Invention starts with the identification of a problem. In this case, the problem is that glare from surface reflections can severely reduce the legibility of displays. The solution is likely obvious to all but very young children: apply a nanocoating to the surface of the display with a structure similar to that found on the surface of a moth’s eye.

Art Berman
Insight Media Consultant

Well, perhaps it is not all that obvious. In fact, it took a team lead by Dr. Frank Burmeister, Project Manager at the Institut für Werkstoffmechanik within the Fraunhofer Institute for Mechanics of Materials IWM (Freiburg, Germany) to develop the process.

Consider the moth. As the sun sets, the insect searches for food. While doing so, the moth must hide from predators and its presence must not be betrayed by reflections from the facets of its eyes. The point worth noting is this: the eyes of most insects shimmer but a moth’s eyes do not.

The reason for the lack of reflection is that there are microscopic structures on the eyes of moths. They are basically tiny protuberances that form a periodic structure on the surface of the eye. Just a few hundred nanometers tall, the protuberances are smaller than the wavelengths of light. The structures creates a gradual transition between the refractive indices of the air and the cornea and serve to suppress reflection.

Knowing of this reflection suppression mechanism, researchers have for years sought to duplicate this anti-reflective texture. The problems have been, first, that it is not easy to duplicate the structure and, second, that the resulting structures are quite fragile. That and the fact that, in order for the moth eye reflection suppressing effect to cut surface reflection down to less than 3%, the structure needs to be almost perfect.

Unfortunately, prior work demonstrated that all of the more obvious methods that could be used to impose the moth’s eye texture on a plastic surface had problems. For example, etching the texture into a plastic surface after molding resulted in a fragile, scratch prone finish.

An alternative is to replicate the desired nanostructure into the surface of the plastic during the component’s manufacturing process. In pursuing this approach, researchers discovered that etching the pattern into the mold itself often resulted in small pieces of the plastic sticking to the mold, causing imperfections in the pattern.

The Fraunhofer team chose to develop an improved version of this second approach. In the new approach, a hard material was used to coat the inside surfaces of the previously patterned injection mold. The thin coating reproduced the surface structure. When the viscous polymer melt was injected into the mold, it picked up the nano-structure pattern of the coating. The thin coating and thus the pattern stays with the polymer after the polymer solidifies.

As a separate, second step, the mold was filled with an ultra-thin layer of polyurethane. This material ran into every crevice and hardened like a two-component adhesive. This second layer is wipe and scratch-resistant and serves to enhance the durability of the nanostructures.

The end result of the new process is a plastic coating that can suppress reflective glare from all types of transparent plastic surfaces. Products to which this might be applied include televisions, cell phones, dashboard gauges, eyewear and panel covers.

Another feature of the process is that, because a second process step is not required, manufacturers can achieve a labor and thus a cost saving as compared to traditional anti-reflective coatings.

Fraunhofer hopes to further develop the technology by working in cooperation with industrial partners.

NOTE: Due to the Memorial Day holiday, there will be no Display Daily article on Monday, May 31.