Castor oil-derived water-based polyurethane coatings: Structure manipulation for property enhancement

Water-based polyurethane coatings give a soft feeling for surface engineering for advanced wear- and friction-resisted purposes. In this work, we synthesized three types of polyols from castor oil applying short- and high-efficiency method for potential soft coating uses, as an appropriate alternative for methods that use a rotary evaporation and a vacuum oven in polyol synthesis. Castor oil was epoxidized in the presence of two types of catalysts, i.e. γ-alumina (ECOAl) and formic acid (ECOF). Although we could not detect unwelcome reactions taking place in the reaction chamber that should reduce the content of oxirane oxygen, the relative percentages of double bond conversion to oxiran for ECOAl and ECOF were theoretically 96% and 74%, respectively. Ring opening reaction of ECOAl was performed by two types of saponified castor oil utilizing Dean–Stark trap, an appropriate alternative for methods that use a rotary evaporation and a vacuum oven in polyol synthesis. The synthesized polyols were characterized by FTIR (ATR), gel permeation chromatography (GPC), differential scanning calorimetry (DSC), viscometer and OH number evaluation. Different weight percentages of the polyol samples having Tg values of ca. -6.5 °C and OH number of 6.2 were applied in synthesis of water-based polyurethane (WPU). It was found that WPU with 40 wt.% polyol has a minimum particle size and minimum poly dispersity index. WPU with 60 wt.% polyol exhibited the highest molecular weight, while it had the lowest viscosity, water absorption, glossiness, hardness, adhesion and Tg. By such structural manipulations we provided a basis for enhancement of properties of coatings for future works.