KURARAY POVAL 5-88 S2

KURARAY POVAL 5-88 S2 is a partially hydrolysed polyvinyl alcohol supplied as a dry, free-flowing powder. The grade designation follows the ISO 15023-1 system: the leading numeral group identifies the nominal 4% aqueous solution viscosity, and the second numeral group identifies the nominal degree of hydrolysis. For this material, the 4% aqueous solution viscosity at 20 °C is controlled within 5.0–5.8 mPa·s when determined by capillary viscometry under ISO 15023-2:2019, and the degree of hydrolysis is controlled within 87.0–89.0 mol%. The S2 suffix identifies a handling-optimized sub-grade within the 5-88 series. Typical release data cited in supplier documentation also list pH of a 4% aqueous solution in the range 4.5–6.5, volatile matter not exceeding 5.0 wt%, and ash not exceeding 0.6 wt% by JIS K6726 or corresponding ISO 15023-2 clauses. The base polymer is a vinyl alcohol–vinyl acetate copolymer with residual acetyl groups distributed along the polymer chain; this composition lowers crystalline order, lowers the aqueous dissolution temperature, and increases cold-water solubility relative to fully hydrolysed polyvinyl alcohol grades.

What Distinguishes the 5-88 S2 Sub-Grade from Standard 5-88 and Fully Hydrolysed POVAL Grades?

The primary distinction between POVAL 5-88 S2 and standard POVAL 5-88 is not a change in the base polymer composition but a change in handling and processing behaviour. Kuraray literature associates the S2 suffix with controlled dry-powder particle size, reduced dusting during bulk transfer, and lower foam entrainment during cold-water batch make-up. Published quantitative differences between 5-88 S2 and standard 5-88 in residual catalyst ash and particle-size distribution remain limited; the certificate of analysis is the authoritative source for lot-specific values. Compared with fully hydrolysed POVAL 5-98, the 5-88 S2 grade carries approximately 12 mol% residual acetyl groups, corresponding to roughly 21 wt% vinyl acetate repeat units. These acetate groups act as internal chain defects, suppressing crystallite density and producing a more open, water-sensitive network. The comparative solution profile is summarised below.

At 88 mol% hydrolysis, the residual acetate content is sufficient to reduce the thermal gelation tendency that is more pronounced in fully hydrolysed grades. The practical consequence is that 5-88 S2 can be dissolved directly in cold water under high shear, while 98–99 mol% hydrolysed grades typically require temperatures above 80 °C for complete dissolution. The trade-off is reduced water resistance after film formation and higher equilibrium moisture uptake under humid conditions.

Aqueous Viscosity and Dissolution Envelope in Industrial Batch Make-up

For cold-water make-up, the powder is dispersed into turbulent cold water before the slurry is heated to 85–90 °C for 30–60 min. Adding dry 5-88 S2 directly to hot water above 70 °C introduces a process risk: the particle surface hydrates rapidly and forms a gelatinous shell that retards water penetration and can produce swollen agglomerates. In a 1,000 L stainless-steel vessel equipped with a 45° pitched-blade turbine and side baffles, a specific power input of 0.15–0.30 kW/kg during the wetting phase is commonly applied, although published data for this precise configuration is limited. The function of the high-shear wetting phase is particle separation, not molecular dissolution. After heating, the solution is cooled and adjusted to the target solids content.

The 4% solution viscosity value is a quality-control reference and does not define the behaviour of concentrated process solutions. At 10 wt% solids, the Brookfield viscosity at 20 °C measured under ISO 2555 is considerably higher than the 4% reference value, and at 20 wt% the solution exhibits pronounced shear thinning. Specific viscosity values at concentrations above 10 wt% must be determined by rheometry because temperature history, pH, and mechanical shear all influence the final solution state. Solutions left unstirred below 15 °C may increase in viscosity and exceed the capability of low-speed centrifugal transfer pumps. Storage above 95 °C for extended periods should be avoided because residual acetate groups can undergo further hydrolysis under sustained thermal load, shifting the effective degree of hydrolysis and altering crystallisation behaviour on cooling.

