By Eman Abdallah Kamel
Eman is a writer and textile engineer. She received her bachelor’s degree in textile science from the Faculty of Applied Arts, Egypt.
In this article, you will learn about reactive dye properties, factors that affect dye baths, methods, risks, and safety tips.
What is a reactive dye?
A reactive dye is a dye that, under suitable conditions, can react chemically with a substrate to form a covalent dye-substrate linkage. The goal of dyeing is to color the entire material so that the dye penetrates the fibers and the visible surface has an equal color. Good penetration of dyes ensures ideal fastness properties.
A reactive can be represented by R-B-X (no matter what class of chemicals it belongs to), where R is the chromogen (the color-producing part), X is the reactive system, and B is the bridging group, connecting the other two. When the dye reacts with the fiber (F), it forms R-B-X-F, in which X-F is the covalent bond. Both atoms (fiber and dye) give an electron to the bond, and the resulting pair of electrons is shared. These bonds share a large amount of energy and require more energy to split them apart again.
There are a variety of types of reactive dyes, including azo, vinyl sulfone, cyanuric chloride, and bifunctional dyes, used to dye a wide range of fibers, such as cotton, silk, linen, wool, rayon, bamboo, and aramid.
Properties
Reactive dye-colored textile materials have:
- Very good lightfastness.
- Moderate to good wash fastness.
- The formation of covalent bonds occurs in an alkaline medium.
- These dyes produce bright shades such as orange, pink, and purple.
- Sweat and air pollution are slightly acidic, may affect textile materials colored with reactive dyes, and cause some fading.
Many factors affect the reactive dye basin:
- Salt Effect
- Alkali effect
- Temperature
- pH Effect
- Time
- Nature of Fiber
1. Salt Effect: The precise and desirable shade cannot be achieved without salt. Both Glauber’s salt and regular salt (sodium chloride) are used. In reactive dyeing:
- Salt increases the affinity of the pigment for the substrate.
- Salt increases the rate of reactive dye exhaustion.
- Reactive dyes require more inorganic salt to speed absorption because of their lower affinity. High-fixation dyes can help reduce the amount of inorganic salt used.
2. Effect of Alkali: The reactive dyes react with fibers in the presence of alkalis to form a strong covalent chemical bond between the carbon atom of the dye molecule and the oxygen atom of the hydroxyl group in cellulose.
A liquid alkaline composition for use in reactive fiber dyeing of cotton and cotton blends. Liquid alkali is a sodium hydroxide, potassium hydroxide, and potassium carbonate formed by carbon dioxide with a potassium hydroxide solution and adding sodium hydroxide to the resulting mixture. The resulting solution has sufficiently high alkalinity to achieve the reaction between the dye, including vinyl sulfone dyes, and the fiber. The composition preferably includes potassium carbonate, potassium citrate, and potassium polyacrylate. Citrates and polyacrylates act as dispersants in the dyeing process and as crystallization inhibitors in solution. Carbonate compounds instead of silicates or phosphorus prevent the discharge of untreatable toxic wastewater into natural waterways. To learn more about this invitation, visit liquid alkali for reactive dyeing of textiles.
3. Temperature: According to a study, samples tied and dyed with standardized dyeing conditions of reactive red dye exhibited excellent wash fastness, both in color change and color staining, for the samples dyed with 20 g/l sodium carbonate at 60 °C.
4. Effect of pH: Using Remazol Yellow RR, researchers assessed the colorfastness and spectrophotometric characteristics of several dye baths ranging in pH from 2 to 10. To achieve the pH, acetic acid, soda ash, and caustic soda were added in that order. The impact of pH is analyzed based on color strength, chromaticity, hue angle, spectrophotometric image, reflectance percentage, and color fastness tests. Gradually, elevated pH boosts color strength and chromaticity, while at the same time, downgrades in hue angle result in an orangish tone in the dyed fabric. Even more, the lightness of dyed fabric decreases and redness rises with the increase in dye bath pH, which results in an orangish-red darker tone of dyed fabric. As a result, fabric dyed at an acidic bath pH exhibits higher resistance to color change and color staining from dyed fabric and multifiber fabric, respectively, and vice versa.
5. Time Affect: Reactive dye solutions should not be kept for extended periods, as this can lead to hydrolysis and a loss of color. The duration of dyeing should be shorter, especially when the process is carried out at higher temperatures or with more alkalis. The time for exhaustion and fixation must be optimized because running the process beyond the calculated time may attract further hydrolysis.
6. Nature of Fiber: The accessible free volume in the fiber influences dye absorption. The larger the volume, the more dye is absorbed. Polished cellulose has the lowest free volume, bleached cellulose is slightly higher, and mercerized cellulose has the largest free volume due to the removal of impurities. Viscose has a larger free volume than mercerized cellulose.
