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

460 Plastics Engineering

Traditionally, plastics have been divided into a number years. The concept of combining different materials
of specific categories. Although these tend to be some- to bring together their desirable attributes into one
what imprecise in that many plastics can belong to several material has been utilized with metals for centuries,
categories, the terminology does help to give a broad in- but its exploitation has been slow with plastics. ABS
dication of the potential uses of individual plastics. (acrylonitrile-butadiene-styrene) was one of the first
commercially available plastic alloys but its success
1. Engineering plastics: Although most plastics can be could not be generally extended to other materials due
used in engineering applications, one group has been dis- to the inherent incompatibility of most plastics. A recent
tinguished by the description engineering plastics. These revival of interest in polymer alloys has been caused by
are polyamides, polycarbonates, polyacetals, modified two factors. The first is the result of breakthroughs in the
polyphenylene ethers, and thermoplastic polyesters. The blending technology, so that some very interesting and
use of the term engineering plastic probably arose when exciting combinations have become possible. The second
it was established that these materials could be used as factor is that some lucrative market sectors (for example,
successful substitutes for metals in light engineering ap- the automotive industry) are demanding property specifi-
plications (for example, gear wheels, pulleys, etc.). cations not available in a single plastic. As a result, there
2. Speciality plastics: Other high-performance plas- is an ever increasing range of commercially available
tics, such as polysulphone and polyphenylene sulphide, alloys, and the material suppliers are prepared to listen to
are sometimes included in the list of engineering plastics, requests for combinations not yet available. Some of the
but the consumption of these materials is relatively small most interesting and successful alloys are polyphenylene
because economic factors restrict them for the more de- oxide/polystyrene (modified PPO), polycarbonate/ABS,
manding types of application. Hence, these materials, polyamide/polypropylene, polyamide/polyphenylene
along with exciting recent arrivals such as polyimides, ether, ASA/polycarbonate, and PBT/polycarbonate.
fluoropolymers, polyetherketones, and polyarylates, tend 5. Structural foam: Many plastics can be foamed by
to be more accurately described as speciality plastics. A the introduction of a blowing agent so that after mold-
particular feature of this type of plastic is the ability to ing the product consists of a cellular foam core and a solid
be used continuously at service temperatures up to 300 C skin.Thistypeofstructureisveryefficientintermsofstiff-
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in circumstances where some metals cannot even be con- ness per unit weight. The foam effect is normally achieved
sidered candidate materials. Typical properties of these by preblending the plastic granules with a heat-activated
materials are shown in Table I. blowing agent. Polycarbonate, polypropylene, and modi-
3. Elastomers (thermoplastic rubbers): Conventional fied PPO are popular materials for foam molding.
vulcanized rubbers possess a range of very desirable 6. Liquid-crystal polymer (LCP): These polymers ar-
properties, such as resilience and flexibility, over a wide rived on the materials scene in the mid-1980s. They
temperature range and resistance to oils, greases, ozone, have an exciting new type of structure that is highly or-
etc. However, they require careful, relatively slow process- dered, even in the molten state. The structure has been
ing, and it is not possible to reuse any waste material. To likened to a stack of uncooked spaghetti: if subjected to
overcome the latter disadvantages, in recent years a range stress the stiff rods can slide past one another, but the
of thermoplastic rubbery materials has been developed. same ordered orientation is retained. It is this retention
These exhibit the familiar feel and performance of rubbers of structural order in LCPs that gives them exceptional
but have the ease of manufacture associated with thermo- properties. They have outstanding dimensional stability,
plastics. There are several basic types of thermoplastic high strength, stiffness, toughness, and excellent chemi-
rubber (TPR). These differ in the way they impart rubber- cal resistance. They also have very good high-temperature
like properties to the thermoplastic. In the polyurethane, performance (up to 300 C), and they are easy to process.
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styrenic, and polyester TPRs the chemists have grafted 7. Oriented polymers: When the chain-like structure
thermoplastic molecules on the rubber molecules. In the of polymers is drawn out so that the chains become highly
olefinic TPR there is a polypropylene matrix with fine oriented, it is possible to achieve very high strength and
rubber particles embedded in it to provide the elastomeric stiffness values. For example, the modulus of polyethy-
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properties. Other types of TPR are based on polyamide lene can readily be increased from under 1 GN/m to
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and are essentially alloys. New advancements in poly- about 70 GN/m . Hot stretching of the plastic can bring
mer chemistry have created other families of elastomers— about some improvement in properties, but the maximum
plastomers, metallocene-based EPDM alloys, styrene- benefit is gained by cold drawing. In addition to me-
ethylene block copolymers, and other blends and alloys. chanical properties, it has been shown that conductivity,
4. Polymer alloys: Blends and alloys have become an diffusion/solubility, and piezoelectric properties are also
important part of the plastics materials scene in recent enhanced,andthisopensthedoortonewapplicationareas.