Dyeing is a crucial process for imparting color to fabrics and fiber yarns, and it plays a key role in enhancing the value of textiles. Different types of textiles are suited to different dyeing methods. Let’s take a look below.

 

1. Direct dyes

Direct dyes can be used for dyeing a wide range of cotton and viscose textiles. During dyeing, the fabric is first wetted with water, the dye solution is added, the temperature is raised to 95°C, and a neutral salt is introduced to promote dye uptake; the dyeing time is 60 minutes. After dyeing, the fabric is rinsed thoroughly, and, if necessary, treated with a color-fixing agent to enhance color fastness.

 

For silk dyeing, direct dyes with high exhaustion and good color fastness are often selected to complement the limited color range of acid dyes, particularly for deep shades such as browns, blacks, and dark greens. Dyeing can be carried out in neutral or slightly acidic baths, and salt is frequently added to enhance dye uptake. To improve the hand feel after dyeing, the fabric may be treated for a specified period in a dilute acetic acid bath.

 

2. Vat Dyes and Soluble Vat Dyes

 

① Dyeing with vat dyes

 

Vat dyes exhibit vivid hues, a complete color spectrum, and excellent fastness, making them high-grade dyes for cellulose fiber dyeing. Vat dyeing can be carried out by two distinct processes: leuco dye immersion and suspension padding. In leuco dye immersion, the dye is first treated with sodium hydrosulfite (a strong reducing agent) and caustic soda to reduce it to its leuco form, which is then dissolved in the alkaline bath. The fabric is subsequently introduced into the bath, with temperature controlled and the amount of common salt adjusted to ensure an optimal dye uptake; if necessary, a leveling agent may be added to regulate the dyeing rate. Upon completion of dyeing, an oxidation treatment is required to convert the leuco form back into the water-insoluble, original dye state, thereby fixing it firmly onto the fibers.

 

Finally, soap washing is carried out to enhance the color brilliance of the dyed fabric and improve its fastness. Suspension padding dyeing is performed on continuous padding equipment. The dye is not reduced; instead, it is formulated into a dispersion containing finely divided particles, which is then applied to the fabric by padding, followed by drying. The fabric is subsequently padded again with an alkaline reducing bath containing sodium hydrosulfite, after which it is fed into a sealed steaming chamber to promote dye reduction, dissolution, diffusion, and penetration. Finally, oxidation and soap washing are conducted in a flat-washing unit.

 

② Dyeing with temporarily soluble vat dyes

 

Soluble vat dyes are predominantly sulfate ester salts of the leuco forms of vat dyes; they are directly soluble in water and can dye fibers without requiring a reduction step. During dyeing, the bath contains the dye, common salt, and the oxidizing agent sodium nitrite. After immersion or pad–dry dyeing, the fabric is treated with a dilute sulfuric acid solution, which hydrolyzes the dye into its leuco acid form; this leuco acid is then oxidized by the oxidizing agent or by atmospheric oxygen to develop color.

 

3. Sulfur Dyes

Sulfur dyes are commonly used for dyeing cotton textiles in deep, intense shades of black and blue. The dyeing process is similar to that for vat dyes: the dye must first be reduced to a soluble leuco form, applied to the fabric, and then oxidized to develop color. However, sodium sulfide, which is relatively inexpensive, is typically used as the reducing agent, and oxidation is generally easier than with vat dyes. Taking roll dyeing of cotton fabric as an example, the fabric is immersed in a leuco dye solution at elevated temperature for a specified period, followed by water washing and oxidation; finally, sodium acetate is applied as a anti-creasing treatment.

 

4. Insoluble Azo Dyes

 

Insoluble azo dyes consist of two components: a coupler and a developer. The dyeing process is essentially the reaction of these two water-soluble components on the fiber to form an insoluble color precipitate. During dyeing, the fabric is first impregnated and rolled with an alkaline solution of the coupler—commonly referred to as “base treatment”—and then dried at low temperature. It is subsequently impregnated and rolled with a developer solution for color development; after airing or steaming for a specified period, the fabric undergoes water washing and soap boiling, followed by thorough rinsing with clean water. This dye can also be applied by immersion dyeing, which likewise involves two steps: “base treatment” and “color development.”

 

5. Reactive Dyes  

 

Reactive dyes contain reactive groups that can form covalent bonds with specific functional groups on fibers, resulting in excellent wash and rubbing fastness. Moreover, they are suitable not only for dyeing cellulosic fibers but also for dyeing wool, silk, and nylon.

 

① Dyeing of cellulose fibers

 

The hydroxyl groups on cellulose fibers can react with reactive dyes under alkaline conditions; therefore, the presence of an alkali is essential during dyeing. In both batch dyeing and continuous pad–dyeing processes, the fabric is typically first treated with a dye bath that does not contain an alkali, after which an alkali is added to the dye bath or an alkaline solution is applied directly to the fabric to facilitate the reaction between the dye and the fiber. Finally, the fabric is thoroughly washed with water and subjected to soap scouring to remove unfixed dye.

