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TypeInternal Bond StrengthTAPPI T 569Taber Stiffness

Fibre Length Distribution Degradation in Recycled Boxboard

Fibre length degradation in recycled boxboard reduces sheet stiffness and score integrity, requiring chemical bonding additives or higher basis weight to preserve performance.

29.08.26 19 min
A wound spool of fibrous recycled paper pulp rests on a metal platform beside aligned rows of dark industrial feedstock pellets.
Vertical stack of varied rigid substrate samples stands against a compressed bale of corrugated waste in a warehouse utility space.

Fibre

Suspended white paper sheets float above a strapped bale of compressed recycled fiber layers in a digital illustration of material circularity.

Fibre Length Degradation Mechanisms in Recycled Stock

Recycled containerboard and boxboard furnish degrades physically with every cycle of collection, repulping, and refining. Mechanical stress in hydrapulpers, high-density cleaners, pressure screens, and refiners shortens arithmetic and weighted mean fibre lengths while thinning z-directional cell walls. Softwood kraft fibres starting between 2.5 mm and 3.5 mm suffer transverse cuts, micro-fibrillar collapse, and irreversible lumen closure (hornification).

Hardwood fibres, originally 0.8 mm to 1.4 mm, break down rapidly into micro-fines that pass a 200-mesh screen. The resulting fibre length distribution loses its log-normal shape, shifting toward a population dominated by fragments below 0.2 mm.

Chemical changes compound the mechanical damage. As hydrogen bonds inside the cell wall collapse during steam cylinder drying, boxboard fibres lose swelling capacity, locking their fibrillar structure in a rigid state. Repulping alone cannot recover the lost water retention value, so mills refine the stock to rebuild swelling and inter-fibre bonding.

Refining strips open the primary wall and secondary S1 layer to uncover the S2 layer. This recovers swelling, but it also shortens fibres further and creates loose fines that clog drainage on the wet end.

Fibre length distribution dictates sheet formation and structural performance. Long fibres form the primary load-bearing network that distributes tensile stress across the web. Short fibres and fines pack into the intervening spaces, increasing density and surface smoothness at the expense of tear strength, z-directional internal bonding, and score fatigue resistance.

If the recycled furnish contains too high a fraction of fibres under 0.5 mm, the network lacks the inter-fibre overlap needed to distribute local converting stresses.

A white paper card attaches with a binder clip to a grey sheet resting upon a heavy beige substrate marked by a horizontal purple stripe.

Fractionation Behavior in Bauer-McNett Classifiers

Evaluating fibre degradation in the lab relies on physical separation across standard wire meshes. Bauer-McNett classification under TAPPI T 233 splits furnish samples into morphological fractions through a cascade of submerged screening chambers. The long-fibre fraction retained on the 14-mesh screen carries most of the load in boxboard.

As material goes through repeated recycling loops, the mass retained on 14-mesh and 28-mesh screens drops off, shifting pulp weight into the 100-mesh, 200-mesh, and passing-200-mesh fines fractions.

A large proportion of fines passing 200-mesh indicates an exhausted furnish. These fines come in two forms: primary fines from parenchyma cells and vessel elements, and secondary fines peeled off cell walls during refining. Primary fines contribute almost no bonding and mostly wash through the forming wire into the white water.

Secondary fines have high specific surface area and exposed hydroxyl groups, which increases drainage resistance on the wire while giving a minor boost to sheet density. Recirculating secondary fines builds up hydraulic resistance across the wet end, slowing machine speeds and demanding higher doses of retention chemicals.

Under ISO 16065-2 testing, the length-weighted mean fibre length of post-consumer recycled boxboard furnish drops below 1.20 mm once the fine fraction passing 200-mesh exceeds 28 percent by dry weight.

Morphological analysis reveals that mechanical shortening hits different fibre types unequally. Thick-walled latewood fibres break transversely far more often during stock prep than thin-walled earlywood fibres. Being stiff, latewood fibres bridge across refiner bar edges where shear concentrates.

Earlywood fibres flatten under load instead, absorbing energy by collapsing longitudinally rather than snapping across the axis. This preferential breakage depletes the stiff long fibres critical for bending stiffness and compressive strength in structural plies.

Various paper substrates and material swatches surround a textured fibrous sheet holding a small rooted plant and laboratory glass instrument on stone.

