A Complete Guide To Acrylic Fiber Dyeing: Methods, Dyes & Best Practices

Acrylic fiber, due to its soft feel, good warmth retention, and wool-like appearance, is widely used in knitting, sweaters, blankets, and home textiles. However, the dyeing process of acrylic fiber has always been considered one of the more challenging among synthetic fibers. Its dyeing process is highly sensitive to water quality, auxiliary agent selection, temperature control, and dye performance; slight errors can easily lead to problems such as uneven coloring, color spots, and color deviation.

Currently, cationic dyes remain the most important and effective type of dye for acrylic fiber dyeing. To achieve good level dyeing and stable dyeing quality, systematic management is necessary across multiple stages, including dyeing water, pretreatment, dyeing formulation design, and dyeing process control. This article will, based on practical production experience, systematically introduce the key control points and solutions to common problems in acrylic fiber dyeing, providing a practical reference for dyeing plants and technicians.

1. Dyeing Water

Because cationic dyes adsorb onto strongly acidic groups, acrylic fibers are easily affected by metal ions such as calcium ions in the water. Furthermore, some cationic dyes are also susceptible to the effects of metal ions. If the water quality is unsuitable for dyeing, appropriate treatment must be performed first to mitigate the adverse effects of the water on dyeing.

In addition, if tap water contains active chlorine, it must be removed before use.2.Scouring

Simple scouring is performed as needed to remove spinning oils, sizing agents from previous processes, and various contaminants from the acrylic fibers, ensuring smooth operation of subsequent processes such as dyeing and finishing.

The following simple scouring treatment method can be used for acrylic fibers:

Nonionic surfactant 0.5～1.0g/L

Treatment temperature 60℃

Treatment time 20～40min.

Many types of scouring auxiliaries are available; selection can be made from a wide range of products produced by various manufacturers.

3. Bleaching and Whitening

The whiteness of acrylic fibers is already quite high. If even higher whiteness is required, optical brightening treatment or a combination of optical brightening and chemical bleaching is necessary.

Optimal brightening with optical brighteners is a simple and widely used method. There are two types of optical brighteners: cationic and dispersed. Brightening is applied according to the dyeing method for cationic or dispersed dyes. Dispersed optical brighteners are more commonly used because they offer better leveling properties. Many manufacturers produce optical brighteners, allowing users to choose.

4.Methods for Formulating Cationic Dye Dyeing Processes

（1）Determining the Dyeing Formula

The dyeing formula is determined based on the practical properties of the final product, such as colorfastness, and the type of dyeing machine. Generally, the formula can be determined in the following order:

a) Prepare a small sample of dye by appropriately mixing the dye according to the sample. Repeatedly mix colors until a color matching the sample is obtained;

b) Once the mixed color matches the sample, determine the amount of dye to use. Simultaneously select a retarding agent and calculate the optimal dosage using the DC value calculation method described in Chapter 1;

c) Set the dyeing temperature program. For steps a) and b) above, it is best to conduct an immersion dyeing experiment first to confirm whether the leveling effect meets the requirements;

d) Confirm whether the colorfastness meets the requirements using a small dyeing machine;

e) If the dyeing uniformity and colorfastness do not meet the requirements, continue the experiment until the required formula is obtained.

(2).Role of Several Auxiliary Agents

Besides retarding agents, another type of surfactant that plays a leveling role in the dyeing process is a migration-type leveling agent, such as the domestic leveling agent XFM-2. This type of leveling agent belongs to low molecular weight cationic quaternary ammonium compounds and has good migration properties. It can be used for dyeing M-type cationic dyes and dyeing and color correction of dyes with K=5. Due to its low molecular weight, it generally does not occupy the dye site and has no effect on Sf.

Acetic acid is the main auxiliary agent for dyeing acrylic fibers with cationic dyes. It provides hydrogen ions in the dye bath to help the dye adhere. In most cases, acetic acid forms a buffer solution with sodium acetate to stabilize the pH of the dye bath. Acetic acid also dissolves cationic dyes; it is used to prepare the slurry during dyeing and then dissolved in boiling water.

In many cases, urea is added to aid dissolution. This is because the acyldiamino group in the urea molecule can break the intermolecular forces and hydrogen bonds between cationic dye molecules, causing the dye aggregates to rapidly dissociate into unimolecular states. Sufficient dye dissolution helps prevent color spots. Because urea contains amino groups, its cationic content is 2.5 times that of sodium, resulting in better leveling effects than ammonium sulfate.

Ammonium sulfate is an acid-releasing agent. As the dyeing temperature increases, ammonia gas is gradually released, leaving acid in the dye bath, lowering the pH and reducing the dyeing speed, which is beneficial for achieving level dyeing. For dyes where the dyeing process is excessively concentrated in the high-temperature region, an appropriate amount of ammonium sulfate can be added to moderate the dyeing process.

3. Control of Dyeing Temperature For acrylic fibers

High molecular chain segment movement only occurs above the glass transition temperature. The space created by this chain segment movement allows dye molecules to penetrate, diffuse, and fix within the fiber, thus completing the dyeing process.

The chain segment movement of polymers is greatly affected by temperature. Above the glass transition temperature, the degree of chain segment movement increases significantly for every 1°C increase. This explains why, above the glass transition temperature, the dyeing rate increases sharply with increasing temperature, even reaching a 30% increase for every 1°C increase. For acrylic fibers, it is precisely the intense chain segment movement of their polymers and the strong Coulomb forces between the acidic groups within the molecules and the cationic groups in the dye molecules that cause a large amount of concentrated dyeing within a narrow temperature range.

