US20090007990A1

US20090007990A1 - Method for passivating metallic surfaces by using itaconic acid homopolymers or copolymers

Info

Publication number: US20090007990A1
Application number: US11/574,021
Authority: US
Inventors: Frank Klippel; Matthias Kluglein; Gunnar Schornick; Alexander Gothlich; Frank Dietsche; Helmut Witteler
Original assignee: Individual
Current assignee: BASF SE
Priority date: 2004-08-24
Filing date: 2005-08-04
Publication date: 2009-01-08
Also published as: JP4514794B2; DE102004041142A1; EP1786953A1; JP2008510603A; CN100554509C; CA2575008A1; CN101006201A; WO2006021309A1

Abstract

A process for passivating a metallic surface by treating it with an acidic aqueous preparation comprising at least one itaconic acid homo- or copolymer. Passivating layers and metallic surfaces obtainable by means of the process.

Description

The present invention relates to a process for passivating metallic surfaces by treating them with an acidic, aqueous preparation which comprises at least one homopolymer or copolymer of itaconic acid. The invention further relates to passivating layers and to metallic surfaces which are obtainable by means of the process.
The corrosion protection treatment of modern metallic materials normally takes place in multistage operations, and the surface of treated metals normally has a number of different layers.
The protection of metallic components against corrosion is of great economic importance. At the same time the requirements imposed on the corrosion protection are also becoming ever more stringent. An example of this is that the newer models of automobile are nowadays warranted with a guarantee of up to 12 years against rust perforation.
Of particular importance both technically and economically is the corrosion protection treatment of aluminum surfaces and also of the surfaces of galvanized metals, especially electrochemically galvanized or hot-dip-galvanized iron and steel. The corrosion protection afforded by the zinc is based on the fact that it is baser than the metallic material itself and therefore to start with undergoes corrosion itself. The metallic material itself remains intact as long as it is still covered by a continuous layer of zinc.
In the presence of atmospheric oxygen a thin oxide layer forms initially on the surface of Zn or Zn alloys, Al or Al alloys and slows the corrosive attack on the underlying metal to a greater or lesser degree depending on the external conditions.
In order to strengthen the protective effect of such an oxide layer, surfaces of Al and Zn are regularly subjected to an additional passivating treatment. In the course of such treatment a fraction of the metal to be protected dissolves and is immediately reincorporated into an oxide film on the metal surface. This film is similar to the oxide film which is present in any case, but it offers greater protection. It is normally referred to as a passivating layer. In many cases it also improves the adhesion of paint layers applied to the metal. Instead of the term “passivating layer”, therefore, the term “conversion coat” is often used synonymously, and sometimes also the term “pretreatment layer” or “posttreatment layer”, depending on the point in the production process at which the treatment takes place. Passivating layers are comparatively thin and normally have a thickness of not more than 3 μm.
In order to reinforce the corrosion protection it is common to apply additional (paint) layers to the passivating layer. Such systems usually comprise a combination of two or more paint layers each of which serve different purposes. They serve to protect the passivating layer and the metal against corrosive gases and/or liquids and also against mechanical damage, such as stone chipping, for example, and of course also serve esthetic purposes. Paint layers are normally much thicker than passivating layers. Typical thicknesses range from 5 μm to 400 μm.
The passivation can be employed for permanent corrosion protection or else only for temporary corrosion protection. Temporary protection is used only for the storage or transportation of a metal sheet or other metallic workpiece and is removed again before final processing.
Passivating layers on zinc or aluminum surfaces have generally been obtained to date by treating the workpiece requiring protection with acidic aqueous solutions of CrO₃. The mechanism of such passivation is complex. It includes the dissolution of metallic Zn or Al from the surface and its reprecipitation in the form of amorphous zinc-chromium oxides or aluminum-chromium oxides, respectively. The layers may, however, also comprise extraneous ions and/or further components from the treatment solution. In the case of treatment with chromic acid in particular it is impossible to rule out the incorporation into the passivating layer of a certain fraction of Cr(VI).
In order to avoid treatment with carcinogenic Cr(VI) solutions proposals have been made to carry out treatment of metallic surfaces with acidic, aqueous Cr(III) solutions. By way of example reference may be made to U.S. Pat. No. 4,384,902 or WO/40208. Increasingly, however, there are customers on the market who require completely chromium-free processes for passivating, In order to avoid the use of Cr(VI) and Cr(III), therefore, the use of polymers is increasingly gaining in importance.
DE-A 195 16 765 discloses a chromium-free and fluoride-free process for producing conversion coats of metallic surfaces of Zn or Al. The acidic solution used for passivation comprises a water-soluble polymer, phosphoric acid, and Al chelate complexes. As an option it is also possible to use polymers and copolymers of (meth)acrylic acid.
DE-A 197 54 108 discloses a chromium-free aqueous corrosion protection composition which comprises hexafluoro anions of Ti(IV) and/or Zr(IV), vanadium ions, cobalt ions, and phosphoric acid. As an option it is also possible for various film-forming polymers to be added as well, including carboxyl-containing copolymers such as acrylic acid/maleic acid copolymers.
JP-A 2001-164377 discloses the production of black steel sheets for the production, for example, of black casings for consumer electronics. The metal sheet is coated with a paint comprising metal ions, a water-soluble polymer, an aqueous polymer dispersion, and an acid. One example mentions a water-soluble polymer of 70% acrylic acid and 30% itaconic acid. The passivation of metals by means of an itaconic acid copolymer is not disclosed by the document.
JP-A 2001-158969 discloses a final coating for metallic surfaces, particularly of said black casings, with 50% to 98% by weight of an aqueous polymer dispersion, metal ions, a water-soluble polymer, and an acid. One example mentions a water-soluble polymer of 70% acrylic acid and 30% itaconic acid. The passivation of metals by means of an itaconic acid copolymer is not disclosed by the document.
JP-A 2003-027202 discloses a process for treating a galvanized steel sheet with a composition composed of a metallic compound, a water-soluble organic resin, and an acid. The polymer may comprise, among others, a copolymer formed from a carboxyl-containing monomer and also further carboxyl-containing monomers or OH-containing monomers. The examples mention a copolymer of 70% acrylic acid and 30% itaconic acid. The document contains no indications of how the polymer is prepared.
Our as yet unpublished application DE 103 53 845, unpublished at the priority date of the present specification, discloses a process for passivating metal surfaces using copolymers of 50% to 99.9% by weight (meth)acrylic acid, 0.1% to 50% by weight acidic comonomers, including ethylenically unsaturated dicarboxylic acids, and, optionally, 0% to 30% by weight further comonomers. The dicarboxylic acid may for example be itaconic acid. The document, however, contains no indications of particular preparation processes for itaconic acid polymers.
As well as achieving very good corrosion protection, a process for passivating, particularly a chromium-free process, is also required to meet a series of technical requirements.
The passivation can be performed by immersing the workpieces requiring passivation in a passivating solution. Loose workpieces (screws, for example) can be placed in a drum for this purpose, and the drum immersed. Larger workpieces can also be mounted on a suitable frame, and the frame immersed. With the dipping method the skilled worker is comparatively free to determine the contact time between the passivating solution and the workpiece, and hence even quite thick passivating layers can be obtained. The contact time may well be of the order of minutes. Where this technique is employed, more complex workpieces are usually assembled first—welded together from steel parts, for example—and then galvanized and passivated as a whole.
For producing sheetlike metallic workpieces such as automobile parts, bodywork parts, instrument casings, façade cladding, ceiling panels or window profiles, for example, metal sheets are shaped by means of suitable techniques such as punching, drilling, folding, profiling and/or deep drawing. Larger components, such as automobile bodies, for example, are assembled if appropriate by welding together a number of individual parts. The raw material for this purpose normally comprises long metal strips, which are produced by rolling the metal and which for the purposes of storage and transportation are wound up to form what are called coils.
The galvanizing and passivation of such metal strips is carried out industrially in continuous plants. For galvanizing, first of all, the metal strip is run through a galvanizing apparatus, such as a trough of molten zinc, for example, and then through a further, passivating apparatus, again a trough, for example, or a rinsing apparatus. As a general rule, further process steps are carried out continuously: cleaning or rinsing steps, for example, or else the application of a first paint layer to the passivating layer. Typical speeds at which metal strips are run through the continuous plants are 50 to 100 m/min. This means that the contact time between the metallic surface and the preparation used for passivating is short. Normally only a few seconds are available for the treatment. A process suitable industrially must therefore provide adequate results even with only short contact times.
It was therefore an object of the invention to provide an improved, preferably chromium-free process for passivating metallic surfaces of Zn, Zn alloys, Al or Al alloys which affords improved corrosion protection as compared with the prior art and in which only short contact times between the metallic surface and the preparation used for passivating are required in order to achieve a result which is nevertheless satisfactory. In particular it ought also to be possible to implement the process continuously.
The invention accordingly provides a process for passivating a metallic surface, in which said surface is treated with an acidic, aqueous preparation comprising at least one water-soluble itaconic acid homo- or copolymer, the polymer having been constructed from the following monomeric units:

