Page 88 - Academic Press Encyclopedia of Physical Science and Technology 3rd Polymer

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Encyclopedia of Physical Science and Technology EN012K-587 July 26, 2001 10:35

458 Plastics Engineering

In addition, although the absolute mechanical property if crystallization is prevented (by, for example, rapid cool-
values of plastics are less than those for metals, the low ing from the melt) transparent moldings can be produced.
density of plastics means that their specific properties The classic example of this is the manufacture of bottles
compare very favorably with other materials. When one from polyethylene terephthalate (PET).
adds on the other attractive properties of plastics such The semicrystalline and amorphous types of plastic re-
as chemical and environmental resistance, toughness, and ferred to above are more generally known as thermoplas-
resilience, and in particular, ease of processing into com- tics. This is because they are capable of going through
plex shapes, it is clear that the engineering applications an almost indefinite cycle of being softened by heat and
for plastics will continue to increase. becoming solid again when the heat is removed. The most
distinguishing feature of this type of plastic is that the
polymer chains remain linear and separate after molding.
I. GENERAL TYPES OF PLASTICS In contrast to this it is possible to have thermosetting plas-
tics, which can be softened only once to take up the shape
The most characteristic feature of the structure of plastics of the mold. Once these materials have solidified they
is that they consist of relatively long molecules. These cannot be softened by the application of heat or by any
molecules are usually referred to as “chain-like” because other method. When heat and pressure are applied to a
during the manufacture of the plastic a large number of thermosetting plastic during the initial molding process,
small units have been linked together. In most cases the the structure undergoes a chemical reaction that locks it
molecular chain has a carbon backbone, and it is the vary- into a three-dimensional network. This is called cross-
ingnatureofthesidegroupsoffthebackbonethatgivesthe linking; it is initiated by heat, chemical agents, irradiation,
different types of plastics their individual characteristics. or a combination of these. As a result of this cross-linking,
The forces between molecular chains are relatively thermosets have an obvious advantage in that they will not
weak, so that when a plastic is heated the thermal energy is soften in high-temperature environments. Thermosets also
capable of moving the chains apart. In this state the struc- have improved resistance to chemical attack, stress crack-
ture has a random configuration, and the plastic can easily ing, and creep. However, they are not so easy to mold
flow into a mold or through a die to take any desired shape. as the thermoplastics, and generally they cannot offer the
When heat is extracted from the plastic it is transformed same level of toughness.
from a melt to a solid and retains its new shape. Examples of thermoplastics are polypropylene, poly-
If the side groups on the molecular chain are simple, carbonate, polyvinyl chloride, ABS, polystyrene, acetal,
it is possible for the structure of the plastic to attain a and nylon (polyamide). Examples of thermosets are epox-
very ordered structure. If this occurs the plastic is said to ies, phenolics, ureas, and melamines. Polyesters and
be crystalline or, perhaps more accurately, semicrystalline polyurethanes can be available as both thermoplastic and
since no plastic is capable of becoming totally crystalline. thermosetting materials. It should be noted that some
The degree of crystallinity can be controlled, to some ex- thermoplastics, such as polyethylene, are now commer-
tent, during molding operations, and it is important that cially available in cross-linkable grades, so the distinction
the molder is aware of the effects on properties that can be between a thermoplastic material and a thermosetting ma-
produced. Crystallinity in a plastic improves some of its terial is becoming less clear-cut.
mechanical properties (such as modulus) but shrinkage is The types of plastics used successfully in engineering
greater, and in a particular plastic the toughness is reduced applications cover the full range of thermoplastics and
as the degree of crystallinity increases. Chemists can also thermosets that are available. This means that the designer
take steps to control crystallinity; this can be used as one has access to materials ranging from polyethylene to
method to obtain, for example, high-temperature plastics. polyetheretherketone (PEEK). In the former case the
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Some plastics have a molecular structure that does not elastic modulus is low (300 to 400 MN/m ), but with
permit any type of crystalline structure. These plastics proper design it may be used in critical, demanding
are referred to as amorphous in that they have a random applications—for example, nationwide pressurized gas-
molecular structure in both the molten and the solid states. distribution pipes. The modulus of polyethylene can also
Amorphous plastics are always capable of being transpar- be increased by a factor of 100 if its structure is highly
ent, although the addition of fillers and pigments results oriented by stretching the material under controlled
in their being available in a wide range of opaque col- conditions in the solid state. In the case of PEEK, it is
ors. Semicrystalline plastics contain both amorphous and an expensive material that possesses unique properties.
crystalline regions in their structure. The way that these When combined with carbon fibers, it provides a material
regions co-exist has a major effect on the properties of the that is stronger and has a higher modulus than many metals
plastic. Semicrystalline plastics are naturally opaque, but (see Table I).