EP2201150A1

EP2201150A1 - Method for coating zinc-plated steel strips with aqueous formulations of acidic polymers

Info

Publication number: EP2201150A1
Application number: EP08805018A
Authority: EP
Inventors: Walter Bertkau
Original assignee: BASF SE
Current assignee: BASF SE
Priority date: 2007-10-05
Filing date: 2008-10-02
Publication date: 2010-06-30
Also published as: KR20100087299A; WO2009047209A1; JP2010540237A; US20100224286A1

Abstract

The invention relates to a continuous method for passivating zinc-plated steel strips using acidic aqueous formulations that comprise at least one water-soluble polymer containing acidic groups.

Description

Process for coating galvanized steel strips with aqueous formulations of acidic polymers

description

The invention relates to a continuous process for passivating galvanized steel strips with acidic aqueous formulations comprising at least one water-soluble polymer comprising acidic groups.

For the production of sheet-like workpieces made of steel, such as automotive parts, body parts, equipment panels, cladding, ceiling panels or window profiles are used today as raw material usually long steel strips produced by hot rolling and / or cold rolling of steel blocks and for storing and transporting to roles (so-called The steel strips can then be cut up and processed into the desired shaped parts.

The steel strips are usually protected by suitable measures against corrosion. These are generally multilevel processes. In a first step, the steel strips are usually coated with zinc or zinc alloys. The effect of zinc is based on the one hand, that it is less noble than steel and therefore first itself corroded. The steel surface remains intact as long as it is still covered with zinc throughout. Furthermore, in the presence of atmospheric oxygen on the surface of Zn or Zn alloys, a thin oxide layer is formed, which more or less slows down the corrosive attack on the underlying metal, depending on the external conditions.

In order to enhance the protective effect of such an oxide layer, the Zn surfaces are usually subjected to an additional passivation treatment. In the course of such treatment, a portion of the metal to be protected dissolves, and is at least partially incorporated into a film on the metal surface. Instead of the term "passivation layer", the terms "conversion layer", "aftertreatment layer" or "pretreatment layer" are also used interchangeably.

It is known to perform such a passivation by treating the galvanized steel surface with acidic Cr (VI) and / or Cr (III) solutions. Increasingly, chromium-free formulations are also used for this purpose, e.g. phosphate-containing formulations or formulations containing various polymers.

An important class of polymers which can be used in formulations for passivation treatments are strongly acidic, water-soluble polymers, such as for example, polyacrylic acid or copolymers of acrylic acid with other monomers, in particular other acidic monomers such as vinylphosphonic acid, maleic acid or itaconic acid. The use of such polymers for passivation is disclosed, for example, in WO 2004/074372, WO 2005/042801, WO 2006/021308, 2006/021309, WO 2006/1341 16, WO 2006/1341 17 or WO 2006/134118. The polymers in each case form the main constituent of the formulations, in particular further acids, for example phosphoric acid, are present in the formulation only in small amounts, if at all. In the course of passivation, the acid groups of the polymer dissolve zinc and possibly other metals with hydrogen formation from the surface. The zinc ions formed on the one hand re-deposit on the surface, but can also crosslink the polymer layer via complex formation with the acid groups of the polymer and optionally other functional groups of the polymer, so that a very dense passivation layer is obtained.

The passivation of galvanized steel strips is usually carried out by means of a continuous process, which is shown schematically in Figure (1). A galvanized steel strip (1) is moved by means of drive rollers (not shown) in the direction (2). By means of a spray station (3), the formulation is sprayed onto the steel strip for passivation and forms a moist film (7). By means of two squeezing rollers (4) and (4 '), excess treatment solution is squeezed off. The result is a thin, damp film (8), which is finally dried in a dryer (6). This gives a galvanized steel strip with passivation layer (9). The squeezed formulation can be collected (5) and used again for coating.

This process has a number of disadvantages when passivated with formulations of strongly acidic, water-soluble polymers. To achieve a satisfactory passivation result, the acidic polymers should be used in concentrations of at least 10% by weight with respect to the formulation used. At lower concentrations, the pH of the formulations is too high so that a sufficiently rapid acid attack on the zinc surface can not take place and poorer passivation layers are obtained. Furthermore, after the application of the passivation layer, unreacted acidic groups may subsequently react further over time, which may result in white discoloration of the passivation layer (so-called "staining") .Such discoloration is highly undesirable.

Formulations containing more than 10% by weight of strongly acidic, water-soluble polymers have a significantly higher viscosity than formulations which contain predominantly low molecular weight components, for example customary formulations based on Cr (VI), Cr (III) or phosphoric acid. When using such highly viscous formulations, a bead (8) is formed in front of the squeeze roller (FIG. 1, (4)). However, the formation of a bead is associated with a number of disadvantages: Since the residence time of individual polymer molecules in the bead is not constant but statistically distributed, the passivation reaction in the bead is uncontrolled. As a result of the reaction with the zinc surface, zinc increasingly settles in the bead. Since zinc increasingly cross-links the polymer formulation via complex formation with the acid groups of the polymer, the viscosity of the formulation changes in the bead. In addition, the squeezed-out formulation becomes contaminated with zinc, which ultimately can not be reused. Due to the viscosity in the bead, there is also an increasing danger that air bubbles will form which, when passing through the squeeze rolls, cause some areas on the strip surface to no longer be covered by a passivation layer. Due to the conditions mentioned, it is very difficult to apply thin homogeneous passivation layers, which ensure good corrosion protection. The dry film thickness of the formed passivation layers is in the illustrated process engineering usually more than 1 micron, the thickness is hard to reduce even by harder rolls.

