PVAc Sealing Adhesive

Type PVAc-S 7040 is an aqueous polyvinyl acetate dispersion intended for rotary carton side-seam, corner, and edge sealing on porous cellulose substrates. The product is supplied at 54 ± 1% solids mass fraction, with pH 4.8–5.8 and Brookfield RVT viscosity 12,000–18,000 mPa·s at 20 °C using spindle 6 at 20 rpm. Density according to ISO 2811-1 is 1.08 ± 0.02 g/cm³, and minimum film formation temperature according to ISO 2115 is 7 °C. The glass transition temperature determined by differential scanning calorimetry under ISO 11357-2 is 28 °C; this yields a rigid thermoplastic dried film at ambient temperature while permitting heat activation above 60 °C for subsequent sealing steps. Median particle size measured by dynamic light scattering under ISO 22412 is 0.8–1.5 µm, with residual vinyl acetate monomer below 100 ppm. Total volatile organic content is below 50 g/L when tested by US EPA Method 24, placing the product below common regulatory low-VOC thresholds for adhesives. Freeze-thaw stability is limited; storage at 5–35 °C for up to 12 months is required because one freeze-thaw cycle can increase particle agglomeration and raise Brookfield viscosity beyond 25,000 mPa·s.

A lower-viscosity variant, PVAc-S 5020, is available for rotary spray and slot-die lines where pump shear is limited. PVAc-S 5020 is supplied at 50 ± 1% solids and Brookfield RVT viscosity 2,500–5,000 mPa·s at 20 °C. It provides faster penetration into porous kraft but lower initial wet tack than PVAc-S 7040 and is not recommended for side-seam applications on machine speeds above 80 m/min without a thickener adjustment. The selection between the two models is based on application equipment shear rate and substrate absorbency, not solely on final bond strength.

When metered through a ceramic anilox roller with cell volume 45–55 cm³/m², wet film weights are typically 18–22 g/m² at line speeds of 60–120 m/min. Published data for this specific configuration is limited, but production operators commonly observe that film weights below 12 g/m² fail fibre-tear coverage on uncoated kraft, whereas deposits above 30 g/m² extend setting time and can cause side-seam strike-through when board caliper is below 250 µm. Quality-control sampling on a 500 kg bulk tank should be performed after 20 min of low-shear mixing and before filling. A lot is released when Brookfield viscosity remains within ±10% of the target and pH is within 4.8–5.8. Viscosity drift of ±500 mPa·s between top and bottom samples may indicate settling; if observed, homogenization with a low-speed propeller is required before use.

Rheological characterization on a Brookfield CAP 2000 viscometer at 25 °C shows shear thinning from approximately 14,000 mPa·s at 100 s⁻¹ to 3,500–5,000 mPa·s at 1,000 s⁻¹. This behaviour is consistent with the anionically stabilized PVAc dispersion structure and supports clean transfer from engraved rolls without excessive foaming. However, high-shear dispersion heads with circulation rates above 20 L/min require defoaming control because the product develops microfoam when air entrainment exceeds 2% by volume; silicone-free defoamers based on mineral oil at 0.1–0.3% addition are compatible without reducing wet tack. Recirculation lines should be flushed after stoppages longer than 30 min; dried adhesive can form irreversible deposits in doctor blade chambers. Cleaning water should be collected because the aqueous polymer dispersion can raise biological oxygen demand in wastewater; local discharge permits often limit BOD5 to 2,000 mg/L before dilution or treatment.

What Distinguishes PVAc Sealing Adhesive from VAE Dispersions and Hot-Melt Sealants?

