Wanwei PVA 24-88 (L) (PVA 088-50)

Commodity-grade partially hydrolyzed polyvinyl alcohol marketed under the designation Wanwei PVA 24-88(L) — equivalently coded as PVA 088-50 in some regional indices — is produced by controlled alcoholysis of a polyvinyl acetate precursor with a mean degree of polymerization centered at approximately 2400. The suffix “88” denotes a nominal degree of hydrolysis in the range 87.0–89.0 mol%, yielding a cold-water-soluble film former that retains a fraction of acetate sidegroups sufficient to depress crystalline ordering while preserving strong intra- and inter-molecular hydrogen bonding. A 4% (w/w) aqueous solution at 20°C exhibits a Brookfield viscosity (spindle No. 2, 20 rpm) typically between 48 and 55 mPa·s, as per ISO 15023-2, designating this grade as a high-viscosity, partially-hydrolyzed polyvinyl alcohol. The “(L)” identifier often denotes a low-ash, low-methanol variant suited to adhesives and aqueous coatings where ionic contaminants must be minimized to avoid interference with surfactant or crosslinker stability. The product is supplied as a white-to-creamy granular powder with a bulk density of 0.45–0.55 g/cm³, a volatile matter content not exceeding 5.0% (ISO 3251, 105°C, 3 h), and a residual sodium content controlled to below 0.7% as Na₂O via ISO 3451-1. Aqueous solutions of 4% by weight display a pH between 5.0 and 7.0 and a light transmittance at 650 nm of at least 90%, as determined by the methodology of JIS K6726.

How Does 24-88(L) Differ from Lower-Molecular-Weight Partially Hydrolyzed Grades?

Compared with the medium-viscosity type 17-88 (nominal viscosity 20–28 mPa·s at 4%) or the low-viscosity 05-88 (5.0–6.5 mPa·s), the 2400 DP backbone of 24-88(L) delivers markedly higher film tensile strength—above 40 MPa at 23°C and 50% RH when tested per ASTM D882-18—and superior cohesive energy in remoistenable adhesives. The high molecular weight restricts chain mobility during drying, producing a more oriented film structure that resists creep under sustained load ≥2.0 MPa. In contrast, 17-88 is preferred where lower solution viscosity facilitates high-solids sizing formulas (20–25%) with reduced wet pickup, while 05-88 serves niche release coating roles requiring minimal film strength. Unlike fully hydrolyzed PVA such as 24-99 (>98 mol% hydrolysis), the 24-88(L) retains sufficient acetate content to remain fully soluble in cold water without the need for hot cook-up above 80°C, but the film’s water resistance and gas barrier properties are correspondingly lower: oxygen permeability at 0% RH is approximately 8–12 cm³·µm/m²·d·kPa compared with 3–5 cm³·µm/m²·d·kPa for the fully hydrolyzed analog.

Water Solubility Profile and Viscosity Build in Aqueous Systems

At 20°C the polymer hydrates via swelling of amorphous regions without requiring external heat, reaching 90% dissolution within 30 min under mechanical agitation at 200–300 rpm in a baffled vessel. However, complete molecular disentanglement and plateau viscosity are obtained only after 60–90 min of gentle stirring, since prolonged high-shear dispersion above 1000 s⁻¹ can induce foaming and entrap air that adversely affects film clarity. Aqueous solutions of 24-88(L) at concentrations above 8% display pronounced pseudoplasticity; Brookfield LVT readings taken at 6 rpm can be 2.5 to 3 times higher than at 60 rpm. This shear-thinning character is beneficial in roller coating and size-press applications where high transfer efficiency depends on rapid viscosity drop under shearing conditions of 10⁴–10⁵ s⁻¹. Stock solutions formulated at 10% solids and stored at 25°C are prone to skin formation within 48 h unless a small headspace preservative (e.g., 0.1% sodium benzoate) is added. Viscosity drift during storage is typically ≤3% over 72 h when the container is sealed and kept away from direct UV exposure, which can trigger chain scission and gradual molecular-weight reduction.

