Low-density polyethylene (LDPE) is distinguished from linear low-density polyethylene (LLDPE) and high-density polyethylene (HDPE) by a randomly branched macromolecular architecture produced in high-pressure autoclave or tubular reactors. This branching suppresses lamellar thickening during crystallization; the resulting solid-state density measured at 23 °C in accordance with ISO 1183-1:2019 or ASTM D1505-18 occupies the range 0.910–0.940 g/cm³. Commercial blown-film grades rarely span that full window; most extrusion grades are released at 0.917–0.923 g/cm³, while certain extrusion-coating and sealant grades may be specified at 0.915–0.920 g/cm³. A two-phase density model with amorphous and crystalline reference densities of 0.855 g/cm³ and 1.000 g/cm³ converts a density of 0.918 g/cm³ to approximately 46% crystallinity. This level of crystallinity is directly responsible for the characteristic balance of low small-strain modulus, moderate yielding, and high extensibility that separates LDPE from more linear polyethylenes.

Because density and melt flow index are release metrics, they appear on certificates of analysis with reference to specific methods. Density is not an intrinsic material property in the thermodynamic sense; it shifts with cooling rate, orientation, and filler content. A density-gradient column conforming to ASTM D1505-18 Method A resolves differences of 0.0001 g/cm³ when the column is maintained within ±0.1 °C, but laboratories may also use the displacement method of ISO 1183-1:2019 Method B for routine lot acceptance. The choice of method affects reported density by up to 0.0005 g/cm³ because of wetting, air bubbles, and thermal equilibration. Consequently, a density specification of 0.920 ± 0.003 g/cm³ must be linked to the originating standard before it is used for incoming resin acceptance.

For mechanical property interpretation, density should be read alongside molecular weight and long-chain branching. Decreasing density from 0.923 g/cm³ to 0.918 g/cm³ does not automatically imply a proportional loss of tensile strength; film fabrication conditions, thickness uniformity, and test temperature can generate larger shifts in tensile values than the density change itself. This is why tensile strength and tear strength are never specified independently of thickness, specimen type, conditioning, and test speed.

What Does the 0.910–0.940 g/cm³ Density Window Control in LDPE Grades?

The density window controls the amorphous-phase volume fraction, which governs the small-strain modulus and the optical clarity of an extrudate. In blown film, a reduction from 0.923 g/cm³ to 0.918 g/cm³ corresponds to an increase in the volume fraction of non-crystalline material; the result is a higher puncture and dart-impact energy and a lower 1% secant modulus when tested per ISO 527-3:2018 or ASTM D882-18. However, the relationship is not linear across the entire 0.910–0.940 g/cm³ range because long-chain branching and molecular weight distribution also influence tie-molecule concentration. Published data for exact grade-to-grade haze comparisons is grade-dependent; the relevant industrial observation is that lower-density autoclave LDPE grades generally produce higher transparency than higher-density tubular LDPE grades of equivalent melt flow index under identical blown-film conditions.

The density window also sets practical limits on secondary operations. Ethylene-vinyl acetate copolymers and LLDPE blends are used when density alone cannot deliver the required combination of low-temperature sealability and puncture resistance; LDPE homopolymer at 0.917–0.920 g/cm³ remains selected for high-clarity shrink packaging and heavy-duty sheeting where a balance of extensibility and low gel count is required. Density is therefore not a target by itself; it is a boundary condition for the crystallinity-dependent properties that follow.

On a 90 mm single-screw blown-film extruder with a 24:1 L/D barrier screw, a change in melt flow index from 2.0 g/10 min to 0.8 g/10 min alters melt pressure at the screen changer and changes bubble stability. Melt flow index is measured at 190 °C under a 2.16 kg load and expressed in grams per 10 min; the test procedures are ISO 1133-1:2022 Method A and ASTM D1238 Procedure B. Lower MFI values indicate higher average molecular weight. They increase extensional viscosity and melt strength, which improves bubble stability at high blow-up ratios, but they also raise extruder melt temperature through viscous dissipation and reduce output at constant screw speed. The processing window is therefore bounded at the low-MFI end by excessive melt pressure and at the high-MFI end by bubble sag and poor gauge control.

