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Conducting Polymers
 

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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