SELVOL Polivinil Álcool 523S

SELVOL Polyvinyl Alcohol 523S is a partially hydrolysed polyvinyl alcohol resin produced as a free-flowing granular solid. Lot-release data define a 4% aqueous solution viscosity of 22.5–27.5 mPa·s at 20°C and a hydrolysis degree of 87.0–89.0 mol% under JIS K6726. The residual acetate groups lower crystallinity, increase segmental mobility, and provide greater surface activity than fully hydrolysed PVOH grades. The product occupies the medium-viscosity region of the SELVOL PVOH range and is selected when a formulation requires stronger colloidal stabilisation than a low-viscosity grade can supply without the high thickened viscosity of a high-molecular-weight grade.

Bulk make-down in aqueous systems is usually handled by pre-slurrying the granules in cold water at 20–30°C and then heating to 85–90°C under low-shear agitation. A pitched-blade turbine operating at 300–600 rpm in a baffled stainless tank is sufficient; high-shear rotor-stator mixers are not required and may introduce entrained air and foam. Dissolution endpoint is confirmed by casting a thin wet film on glass and inspecting it under 100× magnification for translucent gel specks. At storage relative humidity above 60%, open bags can soften and agglomerate; keeping storage below 60% RH and resealing partially used containers preserves hopper flow.

Solution viscosity rises nonlinearly with concentration. A 10 wt% aqueous solution of 523S at 25°C typically develops a Brookfield LVF viscosity of 2,500–4,000 mPa·s at 60 rpm. The solution is shear thinning above 5 wt% solids, so viscosity data must always report spindle, speed, and temperature. For quality control, the 4% solution viscosity under JIS K6726 is the reference value; use of other concentrations is common but not directly comparable between laboratories.

What separates 523S from adjacent viscosity grades in the partially hydrolysed series?

The viscosity ranges demonstrate that 523S sits between 205S and 540S at equivalent hydrolysis. At 4% solids, the difference between 5.0–6.0 mPa·s and 22.5–27.5 mPa·s may appear small, but at 10 wt% solids the practical viscosity difference can exceed an order of magnitude because of the nonlinear concentration dependence. Coating heads that must operate below 4 bar at 50°C generally accept 523S at a lower solids level than 205S. For applications requiring higher film tensile strength without higher solids, 540S is substituted, but its dissolution time is longer and its solution viscosity requires larger heating and pumping capacity.

Compared with fully hydrolysed 103S, the 87–89 mol% hydrolysis of 523S lowers crystallinity and permits clear dissolution at 80–90°C, whereas fully hydrolysed grades above 98 mol% may require 90–95°C and can form gel if not handled properly. The trade-off is moisture sensitivity; plasticised 523S film retains more moisture at 65% RH than fully hydrolysed film and is not specified for water-resistant barriers unless crosslinked or topcoated.

Colloidal stabilisation in vinyl acetate polymerisation imposes a dose window.

Partially hydrolysed PVOH with 87.0–89.0 mol% hydrolysis functions as a protective colloid in vinyl acetate and vinyl acetate-ethylene emulsion polymerisation. In batch reactions, 523S is usually introduced as an aqueous solution at 2–5 wt% on total monomer. Its medium molecular weight provides a thicker hydrodynamic layer around polymer particles than a low-viscosity grade such as 205S, improving shear and storage stability. In a jacketed reactor at 55 wt% solids, latex Brookfield viscosity at 25°C may fall between 3,000 and 8,000 mPa·s, depending on particle size and residual monomer. If the viscosity exceeds the heat transfer capacity of the reactor, dilution or a reduced stabiliser level is required; lowering the 523S dose below 1 wt% can produce coagulum on baffles and temperature probes. Borate salts must not be added before polymerisation because borate ion crosslinks 1,3-diol units and forms intractable gels. Glyoxal-based post-treatments should be evaluated separately for pot life and latex stability.

Scale-up from a 2 L laboratory reactor with a 250 rpm pitched-blade agitator to a 5 m³ production vessel is not linear. Equivalent tip speed does not reproduce the same shear rate distribution or recirculation time. Production batches using 523S require pilot confirmation of the minimum stabiliser dose for a target particle size between 250 nm and 450 nm. Published data for exact dose-response curves across reactor geometries are limited; control charts of coagulum collected on 0.180 mm filter bags provide a more reliable indicator of colloidal stability than bench opacity alone.

