Synthesis and flame retardant efficiency study of two phosphorus-nitrogen type flame retardants containing triazole units

Epoxy resin (EP) is one kind of thermosetting polymer with prominent physical and chemical properties and has been extensively used in various aspects of the world. Nevertheless, its intrinsic inflammability impedes its application in some occasions. Therefore, the flame retardant treatment of EP is of much significance. In this work, two novel phosphorus-nitrogen type flame retardants N-DOPO and PPDAT based on triazole were prepared. The distribution of molecular weight of the as-prepared polymeric flame retardant PPDAT was characterized by gel permeation chromatography (GPC) analysis. The number-average molecular weight (Mn), the weight-average molecular weight (Mw) and the distribution index (D) value of PPDAT were 8849, 9324 and 1.05, respectively. The results of flame retardant tests evinced that after introducing 7.5 wt% N-DOPO, a V-0 rating in the UL-94 test and a desirable limiting oxygen index (LOI) value of 33.5% were achieved. With 7.5 wt% PPDAT added, this composite only reached V-1 level with a LOI value of merely 30%, indicating that in contrast with PPDAT, N-DOPO presents better flame retardant efficiency for epoxy resin. Through the analysis of the products in combustion process, it was determined that EP/PPDAT composite could catalyze the formation process of dense continuous carbon layers, with efficient flame retardant effect in the condensed phase. This work provided the strategy of introducing triazole units into flame retardants and would help sprout a new series of flame retardants.