At a nominal coating weight of 15 g/m², low-density polyethylene applied by extrusion coating onto liquid packaging board forms a melt film of approximately 16.3 µm when the polymer density is 0.918 g/cm³ in accordance with ISO 1183-1:2019. This thickness is not a simple arithmetic scaling of higher-coat-weight process conditions because the thermal capacity of the melt curtain is extremely low, the quench front from the chill roll reaches the board interface within milliseconds, and the draw ratio between the die gap and the final coating is severe. A die gap of 0.7 mm produces a draw ratio of approximately 43:1; a 0.5 mm gap produces approximately 31:1. Under these conditions, adhesion is governed by the interaction of oxidative melt degradation products, board surface topography and chemistry, water vapour evolution from the sheet, nip contact pressure, and the cooling rate. Typical liquid packaging board substrates have grammages between 250 g/m² and 350 g/m², moisture contents of 6% to 8%, Cobb water absorptiveness values from 40 g/m² to 60 g/m² per ISO 535:2014, and Bendtsen roughness values from 200 mL/min to 800 mL/min per ISO 8791-2:2013. These board properties are not passive; they determine whether the polymer melt penetrates the fibre network, fills surface pores, and creates mechanical interlocking. In production-scale extrusion coating lines, the extruder is typically a single-screw machine with a screw diameter of 90 mm to 120 mm, an L/D ratio of 30:1 to 33:1, and a barrier screw design capable of delivering melt pressure of 180 bar to 240 bar at the die. The slot die commonly has a nominal gap of 0.5 mm to 0.8 mm, with internal deckle blades that allow the wet width to be adjusted before trimming. At a line speed of 400 m/min and a coating width of 1200 mm, the required throughput is approximately 432 kg/h, which represents a high-output condition that places the melt film, die lands, and cooling system under thermal stress. The melt temperature measured at the die exit is commonly set at 320 °C to 330 °C for LDPE; this is significantly above the polymer melting point and is necessary to generate the carbonyl, hydroxyl, and carboxyl polar species that contribute to adhesion against the board. A lower melt temperature of 290 °C to 300 °C may reduce neck-in but usually produces a clean interfacial peel instead of fibre tear, particularly on clay-coated liquid packaging board. The selection of a resin with a melt mass-flow rate of 7 g/10 min to 8 g/10 min at 190 °C/2.16 kg per ISO 1133-1:2022 and density of 0.915 g/cm³ to 0.918 g/cm³ is typical for extrusion coating; resins with narrower molecular weight distribution and lower long-chain branching give higher neck-in and lower draw stability. In this unlabelled opening scenario, the central processing problem is the simultaneous demand for low coat weight, high adhesion, and acceptable edge loss. The process is not bounded by a single variable; it operates at the intersection of melt rheology, oxidation chemistry, substrate moisture, and high-speed web handling.

Adhesion measurement is confounded by the anisotropic and fibrous nature of the board. A peel test conducted with a flexible polymer strip bonded to a rigid paperboard substrate can follow ISO 8510-2:2007, using a 15 mm or 25 mm strip and a 180° peel angle. The measured force is not an intrinsic interfacial property because the polymer may tear, the board may delaminate, or the peel front may oscillate between the polymer-board interface and a subsurface fibre layer. In industrial practice, fibre tear is regarded as the minimum acceptable failure mode, while clean peel at the interface indicates insufficient oxidation, excessive moisture, or board surface contamination. Peel force values in the range of 1.5 N/15 mm to 3.5 N/15 mm are commonly specified for liquid packaging board, but published data for this specific 15 g/m² configuration is limited and should not be extrapolated from laminating adhesives. The measured peel force also depends on the coating thickness; because the LDPE film at 15 g/m² is only 16.3 µm thick, cohesive polymer deformation is small, and the peel test more directly reports the interfacial bond. The standard deviation on a single reel can exceed 0.5 N/15 mm if the board moisture varies across the web or if the clay coating is uneven. Therefore, adhesion data must always be reported together with the board type, Cobb value, moisture content, melt temperature, and line speed.