(A) 0.1% to 100% by weight of itaconic acid,
(B) 0% to 99.9% by weight of at least one monoethylenically unsaturated monocarboxylic acid, and
(C) 0% to 40% by weight of at least one further ethylenically unsaturated monomer, other than (A) and (B), which contains acid groups,
- and/or
(D) 0% to 30% by weight of at least one further ethylenically unsaturated monomer, other than (A), (B) and (C),
- the amount being based in each case on the total amount of all of the monomers incorporated in the copolymer,
  where the copolymer is obtainable by means of free-radical polymerization in aqueous solution at a temperature of less than 120° C.

In one preferred embodiment of the invention the metallic surface is the surface of a strip metal and with further preference the passivation is performed by means of a continuous process.
The invention further provides a passivating layer on a metallic surface, which is obtainable by means of the process, and also provides metallic surfaces comprising such a passivating layer.
Surprisingly it has been found that the metal surfaces obtained by means of the process of the invention using itaconic acid polymers are significantly more resistant to corrosion than when using known polymers, such as acrylic acid-maleic acid copolymers, for example. Surprisingly in particular, the itaconic acid copolymers synthesized inventively at less than 120° C. exhibit a much better corrosion protection effect than itaconic acid copolymers which have been synthesized at higher temperatures.
The acrylic acid-itaconic acid copolymers prepared in accordance with the invention result in a significantly improved passivation in comparison to acrylic acid-maleic acid copolymers. The itaconic acid copolymers have lower residual monomer contents than corresponding maleic acid copolymers. By substituting itaconic acid for maleic acid it is possible to obtain a higher dicarboxylic acid fraction in the polymers of the invention, which is likewise beneficial to the passivating properties.
Details of the invention now follow:
The metallic surfaces which can be passivated by means of the process of the invention comprise, in particular, the surfaces of base metals. The surface in question may be, for example, that of iron, steel, Zn, Zn alloys, Al or Al alloys.
The process of the invention is particularly suitable for passivating metallic surfaces of Zn, Zn alloys, Al or Al alloys. These may be the surfaces of structures or workpieces composed entirely of said metals or alloys. Alternatively they may be the surfaces of structures coated with Zn, Zn alloys, Al or Al alloys, it being possible for the structures to be composed of other materials: other metals, alloys, polymers or composites, for example. The surface in question may in particular be that of galvanized iron or steel. In one particular embodiment of the process it is the surface of a strip metal, in particular electrolytically galvanized or hot-dip-galvanized steel.
Zn alloys or Al alloys are known to the skilled worker. The skilled worker selects the type and amount of alloying constituents in accordance with the desired end application. Typical constituents of zinc alloys for hot-dip processes comprise, in particular, Al, Pb, Si, Mg, Sn, Cu or Cd. Zinc alloys which are deposited electrolytically typically comprise Fe, Co, Ni or Mn. Method comprise Typical constituents of aluminum alloys comprise, in particular, Mg, Mn, Si, Zn, Cr, Zr, Cu or Ti. The alloys in question can also be Al/Zn alloys, in which Al and Zn are present in approximately equal amounts. Steel coated with such alloys is available commercially.
The preparation used for passivating comprises one or more homopolymers and/or copolymers comprising itaconic acid units and also, if appropriate, monoethylenically unsaturated monocarboxylic acids and also, optionally, further monomers as structural units.
The polymers used are water-soluble or at least water-dispersible. The term “water-soluble” for the purposes of this invention is intended to denote that the copolymer or copolymers used should be homogeneously water-soluble. The term “water-dispersible” means that, although the solution is not completely clear, the polymer is distributed homogeneously therein and also does not settle. Preference is given to polymers which are water-soluble.
The copolymers used ought preferably to be infinitely miscible with water, even if this is not absolutely necessary in every case. They must, however, be water-soluble at least to an extent such that passivation by means of the process of the invention is possible.
As a general rule the copolymers used ought to have a solubility of at least 50 g/l, preferably 100 g/l and more preferably at least 200 g/l.
The skilled worker in the field of water-soluble polymers is aware that the solubility of COOH-containing polymers in water may be dependent on the pH. The reference point chosen should therefore in each case be the pH which is desired for the particular end use. A copolymer which at one pH has a solubility which is inadequate for the intended use may have an adequate solubility at a different pH.
The monomer (A) for preparing the homopolymer or copolymer used in accordance with the invention is itaconic acid:
The itaconic acid may also be used in the form of its salts: for example, as an alkali metal salt or ammonium salt. Additionally it is also possible to use derivatives of itaconic acid which readily hydrolyze to itaconic acid in aqueous solution, such as, for example, the corresponding anhydride, monoesters or diesters or acid amides. It will be appreciated that mixtures of such derivatives can also be used.
The polymer used in accordance with the invention may be a homopolymer or, preferably, a copolymer of itaconic acid.
The amount of itaconic acid in the polymers is 0.1% to 100% by weight, preferably 10% to 50% by weight, more preferably 15% to 45% by weight, very preferably 20% to 40% by weight and, for example, 25% to 35% by weight, this figure being based on the sum of all of the monomers in the polymer.
The polymer used in accordance with the invention may further comprise up to 99.9% by weight of one or more monomers (B). These are monoethylenically unsaturated monocarboxylic acids.
Examples of suitable monoethylenically unsaturated monocarboxylic acids (B) comprise acrylic acid, methacrylic acid, crotonic acid, vinylacetic acid or else C₁-C₄monoesters of monoethylenically unsaturated dicarboxylic acids. Preferred monomers are acrylic acid and methacrylic acid, particular preference being given to acrylic acid.
It will be appreciated that mixtures of two or more different monoethylenically unsaturated monocarboxylic acids can also be used.
The amount of all monomers (B) together is preferably 50% to 90% by weight, more preferably 55% to 85% by weight, very preferably 60% to 80% by weight, and, for example, 65% to 75% by weight.
Optionally the copolymer of the invention may further include 0% to 40% by weight of at least one further ethylenically unsaturated monomer (C), different from (A) and (B).
The monomers (C) have in each case at least one acidic group. With particular preference they are in each case monoethylenic monomers. The monomers (C) are free-radically polymerizable.
The monomers (C) may be, for example, carboxyl-containing monomers (C1), monomers (C2) comprising phosphoric acid groups and/or phosphonic acid groups, or monomers (C3) comprising sulfonic acid groups.
The monomers (C) can also be used in the form of their salts: for example, as alkali metal salts or ammonium salts. Additionally it is also possible to use derivatives of the monomers (C) which readily hydrolyze to the free acids in aqueous solution, such as anhydrides, monoesters or diesters or acid amides, for example. It will be appreciated that mixtures of such derivatives can also be used.
Examples of carboxyl-containing monomers (C1) comprise, in particular, ethylenically unsaturated dicarboxylic acids such as maleic acid, mesaconic acid, citraconic acid, fumaric acid or methylenemalonic acid. A preferred monomer (C1) is maleic acid and/or maleic anhydride.
Examples of suitable monomers (C2) comprise vinylphosphonic acid, monovinyl phosphate, allylphosphonic acid, monoallyl phosphate, 3-butenylphosphonic acid, mono-3-butenyl phosphate, mono(4-vinyloxybutyl) phosphate, phosphonoxyethyl acrylate, phosphonoxyethyl methacrylate, mono(2-hydroxy-3-vinyloxypropyl) phosphate, mono(1-phosphonoxymethyl-2-vinyloxyethyl) phosphate, mono(3-allyloxy-2-hydroxypropyl) phosphate, mono(2-allyloxy-1-phosphonoxymethylethyl) phosphate, 2-hydroxy-4-vinyloxymethyl-1,3,2-dioxaphosphole, and 2-hydroxy-4-allyloxymethyl-1,3,2-dioxaphosphole. It is also possible to use salts and/or esters, especially C₁to C₈monoalkyl, dialkyl and, if appropriate, trialkyl esters, of the monomers containing phosphoric acid and/or phosphonic acid groups.
One preferred monomer (C2) is vinylphosphonic acid, or hydrolyzable esters thereof.
Examples of monomers (C3) containing sulfonic acid groups comprise allylsulfonic acid, methallylsulfonic acid, styrenesulfonate, vinylsulfonic acid, allyloxybenzenesulfonic acid or 2-acrylamido-2-methylpropanesulfonic acid, 2-(methacyloyl)ethylsulfonic acid and the alkali metal salts thereof.
