Dyeing with Indigo - Natural Fermentation Vat

Why Indigo?

Indigo is a dye different than any other. It does not

require any mordant. Rather it is dyed through a living

fermentation process. The process "reduces" the

Indigo, changing it from blue to yellow. In this state, it

dissolves in an alkaline solution. The fibre is worked in

the solution, or "vat". When brought out to the air, it is

a bright green. Slowly the air changes it to the

beautiful deep and rich blue of Indigo.

Indigo in some form is used in all traditional cultures, for it is the only clear and

fast natural blue. Indigo dyeing was one of the first speciality professions. Yet it

is easy to keep a home pot going, and most colonial homesteads had one. This

recipe is the one most commonly used for home dyeing. It contains no harsh

chemicals nor toxic metals. It can be used to dye any natural fibre.

An additional beauty of dark Indigo is that when ironed or pounded, the blue

cloth takes on a beautiful coppery sheen - the same sheen that is seen on the

well reduced Indigo vat, when it is ready for dyeing.

Indigo: Natural Vermentation Vat

NOTE: requires advance preparation of about one week.

4 oz. ground Indigo

2 oz. ground Madder

2 oz. wheat bran (buy at any health food store)

12 oz. washing soda ("soda ash")

(above amounts are by weight ounces, not volume ounces.)

Combine in about a three gallon pot of warm water.

Always add these amounts in proportion. A larger vat can be made, for example

with: 1 lb. ground indigo, 1/2 lb ground madder, 1/2 lb ground bran and 3 lbs

washing soda in about a 10 gallon plastic tub. However, I advise starting small,

till you are comfortable with the process. The size of the pot is determined by the

amount of fobre you need to dye at one time. A three gallon pot is good for yarn

skeins of 4 to 6 oz., while a 10 gallon or larger tub will be needed for yards of

fabric.

WARMTH: It is necessary to keep the vat warm, but not hot, around 100 - 110°

Fahrenheit. It is the same temperature for raising bread or making yogurt. It

should feel pleasantly warm to the hand.

click to enlarge

Buy Indigo

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To keep it warm, a light bulb in a reflector can be put under the vat, with a

blanket over it to keep in the heat. (See illustration, next page.) In a warm

climate no additional heat is needed, but be sure the vat is out of direct sun so it

does not overheat.

TIME is very important. It takes time for the vat to ferment and it does no good

to try to rush the process. The first time, it takes about a week for the vat to

ferment and be ready to dye. With "renewals" the time needed is a bit less, four

or five days.

It takes time to do the dyeing. The fibre is "dipped" several times to build up a

dark color, with airing between each dip.

The vat itself lasts a long time. I have had my current vat over fifteen years. In

traditional cultures there are vats over 100 years old. The vat is "renewed" with

more Indigo and the other ingredients in proportion, whenever the dye value

weakens. Then let sit a few days to re-ferment. Indigo dyeing by this natural

fermentation method is a slow-steady process. It is good meditation.

Stir the vat once a day. The idea is to integrate the undissolved Indigo, madder

and bran that settles to the bottom, back into solution. And to do this without

incorporating air into the vat. So stir gently.

Keep the vat covered. Air is the enemy of a good Indigo vat. The level of

liquid in the pot should just allow room for the yarn dyeing, without spilling. The

less air between surface and lid the better. I use a domed lid, turned upside

down. If you leave more than 2" of air at the top of the vat, it will not reduce

properly.

The vat is ready for dyeing when:

It develops a coppery film on the top of the vat.

The liquid, lifted carefully in a glass jar, will appear green.

A test piece of fibre or paper will emerge green and turn blue in the air.

Now is the exciting time to begin Indigo dyeing!

1. Wet your fibre out very well in warm water. It must be well wet out. Any

air remaining in the fibre will oxidize Indigo in the vat, and this must be

avoided.

2. Put on rubber gloves! You don't want to scare people with blue hands;

also the strongly alkaline vat may irritate your skin.

3. Enter the fibre (yarn/fabric) into the vat very carefully, to avoid adding

any air to the vat. Now the fibre must be "worked" in the vat, under the

surface. It should not be stirred, but with your gloved hands, gently,

slowly and deliberately squeeze the liquid through the fibre while you

hold it under the surface. Any time you break the surface you introduce

air into the vat and this you do not want to do.

4. After you have worked it several minutes, carefully and slowly raise it out

of the vat, squeezing the excess Indigo solution back into the vat. Do this

squeezing as close to the surface as you can, as dropping liquid will bring

air into the vat.

5. The fibre should be a bright clear green. It will start to turn blue in the

air immediately. Lay it out on newspaper and let it air for 20 minutes.

Repeat the dips up to five times for dark Indigo Blue. Air between each

dip. For lighter shades, fewer dips are needed.

6. Rinse well. Then leave the fibre to air overnight. Soak and do a final rinse

in the morning.

The vat lasts indefinitely. It is begun with a certain amount of Indigo, and all

other ingredients as given, in proportion. Dyeing is begun, with the darkest color

dyed first, then medium, then lights. Between dyeings the vat must rest

overnight or an extra day. This is because, during dyeing a certain amount of the

Indigo is oxidized in the vat. Allowing it to rest lets it re-reduce that Indigo. An

oxidized (blue colored) vat won't dye well. The Indigo color will only wash out

and rub off too quickly.

When the vat is "exhausted", and will only dye light shades, it is time to renew it.

All ingredients are again added, again in correct proportion. The vat is let to

ferment for several days, and is ready to dye when it shows the proper signs. In

this way a vat can be kept going for many years.

If one wishes to rest from dyeing for several weeks, simply turn off the heat

source, and keep the vat cool for that period. Stir it vigorously on occasion.

When ready to dye again, warm it up, renew it with the ingredients, and proceed

as before.

It is not good to leave a vat unused for too long, as it is a living process and may

then get cranky about starting up again. Also it is important to exhaust the vat

before leaving it, or it may over-ferment and ruin any Indigo remaining in it.

Over time a deposit of sludge will develop at the bottom of the vat. You may

want to gently lower a screen into the vat before dyeing, to keep your fibre from

pickinging itup during the dye process. Be sure to remove the screen after the

day's dyeing, so you can stir the vat before closing it.

For greens, dye you fibre Indigo first, then rinse well and overdye with alum

mordant and your chosen yellow dye. For purples, dye the Indigo first, rinse well,

then mordant and dye over with any red dye.

The indigo vat is very alkaline. It is important to rinse out all the alkalinity. Just

to be on the safe side, I always double rinse my indigo dyed textiles. First I rinse

well just after dyeing, then I let air overnight. Next day I soak in two successive

waters for about an hour each time, rinse again, wring and dry.

Squeeze solution through yarn for best penetration. Always work under

the surface of the vat. Always wear rubber gloves.

More About Indigo

Indigofera is a legume. The plant looks similar to alfalfa, but is usually larger. It

is an excellent rotational crop for increasing soil fertility. In southern Mexico,

where some of the current Indigo of commerce originates, it is naturalized and

grows in fallow fields, so no effort is spent cultivating it.

Indigo dye must be prepared from the fresh plant in an exacting and elaborate

process that takes about a month. The Indigo plants are harvested and brought

to a central location. They are soaked in water and allowed to ferment. This

separates the dyestuff from the plant. The solution is then beaten to oxidize and

precipitate the Indigo. Excess water is poured off and the sludge is dried. This

sludge, packed into balls or patties and fully dried, is the Indigo dye of

commerce.

This Indigo comes to us in the form of a hard, dark blue colored cake. It must be

ground to be used for dyeing. Very small amounts can be ground in a mortar and

pestle. Use a bit of water to facilitate grinding and keep down the dust. A Corona

Corn mill is what I use. Meat grinders also work. A zip-loc baggie cinched over

the grinding plates catches all the powder and keeps blue dust from getting

everywhere.

In most traditional cultures, the color(s) of ones clothing indicates ones status or

class. Indigo blue has long been associated with the less than aristocratic

classes. Indigo blue has still the association of "The Working Class". We use the

distinction as "Blue Collar Workers" and "Blue Jeans". These clothes were

originally dyed with indigo. In the past, Indigo has been a prolific dyestuff. It is

relatively easy to grow and dye, and is quite fast. It withstands well the many

washings that work clothes require.

In most cultures, Indigo dyeing is or was a specialty. The dye process is unique,

and the facilities require a stable set-up. Vats made of great clay pots set in the

ground are commonly used in warmer climates. If more heat is needed, pits for

burning charcoal are placed between clusters of the vats.

Indigo dyeing is practiced today in Japan, Southern China, Tibet, India,

Indonesia, Indo China, Africa, especially Nigeria, Southern Mexico and

Guatemala, and it has recently been reintroduced to Turkey. Traditional

fermentation methods are used. However, many of these cultures now use

synthetic Indigo, manufactured from coal tar or petroleum.

Natural Indigo contains several related dye chemicals that give different shades

of blue. As much as twenty percent of the dye may be a violet tone called Indigo

Red. These complexities give Natural Indigo nuances and depths that cannot be

achieved with the synthetic substitute. Here is a page with more about the

chemical properties of indigo.

An additional beauty of dark Indigo Blue, is that when ironed or pounded, cloth

so dyed takes a coppery sheen - the same sheen we see on the top of the well

reduced Indigo vat.

PO Box 14 • Somerset, MA 02726

orders 1-800-2-BUY-DYE technical support 508-676-3838

fax 508-676-3980 e-mail • promail@prochemical.com

www.prochemical.com

Indigo

Please read the directions carefully before starting.

Indigo belongs to a class of dyes called vat dyes, which are among the oldest natural coloring substances used for textiles. Until the beginning of this century, indigo could be obtained only from plants. Two things are required to make indigo work, Thiox, the reducing agent, and Lye, the alkali. The dye vat is prepared in two steps: The stock solution and the dye vat. The stock solution then added to the dye vat. Always do test samples before working on a large project.

While Indigo and the chemicals used are comparatively safe and non-toxic, it is best to treat them all with caution. Wear rubber gloves to minimize contact with hands. Eye protection is urged as you are working with alkalies and strong reducing agents. Always work in a well ventilated area. Good house-keeping is essential to good results. Utensils used for dyeing should never be used for food preparation. See caution for Lye (Sodium Hydroxide) below.

Supplies:

For Cotton, linen, rayon, and silk For Wool

PRO Indigo grains PRO Indigo grains

Lye (Sodium Hydroxide) Lye (Sodium Hydroxide)

Thiox (Thiourea Dioxide) Thiox (Thiourea Dioxide)

Metaphos Synthrapol SP

Non-Iodized Salt Unflavored Gelatin

Distilled White Vinegar Clear Household Ammonia

Ivory Bar Soap or Ivory Flakes Distilled White Vinegar

Ivory Bar Soap or Ivory Flakes

Procedure for dyeing cotton, linen, rayon and silk

1. Determine whether you wish to make a 20 gallon (80 liter) full size vat or 4 gallon (16 liter) mini vat and prepare the stock solution.

