Polyethylene vs Polypropylene: Density, Stiffness and Where Each Wins

Polyethylene (PE) is specified across converting industries by density, melt mass-flow rate, molecular weight distribution and branching architecture. Direct substitution between PE and polypropylene (PP) is rarely decided on a single property; the selection usually pivots on the interaction of density measured according to ISO 1183-1:2019 or ASTM D792-20 and flexural modulus measured according to ISO 178:2019 or ASTM D790-17. PE grades range from branched low-density resins at 0.910 g/cm³ to high-density ethylene homopolymers above 0.940 g/cm³, while PP homopolymer typically occupies a lower density band of 0.900 g/cm³ to 0.910 g/cm³. The density overlap occurs mainly between LLDPE/LDPE and PP random or impact copolymers. Stiffness, however, does not track density identically across the two polymer families: PP at 0.905 g/cm³ can exhibit higher flexural modulus than HDPE at 0.950 g/cm³ because PP crystallises in a helical conformation with restricted chain slip and a higher glass transition region.

What Separates PE and PP Density at the Resin Specification Stage?

The density envelope for unfilled PE is controlled by short-chain branch frequency, comonomer type, crystallinity and thermal history. Linear low-density polyethylene polymerised with 1-butene or 1-hexene comonomer typically spans 0.915 g/cm³ to 0.940 g/cm³; long-chain branched LDPE under autoclave or tubular reactor conditions spans 0.910 g/cm³ to 0.930 g/cm³; HDPE produced with Ziegler-Natta, chromium or metallocene catalysts spans 0.941 g/cm³ to 0.970 g/cm³. Ultra-high-molecular-weight polyethylene is not density-classified beyond the 0.930 g/cm³ to 0.940 g/cm³ range, but its very high melt viscosity places it in a separate processing category. PP homopolymer density is lower, generally 0.900 g/cm³ to 0.910 g/cm³, and PP impact copolymers with ethylene-propylene rubber phases can drop to 0.890 g/cm³. Density is not a direct proxy for stiffness across the two families. In a density-gradient column run per ISO 1183-1:2019, an HDPE closure compound at 0.952 g/cm³ may show flexural modulus near 1,200 MPa, while a nucleated PP homopolymer at 0.905 g/cm³ may reach 1,600 MPa in ISO 178:2019 three-point bending. The specification decision therefore requires simultaneous review of density for part mass and modulus for load-bearing deflection.

In blown-film extrusion, the density-stiffness trade-off appears as a direct tension between dart impact resistance and film modulus. LLDPE blown film extruded on a single-screw line with L/D 24:1, die gap 1.8 mm and blow-up ratio 2.5:1 retains high dart drop values in ASTM D1709-16a Method A because the short-chain branches distribute tie molecules across the semicrystalline lamellae. When HDPE is substituted to increase modulus, dart impact falls sharply: a resin shift from 0.918 g/cm³ LLDPE to 0.948 g/cm³ HDPE can raise ASTM D882-18 secant modulus from 250 MPa to 850 MPa while reducing ASTM D1709-16a failure energy by approximately 60% under comparable gauge control. Cast PP film at 0.900 g/cm³ provides higher tensile modulus and moisture-barrier contribution than LLDPE, but its low-temperature impact resistance is inferior; PP homopolymer film typically embrittles near 0 °C, whereas LLDPE remains ductile below -40 °C. The appropriate PE grade for flexible film is therefore lower in density and modulus when puncture and dart resistance govern, while PP is preferred for stiffness-driven packaging where low density and moisture barrier outweigh sub-zero toughness.

Flexural Modulus Thresholds in Injection-Moulded Closures

Injection-moulded closures are process-limited by melt temperature, mould temperature, cooling time and gate geometry. HDPE closure compounds with melt flow rate 1.5 g/10 min to 3.5 g/10 min at 190 °C/2.16 kg per ISO 1133-1:2022 are moulded on high-cavitation tools at injection pressures of 90 MPa to 120 MPa and clamp forces up to 2,500 kN. PP homopolymer closure grades run at 230 °C/2.16 kg with similar flow classification but crystallise faster and permit shorter holding-pressure time. The stiffness difference is measurable in top-load or side-load testing using ASTM D2659-16 or a universal testing machine with a 500 N load cell. A nucleated PP closure with side-wall flexural modulus near 1,450 MPa accepts higher strip torque than an HDPE closure at 950 MPa without dimensionally changing the tamper-evident band. PE wins in caps requiring high environmental stress crack resistance under detergent or oil contact, because HDPE tested per ASTM D1693-21 Condition A can reach 100 h to 1,000 h failure time in 10% Igepal CO-630 at 50 °C, whereas PP tends to fail earlier in the same geometry when not impact-modified. The processing rule is: PP is selected for dimensional stability and lower part density at equivalent stiffness; HDPE is selected for ESCR-limited sealants and low-temperature abuse resistance.

