Azo dyeDyes:

A dye is 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 more than others. In contrast with a dye, a pigment generally 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.

Dyed flax fibers have been found in the Republic of Georgia dated back in a prehistoric cave to 36,000 BP. Archaeological evidence shows that, particularly in India and Phoenicia, dyeing has been widely carried out for over 5000 years. The dyes were obtained from animal, vegetable or mineral origin, with no or very little processing. By far the greatest source of dyes has been from the plant kingdom, notably roots, berries, bark, leaves and wood, but only a few have ever been used on a commercial scale.

History:Ever since the beginning of humankind, people have been using colorants for painting and dyeing of their surroundings, their skins and their clothes. Until the middle of the 19 century, all colorants applied were from natural origin. Inorganic pigments such as soot, manganese oxide, hematite and ochre have been utilised within living memory.Palaeolithic rock paintings, such as the 30,000 year old drawings that were recently discovered in the chauvet caves in france, provide ancient testimony of their application.

Organic natural colorants have also a timeless history of application, especially as textile dyes. These dyes are all aromatic compounds, originating usually from plants (e.g. the red dye alizarin from madder and indigo (Figure 1.1) from woad) but also from insects (e.g.the scarlet dye kermes from the shield-louse kermes vermilio),fungi and lichens. Synthetic dye mcanufatuing started in 1856, when the English chemist W.H . perkin, in an attempet to synthesise quinine , obtained instead a bluish substance with excellent dyeing properties that later became known as aniline purple, Tyrian purple or mauveine. Perkin, 18 years old, patented his invention and set up a production line. This concept of reseearch and development was soon to be followed by others and new dyes began to appear on the market, aprocess that was strongly stimulated by kekule discovery

of the molecular structure of benzene in 1865. In the beginning of the completely supplanted natural dyes.

Figure 1.1 Indigo

Dye classification:All aromatic compounds absorb electromagnetic energy but only those that absorb light with wavelengths in the visible range ( 350-700 nm) are coloured. Dyes contain chromophores, delocalised electron systems with conjugated doubl bonds, and auxochromes, electron-withdrawing or electron-donating substituents that cause or intensify the colour of the chromophore by altering the overall energy of the electron system . Usual chromophores are -C=C-,-C=N-,C=O, -N=N-, -NO2 and quinoid rings, usual auxochromes are -NH3, -COOH, -SO3H and -OH.

Based on chemical structure or chromophore, 20-30 different groups of dyes can phthalocyanine and triarylmetane dyes are quantitatively the most important nxanthene , nitro , nitroso , methane , thiazole , indamine , indophenol , lactone , aminoketone and hydroxyketone dyes and dyes of undetermined structure (stilbene and sulphur dyes ) .

The vast array of commercial colorants is classified in terms of colour, structure and application method in the colour index (C.I .) which is edited since 1924 (and revised every three months) by the society of Dyers and colourists and the American Association of textile chemists and colorists. The colour index lists about 28,000

commercial dye names , representing 10 , 500 different dyes , 45,000 of which are currently produced . Each different dye is given a C.I . generic name determined by its application characteristics and its colour.

Dyes may be classification according to chemical structure or by their application method:

Dyes

Chemical structure Application Azo dye Acid dye Anthraquinone Reactive dye

Phthalocyanine Metal complex dyes Direct dyes

Basic dyes

Mordant dyes

Disperse dyes

Pigment dyes

Vat dyes

Anionic and ingrain dyes

Sulphur dyes

Solvent dyes

Fluorescent dyes

-chemical classification:The most appropriate system for the classification of dyes is by chemical structure , which has many advantages . First, it readily indentifies dyes as belonging to a group that has characteristic properties , for example, azo dyes (strong , good all-round properties , cost-effective) and anthraquinone dyes (weak , expensive ). Second , there are a manageable number of chemical groups (about a dozen ). Most importantly , it is the classification used most widely by both the synthetic dye chemist and the dye technologist. Thus , both chemists and technologists can readily identify with phrases such as an azo yellow , an anthraquinone red , and a phthalocyanine blue .