It will be appreciated that mixtures of two or more different monomers (C) can also be used. Preference is given to monomers (C1) comprising carboxyl groups and also to monomers (C2) comprising phosphoric acid groups and/or phosphonic acid groups.
If monomers (C) are present their amount together is preferably 0.1% to 20% by weight, more preferably 0.2% to 15% by weight, and very preferably 0.5% to 10% by weight.
If monomer (C) comprises monomers comprising phosphoric acid groups and/or phosphonic acid groups, and particularly vinylphosphonic acid, amounts of 5% to 40% by weight, preferably 10% to 30% by weight, more preferably 12% to 28% by weight, and very preferably 20% to 25% by weight have proven appropriate.
Furthermore, optionally, the copolymer may comprise 0% to 30% by weight of at least one further free-radically polymerizable, ethylenically unsaturated monomer (D), different from (A), (B) and (C). Over and above this no further monomers are used. The monomers (D) serve to fine-tune the properties of the copolymer. They are selected by the skilled worker in accordance with the desired properties of the copolymer. The monomers (D) are likewise free-radically polymerizable.
Preferably they are likewise monoethylenically unsaturated monomers. In particular cases, however, it is also possible to use small amounts of monomers having two or more polymerizable groups. This allows the copolymer to be crosslinked to a slight extent.
Examples of monomers (D) comprise C₁to C₈alkyl esters or C₁to C₄hydroxyalkyl esters of (meth)acrylic acid, such as methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, isopropyl (meth)acrylate, butyl (meth)acrylate, hexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate or butane-1,4-diol monoacrylate. The alcohol components in the (meth)acrylic esters may also be alkoxylated alcohols. Mention may be made here in particular of alkoxylated C₁to C₁₈alcohols which have 2 to 80 mol of ethylene oxide, propylene oxide, butylene oxide or mixtures thereof. Examples of such alkoxylated products comprise methyl polyglycol (meth)acrylate or (meth)acrylic esters of C₁₃/C₁₅oxo alcohol reacted with 3, 5, 7, 10 or 30 mol of ethylene oxide, and/or mixtures thereof, (methyl)styrene, maleimide, N-alkylmaleimide, maleic acid monoamides or maleic monoesters.
Also suitable are vinyl or allyl ethers such as, for example, methyl vinyl ether, ethyl vinyl ether, propyl vinyl ether, isobutyl vinyl ether, 2-ethylhexyl vinyl ether, vinyl cyclohexyl ether, vinyl 4-hydroxybutyl ether, decyl vinyl ether, dodecyl vinyl ether, octadecyl vinyl ether, 2-(diethylamino)ethyl vinyl ether, 2-(di-n-butylamino)ethyl vinyl ether or methyldiglycol vinyl ether, and the corresponding allyl compounds. Use may likewise be made of vinyl esters such as vinyl acetate or vinyl propionate, for example.
Examples of basic monomers comprise acrylamides and alkyl-substituted acrylamides, such as acrylamide, methacrylamide, N-tert-butylacrylamide or N-methyl(meth)acrylamide. Additionally it is also possible to use basic monomers such as 1-vinylimidazole and N-vinylpyrrolidone.
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 butanediol divinyl ether.
It will be appreciated that a mixture of different monomers (D) can also be used. The amount of all monomers (D) used together is 0% to 30% by weight, based on the total amount of all of the monomers used for the process. Preferably, the amount is 0% to 20% by weight, more preferably 0% to 15% by weight, and very preferably 0% to 10% by weight. If crosslinking monomers (D) are present their amount should generally not exceed 5%, preferably 2% by weight, based on the total amount of all of the monomers used for the process. The amount can be, for example, 10 ppm to 1% by weight.
The skilled worker chooses the nature and amount of the monomers used in accordance with the desired passivation. In doing so, he or she will take account of the fact that the polymer is to be water-soluble or water-dispersible. Monomers which might impair the water-solubility of the polymer are therefore used by the skilled worker only in amounts such that no adverse effects can occur.
Particular preference is given to copolymers of about 25% to 30% by weight itaconic acid and about 70% to 75% by weight acrylic acid, in particular about 26% to 28% by weight itaconic acid and about 72% to 74% by weight acrylic acid.
Particular preference is given, moreover, to terpolymers of about 23% to 30% by weight itaconic acid, about 67% to 75% by weight acrylic acid and about 0.1% to 10% by weight vinylphosphonic acid, in particular about 25% to 27% by weight itaconic acid, about 68% to 70% by weight acrylic acid and about 3% to 5% by weight vinylphosphonic acid.
In a further embodiment of the invention terpolymers of about 10% to 30% by weight itaconic acid, about 50% to 70% by weight acrylic acid, and about 10% to 30% by weight vinylphosphonic acid, in particular about 15% to 25% by weight itaconic acid, about 55% to 65% by weight acrylic acid, and about 15% to 25% by weight vinylphosphonic acid, have proven appropriate.
The monomers used are polymerized free-radically in aqueous solution.
The term “aqueous” means that the solvent or diluent used has water as its main constituent. Besides water, however, there may also be fractions of water-miscible organic solvents. This may be necessary, for example, in order to improve the solubility of certain monomers, particularly the monomers (D), in the reaction medium.
The solvent or diluent used accordingly contains at least 50% by weight of water relative to the total amount of the solvent. Besides this there may be one or more water-miscible solvents used. Mention may be made here in particular of alcohols, examples being monoalcohols such as ethanol, propanol or isopropanol, dialcohols such as glycol, diethylene glycol or polyalkylene glycols, or derivatives thereof. Preferred alcohols are propanol and isopropanol. The water fraction is preferably at least 70%, more preferably at least 80% and very preferably at least 90% by weight. With very particular preference water exclusively is used.
The conduct of the free-radical addition polymerization is known in principle to the skilled worker.
The free-radical addition polymerization is preferably initiated by using suitable polymerization initiators. Alternatively, however, it can also be triggered by means, for example, of appropriate radiation. The free-radical initiators ought to be soluble in the reaction solvent, preferably water-soluble.
Among the thermally activable polymerization initiators preference is given to initiators having a decomposition temperature in the range from 30 to 150° C., in particular from 50 to 120° C. This temperature figure refers, as usual, to the 10 h half-life.
As initiators it is possible to use all of the compounds which break down into free radicals under the polymerization conditions, such as, for example, inorganic peroxo compounds, such as peroxodisulfates, especially ammonium peroxodisulfate, potassium peroxodisulfate and preferably sodium peroxodisulfate, peroxosulfates, hydroperoxides, percarbonates, and hydrogen peroxide and what are called redox initiators. Preference is given to the use of water-soluble initiators. In certain cases it is advantageous to use mixtures of different initiators, examples being mixtures of hydrogen peroxide and sodium or potassium peroxodisulfate. Mixtures of hydrogen peroxide and sodium peroxodisulfate can be used in any desired proportion.
Suitable organic peroxo compounds are diacetyl peroxide, di-tert-butyl peroxide, diamyl peroxide, dioctanoyl peroxide, didecanoyl peroxide, dilauroyl peroxide, dibenzoyl peroxide, bis(o-toluoyl) peroxide, succinyl peroxide, tert-butyl peracetate, tert-butyl permaleate, tert-butyl perisobutyrate, tert-butyl perpivalate, tert-butyl peroctoate, tert-butyl perneodecanoate, tert-butyl perbenzoate, tert-butyl peroxide, tert-butyl hydroperoxide (water soluble), cumene hydroperoxide, tert-butyl peroxy-2-ethylhexanoate, and diisopropyl peroxydicarbamate.
Preferred initiators are, moreover, azo compounds. These may be soluble in organic solvents, as is the case, for example, for 2,2′-azobis(4-methoxy-2,4-dimethylvaleronitrile), dimethyl 2,2′-azobis(2-methylpropionate), 1,1′-azobis(cyclohexane-1-carbonitrile), 1-[(cyano-1-methylethyl)azo]formamide, 2,2′-azobis(N-cyclohexyl-2-methylpropionamide), 2,2′-azobis(2,4-dimethylvaleronitrile), 2,2′-azobis(2-methylbutyronitrile), 2,2′-azobis[N-(2-propenyl)-2-methylpropionamide], 2,2′-azobis(N-butyl-2-methylpropionamide). Preferably, however, they are soluble in water, as is the case for example for 2,2′-azobis[2-(5-methyl-2-imidazolin-2-yl)propane]dihydrochloride, 2,2′-azobis[2-(2-imidazolin-2-yl)propane] disulfate dihydrate, 2,2′-azobis[N-(2-carboxyethyl)-2-methylpropionamidine] tetrahydrate, 2,2′-azobis{2-[1-(2-hydroxyethyl)-2-imidazolin-2-yl]propane} dihydrochloride, 2,2′-azobis{2-methyl-N-[1,1-bis(hydroxymethyl)-2-hydroxyethyl]propionamide, 2,2′-azobis[2-methyl-N-(2-hydroxyethyl)propionamide], 2,2′-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride, 2,2′-azobis(2-methylpropionamide) dihydrochloride, 2,2′-azobis[2-(3,4,5,6-tetrahydropyrimidin-2-yl)propane] dihydrochloride, 2,2′-azobis[2-(2-imidazolin-2-yl)propane], 2,2′-azobis{2-methyl-N-[2-(1-hydroxybutyl)]propionamide}.
