Poly(butylene succinate) extrusion coating onto unprimed kraft paper replaces LDPE only when the peel adhesion test is treated as a failure-path distribution rather than a single force value. The absence of a primer eliminates silane or polyethyleneimine coupling layers and leaves the PBS melt dependent on mechanical interlocking into surface fibers, disruption of weak boundary layers, and polar interactions with cellulose and hemicellulose. On a production-scale extrusion coating line equipped with a 90 mm diameter single-screw extruder, 33:1 L/D barrier screw, and 1200 mm flat die with internal deckles, the PBS is processed at a melt temperature between 220 °C and 245 °C, discharged through a die gap of 0.5 mm to 0.8 mm, and drawn across an air gap of 120 mm to 220 mm before contacting unprimed kraft paper at the nip. Coating weights between 10 g/m² and 40 g/m² are common where the PBS layer serves moisture-barrier or heat-seal functions; lightweight webs below 15 g/m² are more sensitive to interfacial defects because less polymer mass is available for fiber penetration. Peel adhesion is measured according to TAPPI T 540 om-20 or ISO 11339:2022, with 25 mm wide specimens peeled at 100 mm/min after conditioning at 23 °C and 50 % RH according to ISO 187. Published data for this specific PBS/kraft configuration remains limited; however, pilot coater trials show that adhesion values are governed less by the bulk tensile properties of PBS than by coating weight, paper moisture, melt temperature, air-gap residence time, and furnish composition.

Failure classification designates a peel as fiber tear when more than 70 % of the peeled interface retains paper fibers, or as interfacial peel when the PBS separates cleanly from the paper surface. The threshold between these modes is often expressed as the minimum peel force required for fiber tear, but the threshold shifts with paper topography and coating weight. Because PBS has a density near 1.25 g/cm³ and a melting peak near 114 °C measured by differential scanning calorimetry according to ISO 11357-3, the coating must retain sufficient heat at the nip to penetrate the fiber mat before crystallization. In contrast, LDPE extrusion coatings typically rely on oxidative degradation products generated in the air gap and on low melt viscosity to promote fiber penetration. PBS already contains ester carbonyl groups, so thermal oxidation does not improve adhesion in the same way as LDPE. Instead, the interfacial bond on unprimed kraft depends on the rheological and thermal history of the melt during the millisecond interval between die and nip. A high melt temperature combined with a short air gap promotes penetration but may also induce hydrolysis and melt strength loss. A low melt temperature preserves molecular weight but yields a viscous melt that cannot enter paper capillaries. The resulting process conflict defines the entire substitution problem.

Does unprimed kraft fiber morphology change PBS wetting more than melt surface tension does?

Across bleached and unbleached kraft furnishes, the lignocellulosic surface presents an adhesion response governed by fiber species, refining energy, internal sizing, surface starch, and the topography of machine-wire and felt marks. Softwood kraft from Scandinavian pine or spruce has longer fibers and higher tear resistance, while hardwood eucalyptus kraft produces a smoother and more closed surface. Surface free energy measurements made by contact angle with diiodomethane and water according to ASTM D5946 typically place unmodified cellulose between 42 mN/m and 55 mN/m, but commercially sized kraft can fall below 38 mN/m when alkyl ketene dimer or rosin size is present. PBS melt surface tension at 230 °C is reported in the range of 36 mN/m to 40 mN/m depending on molecular weight and end-group concentration, which provides only a marginal thermodynamic driving force for spontaneous wetting. The practical adhesion therefore relies on pressure-driven penetration into surface capillaries. Paper roughness measured as Parker PrintSurf according to ISO 8791-4 affects the apparent contact area; a rougher surface creates more mechanical anchoring sites but also more void volume where trapped air can inhibit complete filling. The optimal furnish for unprimed PBS coating is an unsized or lightly sized kraft with sufficient internal porosity to accept the melt under nip pressure. In contrast, a highly sized or surface-starched sheet may pass the same dyne-level test before coating but still produce interfacial peel because the weak boundary layer prevents the melt from reaching load-bearing fibers. This divergence between surface energy and peel adhesion explains why dyne testing alone is not predictive. Production audits must include the paper machine post-cure sections and size press pickup, because the same nominal kraft grade can vary in surface energy by more than 10 mN/m across reels.

