Novel bioderived cross-linked polyphosphazene microspheres decorated with FeCo-layered double hydroxide as an all-in-one intumescent flame retardant for epoxy resin

A green and facile strategy for preparing high-efficiency flame retardants is highly desirable given today's increasingly environmentally friendly requirements. In this study, novel bioderived cross-linked polyphosphazene microspheres (PHDTs) were synthesized through polycondensation with tannic acid and 4,4'-dihydroxybiphenyl as co-monomers and decorated by layered double hydroxide (PHDT@FeCo-LDH) to improve the flame retardancy of epoxy resin (EP). Comparison with pure EP revealed that the EP containing 4.0 wt% PHDT@FeCo-LDH exhibited the highest LOI of 29.7 and a UL-94 V-0 rating. Furthermore, the peak heat release rate, total heat release, and total smoke release of the modified EP were significantly reduced and were superior to most of its previously reported counterparts. The PHDTs could be regarded as an all-in-one bioderived intumescent flame retardant for catalyzing the formation of the highly graphitized char layer of a phosphorus containing cross-linking network in the EP matrix. The transformation of the LDH shell into refractory oxides on the surface of the matrix during combustion further strengthen the structure and thermal stability of the char layer, which effectively inhibited the release of toxic and flammable gases. The flame retardant performance of the modified microspheres was mainly attributed to catalytic charring, free-radical scavenging, and physical barrier effects. Notably, the mechanical properties of the EP composites were improved to meet the requirements of industrial application. This work provides a new methodology for the preparation of high-performance EP composites.