The selection of LDPE grade for 15 g/m² extrusion coating is constrained by the conflicting requirements of high melt strength for low neck-in and sufficient oxidation potential for adhesion. Tubular reactor LDPE grades with a broad molecular weight distribution and high long-chain branching generally exhibit lower neck-in and better draw stability than autoclave LDPE grades of equivalent melt index, because the long-chain branches increase melt extension viscosity without increasing shear viscosity proportionally. Lower-MFI grades of 4 g/10 min reduce neck-in but may generate excessive melt pressure and reduce line speed; higher-MFI grades above 15 g/10 min increase neck-in and may cause edge instability. Lower density copolymers improve adhesion but can reduce heat resistance; higher density HDPE blends reduce neck-in but usually lose board adhesion and require higher melt temperatures. The coating line therefore operates within a narrow resin property window that must be defined by the converter and the resin supplier together.

Why Does Neck-In Become Unstable When the Air Gap Exceeds 250 mm at Low Coat Weight?

Neck-in in extrusion coating is the reduction in width of the molten web between the die exit and the point of contact with the chill roll. For a die slot width of 1200 mm, a final coated width of 1140 mm corresponds to a total neck-in of 60 mm, or 30 mm per edge. At 15 g/m², this value is not stable because the melt curtain has only 16.3 µm nominal thickness under a draw ratio of 43:1 and is extremely sensitive to air currents, web tension, die pressure distribution, and board surface roughness. The primary driving force for neck-in is the elastic recovery of the polymer after it exits the die lips; the melt expands, then is drawn down, and the edges retract under surface tension. The magnitude is controlled by the melt extension viscosity, the residence time in the air gap, the melt temperature, the die gap, and the draw ratio. When the air gap is increased from 150 mm to 250 mm, the melt curtain remains molten for a longer time, but the residence time at a line speed of 400 m/min increases only from 22.5 ms to 37.5 ms. Despite the small absolute value, the additional time allows surface tension to pull the edges inward, especially because the web is thin and its bending stiffness is negligible. Edge bead formation is a related phenomenon: at the edge, the melt web is slightly thicker and cools more slowly, producing a localised ridge that must be trimmed. The edge bead thickness at 15 g/m² can be 3 to 8 times the nominal thickness, and trim widths of 8 mm to 15 mm per edge are common on production lines. Published data for this specific configuration is limited; the values given here are typical industrial observations rather than universal limits.

The instability described as neck-in becomes irregular when the air gap exceeds 250 mm because the melt curtain is no longer attenuated in a uniform manner. The central portion of the web continues to draw down under the haul-off force, while the edges, which have lower temperature due to radiant heat loss and higher surface-to-volume ratio, draw less. This creates a differential strain across the web and can generate edge flapping or draw resonance. Draw resonance appears as periodic thickness fluctuations in the machine direction, typically in the range of 2 Hz to 10 Hz when the critical draw ratio is exceeded. At 15 g/m², the draw ratio is already 43:1 for a 0.7 mm die gap, so the process operates close to the critical draw ratio; any increase in air gap or melt temperature can push the edges into oscillatory behaviour. The width of the coated board then fluctuates, and the edge trim knives must be set wider to avoid missing the edge, which increases waste. The back pressure from the nip also influences neck-in because the melt web is quenched at the chill roll under mechanical constraint. A soft pressure roll with Shore A hardness of 70 to 80 and a nip load of 20 N/mm to 60 N/mm width forces the melt into the board surface but can also spread the melt slightly if the pressure is too high. This spreading is not a substitute for neck-in control; it occurs only after the melt has contacted the chill roll, and it cannot restore lost width.