Furnish changes are more disruptive than melt temperature changes. When a converting line switches from bleached softwood kraft to a recycled furnish containing calendered sheets or mineral fillers, the surface pores close and the adhesion drops even at identical coating parameters. The extent of mechanical interlocking cannot be recovered by increasing melt temperature beyond the PBS degradation threshold. This is a critical operational limitation: LDPE converters commonly raise melt temperature to compensate for substrate variability, but PBS does not tolerate equivalent thermal escalation. Therefore, substrate qualification must be performed before coating. A simple water drop absorption test or contact-angle sweep across the reel width identifies sizing streaks and dryer-limit variation. Peel specimens taken from the front, center, and back of the reel show greater adhesion variability on unprimed kraft than on corona-treated film substrates. Published data for the quantitative relationship between Parker PrintSurf roughness and PBS peel force on this exact substrate is limited, so production sorting based on measured surface roughness remains empirical.

Between the die exit and the pressure nip, the PBS melt curtain is exposed to ambient air and line-speed-dependent strain. The air gap residence time for a 150 mm gap at 120 m/min is approximately 75 ms; at 250 mm and 60 m/min, it increases to 250 ms. Longer residence allows greater surface oxidation and heat loss, but also increases neck-in and melt curtain sag. For LDPE, line operators routinely raise melt temperature to 310–325 °C to compensate for cooling across the gap; PBS undergoes hydrolysis and transesterification at such temperatures. The practical PBS melt temperature is therefore kept below 245 °C, which narrows the available thermal window. Oxidation in the air gap can be monitored indirectly by the change in melt surface free energy after corona and flame treatment; however, the unprimed kraft surface itself is the larger variable. A fiber mat with high surface roughness provides mechanical anchoring even when interfacial chemistry is weak. Conversely, the presence of alkyl ketene dimer or rosin size at 0.01–0.2 kg/tonne of fiber can reduce paper surface energy below 40 mN/m and suppress mechanical adhesion despite adequate melt temperature. Production lines therefore require a previous fiber furnish and sizing audit before stable PBS adhesion is assumed. The nip loading itself is typically set between 40 N/mm and 80 N/mm across the web width; lower settings leave unbonded channels along caliper valleys, while higher settings can densify the paper and pull the melt curtain sideways. Nip roll hardness and cover material also influence the pressure distribution. A silicone rubber roll with Shore A hardness between 70 and 85 is used to accommodate paper caliper variations of ±0.05 mm, while a hard chilled steel roll improves heat removal but may not conform to low-area paper highs.

When residual paper moisture surpasses 6.5%, does the peel interface fail cohesively in the paper?

Conditioning the kraft substrate to a moisture content below 6.5 % prevents steam formation at the nip. Paper reels stored in an uncontrolled warehouse at relative humidity above 60 % RH can reach 8–10 % moisture, which at nip temperatures exceeding 100 °C flashes into vapour. The resulting vapour layer retards melt penetration and creates pinholes at the coating/paper interface. Pre-drying in an infrared or hot-air tunnel at 90–110 °C for 20–40 s is required if the reel moisture exceeds 6.5 %. This operational boundary is not encountered with LDPE under identical conditions, because LDPE processing at temperatures between 310 °C and 325 °C rapidly flashes excess moisture but still permits more aggressive oxidation; PBS operates at less than 245 °C and cannot remove excess moisture before quenching. Batch-to-batch variance in kraft moisture is one of the largest sources of peel adhesion drift on production coaters. Moisture measurement is performed on-line by near-infrared reflectance calibrated against ISO 287 oven-dry weight. A change from 5.8 % to 7.2 % moisture can shift the failure mode from 80 % fiber tear to less than 20 % fiber tear without any change in extruder settings. This shift is partly reversible by pre-drying, but not fully reversible if a hydroxyl-rich skin has already formed on the reel edge. Edge trim areas often show higher moisture due to differential storage, and peel adhesion maps across the web reveal low-force bands near the reel edges. The practical conversion control is to order kraft reels with a moisture content of 5.5 % to 6.0 % and to hold them in a conditioned staging room for 24 h before coating. Coating lines without moisture control cannot qualify this substitution reliably.