Full size vat Mini vat

PRO Indigo 1 cup (70 gm) 3 Tbl (15 gm)

Cold water 6 cups (1.5 liters) 1½ cups (375 ml)

Lye 1 cup (215 gm) 5 tsp (23 gm)

Thiox 2 Tbl (25 gm) 1 tsp (4 gm)

2. Mix Indigo with enough luke warm water to make a lump free paste.

3. Measure the COLD water into a separate container. Carefully add the lye and set aside to cool.

4. In a third container, add the Thiox to 1 cup (250 ml) of warm water (Mini Vat use ½ cup (125 ml) water). Stir gently to dissolve.

5. Slowly add the lye solution to the pasted Indigo and stir to make a smooth mixture.

6. Next, slowly add the thiox solution. Avoid creating air bubbles.

7. Stir this mixture gently from time to time, until reduction is complete. Reduction is complete when the stock solution turns yellow. Otherwise the surface is a deep blue from oxygen coming in contact with the dye. Set the jar of stock solution in a pan of HOT Water, if necessary. Raise the temperature no higher than 135oF (57oC) for 15 to 30 minutes or until reduction takes place.

8. Make the dye vat.

Full Size Vat Mini Vat

Warm Water 120oF(49oC) 20 gallons (80 liters) 3 gallons (12 liters)

Metaphos 3 Tbl (63 gm) 1 tsp (7 gm)

Salt 2 cups (600 gm) 1/4 cup (75 gm)

Thiox 4 tsp (15 gm) ½ tsp (2 gm)

9. Measure warm water into dye vat container. 10. Stir in the Metaphos and salt.

11. Dissolve the Thiox in a small amount of warm water and add it to the dye vat.

12. Add reduced stock solution by carefully lowering the container into the Dye Vat and sliding the liquid out at an angle. Stir gently.

13. After 30 to 60 minutes the vat should be clear greenish-yellow with a shiny, dark blue metallic surface. The vat is then ready to use. If the vat is not clear and greenish-yellow in color wait an additional 30 to 60 minutes. It can take as long as 6 hours for proper reduction.

Maintaining the Indigo vat

* Every precaution MUST be taken to keep oxygen out of an Indigo vat!!!

* For your vat, use a deep vessel with a narrow top to minimize exposure to air.

* When adding additional Stock Solution or dissolved chemicals to the vat, do not pour them in. Lower the container into the vat and slide the liquid out at an angle.

Using the Indigo vat

*Before dyeing, machine wash the fabric on HOT cycle with a minimum temperature of 140oF (60oC) water OR by hand in a pot on the stove with ½ tsp (2 gm) PRO Dye Activator or Soda Ash and ½ tsp (2.5 ml) Synthrapol per pound (454 gm) of fabric. Rinse thoroughly.

* Thoroughly wet fabrics before dipping in vat. A warm water soak for 15 to 30 minutes is recommended. Wearing gloves, squeeze out excess water evenly.

* Gently push the dark blue scum aside before entering fabric in the vat. Where scum clings to the cloth it will look dark blue. However, after dyeing is finished the spot will wash off the surface, leaving a light undyed spot.

* Lower fabric into the vat very gently, without splashing.

* Slowly manipulate the cloth while working below the surface of the vat. This helps the dye penetrate. Keep fabric submerged for the duration of each dip, 2 to 3 minutes. DO NOT SWISH the fabric around in the vat.

* Gently squeeze out excess dye BELOW the surface and remove fabric from the dye vat. DO NOT allow fabric to drip into the vat after dyeing.

*Let fabric oxidize (turn blue) for approximately 15 minutes. Repeat dipping and oxidizing until you’ve reached the desired depth of blue. If you’ve gotten some of the dark blue scum on your fabric, then give it a rinse in plain room temperature water. This way the fabric that is underneath the scum can oxidize. Let the fabric air dry before washing. Remember that after washing, the final color will be one to two shades lighter. If you’ve done a bound resist dyed fabric it’s best to let the fabric dry completely before untying so the threads won’t rip the fabric.

Washing the fabric

Wash the fabric in hot 135oF to140oF (57oC to 60oC) water for 10 minutes in a generous bath of Ivory bar soap or Ivory Flakes. With a knife, it’s easy to flake off a 1/8 inch thick peel of soap. Stir the wash bath occasionally and rinse the fabric until the water runs clear. Hang cotton and rayon to dry.

If you’re dyeing silk then soak it for 10 minutes in a bucket of acid soak. Make the acid soak by mixing 2 Tbl (30 ml) of White Distilled Vinegar in 1 gallon (4 liters) of room temperature water. Then rinse the silk thoroughly in plain water.

Troubleshooting the dye vat

* If the vat appears grayish and watery, it is exhausted. This means that all the indigo has been used. An addition of Stock Solution is needed.

* If the vat has been left for a few days, it may need to be "Sharpened" with a small amount (½ tsp (2 gm) for full size vat) Thiox, dissolved in water.

* If the vat changes from yellow-green to blue, or if blue specks appear, more Thiox is needed. Add a small amount (½ tsp (2 gm) for full size vat) Thiox, dissolved in water. Stir gently. Wait 15 minutes and check vat again before dyeing.

* If white specks appear or the vat appears "milky" add small amount (1 tsp for full size vat) of Lye, dissolved in ½ cup water. Stir gently. Wait 15 minutes and check vat again before dyeing.

* Occasionally, more than one addition is required to revive a vat. Use small amounts, and wait 15 to 20 minutes between additions, testing each time. Excess alkali or reducing agent can unbalance the vat, making it impossible to build up deep shades. Be patient, and the vat will have a long life.

* Always dissolve Thiox and Lye in water BEFORE adding to the dye vat. DO NOT add dry flakes.

* Cover with a tight fitting lid when not in use.

Troubleshooting fabric

* Dye washes off of fabric: too little reducing agent.

* Crocking off (Dye rubs off): too little alkali

Procedure for dyeing wool

1. Prepare the stock solution.

Mini Vat

PRO Indigo 3 Tbl (15 gm)

Lye 5 tsp (23 gm)

Thiox 1 tsp (4 gm)

2. Mix Indigo with enough luke warm water to make a lump free paste.

3. Measure 1½ cups (375 ml) of COLD water into a separate container. Carefully add the lye and set mixture aside to cool.

4. In a third container add the Thiox to ¼ cup (60 ml) water. Stir gently until it’s dissolved.

5. Slowly add the Lye solution to the pasted Indigo and stir to make a smooth mixture.

6. Add the Thiox solution slowly to avoid making air bubbles.

7. Stir gently from time to time until reduction is complete. Reduction is complete when the stock solution turns yellow. If necessary set the jar of stock solution in a pan of HOT

Water. Raise the temperature no higher than 135oF (57o) for 15 to 30 minutes or until reduction takes place.

8. Make the dye vat.

Mini Vat

Warm Water 120oF (49oC) 3 gallons (12 liters)

Unflavored Gelatin powder 1 tsp (2 gm)

Synthrapol 1 tsp (5 ml)

Clear Household Ammonia 2 Tbl (30 ml)

Thiox 1 tsp 4 gm)

9. Measure warm water into dye pot. Use non-reactive metal such as stainless steel or un-chipped enamel as your dye pot.

10. Mix the remaining ingredients in the order listed above making certain each item is thoroughly mixed before adding the next.

11. Add reduced stock solution by carefully lowering the container into the dye vat and sliding the liquid out at an angle. Stir gently.

12. After 30 to 60 minutes the vat should be clear greenish-yellow with a shiny, dark blue metallic surface. The vat is now ready to use.

13. Maintain a 120oF(49oC) temperature throughout the dye process. Soak your wool for at least 15 minutes in 1 gallon (4 liters) of 120oF (49oC) water with 1 tsp (5 ml) of Synthrapol. The wool should sink in the Synthrapol soak, not float. Squeeze out excess water evenly from the wool. Gently push the dark blue scum aside before putting your wool into the vat. Wool that is not thoroughly wet carries large quantities of air which quickly oxidize the reduced Indigo and destroy the vat.

14. Soak the wool in the dye vat for 30 minutes with gentle, intermittent stirring, making sure all the wool remains below the surface of the vat.

15. After 30 minutes, remove the wool, squeeze excess liquid back into the vat while holding the wool close to the surface to avoid introducing air into the vat.

16. Let the wool oxidize (turn blue) for approximately 15 minutes.

17. Repeat dipping and oxidizing until the desired depth of blue is obtained.

18. After the final dip and the fiber is fully oxidized, gently wash the wool in a warm 120oF (49oC) water bath for 10 minutes in a generous bath of Ivory bar soap or Ivory Flakes. With a knife, it’s easy to flake off a 1/8 inch thick peel of soap. Gently stir the wash bath occasionally and rinse the fiber until the water runs clear.

Then soak the wool for 10 minutes in a bucket of acid soak. Make the acid soak by mixing 2 Tbl (30 ml) of White Distilled Vinegar in 1 gallon (4 liters) of room temperature water. Then rinse the wool thoroughly in plain water and hang to dry.

© Copyright 2002

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color-wash-fastness-and-fashion-effects/

Indigo Dyeing Methods – Engineering Color, Wash Fastness And Fashion Effects

Indigo Dyeing

Indigo dyeing is unique and because of the complex chemical reactions should be correctly viewed a a form of chemical engineering.

Only Indigo dyeing requires multiple dye applications for a dark shade. Color consistency of Indigo in recent decades has been unsatisfactory as a result of machine

designs that do not apply basic principles of fluid mechanics properly and unstable dye mixes. Commonly, a single dye lot will have between 8 and 15 visually different shades from beginning

to end and also have shade differences from one side to the other.

Indigo Dyeing Methods

Indigo dyeing follows the same basic steps regardless of machine design. Scour or dye bottoming in a heated tank, washing tanks, dyeing(1-20),a heated tank for topping (optional) and wash tanks. In different areas of the world,the same color is produced using 1.8, 2.0 or 4% Indigo depending

on dyeing method.

Dark Indigo(1.8%) 1. 15% caustic cold

2.Wash 60°C

3.Wash 60°C

4. Wash cold

Drying cylinders hot

Steamer cold

Boxes 5-12 Indigo

Steamer cold

13.Wash 50°C

14. Wash 50°C

15. Wash 50°C

16.Wash 50°C /Softener

Stock Mix 80 g/l Indigo

Pure 100 g/l 50% Caustic

70 g/l Hydro powder.

Chemical Feed 120 g/l 50%

caustic 60g/l Hydro powder

Feed 1.4 liters per minute

Dark Indigo Color

This was an example of a typical method used in the U.S. for a very dark shade. In order to produce the same depth of color as 1.2% in the U.S., in Latin America 2.0% is used

and in Asia from 2.4 to 2.8%. The U.S. method results in more surface (ring dyeing), which loses color faster.

Darkest Indigo Shades

Very dark shades of Indigo are in demand currently around the world. Many companies use 4% or more Indigo on weight of yarn, which is expensive. 2% Indigo will produce the same depth if low levels of caustic are used(0-0.4%) For dark Indigo that does not lose color 2% applied normally, with an Indigo bottom.

