VAc-Acrylate Emulsion for Paper Coating

In coated woodfree offset grades, a carboxylated vinyl acetate–acrylate emulsion with a measured glass transition temperature of −5 °C to +15 °C and an average particle size of 120–220 nm is metered into a topcoat colour based on 60–80 pph delaminated Brazilian kaolin and 20–40 pph fine ground calcium carbonate. The topcoat colour is held at 60–68% solids and 800–1500 mPa·s Brookfield viscosity at 25 °C using an RVT spindle 4 at 100 rpm, while high-shear Hercules viscosity is maintained between 35 and 80 mPa·s at 4,400 s⁻¹ to prevent blade spatter on a Short Dwell coater running at 900–1400 m/min. The binder demand of 8–12 pph dry pigment balances optical efficiency against dry pick resistance, because each additional 1 pph of carboxylated VAc–acrylate reduces coating porosity by approximately 0.02–0.04 cm³/g as mercury intrusion data for similar mineral systems show, while lifting dry pick strength on an IGT AIC2-5 measured by ISO 3783 from below 1.2 m/s to above 2.0 m/s with medium-viscosity pick test oil. The emulsion is added under low-shear agitation after pigment dispersion to avoid shear-induced destabilization above 10,000 s⁻¹; the anionic carboxylated surface charge density of 0.3–0.6 meq/g provides electrostatic compatibility with sodium polyacrylate dispersants but limits the use of cationic wet-strength resins above 0.2% solids in the same mix tank. Drying relies on gas-fired IR zones reaching web surface temperatures of 70–85 °C, followed by air-flotation dryers with supply air at 120–160 °C; film formation occurs when the coating surface exceeds the emulsion’s minimum film-forming temperature by at least 10–15 °C, and premature coalescence in the blade pond is controlled by pH buffering at 8.5–9.2 with ammonia or AMP-90. Calendering at 120–140 °C and linear loads of 200–350 kN/m on a 12-nip supercalender reduces Parker Print Surf roughness to 0.8–1.2 µm under ISO 8791-4, producing a sheet where the binder neither smears on the calender bowls nor generates fibre picking during sheet-fed offset, but the low wet-rub resistance of the uninsolubilized film remains an operational boundary if the printed sheet is exposed to fountain solution for more than 15–20 min before drying.

What Limits Wet Pick Resistance in Sheet-Fed Offset Cartonboard Coating?

Wet pick resistance in sheet-fed offset cartonboard coating is governed less by dry binder content than by the degree of carboxyl group insolubilization at the coating-air interface after applying fountain solution with a film thickness of 1.5–3.0 µm on the printing blanket. A carboxylated VAc–acrylate latex at 10–14 pph in a topcoat formulated with 55–70 pph fine kaolin and 30–45 pph ground calcium carbonate provides a dry pick velocity above 2.2 m/s under ISO 3783, but the unmodified film swells and loses wet rub resistance after contact with an acid fountain solution at pH 3.8–5.5. Production-scale sheet-fed trials on a six-colour lithographic press show that wet picking appears on the trailing edge of the second impression unit when the coating’s water absorption exceeds 35 g/m² by ISO 535 Cobb 60 s, particularly at press speeds above 8,000 sheets/h. To limit this failure mode, 0.3–1.0 pph ammonium zirconium carbonate is post-added to the coating colour at pH 8.6–9.0, crosslinking the carboxylated latex through coordinate bonding with Zr⁴⁺ during drying; the reaction becomes measurable as a reduction in extractable binder from 0.8 mg/cm² to below 0.2 mg/cm² after 24 h ageing at 20 °C. Wet rub resistance is evaluated with a motorized Sutherland rub tester under ASTM D5264 using 10 wet cycles, and acceptable values for litho cartonboard are typically above 4 dry/wet ratio when the coating is fully insolubilized. The binder’s anionic nature forbids direct combination with cationic poly-DADMAC strength additives above 0.1% solution concentration unless a bridging dispersant is introduced; mixing with aluminium sulfate beyond 0.5% solids causes visible curding and blade scratch defects. The finished board must also meet indirect food-contact migration limits under FDA 21 CFR 176.170 and 176.180 when used for dry bakery or frozen food packaging, and residual vinyl acetate after optimized stripping is typically held below 500 ppm by headspace gas chromatography.

