Synthesis and characterization of low-viscosity bio-based styrene alternatives for bisphenol A vinyl ester thermosetting resins

Methacrylated plant oils have been investigated as potential styrene replacements in high-performance thermosetting applications due to the environmental and health hazards presented by styrene. Carvacrol, thymol, and menthol derived from oregano, thyme, and peppermint, respectively, are unique bio-based candidates for methacrylation due to their aromatic and cycloaliphatic structures. When methacrylated, these plant oils exhibited low viscosities-6, 8, and 5 cP for carvacryl methacrylate (CMA), thymyl methacrylate (TMA), and menthyl methacrylate (MMA), respectively. When used as reactive diluents for the diglycidyl ether of bisphenol A (DGEBA)-based vinyl esters (VE), they produced viscosities greater than styrene blends but still within the operating window for standard liquid molding processes. CMA, TMA, and MMA were much less volatile than styrene, each losing less than 5% of its mass over 10 h at 30 °C. Free radical polymerization of VE blends prepared with these plant-based diluents yielded glass-transition temperatures (T_g's) in the range of 132−150 °C (tan δ). The MMA blend had the highest T_g (150 °C), which was slightly higher than that of the styrene control, and showed an enhanced room temperature modulus (E′_RT) of 3.06 GPa. This work demonstrates that CMA, TMA, and MMA are potential candidates to replace styrene as reactive diluent VE systems without sacrificing key aspects of resin performance.