KURARAY POVAL 5-88

KURARAY POVAL 5-88 is a low-viscosity, partially hydrolyzed polyvinyl alcohol grade produced by controlled saponification of polyvinyl acetate. The grade designation encodes two control variables: the first digit denotes a nominal 4% aqueous solution viscosity class of 5 mPa·s, and the suffix 88 denotes a degree of hydrolysis of 88 mol%. The release specification for viscosity is 4.8–5.8 mPa·s at 20 °C for a 4% aqueous solution when measured according to JIS K6726 and corrected to dry solids. The hydrolysis window is 86.5–89.0 mol%, leaving 11.0–13.5 mol% residual vinyl acetate repeat units along the polymer chain. The product is identified by CAS number 25213-24-5 and is supplied as a white to pale-yellow granulate with typical release limits of volatile matter ≤5.0%, ash ≤0.5%, and 4% solution pH 5.0–7.0. Compared with a fully hydrolyzed grade of the same viscosity class, the partial hydrolysis reduces hydrogen-bond-driven crystallinity, lowers the temperature required for complete dissolution, and increases interfacial activity. The low molecular weight places 5-88 below higher-viscosity partially hydrolyzed grades such as 18-88 and 22-88 in terms of solution viscosity and gel strength. Published tensile-property data for this specific grade is limited; however, the structural features are consistent with films of moderate water sensitivity and lower ultimate tensile strength than fully hydrolyzed PVA at equivalent chain length. The grade is used as a protective colloid, water-sensitive binder, or cold-process formulation aid where low solution viscosity is the controlling parameter.

How Does the 88 mol% Hydrolysis Window Alter Solubility and Crystallization?

At 88 mol% hydrolysis, the residual acetate groups interrupt polyvinyl alcohol crystalline registry. In a fully hydrolyzed grade above 98 mol%, vinyl alcohol sequences form extensive intramolecular and intermolecular hydrogen bonds; in 5-88, approximately 12 mol% acetate units reduce this order. The practical consequence appears in dissolution behavior. A 10% aqueous stock solution can be prepared by wetting the powder in cold water at 20–30 °C and heating to 80–85 °C for 60–90 min; a fully hydrolyzed low-viscosity grade typically requires a holding temperature above 90 °C to destroy residual microgel clusters. The solubility advantage is accompanied by higher moisture uptake and lower tensile strength in the dried film. At 50% relative humidity, partially hydrolyzed PVA films absorb more equilibrium moisture than fully hydrolyzed films because acetate groups are less strongly hydrogen-bonded than hydroxyl groups. Quantitative film-property data for 5-88 is limited in published technical literature, but the directional difference is confirmed across the 86–89 mol% hydrolysis range. The grade should not be selected for applications needing hot-water resistance or low equilibrium moisture content; its cold-water solubility is an operational advantage when process temperature must remain below 60 °C.

In vinyl acetate and vinyl acetate-ethylene emulsion polymerization, a 10–15% aqueous solution of 5-88 is introduced into the aqueous phase before monomer metering. The PVA acts as a protective colloid and as a secondary grafting substrate because the residual acetate groups can participate in radical grafting reactions with growing polyvinyl acetate chains. The low molecular weight of 5-88 keeps continuous-phase viscosity lower than that of an equivalent loading of 18-88; this difference is measurable on a Brookfield RVT viscometer at 20 rpm and becomes operationally significant at latex solids above 55%. In a baffled jacketed reactor fitted with a pitched-turbine impeller, a 10% PVA stock solution is typically screened through an 80 mesh in-line filter before charging to remove gel specks; low-viscosity 5-88 passes through this filtration step with lower pressure drop than higher-molecular-weight grades. Dry powder addition directly into a high-shear rotor-stator is not recommended because localized heating and rapid hydration can create fisheye agglomerates that survive downstream filtration. Instead, the powder is wetted in cold water under low-shear anchor agitation, heated to 80–90 °C for 60 min, and cooled to 25–35 °C before use. The protective colloid concentration in the final emulsion is typically between 1% and 5% of total monomer, depending on target particle size and mechanical stability. Redox initiation at 60–65 °C is preferred over thermal initiation above 75 °C when minimizing premature PVA grafting is a process requirement. In comparison with higher-viscosity PVAs, 5-88 may contribute lower latex viscosity and less pseudoplasticity at equivalent PVA content, but it may also provide less steric stabilization under ionic stress; the electrolyte threshold depends on pH, initiator residue, and surfactant package. Mechanical stability of the finished latex is commonly checked by high-speed mixing at 3,000 rpm for 10 min and measuring coagulum retention on a 100 mesh screen.