Light Indigo Shades

Dyeing Indigo in light shades results in a sky-blue impossible with any other dye. This is useful for shirting fabrics that are not strong enough for stonewashing, bleaching or

cellulase treaments. Special procedures are necessary in order to avoid colorfastness problems.

Light Indigo 0.4% 1. 4% caustic 90°C

2.Wash 60°C

3.Wash 60°C

4. Wash 60°C

Bypass drying cylinders

Bypass steamer

Close off boxes 5-8

Boxes 9-12 Indigo

13. Wash 50°C

14. Wash 50°C

15. Wash 50°C

16.Wash 50°C/softener

Control Of Sulphur Bottoming

The typical methods used for dyeing sulfur bottoms result in denim shade differences. When applied as light colors, sulfur dyes should be dyed at temperatures <60 C, If dextrin reducing agents are used, which require 85 C, there will be variation. Sulfur bottoms are an exception to the normal pH for sulfurs(11), requiring 12.

Sulfur Bottom 1. Pad sulfur(cold)

Steamer hot

2.Wash cold

3.Wash 50°C

4.Wash 50°C

Boxes 5-10 Indigo

11. Indigo or wash 50°C

12. Indigo or wash 50°C

Bypass steamer

13. Wash 50°C

14. Wash 50°C

15. Wash 50°C

16.Wash 50°C or softener

Sulphur Topping

In topping the sulfur dye is applied after the Indigo dyeing. Sulfur topping permits much darker color than a sulfur bottom, but is duller. Sulfur topping colors include black, blue-black, yellow brown and green. Sulfur toppings are used to produce slub appearances in normal yarn.

Sulfur Top 1. Pre-wet 2% caustic 90°C

2.Wash 60°C

3.Wash 60°C

4. Wash cold

By pass drying cylinders

Bypass steamer

Boxes 5-10 Indigo

11. Wash 60°C

12. Pad sulfur topping

Steamer hot

13. Wash cold

14. Wash 50°C

15. Wash 50°C

16.Wash 50°C/Softener

Reactive Dyes in Indigo Dyeing

Reactive dyes can be applied on specially-Designed Indigo machines. Small 150 liter boxes are inserted inside the larger dye tanks for Indigo and sulfur. Steamers, drying units near the front of the machine and high-quality dye padders are required

for quality dyeing.

Pad-Dry Chempad- Steam

Reactives

1. Pre-scour wetter plus chelate

90°C

2.Wash 50°C

3. Pad monochlortriazine dye

cold, neutral pH

Drying cylinders hot

Pad caustic in salt brine

Steamer hot

Bypass boxes 5-10

11. Soap 90°C

12. Soap 90°C

Steamer hot

13. Wash 60°C

14. Wash 60°C

15. Wash cold

16.Wash cold/softener

Pad Steam Reactive Topping 1. Pre-wet 10% caustic 90°C

2.Wash 60°C

3.Wash 60°C

4. Wash cold

By pass drying cylinders

Bypass steamer

Boxes 5-10 Indigo

11. Wash 60°C

12. Pad Dichorotriazinyl cold

with bicarbonate

Steamer hot

13. Wash cold

14. Wash 50°C

15. Wash 50°C

16. Wash 50°C / softener

Control Of Indigo Dyeing

The Indigo dyeing process begins with a concentrated mixture of Indigo, sodium hydroxide and

reducing agent. This concentrated mixture (70-90 g/L Indigo) is delivered by pipes to the

Indigo dye tanks where the dye concentration is reduced to 1-4 g/L for dyeing the cotton.

Dye Mixing Procedures

Many denim companies find it difficult to control original and washed Indigo shades. The primary source of color differences is the instability and inconsistency of Indigo mixtures. As the concentration of reducing agent going to the dye machine changes, the color changes.

Uniform Indigo Mixtures

For consistent Indigo dyeing, the mixture must have consistent concentrations of Indigo, sodium hydroxide and reducer from the top of the mixture to the bottom.

The main cause of inconsistent Indigo mixtures relates to concentration levels. Instability of Indigo mixtures results from the decomposition of sodium hydrosulfite.

Consistency of Concentration

There is a limit to the amount of any chemical that can be dissolved in water. When the limit of solubility of any chemical •In water is exceeded, precipitation occurs. Indigo mixes should not have more than 20% solids. At higher levels, chemicals and dye sink to

the bottom of the tank.

Improving Dyeing Consistency

When reducing agent sinks to the bottom of the tank, there is a higher concentration than in the top of the tank. As the dye enters the machine, the higher concentration results in a lighter, greenercolor and as the dye from the top of the tank enters the machine, the color is darker and redder.

Dye Control In Feeding Tank

Stirring the tank for 2 minutes will improve dye uniformity between top and bottom. To avoid settling of dye and chemicals the total solids should not exceed 20%. The “glass plate” test can be used to test concentrations of hydrosulfite in the top and bottom.

If dye requires 50 seconds to oxidize, there is about 50 g/L of reducer.

Buffers In Indigo Dyeing

Alkaline buffers have been used to make very dark shades of Indigo with as little as 1% dye, more ring-dyed, faster fading.

Reductive buffers can eliminate color differences in Indigo-dyed denims and can reduce hydrosulfite use by 30-50%.

Indigo Dyeing – Problems And Potential -Part 1 October 24th, 2011 by Harry Mercer | Filed under Manufacturing Process.

This is a highly technical article on Indigo dyeing by Harry Mercer. Read on if you are technically

oriented..

This is the first of a series of 4 articles addressing the problems and potential of Indigo dyeing. The

Indigo color is the principal source of the almost magical appeal of denim. The dyeing process is

unique among all methods of commercial dyeing, with the unusual design that is necessary for cotton

dyeing with Indigo. Indigo has been used for thousands of years, principally on wool and silk

fibers for which Indigo is more suitable The difficulties in dyeing cotton with Indigo are apparent

with the numerous different shades that result, up to 15 per dye lot and also with side-center side

variation. Elimination of this variation has been accomplished, but it requires a deep understanding of

the unusual variables of Indigo dyeing. The 2 keys to success in manufacturing denim is firstly the

dyeing, then the finishing, both of which are more complex to conduct at a high level of quality. The

failure of most denim companies to overcome the challenges in denim wet-processing is the reason

why they are held hostage to low profit margins.

Part 1 of 4 MACHINERY

Indigo dyeing is a unique process that makes denim special and distinguishes denim operations from

all other types of cotton fabrics. No other method of cotton textile dyeing requires the multiple

application of dye to achieve a dark color, thousands of liters of dye bath, slow production speeds and

extremes of color variation and color-fastness. Indigo dyeing has been conducted without these

problems. The 2 most significant sources of Indigo dyeing are the

control of chemical concentrations, which will be addressed in future articles, and the machine itself.

Machine factors that affect Indigo dyeing results

1) Circulation system design: Indigo dye in its reduced form consists of dye particles that have

been partially solubilized and exists in the form of charged colloidial particles. Colloidial dispersions will

sink due to the influence of gravity and require some agitation to keep them uniformly dispersed in

the dye box. If sample are collected from different parts of an Indigo dye box(top, bottom, front and

back), the concentrations are usually different. The uneven distribution of dye in the box as the

machine operates contributes to color variation. For many years, BASF, a leader in indigo dye for most

of the 20th century, recommended that the volume of the dye box be “turned over” 2 or 3 times an

hour. This means that if the box volume is 2000 liters that 4-6000 liters of flow into and out of

the dye box is needed to prevent low concentrations in part of the box and high in others. Indigo

machines produced in recent decades have been furnished with pipes that are too small to deliver the

right kind of flow. In terms of Reynold‟s number , the flow should be slightly beyond laminar, in the

low transitional range to ensure uniform disper-sion while avoiding turbulence that would destabilize

the dye. Also, for uniform dispersion of Indigo, the entry line should be positioned in the yarn exit side

of the box near the top, while the exit line should be at the yarn entry side near the bottom. Many

Indigo machines have the dye entry line on one side and the exit line at the yarn entry, which is a

cause of cross-shade variation.

2) Dye box design: In a previous article I discussed the effect of dye box design on the color

consistency of Indigo. In most indigo machines the box design is responsible for massive losses

of hydrosulfite at the surface of the dye boxes during operation which results in economic losses as

well as variations in hydrosulfite concentrations in the machine which leads to color variation. The

principle is known as Specific Surface Area which means that the larger the surface area of the

Indigo box to the volume, the faster the hydrosulfite is lost. So, in a 2000 liter box with 2 square

meters of surface area , the hydrosulfite will decompose at twice the rate of a 2000 liter box with 1

square meter of surface area. The total hydrosulfite losses in a typically larger box of a rope range will

average around 15%, while in the smaller boxes of a typical sheet range the losses will be from 45-

70%.

3) Tension Control: High yarn tensions on continuous Indigo dyeing machines has 2 significant

effects- the yarn loses strength and the ability of the dye to penetrate the cotton fibers is reduced.

The loss in yarn strength results in higher warp breaks in weaving, meaning lower efficiency and

higher weaving off-quality. The reduced penetration of Indigo into the fibers results in rubbing

fastness problems and a higher per cent Indigo on weight of yarn for a specific depth of shade.

4) Immersion time: Indigo dyeing is a form of wet-on wet processing. The yarn is normally

scoured and washed before entering the dyeing section, which means that it is already wet. In order

for the Indigo dye to enter the wet yarn efficiently‟ a process known as “liquor exchange” is

necessary in which the Indigo dye/water displaces the water already in the yarn. This is a slow

process and the longer the immersion time , the more easily the dye penetrates into fibers and yarn,

resulting in better colorfastness and darker color. Until the 1970‟s, Indigo machines operated at

speeds of 12 meters per minute through the much larger boxes of rope ranges, so the immersion time

was 2-3 times longer than on modern sheet ranges. The dye penetration was complete which resulted

in the darkest possible color with 3% Indigo and that would never fade.

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Indigo Dyeing : Problems And Potential–Part 2 November 11th, 2011 by Harry Mercer | Filed under Manufacturing Process.

This is a guest post by Harry Mercer on Indigo dyeing. It is second part of the article in series. The

first part can be seen by clicking here

Preparation For Dyeing

In the previous article, the basic machine factors in

Indigo dyeing were discussed. There are many other

details required to achieve the highest quality Indigo

dyeings , but ultimately the most important factors

involve the preparation of Indigo and chemical feeds to

the machine. Approximately 80% of Indigo dyeing

control depends on the stability and consistency of the

dye and chemicals being sent to the machine.

1) Raw materials: The basic ingredients for Indigo dyeing are the Indigo dye, sodium hydroxide and

the hydrosulfite (sodium dithionite). Indigo is an insoluble vat dye which means that

it cannot enter the cotton fiber until it is made soluble by the process of reduction

.Reduction is basically a process where hydrogen is produced which opens up the

Indigo dye molecule allowing it to attach to a water molecule which carries the vat

dye into the fiber. The most commonly-used

2) The reducing chemical is known as “sodium hydrosulfite”, but this

nomenclature is incorrect because the molecule does not contain hydrogen. The

“hydrosulfite acts on the sodium hydroxide to split it into NaO and hydrogen, both of

which attach to the dye molecule in the reduction process.

