What Limits Stickies Removal During Old Corrugated Container Repulping?

In recycled containerboard mills operating on EN 643:2013 grade 1.04 and 1.05 old corrugated containers, the principal source of deposition and sheet defects is fragmented hydrophobic adhesive mass released during drum pulping. The enzyme preparation, standardised for esterase activity against tributyrin substrate at pH 7.0 and 40°C, is added continuously to pulper dilution water at 0.10–0.35 kg per tonne of oven-dry recovered fibre. The effective upper boundary for this enzyme class is 0.6 kg/t oven-dry fibre, beyond which anionic trash load rises and cationic retention aid demand increases without a corresponding stickies reduction. The enzyme cleaves acetyl ester bonds in polyvinyl acetate and acrylic-ester copolymers, converting tacky hydrophobic domains into water-dispersible polyvinyl alcohol moieties and acetate. It does not hydrolyse the carbon-carbon backbone of styrene-butadiene rubber, ethylene-vinyl acetate, polyolefin, or crosslinked polyacrylate hot melts. Process placement is constrained by temperature and pH stability: dosing into the pulper must occur where stock temperature remains within 35–55°C and pH 6.5–8.0 for a retention time of 20–45 min. Injection after high-density cleaning into an alkaline chest above pH 9.0 reduces enzyme half-life to less than 15 min under typical mill conditions. The downstream sequence—0.15 mm slotted fine screens, forward centrifugal cleaners, thickening, and disc dispersion at 80–100°C—removes non-degraded film fragments but cannot return micro-stickies already deposited on fibre surfaces. Macro-stickies concentration in the accepts is monitored by TAPPI T 277; image analysis for visible stickies and plastics follows ISO 15360-2:2015. Finished products include recycled testliner and corrugating medium for containerboard. Food-contact containerboard compliance is verified through extractive limits in FDA 21 CFR 176.170(c) and EU 1935/2004; the enzyme itself is handled under the REACH eSDS. Simultaneous addition of sodium hypochlorite or peracetic acid in the same injection point is incompatible because oxidative residuals inactivate the esterase before it can act on adhesive surfaces.

In tissue mills operating on 100% recovered paper, adhesive-based defects appear as fibre protuberances and holes at the Yankee creping blade rather than as visible sheet dirt. The esterase-containing preparation is dosed into the high-consistency pulper or coarse screening feed chest at 0.15–0.30 kg per tonne of oven-dry furnish when the incoming bale mix contains pressure-sensitive label stock and recycled office waste. The dose is adjusted downward for bales dominated by silicone release liners, ethylene-vinyl acetate backings, or oxidised hot melt fragments because these substrates are not enzyme-degradable. The downstream tissue process—pulping at 4–5% consistency, deinking flotation for white grades, disk refinement, crescent former wet end, and Yankee dryer with creping doctor—generates hood temperatures above 350°C, but the enzyme has already been denatured in the dryer section and its reaction is confined to liquid-phase stock. Compliance for low-basis-weight tissue, kitchen towel, and napkin grades that may contact dry foods follows FDA 21 CFR 176.170 and EU 1935/2004; finished article total nonvolatile extractive testing is conducted under FDA 21 CFR 176.170(c). The enzyme itself is handled under the REACH eSDS. When pulping is carried out above pH 8.5 or at temperatures above 65°C, enzyme half-life falls below 20 min, and a split-dose strategy into the pulper and post-screen chest is required to maintain deposit control on the Yankee surface.

Corrugator Starch Adhesive Viscosity Drift, Gel-Point Kinetics, and Enzyme Inactivation

Corrugated board production consumes a two-component Stein-Hall starch adhesive in which the carrier starch is gelatinised with caustic soda and the raw starch slurry is mixed at the corrugator. The thermostable α-amylase component of the glue control enzyme range is used to reduce carrier starch molecular weight, but residual activity after gel point is the principal processing hazard. The addition ratio is set at 0.15–0.35 kg of enzyme per tonne of dry starch in the carrier phase, or 0.02–0.05% on dry starch, and is trimmed against a target Stein-Hall viscosity of 30–50 s at 61°C. The enzyme cleaves α-1,4-glycosidic linkages, lowers gelatinised carrier viscosity, and permits increased total starch solids without exceeding glue roll pick-up limits. However, the same activity must be fully denatured before the adhesive film reaches the single-facer glue roll; otherwise viscosity continues to fall after application, producing shallow bond lines, dry bond strength loss, and washboarding on the corrugator. Inactivation requires the carrier temperature to exceed 85°C for at least 10 min during cooking, or the addition of a food-grade acidulant to reduce pH below 5.0 after the carrier is fully hydrated. Equipment includes a jacketed carrier tank with turbine agitation, Stein-Hall cup, Brookfield RVT viscometer at 60 rpm, and automatic temperature logging. Finished product types are single-wall, double-wall, and triple-wall corrugated boards for food and industrial packaging. Food-contact corrugated board is subject to FDA 21 CFR 175.105 for the adhesive and FDA 21 CFR 176.170 for the paperboard, while EU member states require compliance with EU 1935/2004. Dry pin adhesion is assessed by TAPPI T 821; viscosity drift is tracked by Stein-Hall cup repeatability of ±2 s. Sodium hypochlorite residuals above 10 ppm in dilution water inactivate the enzyme before starch conversion and produce inconsistent carrier viscosity.