In the area of coating with lacquers, multi-roll systems for application with 2, 3 or more rolls for applying the lacquer are known (see, for example, "coil coating" in Rompp Lexikon Lacke und Druckfarben, page 55, Georg Thieme Verlag Stuttgart, New York, 1998) Multi-roll systems are very complex and expensive.

In the field of paper coating devices are known, with which one applies paper coating slips, such as adhesive layers, on a coating roller and even before contacting the paper by means of a suitable squeegee device scraps off excess material. For example, reference is made to DE 37 35 889 A1, DE 198 00 955 A1 or US Pat. No. 2,970,564. The use of such techniques for the passivation of galvanized steel strips with formulations of strongly acidic polymers has not hitherto been known.

The object of the invention was to provide a continuous process for passivating galvanized steel strips with aqueous formulations of strongly acidic, water-soluble polymers, in which formulations with at least 10% by weight of polymers can be processed satisfactorily into thin passivation layers. It should be as simple as possible process technology with which existing systems for passivation of galvanized steel strips can be easily retrofitted.

Surprisingly, it has been found that known techniques from the field of paper and board web coating are also suitable for applying passivation layers to galvanized steel strips. As a result, it is also possible to apply very thin passivation layers of high quality. Accordingly, a continuous process has been found for passivating galvanized steel strips with an acidic aqueous formulation comprising at least one water-soluble, acidic group-containing polymer X, wherein

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

• the pH of the formulation is not more than 5, and

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

and the galvanized steel strip is moved in its longitudinal direction through an adjustable gap (12) between two opposing rolls (4), (4 ') rotating in the running direction (2) of the steel strip, and wherein

^■ the acidic aqueous formulation of at least one of the two rolls by means of an applicator device (10) is applied and in the direction of rotation of the roll between the applicator device (10) and the gap (12) at least one doctor blade device (1 1) is arranged, with the by separation excess formulation, the amount of aqueous formulation per unit area of the roll is adjustable,

^■ the film on the roller is completely or partially transferred to the galvanized surface on contact with the galvanized surface of the steel strip, and

^The dried film is dried in a drying device (6) arranged behind the pair of rollers in the running direction (2) of the belt.

List of pictures:

Fig. 1: Schematic representation of a passivation process according to the prior art

Fig. 2: Schematic representation of the passivation process according to the invention for the case of a one-sided passivation

Fig. 3: Schematic representation of the passivation process according to the invention for the case of a passivation on both sides More specifically, the following is to be accomplished for the invention:

For passivation by means of the process according to the invention, an acidic aqueous formulation is used which comprises at least one water-soluble, acid group-comprising polymer X. The polymers X used may be homopolymers or copolymers. Of course, mixtures of several different polymers X can be used.

As a solvent, the formulation preferably comprises only water. It may also comprise water-miscible organic solvents in small quantities. Examples include 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 rule, however, the amount of water is at least 80% by weight, preferably at least 90% by weight and very particularly preferably at least 95% by weight. The data refer to the total amount of all solvents.

According to the invention, the polymers X used have at least 0.6 mol of acid groups / 100 g of the polymer. This quantity refers to the free acid groups. The polymers preferably have at least 0.9 mol of acid groups / 100 g, more preferably at least 1 mol / 100 g and very particularly preferably at least 1.2 mol / 100 g.

The formulation used according to the invention comprises 10 to 30% by weight, preferably 12 to 30% by weight, more preferably 15 to 30% by weight and most preferably 18 to 28% by weight of the polymers X, based on the amount of all components of the formulation (including the solvents).

The term water-soluble in the context of this invention is intended to mean that the polymer or polymers X used are homogeneously water-soluble. Aqueous dispersions of crosslinked polymer particles of water-insoluble polymers as polymer X do not belong to the scope of this invention. Preferably, the polymers used X should be completely miscible with water. However, they must be water-soluble at least to such an extent that the stated concentrations in the formulation can be achieved.

The formulation used has a pH of not more than 5, in particular a pH of 0.5 to 5, preferably 1, 5 to 3.5. The pH of the preparation naturally depends on the type and concentration of the polymers X used according to the invention. It can still be influenced by other basic or acidic components in the formulation. Thus, in addition to the acidic polymers, the formulation used may also contain inorganic or organic acids or mixtures thereof. The choice of such acid is not limited, provided that there are no adverse effects along with the other components of the formulation. The skilled person makes a corresponding selection. Examples of suitable acids include phosphoric acid, phosphonic acid or organic phosphonic acids, such as 1-hydroxyethane-1, 1-diphosphonic acid (HEDP), 2-phosphono-1-butan-1, 2,4-tricarboxylic acid (PBTC), aminotri (methylenephosphonic acid) ( ATMP), ethylenediaminetetra (methylenephosphonic acid) (EDTMP) or diethylenetriaminepenta (methylenephosphonic acid) (DTPMP), sulphonic acids such as methanesulphonic acid, amidosulphonic acid, p-toluenesulphonic acid, m-nitrobenzenesulphonic acid and derivatives thereof, nitric acid, formic acid or acetic acid. Phosphorus-containing acids such as H 3 PO 4, phosphonic acid are preferably the organic phosphonic acids mentioned, HNO 3 or methanesulfonic acid. It may preferably be H3PO4 or another phosphorus-containing acid.

However, the acidity of the formulation used should be essentially due to the acid groups of the polymer. The amount of additional acids in addition to the polymers X in the formulation should therefore as a rule not exceed 50% by weight with respect to the amount of all polymers X in the formulation together. Preference should be given to 30% by weight, particularly preferably 20% by weight and most preferably 10% by weight are not exceeded. In a second, particularly preferred embodiment of the invention, no additional acids are present.

