Classification of Dyes

There are several ways for classification of dyes. 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. Some of the prominent ways of classification is given hereunder.

• Classification based on the source of materials
• Chemical classification of the Dyes- Based on the nature of their respective chromophores.
• Dyes according to the nuclear structure
• Industrial Classification of the Dyes

Classification based on the source of materials:

A very common classification of the dyestuff is based on the source from which it is made. Accordingly the classification could be:
• Natural Dyes
• Synthetic Dyes

1.Natural dyes:

Colouring materials have been used for many thousands of years by man. Leather, cloth, food, pottery and housing have all been modified in this way. The two old ways were to cover with a pigment (painting), or to colour the whole mass (dyeing). Pigments for painting were usually made from ground up coloured rocks and minerals, and the dyes were obtained from animals and plants. Today, many of the traditional dye sources are rarely, if ever, used (onion skins, for instance). However, some of our most common dyes are still derived from natural sources. These are termed natural dyes. The Colour Index uses this as a classification and naming system. Each dye is named according to the pattern:

–Natural + base colour + number

These dyes are thereby specifically identified as dyes of the stated colour, and which may still be derived from animals or plants. Note that this is a classification based on the dye’s source and colour. It contains no chemical information; neither does it imply that dyes with similar names but unique numbers are in any way related. It gives no information about the mechanism by which staining occurs.
Natural dyes are often negatively charged. Positively charged natural dyes do exist, but are not common. In other words, the coloured part of the molecule is usually the anion. Although the molecular charge is often shown on a specific atom in structural formulae, it is the whole molecule that is charged. Many, but by no means all, natural dyes require the use of a mordant.
The use of dyes is very ancient. Kermes (natural red 3) is identified in the bible book of Exodus, where references are made to scarlet coloured linen. Similar dyes are carmine (natural red 4) and lac (natural red 25). These three dyes are close chemical relatives, obtained from insects of the genus Coccus. All require a mordant. The most commonly used natural dye is undoubtedly hematein (natural black 1), obtained from the heartwood of a tree. This dye also requires a mordant.

Saffron (natural yellow 6), is obtained from the stigmata of Crocus sativus, and is used without a mordant, staining as an acid dye. Although its use is very ancient, it is more common now as a colouring and spice for food than for dyeing, due to its expense.

Synthetic Dyes
Dyes derived from organic or inorganic compound are known as synthetic dyes. Examples of this class of dyes are Direct, Acid, Basic, Reactive, Mordant, Metal complex, Vat, Sulphure , Disperse dye etc. However using general dye chemistry as the basis for classification, textile dyestuffs are grouped into 14 categories or classes:

Group:	Application:
Direct	Cotton, Cellulosic and Blends
Vat dyes	Cotton, Cellulosic and Blends
Sulphur	Cotton, Cellulosic fibers
Organic pigments	Cotton, Cellulosic, Blended Fabrics, paper
Reactive	cellulosic fibers and fabric
Dispersed dyes	Synthetic fibers
Acid Dyes	Wool, Silk, Synthetic fibers, leather
Azoic	Printing inks and pigments
Basic	silk, wool,cotton
Oxidation dyes	Hair
Developed Dyes	Cellulosic fibers and Fabric
Mordant dyes	Cellulosic fibers and Fabric, Silk, Wool
Optical/Fluorescent Brighteners	synthetic fibers, leather, cotton, sports goods
Solvent dyes	Wood Staining, solvent inks, waxes, colouring oils

Chemical classification of the Dyes:

According to a system of chemical classification, dyes can be divided according to the nature of their Chromophore:

Chromophoric Group	Textiles, leather
Acridine dyes, derivatives of acridine >C=N-and >C=C	Texties
Anthraquinone dyes, derivatives of anthraquinone >C=O and >C=C
Arylmethane dyes; Diarylmethane dyes, based on diphenyl methane, Triarylmethane dyes, based on triphenyl methane Azo dyes, based on a -N=N- azo structure Cyanine dyes, derivatives of phthalocyanine Diazonium dyes, based on diazonium salts Nitro dyes, based on the –NO2 nitro functional group
Nitroso dyes, are based on a –N=O nitroso functional
Phthalocyanine dyes, derivatives of phthalocyanine >C=N	Paper
Quinone-imine dyes, derivatives of quinine	Wool and paper
Azin dyes; -Eurhodin dyes, -Safranin dyes, derivatives of safranin -C-N=C- -C-N-C	Leather and textile
Xanthene dyes, derived from xanthene -O-C6H4-0	Cotton, Silk and Wool
Indophenol dyes, derivatives of indophenol >C=N-and >C=O	Colour photography
Indophenol dyes, derivatives of indophenol >C=N-and >C=O	Colour photography
Oxazin dyes, derivatives of oxazin -C-N=C =C-O-C=	Calico printing
Oxazone dyes, derivatives of oxazone
Thiazin dyes, derivatives of thiazin
Thiazole dyes, derivatives of thiazole >C=N- and -S-0=	Intermediate
Fluorene dyes, derivatives of fluorine	Intermediate
Rhodamine dyes, derivatives of rhodamine Pyronin dyes

