Sizing: Applying a Coating to Yarns to Enhance Strength and Processability

What is Sizing 

Sizing is a crucial step in the textile industry, particularly in the weaving process, where it involves applying a protective coating or finish to yarns (typically made of cotton, wool, or synthetic fibers) to improve their strength and performance during further processing. The main goal is to make the yarns easier to handle, reduce friction, and prevent breakage during weaving or knitting. Sizing refers to the process of applying a protective coating (called size) to yarns before they are woven into fabrics. The coating is usually a mixture of various starches, synthetic resins, or other chemicals. These substances bond to the yarns, providing a temporary coating that helps increase yarn strength, reduce breakage, and enhance weaving efficiency. It acts as a barrier that protects the yarns from physical stress during the manufacturing process, particularly during the weaving and knitting stages. 

Purpose of Sizing 

• Increased Yarn Strength: 
  The size coating helps to strengthen the yarns, particularly when working with weaker fibers. This makes them more resistant to breakage during the weaving process, where yarns can be subjected to high tension and friction. Synthetic fibers like polyester or nylon, which have less natural flexibility, benefit significantly from sizing as it prevents fiber abrasion and breakage. 

• Improved Processability: 
  Yarns that are coated with size are smoother and more lubricated, reducing friction between the yarns during the weaving process. This enables smoother weaving, reducing the likelihood of snagging and abrasion. This is especially important in high-speed weaving operations, where any friction or friction-related damage can lead to downtime or a lower-quality fabric. 

• Protection Against Wear and Tear: 
  The sizing helps to protect the yarns from the stresses they undergo during weaving, including abrasion and tensile stress, by adding a protective layer that keeps the yarn intact. In high-stress weaving environments, like those used for heavy-duty fabrics, the added protection ensures longer-lasting yarns. 

• Enhanced Yarn Handling: 
  Sizing also improves the handling characteristics of yarns, making them easier to manage, spool, and transport during the manufacturing process. It can make yarns less prone to knotting or tangling, which can significantly improve the efficiency of textile production. In large-scale operations, this also helps prevent issues like spool entanglement, which can cause delays or damage to the product. 

• Prevention of Hairiness: 
  It helps in reducing the production of yarn "hairiness," where individual fibers separate from the yarn and cause texture or unevenness. This is particularly important when creating fabrics that require a smooth or uniform appearance, such as woven upholstery fabrics or fine textiles. 

Types of Size (Coatings Used in Sizing) 

• Starches: 
  Traditional starches (like corn starch, wheat starch, and rice starch) are commonly used because they are inexpensive and effective for most natural fibers. They provide a good level of adhesion and are easy to apply. Starch-based sizing is often used for cotton and wool yarns in high-volume textile production. 

• Synthetic Resins: 
  Synthetic polymers (such as polyvinyl alcohol (PVA)) are sometimes used in modern sizing for more durable coatings. They offer greater strength and flexibility, especially for synthetic fibers. Resins are also more resistant to moisture, making them suitable for outdoor fabric applications where exposure to weather is expected. PVA is a water-soluble polymer commonly used in the production of high-strength yarns and durable fabrics. 

• Wax-Based Coatings: 
  Waxes can be used to provide a smooth finish, reduce friction, and improve the overall handling of yarns. Paraffin wax is one such example. Wax coatings help create a lubricant layer around the yarn, reducing the friction between fibers. Wax-based sizing is common in high-performance weaving, where slickness is required to prevent friction-induced wear. 

• Acrylics and Other Chemicals: 
  Acrylics, silicone, and other polymer-based substances are sometimes applied as sizing agents, particularly for more specialized applications like high-performance fabrics. Silicone-based sizing helps improve the softness and pliability of yarns without compromising their strength.
  Acrylics are widely used in synthetic textiles, such as those used in sportswear or outdoor equipment. 

Sizing Application Process 

• Preparation of Size Solution: 
  The size is prepared by dissolving the chosen coating material (e.g., starch, resin) in water or other solvents. The concentration of the size material depends on the type of yarn and the intended application. A higher concentration is used for high-stress applications.
  The solution must be well-mixed to ensure an even coating on the yarns. 

• Application: 
  The size solution is typically applied to yarns using methods such as dipping, spraying, or padding. For continuous yarns, a size box or padding mangle may be used to ensure the yarns are evenly coated. Dipping is the most common method for applying starch-based sizing, while spraying is preferred for resin-based coatings. 

• Drying: 
  After the yarns are coated, they need to be dried, often in heated chambers or drying ovens, to set the size and ensure the coating adheres to the yarn. Drying can occur at varying temperatures depending on the type of size used. The drying process helps solidify the protective layer and prevents clumping of yarns. 

• Curing (if needed): 
  For certain synthetic sizes, curing may be necessary to harden the size and improve its bonding with the yarns. This step can involve heat or chemical processes. Curing is particularly important for resins and acrylic-based sizes, which may need to undergo a chemical cross-linking process for optimal bonding. 

Challenges in Sizing 

• Over-sizing: 
  Too much size can result in excessive stiffness or difficulty in weaving, and can also create problems in subsequent finishing processes, like dyeing or printing. Over-sized yarns may also lead to difficulty in dye uptake during the post-weaving stages. 

• Under-sizing: 
  If the yarns are insufficiently sized, they may be too weak or prone to breakage, which can cause weaving defects or machine downtime. Under-sizing can result in a lower-quality final fabric, particularly for high-end textile products. 

• Compatibility: 
  Sizing agents must be carefully selected to be compatible with the type of yarn and the specific weaving or knitting processes. For example, the size used for cotton yarns might differ from those used for synthetic fibers, and high-performance fibers like Kevlar or carbon fibers may require specialized sizing formulations. 

• Environmental Impact: 
  Some sizing agents can contribute to waste and pollution, particularly when they are not biodegradable. Sustainable sizing agents are being researched to reduce environmental impact, and eco-friendly alternatives like bio-based starches and recyclable resins are emerging. The washing out of sizing materials during textile finishing can cause water pollution, especially with synthetic resins. Therefore, research is focusing on eco-friendly sizing agents that have a minimal environmental footprint. 

Conclusion 

Sizing is an essential process in textile manufacturing that enhances the durability, ease of processing, and performance of yarns. Whether using starch, synthetic resins, or wax-based coatings, sizing ensures that the yarn can withstand the mechanical stresses of weaving, knitting, or other fabric-making processes. The choice of sizing materials and the application method is critical in achieving the desired performance of the final fabric, while balancing cost, environmental impact, and quality.