SELVOL Álcool Polivinílico 840

SELVOL Polyvinyl Alcohol 840 is a partially hydrolysed polyvinyl alcohol resin supplied as granular solid or powder. Material specifications reported under ISO 15023-2 place the degree of hydrolysis at 87.0–89.0 mol% and the viscosity of a 4% aqueous solution at 20 °C in the range 45–55 mPa·s. Aqueous-solution pH is controlled within 4.5–6.5, volatile content is specified at not more than 5.0%, and ash expressed as Na₂O is specified at not more than 0.7%. These values place the product in the medium-high viscosity segment of the partially hydrolysed Selvol series. Residual acetate groups reduce crystallinity relative to fully hydrolysed grades such as Selvol 103 and Selvol 107, providing lower-temperature water solubility but higher equilibrium moisture uptake and lower film water resistance. The molecular weight contribution places Selvol 840 above low-viscosity partially hydrolysed grades such as Selvol 205 in solution viscosity and cohesive strength, while retaining the same general hydrolysis window. The product is therefore selected where higher green strength, higher emulsion stabilising viscosity, or greater adhesive tack is required without moving to a fully hydrolysed grade. Film properties, solubility, and viscosity can be verified by lot-specific test certificates using ISO 15023-2 methods.

The ranges in the table are typical specification values, not substitutes for a certificate of analysis for a specific lot.

What Limits Cold-Water Dissolution of Selvol 840 in High-Shear Make-down?

In aqueous make-down for adhesives and coatings, the resin is first dispersed in water at 20–35 °C and then heated to 85–95 °C in a jacketed vessel fitted with a turbine agitator or high-shear rotor-stator device. The 87.0–89.0 mol% hydrolysis window leaves enough residual acetate groups to suppress the strong interchain hydrogen bonding found in fully hydrolysed material, so complete dissolution is possible without caustic addition. However, the medium-high molecular weight creates a viscosity peak during the gel phase. If the powder is added as a dry slug, the particle surface hydrates into a gel skin that prevents water penetration and forms fish-eye defects. Production-scale make-down tanks from 500 L to 2000 L are typically operated with controlled powder addition through an eductor or venturi mixer, and the final solids content is held at or below 10 wt% unless a high-torque anchor agitator is available. After the temperature reaches 85–95 °C, a hold time of 30–60 min is commonly required to clear haze and reduce undissolved gel counts. Cooling to 25–40 °C then produces the final working viscosity. Failure to control the addition rate creates a processing conflict: the higher molecular weight that improves dry adhesion and film strength also increases the dissolution viscosity and extends the hydration time. The same grade will therefore require a longer make-down cycle than Selvol 205 or Selvol 523 at equal solids. The benefit is a higher final solution viscosity at equal concentration, allowing formulators to reduce binder solids without losing film strength. Plant quality control often uses a Brookfield RVT viscometer at 20 rpm with spindle No. 3 for rapid batch verification, but the reference is the falling-ball method under ISO 15023-2.

Water-based adhesives formulated with Selvol 840 are applied by rod, roll, or slot-die coaters. The higher solution viscosity of the grade relative to Selvol 523 at equal solids allows a reduction in total binder solids, which shortens drying time on thermally sensitive substrates. When applied to paper or board at coat weights from 5–15 g/m², the dry film contributes fibre-tear adhesion under TAPPI T499 or equivalent tensile adhesion tests. The partial hydrolysis level provides better cold-water re-wettability than fully hydrolysed grades, which is useful in remoistenable adhesive applications. The film remains water-sensitive in humid environments, so packaging adhesives may require a water-resistant overcoat or blending with fully hydrolysed PVOH or starch.

When Selvol 840 is applied as a remoistenable adhesive on envelope and label stock, the dried film re-wets at 20–40 °C and develops tack within 2–5 s of contact. The partially hydrolysed structure provides higher re-wet speed than fully hydrolysed grades because water enters the film more readily. The higher molecular weight relative to Selvol 205 imparts higher dry tack, but the adhesive viscosity must be controlled by dilution because the solution can exceed the operating range of high-speed roll coaters. Batch-to-batch variation in the 45–55 mPa·s reference viscosity can produce measurable viscosity differences in the final adhesive; therefore incoming lot testing under ISO 15023-2 is used to adjust dilution factors.

