Application of Pigment, Developing Sulphur, and Vat Dyes

Pigment Dyes

One important piece of information concerning the use of pigment dyes is that in this class of dyes there are certain colors which are insoluble in water and in most solvents can be transferred to fibers by a method of chemical synthesis. Such colors are very fast to light and washing and are not easily removed from the fibers.

Aniline black, for example, can be used to dye cotton fiber by means of the oxidation of salts of aniline when bichromates or chromates are used in connection with certain so called oxygen carriers, such as some of the salts of iron, vanadium, or copper. Other semi oxidation colors are the Ursols, the Furols, and Paramine Brown.

Pigment dyes are derived from four of the chemical groups which follow:

The azo
The nitro
The azine (which contains Aniline Black)
The oxazine classes

In dyeing, the pigment colors are transferred to the fibers being dyed by a chemical synthesis and from colorless chemical compounds.

Developing Dyes

The developing colors in the classification of developing dyes resemble those of the pigment colors in that they are both insoluble and must be transferred and formed on the films of the material being dyed. The developing colors are not colorless dyestuffs, but react differently from ordinary dyes because a chemical reaction causes a coloring-matter to be developed and formed on the fillers.

Sulphur Dyes

These colors of sulphur dyes are also insoluble in pure water, but, if they are dissolved in an aqueous solution of sodium sulphide and heated, they will dye vegetable fibers, especially cotton, in a direct bath. These colors are not used very often on animal fillers. The resultant hues produced on fabrics are not bright, but appear as dull shades of green, brown, blue, and black.

These dyes contain sulphur in the molecules, but their structure is rather indefinite. Sulphur dyes are used quite a lot, however, for dyeing cotton cloth, and also for yarns and hosiery. An after-treatment with a salt of copper, zinc, or chromium is sometimes used to \"fix\" the sulphide colors.

Vat Dyes

The term vat dyes is used to designate certain dyestuffs that are themselves insoluble in water but which can be used in the form of their leuco compounds and then oxidized in the air, thus bringing out the colors. A leuco compound in chemistry is one that is colorless and is derived from a dye by reduction, with an unsatisfactory affinity for nitrogen. By oxidation or the taking on of oxygen, leuco compounds regain color.

For many years indigo was the chief vat dye, but at the present time other indigoid vat dyes are commonly used as well, such as the Ciba series, the Bromindigos, the Thioindigo series, and Brilliant Indigos. There are also anthracene and thiazine vat dyes on the market and are used in the textile industry and at home for occasional use in dying garments and drapery.

The developing colors are not colorless dyestuffs, but react differently from ordinary dyes because a chemical reaction causes a coloring-matter to be developed and formed on the fillers.