Cotton and other cellulosic substrates comprise over 40% of world textile production. Of all the dyes used for the colouration of cellulosic fibres, reactive dyes find the greatest use with over 50% of world consumption. Reactive dyes are desirable because of their excellent wash fastness which arises through the covalent bond formed between the fibre and the dye. Reactive Dyeing essentially has 2 stages. In the 1st stage the dye absorbs onto the cellulosic substrate through hydrogen bonding and the dyes are applied in aqueous form containing electrolytes which are needed to overcome some of the chemistry and physical barriers as well as some of the chemistry changes in the cotton substrate caused by mercerising or bleaching. Without this addition Reactive Dyes (which are very soluble in water) absorption of the dye on the fibre will not occur.
The 2nd stage requires an alkali to be added to achieve a pH of around 11 in order to generate sufficient cellulosate anions within the substrate for covalent bond formation between the dye and the fibre (fixation). However this alkaline environment produces hydroxyl ions in the dyeing solution with which the reactive dye is able to react, in competition with the cellulosate anions in the fibre to form hydrolysed dye. As much as 40% of the dye in solution may be hydrolysed in the dyeing process and whilst having a strong affinity for the cellulosic substrate, will not be covalently bonded.
Accordingly, these reactive dyeings require a multistep wash off process after dyeing involving various aqueous rinses and washings, in order for the dyeing to achieve the characteristic very high wash fastness. This wash off and subsequent effluent treatment, to remove the resultant colouration pollution, can account for up to 50% of the total cost of reactive dyeing.
Another extremely important sustainability consideration is the amount of water used in the wash off process where 7 separate rinsing stages is not uncommon. High volumes of water and numerous repeated individual washes are often required to effect a dilution in electrolyte and alkali concentration in the wash off bath at various temperatures and as such using a high level of energy to bring the water up to the correct temperature.
The use of Dye Transfer Inhibitors (DTI) in domestic laundering detergent compositions is well known. The stain inhibition character of these auxiliaries is effective on both cotton and polyamide fibres in formulations containing nonionic and/or anionic surfactants. Suitable formulations that reduce the problem of dye staining in domestic laundering have been developed. Such DTIs include polymers such as polyvinyl lpyrrolidone, polyvinyl pyridine betaine and polyvinyl pyridine N oxide. These polymers can hold soils and vagrant dye in solution in the laundering process and may be washed from the fabric preventing their redeposition onto any other area of the fabric. Currently these DTIs are used in domestic washing formulations (powder, liquid, and tablet) at concentrations of 0.2 to 1.0% mass of the total formulation.
From a sustainable chemistry and engineering perspective, water consumption and energy usage are arguably the biggest issues in textile dyeing. Existing and developmental DTIs were employed to remove the unfixed hydrolysed dyes following reactive dyeing of cotton with the object of gaining a significant cost saving and sustainability benefit as a result of a drastic wash off process procedure, which will save water, time and energy.
The fabrics used were all 100% cotton, plain weave and 150g/m2. The dyes used were all known to be problematic with respect to removal of hydrolysed dyes, hence, requiring extensive wash off treatments. Commercial samples of the Procion H-E Dyes C.I Reactive Yellow, C.I Reactive Red, C.I Reactive Blue were the dyes used. The cotton samples were dyed in a Laboratory scale dyeing machine according to the dye manufacturer’s recommended method.
A sample of the fabrics were washed off following the traditional 6 stage wash off routine and the other samples were subjected to the novel 3 stage process using the DTI polymer in the 2nd stage. For comparison to Industrial processes where high electrolyte content is carried over the 3rd stage involved the use of anhydrous sodium sulfate as is the norm in traditional dyeing. The samples from both process types were analysed using a spectrophotometer for colour depth and saturation. Wash fastness tests were also carried out under ISO 105:C06/C2S wash test parameters and then assessed using the ISO 105:A03 test protocol for cross staining. The amount of energy consumed was also calculated for each stage.
Results indicated there were minor differences in favour of using the DTI strategy versus the standard wash off procedure and in some cases the DTI solution gave a complete level of improvement on the 1-5 scale used for staining comparisons ISO.