Evaluation of Polyvinyl Alcohol Size Pick up on Flax Fibers

Bath

Liquor formulation during warp preparation determines how deeply synthetic polymers encapsulate bast fiber bundles. Flax yarns possess unique morphological characteristics, including irregular bundle cross-sections, low intrinsic elongation, and residual surface pectins. When passing through a sizing box, untwisted or low-twist flax strands absorb chemical solutions through capillary action between individual elementary fibers.

Polyvinyl alcohol (PVA) serves as a primary film-forming agent, but its pickup behavior relies on specific thermal and concentration parameters established inside the slasher trough.

Sizing liquor concentration controls the absolute dry solids deposited onto the yarn structure. A low-viscosity PVA formulation penetrates deep into the core of the yarn, anchoring internal fibers together. Higher viscosity grades remain on the outer sheath, creating a protective sleeve.

Balancing core penetration against surface coating represents the fundamental technical objective during flax warp preparation.

Viscosity and Penetration Kinetics

Slurry rheology governs how deeply liquor migrates into spun yarn cores, which is critical because flax fibers exhibit low tensile elongation.

Low elongation limits the yarn’s ability to stretch under tension during high-speed weaving. An inadequate size coating allows inter-fiber slipping, resulting in warp end breaks under the cyclic loading of the loom shed. Raising the size box temperature from 75 degrees Celsius to 85 degrees Celsius reduces PVA solution viscosity, encouraging deeper migration into dense Nm 26 bast yarn structures.

High concentrations of solids increase film thickness but risk yarn encasement, turning flexible strands into brittle rods.

A dry size pickup of 8.5 percent on Nm 39 flax warp yarn increases tensile strength by 28 percent while reducing elongation by less than 0.4 percent under standard atmospheric conditions.

Squeeze Roll Pressure

Nip load configuration dictates the ratio between absorbed chemical weight and surface film thickness. Squeezer rolls operating at pneumatic pressures between 15 kilonewtons and 35 kilonewtons per meter squeeze out excess liquid, forcing the remaining PVA binder into voids between elementary fibers. Excessive nip pressure strips the wet film off the yarn perimeter, exposing protruding fiber ends that create high friction against heddle eyes during shedding.

Improper liquor adjustment generates predictable structural failures across the warp beam:

• Excessive core penetration starves the outer fiber layer, leaving surface hairs exposed to abrasive wear against adjacent warp ends.
• Insufficient nip pressure leaves thick chemical crusts on the yarn exterior, causing size shedding and clogged reed dents during shedding.
• Thermal bath fluctuation alters solution viscosity, creating batch-to-batch variance in pickup percentages across wide warp beams.
• High immersion speeds reduce liquor residence time, preventing uniform wetting of hydrophobic residual lignins on unbleached flax.

Fluctuations in yarn moisture regain explain unexpected absorption variances during high-speed runs.

Adhesion

Interfacial binder strength between polyvinyl alcohol and natural cellulosic fibrils relies on molecular hydroxyl group alignment. Flax fibers contain up to 75 percent cellulose alongside hydrophobic waxes, hemicellulose, and lignins. The polar chemical structure of PVA enables extensive hydrogen bonding with accessible cellulose hydroxyl sites.

Residual natural oils on grey flax reduce available bonding sites, lowering overall adhesive power compared to scoured or bleached yarns.

Polymer hydrolysis levels directly alter binder affinity. Fully hydrolyzed PVA grades contain a high concentration of hydroxyl groups, establishing strong inter-molecular bonds with purified cellulose. Partially hydrolyzed grades contain residual acetate groups that increase solubility in warm water while slightly altering film tenacity on bast fiber substrates.

Polymer Hydrolysis Levels

Chemical saponification ratios dictate how water-soluble synthetic size resins interact with raw bast yarn impurities. Partially hydrolyzed variants contain approximately twelve percent acetyl groups. These hydrophobic acetyl groups disrupt hydrogen bonding within the polymer matrix, yielding higher film flexibility and faster cold-water dissolution.

Fully hydrolyzed variants feature minimal acetyl content, maximizing chemical bonding across the polymer chain. These films possess superior tensile strength, making them effective binders for fine linen yarns like Nm 60. Their rigid structure requires careful plasticization with lubricants to prevent brittleness during dry loom shed operation.

