Effect of the fire-retardant, ammonium polyphosphate, on the thermal decomposition of aliphatic polyamides. I: Polyamides 11 and 12

The results of a mechanistic and kinetic study show that ammonium polyphosphate (APP) strongly modifies the thermal degradation process in aliphatic polyamides 11 (PA-11) and 12 (PA-12) by lowering their temperatures of decomposition and changing the composition of the resulting volatile products. From both PA-11 and PA-12, α- and β-unsaturated nitriles are the major volatile products of degradation in the presence of APP. On the other hand, α-unsaturated nitriles are evolved from both pure PA-11 and pure PA-12, together with dodecalactam from PA-12. Minor amounts of hydrocarbons, CO, CO2, are also evolved from the pure polyamides, their formation being suppressed in mixtures with APP. A charring process occurs in parallel with the volatilization of PA-11 and PA-12 on which APP seems not to have a significant effect. A mechanism is proposed for the interaction of APP with PA-11 and PA-12, involving the formation of intermediate phosphate ester bonds whose further decomposition accounts for the thermal behaviour of the mixtures. The intumescent behaviour acquired by the char formed in the presence of APP can explain its fire-retardant action in PA-11 and PA-12