Swelling and Hornification Mechanics

Hornification shrinks internal surface area through irreversible cross-linking during drying and heating. As water evaporates from inter-fibrillar spaces within the cell wall, capillary tension pulls adjacent microfibrils together, allowing hydroxyl groups on neighboring cellulose chains to form direct hydrogen bonds. When re-wetted in the pulper, water fails to break these rigid internal bonds, leaving the fibre wall stiff and unswollen.

This loss of swelling capacity directly impairs fibre conformability during sheet consolidation. Flexible fibres bend around each other under couch roll pressure and wet pressing, establishing broad contact zones that develop into strong hydrogen bonds during drying. Hornified fibres remain stiff tubes that contact only at crossover points.

The resulting drop in bonded area weakens tensile index, burst strength, and internal bonding in the finished board, forcing higher reliance on synthetic strength resins or surface sizing.

Cell wall collapse is quantified through Water Retention Value testing under ISO 23714. Virgin unbleached softwood kraft pulp typically registers water retention values over 1.80 grams of water per gram of dry fibre. After five drying and repulping cycles without intermediate refining, that value drops below 1.10 g/g ~ a decline that correlates directly with lower wet-web strength on the machine and poor internal bond performance during converting.

A compressed bale of corrugated cardboard sits beside a large circular water filled hydrapulper inside a modern paper recycling facility.
Honeycomb paper cylinder wrapped in textured substrate rests on a matte desk alongside a notebook and metal stationery accessories in a digital render.

Refining

A hydraulic press applies extreme vertical pressure to a dense stack of grey paper sheets and square cut waste fragments.

Mechanical Energy Application and Bar Drag Physics

Stock preparation in recycled boxboard mills must balance strength development against further shortening of already damaged fibres. Low-consistency systems at 3.5 to 4.5 percent solids use double-disc or conical refiners with bar patterns designed to impart shear. The mechanical intensity delivered to the slurry depends on Specific Edge Load (Joules per meter) and Specific Refining Energy (kilowatt-hours per metric tonne of dry fibre).

High edge loads chop fibres, whereas low edge loads compress and fibrillate cell walls without severe loss of length.

Recycled furnish requires low specific edge loads between 0.4 and 0.8 J/m to protect fragile fibres. Pushing refining systems above 1.2 J/m sharply accelerates arithmetic mean length loss. Plate patterns need narrow bars (1.5 mm to 2.0 mm) and tight grooves to maximize impact frequency while keeping energy per impact below the failure threshold of weakened cell walls.

Kraft corrugated cardboard cartons are stacked in a pyramidal structure on a dark steel table surrounded by circular sample housings.

Fibrillation versus Transverse Cutting Mechanics

Refining alters fibre morphology through external fibrillation and transverse cutting. Fibrillation peels thin ribbons from the outer secondary wall, raising micro-fibrils that project into the surrounding slurry. These projections increase the surface area available for hydrogen bonding during drying without shortening the main shaft.

Transverse cutting occurs when a fibre is caught perpendicular to crossing bar edges, snapping the main axis and permanently shortening the fibre population.

The balance between fibrillation and transverse cutting establishes the mechanical limits of recycled board. Low-intensity, high-shear refining promotes fibrillation, preserving the long-fibre backbone required for tear strength and bending stiffness. Excessive normal force from worn bar edges or tight plate gaps crushes fibres instead, destroying length and producing inert fines.

Free fines generated by aggressive cutting reduce web permeability on the forming wire, restricting drainage and driving up dryer steam demand.

Enzymatic pre-treatment prior to refining offers a method to limit mechanical fibre cutting. Endoglucanases selectively hydrolyze amorphous cellulose on hornified fibre surfaces, softening the cell wall matrix. Subsequent refining can then expand the cell wall at lower edge loads, attaining target freeness without heavy transverse cutting.

Refining recycled boxboard furnish at a Specific Edge Load above 1.2 J/m reduces average fibre length by over 30 percent while yielding lower burst strength than controlled low-intensity processing at 0.5 J/m.
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Drainage and Strength Trade-Offs on the Fourdrinier Wire

Fibre shortening directly cuts the hydraulic conductivity of the wet web during sheet formation. Schopper-Riegler and Canadian Standard Freeness tests measure how rapidly water drains through a consolidated pulp pad under gravity. Degraded recycled stock loses freeness rapidly during refining because short fragments and fines pack into the voids between longer fibres, building a dense filter cake on the forming wire.