Aqueous inkjet receptive papers require a binder film that creates microporous dye absorption rather than high mineral gloss, and a VAc–acrylate emulsion is used only at 20–40 pph on fumed silica with a BET surface area of 250–380 m²/g because excess hydrophobic acrylate blocks dye absorption and increases banding. The high surface area silica is predispersed at 18–25% solids with a cationic dye-fixing agent such as poly-DADMAC at 5–10% on pigment; the anionic carboxylated latex is added slowly at pH 4.5–5.5 to avoid coagulation caused by zeta-potential reversal above +25 mV. The coating colour is applied by air-knife or curtain coater at 22–35 g/m² dry coat weight on base paper with a Cobb 60 s value of 20–30 g/m² under ISO 535, and dried at 60–80 °C web temperature to retain pore volume above 0.6 cm³/g. Print quality is assessed by optical density at 1.4–1.6 for dye-based black and by 24 h wet fastness to deionized water under ISO 18935:2018; the VAc–acrylate portion contributes film cohesion after dye absorption but cannot provide full water-fastness without cationic fixatives or a polyurethane overcoat. Binder film formation at an MFFT of 5–15 °C is adequate for air-dry conditions at 23 °C and 50% relative humidity, but below 12 °C coating temperature the polymer remains as discrete particles and dry rub resistance drops by more than 50%. Blade application is not recommended for these low-solids silica formulations because high-shear viscosity rises above 150 mPa·s at 4,400 s⁻¹ and leads to streaking; curtain and slide coaters with slot gap settings of 300–500 µm are preferred. The dry coating must be calendered at low load 80–120 kN/m to avoid pore collapse, and surface roughness under ISO 8791-4 is maintained at 1.0–1.5 µm for water-based dye inks, while residual vinyl acetate is controlled below 500 ppm by steam stripping to limit VOC emissions under REACH compliance obligations.

Food-Contact Mineral Barrier Coating Constraints

Food-contact mineral barrier coatings based on platy talc, delaminated kaolin, and carboxylated VAc–acrylate emulsions at 7–12 pph total pigment are applied to bleached kraft board in a single layer of 8–20 g/m² dry coat weight to reduce water vapour transmission rate from uncoated values of 600–900 g/m²·24 h to 100–300 g/m²·24 h at 23 °C and 85% relative humidity under ISO 15106-3. This barrier is not equivalent to polyethylene lamination, and published data for VAc–acrylate-specific configurations at high relative humidity show vapour transmission remains moisture-dependent because the polymer film itself has a water vapour permeability above 2.0 g·mm/m²·day·kPa. The binder is selected over styrene–acrylate where low odour and low glass transition are required, but the carboxylated anionic latex loses film integrity at pH below 4.0, which excludes direct contact with acidic foods such as citrus juice without an additional extruded or waterborne overprint. Compliance requires extraction testing according to FDA 21 CFR 176.170 and 176.180, European framework regulation (EC) 1935/2004, and BfR Recommendation XXXVI for paper and board; residual monomers are controlled below 1,000 ppm total by offline stripping, and the emulsion must be manufactured with no alkylphenol ethoxylate surfactants to satisfy REACH Annex XVII entry 46 restrictions. On a pilot blade coater running at 600 m/min, edge bleed at the backing roll was observed when the coating colour exceeded 1,200 mPa·s Brookfield viscosity, and the defect transferred to the board as variable coat weight across the web. Repulpability under TAPPI T 275 remains acceptable when the coating is less than 5% of board mass, but the polymer can accumulate as tacky fines in mill white-water loops above 2.0% solids, requiring wash cycles with nonionic dispersants. The operational boundary for food-contact use is therefore a mineral coating with limited barrier performance, no direct acidic liquid contact, and strict residual monomer and surfactant control rather than a drop-in replacement for film lamination.