In vinyl acetate and vinyl acetate-ethylene emulsion polymerisation, Selvol 840 is metered into the aqueous phase as a protective colloid at concentrations from 2.0 wt% to 6.0 wt% relative to monomer. The 45–55 mPa·s solution viscosity of the 4% reference solution translates into a higher reactor-phase viscosity than lower-viscosity partially hydrolysed grades, which increases particle size stability and reduces coalescence under stirred conditions. Production reactors using retreat-curve impellers at 80–120 rpm show that selection of this grade can reduce coagulum formation, but the viscosity increase also reduces heat-transfer coefficients at the reactor jacket. The process window is therefore bounded by the need for stable particle formation at the upper end and by the need for adequate heat removal at the lower end. The residual acetate groups alter grafting efficiency with vinyl acetate and shift the distribution of stabiliser between the aqueous phase and particle surface. Published data for this specific configuration is limited in open literature; pilot-scale reactor trials are necessary to determine the optimum dosage because comonomer composition, initiator type, and agitation pattern interact with molecular weight.

Suspension and Solution Viscosity in Ceramic Green Tape Casting

Ceramic green tape casting on slot-die lines uses Selvol 840 when the binder must raise green strength without requiring a fully hydrolysed grade. Slurries compounded at 35–50 vol% barium titanate or alumina solids use the polymer solution as a binder and rheology modifier. The 45–55 mPa·s reference viscosity indicates a longer chain than lower-viscosity partially hydrolysed grades, which increases interparticle bridging and green tensile strength. The trade-off is slurry viscosity: to maintain a casting viscosity of 1500–3000 mPa·s at 100 s⁻¹, solids loading may be reduced by 1.5–2.5 percentage points compared with a Selvol 205 formulation. Tape cast from Selvol 840 exhibits lower cracking during punching and winding when residual moisture is controlled below 1.0 wt%. The ≤0.7% ash specification is critical for multilayer ceramic capacitor and thick-film dielectric applications, because inorganic residues after binder burnout can increase loss tangent and compromise insulation resistance.

On film-transfer size presses running at 40–60 m/min, Selvol 840 is blended with oxidised starch at dry ratios from 1:10 to 1:4 to increase surface strength without creating a separate water-resistant barrier. The size solution is held at 60–70 °C to control viscosity and avoid gel formation. Surface strength is assessed by IGT pick testing under ISO 3783, and the higher molecular weight of Selvol 840 can improve pick strength at equal solids compared with lower-viscosity partially hydrolysed grades. The residual hydrolysis window leaves the film water-sensitive, so it is not selected when wet-rub resistance or low Cobb values below 30 g/m² under ISO 535 are the primary requirement. In those applications, fully hydrolysed grades provide better water resistance, but their solution make-down requires higher temperature and more aggressive agitation.

When Textile Warp Sizing Demands Hot-Water Removability Without Enzymatic Desizing

On slasher ranges operating at 30–80 m/min, Selvol 840 is used in warp size formulations with modified starch and acrylic copolymer. The partial hydrolysis window permits desizing with hot water at 80 °C, eliminating enzymatic or oxidative desizing in plants that do not run cellulosic fibre blends. Compared with fully hydrolysed grades, the lower crystallinity reduces hard deposits on reeds and heddles. The film absorbs atmospheric moisture, so warp beams stored at relative humidity above 60% can develop blocking, and pre-drying of the warp sheet is required before winding. Boric acid or borate crosslinkers should be used with controlled addition, because gelation can occur rapidly and increase size-box viscosity beyond the pumping limits of conventional slasher size boxes.

Mould release films and temporary protective coatings based on Selvol 840 are typically deposited from 2.0–5.0 wt% aqueous solutions and dried at 60–80 °C. The pH specification of 4.5–6.5 and ash limit of ≤0.7% reduce the risk of interaction with acid-catalysed tooling surfaces and leave lower inorganic residue after film peeling. Films made from partially hydrolysed Selvol 840 exhibit higher elongation and lower tensile strength than films from fully hydrolysed grades, a difference that reduces brittle cracking when the film is stretched over complex mould features. The trade-off is lower water resistance, so the material is used where water rinsing after demoulding is acceptable. For epoxy or unsaturated polyester tooling, release behaviour must be tested on production surfaces under actual curing conditions, because surface energy and tooling release chemistry can dominate film performance.

Bulk Storage Stability Depends on Moisture Uptake and Flow Properties

The resin is hygroscopic. Storage in silos or bulk bags at relative humidity above 60% can raise volatile content and alter powder flow, leading to bridging in hoppers and variability in dissolution time. Silo discharge equipment should include dry-air sweeps or desiccant breathers, and the volatile content should be checked by ISO 15023-2 before use when storage conditions are uncontrolled. Dust-air mixtures of polyvinyl alcohol powder are combustible, so transfer lines, dust collectors, and charging stations must be grounded and provided with local exhaust ventilation. The product should not be blended with strong oxidisers or exposed to open flame. Regulatory status for food-contact use is not uniform across jurisdictions, and compliance must be confirmed for the specific article and conditions of use under FDA 21 CFR or the applicable regional legislation before that application is attempted.