LDPE Density Explained: 0.910–0.940 g/cm³ and What It Decides

Low-density polyethylene is specified by a density window of 0.910–0.940 g/cm³ when measured by density-gradient column under ASTM D1505-18 or by immersion under ISO 1183-1:2019 at 23°C. The numerical span is not a formulation curiosity; it encodes the crystalline volume fraction. Reported amorphous polyethylene density is approximately 0.855 g/cm³, and the orthorhombic crystalline phase approximates 1.000 g/cm³. A first-order estimate of crystalline volume fraction is Xc = (ρ measured − 0.855 g/cm³) / (1.000 g/cm³ − 0.855 g/cm³). On that basis, a 0.918 g/cm³ blown film grade corresponds to roughly 42–45% crystallinity, while a 0.935 g/cm³ injection moulding grade approaches 55–58%. The crystallinity difference controls thermal, mechanical, barrier, sealing, and solidification behaviour observed on manufacturing lines, and it works in combination with melt mass-flow rate measured at 190°C/2.16 kg under ISO 1133-1:2022.

Because density is thermal-history dependent, specimens should be conditioned at 23°C ± 2°C and 50% ± 10% RH for not less than 40 h under the procedural notes of ISO 291:2008 before measurement. In a density-gradient column operated according to ASTM D1505-18, the gradient must be calibrated with glass floats of known density, and the specimen must be fully wetted without entrapped air. A column gradient steeper than 0.0002 g/cm³/mm can reduce resolution; therefore a 0.002 g/cm³ difference between two LDPE lots may be outside measurement uncertainty only when the column is properly thermostatted and the specimen is free of moulded-in stress.

Why Does a 0.910 g/cm³ Base Grade Differ from a 0.940 g/cm³ Resin at the Molecular Level?

High-pressure tubular and autoclave reactors produce low-density polyethylene with short-chain branches and long-chain branches. A grade near 0.910 g/cm³ contains a higher short-chain branch frequency per 1,000 carbon atoms than a grade near 0.940 g/cm³; the branches interrupt crystallisable ethylene sequences and reduce lamellae thickness. Differential scanning calorimetry under ASTM D3418-21 reflects this change as a melting peak shift from approximately 105–108°C for a 0.918 g/cm³ film resin toward 112–118°C for a 0.940 g/cm³ grade. The higher-density resin also begins to solidify earlier during cooling. On a 60 mm grooved-feed extruder with a barrier screw, this early solidification increases torque demand at the die lip and haul-off unless the temperature profile is raised by 10–15°C. Because the crystalline phase is stiffer and less permeable than the amorphous phase, the same density span also changes die swell, drawdown, and heat-seal response before any additive package is considered.

Values are representative ranges compiled from public resin supplier technical data sheets and laboratory test summaries; they are not specification limits for any single commercial grade. Molecular weight distribution, long-chain branching, and comonomer content can shift a specific resin outside these bands.

On a 90 mm extrusion coating line running a 0.918 g/cm³ autoclave LDPE with melt flow rate 20 g/10 min at 310°C, the lower density extends the molten curtain and allows higher line speed before edge tear. Neck-in from a 1,000 mm coat-hanger die is typically lower for a 0.928 g/cm³ grade by 15–25% because the melt curtain develops a more elastic, skin-solidified boundary earlier. The trade-off appears in drawdown: a 0.918 g/cm³ grade can deliver a 12 µm coating at 60 m/min without melt fracture on the same deckle, while a 0.930 g/cm³ grade often requires 18–20 µm to avoid edge weave and pinholes at the same melt temperature. Corona treater settings must also be adjusted because higher crystallinity at the substrate interface increases surface wetting tension decay after treatment; surface wetting tension measured by ISO 8296:2003 should be checked within the same shift to avoid loss of adhesion to aluminium foil or board.

When a Blown Film Line Runs a 0.925 g/cm³ Grade at a 2.5:1 Blow-Up Ratio

Density-driven crystallization kinetics set the frost line height and bubble diameter stability. A 0.925 g/cm³ film grade extruded on a 55 mm grooved-feed blown film line with a 150 mm die and 1.0 mm die gap typically shows a frost line height 15–30% lower than a 0.916 g/cm³ grade at identical output, because the higher orthorhombic fraction solidifies at a higher temperature. Dart drop impact energy measured by ISO 7765-1:2004 Method A declines as density rises; a 40 µm film from a 0.918 g/cm³ resin may produce F50 values of 130–180 g, while the same thickness at 0.928 g/cm³ often falls to 80–110 g. Elmendorf tear in the machine direction measured by ASTM D1922-23 also decreases with density because thinner amorphous interlamellar regions limit energy dissipation during tear propagation. Processors compensate by raising blow-up ratio from 2.2:1 to 2.8:1 or by blending with a metallocene-catalysed polyethylene, but the density specification still defines the minimum dart impact and puncture resistance envelope for a given thickness.

