Enhanced fire retardant properties of glass-fiber reinforced Polyamide 6,6 by combining bulk and surface treatments: Toward a better understanding of the fire-retardant mechanism

Polyamide 6,6 (PA6,6) is usually fire retarded in bulk, using aluminum diethylphosphinate (AlPi). In one of our recent papers, it was shown that good fire-retardant properties (e.g. V0 at the UL94 test) could also be achieved by applying an intumescent varnish on the PA6,6 surface. Excellent fire-retardant properties were even obtained combining 5% of AlPi in the bulk with an intumescent coating applied on the polymer surface. The Glow Wire Flammability Index (GWFI) test was validated at 960 °C whereas it was only validated at 750 °C without the AlPi. This paper first aims to describe the mechanism of action of an intumescent coating to protect a polymer, as it was never previously studied in the literature. It was evidenced, analyzing with 13C, 31P and 27Al solid state NMR spectra of the materials at different combustion times that during burning one part of the virgin polymer begins to degrade and is mixed with the semi-viscous charring intumescent layer. In some way, due to this mixture of both phases, it is as if the polymer was fire retarded in bulk during burning. The second objective was to investigate the potential synergy between the gas phase fire-retardant mechanism of AlPi and the condensed phase fire protective mechanism of the intumescent coating to explain the enhanced fire-retardant properties. Similarly, using NMR technique, the interest to combine the bulk treatment in low amount and the intumescent coating was evidenced: the AlPi cannot completely sublimate because of the protective coating, and is probably condensed inside the intumescent structure pores. As it is trapped in the condensed phase, it then degrades into aluminophosphates, increasing the heat barrier efficiency of the expanded char layer.