Maintaining hand and improving fire resistance of cotton fabric through ultrasonication rinsing of multilayer nanocoating

Thin films of environmentally benign polyelectrolytes, cationic chitosan (CH) and anionic poly(sodium phosphate) (PSP), were deposited on cotton fabric via layer-by-layer (LbL) assembly to reduce flammability. This CH–PSP nanocoating promotes charring of the cotton, rendering the fabric self-extinguishing. The coated fabric was rinsed in an ultrasonication bath between deposition steps to improve the softness (i.e., hand) of the coated fabric. Ultrasonication is believed to remove weakly adhered polyelectrolyte, preventing the fabric from becoming stiff, while improving anti-flammable behavior at a given coating weight. At 17 bilayers, only 9.1 wt% was added to the cotton, yet the coated cotton consistently passed vertical flame testing. Electron microscopy provides evidence of intumescence and confirms the cleaner deposition afforded by ultrasonication. The reduction in peak heat release rate and total heat release, as measured by micro cone calorimetry, were 73 and 81 % respectively, which is a new benchmark in LbL flame retardant coating on cotton. The mechanical properties of the fabric were measured using the Kawabata evaluation system, which showed that ultrasonication rinsing significantly improved the hand. The ability to render cotton fabric self-extinguishing, while maintaining a soft hand, marks a major milestone in the development of these environmentally-benign nanocoatings.