Optically Transparent Protective Coating for Plastics Using Dual Spray and Atmospheric Plasma Deposition

The successful processing of bilayer protective coatings on plastics using a combined spray and atmospheric plasma deposition method is shown. The base layer is a spray deposited coating with high adhesion using (3-glycidyloxypropyl) trimethoxysilane and tetrapropyl zirconate (TPOZ) precursors. The top dense layer is deposited by atmospheric plasma deposition with a tetraethyl orthosilicate precursor. The coating deposition rate, chemical composition, elastic modulus, hardness, and adhesion to poly(methyl methacrylate) (PMMA) substrates are investigated. The adhesion to the polymer substrate is found to decrease with increasing TPOZ content in the precursor solution, while the elastic modulus and hardness of the base layer increase. A silane surface pretreatment of the PMMA substrate is shown to significantly increase the coating adhesion. The adhesion of the optimized coating is so high that it forces the debond interface change from adhesive failure at the coating/PMMA interface to cohesive failure within the PMMA substrate. The combined bilayer structure exhibits a >90% transparency in the visible wavelengths, eightfold increase in adhesion energy and fourfold increase in Young's modulus compared to commercial sol-gel polysiloxane coatings. The approach provides a strategy for an unprecedented combination of adhesion and mechanical properties.