies just below the more obviously reactive elemental metals potassium, sodium, calcium and magnesium. The reason for this inert behaviour lies in the nature of the metal surface. All aluminium in ambient conditions is coated by a film of aluminium oxide, estimated at only 3-5 nm in thickness. This tenacious coating helps to protect the underlying metal from chemical attack, but is itself a limited conductor of electricity. Various ingenious attempts have been made to render aluminium flake conductive, often for suppression of electromagnetic interference (see section 15.4) or merely for antistatic performance (see section 3.3.4). Sternfield reviewed such aluminium pigments, including 1-1.4 mm long, 25-40 µm thick, quench cooled flakes manufactured by Transmet Corporation [8]. Typical formulations required 18-22% concentration by volume to change conductivity suddenly from low to high. The Thermofil company in the USA has developed an aluminium pigmented, conductive Nylon 66 blend [9]. Charles claimed certain organo aluminates, titanates and zirconates as agents capable of enhancing the electrical conductivity of metal particulates [10]. The electrical resistivity of a mixture of sub 45 µm particulate aluminium, a phosphorus-containing titanate and a polyethylene glycol was 26 ohm/cm, compared to 31,850 ohm/cm when the titanate was omitted. Similar results were claimed for copper, nickel, tin and zinc. Deguchi [11] disclosed an electrically conductive thermoplastic resin composition. It comprises a thermoplastic and 8-15% by volume of a dispersed phase consisting of at least 2% by volume of metallic fibres of diameter 10-150 µm and length 0.5-10 mm and at least 2% by volume of carbon fibres. The metal fibres may be of steel, gold bronze, copper, aluminium and their alloys.

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Figure 15.1 Use of conductive ink in a calculator keypad Reproduced with permission of Chris Williams

The noble metals, silver and gold, in particulate or flake form are used in electrically conductive inks for polymeric or resinous printed circuit boards. For example, McGowan of the then Ciba-Geigy Corporation [12], disclosed such a coating containing fine, particulate silver and glass frit. Silver flakes also find a use in the defrosting elements of car windscreens. Fine particle size conductive metals, such as nickel and silver, are used in inks for touch keys in calculators, TV and video controllers and the like (see Figure 15.1). Such conductive coatings are generally produced in solvent based resin systems, however nickel flakes are also suitable for conductive water-based systems due to their good resistance to corrosive attack.

15.4 EMI shielding This section follows logically from the last, as EMI shielding requires a degree of electrical conductivity. Wherever an electrical current flows, an electric field exists. Electronic devices are capable of both emitting and receiving extraneous electromagnetic radiation. This generates electro-magnetic interference, more usually referred to as EMI.

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15.4.1 Origin and measurement There are numerous sources of EMI. These include natural atmospheric noise, electric storms, fluorescent lights, vehicle ignition systems, unsuppressed motor commutators, high voltage overhead power lines and unwanted radio transmissions. The effects generated by these emissions range from the annoying, such as poor radio or TV reception, to the disruptive, for example the wiping of a computer’s memory, to the dangerous, where emergency services or air traffic systems are disrupted. In the modern world of instant electronic communication, increased use of limited bandwidth airways has only exacerbated the problem. The very rapidly rising number of mobile telephones increasingly need to be protected against absorption of spurious radiation. Coincidentally, they may need to be protected from radiating interference from their own circuits.

15.4.2 Legislative requirements The need for government regulation of electromagnetic emissions has been recognised for some time, but its introduction in Europe has been delayed. Almost twenty years ago, the United States Federal Communication Committee (FCC) [13] issued regulations limiting the field strength of emitters to between 30 MHz and 1 GHz. In Europe the long awaited EEC Electromagnetic Compatibility Directive 89/336/EEC [14], as amended by 92/31/EEC finally came into force on 1st January 1996. The directive requires almost all items of electrical equipment sold in the European Community to ‘be so constructed that they do not cause excessive electromagnetic interference and are not unduly affected by electromagnetic interference.’ These requirements are met by what is now generally described as EMI shielding or simply EMS (Electro-Magnetic Shielding).

15.4.3 Shielding principles and techniques Several textbooks collectively provide a comprehensive introduction to the subject. ‘Grounding and Shielding Techniques in Instrumentation’ [15] introduces the properties of electrical fields before moving on to describe the applications of electrostatics to the practical problem of shielding instruments. ‘Noise Reduction in Electronic Systems’ [16] considers cabling, grounding, filtering and shielding techniques. It includes a useful section on electrostatic discharge and in an appendix, a very practical noise reduction checklist. ‘Controlling Radiated Emissions by Design’ [17] contains a chapter devoted to box shielding, including the shielding effectiveness of materials such as conductive plastics. The strength of this text is that it teaches the principles of cost-effective EMS design, starting with how to determine box attenuation requirements.

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Metallic Pigments in Polymers Ideally, shielding should completely enclose all sensitive components, forming a so-called Faraday Cage of highly conducting material. The obvious route of using fabricated metal cages tends to be expensive and is undesirable in weight-critical applications such as mobile phones. Designers prefer plastics, which as well as being lightweight, are more easily formed into complex shapes. They also offer corrosion resistance, durability and a good surface finish without the need for machining.

15.4.4 Shielding of polymers Unfortunately, unlike metals, polymers are virtually transparent to electromagnetic radiation. They must therefore be shielded. There are several techniques available to achieve this. Early solutions involved metal foil, later adapted for attachment directly to the polymer during injection moulding. Alternatively, the surface of the article may be prepared to take an electrocoat. Zinc arc spraying is another method in which zinc wire is continuously fed through an electric arc. The melted zinc is then sprayed onto the substrate by compressed air. The most widely used pigment is conductive carbon black. In the finest particle sizes it has very high surface area to establish a conductive path. It is also relatively inexpensive. Its main disadvantage is its colour. Carbon black is therefore unsuitable for transparent or coloured finishes. A further problem is its strong absorption in the IR region of the electromagnetic spectrum. On exposure to sunlight, heating of the coated or mass pigmented article can occur, which can affect the performance of enclosed electronic circuits. Shielding methods relevant to metal pigments are coating and mass pigmentation. Coating the plastic component with a conductive paint became the favoured route in the 1970s and remains widely used today. Conductive paints containing fine nickel, copper, silvercoated nickel or even pure silver flakes are applied by spraying using conventional spray painting equipment. The mass pigmentation route uses conductive carbon black, as powder or fibre, or metal fibres or flakes, especially those derived from stainless steel. Table 15.1 shows the signal attenuation effectiveness of these various techniques. Several coating and mass pigmentation techniques for EMI shielding involving metal flakes have been proposed. The Novamet Specialty Products Corporation and TBA Industrial Products Ltd., are firmly associated with such products.

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Table 15.1 Effectiveness of various shielding techniques Shielding system

Shielding effectiveness (dB)

In-mould foiling

40-90

Metal flake

40-60

Metallised fibre

40-80

Nickel paint

30-80

Vacuum metallising

50-90

Zinc arc spraying

30-60

15.4.5 Coating techniques The earliest air drying acrylic paints had values around 1 ohm/cm2, but advances in flake orientation and drying characteristics have reduced this to below 0.5 ohm/cm2. Hart has recently provided a summary of the requirements and advantages of the coating route, especially in relation to the Compatibility Directive [18]. Metal loadings of over 40% w/w are required in dry film for the highest degree of shielding effectiveness. This places considerable demands on the formulation of the coating, especially to avoid settling of relatively heavy metal particles in low viscosity media. Low shear incorporation is necessary to avoid folded and broken flakes that can adversely affect the smoothness of the dry film. A continuous, uniform film is important. Any flakes protruding through the film surface can be removed by abrasion, with a corresponding reduction in shielding efficiency. There is little requirement for a good appearance since the coating is normally applied to the inside of the enclosure. The range of suitable metal ‘pigments’ is limited by the requirement for high electrical conductivity, corrosion resistance and high aspect ratio. In practice, copper and nickel are the most widely used. Although it has the better conductivity, copper tends to tarnish thereby reducing its effectiveness. In recent years more resistant forms have been formulated. Nickel has excellent corrosion resistance. Its magnetic properties are also an advantage in attenuating low impedance waves. The variety of geometries available increases the metal’s versatility. The dendritic (see figure 15.2) and flake forms (see figure 15.3) are more effective than the spherical type illustrated in figure 3.6. (see figure 15.4) Though popular for its cost-effectiveness amongst coating techniques, there are concerns over nickel’s toxicity, especially as it tends to be supplied as a dry powder (see Chapter 16).

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Figure 15.2 Electron micrograph of dendritic nickel powder [1 cm = approx. 7 µm] Reproduced with permission of Hart Coating Technology and Novamet

Figure 15.3 Electron micrograph of conductive nickel flake [1 cm = approx. 3 µm] Reproduced with permission of Hart Coating Technology and Novamet

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Figure 15.4 The effect of pigment loading on the resistivity of various conductive nickel systems. Reproduced with permission of Hart Coating Technology and Novamet

The corrosion resistance of nickel makes it suitable for water-based coatings, giving very good overall performance. Stainless steel flake is an alternative, since like nickel it is corrosion resistant (see Figure 3.7). Nickel-coated graphite is an angular particulate (see Figure 15.5). It provides particularly good electrical properties when incorporated into silicone resins used to produce gaskets for shielding applications. These grades are available containing 25%, 62% and 75% nickel, the two higher nickel content materials being preferred for electrical conductivity. Nickel-coated mica at 10-15% loading has also been found to give good shielding effectiveness [19]. Where better impact strength is required, blends with stainless steel fibres are suggested. For the greatest attenuation (reduction in electromagnetic transmission = shielding effectiveness) in the most demanding applications, silver flake is used despite its high cost. A less expensive compromise, that still retains the conductivity of silver is silvercoated nickel flake. Friedli and Lau at the Dow Chemical Company [20] patented an aqueous coating composition for EMI and radiofrequency (RF) shielding. Flakes or powders of nickel,

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Metallic Pigments in Polymers

Figure 15.5 Electron micrograph of conductive nickel-coated graphite [1 cm = approx. 40 µm]. Reproduced with permission of Hart Coating Technology and Novamet

copper, aluminium or combinations of these are combined with a zwitterionic, bisphenolA based monomer, a polyacrylamide, a non-ionic surfactant and water. Other envisaged applications include conductive tapes, adhesives and inks for printed circuits. Related work by the Honda Motor Company is described by Monte [21]. Surface oxide-free zinc, prepared by friction milling, when formulated in an acrylic coating and applied and cured on ABS, offers an electromagnetic compatibility comparable to that of a molten zinc coating. Where the weight of an enclosure is important, a coating has the advantage over mass pigmentation that it concentrates the flakes in a relatively thin film. It also allows flow and weld lines to be avoided where the aesthetic appearance of the part is important. Coating however, unlike mass pigmentation, is a multistage process. It tends to be labour intensive and processing time is extended. Spray equipment, now increasingly required to have provision for volatiles capture, is expensive and does not always adequately coat parts with complicated geometry. Furthermore, the coating may peel or wear, thus destroying the conductivity essential for effective EMI shielding. For these reasons, mass pigmentation has been investigated as an alternative. At the time of writing however, commercial acceptance of mass pigmentation has been limited.

