Thermo-oxidative decomposition of α-amino acid-derived polyamidoamine (PAA)-impregnated cotton: temperature effects, protective role, and insights into PAA degradation mechanisms

Natural α-amino-acid-derived polyamidoamines (PAAs), based on glycine (M-GLY), cystine (M-CYSS), and glycine/cystine (M-GLY50-CYSS50) are recognized as intumescent flame retardants for cotton. The structure and composition of chars obtained from thermo-oxidized PAA-impregnated cotton, pristine PAAs and cotton were analyzed to elucidate the condensed-phase flame-retardant action mode. Thermo-oxidation was performed at 300, 350, and 420 °C, and the resulting chars were characterized by solid-state NMR and ATR-FTIR spectroscopies. Spectroscopic analyses revealed structural transformations of PAAs consistent with decomposition pathways initiated by early depolymerization and homolytic cleavage, followed by decarboxylation, deamination, and cyclization, leading to the progressive enrichment in aliphatic structures devoid of heteroatoms and to the formation of aromatic domains. In cotton/PAA systems, PAAs were found to form a protective char layer on the cotton surface at 300 °C, while the cotton substrate itself remained largely unaffected. At 350 °C, PAAs slowed the oxidative decomposition of cotton, most likely by limiting heat and mass transfer (oxygen and volatiles), with protection efficiency following the order M-GLY > M-CYSS > M-GLY50-CYSS50. At 420 °C, distinct PAA-derived char layers were observed on the cotton surface. M-GLY produced a char characterized by larger aromatic domains compared to M-CYSS and M-GLY50-CYSS50, as evidenced by dipolar-dephased 13C CP NMR experiments, and by smaller pores, as indicated by ¹H MAS NMR measurements. These structural features may account for its recognized superior protective performance towards cotton in horizontal flame tests.