Page 25 - Plastics Engineering

8 General Properties of Plastics

and miniaturisation in the electronics industry. However, thermosets are now
fighting back and have a very much improved image as colourful, easy-flow
moulding materials with a superb range of properties.
Phenolic moulding materials, together with the subsequently developed easy-
flowing, granular thermosetting materials based on urea, melamine, unsaturated
polyester (UP) and epoxide resins, today provide the backbone of numerous
technical applications on account of their non-melting, high thermal and
chemical resistance, stiffness, surface hardness, dimensional stability and
low flammability. In many cases, the combination of properties offered by
thermosets cannot be matched by competing engineering thermoplastics such as
polyamides, polycarbonates, PPO, PET, PBT or acetal, nor by the considerably
more expensive products such as polysulphone, polyethersulphone and PEEK.

1.33 Composites
One of the key factors which make plastics attractive for engineering applica-
tions is the possibility of property enhancement through fibre reinforcement.
Composites produced in this way have enabled plastics to become acceptable
in, for example, the demanding aerospace and automobile industries. Currently
in the USA these industries utilise over 100,OOO tonnes of reinforced plastics
out of a total consumption of over one million tonnes.
Both thermoplastics and thermosets can reap the benefit of fibre reinforce-
ment although they have developed in separate market sectors. This situation
has arisen due to fundamental differences in the nature of the two classes of
materials, both in terms of properties and processing characteristics.
Thermosetting systems, hampered on the one hand by brittleness of the
crosslinked matrix, have turned to the use of long, indeed often continuous,
fibre reinforcement but have on the other hand been able to use the low viscosity
state at impregnation to promote maximum utilization of fibre properties. Such
materials have found wide application in large area, relatively low productivity,
moulding. On the other hand, the thermoplastic approach with the advantage
of toughness, but unable to grasp the benefit of increased fibre length, has
concentrated on the short fibre, high productivity moulding industry. It is now
apparent that these two approaches are seeking routes to move into each other’s
territory. On the one hand the traditionally long-fibre based thermoset products
are accepting a reduction in properties through reduced fibre length, in order to
move into high productivity injection moulding, while thermoplastics, seeking
even further advances in properties, by increasing fibre length, have moved
into long-fibre injection moulding compounds and finally into truly structural
plastics with continuous, aligned fibre thermoplastic composites such as the
advanced polymer composite (APC) developed by IC1 and the stampable glass
mat reinforced thermoplastics (GMT) developed in the USA.
Glass fibres are the principal form of reinforcement used for plastics because
they offer a good combination of strength, stiffness and price. Improved