The definition of "hi-tech"
seems to be shifting gears from 'dot coms',
'killer tech stocks', wireless' & 'infrastructure
to the world of new wonder materials called
'conductive polymers'. All carbon based Organic
Plastics have traditionally been regarded as
'insulators'. This is no longer true. The entire
field of 'conducting polymers' was given a scientific
blessing when Alan Heeker, Hideki Shirikawa
and Alan MacDiarmid won the Nobel Prize in 2000
in Chemistry for their work in conducting polymers.
The concept paper attempts to capture some novel
and exciting business applications of conductive
polymers. Conductive polymers are beginning
to invade areas in applications such as display
devices; photographic films, sensors and even
artificial nerves and muscles are some far-fetched
futuristic vision. Exactly where these materials
are going, in the coming years, is most difficult
to predict at the present state of market acceptance.
The hottest thing, which is likely to cause
a ‘revolution’, may be "organic
LEDS".
Until 30 years ago, all carbon-based polymers
were rigidly regarded as 'insulators'. The notion
that plastics could be made to conduct electricity
would have been considered to be absurd. Plastics
have always been extensively used by the electronics
industry for this very property. This very narrow
perspective is rapidly changing as a new class
of polymers known as 'intrinsically conductive
polymers' or 'electroactive polymers' are being
discovered and commercialized. Although this
class of polymer is in its infancy, the potential
uses of these polymers are quite significant.
The first conducting plastics were discovered
by accident at the Plastics Research Laboratory
of BASF in Germany. Scientists at this laboratory
made polyphenylene and polythiophenes, which
showed electrical conductivity of the order
of 0.1scm-1. Since then, other conducting polymers
have been discovered.
There are two main groups of applications for
these polymers. The first group utilizes their
conductivity as its main property. The second
group utilizes their electroactivity.
The molecular structure of these polymers makes
them highly susceptible to chemical or electrochemical
oxidation or reduction. These alter the electrical
and optical properties of the polymer, and by
controlling this oxidation and reduction, it
is possible to precisely control these properties.
Since these reactions are often reversible,
it is possible to systematically control the
electrical and optical properties with a great
deal of precision. It is even possible to switch
from a conducting to insulating state. The two
groups of applications are shown below, Group
1 applications use just the polymer's conductivity.
The polymers are used because of either their
lightweight, ease of manufacturing and cost.
Group 2 applications, as stated earlier utilize
the electoractivity to yield properties for
specific applications.:
Group 1
Group
2
§ Electrostatic materials §
Molecular electronics
§ Conductive adhesives §
Electric displays
§ Electromagnetic shielding §
Chemical bio-chemical and thermal sensors
§ Printed circuited boards §
Rechargeable batteries and solid electrolytes
§ Artificial nerves
§ Drug release systems
§ Antistatic clothing §
Optical computers
§ Piezo ceramics §
Ion exchange membranes
§ Active electronics (diodes, transistors)
§ Electromechanical activators
§ Aircraft structures §
'Smart' structures
§ Switches
Historical Anecdotes about Conductive
Polymers
Like all major discoveries in polymers,
'serendipity' had its share to play in the invention
of conductive polymers and plastic batteries.
Until 1987, the billions of batteries that had
been marketed in myriad sizes and shape all
had one thing in common. To make electricity,
they depended exclusively upon chemical reactions
involving metal components of the battery. But
today, a revolutionary type of battery is available
commercially. It stores electricity in plastic.
Plastic batteries are the most radical innovation
in commercial batteries since the dry cell was
introduced in 1890. The development of plastic
batteries began with an accident. In the early
70s, a graduate student in Japan was trying
to repeat the synthesis of "polyacetylene",
a dark powder made by linking together the molecules
of ordinary acetylene welding gas. After the
reaction was over, instead of a black powder,
the student found a film coating the inside
of his glass reaction vessel that looked much
like aluminum foil. He later realized that he
had inadvertently added much more than the recommended
amount of catalyst to cause the acetylene molecules
to link together.
News about the foil-like film reached Alan Mac
Diarmid of the University of Pennsylvania. He
was interested in non-metallic electrical conductors.
