Layer-by-layer assembled diatomite based on chitosan and ammonium polyphosphate to increase the fire safety of unsaturated polyester resins

In order to develop an environmentally benign flame retardant for unsaturated polyester resins (UPR), an effective intumescent flame retardant architecture based on chitosan (CH) and ammonium polyphosphate (APP) was successfully deposited on the surface of diatomite particles by Layer-by-layer (LbL) assembly. With nine bilayers of CH/APP coating, UPR/9BL@Dia composites exhibiting LOI value of 25.7% and V-0 rating. Cone calorimetry test reveals that 9BL@Dia reduces the peak heat release rate and total heat release of UPR by 40.8% and 18.1%, respectively. Due to the unique morphology of 9BL@Dia particles, UPR/9BL@Dia composites achieves greater thermal stability and higher char yields than pure UPR. Based on the investigation of the gas and condensed phase, flame retardant mechanism of nBL@Dia was proposed as that APP/CH was heated to form phosphate esters, and then combined with diatomite to form a more stable and firm carbon structure, which can improve the fire safety of UPR composites.