GB2346888A

GB2346888A - Forming gasket material

Info

Publication number: GB2346888A
Application number: GB9903867A
Authority: GB
Inventors: Alan William Atkinson; Stephen Peter Bond; Darryl Alfred Turland; Adam Martin West
Original assignee: Federal Mogul Technology Ltd
Current assignee: Federal Mogul Technology Ltd
Priority date: 1999-02-20
Filing date: 1999-02-20
Publication date: 2000-08-23
Also published as: GB9903867D0

Abstract

A method of forming a sealing-enhancing material for use in a gasket. The method comprises forming a water-borne mixture of flaky particles of chemically-exfoliated vermiculite, and ion-providing particles which provide a source of metallic ions capable of causing flocculation of the chemically-exfoliated vermiculite. The method also comprises forming the water-borne mixture into a layer, and drying said layer. The method also comprises preventing formation of said metallic ions until after formation of said layer, and causing formation of said ions after formation of said layer thereby causing said flocculation.

Description

FORMING GASRET MATERIAL This invention is concerned with forming gasket material. In particular, the invention is concerned with forming gasket material which can be utilised as a sealing enhancing layer in a gasket.

Gaskets are used for sealing between two members, eg two portions of the exhaust system of an internal combustion engine, and provide a seal around a passage which passes from one member to the other. Accordingly, a gasket has to be resilient in order to press against the members and provide a fluid-tight seal. In the case of an exhaust gasket, the seal is to prevent escape of exhaust gases and entry of air. The resilience can be provided by utilising a relatively thick layer of resilient material such as a resilient rubber-based material or graphite but, where high temperatures (above 300 C) are experienced, such as in an exhaust system, many resilient materials would degrade and are not commonly used. Instead, the gasket comprises a sheet of metal, eg stainless steel, which is formed into resilient ridges (called"beads") which provide the seal. Thus, when such a gasket is clamped between two members, the clamping force compresses the beads which are resiliently deformed and press against the members along the lines of the beads. However, such gaskets do not provide as good a seal as is desirable because the beads are unable to enter into small cracks and fissures in the members so that gases and liquids can escape past the beads. It is known, in gaskets not utilised at such high temperatures as are experienced by exhaust gaskets, to provide metal beads with a thin layer (typically less than 200 microns in thickness) of a sealing-enhancing layer which will deform under clamping pressure to fill cracks and fissures. Known sealing-enhancing layers, however, degrade at higher temperatures so that they cannot be used on, eg, exhaust gaskets for internal combustion engines.

It is known to utilise chemically-exfoliated vermiculite (hereinafter referred to as CEV) in a sealing layer, ie the layer which provides the sealing force by compression thereof, of a sheet gasket, eg an automotive head gasket. For example, 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 microns, and a dispersed organic polymeric material, eg acrylic polymer, acrylonitrile-butadiene copolymer, epoxy resin, or natural rubber. However, these flexible sheet materials are not suitable for use as a fissure-sealing layer on an exhaust gasket because the polymeric material would degrade at high temperatures and the layer would become less effective.

The present applicants have developed a sealingenhancing layer of the type referred to above which comprises chemically-exfoliated vermiculite. CEV is a known heat-resistant resilient material which provides sealing and possibly binding properties at high temperatures (eg above 300 C). Exfoliated vermiculite is conventionally formed by expanding mineral vermiculite using gas (usually steam). CEV is a form of exfoliated vermiculite which 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 often below 50 microns) vermiculite particles.

The sealing layers containing CEV developed by the present applicants are heat-resistant to above 300 C but the CEV is susceptible to attack by moisture, eg cooling water in the case of head gasket or condensation in the case of an exhaust gasket.

It is an object of the present invention to provide a method of forming a sealing-enhancing material comprising CEV which is less susceptible to attack by moisture.

The invention provides a method of forming a sealingenhancing material for use in a gasket, the method comprising forming a water-borne mixture, the mixture comprising flaky particles of chemically-exfoliated vermiculite, at least 90% by weight of said particles having a thickness of no more than 30 microns, and no dimension greater than lmm, the particles forming 10 to 90 wt% of the solids content of the mixture, the mixture also comprising ion-providing particles which provide a source of metallic ions capable of causing flocculation of the chemically-exfoliated vermiculite, the method also comprising forming the water-borne mixture into a layer, and drying said layer, wherein the method also comprises preventing formation of said metallic ions until after formation of said layer, and causing formation of said ions after formation of said layer thereby causing said flocculation.

