ENAMEL COMPOSITIONS, PROCESS FOR THEIR PREPARATION AND THEIR USE

Abstract: This invention relates to particulate mixtures of oxides (enamel mixtures), which are suitable for the production of self-cleaning enamel layers on steel, a process for their preparation and their use.

Description: This invention relates to particulate mixtures of oxides (enamel mixtures), which are suitable for the production of self-cleaning enamel layers on steel, a process for their preparation and their use. Self-cleaning enamels have been known for a long time and are described in various documents. Thus, for example, US 3460 523 describes a composition which is useful for the coating of ovens and from which a film as porous as possible, containing catalytic materials, is produced. Many catalyst materials have been added to these compositions in order to enhance the decomposition of the fatty and oily residues in the layer and thereby finally to clean the oven (US 3 576 667, DE-B 2 221 135, DE-A 2 614 696, DE-A 3 626 424, DE-A 2 117 032). However, the self-cleaning enamels known hitherto can be tested by the self- cleaning cycle in accordance with DIN 51 171 only about 5 times, before their property of self-cleaning is lost. The test is used to determine the capacity of self-cleaning enamelling firstly to take up fats or splashes of oil in the form of droplets and then to burn the greater part of them under the action of heat. Another disadvantage of these known self-cleaning enamels is that regeneration by means of pyrolysis is not possible at low temperatures (US 3 930 062). Neither can the known self-cleaning enamels, in contrast to other enamels, be obtained by dry electrostatic powder application to the objects to be coated. The object was accordingly to provide enamel frits which permit the production of self-cleaning enamel layers having improved properties, and which moreover can be applied by means of electrostatic powder application to the objects to be enamelled. This object was achieved by the particulate mixtures of oxides according to the invention, which are a mixture of different enamel frits, optionally with other raw materials. This invention provides particulate mixtures of oxides for the production of self- cleaning enamel layers on steel, which have an electrical resistance of between 30 x 10 13 and 700 x 10 13 ohm-cm, measured on the particulate mixture of oxides (H. Kyri "Handbook for Bayer Enamels", Bayer Rickmann GmbH, 1975, pages 267-270) and a particle size of between 0.1 and 100 μm, consisting of A) at least two enamel frits A and B and optionally other raw materials, wherein A is present in a quantity of from 10 to 70 wt.% and has the following composition: 35 to 50 wt.% SiO 2 3 to 10 wt.% B 2 O 3 0 to 5 wt.% AI 2 O 3 and/or Sb 2 O 3 0 to 5 wt.% RO, wherein R denotes Mg, Ca, Sr and/or Ba 15 to 30 wt.% R' 2 O, wherein R' denotes Na, K and/or Li 20 to 30 wt.% TiO 2 0.1 to 10 wt.% ZnO 0 to 10 wt.% Fe 2 O 3 , Cr 2 O 3 , NiO, MnO, CoO and/or CuO and B is present in a quantity of from 30 to 90 wt.% and has the following composition: 40 to 60 wt.% SiO 2 3 to 10 wt.% B 2 O 3 10 to 25 wt.% AI 2 O 3 10 to 30 wt.% RO, wherein R denotes Mg, Ca, Sr and/or Ba 0.1 to 10 wt.% R' 2 O, wherein R' denotes Na, K and/or Li 0 to 10 wt.% Fe 2 O 3 , Cr 2 O 3 , NiO, MnO, CoO and/or CuO and the raw materials are present in a quantity of from 0 to 10 wt.% and are selected from the group comprising quartz, feldspar, zirconium dioxide, wollastonite, zirconium silicate, dolomite, nepheline, syenite, fluorspar, aluminium oxide and conventional dyes from the group comprising Fe 2 O 3 , Cr 2 O 3 , NiO, MnO, CoO, CuO, TiO 2 and mixtures thereof, the frits A and B and the raw materials being mixed so that the following overall composition Z is obtained: 35 to 60 wt.% SiO 2 3 to 10 wt.% B 2 O 3 3 to 25 wt.% AI 2 O 3 1.5 to 20 wt.% R' 2 O, wherein R' denotes Na, K and/or Li 2 to 30 wt.% TiO 2 10 to 30 wt.% RO, wherein R denotes Mg, Ca, Sr and/or Ba 0 to 7 wt.% ZnO 0 to 3.5 wt.% Sb 2 O 3 0.1 to 15 wt.% Fe 2 O 3 , Cr 2 O 3 , NiO, MnO, CoO and/or CuO and B) 0.01 to 0.5 wt.