Stock material or miscellaneous articles – Composite – Of silicon containing
Reexamination Certificate
2001-05-16
2003-03-04
Dawson, Robert (Department: 1712)
Stock material or miscellaneous articles
Composite
Of silicon containing
C428S450000, C427S488000, C427S489000, C427S490000
Reexamination Certificate
active
06528170
ABSTRACT:
SPECIFICATION
The invention concerns a process for the corrosion-resistant coating of metal substrates by means of plasma polymerization. The process is especially suitable for the corrosion-resistant coating of aluminum and aluminum alloys.
Ever since research began addressing the production of plasma-applied polymer layers by means of polymerization processes which produce the energy needed for polymerization by adding gaseous monomers to a gas discharge process, there has been no lack of attempts to deposit these layers in such a manner that they are able to protect the coated surface from different types of attack. This function is by no means trivial, since plasma-applied polymer layers are decidedly thin layers, measuring in the nanometer range up to a few micrometers. Not only were scratch-resistant layers developed, e.g. for optical functional elements made of plastic (WO-A-8504601), but there were also attempts, with moderate success, to use this type of layer to protect metallic materials. Even types of attack that must be considered less than severely corrosive were withstood by these layers for only very short periods of time.
In all the experimentation that is known at this time involving aluminum materials, oxide layers are used to promote adhesion in oxidizing plasma arrangements, and this is analogous to conventional lacquering processes, however, it is also analogous to the preparation of surfaces before gluing, where an oxide layer, which has usually been produced by anodic oxidation, is used. Activation of the boundary layer, which is desirable for good adhesion, is achieved, if at all, by intercalating substances of a foreign nature. Bonding is frequently carried out solely by means of adhesive forces. Experience has shown that such coating or gluing systems exhibit only moderate imperviousness to infiltration, because water vapor arising by diffusion or permeation processes weakens the bond between the material and the coating.
Plasma polymerization, on the other hand, is a process that is capable of coating solid objects by the action of a plasma on an organic molecule in the gas phase, whereby the coatings created in this manner are primarily organic in character and have excellent properties. Plasma polymerization belongs to the category of low-pressure plasma processes, and is used increasingly for industrial purposes. The great interest in this technology derives from the advantages of a rapid, contact-free, dry chemical coating process, which furthermore puts little stress on the work piece.
Plasma-applied polymer layers deposited by low-temperature plasmas, hereinafter referred to as plasma polymers, are distinguished by the following characteristics:
Plasma polymers are often three-dimensionally highly cross-linked and insoluble and swell only slightly or not at all, and are potentially good barriers to diffusion.
Compared to conventionally manufactured polymers, their high degree of cross-linking makes them unusually stable thermally, mechanically, and chemically.
The layers adhere well to most substrate materials, and have a high density and are free of micropores.
The layers are usually amorphous in structure, with a smooth surface that conforms to the shape of the substrate.
The layers are very thin, and the thickness of the layer amounts to only a few 100 nm down to 10 nm.
The process temperatures are low, i.e. room temperature up to approximately 100° C., especially up to approximately 60° C.
On the other hand, no processes are yet known with which metal substrates, especially substrates comprising aluminum materials, can be made corrosion-resistant by coating with a plasma polymer.
Ribbed pipes made from the material AlMgSi0.5 are frequently used in condensing boilers. When used under extreme conditions and in areas approaching the limits for allowable gas composition, such ribbed pipes do not always exhibit sufficient corrosion resistance.
The formation of corrosion products results in malfunctions on the gas side in the vicinity of the pipe ribs and, in advanced stages, a reduction of the heat exchanger surface on the combustion gas side, as well.
Conventional means for protecting against corrosion, which are state of the art, cannot be adopted for several reasons. Processes such as phosphatizing or chromizing bring about the continuous emission of heavy metal ions into the environment and must be excluded due to the likelihood of more restrictive anticipated legislation on waste water disposal.
Lacquer systems do not constitute a viable alternative, either. Lacquers used as a means for protecting surfaces compromise thermal conductivity, which in the present case can be tolerated only within very narrowly defined limits. Furthermore, in conventional lacquer coatings, the diffusion of water vapor leads to infiltration of the protective layer. Subsequent condensation on the metal surface causes the layer to separate in such systems, thereby accelerating the process of corrosion, as is known for localized types of corrosion.
Coating such ribbed pipes for heat exchangers with a plasma polymer would, in and of itself, be desirable. However, in experimental trials, in this connection, corrosion-resistant coatings were not achieved. As a rule, the plasma polymers were found not to adhere firmly enough to the metal surface, and infiltration of the coating was found to occur more or less rapidly, with the result that it soon showed signs of separating.
A process for the surface coating of silver objects is known from [the German patent application] DE-A-42 16 999, in which the surface is first treated with a stripping plasma, and the surface is then coated with a plasma polymer, whereby an initial coupling layer, a surface layer to prevent permeation on top of that, and finally a sealant layer are produced. Ethylene and vinyltrimethylsilane are especially used for the coupling layer, ethylene for the layer preventing permeation, and, for the sealant, hexamethyldisiloxane in conjunction with oxygen as a plasma forming monomer, whereby a continuous transition occurs between the plasma forming monomers. The coatings are largely scratch resistant, and they provide good protection against tarnish; in certain formulations, however, they can be susceptible to removal by cleansers. A coating on aluminum substrates fails to provide corrosion-resistant layers.
On the whole, it would be desirable to have a process available with which a long-lasting and corrosion-resistant plasma polymer coating can be applied to metal materials, especially aluminum materials.
This goal is achieved by a process of the type mentioned at the outset, where the substrate undergoes a pre-treatment step of smoothing by mechanical, chemical, and/or electrochemical means, after which, at a temperature of less than 200° C. and a pressure of 10
−5
to 100 mbar, it is exposed to a plasma, whereby, in an initial step, the surface is activated in a reducing plasma and, in a second step, the plasma polymer is deposited from a plasma that optionally contains at least one hydrocarbon or organosilicon compound, which can be vaporized under the conditions of the plasma, containing oxygen, nitrogen, or sulfur, and which may contain fluorine atoms.
A surprising finding was that the problem of insufficient adhesion of the coating to the metal surface is solved by the combination of a smoothing pre-treatment of the metal substrate which is to be coated with a plasma treatment. The plasma treatment, in turn, consists of 2 steps, namely, first treatment of the surface by a reducing plasma which acts as a surface stripper, and a second step, in which the actual coating is applied directly to the metal layer that has been pre-treated by the plasma.
The pre-treatment, especially the smoothing of the surface of the metal substrate, can be carried out by mechanical, chemical, or electrochemical means. Especially preferred are combinations comprising both mechanical and chemical smoothing. In any event, electrochemical smoothing can be undertaken after mechanical and/or chemical smoothin
Baalmann Alfred
Hufenbach Hartmut
Semrau Wolfgang
Stuke Henning
Vissing Klaus-Dieter
Dawson Robert
Feely Michael J
Fraunhofer-Gesellschaft zur Forderung der ange-wandten Forschung
Weingarten Schurgin, Gagnebin & Lebovici LLP
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