Dyeing Equilibrium - A Practical Dyer's Guide to Reactive Dyeing of Cotton - Part-4

In the previous post we have discussed the chemical aspects only and the processes of reaction with cellulose and the water have been treated as if they were occurring quite separately. But in fact this is not the case and we have just seen the importance of exhaustion in obtaining a good efficiency.

We will now examine what happened in a two stage dyeing process in which the dyestuff is exhausted from the neutral dye bath in the first stage and then at a later stage the solution is made alkaline so that the reaction begins.

In the first stage of neutral dyeing, no decomposition of dye takes place and the process is exactly the same as the dyeing of a direct dye. The only difference is the lower degree of exhaustion of reactive dyes. At the end of the 1st stage we have two equilibrium.

When alkali is added to the system, chemical reaction begins. In the dye bath hydrolysis with water occurs, while in the fibre, the dissolved dye will also hydrolyse, but the adsorbed dye will mainly react with the fibre although the possibility of aqueous hydrolysis cannon be excluded. The hydrolysed dye (DOH) will have similar properties to the parent dye ( or direct dye) and will not get adsorbed on the fibre surface. Finally, when all the reactive dye present has been destroyed one way or another, new equilibrium will be set up between the hydrolysed dye in the dye bath, in solution inside the fiber and adsorbed on cellulose while the combined dye is present as a separate component, not taking part in this.

Reaction Rates of Reactive Dyes: A Practical Dyers's Guide to Reactive Dyeing Cotton - Part-3

The rate of reaction with cellulose may be written:
Rate = Kc (D.Cl)f (Cell O)
The rate constants of reactive dye with cellulose may not be available but the rate constant with water is known and since the two are proportional, may be used as a guide to behavior. The rate constants of some procion dyes are shown in the table below. The rate of reaction of Procion “H” at 20°C is extremely low ad difficult to measure. The values shown in the table are extrapolation from the values at 50°C and 70°C.

Dyes	Bi molecular reaction
	Constant 20°C	1 gram mloe/c- 50°C	1 gram mloe/c- 50°C
Orange 2R	13.5
Red 8B	13.2
Yellow GR	11.0
Blue 3G	3.22
Yellow 6G	2.82
Scarlet G	2.06
Orange G	1.61
Rubine HB	0.18	1.99	9.99
Yellow H3B	0.054	0.81	5.06
Yellow H3G	0.034	0.55	3.62
Blue HB	0.044	0.46	2.23
Blue HGR	0.027	0.34	1.89

It will be observed that there are significant differences between the reactivity of individual dyes in each group the most reactive being roughly 10 times more reactive than the least reactive. However the difference between hot brand and cold brand is even more marked, the latter being 50 times as active as the former.

The rate constant of a chemical reaction increases with increasing temperature by between two and three times for every 10↑8C increase in temperature. Clearly then an increase in temperature of 50° (from 20 to 70°) may be expected to increase the rate 50 times. This is seen in the above table, where rate constants pf Procion H dyes at 70°C are similar to those of Procion dyes at 20°C.

Rate of reaction can be changed by varying the concentration of cellulose ions in the fibre, by changing the pH of the external bath.

If 1 unit pH of the dye bath is increased, the concentration of cellulose ions will increase tenfold. An increase of 1.7 pH units will increase the concentration 50 fold and the rate of reaction similarly. Thus Procion H dyes at pH 12.5 should react at the same rate as the Procion M dyes at pH 10.5. This proves to the case but the yield of the combined dye is relatively low.

The reason is, if the pH exceeds 12, the exhaustion of dye bath falls rapidly. Below pH 11, the concentration of cellulose ions is small compared with that of dye, at pH 11 it is roughly equal and at pH 12 it is considerably greater than that of the dye. Because of the increasing ionization the fibre acquires a large negative charge that depresses the absorption of the dye.

Thus the degree of exhaustion at pH 12 is so low that though the reaction takes place with cellulose in cold in one hour, the efficiency is low.

