"Cotton Wet Processing Explained: All-in-One Guide from Desizing to Finishing with Recipes, Reactions & Technical Insights"
Introduction: What Happens Before Cotton Turns Colourful?
Every cotton shirt, bedsheet, or saree you wear goes through more than just weaving and stitching. Before cotton accepts colour, it has to be cleaned, softened, and chemically treated. This entire preparation journey is called wet processing.
Wet processing in cotton isn’t just a set of industrial steps—it’s a science where chemistry meets fibre behaviour. From desizing to finishing, each stage prepares the fabric to look brighter, feel smoother, and last longer.
In this detailed blog, we’ll explore:
- Each stage in cotton wet processing
- What exactly happens chemically in each step
- Industry-used recipes and technical parameters
- Innovations and eco-friendly alternatives
- Tips and insights useful for both students and textile professionals
Let’s walk through this journey from raw grey fabric to clean, white, and ready-to-dye cotton.
1. Desizing: Removing the Size
Purpose:
Sizing agents (like starch) are applied on yarns during weaving to strengthen them. Desizing removes this coating to allow penetration of scouring agents and dyes.
What Happens:
Desizing breaks down starch (or synthetic sizing agents) into water-soluble compounds using enzymes or acids.
Starch Breakdown:
(C₆H₁₂O₅)ₙ → (C₆H₁₂O₅)ₓ → (C₁₂H₂₂O₁₁) → (C₆H₁₂O₆)
Key:
Starch → Dextrin → Maltose → Glucose
Common Recipe (Enzymatic Desizing):
- Amylase enzyme: 2%
- M:L ratio: 1:20
- Common salt: 1%
- Wetting agent: 1 to 3 gpl
- pH: 6–7
- Temperature: 40-60°C
- Time: 60 minutes
Purpose of chemicals used:
- Enzyme - Breaks down starch (size) into water-soluble sugars (maltose, glucose)
- Common salt - To maintain ionic strength, improving enzyme activity and stability and protects enzyme in hard water.
- Wetting agent - Reduces surface tension of the desizing bath, allowing chemicals/enzymes to penetrate the size film.
Alternative (Acid Desizing):
- The acid hydrolysis the starch present in the sized fabric
- HCl (0.5% solution) or sulphuric acid at room temperature with 100% expression.
- Then the roll is covered with polythene film and kept aside in a room for 8-12 hrs.
- The roll is rotated slowly, so that it remains uniformly wet during the de-sizing action.
- If the fabric is dried then the water will be evaporated from the fabric before washing, this increases the concentration of acid and forms hydrocellulose which can damage the cotton.
- Not preferred due to fibre damage risk.
Key Notes:
- Enzymatic desizing is safer and more fibre-friendly.
- Proper desizing ensures even scouring and bleaching.
2. Scouring: Removing Natural Impurities
Purpose:
Raw cotton contains waxes, pectins, oils, and dirt that resist dyeing. Scouring removes these to improve absorbency.
What Happens:
Alkali breaks down waxes and oils into soap-like compounds that can be washed away. This increases wettability.
Scouring Recipe (for grey fabric):
- Caustic soda (NaOH): 2%
- Sodium carbonate: 1%
- Wetting agent: 1%
- Detergent: 1%
- Sequestering agent (EDTA): 1%
- Temperature: 95–100°C
- Time: 60–90 minutes
- pH: 11–12
Mechanism of scouring:
- Oils & Fats → Turned into soap (saponifiable oils) or broken into tiny droplets (unsaponifiable oils/waxes).
- Proteins → Broken down into water-soluble bits.
- Pectins → Converted into soluble pectic acid salts.
- Amino Acids → Dissolved or turned into ammonia.
- Minerals → Dissolved in water.
- Dirt → Loosened, suspended in the scouring solution, and washed away.
