Low-density polyethylene (LDPE) is classified under ISO 1183-1 and ASTM D1505 with a typical density of 0.910–0.925 g/cm³. Linear low-density polyethylene (LLDPE) occupies 0.916–0.940 g/cm³, while high-density polyethylene (HDPE) is specified at 0.941–0.970 g/cm³. These ranges reflect short-chain branch concentration and the resulting volume fraction of orthorhombic crystalline lamellae. Density remains the primary rapid screening parameter for flexural modulus and seal initiation temperature because both properties shift with crystallinity. A converter selecting among LDPE, LLDPE, and HDPE must resolve a stiffness-seal conflict: lower density improves low-temperature seal initiation and hot tack but reduces secant modulus and long-term creep resistance. Test values are meaningful only after conditioning per ASTM D618-13 at 23°C ± 2°C and 50% ± 10% relative humidity for 24 h.

Density is measured by gradient column according to ISO 1183-1 or ASTM D1505 using a liquid column calibrated with glass floats. Single-laboratory repeatability is often ±0.001 g/cm³, but interlaboratory reproducibility may be larger. The density value cannot distinguish between long-chain branched LDPE and linear short-chain branched LLDPE at overlapping densities. For thin films, stiffness is expressed as tensile secant modulus at 1% strain under ASTM D882-18, not flexural modulus; a 50 µm LDPE film of 0.923 g/cm³ may show a secant modulus of 180–260 MPa, whereas a 50 µm HDPE film of 0.955 g/cm³ may show 700–1100 MPa.

LDPE, LLDPE and HDPE Density Ranges: Choosing by Stiffness and Seal

Density ranges for LDPE, LLDPE, and HDPE are defined by base resin composition and branching architecture, not by melt index. LDPE contains long-chain branches; LLDPE contains short-chain branches from butene, hexene, or octene comonomer; HDPE contains minimal branching. At equivalent density, LLDPE provides higher tensile elongation and puncture resistance than LDPE, while LDPE provides higher melt strength and lower neck-in during extrusion coating. Table 1 lists representative values compiled from resin supplier technical datasheets and ASTM/ISO methods. These are not specification minima for any individual commercial grade.

A single density measurement should be read with comonomer type and molecular weight distribution because a 0.918 g/cm³ octene-based metallocene LLDPE can initiate a seal at a temperature 5–10°C lower than a 0.920 g/cm³ Ziegler-Natta LLDPE with a different branch distribution. Melt flow rate measured under ASTM D1238-20 at 190°C and 2.16 kg is not a reliable indicator of seal performance; two resins with a melt flow rate of 1.0 g/10 min can differ in hot tack by 2–4 N/25.4 mm.

Does Lower Density Always Improve Seal Integrity in Monolayer Film?

Seal initiation temperature does not vary linearly with density across LDPE, LLDPE, and HDPE. In monolayer film, seal strength is controlled by amorphous chain interdiffusion across the interface after the sealing bar raises the film above its crystalline melting range. LDPE with a density of 0.923 g/cm³ typically reaches a seal initiation force of 0.5 N/25.4 mm at 90–100°C under ASTM F1921-18 Method B, whereas an HDPE film of 0.955 g/cm³ reaches the same threshold at 120–130°C. The lower initiation temperature is associated with a broader melting range of LDPE, commonly 105–115°C by ASTM D3418.

Hot tack force is more sensitive to molecular weight distribution and chain linearity than to density alone. A bimodal LDPE with density 0.918 g/cm³ may produce a maximum hot tack force at 110–120°C that exceeds the hot tack of a conventional LDPE at the same density because the high-molecular-weight fraction increases terminal relaxation time and resists seal peeling while the seal remains molten. The converse limitation is that low-density material also yields thin seal edges; at seal bar pressure above 0.35 MPa, local film thinning can reduce final seal strength measured under ASTM F88-21. Seal failure modes observed on production lines include peel, delamination, and burn-through, each requiring different corrective action. Burn-through is often mistaken for a seal-strength failure but is actually caused by heat transfer beyond the melt temperature of the backing layer.

For hot fill and retort applications, the selected LDPE seal layer must retain seal strength after exposure to 85°C water or 121°C retort for 30 min. In such conditions, a low-density sealant can lose measurable seal strength if the sealing layer contains more than 3% low-molecular-weight extractables; solvent extraction per ASTM D5227 should be specified as a release criterion.

