How to Read an LDPE Datasheet: Six Numbers That Decide the Application

Low-density polyethylene is specified through a compact set of physical and rheological values. A datasheet can appear interchangeable across suppliers because the base resin family is defined by a narrow density band and a high-pressure polymerisation architecture, but six numerical entries—density, melt mass-flow rate, tensile yield stress, elongation at break, Vicat softening temperature, and environmental stress crack resistance—are read as a system rather than as isolated targets. The practical selection logic is governed by ISO 1183-1:2019, ISO 1133-1:2022, ISO 527-2, ISO 306, and ASTM D1693. The following sections examine how each number interacts with the downstream equipment that the grade will actually encounter.

Density at 23 °C: The First Sorting Gate in ISO 1183-1:2019

The density value on an LDPE datasheet is not a purity indicator. In high-pressure autoclave and tubular reactors, short-chain and long-chain branching reduce crystallite packing; commercial LDPE therefore falls between 0.917 g/cm³ and 0.930 g/cm³ when measured using ISO 1183-1:2019 Method A or ASTM D1505. Grades in the 0.918–0.922 g/cm³ window are assigned to blown film for clarity and low hot-tack temperature, whereas densities from 0.925 g/cm³ to 0.930 g/cm³ are specified for extrusion coating, laminating, and caps where flexural rigidity and heat-seal separation are more important than tear propagation. Density is not read in isolation; the same nominal density from an autoclave and a tubular grade can show different die swell and bubble stability because long-chain branching frequency is not captured by ISO 1183.

On a blown-film line with a 90 mm grooved-feed single-screw extruder at 30:1 L/D, a move from 0.919 g/cm³ to 0.924 g/cm³ changes the frost line height by 2–3 die diameters when blow-up ratio and die gap remain at 2.8:1 and 1.2 mm. The higher-density film resists stretching at the freeze line, raises bubble internal pressure, and can generate gauge variation outside the ±5% control limit under ISO 4593 if the lower-density grade’s air-ring settings are retained. For extrusion coating onto paperboard at 12–20 g/m², density near 0.928 g/cm³ shortens heat-seal dwell time in a converter’s jaw because the melting range is narrower; however, the same value can reduce hot-tack strength at 115 °C when tested under ASTM F1921-20. Published data for exact property-density correlations across all reactor types is limited, and supplier data must be consulted for batch-to-batch variation.

Before any comparison of film-blowing or injection-moulding grades is attempted, the melt mass-flow rate obtained at 190 °C under 2.16 kg is read from ISO 1133-1:2022 or ASTM D1238-23. The MFR is the mass extruded through a standard capillary in 10 min, not a molecular weight. Commercial LDPE spans roughly 0.2 g/10 min for heavy-duty shrink film and 70 g/10 min for thin-wall injection moulding. For blown film, a grade at 0.25–0.5 g/10 min provides the melt strength required for a bubble with a 2.5:1–3.5:1 blow-up ratio on a 25:1–30:1 L/D extruder, but the higher shear heating may raise melt temperature from 170 °C to 190 °C at the die exit and reduce the drive’s margin if back pressure exceeds 350 bar. Extrusion coating grades at 4–8 g/10 min are selected because they can be drawn from a flat die at melt temperatures of 300–320 °C and line speeds of 150–300 m/min without excessive neck-in; the same grades are processed at screw speeds that deliver 180–250 kg/h on a 90 mm extruder. Injection-moulded closures at 20–50 g/10 min fill thin sections under clamp force settings of 2.5–4.0 kN per cm² of projected area. A grade at 8 g/10 min that is substituted without raising melt temperature can produce short shots or gate-stringing.

Capillary rheometry under ISO 11443:2021 is required when two LDPE grades have the same MFR but different long-chain branching. LDPE displays shear-thinning with a power-law index in the range 0.3–0.6 at processing shear rates; therefore the single MFR point under-represents extrusion pressure. Tubular reactor grades can show greater elongational strain hardening than autoclave grades at identical density and MFR, which stabilises the bubble in blown film and reduces draw resonance in cast film. A cast-film trial at 10 µm thickness and 300 m/min line speed will separate grades with the same MFR but different molecular architecture through edge-bead and thickness variation measured under ISO 4593.

What Does the Vicat Softening Point Change in Sealing and Warm-Fill Operations?

