Low-Density Polyethylene (LDPE) is a long-chain branched polyolefin produced by high-pressure free-radical polymerization in stirred autoclave or tubular reactors at pressures from 100 MPa to 300 MPa and temperatures from 160°C to 300°C. The density range is 0.917 g/cm³ to 0.930 g/cm³ under ISO 1183-1:2019, and the crystalline fraction is typically between 40% and 55%. High-density polyethylene (HDPE) produced by low-pressure coordination catalysis exhibits a crystalline fraction of 60% to 80% and a density range of 0.941 g/cm³ to 0.965 g/cm³. Because crystallinity sets tensile yield, flexural modulus, barrier, thermal softening, and surfactant resistance, the selection problem described as LDPE vs HDPE: Which Polyethylene Fits Your Product is a comparison of crystal network density and chain relaxation time more than a simple density label.

Commercial LDPE extrusion film grades are specified with melt flow indices from 0.2 g/10 min to 2.0 g/10 min at 190°C and 2.16 kg under ISO 1133-1:2022. High-pressure autoclave grades show melt flow ratios I21.6/I2.16 in the range 40 to 70, whereas linear HDPE film grades with similar melt index commonly show melt flow ratios below 25. The difference corresponds to long-chain branching density and molecular weight distribution breadth; it does not scale with density alone. The processing consequence is that LDPE exhibits pronounced shear thinning and elongational strain hardening, while HDPE exhibits less extensional thickening and therefore requires different die design, temperature profiling, and haul-off control.

The crystallographic melting peak for LDPE measured by ISO 11357-3:2018 is usually between 102°C and 115°C, while HDPE homopolymer peaks fall between 125°C and 140°C. This difference of approximately 20°C–30°C shifts the heat-seal initiation temperature and hot-fill threshold. LDPE sealants begin forming peelable seals at 110°C–125°C; HDPE surfaces typically require 130°C–150°C for comparable seal strength at 0.3 MPa bar pressure and 0.5 s dwell.

What Does Long-Chain Branching Do to Melt Flow Ratio and Neck-In?

Long-chain branching in LDPE contributes directly to extensional viscosity growth and reduced neck-in during extrusion coating. On a 90 mm single-screw extruder with 30:1 L/D and a 1200 mm slot die coating 20 μm LDPE onto paperboard at line speeds of 300 m/min to 500 m/min, neck-in is commonly controlled below 15 mm/side at melt temperatures from 290°C to 320°C. The same die running linear HDPE with an equivalent melt index may require a lower melt temperature between 260°C and 300°C and a narrower die gap because lower melt elasticity reduces transverse stabilization force. Draw resonance amplitude measured by online thickness profilometry can exceed ±5% at line accelerations above 80 m/min/s for linear HDPE, while branched LDPE typically remains within ±2% thickness variation under identical haul-off rates. Published data for specific line configurations is limited, but the relative stability difference is reproducible across standard coextrusion coating trials.

On blown-film lines, LDPE exhibits a broader bubble stability window than HDPE when gauge variation is measured by ASTM D8136 or capacitive thickness scanning. A 45 mm single-screw blown-film line with 24:1 L/D, 200 mm spiral mandrel die, and 1.0 mm die gap can process LDPE at barrel profiles from 160°C to 190°C and die temperatures from 180°C to 200°C. Blow-up ratio is maintained between 2.0:1 and 3.0:1, with frost line height set at 300 mm to 600 mm above the die. Under these conditions, long-chain branching stabilizes the bubble against helical instability and permits haul-off speeds up to 60 m/min at a 25 μm film thickness. HDPE blown film with density 0.949 g/cm³ and similar melt index requires barrel and die set points between 200°C and 230°C, and blow-up ratio is typically reduced to 1.8:1–2.5:1 for balanced machine-direction and transverse-direction tear resistance. High-stalk HDPE configurations with stalk heights above 10 die diameters can produce gauge variation exceeding ±8%, forcing either internal bubble cooling or lower haul-off speeds. Published data for this specific line configuration is limited, but the directional orientation balance measured by tear resistance under ISO 6383-2:2015 differs enough that film designers grade the two polymers separately.