Saponification:
- Vegetable oil + NaOH ----- glycerine (Soluble in water) + fatty acid
- Fatty acid+NaOH ----- soap or sodium salt of fatty acid (Which is also soluble in water)
Emulsification:
- Waxes+soap ---- emulsion
Innovative Eco Option:
Enzymatic scouring uses pectinase or lipase instead of alkali to reduce pollution and fibre damage.
Indicators of Proper Scouring:
- Drop of water should be absorbed within 3 seconds.
- Fabric should feel soft, not slippery.
3. Bleaching: Whitening the Fabric
Purpose:
Cotton naturally contains colouring matter (mainly flavonoids) that gives it a creamy hue. Bleaching removes these to make the fabric white and dye-ready.
What Happens:
Oxidizing agents like hydrogen peroxide break down chromophores in the fibre.
Typical Bleaching Recipe:
- Hydrogen peroxide (H₂O₂, 35%): 4–6 g/L
- Sodium silicate (stabiliser): 4 g/L
- Caustic soda: 0.5%
- Sodium carbonate: 0.5%
- Temperature: 80–85°C
- Time: 60–90 minutes
- pH: 10–11
Use of chemicals:
- Sodium carbonate - Buffer solution to maintain pH.
- Hydrogen peroxide - Bleaching agent
- Sodium silicate - Peroxide stabilizer
- Sodium hydroxide - Creates a high-pH environment (pH 10.5–11.5) to activate hydrogen peroxide
Sodium hypochlorite bleaching:
- M:L ratio: 1:20
- Sodium hypochlorite: 3 gpl available chlorine
- Soda ash: 1%
- Time: 1 hour
- Temperature: Room temperature
- pH: 9.5 to 10
Alternative (Low-Temp Bleaching):
- Use stabilised peroxide systems at 70°C to save energy
New Innovation:
- Ozone bleaching and enzymatic bleaching are being explored to reduce effluent load and energy use.
4. Mercerization: Enhancing Strength and Lustre
Purpose:
Mercerization gives cotton higher strength, better lustre, improved dye uptake, and dimensional stability.
What Happens:
Fabric is treated with concentrated caustic soda while under tension, causing the cellulose structure to swell and re-orient.
Mercerizing Recipe:
- Caustic soda (NaOH): 20–25% (200–250 g/L)
- Temperature: 20–25°C
- Time: 30–60 seconds
- Tension: High (to prevent shrinkage)
- Washing: Multiple stages with neutralization
Visible Effect:
After mercerizing, cotton fibres become more round and reflective—giving that signature sheen.
5. Dyeing: Adding Colour
Purpose:
To impart desired shades using various classes of dyes. Cotton is usually dyed with reactive, vat, sulphur, or direct dyes.
What Happens:
In reactive dyeing, a covalent bond forms between dye and fibre cellulose. This gives high fastness properties.
Reactive Dyeing Recipe (Exhaust method):
- Reactive dye: 2–3% (owf)
- Glauber’s salt or common salt: 30–60 g/L
- Soda ash (fixing agent): 10–15 g/L
- Temperature: 60°C
- Time: 45–60 minutes
- pH: 10–11
1. Reactive Dyes
Most popular for cotton – form covalent bonds with cellulose.
Types:
- Cold brand (e.g., Procion MX) – dyes at 20–40°C.
- Hot brand (e.g., Remazol) – dyes at 60–80°C.
- Vinyl sulfone (e.g., Drimaren) – high wash-fastness.
- Pros: Brilliant colors, excellent wash-fastness.
- Cons: High salt/alkali usage, effluent issues.
2. Direct Dyes
- Simple and economical – attracted via hydrogen bonding.
- Examples: Congo Red, Direct Black 19.
- Pros: Easy to apply, no mordant needed.
- Cons: Poor wash-fastness (requires after-treatment).
3. Vat Dyes
- Premium choice for durability – insoluble pigments reduced to soluble form.
- Examples: Indigo (denim), Cibanone.
- Pros: Exceptional light/wash-fastness.
- Cons: Complex process (reduction/oxidation), expensive.
