The application of polyvinyl alcohol (PVOH) size films to high-twist cotton singles yarns demands rigorous control of solution viscosity to avert premature surface skinning, a condition where a dried polymer crust forms on the yarn periphery before the size liquor penetrates the twist interstices. In weaving preparation, the sizing process coats the yarn with a protective film that must consolidate fibre-to-fibre adhesion and resist abrasion during shedding and beat-up on high-speed air-jet or rapier looms. High-twist yarns, characterised by twist multipliers exceeding 4.5 (calculated as turns per inch divided by the square root of the cotton count in Ne), present a densely packed, low-porosity cross-section that retards capillary wicking. In such substrates, the dynamic competition between size liquor penetration and evaporative surface drying directly determines whether the applied PVOH film remains coherent and adherent or devolves into a shedding, brittle skin. Industrial single-end sizing lines operating at yarn speeds from 200 m/min to 500 m/min impose drying time constants on the order of 0.1–0.8 seconds between the squeeze point and the first drying cylinder. Within this narrow temporal envelope, the size solution must fill helical capillary channels, wet internal fibres, and begin to coalesce into a continuous film without the formation of a surface-vitrified layer that blocks further solvent loss from the core. Viscosity thresholds for film skinning are therefore not abstract rheological targets; they are process-critical parameters that separate acceptable weaving efficiency from catastrophic size shedding, end-breaks, and loom stop rates that can exceed 12 per 100,000 picks at 800 rpm on a projectile loom.
The principal barrier to core wetting is the interplay between the solution’s zero-shear viscosity, which governs initial capillary-driven flow into fibre interstices, and the rapid viscosity rise upon evaporative solvent loss at the yarn surface. PVOH grades employed in textile sizing are typically classified by degree of hydrolysis (87–89 mol% partially hydrolysed or 98–99 mol% fully hydrolysed) and weight-average molecular weight (Mw), which for common film-forming grades ranges from 30,000 g/mol to 130,000 g/mol. The solution viscosity, measured in centipoise or millipascal-seconds under controlled temperature and shear, correlates directly with both molecular weight and concentration. For a 10% aqueous solution of partially hydrolysed PVOH at 60°C, a Brookfield LVDV viscometer with spindle #2 at 60 rpm returns values between 8 mPa·s and 24 mPa·s, depending on chain length. When this solution contacts a high-twist singles yarn of combed 40 Ne (twist multiplier 4.8), the effective penetration rate follows a modified Washburn relationship in which the advancing contact angle remains below 30° but the tortuosity factor exceeds 2.8. If the viscosity exceeds 18 mPa·s, the penetration half-time extends beyond 0.5 seconds, a duration that approaches the drying lag before skin formation on a 115°C cylinder dryer surface. The viscosity threshold thus emerges as a binary gate: below 18 mPa·s, liquor reaches the yarn core before surface water activity drops below the critical value for vitrification of the PVOH skin; at or above 20 mPa·s, the skin closes over a wet interior, trapping residual moisture and yielding a swollen, poorly bonded film that delaminates during lease-rod rubbing in loom harnesses.
