SELVOL Polivinil Álcool 805

SELVOL Polyvinyl Alcohol 805 is a partially hydrolyzed polyvinyl alcohol with a typical 4% aqueous solution viscosity of 5.0–6.2 mPa·s at 20 °C determined by ISO 1652, hydrolysis of 87.0–89.0 mol%, pH in the range 4.5–6.5 at 25 °C for an 8% solution, ash as Na₂O not exceeding 0.5%, and volatile matter not exceeding 5.0%. In semi-batch vinyl acetate homopolymer and vinyl acetate-ethylene copolymerization, this grade functions as a protective colloid at concentrations from 2.0 wt% to 5.0 wt% based on vinyl acetate monomer. The partially hydrolyzed structure retains a controlled residual acetyl content that suppresses excessive hydrophilic block length and promotes interfacial adsorption at the monomer-water boundary without producing the low-shear viscosity surge observed with fully hydrolyzed PVOH at equal molecular weight. Pre-dissolution in deionized water at 90–95 °C for 30–45 min under low-shear agitation in a stainless-steel or glass-lined make-down tank is required before charging to the reactor, because undissolved gel seeds at ambient temperature disperse slowly and can appear as grit in the finished latex when filtered through a 150 μm sieve according to ASTM D5097.

During polymerization at 65–75 °C with ammonium persulfate or potassium persulfate metered at 0.1–0.3 wt% of monomer, PVOH 805 undergoes chain-transfer and grafting reactions with vinyl acetate radicals. The grafting yield is influenced by hydrolysis degree, radical flux, and monomer feed rate. Because 805 has an intermediate hydrolysis, it provides a balance between graft stabilization and finished-film water resistance. The reactor is typically a baffled stainless-steel vessel with an anchor or pitched-blade impeller operating at 80–150 min⁻¹; cooling capacity must be sized so that the exotherm does not exceed ±5 °C from the setpoint during the main monomer feed, because the graft layer thickness and resulting latex particle size distribution are temperature-sensitive. Inline or periodic sampling with a rotational viscometer under ISO 2555 is used to track viscosity drift, while off-line particle size distribution is measured by laser diffraction under ISO 13320 or dynamic light scattering under ISO 22412 after dilution with deionized water. Particle size shifts more strongly with reactor shear profile and monomer composition than with colloid concentration alone, so batch records should include agitator speed and monomer feed rate rather than relying on PVOH concentration as the sole predictor of latex particle size.

Latex made with PVOH 805 tends to exhibit shear-thinning behavior in the wet state and low grit formation when the pH is held between 4.0 and 5.5. The addition of sodium bicarbonate or sodium acetate buffers is common to prevent pH drift caused by persulfate decomposition products. If the pH rises above 7.5, the partially hydrolyzed PVOH can become increasingly water-sensitive after film formation and the viscosity may rise sharply in the reactor because of reduced colloidal stability. The latex can be evaluated for food-contact adhesive applications under 21 CFR 175.105, and for paper and paperboard surface applications under 21 CFR 176.170 and 21 CFR 176.180, provided that migration limits for residual vinyl acetate and methanol are met. For converters requiring low-VOC formulations, the absence of alkylphenol ethoxylate surfactant in the protective colloid system is an advantage, but the formulation must still be validated by specific migration testing when used as an indirect food-contact material.

A production-scale failure mode in emulsion polymerization is batch-to-batch drift in latex viscosity when the PVOH solution is not fully dissolved or is stored for more than 24 h without agitation. Microbial growth in a 10–12 wt% PVOH stock solution can reduce viscosity and introduce organic acids that alter reactor pH. Therefore, stock solutions are generally used within 24 h of cooling, or preserved with 0.1–0.2 wt% of a suitable biocide, provided the biocide is compatible with the initiator system. Another operational boundary is the incompatibility of PVOH 805 with strongly cationic additives below about pH 5; quaternary amine-based wetting agents can reduce colloidal stability in the latex, so anionic or nonionic wetting agents are preferred in formulations containing this grade. Published particle-size curves for this exact PVOH grade under all commercial reactor configurations are limited, so pilot-scale runs with the specific monomer composition and impeller type are required before transfer to full production.

Why Does Borate Complexation Create a Viscosity Ceiling in Laminating Adhesives?

