US20100224286A1

US20100224286A1 - Method for coating zinc-plated steel strips with aqueous formulations of acidic polymers

Info

Publication number: US20100224286A1
Application number: US12/681,568
Authority: US
Inventors: Walter Bertkau
Original assignee: BASF SE
Current assignee: BASF SE
Priority date: 2007-10-05
Filing date: 2008-10-02
Publication date: 2010-09-09
Also published as: WO2009047209A1; KR20100087299A; JP2010540237A; EP2201150A1

Abstract

Continuous method of passivating galvanized steel strips with acidic aqueous formulations which comprise at least one water-soluble polymer comprising acidic groups.

Description

The invention relates to a continuous method of passivating galvanized steel strips with acidic aqueous formulations which comprise at least one water-soluble polymer comprising acidic groups.
The raw material used for producing sheetlike steel workpieces, such as, for example, automotive components, bodywork components, equipment casings, exterior architectural facings, ceiling panels or window profiles, presently comprises typically long steel strips which are produced by hot rolling and/or cold rolling from steel slabs and which are wound into coils for the purposes of storage and transportation. The steel strips may subsequently be divided up and processed to the desired shaped parts.
The steel strips are generally protected from corrosion by means of appropriate measures. This generally involves multistage operations. In a first step the steel strips are typically coated with zinc or with zinc alloys. The action of zinc is based on the one hand on the fact that it is baser than steel and thus initially undergoes corrosion itself. The steel surface remains intact as long as it is still covered continuously with zinc. Moreover, in the presence of atmospheric oxygen, a thin oxide layer forms on the surface of Zn or of Zn alloys, and, depending on the external conditions, slows down, to a greater or lesser extent, the corrosive attack on the underlying metal.
In order to boost the protective effect of such an oxide layer, the Zn surfaces are generally subjected town additional passivating treatment. In the course of such a treatment, some of the metal to be protected dissolves and is incorporated at least partly into a film on the metal surface. Instead of the term “passivation coat” the terms “conversion coat”, “aftertreatment coat” or “pretreatment coat” are also used synonymously.
The performance of such passivation by treatment of the galvanized steel surface with acidic Cr(VI) and/or Cr(III) solutions is known. Increasingly, however, use is also being made for this purpose of chromium-free formulations, examples being phosphate formulations or else formulations which comprise various polymers.
One important class of polymers which can be used in formulations for passivating treatments comprises strongly acidic, water-soluble polymers, such as, for example, polyacrylic acid or copolymers of acrylic acid with other monomers, more particularly with other acidic monomers such as vinylphosphonic acid, maleic acid or itaconic acid. The use of polymers of this kind for passivation is disclosed in, for example, WO 2004/074372, WO 2005/042801, WO 2006/021308, 2006/021309, WO 2006/134116, WO 2006/134117 or WO 2006/134118. The polymers in each case form the main constituent of the formulations—more particularly, further acids, such as phosphoric acid, for example, are present in the formulation in small amounts, if at all. In the course of the passivation the acid groups of the polymer detach zinc and also, possibly, further metals from the surface, with formation of hydrogen. The zinc ions that have formed on the one hand are redeposited on the surface, but may also crosslink the polymer layer by forming complexes with the acid groups of the polymer and also, possibly, with other functional groups of the polymer, thus producing a very dense passivation coat.
The passivation of galvanized steel strips is typically performed by means of a continuous method, as is shown diagrammatically in FIG. 1). A galvanized steel strip (1) is moved by means of drive rollers (not shown) in direction (2). By means of a spraying station (3), the formulation for passivating is sprayed onto the steel strip and forms a wet film (7). Two squeeze rolls, (4) and (4′), squeeze off excess treatment solution. The result is a thin, wet film (8), which finally is dried in a dryer (6). This gives a galvanized steel strip with passivation coat (9). The formulation squeezed off can be collected (5) and used again for coating.
When passivation is carried out using formulations of strongly acidic, water-soluble polymers, this method exhibits a series of disadvantages: In order to achieve a satisfactory passivation outcome, the acidic polymers ought to be used at concentrations of at least 10% by weight, based on the formulation employed. At lower concentrations, the pH of the formulations is too high, with the consequence that the zinc surface cannot be attacked by acid with sufficient speed, and the resulting passivation coats are worse in quality. Furthermore, after the application of the passivation coat, acidic groups which have not yet reacted may react in the course of time, possibly leading to a whitening of the passivation coat (referred to as “staining”). Such discoloration is highly undesirable.
Formulations with more than 10% by weight of strongly acidic, water-soluble polymers have a much higher viscosity than formulations which comprise predominantly low molecular mass components, such as typical formulations based on Cr(VI), Cr(III) or phosphoric acid, for example. When high-viscosity formulations of this kind are used, a build-up (8) is formed ahead of the squeeze roll (FIG. 1, (4)).
The development of a build-up, however, is associated with a series of disadvantages: Since the dwell time of individual polymer molecules in the build-up is not constant, but instead is randomly distributed, the passivation reaction in the build-up proceeds uncontrolledly. As a result of the reaction with the zinc surface, zinc increasingly accumulates in the build-up. Since zinc, by forming complexes with the acid groups of the polymer, increasingly crosslinks the polymer formulation, there is a change in the viscosity of the formulation in the build-up. Moreover, the formulation squeezed off becomes contaminated with zinc, and as a result, ultimately, it can no longer be used. Because of the viscosity in the build-up, there is also, increasingly, a risk of air bubbles forming which, on passing through the squeeze rolls, lead to some sites on the strip's surface no longer being covered with a passivation coat. On the basis of the conditions stated, it is very difficult to apply thin, homogeneous passivation coats which ensure effective protection from corrosion. With the process technology depicted, the dry film thickness of the passivation coats formed is, as a general rule, more than 1 μm, a thickness which can hardly be reduced even by means of harder rolls.
Within the field of coil coating with paints, multiple-roll systems for application ere known, with 2, 3 or more rolls for applying the paint (see, for example, “Coil coating” in Römpp Lexikon Lacke and Druckfarben, page 55, Georg Thieme Verlag Stuttgart, New York, 1998). Multiple-roll systems of thiskind, however, are very complicated and expensive.
In the field of paper coating there are devices known with which paper coating compositions, such as adhesive layers, for example, are applied to a coating roll and, even prior to contacting with the paper, excess material is doctored off using a suitable doctor means. Reference may be made, for example, to DE 37 35 889 A1, DE 198 00 955 A1 or U.S. Pat. No. 2,970,564. The use of techniques of this kind for passivating galvanized steel strips with formulations of strongly acidic polymers was unknown to date.
It was an object of the invention to provide a continuous method of passivating galvanized steel strips with aqueous formulations of strongly acidic, water-soluble polymers, in which even formulations with at least 10% by weight of polymers can be satisfactorily processed to thin passivation coats. The process technology involved in this method should be extremely simple, and should allow even existing plant for passivating galvanized steel strips to be easily retrofitted.
Surprisingly it has been found that technologies known from the field of the coating of paper and cardboard webs are also suitable for the application of passivation coats to galvanized steel strips. By means of these techhologies it is possible to apply even very thin passivation coats with high quality.
Found accordingly has been a continuous method of passivating galvanized steel strips with an acidic aqueous formulation comprising at least one water-soluble polymer X comprising acidic groups,

