DOPED ALKALI SILICATE PROTECTIVE LAYERS ON METAL

Abstract

A method for producing a glass-like protective layer on an optionally pre-coated metal or glass substrate. The method comprises: (a) mixing one or more defined silicon compounds with NaOH and KOH, (b) adding water to the mixture obtained in (a) to hydrolyze the silicon compound(s), (c) adding at least one defined compound of formula MY.sub.m,
where M is Pb, Ti, Zr, Al or B, to the hydrolyzed mixture obtained in (b), wherein the molar ratio M/Si is from 0.01/1 to 0.04/1, to obtain a coating sol, (d) applying the coating sol obtained in (c) to the substrate, and (e) thermal densification of the coating sol applied in d) at a temperature of from 300° C. to 500° C. to form the glass-like protective layer.

Claims

1.-18. (canceled)

19. A method for producing a glass-like protective layer on a metal substrate or glass substrate or a pre-coated metal substrate or glass substrate, wherein the method comprises (a) mixing with NaOH and KOH one or more silicon compounds of formula (I)
R.sub.nSiX.sub.4-n   (I) in which R is independently alkyl, aryl, alkaryl or arylalkyl, X is independently alkoxy or acyloxy, and n is 0, 1 or 2, with the proviso that at least one silicon compound of formula (I) in which n is 1 or 2 is used, (b) adding water to a mixture obtained in (a) to hydrolyze the silicon compound(s) of formula (I), (c) adding at least one compound of formula (II)
MY.sub.m   (II) in which M is Pb, Ti, Zr, Al or B, Y is independently alkoxy, and m corresponds to a valence of M, wherein one or more alkoxy groups may be replaced by one or more chelate formers and/or two alkoxy groups may be replaced by an oxo group, to a hydrolyzed mixture obtained in (b), a molar ratio M/Si being from 0.01/1 to 0.04/1, M being a molar amount of M in the at least one compound of formula (II) and Si being a molar amount of Si in the at least one silicon compound of formula (I), to obtain a coating sol, (d) applying the coating sol obtained in (c) to the metal substrate or glass substrate or the pre-coated metal substrate or glass substrate, and (e) thermal densification of the coating sol applied in (d) at a temperature of from 300° C. to 500° C. to form the glass-like protective layer.

20. The method of claim 19, wherein in the silicon compound of formula (I) the group R is selected from C.sub.1-4 alkyl or phenyl, and/or the group X is selected from C.sub.1-4 alkoxy, and/or wherein in the compound of formula (II) the group Y is selected from C.sub.1-8 alkoxy.

21. The method of claim 19, wherein in the silicon compound of formula (I) the group R is selected from methyl, ethyl or phenyl, and/or the group X is selected from methoxy or ethoxy, and/or wherein in the compound of formula (II) the group Y is selected from C.sub.1-4 alkoxy.

22. The method of claim 19, wherein the one or more silicon compounds of formula (I) comprise at least one silicon compound A of formula (I), in which n is 1 or 2, and at least one silicon compound B of formula (I), in which n is 0.

23. The method of claim 22, wherein a molar ratio of the at least one silicon compound A to the at least one silicon compound B is from 1 to 10.

24. The method of claim 22, wherein a molar ratio of the at least one silicon compound A to the at least one silicon compound B is from 3 to 5.

25. The method of claim 19, wherein a molar ratio Si/(Na+K) is from 20 to 7, Si being a molar amount of Si in the at least one silicon compound of formula (I) and (Na+K) being a total molar amount of Na and K in NaOH and KOH.

26. The method of claim 19, wherein the metal substrate is selected from a substrate of steel, aluminum, aluminum alloy, titanium, titanium alloy, magnesium or magnesium alloy.

27. The method of claim 19, wherein the molar ratio M/Si is from 0.015/1 to 0.035/1.

28. The method of claim 19, wherein the molar ratio M/Si is from 0.021/1 to 0.03/1.

29. The method of claim 19, wherein water added in (b) is added in an amount of from 0.2 to 0.5 mol per mol of hydrolyzable groups present in the silicon compound(s) of formula (I).

