Exploring the flame retardant mechanism of α-amino acid-derived polyamidoamine-treated cotton: An EPR study

Coatings composed of polyamidoamines (PAAs) derived from natural α-amino acids act as intumescent flame retardants for cotton fabrics. In this work, chars obtained from the thermo-oxidation of cotton impregnated with PAAs based on glycine, cystine, and a combination of the two, as well as chars obtained from pure cotton and neat PAAs, were investigated by means of EPR with the aim of gaining insight into the flame retardancy mechanism exhibited by such impregnated cotton fabrics. The explored charring temperatures were 300, 350 and 420 °C. Clear trends in radical concentration, g-value, and spectral line shape were observed in the chars as a function of charring temperature. Surface and bulk radicals were distinguished by monitoring the signal increase after nitrogen exposure, offering insight into char porosity. PAAs significantly influence both the EPR spectral line shape and cotton char porosity at 350 °C. These findings suggest that, at this temperature, PAAs retard the decomposition of cotton. The relative amounts of internal and surface radicals of the chars at 420 °C indicate that cystine provides a higher protection to cotton than glycine. The PAA derived from a combination of the two amino acids has an intermediate protective effect. • Polyamidoamines reduce oxygen access to cotton surface at 350 °C • Polyamidoamines retard cotton degradation during thermo-oxidation at 350 °C • EPR distinguishes internal vs. surface radicals, offering insight into char structure • Cystine is superior to glycine in protecting cotton during thermo-oxidation at 420 °C • An intumescent layer made by polyamidoamines insulates cotton from oxygen and heat