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

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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.

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