Method for coating metallic surfaces with an aqueous composition

Abstract

The invention relates to a process for coating metallic surfaces with a composition containing at least one of a silane, silanol, siloxane or polysiloxane that is capable of condensation, water and optionally an organic solvent. The composition also contains compound containing Ti, Hf, Zr, Al or B; and at least one type of cation or an organic compound d) selected from monomers, oligomers, polymers, copolymers and block copolymers. The coating freshly applied with this composition is rinsed with a fluid and is not dried thoroughly before this rinsing step so that the compound capable of condensation does not condense substantially before the rinsing step.

Claims

1. A process for forming a silane pretreatment layer, the process comprising contacting a metallic surface with a first composition to form a first coating, wherein said first composition comprises: water, and a) a compound capable of condensation selected from the group consisting of a silane, a silanol, a siloxane and a polysiloxane, and at least one of b) a compound containing titanium, hafnium, zirconium, aluminum or boron, or c) a cation selected from a metal of subgroup 1 to 3 and 5 to 8, which subgroups include the lanthanides, and of main group 2 of the periodic table of the elements; and d) a hydrophobicity adjusting agent; and rinsing the first coating while it is wet with an aqueous surfactant composition to form the silane pretreatment layer.

2. The process of claim 1, further comprising applying a subsequent coating to the silane pretreatment layer.

3. The process according to claim 2, wherein the subsequent coating is selected from the group consisting of a conversion coating, a coating resulting from the application of an after-rinsing solution, a coating based on a lacquer, a primer and an adhesive.

4. The process of claim 2, further comprising applying the subsequent coating within three minutes of the rinsing step that forms the silane pretreatment layer.

5. The process according to claim 1, wherein the first composition has a pH greater than 1.5 and less than 9.

6. The process according to claim 1, wherein compound a) is in the range from 0.005 to 80 g/l, calculated as the corresponding silanols.

7. The process according to claim 1, wherein compound a) comprises at least one of an amino group, an urea group or an ureido group.

8. The process of claim 1, wherein prior to the contacting of the metallic surface and the first composition, the metallic surface is pre-rinsed and/or a first silane coating is applied to the metallic surface by contacting the metallic surface with a first aqueous composition containing at least one silane, silanol, siloxane, and/or polysiloxane, at least one compound selected from fluoride-free compounds of titanium, hafnium, zirconium, aluminum and boron, at least one alkali solution, or at least one complex fluoride.

9. The process of claim 1, wherein the coating comprises at least one of organic monomers, oligomers, polymers, copolymers and block copolymers each comprising silicon.

Description

EXAMPLES AND COMPARATIVE EXAMPLES

(1) The Examples according to the invention (E) and Comparative Examples (CE) described below are intended to illustrate the subject matter of the invention in greater detail.

(2) The aqueous bath compositions are prepared as mixtures according to Table 2 using already prehydrolysed silanes. They each contain predominantly one silane and optionally also have small contents of at least one other similar silane, where here again the word silane is used rather than silane/silanol/siloxane/polysiloxane by way of simplification, and where normally these various compounds, sometimes in a larger number of similar compounds, also pass through into the formation of the coating, so there are often several similar compounds present in the coating as well. Depending on the silane, the prehydrolysis step can also take several days at room temperature, with vigorous stirring, if the silanes to be used are not already present in prehydrolysed form. The prehydrolysis of the silane is carried out by placing the silane in excess water and optionally catalysing with acetic acid. Acetic acid was added in only a few embodiments for the sole purpose of adjusting the pH. In some embodiments, acetic acid is already present as a hydrolysis catalyst. Ethanol is formed in the hydrolysis, but is not added. The finished mixture is used fresh.

(3) Then, for each test, at least 3 sheets of cold-rolled steel (CRS), aluminium alloy Al 6016, steel hot-dip galvanized or electrogalvanized on both sides, or Galvaneal (ZnFe layer on steel), previously cleaned with an aqueous alkaline cleaner and rinsed with industrial water and then with demineralized water, are brought into contact on both sides with the appropriate treatment liquid at 25 C. by spraying, dipping or rollcoater treatment. Immediately thereafter, the sheets pretreated in this way are briefly rinsed with demineralized water. The sheets of the Comparative Examples are dried at 90 C. PMT and then lacquered with a cathodic automobile dip lacquer (CDL). The sheets of the Examples according to the invention, however, are rinsed immediately after the silane pretreatment and dipped in the CDL bath immediately after rinsing. These sheets were then provided with a complete commercial automotive lacquer system (filler, covering lacquer, transparent lacquer; overall thickness of stacked layers, including CDL, approx. 105 m) and tested for their corrosion protection and lacquer adhesion. The compositions and properties of the treatment baths and the properties of the coatings are collated in Table 2.

(4) The organofunctional silane A is an amino-functional trialkoxysilane and has one amino group per molecule. Like all the silanes used here, it is in extensively or almost completely hydrolysed form in the aqueous solution. The organofunctional silane B has one terminal amino group and one ureido group per molecule. The non-functional silane C is a bis-trialkoxysilane; the corresponding hydrolysed molecule has up to 6 OH groups on two silicon atoms.

