What Is Polyethylene? Structure, Crystallinity and How It Is Made

Polyethylene (PE) is a semi-crystalline thermoplastic polyolefin produced by polymerizing ethylene, with the repeat unit -[CH₂-CH₂]-. The homopolymer is identified by CAS registry number 9002-88-4. Commercial grades are classified by density under ASTM D1505: high-density polyethylene (HDPE) spans 0.941 g/cm³ to 0.970 g/cm³; low-density polyethylene (LDPE) spans 0.910 g/cm³ to 0.925 g/cm³; linear low-density polyethylene (LLDPE) typically spans 0.915 g/cm³ to 0.940 g/cm³. Density is not a direct molecular weight descriptor. Two HDPE grades with identical density can have weight-average molar mass values differing by a factor of two when compared by high-temperature gel permeation chromatography in 1,2,4-trichlorobenzene at 150 °C. Melt flow rate is reported as MFR in g/10 min under ISO 1133-1:2022 at 190 °C with 2.16 kg or 5.0 kg loads; high-load measurements for high-viscosity extrusion grades use 21.6 kg.

The structural basis of polyethylene is a linear hydrocarbon chain with controlled branch populations. HDPE has fewer than 2 short-chain branches per 1000 carbon atoms, whereas LLDPE carries short-chain branches derived from 1-butene, 1-hexene or 1-octene at comonomer incorporation levels of 1 mol% to 4 mol%. LDPE produced under high-pressure radical conditions contains short and long branches; typical film grades report long-chain branch frequency of 0.5 to 3 per 1000 carbon atoms. Branch points are excluded from the orthorhombic crystal lattice, which has unit-cell parameters a = 0.742 nm, b = 0.494 nm, and c = 0.254 nm. A higher branch frequency restricts lamellar thickness and reduces the density of the semi-crystalline solid. Weight-average molar mass and molecular weight distribution are controlled by reactor conditions and catalyst selection. Bimodal HDPE resins for pressure pipe may have dispersity 15 to 30, while metallocene-catalysed LLDPE may show dispersity 2.0 to 3.0. The density classification therefore identifies a microstructural endpoint rather than a single property guarantee.

Crystallinity in polyethylene is the mass fraction of crystalline phase measured by differential scanning calorimetry (DSC). The enthalpy of fusion of fully crystalline PE is 293 J/g; percent crystallinity is calculated as the measured heat of fusion divided by this reference value under ISO 11357-3:2018 or ASTM D3418. HDPE typically exhibits crystallinity of 60% to 80%, LDPE 35% to 55%, and LLDPE 35% to 60%. Melting peaks are 130 °C to 137 °C for HDPE, 120 °C to 125 °C for LLDPE, and 105 °C to 115 °C for LDPE. The glass transition temperature is commonly reported between -120 °C and -80 °C. Crystallinity governs tensile yield stress, modulus, oxygen permeation and slow crack growth resistance. A moulded HDPE specimen of density 0.945 g/cm³ and crystallinity 65% may show a tensile yield stress near 20 MPa when tested as an ISO 527-2 Type 1A specimen at 50 mm/min. The corresponding LDPE with lower crystallinity generally yields at 7 MPa to 12 MPa.

Density-based crystallinity uses the two-phase relation with crystalline density 1.000 g/cm³ and amorphous density 0.855 g/cm³. Wide-angle X-ray diffraction with Cu Kα radiation shows orthorhombic 110 and 200 reflections at approximately 21.5° 2θ and 23.9° 2θ. The amorphous halo is broad; crystallinity by WAXD correlates with DSC values within ±5% for compression-moulded sheets. These independent methods are used because cooling rate, molar mass and branch distribution all shift the crystalline mass fraction without changing the chemical repeat unit.

Molecular weight distribution and melt rheology are measured because conversion, film bubble stability, and pipe extrusion are sensitive to shear response. High-temperature gel permeation chromatography in 1,2,4-trichlorobenzene at 150 °C provides Mw/Mn values. Capillary rheometry under ISO 11443 at 190 °C is used to generate shear viscosity curves for extrusion grades; a blow-film grade may show a shear viscosity of 300 Pa·s to 800 Pa·s at 100 s⁻¹. Ultra-high-molar-mass polyethylene (UHMWPE) is a linear PE with molar mass above 1 000 000 g/mol. It is processed by ram extrusion or compression moulding because melt flow rate under 190 °C/21.6 kg is below 0.1 g/10 min. High entanglement density produces high abrasion resistance, evaluated under ASTM G65, but limits screw extrusion. On a 30:1 L/D barrier-screw blown-film extruder with a 100 mm die, high output rates above the melt fracture threshold are controlled by die-lip geometry and polymer MW distribution. Batch-to-batch variance in MFR and density should be tracked against ISO 1133-1:2022 and ISO 1183-1, respectively.

Hydrogen-to-ethylene ratio is the primary molecular weight control in catalytic PE. In gas-phase and slurry processes, increasing the H2/C2 molar ratio from 0.1 to 1.0 raises MFR by several decades depending on catalyst hydrogen response. In Phillips chromium systems hydrogen response is weaker; molecular weight is adjusted through reactor temperature and comonomer. In high-pressure radical PE, molecular weight is controlled by pressure, temperature, and chain-transfer agent additions such as propylene or propane. These controls are not independent because temperature also changes short-chain branching and melt flow.