Flexible packaging laminates commonly place LDPE as the sealing skin because the lower melting peak and lower crystallinity depress seal initiation. A coextruded sheet with an LDPE skin of 15 μm and HDPE core of 30 μm can form a seal at 110°C–125°C at 0.5 s dwell and 0.3 MPa pressure, while an HDPE skin may require 130°C–150°C under the same conditions. Heat-seal strength for an LDPE skin measured under ASTM F88/F88M-21 at 120°C often exceeds 10 N/25 mm, whereas HDPE film at 120°C may remain below 3 N/25 mm. This difference controls packaging machine speed and energy input on horizontal form-fill-seal lines running at 60 packs/min to 120 packs/min. When specific film structures are qualified, seal initiation is confirmed by gradient seal testing per ASTM F2029-16 rather than inferred from DSC alone.

Thermal Softening and Vicat Onset in Rigid Packaging

Vicat softening temperature measured at 50°C/h and 50 N under ISO 306:2014 Method A50 places LDPE at 85°C to 95°C and HDPE at 115°C to 130°C. Heat deflection temperature at 0.455 MPa under ASTM D648-18 follows at 40°C to 55°C for LDPE and 65°C to 85°C for HDPE. The operational boundary appears in hot-fill: LDPE is excluded from continuous wall contact above 60°C because seal creep and stack deformation become measurable; HDPE is used up to 80°C in dishwasher-tested and hot-filled containers. Oxidation induction time at 200°C under ISO 11357-6:2018 for unstabilized LDPE can be below 5 min; compounded antioxidant systems commonly lift oxidation induction time beyond 30 min, but the thermal aging window cannot exceed the polymer melting and crystallinity limits.

Injection molding of closures and dispensing closures uses LDPE melt flow indices from 20 g/10 min to 85 g/10 min at 190°C/2.16 kg. A 250 t clamp force toggle machine with 50 mm screw and 22:1 L/D processes LDPE at melt temperature 180°C–220°C and mold temperature 10°C–30°C, with cycle time below 10 s for 0.8 mm wall thickness. HDPE closure grades with similar mold cooling and higher melt temperatures 200°C–250°C provide tensile yield stress of 20–32 MPa under ASTM D638-14 compared to 8–12 MPa for LDPE. Flexural modulus under ISO 178:2019 is 150–350 MPa for LDPE and 800–1500 MPa for HDPE, which changes unscrewing torque retention in child-resistant closures and snap-fit assembly force.

Compression force on a 20 mL ophthalmic squeeze bottle measured with a universal testing machine at 10 mm/min crosshead speed is commonly 15–30 N for LDPE and 40–70 N for HDPE at 3 mm deflection. The lower modulus of LDPE allows drop dispensing without excessive hand force; HDPE is selected only when the closure requires high retained backpressure or the drug product demands additional moisture barrier. Published data for product-specific configurations is limited, but the mechanical compliance difference remains measurable across lot-to-lot production trials.

When the Application Requires Stress-Crack Resistance Beyond LDPE Limits

Environmental stress-crack resistance measured by ASTM D1693-15 Method B in 10% Igepal CO-630 at 50°C creates a hard selection boundary. Many high-pressure LDPE film and coating resins reach F50 failure below 100 h, while HDPE pipe, bottle, and engineered packaging resins routinely exceed 500 h to F50; higher molecular weight HDPE grades can pass 1000 h without fracture. For surfactant-loaded contents such as liquid detergents, motor oil, agricultural chemicals, and bleach, HDPE is specified because the combined tensile stress at molded-in corner radii and the polar surfactant environment accelerate craze formation in LDPE. The operational limit for LDPE is therefore explicit: containers with molded-in stress risers and long-term exposure to surface-active fluids should be avoided unless an internal polyolefin tie layer or fluorination treatment is validated. HDPE squeeze bottles may reach flexural modulus above 800 MPa, but stress-crack resistance remains controlled by molecular weight and comonomer placement, not by density alone.

Both LDPE and HDPE homopolymers are covered by FDA 21 CFR 177.1520 for direct food contact when the finished article meets extractable fraction limits in paragraph (c) and the intended condition of use is specified. Under EU 10/2011, overall migration must not exceed 10 mg/dm² for food-contact materials; specific migration limits for certain additives and monomers may be lower. REACH registration obligations do not apply to polymers as such under EC 1907/2006, but imported articles must not contain candidate-list SVHCs above 0.1 wt% at article level. RoHS 2011/65/EU applies to electrical and electronic equipment rather than general packaging, but polyethylene formulations used in cables or tool housings must comply with maximum concentration values of 0.1 wt% for lead, mercury, hexavalent chromium, polybrominated biphenyls, and specific phthalates in homogeneous material. For molded packaging items, ISO 15270:2008 and ASTM D7611/D7611M-20 provide resin identification coding as code 4 for LDPE and code 2 for HDPE, which affects post-consumer recycling streams but not polymer density specification.