Substrate Adhesion Variables in Low Density Polyethylene Extrusion Coating

Under industrial extrusion coating conditions, adhesion between low density polyethylene and paper, paperboard, aluminium foil, or corona-treated polymer films is not controlled by a single surface energy value but by the coupled effect of melt oxidation, melt rheology, air gap residence time, substrate preparation, chill roll heat transfer, and nip mechanics. Commercial LDPE coating grades are typically characterised by a melt flow rate of 4 g/10 min to 15 g/10 min at 190°C under 2.16 kg load according to ISO 1133-1:2022 or ASTM D1238-23, with density between 0.915 g/cm³ and 0.925 g/cm³ measured according to ISO 1183-1:2019 or ASTM D1505-18. These grades are processed on single-screw extruders with screw diameters commonly between 60 mm and 150 mm and barrel length-to-diameter ratios of 24:1 to 30:1, fitted with coat-hanger or T-slot dies having internal deckle adjustment. The melt curtain falls through an air gap of 100 mm to 250 mm before meeting the substrate at the nip, where the draw-down ratio is typically between 40:1 and 100:1 and line speeds range from 80 m/min to 350 m/min. At these conditions, the time available for molten polymer to wet the substrate and penetrate surface porosity is often below 100 ms, so adhesion development must be understood as a transient non-isothermal process. The two principal mechanisms are mechanical interlocking into fibre cavities or etched surface features and polar interaction between oxidised species on the LDPE surface and the substrate. The oxidised species are generated primarily by free-radical thermo-oxidation in the air gap and at the die exit, producing carbonyl, carboxyl, and hydroxyl groups that raise the surface energy of the melt. Consequently, any variable that reduces melt temperature, shortens air gap residence time, suppresses oxidation through antioxidant additives, or slows wetting by increasing melt viscosity will reduce peel strength and shift failure mode from cohesive tearing to interfacial delamination. Peel adhesion is commonly evaluated by T-peel according to ASTM D1876-08(2015)e1 or by a 180° peel test according to ASTM D903-98(2017), while surface preparation is checked by wetting tension according to ASTM D2578-23 and contact angle according to ASTM D5946-17. Full-width production lines further introduce lateral variation from die temperature non-uniformity, roll deflection, and substrate moisture profile, all of which must be controlled independently of the polymer grade itself.

What Limits Thermal Oxidation and Peel Strength on Aluminium Foil and Paperboard?

The practical melt temperature window in extrusion coating is bounded at the low end by insufficient thermal oxidation and high melt viscosity, and at the high end by excessive chain scission, fuming, odour, and the formation of a weak boundary layer of low molecular weight polar fragments. Barrel set points on a typical line are staged from 220°C to 240°C in the feed zones to 305°C to 325°C at the die adapter, while the actual melt temperature is monitored by a flush-mounted insertion thermocouple at the die entry. At die lip shear rates of 10³ s⁻¹ to 10⁴ s⁻¹, LDPE coating grades exhibit apparent melt viscosities typically between 20 Pa·s and 60 Pa·s, depending on melt index and die temperature. For aluminium foil lamination, industrial technical bulletins report that T-peel adhesion values may remain below 1.0 N/15 mm at a die melt temperature of 285°C when no ozone or primer is used, but rise to 4.0 N/15 mm or higher when the melt temperature is increased above 305°C at an air gap of 150 mm to 200 mm. The increase is attributed to carbonyl group generation at the melt surface and to reduced melt viscosity, which improves penetration into the aluminium oxide morphology. On paperboard, acceptable adhesion is normally assessed not as a numerical peel force but as 100% fibre tear at the peel interface; failure to achieve fibre tear indicates a shift in the adhesive failure mechanism toward interfacial debonding. Air gap simultaneously influences both oxidation time and melt temperature at the nip. Increasing the air gap from 100 mm to 200 mm may increase oxidation time by 30 ms to 90 ms depending on line speed, but it also reduces the melt temperature at the chill roll by 20°C to 45°C due to convective and radiative cooling of the thin curtain. The apparent activation energy for the relevant melt-phase oxidation process is reported in the range of 90 kJ/mol to 110 kJ/mol, meaning that a small increase in melt temperature causes a disproportionately large increase in oxidation rate. The processing window can therefore narrow to ±5°C in very thin coatings below 15 g/m² on high-gloss paper or foil, where the thermal mass of the melt is insufficient to maintain nip contact temperature and the water vapour pressure from substrate moisture may generate local delamination. Antioxidant packages also interact with adhesion because hindered phenol primary antioxidants and phosphite secondary antioxidants scavenge the same free radicals responsible for polar oxidation. Over-stabilised grades with antioxidant loadings above 1000 ppm total can delay adhesion development and require higher melt temperatures or ozone assistance. The oxidative state of the melt surface can be monitored semi-quantitatively by Fourier transform infrared spectroscopy as a relative carbonyl index, typically expressed as the absorbance ratio of the carbonyl band near 1715 cm⁻¹ to the methylene reference band near 1465 cm⁻¹. A very low carbonyl index indicates insufficient oxidation, while a very high carbonyl index can indicate chain scission and weak boundary-layer formation. Thus the adhesion window is not a single temperature but a kinetic and thermal balance unique to each extruder, die, and substrate combination.

