Indigo plant, named as “Indigofera tinctoria”, was discovered around 1600s (B.C.). Indigo dyestuff, extracted from its leaves, had been used in various primitive dyeing processes for years. In 1880’s, the first synthetic (unnatural) indigo dye was developed by German chemist Adolf von Baeyer. He identified the chemical structure of indigo. After 1900s, synthetic indigo was marketed. This has triggered off the indigo dyeing developments as a result of improvement in denim wear. In ancient years, indigo dyestuff was fermantated in wooden vats. This process which is called as “vatting”, is supposed to be the origin of vat dyes. Fastness is the resistance of fabric to the action of external influences, such as light, washing, crocking, perspiration, acids, alkalies, etc. Indigo is supposed to have poor washing fastness, fair light fastness, good perspiration fastness and poor crocking fastness.
Properties of Indigo Dyestuff
Indigo dyestuff which is classified as vat dye is insoluble in water and has no affinity to the fibre. They have poor washing fastness which lets the color of denim fabric to change naturally. Indigo creates living colours on fabrics. Indigo dyestuff can never fully penetrate into the fibre, since its molecule is so big and it only adheres to the surface and remains at outer surface of the fibre. The inside stays white. It abrades or fades continually. This character of indigo lets denim fabric to have its final look with different types of washing and finishing applications. It enables denim fabric to response to finishing applications that gives a real life to the fabric. Indigo dye should be classified into two different chemical forms:
- Natural form, insoluble in water (cannot dye the fibre)
- Leuco form, soluble in water (can dye the fibre)
In natural form, indigo dyestuff has a color of blue but after reduced to leuco form, the color of the solution turns to yellow.
Pad Dry Pad Steam: This process can be performed with reactive, indanthren and pigment dyestuff and has 4 main steps. At first step, dyestuff and auxilary chemicals are fed into the dye pad and fabric picks up the dyestuff on itself. Second step is drying. After drying, fabric goes into the chemical pad at third step. Finally, dyestuff gets fixed on the fabric at the steamer. The amount of feeding and auxilary chemicals might be changed according to the dyestuff used.
Pad Steam: This process is performed with sulphur dye. “Pad – Steam” which is a part of PDPS dyeing method is used for this process. Fabric picks up chemicals and dyestuff from the same pad and goes to the steamer for fixation.
Indigo dyeing is built on “continuous warp dying”. Basically there are 2 main methods of indigo dyeing.
- Classic method: beaming, dyeing the warp yarns in rope form, rebeaming and sizing
- Open-width method: Warp yarns are dyed and sized respectively
Rope dyeing: This is the oldest way to dye warp yarns (ropes) and does not have any risk concerning “side to side” problem. Moreover, dyestuff absorption is almost the same since that all the ropes have the same tension during the process.
Loop Dyeing: Warp yarns are dipped into the unique pad many times.
Slasher Dyeing: Warp yarns are dyed as open-width form and dyeing, drying, sizing processes are performed in the same machine continously.
Advantage of Rope Dyeing against Slasher Dyeing:
- Large quantities can be dyed continuously.
- In rope dyeing, ropes are dipped into the dye pads with identical tension and angle, therefore there is not any risk of “side to side” problem.
- Dyeing machine does not have to stop while feeding new dyeing parties which means energy saving.
- Yarn wastage is not that much.