KURARAY POVAL 3-98

Kuraray Poval 3-98 is a fully hydrolysed polyvinyl alcohol (PVOH; CAS 9002-89-5) manufactured by controlled alcoholysis of polyvinyl acetate. The grade designation separates two governing properties: the first numeral 3 denotes a nominal viscosity class for a 4% aqueous solution at 20°C, and the suffix 98 denotes a degree of hydrolysis of 98.0–99.0 mol%. Under JIS K6726, the material is released against degree of hydrolysis, solution viscosity, volatile matter, and ignition residue. The 98.0–99.0 mol% hydrolysis level leaves only a small residual acetyl fraction; this low acetyl content distinguishes fully hydrolysed PVOH from partially hydrolysed grades and produces a denser hydrogen-bonded network, higher crystallinity, and reduced cold-water solubility of the dried film. The product is supplied as a white to light-yellow granulate or powder. Because the powder is hygroscopic and can compact under silo mass flow, gravimetric feeding lines may require hopper liners, aeration pads, and moisture ingress control; particle-size distribution is not typically a certificate-of-analysis parameter under JIS K6726 and must be qualified separately for continuous dosing.

The 3.2–4.0 mPa·s viscosity at 4% solids is the low end of the fully hydrolysed 98-series viscosity range. This low viscosity is not simply a concentration effect; it reflects the shorter chain length and lower hydrodynamic volume of the grade. The practical consequence is that Poval 3-98 is selected when the dried film must retain the higher strength and water resistance of a fully hydrolysed PVOH, but the liquid feed system cannot tolerate the solution viscosity of 4-98, 5-98, or 6-98 at the same solids. The reverse consequence is lower film tensile strength and lower solution extensional viscosity than in longer-chain grades; this must be considered in curtain coating, roll transfer, and adhesive fibre formation where extensional viscosity controls film splitting and web transfer.

What controls dissolution, solution stability, and gelation in Poval 3-98?

Fully hydrolysed PVOH chains contain a dense sequence of secondary hydroxyl groups. The resulting intra- and inter-chain hydrogen bonds increase crystallite density; water penetrates the amorphous regions but does not dissolve the crystallites at room temperature. A standard preparation procedure begins with dispersion of the powder in water at 20–25°C under low-shear agitation at 200–500 rpm. The slurry is then heated in a jacketed stainless-steel vessel at 1–2°C min⁻¹ to 85–95°C and held for 30–60 min. For precision coating use, the solution is filtered through a 100–200 mesh screen; residual microgels are more common if the initial dispersion is too rich or if direct steam injection creates local overheating. Once dissolved, the solution should be held at 60–80°C with slow agitation. Fully hydrolysed PVOH solutions can form structured gels, skins, or crystalline domains during standing at room temperature, particularly at solids above 10 wt%.

Rheology is nearly Newtonian at the 4% reference viscosity and becomes shear-thinning at higher solids. Low-viscosity Brookfield spindles should be calibrated at 20°C because the 4% solution is near the lower torque reliability threshold of high-viscosity spindles. The solution pH is typically 5.0–7.0, but pH affects long-term hydrolysis stability; strong acids or bases can hydrolyse residual acetate groups and alter viscosity over time. Sulphates and phosphates can reduce hydration and raise clouding; borate ions cause reversible di-diol crosslinking, producing a sharp viscosity increase that may be beneficial in gelled adhesives but is an incompatibility in standard size presses and paper coating colour systems.

In comparison with higher-viscosity fully hydrolysed grades such as 4-98, 5-98, and 6-98, Poval 3-98 produces the lowest aqueous-solution viscosity at constant solids. This permits solids increases from 8 wt% to 12 wt% or more before the viscosity exceeds the operating window of a metering size press or positive-displacement pump. The lower chain length also lowers dry-film tensile strength, elongation at break, and solution extensional viscosity. When films are conditioned at 50% relative humidity and 23°C and tested under ISO 527-2 or ASTM D638-14, longer-chain fully hydrolysed grades generally produce higher tensile values; the difference becomes more pronounced at high strain rates. Against partially hydrolysed grades such as 22-88, the 98.0–99.0 mol% hydrolysis level raises the crystalline fraction and reduces equilibrium moisture uptake at 65% relative humidity. The fully hydrolysed film is therefore stiffer, stronger, and more resistant to polar-oil pickup, but it is less cold-water-soluble and has lower elongation unless plasticised. Glycerol, triethylene glycol, or sorbitol at 5–15 phr lowers the glass-transition and increases elongation; the trade-off is a reduction in tensile strength and an increase in moisture uptake. Published comparative data for all substrate and plasticiser combinations is limited; drawdowns on the intended substrate remain necessary.