Additionally preferred initiators are, moreover, redox initiators. The redox initiators comprise as oxidizing component at least one of the peroxo compounds indicated above and as reducing component, for example, ascorbic acid, glucose, sorbose, ammonium or alkali metal hydrogen sulfite, sulfite, thiosulfate, hyposulfite, pyrosulfite or sulfide or sodium hydroxymethylsulfoxylate. As a reducing component in the redox catalyst it is preferred to use ascorbic acid or sodium pyrosulfite. Relative to the amount of monomers used in the polymerization the amount of reducing component used in the redox catalyst is, for example, 1×10⁻⁵to 1 mol %.
In combination with the initiators and/or the redox initiator systems it is possible in addition to use transition metal catalysts, examples being salts of iron, cobalt, nickel, copper, vanadium, and manganese. Examples of suitable salts include iron(II) sulfate, cobalt(II) chloride, nickel(II) sulfate, and copper(I) chloride. The reductive transition metal salt is usually used in an amount of from 0.1 to 1 000 ppm, based on the sum of the monomers. Particularly advantageous combinations are, for example, those of hydrogen peroxide and iron(II) salts, such as a combination of from 0.5% to 30% by weight of hydrogen peroxide and from 0.1 to 500 ppm of FeSO₄x7H₂O, based in each case on the sum of the monomers.
Examples of suitable photoinitiators are acetophenone, benzoin ethers, benzyl dialkyl ketones, and derivatives thereof. They are insoluble or virtually so in water but probably soluble in alcohols, so that a dispersing can be achieved.
It is preferred to use thermal initiators, with preference being given to water-soluble azo compounds and water-soluble peroxo compounds. Particular preference is given to inorganic peroxo compounds, especially hydrogen peroxide and in particular sodium peroxodisulfate or mixtures thereof, optionally in combination with from 0.1 to 500 ppm of FeSO₄x₇H₂O. Especial preference is given to hydrogen peroxide.
It will be appreciated that mixtures of different initiators can also be used subject to the proviso that they do not influence one another negatively. The amount is determined by the skilled worker in accordance with the desired copolymer. As a general rule from 0.05% to 20%, preferably from 0.1% to 15%, and more preferably from 0.2% to 8% by weight is used of the initiator, relative to the total amount of all monomers.
It is also possible, furthermore, in a manner which is known in principle, to use suitable regulators, such as mercaptoethanol, for example. Preferably no regulators are used.
The polymerization may also be performed in the presence of a base. By this means the acidic groups, particularly the carboxyl groups of the monomers, are wholly or partly neutralized.
Bases which can be used include, for example, NaOH, KOH or NH₃. Amines as well are suitable.
Examples of suitable amines comprise linear, cyclic and/or branched C₁-C₈mono-, di- and trialkylamines, linear or branched C₁-C₈mono-, di- or trialkanolamines, especially mono-, di- or trialkanolamines, linear or branched C₁-C₈alkyl ethers of linear or branched C₁-C₈mono-, di- or trialkanolamines, oligoamines and polyamines such as diethylenetriamine or polyethyleneimines, heterocyclic amines such as morpholine, piperazine, imidazole and piperidine, or else certain aromatic amines, such as benzotriazole or tolyltriazole, for example. The amines can also be alkoxylated, in particular ethoxylated. By this means it is possible with advantage to increase the water solubility of amines having relatively long alkyl chains.
The skilled worker makes an appropriate selection from among the amines.
Preference is given to linear or branched C₁-C₈mono-, di- or trialkanolamines, particular preference being given to mono-, di- and triethanolamine and/or the corresponding ethoxylated products.
If bases are used the degree of neutralization should generally not exceed 30 mol %, based on the total amount of all of the acidic groups of the monomers. Preferably the degree of neutralization is not more than 20 mol %, more preferably not more than 10 mol %.
With very particular preference no base is used.
The bases can be added before or during the polymerization. Preferably it is added before the polymerization or no later than at the beginning of the polymerization. The base can be added either all at once or within a time interval which corresponds at most to the entire reaction period. The base can be added to the monomer feed. Preferably the base is included in the initial charge before the polymerization is commenced.
In accordance with the invention the polymerization is performed at a temperature of less than 120° C. Apart from this the temperature can be varied by the skilled worker within wide limits, in accordance with the nature of the monomers used, the initiator, and the desired result. Established in this context is a minimum temperature of approximately 60° C. The temperature can be held constant during the polymerization or else it is possible to operate temperature profiles. The polymerization temperature is preferably 75 to 115° C., more preferably 80 to 110° C., with particularly preference 90 to 108° C., and very preferably 95 to 105° C.
The polymerization can be performed in usual apparatus for free-radical addition polymerization. When operating above the boiling temperature of the water or of the mixture of water and further solvents operation takes place in a suitable pressure vessel, while otherwise operation may take place at atmospheric pressure. The polymerization is preferably performed at atmosphere pressure. Polymerization can be carried out, for example, under reflux.
In connection with the polymerization it is generally proven appropriate to include itaconic acid and/or derivatives thereof in aqueous solution in the initial charge. Thereafter it is possible to meter in the monocarboxylic acid and also the initiator, appropriately likewise in aqueous solution. Feed times which have proven appropriate are from 0.5 h to 24 h, preferably 1 h to 12 h.
In this way the concentration of the more reactive monocarboxylic acids in the aqueous solution is kept relatively low. This reduces the tendency of the monocarboxylic acid to react with itself and produces a more uniform incorporation of the itaconic acid units into the copolymer. Where monomers (C) and/or (D) used optionally are slow to react, it is likewise advisable to include them in the initial charge together with the itaconic acid. It will be appreciated, however, that they can also be added dropwise later. After all of the monomers have been fed in there may also be an afterreaction time, of from 0.5 to 3 h, for example. This ensures that the polymerization reaction proceeds as far as possible to completion.
The skilled worker, of course, can also perform the polymerization in another way.
The synthesized polymers can be isolated from the aqueous solution by means of customary methods known to the skilled worker: for example, by evaporating down the solution, by spray drying, by freeze drying or by precipitation.
With particular preference, however, the polymers after the polymerization are not isolated from the aqueous solution at all; instead, the resulting polymer solutions are used as they are.
In order to make such direct further use easier the amount of aqueous solvent ought right from the start to be such that the concentration of the polymer in the solvent is suitable for the application. A concentration which has proved particularly appropriate is that from 15% to 70% by weight, relative to the sum of all of the components; preferably from 20% to 65% by weight, more preferably from 25% to 60% by weight, and, for example, from 45% to 55% by weight.
The polymers of the invention are soluble or at least dispersible in water or in aqueous solvent mixtures with a water content of at least 50% by weight, the skilled worker being aware that the solubility of COOH-rich polymers can be highly pH-dependent. Reference is therefore made here to the pH values at which the polymers are used for passivation, in other words to an acidic solution, in particular to the pH range from 0.5 to 6. The term “water-dispersible” means that, although the solution is not entirely clear, the polymer is nevertheless homogeneously distributed therein and also does not settle. The polymers in question are preferably polymers which are water-soluble.
The pH of the polymer solution is generally less than 5, preferably less than 4, and more preferably less than 3.
The molecular weight M_w(weight average) of the copolymers is from 5000 to 2 000 000 g/mol, preferably at least 10 000 g/mol, more preferably at least 15 000 g/mol. In general M_wis from 20 000 g/mol to 1 000 000 g/mol, preferably from 30 000 g/mol to 900 000 g/mol, more preferably from 40 000 g/mol to 800 000 g/mol, and very preferably from 50 000 g/mol to 700 000 g/mol. It is determined by the skilled worker in accordance with the desired end use.
The preparation of the polymer or polymers that is used in accordance with the invention is an aqueous acidic formulation.
The polymers of the invention can be used in particular for treating metallic surfaces. For this purpose the polymers of the invention can be used in particular as components of corresponding formulations: for example, as components of cleaners, pickling solutions, corrosion control compositions and/or formulations for passivating.