By the nature of their chromophores , dyes are divided into:-Acridine dyes , derivates of aceridin .

-Anthraquinone dyes , derivates of anthraquinone .

-Arylmethane dyes .

-Diarylmethane dyes , based on diphenyl methane .

-Triarylmethane dyes , derivates of triphenyl methane .

-Azo dyes , based on -N=N- (azo structure) .

-Diazonium dyes , based on diazonium salts .

-Nitro dyes , based on a -NO2 (nitro functional group).

-Nitroso dyes , based on a -N=O (nitroso functional group).

-Phthalocyanine dyes , derivates of phthalocyanine .

-Quinone-imine dyes , derivates of quinone .

-Azin dyes .

-Eurhodin dyes .

-Safranin dyes , derivates of safranin .

-Indamins .

-Indophenol dyes , derivates of indophenol .

-Oxazin dyes , derivates of oxazin .

-Oxazone dyes , derivates of oxazone .

-Thiazin dyes , derivates of thiazin .

-Thiazole dyes ,derivates of thiazole .

-Xanthene dyes , derived from xanthene .

- Fluorene dyes, derivates of fleuore .

-Pyronin dyes .

-Fluorone dyes , based on fluorone .

-Rhodamine dyes , derivates of rhodamine .

Azo dye:Azo compounds are compounds bearing the functional group R-N=N-R', in which R and R' can be either aryl or alkyl. IUPAC defines azo compounds as: "Derivatives of diazene (diimide), HN=NH, wherein both hydrogens are substituted by hydrocarbyl groups, e.g. PhN=NPh azobenzene or diphenyldiazene.". The more stable derivatives contain two aryl groups. The N=N group is called an azo group.

Anthraquinone:

Anthaquinone dyes are derived from anthracene, They have thegeneral formula:

Phthalocyanine:Phthalocyanine is an intensely blue-green coloured macrocyclic compound that is widely used in dyeing. Phthalocyanines form coordination complexes with most elements of the periodic table. These complexes are also intensely colored and also are used as dyes or pigments.

Application classification:

It is advantageous to consider the classification of dyes by use or method of application befor considering chemical structure in detail because of the dye nomenclature and jargon that arises from this system . classification by application is the principal system adoptede by the colour index . Because the most important textile fibers are cotton and polyester , the most important dye types are those used for dyeing these two fibers , including : polyester-cotton blends . Other textile fibers include : nylon , polyacrylonitrite , and

cellulose acetate .

Acid dyes:

The largest class of dyes in the colour index is referred to as Acid dyes (∼ 2300 different acid dyes listed , ∼ 40% of them are in current production). Acid dyes are anionic compunds that are mainly used for dyeing nitrogen-containing fabrics like wool, polyamide, silk and modified acryl. They bind to the cationic NH4 +-ions of those fibres. Most acid dyes are azo (yellow to red, or a broader range colours in case of metal complex azo dyes), anthraquinone or triarylmethane (blue and green) compounds.The adjective .acid. refers to the pH in acid dye dyebaths rather than to the presence0f acid groups (sulphonate, carboxyl) in the molecular structure of these dyes.

Reactive dyes:Reactive dyes are dyes with reactive groups that form covalent bonds with OH-, NH-, or SH-groups in fibres (cotton, wool, silk, nylon). The reactive group is often a heterocyclic aromatic ring substituted with chloride or fluoride, e.g. dichlorotriazine. Another common reactive group is vinyl sulphone (asin Reactive Orange 7). The use of reactive dyes has increased ever since their introduction in 1956, especially in industrialised countries. In the Colour Index, the reactive dyes form the second largest dye class with respect to the amount of active entries: about 600 of the ∼1050 different reactive dyes listed are in current production. During dying with reactive dyes, hydrolysis (i.e. inactivation) of the reactive groups is an undesired side reaction that lowersthe degree of fixation. In spite of the addition of high quantities of salt andureum (up to respectively 60 and 200 g/l) to raise the degree of fixation, it is estimated that 10 to 50% will not react with the fabric and remain .hydrolysed. in the water phase. The problem of coloured effluents is therefore mainly identified with the use of reactive dyes. Most (∼80%) reactive dyes are azo or metal complex azo compounds but also anthraquinone and phthalocyanine reactive dyes are applied, especially for green and blue.

