Effect of the fire-retardant ammonium polyphosphate on the thermal decomposition of aliphatic polyamides. Part III: Polyamides 6.6 and 6.10

Polyamides 6.6 (PA-6.6) and 6.10 (PA-6.10) decompose on heating by scission of the N-alkylamide bond (CH2NHCO), with the formation of vinyl chain ends and primary amide groups which dehydrate to nitriles at the temperature of degradation. In PA-6.6 scission of the alkylamide bond (CH2CONH) also takes place, resulting in methyl and isocyanate chain ends which can further dimerize to carbodiimides with elimination of CO2 or trimerize to isocyanurate structures. In the presence of ammonium polyphosphate (APP), flammability, as measured by oxygen index, decreases because APP induces intumescent behaviour. The chemical interaction between APP and PA-6.6 and PA-6.10 leads to chain scission of the polyamide, with formation of a primary amide group and a phosphate ester which thermally degrades to give a vinyl chain end. APP also catalyses the alkylamide bond scission in PA-6.6. The intumescent behaviour might be due to blowing of the thermally stable polyphosphoric acid layer on the surface of the material by the gases evolved by the degradation of the polyamides, which occurs at lower temperature in the presence of APP.