Allyl-modified epoxy networks that break and reorganize at high temperatures to avoid T reduction

Epoxy resins often age during high-temperature use, causing a drop in Tg and a number of safety problems. Currently, no specific reports addressing this Tg drop are available. Here, we proposed that the Tg drop could be prevented by allyl modification, and designed a series of epoxy monomers containing allyl to demonstrate this view. First, the influence of allyl groups on epoxy monomers was evaluated. It was found that allyl groups could reduce the viscosity of the monomers and the curing rate. These monomers were cured by DDM into resin (virgin resin) and treated at 285 °C for 6 h (treated resin) until the double bond was consumed. By comparison, it was found that although the introduction of allyl groups reduces the Tg of the virgin resin, it does not adversely affect the thermal degradation temperature (Td5%). The Td5% and Tg of the allyl-containing treated resin did not drop significantly like those of the treated resin without allyl. Specially, the Td5% and Tg of an allyl-rich resin increased by + 49 and + 289 °C, respectively. According to further study of the network generated by allyl groups, the allyl group in the para position (relative to the glycidyl ether group) would produce greater cross-linking, whereas the intramolecular ring formation in the ortho position results in an imperfect network of the new resin. Finally, through qualitative analysis of the entire resin network, we believe that this is a result of the high-temperature destruction of the epoxy network and the formation of an allylic network.