Metal complex dye:Among acid and reactive dyes, many Metal complex dyes can be found (not listed as a separate category in the Colour Index). These are strong complexes of one metal atom (usually chromium, copper, cobalt or nickel) and one or two dye molecules, respectively 1:1 and 1:2 metal complex dyes. Metal complex dyes are usually azo compounds. About 1/6 of all azo dyes listed in the Colour Index are metal complexes but also phthalocyanine metal complex dyes areapplied.

Direct dyes:Direct dyes are relatively large molecules with high affinity for especially cellulose fibres. Van der Waals forces make them bind to the fibre. Direct dyes are mostly azo dyes with more than one azo bond or phthalocyanine, stilbene or oxazine compounds. In the Colour Index , the direct dyes from the

second largest largest dye class with respect to the amount of different dyes: About 1600 direct dyes are listed but only ∼30% of them are in current production.

Basic dyes:Basic dyes are cationic compounds that are used for dyeing acid-group containing fibres, usually synthetic fibres like modified polyacryl . Theybind to the acid groups of the fibres. Most basic dyes are diarylmethane,triarylmethane, anthraquinone or azo compounds. Basic dyes represent ∼5% of all dyes listed in the Colour Index.

Mordant dyes:Mordant dyes are fixed to fabric by the addition of a mordant, a chemical that combines with the dye and the fibre. Though mordant dyeing is probably one of the oldest ways of dyeing, the use of mordant dyes is gradually decreasing:only ∼23% of the ∼600 different mordant dyes listed in the Colour Indexare in current production. They are used with wool, leather, silk, paperand modified cellulose fibres. Most mordant dyes are azo, oxazine ortriarylmethane compounds. The mordants are usually dichromates or chromium complexes.

Disperse dyes:Disperse dyes are scarcely soluble dyes that penetrate synthetic fibres (cellulose acetate, polyester, polyamide, acryl, etc.). This diffusion requires swelling of the fibre, either due to high temperatures (>120 °C) or with the help ofchemical softeners. Dying takes place in dyebaths with fine dispersesolutions of these dyes. Disperse dyes form the third largest group of dyes in the Colour Index: about 1400 different compounds are listed, of which ∼40%is currently produced. They are usually small azo or nitro compounds (yellow to red), anthraquinones (blue and green) or metal complex azo compounds (all colours).

Pigment dyes:Pigment dyes (i.e. organic pigments) represent a small but increasing fraction of the pigments, the most widely applied group of colorants. About 25% of allcommercial dye names listed in the Colour Index are pigment dyes but these∼6900 product names stand for less than 800 different dyes. These insoluble,non-ionic compounds or insoluble salts retain their crystalline or particulate structure throughout their application. Pigment dyeing is achieved froma dispersed aqueous solution and therefore requires the use of dispersing agents. Pigments are usually used together with thickeners in print pastesfor printing diverse fabrics. Most pigment dyes are azo compounds (yellow, orange, and red) or metal complex phthalocyanines (blue and green). Also anthraquinone and quinacridone pigment dyes are applied.

Vat dyes:Vat dyes are water-insoluble dyes that are particularly and widely used for dyeing

cellulose fibres. The dyeing method is based on the solubility of vat dyesin their reduced (leuco) form. Reduced with sodium dithionite, the soluble leuco vat dyes impregnate the fabric. Next, oxidation is applied to bring backthe dye in its insoluble form. Almost all vat dyes are anthraquinones or indigoids. Indigo itself is a very old example of a vat dye, with about 5000 yearsof application history. .Vat. refers to the vats that were used for the reductionof indigo plants through fermentation.