Dyes according to the nuclear structure:

Though not very popular but dyes can be categorized into types by using this method of classification:
• Cationic Dyes
• Anionic Dyes

Industrial Classification of the Dyes:

As globally majority of the dyestuff is primarily consumed by the textile industry. So, at this level a classification can be done according to their performances in the dyeing processes. Worldwide around 60% of the dyestuffs are based on azo dyes that gets consumed by in the textile finishing process. Major classes of dyes in textile finishing are given here. Major Dye classes and the substrates:
• Protein Textile Dyes
• Cellulose Textile Dyes
• Synthetic Textile Dyes

Cellulose Textile Dyes:

• Direct dyes:

The name ‘direct dye’ alludes to the fact that these dyes do not require any form of ‘fixing’. They are almost always azo dyes, with some similarities to acid dyes. They also have sulphonate functionality, but in this case, it is only to improve solubility, as the negative charges on dye and fibre will repel each other. Their flat shape and their length enable them to lie along-side cellulose fibres and maximise the Van-der-Waals, dipole and hydrogen bonds. Below is a diagram of a typical direct dye. Note that the sulphonate groups are spread evenly along the molecule on the opposite side to the hydrogen bonding -OH groups, to minimise any repulsive effects.

The main problem with direct dyes is their lack of fastness during washing. However, they are cheap, so are popular for items which are less likely to require fastness during washing. Wash fastness may be improved, though, by the application of direct and developed dyes, which contain -NH2 functional groups as well as sulphonate groups. In this process, the dyed fabric is treated with sodium nitrite, which causes the dye to be converted to a diazo salt. It is then treated with a coupling compound such as 2-napthol. The resultant larger azo molecule now has more affinity for the fibres, and is less soluble.

• Vat dyes:

Vat dyes are a good example of the cross-over between dyes and pigments. Large, planar and often containing multi-ring systems, vat dyes come exclusively from the carbonyl class of dyes (for example, indigo). The ring systems of the vat dyes help to strengthen the Van-der-Waals forces between dye and fibre.

Vat dyes are insoluble in water, but may become solublised by alkali reduction, for example sodium dithionite (a reducing agent) in the presence of sodium hydroxide. For this reason, they tend not to contain many other functional groups which may be vulnerable to oxidation or reduction. The leuco form produced by alkali reduction is absorbed by the cellulose and, once there, can be oxidised back to its insoluble form. Oxidation is usually performed using hydrogen peroxide, but occasionally with atmospheric oxygen under the correct conditions. Treating the dyed textile with a soap completes the process, since the soap molecules encourage the dye molecules to clump together and become crystalline.

The other types of dyes, for example the azo class, undergo a non-reversible change on reduction.

• Basic dyes:

Basic dyes possess cationic functional groups such as -NR3+ or =NR2+. The name ‘basic dye’ refers to when these dyes were still used to dye wool in an alkaline bath. Protein in basic conditions develops a negative charge as the -COOH groups are deprotonated to give -COO-Basic dyes perform poorly on natural fibres, but work very well on acrylics. A general structure of an acrylic type polymer is shown below. It is simplified, and doesn’t show any anionic groups which are often present.

The most common anionic group attached to acrylic polymers is the sulphonate group, -SO3-, closely followed by the carboxylate group, -CO2-. These are either introduced as a result of co-polymerisation, or as the residues of anionic polymerisation inhibitors. It is this anionic property which makes acrylics suitable for dyeing with cationic dyes, since there will be a strong ionic interaction between dye and polymer (in effect, the opposite of the acid dye-protein fibre interaction). An example of a basic dye is shown below:

• Fibre-Reactive Dyes:

A fibre-reactive dye will form a covalent bond with the appropriate textile functionality. This is of great interest, since, once attached, they are very difficult to remove.
Early fibre-reactive dyes; The first fibre-reactive dyes were designed for cellulose fibres, and they are still used mostly in this way. There are also commercially available fibre-reactive dyes for protein and polyamide fibres. In theory, fibre-reactive dyes have been developed for other fibres, but these are not yet practical commercially. Although fibre-reactive dyes have been a goal for quite some time, the breakthrough came fairly late, in 1954. Prior to then, attempts to react the dye and fibres involved harsh conditions that often resulted in degradation of the textile.

