Low-Density Polyethylene (LDPE) Extrusion Coating Resins: Dow LDPE 722 and Cosmothene L420

Low-density polyethylene (LDPE) for extrusion coating is a high-pressure free-radical homopolymer containing long-chain branching and a broad molecular weight distribution. In extrusion coating, the polymer is melted in a single-screw extruder, delivered to a coat-hanger slot die, drawn through an air gap, and laminated against paper, paperboard, aluminium foil, or oriented polymer films. The resins Dow LDPE 722 and Cosmothene L420: Extrusion Coating Resins belong to this category and are used where the melt curtain must remain stable under high draw ratios. Selection depends on melt mass-flow rate, extensional viscosity, neck-in, draw-down, adhesion, seal behaviour, and barrier contribution. Processing is commonly performed on lines with die widths from 600 mm to 3,200 mm, line speeds from 50 m/min to above 300 m/min, and melt temperatures between 280°C and 330°C. LDPE coating grades typically have densities from 0.915 g/cm³ to 0.925 g/cm³ and melt mass-flow rates from 4.0 g/10 min to 12.0 g/10 min when measured at 190°C under 2.16 kg load according to ISO 1133-1:2022 or ASTM D1238.

Processability Parameters for LDPE Extrusion Coating Lines

On single-screw extrusion coating lines with screw diameters from 60 mm to 150 mm and L/D ratios between 24:1 and 30:1, the resin is melted and conveyed to a coat-hanger die with an adjustable lip gap of 0.5 mm to 1.0 mm. Melt temperature is controlled at the adapter and die zones because oxidative modification of the LDPE surface in the air gap is time- and temperature-dependent. Typical melt temperatures range from 280°C to 330°C, with higher set points used for aluminium foil adhesion and low coat weights. Die inlet pressure is monitored with a melt pressure transducer and commonly remains between 5 MPa and 15 MPa depending on throughput, melt temperature, and grade melt index. Chill roll temperature is held between 15°C and 25°C for gloss development, or up to 40°C for improved adhesion to paper surfaces. Air gap distance is typically 100 mm to 250 mm. These parameters are not fixed; they are adjusted for substrate type, line speed, and target coating weight.

Melt curtain stability is not captured by melt index alone. LDPE long-chain branching produces strain hardening in extensional flow; capillary rheometry per ISO 11443 and extensional viscosity measurements show Trouton ratios above 3.0 at strain rates relevant to the air gap. Resins with a nominal melt index near 8.0 g/10 min, such as Dow LDPE 722, provide a balance between screw pumping and curtain stability. A lower melt index increases melt strength but raises melt pressure and thermal load. A higher melt index improves flow but can reduce draw-down and increase neck-in under some conditions. Because molecular weight distribution and branching architecture differ between reactor technologies, two resins with the same melt index can behave differently on the same line. Melt curtain edge position, neck-in per side, and draw resonance onset are therefore evaluated on production-scale dies rather than inferred solely from melt flow data.

What Limits Adhesion to Aluminium Foil and Paperboard?

Adhesion to aluminium foil and paperboard is influenced by melt oxidation, substrate surface energy, and chill roll contact. During the air gap, the LDPE melt surface reacts with oxygen to form carbonyl, hydroxyl, and hydroperoxide species. Longer air gaps of 150 mm to 250 mm increase oxidation and can improve adhesion to paper and board, but excessive oxidation may impair heat seal performance and generate off-taste compounds. For foil structures, LDPE homopolymer lacks primary bonding to aluminium; adhesion is mechanical and polar. Corona pre-treatment of film substrates is typically set to a wetting tension of 38 mN/m to 44 mN/m according to ISO 8296. T-peel adhesion of the coated substrate is measured according to ASTM D1876, and heat seal strength according to ASTM F88/F88M. Low adhesion at the foil surface is often traced to insufficient melt temperature, short air gap, or low chill roll temperature. When fibre tear is observed in paperboard peel tests, the cohesive failure is considered sufficient for many converting operations.

