Synthesis and properties of castor oil–based cationic waterborne polyurethane modified by epoxy resin

Waterborne polyurethane (WPU) has gained widespread attention. However, conventional linear WPU emulsions have lower mechanical properties, poor water resistance, and other disadvantages. In this study, polyurethane prepolymers containing branched chains were successfully synthesized by using diphenylmethane diisocyanate (MDI), polytetramethylene ether glycol (PTMG) and castor oil (CO) as the main raw materials. On this basis, epoxy resin was used to further modify the obtained polyurethane prepolymers, which results in an increase in mechanical properties, water resistance, and heat resistance due to the formation of a network cross-linked structure during the film formation. The type and addition amount of epoxy resin were determined by centrifugal stability and particle size of WPU emulsions, water absorption, and mechanical properties of WPU films. The results showed that the prepared epoxy resin–modified WPU (EWPU) emulsions have excellent centrifugal stability. If 7% epoxy resin was added after the polyurethane chain expansion, the corresponding EWPU films have good mechanical properties (the tensile strength of 28.7 MPa and the elongation at break of 379%), water resistance (water absorption of 4.21%), and the thermal stability due to the cross-linked structure in EWPU. This kind of EWPU emulsions may have promising applications in coating material, adhesives, and the textile industry.