What is CI constitution number?
More than 7500 commercial dyes are being made all over the world by the dyestuff manufacturers in various countries. To categorize these dyes on the basis of chemical structure of their main constituents, CI constitution numbers are introduced.
What is CI generic number?
Dyes are classified according to the application or usage and dyes in each class are subdivided depending upon hue of the dye. They are divided in different dye classes viz. vat, direct, reactive etc. and dyes in each group are subdivided as yellow, orange, red, violet, blue, green, brown and black dyes.
Are all Azo dyestuffs banned due to the regulation on banned amines?
No. Not all Azo dyes are banned. Only those dyestuffs are subjected to the regulations, which when absorbed by the human body undergo reduction, decomposition due to the presence of enzymes with reduction properties in the body. They produce carcinogenic amines therefore they are banned.
Why reproducibility in dyeing is difficult to achieve? Is it possible to achieve reproducibility?
Dyeing is a complex process, where number of variables are involved. Dyeing process is broadly governed by fabric type, dye type, machine type, MLR time, temperature, pH of the fabric and liquor, type of auxiliary used etc. Any minor variation in any of these variables causes problem in dye reproducibility, though it is possible to achieve reproducibility in dyeing results. By standardizing each and every variable we can achieve consistent reproducible results. At Shivitex, all our knowledge centres work to satisfy customer needs by making their process proficient.
What is the role of alkali in reactive dyeing?
Reactive dyes are water-soluble dyes. These water-soluble dyes get exhausted on the cellulosic fibre in the presence of Glauber's salt at neutral pH. Under alkaline conditions fixation is carried out in which covalent bond forms between reactive sites of fibre and cellulosic material. During fixation stage alkali is necessary as it acts as a catalyst between the dyestuff and the fibre. The important point is not the type or amount of alkali, but rather the pH of the bath. In reactive dyeing for fixation purpose different alkali is used i.e. soda ash, sodium hydroxide and sodium phosphate.

As these alkali puts load on effluent treatment plant, alkaline agents introduced specifically during the dyeing helps to adjust the pH of the bath and to get level dyeing. At the same time it reduces load on effluent treatment plant by reducing TDS.

What is the role of a dye-bath conditioner in reactive dyeing?
Water is an essential part of wet processing of textiles. Water used for dyeing must have hardness below or around 10 ppm. Metallic ions present in the water cause uneven dyeing. Metallic impurities can be introduced through impurities present in Glauber's salt and common salt even if soft water is used. In dyeing these ions can lead to uneven dyeing. So it is essential to chelate these ions so that they will not interfere in the dyeing process. These chelating agents / sequestering agents have practically no effect on metal ions in the chromophore of the dye.
Why chlorine fastness is carried out for dyed goods?
Drinking water contains 1 ppm of activated chlorine and swimming pool water contains 5 ppm of activated chlorine, which is used for killing bacteria for disinfection. This water can cause discoloration and fading when it comes into contact with the dyed material. This is due to oxidation caused by active chlorine. This problem occurs more in cotton material as it readily absorbs active chlorine.

Tests for colourfastness to chlorinated water include JIS 0084 (Strong test: 20 ppm, weak test: 10 ppm) and ISO 105 - E03 (available chlorine contains 20 ppm), both of which are widely used).

Why NOx gas fastness is carried out for reactive dyes?
Major source of Nitrogen Oxide is automobile exhaust. Cotton products dyed with reactive dyes can suffer fading due to these gases. Nitrogen oxides are responsible for change in colour due to nitrosification of the amino group in the dyestuff molecules.
Why peroxide based washing fastness is carried out for dyed goods?
Oxidizing agents. These oxygen based oxidising components imparts relatively mild oxidizing effects on dyed goods. Though it hardly interact, chromophore of the dyestuff may affect over a repetitive washings.

ISO 105 and ISO 105-109 are the testing methods for testing fastness to peroxide based washings.

What is an enzyme? What types of enzymes are used in textiles?
Enzymes are the higher molecular weight protein ferments secreted by living organisms capable of catalysing the chemical reactions of the biological process. These protein complex is composed of about 200 to 250 amino acids.
Enzyme types:
Amylase - For desizing starch based sizes.
Proteases: For scouring.
Catalases: For scavanging residual peroxide
Cellulases: For biopolishing purpose.
What is colour?
Colour in scientific term and is a physical sensation in the human dye-producing stimulus in the form of pulses by the photoreceptors and is analyzed by the human brain giving subjective results. It remains in the mind of the observer for a short time, but cannot be expressed or described explicitly. Colour means different sensation to different individual.
What is meant by "isomeric matching" and "metameric matching"?
When a pair of standard and sample match under all conditions, particularly illuminant and observer, then the pair is called as isomeric pair and matching is called as "isomeric matching".