The acid groups of the polymer X should preferably be present as free acid groups. However, they can also be neutralized to a slight extent by bases such as ammonia, amines, amino alcohols or alkali metal hydroxides. Such partial neutralization can be done for pH adjustment. But it can also inevitably result in the course of the production of the polymer. It is known to the skilled person, for example, that it may be necessary in the course of

Preparation of acid group-rich polymers to neutralize a portion of the acid groups to force incorporation of the monomers into the polymer.

The degree of neutralization should, however, by no means be too high to ensure good acid attack on the zinc surface. As a 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 particularly preferably not more than 12 mol%.

The acidic groups of the polymers X are generally selected from the group of carboxyl groups, sulfonic acid groups, phosphoric or phosphonic acid groups. They are preferably carboxyl groups, phosphoric or phosphonic acid groups. pen. With particular preference, the polymer X used is a copolymer of at least two different acid group-containing monomers.

Particularly preferred for carrying out the invention homo- or copolymers are used which comprise (meth) acrylic acid units.

In a particularly preferred embodiment of the invention, the polymer X is one or more water-soluble copolymers X1 of (meth) acrylic acid units (A) and different monoethylenically unsaturated monomers with acidic groups (B). Optionally, OH-containing (meth) acrylic acid esters (C) and / or further monomers (D) may also be present as structural units. In addition, no further monomers are present.

The amount of (meth) acrylic acid (A) in copolymer X1 is from 30 to 90% by weight, preferably from 40 to 80% by weight and more preferably from 50 to 70% by weight, this information based on the sum of all monomers in the polymer is related.

The monomer (B) is at least one monoethylenically unsaturated monomer other than (A) but copolymerizable with (A) which has one or more acidic groups. Of course, several different monomers (B) can be used.

The acidic groups may preferably be a group selected from the group of carboxyl groups, phosphoric acid groups, phosphonic acid groups or sulfonic acid groups. It is preferably a group selected from the group of carboxyl groups, phosphoric acid groups or phosphonic acid groups.

Examples of such monomers include crotonic acid, vinylacetic acid, C 1 -C ₄ -halocid esters of monoethylenically unsaturated dicarboxylic acids, styrenesulfonic acid, vinylsulfonic acid, allylsulfonic acid, 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS), vinylphosphonic acid, monovinyl phosphonate, maleic acid, Fumaric acid or itaconic acid.

The amount of monomers (B) in the copolymer X1 is 10 to 70 wt.%, Preferably 20 to 60 wt.%, And particularly preferably 30 to 50 wt.%, Each based on the sum of all monomers in the polymer.

In a 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) include maleic acid, fumaric acid, methyl fumaric acid, methylmaleic acid, dimethylmaleic acid, methylenemalonic acid or itaconic acid. If appropriate, the monomers can also be used in the form of the corresponding cyclic anhydrides. Preference is given to maleic acid, fumaric acid and itaconic acid, particularly preferably maleic acid or maleic anhydride.

Examples of monomers (B2) include vinylphosphonic acid, phosphoric acid monovinyl ester, allylphosphonic acid, phosphoric acid monoallylester, 3-butenylphosphonic acid, mono (3-butenyl) phosphoric acid, (4-vinyloxybutyl) phosphoric acid, phosphonoxyethyl acrylate, methacrylic acid (phosphonoxyethyl) esters, mono (- 2-hydroxy-3-vinyloxypropyl) phosphoric acid, phosphoric acid mono- (1-phosphonoxymethyl-2-vinyloxy-ethyl) esters, phosphoric mono (3-allyloxy-2-hydroxypropyl) esters , Mono-2- (allylox-1-phosphonoxymethyl-ethyl) phosphoric acid, 2-hydroxy-4-vinyloxymethyl-1,3,2-dioxaphosphole, 2-hydroxy-4-allyloxymethyl-1,3,2-dioxaphosphole. Preference is given to vinylphosphonic acid, phosphoric acid monovinyl ester or allylphosphonic acid, particularly preferably vinylphosphonic acid.

In addition, the copolymer X1 may optionally contain at least one (meth) acrylic acid ester (C) containing OH groups. Preference is given to monohydroxy (meth) acrylates.

The monomers (C) are preferably at least one (meth) acrylic acid ester of the general formula H ² C =CHR ¹ -COOR ² , where R ^{1 is} in a manner known in principle to be H or methyl and R ^{2 is selected} from the group R ^2a , R ^2b or R ^{2c is} selected.

The radicals R ^2a are radicals of the general formula - (R ³ -O-) _n -H. Here, n is a natural number from 2 to 40. Preferably, n is 2 to 20 and more preferably 2 to 10. The radicals R ³ are each independently of one another for a divalent, straight-chain or branched alkyl radical having 2 to

4 C atoms. Examples include in particular 1, 2-ethylene radicals, 1, 2-propylene radicals, 1, 2-butylene radicals and 1, 4-butylene radicals. Of course, it may also be mixtures of different radicals. It is preferably 1, 2-ethylene and / or 1, 2-propylene radicals. Particularly preferred are only 1, 2-ethylene radicals. Preference is furthermore given to radicals (R ^2a ) which have both 1,2-ethylene and 1,2-propylene radicals, the amount of the ethylene radicals being at least 50%, preferably at least 70% and particularly preferably at least 80%, based on the total number of all radicals R is ³ . Examples of R ^2a include -CH ₂ -CH ₂ -O-CH ₂ -CH ₂ -O-CH ₂ -CH ₃ , -CH ₂ -CH (CHS) -O-CH ₂ -CH (CH ₃ ) OH, -CH ₂ -CH ( CHS) -O-CH (CHS) _-CH2 OH, - CH (CHs) -CH ₂ - 0 CH (CHs) -CH ₂ OH or -CH (CHS) -CH ₂ -O-CH ₂ -CH ( CHS) OH. The radicals R ^2b are radicals of the general formula -R ⁴ - (OH) _m . Here, m is a natural number of 1 to 6, preferably 1 to 4, more preferably 1 to 3 and for example 1 or 2. The radical R ⁴ is an (m + 1) -valent, straight-chain or branched alkyl radical 2 to 10 C-atoms, preferably 2 to 6 C-atoms and more preferably 2 to 4 C-atoms.

