Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
2000-01-06
2003-08-26
Cain, Edward J. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C524S442000, C524S444000
Reexamination Certificate
active
06610771
ABSTRACT:
The present invention is concerned with gaskets, in particular with gaskets having a sealing layer with enhanced properties which is based upon chemically exfoliated vermiculite.
Exfoliated vermiculite is a known heat-resistant resilient material. Exfoliated vermiculite is conventionally formed by expanding mineral vermiculite using gas, this material being referred to herein as “gas-exfoliated vermiculite”. The gas may be thermally generated, in which case the product is called “thermally-exfoliated vermiculite” (TEV). TEV may be made by flash-heating mineral vermiculite to 750-1000° C., at which temperature the water (free and combined) in the ore vaporises rapidly and the steam generated forces apart the silicate sheets which form the raw material, so bringing about an expansion of 10-20 times perpendicular to the plane of the sheet. The granules formed have a chemical composition which (apart from the loss of water) is virtually identical to that of the raw material. Gas-exfoliated vermiculite may also be made by treating raw vermiculite with a liquid chemical, eg hydrogen peroxide, that penetrates between the silicate sheets and subsequently evolves a gas, eg oxygen, to bring about exfoliation.
A different form of exfoliated vermiculite is known as “chemically-exfoliated vermiculite” (CEV) and is formed by treating the ore and swelling it in water. In one possible preparation method, the ore is treated with saturated sodium chloride solution to exchange magnesium ions for sodium ions, and then with n-butyl ammonium chloride to replace sodium ions with n—C
4
—H
9
NH
3
ions. On washing with water swelling takes place. The swollen material is then subjected to high shear to produce an aqueous suspension of very fine (diameter below 50 &mgr;m) vermiculite particles.
It is known to utilise exfoliated vermiculite as a layer of a sheet gasket, eg an automotive head gasket, and for other purposes. For example, GB 2 193 953 B discloses forming sheet-like gaskets formed from particles of gas-exfoliated vermiculite. Because such particles do not cohere well, they are bound together by fine particles of CEV. The use of CEV as a binder retains heat resistance and resilience, whereas the use of other inorganic binders could result in an incompressible structure. However, although exfoliated vermiculite has excellent heat resistance and a high degree of resilience, it has poor water resistance. Furthermore, such products were manufactured using CEV with a high water content at low solids content and considerable drying problems are encountered during production due to the tendency of CEV containing materials to form a surface which prevents the further escape of moisture.
GB 2 123 034 B describes making a flexible sheet material, eg for a gasket, by subjecting an aqueous suspension to electrophoresis. The suspension contains an expanded layer silicate, eg CEV with a particle size below 50 &mgr;m, and a dispersed organic polymeric material, eg acrylic polymer, acrylonitrile-butadiene copolymer, epoxy resin, or natural rubber. However, high levels of polymer are disclosed to effect sufficient hydrolysis resistance and such levels of polymer give gasketing problems due to a loss of stress retention and gasket creep in use.
A sealing element for a gasket for the exhaust system of an internal combustion engine is disclosed in GB 2 217 742 A. This sealing element comprises relatively coarse particles of TEV (passing a 2 mm sieve) bonded together by fine CEV particles (about 100 &mgr;m in size). This element is stated to disintegrate quickly if exposed to water as the fine CEV particles are readily dispersed in water. In order to improve water-resistance, GB 2 217 742 A proposes bringing the element into contact with a solution of an aluminate or a zirconyl salt. Further improvement is achieved by treatment with a solution of a silicone elastomer. An example is given of impregnation of a sheet (which had already been treated with sodium aluminate) by a 15% solution of silicone elastomer is toluene, the solids uptake being 3% by weight. However, the product components suffer from a low solids content which result in gasketing materials which are difficult to dry. Furthermore, the strength of the components is insufficient for some applications both in terms of processing and water resistance. Numerous attempts have been made to solve this problem.
GB 2122699A discloses the use of electrophoresis to remove water from the product during production but it is noted that in examples 1 and 2 pure vermiculite is deposited on a metal plate but such products were not boiling water resistant.
U.S. Pat. No. 4,677,551 (Hercules) is directed to ionic bonding using onium ions of CEV. It describes mixing CEV with polymers and indicates that large quantities of organic materials are necessary to introduce sufficient binding properties into the CEV and that such large quantities are unsatisfactory. The document is directed to improving the binding properties using the onium ions.
U.S. Pat. No. 4,762,643 (Armstrong) is directed to improving the properties of CEV by further treatment using guanidine. In addition, the guanidine derived product is further strengthened using fibres. Example 12 of the said document discloses a high level of latex (42%) in order to impart sufficient binding properties of the CEV. Such high levels of latex cause a loss in stress retention and high creep in the final product. Such properties are generally unacceptable in gaskets.
U.S. Pat. No. 4,655,482 is directed to improving the properties of vermiculite dispersions by the use of citrate anions. The citrate anions function as swelling agents for the vermiculite and enhance the rate and extent of swelling to an aqueous medium. The swollen vermiculite is usually delaminated by shearing to provide the inventive dispersions, which comprise a suspension of the delaminated platelets and citrate anions.
It is an object of the present invention to provide a gasket comprising a sealing layer with improved water resistance. It is a further object of the present invention to provide a gasket with a sealing layer with reduced loss in stress retention and low creep.
According to a first aspect of the present invention there is provided a gasket comprising a sealing layer and a support layer, the sealing layer being formed from a resilient material which comprises a CEV component in a proportion of at least 25% w/w of the sealing layer, the said CEV component being at least partially derived from dry CEV, and a hydrolysis resistant polymer to improve the water resistance of said sealing layer wherein the proportion of the said polymer does not exceed 20% w/w of the sealing layer.
For the avoidance of doubt, a gasket of the present invention may provide conventional sealing between static parts and sealing between moving parts such as valves where sealing is only required intermittently. An example of the latter would be valve stem sealing.
Preferably, the proportion of CEV is at least 25% w/w of the sealing layer, more preferably at least 35% w/w of the sealing layer.
Typically, the level of CEV falls within the range 25-80% w/w of the sealing layer, more typically, 30-75% w/w of the sealing layer, most typically 35-70% w/w of the sealing layer.
Preferably, the proportion of the said polymer is less than 15% w/w of the sealing layer, more preferably, less than 10% w/w. Especially preferred is a level of polymer less than 7.5% w/w, more especially preferred is a level of polymer in the range 2.0-7.5% w/w of the sealing layer.
The known prior art products contain high levels of hydrolysis resistant polymer as, hitherto, such high levels were required in order to provide the level of hydrolysis resistance required. Unfortunately, such levels of polymer resulted in a loss of stress retention and unsatisfactory levels of creep, in use. By the use of dry CEV particles in the wet dough composition, it has been surprisingly discovered that much lower levels of hydrolysis resistant polymer can be utilised whilst still providing the necessary levels of hy
Atkinson Alan W.
Bond Stephen P.
Hoyes John R.
West Adam M.
Cain Edward J.
Flexitallic Investments, Inc.
Haynes and Boone
Montgomery John W.
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