Articles
Articles
Articles
In recycled board furnishes derived from old corrugated containers (OCC), mixed recovered fibre, and deinked pulp, the aqueous phase accumulates soluble and colloidal anionic material known collectively as anionic trash. The dominant species include oxidised lignin fragments from residual neutral sulphite semichemical fluting, polygalacturonic acids released by alkaline swelling of fibre fines, fatty acid and resin acid soaps formed by alkaline hydrolysis of unremoved synthetic adhesives, polyacrylic dispersants from recovered coated broke, and silicates from peroxide bleaching of deinked grades. The resulting cationic demand, measured by poly-diallyldimethylammonium chloride titration in a Mütek PCD-04 with a 0.001 N titrant and expressed in microequivalents per litre, typically fluctuates between 350 µeq/L and 900 µeq/L for 100% OCC liner mills operating at fresh water intakes below 4 m³/tonne. In closed-loop systems where clarified white water represents more than 70% of the machine shower water, published mill data indicate cationic demand values above 1,200 µeq/L and conductivity values above 4,000 µS/cm are not uncommon; these conditions reduce the performance of cationic starch and polyacrylamide retention aids through charge neutralisation and competitive adsorption. Alkenyl succinic anhydride sizing is particularly sensitive to this matrix because ASA reacts with water and anionic species at rates that are competitive with the desired esterification to cellulose hydroxyl groups. The hydrolysis product of ASA—alkenyl succinic acid—is a surface-active diacid that can increase stickies deposition on forming fabrics and press felts.
Under such conditions the wet-end control strategy should prioritise anionic trash inactivation with low-charge cationic polyamine or poly-DADMAC polymers upstream of the ASA emulsification point. A practical target for headbox cationic demand is below 600 µeq/L, with below 350 µeq/L desirable for lightweight recycled board under 150 g/m². High cationic demand consumes cationic starch at the ASA droplet interface, destabilises emulsion particles, and creates macro-emulsion droplets that migrate to foam and deposits. Charge neutralisation at the droplet surface can be monitored by streaming current detection or zeta potential measurement on a Mütek SZP-10; a streaming current value of 0 ± 50 mV is a frequently used target, but this target must be re-established after each fibre furnish change because fibre fines charge is not constant. The esterification reaction itself, which creates a covalent linkage between ASA and the cellulosic hydroxyl group, is favoured at drying temperatures above 80°C; therefore the initial adhesive binding of the retained ASA-starch complex to the fibre surface and the subsequent thermal activation in the dryer determine the final Cobb value measured according to TAPPI T 441 om-20 or ISO 535:2014.
ASA sizing efficiency in recycled board wet ends is controlled by the competition between hydrolysis and esterification, by the particle size distribution of the emulsified ASA, and by the electrical charge environment of the fibre surface. Alkenyl succinic anhydride is not a surface-sizing agent; it must be emulsified with cationic starch or a synthetic cationic polymer before injection. The emulsified droplets carry positive surface charge only when the cationic starch protective colloid remains adsorbed at the oil-water interface after the emulsion passes through the machine screen and the fan pump. In a closed recycled-fibre water loop, the high anionic trash concentration and the elevated alkalinity reduce the effective positive charge on the droplet surface and accelerate hydrolysis to alkenyl succinic acid. Machine-specific data for this exact recycled furnish combination remain limited, but process trials on OCC liner machines indicate that a closed-loop white water system with total alkalinity above 400 mg/L as CaCO₃ can force ASA dosage increases of 20–40% to maintain equivalent Cobb 60 s values.
The emulsion particle size itself is a decisive parameter. Rotor-stator emulsifiers operating at tip speeds of 18–25 m/s and back-pressures of 1.5–2.0 bar with cationic starch solution at 15–20% solids typically produce a D50 in the range of 1.0–2.5 µm when the starch-to-ASA ratio is maintained at 2:1 to 4:1 on active solids. Emulsions with D50 above 4.0 µm exhibit lower sizing development per unit of ASA because the available surface area for esterification decreases and because larger droplets are more readily detached by hydrodynamic shear in the forming zone. Conversely, emulsions below 0.7 µm may be over-emulsified, generating excessive interfacial area that accelerates hydrolysis and consumes starch. Laser diffraction using a Malvern Mastersizer 3000 equipped with a liquid dispersion unit is commonly used to track particle size; the span of the distribution, defined as (D90-D10)/D50, should remain below 1.5 for stable machine operation.
