Plasticization effects of dihydroxyl soybean oil improve flexibilities of epoxy‐based films for coating applications

Novel bio‐based coating materials were developed through cationic ring‐opening photopolymerization of dihydroxyl soybean oil (DSO) with commercial epoxy monomers [i.e., epoxidized soybean oil (ESO) and 3,4‐epoxycyclohexylmethyl‐3,4‐epoxycyclohexanecarboxylate (ECHM). The ether cross‐linking and post‐polymerization of the polymeric network were observed using Fourier transform infrared spectroscopy. Thermal properties of the bio‐based coating materials and their copolymerization behaviors were examined using differential scanning calorimetry and a thermogravimetric analyzer. Cross‐link density and molecular weight between cross‐link were obtained from dynamic mechanical analysis. ECHM/DSO (1 : 1.43 weight ratio) films showed the highest elongation at break (49.2%) with a tensile strength of 13.7 MPa. After 2 months of storage, the elongation at break and tensile strength of films were 32% and 15.1 MPa, respectively. ESO/DSO films (w/w ratios of 1 : 0.1, 1 : 0.15, and 1 : 0.2) exhibited stable flexibility of 11–13% of elongation at break without significant reductions in tensile strength (2.5–4.4 MPa) during a 2‐month shelf life. Optical transparencies of the films were comparable to commercial glass and polymers, and water uptake properties (0.72% and 2.83%) were significantly low.