The role of auxiliaries in the immersion dyeing of textile fibres: Part 3 theoretical model to describe the role of inorganic electrolytes used in dyeing cellulosic fibres with direct dyes

The role of auxiliaries in the immersion dyeing of textile fibres: Part 5 practical aspects of the role of inorganic electrolytes in dyeing cellulosic fibres with direct dyes

“…Results are presented of dyeings on cotton that were undertaken using commercial grade direct dyes, in which the liquor ratio employed for dyeing was varied over a wide range and dyeing was carried out in both the absence and presence of a constant concentration of added NaCl. The results of these practical dyeings are interpreted in terms of the theoretical model of direct dye uptake (3,4), according to which, the effects of both added inorganic electrolyte and reduction in liquor ratio can be explained in terms of the combined effects which increased dye aggregation and reduced dye solubility have upon shifting the predisposition of the dye to favour the fibre phase rather than the aqueous dyebath phase. Subsequent parts of the paper will consider the role of auxiliaries that are utilised in the application of reactive dyes to cellulosic fibres as well as other classes of dye to other types of textile substrate.…”

“…As mentioned, from a consideration of the manifold theories that have been proposed over many decades to describe the role of added inorganic electrolyte in the immersion dyeing of cellulosic fibres with direct dyes, it was concluded (2,3) that the promotional effect imparted by added NaCl or Na 2 SO 4 on dye uptake can be attributed to the marked propensity of direct dyes to aggregate in aqueous solution via coplanar association and the facts that this particular characteristic of direct dyes in aqueous solution is encouraged at both high dye concentrations and the addition of electrolyte. In essence, the characteristically high aqueous solubility of direct dyes, which is commonly conferred by the presence of sulfonate groups, is responsible for the inherently low substantivity displayed by direct dyes towards cellulosic fibres in the absence of added electrolyte; such high dye solubility is also the reason why added inorganic electrolyte is so effective in promoting dye uptake (3). From the viewpoint of immersion dyeing, the high aqueous solubility of direct dyes means that when dissolved in the dyebath in the absence of added electrolyte, the dyes display an innate tendency to remain in the aqueous dyebath, rather than transfer to the fibre phase; thus, in the absence of added electrolyte, the extent of dye uptake is low.…”

“…Briefly, the concept of interstitial water in dyeing processes (3,4), which has been used to develop both novel dyeing (10) and wash-off (11)(12)(13)(14) processes for various dye-fibre systems as well as an innovative commercial clothes washing system (15)(16)(17)(18), is based on the premise that of the water used in exhaust dyeing, only a very small proportion that resides within the interstices of the fibrous substrate is required to saturate the textile substrate and impart the crucially important actions of fibre wetting and fibre swelling. In accordance with the concept of free and bound sorbed water molecules and electric double-layer theory, it is considered (3) that interstitial water will differ to that which resides within the surrounding bulk dyebath because of the influences of the charged textile fibre surface and the reduced mobility of the bound water molecules on the distribution of sorbed ions/molecules.…”

“…The volume of interstitial water present within cotton and other cellulosic substrates is assumed (3,4) to correspond to the moisture regain of the substrate at 100% relative humidity (RH), which is commonly reported as 0.22 l kg -1 (19). The crucial role of interstitial water in immersion dyeing is the subject of a subsequent part of this paper.…”

“…In terms of the dyeing of cellulosic fibres with direct dyes, it is possible to explain the promotional effect of both reduced liquor ratio and added inorganic electrolyte on dye uptake in terms of the interstitial dyeing mechanism; the process depicted in Scheme 1 can be considered to apply (3,4).…”