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15.4.6 Mass pigmentation techniques A text on the electrical properties of metal-filled polymer composites is provided by Bigg [22] of Battelle Columbus Laboratories. Very large aluminium flakes with an aspect ratio of 16.7:1 were found to become conductive at 10% by volume when compression moulded. Considerable flake damage was reported when the same formulation was injection moulded. This increases the critical concentration for EMI performance. Chen [23] investigated PP compounded with carbon black and aluminium flake, not only for EMI performance, but also for processability and mechanical properties retention. The conductivity threshold was about 5 vol.% (20 wt.%) for conductive carbon black, but 5-10 vol.% (20-40 wt.%) for aluminium flake. At the maxima within these ranges, shielding effectiveness reached 30 and 35 dB, respectively. This performance is comparable with that of 1 mm copper sheet or of a continuous nickel coating. Chen found that in general, conductive fillers with a high aspect ratio have greater statistical likelihood of forming the necessary conductive paths in the polymer matrix. Kanda of Showa [24] disclosed an EMI shielding material comprising a thermoplastic resin, an aluminium particle, preferably a very large flake, and electrically conductive carbon black. A wide range of polymers with 0.1–5 mm aluminium flake and a carbon black having a specific surface area of 20-1800 m2/g is claimed. The aluminium flake and carbon black are mixed in approximately equal parts to provide 10-60% of the composition by volume. Attenuations measured for the examples range from 25 to 43 dB. Also at Showa [25], an olefin-based rubber was combined with aluminium or alloy in powder, fibre or flake form, a vulcanising agent and the carbon black. Attenuation was in the range 35-50 dB. A key disadvantage of mass pigmentation is the high concentration of metal flake or fibre required. As well as increasing the cost of the moulding, such loadings also compromise its mechanical properties. Although more effective, high aspect ratio flakes are also more prone to damage during incorporation than those of low aspect ratio. Inclusion of low concentrations of silane, titanate or zirconate coupling agents enhances EMI shielding efficiency in filled systems, according to Monte [26]. This is achieved by lowering melt viscosity during processing. This deagglomerates the conducting particles, improving their dispersion and increasing the adhesion between filler and matrix. In turn this limits the loss of mechanical properties. The main use of spherical nickel and silver-coated nickel powders is to produce composites for electromagnetic shielding applications. Because of their controlled size range they can be incorporated into media such as silicone elastomers or epoxy resins that are used to make conductive gaskets or adhesives. They are not normally used in coatings because 191

Metallic Pigments in Polymers the filamentary (thread-like) powder and flake forms are more effective, providing higher levels of conductivity at lower pigment loadings than the spherical product. Nickel-coated carbon fibre is suitable for incorporation into thermoplastic materials for a wide range of electrically conductive applications, including EMI shielding. A potential disadvantage of the mass pigmentation method is that surface finish may be adversely affected, especially if large fibres or flakes are used. It may then be necessary to paint the resulting mouldings to mask the defects and provide an aesthetically pleasing moulding. If this is the case, it may well be more cost-effective to revert to applying an EMI coating to the inside of the component. Alternatively co-injection moulding can be used to apply a thin skin of aesthetically acceptable polymer simultaneously over a core containing the conductive pigment.

15.5 Light exclusion Carbon black remains the most cost-effective means of rendering a polymer opaque to light. Where a more aesthetically pleasing effect is also required, such as in the packaging of light sensitive foods, the high covering power of fine grades of aluminium flake pigments makes them a good choice. A recent development combining the aesthetic attractions of aluminium and gold bronze pigments with the functional attribute of high opacity is known as ‘scratch cards’. These are cards issued as lottery tickets, promotional or contest forms or the like. A winning ticket is identified when the opaque coating covering printed information is removed by scratching it off with a fingernail or coin. Opacity and inter-coat adhesion are essential to dependably conceal this underlying information until the coating is removed. Construction of the card is complex to prevent tampering (see Figure 15.6). To a base of paper stock, a thin film of metal foil may be attached for added security. The design is printed down next. This is the layer ultimately revealed by the act of scratching off the overlying coatings. A clear overlacquer or lamination follows, then the all-important metal pigmented obliterating patches. Finally, optional additional personalisation may be printed onto the obliterating layer. The obliterating layer is typically formed from a fine particle size, high opacity aluminium or gold bronze flake pigment in resin and solvent. Alternatively it may be water-based, using a polymer latex. Carrick [27] describes a scratch-off coating composition comprising an aqueous acrylic polymer resin, a pigment such as aluminium flake and a powdered filler such as calcium carbonate. 192

Non-colouristic Applications

Figure 15.6 Structure of a lottery scratch card Reproduced with kind permission of Chris Williams

15.6 Heat and light reflection The reflective nature of aluminium flakes also makes them a good choice for light and heat reflective polymer film. Reflectivity is enhanced by the orientation of flakes which takes place during mono- or bi-axial stretching of the film. A typical application is crop ripening films, usually made of polyolefins (see section 11.3). A potential, but unproved application is use of metal flake coated PET film to replace the vacuum aluminised PET film that is bonded to plasterboard to enhance thermal reflectance and control vapour permeability. Very high quality leafing aluminium flakes are required to approach the degree of reflectivity provided by vacuum deposition. The most commercially developed application in this category is reflectors for vehicle light cluster coatings. Again the highest reflectance aluminium flakes are necessary. An unusual patent application by the 3M Company [28] describes light reflective articles in which metal flake is used to enhance the reflectivity of transparent spheres. A coating containing, for example, aluminium flakes in a transparent resin system is applied to a substrate. It is then overcoated with spheres, such as glass beads. When partly pressed into the surface, the metal flakes take the shape of the sphere’s surface, enhancing reflectivity. The technique is used for reflective road and pavement markings.

193

Metallic Pigments in Polymers Aluminium flake is also used to control the opacity of transparent polymers by imparting opacity. When incorporated in PC sheet used as a glass substitute, the flakes regulate solar transmission in proportion to their concentration. Flakes in the 30-70 µm diameter range that are free of fines are the most effective. They reduce solar transmission without preventing visibility. A further specialised use of aluminium flake is to enhance the insulation value of foamed PE pipe insulation. The flake particle size appears to control the nucleation of the PE. This determines the cell size, on which the insulation value depends. As aesthetic properties are of secondary interest, low cost, fine particle size grades are generally used. Aluminium flake is also used in a novel process to prevent photographic or electrostatic copying of documents by light reflection [29]. An orange, red or brown coating on white paper, overlaid with a pattern of aluminium ink, prevents photocopying. The coating ‘blinds’ the copier and a black image is obtained. The invention is intended to maintain the security of sensitive documents.

15.7 Thermal conductivity The excellent thermal conductivity of metal flakes is demonstrated by the observation that even at the relatively low concentration used for purely decorative effects in polymers, it is often possible to reduce the temperature of the processing equipment by several degrees. The improvement in thermal conductivity of polymer pigmented by large metal flakes can be quite marked. Cullen of Transmet Corporation [30] in the USA claimed that composites formulated with up to 40 per cent by weight of high aspect ratio aluminium flakes have a heat transfer effectiveness of 80 to 95% of that of the pure metal. Spherical metal powders will increase heat transfer through polymers, but because of their low aspect ratios and high density, high loadings, often over 50 weight percent, have to be used. At such concentrations, mechanical integrity is poor and injection moulding becomes difficult. Flakes are therefore preferred to atomised powder not only due to their higher aspect ratio, but also their greater surface area for heat transfer. Potential applications of metal filled polymers include any injection-moulded components that have complex shapes, expensive to manufacture by casting or machining of the pure metal. A good example is semiconductor heat sinks for the electronics industry, but car engine components are also of interest, particularly since weight-saving is an incidental bonus. Account must be taken of the effect on the mechanical strength of the polymer at

194

Non-colouristic Applications such high metal loadings. Nevertheless, metal filled plastics combine the conductivity of metal with the low cost and easy processing of plastics. The thermal conductivity of a material is the rate of heat flow in a temperature gradient. Put more simply, it is the speed at which heat can move through a material. In the following formula, thermal conductivity (K) is given by: K = QD/A(T1-T2) where:

Q is the rate of heat flow D is the sample thickness A is the area (T1-T2) is the temperature gradient, T1 being the temperature of the warm surface and T2 the temperature of the cold surface.

Measurement of thermal conductivity is described in ASTM C-177 [31]. In this test two flat plates of material of uniform thickness are placed one on each side of a heater. This assembly is then placed between two cooling blocks, and surrounded by insulation. Temperature measurements are made at the interface between the material and the cooling block. The temperatures, and the thickness and area of the material are known, as is the output of the heater, which is determined electrically. From these measurements, the thermal conductivity can be calculated. Using ASTM C-177, it is possible to measure thermal conductivities of both pigmented and unpigmented polymers, and rank them. The Transmet workers were also able to show inherent differences in thermal conductivity of polymers alone, PVC being superior to polyolefins, which in turn were more conductive than ABS or PS. To dramatically increase thermal conductivity, a metal with as high an aspect ratio as possible must be incorporated into the base polymer. The Transmet work was directed to rapidly quenched aluminium flakes. Such flakes are manufactured by a melt spinning process. The very rapid cooling stage limits oxidation, allowing the metal to retain more electrical as well as thermal conductivity. For commercially useful thermal conductivity, a loading of 18 to 22 volume% (approx 40 weight%) is necessary. Loadings of 35 volume% were achieved and in some cases higher loadings were possible. Moulding could still take place on any injection moulding machine.

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Metallic Pigments in Polymers

15.8 Lubrication and wear reduction The current use of copper and aluminium flakes in high temperatures greases suggests that they may also be incorporated in polymers to extend the life of plastic bushes and bearings. To prevent flakes from abrading from the polymer surface, it may be necessary to bond them into the polymer matrix using coupling agents such as titanates, zirconates or silanes. The heat dissipation property of metal flake is a further advantage in extending bearing life. Activity in this area is at the research stage. Nickel flakes are used in a number of specialised applications such as anti-seize additives for high temperature lubricants and in others that rely on magnetic properties of the metal. Stainless steel powders are produced primarily for powder metallurgical and thermal spray applications and for use in controlled porosity filtration systems. They are incorporated into GRP mouldings to increase the wear resistance of the surface layer.

