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

Plastics Engineering 461

Materials that have shown themselves to be amenable to generally the least expensive, and they improve the creep
high degrees of structural orientation include polyethy- resistance, thermal conductivity, and high-temperature ca-
lene, PVC, and PMMA. pability of the plastic at short and long times. The extent
8. Composites: Although unreinforced plastics can be of the improvement depends on the efficiency of the bond
used in engineering applications, it is only when fiber between the fiber and the base resin. This is where aramid
reinforcement is added that they become serious chal- fibers have had difficulty in the past. Although they of-
lengers for metals in the more demanding applications. fer a better property enhancement than glass, the high
Both thermoplastics and thermosets can benefit from fiber surface energy of the lightweight fibers makes wetting
reinforcement. In the past these two types of composites the fibers and dispersion of the base resin difficult. These
tended to develop in separate ways and targeted different problems have to a large extent been overcome by the de-
marketsectors.However,inrecentyearsthereinforcement velopment of special proprietary sizing/coupling systems.
technologies have moved closer together and now they of- Characteristic features of aramid-reinforced plastics are
ten compete directly for the same applications. When fiber low warpage and thermal expansion and uniform proper-
reinforcement was first identified as a means of obtaining ties in all directions.
very significant improvements in the mechanical proper- Carbon fibers offer the best improvements in strength,
ties of plastics, it was continuous-fiber reinforcement that stiffness, and thermal conductivity, albeit at a price. There
attracted most attention. As the very large surface area of are also very significant increases in the heat-deflection
the continuous fibers needed to be completely wetted, it temperature, creep resistance, and fatigue endurance.
was necessary to use the low-molecular-weight resins that Carbon fibers also offer the ability to dissipate static elec-
could be subsequently polymerized when the structure had tric charges very effectively; this is becoming a crucial fea-
been shaped. Hence, thermosetting resins dominated the ture in many new application areas for plastics. The resis-
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continuous-fiber, high-performance sector, but the manu- tivity of most plastics is in the region of 10 –10 16 cm,
facturing processes were slow. In contrast, the thermoplas- which means that they are excellent electrical insulators.
tic industry concentrated on their fast production methods, However, there is a developing market in areas such as
suchasinjectionmolding,butwereobligedtoacceptlower electronic component housings, EMI shielding, and bat-
property enhancement through the use of short fibers. In tery construction where some degree of electrical conduc-
recent years this situation has changed in that the ther- tivity in plastics would be desirable. The simplest way to
mosetting industry is prepared to sacrifice properties, that achieve this is to load the plastic with a conductive filler
is, use shorter fibers to speed up production rates. Simi- such as metal or carbon. The developing technology is to
larly the thermoplastic industry has devised ways of wet- devise structures in plastics that are intrinsically conduc-
ting continuous fibers with thermoplastic resins to get into tive, and some exciting advances have already been made
the more lucrative high-performance application areas (for in this field.
example, aerospace). The main advantages that thermo- Natural fibers, such as jute, sisal, flax and hemp, are
plastics have over thermosets are that they offer better also used in plastics. There are attractive because they
toughness (damage tolerance) and they have better water are low-cost, low-density materials offering good spe-
resistance. It is evident that for thermosets it is possible to cific properties and they are friendly to the environment.
improve either one of these characteristics to acceptable However, there are also problems to be overcome. The
standards but not both at the same time. poor moisture resistance of natural fibers and their in-
compatibility with the matrix resin are problems that
The continuous-fiber reinforced thermoplastics are nor- attract a lot of research activity in some parts of the
mally available as a flat sheet containing the reinforce- world.
ment. This is heated and shaped between matched halves An exciting development area in plastics is nanocom-
of a mold to produce the desired shape. Also available in posites. This involves the addition of 2–5% of nanometer-
the thermoplastic sector are long (as opposed to continu- size particles into a plastic to enhance fire resistance,
ous) fibers for reinforcement. The raw material is available improve mechanical properties, reduce thermal conduc-
as injection-moldable pellets containing wetted fibers that tivity, and reduce permeability. The low addition levels of
are the full length of the pellet, typically 10 mm. Normally, these very fine particles does not alter density of trans-
short fibers have lengths in the range of 0.75 to 0.5 mm parency. The automotive industry has been one of the first
so that the long fiber-reinforced thermoplastics have very to utilize these materials for under-the-hood components.
significantly improved properties while retaining the easy, The original nanocomposites were produced by adding
fast processing associated with injection molding. the small particles at the reactor stage but recent success
The type of fiber used in plastics has a major effect on has been achieved by compounding them into the plastic
the properties and economics of a product. Glass fibers are using and extruder.