Synthesis of a novel phosphorus-nitrogen type flame retardant composed of maleimide, triazine-trione, and phosphaphenanthrene and its flame retardant effect on epoxy resin

In this paper, a novel phosphorus/nitrogen-containing compound (DMT) constructed by maleimide, phosphaphenanthrene and triazine-trione was successfully synthesized. The chemical structure of DMT was characterized by Fourier transform infrared spectroscopy (FTIR), 1H and 31P nuclear magnetic resonance (NMR). DMT was then blended with diglycidyl ether of bisphenol-A (DGEBA) to prepare a series of flame-retardant epoxy resins. The flame retardancy and combustion behavior were evaluated using limited oxygen index (LOI), vertical burning (UL94) and cone calorimeter test. The results indicated that DMT dramatically enhanced the flame retardancy of epoxy resin. When the phosphorus content was only 1.0%, the EP/DMT-1.0 sample had a LOI value of 35.8%, and achieved UL94 V-0 rating. The combustion parameters disclosed that DMT obviously weakened the heat release intensity. Compared with the neat EP sample, the peak of heat release rate (pk-HRR), average of heat release rate (av-HRR) and total heat release (THR) of EP/DMT-1.25 sample were decreased by 59.4%, 28.2% and 27.4%, respectively. The decreased av-EHC of EP/DMT thermosets indicated that DMT inhibited the gaseous phase combustion due to the gaseous phase quenching effect of phosphorus-containing free radicals derived from the decomposed DOPO group. The condensed phase studies indicated that DMT promoted the charring of EP matrix and formation of phosphorus-containing char layer with compact and intumescent structure. DMT exerted bi-phase flame retardant effect.