Synthesis of a novel highly effective flame retardant containing multivalent phosphorus and its application in unsaturated polyester resins

Abstract

Unsaturated polyester resin (UP) as one of the most important thermoset materials often exhibits serious fire hazards. Conventional phosphorus-containing reactive flame ratardants could overcome the deficiency of additive flame retardants in UP, but they always present low flame-retardant efficiency. Herein, a novel reactive flame retardant containing multivalent phosphorus (−1 and +5), 2-((((6-oxidodibenzo[c,e][1,2]oxaphosphinin-6-yl)methoxy)(phenoxy)phosphoryl)oxy)ethyl acrylate (DHP), was successfully synthesized and introduced into unsaturated polyester resin (UP). The structure of DHP was confirmed by Fourier transform infrared spectroscopy (FTIR) and Nuclear Magnetic Resonance (NMR). The introduction of DHP (≤20 wt%) significantly improved the flame retardancy of UP/DHP composites, e.g., it increased the limiting oxygen index (LOI) from 23.0% to 29.0%, decreased the peak heat release rate (PHRR) by 37.2% and passed the UL94 V-0 rating. Thermal degradation analysis by thermogravimetric analysis (TGA) and thermogravimetric analysis-infrared spectrometry (TG-IR), combined with the char analysis by Scanning Electron Microscopy (SEM) demonstrate that the presence of multivalent phosphorus makes DHP in the composites not only reduce flammable gas amounts and capture free radicals in the gas phase, but also catalyze char formation in the condensed phase. This work, which combines gas phase strategy and condensed phase strategy in one flame retardant, provides a promising approach for the flame retardance of polymers, and would also stimulate more efforts to explore highly-effective flame retardants.