An MD&DI July 1999 Column
The inherent properties of electrically conductive films have increased their importance as components in today's smaller, lighter products.
As the 21st century approaches, the emergence of a variety of new portable electronic devices continues at a rapid pace. These devices are being employed in the manufacturing, communications, and transportation sectors, in the home and office, and in education. Examples of such products include the now ubiquitous cellular phone, pagers, laptop computers, and digital cameras. These portable devices and others are also useful in medical applications that include patient monitoring, diagnosis and treatment, and emergency calling.
Devices using conductive films include medical electrodes. Photo courtesy of Vermont Medical Electrodes (Bellows Falls, VT).
Although the configuration and function of new, yet-to-be-released electronic devices remain uncertain, a number of trends have clearly evolved. These include increased computational power in smaller, thinner, lighter packages; enhanced resolution in digital photography; smaller size in camcorders; and improved ergonomics in all devices. Additional significant trends are portability and heightened environmental awareness, including all aspects of disposability and recycling.
Following these trends, components designed for use in these devices will be even thinner, lighter, smaller, and more environmentally friendly in the future. In addition, the manufacturing practices used to fabricate such components will require improved standards of precision, cleanliness, and environmental responsibility.
Conductive films, which are made by dispersing electrically conductive carbon black into a polymer matrix, are already filling an important role in the fabrication of precision components in today's electronic devices. The use of these conductive films is likely to accelerate in the future because such films are inherently thin, relatively inert, and lightweight. The films have a large apparent and actual surface area available for diverse functions, such as acting as a substrate for electrochemical reactions, conductivity, or charge storage. Moreover, there exists a wide range of particle/polymer compositions that can be designed as laminates or coatings with the conductive carbon film substrate. These particle/polymer compositions, when dispersed in a film former and applied to a carbon film substrate, can absorb specific types of radiation, accept and store electric charge, form conductive paths for electrons, enter into electrochemical reactions (as anodes or cathodes in batteries), or even participate in more complicated biochemical reactions such as glucose sensing.
Thin, precisely coated or printed dispersions of particles are already being used in electrochemical communications, medical, and military applications, including:
- Lithium ion and lithium polymer batteries, which employ printed electrodes and current collectors.
- EMI/RFI shielding.
- Static dissipation.
- Digital white boards.
- Iontophoresis electrodes.
- Defibrillation electrodes.
- ECG electrodes.
- TENS electrodes.
One particularly useful type of conductive film is composed of electrically conductive carbon-black particles dispersed in a nonconductive polymeric matrix in a manner whereby the carbon-black particle structure, concentration, and degree of dispersion permits interparticle contact of the carbon particles. Such films exhibit bulk conductivity and are conductive along the x, y plane as well as in the transverse or z direction. Depending on the type of polymeric matrix used, they can be made flexible and tough enough to be processed as a freestanding film in sheet or roll form.
Although dry carbon black has a resistivity of about 10–2/square, carbon black in the dry form has limited utility and must be combined with a film former to achieve useful films. The practical lower limit for resistivity in carbon/polymer mixtures is about 0.75-cm. Other conductive fillers could be used alone or in combination with carbon (or graphite); however, the advantage of carbon and graphite is that they are relatively inert in the presence of many aqueous and organic electrolytes and form a chemically stable interface in contact with metals, metal salts, electrolytes, and ionically conductive hydrogels. This is not true of particulate metallic fillers such as iron, copper, or aluminum.
Because a preferred method of making conductive carbon films is to cast them from solvent or aqueous dispersions, it is possible to use a wide variety of polymers in conductive films. The choice of the film-forming polymer depends on the way the conductive film is to be used, including anticipated temperature exposure extremes and any contact with aqueous or organic electrolytes, metals, or salts that might react with, swell, dissolve, or otherwise degrade the polymer. Swelling in contact with electrolyte is particularly important to avoid since this decreases the conductive carbon volume relative to the polymer and can dramatically increase the resistance of the conductive film as well as change its physical properties.