Anionic dyes and ingrain dyes:Azoic dyes and Ingrain dyes (naphthol dyes) are the insoluble products of a reaction between a coupling component (usually naphthols, phenols or acetoacetylamides; listed in the Colour Index as C.I. azoic coupling components) and a diazotised aromatic amine (listed in the Colour Index as C.I. azoic diazo components). This reaction is carried out on the fibre ,All naphthol dyes are azo compounds.

Sulphur dyes:Sulphur dyes are complex polymeric aromatics with heterocyclic S-containing rings. Though representing about 15% of the global dye production, sulphur dyes are not so much used in Western Europe. Dyeing with sulphur dyes involves reduction and oxidation, comparable to vat dyeing. They are mainlyused for dyeing cellulose fibres.

Solvent dyes:Solvent dyes (lysochromes) are non-ionic dyes that are used for dyeing substrates in which they can dissolve, e.g. plastics, varnish, ink, waxes and fats. They are not often used for textile-processing but their use is increasing. Mostsolvent dyes are diazo compounds that underwent some molecular rearrangement. Also triarylmethane, anthraquinone and phthalocyanine solvent dyes are applied.

Fluorescent dyes: Fluorescent brighteners (or bluing agents) mask the yellowish tint of natural fibres by absorbing ultraviolet light and weakly emitting visible blue. They arenot dyes in the usual sense because they lack intense colour. Based on chemical structure, several different classes of fluorescent brighteners are discerned: stilbene derivatives, coumarin derivatives, pyrazolines, 1,2-ethene derivatives,naphthalimides and aromatic or heterocyclic ring structures. Many fluorescent brighteners contain triazinyl units and water-solubilising groups.

Other dye classes:Apart from the dye classes mentioned above, the Colour Index also lists Food dyes and Natural dyes. Food dyes are not used as textile dyes and the use ofnatural dyes (mainly anthraquinone, indigoid, flavenol, flavone or chromancompounds that can be used as mordant, vat, direct, acid or solvent dyes)in textile-processing operations is very limited.

Group Application

Direct Cotton, cellulosic and blended fibres

Vat dyes Cotton, cellulosic and blended fibres

Sulphur Cotton, cellulosic fibre

Organic pigments Cotton, cellulosic, blended fabric, paper

Reactive Cellulosic fibre and fabric

Disperse dyes Synthetic fibres

Acid Dyes Wool, silk, paper, synthetic fibres, leather

Azoic Printing Inks and Pigments

Basic Silk, wool, cotton

Uses of dyes: Textiles . Cosmetics . Leather . Pigments find regular

application in the following sectors:

Electronics industry Inkjet Inks Paints Industry Plastics

Construction Industry Wood working Cement Industry

Dye laser pumping

Inkjet dyes

Dyes We hold expertise in offering high quality Direct Dyes that are highly effective and colorfast. The dyes offered by us are free from benzidine and exhibit excellent chemical properties. Extensively used for various industry applications, these dyes are highly demanded by the global clients. Clients can avail these dyes from us at market leading prices.

Natural dyes for textile

Used dye panels retain a viewable image of the printed document , and an example of wasted dye that cannot be reused………………………………………………......................................................

A selection of dye-sensitized solar cells.

What makes the Dyes coloured?This is a very common question that occurs in everybodies mind. The answer to which is explained by the presence of a substance called Chromophore in the dyes. By definition dyes are basically aromatic compounds. Their structures have aryl rings that has delocalised electron systems. These structures are said to be responsible for the absorption of electromagnetic radiation that has varying wavelengths, based upon the energy of the electron clouds.

It is actually because of this reason that chromophores do not make dyes coloured. Rather it makes the dyes proficient in their ability to absorb radiation. Chromophores acts by making energy changes in the delocalised electron cloud of the dye. This alteration invariably results in the compound absorbing radiation within the visible range of colours and not outside it. Human eyes detects this absorption, and responds to the colours.

