The production of HDPE is much less energy intensive than that of LDPE. Reaction temperatures can be as low as 60°C and pressures as low as 1 bar. Nonetheless, temperatures of 130–270°C and pressures of 10–160 bar are used commercially. Conversion per pass approaches 100%.
High-density polyethylene is manufactured in solution, slurry, or fluidized-bed processes. In the slurry process, the catalyst is dispersed in a solvent such as hexane, and the ethylene is polymerized batchwise in a series of reactors.
The gas phase, fluidized-bed processes were devised first by Union Carbide and subsequently by BP and BASF. Small HDPE particles are fluidized by gaseous ethylene and comonomer (e.g., 1-butene) at 85–105°C and 20 bar.
Catalyst is continuously sprayed into the reactor. The ethylene and comonomer copolymerize around the preformed polymer particles. At the same time, the gaseous ethylene removes the heat of reaction.
The initial particles grow to an average diameter of 500 microns over a period of three to five hours, during which time only about 2–3% of the ethylene polymerizes. The unconverted reactants are recycled.
Polyethylene, once prepared, is melted, mixed with stabilizers and other additives, and extruded to form spaghetti-like rods, which are then cut into small pellets. The extrusion is an energy-intensive operation.
An objective of the fluidized bed process (Carbide’s Unipol process), not achieved initially, was to obtain the polymer as a powder with uniform-sized particles that could be used as such for molding and extrusion.
Further development has apparently made this possible, although the value of the powder is questionable, because its low-bulk density increases shipping costs. But, serendipitously, the gas-phase process has proved to be an economical way to prepare both high- and linear low-density polyethylenes. It has been licensed extensively and is the most important process in use today.
