Abstract:
An adhesive composition containing an aqueous butadiene polymer latex, and an acid scavenging compound, wherein the adhesive comprises essentially no phenolic resin or methylene donor compounds. The aqueous covercoat adhesive is particularly well suited for rubber to metal bonding applications. The adhesive have shown excellent adhesion to a number of compounds but perform best on natural rubber compounds and prefer higher cure temperatures to lower cure temperatures. Further, the formulation has been developed without the need for a methylene donor such as 2-polyoxymethylene dimethylether (gamma POM).

Description:
CROSS REFERENCE 
     This application claims the benefit of, and incorporates by reference, U.S. Provisional Patent Application No. 60/791,825 filed Apr. 13, 2006. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to compositions of matter useful for bonding various substrates such as elastomeric materials. More specifically, the present invention relates to environmentally resistant aqueous adhesive compositions based on polyvinyl alcohol-stabilized butadiene polymer latices. 
     BACKGROUND OF THE INVENTION 
     In light of the increasing awareness for environmental protection and workplace safety, a current major thrust of the adhesives industry is to develop an adhesive composition which avoids the use of highly volatile organic solvents which can cause damage to the environment and potentially affect the health of workers exposed to the solvent. It has thus far been relatively difficult to develop an aqueous adhesive which performs at a level equal to traditional solvent-based adhesives. One major problem associated with bonds formed from aqueous adhesives is the relative susceptibility of the bonds to high temperature fluids and corrosive materials. In applications involving the bonding of elastomeric substrates to surfaces such as metal surfaces, an aqueous adhesive must exhibit an affinity for the elastomeric substrate as well as possess the ability to withstand degradation by high temperature fluids or corrosive materials. 
     Various aqueous adhesives for bonding elastomeric materials have been developed in a continuing effort to obtain the ultimate aqueous adhesive for bonding elastomeric substrates. For example, U.S. Pat. No. 4,167,500 describes an aqueous adhesive composition that contains a water dispersible novolak phenolic resin, a methylene donor such as an acetal homopolymer or acetal copolymer, and water. The phenolic resins described are primarily derived from resorcinol and alkylphenols such as p-nonylphenol although various other polyhydroxy phenols are mentioned, such as phloroglucinol and pyrogallol. 
     U.S. Pat. No. 4,483,962 describes a terpolymer latex of an emulsion polymerized terpolymer of at least one 2,3-dihalo-1,3-butadiene monomer, at least one monoalkenyl aromatic alkylhalide monomer, and at least one olefinically unsaturated monomer. The terpolymer latex utilizes a surfactant such as an anionic surfactant or a mixture of an anionic surfactant and a non-anionic surfactant. 
     U.S. Pat. No. 4,988,753 describes an aqueous bonding composition containing (1) a mixture of chlorosulfonated polyethylene and vinyl chloride/vinylidene chloride/acrylic acid copolymer, (2) an organic polynitroso compound, and (3) a coreactive compound selected from diallyl acrylamide and phenylene bis-maleic acid imide. The adhesive composition may also optionally contain adhesion promoters, fillers, and processing aids. 
     U.S. Pat. No. 5,036,122 describes an aqueous adhesive composition which is a blend of a latex of a polymerized conjugated diene, a poly-C-nitroso compound, and a polymaleimide compound which is a polymer of a bismaleimide. 
     U.S. Pat. No. 5,496,884 relates to an aqueous adhesive composition comprising a polyvinyl alcohol-stabilized butadiene polymer latex and a methylene donor compound wherein the butadiene polymer latex is prepared by an emulsion polymerization in the presence of polyvinyl alcohol. The methylene donor comprises any compound which is compatible with the other ingredients of the adhesive composition and which is capable of forming a methylene bridge or linkage between the free hydroxyl groups of the polyvinyl alcohol-stabilized butadiene latex at elevated temperatures. U.S. Pat. No. 5,496,884 is hereby incorporated by reference. 
     Many of the previously developed aqueous adhesive compositions such as those described above do not provide adhesive performance at the same level as traditional solvent-based adhesive compositions. A need currently exists for an aqueous adhesive composition that will exhibit substantial affinity for an elastomeric substrate as well as produce a bond that will withstand harsh environmental conditions such as those caused by high temperature fluids and corrosive materials. 
     SUMMARY OF THE INVENTION 
     A first embodiment of the present invention is directed toward an aqueous covercoat adhesive particularly well suited for rubber to metal bonding applications. In one embodiment of the present invention, the adhesives have shown excellent adhesion to a number of compounds but perform best on natural rubber compounds and prefer higher cure temperatures to lower cure temperatures. Further, the formulation has been developed without the need for a methylene donor such as 2-polyoxymethylene dimethylether (gamma POM). Optimized formulations within the scope of this invention have provided excellent results in both primary adhesion tests and some long-term environmental tests. Strengths include higher temperature bonding. The adhesive performs best at around 340° F. and higher although depending on the compound performance at 320° F. can be excellent as well. 
     Gamma POM, the methylene donor used in previous commercially available adhesives is also eliminated from the formulations of the present invention. It was previously believed to provide some cross linking of the PVOH stabilized butadiene latex helping the formulation in both primary adhesion and environmental resistance. However, removing the methylene donor helps eliminate foaming, removes a source of formaldehyde in the formulation and improves the appearance of the adhesive since the methylene donor chosen is difficult to grind down to the desired particle size. Particular advantages of the new adhesive formulation include excellent high temperature performance, excellent prebake resistance, and excellent environmental performance. 
