GB2325496A - Gaskets - Google Patents

Abstract

A gasket comprises a sealing layer formed from a resilient material which comprises particles of chemically-exfoliated vermiculite bonded together, the layer being at least 200 microns in thickness. The layer also comprises 1-90% by weight of a plate-like filler. The particles of the plate-like filler tend to orientate themselves into the plane of the layer and act like a large number of tiny leaf springs to improve sealing. The plate-like filler may be selected from the group consisting of talc, molybdenum disulphide, hexagonal boron nitride, soapstone, pyrophyllite, milled thermally exfoliated vermiculite, mica, fluoromica, powdered graphite, glass flake, metal flake, ceramic flake or kaolinites. The layer may also have an organic binder in the form of a hydrolysis-resistant polymer coupled to the vermiculite by a coupling agent.

Description

GAS KETS This invention is concerned with gaskets, in particular with gaskets having a sealing-enhancing layer which is based on 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 "gasexfoliated vermiculite". The gas may be thermally generated, in which case the product is called thermally exfoliated vermiculite" (TEV). TEV may be made by flashheating 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 sheets. The granules formed have a chemical composition which (apart from the loss of water) is virtually identical to that of the raw material. Gasexfoliated 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-C4H9NH3 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 microns) 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 gasexfoliated vermiculite.

It is an object of the present invention to provide a gasket comprising a layer of sealing enhancing material which is based on exfoliated vermiculite, the layer containing a cost-reducing filler which does not significantly reduce the effectiveness of the layer.

Preferably, the filler should be halogen-free and should reduce the posssibility of heat damage.

The invention provides a gasket comprising a sealing layer formed from a resilient material which comprises particles of chemically-exfoliated vermiculite bonded together, the layer being at least 200 microns in thickness, wherein the layer also comprises 1-90% by weight of a plate-like filler.

In a gasket according to the invention, it is found that the particles of the plate-like filler tend to orientate themselves into the plane of the layer and act like a large number of tiny leaf springs, thereby improving sealing.

The plate-like filler may be selected from the group consisting of talc, molybdenum disulphide, hexagonal boron nitride, soapstone, pyrophyllite, milled thermally exfoliated vermiculite, mica, fluoromica, powdered graphite, glass flake, metal flake, ceramic flake, or kaolinites.

In general, a plate-like filler has an average width of plates of at least three times the average thickness.

In a gasket according to the invention, the layer may comprise 5-80%, eg 40-60t, by weight of the plate-like filler.

In a gasket according to the invention, the layer may also comprise an organic binder. The organic binder may be a hydrolysis-resistant polymer coupled to the vermiculite by a coupling agent. Such a polymer may be selected from nitrile butadiene rubbers, styrene butadiene rubbers, natural rubber, butyl rubber, and ethylene propyldiene monomer. Diene-based polymers are suitable because they are flexible and hydrolysis-resistant. The coupling agent may be a silane, eg a vinyl functional silane such as triethoxy vinyl silane ((CH3CH2O)3SiCH=CH2 ).

Since CEV is a relatively expensive material compared with gas-exfoliated vermiculite, eg TEV, in a gasket according to the invention, the resilient layer may also comprise particles of gas-exfoliated vermiculite, eg the layer may comprise particles of gas-exfoliated vermiculite bonded together by particles of CEV. The gas-exfoliated vermiculite may be milled to a particle size of less than 50 microns.

Said sealing layer may be mechanically bonded to a sheet of the gasket, eg by tangs projecting from the sheet into the layer. The sheet may be of stainless steel or carbon steel.

There now follows a detailed description of three illustrative examples according to the invention.

Example 1 In the example 1, a tanged stainless steel sheet was first prepared. This sheet was 100 microns in thickness.

The sheet was tanged by perforating it with square holes, each hole being 1.5 mm square and the hole centre-spacing being 3 mm. Half the holes were perforated by passing a tool through the sheet in a first direction and the remaining half, which alternated with the first-mentioned half, were perforated by passing a tool through the sheet in the opposite direction. The edges of the holes, thus, formed tangs projecting from the sheet in opposite directions. The tangs projected by about 1 mm.