The copolymers of the invention can be used with particular advantage for passivating metallic surfaces or for forming passivating layers on metals. Instead of the term “passivating layer” the term “conversion coat” is often used synonymously, and sometimes also the term “pretreatment layer”. The copolymers of the invention are particularly suitable for chromium-free passivation.
Any metallic surfaces can be treated, and in particular passivated, by means of the polymers of the invention. Preferably, however, the surfaces are those of low- or high-alloy steel or those of Zn, Zn alloys, Al or Al alloys. These may be the surfaces of structures or workpieces composed entirely of said metals and/or alloys. Alternatively they may be the surfaces of structures coated with Zn, Zn alloys, Al or Al alloys, it being possible for the structures to be composed of other materials: for example, other metals, alloys, polymers or composites. The surface in question may in particular be that of galvanized iron or steel. In one particular embodiment of the process it is the surface of a strip metal, in particular electrolytically galvanized or hot-dip-galvanized steel. In a further-preferred embodiment the surface in question may be that of an automobile body.
Zn alloys or Al alloys are known to the skilled worker. The skilled worker selects the type and amount of alloying constituents in accordance with the desired end application. Typical constituents of zinc alloys for hot-dip processes comprise, in particular, Al, Pb, Si, Mg, Sn, Cu or Cd. Typical alloying components in Zn alloys which are deposited electrolytically are Ni, Fe, Co and Mn. Typical constituents of aluminum alloys comprise, in particular, Mg, Mn, Si, Zn, Cr, Zr, Cu or Ti.
The alloys in question can also be Al/Zn alloys in which Al and Zn are present in approximately equal amounts. Steel coated with such alloys is available commercially.
The solvent or diluent used for the copolymers is water or an aqueous solvent mixture comprising at least 50% by weight of water. If an aqueous mixture is employed the mixture preferably comprises at least 65% by weight, more preferably at least 80% by weight and very preferably at least 95% by weight, of water. The amounts are based in each case on the total amount of all solvents. Further components of a mixture may be water-miscible solvents. Examples comprise monoalcohols such as methanol, ethanol or propanol, higher alcohols such as ethylene glycol or polyether polyols, and ether alcohols such as butyl glycol or methoxypropanol.
Preferably only water is used as solvent.
The concentration of the copolymers in the preparation is determined by the skilled worker in accordance with the desired end application. The thickness of the passivating layer depends, for example, on the chosen process technology, but may also depend on the viscosity of the composition used for passivating. Generally speaking a concentration which has proven appropriate is that from 0.01 g/l to 500 g/l, preferably from 0.1 g/l to 200 g/l and more preferably from 0.5 g/l to 5 g/l. The stated concentrations are based on the preparation in ready-to-use form. Generally it is possible first to prepare a concentrate, which only in situ is diluted to the desired concentration using water or, optionally, other solvent mixtures.
The preparation used in accordance with the invention is acidic. It generally has a pH of from 0.5 to 6, with the choice of narrower pH ranges being possible in accordance with the substrate and mode of application and also with the time during which the surface is exposed to the preparation. By way of example the pH is adjusted preferably to the range from 2 to 4 for the purpose of treating aluminum surfaces and to the range from 1 to 5 in the case where zinc or galvanized steel is being treated.
The pH of the preparation may be controlled on the one hand through the nature and concentration of the COOH-containing polymers or copolymers and, accordingly, comes about automatically. In this context it should be borne in mind that as a result of preparation the COOH groups in the polymer may in certain circumstances have been fully or partly neutralized.
As an alternative option the preparation may further comprise at least one organic or inorganic acid or mixtures thereof. Examples of suitable acids comprise phosphorus, sulfur or nitrogen acids such as phosphoric acid, phosphonic acid, sulfuric acid, sulfonic acids such as methanesulfonic acid, amidosulfonic acid, p-toluenesulfonic acid, m-nitrobenzenesulfonic acid, and derivatives thereof, nitric acid, hydrofluoric acid, hydrochloric acid, formic acid or acetic acid. The acid is preferably selected from the group consisting of HNO₃, H₂SO₄, H₃PO₄, formic acid and acetic acid. Particular preference is given to H₃PO₄and/or HNO₃. It will be appreciated that mixtures of different acids can also be used.
Conversely, bases may also be used to increase the pH, if appropriate.
Examples of phosphonic acids comprise 1-hydroxyethane-1,1-diphosphonic acid (HEDP), 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTC), aminotri(methylenephosphonic acid) (ATMP), ethylenediaminetetra(methylenephosphonic acid) (EDTMP) or diethylenetriaminepenta(methylenephosphonic acid) (DTPMP).
The nature and concentration of the acid in the preparation is determined by the skilled worker in accordance with the desired end application and pH. Generally speaking, a concentration which has proven appropriate is that from 0.01 g/l to 30 g/l, preferably from 0.05 g/l to 3 g/l and more preferably from 0.1 g/l to 5 g/l.
The preparation may also comprise, optionally, further components beyond those specified.
The components present optionally may be, for example, transition metal ions and transition metal compounds, examples being those of Ce, Ni, Co, V, Fe, Zn, Zr, Ca, Mn, Mo, W, Ti, Zr, Hf, Bi, Cr and/or of the lanthanides. They may also be compounds of main group elements, such as Si and/or Al, for example. The compounds can be used for example in the form of the respective aqua complexes. They may also, however, be complexes with other ligands, such as fluoride complexes of Ti(IV), Zr(IV) or Si(IV), or oxometallates such as MoO₄ ²⁻or WO₄ ²⁻, for example. It is also possible, moreover, to use complexes with typical chelate-forming ligands such as ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA), hydroxyethylethylenediaminetriacetic acid (HEDTA), nitrilotriacetic acid (NTA) or methylglycinediacetic acid (MGDA).
Further optional components comprise surface-active compounds, corrosion inhibitors or typical electroplating auxiliaries. The skilled worker makes an appropriate selection from among the optional components that are possible in principle, with respect also to their amounts, in accordance with the desired application. Examples of particularly preferred corrosion inhibitors which can be used in combination with the itaconic acid polymers comprise benzotriazole and/or tolyltriazole.
For the purpose of fine tuning its properties the formulation may also comprise further water-soluble polymers as additional components. Examples of such polymers comprise in particular polymers comprising carboxylate groups which merely do not correspond to the above definition of the composition. Examples that may be mentioned include poly(meth)acrylic acids and also copolymers of (meth)acrylic acid with other monomers containing acid groups, such as maleic acid, fumaric acid, crotonic or vinylacetic acid, for example. The amount of such additional (co)polymers is determined by the skilled worker in accordance with the desired properties of the passivating layer. The amount in general, however, should not exceed 30% by weight, preferably 20% by weight and more preferably 10% by weight, based on the amount of all the polymers used.
The passivation in question is preferably a substantially chromium-free passivation. This is intended to denote that small amounts, at best, of chromium compounds could be added in order to fine-tune the properties of the passivating layer. The amount should not exceed 2%, preferably 1% and more preferably 0.5% by weight of chromium, based on all constituents of the composition. If chromium compounds are to be employed then it is preferably Cr(III) compounds that should be employed. In each case, however, the Cr(VI) content should be kept so low that the amount of Cr(VI) on the passivated metal does not exceed 1 mg/m².
With particular preference the passivation is a chromium-free passivation; in other words, the preparation employed contains no Cr compounds at all. The expression “chromium-free”, however, does not rule out the indirect and per se unintended entrainment of small amounts of chromium into the process. Indeed, if the process of the invention is used to passivate alloys which comprise chromium as an alloying constituent, Cr-containing steel for example, it is always within the bounds of possibility for small amounts of chromium in the metal to be treated to be dissolved by the preparation used for the process and, accordingly, to pass into the preparation unintentionally per se. Even in the case where such metals are used, with the resultant consequences, the process should still be regarded as “chromium-free”.
