Chevron Phillips Marlex® 7308DK is processed as a linear low-density polyethylene film resin. Published technical literature lists a nominal density of 0.918 g/cm³ when determined in accordance with ASTM D1505, and a nominal melt index of 0.80 g/10 min at 190 °C with a 2.16 kg load per ASTM D1238. In blown film installation, the resin is commonly introduced through a 24:1 to 30:1 L/D single-screw extruder with a barrier screw and a Maddock mixing section. Melt temperature at the die is controlled within 190 °C to 220 °C. A die gap of 1.8 mm to 2.5 mm is typical. The bubble is expanded at a blow-up ratio of 2.0:1 to 3.0:1. On a 300 mm annular die, the frost line height is normally held between 0.65 m and 1.10 m above the die face. The combination of die gap, blow-up ratio, and frost line height determines the orientation balance and the final drop mass for thin-gauge film. The grade is suited to applications where puncture resistance and seal behaviour must be balanced against draw-down and bubble stability.

What Limits Output When the Frost Line Is Held Above 1.2 m on High-Stalk Bubbles?

On high-stalk grocery sack lines, the practical output ceiling is controlled by bubble cooling rather than by screw capacity. If the frost line is raised above 1.2 m, the unsupported molten bubble becomes sensitive to ambient air currents and die-lot chatter. The result is transverse-direction gauge variation. Film produced with a target gauge of 25 µm on a 300 mm die at a blow-up ratio of 2.5:1 can drift from 22 µm to 28 µm across the circumference when the frost line is not stabilised. High-stalk configurations use internal bubble cooling and a close-fitting outer air ring. The extruder back pressure on a 30:1 L/D barrier screw typically falls between 18 MPa and 30 MPa at 80 rpm. Increasing screw speed beyond the point where the bubble fails to cool causes the stalk to oscillate at 0.5 Hz to 1.5 Hz. This oscillation is transferred into the collapsing frame and becomes visible as film sag in the haul-off. Melt fracture on the inner or outer bubble surface occurs if the die gap is reduced below 1.6 mm while the melt temperature is held below 190 °C. The material’s melt fracture threshold under these conditions is influenced by die land length. A land length of 10 mm to 15 mm on the annular lip is commonly used. For stable high-stalk operation, the melt temperature is raised to 210 °C to 220 °C, but this raises total heat load and reduces bubble stiffness. Output per die circumference is typically not pushed beyond 0.35 kg/h per mm die diameter when the target gauge is thin. Dart impact of the resulting 25 µm film, measured by ASTM D1709-15A Method A, is routinely specified by converters as ≥ 90 g for grocery sacks, while some converter specifications require ≥ 120 g for high-stalk LLDPE. The specific value for 7308DK will depend on die gap, blow-up ratio, and internal bubble cooling; published data for this specific configuration is limited.

Blending 7308DK with high-pressure LDPE in the range of 20 wt% to 30 wt% is a production-scale method for stabilising heavy-duty sack film. The addition of LDPE increases melt strength and widens the operating window at high gauge. On a 90 mm grooved-feed extruder with 30:1 L/D, the compound is run at melt temperatures of 200 °C to 230 °C. The die gap is held at 2.0 mm to 2.8 mm. Back pressure at 95 rpm is typically 25 MPa to 35 MPa, depending on the screen pack configuration. A screen pack of 60/80/100 mesh is replaced at intervals of 8 h to 12 h when recycled edge trim exceeds 10 wt%. Film gauge for heavy-duty sacks is commonly set at 75 µm to 150 µm. At these thicknesses, heat removal through the bubble becomes the limiting factor. Increasing throughput by raising melt temperature above 230 °C causes the bubble to sag and reduces transverse-direction tension. The bubble is operated at a blow-up ratio of 2.2:1 to 2.8:1. The frost line is kept low, at 0.5 m to 0.8 m, to increase transverse-direction orientation and improve Elmendorf tear measured per ASTM D1922-09. In practice, the MD/TD tear imbalance is maintained below 1.5:1 by adjusting the collapsing frame height. Heavy-duty sack converters often require a dart impact value of ≥ 200 g for 100 µm film tested under ASTM D1709-15A Method B. This value is not a single-material constant; it is controlled by blending ratio, die gap, and cooling rate. A blend containing 25 wt% LDPE and 75 wt% 7308DK is a common starting point on high-output lines. Adding more than 30 wt% LDPE is avoided in some plants because it increases low-tension transverse-direction splitting during sack opening. Batch-to-batch variation in the melt index of the LDPE component can shift the bubble’s neck height by 50 mm to 100 mm; therefore the extruder operator must re-check the frost line after resin lot changes.

