Textile Wet Processing for Knit Fabrics – A Basic Review
Wet processing in textiles refers to all processes where water or chemicals are used to improve the appearance, texture, and performance of fabrics. In the case of knit fabrics, which are soft, stretchable, and loop-structured, wet processing requires special care to preserve the fabric's structure and properties.
1. Stages of Wet Processing for Knit Fabrics
Wet processing typically involves the following three main stages:
a. Pretreatment
Purpose: Remove natural and synthetic impurities to prepare the fabric for dyeing or finishing.
Common steps include:
- Desizing (if applicable): Not usually required for knits.
- Scouring: Removes oils, waxes, dirt, and knitting lubricants.
- Uses alkali (e.g., NaOH) and detergent.
- Bleaching: Removes natural color (especially in cotton knits).
- Uses hydrogen peroxide (H₂O₂) under controlled pH and temperature.
- Bio-scouring (eco-friendly): Uses enzymes to replace harsh chemicals.
b. Dyeing
The stage where color is imparted to the fabric. For cotton knits, reactive dyes are most commonly used.
Dyeing methods include:
- Jet dyeing: Suitable for delicate knits.
- Soft-flow dyeing: Ensures minimal stress and good dye penetration.
- Winch dyeing: Used for tubular knits in loose rope form.
- Package dyeing (for yarns used in knits).
Key factors:
- Temperature control
- pH (critical for dye fixation)
- Time and liquor ratio
c. Finishing
Improves the functional and aesthetic properties of the fabric.
Common finishes:
- Softening: Silicone or fatty acid-based softeners for a smooth hand feel.
- Enzyme finishing: Biopolishing to remove protruding fibers and reduce pilling.
- Heat setting (for synthetic blends): Stabilizes dimensions and shape.
- Moisture management finishes: For sportswear knits.
2. Special Considerations for Knit Fabrics
- Structure Sensitivity: Knits can stretch, distort, or shrink during processing.
- Low Tension Handling: Machines and processes are designed to avoid stress.
- Tubular vs. Open Width: Tubular knits are processed in rope form; open-width knits are slit and processed flat.
- Color Consistency: Due to stretchability and high surface area, dye penetration and uniformity must be carefully managed.
3. Machinery Used
- Jet dyeing machines: Ideal for lightweight and delicate knits.
- Soft-flow machines: For moderate loads and uniform dyeing.
- Open-width bleach and dyeing ranges: For wide fabric processing.
- Compactors: Used after finishing to control shrinkage and GSM.
- Relax dryers: Minimize residual shrinkage and control fabric dimensions.
4. Environmental Aspects
- Water-intensive processes: Knit processing consumes high water volumes.
- Chemical management: Shift toward biodegradable, low-impact chemicals.
- Effluent treatment: Vital for managing dyes, alkalis, and auxiliaries.
- Sustainability focus: Enzyme treatments, low-liquor ratio machines, and recycled water systems are gaining popularity.
Conclusion
Textile wet processing of knit fabrics is a complex yet essential part of fabric production, requiring careful control of chemical, mechanical, and thermal factors. With the rising demand for high-quality, sustainable knitwear, modern wet processing emphasizes low-impact chemicals, energy-efficient machinery, and eco-friendly techniques.