Rigid-container liners and heavy-duty refuse sacks converted from Chevron Phillips 7120ST are processed on grooved-feed single-screw blown film extruders with a barrier screw and 25:1 to 30:1 L/D. The nominal 0.918 g/cm³ density and 1.0 dg/min melt index of the hexene copolymer place the resin at the high-toughness end of LLDPE blown film grades. For liner gauge between 50 μm and 80 μm, the die gap is held at 1.8–2.5 mm to delay melt fracture and to preserve transverse direction tear. A blow-up ratio of 2.0:1 to 2.6:1 is maintained, with melt temperature 190–225 °C across the adapter and die. Screen packs at 40/80/100 mesh protect the die from carbonized gels and metal fines, but screen blinding above 320 bar pressure drop is the more frequent production limit than motor torque on 80–90 mm lines. Slip-antiblock packages using erucamide at 800–1200 ppm and natural silica at 2500–5000 ppm are metered at the feed throat after a dry-blend or masterbatch calibration. These additive levels are starting points, not supplier limits, because pellet humidity, regrind level and screw shear alter surface bloom. Dart impact for a 50 μm industrial liner is commonly specified at 250–400 g under ASTM D1709 Method B, while Elmendorf tear in the machine direction is quantified by ASTM D1922. The hexene branch architecture gives lower low-temperature brittleness than butene LLDPE at equal density, which is the technical rationale for selecting 7120ST for liners exposed to frozen product discharge. ESCR under ASTM D1693 Condition A at 50 °C in 100% Igepal CO-630 is the accepted predictor for detergent and chemical-containing sack liners, with many end-user specifications requiring 50% failure at 500 h or longer. Film for printed industrial liners is corona-treated inline to 38–42 mN/m and wound with edge-guide tension control to avoid bag-side dart failures caused by surface scratches.

What Restricts Heat Seal Initiation Temperature in 7120ST/LDPE Blends?

Heat seal initiation is dominated by the comonomer type and density rather than by melt index alone. In dry-food and bakery packaging webs, 7120ST is blended with high-pressure LDPE at 80/20 or 70/30 ratios by weight. The purpose is not only to lower seal initiation but also to improve bubble stability at high throughput and to widen the sealing window on rotary and pouch machines. Seal initiation temperature is measured by ASTM F2029, while hot tack is measured by ASTM F1921 on a J&B or equivalent hot-tack test apparatus. A 70/30 7120ST/LDPE blend commonly shifts the seal initiation point downward by 5–10 °C relative to the unblended C6 LLDPE, but published data for this exact blend is limited and must be re-established on the target extrusion line because die gap and frost line height shift seal performance. The converting run uses a die gap of 2.0 mm, melt temperature 210–230 °C, and blow-up ratio 2.0:1–2.5:1. Fluoropolymer-based processing aid at 200–500 ppm is added when surface melt fracture appears at high output. For food-contact applications, the olefin polymer layer is controlled by FDA 21 CFR 177.1520(c) and EU Regulation 10/2011, with overall migration limited to 10 mg/dm² under the assigned food simulant and contact condition. Seal jaw temperatures above 145 °C may create polyethylene smearing and transfer to the jaw face, which reduces seal uniformity and increases seal initiation variability. Regrind addition above 20 wt% is avoided for seal-critical webs because heat history and gel formation alter the seal plateau and reduce hot tack. Terminal packages include 25–50 μm pillow pouches, bread bags, biscuit overwrap and rice or pasta liners. The seal area must be free of migrating slip or antiblock agglomerates, because local surface bloom above 0.2 mg/100 cm² can prevent consistent heat transfer.

The ranges are screening values for line startup and are not a release specification. Actual seal temperatures depend on die gap, frost line height, seal jaw pressure, and dwell time on the packaging machine.