Seal Initiation Temperature and Hot Tack in Vertical Form-Fill-Seal Operations

In vertical form-fill-seal service, the lower melting point of LLDPE compared with high-density polyethylene creates a broad seal window but also demands careful jaw temperature control. Film from 7308DK is converted at gauge 50 µm to 80 µm. The seal initiation temperature on a standard laboratory hot-tack tester in accordance with ASTM F1921 is typically recorded between 95 °C and 105 °C for LLDPE film. Production seal jaws are set at 115 °C to 130 °C, with dwell times of 0.3 s to 0.6 s and seal pressure from 0.25 MPa to 0.50 MPa. Because the material is a linear resin, its hot tack strength after 0.5 s dwell is generally lower than an ionomer or a metallocene LLDPE with a higher comonomer fraction. This places a constraint on packaging speeds above 80 packs/min on vertical machines when product mass exceeds 1 kg. The failure mode shifts from cohesive peel to interfacial release if the seal jaw temperature falls below 100 °C. On start-up, thermally induced seal contamination on the jaws appears when the film is sealed above 140 °C for periods longer than 2 s. For food-contact packaging, the formulated film must meet the extractives limits of FDA 21 CFR 177.1520 and the overall migration limit of 10 mg/dm² specified in EU Regulation (EU) No 10/2011. Slip and antiblock masterbatches are often introduced at 2 wt% to 4 wt% to control coefficient of friction without exceeding the migration limits. The seal strength on packaging is commonly specified as ≥ 15 N/25 mm when tested at 200 mm/min jaw separation speed. However, the precise value is affected by the seal jaw profile and chill-roll finish, and published data for this specific configuration is limited.

Melt oxidation is the primary adhesion mechanism in extrusion lamination and coating of 7308DK to primed aluminium foil and polyester films. The resin is processed through a 90 mm to 120 mm single-screw coating extruder at melt temperatures of 300 °C to 320 °C, measured at the adapter. The air gap between the die and the chill roll is maintained at 200 mm to 300 mm. At line speeds of 150 m/min to 300 m/min, the coating weight is controlled between 15 g/m² and 25 g/m². The high processing temperature accelerates carbonyl formation on the melt surface, which improves adhesion to a polyurethane or polyethylene imine primer. If the melt temperature falls below 300 °C, adhesion to aluminium foil can drop below 1.5 N/15 mm as measured by ASTM D1876, which is insufficient for liquid-pouch laminations. Raising the temperature above 320 °C creates odour and smoke that can contaminate food-contact laminates. In addition, the melt curtain loses tensile strength at high temperature, causing draw resonance at speeds above 250 m/min. This is observed as periodic thickness bands at 10 Hz to 20 Hz on the coated web. To suppress draw resonance, the die lip gap is adjusted to 0.4 mm to 0.7 mm and the melt temperature is reduced to 305 °C to 310 °C. The use of a backup roll with a matte finish is common when the coated surface must receive water-based ink. The substrate must be pre-treated to a surface energy of 38 mN/m to 42 mN/m. For woven polypropylene sack lamination, a coating layer of 20 µm to 30 µm is applied directly to the fabric. The fabric must be dried to a moisture content below 0.1 wt% to prevent steam-driven pinholes in the coating. Published data for 7308DK in this exact configuration is limited, but processing requirements are broadly consistent with LLDPE coating grades.

When UV-Stabilised Masterbatch Is Added Above 6 wt% in Silage Cover Film Extrusion

Extending gauge to 150 µm to 200 µm is standard for agricultural silage cover and greenhouse film. At these thicknesses, the addition of a UV stabiliser masterbatch containing hindered amine light stabilisers is necessary for a 24-month outdoor service life. The masterbatch is dosed at 6 wt% to 12 wt%, depending on pigment loading and the ultraviolet index of the installation site. Feeding accuracy must be maintained within ±0.5 wt% to avoid local degradation of mechanical properties. On a 75 mm grooved-feed extruder, the higher masterbatch loading can cause slip in the feed zone if the temperature of the first barrel zone exceeds 200 °C. Barrel zone temperatures are therefore held at 175 °C to 190 °C in the feed section, while the downstream zones operate at 200 °C to 220 °C. Die pressure rises by 2 MPa to 4 MPa when masterbatch level moves from 6 wt% to 12 wt%. This indicates that viscosity dilution by the masterbatch carrier is not sufficient to offset particle-related flow restriction. The pressure rise is monitored by the melt transducer before the screen changer. The screen pack is upgraded to 40/60/80 mesh to protect the die lips from agglomerated stabiliser particles. Film produced at 180 µm is tested for tensile break strength under ASTM D882, with an acceptance value generally ≥ 25 MPa in both machine and transverse directions. After accelerated weathering under ISO 4892-2, retention of elongation at break above 50% after 3000 h is used as a specification benchmark by some agricultural film buyers. The actual value for 7308DK is masterbatch-dependent. If the UV masterbatch is based on a low-melt-index LDPE carrier, the bubble’s frost line shifts upward by 50 mm to 150 mm; if the carrier is a high-MI LLDPE, the frost line drops. This difference in thermal behaviour forces operators to re-adjust internal bubble cooling air volume after each lot of masterbatch. Anti-fog additives used in greenhouse film are added at 1 wt% to 3 wt% and must be pre-dispersed to prevent surface bloom streaks. The same film structure, when used as silage cover, must resist puncture from protruding feed stems. A dart impact of ≥ 300 g at 200 µm film thickness by ASTM D1709-15A Method B is often demanded. This requirement is achievable only when the film is cooled symmetrically; asymmetric air-ring flow will reduce dart impact by 30% to 40% without changing the average gauge.

Export documentation requires a measured property matrix for LLDPE film.

For export documentation, the following test designations and compliance statements are required. The actual values must be taken from the batch certificate of analysis rather than from generic literature.