Slow drainage on the table limits the basis weight a single wire can form at target machine speeds. Mills running high percentages of degraded furnish must lower headbox consistency ~ often below 0.5 percent solids ~ to maintain formation quality and prevent flocculation. Lowering headbox consistency means handling far more water per tonne of production, which overloads flat suction boxes and forces machine slowdowns.

Chemical programs help control fines-driven drainage resistance. Dual-component retention systems using high-molecular-weight cationic polyacrylamides and colloidal silica aggregate fines onto longer fibres. This prevents loose fines from migrating deep into the web, keeping capillary channels open for water removal without disrupting formation.

Molded pulp packaging, white foam cushioning, and greyboard sheets rest upon dark paperboard substrates within a pale studio setting.
Nested corrugated cardboard boxes sit within a metal circular containment collar surrounded by loose fill packing peanuts on a gray workstation.

Forming

A digital graphic composite displays compacted bales of recycled paper alongside shipping containers and an open white industrial box.

Multi-Ply Architecture Strategy for Degraded Stock

Modern recycled paperboard machines ~ producing Coated Recycled Board or multi-ply Folding Boxboard ~ use multi-ply forming sections to optimize furnish allocation. With three to five headboxes depositing separate stock layers onto a combined wet web, mills can place short, degraded recycled fibres in inner core plies while reserving long virgin fibres or clean post-industrial stock for outer top and bottom plies.

Placing short-fibre stock in central core plies preserves sheet bulk at lower raw material cost. Bending stiffness scales with the third power of caliper and the elastic modulus of the outer plies. Structuring the sheet like an I-beam ~ high-modulus, long-fibre stock on the outer faces and lower-density stock in the center ~ lets the core carry shear stress while outer plies handle tensile and compressive loads under flexure.

  • Top Liner Ply ~ Virgin bleached hardwood or softwood kraft furnish providing high surface smoothness, brightness, and ink holdout without structural weakness.
  • Under Liner Ply ~ De-inked post-industrial pulp or clean post-consumer furnish acting as a barrier to hide dark core plies and prevent bleed-through.
  • Filler Core Plies ~ Low-grade post-consumer newsprint and mixed recycled boxboard with high fines content to build bulk and cross-direction caliper profile uniformity.
  • Back Liner Ply ~ Unbleached post-consumer recycled pulp or long-fibre unbleached kraft providing crack resistance during box scoring and folder-gluer operations.
A micrometer assesses the thickness of a white sheet of paper substrate staged in front of stacked bales of recycled fibre in an industrial yard.

Z-Directional Orientation and Internal Bond Strength

Fibre orientation within plies and across ply boundaries dictates internal bond strength, measured by Scott Bond under TAPPI T 569. Jet-to-wire speed ratios at the headbox determine whether fibres align predominantly along the machine direction or remain randomly oriented. Strong machine-direction orientation boosts tensile strength along the web axis, but weakens fibre entanglement across the sheet thickness.

Inter-ply adhesion relies on fibres entangling across wet-web interfaces before the couch press. When core plies carry high proportions of short, hornified recycled fibres, inter-ply bonding drops because short fragments fail to bridge into adjacent wet webs. Mills frequently spray cooked cationic potato or corn starch between forming webs to achieve target Scott Bond values above 150 J/m² on multi-ply CRB grades.

Wet press configurations also shape z-directional density. Shoe presses with extended nip dwell times up to 30 milliseconds consolidate the wet sheet structure without crushing fragile core fibres. This extended nip pressing brings semi-swollen fibre surfaces into close contact, promoting hydrogen bonding and helping compensate for the lost swelling capacity of hornified stock.

Digital render displays disintegrated fiber pulp in a metal sieve alongside cracked substrate panels on a dark testing bench surface.
Multiple stacks of folded corrugated boxes and flat paperboard sheets rest on a steel manual pallet jack within an industrial warehouse setting.

Bench

A dark textured fiber strip hangs above a mechanical feeding system beside a mound of recycled organic pulp used in sustainable substrate production.

