Dyeing of Acrylic Fibers with Basic Dyes

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.

Here, you will learn about dyeing acrylic with basic dye, including fabric preparation, dye bath preparation, the dyeing process, and the effect factors.

Introduction

Acrylic is a synthetic polymer (polyacrylonitrile). The fibers are produced by dissolving the polymer in a solvent such as N, N-dimethylformamide or aqueous sodium thiocyanate. Acrylonitrile polymer contains sulfates and sulfonated terminal groups, which give it an acidic character. Other acidic groups may be present due to acidic co-polymers such as allyl sulfuric acid or acrylic acid. Acrylic is lightweight and has a soft touch.

Basic dyes are salts of organic bases in which a chromophoric system is linked to a cation, while the anion is colorless. Acrylic fibers have an affinity for basic dyes due to the anionic nature of their nitrile groups.

Process

The process of dyeing acrylic includes three stages:

Preparation of the fabric.
Preparation of dyeing path.
Dyeing Process

1. Preparation of the Fabric

Follow these steps for better dyed fabric.

1. The preparation of acrylic fibers involves destarching the woven fabrics, cleaning, and bleaching. Dilute alkaline solutions of ammonia or sodium pyrophosphate (Na₃HP₂O₇) can be used for cleaning. The material can be bleached using sodium chlorite (NaClO₂) and formic acid.

Remember

The use of a non-ionic detergent is essential; cationic auxiliaries can block the anionic groups in the fibers and hinder dyeing, while residues of the anionic product may react with the cationic dyes in the solution and even precipitate. Sometimes, acrylic materials acquire a pale yellow tint, which usually indicates that the drying process was too harsh.

2. Some cationic dyes are highly sensitive to chlorine, causing their colors to fade quickly. Fabrics bleached with sodium chlorite may require treatment with an antichlorine agent. Adding small amounts of sodium bisulfate or sodium thiosulfate to the dyeing solution can also prevent cationic dyes from becoming sensitive to chlorine.

2. Preparation of Dyeing Bath

1. Powdered dyes are typically made into a paste using acetic acid, then mixed with boiling water. For brightly colored cationic dyes, it is essential to prevent dust from escaping the powder. These dyes’ solid forms are often difficult to dissolve because they tend to form a viscous material. Preparing a paste using methanol and adding hot or warm water is sometimes an effective alternative. Concentrated liquid dyes, however, do not suffer from this problem.

2. Some cationic dyes are unstable in boiling water; many react with alkalis to produce colorless compounds, such as free amines resulting from the neutralization of the ammonium ion group, or carbene resulting from the reaction of the cation group with the hydroxide ion. Therefore, cationic dyeing is carried out in a weakly acidic solution.

The mechanism of acrylic dyeing using basic (cationic) dyes.

3. Dyeing Process

1. The dye solution is typically prepared by mixing it with acetic acid. A stable pH between 4.5 and 5.5 can be achieved by adding sodium acetate.

Did Yoy Know?

If the dye does not spread evenly at the beginning, it will be difficult to distribute it evenly, so it is extremely important to control its initial spreading rate. This rate is controlled using retarding agents.

2. The level dyeing process necessitates precise control of the cationic retarding agent concentrations. These agents initially block anionic sites in the fibers and are then gradually replaced by other agents. Adding up to 2.5 g/L of anhydrous sodium sulfate helps neutralize the negative surface charge, and sodium ions also weakly block anionic sites in the fibers. Both effects reduce the initial dye absorption rate. Sodium sulfate is less effective than cationic retarding agents, which have some substantivity for the fiber.

3. The dye bath is then slowly cooled to 50-60°C. Rapid cooling by adding cold water to the dye bath can be harmful because it causes the fabric to wrinkle immediately. The fabric is then gently cleaned with a non-ionic detergent containing a small amount of acetic acid and fabric softener.

Factors Affecting the Dyeing Process

1. Retarder: The ideal acrylic retarder provides even, uniform dyeing and allows for complete dye saturation. There are two types of retarding agents:

Retarding agents that form a complex with the dye, thus keeping it in the dye bath.
Retarding agents that compete with the dye for ionic dye sites in the ion-exchange dyeing process.

2. Temperature: Temperature control is crucial when dyeing acrylics with basic dyes. Therefore, the following should be taken into account:

Precise temperature control is essential when dyeing acrylic fibers with cationic dyes to avoid uneven coloring.
Once the bath temperature reaches 70-75°C, the heating rate typically decreases significantly.

Sources

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