Polyethylene Plastic Explained: Properties, Forms and Everyday Uses

Polyethylene (PE) is a semicrystalline thermoplastic synthesised by addition polymerisation of ethylene. Density is the primary classification parameter and is measured according to ISO 1183-1:2019; commercial values extend from 0.910 g/cm³ to 0.970 g/cm³ across LDPE, LLDPE, HDPE, and UHMWPE. The crystalline fraction, which can range from 40% to 80%, governs tensile modulus, permeation, and chemical resistance. Water absorption at saturation is below 0.01% when tested to ISO 62:2008. Resistance to dilute acids, alkalis, and aqueous salt solutions is retained up to 60°C; however, chlorinated solvents, aromatic hydrocarbons, and certain polar surfactants can cause swelling or environmental stress cracking.

Molecular architecture separates the principal commercial forms. LDPE is produced in high-pressure autoclave or tubular reactors at 150–350 MPa and 150–300°C and contains long-chain branches that reduce density and increase melt strength. LLDPE is copolymerised with but-1-ene, hex-1-ene, or oct-1-ene under low-pressure Ziegler-Natta or metallocene catalysis, producing short-chain branches that lower heat-seal initiation temperature and improve puncture resistance. HDPE is a linear low-pressure homopolymer or copolymer with comonomer content typically below 2 wt%. UHMWPE reaches molecular weights of 3.5–7.5×10⁶ g/mol and is processed by ram extrusion or compression moulding rather than conventional melt extrusion.

What Distinguishes Low-Density from High-Density and Linear-Low-Density Grades in Service?

Standardised mechanical and rheological data show the service boundaries. Tensile modulus for LDPE is typically 100–300 MPa; for HDPE it reaches 600–1400 MPa when tested to ISO 527-2:2012. Tensile strength at yield ranges from 7–12 MPa for LDPE to 20–35 MPa for HDPE and 35–45 MPa for UHMWPE. Melt flow index, measured at 190°C with a 2.16 kg load according to ISO 1133-1:2022, is 0.2–2.0 g/10 min for LDPE film grades and 4–20 g/10 min for HDPE injection-moulding grades; pipe-extrusion grades are restricted to 0.2–0.6 g/10 min. Tensile properties may also be reported under ASTM D638-14; values are not directly interchangeable with ISO data because of specimen geometry differences.

Environmental stress crack resistance differentiates pipe-grade HDPE from lower-molecular-weight injection grades. ESCR measured by ASTM D1693 under 100% Igepal CO-630 at 50°C typically exceeds 500 hours for pipe extrusion grades, whereas thin-wall injection-moulding grades may fail before 50 hours. Vicat softening temperature to ISO 306:2022 ranges from 85–100°C for LDPE and 115–135°C for HDPE. Optical haze measured to ASTM D1003 in 25 µm LDPE blown film is typically 4–10%; HDPE bottle walls can exceed 50% haze due to crystallite light scattering.

When Melt Temperature Exceeds 230°C in Extrusion and Injection Moulding

On single-screw extruders with barrier screws and L/D ratios of 30:1 to 44:1, HDPE pipe grades are processed with barrel temperatures from 180°C to 220°C and die-head temperatures held at 190–210°C. Residence time above 230°C should not exceed 30 minutes; beyond that limit, auto-oxidation generates gel particles that appear as fish-eye defects in film at concentrations above 10 particles/m². Blown film lines running LDPE/LLDPE operate at melt temperatures of 160–200°C, die gaps of 0.8–2.5 mm, and blow-up ratios between 2.0:1 and 3.5:1. HDPE blown film requires a high-stalk bubble configuration and blow-up ratios of 3.0:1 to 5.0:1 because the lower melt strength of HDPE requires extended cooling time to stabilise the bubble before frost-line crystallisation.

