Polyethylene (PE) is a semicrystalline thermoplastic polyolefin obtained by polymerization of ethylene. Commercial grade designations include LDPE, LLDPE, MDPE, HDPE, HMW-HDPE, UHMWPE, PEX, PE100, and PE4710. Density spans 0.890 g/cm³ to 0.970 g/cm³ under ISO 1183-1:2019 or ASTM D792; melt flow rate ranges from below 0.1 g/10 min to above 50 g/10 min at 190 °C/2.16 kg per ISO 1133-1:2022 or ASTM D1238. The product is supplied in pellet or powder form, with powder grades used in rotational moulding and ram extrusion of UHMWPE sheet. Primary specification points are density, melt flow rate, molecular weight distribution, comonomer type, additive package, and oxidative induction time. Polyethylene has a non-polar hydrocarbon backbone, giving water absorption below 0.05% after 24 h per ASTM D570 and a linear coefficient of thermal expansion of 100–200 × 10⁻⁶ K⁻¹ per ASTM D696. The same non-polar structure limits mechanical reinforcement and oxygen barrier relative to condensation polymers such as PET.

How Do Molecular Weight Distribution and Comonomer Type Influence the Processing–Property Balance?

Low-density polyethylene produced in tubular or autoclave reactors contains long-chain branches that increase shear sensitivity and melt strength. A film-grade LDPE typically has density 0.912–0.935 g/cm³, a melt flow rate of 0.2–8.0 g/10 min at 190 °C/2.16 kg, and tensile yield strength of 8–12 MPa when tested by ASTM D638 Type IV. High-pressure LDPE crystallinity is generally 45–55%, which limits modulus and barrier but improves optical clarity and drawdown. Linear low-density polyethylene uses butene, hexene, or octene comonomer to produce short-chain branches. A hexene-based LLDPE film resin may have density 0.915–0.940 g/cm³ and a melt flow rate of 0.5–2.5 g/10 min. The linear backbone with controlled short-chain branching gives higher dart impact and tear strength than LDPE at equivalent density. High-density polyethylene is produced with minimal short-chain branching. Its density is 0.945–0.970 g/cm³, crystallinity is 60–80%, and tensile yield strength is commonly 23–30 MPa per ASTM D638. Flexural modulus ranges from 900 MPa to 1600 MPa per ASTM D790. Bimodal HDPE pipe grades combine a high-molecular-weight fraction for slow crack growth resistance with a low-molecular-weight fraction for extrusion. The ratio of weight-average to number-average molecular weight in film and blow moulding grades is often between 3 and 25. Narrow distributions from metallocene grades improve puncture and sealing, but can reduce melt strength. Processing–property balance therefore shifts with branching, comonomer length, and molecular weight distribution rather than with density alone.

Pressure-pipe polyethylene grades are specified through long-term hydrostatic strength rather than single-point tensile data. A PE100 compound must demonstrate a minimum required strength of 10 MPa at 20 °C and 50 years per ISO 9080. In North American practice, PE4710 represents the corresponding high-performance designation under ASTM D3350 with a typical cell classification of 445574C and a hydrostatic design basis of 1600 psi at 23 °C. PE100 and PE100-RC resins are bimodal HDPE or MDPE grades with density 0.945–0.960 g/cm³ and a melt flow rate below 0.5 g/10 min at 190 °C/5 kg. Pipe extrusion is performed on grooved-feed extruders with 30:1 to 37:1 L/D ratios, melt temperatures of 200–220 °C, and vacuum sizing. Process failure modes include weld-line ovalization, internal melt fracture at high output, and sag in large-diameter thick walls. Slow crack growth resistance is measured by the full notch creep test per ISO 16770 or the Pennsylvania notch test per ASTM F1473. PE100-RC grades are selected for trenchless installation and rough bedding because they exhibit higher resistance to point loads. Hydrostatic pressure tests at 20 °C, 80 °C, and 95 °C establish the pipe pressure rating per EN 12201. Oxidative stability is verified by oxidation induction time at 200 °C per ISO 11357-6; a typical specification for pipe compound is an OIT above 20 min. Outdoor weathering requires carbon black content of 2.0–2.5 wt% and dispersion per ISO 18553.

Blown Film Bubble Stability Parameters in LLDPE-Dominant Structures

Blown film lines running LLDPE-rich formulations encounter lower melt strength than LDPE-rich lines, which narrows the bubble stability window. To compensate, 10–20 wt% LDPE is melt-blended into LLDPE film grades. The extruder is typically a single-screw barrier design with 24:1 to 30:1 L/D, a die gap of 1.5–2.5 mm, and a blow-up ratio of 2.0:1 to 3.0:1. Barrel temperatures are 180–230 °C from feed to die. Bubble stability is controlled through frost line height, internal bubble cooling, and air-ring velocity. Melt fracture, often called sharkskin, appears when the critical shear rate is exceeded; LLDPE is more prone because of its narrow molecular weight distribution and linear architecture. A 25 µm monolayer LLDPE film generally achieves dart drop impact of 100–300 g by ASTM D1709 and Elmendorf tear strength of 3–10 N by ASTM D1922. Tensile properties are tested by ASTM D882; yield strength in the machine direction is typically 10–20 MPa. Seal initiation temperature and hot-tack strength are measured on a heat-seal tester under ASTM F1921. Haze is measured by ASTM D1003, while moisture vapour transmission rate is tested by ASTM F1249 at 38 °C and 90% RH. Because LLDPE has lower melt strength, edge trim and internal bubble cooling become more critical at high output. Die lip deposit, antioxidant migration, and gel count must be controlled to avoid printing and sealing defects.