3) The reduction of Indigo with sodium hydroxide and sodium dithionite is known as

vatting and has been used for thousands of years. Vatting refers to mixing the dye and chemicals into

a tank or “vat” with some stirring and then waiting from 1-4 hours usually for the complete reduction

of the dye to occur which is noted when the solution color is a clear, yellow-brown.. The solution then

is referred to as “leuco” Indigo, a Greek word meaning “without color”. The concentrated Indigo mix

is then ready to pump into the dye machine for dyeing.

4) Most of the variation in Indigo dyeing is a result of instability in this concentrated mix. Sodium

dithionite can be extremely unstable,

with the concentrations in this feeding

mixture becoming smaller with the

passage of time. For example, the initial recipe may specify 100 grams per liter of sodium dithionite,

but by the time the last liter goes into the machine, the concentration often drops to 20 to 30 grams

per liter and each 5 gram per liter loss in dithionite concentration produces a small Indigo color

variation. This is evidenced in many denim operations that suffer 10-15 colors after fabric washing

per dye lot.

5) There are several causes for the decomposition and strength losses of the reducing agent in the

feeding mix: Oxidation at the surface of the tank, unnecessary stirring and high concentrations of

ingredients. The stirring should be only enough to maintain consistent concentrations of dye and

chemicals from the top of the feeding tank to the bottom. Stirring beyond that will result in more

reducing agent being oxidized. Also, in many Indigo operations the stirring units are badly designed

with small propellers that turn at high speeds. The Indigo feeding mix is of very high viscosity and in

order to stir the entire mix out to the edge of the tank, large propellers that cover the tank diameter

are needed. These stirrers should turn at only 10-15 RPM in order to avoid turbulence that would

lower the strength of the mix. With regard to concentrations, if the viscosity of the dye mix is too

high, the reduced Indigo will not disperse uniformly resulting in areas of varying concentration in the

tank that will cause color change as the mix is fed to the machine. Concentrations above 23% solid

have a tendency to settle, so that there are very high concentrations of reducer in the bottom of the

tank, making a greener Indigo tone when pumped to the dye boxes, and lower concentrations of

reducer at the top of the tank, making a redder Indigo tone later in the dyeing. No more than 80

grams per liter of indigo should be added to a feeding mix as this is the maximum amount that

has long been proven that can be completely reduced. The amount of reducing agent should also be

limited to 80 grams per liter since greater amounts will cause more rapid decomposition due to

aerobic and anerobic decay.

6) The concentrations of indigo and reducing agent must be actively managed so that the same

concentrations of dye and reducer are feeding to the machine every minute,otherwise the color will

change. Management of the feeding mix requires an understanding of the chemistry of reduced dye

solutions, measurement of concentrations and skill in correcting strength losses of ingredients in the

feeding mix especially of reducing agent and alkali. There are 2 simple , but special test methods

to measure the concentration of alkali and sodium dithionite in the feeding mix: a 2-endpoint

titration for alkali and the glass plate test which have been in use by the best denim companies for

over a century and will be covered in a future article.

Conclusion:

The problem of Indigo color variations is principally a result of inconsistent dye and chemical

concentrations going to the machine. A glance at the design of flow of dye and chemicals into Indigo

dye machines should make this obvious. The multiple dye box arrangement and circulation in the

dyeing section of indigo machines allow the blending of indigo and reducing agents, so the problem of

variation obviously starts at the mixing tank.

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Indigo Dyeing With Loop Dyeing Machinery November 4th, 2010 by Harry Mercer | Filed under Manufacturing Process.

The name “Loopdye” results from the method of skying or air passage for oxidizing the Indigo-dyed

yarn and the method of passing through the Indigo dye. On the other 2 important Indigo machine

types, the dyed yarn is passed through from 6-8 Indigo boxes on rope machines or 6-20 Indigo boxes

on slasher (sheet) Indigo machines, multiple dye boxes being necessary for dark shades because only

a small amount of Indigo can be applied in each immersion. After immersion in each Indigo dye box,

the yarn is conducted through the air after each box, where the reduced Indigo (yellow-green) is

oxidized or “fixed” by oxygen in the air returning to the original blue, then the yarn enters the next

dye box, passes into the air and so forth until the required depth of shade is developed.

In the case of rope and sheet ranges, this oxidation takes place above each dye box. In the Loopdye

process, there is only a single Indigo box through which the yarn passes 4-5 times. The white

cotton is pulled into the front of the machine and passes first through the pre-treatment boxes, then

moves through a reactor which can be used for steaming or additional reaction time for sulfur-

bottoming or Mercerization, followed by washing. The wet yarn then enters the Indigo dye box. When

the yarn exits the dye box, instead of moving forward, the yarn is carried to the rear of the machine,

around the top and rear of the yarn creel from where it started, passes under the yarn creel where it

is returned to the Indigo box for another dye passage. This continuous passage of yarn between

the yarn creel and the dye box is in the form of a “loop” which is almost circular. After

making multiple loops through the Indigo dye box the yarn is conducted through wash boxes and on

to drying cylinders. The Loopdye machine is a simplified version of a “sheet” or “slasher” Indigo

machine. After drying the Indigo-dyed yarn, the yarn passes directly to sizing where the yarn is

prepared for weaving. Because the sizing part of the machine must stop in order to remove a

completed weaving beam, in order to prevent the dyeing unit from stopping as well, there is a yarn

accumulator between the drying cylinders at dyeing and the wet-size boxes. When the yarn stops

moving on the sizing unit, a series of parallel cylinders begin to move apart allowing the yarn from the

dye unit to continue through dyeing and allows the size machine approximately 2 minutes of time to

install an empty weaving beam and re-start the sizing machine.

Loopdye Machines in the Denim Industry

In the early 1990‟s, thee were approximately 30 Loopdye machines in use. Currently, the number is

reported to be 60 or so. The biggest concentration of these machines is in Brazil. Vicunha employs 11

of these machines, Canatiba, Santana and Cedro have 2 units each, while Tavex, Tear, Textil

Kafi, Santista have 1 each. There are 9 of these machines that have been equipped with

nitrogen units which use nitrogen gas as protective blanket over the surface of the Indigo dye. The

nitrogen gas prevents oxygen in the air from attacking sodium hydrosulfite resulting in more

consistent dyeing and reducing consumption of hydrosulfite, lowering costs and pollution. There are

other claimed advantages such as higher speeds and darker Indigo color.

Advantages and Disadvantages

1. Productivity – When compared to a multi-box slasher machine, productivity is essentially equivalent since the yarn loading, start-up times and speeds are similar. Rope dyeing machines can produce up to 4 times as much dyed yarn.

2. Capital Investment – The Loopdye machine has the lowest initial costs of continuous Indigo dyeing machinery, currently reported to be approximately 25% less than 8 dyebox slasher machine.

3. Operating Costs – Maintenance and energy costs are reported to be approximately 20% lower with Loopdye when compared with slasher dyeing and even lower than with rope dyeing.

4. Space requirements – The Loop machine with a single dye box requires less floor space than either sheet dyeing or rope dyeing. Rope machines also require higher ceilings because of the design of the airing arrangement.

5. Indigo Dyeing Quality – The newer designs of Loopdye are reported to have little of the problems with Cross-Shade (side-to-side) shading than with slasher dyeing equipment. Indigo consistency from the start-to-finish of dyeing can be expected to be better with the inclusion of nitrogen units. Rope machines still have an overall advantage in terms of Indigo dyeing quality, but this may be overcome by employing improved chemical blending.

6. Sulfur dyeing – The Loopdye machine can be equipped with a steamer for cold-pad sulfur bottoming which will provide greater consistency than a hot application in the 1st box. The Loop machine is not provided with enough boxes after Indigo dyeing for sulfur topping as the slasher dyeing is. With the newer methods for cold-sulfur dyeing, the Loop machine is ideal for sulfur colors since it the dye can be applied in only one box, which allows for faster color changes and less dye discarded after the dye lot is finished. Rope machines still have the greatest flexibility with regard to producing a full range of denim colors.

7. Weaving Efficiency – The methods of dyeing, especially of sulfurs, has a direct effect on warp yarn breakage in weaving, which lowers operating efficiency as well as fabric and garment quality. Experience with the older design of Loopdye machines demonstrated higher levels of warp breaks in weaving than other Indigo machines. Rope dyeing results in the lowest-level of weaving stops, largely because yarn breaks in dyeing can be repaired at long-chain re-beaming.

8. Versatility – In the higher denim fashion market, some companies like Vicunha have had success using a combination of Loopdye and slasher dyeing. Overall, the slasher dyeing with its greater number of application boxes offers more flexibility in product development, while rope dyeing provides the greatest flexibility for denim product development.

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Rope Dyeing Vs Slasher (Sheet) Dyeing May 19th, 2011 by Harry Mercer | Filed under Manufacturing Process.

This is a guest post by Harry Mercer

Until 1915, most Indigo dyeing was conducted in skein machines for cotton or loose fiber dyeing for

wool. Skein dyeing of Indigo is still the best method for dyeing Indigo on very fine yarns for the

delicate high-fashion fabrics. In 1915, the first rope dyeing machine appeared and only in the 1970‟s

was sheet dyeing introduced. The relative advantages of rope as opposed to sheet Indigo machines is

a common subject of debate. Based on my 30 years of experience in this area, including as a

consultant in about 40 denim operations worldwide, here are some basic observations that I have

made in companies that had only sheet or rope dyeing, but also in many denim companies that

employed both. These evaluations included mass-balance studies, benchmarking, weaving efficiency

and overall fabric quality.

A. Lower consumption of reducing agent per kilogram of yarn.

The primary reducing agent utilized in Indigo dyeing is sodium dithionite, commercially known as

sodium hydrosulfite. The amounts of this reducer that are consumed in Indigo dyeing are greatly in

excess of what is necessary for the Indigo dyeing itself. In explanation, in order to reduce 100

kilograms of pure Indigo so that dyeing can proceed, only about 66 kilograms of 100% hydrosulfite

are required for the basic reaction. The amount of hydrosulfite actually consumed in Indigo dyeing is

often3 or 4 times this amount. There is often hydrosulfite wasted incurred in the initial mixing of the

stock mix, due to excessive stirring or concentrations of hydrosulfite greater than 80 grams per

liter which promotes anerobic decomposition.

A great deal of hydrosulfite is lost because of contact with atmospheric oxygen at the surface of Indigo

dye boxes as a result of aerobic decomposition. The surface losses of hydrosulfite are related to

the volume and surface area of the dye boxes, with approximately 15% loss in larger Indigo

boxes found on rope ranges and 50% or more in the smaller dye boxes found on sheet ranges.

The scientific explanation for this phenomenon is related to what is known as Specific Surface Area

(SSA). The greater the SSA (the quotient of the surface area and volume), the more rapidly the

sodium hydrosulfite is oxidized. The time for half-oxidation (50% loss) is inversely proportional to the

SSA, which means that decomposition is slowest in a large dye tank with a relatively small surface

area. There are other factors involved such as the initial concentration of hydrosulfite in the dye boxes

– a higher initial concentration decomposes more slowly. However the most significant source of loss

is through surface contact and air brought into the dye tank by yarn.

