PVA Viscosity & Hydrolysis Matrix: Full Grade Chart

The property landscape of polyvinyl alcohol (PVA, CAS 9002-89-5) is defined by two independent specification axes: the nominal 4% aqueous solution viscosity at 20 °C, expressed in mPa·s and determined by rotational viscometry under JIS K 6726 or ISO 15023-2, and the degree of hydrolysis, expressed as mol% vinyl alcohol units. The full grade chart spans nominal viscosities from approximately 3 mPa·s to 70 mPa·s and hydrolysis ranges from approximately 80 mol% to 99.9 mol%. These parameters are not independent: for a given hydrolysis level, increasing viscosity reflects higher degree of polymerisation and greater chain entanglement, while for a fixed viscosity, increasing hydrolysis raises intramolecular and intermolecular hydrogen bonding, crystallinity, tensile strength, and aqueous dissolution temperature. Grade selection is therefore a matrix decision in which single-property optimisation is rarely sufficient.

The most commonly referenced commercial series—Kuraray Poval, Sekisui Selvol, Denka Poval, and Mitsubishi Chemical PVA families—use similar viscosity-grade nomenclatures, but numerical designations are not identical across producers. A full grade chart is typically constructed from the manufacturer’s certificate of analysis, with viscosity reported as the mean of a controlled 4% aqueous solution at 20 °C and hydrolysis measured by saponification titration or infrared spectroscopy. The table below uses representative Kuraray Poval values because those technical bulletins are publicly available and widely used for qualification.

What Distinguishes Partially Hydrolysed Grades in Emulsion Polymerisation?

Partially hydrolysed PVA grades, typically in the 80–89 mol% range, retain a blocky distribution of residual acetate groups. This structural feature imparts surface activity and cold-water solubility. In vinyl acetate-ethylene emulsion polymerisation, PVA functions as a grafted protective colloid rather than a simple thickener. Low-viscosity partially hydrolysed grades such as Poval 205, with a nominal viscosity of 5.0–6.0 mPa·s and hydrolysis of 87–89 mol%, generate low continuous-phase viscosity during nucleation, which permits high-shear agitation to produce fine-particulate dispersions. The same chemistry at higher viscosity, such as Poval 224 with 30.0–38.0 mPa·s, increases emulsion yield stress and may shift the particle size distribution upward if nucleation and growth become diffusion-limited in the aqueous phase. Batch reactors equipped with radial impellers and baffles typically show greater sensitivity to this viscosity shift than unbaffled tanks because shear rate distribution widens as the vortex depth decreases.

The distribution of residual acetate groups is not random. Controlled alkaline saponification of polyvinyl acetate produces block copolymers with measurable sequence length, and this blockiness influences surface activity, foaming, and cold-water solubility more than the gross mol% value alone. Two grades with identical 88 mol% hydrolysis but different block character can show different emulsion nucleation behaviour. The residual acetyl content also controls the lower critical solution temperature and grafting efficiency during radical initiation. As hydrolysis falls from 89 mol% toward 80 mol%, cold-water solubility improves but mechanical film strength and oxygen barrier decline. Grades in the 80–83 mol% range, such as Poval 405 and Poval 417, are therefore reserved for specialty emulsifiers, low-temperature dissolvable packaging, or applications where the film must disintegrate in cold water without external heating. The operational limit is that lower hydrolysis increases residual organic hydrophobicity; in systems regulated under FDA 21 CFR 176.170 or EU Regulation 10/2011, the final extractives and residual vinyl acetate monomer must still be validated, not inferred from the grade chart alone.

The compiled matrix below is the full grade chart for representative Kuraray Poval series. Viscosity values are nominal manufacturer ranges for 4% aqueous solutions at 20 °C; hydrolysis values are the listed control bands. Other producers use comparable bands, but interchange requires qualification because molecular weight distribution, ash content, and process residues differ.

Grades with hydrolysis between 90 mol% and 97 mol% are less common in commercial grade charts because the saponification process is typically controlled to either partial hydrolysis near 87–89 mol% or full hydrolysis above 98 mol%. Published data for this intermediate configuration is limited, and formulators often achieve intermediate performance by blending fully and partially hydrolysed grades rather than specifying a single mid-hydrolysis product.

