Álcool polivinil PVOH 823G

Polyvinyl Alcohol PVOH 823G is a partially hydrolyzed grade with a degree of hydrolysis in the range 87.0–89.0 mol% and a 4% aqueous solution viscosity of 23.0–29.0 mPa·s at 20°C measured per Brookfield LVDV methodology (ASTM D3596). Volatile content is ≤5.0 wt%, and sulfated ash residue holds at ≤0.5 wt%. Produced by the alkaline alcoholysis of vinyl acetate homopolymer, the grade retains sufficient residual acetyl content to enable cold-water solubility—complete dissolution at 10–15°C under low-shear agitation—and surfactant-like interfacial activity that is absent from fully hydrolysed (≥98 mol%) PVOH types. These characteristics establish a performance profile weighted toward intermediate adhesive strength, controlled water sensitivity, and film flexibility, situating 823G in segments such as paper packaging adhesives, ceramic powder binding, and textile warp sizing where an equilibrium between bond integrity and removability is functionally critical.

How Does Partial Hydrolysis Alter the Performance Envelope Relative to Fully Hydrolysed PVOH?

The defining structural difference is the residual acetate group distribution along the vinyl alcohol backbone. At 87.0–89.0 mol% alcoholysis, the polymer chain presents a random or block-like acetate sequence that sterically interrupts inter-chain hydrogen bonding. This disruption suppresses crystallinity to 25–35% (as measured by differential scanning calorimetry against a fully hydrolysed reference at 45–55%), lowers the glass transition temperature to approximately 45–50°C, and increases equilibrium moisture uptake at 50% RH to 8–10 wt%. Cold-water solubility—a direct consequence of reduced crystallite size—permits processing without thermal input, while the hydrophobic acetate moieties impart a moderate surface activity that reduces the aqueous solution surface tension to 48–52 mN/m at 1% concentration (ASTM D1331, Du Noüy ring method). In bulk film form, PVOH 823G delivers a tensile strength of 40–50 MPa and elongation at break of 150–250% (ASTM D882, 50 µm cast film, 50% RH conditioning), whereas its fully hydrolysed counterpart reaches tensile values above 60 MPa with elongation below 100%. The trade-off is a pronounced sensitivity to liquid water: 823G films dissolve fully in 20°C water within 60–120 s, while 98 mol% films require temperatures exceeding 80°C for equivalent dissolution.

Addition of 2.0–4.0 wt% PVOH 823G to paper sack and carton-sealing adhesives based on borax-modified dextrin or glyoxal crosslinkers extends wet tack development time by 20–40 s compared to starch-only controls, as tracked by loop-tack adhesion (ASTM D6195). The mechanism involves PVOH chain entanglement and hydrogen bonding with cellulosic furnish, which elevates the initial shear adhesion failure temperature (SAFT) of the joint. However, the process window narrows sharply with borax addition. At sodium tetraborate decahydrate levels above 0.5–1.0 wt% on dry binder, an ionic gelation threshold is crossed: Brookfield viscosity at 10 rpm can spike from 3,000–5,000 mPa·s to over 50,000 mPa·s within 30 min, forming a chelate network that is thermodynamically irreversible at ambient shear. In roll-coating equipment using enclosed doctor chambers, this gel body leads to filtration blinding of 100 µm stainless-steel wedge-wire screens and starved coating weight fluctuation exceeding ±1.5 g/m². Maintenance logs from production-scale adhesive coaters operating at 120–180 m/min web speed document that gel-related screen replacements triple in frequency when 823G additions exceed 3.5 wt% without proportional borax reduction. Process engineers mitigate this by maintaining bath temperature at 25–35°C and incorporating 0.1–0.3% polyether-siloxane defoamer (active solids on formulation) to counteract the increased surface foaming tendency inherent to the grade’s interfacial activity; without defoamer, entrained air volume fractions above 8% are recorded in the adhesive delivery loop, causing cavitation in positive-displacement pumps.