To conclude chromophores are the atomic configurations which has delocalised electrons. Generally they are represented as carbon, nitrogen, oxygen and sulphur. They can have alternate single and double bonds.

How can the colour of the Dyes be altered?

The answer lies in the Modifiers. Colour modifiers like methyl or ethyl groups can actually alter the colour of dyes. They do so by altering the energy in the delocalised electrons. It has been found that by addition of a particular modifier there is a progressive alteration of colour. An example can be given for methyl violet series.

The following diagram explains what happens to the colour of the dyes when modifiers are added.

Step A: When no methylgroup is added the original dye Pararosanil as it is called is red in colour.

Step B: As Four Methyl groups are added the reddish purple dye Methyl Violet is got..

Step C: With the addition of more groups a purple blue dye Crystal Violet is obtained. It has in it six such groups.

Step D: Further addition of a seventh methyl group the dye that is got is called Methyl green.

What gives the Dyes Solubility and Cohesiveness?

The answer to this riddle lies in substance called Auxochrome. Moreover the Auxochromes has also the abilty to intensify colours. It is a group of atoms which attaches to non-ionising compounds yet has the ability to ionise. Auxochromes are of two types, positively charged or negatively charged.

Azo dye:Azo dyes , which represent about one-half of all dyes in common use , are employed as coloring agents in the food , pharmaceutical , and textile

industries. The popularity and widespread use of azo dyes is due to several factors . As a group , they are color-fast and encompass the entire visible spectrum , and many are easily synthesized from inexpensive and easily obtained starting materials . Also azo dyes are typically amenable to structural modification , and can be made to bind to most synthetic and natural textile fibers .

What is an azo group?The azo compound class accounts for 60-70% of all dyes.  As you might expect, they all contain an azo group, -N=N-,  which links two sp2 hybridised carbon atoms.  Often, these carbons are part of aromatic systems, but this is not always the case.  Most azo dyes contain only one azo group, but some contain two (disazo), three (trisazo) or more.

In theory, azo dyes can supply a complete rainbow of colours.  However, commercially they tend to supply more yellows, oranges and reds than any other colours.  Research is always continuing, though, so that now there are some viable blue azo dyes on the market2.  The relationship between the colour of an azo dye has been more fully discussed in The Basis of Colour.

General chemical formula of azo compounds

Aryl azo compoundsAryl azo compounds are usually stable, crystalline species. Azobenzene is the prototypical aromatic azo compound. It exists mainly as the trans isomer, but upon photolysis, converts to the cis isomer. Aromatic azo compounds can be synthesized by using an azo coupling reaction, which entails an electrophilic substitution reaction where an aryl diazonium cation attacks another aryl ring, especially those substituted with electron-releasing groups. Since diazonium salts are often unstable near room temperature, the azo coupling reactions are typically conducted near ice temperatures. The oxidation of hydrazines (R-NH-NH-R') also gives azo compounds. Azo dyes derived from benzidine are carcinogens; exposure to them has classically been associated with bladder cancer. Accordingly, the production of benzidine azo dyes was discontinued in the 1980s "in the most important western industrialized countries".

Alkyl azo compounds:

Aliphatic azo compounds (R and/or R' = aliphatic) are less commonly encountered than the aryl azo compounds. One example is diethyldiazene, EtN=NEt. At elevated temperatures or upon irradiation,

the carbon-nitrogen (C-N) bonds in certain alkyl azo compounds cleave with the loss of nitrogen gas to generate radicals. Owing to this process, some aliphatic azo compounds are utilized as radical initiators. Representative is Azobisisobutylonitrile (AIBN) which is widely used as an initiator in polymerization. Because of their instability, especially for aliphatic ones, care should be taken with the handling of azo compounds or an explosion may occur.

Azo dye properties:Azo dyes give bright, high intensity colours, much more so than the next most common dye class (anthraquinones).  They have fair to good fastness properties, but not so good as the carbonyl and phthalocyanine classes.  Their biggest advantage it their cost-effectiveness, 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.

