Dyeing Problems with Reactive, Direct, Sulphur and Vat Dyes

Dyeing Problems:
There are a number of problems of the reproducibility and difficulties in obtaining right first time dyeing. Two important aspects of dyeing, namely dye variables and system variables, along with important characteristics of dyeing such as exhaustion, migration and levelling, fixation and color yield, and washing-off and fastness are responsible for this problem.

Many problems take place during dyeing operation. Some problems are given below:

1. Aggregation of dyes

2. Unlevelness (due to material)

Barriness

3. Unlevelness (due to other reasons)

Skitterness
Listing
Pale areas after dyeing

4. Reproducibility

Deviation of shade
Dyeing too pale
Precipitates in the dyebath
Change of shade

5. Fastness properties

Unexpectedly poor light fastness
Unexpectedly poor wet and rub fastness

6. Spots, marks

Precipitates in the dyebath
Singeing droplets
Change of shade (e.g. blue spots in brilliant red shades)
Dark spots
Specks
Dirt spots

7. Appearance of the goods

Dimensional stability (shrinkage)
Creasing
Chafe marks
Stitch distortion (knits)
Moire effects (on beam)
Lustre
Handle
Pilling (staple fibers)

8. Other problems

Difficulty with heavy shades on fine denier yarn
Soiling of machine
Frosting on goods
Poor running of material in subsequent processing

9. Thermosol dyeing problems

Listing
Two-sidedness
Dark or pale selvedges
Barry dyeings
Spots, specks
Ending
Pale points of intersection
Foam formation
Frosting

10. Unlevelness (due to dyeing conditions)

Ending problems
Cloudy dyeing
Pale areas

Dyeing problems with different dyes are described below:

Problems in dyeing with reactive dyes:
In case of reactive dyes, there is a wide variety in terms of their chemical structure, and unless one understands the chemistry, he/she shall not be able to design the correct process to get the required results.

The two most important components of a reactive dye are the chromophore and the reactive group. 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 fibers.

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 the electrolyte, lower temperature, higher pH (up to 11) and lower liquor-to-goods ratio.

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.

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.

Dyes with higher diffusion coefficients usually result in better levelling and more rapid dyeing. Diffusion is hindered by the dye that has reacted with the fiber and the absorption of active dye is restrained by the presence of hydrolysed dye. Different types of dyes have different diffusion characteristics.

Dyes with better solubility can diffuse easily and rapidly into the fibers, 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.

It is observed that a higher temperature in dyeing with reactive dyes results in a higher rate of dyeing, lower color yield, better dye penetration, rapid diffusion, better levelling, easier shading, a higher risk of dye hydrolysis and lower substantivity.

While dyeing, the pH of the dyebath will not remain same and will reduce significantly as the dyeing proceeds. 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.

The addition of electrolyte results in an increase in the rate and extent of exhaustion, increase in dye aggregation and a decrease in diffusion. At lower liquor ratios, there is a higher exhaustion and higher color strength. It is possible to enhance dye uptake on cellulosic fibers with the aid of suitable surfactants. The factors that affect the fastness of reactive dyes are the chromophore group, the stability of the dye–fiber bond and the completeness of the removal of the unfixed dye. To maximize wet fastness, particularly in deep shades, it is advisable to apply cationic agents for after-treatments. It is, therefore, very necessary to understand the dyes and prepare the process sheet.

Problems in dyeing with direct dyes:
Direct dyes represent an extensive range of colorants that are easy to apply and also are very economical. There are three common ways to classify direct dyes, namely, according to their chemical structure, dyeing properties and fastness properties.

Classification of direct dyes by the Society of Dyers and Colorists is based upon the compatibility of different groups of direct dyes with one another under certain conditions of batch dyeing; there are three classes of direct dyes: A, B and C. Class A consists of self-levelling direct dyes. Dyes in this group have good levelling characteristics and are capable of dyeing uniformly even when the electrolyte is added at the beginning of the dyeing operation. They may require relatively large amounts of salt to exhaust well. Class B consists of salt-controllable dyes. These dyes have relatively poor levelling or migration characteristics. They can be batch-dyed uniformly by controlled addition of electrolyte, usually after the dyebath has reached the dyeing temperature.

Class C consists of salt- and temperature-controllable dyes. These dyes show relatively poor levelling or migration and their substantivity increases rapidly with increasing temperature. Their rate of dyeing is controlled by controlling the rate of rise of temperature as well as controlling the salt addition. The dyebath variables that influence the dyeing behaviour of direct dyes include temperature, time of dyeing, liquor ratio, dye solubility, and the 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 as pad-batch or padroll) and by continuous methods (such as pad-steam). Many direct dyes are suitable for application by combined scouring and dyeing. In this process, the usual practice is to employ soda ash and nonionic detergent. However, dyes containing amide groups are avoided because of the risk of alkaline hydrolysis. Direct dyes vary widely in their fastness properties and staining effects on various fibers. Most direct dyes have limited wet fastness in medium to full shades unless they are after-treated.

The fastness of selected direct dyes can be improved in several ways, such as 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, diazotization and development and 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 fiber neps, which depends on 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 neps coverage than those having higher molecular weight.

