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 degrees 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 degrees 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 degrees 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.