Synthesis of a hydrogen-bonded complex intumescent flame retardant through supramolecular complexation and its application in LDPE foam

In this paper, a novel nitrogen-phosphorus hydrogen (H)-bonded complex intumescent flame retardant (IFR)-melamine (ME) phosphoric acid (PA) pentaerythritol (PER) (MPPR) was synthesized through a supramolecular complexation method by reacting a PA PER H-bonded complex intermediate with ME. The PA PER H-bonded complex intermediate was obtained by conducting an H-bonded complexation reaction between PA and PER in aqueous solution. FT-IR measurements and two-dimensional correlation infrared (2D IR) analysis were used to carefully investigate the PA PER H-bonded complexation reaction and the related mechanism, and verify H-bond formation between PA and PER, of which the strength is different from that of PA and PER themselves. The results show that an appropriate PA/PER molar ratio could help to maintain the good solubility of PER in the system and the stability of the formed H-bonds. In addition, the reaction temperature proves to have a significant influence on H-bond formation in the system. 2D IR investigation indicates that in the full temperature range of 25-210 °C investigated, with increasing temperature, the formed H-bonds between PA and PER would be first broken to generate the free functional groups including P-O-H and C-O-H groups, which would then react with each other to form cyclized ester products at higher temperature than 150 °C. The obtained MPPR was finally incorporated into LDPE foam and imparted a good flame retardancy to the prepared foam, which is attributed to its good char forming capability and higher char yields. A supramolecular complexation method was used to synthesize a nitrogen-phosphorus hydrogen-bonded complex intumescent flame retardant, which can impart a good flame retardancy to LDPE foam.