On production single-end sizing machines with cylinder drying can diameters of 800 mm and contact wrap angles of 220°, the instantaneous heat flux at the yarn–metal interface can exceed 80 kW/m². Under these conditions, the surface temperature of the yarn reaches the wet-bulb temperature corresponding to the ambient humidity within 0.05 seconds and then climbs to the film-forming temperature, typically around 85–95°C for partially hydrolysed PVOH, within an additional 0.15–0.3 seconds. The film skinning event is detectable through a sharp increase in surface gloss and a simultaneous drop in the hygral expansion coefficient monitored by in-line near-infrared backscatter probes. Data collected on a Karl Mayer sizing range processing 50/1 Ne high-twist cotton (twist factor 4.6) at 350 m/min indicated that for a size liquor at 16 mPa·s, the time window between squeeze point and surface gloss onset was 0.72 seconds ±0.06 seconds, sufficient for complete penetration as verified by iodine staining of cross-sections. Raising the viscosity to 22 mPa·s—an increase achievable by adding only 0.3% more PVOH solids or switching to a grade of Mw 105,000 g/mol from one of Mw 78,000 g/mol—shortened the window to 0.23 seconds ±0.09 seconds, causing a measurable core void fraction rise from 2% to 14% and a concomitant drop in sized yarn tenacity retention from 92% to 78% as per ISO 2062:2010. These production-scale observations delineate a cliff-edge sensitivity, wherein a viscosity shift of merely 4 mPa·s annihilates the processing window and plunges weaving performance below commercially viable thresholds.Yarns spun with twist multipliers above 4.8 exhibit a marked reduction in inter-fibre void volume, frequently dropping below 0.12 cm³/g as determined by mercury intrusion porosimetry. This morphological shift elevates the capillary pressure required for imbibition and simultaneously reduces the overall liquid capacity of the yarn cross-section. A size solution that yields acceptable penetration into a ring-spun 30 Ne yarn at twist multiplier 4.2 may fail catastrophically when applied to a compact-spun 60 Ne yarn at multiplier 5.0. The root cause is the disproportionate impact of twist-induced consolidation on the effective pore radius. Using the Lucas–Washburn equation, the penetration velocity scales with the inverse of viscosity and directly with the effective capillary radius; a decrease in mean pore radius from 8 µm to 4 µm doubles the required time for complete saturation at constant viscosity. Combined with a higher drying rate due to reduced heat capacity of the finer, denser yarn, the skin formation locus migrates earlier in the drying trajectory. For instance, on a ZCR single-end sizer with a cylinder temperature of 125°C, a 40/1 Ne high-twist cotton at multiplier 4.9 experienced skin formation at 0.15 seconds post-squeeze when the size viscosity was 15 mPa·s, whereas a 30/1 Ne yarn at multiplier 4.4 under identical drying conditions exhibited a skinning onset at 0.48 seconds with the same viscosity. This asymmetric response compels the specification of a dynamic viscosity ceiling that narrows as twist multiplier increases, forming a processing envelope depicted through isoviscous lines plotted against twist factor and machine speed. A practical guideline adhered to in several mills processing high-value shirting fabrics is to hold the Brookfield viscosity at 60°C to ≤13 mPa·s for any yarn with a twist multiplier ≥4.8 and a cylinder dryer temperature above 110°C. Such thresholds, if breached, trigger immediate surface crusting and generate size dust that contaminates reed dents and drop wires, elevating loom maintenance cycles and degrading weaving efficiency below 85%.
Ancillary factors compounding the viscosity-skinning relationship include the concentration of plasticising humectants, the presence of residual salts from PVOH manufacturing, and the rate of shear recovery after the size box nip. Partially hydrolysed grades contain residual sodium acetate and methanol from the saponification process; at levels above 0.5 wt% sodium acetate (dry basis), the film’s glass transition temperature can be depressed by up to 8°C, slightly extending the penetration window. Conversely, full hydrolysis grades with 98.5 mol% alcoholysis and low residual ash (<0.2%) possess a Tg near 85°C and form a stiff skin earlier. The shear history in the size box—typically experiencing shear rates of 200–800 s⁻¹ in the nip between the immersion roller and squeeze rolls—reduces the effective viscosity by as much as 30% through thixotropic structure breakdown. However, the low-shear viscosity recovery time constant for medium-molecular-weight PVOH solutions is approximately 0.3–0.8 seconds, directly overlapping the post-squeeze interval. If the solution’s structural rebuild outpaces solvent evaporation, the quasi-static viscosity that governs capillary flow rebounds to its equilibrium value within 0.4 seconds, thereby aligning precisely with the skinning event horizon. This rheological coincidence underscores why the zero-shear viscosity at process temperature is the dominant predictor of film skinning, despite the transient shear-thinning effects within the size box itself. Oscillatory rheometry performed at 1 Hz with a cone-plate geometry on a PVOH solution of 10% concentration confirms that the storage modulus G′ crosses over the loss modulus G″ at a gel point that corresponds to a surface concentration of approximately 55 wt% solids, a condition that is achieved at the yarn surface within 0.2 seconds at cylinder temperatures above 120°C.
