Long Way Education

By Eman Abdallah Kamel

Contact the author: emanabdallahkamel@gmail.com

Eman is a writer and an engineer. She received her bachelor’s degree in textile science from the Faculty of Applied Arts, Egypt.

Through this article, you will learn the meaning of direct dyes, their properties and methods, and the direct dyeing process of cellulosic fibers.

Direct Dyes

The direct dyes belong to the azo dye group and are soluble in water. They are mainly used to color cellulosic materials in alkaline or neutral solutions.

Direct dyes are widely used in dyeing natural fabrics such as cotton, linen, and jute. They are also better for dyeing semi-cellulosic fibres such as rayon.

Direct dyes are widely used in fabrics, upholstery, and clothing. However, it does not show the highest standards of wet stability.

There are many applications in which individual dyes have very suitable fixing properties. Dye choice needs great care because of the differences between the fastness properties of individual dyes.

During the 1980s, more than 200 direct dyes were listed in the AATCC Buyer’s Guide by their color index names, with chemically distinct structures. About 25% of these dyes are blue, and 20% are red and yellow. 10% are black, and 4% are green.

The classification of direct dyes by the Dyeing and Colorists Association is based on the compatibility of different groups of direct dyes under certain dyeing conditions.

What are Azo Dyes?

Azo dyes are organic compounds with the functional group R−N = N−R′, where R and R′ are usually aryls.

The Society of Dyers and Colorists Classification

The Dyers and Colorists Association classified direct dyes into three categories:

• Class A

Self-leveling direct dyes: This group has good leveling properties and can dye uniformly even when an electrolyte is added at the start of the dyeing process. However, salt may be required in large amounts to exhaust well.

• Class B

Dyes controlled with salt: These dyes have poor settling or migration properties. They can be dyed in batches uniformly by adding the electrolyte in controlled amounts after the dyebath reaches the dyeing temperature.

• Class C

Salt- and Temperature-Controllable Dyes: These dyes show poor migration. The dyeing rate is controlled by the rising rate of dyeing temperature and the addition of salt.

Did You Know?

Leveling is the transfer of dye from higher to lower concentrations on the fibers through a dye bath. Migration is the transfer of dye from fibers to the solution.

Properties of Direct Dye

• Fastness: Direct dyes have poor wash and lightfastness. Fixing agents can improve their washing fastness, but this treatment reduces their lightfastness.
• Water Effect: There are many metal ions present in small amounts, such as calcium, magnesium, copper, and iron. They can cause shade change problems with direct dyes. Regular means of removing these metal ions include sequestrants such as ethylenediaminetetraacetic acid or nitrilotriacetic acid. Sodium hexametaphosphate is the preferred material for direct dyes. Small amounts of residual chlorine-containing chemical types used to purify water can affect many dyes. So anti-chlorines, such as sodium bisulfite (NaHS03) or sodium thiosulfate (Na2S203), can be added to all water in which dyes will be dissolved.
• Time Effect: Usually, longer dyeing times increase dye uniformity.
• Temperature: Depending on the temperature, some dyes can hydrolysis above 130°C (265°F). Direct yellow 105, orange 39, and blue 80 are all suitable for high-temperature stability. In addition, there are some unsuitable dyes, including direct yellow 44, red 80, and red 83.
• Electrolyte Effect: The presence of sodium anions causes a high chemical potential for direct dyes in the solution. Sodium chloride and sodium sulfate can be used. Although sodium chloride is more economical, it has a much higher tendency to corrode stainless steel under the high-temperature conditions possible in jet dyeing or dyeing machines. Sodium sulfate is preferred for these applications, despite being slightly more expensive.
• pH Effect: Direct dyes are usually applied at or around pH 7. The solubility of some direct dyes increases at an alkaline pH. Alkaline dyeing may slow the rate of exhaustion. There is a possibility that some cellulose ionization will occur to produce cellulose ions. These carry a negative charge and can repel dye anions.
• Fixing Agents: Cationic stabilizing agents are organic chemicals with large molecules that dissolve in water. They dissociate into a large positively charged, or cationic, fraction and a small negative ion, such as a chloride ion. They may be resinous derivatives of cyanamide or quaternary ammonium compounds with long hydrocarbon chains. The positive ions are attracted to the direct dye anion to form a large complex salt molecule with very low solubility, thus improving wet fastness. Possible problems include shade changes and reduced lightfastness.

According to a 2020 study, multifunctional stabilizing agents have been developed that bear reactive groups capable of forming more permanent bonds with other suitable groups in the dye or fiber.
To enhance the dyeability of cellulosic fibers using direct dyes by pretreatment with a large variety of cationic products, usually based on nitrogen. A modification of cellulosic fibers with cationic agents increased the intrinsic properties of anionic dyes on cellulosic fibers by creating new cationic sites.

