DE19964627B4 - Coating for sealing substrates, especially for production of cylinder head gaskets, contains thermoplastic fluoropolymer and shows decreasing hardness from the coated surface outwards - Google Patents

Abstract

The coating comprises thermoplastic fluoroplastic(s) (I) and shows a hardness which decreases from the surface in contact with the substrate to the other surface away from the substrate. Independent claims are also included for: (a) a method for the production of a coating as described above by applying layers of plastic of decreasing hardness to a base, and then sintering or melting the layers by heating; and (b) a seal, especially a cylinder head gasket, consisting of or provided with a coating as described.

Description

The invention relates to a seal, in particular a cylinder head gasket, and a method for the production thereof.

From the DE 38 05 766 A1 is a multilayer coating based on a partially filled fluorine-containing thermoplastic known. From the DE 36 18 786 A1 is a flat gasket made of a mixture of PTFE and a refractory thermoplastic known. From the EP 0 268 233 B1 Flat gaskets based on filled fluoroplastic are known. From the DE 689 11 265 T2 is a sintered three-layer composite of partially filled PTFE layers known. From the WO 96/13676 A1 is known a three-layer sealing film whose outer layers are formed as porous PTFE membranes. From the EP 607 934 A1 For example, a paint formulation for a non-stick coating comprising PTFE and a PAI resin is known. From the WO 98/04853 A1 For example, a solid sealing element comprising PFA, PEEK, PPS, or other injection molding grade plastic is known.

In seals, coatings often serve not only to protect the coated materials from weathering or the like, but also to improve the sealing properties of the seal. Above all, this requires a high adaptability of the coating to the counter surface to be sealed, so as to be able to compensate for roughness or unevenness in the counter surface. For this purpose, the coating must have a high deformability.

However, there are also cases in which, in addition to good mating surface adaptability, good continuous sliding properties are desired. However, good continuous conduction requires low flow properties of the coating and low deformability, ie properties that run counter to good adaptability of the coating to the counterface.

An example of a gasket, which should have both a good Dauerergleitung as well as a high adaptability to the counter-surfaces to be sealed, is a cylinder head gasket. So far, cylinder head gaskets are usually provided with a thin coating of about a few microns thick, which should improve the adaptability of the seal to bumps and roughness of the sealed mating surfaces (engine block and cylinder head).

Such conventional coatings consist for example of rubber, soft metals or molybdenum disulfide. Although these coatings improve the adaptability of the cylinder head gasket to the mating surfaces, they are unsatisfactory in terms of continuous lubrication. Rather, the conventional coatings flow laterally away in the areas in which they are exposed to a particularly high surface pressure, so that in these areas the cylinder head gasket immediately after a certain break-in time comes into direct contact with the counter surface to be sealed.

There was therefore a need for a coating which combines a good adaptability to the sealed counter surface on the one hand and good continuous sliding properties on the other hand and which retains these properties even at high surface pressure and high temperature.

The object of the invention is to provide a seal comprising such a coating and a method for the production thereof.

The solution of this problem is achieved with the seal according to claim 1. Preferred embodiments and a method for producing the further claims can be found.

The combination of the properties of continuous conduction with high adaptability is combined with; a seal with a coating according to the features of claim 1 according to the invention achieved. The softest region of the coating of the seal according to the invention is located on that surface which is located away from the substrate to which the coating is to be applied. Towards the substrate to be coated, the hardness of the coating increases. Due to the increasing hardness, the flow properties and deformability of the seal according to the invention comprising the coating are reduced. Thus, the seal according to the invention comprising the coating has a hardness gradient which is so designed that the coating is particularly soft in the region of one of its surfaces, while its hardness increases in the direction of the other surface. This soft surface allows excellent adaptation to a counter surface to be sealed. By using a thermoplastic fluoroplastic, this surface also has an extremely low friction and high Chemical resistance on. The temperature resistance is excellent. However, unlike conventional coatings, the coating does not flow away completely even at high surface pressure and high temperature since its hardness increases toward the first surface adjacent to the substrate to be coated. This reduces deformability and flow properties. This increase in hardness is expediently carried out continuously. The hardness of a plastic can be determined, for example, on the basis of the ball compression hardness (DIN 53456) or the compressive strength according to ASTM D695.

A hardness gradient in the coating can be achieved by coating the seal according to the invention, however, even at high surface pressure and fluoroplastic at least one filler or reinforcing material is added, the concentration of which increases towards the first (substrate) surface. The amount of fillers or reinforcing materials depends primarily on the requirements that are placed on the coating. Usual filler amounts for the soft side of the coating are generally between 0 and 10% by volume, in particular 0 to 5% by volume. On the hard (substrate) side, the fillers or reinforcing materials are usually used in a proportion of 20 to 40, in particular 30 to 40,% by volume.

In the invention, the coating comprises, in addition to the fluoroplastic, at least one thermoplastic polycondensate which has a greater hardness than the fluoroplastic. The concentration of the thermoplastic polycondensate increases toward the first (substrate) surface of the coating. Again, the increase in concentration and thus the increase in hardness takes place essentially continuously.

