Synergistic effect of zeolite in an intumescence process: Study of the carbonaceous structures using solid-state NMR

This work deals with the chemical effect of zeolite 4A added to the intumescent ammonium polyphosphate (APP)-pentaerythritol (PER) system [fire retardant (FR) polyethylene-based formulation]. The study of the thermal behaviour of the system using thermogravimetric (TG) analysis shows that the presence of the zeolite leads to an increase in the stability of the material at high temperature (T > 500 degrees C). Chemical analysis and cross-polarisation dipolar-decoupled magic-angle spinning (CP-DD-MAS)C-13 NMR reveals that the materials resulting from the thermal treatment of the APP-PER and APP-PFR/4A systems are formed by carbonaceous and phosphocarbonaceous species and that the zeolite enhances the stability of the phosphocarbonaceous species. DD-MAS P-31 NMR indicates that this stability may be due to an absence of pyrophosphate species in the heat-treated APP-PER/4A system. Micro-Raman spectroscopy and H-1 NMR of the solid state provides information on the structure of the carbonaceous material. Thermal treatment led to the formation of a 'turbostatic carbon' with a local structure of this 'carbon species', with the zeolite permitting retention of the comparatively unorganised carbon species in the high-temperature range. Moreover, a low-resolution solid-state H-1 NMR study of the spin-lattice and spin-spin relaxations of the samples allows an evaluation of the size of the slow relaxation domains using the Goldman-Shen procedure. It is shown that the zeolite hinders the formation of small molecules in the polyaromatic network. Finally, the relationship between the formation of a coherent macromolecular network and the improved FR performance of the APP-PER/4A system is proposed.