15.9 Gas and moisture barrier Formulations providing gas and moisture barrier properties have some features in common with mechanical reinforcement. The absence of bonding between conventional flakes and the polymer matrix leaves capillary channels that promote rapid migration of gases and moisture. Although the presence of metal flakes may increase the path length for migration from one side of a pigmented polymer film to the other, the net result, even at metal loadings up to 10% w/w, is little change in barrier properties. Eliminating capillary channels by the use of surface treated metal flakes bonded into the polymer matrix, combined with good orientation of the flake by film stretching during manufacture, have shown improved barrier properties in recent tests. The cost-in-use of these pigmented films, compared to existing barrier films such as vacuum deposited metallised film and films co-extruded with barrier polymer layers, is the subject of current research and commercial interest.

15.10 UV protection Aluminium reflects approximately 90-95% of incident UV light [32]. This has implications for the protection of UV sensitive polymers such as polyolefins from embrittlement in exterior applications. The crop ripening films described in section 11.3 are a good example. Unpublished work carried out by Silberline in the USA compared 1%, 3% and 6% loadings of a fine particle size aluminium pigment in high density PE with the performance

196

Non-colouristic Applications of unpigmented polymer. All samples were exposed for up to 1200 hours in an Atlas Weatherometer. Even a 1% loading provided a modest improvement in tensile strength retention. At 3%, the useful life of the polymer was extended by a factor of 3. When the loading was increased to 6%, over 80% of the tensile yield and break strengths were retained after 1200 hours exposure. In comparison, the unpigmented polymer lost over 50% of both properties after less than 600 hours.

15.11 Laser marking As well as the familiar cutting applications, modern industrial lasers have recently found use in marking a very wide range of materials, including polymers. The main advantages are speed, precision, cost-effectiveness and access without mechanical contact. The technique is very flexible, easily automated and environmentally friendly since there are no solvent emissions. Moreover, since the laser beam writes into, rather than onto, the surface, the marking is fade and abrasion resistant, effectively providing a permanently readable surface. A typical installation has a CO2 or higher intensity neodymium doped, yttrium aluminium garnet (Nd:YAG) laser whose beam is directed onto the workpiece by galvanometer mirrors. A computer controls the marking process to produce a wide range of characters, symbols, logos and graphics. CAD and graphics files may be directly imported from other applications. Marking speeds up to several metres per second are achievable with an accuracy of 0.1 mm. There are three marking mechanisms, depending on the nature of the polymer. All are equally durable. The first method uses a laser to heat the material, creating a colour change at the surface, in a process called annealing marking. Surface contours are retained. Alternatively, material can be physically removed to achieve laser engraving. This is useful where infill coloration is subsequently applied. Laser marking of polymers is technically more challenging than that of most other substrates. The third technique, pigment marking, specifically refers to the incorporation of special pigments, such as mica flakes into mass pigmented polymers to create a colour change under a laser beam. For example, white polyolefins can be coloured black by carbonising the polymer. The mica is believed to absorb the laser energy at its surface, which in turn causes carbonising of the local polymer. By a similar mechanism, black pigmented polymer can be marked white by foaming the polymer.

197

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Figure 15.7 The effectiveness of aluminium flake in laser marking. Unpigmented PP, left, exhibits less definition than 2% mica pigmented PP, centre. 2% SILVET LR 30 aluminium flake filled PP, right, shows the highest definition

The main challenges in laser marking of polymers are definition quality and coloration strength. The visual contrast between image and non-image areas of unpigmented polymers is generally insufficient, which explains the interest in colour enhancing pigments such as mica. Most recently, work on aluminium flakes via the pigment marking process has yielded very encouraging results, prompting Silberline to release a range of aluminium pigments in granular form [33]. These ‘LR’ or laser receptive grades provide a higher degree of both fine coloration and definition than mica types, as shown in Figure 15.7.

15.12 Magnetic applications The magnetic properties of iron particles, in powder or flake form, are applied as moulded alternatives to permanent magnets. High loadings are required, incorporated in polymers by mass pigmentation. Although lacking aesthetic properties, the resulting mouldings can have complex shapes, difficult or impossible to produce in a conventional permanent magnet. One of the more prominent applications is in elastomeric door sealing strips for refrigerators. Patterns in coatings can be achieved by applying liquid coating systems, pigmented by magnetically susceptible flakes, to substrates under a magnetic field. The flakes orient along lines of magnetic force to create a decorative effect.

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15.13 Corrosion resistance Zinc powder and flake, and aluminium, nickel and stainless steel flakes are used in a variety of anticorrosive and other protective coatings [34]. Hare [35], in several papers, has evaluated both elemental metal pigments and pigments free of elemental metals, such as micaceous iron oxide, in barrier coatings. Binder selection is generally confined to those polymers with inherently low moisture and oxygen transmission rates and high dielectric constants. A polymer bearing carboxyl, hydroxy and backbone ester groups does not usually perform as well as a polymer having a carbon-carbon, carbon-oxygen or aromatic backbone. On the other hand, some polarity improves adhesion. Vinyl and coal tar systems make excellent vehicles, followed by epoxies and urethanes. Section 13.5 contains a typical starting point formulation. In pigmentation trials involving extended outdoor exposure, leafing aluminium flake was found to be particularly effective [36]. The metal appears to provide some UV protection to the binder. Formulations are also relatively insensitive to flake particle size or leafing value. Aluminium gives uniformly excellent protection and good aesthetics, regardless of vehicle type or test environment. Hare [37] found that comparative tests with glass flake and mica gave very poor results. Stainless steel and micaceous iron oxide, though better, showed neither the degree of performance nor the universality of aluminium. The only weakness of aluminium was its sensitivity to extremes of pH.

15.14 Flame retardation Inclusion of aluminium flake pigment in certain polymers has been shown to generate some flame retarding activity. Unfortunately, loadings above normal commercial colouring concentrations are necessary, typically from 5% w/w on polymer upwards. There is some evidence that finer grades are more efficient than coarse types. Irrespective of flake size however, the effect is insufficiently marked to allow a recognised flame retarding agent to be omitted from formulations in which it is otherwise required. The most amenable polymers are polyolefins, ABS, polystyrene and polycarbonate. Polyamide of the common polymers shows least activity. Apart from a slowing of the rate of burn, structural integrity is better maintained and dripping of burning polymer reduced. A possible explanation is that aluminium oxide generated during combustion coats and helps to protect unburned material. Aluminium oxide is a component of commercial flame retarding agents. Unburned aluminium may conduct away the heat of combustion, which is higher than it otherwise would be, because oxidation of aluminium is very exothermic.

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15.15 Radiation absorption Another specialised non-colouristic application involves the use of lead particles in polymers intended for high energy radiation shields. A typical example is the moulded lead aprons worn by radiographers for protection from the cumulative effects of continued daily exposure to X-rays in the course of their work.

References 1.

K. A. Pollart and T. P. Lafferty, inventors; James River Corporation of Virginia, assignee, US Patent 5,002,826, 1991.

2.

O. E. Seiferth, inventor; James River Corporation, assignee, US Patent 4,641,005, 1987.

3.

H-F Huang and D. E. Plorde, inventors; E.I. Du Pont de Nemours, assignee, European Patent 0,242,952 B1, 1987.

4.

M. Paleari, inventor; W. R. Grace & Co.-Conn, assignee, Canadian Patent 2,149,097, 1995.

5.

R. R. Hartman, B. D. Berger and K. J. DeHaan, inventors; James River Corporation of Virginia, assignee, US Patent 4,982,064, 1991.

6.

K. Goddard, Food Manufacture, 1990, September, 49.

7.

The Influence of Microwave Radiation on Aluminium Flake Pigments, Product Information, Silberline Ltd., UK, 1991.

8.

A. Sternfield, Modern Plastics International, 1982, 12, 7, 48.

9.

Survey of commercially available polymers in Modern Plastics, Mid-November 1994, B-169.

10. H. Charles, inventor; no assignee, US Patent 4,374,760, 1983. 11. R. Deguchi, inventor; Ube Industries Ltd., assignee, US Patent 4,569,786, 1986. 12. E. McGowan, Jr., inventor; Ciba-Geigy Corporation, assignee, US Patent 4,369,063, 1983. 13. United States Federal Communication Committee Rules and Regulations, July 1981, 11, Part 15, Docket No. 20780. 200

Non-colouristic Applications 14. EC Directive 89/336/EEC, relating to electromagnetic compatibility, 1992. 15. R. Morrison, Grounding and Shielding Techniques in Instrumentation, 3rd Revised Edition, John Wiley and Sons Inc., Chichester, 1986. 16. H. W. Ott, Noise Reduction in Electronic Systems, 2nd Revised Edition, John Wiley and Sons Inc., Chichester, 1988. 17. M. Mardiguian, Controlling Radiated Emissions by Design: EMI/RFI Reduction, Van Nostrand Reinhold, New York, 1992. 18. A. C. Hart, Polymers Paint Colour Journal, 1996, 186, 4378, S3. 19. S. Naik, M. Carmel and M. Fenton, Proceedings of the SPI 41st Annual Conference, The Reinforced Plastics/Composites Institute, Atlanta, USA, 1986, Paper No.15E. 20. H. R. Friedli and P. Y. Lau, inventors; The Dow Chemical Company, assignee, US patent 4,556,506, 1985. 21. S. J. Monte, Ken-React Reference Manual, Kenrich Petrochemicals Inc., 2nd Revised Edition, 1993, 63-5. 22. D. M. Bigg in Metal-Filled Polymers, Ed., S. K. Bhattacharya, Marcel Dekker Inc., New York, 1986, 165-217. 23. D. K. Chen, C. H. M. Ma and A. T. Hu, Proceedings of the 36th International SAMPE Symposium, San Diego, USA, 1991, 1474. 24. M. Kanda, T. Hatakeyama and Y. Morito, inventors; Showa Denko KK, assignee, US Patent 4,508,640, 1985. 25. M. Maeda, N. Watanabe and K. Fujitani, inventors; Showa Denko KK, assignee, US Patent 4,557,859, 1985. 26. S. J. Monte, Ken-React Reference Manual, Kenrich Petrochemicals Inc., 2nd Revised Edition, 1993, 65. 27. B. W. Carrick, inventor; GTECH Corporation, assignee, US Patent 5,215,576, 1993. 28. K. Hachey, inventor; Minnesota Mining and Manufacturing Co., assignee, European Patent 0,683,403 A2.