30. The method of claim 19, wherein water is added in (b) at a rate such that a temperature of the mixture does not exceed 40° C.

31. The method of claim 19, wherein the mixture heated by the hydrolysis reaction is allowed to cool to a temperature of from 15° C. to 25° C. after addition of the water before at least one compound of formula (II) is added in (c).

32. The method of claim 19, wherein at least one pigment and/or at least one chelate former is added to the coating sol or a precursor thereof.

33. The method of claim 19, wherein the thermal densification is carried out at a temperature of from 300° C. to 475° C.

34. The method of claim 19, wherein the thermal densification is carried out in two stages, the applied coating sol being first heated in an atmosphere having an oxygen content of at least 1 vol. % to a temperature of from 150° C. to 200° C. and being subsequently heated in an inert gas atmosphere to a temperature of from 300° C. to 500° C.

35. The method of claim 19, wherein the glass-like protective layer is applied as the only coating on the metal substrate or glass substrate.

36. The method of claim 19, wherein the coating sol is applied to a metal substrate or glass substrate which is pre-coated with an enamel layer.

37. The method of claim 19, wherein the layer thickness of the glass-like protective layer after thermal densification is from 2 μm to 10 μm.

38. A metal substrate or glass substrate having a glass-like protective layer, obtained by a method according to claim 19.

Description

EXAMPLES

Synthesis Example 1

[0077] Step a) 593 g tetraethoxysilane and 2032 g methyltriethoxysilane are placed in a flask and 36 g sodium hydroxide and 49 g KOH are added successively while stirring. The mixture is stirred until the salts are completely dissolved. Then, 290 ml of water are added dropwise in the manner known to a person skilled in the art so that a temperature of 36° C. is not exceeded.

[0078] Step b) Once the addition of water is complete, the mixture is allowed to cool and 900 g of 2-propanol are added. 117 g of tetrabutyl titanate (TBOT) are added slowly, dropwise, to the mixture, which is now stirred at 250 rpm. A rest period before addition of the titanium compound is not necessary. The reaction solution is then stirred for a further 2 hours before a sol suitable for coating is obtained. The Ti/Si molar ratio in the coating sol is 0.024/1.

Comparative Synthesis Example 1

[0079] 593 g tetraethoxysilane and 2032 g methyltriethoxysilane are placed in a flask and 36 g sodium hydroxide and 49 g KOH are added successively while stirring. The mixture is stirred until the salts are completely dissolved. Then, 290 ml of water are added dropwise in the manner known to a person skilled in the art so that a temperature of 36° C. is not exceeded.

[0080] Once the addition of water is complete, the mixture is allowed to cool and 900 g of 2-propanol are added. 231.3 g of tetrabutyl titanate (TBOT) are added slowly, dropwise, to the mixture, which is now stirred at 250 rpm. A rest period before addition of the titanium compound is not necessary. The reaction solution is then stirred for a further 2 hours before a sol suitable for coating is obtained. The Ti/Si molar ratio in the coating sol is 0.048.

Comparative Synthesis Example 2

[0081] 593 g tetraethoxysilane and 2032 g methyltriethoxysilane are placed in a flask and 36 g sodium hydroxide and 49 g KOH are added successively while stirring. The mixture is stirred until the salts are completely dissolved. Then, 290 ml of water are added dropwise in the manner known to a person skilled in the art so that a temperature of 36° C. is not exceeded.

[0082] Once the addition of water is complete, the mixture is allowed to cool and 900 g of 2-propanol are added. 492 g of tetrabutyl titanate (TBOT) are added slowly, dropwise, to the mixture, which is now stirred at 250 rpm. A rest period before addition of the titanium compound is not necessary. The reaction solution is then stirred for a further 2 hours before a sol suitable for coating is obtained. The Ti/Si molar ratio in the coating sol is 0.10.

