Reactive dyes work by forming a covalent chemical bond with the fiber molecule, essentially becoming part of the fiber. This creates a very strong and permanent color. Here's a breakdown of the process:
1. Chemical Reaction:
The key characteristic of reactive dyes is their ability to chemically react with the fiber. This reaction typically involves the dye molecule forming a strong, covalent bond with hydroxyl (-OH) groups in cellulose fibers (like cotton) or amino (-NH2) groups in protein fibers (like wool or silk).
2. Application and Fixation:
- Application: The dye is applied to the textile material, often in a solution.
- Alkaline Conditions: Reactive dyes generally require an alkaline (high pH) environment to facilitate the reaction with the fiber. This is often achieved by adding alkali (like sodium carbonate) to the dye bath. Some dyes are applied in neutral solutions which are subsequently alkalized.
- Fixation: Under alkaline conditions, the reactive group on the dye molecule reacts with the fiber, forming a stable chemical bond. This "fixes" the dye permanently to the fiber.
3. Washing and Aftertreatment:
After the fixation process, the textile is thoroughly washed to remove any unfixed dye and auxiliaries. This step is crucial to ensure good wash fastness (resistance to color fading during washing).
4. Influencing Factors:
Several factors affect the efficiency of the dyeing process:
- Temperature: Temperature influences the rate of the reaction. Optimal temperatures vary depending on the dye and fiber type. Some shades may even be intensified or altered by heat.
- pH: As mentioned, alkaline pH is generally essential for the dye to react with the fiber.
- Time: Sufficient time is needed for the reaction to complete.
- Liquor Ratio: The ratio of dye solution to textile material also affects the dyeing process.
- Electrolytes: The addition of salts like sodium chloride can improve dye exhaustion (the amount of dye that is absorbed by the fiber).
Here's a table summarizing the key aspects:
| Aspect | Description |
|---|---|
| Mechanism | Covalent bond formation between dye and fiber |
| Fibers | Primarily cellulose (cotton, linen) and protein (wool, silk) fibers |
| Conditions | Alkaline pH (usually) |
| Fastness | Excellent wash and light fastness due to strong chemical bond |
| Application | Dye bath, padding, or printing |
| Fixation | Chemical reaction under alkaline conditions, sometimes aided by heat |
In conclusion, reactive dyes achieve permanent color by chemically bonding to the fiber, creating a durable and colorfast result. This process typically requires alkaline conditions to facilitate the reaction.
