Advancements in Reactive Textile Dyes
Classifications, characteristics, developments, and advancements in Reactive Textile Dyes
The market for reactive dyes will continue to increase. This will arise partly from a marginal increase in the production of cellulosic fibres, essentially cotton, and more importantly from the replacement of other classes of cellulose dye, such as azoic and sulphur dyes, by reactive dyes.
Development in Reactive Dyes
Need for Development
The earlier reactive dyes contained dichlorotriazine and vinyl sulphone groups. These and other dyes were synthesized based on,
- There low affinity for textile especially cellulose fibres.
- On their reactivity towards fibres.
- The pH of the fixation reaction of the dye with the fibres.
- The optimum temperature of the reaction.
- Ease of washing out the hydrolyzed reactive dye from the dyed fibres.
Recently, three more parameters have been added the carcinogenicity of the dye, their reduction products, the sensitizing or the allergenic potential (causing constant dermatitis) and ease of treatment of the effluent containing these dyes, in view of the awareness of the requirement of the producing and offering to the end-user-Eco-friendly textile.
The reaction of these dyes with the functional group of the fibres like primarily hydroxyl group of cellulose and amino group of protein fibres is brought about by an alkali, such as sodium carbonate, sodium hydroxide, sodium silicates etc.
The dyeing of these dyes on the cellulosic fibre materials involves three distinct steps: 3
- Exhaustion of the dye in a neutral medium by the addition of common salt of the Glauber’s salt at a temperature lower than 1000
- Fixation if the exhausted dye by the addition of alkali, leading to the formation of a covalent bond between the dye and the fibre.
- Through washing out of the hydrolyzed reactive dye, which is loosely held on and in the fabric.
The third step is necessitated by the fact that, whereas a large portion of the reactive dye, taken for dyeing reacts with the fibre, some portion of dye does react with the water in the dye-bath under alkaline condition losing its reactivity toward the fibre and poor affinity for the fibre. If the step is not carried out properly, the washing fastness of the final dyeing becomes poor due to the presence of the hydrolyzed dye on fabric. While the dye reacted with the fibre contributes to the high washing fastness.
When a substantial amount of the dye is hydrolyzed during dyeing and washed out from the dyed fabric subsequently, it pollutes the dye-house effluent, leading to difficulties in the effluent treatment. Hence, high exhaustion reactive dyes have been made, so that very little amount of the dye gets hydrolyzed, leading to a less affluent problem.
Theoretically, it would be possible to achieve the objective of high fixation by use of dyes containing groups capable of reacting with cellulose without substantial hydrolysis. The problem has already been studied in case of reactive dyes for wool, where dyes containing acryloylamino groups (-CH2-CH-CO-NH2) suffer very little if any hydrolysis in a dye bath. Unfortunately, such groups although satisfactory in wool dyes are insufficiently active to provide the degree of reactivity usually considered desirable in reactive cellulose dyes.
The other possibility is to used dyes containing more than one reactive system the principle on which high fixation of dyes containing more than one reactive group is based is easily understood when dyeing process is regarded as acylation of alcohol that is cellulose conducted in water. A dye molecule with only one reactive centre that is a mono-functional acylation agent may react either with alcohol (cellulose) or with water, its reactivity is completely lost and it is in then incapable of reacting with former.
In the instance of a dye having more than one reactive centre, corresponding to a polyfunctional acylation agent, that part of a dye, which has reacted with water, is still capable of further reaction with cellulose is still capable of fixation on fibre.