Lot-to-lot variation in melt mass-flow rate for extrusion coating resins is monitored on production lines. A shift of ±0.5 g/10 min can change the screw speed required to maintain a 20 g/m² coat weight and can move the melt curtain edge position. On lines running below 15 g/m², thickness uniformity depends on die gap, melt temperature, and draw ratio. If the melt index falls to the lower end of the grade specification, melt pressure at the die increases and edge tear may occur; if it rises to the upper end, curtain instability and pinholing can increase. These interactions are observed on single-screw extruders with 30:1 L/D and barrier screws when throughputs exceed 150 kg/h. Thermal degradation is also relevant because the melt is held above 300°C in many coating processes. LDPE undergoes chain scission and crosslinking simultaneously; the balance depends on oxygen concentration and residence time. Degradation is monitored by melt pressure drop, gel count in the coated web, and off-odour in coated board.

When Dow LDPE 722 Is Used in High-Speed Liquid Packaging Coating

For high-speed liquid packaging board, Dow LDPE 722 is processed at melt temperatures between 300°C and 320°C. The grade is specified with a nominal density of 0.918 g/cm³ and a nominal melt mass-flow rate of 8.0 g/10 min at 190°C/2.16 kg per ISO 1133-1:2022. Coating weights of 10 g/m² to 25 g/m² are common. At line speeds above 150 m/min, the melt curtain is drawn to final thickness before chill roll contact; the draw ratio may exceed 40:1. Pinhole detection is performed by dye penetration or light table methods because pinholes in liquid packaging allow direct liquid contact with the board. The resin must maintain low neck-in to preserve edge trim and minimize waste. Pilot-line trials using an 800 mm die, 200 mm air gap, and 320°C melt temperature are used to compare grades; published comparative data for this specific configuration is limited. Draw resonance appears as periodic machine-direction thickness oscillation when draw ratio exceeds a critical value. Long-chain branching and extensional strain hardening suppress draw resonance, but the critical draw ratio shifts with melt temperature, die gap, and air gap length. Edge pinning devices and vacuum box adjustments are used to stabilise the curtain and reduce edge thinning.

Vapour Barrier and Seal Integrity in Flexible Packaging Structures

Water vapour transmission rate of unfilled LDPE extrusion coating at 25 µm is generally reported between 15 g/m²/day and 20 g/m²/day at 38°C and 90% relative humidity according to ASTM F1249. Oxygen transmission is high relative to barrier polymers; LDPE is used as a sealant and moisture barrier rather than an oxygen barrier. Heat seal initiation temperature for LDPE extrusion coating grades typically falls between 95°C and 110°C at pressures of 0.5 N/mm² and 1 s dwell when measured by ASTM F2029. Hot tack is measured per ASTM F1921 to evaluate seal strength before cooling. Seal through contamination is limited; converters apply corona or flame treatment and maintain roll cleanliness. Cosmothene L420, when processed on the same equipment, must be evaluated for seal initiation and hot tack because grade-specific additive packages and molecular architecture can shift seal behaviour by several degrees. Substrate pre-treatment is determined by dyne solutions and contact-angle measurement; film substrates with surface energy below 36 mN/m frequently show poor wetting and uneven coating distribution.

Food-contact status for LDPE extrusion coating resins is assessed under FDA 21 CFR 177.1520 and EU Regulation 10/2011. The FDA regulation covers olefin polymers with density between 0.85 g/cm³ and 1.00 g/cm³ and specifies extraction limits for food types. EU Regulation 10/2011 applies an overall migration limit of 10 mg/dm² for food-contact plastics. REACH registration and RoHS Directive 2011/65/EU compliance apply where electrical and electronic packaging waste is involved. Converters should obtain raw material compliance statements and migration test reports from the resin supplier because specific grades include antioxidant and slip packages that affect organoleptic and migration profiles.

Before substituting Cosmothene L420 for Dow LDPE 722 or blending reclaimed LDPE into the coating layer, converters perform pilot runs with the same die, air gap, and chill roll settings. Infrared thermography of the melt curtain identifies edge cooling and curtain thinning. Melt pressure deviation greater than ±1.0 MPa from the qualified set point triggers die gap adjustment or melt temperature correction. Because no laboratory melt-flow test fully predicts neck-in and draw-down, grade substitution is validated on production-scale equipment under the actual line speed and coat weight range.