Chemical and physical changes during the homopolymerization of DGEBA

The curing reaction of the diglycidyl ether of bisphenol-A (DGEBA) with boron trifluoride monoethylamine (BF₃-MEA) was investigated. The purpose of the work was to model kinetics and network growth so that physical properties could be related to time, temperature and reactant concentrations. A tetrafunctional epoxy system with a simple reaction mechanism was chosen for this purpose. Results of experiments to confirm the BF₃-MEA reaction mechanism, to measure rheology before and during gelation are discussed. BF₃-MEA undergoes a rapid thermal conversion to HBF₄, which is the catalytic species. After the thermal conversion, there is a fast complex formation reaction between HBF₄ and the epoxy, which produces OH absorbances in the FTIR spectrum, and a carbocation from which the epoxy homopolymerization proceeds. The curing mechanism results in a non-free radical system where gel formation occurs at 7%, much less than the 33% expected for a tetrafunctional monomer which is not free-radical cured. An explanation of why the network develops as it does is presented. Significantly, the initial appearance of the gel does not result in a rise in viscosity; macroscopic gelation occurs at 34% conversion, close to the theoretically predicted value of 33%.