Since polyacetylene in its new guise looked
so much like a metal, MacDiarmid speculated
that it might be able to conduct electricity
like a metal as well. McDiarmid invited the
student’s instructor to join his team
in the United States and this collaboration
soon led to further findings. The University
of Pennsylvania investigators confirmed that
polyacetylene exhibited surprisingly high electrical
conductivity.
Scientists recognize that various materials
can conduct electricity in different ways. In
metals, electricity is simply the manifestation
of the movement of free electrons that are not
tightly bound to any single atom. In semi-conductors,
like those that make up transistors and other
electronic devices, electricity is the drift
of excess electrons to form negative current.
Alternatively, the drift of missing electrons
or positive "holes" in the opposite
direction to form a positive current. Typically,
impurity or dopant atoms donate the excess electrons
or the holes.
MacDiarmid's team reasoned that the ability
of polyacetylene to conduct electricity was
probably promoted by trace impurities contributed
by the catalysts involved in the Japanese student's
process In their laboratory, MacDiarmid's team
confirmed that it was possible to chemically
dope polyacetylene to create either mobile excess
electrons or holes. That these electrons and
holes could move explained how polyacetylene
was able to conduct electricity.
When polyacetylene was exposed to traces of
iodine or bromine vapor, the thin polymer film
exhibited still higher electrical conductivity.
The researchers discovered that by deliberately
adding selected impurities to polyacetylene,
its electrical conductivity could be made to
range widely - behaving as an insulator, like
glass, to a conductor, like metal. The discovery
that plastics can behave like metallic conductors
and semi-conductors was chemistry first.
The key breakthrough leading to practical applications
as batteries occurred in 1979 when one of Prof.
MacDiarmid's graduate students was investigating
alternative ways for doping polyacetylene. He
placed two strips of polyacetylene in a solution
containing the doping ions and passed on electric
current from strip to another strip. As expected,
the positive ions migrated to one strip and
the negative ions to the other. But when the
current source was removed, the charge remained
stored in the polyacetylene polymer. This stored
change could then be discharged if an electrical
load was connected between the two strips, just
as in a conventional battery.
Polyacetylene, however, is not an ideal battery
material. It degrades in air, is chemically
stable only in liquid solutions and is brittle
and not amenable to injection moulding methods
used for forming plastic parts in production.
The University of Pennsylvania team, along with
industrial associates, licensed to use their
technology searched for conducting polymers
of higher structural strength, thermoplasticity,
flexibility, and lower costs. Allied corporation
synthesized a new material, polyparaphenylene,
and a black powder capable of being formed into
plates by hot pressing, which could be doped
to conduct electricity. Several other potentially
suitable plastics were discovered thereafter.
One such material was polyaniline. In 1984 and
1985, the University of Pennsylvania group received
patents on the use of this material for rechargeable
batteries. It is inexpensive and unlike polyacetylene,
it is stable in both air and water. Polyaniline
is the material used in the plastic batteries
that first became commercially available in
1987.
In just 8 years, plastic batteries went from
laboratory discovery to commercial availability
an unbelievable revolution! Alan MacDiarmid
shared the 2000 Nobel Prize in Chemistry with
Alan J. Heeker of the University of California
at Santa Barbara and Hideki Shirakawa, University
of Tsukuba, Japan for the discovery and development
of conductive polymers.
Commercially Available Polymers: Properties
& Suppliers
* Polypyrrole-based textiles and fibers
are available from Milliken Research Corporation.
They can be made with different surface resistivity
values based on the application. A conductivity
gradient can also be obtained on the same fabric
for radar dissipation applications.
* Poly (ethylene-dioxythiophene) or PEDT is
available from Bayer in different formulations;
in addition to the monomer and oxidant for those
who want to make their polymer, Bayer sells
water-based dispersions that contain PEDT doped
with polystyrene sulfolnate (PEDT/PSS) in two
different grades. The standard grade and the
electronic grade. An additional formulation
containing PEDT/PSS has a urethane component.
