Mechanistic insights into the thermal decomposition of linear polyamidoamines via coupled TG–FTIR analysis

• TG–FTIR identified volatile species evolved during PAA thermal decomposition. • M-GLY and M-ARG display different gas-evolution pathways during decomposition. • M-MEP releases flammable 2-methylpiperazine, explaining poor flame retardancy. • TG–FTIR clarifies gas-phase inhibition via oxygen and flame-radical dilution. Linear polyamidoamines (PAAs), synthesized via the polyaddition of α-amino acids with N,N′-methylenebisacrylamide, act as effective intumescent flame retardants for cotton. Among the investigated systems, the glycine-derived PAA (M-GLY) achieved flame extinguishment in horizontal flame spread tests at add-on levels of 5–7%, whereas the arginine-derived analogue (M-ARG) required a significantly higher add-on (19%) to reach the same performance. Thermogravimetric analysis (TGA) identified three main thermo-oxidative degradation stages, occurring in the ranges 50–250°C, 250–400°C, and 400–800°C, corresponding to the onset of decomposition, completion of the primary mass-loss event, and the intumescence region, respectively. Hyphenated TGA–Fourier transform infrared spectroscopy (TG–FTIR), performed between 50 and 800°C under both inert and oxidative atmospheres, enabled identification of the volatile species evolved within these temperature intervals. The results highlight subtle yet significant differences in the thermal decomposition pathways of the PAAs, enabling the proposal of a well-defined gas-phase protective mechanism during cotton combustion. The decomposition pathways involve an initial depolymerization step, followed by decarboxylation, deamination, and homolytic bond cleavage reactions, leading to the evolution of gaseous products that contribute to flame inhibition through dilution of oxygen and flame radicals. The thermal and thermo-oxidative decomposition of M-GLY and M-ARG was compared with those of M-MEP – deriving from N,N′-methylenebisacrylamide and 2-methylpiperazine, a non-self-extinguishing PAA adopted as a reference material. The proposed decomposition mechanism of M-MEP involves the extensive release of the volatile and combustible amine monomer 2-methylpiperazine, which accounts for its lack of protective effectiveness toward cotton during combustion.