Surface Technology Metallic Effect Pigments Provide Flawlessly Shiny Plastic Surfaces

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Fraunhofer-Institut für angewandte Polymerforschung

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Metallic shiny plastic surfaces are trending—whether in automotive exterior and interior design, toys, or household appliances. However, injection-molded products have a drawback: they often display so-called flow lines—unsightly streaks and lines. A new method for producing metallic effect pigments is now set to change this.

For decades, experts have been researching how to optimize metallic effect pigments: so far, the conventionally used pigment particles with plate or flake geometry, which are incorporated into the plastic for the metallic effect, create visible flow lines during injection molding processing. A new method for producing metallic effect pigments, developed at the Technical University of Berlin (Germany)  and further optimized at the Fraunhofer Institute for Applied Polymer Research IAP, enables a flow line-free metallic effect on plastic surfaces—and without coating.
For this purpose, scientist Nils Demski at TU Berlin (Germany) developed tetrahedral particles. This geometry makes it possible to precisely control the degree of particle alignment in the plastic melt. Furthermore, both the formation of flow lines and the brightness flop, i.e., the change in brightness depending on the viewing angle, can be influenced. However, the production of these solid aluminum pigments with tetrahedral geometry is complex and associated with a high rejection rate.

Efficient Production through UV Imprint Lithography

Nils Demski, now project manager in the Polymer Composites PYCO research division at the Fraunhofer IAP, therefore adopted a new approach with his team: the production of tetrahedral metallic pigment particles based on UV imprint lithography. Their goal: to minimize pigment waste and simultaneously achieve high automatability through roll-to-roll processes. Instead of aluminum, the pigment particles are made of a UV-cured, subsequently metallized thermoset that meets thermal and mechanical requirements to retain its shape and optical properties during injection molding. The new process reduces material consumption and significantly lowers the rejection rate compared to conventional production methods. The corresponding patent is ready for licensing.
"With the new process, on the basis of which we are currently building a pilot plant for automated pigment production, we can produce metallic effect pigments with high shape accuracy and consistent quality. In addition, the size and, to some extent, the geometry of the pigment particles can be adjusted during the process," explains Demski.
Furthermore, the researchers aim for a monodisperse pigment with a novel tetrahedral particle shape, where all particles have the same geometry and size. This allows for very precisely adjustable optical effects. The production costs are comparable to those of PVD pigments, i.e., very thin flake pigments. In a joint project with BMW, the adhesion of the metallic particle coating was also improved. This led to increased resistance of the pigments to mechanical stress.

Uniform Effects with Reduced Environmental Impact

During the injection molding process, the novel pigment particles in the plastic exhibit a uniform, statistically isotropic orientation across the entire component. This results in flow-line-free and uniformly metallic-glossy surfaces, which, unlike conventional plate-shaped pigments, are not oriented depending on the flow direction. At the same time, energy and raw materials are saved, as the pigments can be produced more resource-efficiently than conventional aluminum flakes made through metal atomization followed by grinding and sieving.
The new production method for metallic effect pigments combines appealing aesthetics with stable and environmentally friendly processes. It is suitable wherever surface quality, cost efficiency, and sustainability are equally important.