Comparative Synthesis Example 3

[0083] Analogous to Synthesis Example 1, methyltriethoxysilane and tetraethoxysilane are placed in a flask and sodium hydroxide and potassium hydroxide are added. After the added alkali hydroxide has completely dissolved, the amount of tetrabutyl titanate mentioned in Synthesis Example 1 is slowly added, dropwise, to the reaction mixture. Immediately after the start of hydrolysis (when water is added), the TiO.sub.2 precipitates as a white precipitate.

[0084] A mixture is obtained that is unsuitable as a coating.

Synthesis Example 2—Synthesis Example for Pigmented Sol

[0085] 3200 g of the sol prepared according to Synthesis Example 1, step a) are mixed, after cooling, with 3186 g of a solution mixture consisting of equal parts by weight of ethanol, isopropanol and 1-butanol. 288 g of tetrabutyl titanate (TBOT) are slowly added, dropwise, to the mixture, which is now stirred at 250 rpm. The reaction solution obtained is stirred for at least 30 minutes.

[0086] Then, 500 g of an aluminium-containing pigment (STAPA IL HYDROLAN S 2100 Eckart) are added to this mixture within 10 minutes while stirring at 250 rpm. The mixture obtained is stirred for at least 30 minutes before a coatable sol is obtained.

[0087] The molar ratio Ti/Si in the coating sol is as in Synthesis Example 1.

Example 1

[0088] The coating sol prepared according to Synthesis Example 1 was applied to a titanium substrate by means of a spraying process. The coating sol can be applied as described in Synthesis Example 1 without further dilution steps (by adding solvent).

[0089] The titanium substrate coated with the coating sol according to Synthesis Example 1 is heated to 350° C. in a heating oven under standard atmosphere, i.e. air, at a rate of 10 K/min and held at this temperature for 30 min after reaching the temperature. The temperature is then cooled at max. speed to at least 200° C. before the cured substrates can be removed from the oven.

[0090] The glass-like protective layer obtained after curing is crack-free.

Comparative Example 1

[0091] The coating sol prepared in accordance with Comparative Synthesis Example 1 was applied to a titanium substrate by means of a spraying process as in Example 1, and the titanium substrate coated with the coating sol according to Comparative Synthesis Example 1 was cured according to the densification method described in Example 1.

[0092] The glass-like protective layer obtained after curing shows cracks.

Comparative Example 2

[0093] The coating sol prepared in accordance with Comparative Synthesis Example 2 was applied to a titanium substrate by means of a spraying process as in Example 1, and the titanium substrate coated with the coating sol according to Comparative Synthesis Example 2 was cured according to the densification method described in Example 1.

[0094] The glass-like protective layer obtained after curing shows cracks.

Example 2

[0095] The coating sol prepared in accordance with Synthesis Example 1 was applied to a stainless steel substrate by means of a spraying process and cured according to the compaction process described in Example 1.

[0096] The glass-like protective layer obtained after curing is crack-free. The substrate shows a certain tarnish colour after curing.

Example 3

[0097] The coating sol prepared in accordance with Synthesis Example 1 was applied to stainless steel substrates as in Example 2. The coated stainless steel substrates (samples) are subjected to a two-stage compression process.

[0098] For this purpose, the furnace chamber containing the samples is heated to 150-200° C. After reaching this temperature, the sample chamber is evacuated until a pressure of at least 10 mbar is reached. After evacuation, the sample chamber is flooded with N.sub.2 and flushed until the oxygen content is below 800 ppm.

[0099] Alternatively, this value can also be reached by repeated evacuation and flooding. As soon as the O.sub.2 value is below 800 ppm and the sample chamber is sufficiently flooded with inert gas, the temperature is raised to 500° C. at a rate of 10 k/min. The target temperature is maintained for 10 min. The temperature is then cooled to at least 200° C. at max. speed before the cured substrates can be removed from the oven.

[0100] The glass-like protective layer obtained after curing is crack-free. The substrate shows virtually no tarnish colour after curing. In order to reduce the discolouration of stainless steel substrates during the curing step, an inerting step can therefore be added during densification.

