Quantifying Micro Sticky Agglomeration Kinetics in Recycled Wet End White Water Systems

Colloid

A 100-micrometre laboratory slotted plate divides recovered paper contamination into macro contaminants and suspended micro contaminants. Screen baskets inside coarse and fine pulping lines easily reject macro stickies, yet particles passing the 100-micrometre slot circulate through wet end white water loops indefinitely. Dispersed acrylates resist standard gravity separation.

These suspended fractions consist of poly(vinyl acetate), styrene-butadiene rubber, polyacrylate adhesives, and ethylene-vinyl acetate copolymers derived from pressure-sensitive labels and hot-melt bindings. In an unperturbed white water stream at neutral pH, these polymer globules carry an anionic surface charge ranging from -25 to -45 millivolts, maintaining a metastable dispersion through electrostatic repulsion.

Soluble fractions remain invisible to optical scanners. When mills close their water circuits to reduce fresh water consumption below 8 cubic metres per metric tonne of finished board, ionic strength rises and mechanical shear increases. White water temperatures consistently exceed 50 degrees Celsius.

The elevated temperature pushes synthetic polymers past their glass transition points, transforming rigid submicron particles into tacky droplets prone to coalescence upon physical contact.

Quantification of micro sticky concentration demands rigorous isolation from wood pitch, sizing agents, and lignocellulosic fines. Unchecked accumulation leads directly to machine downtime, sheet defects, and converting failures.

• Structural pinholes form when agglomerated particles transfer from the web to dryer surfaces, pulling fibres out of the base sheet.
• Press felt compaction escalates void closure. The adhesive mass fills open felt capillary structures, driving up hydraulic press pressure and web crushing.
• Calender wrap breaks occur as tackified polymer complexes adhere to hot calender rolls during final smoothing.
• Converting delamination manifests when heat-sealing jaws or corrugator hot plates soften embedded particles within the inner plies.

The transition from a stable colloidal phase to disruptive macroscopic stickies depends on particle collision frequencies and surface adhesion efficiency. Operating mills that monitor only macro sticky volume miss the true precursor phase circulating silently through the short circulation loop.

Surfactant-stabilized acrylate dispersions maintain zeta potentials below negative thirty millivolts until divalent cation concentrations exceed four hundred milligrams per litre at sixty degrees Celsius.

Measuring this inventory requires centrifugation to separate fibres, followed by solid-phase extraction with dichloromethane or methyl tert-butyl ether to isolate the ester-rich organic fraction before gravimetric determination. The extracted residue yields the total potential sticky burden per dry tonne of furnish.

Collision

Particle transport mechanics govern the rate at which micro sticky droplets meet. Perikinetic diffusion governs submicron particle transport. Thermal energy drives continuous random Brownian movement, producing encounters between particles under one micrometre in diameter.

The perikinetic collision rate scales with the square of particle number concentration, independent of fluid velocity gradients.

Once particles grow beyond one micrometre through initial coalescence, orthokinetic transport dominates the agglomeration path. In the approach piping, fan pumps, and headbox slice, velocity gradients deform fluid elements and force adjacent suspended bodies together. The shear-induced collision frequency scales with the local shear rate, particle number concentration, and the cube of the collision radius.

Is Equilibrium Sieve Retention Predictive across Shear Fields?

Testing laboratory samples under stagnant or laminar conditions misrepresents turbulence inside industrial stock approach systems. High shear environments break loose flocs. In turbulent pipe flow where dissipation rates exceed 1000 reciprocal seconds, the collision frequency rises sharply, yet hydrodynamic drag forces simultaneously attempt to rupture developing adhesive doublets.

1. Diffusion dominated transport generates initial submicron doublets through thermal kinetic energy within quiet storage chests.
2. Shear driven contact forces larger micron-scale droplets into direct physical contact across the velocity gradients of the fan pump.
3. Viscous deformation flattens colliding tacky polymer envelopes, displacing water films and establishing permanent inter-particle adhesive bonds.
4. Hydrodynamic fragmentation cleaves unstable clusters when turbulent shear stresses overcome the internal cohesive tensile strength of the sticky mass.

