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Aluminum in a blaze of color

Bayer MaterialScience AG : 08 November, 2004  (Company News)
Aluminum is a popular material in the home. The reasons: this metal is lightweight, robust, weather-resistant and suitable for many different applications.
The one drawback is that the surface of aluminum is perceived as coldly metallic. It is therefore difficult to make pure aluminum toys, light fittings, bicycle frames, household appliances or trays that satisfy the esthetic demands of modern consumers. In the case of kitchen units and doors, for example, consumers will only accept warm wood tones with a genuine-looking grain effect.

However, aluminum components can be given a highly decorative and individual surface finish with the aid of a simple and elegant process: the patented Decoral® system developed by the Italian company of the same name. In this process, a special powder coating formulated with raw materials supplied by Bayer MaterialScience AG is first applied on the aluminum and polymerized. The desired pattern is transferred from a pre-printed film onto the coated component which is heated again for this purpose. The process offers designers virtually unlimited scope for decorating three-dimensional aluminum components with wood grain, marble, granite or fantasy effects.

Only certain powder coatings can be used for the Decoral® process because they must be capable of withstanding reheating to 200 °C undamaged after polymerization. “They must have good coloring properties and must not become tacky at high temperatures otherwise the film could either not be removed at all or only with damage to the coating,” says Michael Grahl, a powder coating expert at Bayer MaterialScience. “Polyurethane powder coatings based on Crelan® hardener from Bayer MaterialScience have proven ideal for this application.”

This raw material is a uretdione hardener which crosslinks without cleavage or emissions. Powder coatings formulated with Crelan® are also characterized by their very good weather stability and resistance to solvents and other chemicals, making them perfect for application inside and outside the home. Depending on the decor to be applied, the powder coating is supplied as a pale or dark brown base coat, as a white coating or as a clearcoat.

Bayer MaterialScience also supplies the polyester raw materials that are used as the second component in polyurethane powder coatings. These are specially developed hydroxyfunctional polyesters from the Rucote® product family.

The pattern to be transferred onto the powder coating is printed positively on a film or fine paper. This is wrapped round the powder-coated component and pressed onto it with the aid of vacuum suction. The component is then heated for several minutes to about 200 °C in a special oven. At this temperature, the dyestuff changes directly from the solid to the gaseous phase without passing through the liquid phase. This process is called sublimation. The gaseous dyestuff then diffuses and is fixed into the heated powder coating. The direct change in state of the dyestuff is what ensures the very sharply contoured reproduction of the pattern on the powder coating. Indeed, it is sometimes so realistic that it is only possible to tell what the material actually is by touching it.

The consolidation of the Decoral® process is ensured by the Sinal-accredited Decoral System Laboratory, which monitors both the raw materials and the end products, and which is also responsible within Decoral® System for researching and improving the coating products used in the production cycles.
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