Mixed office waste deinking lines that run alkaline pulping at pH 9.5–11.0 create a process window in which esterase performance is sharply reduced unless the formulation is alkali-tolerant. The glue control enzyme is applied at 0.20–0.40 kg per tonne of oven-dry recovered fibre into the buffered chest after coarse screening where pH is adjusted to 8.0–8.8 and temperature is held below 55°C. This placement avoids the strongest alkaline pulper environment and extends enzyme half-life. The mechanism reduces tack of polyvinyl acetate label residues and supports subsequent flotation removal by collector chemicals, but it does not substitute for alkaline soap flotation or post-flotation screening. Process equipment includes medium-consistency drum pulpers, coarse screens, multiple dissolved air flotation cells, forward cleaning, disc dispersion at 25–30% consistency, and reductive bleach towers. The production output is recycled copy paper, magazine stock, and preprint fillers. Compliance is verified through ISO 15360-2:2015 for residual visible stickies and EN 643:2013 for incoming recovered paper grades; where recycled content enters packaging grades, FDA 21 CFR 176.170 applies. A process conflict occurs when enzyme is dosed directly into the alkaline pulper: half-life falls below 15 min, and the resulting acetic acid release can transiently depress pH but not enough to compensate for activity loss. Published data for this specific configuration on mixtures of toner, hot melt, and polyvinyl acetate label residues remains limited; mill trials are required because toner and ethylene-vinyl acetate fragments are not enzyme substrates.

When Molded Fiber Food Trays Demand Zero Visible Adhesive Specks

Molded fibre plants converting recovered newsprint and kraft trimmings into egg cartons, cup carriers, and fruit trays face a specific specification: visual inspection rejects adhesive specks larger than 0.5 mm because brand owners require a clean food-contact surface. The esterase preparation is dosed at 0.10–0.25 kg per tonne of oven-dry fibre in the pulper or stock chest at 40–55°C and pH 6.5–7.8; the dose is selected using TAPPI T 277 macro-stickies counts after fine screening and TAPPI T 213 dirt count for visible specks in the formed sheet. The downstream process includes high-turbulence pulping at 3–4% consistency, pressure screening with 0.15–0.20 mm slots, vacuum forming into shaped wire moulds, and hot press drying at 160–220°C for 40–60 s. Enzyme action reduces tack of polyvinyl acetate-based label residues so they pass screens before they can smear across the mould wire; it does not remove charred or oxidised hot melt specks that have already been thermally fixed during upstream recycling. Compliance is dominated by FDA 21 CFR 176.170(c) for aqueous and fatty foods, EU 1935/2004, and BfR Recommendation XXXVI for paper and board food-contact materials. Terminal product types are egg cartons, cup carriers, food trays, and vineyard trays. An incompatibility exists with cationic starch and high-charge polyamine retention aids: dosing the enzyme into the machine chest while polyamine is present above 0.5 kg/t can form flocs that reduce sheet appearance, so enzyme addition should be completed before final retention aid application.

Continuous deposition on forming fabrics and press felts in recycled board machines can be reduced by applying diluted enzyme directly to the return run of the forming fabric through low-pressure fan nozzles at 0.02–0.05 kg/t of total production, diluted at 1:500–1:1000 in mill water. The spray is applied before the suction boxes to allow contact with adhesive deposits while the wire returns to the headbox loop. This approach does not replace stock preparation enzyme dosing but reduces the interval between shutdowns for high-pressure shower cleaning of wet-end clothing. Typical process parameters are shower water temperature 35–50°C, pH 6.5–7.5, nozzle spacing 150–300 mm, and spray pressure 2–4 bar. The application is limited to machine clothing that is chemically compatible with enzyme solutions; polyurethane-based forming fabrics show no measurable hydrolysis under these short contact times, but repeated exposure above 60°C is avoided. Compliance for worker safety is managed under REACH (EC) No 1907/2006 and EU CLP Regulation (EC) No 1272/2008; the preparation is classified as a mist inhalant sensitiser, so enclosed dosing skids and local exhaust ventilation are required. Terminal products include all recycled board and paper grades produced on the machine, including containerboard, coreboard, and plasterboard liner. Published data for this specific configuration is limited because deposit reduction depends on fabric age, surface porosity, line speed, and the ratio of fresh to oxidised adhesive; therefore the dosage is established by monitoring fabric permeability and press section draw tension.