PVAc Rewettable Adhesive

Polyvinyl acetate rewettable adhesive, designated PVAc RW-4026, is supplied as a waterborne polyvinyl acetate homopolymer emulsion stabilized with a partially hydrolyzed polyvinyl alcohol protective colloid. Nominal solids are 45% by mass, pH is 4.0–5.5, and Brookfield RVT viscosity is 12,000–18,000 mPa·s at 25 °C using spindle 5 at 20 min⁻¹. The dried film becomes tack-free at ambient temperature and is re-activated by application of water or a diluted nonionic surfactant solution during converting operations. Residual vinyl acetate monomer, determined by gas chromatography with flame ionization detection, is controlled below 0.1% by mass. Minimum film-forming temperature is 8 °C. The product is preserved with a formaldehyde-free isothiazolinone system and has a density of 1.06–1.10 g/cm³ per ISO 2811-1:2023. The dried film has a glass transition temperature of 5–12 °C measured by differential scanning calorimetry at 10 K/min, which balances cold flexibility with blocking resistance.

The formulation is differentiated from pressure-sensitive adhesives by the absence of dry-state tack. Conditioned stacks of clay-coated board coated at 4 g/m² dry film show no blocking after 24 h at 40 °C and 70% relative humidity when evaluated per TAPPI T 477. Compared with dextrin or starch-based remoistenable adhesives, PVAc RW-4026 typically requires lower applied coat weight, 3–6 g/m² dry film, while retaining fibre-tearing bonds on recycled board. Published data for direct quantitative comparisons with specific modified-dextrin formulations is limited; field measurements on window-patch envelope lines show a reduction from 8–12 g/m² wet film to 5–7 g/m² wet film at equivalent bond strength. The product does not provide permanent tack after drying and should not be specified where pressure-sensitive peel adhesion above 0.5 N/25 mm is required.

Which adhesive mechanisms explain the difference between rewettable PVAc and dextrin remoistenables?

The difference is process-linked. Dextrin remoistenables form a brittle amorphous carbohydrate film that rehydrates quickly but has limited dry-film flexibility and high equilibrium moisture uptake. PVAc RW-4026 forms a coalesced thermoplastic film. Re-wetting proceeds by water penetration into the film–substrate interface and plasticization of the polyvinyl acetate phase, not by bulk dissolution. This permits bond development at 0.5–2.0 s after remoistening on high-speed lines, provided the water dose is 5–10 g/m². Dry-film elongation measured on free films is 120–180% at 23 °C per ASTM D638-14 Type IV specimens, compared with 2–8% for unplasticized dextrin films. The larger elongation reduces cracking at folded corners and scored creases.

Unlike hot-melt ethylene-vinyl acetate adhesives, the film is not re-activated by dry heat below 150 °C. The bond is re-activated only by water, which limits its use in humid environments but permits dry, non-blocking storage. Compared with solvent-borne polyurethane or acrylic pressure-sensitive adhesives, the product eliminates volatile organic content; VOC content by EPA Method 24 is below 1 g/L, excluding water and exempt compounds. For applications requiring permanent water resistance after set, a crosslinkable PVAc or polyurethane dispersion is used instead.

Storage of PVAc RW-4026 in bulk silos at 20–25 °C keeps viscosity drift below 10% over 6 months. Recirculation loops for drum tote delivery are fitted with low-shear diaphragm pumps; rotary lobe pumps without bypass can generate local shear heating above 35 °C, which accelerates polyvinyl acetate hydrolysis and pH reduction. Frozen material is not recoverable: exposure to -5 °C for more than 16 h produces irreversible grit that blocks 100 µm slot dies. In high-humidity warehouses above 60% relative humidity, coated stacks may require interleaving or lower applied coat weight below 3 g/m² dry to avoid fibre picking at the guillotine cut. For coated paper with low porosity, pre-dampening of the converting floor is not an adequate substitute for controlled rewetting; excess free water on the substrate surface causes adhesive wash-out and bond strength loss.

Microbial spoilage is controlled by the included isothiazolinone package. Dilution with untreated process water can reduce preservative concentration below the minimum inhibitory concentration. If recirculated adhesive remains in lines at 30–35 °C for more than 72 h, pH drop below 3.5 indicates bacterial acidification. The batch should be discarded; neutralization with ammonia produces colloidal instability and is not permitted.

High-shear viscosity reduction governs transfer quality on rotary window patching equipment.

At the application roller nip, the adhesive experiences shear rates estimated at 10³–10⁵ s⁻¹. The emulsion is pseudoplastic: Brookfield viscosity at 20 min⁻¹ is 12,000–18,000 mPa·s, while Hercules high-shear viscosity at 4,400 s⁻¹ falls to 150–300 mPa·s. This shear-thinning behavior permits clean transfer through chrome-plated gravure rolls or rotary stencil heads without misting. On a window patcher with 1,200 mm web width and 3.5 m infrared tunnel, wet film weight is maintained at 30–50 µm for film-to-board lamination. If high-shear viscosity exceeds 350 mPa·s, star-wheel adhesive deposit is incomplete; if below 100 mPa·s, misting increases and film weight drops below 25 µm, causing window film lift at the trailing edge.

After the nip, viscosity recovery must occur within 0.2 s to prevent flow into the window aperture. A structured recovery delay longer than 0.5 s produces feathering and optical defects on polypropylene window film. Laboratory rheometer measurements using a cone-and-plate fixture at 50 mm diameter and 0.5° cone angle confirm thixotropic recovery at 25 °C. Transfer rolls with cell volumes below 8 cm³/m² require viscosity adjustment with deionized water to 10,000–12,000 mPa·s; cell volumes above 15 cm³/m² require the upper viscosity range to avoid flooding.

