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Fluorosilicone and conductive silicones.

Fluorosilicone and conductive silicones

Fluorosilicone and conductive silicone elastomers play unique roles in the very demanding specialty applications of today's high-tech market. Both materials have been used over the past few years in various forms such as high consistency elastomers, sealants and dispersions, and in a variety of applications including all types of gaskets, connectors and cable jacketing. The increasing popularity of these materials is due in part to the inherent silicone advantages each can offer. These include:

* flexibility over organic materials

* stability of the material over a wide temperature range

* oxidation resistance

* low compression set

* biocompatability in a world that is becoming increasingly concerned with the health and environmental impact of today's products.

In addition to these "inherent" silicone advantages, fluorosilicones provide excellent resistance to fuels, oils and most solvents and have superior thermal properties (figure 1). Conductive silicone elastomers can be formulated over a broad conductivity range, have excellent thermal stability and can be formulated with any of the existing silicone polymers such as fluoropolymers for fuel resistance or phenyl polymers to meet low temperature requirements. New and tougher requirements in key areas have extended many existing fluoro and conductive products to their limits. The purpose of this article is to review the new fluorosilicone and conductive elastomer technology and products developed to meet today's application criteria.

New Development in flurosilicone elastomers

Fluorosilicone products have been used in specialty, critical need areas for a number of years. Today, new and tougher requirements in the aerospace industry (specifically in the area of fuel containment and high temperature stability) and in the electronic industry (where dramatic growth in new application areas, and complex new systems, have exposed these materials to new substrates, harsher environments and higher temperatures) have placed a renewed emphasis on the development of new fluorosilicone products.

New fluorosilicone development has been directed at four key areas:

Increased thermal stability. It is generally recognized that fluorosilicone elastomers will give long lasting dependability in both static and dynamic applications over a wide temperature range, -68 [degrees] C - 232 [degrees] C. In addition, the properties of fluorosilicone materials at high temperatures can be better than other materials which have better properties at room temperature. The long term thermal stability as well as the short term high temperature stability of fluorosilicone elastomers can be significantly improved by compounding materials with new fillers and other additives (figure 2). In dynamic applications, maintaining physical properties over a broad range of service conditions is critical to product performance (figure 3 and 4). In one case an experimental fluorosilicone compound was heat aged for 24 hours at 265 [degrees] C. The experimental material was still soft and flexible and retained 80% of its original properties, while the standard material embrittled and showed polymer reversion.

Improved solvent resistance. New fluorosilicone compounds and copolymers are being developed with improved resistance to the traditional materials, such as fuels, nonpolar solvents and hydraulic fluids, and with reduced swelling in materials such as ketones and esters.

Ultra high strength. New materials with tensile strengths approaching 2000 psi could significantly impact the use of these compounds in key aerospace applications.

New fluoro specialty products. The introduction of new fluoro specialty products such as conductive fluorosilicones, fluoro adhesives and conductive-fluoro adhesives will significantly increase the new market opportunities for fluorosilicone elastomers.

The new fluorosilicone elastomer technology is really a combination of a number of different aspects, all of which affect the ultimate performance of the material. It is a blend of new and old compound and formulation technology, as well as an increased understanding of key processing parameters on product performance.

For many years the compounding and catalyzing of materials, fluorosilicone elastomers included, was not recognized as having much if any effect on the properties of the final product. Today, however, new process studies have shown that key areas of the process, including choice of manufacturing equipment, the amount of shear encountered, the order and length of time at various stages in the compounding, the effect and method of catalyzing and the straining/purity of the final material will significantly alter the property profile of the product. Each of these process variables can be used to affect the material in different ways, depending on which properties are most important for the application in question. One example of how process variables can affect properties is shown in figure 6 where different shear levels have resulted in different physical property profiles with the same formulation. Another area where old technology has been improved, at least in part, because of new process understanding, is the use of increased filler levels and the better use of plasticizers.

New technology currently being used in fluorosilicone development activities includes:

* new gums and polymers, as well as copolymers and blends, that can offer unique combinations of physical and thermal properties in addition to improved fuel resistance and thermal and electrical conductivity

* new thermal stability additives

* new fillers including higher surface area fillers and new treated fillers.

Because of these options the new fluorosilicone materials will truly be a specialty product. In many cases they will be a custom formulated material that will take advantage of every available formulation and processing trick to achieve the ultimate product (table 1).

Conductive silicone elastomers

One of the fastest growing specialty product lines today is that of conductive elastomers. A proliferation of electronic devices and new regulations affecting both the consumer and industrial markets have generated the need for a variety of conductive materials. In many cases, the success of these materials and their use in specific applications is dictated as much by other product requirements and properties as it is by the conductivity of the material itself. Some properties impacting the product development include thermal stability requirements, non-metallic requirements, adhesive properties, elasticity over a broad temperature range and compression set.

Generally, conductive elastomers fall into one of three categories. These are:

* silver filled materials with volume resistivities ranging from [10.sup.-3] to [10.sup.-4] ohm-cm

* other metallic filled materials with volume resistivities between [10.sup.-3] and [10.sup.-1] ohm-cm

* non-metallic, carbon filled materials [is greater than] 0.5 ohm-cm resistivity.