Fibre-Reactive Dyes - A Practical dyer's Guide to reactive Dyeing - Part-2

What are Fibre-Reactive Dyes?
Definition
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:

Note the four different components of the dye
The chromogen is as mentioned before (azo, carbonyl or phthalocyanine class).
The water solubilising group (ionic groups, often sulphonate salts), which has the expected effect of improving the solubility, since reactive dyes must be in solution for application to fibres. This means that reactive dyes are not unlike acid dyes in nature.
The bridging group links the chromogen and the fibre-reactive group. Frequently the bridging group is an amino, -NH-, group. This is usually for convenience rather than for any specific purpose.
The fibre-reactive group is the only part of the molecule able to react with the fibre. The different types of fibre-reactive group will be discussed 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 favorable p-electron system. However, nucleophilic substitutions are not very common on aromatics, given their already high electron density. To encourage nucleophilic 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 dye2: (This is the same as the chime molecule at the top of the page)

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 stabilized 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 groups on the cellulose, the fibre-reactive group may react with the HO- ions in the alkali solution and become hydrolyzed. The two reactions compete, and this unfavourable because the hydrolyzed 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 p-bond. They normally undergo electrophilic addition reactions. Again, nucleophilic additions are less favored generally, because of the repulsion between the Nu- and the electron-rich p-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 stabilized, but this has not been shown.

A Practical Dyer's Guide to Reactive Dyeing of Cotton - Part-1

Introduction:
Reactive dyes are probably the most popular class of dyes to produce 'fast dyestings' on piece goods. These were first introduced a little over 40 years based on a principle which has not been used before. These dyes react with fibre forming a direct chemical linkage which os not easily broken.
Their low cost, ease of application, the bright shades produced by them coupled with good wash fastness make them very popular with piece good dyers. Even in thereads these classes are gaining in popularity for cotton sewings.

Chemistry of reactive dyes:
Reactive dyes differ from other colouring matters in that they enter in to chemical reaction with the fibre during the dyeing process and so become a part of the fibre substances.
A reactive dye may be represented as
R - B - X
where R - Chromogen
B - Bridging group
X - Reactive system
When this reacts with the fibre, F, it forms
R - B - X - F
The wet fastness of the dyesings produced depends on the stability of the true covalent bond X-F.

Reactive Systems:
Some of the popupar reactive systems is use today:
Reactive dyes are based on Cyanuryl chloride. The cold brand dyes (M brand) are based on dichloro triazinyl derivatives whereas the "H" brands are monochloro triazinyle derivatives.
The reactivity of the chlorine atoms decreases greatly as they are successively substituted. Thus the dichloride derivative (M) is more reactive than the mono chloro reactive (H) dyes. This is shown by the fact that "M" dyes will react readily with cellulose at room temperatire in the presence of mild alkalies such as sodium carbonate, whwere as "H" dyes need to be heated at least to 60°C and require more strongly alkalines before reaction will take place at a reasonable rate.
The other popular systems are based on Vinyl suplhones (Remazols) and trichloro pyrimidyl. The Remazols are very popular for discharge printing and can given excellant white discharge from a dark base.
Reference: Textile Processing Guide

Color Fastness - What does it mean?

In order to clarify what fastness means, we shall examine various fastness properties that a thread or yarn may be required to have. We shall study how grades of fastness are established and what they signify and finally we should take a look at what fastness you can expect from various classes of dyestuff on the major substrates of interest to us.

Fastness Properties:

The most desirable fastness properties in any thread or yarn is arguably of wash fastness.

In general, the dyers used to carry out two main wash fastness tests, viz.,

Test-1: A length of coloured thread is plaited with a white partner and treated at 95°C in an alkaline soap solution for 30 minutes. The degree of staining on to the adjacent white thread ( which can be of one or more white substrates) is assessed as in the change of shade of the original colour. The test is commonly known as ISO4.

Test-2: As in Test-1 above but treatment is only at 60°C. The test is called ISO3.

Another popular fastness demand is to rubbing both wet and dry with the sample being hand or machine rubbed. Only the staining on the white calico test fabric is recorded.

Fastness to light is either carried out in sunlight ( a low method) or in an artificial Xenon lamp tester (much faster). Along with the test sample are eight blues of known light fastness which fades to the same degree as the sample gives the light fastness grading of the sample, only change of the shade is recorded.

Fastness to bleach, either peroxide or hypochlorite are severe tests where normally only change of shade need be recorded.

A less severe bleach type test is fastness to chlorinated water which is meant to represent the effect of swimming pool waters on textiles; usually for swim and beach wear garments.