- the polymer X having at least 0.6 mol of acid groups/100 g of polymer,
- the pH of the formulation being not more than 5, and
- the amount of all the polymers X together being 10% to 30% by weight, based on the amount of all the components of the formulation,
  and the galvanized steel strip being moved in its longitudinal direction through an adjustable gap (12) between two opposing rolls (4), (4′) which rotate in the direction of travel (2) of the steel strip, and wherein
- the acidic aqueous formulation is applied to at least one of the two rolls using an applicator means (10), and between the applicator means (10) and the gap (12) in the direction of rotation of the roll there is at least one doctor means (11) with which, by removal of excess formulation, it is possible to set the amount of the aqueous formulation per unit area of the roll,
- the film located on the roll, on contact with the galvanized surface of the steel strip, is transferred wholly or partly to the galvanized surface, and
- the wet film is dried in a drying means (6) disposed downstream of the pair of rolls in the direction of travel (2) of the strip.

Index to the figures:

FIG. 1: Diagrammatic representation of a prior-art passivating method

FIG. 2: Diagrammatic representation of the passivating method of the invention for single-sided passivation

FIG. 3: Diagrammatic representation of the passivating method of the invention for double-sided passivation

Details of the invention now follow:
Passivation by means of the methdd of the invention is accomplished using an acidic aqueous formulation which comprises at least one water-soluble polymer X comprising acidic groups. The polymers X employed may be homopolyme'rs or copolymers. It will be appreciated that mixtures of two or more different polymers X can also be used.
As solvents the formulation comprises preferably just water. It may additionally comprise water-miscible organic solvents in small amounts. Examples comprise 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, however, the amount of water is at least 80%, preferably at least 90%, and very preferably at least 95% by weight. The figures are based in each case on the total amount of all the solvents.
In accordance with the invention the polymers X employed have at least 0.6 mol of acid groups/100 g of polymer. This quantity figure is based on the free acid groups. Preferably the polymers have at least 0.9 mol of acid groups/100 g, more preferably at least 1 mol/100 g, and very preferably at least 1.2 mol/100 g.
The formulation employed in accordance with the invention comprises 10% to 30%, preferably 12% to 30%, more preferably 15% to 30%, and very preferably 18% to 28% by weight of the polymers X, based on the amount of all the components of the formulation (including the solvents).
The term “water-soluble” for the purposes of this invention is intended to denote that the polymer or polymers X employed are homogeneously water-soluble. Aqueous dispersions of crosslinked polymer particles of inherently water-insoluble polymers as polymer X are not part of the scope of this invention. The polymers X employed ought preferably to be infinitely miscible with water. They must at least, however, be water-soluble to an extent such that it is possible to attain the stated concentrations in the formulation.
The formulation employed has a pH of not more than 5, more particularly a pH of 0.5 to 5, preferably 1.5 to 3.5. Naturally, the pH of the preparation depends on the nature and concentration of the polymers X used in accordance with the invention. It may additionally be influenced by further basic or acidic components in the formulation.
Accordingly, as well as the acidic polymers, the formulation employed may further comprise organic or inorganic acids or mixtures thereof. There is no limit on the selection of such acid, provided that no negative effects arise together with the other components of the formulation. The skilled person will make an appropriate selection. Examples of suitable acids comprise phosphoric acid, phosphonic acid or organic phosphonic acids such as 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), 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 acids such as H₃PO₄, phosphonic acid, the stated organic phosphonic acids, HNO₃or methanesulfonic acid. With preference the acid in question may be H₃PO₄or another phosphorus acid.
The acidity of the formulation employed ought, however, to be produced substantially by the acid groups of the polymer. The amount of additional acids, besides the polymers X, in the formulation ought therefore not to exceed, in general, 50% by weight, relative to the amount of all the polymers X in the formulation together. Preferably 30%, more preferably 20%, and very preferably 10% by weight ought not to be exceeded. In a second, particularly preferred embodiment of the invention there are no additional acids.
The acid groups of the polymer X ought preferably to be in the form of free acid groups. However, to a small extent, they may also be neutralized by means of bases, such as ammonia, amines, amino alcohols or alkali metal hydroxides, for example. A partial neutralization of this kind may be performed for the purpose of pH adaptation. Alternatively it may, come about automatically in the course of the preparation of the polymer. The skilled worker, for example, is aware that it can be necessary, in the course of the preparation of polymers rich in acid groups, to neutralize some of the acid groups in order to force incorporation of the monomers into the polymer.
In no way, however, should the degree of neutralization be too high to ensure effective attack of acid on the zinc surface. As a general rule, therefore, not more than 25 mol % of the acid groups present in the polymer X should be neutralized, preferably not more than 20 mol %, and more preferably not more than 12 mol %.
The acid groups of the polymers X are selected in general from the group consisting of carboxylic groups, sulfonic add groups, phosphoric or phosphonic acid groups. The groups in question are preferably carboxyl groups, phosphoric or phosphonic acid groups. The polymer X employed is more preferably a copolymer of at least two different acid-functional monomers.
