Page 30 - Plastics Engineering

General Properties of Plastics 13
expensive but as it is much stronger and stiffer it finds numerous applications in
such things as dustbins, bottle crates, general purpose fluid containers and pipes.
One of the most exciting recent developments in this sector has been the
introduction to the marketplace of metallocene-based polyethylenes. Metal-
locenes have been recognised as suitable catalysts for the manufacture of
polyethylenes since the 1950s. However, it is only recently that their use has
been perfected. Their big advantage is that they are single site catalysts so
that the polymer molecules which are produced tend to be all the same - a fact
which offers an array of superior properties. Traditional catalysts for polyethyl-
ene (Ziegler Natta catalysts) are multi-sited so that they produce polymers with
short, medium and long molecules. In the new metallocene grades of polyethyl-
ene, the absence of low molecular weight species results in low extractables,
a narrow melting range and free-flowing material even at low densities. The
absence of high molecular weight species contributes excellent melting point
control, clarity and improved flexibility/toughness at low temperatures.
Metallocene-based polyethylene does not offer the lower production costs
associated with LLDPE. Hence there will be a price premium for the new
materials but this is felt to be justified in view of their improved property
profile.
Cross-linked Polyethylene (XLPE). Some thermoplastic materials such as
polyethylene can have their structure altered so that the molecular chains
become cross-linked and the material then behaves like a thermoset. In the case
of polyethylene, a range of cross-linking methods are available. These include
the use of radiation, peroxides and silanes. In some cases the cross-linking can
occur during moulding whereas in other cases the end-product shape is created
before the cross-linking is initiated. The action of cross-linking has a number
of beneficial effects including improved stress crack resistance, improved creep
resistance, better chemical resistance, improved toughness and better general
thermo-mechanical stability.
Polypropylene (PP). Polypropylene is an extremely versatile plastic and is
available in many grades and also as a copolymer (ethylene/propylene). It has
the lowest density of all thermoplastics (in the order of 900 kg/m3 and this
combined with strength, stiffness and excellent fatigue and chemical resistance
make it attractive in many situations. These include crates, small machine
parts, car components (fans, fascia panels etc), chair shells, cabinets for TV,
tool handles, etc. Its excellent fatigue resistance is utilised in the moulding of
integral hinges (e.g. accelerator pedals and forceps/tweezers). Polypropylene
is also available in fibre form (for ropes, carpet backing) and as a film (for
packaging).
Polyamides (nylon). There are several different types of nylon (e.g. nylon
6, nylon 66, nylon 11) but as a family their characteristics of strength, stiffness
and toughness have earned them a reputation as engineering plastics. Table 1.3
compares the relative merits of light metal alloys and nylon.