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 (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