Dyeing of cotton with reactive dyes quality q&a

Dyeing of cotton with reactive dyes

R.B.Chavan

Department of Textile Technology

Indian Institute of Technology

Hauz-Khas, New Delhi 110016

Manufacturere and dye quality

Dye manufacture• Well reputed large scale manufacturer Like IDI, Clariant,

Dyestar• Large scale marketing: manufacture by small scale

manufacturere, marketed under reputed brand name • Small scale manufacturer: Marketed by non-brand

name• Foreign reputed company : product manufactured in

developing country and mrketed by foreign company• Dye quality varies at the manufacturing stage.

Dye variables

• Main Colouring

• Colouring matter due to side reactions

• Unreated intermediates

• Insoluble matter

• Diluents used for standardization

Quality checks

• Concentration of main colouring component • Prepare a solution of known concentration sample to be tested and

the standard• Compare the optical density and and wave length of maximum

absorption• Other colouring matter• Plot OD against wavelength in the visible region. The standard and

test sample should match if both are identical products• Dyeing test• Carry out dyeing of standard and test sample under identical

conditions. • Compare • Visual appearance and depth of shade• Determine k/s• Determine Delta E (Colour difference, Acceptable range 1-1.2)• Determine L a b Tonal differences

Quality checks for dyes

• Insoluble matter• Dissolve 500-1000 g dye in water, Filter through fine fabric,

Separate the insoluble residue and weigh• Place a spot of filtered dyestuff solution on filter paper to check

its spread. • The appearance of a small lump in the center is a cause for

concern. • Packing • Check the condition of the dyestuff's packaging and the material

from which it is made. Because dyestuff absorbs moisture, • if a thin polyethylene bag is used in the packing, there is the

possibility that the dyestuff has absorbed moisture during transport or storage. The state of the seals should also be checked.

Colour Index No. and dye quality

• Color index categorizes dyes according to the chemical structure of their main constituents,

• Does not express the entire contents of the dye, which can contain additives, impurities and isomers resulting from the raw materials and the method of synthesis.

• Dyes of same Colour Index may not show same dyeing results.

Reproducibility of laboratory recipe on bulk

• following points should be addressed. • Due to mechanical restrictions, and the agitation speeds of the

dye baths are different. • liquor ratio: should be as close as possible• Difference in the material to be dyed: Material dyed in laboratory

and bulk should be same.• Difference in the dye sample used for laboratory and bulk dyeing:

dye sample should be same and from the same lot of dye supply.• Laboratory sample often not stored properly and likely to absorb

more moisture and thus showing less concentration for a given weight

• Difference in dyeing auxiliary: Use same dyeing auxiliary • Difference in water quality• Difference in dyeing parameters, temp., time, pH etc. • If CCM is available, the colour difference between laboratory and

bulk dyeing for a given shade over different trials can be calculated and a correction factor can be determined.

Reproducibility between lots in factory dyeing • Two situations• Dyeing 10,000 meters in 10 lots within a short interval• Repeat dyeing after long interval • Change in the dyeing machine used for diff. lots: this gives

variations in the mechanical conditions. • differences in the weight of the fabric • changes in the liquor ratio, • differences in the degree of preprocessing such as scouring,

fluctuations in the water quality, • Inappropriate dye selection • differences in the weighing of the dyes. • In repeat dyeing , the dyestuff used can often be of a different lot

to that used in the previous dyeing, so attention must be given to variations in concentration from lot to lot.

• If the reproducibility of the dyeing is less than ideal, • determination of the trends of fluctuating colors (through CCM, etc)

Reproducibility between lots in factory dyeing

• If the reproducibility of the dyeing is less than ideal, • determination of the trends of fluctuating colors

(through CCM, etc) • identification of the dyes which display color

fluctuations • through analysis of the dyeing characteristics of

those dyes, the reasons for the fluctuations can often be surmmerised.

• In practice, however, it is recommended that another dye be substituted for one that displays large fluctuations.

Recipes for exhaustion dyeing • The optimum dyeing temperatures and dyeing

conditions for reactive dyes differ according to the type of reactive group involved, so the first thing that must be decided is the type of dye that will be used.

• The substantivity to cellulose of reactive dyes is lower that that of direct dyes,

• but the addition of inorganic salts to the dye bath can raise its substantivity. Usually, Glauber's salt/common salt is used at a conc. of 50g/L,

• but this conc. should be increased in the dyeing of deep shades,

• and can be reduced for low salt reactive dyes. Inorganic salts can be added incrementally to dye baths for level dyeing,

Recipes for exhaustion dyeing

• Alkali is employed as the catalyst in the reaction between the dye and the fiber and

• the important point here is not the type of alkali used but

• the need for the pH of the bath after addition of the alkali to be around 11.5.

