Patent Application
20120080120
The present invention relates to a method for passivating metallic surfaces using an aqueous, acidic composition which comprises a polymer having acidic groups and at least one cationic polyamine.
The present invention further provides an aqueous, acidic composition for passivating a metallic surface, the composition comprising a film-forming polymer having acidic groups, and at least one polyamine, and the composition having a pH of less than 2.
The invention also relates to the use of the stated aqueous, acidic composition for passivating a metal surface and to a method for producing the aqueous acidic composition. The present invention likewise relates to a coating on a metallic surface, obtainable by the inventively described method.
Metallic materials, more particularly iron and steel, are typically galvanized in order to protect them from corrosive environmental influences. The corrosion protection afforded by the zinc derives from the fact that it is baser than the metallic material itself, and therefore is initially corroded itself. The zinc is able to function as what is called a sacrificial electrode relative to the metallic surface. Since the zinc layer itself is also subject to corrosion (white rust), the corrosive attack on a zinc coat of this kind is frequently retarded by the application of what is called a passivating coat. The passivating coat shall retard the corrosive attack on the metal surface and may at the same time serve to improve the adhesion of any coats of coating material that are to be applied. Instead of the term “passivating coat”, the term “conversion coat” is frequently used synonymously.
The application of passivating coats takes place, for example, for galvanized metal parts (e.g., electrogalvanized or hot-dip galvanized supports) which are subsequently coated. Application also takes place for parts which are employed without coating. Similarly, metallic surfaces of aluminum or of aluminum alloys are frequently provided with a passivating coat, especially when they are to be coated subsequently.
The raw material used for the production of sheetlike metallic workpieces, such as, for example, automobile parts, bodywork parts, equipment linings, façade claddings, ceiling claddings or window profiles, are at present typically long metal strips which, by means of suitable techniques, are shaped into the desired parts and/or joined. The corrosion control treatment of metallic materials of these kinds is accomplished typically in multistage operations. The surface of treated metals often has a number of different layers. A corrosion control treatment may be performed at various points in the production operation. This corrosion control may be temporary or permanent. Temporary control, for example, is applied only for the storage or transportation of the metallic workpiece, such as the metal strip, for example, and is removed again before ultimate processing.
Of particular technical and economic importance are strips having a galvanized surface, more particularly strips of electrogalvanized or hot dip galvanized iron or alloys of iron such as steel, for example. Also of importance are metal strips comprising aluminum or alloys of aluminum.
In general, one or more additional coating-material coats are applied to the passivated surface. Examples of the purposes of these coating-material coats are to protect the passivation coat and the metal from corrosive gases and/or liquids, and/or from mechanical damage (such as stone chipping, for example). Such a coat may also serve esthetic purposes. Coating-material coats are typically much thicker than passivation coats. Typical thicknesses for a coat of coating material range from 4 μm to 400 μm.
In the prior art, passivation coats on surfaces of zinc or of aluminum have been obtained typically by treating the workpiece to be protected with aqueous acidic solutions of chromates (e.g., CrO3) or with acidic aqueous solutions of Cr(III) salts (see EP-A 0 907 762).
In the presence of atmospheric oxygen, the surface of zinc or alloys of zinc or of aluminum or alloys of aluminum in general first develops a thin oxide layer which retards corrosive attack on the underlying metal. Typically in the passivating methods based on chromium compounds, this existing oxide film, along with some of the metal to be protected, is dissolved, and is incorporated at least partly into a film on the metal surface. This film resembles the naturally existing oxide film and in general comprises specifically introduced phosphate, heavy metals and/or fluorides. The resulting passivation or conversion coats shall effectively protect the underlying metal from corrosive attack.
More recently, passivation methods, for metal-coated steel panels and piece goods (e.g., hot-dip galvanized steel), for example, have also been developed that are based on polymers as organic film-formers. In these methods, usually acidic aqueous solutions of various film-forming polymers, containing carboxyl, phosphoric acid and/or phosphonic acid groups, for example, are used to form the passivation coat. Following the application of the acidic formulation, there is typically partial dissolution of the metal surface (e.g. zinc) and polyvalent metal ions (e.g., Zn2+) are released. Typically, there is an increase in pH in the vicinity of the metal surface. Crosslinking and film-forming of the acidic polymers with the polyvalent metal ions typically take place.
Since, with the passivation methods described, using acidic polymers, the use of heavy metals such as chromium can be largely avoided or reduced, these polymer-based passivation methods are gaining steadily in importance. Known in the prior art are various—mostly acidic—passivating preparations, which in general comprise a water-soluble, film-forming polymer containing acid groups. Methods for passivation using these preparations have also already been described.
DE-A 195 16 765 relates to a method for generating conversion coats on surfaces of zinc or of aluminum by treatment with an acidic solution which comprises an organic film-former and also aluminum ions in the form of a water-soluble complex with chelate-forming carboxylic acids and phosphoric acids. Organic film-formers cited include carboxyl-containing polymers, more particularly homopolymers and/or copolymers of acrylic and/or methacrylic acid.
WO 2004/074372 describes the passivation of metallic surfaces using copolymers comprising acrylic acid and vinylphosphonic acid and/or maleic acid, the passivating formulation possibly comprising further components.
WO 2008/012248 describes acidic preparations for passivating metallic surfaces, comprising copolymers synthesized from monomethacrylic esters having hydrophobic groups (e.g., hydroxyethyl acetate), monomers with phosphonic acid groups (e.g., vinylphosphonic acid), and monomers with carboxyl groups (e.g., acrylic acid).
