| HS Code | 232848 |
| Product Name | KURARAY POVAL 28-98 |
| Chemical Name | Poly(vinyl alcohol) |
| Cas Number | 9002-89-5 |
| Physical Form | Granular powder |
| Appearance | White to slightly yellowish granules |
| Viscosity 4 Percent Solution 20c | 28 mPa·s (nominal) |
| Degree Of Hydrolysis | 98.0 - 99.0 mol% |
| Ph 4 Percent Solution | 5.0 - 7.0 |
| Ash Content | ≤ 0.7% |
| Volatile Content | ≤ 5.0% |
| Average Degree Of Polymerization | 1700 |
| Density | 1.25 g/cm³ |
| Solubility | Soluble in hot water; requires heating for complete dissolution |
| Melting Point | Approx. 200°C |
| Refractive Index | 1.49 - 1.53 |
As an accredited KURARAY POVAL 28-98 factory, we enforce strict quality protocols—every batch undergoes rigorous testing to ensure consistent efficacy and safety standards.
| Packing | KURARAY POVAL 28-98 is supplied in 25 kg multi-wall paper bags with polyethylene liner, ensuring safe handling and moisture protection. |
| Container Loading (20′ FCL) | Container Loading (20′ FCL): KURARAY POVAL 28-98 shipped as full container load, securely packed in a 20-foot container. |
| Shipping | KURARAY POVAL 28-98 ships as a fine, free-flowing polyvinyl alcohol powder in sealed multi-layer bags or drums. It is non-hazardous for transport under normal conditions, but should be kept dry and protected from moisture. Avoid dust generation, use grounded equipment, and store in a cool, ventilated area. |
| Storage | Store KURARAY POVAL 28-98 in a cool, dry, well-ventilated area away from direct sunlight, heat, and open flames. Keep containers tightly sealed to prevent moisture absorption and contamination. Avoid dust accumulation; use appropriate bonding/grounding for transfers. Maintain temperature below 40°C and separate from oxidizing agents. |
| Shelf Life | Store in a cool, dry place. Shelf life is typically two years from the date of manufacture when kept sealed. |
| Application zone | Standard or regulation | Clause or test method designation |
|---|---|---|
| Paper and board for food contact | FDA 21 CFR 176.170 | Components of paper and paperboard in contact with aqueous and fatty foods |
| Adhesives for indirect food contact | FDA 21 CFR 175.105 | Adhesives and components of coatings |
| Oxygen barrier packaging film | ASTM D3985-17 | Oxygen gas transmission rate through dry film |
| Paper surface strength | ISO 3783:2006 | IGT pick velocity |
| Size press absorption | ISO 535 | Cobb60 water absorptiveness |
Inside a 12,000 L enameled reactor fitted with a Pfaudler retreat-curve impeller, vinyl acetate is fed at a constant rate over 240 min with potassium persulfate at 0.10 parts per 100 parts monomer and sodium bicarbonate at 0.05 parts per 100 parts monomer. A 10.0% aqueous solution of 28-98, prepared at 90°C and filtered through a 200 µm bag, is charged at 5.0 kg dry PVOH per 100 kg vinyl acetate. The 98.0–99.0 mol% hydrolysis level suppresses interfacial migration of the colloid compared with an 87–89 mol% grade, so oligomer-particle grafting occurs later in the droplet stabilisation sequence and the average particle size shifts upward when agitation speed is held at 120 rpm. Laser diffraction under ISO 13320 typically records a D50 between 1.2 µm and 2.4 µm for a 55% solids homopolymer prepared with this high-DP grade, whereas a 4.0 mPa·s partially hydrolysed grade at the same 5.0 phm charge yields a D50 below 1.0 µm. The practical ceiling is not monomer conversion but emulsion viscosity at final solids. Brookfield RVT readings at 20 rpm and 25°C commonly exceed 8,000 mPa·s when solids pass 56%; the retreat-curve impeller becomes starved at the shaft centre, and the heat transfer coefficient at the jacketed wall falls because the product at the probe reaches gel strength. A production-scale remedy is to split the 28-98 charge into 70% initial and 30% delayed fractions, or to substitute 0.5–1.0 phm with a 4.0 mPa·s grade, but either step lowers wet film tensile strength. Conducting polymerisation at 72°C rather than 68°C reduces final Brookfield viscosity by approximately 12–18% because the grafted colloid layer develops a more compact hydrodynamic radius, though published data for this specific configuration is limited. The resulting homopolymer is designed for wet-lamination adhesives where high shear melt viscosity and clean machining on roller coaters dominate; it is not optimised for low-viscosity, high-solids architectural coatings.