Moisture also alters the quench rate. Water vapour condensing at the interface raises the local cooling rate and accelerates the formation of a thin amorphous PBS skin that is weak under peel stress. The same phenomenon is known in extrusion coating of polyesters onto paperboard, where damp substrates produce a white, hazy film edge rather than a clear fiber-tearing bond. For unprimed PBS, this amorphous skin is more critical than for LDPE because the glass transition temperature of PBS is around -32 °C; the skin remains ductile at room temperature but still acts as a distinct layer. Analysis by microtome cross-section under polarized light shows a birefringent outer layer and an isotropic interfacial layer when moisture exceeds the limit. Published data for the specific PBS/kraft interface under the 6.5 % moisture threshold is limited, so process boundaries are set by on-site trials rather than by general polymer data.

Corona discharge energy and ozone venting constraints

Unprimed kraft paper can be pre-treated in-line with a corona discharge unit positioned before the nip, but the energy density must be limited to avoid surface fibrillation and dust. Typical corona doses for paperboard range from 1.0 W·min/m² to 3.0 W·min/m²; higher doses oxidize surface cellulose to low molecular weight fragments that act as a weak boundary layer. The discharge is conducted at a frequency of 15–30 kHz with an electrode gap of 1.5–2.5 mm. In humid conditions above 60 % RH, water vapour in the air gap quenches ozone production and reduces the treatment efficiency. Surface energy after treatment is verified with dyne solutions according to ASTM D2578 or by contact angle according to ASTM D5946. However, a dyne value above 42 mN/m does not guarantee peel adhesion on unprimed PBS-coated kraft because the weak boundary layer created by over-treatment can remain undetected by dyne solution spreading. The better predictor is the presence of paper dust on the backside or in the machine drawers after corona; an accumulation of short fibers indicates that the surface has been burned and mechanically weakened. Unlike polyethylene, PBS lines cannot tolerate loose paper dust because it adsorbs moisture and can hydrolyze the melt inside the nip. Exhaust ducts must vent ozone to below 0.1 ppm occupational exposure limits. If corona pre-treatment is omitted entirely, adhesion on an unsized kraft may still be acceptable when the melt temperature and nip pressure are high, but the operating window narrows by approximately 10 °C. This trade-off is undesirable because the PBS melt temperature window is already limited by degradation. Therefore, low-dose corona or flame treatment is generally applied, but the treatment setting is not the controlling variable for peel adhesion; paper moisture, surface sizing, and melt temperature dominate the failure mode.

Backside treatment is a frequent source of converting-line contamination. The corona treater may be positioned against the backside of the paper to avoid dust generation on the coating side, but some energy inevitably reaches the edges and creates differential wetting. Peel specimens from the edge positions then fail interfacially while the center reaches fiber tear. A split-electrode system with center and edge control reduces this variation. Published documentation of PBS-specific edge effects on unprimed kraft is limited; converting plants are therefore advised to map peel adhesion across the web before releasing production volumes.

When peel test laboratories compare LDPE and PBS on the same unprimed kraft substrate, the test method must be fixed before any force value can be interpreted. Table 1 lists the standard methods and fixture conditions used for this application.

Reported peel force values should always be normalized to specimen width and accompanied by the failure mode designation. A peel force of 2.0 N/25 mm with 100 % fiber tear represents a stronger paper subsurface than a peel force of 3.0 N/25 mm with 20 % interfacial transfer, because the latter indicates a bond that is not propagating into the sheet. Table 2 summarizes the process parameter ranges observed during pilot coating trials on a 90 mm extruder and the associated failure thresholds for PBS relative to LDPE.

Does thermal oxidation of the PBS melt create a brittle interfacial layer that masks fiber-tear adhesion?