Control of neck-in at 15 g/m² involves reducing the air gap to 150 mm to 200 mm, lowering the melt temperature within the oxidation window, selecting a tubular LDPE with broad molecular weight distribution and higher long-chain branching, reducing the die gap to 0.5 mm, and using internal deckle settings that pre-narrow the cast web. Some lines use edge air knives or edge cooling nozzles to quench the edge bead and limit inward movement. The use of edge encapsulation or coextrusion with a lower-MFI edge layer is not typical for monolayer liquid packaging applications because it adds complexity and cost. When the line speed is increased from 300 m/min to 600 m/min, the residence time in the air gap is halved, and neck-in per unit time decreases, but the mechanical forces on the melt curtain are higher. The optimum line speed for 15 g/m² must therefore be established empirically on each line; published data for this specific configuration is limited, and adjusting based on visual observation of the melt curtain is still common in production.

When Melt Temperature Is Reduced to 300 °C, Adhesion Falls Unless Ozone Compensation Is Applied

Reducing the die exit melt temperature from 330 °C to 300 °C is a known method for reducing neck-in because the viscosity and elastic modulus of the melt increase, and the edge retraction rate decreases. However, the oxidative reactions that create polar adhesion-promoting carbonyl, carboxyl, and hydroxyl groups are exponentially retarded at lower temperature. The residence time of the melt curtain in a 200 mm air gap at 400 m/min is only 30 ms, and at 300 °C the rate of oxygen uptake is insufficient to form a continuous polar surface layer. The result is a sharp drop in peel force and a transition from fibre tear to clean interfacial peel, particularly on clay-coated liquid packaging board. This is the central processing conflict for 15 g/m² LDPE coating: the conditions that reduce neck-in also reduce adhesion. Ozone compensation is therefore used to decouple the two variables. Ozone injected into the air gap increases the concentration of reactive oxygen at the melt surface without requiring high melt temperature. The ozone reacts with the LDPE melt to form free radicals and oxygenated functional groups during the short air gap residence time. Industrial ozone generators used in extrusion coating are positioned across the die width and can be adjusted in output; published dose-response data for 15 g/m² liquid packaging board is limited, but the application range is commonly established on each line by measuring peel adhesion and odour. Ozone is normally applied to the board side of the melt curtain, not to the chill roll side, because adhesion is required at the polymer-board interface. If the ozone dose is too high, the surface of the LDPE becomes excessively oxidised, and odour, taste, or off-flavour issues can arise in liquid packaging; this is governed by organoleptic requirements and food-contact regulations.

On a production line, the die is mounted at an adjustable height above the chill roll nip. The air gap is typically set between 150 mm and 250 mm. Ozone injection tubes or a distribution manifold are mounted in the air gap, usually 20 mm to 40 mm above the melt surface, and the ozone-laden air is directed toward the board side. The ozone generator output is commonly balanced with the melt temperature; a line running at 320 °C may need only minimal ozone, while a line running at 300 °C may need a higher dose. The exact dose cannot be specified universally because board moisture, air gap length, melt film thickness, and line speed all affect the concentration of reactive oxygen at the interface. In addition, the board surface may be pre-treated by corona or flame, although corona treatment of paperboard is less common than ozone treatment during LDPE extrusion coating. Pre-corona treatment of the board can increase surface free energy and remove weak boundary layers, but it may also cause surface degradation of the clay coating and increase dust. If used, corona dosage must be kept below the level at which the board surface becomes scorched; typical values for paperboard are lower than those for polymer film, and published data for this specific configuration is limited.

Adhesion promotion by ozone is not a complete replacement for thermal oxidation. Ozone-generated functional groups are concentrated at the outer surface of the melt web, but the melt web is only 16.3 µm thick, so the functional groups can migrate to the board interface during nip contact. At higher coating weights, the ozonated surface remains at the interface because the board contacts the ozonated side before the chill roll; the thickness does not dilute the effect. For 15 g/m², the melt web may be so thin that the ozonated surface and the chill roll side are not distinct, and oxygenated species can affect the entire cross-section. This is an advantage for adhesion but a risk for organoleptic properties because polar groups may absorb flavour compounds. The converter must therefore specify both the adhesion and the sensory performance of the coated board. Adhesion tests per ISO 8510-2:2007 should be supplemented by total migrated substances testing according to EU No 10/2011 and, where applicable, FDA 21 CFR 177.1520 for the LDPE resin.

