Alkyd-urethane resins based on castor oil: Synthesis, characterization and coating properties

Abstract

Castor oil (CO) and dehydrated castor oil (DCO) were used for the synthesis of new alkyd-urethane resins via a two-stage process: synthesis of the alkyd resin flowed by urethanization. 1HNMR and FTIR analyses showed that the synthesized DCO and CO alkyd resins still have a large amount of hydroxyl groups that make then useful as polyols for the further urethane linkages. FTIR analyses confirmed the formation of the urethane bonds when the previous alkyd resins reacted with isophorone diisocyanate. The cured films from both alkyd-urethane resins exhibited similar hardness, adhesion, chemical and water resistance, and gloss. Gouge hardness and scratch hardness were 3H and 4H, respectively. The adhesion of both films was good. Cured films were resistant to water, xylene and H2SO4, but they were slightly affected by NaOH and strongly affected by acetone. The films exhibited significantly high gloss values (120.1 at 60°), associated with the urethane linkages. DCO resin showed lower drying times than those of CO resin and the addition of dryers diminished these times, indicating a strong contribution of double bonds on the drying. Acetone as solvent further improved the drying times due to a better mobility of the molecules. Drying of the resin is due to the formation of more urethane linkages andto the double-bond curing by air-oxidation. A higher synthesis temperature favors the formation of urethane linkages, making the resin more rigid and this hinders the final double-bond curing. Drying times significantly decreased by increasing the drying temperature from 25 to 100 °C. TGA indicated that the cured films were stable up to 220 °C.