Polyethylene Pe The Complete Material Guide To Grades Forms And Uses
Polyethylene (PE): The Complete Material Guide to Grades, Forms and Uses
Polyethylene (PE) is a semi-crystalline thermoplastic family produced by addition polymerisation of ethylene, with optional alpha-olefin comonomers such as 1-butene, 1-hexene, and 1-octene. Density, molecular weight, molecular weight distribution, short-chain branching, and long-chain branching separate commercial grades into LDPE, LLDPE, MDPE, HDPE, and UHMWPE. Processing routes, mechanical properties, and environmental stress cracking resistance follow directly from these molecular variables. The term “form” includes pellet, rotomoulding powder, sheet, blown film, cast film, pipe, foam, fibre, and consolidated stock.
What Governs the Distinction Among LDPE, LLDPE, MDPE, HDPE, and UHMWPE?
Molecular architecture differentiates commercial PE resins. Short-chain branching derived from comonomers such as 1-butene, 1-hexene, and 1-octene reduces lamellar thickness and therefore density. High-pressure LDPE contains long-chain branching that increases melt strength and drawability. LLDPE produced with Ziegler-Natta or metallocene single-site catalysts has controlled short-chain branching and a narrower composition distribution. HDPE homopolymer exhibits minimal branching and crystallinity of 60–80 %. UHMWPE has viscosity-average molecular weight exceeding 1 million g/mol and no measurable melt index under ASTM D1238.
Typical property ranges based on published resin manufacturer datasheets are listed in Table 1.
| Grade | Density range | Melt index (190 °C/2.16 kg) | Tensile yield strength | Flexural modulus | Primary forming routes |
|---|---|---|---|---|---|
| LDPE | 0.910–0.925 g/cm³ | 0.2–70 g/10 min | 8–16 MPa | 150–400 MPa | Blown film, extrusion coating |
| LLDPE | 0.915–0.935 g/cm³ | 0.5–20 g/10 min | 15–25 MPa | 300–700 MPa | Blown and cast film, rotomoulding |
| MDPE | 0.926–0.940 g/cm³ | 0.1–5.0 g/10 min | 18–25 MPa | 500–800 MPa | Gas pipe, rotational moulding |
| HDPE | 0.941–0.960 g/cm³ | 0.03–30 g/10 min | 22–31 MPa | 1000–1550 MPa | Blow moulding, injection moulding, pipe |
| UHMWPE | 0.930–0.935 g/cm³ | Not measurable | 39–48 MPa | 550–750 MPa | Ram extrusion, compression moulding |
Catalyst and reactor selection further define grade architecture. Gas-phase fluidised bed processes using Ziegler-Natta catalysts produce LLDPE and HDPE with melt indices from 0.5 to 50 g/10 min. Slurry loop polymerisation yields HDPE with controlled particle size and comonomer placement for injection and pipe. High-pressure autoclave or tubular reactors at 150–310 MPa and 150–330 °C produce LDPE with high clarity and low neck-in in extrusion coating.
Density, Crystallinity, and Melt Rheology: Specification Boundaries
Density is measured by ASTM D1505 or ISO 1183-1 on compression-moulded plaques conditioned at 23 °C ± 2 °C. Crystallinity by differential scanning calorimetry according to ISO 11357-3 shows HDPE melting peaks at 130–137 °C, LLDPE at 120–125 °C, and LDPE at 105–115 °C. Melt index is a single-point flow parameter measured at 190 °C/2.16 kg under ASTM D1238 or ISO 1133-1:2022. High-load melt index at 21.6 kg is used to compute melt flow ratio MFR = I21/I2. Blow-moulding grades often require MFR 50–80 to resist parison sag; film grades may use MFR 15–30 to balance bubble stability and drawdown. Oscillatory shear rheology in parallel-plate geometry according to ISO 6721-10 is required to resolve differences in molecular weight distribution that a single melt index cannot capture.
Blown film lines for LLDPE operate with a 1.2–2.0 mm die gap, a blow-up ratio of 2:1 to 3:1, and melt temperatures of 190–230 °C. Compared with LDPE, LLDPE displays higher extensional viscosity at high draw and is prone to sharkskin surface melt fracture. A fluoropolymer process aid at 400–1000 ppm is used to reduce die lip buildup and delay melt fracture. Frost line height is set to maintain bubble stability and a haze value below 10 % measured by ASTM D1003 for high-clarity packaging grades. Corona treatment to a surface energy of 38–42 mN/m is applied before printing or lamination.
Extrusion coating and lamination use LDPE with melt index 4–8 g/10 min and melt temperature of 300–330 °C to promote adhesion to paperboard, aluminium foil, or polyester film. Low neck-in and draw resonance suppression at line speeds above 200 m/min require tailored long-chain branching. Adhesion is measured by peel strength under ASTM F88 or ASTM F904. Typical coating weights of 10–25 g/m² are applied for liquid packaging.
Cast film grades with melt indices of 4–12 g/10 min are processed on chill roll lines at 220–260 °C. Higher chill roll temperatures of 20–35 °C reduce crystallinity and improve optical clarity. Gauge variation is controlled by automatic die bolt adjustment to ±2 % or better.
