Patent Application
20220282381
The present invention relates to a method for phosphating metal surfaces in a layer-forming manner using a colloidal aqueous solution as an activation stage, containing a dispersed particulate constituent, the particulate constituent containing, in addition to dispersed inorganic compounds of phosphates of polyvalent metal cations, polymeric organic compounds as dispersing agents which are composed at least partially of styrene and/or an α-olefin having no more than 5 carbon atoms and partially composed of maleic acid, its anhydride and/or its imide, the polymeric organic compounds additionally comprising polyoxyalkylene units. The cleaning and rinsing stages preceding the activation stage as well as the activation stage itself can be carried out using service water in a resource-saving manner without any loss of activation performance, the colloidal aqueous solution containing at least 0.5 mmol/L of alkaline-earth metal ions dissolved in water.
Layer-forming phosphating is a method for applying crystalline anti-corrosion coatings to metal surfaces, in particular to materials of the metals iron, zinc and aluminum, which has been used for decades and has been studied in depth. Zinc phosphating, which is particularly well established for corrosion protection, is carried out using a layer thickness of a few micrometers and is based on corrosive pickling of the metal material in an acidic aqueous composition containing zinc ions and phosphates. In the course of the pickling process, an alkaline diffusion layer forms on the metal surface, which extends into the interior of the solution and within which sparingly soluble crystallites form, which crystallites precipitate directly at the interface with the metal material and continue to grow there. To support the pickling reaction on materials of the metal aluminum and to mask the bath poison aluminum, which in dissolved form disturbs the layer formation on materials of the metal, water-soluble compounds which are a source of fluoride ions are often added. Zinc phosphating is always initiated by activation of the metal surfaces of the component to be phosphated. Wet-chemical activation is carried out conventionally by means of contact with colloidal aqueous solutions of phosphates (“activation stage”), which, insofar as they are immobilized on the metal surface, are used in the subsequent phosphating as a growth nucleus for the formation of the crystalline coating within the alkaline diffusion layer. Suitable dispersions in this case are colloidal, mostly neutral to alkaline aqueous compositions based on phosphate crystallites, which have only small crystallographic deviations in their crystal structure from the type of zinc phosphate layer to be deposited. In this connection, WO 98/39498 A1 for example teaches in particular bi- and trivalent phosphates of the metals Zn, Fe, Mn, Ni, Co, Ca and Al, it being technically preferred for phosphates of the metal zinc to be used for activation for subsequent zinc phosphating.
An activation stage based on dispersions of bi- and trivalent phosphates requires a high level of process control in order to keep the activation performance constantly at an optimal level, in particular when treating a series of metal components. To ensure the method is sufficiently robust, foreign ions carried over from previous treatment baths or aging processes in the colloidal aqueous solution must not lead to the activation performance deteriorating. A deterioration is initially noticeable in increasing layer weights in the subsequent phosphating and ultimately leads to the formation of defective or inhomogeneous phosphate layers. When the process is carried out continuously, it is therefore necessary to prevent foreign ions from being carried over, and therefore the rinsing stages preceding the activation stage and the activation stage itself must be carried out with deionized water or additives must be added in order to increase the bath life and/or reduce the consumption of colloidal active components to such an extent that the phosphating based on colloidal aqueous solutions can be carried out economically. Until now, it has always been necessary to add additives and, in the case of activation using particulate phosphates, this step has usually included the addition of condensed phosphates for colloid stabilization and, additionally or alternatively, the use of other complexing agents for masking water hardness or polyvalent metal ions carried over from previous cleaning and rinsing stages, in order to counteract accelerated formation of colloid agglomerates and thus the sedimentation of the colloidally dispersed bath species. The dosing of condensed phosphates and/or complexing agents requires precise analytical monitoring, since there is both a process-critical minimum amount to be maintained and a system-specific upper limit, the amount of condensed phosphates and/or complexing agents falling below this minimum amount or exceeding this upper limit having a negative impact on activation performance.
Therefore, there is a need to make the activation stage of a pretreatment line for phosphating, in particular zinc phosphating, intended to be carried out on the basis of colloidal aqueous solutions of phosphates, more efficient with regard to its property of activating metal surfaces for the phosphating, and to make said stage more robust in terms of process. This relates above all to the ability of the colloidal aqueous solution to bring about activation of the metal surfaces to be phosphated both as uniformly as possible and in a comprehensive manner, with a comparatively low amount of material being used, and thus to bring about the formation of homogeneous, finely crystalline coatings in the phosphating stage, such that a high electrical charge transfer resistance and thus a correspondingly good grip of the coating is achieved in the subsequent electrocoating in addition to excellent coating adhesion properties. Furthermore, for the aspect of improved robustness of such a method, it is necessary to ensure that there is a higher tolerance to carried-over and accumulated foreign ions and high stability with regard to the structural and chemical state of the colloidal constituent. The aim of all this is to establish a pretreatment line for phosphating, in particular zinc phosphating, that can be carried out continuously in a resource-saving manner and with low technical complexity.
This complex task profile is surprisingly addressed by the use of a specific polymeric dispersing agent for stabilizing the colloidal constituent of an activation stage based on particulate phosphates. Due to the extremely efficient stabilization of the particulate constituent which brings about the activation, the special dispersing agent ensures that even comparatively low proportions of colloids are able to produce homogeneous closed phosphate coatings, without a significant drop being observable in activation performance after the stationary state of a pretreatment line has been reached, with a maintained and consistent colloid content. The use of the specific dispersing agent therefore makes it possible to dispense entirely with the addition of condensed phosphates, and therefore the technical complexity of carrying out a phosphating method continuously can be significantly reduced.
