Important Considerations in Dyeing with Vat Dyes

Vat dyes are the primary choice where the highest degree of fastness to industrial laundering, weathering and light are required. Based on temperature, 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; and
• IN Special dyes require more caustic soda and higher temperature than IN dyes.

Generally speaking, vat dyes have a very rapid strike, a good degree of exhaustion and a very low rate of diffusion within the fibre. Vat dyes of different chemical structure may 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, colour yield decreases with increasing particle size. The effect is generally dye-specific.

The main stages in the dyeing of cotton with vat dyes are as follows:

• Conversion of insoluble vat pigment into soluble sodium leuco-vat anions [reduction]
• Diffusion of sodium leuco-vat anions into cellulosic fibres
• Removal of excess alkali and reducing agents by washing off
• Oxidation of the soluble dye into insoluble pigmentary form within the cellulosic fibres, and
• Soaping, during which the isolated molecules of vat pigments are re-orientated and associate into a different, more crystalline form

Important requirements of vat dye reducing agent are as follows:

• A level of reducing power (reduction potential) sufficient to reduce all commercial vat dyes to their water soluble form, quickly and economically
• Conversion of the vat dyes into products from which the original pigment can be restored (no over-reduction)

Various reducing systems for vat dyes have been proposed and used. The most common type of reducing agent used for dyeing with vat dyes is sodium hydrosulphite, commonly known as hydros but more correctly known as sodium dithionite, which has the chemical formula Na2S2O4. Although a part of the hydros is used up in the reduction of vat dyes, a large part of it may be destroyed by its reaction with oxygen in the air (oxidation), particularly at higher temperatures.

The rate of reduction of vat dyes depends upon various factors, such as the following:

• Particle size of the dye,
• The temperature, time and pH during reduction, and
• Concentration of the reducing agent

The stability of alkaline solutions of reducing agents may decrease with increased temperature, greater exposure to air, greater agitation and lower concentration of the reducing agent. Vat dyes of the Indanthrene type may 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 include the following:

• Type of the substrate,
• Temperature,
• Liquor ratio,
• Concentration of dye, and
• Concentration of electrolyte

Mercerized cotton gives a higher rate of dyeing as compared to un-mercerized cotton which in turn gives higher rate than the 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 might decrease levelness but the rate of diffusion is high. Maximum exhaustion, penetration and levelness can be obtained by starting the dyeing at low temperatures in the leuco stage and slowly raising the temperature. Some dyes may not be stable to 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 electrolyte, the higher is the rate of dyeing.

The purpose of rinsing before oxidation is to remove any loose dye, excess of reducing agent and alkali to lower the pH and establish conditions favourable for oxidation. The higher the temperature and/or pH of the rinsing bath, the lower is the colour strength. Very high pH and temperature during rinsing may also result in the dulling of the shade. The ideal is to do rinsing thoroughly at low temperature at a rinsing bath pH value of 7.

The purpose of oxidation is to convert the water-soluble leuco from of the vat dye, back into the insoluble pigment form. The important variables for the oxidizing step are:

• The type and concentration of oxidising 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 i.e., 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. A pH below 7.5 should be avoided to prevent the possible formation of acid leuco form 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 fibre and is easily rinsed out. The higher the temperature, the faster is the oxidation, the optimum temperature being 120-140 °F.

The purpose of soaping after oxidation is to remove any dye that is not diffused into the fibre and to stabilise the final shade. This results in improved fastness properties and resistance to any shade change to a resin or other finish or to consumer use. Important soaping parameters are:

• Time,
• Temperature,
• Type and concentration of soaping auxiliaries

Even when no detergent is used, the dyeings exhibit good colour strength and good fastness properties. Washing with water alone tends to give a slightly higher colour yield. It is best to carry out soaping without any detergent at boiling temperature. After soaping the fabric is rinsed and dried

Both exhaust and continuous dyeing methods are used to apply vat dyes. Exhaust dyeing processes are mainly used for dyeing of loose stock, yarn and knitted fabrics. Woven fabrics can also be dyed by exhaust method but for large batch sizes, the continuous method is mostly used.

Pad dyeing methods are usually a preference in case of woven fabrics particularly, if these are in large batches. The commonly used pad dyeing methods are pad-jig, pad steam and pad thermosol. The most popular method for dyeing woven fabrics in a continuous manner is pad-dry-pad-steam method, consisting of the following key steps:

• Impregnation of the fabric in a bath containing vat dye, dispersing agent, anti-migrant and a non-foaming wetting agent
• Squeezing the impregnated fabric to a given pick up level
• Drying the fabric to achieve a uniform distribution of the vat pigment throughout the fabric
• Impregnating the fabric with a solution of caustic soda and sodium hydrosulphite, with the optional use of salt
• Expressing the impregnated fabric to a given pick up level
• Steaming the fabric to bring about reduction of the dye to the soluble leuco form and to promote diffusion of the dye into the cellulosic fibres, and
• Rinsing, oxidation, soaping, rinsing and drying the fabric

Intermediate drying is one of the most important steps in the pad-dry-pad-steam process where the most common problem ‘migration’ can take place. Important factors on which migration depends are as follows:

• Dye constitution,
• Dye formulation,
• Pick-up,
• Additives in the dye padder,
• Residues of wetting agents and lubricants on the fabric,
• Fabric structure, and
• Drying conditions

After drying, the fabric is padded with an alkaline solution of sodium hydrosulphite, after which the fabric undergoes steaming. Almost 40 % of vat dyeing problems are related to improper steaming conditions. Ideal steaming conditions are controlled temperature and moisture, freedom from air, and sufficient dwell time. After steaming, the fabric undergoes rinsing, oxidation and soaping.

The most important control steps in vat dyeing are reduction, absorption and oxidation. The reduction and oxidation can best be controlled by metered addition of chemicals. The advantages of metered addition of hydrosulphite are as follows:

• Better levelling by slower vatting
• No need of levelling agent
• Protection from over-reduction
• Control of initial rate of dyeing (strike)
• Possibility of warm pre-pigmentation to give optimum fabric/liquor movement
• Good reproducibility
• Reduction of sulphite/sulphate effluent pollution, and
• Automatic monitoring of vat state and the redox potential by means of measuring and regulating technology

The dosage of hydrogen peroxide in the oxidation tank, by measuring and controlling the pH has the advantages of constant pH during the production run, the presence of enough peroxide for oxidation and controlled speed of oxidation.