Die gaps between 0.8 mm and 2.0 mm, blow-up ratios from 2:1 to 4:1, and frost-line heights from 3 to 8 die diameters are typical for LDPE blown-film lines; the exact combination depends on the MFI and the molecular architecture. Lower MFI film grades at 0.2–0.7 g/10 min are used in heavy-duty shrink film where high stalk heights and high melt strength are required; general-purpose film grades at 0.8–2.0 g/10 min balance output and gauge stability.

Melt Flow Index, Shear Sensitivity, and Molecular Weight Distribution in Extrusion and Injection Molding

Melt flow index is a single-point viscosity index, not a complete rheological description. LDPE is strongly shear thinning because of long-chain branching; the non-Newtonian index at 100 s⁻¹ is typically below 0.6 for high-pressure autoclave grades. As a result, two LDPE grades with identical MFI can show different melt pressure in a blown-film line or different injection filling at identical screw-stroke settings. The flow-rate ratio, usually reported as I21.6/I2.16, provides an indication of shear sensitivity; commercial LDPE grades often show FRR values in the 40–80 range. This is one reason MFI cannot be translated directly to viscosity without capillary rheometry data.

The table is not a substitution for capillary rheometry. On a 120 mm extruder with 30:1 L/D, a reduction in MFI from 2.0 g/10 min to 0.8 g/10 min can raise melt temperature through shear heating at constant screw speed; the actual value depends on screw design, backpressure, and barrel profile. Conversely, injection molders compensate for higher MFI by reducing injection pressure and dwell time, but low-density grades with MFI above 30 g/10 min may exhibit jetting and weld-line weakness if the melt temperature is not raised above 220 °C.

For blow-film gauge in the range 25–100 µm, tensile properties are conventionally determined under ASTM D882-18 or ISO 527-3:2018, not ASTM D638, because the specimen behaves as a thin membrane. A Type 2 film specimen with a 50 mm gauge length and a crosshead speed of 500 mm/min is common for high-elongation LDPE; the exact speed must match the resin specification because LDPE tensile strength at break is strain-rate sensitive. Machine-direction tensile strength is generally higher than transverse-direction tensile strength in blown film, but the difference narrows as blow-up ratio approaches 1:1 and widens when the film is drawn at high stalk heights. The orientation effect is not uniform through the thickness; surface layers may contain more orientated chains than the core, which is one reason tensile results on LDPE film show greater scatter than rigid polyethylene samples.

At constant density, tensile strength at break typically decreases as MFI increases above 2.0 g/10 min because lower molecular weight reduces the number of load-bearing tie chains. At constant MFI, density reduction from 0.923 g/cm³ to 0.918 g/cm³ may lower yield strength but can increase elongation at break; the effect is smaller than orientation-induced differences.

When Tear Strength Is Specified by ASTM D1922, Gauge Variation Becomes the Dominant Error Source

Elmendorf tear strength of LDPE film is measured under ASTM D1922-15(2019) or ISO 6383-2:1983, with the result reported in newtons or grams-force. The test measures the force required to propagate a precut tear across a specimen; because LDPE is highly extensible, the tear does not propagate as a clean crack. Plastic deformation ahead of the tear tip consumes energy and makes tear strength highly sensitive to film thickness. Consequently, tear values on a 25 µm film cannot be multiplied to predict tear values on a 75 µm film. Published data for universal thickness scaling factors is limited; a new test at the intended gauge is required.

A calibrated Elmendorf tester with a 3200 gf or 6400 gf pendulum is used depending on tear force; the specimen notch radius and clamping pressure must be controlled because LDPE film creeps under excessive clamping load. A high-MFI, low-density grade may produce lower Elmendorf values because the melt has lower molecular weight, but blown-film orientation can override molecular weight effects. This is why tear strength cannot be read from density or MFI alone.

Injection molding of LDPE at densities above 0.922 g/cm³ and MFI below 8.0 g/10 min presents a different set of boundaries. Mold shrinkage measured by ASTM D955 generally falls between 1.5% and 2.5% for unfilled LDPE, and is lower when packing pressure and gate freeze time are increased. Lower density grades produce lower as-molded stiffness and can be extracted from deep-draw cores with less ejection force, but they may exhibit sink marks if the MFI is too high and the screw cushion is not held. The processing window is bounded on one side by short-shot formation at low MFI and on the other by flash and part-weight variation at high MFI. These limitations are not eliminated by density selection alone.