WO 2006/134116 describes a method for passivating metallic surfaces by treatment thereof with an aqueous composition comprising acid-group-containing polymers and polyvalent cations such as zinc, calcium, magnesium or aluminum ions.
WO 2006/134117 describes a method for passivating metallic surfaces by treatment thereof with an aqueous composition comprising acidic polymers, with addition of waxes (e.g., polyethylene waxes).
Aqueous passivating solutions of this kind, for the purpose of improving the corrosion control, are often applied to the galvanized steel strip directly after the galvanizing line (e.g. after hot dip galvanizing). Application is generally accomplished by means of roll technology, with, for example, simple squeeze rolls or more technologically sophisticated roll coaters being employed.
In the case of the squeeze roll, which is technologically simple and inexpensive to realize, the passivating solution is typically applied to the galvanized steel strip (by spraying, for example) and then squeezed off using a roll. Using the squeeze-roll technology it is frequently possible only to obtain substandard, i.e., less uniform, passivation coats. If the applied coats are subject to particular requirements, the more sophisticated and more costly roll-coater technology is employed, where the passivating solution is first applied to one or more rolls and then transferred to the steel strip. With this method it is usually possible to obtain more homogeneous passivation coats with a relatively uniform thickness.
The time available for film-forming, i.e., crosslinking the acidic polymer is normally comparatively short. In the case of the continuous passivation of a steel strip, the time remaining from the application of the formulation to the steel strip until the drying of the coated strip in the drier, depending on line speed, is mostly just a few seconds—typically, for example, 2 to 10 seconds.
WO 2009/047209 describes a continuous method for the coating of steel strips, where the passivating composition is first applied to a coating roller, taken off by a doctor blade, and then transferred to the steel strip.
Polyvalent cations, such as zinc ions (Zn2+) or aluminum ions (Al3+), for example, normally play a part as crosslinkers for acidic polymers in the formation of the passivation coat. In this process, the ions released from the metal surface (e.g., zinc ions), which dissolve as a result of the acid following the application of the acidic formulation, may also make a contribution to the crosslinking. If there is a rise in pH in the vicinity of the surface, there may be (partial) deprotonation of the acidic groups (e.g., formation of COO− ions).
Typically it is possible to observe crosslinking by the zinc ions (Zn2+) present, when the pH increases above about 2 to 2.5, with the acidic groups of the polymer. An insoluble crosslinked polymer network is formed.
Described in the prior art is the addition of polyvalent ions as crosslinkers in polymeric passivation coats. For instance, WO 2006/134116 discloses a method for passivating metallic surfaces by treatment of the surface with an aqueous composition comprising polymers containing acid groups, and polyvalent cations such as zinc, calcium, magnesium or aluminum ions.
With the known methods, in accordance with the mechanism described above, the degree of crosslinking decreases as the distance from the zinc surface increases. At the same time, the formulation dries in general up from the top. What remains, then, is a topmost polymer layer which is uncrosslinked, or is inadequately crosslinked, and which, accordingly, remains water-soluble. This topmost layer may be removed by dissolution; the corrosion resistance decreases, and the optical qualities of the surface are unfavorable. These inadequately crosslinked passivation coats produce, for instance, unfavorable values in the test known as the stack test, in which the coated metal surfaces are stacked atop one another and exposed to water over a period of several days. Even when polyvalent ions are added, however, it is often not possible to achieve sufficient crosslinking of the acidic polymers over the entire layer thickness.
It is an object of the present invention to provide compositions and methods for the treatment of metal surfaces that are suitable for the formation of a passivation coat on metal surfaces, the intention being to achieve an extremely thoroughly crosslinked, stable passivating film coat.
The passivation coat obtained is to be suitable not only for workpieces which are subsequently coated but also for those which are employed without coating. In this context, in the event of subsequent coating of the workpieces, it must be borne in mind that sufficient adhesion of the subsequent coating-material coat to the passivation coat is ensured. This means that the adhesion of coating material to the passivation coat should improve (or at least not deteriorate). The quality of coating-material adhesion can be determined using the cross-cut test (see also DIN ISO Standard 2409) on a planar metal surface and/or on a metal surface with defined denting. For this purpose, a defined cross-cut pattern is insized into the coating down to the coated substrate. A defined adhesive tape is then placed over the cross-cut and peeled off. The delamination of the coated cross-cut elements is then evaluated visually and reported using characteristic cross-cut values, on a scale from 0 to 5.
Also of importance are the optical qualities of polymer-containing passivation coats, with clear and transparent coats being desired. The optical qualities may be adversely affected by the phenomenon known as “chalking”. When affected by this phenomenon, the coats are no longer completely clear and transparent, but instead more or less nontransparent, white spots. “Chalking” can easily be confused with the formation of white rust, and may make quality control more difficult. Therefore, the passivation coats produced with the method of the invention or by means of the composition of the invention ought to have clear, transparent optical properties and a low propensity toward chalking.
It has surprisingly been found that, with the aid of passivating compositions based on polymers with acidic groups (more particularly copolymers comprising acrylic acid, vinylphosphonic acid, hydroxyethyl acrylate copolymers) which comprise cationic polyamines (e.g., polyethyleneimines) as cationic crosslinkers, it is possible to obtain highly resistant and thoroughly crosslinked passivation coats. It has been found, moreover, that film formation when using cationic polyamines (e.g., polyethyleneimines) as cationic crosslinkers begins even at very low pH levels (from 0.7 to 1.5, approximately).