Surface Preparation and Corona Discharge Are Not Sufficient Without Wetting Tension Control

Corona discharge treatment raises the surface energy of nonpolar polymer films by generating reactive oxygen and nitrogen species that create polar functional groups, including hydroxyl, carbonyl, and carboxyl groups, on the exposed surface. An untreated LDPE film typically has a wetting tension of 31 mN/m to 34 mN/m; after corona treatment the surface should reach 38 mN/m to 44 mN/m when tested according to ASTM D2578-23 using mixed glycol ether solutions, or a water contact angle below 70° when measured according to ASTM D5946-17. Biaxially oriented polypropylene and polyethylene terephthalate films used in flexible packaging may require wetting tension values of 40 mN/m to 48 mN/m because their base surface energy is lower and their crystalline surfaces are less receptive to mechanical interlocking. Corona treatment is not permanent: the introduced polar groups migrate into the film or are buried by slip additives such as erucamide, and treated surface energy can decay from 42 mN/m to below 36 mN/m within 30 days under uncontrolled ambient storage. This decay is measured by X-ray photoelectron spectroscopy as a reduction in surface oxygen-to-carbon ratio, which may fall from 0.10 to 0.05 depending on additive migration kinetics. For paper and paperboard, dyne solution testing is invalid because the liquid is absorbed rather than forming a stable droplet; adhesion is controlled instead by moisture content, surface roughness, and preheating. Paper moisture above 8 wt% measured by oven drying according to ISO 287:2017 forms steam at the nip and disrupts wetting, while preheating the web to 85°C to 100°C removes surface water and softens the fibre network. Aluminium foil requires removal of rolling lubricants and may be annealed to develop a controlled aluminium oxide surface; the equivalent surface energy of clean foil is above 44 mN/m, but oil contamination can depress this below 38 mN/m and cause adhesive failure. The following table summarises the substrate preparation conditions that are commonly required before LDPE extrusion coating.

Adjusting Chill Roll Temperature, Nip Pressure, and Quench Rate for Mechanical Anchoring

The chill roll is not merely a cooling surface; its temperature, surface roughness, and the dynamic pressure profile at the nip determine whether the molten polyethylene penetrates the substrate topography before solidification, and therefore whether adhesion is governed primarily by mechanical interlocking or by weak polar contact. In production laminators, the backing roll is usually a rubber-covered steel cylinder with Shore A hardness between 70 and 85, and the applied nip force is commonly 70 kN/m to 100 kN/m across the web width. The resulting contact length in the nip is on the order of 10 mm to 20 mm, and non-uniform pressure caused by roll deflection or uneven rubber ageing can produce transverse adhesion bands even when the melt temperature and substrate treatment are constant. Chill roll inlet water temperature is typically maintained at 10°C to 25°C; lower temperatures raise quenching rate and improve optical clarity but can freeze the melt before it fills deep fibre cavities or etched foil pores, shifting failure toward interfacial delamination. Higher chill roll temperatures above 30°C increase the time available for melt penetration and can improve adhesion to rough paper, but they also increase blocking tendency, curl, and the risk of polyslip or wax migration to the coating surface. The chill roll surface finish controls both adhesion and appearance: a matte finish with arithmetic mean roughness Ra 0.4 μm to 0.8 μm imprints a microtexture that increases mechanical anchorage, while a polished finish below Ra 0.05 μm produces high gloss but provides fewer micro-scale locking sites. The quench rate also controls crystal morphology; rapid cooling suppresses spherulite growth and yields a lower-density, more amorphous surface layer, while slower cooling increases crystalline content and may alter the cohesive strength of the coating. Coating weight and line speed interact with these variables: at coating weights below 10 g/m² and line speeds above 250 m/min, the thermal mass of the melt is so low that even small variations in chill roll temperature or nip pressure can change the peel force by more than 50%. Therefore, adhesion to paperboard is often maximised not by setting the chill roll as cold as possible, but by balancing chill roll temperature, nip load, and substrate preheating so that the melt remains in a penetrating state for the entire nip dwell while still being solidified before winding.

