Synthesis Of Cyclotriphosphazene Derivatives And Research Of Their Flame Retardant For Epoxy Resin

Epoxy resins have been widely used in surface coatings, composite materials, semiconductor packages and electronic/electrical industries for its outstanding properties, such as good mechanical properties, chemical resistance and easy molding. However, the flammability of epoxy resins lead to fire disaster inevitably, the epoxy resins are subjected to various mandatory controls for safety reasons. The phosphorus-containing flame retardants present excellent flame retardance, safety and environment protection on epoxy resins without producing corrosive and obscuring smoke during the combustion process.In this work, the epoxy resin, CTP-EP, based on cyclotriphosphazene group was designed and synthesized and its chemical structure was characterized by Fourier transform infrared spectroscopy (FTIR),1H,13C and31P nuclear magnetic resonance (NMR) spectroscopy. The synthesized CTP-EP was cured with the curing agents of diaminodiphenyl methane (DDM), diamino diphenyl sulfone (DDS) and m-phenylenediamine (mPDA), respectively. The flame retardant properties, thermal degradation behaviors and flame retardant mechanism of the cured CTP-EP thermosets were investigated by limiting oxygen index (LOI), vertical burning test (UL-94) and cone calorimeter test (CONE). Results showed that the LOI values of all the cured CTP-EP samples got more than30%and the samples with the thickness of3.0mm and1.6mm successfully passed UL-94V-0rating, which demonstrated that CTP-EP thermoset possessed excellent flame retardancy. According to the cone test, heat release rate (HRR), total heat release (THR) and total heat production (TSP) of the CTP-EP/DDM system decreased significantly compared to that of pure epoxy resin (DGEBA). The results of the TGA showed that, compared with pure DGEBA, the initial decomposition temperature of CTP-EP thermoset decreased, but the decomposition intensity was low which led to the thermal stability at a high temperature of the thermoset. The high char yield of39.0wt%at700℃indicated that the CIP-EP system had a significant charring ability. The results of the SEM and FTIR tests for the char residues revealed that the CTP-EP system formed an expanding, homogeneous and sealing char layer. The results of TGA-FTIR test showed that the incorporation of the phosphazene groups promoted the CTP-EP thermoset decompose at lower temperature and form the char layer which suppressed the thermal conductivity and led to the high flame retardant efficiency and thermal stability at high temperature of CTP-EP thermoset.The epoxy resin flame retardant, CTP-DOPO with phosphazene and phosphaphenanthrene double functional groups, based on cyclotriphosphazene group was designed and synthesized at the same time. After the characterization of its chemical structure, it was blended with pure DGEBA and curing agent DDS to prepare flame retardant epoxy resins. The tests showed that the flame retardant sample with the CTP-EP content of10.6%, as the phosphorus content of1.1%, with the thickness of3.0mm could successfully pass the UL-94V-0, and gain the LOI value of36.6%, revealing the excellent flame retardant efficiency of the CTP-DOPO. According to the cone test, heat release rate (HRR), total heat release (THR) and total heat production (TSP) of the flame retardant epoxy resin decreased significantly compared to that of pure epoxy resin (DGEBA). The results of the TGA showed that, compared with pure DGEBA, the initial decomposition temperature of the flame retardant sample decreased, but the decomposition intensity was low. The high char yield of29.2wt%at700℃indicated that the addition of CTP-DOPO enhanced the charring ability of the epoxy resin. The results of the SEM and FTIR tests for the char residues revealed that the incorporation of CTP-DOPO made the thermoset decompose and form a sufficient, compact and stable char layer on the surface of the thermoset at a lower temperature, which acted as a barrier and protection of the inner material. Thus, the thermoset containing CTP-DOPO exhibited good thermal stability at high temperature and obtained excellent flame retardance.