The distinction between LDPE film vs LLDPE film: stretch, seal strength and tear resistance is governed by branching architecture, comonomer type, and solid-state orientation. Low-density polyethylene is produced in high-pressure autoclave or tubular reactors at 1,400–3,500 bar and contains long-chain branches; linear low-density polyethylene is produced in gas-phase, solution, or slurry processes and contains short-chain branches from butene, hexene, or octene. Film-grade LDPE density measured by ASTM D1505 commonly falls between 0.917 g/cm³ and 0.930 g/cm³, while LLDPE spans 0.915 g/cm³ to 0.940 g/cm³. Melt index measured at 190 °C under 2.16 kg load per ASTM D1238 is typically 0.2–2.0 g/10 min for both resin families. The long-chain branching in LDPE produces pronounced strain-hardening extensional viscosity and higher melt strength, while LLDPE exhibits lower melt strength but higher solid-state drawability because of higher tie-chain formation during orientation. These molecular differences propagate into converting lines, affecting bubble stability, seal dwell tolerance, and tear propagation on packaging machines.

What Limits Machine-Direction Stretch and Strain Hardening in Blown Film?

Tensile properties determined by ASTM D882 or ISO 527-3 on 25 µm monolayer blown films show that LDPE typically develops lower machine-direction elongation at break than LLDPE. LDPE grades commonly fall between 250% and 500% in the machine direction and 350% to 600% in the transverse direction; LLDPE grades frequently reach 500% to 800% machine-direction elongation and can exceed 700% transverse-direction elongation. The higher ultimate stretch of LLDPE is attributed to linear chains with short-chain branches that form more tie-chains across adjacent lamellae during bubble expansion. LDPE long-chain branches increase melt-state extensional viscosity, allowing stable deep draw-down on large blown-film dies at blow-up ratios above 3.0:1, but they limit terminal drawability in the solid state. On single-screw blown-film extruders with L/D 24:1–30:1 and die gaps of 1.8–2.5 mm, LDPE permits stable bubbles at high frost-line heights and melt temperatures of 160–190 °C; LLDPE generally requires dual-lip air rings or internal bubble cooling to maintain stability under similar conditions because its lower melt strength increases susceptibility to bubble sag and edge oscillation.

Stretch film lines employing powered prestretch equipment evaluate machine performance by prestretch ratio and load retention rather than only tensile elongation. LLDPE cast film is routinely stretched to 200–300% pre-stretch on A- and B-stretch wrappers, while LDPE is more often blended at 5–15 wt% as a cling-layer resin in coextruded stretch wrap. Cling performance of such films is assessed by ASTM D5458, where a high-cling LDPE skin reduces z-axial separation under pallet load vibration, but published data for this specific configuration is limited because tackifier type and loading dominate the response.

Heat-seal strength measured by ASTM F88/F88M on monolayer films is determined by seal bar temperature, jaw pressure, dwell time, and polymer melting range rather than by film density alone. For a 50 µm monolayer film sealed with flat heated jaws at 0.28 MPa and 1.0 s dwell, LDPE typically reaches a seal initiation temperature of 100–115 °C, defined as the jaw temperature required to generate 2.0 N/25 mm seal strength. Ziegler-Natta LLDPE based on butene often initiates at 105–125 °C, while metallocene-catalyzed hexene or octene LLDPE can initiate at 90–105 °C and achieve higher ultimate seal strength. Reported ultimate seal strengths for 50 µm films are commonly 8–14 N/25 mm for LDPE and 12–20 N/25 mm for hexene LLDPE, depending on film orientation and gauging variation. The broad melting distribution of LDPE supports a wider seal-bar temperature tolerance on intermittent vertical-form-fill-seal machines, where jaw dwell can be shorter than 300 ms. LLDPE narrower melting distribution can produce higher hot-tack force but requires tighter jaw alignment and pressure uniformity to avoid channel leakers at low seal-bar temperature.

Differential scanning calorimetry by ASTM D3418 at 10 °C/min shows LDPE film grades with a broad melting endotherm peaking between 105 °C and 115 °C and a tail extending to 80–90 °C, while hexene LLDPE typically peaks between 120 °C and 125 °C with a sharper onset. The broad low-temperature tail of LDPE permits partial molten interdiffusion across the seal interface over a wider jaw-temperature range, even though its ultimate crystalline strength is lower. This explains why LDPE often performs better than LLDPE on poorly aligned seal bars with temperature variation exceeding ±10 °C.

