This invention relates to a self-cleaning plate of a metallic porous body having a three dimensional network structure and a specific surface area of at least 2,000 m.sup.2 /m.sup.3 or more, characterized by having oxidation catalyst particles uniformly distributed throughout the binder with which the framework of said porous body is coated and with which the porosities are filled, said binder being preferably any one of an alkali metal silicate, a colloidal silica, a colloidal alumina, a monobasic metal phosphate, and a silicone resin, said oxidation catalyst being preferably at least one selected from the group of metals Pt and Pd or the metal oxides of Mn, Cr, Ni, Co, Cu, and Fe.

BACKGROUND OF THE INVENTION 
The present invention relates to self-cleaning plates which are applied in 
cooking appliances as cooking members such as the inner walls of the 
cooking appliance, and which at cooking temperature can automatically 
clean off dirt such as the fat and grease that spatters from food during 
cooking. 
As to methods for automatically cleaning off the dirt that spatters onto 
the inner wall surfaces of cooking appliances when food is being cooked by 
a cooking appliance such as an oven, an oven toaster, or a microwave oven, 
there have been disclosed by, to begin with, U.S. Pat. No. 3,266,477, as 
well as U.S. Pat. Nos. 3,547,098, 3,580,733, 3,598,650, 3,671,278, and 
3,759,240, and Japanese Patent Publication No. 17832/1972, and a number of 
others besides. The self-cleaning method which forms the mainstream of 
these disclosures is that of oxidative destruction of the dirt that is 
spattered from food at cooking temperature, by the action of an oxidation 
catalyst contained in a porous layer formed using a glass frit as a 
binder, that is to say, as a coating layer forming material, and forming a 
porous layer by coating and baking it on a base layer of enamel coated 
onto a metal substrate. By the action of this oxidation catalyst contained 
in the porous layer, the dirt spattered from the food during cooking is 
oxidatively destroyed. However, there are defects such that at high 
temperatures of 800.degree. C. or more, these calcine, and so the fusion 
and sintering of the particles of oxidation catalyst occur, reducing the 
cleaning performance, such that if the sheet is not of a certain thickness 
it may distort, it use a large amount of energy, a high temperature 
calcining furnace is needed and the catalyst layer cannot be made thick. 
To remedy these defects substances using an alkali silicate as the binder, 
such as, for instance, in Japanese Patent Publication No. 28120/1974, have 
been proposed. As a binder type, this belongs to the heat resistant 
coating group, and a calcining temperature of about 260.degree. C. to 
316.degree. C. (500.degree. F. to 600.degree. F.) is sufficient, and so it 
could be said that it remedies the aforementioned defects. In the 
inventor's experiments it was also confirmed that inorganic coating agents 
of silicas, aluminas or phosphates can be used as binders with the 
aforementioned advantages and which are capable of forming a porous 
coating with heat resistant properties. However, the facts are that when a 
binder containing catalytic particles is coated onto a substrate metal 
(steel plate, stainless steel plate, aluminized steel plate, etc.) it has 
poor adhesion to the substrate, produces cracks due to heat shock and 
steam, etc., and peels off easily, and when it is used in a cooking 
appliance, a long life for the coating layer cannot be guaranteed. 
The present invention, giving full consideration to the aforementioned 
circumstances presents a self-cleaning plate of high cleaning capability 
and which eliminates the defects present in the prior art. 
SUMMARY OF THE INVENTION 
An object of this invention is to provide a self-cleaning plate of a 
metallic porous body characterized by having oxidation catalyst particles 
uniformly distributed throughout the binder with which the framework of 
said porous body is coated and with which the porosities are filled. 
Said metallic porous body preferably has a three dimensional network 
structure and a specific surface area of at least 2,000 m.sup.2 /m.sup.3 
or more. 
The main component of said binder is preferably any one of an alkali 
silicate, a colloidal silica, a colloidal alumina, a monobasic metal 
phosphate, and a silicone resin. 
Said oxidation catalyst is preferably at least one selected from the group 
of metals Pt and Pd or the metal oxides of Mn, Cr, Ni, Co, Cu, and Fe.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Hereinbelow the present invention is explained in detail with reference to 
the drawings and embodiments. 
FIG. 1 is an enlarged cross-sectional sketch of a prior art heat resistant 
coating group self-cleaning type coating layer of a substrate metal 
(aluminized steel plate) (1) on which a coating (2) of an alkali silicate, 
as a binder, is provided. (3) Represents the oxidation catalyst particles 
distributed and contained in the alkali silicate coating (2). 
FIG. 2 is an enlarged cross-sectional sketch showing a self-cleaning plate 
according to the present invention. (4) Is a metallic porous body which 
serves as a substrate and serves to firmly bind the coating (2), and it 
employs a material having a three dimensional network structure. The 
material is Ni. Apart from Ni, as materials for a metallic porous body, 
Cu, Fe, Ag, Al, Ni-Cr alloys, and Fe-Cr alloys are available, and as long 
as there is corrosion resistance and heat resistance to about 400.degree. 
