KURARAY POVAL 28-98 S2

KURARAY POVAL 28-98 S2 is defined by a nominal degree of hydrolysis of 98.0–99.0 mol% and a 4% aqueous solution viscosity of 62–72 mPa·s at 20 °C under JIS K6726. The corresponding nominal degree of polymerisation is near 2800. Dissolution requires mechanical agitation at or above 85 °C; below that temperature the polymer swells but does not form a particle-free solution within conventional cook times. Because the residual acetyl content is below 2 mol%, film crystallinity develops rapidly on cooling and drying, which lowers equilibrium moisture uptake at 50% RH and raises the film's resistance to redissolution at 25 °C. These properties assign the grade to applications where hot-water resistance, high green strength, or high solution viscosity are functional, and exclude it from cold-water-soluble packaging or low-viscosity high-solids coating. The S2 suffix denotes the controlled particle-size and dust-reduced form intended for bulk handling and improved wet-out in high-shear mixing systems; the solution behaviour remains that of the 28-98 base polymer.

What limits the substitution level of 28-98 S2 in oxidised starch surface sizing on high-speed metering size presses?

On a Valmet or Voith metering size press operating above 1200 m/min, substitution of oxidised starch with 28-98 S2 is constrained by the viscosity of the mixed size rather than by film strength. A 10% total solids size made from a low-viscosity oxidised starch and 10 wt% PVOH on starch solids typically holds at 80–120 mPa·s at 60 °C; raising PVOH to 20 wt% shifts the same bath to 180–260 mPa·s, at which point metering blade oscillation and skip-coating defects appear on lightweight coated paper. The fully hydrolysed grade contributes high surface strength: IGT pick values on woodfree base paper increase by 0.4–1.0 m/s at a given Cobb value when PVOH is present, but the gain flattens above 15 wt% of total solids because the size film becomes less uniform. This is the practical substitution ceiling unless the mill increases size-bath temperature above 70 °C or installs larger-diameter metering rods.

The S2 particle form reduces lumping when the PVOH is dry-blended with starch before the jet cooker, but it does not eliminate the need for a separate PVOH cook tank when addition exceeds 5 wt%. Fully hydrolysed PVOH undergoes retrograde association with starch upon cooling; size solutions held below 55 °C for more than 4 h can form weak gels that pass through pressure screens but destabilise at the blade. Mills using calcium chloride or ammonium zirconium carbonate as insolubiliser must keep pH below 8.0 because alkaline hydrolysis of starch and PVOH accelerates in the hot hold tank. Testing under TAPPI T 448 and ISO 535 shows that the fully hydrolysed grade improves dry pick and reduces water absorbency only when the size film is continuous; discontinuous films caused by high viscosity negate the barrier contribution. Food-contact verification for sized board requires the supplier documentation under FDA 21 CFR 176.170 and FDA 21 CFR 176.180 to be reviewed before the size formulation is approved.

Alumina and barium titanate tape casting positions 28-98 S2 as a temporary organic binder that must survive drying and handling but leave an ash residue below 0.5 wt% after burnout at 600 °C. The high degree of hydrolysis is less relevant to green strength than the high molecular weight, which raises the binder film's tensile strength after the solvent phase, often a water/ethanol azeotrope, has evaporated. A ceramic slip with 3 wt% PVOH on dry ceramic powder can produce green tape tensile strengths of 2–4 MPa when measured by ASTM D638-14 at a strain rate of 10 mm/min, but the same slip shows a shear viscosity near 1500–2500 mPa·s at 100 s⁻¹, which can exceed the deairing capacity of a bench-scale tape caster. The high-DP grade is therefore used at the lower end of the binder addition range when high solids loading is required for densification.

Thermal removal is not a single event. Differential scanning calorimetry under nitrogen shows an endothermic side-group elimination beginning near 230 °C, followed by main-chain decomposition between 350 °C and 500 °C. If the first ramp to 400 °C exceeds 2 °C/min, the evolution of volatiles can blister multilayer ceramic capacitors and thick-film substrates. The low-ash grade matters for dielectric applications because sodium and calcium residues above 100 ppm on ceramic solids reduce the breakdown strength of sintered alumina; therefore, the binder solution is prepared with demineralised water rather than plant water. In an industrial spray dryer, the S2 fine-particle form disperses more rapidly into the ceramic slurry, but it also increases dusting during manual addition unless an enclosed induction hopper is used. Operational boundaries include avoiding premix with borate-containing additives, which crosslink the fully hydrolysed grade and create insoluble gel domains that survive the burnout ramp and appear as pinholes after sintering.

