Coloured aluminiums provide sparkle for automotive finishes.

'Twinkle, twinkle, little star. How I wonder what you are... ' So starts one of our earliest childhood rhymes. It is not surprising. We like sparkle. It is appealing. It catches our eye and attracts us. We see it everywhere in our daily lives, in nature, in jewellery, in cosmetics, and even in

A rainbow of colours for automotive finishes has been available for a long time, at least since the distant days of 'any colour you want as long as it's black'. But sparkle in paint is a more modern creation and it is here to stay. Approximately 80% of all passenger cars in the United States of America now have metallic and pearlescent finishes, and there is no indication that this number will decrease. Independent consultant to the pigments and coatings industries Alexander W Kennedy charts a modern phenomenon.

Colour trends change from year to year and from vehicle to vehicle. A well-documented 1998 North American survey shows that the most popular colours are white and green. Colour popularity is a function of the category of the vehicle itself: compact car owners favour, in order, white, teal and green; luxury cars owners select green, white and gold. Trucks and minivans utilise white as the leading colour, with green a close second. These top choices will undoubtedly be replaced as personal tastes change and as car/truck designs are modified. However, the common bond for all finishes will continue to be metallic lustre.

Aluminium flakes

The sparkle in automotive finishes comes from aluminium-flake and/or mica-flake particles dispersed throughout the paint layer. These small, thin, flat particles reflect light and provide the luster that we are accustomed to seeing in new automotive finishes.

The exact shape of the aluminium flake or mica particle determines its visual contribution to the final coating. Properties such as particle size, thickness, flatness, and particle size distribution are key determining factors. The finer the flake, the smoother the appearance. The coarser the flake, the greater the sparkle effect.

Although a smoother appearance is generally more desirable, there is a trade-off. As the particles become smaller, the ultrafine end of the distribution tends to turn the colour to an unattractive, dirty grey. This contrasts to the white, bright and clean look of larger particles. This greying problem can be minimised by providing flake products with narrow particle size distributions.

Aluminium flake is produced by ball milling fine aluminium particles, either atomised powder or chopped foil, in a slurry with mineral spirits and a lubricant, for example, stearic acid for leafing grades of aluminium paste or oleic acid for the nonleafing grades required for automotive finishes. The parameters of the ball-milling operation, such as mill diameter, ball size, milling time, feed material and the ratio of aluminium to mineral spirits, all determine the exact particle size and shape. The final material, after filtering and screening, is adjusted to about 65% non-volatile content to provide manufacturers with a safe, easy-to-handle paste.

The aluminium flake that is used in the automotive paint market accounts for less that 20% of the total aluminium flake produced, but the value of this grade of aluminium is nearly 50% of the aluminium market's revenue. Automotive grade aluminiums are designed for specific effects and the sophisticated manufacturing processes that are required yield pigments with high added value.

As the market sought new and different finishes, it became flooded with a multitude of similar pigments. In recent years, paint companies found themselves with as many as 80 different grades of aluminium pastes for their formulations. It is quite subjective as to whether all of these grades can truly be distinguished from one another. Paint companies are now trying to reduce these inventories to a more manageable number and will therefore consider adapting new pigments only if they offer special value.

Mica flakes

Using mica-flake pigments has offered automotive stylists and designers new options. They can provide the smooth, velvety look often sought while also introducing colour to the flake. Mica flakes are inert, which allows them to be formulated into water-based paint without the problem of gassing.

The micaceous products have the inherent disadvantage of providing less hiding power than aluminium flake. With certain application methods used in the automotive industry and in certain paint formulations (especially water-based paint), the thickness of the applied wet film is quite limited. In such cases, the hiding power can become a most important property and often dictates which class of pigmentation is used.

The first of the micaceous-type pigments were actually made from fish scales. These provided the pearlescent luster that is often found in nature, such as butterfly wings, pearls, and oyster shells. Today, the micaceous pigments are produced from synthetic materials in much the same way as aluminium-flake pigments. Ball milling, very fine screening techniques, and precise classification are used. The colorant is added onto the particles in a wet process.

Coloured aluminium flakes

Coloured aluminium-flake pastes are a more recent development. In general, there are two methods of providing colour. One process is carried out in a fluid bed reactor at elevated temperatures, the colour being the results of a chemical reaction between aluminium flakes and iron carbonyl. Colour can be controlled by parameters such as reactor temperature and oxide-coating thickness. This is a costly process and it carries with it the explosion hazards associated with fine aluminium particles at elevated temperatures.

