Study on the species and stability of free radicals in bisphenol-A based epoxy resin induced by irradiation up to 1000 kGy
AUTHORS: Ji, Zhenyan; Zhang, Fan; Chen, Hongbing; Shen, Rongfang; He, Weirong; Xing, Zhe; Wu, Guozhong
Epoxy resins are radiation-resistant polymers that are widely employed in aerospace and nuclear facilities; however, free radicals formed by ionizing radiation may lead to changes in their physicochemical properties. In this study, bisphenol-A based epoxy resin samples in both air and vacuum environments were subjected to gamma irradiation with doses up to 1000 kGy and the generated free radicals were investigated by electron spin resonance (ESR). Four radicals, namely cyclohexadienyl-type radicals, phenoxy radicals, peroxy radicals, and alkyl radicals, were formed in epoxy resin in both environments after gamma irradiation. The ESR spectra showed that the free radicals yield produced in the air was significantly larger than that produced in a vacuum. The radical concentration of the EP increased until a dose of 500 kGy and approached saturation upon increasing the dose to 1000 kGy. The ESR spectra of the EP annealed between 50 and 160 degrees C indicated that the radical concentration decreased with increasing temperature. The radicals in the EP samples irradiated in air disappeared at an annealing temperature of 130 degrees C, close to the glass transition temperature of the EP. The radicals in the EP irradiated in vacuum still remained even at 160 degrees C, which can be attributed to the stable phenoxy radicals. The free radical reaction mechanism was also developed for the bisphenol-A based EP irradiated by gamma-ray in different atmospheres.