The Development of Epoxidized Hemp Oil Prepolymers for the Preparation of Thermoset Networks

Epoxy thermosets comprised of plant oils along with simple curing agents are sustainable and environmentally friendly polymers. The curing agent selected, and its compatibility with epoxy monomers, strongly affects the curing kinetics, the extent of curing, and the final properties of an epoxy polymers. The goal of this work is to expand the application of epoxidized oils in formulating biobased thermoset polymer systems. Epoxidized hemp oil (EHO) was produced with 8% oxirane oxygen content (OOC) after 24 hours using in situ generated performic acid. Two model curing agents-one aromatic (trimellitic anhydride, TMA) and one biobased nonaromatic (citric acid, CA)-with similar molecular weights were selected to study the cure behavior of EHO in acetone. Both curing agents are insoluble in EHO. The prepolymerization curing reaction behavior was monitored via the OOC, attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, and gel permeation chromatography. It was demonstrated that at 50 degrees C, the reaction of EHO with TMA was extremely fast to form esters of TMA, while the reaction of EHO with CA was slower and followed different pathways. The cured EHO/TMA epoxy network is rigid and has a high alpha relaxation temperature (T-alpha) of 89 degrees C, which is associated with the glass transition temperature (T-g), while the cured EHO/CA network system is semirigid with a T-alpha of 40 degrees C. In addition, TGA analysis showed that the EHO/TMA resin system represents a more homogenous structure compared to the EHO/CA system, as indicated by the presence of lower-temperature decompositions of citric acid derivatives.