Low-pressure ethylene polymerization is performed in slurry-loop, gas-phase fluidised-bed, and solution processes. A slurry-loop process operates at 70 °C to 110 °C and 30 bar to 40 bar with isobutane as diluent. Ethylene, hydrogen for molecular weight control, and 1-butene or 1-hexene are fed into the loop; titanium tetrachloride supported on magnesium chloride and activated by triethylaluminum is a typical Ziegler-Natta system. Phillips-type chromium oxide on silica, activated at 500 °C to 700 °C, is used for HDPE with broad molecular weight distribution and high environmental stress-crack resistance. Gas-phase fluidised-bed reactors operate at 80 °C to 110 °C and 20 bar to 25 bar with superficial gas velocities of 0.6 m/s to 0.9 m/s; static charge accumulation on the distributor plate and fines carry-over are controlled by continuous antistatic dosing and cyclone separation. Metallocene catalysts such as bis(cyclopentadienyl)zirconium dichloride activated with methylaluminoxane yield narrow molecular weight distribution and homogeneous comonomer placement. Solution processes operate above 140 °C to keep the polymer dissolved in a hydrocarbon solvent; the higher temperature allows short residence times and rapid grade transitions. Published data for specific licensed lines is limited; the ranges stated are representative of multiple technology packages.

High-pressure LDPE production uses autoclave or tubular reactors at 1000 bar to 3000 bar and 150 °C to 300 °C. Free-radical initiation is achieved with oxygen or organic peroxides such as tert-butyl peroxybenzoate injected at multiple zones. The propagation reaction releases approximately 93 kJ/mol of ethylene; heat removal in tubular reactors is managed through jackets and interstage coolers, while autoclave reactors use continuous stirred-tank mixing. Unreacted ethylene is separated in high-pressure separators and recycled; the molten polymer is fed to an extruder and underwater pelletizer. Branching in LDPE is generated by intramolecular backbiting and intermolecular transfer, producing long-chain branches that broaden MW distribution and increase shear thinning. Melt temperature during extrusion is normally maintained between 180 °C and 220 °C; sustained operation above 240 °C is avoided because oxidative chain scission and crosslinking compete. Stabilization packages are evaluated by oxidation induction time per ISO 11357-6 and oven aging per ISO 4577.

Designation of HDPE pipe materials follows ISO 12162 using minimum required strength at 20 °C for 50 years. A PE 100 resin has an MRS of 10 MPa; a PE 80 resin has an MRS of 8 MPa. Slow crack growth is assessed by notched pipe test ISO 13479, and environmental stress-crack resistance is evaluated by ASTM D1693 in 10% Igepal CO-630 at 50 °C. These are long-term tests; short-term density or melt-flow data do not substitute for hydrostatic classification. The tie-molecule fraction between lamellae is central to slow crack growth resistance; at crystallinity above 70%, tie-molecule concentration decreases and brittle failure can occur.

For blown film lines, external cooling air is maintained at 10 °C to 15 °C, and die gaps of 1.4 mm to 2.5 mm are common for LLDPE and LDPE. Bubble stability is controlled by internal bubble cooling and by maintaining a melt pressure below 40 MPa at the die entry. Dart drop impact is measured under ISO 7765-1 method A; Elmendorf tear is measured under ISO 6383-2. On a 250 mm die, output rates above 180 kg/h can induce melt fracture when die gaps fall below 1.2 mm; the frost-line height is then increased or the die gap is widened. Published data for this specific configuration is limited, but the processing boundary is consistent across polyethylene blown-film lines.

Injection moulding grades are selected with MFR values of 8 g/10 min to 20 g/10 min under ISO 1133-1:2022 at 190 °C/2.16 kg. For multicavity thin-wall containers, clamp force ranges of 450 to 650 tonnes are employed with melt temperatures of 180 °C to 220 °C and mould temperatures of 10 °C to 40 °C. Post-moulding shrinkage is specified per ISO 294-4; HDPE can shrink from 1.5% to 3.0%, while LDPE typically shrinks from 1.0% to 2.5%. Dimensional variation is controlled by mould cooling uniformity and by selecting a density of 0.952 g/cm³ to 0.965 g/cm³ for structural containers. In thin-wall packaging, a narrow molecular weight distribution reduces warpage but also lowers melt strength; the resin specification therefore includes both MFR and shear viscosity under ISO 11443.

Crystallization kinetics and additive dispersion can be controlled during compounding. Isothermal crystallization half-time at 120 °C for HDPE is typically 1 min to 10 min, depending on molecular weight and nucleating additives. Talc and calcium stearate reduce spherulite size and raise the crystallization temperature. Compounding is carried out on twin-screw extruders with L/D ratios of 30:1 to 45:1 and specific energy input of 0.15 kWh/kg to 0.25 kWh/kg. Pre-drying is unnecessary below 0.05 wt% surface moisture, but hopper magnets and screen packs are required to protect the die plate from metallic contamination. For UV-stabilized outdoor grades, carbon black is added at 2.0 wt% to 2.5 wt% and the dispersion is checked by ISO 18553.

For food-contact compliance, hexane extractables are determined under 21 CFR 177.1520 and European Regulation (EU) No 10/2011. Metallocene LLDPE typically reduces the low-molecular-weight extractable fraction compared with Ziegler-Natta LLDPE because comonomer is placed more uniformly across the molar mass distribution. Extractable ceilings depend on temperature and food type; published data for specific catalyst systems should be verified against the finished film because downstream processing can alter the extractable profile.