The alkyl radical is substituted with at least one OH group, with the proviso that no more than one OH group X per C atom is present in R ⁴ . Examples of suitable radicals R ^2b having OH groups include linear radicals of the general formula - (CH ₂ ) _m -OH, such as -CH ₂ -CH ₂ -OH, -CH ₂ -CH ₂ -CH ₂ -OH, -CH ₂ -CH ₂ -CH ₂ -CH ₂ -OH or -CH ₂ -CH ₂ -CH ₂ -CH ₂ -CH ₂ -CH ₂ -OH.

Particularly preferred radicals R ^2b for carrying out the invention are radicals selected from the group of -CH ₂ -CH ₂ -OH, -CH ₂ -CH ₂ -CH ₂ -OH, -CH ₂ -CH ₂ -CH ₂ -CH ₂ - OH, -CH ₂ -CH (CH ₃ ) -OH, -CH (CHs) -CH ₂ -OH or -CH ₂ -CH (OH) -CH ₂ -OH.

In a further preferred embodiment of the invention, at least one of the radicals R ^2b is a branched alkyl radical of the general formula - R ⁵ -CH (R ⁶ ) OH. Here, R ⁵ and R ⁶ each represent a linear or branched alkyl radical having 1 to 8 C atoms, preferably 1 to 6 C atoms and particularly preferably 1 to 4 C atoms, with the proviso that the sum of the C atoms in R ⁵ and R ^{6 is} not more than 9. R ⁵ and R ^{6 are} each preferably linear alkyl groups. R ⁶ is particularly preferably a methyl group. For example, it may be -CH ₂ -CH (CHs) -OH. In such branched (meth) acrylic acid esters, the tendency of the OH group to form further ester bonds with other COOH-containing monomers is significantly reduced. Very particularly preferred are -CH ₂ -CH (CHs) -OH and / or -CH (CHs) -CH ₂ -OH, in particular a mixture of the two radicals. (Meth) acrylic acid esters with such radicals can in a simple manner, for. B. by esterification of (meth) acrylic acid with 1, 2-propylene glycol.

The radicals R ^2c are mono- or oligosaccharide radicals, preferably monosaccharide radicals. It can in principle be all kinds of saccharides. Preference is given to using radicals derived from pentoses and hexoses, in particular from hexoses. Examples of suitable monosaccharides include glucose, mannose, galactose, fructose or ribose. Preferably, glucose-derived radicals can be used. They may also be derivatives of the saccharides, for example products resulting from the reduction or oxidation of the saccharides. In particular, these may be sugar acids such as, for example, gluconic acid. The amount of the monomers (C) in the copolymer X1 is 0 to 40% by weight, preferably 1 to 30% by weight.

In addition to the monomers (A), (B), (C) and optionally (D), optionally 0 to 30% by weight of at least one further ethylenically unsaturated monomer other than (A), (B) and (C) (D. ) are used. In addition, no other monomers are used.

The monomers (D) serve to fine-tune the properties of the copolymer X1. Of course, several different monomers (D) can be used. They are selected by the skilled person depending on the desired properties of the copolymer and further with the proviso that they must be copolymerizable with the monomers (A), (B) and (C).

They are preferably monoethylenically unsaturated monomers. In special cases, however, small amounts of monomers having a plurality of polymerizable groups can also be used. As a result, the copolymer can be crosslinked to a small extent.

Examples of suitable monomers (D) include 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. Also suitable are vinyl or allyl ethers, e.g. Methyl vinyl ether, ethyl vinyl ether, propyl vinyl ether, 2-ethylhexyl vinyl ether, vinylcyclohexyl 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 or the corresponding allyl compounds. Also used may be vinyl esters such as vinyl acetate or vinyl propionate. It is also possible to use basic comonomers, for example acrylamide and alkyl-substituted acrylamides.

Examples of crosslinking monomers include molecules having a plurality of ethylenically unsaturated groups, for example di (meth) acrylates such as ethylene glycol di (meth) acrylate or butanediol-1,4-di (meth) acrylate or poly (meth) acrylates such as trimethylolpropane tri (meth) acrylate or di (meth) acrylates of oligo- or polyalkylene glycols such as di-, tri- or tetraethylene glycol di (meth) acrylate. Other examples include vinyl (meth) acrylate or butanediol divinyl ether.

The amount of all monomers (D) used together amounts to 0 to 30% by weight, based on the total amount of the monomers used. Preferably, the amount is 0 to 20 wt.%, Particularly preferably 0 to 10%. If crosslinking monomers (D) are present, their amount should as a rule not exceed 5%, preferably 2% by weight, based on the total amount of all monomers used for the process. It may be, for example, 10 ppm to 1 wt.%. In a first preferred embodiment of the invention, the copolymer X1 contains, in addition to (A), at least one monomer (B). In addition to the monomers (A) and (B), other monomers (C) or (D) are preferably present. The amount of (A) in this embodiment is preferably 60 to 90% by weight and the amount of (B) 10 to 40% by weight. Particularly preferred in this embodiment is a copolymer X1 of acrylic acid and maleic acid or acrylic acid and itaconic acid in the abovementioned amounts.

