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

Plastics Engineering 473

Plastics are inherently electrical insulators and they are some plastics are already well established in this field
often used to exploit this characteristic. However, conduc- is further confirmation that they have come of age as
tive thermoplastics offer parts designers extra freedom in engineering materials. Plastics and composites are now
the control of static (ESD) and EMI/RFI. The mechanism widely used for primary and secondary components in
of conductivity in plastics is similar to that of most other civil and military aircraft. The Lear Jet and the Beech
materials. Electrons travel from point to point when under Starship are two examples of what can be achieved in that
a driving force, following the path of least resistance. Con- the complete outer structure of the planes utilize carbon-
ductive modifiers with low resistance can be melt blended fiber-reinforced plastic (CFRP). Another very significant
with plastics to alter the polymers inherent electrical resis- example is the vertical and horizontal tail-plane of the Air-
tance. At a threshold concentration unique to each conduc- bus 320. These structures are predominantly made from
tivemodifierandresincombination,theresistancethrough CFRP in both monolithic and honeycomb-cored sand-
the plastic mass is lowered enough to allow electron move- wich panels. Glass-fiber-reinforced plastics are used for
ment. The speed of electron movement depends on the the bridging layers between the honeycomb cores and the
modifier concentration—in other words, on the separation CFRP skins. The weight saving achieved is about 20%,
between the modifier particles. Varying the percentage or which represents a very considerable reduction in fuel
type of conductive additive used in the compound permits consumption or increase in payload. Another major ad-
onetocontrolthedegreeofelectricalresistivity.Nearlyev- vantage of using composites in this type of application is
ery type of polymer can be compounded with conductive that the fabrication methods used reduce the number of
fillers and most conductive thermoplastics can be made in individual parts that must be assembled. In the case of the
a variety of colors. Airbus 320 the reduction was from about 2000 parts in
The types of conductive additives that can be used in- older designs to less than 100 parts in the latest design.
clude carbon-based powder and fibers, metal powder and Future generations of commercial aircraft will make even
fibers, and metal-coated fibers of carbon or glass. Re- more extensive use of plastic composites.
cently, unique conductive additives such as metal-coated In military and space applications, the advantages of
substrates, intrinsically conductive polymers (ICPs), and composites are utilized to their utmost effect. The cost of
inherently dissipative polymers (IDPs) have found com- launching rockets is so high that a high price can be tol-
mercial use in conductive thermoplastic compounds. erated for any weight saving that can be achieved. As a
Conductive thermoplastic compounds prevent static accu- result, thousands of composite parts have been used suc-
mulation from reaching dangerous levels by reducing the cessfully in spacecraft. The bank of information that has
parts electrical resistance. This allows static to dissipate been built up in this application area has shown that CFRP
slowly and continuously rather than accumulate and dis- can outperform other materials because of their very high
charge rapidly as a spark. Conductive thermoplastic com- specific strengths and stiffnesses, excellent fatigue resis-
pounds provide protection against EMI/RFI by absorbing tance, and low coefficients of thermal expansion. As the
electromagnetic energy and converting it to electrical or trend continues toward longer design lifetimes of space-
thermal energy. They also function by reflecting energy. craft, it seems certain that the use of composites will also
There are numerous application areas for conductive continue to grow rapidly in this application area.
plastics in the computer industry in the form of hous-
ings to protect sensitive electronics, and in industries using
powders there is often a strong dependence on the unique SEE ALSO THE FOLLOWING ARTICLES
characteristics of these materials. In the pharmaceutical
industry, for example, antistatic surfaces, containers, and CHEMICAL PROCESS DESIGN,SIMULATION,OPTIMIZA-
packaging must be used to eliminate dust attraction into TION, AND OPERATION • COMPOSITE MATERIALS • CON-
the product. An example of an application in the medi- STITUTIVE MODELS FOR ENGINEERING MATERIALS •
cal field is the body for asthma inhalers where the correct FRACTURE AND FATIGUE • PLASTICITY • PLASTICIZERS
dosage of the medication is critical and any static “cap-
ture” of the fine-particulate drugs can affect recovery from
a spasm. BIBLIOGRAPHY

Astrom, B. T. (1997). “Manufacturing of Polymer Composites,” Chap-
E. Aircraft/Aerospace Applications man & Hall, London, p. 469.
Berins, M. L., ed. (1991). “Plastics Engineering Handbook,” 5th ed.,
The very stringent demands that the aircraft/aerospace
Hanser, Munich, p. 869.
area makes on materials has made it very much the fi- Crawford, R. J. (1998). “Plastics Engineering,” Butterworth-Hein-
nal frontier for plastics and composites. The fact that emann, Oxford.