DE2504083A1 - POROESE, INORGANIC, NON-METALLIC COATINGS ON A METAL SUBSTRATE FOR SELF-CLEANING SYSTEMS - Google Patents

Description

2:; -'-

BAYER RICKMANN GmbH

Br-her Cologne-Kalk

January 30, 1975

Porous, inorganic, non-metallic coatings on one Metal substrate for self-cleaning systems

The present invention relates to inorganic-non-metallic, up to 50 ° C thermally inert coatings with a defined Porosity for objects made of metal, in particular sheet steel.

They develop during roasting and baking, e.g. in kitchen stoves The dishes mostly contain carbon deposits, which can only be cleaned up poorly with standard household cleaning agents have it removed. More effective cleaning agents and solvents are therefore often used, but not only the deposits, but also often the enamel coating of the oven is attacked and partially damaged. on The subsequent deposits adhere to the roughened enamel surface all the more firmly, so that they are now can hardly be removed completely. The ovens get an unhygienic and unaesthetic appearance.

As the removal of the resinous, carbonaceous deposits - as already mentioned - usually with a

BR 35 - 1 -

6098 3 2/0796

ORIGINAL INSPECTED

There has been no lack of attempts at chemical or mechanical cleaning to replace. For example, cookers are offered whose oven can be switched on after use by switching on a special one Heating level can be heated to approx. 500 ° C. At this temperature, the leftover food and any remaining food burns Ashes can then be removed.

The effort involved in this process, known as "pyrolytic Cleaning "has become known, however, is considerable. Additional heating capacity and safety devices and stronger insulation of the oven make this process relatively expensive. There can also be an unpleasant smell in the kitchen can only be avoided if the combustion gases are discharged via a chimney during the cleaning process will.

It has therefore been suggested that those resulting from baking out to send gaseous combustion products through an additional combustion chamber, which is catalytic acting surfaces is provided. This should completely burn the exhaust gases and avoid unpleasant odors will.

Significant progress has been made in the coating of ovens that are already in use Prevent the formation of deposits. The surfaces of the ovens are covered with layers that promote oxidation which are able to break down droplets of fat as they arise, and therefore to a large extent deposits to avoid. The fat decomposition then takes place on porous layers during the baking and roasting process at temperatures between 160 and 2 & 0 ° C. In this case, one speaks of catalytically self-cleaning or continuous

BR 35 - 2 -

609832/0796

2504033

cleaning systems. The oxidizing, mostly oxidic catalysts can either be on the surface applied to a carrier material or in an enamel coating be embedded. The carrier materials usually consist of oxidation-accelerating, mostly oxidic materials with a glass or ceramic-like composition.

For example, US Pat. No. 3,266,477 describes self-cleaning cooking appliances whose surface has oxidizing catalysts such as platinum or oxides of the subgroup elements such as chromium, manganese, cobalt, nickel, etc. which contain on a catalyst carrier, e.g. aluminum. One disadvantage of this system was that it was inadequate Abrasion resistance of this catalyst layer.

To avoid this disadvantage, the German Offenlegungsschrift 2 117 032 describes an enamel frit with high levels of iron oxide in the melt offset Pritte produced by melting was pretreated after grinding with water, clay and thickening agents Steel sheets applied and after drying at enameling temperatures Burned in together with the sheet steel. These layers show better abrasion resistance, however is the adhesion of this layer when applied directly Insufficient steel sheet.

It was therefore proposed in DT-AS 2 018 54l to apply molten, glass-like, halide-containing frits with the addition of metallic aluminum powder in aqueous suspension to sheet steel and, in the usual way, similar to enamelling, to be burned in. These systems have the advantage of a satisfactory adhesion, a good one

BR 35 -3 -

603832/0798

Abrasion resistance, however, is the rheology of the enamel slip difficult to control due to the decomposition of the aluminum powder in aqueous suspension. In addition, the handling of finely divided metallic aluminum powder because of the easy flammability and the Exclusion of moisture difficult in enamelling works; The development of aluminum dust also leads to error phenomena especially with white enamelling.

The present invention will now be porous, inorganic, non-metallic coatings on metal substrates, particularly steel sheet lor self-cleaning systems, which is characterized in that the coatings in the form of a slurry, the weight addition to the usual for enamelling additives up to the 95th - % (based on solids) of thermally stable inorganic compounds and / or minerals with a particle size between about 20 and 100 microns, applied and then baked at temperatures between about 700 and 850 ° C.