Aqueous laminating adhesives based on PVOH 805 are often compounded at 10–15 wt% solids with 0.05–0.30 wt% sodium tetraborate decahydrate. The borate ion forms reversible diol complexes with the 1,3-diol residues in the PVOH chain. At low borate concentrations, mono-diol complexation increases hydrodynamic radius and wet tack; at concentrations above 0.50 wt%, di-diol crosslinks produce shear-sensitive gels that can exceed 20,000 mPa·s Brookfield viscosity at 20 rpm and become unpumpable in a production adhesive transfer system. Viscosity is measured with a Brookfield RV viscometer using ASTM D1084 or ISO 2555; all samples must be conditioned at 25±0.5 °C because borate complexation is reversible and strongly temperature-dependent. Viscosity decreases with increasing temperature and rebuilds on cooling, so inline viscometer readings without temperature compensation can be misleading when the adhesive is held in a jacketed day tank.

The adhesive pH is maintained between 4.0 and 5.5 using citric acid or fumaric acid. At pH above 7.0, borate ion equilibrium shifts toward tetrahydroxyborate and crosslinking density increases, reducing tack and creating irreversible gel slugs on knife-over-roll coating heads. A production-scale failure occurs when alkaline casein-based defoamers are post-added to a PVOH-borate system; localized pH excursions above 7.5 generate gel particles that lodge in 100 μm slot-die filters, causing pressure spikes above 3.0 bar on the coating line and forcing a shutdown for filter replacement. Amine-based wetting agents that buffer above 7.0 must therefore be excluded from borate-modified PVOH 805 adhesive systems. When increased water resistance is required, 0.5–2.0 wt% of a crosslinker such as glyoxal or melamine formaldehyde is often added, but crosslinker addition must be staged after borate complexation is complete to avoid a dual-network viscosity collapse or gelation.

Open time on clay-coated board is measured by applying a 50 μm wet film with a wire-wound bar and conditioning at 23 °C and 50% RH. Tack is measured by ASTM D2979 probe tack. PVOH 805 at 12 wt% retains measurable wet tack for 15–60 s depending on board porosity and ambient humidity; addition of 5–10 wt% of a plasticizer such as glycerin on dry PVOH extends open time but reduces final T-peel adhesion measured by ASTM D1876. Laminators that operate at high line speeds must therefore evaluate the trade-off between open time and final peel strength with a controlled-humidity drawdown series rather than adjusting plasticizer based on viscosity alone. Because PVOH 805 is a partially hydrolyzed grade, dried adhesive films have lower inherent water resistance than fully hydrolyzed PVOH; water resistance is evaluated by immersion in deionized water at 23 °C for 24 h, with failure defined as loss of bond to the substrate or visible blushing in the adhesive layer.

In blade-coated linerboard and bleached board production, PVOH 805 is used in a puddle or metered film size press at 2.0–6.0 wt% solids. The solution is cooked at 90–95 °C for 30–45 min, cooled to 50–60 °C, and maintained at pH 5.0–7.0; insoluble agglomerates are removed with a 100 μm in-line filter before the size press runback. Because 805 contains residual acetate groups, it reduces interfacial tension with the furnish and improves pick strength at lower dry add-on than oxidized starch, but this effect is strongly dependent on the ratio of PVOH to starch in the size press formulation. At very low addition levels below 0.5 wt% of the size press liquor, the benefit may be masked by the starch film, and surface strength improvements are better captured by IGT pick resistance under ISO 3783 or wax pick number under TAPPI T 459 than by visual inspection alone.

High-shear stability at blade coater speeds above 1200 m/min is evaluated with a capillary viscometer under ISO 11443 or a Hercules high-shear rheometer. At shear rates above 100,000 s⁻¹, the medium molecular weight of 805 limits extensional filament breakage in the film split, reducing orange-peel coating defects on highly calendered substrates. However, if the size press formulation contains more than 2.0 wt% of a cationic starch because of formulation cost pressure, charge incompatibility can produce precipitates that clog the blade tip and reduce coat weight uniformity across the reel. The resulting coat weight nonuniformity must be monitored against reel maps rather than assumed from laboratory drawdowns, because the precipitation is shear-induced and may not appear in low-shear drawdown testing. Optical brightening agents are retained in the size press film because PVOH 805 prevents agglomeration of tetrasulfonated stilbene derivatives; retained OBA is measured by UV fluorescence intensity against a starch-only control to verify the carrier effect.

