Enhancing the flame resistance of cotton by exploiting the interaction between calcium chloride and an aspartic acid-derived polyamidoamine

In this work, the ability of Ca2+ ions to enhance the efficacy of the aspartic acid-derived polyamidoamine M-ASP as a flame-retardant of cotton was investigated. M-ASP bears two carboxyl groups per repeat units that are negatively charged at all pH’s and can ionically interact with Ca2+ ions. Moreover, IR analysis confirmed that Ca2+ ions form Lewis’s acid/base interactions with the amide groups of M-ASP. Thermogravimetric analysis demonstrated that Ca2+ ions improve the thermal and thermo-oxidative stability of cotton treated with M-ASP. The observed effect was greater than previously observed for cotton treated with glycine-derived polyamidoamine M-GLY, which bears only one carboxyl group per repeat unit. In horizontal flame spread tests, M-ASP/CaCl2 protected cotton more efficiently than M-ASP. The effect of Ca2+ ions on cotton/M-ASP was greater than on cotton/M-GLY, both in terms of higher residue and reduced flaming and afterglow combustion times. In vertical flame spread tests, while M-ASP failed to stop cotton combustion, M-ASP/CaCl2 coatings with add-ons 12% and 2%, respectively, inhibited cotton ignition, producing modest afterglow, and leaving an almost intact mass. The efficacy was greater than that of M-GLY/CaCl2 coatings, for which 19% M-GLY add-on was required. These findings demonstrate that the interaction of Ca2+ ions with the cotton/α-amino acid-derived polyamidoamine systems remarkably improves their flame retardancy, to an extent depending on the PAA chemical structure. The high hydrophilicity of M-ASP/CaCl2 and the low add-on adopted allowed obtaining pleasant to the touch coatings and did not noticeably change the hand of the fabrics compared to untreated cotton.