It is, therefore, necessary for the dyer to understand the dyes and the process.

Problems in dyeing with sulphur dyes:
Sulphur dyes have been classified into four main groups: CI Sulphur dyes, CI Leuco Sulphur dyes, CI Solubilized Sulphur dyes and CI Condensed Sulphur dyes. They are available in various commercial forms such as powders, prereduced powders, grains, dispersed powders, dispersed pastes, liquids and water-soluble brands. The various steps in the application of sulphur dyes depend very much on their type and commercial form. The auxiliaries used in sulphur dyeing are reducing agents, antioxidants, sequestering agents, wetting agents, oxidizing agents and fixation additives.

Two special problems in dyeing with sulphur dyes are acid tendering and bronziness. In severe conditions of heat and humidity, some sulphur dyes, notably black, can generate a small amount of sulphuric acid within the cellulosic fibres, leading to tendering. Some sulphur-dyed textile material deteriorates under normal storage conditions. In the event of the dyeing needing subsequent correction, alkylated sulphur dyeings are diffi cult to strip and attempted removal will often entail the destruction of the dye chromogen. Fixation additives, such as alkylating agents based on epichlorohydrin, give dyeings of markedly improved washing fastness but often at the risk of some decrease in lightfastness.

The addition of copper sulphate to batch-wise oxidation baths of sodium dichromate/acetic acid improves the lightfastness but results 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. It is, therefore, necessary to understand the dyes and chemicals and decide the process.

Problems in dyeing with vat dyes:
Vat dyes are preferred where the highest fastness to industrial laundering, weathering and light are required. Based on the temperature and the amount of caustic soda, hydrosulphite and salt used in dyeing, vat dyes can be classified into four main groups:

IN dyes—require high temperature and a large amount of caustic soda and sodium hydrosulphite
IW dyes—require medium temperature and a medium amount of caustic soda and sodium hydrosulphite with salt added
IK dyes—require low temperature and a small amount of caustic soda and sodium hydrosulphite with salt added
IN special dyes—require more caustic soda and higher temperature than IN dyes

Generally, vat dyes have a very rapid strike, a good degree of exhaustion and a very low rate of diffusion within the fiber. They have different chemical structures which differ in the solubility of their sodium leuco-vat, stability towards over-reduction, stability towards over-oxidation, substantivity and rate of diffusion.

Commercial competitive dyes have fairly equal particle sizes. Large particle sizes give dispersions of poor stability. For some vat dyes, color yield decreases with increasing particle size. The effect is generally dye-specific. The rate of reduction of vat dyes depends on various factors, such as the particle size of the dye, the temperature, time and pH during reduction and access to the reducing agent. The stability of alkaline solutions of reducing agents decreases with increased temperature, greater exposure to air, greater agitation and lower concentration of the reducing agent.

Vat dyes of the indanthrene type produce duller or greener shades at dyeing temperatures higher than 60°C due to over-reduction. Over-reduction can be prevented by the use of sodium nitrite if the reducing agent is hydrosulphite. In the case of thiourea oxide, over-reduction cannot be prevented by nitrite. The factors influencing the rate of dyeing with vat dyes are the type of substrate, temperature, liquor ratio and concentration of dye and electrolyte. Mercerized cotton gives a higher rate of dyeing compared with unmercerized cotton, which in turn gives a higher rate than grey material. At low temperature, the rate of exhaustion is low, which might promote levelness but the rate of diffusion is also low.

At high temperature, the rate of exhaustion is high, which decrease levelness, but the rate of diffusion is high. Some dyes are not stable at very high temperatures so the stability of dyes to temperature must be taken into account. The reducing efficiency of sodium hydrosulphite in caustic soda solutions at high temperatures decreases rapidly in the presence of air. The higher the liquor ratio, the slower is the rate of dyeing. Most of the dyes exhaust more rapidly at low concentrations, increasing the risk of unlevel dyeing in light shades. Some have the same rate of dyeing irrespective of the concentration. The higher the concentration of the electrolyte, the higher is the rate of dyeing.

The very high pH and temperature during rinsing result in dulling of the shade. The ideal is to do rinsing thoroughly at low temperature at a rinsing bath pH value of 7.

Oxidation is done in vat dyeing to convert the water-soluble leuco form back into the insoluble pigment form. Normal variables in the oxidizing step are the type and concentration of oxidizing agent, the type of pH regulator and pH during oxidation, and temperature during oxidation. The oxidizing agent must provide a level of oxidation potential sufficient to oxidize the reduced vat dye into insoluble pigment, with no over-oxidation, that is, beyond the oxidation state of the original pigmentary form of the dye. Poor control of pH during oxidation may result in uneven oxidation and a lower temperature may result in slower oxidation.

At pH below 7.5, there is the possibility of formation of acid leuco forms of vat dyes. The optimum pH for oxidation is 7.5–8.5. The acid leuco form of vat dye is difficult to oxidize, has little affinity for fiber and is easily rinsed out. The higher the temperature, the faster is the oxidation, the optimum temperature being 120–140°F.

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