Epoxy resins. I. The stability of the epoxy–trimethoxyboroxine system

The useful life of a material depends on its environmental exposure. The diglycidyl ether of bisphenol A (DGEBA) cured with trimethoxyboroxine (TMB) was evaluated under various aging conditions. For isothermal aging, the main factor controlling weight loss appeared to be related to the diffusion of the degradation products (E_act = 22.1 kcal/mole). Chemical decomposition kinetic parameters were obtained using vacuum thermogravimetric analysis (TGA) on powder samples. The thermal decomposition activation energy and the reaction order of cured DGEBA were 37.5 kcal/mole and 1.05, respectively. The hydrolytic aging of this material was also kinetically analyzed, and it was concluded that the weight change was controlled by both water diffusion into the sample and diffusion of hydrolysis products from the sample. During hydrolytic aging below the glass transition temperature, the specimens gained weight up to 0.05 g based on 1-g unaged cured resin and then leveled off. At higher temperatures, the specimens initially gained weight and then began to lose weight, reaching a constant weight gain. The activation energies for water diffusion into the cured resin are 19.5 kcal/mole at temperatures above T_g and 21.5 kcal/mole at temperatures below T_g. The main hydrolysis product was boric acid from reaction of the boroxine ring with water. The time-temperature superposition principle was used for the weight loss study on isothermal and isothermal hydrolytic aging. The scale factor in this approach was found to be the ratio of the diffusion coefficient at the temperature of interest to that at a reference temperature.