Patent Application
20070054056
The present invention relates to a composition based on silanes for coating metal to protect against corrosion, to its preparation and use.
Anti-corrosion compositions for metals and also adhesion promoters (referred to hereinbelow as primers) based on organosilanes or SiO2 sol have been known for a long time and have been described in diverse form. Unfortunately the known systems still leave something to be desired.
Common methods of producing anti-corrosion layers on metallic surfaces include chromating, phosphating, coating, and anodizing.
The industrial significance of chromating is at the present time continuing to decline owing to toxicological problems and corresponding reclassification of the chromates (EU Directive 2000/53/EC). Despite the good corrosion protection properties of chromating, therefore, the development of new, chromium-free systems is of generally high interest.
Methods of phosphating iron, steel, zinc, and aluminum have long been state of the art. Phosphating in combination with a coating is enough to afford an entirely acceptable protection against corrosion, whereas the two systems alone do not lead to the desired results.
Electrolytically produced protective oxide films on metals, such as aluminum or titanium, despite affording adequate protection against corrosion, exhibit brittleness as the thickness of the oxide film increases, and require a large amount of energy for their production.
Present developments are attempting to deposit perfect monolayer films on the metal surfaces, or are working with conversion coats of cerium [Galvanotechnik 92 (12), 2001, 3243].
Further approaches to avoiding chromates(VI) are formulations which contain chromium(III) and which therefore cannot be considered to be free of heavy metals.
Eco-friendly anti-corrosion layers can be obtained with silanes and silane-based sol-gel condensates (U.S. Pat. No. 5,200,275). For the coating of the surfaces the metal substrates are first degreased and cleaned. The cleaning steps may include treatment with organic solvents and also alkaline and acidic pickling operations [Metalloberfläche, 29 (10), 1975, 517]. The silanes are hydrolyzed in aqueous or at least water-containing solutions and then contacted with the metal surface by immersion, spraying and/or spin coating application. Curing or condensation takes place in air at room temperature or elevated temperature. The film thicknesses achieved are normally between 10 and 100 nm (U.S. Pat. No. 5,750,197).
Sol-gel systems (also referred to hereinbelow as sol) are prepared by means of an acidic or basic catalyst from hydrolyzable silicon compounds, often in combination with corresponding Al, Ti and/or Zr precursors. For this purpose the silanes are hydrolyzed together in a solvent, the solvent generally being an alcohol which is also formed during the hydrolysis of the compounds. The coating composition can be applied to the metallic substrates again by a coating method already mentioned above. The coating composition is applied preferably in dry film thicknesses of from 1 to 50 μm without any final coating with a topcoat material being envisaged (DE 198 13 709).
EP 1 130 066 discloses corrosion-protected metal surfaces which are pretreated with a silane composition and subsequently extrusion coated with a polyamide compound. The water- and alcohol-containing composition employed is based, inter alia, on hydrolyzates, condensates and/or cocondensates, starting from alkoxysilanes Q which carry functional groups, such as aminoalkylalkoxysilanes, and alkoxysilanes M, such as alkylalkoxysilanes, alkenylalkoxysilanes and tetraalkoxysilanes, the sum of components M and Q being present in a molar ratio of 0≦M/Q<20.
It is an object of the present invention to provide a further composition for producing a film on a metallic surface for protection against corrosion. A particular concern was to protect specifically the surface of aluminum or an aluminum alloy against corrosion by means of a coating. A further concern was to find a highly effective and nevertheless eco-friendly solution.
This object is achieved in accordance with the invention as per the details in the patent claims.
Surprisingly it has been found that the morphology of hydrophobic sol coatings can be enhanced by adding small amounts of a hydrophilic component. While the coatings constructed only from hydrophobic components exhibit pores, the addition of small amounts of a hydrophilic compound, especially aminosilanes, makes it possible to produce highly coherent films which have a low pore content or are pore-free. The hydrophilic component is appropriately added to the coating sol only at very low concentrations, ≦15% by weight, based on the SiO2 content of the sol, since otherwise the film swells in aqueous media and corrosion may occur.
The mode of action of the present teaching can also be recognized from the fact that, in a system having a relatively high pore content, simply increasing the film thickness does indeed increase the electrical volume resistance, but the porosity is maintained.
Coatings for protecting metals against corrosion, obtainable using a composition of the invention, are generally very thin, hydrophobic, transparent films with a barrier effect. They constitute in particular an especially effective barrier film for electrolyte solutions and so advantageously bring about passive protection for metals against corrosion, especially for aluminum and aluminum alloys.