Unlike vinyl acetate-ethylene (VAE) dispersions, PVAc homopolymer produces a harder dried film with higher room-temperature cohesive strength and a measurable glass transition temperature of 28 °C under ISO 11357-2. VAE typically exhibits glass transition temperature below 0 °C depending on ethylene content, which improves flexibility and water resistance but reduces clean-cut edge definition and heat resistance. PVAc is therefore selected for rigid paperboard carton side seams where dimensional stability and crisp edge scoring are required. Compared with polyvinyl alcohol solutions, the dried PVAc film is not redispersible in cold water; however, it is not classified as waterproof. Durability under EN 204 is generally D2, not D3 or D4, after 24-hour cold-water contact. Against EVA hot-melt sealants, PVAc provides an open time of 3–8 min at 23 °C and 50% RH, whereas hot-melt formulations set in 1–3 s; PVAc also avoids requiring 160–180 °C application temperatures but demands longer accumulator or stack drying before bonding. Compared with solvent-borne polychloroprene sealants, PVAc does not require explosion-proof coating lines because it is non-flammable and contains no chlorinated solvent. It cannot match polychloroprene oil resistance or initial tack on synthetic rubber gaskets, so it is limited to porous cellulose substrates and some open-pored wood assemblies. The application equipment uses water-based washdown instead of solvent recovery, and air emissions testing under US EPA Method 18 often shows non-detect vinyl acetate when exhaust concentration is below 5 ppm. Unlike two-component polyurethane or epoxy systems, PVAc does not require mixing ratio control or a short pot-life shutdown schedule; sealed containers retain usable rheology for the full shelf life.

Adhesion Thresholds on Clay-Coated SBS Board

On clay-coated solid bleached sulphate board with a surface energy below 38 mN/m, wetting of the dispersion is the primary limiting variable. The adhesive can be applied at 14–18 g/m² wet film weight if the substrate surface energy is above 38 mN/m and the coating contains no wax or silicone release layer. At lower surface energy, transfer defects and crawler lines appear within the first 500 m of production. Corona treatment is not normally required for uncoated kraft but may raise surface energy from 34 mN/m to 44 mN/m on polymer-coated board; treatment should be verified with dyne pens and kept below 2 kW/m to avoid surface degradation. The dried adhesive forms a clear to slightly hazy film; on clay-coated board, linear shrinkage during drying is approximately 0.7–1.0%, but published data for this specific configuration is limited. Film defects such as orange peel can occur when wet film thickness exceeds 30 g/m² and dryer air temperature is above 70 °C, because surface skinning traps water and generates blisters. Lap-shear adhesion on uncoated kraft with a 25 mm overlap at 0.2 MPa pressure for 30 s typically reaches fibre tear in 2–4 h under ambient conditions. Final bond development should be evaluated on the actual board grade, because recycled liner furnish can shift caliper, oil absorption, and fibre-tear response independently of adhesive solids.

When Relative Humidity Exceeds 70% During Application

Water evaporation slows markedly above 70% RH. At 23 °C and 80% RH, drying is approximately 2–3 times slower than at 50% RH, which increases open time but also increases the interval before a dry, heat-sealable film is obtained. For carton side-seam operations running at 80–120 m/min, this may force a reduction in line speed to 50–70 m/min unless an infrared pre-dryer or humidity-controlled hood is installed. The dispersion remains stable under high ambient humidity, but wet films may absorb moisture from air and retain a slightly tacky surface for 15–30 min; recovered wet tack is not a reliable indication of final bond strength. Production-scale equipment using a 300–400 mm wide slot-die applicator should maintain die temperature below 35 °C because localized drying at the lip increases particle deposition. Dilution with water up to 5% by mass is permitted only with slow agitation at <100 rpm; high shear mixing can generate microfoam.

Because the stabilizer system is anionically charged, the product can coagulate when combined with cationic wetting agents, cationic starches, or high concentrations of trivalent metal salts such as aluminium sulfate. Compatibility tests should include a 24-hour sediment and viscosity check at 25 °C before any production trial. The wet adhesive is cleaned with water; dried films can be removed with warm water above 40 °C or with propylene carbonate-containing cleaners. Tightly sealed original containers are required after opening because skin formation occurs within 30–60 min at 35 °C and 20% RH. Equipment contact surfaces of aluminium and brass are not recommended for extended contact, as acetic acid release during film formation at acidic pH may cause corrosion over multi-shift operation. Dilution with recovered wash water is not recommended unless screened through a 100 µm filter and monitored for pH drift below 4.5.