Dyes and its characteristics Dyes are substances used to impart colour to textiles, leather, paper, etc. Compounds used for dyeing (dyestuffs) are generally organic compounds containing conjugated double bonds. The group producing the colour is the chromophore; other noncoloured groups that influence or intensify the colour are called auxochromes. It is generally applied in an aqueous solution, and may require a mordant to improve the fastness of the dye on the fiber. Some important chromophores areO || N = N Azo .. .. C=O Carbonyl .. C=S Thio carbonyl

N > O Nitro

Some important auxochromophores areAcidic: OH, Hydroxy Basic: NH2, Amino SO3H, Sulphonic NHR, Alkylamino COOH Carboxylic NHR2 Dialyliamino

Both dyes and pigments appear to be colored because they absorb some wavelengths of light preferentially. A dye works by having its molecules absorbed by the material. The most permanent dyes are those whose molecules form chemical bonds with the molecules of the materials being dyed. A colored substance is termed as a dye if it fulfills the following conditions, It must have a suitable color. It can be fixed on the fabric either directly or with the help of mordant. When fixed it must be fast to light and washing, i.e., it must be resistant to the action of water, acids and alkalies, particularly to alkalies as washing soda and soap have alkaline nature.

Types of dyes Dyes can be classified according to the chemical structure of the dye molecule. For example, azo dyes contain the N=N group (see azo compounds). In practice, they are classified according to the way in which the dye is applied or is held on the substrate. 1. Acid dye - Acid dyes are compounds in which the chromophore is part of a negative ion (usually an organic sulphonate RSO2O). They are water-soluble anionic dyes that are applied to fibers such as silk, wool, nylon and modified acrylic fibers using neutral to acid dye baths. Attachment to the fiber is attributed, at least partly, to salt formation between anionic groups in the dyes and cationic groups in the fiber. They can be used for protein fibres (e.g. wool and silk) and for polyamide and acrylic fibres. Originally, they were applied from an acidic bath. Metalized dyes are forms of acid dyes in which the negative ion contains a chelated metal atom. 2. Basic dye - Basic dyes have chromophores that are part of a positive ion (usually an amine salt or ionized imino group). They are used for acrylic fibres and also for wool and silk, although they have only moderate fastness with these materials. They 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. Basic dyes are also used in the coloration of paper. Basic dyes perform poorly on natural fibres, but work very well on acrylics. 3. Direct or substantive dye - 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). They have a high affinity for cotton, rayon, and other cellulose fibres. Direct dyes are used on cotton, paper, leather, wool, silk and nylon. They are also used as pH indicators and as biological stains. Like acid dyes, they are usually sulphonic acid salts but are distinguished by their greater substantivity (affinity for the substrate), hence the alternative name substantive dyes. The name 'direct dye' alludes to the fact that these dyes do not require any form of 'fixing'. 4. Mordant or adjective dye - Mordant dyes are those dyes which require a mordant in their application and which upon combination with the mordant deposit insoluble color on the substrate, e.g., dyes with metal chelating groups. Mordants are substances of organic or inorganic origin which combine with the coloring matter and are used to fix the same in the production of the color. The mordant substances include such acids as tannic acid, sumac, gall nuts, bark extracts, oleic