If PVOH 823G is Substituted for Oxidized Starch in Surface Sizing of Recycled Linerboard

Replacing oxidized starch at the size press with a 6–8 wt% solution of PVOH 823G on a metering-rod applicator raises internal bond strength (Scott Bond, TAPPI T569) by 25–40% and reduces surface picking velocity (IGT AIC2-5) by 0.5–1.2 m/s. The Cobb₆₀ water absorbency (ISO 535) decreases to 18–25 g/m² from typical starch-based values of 30–40 g/m², indicating a significantly tighter sheet surface. Gains in printing ink holdout come with a documented hygro-expansion penalty: sheets conditioned from 50% to 80% RH exhibit a dimensional change of 0.35–0.50% cross-direction, versus 0.20–0.30% for starch-sized sheet, creating curl in lightweight grades below 80 g/m². On an industrial Valmet OptiSizer running at 1,200 m/min, film splitting and misting are observed when the metering rod pressure exceeds approximately 12 kN/m, a threshold that is 15–20% lower than that tolerated by oxidized starch at equivalent solids. The mitigation approach is dynamic dilution to keep application solids below 7.5 wt% and pre-wetting of the running rod with a water spray to prevent dry-film build-up. Wet-end retention effects in closed water loops also require attention: anionic trash carryover from PVOH size press broke can reduce first-pass retention of fine filler by 3–5 percentage points unless countered with a cationic fixing agent dosed at 0.05–0.10% on dry furnish.

Ceramic Green Body Processing with PVOH 823G

In tape casting of alumina substrates (≥96% Al₂O₃, 0.5–1.5 µm d₅₀ particle size), PVOH 823G serves as a temporary binder at addition levels of 1.5–3.5 wt% on dry ceramic mass. The grade’s medium molecular weight builds green body flexural strength to 3–6 MPa (ASTM C1161, three-point bending) without causing lamination interfaces after drying. Thermogravimetric profiles on a 10°C/min ramp in air show decomposition onset at 210–230°C and a residual ash mass at 600°C of 0.2–0.4%—sufficiently low to avoid carbonaceous macro-defects in sintered dielectric layers. High-shear dispersion of the binder with zirconia milling beads (1.0–1.2 mm YTZ®) can, however, induce mechano-chemical chain scission; a 20–30% drop in solution intrinsic viscosity after 30 min of bead milling has been recorded in laboratory batch attritors. For this reason, low-shear planetary mixing, followed by de-aeration under vacuum (50–100 mbar absolute), is recommended for slurry formulation. When combined with ammonium polyacrylate dispersants, the binder’s residual acetyl groups contribute to a weak steric stabilization effect that slightly retards the setting rate, extending the tape casting window by 15–20 min before pseudo-plastic viscosity rise exceeds 1,500 mPa·s at 10 s⁻¹.

Aqueous solutions of PVOH 823G stored without preservative exhibit heavy bacterial growth after 48–72 h at ambient temperatures above 28°C, evidenced by pH drift toward 6.5–6.9 and an unpleasant odour; to maintain product integrity in hold tanks, a broad-spectrum biocide such as Kathon™ LX 1.5% at a dosage of 0.1–0.2% on solution weight must be circulated within 24 h of make-up. Storage drums of dry powder require a sealed, <30°C, <60% RH environment: the powder’s equilibrium moisture content can rise to 3–5 wt% within 8 h of exposure to 70% RH air, causing caking and impairing metered feeding from volumetric screw equipment. Mixing water temperature in batch dissolution should be maintained at 10–20°C initially to prevent the “fish-eye” gelling that occurs when powder contacts hot water directly; full dissolution under gradual heating to 40–50°C over 30–40 min with a high-speed disperser at 800–1,200 rpm yields a clear, particle-free solution. Incompatibilities arise with polyvalent metal salts (aluminium sulphate, calcium chloride) at concentrations above 0.5%, where insoluble PVOH-metal complexes precipitate and can plug inline strainers within 15 min of circulation.