S Vala - Department of Polymer Eng and Color Technology., Amirkabir University of Technology
M Mohseni
A Karbasi

The basecoat/clear coat automotive paint systems have been used increasingly in recent years because of their attractive appearance and due to their excellent gloss retention they impart compared with the conventional mono coat systems. However, coating failures may usually occur upon exposure to aggressive environment such as car washing, sun and other damaging sources. It has been shown that the scratch and mar resistance of automotive clear coats can be improved by dispersing an exsitu prepared nano silica into the clear coat leading to increased protection from environmental factors without disturbing the optical clarity. However, the difficulty in this dispersion process as well as an increased viscosity are the main challenges to be overcome. This work was an attempt to enhance the scratch resistance of a solvent-based clear coat through an in situ production of silica particles using a solgel processing. Different ratios of TEOS and methacryloxypropyltrimethoxy silane (MEMO) with various hydrolysis conditions were mixed with the clear coat to obtain organic-inorganic hybrid systems. Vickers hardness, nano indentation coupled with AFM technique, as well as a simulated car washing study were preformed on the prepared samples. It was revealed that the coating composed of 2 wt% silica, based on the complete hydrolysis and condensation of the starting precursors, in which the ratio of TEOS:MEMO was 1:1, gave rise to fully transparent hybrid nano clear coats. Silica particles were shown to be well below 100 nm in diameter. The scratch behavior of the samples shown to be different in nano and macro scales and this was discussed based on the viscoelastic properties of the coatings and their ability to heal the scratched area after exposure to different indenter tips. The highest improvement in overall coating properties and the lowest gloss loss in a simulated car washing test were observed for the clear coat containing non-hydrolyzed TEOS and hydrolyzed MEMO.

Scratch resistant, Nano, Clearcoat