Alcohol polivinílico (PVA) para el tamaño de algodón e hilo mezclado

Polyvinyl alcohol (PVA) for warp sizing of cotton and blended yarns is a water-soluble synthetic polymer whose size film performance is determined by two molecular parameters: the degree of hydrolysis and the degree of polymerization. When fully hydrolyzed grades (98–99 mol%) are applied to ring-spun cotton, the film exhibits high tensile strength but limited cold-water solubility, demanding elevated desizing temperatures. Partially hydrolyzed grades (86–89 mol%) retain sufficient crystallinity for slasher drying while cold-water-soluble residues enable desizing at 40–60 °C, a critical advantage in continuous open-width preparation ranges. Unlike starch-based sizes, PVA does not support microbial growth in the size box, and unlike carboxymethyl cellulose (CMC), its film does not re-wet into a gel but dissolves progressively, reducing shedding on modern high-speed looms.

Why Do Hydrolysis Degree and Polymerization Degree Dictate Size Film Cohesion?

The balance of acetate and hydroxyl functionality in the polymer backbone determines the size film’s adhesion to cotton cellulose and its response to ambient humidity. Fully hydrolyzed grades (hydrolysis ≥98.5 mol%) achieve the highest hydrogen-bond density with cotton hydroxyls, yielding an adhesive bond measured by pull-off adhesion tests that can exceed 12 MPa on polished cotton fabric under 65% RH per an adaptation of ASTM D4541. However, the same high hydroxyl density causes the film to plasticize and lose modulus above 80% RH, which is frequently encountered in weaving sheds without climate control. The polymerization degree, typically expressed as viscosity of a 4% aqueous solution at 20 °C per ASTM D1084, controls the mechanical coherence of the cast film. A low-viscosity grade (5–7 mPa·s) may provide adequate binding for coarse open-end yarns but results in excessive penetration and low film thickness on the yarn surface when sizing Ne 30–40 combed cotton. Commercial sizing grades are routinely classified by this viscosity window, and the selection matrix is summarized in Table 1.

Table 1. Typical PVA Sizing Grades and Viscosity-Application Correspondence

Grade Designation	Hydrolysis (mol%)	Viscosity, 4% aq. (mPa·s at 20°C)	Approximate D.P.	Primary Application
PVA 1788	87–89	20–26	1700	Polyester-cotton blends, cold-water desize
PVA 1799	≥98.5	25–31	1700	100% cotton, high-speed air-jet weaving
PVA 2099	≥98.5	35–42	2000	Fine-count cotton, warps above 100 ends/cm
PVA 2488	87–89	44–55	2400	Blended ring yarns requiring high abrasion resistance

The degree of saponification does not simply trend with adhesion; partially hydrolyzed grades introduce acetate groups that disrupt crystallinity and reduce the dry film’s glass transition temperature (T_g) to approximately 60–65 °C for the 88 mol% hydrolyzed variant, compared to 80–85 °C for the fully hydrolyzed analogue. This depression can cause blocking when sized beams are stored in hot warehouses, a failure mode reported on beams stacked more than three rolls high in ambient temperatures above 35 °C. The risk is mitigated by limiting the add-on of partially hydrolyzed grades to 8–10% on pile and specifying a wax content of 0.3–0.5% based on polymer mass in the size mix.

Slasher Deposits and the Critical Role of Additive Compatibility in Reuse Systems

In multi-cylinder slashers operating with size-box temperatures maintained at 88–92 °C for fully hydrolyzed PVA, film-forming on the surface of the size box can generate insoluble skins that transfer to the warp and create hard deposits on drying cylinders. These deposits carbonize and cause warp streaks that are detectable only after desizing and dyeing as depth-of-shade variations. Plant observations from a 32-cylinder sizing machine processing 36-ends/cm sized beams confirm that covering the size box and maintaining a relative humidity above 50% in the preparation area reduces skinning frequency by approximately 70%, but the residual buildup demands weekly alkaline boil-out with 2–3% NaOH at 95 °C. When PVA is combined with oxidized starch in reuse cooking systems — a common cost-reduction practice — the leached starch component fails to solubilize fully in the recovered size liquor if the cooking kettle temperature drops below 98 °C for more than 15 minutes. This leads to a progressive enrichment of PVA in the circulating size and a simultaneous drop in starch concentration, altering the film from a semi-flexible starch-predominant composite to a stiff PVA-rich coating. Continuous refractometer monitoring of the size concentration (% solids) alone does not reveal the composition shift; only periodic determination of the Brix-to-viscosity ratio via a laboratory suction-filter method can flag the drift before loom breakages escalate.