Fastness to hot pressing at a wide range of temperatures with both change of shade and staining being relevant. Disperse dyes on polyester can sublime ( literally evaporate) on some severe permanent pleating processes and even at low iron heats many classes of dye will stain ( but may not do so on ISO3 wash tests) white fabric on contact.

There are some exotic fastness requirements like fastness to vulcanizing , a process used to cure rubber footwear or fastness to stone washing, a fickle process used to fade cotton denim jeans.

Fastness Grades:

Nearly all fastness properties are assessed on a scale of 1 to 5 with 5 being the best rating and 1 the worst. The Grey Scale I can be used to assess the change in shade. Staining scales are slightly different but the same usage principle applies.

Exceptionally, light fastness is measured on a scale of 1 to 8 with 8 being the best and 1 the worst.

Reference:

Textile testing procedures

Package Dyeing (HT HP) - Cheese Yarn Dyeing

Machinery details:
In its simplest form a package-dyeing machine is a vessel capable of containing packages of textile material through which heated dye liquor is passed by means of a circulation pump. Later developments accelerated by the need to dye polyester at temperatures above the boil lead to enclosing and strengthening such vessels so that they could operate upto 140° c at pressures around 70 psi (4.95 kgs per/ sq. cm.)

Accessories were added to allow samples to the extracted without depressurizing the whole system and to inject dyes and chemicals from out with the main circulation circuit. Later still, simple controls of time and temperature were replaced with fully automatic programmes based on sophisticated microprocessors that reduced operator involvement in the dyeing process to a minimum and elevated LOA (Limits Of Accuracy) sophistication previously unattainable levels.

Liquor flow:
We expect a main circulating pump to deliver 30 litres of liquor per kg of thread at 1.26 kgs per sq.cm. pressure which usually means the bath is pumped through the thread load upto four to five times per minute.

Exceptionally, cheese-bleaching machines need only deliver half of the discharge tobe effective.

Many of us, when faced with an unlevel cheese dye lot, blame our troubles on poor liquor flow which, because the dyeing process, by necessarily, is unobservable and because there is no instrument to read out the flow. Is hard to prove one way or another.

But a small one, however, can interpret the evidence available to him e,g,, here are a few tips on how to ascertain whether or not abnormal liquor flow is the source of Unlevelness.

1. Check the in-out and out-in pressure gauges and compare the readings with your past
experience. Your Dalal and Staffi machines with their modest pumps should register a pressure differential of around 0.5 kg per sq.cm. If the differential is significantly lower than this value, liquor may be freewheeling or channeling through a badly seated carrier, a sprung cap or a loosely loaded column of cheeses.

2. Likewise pressure differential higher than 0.5 kgs per sq.cm could indicate that something is causing unduly high back pressure e.g. very dense cheeses.

3. Unlevelness on a number of cheeses which represent one spindle or multiples of one spindle might indicate poor sealing of the number(s) of spindles involved.

4. Unlevelness on a number of cheeses that represent one complete layer as horizontal cross section of a carrier load of cheeses may mean that the machine has (leveled out) for sampling or during a power failure exposing the top most layer of cheeses to oxidation or differential dye uptake.

5. Loss of air pad pressure in one way low liquor dyeing can cause reduced liquor flow.

Open expansion tank:
This tank is sized so that the top row of cheeses is exposed when liquor is leveled bag to the expansion tank from the kier by gravity.

The tank feeds the suction side of the secondary pump, which normally discharges into the main pump housing via the non-return valve.

The expansion tank is an invaluable aid to level dyeing as it allows controlled additions of chemicals and redip dyes, when pressurized.

Extraction rate from the expansion tank is usually 5 to 25 litres per minute with the pump running at a pressure of around 3.6 kgs. per sq. cm.

It is important that the right balance between expanding main kier liquor and expansion tank injection rate is struck otherwise liquor flow may be affected. This balance is obtained by drilling out the orifice plate on the cooled liquor return from the main kier to the expansion tank.

The efficiency of the back cooler or condenser is also important since if the temperature in the expansion tank is allowed to rise about 80 to 85°C, the adversely secondary pump may cavitate thus affecting the flow characteristics of the dyeing system. If the liquor is over cooled, energy is wasted in reheating it in the main kier and of course cooling water volumes are unnecessarily high.