In a method according to the invention, the CEV is flocculated giving it water-resistance but the flocculation takes place after the layer has been formed, since it would be difficult, if not impossible to form an even layer of a mixture comprising flocculated CEV. The term "flocculation"is used herein to mean the addition of species that destroy or reduce the tendency of particles to repel one another and so remain in or enter into a dispersed state. Flocculating mechanisms include neutralisation of surface charges by absorption of ions having the opposite charge or the formation of chemical bridges between particles. Polyvalent cations (eg Al3+) are particularly effective.

One possible use for a material made by a method according to the invention is to provide a layer which enhances sealing by filling small cracks and fissures in the surface of the gasket and/or of the member against which the gasket seals.

Another possible use for a material made by a method according to the invention is as the filler strip of a spirally-wound gasket, the filler strip being interposed between successive coils of a steel strip.

In a method according to the invention, the formation of said metallic ions may be caused by applying an ionising agent to said ion-providing particles. Said ionising agent may be included in said mixture but be prevented from forming said metallic ions by a barrier material on said ion-providing particles, said barrier material being removed to cause formation of said ions. For example, the ionising agent may be an acid or an alkali, eg sodium hydroxide or lithium hydroxide, the ion-providing particles may be aluminium, and the barrier material may be a stearate-based coating on the aluminium.

In another possibility, said ionising agent may be included in said mixture but be prevented from forming said metallic ions by a barrier material which encapsulates said ionising agent, said barrier material being removed to cause formation of said ions. For example, the ionising agent may be encapsulated in a resin.

The barrier material may be removed by heating or by applying an agent which reacts with said barrier material to the layer. For example, said agent may be sprayed on to the layer before or after it dries. Other possibilities for removing said barrier material include applying a crushing/clamping force, eg when the gasket is clamped in position, electrolysis, and exposure to automotive engine fluids such as coolant or oil.

In another possibility, said mixture may also comprise a precursor of an ionising agent, eg a sodium salt of an organic acid, and the precursor may be heated or otherwise caused to react to form the ionising agent after formation of said layer.

The mixture may also comprise a heat-resistant organic polymer binder. For the present purposes, an organic polymer binder is considered to be heat resistant to a particular temperature if, when the binder is formed into a film lmm or less in thickness and heated to that temperature in free air for 24 hours, it either does not decompose or decomposes leaving a residue of at least 20% by weight of the film. The polymer binder is preferably a silicon-containing polymer, eg a silicone resin or a siliconate. Also possible are PTFE, phenolics, and fluoroelastomers.

The layer may also comprise a flaky filler, eg mica, or milled thermally-exfoliated vermiculite. A suitable selection of filler can enhance the drying of the layer after it is applied.

The layer also, optionally, comprises a supplementary inorganic binder/adhesion promoter, eg a water-soluble alkali silicate, especially lithium silicate.

A method according to the invention may form a dried layer having a thickness of up to 100 microns when the layer is to be used for fissure-sealing, eg the thickness may be up to 80 microns, eg between 50 and 75 microns. For other purposes, eg as filler strip in a spirally wound gasket, the layer may have a thickness of up to 3mm, eg 0.1 to 1. 5mm.

A layer according to the invention may be used on a bore seal of an embossed steel gasket sheet and is suitable for use in an exhaust gasket of an internal combustion engine. It may also be used as the filler in a spirally wound gasket, in which case, the layer may either be bonded to a supporting substrate, eg a metal strip, or be a freestanding strip.

Where relatively low clamping forces are employed, eg in an automotive exhaust gasket, in order to increase resilience, a layer made by the method according to the invention may have a density of below 70% of the theoretical density of the material forming the layer. The density of the layer may be below 50% of said theoretical density. For spiral-wound gaskets, where higher clamping means are employed and better sealing is needed, the density should be higher ( > 1,200 kg/m3).

A layer made by a method according to the invention may be applied to at least a portion of a metal sheet, the sheet may be embossed with the layer to form at least one resilient ridge therein with the layer extending across said ridge, and heating applied to the embossed sheet to a temperature of at least 350 C to temper said sheet.

Preferably, said metal sheet is made of stainless steel.

The layer may be applied to both sides of the sheet, including possibly to both sides of the sheet in the region of the ridge, ie the layer borders the trough created in the other side of the sheet by the formation of a ridge.

There now follows a detailed description of four methods of forming a sealing-enhancing material for use in a gasket, the methods being illustrative of the invention.