Agglomeration rate constants scale quadratically. The Smoluchowski collision kernel describes the rate of change in particle count, modified by a collision efficiency factor, alpha. For fully repulsive, charge-stabilized systems, alpha approaches zero.

As electrolytes compress the electrical double layer or when non-ionic polymer bridges form, alpha approaches unity, accelerating particle enlargement.

Double-layer thickness contracts in direct proportion to the square root of white water ionic strength.

Centrifugal forces detach weakly bound clusters. In a clean recirculating system, total particle number drops while mean particle diameter shifts toward the 100-micrometre boundary. Centrifugal cleaners miss neutral density particles.

The persistence of micro stickies through cleaner banks guarantees repeated passes across high-shear pump impellers, compounding growth cycles until the secondary macro sticky threshold is crossed. This dynamic leaves open the unresolved problem of predicting the exact shear threshold where aggregate breakage perfectly balances shear-induced coalescence across fluctuating headbox consistencies.

Gel

Polymer tack governs whether an physical collision yields permanent cohesion. Tack increases sharply above forty degrees. Recycled packaging mills running closed wet end white water circuits operate between 45 and 65 degrees Celsius.

This temperature range sits far above the glass transition temperature of 2-ethylhexyl acrylate and butyl acrylate, two universal monomer components of modern pressure-sensitive label adhesives.

Will Thermal Softening Accelerate Agglomerate Growth Rates?

Thermal transitions alter the viscoelastic loss modulus of the dispersed adhesive droplets. Tack reaches its maximum when the test temperature exceeds the polymer glass transition value by thirty to forty degrees. At this point, the polymer exhibits sufficient fluidity to wet contacting surfaces, combined with enough cohesive elasticity to resist immediate separation.

Double layers collapse under divalent cations. Calcium ions originating from calcium carbonate fillers and gypsum process aids accumulate rapidly in closed loops. When calcium concentrations exceed 500 milligrams per litre, divalent cations screen anionic charges on the acrylate surfaces, collapsing the electrical double layer from fifteen nanometres down to less than two nanometres.

• Dissolved calcium threshold values above four hundred parts per million neutralize carboxylate functional groups, eliminating steric repulsive charges.
• Specific conductivity elevation beyond 3500 microsiemens per centimetre accelerates coagulation by thinning the protective hydration shell.
• Surface tension suppression from residual deinking surfactants reduces the thermodynamic work of adhesion, promoting droplet engulfment.
• Furnish pH shifts toward the acidic range protonate carboxylate groups, destroying the electrostatic barrier that maintains colloid dispersion.

Fibre fines capture destabilized pitch droplets. Micro stickies do not agglomerate solely with each other; they deposit onto fines and mineral filler particles, generating composite agglomerates. These hybrid aggregates exhibit variable density and irregular morphology, complicating removal by conventional centrifugal cleaning.

Agglomeration rates accelerate rapidly when process temperatures cross the adhesive glass transition boundary.

Specific surface area drops during coalescence. The loss of surface area expels bound water from the organic matrix, forming a dense, hydrophobic gel mass. Saturated hydrophobic particles migrate to the water-air interface in flotation units and white water trays, forming floating scums that break away as large adhesive clumps into the headbox feed.

Balance

Process chemistry balance determines whether micro stickies remain harmlessly dispersed, fix onto pulp fibres, or coalesce into destructive secondary macro stickies. Cationic demand tracks organic loading tightly. When recycled mills introduce coagulants and fixatives into white water loops, dosage precision governs runnability.

Under-dosing leaves adhesive droplets free to agglomerate under shear. Over-dosing flips the colloidal zeta potential from negative to positive, re-dispersing the contaminants as cationic colloids that foul polyacrylamide retention aid chains.

A rigorous mass balance evaluates chemical treatment against furnish loss and downtime costs. Synthetic polyamines, polyDADMAC, and inorganic bentonite microparticles compete for sticky adsorption sites. Bentonite provides high specific surface area to detackify polymer droplets by passivating their outer surfaces, while high-charge-density, low-molecular-weight coagulants pin submicron stickies onto the cellulosic fibre surface for export out of the mill loop within the finished paperboard web.

