Which LLDPE Grade for 15 µm Stretch Film vs 100 µm Heavy-Duty Bag?

Cast stretch film at 15 µm and blown heavy-duty bag at 100 µm are both LLDPE conversion processes but occupy opposite ends of melt flow, melt strength, cooling rate, and mechanical damage tolerance. The first is produced on high-speed cast lines with chill-roll quenching, vacuum-box edge pinning, and in-line slitting; the second is produced on blown-film towers with internal bubble cooling, gusseting, and surface or center winding. Grade selection is therefore not a single LLDPE compromise but a matched resin window. A 15 µm cast stretch film typically requires a 1-octene or 1-hexene LLDPE with melt index 2.5–4.0 g/10 min (ASTM D1238, 190°C/2.16 kg) and density 0.917–0.921 g/cm³ (ASTM D1505). A 100 µm heavy-duty bag typically uses a 1-hexene or 1-octene blown-film grade with melt index 0.5–1.0 g/10 min and density 0.920–0.925 g/cm³. The melt-index gap is driven by cast-line shear and drawdown versus blown-film bubble stability.

Does Down-Gauging to 15 µm Require Octene Over Butene in Cast Stretch Resins?

Butene-based LLDPE is economically attractive but has insufficient tie-chain probability at equivalent density for 15 µm cast stretch film. In downgauged films, puncture and dart resistance are controlled by the frequency of tie molecules bridging adjacent crystals; 1-octene and 1-hexene short-chain branches are more effective than 1-butene because the branch does not co-crystallize and generates a broader interlamellar region. Comparative supplier data evaluated under ASTM D5748 puncture and ASTM D1922 Elmendorf tear show that octene-based cast stretch grades retain tear and puncture at 15 µm better than butene-rich grades. Commercial metallocene octene cast stretch resins, typically with MI 2.5–3.5 g/10 min and density 0.917–0.920 g/cm³, are specified with gel counts below 10 gels per m² and haze below 1.5% (ASTM D1003) for high-clarity wrap lines.

On a cast stretch line, the grade is plastified in a 90–120 mm single-screw extruder with 30:1 L/D and barrier screw geometry, then delivered through a gear pump to a flat die with 0.5–0.8 mm die gap. Melt temperature is held at 245–270°C, and the web is quenched on a chill roll maintained at 15–25°C. At line speeds above 400 m/min, gauge variation below ±5% requires automatic die-bolt control and edge-pinning vacuum boxes; neck-in and draw resonance become limiting if the resin lacks sufficient melt strength or if the melt index exceeds 4.0 g/10 min. High-shear viscosity is also relevant: a metallocene octene grade with narrow molecular weight distribution may show lower extruder head pressure than a Ziegler-Natta C4 grade of identical MI, requiring screw-speed and die-gap adjustments to maintain 15 µm gauge uniformity. Tensile properties measured by ASTM D882 typically show machine-direction elongation at break above 350% for cast stretch grades, while transverse-direction elongation may be lower but sufficient for wrap integrity.

Production of 100 µm heavy-duty bag film on conventional blown-film towers uses 2.0–2.5:1 blow-up ratio and 8–10 die-diameter frost line height. A C6 or C8 LLDPE with MI 0.5–1.0 g/10 min and density 0.920–0.925 g/cm³ provides bubble stability and low sag. Internal bubble cooling with die diameter 200–350 mm and die gap 1.5–2.5 mm allows high output while maintaining dart impact. For 100 µm monolayer heavy-duty sacks, converter specifications commonly require ASTM D1709 Method A dart impact above 700 g and ASTM D1922 Elmendorf tear above 12 N/mm in machine direction. Published data for a specific line configuration is limited, because frost line height, bubble cooling, and die gap shift values by ±15%. Adding 10–20% high-pressure LDPE improves bubble stability and tear anisotropy but reduces dart impact and stiffness; this trade-off is accepted only when puncture requirements are moderate.