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Metallic Pigments in Polymers 29. K. Nagafuchi, inventor, Kisokaseisangyou, Co., Ltd., assignee, US Patent 5,114,782, 1990. 30. D. L. Cullen, M. S. Zawojski and A. L. Holbrook, Plastics Engineering, 1988, 44, 1, 37. 31. ASTM C 177-97 Test Method for Steady-State Heat Flux Measurements and Thermal Transmission Properties by Means of the Guarded-Hot-Plate Apparatus. 32. J. H. Tundermann and J. H. Harrington, inventors; The International Nickel Company, assignee, US Patent 3,941,584, 1976. 33.

‘SILVET LR’ aluminium flake pigments, Product Information, Silberline Ltd., UK, 1998.

34. T. Higashiyama and M. Nakazato, inventors; Nippon Dacro Shamrock Co., assignee, European Patent 0,468,883, B1, 1996. 35. C. H. Hare, Journal of Protective Coatings and Linings, 1989, February, 59. 36. C. H. Hare, Modern Paint and Coatings, 1985. 37. C. H. Hare and M. G. Fernald, Modern Paint and Coatings, 1984, 136.

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16

Health, Safety and Handling

This chapter contains health, safety and environmental information on the common metal pigments from a UK perspective. It includes specific guidance on correct handling, storage, disposal and fire fighting techniques. Under each of these headings, information common to all metal pigments precedes any guidance for particular metal types. Much useful information is contained in a reference work by Lewis [1]. Health aspects of aluminium and gold bronze pigments, not well covered in the period up to about 1980, are well summarised by Wendon [2]. Several metals appear in the Approved Supply List, part of the European Commission (EC) Directive 67/548, known as the Substances Directive. Its text has been incorporated into the national law of the member states and revised and extended on numerous occasions. The most recent UK enactment is the Chemicals Hazard Information and Packaging for Supply or ‘CHIP’ 98. This provides a method for classifying the risk posed by a chemical substance and expressing it via a group of numbered Risk, or ‘R’ Phrases of prescribed text. The Risk Phrases are complemented by Safety, ‘S’ Phrases, which describe how to respond to the stated hazard. In addition, substances and preparations are classified by the hazards they present, such as flammable, harmful or toxic. Table 16.1 contains the hazard classification, R and S phrases for the common metal pigments in dry flake form, together with their European Inventory of Existing Chemical Substances (EINECS) and Chemical Abstracts Service (CAS) identification numbers. Stainless steel is not included due to its minimal hazard and variability of composition. CHIP also contains rules governing the risk and safety classifications of preparations, defined as compositions containing two or more substances. It is impractical to list classifications of the very large number of preparations based on metal pigments. Traditional aluminium pastes however, consisting of around 60-75% metal, with a balance of high boiling aliphatic and/or aromatic hydrocarbons, are generally classified as flammable. Only fine grades of copper and gold bronze are so classed. The remaining metal pigments are not regarded as flammable. Due to their physical form and absence of solvent, commercially available plasticiser dispersions and metal pigment granules do not usually present a hazard and are therefore not classified under CHIP. There is then no obligation to provide a Material Safety Data Sheet under the Data Sheets Directive, 91/155/EC [3].

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Metallic Pigments in Polymers

Table 16.1 Hazard classifications of metals in flake form

used as pigments Metal

Hazard

R

S

EINECS

CAS No.

Aluminium

F

10

7/8,43

231-072-3

7429-90-5

Gold bronze

F*

(10)

(7,43)

None

12597-71-6

Copper

F*

(10)

(7,43)

231-159-6

7440-50-8

NC**

-

-

231-096-4

7439-89-6

Xn

40,43

(Z)22,36

231-111-4

7440-02-0

231-117-3

7440-66-6

Iron Nickel Zinc

NC**

* Flammable below about 10 µm ** NC = Not classified

Another important piece of legislation is the UK Control of Substances Hazardous to Health Regulations 1999, (COSHH) [4] which update the 1994 Regulations in line with the directive 96/55/EC [5] on the marketing and use of dangerous substances and preparations. COSHH requires risk assessments to be prepared, which relate to the actual use of the products. All metal pigments present a low degree of risk, provided they are handled correctly. This chiefly means taking precautions to minimise dust formation and maintaining good housekeeping practices (see section 16.2).

16.1 Health Health aspects of metal pigments arise from two sources; those concerned with particle geometry and those related to the toxicology of the metals themselves. Particle geometry is relevant because the flake form, even of the heavier metals, is easily carried in air currents. The smaller the particle size and the lower the density, the greater the potential for dusting. Dry flake products present the greatest problem in this respect. Pastes that have dried out will tend to aggregate, but inevitably some particles will also break free. Both product types can therefore have the properties of nuisance dusts, with assigned occupational exposure standards, (OES) defined in EH40 [6]. OES values for aluminium particulates are 10 mg/m3 for inhalable dust and 4 mg/m3 for respirable dust. These limits do not apply to exposure to aluminium coated with mineral oil. Particles with a largest dimension of less than about 5 µm pass the hairs which line the nose and are drawn into the lungs. The hazards here depend on the metal’s toxicology and its ability to pass into the body tissues rather than being excreted.

204

Health, Safety and Handling The hydrocarbon solvents traditionally used as carriers for metal pigment pastes defat the skin and with repeated contact can cause irritation and dermatitis. Habitual skin contact should therefore be avoided. Fatty acid lubricants are nowadays generally obtained from vegetable sources. When used in products intended for food contact, this eliminates any possibility that the brain disease CJD could be contracted. Traditionally these lubricants were derived from animal sources. Plasticiser pastes are solvent-free, but phthalate esters, used for many years as inexpensive carriers of low level of acute toxicity, are coming under scrutiny. Di-2-ethyl hexyl phthalate, more commonly known as DOP (dioctyl phthalate), is a confirmed carcinogen in animals [7]. A link with male testicular cancer and female breast cancer, as well as declining sperm counts, is suspected [8]. Products containing DOP, emanating from the USA, are required to display a carcinogen warning label. In Europe, DOP is classed as Xn (harmful) according to the Substances Directive and is required to carry the risk phrases R62 (‘possible risk of impaired fertility’) and R63 (‘possible risk of harm to the unborn child’). Though there is rebuttal of a cancerous link in man, phthalate substitutes are being researched and introduced. Concurrently, a Dutch research group has produced evidence to allay the apprehensions of the EU Scientific Committee for Toxicity, Ecotoxicity and the Environment (CSTEE) [9, 10]. At the time of writing the future health and safety status of DOP is unclear. It is worth noting, however that its hazard classification has remained unchanged in CHIP 98. Solid carriers such as low molecular weight polyolefins, used for granular product forms, are generally unclassified for any health or safety risk. Granular products, being nondusting and virtually solvent-free are in any case unlikely to be accidentally ingested. Indeed some are food contact approved, as are many mineral oil carriers. They therefore present no significant health concerns.

16.1.1 Aluminium Although found in the body, aluminium is not thought essential to any bodily function. It has also long been accepted that aluminium flake does not present a health hazard in manufacture or use. Wendon [11] documents early studies on workers exposed to aluminium dusts during manufacture. There was no indication that aluminium should be categorised as a health hazard providing a conventional long chain fatty acid lubricant is used in the manufacturing process.

205

Metallic Pigments in Polymers Aluminium enters the body in food, drink and by inhalation. There is an effective mechanism in the gut to limit absorption and in the kidneys to control excretion. In a healthy adult, intake and excretion are in balance. Only where there is impairment of bodily functions does the metal accumulate. Even then, cases of aluminium toxicity are exceedingly rare [12]. Suggestions were first made of a link between aluminium and the degenerative brain disorder Alzheimer’s disease in the mid 1970s, following the discovery of abnormally high levels of aluminium in the brains of sufferers. The metal was also implicated in dialysis dementia, a consequence of kidney failure and in a bone disease called osteomalacia. The Aluminum Association in the USA as long ago as 1955 had carried out an exhaustive survey of over 800 sources of information on aluminium and health. The review was updated in 1974 and again in 1981. Thereafter, a team at the University of Cincinnati took up the monitoring process. Research programmes were put in place by the Aluminum Association to establish where and how aluminium enters the body, how much is absorbed and where it accumulates. The state of knowledge was summarised for the layman by Epstein [13], who concluded that the cause of Alzheimer’s disease had not been established. Neither was the biological significance of aluminium in the brain understood. Absorption by the body is poor and ordinary environmental exposure to aluminium is safe. Indeed a patent application has recently been made for the use of iron oxide coated aluminium flake as a food colorant [14].

16.1.2 Gold bronze and copper Gold bronze is apparently non-toxic to man [15]. Both its copper and zinc constituents are essential trace elements in the diet and a slight deficiency is not uncommon. Due to its high copper content, inhalation of large quantities of gold bronze flake will produce metal fume fever, which causes an influenza-like illness. The effect is transient, typically lasting only 24 hours. There are no known adverse long-term effects of exposure. It is not persistent in the lungs and is believed to be absorbed into the body in soluble form. For this reason it does not lead to lung diseases such as pneumoconiosis that are characteristic of some mineral dusts. Concerns over the safety of gold bronze in packaging inks have been largely allayed by Warnes [16].

206

Health, Safety and Handling The health and safety status of gold bronze is less of an issue than its environmental impact. Its substantial copper content is often closely regulated in aqueous effluent by environmental agencies. As well as producing metal fume fever, copper also causes irritation of the upper respiratory tract, a metallic taste and discolouration of hair and skin. The body is able to eliminate any excess copper that is absorbed. Nevertheless, the 8 hour Time Weighted Average (TWA) exposure limit for copper dust under the UK EH40 legislation is a very low 1 mg/ m3. The intraperitoneal LD50 in mice is also low at 3.5 mg/kg [17]. The relative absence of health effects for gold bronze, which as noted previously is predominantly copper, may be attributed to the protective effects of the milling lubricant.

16.1.3 Other metal pigments The International Agency for Research on Cancer, IARC, classifies nickel as a carcinogen. It is a poison by intratracheal, intraperitoneal, subcutaneous and intravenous routes [18]. EC Directive 67/548 also classifies nickel as a carcinogen in Class 3, which requires the hazardous designation Xn. The metal has a Maximum Exposure Standard or MES with an 8 hr Time Weighted Average (TWA) of 0.1 mg/m3 [19]. This section of EH40 now contains the former schedule 1 of COSHH, the Control of Substances Hazardous to Health Regulations, 1994, amended in 1996, 1997 and 1999 [20]. Skin contact should be avoided. A form of dermatitis known as ‘nickel itch’ may be contracted and nickel has also been found in the hair of workers exposed to the oxidised dust. Iron and zinc are not inherently toxic materials [21, 22]. Iron dust can cause conjunctivitis. When oxidised by heating, zinc causes a disease called ‘brass founders’ ague’ or ‘brass chills’. It is not persistent. Type 316 stainless steel is classified in the same way as nickel, because of its 12-14% concentration of the metal. Silver and gold have no harmful effects other than the transient eye irritation to be expected of flake particulates.

