Preparation of sol–gel-based nanostructured hybrid coatings; part 1: morphological and mechanical studies

Abstract

Attempts have been made using sol–gel-based precursors to produce hybrid organic–inorganic clearcoats. To this end, a typical automotive acrylic/melamine clearcoat with tetramethyl ortosilicate (TEOS) and methacryoloxy propyltrimethoxysilane (MEMO) were used to obtain nanostructured silica clusters produced in situ embedded in the polymeric matrix. Microscopic techniques including scanning electron microscope (SEM), atomic force microscope (AFM), and transmission electron microscope (TEM) were utilized to investigate the morphology of coatings. The effect of each precursor on coating mechanical properties was also studied using dynamic mechanical thermal analysis (DMTA) as well as micro and nanoindentation techniques. It was found that using TEOS and MEMO (in non-hydrolyzed state), the mechanical properties of the resulting films were negatively influenced. The decreased hardness, lower T _g and cross-linking density, and reduced elastic modulus were observed with non-hydrolyzed precursors. In addition, the phase separation of organic and inorganic domains occurred in the presence of pristine sol–gel precursors. However, using hydrolyzed precursors (HTEOS and HMEMO), the mechanical properties were notably improved. While HTEOS resulted in an increase in coating T _g, and cross-linking density as well as improved elastic modulus and hardness, HMEMO caused an increase in coating hardness but lowered coating T _g and cross-linking density.