Jigger dyeing machine and winch dyeing machine. Image source: wikimedia.org

Direct Dyeing Methods

Direct dyeing methods are divided into Batch dyeing and Pad dyeing

1. Batch dyeing:

In this process, dye is gradually transferred from a dye bath onto the fabric inside the same piece of equipment. There are different methods of batch dyeing, depending on the type of machine. Batch dyeing machines include jigger dyeing machines, winch dyeing machines, jet dyeing machines, and beam dyeing machines.

2. Pad dyeing:

During pad dyeing, fabrics are passed between rollers to apply the dye. Pad dyeing is divided into semi-contentious and contentious processes.

The semi-continuous dyeing process consists of pad-batch and pad-jig.

1. Pad-batch process: The fabric is padded in the dye bath. It then shrinks to less than 100%. After that, the cloth is wrapped with a sheet of polythene. For cold batching, the fabric batch is allowed to roll at ambient temperature for 8–24 hours, depending on the depth of shade. For hot batching, the cloth is heated to 80–90°C using infrared rays. Then the cloth batch is left to roll in a closed room for 1–8 hours. To prevent the top layer of fabrics from drying out, it is important to keep the humidity constant.
2. Pad-jig process: To improve the solubility of deep shades, the fabric is padded with a solution containing a dye-wetting agent, an anti-foam agent, and urea. Afterward, the cloth is pressed to get 60–80% pickup. In a jigger containing 5–15 g/L salt and 10–40 mL/L dyebath pad (for deep shading). The temperature of the jig is raised to a boil and maintained for 20–30 minutes.

Continuous padding operations: pad-dry and pad-steam.

1. Pad-dry process: Fabric that has been padded in a dye bath is then dried. The drying can be done in two stages: first, IR drying (with a residual moisture of 5–10%). Then a hot flue-drying and fixing process is applied (100–150°C) for half a minute to two minutes. This depends on the depth of the shade and the fabric GSM.
2. Pad-steam process: After padding, the fabric is exposed to saturated steam at 100–105°C. The addition of urea (as a disaggregating agent): Urea is added to the padding in an equal amount of dye so that the dye is soluble and easier to diffuse during steaming. Urea should be increased if the steaming temperature is less than 100°C. In addition, sodium alginate should be added to prevent migration. After steaming, in the case of deep shadows, the fabric is passed through 1-2 boiling water baths containing 10–20 g/L of salt and then washed.

Direct Dyeing Process of Cellulosic Fibers

Dyeing cellulosic fiber with direct dyes is the simplest dyeing process. However, great care must be taken when dyeing semi-cellulose fibers because the larger pores of viscose compared to natural cellulose speed up the absorption phase. As a result, it is more likely that the dye will not be uniformly distributed.

When dyeing cellulose fibers such as cotton or linen, keep the following points in mind:

1. Water Quality: It is necessary to adjust the amount of water to ensure easy circulation according to the nature of the fabric and the model and design of the dyeing machine. If pure water is not available, desalting agents must be added to prevent mineral or lime deposits from forming on the fabric. Due to these deposits, the fabric is unable to absorb dye readily.

2. Adding Agents: For the correct preparation of the dye bath, some agents are required, such as:

• Wet agents: They increase dye penetration into fiber depth.

• Scattering agents: To prevent dyes from accumulating due to the colloidal effect of the dye ions.

• Leveling agents: They help increase migration rates.

3. Dissolving and Adding the Dye: The dye is first dissolved in warm water, then the rest of the water is added while stirring. Soda ash, or urea, is also necessary for good dissolution. A solution is filtered and then added slowly and carefully to the dye bath.

4. Addition of Salt: Salt is usually added to increase dye migration rates. Salt is usually sodium sulfate or sodium chloride (table salt).

5. Addition of alkali: Some direct dyes require the addition of alkali, usually 2 g per liter of sodium carbonate.

6. High Temperature: After adding the dyeing bath materials, it is stirred or rotated for 5–10 minutes at a cold temperature to ensure the highest exhaust of the bath dye. The temperature is raised according to the following:

• Model of dyeing machine, water rate, and fabric rotation speed.

• Leveling and percentage of dye.

In proportion to the amount of dye, boiling continues until dye penetration of the fabric reaches leveling and stability.

7. Cold Water: Changing the dye bath water without stopping the fabric’s rotation to prevent staining the fabric. Cold water is pumped from the bottom of the dye bath and drained from the top.

8. Re-leveling: This process is usually done by re-boiling in the presence of higher amounts of the equalizing agent. The boiling process continues until the appearance of non-leveling disappears to the naked eye.

Sources

• chemicalbook.com Product Catalog.
• academia.edu… Dyeing with Direct Dyes.

©Eman Abdallah Kamel, 2023

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