Surprisingly, it has now been found that an at least equivalent cleaning effect against food residues and Fats can be achieved if, instead of the catalytic systems described above, porous layers are used applied from inert materials of defined particle size to metal substrates. Can be used as inert materials all inorganic, non-metallic compounds and minerals are used that are at normal enamelling temperatures do not change thermally up to approx. 850 C, i.e. neither melt, sinter nor decompose. Materials such as quartz powder, feldspars, clay slate, rutile, ilmenite and aluminum oxide have proven to be particularly suitable.

BR J55 - 4 -

6Q9B32 / Q796

The porous layers built up with these substances develop their optimal cleaning ability compared to organic ones Impurities, e.g. fat droplets, if the particle size of these compounds is between about 20 and ICO, preferably between 20 and bO microns. the best results are achieved between 25 and 60 microns.

These inert materials are known to have neither oxidative properties Properties nor a catalytic effect can be ascribed to them. Nevertheless, they show in the cleaning comparison test at least the same effect as the currently known and described catalytically self-cleaning agents Systems.

For the production of the porous layer according to the invention, quartz powder, feldspar and coring in the particle size mentioned are particularly suitable. However, mixtures of the substances mentioned can also be used. A mixture of 90 parts of feldspar and 10 parts of amygonite (5-6% by weight LiOp, 20 % POh, remainder AlpO ^) has proven to be particularly suitable. This mixture has the following chemical composition: ι

SiO ₂ 60 - 80 G Al ₂ O ₃ 0 - 20th It P ₂ O ₅ 0 - 10 t! Li ₂ O 0 - 5th IT F. 0 - 5th Il κ ₂ ο 10 - 20th Il

The ceramic layer does not contain any additions of oxidic, catalytically active substances such as heavy metal oxides.

BR 35 - 5 -

609832/0796

The ceramic layer is applied to the steel substrate in a similar way to enamelling in Form of an aqueous suspension. To make a The inert substances, such as the feldspar-amblygonite mixture, mixed homogeneously with water, clay and borax using an intensive stirrer. This slip will using known methods of stitching technique by painting, Dipping or spraying applied to the metallic substrate.

Before coating the metal substrate with the porous In a preferred embodiment, these can be coated in the usual way with a base enamel layer will. The layer can also be applied directly to the sheet steel; here, however, the sheet steel, as with the Direct white enamelling is common, e.g. pretreated by dip nickel plating, with the nickel plating at least Should be 1 g / qm. It has proven to be particularly advantageous to apply a base enamel slip to the steel substrate to apply, to dry this, then to apply the inert substances described, also in aqueous suspension and to burn both layers in a fire after drying.

In the case of the previous basic enamelling, the hardness is of the base enamel should be selected so that the porous layer does not sink into the base enamel layer when it is fired, since in this case, the cleaning effect is deteriorated. In the case of direct burning of the inert material slip on the nickel-plated steel substrate, the stoving temperature in conventional enamelling ovens is between about 800 and

BR 35 - 6 -

£ 09832/0 * 798

2504Q83

After baking, a gray birch is disfigured on the steel substrate. gray-black coating that is porous and absorbent is. An applied drop of water is absorbed after a few seconds without leaving a trace. These Absorbency is a measure of the porosity and, in a broader sense, also of the lowering effect on fat droplets.

The porous layer is abrasion-resistant and scratch-resistant. Rub in with a wooden scraper leaves no visible trace. Only after vigorous handling with a steel pen the porous covering is damaged. :

Check the adhesion of the covering with the usual adhesion tests for enamelling, e.g. with the Erichsen device or the falling weight, after the known deformation of the steel sheet, no flaking is noticeable. bar.

However, in order to achieve the good adhesion described, the stoving temperature of the layer must be adapted to the inert material. It is generally between about 700 and 85O ⁰ C, preferably ^780-0

The porous ceramic layer can be made up of color bodies, such as those used when coloring enamels or ceramic glazes are common to be stained. For coloring in darker tones, spinel pigments, for the black coloring an addition of thermostable! - based iron oxide.

The color body additives range between 2% and Λο% depending on the intensity of the color.