The present invention therefore relates to a method for anti-corrosion pretreatment of a metal material selected from zinc, iron or aluminum or of a component which is composed at least partially of such metal materials, in which method the metal material or the component undergoes firstly activation (i) and then phosphating (ii), in particular zinc phosphating, in consecutive method steps, the activation in method step (i) being carried out by bringing the metal material or the component into contact with a colloidal aqueous solution containing, in the dispersed particulate constituent (a) of the solution,
(a1) at least one particulate inorganic compound which is composed of phosphates of polyvalent metal cations at least partially selected from hopeite, phosphophyllite, scholzite and/or hureaulite, and
(a2) at least one polymeric organic compound which is composed at least partially of styrene and/or an α-olefin having no more than 5 carbon atoms and is composed at least partially of maleic acid, its anhydride and/or its imide, the polymeric organic compound additionally comprising polyoxyalkylene units, wherein the colloidal aqueous solution contains at least 0.5 mmol/L of alkaline-earth metal ions dissolved in water.
The dispersed particulate constituent (a) of the colloidal aqueous solution in the activation (i) of the method according to the invention is the solids content that remains after drying the retentate of an ultrafiltration of a defined partial volume of the aqueous dispersion having a nominal cutoff limit of 10 kD (NMWC: nominal molecular weight cutoff). The ultrafiltration is carried out by adding deionized water (κ<1 μScm−1) until a conductivity of below 10 μScm−1 is measured in the filtrate.
In the context of the present invention, an organic compound is polymeric if its weight-average molar mass is greater than 500 g/mol. The molar mass is determined using the molar mass distribution curve of a sample of the relevant reference value, which curve is established experimentally at 30° C. by means of size-exclusion chromatography using a concentration-dependent refractive index detector and calibrated against polyethylene glycol standards. The analysis of the average molar masses is carried out with the aid of a computer according to the strip method with a third-order calibration curve. Hydroxylated polymethacrylate is suitable as a column material, and an aqueous solution of 0.2 mol/L sodium chloride, 0.02 mol/L sodium hydroxide, and 6.5 mmol/L ammonium hydroxide is suitable as an eluent.
The high tolerance of the method according to the invention with respect to carried-over foreign ions also makes it possible for the cleaning stages and rinsing stages carried out before the activation stage and also the activation stage itself to be carried out with service water instead of deionized water. In this way, the method according to the invention is carried out in a particularly resource-saving manner. It is therefore preferred according to the invention for the colloidal aqueous solution in the activation to contain at least 1.0 mmol/L, particularly preferably at least 1.5 mmol/L, of alkaline-earth metal ions dissolved in water.
This advantage over conventional activation baths is particularly apparent when phosphating components in series, i.e., during continuous operation of a pretreatment line for phosphating. In a preferred embodiment of the method according to the invention, a large number of specific components, which at least partially consist of zinc, iron or aluminum, are therefore treated in series. Pretreatment in series is when the large number of components are brought into contact with the colloidal aqueous solution located in the system tank of the activation stage, the individual components being brought into contact with said solution one after the other and thus at different times, and the components being subsequently supplied to the phosphating process. In this case, the system tank is the container in which the colloidal aqueous solution is located for the purpose of activation for phosphating in series.
Should the tolerance of the method according to the invention reach the system-specific limits in each case with exceptionally high ionic strengths, e.g., high permanent water hardness and at the same time a high content of carried-over foreign ions from previous cleaning stages, organic complexing agents can be added to mask the foreign ions in order to maintain a long bath life. In this case, it must be assessed whether the economic advantage of it being possible to carry out the activation stage and, where necessary, preceding cleaning stages and rinsing with service water is not impeded by the addition of organic complexing agents and their technical monitoring in the system tank of the activation stage. Suitable organic complexing agents which are preferred in this context are selected from α-hydroxycarboxylic acids, which in turn are preferably selected from gluconic acid, tartronic acid, glycolic acid, citric acid, tartaric acid, lactic acid, very particularly preferably gluconic acid, and/or organophosphonic acids, which in turn are preferably selected from etidronic acid, aminotris(methylenephosphonic acid), aminotris(methylenephosphonic acid), phosphonobutane-1,2,4-tricarboxylic acid, diethylenetriaminepenta(methylenephosphonic acid), hexamethylenediamine tetra(methylenephosphonic acid) and/or hydroxyphosphonoacetic acid, particularly preferably from etidronic acid.
To maintain a stable activation performance, organic complexing agents should only be added to such an extent that the amount thereof in the colloidal aqueous solution is preferably no more than twice, particularly preferably no more than 1.5 times, the amount of alkaline-earth metal ions, and is very particularly preferably no more than equimolar to the amount of alkaline-earth metal ions.
In the method according to the invention, the phosphating can be carried out in a stable manner, i.e., largely without the addition of complexing agents, up to a water hardness corresponding to 10 mmol/L of alkaline-earth metal ions in the activation stage. In a preferred embodiment of the method according to the invention, the colloidal aqueous solution therefore preferably contains no more than 10 mmol/L, particularly preferably no more than 5 mmol/L, of alkaline-earth metal ions dissolved in water.
Another advantage of the method according to the invention compared with conventional activation methods is that the addition of condensed phosphates in the activation stage can be dispensed with. Condensed phosphates dissolved in the aqueous phase of the activation fulfill the task of masking permanent water hardness and, based on experience, the specific task of stabilizing the content of the phosphates hopeite, phosphophyllite, scholzite and/or hurealite at a colloidal level, in particular during continuous operation of a pretreatment line, and thus keeping said phosphates permanently available for the activation. It is remarkable and surprising for a person skilled in the art that, in methods according to the invention which are based on an activation stage based on the particulate constituent (a), the addition of condensed phosphates can be dispensed with.
Overall, in the context of the present invention, the addition of condensed phosphates can thus be entirely dispensed with, and therefore the activation only involves small amounts of condensed phosphates that make it to the activation stage from previous cleaning stages involving the component to be pretreated, in particular when treating a large number of components in series. In a preferred embodiment of the method according to the invention, the content of condensed phosphates dissolved in water in the colloidal aqueous solution is less than 0.25, preferably less than 0.20, particularly preferably less than 0.15, and very particularly preferably less than 0.10, based on the phosphate content of the at least one particulate compound (a1), in each case based on the element P.
