Coating processes – With post-treatment of coating or coating material – Heating or drying
Reexamination Certificate
1999-08-30
2002-07-23
Dawson, Robert (Department: 1712)
Coating processes
With post-treatment of coating or coating material
Heating or drying
C427S515000, C427S517000, C522S025000, C522S026000, C522S027000, C522S031000, C522S065000, C522S066000, C522S067000, C522S068000, C522S099000, C528S013000, C528S019000, C528S021000, C528S023000, C528S032000, C528S037000
Reexamination Certificate
active
06423378
ABSTRACT:
The area of the present invention is that of silicone compositions based on organofunctional polyorganosiloxanes (POS) crosslinkable by the cationic route under the effect of activation obtained via light radiation, preferably UV radiation, and/or via an electron beam and/or via thermal energy.
More specifically, the invention relates to the use of such compositions for the treatment of sheet gaskets, in particular for impregnating or varnishing cylinder head gaskets of internal combustion engines.
It relates in particular, using these compositions crosslinkable by the cationic route under UV radiation, e.g., to impregnating sheet gaskets (cylinder head gaskets) and/or to preparing antiadhesive coatings at the engine block/cylinder head interface of heat engines, the said interface preferably being the cylinder head gasket itself.
The silicone compositions used for this purpose were not until now silicone compositions crosslinkable by the cationic route under UV radiation.
The functioning of an internal combustion engine requires strong cohesion of the stationary components, thorough lubrication of the moving components and efficient cooling of the engine block assembly. The cylinder head gasket has a two-fold role; it provides for the cohesion of the engine block/cylinder head dismountable assembly and for the leaktightness of the intersecting circulations of the cooling liquid (water to which glycol and alkaline agents have been added), oil and gases in the course of combustion.
Cylinder head gaskets can be made of highly complex composite materials comprising a metal core made of finely perforated sheet metal, a board (preferably devoid of asbestos) composed of organic fibres and of inorganic fillers, crimped firing rings opening on the explosion chamber, and a kerb made of silicone elastomer to contain possible leaks onto the surface.
Cylinder head gaskets can also be composed simply of one or more metal sheets optionally covered with an NBR rubber elastomer coating or fluorinated elastomer coating of the Viton type (cf., for example, JP-A-082 09 113).
Cylinder head gaskets are generally subjected, prior to their use, to two types of treatment using silicones:
1—an impregnation treatment intended to fill the voids in order to render the board water-repellent and to improve its thermal behaviour,
2—and a varnishing treatment intended to lower the coefficient of friction of the gasket and to bring about good antiadhesiveness.
In addition to these treatments, a kerb made of silicone elastomer is also deposited, by silk-screen printing, on cylinder head gaskets.
The impregnation of cylinder head gaskets with crosslinked silicones is recommended in the case where the support is porous. This makes it possible to ensure leaktightness with regard to the various operating fluids of the engine (oils, petrol, fuel oil, cooling liquid or combustion gases). Furthermore, the impregnation provides the gasket with a degree of compressibility which it has to have during the flattening to which it is subjected when the cylinder head is tightened.
When the cylinder head is taken off, the gasket must be removed very easily without adhering to the surfaces of the cylinder head. It is therefore essential for both faces of the gasket to be antiadhesive and this is one of the essential roles of the varnishing by crosslinked silicone compositions. Varnishing with the silicone contributes to rendering leakproof but also to protecting the gasket with respect to possible damage during the operation of the engine (mechanical stress: thermal stress and/or shearing: heating/cooling).
The impregnating agents and varnishes conventionally used are compositions composed of silicone oils or resins of low viscosity comprising SiOH or SiH functional groups (U.S. Pat. No. 4,720,316; EP-A-272,282) or SiVinyl functional groups (DE-A-3,731,032; U.S. Pat. No. 4,499,135) or of mixtures of a silicone oil or resin comprising SiH functional groups and of a silicone oil or resin comprising SiVinyl functional groups (DE-A-3,718,599; EP-A-471,979; DE-A-3,544,740) capable of crosslinking at high temperature (generally greater than 100° C.) in the presence of a metal catalyst, such as tin, titanium, sirconium or platinum salts, or of a peroxide.
Compositions based on silicone oils or resins comprising SiOH, SiH, SiVinyl or SiH/SiVinyl functional groups generating exhibit at least one of the following disadvantages:
the mixture composed of the silicone matrix and the catalyst is unstable at ambient temperature (in particular in the absence of solvent), this instability being reflected by an increase in viscosity, indeed even by a gelling, of the impregnation or varnishing bath, which requires frequent replacement of the impregnation or varnishing bath or treatment in two stages; for example, Patents U.S. Pat. No. 4,720,316 and EP-A-272,382 recommend the use of the catalyst during the manufacture of the board and then impregnation of the dried board with a silicone oil possessing SiH groups;
it is often necessary to disperse the silicone+catalyst mixture in an organic solvent in order to increase the stability and to decrease the viscosity of the said mixture; this results in the disadvantages related to the use of solvents (toxicity, provision of safety devices, of devices for recycling the solvent, and the like)
the cost price of the starting materials is high, in particular in the case of vinylated oils or resins optionally used in combination with oils or resins possessing SiH functional groups and with a platinum catalyst; these mixtures can be very stable at ambient temperature, in particular in the presence of a platinum inhibitor, but are expensive because of the use of vinylated oils or resins.
In the quest for a novel silicone composition for the treatment of cylinder head gaskets having the object of improving the abovementioned known compositions (Rhône-Poulenc French patent application on cylinder head gaskets—U.S. Pat. No. 2,697,532), provision has been made to use a silicone composition crosslinkable by hydrosilylation comprising an &agr;,&ohgr;-diOH POS, a POS possessing an SiH unit of the PMHS type, and ethynylcyclohexanol, the latter compound making it possible to form resins possessing a silanol functional group; the ratio of SiH to SiOH of the POS concerned being of the order of 10/1 to 30/1 and the composition also comprising a polyaddition catalyst of the Karstedt type.
Even if such a composition effectively leads to improvement with regard to stability, reactivity and flexibility of use, it still remains the case that the crosslinking specific to this type of composition comprises a restrictive stage of heating at high temperature (160° C.) for 5 to 10 minutes. Such a stage is particularly penalizing with regard to the productivity of an industrial process for the treatment or coating of cylinder head gaskets using silicones. It may thus be necessary, for example, to resort to the use of sophisticated and expensive equipment, such as long heating tunnels. It therefore seems that these silicone compositions, crosslinkable by polyaddition and used for the coating of cylinder head gaskets, still do not contribute an entirely satisfactory solution to the problem of carrying out silicone impregnation and treatment of cylinder head gaskets which meet the specifications of antiadhesiveness and of compressibility, as well as the requirements of productivity and of profitability for large scale production.
French Patent Application No. 2,722,203 relates to the application of a crosslinkable fluorosilicone coating to a cylinder head gasket. Such a coating is obtained from a silicone composition crosslinkable, under the effect of heat, of UV radiation or of an electron beam, by hydrosilylation. This composition comprises a vinylated fluorosilicone, a POS carrying SiH units, a platinum hydrosilylation catalyst, a condensation catalyst of the peroxide type and, optionally, a solvent of the halogenated solvent, ester or acetone type. The support to which this composition can be applied is described as preferabl
Cotting Marie-Christine
Joubert Gérard
Loubet Olivier
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