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issue: May 2004 APPLIANCE Magazine

Electrical Manufacturing
Conductive Plastics


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by Barry M. Lunt and A. Brent Strong, Brigham Young University

Plastics have been widely used in electrical manufacturing for several decades, due to their many advantageous properties, including moldability, ruggedness, and low cost.

For applications in electricity and electronics, applications for plastics have always been as dielectrics, due to their inherent property as electrical insulators. The invention of conductive plastics has enabled the creation of many products that have previously not existed. This paper describes the invention of conductive plastics and several of the products they have made possible and some that will be possible in the future.

Three scientists recently shared the 2000 Nobel Prize in Chemistry for their discovery and development of conductive plastics (polymers). The Nobel Prize was awarded because conductive polymers are so different from any other polymeric material in their conductivity, but are similar to other plastics in their weight, cost, moldability, and general physical properties. The advantages of plastics have always been associated with the typical plastic properties that have included a low electrical conductivity[1]. For some applications, such as use in electronic packaging and as circuit board material, this low conductivity was favorable. However, there are many other applications where a conductive polymer would be highly advantageous.

Applications of Conductive Polymers

The following materials have either reached some minimal commercial viability or are on the verge of becoming commercial.

Static Suppression

Electrostatic discharge (ESD) is a serious problem in the electronics industry since voltages as low as 50 V can readily damage sensitive electronic components, and this range of voltage is extremely common on equipment and personnel that are not properly protected. Today’s equipment used for ESD protection is commonly made from polymeric materials that have been made somewhat conductive using fillers and weaving with metal or carbon fibers (for fabrics). However, because polymeric materials are inherently non-conductive, they must be tested regularly to assure their continued conductive properties. If future ESD protection equipment is made from conductive polymers, the native material will be conductive, and there will be no need for periodic testing to ensure its proper protective operation. Conductive polymers have been used to suppress static on computer screens, on films, and in windows.

Batteries

In an age of massive portability in electronics, the need for improved batteries is critical. Modern society seems to be constantly moving and taking their electronics with them, and we are seeing tremendous growth in laptop computers, cellular phones, and personal digital assistants (PDAs). Electronics are being put in every place that we might spend our time. Therefore, replacing heavier metal components with lightweight polymers would seem to be highly desirable.

The electrodes of all common batteries are made of metals. (Car batteries are lead, flashlight batteries are nickel/cadmium, and button cells are lithium.) By replacing these metals with conductive polymers, the following advantages have been shown: lower weight, lower cost, more charge/discharge cycles, lower toxicity, and improved recyclability. Hence, the promise of substituting conductive polymers for these metals is highly desirable and has already proven to be commercially successful. We should also note that originally obtaining the metals, usually from mines, is far more costly and damaging to the environment than is the obtaining of polymer raw materials from natural gas, deep oil wells, or now, increasingly, from residual plant materials.

Silicon Replacement

The ultimate goal is to make integrated circuits and other electronic devices for low-cost applications by avoiding the use of silicon. The problem with silicon is that it must be highly purified and exist in an absolutely crystalline form before it can be doped for use in electronic devices. This need for purity and crystal exactness drives the price of electronics-grade silicon to an estimated U.S. $500 per pound. It is estimated that the price of polymeric conductors would, at large volume, be no more than $10 per pound.

Light-Emitting Diodes

Conductive polymers have been made into devices that provide an alternative to conventional backlit LCD displays. The devices (termed OLEDs, for organic light-emitting diodes), which use conductive polymers, are sandwich-type structures where the active polymeric film layer is positioned between a semi-transparent anode and a back row cathode. The devices emit uniformly over the entire device. Such devices have already found application in displays for cellular telephones, camcorders, PDAs, and numerous industrial devices needing a readout display. Small (3-in diagonal) full-color OLED displays are available in limited volumes. Their present advantages over LCD backlit displays include lower power, lighter weight, increased durability (no glass), wider viewing angle, and increased brightness; their future advantage of lower cost is also promising [2].

Electronic Ink

Another potential application, closely related to OLEDs, is the development of electronic ink. Using the same thin, flexible, and inexpensive plastic substrates as OLED displays, organic transistors can be made to move an electrophoretic ink (an ink which is sensitive to electric fields) so that it is either visible or not visible [2]. The ink stays wherever it is moved. This gives us electronic ink, or electronic paper. It means a user could buy an inexpensive e-paper display, and each day download the daily news onto it. Only the downloading and storing actually use power; the image remains when the display is powered off.

Microtools

One interesting property of many conductive polymers is that they swell when they conduct. This means that conductive polymers can change electrical signals into mechanical energy, similar to piezoelectric materials. However, in contrast to piezoelectric films, conductive polymeric films work well at low voltages, thus expanding the areas of applicability for such devices.

