Colour is a very important property of materials around us but we often neglect its study socially and scientifically. We perceive colour just as we perceive things like taste, smell, and touch etc. colour can influence our emotions, actions and how we respond to various things, people and ideas. Colour is extremely versatile in its uses. It can be used to make a statement, create an atmosphere, or call forth a response. Colour expresses outwards towards the world, but it also helps us to travel inwards towards spiritual states, towards our true self. We can use colour to get what we want from the world, or we can use it to find something in ourselves.
Colour provides a vial enhancement to the world in which we live. Every day 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 colours around our lives; Various shades of green in the forest, various colours of flowers around our houses and even the differing colour of our skins.
Concept of Colour
Colour 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 colour we see in the rainbow. When light hits an object, some of the wavelength are absorbed while others reflected, depending on materials in the object. The reflected wavelength is what we see as the object colour.
Newton’s prism experiment proves to be very helpful in understanding colour. Newton realized that colours other than those in the spectral sequence do exist, but he noted that all the colours in the universe which are made by light, and depend not on the power of imagination, are either the colours of homogeneal lights [i.e., spectral colours], or compounded of these.
Newton also recognized that: rays, to speak properly, are not coloured. In them there is nothing else than a certain power…to stir up a sensation of this or that colour.
Fig 1: The table below gives a summary of the colour perception by human eye:
Colour is obtained in textiles and other materials by the use of colourants (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 coloured 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 coloured
- 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 very unique chemistry, structure and particular way of bonding. While some dyes can react chemically with the substrates forming strong bonds in the process, others can be held by physical forces. The table below gives a summary of textile dyes and their suitable substrates/area of application.
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 mlange, 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) Jig 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:
Diffusion in Solution – Dye must move or diffuse through the dyebath in order to establish contact with the textile material being dyed.
Adsorption on fiber surface – dye molecules are attracted to the fibre and are initially deposited on the fibre surface.
Diffusion into the fibre – 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 fibre. This migration tends to have a levelling effect on the dye application. Dyes which migrate readily are easy to apply uniformaly. However, dyes which migrate and level easily also tends to have poorer wash fastness than dyes which do not level easily.
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 in order to be small enough to diffuse into the fibre.
Fastness of Textile Dyes
Colour fastness is defined as the resistance of coloured 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 colour 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.
In the test, the material is treated with/under the influence of the agent under consideration for a specific time and the effect assessed using the grey scale for assessing change in colour. The extent of staining can also be assessed using the grey scale for assessing staining.
The colour 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.
A white source such as the tungsten lamp usually provides the light for the spectrophotometer. The monochrometer is a prism, spectrum filter, or diffraction grating which spreads the light from the source into a spectrum. Although, the word “monochromatic” means one colour or one wavelength, the monochrometer and slit select a narrow band of light rather a single wavelength to be measured. The slit scans across the spectrum selecting the band of light to be measured. The monochrometer and slit may be placed either before the sample position giving the monochromatic illumination or after the sample position, giving polychromatic illumination. However, only polychromatic illumination followed by separation the separation of the reflected light by a monochrometer gives an accurate spectrophotometric curve if the sample is fluorescent. The detector is a photoelectric device which converts the transmitted or reflected light to an electrical signal in a chart or computer records.
An abridged spectrophotometer measures a few narrow bands across the spectrum rather than scanning wavelength by wavelength. Typically about 16 bands each about 20 nm wide are measured. These instruments use series of filters to select the bands to be measured or may use a series of detectors to measure the reflectance at all of the wavelengths simultaneously. Abridged spectrophotometers are simpler and less expensive than scanning spectrophotometers, but provide less information.
Courtesy: Yusuf Maigari
Department of Textile Engineering
Ahmadu Bello University, Nigeria
- Warren S.P., Textile colouration and finishing , Carolina Academic press, 700 kent street, durham, north Carolina.
- Encyclopedia Britannica Inc., 1994