     In a further embodiment of the present invention, the aqueous adhesive can be prepared without a commonly used nitroso or maleimide crosslinker, dinitrosobenzene (DNB). The omission of DNB in the formulation eliminates or greatly reduces a number of problems found in the prior art. For example, DNB is known to sublime at elevated temperatures. It will stain molds causing parts to hang up and when this happen usually the mold will be taken out of service and be cleaned. The fumes also can cause knit-lines on bonded parts where DNB fumes cure the surface of the rubber as it enters the mold cavity. Knit-lines can cause a bonded part to fail prematurely if the problem is severe enough. Adhesive formulations of this invention have far greater resistance to prebake compared to adhesives containing DNB. 
     The present invention comprises an aqueous adhesive composition that exhibits substantial affinity for an elastomeric substrate and that produces an environmentally resistant adhesive bond. One embodiment of the present invention provides an adhesive composition comprising a polyvinyl alcohol-stabilized butadiene polymer latex in combination with an acid scavenger and a cross-linking agent. The latices utilized in the present invention can be prepared by the emulsion polymerization of butadiene monomers, or a combination of butadiene monomers and additional copolymerizable monomers in the presence of polyvinyl alcohol. More specifically, the latices of the present invention are prepared by initiating and maintaining free radical polymerization of the appropriate monomers in an aqueous suspension of polyvinyl alcohol. A butadiene homopolymer may be prepared by utilizing a butadiene monomer such as 2,3-dichloro-1,3-butadiene, or a copolymer or terpolymer of polybutadiene monomers may be produced by copolymerizing a combination of butadiene monomers or by copolymerizing one or more butadiene monomers with other copolymerizable monomers. Examples of other copolymerizable monomers include α-haloacrylonitrile, acrylic acid, methacrylic acid, and styrene sulfonic acid. 
     In another embodiment of the present invention, it has been discovered that the combination of the acid scavenger and a cross-linking agent with the polyvinyl alcohol-stabilized butadiene polymer latex results in an adhesive composition which exhibits an unexpected improvement in bonding performance, particularly with respect to resistance to high temperature fluids and corrosive materials without the need for a methylene donor compound. 
     Embodiments of the present invention provide an aqueous adhesive that when applied over the appropriate primer can bond a variety of rubber compounds such as natural rubber to a variety of substrates that include steel, plastic, and other metals. Unlike prior aqueous adhesives, some embodiments of the present invention don&#39;t require a nitroso compound and all embodiments eliminate the need for a methylene donor compound. Additionally, the adhesives of the present invention do not contain phenolic materials, yet achieve exemplary bonding when used with a phenolic containing primer. Elimination of these undesirable compounds solves many problems encountered by customers and provides a safer product for the worker that uses the adhesive. 
     The adhesives of the present invention will bond rubber to metal during the cure cycle of the rubber compound if applied over the appropriate and suggested primer system. Being aqueous this adhesive is environmentally friendly and yet provides excellent adhesion data in laboratory testing, especially in 40 hour boiling water tests. 
     Thus, there has been outlined, rather broadly, the more important features of the invention in order that the detailed description that follows may be better understood and in order that the present contribution to the art may be better appreciated. There are, obviously, additional features of the invention that will be described hereinafter and which will form the subject matter of the claims appended hereto. In this respect, before explaining several embodiments of the invention in detail, it is to be understood that the invention is not limited in its application to the details and construction and to the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. 
     It is also to be understood that the phraseology and terminology herein are for the purposes of description and should not be regarded as limiting in any respect. Those skilled in the art will appreciate the concepts upon which this disclosure is based and that it may readily be utilized as the basis for designating other structures, methods and systems for carrying out the several purposes of this development. It is important that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention. 
     DETAILED DESCRIPTION 
     The butadiene latices of the present invention can be prepared by polymerizing appropriate monomers in an aqueous solution of polyvinyl alcohol. The butadiene polymer of the present invention can be prepared from butadiene monomers alone or from a combination of butadiene monomers and other copolymerizable monomers described in more detail below. “Butadiene polymer,” therefore, herein refers to butadiene homopolymers, butadiene copolymers, butadiene terpolymers and higher polymers. 
     The butadiene monomers useful for preparing the butadiene polymer of the latex of the present invention can essentially be any monomer containing conjugated unsaturation. Typical monomers include 2,3-dichloro-1,3-butadiene; 1,3-butadiene; 2,3-dibromo-1,3-butadiene; isoprene; 2,3-dimethylbutadiene; chloroprene; bromoprene; 2,3-dibromo-1,3-butadiene; 1,1,2-trichlorobutadiene; cyanoprene; hexachlorobutadiene and combinations thereof. It is particularly preferred to use 2,3-dichloro-1,3-butadiene as the butadiene monomer of the present invention since butadiene homopolymers derived from 2,3-dichloro-1,3-butadiene or butadiene copolymers wherein a major portion of the polymer contains 2,3-dichloro-1,3-butadiene monomer units have been found to be particularly useful in adhesive applications due to the excellent bonding ability and barrier properties of the 2,3-dichloro-1,3-butadiene-based polymers. 