In remoistenable tape coating on 40 g/m² machine-glazed paper, the adhesive is prepared at 15.0% solids by dissolving 28-98 at 92°C for 30 min, then cooling to 45°C before adding 5.0 parts glycerol and 0.2 parts methylparaben per 100 parts dry PVOH. Because the fully hydrolysed backbone crystallises on drying, the coated tape remains block-free in a 40°C, 60% RH storage trial, whereas a partially hydrolysed grade at the same plasticiser level may transfer adhesive to the backing under the same conditions. The upper viscosity limitation appears at the coating station: a Meyer rod coater running at 120 m/min lays down 18 g/m² wet film without foaming when the solution Brookfield RVT reading at 50°C is below 1,500 mPa·s; above that value, air entrainment at the rod edge creates continuous crater defects. The high-DP grade resists mechanical degradation during 8 h of recirculation through a diaphragm pump, as shown by stable viscosity at 60°C, but the retained molecular weight increases drying demand because the cast film requires a three-zone dryer set to 75°C, 90°C, and 110°C to reach residual moisture below 2.0%. Rewetting is measured by applying a standard 10 µL water droplet to the coated surface and recording time to visible gloss loss; 28-98 films take 5–8 s longer than a 4.0 mPa·s partially hydrolysed control due to their higher crystalline fraction. Adhesive bond strength to envelope paper, tested in a 180° T-peel after remoistening, is 30–40% higher than the control when conditioned at 23°C and 50% RH for 24 h. The formulation falls under FDA 21 CFR 175.105 for indirect food-contact adhesives, provided the final adhesive is separated from food by a functional barrier; European converters should also verify that total migration into simulant D1 does not exceed 10 mg/dm² under EN 1186-1.
Because 28-98 is fully hydrolysed and high in DP, warp sizing on 40 Ne combed cotton run at an air-jet loom speed of 1,100 picks/min requires steam injection at the size box sump rather than direct steam into the immersion bath to prevent local overshearing. A production formulation on a seven-box slasher uses 7.0% 28-98, 7.0% oxidized maize starch, 0.3% polyester wax, and 0.05% non-silicone defoamer on dry solids at an add-on of 12.0% by weight on yarn. The resulting size film, cast and tested by ASTM D638-14 Type IV at 23°C and 50% RH, shows tensile strength in the range 58–65 MPa and elongation at break 120–160%; these values exceed those of a low-DP, 88 mol% hydrolysed grade and reduce loom stops at the centre shed by increasing inter-fibre cohesion. The high molecular weight creates a risk of “hard size” if the slasher can temperature drops below 80°C mid-run, because the size liquor begins to form a surface skin that transfers to the split rods and accumulates on drying cans. Desizing of 28-98-based film from fabric woven at warp insertion above 1,000 m/min is achieved by a three-stage wash at 85°C, 90°C, and 95°C with 0.5 g/L non-ionic wetting agent; residual PVOH is checked by iodine-polyvinyl alcohol complex colour, and hot-water washing efficiency is more dependent on bath turbulence than on residence time. Effluent from the desizing range carries a COD load governed by final size add-on and desize liquor transfer rate; production sites using biological treatment must verify that mixed-liquor acclimation for PVOH-degrading bacteria is not shorter than 7–14 days under EU BAT reference conditions, since the fully hydrolysed grade mineralises more slowly than starch in an activated sludge system at 30°C.In a Stein Hall system mixing 18% starch carrier with 82% starch raw slurry, addition of 0.8% 28-98 on total dry starch shifts the gel point from 62°C to 67°C at 16% total solids, measured by Brabender Amylograph under 75 rpm shear. This change extends the open time between glue roll application and the pressure roll nip on a corrugator running 250 m/min. The 28-98 network increases green bond and reduces washboarding on 150 g/m² clay-coated white-top liner, but the adhesive must be pumped through 50 mm pipes; viscosity at 35°C rises from 22 s to 34 s in a Stein Hall cup, and the storage tank requires a slow sweep agitator rather than a high-shear mixer to avoid mechanically degrading the high-DP PVOH. The high hydrolysis level renders the PVOH film insoluble in the alkaline starch medium at room temperature, so bonding to the calcium carbonate coating is maintained after the board exits the hot section and cools to 30°C.
