Polyethylene (PE) is a semicrystalline thermoplastic produced by polymerisation of ethylene, available in four principal commercial models: PE-LD, PE-LLD, PE-HD, and PE-UHMW. Density measured according to ASTM D1505 or ISO 1183-1 separates the families: PE-LD spans 0.910–0.925 g/cm³, PE-LLD spans 0.910–0.940 g/cm³, PE-HD spans 0.941–0.965 g/cm³, and PE-UHMW commonly occupies 0.930–0.945 g/cm³. Melt mass-flow rate is determined at 190 °C under 2.16 kg per ISO 1133-1 or ASTM D1238, with high-load 21.6 kg used for high-molecular-weight grades. The material is not a single specification but a family of compounds whose density, melt flow rate, comonomer type, stabiliser package, filler loading, and additive package are controlled by the grade datasheet. Polyethylene is used in blown film, blow moulding, injection moulding, pressure pipe, wire and cable insulation, and rotational moulding. Specification conformance is generally anchored to ASTM D4976 for moulding and extrusion materials and to ISO 1872-1 for the international designation system.

For pressure piping, the relevant designations are PE80 and PE100 under ISO 12162, where the number denotes a minimum required strength at 20 °C for 50 years of 8 MPa and 10 MPa, respectively, based on ISO 9080 hydrostatic testing. Crosslinked grades are classified separately as PE-X under ISO 15875, with the crosslinking method designated as peroxide (PEX-A), silane (PEX-B), or electron beam (PEX-C). These standard designations function as product models and allow direct substitution of materials from different suppliers only when the full data block, not merely the density and melt flow rate, is matched.

Which ISO 1872 Designation Governs PE-HD and PE-LD Selection?

Under ISO 1872-1, a designation data block encodes the polymer family, density, melt flow rate, and additive or processing stabilisation. A PE-HD grade with density 0.950 g/cm³ and MFR 8 g/10 min is specified through the standard cell structure rather than through a trade name alone. In ASTM D4976, PE materials are specified by property cells covering density, melt index, tensile strength, slow crack growth resistance, and dielectric strength. The cell classification does not imply interchangeability; a grade meeting the same cell class may differ in catalyst residue, antioxidant package, die swell, or organoleptic profile. Selection of PE-HD versus PE-LD is governed by mechanical load: PE-HD is selected when tensile yield strength above 18 MPa and modulus above 600 MPa are required, while PE-LD is selected for flexible films and closures where yield strength below 15 MPa and low modulus ease orientation. The high-load melt index ratio, calculated as HLMI divided by MI, is used to infer molecular weight distribution; a ratio above 50 typically indicates broad molecular weight distribution for blow moulding. PE-LD has long-chain branching, giving pronounced shear thinning and processability in blown film at melt temperatures of 160–220 °C.

Melt Flow Classification, Shear Viscosity, and Extruder Screw Selection

Melt flow classification is an inverted viscosity index; higher MFR values indicate lower molecular weight and lower viscosity. HDPE injection grades commonly range from 4 g/10 min to 30 g/10 min, HDPE blow moulding grades from 0.2 g/10 min to 1.0 g/10 min, and LLDPE film grades from 0.5 g/10 min to 2.0 g/10 min. For blown film, LLDPE requires higher extruder torque and melt temperature settings than LDPE at the same MFR because the linear backbone produces higher shear viscosity at extrusion rates. Single-screw extruder suppliers commonly specify barrier screws with 24:1–30:1 L/D and compression ratios of 2.5:1–3.5:1 for LLDPE, and 20:1–24:1 L/D for LDPE. Die gaps in blown film lines are typically 0.8–2.0 mm. Melt temperatures are held at 160–220 °C for LDPE and 190–240 °C for LLDPE/HDPE, with barrel profiles increasing toward the die.

On a 1,500 kN injection moulding press running a 0.950 g/cm³ HDPE grade with an MFR of 8 g/10 min, typical supplier bulletins describe barrel settings from 180 °C at the feed throat to 240 °C at the nozzle, with mould temperature held at 20–50 °C. Clamp force requirements for HDPE are commonly estimated at 3–5 kN/cm² of projected area. Melt injection pressures are generally 70–110 MPa, with pack pressures 40–70 MPa. Mould shrinkage after 24 h, measured per ASTM D955, ranges from 1.5–3.0% for HDPE and 1.5–4.0% for LDPE. Pre-drying is not required when resin is stored below 60% RH; if surface condensation is present, drying at 70–80 °C for 1–2 h is applied. Processing PE after PVC in a shared extruder requires a purge sequence because residual PVC releases hydrogen chloride and accelerates corrosion.