* In addition to the use of PEDT/PSS on photographic
films, Agfa has developed ORGACON EL, a PEDT
foil for use as an electrode in electroluminescent
lamps (ACPLED), as an alternative to ITO foils.
This material is commercially available from
Agfa.
* Polyaniline powders and aqueous and non-aqueous
dispersions as well as solid dispersions are
available from Ormecon Chemie
* Polyaniline - and Polypyrrole-coated carbon
powders are available from eeonyx Corp. Compounds
based on these powders can also be available
from the RTP Company.
* Polypyrrole dispersions having different core
materials are available from DSM
* Polypyrrole-coated fibers for Antistatic applications
are available from Sterling Fibers Inc.
* GeoTech Chemical Co., LLC, has developed new
anti-corrosion products. Poly (phenylene vinylene)-based
polymers for light emission applications are
available from the Frankfurt-based Covion, a
joint venture between Aventis and Avecia. Some
of its clients include Philips and Uniax (purchased
by DuPont).
Technological & Commercial Applications
of Conductive Polymers
Based on the available published information,
a few important technological and commercial
applications are discussed in this section.
Battery Applications:
Chemically, the plastic battery is different
from conventional metal-based rechargeable batteries
in which material from one plate migrates to
another plate and back in a reversible chemical
reaction. In a conducting plastic battery, only
the stored icons of the solution move, the plates
are not consumed and reconstituted. Since conventional
battery life is limited by the number of times
the plates can be reconstituted, this difference
portends a longer recharge-cycle lifetime for
the plastic batteries.Probably, the most significant
commercialization of conductive polymers was
for flexible, long-lived batteries that were
produced in quantity by Bridgestone Corp., and
Seiko Co., in Japan and by BASF/Varta in Germany.
One potential application for polymer batteries
is in battery-powered automobiles. Two key measures
of a battery's suitability for automotive application
are the power density (which determines acceleration
and hill climbing ability) and the energy density
(which determines the number of miles that can
be driven between charges).
Polyacetylene's power density is 12 times that
of ordinary lead acid batteries. Its energy
density is also higher - about 50 wh/kg Vs 35
for lead acid batteries. Although plastic batteries
are competing against other advanced development
batteries with similar capability for this application,
they have the unique potential to be made of
low-cost environmentally benign materials. Supporters
of this technology feel that a polymer battery
can be part of the battery-powered car of the
future.
Companies are testing new shapes and configurations
including flat batteries, which can be bent,
like cardboard. Researchers feel that the new
technology will free electronic designers from
many of the constraints imposed by metal batteries.
Conductive Polymers in Photography:
Engineers at the well-known photographic
firm AGFA, Germany were facing a critical problem
with the production of photofilm in the late
80s. Static discharges were ruining the huge,
costly rolls of the company's film; induced
by friction; the little electric sparks generated
huge losses. The engineers' investigation showed
that the inorganic salts AGFA traditionally
used as an Antistatic coating failed to work
when the humidity dropped below 50%. These water-soluble
ionic compounds also washed away after developing,
again leaving the photofilm vulnerable to stray
sparks.
AGFA turned to parent company BAYER A.G., to
see whether its central research arm could develop
a new low cost Antistatic agent. The Antistatic
coating had to operate independent of air humidity,
surface resistance greater than 108-ohm square,
had to be transparent and free of heavy metals
and had to be produced from a water-borne solution.
Following a thorough development effort involving
the selection of the ideal polythiophene derivative,
its subsequent synthesis and its polymerization,
the BAYER research team succeeded in inventing
an aqueous processing route for plastic coating.
Today, more that 10,000 square meters of AGFA
photographic film has been coated with the conducting
polymer, polythiophene.
Conductive Polymers in Display Devices
Polymer Light-Emitting Diodes for Backlights
& Displays:
A polymer Light-Emitting Diode (LED) is a thin
light source in which a polymer is used as the
emissive material. The LED’s are attractive
for a host of consumer applications as they
operate at a low bias voltage. They enable large
area devices to be fabricated inexpensively.