Fully Hydrolysed Grades and Hydrogen Bonding Density

At hydrolysis levels above 98 mol%, vinyl alcohol sequences become long enough to develop crystalline domains, and aqueous dissolution temperature rises to approximately 80–95 °C depending on molecular weight. Low-viscosity fully hydrolysed grades, such as Poval 4-98 and Poval 5-98, provide high solids paper sizing and low-tack adhesive bases; high-viscosity fully hydrolysed grades, such as Poval 28-98 and Poval 117, provide higher tensile strength and barrier function. Tensile property shifts are measured not by viscosity alone but by solution-cast film testing under ASTM D882-18, commonly after conditioning at 50% RH. A film cast from Poval 10-98 at 10.0–11.6 mPa·s will generally show lower yield stress and elongation than one cast from Poval 28-98 at 26.0–30.0 mPa·s, but the exact difference depends on casting thickness, drying rate, and residual moisture.

Fully hydrolysed PVA crystalline domains create a melting-like dissolution endotherm. The dissolution temperature is a function of both hydrolysis and viscosity: for Poval 4-98, dissolution may occur near 80 °C, while high-viscosity Poval 117 may require 90–95 °C. The exact temperature is measured by hot-stage microscopy or by viscosity development during controlled heating. Processing below the dissolution endotherm yields swollen gels rather than homogeneous solutions.

The gas barrier of fully hydrolysed PVA is characterised by very low oxygen transmission at dry conditions; testing under ASTM D3985-17 at 0% RH is common because barrier performance deteriorates sharply as relative humidity rises. This humidity sensitivity is the main trust boundary for barrier packaging. Below 40% RH, a thin PVA layer can reduce oxygen permeability by orders of magnitude relative to polyolefins, but above 60% RH the film absorbs water, plasticises, and oxygen transmission increases. Industrial structures therefore bury PVA between hydrophobic layers or use solution coating at low moisture load; otherwise the barrier specification cannot be guaranteed. On converting lines, the practical boundary is that fully hydrolysed PVA solution must be held above its dissolution temperature during transfer; chilled transfer lines can induce gelation and create die streaks.

When Viscosity Exceeds 40 mPa·s in Slot-Die Coating

In roll-to-roll slot-die coating of PVA solutions, high-viscosity grades such as Poval 48-88 at 40.0–48.0 mPa·s, Poval 235 at 50.0–60.0 mPa·s, and Poval 245 at 60.0–70.0 mPa·s shift the processing window from simple solution delivery to pressure-controlled positive-displacement pumping. A 10–15 wt% solution of Poval 245 at 20 °C may exceed 3,000 mPa·s at low shear, requiring heated feed tanks, jacketed transfer lines, and degassing before the die. Shear heating in gear pumps becomes measurable, and viscosity drift across the coating width can appear if die body temperature varies by more than ±2 °C. The defect signature is not gross coating failure but a transverse thickness profile drift because viscosity at the die lips controls the wet film pressure drop. Production equipment with closed-loop slot-die lip adjustment and capacitance thickness scanning can correct for this drift, but the correction range is limited by the shear-rate dependence of PVA solutions. At high shear, PVA exhibits pseudoplastic behaviour; apparent viscosity falls, but first normal stress difference can rise, leading to die swell and edge bead instability.

The same high-viscosity grades are useful in cast film because they permit higher wet thickness at lower solids, but drying load increases. A high-viscosity PVA film often requires staged drying with the first zone at low air impingement to prevent skinning over a wet core. Skinning traps water and produces haze or microvoids. The practical limit for high-speed slot-die lines is therefore not the coating station but the drying tunnel length and equilibrium moisture content at the winding roll. If winding is attempted above the glass transition of the conditioned PVA layer, blocking can occur; conditioning to 20–30% RH and cooling to below 35 °C is typical for continuous winding. The shear-rate dependence can be approximated by a power-law model, but for high-viscosity PVA solutions the power-law index is not constant across three decades of shear. Low-shear Brookfield data at 20 °C are insufficient for die design, which requires capillary or slit rheometry at process temperature. Published data for specific formulations is often limited, so pilot-scale slot-die trials at target coat weight and line speed are required. Limiting process conditions are usually air entrainment at high line speed and die lip buildup from partial drying at the gap.

