Re-examination of the intumescence mechanism of ammonium polyphosphate/pentaerythritol/zeolite 4A fire-retarded formulation using advanced spectroscopic techniques

The mixture of ammonium polyphosphate and pentaerythritol is a very efficient intumescent system suitable for polyolefins, especially polypropylene. In this article, the intumescence mechanism of this intumescent system with and without zeolite 4A used as a synergy agent is revisited. The intumescent system was investigated in depth using continuous-wave electron paramagnetic resonance spectroscopy, solid-state nuclear magnetic resonance, and the advanced technique, namely hyperfine sublevel correlation pulsed electron paramagnetic resonance. It was observed that the char generated between 250°C and 350°C is made of polycyclic heterocyclic radicals with nitrogen atoms and that free radicals are mainly generated at these temperatures with a spin concentration relatively stable at least up to 500°C. Moreover, the presence of hydrogen, carbon, nitrogen, and phosphorus was clearly evidenced in the chemical environment of free electrons at 350°C (hyperfine sublevel correlation pulsed electron paramagnetic resonance). Besides, it was also evidenced that 4A totally collapses below 250°C. Contrary to previous works suggesting the presence of aluminosilicophosphate complexes, this work demonstrated that distinct alumino- and silicophosphate complexes are generated and protected the residue at high temperatures.