Solder

Solder is used heavily in the electronics industry to physically and electrically attach electronic devices to printed wiring boards that connect them together. The solder used is made of a mixture of tin and lead and has served the industry extremely well for many decades. However, in recent years there has been increasing concern that the lead in electronic products could end up in landfills, where it could eventually leak into the water supply. Although this is not presently a problem, industry should proceed as quickly as possible to find alternatives. Many metal compounds have been investigated for this application, and much progress has been made. There is even a small niche of the electronics market that has begun to use conductive adhesives for a solder replacement. These adhesives are typically made from epoxies loaded with conductive filler material. One disadvantage of conductive adhesives is that they are thermosets and are not reworkable (repairable). The new class of conductive adhesives are generally thermoplastics; these have a great potential to replace currently used conductive adhesives.

Printed Circuit Boards

Today’s electronic devices are attached to printed circuit boards made from a composite of epoxy and fiberglass to which a thin layer of copper has been attached and etched to form the appropriate conductive patterns. This process involves the use of expensive photographic equipment, along with additional processing equipment for hazardous materials used in the process. The end product is very reliable and reasonably inexpensive, but there is much room for improvement. With conductive polymers, the possibility exists to make circuit boards that are truly printed (current ones are actually etched, not printed). Changing to a printing technology would dramatically lower the costs associated with making these boards, allowing for many applications that today are only ideas.

Limitations of Conductive Polymers

The best currently available conductive polymers do not have the precision in conductance and other electrical applications that engineers have come to expect from metallic and semi-conductive (silicon) materials. Also, conductive polymers do not conduct electricity at the same speed as silicon chips. Polymers are, therefore, limited to those applications where gross or relatively slow changes occur. The conductive polymers are, at least at present, to be viewed as complementary to silicon and metal devices rather than competitive.

Conductive polymers are still polymers. While that has some significant advantages in moldability and elongation, the polymer nature also has some disadvantages. For instance, the conductive polymers are still much weaker in mechanical strength when compared to metals, although the polymers are better than silicon-based devices. Also, the polymer materials are softer and, therefore, more likely to be damaged by scratching and abrasion when compared to metals. Moreover, many of the conductive devices are currently encased in glass to prevent physical scratching or handling damage. Hopefully, this abrasion problem will be solved in other ways, as glass sometimes makes the devices quite fragile.

Lastly, polymeric devices are mostly conductive in only one or two dimensions, whereas metals are fully conductive in three dimensions; that is, they are anisotropic conductors. The dimensionality restriction of the polymers (anisotropy) is because polymers are linear or, occasionally, planar structures, and the delocalized electrons follow the shape of the polymer network. Designers need to be aware of this difference in directional conductivity. It can be a problem but, in some applications, it might also be an advantage to have a significantly reduced conductivity in a specific direction. In fact, anisotropic conductors are used in many applications in electronics, including inexpensive digital watches.

The Future of Conductive Plastics

Some envisioned applications for conductive polymers seem too unconventional to be possible but are being widely reported by the American Plastics Council. For instance, applications for electronic paper include the delivery of the daily news, displays that could be updated with data via computer (perhaps using a PDA) linked to the Internet or some wireless communication device and then rolled up and carried to a presentation, or even paintings in a home—users could, of course, have several of these mounted in their home and then change the display from one painting to another to fit their mood.

Medical researchers are excited about possibilities they see in conductive polymers. Scientists at the University of Texas (Austin, TX, U.S.) reported in a recent meeting of the American Chemical Society some encouraging success in mending damaged nerves by fitting the severed ends into an electrically conductive plastic sleeve packed with sugar. The sugar apparently encourages blood vessel growth, which in turn, stimulates nerve regeneration. The electrical properties of the plastic, which dissolves over several weeks, appear to have a beneficial effect on the nerves.

Some researchers have embarked on a study of conductive polymers as a new method for storing electronic information, perhaps optically. These could be developed into very fast storage and retrieval devices. Others see conductive polymers as light-detecting devices that could be configured into large arrays for military and commercial applications.

Conclusion

Surely conductive polymers are exciting developments. As they become more common, they have become part of many products with which we are already familiar and will certainly enable many advances in future products.

References

1. De Gaspari, John, “New alternatives In Conductive Plastics,” Plastics Technology, November 1997, p. 13-15.

2. Moore, Samuel K., “Just One Word—Plastics,” IEEE Spectrum, September 2002, p. 55-59.

 
About the Authors

A. Brent Strong is a professor of Manufacturing Engineering Technology and the Lorin Farr Professor of Entrepreneurial Technology at Brigham Young University. Dr. Strong received his B.A. degree in Chemistry and his Ph.D. in Physical Chemistry from the University of Utah (Salt Lake City, UT, U.S.). He has been at Brigham Young University for 16 years and prior to that was a research chemist for the DuPont Company and president of Hardie Irrigation, a plastics molding and extrusion company. His emphasis is plastics and composites.

Barry M. Lunt is an associate professor of Information Technology at Brigham Young University in Provo, UT, U.S. Dr. Lunt received a B.S. and M.S. degree in Electronics Engineering Technology from Brigham Young University and a Ph.D. in Occupational and Adult Education from Utah State University in Logan, UT, U.S. He has spent 7 years in industry as a design engineer, and 16 years in college education at three institutions. His present research emphases are the physical design of electronic circuits and systems, and engineering education.

 

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