During the 18th century apparels made of Denim became very popular for the first time. It was due to its durability and the resistance to tearing with a fashionable look. Now the denim has become a vivid part of our modern fashion and an essential element of our personal attire.
What is Denim?
Denim is a type of woven fabric which has colored warp yarns but the weft yarns are white in color as these are not dyed. It is a rugged and sturdy fabric in feel. It has generally a twill weaving which produces a diagonal ribbing that distinguishes it other fabrics. Denim is dominantly made of cotton fibers and in some cases, it is blended with other types of fibers to add special features.
A denim predecessor known as Dungaree had been produced in India for hundreds of years before the advent today’s denim in Nîmes a city of France.
The word ‘Denim’ is derived from the French phrase ‘Serge de Nimes’ which means Serge from Nimes.
Color of denim:
The color of a denim fabric is the color imparted on the warp yarns of the fabric. The color of the warp may be blue, black, green, and so on. And the weft yarns are always in its natural color the white. The warp yarns go through a Surface Dyeing where colors are only attached on the surface of the fabric or yarn instead of getting into to the core of the yarn. It helps easy fading of the fabric which bring the desired look of a denim garment after different types of washing.
The coloration of weft yarn is done using whether Indigo dye or the Sulphur dye. Indigo dyeing produces conventional blue color and shade alike to blue color. Sulphur dyeing, which also called color denim is used to produce particular colors like black, cherry, grey, rust, mustard and lime, and so on.
Both of them are vat dyestuff. They are insoluble in water and have a very poor affinity to cellulose fibers like cotton fiber. In normal situation, vat dyes will not attach on cotton fiber. For dyeing of cotton yarn, vat dyes should be transformed into water-soluble form via chemical reduction process, in which hydrogen is liberated. The hydrogen reacts with the dye and allows a water molecule to attach to the dye. The dyes are then transported into cotton fiber by the media of water. The dye is attached onto cotton fiber by the water. These reduced dyes are then oxidized. Oxygen reacts with the hydrogen to produce water. Removing the hydrogen makes the dye insoluble and results in the dye becoming physically trapped inside the fiber. Some oxidizing agents like Hydrogen Peroxide, Sodium bichromate are also used.
Following auxiliaries are used with the dyes:
| Reducing Agent | It makes the insoluble dye soluble by reduction. The most commonly used reducing agents are sodium dithionite (Na2S2O4), Sodium hydrosulfite, etc. |
| Alkali | It helps to maintain a higher pH of 10.5-11.5 to obtain the ionized form of the dye. Mainly, Caustic soda (NaOH) is used. |
| Dispersing Agent | It prevents the agglomeration and aggregation of undissolved particles. |
| Wetting Agent | It helps to wet the fabric by reducing the surface tension of water. |
| Sequestering Agent | It makes hard water soft by deactivating the metal ions present in water. |
For a good quality of dyeing following parameters should be maintained properly:
- Dye-bath pH
- Amount of Reducing Agent
- Immersion Time
- Dye-bath Temperature
- Time of Airing/oxidation
Dyeing Methods
Following dyeing methods are commonly used to dye the warp yarns of a denim fabric before weaving them with the weft yarns.
- Rope/Ball warp/ Chain dyeing
- Slasher/ Open warp/ Sheet dyeing
Rope Dyeing
It was first started in USA at 1915. It is a superior dyeing technology in which the warp yarns are pre-treated and dyed in rope form.
Dyeing Process:
- The warp yarns are passed through a special comb and lease rods and wound in the form of ropes in ball warp.
- The rope length generally remains up to 2500 meters. Each rope contains 350-400 ends.
- Machine capacity is up to 36 ropes and 14400 threads depending on the size and the width.
- The ropes are dipped into 6-8 dye baths.
- Multiple dipping of the ropes are carried out for ensuring better penetration.
- Skying of the ropes are carried out after each dip for air-oxidation. The shade is built up due to this.
- The yarn count ranges from 1-16 Ne
| Advantages of rope dyeing | Disadvantages of rope dyeing |
| No cross-shade variation
Wastage of thread is low Productivity is high and flexible production Less reducing agent consumption No time loss during lot change Versatility in denim production |
A lot of space is required
Immersion time and oxidation time is comparatively higher An additional step of opening ropes after dyeing is necessary Less flexibility in changing color The production cost is high |
Slasher Dyeing
This continuous process was first introduced in 1970. The warp yarns are pre-treated and dyed in the form of a yarn sheet. This is multi-dip, multi-nip, and multi-airing indigo dyeing.
Beam Warping ⇒ Pre-Treatment ⇒ Washing ⇒ Warp Sheet Dyeing ⇒ Airing for Oxidation ⇒ After-Wash ⇒ Pre-Drying ⇒ Warp Yarn Sizing in Sheet Form ⇒ Weaving ⇒ Finishing
Dyeing Process:
- 6-10 dye-baths per vat. Less time is required as each yarn is independently subjected to treatment.
- Warp beams supply warp yarns. The machine capacity is 9500 warp yarns having 300-750 warp yarns per beam.
- Yarn count and denim fabric design determine the number of total warp yarns.
- The yarn count ranges from 1-30 Ne.
| Advantages of slasher dyeing | Disadvantages of slasher dyeing |
| Less space is required due to compact design
Oxidation and immersion times are less Continuous process Flexibility in changing color Production cost is low |
Possibility of cross-shade variation
Possibility of yarn rupture Productivity and flexibility in production are low Extra time needed for lot change No versatility in denim production Reducing agent consumption is high |
Conclusion:
Beside these common dyeing methods some advanced dyeing methods such as Bead dyeing, Foam dyeing, Loop dyeing are also being adopted by the dye houses due to their diverse feature in terms of dyeing denims.
With the combination of Indigo and Sulphur dyes topping, bottoming and ring effect can be imparted on the denim yarns.
These affects add extra elegance to the denims, which makes the denim products more fashionable.