Emulsion polymerisation with 28-98 S2 as the primary protective colloid under ethylene pressures above 40 bar

The protective colloid requirement in a 20 m³ stirred-tank reactor producing vinyl acetate-ethylene (VAE) emulsion is twofold: maintain latex particle size during nucleation and supply grafting sites for vinyl acetate radicals without generating excessive water sensitivity in the dry film. Fully hydrolysed PVOH has fewer hydrophobic acetyl sequences than partially hydrolysed grades, so its initial interfacial activity is lower. At 2 wt% colloid on monomer, 28-98 S2 tends to produce a final particle size in the range of 1200–1800 nm, larger than the 400–700 nm typical of an 88 mol% hydrolysed grade under the same agitation and ethylene pressure. This coarse distribution lowers latex viscosity and raises shear stability of the finished adhesive when measured by ASTM D2196.

Process constraints dominate at the feed stage. A 15% PVOH solution is near the upper viscosity limit for continuous dosing through a positive-displacement pump at 40–50 °C; at 20 wt% solids, the solution viscosity exceeds 5000 mPa·s and can cavitate the pump head or starve the monomer feed mixing zone. The polymer is usually pre-dissolved in a separate cook vessel at 90 °C, then cooled to 50 °C before addition. Residual monomer stripping at 60–70 °C under vacuum is not impaired by the high molecular weight, but foam formation is more persistent with fully hydrolysed grades because the colloid migrates to the air-water interface as the latex cools. Defoamer addition must be kept below 0.1 wt% to avoid coagulum formation, which is observable as screen residue on a 250 μm mesh at the discharge line.

A cotton/polyester warp sizing process running at 80–100 m/min should be treated as a concentrated-solution rheology problem rather than a simple film-strength problem

The size-box solids for a cotton/polyester warp are typically 8–12%, and 28-98 S2 may replace 30–50 wt% of the starch component to reduce shed lint and improve weaving efficiency. The Brookfield viscosity of an 8% PVOH-only solution at 90 °C can reach 800–1200 mPa·s, which is within the operating range of a two-roll size box but above the limit for a spray-jetted size application. After the drying cans, the PVOH film is highly crystalline and may skin-flake at the reed if the add-on exceeds 12 wt% of yarn mass without a post-humidification chamber. The desizing operation requires hot water above 60 °C and, for tight-weave constructions, an oxidative desizing agent such as sodium persulphate at 2–5 g/L. Weaving abrasion resistance is commonly assessed by the number of cycles to yarn break under constant tension rather than by tensile strength alone, and the fully hydrolysed high-DP grade provides a measurable reduction in shed lint relative to low-viscosity grades at equal add-on.

Remoistenable adhesive formulations based on 28-98 S2 require a deliberate wetting-time window because the fully hydrolysed grade develops a more thermally stable film than partially hydrolysed counterparts. At 20–25% solids and 25 °C, the solution viscosity is typically above 20 000 mPa·s, restricting application to roller or slot coating rather than high-speed gravure. The dried film re-wets in 3–8 s depending on coat weight and relative humidity; above 60% RH, tack development becomes unpredictable because the film absorbs moisture and begins to block before the converting line reaches the rewinder. This limitation is why such grades are selected for paper tube lamination and envelope window adhesives where dry tack is not the primary requirement, and not for high-speed label remoistening.

When 28-98 S2 is cast into water-soluble film, the drying temperature profile dictates whether the final film dissolves below 30 °C or above 90 °C

Fully hydrolysed PVOH films are not inherently cold-water soluble. A solution-cast film dried below 80 °C retains enough amorphous phase to disintegrate in water at 25 °C within 60 s if the thickness is below 40 μm; however, drying or annealing above 120 °C increases crystallite size and shifts complete dissolution to above 90 °C. The high molecular weight of 28-98 S2 raises the film's tensile strength but also raises the viscosity of the casting solution. At 8% solids and 70 °C, the solution viscosity may exceed 3000 mPa·s, which makes precision metering through slot dies difficult without bubble entrainment. For agricultural water-soluble packaging, the film must meet dissolution and mechanical criteria under ASTM D882 and ISO 527-3; published data for this specific S2 film configuration is limited, so pilot trials are required to establish the drying window for a given cast film line.