%, based on the overall composition, of at least one hydrophobic, surface-active agent. The particulate mixtures of oxides according to the invention, in contrast to the compositions known hitherto which are used for the production of self-cleaning enamel layers, can be applied by means of electrostatic powder application to the objects to be enamelled. The electrostatic powder application is known per se and need not be described again in more detail here (see, for example, DE- A 2 015 072, US 3 930 062). The self-cleaning enamelling produced with the particulate mixtures of oxides according to the invention can be repeatedly regenerated by pyrolysis at low temperature (350°C to 600°C), without this having an influence on their properties or on their appearance. In addition, the so-called self-cleaning cycle in accordance with DIN 51 171 can be operated considerably more often than can the hitherto known self-cleaning enamelling. The present invention also provides a process for the preparation of the particulate mixtures of oxides according to the invention, which is characterised in that at least two enamel frits A and B and optionally the raw materials, which have already been described in more detail above, are mixed with one another and the mixture is ground to a particle size of from 0.1 to 100 μm with the addition of at least one hydrophobic surface-active agent. The particulate mixtures of oxides according to the invention are used for the production of self-cleaning enamel layers on steel, being applied preferably by electrostatic powder application to the steel object to be enamelled, and namely in a quantity which is adequate to obtain a self-cleaning enamel layer of from 60 to 250 μm in thickness after the burning process, and then being burnt at 760°C to 850°C for 3 to 6 minutes. The invention is explained in more detail by means of the following Example. Example A steel sheet of 1 mm in thickness, to which a basecoat enamel (thickness: 80 to 100 μm) had already been applied, was provided for the application of the particulate mixture of oxides. In this Example, the enamel primer used was a frit of the following composition. The frit was applied electrostatically. AI 2 O 3 3.3 wt.% B 2 O 3 13.4 wt.% BaO 1.6 wt.% CaO 7.0 wt.% CoO 0.5 wt.% CuO 0.4 wt.% F 4.1 wt.% Fe 2 0.4 wt.% K 2 O 1.8 wt.% MnO 12 wt.% Na 2 O 15.2 wt.% NiO 1.6 wt.% SiO 2 47.3 wt.% ZrO, 3.6 wt.% Frits for basecoat enamels may be applied both wet and electrostatically. Preferably the particulate mixture of oxides according to the invention is then applied directly following the application of the enamel primer, without previous burning of the enamel primer (so-called two-layer-one-burn process). The particulate mixture of oxides had the following composition: Frit A Frit B SiO 2 [wt.%] 41 55 B 2 O 3 [wt.%] 7.5 7.2 AI 2 O 3 [wt.%] 0.4 14.1 CaO [wt.%] - 22.3 MgO [wt.%] - 0.5 ZnO [wt.%] 3.1 TiO 2 [wt.%] 24.2 Na 2 O [wt.%] 5 0.2 K 2 O [wt.%] 15.7 0.5 Li 2 O [wt.%] 2 0.2 Sb 2 O 3 [wt.%] 1.1 The two frits A and B and the dye FA 1216 from the firm Rhenania (black spinel based on Co/Cr/Fe/Mn/Ni) were mixed together and ground. 0.3 wt.% silicone oil (Baysilon MH 15 from Bayer AG, Leverkusen) was added during the grinding process. The composition contained 30 wt.% frit A, 70 wt.% frit B, 2 wt.% of the dye and 0.3 wt.% Baysilon MH 15. The particle size was less than 100 μm (15 wt.% > 43 μm). The electrical resistance of the powder was 100-10 13 ohm-cm. The powder was applied to the enamel primer electrostatically. This was followed by burning for 4 minutes at 820°C. The thickness of the self-cleaning enamel layer after the burning process was 200 μm. The enamelling was tested in accordance with DIN 51 171. The self-cleaning cycle could be operated at least 9 times. For the regeneration, the sample was pyrolysed at 400°C for a period of 2 hours. The enamelling is therefore a very satisfactory self-cleaning enamel layer which has a considerably better self- cleaning action than that of prior art. Moreover it can be applied electrostatically to the enamel primer.