     “Copolymerizable monomers” herein refers to monomers which are capable of undergoing copolymerization with the butadiene monomers described above. Typical copolymerizable monomers useful in the present invention include a-haloacrylonitriles such as α-bromoacrylonitrile and α-chloroacrylonitrile; α,β-unsaturated carboxylic acids such as acrylic, methacrylic, 2-ethylacrylic, 2-propylacrylic, 2-butylacrylic and itaconic acids; alkyl-2-haloacrylates such as ethyl-2-chloroacrylate and ethyl-2-bromoacrylate; styrene; styrene sulfonic acid; α-halostyrenes; chlorostyrene; α-methylstyrene; α-bromovinylketone; vinylidene chloride; vinyl toluenes; vinylnaphthalenes; vinyl ethers, esters, and ketones such as methyl vinyl ether, vinyl acetate, and methyl vinyl ketone; esters, amides, and nitriles of acrylic and methacrylic acids such as ethyl acrylate, methyl methacrylate, glycidyl acrylate, methacrylamide, and acrylonitrile; and combinations of such monomers. 
     The copolymerizable monomers, if utilized, are preferably α-halo-acrylonitrile and/or α,β-unsaturated carboxylic acid monomers. The copolymerizable monomers are utilized in an amount ranging from about 0.1 to 30, percent by weight of the total monomers utilized to form the butadiene polymer. 
     Two butadiene polymers which have been found to be particularly useful in adhesive applications include a butadiene copolymer prepared from butadiene monomers and α-halo-acrylonitrile monomers wherein the α-haloacrylonitrile monomers comprise from about 1 to 29, preferably about 5 to 20, percent by weight of the total monomers, and a butadiene terpolymer prepared from butadiene monomers, α-haloacrylonitrile monomers, and α,β-unsaturated carboxylic acid monomers, wherein the α-haloacrylo-nitrile monomers comprise from about 1 to 29, preferably about 5 to 20, percent by weight and the α,β-unsaturated carboxylic acid monomers comprise from about 0.1 to 10, preferably about 0.1 to 1, percent by weight of the total monomers utilized. 
     The polyvinyl alcohol (PVA) of the present invention can be any PVA, commercially or otherwise available, which will dissolve in the present aqueous polymerization system at the temperature of the polymerization. Such PVA will usually be the product of hydrolysis of polyvinyl acetate, wherein the degree of hydrolysis is preferably about 80-99 percent. The average degree of polymerization of the PVA will be about 350-2,500. For a general discussion of various PVAs, see The Encyclopedia of Polymer Science and Technology, Interscience Publishers, Vol. 14, pp. 149ff, (1971). The preferred proportion of PVA is about 3 to 12, preferably about 6 to 8, parts per 100 parts by weight of total monomers. The PVA acts as an emulsion stabilizer during the polymerization. 
     It is presently preferred to utilize a stabilizing solvent in combination with the polyvinyl alcohol when preparing the latices of the present invention. The stabilizing solvent useful for preparing the improved latices of the present invention which are useful in adhesive applications can essentially be any organic solvent capable of exhibiting miscibility with water. The solvent is preferably an organic alcohol such as methanol; ethanol; isopropanol; butanol; 2-(2-ethoxy ethoxy)ethanol; 2-(2-butoxy ethoxy)ethanol; 2-(2-methoxy ethoxy)-ethanol; 2-methoxy ethanol; 2-butoxy ethanol; 2-ethoxy ethanol; 2-butoxy propanol; 2-butoxy ethoxy propanol and the propoxy propanols; also useful are known glycols including ethylene and propylene glycols, and glycol derivatives such as ethylene glycol mono-methyl ether and propylene glycol monomethyl ether. Various combinations of the foregoing solvents may also be utilized. Preferred stabilizing solvents useful for the present invention include methanol, ethanol, isopropanol, butanol, ethylene glycol monomethyl ether, and propylene glycol monomethyl ether. The stabilizing solvent is typically utilized in an amount ranging from about 10 to 100, preferably from about 30 to 50, parts by weight per 100 parts by weight of total monomers. The stabilizing solvent is typically vacuum-stripped from the latex after the emulsion polymerization so as to avoid the presence of any volatile material in the final latex. In the case of higher boiling solvents, the solvent may remain in the latex so as to yield a latex which does not require post polymerization stripping. 
     In carrying out the emulsion polymerization with the preferred stabilizing solvent to produce the latex of the present invention, other optional ingredients may be employed during the polymerization process. For example, conventional anionic and/or nonionic surfactants may optionally be utilized in order to aid in the formation of the latex. Typical anionic surfactants include carboxylates, such as fatty acid soaps from lauric, stearic, and oleic acid; acyl derivatives of sarcosine, such as methyl glycine; sulfates, such as sodium laurel sulfate; sulfated natural oils and esters, such as Turkey Red Oil; alkyl aryl polyether sulfates; alkali alkyl sulfates; ethoxylated aryl sulfonic acid salts; alkyl aryl polyether sulfonates; isopropyl naphthalene sulfonates; sulfosuccinates; phosphate esters, such as short chain fatty alcohol partial esters of complex phosphates; and orthophosphate esters of polyethoxylated fatty alcohols. Typical nonionic surfactants include ethoxylated (ethylene oxide) derivatives, such as ethoxylated alkyl aryl derivatives; mono- and polyhydric alcohols; ethylene oxide/propylene oxide block copolymers; esters, such as glyceryl monostearate; products of the dehydration of sorbitol, such as sorbitan monostearate and polyethylene oxide sorbitan monolaurate; amines; lauric acid; and isopropenyl halide. A conventional surfactant, if utilized, is employed in an amount ranging from about 0.01 to 15, preferably from about 0.1 to 5 parts by weight per 100 parts by weight of total monomer. 
     Chain transfer agents may also be employed with the stabilizing solvent during the preferred emulsion polymerization of the present invention in order to control the molecular weight of the butadiene polymers and to modify the physical properties of the resultant polymers as is known in the art. Any of the conventional organic sulfur-containing chain transfer agents may be utilized such as alkyl mercaptans, dialkyl xanthogen disulfides and sodium thioglycolate. Typical alkyl mercaptans include dodecyl mercaptan, octyl mercaptan, tert-dodecyl mercaptan, tridecyl mercaptan, and mixtures of mercaptans derived from coconut oil (often called lauryl mercaptan), with dodecyl mercaptan being preferred. 
     The dialkyl xanthogen disulfides can be represented by the structure 
                                
