Synthesis of silane-functionalized polyols from natural rubber by thiol-ene click reaction and preparation of waterborne polyurethane films

Abstract

A new bio-polyol silane-functionalized hydroxyl telechelic natural rubber (SHTNR) was successfully synthesized from hydroxyl telechelic natural rubber (HTNR) with molecular weight 3500 g.mol⁻¹ using (3-mercaptopropyl) trimethoxysilane (MPTMS) and Darocur1173 as photoinitiator, via thiol-ene click reaction. The photoinitiator content, reaction time, and the MPTMS content for the SHTNR synthesis were studied. The chemical structure of SHTNR was confirmed by FTIR, ¹H NMR, ¹³C NMR, and ²⁹Si NMR spectroscopy. The kinetics of thiol-ene click reactions were monitored by ¹H NMR. The percentage of double bond conversion and the molecular weight of SHTNR increased with photoinitiator content, with reaction time and depended on MPTMS amount. Waterborne polyurethane (WPU) was synthesized from HTNR by emulsion polymerization. In addition, a series of SHTNR were used as a functional bio-polyol in the preparation of hybrid waterborne polyurethane (HWPU) for comparison with WPU, giving stable dispersions and a total solid content of about 15 %. The particle size and the negative zeta potential values increased with the degree of silane functionalization in the HWPU, with a shelf-life of less than 3 months. The shape and morphology of WPU and a series of HWPU particles were observed by Transmission Electron Microscopy, an average diameter of 180 nm was measured. The HWPU films exhibited better tensile strength and thermal stability than WPU ones. Moreover, the activation energy of the degradation increased with increasing degree of silane functionalization in HWPU film. In addition, the incorporation of silanol group in HWPU films was confirmed by SEM-EDX spectroscopy. The experimental results demonstrated the benefit of using natural rubber from renewable resources and thiol-ene reaction to form a new bio-polyol to produce hybrid polyurethane dispersions that could serve as an alternative in green coating applications.