A prerequisite for a passivation method of this kind is that it is possible to prepare clear, storage-stable solutions of the two inherently incompatible components represented by the polyamine (polycation) and the acidic polymers (polyanion). This is accomplished by carrying out neutralization of the polyamines in the first step, using strong acids such as sulfuric acid, phosphoric acid or methanesulfonic acid, for example, and adjusting the pH to the same level as that of the aqueous solution of the acidic polymer (typically a pH of less than 2). In the second step, the two aqueous, acidic polymer solutions are combined.
With the increase in the pH, the polyammonium compounds bring about crosslinking via the acidic groups of the polymer (e.g., carboxylate, phosphonate), it being possible for the crosslinking to take place even at very low pH levels below a pH of 2, more particularly even from a pH of as low as 0.7. The crosslinking of corresponding acidic polymers with polyvalent ions, such as Zn2+, takes place in general only at higher pH levels of 2 to 2.5. It is therefore advantageous that the crosslinking of the passivation coat in the method of the invention takes place more rapidly and hence more completely, with addition of a polyammonium compound, even in those regions which are at a greater distance from the zinc surface.
With the aid of the inventive combination of water-soluble polymers comprising acid groups (especially the copolymers described comprise or are composed of acrylic acid and vinylphosphonic acid) and a cationic polyamine, it is possible to obtain a significant improvement in the corrosion protection of metal surfaces by comparison with a known treatment using polyvalent cations as crosslinkers.
The passivating composition of the invention is, furthermore, preferably free from heavy metal compounds. A further advantage is the improved coating-material adhesion, relative to unpassivated, metallic surfaces and/or to passivation coatings of the prior art, on the metallic surfaces coated by the method of the invention.
It has been found, in addition, that the addition of surface-active substances, such as of cationic surfactants (e.g., alkylammonium compounds), for example, to the aqueous passivating composition makes it possible to achieve further improvements in the corrosion resistance of the passivation coat.
The present invention pertains to a method for passivating a metallic surface (the metallic surface often consists essentially of one or more metals selected from the group consisting of zinc, aluminum and magnesium), wherein the metallic surface is contacted with an aqueous composition comprising (or consisting of) the following components:
The present invention pertains more particularly to a method for passivating a metallic surface, as described above, wherein a composition is employed comprising (or consisting of):
The invention also relates to the formulations and compositions themselves and to methods for their preparation.
For passivation by means of the method of the invention, an acidic aqueous formulation is used which comprises at least one water-soluble polymer (X) comprising acidic groups. The polymers (X) employed may be homopolymers and/or copolymers. It is also possible for mixtures of two or more different polymers to be employed.
The formulation (or composition) employed in accordance with the invention comprises preferably 10% to 40%, more preferably 15% to 35%, very preferably 15% to 30%, and frequently 18% to 25% by weight of the polymer or polymers (X), based on the amount of all of the components in the formulation (including the solvents).
The term “water-soluble” in the sense of this invention means that the polymer or polymers (X) employed are homogeneously water-soluble, in the amounts stated for the compositions. The polymers (X) employed ought preferably to be of infinite (unrestricted) miscibility with water. More particularly, the polymers employed ought to have a solubility of at least 50 g/l, preferably 100 g/l, and more preferably at least 200 g/l, in water at room temperature and a pH of 7. It should be borne in mind, however, that the solubility of the polymers (X) described, comprising acidic groups, in water is also dependent on the pH. A polymer which at one particular pH has an insufficient solubility for the intended purpose may, at another pH, have a solubility which is still sufficient.
In accordance with the invention, the polymers (X) employed contain at least 0.6 mol of acid groups/100 g of polymer. These figures relate to the free acid groups. The polymers preferably contain at least 0.9 mol of acid groups/100 g, more preferably at least 1 mol of acid groups/100 g, and frequently in fact at least 1.2 mol of acid groups/100 g. In one embodiment of the invention, not more than 25 mol % of the acid groups present in the polymer X ought to be neutralized, preferably not more than 20 mol %, and more preferably not more than 12 mol %.
The acidic groups of the polymers (X) are generally selected from carboxyl groups, sulfonic acid groups, phosphoric acid groups and/or phosphonic acid groups. The acidic groups are preferably selected from carboxyl groups, phosphoric acid groups, and phosphonic acid groups. With particular preference the polymer (X) employed is a copolymer (X1) synthesized from at least two different monomers containing acid groups, more particularly from monomers comprising carboxyl groups and from monomers comprising phosphonic acid groups.
For performing the invention it is particularly preferred to use homopolymers or copolymers which comprise acrylic acid and/or methacrylic acid units.
More particularly the polymer (X) comprises one or more water-soluble copolymers (X1) synthesized from acrylic and/or methacrylic acid monomer units (M1) and from monoethylenically unsaturated monomers with acidic groups (M2), these monomers being different from (M1). Optionally, furthermore, there may be OH-containing acrylic esters or methacrylic esters (M3) present. Optionally there may be further monomers (M4) present as structural units.
In one preferred embodiment of the invention, the water-soluble polymer (X) is a copolymer (X1) which is synthesized from (“which comprises”) the following monomers:
These % by weight figures are based on the sum total (100% by weight) of all of the monomers in the copolymer (X1).
Furthermore, the preferred polymers (X) described in WO 2009/047209 that comprise acidic groups may be used for the purposes of the present invention. For the polymers (X) employed in the method of the invention it is possible to use the embodiments described in WO 2009/047209 in relation to the monomers (M1), (M2), (M3), and (M4).
The amount of acrylic acid and/or methacrylic acid (M1) in the copolymer (X1) is 30% to 90%, preferably 40% to 80%, and more preferably 50% to 70%, by weight, this figure being based on the sum total of all of the monomers in the polymer.
