PROCESS FOR COATING ELECTROCONDUCTIVE SUBSTRATES

Abstract

The present invention relates to a process for the at least two-stage coating of an electrically conductive substrate, the at least two-stage coating being carried out in a single dip-coating bath, the dip-coating bath comprising a coating material composition which comprises at least one cathodically depositable film-forming polymer and also an anodically depositable component, the anodically depositable component comprising anions of at least one phosphorus oxoacid; in a first stage, the electrically conductive substrate for coating is connected as anode in said dip-coating bath, and in a subsequent stage the now precoated substrate is connected as cathode in said dip-coating bath. The invention further relates to a substrate coated by the process of the invention.

Claims

1: A process for an at least two-stage coating of an electrically conductive substrate, the process comprising: in a single dip-coating bath comprising a coating material composition which comprises at least one cathodically depositable film-forming polymer and an anodically depositable component comprising anions of at least one phosphorus oxoacid, connecting the electrically conductive substrate for coating as an anode in a first stage to obtain a precoated substrate, and connecting the precoated substrate as cathode in a subsequent stage.

2: The process as claimed in claim 1, wherein the anions of the phosphorus oxoacid are anions of phosphoric acid, anions of phosphorous acid, anions of diphosphoric acid, anions of diphosphorous acid, anions of linear or cyclic oligophosphoric acids having 3 to 10 phosphorus atoms, or any mixtures thereof.

3: The process as claimed in claim 1, wherein the anions of the at least one phosphorus oxoaxid are present in a concentration of 0.1 to 40 g/l in the coating material composition of the dip-coating bath.

4: The process as claimed in claim 1, wherein a voltage in the range from 1 to 100 volts is applied in the first stage between the electrically conductive substrate connected as anode and a counterelectrode.

5: The process as claimed in claim 1, wherein the electrically conductive substrate in the first stage is connected as anode for a period in the range from 5 to 240 seconds.

6: The process as claimed in claim 4, wherein the counterelectrode constitutes an electrically conductive substrate which in a preceding stage was connected as anode in the dip-coating bath.

7: The process as claimed in claim 1, wherein the electrically conductive substrate connected as anode in the first stage is connected as cathode in the subsequent stage in the dip-coating bath, relative to an anode with a voltage in the range from 50 to 500 volts applied between the electrically conductive substrate and the anode.

8: The process as claimed in claim 7, wherein the voltage between the electrically conductive substrate connected as cathode and the anode is maintained for a period of 10 to 300 seconds at a level within the stated voltage range.

9: The process as claimed in claim 1, wherein the electrically conductive substrate last connected as cathode is withdrawn from the dip-coating bath and, with or without intermediate rinsing with water, is contacted with an aqueous solution of a crosslinking catalyst for a reaction between epoxy resins and/or acrylate resins with crosslinking agent selected from the group consisting of a blocked polyisocyanate, an amino resin, a phenolic resin, a polyfunctional Mannich base, a melamine resin a benzoguanamine resin, an epoxide, a free polyisocyanate, and a mixture thereof.

10: The process as claimed in claim 9, wherein the aqueous solution of a crosslinking catalyst is a solution of a water-soluble bismuth compound.

11: The process as claimed in claim 9, wherein a cathodic voltage is applied relative to an anode to the electrically conductive substrate during contact with the aqueous solution of the crosslinking catalyst.

12: The process as claimed in claim 1, wherein the voltage is 5 V to 100 V.

13: The process as claimed in claim 1, wherein the electrically conductive substrate has no crystalline metal phosphate layer at least on part of its surface.

14: The process as claimed in claim 1, wherein the substrate is a vehicle body or a part of the vehicle body.

15: The process as claimed in claim 1, wherein a resulting electrocoat film is cured at a temperature of >80 C.

16: A substrate coated by the process as claimed in claim 1.

17: The substrate as claimed in claim 16, coated additionally with at least one surfacer and/or at least one basecoat material and/or at least one clearcoat material, wherein the coats applied are cured individually or jointly.

18: The substrate as claimed in claim 16, which is a vehicle body or a part of the vehicle body.

Description

EXAMPLES

Methods of Determination

1. Copper-Accelerated Acetic Acid Salt Spray Mist Test to DIN EN ISO 9227 CASS (for Short: CASS Test)

[0053] The copper-accelerated acetic acid salt spray mist test serves for determining the corrosion resistance of a coating on a substrate. The copper-accelerated acetic acid salt spray mist test is carried out according to DIN EN ISO 9227 CASS for the metallic aluminum substrate (AA6014 (ALU)) coated by the method of the invention or by a comparative method. It involves the samples under investigation being in a chamber in which misting is performed continuously over a duration of 240 hours and at a temperature of 50 C. with a 5% strength common salt solution of controlled pH, with copper chloride and acetic acid being added to the salt solution. The mist is deposited on the samples under investigation, coating them with a corrosive salt water film.