• Soda ash is easily used as the alkali because the addition of 20g/L yields a pH of around 11.5.

• Caustic soda is also convenient, but because it is a strong alkali, adjusting the pH to around 11.5 is extremely difficult, and

Recipes for exhaustion dyeing

• Because the dyeing rate of reactive dyes depends on a combination of exhaustion through substantivity and fixing through reaction, quantitative adjustments through the control of the dyeing rate by adjusting the rate of temperature increase, as are used for other classes of dyes, are not effective,

• but adjustments with the incremental addition of inorganic salts or alkalis (pH) are effective.

• Consequently, isothermal (at constant temperature) dyeing through the incremental addition of alkali or alkali dosing is used. The specifics of incremental addition and dosing are decided by the agitation efficiency of the dyeing apparatus.

• if hard water is used in the dyeing process, a sequestering agent can be added.

Level dyeing with Br. Blue R• C.I. Reactive blue 19 (Br. Blue R) is a representative trichhromatic

blue dye that is widely used throughout the world. • This dye is vinyl sulfone-based, • and is used in dyeing in the form of sulfate ethyl sulfone, • but with the addition of alkali, it becomes vinyl sulfone and fixes to

the fiber by reacting with it• Because Blue 19 in the form of sulfate ethyl sulfone has a high

solubility and a very low affinity, it displays a low primary exhaustion rate,

• but in the form of vinyl sulfone, its solubility decreases dramatically and it displays a high affinity, and thus the exhaustion rate increases rapidly.

• Consequently, under typical dyeing conditions, dyeing advances rapidly after the addition of alkali,

• Resulting in high risk of uneven dyeing

Level dyeing with Br. Blue R

• In order to ensure level dyeing, an anion surfactant can increase the solution stability of the vinyl dye,

• and the incremental addition or dosing of inorganic salt and alkali can prevent the rapid exhaustion of the dye.

• and conventional application methods of Blue 19 are not recommended because precipitation of the dye occurs after the first addition of alkali, thus causing uneven dyeing

Dyeing mercerized yarn

• In comparison with unmercerized cotton yarn, mercerized cotton yarn exhibits the following dyeing properties:

• Visibly deeper colors (less dyestuff is required for color matching with the same color sample).

• Faster absorption and dyeing rates. • To control the rate of exhaustion, it is important

incremental addition of inorganic salt and alkaline, • In the dyeing of mercerized cotton yarn, the amount of

inorganic salt and alkali added in the first addition should be reduced and

• the number of times addition occurs should be increased.

• the amount of alkali can be reduced. (For example, the amount of soda ash used is 20g/L for unmercerized yarn but 15g/L for mercerized yarn.

Level dyeing with phthalocyanine Turq Blue

• The chromophore in brilliant turquoise blue reactive dyestuffs is usually copper phthalocyanine. Some of the characteristics of phthalocyanine dyes are as follows:

• Large molecular weight in comparison to other reactive dyes.

• Associate easily while also being water soluble, existing in a colloidal state.

• Chemically, include a large number of isomers, and even for dyestuffs with the same color index number, the structure and composition can vary according to maker or lot.

• the use of dyestuffs of consistent quality is recommended.

• Because of these characteristics, these are some of the more difficult dyestuffs to use.


• For example, the well-known C.I. Reactive Blue 21 is widely used for producing turquoise blue and green.

• This dyestuff is vinyl sulfone-based and has copper phthalocyanine as its chromophore.

• it has a high affinity and displays a high primary exhaustion rate.

• Furthermore, it associates easily, and its low solubility has a negative effect on level dyeing.

• the dyeing temperature is set not at 50‹C, the �standard temperature for vinyl sulfone-based dyes, but at the higher temperature of 70‹C. �

• The absorption rate is adjusted through the incremental addition of Glauber's salt.

•


• The incremental addition of alkali is not very effective because the level of absorption in the primary absorption process is great and the contribution of the secondary absorption is minimal.

• The diagram displays an example dyeing program including the initial addition of alkali.

•

The role of alkali in reactive dyeing

• In the dyeing of cellulose with reactive dyes, alkali is necessary because it acts as a catalyst in the reaction between the dyestuff and the fiber. The important point is not the type or amount of alkali but rather the pH of the dyebath, which must be closely supervised.

• The most suitable pH for dyeing varies with the temperature, being approx. 11.5 for common warm dyeing (dyeing at approx. 60), 10-11 for hot dyeing (80) and 12.5 for cold dyeing (40). The relationship between temperature and the optimum pH is shown in the following graph.


•


• Soda ash is often used because 20g/l usually produces a pH of around 11.5.

• However, it can be used with sodium phosphate or caustic soda when a higher pH is required, or

• with sodium bicarbonate (baking soda) when a lower pH is required.

• Caustic soda is not often used because it is a strong alkali and achieving a pH in the range of 10-12 is difficult.