wherein R and R′ independently are alkyl radicals having 1-8 carbon atoms. Examples of suitable alkyl radicals are methyl, ethyl, propyl, isopropyl, and the various isomeric butyl, amyl, hexyl, heptyl and octyl radicals. The preferred dialkyl xanthogen disulfides are those in which each alkyl radical has 1-4 carbon atoms, especially diisopropyl xanthogen disulfide.
 
     A chain transfer agent, if utilized, is typically employed in an amount ranging from about 0.1 to 2, preferably from about 0.2 to 1, parts by weight per 100 parts by weight of total monomer. 
     As stated above, the formation of the stable latices of the present invention is preferably carried out by emulsion polymerizing the appropriate monomers in the presence of the polyvinyl alcohol and the stabilizing solvent. Specifically, an aqueous emulsification mixture of water, the polyvinyl alcohol and the stabilizing solvent, is formed to which is added the appropriate monomers. The emulsification mixture typically contains from about 40 to 80, preferably about 50 to 70, percent by weight of water. 
     The preferred emulsion polymerization with the stabilizing solvent of the present invention is typically triggered by a free radical initiator. Typical free radical initiators useful in the present invention include conventional redox systems, peroxide systems, azo derivatives, and hydroperoxide systems. The use of a redox system is presently preferred for use in the invention and examples of such redox systems include ammonium persulfate/sodium metabisulfite, ferric sulfate/ascorbic acid/hydroperoxide and tributylborane/hydroperoxide. Presently, the most preferred redox system utilizes (NH 4 ) 2 S 2 O 8  (ammonium persulfate) and Na 2 S 2 O 5  (sodium metabisulfite). When utilizing this particular redox system, the Na 2 S 2 O 5  is utilized to prepare the emulsification mixture. The (NH 4 ) 2 S 2 O 8  is then added to the emulsification mixture along with the appropriate monomers to initiate polymerization. Both the Na 2 S 2 O 5  and the (NH 4 ) 2 S 2 O 8  are utilized in an amount ranging from about 0.1 to 3, preferably about 0.2 to 1, parts by weight per 100 parts by weight of total monomer. 
     The preferred emulsion polymerization with the stabilizing solvent is typically carried out at a temperature in the range from about 30° C. to 90° C., preferably about 40° C. to 60° C. Monomer conversion typically ranges from about 70 to 100 percent, preferably from about 80 to 100 percent. 
     The polyvinyl alcohol-stabilized butadiene latices of the present invention typically have a solids content of between about 30 and 70 percent, more typically between about 40 and 60 percent; a viscosity of between about 50 and 10,000 centipoise, preferably between about 100 and 1,000 centipoise; and a particle size between about 100 and 300 nanometers. The butadiene polymer of the latex typically has a molecular weight of between about 3,000 and about 300,000, preferably between about 35,000 and 100,000 Mn. 
     The polyvinyl alcohol-stabilized butadiene latices of the invention may also be prepared according to methods such as those disclosed in U.S. Pat. Nos. 3,920,600 and 4,128,514; and in British Patent No. 1,469,993. 
     The polyvinyl alcohol-stabilized butadiene polymer latex is typically utilized in the adhesive composition of the present invention in an amount ranging from about 50 to 99, preferably from about 80 to 97 percent by weight of the essential components of the adhesive composition. 
     The adhesive compositions of the present invention comprise an acid-scavenging compound for purposes of consuming any acid compound by-products produced during the bonding process. The acid-scavenging compound is preferably a metal oxide or a lead-containing compound. The metal oxide of the present invention can be any known metal oxide such as the oxides of zinc, cadmium, magnesium, lead, and zirconium; litharge; red lead; zirconium salts; and combinations thereof. Various lead-containing compounds may also be utilized as an acid-scavenging compound in lieu of, or in addition to, the metal oxide. Examples of such lead-containing compounds include lead salts such as polybasic lead salts of phosphorous acid and saturated and unsaturated organic dicarboxylic acids and acid anhydrides. Specific examples of lead salts include dibasic lead phthalate, monohydrous tribasic lead maleate, tetrabasic lead fumarate, dibasic lead phosphite, and combinations thereof. Other examples of lead-containing compounds include basic lead carbonate, lead oxide and lead dioxide. For environmental reasons, metal oxides are preferred over lead-containing compounds for purposes of the invention. 
     The acid-scavenging compound is typically utilized in an amount ranging from about 0.1 to 40 percent by dry weight of the total adhesive composition. 
     In a further embodiment of the present invention, the adhesive composition further comprises a vulcanizing agent. The vulcanizing agent of the present invention can be any known vulcanizing agent which is capable of crosslinking elastomers at molding temperatures (140° C.-200° C.). Preferred vulcanizing agents for use in the invention are selenium, sulfur, and tellurium, with selenium being most preferred. 
     In embodiments of the present invention comprising a vulcanizing agent, it is typically utilized in an amount ranging from about 1 to 15 percent by dry weight of the total adhesive composition. 