For the performance of the invention it is particularly preferred to use homopolymers or copolymers which comprise (meth)acrylic acid units.
In one particularly preferred embodiment of the invention, the polymer X comprises one or more water-soluble copolymers X1 of (meth)acrylic acid units (A) and different monoethylenically unsaturated monomers with acidic groups (B). Optionally, furthermore, (meth)acrylic esters (C) containing OH groups, and/or further monomers (D) may be present as structural units. Over and above these there are no further monomers present.
The amount of (meth)acrylic acid (A) 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 of all the monomers in the polymer.
The monomer (B) is at least one monoethylenically unsaturated monomer which is different from (A) but copolymerizable with (A) and which contains one or more acidic groups. It will be appreciated that two or more different monomers (B) can also be used.
The acidic groups in question may preferably comprise a group selected from the group consisting of carboxyl groups, phosphoric acid groups, phosphonic acid groups or sulfonic acid groups. The group in question is preferably selected from the group consisting of carboxyl groups, phosphoric add groups or phosphonic acid groups.
Examples of monomers of this kind comprise crotonic acid, vinylacetic acid, C₁-C₄monoesters 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.
The amount of the monomers (B) 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 of all the monomers in the polymer.
In one preferred embodiment of the invention the monomers (B) are monoethylenically unsaturated dicarboxylic acids having 4 to 7 carbon atoms (B1) and/or monoethylenically unsaturated phosphoric and/or phosphonic acids (B2).
Examples of monomers (B1) comprise maleic acid, fumaric acid, methylfumaric acid, methylmaleic acid, dimethylmaleic acid, methylenemalonic acid or itaconic acid. The monomers may if desired also be used in the form of the corresponding cyclic anhydrides. Preference is given to maleic acid, fumaric acid, and itaconic acid, particular preference to maleic acid and/or maleic anhydride.
Examples of monomers (B2) 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-vinyloxypropylphosphate, mono-1-phosphonoxymethyl-2-vinyloxyethyl phosphate, mono-3-allyloxy-2-hydroxypropyl phosphate, mono-2-allylox-1-phosphonoxymethylethyl phosphate, 2-hydroxy-4-vinyloxymethyl-1,3,2-dioxaphosphole, and 2-hydroxy-4-allyloxymethyl-1,3,2-dioxaphosphole. Preferably the monomer is vinylphosphonic acid, monovinyl phosphate or allylphosphonic acid, particular preference being given to vinylphosphonic acid.
The copolymer X1 may further optionally comprise at least one (meth)acrylic ester (C) having OH groups. The monomers in question are preferably monohydroxy (meth)acrylic esters.
The monomers (C) preferably comprise at least one (meth)acrylic ester of the general formula H₂C═CHR¹—COOR², where R¹, in a manner known in principle, stands for H or methyl and where R²is selected from the group consisting of R^2a, R^2bor R^2c.
The radicals R^2aare radicals of the general formula —(R³—O—)_n—H. In this formula n stands for a natural number from 2 to 40. Preferably n stands for 2 to 20 and more preferably for 2 to 10. The radicals R³, in each case independently of one another, stand for a divalent, straight-chain or branched alkyl radical having 2 to 4 C atoms. Examples comprise, more particularly, 1,2-ethylene radicals, 1,2-propylene radicals, 1,2-butylene radicals, and 1,4-butylene radicals. It will be appreciated that they may also comprise mixtures of different radicals. Preference is given to 1,2-ethylene and/or 1,2-propylene radicals. Particular preference is given exclusively to 1,2-ethylene radicals. Further preference is given to radicals (R^2a) which feature not only 1,2-ethylene but also 1,2-propylene radicals, the amount of the ethylene radicals being at least 50%, preferably at least 70%, and more preferably at least 80%, based on the total number of all the radicals R³. Examples of radicals R^2acomprise —CH₂—CH₂—O—CH₂—CH₂—O—CH₂—CH₃, —CH₂—CH(CH₃)—O—CH₂—CH(CH₃)OH, —CH₂—CH(CH₃)—O—CH(CH₃)—CH₂OH, —CH(CH₃)—CH₂—O—CH(CH₃)—CH₂OH or —CH(CH₃)—CH₂—O—CH₂—CH(CH₃)OH.
The radicals R^2bare radicals of the general formula —R⁴—(OH)_m. In this formula m is a natural number from 1 to 6, preferably 1 to 4, more preferably 1 to 3, and, for example, 1 or 2. The radical R⁴stands for an (m+1)-valent, straight-chain or branched alkyl radical having 2 to 10 C atoms, preferably 2 to 6 C atoms, and more preferably 2 to 4 C atoms.
The alkyl radical is substituted by at least one OH group, with the proviso that there is not more than one OH group X per C atom in R⁴. Examples of suitable radicals R^2bwith OH groups comprise linear radicals of the general formula —(CH₂)_m—OH such as —CH₂—CH₂—OH, —CH₂—CH₂—CH₂—OH, —CH₂—CH₂—CH₂—CH₂—OH or —CH₂—CH₂—CH₂—CH₂—CH₂—CH₂—OH.
Particularly preferred radicals R^2bfor performing the invention are radicals selected from the group consisting of —CH₂—CH₂—OH, —CH₂—CH₂—CH₂—OH, —CH₂—CH₂—CH₂—CH₂—OH, —CH₂—CH(CH₃)—OH, —CH(CH₃)—CH₂—OH or —CH₂—CH(OH)—CH₂—OH.
In a further, preferred embodiment of the invention at least one of the radicals R^2bis a branched alkyl radical of the general formula —R⁵—CH(R⁶)OH. In this formula R⁵and R⁶each stand for a linear or branched alkyl radical having 1 to 8 C atoms, preferably 1 to 6 C atoms, and more preferably 1 to 4 C atoms, with the proviso that the sum of the C atoms in R⁵and R⁶is not more than 9. Preferably R⁵and R⁶are each linear alkyl groups. More preferably R⁶is a methyl group. The radical in question may be, for example, —CH₂—CH(CH₃)—OH. In the case of branched (meth)acrylic esters of this kind, the tendency of the OH group to form further ester bonds with other COOH-containing monomers is significantly reduced. Very particular preference is given to —CH₂—CH(CH₃)—OH and/or —CH(CH₃)—CH₂—OH, more particularly to a mixture of both radicals. (Meth)acrylic esters with such radicals may be obtained in a simple way, as for example by esterification of (meth)acrylic acid with 1,2-propylene glycol.