The first illustrative method comprises forming a water-borne mixture by mixing the following together: (a) 50g of flaky particles of chemically-exfoliated vermiculite suspension (15% solids in water). This was obtained from W R Grace & Co under the designation Microlite HTS. The particles of CEV were flaky and had a size distribution such that at least 90% by weight had a thickness of no more than 30 microns and no dimension greater than lmm. Specifically, the vermiculite particles have less than 33% above 45 microns diameter and an aspect ratio of at least 100.

(b) 5g of graphite flake particles (grade Hart 400 from Colin Hart Minerals). This graphite is milled so that 96% of the particles pass through a 37 micron sieve.

These particles were included to provide a solid lubricant.

(c) 5g of aluminium powder (grade Fine 124 from Ronald Britton & Co). The mean particle size of the aluminium is 16-24 microns with less than 1% being retained on a 160 micron sieve. The aluminium particles provided ion-providing particles which are a source of metallic ions capable of causing flocculation of the CEV. The particles were coated with a thin layer of a stearate which acts as a barrier material which prevents ionisation of the aluminium during formation of the mixture and during formation of the sealing-enhancing material.

(d) 5g of mica particles (SX 300 from Microfine Minerals).

1-4% of the mica particles are retained by a 20 micron sieve. The mica was included as an inorganic filler.

(e) lOg of methyl phenyl silicone resin emulsion (MP 42E from Wacker). This was a suspension in water at 42% solids and provided an organic polymer binder. This resin has an ignition point of 465 C. This binder, when formed into a film 1mm or less in thickness and heated to 300 C in free air for 24 hours decomposes leaving a residue of at least 20% by weight of the film, the residue being silica which has some binding properties.

(f) 5g of soluble lithium silicate solution (23% solids).

This was obtained from Crossfield under designation L40 and provided an inorganic binder in addition to the organic binder provided by the silicone resin and the CEV. The lithium silicate takes over some of the binding function at higher temperatures (at which the silicone resin undergoes thermally-induced chemical changes). The lithium silicate may also promote bonding to steel substrates. This solution also contains lithium hydroxide.

Next in the first illustrative method, the mixture was sprayed onto the surface of a stainless steel sheet in order to form the water-borne mixture into a layer. The layer was then dried forming a dry layer over one surface of the sheet having a thickness of approximately 50 microns. The layer was heated to 200 C for 10 minutes to remove most of the stearate coating from the aluminium particles. The layer had a density of between 700 and 800 kg/m3, the theoretical density of the material forming the layer being approximately 2000 kg/m3. During formation of the layer and drying thereof, the stearate coating on the aluminium particles prevented formation of metallic ions.

Once the layer had been formed as described above, the lithium hydroxide was, on subsequent exposure to water, able to attack the aluminium particles, causing the formation of aluminium ions. Thus, the lithium hydroxide acts as an ionising agent for ionising the aluminium particles once the stearate coating is removed from the aluminium particles. The aluminium ions then caused the CEV to flocculate, thereby water-proofing the CEV.

Next in the first illustrative method, the sheet with the layer thereon was embossed to form sealing beads which were covered at their crests by the layer.

Next in the first illustrative method, the embossed coated sheet was heated to 350 C for one hour to temper the steel with the embossments at layer in situ. The sheet was then incorporated in a multi-layer steel gasket. It was found that the layer performed well in micro-sealing.

In the second illustrative method, the same procedure was utilised except for the formulation of said mixture which comprised: (a) 50g of chemically exfoliated vermiculite as mentioned in the first illustrative method.

(b) 30g of methyl silicone resin emulsion (M50 E from Wacker). This was a suspension in water at 50% solids and provided an organic polymer binder.

(c) 5g of lithium silicate as disclosed in the first illustrative method.

(d) 5g of graphite flake particles as disclosed in the first illustrative method.

(e) 5g of mica particles as disclosed in the first illustrative method.

(f) 0.25g of aluminium powder as disclosed in the first illustrative method.

(g) 30g of water.

The gasket layers formed by the first and second illustrative methods both gave enhanced sealing layers which were capable of micro-sealing at temperatures above 300 C and were water resistant. In both cases, the gasket produced could be immersed in water without the layer disintegrating. In contrast, compositions which omitted the aluminium particles or the lithium silicate lost much of their integrity on heating and immersion in water.

In the third illustrative method, the water-borne mixture comprised the following: (a) 49g of thermally-exfoliated vermiculite (DSF grade from Dupre Minerals).

(b) 50g of CEV in suspension (as disclosed above).

(c) lg of aluminium flake (not coated with stearate).