     The adhesive compositions of the present invention may utilize one or more various optional ingredients selected from the group consisting of a supplemental polymeric film-forming component, a nitroso compound crosslinker, and other additives, all of which are described in more detail hereinbelow. 
     In addition to the polyvinyl alcohol-stabilized butadiene latices, the adhesive compositions of the present invention may contain a supplemental polymeric film-forming component which is preferably a latex of a halogenated polyolefin. The halogenated polyolefin of the latex can essentially be any natural or synthetic halogenated polyolefin elastomer. The halogens employed in the halogenated polyolefinic elastomer are typically chlorine or bromine, although fluorine can also be used. Mixtures of halogens can also be employed in which case the halogen-containing polyolefinic elastomer will have more than one type of halogen substituted thereon. The amount of halogen does not appear critical and can range from as low as about 3 weight percent to more than 70 weight percent, depending on the nature of the base elastomer or polymer. Halogenated polyolefins and their preparation are well-known to those skilled in the art. 
     Representative halogenated polyolefins include chlorinated natural rubber, chlorine- and bromine-containing synthetic rubbers including polychloroprene, chlorinated polychloroprene, chlorinated polybutadiene, hexachloropentadiene, butadiene/halogenated cyclic conjugated diene adducts, chlorinated butadiene styrene copolymers, chlorinated ethylene propylene copolymers and ethylene/propylene/non-conjugated diene terpolymers, chlorinated polyethylene, chlorosulfonated polyethylene, brominated poly(2,3-dichloro-1,3-butadiene), copolymers of α-haloacrylo-nitriles and 2,3-dichloro-1,3-butadiene, chlorinated poly(vinyl chloride), as discussed above, and the like, including mixtures of such halogen-containing elastomers. Thus substantially any of the known halogen-containing derivatives of natural and synthetic elastomers can be employed in the practice of this invention, including mixtures of such elastomers. 
     The latex of the halogenated polyolefin of the present invention can be prepared according to methods known in the art such as by dissolving the halogenated polyolefin in a solvent and adding a surfactant to the resulting solution. Water can then be added to the solution under high shear to emulsify the polymer. The solvent is then stripped to obtain a latex having a total solids content of from about 10 to 60, preferably 25 to 50, percent by weight. The latex can also be prepared by emulsion polymerization of chlorinated ethylenically unsaturated monomers. 
     Although less preferred, the supplemental polymeric film-forming component may also be a latex, dispersion, emulsion or other aqueous form of a non-halogenated polymeric material. Examples of such non-halogenated polymeric materials which may be utilized in aqueous form include epoxy resins, resorcinol resins, melamine resins, styrene-butadiene copolymer rubber, natural rubber, polyacrylates, polybutadienes and polyvinylacetates. It should be noted that in addition to acting as a supplemental film-former, an epoxy resin may also act as an acid-scavenging compound as described hereinafter. 
     If employed, the supplemental polymeric film-forming component of the present invention is typically utilized in an amount ranging from about 0.1 to 50, preferably from about 5 to 20 percent by dry weight (excluding solvent and water) of the total adhesive composition. 
     In a further embodiment of the present invention, the adhesive composition optionally comprise a nitroso compound crosslinker. The nitroso compound crosslinker may be a nitroso compound per se, or a nitroso compound precursor. The nitroso compound useful as an nitroso compound crosslinker of the present invention can be any aromatic hydrocarbon, such as benzenes, naphthalenes, anthracenes, biphenyls, and the like, containing at least two nitroso groups attached directly to non-adjacent ring carbon atoms. More particularly, such nitroso compounds are described as aromatic compounds having from 1 to 3 aromatic nuclei, including fused aromatic nuclei, having from 2 to 6 nitroso groups attached directly to non-adjacent nuclear carbon atoms. The present preferred nitroso compounds are the dinitroso aromatic compounds, especially the dinitrosobenzenes and dinitrosonaphthalenes, such as the meta- or para-dinitrosobenzenes and the meta- or para-dinitrosonaphthalenes. The nuclear hydrogen atoms of the aromatic nucleus can be replaced by alkyl, alkoxy, cycloalkyl, aryl, aralkyl, alkaryl, arylamine, arylnitroso, amino, halogen, and like groups. The presence of such substituents on the aromatic nuclei has little effect on the activity of the nitroso compounds in the present invention. As far as is presently known, there is no limitation as to the character of the substituent, and such substituents can be organic or inorganic in nature. Thus, where reference is made herein to nitroso compound, it will be understood to include both substituted and unsubstituted nitroso compounds, unless otherwise specified. 
     Particularly preferred nitroso compounds are characterized by the formula:
 