The point of addition relative to the fan pump and screen is equally important. ASA emulsion should be injected after the pressure screen and centrifugal cleaners but before the fan pump; this placement minimises residence time in high-shear zones and avoids contact with hot, acidic or highly conductive streams. Contact time from the emulsifier outlet to the headbox should be maintained below 30 s, and the temperature of the emulsion should be kept below 35°C. If the distance between the emulsifier and the fan pump suction exceeds 20 m, an intermediate static mixer is required to prevent cream separation and droplet coalescence. Process-scale failure modes observed on recycled board machines include deposition of tan-coloured alkenyl succinic acid calcium salts on the top wire return roll and on the first dryer cans, foam accumulation at the suction couch roll because of free alkenyl succinic acid, and uneven sizing across the web when the emulsion feed line is not adequately heat-traced.
Because quaternary ammonium starch functions simultaneously as an emulsifier, a retention-promoting cationic donor, and a dry-strength additive, its degree of substitution, molecular weight, and acid-thinned viscosity govern both emulsion stability and machine retention. Wet-end cationic starches are typically potato, corn, or waxy maize starch derivatives modified by reaction with 3-chloro-2-hydroxypropyltrimethylammonium chloride, yielding a quaternary ammonium group that remains cationic across the pH 4–9 range. The degree of substitution for ASA emulsification grades is usually 0.035–0.050 per anhydroglucose unit; lower values below 0.025 provide insufficient droplet surface charge, while higher values above 0.060 can generate excessive soluble cationic charge that destabilises the emulsion and increases biological growth. Starch cooks are prepared in batch cookers at 95–98°C for 30 min, or in continuous jet cookers at 120–130°C with a residence time of 1–2 min; complete gelatinisation is essential because ungelatinised granules create microscopic hydrophobic deposits that retain as sticky material rather than as fibre-bound starch.
The molecular weight of the starch fraction used at the emulsifier should be moderate. High molecular weight cationic starch improves bridging but can overflocculate the stock and degrade formation when the same starch is overfed to the machine chest. In recycled board production, a split addition strategy is therefore common: 50–70% of the total cationic starch is added as cooked starch to the machine chest or blend chest for dry strength and baseline charge, while the remaining 30–50% is used as the ASA emulsifying starch. This split improves emulsion stability without overloading the wet end with soluble cationic polymer that is consumed by anionic trash. The starch solution used at the emulsifier should have a Brookfield viscosity in the range of 150–300 mPa·s at 60°C and 10% solids; batch-to-batch excursions outside this range can alter the droplet size distribution at constant emulsifier settings. Cationic starch storage and handling boundaries include maintaining cooked starch above 60°C to prevent retrogradation, avoiding storage beyond 8 h, and using biocide only where compatibility with the ASA emulsion has been confirmed, because some oxidative biocides can destabilise the starch film at the droplet surface.
Retention of the ASA-starch complex occurs through a combination of cationic adsorption to anionic fibre surfaces and heteroflocculation by high molecular weight cationic polyacrylamide. The cationic starch on the droplet surface provides the initial anchoring, but in recycled board furnishes with high fines content, a single polymer is rarely sufficient. A dual or microparticle retention programme is usually configured with a low-charge polyamine or poly-DADMAC added before the machine screen to neutralise dissolved anionic trash, a cationic polyacrylamide of high molecular weight added after the screen, and bentonite or silica microparticles added after the fan pump. The bentonite dosage typically ranges from 2.0–4.0 kg/t on a dry fibre basis, while the cationic polyacrylamide dosage ranges from 0.05–0.15 kg/t active. The microparticle system improves retention without the severe overflocculation that would occur if cPAM dosage alone were increased to overcome anionic trash. This is particularly important for recycled board because retention aid overfeed produces hard flocs, poor formation, and uneven ASA distribution, which appears as patchy Cobb values on the finished reel.
