Reactive Dye – An Overview
Reactive dyes are a class of highly colored organic substances that form a covalent bond with the fiber, especially cellulosic fibers like cotton, rayon, and linen. This permanent bonding makes reactive dyes one of the most wash-fast dye classes available.
1. Characteristics of Reactive Dyes
- Covalent Bond Formation: They react chemically with the hydroxyl groups in cellulose.
- Bright Shades: Reactive dyes provide a wide range of vibrant colors.
- Excellent Fastness: High wash, light, and rubbing fastness due to chemical bonding.
- Water-Soluble: Most are applied from aqueous solution.
- Eco-friendly Potential: When used and fixed efficiently, they are among the more environmentally sustainable options.
2. Types of Reactive Dyes
Reactive dyes are classified based on their reactive group:
a. Based on Reactive Group
- Monofunctional: One reactive group (e.g., monochlorotriazine)
- Bifunctional: Two reactive groups, improving fixation and fastness (e.g., MCT + VS type)
- Multifunctional: More than two reactive sites for higher reactivity and performance
b. Based on Application Conditions
- Cold brand: Applied at room temperature (20–40°C)
- Hot brand: Applied at higher temperatures (60–80°C)
- High-temperature brand: Requires 80–90°C
3. Application Process
The reactive dyeing process includes the following steps:
a. Exhaust Dyeing Method
- Fabric is immersed in dye bath
- Addition of salt (e.g., Glauber’s salt) to promote exhaustion
- Alkali (e.g., sodium carbonate) is added to fix the dye
- After-treatment includes washing off unfixed dye and soaping
b. Pad Batch or Pad Dry Method
- Fabric is padded with dye liquor
- Batched and kept at room temperature (pad-batch)
- In pad-dry, followed by drying and curing at higher temperature
c. Continuous Dyeing
- Used in large-scale operations with thermosol or pad-steam processes
4. Advantages of Reactive Dyes
- Strong bonding results in excellent durability
- Wide range of shades, including pastels and brights
- Suitable for printing as well as dyeing
- Good reproducibility in batch processes
5. Disadvantages
- Hydrolysis: Unreacted dye reacts with water, forming hydrolyzed dye that doesn’t bond with fiber
- Requires thorough washing-off to remove unfixed dye
- High water consumption and potential for pollution if effluent is not treated
- Sensitive to pH and temperature variations
6. Environmental Considerations
Though reactive dyes are more stable than many alternatives, issues include:
- Large volumes of effluent
- Presence of salts and alkalis in wastewater
- Development of low-salt or salt-free reactive dyes is a growing trend
- Membrane filtration, oxidation, and biological treatments are used in effluent treatment