1 Polyethylene

Polyethylene is a predominantly linear polymer of the monomer ethene (ethylene), i.e.:

It is currently the largest tonnage plastic material in the world, having first been produced commercially as long ago as 1939, when it was first marketed by ICI in the UK as an electrical insulator.1 The most common grades have number average molecular weights in the range 10000 40000, though there are a number of special purpose grades on either side of this range.2 In particular, the special grade known as ultrahigh molecular weight polyethylene, UHMWPE, used in medical implants, has molecular weights in the range of millions. These grades are prepared by the Ziegler process, i.e. using co-ordination reactions with metal alkyls as catalysts.1 They are difficult to process because they cannot be melted without decomposition, and must be formed into useful articles in the rubbery phase.2

Ultrahigh molecular weight polyethylene, UHMWPE, is defined by ISO 11542 as having a molecular weight greater than 1 10 ⁶; by contrast, the ASTM D 4020 definition states that it is a linear polymer with a molecular weight greater than 3.1 10 ⁶. However, the nomenclature of this group of polymers has changed considerably in the last 30 years, as explained by Kurtz et al.,3 making comparisons of clinical results across the years difficult. Whatever the precise details of the molecular weight, the material consists of predominantly linear molecules that, because of their large size, are relatively poorly crystalline. Consequently, they are of very low density, typically...