KURARAY POVAL 28-98

KURARAY POVAL 28-98 is a fully hydrolysed polyvinyl alcohol grade manufactured by Kuraray Co., Ltd. The designation identifies a nominal aqueous solution viscosity of 28 mPa·s at 4% solids and 20°C, and a degree of hydrolysis of 98.0–99.0 mol%. Specification values are determined according to JIS K6726 and ISO 15023-2. The product is supplied as a white granular solid with volatile matter limited to ≤5.0 wt%, ash content to ≤0.5 wt%, and pH of a 4% aqueous solution in the range 5.0–7.0. The viscosity-average degree of polymerization is approximately 1700, corresponding to a molecular weight that supports flexible but mechanically strong films. Under ISO 15023-1, the grade is classified by its viscosity number and hydrolysis level, allowing specification-based substitution against other fully hydrolysed PVOH resins.

The residual acetyl content of 1.0–2.0 mol% is low enough to provide high crystallinity and strong hydrogen bonding after drying, but slightly higher than a ≥99.0 mol% grade, which reduces the dissolution temperature slightly and improves melt processability in plasticized systems. This molecular difference is a primary driver of grade selection when comparing 28-98 with 28-99 or with partially hydrolysed copolymers.

In paper surface sizing, 28-98 is cooked to 5–8 wt% solids and applied on a puddle size press, gate-roll applicator, or film press. Dry pickup is typically held at 0.8–1.5 g/m² per side. Surface strength is assessed by IGT pick resistance under ISO 3783; an increase of 0.2–0.5 m/s in pick velocity is a typical target when replacing oxidized starch with PVOH at equal solids. The same grade is used in textile warp sizing of spun cotton, polyester/cotton blends, and regenerated cellulose. A 9–12 wt% solution is applied at 50–60°C on multi-cylinder sizing machines with drying cylinder surface temperatures of 110–130°C. The film forms a clear, tough coating that reduces warp breakage in weaving and can be desized by hot-water washing without enzymes.

Why Does the 28 mPa·s Viscosity Class Alter Size Press Pickup and Doctor Blade Loading?

A medium-viscosity PVOH grade in the 28–31 mPa·s class creates a thicker hydrodynamic film during roll metering than a lower-viscosity grade at equal solids. Film-splitting thickness in a puddle size press is proportional to the viscous shear force; higher viscosity increases the wet film thickness on the roll before transfer. If the paper machine operates at 800–1200 m/min, a change from a 20 mPa·s grade to 28-98 may require an increase in metering pressure or a reduction in size solids to maintain target dry pickup. The effect is amplified when the solution is cooled near 30°C, where non-Newtonian viscosity rises. On gate-roll applicators with ceramic-coated metering rolls, doctor blade loading must be checked after grade substitution because torque demand on the metering drives increases. Control is normally based on continuous pickup measurement and Brookfield viscosity at 20°C, rather than on solids alone. Published data for this specific configuration is limited.

On production-scale sizing machines, add-on of 28-98 is controlled by squeeze roll pressure and yarn count. For a Ne 30 cotton warp, a size add-on of 10–12% by dry yarn weight is common. The low ash content and low volatiles reduce deposit formation on slasher dryer cans, which is a batch-to-batch variance problem with lower-purity PVOH. The dried size film is soluble in hot water above 90°C; desizing efficiency is tested by starch-iodine or similar mill procedures, not by an international standard. Because 28-98 contains no added surfactant, foam generation is lower than in partially hydrolysed grades, but defoamer may still be required when high-shear mixing is used.

In vinyl acetate emulsion polymerization, 28-98 is used as a protective colloid at 4–8 phr on monomer. The high hydrolysis level lowers the cloud point and increases grafting activity relative to partially hydrolysed grades; therefore, the reactor charge may require a surfactant supplement to maintain colloidal stability during monomer addition at 70–80°C. The resulting emulsion viscosity at 55% solids can reach 2000–4000 mPa·s, depending on particle size distribution. Above 8 phr, high shear viscosity may restrict heat transfer through the reactor jacket.