Furthermore, it is preferred in this context for the content of condensed phosphates dissolved in water in the colloidal aqueous solution, calculated as P, to be less than 20 mg/kg, preferably less than 15 mg/kg, particularly preferably less than 10 mg/kg, based on the colloidal aqueous solution.
In the context of the present invention, condensed phosphates are metaphosphates and polyphosphates, preferably polyphosphates, particularly preferably pyrophosphate. The condensed phosphates are preferably in the form of compounds of monovalent cations, preferably selected from Li, Na and/or K, particularly preferably Na and/or K.
The content of condensed phosphates can be determined analytically from the difference in the total phosphate content in the non-particulate constituent of the colloidal aqueous solution with and without oxidative digestion, for example by means of peroxodisulfate, the dissolved orthophosphate content being quantified by means of photometry. Alternatively, if polyphosphates are used as condensed phosphates, enzymatic digestion with a pyrophosphatase can take place instead of oxidative digestion. The non-particulate constituent of the colloidal aqueous solution is the solids content of the colloidal aqueous solution in the permeate of the above-described ultrafiltration after it has been dried to constant mass at 105° C.—that is, the solids content after the particulate constituent (a) has been separated by means of ultrafiltration.
The colloidal aqueous solution in the activation (i) of the method according to the invention preferably has an alkaline pH, particularly preferably a pH above 8.0, more particularly preferably above 9.0, but preferably below 11.0, it being possible to use compounds which influence the pH, such as phosphoric acid, sodium hydroxide solution, ammonium hydroxide or ammonia, to adjust the pH. The “pH” as used in the context of the present invention corresponds to the negative common logarithm of the hydronium ion activity at 20° C. and can be determined by means of pH-sensitive glass electrodes.
For good activation performance, it is necessary to use polyvalent metal cations in the form of phosphates which should be contained in the dispersed particulate constituent (a) for activation in a correspondingly high proportion. Accordingly, the content of phosphates contained in the at least one particulate inorganic compound (a1), based on the dispersed particulate constituent (a) of the colloidal aqueous solution, is preferably at least 25 wt. %, particularly preferably at least 35 wt. %, more particularly preferably at least 40 wt. %, very particularly preferably at least 45 wt. %. The inorganic particulate constituent of the colloidal aqueous solution is, in turn, that which remains when the particulate constituent (a) obtained from the drying of the retentate of the ultrafiltration is pyrolyzed in a reaction furnace by supplying a CO2-free oxygen flow at 900° C. without admixture of catalysts or other additives until an infrared sensor provides a signal identical to the CO2-free carrier gas (blank value) in the outlet of the reaction furnace. The phosphates contained in the inorganic particulate constituent are determined as phosphorus content by means of atomic emission spectrometry (ICP-OES) after acid digestion of the constituent with aqueous 10 wt. % HNO3 solution at 25° C. for 15 min, directly from the acid digestion.
The active components of the colloidal aqueous dispersion which effectively promote the formation of a closed phosphate coating on the metal surfaces and in this sense activate the metal surfaces are, as already mentioned, composed primarily of phosphates, which in turn result in the formation of finely crystalline coatings, and are therefore at least partially selected from hopeite, phosphophyllite, scholzite and/or hureaulite, preferably at least partially selected from hopeite, phosphophyllite and/or scholzite, particularly preferably at least partially selected from hopeite and/or phosphophyllite and very particularly preferably at least partially selected from hopeite. An activation within the meaning of the present invention is thus substantially based on the phosphates in particulate form contained in the activation stage. Without taking into account water of crystallization, hopeites stoichiometrically comprise Zn3(PO4)2 and the nickel-containing and manganese-containing variants Zn2Mn(PO4)3, Zn2Ni(PO4)3, whereas phosphophyllite consists of Zn2Fe(PO4)3, scholzite consists of Zn2Ca(PO4)3 and hureaulite consists of Mn3(PO4)2. The existence of the crystalline phases hopeite, phosphophyllite, scholzite and/or hureaulite in the aqueous dispersion according to the invention can be demonstrated by means of X-ray diffractometric methods (XRD) after separation of the particulate constituent (a) by means of ultrafiltration with a nominal cutoff limit of 10 kD (NMWC: nominal molecular weight cutoff) as described above and drying of the retentate to constant mass at 105° C.
Due to the preference for the presence of phosphates comprising zinc ions and having a certain crystallinity, it is preferred for the formation of firmly adherent crystalline zinc phosphate coatings after successful activation, in the method according to the invention, for the colloidal aqueous dispersion to contain at least 20 wt. %, particularly preferably at least 30 wt. %, more particularly preferably at least 40 wt. %, of zinc in the inorganic particulate constituent of the colloidal aqueous solution, based on the phosphate content of the inorganic particulate constituent, calculated as PO4.
Another advantage of the method according to the invention is that even small proportions of the particulate inorganic compound (a1) in the activation (i) are sufficient for achieving the full activation performance on the materials zinc, aluminum and iron. It is therefore preferred according to the invention for the content of the dispersed particulate constituent (a) of the colloidal aqueous solution to be at least 0.05 g/kg, preferably at least 0.1 g/kg, particularly preferably at least 0.2 g/kg, but preferably no more than 10 g/kg, particularly preferably no more than 2 g/kg, very particularly preferably no more than 1 g/kg, in each case based on the colloidal aqueous solution.
However, activation within the meaning of the present invention is preferably not achieved by means of colloidal solutions of titanium phosphates, since otherwise layer-forming zinc phosphating on iron, in particular steel, is not reliably achieved. In a preferred embodiment of the method according to the invention, the content of titanium in the inorganic particulate constituent of the colloidal aqueous solution is therefore less than 0.01 wt. %, particularly preferably less than 0.001 wt. %, based on the colloidal aqueous solution. In a particularly preferred embodiment, the colloidal aqueous solution of the activation (i) contains a total of less than 10 mg/kg, particularly preferably less than 1 mg/kg, of titanium.
