AQUEOUS COATING COMPOSITION FOR DIPCOATING ELECTRICALLY CONDUCTIVE SUBSTRATES CONTAINING BISMUTH AND LITHIUM

Abstract

Described herein is an aqueous coating composition (A) for at least partly coating an electrically conductive substrate with an electrocoat material, including (A1) at least one cathodically depositable resin binder, (A2) at least one crosslinking agent, (A3) at least 100 ppm of bismuth, based on the total weight of the coating composition (A), and (A4) lithium, in a form dissolved in (A), the lithium not exceeding a fraction of 300 ppm, based on the total weight of the coating composition (A). Also described herein are a method for producing (A), a coating method, and an at least partly coated substrate obtainable by the method.

Claims

1. An aqueous coating composition (A) for at least partly coating an electrically conductive substrate with an electrocoat material, comprising (A1) at least one cathodically depositable resin binder, (A2) at least one crosslinking agent, and (A3) at least 100 ppm of bismuth, based on the total weight of the coating composition (A), wherein (A4) the coating composition comprises lithium, in a form dissolved in (A), said lithium not exceeding a fraction of 300 ppm, based on the total weight of the coating composition (A).

2. The aqueous coating composition (A) according to claim 1, wherein the fraction of lithium (A4) is from 2.5 to 250 ppm based on the total weight of the coating composition (A).

3. The aqueous coating composition (A) according to claim 1, which comprises a bismuth-based crosslinking catalyst.

4. The aqueous coating composition (A) according to claim 3, comprising the bismuth-based crosslinking catalyst in component (A3).

5. The aqueous coating composition (A) according to claim 3, wherein (A5) the fraction of phosphorus does not exceed an amount of 100 ppm based on the total weight of the coating composition (A).

6. The aqueous coating composition (A) according to claim 1, which comprises (A6) copper.

7. The aqueous coating composition (A) according to claim 6, wherein (A6a) copper is included in a form dissolved in (A) and the amount (A6a) is from 5 to 1000 ppm, based on the total weight of the coating composition (A).

8. The aqueous coating composition (A) according to claim 1, wherein the coating composition (A) comprises a total amount of at least 300 ppm of bismuth, based on the total weight of the coating composition (A), including (A3a) at least 100 ppm of bismuth, based on the total weight of the coating composition (A), in a form in which it is in solution in the coating composition (A), and (A3b) at least 200 ppm of bismuth, based on the total weight of the coating composition (A), in a form in which it is not in solution in the coating composition (A).

9. The aqueous coating composition (A) according to claim 1, wherein the total amount of the bismuth present in the coating composition (A) is in a range from at least 500 ppm to 20 000 ppm and the coating composition (A) comprises at least one at least bidentate complexing agent (A3aa) suitable for the complexing of bismuth.

10. A method for at least partly coating an electrically conductive substrate with an electrocoat material, comprising at least a step (1), (1) contacting the electrically conductive substrate, connected as cathode, with the aqueous coating composition (A) according to claim 1 step (1) being carried out in at least two successive stages (1a) and (1b), namely (1a) at an applied voltage in a range from 1 to 50 V, which is applied over a duration of at least 5 seconds, and (1b) at an applied voltage in a range from 50 to 400 V, with the proviso that the voltage applied in stage (1b) is greater by at least 10 V than the voltage applied in stage (la).

11. The method according to claim 10, wherein the voltage applied in stage (1a) is applied over a duration in a range from at least 5 to 300 seconds, and the voltage applied in stage (1b) in the range from 50 to 400 V takes place in a time interval of 0 to 300 seconds after implementation of stage (1a) and is maintained for a period in the range from 10 to 300 seconds at a value within the stated voltage range of 50 to 400 V.

12. The method according to claim 11, wherein the electrically conductive substrate in relation to the type of metal has different regions.

13. The method according to claim 12, wherein the aqueous coating composition (A) comprises lithium, in a form (A4) dissolved in (A), and copper, in a form (A6a) dissolved in (A), and the fraction of (A4) is from 12.5 to 70 ppm, and the fraction of (A6a) is from 20 to 250 ppm.

14. A coated substrate coated by the method according to claim 10.

15. A component or article, which comprises the coated substrate according to claim 14.

16. The aqueous coating composition (A) according to claim 1, wherein the fraction of lithium (A4) is from 12.5 to 70 ppm based on the total weight of the coating composition (A).