For a living hinge with a thickness of 0.25 mm to 0.40 mm, PP is the default material because the hinge ligament undergoes repeated flexural strain that PE cannot survive without rapid craze formation. PP homopolymer tested in a repeated flexural bending jig at ±180° deflection can exceed 10⁴ cycles before yield-line fracture, whereas HDPE at the same ligament thickness typically fails before 10³ cycles under identical fixture kinematics. The governing resin property is not static flexural modulus but fatigue crack propagation resistance in the oriented hinge zone, which is developed by post-mould flexing while the part is still above the PP crystallisation temperature. Equipment for hinge conditioning includes a servo-driven flexing station with 12 cycles/min to 30 cycles/min. PE wins the closure application when the hinge is absent or when the part is an induction-sealed liner requiring high ESCR and low seal initiation temperature. In a high-speed induction sealing line, LLDPE foam or LDPE film sealants are preferred because seal initiation begins at 10 °C to 15 °C lower than PP, reducing hot-tack failure on continuous fillers running 450 m/min.

When a Living Hinge Demands PP and a Chemical Bottle Demands HDPE

Chemical compatibility and permeation resistance separate PE and PP in blow-moulded packaging. HDPE extrusion blow-moulded containers with wall thickness 0.8 mm to 1.5 mm are used for bleach, agricultural chemicals and detergent because HDPE offers a combination of stress crack resistance and low water vapour transmission. Permeation testing per ASTM E96/E96M-22 shows HDPE water vapour transmission rate below 1.5 g·mm/m²·day at 38 °C and 90% relative humidity, while PP homopolymer transmits slightly higher water vapour but has lower oxygen permeability when oriented. The selection shifts to PP when the bottle must withstand hot-fill temperatures above 85 °C or when a retort cycle at 121 °C is required. HDPE under a sustained load above 65 °C begins to lose hoop strength; PP with a Vicat softening point near 150 °C per ISO 306:2022 Method A50 retains better dimensional stability. PE therefore wins in aggressive surfactant environments at ambient temperature, while PP wins in hot-fill or microwaveable containers subject to thermal distortion limits.

Long-term pressure performance in pipe extrusion demonstrates the structural advantage of HDPE in buried cold-water and gas distribution when measured by ISO 9080:2012 regression. A PE100 compound classified under ISO 12162:2009 carries a minimum required strength of 10 MPa at 20 °C for 50 years; a PE80 compound is limited to 8 MPa under the same extrapolation. PP random copolymer pipe evaluated under ISO 15874-2:2013 is preferred in hot-water plumbing because it maintains long-term hoop stress at 70 °C with a design stress near 3.2 MPa, while HDPE loses useful long-term strength above 60 °C. The extrusion melt temperature window illustrates the processing contrast: HDPE pipe extrusion on a single-screw line with L/D 30:1 is stable at 200 °C to 220 °C, while PP-R pipe requires 230 °C to 250 °C and tighter downstream calendration because of lower melt strength. PE wins underground gas and water at lower temperatures; PP wins pressurised hot-water systems where the service temperature exceeds the long-term oxidative resistance of PE.

Rotomoulding and Pipe Pressure: Long-Term Failure Comparisons

Rotational moulding is a low-shear process in which the density and sintering window of the powder control wall-thickness uniformity and bubble removal. LLDPE and HDPE rotomoulding powders are ground to 35 mesh to 50 mesh and processed at oven temperatures of 250 °C to 280 °C on biaxially rotating arms. During the cast-iron or aluminium tool rotation, PE powder densifies through air release and polymer melt sintering; HDPE grades with melt flow rate 4 g/10 min to 8 g/10 min at 190 °C/2.16 kg provide adequate flow for complex cavities. PP rotomoulding is less common because PP has a narrow sintering window above its melting point and is more prone to oxidative degradation at the long oven residence times of 20 min to 45 min. PE wins in large storage tanks requiring low-temperature impact resistance and weldability, while PP rotomoulding is limited to smaller components where higher flexural modulus and chemical resistance justify the tighter process window.

Molecular weight distribution controls the final processing split in extrusion blow moulding and sheet extrusion. A bimodal HDPE with melt flow rate 0.3 g/10 min to 0.45 g/10 min at 190 °C/2.16 kg measured per ISO 1133-1:2022 provides melt strength adequate for parison hang times of 12 s to 18 s on accumulator-head machines. The high-molecular-weight fraction raises zero-shear viscosity at 190 °C to approximately 5 × 10⁴ Pa·s, reducing parison sag and wall-thickness variation. PP substituted in the same extrusion blow moulding tooling at the same melt temperature produces lower die swell and requires higher extrusion back pressure; the lower melt strength also reduces wall-thickness uniformity across parting-line corners by 8% to 15% when mapped by ultrasonic wall-thickness inspection. In thermoformed sheet, PP wins when the part is thin-walled and stiffness-limited; HDPE is selected when the formed container must survive cold-chain impact and panel flex without stress whitening. The density and stiffness relationship is therefore not a single comparative ranking but a tooling- and test-standard-dependent selection between two semicrystalline polyolefin families with different failure modes.