4. Sulfur Dyes
- Cost-effective for dark shades – similar to vat dyes but uses sulfur.
- Examples: Sulfur Black 1.
- Pros: Cheap, good wash-fastness.
- Cons: Limited color range (blacks/browns), sulfur odor.
5. Azoic Dyes (Naphthol Dyes)
- In-situ formation – combines naphthol + diazo compound on fabric.
- Examples: Para Red, Naphtol AS.
- Pros: Bright reds/oranges, high color yield.
- Cons: Complex application, carcinogenic intermediates.
6. Natural Dyes
- Eco-friendly but less common – plant/animal/mineral-based.
- Examples: Indigo (plant), Madder (root), Cochineal (insect).
- Pros: Sustainable, non-toxic.
- Cons: Low color fastness, inconsistent shades.
7. Pigment Dyes
- Not true dyes – mechanically bonded with binders.
- Examples: Acrylic-based pigments.
- Pros: Applicable to all fibers, no water needed.
- Cons: Stiffens fabric, poor rub-fastness.
After Treatment:
Washing, soaping, and neutralisation are essential to remove unfixed dyes and improve fastness.
Latest Trends:
Cold pad batch dyeing, dope dyeing, and digital reactive inkjet printing reduce water and energy usage.
6. Finishing: Giving Functional Properties
Purpose:
To improve feel, performance, and user-specific properties. Finishing may be mechanical, chemical, or both.
Types of Finishes:
- Softening finish (for feel)
- Crease-resistant finish (using DMDHEU resins)
- Antibacterial finish (for hygiene)
- Water-repellent finish (silicone-based)
- Flame-retardant finish (using PROBAN or THPC-based systems)
Basic Soft Finish Recipe:
- Cationic softener: 4–6 g/L
- Acetic acid (pH adjust): 1 g/L
- Temperature: 40–50°C
- Time: 20 minutes
- pH: 5–5.5
Innovative Finishes Include:
- Herbal antimicrobial finishes (tulsi, neem extracts)
- Wrinkle-free finishes with eco-friendly crosslinkers
- PCM finishes (Phase Change Materials) for temperature control
Water and Effluent Treatment in Wet Processing
Each wet process consumes significant water and discharges effluents containing:
- Alkali and acids
- Oxidising agents
- Dye molecules
- Surfactants and salts
Modern mills integrate ETPs (Effluent Treatment Plants) with:
- Primary filtration
- Biological treatment
- Reverse osmosis or Zero Liquid Discharge (ZLD)
Eco-labeling standards (GOTS, OEKO-TEX) demand proof of low-impact processing and controlled discharges.
Exam-Oriented Tips & Technical Points to Remember
- Desizing pH: 6–7
- Scouring temperature: 100°C
- Hydrogen peroxide pH: 10–11
- Mercerisation causes fibre swelling
- Reactive dyes form covalent bonds
- Finishing pH: 4.5–5.5
- Fabric should absorb water in 3 seconds after scouring
- Bleaching indicator: reflectance or whiteness index
- M:L ratio in dyeing: typically 1:10
- Mercerized cotton has higher dye affinity than raw cotton
Did You Know? (Interesting Facts)
- Mercerization increases dye uptake by almost 25–30%
- Enzymatic scouring saves 30–40% water and energy
- Low liquor ratio jet dyeing machines use only 1/3rd water compared to conventional methods
- Ozone bleaching can reduce BOD and COD by over 60%
- Crease-resistant finishes can survive 50+ washes with the right curing
Conclusion: Why Cotton Wet Processing Still Matters
Despite the rise of synthetic and blended fabrics, cotton remains the backbone of the Indian textile industry. Understanding wet processing—from desizing to finishing—is essential for anyone in the field.
This knowledge doesn’t just help in exam preparation—it’s what mills, dye houses, and eco-certification bodies look for in real-world work. Whether you are a student, technician, merchandiser, or researcher, a strong command of these processes will give you a clear edge.