Method for Bonding Elastomers to Metals

This invention relates to bonding of elastomeric compositions. More particularly, the invention is concerned with methods for bonding natural and synthetic elastomers to metal substrates and the resulting composite structures.

Laminates and composite structures wherein natural and synthetic elastomers are adhesively bonded to metal substrates are well-known articles of commerce. A common problem of such structures is their undesirably poor resistance to hostile environments such as saline conditions and/or high humidity. Under such aggressive conditions, underbond corrosion can begin, leading to the deterioration of the bonded article. To counter the problem, it is a common practice to protect the metal by such techniques as phosphatizing, passivation, galvanizing, tinning, aluminizing, sherardizing, chromizing and other chemical conversion coatings. Such techniques suffer from the disadvantage that they are commonly "wet processes", that is, they are applied from solution, become readily contaminated and provide handling and health hazards. In addition, the use of such processes poses severe environmental waste disposal problems and requires large capital equipment investments to suitably dispose of the chemical wastes resulting from these surface treatments. There is a need for methods of improving environmental resistance of rubber-metal adhesively-bonded assemblies, and particularly for such methods which do not require the disposal of spent treating solutions.

The present invention provides a novel and unobvious method of bonding vulcanizable natural and synthetic rubbers to metal substrates to provide an adhesive bond having an improved resistance to adverse and aggressive environments. More particularly, the bonding method of the present invention generally comprises treating in a reactive monomer gas plasma one or more metal elements, applying an adhesive composition suitable for bonding vulcanizable natural and synthetic rubbers to metal substrates to the plasma-treated surfaces, contacting the adhesively-coated substrate with a vulcanizable natural or synthetic rubber, and subjecting the resultant assembly to conditions of heat and pressure for a time sufficient to effect vulcanization of the rubber composition and curing of the adhesive composition. Broadly, the present invention provides a gas phase plasma coating process wherein there is deposited on the metal substrate an impervious polymeric film. The thin polymeric film exhibits excellent adhesion to the metal and provides a protective primer coating for adhesively bonded products. The deposited coating is compatible with the conventional rubber-metal adhesives to provide bonded articles which are resistant to attack by hostile environments, such as salt spray and water.

FIG. 1 discloses types and properties of known electrical discharges; and

FIG. 2 is a schematic diagram of an apparatus which can be employed in the practice of the invention.

Plasma chemistry deals with chemical reactions which occur in an at least partially ionized gas which is composed of ions, free radicals and neutral fragments. This "fourth state of matter" can be achieved by the action of very high temperatures or strong electrical or magnetic fields. The present invention is concerned with ionized gases produced by an electrical discharge in a gaseous medium. In such a discharge, free electrons gain energy from the applied electrical field and can collide with neutral fragments. In the process, energy is transferred to the neutral particles and leads to a plethora of new specie, including free radicals (molecular specie with unpaired electrons), ions (specie with whole positive or negative charges), atoms and metastables (energetic fragments which are transient and decay to lower energy states). These various specie are available to participate in different chemical and physical processes.

Of the several types of electrical discharges which can create plasmas, see FIG. 1, the present invention utilizes those plasmas generally characterized as glow discharges and which are defined by electron energies of 1-10 eV and electron densities of 10.sup.9 -10.sup.12 cm.sup.-3. Additionally, such plasma lack equilibrium between the electron temperature Te and the gas temperature Tg. Typically, ratios for Te/Tg lie in the range 10-10.sup.2. This lack of thermal equilibrium can provide a plasma in which the gas temperature is near ambient value but in which electron energies are sufficiently high to rupture molecular bonds and kinetically cause other transformations. Depending on the gaseous medium, plasmas may be chemically active, that is, the plasma gas components may become involved in substrate surface reactions, or non-reactive, that is, the plasma gas components do not become involved in such substrate reactions. The non-reactive plasmas are usually regarded as predominantly physical processes in which energetic ions, though unable to react chemically with the substrate surface, can impart kinetic energy and momentun to surface atoms or molecules to eject from the surface.