Products, which are currently being developed,
are small emissive displays and Backlights for
small Liquid-Crystal Displays (LCD’s).
Phillips Research, the inventor in this field
had demonstrated the backlight for a mobile
phone LCD as an example of one of tomorrow's
devices.
The simplest polymer LED consists of a polymer
layer, which is sandwiched between two electrodes.
The bottom electrode (anode) is a thin indium-tin
oxide (ITO) layer that is deposited onto a glass
substrate. A vacuum-deposited metal electrode
serves as the top electrode (cathode).
From Green to Orange
A soluble derivative of the poly-phenylene-vinylene
polymer is used as the emissive material. The
polymer is spin-coated onto the ITO, which allows
the fabrication of large-area devices. By changing
the chemical structure of the polymer, the emission
colour of the device can be varied from green
to orange-red. The yellow-orange backlight,
currently in use by Phillips Research, requires
3.5 V & 5 mA/cm2 to achieve the high brightness
of a computer screen, i.e., and 100 cd/m2. The
lifetimes of these devices (measuring 8 cm2)
are typically 30,000 hrs. It has to be noted
that the lifetime of a device depends greatly
on its operating brightness and its size.
New Plastic Circuits
Phillips Research is working on a new
technology, in which polymers can replace silicon
in contact less, readable bar code labels. The
resulting IC's are lower in cost when compared
with their silicon counterparts and as they
still operate when the foils are sharply bent,
they are ideally suited for integration into
product wrappings for soft packages. Only a
limited number of process steps are needed to
produce these low-cost disposable identification
devices. Phillips Research has already demonstrated
the all-polymer approach by manufacturing prototypes
of a complete radio frequency (RF) identification
tag with programmable code generator and anti-theft
sticker.
Semi-conductive polymers have been previously
used as the active component in Metal-Insulator-Semi-
conductor, Field Effect Transistors (MISFETs).
In this technology, developed by Phillips Research,
this is the first time the conductive and insulating
parts of the transistor have also been made
from polymers.
The substrate used in the new all-polymer process
is a polyimide foil with a conducting polyaniline
layer containing a photo initiator. This layer
is exposed to deep-UV light to create the shaping
of interconnects and electrodes. The process
reduces the conducting polyaniline to non-conducting
leucomeraldine. A 50mm semi-conducting layer
of polythienylene-vinylene is then applied by
spin coating and converted at an elevated temperature,
using a catalyst. A polyvinylphenol spin coated
layer is used as gate dielectric and as insulation
for the second layers of interconnect. This
interconnect is created in the top polyaniline
layer using a second mask. Vertical interconnects
(vias) needed to link transistors in logic circuits
are made by punching-through overlapping contact
pads in bottom and top layers using a mask as
guidance. Stack integrity is assured and the
process does not imply a temperature hierarchy.
Logic functionality comprises a programmable
code generator, which produces a data stream
of 15 bits at 30 bits per second. The generator
is 27 sq., 326-transistor, 300-via circuits
with on-board clock. It has been combined with
a proprietary anti-theft device, which enables
the label to be interrogated from a distance.
Future research is aimed at reducing cost and
increasing the bit-rate by improving the charge
carrier mobility of the semi-conductor and by
scaling-down the lateral dimensions.
Conducting Polymers in Sensors
The chemical properties of conducting polymers
make them very useful for use in sensors. This
utilizes the ability of such materials to change
their electrical properties during reaction
with various redox agents (dopants) or via their
instability to moisture and heat. An example
of this is the development of gas sensors. It
has been shown that Polypyrrole behaves as a
quasi 'p' type material. Its resistance increases
in the presence of a reducing gas such as ammonia
and decreases in the presence of an oxidizing
gas such as nitrogen dioxide. The gases cause
a change in the near surface charge carrier
(here electron holes) density by reacting with
surface adsorbed oxygen ions. Another type of
sensor developed is a "biosensor".
This utilizes the ability of triiodide to oxidize
polyacetylene as a means to measure glucose
concentration. Glucose is oxidized with oxygen
with the help of glucose oxidase. This produces
hydrogen peroxide, which oxidizes iodide ions
to triiodide ions. Hence, conductivity is proportional
to the peroxide concentration, which is proportional
to the glucose concentration.