In mills where clarified white water is returned with minimal blowdown, bicarbonate and carbonate ions accumulate from calcium carbonate filler dissolution and from the alkaline hydrolysis of recycled fibre extractives. At pH 7.5 and total alkalinity of 400 mg/L as CaCO₃, the bicarbonate buffering capacity is sufficient to hold the wet-end pH above 7.0 for long periods, and the base-catalysed hydrolysis of ASA becomes the dominant reaction. The pseudo-first-order rate constant for ASA hydrolysis increases by roughly a factor of 10 for each unit increase in pH between pH 6 and pH 8; published data for this specific configuration is limited, but the observed consequence is that the retained fraction of ASA can fall below 40% of the fed amount when contact time exceeds 45 s. The resulting alkenyl succinic acid has no esterification activity and migrates to the wet-end foam, forming calcium soaps when calcium hardness exceeds 600 mg/L as CaCO₃.
Under this alkalinity regime, the injection point must be moved as close as possible to the headbox. A static mixer at the fan pump suction followed by a contact time of 10–15 s to the slice is the standard control measure, but this is not possible on every machine because of long approach piping. An alternative is to reduce total alkalinity below 250 mg/L as CaCO₃ by increasing fresh water purge or by controlled addition of acid; however, acid addition must be applied to a side stream and not directly to the ASA emulsion. Direct acidification of the ASA emulsion to below pH 6 can result in acid-catalysed hydrolysis and a hydrolysis exotherm in the emulsifier. Process conditions should therefore be maintained at pH 6.5–7.5 at the injection point, with total alkalinity below 300 mg/L as CaCO₃ for optimal mill control.
The practical detection of hydrolysis-driven sizing loss in recycled board is usually delayed because the Cobb value may not rise until the ASA dosage has been increased several times and the hydrolysis products have begun to collect on the dryer can surfaces. Online monitoring of emulsion particle size and zeta potential at the white water tray can provide an earlier warning than off-line Cobb testing. Zeta potential values that drift below −10 mV at the tray may indicate that free anionic hydrolysis products have saturated the cationic starch and that the retained ASA droplets have lost their positive charge. The corrective action in that state is to reduce ASA feed for 1–2 h, add a poly-DADMAC trash catcher, and purge the white water through the saveall; continuing to increase ASA under such conditions typically worsens deposition and yields no improvement in sizing.
| Control parameter | Test method or instrument | Operating window | Failure boundary and corrective action |
|---|---|---|---|
| Headbox cationic demand | Mütek PCD-04 with 0.001 N poly-DADMAC | 250–600 µeq/L | Above 800 µeq/L; add polyamine or purge white water |
| ASA emulsion particle size D50 | Malvern Mastersizer 3000 laser diffraction | 1.0–2.5 µm | Above 4.0 µm; check emulsifier tip speed, starch ratio, back-pressure |
| Cobb 60 s water absorption | TAPPI T 441 om-20 / ISO 535:2014 | 35–70 g/m² for recycled liner | Above 100 g/m²; verify hydrolysis and retention |
| Contact time from emulsifier to headbox | PLC tracer test with dye or conductivity spike | 10–30 s | Above 45 s; relocate injection to fan pump suction |
| pH at ASA injection | inline pH electrode | 6.5–7.5 | Above 8.0; move acid addition to side stream, increase purge |
| Total alkalinity | ISO 9963-1:1994 water quality alkalinity | 100–300 mg/L CaCO₃ | Above 400 mg/L CaCO₃; purge saveall, reduce white water reuse |
| Conductivity | inline conductivity cell | 1,500–4,000 µS/cm | Above 5,000 µS/cm; increased fresh water make-up |
| Cationic starch degree of substitution | Kjeldahl nitrogen or colloid titration | 0.035–0.050 | Below 0.025; replace with higher-DS starch for ASA emulsification |
Across twin-wire gap formers operating above 900 m/min, retention time from the point of ASA injection to the first vacuum flatbox may be less than 15 s; this short residence time means the adsorbed starch-stabilised ASA droplets must be fixed by a retention aid before hydrodynamic shear removes them. The first-pass retention target in recycled board production is typically 75–85% for the total stock, with ash retention values often 30–50% depending on filler loading. In a dual polymer system, the cationic polyacrylamide is introduced after the pressure screen because high shear after flocculation breaks the polymer bridges; the bentonite or silica microparticle is then added after the fan pump to re-densify the flocs through charge neutralisation. The sequence is critical: adding bentonite before cPAM results in competing adsorption and consumes the microparticle without retention gain, while adding cPAM too early exposes the flocs to screen shear and centrifugal cleaning. Process-scale machines running recycled board with 100% OCC and a closed white water system frequently report that a single-polymer retention aid programme produces acceptable first-pass retention but poor ASA sizing variability because ASA droplets are not retained evenly across the sheet. The standard corrective pattern is to add a low-charge anionic trash catcher at 0.3–1.0 kg/t active, followed by cPAM at 0.05–0.15 kg/t active, and then bentonite at 2.0–4.0 kg/t.
