Investigation of factors influencing the cationic polymerization of epoxy resins

E-beam-induced polymerization of epoxies proceeds via a cationic mechanism in the presence of an appropriate photoinitiator. Despite good thermal properties and significant processing advantages, epoxy-based composites manufactured using e-beam-curable systems suffer from low compressive strength, poor interlaminar shear strength, and low fracture toughness. A detailed understanding of the reaction mechanisms of e-beam-induced cationic polymerization as it pertains to network formation is required to properly address the shortcomings associated with e-beam-curable systems. This work investigated the effect of hydroxyl-containing materials on e-beam-induced cationic polymerization of epoxy resins. Such hydroxyl moieties can be found as water, part of the monomer, the reaction products, or additives in the form of alcohols. Two types of experiments were conducted: (1) an investigation of the e-beam-induced cationic reaction of the model monoepoxy system, phenyl glycidyl ether (PGE) in the presence of a primary aliphatic alcohol, 2-cyclohexyl ethyl alcohol (CHEA) and phenol was conducted to determine relative reaction rates; and (2) an investigation of the effect of CHEA and phenol on the network formation of diglycidyl ether of bisphenol A (DGEBA) (Epon 825). The alcohols were found to take part in the cationic polymerization of epoxies, with the aliphatic hydroxyl group being incorporated into the polymer more readily than the phenolic hydroxyl counterpart. Furthermore, it was determined that the high reactivity of the hydroxyl group disrupts the development of a crosslinked network in the Epon 825 system and that the degree to which this occurs depends on the intrinsic reactivity of the alcohol This suggests that the potential exists to tailor network formation in such systems based on the judicious selection of monomers.