The Effect of Weathering on Viscoelastic and Mechanical Properties of Automotive Clearcoat Containing Hydrophobic Nano-Silica

Automotive coatings tolerate various weathering degradations during their service life, which leads to variations in all aspects of these coatings. The study of such variations during weathering; therefore, seems to be of utmost importance, especially for newly developed coatings. The results of our previous work showed that the incorporation of hydrophobic nano-silica (Nanobyk-3650) into an acrylic/melamine automotive clearcoat improved its mar and scratch resistance. In the present study, the effect of weathering on viscoelastic and mechanical properties of such clearcoats was investigated. Various clearcoat formulations containing 0, 2.5, 3.75, and 6 % hydrophobic nano-silica were prepared and exposed to artificially weathering conditions. To this end, an accelerated weathering tester was used to simulate natural weathering. The weathered clearcoats after various exposure times (0, 100, 500, 1000 hours) were characterized. Tensile and nanoindentation, being the most useful and reliable tests, were carried out to study mechanical and viscoelastic properties of the coating such as hardness, elastic modulus and elongation at break. DMTA and FTIR spectroscopy were carried out to follow variations in basic characteristics of the coatings in order to relate these to mechanical and visoelastic properties of such clearcoats. DMTA results, before exposure, showed that the nano silica containing clearcoats had lower cross-linking density compared to the pristine clearcoat, indicating incomplete curing processes for these coating. Also, coatings containing low loads of nano silica experienced post curing at early stages of the weathering process, leading to increased crosslinking and Young’s modulus, as proved by tensile tests. Hardness and elastic recovery of the coatings were also increased during such weathering tests. In addition, the FTIR results exhibited that nano silica particles could improve the weathering performance of the clearcoat especially at higher loads.

Keywords:

Weathering , Nano-silica , Automotive coatings , Nano-indentation , Mechanical and viscoelastic properties