PVA for Textile Warp Sizing: Weaving Mill Guide

Polyvinyl alcohol (PVA) specified for warp sizing is a water-soluble synthetic polymer produced by partial alcoholysis of polyvinyl acetate. In weaving mill formulations the functional grade is not fully hydrolysed; it retains residual acetyl groups that control solubility, film flexibility, and hot-water desizing. A standard partly hydrolysed sizing grade is characterised by an alcoholysis degree of 86.0–89.0 mol% and a 4% aqueous solution viscosity of 20.0–30.0 mPa·s at 20 °C, measured in accordance with ISO 15023-1:2017. Typical supply specifications also include volatile matter of 1.0–5.0%, ash below 0.5%, and pH of a 4% solution between 5.0–7.0. These boundaries are not commercial preferences; they determine whether the size film will split cleanly at the slasher lease rods and survive shedding on high-speed looms.

Partially hydrolysed PVA belongs to the small group of synthetic sizes that can form a coherent film with sufficient elongation to follow yarn deformation without immediate crack propagation. On a ring-spun cotton warp of 20–30 Ne, the size film is not merely lubricating the yarn surface; it penetrates the outer fibre layer and cements protruding fibres into the yarn body. The film must therefore balance two contradictory requirements: high enough tensile strength to resist abrasion at heald frames and reed, and low enough brittleness to avoid dusting under cyclic flexure. When the alcoholysis degree is lowered below 85.0 mol%, the film becomes softer and tackier, causing size build-up on drying cylinders. When the alcoholysis degree exceeds 90.0 mol%, solubility decreases sharply and desizing temperatures must be raised above 85–90 °C.

What limits PVA substitution in viscose and regenerated cellulose warp sizing?

For regenerated cellulose yarns, the primary substitution limit is moisture sensitivity rather than interfacial adhesion. A PVA film conditioned at 50% RH and tested to ASTM D882-18 typically shows tensile strength in the range 40–60 MPa and elongation at break of 120–200%. At slashing-room relative humidity below 35%, equilibrium moisture in the film falls under 2.0%, and the elongation can drop by more than half, converting the film from ductile to brittle. Viscose warps are hydrated by the size bath and can lose wet strength; a brittle surface film adds bending stiffness to a yarn that is already sensitive to wet abrasion. This combination produces visible film cracks at lease rods and higher end breaks on the loom.

Furthermore, viscose yarns may release alkali during hot-wet processing. In a size box held at 75–80 °C, alkaline carry-over can raise the bath pH above 8.0. Under those conditions, residual acetyl groups in partially hydrolysed PVA undergo slow hydrolysis, changing solution viscosity and reducing film cohesion during long slashing runs. For viscose warps, a PVA grade with alcoholysis degree nearer to 88–89 mol% and a relatively low ash content is preferred because it is less tacky and more dimensionally stable in the wet state, but published data for this specific configuration is limited and mill validation remains necessary.

Production-scale PVA cooking is preferably performed by first dispersing the granular product in cold water at 20–25 °C using a high-shear mixer operating at 900–1500 min⁻¹. This pre-slurry is then pumped to a steam jet cooker and held at 90–95 °C for 25–35 min. In pressure cooking systems, 110–120 °C for 10–15 min is sufficient for low-viscosity grades. Undercooking leaves gel particles that block slasher filter screens of 125–150 µm mesh, while overcooking in alkaline process water above pH 8.0 can cause progressive deacetylation and lower film strength by an estimated 10–20%. A fully cooked solution is clear, free of visible gel, and should be passed through a screen before delivery to the size box.

Slashing viscosity stability and wet split retention in PVA size boxes

At the size box working temperature of 75–85 °C, a PVA solution with 8–12% solids typically shows a viscosity of 10–20 mPa·s. This viscosity window is critical for wet split control. If viscosity rises above 30 mPa·s, the size can penetrate insufficiently into the yarn core and the wet split at the final lease rods becomes difficult, producing stuck ends and size film disruption. If viscosity falls below 8 mPa·s, deep penetration may occur, but surface film thickness becomes insufficient for loom abrasion resistance. Squeeze roller pressure in a two-box slasher is usually maintained between 3.0 bar and 5.0 bar in the second box. Electrical load on the slasher drive and beam hardness are secondary indicators of add-on; both change rapidly when size box solids drift by more than 1–2%.

