PU Anti-reflective Coating
Crystal clear plastics are frequently used for eyeglass lenses, displays and instrument covers. However, such plastics are not completely transparent, since some of the incident light rays are reflected at the surface. These reflections are often disturbing, and the resultant dazzle may, under certain circumstances, create hazardous situations. Such applications thus require special coatings to minimize reflection from the surface.
Until now, applying this anti-reflective coating involved a second step following the actual production of the part. The Fraunhofer Institute for Mechanics of Materials has now developed a new method to achieve an anti-reflective effect directly during injection molding. To this end, the injection molds received a special abrasion-resistant coating. To transfer the nanostructures produced to the surface of the molded part, a very thin film of polyurethane (PU) flows over the thermoplastic part. Because of the very low viscosity, the PU is able to replicate the microscopically fine surface structure. The PU surfaces are wipe- and scratch-resistant.
When designing the surface structure, the researchers mimicked the natural multi-faceted eye of moths: miniscule projections smaller than the wavelength of light on the surface their eyes form a periodic structure. This nanostructure creates a gradual transition between the index of refraction of air and that of the cornea, so that reflection of light is minimized. This provides a two-fold benefit for the moths: it increases their own visual acuity by allowing them to process almost all remaining light, and at the same time makes them less visible to predators by preventing reflection from their own eyes.