The activation stage in the method according to the invention can additionally be characterized by its D50 value, above which the activation performance decreases significantly. The D50 value of the colloidal aqueous solution is preferably below 1 μm, particularly preferably below 0.4 μm. In the context of the present invention, the D50 value denotes the particle diameter which is not exceeded by 50 vol. % of the particulate constituents contained in the colloidal aqueous solution. According to ISO 13320:2009, the D50 value can be determined at 20° C. from volume-weighted cumulative particle size distributions by means of scattered light analysis according to Mie theory immediately after the sample has been taken from the activation stage, using spherical particles and a refractive index of the scattering particles of nD=1.52−i·0.1.
Within the meaning of the present invention, the polymeric organic compounds (a2) which are used as dispersing agents and have polyoxyalkylene units are composed at least partially of styrene and/or an α-olefin having no more than 5 carbon atoms and of maleic acid, its anhydride and/or its imide, and cause the extremely high stability of the colloidal aqueous solution in the activation stage of the method according to the invention.
The α-olefin in this case is preferably selected from ethene, 1-propene, 1-butene, isobutylene, 1-pentene, 2-methyl-but-1-ene and/or 3-methyl-but-1-ene and particularly preferably selected from isobutylene. It is clear to a person skilled in the art that the polymeric organic compounds (a2) contain these monomers as structural units in unsaturated form covalently linked to one another or to other structural units. Suitable commercially available representatives are, for example, Dispex® CX 4320 (BASF SE), a maleic acid-isobutylene copolymer modified with polypropylene glycol, Tego® Dispers 752 W (Evonik Industries AG), a maleic acid-styrene copolymer modified with polyethylene glycol, or Edaplan® 490 (Münzing Chemie GmbH), a maleic acid-styrene copolymer modified with EO/PO and imidazole units. In the context of the present invention, polymeric organic compounds (a2) which are composed at least partially of styrene are preferred.
The polymeric organic compounds (a2) used as dispersing agents have polyoxyalkylene units which are preferably composed of 1,2-ethanediol and/or 1,2-propanediol, particularly preferably of both 1,2-ethanediol and 1,2-propanediol, the content of 1,2-propanediols in the entirety of the polyoxyalkylene units being preferably at least 15 wt. %, but particularly preferably not exceeding 40 wt. %, based on the entirety of the polyoxyalkylene units. Furthermore, the polyoxyalkylene units are preferably contained in the side chains of the polymeric organic compounds (a2). A content of the polyoxyalkylene units in the entirety of the polymeric organic compounds (a2) of preferably at least 40 wt. %, particularly preferably at least 50 wt. %, but preferably no more than 70 wt. %, is advantageous for the dispersibility.
For anchoring the dispersing agent with the inorganic particulate constituent of the colloidal aqueous solution, which is at least partially formed by polyvalent metal cations in the form of phosphates selected from hopeite, phosphophyllite, scholzite and/or hurealite, the organic polymeric compounds (a2) preferably also contain N-heterocycle units, which in turn are preferably selected from pyridine, imidazole, imidazoline, morpholine, pyrrole and/or pyrrolidone units, particularly preferably from imidazole and/or imidazoline units, more particularly preferably from imidazole units. These N-heterocycle units are each preferably part of the side chains of the polymeric organic compound (a2) and, within the side chain, are preferably aliphatically connected to the main chain via preferably at least 3 carbon atoms, particularly preferably in such a way that the polyoxyalkylene units of the polymeric organic compounds (a2) are at least partially end-capped with an N-heterocycle, and therefore, in the preferred embodiment, there are terminal N-heterocyclic groups in the polyoxyalkylene side chain. The N-heterocycle units are preferably covalently linked in the side chains of the polymeric organic compound (a2), preferably the side chains which have polyoxyalkylene units, via a nitrogen atom of the heterocycle. The N-heterocycle units are preferably present in an at least partially quaternized form, particularly preferably as N-alkylated quaternary N-heterocycle units.
In a preferred embodiment, the amine value of the organic polymeric compounds (a2) is at least 25 mg KOH/g, particularly preferably at least 40 mg KOH/g, but preferably less than 125 mg KOH/g, particularly preferably less than 80 mg KOH/g, and therefore, in a preferred embodiment, the entirety of the polymeric organic compounds in the particulate constituent (a) also have these preferred amine values. The amine value is determined in each case by weighing out approximately 1 g of the relevant reference value—organic polymeric compounds (a2) or the entirety of the polymeric organic compounds in the particulate constituent—in 100 mL of ethanol, titration being carried out using 0.1 N HCl titrant solution against the indicator bromophenol blue until the color changes to yellow at a temperature of the ethanolic solution of 20° C. The amount of HCl titrant solution used in milliliters multiplied by the factor 5.61 divided by the exact mass of the weight in grams corresponds to the amine value in milligrams KOH per gram of the relevant reference value.
The presence of maleic acid, insofar as it is a constituent of the organic polymeric compound (a2) as a free acid and not in the form of the anhydride or imide, can impart increased water solubility of the dispersing agent, in particular in the alkaline range. It is therefore preferred for the polymeric organic compounds (a2), preferably also the entirety of the polymeric organic compounds in the particulate constituent (a), to have an acid number according to DGF CV 2 (06) (as of April 2018) of at least 25 mg KOH/g, but preferably of less than 100 mg KOH/g, particularly preferably of less than 70 mg KOH/g to ensure a sufficient number of polyoxyalkylene units. It is also preferred for the polymeric organic compounds (a2), preferably also the entirety of the polymeric organic compounds in the particulate constituent (a), to have a hydroxyl number of less than 15 mg KOH/g, particularly preferably of less than 12 mg KOH/g, more particularly preferably of less than 10 mg KOH/g, determined according to method A of 01/2008:20503 from European Pharmacopoeia 9.0 in each case.