The first fibre-reactive dyes contained the 1,3-5-triazinyl group, and were shown by Rattee and Stephen to react with cellulose in mild alkali solution. No significant fibre degradation occurred. ICI launched a range of dyes based on this chemistry, called the Procion dyes. This new range was superior in every way to vat and direct dyes, having excellent wash fastness and a wide range of brilliant colours. Procion dyes could also be applied in batches, or continuously.
The general structure of a fibre-reactive dye is shown below:

A cellulose polymer has hydroxy functional groups, and it is these that the reactive dyes utilise as nucleophiles. Under alkali conditions, the cellulose-OH groups are encouraged to deprotonate to give cellulose-O- groups. These can then attack electron-poor regions of the fibre-reactive group, and perform either aromatic nucleophilic substitution to aromatics or nucleophilic addition to alkenes.

Nucleophilic substitution; Aromatic rings are electronically very stable, and will attempt to retain this. This means that instead of the nucleophilic addition that occurs with alkenes, they undergo nucleophilic substitution, and keep the favourable -electron system. However, nucleophilic subsitutions are not very common on aromatics, given their already high electron density. To encourage nucloephilic substitution, groups can be added to the aromatic ring which will decrease the electron density at a position and facilitate attack. For example:

But this requires harsh conditions. To improve the rate under mild conditions, powerful electron-withdrawing groups such as -NO2 may be added.

However, this will only work if there is a good leaving group, such as -Cl or -N2. The major fibre-reactive group which reacts this way contains six-membered, heterocyclic, aromatic rings, with halogen substituents. For example, the Procion dye:

Where X = Cl, NHR, OR. Nucleophilic substitution is facilitated by the electron withdrawing properties of the aromatic nitrogens, and the chlorine, and the anionic intermediate is resonance stabilised as well. This resonance means that the negative charge is delocalised onto the electronegative nitrogens:

One problem is that instead of reacting with the -OH grous on the cellulose, the fibre-reactive group may react with the HO- ions in the alkali solution and become hydrolysed. The two reactions compete, and this unfavourable because the hydrolysed dye cannot react further. This must be washed out of the fabric before use, to prevent any leakage of dye, and not only increases the cost of the textile, but adds to possible environmental damage from contaminated water.

Nucleophilic addition;Alkenes are quite reactive due to the electron-rich -bond. They normally undergo electrophilic addition reactions. Again, nucleophilic additions are less favoured generally, because of the repulsion between the Nu- and the electron-rich bond. However, they will occur if there are sufficient electron withdrawing groups are attached to the alkene, much as before, with aromatic substitution. In this case, the process is known as Michael addition or Conjugate addition.

For this reaction type, the most important dye class is the Remazol reactive dye. This dye type reacts in the presence of a base such as HO-. The mechanism for the reaction of one of these dyes is shown below:

As before, the intermediate is resonance stabilised, but this has not been shown.

Protein Textile Dyes:

Acid dyes:

Acidic dyes are highly water soluble, and have better light fastness than basic dyes. They contain sulphonic acid groups, which are usually present as sodium sulphonate salts. These increase solubility in water, and give the dye molecules a negative charge. In an acidic solution, the -NH2 functionalities of the fibres are protonated to give a positive charge: -NH3+. This charge interacts with the negative dye charge, allowing the formation of ionic interactions. As well as this, Van-der-Waals bonds, dipolar bonds and hydrogen bonds are formed between dye and fibre. As a group, acid dyes can be divided into two sub-groups: acid-leveling or acid-milling.

Acid-leveling dyes: 

These planar dyes tend to be small or medium sized, and show moderate inter-molecular attractions for wool fibres. This means that the dye molecules can move fairly easily through the fibres and achieve an even colour. This is somewhat similar to the process that occurs during chromatography- the molecules with the strongest affinity for the substrate move the least distance from the point of origin whereas molecules with less affinity move much further. However, the low affinity means that these dyes are not always very resistant to washing.

Acid-milling dyes: 

Acid-milling dyes are larger than acid-leveling dyes, and show a much stronger affinity for wool fibres. Because of this, the resultant colour may be less even (see explanation above), but they are much more resistant to washing. As well as intermolecular interactions, intramolecular interactions play an important part in the properties of the dye. Compare the two molecules shown below. They are isomers, but the one on the right (with hydrogen bonding) shows a much greater resistance to washing in alkali, and much increased light fastness.

