Synthesis of a cross-linked triazine phosphine polymer and its effect on fire retardancy, thermal degradation and moisture resistance of epoxy resins

Abstract A novel cross-linked triazine phosphine polymeric flame retardant additive polymelamine tetramethylene phosphonium sulfate defined as PMTMPS was synthesized by the condensation polymerization between melamine and tetrahydroxymethyl phosphonium sulfate. Its chemical structure was well characterized by Fourier transform infrared (FTIR) spectroscopy, 13 C and 31 P solid-state nuclear magnetic resonance. The synthesized PMTMPS and curing agent m -phenylenediamine were blended into epoxy resins to prepare flame retardant epoxy resins thermosets. The effects of PMTMPS on fire retardancy and thermal degradation behavior of the cured EP/PMTMPS composites were investigated by limiting oxygen index (LOI), vertical burning test (UL-94), cone calorimeter measurement (cone) and thermogravimetric analysis (TGA) tests. The morphologies and chemical compositions of char residues for cured epoxy resins were investigated by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS), respectively. Moisture resistant properties of epoxy resins thermosets were evaluated by putting the samples into distilled water at 70 °C for 168 h. The results demonstrated that the cured EP/11 wt % PMTMPS composites successfully passed UL-94 V-0 flammability rating and the LOI value was as high as 32.5%. The TGA results indicated that the incorporation of PMTMPS stimulated epoxy resins matrix decomposed and char forming ahead of time, which led to a higher char yield and thermal stability for epoxy resins thermosets at high temperature. The morphological structures and the analysis results of XPS for char residues of thermosets revealed that the introduction of PMTMPS benefited to the formation of a sufficient, more compact and homogeneous char layer with rich flame retardant elements on the materials surface during burning, which prevented the heat transmission and diffusion, limited the production of combustible gases, inhibited the emission of smoke and then led to the reduction of the heat release rate and smoke produce rate. The results of water resistance tests shown the moisture absorption of EP/PMTMPS thermosets was lower than that of pure epoxy resins thermosets and the samples still remained excellent flame retardancy after water resistance tests.