Synthesis of a novel flame retardant based on cyclotriphosphazene and DOPO groups and its application in epoxy resins

A novel flame retardant additive with phosphazene and phosphaphenanthrene groups hexa-[4-(p-hydroxyanilino-phosphaphenanthrene-methyl)-phenoxyl]-cyclotriphosphazene defined as CTP-DOPO was successfully synthesized from hexachlorocyclotriphosphazene, p-hydroxybenzaldehyde, 4-aminophenol and 9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide (DOPO). Its chemical structure was well characterized by Fourier transform infrared (FTIR) spectroscopy, 1H nuclear magnetic resonance (1H NMR) and 31P nuclear magnetic resonance (31P NMR). The prepared flame retardant additive CTP-DOPO was incorporated into diglycidyl ether of bisphenol-A (DGEBA) to prepare flame retardant epoxy resins thermosets. The flame retardant properties, thermal degradation behavior and combustion behavior of the DGEBA thermosets cured by 4, 4′ -Diamino-diphenyl sulfone (DDS) were investigated by limiting oxygen index (LOI), vertical burning test (UL-94), thermogravimetric analysis/infrared spectrometry (TG-IR) and cone calorimeter tests. The structure and surface morphology of the char residues after cone calorimeter tests was measured by FTIR and scanning electron microscopy (SEM). The results revealed that DGEBA/CTP-DOPO thermosets successfully passed UL-94 V-0 flammability rating and its LOI value dramatically increased from 21.7% for cured pure DGEBA to 36.6% when the loading amount of CTP-DOPO was 10.6 wt % and the phosphorus content was only 1.1 wt % in thermosets, which indicated that the prepared CTP-DOPO possessed excellent flame retardancy for DGEBA thermoset. The TG-IR results indicated that the introduction of CTP-DOPO stimulated epoxy resins matrix decomposed and char forming ahead of time, which led to a higher residual char and thermal stability for epoxy resins thermosets at high temperature. The cone calorimeter tests revealed that the incorporation of CTP-DOPO effectively reduced the combustion parameters of DGEBA thermosets, such as heat release rate (HRR), total heat release (THR), smoke production rate (SPR) and total heat production (TSP), and so on. The SEM results showed that the phosphazene and phosphaphenanthrene groups in CTP-DOPO obviously stimulated the formation of the intumescent, compact and strong char layer, which enhanced the flame retardany of the DGEBA matrix during combustion. Therefore, the underlying materials were protected from further degradation and combustion and resulted in the efficient flame retardancy for EDGEBA thermoset.