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

This is a guest post by Harry Mercer. This is a very technical description – read on if you are technically oriented..

Here are given some important dyeing processes related to indigo dyeing – specially on Rope Dyeing .

Pre Treatment

Pre-treatment is conducted in the 1st tank. The most common pre-treatments are :

1.  Sulfur bottoming

2.  Scouring with sodium hydroxide

3.  Causticizing or Mercerizing

Pre- Treatment : Sulphur Bottoming

•The purpose of sulfur bottoming was to:

  • Originally to produce a dark shade on denim using less Indigo for lower costs
  • In the U.S. sulfur bottoms were dyed using a combination of blue and black dyes
  • In denim operations outside the U.S. the bottom is normally dyed with sulfur black

Pre – Treatment : Cotton Scouring

Cotton fibers contain impurities like waxes, pectins and minerals that will interfere with Indigo dyeing and result in streaks.-Sodium hydroxide at low concentrations (<5%) are applied at high temperatures (>85 C) in order to remove impurities and melt natural cotton waxes.

Pre – Treatment : Causticizing

  • Causticizing generally refers to using sodium hydroxide at below Mercerizing concentrations (<18%).
  • Cold causticizing of cotton yarn results infaster Indigo dye fading from laundryabrasion, darker Indigo color with the same % of Indigo and unique washdowns.
  • Hot causticizing improves colorfastness

Pre – Treatment : Mercerizing

Mercerizing is the use of strong sodium hydroxide (18-30%) to swell surface fibers.

Caution:

  • When using strong sodium hydroxide it is important to remove all of it.
  • If sodium hydroxide is on the yarn as it enters the Indigo tanks, the yarn color will change.
  • Concentrations of sodium hydroxide more than 18% are not a solution, but a gel and are difficult to remove.
  • Mercerized yarns are more ring-dyed and dye more darkly than non-Mercerized.
  • Mercerizing is normally conducted at low temperatures, but hot Mercerizing can be employed for a more abraded appearance after garment laundering.

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

Spectrum_single_thread

  • 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.

 

indigo dyeing methods

 

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.

indigo dye baths

 

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.

indigo machine

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.

indigo dyeing machine

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.

image 

 

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

Vat Dyeing

  • Indigo and sulfurs are types of vat dyes.
  • In non-denim cotton dyeing, another class of vats, anthaquinoids are used to produce a full range of colors that are colorfast.
  • Some of these vat dyes can be blended with Indigo or applied using standard procedures on specially designed machines.

 indigo dyeing machine

Pad-Dry Chempad Vats

1. Pre-wet 4% caustic 90°C
2.Wash 60°C
3. Pad vat dye cold
Drying cylinders hot
4. Chempad caustic/hydro cold
Steamer hot
Bypass boxes 5-10
11. Wash 60°C
12. Oxidize
13. Soap with anti-oxidant
Steamer hot
14. Wash 50°C
15. Wash 50°C
16. Wash 50°C / softener

 

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Pad-Dry Chempad Steam Vat Bottom

1. Pre-wet 4% caustic 90°C
2. Wash 60°
3. Pad vat dye cold
Drying cylinders hot
4. Chem-pad caustic/hydro cold
Steamer hot
Boxes 5-10 Indigo
11. Wash 60°C
12. Soap 90°C
Steamer hot
13. Wash cold
14. Wash 50°C
15. Wash 50°C
16. Wash 50°C

 2-1_indigo_washer

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%.

Cold Dyeing Methods

  • Sulfur colors can be dyed at low temperatures with specific buffers which produce more colorfast dyeings with no color variation.
  • Cold dyeing methods have been used to blend Indigo and sulfurs, eliminating the need for separate bottoming and topping, while eliminating shade changes in both.

Special Dyeing Techniques

  • On rope ranges, space dyeing techniques can be simple and produce a wide range of special effects in denim.
  • By dyeing part of the yarns with a sulfur top and leaving the rest un-dyed, many companies produce a slub appearance with regular yarns.
  • Blending ring yarns of different sizes also produces a slub appearance.

imageThis is a guest post by Harry Mercer. Mr. Mercer has 30 years experience in the denim business including 3 prominent U.S. denim companies. He is an expert colorist for measurement and color matching as well as textile testing.

  1. Your articles in dyeing and washings are very much useful.I would
    like know more about zero percent discharge from dyeing process.
    In Tirupur,India,struggle to over come the problem with any
    right solution. Your valuable advice and guidance can make
    market size in Tirupur say around 18000 crore rupees will be
    gradually diluted into other areas in India and later vanishing
    from the textile map.
    Please forward your expertise advice to our Tirupur Exporters
    Association.
    Warm regards,
    Sathyanarayanan iyer pv,Tirupur

  2. Hi:
    I think that I have already responded to you on this matter, but as I expalined, I need some information.

    Regards,

    Harry Mercer

  3. I am astounded at the considerable use of seemingly environmentally unfriendly chemicals in the production of denim.
    Organic cotton and herbal indigo dyeing is prohibited by cost,but I think premium denim jean manufactures should make a more conserted effort to promote their use.

  4. Hi.
    What is in general consumption of hydros in % per Kg of denim fabric?
    Please help and guide to understand in detail.
    thanks,
    NIRAJ

  5. MR.HARRY MERCER. THANKS FOR YOUR GUIDANCE.
    I AM SATHISHKUMAR TIRUPUR. WE HAVE A DYING UNIT IN TIRUPUR, INDIA.MACHINARY WAS OWN ASSEMBLED ESPECIALLY FOR VAT INDIGO CONTINUOUS DYEING PROCESS IN WOVEN 100% COTTON FABRICS AND 100% VISCOSE WOVEN FABRICS. WE ARE IN THIS FIELD FOR LAST 7 YEARS.FOR THE LAST TWO YEARS OUR UNIT WAS UNDER CLOSURE DUE TO ZERO LIQUID DISCHARGE PROBLEM. RECENTLY BEFORE FEW DAYS WE ARE PERMITTED FOR REOPENING AND NOW THE WORK IS GOING ON. WE ARE USING VAT INDIGO 60% GRAINS ALONGWITH CAUSTIC SODA FLAKES AND HYDROS.THE PROBLEM IS IN THE FINISHED FABRICS SOME POWDER IS RAISING WHEN IT IS UNDER MACHINE FOLDING AFTER FRICTION CALENDERING. WE WANT TO KNOW HOW TO CONTROL THAT POWDER ARISING. WE HUMBLY REQUEST YOU TO GUIDE IN THIS REGARDS.KINDLY CONVEY ALSO THE RATIO AMONG THE DYES AND CHEMICALS. WE ARE EXPORTING THIS FABRICS TO SAHARA DESERT ORIENTED COUNTRIES, WHERE THEY ARE USING AS “TUAREG”.
    THANKS SIR

  6. Hi Mr.Harry Mercer

    I am srujana sadula running a dyeing unit in tirupur, India. We are trying to develop denim effect in knitted fabric through vat dye. we are facing problems in color fastness to air and light. please give you suggestions.

  7. dear herry mercer you are very good technocrate perticularly in denim as you have worked with leading industry. as my request to sent the details of all seminar content in drief for better quality denim fabric mfg stage, trouble shooting in indigo , sbit , ibst, foam finish, coating,value addition in fabric , garment etc , & all relevent information to oue e mail for update our knowledge , up course free of charge. your valuable support will be highly apppriciated ,pl share your nowledge
    rgds

    r b gandhi ex arvind mill denim – india

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