17. The aqueous coating composition (A) according to claim 3, wherein (A5) the fraction of phosphorus does not exceed an amount of 45 ppm based on the total weight of the coating composition (A).

18. The method according to claim 11, wherein the electrically conductive substrate in relation to the type of metal has at least one region which is steel-based and at least one further region which is aluminum-based.

19. The method according to claim 12, wherein the aqueous coating composition (A) comprises lithium, in a form (A4) dissolved in (A), and copper, in a form (A6a) dissolved in (A), and the fraction of (A4) is from 12.5 to 50 ppm, and the fraction of (A6a) is from 20 to 250 ppm.

20. An automobile body, which comprises the coated substrate according to claim 14.

Description

INVENTIVE AND COMPARATIVE EXAMPLES

[0298] 1a. Production of Inventive Aqueous Coating Compositions and of a Comparative Coating Composition

[0299] The CathoGuard® 800 pigment paste from BASF that is used for producing the exemplary inventive coating compositions and the comparative coating composition V1 below comprises bismuth subnitrate. The skilled person knows of the production of such pigment pastes from, for example, DE 10 2008 016 220 A1 (page 7, table 1, variant B).

[0300] Comparative Coating Composition (A)V1

[0301] 2129 g of an aqueous dispersion of a binder and a crosslinking agent (commercially available product CathoGuard® 800 from BASF with a solids content of 38.0 wt %), 302 g of a pigment paste (commercially available product CathoGuard® 800 from BASF with a solids content of 65.5 wt %), 1258 g of water, and 1309.5 g of a solution of bicine (N,N′-bis(2-hydroxyethyl)glycine) in water (59.5 g bicine+1250 g water) were combined to form a comparative coating composition (A)V1. In this case the bicine solution was prepared first and then added to the initial charge, comprising the binder and the paste. The mixture was stirred at 18-23° C. for 24 hours.

[0302] Coating Composition (A)E1

[0303] The coating composition (A)E1 was produced as for (A)V1, except that additionally 250 g of a solution of lithium acetate dihydrate in water (7.2 g of lithium acetate dihydrate in 892.8 g of water, then 250 g of this solution) were admixed and the amount of water added was reduced correspondingly by 250 g. The fraction of lithium (A4) in (A)E1 therefore corresponded to a fraction of 27 ppm, based on the total amount of (A)E1.

[0304] Coating Composition (A)E2

[0305] The coating composition (A)E2 was produced as for (A)E1, except that additionally 250 g of a solution of copper(II) nitrate trihydrate in water (7.6 g of copper(II) nitrate trihydrate +992.4 g of water, then 250 g of this solution) were admixed and the amount of water added was reduced correspondingly by 250 g. The fraction of copper (A6a) in (A)E2 therefore corresponded to a fraction of 100 ppm, based on the total amount of (A)E2.

[0306] For all three stated coating compositions, determinations were made of the amounts of dissolved lithium (A4), the amount of dissolved phosphorus (A5) (for the control), the amount of dissolved copper, and the amounts of bismuth (A3a) and (A3b). Table 1a provides an overview of the resultant inventive coating compositions (A)E1 and (A)E2 and also of the comparative coating composition (A)V1. Also stated, where appropriate, are the pH and the conductivity. The respective pH values and conductivities of table 1a are each ascertained, if determined at all, at a temperature in the region of 20° C.

TABLE-US-00001 TABLE 1a (A)V1 (A)E1 (A)E2 Fraction (A3a)*  980 ppm  980 ppm  980 ppm Fraction (A3b)* 1520 ppm 1520 ppm 1520 ppm Fraction (A4) —   27 ppm   27 ppm Fraction (A5) — — — Fraction (A6a) — — 100 ppm pH 5.44 5.58 5.47 Conductivity 2.03 mS/cm 2.26 mS/cm 2.26 mS/cm *Amounts determined by way of example for (A)E2; for (A)V1 and (A)E1 exactly the same amounts and fractions of the bismuth-containing components were used.

[0307] 1b. Production of Further Coating Compositions with Varying Fraction of Lithium (A4)

[0308] Further coating compositions were produced as for (A)E2, but with variation in the fraction of lithium (A4). For this purpose, the abovementioned solution of lithium acetate dihydrate in water (7.2 g of lithium acetate dihydrate in 892.8 g of water) was io admixed in correspondingly different amounts. By varying the fraction of water added, the total amount of sample produced was again kept constant.

