Thermo-oxidative degradation of cotton treated with flame retardants: a joint solid-state NMR and EPR investigation

Cotton is naturally flammable, posing a serious risk in the event of a fire, especially when used in indoor furnishings. Its fire resistance can be improved with flame-retardant treatments. Among emerging bio-based solutions, polyamidoamines (PAAs) derived from natural α-amino acids stand out as sustainable, effective, and smoke-free alternatives. This study explores the degradation of cotton treated with PAAs under fire-like conditions. Since combustion is a fast and complex process, controlled thermo-oxidation was used to simulate it, allowing the characterization of intermediate products. To date, no detailed mechanism has been reported for the thermo-oxidation of cotton in the presence of PAAs. To gain insight into this process, 1H and 13C solid-state NMR and EPR spectroscopies were employed to analyze the char residues of cotton treated with PAAs derived from glycine and cystine, as well as the residues of the chars from pure PAAs. The combined use of these spectroscopies suggests that the PAAs form a protective layer inaccessible to oxygen which effectively shifts the thermal degradation pathway of the impregnated cotton towards a pyrolysis-like process. As an example, Fig. 1 shows that pyrolyzed cotton and cotton impregnated with the PAA and thermo-oxidized exhibit similar spectra, which are markedly different from that of thermo-oxidized pure cotton.