A novel efficient flame-retardant curing agent for epoxy resin based on P-N synergistic effect: Bio-based benzoxazine phosphate ester

Abstract

Most epoxy resins on the market have low thermal stability and flammable defects. As a potential curing agent for epoxy resin, polybenzoxazine (PBz) can effectively improve the thermal properties of the resin due to its low heat release capacity (HRC) and rigid aromatic ring support, and its raw materials can be derived from renewable resources. Here, we have successfully developed a molecular design strategy to obtain a trifunctional benzoxazine monomer with phosphate ester. Firstly, guaiacol-ethanolamine benzoxazine monomer (GE) was synthesized by Mannich condensation using guaiacol and ethanolamine as raw materials. Then trifunctional benzoxazine phosphate ester (TBP) was synthesized by the nucleophilic reaction of GE with phosphorus oxychloride. Finally, the epoxy resin DGEBA was cured by TBP to obtain a PBz modified epoxy resin (EP-TBP polymer) with Tg of 113.7 °C and char yield of 37.4 %. Due to the introduction of TBP rich in phosphorus and nitrogen, it releases phosphorus radicals and nitrogen-containing inert gases during combustion, which leads to quenching effect and dilution effect. In addition, TBP can also promote the dehydration and dehydrogenation of the polymer and accelerate the formation of the aromatic hybrid char layer. Therefore, the THR of TBP modified epoxy resin is 49.3 % lower than that of epoxy resin without TBP, and the char yield is 47 times higher. And when the phosphorus content is 1.76 %, the flame-retardant grade can reach V-0. In addition, the introduction of TBP also enhanced the mechanical properties, adhesion properties and hydrophobicity properties of the polymer. It is worth mentioning that under alkaline conditions, the phosphate ester in the structure of EP-TBP polymer will undergo ammonolysis reaction with ethanolamine, and the resulting oligomers will be dissolved in it, thereby achieving rapid degradation of EP-TBP polymer. This work is to develop a high-performance multifunctional epoxy resin flame retardant curing agent based on renewable raw materials, and provide important insights for the subsequent development of halogen-free bio-based PBz flame retardant materials.