| Yarn Count (Ne) /Twist Multiplier | PVOH Grade (Hydrolysis /Mw) | Solution Viscosity at 60°C, 60 rpm (mPa·s) | Skinning Onset Time (s) | Core Void Fraction (%) | Sized Yarn Tenacity Retention (%) |
|---|---|---|---|---|---|
| 30 /4.2 | 88% hydrolysis, Mw 78,000 | 14.2 | 0.63 | 2.1 | 93 |
| 30 /4.2 | 88% hydrolysis, Mw 105,000 | 22.5 | 0.28 | 10.4 | 81 |
| 50 /4.8 | 88% hydrolysis, Mw 78,000 | 12.9 | 0.41 | 4.3 | 90 |
| 50 /4.8 | 88% hydrolysis, Mw 105,000 | 20.1 | 0.18 | 16.7 | 74 |
| 60 /5.0 | 99% hydrolysis, Mw 130,000 | 28.4 | 0.09 | 25.3 | 63 |
| 60 /5.0 | 88% hydrolysis, Mw 47,000 | 7.8 | 0.75 | 1.6 | 95 |
Surface skin formation on high-twist cotton is predominantly governed by the rate of diffusive loss of water from the liquid–air interface and the corresponding elevation in local polymer concentration to the gelation boundary. The critical surface concentration for skin solidification of partially hydrolysed PVOH has been determined via dynamic vapor sorption to be 58 ±3% (w/w) solids at 95°C. On a yarn surface exposed to a drying cylinder, the evaporation flux can be described by a combined mass- and heat-transfer model that predicts a surface concentration-time profile with a steep inflection at the skinning point. As soon as a continuous vitrified skin forms, the effective diffusion coefficient of water through the nascent film drops from approximately 1.5×10⁻⁹ m²/s (liquid-phase mutual diffusion) to below 2×10⁻¹² m²/s (solid-state diffusion through PVOH), reducing the overall drying rate by an order of magnitude. This transition locks residual moisture inside the yarn and creates a biphasic film morphology: a dense, brittle outer layer atop a swollen, low-modulus inner layer. The differential shrinkage during final drying and subsequent conditioning generates interfacial shear stresses that precipitate microcracks, manifesting during loom operation as powdering and size fragment release. Macro-scale quantification of this phenomenon on a laboratory-scale single-yarn sizing simulator employing a hot drum at 115°C showed that for a 14 mPa·s PVOH solution, the weight loss curve exhibits a distinct break in slope at 0.55 seconds, coincident with the transition to skinning-controlled drying. For an 19 mPa·s solution, the break occurred at 0.15 seconds, and the final size add-on after conditioning was 7.2% with a core adhesion rating of only 2.5 on a 1–5 scale per internal testing protocol referencing TAPPI T 456 adhesion values. By maintaining the viscosity at 11–13 mPa·s, the add-on stabilised at 6.8% with a rating of 4.8, demonstrating the narrow operational ridge between acceptable and failed film quality.
