Coolant resistant and thermally stable primer composition

[0002] The invention relates to primer compositions that are useful for bonding elastomers to various substrates, such as metals, ceramics, fiberglass, and organic polymers. The primer compositions of the invention contain organic-functional silane compounds.

[0003] Since the late 1980's, elastomer coated stainless steel coils have been used to design multi-layered steel engine head gaskets. In particular, these stainless steel coils have been coated with elastomers such as nitrile butadiene rubber or fluoroelastomer. Application of a thin primer between the rubber coating and stainless steel surface is a common industrial practice since an elastomer coating alone does not provide the desired wet adhesion on stainless steels. Generally speaking, the resistance of the primer to the surrounding media, such as engine oil, coolant, fuel and heat, in a great degree, decides the performance and durability of the gasket.

[0004] Numerous silane adhesive compositions have previously been developed for the purpose of bonding elastomeric materials to various substrates. For example, U.S. Pat. Nos. 5,532,314 and 5,907,015 relate to silane adhesive compositions which have various organic-functional silanes as a bonding agent. Primers based on silanes generally show good dry adhesion in bonding organic coatings to steel or other metal substrates. However, some of the silane based primers have been found to not have the desired durability in hot coolant immersion.

[0005] Phenolic resoles toughed by elastomers or flexible organic resins are widely used as primers or structural adhesives to bond metals. The adhesion between the phenolic resole based primers and the metal surface can be enhanced by addition of organic silanes. In many applications, satisfactory adhesion is achieved using a phenolic/silane based primer. However, the adhesion is gradually lost under aggressive wet environments such as boiling water exposure or hot coolant immersion.

[0006] Thus, there is a need in the art for a primer or adhesive composition for bonding elastomers to substrates which exhibits good adhesion in both dry and wet applications. There further is a need in the art for a primer or adhesive composition which shows robust dry and wet adhesion to metallic or other surfaces under harsh environments such as hot oil, hot coolant and thermal aging. Additionally, there is a need in the art for a coolant resistant primer or adhesive for bonding elastomers to substrates which also is thermally stable. Moreover, there is a need in the art for a primer composition which can bond elastomers to substrates without surface treatment of the substrate. This invention provides such a primer and a method for bonding an elastomer to a substrate with the primer of the invention.

[0007] The present invention provides primer compositions which bond elastomers to substrates such as metals, ceramics, fiberglass, or organic polymers. In particular, the invention is directed to a primer composition which includes an organic-functional silane adduct. In one embodiment, the organic-functional silane adduct is prepared by reacting at least one amino-functional silane with at least one isocyanato-functional silane. In another embodiment, the primer composition comprises a phenolic resin, a phenoxy resin, and an organic-functional silane adduct, preferably an organic-functional silane adduct wherein the silane adduct is prepared by reacting at least one amino-functional silane with at least one isocyanato-functional silane. In a preferred embodiment, the primer composition further comprises talc.

[0008] The invention further is directed to a method of bonding an elastomer to a metal comprising coating the metal with a primer composition comprising a phenolic resin, a phenoxy resin, and an organic-functional silane adduct, drying the primer composition coating, applying an elastomer coating to the primer composition coating, and curing the elastomer coating with heat.

[0009] It has been found that the primer compositions of the present invention containing an organic-functional silane adduct provides robust dry and wet adhesion, particularly of elastomers to substrates such as metals, when these adducts are used in compositions as primers, adhesives or as a protective coating on metals.

[0010] The silane adducts used in accordance with the present invention are organic-functional silane compounds. An organic-functional silane adduct as used herein is a reaction product of two or more organic functional silane compounds. Preferably, the silane adducts are prepared by reacting at least one amino-functional silane with at least one isocyanato-functional silane. It has unexpectedly been found that the reaction products of these types of organic-functional silanes provide good adhesion in both wet and dry applications. The reaction of an amino-functional silane and an isocyanato-functional silane results in a hexaalkoxylsilane bonded through a urethane or substituted urea linkage. The resulting adduct provides excellent properties when used as a primer, even in aggressive wet environments.

