Preparation of hydrophobic nanosilica-filled polyacrylate hard coatings on plastic substrates

Colloidal silica nanoparticles synthesized from tetraethoxysilane via a sol-gel process were surface-modified first by 3-(trimethoxysilyl)propyl methacrylate, and then by trimethylethoxysilane (TMES). The former agent acts both as a coupling agent and as a C=C provider, whereas the latter agent is used to prevent particle aggregation and to increase hydrophobicity of the coating. The modified silica particles were UV-cured together with the monomer, 2-hydroxyethyl methacrylate (2-HEMA), and the crosslinking agent, dipentaerythritol hexaacrylate (DPHA), to form highly transparent hard coatings on plastic (PMMA and PET) substrates. Both differential scanning calorimetric and thermal gravimetric analyses of the hybrid materials indicated enhanced thermal stability with respect to the neat HEMA-DPHA copolymer. Furthermore, due to the incorporation of TMES, hydrophobicity of the hybrid coating increased considerably with increasing modified silica content. In the extreme case, an antiabrasive hard coating (7H on PMMA) with a water contact angle of 99A degrees was obtained at the silica content of 15 wt%.