Problem Solving in Dyeing with Direct Dyes for Cotton Textile Materials

Problems in Dyeing With Direct Dyes

Direct dyes represent an extensive range ofcolorants that are easy to apply and also are very economical. There are threecommon ways to classify direct dyes, namely, according to their chemicalstructure, according to their dyeing properties, and according to theirfastness properties.

Of these three possible ways of classifying directdyes, the first is of least importance to the dyer, although of considerableimportance to those interested in dye chemistry. According to the Society ofDyers and Colourists' classification, which is essentially based upon thecompatibility of different groups of direct dyes with one another under certainconditions of batch dyeing, there are three classes of direct dyes: A, Band C.

Class Aconsists of self-leveling direct dyes. Dyes in this group have good levellingcharacteristics and are capable of dyeing uniformly even when the electrolyteis added at the beginning of the dyeing operation. They may require relativelylarge amounts of salt to exhaust well.

Class Bconsists of salt-controllable dyes. These dyes have relatively poor levellingor migration characteristics. They can be batch dyed uniformly by controlledaddition of electrolyte, usually after the dye bath has reached the dyeingtemperature.

Class Cconsists of salt- and temperature-controllable dyes. These dyes show relativelypoor leveling or migration and their substantivity increases rapidly withincreasing temperature. Their rate of dyeing is controlled by controlling therate of rise of temperature, as well as controlling the salt addition.

Important dye bath variables that influence thedyeing behaviour of direct dyes include temperature, time of dyeing, liquorratio, dye solubility, and presence of electrolyte and other auxiliaries.

Direct dyes can be applied by batch dyeing methods(on jigs, jet or package dyeing machines), by semi-continuous methods (such aspad-batch or pad roll) and by continuous methods (such as pad-steam). Manydirect dyes are suitable for application by combined scouring and dyeing. Inthis process the usual practice is to employ soda ash and non-ionic detergent.However, dyes containing amide groups are avoided because of the risk ofalkaline hydrolysis.

Direct dyes vary widely in their fastnessproperties, and staining effects on various fibres. Most direct dyes, however,have limited wet fastness in medium to full shades unless they areafter-treated.

The fastness of selected direct dyes can beimproved in several ways, such as the following:

• Treatment with cationic fixing agents
• Treatment with formaldehyde
• Treatment with copper salts such as copper sulphate

• Treatment with cationic agents and copper sulphate in combination
• Diazotisation and development
• Treatment with cross linking agents or resins

An important consideration in dyeing with direct dyes is the ability of the dyes to cover the immature cotton fibre neps, which has been explained, in most cases, in terms of both the molecular weight and hydrogen bond formation capacity of the dye molecules. Given a similar capacity to form hydrogen bonds, dyes having lower molecular weight show proportionately better nep coverage than those having higher molecular weight.

Problems in Dyeing With Sulphur Dyes

Despite their environmental concerns, which are constantly being addressed, sulphur dyes occupy an important place for dyeing of inexpensive black, blue, brown and green shades in medium to heavy depths on cellulosic fibres.

The history, development and application of sulphur dyes have been widely reviewed by various authors. Sulphur dyes have been classified into four main groups: CI Sulphur dyes; CI Leuco Sulphur dyes; CI Solublised Sulphur dyes; and CI Condensed Sulphur dyes.

CI Sulphur dyes are water-insoluble, containing sulphur both as an integral part of the chromophore and in attached polysulphide chains. They are normally applied in the alkaline reduced form from a sodium sulphide solution and subsequently oxidized to the insoluble form on the fibre.

Sulphur dyes differ from the vat dyes in being easier to reduce but more difficult to re-oxidise, different oxidants producing variations in hue and fastness properties. A leuco sulphur dye has the same CI constitution number as the parent sulphur dye but exists as the soluble leuco form of the parent dye together with a reducing agent in sufficient quantity to make it 8llitable for application either directly or with only a small addition of extra I8ducing agent.

A solublised sulphur dye has a different constitution number because it is a chemical derivative of the parent dye, non-substantive to cellulose but converted to the substantive form dyeing.

Condensed sulphur dyes, although containing sulphur, bear little resemblance to traditional sulphur dyes -their constitution and method of manufacture.

Sulphur dyes are available in various commercial forms such as powders, pre-reduced powders, grains, dispersed powders, dispersed pastest, liquids and water soluble-brands.

The various steps in the application of sulphur dyes depend very much on their type and commercial form. The main steps in the application of water-insoluble sulphur dyes are as follows:

• Reduction, whereby the water insoluble dye is converted into water-soluble form.
• Application, whereby the solubilised dye is applied onto the substrate by a suitable exhaust or continuous method.
• Rinsing, whereby all loose colour is removed before the oxidation stage.
• Oxidation, whereby the dye absorbed by the substrate is oxidized back into water-insoluble form.
• Soaping, this results in an increase in brightness as well as improved fastness of the final shade.

The auxiliaries used in sulphur dyeing are: reducing agents, antioxidants, sequestering agents, wetting agents, oxidizing agents and fixation additives.

The two most important reducing agents for sulphur dyes are sodium sulphide [Na2S] and sodium hydrosulphide [NaHS]. Caustic soda/ sodium dithionite are conventional chemicals for vat dye reduction but this system is difficult to control in the application of sulphur dyes and tends to give inconsistent results except with certain sulphur vat dyes.

A sodium carbonate/sodium dithionite mixture is too weakly alkaline for the water insoluble type sulphur dyes and requires careful control if over reduction and consequent low colour yield are to be avoided. Glucose in the presence of alkali, usually caustic soda or a caustic soda/soda ash mixture has been used as another possible sulphur dye reducing agent, but it is a weak reducing agent as compared to sodium sulphide or sodium hydrosulphide.

Other reducing agents such as thioglycol, hydroxyacetone and thiourea dioxide, have had limited success. Sodium polysulphide and sodium borohydride can be used as antioxidants to inhibit premature oxidation, promote better dyebath stability and lessen the risk of bronzing, poor rubbing fastness and dark selvedges. Sequestering agents are used where water quality is poor or variable, to avoid poor rubbing fastness or unlevelness in the presence of multivalent ions in the dye liquor or in the substrate.

Wetting agents may be used to improve the wet ability of the substrate. Although the majority of sulphur dyes are unaffected by most wetting agents, some non-ionic wetting agents may inhibit the dye uptake in exhaust dyeing or precipitate the dye as a tarry leuco product.

Traditionally, the most preferred oxidizing system has been sodium dichromate/acetic acid because of its ability to rapidly and completely oxidize all reduced sulphur dyes, resulting in good colour yield and fastness properties. Nevertheless, it has been criticized increasingly on environmental grounds and for its effects on handle and sews ability, especially with sulphur blacks. The addition of 1 g/l copper sulphate to batch wise oxidation baths of sodium dichromate/acetic acid improves the light fastness but may result in dulling of the shades, as well as harsher handle. It is not recommended with sulphur blacks, where the presence of copper promotes acid tendering.

Other oxidizing agents that have been tried as alternatives to sodium dichromate/acetic, with various degrees of success, include: potassium iodate/acetic acid; sodium bromate; hydrogen peroxide and peroxy compounds; and sodium chlorite. Fixation additives, such as alkylating agents based on epichlorohydrin, give dyeing of markedly improved washing fastness but often at the risk of some decrease in light fastness. Moreover, in the event of the dyeing needing subsequent correction, alkylated sulphur dyeing are difficult to strip and attempted removal will often entail destruction of the dye chromogen.

Image Courtesy:

1.      Indiamart.com

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