(R)m-Ar—(NO) 2  
 
wherein Ar is selected from the group consisting of phenylene and naphthalene; R is a monovalent organic radical selected from the group consisting of alkyl, cycloalkyl, aryl, aralkyl, alkaryl, arylamine, and alkoxy radicals having from 1 to 20 carbon atoms, amino, or halogen, and is preferably an alkyl group having from 1 to 8 carbon atoms; and m is zero, 1, 2, 3, or 4, and preferably is zero.
 
     A partial non-limiting listing of nitroso compounds which are suitable for use in the practice of the invention include m-dinitrosobenzene, p-dinitrosobenzene, m-dinitrosonaphthalene, p-dinitrosonaphthalene, 2,5-dinitroso-p-cymeme, 2-methyl-1,4-dinitrosobenzene, 2-methyl-5-chloro-1,4-dinitrosobenzene, 2-fluoro-1,4-dinitrosobenzene, 2-methoxy-1-3-dinitroso-benzene, 5-chloro-1,3-dinitrosobenzene, 2-benzyl-1,4-dinitrosobenzene, 2-cyclohexyl-1,4-dinitrosobenzene and combinations thereof. Particularly preferred nitroso compounds include p-dinitrosobenzene and m-dinitroso-benzene. 
     The nitroso compound precursor which can function as a nitroso compound crosslinker for purposes of the present invention may be essentially any compound that is capable of being converted, typically by oxidation, to a nitroso compound at elevated temperatures, typically in the range from about 140° C. to 200° C. The most common nitroso compound precursors are derivatives of quinone compounds. Examples of quinone compound derivatives useful as nitroso compound precursors in the present invention include quinone dioxime, dibenzoquinone dioxime, 1,2,4,5-tetrachlorobenzoquinone, 2-methyl-1,4-benzoquinone dioxime, 1,4-naphthoquinone dioxime, 1,2-naphthoquinone dioxime, and 2,6-naphthoquinone dioxime. 
     If employed, the nitroso compound crosslinker is typically utilized in an amount ranging from about 0.1 to 50, preferably from about 2 to 35, percent by dry weight of the total adhesive composition. 
     The adhesive composition of the present invention may also contain a maleimide compound crosslinker. The maleimide compound crosslinker can essentially be any compound containing at least two maleimide groups. The maleimide groups may be attached to one another or may be joined to and separated by an intervening divalent radical such as alkylene, cyclo-alkylene, epoxydimethylene, phenylene (all 3 isomers), 2,6-dimethylene-4-alkylphenol, or sulfonyl. An example of a maleimide compound wherein the maleimide groups are attached to a phenylene radical is m-phenylene bismaleimide and is available as HVA-2 from E.I. Du Pont de Nemours &amp; Co. 
     The maleimide compound crosslinker may also be an aromatic polymaleimide compound. Aromatic polymaleimides having from about 2 to 100 aromatic nuclei wherein no more than one maleimide group is directly attached to each adjacent aromatic ring are preferred. Particularly preferred aromatic polymaleimide compounds have the formula: 
                                
wherein x is from about 0 to 100. Such aromatic polymaleimides are common materials of commerce and are sold under different trade names by different companies, such as BMI-M-20 and BMI-S aromatic polymaleimides supplied by Mitsui Toatsu Fine Chemicals, Incorporated.
 