PVA solutions do not undergo the sharp retrograde thickening observed with many native starches, but they are not infinitely stable. Holding a PVA solution in an open size box at 80 °C for 8 h can produce viscosity drift of approximately ±5% when pH is maintained between 6.0–7.5. At pH above 8.5, drift may become larger and less predictable because of hydrolysis. Size batch records should therefore log pH, temperature, solids refractometer readings, and drain time through a Zahn cup 2 at the soup tank. Those measurements provide more immediate control than laboratory viscosity alone.

When air-jet loom speeds exceed 850 rpm on cotton-polyester warps

At loom speeds above 850 rpm, the warp sheet is subjected to high-frequency tensile loading that makes film abrasion the dominant failure mode. In this regime, partially hydrolysed PVA with a degree of polymerisation of 1700–2000 provides better yarn-to-metal abrasion resistance than low-viscosity grades with degree of polymerisation 500–1000, but the same high-molecular-weight films are slower to desize. For 20–30 Ne cotton-polyester warps, the validated compromise is usually a degree of polymerisation of 1700–1800 combined with 10–20 wt% acrylic copolymer solids in the size recipe. The acrylic component improves adhesion to hydrophobic polyester, but increases effluent COD and can lower the glass transition temperature of the dried film if the acrylic grade is soft.

Film thickness becomes a process cliff-edge at high loom speed. On a sized yarn of 20 Ne cotton with a size add-on of 8–14%, the surface film is thick enough to prevent protruding fibre entanglement. When add-on falls below 8%, mill trials show an increase in end breaks in the shed zones, although published data for this specific configuration is limited because loom type, shed geometry, and yarn quality dominate the result. Above 14–16% add-on, weaving efficiency tends to plateau or decline because the yarn becomes stiff, the beam may block in humid weather, and desizing costs increase without additional abrasion protection.

Storage of sized beams between slashing and weaving is a process boundary that deserves direct specification. When the weaving shed relative humidity remains below 40%, the dried PVA film loses moisture and begins to dust at the back rest and drop wires. When relative humidity exceeds 70%, the film absorbs enough water to become tacky, and adjacent yarns can block on the beam, causing uneven tension and shedding hard ends. For storage periods longer than 48 h, conditioned beam stores should be held at 55–65% RH and 24–30 °C. If such storage is not available, the beam must be wrapped in film or scheduled for immediate use. This is not a theoretical concern; it changes warp break rate and slasher production economics on the same weaving machine.

Desizing effluent limitations and ultrafiltration recovery on textile processing lines

Desizing of partially hydrolysed PVA is performed in continuous open-width washers at 60–80 °C. Fully hydrolysed PVA requires 90–95 °C because the more crystalline structure resists dissolution. A counter-current range with three to five wash boxes and a residence time of 2–4 min per box usually reduces residual size on fabric below 0.5%. Residual PVA can be detected by the iodine-boric acid colour complex, with absorbance measured at 690 nm after standard extraction; the method is sensitive but requires calibration against the same PVA grade because colour yield varies with acetate content.

The chemical oxygen demand of PVA measured to ISO 6060 is approximately 1.6–1.7 g O₂/g polymer. A segregated desizing effluent containing 10 g/L PVA therefore contributes roughly 16–17 g/L COD. Under standard activated sludge conditions, high-molecular-weight PVA is not readily biodegradable in short retention times; OECD 301B screening data often show degradation below 20% in 28 d. Recovery by ultrafiltration is therefore the least polluting route in mills that use PVA continuously. Polyethersulfone spiral membranes with molecular weight cut-off between 30–50 kDa, operated at 2–4 bar, can recover 85–95% of PVA from hot desizing liquor, but fibre fragments, starch size, and waxes reduce flux by 30–50% unless pre-filtration is used. The economic boundary is concentration: at PVA concentrations below 5 g/L, recovery on production-scale equipment may not be justified because permeate energy and membrane area make the operation cost-ineffective. Desizing effluent for membrane recovery should therefore be segregated from scouring and bleaching waste streams.