Optical Fibre Analysis Protocols and Calibration

Laboratory measurement of fibre degradation relies on automated optical image analyzers operating under ISO 16065-1 and ISO 16065-2. These instruments evaluate thousands of suspended fibres passing through a capillary flow cell illuminated by polarized light or a laser diode. High-resolution CCD cameras capture two-dimensional projections to calculate arithmetic, length-weighted, and weight-weighted mean lengths, alongside mean width, curl index, and kink index.

Calibration requires strict adherence to standardized cut-off limits. Optical counters must evaluate particles down to 0.05 mm to capture fines, though fragments under 0.20 mm are typically excluded from length-weighted metrics so they do not distort the data. Sample preparation demands complete pulp disintegration without mechanical cutting; ISO 5263 disintegration using 30,000 revolutions in water separates fibre bundles without altering the underlying length distribution.

Various material samples including granite, metal, leather, and oxidized copper sheets rest on stacked paper substrates inside a steel environmental test chamber.

Dissecting Length-Weighted and Weight-Weighted Distribution Statistics

Selecting appropriate statistical metrics is critical when reporting fibre degradation. Arithmetic mean length overstates the abundance of fine particles, masking the loss of structural long fibres. Length-weighted average length multiplies each length class by its relative frequency, yielding a metric that tracks tensile development.

Weight-weighted average length squares the length variable, placing heavy emphasis on the longest fibre fractions that govern tear resistance and bending stiffness.

A widening gap between length-weighted and weight-weighted mean lengths signals selective fibre fracture. This divergence reveals a bi-modal distribution ~ remnant long virgin fibres mixed with heavily chopped recycled fragments. Bi-modal distributions cause erratic web behavior during high-speed converting because localized stresses concentrate in regions lacking long structural fibres.

  1. Sample Preparation ~ Disintegrate 5.0 grams of dry board sample in 2.0 liters of deionized water at 20°C per ISO 5263 until free of flakes.
  2. Dilution Protocol ~ Pipette 100 ml of stock slurry into a secondary flask and dilute to 0.01% consistency to prevent fibre overlap in the analyzer cell.
  3. Optical Acquisition ~ Pass the diluted suspension through the optical analyzer at a flow rate of 2.5 ml/s, acquiring images for at least 10,000 discrete objects.
  4. Data Separation ~ Calculate length metrics using cut-offs at 0.20 mm for fines separation and 1.50 mm for long-fibre isolation.
The ratio of weight-weighted to length-weighted mean fibre length provides a direct measure of furnish polydispersity, with values exceeding 1.35 indicating severe bi-modal degradation from mixed waste streams.
A textured gray fibrous sheet travels along a conveyor into rollers to meet a smooth white substrate layer for integrated production.

Standard Conditioning Constraints and Laboratory Pitfalls

Physical testing of recycled paperboard must take place under standard atmospheric conditioning per ISO 187 or TAPPI T 402 (23.0°C ± 1.0°C and 50.0% ± 2.0% relative humidity). Strict climate control is mandatory because cellulose moisture sorption displays notable hysteresis. Having lost internal swelling capacity to hornification, recycled fibres reach equilibrium at lower absolute moisture contents than virgin fibres under identical ambient conditions.

Testing improperly conditioned board samples produces misleading strength data. Dropping ambient relative humidity from 50% to 30% inflates tensile strength while depressing tear resistance and score foldability. Dry, hornified fibres fracture cleanly when bent, causing score lines to burst open on automated packaging equipment.

A digital render shows thermal degradation of a tested substrate resting on holographic barrier film inside a dark laboratory material research setting.
A brown kraft paper padded envelope and a short length of frayed natural fiber rope lie on a metal work surface.

Converting

An articulated robotic arm positions a molded fiber component above compressed stacks of dark recycled paper substrate inside a manufacturing facility.

Score Line Fracture and Failure Modes in Packaging Production

Converting recycled boxboard on high-speed carton lines exposes mechanical weaknesses created by fibre degradation. Creasing matrix tools deform the board along defined channels, creating internal ply separation along the z-axis without breaking the outer linerboard surfaces. Long fibres in liner plies stretch under tension on the outer score bead, while shorter fibres in core plies accommodate shear displacement.

