Dyes

Colorants for textile materials may be classified as either dyes or pigments. The terms dye and pigment, while almost interchangeable in common use, have distinctly different meaning in coloration of textile. A dye is a substance affinity for the textile material. Dyes are soluble in the dyeing medium during or at least in some stage of the dyeing process. A pigment is simply a substance used to impart colour and which does not have inherent affinity for the textile material. Both dyes and pigments can be used to colour textile materials. Dyes can diffuse into fibers and interact with the polymer structure of the fiber. Pigments are simply bonded to the surface of the fiber, fabric, or yarn by other chemical agents. Pigments can be either organic or inorganic substances. All textile dyes are organic chemicals.

Dyes may be classified according to chemical structure or according to their method of application. Classification of dyes according to chemical structure is most useful to the dye chemist who may be interested dye synthesis and the relationship between chemical structure and properties of dye. Classification according to method of application is most useful to the technologist concerned with coloration of textile products. Understanding of both of these methods of classification is useful to the textile engineer or textile chemist.

Dye Classes by Application Method

Eight major dye classes according to method of application are commonly used in textile. The five classes used mainly on cellulose fibers are direct dyes, sulphur dyes, azoic dyes, reactive dyes, and vat dyes. The three classes used mainly for protein and synthetic fibers are acid dyes, basic dyes, and disperse dyes. All of these dyes are synthetic organic compounds that have been discovered since the synthesis of the first synthetic dye by W.H. Perkins in 1856. Several important synthetic dyes produced today have natural counterparts, but synthetic manufacture of the products is more economical than collecting the naturally occurring dyes . Following is a brief introduction to these eight major classes of dyes.

Direct dyes are so named because they have natural affinity for cellulose and can be applied without using any auxiliary chemicals . In practice, the dyeing rate and colour yield can be greatly improved by adding inorganic salts such as sodium chloride or sodium chloride or sodium sulfate to the dye bath. The direct dyes are widely used on cotton and rayon. The greatest limitation of direct dyes is that their fastness to washing is not good enough for some purpose Wash fastness of direct dyes is improved by resin treatment of dyed is improved by resin treatment of dyed fabrics or by various after treatments.

Sulphur dyes are complex organic compounds synthesized by heating simple amines or phenolic compounds in the presence of sulphur. Sulfur dyes exist as a pigment form which does not have affinity for sulfur dyes exist as pigment from which does not have affinity for cellulose by treatment with a reducing agent under alkaline conditions. After application to the fibers, sulphur dyes must be oxidized back to their pigment form. The greatest advantage of sulphur dyes is relatively low cost. The biggest limitation of sulphur dyes is that they are not bright enough in colour for many uses.

Dyes Continued

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