One bath dyeing of cellulosic blends

A number of one bath methods have been developed for cellulosic blend dyeing. Disperse/direct dyeing of cotton/nylon or polyester is well known although of little practiced use because of the poor wet fastness, except in the pale shades.

Disperse/Reactive:
Methods based on hot dyeing reactive dyes in which the disperse and reactive dyes are added to the bath with 5 gram/liter of Resist salt L (m-nitro-sodium-benzene-sulphuric acid) which prevents hydrolysis of reactive group. Dyeing of polyester is first carried out at 120°C. Then the bath is cooled to 80°C. Electrolyte is added and dyeing of the cotton proceeds in the usual way.

Disperse/Vat:
Disperse and vat (pigment) dyes are added to the bath plus a large quantity of dispersing agent. Dyeing at 100° to 130°C proceeds. Then the bath is cooled to 80°C and caustic soda and hydros are added and dyeing of the cotton carried out. There are several Union dyes (Cottestren) on the market using this principle. These commercial blends have to be formulated for a predetermined fibre mix and may turn out to be uneconomic for a customer's particular end use and further more may not give solid shade dyeing under adverse conditions of applications.

Nylon/Cotton:
A Hoechst patent for single bath application reactive/metal complex dyes has the following method. A dyestuff and alkali to give a pH of 8 to 12 and raise temperature to 80°C to dye the cotton. The pH is then reduced to 6.5 to 7 by the addition acid. The temperature is raised to 95°C and the nylon portion is dyed. Acid dyes will precipitate under these conditions, metal complex dyes will not.

Azoic Dyeing of cotton yarn

Naphthol dyestuff have traditionally been applied by the multi stage route of impregnation, hydro extraction/rinsing and development.
German and Swiss manufacturers have now developed a one bath application process which offers a real rationalization of the dyeing process. This one bath process is cotton in hank in open becks and spray dyeing units and cotton piece on the winch.
The basis of the method is to retain the bath after impregnation and maintain the color pigment which is formed during development in highly dispersed form, by means of a special auxiliary. The one bath method consist of

1. impregnation for about 20 minutes. at 20 to 30°C.
2. addition of acid and dizao solution without letting off the bath.
3. coupling the dyestuff of about 30 minutes
4. cleansing after treatment.

A fairly wide selection of naphthol/base combinations are suitable for this process.

Liquid Ammonia Treatment of textile yarn

Liquid ammonia can be regarded simply as a new medium for tailoring the dimensions and properties of cellulosic materials to shrink, swell, stretch and relax and can therefore be used to obtain a variety of effects on many materials.
The economics of cotton yarn manufacture hinge on the price of raw material comprising it, e.g. more than 25% of the cost of a cotton sewing thread is accounted for by the raw cotton price. An accepted yardstick of a cotton is the strength it produces in yarn and thread forms and it was to this end that much of the development work of the liquid ammonia process was designed.

Properties of Liquid Ammonia Treated yarn:
The following properties have been established for liquid ammonia treated yarns:

1. Tensile strength significantly increases.
2. The elongation at break is only about 2/3 that of untreated yarn.
3. Loop strength and knot strength increases slightly.
4. Abrasion resistance is reduced but this decrease is less than caustic soda mercerising.
5. After bleaching or dyeing, treated yarns have virtually zero shrinkage when treated in boiling water.
6. A pleasing lustre is imparted to the treated yarns albeit slightly less than for caustic mercerising.
7. Dye affinity is increased by 3/4 of the amount attained by caustic soda mercerising.
8. Moisture absorption is increased but again to some what lesser degree than for caustic mercerising.
9. The heat resistance is substantially increased.

Liquid ammonia treated yarns are significantly cheaper in price than caustic mercerised yarns.

The elimination of hank winding is possible, due to the high speed reaction in liquid ammonia which permits package to package processing.

Maximum strength increases, require maximum stretch in the ammonia moving zone but this is difficult to apply without breakage to yarns. However, if the stretch is reduced and more modest strength increases accepted ( of the order of 20% - 30%) is readily possible to liquid ammonia treat singles yarn. This is a sharp contrast to the difficulties in processing singles yarn by mercerising.