When UHMWPE Processing Diverges from Conventional Melt-Flow Routes
UHMWPE cannot be screw-extruded or injection-moulded because its molecular weight suppresses melt flow. Compression moulding at 190–220 °C under 10–30 MPa with cooling under pressure at 5–15 °C/min prevents void formation. Ram extrusion uses a reciprocating plunger and heated die; surgical implant stock is machined from consolidated forms specified as ASTM F648 Type 1 or Type 2. Gamma irradiation at 50–100 kGy in inert packaging induces crosslinking and lowers wear rate in orthopaedic inserts, measured by pin-on-disc testing under ASTM F732. Thermal exposure above 250 °C in the presence of oxygen causes chain scission and discolouration.
Environmental Stress Cracking Resistance in HDPE Pipe and Geomembranes
ESCR is evaluated by ASTM D1693 condition B in 10 % Igepal CO-630 at 50 °C. Pipe-grade HDPE compounds with high molecular weight and 1-hexene comonomer often exhibit ESCR values above 1000 h, but published values for specific PE100 formulations vary with density and melt index. PE80 and PE100 classifications define minimum required strength at 20 °C for 50 years of 8.0 MPa and 10.0 MPa respectively under ISO 4427. Slow crack growth resistance is measured by the notched pipe test ISO 13479, and rapid crack propagation resistance is assessed with the S4 test ISO 13477.
Grooved-feed single-screw extruders with 30:1–36:1 L/D produce HDPE pipe because the grooved barrel section increases conveying efficiency and maintains throughput at melt temperatures of 200–230 °C. Vacuum calibration tanks with sizing pressure of −0.02 to −0.08 MPa set the outer diameter and wall thickness. Cooling water kept below 20 °C controls residual thermal stresses and diametrical shrinkage. Butt fusion joining according to ISO 21307 uses heater plate temperatures of 200–220 °C and interfacial pressure of 0.15 MPa during heating and cooling.
Compounds for outdoor HDPE pipe contain carbon black at 2.0–2.5 wt% for ultraviolet resistance, with dispersion assessed according to ISO 18553. Oxidative induction time at 200 °C is determined by ISO 11357-6; pipe compounds typically require values above 20 min. Calcium stearate or zinc stearate acid scavengers at 0.05–0.15 wt% are used to neutralise catalyst residues.
Rotational Moulding Powder Specifications and Injection Moulding Shrinkage Management
Rotational moulding PE grades require powder with 95 wt% passing 500 µm and a melt index of 3–5 g/10 min. Biaxial rotational moulding machines use a primary-to-secondary axis speed ratio of 4:1; oven air temperature of 280–320 °C and cycle time of 20–45 min are common. Shrinkage and warpage are controlled by cooling rate and mould release. Post-mould dimensional checks for gravity-flow tanks follow ASTM D1998.
Injection moulding of HDPE uses clamp force of 3–5 kN/cm² projected area. Linear mould shrinkage for HDPE ranges from 1.5–2.5 %, while LDPE can reach 2.0–3.0 %. Weld lines and sink marks are mitigated by gate placement, packing pressure, and cooling time. Notched Izod impact is measured under ASTM D256, and tensile properties under ASTM D638.
HDPE blow moulding of household and industrial containers uses accumulator-head machines with parison programming. Grades selected for this route typically have melt index 0.2–1.0 g/10 min and MFR 50–70. Top-load strength is tested under ASTM D2659, and drop impact under ASTM D2463. Stress cracking against aggressive detergent formulations is screened with ASTM D1693 condition A or B.
Coextruded barrier films combine HDPE or LLDPE skin layers with ethylene vinyl alcohol copolymer barrier cores and maleic anhydride grafted PE tie layers. Melt temperatures for the PE layers are held at 190–230 °C, while the barrier polymer is processed within its own thermal stability window. Oxygen transmission rate for a 50 µm film is measured under ASTM D3985 and depends on barrier layer thickness and relative humidity. This structure is used for modified atmosphere packaging and retort applications.
Compliance depends on end market and regional regulation. Table 2 lists the principal standards applicable to PE compounds and fabricated articles.
| Standard | Scope |
|---|---|
| FDA 21 CFR 177.1520 | Olefin polymers for food contact use |
| EU Regulation (EU) No 10/2011 | Plastics in food contact materials; overall migration |
| REACH Regulation (EC) No 1907/2006 | Registration and SVHC restrictions in the European Union |
| RoHS Directive 2011/65/EU | Restriction of hazardous substances in electrical/electronic components |
| ISO 4427 / EN 12201 | Polyethylene pressure pipe systems |
| ASTM D3350 | PE pipe and fitting materials cell classification |
| IEC 60502 | XLPE-insulated power cable requirements |
| ASTM F648 | UHMWPE powder and fabricated forms for surgical implants |
Low-density polyethylene and LLDPE are used as primary insulation in communication and power cable. Peroxide crosslinking converts LDPE to crosslinked polyethylene with a gel content above 70 % measured by extraction under ASTM D2765; continuous vulcanisation lines operate at 280–350 °C under nitrogen. Silane-grafted LLDPE cures by moisture after extrusion and is evaluated for thermal ageing under IEC 60811. For automotive wire jacketing, LLDPE grades with melt index below 2.0 g/10 min are selected for abrasion resistance.