Application of azo dyes:

Azo dyes have been involved in a wide range of application:- Yellow azo dyes consequence of п-delocalization , aryl azo compounds have vivid colors, especially reds, oranges, and yellows. Therefore, they are used as dyes , and are commonly known as azo dyes, an example of which is Disperse oranges 1. Some azo compounds, e.g.methyl orange, are used as acid-base indicators due to the different colors of their acid and salt forms.

-Most DVD-R/+R and some CD-R discs use blue azo dye as the recording layer. The development of azo dyes was an important step in the development of the chemical industry.

-Azo pigments are colorless particles (typically earths or clays), which have been colored using an azo compound. Azo pigments are important in a variety of paints including artist's paints. They have excellent coloring properties, again mainly in the yellow to red range, as well as lightfastness. The lightfastness depends not only on the properties of the organic azo compound, but also on the way they have been adsorbed on the pigment carrier. Many azo pigments are non-toxic, although some, such as dinitroaniline orange, ortho-nitroaniline orange, or pigment orange 1, 2, and 5 have been found to be mutagenic. Likewise, several case studies have linked azo pigments with Basal Cell Carcinoma.

-Azo dyes account for approximately 60-70% of all dyes used in food and textile manufacture.

-Four kinds of azo dyes and their meta-azo dyes were synthesized in order to obtain a high reflective index and better thermal stability as a recording material for DVD-R .

Isomerism in azo dyes:Geometrical isomerism

As with any double bond, the planar -N=N- bond shows geometrical isomerism:

This change from trans (preferred) to cis can be effected by exposure to UV radiation.  This can lead to photochromism, a light-induced reversible colour change in some dyes, for example C.I. Disperse Red 1.  This effect was considered a nuisance and has largely been eliminated by careful development of more stable dyes.  But photochromic dyes are beginning to make a comeback in technology like sunglasses and sunroofs in cars.

Tautomerism:

This involves the removal of a hydrogen from one part of the molecule, and the addition of a hydrogen to a different part of the molecule.  This is common when there is an -OH group ortho or para to the azo group:

Tautomeric forms can be identified form their characteristic spectra.  Ketohydrazones are normally bathochromic compared to their counterpart hydroxyazo forms.  Ketohydrazones also have higher molar extinction co-efficients.  However, not all azo dyes show tautomerism, and some tautomeric forms are more stable than others.

Synthesis of azo dyes:

An overview of azo dye synthesis is shown below:

Stage 1- Diazotisation:

This involves a primary aromatic amine, called the diazo component.  It is treated in low temperature, acid conditions with sodium nitrite to form an unstable diazonium salt.

Stage 2- Azo coupling:

The diazonium salt is reacted with a coupling component (for example a phenol or an aromatic amine).  This forms the stable azo dye.

Mechanism:

Experimental: ( Dye1)

Chemicals MWT Amount

P-amino phenol 1gm2-(methyl phenyl amino)

ethanol1gm

Sodium nitrite (NaNo2) 1,1gmSodiumacetate (CH3CooNa ) Any

amount

Conc.HCL 2mlCH3COOH 5ml

Diazotization and coupling:-The diazotization of P-Amino phenol and coupling with 2-(methyl phenyl amino ) ethanol .

Procedure:-1gm of P-Amino phenol was dissolved in a mixture of (15ml H2O + 2ml HCL ) . The solution was then cooled to 0-5 C in an ice-brine bath and maintained at this temperature . Sodium nitrite1,1gm (0,015 mol ) was dissolved in 5ml H2O, was added as a period of 20 min and stirring was continued for 30 min , keeping the temperature between 0-5 C .

-2-(methyl phenyl amino ) ethanol (1gm) was dissolved in a mixture 5ml acetic acid and sodium acetate was then added to maintain PH about 5.5 . The solution was cooled to 0-5 C in an ice-brine bath .

-The diazonium solution was added portionwise to the coupling component , and maintained at 0-5C for 2h and leave in fridge .

-After that ,The mixture was filteration .