and stearic acids, and Turkey red oil; and metallic substances such as various combinations or soluble salts of chromium, aluminum, iron, copper, and tin. The latter, the metallic mordants, are more used than the acid mordants. Mordant improves the fastness of the dye on the fibre such as water, light and perspiration fastness. The choice of mordant is very important as different mordants can change the final colour significantly. The most commonly used mordant dyes have hydroxyl and carboxyl groups and are negatively charged, i.e. anionic. 5. Vat dye - Vat dyes are insoluble substances used for cotton dyeing. They usually contain keto groups, C=O, which are reduced to C-OH groups, rendering the dye soluble (the leuco form of the dye). The dye is applied in this form, then oxidized by air or oxidizing agents to precipitate the pigment in the fibres. Indigo and anthroquinone dyes are examples of vat dyes. Vat dye is obtained through oxidation. It is usually very bright and will hold up better when bleached than most other dyes. The process is very colorfast in all respects. This is an expensive procedure and is used mainly on high-end products. Vat dyes include the natural dyestuff, indigo, and the artificial dyes called by the trade names, indanthrene, and flavanthrene. They are called vat dyes because, being originally insoluble in water, they undergo special preparation in large vats before the cloth is introduced; here they are made soluble, usually by the adding of caustic soda and hyposulphite. In this mixture or dye liquor the textiles are soaked. Certain chemicals are thereafter added, changing the dyestuff back to the insoluble form in the cloth or fiber. This is called the fixing process. 6. Disperse dye - Disperse dyes were originally developed for the dyeing of cellulose acetate, and are water insoluble. The dyes are finely ground in the presence of a dispersing agent and sold as a paste, or spray-dried and sold as a powder. Their main use is to dye polyester but they can also be used to dye nylon, cellulose triacetate, and acrylic fibres. In some cases, a dyeing temperature of 130 C is required, and a pressurised dyebath is used. The very fine particle size gives a large surface area that aids dissolution to allow uptake by the fibre. The dyeing rate can be significantly influenced by the choice of dispersing agent used during the grinding. Disperse dyes have low solubility in water, but they can interact with the polyester chains by forming dispersed particles. The general structure of disperse dyes is small, planar and non-ionic, with attached polar functional groups like NO2 and -CN. The shape makes it easier for the dye to slide between the tightlypacked polymer chains, and the polar groups improve the water solubility, improve the dipolar bonding between dye and polymer and affect the colour of the dye. However, their small size means that disperse dyes are quite volatile, and tend to sublime out of the polymer at sufficiently high temperatures2.

7. Sulfur dye - Sulfur dyes are the most commonly used dyes manufactured for cotton in terms of volume. They are cheap, generally have good wash-fastness and are easy to apply. The dyes are absorbed by cotton from a bath containing sodium sulfide or sodium hydrosulfite and are made insoluble within the fiber by oxidation. During this process these dyes form complex larger molecules which are the basis of their good wash-fastness. These dyes have well all round fastness except to chlorine. Due to the highly polluting nature of the dye-bath effluent, slowly sulfur dyes are being phased out. Sulfur dyes are primarily used for dark colors such as blacks, browns, and dark blues. The deep indigo blues of denim blue jeans are a product of sulfur dyes. Sulfur dyes are water insoluble. They have to be treated with a reducing agent and an alkali at temperature of around 80 degrees Celsius where the dye breaks into small particles which then becomes water soluble and hence can be absorbed by the fabric. Heating and adding a substance like common salt facilitates the absorption. After this the fabric is removed from the dye solution and then taken for oxidation. During the oxidation step the small particles of dye once more form the parent dye which is insoluble in water. This oxidation can be done in air or by using oxidizing agents like hydrogen peroxide or sodium bromate in a mildly acidic solution. Now as the dye has become water insoluble in fiber so it will not bleed in water when washed and will not stain other clothes.

CHEMICAL CLASSIFICATION OF DYES 1. Nitro or Nitroso dye -These dyes contain nitro or nitroso groups as the chromophores and OH as auxochrome. They form only a tiny fraction of commercial synthetic dyes. They are cheap and mainly used for coloring in paper. These dyes are polygenetic (i.e. form different colored complexes with different materials), but only the green iron complexes due to their good fastness of light have found application as dyes. They have the general formula

Some important examples of nitroso dye

Picric acid

2. Triarylmethane dye -In these dyes, the central carbon is bonded to three aromatic rings. One of which is in the quinonoid form (the chromophore). Malachite green or 4-[(4-dimethylaminophenyl) phenyl-methyl]-N, Ndimethylaniline is a typical example of this class.

