ROPE DYEING

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Most denim is yarn-dyed fabric with the warp yarns dyed with indigo dye and the filling yarns left undyed. There are a number of modifications or alternatives in the dyeing process that are routinely used to change the overall look or performance of the fabric. With the advent of denim garment washing techniques, the consistencies of the indigo dyeing process and its modifications have become crucially important in determining the quality and performance of indigo denim products.

The properties of the indigo dye account for the wide variety of color designs that are available on denim materials. Indigo is unique as a major textile dye, because it has a very low affinity for the cotton fiber. Because of the low substantively of the indigo, the ball warps dyeing process ring dyes cotton. Unlike almost all other commercially successful dye-stuffs, the indigo dye concentrates in the outer layers of the cotton yarn and fiber during the dyeing process. This produces an intense ring of color around a white core in the cotton yarn and the cotton fiber thus the name ring dyeing. When using most other dyes, if the ring-dyeing effect occurs, it would be considered a dyeing defect.

This ring diameter depends on pH and ORP or can be adjusted by these two factors

                                    pH      +     ORP

                    NAOH                                        Na2S2O3

Indigo dye in its normal form is a vibrant blue, it is insoluble in water, and it will not dye cotton fiber. In order to dye cotton, the indigo must be converted to a water-soluble “leuco” form and then applied to the cotton. This process is known as chemical reduction. Reducing agents such as sodium hydrosulphite with sodium hydroxide chemically convert the indigo dye to its soluble form. This also temporarily converts the dye from its blue color to a very pale greenish yellow color. The leuco form of indigo is readily absorbed by the outer layers of the cotton yarn. Once in the fiber/yarn, the indigo is made insoluble by oxidizing the yarn by passing the yarn through the air (skying). In fact, the dye will start to oxidize immediately when exposed to the air. The oxygen in air converts the dye back to its original blue and insoluble form. Thus the dye becomes trapped inside the outer layers of the cotton yarn. This results in a small amount of dye being deposited on the surface resulting in only light blue dyed yarn. In order to obtain deep blue indigo dyed yarns, the color must be built in layers.

The dye is layered by using multiple passes of the rope of yarn into the soluble dye and then exposing it to the air for oxidation. This multiple passing of yarn into dye is called dips. Normally, this process is repeated from three to twelve times to build up a deep indigo blue color. The number of dips is limited to the number of dye boxes on the dye range. If the concentration of indigo dye in the dye boxes is doubled, this will result in slightly darker denim. This acts as a multiplier when labeling the denim. A double concentration of dye in nine dye boxes makes it 18-dip denim. Tripling the concentration makes it 27-dip denim. When even darker shades are desired, a sulfur black or blue dye can be applied to the yarn before indigo dyeing. 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.

In rope dyeing, ball warps are continuously fed into the rope or chain-dyeing range for application of the indigo dyeing. Typically, 12-36 individual ropes of yarn are fed side-by-side simultaneously into the range. The ropes are kept separate from each other throughout the various parts of the dye range. For example, if the total number of ends on the loom beam is 3,456, and each ball would have 288 ends, then the dye set would have a total of 12 ball warps. If there can only be a multiple of 10 balls on the dye range, then there would be 345 ends on 9 balls and 351 ends on the tenth ball.

The ropes are first fed into one or more scouring baths, which consist of wetting agents detergents and caustic. The purpose of these baths is to remove naturally occurring impurities found on the cotton fiber such as dirt, minerals, ash, pectin, and naturally occurring waxes. It is very important to remove these materials to guarantee uniform wetting and uniform dyeing. The ropes are subsequently fed into one or more water rinsing baths.

If a sulfur bottom is required at this point, the ropes of yarn are fed into a bath of a reduced sulfur dye. Similar to indigo, sulfur dyes are water insoluble. They must be reduced to a water-soluble form before applying to cotton. Unlike indigo, the sulfur dye can penetrate into the core of the cotton fiber/yarn. The purpose of this process is to give the indigo dyed yarns a much deeper and darker shade or to slightly change the shade of the blue yarn to make it unique.

Once the reduced sulfur dye is applied to the ropes, they are skied to allow the dye to oxidize into its normal water insoluble form.

The ropes of yarn are then fed into the indigo dye baths and skied after each dip. The ropes of yarn are rinsed in several water baths to remove any unfixed dye. If a variant type of yarn color is desired, sulfur dye can be added at this point. Similar to the bottom-dyeing process discussed previously, this process is known as a sulfur top. Although the sulfur dye will migrate towards the core of the fiber/yarn, the sulfur top gives a different type of yarn color performance when garment washed than a sulfur bottom. The sulfur top process is then followed by a water rinse to remove any unfixed dye.

After either rinsing following indigo dyeing or rinsing following sulfur topping, the yarn ropes pass through squeeze rolls to mechanically extract water. The yarns are then dried and coiled into large tubs. The typical type of drying apparatus is a multiple stack of drying cans. These metal cylinders, which in most cases are Teflon® covered to prevent the yarn from sticking, are filled with steam under pressure. Maintaining a consistent pressure of steam within the cylinder can accurately control the temperature of the surface of each cylinder. Care must be taken not to attempt to dry the rope of yarn too quickly, which causes the dye to migrate to the surface of the rope. Additionally, if the surface of the drying can is too hot, the yarn can be over stressed producing an undesirable glazed appearance that reduces absorbency in later processing. Over-drying of the yarns will weaken them considerably adversely affecting re-beaming, sizing, and weaving.

After drying, the color of the yarn is checked either visually or instrumentally. With many modern indigo dye ranges, the color of the yarn is continuously monitored by instruments, which are electronically linked to the controls of the indigo dye baths. This type of control system can automatically adjust the dynamics of the process to obtain the most consistent color from the beginning to the end of the many thousands of yards of yarn contained within a single dye lot.

In order to minimize the color variability between denim fabric panels after garment washing, denim manufacturers employ a technique known as sequential dyeing. Basically, this method is based on the concept that the color properties of indigo-dyed yarn processed at a specific time, most closely resemble the color properties of the indigo yarn processed just before and just after that lot. This method has proven much more effective at minimizing color variability in garment washing when compared to the technique of shade sorting alone.

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