DSM Research, Geleen, The Netherlands

Innovation at DSM: State of the Art Single Layer Anti-Reflective Coatings

Optical reflection is a fundamental phenomenon that occurs when light propagates across a boundary between two media with different refractive indices. In principal, there are two approaches to lower the reflection of a substrate: the application of an interference-type multiple layer coating system or the use of one single porous graded index coating. Although the application of a single-layer anti-reflective (AR) coating is the most efficient of both approaches, it is not yet commonly used in industry. The reason for this paradox is the high level of porosity required to obtain good AR properties with this technology. This is usually accompanied by a high surface roughness which causes poor abrasion resistance, a high degree of optical fouling and problems with cleaning.

DSM started a research program on single layer AR coatings in 2002 with the development of AR coatings for the display market. In the course of this program a UV cured organic/inorganic hybrid coating was developed (OptoclearTM). This technology is based on inorganic nanoparticles modified with polymerisable acrylate groups. A multi-functional acrylate is used for network formation. The necessary level of porosity in these coatings is achieved by the packing of the modified nanoparticles. Depending on the size and morphology of the particles, levels of porosity between 25% and 50% are achieved. The resulting coating shows a low rest reflection (£ 1.5%) and a good abrasion resistance.

In 2005, DSM decided to extend the research program with a project on AR coatings for glass substrates. This led to the development of ®claryl, DSM’s picture glass, which was launched in October 2007. This AR glass contains a state of the art single-layer anti-reflective coating and is produced in a straightforward dip-coating process. By controlling the balance of surface roughness and internal porosity DSM managed to overcome the typical drawbacks mentioned above providing a mechanically robust and easy-to-clean anti-reflective glass with a transmission of 98% or higher and a low level of rest reflection (£ 1.2%).

Further optimization of the balance of surface roughness and internal porosity provides the coating with an excellent outdoor stability. This makes the glass suitable for outdoor applications like solar cells and greenhouses. Studies towards further improvement of the outdoor stability of our coating are currently ongoing in our laboratories.