Stock material or miscellaneous articles – Composite – Of epoxy ether
Reexamination Certificate
2001-12-07
2004-06-08
Dawson, Robert (Department: 1712)
Stock material or miscellaneous articles
Composite
Of epoxy ether
C428S408000, C428S414000, C428S103000
Reexamination Certificate
active
06746771
ABSTRACT:
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The invention relates to a synthetic resin-impregnated body made of expanded or at least partially recompressed expanded graphite, a process for producing such a body and a sealing element, a fuel cell component and a heat-conducting element employing the body. In this context, the term “synthetic resin-impregnated body” is understood to mean a body made of expanded graphite which is impregnated by synthetic resin.
Material composites of graphite and plastics are widely used in many technical applications. For example, particles of electrographite are processed with fluoroplastics into highly corrosion-resistant components for the construction of chemical apparatus, but they are comparatively expensive due to the costs of the fluoroplastics and of the processing technique that is required. A subject which in terms of content is even closer to the present application is set out in U.S. Pat. No. 4,265,952: expanded graphite is mixed, for example, with fine PTFE powder and subsequently compressed. To that extent, the production technique differs from the impregnating technique described in the present application.
Another example of a material composite of graphite and plastics material is superficially resin-impregnated foils made of natural graphite, which are predominantly employed in the form of flat seals against particularly aggressive media. Many references to that second example are found in the technical literature.
Today, thousands of tons of foils made of natural graphite are produced worldwide every year. The process used therefor is described in European Patent Application 0 087 489 A1, U.S. Pat. Nos. 3,404,061 and 3,494,382. Those disclosures are incorporated by reference in the present application. The teaching of those references can be summarized as follows: an intercalating agent such as concentrated sulfuric acid, for example, acts on natural graphite, preferably platelike or flaky natural graphite, in the presence of an oxidizing agent such as concentrated nitric acid or hydrogen peroxide, for example. That results in graphite intercalation compounds that are likewise flaky or have a platelike shape. The flakes are thermally decomposed and, as a result of the gas pressure arising in their interior during that decomposition process, puff up to form loose graphite particles with a wormlike shape by brief heating, for example by introduction into the flame of a gas burner. That product is also referred to as “expanded” graphite or as graphite expandate.
Expanded graphite is extremely plastic and can be readily shaped without the aid of a special binder while being compressed to a greater or lesser degree. Economically, the most important product thus produced is a flexible graphite foil, which can be produced efficiently on calender belts. Such products have typical bulk densities of between 0.7 and 1.3 g/cm
3
. However, other parts having different geometry, for instance individual sealing bodies which, on average, are compressed to a greater degree and have bulk densities of 1.0 to 1.8 g/cm
3
, are also possible. There are also sponge-like parts, on average having low bulk density, with values of 0.1 to 1.0 g/cm
3
. All of those bodies with different shapes and different bulk densities have an open pore system. They are referred to hereinbelow as a “primary product”.
Material composites formed of such a primary product and synthetic resins or plastics materials perform a variety of tasks. Synthetic resins or plastics materials lower the permeability, improve the surface properties, for example the scratch resistance, increase the strength to a small extent, lower the thermal stability of a material composite containing expanded graphite, and can also reduce the electrical conductivity or modify the resistance to media. An expedient technique for the production of such material composites is impregnation.
According to German Patent DE 32 44 595 C2, corresponding to UK Patent Application GB 2 131 500 A, the sticking action of graphite foils to metal surfaces can be reduced by impregnating the primary product with furan resin in regions close to the surface.
According to the prior art, the substantial impregnation of shaped bodies made of expanded and partially recompressed graphite is difficult. In order to overcome the difficulties, International Publication No. WO 99/16141, corresponding to U.S. Pat. No. 6,037,074, teaches that such a body can be satisfactorily impregnated when mineral fibers are interspersed therein. Those fibers also pass through the surface of the particular bodies. In that way, small channels are formed along those mineral fibers, in which the resin can flow into the interior of the bodies during the impregnation. In that specification, a phenolic resin dissolved in acetone, i.e. a solvent-containing thermosetting resin with condensation reactions during the curing, is named as the impregnating agent.
Another method for achieving good impregnation of bodies made of expanded graphite resides in converting the desired resins through the use of solvents into low-viscosity liquids, whereby the impregnation becomes more complete. In Published Japanese Patent Application JP 11 00 040 A2, the thermosetting resins named are based on phenols, epoxides, polyimides, melamines, polyesters and furans, which are used in a mixture solution with polyvinylbutyral.
Published Japanese Patent Application JP 1 308 872 A2 describes the solution to other problems. A material composite formed of a glass fiber nonwoven fabric and an expanded graphite foil is produced in order to thus strengthen the latter and overall obtain a liquid-tight material. That is achieved by impregnating with epoxy resin. The resin penetrates the nonwoven fabric, with the composite material being formed during the subsequent curing of the support part. At the same time, the resin also penetrates into the surface, i.e. partially into the foil, and seals the latter.
The impregnation of expanded graphite foil with phenolic resin or epoxy resin, which is set out in Published Japanese Patent Application JP 60 24 2041 A2, corresponding to German Patent DE 35 12 867 C2, serves similar purposes, namely to improve strength and gas-tightness. The special feature in that case lies in a degassing process for the liquid resins and the foil present therein which is repeated a number of times, presumably with the aim of improving the quality of the impregnation.
German Published, Non-Prosecuted Patent Application DE 43 32 346 A1 describes the impregnation of the expanded graphite foils for the purpose of improving adhesion to elastomer layers lying thereon. The viscosity of the epoxy resins used in that case is 2100 to 2400 mPa·s.
Published Japanese Patent Application JP 11 35 4136 A2 entitled “Fuel Cell, Separator for Fuel Cell, and Manufacture Therefor” describes the production of expanded graphite in sheet-like form. That partially recompressed expanded graphite is subsequently comminuted (pulverized) and then mixed selectively with resins, solvent-free epoxy resin, solid epoxy resin, melamine resin, acrylic resin, phenolic resin, polyamide resin, and the like. That mixture is subsequently shaped. As will be shown later, that technique differs from the bodies according to the present invention which have an entirely different structure in that the resins are mixed into an expanded graphite granulate.
The prior art set out above discloses various synthetic resin-containing bodies produced by using expanded graphite as well as processes for their production. That it is difficult to produce high-quality, synthetic resin-containing graphite bodies from recompressed, expanded graphite is easy to see. All of the processes described have disadvantages, some of which are serious: if resins diluted by solvents and thus having lower viscosity are used during the impregnation, it is true that the impregnation is easier. However, the vapors from the, in most cases, readily volatile solvents cause serious problems during the impregnation itself, especially durin
Bacher Jürgen
Langer Werner
Öttinger Oswin
Dawson Robert
Greenberg Laurence A.
Keehan Christopher
Locher Ralph E.
SGL Carbon AG
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