(5) The complex fluorides of aluminium, silicon, titanium or zirconium are used extensively in the form of an MeF.sub.6 complex, but the complex fluorides of boron are used extensively in the form of an MeF.sub.4 complex. Manganese is added to the particular complex fluoride solution as metallic manganese and dissolved therein. This solution is mixed with the aqueous composition. If no complex fluoride is used, manganese nitrate is added. The silylated epoxy polymer has a low content of OH.sup. and isocyanate groups, so it can also subsequently be chemically crosslinked at temperatures above 100 C.

(6) The silanes present in the aqueous compositionconcentrate and/or bathare monomers, oligomers, polymers, copolymers and/or reaction products with other components due to hydrolysis reactions, condensation reactions and/or other reactions. The reactions take place especially in the solution, during drying or optionally also during curing of the coating, especially at temperatures above 70 C. All the concentrates and baths proved to be stable for one week without undergoing changes or precipitations. No ethanol was added. Ethanol contents in the compositions originate only from chemical reactions.

(7) In the majority of Examples and Comparative Examples, the pH is adjusted with ammonia if at least one complex fluoride is present and with an acid in other cases. All the baths have a good solution quality and almost always a good stability. There are no precipitations in the baths. In the Examples according to the invention and the Comparative Examples, the coating pretreatment with the silane-containing solution is immediately followed firstly by one brief rinse with demineralized water. The freshly applied wet film was not able to dry on more substantially because it was rinsed within 5 seconds of application of the silane-containing coating. Both the freshly coated substrate and the rinsing water were at room temperature. A rinse was necessary to prevent substances in the pretreatment solution from being introduced into the subsequent lacquer bath. The freshly rinsed, coated substrate was then immediately dipped in the cathodic dip lacquer, so no further drying could occur. Whereas the coated sheets of the Examples according to the invention, were coated with a cathodic dip lacquer immediately after rinsing, without intermediate drying, those of the Comparative Examples were dried in an oven for 5 minutes at 120 C. immediately after rinsing.

(8) Because of the interference colours, only the coatings on steel can be significantly examined visually, allowing an assessment of the homogeneity of the coating. The coatings without any complex fluoride content are very inhomogeneous. Surprisingly, a coating with titanium complex fluoride and zirconium complex fluoride proved to be markedly more homogeneous than when only one of these complex fluorides had been applied. An addition of nitroguanidine, nitrate or nitrite likewise improves the homogeneity of the coating. In some cases the layer thickness increases with the concentration of these substances.

(9) TABLE-US-00002 TABLE 2 Bath compositions in g/l, based on solids contents or, in the case of silanes, on the weight of the hydrolysed silanes; residual content: water and usually a very small amount of ethanol; process data and properties of the coatings Example/CE CE 1 E 1 CE 2 E 2 CE 3 E 3 CE 4 E 4 CE 5 E 5 CE 6 E 6 CE 7 E 7 CE 8 E 8 CE 9 E 9 Organofunct. 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.1 0.1 0.3 0.3 0.2 0.2 0.2 0.2 silane A H.sub.2TiF.sub.6 as Ti 0.2 0.2 0.2 0.2 0.2 0.2 0.1 0.1 0.3 0.3 0.2 0.2 0.2 0.2 H.sub.2ZrF.sub.6 as Zr 0.2 0.2 0.2 0.2 0.2 0.2 0.1 0.1 0.3 0.3 0.4 0.4 0.2 0.2 Mn 0.3 0.3 0.3 0.3 Silylated epoxy 1.0 1.0 polymer pH 10.5 10.5 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 Layer weight, 10- 10- 20- 20- 20- 20- 20- 20- 20- 20- 10- 10- 30- 30- 30- 30- 50- 50- mg/m.sup.2, of silanol 20 20 50 50 50 50 50 50 60 60 40 40 80 80 80 80 100 100 and metal BMW cross-cut test: score Steel 4 3 5 5 3 2 2 1 1 0 2 1 1 1 1 0 1 1 E-zinc on steel 3 3 4 3 4 3 1-2 1 1 0 1 1 1 1 1 1 0 0 Hot-dip zinc on 2 2 4 3 4 3 1 0 0 0 1 0 0-1 0-1 0-1 0 0 0 steel Al 6016 2 2 2 2 2 2 1 1 1 0 2 1 1 1 1 0 1 1 Galvaneal 1 1 1 1 2 1 1 0 1 0 1 0 0 0 0 0 0 0 10 VDA cycles, mm disbonding Steel 8 8 7 5 4 3 3 2.5 2 1 3.5 2 1.5 1.5 1.5 <1 2.5 2 E-zinc on steel 5 4 3 2.5 4 4 2 1 1 <1 3 1.5 1.5 1 1 <1 1 <1 Hot-dip zinc on 4 4 2.5 2 3.5 3 <1 <1 <1 <1 1.5 1 1 1 1 <1 <1 <1 steel Galvaneal 2 2 2 1 1.5 1.5 <1 0 <1 <1 1 1 <1 <1 <1 <1 0 0 Stone chip resis- tance after VDA stress: score Steel 5 5 4 4 4 3 2-3 1 1 1 2 2 2 1 1-2 1 1 0-1 E-zinc on steel 5 4 3 2 4 3 2 1 1 1 2 1 1-2 0 1 0 1 0-1 Hot-dip zinc on 5 4 3 2 4 3 1 0 1 0 1 1 1 0 1 0 0 0 steel Galvaneal 4 4 2 2 3 2 1-2 0 1 0 1 0-1 0 0 0 0 0 0 Salt spray test, 1008 h Steel 7 6 4 4 3.5 3 2 1.5 1.5 <1 2.5 2 1.5 <1 1.5 <1 1 1 CASS test, mm disbonding Al 6016 6 6 3.5 3 3.5 3 2.5 2.5 1.5 1 2.5 2 1.5 1 1.5 1 1.5 1