When processing polyester-cotton blends with open-end spun yarns, the size recipe must balance adhesion to the hydrophobic polyester component and flexibility to survive reed beat-up without cracking. A typical formulation for a 65/35 polyester-cotton rotor-spun warp uses a blend of partially hydrolyzed PVA (grade 1788) and a fully hydrolyzed grade (1799) in a 70:30 ratio, with a total solids pick-up of 7.5–9.0% on bone-dry warp weight. The partially hydrolyzed component plasticizes the film and lowers the dry-state modulus to 1.2–1.8 GPa, measured by tensile testing of free films per ASTM D882, which is compatible with the elongation-at-break requirements of rotor yarns that rarely exceed 6%. Attempting to replace this blend entirely with a single intermediate hydrolysis grade (92–94 mol%) risks film embrittlement if the slasher drying rate causes surface skin formation before full water removal, a phenomenon documented by thermal gravimetric analysis of size films showing a skin vitrification threshold near 105–110 °C on the first cylinder.

Comparative Size Film Mechanical Properties and Desizing Effluent Load

The selection of PVA over oxidized starch or CMC is frequently driven by the mechanical durability of the size film during high-speed weaving, but the environmental profile of the desize effluent imposes a trade-off. Table 2 summarizes key performance indicators derived from laboratory-prepared films and industrial slasher runs documented in published technical service bulletins.

Table 2. Performance Comparison: PVA vs. Starch and CMC Size Films on Cotton Warp

Property /Endpoint	Fully Hydrolyzed PVA (1799)	Oxidized Corn Starch	Technical CMC (DS 0.7–0.9)	Standard /Note
Tensile strength of cast film (MPa)	40–60	10–18	30–45	ASTM D882; 50% RH, 23°C
Elongation at break (%)	150–250	2–4	8–15	ASTM D882
Adhesion to cotton fabric (N/25mm)	15–20	6–10	10–14	Modified ASTM D1876; T-peel
Sized yarn tensile retention (% of grey strength)	110–130	95–105	105–115	ASTM D2256; Ne 30 ring-spun
Desizing efficiency (weight loss after 10 min at 90°C with 0.5% enzyme/wetting agent)	>98%	>99%	85–92%	Simulated pad-batch desize
BOD5 of pure size agent (mg O₂/g)	20–30	600–800	10–15	OECD 301F; ready biodegradability
COD of size agent (mg O₂/g)	1600–1700	900–1100	900–1050	Standard potassium dichromate method

The data underscore the core difference: PVA delivers a mechanically superior film that permits lower size add-on — typically 6–9% on cotton versus 12–16% for oxidized starch — but generates a desizing waste stream with a higher COD:BOD ratio, indicating resistance to rapid biological oxidation in conventional effluent treatment plants. Industrial installations recirculating desize liquor through ultrafiltration modules (molecular weight cut-off of 20–30 kDa) recover up to 85% of the PVA for reuse, reducing the net COD load per kilogram of fabric below that of starch-based systems when assessed over a full cycle. Without such recovery, direct discharge of PVA-containing effluent can elevate biological treatment sludge age and requires extended aeration times beyond 24 hours. The difference from other synthetic sizes such as polyacrylates or polyester resin dispersions is primarily economic and in film re-wettability. Polyacrylate sizes offer superior adhesion to pure polyester but generate a much harder film that can fracture during loom shedding and are significantly more costly, with commercial pricing typically 2.5–3.0 times that of PVA on a dry-weight basis. Polyester dispersions are limited to continuous-filament sizing due to their blocked surface chemistry and are incompatible with starch blends. PVA, in contrast, forms clear, flexible films that can be blended with starch or used alone, and it does not require catalysis or curing.

In practical slashing, the pre-blending sequence is critical. PVA must first be dispersed in cold water under agitation to prevent lump formation, then heated to 90–95 °C for 30–45 minutes to achieve full dissolution. When jet cooking at 130–140 °C for 2–3 minutes, a 20% solids stock paste remains stable without viscosity breakdown, provided that shear rates in the cooker remain below 3000 s⁻¹; exceeding this threshold can shear-degrade the high-molecular-weight fraction, permanently lowering the solution viscosity by 10–15% and reducing size film cohesion. Once cooked, the size must be maintained at 80–85 °C in a jacketed supply tank fitted with a slow-speed propeller agitator (30–40 rpm) to avoid aeration. Foam entrapment, if not suppressed with a non-silicone defoamer at 0.05–0.1% on size liquor weight, deposits microvoids in the dried film that act as stress concentrators and initiate cohesive failure during weaving.