KURARAY POVAL 18-88 LA

KURARAY POVAL 18-88 LA, a partially hydrolysed polyvinyl alcohol with nominal 4% aqueous solution viscosity of 18 mPa·s at 20°C and hydrolysis degree of 87–89 mol%, functions in aqueous vinyl acetate emulsion polymerization as a steric stabiliser and grafting substrate. The protective colloid mechanism depends on partial grafting of vinyl acetate radicals onto the PVOH backbone during polymerisation, producing a chemically bound stabiliser shell around the latex particle; the residual acetate groups at 88 mol% hydrolysis reduce the stabiliser shell hydrophilicity enough to control particle size without causing excessive viscosity. In a 10,000-litre glass-lined reactor equipped with an anchor impeller running at 120–180 rpm and jacket temperature controlled to 72–78°C, the aqueous phase is charged with a 10% PVOH solution pre-dissolved at 85°C and cooled to 35°C before initiation. The PVOH dosage is maintained at 3–6 parts per 100 parts vinyl acetate monomer; a seed charge of 5–10% of total monomer is polymerised before the remaining monomer and redox initiator, typically hydrogen peroxide and sodium erythorbate, are delayed over 3.5–4.5 h. The pH is held between 4.0 and 5.0 with sodium bicarbonate because decomposition products of hydrogen peroxide progressively acidify the dispersion and can destabilise the PVOH protective layer. Final dispersion viscosity is measured at 25°C on a Brookfield RVT viscometer according to ISO 2555; acceptable polyvinyl acetate homopolymer and VA/ethylene batches typically fall between 5,000 and 12,000 mPa·s depending on PVOH dosage, total solids, and ethylene content. Coagulum retained on a 100-mesh screen after wet filtration is the most sensitive manufacturing control: below 2.5 parts PVOH per 100 parts monomer, coagulum rises above 0.5% of wet dispersion, while above 6.5 parts PVOH the final dispersion viscosity exceeds 15,000 mPa·s and causes cavitation in the letdown pump. The LA-grade reduced ash specification, controlled below 0.2% by the manufacturer, limits ionic interference with emulsion stability and reduces residue in dried films. Polyvalent metal ions, especially Fe³⁺ and Al³⁺, must be excluded from the aqueous phase to prevent premature crosslinking of PVOH and stringy coagulum on reactor internals. Storage of the pre-dissolved 10% solution beyond 48 h at 30–35°C can result in viscosity drift of ±10% because of trace aldehyde formation; the solution is therefore prepared in demineralised water and kept under nitrogen blanketing.

Vinyl acetate-ethylene systems operating under ethylene overpressure are more sensitive to PVOH dosage because ethylene reduces particle polarity and lowers graft density at the same stabiliser level. In those formulations, the PVOH charge is commonly biased toward the upper end of the 4–6 part range while the initiator feed is reduced to avoid low-graft sol fractions that thicken during storage. The final VAE dispersion used in wood adhesives is typically adjusted to 55% solids and 8,000–10,000 mPa·s Brookfield RVT viscosity at 25°C, with coagulum below 0.1% of wet dispersion. The LA-grade ash ceiling below 0.2% is particularly relevant for pressure-sensitive adhesive and clear film applications where extractable ions contribute to polymer specking and reduced optical clarity. Equipment experience indicates that batch-to-batch viscosity deviation in the finished dispersion is most commonly caused by drift in the PVOH solution feed viscosity; therefore the solution is metered through mass flow rather than volumetric control. High-shear dispersion of the finished latex through a 40-mesh filter before drumming removes residual oversize polymer that would otherwise clog transfer lines. The use of partially hydrolysed PVOH in this application is bounded by high-shear instability at addition above 6.5% on monomer and by low-molecular-weight stabiliser limitations at reactor temperatures above 80°C.

Why Does 88 mol% Hydrolysis Control Both Laundry Pack Integrity and Cold-Water Dissolution?