Malachite Green

Malachite green is prepared by the condensation of bezaldehyde and dimethylaniline to give leuco malachite green (LMG): C6H5CHO + 2 C6H5N(CH3)2 C6H5CH(C6H4N(CH3)2)2 + H2O

Second, this colourless leuco compound, a relative of triphenylmethane, is oxidized to the cation that is MG: C6H5CH(C6H4N(CH3)2)2 + HCl + 1/2 O2 [C6H5C(C6H4N(CH3)2)2]Cl + H2O

Hydrolysis of MG gives the carbinol form: [1] [C6H5C(C6H4N(CH3)2)2]Cl + H2O C6H5C(OH)(C6H4N(CH3)2)2 + HCl

3. Phthalein dye - Products obtained by condensation of phthalic anhydride with phenols in presence of dehydrating agents like conc. H2SO4 or anhydrous zinc chloride are called phthaleins. The other important dyes of this class are

4. Xanthene dye - Xanthene dyes are derived from xanthene. This class of dyes is divided into three subgroups: Fluorenes, fluorones and rhodols. Fluorenes and fluorones contain dyes of importance in histotechnology, the rhodols do not. The fluorenes are further subdivided into five groups. Of these the pyronins and rhodamines include dyes we use. The others do not. Xanthene dyes tend to be fluorescent, yellow to pink to bluish red, brilliant dyes. Many xanthene dyes can be prepared by condensation of derivates of phthalic anhydride with derivates of resorcinol or 3-aminophenol.

Xanthene

5. Anthraquinono dye - Anthaquinone dyes are derived from anthracene

Anthracene

They have the general formula

The general formula shows the chromophore to be a quinoid ring. This class of dyes may have either hydroxyl groups or amino groups attached to the general structure. Those that have hydroxyl groups (hydroxyanthraquinones) are the more

numerous for histological purposes, and include the mordant dyes alizarin and alizarin red S, both used to demonstrate calcium. Dyes having amino groups (aminoanthraquinones) are less common, but include nuclear fast red, a mordant dye useful for staining nuclei red. Alizarin or 1,2-dihydroxyanthraquinone is an organic compound with formula C14H8O4 is an example of anthraquinone dye.

Alizarin

6. Azo dye - The azo dyes contain one or more azo groups N=N, as the chromophore. Azo dyes constitute the largest and most important group of synthetic dyes. These can be prepared by diazotising an aromatic amine and subsequent coupling with a suitable aromatic phenol or amine.

The important azo dyes are the following

Azo dyes give bright, high intensity colours, much more so than the next most common dye class (anthraquinones). Azo dyes can be further divided into acid, basic, direct, ingrain or developed dyes, etc., on the basis of mode of application.They have fair to good fastness properties, but not so good as the carbonyl and phthalocyanine classes. Their biggest advantage it their costeffectiveness, which is due to the processes involved in manufacture. The general formula for making an azo dye requires two organic compounds- a coupling component and a diazo component. Since these can be altered considerably, an enormous range of possible dyes are available, especially as the starting molecules are readily available and cheap. Furthermore, the simplicity of the reactions mean that the process can be scaled up or down very easily, which is always a key factor in the cost of chemicals. Energy requirements for the reaction are low, since most of the chemistry occurs at or below room temperature. The environmental impact is reduced by the fact that all reactions are carried out in water, which is easy and cheap to obtain, clean and dispose of. As other dye classes become less viable from either an environmental or economic reasons, azo dyes become ever more attractive options.

Applications of dyes Constant researches done over the 20th century and thereafter have resulted into every imaginable form of color of dye. Modern dyes serve more than just being pretty. They have become indispensable tools for a variety of industries. From acting as colorants for plastics, Textile dyeing industries and the highly sophisticated biotechnology industry dyes are touching our life everywhere. Dyes are also used by industries for inks and tinting. Today various dyes are manufactured to meet the requirements of each type of industries. Dyes are available in various forms. Examples are dry powders, granules, pastes, liquids, pellets, and chips. Other industries where dyes are used in a variety of products include paper and pulp, adhesives, art supplies, beverages, ceramics, construction, cosmetics, food, glass, paints, polymers, soap, wax biomedicine etc. Synthetic dyes are used in a multitude of industries that includes the following besides textiles. Medicine, chemistry, and other related fields. Commercial products like plastics, paint, printing ink, rubber, cosmetics etc. Dyes are now an important ingredient in many of the medical tests. Many of the tests that are carried out on patients use the dye to get accurate results. One such example is that of Fluorescein angiography. Fluorescein angiography derives its name from fluorescein, the dye that is used is very successfully for carrying out the tests. Angiogram is a very valuable test that gives information about the circulatory system.

Preparation of organic dyes