The amount of the monomers (M2) in the copolymer (X1) is 10% to 70%, preferably 20% to 60%, and more preferably 30% to 50%, by weight, based in each case on the sum total of all of the monomers in the polymer.
The monomer (M2) comprises at least one monoethylenically unsaturated monomer which is different from (M1) but is copolymerizable with (M1) and which contains one or more acidic groups, the acidic groups being selected from carboxyl groups, phosphoric acid groups, phosphonic groups or sulfonic acid groups. It is possible to use two or more different monomers (M2).
Concerning the preferred embodiments of monomer (M2), reference is made to document WO 2009/047209.
Examples of such monomers (M2) comprise crotonic acid, vinylacetic acid, C1-C4 monoesters of monoethylenically unsaturated dicarboxylic acids, styrenesulfonic acid, vinylsulfonic acid, allylsulfonic acid, 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS), vinylphosphonic acid, monovinyl phosphate, maleic acid, fumaric acid or itaconic acid. It is preferred to use vinylphosphonic acid as monomer (M2).
The copolymer (X1) may further comprise, optionally, at least one OH-containing acrylic ester and/or methacrylic ester as monomer unit (M3). The monomers in question are preferably monohydroxy acrylic esters and/or monohydroxy methacrylic esters. It is preferred to use hydroxyethyl acrylate as monomer (M3).
The amount of the monomers (M3) in the copolymer (X1) is 0% to 40%, preferably 1% to 30% by weight. With regard to the further preferred embodiments of polymer (M3), reference is made to the document WO 2009/047209.
Besides the monomers (M1), (M2), and, optionally, (M3), it is possible optionally for 0% to 30% by weight of at least one further ethylenically unsaturated monomer (M4), different from (M1), (M2), and (M3), to be employed. Furthermore, preferably, no other monomers are employed.
The monomers (M4) may serve to fine-tune the properties of the copolymer (X1). It is also possible for two or more different monomers (M4) to be used. They are selected by the skilled person in accordance with the desired properties of the copolymer, with the proviso that they must be copolymerizable with the monomers (M1), (M2), and (M3). They are preferably monoethylenically unsaturated monomers. In special cases, however, small amounts of monomers having two or more polymerizable groups may also be employed. By this means the copolymer can be crosslinked to a small extent. Examples of suitable monomers (M4) comprise, in particular, aliphatic alkyl esters of (meth)acrylic acid, such as methyl(meth)acrylate, ethyl(meth)acrylate, butyl(meth)acrylate or 2-ethylhexyl (meth)acrylate. Additionally suitable are vinyl ethers or allyl ethers such as, for example, methyl vinyl ether, ethyl vinyl ether, propyl vinyl ether, 2-ethylhexyl vinyl ether, vinyl cyclo-hexyl ether, vinyl-4-hydroxybutyl ether, decyl vinyl ether, 2-(diethylamino)ethyl vinyl ether, 2-(di-n-butylamino)ethyl vinyl ether or methyldiglycol vinyl ether, and the corresponding allyl compounds. Likewise possible for use are vinyl esters such as, for example, vinyl acetate or vinyl propionate. It is also possible to use basic comonomers, such as acrylamide and alkyl-substituted acrylamides, for example.
Examples of crosslinking monomers comprise molecules having two or more ethylenically unsaturated groups, examples being di(meth)acrylates such as ethylene glycol di(meth)acrylate or butane-1,4-diol di(meth)acrylate or poly(meth)acrylates such as trimethylolpropane tri(meth)acrylate or else di(meth)acrylates of oligoalkylene or polyalkylene glycols, such as di-, tri- or tetraethylene glycol di(meth)acrylate. Further examples comprise vinyl(meth)acrylate or butane diol divinyl ether. The term “(meth)acryl” as used herein refers to either an acrylic group or a methacrylic group.
The amount of all of the monomers (M4) employed, together, is 0% to 30% by weight, based on the total amount of the monomers employed. The amount is preferably 0% to 20%, more preferably 0% to 10%, by weight. If monomers (M4) with crosslinking activity are present, their amount ought in general not to exceed 5%, preferably 2%, by weight (based on the total amount of all of the monomers used for the method). The amount may be, for example, 10 ppm to 1% by weight.
Concerning the preferred embodiments of monomer (M4), reference is made to the document WO 2009/047209.
One preferred embodiment relates to the above-described method wherein the water-soluble polymer (X) is a copolymer (X1) which is synthesized (or which comprises) the following monomers:
The polymers (X) and copolymers (X1) may be prepared by methods known to the skilled person. The copolymers are prepared preferably by radical polymerization of the stated components (M1), (M2), and, optionally, (M3) and/or (M4) in aqueous solution. Details relating to the conduct of a radical polymerization are known to the skilled person. Preparation processes for the copolymers (X1) are described in, for example, WO 2006/021308 or WO 2006/134116.
The synthesized copolymers (X1) can be isolated from the aqueous solution by means of typical methods known to the skilled person, as for example by evaporating the solution, spray drying, freeze drying or precipitating. Preferably, the copolymers (X1), after the polymerization, are not isolated from the aqueous solution; instead, the resultant solutions of the copolymers are used as they are (optionally after addition of further adjuvants) for the method of the invention. In order to facilitate such direct further use, the amount of the aqueous solvent used for the polymerization ought to be made from the start such that the concentration of the polymer in the solvent is suitable for the application. It is also possible first to prepare a concentrate, which is diluted only on site with water or, optionally, other solvents to the desired concentration.
