Examining the influence of carboxylic anhydride structures on the reaction kinetics and processing characteristics of an epoxy resin for wind turbine applications

The cure of a low molecular weight (approximate EEW = 184 g/mol), difunctional epoxy resin based on bisphenol A has been studied in the presence of three carboxylic anhydrides: 3- or 4-methyl-1,2,3,6-tetrahydrophthalic anhydride, 3- or 4-methyl-hexahydrophthalic anhydride, and methyl-3,6-endomethylene-1,2,3,6-tetrahydrophthalic anhydride, and a tertiary amine (Ancamine K54). The formulated blends display complex viscosities ranging from 36 to 58 mPa.s and at 75 °C, the blends take between 56 and 73 min to reach gelation, with the highest viscosity and longest gel time observed for the blend containing methyl-3,6-endomethylene-1,2,3,6-tetrahydrophthalic anhydride. Rate constants of 6.8 to 14 s⁻¹ at 75 °C and activation energies of 69 to 78 kJ/mol are determined using dynamic differential scanning calorimetry. Glass transition temperatures for the cured blends are similar, at 100 °C, with conversions of 83 to 89% observed. The cured blend containing methyl-3,6-endomethylene-1,2,3,6-tetrahydrophthalic anhydride displays the poorest thermal stability in terms of the onset of degradation, while yielding the highest char yield of the blends studied.