In the process of the invention for passivating metallic surfaces the surface of the metal is treated with the preparation by means, for example, of spraying, dipping or rolling. After a dipping operation excess treatment solution can be removed from the workpiece by allowing it to drip dry; in the case of metal sheets, metal foils or the like excess treatment solution can alternatively be removed by squeezing off or squeegeeing, for example. In the course of the treatment parts at least of the polymer used and also further components of the preparation are chemisorbed by the surface of the metal, so that a solid bond comes about between the surface and the component. Treatment with the preparation takes place generally at room temperature or above, although this is not intended to rule out the possibility of lower temperatures in principle. As a general rule the treatment takes place at from 20 to 90° C., preferably from 25 to 80° C. and more preferably from 30 to 60° C. For that purpose the bath containing the formulation can be heated, although an elevated temperature may also come about automatically, by the immersion of hot metal into the bath.
It is possible to rinse the surface, after treatment, with a cleaning liquid, in particular with water, in order to remove residues of the preparation used in accordance with the invention from the surface.
The treatment may alternatively be what is called a no-rinse operation, in which the treatment solution is dried directly in a drying oven immediately following its application, without rinsing.
The treatment of the metal surface with the preparation and the crosslinker can take place discontinuously or, in particular, continuously. A continuous process is particularly suitable for treating strip metals. The metal strip is run through a trough or a spraying apparatus with the preparation and also, optionally, through a trough or spraying apparatus, and also, optionally, through further pretreatment or aftertreatment stations.
The treatment time is specified by the skilled worker in accordance with the desired properties of the layer, the composition used for the treatment, and the technical boundary conditions. It may be considerably less than one second or may be two or more minutes. In the case of the continuous process it has proven particularly appropriate to contact the surface with the preparation for a time of from 1 to 60 s.
Following the treatment the solvent used is removed. It can be removed at room temperature by simple evaporation in air at room temperature.
Alternatively the removal of the solvent may be assisted by means of suitable auxiliaries, such as by heating and/or passing streams of gas, particularly streams of air, over the treated surface. The evaporation of the solvent can be assisted, for example, by means of IR emitters, or else, for example, by drying in a drying tunnel. For the purpose of drying a temperature which has proven appropriate is that from 30° C. to 160° C., preferably from 40° C. to 100° C. and more preferably from 50° C. to 80° C. This refers to the temperature on the metal surface; it may be necessary to set the dryer temperature at a higher level, which is chosen appropriately by the skilled worker.
The process of the invention may optionally comprise one or more pretreatment steps. For example, prior to passivation, the metallic surface can be cleaned with the preparation used in accordance with the invention, in order for example to remove greases or oils. It is also possible to pickle the surface prior to passivation, in order to remove oxide deposits, scale, temporary corrosion protection, and the like. It is additionally necessary to rinse the surface, with water if appropriate, after and between such pretreatment steps, and to remove the residues of rinsing solutions or pickling solutions.
The passivating layer may additionally be crosslinked. For this purpose it is possible, for example, to admix a crosslinker to the preparation used, provided said crosslinker does not react while still in the preparation. An alternative is first to treat the metal with the preparation and thereafter to treat the layer with a suitable crosslinker for example, to spray it with a solution of a crosslinker.
Suitable crosslinkers should be water-soluble or at least soluble in the aforementioned aqueous solvent mixture. Examples of suitable crosslinkers comprise in particular those which have at least two crosslinking groups selected from the group of azirane groups, oxirane groups or thiirane groups. Further details of suitable crosslinkers are disclosed in our application WO 2005/042801, hereby expressly incorporated by reference.
The process of the invention makes it possible to obtain a passivating layer on a metallic surface. The precise structure and composition of the passivating layer are unknown to us. However, in addition to the customary amorphous oxides of aluminum or of zinc and also, if appropriate, of other metals, said layer comprises the reaction products of the polymer and also, where appropriate, of the crosslinker and/or of further components of the formulation. The composition of the passivating layer is generally not homogeneous; rather, the components appear to exhibit concentration gradients.
The thickness of the passivating layer is adjusted by the skilled worker in accordance with the desired properties of the layer. In general the thickness is from 0.01 to 3 μm, preferably from 0.1 to 2.5 μm, more preferably from 0.2 to 2 μm, most preferably from 0.3 to 1.5 μm, and for example from 1 to 2 μm. The thickness can be influenced, for example, by way of the nature and amount of the components applied, and also by way of the exposure time. In addition, it is possible to use technical parameters of the process to influence the thickness: by using rollers or squeegees to remove treatment solution applied in excess, for example.
The thickness of the layer is determined by differential weighing before and after exposure of the metal surface to the composition used in accordance with the invention, on the assumption that the layer has a specific density of 1 kg/l. In the text below, “layer thickness” always refers to a variable determined in this way, irrespective of the actual specific density of the layer. These thin layers are enough to obtain outstanding corrosion protection. Thin layers of this kind ensure that the dimensions of the passivated workpieces are maintained.
The present specification further provides a metallic surface which comprises the passivating layer of the invention. The passivating layer is applied directly on the actual metal surface. In one preferred embodiment the metal surface in question is that of a strip metal of steel which comprises a coating of Zn or of Zn alloy and on which a passivating layer of the invention has been applied. The surface in question may also be that of an automobile body which has been coated with the passivating layer of the invention.
The metallic surface with its passivating layer may in principle be overcoated in a known manner with one or more color or effect paint layers. Typical paints, their composition, and typical layer sequences in the case of two or more paint layers are known in principle to the skilled worker.
The passivation of the invention can be employed at different processing stages. It can be undertaken, for example, at the premises of a steel maker. In this case a steel strip can be galvanized in a continuous process and immediately after having been galvanized can be passivated by treatment with the formulation used in accordance with the invention. Passivation at this stage is frequently referred to by the skilled worker as “aftertreatment”.
The passivation in question may be only temporary, serving to protect against corrosion in the course of storage and transport and/or during further process steps, but removed again before the permanent corrosion protection is applied. The acidic copolymers can be removed from the surface again by cleaning with aqueous alkaline solutions.
Alternatively the treatment may be a permanent corrosion protection treatment, which remains on the strip or on the fully-formed workpiece and is provided with additional paint coats. Passivation at this stage is frequently referred to by the skilled worker as “pretreatment”.
The passivated and, if appropriate, painted metal sheets, strips or other semifinished metallic products can be processed further to form metallic workpieces, such as an automobile body, for example. This generally necessitates at least one separating step and one forming step. Larger components can be assembled subsequently from individual components. Forming involves changing the shape of the material, generally in contact with a tool. Forming may, for example, involve compressive forming, such as rolling or embossing, tensile compressive forming, such as cold drawing, deep drawing, roll-bending or press-bending, tensile forming such as lengthening or widening, flexural forming such as bending, edge-rolling or edging, and shearing forming such as twisting or dislocating.
The process of the invention for passivating achieves much better corrosion protection than when using itaconic acid copolymers which have been obtained at relatively high temperatures.
The examples which follow are intended to illustrate the invention in more detail:

Measurement Methods:

The K values were measured by the method of H. Fikentscher, Cellulose-Chemie, vol. 13, pp. 58-64 and 71-74 (1932) in 1% strength by weight aqueous solution at 25° C. M_wvalues were determined by means of gel permeation chromatography.

Preparation of the Polymers

COMPARATIVE EXAMPLE 1

Copolymer of 75% by weight acrylic acid and 25% by weight maleic acid
A 6 l pressure reactor with anchor stirrer, temperature monitoring and nitrogen inlet is charged with 480 g of maleic anhydride and 22.5 mg of iron sulfate in one liter of deionized water.
This initial charge is heated at 115 to 120° C. under a nitrogen atmosphere. When this temperature has been reached 1670 g of acrylic acid in 1.2 l of deionized water are metered in at a uniform rate over the course of 4 h and 115 g of 30% strength hydrogen peroxide solution and 300 ml of deionized water are metered in at a uniform rate over the course of 5 h. During the polymerization the pressure is maintained at from 3 to 4 bar by careful release of pressure.
Cooling gives a yellowish, clear polymer solution having a solids content of 45.6% and a K value (1% in deionized water) of 26.0.

COMPARATIVE EXAMPLE 2

Copolymer of acrylic acid/itaconic acid (73/27), polymerization at 120° C. in a pressure reactor
A 6 l pressure reactor provided with anchor stirrer, temperature monitoring, nitrogen inlet and 2 feed ports is charged with 444 g of itaconic acid, 15.5 mg of iron sulfate heptahydrate and 889 g of deionized water.
This initial charge is heated to 120° C. under a nitrogen atmosphere. When this temperature has been reached feed stream 1, consisting of 1188.0 g of acrylic acid and 926 g of deionized water, is metered in at a uniform rate over the course of 5 h and also feed stream 2, consisting of 81.6 g of hydrogen peroxide (30% strength) and 177 g of deionized water, is metered in at a uniform rate over the course of 6 hours. After the end of feed stream 1 a further 133 g of deionized water are added. The reaction mixture is stirred at 120° C. for a further 2 hours. During the polymerization the pressure is held at from 3 to 4 bar by careful release of pressure. The batch is diluted with water.
Cooling gives a yellowish, clear polymer solution having a solids content of 17.9% and a K value (1% strength in deionized water) of 83.5. No free itaconic acid was detectable by means of the ¹H NMR spectrum.

EXAMPLE 1

Copolymer of Acrylic Acid and Itaconic Acid (73/27), Unpressurized Polymerization at 100° C.

In a stirring pot with blade stirrer and internal thermometer 111.0 g of itaconic acid are dissolved with 250 g of deionized water and, following the addition of 3.875 mg of iron(II) sulfate heptahydrate, the solution is heated at slight reflux (internal temperature: 98° C.). Subsequently, over the course of 5 h, feed stream 1, consisting of 297.0 g of acrylic acid and 398.0 g of deionized water, and, over the course of 6 h, feed stream 2, consisting of 6.12 g of sodium peroxodisulfate in 180 g of water, are added. After the end of feed stream 1 the system is stirred at 98° C. for further 2. This gives a pale yellow, clear polymer solution having a solids content of 36.4% and a K value (1% strength in deionized water) of 59.6. No free itaconic acid was detectable by means of the ¹H NMR spectrum.

EXAMPLE 2

Copolymer of Acrylic Acid and Itaconic Acid (73/27); Unpressurized Polymerization at 100° C.

In a stirring pot with blade stirrer and internal thermometer 111.0 g of itaconic acid are dissolved with 250 g of deionized water and, following the addition of 3.875 mg of iron(II) sulfate heptahydrate, the solution is heated at slight reflux (internal temperature: 98° C.). Subsequently, over the course of 5 h, feed stream 1, consisting of 297.0 g of acrylic acid and 398.0 g of deionized water, and, over the course of 6 h, feed stream 2, consisting of 20.4 g of hydrogen peroxide (30% strength) in 180 g of water, are added. After the end of feed stream 1 the system is stirred at 98° C. for further 2. The batch is diluted in a number of portions with a total of 750 g of water. This gives a pale yellow, clear polymer solution having a solids content of 15.0% and a K value (1% strength in deionized water) of 115.2. No free itaconic acid was detectable by means of the ¹H NMR spectrum.

EXAMPLE 3

Terpolymer of Acrylic Acid, Itaconic Acid and Vinylphosphonic Acid (69/26/5); Unpressurized Polymerization at 100° C.

In a stirring pot with blade stirrer and internal thermometer 111.0 g of itaconic acid are dissolved with 250 g of deionized water and, following the addition of 3.875 mg of iron(II) sulfate heptahydrate, the solution is heated at slight reflux (internal temperature: 98° C.). Subsequently, over the course of 5 h, feed stream 1, consisting of 297.0 g of acrylic acid, 22.7 g of vinylphosphonic acid and 346.0 g of deionized water, and, over the course of 6 h, feed stream 2, consisting of 42.8 g of hydrogen peroxide in 180 g of water, are added. After the end of feed stream 1 the system is stirred at 98° C. for further 2. The batch is diluted in a number of portions with a total of 700 g of water. This gives a pale yellow, clear polymer solution having a solids content of 21.2% and a K value (1% strength in deionized water) of 37.8.