Reaction temperature in vinyl acetate polymerisation is typically 65–80°C. The 523S solution must be fully dissolved before addition; undispersed granules can act as seed sites and create clear gel domains in the latex. Post-reaction filtration through a bag with 0.250 mm opening removes skin and incidental coagulum. If residual PVOH content in the latex remains too high, water resistance of the dried film can be compromised, necessitating a post-crosslinker.

When feed moisture is not controlled but pre-drying is not required

Because water is the intended solvent, 523S is not routinely pre-dried before aqueous processing. However, moisture content influences gravimetric and volumetric feeding. The volatile specification of ≤5.0 wt% means that material exposed to humid air may retain surface moisture. In a loss-in-weight feeder with a 500 kg/h capacity, a shift from 3 wt% to 5 wt% moisture changes bulk density sufficiently to disturb volumetric dosing if the feeder is not recalibrated. Gravimetric feed control or a Karl Fischer moisture check on the day of use is recommended for continuous make-down systems. Airborne dust should be controlled below 15 g/m³; dust collectors and conductive earthing should conform to NFPA 654 or a local equivalent.

Viscosity-controlled penetration into paper and textile fibres

Paper surface sizing with 523S is usually run at 3–8 wt% solids in a size press maintained at 40–60°C. The medium viscosity restricts capillary penetration into the sheet, retaining more of the PVOH at the surface for oil hold-out and smoothness. At equal solids, a lower-viscosity grade such as 205S penetrates further into the web and may require a higher dose to achieve equivalent surface film. Size-solution viscosity is often measured at 60°C with a Brookfield LV spindle at 60 rpm; values of 100–400 mPa·s are common depending on co-binder and pigment content. At size press speeds above 1,000 m/min, lower-viscosity grades may be preferred; 523S can still be used if solids are reduced and the press roll gap is optimised. Film split patterns and misting are affected by extensional viscosity, and 523S can produce more visible misting than 205S at equal speed and solids.

Textile warp sizing with 523S is applied at 8–12 wt% solids from a size box held at 70–80°C. The dried film reduces warp break rate during weaving on shuttle and rapier looms. Compared with 540S, 523S gives slightly lower film tensile strength but easier size removal in desizing, which reduces downstream handling cost. The selection between 523S and 540S is therefore a balance between loom efficiency and desizing severity.

Controlling open time in remoistenable adhesives without increasing tack

Remoistenable adhesive films made from 523S at 10–20 wt% solids with glycerol or sorbitol plasticiser are tack-free at 23°C and 50% RH, but rewet rapidly when passed through a water station. Open time is shorter than for 540S because the lower molecular weight permits faster water release at a given coat weight. On roll-to-roll coating lines above 150 m/min, 523S is generally preferred over 540S to avoid ribbing defects from excess viscosity. Drying at air temperatures of 70–90°C is followed by conditioning to ≤50% RH before slitting to prevent blocking. Below 40°C drying is too slow, while above 90°C the surface can form a dried layer that traps water and creates bubbles.

Aqueous barrier coatings for grease-resistant paper can be formulated with 523S at 6–10 wt% solids. Oil hold-out is assessed by TAPPI/ANSI T 454 or ISO 16532-1; the kit value improvement depends on coat weight and substrate porosity and cannot be predicted from PVOH film properties alone. The partially hydrolysed grade has limited water resistance and is not suitable for prolonged water contact unless crosslinked or topcoated. For lower cold-water solubility or higher moisture resistance, fully hydrolysed grades above 98 mol% hydrolysis are substituted, but they require higher dissolution temperatures and may lose surface activity on hydrophobic substrates.

In coextruded water-soluble film structures, 523S can serve as an intermediate-viscosity layer between 205S and 540S. Dissolution at 20°C is slower than 205S and faster than 540S; a 20 mm unstirred film specimen in water may dissolve completely between 15 min and 60 min depending on thickness and plasticiser content. At 60°C, dissolution time typically falls below 10 min. Because these values are configuration-dependent, film converters should verify them for each multilayer structure and plasticiser system.