GB2344773A - Gasket coating - Google Patents

Abstract

A sealing-enhancing coating of a gasket or a portion of a gasket is based on a silicone resin. The coating also comprises a siliconate dispersed within said resin, e.g. alkali metal, ammonium or potassium methyl siliconate preferably in a proportion to the resin of 1:25 to 1:5 by dry weight.

Description

2344773 1 GASKET COATING This invention is concerned with a gasket

coating. The invention is applicable to a gasket coating which enhances sealing by filling small cracks and fissures in the surface of a sheet of the gasket and/or of the member against which the gasket seals ("micro-sealing").

Gaskets are used for sealing between two members, eg the block and head 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. The resilience can be provided by utilising a relatively thick layer of resilient material such as a resilient rubber-based material. The resilience can alternatively be provided by resilient beads running along lines where sealing is required, the beads being formed of rubber-based material and being mounted on a sheet of the gasket. Where the gasket comprises a sheet of metal, eg stainless steel, the resilience can be provided by embossing the sheet to form resilient ridges which provide the seal. Thus, when such a gasket is clamped between two members, the clamping force resiliently deforms the resilient portions of the gasket which, therefore, press against the members to form the seal.

In gaskets where the seal is provided by embossed metal ridges, it is found that the seal may be compromised because the ridges are unable to enter into small cracks and fissures in the members so that gases and liquids can escape past the ridges. In order to overcome this problem, it is known to apply a thin coating (typically less than 2 microns in thickness) of a sealing- enhancing coating to at least the crests of the ridges. The coa ing is designed to deform under clamping pressure to f 11 cracks and fissures. Known seal ing- enhancing coatin s are based on elastomers, such as nitrile rubbers and flu ro-rubbers, and are often applied in a solvent-borne form.

Because fluoro-elastomers, nitrile rubbers and silicone elastomers all degrade or bec me brittle at temperatures above 250'C, sealing-enhanci g coatings f or micro-sealing at temperatures above 2500C ave not, to the applicants' knowledge been based on these materials. For example, silicone elastomers start to de ompose at 200250'C and either disintegrate or become ve y hard.

It is an object of the present invention to provide a sealing- enhancing coating which is based on a silicone resin and which can be used at temperature above 250'C.

The invention provides a sealing- enh& icing coating of a gasket or a portion of a gasket, the coa:ing being based on a silicone resin, wherein the coating comprises a siliconate dispersed within said resin.

It is, surprisingly, found that a coating according to the invention remains effective after heating to a temperature exceeding 400'C. It is su -mised that the siliconate promotes a higher degree of cross-linking during I s curing of the coating leading to higher eat resistance. The coating is also resistant to the fluid, associated with internal combustion engines, ie oil, wat r, anti-freeze, and fuel. Siliconates are, of course, d fferent to both silicates and silicones (including siloxa es and silanes). Since the 19501s, siliconates have been sed for coating masonry to inhibit mould growth, prevent alt ingress and prevent frost damage. Alkyl siliconates are produced by the hydrolysis of alkyl chlorosilanes t:) produce a gel 3 which is alkyl silane triol. The siliconate is produced by dissolving the gel in an aqueous alkaline solution, eg potassium hydroxide. The siliconate clearly has to be soluble in order to distribute it in the coating.

The siliconate used in the invention is preferably an alkali metal or ammonium siliconate, for example potassium methyl siliconate (CAS No 16589-43-8). Other potassium, sodium or lithium siliconates are possible. Preferably, the proportion of siliconate to resin is between 1:25 and 1:5 by dry weight.

The silicone resin may be a polymeric alkoxyfunctional siloxane, for example a polymeric ethoxy functional siloxane.

It is surprisingly found that, in a significant number of applications, a coating according to the invention, after drying and in a semi-cured state, is flexible enough to survive an embossing process and also heatresistant enough to still provide a functional coating after a tempering step (eg heating to 42CC for one hour). This makes it feasible to apply the coating to a planar steel sheet, then emboss the sheet to form the ridges, and then temper the steel.

A coating according to the invention may also be used on gasket sheets which are absorbent so that the coating may be impregnated into the material of the sheet to key the coating to the sheet.

It is a further object of the present invention to provide an improved method of forming a gasket having an improved sealing-enhancing coating.

The invention also provides a method of forming a gasket having a sealingenhancing coating, the method 4 comprising forming a spreadable mixture comr rising silicone resin and a siliconate solution, spreading 1 he mixture over at least a portion of a sheet of a gasket, and curing the mixture.