It is therefore possible to produce this means a lustrous singles yarn for use in weaving and knitting applications.

From ecological view point also, ammonia is more readily and cheaply recoverable than caustic soda mercerising liquors which produce effluent and which has to be disposed of. The problem of caustic liquor discharge to rivers is so acute in some countries that permission to erect mercerising plants is difficult to obtain.

Early difficulties of dye affinity variations between packages of liquor ammonia treated yarns have now been eliminated by improved control of the treatment process.

The technological difficulties of converting pressurised liquid ammonia and recovering pressurised liquid ammonia from the gas evolved during the process, have been successfully overcome.

Yarn Mercerisation - Part-2

Mercerisation:
The mercerising process introduces changes to cotton yarns e.g. increase in lustre, increase in tensile strength and increase in dye affinity.
If tension is applied, mercerisation generally causes increase in strength from 10% to 40% depending on the yarn construction.
The moisture content based on the dry weight of cotton increases with the concentration of caustic soda used. The percentage moisture present ranges from 6-12%.

Affinity for Dyestuffs:
The weight of dyestuff absorbed increases with increasing concentration of caustic soda up to 13.5% (30°TW) and thereafter the increase is less rapid. Tension applied and drying also have an effect on the affinity.. The effect of tension is to decrease the amount of dye absorbed when compared with yarn mercerised without tension. Drying a mercerised yarn decreases the affinity for dyestuff, this decrease being greater the higher the temperature of drying.

Due to this change in affinity for dyestuffs, air drying of mercerised cotton must be avoided after the yarn has been mercerised and before it is passed on to the next process. Precautions must be taken to keep such yarn wet, otherwise unlevelness in dyeing is likely to occur.

Wetting Agents in Mercerising:
Wetting agents are added to mercerising liquors on order to obtain quick penetration of the caustic soda solution. Several types of such agents are available. The most commonly used is Cresol which is relatively cheap in price. A small amount of a higher alcohol such as Butyl carbitol or butyl cellosolve is added to assist penetration and as an anti-foam.
To get the best use of a wetting agent it should be soluble and have good stability in caustic soda solution of mercerising strength (53-54° Tw). In some cases an added advantage is found if the wetting agent is soluble n strong caustic solution (76-80°Tw) used as feed liquor to the mercerising machine. By this means, the necessity for adding wetting agent to each of the machine tanks can be avoided, if facilities exist for providing a strong liquor feed line to each machine.
The quantity of wetting agent used is about 1to 2% by volume. This quantity should be sufficient to give a wetting time of 4-5 seconds under the standard conditions for wetting out.
Tests to check the wetting out properties of the mercerising liquor should be carried out every 4 hours or as necessity demands.
Nowadays use of cresylic wetting agents are prohibited by local authorities due to contamination drainage areas with phenolic compounds which exist in the wetting agent.

Fungus formation in yarn before and after dyeing

Fungus formation is the common problem all dyers would have come across. If the yarn is kept stored under moist conditions for more than 24 hours, there is every possibility of it getting afflicted by natural fungus formation.

These fungus would appear at the beginning like yellow patches and spots and slowly turn in to green or blue and end up in black color. These are both aerobic and anaerobic. The portions affected by fungus, if left unnoticed may become weak and tender. If it is a ready for dyeing fabric, then that portion would be dyed lighter or darker according to the nature of dye.

The best solution to avoid the formation of fungus on cotton yarn or fabric, is to keep either completely dry before storage or it it needs to be kept under wet condition, it should be kept in slightly alkaline condition - pH 8 to 9, because all fungus has a tendency to grow in acidic medium only.