Controlling Particle Size in Emulsion Polymerization Protective Colloid Use

Emulsion polymerisation utilises 5-88 S2 as a protective colloid in vinyl acetate, vinyl acetate-ethylene, and acrylic ester polymerisations. The polymer adsorbs at the monomer–water interface and provides steric stabilisation during nucleation and particle growth. The 87–89 mol% hydrolysis band gives sufficient amphiphilic character without the excessive interfacial rigidity that can occur with highly hydrolysed grades. For a 45–55 wt% solids vinyl acetate homopolymer or copolymer latex, protective colloid addition levels are commonly 2–6 wt% based on monomer. Typical final emulsion Brookfield viscosity ranges from 500–10,000 mPa·s, depending on reactor temperature, initiator type, and the residual monomer level. Continuous stirred-tank reactors operating at 70–80 °C with mean residence times of 2–4 h are representative, though published data for the S2 sub-grade in a specific reactor configuration is limited.

At protective colloid levels below 1.5 wt% on monomer, particle size control can deteriorate and coagulum formation may increase. Above 8 wt%, the continuous-phase viscosity may restrict monomer diffusion and reduce overall conversion. The residual 5-88 S2 remaining in the dried latex film contributes wet tack and dry adhesion but also increases water sensitivity. In addition, the ash and volatile matter limits are relevant in polymerisation because residual inorganic species can alter initiator decomposition and pH drift during batch hold. Grade-to-grade ash variance is therefore monitored by lot certificate rather than assumed from the nominal POVAL designation.

Paper surface sizing is another established use of 5-88 S2. The partially hydrolysed grade is selected over fully hydrolysed PVOH when the size-press formulation requires low foaming and reduced film blocking. The 5.0–5.8 mPa·s 4% viscosity permits low-viscosity circulation at 4–10 wt% solids in a size press. The S2 handling-optimized form is reported to lower foam entrainment during continuous metering; quantitative foam-height reduction data are supplier-specific and should be confirmed by a dynamic foam test such as ASTM D1173. Typical size-press pick-up on bleached board is 0.5–1.5 g/m², but the appropriate addition level depends on substrate porosity, surface energy, and the wet-end cationic demand of the machine.

When Partially Hydrolysed PVOH Replaces Fully Hydrolysed Grades in Adhesive Compounding

Substituting 5-88 S2 for a fully hydrolysed grade such as 5-98 in remoistenable adhesives shifts end-use properties in two directions. The 87.0–89.0 mol% hydrolysis gives cold-water rewettability and lower film blocking, which are functional advantages in envelope and label adhesives. The same residual acetate content increases equilibrium moisture uptake and reduces wet bond strength after water exposure. In borate-containing formulations, 5-88 S2 reacts with boric acid or sodium tetraborate through reversible didiol crosslinks. At pH 8–9 and 5 wt% PVOH, borax additions below 0.5 wt% can produce a sharp viscosity increase; at pH below 5, crosslink density remains low. Uncontrolled borate addition is a process boundary because irreversible gel lumps can form rapidly in high-shear mixing zones.

Alkaline additives, including caustic and certain alkanolamines, accelerate saponification of residual acetate groups in 5-88 S2 and should be buffered if pH stability is required. Amine-based additives are not recommended in unbuffered formulations where long-term viscosity stability is critical. The material is not the preferred choice for blown-film oxygen-barrier structures because the partially hydrolysed grade does not provide the oxygen transmission resistance of fully hydrolysed PVOH or ethylene-vinyl alcohol copolymer. Melt processing without plasticiser is not recommended because the onset of thermal decomposition is near 200 °C; compounding with polyolefins requires glycerol or polyol plasticisers and may still result in phase separation. Regulatory status for food-contact uses should be verified against the specific residual monomer and catalyst profile. Polyvinyl alcohol is listed in FDA 21 CFR 175.105 for adhesive components and FDA 21 CFR 176.170 for paper and paperboard components, subject to extractives limitations. REACH registration applies to the polymer as a registered substance in the EU.