Optimizing Aqueous Dispersion Coatings for Standard Paperboard Repulping Operations

Chemistry

Polyolefin extrusions form continuous, insoluble sheets that must be peeled from fiber and screened out in large pieces. Water-borne dispersion coatings work differently: their barrier films ~ most often copolymers of styrene-butadiene, styrene-acrylate, or ethylene-acrylic acid ~ are engineered to hold off liquid water during use but disperse or fragment once exposed to hot alkaline pulping liquor. How well that works comes down to the carboxylated monomer content in the polymer backbone, which sets the rate of swelling and break-up in the hydrapulper.

When dispersion-coated paperboard enters an industrial hydrapulper, water cuts through pinholes and raw cut edges into the underlying fibers. Deprotonation of carboxyl groups at alkaline pH generates electrostatic repulsion throughout the polymer chains, forcing the film to swell and lose cohesion. Pulper rotor shear then tears the swollen barrier into discrete flakes rather than stringy ribbons.

That fragmentation pattern determines the fate of the coating downstream: either the fragments are large enough to lodge on pressure screens, or they shatter into micro-stickies that slip through and deposit on machine wires.

Glass transition temperature marks the dividing line between clean mechanical screening and adhesive fouling. If the binder’s Tg sits well below the pulping water temperature, the polymer softens under shear, stretching into deformable bits that extrude through screen slots and settle as sticky deposits in the white water loop. Resins with a Tg higher than the pulper operating bath stay rigid, shattering into flat, stiff flakes that present a wide cross-section to slotted screen baskets.

At an operating pH of 8.5 and a pulping temperature of 50 °C, styrene-acrylic coatings containing 6 wt% acrylic acid maintain a mean screenable fragment size of 210 µm.

Surfactants stabilize the emulsion during coating and leveling, but carryover into the pulper impairs recycled sheet strength. High residual loadings of non-ionic alkylphenol ethoxylates or ionic emulsifiers drag pulper liquor surface tension down below 45 mN/m, adsorbing onto exposed cellulose surfaces and interfering with fiber-to-fiber bonding. Crosslinkers like zinc ammonium carbonate or zirconium complexes present the opposite headache: while they secure water resistance on the dry box, they form covalent networks that resist alkaline swelling entirely, leaving the dispersion behaving much like low-density polyethylene.

• Excessive crosslinking density blocks water penetration into the polymer matrix, turning the film into an unyielding layer that resists hydraulic shear.
• Low glass transition temperature causes the resin to soften at standard pulping temperatures, forming tacky specks that smear across screen surfaces and wire meshes.
• Surfactant wash-off pulls surface tension down in the pulper vat, generating persistent foam that carries usable fines out with clarifier skimmings.
• Hydrophobic filler agglomeration seals off alkaline swell pathways when high loadings of calcium carbonate or talc clump within the binder.

Mill pulping temperatures are routinely cited as the primary cause of repulping failures, though low carboxyl content or an overly soft monomer ratio in the coating binder is often the real driver.

Slurry

Defibering dispersion-coated board in an industrial hydrapulper balances hydraulic shear against stock consistency, bath temperature, and pH. Running at high consistency (12% to 16% solids) relies on board-on-board attrition to roll the barrier film off the substrate in broad patches. Low-consistency pulping below 6% depends almost entirely on fluid turbulence off the rotor, which tends to mince brittle barrier films into fragments too small for standard screening.

Heating the pulper bath past the resin’s thermal softening point causes the coating to ball into elastic globules that snare suspended wood fibers. Holding the slurry at least 5 °C below the resin’s glass transition temperature or minimum film formation temperature keeps the film brittle enough to fracture cleanly. Dosing sodium hydroxide or sodium silicate to push pH into the 8.5 to 9.5 range ionizes carboxyl groups along the polymer chain, driving rapid osmotic swelling along the fiber-coating boundary.

Maintaining pulper slurry temperature below the polymer glass transition point keeps film fragments rigid for mechanical screening.

