Straightforward synthesis of novel chitosan bio-based flame retardants and their application to epoxy resin flame retardancy

Epoxy resin (EP) is widely used in various industries. However, its inherent flammability characteristics limit its application, so there is an urgent need to develop bio-based flame retardants for efficient flame retardant EP. Chitosan (CS) is one of the most commonly used bio-based raw materials, but most chitosan-based flame retardants are synthesised using large amounts of organic solvents, with low yields and not in line with the theme of green and sustainable development. In this paper, we report a facile, eco-friendly, and solvent-free approach to chitosan-based flame retardants. A novel full biomass flame retardant, chitosan-based ammonium phytate (PUCS), was synthesised without the organic solvent by using chitosan and phytanic acid as raw materials and the electrostatic adsorption interaction between them and urea, and it was used as a flame retardant additive for EP. The results showed that the EP samples with 7.5 wt% PUCS added increased the limiting oxygen index from 22.2 % to 32.2 % and the vertical burning rating from no rating to V-0. Moreover, the cone calorimeter test results showed that the EP/PUCS composites with adding PUCS showed a significant decrease in the peak heat release rate, and it has excellent flame retardancy. It is worth mentioning that the mechanical property test results showed that the mechanical properties of EP/PUCS composites with adding PUCS could still reach the scope of the needs for engineering applications. This work provides an economical, convenient, and eco-friendly feasible route for bio-based flame retardants. [Display omitted] •A novel layer-by-layer self-assembly method of biobased phytic acid, chitosan, and urea for the synthesis of PUCS flame retardants.•PUCS has excellent flame-retardant properties and can catalyze the formation of a coke layer in epoxy resins.•PUCS can achieve excellent flame retardant performances at lower addition than the commonly used intumescent flame retardants.