Coating processes – Applying superposed diverse coating or coating a coated base – Synthetic resin coating
Reexamination Certificate
2000-03-21
2002-04-30
Nakarani, D. S. (Department: 1773)
Coating processes
Applying superposed diverse coating or coating a coated base
Synthetic resin coating
C427S410000, C427S413000
Reexamination Certificate
active
06379752
ABSTRACT:
This invention relates to a process for producing a rubber-metal composite, the innovation consisting in the application of a layer of a self-depositing resin to the metal before the rubber is vulcanised-on in a subsequent step. The present invention further relates to a composite structure prepared from metal and rubber, which contains between the metal surface and the rubber a cured layer of a self-depositing resin. Such composite structures are applicable in many areas of industry. Vehicle and machine manufacture are examples.
One requirement which rubber-metal composite structures must naturally meet is sufficiently firm adhesion between the rubber and the metal. Adhesion is sufficiently firm when adhesion tests in which the composite prepared from rubber and metal is torn apart result in fracture within the rubber compound, not between the rubber and the metal. However, the corrosion resistance of the rubber-metal composites is a serious problem in many applications. The composites may come into contact with corrosive media, such as salt water, and must display correspondingly adequate corrosion resistance. In principle, the entire rubber-metal composite could be over-lacquered after it has been produced. However, lacquers which must be stoved at a temperature at which rubber sustains damage cannot be used for this purpose. Lacquers which require no stoving do not, however, afford adequate corrosion protection. Even with lacquers which offer good corrosion protection, corrosion problems may arise if the lacquer cracks or flakes off owing to mechanical deformation of the composite.
The prior art proposes various possibilities for improving the corrosion protection of rubber-metal composites. DE-A-27 48 686 describes a process for increasing the corrosion resistance of a rubber-metal structure, in which an epoxy resin-based powder coating is applied to the metal surface before it is bonded with the rubber. The latter coating has the drawback that it softens at rubber vulcanising temperatures. Since vulcanising is generally carried out under pressure, there is a risk of displacement of the rubber on the softened base. The same risk arises when it is subsequently subjected to load at a temperature higher than the softening point of the powder coating (upwards of about 50° C.). Such temperatures may easily be reached in a motor vehicle which is parked in the sun, for example.
EP-A-54 861 proposes coating the metal by cataphoretic dip coating before the rubber is applied. Firstly, this is costly in plant terms, because, before cataphoretic dip coating, the metal surface must be pre-treated chemically, for example, by phosphating and post-rinsing. This necessitates the introduction of a number of process steps upstream, and hence a number of treatment baths. Secondly, cataphoretic dip coating is heavy on energy consumption and thus has economic disadvantages.
An object of the present invention is to provide a novel process for producing a rubber-metal composite. The metal should here be coated with a protective coating offering a known good corrosion-protecting effect before the rubber is applied. The latter protective coating should be able to be applied in a manner which is technically simple and hence economical, and should not soften under the conditions which prevail during rubber vulcanising.
Accordingly, a first embodiment of the present invention relates to a process for producing a rubber-metal composite on a metal, characterised in that:
(a) a self-depositing resin is deposited on the metal and cured;
(b) if desired, a primer is applied to the resin;
(c) a binder is applied to the primer or the self-depositing resin;
(d) a natural or synthetic rubber is applied to the binder; and
(e) the rubber is vulcanised at a temperature within the range from 90 to 220° C.
Metals which are suitable as the metal substrate are those whose ions bring about coagulation and deposition of the self-depositing resin. Metals currently considered for this purpose are in particular cast iron, steel or other iron-containing substrates. The process is accordingly preferably carried out with the use of iron-containing substrates. However, it may also be carried out on zinc or galvanised steel if baths of self-depositing resins are selected which are suitable for the purpose, and/or suitable pre-rinses are used. Other metal substrates are also considered where self-depositing resins are available for them.
The self-depositing resins which are usable within the meaning of the present invention are also designated in the art “autophoretic resins or “autophoretic lacquers”. The expression “Autophoretic® Coating Chemicals” is common parlance in the English-speaking world, where the abbreviation-“ACC®” is frequently used. The principle of autophoretic lacquer deposition is as follows: an acid aqueous emulsion of an organic polymer is prepared. When a metal surface is brought into contact with such an emulsion, the acid acts to dissolve metal ions out of the surface. The metal ions bond with the polymer particles and bring about coagulation of the latter. As this process takes place directly on the metal surface, the coagulated polymer is deposited as a coating on the metal surface. When the metal surface is covered completely with polymer, the process comes to a halt. The layer thicknesses obtained in this process are generally within the range from about 15 to about 30 &mgr;m. When coating is concluded, the metal parts are removed from the treatment bath, and excess treatment emulsion is rinsed off with water. A reactive post-rinse frequently follows, which improves both the adhesion of the autophoretic lacquer to the metal and also the corrosion protection. Solutions of chromic acid and/or of chromates are, for example, considered here. The resin is then cured by heating to a temperature within the range from 140 to 250° C., preferably 150 to 180° C.
The self-depositing resins which are usable in the process according to the present invention are known as such in the prior art of coating metal parts. The corrosion-protecting effect thereof is sufficiently well-tried. Examples of self-depositing resins such as may be used in the process according to the present invention are listed in WO 93/15154. This publication names as examples urethane resins, epoxy resins, polyester resins and resins based on various acrylates. Specific examples of acrylic resins are those such as contain one or more of the following monomers: methyl acrylate, ethyl acrylate, butyl acrylate, 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, 2-ethylhexyl acrylate, methyl methacrylate, ethyl methacrylate, n-butyl methacrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate, glycidyl acrylate, glycidyl methacrylate, acrylamide, methacrylamide, acrylic acid and methacrylic acid, and acrylic-alkyd resins. The latter acrylates may be present as copolymers with ethylene, styrene, vinyl chloride, vinylidene chloride and vinyl acetate.
Epoxy-based resins which may also be used within the framework of the process according to the present invention are described, for example, in WO 97/07163. Apart from pure epoxy resins, epoxy acrylate-based resins are suitable.
In addition to the self-depositing resin and the acid, the emulsions frequently contain oxidants and/or fluoride ions. These improve the deposition process. Examples of such process variants which may be used in the cycle of the process according to the present invention are: EP-A-32 297, EP-A-374 772, WO 93/15154 and WO 93/16813.
As is conventional when coating metal parts for corrosion protection, it is also preferred in the process according to the present invention to carry out an intermediate rinse using an aqueous solution of chromic acid or of chromates between deposition of the self-depositing resin and curing.
The self-depositing resin is cured at a temperature within the range from 140 to 250° C., in particular 150 to 180° C.
If desired, a so-called primer may now be applied to the self-depositing resin. This part step is not, however, absolutely necessary t
Lange Ilona
Schelbach Ralf
Harper Stephen D.
Henkel Kommanditgesellschaft auf Aktien
Nakarani D. S.
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