Quality control for 15 g/m² LDPE extrusion coating on liquid packaging board requires simultaneous verification of coat weight, adhesion, pinhole integrity, surface tension, and neck-in edge loss. Coat weight is measured on-machine by beta or X-ray gauge and cross-checked by gravimetric solvent extraction or burn-off. The tolerance is typically ±1.5 g/m² across the web, but at 15 g/m² the relative tolerance is 10%, which is significant for barrier and adhesion. Adhesion testing uses 180° peel with a 15 mm or 25 mm strip, per ISO 8510-2:2007 or an equivalent internal method; the failure mode is classified as fibre tear, combined fibre tear/interfacial peel, or clean peel. A clean peel at 15 g/m² with low peel force indicates insufficient oxidation, board contamination, or moisture. Pinhole and skips can be assessed by optical scanning, solvent dye penetration, or spark testing; the chosen method depends on the board thickness and production speed. Surface tension of the coated side for subsequent printing or lamination should be at least 38 mN/m according to ISO 8296:2003. This unlabelled section is not an afterthought; the measuring methods themselves interact with the thin coating. Destructive peel testing of 16.3 µm LDPE on a rough board is sensitive to the peel angle and speed, and the board may split before the coating peels.

Board conditioning before coating is an operational boundary. If the relative humidity exceeds 60%, paperboard absorbs moisture unevenly, and the moisture content can exceed 7%. When the molten LDPE contacts the board, water vapour expands at the interface and creates steam blisters that reduce adhesion. The board must therefore be stored and unwound under controlled humidity, usually 45% to 55% RH, and the roll should be allowed to equilibrate for at least 24 h before coating. If the board is too dry below 4%, electrostatic charges increase and the board surface may dust, causing skips. The same humidity control applies to the coated reels before printing or sheeting, because the LDPE layer is a barrier to moisture loss from the board faces but not across the cut edges. In practice, liquid packaging board producers monitor Cobb values and moisture content by near-infrared sensors in the unwind and rewinding units. These are not optional observations; they are essential because the thin 15 g/m² coating cannot absorb or compensate for substrate variability that a 30 g/m² coating might mask.

Chill Roll Finish, Backing Roll Pressure, and Edge Bead Dynamics

The chill roll surface finish and the backing roll pressure are not independent parameters; they determine the contact area, the quench rate, and the localised pressure at the board-polymer interface. A polished chrome chill roll with a surface roughness below 0.05 µm Ra produces a high gloss LDPE surface but transfers heat less effectively if the contact is uneven. A matte-finish chill roll with a surface roughness of 0.3 µm Ra to 0.6 µm Ra increases the contact area and helps remove air pockets, but the resulting coating surface has lower gloss. For liquid packaging board, a gloss surface is usually required for aesthetic and barrier reasons, and the chill roll temperature is set at 15 °C to 25 °C. The backing roll is typically a silicone rubber roll with a Shore A hardness of 70 to 80; the nip pressure is adjusted to ensure contact without crushing the board. Nip loads of 20 N/mm to 60 N/mm of width are common, but the actual pressure depends on the board caliper and the rubber deformation. At 15 g/m², the nip pressure must be uniform across the web; any localised low-pressure area leaves the melt curtain incompletely bonded, and the thin coating cannot redistribute itself. The edge bead is a persistent defect at the junction between the coated and uncoated board; its thickness can be several times the nominal coating, and it cools more slowly because of its higher mass. If the edge bead is not trimmed cleanly, it can crack and generate flakes that contaminate the coating, or it can cause blocking when the coated reel is wound. Edge trim knives must be inspected at intervals and set to remove the entire bead plus a margin of at least 5 mm to 10 mm. The trim is normally recycled as in-plant regrind if permitted by food-contact requirements, but the oxidation level of the trim may be high and must be controlled.

