Long Way Education

By Eman Abdallah Kamel

Eman is a writer and engineer. She received her Bachelor’s degree in Textile Science from the Faculty of Applied Arts in Egypt.

In this article, you will learn about the main eight dyes that can be used in dyeing cotton fibers.

Introduction

Cotton is a soft fibre that develops in bolls around the seeds of the cotton plant, which belongs to the Malvaceae family. Cotton fibres, which are mostly made of cellulose, are vital to the economies of many countries worldwide.

The process of applying colour to a textile material that has a certain level of fastness is called dyeing. The substances that impart color are known as colorants. Colourants are called dyes when they naturally attach to and remain on textiles.

Cotton dyes

Let’s explore the main dyes for cotton.

1. Direct dyes
2. Reactive Dyes
3. Sulphur Dyes
4. Vat Dyes
5. Disperse Dyes
6. Naphthol Dyes
7. Basic Dyes
8. Pigments

1. Direct Dyes

Direct dyes were developed after the discovery of Congo red in 1884. The couple of the united amine and 1-naphthylamine-4-sulfonic acid forms the Congo red. In the 19th century, many different amino acid naphthols began industrial production, and their varieties were greatly enriched.

This type of dye is mostly composed of sodium salts of aromatic compounds. It is soluble in water, and its solubility increases with increasing temperature. Direct dye dissociates into anions in water and is mainly used in exhaust dyeing.

Did You Know?

Direct dye does not react with the fiber structure but relies on dye-to-fiber associations such as hydrogen bonding and dipole moments to help with colorfastness after dyeing.

Its Properties,

1. Direct dyes use less chemistry, water, and time.
2. Its lightfastness and soaping fastness are relatively poor.
3. After dyeing, if you treat them with a fixing agent, you can improve their fastness.
4. Direct dyes are sensitive to acid, and adding acid can cause them to deposit.

To learn more about direct dyes, visit Direct Dyes: Properties, Methods, and Process on Cellulosic Fibers. 

2. Reactive Dyes

Reactive dyes were invented in 1956 and provide the widest and brightest color range for cotton. They work well for both continuous and exhaust dyeing and are highly water-soluble.

This type of dye reacts directly with the chemical structure of cotton (cellulose), forming a strong covalent bond between the fiber and the dye, giving excellent wash fastness.

Many new reactive dyes react in two locations on the fiber, known as “bi-reactive dyes.” These dyes have excellent fastness properties but are more difficult to process.

Keep in Mind

The cold dyeing can dye cotton at low temperatures and is used in the cold pad-batch energy-saving dyeing technique. Hot dyeing, on the other hand, is more difficult to exhaust into the fiber and requires higher dye bath temperatures for extended periods.

Visit Types of Reactive Dyes to discover more about reactive dye types.

Properties,

1. Most reactive dyes have lightfastness.
2. They also have excellent wash stability.
3. Reactive dyes require large amounts of salt, such as sodium sulfate, as a dyeing agent, which can be an environmental concern in wastewater.
4. These dyes are relatively expensive.

Remember

Water may react with some reactive dyes in the bath during the dyeing process, rendering them inactive. This inactive dye cannot be recovered and becomes a colored wastewater pollutant.

To learn more about reactive dyes, visit Reactive Dye Methods.

3. Sulphur Dyes

Sulfur dyes are non-ionic (water-insoluble). The dyes are applied in a reduced, dissolved anionic state, with an affinity for cellulose. After dyeing, an oxidation process is carried out to restore the original non-ionic dye structure; the sulfur dye’s bond to cellulose is stabilized by physical forces, with the dye aggregating in situ.

Dyeing Mechanism,

At a high temperature (80°C), with common salt and with an alkaline pH and reducing agent (sodium sulphide or sodium hydrosulphide), the dye particles break down and become soluble in water, allowing the fabric to absorb them.

After removing the fabric from the dye solution, it is left in the air, where the dye is renewed by oxidation. The original, renewed dye is insoluble in water. Oxidation can also be carried out with hydrogen peroxide or sodium bromate in a mildly acidic solution.

Properties,

1. Sulphur dyes have a fairly broad shade range but produce muted colours.
2. It usually has poor lightfastness in light shades.
3. The lightfastness of these dyes is good in dark shades.
4. Sulfur dyes have good wash fastness.
5. They might not have good friction stability.
6. Sulfur dyes are bleached with hypochlorite bleach.
7. During storage, especially in hot and humid conditions, sulfur-dyed products may form sulfuric acids, which attack and destroy cellulose.
8. They are inexpensive dyes.

Remember

Sodium sulfide, the most effective reducing and dissolving agent, contaminates wastewater, necessitating the use of alternative reducing systems, such as hydroxyacetone, glucose, and reducing sugars.

4. Vat Dyes

Vat dyes are complex, water-insoluble dyes, and the most important of these were invented in the 20th century. Vat dyes play an important role in both dyeing and printing of cotton fibers. Examples of these dyes include benzanthrene, amaranthrene, carbenthrene, solanthrene, navinon, and indanthrene.

