Sizing is a protective coating applied to yarns before they’re woven into fabric. Manufacturers coat warp yarns (the lengthwise threads on a loom) with a thin film of starch, synthetic polymers, or other adhesive substances to make them stronger, smoother, and less likely to snap during the high-friction process of weaving. If you’ve ever bought new fabric or clothing and noticed it feels stiffer or crisper than expected, that stiffness is almost certainly residual sizing.
Why Manufacturers Apply Sizing
Weaving puts enormous stress on yarn. Warp threads are pulled taut on the loom and repeatedly struck by the shuttle or rapier carrying the weft thread across them. Without protection, the friction and tension would cause individual fibers to fray, weaken, and break, slowing production and creating flawed fabric.
Sizing solves this by doing three things at once. First, it glues down the tiny surface fibers that stick out from the yarn, creating a smoother profile that slides more easily against neighboring threads and loom parts. Second, it adds a degree of rigidity so the yarn holds its shape under tension rather than stretching unevenly. Third, it increases the yarn’s tensile strength, meaning it can withstand more pulling force before it breaks. The result is faster, more efficient weaving with fewer defects in the finished cloth.
What Sizing Is Made Of
The ingredients depend largely on the type of fiber being woven. For natural fibers like cotton and viscose, starch is the traditional go-to sizing agent. Starch’s molecular structure bonds well with plant-based fibers because both are hydrophilic (water-attracting), which helps the coating grip the yarn surface. Modified starches, including starch ethers and starch acetates, offer improved performance over plain starch and are widely used in commercial mills.
Synthetic and blended fabrics call for different chemistry. Polyvinyl alcohol (PVA) is one of the most common synthetic sizing agents. Higher-grade PVA works well on natural fibers, while lower-grade versions are better suited for synthetic fibers like polyester and nylon. For polyester specifically, water-soluble polyester sizing agents have a natural chemical affinity for the fiber, producing strong adhesion. Acrylic-based polymers, derived from acrylic acid and its variants, also appear in sizing formulations, often blended with starch derivatives to fine-tune performance.
Cellulose derivatives like carboxymethyl cellulose round out the lineup, offering low-viscosity solutions that coat evenly at lower concentrations. In practice, most commercial sizing recipes are blends of two or more of these ingredients, tailored to the specific yarn and loom setup.
How Sizing Differs by Fiber Type
Cotton and other cellulose-based yarns are sized primarily with starch or starch blends. The hydroxyl groups in starch form hydrogen bonds with the hydroxyl groups in cotton, creating a strong, even film. Acrylic polymers are sometimes added to boost adhesion and flexibility.
Polyester yarns need sizing agents with a chemical affinity for hydrophobic (water-repelling) surfaces. Water-soluble polyester copolymers work best here because their molecular structure closely matches the fiber itself. PVA with a lower degree of polymerization also bonds well to polyester. Cotton-polyester blends typically get a mixed sizing formulation that addresses both fiber types, often combining starch with PVA or a polyester-based agent.
How Sizing Gets Removed
After weaving, fabric destined for dyeing or finishing goes through a process called desizing. The sizing has done its job on the loom and now needs to come off so the fabric can absorb dyes, softeners, and other treatments evenly.
For starch-based sizing, enzymatic desizing is the industry standard worldwide. Mills use an enzyme called alpha-amylase that breaks starch down into simple sugars, which dissolve in water and rinse away. The process typically runs at about 60°C (140°F) for one hour. It’s favored because it’s effective and produces no toxic byproducts, making it more environmentally friendly than chemical alternatives.
When enzymatic methods aren’t effective, acidic desizing with dilute hydrochloric acid can break down stubborn starch. Oxidizing agents offer yet another route, converting sizing polymers into water-soluble fragments. Synthetic sizing agents like PVA are generally water-soluble on their own, so they wash out during the standard scouring step without needing enzymes or acid.
For home sewers and garment buyers, the practical version of desizing is simply washing. A regular machine wash in warm water with detergent removes most residual sizing from new fabric or clothing. If fabric feels unusually stiff or has a slightly waxy hand, a second wash usually takes care of it.
Sizing and Skin Sensitivity
Residual sizing chemicals in new clothing can occasionally cause skin irritation, particularly for people with sensitive skin or existing conditions like eczema. Finishing chemicals, including sizing agents and the resins sometimes used alongside them, have been identified as risk factors for contact dermatitis among textile workers with prolonged exposure. For the average consumer, the risk is much lower, but washing new garments before wearing them is a simple way to remove any leftover sizing and reduce the chance of a reaction.
Environmental Impact of Sizing
Desizing generates a significant portion of a textile mill’s wastewater pollution. When starch-based sizing washes off fabric, it creates wastewater with high biological oxygen demand (BOD), meaning the dissolved organic material consumes oxygen as it breaks down in waterways. Studies of textile wastewater have recorded BOD levels ranging from under 10 mg/L to over 1,700 mg/L depending on the production stage, with desizing being one of the heaviest contributors.
This is one reason the industry has shifted toward enzymatic desizing and toward sizing agents that require less material per yard of fabric. Some newer formulations are designed to be recovered and recycled from the wash water rather than discharged, reducing both chemical costs and environmental load. PVA, despite being effective, has drawn scrutiny because it biodegrades slowly in wastewater systems, prompting ongoing work on more easily degradable synthetic alternatives.
Why It Matters for Home Sewers and Consumers
If you’re buying fabric for sewing, sizing affects how the material handles. Heavily sized fabric feels crisp and holds its shape well for cutting and pinning, which is why quilting cottons often retain some sizing. But the finished garment may feel stiffer than expected until you wash it. Pre-washing your fabric before cutting removes sizing, gives you a truer sense of the fabric’s drape, and accounts for any shrinkage that would otherwise happen after your first launder.
For ready-made clothing, that “brand new” crispness is often sizing at work. It gives shirts and sheets a smooth, polished look on the shelf, but it washes out over time. If a new garment feels scratchy or triggers mild irritation, a wash cycle is usually all it takes to strip the sizing and soften the hand of the fabric.