Research on single-molecule intumescent flame retardant to improve the flame retardant performance of polypropylene

The intrinsic flammability of polypropylene （PP） limits its use in applications requiring high safety standards， making the enhancement of its flame-retardant performance crucial for broadening its utility. In this study， PP composites with excellent flame-retardant properties were developed by compounding a self-synthesized monomolecular intumescent flame retardant， PPOPEPA， with ammonium polyphosphate （APP）. PPOPEPA was synthesized by using phosphorus oxychloride， pentaerythritol phosphate （PEPA）， and anhydrous piperazine. The successful synthesis of PPOPEPA was confirmed by Fourier transform infrared spectroscopy （FTIR） and nuclear magnetic resonance （NMR） spectroscopy. Burning testing demonstrated that the incorporation of the PPOPEPA/APP flame retardant system significantly enhanced the flame retardancy of PP. Under a nitrogen atmosphere， thermogravimetric analysis revealed that the initial decomposition temperature （5% weight loss） increased by 18 °C， while the maximum decomposition rate temperature shifted from 343 °C to 376 °C. The residual char yield at 700 °C increased from 0% to 28.8%， indicating a strong condensed-phase flame retardant effect. When the total loading of PPOPEPA and APP reached 30% at a mass ratio of 1∶2， the limiting oxygen index （LOI） of the flame-retardant PP increased to 34%， and the UL-94 vertical burning test achieved a V-0 rating. Cone calorimetry tests further demonstrated that this flame-retardant system reduced the peak heat release rate （PHRR） and total heat release （THR） by 90.8% and 23.8%， respectively. The smoke production rate （SPR） and total smoke release （TSP） also showed significant reductions. Additionally， the fire growth rate index （FIGRA） was markedly lowered compared to the neat PP， indicating improved fire safety performance. Scanning electron microscopy （SEM） of the char residues revealed that PPOPEPA and APP synergistically formed a dense， continuous char layer， effectively insulating the material by blocking heat and oxygen transfer. The structural integrity of this char layer was attributed to APP promoting dehydration and carbonization， while PPOPEPA facilitated char expansion via gas-phase radical quenching. Moreover， the CO production rate during combustion was significantly reduced， indicating lower toxicity of the flame-retardant composite. In conclusion， the PPOPEPA/APP flame retardant system not only effectively mitigates the inherent flammability of PP， but also reduces smoke and toxic gas emissions. This environmentally friendly and low-additive formulation holds great potential for high-safety applications， such as in the automotive and electronics industries. The findings of this study provide theoretical support for the development of sustainable and efficient flame-retardant systems for polypropylene.