16.2 Safety Safety precautions should address avoidance of both fire and personal contact, the latter particularly by inhalation. The degree of fire risk is broadly proportional to the position of the metal in the electrochemical series; in effect how prone to oxidation it is. Thus, dry aluminium flake is regarded as a potent fire and explosion hazard, whilst silver is non-flammable.

207

Metallic Pigments in Polymers Classification of the flammability of metal pigments for transport is according to tests prescribed in the UN Recommendations on the Transport of Dangerous Goods [23]. The procedure involves spreading a carefully formed track of the product on an inert surface, igniting one end of the track and determining the distance covered by the flame front in two minutes (or 20 minutes in the case of metallic powders). If the distance is less than 20 cm, the material is declared non-flammable. Above this figure, a more stringent set of tests must be followed, potentially requiring the product to carry a flammable designation. Safety precautions should be directed at preventing contact of the metal pigment product with any part of the body. In accordance with good industrial practice, basic personal protective equipment comprising PVC gloves, eye protection and impervious clothing should be worn. A mask suitable for nuisance particulates should be worn if conditions are dusty. In warm conditions in confined spaces, the handling of large quantities of solvent pastes may require a suitable respirator. In the case of metal pigment compositions, account must be taken of the other components, for example the flammability of the solvent components of pastes. Contacting metal pigments with certain other commonly used materials can give unwanted chemical reactions, or even pose an explosion hazard. Table 16.2 provides a list of materials that should not be stored with, or allowed to come into contact with, the various metal pigments in unpassivated form.

Table 16.2 Materials that should not be contacted with metal pigments Metal

Materials to avoid

Aluminium

Acids, alkalis, chlorinated hydrocarbons, oxidising agents, iron oxide.

Gold Bronze

Acids, acetylene, chlorates, bromates, iodates and oxidising agents.

Copper

Acetylene, oxidising agents and halogens.

Nickel

Acids.

Iron

Strong oxidising agents and acids.

Zinc

Acids, nitrates, chlorates, sulphur.

16.2.1 Aluminium There is a tendency to underestimate the potential of aluminium to burn, probably because in the bulk form and due to its thin, protective oxide coating, it appears inert. Powders

208

Health, Safety and Handling and solvent-based pastes are classified as flammable under the UN recommendations mentioned previously [23]. In the case of pastes, it is common for the solvent component to burn to the track length required by the UN test without ignition of the metal. Metal ignition, where it occurs, is characterised by an intense white light. Depending on the nature of the carrier, some granular products are non-flammable. This is probably because the mainly hydrocarbon carrier burns to a rigid carbon residue that coats the metal flakes, thereby preventing their ignition. When airborne in finely divided powder or especially in flake form, the high surface area of typically a few m2 per gram, provides enormous explosive potential, generating a rate of pressure rise of over 20,000 psi/s. The quantity suspended in the atmosphere required for this to occur is small, at around 10 g/m3, as is the few mJ required to initiate an explosion. Such a low energy level can be generated by many apparently mundane sources, for example loose electrical wiring, electric motor commutators or poorly lubricated, overheating bearings; even static caused by pouring non-earthed powders. An atmosphere containing as little as 9% oxygen in nitrogen will support combustion. Aluminium flake in any non-passivated product form should not be allowed to come into contact with water, especially at elevated temperature. The reaction between the two generates extremely flammable hydrogen gas. For this reason an aluminium fire should never be tackled with water (see section 16.6). Contact with oxidising agents and halogens should be avoided. Iron oxide is another substance that should be prevented from coming into contact with aluminium flake, since a highly exothermic chemical reaction known as the thermite reaction may occur [24].

16.2.2 Gold bronze Gold bronze powder is combustible but not explosive. Grades with median particle diameters below about 30 µm are classed as flammable for supply and carry the CHIP risk phrase R10. They are also flammable for transport. Coarser grades are generally regarded as non-flammable for both transport and supply. Because much gold bronze is still sold in the dry flake powder form, dusting should be avoided by use of good handling techniques. Airborne material has the properties of a nuisance dust. There is no OES, but an 8 h TWA exposure limit of 10 mg/m 3 is recommended [Manufacturer’s recommendation; Wolstenholme International Ltd.]. The material is stable under normal conditions, but contact with acetylene, chlorates, bromates, iodates and oxidising agents, which could induce a violent reaction, should be avoided. The solvents influence the safety classifications of paste forms in much the same way as for aluminium pastes.

209

Metallic Pigments in Polymers

16.2.3 Other metal pigments Copper flake can react potentially explosively with acetylenic compounds, oxidising agents and halogens. Copper dust has an OES (8 h TWA) of only 1 mg/m3. Iron, stainless steel and zinc pose no significant safety hazard. Due to its classification as a carcinogen, measures must be taken to limit inhalation of nickel flake dusts. It is also important to keep nickel out of wounds.

16.3 Health and safety in use When incorporated in polymers, whether by mass pigmentation or as part of a coating, the explosion hazards posed by dry metal pigments in particulate form are eliminated. Health and safety issues then refocus on the properties of the metals themselves and on the carriers, both liquid and solid, used to convey them into application media. One of the most frequently asked questions on the health and safety of metal pigments relates to food contact acceptability. The US FDA sets down in numbered paragraphs, conditions and concentrations under which substances, but not preparations, are to be regarded as safe in contact with food. Aluminium, for example, is approved as a pigment and colorant in Title 21, Paragraph 26, Page 154 of Section 175.300 of the US Code of Federal Regulations. In the UK the relevant legislation is the Statutory Instrument 1998 No. 1376, the Plastic Materials and Articles in Contact with Food Regulations. Gold bronze is not approved for direct food contact applications. Directive 95/45/EC [25], which came into force in the member states in July 1996, builds on 89/107/EC [26] and 94/34/EC [27] to define the purity of colorants used in foodstuffs for human consumption. Aluminium (E173), silver (E174) and gold (E175) pigments are listed. Also listed in this Directive are the red, yellow and black oxides of iron used as coloured coatings on metal flake surfaces. A further EC Directive, 94/62/EC [28], on recovery, recycling and re-use of packaging and packaging waste also impacts on metal pigments. Article 11 requires that the sum of the concentrations of lead, cadmium, mercury and hexavalent chromium shall not exceed 600 ppm, 250 ppm and 100 ppm by 30 June of the years 1998, 1999 and 2001, respectively. In practice, the high purity metals used in modern pigments comfortably comply with this legislation, even at loadings well above commercial limits. Another key piece of European legislation, applicable to both mass pigmented polymers and coatings, is EN71 Part 3, (1994) [29], produced by the European Committee for Standardisation. These so-called ‘Toy Regulations’ specify limiting concentrations of certain elements, mostly heavy metals, extractable from toys. It is important to note that the Standard does not specify limits for the elements themselves, but deals with

210

Health, Safety and Handling extractability or migration of these impurities from the finished toy material. Adoption of the Standard, which supersedes that of 1988, is mandatory in all CEN member states. As with the Packaging Directive mentioned previously, the purity of metal pigments is generally sufficiently high for compliance, even above normal commercial loadings.

16.3.1 Mass pigmentation Metal pigments incorporated by mass pigmentation present no safety concerns related to their physical form. Of more interest is conversion temperature since it is this parameter that differentiates mass pigmented systems from coatings. Carriers that are of little safety concern at ambient temperatures can become a risk at processing temperatures. It is therefore important not to exceed the temperature ceiling defined by the manufacturer. Precautions should also be taken to avoid hot melt coming into contact with the skin. Heat resistant gloves or gauntlets should be worn.

16.3.2 Coatings Hazards associated with metal pigments in coating media are more associated with the carriers for the metal than the metals themselves. These are typically low flash point ink solvents, capable of providing the necessary fast solvent release. Volatile organic solvents in general are coming under increasing scrutiny. In 1996 the EC proposed a Council Directive on ‘Limitation of emissions of volatile organic compounds (VOC) due to the use of organic solvents in certain industrial activities’. A VOC is defined as ‘any organic compound having at 293.15K a vapour pressure of 0.01 kPa or more, or having a corresponding volatility under the particular conditions of use’. The ramifications of this proposed legislation are explored by Kershaw [30]. At the time of writing there is no convergence. Each EU country applies its national legislation. In the UK, the 1990 Environmental Protection Act, amended in 1995, introduced a broad control of air, water and land pollution. Guidance notes on compliance include PG6/10, describing operating limits for coatings manufacture, and PG6/23, relating to coatings on metal and plastics in enclosed conditions, such as a spray shop. The VOC limit for primers and undercoats is 250 g/l and 420 g/l for topcoats. A related, and potentially more far reaching EC directive, is the Integrated Pollution Prevention Control (IPPC) Directive, 96/61/EC [31]. It must be transposed into UK legislation by October 1999. When fully effective in 2007 it will control the release of solvent to air, water and land by large users.

211

Metallic Pigments in Polymers

16.4 Environment Environmental considerations applicable to metal pigments relate both to the metals themselves and to their carriers. All metal flakes are persistent and will accumulate. The more chemically active will slowly convert to soluble or insoluble compounds. Although persistent in the environment, aluminium- and copper-based flakes degrade into non-bioavailable forms. Traditional solvent pastes generally contain sufficient hydrocarbon solvents to classify them as flammable for transport and as marine pollutants. Many are required to carry the relatively new EC risk phrase R65 - ‘hazardous for the environment’. No metal pigment in any delivery form should be allowed to enter drains or watercourses. Although naturally present in the environment, high concentrations of copper and zinc may have an effect on aquatic organisms. Copper is regarded as an environmentally hazardous substance, with the UN classification 3077. It must be so labelled for carriage. Copper is designated a marine pollutant or ‘marpol’ by the International Maritime Organisation (IMO). Some aluminium pastes can also carry the marpol designation, but by virtue of their solvent content, rather than due to the metal itself. Aqueous copper-based effluents can normally be discharged into sewers. Consented limits vary according to local circumstances and relevant legislation. Values are around 10 mg/l, approximately the same as for nickel and zinc.