When 5-88 Replaces Higher-Viscosity Partially Hydrolyzed Grades in Paper Coating and Adhesive Compounding

In paper coating color preparation, the lower aqueous viscosity of 5-88 permits a higher total solids content at a target low-shear viscosity than would be possible with 18-88 at the same PVA addition. A metering size press or blade coater system typically operates within a low-shear Brookfield viscosity window of 100–1,500 mPa·s at 20 rpm; the lower molecular weight of 5-88 widens formulation latitude before the upper limit is exceeded. The trade-off appears in coated paper surface strength and wet pick resistance. Because 5-88 contributes less chain entanglement and lower film cohesion than higher-viscosity grades, coated paper surface strength measured by IGT pick velocity may be lower; substitution is limited to formulations where runnability and binder migration control are more critical than maximum dry strength. In adhesive compounding, 5-88 is compatible with cold-process starch/PVA blends and can be added as a 10–20% aqueous solution without pre-cooking the starch phase at elevated temperature. The low viscosity also supports spray application and curtain coating where nozzle atomization and curtain stability depend on extensional viscosity. These substitution benefits are conditional: if the adhesive bond will be exposed to water or high humidity, a fully hydrolyzed grade or a crosslinker is required because residual acetate groups in 5-88 increase moisture sensitivity. The same logic applies when replacing higher-viscosity partially hydrolyzed grades in emulsion polymerization; low-viscosity PVA lowers continuous-phase viscosity but may require an increase in PVA content or a secondary surfactant to maintain shelf-life stability.

Specification Boundaries and Certificate-of-Analysis Parameters

The release ranges in the table below are typical for the grade and are corrected to dry solids where applicable. Regional certificate limits may differ.

Comparative positioning within representative Poval grades is summarized below. Values are representative ranges from Kuraray technical literature, not certificate limits for all regions.

The comparative table confirms that 5-88 differs from 4-88 mainly by a slightly higher viscosity window, from 18-88 mainly by molecular weight, and from 5-98 mainly by hydrolysis level. The choice between these grades is determined by the balance of continuous-phase viscosity, film water resistance, and dissolution temperature.

In textile warp sizing for spun yarns, a 8–12% aqueous 5-88 solution is applied in a slasher sizing box at 40–60 °C. The low-molecular-weight polymer penetrates the yarn bundle with lower size-box viscosity than 18-88, reducing size add-on at equivalent squeeze pressure. Desizing is accomplished at 60–70 °C with less aggressive washing than fully hydrolyzed PVA because the film dissolves more readily. Published data for 5-88 in this specific configuration is limited; however, the dissolution and viscosity differences support the substitution in low-add-on textile formulations. The grade is generally not selected as the sole film former in water-soluble packaging. Higher-molecular-weight partially hydrolyzed grades are preferred where film toughness and controlled dissolution are required. Published data for water-soluble film mechanical properties of 5-88 is limited; the low viscosity indicates a lower degree of chain entanglement and therefore lower tensile modulus than 22-88 cast film.

Handling Aqueous 5-88 Stock Solutions Under Shear and Heat

Preparation of a 10–20% stock solution requires a controlled heat-and-shear sequence. The powder is first dispersed in cold water under low-shear agitation to allow individual particles to wet. High-shear mixing at the early hydration stage is not recommended because localized viscous heating can form a gelatinous skin on the particle surface, slowing subsequent dissolution and producing filterable gel specks. After complete wetting, the batch is heated to 80–90 °C and held under anchor agitation for 60–90 min; the vessel is then cooled to 25–35 °C before transfer. A solution of 5-88 at 10% solids has low room-temperature viscosity, but the viscosity rises during cooling and may shift with pH if acidic additives are introduced. Inline filtration through a 80–120 µm sintered metal filter removes residual gel specks before the solution enters a meter pump or reactor. The product is incompatible with borate ions, which crosslink the 1,3-diol structures and can produce a gel at low borate concentrations. Strong acids and oxidizing agents degrade the polymer and lower solution viscosity over time. Storage of the dry granulate should be maintained below 30 °C and 60% relative humidity; moisture uptake above the ≤5.0% volatile-matter limit can reduce free-flowability in loss-in-weight feeding systems and shift the weight basis of batching. These boundaries apply equally to adhesive, coating, and emulsion polymerization operations.

For indirect food-contact adhesive applications, polyvinyl alcohol is listed in FDA 21 CFR 175.105 as a permitted adhesive component, subject to good manufacturing practice. This listing is specific to the adhesive function and does not confer direct food-contact approval. Under REACH, polyvinyl alcohol is a polymer; compliance obligations depend on registration status and tonnage band. Other jurisdiction-specific positive lists require independent confirmation. Published data for 5-88 in direct food-contact films is limited; such use is not recommended without further regulatory evaluation.