In a second preferred embodiment of the invention, the copolymer X1 contains (A) at least one monomer (B1) and at least one monomer (B2). Further particularly preferred, apart from the monomers (A), (B1) and (B2), no further monomers (D) are present. The amount of (A) in this embodiment is preferably 50 to 90 wt%, the amount of (B1) 5 to 45 wt%, the amount (B2) 5 to 45 wt%, and the amount of (D) 0 up to 20% by weight. Particular preference is given to a copolymer X1 of acrylic acid, maleic acid and vinylphosphonic acid in the abovementioned amounts.

In a third, preferred embodiment of the invention, the copolymer X1 contains (A) at least one monomer (B2) and at least one monomer (C). The amount of (A) in this embodiment is preferably 20 to 60 wt%, the amount (B2) is 20 to 60 wt%, the amount of (C) is 1 to 40 wt%, and the amount of (D) is 0 up to 20% by weight. Particular preference is given to a copolymer X1 of acrylic acid, vinylphosphonic acid and hydroxyethyl acrylate and / or hydroxypropyl acrylate.

The preparation of the polymers X can be carried out by methods known to the person skilled in the art. The copolymers are preferably prepared by free-radical polymerization of the stated components (A), (B) and optionally (C) and / or (D) in aqueous solution. Details for carrying out a radical polymerization are known to the person skilled in the art. Preparation processes for the copolymers X1 are described, for example, in WO 2006/021308 or in WO 2006/1341 16, page 9, line 38 to page 13, line 24.

The synthesized copolymers X1 can be isolated from the aqueous solution by conventional methods known to those skilled in the art, for example by evaporation of the solution, spray drying, freeze drying or precipitation. However, the copolymers X1 are preferably not isolated at all from the aqueous solution after the polymerization, but the solutions of the copolymers obtained are - gf. after addition of further additives, used as such for the process according to the invention. In order to facilitate such direct reuse, the amount of aqueous solvent used for the polymerization should be from the beginning so dimensioned that the concentration of the polymer in the solvent is suitable for the application. It can also be first prepared a concentrate, which only on site with water or optionally diluted with other solvent mixtures to the desired concentration.

The molecular weight M _w (weight average) of the polymers X or copolymers X1 used for the process according to the invention is determined by the person skilled in the art according to the desired application. For example, polymers having a molecular weight M _w of 3000 to 1 000 000 g / mol can be used. Polymers having from 5000 g / mol to 500 000 g / mol, preferably from 10 000 g / mol to 250 000 g / mol, more preferably from 15 000 to 100 000 g / mol and very particularly preferably from 20 000 to 75 000 g, have proven particularly useful / mole.

The formulation used may optionally contain further components via the polymer X and the acid mentioned.

For example, the formulation may optionally include metal ions or metal compounds. If metal ions or metal compounds should be present, however, they are preferably formulations which contain no chromium compounds. Furthermore, preferably no metal fluorides or complex metal fluorides should be present. The passivation according to the invention is therefore preferably a chromium-free passivation, more preferably a chromium- and fluoride-free passivation.

The metal ions may be metal ions selected from the group of Zn ²⁺ , Mg ²⁺ , Ca ²⁺ or Al ³⁺ . It is preferably Zn ²⁺ or Mg ²⁺ and most preferably Zn ²⁺ . The preparation next to it preferably does not comprise any further metal ions. The ions may be present as hydrated metal ions, but they may be in the form of dissolved compounds, for example as complex compounds. In particular, the ions may have complex bonds to the acidic groups of the polymer.

If present, the amount of metal ions from the group of Zn ²⁺ , Mg ²⁺ , Ca ²⁺ or Al ^{3+ is from} 0.01% by weight to 20% by weight, preferably from 0.5% to 18% by weight and more preferably 1 to 15% by weight, based in each case on the total amount of all polymers X in the formulation.

The formulation may further comprise at least one dissolved phosphate ion. These can be all types of phosphate ions. For example, they may be orthophosphates or diphosphates. It is clear to the person skilled in the art that in aqueous solution, depending on the pH and concentration, there may be an equilibrium between the different dissociation stages of the ions. Furthermore, the formulation may include methanesulfonate ions. If phosphate ions are present, the metal ions and phosphate ions can preferably be used in the form of soluble salts containing both ions. Examples of such compounds include Zn3 (PO4) 2, ZnHbPO _4, Mg3 (PO4) 2 or Ca (H2PO4) 2, or corresponding hydrates thereof.

However, the metal ions and phosphate ions can also be added separately to one another. For example, the metal ions can be used in the form of the corresponding nitrates, alkanesulfonates or carboxylates, for example acetates, and the phosphates can be used in the form of phosphoric acid. It is also possible to use insoluble or sparingly soluble compounds, such as the corresponding carbonates, oxides, oxide hydrates or hydroxides, which are dissolved under the influence of acid.

Similarly, metal ions and methanesulfonate ions may be used together as metal salts of methanesulfonic acid such as Zn (CH 3 SO 3) 2, or else separately in the form of other metal salts and methanesulfonic acid.

The amount of phosphate ion and / or methane sulfonate ion in the formulation will be determined by those skilled in the art according to the desired properties of the formulation. If present, it is generally 0.01% by weight to 20% by weight, preferably 0.5 to 25% by weight, particularly preferably 1 to 25% by weight, in each case calculated as orthophosphoric acid and in each case based on the polymers X.

