Synthesis and characterization of a novel epoxy resin based on cyclotriphosphazene and its thermal degradation and flammability performance

Abstract

The cyclotriphosphazene-based epoxy resin hexa-[4-(glycidyloxycarbonyl) phenoxy]cyclotriphosphazene defined as CTP-EP was successfully synthesized from hexachlorocyclotriphosphazene, p-hydroxybenzaldehyde and epichlorohydrin. Its chemical structure was characterized by Fourier transform infrared (FTIR) spectroscopy, 1H NMR, 13C NMR and 31P NMR spectroscopy. The prepared compound was cured with diamino diphenyl methane (DDM), diamino diphenyl sulfone (DDS) and m-phenylenediamine (mPDA), respectively. The flame retardant properties, thermal degradation behavior and flame retardant mechanism of the cured CTP-EP were investigated. The LOI values of all cured CTP-EP samples increased from 19.9% for cured diglycidyl ether of bisphenol-A (DGEBA) to above 30% and successfully passed UL-94 V-0 rating, which demonstrated that the prepared CTP-EP thermoset exhibited excellent flame retardancy. The dynamic mechanical thermal analysis (DMA) test demonstrated that the glass transition temperature (Tg) enhanced from 155 °C for the cured DGEBA to 167 °C for CTP-EP thermoset. The results of thermogravimetric analysis/infrared spectrometry (TG-IR) tests suggested that the phosphazene group and high aromatic content in CTP-EP thermoset stimulated the dehydration of the matrix at earlier degradation process and the formation of char layer, which retarded the release of pyrolysis gas and enhanced the thermal stability of the cured CTP-EP at high temperature. The cone calorimeter tests (cone) test revealed that heat release rate (HRR), total heat release (THR), smoke production rate (SPR) and total heat production (TSP) of the cured CTP-EP decreased significantly compared with the cured DGEBA. The scanning electron microscopy (SEM) results showed that the phosphazene groups in CTP-EP obviously stimulated the formation of the intumescent, compact and stable char layer, which enhanced the flame retardancy of the matrix during combustion. Consequently, the underlying materials were protected from further degradation and combustion during a fire and result in the efficient flame retardancy of CTP-EP thermoset.