The Four Basic Dyes for Dyeing Wool

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.

There are many types of wool dyes, but in this article, you will learn how to dye wool with four basic dyes: acid, reactive, chrome, and metal complex dyes.

Image source: istockphotos.com

Introduction

Wool is a fiber removed from sheep and other animals. Wool is a protein fiber, and its physical and chemical properties vary among many factors, such as breeds and sheep within the same breed. Other factors, such as diet, the sheep’s condition, and the growing environment, significantly influence wool’s dyeing properties.

A dye is a substance or liquid used to change the material’s color. Dyes are divided into natural dyes and synthetic dyes.

Dyes used to color wool fabrics are available in powder, granular, or liquid form. Wool dyes are water-soluble, so most wool dyeing recipes are water-based.

Protein Fibres

Protein fibers are polypeptides, long polymer chains terminated at one end with an amino group and at the other with a carboxylic acid group. These terminal groups occur in nature as charged pairs, i.e., an ammonium cation and a carboxylate anion. These ionic pairs are also known as zwitterion pairs. The polymer chains carry no net charge because each ammonium cation is balanced by a neighboring carboxylate anion.

Wool Dyes

The four main categories of dyes used in dyeing wool fabrics are,

Acid Dyes
Reactive Dyes
Chrome Dyes
Metal Complex Dyes

1. Acid Dyes

Acid dyes are anionic and dissolve in water. They are cheap dyes.

Did You Know?

In the past, protein fibers were first treated with acid before dyeing. This process freed up the dye sites and increased the attractive forces between the fibers and the acid dye.

How to dye wool with acid dyes:

1. Acid dyeing processes begin with warm water around 50°C and gradually increase the temperature to boiling.

Keep in Mind

Dye dries quickly and produces an inconsistent final product if the dye bath is too hot to begin with or heated quickly because the dye does not have enough time to absorb and diffuse to be evenly distributed throughout the fibers.

2. Glauber’s salt (sodium sulphate) aids dye migration, especially when pastel hues are required. It is not usually used with dark colors.

3. Most dye manufacturers develop levelling agents for their acid dye collections. Eugene MF, for instance, is made to work with acid dyes. These chemicals enhance dye penetration into the fibers and are particularly effective in reducing color variation between the roots and ends of wool fibers.

Mechanism of Acid Dyebath:

The acid cations in the dye bath titrate the carboxylate anions, which transforms them back into carboxylic acid groups.
The fibers are not positively charged because the anti-acid anion, the chloride anion, balances the charge. According to Coulomb’s Law, the ammonium cations can act as dye sites to attract the negatively charged dye anions.
The dye anion is more hydrophobic than the chloride anion. Thus, the dye anion gradually displaces the chloride anions from the ammonium cation dye sites.
When sodium sulfate is used as a levelling agent, the sulfate anions compete with the dye anions for dye sites, slowing down the dyeing process and giving the dye time to spread evenly throughout the wool fibers.

2. Reactive Dyes

Reactive dyes’ broad colour spectrum, application versatility, and superior fastness qualities make them effective synthetics. Reactive dyes are more expensive than acid dyes.

Did You Know?

Reactive dyes form much stronger bonds with fibers than acid dyes. Once interacted with, they remain fixed and immobile. Because they react chemically with wool, either through substitution or nucleophilic addition reactions, forming a covalent bond between the reactive dye and the wool fibers.

How to dye wool with reactive dyes:

Lukewarm water (40°C) is typically used as the starting point for hot dyeing with reactive dyes to prevent the dye from adhering to the wool fibers too quickly at the beginning of the process. The dye bath is then slowly heated to about 65°C, left at this moderate temperature for approximately 20 minutes, and then slowly brought to a boil. This ensures the dye is evenly distributed throughout the dye bath and fibers.
Glauber’s salt gives a powerful restraining effect on the reactive dyes because it reduces the rate at which the dyes fix to the fibre, thus improving the ability to obtain even dyeing.
Levelling agents such as Lyogen FN have been developed for fiber-reactive dyes. These slow down the dyeing process and improve the evenness of the dye before setting.

Did You Know?

Many countries now impose strict limits on AOX (absorbable organohalogens) levels.

Examples of nucleophilic addition reactions include reactive dyes based on beta-sulfoethyl sulfone.

High temperatures activate the reactive group even under mildly acidic conditions.
The nucleophile in the wool, the –NH₂ amino group, reacts with the vinyl sulfone reactive group by nucleophilic addition.
Don’t forget to use levelling agents.
The reactive dye is now permanently fixed to the wool substrate.

Reaction Mechanism:

D represents the chemical structure of the chromophore in the dye.

Chrome dyes, also called mordant dyes. This photo is for C.I. Mordant Black 1.

3. Chrome Dyes

Chromium dyes have chemical structures similar to acid dyes but can form stable complexes with chromium ions (Cr+++). This structure’s distinguishing feature is the presence of two hydroxyl groups in the ortho positions of the azo group. Chromium dyes can be used alone or in combination with a chromium salt.

How to Dye:

At first, the dye is applied as an acid dye, using acetic acid at the boil.
Then the dye bath is cooled to approximately 80°C, after which the dichromate is added. The temperature is then increased to boiling to complete the chelation process.

Keep in Mind

Chromium is added to the dye bath in the form of sodium or potassium dichromate. Sodium dichromate is hygroscopic. Potassium dichromate is non-hygroscopic, but it is expensive. Wool absorbs the dichromate anions; a dye-metal complex with chromium(III) is formed. Thus, wool must reduce the dichromate anions.

Mechanism of dichromate anion reduction:

Remember,

Chromium(III) combines with the dye molecules and wool, making chrome dyeing very fast in both wet and light processes.
Using chromium salts to form complex materials leads to heavy metal problems in wastewater from the dyeing process.

4. Metal Complex Dyes

A dye manufacturer has synthesized metal complex dyes by adding a chelating metal atom. For wool, the metal is typically chromium.

There are two categories for metal complex dyes:

1:1 Metal Complex Dyes: This type of dye has one dye molecule and one metal atom. The dyebath is often an extremely acidic bath (pH 2).
2:1 Metallic Complex Dye: This type of dye contains two dye molecules and one metal atom. The dye bath is typically neutral to slightly acidic (pH 6-7).

Remember

The molecular size of 2:1 complex metal dyes is twice that of 1:1 complex metal dyes and therefore is difficult to level on wool.

Metal Complex Dyes Characterize:

The colorfastness of metal complex dyes is good when wet.
Their colorfastness against light is good.

Sources

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