Denim Fabric Dyes

By Eman Abdallah Kamel

Eman is a writer and engineer. She holds a bachelor’s degree in textile science from the Faculty of Applied Arts.

Denim is a strong fabric. In this article, you will learn about the dyes used to dye denim, including indigo, direct, reactive, vat, and sulfur dyes.

Indigo, a historical dye collection belonging to the Dresden University of Technology, Germany. Image source: wikimedia.org

Denim Fabric

Denim is a durable cotton fabric with a diagonal weave, where the weft thread passes under two or more warp threads. This results in the distinctive diagonal ribbing of the fabric, which is characteristic of denim. One of the most commonly used colors on the fabric is indigo blue.

There are two main types of denim dyeing: indigo dyeing and sulfur dyeing. Indigo dyeing produces the traditional blue color and its various shades. Sulfur dyeing is used to produce specific colors such as black, cherry, gray, mustard, and lime, as well as to improve the fabric’s quality.

Both types are vat dyes. They are insoluble in water and have little adhesion to cellulose fibers. To dye cotton yarns, vat dyes must be converted to a water-soluble form using a chemical reduction process that releases hydrogen. The hydrogen reacts with the dye, causing a water molecule to bind to it. The dye is then transferred to the cotton fibers by the water. Sodium hydroxide is among the reducing agents used to convert the dye to its soluble form. The dye then binds to the cotton fibers with the water. These reduced dyes must then be oxidized. Oxygen reacts with the hydrogen to produce water. The removal of hydrogen makes the dye insoluble, causing it to become trapped within the fibers.

Denim Dyes

Let’s take a look at some of the different dyes for denim.

Indigo,
Direct Dyes,
Reactive Dyes,
Vat Dyes,
Sulfur Dyes.

1. Indigo

When denim is mentioned, the indigo dye is often the first thing that comes to mind. Since the advent of denim, indigo dye has been associated with it. Indigo was a valuable plant in Egypt, Babylon, Greece, Rome, China, and India. In Islamic Spain, indigo cultivation began in the 11th century. Industrial production of indigo dyes has supplanted plant dyes in modern times.

Nevertheless, indigo is unique in its ability to impart surface color by partially penetrating the cotton fibers. When the indigo-dyed cotton yarn loosens, the inner layers remain uncolored.

Indigo’s chemical formula is C₁₆H₁₀N₂O₂. Indigo is a dark blue powder that sublimates between 390 and 392°C (734 and 738°F). Indigo dyes are not soluble in water, ether, or alcohol but are instead soluble in nitrobenzene, chloroform, and concentrated sulfuric acid.

The unique feature of indigo-dyed denim is that it can achieve wash effects on repeated washes without losing the freshness of the color. Another important advantage of indigo is that, unlike many other dyes, denim-dyed indigo poses no health risks.

Technical features of indigo include the following:

The ability to dye at room temperature.
Preferred shades range from black to dark blue, with a non-glossy finish.
A low-iodine solution that is not sensitive to water hardness—this allows for a gray dye or minimal pretreatment of the cotton.
Repeated washing removes the dye slowly without losing its freshness.

2. Direct Dyes

Direct dyes are used to dye jeans in light colors for the higher fashion market. Also, direct dyes can be used to color denim fabrics by adding them to the final formula.

3. Reactive Dyes

There have been attempts to apply reactive dyes to indigo machines. However, dye control is nearly impossible due to the large dye boxes in most indigo equipment, as much of the dye is decomposed before it interacts with the cotton. Therefore, reactive dyes should be placed in a small box, then dried, chemically primed, and evaporated. The only practical alternative for most denim companies is to apply reactive dyes during sizing with special procedures.

4. Vat Dyes

There are a few vat dyes that can be mixed with indigo for casting. However, pure tub colors are best applied to denim with specially designed rope ranges with an extra set of drying cylinders, steam bowls, and small boxes for dye pads, dry chemical pads, and steam methods.

Did You Know?

Vat dyes produce shades with high fastness properties. Thus, the classic denim fabric look is hard to come by.

5. Sulfur Dyes

Sulfur dyes have a more natural look than reactive or direct dyes, have a softer appearance, and allow for versatile washing effects in wash operations. Therefore, among the different dye classes other than indigo, sulfur dyes are the most popular and are used for the wrapping, decorating, or dyeing of indigo-dyed garments to produce a variety of shades with a great look. Recent advances in dyeing techniques have allowed the replacement of toxic sulfide-reducing agents. Glucose is now used in the primary solution, and both low-sulfide and non-sulfide products are available. Sulfur dyes are characterized by:

Low-cost,
Can be applied to indigo machines,
Pad dyeing machines,
And garment dyeing machines with properly designed methods.

Sources

Introduction of denim dyeing… pdf
Dyeing with Indigo Dye

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