Cooling water flow and delta T across the chill roll are critical for stable coating. At 15 g/m², the heat load from the melt is relatively low compared with heavier coatings, but the line speed is high, and the chill roll must remove heat fast enough to solidify the film before it leaves the roll. The chill roll surface temperature is maintained by chilled water at 10 °C to 18 °C, with a return temperature 2 °C to 5 °C higher. If the chill roll is too cold, condensation can form on the roll surface, causing water spots and intermittent adhesion loss. If the chill roll is too warm, the LDPE surface remains tacky at the stripping point, and the coating may stick to the roll or pick off onto the backing roll. The stripping angle is also important: the coated board is pulled from the chill roll at an angle that must be constant; variations in stripping angle cause transverse lines and can peel the coating from the board before it has fully developed adhesion. On high-speed lines, an air knife or doctor blade is often used to assist release without applying excessive tension to the thin coating.

Backing roll pressure interacts with board surface roughness. A clay-coated liquid packaging board has a compressible surface layer that can be damaged if the backing roll pressure exceeds the crush strength of the board. At 15 g/m², excessive nip pressure can force the polymer into the board and leave a translucent or glossy spot on the reverse side, but does not increase adhesion because the film is too thin. Insufficient nip pressure allows air to be trapped at the interface, producing white spots that fail adhesion testing. The correct pressure must be determined by peel testing and visual inspection across the web; published data for this specific configuration is limited. The edge bead also responds to nip pressure: a higher pressure can flatten the bead and spread it inward, increasing the trimmed width and reducing the usable coated area. Therefore, neck-in measurement should be taken after the nip, not just at the die, because the nip can alter the final edge definition.

Adhesion Failure on Clay-Coated Board Occurs as Clean Interfacial Peel

On clay-coated liquid packaging board, adhesion failure manifests as a clean interfacial peel in which the LDPE separates from the mineral coating without fibre tear. This failure mode is diagnostically distinct from failure on uncoated board, where the polymer may drag fibres with it. The clay-coated surface consists of kaolin, calcium carbonate, and latex binder; it is smoother and less porous than uncoated fibre, and its surface energy is influenced by the binder chemistry and any surface sizing. At 15 g/m², the thin melt film has limited ability to penetrate the mineral layer, so adhesion depends more on the polar interactions between oxidised LDPE and the functional groups on the clay coating. If the board surface is contaminated with silicone, mineral oil, or fatty acid derivatives, the contact angle is high and the peel force drops. The board must be tested for surface irregularities using a tape pull test or a surrogate peel test before coating. In extrusion coating, a small amount of board dust or coating particles can be picked up by the melt web and create pinholes; the melt curtain then breaks at the point of contamination, and the line must be stopped. This is particularly problematic at 15 g/m² because the melt web is so thin that even a 20 µm particle can cause a break.

The oxidation products at the LDPE surface are polar but not strong bonding groups. They form hydrogen bonds, acid-base interactions, and perhaps covalent bonds with hydroxyl groups on the clay and cellulose. The number of such groups is limited by the short air gap and the thin film. When the melt temperature is high and the air gap is long, the polar group concentration increases, but the melt web may also become brittle and generate odour. Adhesion therefore has an optimum, not a monotonic trend. On clay-coated board, this optimum is narrower than on uncoated board; the processing window may be as small as ±5 °C in melt temperature and ±20 mm in air gap. This is a deep-dive zone for process control: published data for this specific configuration is limited, but production experience on 90 mm to 120 mm extrusion lines indicates that the transition from fibre tear to clean peel can occur within a few degrees of melt temperature. The operator must adjust the temperature profile and ozone output gradually while monitoring peel force and odour. A temperature reduction of 5 °C can reduce neck-in by a few millimetres per edge but can simultaneously lower peel force; the exact relationship is line-specific and published data for this specific configuration is limited.