Low-salt dyeing

The importance osurfactant/lubricant in reactive dyeing

• In the dyeing of cotton with reactive dyes, does not require the use of surfactants.

• However, in the dyeing of cloth in rope form in jet dyeing machine, dyed in rope-form,

• but because it is sometimes difficult to circulate the liquid over the cloth, rope marks can easily form.

• The use of a dyebath lubricant is recommended as a method for dealing with this problem.

The importance of surfactant/lubricant in reactive dyeing

• However, when the solubility of a reactive dyestuff is reduced due to factors such as association, the use of a surface-active dyebath lubricant can result in a dispersion effect.

• Furthermore, when a small amount of wax still remains on the substrate due to insufficient scouring, the use of a dyebath surfactant can promote the penetration of the dye. Thus to lower the degree of risk in dyeing, the use of an appropriate dyebath surfactant/lubricant is recommended.

The role of sequestering agents in reactive dyeing • When the water of the dyebath contains metallic ions, • there is the danger that uneven dyeing, such as specking, • or reduction of the concentration of the dye will be caused by the

dyestuff's coagulation and reduced solubility. • metallic ions can also be introduced by impurities in Glauber's salt

or common salt, • so even when soft water is used in the dyeing, the presence of

metallic ions can lead to problems. • the use of sequestering agents is very important. • In the past, traditional thinking has been that "if the chromophore

of a dyestuff is a metallic complex salt, then sequestering agents should not be used.

• " Recently, however, it has been ascertained that sequestering agents added to the dyebath have practically no effect on the metal in the chromophore of the dye.

• Sequestering agents effective in neutral to alkaline conditions are preferable

• The use of sodium hexa meta phosphate is common

Dispersing leveling agents • Since reactive dyestuffs are water-soluble, anionic surfactants,

which are common dispersing agents, are not used. however, the solubility of some dyestuffs is reduced and some dyes may coagulate.

• In these cases, the use of a dispersing leveling agent can be

advantageous. • In an alkaline bath, a typical dyestuff that suffers reduced

solubility is C.I. Reactive Blue 19.• While this dyestuff displays good solubility in its commercial

form of sulfate ethyl sulfone, • in its vinyl-sulfone form after the addition of alkali to the dyebath,

its solubility is dramatically reduced and it may coagulate.

Dispersing leveling agents

• The structural transformation of C.I. Reactive Blue 19 in an alkaline bath

• In this case, use of an anionic dispersing leveling agent produces even dispersion and good solubilization, ensuring level dyeing and good reproducibility.

• An example of a typical dyestuff which tends to coagulate is phthalocyanine-based Turq Blue. (eg. C.I. Reactive Blue 21). Use of a dispersing leveling agent raises the level of solubility and improves the levelness of dyeing.

• For dyes of this type, however, since dispersing leveling agents display a large retarding effect, addition of too much dispersing leveling agent can reduce the concentration of the dye fixing.

• Due to this, the quantity used should be carefully supervised. • In addition, when the water used in the dyebath contains metal

ions, reactive dyestuffs in general tend to suffer reduced solubility due to their influence. In this case too, use of a dispersing leveling agent is advantageous.

• An example of a dispersing leveling agent that can be used for realizing the above effects is Sumipon SE.

Setting the dyeing temperature • optimum dyeing temperatures can be divided into the following

three groups: • Cold dyeing (30-40): eg. dichlorotriazine, difluorochloropyrimidine

dyestuffs • Warm dyeing (50-60): eg. vinyl sulfone, monofluorotriazine

dyestuffs • Hot dyeing (80-90): eg. monochlorotriazine, trichloropyrimidine

dyestuffs • Traditional reactive dyestuffs use a chromophore of relatively low

molecular weight similar to that of acid dyestuffs. Thus while the dyeing temperature for most dyestuffs can still be decided according to the reactant as outlined above,

• some recently developed dyestuffs have higher molecular weights approaching those of direct dyestuffs and increased affinity which results in a higher degree of fixation.

• For these dyes, the temperature as traditionally determined according to the type of reactive group may not necessarily be the most suitable.

• higher dyeing temp is recommended for bifuntional reactive dyes

Liquor ratio and dyeing properties • Reactive dyestuffs tend to have a low substantivity and are thus

readily influenced by the liquor ratio. • rise in the liquor ratio, will lower the degree of exhaustion. • Reactive dyestuffs have a lower substantivity and thus dyeing

with these dyes tends to be more readily influenced by the liquor ratio.

• In winch and jet dyeing, the liquor ratio must be decided in view of the fact that any reductions in the liquor ratio increase the risk of uneven dyeing.

• Since cotton knits are held at conditions of low tension during the jet dyeing process, the weight of dye solution held by the substrate is twice the weight of the substrate itself.

