Authors: Yusuf Maigari
Color is a very important property of materials around us but we often neglect its study socially and scientifically. We perceive color just as we perceive things like taste, smell, and touch etc. Color can influence our emotions, actions and how we respond to various things, people and ideas. Color is extremely versatile in its uses. It can be used to make a statement, create an atmosphere, or call forth a response. Color expresses outwards towards the world, but it also helps us to travel inwards towards spiritual states, towards our true self. Color provides a vial enhancement to the world in which we live. Everyday materials we use Textiles, paints, plastics, paper, and foodstuffs are especially appealing if they are colorful. So, imagine a world where all the materials are black, white, or grey; how will that world look like? Nature too presents a kaleidoscope of colors around our lives, various shades of green in the forest, various colors of flowers around our houses and even the differing color of our skins. Concept of Color: Color is the bye-product of the spectrum of light, as it is reflected or absorbed, as received by the Human eye and processed by the human brain. The world is full of light. Visible light is made of seven wavelength groups. These are the color we see in the rainbow. When light hits an object, some of the wavelengths are absorbed while others reflected, depending on materials in the object. The reflected wavelength is what we see as the object color. Newtons prism experiment proves to be very helpful in understanding color. Newton realized that colors other than those in the spectral sequence do exist, but he noted that all the colors in the universe which are made by light, and depend not on the power of imagination, are either the colors of homogeneal lights [i.e., spectral colors, or compounded of these. Textile Dyes: Color is obtained in textiles and other materials using colorants (Dyes and pigments). Indigo and alizarin obtained from the tree Tinctoria indigofera and the root of Madder respectively, were used by the ancients for dyeing since the beginning of recorded history. However, from the year 1856 when William Perkin produced Mauviene from simple organic compounds obtained from petroleum and coal tar distillates, the use of these natural pigments was jeopardized. A dye can be referred as a water-soluble colored organic compound that has affinity for the substrate whereas pigments are usually water insoluble. For a compound to be a dye, it must fulfill the following conditions: Must be soluble in aqueous either permanently or during application. Must be intensely colored, must have substantivity (ability to be absorbed by the substrate) or be able to react with the substrate chemically. Must possess reasonable fastness properties. Classification of Textile Dyes: There are various criteria used in the classification of dyes. These include Origin, chemical structure, methods of application, nuclear structure, and industrial classification. However, it should be noted that each class of dye has a unique chemistry, structure, and particular way of bonding. While some textile dyes can react chemically with the substrates forming strong bonds in the process, others can be held by physical forces. Techniques of Dyeing: 1) Bale Dyeing: This is a low-cost method to dye cotton cloth. The material is sent without scouring or singeing, through a cold-water bath where the sized warp has affinity for the dye. Imitation chambray and comparable fabrics are often dyed this way. 2) Batik Dyeing: This is one of the oldest forms known to man. It originated in Java. Portions of the fabric are coated with wax so that only un-waxed areas will take on the dye matter. The operation may be repeated several times and several colors may use for the bizarre effects. Motifs show a melange, mottled or streaked effect, imitated in machine printing. 3) Beam Dyeing: In this method the warp is dyed prior to weaving. It is wound onto a perforated beam and the dye is forced through the perforations thereby saturating the yarn with color. 4) Burl or speck Dyeing: This is done mostly on woolens or worsteds; colored specks and blemishes are covered by the use of special colored links which come in many colors and shades. It is a hand operation. 5) Chain Dyeing: This is used when yarns and cloth are low in tensile strength. Several cuts or pieces of cloth are tacked end-to-end and run through in a continuous chain in the dye color. This method affords high production. 6) Cross Dyeing: This is a very popular method in which varied color effects are obtained in the one dye bath for a cloth which contains fibers with varying affinities for the dye used. For example, a blue dyestuff might give nylon 6 a dark blue shade, nylon 6, 6 a light blue shade, and have no affinity for polyester area unscathed or white. 7) Jigger Dyeing: This is done in a jig, kier, vat, beck, or vessel in an open formation of the goods. The fabric goes from one roller to another through a deep dye bath until the desired shade is achieved. 8) Piece Dyeing: The dyeing of fabrics in the cut, bolt or piece form is called piece dyeing. It follows the weaving of the goods and provides a single color for the material, such as blue serge, a green organdy. 9) Random Dyeing: Coloring only certain designated portions of the yarn. There are three ways of doing this type of coloring. Skeins may be tightly dyed in two or more places and dyed at one side of the dye with one color and at the other side with another one. Color may be printed onto the skeins which are spread out on the blanket fabric of the printing machine. Adsorption of Dye from the Dyebath: Several distinct and identifiable events take place in the dyeing of a textile material. The events are as follows: a) Diffusion in Solution Dye must move or diffuse through the dyebath to establish contact with the textile material being dyed. b) Adsorption on fiber surface dye molecules are attracted to the fiber and are initially deposited on the fiber surface. c) Diffusion into the fiber dye deposited on the surface creates a concentration gradient which is the driving force for movement of dye from the surface towards the interior of the fiber. During diffusion, dye molecules migrate from place to place on the fiber. This migration tends to have a levelling effect on the dye application. Textile dyes which migrate readily are easy to apply uniformly. However, dyes which migrate, and level easily also tends to have poorer wash fastness than dyes which do not level easily. Color Measurement: The color of textile dyes is measured using the spectrophotometer. There are different spectrophotometers for different types of light e.g. Infrared spectrometer, ultraviolet- visible spectrometer etc. but the working principle is almost the same. d) Dissolution of the dye in the dyebath dyes which are only sparingly soluble in water may have to dissolve from a dispersion of highly aggregated particles to be small enough to diffuse into the fiber. Fastness of Textile Dyes: Color fastness is defined as the resistance of colored materials to fading or running during processing or in subsequent useful life. The term is usually used in the context of clothes. The fading or color running is brought about by Washing, rubbing, action of light, hot pressing and perspiration. Therefore, fastness tests include washing fastness, rubbing fastness, light fastness, fastness to hot pressing and perspiration fastness.