Adhesion failure at 15 g/m² is also influenced by the board moisture and the temperature of the board at the nip. If the board is too cold, the melt film solidifies before it can flow into surface pores; if the board is too hot, the water vapour pressure at the interface can disrupt bonding. Preheating the board with infrared lamps or a heated backing roll is sometimes used, but it is not standard for all liquid packaging board grades. The board surface temperature at the nip is commonly between 20 °C and 40 °C; above 50 °C, steam blister risk increases. The exact boundary depends on the board moisture and the sealing of the fibre network. Therefore, the operator must condition the board and maintain the unwind area at 50% RH and 23 °C.

Electrical discharge treatment of the coated surface, often called corona treatment, is used after cooling and before winding if the coated board is intended for printing, laminating, or liquid packaging sealing. The treatment raises the surface tension of the LDPE from an untreated level of 31 mN/m to 38 mN/m or higher, per ISO 8296:2003. However, corona treatment cannot compensate for poor adhesion at the board interface because it is applied to the exposed chill-roll side of the LDPE, not the board side. The board-side interface is fixed at the moment of nip contact; if the chemistry is not right at that point, no downstream treatment can improve it. This distinction is critical for troubleshooting. When a converter reports low peel force or clean peel, the first variables to check are the melt temperature at the die, the air gap, the ozone output, the board moisture, and the nip pressure. If the failure is intermittent in the cross-machine direction, the die gap uniformity or the pressure roll alignment should be verified. If the failure is periodic in the machine direction, the melt curtain may be surging due to extruder speed instability, temperature fluctuation, or draw resonance. At 15 g/m², a thickness variation of ±1 µm is enough to change the local draw ratio and produce adhesive defects. Thickness gauges with a scanning frame should be used to map the coating profile; the profile should be evaluated after edge trimming but before printing.

Process boundaries for this application are defined by adhesion, neck-in, pinhole integrity, and organoleptic suitability. Adhesion improves with increasing melt temperature, longer air gap within limits, higher board surface roughness, lower board moisture, and ozone addition. Neck-in improves with lower melt temperature, shorter air gap, narrower die gap, higher melt elasticity, and higher line speed. These two groups are in direct conflict: the conditions that improve adhesion generally worsen neck-in, and the conditions that reduce neck-in generally reduce adhesion. The process window at 15 g/m² is therefore a compromise that must be re-established for each board grade and coating line. Pre-drying of the board is required when the storage RH exceeds 60%; if the board moisture exceeds 7%, adhesion loss is likely, and the reel should be returned to a conditioned area. The LDPE resin should be purged with a lower-MFI or linear low-density polyethylene after prolonged shutdowns at high melt temperature to avoid carbonized material in the die. Avoid combining LDPE with amine-based slip additives or certain colour concentrates that can interfere with oxidation and reduce adhesion. The system is not tolerant of uncontrolled process variation; at 15 g/m², the coating thickness is only 16.3 µm, and the margin for error is small.

Adhesion data generated from laboratory peel tests must be interpreted with the board type and test speed documented. A high peel force with full fibre tear is acceptable, but a very high peel force may indicate that the board itself is weak and splitting; this is not necessarily a strong interface. Conversely, a low peel force with clean interfacial peel is unacceptable for liquid packaging because the polymer layer can delaminate during sealing, forming skips and leaks. The quantitative acceptance criterion should be set by the converting operation based on internal capability studies and cannot be replaced by a universal value. Published data for this specific configuration is limited, and the values presented here are intended as typical industrial starting points rather than guaranteed limits. The converting line must establish control charts for coat weight, peel force, neck-in, and edge trim width to maintain process capability under high-speed production conditions.