KURARAY POVAL 95-88

KURARAY POVAL 95-88 is a polyvinyl alcohol resin obtained by controlled alcoholysis of polyvinyl acetate. The grade designation encodes two independent specifications: the first numeric segment, 95, refers to the nominal viscosity of a 4% aqueous solution at 20°C, stated as 95 mPa·s when measured with a Höppler falling-ball viscometer according to DIN 53015; the second segment, 88, identifies a nominal degree of hydrolysis of 88 mol% when tested by titration according to ISO 15023-2:2019. The residual vinyl acetate fraction of approximately 12 mol% is not an impurity but a structural modifier: it interrupts chain regularity, reduces crystallite size, and lowers the thermodynamic barrier to cold-water swelling. The product is supplied as white to off-white granules or powder, with a bulk density typically in the range 0.40–0.55 g/cm³ and a moisture content below 5 wt% on the certificate of analysis. Because both viscosity and hydrolysis are nominal values, batch-to-batch variation is controlled within the manufacturer’s release window; incoming inspection for critical applications should verify the lot certificate rather than rely on the grade name alone.

The molecular weight distribution of this high-viscosity grade is broader than that of low-viscosity PVOH types; aqueous solutions display pronounced chain entanglement. The critical overlap concentration is reached below 1 wt% at 20°C for a 95 mPa·s grade, which means that most application concentrations are in the semi-dilute regime. This rheological state controls pump sizing, heat transfer, and levelling behaviour.

Aqueous Solution Behaviour and Film Properties

Dissolution of POVAL 95-88 proceeds through a two-stage process: cold-water dispersion at 20–25°C with vigorous mixing, followed by heating to 85–90°C for 30–60 min. The resulting 4% solution has a pH of 5–7 by ISO 976 and a viscosity of approximately 95 mPa·s by DIN 53015. At 10% solids, Brookfield viscosity at 20 rpm exceeds 1,500 mPa·s; the solution is pseudoplastic, and viscosity at 100 rpm may be only 40–50% of the low-shear value. This shear-thinning behaviour is exploited in size-press and coating equipment, where shear rates at the metering nip exceed 10,000 s⁻¹. Films cast from a 10% aqueous solution and dried at 40°C for 24 h are transparent and flexible. When conditioned at 23°C and 50% relative humidity, tensile strength measured by ASTM D882-18 is commonly between 30 MPa and 45 MPa, with elongation at break between 150% and 300%. The presence of 12 mol% residual acetate reduces tensile modulus and increases elongation compared with fully hydrolysed PVOH. Oxygen permeability measured according to ASTM D3985-24 at 0% relative humidity is typically below 0.5 cm³·mm/(m²·day·atm); at 80% relative humidity the value can exceed 20 cm³·mm/(m²·day·atm). This order-of-magnitude increase is caused by plasticisation of the amorphous phase and is the principal limitation for barrier packaging without a moisture-barrier layer.

How Does 95-88 Compare with Lower-Viscosity and Fully Hydrolysed Grades?

Grade selection in polyvinyl alcohol systems is governed by the interaction between viscosity, degree of hydrolysis, and the thermal history of the final product. POVAL 95-88 combines high molar mass with partial hydrolysis; the high molar mass contributes to film strength and aqueous thickening, while the residual acetate content preserves some cold-water compatibility. A fully hydrolysed grade with 99 mol% hydrolysis, even at a similar viscosity, will dissolve only above 70°C and will yield a more crystalline, more water-resistant film. A low-viscosity partially hydrolysed grade such as 5-88 dissolves rapidly in cold water but produces films with lower tensile strength and little thickening at the same solids. In applications where the polymer must be added at more than 5 wt% solids, the high viscosity of POVAL 95-88 becomes a practical constraint because solution viscosity may be too high for conventional pumps and filters. The comparison is summarised in the table below.

Vinyl chloride suspension polymerisation uses POVAL 95-88 as a primary protective colloid at charge levels of 0.05–0.15 g/100 g monomer. The combination of 88 mol% hydrolysis and high solution viscosity modifies the equilibrium between droplet break-up and coalescence in the pre-gelation stage. In a 13 m³ stirred reactor fitted with a 2.0 m diameter turbine impeller and operated at 250–300 rpm, an aqueous phase containing 0.10 wt% POVAL 95-88 at 25°C develops a Brookfield viscosity in the range 120–180 mPa·s at 20 rpm. That viscosity increase reduces jacket heat-transfer efficiency; operators sometimes compensate with 10–15% higher coolant flow to hold the polymerisation exotherm within the target band. At 0.20 wt% or greater, the aqueous-phase viscosity can rise above 250 mPa·s, producing persistent foam in the overhead condenser and a broader grain-size distribution. For reactor systems where the jacket-to-reactor volume ratio is lower than 0.8, published data for this specific grade is limited; pilot-scale verification under the intended agitation profile is required. The partially hydrolysed structure also influences grain porosity and monomer absorption. Compared with fully hydrolysed protective colloids, POVAL 95-88 shifts the mean particle size lower at equivalent stirring speed when measured by laser diffraction according to ISO 13320, but the magnitude depends on buffer concentration, initiator selection, and monomer purity.

