The objective of this work was to determine the optimum concentration of nitrogen- phosphorus-based flame retardant to achieve flame-retardant properties of cotton fabrics that meet automotive fire safety regulations, and to investigate the effect of the concentration variation on the mechanical properties of cotton fabrics. The market product Eco-Flam CL (Devan Chemicals, Belgium) was applied to 100% cotton fabrics in an impregnation process at six different concentrations ranging from 100 to 150 g/l. The morphological and chemical properties of the investigated samples were determined by scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). The functional flame retardant activity was determined by a horizontal flammability test, while the influence of the flame retardant on the mechanical properties of the cotton fabric was investigated by measuring stiffness, breaking strength and elongation. Application of flame retardant finish reduced the flammability and burning rate of cotton fabrics even at the lowest concentration applied, but at higher concentration of 110–130 g/l, a slight increase in flammability of these samples was observed. However, at the highest concentration of 150 g/l, the flammability of cotton decreased drastically and significant flame retardancy was obtained. The presence of flame retardants at lower concentrations of 100–120 g/l decreased the overall stiffness of cotton fabrics, while further increase in concentration to 130–150 g/l resulted in impairment of flexibility and handling properties of cotton fabric. The same trend as for the stiffness was observed in the tensile strength and elongation of the tested samples, where the samples with the highest concentration of a flame retardant had about 5 and 15 times higher tensile strength and elongation in the warp direction, respectively, compared to the untreated sample. In the weft direction, variation of the flame retardant concentration from 100 to 150 g/l had no significant effect on the tensile strength and elongation of the finished samples.
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