The molecular weight, more particularly based on the weight-average molecular weight Mw, of the polymers (X) and copolymers (X1) used for the method of the invention is specified by the skilled person in accordance with the desired application. Use may be made, for example, of polymers having molecular weight Mw of 3000 to 1 000 000 g/mol. Especially established are polymers with 5000 g/mol to 500 000 g/mol, preferably 10 000 g/mol to 250 000 g/mol, more preferably 15 000 to 100 000 g/mol, and very preferably 20 000 to 75 000 g/mol.
The above-described aqueous composition comprises at least one polyamine (P) comprising at least two ammonium groups.
Ammonium groups for the purposes of the present invention are primary, secondary, tertiary and/or quaternary amino groups which under the prevailing conditions (aqueous composition with pH of 0.5 to 2) have a cationic charge.
The polyamine component (P) is present in the aqueous composition used in the method of the invention in an amount of 0.5% to 9.5%, preferably 0.5% to 5%, with particular preference 1% to 3%, by weight.
A polyamine or a polyamine compound for the purposes of the present invention is a saturated or unsaturated, open-chain or cyclic organic compound comprising at least two (preferably at least five) amino groups, it being possible for the amino groups to be selected from primary, secondary, tertiary, and quaternary amino groups.
Use may also be made more particularly of alkoxylated, preferably ethoxylated and/or propoxylated, polyamine compounds in the context of the present invention. Use may be made more particularly of alkoxylated polyamine compounds comprising 1 to 1000, preferably 1 to 100, preferably 1 to 50, preferably 1 to 10 alkoxy units in the context of the present invention.
Use may also be made more particularly of quaternized polyamine compounds in the context of the present invention that have at least one quaternary ammonium group, which may be obtained more particularly by substitution on the amino group by one or more radicals selected from C1-C6-alkyl and benzyl, starting from the unquaternized polyamine compound.
In the method of the invention it is preferred to use at least one polyamine (P) selected from the group consisting of polyalkylimines (polyiminoalkylenes, e.g., polyethyleneimines PEI), alkoxylated polyalkylimines (e.g., alkoxylated polyethyleneimines), quaternized polyalkylimines (e.g. quaternized polyethyleneimines), polyvinylimidazoles (polyimidazoles), quaternized polyvinylimidazoles, alkoxylated polyvinylimidazoles, polyvinylamines, quaternized polyvinylamines and alkoxylated polyvinylamines.
In the method of the invention it is preferred to use at least one polyamine selected from the group consisting of polyalkylimines (polyiminoalkylenes, e.g., polyethyleneimine PEI), alkoxylated polyethyleneimines, polyvinylimidazoles (polyimidazoles), quaternized polyvinylimidazoles, and polyvinylamines.
As polyamine (P) use may be made more particularly of polyalkylimines, such as polyethyleneimines (PEI) or polypropyleneimines.
Polyethyleneimines may be described more particularly by the structural unit of the following general formula (P1)
where
The molecular weight of the polyethyleneimines used is situated more particularly in the range from 100 to 800 000 g/mol, preferably in the range from 100 to 500 000 g/mol, preferably in the range from 100 to 50 000 g/mol, more preferably in the range from 500 to 10 000 g/mol. These figures may relate alternatively to the number-average molecular weight, to the weight-average molecular weight or to the viscosity-average molecular weight; in particular, the figures relate to the number-average molecular weight.
The compounds in question may more particularly be branched polyethyleneimines. Additionally possible is the use of alkoxylated polyethyleneimines (e.g., ethoxylated or propoxylated polyethyleneimines) preferably having 2 to 20 alkoxy units.
A further possibility is to use a copolymeric polyamine wax as polyamine component (P). Polyamine waxes represent copolymers constructed from vinylamine units and alkyl units, such as ethyl and propyl. These copolymers may be obtained, for instance, by copolymerization of vinylamine-analogous monomer building blocks (e.g., vinylformamide) and ethylene and/or propylene.
As polyamine component (P) it is possible more particularly to employ polyvinylamines. Polyvinylamines may be described, in particular, by reference to the structural unit of the general formula (P2)
where
Branched polyvinylamines are used more particularly in the context of the present invention.
The molecular weight of the polyvinylamines used is situated more particularly in the range from 100 to 500 000 g/mol, preferably from 500 to 250 000 g/mol, preferably in the range from 1000 to 250 000 g/mol. These figures may relate alternatively to the number-average molecular weight, to the weight-average molecular weight or to the viscosity-average molecular weight; in particular, the figures relate to the number-average molecular weight.
In another embodiment of the invention it is possible to use polyvinylimidazoles as polyamine component (P). Polyvinylimidazoles may be described in particular by a structural unit of the general formula (P3):
where
In one preferred embodiment, the aqueous composition described above comprises a quaternized polyvinylimidazole. Quaternization may take place in particular by addition of a C1-6 alkyl radical and/or of a benzyl radical to at least one of the nitrogen atoms in the polyvinylimidazole; in particular, the quaternization may take place with a commonplace methylating reagent (e.g., methyl halide).
The molecular weight of the polyvinylimidazoles used is situated in particular in the range from 200 to 1 000 000 g/mol, preferably in the range from 1000 to 500 000 g/mol, preferably in the range from 1000 to 200 000 g/mol. These figures may relate alternatively to the number-average molecular weight, to the weight-average molecular weight or to the viscosity-average molecular weight; in particular, the figures relate to the number-average molecular weight.