In adhesive and paper sizing applications, the hydrolysis-viscosity matrix controls penetration depth, binder migration, and dry tensile strength. Low-viscosity fully hydrolysed grades penetrate paper substrates and bind surface fines; high-viscosity partially hydrolysed grades remain at the surface and contribute film strength. For corrugated board adhesive formulations, PVA is usually combined with starch or borate-modified starch, and the PVA grade must be selected for compatibility with the starch cook temperature. Borate additives crosslink the 1,2-diol segments of PVA and can produce a shear-thinning gel; this is beneficial in controlled doses but causes irreversible lump formation if the borate solution is added too rapidly or at high pH. The viscosity-hydrolysis chart is therefore used to pre-screen grades, but final qualification requires pilot-scale drawdown on the actual paper stock because fibre furnish and calender pressure alter adhesive demand.

Dissolution Kinetics, Gelling Additives, and Equipment Boundaries

Dissolution of PVA is a two-stage process: suspension in cold water to separate granules, followed by heating to complete solvation of crystalline domains. Partially hydrolysed grades with 80–89 mol% hydrolysis dissolve in water at 20–40 °C, whereas fully hydrolysed grades require 80–95 °C for complete dissolution. If fully hydrolysed PVA is added directly to hot water without cold dispersion, granules gelatinise at the surface and form fisheyes. High-shear eductor systems and batch make-down vessels with a lower impeller near the tank bottom reduce this risk. Finished solution viscosity is temperature-dependent; a solution prepared at 90 °C will increase in viscosity as it cools, and the rate of increase is steeper for high-viscosity fully hydrolysed grades than for low-viscosity partially hydrolysed grades. Continuous process operations therefore specify viscosity at the point of use, not at the make-down vessel. This distinction is often missed when a grade chart lists only nominal viscosity at 20 °C.

Although PVA solutions are generally compatible with nonionic surfactants, hard water ions, and low levels of glycol plasticisers, operational boundaries exist. Gelling additives based on borate or certain transition metal salts crosslink PVA and must be metered in a separate low-shear zone. Chlorine-based oxidising agents can degrade PVA and reduce viscosity during extended storage; concentrated mineral acids hydrolyse residual acetate groups and may change effective hydrolysis level. If a formulation requires amine-based additives, specific compatibility testing is required because alkaline conditions can accelerate saponification of residual acetate groups and alter solubility. Under ISO 15023-2, specimens should be prepared and conditioned with controlled moisture because PVA is hygroscopic; stored at RH > 60%, powder can gain moisture and feed poorly in continuous dosing systems.

Incoming lot acceptance for PVA usually specifies viscosity, hydrolysis, ash content, volatile matter, and solution clarity. JIS K 6726 defines test methods for viscosity, saponification degree, ash, and volatile content. Batch-to-batch viscosity variation within a grade band can shift downstream rheology, particularly in high-viscosity grades where a ±2 mPa·s lot difference produces a measurable change in low-shear solution viscosity at 10 wt%. Hydrolysis variation of ±0.5 mol% may alter cold-water dissolution, cloud point, and surfactant behaviour. Because nominal viscosity is measured on a 4% solution, lot-specific variation in moisture affects as-is powder weight. Powder stored above 60% RH can show apparent viscosity lower than the certificate of analysis because polymer weight fraction was not corrected for moisture. Weighing by dry solids and checking moisture by loss-on-drying is mandatory for high-viscosity grades used in controlled coating thickness. Conversion lines that blend by weight only, without viscosity-adjusted solids, may experience coating-weight drift. For medical or food-contact grades, vendor documentation must include migration test data or reference a specific regulatory listing; the grade chart alone does not establish compliance under FDA 21 CFR 175.105, FDA 21 CFR 176.170, or EU Regulation 10/2011. The final selected grade is therefore qualified against the process window: solution make-down temperature, hold time, coating or mixing shear rate, drying profile, and end-use environmental humidity.