The thermo-oxidative behavior of cotton coated with polyamidoamines: a solid state NMR Study

Cotton is not fire-resistant and therefore poses a serious hazard in the event of a fire when used in indoor furnishings. However, cotton can be made flame-retardant through treatments with additives. The development of bio- and eco-friendy treatments using different types of additives is currently a quite active research field. Polyamidoamines (PAAs) derived from α-amino acids are both bio- and eco-friendly flame retardants (FRs) and in addition do not release smoke during combustion. Aim of this study is to investigate the reactions that take place in cotton impregnated with PAAs during a fire. Given the rapidity of the process, it is difficult to follow the transformations occurring in the burning material. Thus, we adopted the strategy of thermal oxidizing the material under controlled conditions and characterizing the intermediate products. No evidence on the mechanism of thermal oxidation has been reported although it would represent an advancement in knowledge, besides being of use in designing novel FRs for cotton textiles. We used 13C solid state NMR to study the products of partial combustion of cotton impregnated with three different PAAs. The PAA structures are based on the reaction of N,N’-methylene bis(acrylamide) with glycine (M-GLY), cystine (M-CYS) and their copolymer (M-CYS-GLY) (Fig. 1). The samples have been obtained after heating the impregnated cotton in air at different temperatures, corresponding to different cotton degradation levels, i.e. prior to, simultaneous to and after the main degradation stage of cotton. Since combustion of organic polymers, including cellulose, generates stable organic radicals, EPR could be highly informative on the mechanism of thermal oxidation.