C., any of these may be used. Also, the metallic porous body may, instead 
of the three dimensional network structure, as in the above embodiment, 
employ a calcined powder and a fibrous metallic porous material. However, 
according to the inventor's experiments, a material with a three 
dimensional network structure gives the highest degree of porosity among 
these porous bodies, which is to say it is possible to raise the effective 
surface area of the oxidation catalyst, and therefore it is the best. In 
use, unless it has a specific surface area of at least 2000 m.sup.2 
/m.sup.3 it is difficult to coat a coating (2) containing an oxidation 
catalyst (3) in a binder on the structure of a metallic porous body as in 
FIG. 2, and to fill up the porosities, and even if, for example, this can 
be done, it is difficult to manufacture a material which is strong and 
durable. However, when a material with a specific surface area of more 
than 2000 m.sup.2 /m.sup.3 is used, the capillary effect becomes very 
strong and when dirt adheres there is plenty of opportunity for it to be 
absorbed inside and so come into contact with the oxidation catalyst 
particles, thus revealing outstanding cleaning performance. Consequently, 
in the present invention, when a metallic porous body with a three 
dimensional network structure is used, the relative surface area should be 
2000 m.sup.2 /m.sup.3 or more. 
Also, with a plate type metallic porous body, whether one uses the brush 
method, the spray method, or the dip method, it is possible to fill the 
porosities not just on one side but on both sides with a compound 
containing oxidation catalyst particles. And, as apparent in FIG. 2, it is 
possible by not coating so that the surface is flat and thick, but by 
coating so that the surface is as thin as possible, retaining the surface 
shape of the metallic porous body, to improve workability and wear 
resistance. 
Next, in order to investigate the adhesion to the substrate metal and 
workability with respect to a prior art heat resistant coating material as 
shown in FIG. 1, coated with an alkali metal silicate, and with respect to 
a self-cleaning plate according to this invention, as in FIG. 2, heat 
cycle tests (400.degree. C..rarw..fwdarw.room temperature each for 1 hour 
being 1 cycle), boiling water tests, and Erichsen tests were carried out. 
The results of these are shown in table 1. As will be observed from the 
results in table 1, the self-cleaning plate of this invention formed as in 
FIG. 2 has a strong bonding of the metallic porous body and the binder 
containing the oxidation catalyst, and does not easily produce peeling or 
cracks. 
TABLE 1 
______________________________________ 
Prior Material of 
Materials this Invention 
______________________________________ 
Heat Partial Nothing unusual 
Cycle peeling at after 50 cycles 
Test 4 cycles 
Boiling Partial Nothing unusual 
Water peeling at after 50 hours 
Test 2 hours 
Erichsen Cracks produced 
Nothing unusual 
Test with 1.5 mm with 3.5 mm 
protrusion protrusion 
______________________________________ 
Adhesion and workability were investigated by the same tests as in table 1, 
with regard to the use of a shot blast steel plate and a steel in which 
the surface was roughened by etching, as the substrate metal, but the 
self-cleaning plate of the present invention was vastly superior to both 
of these. The reasons for this are that in the material of the present 
invention the binder gets inside the vacant pores of the metallic porous 
body and is firmly locked to the metallic porous body as an aggregate. 
Accordingly, as the binder, apart from the alkali silicate used in the 
aforementioned embodiment, even general inorganic coating materials (or 
ceramic coating materials) which are known to easily produce cracks and 
peeling due to the difference of thermal expansivity with the substrate 
metal, may be used. In the inventor's experiments it was found that a 
self-cleaning plate which has a high oxidative catalytic effect yet which 
is as tough as with the alkali metal silicate used in the aforementioned 
embodiment, can be produced using a material in which an inorganic coating 
material of a colloidal silica, a colloidal alumina, or a monobasic metal 
phosphate (for instance, monobasic calcium phosphate, etc.) and a silicone 
resin are coated and then calcined at high temperature (350.degree. C. or 
more). 
Next, some examples of the application of the present invention to 
self-cleaning plate cooking appliances, are given. 
FIG. 3 illustrates an embodiment in which the self-cleaning plate of the 
present invention is applied to the inner wall (8) of a cooking appliance 
chamber, (5) is a cooking tray, (6) is a food, and (7) is a heater for 
cooking. In order to affix the self-cleaning plates to the inner wall of a 
cooking appliance, it may, as shown in FIG. 3, be retained by screws (9). 
It is not shown in FIG. 3, but the self-cleaning plate of the present 
invention may also be applied to the inner ceiling board of a cooking 
appliance chamber, and as such position is difficult to clean, this may be 
considered a very effective application. FIG. 3 shows an example of the 
application of a self-cleaning plate of the present invention to an 
electric type oven, but it goes without saying that it may also be applied 
in the same way to oven toasters, microwave ovens and gas ovens. 
FIG. 4 illustrates an embodiment wherein a self-cleaning plate of this 
invention is applied to the reflector plate (12) employed in a disposition 
above a movable heater (10) in an electric cooking appliance of the type 
where a heater is movable up or down. (11) Is a groove (rail) for the 
upwards and downwards movement of movable heater (10). Reflector plate 
(12) reflects the heat of heater (10), whereby the heating efficiency with 
regard to the food substance (6) is increased, and also serves the 
function of catching dirt from the food substance (6) during cooking 
thereby preventing widespread spattering. 