The mixture was calendered to form a 4.5mm wet thickness layer (equivalent to 3. Omm dry undensified thickness) on either side of a supporting substrate comprising a O. lmm tanged stainless steel sheet. The layers were dried and re-calendered to 2. Omm final dry thickness and a density of 1,100 kg/m3. This sheet was then sprayed with lOOg/m2 of sodium hydroxide solution (40g/l) and re-dried. The sodium hydroxide provided an ionising agent for the aluminium which caused flocculation of the CEV. The sheet showed much improved integrity on soaking in water, compared with similar sheets which either contained no aluminium or were not treated with sodium hydroxide. The sheet was cut into 6mm wide strips and wound, along with stainless steel strip, to form a spirally-wound gasket. The gasket was water-proof and provided good high temperature sealing.

In a variation of the third illustrative method a similar sheet was prepared but without the sodium hydroxide treatment. The sheet was cut into strips and wound into a spiral gasket as described above, the CEV not being flocculated at this stage. The gasket was then immersed in sodium hydroxide solution (as described above) for one hour which caused flocculation to occur and gave the gasket similar water-proof properties to the above mentioned spirally wound gasket.

In the fourth illustrative example, the following were mixed together: (a) 529g of CEV suspension (as disclosed above).

(b) 278g of powdered CEV (Microlite powder).

(c) 193g of milled thermally exfoliated vermiculite.

(d) 6g of aluminium powder.

(e) 70g of 16% solution of nitrile butyl rubber.

(f) 6g of soluble lithium silicate solution (as described above).

The fourth illustrative method continued as disclosed above in relation to the third illustrative method.

Claims

CLAIMS 1 A method of forming a sealing-enhancing material for use in a gasket, the method comprising forming a water-borne mixture, the mixture comprising flaky particles of chemically-exfoliated vermiculite, at least 90% by weight of said particles having a thickness of no more than 30 microns, and no dimension greater than lmm, the particles forming 10 to 90 wt% of the solids content of the mixture, the mixture also comprising ion-providing particles which provide a source of metallic ions capable of causing flocculation of the chemically-exfoliated vermiculite, the method also comprising forming the water-borne mixture into a layer, and drying said layer, wherein the method also comprises preventing formation of said metallic ions until after formation of said layer, and causing formation of said ions after formation of said layer thereby causing said flocculation.
2 A method according to claim 1, wherein formation of said metallic ions is caused by applying an ionising agent to said ion-providing particles.
3 A method according to claim 2, wherein said ionising agent is included in said mixture but is prevented from forming said metallic ions by a barrier material on said ion-providing particles, said barrier material being removed to cause formation of said ions.
4 A method according to claim 2, wherein said ionising agent is included in said mixture but is prevented from forming said metallic ions by a barrier material which encapsulates said ionising agent, said barrier material being removed to cause formation of said ions.
5 A method according to either one of claims 3 and 4, wherein said barrier material is removed by heating.
6 A method according to either one of claims 3 and 4, wherein said barrier material is removed by applying an agent which reacts with said barrier material to the layer.
7 A method according to claim 1, wherein said mixture also comprises a precursor of an ionising agent which is caused to react to form the ionising agent after formation of said layer.
8 A method according to any one of claims 1 to 7, wherein said mixture also comprises 50 to 10 wt% of an organic polymer binder which is heat resistant to at least 300 C.
9 A method according to claim 8, wherein the polymer binder is a silicon-containing polymer.
10 A method according to claim 9, wherein the polymer binder is a silicone.
11 A method according to claim 10, wherein the polymer binder is a siliconate.
12 A method according to claim 8, wherein the polymer binder is selected from PTFE, phenolics, and fluoroelastomers.
13 A method according to any one of claims 1 to 12, wherein the mixture also comprises a flaky filler.
14 A method according to any one of claims 1 to 13, wherein the mixture also comprises a supplementary inorganic binder.
15 A method according to claim 14, wherein the supplementary inorganic binder is lithium silicate.
16 A method according to either one of claims 14 and 15, wherein the mixture also comprises a waterproofing agent acting on the supplementary inorganic binder.
17 A method according to any one of claims 1 to 16, wherein the layer has a thickness up to 100 microns.
18 A method according to any one of claims 1 to 16, wherein the layer has a thickness of between O. lmm and 1. 5mm.
19 A method according to any one of claims 1 to 18, wherein the layer has a density of below 70% of the theoretical density of the material forming the layer.
20 A method substantially as hereinbefore described with reference to the illustrative examples.