Suitably, the proportion of fluoroplastic in the region of the first surface 0 wt .-%, since in this way the adhesion to the substrate can be improved. In the region of the second surface, which faces away from the substrate, preferably 80 to 100 wt .-% fluoroplastic and in particular 95 to 100 wt .-% present.

Also, at least one filler or reinforcing material may be added to the thermoplastic polycondensate. Suitably, this filler or reinforcing material is used to adjust the hardness in the polycondensate according to the requirements. In principle, all conventional fillers or reinforcing materials can be used. Particularly preferred are graphite, carbon, carbon fibers or glass fibers.

The thermoplastic polycondensate of the coating of the seal according to the invention has an upper continuous service temperature of at least 200 ° C.

For the same reasons, it is expedient for the thermoplastic polycondensate to have a dimensional stability under heat (measured according to ASTM D648 at 1.8 N / mm ² ) of at least 140 ° C., in particular of at least 180 ° C. and preferably of at least 190 ° C. having.

Also, for use in cylinder head gaskets or similar uses, high hydrolysis resistance, even at high temperatures, and good oil and grease resistance is required. It is therefore used as a thermoplastic polycondensate in the coating of the seal according to the invention one which is resistant to hydrolysis and resistant to hot water vapor to at least 130 ° C. The oil and grease resistance of the thermoplastic polycondensate should preferably be up to a temperature of at least 150 ° C.

It is also advantageous if the thermoplastic polycondensate used has good sliding friction properties. It preferably has a coefficient of sliding friction μ at 40 ° C. of ≦ 0.7 and in particular ≦ 0.55.

The thermoplastic polycondensate comprises polyaryletherketone, preferably selected from polyetherketone (PEK) and polyetheretherketone (PEEK).

Thermoplastic polycondensates which are further suitable for the coating are a polysulfone (PSU), polyphenylene sulfide (PPS), polyphenyl ether sulfone (PPSU), polyethersulfone (PES). The above-listed properties can optionally be achieved by appropriate addition of fillers or reinforcing agents or by mixing various thermoplastic polycondensates.

Other thermoplastic polycondensates such as polyamides, polyimides or polyesters are less suitable for use in the coating of the seal according to the invention, since they tend to hydrolysis, especially at elevated temperature. Due to their outstanding mechanical and thermal properties as well as their excellent oil, grease and hydrolysis resistance even at high temperatures PPS, PEK and PEEK, filled or unfilled, as well as their mixtures to the inventively preferred thermoplastic polycondensates.

The fluoroplastic of the coating of the gasket of the present invention is suitably selected from polytetrafluoroethylene (PTFE), tetrafluoroethylene / hexafluoropropylene copolyer (FEP) or perfluoroalkoxy copolymer (PFA), which can be used filled or unfilled. It is also possible to use mixtures of these fluoropolymers.

The adhesion of the coating of the seal according to the invention to the substrate which is to be coated is usually sufficient, in particular if no fluoroplastic is present in the region of the surface of the coating facing the substrate. However, it is also possible to add at least one adhesion promoter and / or at least one adhesive in the region of the substrate surface. The choice of these substances is not particularly limited. It only needs to be taken care of since they are compatible with both the coating and the substrate.

The coating is conveniently prepared by the method described in claim 13. For this purpose, successively layers of plastic with decreasing hardness are applied to a pad, and the individual layers are sintered or melted under heating and thus joined together.

In the case of a coating in which the hardness is varied over the concentration of fillers or reinforcing agents, for example, various fluoroplastics are provided with different content of filler or reinforcing material. The plastic with the largest amount of filler is first applied to the substrate and sintered. Subsequently, a layer of the fluoroplastic with the next lower filler content is applied and sintered again. The process continues up to the fluoroplastic with the least or no filler content. The sintering process not only leads to a connection of the individual layers, but also to a homogenization of the Füllstoffgefälles within the coating.

The procedure is analogous to a coating of the seal according to claim 1. Thus, batches of a thermoplastic polycondensate are provided which have different proportions of fluoroplastic. The batch is first applied and heated, which contains the least amount of fluoroplastic. The topmost layer is the one with the highest fluoroplastic content. The hardness of the individual layers can also be varied by further additions such as different proportions of fillers or reinforcing materials, admixing of a further thermoplastic polycondensate, etc.

The heating temperature depends on the components of the coating used. For the specified as preferred components, a temperature range of 350 to 400 ° C has been found suitable.

The layered application and heating can be done directly on the substrate to be coated as a base. Alternatively, however, it is possible to prepare the coating according to the invention independently of the substrate to be coated. Thus, a suitable substrate is used which is not the substrate to be coated. The finished coating is removed from the substrate and may optionally be provided with an adhesive layer to facilitate later application to the substrate to be coated. In the case that the base on which the coating of the gasket according to the invention is produced is not the substrate to be coated, it is basically possible to build up the coating from the soft to the hard side. It is more convenient, however, to start applying the hardest layer to the substrate.

The coating is used for a gasket and in particular a cylinder head gasket. In one aspect, the coatings replace those coatings heretofore used in known seals or cylinder head gaskets.