When POVAL 95-88 Is Used in Aqueous Adhesive and Paper Sizing Systems

Adhesive formulators select POVAL 95-88 when higher green strength and film toughness are required at low addition levels. In starch-based corrugating adhesives containing 0.5–1.0 wt% POVAL 95-88 based on total adhesive mass, the time to reach a T-peel green bond of 50 N/m at 20°C is typically reduced by 15–30% relative to the same formulation without PVOH. This measurement is performed on a tensile tester at 100 mm/min crosshead speed using 25 mm wide strips of kraft liner. Borax is a common additive in PVOH-based adhesives, but the high viscosity of POVAL 95-88 magnifies gelation: adding 1.0 wt% borax decahydrate to an 8% PVOH solution at 25°C can raise Brookfield viscosity tenfold within 10 min. At borax levels above 2 wt%, the system forms a rubbery gel that undergoes syneresis and is no longer transferable by gear pump. For surface sizing of linerboard, a 6–8 wt% solution at 50–60°C is metered with a rod coater to achieve 2–4 g/m² dry pick-up. Surface strength and printing performance are measured by IGT pick tests according to ISO 3783; typical improvement above the starch control is 20–40% at 4 g/m² dry pick-up. The main operational constraint is misting from high-viscosity solutions at rod-coater speeds above 1,500 m/min; the use of high-solids starch-PVOH blends reduces the issue but requires shear-stability screening.

Textile warp sizing with POVAL 95-88 requires a lower solids setting than low-viscosity grades because of the high solution viscosity. A size liquor at 80°C containing 3–5 wt% POVAL 95-88 and 0.5–1.0 wt% synthetic lubricant exhibits a viscosity of 50–100 mPa·s and is compatible with conventional slasher equipment. Warp-hairiness reduction and abrasion resistance on air-jet looms are assessed by the Kapur yarn abrasion method and by loom stop counts rather than by a single ASTM standard; line trials at 700–900 picks/min have shown that adequate film strength is maintained at size add-on of 8–12% on ring-spun cotton. Desizing is performed with hot water at 80–90°C because the 88 mol% hydrolysis allows enzymatic or oxidative desizing within 30–45 min; fully hydrolysed PVOH would require more aggressive wash conditions. Published data for the specific grade in all weave constructions is limited; slasher trials are recommended.

Storing POVAL 95-88 Below 60% Relative Humidity Maintains Flowability

KURARAY POVAL 95-88 is hygroscopic. Equilibrium moisture at 20°C and 60% relative humidity is between 5 wt% and 7 wt%; at 80% relative humidity, moisture uptake can exceed 12 wt% and the free-flowing powder may cake. Moisture content is determined by Karl Fischer titration according to ISO 15512:2019; ash content by ISO 3451-1:2019 is typically below 0.5 wt%. Store sealed bags below 25°C and 60% relative humidity; opened bags should be resealed under nitrogen or consumed within 24 h to avoid moisture absorption. Aqueous solutions of this grade at 1–10% solids should be preserved if held longer than 24 h at ambient temperature; unpreserved solutions can develop microbial colonies within 48–72 h. Methylisothiazolinone or benzisothiazolinone at 0.05–0.15 wt% on solution mass controls bacterial growth. Strong acids, strong oxidisers, and aldehydes are incompatible with PVOH. Dry resin heated above 200°C under air can discolour and degrade; aqueous solutions should not be held above 90°C for more than 2 h because chain scission reduces viscosity.

Regulatory evaluation of KURARAY POVAL 95-88 is end-use specific. For paper and paperboard intended for food contact, compliance is assessed under U.S. FDA 21 CFR 176.170 and 21 CFR 176.180. For plastic materials and articles in the European Union, the resin may be evaluated under Commission Regulation (EU) 10/2011 as a component within the final article; specific migration limits depend on the article composition and food simulant. The supplier certificate of analysis reports residual vinyl acetate monomer, typically below 5 mg/kg, and heavy-metal concentrations for lead, cadmium, mercury, and arsenic below 10 mg/kg, 1 mg/kg, 0.1 mg/kg, and 1 mg/kg, respectively, by inductively coupled plasma optical emission spectrometry after acid digestion. These values are typical industrial acceptance criteria; the absence of a release number on a lot certificate should be resolved before use in regulated contact applications.