Pulping duration governs both defibering yield and contaminant particle size. Leaving stock in the tub past the point of fiber liberation yields diminishing returns while rotor attrition steadily grinds coating flakes down to sub-screenable debris. The operating window generally targets complete fiber release within 12 to 18 minutes, followed by an immediate dump through a coarse extraction plate to trap large fragments before secondary breakdown begins.

1. Fill the pulper tub with process white water and bring the temperature to 45 °C via low-pressure steam injection.
2. Add sodium hydroxide solution to establish an initial pH of 9.0 before charging the paperboard.
3. Charge dispersion-coated board to hit a target operating consistency of 14% dry solids.
4. Run the rotor at pulping speed for 15 minutes to promote fiber-on-fiber rub without grinding down film flakes.
5. Dilute the slurry to 4% consistency with cold white water, pulling batch temperature below 35 °C before dumping.
6. Pump the diluted stock through a 6 mm extraction plate to hold coarse coating skins inside the tub.

Operating outside verified consistency and temperature windows shreds the barrier layer into fine micro-stickies that blind press felts and force converting operations into costly unscheduled washdowns.

Screen

Extracting freed dispersion particles from usable pulp takes place in approach-flow pressure screens fitted with narrow slotted baskets. Clean separation depends on exact rotor foil clearance, basket surface velocity, and steady reject rates. Because dispersion flakes are flat, two-dimensional structures, screen face velocities must remain low; excessive hydraulic suction pulses tip flakes on edge, drawing them straight through the slots.

Coarse stages rely on perforated baskets with 1.2 mm to 2.0 mm holes to scalp out unpulped board flakes and large sheets of coating. Stock then flows to fine screens fitted with contoured wedge-wire slots between 0.10 mm and 0.15 mm wide. Surface contouring generates localized micro-vortices that hold flat polymer flakes off the wire face, while individual flexible wood fibers flex under the pressure drop and pass with the accept flow.

According to CEPI Recyclability Test Method guidelines, fine screening passing through a 150 µm slotted laboratory screen must clear at least 95% of total input synthetic coating mass to earn a fully recyclable classification.

Reject rate control dictates whether a screen line runs cleanly or blinds over with fiber matting. Maintaining volumetric reject rates between 15% and 22% on fine stages preserves enough cross-flow velocity to sweep coating flakes toward the reject port before hydraulic shear forces them through the wire. Cascaded screening trains ~ running primary, secondary, and tertiary loops ~ progressively thicken coating rejects into a concentrated waste stream while returning recovered fiber to the headbox feed.

• Slot width selection balances hydraulic throughput against particle interception, requiring a maximum 0.12 mm slot for aqueous dispersions with low shear strength.
• Rotor speed optimization provides periodic pressure pulses to clear screen slots without generating shear forces that fracture soft polymer flakes.
• Contour height profile controls micro-vortices at the screen surface, lifting low-density coating debris away from the accept flow path.
• Mass reject ratio must stay above 18% in primary screening stages to prevent contaminant concentration build-up inside the screen basket zone.

Supply agreements specify that delivered paperboard rolls containing repulpable dispersion coatings must not generate macro-stickie concentrations above 50 mm²/kg when tested on a 150 µm laboratory screen under standard pulping protocols.

Validation

Qualifying a dispersion coating as repulpable requires standardized testing that balances usable fiber yield against residual contaminant loads. Protocols such as PTS-RH 021/97, the CEPI Recyclability Laboratory Test Method, and TAPPI UM 242 pulp coated samples under set conditions, screening the resulting slurry to weigh accepted fiber against retained coating debris.

Macro-stickie quantification isolates debris larger than 100 µm on a Somerville or Haindl laboratory fractionator using a 150 µm slotted plate. Screened fiber is then formed into laboratory handsheets (ISO 5269-1) for visual and hot-press inspection. Drying handsheets under heat reveals thermoplastic carryover directly: polymer particles soften, leaving translucent windowpanes or tacky spots that adhere to drying felts.

Can Standard Laboratory Pulpers Reproduce Industrial Screen Pass Rates?