Optionally, the formulation may further contain at least one wax dispersed in the formulation. Of course, mixtures of different waxes can be used. The term "wax" here encompasses both the actual wax and any auxiliary agents which may be used to form a wax dispersion .Waxes for use in aqueous dispersions are known to the person skilled in the art and suitable choices are made Oxydated polyethylene waxes based on fluorinated polyethylene such as PTFE or other polymers based on C, H and F. The term "polyethylene" is also intended to include copolymers of ethylene and other monomers, especially other olefins such as propylene. Preferably, such ethylene copolymers comprise at least 65 weight percent ethylene. By the addition of waxes, the friction of the surface with the surface of the tools used for forming can be advantageously reduced.

The amount of optionally used waxes is determined by the person skilled in the art according to the desired properties of the passivation layer. As a rule, an amount of from 0.01 to 70% by weight, preferably from 0.5 to 25 and particularly preferably from 1 to 10% by weight, based in each case based on the polymer X, has proven useful. Other optional components for the formulation include surface-active compounds, corrosion inhibitors, complexing agents, typical electroplating aids or also other polymers to be distinguished from the polymers X used according to the invention.

The person skilled in the art will make an appropriate selection of the possible optional components as well as their quantities depending on the desired application. The amount of optional components in addition to the polymer X should, however, as a rule not be more than 20% by weight, preferably not more than 10% by weight and more preferably not more than 5% by weight, based on the polymers X.

The formulations to be used according to the invention can be obtained by simply mixing the components. If waxes are used, they are preferably first dispersed separately in water and mixed as a dispersion with the other components. Such wax dispersions are also commercially available.

To carry out the method according to the invention, galvanized steel strips are used, which are passivated by means of a continuous process in a strip coating plant.

Galvanized steel strips usually have a thickness of 0.2 to 3 mm and width of 0.5 to 2.5 m. Galvanized steel strips are commercially available for a wide variety of applications. The person skilled in the art selects a suitable steel strip depending on the intended use.

Of course, the term "galvanized" also includes steel strips coated with Zn alloys, which may be hot dip galvanized or electrolytically galvanized steel strips, Zn alloys for coating steel are known to those skilled in the art, depending on the desired application Typical components of zinc alloys include in particular Al, Mg, Pb, Si, Mg, Sn, Cu or Cd, preferably Al or Mg. These may also be Al-Zn alloys The coatings can be substantially homogeneous coatings or also coatings with concentration gradients, furthermore preferably Zn-Mg alloys Zn-Mg alloy coated steel, for example galvanized steel, or it may be galvanized steel which additionally steamed with Mg. As a result, a surface Zn / Mg alloy can arise. It may be unilaterally the double-sided galvanized bands. For both sides galvanized bands both sides or even one side can be passivated by the method according to the invention.

For carrying out the method according to the invention, a coil coating system is used. The part of the plant in which the strip is coated is shown schematically in Figure 2. Of course, the strip coater, in addition to the illustrated parts, also includes other common components, such as a device for unwinding the raw steel strip from roll, a device for winding up the coated strip and a drive device for the strip. It may also optionally include other common ingredients, such as cleaning stations. The steel belts can, for example, be driven through the plant at a speed of 30 to 200 m / s, preferably 50 to 150 m / s.

For coating purposes, the system used comprises two rollers (4) and (4 ') arranged opposite one another. Between the two rollers (4), (4 ') remains a gap (12), which is adjustable in width. For this purpose, one or both rollers may be provided with a corresponding mechanism for adjusting the roller position. To carry out the method, the galvanized steel strip is moved in its longitudinal direction through the adjustable gap (12) between the rolls (4), (4 '). The rollers rotate in the direction (2) of the steel strip.

The rollers (4), (4 ') can comprise a steel core in a manner known in principle, which has a plastic coating on its outside. A plastic coating may for example consist of EPDM rubber and ground if necessary. Rollers with a harder surface, for example those having a Shore A hardness of at least 60, preferably at least 70 and particularly preferably at least 80, have proven particularly suitable for carrying out the process according to the invention. For example, a roll having 90 to 98 Shore A can be used become. The surface roughness of the rolls can be selected by the skilled person depending on the desired result. The drive of the rollers should preferably be controllable independently of the drive of the steel strip. In this way, the rollers can also move at a different speed than the belt, so that a certain slip between the steel belt and the rollers (4), (4 ') is adjustable. Due to the adjustability of the gap (12) between the two rolls, the contact pressure of the rolls can be varied.

According to the invention, the formulation described for passivation is applied to at least one of the two application rollers (4) and / or (4 ') by means of a suitable application device (10). Figure 2 shows schematically the case that only one side is passivated. Of course, but can also be applied to both rollers so that both sides of the galvanized steel strip can be passivated. In this case, the second roller (4 ') on an applicator device (10'). This is shown schematically in Figure 3.

The type of application device (10) or (10 ') is not limited in this case. For example, the formulation can be sprayed onto the roll by means of suitable nozzles or transferred to the roll by means of a casting device by means of a casting 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 those skilled in the field of coating paper or board webs and are shown, for example, in US Pat. Nos. 2,970,564, DE 34 17 487 A1 or DE 37 35 889 A1. It is also a roller applicator, in which the formulation is removed from a storage vessel by means of a suitable scooping roller and transferred directly or via further rollers to the applicator rollers (4), (4 '). Preference is given to processes in which the formulation is applied directly to the rolls (4), (4 ') and is not transferred indirectly via further rolls from the storage vessel to the applicator rolls (4), (4').