[0054] Prior to the copper-accelerated acetic acid salt spray mist test to DIN EN ISO 9227 CASS, the samples under investigation are scored down to the substrate with a knife cut, allowing the samples to be investigated for their degree of corrosive undermining in accordance with DIN EN ISO 4628-8, since during the copper-accelerated acetic acid salt spray mist test to DIN EN ISO 9227 CASS, the substrate is corroded along the line of scoring. The progressive process of corrosion causes greater or lesser undermining of the coating in the course of the test. The degree of undermining in [mm] is a measure of the resistance of the coating.

2. Filiform Corrosion to DIN EN 3665 (for Short: Filiform Test)

[0055] Determining the filiform corrosion is used to ascertain the corrosion resistance of a coating on a substrate. This determination is carried out according to DIN EN 3665 (date: Aug. 1, 1997) for the electrically conductive substrate aluminum (ALU), coated with an inventive coating composition or with a comparative coating composition, over a duration of 1008 hours. In the test, the respective coating, starting from a line of induced damage to the coating, is undermined by corrosion that takes the form of a line or thread. The average and maximum thread lengths in [mm] can be measured according to DIN EN 3665 (Method 3), and are a measure of the resistance of the coating to corrosion. Also determined is the undermining in [mm] according to PAPP WT 3102 (Daimler) (date: Dec. 21, 2006).

3. VDA Alternating Climate Test to VDA 621-415 (for Short: ACT-VDA) [VDA=German Automakers Association]

[0056] This alternating climate test is used for determining the corrosion resistance of a coating on a substrate. The alternating climate test is carried out for the correspondingly coated cold-rolled steel (CRS) substrate. The alternating climate test is carried out in 10 cycles. One cycle consists of a total of 168 hours (1 week) and encompasses [0057] a) 24 hours of salt spray mist testing to DIN EN ISO 9227 NSS (date: Sep. 1, 2012), [0058] b) followed by 8 hours of storage, including warming, as per DIN EN ISO 6270-2 of September 2005, AHT method, [0059] c) followed by 16 hours of storage, including cooling, as per DIN EN ISO 6270-2 of September 2005, AHT method, [0060] d) 3-fold repetition of b) and c) (hence in total 72 hours), and [0061] e) 48 hours of storage, including cooling, with an aerated climate chamber as per DIN EN ISO 6270-2 of September 2005, AHT method.

[0062] If, still prior to the alternating climate test being carried out, the respective baked coating of the samples under investigation is scored down to the substrate with a knife cut, the samples can be investigated for their degree of corrosive undermining according to DIN EN ISO 4628-8 (date: Mar. 1, 2013), since the substrate is corroded along the scoring line during the implementation of the alternating climate test. The progressive process of corrosion causes greater or lesser undermining of the coating during the test. The degree of undermining in [mm] is a measure of the resistance of the coating.

4. VW Alternating Climate Test, PV 1210 (for Short: ACT-VW)

[0063] This alternating climate test is used to ascertain the corrosion resistance of a coating on a substrate. The alternating climate test is carried out for the electrically conductive cold-rolled steel (CRS) substrate, coated by the method of the invention or by a comparative method. This alternating climate test is carried out in 30 cycles. One cycle (24 hours) consists of 4 hours of salt spray mist testing to DIN EN ISO 9227 NSS (date: Sep. 1, 2012), 4 hours of storage, including cooling, according to DIN EN ISO 6270-2 of September 2005 (AHT method), and 16 hours of storage, including warming, according to DIN EN ISO 6270-2 of September 2005, AHT method, at 403 C. and a humidity of 100%. After every 5 cycles there is a pause of 48 hours, including cooling, according to DIN EN ISO 6270-2 of September 2005, AHT method. 30 cycles therefore correspond to a duration of 42 days in all.

[0064] If, still prior to the alternating climate test being carried out, the coating of the samples under investigation is scored down to the substrate with a knife cut, the samples can be investigated for their degree of corrosive undermining according to DIN EN ISO 4628-8 (date: Mar. 1, 2013), since the substrate is corroded along the scoring line during the implementation of the alternating climate test. The progressive process of corrosion causes greater or lesser undermining of the coating during the test. The degree of undermining in [mm] is a measure of the resistance of the coating.

