Influence of monomer reactivity on radiation grafting of phosphorus flame retardants on flax fabrics

Abstract Flax fabrics were modified by radiation grafting method to improve their flame retardancy. Several phosphonated monomers with different carbon-carbon double bond reactivity were grafted. According to 1 H NMR carried out on irradiated monomers solubilized in water, the reactivity is the highest for (acryloyloxy)methyl phosphonic acid (hPAAPC1), and the lowest for allyl phosphonic acid (APA). Nevertheless, grafting yield on flax, assessed by X-ray fluorescence and scanning electron microscopy showed a different tendency. Especially (methacryloyloxy)methyl phosphonic acid (hPMAPC1) and vinyl phosphonic acid (VPA) appear to be highly grafted. In all cases, diffusion of the molecules into the flax elementary fibers bulk was observed. The choice of the solvent in washing step after the irradiation step showed to be effective to control the final phosphorus content in flax fabrics. The effect of phosphorus grafting on thermal properties and fire behavior was studied using thermogravimetric analysis, pyrolysis combustion flow calorimetry and a preliminary fire test. According to the latter, self-extinguishing and non-flammable fabrics were obtained for phosphorus content of 0.5 wt% and 1.2 wt% respectively. Correlations with flammability at microscale were pointed out. Additional cone calorimeter tests highlight the influence of flame retardant treatment on flammability under forced flaming conditions.