For sufficient dispersion of the inorganic particulate constituents in the colloidal aqueous dispersion, it is sufficient for the content of the polymeric organic compounds (a2), preferably the entirety of the polymeric organic compounds in the particulate constituent (a), based on the particulate constituent (a), to be at least 3 wt. %, particularly preferably at least 6 wt. %, but preferably not exceeding 15 wt. %.
In a further aspect, the present invention relates to a method for anti-corrosion pretreatment based on phosphating and involving an aqueous dispersion. Such a method according to the invention relates to the anti-corrosion pretreatment of a metal material selected from zinc, iron or aluminum or of a component which is composed at least partially of such metal materials, in which method the metal material or the component undergoes firstly activation (i) and then phosphating (ii), in particular zinc phosphating, in consecutive method steps, the activation in method step (i) being carried out by bringing the metal material or at least one metal material of the component into contact with a colloidal aqueous solution as described above which can be obtained as an aqueous dispersion diluted by a factor of 20 to 100,000, and comprises:
For the dispersed particulate constituent (A) as well as the at least one particulate inorganic compound (A1) and polymeric organic compound (A2), the same definitions and preferred specifications apply as those given above for the colloidal aqueous solution.
Due to the excellent colloid stability of the particulate constituent (A) by means of the polymeric organic compound (A2) as a dispersing agent, the dilution is preferably carried out with deionized water (κ<1 μScm−1), particularly preferably with service water, in order to make the method according to the invention as resource-saving as possible. In the light of the underlying technical application, service water contains at least 0.5 mmol/L of alkaline-earth metal ions.
The presence of a thickener according to component (B) gives the aqueous dispersion, in combination with its particulate constituent, thixotropic flow behavior and thereby contributes to preventing the irreversible formation of agglomerates in the particulate constituent of the dispersion, from which primary particles cannot be detached. The addition of the thickener is preferably to be controlled such that the aqueous dispersion has a maximum dynamic viscosity of at least 1000 Pa·s, but preferably of below 5000 Pa·s, at a temperature of 25° C. in the shear rate range of from 0.001 to 0.25 reciprocal seconds, and preferably exhibits shear thinning behavior at 25° C. at shear rates above that which is present at the maximum dynamic viscosity, i.e., a decrease in viscosity as the shear rate increases, such that the aqueous dispersion as a whole has thixotropic flow behavior. The viscosity over the specified shear rate range can be determined using a cone and plate viscometer with a cone diameter of 35 mm and a gap width of 0.047 mm.
A thickener according to component (B) is a polymeric chemical compound or a defined mixture of chemical compounds which, as a 0.5 wt. % constituent in deionized water (κ<1 μScm−1) at a temperature of 25° C., has a Brookfield viscosity of at least 100 mPa·s at a shear rate of 60 rpm (=rounds per minute) using a size 2 spindle. When determining this thickener property, the mixture should be mixed with water in such a way that the corresponding amount of the polymeric chemical compound is added to the water phase at 25° C. while stirring and the homogenized mixture is then freed of air bubbles in an ultrasonic bath and left to stand for 24 hours. The measurement value of the viscosity is then read within 5 seconds immediately after application of a shear rate of 60 rpm by the number 2 spindle.
An aqueous dispersion according to the invention preferably contains a total of at least 0.5 wt. %, but preferably no more than 4 wt. %, particularly preferably no more than 3 wt. %, of one or more thickeners according to component (B), the total content of polymeric organic compounds in the non-particulate constituent of the aqueous dispersion preferably also not exceeding 4 wt. % (based on the dispersion). The non-particulate constituent is the solids content of the aqueous dispersion in the permeate of the above-described ultrafiltration after it has been dried to constant mass at 105° C.—that is, the solids content after the particulate constituent has been separated by means of ultrafiltration.
Certain classes of polymeric compounds are particularly suitable thickeners according to component (B) and are also readily commercially available. In this connection, the thickener according to component (B) is above all preferably selected from polymeric organic compounds, which in turn are preferably selected from polysaccharides, cellulose derivatives, aminoplasts, polyvinyl alcohols, polyvinylpyrrolidones, polyurethanes and/or urea urethane resins, and particularly preferably from urea urethane resin.
A urea urethane resin as a thickener according to component (B) of the preferred method according to the invention for providing a colloidal aqueous solution starting from the aqueous dispersion is a mixture of polymeric compounds which results from the reaction of a polyvalent isocyanate with a polyol and a mono- and/or diamine. In a preferred embodiment, the urea urethane resin results from a polyvalent isocyanate, preferably selected from 1,4-tetramethylene diisocyanate, 1,6-hexamethylene diisocyanate, 2,2(4),4-trimethyl-1,6-hexamethylene diisocyanate, 1,10-decamethylene diisocyanate, 1,4,-cyclohexylene diisocyanate, p-phenylene diisocyanate, m-phenylene diisocyanate, 2,6-toluene diisocyanate, 2,4-toluene diisocyanate and mixtures thereof, p- and m-xylylene diisocyanate, and 4-4′-diisocyanatodicyclohexylmethane, particularly preferably selected from 2,4-toluenediisocyanate and/or m-xylylene diisocyanate. In a particularly preferred embodiment, the urea urethane resin results from a polyol selected from polyoxyalkylene diols, particularly preferably from polyoxyethylene glycols, which in turn are preferably composed of at least 6, particularly preferably at least 8, more particularly preferably at least 10, but preferably less than 26, particularly preferably less than 23, oxyalkylene units.