When a 100 µm Bag Needs Both High-Speed Output and Low Gauge Variation

The resin molecular architecture determines whether a 100 µm heavy-duty bag can be produced at output rates beyond 120 kg/h without bubble instability. A 1-hexene LLDPE with broadened molecular weight distribution, produced either by Ziegler-Natta dual-reactor technology or a single-site catalyst with long-chain branching, demonstrates higher melt strength and less draw sensitivity than a narrow-MWD metallocene octene grade. However, the broader distribution may raise soluble fraction and lower dart impact. The process conflict is most visible when BUR exceeds 2.5:1: the bubble enters the heavy-shoulder instability region, gauge variation increases to ±10%, and ASTM D1709 dart impact drops nonlinearly. On such lines, converters use a coextruded structure with a high-melt-strength LDPE skin layer and a C6/C8 LLDPE core to decouple bubble stability from toughness. The core layer may be 60–80% of total thickness, with the LDPE skins not exceeding 20% total.

At 100 µm, the influence of density is nonlinear. When density rises from 0.920 g/cm³ to 0.925 g/cm³ in a C6 LLDPE, dart impact can fall by 20–30% due to reduced tie-chain density, while stiffness increases. For bags requiring ASTM D1709 dart impact above 1,000 g, density is usually held at or below 0.922 g/cm³ with a C8 or C6 metallocene. For less demanding industrial liners, butene-rich blends with 20–30% LDPE may be acceptable. The heavy-duty bag grade must also resist crease cracking at folds, where environmental stress crack resistance measured by ASTM D1693 becomes relevant. A C4 LLDPE at 100 µm often fails to meet high-dart and crease-resistance requirements unless blended with a tougher C6 or C8 resin.

Typical commercial datasheet and converter specification ranges for the two applications are summarized in the following comparison table. Values are not universal resin limits but represent the window in which most industrial lines are operated.

Melt Rheology, Comonomer Distribution, and Gel-Performance Boundaries

Capillary rheometry at 190°C distinguishes the two grades by shear viscosity ratio and extensional behavior. A cast stretch octene grade with MI 3.0 g/10 min may show apparent shear viscosity near 120–180 Pa·s at 1,000 s⁻¹, while a blown-film heavy-duty grade with MI 0.5 g/10 min may be above 300 Pa·s at the same shear rate. The lower viscosity of the cast grade permits thin gauge at high line speed but reduces melt strength and increases neck-in. In heavy-duty film, the higher viscosity grade stabilizes the bubble but increases extruder torque and melt temperature. Comonomer distribution measured by crystallization elution fractionation shows that heavy-duty bag grades often have a bimodal or broad short-chain branching distribution to balance stiffness, seal initiation, and dart; cast stretch grades are narrower to minimize gels and improve cling-layer clarity.

Additive selection is application-specific. Cast stretch film relies on a cling layer containing 0.5–2.0 wt% polyisobutylene or specialty tackifier at the chill-roll side; levels above 2.0 wt% may cause screw slippage and die-lip buildup on high-speed cast lines. Cling force is measured by ASTM D5458; reported values in supplier literature for 15 µm machine film typically range from 20–60 g under standardized conditions. Heavy-duty bag film uses silica antiblock at 2,000–5,000 ppm and erucamide slip at 200–500 ppm to control blocking and coefficient of friction measured by ASTM D1894. Overloading slip above 500 ppm in a 100 µm heavy-duty bag reduces heat-seal strength and increases seal initiation temperature in bag conversion.

Extrusion Equipment, Scaling, and Operational Boundaries

A converter attempting to run a 15 µm cast stretch grade in a 100 µm blown-film line with 2.5:1 BUR commonly observes bubble instability, poor dart impact, and low melt strength at typical blow-up ratios. Conversely, a 0.5 g/10 min heavy-duty bag grade run on a high-speed cast line may generate melt fracture, excessive die pressure, and poor chill-roll wet-out. These are operational boundaries, not resin defects. For cast stretch, the lower practical MI is approximately 1.5 g/10 min; below this, melt fracture and gear-pump pressure rise become severe. For blown heavy-duty bags, the upper practical MI is approximately 1.5 g/10 min; above this, bubble instability at BUR 2.5:1 becomes difficult even with internal bubble cooling.

For food-contact heavy-duty liners, selected LLDPE grades must comply with 21 CFR 177.1520 or EU Regulation 10/2011; for industrial stretch wrap, regulatory compliance is normally limited to REACH and packaging waste directives. In both cases, the specific grade certificate must list the base resin density, MI, comonomer type, and additive package. Processors should avoid using a C4 LLDPE at 100 µm when dart impact above 700 g is required without an LDPE or C6/C8 blend, because the butene branch does not generate sufficient tie-molecule density. Published data for this specific configuration is limited; converter trials at fixed BUR and frost line height remain the final selection method.