Coatings obtained using the present composition are further outstandingly suitable for primer applications and can be coated—either directly or following further treatment with a corresponding functional silane—with an additional topcoat and/or color coating material. At least one functional group of the silane should preferably be chosen so that chemical attachment to the coating material can take place: suitably, aminosilanes for urethane or epoxy coating materials, such as 3-aminopropyltrimethoxysilane or else 3-glycidyloxypropyltrialkoxysilanes, and silanes containing double bonds, such as 3-methacryloyloxypropyltrialkoxysilane or vinyltrialkoxysilanes, for free-radically curing coating systems, alkoxy here and below preferably being methoxy, ethoxy, propoxy, butoxy or 2-methoxyethoxy.
With such a coating, obtainable using the composition of the invention, it is observed in a corrosion test, particularly in the case of aluminum, that there is virtually no “scribe creep”, or corrosive migration beneath the coating material starting from the scribe mark. Accordingly, through the formation of very thin barrier films, the coatings of the invention afford good passive protection against corrosion. This inventively thin anti-corrosion layer also effects astonishingly good coverage of rough surfaces, such as mechanically ground aluminum sheets, for example. Consequently, films of this kind may replace not only chromating but also the cathodic electrocoating which normally follows.
The sol-gel coating of the invention can likewise be used as a stable carrier system for other and/or further silane-based coating systems, such as perfluoroalkylsilane films which repel water and dirt, using for example, inter alia, tridecafluoro-1,2,2-tetrahydrooctyltriethoxysilanes, corresponding formulations, condensates or cocondensates, such as may also be found in EP 0 838 467 B1, EP 0 846 717 B1, EP 0 846 716 B1, EP 1 033 395 A2 and EP 1 101 787 A2.
The advantages of the present composition and of the substantially pore-free coatings obtained using the composition are as follows:
The present invention accordingly provides a composition for coating metals to protect against corrosion, comprising
(i) a sol based on silicon compounds,
(ii) at least one aminoalkyl-functional alkoxysilane and/or
(iii) at least one reaction product of components (i) and (ii), the amount of aminoalkyl-functional silane component from (ii) and (iii) being in total from 0.01 to 15% by weight, calculated as the particular aminoalkylalkoxysilane employed and based on the SiO2 content of the sol as per (i).
The amount of aminoalkyl-functional silane component from (ii) and (iii) in the composition of the invention is preferably from 0.02 to 10.0% by weight, more preferably from 0.04 to 4.0% by weight, very preferably from 0.06 to 1.6% by weight, and in particular from 0.08 to 1.0% by weight, calculated as the particular aminoalkylalkoxysilane employed and based on the SiO2 content of the sol as per (i).
The composition of the invention suitably includes at least one aminoalkyl-functional alkoxysilane from the group consisting of 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, N-(2-aminoethyl)-3-aminopropyltrimethoxysilane, N-(2-aminoethyl)-3-amino propyltriethoxysilane, N-(2-aminoethyl)-3-aminopropyl-methyidimethoxysilane, N-(2-aminoethyl)-3-aminopropylmethyidiethoxysilane, N,N-di(2-aminoethyl)-3-aminopropyltrimethoxysilane, N,N-di(2-aminoethyl)-3-aminopropyltriethoxysilane, N-[N′-(2-aminoethyl)-2-aminoethyl)]-3-amino-propyltrimethoxysilane, N-[N′-(2-aminoethyl)-2-aminoethyl)]-3-aminopropyl-triethoxysilane, N-methyl-3-aminopropyltrimethoxysilane, N-methyl-3-aminopropyl-triethoxysilane, N-(n-butyl)-3-aminopropyltrimethoxysilane, N-(n-butyl)-3-aminopropyltriethoxysilane, N-cyclohexyl-3-aminopropyltrimethoxysilane, N-cyclohexyl-3-aminopropyltriethoxysilane, N-phenyl-3-aminopropyltrimethoxy-silane, 3-aminopropylmethyldimethoxysilane, 3-aminopropylmethyidiethoxysilane, bis(3-trimethoxysilylpropyl)amine, and bis(3-triethoxysilylpropyl)amine.
In the composition of the invention the amount of component (i) is preferably from 0.25 to 40% by weight, more preferably from 5 to 30% by weight, very preferably from 8 to 25% by weight, and in particular from 10 to 15% by weight, calculated as SiO2 and based on the overall makeup of the composition, all of the constituents of the composition adding up to 100% by weight.
