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.

Acid Dyes

Acid dyes are large dyes that contain one or more functional groups, such as sulfonic or carboxylic acid salts. Acid dyes are applied to fibers from acidic solutions, where the accumulation of positive charge within the fiber in these solutions drives the diffusion and migration of the dye. Acid dyes are only suitable for dyeing fibers that acquire a positive charge in the presence of acid, such as protein fibers, nylon, and some modified synthetic fibers.

Did You Know?

Most commercially available acid dyes are azo, anthraquinone, or triphenylmethane dyes. The azo group is the most common color group in acid dyes. Following the discovery of the diazo reaction by Gris in 1858, mono- and diazo dyes were produced. After the discovery of the diazo reaction by Griess in 1858, both monoazo and bisazo dyes were produced. The first azo acid dye was formed by diazotizing sulphanilic acid and coupling with β-naphthol.

Properties

1. Acid dyes are characterized by their solubility in water due to their containing sulfo or carboxyl groups.
2. The molecular weights of acid dyes range from 200 to 900. Most contain one or two -SO₃Na groups, are water-soluble, and can bind to fibers with cationic sites.
3. Acid dyes applied to fibers have acceptable colorfastness to light and washing, but fixation (increasing the dye’s insolubility through a reaction with a metal salt) improves overall colorfastness properties.
4. Fixation may have a slight effect on the dye’s color. Premetallized acid dyes, on the other hand, are a subset of acid dyes that have been treated with a mordant before dyeing and have sufficient solubility to be dyed under the conditions typically used for acid dyes.
5. These dyes impart a wide range of bright colors to textiles, especially when using monoazo and anthraquinone structures.
6. Some anthraquinone greens and violets are brighter than azo dyes.
7. Acid dyes based on the triphenylmethane structure and its heterocyclic forms are uncommon, despite their reputation for brightness.
8. Carbolan dyes are characterized by their high stability to milling and washing processes.

Did You Know?

Acid dyes are generally considered non-toxic, except for two—C.I. Acid Orange 150 and 165, which ETAD classifies as toxic. C.I. Acid Violet 17 has been reported to have an allergenic effect. The presence of acid dyes in wastewater is low due to their high degree of depletion and fixation.

Classification

1. Classification Based on Dyeing Properties
2. Classification Based on Leveling Properties
3. Classification Based on Acidic Power

1. Classification Based on Dyeing Properties

• Equalizing/Leveling Acid Dyes: Uniform acid dyes are designed to provide a consistent color level throughout the fabric. This class of dyes is crucial when achieving uniformity is paramount, such as in the production of single-color fabrics or garments.
• Milling Acid Dyes: Dyes are intended for applications that require precise particle size control and excellent dispersion. This class of pigments is used in dyeing wool and silk, where it offers superior color penetration and excellent fastness.
• Fast Acid Dyes: These dyes are characterized by their exceptional colorfastness to light and washing and are used to make textiles that will be washed frequently or exposed to sunlight for an extended period of time.
• Metal Complex Acid Dyes: These dyes are characterized by their ability to form compounds with metal ions, which enhances their affinity for fibers. This unique property enables them to produce a wide variety of shades and colors, making them versatile in various textile applications.

2. Classification Based on Leveling Properties

• Good Leveling Properties: The dyes in this category have exceptional abilities to ensure an even and uniform distribution of color on the fibers.
• Average Leveling Properties: This category includes pigments that provide a medium level of color leveling. While not as precise as pigments with excellent leveling properties, they offer satisfactory uniformity in color application.
• Poor Leveling Properties: Dyeings with poor color distribution properties exhibit an uneven distribution of colors, requiring careful control during the dyeing process to achieve a satisfactory result.

3. Classification Based on Acidic Power

• Neutral: These dyes have a unique position. They strike a balance between weak and strong dyes, making them versatile in applications requiring neutral acidity.
• Weak: Weak acid dyes offer flexibility in handling delicate fibers. These dyes are ideal for applications where fabric integrity must be preserved. While they may require a longer dyeing time, the result is a more accurate color application.
• Strong: This type of acid dye is ideal for quick dyeing. Industries looking for bold, deep colors, such as the manufacture of vibrant clothing or accessories, often turn to strong acid dyes.

Dyeing Process

• Preparing Material
• Preparation of Acid Dye Solutions

1. Preparing Material

The acidic conditions required are achieved using one of the following methods:

1. Immersion dyeing: Soak the fibers, threads, or fabric in water overnight to allow the dye to penetrate. Add a small amount of vinegar to the dye solution. 
2. Space-dyeing: Soak the fibers or fabric overnight (at least 48 hours for silk fibers) in plenty of water, with a small amount of dish soap and 50 ml of vinegar per 100 g of fiber/thread.

Prepare the dyes, then squeeze out the excess water until the thread/fiber is thoroughly damp but not wet.

Remember

Acid dyes are used as 1% stock solutions for most dyeing applications. If possible, prepare the entire pot of powder at once. This minimizes the spread of dust particles and the need for scales that accurately weigh small amounts.

2. Preparation of Acid Dye Solutions

1. To make a 1% solution, use 100 milliliters of water for every one gram of dye powder. As a result, 2.5 liters are required to prepare a container with 25 grams of dye powder, and 5 liters for a container with 50 grams.
2. An alternative that saves storage space is to prepare the dyes in a stronger solution (e.g., 2% or 5%) and then dilute them to 1% as needed.
3. Carefully empty the powder into a measuring jug, then add enough cold water to combine and form a paste.
4. Add hot water and stir until the powder is completely dissolved.
5. Add cold water until it reaches the required volume, stir well, then pour it into a container.
6. Label the container with the color of the dye and the strength of the solution (e.g., 1% or 2%).

Methods of using concentrated dye solutions

1. Immersion Dyeing: For immersion dyeing methods (including dip dyeing and two-color spherical dyeing), follow these steps:

• Add 300-400 ml of a 1% concentrated solution to the dye container, which contains 50 ml of vinegar and enough water to cover the threads and allow them to move freely in the container.
• Heat the solution gently until it boils, then leave it for 20-30 minutes, stirring occasionally.

2. Space Dyeing: Applying the dye directly to the fibers by pouring, coating, or sponge application requires slightly different quantities and concentrations. The following are approximate amounts of dye solution absorbed by 100 grams of each type of wet fiber:

• Wool fibers: Most breeds absorb approximately 500-600 ml of the concentrated solution per 100 grams. Prepare 600 ml of a 1% solution for intense colors. For muted colors, use a small amount of the concentrated solution and add water until the volume reaches 600 ml.
• Silk: It typically absorbs dye about half as much as wool, so you’ll need approximately 250-300 ml of dye. To achieve the same color intensity as with wool, use 250 ml of a 2% solution for strong, vibrant colors. For lighter colors, measure a small amount of concentrated dye and add water to reach 250 ml.

Sources

• Acid Dye
• The Basics of Acid Dyes: Types, Properties, and Applications
• Acid Dyes.pdf

©Eman Abdallah Kamel, 2025

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