[0309] Table 1 b provides an overview of the coating compositions produced. For greater ease of comparison, the coating composition (A)E1 is also shown.

TABLE-US-00002 TABLE 1b (A)E2 (A)E1 (A)E3 (A)E4 (A)E5 (A)V2 Fraction (A4)   17 ppm   21 ppm   27 ppm   38 ppm   49 ppm  350 ppm pH 5.24 5.34 5.58 5.60 5.58 5.96 Conductivity 2.47 2.47 2.26 2.40 2.58 4.55 [mS/cm]

[0310] 1c. Production of Further Coating Compositions with Varying Fraction of Lithium and Phosphorus

[0311] Further coating compositions were produced as for (A)E1, but using lithium phosphate instead of a solution of lithium acetate dihydrate in water. The lithium phosphate here was incorporated by grinding as a constituent of the pigment paste. Table lc provides an overview of these coating compositions. For greater ease of comparison, the coating composition (A)E1 is also shown.

TABLE-US-00003 TABLE 1c (A)E1 (A)E6 (A)E7 (A)E8 (A)E9 Fraction lithium — 100 ppm 150 ppm 250 ppm  350 ppm phosphate Fraction (A4) 27 ppm  27 ppm  27 ppm n.d. n.d. Fraction (A5) —  27 ppm  40 ppm  67 ppm 93.5 ppm pH 5.58 5.31 5.24 5.50 5.60 Conductivity 2.26 2.34 2.29 2.32 2.45 [mS/cm] n.d.—not determined

[0312] 2. Production of Coated Electrically Conductive Substrates by Means of One of the Coating Compositions (A)

[0313] The aqueous coating compositions described under 1. were each applied as dipped coatings to various substrates. Each of the compositions here is applied to the various substrates immediately after its production as described above.

[0314] Three kinds of metal test panels are used, these being T1 (hot-dip-galvanized steel (HDG)) and T2 (aluminum (ALU)) and also T3 (cold-rolled steel (CRS)).

[0315] These panels are first of all each cleaned by immersion into a bath comprising an aqueous solution comprising the commercially available product Gardoclean S5160 from Chemetall and also water (97.7 wt %) for a duration of 2 minutes at a temperature of 60° C.

[0316] The substrates cleaned in this way are subsequently rinsed with water.

[0317] Immediately thereafter, one of the inventively employed aqueous coating compositions is applied to each panel T1, T2 or T3, with the respective panel being immersed in each case into a corresponding dip-coating bath comprising one of the compositions. This dip-coating bath has a respective temperature of 30° C.

[0318] Coating in the dip-coating bath here is carried out by means of a two-stage deposition step and coating step, which provides two stages (1a) and (1 b), where first of all, galvanostatically, current strengths in the range from 0.02 to 0.32 A or, potentiostatically, a voltage of 4 V are applied, in each case over a duration of 120 seconds (corresponding to stage (1a)).

[0319] Subsequent to this, for the substrates obtained after stage (1a), stage (1b) of step (1) of the method of the invention is carried out, with application either potentiostatically of a voltage of 4 V or galvanostatically of current strengths in the range of 0.12 to 0.28 A, which in each case within stage (1b) are increased continuously, linearly, to a voltage in the range of 200-220 V, in each case over a duration of 30 seconds, by means of a voltage ramp. This respective voltage is then maintained for a duration of 90 seconds (hold time) to give (after the subsequent curing) a coating of the respective substrate in a dry film thickness of 17 to 22 micrometers. The test panels are subsequently cured for 25 minutes in an oven (175° C. unless explicitly stated otherwise).

[0320] 3. Investigation of the Corrosion Prevention Effect of the Coated Substrates

[0321] The substrates coated with one of the coating compositions are investigated by means of the measurement methods described earlier on above.

[0322] 3a Investigation of Coatings Produced Using the Coating Compositions Listed in Table 1a

[0323] The coatings produced using the coating compositions listed in table 1a were investigated for their corrosion resistance. It should be pointed out that outstanding resistance is achieved for average underminings of around 1 mm. Moreover, differences in the absolute range by about 1 mm are difficult to evaluate technically and are therefore not meaningful. Table 3a shows the results.

