DYEING

DYEING

DYEING OVERVIEW

Dyeing is the process of imparting colours to a textile material in loose fibre, yarn, cloth or garment form by treatment with a dye.A dye can generally be described as a colored substance that has an affinity to the substrate to which it is being applied. The dye is generally applied in an aqueous solution, and may require a mordant to improve the fastness of the dye on the fiber.Both dyes and pigments appear to be colored because they absorb some wavelengths of light preferentially. In contrast with a dye, a pigmentgenerally is insoluble, and has no affinity for the substrate. Some dyes can be precipitated with an inert salt to produce a lake pigment, and based on the salt used they could be aluminum lake, calcium lake or barium lake pigments.Textile dyeing is concerned with organic (that is, carbon-based) compounds that can be dissolved in appropriate solvents, usually water. The dyes in solution are absorbed on the surface of the textile fibre then pass into the interior of the material by a process called diffusion.The process of transferring the dye from solution to the fibre is called exhaustion, with 100% exhaustion meaning that there is no dye left in the dyebath solution. An important property of a dyeing is its levelness, in other words when the same depth of colour can be seen all over the material. Dye molecules are attracted by physical forces at the molecular level to the textile. The amount of this attraction is known as 'substantivity': the higher the substantivity the greater the attraction of the dye for the fibre.Another factor is good penetration, when the dye has penetrated deeply into the structure of the fibre, colouring it from the outer surface of the fibre to its interior

IT'S TYPES


NATURAL DYES

Natural dyes have now been almost entirely superseded by the synthetic products, except for a few specialised uses. Logwood, the only natural dye still in large-scale use, is however used for dyeing not only silk and wool, but also secondary cellulose acetate and nylon.Indigo was the main natural dye used to yield blue shades; its fastness to light was outstanding when compared with other natural dyes. As a result it achieved particular importance. Even so, the bacterial fermentation process used for its extraction from either Indigofera or woad plants was highly unpleasant Indigo blue always keeps its stunning hue even if it grows paler, for this reason the only original colour of the Bayeux tapestry that remains true is the indigo blue of its woad-dyed wools.The dark blue indigo dye has been known for over 4000 years. When the Romans attacked England, they found that the country was populated by people who tattooed and painted themselves with indigo. The name the Romans gave to these people, Briton, means 'painted men'.

SYNTHETIC DYES


The first human-made (synthetic) organic dye, mauveine, was discovered by William Henry Perkin in 1856.Synthetic dyes quickly replaced the traditional natural dyes. They cost less, they offered a vast range of new colors, and they imparted better properties upon the dyed materials.Dyes are now classified according to how they are used in the dyeing processThe first man-made fibre to achieve commercial significance was viscose rayon, in the early 1900s. This is chemically similar to cotton (in other words it is a cellulosic fibre) and so the dyes already available for cotton were used on viscose rayon. At the time these were mainly direct, vat, azoic and sulphur dyes, but since the 1960s fibre-reactive dyes have come to be widely used on all cellulosic fibres.

Acid dyes are water-soluble anionic dyes that are applied to fibers such as silk, wool, nylon and modified acrylic fibers using neutral to acid dyebaths. Attachment to the fiber is attributed, at least partly, to salt formation between anionic groups in the dyes and cationic groups in the fiber

Basic dyes are water-soluble cationic dyes that are mainly applied to acrylic fibers, but find some use for wool and silk. Usually acetic acid is added to the dyebath to help the uptake of the dye onto the fiber.

Direct or substantive dyeing is normally carried out in a neutral or slightly alkaline dyebath, at or near boiling point, with the addition of either sodium chloride (NaCl) or sodium sulfate(Na2SO4). Direct dyes are used on cotton, paper, leather, wool, silk and nylon





Vat dyes are essentially insoluble in water and incapable of dyeing fibres directly. However, reduction in alkaline liquor produces the water soluble alkali metal salt of the dye, which, in this leuco form, has an affinity for the textile fibre.



Reactive dyes utilize a chromophore attached to a substituent that is capable of directly reacting with the fibre substrate. The covalent bonds that attach reactive dye to natural fibers make them among the most permanent of dyes. "Cold" reactive dyes, such as Procion MX, Cibacron F, and Drimarene K, are very easy to use because the dye can be applied at room temperature.