1.        Problem Solving with Reactive Dyes:

Reactive dyes are one of the most commonly used application class of dyes for cotton materials. Two important aspects of reactive dyeing, namely dye variables and system variables, are discussed in this section, along with important characteristics of reactive dyeing such as exhaustion, migration and leveling, fixation and colour yield, and washing off and fastness. A significant portion of this section also deals with the problem of the reproducibility and difficulties in obtaining right-first-time dyeing.

Dye Variable in Reactive Dyeing

The major dye variables that affect reactive dyeing are dye chemistry, substantivity, reactivity, diffusion coefficient, and solubility. Each of these will be briefly discussed below.

Dye chemistry:

Reactive dyes have a wide variety in terms of their chemical structure. The two most important components of a reactive dye are the chromophore and the reactive group. The characteristics governed by the chromophore are colour gamut, light fastness, chlorine/bleach fastness, solubility, affinity, and diffusion. The chromophores of most of the reactive dyes are azo, anthraquinone, or phthalocyanine. Azo dyes are dischargeable. Dis-azo dyes have the disadvantage of being much more sensitive to reduction and many of them are difficult to wash-off.

Anthraquinone dyes exhibit relatively low substantivity and are easier to wash-off. Most of them possess excellent fastness to light and to crease-resistant finishes, but they are not dischargeable. Phthalocyanine dyes diffuse slowly and are difficult to wash off.

Metal complex dyes containing copper possess rather dull hues but show a high degree of fastness to light and to crease-resistant finishes. Their substantivity is fairly high; 1:2 complexes diffuse relatively slowly, so a longer time is needed to wash-out unfixed dye completely.

The dye characteristics governed by the reactive group are reactivity, dye fibre bond stability, efficiency of reaction with the fiber, and affinity. Dyeing conditions, especially the alkali requirement and temperature as well as the use of salt also depend on the type of reactive group. Dyes based on s-triazine do not have good wet fastness properties in acidic media and, due to their high substantivity, have poor wash-off properties.

Similarly, dyes having a vinyl sulphone reactive system have poor alkaline fastness. The chemical bond between the vinyl sulphone and the cellulosic fibre is very stable to acid hydrolysis.


The substantivity of hydrolysed byproducts of vinyl sulphone is low, so washing off is easy. Monochloro triazines have good fastness to light, perspiration and chlorine. The turquoise reactive dye shows an optimum dyeing temperature that is generally about 20C higher than that of other dyes with the same reactive group. The fluorotriazine groups form linkages with cellulose that are stable to alkaline media.


Reactive dyes of dichloroquinoxaline, monochlorotriazine and monofluo triazine types show a lower resistance tendency to peroxide washing and dye-fibre bond stability. A lower sensitivity to changes in dyeing conditions (particularly temperature) is the most important characteristic feature of the monochlorotriazine-vinyl sulphone hetero bi-functional dyes. Dyeing properties of some important reactive groups have been discussed in detail by various authors.


Substantivity:


Substantivity is more dependent on the chromophore as compared to the reactive system. A higher dye substantivity may result in a lower dye solubility, a higher primary exhaustion, a higher reaction rate for a given reactivity, a higher efficiency of fixation, a lower diffusion coefficient, less sensitivity of dye to the variation in processing conditions such as temperature and pH, less diffusion, migration and levelness, a higher risk of unlevel dyeing, and more difficult removal of unfixed dye. Substantivity is the best measure of the ability of a dye to cover dead or immature fibres.


Covering power is best when the substantivity is either high or very low. An increase in the dye substantivity may be affected by lower concentration of the dye, higher concentration of electrolyte, lower temperature, higher pH (up to 11) and lower liquor to goods ratio.


Reactivity:


High dye reactivity entails a lower dyeing time and a lower efficiency of fixation. (To improve the efficiency of fixation by reducing dye reactivity requires a longer dyeing time and is, therefore, less effective than an increase in substantivity.) Also there is a wider range of temperature and pH over which the dye can be applied. Reactivity of a dye can be modified by altering the pH or temperature, or both. By a suitable adjustment of pH and temperature, two dyes of intrinsically different reactivity may be made to react at a similar rate.