Next, in example 1, an aqueous slurry (15% solids) was obtained containing about 0.741 Kg of CEV particles (the slurry was obtained from Grace Construction Products and is designated "Microlite HTS"). To this slurry was added 0.074 Kg of particles of spray-dried CEV having particle size about 45 microns obtained from Grace Construction Products Limited and designated "Microlite Powder". To this, was added 0.148 Xg of Dupre Superfine TEV. To this was added, 37g of Molybdenum disulphide (99% < 2zm powder, ex Aldrich Chemicals). This gave a paste having approximately 37% solids. The mineral content of the paste was 50% CEV and 40% TEV. To this paste was added 3.7 g of a coupling agent (a vinyl functional silane called "Silquest A-151" obtainable from OSi Specialities) and further mixing was carried out.

Next, a hydrolysis-resistant polymer/solvent mixture was prepared. This mixture was 50 g of solid nitrile butadiene rubber (Nippon Zeon N36C80), 250 g of toluene, and 3.1 g of a curing agent ("Dicup 40", dicumylperoxide).

111 g of this mixture (ie 18.5 g of rubber) was added to the above-mentioned paste and mixing was carried out. This gave a paste with approximately 5 % rubber content.

Next, the paste (including the polymer/solvent mixture) was spread over one side of the metal sheet mentioned above. The sheet was then passed between calendering rollers and was dried. Further paste was then spread over the other side of the metal sheet and the calendering and drying was repeated. The sheet was then pressed to densify the resilient material which formed layers approximately 0.75 mm thick on both sides of the metal. Then, it was heated to peroxide cure the rubber.

The completed gasket had two sealing layers formed from a resilient material. The resilient material comprised particles of CEV bonded together, and coupled to the nitrile butadiene rubber by the silane. The gasket was tested to determine its water resistance by boiling in water for 5 hours. The gasket retained its integrity.

Example 2 Example 1 was repeated except that 37g of talc (ex Norwegian Talc (UK) Ltd, grade IT300) was added instead of the molybdenum disulphide.

Example 3 Example I was repeated except that 37g of powdered graphite (ex David Hart Ltd) was added instead of molybdenum disulphide.

Example 4 Example 3 was repeated except that 185g of powdered graphite was added,instead of TEV) giving equal proportions of graphite and CEV.

Example 5 Example 4 was repeated except that 185g of mica was added instead of graphite.

Claims (10)

1A gasket comprising a sealing layer formed from a resilient material which comprises particles of chemically-exfoliated vermiculite bonded together, the layer being at least 200 microns in thickness, wherein the layer also comprises 1-90% by weight of a plate like filler.

2 A gasket according to claim 1, wherein the plate-like filler is selected from the group consisting of talc, molybdenum disulphide, hexagonal boron nitride, soapstone, pyrophyllite, milled thermally exfoliated vermiculite, mica, fluoromica, powdered graphite, glass flake, metal flake, ceramic flake, or kaolinites.

3 A gasket according to either one of claims 1 and 2, wherein the layer comprises 5-80% by weight of the plate-like filler.

4 A gasket according to any one of claims 1 to 3, wherein the layer also comprises an organic binder.

5 A gasket according to claim 4, wherein the organic binder is a hydrolysis-resistant polymer coupled to the vermiculite by a coupling agent.

6 A gasket according to claim 5, wherein said polymer is selected from nitrile butadiene rubbers, styrene butadiene rubbers, natural rubber, butyl rubber, and ethylene propyldiene monomer.

7 A gasket according to either one of claims 5 and 6, wherein the coupling agent is a silane.

8 A gasket according to any one of claims 1 to 7, wherein the sealing layer also comprises particles of gas-exfoliated vermiculite.

9 A gasket according to any one of claims 1 to 8, wherein said sealing layer is mechanically bonded to a metal sheet of the gasket.

10 A gasket substantially as hereinbefore described with reference to any one of the illustrative examples.