The composition of the invention further preferably includes water, methanol, ethanol, isopropanol, butanol, methoxypropanol, butylglycol, or a mixture of at least two of the aforementioned solvents/diluents. Thus the amount of solvent/diluent in the composition of the invention can be from 1 to 99% by weight, preferably from 15 to 80% by weight, more preferably from 30 to 50% by weight, the amount being based on the overall makeup of the present composition.
The composition of the invention may further comprise at least one corrosion inhibitor, examples being inorganic corrosion inhibitors based on borates, phosphorus silicates, and zirconium complexes, or organic corrosion inhibitors, such as ammonium benzoate, benzothiazolylthiosuccinic acid or ammonium salts of fatty acids, to name but a few. The composition of the invention appropriately contains from 0.05 to 20% by weight, preferably from 0.5 to 10% by weight, in particular from 1 to 5% by weight of corrosion inhibitor, the amount being based on the overall makeup of the present composition.
The composition of the invention may also include at least one organic resin and/or at least one polymer, examples being bisphenol A, epoxy resins, such as bisphenol A diglycidyl ether, and polyesters, polyacrylates, polyethers or polyurethanes.
The composition of the invention preferably contains from 0.05 to 20% by weight, more preferably from 0.5 to 10% by weight, very preferably from 1 to 5% by weight of resin, polymer or a mixture of resin and polymer, the amount being based on the overall makeup of the present composition.
The present invention further provides a process for preparing a composition of the invention for coating metal to protect against corrosion, said process comprising
In the first step it is preferred to employ at least one alkoxysilane from the group consisting of tetramethoxysilane, methyltrimethoxysilane, propyltrimethoxysilane, isopropyltrimethoxysilane, octyltrimethoxysilane, hexadecyltrimethoxysilane, octadecyltrimethoxysilane, tetraethoxysilane, methyltriethoxysilane, propyltriethoxysilane, isopropyltriethoxysilane, octyltriethoxysilane, hexadecyltriethoxysilane, and octadecyltriethoxysilane or a mixture of at least two of the aforementioned alkoxysilanes.
For the preparation of the sol the alkoxysilane is subjected to controlled hydrolysis and the hydrolyzate is reacted to form the sol, where appropriate with thorough mixing. The reaction can be conducted where appropriate in the presence of a hydrolysis catalyst, such as an organic or mineral acid, for example, such as acetic acid, citric acid or phosphoric acid, sulfuric acid or nitric acid, or a base, such as sodium methoxide, sodium ethoxide, sodium hydroxide, potassium hydroxide, ammonia or organic amines, such as alkylamines, for example. The hydrolysis catalyst can be introduced in a mixture with the water that is used here for the hydrolysis. It is also possible to add a diluent/isolvent to the hydrolysis mixture or to the resultant sol, said diluent/isolvent being, for example, an alcohol which corresponds to the alkoxy group of the alkoxysilane employed.
Preparation of the sol is suitably carried out with thorough mixing of the reaction batch. The sol is preferably adjusted to a pH of from 1 to 5 by addition of one of the abovementioned acids.
A sol can be obtained, for example, from 50 to 100 parts by weight of at least one alkoxysilane, selected for example from methyltrialkoxysilane and/or tetraalkoxysilane, 0.01 to 5 parts by weight of acid, phosphoric acid for example, 5 to 150 parts by weight of water and, if desired, 10 to 200 parts by weight of alcohol, ethanol for example, hydrolysis and subsequent reaction being carried out appropriately with stirring at a temperature of from 10 to 80° C over a period of from 10 minutes to 5 days.
In the process of the invention the aminoalkylalkoxysilane is then added to the sol from the first step. The batch, appropriately, is mixed thoroughly.
It is further preferred in the second step to add water in an amount of from 0.001 to 100 parts by weight, more preferably from 1.0 to 50 parts by weight, very preferably from 2.0 to 10 parts by weight, per 100 parts by weight of sol from the first step.
Moreover it is possible in the second step to add at least one solvent/diluent from the group consisting of methanol, ethanol, isopropanol, butanol, methoxypropanol, and butylglycol in an amount of from 0.001 to 10 000 parts by weight, preferably from 50 to 900 parts by weight, more preferably from 100 to 300 parts by weight, per 100 parts by weight of sol from the first step.
The process of the invention is generally conducted in such a way that an optionally aqueous and/or alcohol-containing sol is prepared on the basis of alkoxysilanes in a conventional manner, an aminoalkylalkoxysilane is added, and the mixture is reacted.
The product mixture obtained in the second step is suitably reacted additionally at a temperature in the range from 10 to 60° C., preferably from 20 to 30° C., for from 1 minute to 24 hours, preferably from 10 to 60 minutes, with thorough mixing.
