Suede and Nubuck Dyeing Techniques and Dye Knowledge

1. Common Dyes Used for Leather Dyeing
1. Acid Dyes
Acid dyes are mostly sodium sulfonates of aromatic compounds (some contain carboxyl groups -COOH), dispersed in aqueous solution to form colored anions. Acid dye molecules are small, have strong penetration, and easily diffuse into the leather, facilitating thorough dyeing. They are among the most commonly used dyes for leather dyeing. Chemically, acid dyes mainly include aromatic methylene dyes, quinonoid imine dyes, anthraquinone dyes, azo dyes, and nitro dyes. Acid dyes have good lightfastness. Because acid dyes carry a negative charge, they are suitable for dyeing chrome-tanned leather and other metal-tanned leathers with positively charged surfaces. In acidic media, acid dyes can dye protein fibers (leather, fur, wool, silk) and polyamide fibers.
(1) Strong Acid Dyes
Strong acid dyes can dye leather and fur in acidic media. They have simple molecular structures, low molecular weight, contain sulfonic or carboxyl groups, and have low affinity for leather. The dye can migrate evenly on leather, resulting in uniform coloration.
Although strong acid dyes produce bright colors and good uniformity, the dyeing is not deep and wet fastness is poor. Therefore, improvements were made to strong acid dyes, resulting in the development of weak acid dyes.
(2) Weak Acid Dyes
Weak acid dyes have more complex molecular structures than strong acid dyes and have greater affinity for leather. They can dye leather in weakly acidic media. During dyeing, dye molecules bind to leather fibers not only through ionic bonds, hydrogen bonds, and other polar bonds but also through intermolecular nonpolar van der Waals forces.
Weak acid dyes are inexpensive, have good penetration, strong dyeing power, and a complete color range. They can produce bright tones and are the most commonly used dyes for leather dyeing. They can also be combined with other dyes for leather dyeing.
2. Direct Dyes
Direct dyes are also commonly used for leather dyeing. Chemically, most direct dyes are sodium sulfonates of aromatic compounds that dissociate into anionic chromophores in water and dye leather fibers directly under the action of electrolytes. Direct dyes also have affinity for cellulose fibers and amphoteric fibers.
Direct dyes generally have larger and longer linear macromolecules than acid dyes, with lower penetration than acid dyes. Their color brightness is not as good as acid dyes, but they have better covering power and provide excellent surface dyeing effects on chrome-tanned leather. They are the main dyes for "surface coloring" of leather.
Direct dyes have a complete color range, good uniformity, easy color matching, rich color, and are convenient to use. Therefore, they are widely applied in leather processing and often combined with acid dyes for dyeing various leathers.
3. Metal Chrome Complex Dyes
Most of the earliest dyes used for leather were transplanted from the textile industry. With the development of leather chemicals, leather-specific dyes represented by metal complex dyes have emerged. These dyes have high fastness, compatibility, and uniformity, and are increasingly used on leather. According to their molecular structure characteristics, they can be divided into the following two groups:
(1) 1:1 Metal Complex Dyes
Complex dyes formed by one metal atom combined with one dye molecule are called 1:1 metal complex dyes. These dyes have good fastness, rich and bright colors, and can be well mixed with each other to obtain a wide color range. They also have good lightfastness, wash fastness, and uniformity. They are mainly used for dyeing fur, filament, and leather.
(2) 1:2 Metal Complex Dyes
Complex dyes formed by one metal atom combined with two dye molecules are called 1:2 metal complex dyes. Also known as neutral dyes, they mix well with each other and can be used with other types of dyes. This group of dyes has good lightfastness, wet fastness, and sweat fastness, and also has good dyeing properties for vegetable or combination tanned leather. Due to their relatively large molecules, their penetration ability is reduced. 1:2 metal complex dyes come in powder and liquid forms (the liquid form is called dye liquor in laundries). The advantage of the liquid form is that it does not produce dye dust, is easy to measure and add, and more importantly, the liquid dye contains low salt content.
Leather-specific dyes represented by metal complex dyes have high fastness, compatibility, and uniformity, and are increasingly used on leather.
Other dyes that can be used for leather dyeing include mordant dyes, reactive dyes, basic dyes, sulfur dyes, oxidation dyes, and dyes soluble in organic media.
2. Basic Requirements for Dyes in Leather Dyeing
Since leather is a special bio-modified material different from woolen, cotton, paper, and textiles, there are some basic requirements for the dyes used.
1. Alkali Resistance
Dissolved dyes should have certain stability against dilute alkalis such as soda ash or ammonia water and should not form color precipitates.
2. Acid Resistance
Different dyes have varying sensitivities to acids. For example, acid dyes are more sensitive to acids; adding acid easily causes them to form "lake pigments," which helps fix the color. However, other dyes are less sensitive to acids.
3. Color Strength
Color strength indicates the dye's ability to color the substrate and is an important indicator of the dye. The higher the color strength, the less dye is needed to achieve the same shade.
4. Purity of Dye
The purity of the dye directly affects the purity of the shade. The purer the dye, the fewer the secondary colors, making the dyeing easier to control. Generally, the content of secondary color components should not exceed 5% of the dye.
5. Hard Water Resistance
Leather dyes react to hard water to varying degrees. Therefore, during leather dyeing and fatliquoring, it is best to use soft water or water that has been softened to ensure dyeing quality.
6. Impurity Content in Dye
Dye manufacturing involves many chemical processes and production stages, so dye products always contain some impurities that affect dye strength. The higher the impurity content, the lower the dye strength.
7. Solubility
The solubility of a dye is an important indicator of its use, especially in low-temperature dyeing, powder dyeing, and no-bath dyeing. Poorly soluble dyes can form spotty stains on the grain or flesh side of leather and cause color shifts when mixed. However, if the dye is too soluble, its hydrophilicity makes dye uptake difficult; after acid fixing, it will heavily color the leather surface, causing floating color.
8. Absorption Performance
Dye absorption performance is evaluated by the percentage of dye absorbed by the leather per unit time after dyeing relative to the total original dye amount. Dye absorption depends not only on the dye's chemical structure but also on the leather's tanning method, type and amount of retanning agents, pH, and dyeing temperature. Dye absorption ability is used for coordinating with other dyes; various dyes need similar absorption performance to ensure accurate leather color.
9. Color Fastness
Leather dyes should have good color fastness, including light fastness, dry/wet rubbing fastness, migration resistance, and heat resistance. For washable leather, dyes must also have excellent wash fastness, sweat fastness, alkali resistance, and even dry cleaning fastness.
III. Leather Dyeing Process
Due to the large differences in leather processing methods and tanning agents used, dyeing methods also vary.
1. Dyeing of Chrome-Tanned and Multi-Metal Combination Tanned Leather