16.5 Handling, storage and disposal Dry flake products should be handled gently, using a non-sparking scoop for transfer. Pastes and granules are less prone to dusting, but it is good practice to treat them in the same way. All containers should be kept closed and tightly sealed when not in use. For pastes, apart from avoiding solvent loss to atmosphere and possible dust creation, the application qualities of the product are preserved. Metal pigments should be stored in their original containers in a cool, dry place, away from sources of ignition and the materials listed in Table 16.2. Unless for storing UV products (see section 16.5.3), glass and plastic containers should be avoided as they may build a static charge. Empty containers should be cleaned and recycled or re-used. Scrapped metal pigment should be disposed of in accordance with local regulations. The less chemically active 212

Health, Safety and Handling metals may be melted and re-used. Due to the high oxide content, it is not possible to recover aluminium flake other than by the same electrolytic process used in its production from bauxite. If aluminium waste cannot be re-milled to pigment, it is generally landfilled. Good housekeeping practices should be followed. These include collecting up spillages promptly and not allowing dust to collect on hot electrical equipment, or roof beams or in extraction trunking.

16.5.1 Aluminium In the special case of fine aluminium powder, transfer scoops and containers should be earthed to avoid the possibility of static discharge. Powder and paste should not be stored in areas containing flammable liquids or other combustible materials due to differences in fire fighting methods. Dry aluminium flake should not be allowed to come into contact with rust, as for example provided by rusty containers or implements. Contact only requires an energy source to cause the extremely exothermic thermite reaction. Good housekeeping is particularly important to prevent accumulation of powder, which if disturbed could provide the fuel for a powder explosion. For this reason, the use of local exhaust ventilation (LEV) trunking is discouraged. So also is vacuum cleaning of spillages.

16.5.2 Other metals The other common metal pigments pose no significant handling problems in the forms in which they are commercially supplied. The same avoidance of dust and personal contact, combined with good housekeeping, apply.

16.5.3 UV grades Some metal flakes, especially aluminium, are capable of curing UV monomers in the absence of light. For this reason, unpassivated metal flake and the UV system should be stored separately, only being combined before use. For maximum shelf life, storage of active UV curing coating systems should always be in opaque, preferably plastic packaging. If metal containers must be used, they should be well coated with an inert resin.

213

Metallic Pigments in Polymers

16.6 Fire fighting Only the metals high in the electrochemical series burn. All but aluminium can theoretically be extinguished by water, but dry powder or foam extinguishers are preferred. Particle size influences flammability, the finest particles, with the largest collective surface area being the most likely to burn.

16.6.1 Aluminium Aluminium fires are particularly dangerous. At high temperatures aluminium will combine chemically with water, halogens, nitrogen and carbon dioxide. For this reason an aluminium metal fire is almost impossible to extinguish. Fire fighting therefore consists of trying to prevent metal ignition by controlling the fire in the incipient stages. If dry powder catches fire, it is best left undisturbed. Any attempt to use an extinguisher is likely to create a dust cloud, due to the pressure of the propellant. If the fire is small and confined to a relatively flat area, it may be gently ringed with dry sand and left undisturbed to burn out. Alternatively, a dry powder extinguisher may be directed over the fire, allowing powder to gently rain down on the burning mass. Better still, long handled scoops may be used to carefully add dry powder extinguishing agent, taking care not to disturb the burning material. If the fire is ringed in this way, combustion will generate a crust that will eventually bring the fire under control. This can take several days in the case of a large fire. Caution is required during this time, as the fire can continue to burn under the crust of material, without obvious external indication. Aluminium paste fires start with combustion of the solvents. In most cases the solvent component will burn out without reaching a sufficient temperature to trigger a metal fire. Indeed, it is rare in practice to achieve metal ignition. An aluminium fire, should it occur however, is easily recognised by the spread of an intense, white-hot glow. Under no circumstances should water be used on any form of aluminium fire. There will be two consequences. The water will vapourise almost instantaneously, the expansion throwing the flakes into the air. Secondly, hydrogen gas will be generated by reaction with the metal, providing more highly combustible fuel. The result is a violent explosion. Foam and halogenated fire extinguishers are also unsuitable, again because of the possibility of reaction. Finally it is worth remembering that local fire department personnel may not be trained for the special response needed for an aluminium fire. The site fire team should therefore guide them.

214

Health, Safety and Handling

16.6.2 Other metals Coarse grades of gold bronze powder are poorly supportive of combustion. They are unlikely to catch fire, but if ignited, can be extinguished by excluding oxygen. Irrespective of particle size, sand is the most effective extinguishing agent. Water and foam may be used, but at low pressure. Zinc is classed as flammable. Fires involving zinc and also the similarly combustible nickel should be tackled with dry chemical extinguishers, not water. Iron and stainless steel are all either poorly combustible or non-flammable, unless of extremely fine particle size. Fires can be tackled by normal wet methods. Silver and gold are not flammable, irrespective of particle size.

References 1.

R. J. Lewis Sr., Sax’s Dangerous Properties of Materials, 9th Edn., Van Nostrand Reinhold, New York, 1996.

2.

G. W. Wendon, Aluminium and Bronze Flake Powders, Electrochemical Publications Limited, Ayr, Scotland, 1983, 83-90.

3.

EC Directive 91/155/EC, for the system of specific information relating to dangerous preparations in implementation of Article 10 of Directive 88/379/EEC, 1991.

4.

UK Control of Hazardous to Health (Amendment) Regulations 1999 (COSHH).

5.

EC Directive 96/55/EC, on the approximation of the laws, regulations and administrative provisions of the member states relating to restrictions on the marketing and use of certain dangerous substances and preparations (chlorinated solvents), 1996.

6.

EH40 Occupational Exposure Limits 1998, UK Health & Safety Executive, Sudbury, UK, 1, 3-18.

7.

R. J. Lewis Sr., Sax’s Dangerous Properties of Materials, 9th Edn., Van Nostrand Reinhold, New York, 1996, 1391-2.

8.

Plastics & Rubber Weekly, 1995, 1596, 1.

9.

Plastics & Rubber Weekly, 1998, 1745, 16.

215

Metallic Pigments in Polymers 10. Plastics & Rubber Weekly, 1998, 1755, 1. 11. G. W. Wendon, Aluminium and Bronze Flake Powders, Electrochemical Publications Ltd., Ayr, Scotland, 1983, 85-86. 12. J. T. Hughes, Aluminium and Your Health, Rimes House, Cirencester, UK, 1992. 13. S. G. Epstein, Aluminum and Health, 4th Edn., The Aluminum Association Inc., Washington, DC, USA, 1996. 14. Gerhard Ruth GmbH, assignee, German Patent Application 4,215,367, reported in Chemical Abstracts, 120, (3) : 29819F. 15. G. W. Wendon, Aluminium and Bronze Flake Powders, Electrochemical Publications Ltd., Ayr, Scotland, 1983, 89. 16. P. Warnes, Polymers Paint Colour Journal, 1995, 185, 4364, S1. 17. R. J. Lewis Sr., Sax’s Dangerous Properties of Materials, 9th Edn., Van Nostrand Reinhold, New York, 1996, 913. 18. R. J. Lewis Sr., Sax’s Dangerous Properties of Materials, 9th Edn., Van Nostrand Reinhold, New York, 1996, 2399. 19. EH40 Occupational Exposure Limits 1998, UK Health & Safety Executive, Sudbury UK, 2, 21. 20. Control of Substances Hazardous to Health (COSHH), 1994, amended 1996, 1997 and 1998. 21. R. J. Lewis Sr., Sax’s Dangerous Properties of Materials, 9th Edn., Van Nostrand Reinhold, New York, 1996, 1943 22. R. J. Lewis Sr., Sax’s Dangerous Properties of Materials, 9th Edn., Van Nostrand Reinhold, New York, 1996, 3423. 23. UN Recommendations on the Transport of Dangerous Goods, Manual of Tests and Criteria, Part III, Subsection 33. 24. G. G. Hawley, Condensed Chemical Dictionary, 10th Edn., Van Nostrand Reinhold, New York, 1981, 1015.

216

Health, Safety and Handling 25. Official Journal of the European Communities, Vol. 38, L226, 22 September 1995, 1. 26. EC Directive 89/107/EEC, concerning food additives authorised for use in foodstuffs intended for human consumption, 1988. 27. EC Directive 94/34/EEC, amendment of 89/107/EEC concerning food additives authorised for use in foodstuffs intended for human consumption, 1994. 28. EC Directive 94/62/EC, on packaging and packaging waste, 1994. 29. Safety of Toys, Migration of Certain Elements, EN71 Part 3, 1994. 30. Y. Kershaw, European Coatings Journal, 1998, No.4, 230. 31. EC Directive, 96/61/EC, concerning integrated pollution prevention and control, 1996.

217

Author Index

A Akay, M. 88 Allan, P. S. 98 Anderson, J. L. 148

B Backhouse, A. J. 148 Banba, T. 137 Barkley, D. 88 Barrick, P. L. 85 Bernhardt, A. 93 Bertacchi, G. 93 Besold, R. 169, 175 Bessemer, H. 4 Bevis, M. J. 98 Bigg, D. M. 191 Birch, J. 172 Bittler, K. 27 Blechschmidt, D. 128 Bock, G. 27 Booz, A. D. 13 Boswell, P. 23 Bradley, D. 92 Brown, M. H. 136 Bryce, D. H. 93

Chan, C. H. 87 Chang, D. C. K. 137, 149 Charles, H. 183 Chen, D. K. 191 Chida, K. 29 Clegg, N. E. 27 Cooke, M. 175 Cullen, D. L. 194

D Daly, A. T. 170 Deguchi, R. 183 Dreher, B. 167

E Edwards, J. D. 11, 41

F Fawcett, S. L. 125 Ferguson, R. L. 135, 145 Fetz, A. 26 Frangou, A. 148 Friedli, H. R. 189 Fritz, H. E. 116 Fritzsche, T. 126

C Camelon, M. J. 170 Carpenter, C. W. 137 Carrick, B. W. 192 Catoen, B. 92

G Gardner, G. 97 Gazonnet, J. P. 97 Gessner, M. A. 173

221

Introduction to Rubber Technology Gibeau, R. C. 170 Gilmore, G. D. 86 Goemans, C. 168 Gray, H. R. 24 Green, W. I. 26 Greening, W. G. 27 Grossman, E. M. 95

H Hagerman, E. M. 85 Hall, E. J. 14, 19 Hamada, H. 86 Harakawa, Y. 26 Hare, C. 163 Hare, C. H. 199 Harrington, J. H. 21 Harris, S. T. 171 Hart, A. C. 187 Hartman, R. R. 181 Hashemi, S. 89 Hashizume, Y. 29 Hatch, B. 93 Henninger, F. 125 Herten, J. F. 87 Heuzey, M-C. 81 Hieda, T. 19 Hobbs, S. Y. 85 Holman, R. 139, 161 Huang, H-F. 181