[0065] After the alternating climate test has been carried out, the samples can be investigated for their degree of blistering in accordance with DIN EN ISO 4628-2 (date: Jan. 1, 2004). The assessment is made using characteristic values in the range from 0 (low degree of blistering) to 5 (very high degree of blistering).

5. X-Ray Fluorescence Analysis (XFA) for Film Weight Determination

[0066] The anodic deposition of the anions of the oxoacid of phosphorus is determined by means of wavelength-dispersive X-ray fluorescence analysis (XFA) according to DIN 51001 (date: August 2003). The signals obtained during the implementation of the X-ray fluorescence analysis are corrected to account for a separately measured substrate of an uncoated reference sample. Gross count rates (in kilocounts per second) are determined for each of the elements under analysis (presently: phosphorus). The gross count rates for phosphorus from a reference sample (uncoated substrate) are subtracted from the thus-determined gross count rates for the respective sample, to give the net count rates of the elements under analysis.

6. Solvent Resistance Test (for Short: SR Test)

[0067] The solvent resistance of the baked electrocoats was ascertained by means of an acetone test. In this test, a cloth soaked with acetone was rubbed over the coating, this being recorded in the form of double rubs (back and forth). The double rub values reported indicate the number of double rubs necessary to expose the metallic substrate beneath the coating. This analysis was carried out only up to a maximum of 50 double rubs; a report of 50 double rubs means that the coating is solvent-resistant for the purposes of this test.

Electrocoat CG 520

[0068] The electrocoat material identified below as electrocoat CG 520 is obtained by mixing 4840 g of fully demineralized water, 4590 g of the binder CG 520 (manufacturer: BASF Coatings GmbH; solids content: 40%), and 630 g of pigment paste CG 520 (manufacturer: BASF Coatings GmbH; solids content: 65%) with one another with stirring. The conditions of cathodic deposition with this electrocoat material and with the electrocoat materials produced therefrom and used in accordance with the invention were typically 32 C. for a duration of 120 seconds, unless otherwise indicated. Unless otherwise indicated, the voltage was set in the range from 160 V to 240 V so as to give a dry film thickness, for the baked coating material, of 20 m.

In-House Binder Dispersion D1

[0069] A crosslinking agent V1 was prepared as described in DE 102007038824, paragraph [0028] in example 1.1.

[0070] An aqueous binder dispersion D1 was prepared as described in DE 102007038824, paragraphs [0029 to 0030] in example 1.2.

[0071] The subsequent workstep in paragraph [0031] was modified as follows:

[0072] 2400 parts of the resulting mixture are dispersed immediately into an existing mixture of 2173 parts of demineralized water and 25.7 parts of glacial acetic acid. Addition of a further 751 parts of demineralized water gives a stable dispersion.

In-House Pigment Paste

[0073] The pigment paste was prepared by the method described in DE 10 2008 016220 A1 (page 7, Table 1, variant B).

Examples 1a to 1f

Deposition from Aqueous Solutions

[0074] Sheets of cold-rolled steel (CRS sheets), available under the Gardobond MBS designation from Chemetall, were first subjected to dip cleaning in a solution of Ridoline 1565 (3%) and Ridosol 1400 (0.3%), according to the use instructions, for 5 minutes at 60 C. (Ridoline 1565 and Ridosol 1400 are available from Henkel). This was followed by dip rinsing with service water (1 min) and then with fully demineralized water (1 min).

[0075] After this cleaning, the cold-rolled steel sheets were treated under various conditions (see Table 1) anodically for 30 seconds with an acetate-buffered (5 g/l acetate buffer pH 5.2), aqueous solution of sodium dihydrogen phosphate (0.9 g/l) in a dip-coating bath.

[0076] Thereafter the sheets were rinsed for 1 minute by spraying with fully demineralized water, followed by drying.

[0077] The deposition of phosphorus was evaluated by XFA (X-ray fluorescence analysis). The phosphorus add-on was determined from the intensity of the phosphorus signal in kilocounts per second (kcps) (higher values correspond to a higher deposition rate). The results are presented in Table 1.

TABLE-US-00001 TABLE 1 Phosphorus Example Dipping conditions add-on in kcps 1a electroless 3.76 1b 20 V (continuous) 17.15 1c 30 V (continuous) 17.7 1d 10 V (pulse method: 1 sec on, 8.45 0.1 sec off) 1e 20 V (pulse method: 1 sec on, 11.6 0.1 sec off) 1f 20 V (pulse method: 2 sec on, 19.03 0.1 sec off)

[0078] The investigations show that when an anodic current was used, the deposition of phosphorus found on the metal sheet was higher by comparison to electroless immersion into the solution.

