Utilisation of rice bran soft wax for the development of water-based polyurethane dispersions for coating applications

Rice bran soft wax (RBSW) is a by-product obtained during the purification of rice bran wax (RBW). In the current work, RBSW has been analysed for its composition and subjected to chemical transformation through epoxidation and ring opening reaction. The resulting polyol carrying hydroxyl groups was characterised for its physicochemical properties and used as a starting material for the development of aqueous polyurethane dispersion. Isophorone diisocyanate was reacted with RBSW polyol and dimethylolpropionic acid as the internal emulsifier to prepare the prepolymer terminated with isocyanate groups. The chain extension of the prepolymers was carried out after dispersing them in water, reacting with adipic dihydrazide to give stable waterborne dispersions. NCO/OH ratio was maintained at 2.75 and 3.0 while the ionic content was varied from 4 to 7 in both the cases to get stable dispersions. The dispersions had a shelf life of 6 months, pH of 8, and a solid content of 20%. While the particle size and viscosity increased with increasing ionic content for an NCO/OH ratio of 2.75, both the values reduced with increasing ionic content for an NCO/OH ratio of 3.0. Due to phase mixing between the hard and soft segments, the glass transition temperature, tensile strength, and elongation were not significantly different for the compositions with fixed NCO/OH ratio and varying ionic content. However, a vivid drop in the storage modulus and degradation temperature and increase in water contact angle value were observed with increase in ionic content. With a view to utilise the PUDs for leather as a top coat, a comparative study of the properties with the commercial PUD sample was also carried out.