In a further aspect, the gasket according to the invention itself, and in particular the cylinder head gasket, consists entirely of the coating. In this case, therefore, no substrate is present, to which the coating according to the invention is applied. Rather, the coating of the seal according to the invention is produced during its production in the form required for the seal and with the required thickness distribution. This is for example possible because the coating is introduced into a correspondingly formed sintered mold and, if necessary, the layer thickness is varied over the sealing surface.

The invention will be explained in more detail with reference to an example. The proportions are given in percent by weight.

example 1

Production of a coating for a cylinder head gasket

One according to EP 0835399 A1 manufactured one-piece metal seal is provided on both sides with a coating. The procedure is as follows:
Plastic blends of the following composition were provided: Mixture a) 95% PEEK 5% graphite Mixture b) 80% PEEK 10% PTFE 3% PFA 7% graphite Mixture c) 70% PEEK 15% PTFE 8% PFA 7% graphite Mixture d) 60% PEEK 20% PTFE 13% PFA 7% graphite Mixture e) 50% PEEK 25% PTFE 18% PFA 7% graphite Mixture f) 40% PEEK 30% PTFE 23% PFA 7% graphite Mixture g) 30% PEEK 35% PTFE 28% PFA 7% graphite Mixture h) 20% PEEK 40% PTFE 30% PFA 10% graphite Mixture i) 10% PEEK so% PTFE 30% PFA 10% graphite Mixture j) 60% PTFE 30% PFA 10% graphite

First, mixture a) was applied in the form of an aqueous dispersion to the thoroughly degreased surfaces of the cylinder head gasket to be coated. The layer was first dried in an oven at 100 ° C and then sintered at 380 ° C. After cooling, mixture b), likewise as aqueous dispersion, was applied to the first layer and dried and sintered like this. The mixtures C) to j) were applied in succession in the same way.

The total layer thickness was 60 μm. In a long-term test in an engine, the cylinder head gasket provided with the coating according to the invention showed excellent tightness. The coating of the seal according to the invention provided for an excellent adaptation to the counter-surfaces to be sealed, simultaneously high stability and excellent continuous sliding properties.

Claims (14)

Seal comprising a coating, In which the coating contains at least one thermoplastic fluoroplastic and at least one thermoplastic polycondensate which has a greater hardness than the fluoroplastic and comprises polyaryletherketone, and has an upper continuous service temperature of at least 200 ° C., In which the thermoplastic polycondensate is resistant to hydrolysis and is resistant to hot steam up to at least 130 ° C., In which the hardness of the coating decreases continuously from the first surface intended for application to the substrate in the direction of the second surface remote from the substrate, In which the concentration of the thermoplastic polycondensate continuously increases in the direction of the first surface of the coating, - In which the first surface facing bottom layer of the coating has the lowest proportion and the top layer has the highest content of fluoroplastic. Seal according to claim 1, wherein at least one filler or reinforcing material is added to the at least one thermoplastic polycondensate. Seal according to one of claims 1 to 2, wherein the at least one filler or reinforcing material is selected from graphite, carbon, carbon fibers or glass fibers. Seal according to one of claims 1 to 3, wherein in the region of the first surface, the proportion of fluoroplastic 0 wt.% And in the region of the second surface 80 to 100 wt.% And in particular 95 to 100 wt.%. A gasket according to any one of claims 1 to 4, wherein the thermoplastic polycondensate has a heat distortion temperature (ASTM D648: 1.8 N / mm ² ) of at least 140 ° C, more preferably at least 180 ° C and preferably at least 190 ° C having. Seal according to one of claims 1 to 5, wherein the thermoplastic polycondensate is oil and grease resistant and in particular up to a temperature of at least 150 ° C. Seal according to one of claims 1 to 6, wherein the thermoplastic polycondensate has a sliding friction coefficient μ at 40 ° C of ≤ 0.7 and in particular ≤ 0.55. A gasket according to claim 1, wherein the polyaryletherketone is selected from a polyetherketone (PEK) or a polyetheretherketone (PEEK). Seal according to one of claims 1 to 8, wherein the coating comprises at least one further thermoplastic polycondensate, which is selected from a polysulfone (PSU) polyphenylene sulfide (PPS), polyphenyl ether sulfone (PPSU) and polyethersulfone (PES). Seal according to one of claims 1 to 9, wherein the thermoplastic polycondensate is a mixture of filled and unfilled PPS, PEK and / or PEEK. Seal according to one of claims 1 to 10, wherein the fluoroplastic is selected from filled or unfilled polytetrafluoroethylene (PTFE), tetrafluoroethylene / hexafluoropropylene copolymer (FEP), perfluoroalkoxy copolymer (PFA) or mixtures thereof. Seal according to one of claims 1 to 11, wherein the coating in the region of the first surface has at least one adhesion promoter and / or at least one adhesive. A method of producing a gasket comprising a coating according to claim 1, wherein layers of decreasing hardness which successively cover the fluoroplastic and / or the polycondensate are successively applied to a substrate and sintered or fused under heating. A method according to claim 13, wherein heating is carried out at 350 to 400 ° C during sintering.