Post-treatment of reactive dyed cotton fabrics by caustic mercerization and liquid ammonia treatment

The influence of caustic mercerization (C_M) and liquid ammonia (L_A) treatments on the properties of reactively dyed cotton fabrics were schematically studied. The cotton fabrics were dyed using different commercial reactive dyes such as Reactive Red 2 (R₂), Reactive Blue 21 (B₂₁), and Reactive Orange 5 (O₅) via industrial dyeing procedures. The dyed fabrics were then treated using 260 g·L⁻¹ of caustic soda solution at 23 °C for 180 s and pure L_A at −40 °C for 3 min in slack and tension conditions. The dyeing properties of the fabrics such as dye removal percentage, color strength, color uniformity, reflectance, and fastness properties (washing, rubbing, hot pressing and light) were examined. The changes in tensile strength, elongation, bending property, decomposition temperature, crystal structure, and surface morphology of the samples were also investigated using a tensile strength tester, thermogravimetric analyzer (TGA), scanning electron microscope (SEM), and an X-ray diffractometer (XRD). Results revealed that the fabric dyed with O₅ showed the best stability in C_M and L_A treatment, and the color strength and color uniformity of the dyed fabrics increased extensively after both treatments, while the reflectance of the fabrics decreased. The washing and rubbing fastness of the fabrics were also improved, which was mainly attributed to the diffusion of the dye molecules inside the fibers and enhanced bonding between fiber and dye molecules. The fastness to hot pressing and light was unaffected and all the samples retained fastness rating between 4 and 5 after being post-treated with the C_M and L_A, respectively. The tensile strength of the fabrics improved from 287 to 291 and 301 N, and the elongation at break increased from 11 to 21.5 and 26% after C_M and L_A treatment in slack conditions, respectively. Additionally, the tensile strength of the fabrics improved from 287 to 312 and 330 N, and the elongation at break increased from 11 to 12% after applying tension during both treatments. However, the total bending length and total flexural rigidity of the samples slightly increased after both the treatments either treated with or without tension, which was mainly attributed to the orientation of the cellulose microfibril angle along the fiber axis. The surface morphology and X-ray diffraction examination revealed that the treated samples consisted of smooth rod-like structure and the cellulose I allomorph was converted to cellulose II and cellulose III after C_M and L_A treatments. This developed treatment method could have application potential within the textile wet-processing industry, as it could be used to improve the dyeing properties and mechanical properties of cotton fabrics.