The instability of hydrosulfite in smaller Indigo boxes is also the primary cause of color variation in

Indigo dyeing, which on rope ranges is much better controlled. It should also be noted that rope

ranges have the advantage in regards to Cross Shade Variation(CSV), which refers to differences in

color from side-to-side in the fabric. CSV is basically a result of dye circulation system design where

the Indigo enters the dye box from the side instead of the front. In rope ranges that are designed with

that style of circulation there is also some difference in the yarn color from to side-to-side, but unlike

sheet ranges where the yarns are fixed in their final fabric position, the yarn ropes can be blended to

remove the side to side effects. There have been some newer designs of Loop indigo machines which

have greatly improved CSV.

B. VERSATILITY IN DENIM PRODUCT DEVELOPMENT

Rope ranges have been designed to apply the widest range of dyeing techniques. For example, the

Spectrum Dye Machine available from Morrison contains features like additional steamers and drying

sections that allow not only the standard dyeing techniques of sulfur bottoming and topping, but also

consistent application of all other cotton dyes such as vats, reactives and directs in combination with

Indigo or dyeing yarns with these dye classes only.

Also available are specially designed dye boxes that allow the simultaneous dyeing of 2 different sulfur

applications, such as one set of yarn with a sulfur topping and the other set without topping, or with

only a sulfur color, which allows flexibility in production. Rope ranges are also easily adaptable for

random effects such as space dyeing of yarn. With the rope design, yarns from different dyeings such

as Indigo only and sulfur only, can be blended for producing stripe patterns.

C. HIGHER PRODUCTION AND FABRIC QUALITY

Common methods of operating Indigo machines have a damaging effect on yarn quality which results

in very high warp breaks in weaving, lowering efficiency and increasing off quality. Yarn on the

machines is made weaker as yarn tension increases. Sheet Indigo machines, because they are

attached to size machines, have very high levels of yarn tension and therefore higher weaving

breaks than yarn dyed on rope ranges. A yarn quality that would result in 10 warp breaks per million

weft insertions without Indigo dyeing often will have around a break level of 200 with sheet dyeing,

but as low as 15 if processed on rope machines. This is because tension on rope machines is

much lower and can be easily controlled at very low levels.

Another important cause of high weaving breaks in denim is dirty yarn – the cleaner the yarn the

higher the weaving efficiency. This is because chemicals not washed from the yarn after Indigo dyeing

result in bad sizing and lower protection of warp yarns. Wash boxes on rope ranges are typically

more efficient than the smaller wash boxes on sheet ranges that use overflow washing

methods. The importance of washing the yarn dictates that it is better not to apply softeners in the

final box for rebeaming efficiency which is optimal though improved washing and moisture control

after drying.

The need for a separate rebeaming step in rope dyeing is often considered objectionable in rope

dyeing, but this is actually an important advantage, since yarn breaks can be repaired at rebeaming

resulting in higher weaving efficiencies. Yarn breaks from warping and dyeing cannot be

repaired in sheet machines because they are passed directly from dyeing to sizing.

D. FLEXIBILITY IN PRODUCTION

Sheet ranges are usually limited to producing yarn for only 1 weaving set at a time. In a rope range,

normally 12 ropes will produce enough yarn for a weaving set and because rope ranges do not pass

the yarn directly to the size machine, from 1 to 50 ropes can be dyed at one time. Any

combination of yarns can be processed for completely different fabric constructions at the same time

and dyed with the same Indigo color. Also, rope ranges can be operated continuously without

stopping, which avoids the waste of yarn which occurs when sheet ranges must stop in order to

change yarn lots. Because the yarn is sized separately.higher priority fabric orders can be processed

without delays resulting from the need to complete a dye set as with sheet dyeing.

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Dyeing Process basically discusses what you do with the dyes. There are today available latest and state-of-

the-art dyeing methods that effectively colour the various substrates. This is a very critical operation carried out

in the Dye houses in a series of steps. The pages here gives precious information regarding the various dyeing

processes in different industries.

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Dyeing Process » Continuous Dyeing Process

Batch Dyeing

Process

Continuous Dyeing

Process

Semi-continuous Dyeing

Process

Pigment Dyeing

Process

The working of a continuous dyeing process is described here. The textile substrates are feeded continuously into a

dye range. The speeds can vary between 50 to 250 meters per minute. According to Industry estimates Continuous

dyeing is a popular dyeing method and accounts for around 60% of total yardage of the products that are dyed.

A Continuous dyeing process typically consists the following. Dye application, dye fixation with heat or chemicals and

finally washing. Continuous dyeing has been found to be most suitable for woven fabrics. Mostly continuous dye

ranges are designed for dyeing blends of polyester and cotton. The step of padding plays a key role in the operation

of continuous dyeing. Sometimes Nylon carpets are also dyed in continuous processes, but the design ranges for

them is unlike that for flat fabrics. Warps are also dyed in continuous process. Very good examples of such warp

dyeing are long chain warp dyeing and slasher dyeing using indigo.

A continuous dye range has been found useful and economically sustainable for dyeing long runs of a given shade.

One important factor that separates continuous dyeing from batch dyeing is the tolerance factor for color variation.

That is more for continuous dyeing as compared to batch dyeing. This is so because of two reasons a) the speed of

the process. b) presence of a large number of process variables which affects dye application. The process that is

illustrated below is designed for dyeing of blended fabric of polyester and cotton.

Some of the popular methods in continuous dyeing process are Pad-steam, Wet-steam, thermosol dyeing, TAK

dyeing, space dyeing, and pad-steam dyeing long chain warp dyeing etc.

Optimizing the Continuous dyeing Process

Continuous and to some extent semi-continuous dyeing processes both are less prone to water consumption than

batch dyeing, but results in high concentration of residues. If some strict control measures are taken up it is possible

to reduce this losses of concentrated liquor. The following steps may prove useful.

Applying low add-on liquor application systems along with minimising of volume capacity of the dip through

when pad dyeing techniques are in operation.

Adoption of latest dispensing systems, where the chemicals get dispensed on-line as separate streams.

They gets mixed only at the moment just before the delivery to the applicator.

Using any of the following systems for dosing of the padding liquor. Important to know that it should be

strictly according to the measurement of the pick up:

o A proper measurement of the dyeing liquor quantity consumption in comparison to the processed

fabric. The resulting values thus obtained are processed automatically and applied in preparing the

next comparable batch.

o Application of the technique of rapid batch dyeing. Here the dyestuff solution is prepared just in

time, with steps that are based on on-line measurement of the pick up. This proves better than

those dyestuff that is kept prepared already for the whole batch before the commencement of the

dyeing batch.

To increase washing efficiency based on the proven principles like reduction of carry-over and counter-

current washing.

What gives Carpet its vibrant colour?

The carpets that you see in different colour and hues is dyed by a continuous dyeing. A

continuous dye range for carpet typically consist of a steamer and a dye applicator. Generally

acid dyes are used. Carpet manufacturers are very adept in application of dye for producing

special color effects on their product. As a result of this, many variations of dye applicators exist.

Under normal circumstances, a very high liquor ratio is must to produce good quality dyeing of

carpet. Typically, application method is used to meter the dye solution into the carpet. Patterned

effects are produced when the stream of dye that is metered onto the carpet are momentarily

interrupted. Streams of variety color dyes are applied in different patterns to create those special

effects.

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Continuous Processing of Cotton woven fabric: A general process sequence may be as below:

PRETREATMENT PART:

1. Singe & Desize (Semi Continuous process)

2. Scouring or Boiling

3. Peroxide Bleaching

4. Neutralization

5. Drying

6. Mercerising and washing off to a pH of 6

7. Neutralizing

8. Drying

9. Ready For Dyeing (RFD stage).

1. Singeing & Desizing:

a. Recipe: Bactosol HC = 5 g/l

b. Common Salt = 5 g/l

c. Wetting Agent (100%)= 3 g/l

i. Do cold padding with the above chemicals, batch,

and keep rotating the batch for 8 hours;

ii. Desize washing in 4 compartments @

90mts/minute speed.

2. Bleaching (Continuous Process)-

a. Caustic Impregnation:

i. Recipe:

1. Caustic Soda = 70gram/liter

2. Lissopal D paste = 10 gram/liter

a. 4 dips – 1 nip @ 90 mtrs/minute speed.

b. Steaming @ 105°C for 45 minutes in the steaming

chamber.

c. Cold Wash in 2 compartments (4 dips – 1 nip) x 2

b. J-BOX Bleaching optional

i. Recipe:

1. Sodium Hypochlorite = 2 g/l Avl Cl2

2. Dwell time in J box – 30 to 45 minutes at room temp.

3. No washing

c. Peroxide Impregnation:

i. Recipe:

1. Hydrogen Peroxide(50%) = 3 ml/liter

2. Stabilizer = 0.5 gram/liter

a. 4 Dips – 1 Nip at cold.

b. Steam in Peroxide Steamer for 45 minutes.

3. Washing in 5 compartments with each 4 dip and 1 nip.

4. Dry and batch on A frame roller.

3. Mercerisation: (Chainless Merceriser)

i. Recipe:

1. Caustic Soda = 280 to 300 gpl

2. Permenol N = 10 g/l

a. Cold Padding Speed:

i.

Bleached Fabric = 25 mtrs/minute

ii.

Grey Fabric = 15 mtrs/minute

iii.

Scoured Fabric = 20 mtrs/minute

b. Mangle Pressure: 3.2 to 4 kg/cm2

c. Liquor Pickup : 110%

d. Recuperation:

i. 1st compartment – strength = 8 to 27 gpl

ii. 1st compartment – temp = 90 to 95°C

iii. 2nd

compartment – strength = 1.6 to 6.4 gpl

iv. 2nd

compartment – temp = 65 to 95°C

e. Wash Tanks:

i. T

ank 1 – 50 to 55°C

ii. T

ank 2 – Acetic Acid Neutralizing

iii.

Tank 3 – 60 to 70°C

iv.

Wetting Agent room temp

f. Final fabric pH – 5.5.to 6.0

g. Drying:

i. F

abric is dried in cylinder or stenter and batched as

RFD.

4. Continuous Dyeing Method for 100% Cellulose:

a. There are 3 methods:

i. Pad – Dry – Chemical Pad – Steam

1. Dye Liquor Preparation & recipe:

a. Cotfix Dyestuff = x grams/liter

b. Wetting Agent = 2-3 gram/liter

c. Antimigartion agent = 8-10 gram/liter

d. Mild Oxidizing Agent = 5-10 gram/liter

2. Padding Temperature should be < 35°C

3. Liquor Pick up% = 60 to 80

4. Chemical Pad Recipe:

Dyestuff G/L Caustic Soda Salt (g/l) Soda Ash

(g/l)

< 20 Nil 200 20

30 – 40 Nil 250 20

> 40 Nil 250 20

5. Chemical Padding Temperature < 35°C

6. Liquor Pickup 70 to 80%

7. Steaming 45-90 seconds with

Saturated steam (101 to

105°) the steam should be free

from acid.

ii. Pad-Dry-Steam:

1. This is carried out on a padding mangle attached with either Hot

Flue or Flow Dried. To fix the dyestuff, the dried material is

steamed in the normal continuous steamer for 4 to 7 minutes at 100

to 103°C.

