KURARAY POVAL 5-98

In surface-sizing operations on fine paper machines running above 1,200 m/min, the selection of KURARAY POVAL 5-98 as a co-binder with cationic starch is determined less by film formation alone than by the need to hold size-press solids at 6–9 wt% without exceeding 150–250 mPa·s at 55 °C in the transfer-roll nip. The grade is fully hydrolysed at 98–99 mol%, and its low viscosity of 5.0–6.0 mPa·s as a 4 wt% aqueous solution at 20 °C permits high-solids size preparation without the shear-transfer instability that occurs when a 20 mPa·s grade is blended with oxidised starch at 55 °C. In a rod-metered film press with roll hardness 0–5 P&J and rod pressure 0.8–1.2 bar, the size formulation typically contains 0.5–1.5 parts of 5-98 per 100 parts cationic starch on dry basis, corresponding to a dry PVA pick-up of 0.2–0.8 g/m² per side. The size solution is prepared by dispersing POVAL 5-98 in cold water with agitation, heating to 88–95 °C for 30 min, and then cooling to 55 °C before starch addition; this sequence prevents residual gel fish-eyes from forming on the rod and reduces blade scratches. On the downstream sheet, the PVA fraction increases surface strength and reduces lint build-up on offset blanket cylinders, and the film-press runnability window narrows if the starch/PVA blend is held for more than 45 min at 55 °C because of retrogradation-related viscosity drift. Direct mixing with borax or boric acid is not practised because fully hydrolysed PVA forms a three-dimensional diol-borate gel above pH 8.0. Compliance is anchored to ISO 535:2023 Cobb60 for water absorptiveness, ISO 8791-4:2021 Parker PrintSurf for surface roughness, FDA 21 CFR §176.170 for paper and paperboard components intended for aqueous and fatty food packaging, and EU Regulation 1935/2004 Articles 3, 15, and 17 for overall migration and good manufacturing practice. Terminal products from this configuration include inkjet coated base paper, lightweight coated offset grades, envelope base paper, and label facestock. Process limitations are linked to drying capacity; the infrared air-flotation dryers after the size press must reach a sheet surface temperature of 85–105 °C within 0.8–1.5 s to prevent blocking when 5-98 is used at the upper addition level.

What Causes the Viscosity Plateau in 5-98 Remoistenable Adhesive Mixes Above 14 wt% Solids?

In remoistenable adhesive coating for high-speed envelope converting, KURARAY POVAL 5-98 is used at 8–12 wt% of solution solids, with dextrin at 20–35 wt%, urea or glycerin plasticizer at 2–4 wt%, and water to balance. The dried adhesive film typically contains 25–35 wt% PVA on dry solids, with the remainder being dextrin and plasticiser. Application is performed by a roller coater or doctor knife at 15–25 g/m² wet film weight on envelope lines running at 80–150 m/min, followed by infrared drying at 120–140 °C and forced air cooling to 25–30 °C before stacking; if stack temperature exceeds 35 °C, blocking occurs because the fully hydrolysed film remains thermoplastic above its glass transition temperature of 75–80 °C. The remoistening unit then re-wets the adhesive with 5–10 g/m² water, and initial tack develops within 3–8 s. Viscosity control at 25 °C using a Brookfield RVT #4 spindle at 20 rpm should remain at 1,500–3,500 mPa·s; above 14 wt% total solids the mix can enter a hydrogen-bond-driven viscosity plateau that complicates levelling and increases strikethrough on low-grammage papers, a condition observed on batch mixers where the circulation pump is undersized for the final equivalent diameter of hydrated dextrin-PVA clusters. Formulation compatibility is bounded by borate-containing tackifiers; the fully hydrolysed 5-98 crosslinks rapidly at pH > 8.0, producing a stiff gel that cannot be applied by roller coaters. Compliance for the adhesive route is based on FDA 21 CFR §175.105 for adhesives used in food packaging laminates and EU REACH Annex XVII for restricted substances in consumer articles. Terminal products are remoistenable envelope flaps, stamp gum, paper labels, and wallpaper border adhesive.

Before a polyester/cotton warp enters the weaving shed at speeds of 160–240 m/min, the size formulation applied in the slashing room must produce a continuous film on the yarn surface with sufficient abrasion resistance to survive reed impact and harness friction. KURARAY POVAL 5-98 is metered into the size mix at 30–50 wt% of the total film-former, with acid-thinned starch or oxidised starch comprising 50–70 wt%, wax at 0.2–0.5 wt%, and pH held between 6.5 and 7.5; the size box solids are typically 8–12 wt% depending on yarn count and loom speed. The size kitchen preparation starts with dispersion of POVAL 5-98 in cold water, heating to 88–95 °C in a jet cooker with a 20–30 min hold to eliminate undissolved gel particles, then cooling to 75–85 °C before mixing with starch. On the slasher, the warp passes through a size box with squeeze roll load of 16–22 N/mm², and the wet pick-up is controlled to 80–120% on yarn mass; drying cylinder profiles are set to 105–130 °C to bring residual moisture to 3–5% without overheating the PVA film. Weaving performance depends on the ability to remove the size after greige fabric production; the starch portion is hydrolysed with α-amylase at 60–70 °C, while the PVA fraction requires oxidative desizing with hydrogen peroxide or sodium persulfate at 80–95 °C, and incomplete removal leads to redeposition on reeds and higher warp breaks during subsequent wet processing. Regulatory alignment for the size formulation is based on Oeko-Tex Standard 100 Annex 4 Class I for textile finishes, ZDHC MRSL Version 3.1 for wastewater-relevant sizing agents, and EU REACH Annex XVII entry 72 for restricted substances. Terminal woven products served by this route include cotton/polyester shirting, denim, twill workwear, and filament blend wovens. The operational boundary is sharp: at squeeze roll pressure below 12 N/mm², the size pick-up becomes too high, drying is incomplete, and the warp becomes tacky on the headstock beams, whereas above 22 N/mm² the low-viscosity 5-98 film is over-squeezed and abrasion protection drops measurably on high-speed air-jet looms.

