Preparation of anionic waterborne polyurethane composites with silica from rice husk ash

Anionic waterborne polyurethane (aWPU) composites were fabricated using hydoxytelechelic epoxidized natural rubber (HTENR) and silica, extracted from rice husk ash (RHA). Silica was then modified by poly(diallyldimethyl ammonium chloride) (PDDA), a cationic polymer. The PDDA content attached to the modified particles was about 4.43%. PDDA-SiO2 particles were smaller and the size distribution narrower than pristine SiO2 due to deagglomeration. The chemical structure of PDDA-SiO2 was confirmed by Fourier infrared spectroscopy (FTIR). PDDA-SiO2 particles were then loaded in aWPU at contents of 0.5 to 5.0% wt. using self-assembly by electrostatic interaction and hydrogen bonding between PDDA-SiO2 particle and aWPU. At PDDA-SiO2 loadings of 0.5 to 1.5% wt., aWPU/PDDA-SiO2 composites formed homogenous emulsions and transparent films. PDDA-SiO2 loadings of 3.0% wt. and 5.0% wt. produced PDDA-SiO2 agglomerations and filler clusters were seen in the obtained films due to poor dispersion of PDDA-SiO2 particles in the WPU matrix. PDDA-SiO2 incorporation increased Tg and raised the degradation temperature at 10% weight loss from 276 °C to 300 °C but reduced tensile strength compared to neat aWPU. Due to the insufficient interaction of aWPU and PDDA-SiO2 in case of physical mixing, sedimentation of PDDA-SiO2 particles in the PU matrix was observed but with PDDA modification the particles were uniformly embedded in nano-scale clusters.