Stock material or miscellaneous articles – Self-sustaining carbon mass or layer with impregnant or...
Reexamination Certificate
1999-02-04
2001-07-10
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
Self-sustaining carbon mass or layer with impregnant or...
C428S216000, C428S218000, C428S408000, C277S938000, C277S936000, C277S944000, C277S946000
Reexamination Certificate
active
06258457
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a multilayer sheet having high compressive strength, high heat resistance and low permeability relative to fluids, including layers of graphite foils and metal foils alternatingly disposed one upon another and parallel, with the graphite foils joined to the metal foils.
Such multilayer sheets are used, in particular, in sealing technology and for impermeable linings. U.S. Pat. No. 5,128,209 describes a gasket material being formed of layers of a fluoropolymer, graphite foils and metal foils, with the layers being bonded together by an adhesive. The layers of the fluoropolymer are made of material which is porous and therefore permeable to fluids. They impart a greater stability, toughness, tensile strength and an improved handling to that component of the composite being formed of the graphite foils and the fluoropolymer foils. The disadvantages of that gasket material are firstly the permeability of the fluoropolymer foils to fluids and the presence of adhesives as bonding agents between the layers. The fluoropolymer foils contribute virtually nothing to the impermeability of the overall system, which has an adverse effect especially in the case of gaskets for high and extreme impermeability requirements. The adhesives are a weak point, especially in the case of gaskets subjected to high pressures or stresses, because the layers of the multilayer sheet can slide on them. That ultimately leads to the non-operability of the gasket or, in particular in co-operation with elevated temperatures, to fine cracks occurring in the adhesive layer which impair the sealing effect.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a metal-reinforced graphite multilayer sheet, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known products of this general type, which is to be used for sealing purposes and which has an improved impermeability and an improved compressive strength with a long-term temperature stability of at least 150° C.
With the foregoing and other objects in view there is provided, in accordance with the invention, a multilayer sheet having high compressive strength, high heat resistance and low permeability relative to fluids, comprising alternating parallel layers of at least one graphite foil and metal foils disposed one upon the other and joined to one another; the layers of metal foils including an upper boundary layer and a lower boundary layer each having a flat outwardly-directed surface; and two gas-tight foils made of an organic polymer having a long-term temperature stability of at least 150° C., the gas-tight foils each completely covering and joined to a respective one of the flat outwardly-directed surfaces of the metal foils of the upper and lower boundary layers.
The mode of operation of this multilayer sheet as a gasket material is as follows: The graphite foil or the graphite foils, which has or have an excellent stability at high temperatures, impart to the multilayer sheet a high compressibility, a high spring back capacity and an extremely low tendency to creep under pressure. The likewise temperature-stable metal foils do not creep under pressure and confer a high compressive strength on the multilayer sheet. They form an additional impermeable barrier for fluids. However, they cannot fit tightly to the unevennesses of surfaces against which they lie such as, for example, surfaces of flanges, through the use of which the pressure necessary to achieve a sealing action is to be transferred to the multilayer sheet or to a gasket produced from the sheet. The function of fitting is taken over by the foils made of an organic polymer which form the external boundaries of the multilayer sheet. Due to good flowability under pressure, they adapt very well to unevenness and damage present on such surfaces and thus provide excellent sealing. If they are used in a sufficiently low thickness, their tendency to creep, which per se is disadvantageous for gaskets, and their low spring back capacity, are of no consequence. That is because the adverse effects caused thereby are more than compensated for by the other components of the multilayer sheet, in particular by the graphite components.
The joinings between the different layers of the multilayer sheet can be produced by a suitable adhesive, which must have a long-term temperature stability of at least 150° C.
However, in accordance with another feature of the invention, joinings which are completely free of adhesives are preferred, between the graphite foils and the metal foils as well as between the metal foils and the polymer foils.
For the purpose of this invention, the term “graphite foils” means both graphite foils and graphite laminates obtained by known processes by the compression or calendering of expanded, so-called vermicular graphite. Expanded graphite is produced by the sudden decomposition of graphite salts, such as graphite hydrogen sulfate, at elevated temperatures. Vermicular graphite can be produced, for example, by a process disclosed in U.S. Pat. No. 4,091,083.
In accordance with a further feature of the invention, the graphite foils used for the production of multilayer sheets according to the invention and included in these multilayer sheets have a bulk density within the range of 0.1 g/cm
3
to 1.8 g/cm
3
. They have a carbon content preferably within the range of 90 to 99.95 weight per cent. Their thickness is within the range of 0.1 mm to 4 mm.
In accordance with an added feature of the invention, the multilayer sheet is formed of a centrally disposed graphite foil having two flat surfaces each of which is joined to a metal foil, and the two outwardly-directed surfaces of the metal foils are covered with and joined to a polymer foil.
In accordance with another additional feature of the invention, the multilayer sheet is formed of a metal foil which is disposed in the center of the multilayer sheet and has a graphite foil joined to each of its two flat surfaces. The two free flat outwardly-directed surfaces of the graphite foils are each joined to a metal foil and the two free flat outwardly-directed surfaces of the metal foils are joined to a polymer foil.
In accordance with yet another feature of the invention one, or more than one, of the metal foils situated in the interior of the multilayer sheet, which are not joined to polymer foils, can be shaped in the form of a tanged metal sheet, and in this case its tines can be disposed on only one side or on both sides of the tanged metal sheet.
In accordance with yet a further feature of the invention, the metal foils can be joined to the polymer foils by any of the known processes. Preferably, however, joining will be done without the use of adhesives. In the case of the polymer foils which are suitable for this invention, solvent-free joining of the metal foils to the polymer foils by heat welding has proved to be best, and this can be successfully used even for polytetrafluoroethylene foils. This welding is advantageously carried out by additional application of pressure, for which both stamping presses and roll presses can be used. During the welding it is advantageous if the metal foils and the polymer foils have a temperature within the melting range of the polymer foils. In some cases it is advantageous that the metal foils have a temperature which is within the upper region of the melting range of the polymer foils and the polymer foils have a temperature below this range.
The joining of the surfaces of the graphite foils to those of the metal foils can be carried out through the use of adhesives or by pressing the surfaces of the tanged metal sheets inducing anchoring into the graphite foils without the use of adhesives. However, adhesives have the disadvantages described at the outset and tanged metal sheets are preferably used only in the center of the multilayer sheets. The preferred methods for joining the surfaces of the graphite foils to those of the metal foils do not use adhesives. In one method,
Mechen Silvia
Öttinger Oswin
Römmler Mike
Greenberg Laurence A.
Jones Deborah
Lerner Herbert L.
Miranda Lymarie
SGL Technik GmbH
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