Dyeing Mechanism,

Vat dyes are made water-soluble through a chemical reduction process. A reducing agent, such as sodium hydrosulfite, and an alkali (sodium hydroxide). The reduced form of the dye is known as the leuco form. In most vat dyes, the leuco form is highly water-soluble and is almost colorless or light-colored. In the leuco form, the dye is strongly attracted to the cotton fibers and is quickly absorbed into the fiber structure. Once inside the fibers, the dye is oxidized back to its original, water-insoluble form.

Remember

Sodium hydroxide and sodium hydrogen sulfide allow vat dye to be dissolved in water for 15–20 min at 50°C. Adding common salt to the dye bath during cotton dyeing can also loosen the dye.

Properties,

1. Vat dyes are among the most expensive dyes used in cotton dyeing.
2. Vat dyes are limited in their color choices; very few dyes are available in yellow, orange, and red, and they tend to be dull. Black, green, and blue are examples of dark colours that have relatively bright dyes.
3. They have good washfastness.
4. The majority of vat dyes are very resistant to chlorine.
5. Vat dyes are used more in continuous dyeing than in exhaust processing because of the intricate dyeing procedure.

Did you know?

The ancient natural dye indigo also functions as a vat dye. Synthetic indigo is the most widely used dye in the textile industry and is used in denim dyeing. When dyed onto cotton fibers, indigo produces a vivid reddish-blue color.

5. Disperse dyes

Disperse dyes are water-insoluble dyes used in dyeing synthetic fibers. They are typically of poor quality for nylon and medium quality for acrylic fibers. They are usually used in dyeing polyester. During the polyester dyeing process, they are dispersed into particles.

Dyeing Mechanism,

During polyester dyeing, the fibers are swelled by a chemical swelling agent known as a carrier or at a high temperature of 265–275°F. Carrier dyeing is processed with water up to its boiling point (212°F). To bring the water to these temperatures, a high-temperature pressurized dyeing vessel, such as a fabric jet machine or a package machine for yarn dyeing, is required.

The dispersed dye is then dispersed throughout the polyester structure. After cooling, the polyester layer is washed to remove the unfixed dye.

Properties,

1. Disperse dyes have good washfastness,
2. Good light and heat stability,
3. They are used for all types of cotton and polyester blends.
4. Disperse dyes are a mid-priced dye.
5. Some dispersed dyes can sublimate.

What is sublimation?

Sublimation is the property of a solid dye particle changing directly to a gas when exposed to intense heat, without transforming to a liquid or molten state. The dye then immediately returns to its solid state under suitable temperature conditions.

6. Naphthol Dyes

Naphthol dyes are an outdated dyeing system, with a very complex dyeing process and difficult color coordination. Naphthol dyes have lost most of their market share to reactive dyes.

Dyeing Mechanism,

In this class of dyes, a water-insoluble dye is produced within the fibers during the dyeing process. A compound known as naphthol is added to the cotton material. This compound is not a dye and can be removed from the substrate by washing.

Then, a second component, known as a coupling salt, is added to the naphthol in a process similar to exhaust dyeing. The color salt reacts with the naphthol, forming a water-insoluble dye within the fiber.

7. Basic Dyes

Basic dyes are water-soluble dyes that are primarily used to colour acrylic fibres, though they can also be used with blends of cotton and acrylic.

Dyeing Mechanism,

Under dyeing conditions, the dye produces a positive charge, and acrylic fibres produce negatively charged dye sites. Positive-negative electron attraction is the primary driving force for dye transfer from the dye bath to the fiber.

Within the fiber, other dye-fiber interactions affect the final properties of dye colorfastness.

Properties,

1. Basic dyes come in a variety of colours.
2. They have diverse colorfastness properties.
3. They have poor lightfastness.
4. Many of these dyes produce extremely vivid colors.

Keep in Mind

Basic dyes can cause severe staining on cotton, and this should be taken into account when dyeing acrylic and cotton blends.

8. Pigments

Pigments are colorants that are insoluble in water and do not penetrate the fiber structure. They do not react with textile fibers and must be bonded to the surface of the fiber or yarn using a binder. These binders are not specific to a particular fiber and can be used on many fiber types.

Since the colorants are glued to the outer surface of the yarn in the fabric, pigments have limited crockfastness, especially dry crockfastness. The binder systems tend to make the textile feel stiff. The better the pigment fastness properties, the stronger the binder. When applied by padding or spraying, pigments can migrate during drying, leading to uneven color or blotches. During processing, the binder may cause the substrate to adhere to or build up on rolls of machinery.

Properties,

1. Easy to use.
2. Compared to dyes, pigments are less expensive and more economical colorants due to the limited number of processing steps.
3. Pigment colors have good lightfastness.
4. Their lightfastness depends on the choice of binder.
5. In general, they generate vibrant hues in a variety of tones.
6. Pigments are useful in spray coloration, continuous pad, and printing.

Sources

• Textile Dyeing… pdf
• Chemical Book… Direct Dyes
• Sulphur Dye (Wikipedia).
• Direct Dyes (ScienceDirect).
• Vat Dyes.

©Eman Abdallah Kamel, 2025

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