Selecting a size binder with an incorrect hydrolysis level destabilizes the film matrix before the warp beam reaches half its total run length.

Film Elasticity and Yarn Stiffness

Coating flexibility regulates whether treated warp strands bend around loom guide rollers without micro-fracturing. Flax yarn displays an exceptionally high initial modulus. Sizing must not increase flexural rigidity beyond acceptable weaving thresholds, or drop wire action becomes unreliable.

1. Pre-condition raw flax yarn packages at 65 percent relative humidity and 20 degrees Celsius for 24 hours to stabilize moisture content.
2. Prepare a nine percent PVA solution by weight, dissolving the polymer at 95 degrees Celsius for 45 minutes under continuous stirring.
3. Submerge yarn hanks in the bath for 30 seconds at a constant temperature of 85 degrees Celsius.
4. Pass submerged hanks through laboratory squeeze rollers set at a uniform nip pressure of 20 kilonewtons per meter.
5. Dry the sized hanks in a forced-air oven at 105 degrees Celsius until moisture content reaches six percent.
6. Desize test samples in boiling distilled water for 60 minutes to calculate net dry size pickup gravimetrically.

Higher thread counts on coarse flax warps demand greater film elasticity than absolute tensile strength.

Abrasion

Frictional resistance along the yarn path dictates warp survivability in high-speed shedding cycles. As the loom operates, warps experience intensive mechanical wear against drop wires, heddle eyes, and reed wires. Flax strands lack the natural elasticity of wool or synthetic filaments.

Without protective size coatings, protruding flax hairs interlock with adjacent yarns, forming cling zones that lead to complete end breaks.

Size pickup percentages directly correlate with thread breakdown rates under accelerated friction testing. An inadequate pickup level leaves protruding fibers unprotected, while excessive pick-up makes the yarn brittle. The optimal sizing zone provides sufficient film coverage to lay down surface hairs without filling every structural void inside the yarn matrix.

Zweigle Staff Testing

Laboratory friction simulators replicate the mechanical wear caused by drop wires, heddles, and reed dents. A Zweigle staff tester subjects weighted yarn samples to reciprocal abrasion against a standard metal pin. The tester records the number of cycles required to rupture the sample or create an unacceptable level of fiber fuzzing.

Core penetration prevents abrasive shedding. When size stays on the outer surface without penetrating, mechanical friction peels the film away in flakes. This debris accumulates inside heddle eyes, causing tension spikes that snap stiff flax strands during high-speed insertion.

Why Does High Degree of Hydrolysis Limit Desizing Washability?

Fully saponified resins form dense crystalline networks that resist cold solvent penetration. The lack of residual acetyl groups permits tight packing between polymer chains, establishing powerful inter-chain hydrogen bonds. Water molecules cannot easily penetrate this crystalline structure at temperatures below 80 degrees Celsius.

Warm water washing fails to dissolve fully hydrolyzed PVA coatings after weaving. The grey cloth retains hard chemical residues that impede wet processing operations like bleaching or dyeing. Adding synthetic wax or using co-polymer blends lowers the melting temperature of the size film, facilitating water ingress during scouring cycles.

Maintaining balanced pickup criteria requires systematic monitoring on the slasher floor:

• Refractometer solids check performed on fresh batch mixing tanks prior to transfer into slasher storage units.
• Squeeze roll pressure calibration verified at both operator and drive sides using electronic load cells.
• Yarn stretch measurement tracked across the drying section to keep elongation loss below one percent.
• Moisture sensor verification conducted at the headstock to confirm residual water content stays between six and eight percent.

Standard quality control specifications under ISO 105-F10 demand a minimum size film retention of 92 percent following dry warping preparation.

Whether ultra-fine flax warps can achieve zero-shedding performance without supplementary wax additives remains an active industry debate.

Desizing

Polymer extraction after weaving relies on elevated bath temperatures and mechanical agitation rather than enzymatic breakdown. Enzymatic desizing formulations target starch molecules through specific catalytic cleavage. Starch enzymes leave polyvinyl alcohol molecules entirely intact.

Desizing PVA-treated linen requires thermal solubilization in hot water, frequently combined with mild alkaline builders or surfactant wetting agents.

