Synthesis, application and flame retardancy mechanism of a novel flame retardant containing silicon and caged bicyclic phosphate for polyamide 6

A novel flame retardant containing silicon and caged bicyclic phosphate groups, tri(2,6,7-trioxa-1-phosphabicyclo[2.2.2]octane-1-oxo-4-hydroxymethyl) phenylsilane (TPPSi) was successfully synthesized. The chemical structure of TPPSi was characterized by FTIR, 1H NMR and 31P NMR. The application of TPPSi (25 wt%) as a flame retardant in polyamide 6 (PA6) not only gains satisfied flame retardancy and smoke suppression, but also retains the high toughness and inherent appearance of pure PA6. The influence of TPPSi on the decomposition pathway of PA6 was discussed based on TG–FTIR and FTIR analysis. The interaction between TPPSi and PA6 at high temperature alters the decomposition pathway of PA6 resulting in the formation of the residue containing phosphorus and silicon. The heat and smoke release behaviors at different external heat fluxes were measured by cone calorimeter, and the fire residue was analyzed by SEM–EDX. The condensed phase action resulting from the barrier effect of residue is proposed to be the major flame retardancy mechanism of TPPSi in PA6, with the fuel reduction action as the minor.