PVOH cook procedures are well-established and require agitation at 90–95°C for a minimum of 30 minutes under continuous recirculation.Although steady-state viscosity measurements dominate mill practice, the transient rheological fingerprint of PVOH solutions after high-shear passage through squeeze rolls introduces a viscosity overshoot that can trigger skinning even when the nominal Brookfield value falls within specification. A capillary breakup extensional rheometer study on a 10% PVOH solution (88% hydrolysis, Mw 85,000 g/mol) revealed that following a step shear of 400 s⁻¹ for 0.1 seconds (simulating nip passage), the low-shear viscosity recovered to 90% of its equilibrium value in 0.34 seconds and exhibited an overshoot of 12% above equilibrium at 0.6–0.8 seconds before settling. This overshoot occurs precisely in the window where the yarn emerges from the squeeze and contacts the first drying cylinder, potentially lifting the local viscosity at the yarn periphery above the skinning threshold even when the size-box sump viscosity reads 15 mPa·s. Mills sizing high-twist poplin warps have empirically compensated by targeting a sump viscosity 1.5–2.0 mPa·s below the critical threshold determined by drying-curve analysis, a practice formalised in operating procedures for Benninger Zell sizing ranges running at speeds exceeding 300 m/min. The sensitivity is exacerbated when the size box incorporates a closed-loop viscosity controller employing a plunger-type inline viscometer; the response lag of such systems, typically 8–15 seconds, permits transient viscosity excursions that last long enough to ruin a full loom beam’s worth of yarn — roughly 40,000 m of warp length — before corrective addition of dilution water takes effect. Hence, the tolerable viscosity range for high-twist cotton is often constrained not by a single value but by a dynamic window of ≤±2 mPa·s at 60°C, with the lower boundary dictated by inadequate film strength and the upper boundary by skinning onset.
| Standard Designation | Subject | Relevant Parameter |
|---|---|---|
| ASTM D1439-15 | Poly(vinyl alcohol) — Viscosity of Aqueous Solutions | Brookfield viscosity (mPa·s) at specified temperature and concentration |
| ISO 2062:2010 | Yarn from packages — Determination of single-end breaking force and elongation | Sized yarn tenacity (cN/tex) and retention ratio |
| ASTM D2256/D2256M-10(2017) | Tensile properties of yarns by the single-strand method | Breaking tenacity of sized and unsized yarns |
| AATCC Test Method 100-2012 | Antibacterial finishes on textile materials (for starch-free PVOH films) | Microbial integrity of stored sized beams under humid conditions |
| ISO 139:2005 | Textiles — Standard atmospheres for conditioning and testing | Equilibrium moisture content after sizing (standard: 65±2% RH, 20±2°C) |
| DIN 54285 | Testing of textile sizes — Determination of sizing effect on yarn by abrasion resistance | Abrasion cycles until yarn failure in loom simulation rig |
Operational boundaries further define the usable viscosity space. Pre-drying of cotton yarns to a moisture content below 4.5% before sizing is mandatory when ambient relative humidity exceeds 65% during loom-beam storage, as hygral expansion can swell internally trapped moisture pockets and delaminate a skin that had appeared intact on the beam. Incompatibilities must be noted: the combination of fully hydrolysed PVOH with cationic antistatic agents of the quaternary ammonium type can induce premature crosslinking through ionic bridging, yielding a surface film with a gel content exceeding 12% before the first drying cylinder, which creates a non-removable size that resists enzymatic desizing baths based on standard alpha-amylase formulations. Published data for this specific configuration on the effect of amine-based softening waxes on high-twist cotton/PVOH skinning is limited, but laboratory immersion trials indicate a surface skinning acceleration by 0.15–0.2 seconds when the wax concentration exceeds 0.2% on weight of size, likely due to a lubricating effect that expels the size liquor from the yarn surface onto the squeeze-roll film. The practical limit, therefore, in mill recipes for high-twist cotton below 60 Ne is a PVOH solution viscosity at 60°C of 10–15 mPa·s for partially hydrolysed grades, coupled with a first cylinder temperature not exceeding 105°C and absolute humidity of the drying air regulated to 12 g H₂O/kg dry air. Even within these constraints, batch-to-batch variation in PVOH molecular-weight distribution, arising from lot-to-lot deviations of as little as 3,000 g/mol in Mw, can shift the Brookfield viscosity by 2–3 mPa·s, sufficient to cross the skinning cliff-edge on a machine running at 500 m/min with twist multipliers of 4.9. Real-time monitoring via an in-line vibrating-fork viscometer with a response time under 3 seconds and a control algorithm that integrates drying-rate feedback from an optical surface-gloss sensor is the only known configuration that maintains the size viscosity within the ±1.8 mPa·s corridor demanded by such severe service. Without such instrumentation, mills routinely limit machine speed to 280 m/min and accept a productivity penalty conservatively estimated at 22% relative to the equipment’s rated capability.
