Textile Pretreatment: Lecture #3 – Desizing

Welcome back to our series on Textile Pretreatment! After the crucial steps of greige fabric inspection and singeing, the fabric is now ready for its first major wet process: Desizing. This step is critical, especially for woven fabrics made from staple fibers like cotton.

1. Introduction to Desizing

Desizing is the process of removing the sizing agents (or simply “size”) that were applied to the warp yarns before weaving. As we discussed in Lecture #1, sizing agents are starches, synthetic polymers, or blends used to strengthen warp yarns and protect them from abrasion during the high-tension weaving process.

Why is Desizing Essential?

The presence of sizing agents on the fabric can cause numerous problems in subsequent wet processes:

• Poor Absorbency: Most sizing agents, especially starches, are hydrophobic or form a barrier, preventing water and chemical solutions (like those used in scouring, bleaching, and dyeing) from uniformly penetrating the fibers. This leads to unlevel dyeing.
• Reduced Whiteness: Sizing agents can contribute to the fabric’s yellowish tint and hinder the effectiveness of bleaching.
• Harsh Hand Feel: Size imparts a stiff, papery feel to the fabric, which is undesirable for most textile products.
• Impaired Dyeing and Printing: Size acts as a resist, preventing uniform dye uptake and causing blurry prints.
• Environmental Concerns: If not removed, sizing agents can contribute significantly to the biochemical oxygen demand (BOD) in wastewater.

Effective desizing is the first step in achieving a uniformly absorbent, soft, and well-prepared fabric for dyeing and finishing.

2. Types of Sizing Agents

The method of desizing largely depends on the type of sizing agent used:

• Starch and Starch Derivatives:
  • Native Starch: The most common natural sizing agent (e.g., corn starch, potato starch). It’s insoluble in cold water and forms strong films.
  • Modified Starches: Chemically modified starches (e.g., oxidized starch, dextrins) that are more soluble and have lower viscosity.
• Synthetic Polymers:
  • Polyvinyl Alcohol (PVA): Excellent film former, highly soluble in hot water, and easily recyclable.
  • Carboxymethyl Cellulose (CMC): Water-soluble, good film former.
  • Polyacrylates: Offer good adhesion and film formation.
• Blends: Often, a combination of starch and synthetic polymers is used to optimize performance and cost.

3. Desizing Methods

The goal of all desizing methods is to convert the insoluble sizing agent into a water-soluble form that can be easily washed off.

• a) Enzymatic Desizing (for Starch-based Sizes):
  • Principle: Enzymes (amylases) specifically break down starch molecules into smaller, water-soluble dextrins and sugars. Amylases are highly specific, acting only on starch, thus minimizing fiber damage.
  • Chemical Reaction (Simplified): Starch (Insoluble)Amylase Enzyme, Water, Heat​Dextrins + Sugars (Soluble)
  • Process: The fabric is padded (impregnated) with a solution containing an amylase enzyme, a wetting agent, and often a buffer to maintain optimal pH (typically neutral to slightly alkaline). The fabric is then stored (batching/piling) for a period (e.g., 4-16 hours) to allow the enzymes to act. Finally, the fabric is thoroughly rinsed with hot water to wash off the soluble products.
  • Advantages: Environmentally friendly (biodegradable), excellent for starch removal, causes no fiber degradation.
  • Disadvantages: Time-consuming (requires dwelling time), specific to starch, not effective for synthetic sizes.
• b) Oxidative Desizing:
  • Principle: Strong oxidizing agents break down starch and some synthetic sizes into smaller, water-soluble fragments.
  • Reagents: Sodium hypochlorite (NaOCl), hydrogen peroxide (H2​O2​), or ammonium persulfate.
  • Process: Fabric is treated with an oxidative agent solution, often at elevated temperatures.
  • Advantages: Faster than enzymatic desizing, can remove some synthetic sizes, combines desizing with a mild bleaching action.
  • Disadvantages: Can be harsh on cellulose fibers if not carefully controlled (risk of tendering), less specific than enzymes, potential environmental concerns with some oxidizers.
• c) Acid Desizing:
  • Principle: Dilute mineral acids (e.g., sulfuric acid, hydrochloric acid) hydrolyze starch molecules into dextrins and sugars.
  • Process: Fabric is impregnated with a dilute acid solution and then subjected to heat (steaming or hot storage). Followed by thorough rinsing and neutralization.
  • Advantages: Relatively simple process.
  • Disadvantages: High risk of fiber damage (acid tendering) if concentration, temperature, or time are not precisely controlled, environmental concerns with acid effluent. Less common for cotton due to tendering risk.
• d) Hot Water Desizing (for Water-Soluble Sizes like PVA, CMC):
  • Principle: Simply dissolving the water-soluble sizing agents in hot water.
  • Process: Fabric is washed extensively in hot water, often in continuous washing ranges.
  • Advantages: Simple, environmentally friendly, no harsh chemicals.
  • Disadvantages: Only effective for truly water-soluble sizes; not suitable for starch or other insoluble sizes. PVA can be recovered and reused.

4. Factors Influencing Desizing Efficiency

• Type and Concentration of Sizing Agent: Dictates the choice of desizing method and chemical concentrations.
• Temperature: Higher temperatures generally accelerate desizing reactions (enzymatic activity, hydrolysis, oxidation).
• Time: Sufficient dwell time is necessary for the chemicals to act.
• pH: Crucial for enzyme activity (optimal pH ranges) or for controlling acid/oxidative reactions.
• Washing Efficiency: Effective rinsing is paramount to remove the dissolved sizing agents and reaction by-products.

5. Testing for Complete Desizing

It’s vital to ensure that desizing is complete before proceeding to subsequent steps. The presence of residual starch can be detected using a spot test with iodine solution.

• Procedure: A drop of iodine solution is applied to the desized fabric.
• Result:
  • Blue-violet color: Indicates the presence of starch, meaning desizing is incomplete.
  • Yellowish-brown or no color: Indicates effective starch removal.

6. Environmental and Safety Considerations

• Wastewater: Desizing effluent can have high BOD due to dissolved organic matter (starch, PVA, etc.) and may contain residual chemicals. Proper effluent treatment is essential.
• Chemical Handling: Safe handling procedures are necessary for acids, oxidizers, and enzymes.
• PVA Recovery: For PVA, recovery systems can significantly reduce environmental impact and costs.

Desizing is a non-negotiable step for woven fabrics, especially those containing starch-based sizes. It lays the groundwork for uniform and brilliant dyeing results by ensuring the fiber’s maximum and even absorbency.