Production Methods for HDPE and LLDPE
Both HDPE and LLDPE are commonly made using suspension polymerisation and gas-phase polymerisation.

Suspension

HDPE was first produced using a suspension or "slurry" method in which the catalyst and polymer particles remain suspended in an inert solvent. This method is also used to make LLDPE. Solvents used include cyclohexane (C6H12), 2-methylpropane (C4H10) and C8 to C10 hydrocarbons. After extracting the polymer resin, the solvent is recycled.

Some processing of solvent is necessary, and traces must be removed from the polymer resin. Care must also be taken to minimise the risk of release of solvent in the form of liquid or vapour.

There are several different reactor designs, but all allow a continuous process to be operated.

diagram: slurry reaction

Gas-phase

As an alternative, gas phase manufacture is also carried out, using no solvent at all. The reactor runs below 100°C, at which temperature poly(ethene) remains as solid pellets, and the ethene gas is "bubbled" through a bed of these pellets. Catalyst is added higher up, and reaction occurs. Pellets are removed before they get too large. One important advantage of this method is that it removes the need for solvent processing and recovery.

A gas-phase reactor can generally be used for both HDPE and LLDPE production, giving added flexibility.

diagram: gas-phase reactor

  back to top