According to the invention, the strip coater used in the direction of rotation of the roller (4) or (4 ') between the application device (10) and the gap (12) at least one doctor device (1 1) or (11'), with by separating excess formulation the amount of aqueous formulation per unit area of the roller is adjustable. At the same time a more uniform film is achieved on the roller. The separated, excess formulation can be recycled back to the coating. Because the formulation at this stage of the process still has no contact with the zinc surface, the recycled formulation is also not contaminated with detached zinc and thus unchanged in its rheological properties. This is a great advantage in contrast to the passivation methods according to the prior art described above. The recycling of the formulation may, for example, be accomplished by simply recycling excess formulation separated from the squeegee device (11) back into the applicator, or, for example, by collecting the formulation and returning it to the reservoir, for example by pumping. The amount of formulation per unit area transferred to the roller (4) or (4 ') can, of course, be controlled not only by the squeegee device, but optionally also by the setting of the application device (10) and by further parameters, such as the Speed of the application rollers (4) or (4 ').

In principle, doctoring devices known to the person skilled in the art can be used as the doctoring device. The doctor device may be made of metal, plastic, coated metal, glass or ceramic. The doctor device (11) can be, for example, a doctor blade or a doctor blade. It may be a smooth blade, or the blade may also have notches on its surface. The amount of formulation which is to remain on the roller (4) or (4 '), in the usual manner by adjusting the gap between the doctor blade and the roller (4) or (4') take place.

In a preferred embodiment of the invention, the doctor device (11) or (1 1 ') comprises at least one cylindrical roller doctor rod. Rolling doctor bars are generally known to the person skilled in the art. They usually have a diameter of a few cm, for example 1 to 3 cm, without the invention being restricted thereto. The roller doctor rod is rotatably mounted in a suitable device and is pressed against the roller (4) or (4 '), wherein the contact pressure is preferably adjustable. The roller doctor rod can be rotated by the rotation of the rollers (4), (4 '), but preferably it is actively driven off. As a result, the speeds of the rollers (4) or (4 ') and the roller doctor rod can be adjusted separately so that a certain slip between the roller and the roller doctor rod is adjustable. The roller doctor bar (1 1) or (11 ') can in the same or - if actively driven - also in the opposite direction of rotation to the rollers (4) and (4') run. The roller doctor bar is preferably in close proximity to the application device (11). Examples of corresponding arrangements can be found in US Pat. No. 2,970,564, DE 34 17 487 A1 or DE 37 35 889 A1, without the invention being restricted to these embodiments. It is possible, for example, that the roller doctor rod is arranged so that the polymer formulation is metered from the top side directly into the cavity formed by the applicator roll and the doctor blade, for example by casting or spraying.

The adjustment of the amount of formulation which is to remain on the applicator rollers (4) and (4 '), can be done on the nature of the roller doctor rod. Preferably, the inserted rotary doctor bar comprises a corrugation in the surface, i. The surface of the roller doctor rod is not flat, but has recesses, for example, line-shaped or punctiform depressions. About the type of corrugation, such as the type, depth and density of the wells, the amount of formulation, which should remain on the roller, set. For this purpose, a plurality of different rod rods with different corrugation can be provided and, depending on the type of formulation used for passivating, be installed in the band coating system. Alternatively, the roller doctor rod can also be wound with a wire, or it can be a wire spiral.

The film on the roller (4) or (4 ') is completely or partially transferred to the galvanized surface on contact with the galvanized surface of the steel strip. The amount of formulation on the applicator roller (4) or (4 ') tion per unit area is in this case advantageously dimensioned such that no bead or at least no substantial bead forms in front of the gap (12). It should be as complete as possible transfer of the film from the roller (4) or (4 ') carried out on the galvanized surface. The quantity is further calculated so that the desired layer thickness for the respective application results. It has proven useful to use an amount of less than 5 g / m ^{2 of} the aqueous formulation, based on an approximately 20% formulation.

After application of the aqueous formulation, it reacts with the galvanized surface to form hydrogen. The acidic groups of the polymer anchor the polymer to the surface and the detached Zn ²⁺ ions crosslink the polymer layer.

The wet film is finally dried in a running direction (2) of the tape behind the pair of rollers arranged drying device (6). For example, circulating air dryers or RT rockers can be used here. The drying temperature is set by the skilled person depending on the formulation used and the desired properties of the layer. Proves itself has a temperature of 30 to 95 ° C and preferably 40 to 80 ⁰ C, measured as "peak metal temperature". The temperature of the recirculating air in a convection dryer can in this case be of course also be higher.

The time from applying the formulation through the rollers (4) or (4 ') and the entry into the dryer, so the reaction time can be influenced by the skilled worker by the belt speed and / or the distance of the dryer from the application point in principle known ,

After passing through the dryer, a dried passivation layer remains on the galvanized steel strip. The dry layer thickness can be determined by the person skilled in the art about the applied amount of the formulation by means of the measures already described. The layer thickness depends on the desired properties of the layer.

The device used for the method according to the invention may of course comprise further components. Thus, it is in principle possible for the application rollers (4) or (4 ') each have more than one doctor device (1 1). Furthermore, it is possible to provide further devices for smoothing the passivation layer between the application rollers (4), (4 ') around the dryer, for example additional squeegees and / or rollers.

By means of the method according to the invention, dry layer thicknesses of less than 1 .mu.m, preferably less than 0.6 .mu.m and particularly preferably 0.01 to 0.4 .mu.m can be achieved, which are nevertheless very uniform and have high corrosion resistance exhibit. Due to the high use concentration, the polymer reacts very quickly, so that the acidic polymer virtually no longer reacts and, therefore, no white discoloration of the passivation layer takes place any more.

