Low-density polyethylene (LDPE) is a high-pressure, branched thermoplastic produced by free-radical polymerisation in tubular or autoclave reactors at pressures of 140–350 MPa and temperatures of 160–320°C. Initiation by oxygen or organic peroxides creates short-chain branches, mainly ethyl and butyl, and long-chain branches that reduce density to 0.910–0.940 g/cm³ when tested per ASTM D1505 or ISO 1183-1. Crystallinity determined by differential scanning calorimetry per ISO 11357-3 is typically 40–55%, giving a melting peak at 105–115°C. This architecture differs from linear low-density polyethylene (LLDPE), which is a low-pressure catalytic copolymer of ethylene with 1-butene, 1-hexene, or 1-octene and has density 0.916–0.940 g/cm³ but no long-chain branching. High-density polyethylene (HDPE) has minimal branching, density 0.941–0.965 g/cm³, and crystallinity 60–80%. For converters, the selection matrix captured under LDPE vs LLDPE vs HDPE: Property Table for Converters is not a simple density ranking; melt strength, extensional rheology, seal initiation temperature, extrusion head pressure, dart impact, environmental stress crack resistance, and thermal dimensional stability diverge in ways that dictate machine settings and lot-release criteria.

Melt index measured per ASTM D1238 or ISO 1133-1 at 190°C and 2.16 kg is a common but incomplete sorting parameter. LDPE extrusion grades for film are commonly specified at 0.2–2.0 g/10 min, injection moulding grades at 2.0–8.0 g/10 min, and extrusion coating grades at 4.0–8.0 g/10 min. Two lots with identical melt index can differ in long-chain branching concentration and die swell. The branching architecture of LDPE produces strain-hardening behaviour in extensional flow, which is not captured by melt index. A Rosand capillary rheometer with a melt-strength attachment or a Sentmanat extensional fixture is used to quantify melt strength before line trials. LLDPE of similar melt index has lower extensional viscosity and a more linear stress-strain response, while HDPE has higher melt temperature and lower melt strength at equivalent index.

Which Melt Rheology Characteristics Govern LDPE Film Bubble Stability?

Long-chain branching, not merely molecular weight distribution, governs bubble stability on production blown film lines. Melt strength measured with a Rosand RH10 twin-bore capillary rheometer fitted with a melt-strength wheel is typically 8–15 cN for LDPE film grades with MFI 0.8 g/10 min, whereas LLDPE of the same MFI often measures 3–7 cN. HDPE film grades measure lower still and require high-stalk bubble geometry. The property difference is attributed to strain hardening, in which extensional viscosity rises by a factor of 4–10 at Hencky strain rates above 1 s⁻¹ for LDPE. This stabilises the bubble against draw resonance and permits a higher frost line.

On a 90 mm single-screw extruder with a 30:1 L/D barrier screw and a 300 mm die, LDPE with MFI 0.8 g/10 min is processed at melt temperature 160–190°C and die pressure 25–35 MPa. The same line processing a 1-octene LLDPE of equivalent melt index draws 10–15% higher extruder amperage and requires a wider die gap of 2.0–2.5 mm to delay melt fracture. HDPE film grades require melt temperatures of 180–230°C and a lower blow-up ratio unless high-stalk geometry is configured. Field data from converter trials show that batch-to-batch variation of LDPE melt index of ±0.1 g/10 min can shift frost-line height by 50–100 mm on a 300 mm die, requiring screw-speed adjustments of 5–10% to maintain gauge uniformity.

Blown film physical properties are anisotropic because of orientation. Machine-direction tear measured per ASTM D1922 is typically lower than transverse-direction tear for LDPE by a factor of 2–3, whereas LLDPE can be balanced by orienting at higher blow-up ratios. The property table therefore guides material choice, die size, blow-up ratio, and frost-line settings.

LDPE vs LLDPE vs HDPE: Property Table for Converters

Property ranges are typical for conversion grades and do not replace supplier lot-specific certificates.

In blown film conversion, the values above explain why LDPE remains specified for low-haze shrink and overwrap structures despite lower tensile yield and flexural modulus. A 25 µm LDPE film produced at a blow-up ratio of 2.0:1 to 3.0:1 typically exhibits haze below 10% and gloss above 60 GU when measured per ASTM D1003 and ASTM D2457 at 45°. LLDPE of equivalent thickness has higher dart impact and Elmendorf tear but may require blending with 10–30% LDPE to obtain equivalent optical quality. HDPE film provides the highest stiffness and moisture barrier but is limited in applications needing tear initiation resistance and deep-draw seal geometry. For converters running automatic packaging lines, the lower melt temperature and faster phase change of LDPE reduce cooling load on the collapsing frame and allow higher throughput on lines limited by film blocking.

Seal initiation temperature measured on a heat-seal tester at 0.5 s dwell and 0.5 MPa jaw pressure is typically 90–120°C for LDPE, 100–130°C for LLDPE, and 125–140°C for HDPE. Converters with dwell times below 0.3 s must therefore set seal-bar temperatures 10–15°C higher for LLDPE and 25–35°C higher for HDPE. Hot-tack strength measured per ASTM F1921 is also lower for HDPE at equivalent thickness; LDPE and ionomer-modified LDPE grades are specified when a lap seal must resist opening during pouch filling. The thermal conductivity of semicrystalline polyethylene, approximately 0.33–0.52 W/(m·K), determines heat-seal dwell time on high-speed vertical form-fill-seal machines.

