Structural, morphological and mechanical studies of UV cured organic-inorganic sol-gel based nanocomposite films on polycarbonate

Polycarbonate (PC) is one of the most usable thermoplastic engineering plastics because of its high toughness and transparency. However, low crystallinity of PC makes it low scratch resistant, resulting in reduced clarity during its service life. Use of highly cross-linked thermosetting polymeric coatings such as UV curable resins, is a good alternative to overcome this problem. Furthermore, modification of these coatings by inorganic nano particles will enhance the scratch resistance. A versatile chemistry based on sol-gel processing has recently shown that organic-inorganic hybrid coatings can be obtained in which the properties of organic phase can be combined with those of inorganics to produce high scratch resistant nanocomposite films on PC. In this study, nano silica networks prepared using hydrolysis and condensation of silane coupling agents were embedded into UV curable urethane acrylate oligomers. Tetraethyl orthosilicate (TEOS), as network former, and tri-methacryloxypropyl trimethoxy silane (MEMO), as network modifier, were utilized. The effect of molar ratio of TEOS/MEMO and inorganic/organic contents on morphology and mechanical properties of hybrid coatings were investigated. Increasing TEOS/MEMO and inorganic/organic ratios were found effective to enhance mechanical properties and scratch behaviour. Nanoindentation and nano scratch testings were conducted to correlate the interfacial interaction of phases to the mechanical performance of nano composite films. With increasing TEOS/MEMO molar ratio, as well as inorganic /organic weight ratio, penetration depth and width of scratch area decreased, indicating that scratch resistance increased. This behaviour corresponds to the increased elastic modulus due to the ability of coatings to plastically deform. This arised from an increase in glass transition temperature (Tg) as revealed by Dynamic Mechanical-Thermal Analysis (DMTA). FTIR analysis showed that TEOS with 4 alkoxide functional groups could form a more complete structure. MEMO with a longer organic chain having maximum 3 functionality creates an open structure. This more open structure gives rise to higher free volume and decreased Tg..

Keywords: Polycarbonate, Nano Composite Films, Sol-gel, Scratch Resistance