Poly(ethene)

In the manufacture of LDPE, ethene is compressed to between 1000 and 3000 atmospheres, mixed with a polymerisation initiator and heated to about 300°C. Under these conditions the ethene is in the supercritical state and acts as a solvent as well as reactant. LDPE is sometimes referred to as high pressure poly(ethene) because of the conditions under which it is made.

Reaction

The polymerisation reaction is highly exothermic, and cooling is needed. Unreacted ethene is recycled, and the molten polymer is converted to granules ready for further processing and moulding.

diagram: manufacture of LDPE

The polymer produced by this process consists of chains of between 4,000 and 40,000 carbon atoms, with a relatively high degree of branching (up to 30 branches per 1000 carbon-atoms). The polymer chains cannot pack together easily, and so the resulting plastic is easily moulded but not particularly strong.

Mechanism

Polymerisation by this method proceeds as a free radical addition reaction, and is very similar to the mechanism of PVC polymerisation.

Click here for a static diagram of the mechanism

The future of LDPE

In many applications LDPE is being replaced by LLDPE, which can be made using much milder conditions and is a superior product for several applications. Many manufacturers have switched to LLDPE.

Click here for more information on LLDPE.