Once formed, conductive films can be supplied in sheet or roll form either as free-standing films or on a release support. In either case, they are then available for a number of postprocessing manipulations, which can either enhance the conductivity of the carbon film, insulate it from other layers in a composite structure, or add layers to the carbon film that are electrochemically active. These process steps can include:
- Lamination to a metal foil substrate (e.g., to aluminum, copper, zinc, silver, or tin).
- Lamination to other plastic films or metal foils.
- Coatings of electrochemically active metals (e.g., a dispersion of zinc particles or metal salts).
- Coatings of functional layers (e.g., silver/silver chloride mixtures for ECG electrodes).
- Vacuum deposition or sputtering of metals to the conductive film surface.
Figure 1. Conductive film use in the flat-cell zinc/manganese dioxide battery.
A long-standing application for such films is in the flat-cell zinc/manganese dioxide (MnO2) battery. A cross section of this battery is depicted in Figure 1 and serves to illustrate many of the property requirements of conductive films:
- The conductive film must be rugged enough to hold up to the physical stress of the battery-assembly process without damage.
- The film must have sufficient thermal stability to permit the application of anode and cathode from dispersions of zinc and MnO2, respectively, and to allow for adherence to the aluminum end plates.
- The film cannot have any pinholes that would permit the electrolyte to contact and corrode the aluminum plates or to transfer ions across the (internal) bipolar electrode interface.
- The film must withstand contact with the electrolyte, zinc anode, and MnO2 cathode without corrosion or deterioration.
- The film must be capable of heat-sealing to a nonconductive gasket, which contains the electrolyte and prevents the battery from drying out.
- The film must be sufficiently conductive to conduct electrons to the aluminum collector plates without excessively increasing internal resistance.
An interesting comparison can be made between the use of carbon-based conductive films in the above example and the carbon "pencil" electrode used in older, zinc chloride–type flashlight cells.
The battery construction illustrated in Figure 1 is layered, with cells connected in series. Conductive films, however, have enough flexibility to permit the construction of cells in a "jellyroll," flattened roll, or prismatic format.
Many of the properties described above make conductive films extremely useful in medical applications. For instance, certain physiological phenomena of the body produce electrical potentials on the skin. The electrical potential associated with the physiological functioning of the heart is one example. Conductive films can be used with ionically conducting adhesives in ECG electrodes to make intimate contact with the skin and monitor these electrical potentials (Figure 2).
Figure 2. ECG electrode construction.
In this application, a very thin layer of a particulate mixture of silver and silver chloride (AgAgCl) can be applied to one side of the conductive carbon film. Since many silver/silver chloride inks are solvent based, the conductive film used with these inks must have sufficient solvent resistance to prevent degradation until the solvent is removed by drying. Silver/silver chloride layers are often so thin and porous that they do not completely separate the conductive film from the hydrogels typically used in these constructions. Therefore, the conductive film must not react with the silver/silver chloride layer or corrode in the presence of the hydrogels, which obtain their ionic conductivity through the presence of dissolved salts.
Another medical use of conductive films is in the fabrication of TENS electrodes. The typical construction of such an electrode is depicted in Figure 3. In both the ECG and TENS electrodes, a desirable property of the conductive film is that it be flexible enough to conform easily to the patient's skin. At a given carbon loading, this flexibility is determined by a careful selection of the film-forming polymer or polymers that bind the carbon particles together.
Figure 3. TENS electrode construction.
In the TENS electrode, the more-conductive silver metal layer serves to disperse current uniformly in the x, y direction thereby preventing localized hot spots. Once again, the interfaces between carbon, conductive film, electrically conductive metal layer, and ionically conductive hydrogel are stable, and no corrosion takes place at either interface. Such corrosion could create a very high interfacial resistance that would either limit shelf life or prevent the operation of the device entirely.