It is further advantageous to use a combination of at least one polyamine (P) and at least one polyvalent metal ion (M) as cationic crosslinker in the method described above. The polyvalent metal ion may be selected more particularly from the group consisting of Mg2+, Ca2+, Zn2+, Mn2+, Fe2+, Cr3+, Al3+, Ti4+, Zr4+, Ce3+ and Ce4+, preferably Zn2+, Mg2+, Ca2+, Mn2+ and Al3+. The corresponding compound is used for example in an amount of 0.01% to 25%, preferably 0.5% to 10%, and more preferably 1% to 5%, by weight.
The aqueous composition used in the method of the invention may optionally comprise at least one surfactant (T), preferably a nonionic and/or cationic surfactant, more preferably a cationic surfactant (e.g., quaternary alkylammonium compounds).
The aqueous composition may preferably comprise at least one cationic surfactant (T), this being a quaternary alkylammonium compound, and the alkyl radicals in the ammonium compound being selected more particularly from branched or unbranched alkyl radicals having 1 to 20 C atoms, more particularly from the radicals stearyl, palmityl, methyl, ethyl, and butyl. As cationic surfactants it is possible more particularly to employ C12-14 alkyl trimethylammonium sulfates.
Adding a cationic or nonionic surfactant allows the spreading behavior of the aqueous passivating composition on the metal surface to be improved, and the corrosion resistance increased.
In another embodiment, the aqueous composition may optionally comprise at least one surfactant (T) selected from ethoxylated C8-20 alcohols comprising a linear or branched, saturated or unsaturated alkyl radical and comprising 2 to 14, preferably 4 to 10, ethylene oxide units.
The surfactant (T) may be present in the aqueous composition in the range from 0% to 2%, preferably from 0% to 1%, by weight.
Water is preferably used exclusively as solvent (L) in the compositions. Besides water, the solvent may also comprise small amounts of water-miscible organic solvents. Small amounts of organic solvents can be used in particular, selected in particular from the group: monoalcohols such as methanol, ethanol or propanol, higher alcohols such as ethylene glycol or polyether polyols, ether alcohols such as butyl glycol or methoxypropanol, and N-methylpyrrolidone. As a general rule, the amount of water is at least 80% by weight (based on the total amount of solvent), preferably at least 90% by weight (based on the total amount of the solvent), and frequently at least 95% by weight (based on the total amount of solvent).
The solvent is present in the above-described aqueous composition preferably in an amount of 20% to 89% by weight, preferably of 20% to 80% by weight, preferably of 20% to 70% by weight, preferably of 20% to 60% by weight, preferably of 20% to 50% by weight.
As further components, the above-described aqueous composition may comprise at least one of the following components:
The amount of additional components (K) ought in total not to be more than 30% by weight, more particularly not more than 20% by weight, preferably not more than 10% by weight, and with particular preference not more than 5% by weight (based in each case on the overall composition).
The aqueous composition used in the method of this invention generally has a pH (at room temperature) of less than 2, more particularly in the range from 0.5 to 2, more particularly in the range from 0.7 to 1.5, more preferably in the range from 0.7 to 1. The pH of the preparation is dependent on the nature and concentration of the polymers (X) used. It may be influenced, however, by adding further basic or acidic components to the formulation. For instance the formulation used, in addition to the acidic polymers, may further comprise organic or inorganic acids or mixtures thereof. There is no restriction on the selection of such an acid, apart from that it should not give rise to any adverse effects together with the other components of the aqueous composition.
Examples of suitable acids comprise phosphoric acid, phosphonic acid or organic phosphonic acids such as 1-hydroxyethane-1,1-diphosphonic acid (HEDP), 2-phos-phonobutane-1,2,4-tricarboxylic acid (PBTC), aminotri(methylenephosphonic acid) (ATMP), ethylenediaminetetra(methylenephosphonic acid) (EDTMP) or diethylenetriamine-penta(methylenephosphonic acid) (DTPMP), sulfonic acids such as methanesulfonic acid, amidosulfonic acid, p-toluenesulfonic acid, m-nitrobenzenesulfonic acid, and derivatives thereof, nitric acid, formic acid or acetic acid. Preference is given to phosphorus-containing acids such as phosphoric acid (H3PO4), phosphonic acid (H3PO3), the stated organic phosphonic acids, nitric acid (HNO3) or methanesulfonic acid. The acid in question may preferably be phosphoric acid (H3PO4) or another phosphorus-containing acid.
The acidity of the aqueous compositions used may also be brought about substantially through the acidic groups of the polymer (X).
The amount of additional acids besides the polymers (X) in the aqueous composition therefore does not exceed, in particular, 30% by weight (relative to the amount of all of the components in the composition). Preferably it should not exceed 25%, more preferably 20%, and very preferably 10% by weight. In a second, particularly preferred embodiment of the invention, there are no additional acids present. The acid groups of the polymer (X) are preferably in the form of free acid groups.
In one preferred embodiment of the invention, the aqueous composition employed may comprise, as additional component, at least one kind of polyvalent metal ions selected from the group consisting of Zn2+, Mg2+, Ca2+, Mn2+, and Al3+. The ions in question are preferably Zn2+ and/or Mg2+, more preferably Zn2+. The composition preferably comprises no further metal ions. More particularly, the ions may have complex bonds to the acidic groups of the polymer. The amount of added compounds with polyvalent metal ions (selected from the group consisting of Zn2+, Mg2+, Ca2+, Mn2+ and Al3+), if present, is 0.01% to 10% by weight, preferably 0.01% to 2%.
If metal ions or metal compounds are present, the compositions in question are preferably compositions which do not comprise chromium compounds. Furthermore, there ought preferably to be no metal fluorides or complex metal fluorides present. The passivation of the invention, therefore, is preferably a chromium-free passivation, more preferably a chromium- and fluoride-free passivation.
