CLASSIFICATION OF DYES

The English chemist William Perkins accidentally discovered the synthetic dye, mauve, in 1856, while he was trying to synthesize quinine in aniline. Before that time, all coloring materials were extracted from barks, roots, seeds, leaves and shellfish. Most synthetic dyestuffs are made from coal-tar derivatives. Their manufacture is a highly complicated technical operation, which is constantly changing. There is variation in the manner in which different fibers respond to dyestuffs and even the same fibers do not produce a full range of colors with a particular type of dye.

Basic or Cationic Dyes

This group was the first of the synthetic dyes to be taken out of coal-tar derivatives. As textile dyes, they have been largely replaced by later developments. However, they are still used in discharge printing, and for preparing leather, paper, wood, and straw. More recently they have been successfully used with some readymade fibers, especially the acrylics. The name means that these are dyes with an organic material, which is soluble in a simple acid. Basic dyes were originally used to color wool, silk, linen, hemp, etc., without the use of a mordant, or using agent. With a mordant like tannic acid they were used on cotton and rayon. Basic dyes give brilliant colors with exceptional fastness to acrylic fibers. They can be used on basic dyeable variants of nylon and polyester.

Nowadays basic dyes are no longer used to any great extent on cotton or linen and seldom on wool. Since they are cheap, however, they are used for hemp, jute and similar fibers.

Their most important use today is on acrylics. They can also be used on basic dyeable variants of nylon and polyester.

The Direct Dyes

Historically, the direct dyes followed the basic dyes and were widely hailed because they made it unnecessary to use a mordant or binder in dyeing cotton. The colors are not as brilliant as those in the basic dyes but they have better fastness to light and washing, and such fastness can be measurably improved by after treatments (diazotized and developed.) Direct dyes can be used on cotton, linen, rayon, wool, silk and nylon. These dyes usually have azo linkage –N=N- and high molecular weight. They are water soluble because of sulfonic acid groups.

The Acid Dyes

This is a very large and important group of dyestuffs. While an acid dye is a salt the color comes from the acidic component, while in the basic dye it's from the organic base. The first acid dyes were combinations of basic dyes with sulphuric or nitric acid.

Adding metallic salts especially chrome to the dyed fabric in an after-treatment generally has increased colorfastness of acid dyes. Acid dyes cannot be used for wool tops but are used in dyeing wool piece goods, silk, nylon, and some of the other manmade fibers. If a mordant is used they will successfully dye cotton and linen, though this is seldom done today. The ordinary type of acid dye is reserved largely for apparel fabrics and for knitting and rug yarns. A great deal of it is used on nylon carpeting.

The Premetalized Dyes

This is an important group of acid dyes, which have been complexed with metallic ions to improve light fastness on wool and nylon.

The Sulphur Dyes

The sulphur dyes provide very deep shades, which have excellent resistance to washing but poor resistance to sunlight. They will dye cotton, linen, and rayon, but not brightly. A problem with sulphur dyes especially the black colors is that they make the fabric tender, or weaken its structure, so that it breaks easily. Sulphur dyed fabrics therefore usually must be treated with alkalis to neutralize the acids, which have formed.

Azoic Dyes

These dyes are used primarily for bright red shades in dyeing and printing since most other classes of fast dyes are lacking in good red dyes. Azoic dyes, called Naphthols in the industry, are actually manufactured in the fabric by applying one half of the dye. The other half is then put on and they combine to form the finished color. Unless they are carefully applied and well washed, they have poor fastness to rubbing or crocking.

The Vat Dyes

These are perhaps the best known group of dyes in use today because of their all round fastness to washing and sunlight on cotton and rayon.

The term vat comes from the old indigo method dyeing in a vat: indigo had to be reduced to light form. Vat dyes are made from indigo, anthraquinone and carbazole. They are successfully used on cotton, linen, rayon, wool, silk, and sometimes nylon. Vat dyes are also used in the continuous piece of dyeing process sometimes called the pigment application process. In this method the dyes are not reduced before application, but after they have been introduced into the fabric. This makes for a dyeing superior appearance and economy. There are no light red vat dyes.

Soluble Vats: There are no water-soluble preparations for dyes.

Indigo: The oldest known vat dyestuff, formerly made from the indigo plant, but now made synthetically.

Collective Dyes

Collectives are the latest dyestuff and because they react chemically with cotton, viscose, linen, wool and silk they are very fast to washing treatments. They can be dyed and printed by many methods and for the first time, the whole spectrum of color can be put onto cloth using just one class of dyes. Substituting a reactive group on a direct dye produces these dyes.

Dyes for Manmade Fibers

Dyeing man made fibers such as acetate, the polyacids, polyesters and acrylics, etc., has proved to be a challenge to dyers. Each new fiber, as it emerges from the laboratory, must be carefully analyzed and tested for its reaction to different dyestuffs. The process has been continuous experimentation with new developments turning up constantly.

To date both basic and acid dyes have been used as well as what is known as disperse colors. A dispersed dye may be any one of a number of slightly soluble dyestuffs dispersed, or held in suspension in the dye bath. Perhaps the best known example is the dispersed dye method for coloring acetate, which cannot be dyed by any other technique. For acetate dyes, the dye substance is derived from anthraquinone and azoic dyes. It is ground in a colloid mill. When dispersed in dye bath (colloidal suspension), the particles are microscopic and cannot be detected by the naked eye.

Alizarin Dyes: These are vegetables dyes, originally derived from the madder plant and now produced synthetically. They are used on wool and sometimes on cotton. They produce a brilliant turkey red, among other colors.

Aniline Black: They are produced from the chemical aniline, and are usually associated with the color black. Aniline black is a fast black, much used on cottons, and is developed by oxidizing the aniline on the fiber. It is very fast tot light, washing, and chlorine.

Chrome Dyes: These are a special type of acid dyes and they are used to color animal fibers, especially woolens and worsteds. They will react well on a fabric with metals such as chromium. The process, however, tends to dull the color brilliance but does provide high light fastness and washfastness.

Neutral Dyes: These are metal containing acid dyes and the metal is added in manufacture.

Acetate of Disperse Dyes: Disperse dyes were originally developed for dyeing secondary cellulose acetate fibers. These dyes are relatively insoluble in water and are prepared for dyeing by being ground into relatively fine powder in the presence of dispersing agents. In the dye bath, a suspension of the dye particle dispersion produces a very dilute solution of the dyes, which are then absorbed by the fibers. This dye class is used to dye polyester, nylon, acetate and triacetate fibers.