Synthesis of novel N-phosphorylated iminophosphoranes and their application in flame-retardant epoxy resin

Abstract

Background

Epoxy resin, one of the most commonly used thermosetting materials, suffers from the more obvious defect of being flammable.

Methods

In this work, three new types of N-phosphorylated iminophosphorane based on phosphorous compounds containing different phosphorus oxidation states (DPP-N-TMP, DOPO-N-TMP and DPPO-N-TMP) was successfully synthesized via the one-pot “Atherton-Todd and Staudinger reactions” approachs, and then introduced separately into EP matrix.

Significant findings

TGA results indicated that the incorporation of these three N-phosphorylated iminophosphoranes can prominently enhance the thermal stability at high temperature. Especially the char yield of EP/2.5 %DPP-N-TMP, EP/5 %DPP-N-TMP and EP/10 %DPP-N-TMP was gradually enhanced from 7.6 % of pure EP to 17.6 %, 22.7 % and 25.2 %, respectively. Notably, EP/10 %DPP-N-TMP possessed a relatively high LOI value of 29.0 % and passed V0 rating in UL-94 test, while EP/10 %DOPO-N-TMP (LOI∼27.5 %) and EP/10 %DPPO-N-TMP (LOI∼24.0 %) only displayed V1 and NR rating respectively. As for cone test results, EP/10 %DPP-N-TMP exhibited the most significant reduction of 63.8 % in PHRR and its THR was also remarkably reduced by 54.5 % compared to the neat EP. The analysis implied that EP/10 %DPP-N-TMP possessed the best flame retardant performance due to its higher phosphorus oxidation state, conducive to forming char layers, which improved fire retardancy.