Polyvinyl Alcohol (PVA, CAS 9002-89-5) is a water-soluble vinyl polymer obtained by alcoholysis of poly(vinyl acetate). It is supplied as white-to-off-white granules or powder with particle sizes that typically fall between 100 µm and 500 µm, although spray-dried grades can be finer. The dissolution of PVA in water is governed by the residual acetate content, the number-average degree of polymerization, the particle size distribution, and the shear field applied during make-down. Fully hydrolysed commercial grades with a degree of hydrolysis of 98.0–99.0 mol% and a 4 % aqueous solution viscosity of 20–60 mPa·s at 20 °C under JIS K6726 do not dissolve reliably in cold water; they require a controlled hot-water process in the range 90–95 °C. Fish-eye defects, defined here as translucent gel particles caused by the incomplete dissolution of polymer grains, are the main batch rejection criterion in PVA solution make-down. This dissolution guide details the 90–95 °C process window and the mechanical, thermal, and charging controls that prevent fish eyes.

How is the 90–95 °C dissolution window defined for fully hydrolysed PVA?

The 90–95 °C window is a practical compromise between the thermal energy needed to disrupt crystalline PVA domains and the need to avoid chain scission, excessive foaming, and wall-scale fouling. Fully hydrolysed PVA contains long sequences of hydroxyl groups that form strong interchain hydrogen bonds. In film and fibre X-ray diffraction, the main crystalline reflection appears near 2θ = 19.5–20.1° using Cu Kα radiation, and differential scanning calorimetry on dry PVA shows melting endotherms in the 220–235 °C range. However, water plasticises the amorphous phase and lowers the effective structural relaxation temperature. Below 85 °C, water penetration is limited to amorphous regions and the particle expands as a swollen gel without complete molecular dispersion. Between 90 °C and 95 °C, industrial agitation supplies sufficient heat transfer and boundary-layer thinning for dissolution to reach a haze-free endpoint. Operation above 95 °C is not generally beneficial: the incremental diffusion gain is small, while evaporation from open atmospheric tanks, foam stabilisation, and the risk of thermal hydrolysis increase. Jacketed vessel control of ±2 °C across the solution is therefore applied in the dissolving tank.

The hold time at 90–95 °C is not uniform across grades. Low-viscosity fully hydrolysed PVA, with a 4 % viscosity below 10 mPa·s, may clarify in 30–45 min. High-viscosity grades at 45–60 mPa·s require 45–90 min and may demand a lower final concentration or staged powder addition. The exact endpoint is grade-dependent, and published data for each specific configuration are limited; therefore, production batches are qualified by filter-residue and optical film checks rather than by temperature alone.

Batch make-down in temperature-controlled 316L stainless steel vessels separates the wetting stage from the high-temperature dissolution stage. Dry PVA is not charged directly into water that is already at 90–95 °C, because rapid surface gelation forms fish eyes. Instead, the powder is dispersed in water at 20–35 °C, then the jacket temperature is ramped at 0.5–1.0 °C/min to 90–95 °C. Table 1 gives a batch sequence for a 1 000 L atmospheric vessel equipped with a bottom-mounted axial-flow impeller.

The cold dispersion step uses water below 35 °C to wet the particle surface without creating the hot, high-viscosity gel skin that blocks water transport. Agitation during powder addition should produce a visible vortex limited to 5–10 % of the liquid height to avoid air entrainment. Typical vendor technical bulletins for 1 000 L vessels recommend impeller tip speeds in the range 0.5–2.5 m/s during dispersion and 1.0–3.0 m/s during hold, with lower speeds for high-viscosity grades to avoid mechanical degradation.

Fish-eye defects, dissolution front kinetics, and corrective charging practice

Fish-eye formation is a diffusion-limited wetting problem rather than a simple solubility failure. When a dry PVA particle contacts water, water molecules penetrate the grain surface and plasticise a thin polymer layer. If the local polymer concentration in that layer becomes high, the zero-shear viscosity rises abruptly and creates a gel skin whose boundary-layer viscosity is orders of magnitude above the bulk solution. This skin retards further water transport to the dry core. The exact concentration at which the surface film becomes diffusion-limiting varies with degree of hydrolysis and molecular weight; published data across all industrial powder grades are limited. In industrial practice, the dissolution endpoint is therefore controlled by feed rate, particle separation, temperature ramp, and shear rather than by a single calculated diffusivity.

Finely divided powder wets more rapidly than coarse granules but also tends to form lumps if added too quickly. Dry powder should be drawn into the liquid through an eductor or vacuum diluter, or added slowly at the sidewall where shear is highest. Charging intervals of 5–10 min between additions prevent localised concentration build-up near the feed point. If fish eyes persist after the standard hold, mechanical rework through a high-shear inline rotor-stator at 60–70 °C can break gel skins, but such rework must be limited because excessive high-shear input can introduce microbubbles and reduce solution viscosity by mechanical chain scission.

When the 90–95 °C process fails for partially hydrolysed PVA grades

For grades with residual acetate contents of 10–13 mol%, the 90–95 °C process is not required and may be counterproductive. The acetate groups interrupt interchain hydrogen bonding and reduce crystallinity; partial grades dissolve under moderate shear at 20–35 °C within 20–40 min. Heating these grades to 90–95 °C can accelerate deacetylation, shift pH, increase foaming, and discolour the solution. For modified PVA grades, such as carboxylated PVA or acetoacetylated PVA, thermal sensitivity is also higher, and process temperatures should follow the manufacturer’s dissolution specification rather than the standard fully hydrolysed hot-water route. Incompatibilities include borate and certain multivalent salts, which can form reversible or irreversible gel networks with PVA at alkaline pH; such additives must not be introduced into the dissolving tank without compatibility testing.

Batch charging without hot-water direct contact at the powder surface

Under production conditions, the most reproducible feed pattern is to charge 80–90 % of the total water at 20–35 °C, begin agitation, and then introduce the weighed PVA portion gradually over 10–20 min. The remaining water is added through the vessel spray ring or bottom manifold after the powder has been thoroughly wetted. This sequence prevents dry powder from floating on the surface and avoids hot spots near the jacket. For twin-tank continuous make-down systems, a cold-water pre-slurry at 5–10 % solids is pumped into a jacketed in-line heater held at 90–95 °C; residence time is set between 20 min and 60 min depending on grade. Published data for specific continuous configurations are limited, so start-up validation relies on downstream filter pressure increase and optical film defect counts.

Finished PVA solutions are prone to microbial growth and gelation during storage. Storage at 10–35 °C in closed 316L or HDPE tanks is typical. Preservatives may be required for storage beyond 24 h; the choice must be validated for interaction with PVA and final product. Filtration through a 40–100 µm bag or cartridge before use removes residual fish eyes and incidental contamination. Degree of hydrolysis and viscosity are measured by titration and rotational viscometry according to JIS K6726; property determinations for specification compliance can follow ISO 15023-2. The filtrate must not be mixed with organic solvents that are nonsolvents for PVA, such as methanol beyond the concentration limits defined by the ternary PVA-water-solvent phase system.