Denim Dyeing Process with Indigo Dyes

Denim Dyeing Process with Indigo Dyes

Pallavi Sunil Gudulkar
Department of Textiles (Textile Chemistry)
DKTE’S Textile & Engineering Institute, Ichalkaranji, India
Intern at Textile Learner


1. Introduction
Denim is a robust, durable fabric in which the warp yarn is dyed but the weft yarn is not. The fabric is usually 3/1 warp-faced twill. Denim differs out from other cotton fabrics because it has a diagonal ribbing on one side. Indigo is used exclusively to produce brilliant blue shades with the desired wash-down effect. Despite the fact that indigo is a vat dye, it has no affinity for cellulosic in reduced and solubilized form, unlike other vat dyes. Exhaust dyeing fails to demonstrate such dye uptake, demanding the use of a multi-dip-nip padding procedure with intermediate airing to achieve a progressive shade build-up. pH is the most important dyeing parameter for achieving surface intensive ring dyeing effect and developing desirable effect.

2. Indigo Dyes for Denim Dyeing
Denim’s almost magical attraction stems mostly from its blue color. Fabric takes on a vivid blue tint as a result of the dye. There is no other dye that produces such a deep blue color with such a little number of carbon atoms in its molecule. Due to partial penetration in cotton fibers, indigo is unusual in its capacity to impart surface color. When indigo-dyed cotton yarn is untwisted, the inner layers remain colorless. The color is also constantly fading and scratching. With several types of washing and finishing processes, indigo allows denim fabric to achieve its ultimate worn look. It allows denim fabric to respond to finishing applications, giving it a more realistic appearance.

Indigo has continued its popularity for denim dyeing despite the availability of numerous other blue dyestuffs. The ability to achieve wash down effects on multiple washings without losing the color’s freshness is a unique quality of indigo dyed denim. Another benefit of indigo dyed denim is that, unlike many other colors, it does not cause health problems. Indigo is so safe for living things that it’s been used to color polyester medical sutures for a long time. Indigo is also utilized as a food color and as an intravenous medicinal indication.

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Indigo’s technical characteristics include the ability to dye at room temperature, favored shades ranging from black to navy to sky blue that are not too brilliant or dull, reduced indigo solution is not sensitive to water hardness, allowing the dyeing of greige or minimally pre-treated cotton, does not strongly bond to the fiber, repeated washings slowly remove the dye without losing freshness, and indigo’s reduced indigo solution is not sensitive to water hardness, allowing the dyeing of shades.

3. Methods Used for Denim Dyeing with Indigo Dyes
Three denim dyeing methods with Indigo are as follows

  1. Rope/Ball warp/ Chain dyeing
  2. Slasher/ Open warp/ Sheet dyeing
  3. Loop dyeing

3.1 Rope dyeing process:
Rope dyeing uses a ball warping machine to pull creel yarns, which are then run through a lease stand with a specific comb and lease rods to ensure accurate end registration. These pulled ends are then merged into a bundle and sent through a condenser tube assembly, which is then passed to make a rope with 350-400 ends. After that, the ropes are wound on drums. The beam is unloaded and put on a dye range creel once it has been fully coiled. Ropes are dyed by passing them through a pre-treatment bath, then multiple dipping in various indigo baths, with intermediate squeezing and skying. Schematic diagram of rope dyeing unit is as below,

Schematic diagram of indigo rope dyeing unit
Figure 1: Schematic diagram of indigo rope dyeing unit

3.1.1 Advantages of Rope Dyeing

  • No cross-shade variation
  • Low thread wastage
  • High productivity and flexibility in production
  • Less reduction agent use
  • No time waste during lot changes
  • Versatility in denim production

3.1.2 Disadvantages of Rope Dyeing

  • There is a lot of space required
  • Immersion and oxidation times are considerably longer
  • An additional step of opening ropes after dyeing is required
  • There is less color flexibility
  • The production cost is high
Indigo Rope Dyeing
Figure 2: Rope Dyeing

3.2 Slasher dyeing:
Slasher dyeing involves pre-treatment of denim yarns in the form of a warp sheet in early compartments, followed by multi dip/nip indigo dyeing; the process is completed with after washing, drying, sizing, and final drying. Because the warp sheet is instantly processed and then transported to the weaving department for conversion to fabric, there is very little yarn handling. A slasher dyeing range can be made up of either a single indigo dyeing range or a continuous indigo dyeing range with a built-in size range. In the first kind, the yarn sheet passes through one or two pre-treatment boxes, going through the phases of dipping, squeezing, and skying; the only aftertreatments are washing and drying. Schematic diagram of slasher dyeing unit is as below,

Schematic diagram of slasher dyeing unit
Figure 3: Schematic diagram of slasher dyeing unit

3.2.1 Advantages of Slasher Dyeing

  • Due to the compact design, less room is required
  • Oxidation and immersion times are reduced
  • The process is continuous
  • Color changeability
  • The production costs are modest

3.2.2 Disadvantages of Slasher Dyeing

  • Cross-shade variation is possible; yarn rupture is possible; and production productivity and flexibility are limited.
  • Change of lot necessitates additional time.
  • There is a lack of adaptability in denim manufacturing.
  • The cost of lowering agent use is high.
indigo slasher dyeing machine
Figure 4: Indigo slasher dyeing machine

3.3. Loop dyeing:
It’s almost like slasher dyeing, but there’s only one dye bath. It involves running ropes of yarn through a vat of indigo dye before releasing them onto the factory’s roof to oxidize before returning them to the dye bath. Loop dyeing produces more consistent indigo colors and takes up less room than other dyeing methods. Multiple dips and layered skying are used in the procedure. Sheet dyeing is used for warp yarns. After each dip, the air is oxidized. The warp sheet rotates in a closed-circuit fashion, like a loop. Because of the significant drop in dye and chemical concentration after each dip, immediate fortification of the bath is essential to obtain the appropriate results. Schematic diagrams of loop dyeing are given below –

Schematic diagram of loop dyeing
Figure 5: Schematic diagram of loop dyeing

3.3.1 Advantages of Loop Dyeing

  • Because it is a compact system, it requires less space.
  • The initial outlay is little.
  • Low maintenance and energy expenses.
  • Water, dyestuff, and chemical consumption are all kept to a minimum.

3.3.2 Disadvantages of Loop Dyeing

  • Selvage center variation and yarn breaking are both possible.
  • During the article change, threads are lost.
  • It takes longer to change a lot.
  • Low productivity and a lack of flexibility.

4. Conclusion
Though Indigo is a type of vat dye, due to its low affinity for cotton, it cannot be dyed using traditional vat dyeing procedures. Instead, multiple dip/nips with intermediate airing must be used. At least 95% of world denim production is done by rope and slasher dyeing.

5. References

  1. Denim: Manufacture, Finishing and Applications Edited by Roshan Paul
  2. Sustainability in Denim Edited by Subramanian Senthilkannan Muthu
  3. Dyeing of denim with indigo by J N Chakraborty and R B Chavan
  4. Loop dyed -Heddles

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