Facile synthesis of a P/N-containing heterocyclic compound for simultaneous enhancement of heat resistance, mechanical properties and fire safety of epoxy resin

Abstract

The demand for flame retardancy in epoxy thermosetting resins (EP) has become increasingly urgent, but the mechanical properties should not be damaged while ensuring flame-retardancy efficiency. To achieve this, a heterocyclic compound (BDP) containing reactive functional groups was successfully synthesized by 2,4-Diamino-6-phenyl-1,3,5-triazine (DPT), paraformaldehyde and 9, 10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) in a one-pot process. From the analysis of the curing behavior, the addition of BDP facilitated the curing reaction of EP by reducing the apparent activation energy (Ec). Dynamic mechanical analysis (DMA) and flexural tests indicated an increase in the glass transition temperature (Tg) and flexural strength of the flame-retardant epoxy resin (FREP) increased by 5.2% and 8.9% respectively, caused by an enhancement of intermolecular forces. As well, a V-0 classification in vertical combustion with a limiting oxygen index (LOI) of 35.2% was attained at a phosphorus content of merely 0.48 wt%. Cone calorimetric (CC) test demonstrated that total smoke production (TSP) and the peak heat release rate (PHRR) of EP-7.5/BDP were reduced by approximately 19.2% and 29.4% respectively in comparison to EP. This positive smoke inhibition and flame retardancy were caused by the blocking and dilution effect presented by the FREP during combustion.