Briefly Description of Rope Dyeing in Denim

Denim Dyeing Process:

There are three processes in the practice for continuous denim dyeing:

1. Rope Dyeing
2. Slasher or Sheet Dyeing
3. Loop Dyeing

Rope Dyeing :

The indigo Rope dyeing technology for denim production is considered a superior dyeing technology, where better uniformity of dyeing is achieved than other Indigo dyeing technologies like slasher dyeing. Indigo rope dyeing was started in USA.

Today rope dyeing accounts for a large percentage of warp yarn dyed for denim production. The system offers highest production, due to continuous process, as there is no stoppages for set changes. In this dyeing method, maximum continuity of shades and minimum danger of center to selvedge shade variation can be achieved. Flow diagram of rope dyeing is shown in Fig. .

During dyeing process, it forms a coating in the outer layers of the cotton yarn and fiber. This produces a ring of color around the cotton yarn, with the core remains white core. This dyeing effect is known as ring dyeing.

Fig. : Rope dyeing and subsequent sizing, Step 1: Ball Warper, Step 2:Rope Dyeing, Step 3: Long Chain Beamer, Step 4: Sizing

Indigo dye is a vat dyestuff. It is insoluble in water and has a very poor affinity to cellulose fibers like cotton fiber. In normal stage, indigo will not dye cotton fiber. For dyeing of cotton yarn, indigo should be converted into water-soluble “leuco” form in chemical reduction process. Reducing agents such as sodium hydrosulfite with sodium hydroxide are used to convert the indigo dye to its soluble form. The reducing agent also initially changes the color of the dye from its blue color to a very pale greenish yellow color. The leuco form of indigo forms a coating on the outer layer of the cotton yarn. The yarn is then exposed to the air, where the indigo become insoluble by oxidation. The oxygen in air converts the soluble indigo dye to its original insoluble form and back to its original blue color. The chemical reaction is shown in Fig. .

Fig. : Simplified description of the reduction/oxidation of Indigo

When the indigo dye oxidized, it becomes trapped into the outer layers of the cotton yarn. In a single passage of indigo dye, a small amount of dye being deposited on the yarn surface which causes light blue shade. In order to obtain deep indigo blue shades, the cotton yarn should be subjected to repeatedly dye with indigo dye. The indigo dye is layered by dyeing in a multiple passes of the rope into the soluble indigo dye and then subsequently exposing it to the oxygen in the air for oxidation. This multiple passing of yarn into dye bath is known as dipping. Normally, the cotton yarns are dyed with indigo dye in at least 4 to 8 passages of the dye bath. After every passage of dye bath the yarns are exposed to air and followed by further dipping in soluble indigo dye bath. After every passage of the dye bath all the oxidized indigo dyestuff fixed on the surface of the cotton fiber/ yarn, only a small portion of indigo is replaced by fresh indigo. This results in the increase in the depth of shades.

In some cases a sulfur black or blue dye can be applied to the yarn before indigo dyeing in order to achieve darker shades. This is known as a sulfur bottom. If the sulfur dye is applied after the yarn has been indigo dyed, it is known as sulfur top.