The "low temperature" or non-equilibrium electric glow discharge reactive plasmas which are utilized in the practice of the invention involve coupling radio frequency or microwave power to a discharge zone, with the former being currently preferred. At RF frequencies, either inductive or capacitative types of coupling can be employed. Suitable plasmas can be formed with a radio frequency discharge operated between 0 to 100 megahertz and 1 to 600 watts or with a microwave discharge operated between 400 to 4000 megahertz and 20 to 400 watts. The wattage considerations are based on a five-inch diameter reactor and will vary somewhat with reactor size. The residence or treating period within the reactor zone is generally from 10 minutes to 20 hours, preferably 30 minutes to 2 hours, although residence times outside these limits can often be advantageous.

Substantially any metal substrate, including substrates which have been treated by any known surface treating process such as chemical conversion coatings, nitriding and the like can be treated in accordance with the invention. The methods of the invention are especially suitable for treating metal surfaces which have been otherwise untreated, except for appropriate cleaning, such as degreased, rinsed and/or sand-blasted. Substantially any metal substrate can be treated in accordance with this invention, with ferrous-based metals being preferred.

Metal substrates are prepared for bonding to vulcanizable natural and synthetic rubbers in accordance with this invention by a treatment comprising exposing at least that portion of the metal substrate which is to be subsequently bonded in a zone of electrical glow discharge plasma to a gaseous medium consisting essentially of at least one reactive sulfur-containing monomer in vaporized form. It is critical that the entire portion of the substrate which is to be bonded be entirely within the visible plasma. In a particularly preferred embodiment, the metal elements are exposed to a non-reactive inert gas plasma treatment prior to being exposed to the reactive sulfur-containing monomer plasma.

The sulfur-containing monomer compounds which are employed in forming reactive gas plasmas in accordance with the invention can be broadly described as cyclic thioethers. More particularly, suitable sulfur-containing monomer compounds are selected from the group consisting of cyclic thioethers having the formula ##STR1## wherein R.sup.0 is selected from the group consisting of hydrogen, halogen, alkyl group having from 1 to 8 carbon atoms, cyclohexyl and phenyl; and R.sup.1 and R.sup.2 can be the same or different and each is individually selected from the group consisting of hydrogen, halogen, alkyl group having from 1 to 8 carbon atoms, cycloaliphatic group having 5, 6 or 7 ring carbon atoms and aryl group having 6 or 10 nuclear carbon carbon atoms. Representative members of such cyclic thioethers include, without limitation, thiophene, 2-chlorothiophene, 2-methylthiophene, 2-ethylthiophene, 2-hexylthiophene, 2-cyclohexylthiophene, 2-phenylthiophene, tetrahydrothiophene, 2-fluorotetrahydrothiopene, 2-isobutyltetrahrothiophene, ethylene sulfide, propyliene sulfide, 2,3-butylene sulfide, and the like.

Non-reactive plasmas for use in preferred embodiments of the invention can be obtained by the ionization of substantially non-reactive, that is, the gaseous ions are substantially non-reactive with the substrate, inert ionizable gases, such as helium, nitrogen, neon, argon, krypton and xenon, with argon being currently preferred.

Vulcanizable rubbers which can be bonded in accordance with the invention include, without limitation, natural rubber, polychloroprene rubber, styrene/butadiene rubber, ethylene/propylene copolymer rubber (EPM), ethylene/propylene/diene terpolymer rubber (EPDM), butyl rubber and polyurethane rubber.

Metal substrates which have been plasma-treated in accordance with this invention are readily bonded to natural and synthetic rubbers employing any adhesive composition which is known to be suitable for bonding vulcanizable natural and synthetic rubbers to metal substrates. Suitable adhesive compositions include both one-coat and two-coat systems, as these terms are interpreted in the adhesive arts, and are so well-known that a detailed discussion is unnecessary. Exemplary rubber-metal adhesive systems are disclosed in Coleman et al U.S. Pat. No. 3,258,388.

