Dyeing 100% Cotton Knit Fabric with Reactive Dye

Dyeing cotton fabrics, especially knit fabrics, with reactive dye is a widely used technique in the textile industry. Cotton, a natural cellulose fiber, pairs exceptionally well with reactive dyes, which establish a chemical bond with the fiber. This results in rich, vibrant, and permanent colors that withstand repeated washing and wear. This guide covers every crucial aspect of dyeing cotton knit fabric with reactive dye, from preparation and dye application to troubleshooting and finishing, to ensure successful and durable dyeing.

 

Why Choose Reactive Dye for Cotton Knit Fabric?

Reactive dye is preferred for dyeing cotton and other cellulosic fibers because it forms a covalent bond with the fiber's cellulose molecules. This chemical bond makes the dye highly resistant to washing and fading, leading to durable, vibrant colors that remain for the lifetime of the fabric. The following properties make reactive dyes ideal for 100% cotton knit fabrics:

• High Wash-Fastness: The dye's chemical bond with cotton ensures the color remains intact despite frequent washing.
• Lightfastness: Reactive dyes maintain their brightness and do not easily fade when exposed to light.
• Broad Color Range: These dyes offer a wide variety of hues, from intense brights to deep, rich colors.
• Eco-Friendly and Cost-Effective: Reactive dyes typically require less energy and water than other types of dyes, which is not only sustainable but also reduces production costs.

These qualities make reactive dye one of the most popular choices for dyeing cotton fabrics used in clothing, home textiles, and various other applications.

 

Pre-Treatment of Cotton Knit Fabric

Before dying, it’s essential to prepare the cotton fabric. Pre-treatment is necessary to ensure uniform dye absorption and to eliminate any impurities that may hinder the dyeing process.

Scouring

• Purpose: Scouring removes natural and synthetic impurities such as wax, oil, or residues that may be present on the cotton. These impurities can prevent the dye from penetrating the fabric evenly.
• Process: The fabric is treated in an alkaline solution (usually caustic soda) at high temperatures, which dissolves these impurities, leaving the cotton fabric in a clean, dye-ready state.

Bleaching

• Purpose: Bleaching lightens the natural color of the cotton, creating a consistent, uniform base that ensures the dye color appears true and even across the entire fabric.
• Process: Hydrogen peroxide or other bleaching agents are commonly used. This step is essential when dyeing lighter shades, as it helps prevent any uneven tones or dullness.

Mercerization (optional)

• Purpose: Mercerization enhances the dye uptake, color depth, and sheen of cotton fabric. This treatment improves the fiber's reactivity by increasing its affinity for dye molecules.
• Process: Cotton is treated with caustic soda under tension, which makes the fibers swell, making them smoother and more lustrous.

 

Reactive Dye Selection and Preparation

Selecting the right type of reactive dye and preparing it properly is crucial to achieving optimal colorfastness and vibrancy. Some commonly used reactive dyes include Procion MX, Remazol, and Drimarene K, which are known for their stability and performance on cotton.

Preparation of Dye Solution

1. Dissolving the Dye: Measure the dye accurately and dissolve it in water. The concentration and volume of the dye solution depend on the desired shade intensity.
2. Salt Addition: Salt plays a significant role in dyeing cotton. It acts as an electrolyte, helping to push the dye onto the fiber and achieve uniform penetration. Commonly, sodium chloride or Glauber's salt is used.
3. Soda Ash Addition: Soda ash (sodium carbonate) is added to maintain an alkaline pH, which is necessary for the dye to react with the cotton fiber. Without proper alkalinity, the dye will not bond, leading to poor color retention.

 

The Dyeing Process: Step-by-Step

1. Setting Up the Dye Bath

• Water Volume: Use enough water in the dye bath to allow the fabric to move freely, which prevents creases and ensures even color.
• Temperature Control: Set the dye bath temperature between 60–80°C, depending on the dye’s specifications. Temperature control is critical, as reactive dyes have optimal fixation temperatures that need to be maintained.