If top or back liners lack sufficient long fibres, outer plies crack along the score line during a 180-degree fold. Score cracking exposes unbleached core fibres or unprinted underlayers, ruining graphics and allowing moisture penetration. Substituting short-fibre post-consumer waste into outer plies drops the critical strain-to-break threshold from 4.5% down below 2.1%, causing carton failures on packaging lines running over 300 cartons per minute.

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Bending Stiffness Loss and Taber Stiffness Recovery

Bending stiffness maintains structural integrity in loaded cartons, retail packaging, and shipping containers. It depends on sheet thickness and elastic modulus ~ both of which suffer from fibre degradation. Short or over-refined recycled fibres produce a dense sheet with low bulk, requiring higher basis weight to match the caliper and stiffness of virgin unbleached kraft board.

Taber stiffness testing under TAPPI T 489 or ISO 2493 measures the bending resistance of 38 mm wide samples deflected through a 15-degree arc. Replacing virgin long-fibre stock with post-consumer recycled furnish lowers Taber stiffness if basis weight remains unchanged. To compensate, mills back off wet-press pressure or widen calender gaps to maintain caliper, though this sacrifices inter-fibre bonding and surface smoothness.

In creased packaging, the ratio of creased to uncreased bending stiffness determines carton opening force. Automated packaging machines use vacuum arms to erect flat cartons into shape. Cartons made from severely degraded stock exhibit unpredictable stiffness recovery, causing panel bowing or resistance to opening that jams high-speed cartoners.

Large rectangular bales of compressed brown cardboard and kraft paper stand vertically stacked outside a dark industrial shed near corrugated metal cladding.

Dust Generation during High-Speed Die-Cutting Operations

Die-cutting boxboard with steel rule dies subjects the internal network to heavy localized compression and shear. As the rule penetrates the sheet, it must shear through individual fibres. Long, flexible fibres cut cleanly and stay anchored in the matrix; short, brittle fragments and loose fines shatter under impact, shedding dust along cut edges.

Die-cutting dust creates persistent problems in downstream converting. Loose particles accumulate on offset printing blankets, causing picking, hickies, and frequent press stops for cleaning. In folder-gluers, edge dust mixes into cold adhesives or hot melts, altering glue viscosity and weakening side-seam bonds.

Vacuum hoods over die-cutting stations capture surface dust, but cannot extract dust trapped inside cut channels. Board mills must manage core ply fines and rely on surface starch sizing to lock short fragments into the sheet matrix.

A mechanical gear assembly shreds a brown paper substrate directly into a laboratory desiccator for chemical analysis of moisture content and material composition.
A dark liquid pours from a black beaker into a glass beaker containing fibrous paper pulp slurry within a laboratory setting.

Ledger

Fibrous recycled material feeds directly into industrial converting equipment as a continuous sheet substrate is prepared for downstream packaging production and distribution.

Furnish Substitution Economics and Chemical Additive Costs

Replacing virgin kraft pulp with post-consumer recycled furnish cuts initial fibre costs per tonne, but introduces chemical and processing expenses to offset lost strength. Switching from virgin softwood unbleached kraft at 820 USD per air-dry metric tonne to Old Corrugated Containers (OCC) at 180 USD per tonne requires additional capital and operational outlay for stock prep, cleaning, fractionation, and chemical dosing.

To match the physical strength of 350 g/m² virgin kraft board with a 100% recycled sheet, mills rely on heavy wet-end chemical additions. Cationic starch at 15 to 25 kg/t adds 22.50 USD to 37.50 USD per tonne. Synthetic dry-strength resins like glyoxalated polyacrylamide (GPAM) or amphoteric polyacrylamides at 3.0 to 6.0 kg/t add another 12.00 USD to 24.00 USD per tonne.

Retention aids, micro-particle drainage aids, and defoamers required to handle high fines loads add an additional 14.00 USD per tonne.

Refining degraded furnish also drives up energy costs. Although recycled fibres require less energy to repulp, low-intensity refining to build strength without severe cutting demands significant power. Running disc refiners at low edge loads increases energy demand by 40 to 80 kWh per tonne, adding roughly 4.80 USD to 9.60 USD per tonne depending on local electricity rates.

A compressed cardboard bale rests on a wooden pallet beneath rows of colored substrate sheets suspended within metal frames in a facility.