Conducting Polymers inside the Human
Body
Due to the biocompatability of some
conducting polymers, they may be used to transport
small electric signals through the body, i.e.,
act as "artificial nerves". Perhaps,
modifications to the brain might eventually
be contemplated. The use of polymers with electroactive
reaction has led to their use to emulate biological
muscles with high toughness, large actuation
strain, and inherent vibration damping. This
similarity gained them the name "Artificial
Muscles" and offers the potential of developing
biologically inspired robots.
Conductive Polymers in Aircraft Industry
Weight is at a premium for aircraft
and spacecraft. The use of polymers with density
of about 1-g cc-1 rather than 10 g cm-1 for
metals is attractive. Moreover, the power ratio
of the internal combustion engine is about 676.6
W/kg. This compares to 33.8 W/kg for a battery-electric
motor combination. A drop in magnitude of weight
could give similar ratios to the internal combustion
engine. Modern planes are often made with lightweight
composites. This makes them vulnerable to damage
from lightning bolts. Coating aircraft with
a conducting polymer can direct the electricity
directed away from the vulnerable internals
of the aircraft.
Polypyrrole has been approved for use in the
U.S. Navy's A-12 stealth attack carrier aircraft
for use in edge card components that dissipate
incoming radar energy by conducting electric
charge across a gradient of increasing resistance
that the plastic material produces.
Antistatic Fabrics
Another promising product incorporating
conducting polymers is ContexÒ, a fibre
that has been manufactured by Milliken &
Co., in Spartanburg. The fibre is coated with
a conductive polymer material called Polypyrrole
and can be woven to create an Antistatic fabric.
Milliken is interested in using this technology
for its carpet products. Milliken also attempted
to market ultralight camouflage netting based
on Contex to help conceal military equipment
and personnel from near infrared and radar detection.
Antistatic fabrics are also being explored for
possible application in clean room applications.
Conductive Polymers for Medical Applications
Suitable for a variety of applications,
conductive thermoplastic compounds can satisfy
the medical industry's need for miniaturized,
high-strength parts. Most can withstand state-of-the-art
sterilization procedures, including autoclave
and many are certified for purity and pre-tested
to minimize ionic contamination. Medical applications
using conductive thermoplastics include:
§ Bodies for asthma inhalers. Because the
proper dose of asthma medications is critical
to relief, any static "capture" of
the fine particulate drugs can affect recovery
from a spasm.
§ Airway or breathing tubes and structures.
A flow of gases creates tirboelectric charge
or decay. A buildup of such charges could cause
an explosion in high-oxygen atmospheres.
§ Antistatic surfaces, containers, packaging
to eliminate dust attraction in pharmaceutical
manufacturing.
§ ESD housings to provide Faraday cage
isolation for electronic components in monitors
and diagnostic equipment.
§ ECG electrodes manufactured from highly
conductive materials. These are x-ray transparent
and can reduce costs compared with metal components.
§ High thermal transfer and microwave absorbing
materials used in warming fluids.
Dust Relief with ICP (Corrosion Control)
Through an exclusive license agreement
by NASA's Kennedy Space Center, GeoTech Chemical
Co., LLC of Tallmadge, OH, has a coating additive
containing the inherently conductive polymer
(ICP) (LignoPANI) TM as a key component of the
company's new corrosion control product line.
This product, known as CatizeTM is the subject
of this first expose on the New World of ICPs
and their use in corrosion-control systems.
The product is currently undergoing commercial
scale-up, and will be available later this year
for solvent and waterborne formulations, providing
coatings thus formulated with the best behavioral
characteristics of both barrier and cathodic
protection, according to GeoTech.
GeoTech's patented corrosion control system
employs the use of Lignosulfonic Acid Doped
Polyaniline (LignoPANI) TM, an ICP also referred
to as a synthetic metal. CatizeTM a polymer
/ metal powder dispersion will be available
to the coatings industry as formulation additive.
