Poly(ethylene glycol)-grafted silica nanoparticles for highly hydrophilic acrylic-based polyurethane coatings

A silane coupling agent bearing poly(ethylene glycol) (PEG) chains was synthesized using triethoxysilylpropyl isocyanate and excess PEG, with molecular weights of 400 or 1000 g/mol (PEG400 and PEG1000) and chemically bonded to SiO2 nanoparticles, to obtain PEG-modified SiO2 nanoparticles (SiO2-PEG) with pendant hydroxyl groups. The PEG modification was demonstrated by Fourier transformed infrared spectroscopy, thermogravimetric analysis, and through the dispersion behavior of SiO2-PEG nanoparticles in water. The SiO2-PEG nanoparticles were further incorporated into acrylic-based polyurethane (APU) coatings. Transparent and crack-free nanocomposite coatings with SiO2-PEG content up to 35 and 40 wt.% were achieved for SiO2-PEG400 and SiO2-PEG1000, respectively. The high quality of the product was owed to the improved compatibility of SiO2 nanoparticles with the APU matrix after PEG modification. Due to the rearrangement of hydrophilic/hydrophobic segments, a highly hydrophilic surface, with a water contact angle (WCA) as low as 38.7°, was attained for the SiO2-PEG embedded coatings after induction with water. Moreover, SiO2-PEG400 was more efficient in enhancing the surface hydrophilicity of the APU coatings than SiO2-PEG1000 at the same SiO2-PEG content. In contrast, pure APU coatings and the APU coatings containing unmodified SiO2 nanoparticles displayed WCAs of 68.2° and 70.1°, respectively. Outdoor exposure experiments showed that the APU coatings containing SiO2-PEG nanoparticles had excellent dirt-resistance.