Roles of degradation and charring in the flame retardancy of polyamide 56 induced by phosphomolybdates with different cations

Polyamide 56 (PA56) is a polyamide based on bio-based chemical and has attracted widespread attentions. In this paper, a series of phosphomolybdates (POMs) with Keggin structure were incorporated into PA56 to modify its flame-retardant properties. Different POMs show different effects on the flame-retardant efficiency in PA56. A small amount of 3 wt% sodium phosphomolybdate (PMoNa) or nickel phosphomolybdate (PMoNi) enable PA56 to achieve the UL-94 V-0 rating, while the samples containing other POMs are not classified in the test. The POMs promote the degradation of PA56 at low temperature and char formation at high temperature, and improves the thermal stability in air. Especially, the PMoNa and PMoNi restrain the release of heat, flammable volatiles and toxic gas by forming more char and changing the pyrolysis pathway during combustion, thus modify the flame retardancy of PA56 effectively. The mechanical properties are deteriorated more or less which need to be modified further.