IntroductionBefore going into the details of dyes and pigments let us review what dyeing is? Dyeing: Process of adding colorant molecules to the textile and then permanently holding them inside the fabric is called Dyeing. The term colorant is widely applied to both the dyes and pigments. Suitable distinction between both will discussed separately. Dyes The term ‘dye’ applies to a coloring organic compound which imparts colour to the textile material. Colorants are usually applied from some fluid medium for example most dyes use water as the application fluid. It is ultimately the solubility of the colorant in the medium that is used to distinguish between dyes and pigments. From application point of view dyes are the “materials which when applied from a suitable medium will transfer from that medium onto a textile substrate and impart coloration which has some degree of permanence.” Two things are important here that how that permanence is achieved to a certain extent and how the color is produced? Dye molecules have elements which bond with polymer molecules of the substrate. They do so by some kind of attractive force such as van der waal’s force, hydrogen bond and even ionic bond depending upon the chemical elements the dye holds in it’s structure. There have been a number of attempts to explain the links between color and the chemical nature of colorants. In 1876 ‘Witt’ explained that color comes from the presence of certain unsaturated chemical groups (>C=O, -N=O or -N=N-). These coloring groups are named as chromophores. It is these chromophores that determine the color of the dye and provided their classification. In addition to chromophores there are also basic or weakly acidic groups that increase the color of the dye. They are termed as Auxochromes. Important dye classes according to chromophores they contain are Azo, Stilbene, Anthranquinone, Thiazole, Indigoid etc. It must be stable in solid form at ambient temperatures. A pigment must have a high tinting strength relative to the materials it colors. A ‘pigment’ is a material that changes the color of reflected or transmitted light as the result of wavelength-selective absorption. This physical process differs from fluorescence.e all pigment are insoluble in the medium from which they are applied. permanence and stability are desirable properties.Some essential characteristics that a dye should possess are. Solubility: They have the ability to dissolve in water. Durable: The dye should provide some durability to a limited number of washes. in which a material emits light. Pigments that are not permanent are called fugitive. Color: Dye molecules must have the desired color. Other factors used to determine the . Many materials selectively absorb certain wavelengths of light. For industrial applications. While solubility is the most widely recognized distinction between dyes and pigments. as well as in the arts. Pigment can be used in dyeing as well as in printing. Dyes on the other hand are generally soluble in the application medium. and other forms of luminescence. however there is some overlap of the categories. Difference Between Dyes and Pigments All colourants are often grouped according to whether they are pigments or dyes. phosphorescence. Pigments The pigments are insoluble colouring compounds and these are opaque. They can be applied with a suitable adhesive such as binder and can applied to all kinds of textile fibers. Substantivity: Dyes should have an affinity/attraction to the fiber in question. The simplest definition of pigment is based on its insolubility in the application medium i. Fugitive pigments fade over time. It is the solubility of the colourant that is used to distinguish between dyes and pigments. Materials that humans have chosen and developed for use as pigments usually have special properties that make them ideal for coloring other materials. difference is the chemical nature: traditionally pigments were inorganic and dyes organic but this has changed as the chemical has produced more organic pigments. whereas pigments do not and therefore require a binder to fix them to the substrate. These differences are important when considering the application properties and use of dyes and pigments. The major differences between dyes and pigments are: Dyes Pigments Solubility Soluble in many liquids Insoluble in water and most of the solvents Number Available in large number Comparatively lesser in number Lightfastness Lower dyes are very much Traditionally pigments have vulnerable. often involving heavy toxic metals . It means that the particles can be seen under a magnifying glass Bonding Dye molecules have electrostatic charges that serve as a method for attaching the dye to the concrete Pigment requires the help if a binder for gluing. Another difference is that dyes have an affinity for the substrate material. An important difference between dyes and pigments: Pigments are used in physical form in which they are made whereas dyes are dissolved in a dye bath and then diffuse onto the textile to produce coloration. it’s like comparing a football (pigment) to say a head of a pin (dye) Pigment particles are about 1-2 microns in size. Lights destroy been found to be more lightfast colored objects by breaking than dyes open electronic