When 805 Replaces Starch Ether in Gypsum Plasterboard Joint Compounds

Joint compounds formulated with PVOH 805 at 0.2–0.6 wt% on total dry solids show water retention values measured by vacuum desorption through a 0.45 μm membrane that fall between those of hydroxypropyl starch ether and carboxymethyl cellulose. The grade is dissolved separately as a 10% stock solution and post-added to the mixer after initial water addition to avoid lumping in the high-shear disperser. A horizontal plow mixer or twin-shaft batch mixer operating at 600–1000 rpm is sufficient for dispersion; direct dry addition into a calcium sulfate hemihydrate dry blend can generate gel particles that survive standard 150 μm sieve checks and require reworking of the batch. Bond strength and crack resistance are tested under ASTM C474, and the measured workability window is reduced if the mixer temperature falls below 20 °C because PVOH 805 dissolution is delayed in cold processing water.

An operational boundary exists at additions above 1.0 wt%: the higher molecular weight fraction of 805 increases the yield stress of the compound and slows trowel smoothing under low-shear conditions, while calcium ion concentration in the gypsum system can reduce the solubility of PVOH aggregates if the pre-mix temperature is below 70 °C. At the same time, 805 contributes less to slump reduction than typical cellulose ethers, so it is not a direct replacement when a formulation is adjusted to compensate for starch ether rheology. Published data for this specific PVOH grade in low-VOC ready-mix joint compounds is limited; batch-scale rheology must be confirmed by cone penetrometer measurements after 24 h aging, and water retention must be rechecked after storage because PVOH can hydrolyze in the alkaline gypsum environment over time. The presence of aluminum sulfate in some accelerator systems can further reduce viscosity stability, so the addition sequence should place 805 before aluminum sulfate to avoid localized acid hydrolysis and gel formation.

Warp Sizing Efficiency in High-Density Polyester Blended Fabrics

PVOH 805 is applied on multi-cylinder size boxes or single-end sizing machines at size add-on from 6% to 12% on warp yarn. The size box temperature is maintained at 80–90 °C for solubility and the solids content is adjusted to 8–12%, with viscosity checked by flow cup rather than Brookfield alone because production control in weaving mills relies on flow time. Partially hydrolyzed grades form flexible films that adhere to hydrophobic polyester blends, but the film strength is lower than fully hydrolyzed grades; therefore 805 is usually combined with starch or acrylic binder at 20–40 wt% of total solids to balance film elongation and abrasion resistance. Weaving performance is monitored by warp breaks per 100,000 picks on air-jet looms; an increase in break frequency after seasonal relative humidity changes below 40% indicates embrittlement of the size film, not necessarily sizing add-on error.

Desizing of 805 from greige fabric is performed at 80–90 °C in a continuous open-width washer with 0.5–1.0 g/L nonionic wetting agent; residual PVOH is verified by iodine-boric acid complex colorimetry at 690 nm after extraction. Because the grade is partially hydrolyzed, desizing is faster than for fully hydrolyzed PVOH at equivalent molecular weight, but residual films can remain in high-density woven constructions if the desizing bath is not agitated sufficiently. In mills that operate a countercurrent washing train, the desize liquor can be recycled through a heat exchanger to maintain wash water temperature; failure to maintain the final wash tank above 70 °C results in redeposition of PVOH onto the fabric and increases dye uptake unevenness in subsequent finishing. Effluent discharge of PVOH desize liquor is subject to local municipal limits; recovery by ultrafiltration with 50,000 Da molecular weight cut-off membranes can concentrate 805 for reuse, but the partially hydrolyzed grade has a lower gelation threshold than fully hydrolyzed PVOH under alkaline conditions.

Cast Film Edge Curl and Microcracking Are Controlled by Drying-Rate Gradients

Aqueous casting solutions of PVOH 805 at 10–15 wt% solids are used in temporary release films and water-soluble sachet applications. Film physical properties are measured on ASTM D882 tensile specimens conditioned at 23 °C and 50% RH; however, published tensile values for this exact grade vary with plasticizer content and residual moisture, so only comparative data against a control formulation should be used for specification. At drying-air relative humidity below 40%, edge regions lose water faster than the film center, producing differential tensile stresses that cause curl and microcracks on the casting belt. The defect can be reduced by staged drying: first zone at 60 °C and 60% RH, second zone at 80 °C and 45% RH, and final conditioning at 23 °C and 50% RH. Glycerin at 10–20 phr on dry PVOH controls flexibility, but above 30 phr the dried film becomes tacky and blocking occurs in roll storage. Solution hold tanks should be blanketed or closed because evaporation at the liquid surface can form a skin that later appears as gel specks in the cast film.