Injection moulding of HDPE uses grades with melt flow rate 4–12 g/10 min at 190 °C/2.16 kg for caps, crates, and housewares. Barrel temperatures range from 200 °C to 250 °C, with nozzle settings of 210–240 °C. Mould temperatures are kept at 10–40 °C. The required clamp force is commonly 25–70 MPa of projected part area, depending on wall thickness and flow length. Mould shrinkage for HDPE ranges from 1.5% to 3.5% per ASTM D955; excessive pack pressure or gate-freeze time can increase cycle time without reducing warpage. In thick sections, void formation and sink marks are mitigated by maintaining holding pressure above 40 MPa and cooling the part uniformly. A melt temperature above 260 °C accelerates oxidative degradation and can cause yellowing or loss of impact strength. Post-mould warpage is assessed by dimensional stability testing per ISO 294-4. Dry blending of colour masterbatch requires a high-shear mixing screw or static mixer to avoid colour streaks. Low-temperature impact of moulded HDPE caps and closures is frequently specified at -20 °C or -40 °C using ISO 179 or ASTM D256. For injection moulding of caps, HDPE with a density of 0.950–0.960 g/cm³ is preferred because higher density improves stiffness and creep resistance under closure seal loads.

Rotational Moulding Grades Require Narrow Particle Size Distribution and Sufficient Thermal Stabilization

Rotational moulding grades are supplied as powder with a melt flow rate of 3–7 g/10 min at 190 °C/2.16 kg and density 0.935–0.945 g/cm³. Particle size distribution is tightly controlled, typically 35–60 mesh (500–250 µm), because dry flow and wall coverage depend on particle packing. Peak internal air temperature is 200–230 °C, while oven set temperature may be 280–320 °C. The stabilizer package is evaluated by oxidation induction time at 200 °C per ISO 11357-6; specifications often require 20–40 min to survive extended mould cycles. Low-temperature impact resistance is measured on flat plaques by falling-weight puncture; published data for specific configurations is limited, but part qualification commonly requires no brittle failure at -20 °C or -40 °C. Warpage is controlled by cooling rate and mould release. The main processing conflict is that longer heating increases part density and impact through full coalescence, but reduces OIT and accelerates colour shift. Rotomoulded tanks and containers also require environmental stress crack resistance testing by ASTM D1693, with many grades rated above 1000 h in 100% Igepal CO-630 at 50 °C. UV-stabilized grades are specified for outdoor service under ISO 4892-3, with radiant exposure requirements based on end-use latitude.

The table below summarizes typical specification ranges for four PE families.

When Polyethylene Is Compared with Polypropylene, PVC, and PET in Rigid Packaging and Chemical Containment

Polyethylene is a non-polar, halogen-free polyolefin with lower density than PVC and PET. HDPE density is 0.945–0.967 g/cm³, while rigid PVC is 1.35–1.45 g/cm³ and PET is 1.38–1.40 g/cm³. Polypropylene has similar density to PE but higher heat deflection temperature and flexural modulus. HDPE Vicat softening is 120–135 °C by ASTM D1525, whereas PP homopolymer commonly reaches 150–160 °C. HDPE is preferred where low-temperature impact and environmental stress crack resistance outweigh stiffness. At -20 °C or lower, HDPE frequently shows ductile failure in notched impact testing, while PP homopolymer can become brittle. Compared with rigid PVC, PE does not require metal soap or organotin stabilizers and does not release hydrogen chloride during combustion. However, PE has lower modulus and higher flammability. PET provides transparency, high tensile strength, and low oxygen transmission; LDPE and HDPE are translucent to opaque and have oxygen permeability values 100–1000× higher than PET in oxygen transmission testing by ASTM D3985. PE has lower water vapour transmission rate: HDPE values are typically below 10 g·mm/(m²·day) at 38 °C/90% RH by ASTM F1249, while PET and PVC are more permeable to moisture. Chemical resistance of PE is high in aqueous acids, alkalis, and salt solutions, but it swells in aromatic hydrocarbons, chlorinated solvents, and some oils. Strong oxidizing acids such as concentrated nitric acid and halogens attack PE, especially above ambient temperature. For food contact, PE grades are cleared under FDA 21 CFR 177.1520 and EU 10/2011. For medical packaging, radiation resistance is specified by ISO 11137, and post-irradiation colour stability is a lot-release criterion.

Crosslinked polyethylene compounds used for medium-voltage cable insulation are based on LDPE or LLDPE with peroxide or silane crosslinking. In peroxide crosslinking, the base resin should have a melt flow rate of 1–3 g/10 min at 190 °C/2.16 kg and low antioxidant interference. Continuous vulcanization lines run with crosslinking initiated above 160 °C in the catenary tube. Gel content after crosslinking is tested by extraction in xylene or decahydronaphthalene; typical values exceed 70% per ASTM D2765. Crosslinked PE insulation is rated for continuous conductor temperatures up to 90 °C under IEC 60502, whereas thermoplastic PE is limited below 70 °C. Silane-grafted PEX pipes for hot-water plumbing are specified by ISO 15875 and ASTM F876; the pipe must survive hydrostatic pressure testing at 95 °C, and crosslink degree is verified by gel content or differential scanning calorimetry. In rotomoulding and cable extrusion, peroxide-induced degradation is controlled by antioxidant packages based on hindered phenols and phosphites. The main incompatibility is with amine-based antidegradants that interfere with peroxide cure and cause premature crosslinking in melt processing. Moisture-curing silane systems require pre-drying at relative humidity below 60% before grafting, and storage of grafted compound must exclude atmospheric moisture to avoid surface scorch. This segment demonstrates the trade-off between processability of thermoplastic PE and heat resistance of network PE, with specifications directed at gel content, hot set elongation, and long-term hydrostatic strength rather than melt flow alone.