Synergism between α-amino acid-derived polyamidoamines and sodium montmorillonite for enhancing the flame retardancy of cotton fabrics

Polyamidoamine (PAA)/sodium montmorillonite (MMT) nanocomposite coatings were investigated as flame retardants for cotton to ascertain whether the addition of clay improved the efficacy of PAAs thanks to its ability to act as insulating shield during combustion. Three amphoteric PAAs were obtained by reacting N,N’-methylenebisacrylamide with natural α-amino acids, namely glycine (M-GLY), arginine (M-ARG) and glutamic acid (M-GLU). These PAAs act as intumescent flame retardants for cotton performing well in horizontal flame spread tests (HFSTs) but failing to inhibit combustion in vertical flame spread tests (VFSTs). All three PAAs have been proven to form strong interactions in water with MMT via their protonated tert-amine groups. The presence of 12.5% MMT did not significantly change the thermal and thermo-oxidative stability of PAA coatings, while 2% MMT add-on affected those of both untreated and PAA-treated cotton fabrics. In HFSTs, substituting 2% MMT for PAA did not significantly change the flame retardant efficacy of the coatings. In VFSTs, the 2% MMT/14% PAA combination inhibited cotton ignition, regardless of the PAA structure, while in the 2% MMT/11% PAA combination M-GLU protected cotton from ignition, M-ARG extinguished the flame and M-GLY burned completely. By further reducing the PAA content to 8%, only M-GLU quenched cotton combustion, leaving an RMF of 82%. The fact that neither MMT nor PAAs alone induced flame extinguishment at the same add-ons used in the adopted formulations suggests a synergistic behavior of MMT and PAAs.