Cotton Pretreatment Guide: The Secret to RFT

The “Hidden” Cost of Shortcuts

There is an old saying on the shop floor that every production manager knows but often ignores:
“Dyeing is where you make your profit, but pretreatment is where you lose your reputation.”

I have seen mills try to cut corners here. They reduce the steaming time by 10 minutes to increase production metres per shift, or they cut the surfactant dosage to save a few paise. Any senior technologist knows this is false economy. If your cotton pretreatment is uneven, your dyeing will be uneven. That is not an opinion—it is process logic.

You cannot “fix” bad preparation in the dye bath; you can only try to hide it, and usually not for long.

If you are seeing patchiness, resist marks, or poor rubbing fastness later in the line, in the majority of recurring cases the root cause sits back in the bleaching department—not in the dye house.

The RFT Equation

In any dyeing setup or vertical mill, Right First Time (RFT) is the only metric that truly keeps the lights on. If a fabric is not properly prepared, you face three major risks:

Poor Absorbency
This leads to superficial dyeing, where the dye molecule sits on the surface rather than penetrating the fibre core.

Residual Impurities
Natural waxes and pectins act as resist agents, creating lighter or cloudy patches.

Strength Loss
Over-bleaching attacks the cellulose chain, resulting in tearing during high-speed garmenting or finishing.

What we actually need is a substrate that is uniformly absorbent (hydrophilic) and, where required, optically clean, without degrading the fibre. This guide focuses on achieving that balance.

The Pretreatment Process Chain

For clarity, this guide explains the chemistry and mechanics of continuous and semi-continuous processing. However, the same principles apply if you are running batch processes in soft-flow machines or jiggers.

1. Singeing: The First Line of Defence

Before wet processing, protruding fibres must be removed to prevent a frosty or hazy appearance after dyeing. Whether you are running a basic singeing machine or a premium Osthoff line, the principle remains the same.

Flame Position
The flame must strike the fabric uniformly, typically at a perpendicular or slightly tangential angle depending on fabric construction.

Speed vs. Intensity
Higher speed requires higher flame intensity. Typical working range: 80–120 m/min, depending on GSM and fabric type.

The Cooling Hazard
Adequate cooling is critical before desizing. Hot sparks carried into the desizing pit can ignite starch residues. This is a real fire risk, not a theoretical one.

2. Desizing: Removing the “Starch” Barrier

Grey fabric arrives loaded with size (starch, PVA, CMC) from weaving. If size is not fully removed, dyes simply will not penetrate uniformly.

The Enzymatic Route

While oxidative desizing exists, enzymatic desizing using amylase is the industry standard for cotton.

Temperature
Typically 60–70°C (always verify with the enzyme supplier’s TDS).

pH Control
Strictly 6.0–7.0. If pH drifts due to water quality or carryover alkali, enzyme activity drops sharply, leading to patchy desizing.

Dwell Time
4–8 hours in pad-batch, pit, or rotation systems. Continuous ranges compensate with higher temperature and controlled dosing instead of time.

The TEGEWA Check
After washing, always perform the iodine test.
A TEGEWA rating ≥ 6 is mandatory before scouring. If the fabric turns blue or violet, do not pass the lot forward.

3. Scouring: The Heavy Lifting Stage

Scouring is the true cleaning phase. Raw cotton contains waxes, pectins, proteins, and residual oils.

Mechanism
Caustic soda (NaOH) saponifies natural waxes and emulsifies oils so they can be washed away.

The Surfactant Role
The detergent must be stable under high alkali and temperature. If it breaks down, removed oils re-deposit on the fabric, creating stains that are extremely difficult to remove later.

4. Bleaching: Achieving Controlled Whiteness

In modern continuous ranges (for example, Benninger or Goller), scouring and bleaching are often combined to save water and energy.

Key Chemicals & Their Functions

Hydrogen Peroxide
The bleaching agent. It oxidizes natural colouring matter in cotton.

Stabilizer
Crucial for controlled peroxide decomposition. Peroxide naturally wants to decompose rapidly into water and oxygen. The stabilizer forces a slow, uniform release. Without it, fibre damage is inevitable.

