Facile synthesis of a novel organic salt containing DOPO and pyrimidine as a reactive additive for high-efficiency flame retarding epoxy resin

A novel organic salt containing DOPO and pyrimidine (AD) was successfully synthesized through a simple acid-base neutralization reaction. The salt was used as a reactive flame retardant for epoxy resin. Non-isothermal DSC tests disclosed that the epoxy resin displayed a lower temperature of onset curing and exothermic peak in the presence of AD. By using only 3 wt% AD, the epoxy resin achieved a UL-94 V-0 rating and a limited oxygen index value of 27.4%, revealing the high flame-retardant efficiency of AD. AD inhibited the release of heat and smoke. The analysis of the flame-retardant mode of action indicated that AD was able to impede combustion not only by the dilution and quenching effects in the gas phase but also by the charring effect in the condensed phase. During UL-94 testing, 3 wt% AD gave the epoxy resin good flame retardancy due to the charring and blowing-out effects formed by adding AD. The blowing-out effect was that the release of strong airflows suppressed or even blew out the flame based on the dilution and quenching effects of phosphorous-containing radicals and numerous non-combustible gases produced by the degradation of AD and epoxy resin matrix. Additionally, the incorporation of AD slightly increased the glass transition temperature and the tensile and flexural strengths of the epoxy resin. This study provided a meaningful attempt to improve the flame retardance of epoxy resin by introducing a reactive pyrimidine-organophosphonic acid salt.