Coextruded Frozen Food Web Structures and Dart Test Methods

Five-layer barrier film for frozen vegetables and seafood uses 7120ST in the heat-seal and outer skins, paired with an EVOH oxygen barrier core. A working structure is PE skin 12–15 μm / tie 3–4 μm / EVOH 3–5 μm / tie 3–4 μm / PE skin 12–15 μm, with total thickness 45–65 μm. The hexene copolymer contributes low-temperature film toughness after frozen distribution at -18 °C, measured by dart impact ASTM D1709 Method B and Elmendorf tear ASTM D1922. The coextrusion die is operated with PE layer melt temperature 190–220 °C and EVOH layer melt temperature 205–220 °C, because viscosity matching at the die entry is critical to avoid interfacial flow instability. 7120ST at 1.0 dg/min provides a melt viscosity closer to standard barrier grades than high-MFR LLDPE grades, but edge trim regrind should be kept below 15 wt% to avoid gel contamination and layer confusion. Blown film lines use a die gap of 1.8–2.4 mm, blow-up ratio 2.0:1–2.5:1, and a dual-lip air ring with internal bubble cooling for gauge uniformity. Food-contact compliance for the PE faces is assessed under FDA 21 CFR 177.1520(c) and EU 10/2011; the EVOH core is assessed under its own food-contact listing or national equivalent. The film is not intended for retort or hot-fill processing, and operational limits are reached above 60 °C sustained surface temperature where low-density polyethylene loses mechanical stiffness and seal creep accelerates. End-use films are converted into pillow bags, stand-up pouches and box liners for frozen vegetables, potato products and battered seafood. The primary failure mode on packaging lines is seal-area deformation caused by frost and product dust, not film puncture, when the skin layer is too thin; maintaining the skin above 12 μm is required for consistent freezer-to-boil abuse resistance.

When High BUR Agricultural Film Demands Antiblock Packages and UV Stabilization

Greenhouse covers, silage pit covers and agricultural lamination base webs run at high blow-up ratios from 2.5:1 to 3.0:1 to balance machine-direction and transverse-direction elongation. 7120ST is formulated with a hindered amine light stabilizer package at 0.2–0.6 wt%, a UV absorber at 0.1–0.3 wt%, and natural silica antiblock at 2000–5000 ppm; these loadings are adjusted by accelerated weathering and film surface tack requirements. The die gap is widened to 2.2–2.6 mm to maintain bubble stability at high BUR, and melt temperature is held at 200–220 °C to minimize additive decomposition. Frost line height is set between 600 mm and 1000 mm depending on cooling air temperature and line speed; a high frost line combined with high BUR increases transverse direction tear but can reduce machine direction tear and promote edge winkles. Tensile properties are quantified by ISO 527-3, tear resistance by ASTM D1922, and dart impact by ASTM D1709. Agricultural films produced from 7120ST are not food-contact materials and do not require FDA 21 CFR compliance, but the resin and additives must meet REACH registration duties and local agricultural plastic regulations where applicable. The film must be wound at low tension below 100 N/m width to avoid blocking and blocking-related surface impressions in the roll. Bubble wander becomes an operational limit above 3.0:1 on single-lip air rings, so internal bubble cooling or improved air-ring lips are used. End-product gauges range from 30 μm for row cover to 150 μm for greenhouse sheeting, with the higher-gauge film requiring additional cooling capacity or reduced output to prevent heat-induced shrinkback in the roll.

In dunnage air bag and heavy shipping sack converting, the selection of 7120ST-rich tubular film is driven by puncture propagation resistance and seal survival under rail vibration. Film for these applications is run at 75–125 μm thickness, frequently at 100% 7120ST or with 10–20 wt% high-pressure LDPE to improve bubble handling and seal flow. The die gap is set at 2.0–2.5 mm, blow-up ratio 1.8:1–2.2:1, and melt temperature 195–215 °C. A lower BUR is used to favor machine-direction tensile and reduce transverse direction stretch during bag forming. Seal strength is measured under ASTM F88, with minimum values set by bag drop and burst tests rather than by packaging appearance. Puncture resistance is evaluated by ASTM D1709 Method B for thin-gauge sections and by slow puncture methods based on end-user specifications for dunnage air bags. The hexene comonomer in 7120ST provides tensile toughness without the high-branching haze of some LDPE alternatives, but the resin is not a barrier material and is unsuitable for sustained oil contact or solvent retention. Continuous service temperature above 60 °C is outside the reliable operational range because the polymer softens and seal creep increases. Converters report that bag-side impact failures occur when film is overstretched in the transverse direction before sealing, so the bubble is collapsed with low nip pressure and edge guides are aligned to prevent gauge bands. REACH registration duties apply to the resin and additive masterbatch; no food-contact certification is assigned to this industrial film. The terminal packages are used as void-fill dunnage bags, FIBC inner liners, contamination-control liners and heavy shipping sacks for granular resins and pigments. The film surface must be clean and dry before sealing, and moisture above 0.15 wt% on regrind is removed before extrusion to prevent steam bubbles and inconsistent gauge in the finished bag.