2. PAD LIQUOR PREPARATION for VS dyes:

Dyestuff G/L 10 20 30 40 50 >50

Urea G/L 0~50 0~50 0~50 50~70 50~75 50~75

Sodium

Bicarbonate

G/L 5 10 15 15-18 20 20-25

Resist Salt G/L 10 10 10 10 10 10

3. Take all the precautions and conditions mentioned for Pad –Dry-

Chemical Pad-Steam.

iii. Pad-Dry-Thermo fix Process:

1. Here again there are 2 processes:

a. Soda Ash Method

b. Sodium Bicarbonate Method

PAD LIQUOR PREPARATION for VS dyes

Dye G/L 2 10 20 40 60

Urea G/L 15 30 45 60 80

Soda

Ash(OR)

G/L 8 12 17 25 33

Sodium

Bicarbonate

G/L 12 18 27 36 45

The material is padded in the above liquor at room temperature, dried & thermo fixed as follows:

Temperature Soda Ash

Process

Sod.Bicarnonate

process

100°C 4-6 minutes 5-7 minutes

120°C 2-4 minutes 3-5 minutes

140°C 40-60 seconds 60-90 seconds

5. After treatment:

a. Overflow Cold wash in soapers 4 dip 1 nip in 2 compartments

b. Neutralization with Acetic Acid in the soaped at cold with 2 ~3 g/l Acetic acid.

c. Hot wash in one compartment (soaper) @ 80° , 4 dip 1 nip in

d. Soaping with 2 g/l of soaping agent (Sandopur RSK) at 90°C

e. 2nd

soaping with 1 g/l soaping agent (Sandopur RSK) at 90°C

f. Hot Wash

g. Hot Wash

h. Cold Wash

i. Acetic Acid Treatment 1 g/l or Formic Acid treatment 0.5 g/l

j. Squeezed on the last mangle and either plaited or wound on a batcher and taken

for drying.

Possible Faults that may occur during the above process and check points/rectification:

1. Tailing: The Alkalinity in the mercerized fabric would be un-uniform ranging

from 0.25 to 1 and above gram/liter due to inefficient recuperation. That

means more and more alkalinity would be carried over on the

mercerized cloth and that can be checked in lab by titration. Any

variation in the alkalinity of the mercerized fabric can show up through

the length by the dye affinity difference and that is tailing.

a. Check Point:

i.

Alkalinity of the mercerized cloth has to be

quantitatively checked in the lab at intervals of 500

meters. It must be uniform.

2. C/S Variation – may arise due to

a. Desizing – Expression of the squeezing mangle is to be checked

at frequent intervals.

b. Caustic Impregnation - Expression of the squeezing mangle is to

be checked at frequent intervals.

c. Peroxide Bleaching:

i. S

team Pressure

ii. S

team Temperature

iii.

Time duration of the steaming are to be controlled.

ii. Mercerising – Impregnation Mangle expression

has to be checked.

1. Recuperator Efficiency has to be checked at every 1000 meters.

2. Final Alkalinity of the mercerized fabric should be less than 0.25

g/l of caustic.

3. Soaper washing – Acid for neutralization should be prepared in

volume and should be fed in to the neutralization tank

continuously.

4. To counter check, the fabric absorbency may be checked in the

center and the two selvedges at regular intervals.

3. Face to back variation

a. Mangle Expression in the dye padding should be 70% and chemical padding

should be 80%.

b. Thermosol temperature difference between faces may result in migration of

dyestuff.

c. In a 2 dip 2 nip mangles, if the expression of both the mangle are not identical,

then back to face variation is possible at any stage, dyeing, finishing etc.

d. In stenters, if the hot air blowers are not functioning, there may be face to back

variation.

e. In jigger dyeing machine, out of the two bottom rollers, one is not rotating

properly then back to face variation is possible.

4. Specky Dyeing:

a. Improper dissolution of dyestuff:

i. The quantity of the dyestuff is to be pasted,

say for 1 kg of dyestuff, 5 liters of soft water should be used for pasting.

Then 1 kg of urea is dissolved in 5 liters of water and this solution is

added in the paste and stirred well to make a uniform lump less slurry.

About 40 liters of hot water at 85°C is added to the slurry which is kept

under a high speed stirrer. The hot water should be added slowly to the

rotating vertex of the dye slurry. The filter this dissolved dyestuff

through a fine gauze.

ii. Temperature of dissolution:

1. M-Brand = 50°C

2. V.S = 80°C

3. ME, XL, HE = 80°C.

iii. The dissolved dyestuff may be spotted on a

filter paper - a uniform circle shows proper dissolution.

http://www.thesmarttime.com/faq/continuous-dyeing.htm

Dyeing of Reactives by Exhaust Method

DYEING OF REACTIVE DYES BY EXHAUST METHOD

REACTIVE DYES

EXHAUSTION PHASE

Primary Exhaustion Phase

Adsorption

Diffusion

Substantivity

REACTIVE DYES AND DIRECT COTTON DYES

Direct Cotton Dye

Reactive Dyes

Role of Electrolyte

Partition /Distribution Coefficient and Degree of Exhaustion

Liquor Ratio

Temperature

Influence of pH.

Influence of Substantivity

Migration phase

Secondary Exhaustion

Hydrolysis of Reactive dyes

Typical Examples

REACTIVE DYES

Choice of Reactive class of Dyes has become indispensable for application of

colours on the cellulosics to provide bright range of shades with reasonably

good fastness features. No other class of colours can boast of the versatile

range of shades with unmatched brilliance, yet economically viable and cost

effective that this class of dyes can offer. Even as Reactive dyes are most

popular for dyeing solid shades it is equally sought after for various resist

and discharge printing styles, thanks to its suitability to be resisted or

discharged readily and effectively

The reaction mechanism is apparently simple in that on just altering the pH

after exhaustion, formation of covalent bonds between the reactive group of

the dye and the OH of cellulose proceeds. For the same reason of ready

reactivity with Cell OH groups, it reacts with Water also to get hydrolyzed in

which state the dye behaves no better than a direct cotton dye. The

management of the various factors/variables that govern the transport of

dye uniformly from an aqueous bath to the cellulose substrate and its

preferential reactivity to the fibre than to water is far more complex and

critical to perform to obtain a satisfactory dyeing. As the shades invariably

are tertiary matchings, the behaviour of individual dyes with different

exhaustion and reactivity characteristics, all the more compounds the

complexity of the problems of differential shade build up, variations, uneven

dyeings, reproducibility, fastness etc multifold.

Though there are other methods of dyeing „Reactives‟ like pad batch, pad –

dry-cure or pad-dry-steam etc exhaust dyeing is practiced widely because of

its flexibility to process fabrics in rope form and in the case of yarn and

other packages, exhaust dyeing is the only alternative as on date. Tubular

knit-ware, by its very physical form is more amenable to exhaust dyeing in

„rope„s form; however, advanced machineries obtainable in recent years

claim satisfactory open width dyeing by Pad Batch technique.

The exhaust method of dyeing would include the following phases

1. Primary exhaustion phase /Migration 2. Secondary exhaustion phase, 3.

Fixation (Reaction) phase -Secondary exhaustion and Fixation can run

concurrently/over lapping. 4. Washing off phase.

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EXHAUSTION PHASE

Primary Exhaustion Phase

Exhaustion of dye from the dye bath to the cellulose during Primary

Exhaustion phase is governed by the following three physical processes and

the phenomenon of substantivity

Adsorption Diffusion, Absorption/ Exhaustion/Migration

Adsorption

It would be relevant to briefly look at cellulose structure with respect to its

Hydrogen bonding behaviour at the surface layers and in the interiors of the

cellulose micro fibrils The interior layers contain both forms - 1Alpha and 1

Beta of Cellulose molecular chains that are packed compactly and there are

intra molecular Hydrogen bonding parallel to the 1.4 Beta Glucoside link

(OH of #2 to #6 of the succeeding glucose unit and #3 OH with the ring O

of the preceding Glucose Unit) that stabilize the cellulose chain.

The other four hydroxyl groups are fully free for Hydrogen bonding. At the

surface layers of cellulose even the O-3 (OH) and 2-6 Hydrogen bondings

are reported to be absent and therefore all the six Hydroxyl groups in the

Cellobiose repeat units at the surface are free to attract Hydrogen bonding

with the water molecules.

Adsorption in an exhaust dyeing process is fundamentally the inter-phase

phenomenon of a dye (solute) in its solution in water coming in to surface

contact with the substrate and forming a surface layer/ coating. That is the

starting phase for the rest of the diffusion and absorption phenomenon. In

the case of Cellulose exposed to a dye solution in water at slightly acidic pH

there is no ionization of cellulose. However, with abundance of „free‟ OH

groups available at the surface (six numbers in each of the repeat Cellobiose

unit), water molecules are drawn in clusters around the cellulose molecules

to form hydrogen bonds causing an overall charge separation. Resultant

surface thus carries a negative charge known as the zeta potential

This surface negative charge would repel the advances of the negatively

charged ionized dyestuff anions. The zeta potential is partially overcome due

to the presence of large amount of dye anions, some of which are forced

across the electron cloud through increase in energy (raise in temperature)

or through mechanical agitation to come within the effective distance for the

inter molecular forces like Wander Vaal‟s forces/secondary valence forces to

facilitate the dye anion to get adsorbed on the surface of cellulose. Presence

of electrolyte also helps in providing the positive charge that can effectively

neutralize the zeta potential and improve the adsorption. (Discussed under

„Role of Electrolyte‟)

Diffusion phenomenon takes over followed by the absorption and migration

of dyestuff across the cellulose membrane. Diffusion is influenced by the

concentration gradient across the interface of cellulose surface and dye

bath, the surface area of the cotton substrate in contact with the dye bath,

temperature and time and the physical characteristics of the substrate. This

is termed as the primary exhaustion phase. The term exhaustion would

include the collective phenomenon of adsorption, absorption diffusion and

migration in that order.

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Diffusion

Diffusion process is explained by the relationship (Ficks Law of Diffusion in

its simplest form.)

F = -D (C1-C2) / L And D = Do e -E/RT

Where

F = Mass flow of dye gms/cm2 sec

D = Diffusion coefficient of the dye m2/sec

D0 = Diffusion Coefficient at Infinite Temperature

C1 = Concentration of dye in the dye bath g/cm3

C2 = Concentration of dye on surface of the fiber g/cm3

L = Thickness of the layer cm

e, E, R = Constants (E activation Energy; e exponential; R Universal Gas

Constant)

T = Temperature Kelvin

Applying the above relationship the following dynamics may be inferred

during the diffusion / exhaustion stages of the dye to the cotton substrate.

F is the dyestuff sorbed across Unit

area of the fiber surface in unit

time (Rate)

Greater the surface area of the

fiber in contact with the dye bath

greater is the dyestuff sorbed.