• This means that dyeing cannot be conducted at liquor ratios lower than 1:2.

• A volume of dye solution equal to that necessary to fill the pipes, pumps and heat exchanger is also necessary.

• Furthermore, in order to enhance the efficacy of the dye circulation to achieve an evenly dyed surface, the volume of dye solution must be sufficient for it to move freely over the surface of the fabric.

Liquor ratio and dyeing properties

• Some atmospheric low liquor ratio jet dyeing machines are marketed as being capable of dyeing at liquor ratios of 1:4,

• but in reality dyeing at a liquor ratio of 1:4 is likely to lead to many problems including that of uneven dyeing.

• Within the limits set by modern technology, a ratio of at least 1:6 is necessary for achieving high-quality dyeing.

• It is important to note that problems can easily arise if pretreatment is conducted in the same low liquor ratio dyeing equipment.

• When the substrate is wound in a compact form as in cheese dyeing, the substrate holds less dye solution and efforts to reduce the internal volume of the dyeing equipment can make dyeing at a liquor ratio of around 1:3 possible.

• In this case, lowering of the liquor ratio leads to an increased number of circulations which allows level dyeing to be achieved

Reactive dyestuffs for ground color in discharge printing

• Discharge printing with reducing agents is a method that uses a reducing agent to decompose and decolor dyestuffs with azo groups as shown in the following diagram.


• The original color of the azo-based dyestuff disappears due to the decomposition of the azo group of the chromophore brought about by the reducing agent in discharge printing.

• The amine component that is produced after decomposition (the groups remaining from the decomposed chromophore) cause a yellow to orange coloration.

• If amines remain bound to the fiber, white discharge printing is not possible, making such dyestuffs unsuitable for ground dyeing.

• It is in this way that a dyestuff's suitability in printing is influenced by chemical structure.

• In white discharge printing, the level of whiteness produced and color changes over time are influenced by the molecular structure of the components combined with reactive groups that remain on the surface of the fabric.

• For example, the structure of red reactive dyestuffs and the possibility of white discharge printing are outlined below.


• In the following example with a monochloro triazine-based dyestuff,

• since the amine remaining after reduction decomposition is a colored amine with a high molecular weight containing naphthalene, the cloth will retain an orange coloring.

• Thus this dyestuff is unsuitable for use in ground dyeing in discharge printing.

• White discharge not possible


• In the following example with a monochlorotriazine-based dyestuff,

• after reduction decomposition the remaining amine is a colored amine with relatively low molecular weight.

• Since the degree of coloring is low, it can be used for pale shades in ground dyeing in discharge printing, but is not suitable for use with medium and dark shades.

• White discharge printing possible with pale shades


• In the following example with a vinyl sulfone-based dyestuff, since, the amine remaining after reduction decomposition is a colorless amine with a low molecular weight, this dyestuff is suitable for ground dyeing in discharge printing.

• White discharge printing possible even with dark shade


• Although the suitability of reactive dyestuffs for discharge printing is not related to the original reactive group,

• since triazine-based and pyrimidine-based reactive dyestuffs are often used with the reactive group introduced into the coupling component,

• colored amines with high molecular weight remain on the fibers after discharge printing

• with the result that many of these types of dyestuff are unsuitable for ground dyeing in discharge printing.

• In vinyl sulfone-based azo dyestuffs, • the reactive group is introduced directly to a benzene-based

amine of low molecular weight and is used as a diazo component• the component remaining on the fibers after discharge printing

has a low molecular weight and practically no coloring. • As a result, many of these dyestuffs are suitable for use in ground

dyeing in discharge printing.

Removal of fixing agent

• when using vinylsulfone-based dyestuffs, the fixing process can be skipped, because they have good wash-off properties.

• However, in cases of having difficulty in complete removal of unfixed dyestuffs such as in dyeing deep to extra-deep shades, fixing is carried out in order to improve wet rubbing fastness.

• De-fixing is carried out when shading or de-coloration is required after fixing,

• complete de-fixing has been found difficult to achieve. • In the event of requiring de-fixing, acid treatment, is generally

used. The following are examples of processing conditions.

Acetic acid (48%) 3-6ml/L or hydrochloric/phosphoric acid 1-3ml/L) Temperature and time: 80°C for 30 minutes

Removal of fixing agnet

• Preliminary tests are required as treatment may accompany color alteration or fading in the substrate.

• Removal of fixing agent to the degree that allows subsequent re-dyeing is often difficult especially when the agent used is cationic,

• in which case re-dyeing can be made possible by blocking the cation groups in the agent through the use of an anionic masking agent.

Masking agent (Senkanol A-300) 2g/L Acetic acid (90%) 1 g/L

temperatures 95-100°C for 30 minutes

Dyeing of cotton with reactive dyes quality q&a

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