Film formed from KURARAY POVAL 18-88 LA with 88 mol% hydrolysis dissolves through progressive swelling and disintegration rather than instantaneous solubilisation; this behaviour is necessary for unit-dose laundry packets because the film must remain mechanically intact during storage, detergent filling, and heat sealing, but release the liquid detergent within the wash cycle. The cast film formulation is prepared as a 12–16% aqueous solution at 85–90°C with plasticiser addition in the range 10–20 parts per 100 parts PVOH. Glycerol gives lower glass transition and faster 10°C dissolution; sorbitol gives higher rigidity and lower blocking under tropical humidity. Casting on a chrome-plated drum at 75–85°C and 15–25% relative humidity yields film with equilibrium moisture 8–12%. Tensile properties are tested at 23°C and 50% RH using ISO 527-3 with type 5 specimens; transverse-direction tensile strength typically falls in the 25–40 MPa range for plasticised film, with elongation at break between 150% and 250%. Heat-seal strength on a vertical form-fill-seal machine with sealing jaw temperature 150–170°C is measured by peel testing under ASTM F88/F88M; seal strengths below 8 N/25 mm are considered inadequate for detergent pack integrity. Dissolution rate is determined in a 10°C water bath with a 1 L/min flow through a 35 mm circular film clamped in a dissolution cell; 50% disintegration for 76 µm film commonly falls between 40 and 90 s depending on plasticiser content. High humidity above 60% leads to surface tack and blocking, so the film must be conditioned and stored in sealed packaging. The LA-grade low ash content is relevant because residual ash contributes to haze and can nucleate pinholes during casting. Formulators should not exceed 25 parts glycerol per 100 parts PVOH because the film blocking force increases rapidly and the machine-direction tensile strength drops below the threshold needed for high-speed drum sealing.

Rod-metered size presses running oxidised starch at 5–10% solids accept partially hydrolysed PVOH as a co-binder to improve surface strength and reduce dusting; the 88 mol% grade is cold-water-soluble but must be pre-dissolved separately at 10–15% solids before blending into the starch cook to avoid gel lumps. A typical size press formulation contains 85–95 parts oxidised starch and 5–15 parts PVOH on dry solids, with total solids 4–8% and application temperature 55–65°C. Size pick-up on fine paper in a film press at 1,000 m/min is controlled at 1.0–2.0 g/m² per side; PVOH increases the IGT dry pick resistance measured by ISO 3783 from the starch-only baseline by 10–20% depending on base sheet. Cobb water absorption under ISO 535 is often maintained in the 25–40 g/m² range because PVOH retains water sensitivity; where lower Cobb values are required, an internal sizing agent such as alkyl ketene dimer is combined at 0.05–0.15% on dry fiber. The LA-grade ash ceiling prevents deposit accumulation on metering rolls, which is the dominant cause of bar marks in long campaigns. Film-split tack and rheology of the size press solution are measured with a Brookfield viscometer at 60°C; values above 200 mPa·s lead to misting at high machine speeds, so the PVOH fraction is reduced or the starch/PVOH solids shifted. Offset printing papers and inkjet base papers produced with the formulation are typically tested for IGT pick resistance and for lint accumulation on offset blankets by TAPPI T 476; the PVOH fraction shifts the failure mode from fibre pulling to coating cracking only at addition above 15 parts per 100 parts starch. Size press solution must be filtered through 60-mesh screens to remove gel particles, and LA-grade ash content below 0.2% reduces the formation of doctor blade deposits on long runs. Where the mill operates closed-loop broke recovery, PVOH-containing broke can be returned at up to 10% of the furnish without significant pitch deposition because the polymer remains water-soluble.