(C1-C2) concentration gradient

during the process of diffusion.

The concentration gradient at the

initial stages would be higher and

therefore the rate of dyestuff

transport to the fibre phase will be

correspondingly higher tending

towards zero at equilibrium.

D Diffusion coefficient Higher the Diffusion coefficient,

lesser the time taken to reach the

equilibrium. Time taken for dyeing

50% of the equilibrium depth of

shade is an index of the speed

Temperature Increase in Temperature increases

Diffusion coefficient.

Since surface area is a factor, the characteristics of the fiber and

construction would influence the diffusion. Nature of cotton from different

sources would have different shape, cross section, micronaire, fineness,

impurities, etc and different packing densities of the cellulose molecular

chains thus altering the surface area characteristics. The corollary is that

thinner the fibre/count and lower the density factor greater is the surface

area available and better would be the diffusion.

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Substantivity

The term substantivity is primarily a measure of the amount of the

molecular dye chromophore that can penetrate/diffuse into the interstices of

cellulose micro fibrils assisted by physical forces from an aqueous dye bath.

This is influenced by the salt concentration in the dye bath, the liquor ratio,

the temperature and the fibre surface area characteristics, besides the

chemistry of the dye chromophore. Substantivity ratio is the unit

concentration of dye on the fibre to the unit concentration of dye in the bath

at the equilibrium state (both expressed in the same units)

The process of primary exhaustion proceeds to its limiting values dictated

by the substantivity beyond which it ceases. In the absence of salt, the dye

uptake by substantivity phenomenon as stated above is around 20 to 40%

of the starting bath concentration or lower, a figure far too low to have any

significant economically feasible colour yield. Therefore, as a general rule,

without salt additions, substantvity by primary exhaustion of Reactive dye

to cellulose cannot be improved or maximized, at the present status of

Colouration technology.

[Efforts are on for reduced salt /salt-less systems based on changes in the

chemistry of the dyes to exhibit reduced anionic behaviour, fibre substrate

modification/sensitization to display cationic behavior to induce exhaustion

with less/no salt, while retaining the reactive system for the ultimate

fixation. Such developments are still in the R&D Labs and not presently

available for bulk]

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REACTIVE DYES AND DIRECT COTTON DYES

Reactive and Direct Cotton dyes sport similar dye chromophoric structures

but for the Reactive groups present in the Reactive dyes as opposed to

Direct cotton dyes. The Reactive dyes are smaller sized more akin to Acid

class of dyes (not necessarily as a general rule) with Reactive groups.

Direct Cotton Dye

Direct Cotton Dyes molecules are engineered to include some or all of the

important features listed below 1. More number of hydrogen bonding

groups, groups that would facilitate inter molecular attraction / diminish

repelling forces and groups that can chelate with hydroxyl groups of the

Cellulose 2. Molecules of sufficiently large enough size and shape that on

aggregation could get trapped in the interstices of the Cellulose molecular

chains thus difficult to be removed/washed off.. 3. Optimized number of

solubilizing groups (invariably „-SO3Na‟), just enough for the dye to go in to

aqueous solution. Dyeing is invariably carried out at boil, to provide the heat

energy to facilitate diffusion and migration. Higher temperatures can also

cause de-aggregation and consequent de-sorption Since the dyes have good

substantivity due to affinity caused by physical forces like Hydrogen

bonding, metal chelation etc. there is less propensity to desorb and higher

temperatures facilitates migration within the substrate forming the same

physical bonding at new sites (High substantivity always causes an initial

„strike‟ – aggregation of colour in most favourable loosely packed sites and

migration to other sites to increase uniformity in dyeing is facilitated only by

imparting energy.) Fastness characteristics are just adequate even for the

most satisfactory dyes of its class due to bonding only by physical forces

that are relatively week to the more powerful covalent bonds.

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Reactive Dyes

Reactive Dyes are capable of forming chemical covalent bonds with the

Hydroxyl groups of cellulose fibre and therefore, better anchored to the

substrate and not depend on the relatively weak physical forces to give

better levels of fastness. All of the features that are desirable for a

reasonably „fast to wash‟ Direct Cotton dyes are not essential for Reactive

class of dyes (because of the more strong covalent bond), though cannot be

totally discarded as undesirable. Some of them could be counter productive.

For example, Reactive Dyes with features listed under I and 2 of the Direct

cotton dyes would exhibit problems of low migration and or difficulty to

wash off the hydrolyzed dye. Certain quantity of Hydrolyzed dye is

inevitable after the fixation stage and non removal of such unfixed dye

would entail bleeding/staining of white during washing. Migration is

facilitated by increase in temperature; but higher temperatures induce

hydrolysis of Reactive dye during the fixation phase and therefore it would

be necessary to bring down the temperature to the most favourble

temperature for the reaction between dye stuff and substrate before alkalie

addition can be made. There fore, in the case of Reactive dyes the following

aspects are most important 1. Degree of Exhaustion of the dye bath on to

the fibre (both primary and secondary) that is directly related to the

substantivity should be maximized /optimized (assisted more by salt

addition than by the physical forces). 2. The migration of the dye within the

substrate during the primary exhaustion phase should be maximized. 3.

Efficiency of reaction of the exhausted dye to the fibre should be maximized

during fixation phase. 4. The kinetics of reactivity has the final influence on

the success of dyeing, irrespective of high levels of success achieved in the

exhaustion stages, though exhaustion is an important (primary and or

secondary) pre-requisite... 5. The above four aspects need to be performed

within a reasonable span of time. 6. The corollary here is that the extent of

hydrolysis of the dye during exhaustion and fixation stages needs to be

minimized.

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Role of Electrolyte

Addition of electrolyte induces exhaustion both its rate and extent. Where

the substantivity is lower the prime driving mechanism for diffusion

/exhaustion of dye into the fibre is the concentration gradient across

fibre/liquor interface and presence of common ion- i.e. electrolyte (Salt).

The electrolyte, say, Sodium Chloride dissociates in water into Na+ and Cl -

and Na+ has higher propensity to travel to the fibre /water interface and

neutralize the negative charge thus facilitating the free transport of dye

anion to be adsorbed onto the surface of the fibre and the subsequent

diffusion/ absorption (exhaustion) to take place. Secondly, the dissociated

NaCl ions are more associated with water than with the large molecular dye

Chromophore with a few SO3Na or other solubilizing groups and thus

occupy the limited available sites in the water effectively displacing the dye

Chromophore.The distribution coefficient of dye therefore shifts towards

fibre. It is not the quantity of the salt but its concentration that influences

the degree of exhaustion. The degree of exhaustion increases with

increasing concentrations of Salt to a limiting concentration. Higher

concentrations of Salt result in aggregation of the dye in the dye bath itself

and hence „it is salted out‟ much in the same manner as in the manufacture

of the dyestuff and less and less monomolecular dyes are available for

reaching the fibre phase The optimal quantity of Salt in terms of

concentration depends on the chemistry of the dye, its molecular size, its

solubilizing groups, quality of water and the fibre substrate etc. Secondly,

dyes displaying higher substantivity in the absence of salt would need lesser

salt concentrations.

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Partition /Distribution Coefficient and Degree of Exhaustion

At a given liquor ratio and bath concentration of dyestuff and salt, the

exhaustion of the dye proceeds from the liquor phase to the solid phase

(cellulose) until it reaches an equilibrium. This state would be different for

different solutes (dyestuffs) and the factors that contribute to this variability

are their molecular size, ionic character, extent of hydrogen bonding groups,

inter molecular forces, temperature etc. Such equilibrium, where the

number of molecules absorbed is equal to the number of molecules

desorbed at the cellulose/dye liquor interface, can safely be assumed to

have been reached in a time span of infinity, i.e. at the end of Exhaustion

phase or Partition of the dye from the liquor phase to the solid phase at a

notional infinite time It is desirable that the exhaustion proceeds at a

satisfactory rate to achieve close to equilibrium exhaustion within a

manageable /practicable time span a condition that is influenced by

diffusion coefficient. Higher the diffusion coefficient faster the exhaustion as

discussed earlier under diffusion...

The Partition/Distribution coefficient of a solute between two phases is

calculated as the ratio of the concentration of the solute in one phase to the

concentration of the solute in the other phase under equilibrium conditions

Interestingly, at the equilibrium state of exhaustion where the

concentrations of dye on fibre and in the final bath tend to become steady

and constant, it is an established fact that as the dye bath concentration is

increased, the concentration in fiber phase at equilibrium though increases,

does not do so linearly but progressively diminishes giving relatively lower

distribution coefficient values.

Degree of exhaustion is the ratio of the total amount of dye present in the

cellulose at the end of exhaustion to the amount of dye present in the

original bath before the start of the exhaustion process.

Degree of Exhaustion in terms of distribution coefficient and liquor ratio is

given by the relationship

Where

E Degree of Exhaustion

K Partition coefficient

L Material Liquor Ratio

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Liquor Ratio

Recipe of x% owf (on weight of fabric) in terms of absolute quantity would

be present in the starting dye bath but its concentration in the dye bath

would vary depending on the liquor ratio

The recipe equivalent dye % on the fabric after the completion of dyeing

would not be x% but would tend towards x% - depending on the efficiency

of dyeing/the substantivity /reactivity of the dye. In an ionic kind of

reactions like Acid dyes on wool the degree of exhaustion would proceed to

almost to .100% subject to the dye present in the dye bath does not exceed

the saturation capacity of the reacting sites present in the substrate.- the

limiting degree of exhaustion in this case.

In a model scenario where the liquor ratio is changed to a higher

one: Amount of dyestuff expressed owf, when present in the higher liquor

ratio would register proportionately a lower concentration of the dye in the

starting bath and consequently lower concentration gradient at the fibre

liquor interface resulting in lesser rate of diffusion of the dye from liquor

phase to fiber phase

Numerical Example

Case Recipe

owf

Substrate

Weight

Amount of

dye on fibre*

Liquor

Ratio

Liquor

Volume

(Wt)

Dye

bath

Concn.

I 1% 100 Kgs 1.0

Kgs 1: 5 500L 2 gpl

2 1% 100 Kgs 1.0

Kgs 1:10 1000L 1 gpl

*Arrow indicates‟ tending towards‟

Only 50% of the dye molecules are available at the interface for adsorption

and diffusion in case 2.and therefore the rate of diffusion will be lowered

and it would take relatively far longer time to reach the equilibrium state.

In case 1 starting from 1:10 going to 1:5, the increased concentration of

dye in the bath would increase the rate of diffusion (increased concentration

gradient) and take shorter time for exhaustion.

The relationship E= K/ (K+L) as discussed under Distribution coefficient (K);

any increase in L would diminish the E –the degree of exhaustion.

Such a situation would entail higher starting concentration of the dye and or

increase in concentration of Salt to „occupy the available sites in water‟ (as

explained earlier under salt concentration) in a larger volume of water to

displace the dye anion to shift the distribution coefficient to the fiber phase.

But increased salt addition cannot always fully compensate for the adverse

exhaustion behaviour but only to a point (as discussed under Role of

Electrolyte) Therefore, not only increase in concentration of the dye, but

also that of salt will be necessary (barring certain marginal cases) -

quantitative aspects governed by the substatnivity characteristics of the

dyestuff.