Downgauging Limits and Weight Penalty Calculations

Loss of fibre length and bulk forces boxboard buyers to accept a basis weight penalty to maintain structural performance. Packaging converters specify minimum bending stiffness targets so cartons withstand automated filling, pallet stacking, and display. Because recycled furnish has a lower specific elastic modulus, achieving equivalent Taber stiffness demands a higher basis weight.

Replacing 310 g/m² virgin unbleached kraft with Coated Recycled Board (CRB) requires increasing grammage to 375 g/m² to match cross-direction Taber stiffness. That 21% jump in basis weight cuts coverage per tonne: one metric tonne of 310 g/m² board yields 3,225 square meters of printable area, whereas a tonne of 375 g/m² board yields only 2,666 square meters.

The higher basis weight pushes additional costs through the supply chain. Freight expenses for moving raw board to converters and finished cartons to brand owners scale directly with weight. In jurisdictions with weight-based Extended Producer Responsibility packaging fees, heavier recycled packaging incurs higher regulatory surcharges, eroding part of the initial fibre savings.

Rusted threaded shafts and worn metal rollers rest inside a steel tray upon a workbench during mechanical maintenance.

Yield Loss and Converting Spoilage Financial Impact

Fibre degradation introduces converting variability that translates directly to waste and downtime. High fines content and short fibres lower tensile strain limits, increasing web breaks on web-fed presses and folder-gluers. On high-speed sheet-fed offset presses, dusting forces stops every two or three pallets for plate wash-ups, reducing overall equipment effectiveness.

A converter processing 500 tonnes of Coated Recycled Board per month evaluates job costs across substrate yield, print spoilage, die-cutting waste, and gluer downtime. A 3.6% increase in spoilage from score cracking and picking directly increases the cost per thousand cartons. When folder-gluer speeds drop from 350 to 240 cartons per minute to prevent score bursting, fixed overhead costs per carton rise accordingly.

Sourcing decisions must evaluate raw board savings against total landed cost. While 100% recycled grades offer lower initial pricing per tonne, basis weight penalties, chemical surcharges, higher freight, packaging fees, and converting waste often tip the balance toward multi-ply grades that combine virgin and recycled fibre strategically.

Nomenclature

Multi-Ply Forming

Layered Construction ~ Stratified construction of heavy paper or board by combining several layers of wet pulp fibers during the initial stage of the paper machine creates specific performance profiles.

Hornification

Structural Phenomenon ~ Irreversible internal pore collapse within the cell walls of wood pulp fibers occurs during repeated drying and re-wetting cycles.

TAPPI T 569

Standard Methodology ~ Standardized testing protocols in the pulp and paper industry establish consistent procedures for measuring the physical and mechanical properties of paperboard.

Specific Refining Energy

Mechanical Workload ~ Kilowatt hours consumed per oven-dry metric tonne of pulp processed describes the net electrical input applied during the mechanical defibrillation stage of a paper manufacturing line.

Taber Stiffness

Bending Resistance ~ Mechanical rigidity governs how flat paperboard responds to folding forces on high-speed cartoning equipment.

Scott Bond

Fibre Adhesion ~ Adhesive cross-linking efficiency defines how effectively a chemical bridge locks cellulose fibres to a synthetic barrier coating during the lamination phase of board production.

Structural Downgauging

Grammage Reduction Engineering ~ Systematic reduction of paperboard basis weight or thickness achieved while preserving the necessary functional and structural performance of the finished package defines structural downgauging.

Coated Recycled Board

Substrate Composition ~ Mineral-coated paperboard composed of multiple layers of recovered fiber provides a surface for high-quality graphics.

Tensile Strain

Elastic Tolerance ~ Tensile strain defines the proportional elongation of a paper substrate under applied stress relative to its original length before permanent deformation or rupture occurs within the material structure.

Cationic Starch

Starch Affinity ~ Modified carbohydrate derivative introduces quaternary ammonium groups directly into polysaccharide chains to establish permanent positive charges.

Recycled Boxboard

Packaging Substrate ~ Paperboard manufactured primarily from recovered paper and board fibres is the standard material for a wide range of secondary retail packaging.

Schopper-Riegler Freeness

Drainage Measurement ~ Empirical test measures the rate at which water drains from a diluted pulp suspension through a standardized wire mesh.

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