When 28-98 is used as a binder in aqueous ceramic green tape or electrode coating slurries, the high molecular weight suppresses migration of the binder to the surface during drying. At 0.5–2.0 wt% PVOH on dry powder, surface enrichment is reduced relative to a low-viscosity grade, producing more uniform green density after calendering. The low ash specification is critical in this application because sodium or chloride residues can interfere with ceramic sintering and electrochemical stability.

Solution Viscosity Remains Stable Only Above 30°C

Fully hydrolysed PVOH solutions develop physical gels on cooling. For 28-98, a 10 wt% solution held at 20°C will exhibit progressive viscosity increase and may form a turbid gel within 24–48 h; at 10°C, gel formation is faster. Transfer lines and storage tanks are therefore maintained at 50–60°C. When viscosity is measured on a 4% solution at 20°C, the sample must be filtered through a 100–150 µm screen to remove microgel and then measured with a calibrated Brookfield LV or RV viscometer. Values above 31.0 mPa·s can indicate bacterial contamination or incomplete dissolution; values below 28.0 mPa·s can result from polymer chain scission during prolonged heating at low pH. For storage longer than 24 h, a non-oxidizing biocide is required because aqueous PVOH supports microbial growth.

When 28-98 Replaces Partially Hydrolysed Grades in Barrier Coextrusion

Substitution of a partially hydrolysed grade with 28-98 in barrier film changes both processing and end-use moisture response. Because the degree of hydrolysis is higher, melting point and crystallinity are higher. Plasticized melt extrusion is performed with 15–25 phr glycerol or a sorbitol/glycerol blend; extrusion barrel temperatures above 220°C promote acetic acid cleavage and yellowing, so the melt temperature should be maintained at 190–210°C. Fully hydrolysed film shows lower oxygen permeability at low moisture. ASTM F1927 oxygen transmission testing on 15 µm cast film at 0% relative humidity gives values below 1.0 cm³/(m²·day·atm) for many fully hydrolysed PVOH grades. Above 60% relative humidity, oxygen barrier deteriorates sharply, and the film must be protected by hydrophobic skin layers or a moisture-barrier coating.

Partially hydrolysed grades in the 87–89 mol% range dissolve with less heating and generate lower solution viscosity, making them easier to handle in cold-water systems, but dried films retain higher water sensitivity and higher oxygen transmission under humid conditions. The difference is not linear: oxygen barrier becomes significant only above roughly 97 mol% hydrolysis, so 28-98 sits near the beginning of the high-barrier region while retaining slightly better solvation than ≥99 mol% grades.

Paper-tube and carton-sealing adhesives based on 28-98 are compounded at 20–35 wt% solids. A representative formulation contains glycerol at 5–15 phr, kaolin or calcium carbonate at 10–20 phr, and defoamer at 0.1–0.3 wt%. High hydroxyl density promotes adhesion to cellulose through hydrogen bonding; after compression, initial tack is sufficient for high-speed carton lines. Wet bond strength is assessed by conditioning bonded specimens at 90% relative humidity and 23°C for 24 h before tensile shear testing. When bonded to rigid carton board, tensile shear testing may follow ISO 4587. Fully hydrolysed PVOH of this viscosity class shows less cold-water re-dissolution than partially hydrolysed adhesives, which is an advantage for moisture-resistant packaging but a limitation when repulpability must be maximized.

Borate-Induced Gelation Thresholds and Additive Boundaries

Borate ions crosslink the 1,3-diol units of PVOH and produce high-viscosity gels. In a 10 wt% solution of 28-98, addition of borax at 0.1–0.5 wt% relative to polymer can shift the solution from a pumpable liquid to a gel; the exact threshold depends on pH, solution concentration, and temperature. This reaction is used in some starch-free corrugating adhesives but creates feed blockages if 28-98 is combined with borated preservatives or recycled paper containing borate. Strong oxidizing agents, concentrated mineral acids, and prolonged heating above 200°C degrade the polymer chain and produce discoloration. Insolubilizers such as glyoxal should be added after pH adjustment to 4.0–5.5 and only at the final mixing stage. For pneumatic handling, the granulate should be screened and stored below 60% relative humidity to prevent caking.