The aqueous compositions for use in accordance with the invention may be obtained by mixing the components described above.
The metal surface that can be employed for treatment is in general any desired metal surface, the surfaces in question being more particularly those of base metals. These surfaces may be, for example, surfaces comprising or consisting substantially of iron, alloys of iron, steel, Zn, Zn alloys, Al or Al alloys, Sn and Sn alloys, Mg or Mg alloys. The steels may be either low-alloy or high-alloy steels. Frequently the metal surface comprises aluminum or alloys of aluminum, or zinc or alloys of zinc, with a surface of zinc or of alloys of zinc being obtained generally by a galvanizing operation on a metallic material such as iron or steel.
The method of the invention is suitable more particularly for passivating metallic surfaces of Zn, Zn alloys, Al or Al alloys. These surfaces may be those of bodies or workpieces composed entirely of said metals and/or alloys. Alternatively they may be the surfaces of bodies coated with Zn, Zn alloys, Al or Al alloys, it being possible for the bodies to be composed of other materials, as for example of other metals, alloys, polymers or composite materials. The surface in question may more particularly be of galvanized iron or steel. The term “galvanized” also encompasses coating with a zinc alloy, more particular hot dip galvanizing with Zn—Al alloys, and electrogalvanizing with Zn—Ni, Zn—Fe, Zn/Mn, and Zn/Co alloys.
The present invention relates preferably to a method for passivating a metallic surface which is a surface consisting substantially of one or more metals selected from the group consisting of zinc (Zn), aluminum (Al), and magnesium (Mg).
Zn alloys or Al alloys are known to the skilled person. The desired end application influences the skilled person's choice of the type and amount of alloying constituents. Typical further constituents of zinc alloys comprise more particularly Al, Mg, Pb, Si, Mg, Sn, Cu, and Cd. Also possible are Al/Zn alloys in which Al and Zn are present in approximately the same amount. The coatings may be largely homogeneous coatings or else coatings having concentration gradients. One possible example is galvanized steel which has additionally be vapor-coated with Mg. As a result of this, a Zn/Mg alloy can be produced superficially. Typical further constituents of aluminum alloys are more particularly Mg, Mn, Si, Zn, Cr, Zr, Cu, and Ti.
In one preferred embodiment of the method the surface in question is that of a strip metal, preferably comprising aluminum or an alloy of aluminum, or comprising iron or steel, especially strips of electrogalvanized or hot dip galvanized steel.
The surfaces in question are often those of shaped bodies which are obtainable from said strip metals by processing procedures such as cutting, working and/or joining. Examples are automobile bodies or parts thereof, truck bodies, paneling for household appliances (such as washing machines, dishwashers, wash driers, gas and electric cookers, microwave ovens, chest freezers or refrigerators, for example), cladding for technical appliances or apparatus (such as machines, switch cabinets, computer housings or the like, for example), components in the architectural sector (such as wall parts, facade elements, ceiling elements, windows or door profiles or partitions), and furniture made of metallic materials (such as metal cabinets or metal shelving).
The metallic surfaces for treatment may also have thin oxidic, hydroxidic and/or carbonatic surface layers or layers of similar construction. Layers of this kind form usually spontaneously on metallic surfaces in contact with the atmosphere, and are included in the term “metallic surface”.
In one preferred embodiment, the method relates to a continuous method for passivating galvanized steel strips on a coil-coating line subsequent to galvanization (e.g., electrogalvanizing or hot dip galvanizing).
The method of the invention can be used to passivate steel strips galvanized on one or both sides. Galvanized steel strips have a thickness of 0.2 to 0.3 mm and widths of 0.5 to 2.5 m. Galvanized steel strips are available commercially for a variety of applications. The skilled person selects a suitable steel strip in accordance with the desired end use.
Generally speaking, the method of the invention can be performed using the known coil-coating lines, based, for example, on squeeze-roll technology or on roll-coater technology. Suitable lines are described in WO 2009/047209, for example.
The method of the invention for passivating galvanized steel strips is performed preferably by means of a continuous process using squeeze rolls. For this purpose, the galvanized steel strip is moved by means of drive rollers. The passivating composition is applied to the steel strip by spraying, using a spraying station, for example, and forms a wet film. One or more squeeze rolls squeeze off excess passivating composition. The result is a thin, wet film, which can be dried subsequently in a drier.
The steel strips may be run through the line typically with a speed of 80 to 200 m/min, preferably 50 to 150 m/min. The treatment time can be specified by the skilled person in accordance with the desired properties of the passivation coat, and with other factors. In the case of continuous processes it is advantageous for the maximum time between application of the aqueous composition to the metallic surface of the steel strip and drying of the film to be 1 to 60 s.
In the method of the invention described above, the metallic surface may be contacted with the aqueous composition, more particularly by spraying, dipping or roll application.
In one preferred embodiment, the method described above for passivating a metallic surface comprises the following steps:
In one preferred embodiment, the invention pertains to a method as described above where the aqueous composition is applied to the metallic surface in the form of a wet film, the aqueous composition being applied at a coat weight in the range from 0.3 to 2 g/m2, preferably 0.3 to 1 g/m2, more particularly 0.4 to 0.8 g/m2 (based on the sum of the solids of the aqueous composition) to the metallic surface.
The passivation coats obtainable with the described method of the invention preferably have a coat thickness in the range from 1 to 3 μm, preferably 1 to 2 μm, and a coat weight in the range from 0.3 to 2 g/m2, preferably 0.3 to 1 g/m2, with particular preference 0.4 to 0.8 g/m2 (based on the sum of the solids in the aqueous composition).