 

An energy-saving dyeing method is the cold pad–batch process, in which the fabric is first impregnated and rolled with a dye having high reactivity while a定量 amount of alkali solution is simultaneously applied; no heat treatment is performed, and the fabric is directly rolled up and left to stand at room temperature for a specified period to allow the dye to complete diffusion and fixation. Finally, the fabric is rinsed with water and boiled in a soap solution.

 

② Dyeing of wool, silk, and nylon

 

Wool and silk fibers contain various functional groups that can react with reactive dyes; some of these groups are already reactive in weakly acidic to neutral media. Consequently, the dyeing conditions for wool and silk differ from those used for cellulosic fibers when employing reactive dyes.

 

Wool dyeing is typically carried out under mildly acidic conditions, with acetic acid used to adjust the pH and sodium sulfate employed as a leveling agent; neutralization and water washing follow dyeing. For silk, various dyeing processes can be used, including weakly acidic or neutral dyeing as well as alkaline dyeing; alternatively, the fabric may first be dyed in a weakly acidic or neutral bath and then fixed in an alkaline bath. Nylon has fewer reactive groups than wool and silk, making it difficult to achieve deep shades with reactive dyes; its dyeing process is therefore similar to that of silk.

 

6. Acid Dyes, Acid Mordant Dyes, and Acid Dyes with Mordants

 

Acid dyes, acid mordant dyes, and acid dyes with mordants are all water-soluble dyes specifically used for wool, silk, and nylon; however, they differ in structure, performance, and application scope. Acid dyes have a simple molecular structure and can be classified according to molecular size into strong-acid dyes (also known as level-dyeing acid dyes, with smaller molecules) and weak-acid dyes (also known as shrink-resistant acid dyes, with larger molecules). The former can dye wool in strongly acidic media, yielding bright colors and excellent level dyeing; the latter can dye wool, silk, and nylon in neutral or slightly acidic media, offering higher color fastness, but inferior level dyeing and color brilliance compared with strong-acid dyes.

 

Acid mordant dyes possess the basic structural features of acid dyes, but their structures also include functional groups that can form complex bonds with chromium ions as mordants. These dyes are primarily used for wool dyeing; they exhibit superior wash and light fastness compared with conventional acid dyes, though their color shades tend to be darker. Dyeing is typically carried out by the immersion method, followed by a mordanting treatment with potassium dichromate for a specified duration.

 

Acid mordant dyes already contain chromium complexes in their molecular structure, eliminating the need for a mordanting step during dyeing and making them convenient to use. There are two types of acid mordant dyes: one is the 1:1 type, in which a single chromium ion is complexed with one dye molecule; the other is the 1:2 type, in which a single chromium ion is complexed with two dye molecules. The former exhibits dyeing behavior similar to that of strongly acidic dyes and is primarily used for wool; the latter is commonly employed for neutral-bath dyeing of wool, silk, and nylon, hence its alternative name “neutral dye.” It offers higher color fastness but produces relatively darker shades.

 

7. Disperse Dyes

 

Disperse dyes are the primary class of dyes used for dyeing synthetic fibers, particularly polyester fibers. They exhibit excellent dyeing performance on polyester and generally offer high color fastness. Disperse dyes lack ionizable functional groups in their molecular structure, classifying them as nonionic, hydrophobic dyes that are insoluble in water. Instead, they remain stably dispersed in the dye bath as fine particulate suspensions. Moreover, their simple structure and relatively small molecular size confer strong affinity for the hydrophobic, tightly packed structure of polyester fibers, enabling efficient diffusion into the fiber interior and resulting in very high dye uptake.

 

When disperse dyes are used to dye polyester, a high-temperature condition of above 120°C is essential. This is because only at such elevated temperatures does the thermal motion of the polyester macromolecular segments become sufficient to create the “transient microvoids” required for dye diffusion. There are three main methods for dyeing polyester with disperse dyes: the carrier method, the high-temperature high-pressure method, and the thermofusion method. Among these, the high-temperature high-pressure method is the most widely used; the dyeing process is carried out in a sealed, pressurized vessel at temperatures up to 130°C, resulting in very high dye uptake. To control shade and ensure uniform dyeing, acetic acid and leveling agents are often added as auxiliaries.

 

8. Cationic Dyes

 

Cationic dyes are the primary dyes used for dyeing acrylic fibers, offering not only vibrant colors and high exhaustion but also excellent colorfastness. This is because the macromolecules of acrylic fibers contain acidic functional groups that ionize in water to carry a negative charge, resulting in strong adsorption of cationic dyes and consequently high dye uptake rates and rapid dyeing kinetics.

 

Cationic dyes exhibit poor migration on acrylic fibers, and coupled with their relatively rapid uptake, they are highly prone to color unevenness defects. Therefore, the dyeing process must be strictly controlled, and leveling agents should be used. Commonly employed leveling agents include acetic acid, sodium sulfate, and cationic surfactants. Dyeing is typically carried out by the exhaustion method: the dye bath pH is adjusted to 4.5 using acetic acid or similar agents, a leveling agent is added, the fabric is introduced into the bath at room temperature, and the heating rate is carefully controlled. After reaching boiling, the dyeing is continued for a specified period, followed by gradual cooling and thorough water washing.