[0011] Silanes previously have been used as adhesion promoters for organic coatings. However, it is known that the siloxane bond formed after silane crosslinking can be hydrolyzed with water. Consequently, most of the silane based primers have good dry adhesion but poor wet adhesion. The primer composition of the present invention has overcome this deficiency of the art by utilizing an organic-functional silane adduct, preferably synthesized by the reaction of an amino-functional silane with an isocyanato-functional silane. While not wishing to be bound by any theory, it is believed that this adduct provides superior wet adhesion for a number of reasons. First, this adduct has a higher molecular weight than that of the individual silane and that of silanes known in the art. The higher molecular weight of the silane adduct is believed to reduce the evaporation rate during the cure of the coating in hot air. Thus, this silane adduct is more effectively involved in the metal and coating adhesion. Secondly, this silane adduct becomes a more effective crosslinker by doubling the methoxy or ethoxy groups of the individual silanes through the reaction. Thus, this silane adduct should have higher crosslinking density at the primer to substrate interface under water attack.

[0012] Amino-functional silanes useful in the present invention may be any amino-functional silane capable of reacting with an isocyanato-functional silane. In one embodiment, the amino-functional silane is an aminoalkyl silane. In another embodiment, the amino-functional silane is a secondary amino-functional silane. In a preferred embodiment, the amino-functional silane is a phenyl-substituted amino-functional silane. Examples of amino-functional silanes which are useful in the present invention include those available under the tradename Silquest™. Examples of useful Silquest™ amino-functional silanes include Silquest™ Y-9669, N-phenyl-gamma-aminopropyltrimethoxysilane, Silquest™ A1170, bis-(gamma-trimethoxysilylpropyl)amine, Silquest™ A1100, gamma-aminopropyltriethoxysilane, Silquest™ A1110, gamma-aminopropyltrimethoxysilane, and Silquest™ A1120, N-(beta-aminoethyl)-gamma-aminopropyltrimethoxysilane. The preferred amino-functional silane for use in the present invention is Silquest™ Y-9669.

[0013] The isocyanato-functional silane used in the primer composition of the present invention may be any isocyanato-functional silane which can react with an amino-functional silane. Generally, the isocyanato-functional silane has at least one isocyanate group. In a preferred embodiment, the isocyanato-functional silane is an isocyanatoalkylalkoxysilane, wherein the alkyl and alkoxy groups have 1-5 carbon atoms. An example of an isocyanato-functional silane useful in the present invention is Silquest™ A-1310, which is ganima-isocyanatopropyltriethoxysilane.

[0014] The relative proportion of amino-functional silane to isocyanato-functional silane present in the composition of the invention may vary somewhat depending on the use intended. Generally, the relative proportion of amino-functional silane to isocyanato-functional silane will be about 1:0.2 to about 1:1. In a preferred embodiment, the relative proportion of amino-functional silane to isocyanato-functional silane will be about 1:0.5 to about 1:0.8.

[0015] The organic-functional silane adduct generally will be present in the primer composition in an amount of about 0.2% to about 15% based on the dry weight of the total composition. Preferably, the silane adduct will be present in the primer composition in an amount of about 0.5% to about 5% based on the dry weight of the total composition. In a preferred embodiment, an excess of amino-functional silane will be present in the primer composition. This excess amino-functional silane is provided to react with any remaining isocyanate from the previous reaction of amino-functional silane with isocyanato-functional silane.

[0016] The phenoxy resin used in the composition of the present invention is provided as a toughener. Any phenoxy resin may be used; however, in a preferred embodiment, the phenoxy resin is crosslinked by a phenolic resin. It has been found that the reaction product of phenoxy resin crosslinked by phenolic resin has satisfactory thermal stability and hydrophobicity, enhancing the wet adhesion of the primer to the substrate. When phenoxy resin and a phenolic resin are provided in the primer composition, heat provided during the curing process in the bonding of an elastomeric material to a substrate with the primer compositions enables the crosslinking of the phenoxy resin by the phenolic resin.