When 18-88 LA Replaces Partially Hydrolysed Starch in Air-Jet Weaving

Warp yarn sizing for 65/35 polyester/cotton blends requires a film-forming binder that remains flexible at high weaving speeds and can be removed in hot water without enzymatic desizing. The sizing liquor is prepared at 8–12% solids with PVOH as the primary binder, 0.3–0.8% of a fatty acid wax dispersion, and 0.1–0.4% of an acrylic co-binder to improve abrasion resistance. The solution is applied on a multi-cylinder sizing machine at 80–85°C, squeeze pressure 20–30 kN/m, and sizing speed 300–500 m/min. Add-on control is measured as size pick-up of 6–10% on yarn dry weight. Sized yarn strength gain over unsized yarn is tested according to ISO 2062; for a 20 tex 65/35 yarn the improvement in breaking force is typically 8–15%, while abrasion cycles on a Reutlingen web tester improve by 30–50%. In air-jet weaving at 600–800 rpm, the main failure mode is size shedding on reed dents and heald eyes; PVOH film toughness reduces warp stops per 100,000 picks compared with starch-only size. Desizing is operated in a continuous wash bath at 80–90°C for 30–60 s; residual PVOH is checked by iodine staining. The low ash content of the LA grade reduces deposits on drying cylinders and prevents yellowing of white goods after heat setting. The 88 mol% grade dissolves at lower temperature than fully hydrolysed grades, but it is more sensitive to high-humidity storage; size film exposed to RH above 70% may become tacky and cause end-breaks. For medium-to-fine counts below 15 tex, size solids are reduced to 7–9% and the PVOH fraction is raised to maintain elongation.

In dry-pressed technical ceramics, the binder system must provide green strength after spray drying, burn out below the sintering onset temperature, and leave minimal ionic residue. Aqueous binder solutions of 88 mol% PVOH are incorporated into ceramic slips at 0.5–2.0% by dry powder weight before spray drying. The 18 mPa·s viscosity grade does not raise slurry viscosity excessively at 10% binder solution concentration, which helps maintain atomisation in a rotary atomiser at 10,000–15,000 rpm. Spray-dried granules with 0.5–1.0% moisture are compacted at 100–200 MPa in hydraulic presses. Green flexural strength is measured by ASTM C1161 four-point bending; compacts with 1.0% PVOH typically show 3–5 MPa, sufficient for automatic handling but below the 8 MPa range where lamination risk from binder migration increases. The low-ash LA specification is critical because residual sodium oxide or chloride from non-LA grades forms low-melting phases that reduce hot strength during the early stage of firing and can cause bloating in dense alumina. The burnout schedule is set at 1–2°C/min to 550°C with a hold of 1–2 h; the 88 mol% PVOH has decomposition onset near 200°C and completes burnout by 500–550°C. Carbon residue from incomplete burnout above 0.05% can reduce whiteness in sanitaryware bodies. During high-humidity storage, green compacts bound with partially hydrolysed PVOH absorb moisture and may soften; green parts are therefore kept below 40% RH before kiln loading. Final articles include alumina electronic substrates, zirconia oxygen-sensor elements, and technical porcelain bodies where trace ash control and edge integrity after green machining are practical production controls.

Remoistenable Adhesive Coatings and FDA 21 CFR 176.170 Compliance

Remoistenable paper coatings for envelope flaps and label stock are formulated with partially hydrolysed PVOH as the adhesion-promoting film former, combined with dextrin or sucrose and a plasticiser. A representative coating formula contains 60–75 parts PVOH, 20–30 parts dextrin, and 5–10 parts glycerol, applied at 4–8 g/m² dry coat weight with a Mayer rod or slot die on coated paper. The coating solution is prepared at 25–35% solids and 65–75°C, then cooled to 40–50°C for application to prevent penetration into the paper substrate. Drying is performed in air-float dryers at 80–95°C; residual moisture of the coated sheet is conditioned to 6–8%. Remoistening time for standard envelope stock is assessed by TAPPI T 441 water absorptiveness method or a laboratory lick-and-stick bond test; the adhesive tack after 1–2 s rewetting is sufficient to give fibre-tearing bonds on paper, with set time below 5 s. High humidity above 75% causes blocking in stacks of coated blanks unless the coating is formulated with a small amount of anti-blocking agent, typically 1–3% of calcium stearate dispersion. The 88 mol% hydrolysis provides a balance between dry surface slip and rapid remoistening; fully hydrolysed grades give higher dry film strength but require longer rewetting times above 10 s. Coating viscosity is maintained at 500–1,000 mPa·s at 40°C with Brookfield LV spindle 3 at 30 rpm; lower viscosity causes penetration into the sheet and loss of surface tack, while higher viscosity leads to ribbing and uneven coat weight on high-speed coaters. All components used in food-contact paper are selected to comply with FDA 21 CFR 176.170 and FDA 21 CFR 176.180, and the LA-grade low ash reduces the risk of extractable metal ions in the final paper matrix.