Development of polymeric anti-reflective coatings for polymer substrates

Abstract

The most frustrating thing about owning a cell phone or digital camera, is not being able to view the screen while outside because of reflected sunlight. This research evaluated technologies for solving this problem at a Iow cost to consumers. The current metal halide anti-reflective coatings are not easily coated onto plastic substrates due to the high temperatures required for the coating process. Modifications have been made to the process to allow for metal halide coatings on plastic materials, but are still not satisfactory. Another disadvantage to using metal halide and metal oxide coatings on plastic substrates is the difference in thermal expansion, resulting in cracking and delamination of the coatings. Ideally the coating needs to have a thermal coefficient similar to that of the substrate. This research has investigated a new class of materials which can be used as an antireflective coating. A fluoropolymer, Cytop™, developed by Asahi Glass Ltd has a refractive index of 1.34 and a coefficient of thermal expansion in the same order of magnitude as that of the plastic substrate. Cytop™ proved itself to be a successful single layer anti-reflective coating. However, the drawback is that it is inherently soft and is easily scratched and abraded from the surface of the substrate. Another fluoropolymer, Teflon AF from DuPont, has also been studied. Fivestigations focused on solving the problem of scratch and abrasion resistance, many different substrate coatings and treatments being evaluated. A fluoroacrylate, Zonyl TA-N, was investigated as a possible adhesion promoter between the substrate and coating. Two main hard coatings were used; methacryloxypropyltrimethoxysilane (MAPTMS) and isobornyl methacrylate. These improved the abrasion resistance of the substrate but did not improve the abrasion resistance of the anti-reflective coating. This was believed to be due to the Iack of bonding between the hard coat and the substrate. These coatings were further modified to try to improve their adhesion to the substrate using 3-aminopropyltriethoxysilane (AMEO), and a Auorosilane (F8261). The coating systems that gave the overall best results for scratch and abrasion resistance were MAPTMS-AMEO/Cytop™ CTX 109A(E) and Zonyl TA-N/Cytop™ CTX 109A(E). MAPTSM-F8261/Cytop™ CTX 109A(E)/F8261 and MAPTMS-F8261/Cytop™ CTX 109A(E) coating systems also improved abrasion resistance but not as significantly as the previous two coatings. It is believed that this might be due to solvent still being present in the Cytop™ coating. The MAPTMS-AMEO/Cytop™ CTX 109A(E) and Zonyl TA-N/Cytop™ CTX 109A(E) coatings had aged and hence the solvent was completely removed. If the other coatings were given this same aging process, it is believed that they would also have the required abrasion resistance.