TABLE-US-00004 TABLE 3a Coating composition (A)V1 (A)E1 (A)E2 Coating B(A)V1 B(A)E1 B(A)E2 Average undermining 4.2 3.7 2.9 NSS (substrate CRS) [mm ] Average undermining 5.7 4.9 3.9 VDA (substrate CRS) [mm] Average undermining 6.7 6.8 5.5 PV1210 (substrate HDG) [mm] Average undermining 0.8 1.2 0.4 CASS (substrate ALU) [mm] Delamination number 124.8 3.2 3.8 NSS (substrate ALU)

[0324] The results show that the system B(A)E1 in comparative to the system B(A)V1 exhibits slightly improved corrosion resistance on steel substrates. While the undermining properties on aluminum are very good for both B(A)E1 and B(A)V1, the io number of delamination points on the panel in the case of B(A)V1 is extremely high, which means that there are numerous nuclei for further corrosive attack. In contrast to this, the number of such delamination points in the B(A)E1 system is extremely small. The inventive system B(A)E2 is further improved in relation to the corrosion inhibition properties.

[0325] In total it is found that the inventive system is ideally suited to offering outstanding corrosion prevention in relation to both common metal types, steel and aluminum, and is therefore ideally suited to substrates in which both types of metal are present.

[0326] 3b Investigation of Coatings Produced Using the Coating Compositions Listed in Table 1 b

[0327] The coatings produced using the coating compositions listed in table 1 b were investigated for their surface structure/surface quality. Table 3b shows the results.

[0328] It was indeed possible to deposit the coating composition (A)V2 by the method of electrophoretic deposition coating. The result, however, was a disturbed surface with lots of holes, which was unacceptable.

TABLE-US-00005 TABLE 3b Coating composition (A)E2 (A)E1 (A)E3 (A)E4 (A)E5 (A)V2 Coating B(A)E2 B(A)E1 B(A)E3 B(A)E4 B(A)E5 B(A)E6 Fraction (A4) 17 ppm 21 ppm 27 ppm 38 ppm 49 ppm 350 ppm Surface quality  1  1-2  1-2  2-3  3-4  5

[0329] Furthermore, the surface quality of the coating B(A)V1 was investigated and was rated with a score of 1.

[0330] Representatively, moreover, a number of the investigations of corrosion resistance described under 3a were also carried out for the coatings B(A)E2, B(A)E3, B(A)E4, and B(A)E5. The results showed that the respective corrosion resistance is in the region of the coating B(A)E1 and is therefore better than the corrosion resistance of the coating B(A)V1.

[0331] In total the results show that the addition of lithium (A4) to the coating compositions leads on the one hand to a significantly improved corrosion resistance. On the other hand an increase in the fraction of (A4) leads to a reduced surface quality. At a fraction of (A4) of 388 ppm it was not even possible any longer to carry out electrophoretic deposition.

[0332] 3c Investigation of Coatings Produced Using the Coating Compositions Listed in Table 1 c

[0333] The coatings produced using the coating compositions listed in table 1c were investigated for their crosslinking properties. Table 3c shows the results.

TABLE-US-00006 TABLE 3c (A)E1 (A)E6 (A)E7 (A)E8 (A)E9 Fraction lithium — 100 ppm 150 ppm 250 ppm  350 ppm phosphate Fraction (A4) 27 ppm  18 ppm  27 ppm n.d. n.d. Fraction (A5) —  27 ppm  40 ppm  67 ppm 93.5 ppm Onset 147° C. 148° C. 150° C. 157° C. 155° C. temperature Offset time 30 min 30 min 36 min 43 min 46 min 160° C. Offset time 20 min 20 min 22 min 27 min 28 min 175° C. Offset time 15 min 15 min 18 min 19 min 18 min 190° C. Tg (CRS) 88 80 77 56 52 Tg (HDG) 86 76 64 57 56 Tg (ALU) 87 84 56 49 48

[0334] The results show that for the preferred system of a coating composition (A) comprising a bismuth-based catalyst, the fraction of phosphorus present in the composition ought preferably to be extremely low. The reason is that, the higher the fraction of phosphorus, the poorer the crosslinkability (apparent from the higher onset temperature, the longer offset times, and the lower glass transition temperatures).