Partially hydrolyzed PVA grades dissolve rapidly in water temperatures between 60 degrees Celsius and 70 degrees Celsius. Fully hydrolyzed variants demand hot wash regimes maintained between 90 degrees Celsius and 95 degrees Celsius. Skimping on water temperature results in redeposited film fragments that cause streakiness during subsequent piece-dyeing.

Effluent Recovery Systems

Filtration units isolate washed synthetic size compounds from wastewater streams for thermal re-concentration. Polyvinyl alcohol possesses high chemical oxygen demand (COD) values while displaying low biological oxygen demand (BOD) rates. Standard municipal biological treatment plants cannot readily degrade dissolved PVA chains within typical hydraulic retention times.

Ultrafiltration technology recovers up to 85 percent of dissolved PVA from wash liquors. Ultrafiltration membranes pass clean hot water while retaining high molecular weight polymers. The concentrated size paste returns to the slasher kitchen for re-formulation, reducing chemical purchasing costs while meeting environmental discharge limits.

Washing Temperature Regimes

Thermal thresholds in continuous scour boxes dictate the solvation speed of dried synthetic coatings, requiring elevated wash temperatures to prevent cold water from shocking the sized fabric and freezing residual polymers into insolubilized gels on the flax fiber surface.

Continuous open-width washers utilize multiple counter-current liquor compartments. The fabric passes through high-pressure water jets that mechanically strip hydrated PVA from the linen matrix. Maintaining alkaline conditions around pH 9.0 accelerates film swelling, speeding up extraction times on heavy weight fabrics.

Purchasing specifications must include mandatory documentation covering post-weaving size removal:

• Chemical composition declaration detailing exact PVA hydrolysis percentages and secondary binder ratios.
• Desizing temperature profile specifying minimum bath heat and residence time needed for complete extraction.
• Residual size test certificate confirming post-wash dry residue remains below 0.3 percent of total fabric weight.
• Effluent discharge impact report listing theoretical COD contribution per thousand square meters of greige cloth.

Incomplete size removal leads to uneven dye penetration, spotty finishing defects, and severe commercial penalties during piece-goods inspection.

Yield

Metre-cost calculations reflect the direct balance between warp sizing expenditure and shed output per hour. Raw flax yarn represents a high-cost raw material input. Warp end breakage directly destroys loom productivity, converting expensive machine time into non-productive idle hours.

Investing in precise size pickup evaluation protects operational margins by maintaining loom efficiency above 90 percent.

Shed time is sold in machine hours while cloth is bought in metres. Rapier looms running 100 percent flax warps at 450 picks per minute incur fixed hourly operational costs including power, loom wear, weaver labor, and facility overhead. Stop time caused by size failure rapidly shifts profitable production runs into net loss orders.

Slasher Efficiency Calculations

Warp preparation machine throughput sets the baseline labor and energy expense per kilogram of processed yarn. Running a slasher line too quickly prevents uniform size absorption, forcing operators to run low solids content to maintain speed. Low solids content yields insufficient pickup, causing end breaks that stall the weaving shed later in the process.

Optimal slasher speed balances drying canopy capacity against squeezer residence time, where squeeze rollers strip excess liquor; if line speed outpaces drying capacity, residual moisture causes mold growth and yarn sticking on the warp beam.

Optimizing size pickup from 5.0 percent to 8.5 percent on a 220 centimeter width linen plain weave reduces net landed fabric cost by 0.088 USD per meter due to eliminated loom stoppage time.

Total Metre Costing Structure

Landed cloth expenses combine raw yarn expenditure, sizing chemical inputs, loom hour charges, and finishing losses. Sizing chemical costs account for less than five percent of total greige manufacturing expenditure. Attempting to reduce overall costs by cutting size solids or using inferior PVA grades creates false savings that are destroyed by reduced loom efficiency.

Because loom stops increase weave costs, a single warp end break requiring operator intervention leaves the loom idle for two to three minutes. Across a 5,000 meter order, high breakage rates add dozens of unproductive machine hours, increasing the final cost per finished meter while missing shipment schedules.

Inserting a mandatory dry size pick-up tolerance of plus or minus one percent into the greige supply contract shifts financial responsibility for loom stoppage time directly back to the slasher operator.