(10) When the different metallic surfaces which have been coated are considered as a whole, all the Examples show a significant improvement in comparison with the respective Comparative Example, the same bath composition being applied each time, in one case with subsequent drying (as Comparative Example, CE) and in the other case without subsequent drying (as Example according to the invention, E).

(11) It was surprising that this improvement, which brings only a limited improvement to already good coating results, is systematically improved, by not drying after application of the aqueous composition. It is therefore possible, surprisingly, to achieve, by not drying, a significant improvement which is universally almost independent of the chemical composition of the aqueous bath. It was further surprising that this improvement occurred both in the case of the solutions containing only silane, and in the case of the solutions containing silane and complex fluoride and optionally also manganese ions. It is therefore assumed that a similar constant improvement from drying to not, drying also occurs in the case of solutions of similar composition or in the case of solutions containing several different substances, based on silane or on silane and complex fluoride. The greater the number of different substances present and the larger their inherently small contents, the better the corrosion resistance and lacquer adhesion can be, provided any optimum is not exceeded.

(12) The layer weight varies not only with the contents of the individual constituents of the aqueous solutions, but also with the particular type of metallic surface being coated. Choosing the bath components and their contents makes it possible overall to achieve a very marked improvement in the corrosion resistance and lacquer adhesion.

(13) Over the short period of use, all the bath compositions are found to be stable and satisfactory to apply. There are no differences in behaviour, visual impression or test results between the different Examples and Comparative Examples which can be attributed to the treatment conditions, e.g. application by spraying, dipping or rollcoater treatment. The films formed are transparent and almost all are extensively homogeneous. They do not colour the coating. The films formed are transparent and almost all are extensively homogeneous. The structure, gloss and colour of the metallic surface appear to be only slightly changed by the coating. If a titanium and/or zirconium complex fluoride is present, iridescent layers are formed, especially on steel surfaces. Combining several silanes has not so far brought about a significant improvement in the corrosion protection, but this cannot be ruled out. Furthermore, a content of H.sub.3AlF.sub.6 was found on aluminium-rich metallic surfaces due to corresponding reactions in the aqueous composition. Surprisingly, however, combining two or three complex fluorides in the aqueous composition has proved extremely beneficial.

(14) The layer thickness of the coatings produced in this way also dependent on the type of application, which was initially varied in specific experimentsranged from 0.01 to 0.16 m and usually from 0.02 to 0.12 m and was often up to 0.08 m, being markedly greater when organic polymer was added.

(15) Given that the development of the zinc/manganese/nickel phosphatizing of car bodies has spanned several decades, the phosphate layers of this type produced today are of extremely high quality. Nevertheless, contrary to expectation, it was possible to achieve the same high-quality properties with silane-containing coatings by means of aqueous silane-containing compositions that have only been in use for a few years, even though a greater effort was required.

(16) The corrosion protection scores in the cross-cut test according to DIN EN ISO 2409, after storage for 40 hours in 5% NaCl solution according to BMW specification GS 90011, range from 0 to 5, 0 representing the best values. In the salt spray/condensation water alternation test over 10 cycles according to VDA test sheet 621-415 with alternating corrosion stress between salt spray test, perspiration water test and drying interval, the disbonding is measured on one side from the scratch outwards and reported in mm, the disbanding ideally being as small as possible. In the stone chip resistance test according to DIN 55996-4, the coated metallic sheets are bombarded with scrap steel after the aforementioned VDA alternation test over 10 cycles: The damage picture is characterized by scores from 0 to 5, 0 representing the best results. In the salt spray test according to DIN 50021 SS, the coated sheets are exposed for up to 1008 hours to an atomized corrosive sodium chloride solution; the disbonding is then measured in mm from the scratch outwards, the scratch being made with a standard gouge down to the metallic surface, and the disbonding ideally being as small as possible. In the CASS test according to DIN 50021 CABS, the coated sheets made of an aluminium alloy are exposed for 504 hours to an atomized special corrosive atmosphere; the disbonding is then measured in mm from the scratch outwards and ideally is as small as possible.

(17) Other experiments on car body elements have shown that the electrochemical conditions of the CDL bath may be very slightly adaptable to the different kind of coating, but otherwise that the outstanding properties obtained in laboratory experiments can be reproduced on car body elements.