The following examples are intended to explain the invention in more detail:

Used polymer solution:

For the experiments, a 25% solution of a copolymer of 70% by weight of acrylic acid and 30% by weight of vinylphosphonic acid in water was used.

Comparative Example:

For the comparative experiment, a commercial squeezing device for the laboratory was used.

The device consists of two steel rollers running against each other (total diameter 100 mm, length 350 mm), each of which is provided with a rubber pad (hardness 80 Shore A). The rolls are pressed together with a line pressure of about 10-15 N / mm and rotated at a rotational speed of about 25 m / min.

For the coating test, a commercially available steel sheet for test purposes (200 mm × 100 mm) was completely immersed in the above solution for about 1 s and then squeezed off immediately by means of the squeezing device by inserting the sheet into the gap between the two rollers so that it was transported by the rollers through the gap. The rolls had previously been wetted with the polymer solution. After squeezing, the treated sheet is immediately dried for a few seconds in an oven at a PMT (peak metal temperature) of 50 ^° C.

The thickness of the polymer layer on the metal after drying was 370 nm.

Examples according to the invention:

For the examples according to the invention, a smooth roller doctor bar (diameter 12 mm) was attached to the upper of the two rollers. The modified device is shown schematically below.

The roller doctor rod is held by a holder such that the contact pressure of the roller doctor rod can be changed to the role. The construction of the roller scraper bar is secured against bending. The polymer solution is metered from the top by means of a metering device directly into the gap between the roller doctor blade and the Abpressrolle in such a way that always a small column of liquid between the rollers, the solution on the other hand does not overflow. The pressure between the two rollers and their speed of rotation remained as above.

For coating, a galvanized steel sheet was inserted into the nip between the two rolls so that it was transported by the rolls through the nip. Subsequently, the sheet was dried as above.

There were 3 tests carried out at different contact pressures of the rotary doctor bar. The results are summarized in the table below.

The examples show that by means of the process according to the invention significantly thinner polymer layers can be achieved than in the conventional process by direct application and squeezing of the superfluous amount. Furthermore, by means of differently superficially structured doctor bar a further layer contact area can be very finely controlled.

Claims

claims

A process for continuously passivating a galvanized steel strip with an acidic aqueous formulation comprising at least one water-soluble, acidic group-containing polymer X, wherein

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

• the pH of the formulation is not more than 5, and • the amount of all polymers X together 10 to 30 wt.% Based on the

Amount of all components of the formulation is,

and wherein the galvanized steel strip is moved in its longitudinal direction by an adjustable gap (12) between two rollers (4), (4 ') rotating in the running direction (2) of the steel strip, characterized

^■ that the acidic aqueous formulation is applied to at least one of the two rolls by means of an application device (10) and at least one doctor device (11) is arranged in the direction of rotation of the roll between the application device (10) and the gap (12) in that the quantity of aqueous formulation per unit area of the roller can be set by removal of excess formulation,

■ the film on the roller is completely or partially transferred to the galvanized surface on contact with the galvanized surface of the steel strip, and

^■ the moist film is dried in a drying device (6) arranged in the running direction (2) of the strip behind the pair of rolls.

2. Method according to claim 1, characterized in that the squeegee device (11) is a device comprising at least one roller doctor rod.

3. The method according to claim 2, characterized in that the roller doctor rod has a corrugation in the surface.

4. The method according to claim 1, characterized in that it is in the

Doctor blade device (1 1) around a device comprising at least one doctor blade.

5. The method according to any one of claims 1 to 4, characterized in that by means of the doctor device (1 1) separated formulation is returned to the coating again.

6. The method according to any one of claims 1 to 5, characterized in that one sets the amount of aqueous formulation per unit area of the roller so that no bead forms before the gap (12).

7. The method according to any one of claims 1 to 6, characterized in that the thickness of the formed, dried passivation layer is less than 1 micron.

8. The method according to any one of claims 1 to 7, characterized in that it is the acidic groups of the polymer X is selected from the group of carboxyl groups, sulfonic acid groups, phosphoric acid groups and phosphonic acid groups.

9. The method according to any one of claims 1 to 7, characterized in that it is the acidic groups of the polymer X is selected from the group of carboxyl groups, phosphoric acid groups and phosphonic acid groups.

10. The method according to any one of claims 1 to 7, characterized in that the acidic, water-soluble polymer X is a (meth) acrylic acid comprehensive homo- or copolymer.

1 1. A method according to any one of claims 1 to 7, characterized in that it is the acidic, water-soluble polymer X is a copolymer X1, which consists of the following monomeric units - in each case based on the amount of all monomers copolymerized in the copolymer- is constructed:

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

(B) 10 to 70% by weight of at least one further monomethylenically unsaturated monomer other than (A) which has one or more acidic monomers

Has groups, as well

(C) optionally 0 to 40% by weight of at least one OH group-containing (meth) acrylic ester,

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

12. Process according to claim 1 1, characterized in that the monomers (B) are monomers selected from the group of

(B1) monoethylenically unsaturated dicarboxylic acids having 4 to 7 carbon atoms, and / or

(B2) monoethylenically unsaturated phosphoric and / or phosphonic acids

is.

13. The method according to claim 1 1 or 12, characterized in that the amount of monomer (C) 1 to 30 wt.% Is.

14. The method according to any one of claims 1 to 13, characterized in that not more than 25 mol% of the acid groups of the polymer X are neutralized.

15. The method according to any one of claims 1 to 14, characterized in that the amount of acids in the formulation 20 wt.% With respect to the amount of all polymers X does not exceed together.

16. The method according to any one of claims 1 to 15, characterized in that the formulation comprises no additional acids in addition to the polymer X.