The product specification set PVA Specification: Degree of Hydrolysis & Viscosity (17-99, 24-88) defines two polyvinyl alcohol grades through the viscosity of a 4% aqueous solution at 20°C and the mole percentage of vinyl acetate units saponified to vinyl alcohol units. The numeric prefix in 17-99 denotes a nominal solution viscosity of 17 mPa·s; the suffix denotes a nominal degree of hydrolysis of 99 mol%. The numeric prefix in 24-88 denotes a nominal solution viscosity of 24 mPa·s; the suffix denotes 88 mol% hydrolysis. Because the first segment is a nominal value rather than a single-point measurement, commercial certificates of analysis list a tolerance band, typically 16.0–20.0 mPa·s for 17-99 and 22.0–26.0 mPa·s for 24-88, with hydrolysis windows of 98.0–99.0 mol% and 87.0–89.0 mol%. The specifications are controlled under JIS K6726 and ISO 15023-2:2019, and the resulting differences in hydrogen bonding, crystallinity, dissolution temperature, and interfacial activity determine grade selection across emulsion polymerization, paper sizing, textile warp size, water-soluble film, and adhesive compounding.

How Are Degree of Hydrolysis and Viscosity Verified for 17-99 and 24-88?

Degree of hydrolysis is determined by saponification of residual acetate groups with excess sodium hydroxide and back-titration with hydrochloric acid. A fully hydrolysed 17-99 grade requires residual acetate content below 2 mol%, with typical commercial material falling within 98.0–99.0 mol% hydrolysis. Partially hydrolysed 24-88 retains approximately 12 mol% acetate functionality, which is specified at 87.0–89.0 mol%. The titration is performed on a dry polymer sample with an automatic potentiometric titrator equipped with a pH electrode, and the result is reported on an ash-free basis to correct for sodium acetate residues from the manufacturing process.

Solution viscosity is measured on a 4% dry-solids aqueous solution after complete dissolution in deionised water. For 17-99, dissolution in a jacketed glass reactor is held at 90–95°C for 30–45 min with a low-shear anchor agitator at 50–100 r/min. For 24-88, the same concentration can be prepared at 70–80°C with a hold time of 30 min. The solution is cooled to 20°C ± 0.1°C and conditioned for 1 h before measurement. Viscosity is read on a Brookfield LV rotational viscometer using spindle No. 1 at 60 r/min after 30 s of rotation, in accordance with JIS K6726 and ISO 15023-2:2019. The higher measured viscosity for 24-88 correlates with a higher molecular weight fraction; viscosity is not a direct molar mass assay but serves as a reproducible rheological proxy for degree of polymerisation.

The hydrodynamic volume of PVA in water is not a linear function of hydrolysis. At 88 mol% hydrolysis, the residual acetate groups increase chain flexibility and lower the theta temperature, so 24-88 solutions remain optically clear at lower temperatures than 17-99 solutions of the same nominal viscosity. Surface tension of a 4% solution at 20°C measured by the Du Noüy ring method is typically 45–50 mN/m for 17-99 and 42–47 mN/m for 24-88; the difference is small and not sufficient to replace a wetting agent in high-speed coating. More significant is foam behaviour: fully hydrolysed 17-99 produces a stable closed-cell foam under high-shear mixing, while 24-88 produces larger bubbles that collapse more rapidly. This distinction is controlled by the surface rheology of adsorbed acetate groups and can be quantified by dynamic surface tension measurements at 1 Hz using a bubble-pressure tensiometer.

When a 10 wt% aqueous solution of 17-99 is prepared in a production-scale jacketed stainless-steel vessel equipped with a three-blade propeller and a scraping surface agitator, the critical processing risk is insufficient hold time at dissolution temperature. Batches discharged before the solution clears can carry gel micro-particles into downstream coating or casting equipment, producing surface defects on film substrates. The solution is heated to 90–95°C and held for at least 30 min; viscosity is then checked with a falling-ball viscometer or a portable rotational viscometer before transfer through a 100 µm filter bag. For 24-88, the lower hydrolysis level disrupts crystallisation, allowing dissolution to progress at 60–70°C; however, dissolved 24-88 retains higher equilibrium water absorption under humid storage and can exhibit viscosity drift if the solution is left at pH < 3 or pH > 9 for more than 8 h. Pre-drying of dry polymer is required when storage relative humidity exceeds 60%, because the powder can absorb atmospheric moisture, altering the dry-solids basis and shifting the apparent viscosity of subsequent batches.

Viscosity Stability and Shear History in Paper Surface Sizing

In metering size-press applications, the process viscosity of a 10–12% solids solution of 17-99 is typically maintained between 50 and 80 mPa·s at the application temperature, which may be 60–70°C on a film press or bent-blade coater. The solution is delivered to the nip through a positive-displacement pump and is filtered through a 150 µm screen. A fully hydrolysed grade reduces water sensitivity of the sized paper surface, as measured by a Cobb test according to ISO 535; the 60 s Cobb value is often lowered by 15–30 g/m² relative to an oxidised starch control at the same coat weight. The limitation of 17-99 in this operation is shear-induced molecular orientation in the metering nip; if the film split generates significant extensional shear, older solutions that have begun to form hydrogen-bonded clusters may deposit on the applicator roll. 24-88 is less prone to cluster formation because the residual acetate groups disrupt interchain association, and it is used where strike-through control must be balanced against lower water resistance.

