Standardisation Of Testing


When a textile material is tested certain things are expected from the results. Some of these are explicit but other requirements are implicit. The explicit requirements from the results are either that they will give an indication of how the material will perform in service or that they will show that it meets its specification. The implicit requirement from a test is that it is reproducible, that is if the same material is tested either at another time, or by another operator or in a different laboratory the same values will be obtained. In other words the test measures some ‘true’ or correct value of the property being assessed. If the test results vary from laboratory to laboratory then the test is not measuring anything real and it is pointless carrying it out. However, the values that are obtained from testing textile materials are not expected to be exactly the same, so that appropriate statistical criteria should be applied to the results to see whether they fall within the accepted spread of values.

The lack of reproducibility of test results can be due to a number of causes.

Variation in the material

Most textile materials are variable, natural fibres having the most variation in their properties. The variation decreases as the production progresses from fibres to yarns to fabrics, since the assembly of small variable units into larger units helps to smooth out the variation in properties. The problem of variable material can be dealt with by the proper selection of representative samples and the use of suitable statistical methods to analyse the results.

Variation caused by the test method

It is important that any variations due to the test itself are kept to the minimum. Variability from this source can be due to a number of causes:

1 The influence of the operator on the test results. This can be due to differences in adherence to the test procedures, care in the mounting of specimens, precision in the adjustment of the machine such as the zero setting and in the taking of readings.

2 The influence of specimen size on the test results, for instance the effect of specimen length on measured strength.

3 The temperature and humidity conditions under which the test is carried out. A number of fibres such as wool, viscose and cotton change their properties as the atmospheric moisture content changes.

4 The type and make of equipment used in the test. For instance pilling tests can be carried out using a pilling box or on the Martindale abrasion machine. The results from these two tests are not necessarily comparable.

5 The conditions under which the test is carried out such as the speed, pressure or duration of any of the factors.

It is therefore necessary even within a single organisation to lay down test procedures that minimise operator variability and set the conditions of test and the dimensions of the specimen. Very often in such cases, factors such as temperature, humidity and make of equipment are determined by what is available.

However, when material is bought or sold outside the factory there are then two parties to the transaction, both of whom may wish to test the material. It therefore becomes important in such cases that they both get the same result from testing the same material. Otherwise disputes would arise which could not be resolved because each party was essentially testing a different property.

This requires that any test procedures used by more than one organisation have to be more carefully specified, including, for instance, the temperature and humidity levels at which the test takes place. The details in the procedure have to be sufficient so that equipment from different manufacturers will produce the same results as one another. This need for standard written test methods leads to the setting up of national standards  for test procedures so making easier the buying and selling of textiles within that country. Even so certain large organisations, such as IWS or Marks and Spencer, have produced their own test procedures to which suppliers have to conform if they wish to carry the woolmark label or to sell to Marks and Spencer.

Most countries have their own standards organisations for example: BS (Britain), ASTM (USA) and DIN (Germany) standards. The same arguments that are used to justify national standards can also be applied to the need for international standards to assist world-wide trade, hence the existence of International Organization for Standardization (ISO) test methods and, within the European Union, the drive to European standards.

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Reasons For Textile Testing


The testing of textile products is an expensive business. A laboratory has to be set up and furnished with a range of test equipment. Trained operatives have to be employed whose salaries have to be paid throughout the year, not just when results are required. Moreover all these costs are non productive and therefore add to the final cost of the product. Therefore it is important that testing is not undertaken without adding some benefit to the final product.

There are a number of points in the production cycle where testing may be carried out to improve the product or to prevent sub-standard merchandise progressing further in the cycle.

Checking raw materials

The production cycle as far as testing is concerned starts with the delivery of raw material. If the material is incorrect or sub-standard then it is impossible to produce the required quality of final product.

The textile industry consists of a number of separate processes such as natural fibre production, man-made fibre extrusion, wool scouring, yarn spinning, weaving, dyeing and finishing, knitting, garment manufacture and production of household and technical products. These processes are very often carried out in separate establishments, therefore what is considered to be a raw material depends on the stage in processing at which the testing takes place. It can be either the raw fibre for a spinner, the yarn for a weaver
or the finished fabric for a garment maker. The incoming material is checked for the required properties so that unsuitable material can be rejected or appropriate adjustments made to the production conditions. The standards that the raw material has to meet must be set at a realistic level. If the standards are set too high then material will be rejected that is good enough for the end use, and if they are set too low then large amounts of inferior material will go forward into production.

Monitoring production

Production monitoring, which involves testing samples taken from the production line, is known as quality control. Its aim is to maintain, within known tolerances, certain specified properties of the product at the level at which they have been set. A quality product for these purposes is defined as one whose properties meets or exceeds the set specifications. Besides the need to carry out the tests correctly, successful monitoring of production also requires the careful design of appropriate sampling procedures and the use of statistical analysis to make sense of the results.

Assessing the final product

In this process the bulk production is examined before delivery to the customer to see if it meets the specifications. By its nature this takes place after the material has been produced. It is therefore too late to alter the production conditions. In some cases selected samples are tested and in other cases all the material is checked and steps taken to rectify faults. For instance some qualities of fabric are inspected for faulty places which are then mended by skilled operatives; this is a normal part of the process and the material would be dispatched as first quality.

Investigation of faulty material

If faulty material is discovered either at final inspection or through a customer complaint it is important that the cause is isolated. This enables steps to be taken to eliminate faulty production in future and so provide a better quality product. Investigations of faults can also involve the determination of which party is responsible for faulty material in the case of a dispute between a supplier and a user, especially where processes such as finishing have been undertaken by outside companies. Work of this nature is often contracted out to independent laboratories who are then able to give an unbiased opinion.

Product development and research

In the textile industry technology is changing all the time, bringing modified materials or different methods of production. Before any modified product reaches the market place it is necessary to test the material to check that the properties have been improved or have not been degraded by faster production methods. In this way an improved product or a lower-cost product with the same properties can be provided for the customer. A large organisation will often have a separate department to carry out research and development; otherwise it is part of the normal duties of the testing department.

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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

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.

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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.

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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.

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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:

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  • 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:

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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:

image

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

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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:

image

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:

image

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:

image

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

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  • 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.

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

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.

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  • 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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