Preparation of polymer/silica hybrid hard coatings with enhanced hydrophobicity on plastic substrates

In this study, an easy method to increase hydrophobicity of the polymer/silica hybrid coating was demonstrated. UV-curable nano-sized colloidal silica was synthesized and surface-modified both by a coupling agent, 3-(trimethoxysilyl)propyl methacrylate (MSMA), and a capping agent, trimethyethoxysiliane (TMES). The formed particles were introduced into the poly(2-hydroxyethyl methacrylate) (PHEMA) matrix to yield PHEMA/silica hybrid hard coatings on plastic substrates via a UV-curing process. Differential scanning calorimetric (DSC) analyses of the hybrids indicated increases of the glass transition temperature (T_g) with increasing silica content in the hybrids; in general, an increase of 23 °C could be achieved for hybrids doped with 15 wt.% silica. Thermal decomposition temperature (T_d), as measured by the thermal gravimetric analyzer (TGA), was found to depend on the silica content in a trend similar to that on T_g. Specifically, a large increase of 25 °C was observed when the sample contained 15 wt.% silica. The pencil hardness of the PHEMA/silica hybrids coated on poly(methyl methacrylate) (PMMA) substrates can reach 5H, in comparison with 2H for pure PHEMA coating. Abrasion resistance was enhanced when silica nanoparticles were incorporated. Furthermore, due to the incorporation of TMES, hydrophobicity of the hybrid coating increased considerably as the TMES content was increased. In the extreme case, a hard surface with a water contact angle (92°) has been obtained.