Examples 2a to 2e

Deposition from Electrocoat-Containing Baths

[0079] Sheets of cold-rolled steel (CRS sheets), available under the Gardobond MBS designation from Chemetall, were first subjected to dip cleaning in a solution of Ridoline 1565 (3%) and Ridosol 1400 (0.3%), according to the use instructions, for 5 minutes at 60 C. This was followed by dip rinsing with service water (1 min) and then with fully demineralized water (1 min).

[0080] After this cleaning, the cold-rolled steel sheets were coated under different inventive conditions (see Table 2), first anodically and then cathodically with a standard-use cathodic electrocoat material (CG 520, BASF Coatings GmbH) to which 0.9 g/l sodium dihydrogen phosphate was added.

[0081] Thereafter the sheets were rinsed for 1 minute by spraying with fully demineralized water, followed by drying, and then baked in a forced air oven at 175 C. for 25 minutes (corresponding to a hold time of around 15 minutes at a substrate temperature of 175 C.).

[0082] The deposition of phosphorus was evaluated by XFA (X-ray fluorescence analysis). The phosphorus add-on was determined from the intensity of the phosphorus signal in kilocounts per second (kcps) (higher values correspond to a higher deposition rate). The results are presented in Table 2.

TABLE-US-00002 TABLE 2 Phosphorus Example Dipping conditions add-on in kcps 2a Anode: 30 sec at 5 V 9.16 Cathode: 120 sec at 200 V 2b Anode: 30 sec at 15 V 9.92 Cathode: 120 sec at 200 V 2c Anode: 30 sec at 30 V 9.83 Cathode: 120 sec at 200 V 2d Anode: 60 sec at 30 V 10.26 Cathode: 120 sec at 200 V 2e Anode: 120 sec at 30 V 10.97 Cathode: 120 sec at 200 V

[0083] The investigations show that an increase in the coating time in the anodic stage leads to an increase in the amount of phosphorus deposited.

Example 3a (Noninventive), 3b (Inventive), 3c (Inventive), and 3d (Inventive)

Effect of Phosphorus Deposition on Corrosion Control

[0084] Sheets of cold-rolled steel (CRS sheets), available under the Gardobond MBS designation, and also sheets of hot-dip galvanized steel (HDG sheets), available under the Gardobond EA designation, and sheets of aluminum, available under the Gardobond Aluminium AA6014 designation, all Gardobond sheets from Chemetall, were first subjected to dip cleaning in a solution of Ridoline 1565 (3%) and Ridosol 1400 (0.3%), according to the use instructions, for 5 minutes at 60 C. This was followed by dip rinsing with service water (1 min) and then with fully demineralized water (1 min).

[0085] The sheets thus cleaned were coated under different inventive conditions (see Table 3), first anodically and then cathodically. In the case of noninventive example 3a, a standard-use cathodic electrocoat material (CG 520, available from BASF Coatings GmbH) was used. In inventive examples 3b and 3c, the samples were likewise coated with the electrocoat material CG 520, to which different amounts (0.3 or 0.9 g/l) of sodium dihydrogen phosphate (examples 3b and 3c) were added. In inventive example 3d, the electrocoat material was prepared by replacing the CG520 binder component with equal parts of dispersion D1. This electrocoating bath was admixed with 1.82 g/l 95% strength orthophosphoric acid.

[0086] Thereafter the sheets were rinsed by spraying with fully demineralized water for 1 minute, followed by drying, and were baked in a forced air oven at 175 C. for 25 minutes (corresponding to a hold time of about 15 minutes at a substrate temperature of 175 C.).

TABLE-US-00003 TABLE 3 Phosphorus compound and Example Dipping conditions concentration 3a Anode: no anodic deposition Cathode: 120 sec at 220 V 3b Anode: 30 sec at 20 V NaH.sub.2PO.sub.4 Cathode: 120 sec at 220 V 0.3 g/l 3c Anode: 30 sec at 20 V NaH.sub.2PO.sub.4 Cathode: 120 sec at 220 V 0.9 g/l 3d Anode: 30 sec at 20 V H.sub.3PO.sub.4 Cathode: 120 sec at 220 V 1.82 g/l

[0087] Table 4 reports the performance results in the corrosion tests described above. The example number has been given a suffix code for the substrate used: CRS, HDG, or ALU.

