Preparation and characterization of durable flame retardant cotton fabrics using phosphorylation reaction of cellulose with ammonium etidronate

Developing an effective flame retardant for cotton is urgently needed to minimize the life and property loss caused by fire hazard. Here, diphosphonate system flame retardant, the ammonium salt of 1-hydroxyethylidene-1,1-diphosphonic acid (AHEDPA) is obtained by reacting urea with etidronic acid (or HEDPA), which is newly synthesized from acetic acid, phosphorus oxychloride and phosphorous acid. We predicted the suitable conditions of phosphorylation reaction for enhancing grafting rate of AHEDPA on cotton fibers, and revealed the mechanism of reaction using dicyandiamide as an accelerant. Through the Fourier transform infrared spectra and scanning electron microscopy analysis, it was found that AHEDPA with P = O(ONH₄⁺)₂ functional groups could be combined with –OH groups of cellulose, creating the P−O−C and P−NH₂ bonds. Limiting oxygen index, vertical flame test and cone calorimetric measurements were carried out on the untreated and treated cotton fabric samples with AHEDPA at different concentrations, demonstrating that the flame retardant cotton fabric with excellent washing durability were successfully obtained. Through the thermogravimetric analysis, it was confirmed that the cotton fabrics treated with AHEDPA exhibited a promotion of dehydration in cellulose and thus an increase of char formation rate, thereby protecting the substrate (cotton) from heat and fuel. The newly proposed method could enhance the grafting rate of cotton and remarkably improved the flame retardancy and durability of cotton fabrics, highlighting a wide prospect in production of flame retardant clothes.