Calcium gluconate-based flame retardant towards simultaneously high-efficiency fire safety and mechanical enhancement for epoxy resin

Flame retardants containing biomass receive growing interest in environmental friendliness and sustainability but usually face the low flame-retardant efficiency and deterioration on mechanical property of matrix. Herein, a calcium gluconate-based flame retardant (CG@APP) was chemically prepared using calcium gluconate (CG) and ammonium polyphosphate (APP) via ion exchange reaction, and enabled the excellent fire safety and mechanical enhancement for epoxy resin (EP). The resulted EP composites containing 6 wt% CG@APP (EP/CG@APP6) exhibited V-0 ratings in UL-94 test. Furthermore, with respect to EP/APP6, the peak of heat release rate (pHRR) and peak of smoke production rate (pSPR) of EP/CG@APP6 decreased by 70.5 % and 50.0 %, respectively. The well synergistic flame-retardant mechanism of CG@APP between gaseous and solid phases was revealed to generate denser and more continuous charring residuals, which could do well work on insulation for heat transfer and fuel diffusion. In addition, the shell rich in hydroxyl group and Ca2+ on the surface of CG@APP well enhanced the interface compatibility through the hydrogen bond and coordinated bond, thus the tensile strength, flexural strength and impact strength of EP/CG@APP6 increased by 18.2 %, 4.5 % and 9.1 % compared with pure EP, respectively. This work provided a simple and sustainable way to construct excellent fire-safety composites.