PRESSURE-SENSITIVE CONDUCTIVE ADHESIVES
A special but very useful type of conductive film employs a tacky, pressure-sensitive polymer as the binder for the carbon particles. The construction of these films is inherently difficult since very low concentrations of conductive filler must be used in order to prevent the drying out of the polymer by the conductive filler, with attendant loss of tack. In addition, if the conductive filler is too finely dispersed, the particles will be insulated by the nonconductive binder and not even z-direction conductivity will occur.
Figure 4. Design of conductive pressure-sensitive adhesive.
The microstructure of a conductive, pressure-sensitive adhesive is depicted in Figure 4. One property of conductive, pressure-sensitive adhesives made in this fashion is that their z-direction conductivity is to a certain extent dependent on the amount of pressure applied. Another is that, in order to achieve conductivity in the x-y plane, it is necessary to incorporate conductive fibers to bridge conductive-particle aggregates (as in Figure 4b) or to add a conductive film bounded on one or both sides by the conductive adhesive (as in Figure 4c). The conductive film can be replaced by a foil such as copper to provide EMI/RFI shielding of sensitive medical electronics.
Although camcorders, portable computers, and cellular phones are devices more often employed in nonmedical settings, they are finding increased use in the recording and transmission of patient data as well as in therapies such as iontophoresis. Like portable ECG, TENS, and defibrillation units, such devices use battery power for their operation. Another medical use of portable power is in battery-powered wheelchairs and other assist devices.
Figure 5. Volumetric and gravimetric energy densities for various battery types.
Figure 6. Lithium polymer battery construction.
The design trend for all of the above-mentioned devices is toward smaller, thinner, and lighter units, which has created a demand for smaller, thinner, lighter, longer-lasting sources of portable power. Conductive films are being used in the design of lightweight, long-lasting lithium batteries, as well as in electrochemical capacitors and fuel cells. Volumetric and gravimetric energy densities for various battery types are compared in Figure 5, in which the advantages of lithium and lithium ion batteries are apparent. The construction of a lithium polymer battery with a conductive carbon-film current collector is shown in Figure 6.
Figure 7. Electrochemical capacitor construction.
Figure 7 depicts the construction of an electrochemical capacitor. In these devices, the charge is stored as a double layer on a very-high-surface-area carbon. When discharged, current is conducted through the conductive film, which again must be inert in the presence of the particular electrolyte used in the device.
Figure 8. Schematic of conductive film manufacturing process.
The manufacture of conductive films requires a considerable investment in process equipment (Figure 8). The steps in the process are generally as follows:
- Dispersion of the conductive particles is generally done separately to an end point determined by the desired agglomerate size.
- During mixing, a binder is dispersed, or dissolved polymer is added to the dispersion, and the mixture stirred until it becomes homogeneous.
- Filtration is used to remove any large carbon agglomerates, undissolved polymer particles, or foreign matter above a predetermined micron limit (e.g., 2-µm particles).
- Precision coating onto a suitable substrate is performed by the preferred method, which can include solvent extrusion, gravure, or various types of roll coating.
- Drying is employed to remove water and/or solvent from the wet coating.
- Incineration of the removed solvent is carried out to prevent its escape into the atmosphere and to comply with state and local regulations regarding volatile organic compounds (VOCs).
- Winding of the dried coating and substrate into large rolls completes the processing. Postprocessing can include striping, laminating, or additional coatings.
The use of thin, flexible conductive films complements the trend toward compact, lightweight medical electronic devices. The formation of these films by coating is a preferred method of manufacture because of the advantages inherent in the formation of fine dispersions of conductive particles in mixtures of polymer and solvent. Such dispersions can be coated by precision methods to produce homogenous films with very low porosities at precisely defined thicknesses.
The key to the successful design and use of conductive films lies in a thorough understanding of the device architecture and consideration of the chemical and physical environment in which the film will need to perform its electrical conductivity function.
Francis J. Kearney is business manager of the conductive films group at Rexam Graphics (South Hadley, MA).