In a method according to the inventiDn, the mixture may be spread by any of the conventional methods such as screen printing, roller coating, etc.

In order to reduce the use of pote-itially harmful solvents, it is preferred that, in a method according to the present invention, the silicone resin Is substantially free of organic solvents and water, ie thE: silicone resin is a high solids liqui d eg a "100% solids Liquid".

It is preferred that the mixture lso contains a curing agent. The curing agent may be sele=ted from one or more of polybutyltitanate and similar organotitanium complexes, organo-metallic tin complexes (eg dibutylin diacetate and dibutylin dilaurate), organo-aluminium complexes (such as aluminium acetate and acetylacetonate), titanium acetylacetonate, silicon acetylace onate, possibly other metallic acetylacetonates, and zinc xide.

In order to reduce the viscosity of 4.

the mixture and thereby improve ease of application, and also to increase pot life, the mixture may comprise a molecular architecture modifier or a react ion- retarding catalyst, or a reactive diluent, or any two or all three of these.

The mixture may also contain a reinforcing filler. The filler may be selected from mica, milled exfoliated vermiculite, wollastonite, graphite, calcined china clay, barium sulphate, calcined alumina, fumed sJ lica, and carbon black.

A method according to the invention may also comprise forming at least one hole through said sheet and the coating to provide a passage through the gasket, embossing at least one ridge into said sheet and coating, the ridge extending around said hole, and tempering said sheet by heating to, eg, 4200C. The hole may be formed by a cutting or a punching operation and the ridge may be formed by pressing between two dies.

There now follow detailed descriptions of four gasket coatings which are illustrative of the invention and of four corresponding methods of forming a gasket having a seal ing- enhancing coating which are illustrative of the method aspects of the invention.

All four illustrative coatings provide a micro-sealing coating on a sheet of a multi-layer steel gasket for use between the head and the block of an internal combustion engine. The coating is 30 to 40 micrometres in thickness and extends over at least the sealing region of embossed resilient ridges formed in a steel sheet of the gasket. All four illustrative coatings comprise a silicone resin, specifically polymeric ethoxy functional siloxane, and a siliconate.' specifically potassium methyl siliconate, dispersed within said silicone resin. The dispersal of the siliconate is achieved by mixing the resin with a siliconate solution before curing.

In the first illustrative method, the first illustrative coating was formed by forming a spreadable mixture comprising the following components:

200g of silicone resin (100% solids)(Wacker SLM 43220 - an ethoxyfunctional dimethylpolysiloxane); 60g of aqueous solution of siliconate in potassium hydroxide (42% solids)(Wacker ES15 - it improves the high temperature performance of the coating and may react into the resin backbone on curing); _,,i 6 22g of curing agent (polybutyl-_itanate (100% solids) (Tilcom PBT - it acts as a crosslinker/catalyst for the resin); and 20g of reinforcing filler (mica (Mi rofine Minerals SX300)).

The nominal solids content of the mixture was 88%. The mixture was spread over one surface of a planar sheet of st ainless steel of the type conventionally used in multi-layer steel gaskets. The spreading was by means of a K-bar (also known as a Mayer Bar) to a thickness which gave a coating thickness of 30 microns aft.,er drying. The mixture was then dried and partly-cured by heating to 200'C for 10 minutes.

Next in the first illustrative method, holes were formed through the sheet and the first illustrative coating. The holes were formed in the appropriate positions to provide passages through thE gasket. Next, the steel sheet and the coating were embossed to give resilient steel ridges coated by the coating, the ridges extended in closed loops around said holes to provide seals around the holes. It was found that the coating remained continuous after the embossing operation.

Finally, in the first illustrative mathod, the steel sheet and coating were heated to 420'C for one hour to temper thesteel sheet in its embossed form. This did not damage the coating.

When the gasket was subjected to thE usual tests, it was found that the first: illustrative coating had retained its micro-sealing ability despite the temrering operation. Furthermore, the coating was found to be iesistant to oil, water, anti-freeze and fuel.

7 Although the first illustrative coating was found to have excellent fluid and heat resistance, its viscosity was higher than is desirable for some spreading methods. Its viscosity was also unstable and rapidly increased. Thus, the pot life of the mixture was found to be short. Accordingly, a second illustrative coating was formed by a second illustrative method.

In the second illustrative method, the first illustrative method was followed except that the mixture contained:

200g of silicone resin (the same resin mentioned in the first illustrative example); 60g of aqueous solution of siliconate in potassium hydroxide (the same solution mentioned in the first illustrative example); 25g of curing agent (the same curing agent mentioned in the first illustrative example); 50g of a molecular architecture modifier (Wacker TES 40 - see below); and 20g of a react ion- retarding catalyst (Wacker catalyst F100% - aluminium acetylacetonate).

