Synthesis and Properties of Alkoxysilane Castor Oil and Their Polyurethane/Urea-Silica Hybrid Coating Films

This paper investigates the structural modifications of castor oil, a renewable resource, to develop functional organic inorganic hybrid coatings. A novel methodology has been developed to introduce hydrolyzable -Si-OCH3 groups in the castor oil backbone that has been used subsequently for the development of polyurethane/urea-silica hybrid coatings. The alkoxysilane functional castor oil (ASCO) was characterized by techniques such as H-1, C-13, and Si-29 nuclear magnetic resonance spectroscopy (NMR), Fourier transform infrared (FTIR) spectroscopy, gel permeation chromatography (GPC), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). The ASCO was further reacted with different ratios of isophorone diisocyanate (IPDI) to get an isocyanate-terminated hybrid polyurethane prepolymer that was cured under atmospheric moisture to get the desired coating films. The glass transition temperatures (Tg) of the hybrid networks were found to be in the range of 29-70 degrees C, and the water contact angles were in the range of 75 degrees-82 degrees. The Tg and hydrophobic character of the hybrid coating films found to increase with an increasing NCO/OH ratio. The thermo-mechanical, viscoelastic, swelling, morphological, and contact angle properties of these films were evaluated. The alkoxy silane-modified castor oil-based coatings have shown better mechanical and viscoelastic properties in comparison to the control (unmodified castor oil) coatings. This work provides an effective and promising way to prepare hydrolyzable silane functional castor oil for high performance hybrid coatings.