Urea urethane resins which are particularly suitable and therefore preferred according to the invention can be obtained by first reacting a diisocyanate, for example toluene-2,4-diisocyanate, with a polyol, for example a polyethylene glycol, so as to form NCO-terminated urethane prepolymers, subsequently further reacted with a primary monoamine and/or with a primary diamine, for example m-xylylenediamine. Urea urethane resins which have neither free nor blocked isocyanate groups are particularly preferred. Such urea urethane resins, as a constituent of the aqueous dispersion from which the colloidal aqueous solution of the method according to the invention can be obtained by dilution, promote the formation of loose agglomerates of primary particles, which, however, are stabilized in the aqueous phase and protected against further agglomeration to such an extent that the sedimentation of the particulate constituent in the aqueous dispersion is largely prevented. To further promote this property profile, urea urethane resins which have neither free or blocked isocyanate groups nor terminal amine groups are preferably used as component (B). In a preferred embodiment, the thickener according to component (B), which is a urea urethane resin, therefore has an amine value of less than 8 mg KOH/g, particularly preferably of less than 5 mg KOH/g, more particularly preferably of less than 2 mg KOH/g, determined according to the method as previously described for the organic polymeric compound (A2) in each case. Since the thickener is substantially dissolved in the aqueous phase and can thus be assigned to the non-particulate constituent of the aqueous dispersion, while component (A2) is substantially bound in the particulate constituent (A), an aqueous dispersion for providing the colloidal aqueous solution of the activation in which the entirety of the polymeric organic compounds in the non-particulate constituent preferably has an amine value of less than 16 mg KOH/g, particularly preferably of less than 10 mg KOH/g, more particularly preferably of less than 4 mg KOH/g, is preferred. It is further preferred for the urea urethane resin to have a hydroxyl number in the range of from 10 to 100 mg KOH/g, particularly preferably in the range of from 20 to 60 mg KOH/g, determined according to method A of 01/2008:20503 from European Pharmacopoeia 9.0. With regard to the molecular weight, a weight-average molar mass of the urea urethane resin in the range of from 1000 to 10000 g/mol, preferably in the range of from 2000 to 6000 g/mol, is advantageous according to the invention and therefore preferred, in each case determined experimentally, as previously described in connection with the definition according to the invention of a polymeric compound.
Without the addition of auxiliaries, the pH of the dispersion for providing the colloidal aqueous solution of the activation of the method according to the invention is usually in the range of 6.0-9.0, and such a pH range is therefore preferred according to the invention. For compatibility with the actual and regularly alkaline colloidal aqueous solution in the activation stage, however, it is advantageous for the pH of the aqueous dispersion to be above 7.2, particularly preferably above 8.0, if necessary as a result of adding compounds that react in an alkaline manner. Since some polyvalent metal cations have an amphoteric character and can therefore be detached from the particulate constituent at higher pH values, the alkalinity of the aqueous dispersion according to the invention is ideally limited, such that the pH of the aqueous dispersion is preferably below 10 and particularly preferably below 9.0.
The above-described aqueous dispersion for providing the colloidal aqueous solution is for its part preferably obtainable by
i) providing a pigment paste by triturating 10 parts by mass of an inorganic particulate compound (A1) with 0.5 to 2 parts by mass of the polymeric organic compound (A2) in the presence of 4 to 7 parts by mass of water and grinding until a D50 value of less than 1 μm has been reached, as determined by means of dynamic light scattering after dilution with water by a factor of 1000, for example by means of Zetasizer® Nano ZS, Malvern Panalytical GmbH;
ii) diluting the pigment paste with such an amount of water, preferably deionized water (κ<1 μScm−1) or service water and a thickener (B) that a dispersed particulate constituent (A) of at least 5 wt. % and a maximum dynamic viscosity of at least 1000 Pa·s at a temperature of 25° C. in the shear rate range of from 0.001 to 0.25 reciprocal seconds is set; and
iii) setting a pH in the range of from 7.2 to 10.0 using a compound that reacts in an alkaline manner,
preferred embodiments of the dispersion being obtained similarly by selecting corresponding components (A1), (A2) and (A) in the provided or required amount as necessary in each case, as described in connection with the colloidal aqueous solution.
The aqueous dispersion can also contain auxiliaries, for example selected from preservatives, wetting agents and defoamers, which are contained in the amount necessary for the relevant function. The content of auxiliaries, particularly preferably of other compounds in the non-particulate constituent which are not thickeners and not compounds that react in an alkaline manner, is preferably less than 1 wt. %. In the context of the present invention, a compound that reacts in an alkaline manner is water-soluble (water solubility: at least 10 g per kg of water with κ<1 μSun′) and has a pKB value of above 8.0 for the first protonation step.
When the treatment of a metal material selected from zinc, iron or aluminum is referenced in the context of the method according to the invention, all materials which contain more than 50 at. % of the relevant element are included. Anti-corrosion pretreatment always relates to the surfaces of the material or component. The material can be a uniform material or a coating. According to the invention, galvanized steel grades consist both of the material steel and of the material zinc, it being possible for surfaces of iron to be exposed at the cutting edges and cylindrical grinding points of, for example, an automobile body which is made of galvanized steel, in which case according to the invention there is pretreatment of the material iron.
The components treated according to the present invention can be three-dimensional structures of any shape and design that originate from a manufacturing process, in particular also including semi-finished products such as strips, metal sheets, rods, pipes, etc., and composite structures assembled from said semi-finished products, the semi-finished products preferably being interconnected by means of adhesion, welding and/or flanging to form composite structures.
There may be a rinsing step between the activation (i) and the phosphating (ii) in order to reduce the carryover of alkaline constituents into the mostly acidic phosphating, but a rinsing step is preferably dispensed with in order to fully maintain the activation performance. A rinsing step is used exclusively for the complete or partial removal of soluble residues, particles and active components that are carried over by adhering to the component from a previous wet-chemical treatment step, from the component to be treated, without metal-element-based or semi-metal-element-based active components, which are already consumed merely by bringing the metal surfaces of the component into contact with the rinsing liquid, being contained in the rinsing liquid itself. For example, the rinsing liquid can simply be city water or deionized water or, if necessary, can also be a rinsing liquid which contains surface-active compounds to improve the wettability by means of the rinsing liquid.
For layer-forming phosphating and the formation of semi-crystalline coatings, which are the aim of the activation of the metal materials, it is preferred for the phosphating in method step (ii) by bringing the surfaces into contact with an acidic aqueous composition containing 5-50 g/kg of phosphates dissolved in water calculated as PO4 and preferably additionally containing at least one source of free fluoride. According to the invention, the amount of phosphate ions includes orthophosphoric acid and the anions, dissolved in water, of the salts of orthophosphoric acid, calculated as PO4.
