| 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.
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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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