For blown film conversion of LDPE, a typical 63.5 mm single-screw extruder with a 24:1 L/D ratio and a barrier feed section operates at a melt temperature of 160–190°C, barrel zone settings from 140–185°C, and head pressure of 15–35 MPa. Die gap is set at 0.8–1.2 mm, blow-up ratio between 2.0:1 and 2.5:1, and frost line height between 5 and 8 die diameters. Excessive melt temperature above 200°C may initiate chain scission, leading to gel formation and reduced seal strength. The lower melt strength of conventional LDPE compared with HDPE makes bubble stability sensitive to ambient air fluctuations; a forced-air internal bubble cooling system with a cooling air temperature of 7–15°C is typically required for throughput above 180 kg/h on a 90 mm die.

Extrusion coating with LDPE uses a different thermal profile. Resin is processed at melt temperatures of 280–330°C through a 0.5–0.7 mm die gap, with draw ratio between 20:1 and 100:1 and line speeds often exceeding 300 m/min. At these temperatures, long-chain branch content provides neck-in of 30–60 mm, which is significantly lower than that of LLDPE at equivalent melt index. However, thermal oxidative degradation at the die lip may form brown specks; therefore an antioxidant package is required and the extruder hopper is often blanketed with nitrogen. The melt index for extrusion coating LDPE is typically 3.5–8.0 g/10 min under ASTM D1238-20 at 190°C and 2.16 kg.

When Coextruded LDPE Seal Layers Must Satisfy 21 CFR 177.1520 and EU No 10/2011

Food-contact status is not automatically conferred by density range. LDPE seal layers intended for direct food contact must comply with FDA 21 CFR 177.1520(c) for olefin polymers, which sets maximum extractable fraction depending on polymer density and end-use temperature. The extraction test uses n-hexane or xylene under specified time-temperature conditions; a 0.920 g/cm³ LDPE seal layer may have a different extractable limit than a 0.950 g/cm³ HDPE layer. In the European Union, plastic materials and articles are subject to EU Regulation No 10/2011, including an overall migration limit of 10 mg/dm² for the final article and restrictions on intentionally added substances listed in Annex I and Annex II.

Converters should not assume that a low-density grade carries inherently higher purity. The same density can be obtained with high catalyst residues or processing aids that affect organoleptic performance. For low-odour film applications, the resin specification should include hexane extractables per FDA 21 CFR 177.1520(d)(1) and a sensory panel limit of ≤ 3.0 on a 1–4 intensity scale when the package is tested at 38°C after 24 h. Published data for this specific configuration is limited, but most supplier datasheets provide extractable fraction only at 21°C, not at the elevated temperature required for hot-fill or retort.

In soap and detergent overwrap structures, LDPE is frequently selected as the seal layer at 12–25 µm thickness, while HDPE or LLDPE provides bulk stiffness in a coextruded film. A three-layer A/B/A structure with outer layers of LDPE at 0.923 g/cm³ and a core of HDPE at 0.955 g/cm³ can achieve a 1% secant modulus above 500 MPa while retaining an ultimate seal strength above 15 N/25.4 mm at 115°C, as measured by ASTM F88-21. When the same structure is produced with an LDPE sealant of 0.918 g/cm³, seal initiation may drop by 5°C, but the film loses approximately 20–30% of stiffness. The choice is therefore a balance between filling line seal dwell time and package stand-up performance on shelf.

For liquid food packaging, a coextruded structure with an LDPE seal layer is used only where seal temperature must remain below 110°C to protect an ethylene vinyl alcohol barrier layer. At sealing dwell times shorter than 0.3 s, the LDPE layer transmits heat to the barrier; an LLDPE sealant would require a 5–8°C higher setpoint, increasing the risk of barrier shrinkage. The operational limit is controlled by a seal bar temperature tolerance of ±3°C across the width of the sealing jaw. Deviation beyond this tolerance produces weak seal spots below 0.5 N/25.4 mm or burn-through at the film edge.

For frozen food packaging, LDPE remains usable at low temperatures because its glass transition is below -80°C; however, stiffness at -20°C is lower than LLDPE. A film with a density of 0.918 g/cm³ may show a tensile impact strength measured by ISO 8256 of 400–600 kJ/m² at -20°C, while an HDPE film of 0.950 g/cm³ may fail at 100–200 kJ/m². The lower modulus of LDPE at freezer temperatures reduces cracking but also permits package deformation under compression. Converters therefore blend 5–15% LLDPE into the LDPE seal layer to lift low-temperature modulus without raising seal initiation beyond 5°C.