At a given concentration, 24-88 produces a higher solution viscosity than 17-99, so the solids content is commonly reduced by 1–2 wt% to match the same Brookfield viscosity. In a twin-roll size press running at 800–1200 m/min, viscosity fluctuations above ±5% of the set point correlate with coat-weight variation in the cross-machine direction. On-line process refractometers are calibrated against laboratory solids with a Karl Fischer moisture balance; this prevents dry-solids drift when recycled size is returned from the applicator pan to the run tank.

If 24-88 Is Used as a Protective Colloid in Continuous Emulsion Polymerization

In continuous vinyl acetate emulsion polymerization, 24-88 is introduced into the reactor pre-mix at 2–5 wt% based on the monomer phase. The 88 mol% hydrolysis level provides a balance between water solubility and interfacial adsorption; the residual acetate segments are sufficiently hydrophobic to anchor at the monomer-water interface, while the hydroxyl sequence extends into the aqueous phase and stabilises the growing polymer particle. The reactor is typically a jacketed stainless-steel stirred tank of 5,000–20,000 L capacity, fitted with a 45° pitched-blade turbine and operating at 60–70°C under reflux or at 1–3 bar gauge pressure. Particle size is monitored on-line by dynamic light scattering or off-line by laser diffraction according to ISO 13320:2020. Batches produced with 24-88 are specified for final Brookfield viscosity in the range of 3,000–12,000 mPa·s, with residual vinyl acetate monomer below 0.5 wt% determined by gas chromatography according to ISO 13741-2.

The process conflict in continuous operation is the tendency of fully hydrolysed 17-99 to form a thicker adsorbed layer and increase latex viscosity at high conversion. In contrast, 24-88 yields lower colloidal stability against high-ionic-strength additives but improves freeze-thaw resistance and pigment compatibility. Trials on a pilot continuous loop reactor with a mean residence time of 30–45 min have shown that replacing 17-99 with 24-88 at the same colloid loading can lower final latex viscosity by 10–20%, although published data for this specific configuration is limited. The operational boundary is electrolyte concentration: above 0.5 mol/L calcium chloride equivalent in post-compounding, 24-88-stabilised lattices may exhibit coagulum formation unless an anionic surfactant is co-fed at 0.5–1.0 wt% on latex solids.

Tensile Strength Increases as Hydrolysis Moves from 88 to 99 mol%

Cast films of 17-99 are evaluated under ISO 527-3 at 23°C and 50% relative humidity. The higher degree of hydrolysis increases crystallinity after drying, producing higher tensile strength and lower elongation at break compared with 24-88. A 30 µm film of 17-99 conditioned for 24 h typically exhibits tensile strength above 50 MPa, while 24-88 films may fail below 30 MPa under the same conditions depending on plasticizer content. The difference is not solely molecular weight; the 88 mol% hydrolysis level reduces hydrogen-bonded crystallite formation, which also raises equilibrium moisture uptake. Water vapour transmission is measured according to ASTM E96/E96M-22 at 38°C and 90% relative humidity. Fully hydrolysed 17-99 films show lower permeability below 100 g·mm/m²·day, but they also require higher dissolution temperature for subsequent use in detergent pods.

The operational boundary for 17-99 film casting is the gelation limit during solution preparation and drying. Above 15 wt% solids, a 17-99 solution may form a skin on the surface of a slot-die coating pan at 60°C, causing die-lip deposition. 24-88 can be cast at higher initial solids, but the dried film absorbs more water, so horizontal form-fill-seal packaging lines specify storage of finished film at ≤55% relative humidity to prevent blocking. Plasticizer addition is required for both grades when elongation at break must exceed 150%; glycerol is typically added at 10–20 phr, and the resulting mechanical values are validated by ASTM D882-18 rather than ISO 527-3 to accommodate thin-film grip conditions.

For food-contact films and coatings, the selected 17-99 or 24-88 grade must be listed under the relevant polymer exemption in FDA 21 CFR 175.300 or 21 CFR 176.170 and comply with migration limits in EU 10/2011; this is a regulatory constraint, not a performance guarantee.

In textile warp sizing, 17-99 is applied from a single-box slasher at 8–10% solids and 80–90°C, with squeeze-roll pressure adjusted to leave 8–10% dry size on yarn. The higher hydrolysis grade provides lower dusting on high-speed air-jet looms running at 800–1000 rpm. 24-88 is preferred for hydrophobic filament polyester because the residual acetate groups improve adhesion to low-polarity fibre surfaces and reduce shedding. The main incompatibility is with borate ions; addition of 0.1 wt% disodium tetraborate to a 4% PVA solution can increase viscosity by an order of magnitude, so borate-containing additives must be segregated from both grades in sizing and adhesive operations. In polyvinyl acetate adhesive compounding, 24-88 is used as a rheology modifier at 2–4 wt% dry basis, where the 24 mPa·s nominal viscosity provides higher low-shear body than 17-99 but requires a preservative when stored in non-sterile water for more than 72 h.