A preparation obtained in this way, which is generally clear or opalescent to milky, can then be used, as obtained or in diluted form, as a composition for coating metal surfaces to protect against corrosion. To this end, the metal surface to be coated can be precleaned, mechanically for example by abrasion or chemically by etching, after which the composition of the invention can be applied, by dipping, brushing, spraying or knife coating, for example. The metal surface thus treated is appropriately dried thermally, preferably at up to 220° C., in particular at up to 200° C., which generally produces a hard and clear coating which adheres well to the metal.
Accordingly, the present invention likewise provides for the use of a composition of the invention as an anti-corrosion composition for a metal from the group consisting of aluminum, aluminum alloys, magnesium, magnesium alloys, zinc, zinc alloys, titanium, titanium alloys, iron, iron alloys, galvanized iron sheet, galvanized iron alloy, tin, tin alloys, copper, copper alloys, and silver and silver alloys.
The invention further provides for the use of a composition of the invention by applying the composition to the optionally cleaned and/or pretreated metal surface, drying and/or curing the applied film, and, if desired, applying one or more paint and/or polymer layers to the coating thus obtained.
The invention additionally provides for the use of a composition of the invention by applying the composition to an optionally cleaned and/or pretreated metal surface, drying and/or curing it, applying to the anti-corrosion layer thus produced a silane-based adhesion promoter, based preferably on an aminosilane, a glycidyloxyalkylsilane, an acryloylsilane, a vinylsilane or a corresponding mixture thereof, drying the applied adhesion promoter (primer), and applying to the coating thus obtained one or more paint and/or polymer layers, such as color coating material and also topcoat materials. The primer is preferably selected from the group consisting of 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, N-(2-aminoethyl)-3-aminopropyltrimethoxysilane, N-(2-aminoethyl)-3-aminopropyltriethoxysilane, N-(2-aminoethyl)-3-aminopropylmethyidimethoxy-silane, N-(2-aminoethyl)-3-aminopropylmethyldiethoxysilane, N,N-di(2-aminoethyl)-3-aminopropyltrimethoxysilane, N,N-di(2-aminoethyl)-3-aminopropyltriethoxysilane, N-[N′-(2-aminoethyl)-2-aminoethyl)]-3-aminopropyltrimethoxysilane, N-[N′-(2-aminoethyl)-2-aminoethyl)]-3-aminopropyltriethoxysilane, N-methyl-3-aminopropyltrimethoxysilane, N-methyl-3-aminopropyltriethoxysilane, N-(n-butyl)-3-aminopropyltrimethoxysilane, N-(n-butyl)-3-aminopropyltriethoxysilane, N-cyclohexyl-3-aminopropyltrimethoxysilane, N-cyclohexyl-3-aminopropyl-triethoxysilane, N-phenyl-3-aminopropyltrimethoxysilane, 3-aminopropylmethyl-dimethoxysilane, 3-aminopropylmethyldiethoxysilane, bis(3-trimethoxysilyl-propyl)amine, and bis(3-triethoxysilylpropyl)amine, in which case the aminosilane can suitably be used as an alcoholic and/or aqueous primer solution having an aminosilane content of from 0.001 to 10% by weight, based on the solution. When employing a polyurethane or epoxy coating material it is additionally possible to use β-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, 3-glycidyloxypropyltrimethoxy-silane or 3-glycidyloxypropyltriethoxysilane as primer or adhesion promoter. Additionally, for free-radically curing coating materials, the adhesion promoter employed may advantageously be 3-methacryloyloxypropyltrimethoxysilane, vinyltrimethoxysilane, vinyltriethoxysilane or a mixture thereof.
The composition of the invention can be used to advantage in particular in automobile construction, in mechanical engineering, in ship building, in aircraft construction, or for house construction components.
The invention accordingly provides a novel composition which advantageously allows the coating of metals to protect against corrosion. In particular the composition of the invention allows the protection of aluminum and aluminum alloys against corrosion, the anti-corrosion layer produced in accordance with the invention possessing excellent adhesion directly to the metallic substrate and, with the further possibility of applying additional layers having outstanding adhesion or further particular properties, such as hydrophobicity and oleophobicity, to said anti-corrosion layer in a simple and economical way.
The present invention is illustrated by the following examples, without restriction of its subject matter.