I Imasoto, Y. 19, 48 Iri, K. 137

J Janicki, S. L. 86 Jong, W. R. 87

222

K Kanda, M. 191 Kazmer, D. O. 97 Keemer, C. B. 29 Kern, G. M. 45, 57 Kerr, J. D. 172 Kerr, S. 92, 107, 161 Kershaw, Y. 211 Kim, J. K. 88 Kim, S. G. 85 Knittel, R. 115 Knowles, R. I. 132 Knox, J. 26 Kondis, T. J. 13 Kramer, E. 13 Kuvshinikov, P. J. 87

L Lalande, F. 88 Lau, P. Y. 189 Lautenbach, S. 87 Levine, S. 24 Lewis, R. J. 203 Lim, J. K. 88

M Malguarnera, S. C. 82, 84, 85 Malloy, R. 97 Mandle, H. H. 13 McAdow, W. R. 24 McGowan, E. 184 McKay, A. 22 McKay, C. 22 Mekhilef, N. 87 Merhar, C. F. 86 Miekka, R. G. 24 Miller, B. 88 Monte, S. J. 190, 191

Author Index Mosle, H. G. 86, 108 Mronga, N. 27 M¸ller, B. 137, 139 Muller, F. 29 Murphy, W. G. 27

Schmid, R. 27, 137 Seo, K. S. 85 Souma, T. 28 Spencer, R. S. 86 Steele, C. D. 168 Suzuki, M. 29, 48

N Nadkarni, S. K. 27

O Oldring, P. 139, 161 Osmond, M. F. 168 Ostertag, W. 27

T Thamm, R. C. 88, 91 Tundermann, J. H. 21

U Uchimura, E. 137

P

W

Paleari, M. 181 Peacock, A. J. 88 Pecorini, T. J. 85 Peters, R. B. 86 Philips, R. W. 28 Piccarolo, S. 86 Plorde, D. E. 181

Wada, A. 98 Walker, W. H. 97 Warnes, P. 206 Watters, C. 92, 107 Wendon, G. W. 11, 12, 203, 205 West, E. J. 131 Wheeler, I. R. 57 Wilkinson, R. 92 Williams, C. F. 173 Wray, R. I. 11, 41 Wu, C. C. 87

R Ravella, A. 27 Rawson, K. W. 98 Richart, D. S. 170 Ringan, E. 22, 92, 107 Roberts, C. B. 24 Roe, D. S. 97 Rolles, R. 136, 170 Rosato, D. V. 92, 114

Y Yeh, J. W. 23 Yokoi, H. 86 Yolles, S. 30

S Sanschagrin, B. 90, 111 Savadori, A. 88

223

Company Name Index

3M Company 193

F

A

Flex Products 28 Ford Motor Company 170 H.B. Fuller Company 170

Akzo Nobel 131 Alcoa 13, 169 Aluminum Association in the USA 163, 206 Asahi Chemical Industry Company 98 Atlantic Powdered Metals 19 Avery Dennison Corporation 24

G General Electric (GE) 60

H B BASF 27, 28, 118, 131, 137, 148, 149, 173 Battelle Columbus Laboratories 191 Binney Smith Inc. 163

C C-Mold 87, 92 Carlfors 13 Ciba-Geigy Corporation 184 Cilas 35

Hartstoff-Metall AG 13 Herberts 131 Honda Motor Company 190 Husky Injection Moulding Systems 92

I ICI 148 Interchemical Corporation 26 International Agency for Research on Cancer 207 International Maritime Organisation (IMO) 212

D Datacolor 44 Dow Chemical Company 86, 189 DSM 60 Du Pont 19, 29, 92, 131, 146, 149, 181

K

E

Loctite Corporation 163

Kenrich Petrochemicals Inc. 66

L

Eastman Kodak Company 148 Eckart-Werke 26, 56, 136, 137, 174

225

Introduction to Rubber Technology

M Macbeth 44 Malvern 35 Mercedes Benz AG 127 Moldflow 87, 92 Mond Nickel Company 30

N Nippon Paint 148 Novamet Specialty Products Corporation 30, 186 NPA 131

SMH AG 127 Sonneborn & Rieck 131

T TBA Industrial Products Ltd. 186 Texas Plastic Technologies 93 Thermofil 183 Thomas de la Rue & Company 158 Toyo Aluminium 29, 136, 137 Transmet Corporation 125, 183, 194, 195

U

Optronik 44

United States Federal Communication Committee 185 University of Cincinnati 206

P

V

Paintmakers Association of Great Britain 175 Pole Européen de Plasturgie 97 Powder Coating Institute 167 PPG Industries 131 Pratt and Whitney Co. 23

Verilac 131

R

X

Revlon 24 Rohm and Engel 118

X-Rite 44

O

S Schlenk 174 Scortec Inc. 95 Senoplast 118 Showa Denko KK 191 Silberline 22, 26, 57, 59, 92, 109, 196, 198 Silberline Inc. 172 SISE 97

226

W Weilburger 131 Wolstenholme International 77, 174, 209

227

Main Index Index Terms

Links

A Absorbency Acid ascorbic citric gallic hydroxycarboxylic stearic Acid dimer Acid number Acrylonitrile-butadiene-styrene Acrylonitrile/styrene/acrylate Additive silicone Adhesion Adhesives Aggregate Aggregation (seeds) Alcohols Aldehyde Alloy Aluminium (see also passivation) absorption flake gas generation oxide spherical toxicity Alzheimer’s disease Amines Antioxidants Application characteristics Applications agricultural film architectural automotive bags bicycle components coatings containers copy prevention corrosion resistance domestic EMI shielding enclosures (cabinets) fibres

160 137 137 139 137 137 14 136 135 59 118 146 146 131 190 56 154 47 119 3 7 163 206 191 135 138 22 206 206 149 12 69 4 125 174 126 125 124 145 126 194 199 174 184 123 128

138

156

137 13 204

135 206

183

199

137 208

138

77

174

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162

228 Index Terms Applications (Cont.) flame retardation floor coverings footwear furniture garden hammer finish heat sinks household laser marking lubrication magnetic mechanical reinforcement microwave heating non-aesthetic pipe insulation plasterboard radiation absorption sacks scratch cards sheet laminate solar transmission sports goods substrate textiles thermal conductivity toys tubs UV protection wear reduction Approved Supply List Area by BET method specific surface water covering Aspect ratio Attenuation

Links 199 123 123 174 123 174 194 123 197 196 198 179 180 4 194 193 200 125 193 128 194 124 134 128 194 123 128 196 196 203 42 42 41 39 189

90

171

187

B Bags carrier garbage Ball mill Barrier gas moisture Barrier coatings Bearings

126 126 11 175 196 196 199 196

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194

195

229 Index Terms BET method Binder Blow moulding parison parison pleating Blown film pinholing Bonding pipes Brightness Bronze gold silver true Building products

Links 42 4 84 84 84 110 115 115 169 182 182 63 7 4 8 4 4 131

12

13

19

126

181

183

186

191

171 152 187

157 193

161

162

163

C Carbon black Carcinogens Carriers acrylics aldehydes coumarone-indene fatty alcohol glycol hydrocarbons ketones polypropylene glycol resins surfactant CAS see Chemical Abstracts Service Cavity additional Cerium Chemical Abstracts Service Chemical resistance Chemical Hazard Information & Packaging for Supply Regulations CHIP ’98 Chromium Chromium oxides Co-extrusion Coating advantages of anticorrosive applications

115 205 138 59 59 59 140 59 59 59 138 59 138

94 137 203 43 203 203 137 136 115 63 168 63 162 145

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230 Index Terms Coating (Cont.) barrier bridges chemical plant curtain dispersion EB cured fabric in-mould (see paintless film) incorporation knife leafing leathercloth light cluster metal on non-metallic substrates non-leafing one pack one pack UV cured oxide paint plastisol powder (see also powder coating) reflective road markings roof solvent-based sprayed storage tanks substrate preparation two pack UV cured water-based Coating formulation Coatings Cold welding Colour bleed concentrate depth detachment Fantasy measurement retention saturation shift uniformity

Links 162 163 163 157 133 139 162 133 157 133 162 193 29 133 139 162 135 145 162 167 193 193 163 134 65 163 131 139 139 135 132 139 5 7 79 59 65 9 60 44 115 76 70 64

168

161

162 15 44

167 69

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231 Index Terms Conductivity electrical thermal Constant Containers active cosmetic passive rotable toiletry Continuous milling Copper Cornflakes Corona discharge Corrosion resistance Cost coatings formulation mass pigmentation Coupling agents Crayons Credit cards Curing EB UV Cut foil glitters

Links 183 183 93 114 174 199 180 181 180 128 180 181 128 13 7 8 38 106 131 146 137 126 7 67 67 75 67 191 163 158 139 174 139 139 25

190 153

203

204

D Data Sheets Directive Deformation Degradation plasticiser resistance to Degreasing Delivery forms compound dry masterbatch dry powder liquid masterbatch paste plasticiser dispersion granules Density Dermatitis Di-isodecyl phthalate Di-octyl phthalate Dipping

203 134 78 39 131 5 60 59 55 59 55 56 57 3 205 57 57 64

55

158 207

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207

208

232 Index Terms Dispersibility Dispersion energy plasticiser resin Disposal paint residues Distinctiveness of image DOI see distinctiveness of image Dosing Dry milling Drying Ductility Dusting Dyestuffs

Links 69 56 133 56 113 56 66 50 60 11 170 3 58 108

114

86 173

E Effluent EINECS Elastomers Electrical conductivity Electro-Magnetic Shielding see EMI shielding Electrostatic spraying Elements metallic Elongation EMI shielding design effectiveness mass pigmentation measurement origin polymers principles Emulsions Encapsulation Environmental hazards (see also Hazards) Equipment metering Esters Ethyl acetate Ethylene-vinyl acetate copolymer EU Scientific Committee for Toxicity, Ecotoxicity

207 203 88 183

191

173 3 88 184 185 187 191 185 185 186 185 138 137 158

185

187

191

191

203

58 47 56 78 205

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233 Index Terms European Inventory of Existing Chemical Substances Explosion risk see Hazards Explosive potential see Hazards Extrusion biaxial stretching mandrel spider’s legs twin screw

Links 203

110 84 84 114

F Fantasy Colour Faraday Cage Fashion Fastness heat light Fibreglass Fibres metal Films crop ripening lidding silage wrapping weed suppression Fire Fire extinguishing agent Fire fighting aluminium gold bronze Flakes aluminium broken coated coloured surface concentration copper cut foil dry electrical charging folded glass glitter inertia of iron mica morphology nickel

