The role of carboxyl and cationic groups in low-level cationised cellulose fibres investigated by zeta potential and sorption studies

The anionic nature of both cellulose fibres and reactive dyes prevents substantial exhaustion of dye from the dyebath, which is at neutral pH before alkali is added to initiate dye fixation. Conventionally, salt is added to minimize the electrostatic repulsions that interfere with dye sorption, but that increases salt loads in effluents. An alternative is to affix cationic agents on the cellulose to overcome the inherent anionic charge, but that has generally been observed to result in uneven dye sorption. The focus of investigations in this work is to examine the influence of the ratio of charges on cellulose (of affixed cationic charges to inherent anionic charges) on the extents and evenness of dye sorption. The cationisation agent 3-chloro-2-hydroxypropyl-N,N,N-trimethylammonium chloride (CHPTAC) was grafted on loose viscose fibres to yield 12 to 185 mmol kg⁻¹ cationic group content on the fibre that exhibited an inherent carboxyl group content of 68 mmol kg⁻¹. Three different dyes (of varying molecular sizes and anionic group content) were employed for examination of sorption profiles. The results from both zeta potential measurements and dye sorption profiles showed evidence of limited dye uptake until the cationic group content in fibres exceeded that of the inherent carboxyl groups. Thereafter, an uptick in dye sorption was observed, with dye sorption levels increasing with rise in degree of cationisation. There were differences between the dyes in their degrees of sorption, which appear correlated with their molecular sizes.