Acid dye colours: 

Usually, yellow, orange and red acid dyes are azo compounds, with blues and greens often come from the carbonyl class, particularly anthraquinones (see the example below).An example of an acid dye is Alizarine Pure Blue B. It is a sulphonated aminoanthraquinone

• Mordant dyes:

Mordant is a Latin word meaning ‘to bite’. Mordants act as ‘fixing agents’ to improve the colour fastness of some acid dyes, which have the ability to form complexes with metal ions. Mordants are usually metal salts; alum was commonly used for ancient dyes, but there is a large range of other metallic salt mordants available. Each one gives a different colour with any particular dye, by forming an insoluble complex with the dye molecules. Chromium salts such as sodium or potassium dichromate are commonly used now for synthetic mordant dyes. The diagrams below show C.I. Mordant Black 1 with and without a chromium (III) ion. Chromium (III) forms 6-coordinate complexes, so two Mordant Black molecules would attach to one ion. Only one is shown below for clarity.

Mordants do not have to be metal salts. Organic molecules such as tannic acid and tartaric acid can be used as well.

2.Synthetic Textile Dyes:

• Disperse dyes:

Disperse dyes have low solubility in water, but they can interact with the polyester chains by forming dispersed particles. Their main use is the dyeing of polyesters, and they find minor use dyeing cellulose acetates and polyamides. 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 tightly-packed 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 temperatures.

The dye is generally applied under pressure, at temperatures of about 130°C. At this temperature, thermal agitation causes the polymer’s structure to become looser and less crystalline, opening gaps for the dye molecules to enter. The interactions between dye and polymer are thought to be Van-der-Waals and dipole forces.

The volatility of the dye can cause loss of colour density, and staining of other materials at high temperatures. This can be counteracted by using larger molecules or making the dye more polar (or both). This has a drawback, however, in that this new larger, more polar molecule will need more extreme forcing conditions to dye the polymer.

Classes of disperse dye: The most important class is the azo class. This class of azo disperse dyes may be further sub-divided into four groups, the most numerous of which is the aminoazobenzene class. This class of dye can be altered as mentioned before, to produce bathochromic shifts. A range of heterocyclic aminoazobenzene dyes are also available. These give bright dyes, and are bathochromically shifted to give blues. The third class of disperse dye is based on heterocyclic coupling components, which produce bright yellow dyes. The fourth class are disazo dyes. These tend to be quite simple in structure. Other than these, there are disperse dyes of the carbonyl
class, and a few from the nitro and polymethine classes.

• Solvent dyes:

Dyes are generally defined along the lines of being coloured, aromatic compounds that can ionise. One class of dyes is an exception to this. These colours are applied by dissolving in the target, which is invariably a lipid or non-polar solvent.

The Colour Index uses this as a classification and naming system. Each dye is named according to the pattern: – solvent + base colour + number

These dyes are thereby specifically identified as dyes of the stated colour, and whose primary mechanism of staining is by dissolving in the target. Note that this is a functional and colour classification. It contains no chemical information; neither does it imply that dyes with similar names but unique numbers are in any way related. It should also be noted that the classification refers to the primary mechanism of staining. Other mechanisms may also be possible, but are rare.

As a general principle, solvent dyes do not ionise. Many are azo dyes which have undergone some molecular rearrangement and lost the ability to ionise. In the process they gained the ability to dissolve in non-polar materials such as triglycerides. They are commonly used to stain such materials in sections. They are frequently called lysochrome dyes. The prefix lyso means dissolve, and chrome means colour. Sudan III (solvent red 23), sudan IV (solvent red 24) and oil red O (solvent red 27) are commonly used for demonstrating fat in sections. Sudan black B (solvent black 3) is also very effective, but can also stain ionically under some circumstances.

Other important dyes:

A number of other classes have also been established, based among others on application that includes the following:

• Leather Dyes – Used for leather.
• Oxidation Dyes – Used mainly for hair.
• Optical Brighteners – Used primarily for textile fibres and paper.
• Solvent Dyes – For application in wood staining and production of coloured lacquers, solvent inks, waxes and colouring oils etc.
• Fluorescent Dyes – A very innovative dye. Used for application in sports good etc.
• Fuel Dyes – As the name suggests it is used in fuels.
• Smoke Dyes – Used in military activities.
• Sublimation Dyes – For application in textile printing.
• Inkjet Dyes – Writing industry including the inkjet printers.
• Leuco Dyes – Has a wide variety of applications including electronic industries and papers.

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