Bio-Based Phthalonitrile Bearing a Schiff Base Moiety as a High-Performance Modifier for Epoxy Resins: Curing Behavior and Thermal Properties

The thermal stability of epoxy resin (EP) constitutes one of the major bottlenecks restricting its high-temperature applications. Bio-based phthalonitrile resin (FUPN) bearing a Schiff base unit was utilized as a reactive modifier for a diglycidyl ether of bisphenol F (DGEBF)/diaminodiphenyl sulfone (DDS) system. The effects of FUPN on the thermal performance of cured epoxy networks were investigated. The glass transition temperature (T_g) of the cured thermosets increased from 161 °C for neat DGEBF/DDS (EP/FU0) to 231 °C for EP/FU50. The char yield at 800 °C under N2 atmosphere rose from 21.4 wt% to 57.7 wt%, while the maximum mass loss rate decreased from 14.3%/min to 3.7%/min. The char yield under air atmosphere at 800 °C was remarkably enhanced from 3.4 wt% to 31.5 wt%. The char residues were further characterized via XPS and Raman spectroscopy. The peak heat release rate and total heat release of the thermoset during combustion were reduced by 76.7% and 54.2%, respectively. Furthermore, the uncured mixture maintained a low viscosity (<2 Pa·s) at moderate temperatures even with a high loading of FUPN, which is highly favorable for practical engineering applications.