How Polyethylene Is Made: From Ethylene to Polymer in Four Industrial Routes

Polyethylene production from ethylene proceeds through two broad kinetic mechanisms: free-radical polymerization at high pressure and coordination polymerization over transition-metal or chromium oxide catalysts. Commercial reactor configurations are routinely grouped into four industrial routes. Each route determines short-chain branching, long-chain branching, molecular weight distribution, comonomer incorporation, catalyst residue content, and the thermal and rheological limits of the final resin. Polymer-grade ethylene is controlled before polymerization because oxygen, carbon monoxide, acetylene, water, sulfur compounds, and heavy hydrocarbons influence catalyst activity or can trigger uncontrolled decomposition in high-pressure plants. Purified ethylene is compressed, metered, and contacted with initiator or catalyst under controlled heat-removal conditions. Commercial grade densities extend from 0.880 g/cm³ for some solution plastomers to 0.965 g/cm³ for high-density homopolymer. Melt mass-flow rates measured by ISO 1133-1:2022 cover approximately 0.1 g/10 min to 100 g/10 min across injection, film, pipe, and coating grades.

The Free-Radical High-Pressure Route

Ethylene is polymerized in the dense phase at pressures between 150 MPa and 350 MPa and temperatures from 150°C to 350°C. Two reactor designs are used. A stirred autoclave with an L/D ratio of 2:1 to 20:1 provides backmixed residence time distribution and high long-chain branching. A tubular reactor with internal diameter 25 mm to 75 mm and length above 1,000 m operates with multiple injection zones and a narrower heat-age distribution. Organic peroxides or controlled oxygen dosing initiate free-radical propagation. Oxygen initiation introduces an induction period and produces broader polydispersity. Molecular weight is regulated by chain transfer to propylene, 1-butene, or aldehydes. Single-pass ethylene conversion is limited to 15% to 40% because exothermic heat release and ethylene decomposition impose strict ceiling conditions.

Ethylene decomposition is the critical thermal hazard. At temperatures above approximately 350°C, ethylene can undergo exothermic decomposition to methane and carbon with rapid pressure rise. Tubular reactors therefore use segmented initiator injection, internal wall thermocouples, peak temperature interlocks, and emergency quench valves. Compression consumes substantial mechanical work; reciprocating hypercompressors raise ethylene pressure through multiple stages with interstage cooling. Polymerization heat is removed through jacketed tube walls and by controlled ethylene preheating. Low-density polyethylene from this route has density 0.915 g/cm³ to 0.935 g/cm³, long-chain branching, high melt strength, and broad molecular weight distribution. Blown-film grades typically maintain melt mass-flow rates of 0.2 g/10 min to 2.0 g/10 min, while extrusion-coating grades are produced at higher flow and lower neck-in. The route is capital and energy intensive but is selected when long-chain branching, processability, and optical performance justify the operational burden.

Why Slurry Loop Stability Depends on Solubility Margins

Slurry polymerization is conducted in continuously circulating loop reactors or stirred isobutane vessels. In loop operation, ethylene, hydrogen, comonomer, isobutane, catalyst, and cocatalyst form a hydrocarbon slurry with polymer solids content of 25 wt% to 35 wt%. Circulation velocity is maintained at 6 m/s to 12 m/s to prevent settling in horizontal reactor legs. Operating temperature is held at 90°C to 110°C and pressure at 3.5 MPa to 4.5 MPa, maintaining isobutane in the liquid state. Diesel-range waxes and high-comonomer fractions dissolve into the diluent. If temperature rises or the target resin becomes too comonomer-rich, the polymer particles swell and become tacky, fouling heat-exchange surfaces and reducing circulation efficiency.

The practical density range for slurry high-density polyethylene is 0.940 g/cm³ to 0.965 g/cm³. Hydrogen is the primary chain-transfer agent. Catalyst systems include titanium tetrachloride supported on magnesium chloride with triethylaluminum cocatalyst, or Phillips chromium oxide on silica. Chromium-based grades produce broad molecular weight distribution and high shear response. Catalyst residues remain in the polymer, so compounding formulations require acid scavengers, phenolic antioxidants, and phosphite stabilizers. The main process conflict is solubility-driven fouling. Very high melt-flow grades and high-comonomer linear low-density resins cannot be produced economically in slurry because the soluble fraction raises loop viscosity and deposits on reactor walls. Proprietary catalyst design widens the operable window, but exact hydrogen response and solubility boundaries remain catalyst-specific.

