Coating processes – With post-treatment of coating or coating material – Heating or drying
Reexamination Certificate
2000-09-18
2002-06-11
Cameron, Erma (Department: 1762)
Coating processes
With post-treatment of coating or coating material
Heating or drying
C427S388100
Reexamination Certificate
active
06403164
ABSTRACT:
The present invention relates to a process for protecting a metallic substrate against corrosion and in particular to a process wherein the formation of a possible corrosion product of the metal is substantially or completely prevented by a competing reaction or interaction of the metal with a species which leads to a product having a more negative formation enthalpy than the corrosion product.
Metals whose position in the voltage series means that they react with water may under certain circumstances require a protective coating that prevents attack by water and/or oxygen. For this purpose, the prior art includes a very wide variety of processes. Whereas, in the case of aluminium, anodic oxidation (Eloxal process) is a widespread process, it is difficult if not impossible in the case of magnesium. Another possibility for preventing attack on the surface of the metal is to prevent the permeation of the corrosion-causing species through the entire coat thickness of the coating material. For this purpose it is necessary to use coat thicknesses of 50 &mgr;m or more. In another known corrosion protection process, a porous sol-gel coat is applied to the substrate, is subsequently impregnated repeatedly with polymers, in this case silicones, and, owing to the high proportion of inorganic structure, provides a relatively good diffusion barrier. A disadvantage of this process, however, is the need for multiple impregnation. Consequently, a protection system of this kind becomes complex to operate and very expensive, and so the principle has not established itself in the market. In the case of ferrous metals, corrosion-inhibiting pigments are generally added.
It was therefore an object of the present invention to develop a (preferably transparent) coating system which provides effective corrosion protection for a very wide variety of metallic substrates, preferably in combination with a very high abrasion resistance.
In accordance with the invention it has been found that this object may be achieved by providing silicic acid (hetero)polycondensates with certain species which are able to enter into a bond or at least an interaction with the metal (ion) in the course of which the free interface enthalpy is lowered sufficiently, and by enclosing or anchoring this species firmly in the structure of the coating by means of an inorganic network. Owing to the inorganic network, the resultant coatings also possess high abrasion resistance, which may be strengthened further by incorporating nanoscale particles. Another effect of incorporating the nanoscale particles is that such coats remain transparent.
In contrast to the prior art, which necessitates phosphating or chromating as a passivation step in the case of conventional corrosion protection coatings, this step may be omitted if the species used in accordance with the invention are incorporated in molecularly disperse form into the (hetero)polysiloxane structure. Through a diffusion process, they reach the interface during the wet coating operation, and develop their stabilizing activities there. Consequently, these species also differ from the anti-corrosion pigments of the prior art. If the desire is not for transparent coats, then the systems (without a loss of their primary corrosion protection activity) may of course be formulated with additional pigments. A further (additional) possibility is the incorporation of fluorinated side-chains (via correspondingly hydrolysable silanes, for example), by means of which such coats provide a low surface energy at the same time.
The present invention accordingly provides a process for protecting a metallic substrate against corrosion resulting in the formation of a species X, derived from the metal, wherein a coating composition based on (hetero)polysiloxanes prepared by hydrolysis and condensation processes is applied to the substrate and cured, and which is characterized in that the coating composition comprises at least one species Z which reacts or interacts with the metal to form a species Y having a more negative formation enthalpy than the species X.
In the text below, the present invention is elucidated further, taking into account preferred embodiments thereof.
The term “corrosion” as used hereinbelow refers to any change in the metal which leads to oxidation (conversion) to the corresponding metal cation with formation of a species X. Such species X are generally (optionally hydrated) metal oxides, carbonates, sulphites, sulphates or else sulphides (for example, in the case of the action of H
2
S on Ag).
The term “metallic substrate” as used in the present description and claims refers to any substrate which consists entirely of metal or has at least one metallic layer on its surface.
In the present context, the terms “metal” and “metallic” embrace not only pure metals but also mixtures of metals and metal alloys, these metals and metal alloys preferably being susceptible to corrosion.
Accordingly, the process of the invention may be applied with particular advantage to metallic substrates comprising at least one metal from the group consisting of iron, aluminium, magnesium, zinc, silver and copper, although the scope of application of the present invention is not restricted to these metals. Among the metal alloys which may particularly profit from the present invention, mention may be made in particular of steel and brass and of aluminium alloys.
The coating composition based on (hetero)polysiloxanes prepared by hydrolysis and condensation processes that is used in accordance with he invention derives preferably from at least one hydrolysable silicon compound of the general formula (I)
R
4−x
SiR′
x
(I)
in which the radicals R, which may be identical or different (preferably identical), are hydrolytically eliminable radicals, preference being given to alkoxy (especially of 1 to 6 carbon atoms, such as methoxy, ethoxy, propoxy and butoxy), halogen (especially Cl and Br), hydrogen, acyloxy (preferably of 2 to 6 carbon atoms, such as acetoxy, for example) and -NR″
2
(in which the radicals R″, which may be identical or different, are preferably hydrogen or C
1-4
alkyl radicals) and particular preference being given to methoxy or ethoxy; R′ is alkyl, alkenyl, aryl, alkylaryl, arylalkyl, arylalkenyl, alkenylaryl (preferably of in each case 1 to 12 and in particular 1 to 8 carbon atoms, and including cyclic forms), it being possible for these radicals. to be interrupted by oxygen, sulphur or nitrogen atoms or by the group -NR″ and to carry one or more substituents from the group consisting of halogens and substituted or unsubstituted amino, amide, carboxyl, mercapto, isocyanato, hydroxyl, alkoxy, alkoxycarbonyl, phosphoric acid, acryloxy, methacryloxy, alkenyl, epoxy and vinyl groups; and x is 1, 2 or 3 (preferably 1 or 2 and especially 1).
Particularly preferred silicon starting compounds are those of the above general formula (I) in which R is methoxy or ethoxy, x is 1 and R′ is an alkyl radical (preferably of 2 to 6 and with particular preference 2 to 4 carbon atoms) which is substituted by a group comprising a grouping which is amenable to a polyaddition reaction (including addition polymerization) and/or polycondensation reaction. With particular preference, this grouping amenable to a polyaddition and/or polycondensation reaction is an epoxy group.
Accordingly, R′ in the above formula (I) is with particular preference an &ohgr;-glycidyloxy-C
2-6
alkyl radical, more preferably a &ggr;-glycidyloxypropyl radical. Specific examples of particularly preferred silicon compounds, accordingly, are &ggr;-glycidyloxypropyltrimethoxysilane and &ggr;-glycidyloxypropyltriethoxysilane.
In addition to the above-described hydrolysable silicon compound(s), the (hetero)polysiloxanes for use in accordance with the invention may further derive from other hydrolysable silicon compounds (known to the person skilled in the art), for example those in which x in the above formula (I) is zero (e.g. tetraethoxysilane), and also from other hydrolysabl
Jonschker Gerhard
Langenfeld Stefan
Schmidt Helmut
Cameron Erma
Heller Ehrman White & McAuliffe LLP
Institut für Neue Materialien gemeinnützige GmbH
LandOfFree
Method for protecting a metallic substrate against corrosion does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method for protecting a metallic substrate against corrosion, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for protecting a metallic substrate against corrosion will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2951053