When coater and size press viscosity ceilings determine grade selection

On high-speed paper machines, film-press and metering size press applications operate within a narrow viscosity band, typically 20–50 mPa·s at process temperature. Poval 3-98 is blended with oxidised starch at 4–8 wt% total solids and 0.5–2.0 wt% PVOH on dry fibre. The low-viscosity PVOH fraction raises film strength and reduces surface dusting without moving the circulating size outside the runnable viscosity window. The size is maintained at 60–70°C to prevent starch retrogradation and PVOH gelation; filters are selected for 100–150 µm retention to protect the metering elements. The sized sheet is evaluated for water absorption by ISO 535 and for tensile strength by ISO 1924-2. The fully hydrolysed PVOH contributes lower equilibrium moisture uptake than a partially hydrolysed grade, but at the low add-on levels used in surface sizing it modifies the fibre surface rather than closing all pores.

In cotton and polyester-cotton warp sizing, the size is cooked in a jet cooker or pressure cooker at 120–130°C and applied at 6–10 wt% solids. The low molecular weight of Poval 3-98 permits a size pickup of 8–14% on warp yarns while maintaining penetration into the yarn bundle. Abrasion resistance is tested on a Zweigle warp-sizing tester, and isolated size films are conditioned at 65% relative humidity and 20°C before tensile measurement. Desizing requires hot water above 80°C; if starch is present, enzymatic or oxidative desizing is used first, followed by hot washing to remove the PVOH fraction.

For remoistenable and hot-water-activated paper adhesives, Poval 3-98 is formulated at 10–20 wt% solids with a plasticiser and, where controlled gelling is needed, a metered borate crosslinker. The low solution viscosity allows higher solids in roller-coating and extrusion-coating feed systems without exceeding the transfer system viscosity limit. The dried film is harder and more water-resistant than a partially hydrolysed PVOH of comparable viscosity class; activation therefore relies on heated rolls or hot water rather than cold-water rewetting. Uncontrolled borate addition creates filter-plugging gels, so borate should be injected only at controlled pH and under continuous viscosity monitoring.

Film mechanics, moisture response, and barrier-related limits

Films cast from Poval 3-98 and dried between 50°C and 80°C develop high crystalline order because the low residual acetyl content permits close chain packing. The effect is increased modulus and tensile strength relative to partially hydrolysed PVOH, but also lower elongation and greater drying stress. At 50% relative humidity, equilibrium moisture uptake is lower than for an 88 mol% hydrolysed grade; at 80% relative humidity, all PVOH films plasticise substantially and lose tensile strength. Plasticisers such as glycerol at 5–15 phr suppress the glass-transition and reduce internal stress, but they also increase moisture uptake and can migrate in contact with paper or board. Without crosslinking, fully hydrolysed PVOH remains soluble in hot water and should not be specified as a permanent water barrier.

Poval 3-98 may be considered where a low-viscosity PVOH is used as an oxygen-barrier coating on oriented polypropylene or polyester at 0.5–2.0 g/m² dry coat weight. Oxygen transmission rate is evaluated by ASTM D3985 at 23°C and 0% relative humidity; barrier performance falls as humidity rises because water acts as a plasticiser. The low chain length of 3-98 permits higher coating solids, but the dry film is slightly weaker than that of 5-98; coating weight or crosslinking may need to compensate. Because the residual acetyl content is very low, surface activity at oil/water interfaces is lower than in partially hydrolysed grades. Poval 3-98 is therefore not the first-choice protective colloid for vinyl chloride suspension polymerisation, where grades in the 70–80 mol% hydrolysis range are typically used.

At storage relative humidity above 60%, the powder absorbs atmospheric moisture and may clump; pre-drying is required when gravimetric dosing or moisture-sensitive compounding is involved. The dry product forms combustible organic dust, and process equipment should be grounded with dust collection complying with local combustible-dust standards. Aqueous solutions are susceptible to microbial growth; preservative selection must be checked for compatibility with PVOH and with the borate/plasticiser system. Food-contact applications require grade-specific regulatory confirmation; polyvinyl alcohol may be referenced in FDA 21 CFR §175.300, §176.170, and §176.180, but the specific end-use temperature and simulant type must be evaluated under current listings and supplier certificates. Melt extrusion of this fully hydrolysed grade without plasticiser is constrained by thermal degradation before flow; solution casting and aqueous coating are the standard conversion routes. Published melt-processing data for Poval 3-98 is limited, and extrusion trials require plasticiser pre-blending and screw configurations designed to minimise shear heating.