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

Abstract

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.