Table 1. Comparative property envelopes for PE grade families based on published supplier data.

The values in Table 1 are not single grade specifications; they represent typical ranges across commercial formulations. For UHMWPE, melt flow rate is not applicable, and intrinsic viscosity measured per ISO 1628-3 is the primary molecular weight indicator.

When Polyethylene Replaces Polypropylene in Low-Temperature Impact and ESCR Service

Polyethylene is selected over polypropylene when low-temperature impact and environmental stress crack resistance are the controlling requirements. PP homopolymer has a glass transition temperature near 0 °C, while HDPE retains ductile behaviour below -40 °C in ASTM D256 Izod impact and ASTM D746 brittleness tests. The density penalty is modest, but the stiffness differential is significant: PP homopolymer tensile modulus is typically 1.0–1.8 GPa, whereas HDPE tensile modulus is 0.6–1.4 GPa. In pressurised pipe, PE100 grades have notched pipe test values under ISO 13479 exceeding 1,000 h, while PP-H and PP-R pipe grades are selected primarily for higher-temperature drainage and hot water lines. For detergent bottle and container applications, HDPE grades with ESCR F50 > 300 h per ASTM D1693 are specified because PP does not fail by the same slow crack growth mechanism but can oxidise under aggressive surfactants at elevated temperature.

Conversely, polypropylene is preferred over PE when continuous service temperature exceeds 80–90 °C; HDPE heat deflection temperature under 0.455 MPa is often 70–100 °C, while PP homopolymer HDT is 90–120 °C per ASTM D648. Chemical resistance also differs: HDPE tolerates many aqueous salt solutions, dilute alkalis, and selected acids at ambient temperature, but swells markedly in aromatic hydrocarbons such as toluene and xylene. PP is similarly attacked by strong oxidising acids, but can withstand some polar organics better at elevated temperature. The decision to replace PP with PE therefore requires derating for stiffness, thermal service, and dimensional retention, not simply matching melt flow rate.

Environmental stress cracking in HDPE is accelerated by polar wetting agents, so container formulations are specified with ESCR testing according to ASTM D1693 and notch pipe testing under ISO 13479 for pressure pipe. Outdoor weathering requires stabilisation with 2–3 wt% carbon black or hindered amine stabiliser packages; without UV additives, tensile elongation retention after xenon arc exposure under ASTM G155 can drop below 50% within several thousand hours. Water vapour transmission through PE is lower than through cellulosic or polyamide films, but oxygen permeability is high; published oxygen permeation data for LDPE at 23 °C are commonly 1,000–2,000 cm³·mm/(m²·day·atm), compared with 30–50 for biaxially oriented PET and 0.5–5 for EVOH. PE is therefore not recommended as an oxygen barrier unless coextruded with EVOH or metallised. Continuous contact with strong oxidising acids, chlorinated hydrocarbon solvents, and high concentrations of hydrogen peroxide above 50 °C should be avoided because chain scission and surface cracking occur.

Compliance Mandates in Food Contact and Medical Packaging Remain Grade-Dependent

Food contact grades are supplied under FDA 21 CFR 177.1520, which covers olefin polymers; EU food contact under Regulation (EU) 10/2011; and general chemical registration under REACH EC 1907/2006. RoHS compliance for electrical equipment is assessed under 2011/65/EU. Medical packaging grades require biological evaluation per ISO 10993-5 and ISO 10993-10, and pharmacopeial testing per USP <87> and USP <88> when the PE contacts injectable or mucosal surfaces. The migration limit for plastic food contact materials under EU 10/2011 is an overall migration of 10 mg/dm² or 60 mg/kg for infant foods, measured with simulant conditions specified in the regulation.

Table 2. Compliance matrix for common PE application classes.

Compliance data is grade-specific and cannot be inferred from density or melt flow rate alone. A PE-HD food contact grade may carry FDA and EU declarations only if the antioxidant and processing-aid package is listed in the relevant positive lists; otherwise the same density and MFR resin is not compliant. For medical applications, lot-to-lot extractables profiles must be evaluated under the final sterilisation condition, because gamma irradiation at 25–40 kGy can generate oxygenated species that alter USP test results. Published data for steam-sterilised PE is limited because PE softens above 120 °C and is not suitable for autoclave cycles above 121 °C for rigid packaging.