The radicals R^2care monosaccharide or oligosaccharide radicals, preferably monosaccharide radicals. The saccharides in question may in principle be all kinds of saccharides. With preference it is possible to use radicals derived from pentoses and hexoses, more particularly from hexoses. Examples of suitable monosaccharides comprise glucose, mannose, galactose, fructose or ribose. With preference it is possible to use radicals derived from glucose. The radicals in question may also be derivatives of the saccharides, as for example products originating from the saccharides through reduction or oxidation. The saccharides in question may more particularly be sugar acids such as gluconic acid, for example.
The amount of the monomers (C) in the copolymer X1 is 0% to 40%, preferably 1% to 30% by weight.
Besides the monomers (A), (B), (C), and, if used, (D), it is possible optionally to use 0% to 30% by weight of at least one further ethylenically unsaturated monomer (D), different from (A), (B), and (C). Over and above these there are no other monomers used.
The monomers (D) serve to fine-tune the properties of the copolymer X1. It will be appreciated that two or more different monomers (D) can also be used. They are selected by the skilled worker in accordance with the desired properties of the copolymer, and additionally on the condition that they must be copolymerizable with the monomers (A), (B), and (C).
The monomers in question are preferably monoethylenically unsaturated monomers. In special cases, however, small amounts of monomers with two or more polymerizable groups may also be used. By this, means it is possible for the copolymer to be crosslinked to a small extent.
Examples of suitable monomers (D) 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 or allyl ethers such as methyl vinyl ether, ethyl vinyl ether, propyl vinyl ether, 2-ethylhexyl vinyl ether, vinyl cyclohexyl ether, vinyl 4-hydroxybutyl ether, decyl vinyl ether, 2-(diethylamino)ethyl vinyl ether, 2-(di-n-butylamino)ethyl vinyl ether or methyl diglycol vinyl ether, and/or the corresponding allyl compounds. It is likewise possible to employ vinyl esters, such as vinyl acetate or vinyl propionate, for example. It is also possible to employ 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 butanediol divinyl ether.
The amount of all the monomers (D) used, together, is 0% to 30% by weight, based on the total amount of the monomers used. Preferably the amount is 0% to 20% by weight, more preferably 0% to 10%. If crosslinking monomers (D) are present, their amount ought as a general rule not to exceed 5%, preferably 2% by weight, based on the total amount of all the monomers used for the method. It can, for example, be 10 ppm to 1% by weight.
In a first preferred embodiment of the invention the copolymer X1 comprises, besides (A), at least one monomer (B). Preferably there are no further monomers (C) or (D) besides the monomers (A) and (B). The amount of (A) in the case of this embodiment is preferably 60% to 90% by weight, and the amount of (B) 10% to 40% by weight. With particular preference, in the case of this embodiment, the copolymer X1 is a copolymer of acrylic add and maleic acid or of acrylic acid and itaconic add in the aforementioned amounts.
In a second preferred embodiment of the invention the copolymer X1 comprises, besides (A), at least one monomer (B1) and at least one monomer (B2). Furthermore, with particular preference, there are no further monomers (D) besides the monomers (A), (B1), and (B2). The amount of (A) in the case of this embodiment is preferably 50% to 90% by weight, the amount of (B1) 5% to 45% by weight, the amount of (B2) 5% to 45% by weight, and the amount of (D) 0% to 20% by weight. With particular preference the copolymer is a copolymer X1 of acrylic acid, maleic acid, and vinylphosphonic acid in the aforementioned amounts.
In a third preferred embodiment, of the invention the copolymer X1 comprises at least one monomer (B2) and at least one monomer (C) besides (A). The amount of (A) in the case of this embodiment is preferably 20% to 60% by weight, the amount of (B2) 20% to 60% by weight, the amount of (C) 1% to 40% by weight, and the amount of (D) 0% to 20% by weight. With particular preference the copolymer in question is a copolymer X1 of acrylic acid, vinylphosphonic acid, and hydroxyethyl acrylate and/or hydroxypropyl acrylate.
The preparation of the polymers X may take place in accordance with methods that are known to the skilled worker. Preferably the copolymers are prepared by free-radical addition polymerization of the stated components (A), (B), and, optionally, (C) and/or (D) in aqueous solution. Details concerning the conduct of a free-radical addition polymerization are known to the skilled worker. Preparation processes for the copolymers X1 are described in, for example, WO 2006/021308 or WO 2006/134116 at page 9 line 38 to page 13 line 24.
The synthesized copolymers X1 can be isolated from the aqueous, solution by means of typical techniques known to the skilled worker, as for example by evaporating down the solution, spray drying, freeze drying or precipitating. Preferably, however, the copolymers X1 after the polymerization are not isolated at all from the aqueous solution; instead, the resulting solutions of the copolymers—following addition of further additives if desired—are used as they are 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 from the start to be calculated 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 only when in situ is diluted with water or, optionally, other solvent mixtures to the desired concentration.
The molecular weight M_w(weight average) of the polymers X and/or copolymers X1 used for the method of the invention is specified by the skilled worker in accordance with the desired application. It is possible, for example, to use polymers having a molecular weight M_wof 3000 to 1 000 000 g/mol. Polymers having proven appropriate include more particularly 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 formulation employed may optionally comprise further components above and beyond the polymer X and the stated acid.