Example 4

[0101] The adhesion of burnt-on standard foods is tested using a food baking test. These standard foods used are apricot jam, egg yolk, egg white, olive oil, tomato puree and mustard. These are each applied to metal substrates (samples A, B and C as described below) and baked at a temperature of 250° C. for 10 minutes. The areas where the food was applied are cleaned mechanically with a white sponge, washing-up liquid and warm water and the result is assessed. [0102] Sample A: enameled substrate [0103] Sample B: enameled substrate according to sample A, in which a coating sol containing alkali silicate according to Example 2 of DE 19714949 A1 is applied to the enamel and bonded to the enamel layer in a baking step at 500° C. analogously to Example 4 of DE 19714949 A1 with the formation of a glass-like layer. [0104] Sample C: enameled substrate according to sample A, in which a coating sol according to the invention is applied to the enamel layer by spray application according to Synthesis Example 1 and is bonded to it in a baking step to form a glass-like layer (alkali silicate layer containing Ti). The coating sol is cured at 350° C. in the baking step analogously to the compression process described in Example 1.

[0105] The obtained samples A, B and C were then subjected to the food baking test with 6 foods each as described above. The results of the tested systems are shown in Table 1.

TABLE-US-00001 TABLE 1 Apricot Tomato Sample jam Egg yolk Egg white Olive oil pulp Mustard A − −−− −− − − − B ± ± ± ± ± ± C ++ ++ ++ +++ +++ +++ Key: −/−−/−−−: slight/medium/severe damage ±: no change compared to enamel; +/++/+++: slight/medium/severe improvement compared to the prior art.

[0106] As can be seen from Table 1, the alkali silicate layers containing Ti according to the invention (sample C) show the best results in all food baking tests performed. In contrast to the pure enamel surface (sample A), which was clearly damaged in all tests, and in comparison to a coating (sample B) corresponding to the prior art, which shows only slightly improved properties compared to the enamel surface, the Ti-containing alkali silicate layer according to the invention passes all tests.

[0107] The protective layer obtained by the method according to the invention at a baking temperature of 350° C. shows the best results compared to the pure enamel layer and the surfaces coated in accordance with the prior art.

Example 5

[0108] According to the prior art, temperature-stressed components (for example for the automotive sector) made of stainless steel 1.4301 are coated with an alkali silicate sol-gel system from the prior art, which contains aluminum pigments as coloring agent. After thermal curing, however, it became apparent that these layers detach locally under certain conditions. These local defects occur especially in the area of shaped surfaces, i.e. in areas where strong mechanical forces were applied for shaping. After initial flaking, the corrosion then progresses as layer corrosion and gradually detaches more and more areas of the sol-gel layer.

[0109] In another experiment, the temperature-loaded components made of stainless steel 1.4301 are coated by the method according to the invention with a coating sol according to Synthesis Example 2, which contains the above-mentioned aluminium pigments as colouring agents, and cured by thermal densification. Surprisingly, with the protective coating produced in accordance with the invention, there is no delamination in unshaped areas. The incorporation of the Ti components improves adhesion, and thus the required corrosion protection is ensured.

[0110] Comparative NMR spectra of the prior-art sol with the sol produced in accordance with the invention unexpectedly showed that the degree of condensation is reduced after the addition of the Ti component compared to the initial system.

Example 6

[0111] The coating sol prepared in accordance with Synthesis Example 1 was applied to titanium substrates to obtain samples. Thermal densification of the coated samples was carried out, on the one hand, at a baking temperature of 350° C. as described in Example 1 and, on the other hand, analogously but at a baking temperature of 500° C.

[0112] Unexpectedly, in both cases the sensitivity to fingerprint marks of the titanium substrate could be reduced by applying the coating sol by the method according to the invention. It was found that at baking temperatures of 500° C., the titanium formed distinct tarnish colors. The reduction of the baking temperature to 350° C., which is possible with the composition according to the invention, completely eliminates the tarnish color while maintaining excellent anti-fingerprint, anti-corrosion and mechanical properties.