TABLE-US-00004 TABLE 4 Filiform test Filiform test CASS undermining thread length ACT- ACT- Example test (max.) (max.) VDA VW 3a-CRS 11.2 9.1 3b-CRS 6.8 9.4 3c-CRS 8.0 7.8 3d-CRS 3.9 3a-HDG 5.2 3b-HDG 5.2 3c-HDG 5.8 3d-HDG 2.2 3a-ALU 2.5 3.1 8.0 3b-ALU 1.7 0.3 1.6 3c-ALU 0.9 0.1 1.2 3d-ALU 1.2

[0088] In virtually all cases on all substrates, the results show a distinct improvement in corrosion control relative to noninventive example 3a. In this case there is generally likewise an increase in corrosion control with increasing phosphate content on the part of the electrocoat material. For example 3d (highest phosphate content), it was found, in a filtration experiment not presented above, that the long-term bath stability was somewhat lower than for the other inventive electrocoat materials. This means that phosphate contents beyond that selected in example 3d may have an adverse effect on bath stability.

Examples 4a (Noninventive), 4b (Inventive), 4c (Inventive), 4d (Inventive), and 4e (Inventive)

Effect of a Bismuth Catalyst Rinse (Electroless)

[0089] Noninventive example 4a corresponds to noninventive example 3a. Inventive example 4b corresponds to inventive example 3d. Inventive examples 4c, 4d, and 4e differ from example 4b only in that immediately after the spray rinse with fully demineralized water and before implementation of the 175 C. baking step, a further rinsing step was carried out electroless with a rinsing solution containing bismuth ions for 1 minute (example 4c), 2 minutes (example 4d), or 3 minutes (example 4e). The dip rinsing solution containing bismuth ions that was used was a solution, regulated to a temperature of 30 C., of 23.3 g of bismuth methane sulfonate in 2.2 l of fully demineralized water, this solution having been adjusted to a pH of 4.3 using ammonium hydroxide solution.

[0090] As well as corrosion control tests, a determination was also made of the solvent resistance of the baked electrocoat materials obtained. The results of the corrosion control tests and of the solvent resistance test have been reproduced in Table 5.

TABLE-US-00005 TABLE 5 CASS ACT- SR Example test VW test 4a-CRS 9.1 50 4b-CRS 3.9 4 4c-CRS 4.1 8 4d-CRS 5.0 10 4e-CRS 5.2 50 4a-HDG 5.2 50 4b-HDG 2.2 5 4c-HDG 2.1 6 4d-HDG 3.1 46 4e-HDG 2.3 49 4a-ALU 2.5 50 4b-ALU 1.2 6 4c-ALU 1.6 45 4d-ALU 1.5 30 4e-ALU 1.2 22

[0091] In all cases on all substrates, the results show a distinct improvement in corrosion control relative to noninventive example 4a. As the period of immersion goes up, there is a marked increase in the solvent resistance of the baked cathodic electrocoats on cold-rolled steel and hot-dipped galvanized steel.

Inventive Examples 5a to 5f: Effect of Bismuth Catalyst Rinsing, with Cathodic Current Applied

[0092] In example 4 it was shown that the use of a bismuth rinse allows the degree of crosslinking or solvent resistance of a cathodic electrocoat system to be increased. In this example, additionally, a cathodic current was applied to the substrate during this rinsing step. It was shown that the cathodic current allows the effect of the bismuth rinse to be increased, and that the application of the cathodic current allows the immersion time to be reduced (see Table 6).

[0093] Inventive example 5a corresponds to inventive example 4c (1-minute electroless after-rinsing). Examples 5b, 5c, and 5d differ from example 5a only in the application of a voltage of 5V (example 5b), 10V (example 5c), and 30V (example 5d) and hence in a cathodic deposition of bismuth during the immersion time of 1 minute. Example 5e differs from example 5a in an increased immersion time of 3 minutes and in the application of a voltage of 30V, while in example 5f (by comparison with example 5e) the immersion time was increased again to 5 minutes.

TABLE-US-00006 TABLE 6 Example SR test 5a-CRS 8 5b-CRS 29 5c-CRS 15 5d-CRS 50 5e-CRS 50 5f-CRS 50 5a-HDG 6 5b-HDG 49 5c-HDG 30 5d-HDG 30 5e-HDG 50 5f-HDG 50 5a-ALU 45 5b-ALU 50 5c-ALU 50 5d-ALU 50 5e-ALU 47 5f-ALU 50