High conductivity and dissolved calcium interfere with this retention programme by shrinking the hydrodynamic radius of the cationic polyacrylamide and by reducing the charge patch effect on the fibre surface. When conductivity exceeds 5,000 µS/cm, cPAM begins to behave as a neutral polymer because the electrical double layer is compressed; retention improvement falls and the polymer may deposit as a sticky film on suction rolls. Dissolved calcium above 600 mg/L as CaCO₃ can also bridge anionic alkenyl succinic acid hydrolysis products into calcium soap deposits, particularly on the first press felt if the fabric is not equipped with a high-pressure reciprocating shower. The operational boundary for calcium hardness therefore depends on the extent of ASA hydrolysis, the white water closure, and the felt conditioning system.
The interaction between sheet formation and ASA sizing uniformity becomes most visible on multi-layer recycled board machines where the top ply contains heavily refined recovered fibre and the middle ply contains coarse OCC. In such configurations, the droplet retention capacity of the top ply is high because of the large specific surface area of fines, but the drainage resistance is also high, causing the top ply to remain wetter at the first press. If ASA hydrolysis products are present, these wetter conditions allow alkenyl succinic acid to migrate to the sheet surface and form a greasy film on the first press felt. The felt conditioning system must therefore be operated with a minimum felt tension of 4.5 kN/m and high-pressure showers at 25–30 bar using clarified white water rather than fresh water only; otherwise the felt becomes contaminated with a calcium salt deposit that is difficult to remove with conventional acid or alkali felt wash.
The moisture content of the sheet entering the size press or blade coater is not the primary driver of ASA sizing, because ASA does not require surface application; however, the extent of thermal activation in the dryer section is. The esterification reaction between ASA and cellulose proceeds at measurable rates only above 80°C, and the reaction is limited by the residence time in the dryer. On machines producing recycled board above 350 g/m², the centre of the sheet may remain below 80°C for much of the dryer section, leading to a condition in which the surfaces are sized but the centre plies are not. A Cobb test on the finished board may appear acceptable while the internal fibre matrix remains hydrophilic, causing edge wicking and water bleed in die-cutting. For this reason, mill quality control should include an internal sizing test such as edge wick testing with low surface tension ink or a Cobb test on a split sheet, rather than relying only on surface contact angle.
Limits for operational control include a maximum board entering moisture of 55% at the first dryer can, a reeling moisture content of 6–8%, and a furnish pH at the headbox of 6.8–7.5. Exceeding these boundaries with ASA present often produces picking at the dryer can and a rise in reel end dust, both of which are traceable to alkenyl succinic acid hydrolysis products rather than to cationic starch alone. When such symptoms occur, the immediate action is to remove the ASA emulsion and purge the system with fresh water for 2 h while maintaining cationic starch feed; this isolates the deposit source from the starch source. Published data for this specific configuration is limited, and the observations below are therefore drawn from machine audits rather than controlled laboratory studies; nonetheless, process-scale observations on recycled board machines indicate that this combination of symptoms is rarely caused by cationic starch retention alone when starch degree of substitution and cooking temperature are within the ranges specified.
Deposit control on recycled board machines using ASA and cationic starch requires recognition that the hydrolysis product is not a single species and that calcium soaps of alkenyl succinic acid have different solubility behaviour than starch-based deposits. The deposits on the first dryer cans are often a mixture of calcium alkenyl succinate, cationic starch, and polymer fines from polyacrylamide. Removing these deposits with organic solvents may dissolve the alkenyl succinic acid component but leave the starch matrix, which is water-sensitive and can be removed only with hot water and alkali. In practice, a rotating high-pressure shower on the fabric and a chemical felt wash sequence at pH 10–11 followed by a rinse at pH 6–7 provides the best balance between deposit removal and felt life. The use of continuous solvent-based wire passivation is not recommended when ASA is present because the solvent can extract the alkenyl succinic acid and spread it across the fabric.