When Doctor-Blade Gap Drifts Beyond 5 µm in Aqueous Alumina Tape Casting

KURARAY POVAL 5-98 enters ceramic tape casting as a green-body binder for aqueous alumina and zirconia slips, where its low solution viscosity of 5.0–6.0 mPa·s at 4 wt% allows binder solutions of 4–6 wt% to be added without pushing slurry viscosity above 2,000–4,000 mPa·s at 5 s⁻¹. The binder is charged at 1.0–2.5 wt% of dry ceramic powder mass, with a plasticiser such as PEG 400 at 20–30 wt% of the PVA dry weight, defoamer at 0.1–0.3 wt%, and dispersant such as Darvan C-N at 0.5–1.0 wt%; the ceramic solids loading is held at 50–55 vol% for aqueous tape formulations. Slurry preparation is carried out in a ball mill with alumina or zirconia media for 16–24 h, followed by deairing under vacuum at 50–100 mbar to prevent pinholes. The tape is cast with a doctor blade gap of 100–500 µm and a carrier speed of 0.5–2.0 m/min; drying at 50–80 °C in a three-zone air flotation oven removes water without forming a surface skin, and the PVA binder bonds the ceramic particles until burnout. Thermal removal of 5-98 is performed in air or nitrogen with a hold at 450–500 °C for 2–4 h, and residual ash must be controlled because the sodium content in some PVA batches can shift sintered dielectric properties. Compliance for finished ceramics is assessed through ASTM C373-18 for water absorption, ISO 14604:2012 for advanced ceramic coatings, and EU REACH for organic binder constituents. Terminal products manufactured from this route include alumina substrates, LTCC multilayer low-temperature co-fired ceramic tapes, zirconia oxygen sensor elements, and porous ceramic membranes. The process window is particularly sensitive to blade gap drift; a movement of more than 5 µm in the doctor blade gap changes wet tape thickness by more than the allowed 10% tolerance and alters green density, which in turn shifts sintered shrinkage and camber on cutting lines.

Gypsum Dry-Mix Open Time and Crusting Behaviour with 5-98 Powder Addition

At 0.3–0.8 wt% of dry mix, KURARAY POVAL 5-98 functions in gypsum-based joint compounds and skim coats as a water-soluble polymer that retards moisture loss at the trowelling face and reduces crusting under hot, low-humidity site conditions. The powder is blended in a forced-action mixer with hydrated calcium sulfate, limestone filler, and cellulose ether; water demand is adjusted to maintain a water-to-dry-mix ratio of 0.45–0.60 depending on spread rate. Application is by trowel over gypsum board joints, with initial setting controlled by the gypsum system rather than the PVA, and the PVA residue at 0.3–0.8 wt% does not form a coalesced latex film because it is water-soluble and is distributed in the hydrated matrix. Compliance is based on ASTM C474-15 for joint treatment materials, EN 13963:2014 for gypsum plaster products, and EU REACH for the polymer constituent. Terminal products are drywall joint compounds, gypsum skim coats, and patching plasters. The boundary condition is that increasing 5-98 above 1.0 wt% can increase open time but may soften the cured surface under humid conditions because the fully hydrolysed film remains hygroscopic.

If Silicone Holdout Falls Below 85% on Clay-Coated Release Base Stock

Release liner converting uses KURARAY POVAL 5-98 as an aqueous pre-coat at 4–7 wt% solids on clay-coated or supercalendered kraft base stock before the thermally cured silicone top coat. The dry pre-coat weight is 0.3–0.8 g/m², and the coating is run on an air-knife or rod coater at 150–300 m/min with drying at 100–120 °C; the PVA film blocks silicone migration into the base paper and provides a uniform surface for the subsequent solventless silicone layer. The downstream process uses a five-roll solventless silicone coater with addition of 0.8–1.5 g/m² thermal silicone, and the cure is run at 120–150 °C for 5–10 s. Release performance is checked by peel force at 180° and 30 cm/min, with typical target release of 5–15 cN/25 mm for label applications, and by indicator stain tests for holdout. If the PVA pre-coat holdout falls below 85%, silicone strike-in causes release force drift and adhesive deadening on pressure-sensitive labels. Compliance includes FDA 21 CFR §176.170 for indirect food contact, EU 1935/2004, and ISO 8791-4 for surface roughness. Terminal products include silicone release liners for pressure-sensitive labels, tape backings, medical plasters, and industrial laminates. The main incompatibility is with cationic water chemistry at pH < 4.5, which can precipitate the PVA and reduce holdout.