The formulation may, for example, optionally comprise metal ions or metal compounds. If metal ions or metal compounds are to be present, however, the formulations in question should preferably not comprise chromium compounds. Moreover, there ought preferably to be no metal fluorides or complex metal fluorides present. In other words, preferably, the passivation of the invention is a chromium-free passivation, more preferably a chromium-free and fluoride-free passivation.
The metal ions may be metal ions selected from the group consisting of Zn²⁺, Mg²⁺, Ca²⁺ or Al³⁺. Preference is given to Zn²⁺ or Me, and especial preference to Zn²⁺. Besides these the preparation preferably comprises no further metal ions. The ions may take the form of hydrated metal ions, or else may take the form of dissolved compounds—complex compounds, for example. More particularly the ions may have complex bonds to the acidic groups of the polymer.
If present, the amount of the metal ions from the group consisting of Zn²⁺, Mg²⁺, Ca²⁺ or Al³⁺ is 0.01% to 20%, preferably 0.5% to 18%, and more preferably 1% to 15% by weight, based in each case on the total, amount of all the polymers X in the formulation.
The formulation may further comprise at least one dissolved phosphate ion. The ion in question may comprise all kinds of phosphate ions. The species in question may be, for example, orthophosphates or diphosphates. For the skilled worker it is clear that in aqueous solution, depending on pH and concentration, there may be an equilibrium between the different dissociation states of the ions. Moreover, the formulation may comprise methanesulfonate ions.
Where phosphate ions are present, the metal ions and phosphate ions may be used preferably in the form of soluble salts comprising both ions. Examples of such compounds comprise Zn₃(PO₄)₂, ZnH₂PO₄, Mg₃(PO₄)₂or Ca(H₂PO₄)₂, and corresponding hydrates thereof.
The metal ions and phosphate ions may alternatively be added separately from one another. For example, the metal ions can be used in the form of the corresponding nitrates, alkanesulfonates or carboxylates, acetates for example, and the phosphates in the form of phosphoric acid. It is also possible to use insoluble or low-solubility compounds, such as the corresponding carbonates, oxides, oxide hydrates or hydroxides, for example, which are dissolved under the influence of add.
Similarly, metal ions and methanesulfonate ions can be used together as, metal salts of methanesulfonic acid, such as Zn(CH₃SO₃)₂, for example, or else separately in the form of other metal salts and methanesulfonic acid.
The amount of the phosphate ions and/or methanesulfonate ions in the formulation is specified by the skilled worker in accordance with the desired properties of the formulation. Where they are present, their amount is generally 0.01% to 20%, preferably 0.5% to 25%, more preferably 1% to 25%, by weight, calculated in each case as orthophosphoric acid and based in each case on the polymers X.
The formulation may optionally further comprise at least one wax dispersed in the formulation. It will be appreciated that mixtures of different waxes can also be used. The term “wax” here comprises not only wax itself but also auxiliaries that may be used to form a wax dispersion. The skilled worker is aware of waxes for use in aqueous dispersions, and will make an appropriate selection. The waxes may, for example, be montan waxes, polyethylene waxes, waxes based on oxidized polyethylene, based on fluorinated polyethylene such as PTFE or other polymers based on C, H, and F. The term “polyethylene” is also intended to comprise copolymers of ethylene and other monomers, more particularly of other olefins such as propylene, for example. Ethylene copolymers of this kind preferably comprise at least 65% by weight of ethylene. Through the addition of waxes it is possible advantageously to lower the friction of the surface with the surface of the tools used for shaping.
The amount of optionally employed waxes is determined by the skilled worker in accordance with the desired properties of the passivation coat. An amount which has proven appropriate is in general from 0.01% to 70%, preferably 0.5% to 25%, and more preferably 1% to 10%, by weight, based in each case on the polymer X.
Further components which can be used optionally for the formulation comprise surface-active compounds, corrosion inhibitors, complexing agents, typical electroplating assistants, or else other polymers to be distinguished from the polymers X used in accordance with the invention.
The skilled worker makes an appropriate selection from the optional components that are possible in principle, and with respect to their amounts as: well, in accordance with the desired application. As a general rule, however, the amount of optional components alongside the polymer X ought not to be more than 20%, preferably not more than 10%, and more preferably not more than 5% by weight, based on the polymers X.
The formulations for use in accordance with the invention can be obtained by simple mixing of the components. If waxes are used they are preferably first dispersed separately in water and, in the form of a dispersion, mixed with the other components. Wax dispersions of this kind are also available commercially.
The method of the invention is performed using galvanized steel strips which are passivated on a coil coating line by means of a continuous process.
Galvanized steel strips typically have a thickness of 0.2 to 3 mm and a width of 0.5 to 2.5 m. Galvanized steel strips are available commercially for a very wide variety of applications. The skilled worker selects a suitable steel strip in accordance with the desired end use.