Guideline Formulation (Continuous Bleaching)

Disclaimer: This is a general guideline. Always validate on a trial lot. Dosages may be expressed per kg fabric or per litre liquor depending on machine configuration.

Ingredient	Concentration (Typical Range)	Function
Caustic Soda Flakes	20 – 40 g/L	Saponification & seed removal
Hydrogen Peroxide (50%)	25 – 45 ml/kg	Bleaching agent
Stabilizer (Organic)	8 – 12 ml/kg	Controls reaction rate
Sequestering Agent	2 – 4 g/L	Neutralizes hardness & iron
Wetting Agent	3 – 5 g/L	Ensures penetration
Temperature	95 – 102°C	Steamer temperature
Time	20 – 45 min	Dwell time in steamer

The Temperature Trap
Many operators set the steamer at 98°C but never verify actual fabric temperature. Steam leakage or poor sealing can drop fabric temperature below 92–94°C, significantly reducing bleaching and seed removal efficiency.

5. Mercerizing (Optional but Strongly Recommended)

For high-value shirting and premium T-shirts where lustre and depth of shade matter, mercerizing remains critical.

Swelling
High-strength caustic (28–30° Bé) causes fibre swelling.

Tension
The fabric must be held under tension to align cellulose chains.

Barium Number
Standard indicator of mercerization efficiency.
Target range: 135–150.

The Pretreatment QC Checklist

Visual whiteness is subjective. Data is not. Implement these checks as SOP, not exceptions.

Absorbency (Drop Test)

• Drop water from 4 cm height on dry fabric
• Pass: < 3 seconds
• Fail: > 5 seconds → rewash

Whiteness Index (Berger)

Measured on spectrophotometer.

• Dark shades: > 65
• Medium shades: > 75
• Pastels/white: > 82

Residual Peroxide

Residual peroxide destroys reactive dyes.

• Use catalase enzyme if required
• Spot test with titanium sulphate
  • Yellow/orange = peroxide present

Fabric Core pH

Extract liquid from fabric core and test.

• Target: pH 6.0–7.0

Surface pH may appear neutral while the core remains alkaline, causing pH shock during dyeing.

Fluidity / DP (Where Facilities Exist)

Monitor cellulose degradation periodically.

• Healthy bleached cotton DP: ~1800–2200

Troubleshooting That Actually Matters

1. Pinholes from Iron Catalysis

Tiny pinholes after bleaching are almost always due to iron contamination. Even 0.1 ppm Fe can catalyse peroxide decomposition locally, causing aggressive fibre attack.

Fix
Use strong chelation targeted for iron. Standard EDTA may be insufficient; phosphonate-based sequestrants perform better.

2. Cloudy Dyeing from Silicate Spots

When sodium silicate is used as a stabilizer, poor washing control causes silicate precipitation.

Fix
First wash after bleaching must remain above 85°C. Below this, silicates gel and deposit on fabric.

3. Crease Marks in Cotton/Lycra

Rapid cooling causes thermal shock, locking creases permanently.

Fix
Introduce controlled cooling:
90°C → 70°C → 50°C
Never quench directly to cold.

FAQs

Q: Can I use optical brighteners during bleaching for dyeing lots?
No. OBA interferes with shade matching and causes metamerism. Use only for full-white finished goods.

Q: Why does my fabric tear after bleaching?
Cellulose degradation (oxycellulose formation), usually from iron contamination or insufficient stabilizer at high temperature. Verify fluidity results.

Q: Is bioscouring better than caustic scouring?
Bioscouring is more sustainable and gentler, but for heavy cottons requiring full wax and seed removal, caustic scouring remains more reliable.

Q: How do I remove silicone contamination?
This is difficult. It requires specialised silicone strippers and often acidic treatment. Prevention through machine hygiene is far cheaper.

Takeaway

Pretreatment is not “washing” fabric. It is engineering the substrate. You are preparing the canvas for the dyer.

Control Time, Temperature, and Titration here, and your dye house will run smoothly.

Tomorrow morning, check residual peroxide on your RFD lots. You may find that many so-called “dyeing faults” are actually bleaching faults. For common dyeing faults and root cause analysis. check other article.