Consider a recycled testliner machine producing 800 metric tonnes per day of 140 grams per square metre containerboard from 100 percent old corrugated containers. The white water circuit runs at a low specific fresh water consumption of 4.5 cubic metres per tonne. Without chemical management, the micro sticky concentration in the primary white water reaches 1450 milligrams per kilogram of dry solids, measured via dichloromethane extraction following TAPPI T 277 filtration.

Secondary stickies deposit on drying cylinders. In the untreated scenario, uncontrolled orthokinetic agglomeration produces secondary macro stickies at a rate of 4200 square millimetres per kilogram of pulp. The machine experiences 4.2 breaks per day in the early dryer section where sheet moisture is 35 to 45 percent.

Each break consumes 22 minutes of production time, generating 12.2 tonnes of broke per incident. At a production cost of 370 USD per tonne and an energy-reprocessing penalty of 45 USD per tonne of re-slushed broke, breaks cost 15,120 USD daily, or 18.90 USD per saleable tonne.

Implementing a dual fixation and passivation program alters this economic equation. Dosing a polyDADMAC coagulant at 0.8 kilograms per tonne alongside activated bentonite at 2.5 kilograms per tonne costs 5.40 USD per tonne. The cationic polymer fixes micro stickies directly to the fibre surface before fan-pump shear triggers agglomeration, while bentonite particles passivate exposed adhesive domains.

Chemical fixatives pin dispersed species onto fibres.

Secondary macro sticky area falls from 4200 down to 210 square millimetres per kilogram, and dryer breaks decline to 0.4 per day. Break-related losses drop to 1.80 USD per tonne. Combining the chemical program expenditure of 5.40 USD with the break loss of 1.80 USD yields a total operational cost of 7.20 USD per tonne, capturing a net saving of 11.70 USD per tonne compared to the untreated regime.

Across 280,000 annual tonnes of output, the verified operational saving totals 3,276,000 USD.

Under section 4.2 of the mill furnish qualification agreement, sticky contamination exceeding two thousand square millimetres per kilogram triggers an automatic debit of twelve dollars per delivered tonne.

Chemical suppliers often state that total charge neutralization guarantees absolute sticky control across all grades. This claim fails to consider the competitive adsorption of dissolved hemicelluloses and lignosulfonates, which consume cationic polymers and leave dispersed synthetic adhesives free to agglomerate during thermal peaks.

Drift

Incoming recovered paper bales exhibit substantial moisture and contaminant variation. Mill receiving docks test bales for moisture and gross non-paper components, yet current dock inspection methods cannot quantify submicron adhesive loads. Sourcing decisions based purely on bale purchase price expose the machine to unmanaged contaminant drift.

Polyisoprene and acrylate concentrations in mixed paper bales vary by more than 300 percent between commercial collection batches and residential post-consumer sources.

Tack remains temperature dependent. When pulping systems receive heavily contaminated bales, the micro sticky concentration in the thick stock rises rapidly, overwhelming baseline chemical dosing schedules. Automatic dose adjustments pegged solely to incoming consistency or gross tonnage fail to account for surges in colloidal surface area.

Online streaming current detectors and cationic demand titrators identify charge drift, providing fifteen to thirty minutes of lead time before secondary stickies emerge in the white water.

A buyer auditing recycled containerboard mills inspects the technical controls deployed to mitigate this risk. Mills lacking automated coagulant pacing tied to white water cationic demand experience wide swings in sheet cleanliness. In converting operations, these swings translate directly into erratic friction coefficients, print mottling from non-receptive polymer specks on the sheet surface, and delamination along corrugator score lines.

Failing to establish verified thresholds for colloidal contaminant kinetics across closed wet end loops shifts the operational burden to the converting plant, where adhesive deposits on hot-plate shoes generate continuous corrugated board blistering, jam high-speed flexographic folder-gluers, and void converter performance warranties.