Effect of cellulose cationisation level and molecular dye structure on sorption and migration behaviour

The occurrence of unlevel colouration in salt-free reactive dyeing of cationised cellulose textiles represents a significant hindrance in the replacement of conventional reactive dyeing processes. The phenomenon is attributed to the rapid adsorption of dye molecules onto cationised cellulose. However, it is hypothesised that the migration ability and the strength of the dye fibre interaction are responsible for poor levelling. In this study, three anionic dyes – C.I. Acid Blue 25, C.I. Reactive Blue 19 and C.I. Reactive Red 120 – were utilised to examine the rates of sorption, the extent of migration, and the relative affinity of the dye molecules to cationised cellulose fibres. It was observed that at low cationisation levels, the sorption rates, especially for larger dye molecules, were slowed down. This phenomenon is attributed to slow intraparticle diffusion. In migration test and dye exchange experiments, it was observed that the relative affinity of a dye molecule to cationised cellulose determines the extent of dye migration and dye exchange. It was concluded that the primary factors contributing to the non-uniformity observed in the dyeing process were the high affinity of dye molecules and thus low migration ability, and the incomplete saturation of cationic groups by dye molecules, i.e. when complete dye exhaustion occurs due to too high cationisation levels for the desired colour shade, rather than the rate of sorption. • Sorption rates of three anionic dyes on cationised cellulose fibres. • Sorption rate depends on dye structure and cellulose cationisation level. • Dye-fibre interaction strength affects diffusion and migration behaviour. • Adsorbed dye molecules can be replaced by other dye molecules.