Rheological data explain die swell and melt fracture limits. LDPE exhibits power-law index values of 0.3–0.6 over shear rates of 10–1000 s⁻¹; HDPE values are 0.5–0.7. LLDPE can exhibit sharkskin melt fracture when wall shear stress exceeds approximately 0.14 MPa; published data for specific resin and die-gap combinations vary. Processing aid fluoropolymers are added at 200–800 ppm to delay melt fracture and reduce die-lip build-up in high-shear film and pipe operations.

Rotational moulding of LLDPE or HDPE powder with a particle-size distribution passing 35 mesh (500 µm) uses peak internal air temperature of 180–190°C. Oven dwell times shorter than 15 minutes leave unmelted particle cores; dwell times longer than 25 minutes accelerate carbonyl index growth. Injection moulding of HDPE closures and crates is performed at melt temperatures of 180–220°C, mould temperatures of 20–60°C, and hydraulic injection pressures of 70–100 MPa. Pre-drying is not required for bulk HDPE because water absorption is below 0.01%; however, surface moisture condensed at relative humidity above 60% can produce silver streaks and is removed with a desiccant hopper at 60–80°C for 2 hours.

In daily-contact packaging, LDPE and LLDPE blown films are converted into grocery sacks, agricultural mulch film, and stretch-wrap. The heat-seal initiation temperature of LLDPE is typically 95–120°C, permitting sealing jaw dwell times of 0.5–2.0 s on horizontal form-fill-seal machines. Blow-moulded HDPE containers for milk and household detergent are produced on shuttle or rotary blow moulders at clamp forces from 50 kN to 200 kN and parison temperatures of 170–190°C. Extrusion coating of paperboard for liquid packaging uses LDPE at melt temperatures of 285–320°C to achieve adhesion without a primer; the same process is not suitable for HDPE because of higher melt viscosity and poor drawdown.

Other daily forms include crosslinked PE insulation and rotomoulded tanks. XLPE cable insulation exhibits a dielectric constant of 2.25–2.40 at 1 MHz when tested to IEC 60250, and volume resistivity above 10¹⁶ Ω·cm per IEC 60093. Rotomoulded HDPE water-storage tanks with wall thicknesses of 3–10 mm are specified with low-temperature impact resistance to ISO 6603-2 at -20°C. UHMWPE rod and sheet stock is machined into guide rails and wear liners where dry sliding wear rate is lower than that of HDPE under ASTM G133 configurations.

Environmental Stress Crack Resistance and Hydrostatic Design Basis in Piping

Pressure pipe represents the most standardised load-bearing use of HDPE. PE100 grades show a minimum required strength of 10 MPa at 20°C and 50 years when tested according to ISO 9080. Slow crack growth resistance is evaluated by the notched pipe test ISO 13479; pipe-grade HDPE formulations with bimodal molecular-weight distribution pass beyond 1,000 hours without brittle failure, while monomodal grades may fail below 500 hours. Melt flow index for PE100 pipe grades is 0.2–0.6 g/10 min, which limits extrusion output but improves ESCR and hydrostatic design basis.

For buried and outdoor pipe, UV stabilization is achieved with carbon black at 2.0–2.5 wt% and dispersion verified according to ISO 18553. Antioxidant packages containing hindered phenol at 0.1–0.3 wt% and phosphite processing stabiliser at 0.05–0.15 wt% are common; these are selected to limit oxidative induction time depression during multiple extruder passes. The following compliance matrix identifies the principal regulatory and specification references for PE in contact and structural uses.

Medical and food-contact grades derived from LDPE, LLDPE, and HDPE are assessed for cytocompatibility under ISO 10993-1:2018 when sterilisation by ethylene oxide or gamma radiation up to 25 kGy is specified. Steam autoclaving at 121°C is not recommended for unmodified LDPE because it exceeds practical service temperature and produces dimensional distortion; HDPE retains shape better but can lose long-term strength. PE is incompatible with aromatic and chlorinated solvents and with certain mineral oils that cause environmental stress cracking in stressed components. Published data for long-term exposure of PE to concentrated nitric acid at temperatures above 60°C show oxidative attack. No adhesion to polar substrates is achieved without corona discharge, flame treatment, or a tie layer.