KURARAY POVAL 8-88

Supplied as a white to pale-yellow granulate, KURARAY POVAL 8-88 is a partially hydrolyzed polyvinyl alcohol resin with a degree of hydrolysis controlled to 87.0–89.0 mol% and a dynamic viscosity of 7.0–9.0 mPa·s when measured as a 4% aqueous solution at 20°C. The grade designation follows the Kuraray nomenclature: the first numeral identifies the nominal viscosity class of 8 mPa·s, and the final two digits identify the nominal hydrolysis level of 88 mol%. The product is manufactured by saponification of polyvinyl acetate and retains a residual acetate content of approximately 11–13 mol%. Typical lot-release data are shown in Table 1. The product is used in emulsion polymerization, paper and textile sizing, water-borne adhesives, and temporary protective coatings where cold-water solubility, controlled viscosity build, and moderate film strength are required.

Table 1. Typical lot-release data for KURARAY POVAL 8-88.

Typical values are determined using methods aligned with JIS K6726 or equivalent internal release methods. Customer specifications should fix the test method and lot-release limits because test-method variation can change the reported viscosity by several tenths of a millipascal-second.

How Does the 87–89 mol% Hydrolysis Window Affect Cold-Water Solubility and Crystallinity?

The residual acetate groups in 8-88 act as steric disruptions along the polyvinyl alcohol chain. They reduce the average crystallite size and lower the melting endotherm compared with fully hydrolyzed grades. The practical consequence is solubility in cold water: a 4% aqueous dispersion can be dissolved under moderate agitation at 20°C within 30–60 min. Heating to 80–90°C shortens the dissolution cycle and reduces residual microgel, which is the preferred industrial protocol even though the grade is classified as cold-water soluble.

Compared with fully hydrolyzed grades at similar viscosity, 8-88 forms films with lower tensile stiffness and higher elongation after conditioning at 23°C and 50% RH. However, published data for plasticizer-free film specimens tested under ASTM D882 is limited; most industrial data are collected on formulated coatings or adhesives rather than on isolated PVOH films. The hydrolysis window also reduces the quantity of long hydrogen-bonded sequences that contribute to water resistance after drying. Consequently, 8-88 is not suitable for permanent water-resistant barrier films unless crosslinked or blended with less water-sensitive polymers.

For bulk dissolution, vertical cylindrical tanks with dished bottoms and center-mounted propellers are used. The powder is added through a mesh screen onto the liquid surface while the agitator operates at 100–300 rpm. If the batch temperature is raised above 60°C before the granules are fully wetted, the surface of the granule can gel and trap undissolved powder. After the temperature reaches 80–90°C, a hold time of 30–60 min is typically required for complete hydration. Dissolved solutions are then filtered through 100–200 µm stainless steel mesh to remove insoluble skins or foreign matter. Above 15 wt% solids, 8-88 solutions become pseudoplastic: the low-shear Brookfield viscosity rises sharply, while the high-shear viscosity remains processable for pumping and metering.

In vinyl acetate homopolymer and vinyl acetate-ethylene emulsion polymerization, 8-88 is charged as a pre-dissolved protective colloid rather than as a bulk rheology modifier. Published formulation data for PVOH-stabilized lattices typically place the protective colloid at 4–10 pphm, with the lower end used for low-viscosity lattices and the upper end for small particle size or high-solids systems. Because the 4% viscosity of 8-88 is 7.0–9.0 mPa·s, the grade builds less reactor viscosity than 18-88 while still providing more colloidal stabilization than very low-viscosity grades such as 3-98.

Production-scale charging is usually performed with an 8–10 wt% aqueous solution prepared in a separate vessel, filtered through 100–200 µm screens, and transferred by low-shear progressing cavity pump. In 10,000 L reactors, rapid addition of dry granulate directly to the hot monomer phase has caused localized swelling and lump formation; field experience indicates that slow addition of pre-dissolved 8-88 below the liquid surface reduces such defects. Mechanical aeration during transfer can increase foam, so the use of a center-mounted turbine at 100–300 rpm is preferred over high-shear rotor-stator devices. Residual acetate groups contribute to interfacial activity, but published data correlating acetate content with final particle size distribution in high-pressure ethylene systems for this specific grade is limited.

When 8-88 Is Selected for Aqueous Coating Applications, What Processing Limits Emerge?

For film-forming and temporary protective coatings, 8-88 is dissolved into water at 8–12 wt% solids for wire-wound rod application. Size press and roll coater baths are often operated at lower polymer concentration, typically 5–8 wt%, because the low molecular weight of the grade produces low solution viscosity and can cause ribbing or streaking when the low-shear Brookfield viscosity falls below 100 mPa·s at 23°C. In pilot blade-coating trials, the use of 18-88 or a rheology modifier has been required above 150 m/min line speed to maintain uniform film weight.

Because the dried film remains cold-water-redispersible, 8-88 is used where the coating is removed by water or where re-wetting is part of the converting process. Oil and grease barrier performance of PVOH-coated paper is routinely evaluated with TAPPI T 454 or ISO 16532-1, but measured values depend on coat weight, drying rate, and plasticizer content. Permanent water resistance is not achieved with 8-88 alone, and typical published data for crosslinked barrier films based on this grade are limited.