The epithet “galvanized” also applies, of course, to steel strips which have been coated with Zn alloys. These may be hot dip galvanized or electrolytically galvanized steel strips. Zn alloys for coating steel are known to the skilled worker. Depending on the desired end application, the skilled worker selects the nature and amount of alloying constituents. Typical constituents of zinc alloys comprise more particularly Al, Mg, Pb, Si, Mg, Sn, Cu or Cd, preferably Al or Mg. The alloys in question may also be Al Zn alloys in which Al and Zn are present in approximately the same amount. The coatings in question may be largely homogeneous coatings or else coatings which exhibit concentration gradients. The alloys may with further preference be Zn Mg alloys. In this case the steel may be steel coated with a Zn Mg alloy, hot dip galvanized steel for example, or may be galvanized steel which has additionally been vapor-coated with. Mg. This may produce a Zn/Mg alloy at the surface.
The strips may be galvanized on one or both sides. In the case of strips galvanized on both sides, the method of the invention can be used to passivate both sides or else only one side.
The method of the invention is performed using a coil coating line. The section of the line in which the strip is coated is shown diagrammatically in FIG. 2. Beyond the sections shown, the coil coating line of course comprises further typical components, such as a means of unwinding the raw steel strip from coil, a means of winding the coated strip, and a drive means for the strip. It may further optionally comprise additional typical components, such as cleaning stations, for example. The steel strips may be run through the line at a speed, for example, of 30 to 200 m/s, preferably 50 to 150 m/s.
To effect coating, the line that is used comprises two mutually opposed rolls (4) and (4′). Between the two rolls (4), (4′) there remains a gap (12) which is adjustable in its width. For this purpose, one roll or else both rolls can be provided with an appropriate mechanism for adjusting the roll position. For the performance of the method, the galvanized steel strip is moved in its longitudinal direction through the adjustable gap (12) between the rolls (4), (4′). The rolls rotate in the direction of travel (2) of the steel strip.
The rolls (4), (4′) may, in a way which is known in principle, comprise a steel core bearing on its outer face a polymeric coating. A polymeric coating may be composed, for example, of EPDM rubber and may be abraded if required. Rolls which have proven particularly appropriate for the performance of the method of the invention are those having a relatively hard surface, examples being those having a Shore A hardness of at least 60, preferably at least 70, and more preferably at least 80. By way of example it is possible to use a roll of 90 to 98 Shore A. The surface roughness of the rolls can be chosen by the skilled worker in accordance with the desired outcome. The drive of the rolls ought preferably to be capable of regulation independently from the drive of the steel strip. In this way the rolls can also move at a different speed from the strip, thereby allowing a certain slip to be set between the steel strip and the rolls (4), (4′). The adjustability of the gap (12) between the two rolls allows the pressure exerted by the rolls to be varied.
In accordance with the invention, for passivation, the formulation described is applied using a suitable applicator means (10) to at least one of the two application rolls (4) and/or (4′). FIG. 2 shows diagrammatically the case where only one side is passivated. Of course, application may also take place to both rolls, thereby allowing both sides of the galvanized steel strip, to be passivated. In that case the second roll (4′) as well has an applicator means (10′). This is shown diagrammatically in FIG. 3.
There is no restriction here on the nature of the applicator means (10) and (10′). For example, the formulation can be applied to the roll by spraying, with the aid of appropriate nozzles, or transferred to the roll using a pouring means, a pouring gap. Furthermore, the formulation can also be transferred to the roll via an open application chamber. Suitable constructions of open application chambers are known to the skilled worker from the field of the coating of paper or cardboard webs, and are shown in, for example, U.S. Pat. No. 2,970,564, DE 34 17 487 A1 or DE 37 35 889 A1. The applicator may also comprise a roll applicator mechanism in which the formulation is taken from a reservoir vessel by means of a suitable Scoop roll and is transferred, directly or via further rolls, to the application rolls (4), (4′). Preference is given to methods wherein the formulation is applied directly to the rolls (4), (4′) and not transferred indirectly via further rolls from the reservoir vessel to the application rolls (4), (4′).
In accordance with the invention the coil coating line used comprises, between the applicator means (10) and the gap (12) in the direction of rotation of the roll (4) and/or (4′), at least one doctor means (11) and/or (11′) with which, by removal of excess formulation, it is possible to set the amount of the aqueous formulation per unit area of the roll. In this case, at the same time, a more uniform film on the roll is obtained. The excess formulation removed can be returned to the coating operation. Since at this stage of the method the formulation is not yet in contact with the zinc surface, the returned formulation is also not contaminated with detached zinc, and is therefore unchanged in its rheological properties. This is a great advantage, in contrast to the above-described prior-art passivating methods. The formulation can be returned by means, for example, of excess formulation removed from the doctor means (11) simply running back into the applicator means, or, for example, by the formulation being collected and conveyed back into the reservoir container, by means of pumps, for example. The amount of formulation transferred to the roll (4) and/or (4′) per unit area may of course be controlled not only by the doctor means, but also, if desired, by the setting of the applicator means (10) and also by further parameters, such as the speed of the application rolls (4) and/or (4′), for example.