At a 120 t hydraulic clamp injection moulding machine with a 40 mm screw and a cold runner plaque tool, the shift from 0.925 g/cm³ to 0.935 g/cm³ changes mould shrinkage and warpage. Linear mould shrinkage measured after 48 h at 23°C under ISO 294-4:2018 can increase from 1.2–1.6% to 1.8–2.2% parallel to flow as density increases. Transverse shrinkage is typically 0.2–0.4 percentage points lower because shear-induced orientation is frozen into the part. Warpage in shallow rectangular lids occurs when thickness-averaged crystallinity differs between gate and end-of-fill regions; the 0.935 g/cm³ grade has a shorter crystallization half-time under shear, so it solidifies earlier near the gate and produces differential volume contraction. Maintaining melt temperature above 190°C and mould temperature below 15°C reduces post-mould shrinkage, but the density-dependent crystallization rate remains the dominant variable in the dimensional tolerance band.

Environmental Stress Crack Resistance Boundaries for 0.930 g/cm³ Moulded Containers

Higher density reduces environmental stress crack resistance because the increased crystalline volume fraction reduces the concentration of load-bearing tie molecules in the amorphous phase. Under ASTM D1693-21 with 10% Igepal CO-630 at 50°C, an 0.918 g/cm³ LDPE with a narrow molecular weight distribution may show F50 values above 100 h; an 0.932 g/cm³ grade of similar melt flow rate can fall below 10 h in the same notched bent strip. The failure mode is localized fibrillar deformation at the tip of a surface flaw, accelerated by surface-active agents, mineral oil, and motor oil. For thin-wall chemical containers, drum liners, and fuel spout caps, grade selection should favour the lowest density that still meets hoop stiffness and top-load requirements. Chemical exposure testing under ASTM D543-21 with the actual packaged fluid is required before substitution; swelling in toluene or carbon tetrachloride under constrained load is expected at ambient temperature, and the probability of environmental stress cracking rises when density exceeds 0.926 g/cm³ and wall thickness falls below 1.0 mm.

Oxygen transmission through 25 µm LDPE film measured by ASTM D3985-20 at 23°C and 0% RH is generally reported in the range 2,500–4,000 cm³/(m²·day·atm); higher-density grades occupy the lower end because crystalline lamellae are impermeable to oxygen diffusion. Water vapour transmission through 25 µm film tested under ASTM E96/E96M-22 at 38°C and 90% RH commonly falls between 10–18 g/(m²·day); density has a weaker effect on water vapour because water sorption at the crystalline surface partially offsets the reduction in diffusion. In heat-seal packaging, a 20 µm sealant layer from a 0.918 g/cm³ LDPE may show seal initiation near 88–95°C under ASTM F1921-18 hot-tack conditions; moving to a 0.928 g/cm³ grade raises seal initiation by 3–7°C and narrows the hot-tack window by reducing molten survival time at the seal bar. This directly limits vertical form-fill-seal line speed, especially when sealing through contaminated surfaces.

In regulated food-contact packaging, the density of LDPE is not a direct compliance marker, but it controls migration-relevant diffusion. The relevant food-contact authorizations are contained in FDA 21 CFR 177.1520 for olefin polymers and in Commission Regulation (EU) No 10/2011 for plastic materials intended to be in contact with food. A higher-density grade with greater crystalline fraction generally exhibits lower total migration in fatty-food simulants under EN 1186-1:2002 contact testing, provided the same molecular weight and additive package are compared. However, processability trade-offs in heat sealing and dart impact remain more restrictive than migration margins. Grades intended for medical or pharmaceutical packaging are additionally evaluated for extractables under Ph Eur 3.1.3 or USP <661.1>; density affects the extraction profile only through crystalline tortuosity and not through the base polymer’s toxicological status.

Tensile Yield, Flexural Modulus, and Vicat Softening Do Not Track Density Linearly

Mechanical response in the 0.910–0.940 g/cm³ range is monotonic but not linear; the steepest stiffness increase normally occurs between 0.915 and 0.930 g/cm³. Tensile yield stress measured under ISO 527-2:2012 on a type 1A specimen at 50 mm/min rises from approximately 8 MPa at 0.916 g/cm³ to 12 MPa at 0.932 g/cm³, while elongation at break falls from above 500% to less than 300%. Flexural modulus under ISO 178:2019 at 23°C increases from 160–220 MPa to 280–350 MPa across the same density band. Vicat softening temperature under ISO 306:2022 Method A50 increases by 10–15 K between 0.918 and 0.940 g/cm³; heat deflection temperature under ISO 75-2:2013 Method B moves from approximately 35–40°C to 45–55°C. These property shifts alter injection clamp force and pressure requirements: a 0.938 g/cm³ grade may require a specific injection pressure of 800–1,000 bar to fill a thin-wall mould that a 0.920 g/cm³ grade fills at 650–800 bar. The density specification therefore interacts with melt flow rate and molecular weight distribution to set the practical filling-pressure envelope and dimensional stability boundary.