In a preferred embodiment of the present invention, the subsequent phosphating is zinc phosphating and the phosphating in method step (ii) is based on an acidic aqueous composition containing 0.3-3 g/kg of zinc ions, preferably on an acidic aqueous composition containing 5-50 g/L of phosphate ions, 0.3-3 g/L of zinc ions and an amount of free fluoride.
A source of free fluoride ions is essential for the process of layer-forming zinc phosphating, insofar as the layer formation on all metal materials selected from zinc, iron or aluminum is desired, and is required, for example, for zinc phosphating of automobile bodies, which are at least also partially made of aluminum. If all surfaces of the metal materials of a component are provided with a phosphate coating, the amount of the particulate constituents in the activation must often be adapted to the amount of free fluoride required for layer formation in the zinc phosphating. In a method according to the invention based on activation (i) followed by zinc phosphating (ii), in which the components to be pretreated are made from metal materials of zinc and iron, in particular steel, it is advantageous for a closed and defect-free phosphate coating for the amount of free fluoride in the acidic aqueous composition to be at least 0.5 mmol/kg. If the component is also made of the metal material aluminum and if its surfaces are also to be provided with a closed phosphate coating, then it is also preferred in the method according to the invention for the amount of free fluoride in the acidic aqueous composition to be at least 2 mmol/kg. The concentration of free fluoride should not exceed values above which the phosphate coatings predominantly have adhesions that can be easily wiped off, since these adhesions cannot be avoided even by a disproportionately increased amount of particulate constituents in the colloidal aqueous solution of the activation. Therefore, it is also economically advantageous, and therefore preferred, for the concentration of free fluoride in the acidic aqueous composition of the zinc phosphating to be below 15 mmol/kg, particularly preferably below 10 mmol/kg and more particularly preferably below 8 mmol/kg, in the method according to the invention based on activation (i) followed by zinc phosphating (ii).
The amount of free fluoride can be determined potentiometrically by means of a fluoride-sensitive measuring electrode at 20° C. in the relevant acidic aqueous composition after calibration with fluoride-containing buffer solutions without pH buffering. Suitable sources of free fluoride are hydrofluoric acid and the water-soluble salts thereof, such as ammonium bifluoride and sodium fluoride, as well as complex fluorides of the elements Zr, Ti and/or Si, in particular complex fluorides of the element Si. In a phosphating process according to the present invention, the source of free fluoride is therefore preferably selected from hydrofluoric acid and the water-soluble salts thereof and/or complex fluorides of the elements Zr, Ti and/or Si. Salts of hydrofluoric acid are water-soluble within the meaning of the present invention if their solubility in deionized water (κ<1 μScm−1) at 60° C. is at least 1 g/L, calculated as F.
In order to suppress what is known as “pin-holing” on the surfaces of the metal materials which are made of zinc, it is preferred, in such methods according to the invention in which zinc phosphating is carried out in step (ii), for the source of free fluoride to be at least partially selected from complex fluorides of the element Si, in particular from hexafluorosilicic acid and the salts thereof. The term pin-holing is understood by a person skilled in the art of phosphating to mean the phenomenon of local deposition of amorphous, white zinc phosphate in an otherwise crystalline phosphate layer on the treated zinc surfaces or on the treated galvanized or alloy-galvanized steel surfaces. Pin-holing is caused in this case by a locally increased pickling rate of the substrate. Such point defects in the phosphating can be the starting point for corrosive delamination of subsequently applied organic coating systems, and therefore the occurrence of pin-holes should be largely avoided in practice. In this context, it is preferred for the concentration of silicon in water-dissolved form in the acidic aqueous composition of the zinc phosphating in method step (ii) to be at least 0.5 mmol/kg, particularly preferably at least 1 mmol/kg, more particularly preferably at least 2 mmol/kg, but preferably less than 15 mmol/kg, particularly preferably less than 12 mmol/kg, more particularly preferably less than 10 mmol/kg and very particularly preferably less than 8 mmol/kg. The upper limits for the concentration of silicon are preferred because above these values, phosphate coatings are favored which mostly have loose adhesions, which cannot be avoided even by a disproportionately high amount of particulate constituents in the colloidal aqueous solution of the activation stage. The concentration of silicon in the acidic aqueous composition in water-dissolved form can be determined by means of atomic emission spectrometry (ICP-OES) in the filtrate of a membrane filtration of the acidic aqueous composition that is carried out using a membrane having a nominal pore size of 0.2 μm.
With regard to the interaction of activation and zinc phosphating, it has been found that the content of particulate constituents contributing to activation has to be adapted to the amount of free fluoride and silicon in the zinc phosphating in order to ensure that the higher amounts of free fluoride for layer-forming phosphating on components comprising aluminum as a metal material contained in the phosphating bath do not have a disadvantageous effect on the layer formation, which is of great importance to a constant quality of the phosphate coatings, in particular when pretreating a large number of components. In this context, a method is preferred according to the invention in which a series of components is to be pretreated, which series comprises components which are at least partially made of the materials zinc and aluminum, and in which the components of the series undergo firstly activation (i) and then zinc phosphating (ii) in consecutive method steps, the activation in method step (i) being carried out by bringing the component into contact with a colloidal aqueous solution as described above which, in a preferred embodiment, can be obtained as an aqueous dispersion as described above which is diluted by a factor of from 20 to 100,000, and the zinc phosphating in method step (ii) being carried out by contact with an acidic aqueous composition containing
(a) 5-50 g/l of phosphate ions,
(b) 0.3-3 g/l of zinc ions, and
(c) at least one source of free fluoride,
wherein the quotient of the concentration of the phosphates in the inorganic particulate constituent of the colloidal aqueous solution of the activation in mmol/kg, based on PO4, with respect to the sum of the concentration of free fluoride and the concentration of silicon in each case in the acidic aqueous composition of the zinc phosphating and in each case in mmol/kg is greater than 0.2, preferably greater than 0.3, particularly preferably greater than 0.4.