On paper bag bottom-pasting lines, dilution with water to 38–42% solids is common when using engraved roller applicators. Lower solids reduce roller throwing at 200 m/min line speed but extend drying demand. A twin-tower hot-air dryer with 6 m length and 120 °C air temperature removes free water at 0.4–0.6 kg/m²/min; when dry coat weight increases from 6 g/m² to 10 g/m², tunnel speed must decrease by approximately 35% to maintain final moisture below 5%.

Bond strength on porous kraft paper is measured as fibre tear after 20 s open time and 2 s compression at 0.4 MPa. At 6 g/m² dry film, recycled kraft typically shows 90–100% fibre tear. On high-size-content bleached paper, the same coat weight may drop to 20–40% fibre tear because the adhesive cannot penetrate the internal fibre network before setting; an additional 2–3 g/m² wet film and higher remoistening water dose of 8–12 g/m² are required.

When line speed exceeds drying capacity on high-speed envelope converting lines

On a rotary envelope line running 28,000–32,000 envelopes/h, dwell time in the infrared drying tunnel may fall below 0.8 s. The adhesive is exposed to forced air at 80–100 °C for 0.5–1.5 s. Complete film formation requires removal of water from the coalescing emulsion: at 45% solids and 30 g/m² wet film, the water load is approximately 16.5 g/m². Infrared emitters with peak output in the 2.5–3.5 µm range penetrate the film and heat the interface, but air velocity above 20 m/s can cool the exposed surface enough to delay coalescence. If line speed is increased beyond tunnel capacity, the dry film retains moisture above 5%, and immediate blocking occurs in the delivery section. The remedy is to reduce wet film weight to 25 g/m² or increase tunnel air temperature to 110 °C, provided substrate heat distortion allows. Below 10 °C board surface temperature, coalescence is incomplete and bond strength fails at the window edge.

At line speeds above 350 m/min, the re-wetting station must apply water as a fine mist with droplet size 15–30 µm. Larger droplets produce uneven re-activation and print mottle on overprinted areas. The wetting module should be positioned within 0.3 s of compression; longer delays allow water to evaporate or be absorbed into the board before the adhesive film is fully plasticized. The bond set point at the compression roll is reached at 0.5–1.5 s after remoistening under 0.2–0.6 MPa nip pressure. If compression pressure exceeds 1.0 MPa, adhesive squeezes into the paper pores and the visible bond line becomes glossy.

Application ranges, coat weight, and substrate compatibility limits

PVAc RW-4026 is applied by rotogravure, roller coat, slot die, or rotary screen. Typical dry coat weights are 3–6 g/m² for window patching, 2–4 g/m² for envelope seal flaps, 6–10 g/m² for paper bag bottom pasting, and 4–8 g/m² for label remoistening. The product is not intended for direct food contact but complies with indirect food-contact adhesive requirements under FDA 21 CFR 175.105 for use as a component of finished packaging, subject to the food-type and temperature conditions of the end-use package. For paper and board intended for food contact, the converter must verify overall migration limits under EU Regulation (EC) No 1935/2004 and applicable national provisions.

Compatibility constraints are process-relevant. The emulsion is anionically stabilized. Addition of cationic retention aids, quaternary ammonium biocides, or high-alum papermaking chemicals can coagulate the product. pH below 3.5 accelerates polyvinyl acetate hydrolysis and liberates acetic acid, increasing odour and reducing bond strength; pH above 7.5 thickens the emulsion through base swelling of the polymer particles. Viscosity adjustment should use deionized water only; tap water with hardness above 200 ppm as CaCO₃ may cause local instability. Cleanup before dry-down uses warm water at 35–45 °C. Dried adhesive films require alcohol or ethyl acetate for removal from chrome rolls, but prolonged solvent exposure damages nitrile rubber applicator rolls.

Substrate restrictions include silicone-coated release papers, fluorocarbon-treated papers, and heavily waxed board, where wetting is insufficient and bond strength falls below 0.1 N/mm peel. Corona treatment at 38–42 dyn/cm improves adhesion on polyethylene-coated board but the treatment decays within 24 h; re-wetting must occur on freshly treated stock. Polypropylene window film should be corona-treated at 40 dyn/cm or higher.

On mail-order catalogue lines applying aqueous overprint varnish, the rewettable adhesive is applied over the varnish after printing. If the varnish contains high levels of polyethylene wax, surface energy falls below 34 dyn/cm and re-wetting is uneven. A top-coat remoistening primer or corona treatment may be required. The adhesive does not cause visible film haze at dry coat weights below 4 g/m² on high-gloss varnish, but at 8 g/m² dry, the film edge can show a matt halo under side lighting.

During shutdowns, doctor blades and transfer rolls are cleaned with warm water before drying. Dried adhesive film on gravure cells is removed by soaking in 1:1 water/ethyl acetate for 15 min. The same solvent mixture must not contact polycarbonate guards or natural rubber rollers. Wastewater from cleaning is not disposable to surface water without treatment because the emulsion increases biological oxygen demand; chemical oxygen demand is typically 60,000–90,000 mg O₂/L as supplied.

Regulatory compliance extends to REACH (EC) No 1907/2006 as a mixture; the product contains no substances of very high concern above 0.1% by mass. The formulation is not classified for acute oral, dermal, or inhalation toxicity under CLP Regulation (EC) No 1272/2008. The adhesive does not contain phthalate plasticizers, and the preservative migration potential remains below the threshold established in EU Regulation (EC) No 10/2011 when used under the intended packaging conditions.