A more recent approach is to physically add pigmentation onto the surface of aluminium flakes using a wet process. The colorant can then be additionally bonded onto the surface by applying a cross-linking acrylic polymer. This affixes the colour and renders the product more stable.

High-chromaticity aluminium pastes

Deeply coloured aluminiums have been receiving a lot of attention by paint manufacturers and are now being commercialised. Since there are essentially no restrictions on the palette of colours that can be provided, there is a multitude of styling options to be explored and developed.

US Aluminium Inc has recently introduced a line of high-chromaticity aluminium pastes under the trade name of FireFlake. These deeply coloured pastes provide more styling options than were previously available with the pastel-shaded products.

The FireFlake pigments are manufactured by Showa Aluminium Powder of Nara, Japan. They are available in North America through US Aluminium and in Europe through their affiliate company, Shamrock Aluminium of Waterford, Ireland.

How coloured aluminiums function

An illustration of how these coloured aluminiums function can be seen by first analysing how colour and appearance is normally achieved by blending conventional pigments and standard, uncoloured aluminium paste. Yellow and blue transparent pigments in combination with aluminium will produce a green, sparkling finish. Imagine if you will the appearance of a coating based on a blend of blue-coloured and yellow-coloured aluminium. As expected, the finish is green, yet discrete speckles of both blue and yellow remain visible in bright sunlight. The physical blending of the coloured aluminiums on a macro-scale, as opposed to the blending of conventional pigments on a micro-scale, accounts for this striking effect.

Blending a standard pigment of one shade with a differently coloured aluminium produces a finish that exhibits a shift in colour as it is viewed from varying angles. This colour flop, or travel, can provide very attractive appearances. For example, a blend of a transparent red pigment with blue-coloured aluminium flake will produce a finish that has a strong blue appearance when viewed head on, but shifts to a red hue when seen at a low angle.

Another example of the effect attained by mixing pigments can be seen in a blend of yellow-coloured aluminium with a transparent blue. This finish will appear bright green when viewed head on and dark blue at an angle nearly parallel to the surface. This is because the contribution of the yellow aluminium is lessened when viewed at a low angle, so the blue dominates.

Benefits

The automotive industry is currently evaluating three-coat finishes which comprise a coloured basecoat, tinted clearcoat and a clearcoat. Such finishes offer deep, rich appearances not readily attainable with the standard basecoat/clearcoat method. This possibly will cost more now, but with constantly improving technology, a three-coat appearance will eventually be achieved in only two coats by using coloured aluminium.

Another benefit of using coloured aluminiums can be found in the theology of the finished paints. In order to achieve colour and hiding with certain conventional transparent pigments, the theology of the finished paint often becomes unworkable. If the colorant is affixed onto the aluminium particles, this problem is obviated.

Commercial applications of coloured aluminiums

For their 1998 model year, Harley-Davidson began offering new colours utilising FireFlake pigments. Harley, always known for its attractive and distinctive colours, has taken advantage of the effects that can be obtained when colorant is distributed throughout the finish.

General Motors will be offering a new colour to its customers on their 1999 model year trucks and vans. The colour, a deep rich green, takes advantage of both the visual contribution of a yellow-coloured aluminium and the improved theology of the paint itself. Many other colours, in all hues and intensities, have already been developed for future model years.

Economics also enter into the picture. Obviously, the process of applying colorant onto aluminium flake particles increases the manufacturing cost. Coloured aluminium pastes that exhibit high chromaticity can increase the pigment cost as much as 50%. The question then becomes one of value assigned to these new appearances. Are they worth the added premiums? Such issues will be resolved in the marketplace as these attractive, versatile aluminium pigments are evaluated and the new styling options are more fully developed. Whatever the exact outcome, we can be sure that our automotive finishes will continue to 'twinkle', just like that little star in our rhyme.

Author: Alexander W Kennedy is an independent consultant to the pigments and coatings industries. He has worked with both US Aluminum and Shamrock Aluminium for more than 10 years. His experience with these companies includes work in the automotive paint market and with a few other manufacturers of anti-corrosion coatings. Previously Mr kennedy spent 22 years with Diamond Shamrock, a large US chemical company.

For further information contact Tom Barry, Shamrock Aluminium, Grannagh, Waterford, Ireland. Tel: +353 51 855511. Fax: +353 51 875843.

This article has been updated from one which was first published in the US journal PCI.