DYEINGFor those who see color, the world is a colorful place. Most humans have color vision and they enjoy and appreciate the wide palette of nature. We use colors to imitate nature, and to decorate our environment, our food, and ourselves. For most colors under human control, a colorant can be included when an item is formed (such as for an extruded plastic object) or combined in some medium that is applied to a surface (eg, paints and cosmetics). For clothing, furnishing, and in many other end-uses, textile materials are ubiquitous. The process of dyeing, in which a colorant penetrates the substrate, is used to color them. The same dyeing process is occasionally used to color paper and leather, and textiles can be colored by nondyeing means, but textiles and dyeing have a high degree of mutual inclusivity.The global consumption of textiles in 2012 was estimated at 83 million tonnes, of which 55 million tonnes was synthetic (including almost 40 million tonnes of polyester) and 23 million tonnes was cotton. Manufactured cellulosic and other natural fibers constituted the remaining 5 million tonnes (1). Fiber use has continued to increase at a rate faster than the growth of population. Dye is applied to these fibers at a rate of approximately 1-2%, ie, the annual production of synthetic dyes is approximately 1 million tonnes (2,3).Dyeing occurs when a soluble colorant is adsorbed at the surface of a fibrous substrate, and then diffuses into the substrate. What follows is a detailed examination of this basic process in both theoretical and practical terms.
Dyes, Pigments, and FibersColorants are typically divided into dyes and pigments. The demarcation between them is primarily based on their solubility; a pigment relies on insolubility of the medium in which it is dispersed, while a dye requires some degree of solubility, maybe low or maybe temporary, that will allow it to diffuse into the polymeric matrix of a textile fiber. In order for a colored substance to be considered as a useful dyestuff, factors beyond solubility are required. A dyestuff must be substantive for a textile and thus be preferentially taken up by the fiber, usually from an aqueous solution. The uptake should be high enough to be economic, and the rate at which the dyeing occurs should be controllable to give a uniform level result. The dyed textile should have satisfactory fastness for the intended end use. The process of dyeing therefore combines chemistry, application technology, economics, and customer needs.1.1. Dyes: Natural vs Synthetic. Dyeing is an ancient practice. Originally, all dyes were of natural origin obtained locally from plants and even animals. Tyrian purple, obtained from a species of whelk, gave rise to the "purple of the emperors," its elevated status arising from the cost and effort involved in extracting useful quantities of colorant. As communication developed, dyes and dyed textiles were among the important items of trade, and technically superior natural dyes often supplanted local materials. Indigo from India and M...
