The role of auxiliaries in the immersion dyeing of textile fibres: Part 7 theoretical models to describe the mechanism by which inorganic electrolytes promote reactive dye uptake on cellulosic fibres

Abstract

This is a repository copy of The role of auxiliaries in the immersion dyeing of textile fibres: Part 7 theoretical models to describe the mechanism by which inorganic electrolytes promote reactive dye uptake on cellulosic fibres.

The role of auxiliaries in the immersion dyeing of textile fibres: Part 9 practical aspects of the role of inorganic electrolytes in dyeing cellulosic fibres with pure reactive dyes

“…A brief introduction to the chemistry and application of reactive dyes that are utilised for dyeing cellulosic fibres has been presented (7). Reactive dyes derive their name from the fact that they comprise a reactive system, containing one or more electrophilic groups, which enable the dyes to form a covalent bond with nucleophilic hydroxyl groups in the cellulosic substrate.…”

“…Scheme 1 representation of reactive dyeing mechanism (7) This approach, which is depicted in Scheme 1 (7), assumes that a very small proportion of the water that is used in immersion dyeing processes resides within the interstices of the fibrous substrate.…”

“…swelling; as such, the volume of interstitial water is assumed to correspond to the moisture regain of the substrate at 100% relative humidity (RH) (3, 4) (~ 0.22 l kg -1 in the case of cotton). It is presumed that because of the presence of different types of sorbed water molecules in porous materials such as textile fibres and based on current views of electric double-layer theory, the properties of interstitial water will differ to that of the water that is located within the surrounding bulk aqueous dyebath in terms of the distribution of sorbed ions/molecules (3,7). The remaining, much greater proportion of the water that is utilised in immersion dyeing resides within the bulk dyebath that surrounds the water-swollen fibre and provides functions such as heating, dye dissolution, etc.…”

“…In This model (7) therefore attributes the characteristically low substantivity displayed by the dyes towards cellulosic fibres, when applied in the absence of added electrolyte to the high aqueous solubility of the dyes; such high aqueous dye solubility was also considered to account for the remarkable ability of added inorganic electrolyte to promote reactive dye uptake (7). Accordingly, the low extent of colour yield ( Figure 5) and dye exhaustion ( Figure 6) observed when dyeing had been carried out in the absence of added electrolyte at high liquor ratios (ie 1:50, 1:20, 1:10) can be attributed to the intrinsically low dye-fibre substantivity created at such high liquor ratios.…”

“…In recognition of the global significance of cotton and other cellulosic fibres, the focus of the paper thus far, has been to elucidate the mechanism by which inorganic electrolyte, in the form of either NaCl or Na 2 SO 4 , promotes the uptake of both direct dyes (2)(3)(4)(5) and reactive dyes (6)(7)(8) on cellulosic fibres. Theoretical models were proposed to explain the role of such inorganic electrolyte in the immersion application of direct dyes (3) and reactive dyes (7), according to which, the addition of inorganic electrolyte to a direct dye or reactive dye dyebath promotes dye uptake owing to the combined effects of induced dye aggregation and reduced dye solubility on the effective concentration of dye in the dyebath. This particular theoretical approach, which invoked the concept of interstitial water, was also used to explain the promotional effect of reducing the liquor ratio used for dyeing on the uptake of both direct dyes (4) and reactive dyes (7) on cellulosic fibres.…”