When Formulating with Glyoxal or Borate Crosslinkers

Addition of glyoxal (0.5–2.0% on PVA weight) to a 4% 24-88(L) solution adjusted to pH 4.0–4.5 with citric acid initiates acetal formation, raising the gel point after 4–6 h at 55°C. Pot life at 25°C without pH adjustment remains 8–12 h, permitting single-component adhesive formulations for spiral-wound paper tubes. Borax (sodium tetraborate decahydrate) at concentrations as low as 0.3% induces immediate thixotropic gelation through diol complexation; the gel strength, measured by a falling ball viscometer, increases exponentially with a borax/PVA molar ratio above 0.05. The partially hydrolyzed grade tolerates a slightly wider crosslinker window than fully hydrolyzed counterparts because residual acetyl groups sterically moderate the diol network, but mixing with amine-based additives must be avoided when glyoxal is present to prevent premature Schiff-base crosslinking that can occur within 5 min. Industrial slushing operations on a 1000-L mixing tank with a turbine impeller at 80 rpm observe that local over-concentration of crosslinker near the dip tube often leads to gel grain formation unless the crosslinker is fed through a static mixer directly into the recirculation loop at a rate ≤0.2 L/min.

Thermomechanical Processing Demands in Film Casting and Extrusion Coating

Melt processing of 24-88(L) without an external plasticizer is constrained by a narrow thermal window between the crystalline melting point (180–190°C by DSC, 10°C/min) and the onset of thermal degradation, which generates acetic acid and unsaturated aldehydes detectable by headspace GC at 205–210°C. On a co-rotating twin-screw extruder with 40:1 L/D and a vented barrel (zone profile 170/185/195/200/195°C), incorporation of 10–15 phr glycerol plus 2–3 phr stearic acid salt lubricant reduces the melt pressure at the die to 12–15 MPa while maintaining a melt strength sufficient for cast film take-up at 15–20 m/min. The low-ash variant 24-88(L) is preferred because residual sodium acetate above 0.8% catalyses depolymerization, shortening the safe residence time at 200°C to less than 90 s. When co-extruded with ethylene-vinyl alcohol as a tie-layer on a three-layer blown film line with a spiral mandrel die (300 mm diameter, 1.5 mm gap), the PVA layer thickness is typically maintained at 15–25 µm to prevent interlayer delamination arising from differences in moisture expansion coefficients — the PVA layer can swell by 8–12% in the machine direction upon water contact, versus 2–3% for the polyolefin substrate.

In high-speed warp slashing operations running at 150–300 m/min, the 24-88(L) grade is applied as an 8–10% aqueous size liquor maintained at 85–90°C in the size box, which pressurizes a squeeze nip at 12–18 kN to achieve a wet pickup of 95–120% on cotton/polyester blend yarn. The latent heat of evaporation during dryer contact (cylinder surface temperature 120–135°C) must be balanced against film skinning: if the surface temperature of the size film exceeds 100°C before the core moisture falls below 15%, a brittle shell forms that later dislodges as dust on the loom, lowering weaving efficiency by 3–5% on air-jet looms at 800 rpm. Desizing of woven grey fabric is subsequently accomplished using oxidative pads with hydrogen peroxide 2.0–3.0% owb at 90°C and pH 11.5, achieving residual size content ≤0.2% within 20 min, as verified by trichromatic iodine staining per AATCC 94-2012. The grade’s high DP minimizes re-deposition of size oligomers during the caustic scouring that follows, a distinct advantage over lower-molecular-weight alternatives whose fragments can redeposit on polyester fiber surfaces and resist rinsing.

Paper surface sizing trials on a puddle-type size press running a 6% solution at 55°C and 80–120 m/min show that 24-88(L) delivers an IGT pick strength improvement of 35–45% over unsized base paper of 70 g/m² when the dry pick-up is held at 1.8–2.2 g/m² per side, measured by ash analysis. Blending with oxidized starch at a 1:3 ratio reduces total sizing cost by 18–22% with a marginal loss in Cobb60 water absorption from 22 g/m² (pure PVA) to 28 g/m² (ISO 535:2014). The partially hydrolyzed grade does not re-crystallize as rapidly as fully hydrolyzed types, so the sized sheets retain higher fold endurance after conditioning at 10% equilibrium moisture, a requirement for multi-pass converting on carton erecting lines that impose 200–250 crease cycles.