Synthesis and effect of hyperbranched (3-hydroxyphenyl) phosphate as a curing agent on the thermal and combustion behaviours of novolac epoxy resin

A novel type of hyperbranched (3-hydroxyphenyl) phosphate (HHPP) with high functionality as a curing agent of epoxy resins was synthesized and characterized by FTIR, ¹H NMR and vapor phase osmometry (VPO). The cured epoxy resin with HHPP possessed improved glass transition temperature. The thermostability and flame retardancy of O-cresol novolac epoxy resin cured with different contents of HHPP were investigated by thermogravimetric analysis (TGA), limiting oxygen index (LOI) and cone calorimetry. The obtained results show that the samples containing a higher percentage of HHPP exhibit relatively lower thermostability at lower temperature while higher thermostability at elevated temperature and more char was formed compared with those containing a lower percentage of HHPP. The LOI value increased from 22.0 to 30.0 when HHPP, instead of 1,3-dihydroxybenzene, was used as a curing agent. The 25 wt% addition of HHPP in the curing agent complex effectively decreased the heat release rate and improved the char yield to the content nearly similar as those of the epoxy resin cured with pure HHPP.