As the doctor means it is possible in principle to use the doctor means that are known to the skilled worker. The doctor means may be composed of metal, plastic, coated metal, glass or ceramic.
The doctor means (11) may for example be a doctor knife or a doctor blade. The blade may be a smooth one or may also have notches on its surface. The amount of the formulation which is intended to remain on the roll (4) and/or (4′) may be accomplished in a typical way by the setting of the gap between the doctor knife and the roll (4) and/or (4′).
In one preferred embodiment of the invention the doctor means (11) and/or (11′) comprises at least one rotating cylindrical doctor rod. Rotating doctor rods are known in principle to the skilled worker. They typically have a diameter of a few cm, 1 to 3 cm for example, without any intention that the invention should be restricted to this. The rotating doctor rod is mounted rotatably in a suitable device and is pressed against the roll (4) and/or (4′), the exerted pressure preferably being adjustable. The rotating doctor rod may be set in rotation through the rotation of the rolls (4), (4′), but preferably is actively driven. As a result of this it is possible to set the speeds of the rolls (4) and/or (4′) and also of the rotating doctor rod separately from one another, thereby allowing a certain slip to be set between the roll and the rotating doctor rod. The rotating doctor rod (1.1) and/or (11′) may run in the same direction or else—where actively driven—in the opposite direction of rotation to the rolls (4) and/or (4′). The rotating doctor rod is preferably closely adjacent to the applicator means (11). Examples of such arrangements are found in. U.S. Pat. No. 2,970,564, DE 34 17 487 A1 or DE 37 35 889 A1, without any intention that the invention should be thereby restricted to those embodiments. It is possible, for example, to arrange the rotating doctor rod such that the polymer formulation is fed from the top side directly into the cavity formed by applicator roll and rotating doctor rod, by pouring or spraying, for example.
The amount of the formulation that is intended to remain on the application rolls (4) and/or (4′) can be set via the nature of the rotating doctor rod. The rotating doctor rod used preferably comprises grooves in the surface—that is, the surface of the rotating doctor rod is not planar but instead has indentations, such as indentations in line or dot format. The nature of the grooves, such as, for example, the nature, depth, and density of the indentations, enables setting of the amount of formulation which is intended to remain on the roll. For this purpose it is possible to hold in stock a plurality of different rotating doctor rods, with different grooves, and to install them on the coil coating line in accordance with the nature of the formulation used for passivating. Alternatively the rotating doctor rod can also be wound with a wire, or may constitute a wire spiral.
The film located on the roll (4) and/or (4′), on contact with the galvanized surface of the steel strip, is transferred wholly or partly to the galvanized surface. Here, the amount of formulation located on the application rolls (4) and/or (4′) per unit area is advantageously such that no build-up, or at least no substantial build-up, is formed ahead of the gap (12). The transfer of the film from the roll, (4) and/or (4′) to the galvanized surface should be very largely complete. The amount is further made such as to result in the film thickness desired for the specific application. It has been found appropriate to use an amount of less than 5 g/m²of the aqueous formulation, based on an approximately 20% formulation.
After the aqueous formulation has been applied, it reacts with the galvanized surface, with formation of hydrogen. The acidic groups of the polymer anchor the polymer to the surface, and the detached Zn²⁺ ions crosslink the polymer coat.
The wet film, finally, is dried in a drying means (6) disposed downstream of the pair of rolls in the direction of travel (2) of the strip. In this context it is possible to make use, for example, of forced-air dryers or IR dryers. The drying temperature is set by the skilled worker in accordance with the formulation used and with the desired properties of the coat. A temperature of 30 to 95° C. and preferably 40 to 80° C. has been found appropriate, in each case measured as the peak metal temperature. The temperature of the forced air in a forced-air dryer may of course also be higher.
The time from application of the formulation by the rolls (4) and/or (4′) to entry into the dryer, in other words the reaction time, can be influenced by the skilled worker, in a way which is known in principle, by the strip speed and/or by the distance of the dryer from the point of application.
After passage through the dryer, a dried passivation coat remains on the galvanized steel strip. The dry film thickness can be determined by the skilled worker via the amount of formulation applied, by means of the measures already outlined. The film thickness is guided in each case by the desired properties of the coat.
The apparatus used for the method of the invention may of course also comprise further components. Thus it is possible in principle for each of the application rolls (4) and/or (4′) to possess more than one doctor means (11). It is possible, furthermore, for further means for smoothing the passivation coat to be provided between the application rolls (4), (4′) and the dryer—additional doctors and/or rolls, for example.
Using the method of the invention it is possible to achieve dry coat thicknesses of less than 1 μm, preferably less than 0.6 μm, and more preferably 0.01 to 0.4 μm, which are nevertheless very uniform and exhibit a high corrosion resistance. On the basis of the high concentration at which it is used, the polymer reacts very quickly, so that there is virtually no longer any afterreaction of the acidic polymer and hence also no longer any whitening of the passivation coat.
The examples below are intended to illustrate the invention in more detail:

Polymer Solution Used:

The experiments were carried out using a 25% strength solution of a copolymer of 70% by weight acrylic acid and 30% by weight vinyiphosphonic acid in water.

COMPARATIVE EXAMPLE

For the comparative example a commercial laboratory squeeze-off device was used.
The device is composed of two counter-rotating steel rolls (total diameter 100 mm; length 350 mm) each provided with a rubber lining (Shore A hardness: 80). The rollers are pressed against one another with a linear pressure of approximately 10-15 N/mm, and rotated with a speed of approximately 25 m/min.
For the coating experiment, a commercial steel sheet for testing use (200 mm×100 mm) was immersed completely into the above solution for approximately 1 s and then immediately squeezed off by means of the squeeze-off device, the metal sheet being introduced into the gap between the two rolls, and so transported through the gap by the rollers. The rollers had been wetted beforehand with the polymer solution. After squeezing off, the treated metal sheet is immediately dried for a number of seconds in an oven at a PMT (peak metal temperature) of 50° C.
The thickness of the polymer film on the metal after drying was 370 nm.

INVENTIVE EXAMPLES

For the inventive examples, a smooth rotating doctor rod (diameter: 12 mm) was installed on the upper of the two rollers. The modified device is shown diagrammatically below.
The rotating doctor rod is held by a mount so that the pressure of the rod on the roll can be altered. By virtue of its construction, the rotating doctor rod is secured against sagging. By means of a metering device, the polymer solution is fed from the top side directly into the gap between the rotating doctor rod and the press-off roll, in such a way that there is always a small column of liquid between the rolls, but the solution does not overflow. The pressure between the two rollers and also their speed of rotation remain as above.
For coating, a galvanized steel sheet was, introduced into the gap between the two rollers, and so was transported through the gap by the rollers. The metal sheet was then dried as above.
Three experiments were carried out with different pressures of the rotating doctor rod. The results are compiled in the table below.


Example	Pressure of rotating doctor rod	Thickness of the polymer film

1	0.25 bar	90 nm
2	0.5 bar	40 nm
3	1.0 bar	30 nm

The examples show that, by means of the method of the invention, it is possible to obtain significantly thinner polymer films than in the case of the conventional method by direct application and squeezing-off of the excess. Furthermore, by means of doctor rods with different surface structuring, it is additionally possible to exert very precise control within a wide range of film add-ons.

Claims

1. A method of continuously passivating a galvanized steel strip with an acidic aqueous formulation comprising at least one water-soluble polymer X comprising acidic groups,

the polymer X having at least 0.6 mol of acid groups/100 g of polymer,

the pH of the formulation being not more than 5, and

the amount of all the polymers X together being 10% to 30% by weight, based on the amount of all the components of the formulation,

and the galvanized steel strip being moved in its longitudinal direction through an adjustable gap between two opposing rolls, which rotate in the direction of travel of the steel strip, and wherein

the acidic aqueous formulation is applied to at least one of the two rolls using an applicator means, and between the applicator means and the gap in the direction of rotation of the roll there is at least one doctor means with which, by removal of excess formulation, it is possible to set the amount of the aqueous formulation per unit area of the roll,

the film located on the roll, on contact with the galvanized surface of the steel strip, is transferred wholly or partly to the galvanized surface, and

the wet film is dried in a drying means disposed downstream of the pair of rolls in the direction of travel of the strip.

2. The method according to claim 1, wherein the doctor means comprises a means comprising at least one rotating doctor rod.

3. The method according to claim 2, wherein the rotating doctor rod has grooves in the surface.

4. The method according to claim 1, wherein the doctor means comprises a means comprising at least one doctor knife.

5. The method according to claim 1, wherein formulation removed by the doctor means is returned to the coating operation.

6. The method according to claim 1, wherein the amount of the aqueous formulation per unit area of the roll is set such that no build-up is formed ahead of the gap.

7. The method according to claim 1, wherein the thickness of the dried passivation coat formed is less than 1 μm.

8. The method according to claim 1, wherein the acidic groups of the polymer X are groups selected from the group consisting of carboxyl groups, sulfonic acid groups, phosphoric acid groups, and phosphonic acid groups.

9. The method according to claim 1, wherein the acidic groups of the polymer X are groups selected from the group consisting of carboxyl groups, phosphoric acid groups, and phosphonic acid groups.

10. The method according claim 1, wherein the acidic, water-soluble polymer X is a homopolymer or copolymer comprising (meth)acrylic acid units.

11. The method according to claim 1, wherein the acidic, water-soluble polymer X is a copolymer X1 which is composed of the following monomeric units—based in each case on the amount of all the monomers copolymerized into the copolymer:

(A) 30% to 90% by weight of (meth)acrylic acid,

(B) 10% to 70% by weight of at least one further monoethylenically unsaturated monomer, different from (A), which has one or more acidic groups, and also

(C) optionally 0% to 40% by weight of at least one (meth)acrylic ester having OH groups,

(D) optionally 0% to 30% by weight of at least one further ethylenically unsaturated monomer, different from (A), (B) or (C).

12. The method according to claim 11, wherein the monomers (B) are monomers selected from the group consisting of

(B1) monoethylenically unsaturated dicarboxylic acids having 20 to 23 carbon atoms, and/or

(B2) monoethylenically unsaturated phosphoric and/or phosphonic acids.

13. The method according to claim 1, wherein the amount of monomer (C) is 1% to 30% by weight.

14. The method according to claim 1, wherein not more than 25 mol % of the acid groups of the polymer X are neutralized.

15. The method according to claim 1, wherein the amount of acids in the formulation does not exceed 20% by weight relative to the amount of all the polymers X together.

16. (canceled)

17. The method according to claim 2, wherein formulation removed by the doctor means is returned to the coating operation.

18. The method according to claim 3, wherein formulation removed by the doctor means is returned to the coating operation.

19. The method according to claim 4, wherein formulation removed by the doctor means is returned to the coating operation.

20. The method according to claim 2, wherein the amount of the aqueous formulation per unit area of the roll is set such that no build-up is formed ahead of the gap.

21. The method according to claim 3, wherein the amount of the aqueous formulation per unit area of the roll is set such that no build-up is formed ahead of the gap.