Operating procedure: First adjust the water temperature, add dye and fatliquor, and stir thoroughly until the dye is completely dissolved in water. Put in the cleaned leather garment and stir and rub quickly for 10 minutes. Add retanning agent (pre-dissolved in hot water) and stir for 20 minutes. Add diluted formic acid (1:20) in two portions at 5-minute intervals, adjust pH to 3.5–3.8, stir for more than 10 minutes, rinse once with cold water, dehydrate, then rinse 1–2 times, dehydrate again, place in a drying machine (if no drying machine, use dry cleaning machine or washing machine without detergent as a substitute, same below) and drum for 10 minutes (no heating) or manually stretch and shape. Hang upside down to dry for 2–3 hours, then hang on appropriately sized hangers in a shaded area without direct sun or fire. When about 80% dry, remove and seal in plastic bags for more than 8 hours. Then take out and put back into the drying machine to drum for 30 minutes or manually soften, shape, and iron.
2. Dyeing of Vegetable-Tanned and Combination Tanned Leather
The process of treating rawhide with vegetable tannins to turn it into leather is called vegetable tanning, and the leather produced is called vegetable-tanned leather, or simply veg-tan leather. Vegetable tannins are extracted from tree bark, so they are also called natural tannins. Vegetable-tanned leather has advantages such as dense structure, firmness, fullness, good formability, strong sweat and water absorption, and environmental friendliness, making it a popular leather type in recent years.
During vegetable tanning, the tannin molecules with negative charges react ionically with the positively charged functional groups in collagen, causing the leather fibers to carry negative charges. Since the leather dyes used are mainly anionic dyes, the same charge makes dyeing difficult. To achieve rich and pure colors, a retanning pretreatment is done before dyeing to artificially give the leather fibers positive charges, improving the uptake and fixation of anionic dyes.
Processing procedure as follows:

Note: Because iron salts can chemically react with vegetable tannins to form black or blue-black precipitates that are difficult to remove, and sometimes cause the leather surface to darken during ironing, vegetable-tanned leather must not come into contact with iron during washing or dyeing.