In a further aspect, the present invention pertains to a composition for passivating a metallic surface, comprising
the pH of the composition being in the range from 0.5 to 2; more particulaly in the range from 0.7 to 1.5; more preferably in the range from 0.7 to 1.
The aqueous composition of the invention preferably comprises 0.5% to 5%, more particularly 1% to 3%, by weight of at least one polyethylenimine as polyamine component (P), the polyethyleneimine having an average molecular weight (based on the viscosity-average molecular weight) of 100 to 500 000 g/mol, more preferably in the range from 500 to 10 000 g/mol, more preferably in the range from 500 to 8 000 g/mol.
Furthermore, for the stated components (polymer (X), polyamine (P), surfactant (T), solvent (L), and further components (K)), the preferred embodiments described earlier on above in connection with the method of the invention are applicable.
The present invention is further directed to a process for preparing an above-described composition, comprising the following steps:
Furthermore, for the stated components (polymer (X), polyamine (P), surfactant (T), solvent (L), and further components (K)), the preferred embodiments described earlier on above in connection with the method of the invention are applicable.
The present invention is further directed to the use of the above-described composition for passivating a metallic surface, more particularly a metallic surface consisting substantially of one or more metals selected from the group consisting of zinc (Zn), aluminum (Al) and magnesium (Mg), more particularly of galvanized steel. The invention relates more particularly to the use of the above-described composition for passivating a galvanized steel strip in a continuous process.
The present invention further provides a coating on a metallic surface, obtainable by contacting the metallic surface with an aqueous composition comprising the following components:
For the stated components (polymer (X), polyamine (P), surfactant (T), solvent (L), and further components (K)), the preferred embodiments described earlier on above in connection with the method of the invention are applicable.
The present invention is elucidated in more detail with reference to the experimental examples below.
Use was made in each case of aqueous solutions of an acid-group-containing polymer (polymer X) formed of about 50% by weight of acrylic acid (monomer M1), about 30% by weight of vinylphosphonic acid (monomer M2), and about 20% by weight of hydroxyethyl acrylate (monomer M3), prepared as described in WO 2008/612248. Initial formulations were prepared from about 20% by weight of the acid-group-containing polymer (polymer (X)), 85% phosphoric acid (H3PO4), magnesium phosphate Mg3(PO4)2*8H2O, by dissolving the components in water and mixing the components. The precise quantities (in % by weight) of the components in the initial formulation are summarized in table 2. The pH of the initial formulation was in the range from 0.5 to 2.
Different amounts in each case of various polyamines (P) were added to the initial formulation. For this purpose, the respective polyamine solution was adjusted to the same pH, using 70% strength methanesulfonic acid (MSA), as the aqueous solution of the polymer containing acid groups (initial formulation described above).
The formulations C1 to C4 and also V1 to V13 were prepared by adding the polyamines (P) described in table 1 to the initial formulation. The precise compositions and quantities are summarized in tables 2 and 3.
In certain cases, a cationic surfactant T1 (quaternized alkylamine) was added as surfactant (T).
Using the compositions X1 to X4 and V1 to V13 described in example 1, coated test panels of hot dip galvanized steel (Gardobond OE HDG 3, 105×190 mm) were produced.
As a pretreatment, the test panels were immersed for around 30 seconds in a mild alkaline cleaner solution (Surtech 133 from Surtech), immediately rinsed off with fully demineralized water, and then dried with nitrogen.
The cleaned panels were immersed at room temperature for 1 second each in the formulations listed in tables 2 and 3, squeezed off with a roller system, and dried in a drying cabinet at 160° C. for 12 seconds. The peak metal temperature (PMT) in the course of drying here did not exceed 50° C.
The coated test panels thus obtained were tested for their corrosion resistance with the aid of various test procedures described in example 3. The test results are summarized in tables 4 and 5.
It is apparent that coatings with the inventive compositions (V1 to V13) in all test procedures exhibit significantly better corrosion resistance in comparison to test panels coated with a comparative solution (C2 to C4) or not coated (C1).
The passivation coat was assessed by performance of the test procedures described below and summarized in tables 4 and 5.
The quality of the corrosion control on the test panels coated as in example 2 was evaluated in the neutral salt spray test as per DIN EN ISO 9227, by awarding evaluation scores of 0 to 10 in accordance with defined standards. The evaluation score (or alternatively degree of evaluation) is a measure of the formation of white rust on the panel. The higher the evaluation score, the lower the proportion of the corroded surface area (erroneous surface area) in % in relation to the entire surface area and the better the corrosion control. The evaluation scores were awarded in accordance with table 6. An average was formed from 5 panels.
Additionally, the panels were tested in the condensation cycling test (CCT) in accordance with DIN EN ISO 6270-2. This test consists of one or more cycles each with two test sections.
In the first section, the test specimens are exposed for 8 hours to a temperature of 40° C. and a relative humidity of 100%; in the second section, they are exposed to a temperature of 18-28° C. and a humidity of below 100% (ambient conditions). The duration of a cycle is 24 hours. The samples were assessed visually in accordance with the following criteria, as an average of 3 panels:
The resistance of the test panels coated as described above was compared against one another in a stack test. For this purpose, three coated panels were divided in the center, wetted with 5 ml of distilled water each, and placed with the test sides against one another. Testing took place in a climatic cycling chamber, in which the stacks were weighted with a 5 kg weight and exposed to a defined number of cycles (DIN EN ISO 6270-2, AHT).
The corrosion was evaluated as the average over 3 panels, in accordance with the following evaluation scheme:
| Number | Date | Country | |
|---|---|---|---|
| 61389729 | Oct 2010 | US |