EXAMPLE 4

Copolymer of Acrylic Acid, Itaconic Acid and Vinyl Phosphonic Acid

In a stirring pot with blade stirrer and internal thermometer 161.3 g of itaconic acid and 152 of vinylphosphonic acid (95%) are dissolved with 200 g of deionized water and, following the addition of 703 mg of iron(II) sulfate heptahydrate, the solution is heated at slight reflux (internal temperature: 98° C.). Subsequently, over the course of 5 h, feed stream 1, consisting of 430.0 g of acrylic acid and 455 g of deionized water, and, over the course of 6 h, feed stream 2, consisting of 42.5 g of sodium peroxodisulfate in 160 g of water, are added. After the end of feed stream 1 the system is stirred at 98° C. for further 2 h. This gives a pale yellow, clear polymer solution having a solids content of 48.8% and a K value (1% strength in deionized water) of 28.2.

Passivation of Metallic Surfaces

Metallic Surface and its Pretreatment:

The tests were carried out using in each case alkalinically galvanized or hot-dip-galvanized steel sheets (approx. 100×190×0.7 mm; 20 μm zinc add-on).
The alkalinically galvanized steel sheets (AV) are immersed in a cleaning solution (0.5% HCl+0.1% Lutensol® AP 10, BASF AG) for about 5 seconds and immediately rinsed with deionized water and dried by blowing.
The hot-dip-galvanized steel sheets (HV) are immersed in an alkaline degreaser at 50° C. for 120 seconds and immediately rinsed with deionized water and dried by blowing.
Employment of the Preparation Used in Accordance with the Invention
The synthesized polymers were dissolved in water (solids content 5% by weight), homogenized and introduced into a dip bath. The cleaned metal sheets are immersed directly for 30 s in the polymer solution, which has been conditioned to a temperature of 50° C., and are dried at RT. The edges of the passivated sheet are masked off.

Differential Weighing

The thickness of the layer is determined by differential weighing before and after the metal surface has been exposed to the composition employed, on the assumption that the layer has a specific density of 1 kg/l. In the text below “layer thickness” always refers to a parameter determined in this way, irrespective of the actual specific density of the layer.
The layer thicknesses determined by differential weighing are between 1.5 and 3 μm. The withstand time is determined in a salt spray mist test.

Test Method

Salt Spray Test

The result of a salt spray test in accordance with DIN 50021 is used as a measure of the corrosion inhibition effect. The withstand time in the corrosion test is defined differently according to what kind of corrosion damage is observed.

- if white spots of generally more than 1 mm in diameter (Zn oxide or Al oxide, known as white rust) are formed, the withstand time reported is the time after which the appearance of the damage corresponds to evaluation level 8 in DIN EN ISO 10289 of April 2001, annex B, page 19.
- if black spots of generally less than 1 mm in diameter form before white rust spots (particularly on zinc provided with a passivating layer), the withstand time reported is the time after which the appearance of the damage corresponds to evaluation level 8 in DIN EN ISO 10289 of April 2001, annex A, page 9.
  Test chamber volume: 400 l
  Spray mist throughput measured according to DIN: 2.2 ml/h

The results of the tests are assembled in table 1.

TABLE 1

Results of the inventive and comparative examples

Polymeri-

Residual

Withstand time [h]

zation

Initiator/

mono.

alkalin-

Acrylic

Itaconic

Further

temperature

Pressure

amount

Fe

dicarboxylic

M_w

ically

hot-dip-

Example

acid

comonomers

[° C.]

[bar]

[wt. %]

[ppm]

K value

acid

[g/mol]

galvanized

Comparative	75	—	25% maleic	120°	3-4	H₂O₂/	2	26	4.5%	85 000	30	18
example 1			acid			1.5
Comparative	73	27	—	120	3-4	H₂O₂/	2	83.5	not	304 000	35	20
example 2						1.5			detectable
									in the
									¹H NMR
Inventive	73	27	—	100	1	NaPS/	2	59.6	not	630 000	52	23
example 1						1.5			detectable
									in the
									¹H NMR
Inventive	73	27	—	100	1	H₂O₂/	2	115.2	not	510 000	50	28
example 2						1.5			detectable
									in the
									¹H NMR
Inventive	69	26	5%	100	1	H₂O₂/	2	37.8	n.d.	610 000	51	35
example 3			vinylphos-			3.0
			phonic acid
Inventive	58	22	20%	100	1	NaPS/	250	28.2	n.d.	n.d.	—	55
Example 4			vinylphos-			3.6
			phonic acid

n.d. not determined

The inventive and comparative examples show that by means of the process of the invention for passivating using itaconic acid polymers with a polymerization temperature of less than 120° C. a much better corrosion protection is achieved than when using itaconic acid polymers which have been prepared by polymerization at higher temperatures. The efficacy is also higher than that of acrylic acid-maleic acid copolymers.

Claims

1-20. (canceled)

21. A process for passivating a metallic surface by treating it with an acidic aqueous preparation comprising at least one water-soluble or water-dispersible itaconic acid copolymer, wherein the copolymer has been constructed from the following monomeric units:

(A) 15% to 45% by weight of itaconic acid,

(B) 55% to 85% by weight of at least one monoethylenically unsaturated monocarboxylic acid,

(C) 5% to 40% by weight of vinylphosphonic acid, and

(D) optionally 0% to 30% by weight of at least one further ethylenically unsaturated monomer, other than (A), (B) and (C),

the amount being based in each case on the total amount of all of the monomers incorporated in the copolymer, and the copolymer being obtainable by means of free-radical polymerization in aqueous solution at a temperature of less than 120° C.

22. The process according to claim 21, wherein the amount of vinylphosphonic acid is 10% to 30% by weight.

23. The process according to claim 21, wherein the polymer is a terpolymer of 67% to 75% acrylic acid, 23% to 30% itaconic acid, and 0.1% to 10% vinylphosphonic acid, by weight.

24. The process according to claim 21, wherein the polymer is a terpolymer of 55% to 65% acrylic acid, 15% to 25% itaconic acid, and 15% to 25% vinylphosphonic acid, by weight.

25. The process according to claim 21, wherein the polymerization is performed at a temperature from 90 to 105° C.

26. The process according to claim 21, wherein said metallic surface is of Zn, Al or alloys thereof.

27. The process according to claim 21, being a substantially chromium-free process.

28. The process according to claim 21, wherein the treating takes place by means of rolling, spraying or dipping methods.

29. The process according to claim 21, wherein the metal surface is the surface of a strip metal.

30. The process according to claim 29, wherein the strip metal is electrolytically galvanized or hot-dip-galvanized steel.

31. The process according to claim 29, wherein the treating is performed by means of a continuous process.

32. The process according to claim 29, wherein the surface is contacted with the preparation for a time of from 1 to 60 s.

33. The process according to claim 21, wherein the formulation further comprises benzo- and/or tolyltriazole.

34. A water-soluble or water-dispersible itaconic acid copolymer constructed from the following monomeric units:

(A) 15% to 45% by weight of itaconic acid,

(C) 5% to 40% by weight of vinylphosphonic acid, and

35. The copolymer according to claim 34, wherein the amount of vinylphosphonic acid is 10% to 30% by weight.

36. The copolymer according to claim 34, wherein the polymer is a terpolymer of 67% to 75% acrylic acid, 23% to 30% itaconic acid, and 0.1% to 10% vinylphosphonic acid, by weight.

37. The copolymer according to claim 34, wherein the polymer is a terpolymer of 55% to 65% acrylic acid, 15% to 25% itaconic acid, and 15% to 25% vinylphosphonic acid, by weight.

38. The copolymer according to claim 34, wherein the polymerization is performed at a temperature from 90 to 105° C.