     If employed, the maleimide compound crosslinker is typically utilized in the present invention in an amount ranging from about 2 to 50, preferably from about 5 to 15, percent by dry weight of the total adhesive composition. 
     The adhesive compositions of the present invention can optionally contain other well-known additives including plasticizers, fillers, pigments, surfactants, dispersing agents, wetting agents, reinforcing agents and the like, in amounts employed by those skilled in the adhesive arts to obtain a desired color and consistency. Examples of optional ingredients include carbon black, silica such as fumed silica, sodium aluminosilicate, and titanium dioxide. 
     The adhesive compositions of the present invention may be prepared by any method known in the art, but are preferably prepared by combining and milling or shaking the ingredients and water in a ball-mill, sand-mill, ceramic bead-mill, steel bead-mill, high speed media-mill, or the like. 
     The adhesive compositions may be applied to a surface to be bonded by spraying, dipping, brushing, wiping, roll-coating or the like, after which the adhesive composition is permitted to dry. The adhesive composition is typically applied in an amount sufficient to form a dry film thickness ranging from about 0.1 to 1.0 mils, preferably from about 0.2 to 0.8 mils. In the case of a two-coat adhesive composition as described more fully hereinafter, the adhesive is applied in a similar manner over the prime coat which has been permitted to completely dry. 
     The adhesive compositions of the present invention are capable of bonding any substrate or surface capable of receiving the adhesive composition. The material, which may be bonded to a surface such as a metal surface in accordance with the present invention, is preferably a polymeric material, including any elastomeric material selected from any of the natural rubbers and olefinic synthetic rubbers including polychloroprene, polybutadiene, neoprene, styrene-butadiene copolymer rubber, acrylonitrile-butadiene copolymer rubber, ethylene-propylene copolymer rubber, ethylene-propylene-diene terpolymer rubber, butyl rubber, brominated butyl rubber, alkylated chlorosulfonated polyethylene and the like. The material is most preferably an elastomeric material such as natural rubber (cis-polyisoprene). The surface to which the material is bonded can be any surface capable of receiving the adhesive such as a glass, plastic, or fabric surface, and is preferably a metal surface selected from any of the common structural metals such as iron, steel (including stainless steel), lead, aluminum, copper, brass, bronze, MONEL® metal alloy (Huntington Alloy Products Div., International Nickel Co., Inc.), nickel, zinc, and the like. Prior to bonding, a metal surface is typically cleaned according to one or more methods known in the art such as degreasing, grit-blasting and zinc-phosphatizing. 
     The present adhesive compositions are preferably utilized to bond an elastomeric material to a metal surface. The adhesive composition is typically applied to the metal surface and the coated metal surface and elastomeric substrate are then brought together under heat and pressure to complete the bonding procedure. In some cases, it may be desirable to preheat the metal surface prior to application of the adhesive composition to assist in drying of the adhesive composition. The coated surface of the metal and the elastomeric substrate are typically brought together under a pressure of from about 20.7 to 172.4 Mega Pascals (MPa), preferably from about 20 MPa to 50 MPa. The resulting rubber-metal assembly is simultaneously heated to a temperature of from about 140° C. to about 200° C., preferably from about 150° C. to 170° C. The assembly should remain under the applied pressure and temperature for a period of from about 3 minutes to 60 minutes, depending on the cure rate and thickness of the rubber substrate. This process may be carried out by applying the rubber substrate as a semi-molten material to the metal surface as in, for example, an injection-molding process. The process may also be carried out by utilizing compression molding, transfer molding or autoclave curing techniques. After the process is complete, the bond is fully vulcanized and ready for use in a final application. 
     The adhesive compositions prepared in accordance with the present invention can be effectively employed in adhesives, coatings, and the like. The compositions of the present invention are preferably utilized in an aqueous adhesive system for bonding an elastomeric substrate to a metal surface or for bonding an elastomeric substrate to a fabric surface such as textile cloth or cord. The present adhesive compositions have a particular affinity for elastomeric substrates, and when utilized in an adhesive system, should be applied at least in substantial contact with the elastomeric substrate. “At least in substantial contact” herein refers to at least minimum physical contact between the adhesive composition and the elastomeric substrate. 
     The metal surface to which the elastomeric substrate may be bonded may optionally have a conventional water-based or solvent-based metal primer applied thereto. Typical water-based primers include METALJACKET® 1100/2110, CHEMLOK® 8007, CHEMLOK® 8006, and Chemlok® 8008 phenolic resin-type primers produced by Lord Corporation. Typical solvent-based primers include CHEMLOK® 205 or CHEMLOK® 207 phenolic resin-type primers produced by Lord Corporation. The adhesive composition is typically applied directly to a metal surface or directly to any primer which has been applied to the metal so as to ensure contact between the adhesive composition and the elastomeric substrate which is brought into contact with the coated metal surface. The fabric surface to which the elastomeric substrate may be bonded may have a conventional resorcinol-formaldehyde/latex (RFL) primer applied thereto. 
     The following examples are disclosed in order to further illustrate and fully disclose the invention and are not intended to limit in any manner the scope of the invention which is defined by the claims. 
    
    
     EXAMPLES 
     Preparation of Polyvinyl Alcohol-Stabilized Latex: 
     A polyvinyl alcohol-stabilized dichlorobutadiene/α-bromoacrylonitrile copolymer latex is prepared with the following ingredients (PHM=parts per hundred parts monomer): 
                                                               Ingredient   Weight (g)   PHM                                    2,3-dichloro-1,3-butadiene (90% in CH 2 Cl 2 )   656.0   95.0           α-bromoacrylonitrile   34.5   5.0           Polyvinyl alcohol   56.0   7.0           Deionized water   1056.0   132.0           Methanol   400.0   50.0           (NH 4 ) 2 S 2 O 8     4.0   0.5           Na 2 S 2 O 5     4.0   0.5                    
The polyvinyl alcohol, methanol, Na 2 S 2 O 5  and 856 grams of water are added to a 3 L flask equipped with stirring, N 2 , heat and a condenser. The mixture is heated to 50° C., after which the two monomers and the (NH 4 ) 2 S 2 O 8  dissolved in the remaining water are added over a 1 hour period. The resulting latex is vacuumed-stripped for 1 hour at 80 mmHg and 50° C. to remove the methanol. The latex has a solids content of 35% and a viscosity of 300 centipoise.
 