When a pair of standard and sample match under one condition and not under the other condition, it is called as metameric pair and matching is called as metameric matching. The phenomenon of metameric matching is called as metamerism.

What are different methods of colour removal from dye house effluent?
Textile process house uses dyes and pigments and about 18% of the total effluent of textile mills come from the Dyeing Department. The Different methods used to treat textile effluents to remove colour are :
  • Conventional biological processes
  • Ozonation
  • Adsorption
  • Chemical coagulation
  • Reverse osmosis.
What is the role of salt in reactive dyeing?
Salts plays important role in reactive dyeing by improving the affinity of the dyestuff towards the fibre and acceleration of the dyestuff's association and lowering its solubility. Normally, Glauber's salt or common salt is used for this purpose. Less common salt is required in place of Glauber's salt as 1 kg of Glauber's salt contains 324 gms of sodium, while 1 kg of common salt contains 393 gms of sodium. The presence of chlorine ion in the common salt may cause corrosion of the equipment. Hence, Glauber's salt is always preferred over common salt. .
Why obverse and reverse side colour differences in the dyeing of cotton knits occur?
In the dyeing of a tubular knit, the substrate is fed into the dyeing M/c inside out, with the obverse surface on the inside. There are number of reasons for this type of problems.
  • Improper pretreatment process can have direct influence on the results of the dyeing. To over come this problem the condition of the fabric after scouring and bleaching should be checked first.
  • Dye selection is also very important requirement for even dyeing on both the surfaces. If dyestuff used has poor penetration property, penetration of the dyestuff will differ between the surface on the outside, which comes into direct contact with the circulating dye solution and the surface on the inside, where the dye solution is not being exchanged to such an extent, resulting in a colour difference between the two sides.
  • Difference in affinity of dyestuff and solubility of dyestuff also reduces differences in dyeing of both the surfaces.
  • Hard water used in processing results in lower solubility of dyestuff resulting in surface dyeing.
To overcome these problems, the dyestuff used should be chosen carefully.
How to overcome poor / uneven absorbency problem in cotton processing?
Cotton contains natural impurities like waxes, pectins, proteins and ash etc., due to which cotton material becomes in absorbent. To make the cotton absorbent, scouring process is carried out. Improper scouring results in poor absorbency. To improve the efficiency of the process surfactants are introduced in this process.

These surfactants help to reduce surface tension of water and improve the penetration of chemicals used for scouring. At the same time surfactants possess detergency property, which avoids the redeposition of dirt.

How to overcome problem of whiteness variation?
Cotton fibre contains natural coloring matter. These colouring matters get destroyed during bleaching process. Mainly oxidizing chemicals are used to remove these natural colouring matter. Now-a-days hydrogen peroxide based bleaching agent is used. If proper pH, temperature and time are not maintained it results in variation in whiteness and and also results in poor whiteness. To overcome these problems peroxide stabilizers or peroxide regulators are introduced. Peroxide regulator regulates the decomposition of peroxide, slowly releases the nascent oxygen and make it available for destroying natural colouring matter.

For processing, if hard water is used, which contains metal ion impurities and heavy earth metals, causes faster decomposition of peroxide and ultimately results in poor whiteness.

To overcome this problem sequestering agents, which work under higher alkaline condition must be incorporated in processing.

(Please visit speciality chemicals page for detailed information about Peroxide Stabilizer and Sequestering agents).
How to avoid silicate-scaling problem on machinery?
Sodium silicate used as peroxide stabilizer also acts as an alkali buffer. Under alkaline condition it gets polymerised and gets deposited on metal surface of machine causing scaling problem. To overcome this problem organic and inorganic stabilizers are introduced in the process of bleaching.
Why lubricating agents are used in the wet processing?
Now-a-days textile material is processed in low liquor ratio. Soft flow, air flow and jet machines, where fabric is introduced in rope form during processing. To avoid fibre to fibre and fibre to metal friction lubricating agents are introduced. Terry towel fabric and heavy fabric variety may cause running crease problems, while processing. Hence to avoid this problem lubricating agents and anticrease agents are used in processing.
How to impart better wash and rub fastness properties to reactive dyed goods?
In reactive dyeing at fixation stage, when alkali is used, it acts as catalyst for formation of covalent between dye and fibre reactive sites. At the same time during alkaline conditions hydrolyzed dyes are formed, which acts as direct dye. This hydrolyzed dye results in poor wash and rub fastness property.

To overcome this problem better washing-off action is needed, such that all hydrolyzed dye gets removed.

Few reactive dyes results in bleeding during home laundering. To overcome this problem dye-fixing action need to be carried out.
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