Such a situation would be more pronounced in the case of low/poor

substantive dyes compared to the dyes with better substantivity. There are

ready reckoners for recipe correction available for changes in liquor ratios

from the dyestuff manufacturers but they are only for guidance. As

individual dyes would behave differently, an intelligent understanding and

application of the given information only can give meaningful results.

The corollary is that a change in liquor ratio would affect the least in dyes

with high substantivity and most in those with poor substantivity

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Temperature

Temperature of the bath is another factor influencing exhaustion As

explained earlier presence of salt increases the substantivity facilitating

aggregation of the dye in the fiber phase Increase in temperature in the

case of high substantive dyes as in the case of direct cotton dyes help in the

migration of the dye within the substrate but in the case of dyes that are

less substantive increase in temperatures could be counterproductive

Temperature up to 50 deg C contributes to de-aggregation of the molecules

of dye, both in fibre and water phases; but relatively less in fiber phase and

more in the water phase. Therefore the net effect is that there are more de-

aggregated monomolecular dye free to move towards the fiber phase than

that is desorbed from the fibre and therefore the exhaustion proceeds.

There is a maxima in the exhaustion curves of dyes of low substantivity at

temperature around 40 to 50 deg C. beyond which increase in temperatures

results in decreasing degrees of exhaustion explained by the higher degree

of de-aggregation of the dye in the fiber phase and lesser physical forces to

resist desorption, unlike in the case of substantive direct cotton dyes;

annulling the influence of salt..

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Influence of pH.

The pH is relevant to the Reactivity aspect and not considered as a factor in

the exhaustion process. Also, the dye bath pH during the exhaustion phase

is maintained at 5.5 to 6. As long as the pH of the bath is slightly acidic, no

reaction can take place and therefore primary exhaustion and bringing the

temperature close to the reaction temperatures can be carried out

conveniently.

Influence of Substantivity

High substantivity facilitates exhaustion process; also requiring less

concentration of salt for exhaustion but for the same reason migration of

the dye would be restricted resulting in unlevel dyeing. However dyes with

medium+ substantivity engineered to provide the balance in the molecular

structure to promote migration and good reactivity that matches the

exhaustion curve (primary and secondary) would give the best results both

in terms of dye yield and washing efficiency. Poor substantive dyes that are

also not sensitive to electrolyte additions are poor builders and therefore will

give poor yields.

High substantivity .dyes with low reactivity (Fixation) falling below the

exhaustion levels would result in high levels of unfixed and hydrolyzed dye

to be washed off and the dye and its hydrolyzed version also being highly

substantive, the washing efforts also will be high requiring more water,

energy and mechanical efforts

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Migration phase

Since fiber surface area is a factor in diffusion process, the exhaustion

would proceed to locations where relatively more surface area is presented

like in the amorphous areas and less densely packed crystalline areas in

that order in the cellulose and therefore the dye concentration within the

cellulose substrate would not be uniform/even. Such a situation would result

in uneven build up of the dye both in hue and intensity. In a trichromatic

mixture the situation could be worse.

The process of Migration of the exhausted dye depends on the molecular

size of the dye its spatial profile (Steric) and the solubilizing groups present.

The other external factors would relate to temperature, machinery used and

the package profiles and densities (in case of package dyeings).

Raising the temperature would provide the required thermal energy; but

cannot be increased arbitrarily due to limitations discussed under

„Temperature‟. Both exhaustion and migrations can be maximized

/improved by better mechanical agitations that would facilitate intimate

surface area contact of the cellulose with dye liquor and by improved flow

designs that facilitate better liquor exchange at the fiber liquor inter-phase.

Migration phase should precede the fixation phase as once the reactive dye

forms a covalent bond with Cell O- it is anchored strongly and cannot be

shifted.

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Secondary Exhaustion

The observations and inferences in the above deliberations related to

primary exhaustion in a Reactive exhaust dyeing process are incomplete

without the final fixation. When Alkali is added, the cellulose ionizes to form

Cell-O- and H+ (Cell O– Na+) and starts forming covalent bonds with the

reactive functional groups of the dye Chromophore. When more and more of

dye anions are covalently bond, the distribution coefficient shifts to fiber

phase effecting further exhaustion due to deficiency of dye anions in the

cellulose phase and dye bath concentration starts depleting further. The

degree of alkalinity in terms of pH plays a major role in shifting the fixation

of dye to its hydrolysis reacting with water. Any exhaustion during this

stage if it is hydrolyzed dye it would be far more undesirable In a reactive

dye system therefore, primary exhaustion alone does not govern the

efficiency of dyeing. The degree of secondary exhaustion also would

influence the efficiency. During the secondary exhaustion when alkalie is

added, there is a second reaction that also sets in motion in parallel ( i.e.

the hydrolysis of the Reactive dye with water) in competition to the fixation

of the dye that is the primary aim. The dye anion is equally facilitated to

react with OH of water to form the hydrolyzed dye in which state the dye is

as good as a direct dye with all its „undesirable‟ characteristics. It is the

reactive group in the dye, pH and temperature that influence the hydrolysis

of dye in preference to reacting with cellulose. It becomes critical that the

hydrolysis is curbed to maximize efficiency. The relationship between

temperature and reactivity is that higher temperatures require lower

alkalinity; to optimize on hydrolysis. They can be broadly grouped under

„High‟ „Medium‟ and „Low‟ categories requiring 40º C. 60 º C and 80º.C

respectively - levels of pH 12.5 for High (cold dyeing), 11.5 for Medium

(Warm) and 10 - 11.0 for Low (Hot Dyeing) for the reaction to proceed

more favorably towards the substrate. The term more reactive is used in the

sense that it requires lesser levels of alkalinity and lower temperatures (and

not the reaction itself. Given the right temperatures, alkalinity and time the

reaction proceeds to completion in all cases.)

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Hydrolysis of Reactive dyes

The most critical part of the Reactive dyeing is the actual fixation where the

covalent bond takes place between the Cellulose O - and the Reactive

group of the Dye Chromophore.

1. Cynuryl chloride based dyes

2. Vinyl Sulphones

Dye-SO2 –CH2-CH2-OSO3H (MINUS) H2SO4 Dye SO2-

CH=CH2

The electron attracting Sulphone group causes electron deficiency on the

terminal carbon atom enabling neucleophylic attack to take place. .

(Addition reaction)

Dye-SO2-CH=CH2 + O-R1- Dye-SO2-CH -- CH2-OR1

H (+) Dye-SO2-CH 2- CH2-OR1

Where [-O-R1] is [-O Cellulose] or [-OH] of water, etc.

The liberated acid in both the two reactions is continuously neutralized by

alkalie for the forward reaction to proceed during the fixation process.

.

Efficiency of Reactive dyeing (Rate of Fixation /Rate of Hydrolysis) for a

given exhaust dyeing process has been expressed in mathematical terms

making use of the competing First order /pseudo first order rate constants

of the reaction of the dye with the cellulose and the dye hydrolysis with

water , the equilibrium concentration of the dye on fabric and concentration

of dye in the aqueous phase (For details please refer Chapter 4 of „The

Dyeing of Cellulosic Fibres‟ by Maurice R Fox and Harry H Sumner Edited by

Clifford Preston 1986 - SDC Publication}

It has also been emphasized that the expression is too ideal and relates to

certain assumptions and conditions that are not practically achievable in the

real situation. However, the broad principles are applicable and the direction

of the reactions proceeds towards the ideal. To whatever extent the

variables can be controlled and maintained, the results achieved could be

optimized and also reproduced maintaining the same conditions and controls

every time.

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Typical Examples

Reviewing the critical variables that govern the dyeing Efficiency in a

Reactive dyeing process, the following few examples will highlight the pros

and cons of the factors discussed. 1. Low Primary exhaustion (P) and high

Reactivity (R) (incidentally higher secondary exhaustion) Where P is low and

Fixation is high

Initial exhaustion phase will not be

critical as less amount of dye is

transported. On addition of alkalie the

reaction starts and the secondary

exhaustion proceeds as more and more

of the dye takes part in the reaction.

During this phase the competing

reaction - hydrolyzation of the un-

exhausted dye close to the substrate

phase and in the dye bath also starts

and it would become critical to control minimize this aspect of the reaction.

The direction and rate of reaction towards covalent bonding with substrate

have to be controlled by careful manipulation of pH and temperature. That

would require precision instruments /plc controls. Secondly since the

exhaustion is low and better part of the dye

exhaustion takes place in the secondary

phase, migration would be affected and the

dyeing would be non uniform.

Where P, Substantivity and R are high

Primary exhaustion would be high and

whatever exhausted would be fixed. In this

case it would be critical during exhaustion phase as the substantivity is high

and migration could be a problem. Higher temperatures need to be resorted

to for migration and that would not be in favour with Dyes of the low

reaction temperatures in view of its high reactivity. Such a situation would

warrant graduated salt additions to avoid initial strike – linear or step wise

in order to facilitate phased migration. It would require cooling if higher

temperatures were to be adopted. Because of the high reactivity pH control

to maintain low and constant alkaline pH through out the reaction/fixation

phase would be critical. Depending on the hot or cold class of colours the

temperature maintenance will be critical.

In the above example where the substantivity before salt addition is

relatively lower but enhanced by salt addition, migration would be better

facilitated. It could be possible to standardize on an isothermal dyeing

sequence starting with salt bath

The desirable features of the dyestuff would be to posess reasonbly good

substantivity and migration capability, a good exhaustion percntage

including the seondary exhustion that are achievabe within a paracticable

time dimension and reactivity that matches the degree of exhaustion so

that all the exhausated dye is fixed.

This would mean ideally that the curves

S and F should super impose at the

concluding stages of the dyeing

process. Such a dyeing would require

least effort for soaping, However such

an ideal system is not practicable but

efforts should be to move towards the

ideal system Dyes with similar

substantivity that are moderate and having good primary exhaustion

(assisted by salt addition) and migration potentials and also a relatively

lower secondary exhaustion with reactivity reaching close to equilibrium

exhaustion would be the most suitable choice where auto dozing and

sophisticated control systems are not available.

Top

Evaluation of Substantivity

A very useful and simple practical method to assess substantivity of the

Reactive dyestuffs in the lab based on chromatographic principles is given in

the article “Effects of Dye Substantivity in the Dyeing of Cotton with

Reactive Dyes” a prize winning article By Canadian Association of Textile

Colourists and Chemists in TCC Nov 91).

The individual process house labs can conveniently assess substantivity of

the dyes and group them for using in their recipe mixtures. The dyestuff

manufacturers themselves recommend colours that have similar

substantivity features; however it would be safe to assess in ones own lab

unless supplied by propriety manufacturers.

Evaluation of Migration Index

Ref.material Practical method to evaluate migration Index

“Reactive Dye Selection and Process Development for Exhaust Dyeing of

Cellulose”

BY M.J. Bradbury, P. S. Collishaw and S. Moorhouse, ZENECA Colours,

Blackley, England. August1995, Vol. 27, No. 8

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