Preparation And Chemicals Required To Color With Reactive Dye

Except in special instances, batchwise preparation before reactive dyeing is carried out in the dyeing machine itself or in equipment of similar design reserved for the preparation stage. The essential requirements are that the material must be made available for dyeing in a neutral, uniform and readily absorbent state. In contrast to the application of vat or sulphur dyes, typical reactive dyeing processes will not eliminate natural or added fats and waxes.

Acceptable results are often possible on knitgoods without lengthy pretreatment, as in the pad–batch or hot batchwise dyeing processes in the presence of a powerful wetting agent. Residual size must always be removed from woven goods because of the risk of dye wastage by reaction with hydroxyl groups in size components. Owing to the brilliant hues of many reactive dyes, sufficient brightness may be attainable on woven cotton without prebleaching. Thoroughly desized and scoured fabrics can be used in many cases. Where bleaching is necessary it is imperative to check that all traces of residual chlorine or peroxy compounds are removed prior to dyeing, otherwise loss of reactivity and even partial destruction of some dyes can occur. There is considerable variation in the ability of reactive dyes to cover dead or immature cotton. For this reason it may sometimes be necessary to causticise or mercerise woven fabrics in order to achieve a satisfactory appearance in certain hues. Such pretreatments give the further advantage of better colour yield.

With few exceptions, reactive dyes have good solubility in water. Although seldom sensitive to neutral hard water, precipitation of hardness constituents results at the alkaline fixation stage and thus soft water should be used for all dissolving and dyebath operations. Sources of water with variable bicarbonate content, as might arise in a water supply with temporary hardness, can adversely affect the reproducibility of the dye fixation conditions. The influence of the bicarbonate anions largely depends on the alkali used for fixation. Alkalis based on sodium hydroxide exhibit higher sensitivity than sodium carbonate systems [1].
The use of either common salt (sodium chloride) or Glauber’s salt (hydrated sodium sulphate), in large amounts, is essential to all batchwise dyeing processes for reactive dyes. The relative prices, purity and availability of these electrolytes vary considerably in different parts of the world and selection must take account of this. Common salt is widely used, but Glauber’s salt is preferred with certain bright royal blues based on anthraquinone chromogens and turquoise or green hues dyed with copper or nickel phthalocyanine derivatives. Common salt is more soluble and easier to dissolve than Glauber’s salt. Electrolyte, from whatever source, must be free from alkali, since the latter causes premature fixation or hydrolysis of the dye.

The impurities that can significantly influence the reproducibility of the dyeing process are the alkaline earth metals calcium and magnesium, as well as the transition elements copper and iron. Their adverse effect on the process can be seen as inferior reproducibility, unlevel dyeing and lower wet fastness. The quality and source of electrolyte have a major influence on the levels of trace metal impurities introduced into the dyebath. Common salt is obtained from underground deposits or from seawater. The levels of metal ion impurities vary according to source and degree of purification [47]. These impurities can be controlled by the appropriate use of sequestering agents, taking into account the pH and temperature of the dyeing process. Uncontrolled use of organic sequestrants, such as ethylenediaminetetra-acetic acid, can lead to problems of hue change and lower light fastness. All metal-complex reactive dyes, with the exception of the phthalocyanine derivatives, will give rise to this effect.

Whilst soda ash (98% anhydrous sodium carbonate) remains the most widely used alkali for reactive dyeing, sodium bicarbonate, sodium silicate, caustic soda and various phosphates are also important. Alkalis of high purity are recommended and additions of solid brands should be well-diluted beforehand. Hot, damp conditions of storage should be avoided and dry scoops should be used when weighing. Sodium bicarbonate should be dissolved at low temperatures and direct heating by steam injection must be avoided. Caustic soda and sodium silicate are normally marketed and used as concentrated liquors of known concentration. In recent years, liquid buffer systems and liquid alkali products such as Alkaflo (Tanatex) have been introduced mainly for automatic dosing and metering systems [48].

Provided goods have been prepared efficiently for exhaust dyeing, it is unnecessary to add wetting or levelling agents to the dyebath. In winch or overflow dyeing of tubular-knitted cotton, however, minimal addition of a wetting agent can provide a lubricating action for the avoidance of rope marks. After any pretreatment with wetting agent, thorough rinsing is advisable before a fresh bath is set for dyeing, in the interest of reproducibility. Unwanted foam in jet or overflow machines can be minimised by careful addition of selected antifoam agents. When dyeing in enclosed machines at temperatures higher than 70°C, certain azo reactive dyes may undergo reduction owing to the combined effects of heat, alkali and aldehydic groups in the cellulose. If this problem is expected to occur, it is advisable to add 1–2 gl-1(grams per liter) of sodium m-nitrobenzenesulphonate as a protecting agent.

References:

1. P S Collishaw, B Glover and M J Bradbury, J.S.D.C., 108 (1992) 13.
2. T D Fulmer, Am. Text. Int., 16 (1987) 69.