When Blade Runnability Falls Below 1,200 m/min in High-Solids Precoats

When blade runnability falls below 1,200 m/min on high-solids precoats, the defect signature is normally a combination of blade scratches, wet matter deposition on the backing roll, and coat weight oscillation between 8 and 14 g/m² at the deckle edges. In such precoats for folding boxboard, the VAc–acrylate emulsion is added at 5–8 pph on a coarse ground calcium carbonate pigment with a mean particle size of 1.0–2.5 µm, and the coating colour is prepared at 62–68% solids with a Brookfield viscosity of 900–1600 mPa·s at 25 °C. The emulsion’s shear stability under a blade tip is not uniform; below 0.3 meq/g carboxylation, the latex shows no visible grit at 20,000 s⁻¹ but loses water retention, while above 0.8 meq/g the high-shear viscosity climbs above 120 mPa·s and the blade deflection must be increased beyond 0.6 mm. Water retention is measured on an AA-GWR water retention meter at 2.0 bar for 90 s; values above 180 g/m² correlate with binder migration into the base sheet and reduced topcoat holdout. The precoat is dried to a moisture content of 4.5–6.0% before topcoating, using infrared dryers with a peak web surface temperature below 75 °C to avoid partial coalescence in the base sheet that would create mottling. Trials on a 2.5 m wide blade coater with a 0.45 mm bevel angle blade showed that lowering emulsion solids from 55% to 50% while adding 0.5 pph carboxymethyl cellulose restores runnability at 1,300 m/min by reducing high-shear viscosity without sacrificing dry pick, but precoat smoothness under ISO 8791-4 increases from 1.8 µm to 2.4 µm. Ionic incompatibility is a recurring failure: if the mill’s calcium ion content in dilution water exceeds 200 ppm hardness, the carboxylated latex forms microgels that deposit on the blade within 20–40 min and show as diagonal scratches. The operating boundary is therefore defined by source water hardness, carboxylation level, and water-retention additives rather than by solids content alone.

Where pressure-sensitive label face stock must reconcile flexographic print density, adhesive wet-out, and die-cutting, the base paper is coated with a clay–VAc–acrylate formulation at 6–10 g/m² dry coat weight on a machine-glazed or supercalendered kraft base of 65–90 g/m². The binder level of 8–12 pph on a pigment blend of 70–85 pph fine kaolin and 15–30 pph calcium carbonate produces a Parker Print Surf roughness of 0.9–1.4 µm under ISO 8791-4 and a Bekk smoothness above 1,200 s, which are required for UV flexo at 4,000–6,000 m/h line speed. VAc–acrylate is preferentially used over styrene–acrylate because its lower glass transition temperature of −5 °C to +10 °C reduces edge cracking during rotary die-cutting at 80–120 m/min, but the same low glass transition temperature reduces surface hardness; Taber abrasion under TAPPI T 476 is acceptable only if the coat weight is kept above 6 g/m². Silicone release liner base made from this coated paper is further extrusion-coated or off-line silicurized after the clay coating is fully dried below 0.8% moisture; residual moisture above 1.2% in the coating at the silicone station has been observed to cause pinholing in platinum-catalysed addition-cure silicone layers. The anionic latex is compatible with calcium stearate lubricant at 0.5–1.0 pph but coagulates with zinc stearate above 1.0% due to divalent ion exchange, so calcium-based additives are used. The coated face stock must meet ISO 536 grammage tolerance of ±2 g/m² and ISO 2470-2 brightness above 88 for backside adhesive transparency, while the absence of crosslinking chemistry keeps the coated broke repulpable under TAPPI T 275 at levels up to 5% of furnish. Data from rotary press lines show that if warehouse relative humidity falls below 35% for more than 12 h, the latex film embrittles and cracks at fold scores; above 70% relative humidity the sheet picks on the adhesive coating head because the coating film softens. These humidity boundaries are operational constraints that must be designed into the label converting schedule.