Insofar as zinc phosphating in method step (ii) is mentioned in the context of the second aspect of the present invention, the preferred pH of the acidic aqueous composition which brings about the zinc phosphating is above 2.5, particularly preferably above 2.7, but is preferably below 3.5, particularly preferably below 3.3. The content of the free acid in points in the acidic aqueous composition of the zinc phosphating in method step (ii) is preferably at least 0.4, but preferably no more than 3.0, particularly preferably no more than 2.0. The content of free acid in points is determined by diluting 10 ml sample volume of the acidic aqueous composition to 50 ml and titrating with 0.1 N sodium hydroxide solution to a pH of 3.6. The consumption of mL of sodium hydroxide solution indicates the point number of the free acid.
The conventional addition of additives for zinc phosphating can also be carried out similarly in the context of the present invention, such that the acidic aqueous composition in method step (ii) can contain the conventional accelerators such as hydrogen peroxide, nitrite, hydroxylamine, nitroguanidine and/or N-methylmorpholine-N-oxide and additionally cations of the metals manganese, calcium and/or iron in the form of water-soluble salts, which have a positive influence on layer formation. In an embodiment that is preferred for environmental hygiene reasons, in total less than 10 ppm of nickel and/or cobalt ions are contained in the acidic aqueous composition of the zinc phosphating in method step (ii).
In the method according to the invention, a good coating primer for a subsequent dip coating, in the course of which a substantially organic cover layer is applied, is produced. Accordingly, in a preferred embodiment of the method according to the invention, the zinc phosphating, with or without an intermediate rinsing and/or drying step, but preferably with a rinsing step and without a drying step, is followed by dip coating, particularly preferably electrocoating, more particularly preferably cathodic electrocoating, which preferably contains water-soluble or water-dispersible salts of yttrium and/or bismuth in addition to the dispersed resin, which preferably comprises an amine-modified polyepoxide.
In the following, the properties of activation involving city water for subsequent zinc phosphating are presented with regard to the phosphate layer weights and corrosion protection results achieved under the same conditions.
To prepare a pigment paste for providing a dispersion for activation, 15 parts by mass of Edaplan® 490 (Münzing Chemie GmbH) were predispersed as dispersing agents in 25 parts by mass of fully deionized water (κ<1 μScm−1) and then mixed with 60 parts by mass of zinc phosphate of quality level PZ 20. This phase was transferred to a KDL type Dyno®-Mill bead mill and the zinc phosphate particles were continuously milled for two hours (milling parameters: 75% bead fill level, 2000 revolutions per minute, 20 L volumetric flow per hour, temperature of the milled material 40-45° C.). The result was an average particle size of approximately 0.35 μm determined using a Zetasizer Nano ZS from Malvern.
2.5 parts by mass of a urea urethane resin solution containing 40 wt. % of the resin based on an amine-modified prepolymer of TDI/XDI and PEG-16 (amine value<1 mg KOH/g; hydroxyl number approximately 40 mg KOH/g) in approximately 64 parts by mass of fully deionized water (κ<1 μScm−1) were then supplied as a thickener, homogenized, and adjusted to pH 9 using 10% sodium hydroxide solution. Then, approximately 33 parts by mass of the pigment paste were added while stirring, adjusted to pH 9 using 1 wt. % NaOH solution and stirred to the point of complete homogenization.
In a 5 L beaker, 5 liters of
For layer-forming phosphating by activation based on the colloidal aqueous solution, sheets of cold-rolled steel (CRS), hot-dip galvanized steel (HDG) and aluminum (AA6014) were:
a) firstly alkaline cleaned while stirring in Düsseldorf city water (pH: 10.2-10.9; 55° C.) by being immersed for 5 minutes in a degreasing bath containing 4 wt. % of Bonderite® C-AK 1565 A and 0.6 wt. % of Bonderite® C-AD 1561, each of which is available from Henkel AG & Co KGaA;
b) subjected to rinsing with Düsseldorf city water and then with fully deionized water (κ<1 μScm−1) for approximately 30 seconds in each case;
c) in a water-wetted state, brought into contact with the activation solution A or, according to the invention, with the activation solution B by immersion for 60 seconds;
d) and immediately afterwards, and without further rinsing steps, immersed into a hydroxylamine-accelerated phosphating bath having a free acid content of 0.9-1.4 points (titrated to a pH of 3.6), a total acid content of 25-30 points (titrated to a pH of 8.5) and a free fluoride content of approximately 150 mg/kg, containing 4.6 wt. % of Bonderite® M-ZN 1994, 0.8 wt. % of Bonderite® M-AD 565, 0.24 wt. % of Bonderite® M-AD 338 and 0.38 wt. % of Bonderite® M-AD 110, each of which is available from Henkel AG & Co KGaA, in fully deionized water (κ<1 μScm−1), for 3 min while stirring at 52° C.;
e) subjected to rinsing with fully deionized water (κ<1 μScm−1) for approximately 30 seconds; and
f) provided with an approximately 20 μm thick layer of an electrocoat of the type Cathoguard® 800 (BASF SE) and then cured at 180° C. for 35 min.
Table 1 summarizes the results of zinc phosphating with regard to layer weight and after aging in the corrosion test. It is apparent that, when activation occurs using city water (B) by comparison with an approach using deionized water (A), homogeneous, closed zinc phosphate coatings are achieved which have a lower layer weight and also improved corrosion protection results.
1 differential gravimetric determination after detaching the phosphate layer in aqueous 5 wt. % chromic acid solution
2 delamination at the scratch after aging in a VW PV 1210 alternating climate test for 6 weeks over 30 cycles
3 longest filiform corrosion thread according to DIN EN 3665
| Number | Date | Country | Kind |
|---|---|---|---|
| 19211407.2 | Nov 2019 | EP | regional |
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/EP2020/082618 | Nov 2020 | US |
| Child | 17664665 | US |