Diffusion coefficient:


Dyes with higher diffusion-coefficients usually result in better leveling and more rapid dyeing. Diffusion is hindered by the dye that has reacted with the fibre and the absorption of active dye is restrained by the presence of hydrolysed dye. Different types of dyes have different diffusion characteristics. For example, the order of decreasing diffusion is: unmetallised dyes, 1:1 metal-complex dyes, 1:2 metal complex dyes; phthalocyanine dyes. An increase in the diffusion is affected by increasing temperature, decreasing electrolyte concentration, adding urea in the bath and using dyes of low substantivity.


Solubility:


Dyes of better solubility can diffuse easily and rapidly into the fibres, resulting in better migration and levelling. An increase in dye solubility may be affected by increasing the temperature, adding urea and decreasing the use of electrolytes.


1.     System Variables in reactive Dyeing


Temperature:


A higher temperature in dyeing with reactive dyes results in a higher rate of dyeing, lower colour yield, better dye penetration, rapid diffusion, better levelling, easier shading, a higher risk of dye hydrolysis, and lower substantivity. Raising the temperature appears to result in an opening-up of the cellulose structure, increasing the accessibility of cellulose hydroxyls, enhancing the mobility as well as the reactivity of dye molecules and overcoming the activation energy barrier of the dyeing process, thereby increasing the level of molecular activity of the dye-fibre system as well as dye-fibre interaction. A comparison of hot and cold reactive dyes has been given in along with some technical advantages of hot reactive dyes over cold reactive dyes.


pH:


The intial pH of the dyebath will be lower at the end of dyeing by one half to a whole unit, indicating that some alkali has been used up during dyeing. The cellulosic fibre is responsible for some of this reduction, while a smaller part is used by the dyestuff as it hydrolyses. In discussing the effect of pH, account must be taken of the internal pH of the fibre as well as the external pH of the solutior. The internal pH is always lower than the external pH of the solution.


 

As the electrolyte content of the bath is increased, the internal pH tends to equal the external pH. Since the decomposition reaction is entirely in the external solution, the higher external pH favours decomposition of the dye rather than reaction with the fibre. pH influences primarily the concentration of the cellusate sites on the fibre. It also influences the hydroxyl ion concentration in the bath and in the fibre. Raising the pH value by 1 unit corresponds to a temperature rise of 20oG. The dyeing rate is best improved by raising the dyeing temperature once a pH of 11-12 is reached. Further increase in pH will reduce the reaction rate as well as the efficiency of fixation. Different types of alkalis, such as caustic soda, soda ash, sodium silicate or a combination of these alkalis, are used in order to attain the required dyeing pH. The choice of alkali usually depends upon the dye used, the dyeing method as well as other economic and technical factors.


Electrolyte:


The addition of electrolyte results in an increase in the rate and extent of exhaustion, increase in dye aggregation and a decrease in diffusion. The electrolyte efficiency increases in the order: KGI < Na2S04 < NaG!. There may be impurities present in the salt to be used, such as calcium sulphate, magnesium sulphate, iron, copper and alkalinity, that can be a source of many dyeing problems.


Liquor ratio:


At lower liquor ratios, Diffusion coefficient: Dyes with be lower at the end of the dyeing by there is a higher exhaustion and higher exhaustion and higher color strength. An increase in colour strength may be attributed to greater availability of dye active species in the vicinity of the cellulose macro molecules, at lower liquor ratio. Surfactants and other auxiliaries: It is possible to enhance dye uptake on cellulosic fibres with the aid of suitable surfactants. Amongst all the systems, the highest dye uptake is obtained with anionic surfactants.


Non-ionic surfactants may result in a decrease in dye exhaustion and colour yield, and a change in shade. Some non-ionic surfactants may slow down the dye hydrolysis. Triethanolamine (TEA) is known to enhance colour strength by enhancing the swell ability and accessibility of the cellulose structure. It may also modify the state of the dye, thereby enhancing its reactivity and increasing the extent of covalent dye fixation.


This article was originally published in the August issue of the magazine, New Cloth Market the complete textile magazines from textile technologists."