Vicat softening temperature, measured under ISO 306:2022 A50 or ASTM D1525-17 with a 50 °C/h heating rate and a 10 N load, is a short-term thermal boundary, not an operating temperature. Commercial LDPE typically falls between 85 °C and 100 °C. Higher Vicat values are used in warm-fill condiment pouches and extrusion-coated paperboard where filling temperatures can reach 60–80 °C and sealing jaws operate at 120–140 °C. A grade reporting 87 °C may seal acceptably at 115 °C but can exhibit cohesive failure when a converter raises jaw temperature to 135 °C and reduces dwell time below 0.4 s. For stressed parts, the practical continuous-use limit is typically below 65 °C. In blown film, Vicat also correlates with blocking after hot winding: rolls stored at 40 °C under high tension can block if hot-tack performance under ASTM F1921-20 is marginal. The value is not a substitute for tensile creep data under ISO 899-1 or dynamic mechanical analysis.

At a jaw separation speed of 50 mm/min, ISO 527-2/1A or ASTM D638-14 reports the tensile yield stress for LDPE in the range 8–12 MPa, with break stress commonly 12–20 MPa in unoriented specimens. The yield stress is used to screen against abuse conditions. A blown-film grade with a yield stress below 9 MPa may survive low-tension converting but is not adequate for heavy-duty shipping sacks if dart impact under ISO 7765-1 is also below 500 g and Elmendorf tear under ISO 6383-2 is marginal. On an extrusion-coating line, machine-direction tensile stress can exceed 20 MPa after orientation at a draw ratio of 80:1; the datasheet value does not replace a tensile test on the coated substrate. For injection-moulded screw caps, yield stress at 10–12 MPa supports hoop retention force after a packer’s top-load test, but dimensions and thread geometry dominate. The selection rule is to compare yield stress only at identical test speed and specimen conditioning under 23 °C and 50% RH.

Percent elongation at break, recorded from the same ISO 527-2 pull or from ISO 527-3 for film thinner than 1 mm, separates stretch-oriented uses from flat packaging. Typical LDPE film values fall between 200% and 800%. The value is strain-rate dependent; a test at 500 mm/min does not predict performance on a pallet wrapper operating at 5 m/s. A blown-film grade with 300% elongation at break can pass a static ASTM D882-18 tensile test but fail on a high-speed wrapper because tears propagate from edge defects. Grades above 600% are usually preferred for collation shrink and stretch wrap; the film is produced on a blown-film line with a 0.8–2.0 mm die gap and a frost line height of 5–8 die diameters to build orientation. For injection moulded parts, elongation below 100% often indicates high gel or excessive post-industrial recycle; the value is therefore monitored as a lot-to-lot purity indicator.

When Environmental Stress Crack Resistance Separates Squeeze Bottles from Collation Shrink

Environmental stress crack resistance is measured under ASTM D1693-21 with a bent strip in 10% or 100% Igepal CO-630 at 50 °C, or under ISO 22088-3:2021. Datasheet values span from 1 h to several hundred hours, but they are specimen-dependent. A grade reporting F50 of 24 h under ASTM D1693 condition B is adequate for general-purpose film but is not sufficient for detergent squeeze bottles with a 0.6 mm wall subjected to surfactant contact at 40 °C. Blow-moulded bottle grades typically require more than 100 h at condition B to survive a 5 °C drop test. In extrusion coating, ESCR is less commonly specified than edge-tear resistance, but liquid packaging board coated with 12–20 g/m² LDPE is evaluated for stress cracking in contact with fatty emulsions. The value only ranks grades when the same test condition and specimen preparation are used; a value from compression-moulded plaques cannot be transferred directly to a blow-moulded part.

For a liquid packaging board line running at 250 m/min with a 0.8 mm coating die and a 300 °C melt temperature, grade selection is constrained by density 0.927–0.930 g/cm³ for adhesion, MFR 7–8 g/10 min for drawdown, and Vicat above 90 °C for sealing. A deviation in any one of these numbers shifts the process: lower density reduces adhesion after flame or corona treatment; lower MFR raises die pressure and promotes edge necking; lower Vicat narrows the heat-seal window at the converter’s jaw. Operators record thickness variation under ISO 4593 and adhesion levels under the converter’s internal top-ply cohesion procedure.