The "process according to the invention" is to be explained in more detail with the aid of the following examples:

BR 35 - -.7 - ^:

Example 1:

Commercially available quartz powder with a SiOg content of at least 99% by weight , as used for the production of glass, ceramic frits and enamels, was broken down into the following sieve fractions from bottom to top with increasing mesh diameter using sieves arranged one above the other:

Particle size / micron specific surface area / qm / g

20 - 35 approx. 1.3

35-42 ■ 1.2

42-60 1.0

6o - 75 0.8

75-100 0.6

100-120 '"0.4

120-150 x 0.2

In each case 100 g of these sieve fractions were mixed with the following additives with the addition of 50 ml of water using an intensive stirrer mixed into a suspension. Their density was 1.89.

Fine tone 7.0 g

Boric acid 3.0 g

Bentonite 0.5 g

NaNO ₂ 0.2 g

thermally stable iron oxide pigment 10.0 g

As usual with enamel slurries, this suspension was applied with the help of a spray gun to a 1 mm thick steel sheet, which in a known manner for a Single-layer enamel pretreated by dip nickel plating

BR 35 - 8 -

6098 32/079

was, so that about 250 g / m solid (measured in dried Condition) on the substrate.

This sheet steel substrate coated in this way was heated at about 120.degree dried in a drying cabinet until most of the water in the suspension was removed from the coating. That coated and dried substrate was baked in a muffle furnace at b20 ° C for 4 minutes and then cooled to room temperature. The thickness of the fired layer was 0.20 mm.

The panels coated with the above grain fractions were subjected to the following cleaning test:

As a criterion for self-cleaning catalytic systems cooking oil is applied to the surface to be tested and its disappearance when stored at higher temperatures observed. The quality will depend on the number of cleaning cycles when the same amount of oil is repeatedly applied the self-cleaning layer.

In a test, coated sheet metal samples 10 × 10 cm in size are inserted into the oven of an electrically heated cooker, which is heated ^{to 250 ° C.} After 15 minutes these test specimens are taken out and coated with commercially available cooking oil using a brush so that 2/3 of the width of the plate is covered by an approx. 1 cm wide line of oil. The oil application must be so thin that no reflective surface remains after the oil has been absorbed through the porous layer. Subsequently, the sample plates are reinstated in the 250 ⁰ C hot oven and stored there for 2 hours.

BR 35 - 9 -

609832 / 079β

After heating, the sheets are cooled down and assessed according to the following evaluation standard:

"satisfactory"
"sufficient"

"insufficient"

The coating shows little or no noticeable at the painted areas Discoloration, spots, or marginal markings;

The enamel shows slight discoloration and clearly developed edges; The enamel is darker in the painted areas and it can be mechanically easily removed, not shiny Detect residues;

The enamel is heavily discolored in the painted areas and is shiny and greasy Residues that are "difficult or impossible to remove mechanically can.

This test is repeated until - as described in the assessment "unsatisfactory" - the coated Places discolour strongly and shiny residues are formed. The number of cleaning cycles with the value standard "good" are therefore standards for the cleanability of the coating.

The following table shows the cleaning effect as the number of test cycles depending on those described with the Test plates produced by sieve fractions:

- 10 -

609832/0796

Particle size / micron cleaning cycles

0-20 1

20-35 '7'

35-42 8

42-60 8

6o - 75 6

75 - 100 4

ICO - 120 2nd

120 - 150 1

This clearly shows that the cleanability of the substrate plates produced with the different quartz grain fractions is different. Those coated with grain fractions smaller than 20 microns and larger than 100 microns Sample panels show with 0-1 cleaning cycle insufficient cleanability. Better values are obtained in the grain size spectrum between 20 microns and 75 microns; optimum cleaning is achieved with grain fractions between 35 microns and 60 microns.

Example 2:; .- '..

To the quartz flour fraction between 35 microns and 60 microns described in Example 1, 10 % metal oxides of the same particle size were added:

A. Cobalt oxide black (at least 99 % CoO)

B. Black nickel oxide (at least 99 % NiO) G. Braunstein · (at least 98 % MnO ₂ )

D. Iron oxide red (pigment quality)

E. Chrome oxide green (pigment quality)

BR 35 - 11 -

ÖD 9832/0 %%

100 g each of these quartz oxide mixtures, designated A. - E. and pure quartz flour F. were made with the following additives with the addition of 50 g of water using a stirrer mixed to (a) homogeneous suspensions, also designated A. - F.:

Fine tone 7.0 G Dentonite 0.5 G Borax decahydrate 1.0 G Sodium nitrite 0.2 G

These email slip-like suspensions were on a 1 mm thick steel substrate, which was prepared in the usual way for single-layer enamelling, by dipping like this applied so that there was approximately 400 g / m 2 on the steel substrate. The coated substrates were at 120 ° C for 10 ' dried for a long time so that most of the adhering water was removed from the covering. It was then turned into a Drum furnace baked in at & 00 ° C for 4 'and then cooled to room temperature.