[0017] The phenoxy resin may be any commercially available phenoxy resin such as Paphen phenoxy resin. The phenoxy resin generally will be present in the primer composition in an amount of about 10% to about 80% based on the dry weight of the total composition. In a preferred embodiment, the phenoxy resin is present in an amount of about 20% to about 50% based on the dry weight of the total composition.

[0018] The phenolic resin useful in the primer composition may be any resole or thermoset phenolic resin. In a preferred embodiment, the phenolic resin generally will be an alkylated thermosetting phenolic resin. In the most preferred embodiment, the phenolic resin is a butylated thermosetting phenolic resin such as GPRI 7590. The phenolic resin generally will be present in the primer composition in an amount of about 5% to about 70% based on the dry weight of the total composition. In a preferred embodiment, the phenolic resin is present in an amount of about 15% to about 45% based on the dry weight of the total composition.

[0019] The primer composition of the present invention also preferably contains a filler component, such as carbon black, metal oxides, organic fillers or talc. In a preferred embodiment, the filler component is talc. Any talc may be used, but the preferred talc is a surface treated talc, such as Mistron ZSC, which is surface treated with zinc stearate to obtain a hydrophobic surface. Due to the flake shape, talc is believed to have much better barrier properties than non-flake type fillers. Moreover, the surface treated talc has been found to have much higher hydrophobicity than untreated talc products. It is further believed that the high hydrophobicity of the treated talc reduces the water absorption rate which, in return, slows down the hydrolysis process of the crosslinked silanes of the present invention. The talc generally will be used in an amount of about 5% to about 60% by dry weight of the total composition. In a preferred embodiment, the talc will be used in an amount of about 20% to about 40% by dry weight of the total composition.

[0020] The primer compositions of the present invention can optionally contain well-known additives including plasticizers, pigments, reinforcing agents, and the like, in amounts employed by those skilled in the primer or adhesive arts to obtain the desired color and consistency. Typical optional additives include titanium dioxide, carbon black, and zinc oxide.

[0021] The dry components of the primer composition of the present invention generally will be mixed with one or more compatible solvents. These solvents may be organic solvents such as ketones, acetates, hydrocarbons, or alkyl alcohols. In one embodiment, the solvent component is a mixture of organic solvents. Preferred solvents include toluene, ethyl acetate or isopropyl alcohol. The solvent component generally will be used in an amount of about 20% to about 90% by wet weight of the total composition. In a preferred embodiment, the solvent will be used in an amount of about 60% to about 80% by wet weight of the total composition.

[0022] The primer compositions of the present invention are typically prepared by mixing the organic-functional silane with the phenoxy resin, the phenolic resin and any remaining ingredients. In a preferred embodiment, the composition is prepared by first reacting an amino-functional silane with an isocyanato-functional silane at room temperature by slowly adding the isocyanato-functional silane into the amino-functional silane under agitation. The remaining ingredients are then added to the silane adduct with mixing and agitation of the combination of ingredients in any conventional manner.

[0023] The materials which may be bonded to a substrate such as metal, ceramic, fiberglass, or organic polymers, with the primer compositions of the present invention, are preferably polymeric materials, including any elastomeric material selected from any of the natural rubbers and olefinic synthetic rubbers including polychloroprene, polybutadiene, neoprene, styrene-butadiene rubber, butyl rubber, brominated butyl rubber, nitrile butadiene rubber, fluoroelastomers, and the like. In a preferred embodiment, the primer composition is used for bonding nitrile butadiene rubber or a fluoroelastomer.

[0024] The substrate to which the elastomer is bonded with the primer composition of the invention can be any solid surface such as metal, ceramic, fiberglass, organic polymer or fabric capable of receiving the primer. Preferably, the primer composition will be used with a metal surface selected from any of the common structural metals such as iron, steel, including stainless steel, lead, aluminum, copper, brass, bronze, titanium, Monel metal, nickel, zinc, phosphatized steel, and the like.