Environmental stress crack resistance measured per ASTM D1693 Condition B shows LDPE and LLDPE are not automatically equivalent. LDPE with low melt index and high molecular weight can exceed 1000 h, but high-melt-index injection grades may fail below 100 h. HDPE homopolymer ranges from below 10 h for low-molecular-weight injection grades to more than 1000 h for bimodal pipe grades. Converters of caps, closures, and rigid containers evaluate the specific grade rather than the polymer class because molecular architecture and comonomer distribution dominate ESCR.

Cast-film converters running LDPE at 15–40 µm thickness set chill-roll temperature at 15–30°C to control film crystallinity and blocking. Slip and antiblock masterbatches are added at 5–15 wt% with erucamide slip loadings of 500–1500 ppm; the effective slip migration is measured by coefficient of friction per ASTM D1894 after 72 h aging. LLDPE may require higher antiblock loading to offset its lower surface smoothness. This is a converter-relevant extension of the property table because surface properties are not bulk mechanical data but determine reel handling and packaging-machine feeding.

When Converters Specify LDPE over LLDPE in Thin-Gauge Applications

Shrink force and free-shrink percentage measured per ASTM D2732 at 110–120°C are common specification points. A 50 µm LDPE shrink film typically reaches 30–50% machine-direction shrinkage and 20–40% transverse-direction shrinkage at 120°C, whereas LLDPE of the same gauge may produce lower free shrink at equivalent temperature because of higher melting peak and lower molecular mobility. LDPE is therefore selected for high-shrink collation and label film when the converter requires a low seal-initiation threshold and controlled shrink force. The operational boundary is the upper service temperature: LDPE film begins to lose dimensional stability above 65–80°C, while HDPE can tolerate short-term exposure up to 90–110°C. Continuous exposure above these thresholds causes creep and relaxation, which should be evaluated by ASTM D2990 tensile creep or ISO 899-1.

Extrusion coating applications impose a different ranking. LDPE grades with MFI 4.0–8.0 g/10 min are pumped through a 32:1 L/D single-screw extruder and a flat die at melt temperature 280–320°C to coat paper, aluminium foil, or oriented polypropylene. Neck-in below 50 mm at 100 m/min line speed on a 1.0 m slot die is typical for LDPE; LLDPE of similar melt index produces greater neck-in and higher haul-off tension unless blended with LDPE. LDPE also promotes adhesion to aluminium foil through polar oxidation products formed at high melt temperature; peel strength measured per ASTM D903 after lamination is typically 2.0–5.0 N/15 mm for 15 µm coatings. Converters report lower odour and taste carry-over when LDPE is processed with a nitrogen-purged hopper and melt temperature sufficiently high to devolatilise residual low-molecular-weight species.

Injection moulding of LDPE is limited to flexible lids, overcaps, and closures with low structural load. Melt temperatures of 160–200°C and mould temperatures of 10–30°C are used; injection pressure is lower than HDPE because of lower viscosity. Shrinkage measured per ASTM D955 after 24 h is typically 2.0–3.5% for LDPE, compared with 1.5–2.5% for HDPE. Dimensional tolerance on large flat lids is therefore adjusted in tool design to compensate for LDPE shrinkage and warpage.

For extrusion lamination of multi-layer structures, LDPE is combined with aluminium foil and sealant films at nip pressures of 0.5–1.0 MPa and corona discharge treatment above 40 dyn/cm measured per ASTM D2578 to raise surface energy. The oxidation products generated at melt temperatures above 300°C improve foil adhesion but can increase odour in very high-speed runs; converters monitor residual volatiles by gas chromatography with headspace sampling. Published data for this specific configuration is limited, so line qualification rather than laboratory data controls acceptance.

Thermal Stability, Chemical Resistance, and Regulatory Boundary Conditions

Thermo-oxidative stability is controlled by branching and stabiliser package. Oxidation induction time measured per ASTM D3895 at 200°C for an antioxidant-stabilised LDPE blow-moulding grade usually exceeds 20 min; unstabilised powder may fail below 5 min. Processing above 240°C accelerates chain scission and gel formation, producing die-lip build-up on cast-film lines and black specks in film. For long production campaigns, melt temperature should be capped at 230°C unless the stabiliser package is qualified by multiple extrusion passes. Repeated extrusion of LDPE at temperatures above 250°C shifts molecular weight distribution and increases gel area in cast film; a gel-count inspection under ASTM D3351 or an equivalent optical method is used to monitor contamination.

Food contact status for LDPE is covered by FDA 21 CFR 177.1520 and European Regulation (EU) No 10/2011; converters must verify overall migration limits of 10 mg/dm² for food simulants. The material is not suitable for sustained contact with aromatic hydrocarbons, chlorinated solvents, or strong oxidising acids, which can cause swelling and environmental stress cracking. LDPE also requires protection from extended UV exposure unless carbon black or hindered-amine stabilisers are incorporated at concentrations typically 2.0–3.0 wt% for outdoor film. Electrical insulation grades are often specified with a dissipation factor below 0.0003 measured per ASTM D150 and a dielectric strength above 20 kV/mm measured per ASTM D149; these properties support use in low-voltage cable jacketing but the maximum continuous conductor temperature remains lower than for crosslinked polyethylene.

In extrusion blow moulding of squeeze tubes and dropper bottles, LDPE with MFI 1.0–2.0 g/10 min is processed at melt temperature 160–190°C with a 25:1 L/D extruder and parison die. Die swell of LDPE is higher than HDPE, requiring parison programmer adjustments to maintain uniform wall thickness. Weight distribution across the parison length is normally adjusted with a 20-point die gap control on automated blow moulders. Because LDPE has lower stiffness, the maximum top-load force measured per ASTM D2659 is lower than HDPE for identical wall thickness, and a 10–20% thicker wall may be required to match top-load specification.