Phosphorus/sulfur-containing aliphatic polyamide curing agent endowing epoxy resin with well-balanced flame safety, transparency and refractive index

Abstract

Aliphatic polyamides and derivatives are widely used as curing agents for epoxy resin (EP) to obtain excellent transparency and refractive index in optical applications. However, these EPs are more flammable than EPs cured using other curing agents. When sufficient curing agents meet requirement for flame safety, optical properties are often diminished. To solve the problem, an intrinsic curing agent (PTDP) consisting of phosphorus and sulfur was designed and prepared from polyethylenimine (PEI), thiourea and diphenylphosphinyl chloride. This curing agent was used to obtain a multi-functional epoxy resin (PTDP-EP). PTDP-EP displayed excellent transparency with transmittance up to 90% due to no change in the amorphous structure of EP. In addition, owing to the introduction of sulfur in PTDP, the refractive index of PTDP-EP increased to 1.591 from 1.495 for reference PEI-EP. Further, the EP with the addition of 30 wt% PTDP displayed a UL-94 V-0 rating and a limiting oxygen index of 30.4%. Moreover, the total heat release and total smoke production were respectively decreased by 61.0% and 64.3% compared with the corresponding values for EP containing no PTDP. Further study confirmed that joint action in the gas and condensed phase led to the fire safety of PTDP-EP during burning. Keywords: Epoxy resin, Fire safety, Transparency, Refractive index, Aliphatic polyamides