Methods
There are many methods for applying reactive dye to cellulosic fibers, such as:
- Exhaust Dyeing Process
- Continuous Dyeing Process
- Semi-continuous Dyeing Process
- One-step Method without Salt
- Infrared Heating Method
A winch and jigger are used in the exhaust dyeing process. The first photo is of a winch dyeing machine, and the second is of a jigger dyeing machine. The third photo is of the exhaust dyeing process.
1. Exhaust Dyeing Process
The basic principle in the exhaust dyeing process is to exhaust as much of the dye as possible onto the fiber using neutral or weakly acidic conditions before initiating the fixation step. The initial dyebath contains a dye dissolved completely in water, a solubilizing agent such as urea, and an electrolyte such as salt or Glauber’s salt. Under these conditions, the dye reacts very slowly, and leveling happens. The pH of the dye bath is then increased by alkali addition so that dye-fiber fixation occurs. The particular type of reactive dye being used usually governs the choice of temperature. The last step is the wash-off process, which removes unfixed dye, alkali, and electrolyte. Unleveled dyeing may happen if the temperature rise and salt addition rate are not carefully regulated.
Did You Know?
According to a study to know the dyeing properties such as exhaustion, fixation rate, migration rating, leveling, and wash fastness of five reactive dyes on 100% cotton fabric by the exhaust method. Drimaren Yellow CL-2R, Drimaren Red CL-B, Drimaren Blue CL-BR, and Drimaren Blue HF-2B showed medium exhaustion and fixation behavior, a good migration rating, level dyeing, and good wash fastness to color. Drimaren Orange CL-3R showed high exhaustion-fixation behavior, poor migration rating, level dyeing, and good wash fastness to color. Drimaren Navy CL-R showed initially low but later medium exhaustion-fixation behavior, a good migration rating, level dyeing, and good wash fastness to color.
2. Continuous Dyeing Process
The basic process involves using a padding unit to impregnate the fabric in an open-width form using a dyebath. There are two possible methods for completing this process: one bath or two baths. In a one-bath process, the alkali is incorporated into the lone-pad bath, whereas in a two-bath process, the alkali is padded from the second bath. Then, the impregnated fabrics undergo the fixation procedure, typically completed with steam fixation or dry heat, and then washed off.
There are various techniques for the continuous dyeing process, such as:
- Pad-dry-steam-process
- Pad-dry-cure-wash
- Pad-dry-pad (alkali)-steam-wash
Pad-dry-steam-process: Reactive dye solution containing urea (50 G/L), Ludigol/Resist salt (10G/L) (to protect the dye from reducing fumes), and soda ash (6–12 G/L) is padded at room temperature. The padded fabrics are then dried in a hot furnace at 60–110 °C, after which the fabrics are steamed for 6–8 min at 100–105 °C. The steamed fabrics are then washed off.
Soda ash should not be added to the padding with dye-liquid until immediately before dyeing starts. In this process, the dyes have less tendency to migrate because of the intermediate drying operation. The fabrics are run dry into the steamer, which requires a longer steaming time (6–8 min).
Pad-dry-cure-wash:
The reactive dye is padded on cellulose in the presence of sodium bicarbonate and urea (150–200 g/L). After drying, dye fixation is performed in the curing chamber at a temperature of 125–1600 °C for 2–5 minutes, depending on the depth of the shade.
In this process, unwanted smoke generation and cellulose yellowing can happen if high-temperature (HT) steam instead of dry heat is used to heat the fabric. On the other hand, advantages such as higher color yield can be obtained in shorter times, lower fixation temperature, brilliance, and shade uniformity.
Pad-dry-pad-steam-wash:
For better dye penetration in heavy-weight fabrics, it is possible to use modified padding mangles. After that, the fabrics are dried in hot furnaces to prevent color migration. Then, a chemical solution comprising electrolyte (250 g/L), caustic soda, soda ash, or a combination of caustic soda and sodium silicate is used to pad the fabrics. 5 g/L of caustic soda is enough for cold brand reactive dyes and 10 g/L for hot branding.
Did You Know?
In a study, cotton fabric was dyed with reactive dye by pad-dry-steam, E-control, and cold pad batches. All the processes have their own merits and demerits. Cold pad-batch dyeing is the most economical but requires more batching time. Pad dry pad steam is the fastest process but requires more chemicals. E-control is the more eco-friendly process and the most significant and unique feature of this process. It does not use a separate fixation step. The pad dry pad steam process gives a maximum depth of any shade.
3. Semi-continuous Dyeing Process
This exhaust method operates at room temperature with very low liquid ratios. The goal is to pad on a solution of dye and alkali and batch the fabrics uniformly. After stuffing, the fabrics are wrapped in plastic to prevent water evaporation and alkali reactions with carbon dioxide in the air.
