Advancements in Reactive Textile Dyes

Introduction

Throughout the long history of the tinctorial arts, the dyer has been confronted with match the performance of has colourants as closely as possible with the useful life of the fabric.

The dyeing of cotton (cellulosic) with direct, vat or azoic dyes depends on adsorption of dye molecules into the fibre, while vat and azoic dyes, the dyes are first adsorbed on to the fibre followed by reaction to convert them into an insoluble form, thus retained on the fibre. The possibility of attaching dyes to fibres by forming covalent bond has for long been attractive to dyestuff chemists because attachment by physical adsorption and mechanical retention has the disadvantages of either low wet fastness or high cost1.

An event which had far-reaching consequences was the discovery made by Rattee and Stephen (ICI) in 1954, of dyes containing a dichlorotriazinyl group which could be applied from aqueous solution and caused to form a covalent bond with cellulose by increasing the pH. This discovery resulted in first commercial reactive dyes for cellulose.

The appearance of reactive dyes on the market gave a great impulse to the investigation of reactive systems other the s-triazine and within a few years, several other ranges of commercial reactive dyes were available. The impact of reactive dyes in the synthetic dyestuffs scenario has been to such an extent that today reactive dyes constitute the largest group of dyes both on a monetary and bulk basis.

The reason for such rapid increases in demand for reactive dyes is primarily due to the excellent characteristic of reactive dyes. Eg. Their brilliant shades, excellent wet fastness of dying and simple dyeing operations, which have increasingly been accepted within the industry.

Despite the obvious advantage associated with reactive dyes, there are many problems with present qualified by reactive dyes have been raised below:

• In exhaust dye the degree of exhaustion and fixation is low and the colouring

• The degree of wastewater is high.

• The problem occurs in levelness and reproducibility of dying.

• More water, energy and a long time are required for washing off.

• In wearing or washing textile goods of reactive dye or printing, colour change and bleeding to white areas can be done which leads to consumer complaints that for such problem many companies have developed the dyes for dyeing theories.

In the single dyeing process the application temperature is fixed by the need to apply the disperse dye at an elevated temperature, and when the chosen cotton dye is a reactive dye this limits the choice to those dyes designed for the host application. An immediate difficulty arises because the satisfactory applications of disperse dye calls for the dyeing process to be operated either at neutrality or under acidic condition. Since alkaline condition lower dispersion stability, whereas the pressure of alkali is normally required for the application of reactive dye in order to ionize the cellulose and promote the dye-fibre reaction. Many developments have been done in this aspect during the last few years.

The research activity in reactive dyes has been mainly directed towards solving the mentioned deficiencies of reactive dyes. Moreover, the use of some new chromophoric system has been disclosed in the patent literature.

Chemistry of Reactive Dyes

The dyeing principle based on the fibre reactivity involves the reaction of a functional group of the dyestuff with a site on the fibre to form a covalent link between the dye molecules and the substrate.2

The four structural feature of a typical reactive dyes molecule are:

• The chromophoric grouping, contributing the colour much of the substantively for cellulose.

• The reactive system, enabling them to dye to react with the hydroxyl group in cellulose.

• A bridging group that links the reactive system to the chromophore

• One or more solubilising group, usually sulphuric acid substituent attached to the chromophoric group for their colour, although the azo chromophore –N=N- is by far the most important

All reactive dye contain sodium sulphonate group for solubility and dissolve in water to give coloured sulphonate anions and sodium cations. Most reactive dyes have from one to four of these sulphonate group-reactive dye molecules. However, do have several specific structural features of their own.