In most cases dirt adheres to the fibres of textile fabrics by 2 kinds of forces – first by sticking to a coating of grease or a dried coating of substances which swell up in water or other solvents (starch, Proteins and other glue like substances); second by direct adhesion because of the physico – chemical character of the fibres and the dirt. Dirt consists of fatty substances, proteins, and dust and soot particles. Such dirt is largely insoluble in water or is water repellent. Used shirt is soiled with greasy substances constituting about 0.25 percent of its weight; collar may contain as much as 1.2 percent of its weight of greasy dirt.
The dirt may be held by the fibres in various ways; mechanically (pigments are jammed between fibres), chemically (fruit oil or ink strains) by absorption (feebler chemical band, dissoluble by detergents) and by electric forces. As most of the dirt is firmly held by the fibres, pure water is not very effective. Chemical cleaning or dry cleaning uses liquids other than water for cleaning of fabrics. In this treatment adhering dirt of the first kind is removed by dissolving the grease or their sticky matter to which the dirt particles are clinging.
A wide range of solvents are employed: carbon tetrachloride, trichloroethylene, tetrachloro ethylene, naphtha (petroleum ether) and benzene. A modern dry cleaning plant comprises a number of specialized machines and appliances. The soiled garments are treated with the solvents in rotating drums.
Contaminated solvent is drained off and purified for re-use. Cleaned garments are dried, impregnated and reshaped. In some cases, however, it is necessary to use water as an additional solvent or swelling agent.
The second type of dirt is dislodged from the fabric by means of detergents added to water. After this wet treatment the fabric is usually treated in weak acid solutions (to revive colours). Rinsed, centrifuged and dried.
Impregnation treatment may be applied at an intermediate stage to stiffen the fabric and make it water- and dirt-repellent. Garments are finally pressed on special machines operated with steam and air.