Mechanism of action of phosphorus-based flame retardants in nylon 6. I: Ammonium polyphosphate

Comparison of oxygen and nitrous oxide indices indicates that ammonium polyphosphate (APP) should act in the condensed phase when added to nylon 6 (PA-6). A detailed study of the thermal degradation mechanism of PA-6/APP mixtures is carried out. A polyphosphate ester is formed on heating, which is the precursor of an intumescent char. Evidence is given of the thermal insulating action of the intumescent char which slows down the rate of combustion of the polyme

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

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

Mechanism of action of phosphorus-based flame retardants in nylon 6. III: Ammonium polyphosphate / manganese dioxide

Partial substitution of ammonium polyphosphate (APP) by manganese dioxide (MnO2) in polyamide 6 (PA-6) fire retarded with 20% of APP strongly increases the fire retardant effect. ‘Linear pyrolysis’ experiments, which are modified cone calorimeter tests, show an increase in the amount and an improvement of the shielding properties of the intumescent char formed on the surface of burning polymer. The enhancement of the yield of aliphatic–aromatic char stable to oxidation was observed in thermogravimetry under air. The fire retardant action of an APP/MnO2 mixture in PA-6 is twofold. On the one hand, this additive promotes involvement of the polymer in the charring and, on the other, the formed manganese phosphate glasses improve the thermo-insulating properties of the intumescent char on the surface of burning PA-6

Effect of the fire-retardant, ammonium polyphosphate, on the thermal decomposition of aliphatic polyamides. Part II: polyamide 6

The thermal degradation of polyamide 6 (PA-6) occurs through competing mechanisms leading to caprolactam, chain fragments with nitrile and carbon-carbon unsaturated chain ends and a small amount of residue stable to 550°C. A chemical interaction is shown to take place between ammonium polyphosphate (APP) and PA-6 on heating, resulting in destabilization of PA-6 and modification of its degradation behaviour. Caprolactam is the major volatile product obtained from the mixtures, whereas the yield of residue seems not to be affected. It is shown that the intumescent behaviour induced by APP in mixtures with PA-6 is responsible for its fire-retardant action