Check Points for flawless dyeing in Cabinet Dyeing Machine

The following points may lead one to a flawless dyeing:

1. Grieg yarn inspection - for oil stains, uneven twist, color variation is Grieg stage, Grieg yarn CSP, weight per bundle, hank weight, yarn count, moisture content of yarn - some of the tests, you can do it in your lab and for some of the textile tests, you have to get the report from an authorised textile lab.
2. Proper dressing of hank yarn - this would be a manual operation. The yarn in hank form from bundles, have to be opened, beaten well on the beating poles to separate each yarn in to loose single thread. If the yarn hanks are not properly prepared, then there is every possibility of getting shade variations between the hanks with in a batch itself. After beating and making each strand parallel, the hank should be hanged on the stainless dye-sticks or poles.
3. Cleanliness of the machine - Check the compartments of the dyeing machine. There should not be any stain of previous shade processed. Inspect thorugly and if necessary clean the machine once again before loading the greig yarn.
4. Loading in to the cabinet dyeing machine - this operation requires minimum two operators. One should carry the poles from the trolley to the person standing near the machine. The loading of yarn should be done with all care, so that it would not get entangled on the side wall slides, or with other hanks.
5. Water filling in to the machine - When you add the water in to the machine make sure, up to which level you should fill to reach the required volume of water. A best practice is to fill the water through a metering pump. Liquor ratio also plays an important role in reducing the batch to batch variation.
6. Cabinet Dyeing Machine propeller wheel speed - The speed of the propeller determines, how much volume of water is being transferred per unit time. Higher the speed, faster the cycles. (High speed induces entanglements in the yarn hanks apart from enhancing the pilling.) If the belt connecting the motor is loose, then the speed will be less and the reverse impact will result.
7. Calibrated thermometers - Dyeing is a chemical process, where the kinetic energy plays an important role. The ambient temperature recommended for each process has to be adhered without fail. The right temperature leads to right fixation and exhaustion as suggested by the dyestuff manufacturers. So keep the thermometer of cabinet dyeing machine frequently calibrated.
8. Process Timings - Each operation is a chemical process, that needs to be done at proper temperature and for proper time. Higher and lower timings for each process creates again batch variations.
9. Calculation mistakes of dyes and chemicals - please recheck your calculations for each dyestuff and chemical you are adding in to the bath. A third person checking is highly recommended.
10. Check and calibrate your weight machines - Have the practice of checking your weight machine, at least once in a day. Calibrate with standard weight. Do not over or under weigh than what is specified in the weighing machine.
11. Dyes and chemicals Weighment - appoint a separate person to inspect the weighments always. Weighing pans should be cleaned every time properly.
12. Dissolving of dyestuff - this is a very important point, everybody is missing. Your lab to bulk reproduction mainly fails here. Use adequate quantity of water for dissolving the dyes and chemicals. Proper dilution during dissolution is very important. In case of vat and reactive dyes, temperature of dissolution is very important.
13. Filtering of Dissolved dyes and chemicals - if it is a dyestuff, filter the dissolved dyestuff through a fine mesh like bolting cloth.
14. Chemical parameters:

a) Physical and chemical properties of dyes and chemicals used to be tested (Quality Assurance) before using it in bulk production.

b) Water Quality - hardness, TDS, Iron and Copper Content - all these parameters should be tested before using the water for dyeing.

c) pH of processing at every stage - during bleaching, during bio-scouring, during peroxide killing, during dyeing, after dyeing, before fixing or softening treatment. Please read the article on the influence of pH in textile dyeing and textile finishing.

If all the check points are kept under control, one can dream on batch repetition

Cabinet Yarn Dyeing - Sequence of operations

Generally in cabinet yarn dyeing machine, cotton, acrylic, jute, flax and coarse natural fibers can be processed in hank form.



For Cotton yarn processing, the general sequence of operations are as below:

(1) Load the machine with yarn -> (2) Scouring and bleaching -> (3) Hot wash -> (4) Cold wash -> (5) Neutralization -> (6) Peroxide kill -> (7) Cold wash -> (8) Dyeing -> (9) Cold wash -> (10) Neutralization -> (11) Cold Wash -> (12) Soaping at boil -> (13) Hot wash -> (14) Cold Wash -> (15) Softening Treatment -> (16) Unload.



Out of the 16 operations, the (1) and (16), do not consume water. The other 14 operations mean, 14 times the machine has to be filled with fresh water. If for example, we dye a 100 kgs batch at 1:15 liquor ratio, then the water consumption will be 1500 liters per bath and for 14 baths, it will be 14 x 1500 = 21000 liters. That mean, for processing 1 kg of yarn, it requires 210 liters of water. This is a very high volume, in these days of water scarcity. So the following measures are being adopted in this industry.