Microbiological slime in starch and ASA feed lines is an additional failure boundary. Cationic starch solutions above 60°C suppress microbial growth, but dead legs and recirculation lines at 40–50°C provide ideal growth conditions for Bacillus and Pseudomonas species. These organisms consume starch and produce organic acids that lower pH and destabilise the ASA emulsion. A biocide programme using an oxidising biocide in the white water loop and a non-oxidising biocide in the starch line should be validated for compatibility with ASA; quaternary ammonium biocides are often incompatible with ASA emulsion stability and should not be injected directly into the emulsifier feed.
Regulatorily, alkenyl succinic anhydride and cationic starch used in recycled board intended for food contact must comply with the positive list framework of the destination market. In the United States, ASA is permitted as a component of paper and paperboard under 21 CFR 176.170 and 21 CFR 176.180, subject to good manufacturing practice and the absence of harmful extractives. In the European Union, paper and board food contact materials are regulated under Regulation (EC) No 1935/2004, with national or BfR Recommendation XXXVI providing presumptive compliance for sized board; ASAs and cationic starches are acceptable only if the finished article does not transfer constituents to food in quantities above the overall migration limit of 10 mg/dm² or the specific migration limits for individual hydrolysis products. The REACH registration duties under Regulation (EC) No 1907/2006 apply to the neat substances ASA and cationic starch, but not to the finished paperboard; however, the alkenyl succinic acid hydrolysis product and residual quaternary ammonium starch should be considered when preparing food contact documentation.
| Requirement | Standard or regulation | Applied to | Boundary or test method |
|---|---|---|---|
| United States food contact | 21 CFR 176.170 | ASA and cationic starch as components of paper and paperboard | Good manufacturing practice; no harmful extractives |
| United States dry food contact | 21 CFR 176.180 | Sized board for dry foods | No migration to food |
| European Union framework | Regulation (EC) No 1935/2004 Article 3 | Finished recycled board | Safety and inertness; overall migration below 10 mg/dm² |
| German BfR recommendation | BfR Recommendation XXXVI | Paper and board for food contact | Positive list compliance; organoleptic tests |
| Cobb water absorption | TAPPI T 441 om-20 / ISO 535:2014 | Sized recycled board | 35–70 g/m² for liner; target by grade |
| REACH registration | Regulation (EC) No 1907/2006 | Neat chemicals ASA and cationic starch | Registration; safety data sheet |
| Recovered paper quality | EN 643 | Furnish | Food-contact grades restricted to suitable recovered paper |
Production-scale compliance monitoring for food contact recycled board normally includes a combination of extraction test methods and surface sizing quality tests. The water absorptiveness of the board is measured by TAPPI T 441 om-20 or ISO 535:2014, and the results are used as an indirect indicator of cross-linking and barrier formation, but these methods do not measure migration. For migration assessment, the applicable analytical methods are those specified in EU Regulation (EC) No 10/2011 for plastics and in BfR Recommendation XXXVI for paper, using food simulants assigned to the intended use. A practical boundary for recycled board containing ASA and cationic starch is the requirement that the extractive content of the sized board, measured after conditioning according to ISO 187 and a solvent extraction method equivalent to TAPPI T 204 cm-17 adapted for paper, should not exceed the level at which organoleptic taint is detectable by an accredited panel. If the furnish contains recovered paper that is not suitable for food contact under EN 643 grade definitions, the mill must demonstrate that the migration of mineral oil hydrocarbons, phthalates, and subsequent hydrolysis products remains below the applicable limits; otherwise the use of ASA sizing and cationic starch retention in that specific recycled board configuration is limited to non-food grades.
Operational limitations arise when the recycled furnish contains recovered paper with unknown adhesive chemistry. ASA will react not only with cellulose but also with hydroxyl groups in starch-based adhesives and with amine-containing contaminants; the latter reaction can release low molecular weight amides that affect odour and taint. Because cationic starch retention is also depressed by high levels of dissolved and colloidal material from pressure-sensitive adhesives, the combined use of ASA sizing and cationic starch is not recommended for recycled board furnishes containing more than 5% visible stickies by mass unless an alkaline stickies control agent and a scavenging bentonite loop are installed. In those cases, published data for this specific configuration is limited, and the mill must validate the finished board against the applicable food contact migration limit before release.