Applications for conductive elastomers

There are two major application areas for conductive elastomers; namely, EMI/RF shielding and electrical/electronic applications. In both of these applications, new FCC regulations for commercial as well as consumer devices have significantly reduced the level of interference allowed. This electromagnetic interference has been shown to jam communication systems, cause navigation errors, erase computer memories and affect medical devices. The most common form of these shielding devices is the conductive gasket that provides continuity of a metallic housing at openings, corners, edges, etc. It is easy to see how these types of requirements dictate the need for a flexible, deformable conductive elastomer.

In the area of electrical/electronic applications there are a variety of different requirements that must be met. One of the most important is the dissipation of static charges. While high conductivity of the material is not necessary (volume resistivity [is greater than] 500 ohm-cm is usually sufficient) this is the largest application area for conductive materials. It is used in hospital flooring, personal comfort and in belts, hoses, mines and other environments where explosive vapors can accumulate. The next largest application area is wire and cable jacketing where conductive materials are used to eliminate voids between the conductive core and insulation, reducing arching and high electrical stress. A relatively new application in the electrical area is the use of carbon filled materials as self-regulating resistors and heating elements. Many medical applications can benefit from this type of material because of the flexibility and the elimination of electrical shock hazards. In the electronic area, new electrochemical applications, including printed wire boards, battery components, electro-plating on plastics and electroplateable resins, are being pursued.

In addition, new materials are being developed as electrical contacts, zebra connectors and as die attach adhesives, replacing the traditional rigid epoxies that can cause chip and board cracking. In the future, silver filled materials may even replace solder in some circuit board applications.

In the past, many of these application areas required the use of silver and/or other metallic filled elastomers that were extremely expensive and had poor physical properties. Today, new advances in carbon filled technology are meeting many of these critical needs at a lower cost and with better physical property performance.

A variety of problems have plagued the use as well as the manufacturing of conductive elastomers. Sensitivity of the conductivity to handling and processing caused lot-to-lot variability. The thermal instability of the materials, especially non-reproducible changes in conductivity with temperature and other environmental changes, have created serious problems with these products in the field. In addition, raw material variability and testing inconsistencies contributed to the poor reputation of conductive elastomers. New technology has eliminated many of these problems and expanded the potential application areas for conductive silicone elastomers.

New conductive technology

New developments in conductive silicone elastomers include advances in the areas of processing, formulation and filler technology. Key to this development has been an increased understanding of the effects and interactions of the fillers and processing variables. Some key aspects that can increase conductivity and improve lot-to-lot variability are:

* decrease filler particle size

* improve filler packing efficiency [is greater than] 100A

* increase the filler surface area

* increase the structure and order of the fillers

* increase the particle size distribution

* reduce surface contaminants that reduce conductivity

* control processing/shear effects and orientation of the particles.

In addition, new polymer systems have been developed and new, improved fillers, incorporating many of the ideas discussed previously, have appeared on the market over the last two years. In response to different organizations and industrial needs, new uniform test methods for semi-conducting materials (ASTM D4496) have been developed.

One of the most dramatic effects of the new conductive technology is the improvement in carbon filled conductivities. In the past, typical carbon filled materials contained 10 - 40% carbon filler to obtain volume resistivities between 10 - [10.sup.6] ohm-cm. At the high carbon loadings, the physical properties were very poor and the handling/processibility of the materials was unacceptable. Today, carbon loadings have been significantly reduced, and at the same time the conductivities have dramatically increased. As the viscosity and/or plasticity of these materials has decreased, the processibility has improved and reinforcing fillers have been added to improve physical properties. In addition, these new materials are less affected by processing variations and environmental changes.

In the areas of silver, silver coated and other metallic fillers, new processing and filler treatments have significantly reduced lot-to-lot variability, improved physical properties, and improved stability - especially thermal stability. The development of complex new circuit boards has sparked the development of new silver filled screen printable adhesives to replace solder in some applications. New filler treatment technology has also revived interest in other metallic fillers, such as aluminum, nickel and copper; and new developments with these and other fillers are being evaluated in many products.

Like the fluorosilicone elastomers, the success of this product line will depend on the flexibility of these materials. Many specialty materials, some with only very small volume, optimized for specific applications will be needed. The ability of the conductive silicone suppliers to respond to this need will have a dramatic effect on the growth of this product line (table 2).

Summary In summary, the improved performance of the new fluorosilicone elastomers and the new conductive elastomers is a combination of a number of factors, including improvements in the use of existing materials and formulations, a better understanding of process variables on product performance and the development of new technology. Equally important, however, is the development of a new philosophy regarding specialty materials, like conductive and fluorosilicone elastomers. It is a philosophy that uses all the new understanding and technology to optimize a material for a particular application without trying to fit a material over a broad application area. [Table 1 and 2 Omitted] [Figure 1, 2, 3, 4, and 6 Omitted]

References [1]"Fabricating with Silastic Silicone Rubber," Dow Corning Corp., 1979. [2]T. Maxson, "Fluorosilicone Chemistry, Compounding, Properties," 1987 Energy Group Education Symposium, Houston, TX, September 1987. [3]"Electrically Conductive Polymers," Center for Professional Advancement, Somerset, NJ, May 1988.
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Author:Kroupa, Laurie
Publication:Rubber World
Date:Jun 1, 1989
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