Low and High Density Polythene

Poly(ethene) can be made in two quite different forms by changing the reaction conditions. The monomer is the same, but the chain shape is different, and that changes the properties.Low-density poly(ethene), LDPE, is made at very high pressure (about 200 MPa) and about 200 °C, with a trace of oxygen as an initiator. It forms by a free-radical mechanism, and the chains are highly branched. The branches stop the chains packing closely, so the van der Waals forces between them are weaker. LDPE has a lower density (about 0.92 g cm^–3), a lower melting point (about 110 °C) and is soft and flexible. Uses: carrier bags, food film, squeezy bottles.High-density poly(ethene), HDPE, is made at low pressure and about 60 °C using a Ziegler-Natta catalyst. The chains have very few branches, so they can line up and pack closely, with many regions where the chains are ordered (crystalline). The van der Waals forces are stronger, so HDPE is denser (about 0.96 g cm^–3), has a higher melting point (about 135 °C) and is stiffer and stronger. Uses: milk bottles, crates, water pipes.Key idea: the strength of a polymer depends on how closely its chains pack. Straight, unbranched chains pack closely and give stronger forces between chains; branched chains are kept apart and give weaker forces.So a product that must be rigid and survive hot washing, such as a crate, is made from HDPE, while a thin, stretchy bag is made from LDPE.