2. Adding Salt to the Dye Bath

• Timing and Method: Add the salt gradually to the dye bath after the water has reached the desired temperature. Stir well to dissolve it fully, as uneven salt distribution can lead to streaks in the final product.

3. Adding the Dye Solution

• Slow Addition and Mixing: Slowly pour the dye solution into the dye bath while continuously stirring. This ensures that the dye is evenly distributed throughout the solution, helping to prevent patchy results.

4. Immersing the Fabric

• Gentle Agitation: Submerge the pre-treated cotton knit fabric in the dye bath and gently agitate it to ensure uniform dye penetration. The fabric should be agitated continuously or at regular intervals to avoid any uneven coloration.

5. Adding Soda Ash

• Gradual Addition: Add soda ash incrementally, usually in three stages, allowing the pH to increase gradually. This controls the fixation rate, helping the dye bond effectively with the cotton fibers.

6. Maintaining Temperature and Agitation

• Stabilizing Conditions: For best results, maintain a stable temperature and keep the dye bath moving. Reactive dyes need constant temperature and movement for even fixation. Typically, the fabric remains in the dye bath for 45–60 minutes.

 

Post-Dyeing Treatment: Rinsing and Washing

Proper post-dyeing treatment is essential to remove unfixed dye and improve colorfastness.

1. Cold Water Rinse

• Rinse the dyed fabric in cold water to remove excess dye that has not bonded to the fiber.

2. Neutralizing the Fabric

• Soap or Detergent Wash: A warm, soapy wash helps to remove any remaining unfixed dye molecules. The use of neutral detergents is ideal as it cleanses the fabric without altering the pH.
• Acidic Neutralization (optional): In some cases, a mild acidic rinse, such as with acetic acid, is used to neutralize residual alkalinity, which can enhance colorfastness.

3. Final Rinse and Drying

• After the soap wash, rinse the fabric with cold water one final time. Then, allow the fabric to air-dry or machine-dry, depending on the equipment available. Avoid direct sunlight, as it can fade the color.

 

Key Factors to Consider in Reactive Dyeing of Cotton

pH Levels

Maintaining an alkaline environment (typically pH 10–11) in the dye bath is crucial for dye fixation. Soda ash is commonly used to achieve this. Monitoring pH levels is essential because if they fluctuate, it can impact the dye’s bonding capacity.

Salt Concentration

Too much or too little salt can lead to uneven results. The salt concentration directly influences the dyeing process; a shortage can prevent the dye from moving onto the fiber, while excess salt may damage it.

Temperature Control

Each type of reactive dye has an ideal temperature range for fixation. High temperatures can lead to hydrolysis (breaking down of dye), while low temperatures may not allow the dye to bond fully.

Agitation

Proper movement of the fabric during dyeing prevents dye pooling, which can result in uneven coloration and streaks.

 

Advantages of Reactive Dyeing on Cotton Knit Fabrics

• Durability and Colorfastness: The covalent bond between dye and fiber ensures that the color does not easily wash out or fade.
• Brilliant Colors: Reactive dyes provide deep, rich colors that appear even more vibrant on cotton knit fabrics.
• Environmental Consideration: Reactive dyes require less water and energy, leading to reduced environmental impact, especially when using closed-loop systems for water treatment.

 

Troubleshooting Common Dyeing Issues

1. Patchiness: Uneven dye absorption often results from insufficient agitation or salt distribution. Keeping the dye bath in constant motion helps prevent this.
2. Color Fading: This issue may occur if pH levels are not maintained correctly or if the fabric isn’t rinsed thoroughly. Checking pH and doing proper rinsing are vital steps.
3. Uneven Shades: Impurities or irregular temperatures in the dye bath can cause shade inconsistencies. Pre-treating the fabric and monitoring temperature carefully can help avoid this issue.

 

Conclusion

Reactive dyeing of 100% cotton knit fabric offers vibrant, durable, and eco-friendly color options. Through careful pre-treatment, dye bath setup, and post-dyeing care, textile manufacturers can achieve consistent, high-quality results that stand the test of time. The ability to create rich, lasting colors using reactive dyes makes it an excellent choice for apparel, home textiles, and beyond.