1.1 Preparation of the Coating Formulation:
120 g of tetraethoxysilane (DYNASIL® A) and 300 g of methyltriethoxysilane (DYNASYLAN® MTES) were charged to a four-necked flask with mechanical stirrer, reflux condenser, dropping funnel, and thermometer. With intensive stirring, 42 g of deionized water were added dropwise over the course of 5 minutes and the mixture was diluted with 138 g of isopropanol. The pH was adjusted to 3.5 using phosphoric acid. The mixture, which was cloudy to start with, was stirred for a further 5 hours. This gave a clear sol.
The concentrated sol was divided into portions, each of which was mixed as per table 1 with N-butylaminopropyltrimethoxysilane (DYNASYLAN® 1189), diluted with isopropanol, activated with deionized water, and stirred for 30 minutes.
*)The solution was adjusted to a pH of 4 using acetic acid.
1.2 Cleaning of the Metals:
All metal sheets were degreased with organic solvents, pickled in dilute (10%) alkaline cleaning solution (CARELA® SP, R. Spane GmbH) at 70° C. for 10 to 20 seconds, and then rinsed off with deionized water. The sheets were further pickled for 5 minutes in dilute (0.0016 mol/l) nitric acid, rinsed with deionized water, and dried.
1.3 Application of the Coatings:
Test sheets (aluminum alloy 6016: AlMg0.4 Si1.2) cleaned according to 1.2 were immersed in the activated coating sols A to E and also in solution F for 5 minutes. The sheets were stood up vertically to allow excess sol to drip off. After brief initial drying at room temperature, curing took place at 200° C. for 10 minutes. This gave a transparent coating with a thickness of less than 1 μm. Thereafter the sheets were referred to as 1A to 1F, depending on the coating solution. Specimen 1A(2) was coated again with the coating sol A.
1.4 Testing of the Sheets:
The sheets 1A to 1F coated in accordance with 1.3 were stored in oxidizing corrosion solutions (solution of NaCl and H202 in DI water as per DIN 50 905) in closed corrosion chambers at 50° C. The corrosion solution was replaced each day. Uncoated sheets were also subjected to the corrosion test, as comparison specimens.
While the coated sheets showed no surface attack, severe oxidation occurred on the untreated comparison specimen after just a few days in the corrosion solution. Even after 30 days in the corrosion solution the metal sheet coated with solution E showed only slight surface corrosion. It was followed, in a graded evaluation made on the basis of inspection, by the metal sheets with the barrier layers D, A, C, B, and F; cf. table 1.
The quality of the coating on the sheets coated according to 1.3 was further investigated by electrochemical impedance spectroscopy (EIS) [Mansfeld, F., “Electrochemical Impedance Spectroscopy (EIS) as a New Tool for Investigation Methods of Corrosion Protection”, Electrochimica Acta 35 (10), pages 1533 to 1544, 1990]. The impedance of the coating on the sheet was measured at different alternating current frequencies.
The addition of the hydrophilic aminosilane component produced a marked change in the course of the plot.
2.1 Application of the Coatings:
Untreated metal test sheets (aluminum alloy 6016: AlMg0.4Si1.2) and superficially abraded aluminum sheets of the same alloy were cleaned as in 1.2. Untreated aluminum sheets were immersed for 5 minutes in the activated coating sol E, while superficially abraded aluminum sheets were immersed in the activated coating sol A. The sheets were stood up vertically to allow excess sol to drip off. After brief initial drying at room temperature, curing was carried out at 200° C. for 10 minutes. To promote adhesion, the sheets were coated a second time with the solution F (primer). The duration of immersion and drying procedure were analogous to those of the 1st coating step. The overall film thickness of the silane system was less than 1 μm. For better comparison, sheets were also coated only with solution F and dried at 200° C. for 10 minutes.
The silane-coated metal sheets were painted with a 2-component polyurethane varnish (Standox GmbH) and dried at 60° C. for 60 minutes. The thickness of the varnish layer was approximately 25 μm.
2.2 Testing of the Metal Sheets:
After 240 hours' exposure to the salt spray mist conditions (CASS-Test, DIN 50 021) the scribe creep was 0.09 mm for the sheets coated with E, 0.17 mm for the abraded sheets coated with A, and 0.41 mm for the sheets coated only with F. According to the CASS test, unsilanized, varnished comparison sheets showed a scribe creep of 2.65 mm. Varnished aluminum sheets with yellow chromating, which were likewise tested, showed a scribe creep of 0.70 mm.
Therefore, in comparison with the other varnish-coated metal sheets, sheets E exhibit virtually no creep and thus give the best result under testing.
| Number | Date | Country | Kind |
|---|---|---|---|
| 103 20 765.1 | May 2003 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP03/10654 | 9/25/2003 | WO | 11/15/2006 |