Batching time can be 17–18 hours for highly reactive dyes and 6–18 hours for less reactive dyes. The fabrics are then efficiently washed off on perforated beams with a slow hot or cold water flow.
The advantages of pad batch over exhaust dyeing are lower initial cost, lower consumption of water, energy, and chemicals, lower control of effluent and labor costs, a significant increase in productivity, and the quality of the dyed fabrics. Another benefit linked with processing woven fabrics is the tendency for pad-batch methods to be smaller, which is a disadvantage in continuous processing, where the minimum economical lot size is considered to be about 5,000 meters.
4. One-step Method without Salt
The method includes a two-dip-one-dry or three-dip-one-dry process, wherein the dye solution used for dipping and drying contains 15 to 25 g/l of dye and 1 g/l of an alkaline agent; pickup is between 55 and 60 percent; then, the dyeing solution is subjected to steaming, washing, hot water washing, and drying. The method does not have special requirements for the dye and does not relate to the modification of fibers or application of a salt-free dye cross-linker; preliminary drying and cylinder drying are saved, and heat energy consumption is greatly reduced. To learn more about this process, visit patents.google.com
5. Infrared Heating Method
According to a study, the chemicals used for cellulosic materials are Glauber salt, NaOH, sodium chloride, and the wetting agent. The reactive dye is Procion H-EXL blue.
There are two possible methods for this dyeing process:
- Scouring: In this method, cotton fabrics are boiled for 1.5 hours in an alkaline solution containing sodium hydroxide (4.0%) and a wetting agent (Triton X-100) using a liquid ratio of 1:50 w/v. Then rinse with hot and cold water and air-dry at room temperature.
- Mercerization: The scoured fabrics are mercerized using NaOH solution (20%); the liquor ratio is 1:50 w/v at 20°C for 2 min. The fabrics are washed with boiling and cold water several times to remove the excess caustic soda. After neutralizing the fabric with a 10% acetic acid solution, it is rinsed in cold water and dried at 80–100 degrees Celsius.
Scoured and mercerized fabrics were dyed with different concentrations (2%, 4%, and 6%) of Procion H-EXL Blue reactive dye using two different heat-dyeing techniques (conventional exhaustion and infrared). Infrared radiation is used as a heating source. After dyeing, soaping was performed using 2 g/L detergent at 90°C for 10 minutes, and then the fabrics were dried.
In both dyeing techniques, mercerized fabrics exhibited better color strength. The colorfastness to washing, rubbing, and perspiration is good to excellent. On the other hand, 45 minutes of IR dyeing at 60 °C with a 4% dye concentration showed the best dyeing conditions. The infrared rays used as a heating source in the dyeing machine are friendly for the environment, as there are no fumes and no resulting pollution.
Risks
Reactive dyes have a high degree of wet fastness because the reactive dye molecule attaches itself to natural materials by a strong chemical bond. Reactive dyes can react in the same way inside the body if inhaled. Sometimes, it affects the body’s immune system. A person’s body reacts dramatically to even a small amount of reactive dye the next time due to changes in the immune system.
Long-term exposure to high dust concentrations can alter lung function by causing particles smaller than 0.5 microns and their debris to accumulate in the lung.
Reactive dyes may be respiratory or skin sensitizers.
Symptoms of allergy:
- Watery eyes or swollen eyelids.
- Sneezing, itching, and a runny nose.
- Asthma symptoms include unusual shortness of breath while running or exercising, coughing, wheezing, and chest tightness.
If a person experiences any of the symptoms of an allergy, follow the next steps:
- Eyes: Rinse cautiously with water for several minutes. If eye irritation persists, get medical attention.
- Inhaled: If breathing is difficult, leave the workplace and go to fresh air to breathe well. If breathing has stopped, immediately administer artificial respiration and seek medical attention.
- Skin: Wash with plenty of soap and water. If irritation occurs, get medical advice.
When starting work on a dye bath, do not forget:
- Put on appropriate personal equipment, such as gloves, face masks, and safety glasses with side shields.
- Store the dye in tightly closed containers when not in use.
Sources
- Effect and Role of Salt in Cellulosic Fabric Dyeing
- The Effect of Temperature Variation on Dyeing Tie-Dye Cotton Fabrics Dyed with Reactive Dye
- Study of some important properties of Reactive Dyeing on 100% cotton fabric
- Chemistry, Classification, and Application of Reactive Dye to Cotton
- Dyeing cotton fabrics with reactive dye using the infrared heating technique
- Reactive dyes: Safe handling in textile finishing
- Safety Data Sheet for a Reactive Black Dye
©Eman Abdallah Kamel, 2024
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