Then the cleanability of the test panels investigated as described in Example 1 by applying edible oil and repeated storage at 250.degree.

The test results are shown in the table below as the mean value of 3 test plates each:

Mix of

A. 90 quartz powder + 10 cobalt oxide: 8 cleaning cycles

B. 90 quartz powder + 10 nickel oxide: 7 cleaning cycles

C. 90 quartz powder + 10 manganese dioxide: δ cleaning cycles

D. 90 quartz powder + 10 iron oxide: S cleaning cycles

E. 90 quartz powder + 10 chromium oxide: 7 cleaning cycles

F. 100 quartz powder: 8 cleaning cycles

BR 35 - 12 -

609832/079

The tests clearly show that the cleaning effect of the quartz flours A. - E. within the error limit of the test is identical to the uncut quartz flour. This results in, that the decomposition of the applied cooking fat alone causes the porous layer. Furthermore, the metal oxides do not have any accelerating or catalytic decomposition Effect. The metal oxide particles act - similar to the quartz grains - as an inert material that forms the basis of the porous Layer forms.

Example J:

Made from commercially available, ground potash feldspar flour, as used in the porcelain and enamel industry, with following chemical analysis:

Na ₂ O 5 %

K ₂ O 10 %

Al ₂ O ^ 18 %

SiO ₂ 64 %

with the grain size distribution:

Grain size / micron proportions in parts by weight

100 71 - 100 38 - 71 20th - 38 20th

38.3 14.2 20.5 12.4 16.4

BR 35 - 13 -

609832/079 *

the fraction between 20 microns and 71 microns insulated. 100 g of this sieve fraction and 100 g of the starting material were with the addition of 50 g of water, 7 g of enameled clay, 1 g of borax and 0.2 g of sodium nitrite were stirred to form an aqueous suspension. With this Suspensions were - as described in Example 2 - coated sheet steel substrates 1 mm thick and - analogously to the example 2 - dried and baked. The cleaning effect was examined with the aid of the oil test already given. Included It was found that the unscreened feldspar of the above composition 5 cleaning cycles until laking, the sieve fractions but between 20 microns and 71 microns yielded 9 cleaning cycles.

Here, too, the grain size build-up is between 20 Microns and 71 microns the grain size spectrum of the starting material superior in terms of cleanability.

Example 4:

Analogously to Examples 1-3, aluminum oxide, rutile flour, Ilmenite and slate flour treated by air sifting so that preferably a particle size between 25 microns and 50 Micron was obtained. These grain fractions were again mixed with water with the aid of clay, borax and sodium nitrite blended into a homogeneous, non-settling suspension. This suspension was once enameled on Steel sheets, on the other hand on steel sheets nickel-plated in the usual way for direct enamelling by means of a Spray gun applied. In all cases, the layer thickness of the covering was between 0.18 mm in the dried state and 0.20 mm, after baking at & 20 ° C between 0.14 mm and 0.16 mm.

BR 35 - 14 -

609-832 / 079®

/ r

The base enamel combination used was adjusted by using relatively hard enamel frits and adding 25 % quartz flour in the mill offset so that it does not react with the porous inert material layer during the second firing. The following table shows the number of cleaning cycles of the cleaning tests described in Examples 1 and 2 as average values of 4 test panels each:

Rutile ilmenite slate flour

on basic email direct 9. 8th 8th - 9 8th 7th 8th 8th

BR 35

- 15 -

609832 / 079β

Claims (2)

Patent claims: \ 1) Porous, inorganic, non-metallic coatings on metal substrates, in particular on sheet steel, characterized in that the coatings are applied in the form of a Schlleker, in addition to the usual additives for enamelling, up to 95 wt .-% (based on solid substance thermal) stable inorganic compounds and / or minerals with a particle size between about 20 and 100 microns, contains, and is then fired at temperatures between about 700 and 85O ⁰ C. 2) coatings according to claim 1, characterized in that the particle size is between 25 and 60 microns. j5) Use of the coatings according to claims 1 to 2 for coating cooking, roasting or baking equipment. BR 35 - 16 -