Standard disintegrators running under ISO 5263 generate rotational fluid shear, completely missing the fiber-on-fiber rub characteristic of mill-scale high-consistency pulpers. Benchtop evaluations routinely misjudge fragmentation as a result. A coating that looks easily screenable in the lab can score artificially high marks, only to disintegrate under the aggressive hydraulic shear of industrial stock pumps and pressure screens into troublesome micro-stickies.

Automated image analysis tallies residual macro-stickies by scanning handsheets dusted with black toner or dyed with hydrophobic stains that adhere to polymer specks. Sub-100 µm micro-stickies pass right through standard lab screens, requiring filtrate chemical oxygen demand (COD) testing or gas chromatography-mass spectrometry to quantify dissolved and colloidal material. A sharp rise in filtrate COD reveals that the dispersion is washing into solution or re-emulsifying rather than holding together as screenable solids.

Views remain split on whether high COD contributions from dissolved coating resin should outright fail a grade, or whether modern dissolved air flotation (DAF) units can clear the soluble load economically.

Recovery

The commercial case for swapping polyolefin extrusions for dispersion coatings hinges on net fiber yield versus effluent surcharges. Recycled mills buy secondary fiber by dry weight, but unusable coating mass, moisture differences, and screening rejects erode what actually reaches the headbox. If a dispersion breaks down into colloidal fines instead of screenable flakes, operating costs climb quickly through extra coagulant dosing, pitch fixatives, and heavier biological oxygen demand in wastewater operations.

Processing 10,000 metric tonnes of board coated with a 15 g/m² aqueous dispersion introduces roughly 500 tonnes of synthetic polymer solids into the stock loop. Even if screening traps 90% of that mass in the reject bins, 50 tonnes of colloidal and micro-particulate resin remain behind in the white water. In closed-loop mills, these suspended solids accumulate, forcing operators to step up cationic coagulant feeds to avoid sheet breaks through the dryer section.

For an SBS packaging line running 40,000 metric tonnes annually with an aqueous dispersion barrier, economics hinge on the balance between incoming substrate weight, usable fiber out the door, and effluent surcharges.

Mills levy steep surcharges or turn away truckloads outright when incoming repulpable board triggers severe foaming, wire deposition, or COD spikes in the clarifier.

1. Mill Acceptance Certificate verifying compliance with CEPI or PTS recyclability thresholds for the specific paperboard grammage and coating weight combination.
2. Safety Data Sheet (SDS) disclosing all functional monomers, residual surfactants, crosslinkers, and biocide formulations present in the liquid coating.
3. Mass Balance Yield MTR detailing dry fiber percentage, ash content, polymer coating weight, and expected screening reject fractions under standard pulping conditions.
4. Effluent Impact Profile declaring chemical oxygen demand, biochemical oxygen demand, and total dissolved solids contribution per tonne of pulped substrate.

Inconsistent pulper chemistry shifts barrier polymers from screenable solids to soluble white water contaminants.

Barrier specifications that overlook COD loading invite mill gate rejections or municipal wastewater penalties, no matter how clean the laboratory fiber yield appears.

Tack

Formulating aqueous dispersion barriers involves an ongoing trade-off between roll-winding performance and hydrapulper breakdown. Converters need tough, moisture-resistant surfaces that will not block on the roll under winding tension or gum up rotary cutting dies. Yet the very cohesion that prevents roll-blocking directly resists the rapid film disintegration needed during mild alkaline pulping.

Core-shell acrylic architectures and paraffin wax additions help resolve this conflict. A hard polymer core maintains block resistance and dimensional stability up to 60 °C, while a carboxyl-rich outer shell ensures rapid swelling in alkaline water. Paraffin wax lowers cured surface energy to prevent face-to-back adhesion in master reels, while simultaneously acting as a release aid during repulping by creating internal shear planes that encourage brittle fracture under pulper agitation.

Tuning monomer ratios controls the glass transition profile across temperature swings. Incorporating hydrolyzable monomers like acetoacetoxyethyl methacrylate keeps the cured film cohesive and water-resistant under neutral converting conditions. Once submerged in hot, alkaline pulper liquor, however, the ester bonds saponify, destabilizing the polymer backbone into screenable flakes before sticky agglomerates can form and foul the machine clothing.