• Wherever possible, the dyeing units have started disposing off the old, high liquor ratio machines and buy new low liquor machines. Now 1:12 MLR cabinets are widely used everywhere in this industry. Straightaway 20% water saving is achieved.


• Wherever possible, the dyeing masters have implemented reduced number of baths, with new auxiliary chemicals. For example, bio scouring and bio-peroxide killing is new emerging process, where you need not do separate cold and hot washes after scouring and there is no need for separate bath for residual peroxide removal. This process eliminates almost 3 baths with a saving of again a 20% water.


• Using suitable good soaping of agents of anionic nature, one can easily acquire the soft feel and finish required by the end user. This also reduces one more bath.

This high volume of water consumption, not only adds to the water cost, but also to many other indirect costs like energy cost - steam to heat the water and power to pump in the water, extra dosage of dyes and chemicals to maintain the bath concentration and effluent treatment of excess water that is being let out.

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Yarn Mercerisation -Part-1

What is mercerisation?
Mercerisation is a process of treating cotton yarn or fabric in concentrated solution of caustic soda under tension for a specified time duration and washing off of the caustic solution under the same conditions. This process bestows the following important properties to cotton.

1) High dye affinity
2) Lusture
3) Improved tensile strength

In this issue we are going to discuss about the machinery required for mercerising the yarn and the process and recipe details of mercerisation.

Machinery Required:
The yarn mercerising machine should have the following facilities to treat the yarn in high concentration of caustic lye.
1) A rotating hank holding device viz hank holding arm
2) A squeezing bowl
3) A shallow trough to hold the required volume of concentrated caustic dye
4) A device to make and keep the hank yarn under suitable tension.
5) A washing arrangement to remove the casutic lye from the yarn after the impregnating time is over.
6) A timer arrangement - either mechanically or electronically controlled device to hold the yarn iside the caustic bath for the required number of minutes and to wash it after exactly after the same number of minutes after impregnation.

Mercerisation Process Sequence:
1) Yarn hanks are evenly placed on the pair of rollers that hold the yarn hanks.
2) The hanks are properly positioned on the rollers. The rollers may be alligned either horizontally or vertically according to the machine design, rotating the rollers, with a view to confirm that the yarn hank does not get ruffled and the yarn strands remain perfectly parallel. Please note that the rollers must be capable of rolling on both directions.
3) Application of tension to the hank yarn, while rotating and raising the merceirising lye tray, till the lye covers the lower part of the rotating hanks. The yarn tension is measured in terms of increase in length between the rollers.
4) At this stage of the operation, the sqeezing roller should commnce sqeezing with a light sqeeze, so that the mercerising lye gets uniformly soaked in the yarn, in turn, an equal overall mercerising effect.
5) The light sqeeze is maintained. The yarn tension is increased to the maximum pre-determinded level and in maintained at this level. The time for which the maximum tension should be retained, depends on the yarn count, twist and the folded state of the yarn.
For each quality of yarn, this dwell time under tension should be worked out before-hand, so that the instructions can be rigourously followed by the operators.
6) At the end of the dwell time, the mercerising-lye-tray should be lowered. Excess lye taken up by the yarn is squeezed-out by applying higher pressure on squeezing rollers.After the squeeze out the lye tray is moved away.
7) The wash tray is brought in to position. Hot water is sprayed over the yarn hanks while it is still under the light sqeeze . The yarn tension should be maintained during the hot wash.
8) While still maintaining the yarn tension, the following measures should be taken:
(a) The squeezing pressure should ne raised in order to squeeze out the still remainling lye in the yarn.
(b) The squeezing pressure should be reduced to keep only a light squeeze.
(c) Cold water should be sprayed over the hank so as to wash out the yarn hank to an alkali free stage.
9) After completing the mercerising of the yarn, as indicated in the above stages, the squeeze and the tension on the hank are released. The hanks are then unloaded from the pair pf rollers for the next process, which may be bleaching, dyeing or plain drying.

If all the above stages are maintianed as said above the degree of mercerisation would be excellant.

Recipe Details:

1) The mercerising lye concentration is maintained at 250 to 300 grams/liter.
2) A powerful wetting agent that does not get affected by the high concentration of caustic lye is used between to 1 to 2 grams/liter.
3) The impregnation time depends on the quality of yarn and the lye strength. Ranges from 2 to 3 minutes.