The adhesive compositions are applied to the plasma-treated substrates in a conventional manner, such as by dipping, spraying or brushing. Preferably, the applied adhesive film, whether comprising a single-coat or a two-or-more adhesive system, is allowed to dry before being contacted with the vulcanizable rubber composition. Subsequent to forming the rubber-metal assembly, with the adhesive disposed between the rubber and metal, the assembly is cured by heating in accordance with conventional practices. The exact conditions will depend on the particular elastomer being bonded and the adhesive system which is employed, and will generally be at a temperature in the range from 120.degree. C. to 200.degree. C. for from 5 to 75 minutes.

The invention will be more fully described with reference to FIG. 2, which is a schematic diagram of an apparatus which can be employed in the practive of the invention.

As shown in FIG. 2, there are provided non-reactive gas reservoir 1 and reactive monomer reservoir 2 with conduit means to deliver either or both non-reactive gas and reactive monomer to reactor 3. A flowmeter 4 is provided for measuring gas flow rates and a vacuum gauge 5 is provided in conduit 11 to monitor the pressure within the reactor system. Valves 14, 15, 16 and 17 are provided in conduits 10 and 11 to regulate the flow rate of non-reactive and reactive monomer materials to reactor 3. Prior to use, reactor 3 is evacuated by opening valve 18 to vacuum source 7. A suitable helical coil 8 is connected to a suitable radio frequency oscillator 9, which can be operated at a frequency in the range from 0 to 100 megahertz. Substrates to be treated in accordance with the invention are placed on an appropriate rack, not shown, in a staggered configuration and disposed within the reaction zone. The bond surface area of the substrates must be located entirely within the visible plasma region. The reactor system also includes a trap 6 and vent conduit 13 and its valve 19.

The following Example is provided to illustrate the invention. It is understood that the Example is not to be construed to unduly limit the invention.

Cold-rolled steel elements are solvent-degreased in trichloroethylene and grit-blasted to clean the exposed surfaces of contaminants and loose scale. The cleaned elements are placed in the plasma reaction zone and arranged in a staggered configuration, with all elements being located within boundaries defined by the helical coil. The reaction chamber is pumped down to a vacuum of 15 microns of mercury and argon is allowed to flow through the chamber at a pressure of 1000 microns Hg. A plasma is initiated by radio frequency discharge at a power of 50 watts and a frequency of 13.5 merahertz. The steel elements are exposed to the argon plasma for 30 minutes. The argon plasma is terminated and the plasma chamber is again evacuated to a pressure of 15 microns Hg. The reactive monomer thiophene is introduced into the plasma reaction zone at a pressure of 20-30 microns Hg. The plasma is activated by radio frequency discharge at a power of 20 watts and a frequency of 13.5 megahertz. The steel substrates are exposed to the thiophene plasma for 4 hours, the plasma is terminated and the plasma reaction zone is purged with nitrogen. The steel elements are coated following the plasma deposition with a brown film which is hydrophobic. The steel elements are placed in boiling water for 16 hours. No corrosion is observed, whereas control non-plasma-treated steel elements corrode in one hour. The plasma-deposited coating is still hydrophobic after the boiling water treatment.

The above procedure is repeated employing 2-ethylthiophene and tetrahydrothiopene reactive gaseous monomers.

The plasma-treated samples are prepared for bonding to vulcanizable natural rubber stock by dipping in an inert organic solvent-based heat reactive phenolic primer. After evaporation of the inert organic solvent, the primed metal elements are dipped in an inert organic solvent-based chlorinated rubber rubber-metal adhesive. After evaporation of the inert organic solvent, the adhesively-coated metal elements are bonded to a sulfur-curable vulcanizable natural rubber composition at a temperature of 150.degree. C. for 20 minutes. The bonded assemblies are tested in accordance with ASTM D429, Method B, modified to 45.degree.. The results are shown in the following Table and compared to non-plasma-treated bonded control samples.