Preparation of Adhesive Composition:
 
     A master batch is prepared by milling the following ingredients for 2-3 hours in a ceramic bead mill to achieve desired particle size: 
                                         Ingredients   Weight                       DNB   15.0 g           ZnO   15.0 g           Carbon black    2.0 g           Dispersing agent (a)      1.0 g           Water   77.0 g           Total   110.0 g                  (a) MARASPERSE CBOS4 (American Can Company)            
110.0 g of the resulting master batch is then added to 188.6 g of the latex described above having a solids level of 35% in water. To this mixture 2.0 grams of 50% solids DAPRO W-77 is added along with 32.73 g of water. The resulting adhesive comprises 30% solids. Ammonium hydroxide is added to adjust the pH to approximately 7. This would represent process for preparing Sample A identified below.
 
Adhesive Tests:
 
     The following formulations are prepared as outlined above with DAPRO W-77, DNB, and optionally gamma POM added to the adhesive formulation. 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
               
               
                 Percent DRY Weight 
               
             
          
           
               
                   
                 SAMPLE 
                 SAMPLE 
                 SAMPLE 
                 SAMPLE 
               
               
                 Raw Materials 
                 A 
                 B 
                 C 
                 D 
               
               
                   
               
             
          
           
               
                 Marasperse (dispersant) 
                 1.0 
                 1.0 
                 1.0 
                 1.0 
               
               
                 ZNO (acid acceptor) 
                 15.0 
                 10.0 
                 5.0 
                 0.0 
               
               
                 Gamma POM 
                 0.0 
                 5.0 
                 10.0 
                 15.0 
               
               
                 (phenolic curative) 
                   
                   
                   
                   
               
               
                 Carbon Black (filler) 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
               
               
                 DNB 
                 15.0 
                 15.0 
                 15.0 
                 15.0 
               
               
                 (rubber curative) 
                   
                   
                   
                   
               
               
                 DCD/a-Bran 95/5 latex 
                 66.0 
                 66.0 
                 66.0 
                 66.0 
               
               
                 (halogenated 
                   
                   
                   
                   
               
               
                 film former) 
                   
                   
                   
                   
               
               
                 Dapro W-77 
                 1.0 
                 1.0 
                 1.0 
                 1.0 
               
               
                 (wetting agent) 
                   
                   
                   
                   
               
               
                 TOTALS 
                 100.0 
                 100.0 
                 100.0 
                 100.0 
               
               
                   
               
             
          
         
       
     
     A conventional aqueous primer is sprayed onto zinc phosphatized steel coupons at a film thickness of 0.30 mils. The adhesive compositions were then prepared as below and then spray applied to the primed metal coupons at a film thickness of 0.70 mils. The coated coupons were then bonded to a natural rubber substrate by compression molding the rubber on to the coupons at 340° F. Different natural rubber compounds identified as Rubber A, Rubber B and Rubber C were tested. 
     Primary Adhesion 
     Bonded parts are pulled to destruction according to ASTM Test D429—Method B. Parts are tested in peel with a peel angle of 45 degrees. The test is conducted at room temperature with a test speed of 2.0 inches per minute. After the bonded part fails, the percent rubber retention on the adhesive coated area of the part is measured. 
     Hot Tear 
     Bonded parts are pulled to destruction immediately upon removal from the mold as close to 340° F. as possible. After the bonded part fails, the percent rubber retention on the adhesive coated area of the part is measured. 
     Boiling Water 
     Boiling water tested parts are pulled at a 90 degree angle with a 2 Kg weight applying a constant stress. After 24-hour test is complete pliers are used to tear remaining rubber from tested part. After the bonded part fails, the percent rubber retention on the adhesive coated area of the part is measured. 
     
       
         
               
               
               
               
               
             
           
               
                   
               
               
                   
                 Sample A 
                 Sample B 
                 Sample C 
                 Sample D 
               
               
                   
               
             
             
               
                 Primary adh. 
                 100R 
                 100R 
                 100R  
                 100R  
               
               
                 Hot Tear 
                 100R 
                 100R 
                 85R 
                 95R 
               
               
                 Boiling Water 
                  85R 
                  70R 
                 85R 
                 90R 
               
               
                 Rubber A 
                   
                   
                   
                   
               
               
                 Boiling Water 
                  20R 
                  15R 
                 15R 
                 15R 
               
               
                 Rubber B 
                   
                   
                   
                   
               
               
                 Boiling Water 
                  85R 
                  65R 
                 83R 
                 45R 
               
               
                 Rubber C 
               
               
                   
               
             
          
         
       
     
     The results of these tests illustrate that the adhesive composition comprising only zinc oxide performs as well as compounds containing gamma POM (polyoxymethylene) for boiling water resistance. 
     Although the present invention has been described with reference to particular embodiments, it should be recognized that these embodiments are merely illustrative of the principles of the present invention. Those of ordinary skill in the art will appreciate that the apparatus and methods of the present invention may be constructed and implemented in other ways and embodiments. Accordingly, the description herein should not be read as limiting the present invention, as other embodiments also fall within the scope of the present invention.