Fluidized-bed gas-phase polyethylene production is operated at 2.0 MPa to 3.0 MPa and 80°C to 110°C, below the sintering onset of the resin. Ethylene, comonomer, hydrogen, and nitrogen are circulated through a distributor plate to fluidize a bed of growing polymer particles. Ziegler-Natta or metallocene catalysts are injected dry or as slurry. Condensed-mode operation introduces isopentane into the cycle gas; evaporation removes reaction heat and increases space-time yield. The reactor is limited by static charge accumulation and wall sheeting. Particle collisions create triboelectric charge, and non-neutral particles adhere to reactor walls, forming agglomerates and chunks. Operators monitor electrostatic probes, bed level, acoustic emissions, and wall temperature. Antistatic agents may be metered, but overdosing can poison catalyst and disrupt fluidization.

Superficial gas velocity is typically 0.6 m/s to 1.0 m/s, and fluidized bed bulk density is commonly 240 kg/m³ to 320 kg/m³. The route produces both linear low-density and high-density polyethylene, with density from 0.915 g/cm³ to 0.965 g/cm³. Metallocene grades made in gas-phase reactors exhibit narrow molecular weight distribution and uniform comonomer incorporation. However, they demand tighter temperature control because resin softening and increased stickiness can occur when bed temperature approaches the melting range. Ethylene conversion per pass is lower than in liquid-pool processes, so unreacted monomer is recovered and recycled at high rate. The gas-phase route avoids solvent recovery and operates at moderate pressure, but static control and bed-level stability define the production window.

When Ethylene and 1-Octene React in a Single-Phase Solution

Solution polymerization is carried out in a hydrocarbon solvent at temperatures from 150°C to 250°C and pressures from 3 MPa to 10 MPa. Because the polymer remains dissolved, the process can produce low-density plastomers and elastomers with density down to 0.880 g/cm³. Reactor residence time is short, commonly 5 min to 10 min, allowing grade transitions with limited off-specification material. High temperature reduces solution viscosity, but polymer concentration is limited to approximately 15 wt% to 25 wt%. Higher solids increase heat-transfer resistance and pump load, especially with high-molecular-weight resins. Metallocene and constrained geometry catalysts incorporate 1-octene or 1-hexene with narrow composition distribution. Hydrogen controls molecular weight, while solvent selection and temperature maintain single-phase operation below the lower critical solution temperature boundary.

After reaction, catalyst residues are deactivated and solvent is removed in devolatilizing extruders. The solution route has a narrower molecular-weight operating window than slurry or gas-phase. Very high-molecular-weight grades become too viscous for homogeneous transport, and very low-molecular-weight wax fractions can precipitate from solution. Final pellets are tested for density by ASTM D792 and melt mass-flow rate by ISO 1133-1:2022. Film and sealant grades typically use melt indices from 0.5 g/10 min to 5.0 g/10 min, while injection-molding grades may exceed 20 g/10 min. The route is selected for precise comonomer placement and low extractables, but the high solvent-recovery energy and viscosity ceiling are the principal operational boundaries.

Pelletizing and stabilization are route-dependent only in the details of residual catalyst neutralization and solvent removal. A typical additive package includes a primary antioxidant, a secondary antioxidant, an acid scavenger, and, for film applications, slip and antiblock agents. Oxidation induction time at 200°C is measured by ASTM D3895. High-density pipe grades are compounded on twin-screw extruders with L/D ratio 30:1 to 48:1. Film-grade LDPE is tested for gel count, haze, and dart impact, with gel levels above specification indicating premature crosslinking or insufficient homogenization. Injection-molding lots are verified by capillary rheometry and tensile testing under ASTM D638-14. Food-contact formulations must comply with FDA 21 CFR 177.1520 and EU 10/2011 migration limits. Polyethylene does not require drying unless surface moisture has condensed during storage at high relative humidity; wet pellets can produce extrusion defects and reduced output. Additive incompatibility can arise when high loadings of acid scavengers plate out on die lips or when incorrect antioxidant chemistry affects organoleptic performance.