Across paper surface sizing, 8-88 is combined with oxidized starch or styrene-acrylate surface sizes at 0.5–2.0% PVOH on total size solids in the size press. The bath temperature is commonly held at 60°C, and the target low-shear viscosity is 20–60 mPa·s to maintain runnability without misting. In such formulations, 8-88 contributes less viscosity build than 18-88 while still reducing surface pick and improving IGT pick resistance as tested under ISO 3783. Surface sizing efficacy is also evaluated by Hercules size test under TAPPI T 530; the change in seconds-to-penetration is sheet-porosity and coat-weight dependent, so a specific value for 8-88 cannot be assigned without trial.

Textile warp sizing with 8-88 is carried out on slasher lines where the size liquor contains a PVOH/starch blend at 30–50% PVOH on dry solids. Because the partial hydrolysis lowers the dissolution temperature, desizing is performed at 60–70°C rather than the 80–90°C often required for fully hydrolyzed grades. Production line experience indicates reduced warp hairiness and fewer broken ends, but published data for specific yarn counts and slasher speeds using 8-88 is limited.

Comparative Position Against Lower- and Higher-Viscosity Partially Hydrolyzed Grades

Table 2 compares 8-88 with 3-98 and 18-88, which are commonly quoted as alternative grades when viscosity or hydrolysis must be changed. Values are typical commercial datasheet ranges, not customer-specific certified limits.

In paper sizing, 8-88 penetrates more than 18-88 at equivalent bath solids but leaves a thinner compacted surface film than 18-88. In adhesive compounding, formulators select 8-88 when open time and redispersibility are more important than maximum tensile strength. In emulsion polymerization, replacing 8-88 with 18-88 raises latex viscosity and can shift the particle size distribution toward coarser particles if nucleation is shear-limited; replacing with 3-98 removes cold-water solubility and may require a post-polymerization surfactant addition. Published comparative data under identical reactor conditions for all three grades is limited, so these differences are directional and should be confirmed by pilot trials.

Storage Stability Limits, Foam Control, and Borate Compatibility

Aqueous solutions of 8-88 are biologically degradable and can develop microbial growth within 24–48 h when stored at 20–30°C without preservative. The native pH range of 4.5–7.0 does not prevent bacterial or fungal proliferation. For storage beyond 48 h, an approved preservative or refrigerated storage below 10°C is required.

Prolonged heating above 95°C may cause acid-catalyzed hydrolysis of residual acetate groups, leading to a gradual increase in water solubility but a decrease in solution viscosity. The recommended dissolution protocol is to add the granulate slowly to cold water under agitation, then heat to 80–90°C for 30–60 min. Excessive vortex formation at impeller tip speeds above 1.5 m/s increases air entrainment and foam; vacuum deaeration or a shear-stable defoamer at 0.05–0.2 wt% on solution weight is used when foam interferes with coating or adhesive transfer.

Borate ions present in borax or boric acid can crosslink the 1,3-diol structures of PVOH and produce sudden gelation. Borate-containing additives should be introduced as a dilute feed with continuous viscosity monitoring using a Brookfield viscometer at 23°C, and a maximum dose should be established for each formulation. Contact with strong oxidizing acids or iron-contaminated vessels can accelerate oxidative chain scission, so stainless steel or lined equipment is specified for long-term storage.

Where converters require indirect food-contact compliance, 8-88 is evaluated against the relevant national listing. In the United States, polyvinyl alcohol may be used as a component of food-contact articles under FDA 21 CFR 177.1670, provided migration levels and end-use restrictions are met. In the European Union, final compliance is assessed under EC 1935/2004 and the applicable plastics measure, not solely from the resin datasheet. The ash content of ≤0.5% and volatile matter ≤5.0% are release parameters, not migration limits. Published extraction data for this specific grade is limited, so converters should qualify the formulated article under the intended food type and temperature condition.

Because 8-88 carries a partial acetate residue, it is used as a secondary film former and rheology modifier in water-borne adhesive systems rather than as a structural binder for load-bearing joints. In starch/dextrin packaging adhesives, addition of 1–5 wt% PVOH on dry solids is used to modify low-shear viscosity and machine stability. Viscosity stability during the production run is monitored with a Brookfield viscometer at 23°C; initial tack on recycled board is formulation-dependent and is evaluated by converter-specific loop tack methods rather than a single standardized test.

Compatibility with borate-crosslinked starch formulations is limited. The same borate dose that gels the starch phase may react with PVOH and produce visible lumping; a pilot pre-test with a Brookfield viscometer at 23°C and a 100 µm filter is used to define the maximum borate dose before production. The product should not be dry-blended with oxidizing agents or stored in unlined carbon steel vessels where iron pickup may accelerate chain scission. In multi-product plants, separate lines or thorough washing protocols are required to avoid contamination with hydrophobic latex polymers that can destabilize the aqueous PVOH solution.