ABSTRACT:Measurements of the rate of heating from a polymerization reaction of a popular Epoxy compound, EPON 828, with piperidine as the initiator were made until no further heat output was detectable. The results were obtained using a microcalorimeter at a temperature of 27.5°C. Three experiments were run where the amine equivalents relative to the epoxy equivalents were 0.034, 0.100, and 0.170. At these ratios, the amine was insufficient to bond with all the epoxide rings. Each amine bond produces an oxide ion, which in turn reacts with other epoxide rings creating another oxide ion. This propagation reaction continues until all the epoxy groups are opened and bonded. The heat rate from the epoxy reaction started at a value proportional to the amine content but then it accelerated rapidly by over an order of magnitude and then decelerated after about 50% completion. The heat rate profiles were found to fit a combination of three mechanisms operating during the polymerization. The first stage is generation of an oxide ion by amine reaction and by a hydrogen ion exchange between the unreacted amine and a hydroxyl group present in the EPON 828 molecule. The second stage is the rapid acceleration in heat rate due to a build up of an ether bond from the reaction of the oxide ion with an epoxy group. The peak heating rate occurred when the epoxy rings and ether bonds were equal. The last stage is a classic diffusion process, which is the only mechanism left to allow reaction after the other mechanisms have dissipated. The paper generates rate equations and discusses specific issues arising from the heat rate database.

The steep autocatalytic feature in a highly accurate DSC study of the heat rate from curing an epoxy resin with piperidine at 27.5 Deg C could not be explained using chemical kinetic power laws usually applied to curing epoxy resin products at higher temperatures. The theory of disordered conjugated organic semiconductors developed in the last decade has been applied to the observed heat rate data. Four heat rate sources have been identified to completely account for the experimental data. Two of the four sources generating 80% of the heat are consistent with mobility change of ion pairs indicating that the low temperature cure follows an organic semiconductor mechanism. It was shown that autocatalysis did not begin until about one fiftieth of the epoxy rings were opened (ignition). After ignition the heat rates of two propagation mechanisms grow exponentially. One charge transport mechanism generates a small heat rate but grows immediately after ignition due to an increase in ion pairs by the dopant (piperidine). The second mechanism appears later but becomes dominant, peaking at 50% completion, where the heat rate is about 50 times higher than the start of the first mechanism. The rate increase is attributed to localized energy sites that lower the LUMO level closer to the HOMO level of the monomer increasing the mobility (heat rate).

No abstract