TES 40 is a mixture of tetraethylorthosilicate monomers and oligomers. It is tetra functional and because of its alkoxy functionality can participate in condensation reactions, and can form a tetra functional branch point in the resin network, thus modifying the molecular architecture. It also lowers the viscosity of the mixture prior to coating and extends pot life.

The second illustrative coating was found to have excellent fluid and heat resistance, reduced and more stable viscosity, and increased pot life. However, further improvement was sought by forming a third illustrative coating by a third illustrative method.

8 In the third il--ustrative metho the second illustrative method was followed except t at the mixture also contained:

50g of a reactive d--luent (Dow 3037 - which is a low molecular weight methoxy functional dime thyl s i loxane, which has a low viscosity. It reduces the viscosity of the coating material prior to application but, because of its alkoxy-functionality, cross-links into the resin structure on curing).

The third illustrative coating was found to have excellent fluid and heat resistance, reduce viscosity, and increased pot life.

In the fourth illustrative meth d, the third illustrative method was followed except that the mixture comprised the following components:

200g of silicone resin as mentione in the f irst illustrative example; 20g of a reaction-retarding cata yst (the same L catalyst mentioned in the secod. illustrative example); 50g of a molecular architecture modifier (the same mentioned in the second illustrative example); 60g of aqueous solution of siliconate in potassium hydroxide (as mentioned in the fiyst illustrative example; 25g of curing agent (the same mentioned in the first illustrative example); and 50g of a reactive dilutent. specifically triethoxyoctylsilane.

The fourth illustrative coating wa found to have excellent fluid and heat resistance, reduced viscosity, and increased pot life.

9 Fillers and pigments can be added to the mixture in a method according to the invention.

other possible reaction-retarders and diluents are other organosilanes, particularly alkyl trialkoxys i lanes, especially methyltriethoxysilanes or octyltriethoxysilane.

Claims (1)

1 A sealing-enhancing coating of a gasKet or a portion of a gasket, the coating being based on a silicone resin, wherein the coating also comprises a siliconate dispersed within.said resin.

2 A seal ing- enhancing coating to clai 1, wherein the siliconate is an alkali metal or ammo ium siliconate.

3 A coating according to either one of claims 1 and 2, wherein the siliconate is potassium MEthyl siliconate.

4 A coating according to any one of claims 1 to 3, wherein the proportion of siliconate to resin is between 1:25 and 1:5 by dry weight.

A coating according to any one of claims 1 to 4, wherein the silicone resin is a polymeric alkoxyfunctional siloxane.

6 A coating according to any one of claims 1 to 5, wherein the silicone resin IS a polymeric ethoxyfunctional siloxane.

7 A sealing-enhancing coating substantially as hereinbefore described with reference to any one of the illustrative coatings.

8 A method of forming a gasket ha ing a sealingenhancing coating, the method comprising forming a spreadable mixture comprising silicone resin and a siliconate soluticn, spreading the mixture over at least a portion of a sheet of a gasket, and curing the mixture.

9 A method according to claim 8, wherein the silicone resin is substantially free of organic solvents and water, A method according to either one of claims 8 and 9, wherein the mixture also contains one or more curing agents.

11 A method according to claim 10, wherein the curing agent is selected from polybutyltitanate, similar organotitanium complexes, organo-metallic tin complexes, organo- aluminium complexes, and zinc oxide.

12 A method according to any one of claims 8 to. 11, wherein the mixture comprises a molecular architecture modifier.

13 A method according to any one of claims 8 to 12, wherein the mixture comprises a react ion- retarding catalyst.

14 A method according to any one of claims 8 to 13, wherein the mixture also comprises a reactive diluent.

A method according to claim 14, wherein the reactive diluent is a silane.

16 A method according to any one of claims 8 to 15, wherein the mixture also contains a filler.

17 A method according to claim 16, wherein the filler is selected from mica, milled exfoliated vermiculite, wollastonite, graphite, calcined china clay, barium sulphate, calcined alumina, and carbon black.

18 A method according to any one of claims 8 to 17, wherein the method also comprises forming at least one 12 hole through said sheet and coating, embossing at least one ridge into said sheet and coating, and tempering said sheet.

19 A method of forming a gasket coating substantially as h ereinbefore described with referenCE to any one of the illustrative methods.