PVOH 745'in

Polyvinyl alcohol grade 745 is a partially hydrolysed, medium-viscosity resin characterised by a nominal degree of hydrolysis of 74.0 mol% (73.0–75.0 mol%) and a viscosity of 4.8–5.8 mPa·s measured as a 4 % aqueous solution at 20 °C per JIS K6726. The residual acetate content imparts controlled hydrophilicity, lowering the dissolution temperature relative to fully hydrolysed grades while suppressing gelation tendency in cold-water dispersions. Ash content is limited to 0.4 % maximum (ISO 3451‑1), volatile matter to 5.0 % maximum, and pH of the aqueous solution falls between 5.0 and 7.0. This grade serves as a functional binder, film former, and sizing agent across paper conversion, adhesive compounding, textile warp sizing, and temporary protective coatings, where the balance between cold-water dispersibility and moderate water resistance is critical.

What Distinguishes Partially Hydrolysed PVOH Grade 745 from Fully Hydrolysed Counterparts?

Partially hydrolysed grades such as 745 depart from fully hydrolysed polyvinyl alcohols (98–99 mol% hydrolysis) in three operationally decisive properties: aqueous solubility mechanism, thermal processability, and interaction with cellulosic substrates. The blocky distribution of residual acetyl groups along the chain lowers the degree of crystallinity, shifting the onset of solution precipitation to lower temperatures. Literature places the lower critical solution temperature (LCST) for a 74 mol% hydrolysed polymer at approximately 35–45 °C; fully hydrolysed types lack a practical cloud point below 100 °C. Consequently, grade 745 dissolves completely in water at 60–70 °C, whereas a fully hydrolysed grade of comparable molecular weight requires 85–95 °C for full dissolution. Films cast from 745 exhibit lower tensile strength—typically 40–55 MPa at 23 °C and 50 % RH per ASTM D882—compared with 60–80 MPa for a fully hydrolysed, similar-viscosity grade such as 505. However, the elongation at break increases from 100–150 % to 200–300 %, favouring applications demanding flexibility over tensile stiffness. The reduced hydrogen‐bonding density also depresses the melt temperature from 220–230 °C (fully hydrolysed) to 180–200 °C, substantially widening the safe processing window for extrusion and injection moulding.

Processing Window and Melt Rheology Constraints

Thermoplastic conversion of grade 745 demands strict moisture management pre‑processing. The resin must be dried in a desiccant dryer at 80 °C for a minimum of 4 h to achieve a residual moisture content below 0.3 wt%. Moisture exceeding 0.5 wt% induces hydrolytic chain scission during melting, generating acetic acid and resulting in bubble‑ridden extrudate with a reduced intrinsic viscosity. Single‑screw extruders with a length‑to‑diameter ratio of 24:1 to 30:1 and a barrier screw design are preferred; the metering zone should be maintained at 190–220 °C, and the melt temperature must not exceed 225 °C. Beyond 230 °C, differential scanning calorimetry records an abrupt exothermic onset attributed to elimination of water and acetic acid and the formation of conjugated polyene sequences, visibly manifesting as amber discoloration. In injection moulding, clamp forces of 3–5 kN/cm² projected area and a mould temperature of 30–50 °C are typical. Because the melt viscosity at the recommended processing shear rates of 100–500 s⁻¹ lies in the range of 200–600 Pa·s, cold‑runner systems must be sized to avoid pressure drops exceeding 30 MPa. Processors blending 745 with plasticisers such as glycerol (10–20 phr) or trimethylolpropane will observe a viscosity depression of 20–40 % and a corresponding reduction of the solid‑state glass transition temperature from 58 °C to as low as 25 °C (measured by DMA per ASTM E1640). The addition level is capped at 25 phr to prevent phase exudation during long‑term storage under humid conditions.

In aqueous solution preparation, the resin must first be dispersed under vigorous agitation in water at room temperature, then heated with continuous shear to 65–70 °C to achieve full dissolution. Direct addition of dry powder to hot water above 40 °C causes premature surface swelling and agglomerate formation—commonly termed “fisheyes”—that will not dissolve without sustained high‑shear mixing at tip speeds above 15 m/s. Solutions held at 40–45 °C for more than 6 h may begin to exhibit turbidity due to the LCST; cooling to 25 °C restores clarity, but repeated thermal cycling past the cloud point gradually increases insoluble fractions. For adhesive formulations, the working viscosity is often adjusted to 1 000–3 500 mPa·s (Brookfield, 20 rpm, 25 °C) to balance penetration into porous substrates with adequate green tack.

When Viscosity at Low Shear Limits Metering in Roll Coating

Roll coating lines applying PVOH 745 as a pigment binder or surface size encounter a critical low‑shear viscosity threshold at 0.1–1 s⁻¹. At 10 % solids, the shear‑rate‑dependent viscosity drops from approximately 2 500 mPa·s at rest to below 300 mPa·s under the dynamic gap shear of a two‑roll nip. If the solution temperature rises above the cloud point, phase‑separated micro‑domains increase apparent viscosity by 15–25 %, causing film‑weight drifts exceeding ±2 g/m² on lightweight papers (30–45 g/m²). Closed‑loop chiller units maintaining the coating pan at 28–32 °C are therefore mandatory. Foaming is controlled through addition of silicone‑free defoamers at 0.05–0.1 % on wet coating weight; excessive defoamer causes cratering visible under ASTM D4062 levelling evaluation.

Adhesion to clay‑coated paperboard as measured by TAPPI T 569 internal bond strength reaches 180–220 J/m² for PVOH 745 applied at 2.5 g/m² dry coat weight, outperforming standard oxidized starch by approximately 30 %. The improvement is attributed to specific hydrogen‑bonding interactions between the residual acetyl oxygen and the clay platelet hydroxyls, which are absent in fully hydrolysed PVOH. In textile warp sizing, 8–10 % concentration at 55 °C yields yarn tenacity increases of 12–18 % (assessed by ISO 2062) and hairiness reduction of 40–50 % (Zweigle tester), with the added advantage that desizing requires water at only 60 °C, substantially lower than the 85–90 °C needed for fully hydrolysed sizes.

PVOH 745 is also utilised as a water‑soluble support material in extrusion‑based additive manufacturing, where its interlayer adhesion and dissolution kinetics are critical. Filament dried to below 0.2 % moisture and extruded at 195±5 °C through a 0.4 mm nozzle onto a bed at 45–55 °C yields a void‑free support structure that dissolves completely in agitated water at 35–40 °C within 90–120 min. Build‑plate adhesion is enhanced by pre‑applying a PVOH slurry of the same grade. Variants of this grade have been formulated into filament with a 1.27–1.29 g/cm³ density and a melt flow index (210 °C, 2.16 kg) of 8–12 g/10 min according to ISO 1133‑1.

Storage of the powder requires sealed containers at 15–30 °C and relative humidity below 60 %; exposure to 70 % RH for 48 h increases moisture content to 4–5 %, necessitating re‑drying before melt processing. The grade is incompatible with amine‑based crosslinkers that catalyse premature acetal formation, leading to gel particles that plug 25–50 µm filtration units in coating head supply lines. No adverse reactivity is reported with common polyol plasticisers, borates, or non‑ionic surfactants up to 3 % w/w admixture.