bonding within the molecule Product resistance Lower as compared to pigments Very high Size Dye molecules are comparatively smaller. As it is an inert substance which si merely suspended in a carrier/binder Imparting colors Dyes can impart color by Pigments impart colors by selective absorption of the dyes either scattering of light or by selective absorption Combustible Properties Combustible Non-combustible Chemical Composition Organic compounds Normally inorganic compounds. For this purpose heat is applied to raise the temperature which opens up the fiber structure to accommodate the dye molecules inside. Direct dyes on the other hand are applied at neutral pH. Considerable amount of repulsion between dye and water is needed in many cases to enable the dye to migrate towards the fabric. Different dyeing processes are used to dye the textile material i. Binders: The binders are high molecular weight film-forming substances that are marketed as aqueous dispersions or as liquids in solvents. the dye bath is supplied with chemicals and auxiliaries to help exhaust the dye to the fabric. Chemically binders are derivatives of acrylic acid particularly their esters as well as butadiene and vinyl acetate Thickeners: . NaCl for example is added in direct dyeing to reduce the long range repulsion between direct dyes (anionic) and cotton which becomes negatively charged in water. Pigments Application of pigments is a simple process and the padding bath consists of a binder and a thickener in addition to the pigment dispersion.Longevity Do not last as long as pigments Last longer than dyes Application Method Dyes As discussed previously dyes are applied from an aqueous medium. To successfully exhaust the dye from water to the fabric. Generally when fabric length is extremely large we use continuous method. form. For example. On heating water of solvent is removed and the binder particles fuse together to form a flexible film that envelops the pigment particles and adheres to the fibers. in application of reactive dyes an alkali is added to shift the pH mainly to improve the exhaustion and fixation of dye.e Exhaust and Continuous. Which method is applied depends upon the type. Furthermore the dye molecules need the space to diffuse into the fiber polymer structure containing both crystalline and amorphous structures. while as acid dyes are mostly applied at acidic pH. amount of material and condition of dyeing. For example in dyeing fiber and yarn exhaust method will be employed while as in dyeing fabric both the exhaust and continuous processes can be used. Different dyes need different pH conditions to diffuse the dye molecules inside the fabric. Certain auxiliaries may also be added to further improve the fastness properties of the pigments and to facilitate their application. The constituents of the bath are briefly discussed below. These have a high thickening power. This process is only used in the wash out or the faded look process and is generally meant for garment dyeing. The synthetic thickeners are basically acrylic acids that are neutralized with ammonia before use. resistance to organic solvent and improvement in the wet fastness can be achieved. digital printing etc. flat printing. latter being the most common. needing a concentration of only 0. Most machines accommodate roller with a maximum 16 inches circumference. which means that the size of the print repeat cannot be larger than 16 inches. air pollution and foul smell.The thickeners are required for a uniform spreading of colour on fabric but the natural thickener are not suitable because these need to be washed off and so interfere with formation of a continuous film. Most important are roller printing. Digital printing consists of printing the design on the fabric directly from the computer with no other additional step which means that after finish creating the designs once has them in repeat. Some ink type may require a post treatment such as steaming or heat setting the print. For proper coloration the fabric should be free of any residual size and oxidizing agent besides being uniformly absorbent. Pigment Printing There are several methods for printing of textile. It is high speed process capable of producing over 8000 yards of printed fabric per hour. In roller printing the design is put onto fabric by copper engraved roller. This is due to association of the white oil with fire hazards. The pigment colors do not have any affinity with fibers and so are unable to form any dye fiber chemical bonds. The normal pigment dying is not fast to dry cleaning but with use of special binders.5% and so have virtually no effect on handle of the finished fabric. . the print on fabric just like the print on paper. transfer printing. Fastness properties of the pigments depend to the large extent on the nature of fabric and its preparation. Pigment Dyeing Pigment Dyeing can be done by Exhaust and Continuous method. Initially an emulsion of white oil in water was used but this has now been replaced by the synthetic thickeners in spite of the batter yields and brighter colours obtained by the former. The padding method of application is very simple and consists of padding fabric with suitably diluted pigment dispersion and binder and drying in a hot flue machine.