[0025] The primer composition of the present invention may be used for a number of different applications, including as a primer, an adhesive, or a protective coating. For example, the primer compositions of the invention may be used as a surface protection coating for stainless steel and other metallic surfaces. Due to the good flexibility and coolant resistance of the primer composition, the primer composition may also be used as a general flexible, ethylene glycol resistant primer for rubber to metal bonding. In a most preferred embodiment, the primer composition of the invention is used for bonding nitrile butadiene rubber and/or a fluoroelastomer to stainless steel and the nitrile butadiene rubber and/or fluoroelastomer coated stainless steel is used to make multi-layer steel engine head gaskets. Such gaskets require aggressive environmental resistance to elements such as hot oil, hot coolant and heat aging. The primer composition of the invention provides all the performance needed for such applications.

[0026] The primer compositions are preferably used to bond an elastomeric material such as nitrile butadiene rubber or a fluoroelastomer to a metal surface. The primer composition may be applied to the metal surface by roll coating, spraying, dipping, brushing, wiping, screen printing, or the like, after which the adhesive is permitted to dry. The drying step may be by air drying or other method known to those of skill in the art. Generally, the air drying step will be conducted at temperatures less than about 300° F.

[0027] After the primer has been subjected to drying, an elastomer is applied to the primer surface by any of the methods known in the art. The elastomer is allowed to dry, also generally by air drying at temperatures less than about 300° F. The elastomer/substrate composite which has been adhered with the primer composition may be then cured with heat. The time and temperature of the cure will depend upon the application for which the elastomer to substrate composite is intended. For example, for some applications, a fast cure is desirable. In those cases, a high temperature between about 400° F. and about 550° F. is used for less than about two minutes. For applications where a longer cure is acceptable, the temperature generally will be about 300° F. to about 350° F. for about two to about 10 minutes. After the process is complete, the bond is fully vulcanized and ready for use in a final application.

[0028] The following examples are given to illustrate the invention and should not be construed to limit the scope of the invention.

[0029] A primer composition according to the present invention was made by first preparing a silane adduct by reacting Silquest A1310, an isocyanato-functional silane, with Silquest Y9669, an amino-functional silane. Silquest A1310 is gamma-isocyanatopropyltriethyoxysilane and Silquest Y9669 is N-phenyl-gamima-aminopropyltrimethoxysilane. The reaction of the isocyanato-functional silane with the amino-functional silane was conducted at room temperature by slowly adding A1310 into Y9669 under agitation. No by-product was formed in this reaction.

[0030] After the silane adduct was formed, the remaining ingredients were combined with the silane adduct. The composition was as follows:

[0031] This composition was draw-down coated on stainless steel coils. After the primer film was air dried at room temperature, a nitrile butadiene rubber coating was applied. After flashing off the solvents, a high temperature cure was conducted at 450° F. for 2 minutes.

[0032] The coated steel was then tested under different environments such as heat aging at 250° F., ASTM#3 oil immersion at 300° F. and coolant immersion at 212° F. for a specified time, such as 500 hours. The performance of the coating and primer was then evaluated by Scotch tape peel after crosshatch. The results were that a 0% peel off of the coating was achieved.

[0033] A primer composition according to the present invention was made by first preparing a silane adduct by reacting Silquest A1310, an isocyanato-functional silane, with Silquest Y9669, an amino-functional silane, as in Example 1. The reaction of the isocyanato-functional silane with the amino-functional silane was conducted at room temperature by slowly adding A1310 into Y9669 under agitation. No by-product was formed in this reaction.

[0034] After the silane adduct was formed, the remaining ingredients were combined with the silane adduct. The composition was as follows:

[0035] This composition was draw-down coated on stainless steel coils. After the primer film was air dried at room temperature, a fluoroclastomer coating was applied. The coated steel was then tested under different environments such as heat aging at 392° F., ASTM#3 oil immersion at 300° F. and coolant immersion at 212° F. for a specified time, such as 500 hours. The performance of the coating and primer was then evaluated by Scotch tape peel after crosshatch. The result was that a 0% peel off of the coating was achieved.

[0036] While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made without departing from the spirit and scope thereof.