When Ozone Treatment Is Used to Raise Adhesion Without Raising Melt Temperature

If the melt temperature cannot be increased because of heat-sensitive substrates, odour and taint constraints, or unsuitable polymer stabilisation, ozone may be injected into the air gap to accelerate polar oxidation on the exposed surfaces of the melt curtain just before nip contact. Ozone generators used for this purpose typically deliver 40 g/m³ to 80 g/m³ of ozone in the treatment air stream, and the ozone dose is commonly expressed in grams per hour per 100 kg/h of melt output. The industrial target range is often 0.6 g/h to 1.5 g/h per 100 kg/h melt output, depending on melt temperature, air gap, line speed, and coating thickness. Ozone reacts rapidly with unsaturated and free-radical sites on the molten polyethylene surface, raising the concentration of carbonyl and carboxyl groups that are responsible for adhesion to aluminium oxide and polar substrates. However, the reaction is confined to the outer melt surface and has a short residence time of only 30 ms to 150 ms in the air gap, so the treatment is highly line-speed dependent. At very high line speeds above 300 m/min, the ozone dose must be increased to compensate for the shorter exposure, but excessive ozone above approximately 3.0 g/h per 100 kg/h can produce surface pitting, pinholes, odour, and extractable oxidation products. Occupational exposure limits for ozone in the workroom air are generally between 0.05 ppm and 0.1 ppm as an eight-hour time-weighted average, and ozone destruction units with catalytic converters or thermal decomposition are required on the exhaust system to prevent release into the plant environment. The following table summarises representative ozone operating ranges and their effects on aluminium foil adhesion in extrusion coating.

Adhesion promoter chemistry in extrusion-coated laminates is selected according to the polarity and thermal stability of the substrate, the intended converting steps, and the final food-contact or pharmaceutical compliance boundary condition. Ethylene-acrylic acid copolymers are widely used as coextruded tie layers or solvent-dispersed primers because the carboxylic acid groups interact with aluminium oxide and with the hydroxyl groups of paper fibres. These copolymers typically contain 6 wt% to 9 wt% acrylic acid and have melt flow rates from 7 dg/min to 12 dg/min, although exact values vary by supplier. Polyethyleneimine primers are applied at very low dry coat weights of 0.01 g/m² to 0.05 g/m² and form strong polar bonds to corona-treated films, but they are moisture-sensitive and require immediate coating after application. Polyurethane dispersion primers are used for polyester and oriented polypropylene films at dry coat weights of 0.05 g/m² to 0.2 g/m² and require drying at 70°C to 90°C before the extrusion coating station. The selection of a specific primer or tie-layer chemistry is constrained by migration kinetics in the polymer matrix and by the regulatory status of the entire laminate structure. In the United States, laminating adhesives and components may be evaluated under FDA 21 CFR 177.1390 or the applicable olefin polymer regulation FDA 21 CFR 177.1520(c), while in the European Union the final food-contact article must comply with Commission Regulation (EU) No 10/2011, including an overall migration limit of 10 mg/dm² and positive-list authorisation for monomers and additives. Published quantitative peel data for proprietary tie-layer formulations is limited because most commercial primer systems are confidential blends; material suppliers provide application-specific adhesion values and migration statements rather than independently standardised datasets. For substrates stored or processed at relative humidity above 60%, predrying is required to prevent interfacial steam formation and hydrolysis of moisture-sensitive primers, and some amine-based adhesion promoters should not be combined with acid-functional tie layers because premature ionic crosslinking can reduce wetting and increase gel particle formation.

On a production line equipped with a 90 mm single-screw extruder and a 1.5 m coat-hanger die, edge-bead instability and transverse die-lip fouling are recurrent sources of adhesion deviation that cannot be corrected by surface treatment alone. Narrow die gaps of 0.5 mm to 0.8 mm are used to create high draw-down ratios, but die-lip residue from over-oxidised polymer can accumulate within 90 min to 120 min of continuous running and distort the melt curtain. The deckle width is normally set 50 mm to 100 mm wider than the substrate to allow for neck-in, but excessive neck-in at long air gaps produces thin edges that cool prematurely and lose adhesion before the nip. Output rate and line speed are coupled through coating weight: for a 1.5 m wide line, output rates between 150 kg/h and 300 kg/h at line speeds of 100 m/min to 300 m/min produce coating weights of approximately 8 g/m² to 25 g/m². Web tension during laminating is commonly maintained at 100 N/m to 300 N/m width, and excessive tension can pull the substrate away from the melt curtain just before the nip, reducing mechanical interlocking on rough paper. Substrate preheating using infrared panels or heated rollers at 60°C to 100°C reduces the temperature difference between the web and the melt and helps remove residual moisture from hygroscopic paper and board. When an adhesion failure is detected, the diagnosis should begin with wetting tension measurement at the failed interface according to ASTM D2578-23; values below 38 mN/m on film substrates indicate inadequate or decayed corona treatment. If wetting tension is acceptable, the next diagnostic step is to evaluate the oxidative state of the coating by Fourier transform infrared carbonyl index or by comparing T-peel values across the web width. Transverse adhesion mapping at intervals not exceeding 500 mm across the web is used to identify die-temperature drift, localised die-lip fouling, or chill roll contact inconsistencies that require mechanical or thermal correction before further polymer or substrate changes are made.