Claims: ClaJms 1. A particulate mixture of oxides for the production of self-cleaning enamel layers on steel, which has an electrical resistance of between 30 x 10 13 and 700 x 10 13 ohm-cm, measured on the particulate mixture of oxides, and a particle size of between 0.1 and 100 μm, consisting of A) at least two enamel frits A and B and optionally other raw materials, wherein A is present in a quantity of from 10 to 70 wt.% and has the following composition: 35 to 50 wt.% SiO 2 3 to 10 wt.% B 2 O 3 0 to 5 wt.% AI 2 O 3 and/or Sb 2 O 3 0 to 5 wt.% RO, wherein R denotes Mg, Ca, Sr and/or Ba 15 to 30 wt.% R' 2 O, wherein R' denotes Na, K and/or Li 20 to 30 wt.% TiO 2 0.1 to 10 wt.% ZnO 0 to 10 wt.% Fe 2 O 3 , Cr 2 O 3 , NiO, MnO, CoO and/or CuO and B is present in a quantity of from 30 to 90 wt.% and has the following composition: 40 to 60 wt.% SiO, 3 to 10 wt.% B 2 O 3 10 to 25 wt.% AI 2 O 3 10 to 30 wt.% RO, wherein R denotes Mg, Ca, Sr and/or Ba 0.1 to 10 wt.% R' 2 O, wherein R' denotes Na, K and/or Li 0 to 10 wt.% Fe 2 O 3 , Cr 2 O 3 , NiO, MnO, CoO and/or CuO and the raw materials are present in a quantity of from 0 to 10 wt.% and are selected from the group comprising quartz, feldspar, zirconium dioxide, wollastonite, zirconium silicate, dolomite, nepheline, syenite, fluorspar, aluminium oxide and conventional dyes from the group comprising Fe 2 O 3 , Cr 2 O 3 , NiO, MnO, CoO, CuO, TiO 2 and mixtures thereof, the frits A and B and the raw materials being mixed so that the following overall composition Z is obtained: 35 to 60 wt.% SiO 2 3 to 10 wt.% B 2 O 3 3 to 25 wt.% AI 2 O 3 and/or Sb 2 O 3 1.5 to 20 wt.% R' 2 O, wherein R' denotes Na, K and/or Li 2 to 30 wt.% TiO 2 10 to 30 wt.% RO, wherein R denotes Mg, Ca, Sr and/or Ba 0 to 7 wt.% ZnO 0 to 3.5 wt.% Sb 2 O 3 0.1 to 15 wt.% Fe 2 O 3 , Cr 2 O 3 , NiO, MnO, CoO and/or CuO and B) 0.01 to 0.5 wt.%, based on the overall composition, of at least one hydrophobic, surface-active agent. 2. A process for the preparation of the particulate mixture of oxides according to claim 1 , characterised in that at least two enamel frits A and B and optionally the raw materials are mixed with one another and the mixture is ground to a particle size of from 0.1 to 100 μm with the addition of at least one hydrophobic surface-active agent. The use of the particulate mixture of oxides according to claim 1 for the production of self-cleaning enamel layers on prime-enamelled steel by electrostatic powder application of the particulate mixture of oxides in a quantity which is adequate to produce a self-cleaning enamel layer of from 60 to 250 μm in thickness after the burning process, and a subsequent burning process at 760°C to 850°C for 3 to 6 minutes.

Inventors: DANGREAU NATHALIE, DHAESE ANTOINE, GLEDITZSCH SIEGFRIED, LAMOTE FRITS, LEVEAUX MARC, DANGREAU, NATHALIE, DHAESE, ANTOINE, GLEDITZSCH, SIEGFRIED, LAMOTE, FRITS, LEVEAUX, MARC
Applicants: BAYER AG, DANGREAU NATHALIE, DHAESE ANTOINE, GLEDITZSCH SIEGFRIED, LAMOTE FRITS, LEVEAUX MARC, BAYER AKTIENGESELLSCHAFT, DANGREAU, NATHALIE, DHAESE, ANTOINE, GLEDITZSCH, SIEGFRIED, LAMOTE, FRITS, LEVEAUX, MARC
Application year: 1997
Main classification: C03C8
IPC codes: C03C8, C23D5