A US local polymer supplier with the participation
of an Aluminum pigment supplier, who provides
the metal particle used as a cathodic element
in the formulation is executing the production
of Ligno-PANI. Much along the same lines is
application of conductive adhesives and inks.
Futuristic Applications
One of the most futuristic applications
for conducting polymers is 'smart' structure.
These are items, which alter themselves to make
themselves better. An example is a golf club,
which adapt in real time to persons' tendency
to slice or undercut their shots. A more realizable
application is vibration control. Smart skis
have recently been developed which do not vibrate
during skiing. This is achieved by using the
force of the vibration. Other applications of
smart structures include active suspension systems
on cars, trucks and bridges; damage assessment
on boats; automatic damping of buildings and
programmable floors for robotics.
Business Forecasts
Conductive polymers (resins and additives)
demand in the US is forecast to increase six
percent per annum to 490 million pounds in the
year 2002 valued at $1.2 billion. Growth factors
include the increased sensitivity and power
of electronic devices, more stringent regulation
of electronic noise, rising raw material costs
and continued electronics diffusion, especially
in higher-end products. ABS, PVC, polyphenylele-based
resins, Polycarbonate and polyethylene resins
account for 70 percent of all conductive polymers
used. ABS will remain the leading resin based
on the material's high impact strength. However,
PVC will present the best opportunities because
of the resin's lower cost and design and processing
ease. Polyphenylele-based resins such as polyphenylene
sulfide will increasingly be used wherein high
temperature and chemical resistance are required,
such as in motor vehicle components. Conductive
polymers are primarily produced by compounding
a base resin with a conductive additive such
as carbon black, metallic fibers / flakes chemical
Antistatic agents. Carbon powders are generally
used only for ESD protection, while fibers are
used for both EMI and RFI shielding and static
control. Best opportunities are anticipated
for fiber-filled conductive polymers because
of performance advantages as well as surface
quality and processing improvements. Product
components will provide the best market opportunities
for conductive polymers due to widespread applications
in housings and enclosures based on advantages
over metal in weight, cost and design features.
Demand will be stimulated by the high levels
of static electricity developed by moving parts
and needs to control EMI/RFI emissions. Antistatic
packaging will exhibit good growth based on
its cost effectiveness in protecting sensitive
electronic devices from static discharges during
all stages of handling, worksurfaces and flooring.
In a move to establish a leading position in
the global market for conductive polymers, DuPont
has signed an agreement with Ormecon Chemie
of Ammersbek, Germany. DuPont will market Ormecon's
polyaniline-based products including anticorrosion
coatings and printed circuit board surface finishes
on a global basis. Polyaniline is an "organic
metal and application areas such as anticorrosion
coatings and printed circuit board finishes
represent a $9-15 billion market.
Ormecon's anticorrosion products, including
an extensive family of products marketed in
Europe, represent a performance breakthrough
in the use of inherently conducting polymers
for corrosion-protection coatings. They provide
superior anticorrosion properties in environmentally
sound, cost-effect compositions. Examples of
successful applications range from railroad
bridges, wastewater treatment system components,
and chemical plant structures, to pipelines,
ocean-going container vessels, and public constructions.
As part of the agreement, Ormecon will carry
out research and development aimed at commercializing
their concepts for even more advanced anticorrosion
coatings and higher-conductivity polyaniline
for electrical shielding applications. DuPont
will also market these future products. The
agreement also provides DuPont a license of
Ormecon's patents covering many applications
and processes for the use of polyaniline-based
products.
They are excited to begin this marketing and
R&D relationship with Ormecon and believe
that Ormecon's existing products open new market
application areas that are not addressed by
other current products. They are also looking
forward to the products that we expect from
the planned R&D and the many new applications
for conductive polymers they will generate.
The relationship with DuPont, a major global
company, firmly validates the more that 20 years
of R&D that Ormecon has invested in developing
their existing products, and positions them
well for the future.
(Dr. Y.B. Vasudeo & Dr. R. Rangaprasad
Product Application & Research Centre, Reliance
Ind Ltd.)
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