Controlling the dyeing temperature within this narrow temperature range ensures uniform dyeing, strictly controlling temperature rise and fall according to process requirements with minimal error.

4. Several Dyeing Methods for Acrylic Fibers

Generally, a reasonable dyeing process can be developed. Common dyeing methods include isothermal rapid dyeing, saturation dyeing, and ordinary dyeing.

The constant-temperature rapid dyeing method refers to starting dyeing at a temperature above the glass transition temperature and below the boiling point, and holding it at this temperature for 45-90 minutes to ensure that most of the dye is absorbed. Then, the temperature is raised to the boiling point for color fixation, with a relatively short fixation time. Finally, the temperature is slowly lowered to 50°C, and the garment is rinsed and removed from the machine. The key to this method is selecting a constant dyeing temperature. The advent of third-generation retarding agents has significantly increased the safety factor of this method. Furthermore, the dyeing formula for this method is relatively simple. The typical composition includes a buffer solution of acetic acid and sodium acetate, less than 0.2% of retarding agent A, and 0.3%-0.5% of leveling agent 1227. When the dye volume is high, 2.0-3.0% urea is added. The garment to be dyed is loaded into the machine, water is added, and the temperature is raised to 5°C below the set temperature. All auxiliaries (except chemical auxiliaries) are added, and the machine is run for 10-15 minutes. Then, fully dissolved dye solution is added, and the machine is run to the set temperature for holding.

Saturation dyeing refers to a dyeing method where the sum of the products of the amounts of dye and retarder in the dyeing formula and their respective f-values is equivalent to the dyeing saturation value of acrylic fibers.

The key to saturation dyeing lies in controlling the amount of second-generation retarder, as the amount directly affects the depth and hue of the resulting color.

Ordinary dyeing combines the advantages of temperature control without retarder and temperature control with retarder, while also utilizing the characteristics of third-generation retarder, making it a widely applicable dyeing method. In this method, leveling agent 1227 and retarder A in the dyeing formula work synergistically. Leveling agent 1227 primarily functions as a diffusion and washing agent. The amount of retarder in the formula is 0.02%–0.4%, and the amount of leveling agent 1227 is 0.2%–0.5%.

Furthermore, with the development of dyes suitable for acrylic fibers, migration-type cationic dyeing methods and disperse-type cationic dyeing methods have also been developed in recent years. These will not be discussed here.

5. Correction of Color and Dye Spots

If discrepancies are found between the dyeing process and the sample during the use of cationic dyes, please handle it as follows: Before adding cationic dye solution, lower the dye bath temperature to 80℃. After adding dye solution, maintain the temperature at 80℃ and stir for 5 minutes. After thorough stirring, reheat to the boiling point to correct the color. It is best to perform a small-scale test beforehand.

When dye spots appear, handle them as described in Chapter 1.

If the dyeing is severely uneven when using cationic dyes, decolorization is required. This involves chlorine bleaching to chemically decompose the dye, followed by re-dyeing.

Despite the above corrective measures, this can still reduce dyeing quality and even damage the reputation of the dyeing factory. Therefore, before actual dyeing, it is essential to carefully perform small-scale tests, even dyeing, and color matching experiments to prevent accidents and minimize losses.

6. Other Precautions

To achieve a darker dyeing color, appropriate measures can be taken in actual production to ensure that the functional groups on the fiber fully react with the dye. Specific measures include:

1) Increasing the pH value of the dye bath to allow the acidic functional groups of the fiber to fully ionize and bind with the dye;

2) Adding a small amount of carrier, such as β-naphthol;

3) Reducing the amount of second-generation cationic retarder;

4) Extending the boiling dyeing time, or using high-temperature dyeing above 100℃.

Level dyeing of acrylic fibers is mainly achieved by controlling the dyeing rate. Different brands of acrylic fibers have different dyeing rates. Therefore, it is necessary to understand the properties of the acrylic fiber to be dyed before dyeing. Then, an appropriate dye can be selected to address this issue.

In addition, the key dyeing temperature range in the dyeing formula can be identified through the dyeing rate curve. Level dyeing can be achieved by maintaining the temperature within this range or slowing down the heating rate.

7.Special Notes

Finally, regardless of the dye, method, or machine used, some operational issues are common and require special attention:

1) Chemical Additives: Some auxiliaries, such as acetic acid, need to be added during pulping;

2) Cooling: Because acrylic fiber dyeing occurs above its glass transition temperature, sudden cooling will result in uneven fiber hardening due to inconsistent cooling rates, severely affecting the hand feel. Therefore, cooling must be done slowly;

3) Softening Treatment: Cationic auxiliaries are commonly used for softening acrylic fibers.

Overall, the dyeing quality of acrylic fibers does not depend on a single factor, but rather on the coordinated control of multiple process steps. From water quality management, scouring and whitening treatments, to the rational selection of cationic dyes, the scientific combination of auxiliaries, and the precise control of the dyeing temperature program, every detail directly affects the final dyeing effect.

In actual production, controlling the dyeing speed is the core of achieving uniform dyeing of acrylic fibers. Different brands and specifications of acrylic fibers exhibit significant differences in dyeing performance; therefore, conducting thorough small-scale trials and mastering the key dyeing temperature range before formal production is crucial. By rationally designing the dyeing process and stabilizing operating conditions, not only can dye spots and color differences be effectively avoided, but production efficiency can also be improved, and the risks of rework and waste reduced.

With the continuous development of cationic dyes and supporting auxiliaries, the dyeing process of acrylic fibers is also continuously optimized. Only with a full understanding of fiber characteristics and dyeing mechanisms can stable, controllable, and high-quality acrylic fiber dyeing effects be achieved.