Thermal and flame retardant properties of epoxy resin cured by a novel phosphorus-containing 4,4′-bisphenol novolac curing agent

GRAPHICAL ABSTRACT ABSTRACT A novel flame retardant curing agent for epoxy resin (EP), i.e., a DOPO (9,10-dihydro-9-oxa-10-phosphaphenan-threne-10-oxide)-containing 4,4'-bisphenol novolac (BIP-DOPO) was synthesized and characterized by Fourier transform infrared (FTIR), 1H NMR, 31P NMR spectroscopy, and gel permeation chromatography. The epoxy resin cured by BIP-DOPO itself or its mixture with a commonly used bisphenol A-formaldehyde novolac resin (NPEH720) was prepared. The flame retardancy of the cured EP thermosets were studied by limiting oxygen index (LOI), UL 94 and cone calorimeter test (CCT), and the thermal properties by thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC). The results show that the cured epoxy resin EPNP/BI/3/1, which contains 2.2% phosphorus, possesses a value of 26.2% and achieves the UL 94 V-0 rating. The data from cone calorimeter test demonstrated that the peak release rate, average heat release rate, total heat release decline sharply for the flame retarded epoxy resins, compared with those of pure ones. DSC results show that the glass-transition temperatures of cured epoxy resins decrease with increasing phosphorus content. TGA indicates that the incorporation of BIP-DOPO promotes the decomposition of epoxy resin matrix ahead of time and leads to higher char yield. The surface morphological structures of the char residues reveal that the introduction of BIP-DOPO benefits to the formation of a continuous and solid char layer on the epoxy resin material surface during combustion.