A bio-based casein flame retardant synergist for improving phosphorus utilization efficiency: Char-forming behavior and flame retardant mechanism

In this paper, a new flame retardant synergist PSil-CS was successfully synthesized, which was combined with flame retardant ammonium polyphosphate (APP) to flame retardant Unsaturated polyester resin (UPR) composites. When the weight ratio of APP: PSil-CS is 30:1 and 15:1, the UL 94 V-0 level can be achieved. When the weight ratio of APP: PSil-CS is 15:1, the UPR showed a good flame retardant promotion effect, the LOI value was up to 35.1 %, the peak heat release rate (pk-HRR) was 70.45 % lower than that of pure UPR sample, and 30.4 % lower than that of UPR/APP-16 % sample. Under the premise that the total amount of flame retardants is unchanged, although the total amount of phosphorus is reduced by replacing APP with PSil-CS, it is found in TGIR test that the addition of PSil-CS can promote the flame retarding effect of phosphorus by generating more PO· and PO2· free radicals. These free radicals can capture the free radicals released by the UPR matrix during the combustion process, thus interrupting the chain reaction of the combustion of UPR composites and reducing the combustion intensity of the composites. Adding PSil-CS can promote the carbonization of the UPR matrix and increase the utilization efficiency of the P element. The silicon element can also form a silicon-containing protective layer on the surface to improve the thermal stability of the surface char layer. At the same time, the addition of phosphosilane-modified PSil-CS can effectively improve the bending strength of the composites.