60 169 3 42 42 120 3 183 180 125 180 125 125 169 214 214 214 215 134 196 109 172 25 106 196 25 113 174 109 199 25 91 107 197 3 153

126

183

172 199

174 206

196

52

79

128

184

196

210

210

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234 Index Terms Flakes (Cont.) orientation rapidly solidified silver-coated nickel size stainless steel with coloured surfaces with pre-formed colorants zinc Flame retardation Flame treatment Flammability Flip Flop Flow line definition origin Fluidised bed Foaming Foil Food colorant Foodstuffs Formaldehyde Formulations flake concentration in deep shades low shear forces in metallic/organic pigment combinations in spherical pigments in

Links 63 23 30 106 153 78 28 175 199 131 209 45 45 5 81 81 173 156 4 206 180 77 105 106 108 109

110

174

64 81

158

11

111

132

108 108

G Gate pin size strip Gel coat Glitter flakes Glycol ethers Gold Gold bronze Gold leaf Granules graphite conductive nickel-coated handling of nickel-coated Greases

89 89 93 93 119 52 48 3 4 204 4 57 190 75 189 196

93

10 12 208

184 13 209

113

146

19

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175

203

235 Index Terms GRP spraying of

Links 120

H Hydrolux Hall process Handling aluminium Hazards environmental explosion explosion potential fire Health and safety aluminium coatings copper gold bronze mass pigmentation Hiding power History Household goods Hydrocarbons Hydrogen

136 14 212 213 158 5 209 169 203 205 211 206 206 211 70 4 123 119 135

22

23

203 12

212 169

204

210

209

209

I Inhibition Injection moulding tool design Inks aluminium bronze replacement flexographic gold gold bronze gravure lithographic non-leafing one pack UV cured paste petroleum distillate polyolefin substrates screen security silver solvent-based water-based water-based leafing

136 85 154 155 158 155 159 155 155 154 157 161 154 154 161 162 157 155 154 156 156

156 156

157 157

159 159

160 160

161 160 157 157 155

157 155 160

159

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161

236 Index Terms Iron Iron oxide micaceous Iron particles Isoelectric point

Links 7 9 199 206 199 198 138

204 209

207

208

48

170

199

J Jetting

84

K Ketones

119

L LALLS Laminates (see also Simulants, mineral) Laser marking Lead Leafing measurement of surface tension in value Leathercloth Legislation electromagnetic emissions environmental health and safety landfill pollution prevention Light exclusion scattering Liquid monomers Localised mould heating Lubricants

35 180 197 200 13 48 48 48 129 185 185 66 210 66 211 192 47 139 98 5

47

162

47

M Magix Magnetism Malleability Manufacturing Marine pollutants Mass pigmentation general techniques interrelationship optimisation Masterbatch dry liquid Maximum Exposure Standard

60 43 3 11 212 63 105 105 105 57 59 59 207

7 168 69

105

59

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237 Index Terms Measurement of colour leafing particle size specific gravity Mechanical properties Meld line Metal fume fever Metal pigmented coatings Metallisation vacuum Metalure Metasheen Meterability Methyl ethyl ketone MFI Mica nickel-coated Microwave browning heating MIE see minimum explosive energy Milling ball continuous dry Mills ball sand Mineral oil Minimum explosive energy Molybdenum Molybdic acid Mould tool design of gates runners sprues texturing Moulding annealing biaxial stretching blow blown containers blown film bright surface fountain flow in-mould decoration

Links 44 48 35 41 71 82 206 131 65 25 25 113 56 108 3 189 180 180

5 13 11 133 133 133 57 175 137 29

11

92 92 92 92 94 85 110 115 116 115 98 90 99

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238 Index Terms Moulding (Cont.) induction heating of injection localised mould heating moving boundary multi-coloured multiple live feed paint-less film moulding (PFM) parison parison pleating pinholing post finishing by punching reversible transient flow rotational stratification tool design voids in Mylar

Links 98 55 98 97 97 95 117 84 84 115 99 97 118 110 85 95 25

127

111

N Nickel carbon fibre carbonyl conductive flake dendritic powder filamentary itch silver-coated spherical Nitro salicylic acid Non-colouristic applications Non-leafing Non-volatile content

7 192 20 188 188 20 207 189 21 136 179 13 18

8

20

187

192

192

194

192 191 191

47

170

O Occupational exposure standards Odour Oleic acid Opacity Orientation biaxial stretching flake sphere weld line Overspray Oxidation Oxidising agents

204 134 14 3 71 116 63 78 87 167 14 209

154 45 149

70 193

171

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204

207

239 Index Terms

Links

P pH see Aluminium, gas generation Packaging corrugated board disposable food gift wrap Paints aerosol automotive test system barrier brushing conductive dry fast drying gold-bronze hammer heat resistant industrial industrial stoving metal miscellaneous OEM refinish silver solvent-based water reducible water-based water-based acrylic Parison Particle size distribution measurement shape Passivation aluminium cerium gallate gold bronze silica specificity Pastes filler Pewter Phosphorus Picture framing

153 160 181 192 153 146 153 146 150 163 148 186 167 151 148 146 151 150 147 7 153 146 146 147 146 150 148 151 117 35 35 35 38 56 135 139 139 139 139 138 55 163 9 137 169

156

160

181

192

147 163 148

148 167 152

150

151

152 147 151

147 147 150

151

111 36

132

135

139

163 153 171

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152

240 Index Terms Pigment aggregates characteristics colour-variable compatibility compounding concentration concentration in coatings dendritic dispersion disposal effect glitter flakes handling incorporation in polymers inorganic loading mica mineral simulant morphology nickel-coated mica nickel-coated carbon fibre nickel-coated graphite orientation pearlescent pearl simulants plasticiser dispersions quality separation spherical storage water-based aluminium zinc Pipes plastic Plasticiser Plastisols Polishing Polyacetal Polybutylene terephthalate Polyester-triglycidyl isocyanurate Polyethylene terephthalate Polymeric media

Links 56 128 114 35 28 77 60 71 128 132 187 113 212 79 126 52 203 212 109 3 71 92 128 35 30 30 30 71 89 3 38 127 56 110 160 22 78 212 56 135 182 205 57 11 77 118 168 24 4

129 22

135

160

108

194

162

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241 Index Terms Polymers ABS cellulose acetate propionate compatibility decomposition dynamic melt EMI shielding fibre reinforced glass filled glass reinforced plastic mass pigmented melt flow index molecular weight multiphase Nylon 6 pellets polyamide polycarbonate polymethyl methacrylate polypropylene polystyrene PP/EPDM blends PP/HDPE/EPDM blends pre-drying processing polyvinyl chloride PTFE relaxation time screening shear minimisation single stage styrene acrylonitrile copolymer styrene-maleic temperature thermosetting transparency virgin viscosity Pot life Powder coating application bonding dry blend encapsulation formulation handling

Links 73 85 77 77 94 186 90 87 119 197 108 138 87 86 114 74 86 86 72 73 88 88 70 112 74 173 86 75 109 66 86 137 65 120 107 113 107 140 167 173 169 169 172 168 175

86

162

168

171

173

175

173

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242 Index Terms Powder coating (Cont.) manufacture metals safety Printing gravure screen Processing air entrapment chemical modification co-extrusion coloration in situ conversion double cone blender extrusion gas phase Hall in-mould decoration injection moulding laser marking melt spinning milling multi stage rapid solidification rotational moulding spinning disc stamping thermoforming vacuum deposition vacuum forming Properties adhesion barrier brightness colour corrosion resistance distinctiveness of image gassing impact strength interrelationships mechanical physical sparkle tensile strength thermal expansion visual weight saving Propyl acetate

Links 168 173 175 153 132 132 70 85 26 117 27 113 69 117 137 14 118 114 197 195 75 66 23 118 23 11 118 24 118 126 145 117 44 44 126 50 135 71 76 4 41 50 71 89 43 126 56

86 118

15

22

23

118

132 175

71

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243 Index Terms Protective equipment PTFE Poly vinyl chloride see Polymers

Links 208 173

R Radiation absorption Radiation sources Rapidly solidified flakes Receptors Recycling Reflection heat light Resins acrylic aldehyde alkyd epoxy film formers ketone phenolic polyester Resinous media Resistance abrasion chemical corrosion degradation heat humidity impact stone chip water weather Risk phrases Runner hot size and location Rusting

200 185 23 180 66 193 193 138 134 134 138 138 154 134 138 138 4 168 168 43 7 171 151 168 168 174 136 174 203

117

212

154 154

154

174 174 168

93 93 9

213

207 208 209 203 97 95

96

175 187

S Safety aluminium gold bronze Safety Phrases SCOREX SCORIM

97

98

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244 Index Terms SCORTEC venting Scratch cards Screening Seed Sheet leathercloth Shelf life Shielding see EMI shielding Silanes Silica fumed Silicon Silver Silver dollars SILVET® Simulants mineral pearl Sink marks Smart car Solvents acetate alcohols aliphatic aromatic ketone polar release toluene xylene Sparkle Specific gravity Specific surface area Spheres aluminium silver-coated nickel Spraying electrostatic Stabiliser lead tin Stainless steel austenitic powders UNS-S 31603 Stamping

Links 94 97 192 193 4 5 132 161 161 18

17

146 170 137 3 18 57 128 127 95 127 133 133 133 134 134 133 133 152 133 133 17 7 42 22 30 167 167 77 77 7 9 196 9 4

7 38 109

10 106

19

184

9 43

21

43

162

22 11

153

154 154 154

50 41

173 173

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163

186

245 Index Terms Standards ASTM C-177 ASTM D480-88 DIN 53196 DIN 53217 DIN 55923 Star-Brite Stearic acid Stoneware Storage Strength tensile Substances Directive Surface uniformity Susceptor

Links 195 48 36 41 48 25 14 128 213 88 203 41 180

T Teletronics Test methods particle size Thermal conductivity Thermal expansion Time Weighted Average Tin Tint strength Titanates Titanium Toiletries Toluene Toxicity Travel Tribo Tufflakes

131 36 194 75 207 10 70 146 10 126 47 204 45 169 19

145

137

171

174

U UN Recommendations on the Transport of Dangerous Goods Urethane acrylates UV protection

208 161 199

V Vacuum deposition Vacuum metallisation Visual properties VOC see volatile organic compounds Volatile organic compounds

24 65 43

153

139

161

167

211

W Water covering area Weight saving

41 179

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246 Index Terms Weld line crack propagation definition origin strength vanishing angle vanishing point wall slip Wetting Wetting agents White spirit

Links 5 81 88 81 81 87 88 87 86 81 71 79 47 56

82

86

22 207

139 208

X X-rays

200

Z Zinc Zirconates

7 199 146

9 204

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162

175

186