Enhancement of the intumescent flame retardant efficiency in polypropylene by synergistic charring effect of a hypophosphite/cyclotetrasiloxane bi-group compound

Abstract

The influence of a hypophosphite/cyclotetrasiloxane bi-group compound (MVC-AlPi) on the efficiency of intumescent flame retardant polypropylene (PP/IFR) consisting of ammonium polyphosphate (APP) and hyperbranched triazine-based charring agent (HTCFA) was investigated through evaluating the fire behavior and thermal stability by limiting oxygen index (LOI), the vertical burning tests (UL94), cone calorimeter and thermogravimetric analysis (TGA). The scanning electron microscopy (SEM), fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) were used to analyze the final residues. The thermal decomposition and char-forming behavior between MVC-AlPi and each IFR formulation was studied by TGA and thermogravimetry-fourier transform infrared spectroscopy (TG-IR) test. Results revealed that a little amount substitution of IFR by MVC-AlPi imparted overall improvement in flame retardancy with reduced heat/smoke release and UL 94 V-0 rating for composites with a total loading of only 18 wt% instead of 25 wt%. The thermal decomposition behavior analysis exhibited a synergistic char-forming effect after the combination of MVC-AlPi, improving high-temperature thermal stability and yield of residual char with phosphorus- and silicon-containing crosslinked structures due to the interactions between MVC-AlPi and IFR, especially APP. The introduction of silicon into the cross-linked structures generated a fishnet-like protective device covering on the char surface inhibiting bursting the foamed carbonaceous structures and buttressed cell walls not to be fractured, leading to the formation of an integrated and thick intumescent char layer, plus the factor of the early and gentle release of non-flammable gases. This lifted flame retardant efficiency mainly contributed to the condense-phase mechanism with not ignored flame inhibition of phosphorus-containing free radicals generated from MVC-AlPi pyrolysis in gas phase. This work provided a potential way to enhance the flame retardant efficiency of IFR system.