Residual moisture in PBS pellets must be dried to below 250 ppm before extrusion, because the ester linkages are hydrolytically unstable at processing temperatures. Drying in a desiccant dryer with a dew point of -40 °C for 3–4 h at 80 °C is typical. If pellets enter the extruder above 500 ppm moisture, chain scission increases the melt flow rate by as much as 40 % and lowers melt strength. The melt flow rate is measured according to ISO 1133-1:2022 at 190 °C with 2.16 kg load; coating grades typically fall between 4 g/10 min and 20 g/10 min. At melt temperatures above 245 °C, the molecular weight distribution broadens and carboxylic acid end groups increase. This degradation can create a low molecular weight polar layer at the melt surface, which is initially more spreadable but subsequently weakens the interfacial boundary. The resulting peel failure is not clean interfacial separation; it is a thin cohesive failure within a degraded PBS skin. Attenuated total reflectance infrared spectroscopy of the failed interface can reveal an increase in carboxylic acid absorption between 3200 cm⁻¹ and 2500 cm⁻¹, but published data for this specific PBS/kraft interface is limited. The safe operating range for one pilot-scale evaluation was 232 °C to 242 °C, a window of 10 °C, before fiber tear fell below 70 % at the edge positions. LDPE offers a much wider effective melt temperature window under the same peel test. The practical consequence is that barrel zone settings for PBS must be tightly controlled and monitored with melt probes, thermocouples, and pressure transducers before the screen changer. A barrel zone override by 5 °C is sufficient to shift the peel failure distribution. Operators must not use the barrel trim heaters to compensate for pressure fluctuations as they would with LDPE.

Screen pack selection also influences the degradation pathway. A breaker plate with 40/60/100 mesh packs may raise shear heating enough to increase melt temperature by 3–5 °C at high backpressure. Replacing a dirty screen pack during a run causes momentary melt temperature and pressure transients that produce a visible adhesive band across the web. The use of a continuous melt filter or a hydraulic screen changer reduces these transients. Extruder throughput and temperature must be restabilized before peel specimens are collected. Published data on long-run PBS coating stability on kraft is limited, but converting-line records show that warm-up and shutdown phases generate off-quality edges that must be removed from subsequent converting operations because the peel test does not capture the entire web-width variation.

Die gap reductions below PBS melt strength limits

Reducing the die gap from 0.8 mm to 0.5 mm increases shear rate and improves gauge uniformity but can destabilize the PBS melt curtain. PBS has lower melt strength than LDPE at similar melt flow rates, and draw resonance appears when the draw ratio exceeds 15:1 to 20:1. The draw ratio is defined as the ratio of line speed to melt velocity at the die exit, which for a 20 g/m² coating at 100 m/min from a 0.5 mm gap can place the melt curtain near this threshold. Instabilities in the curtain create alternating thick and thin bands in the machine direction; thin bands carry less heat to the nip and produce intermittent interfacial peel. The resulting peel force scatter is high even when the average value meets specification. Reducing the die gap also raises melt temperature through viscous dissipation, which may push the PBS above its thermal stability limit. The die gap should therefore not be reduced below 0.5 mm without verifying melt temperature at the die lip and monitoring web thickness across the width. Internal deckles at the die edges generate edge stress and may initiate edge tear; the deckle setting must be adjusted for PBS because its neck-in is lower than LDPE at equivalent coating weight but its edge bead is more brittle. Edge trim waste from PBS-coated paper is also less compatible with standard paper recycling streams when the coating weight exceeds 25 g/m², though this is not a peel adhesion issue directly. The melt curtain stability factor is quantified by oscillation in web tension measured at a load cell before the nip; oscillations greater than ±5 % of set tension indicate that the draw ratio is too high. Published data for PBS-specific draw resonance onset on unprimed kraft is limited, and production settings are developed by incremental line trials.

Adhesion promoters and primers are deliberately excluded from this substitution scenario, but their absence magnifies the effect of paper surface chemistry. On unprimed kraft, the peel adhesion of PBS cannot be increased by adding polar waxes or maleic anhydride grafted polyolefins because the low-melt-temperature processing window and the desire to retain compostability restrict formulation changes. Some commercial PBS coating grades contain chain extenders or nucleating agents that alter the crystallization rate and change the time available for fiber penetration. Chain-extended PBS grades show higher melt strength and lower neck-in, but their higher viscosity at low shear can reduce penetration into paper capillaries. Nucleated grades crystallize at higher temperatures, which shortens the open time at the nip and may lower peel force. The effect of these additives is not uniform across all kraft furnishes. Published data for this specific configuration is limited, so each grade must be evaluated on the actual paper reel rather than on a laboratory cast film. Peel failure maps created from specimens collected across the web and along the machine direction provide the only reliable basis for setting production release limits. The process boundaries identified in the preceding sections define the substitution envelope: melt temperature below 245 °C, paper moisture below 6.5 %, corona dose below 2 W·min/m², and air gap below 220 mm. Operating outside any single boundary produces a peel adhesion collapse that is not predicted by the general data sheets of either the PBS supplier or the paper mill.