Plastics 195
may be converted into films or sheets, solid or hollow three-dimensional
forms, fibres and foams. Early plastics, like the natural materials they were
developed to mimic, were roughly moulded, hand carved and polished; all
resource-demanding activities. As demand for plastics grew, such techniques
were replaced by mechanised, automated processes.
Commercially speaking, the most important conversion processes are cal-
endering, extrusion and injection moulding. During calendering, liquid plas-
tic is rolled between heated metal rollers into thin films. Extrusion moulding
was first developed in the 1830s. Pellets of thermoplastic plastics are fed into
an extrusion mould and heated to melting. The liquid plastic is pressed by a
screw arrangement, into a die. Extrusion moulding is suitable for the manu-
facture of symmetrical shapes of even thicknesses including pipes, fibres and
films, where a uniform shape is required. Injection moulding was first developed
in the 1930s and varies from extrusion moulding; in the latter process molten
plastic is pushed into a closed mould before releasing the cooled, shaped
material. Complex, asymmetric shapes, such as plastic water bottles, can be
formed by injection moulding.
On cooling, plastic materials tend to contract or shrink considerably more
than other materials such as metals, ceramics and glass. For example, a copper
pipe will shrink by 0.01% if the temperature is reduced by 10°C. Under the
same conditions, a high-density polyethylene pipe would shrink by 0.07%,
and polypropylene and hard PVC pipes by 0.04%. In addition, surfaces of
plastic materials cool before their cores. Such a situation leads to the initial
contraction of plastic materials at surfaces, before significant change in dimen-
sionoccurs in the bulk. The skins of moulded plastics tend to be stiffer than
the bulk, so are more prone to degradation by mechanical action, e.g.flexing.
Shaping of foams provides different challenges to shaping of plastics because
the foam is growing and expanding while being shaped. The polyurethane
polymer ingredients are mixed and poured evenly into a moving trough or
Henecke machine. Water and catalysts are then injected into the polymer and
the whole is vigorously stirred. As foaming takes place, the mixture forms an
even block of foam which, when cool and hard, may be cut to size. If a cylinder
of foam is required, the foaming mixture is fed into the bottom of a cylinder
and pushed upwards. Pressure applied above the foaming mixture is used to
control density. The solidified cylinder may be sliced horizontally into discs.
3 Identification of Plastics
It is important to identify the types of plastics present in objects to understand
how, when and why they were produced, and to identify the optimal conser-
vation strategies for them. Methods of identification may be divided into simple
tests, based on measuring their appearance, mechanical and chemical properties,