DYEING
INTRODUCTION
Dyeing is a process in which we impart colour into the fabric. All commercial textile dyeing processes take place by the application of a solution or a dispersion of the dyes to the textile material followed by some type of fixation process. The dye solution or dispersion is almost always in an aqueous medium. A major objective of the fixation step is normally to ensure that the coloured textile exhibits satisfactory fastness to subsequent treatment in aqueous wash liquors. Dyeing is mainly depends on the type of fabric, structure of fabric and the properties of dyes.
CLASSIFICATION OF DYES
For our convenience we make classes of dyes, due to these classes the application of dyes is much easier. The classifications of dyes are as follows:
- • Direct Dyes
- • Reactive Dyes
- • Sulphur Dyes
- • Vat Dyes
DIRECT DYES
Direct dyes have been used to dye cellulose for over 100 years. Because of the simplicity of application and great choice of products available, direct dyes are a popular dye class. Direct dyes ranges from moderate to poor in wash fastness. Light fastness varies from poor to excellent depending on the particular dye. Direct dyes depend on secondary forces such as hydrogen bonds and vander waals forces to bond with cellulose fibres. Since secondary forces are relatively weak. Therefore, direct dye molecules have relatively high molecular weights, typically 400 to 1200. The great majority of direct dyes are azo structures.
REACTIVE DYES
Reactive dyes contain groups that react with the hydroxyl groups in cellulose. The reaction between a reactive dye and the fibre produces a covalent bond. By their nature reactive dyes also react with water. Dye which reacts with the fibre is said to be “fixed” to the fibre. Dye which reacts with water is said to be “hydrolyzed”.
Fixation Reaction
Cell – CH2 – OH + DYE – X Cell – CH2 – O – DYE + HX
Cellulose reactive dye dyed fibre acid
X= reactive group
Hydrolysis reaction
H – O – H + DYE – X DYE – OH + HX
Dye molecules which react with the fibre and become fixed have excellent fastness to washing because of the high strength of the covalent bond. Dye molecules which are hydrolyzed may be weakly attached to the fibre and if not washed out at the end of the dye cycle have very poor wash fastness.
SULPHUR DYES
Sulphur dyes are widely used on cotton mainly because they are economical to use. They have good to excellent wash fastness and good light fastness in dark shades. Light fastness of pale shades is poor. Sulphur dyes are usually dull in shade since the molecular structures are complex. As a class, the sulphur dyes are not resistant to chlorine containing bleaches.
VAT DYES
Vat dyes are mainly on cellulosic fibres, but some can be applied to protein fibres. They usually have outstanding colourfastness properties. Vat dyes are more expensive and difficult to apply than other classes for cellulose such as directs sulphurs, and reactive. Indigo is a special case in the vat dye class. Indigo is attractive for its pleasing blue colour and for the unique fading characteristics of garment dyed with it. Vat dyes are characterized by the presence of a keto group. Vat dyes in keto form are water insoluble pigments.
BASIC STEPS IN THE DYEING PROCESS
The dyeing of cellulosic materials with vat dyes follows a four-step sequence:
- 1. Preparation of the vat containing the leuco forms of the dyes;
- 2. Dyeing of the material, in which the fibers absorb the water-soluble leuco compound;
- 3. Oxidation of the absorbed leuco compound back to the parent pigment inside the fibers
- 4. Soaping of the dyed material to remove pigment loosely adhering to the fiber surfaces and to develop the true shade and fastness properties.
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