Tape casting of 200 µm alumina green sheets at 0.8 m/min with an aqueous slurry at 55 vol% solids uses 28-98 as the sole binder at 4.0 wt% of dry ceramic powder. The PVOH is added as a 10% solution after dispersing the alumina with an ammonium polyacrylate dispersant at 0.6 wt%; the slurry is milled at 25 rpm for 18 h in a 20 L polyethylene jar with 10 mm zirconia media. The high-DP binder increases green sheet tensile strength after drying at 60°C to 4.8 MPa, measured by a three-point bend tester on strips cut to 25 mm × 5 mm × 0.2 mm, but the same macromolecular chain length raises slurry viscosity to 3,500 mPa·s at 10 s⁻¹ and produces edge streaks when the doctor blade gap is below 1.0 mm. A plasticised variant with 1.5 wt% PEG 400 on dry PVOH reduces viscosity to 2,100 mPa·s at the same shear rate and lowers the minimum defect-free blade gap to 0.6 mm. Binder burnout is conducted in a tunnel kiln at 450°C for 2 h in air flow; thermogravimetric analysis at 10 K/min under air shows oxidation begins at 240°C and reaches 95% mass loss by 470°C, so the heating ramp between 200°C and 450°C must not exceed 1 K/min to avoid carbon black residue in the sintered ceramic. The use of 28-98 in this application is not covered by a specific EU ceramics directive, but workplace dust exposure limits for polyvinyl alcohol dust under national occupational exposure legislation should be checked for powder handling.Unmodified KURARAY POVAL 28-98 coatings on corona-treated polypropylene film exhibit a step change in oxygen permeability when conditioning humidity crosses 60% RH. A 4.5 µm dry film applied from an 8.0% aqueous solution by reverse gravure and dried to residual moisture of 0.5% records OTR below 0.5 cm³/(m²·day·bar) at 23°C and 0% RH under ASTM D3985-17; at 70% RH and the same temperature, the oxygen transmission rate increases by an order of magnitude because water acts as a plasticiser within the amorphous domains of PVOH. The fully hydrolysed grade has a higher crystalline fraction than 88 mol% grades before storage, but barrier loss is governed by the plasticised amorphous phase, not by crystallinity. Addition of 5–10 wt% of a high-aspect-ratio sodium montmorillonite or a dialdehyde crosslinker can shift the RH resistance upward, but both modifications increase coating viscosity and reduce maximum coater line speed. In practice the coating weight must be raised to 8–10 µm or laminated against a moisture barrier to maintain OTR below 1.0 cm³/(m²·day·bar) at 80% RH. Substrate wetting on polypropylene is insufficient without corona treatment at 38–44 mN/m; below this surface tension the wet coating reticulates in the gravure cells and dry film continuity is lost.
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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.
| Property | Specification range | Test standard |
|---|---|---|
| Viscosity, 4% aqueous solution, 20°C | 28.0–31.0 mPa·s | JIS K6726 / ISO 15023-2 |
| Degree of hydrolysis | 98.0–99.0 mol% | JIS K6726 / ISO 15023-2 |
| Volatile matter | ≤5.0 wt% | JIS K6726 |
| Ash content | ≤0.5 wt% | JIS K6726 |
| pH, 4% solution | 5.0–7.0 | JIS K6726 |
| Viscosity-average degree of polymerization | approx 1700 | Calculated from viscosity |
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.
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.
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.
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.
| Grade class | Hydrolysis | Aqueous viscosity | Primary processing implication |
|---|---|---|---|
| KURARAY POVAL 28-98 | 98.0–99.0 mol% | 28–31 mPa·s | Hot dissolution above 90°C; high water resistance after drying |
| Fully hydrolysed 28-99 class | ≥99.0 mol% | 28–32 mPa·s | Slightly higher dissolution temperature and crystalline order |
| Partially hydrolysed 20–30 mPa·s class | 87.0–89.0 mol% | 20–30 mPa·s | Cold-water solubility; lower moisture resistance and higher oxygen permeability |
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 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.