When Seal Bar Temperature Falls Below the Hot-Tack Plateau, Failure Moves from Peel to Elongation

Hot-tack force measured by ASTM F1921 on a 25 mm wide specimen after 0.5 s seal time and 0.5 s cooling time shows that LDPE develops a broad usable plateau between 110 °C and 140 °C, although its peak force is lower than that of hexene or butene LLDPE. A typical LDPE monolayer film may reach 3–6 N/25 mm hot-tack force within the plateau, whereas hexene LLDPE can reach 5–9 N/25 mm but often over a narrower 100–120 °C plateau. At jaw temperatures below the plateau, partial melting creates weak interfacial entanglements; failure shifts from the seal interface to the film boundary because the peel front propagates before the seal root yields. This effect is observed on high-speed horizontal flow-wrapping lines when jaw contact time is shorter than 100 ms and film heat transfer limits the depth of fusion. In contrast, at temperatures above the plateau, both materials may fail by cohesive film break at the seal edge, but LDPE typically retains a wider processing window when temperature drift exceeds ±10 °C.

Indicative ranges for 50 µm monolayer blown films sealed at 0.28 MPa and 1.0 s dwell per ASTM F88/F88M.

Elmendorf tear resistance measured by ASTM D1922 on 25 µm monolayer blown films shows that LLDPE generally requires higher tear energy than LDPE in both machine and transverse directions. Reported Elmendorf tear values for LDPE often fall between 100 gf and 350 gf, while LLDPE values are commonly between 250 gf and 700 gf, depending on orientation, gauge, and comonomer length. The improvement is most pronounced in transverse-direction tear because LLDPE forms a more balanced crystalline orientation and higher interlaminar tie-chain density after biaxial orientation. LDPE often exhibits a machine-direction to transverse-direction tear ratio above 1.0, indicating anisotropic stress propagation, while LLDPE can hold the ratio near 0.8–1.2 or invert it depending on frost-line height and blow-up ratio. In cast films, the orientation difference is reduced because machine-direction draw is lower than in blown film; LLDPE cast film still shows higher puncture and tear, but the machine-direction to transverse-direction tear ratio can be closer to 1.0. Trouser tear measured by ISO 6383-1 and dart impact measured by ASTM D1709 show the same ranking: at equal gauge, LLDPE films generally provide higher puncture propagation resistance and impact energy absorption than LDPE.

Indicative ranges compiled from published industrial values for blown monolayer film; orientation and comonomer type can shift values outside these bands.

Coextrusion Stabilisation and Blown-Film Line Transferability

The transition from LDPE to LLDPE on existing blown-film equipment is constrained by die pressure and melt fracture. Dies with 1.0–1.5 mm gaps that run LDPE acceptably may require enlargement to 2.0–2.5 mm for LLDPE to preserve die-lip quality. Extruder barrel temperatures for LLDPE are typically set 10–20 °C higher than for LDPE to reduce shear viscosity, but feed-throat cooling remains critical to prevent pellet bridging at screw speeds above 80 rpm. On a 90 mm grooved-feed extruder with L/D 24:1, LLDPE can generate 15–25% higher melt pressure across the die than LDPE at the same output and melt temperature, depending on melt index and die gap; screen packs must be sized accordingly to avoid pressure-related bubble instability. Commercial solutions commonly include fluoropolymer-based polymer processing aids at 200–600 ppm to reduce die-lip build-up and melt fracture in LLDPE, whereas this is often unnecessary with LDPE because of its lower apparent melt viscosity at die shear rates. Coextruded films commonly use LDPE in the skin layer to improve seal initiation and optics, and LLDPE in the core for mechanical toughness. Layer ratios of 10–20% LDPE skin and 80–90% LLDPE core are typical for heavy-duty sacks and liners, but the optimal ratio remains driven by seal requirements and tear-resistance targets. Food-contact compliance for these structures is anchored to FDA 21 CFR 177.1520 and EU Regulation 10/2011, which set overall migration and specific migration limits for olefin polymers.

LDPE and LLDPE are not direct drop-in substitutes in every film application. LDPE offers better bubble stability and broader seal-temperature tolerance but lower dart impact, lower tear resistance, and lower ultimate elongation than LLDPE at equal thickness. LLDPE provides higher mechanical toughness but can exhibit higher melt pressure, lower melt strength, and greater susceptibility to melt fracture on narrow die gaps. In blends, addition of 10–30% LLDPE to LDPE often increases tear strength and dart impact, but the resulting film may lose optical clarity and require higher seal-bar temperature. Conversely, addition of 10–20% LDPE to LLDPE improves bubble stability and seal initiation but reduces ultimate dart impact and tear resistance. These compromise regions do not follow a linear mixing rule because phase morphology and orientation vary with screw design and die gap. Published data for precise commercial blended-film thresholds is limited and should be established via pilot-line trials using the specific extruder, die, and downstream sealing parameters. Polyethylene films generally require no predrying at ambient humidity, but exposure to high relative humidity can create surface moisture that affects print adhesion and seal consistency if condensation occurs; converters should follow ASTM D2578 for film surface-wetting tension when adhesion problems are suspected. Oxidative embrittlement and gel formation in reprocessed film are managed by limiting regrind addition to 10–30% and monitoring melt filtration pressure upstream of the die.