When a self-cleaning plate manufactured as that in FIG. 2 with aluminum 
phosphate (a monobasic aluminum phosphate) as the binder and manganese 
dioxide as the oxidation catalyst, and applied to cooking appliances as in 
FIGS. 3 and 4, was used repeatedly in cooking, it was confirmed that dirt 
spattering from food substances was thoroughly cleaned off during cooking. 
Also, the dirt cleaning performance of the self-cleaning plate of the 
present invention manufactured as in FIG. 2 was measured in comparison to 
that of the prior art heat resistant coating type, and the enamel type, as 
shown in FIG. 1. The cleaning efficiency characteristics were established 
by adhering about 50 mg of butter to about 10 points on the surfaces of 
the respective plates and measuring the weight change after heating them 
in an electric oven at temperatures at 50.degree. C. intervals from 
150.degree. C. to 350.degree. C. for 20 minutes at each temperature. The 
results thereof are shown in FIGS. 5 and 6. In both, curve (A) shows the 
characteristics of an embodiment of this invention, curve (B) shows the 
characteristics of the prior art heat resistant coating type, and curve 
(C) shows the characteristics of the prior art enamel type. In FIG. 5, Pt 
is used as the oxidation catalyst, and in FIG. 6 a mixture of oxides of Ni 
and Cu is used for the same purpose. Both the present inventive and the 
prior art heat resistant type coating use aluminum phosphate as the 
binder. As will be observed from FIGS. 5 and 6, the cleaning efficiency of 
the self-cleaning plate of this invention is superior to either of the 
prior coating layers at all temperatures. The reasons for the 
deterioration of the cleaning performance of the enamel type have already 
been discussed. The reasons why this invention is superior to the prior 
art heat resistant type coating are felt to be that the porous body 
structure as in FIG. 2 forms a framework and so dirt that adheres to the 
surface permeates into the pores by capillary action and so has plenty of 
opportunity to come into contact with the contained oxidation catalyst, 
and that unlike a plate formed with a coating layer on a flat sheet, the 
thickness of the membrane possessing the oxidative catalytic effect (i.e. 
the thickness of the metallic porous body) can be made as thick as is 
desired (which also gives long life), so as above, there is plenty of 
opportunity for the dirt to come into contact with the oxidation catalyst. 
When cleaning efficiency with regard to salad oil, sauce, sugared water, 
gravy, lard, and fish oil was measured in the same way as in the above 
example, it was confirmed that this invention has a vastly superior 
performance to any of the prior plates. 
Further, on comparing FIG. 5 and FIG. 6, it will be seen that even without 
using an expensive Pt as the oxidation catalyst, as in FIG. 5, a 
substantially high cleaning efficiency is also exhibited at temperatures 
of about 250.degree.-300.degree. C. with oxides of Ni and Cu. Generally, 
as catalysts for the oxidation, dispersions of organic substances, 
platinum group metals and oxides of other metals are known, the former 
having the ability to adsorb and desorb oxygen from the air, and the 
latter by the action of the oxygen in air and the oxygen in their own 
constructions, respectively achieve the oxidative catalytic effect. 
According to the inventor's experiments conducted in the same way as for 
FIGS. 5 and 6, as to oxidation catalysts for cleaning off dirt such as 
grease, butter, salad oil, and seasonings, spattered from food, among 
metal, apart from Pt, metallic particles of Pd, and among metal oxides, 
apart from oxide particles of Ni and Cu, oxide particles of Mn, Cr, Co, 
and Fe, too, have outstanding dirt cleaning efficiency, and it was found 
that they can be used to particular effect in this invention. Also, in 
this invention at least one of the above oxidation catalysts is contained 
together with another additive in a binder. Among these oxidation 
catalysts one which is relatively cheap and high in cleaning efficiency, 
and which is also most desirable from the points of view of hygiene and 
pollution, is an oxide of Mn, and the .gamma. (gamma) type manganese 
dioxide, particularly, is very high in activity and exhibits outstanding 
catalytic effects. However, .gamma.-MnO.sub.2 has the characteristic that 
when it is heated to a temperature of about 400.degree. C. or above, it 
transforms into .alpha. (alpha) type or .beta. (beta) type, and the 
activity is reduced. That is to say, when .gamma.-MnO.sub.2 has been used 
in the coating layers of glass (enamel) frit which at present is the most 
generally and widely used, said phenomenon occurs as the calcining 
temperature is high (normally 800.degree. C. or more, and even in those 
with a low softening point it is about 500.degree. C.), and the catalytic 
performance is diminished. In this point, too, with a self-cleaning plate 
of this invention, as described above, it is possible to use even a binder 
which, like alkali metal silicate, colloidal silica, colloidal alumina, a 
monobasic metal phosphate, or silicone resin, containing an oxidation 
catalyst is difficult to form into a membrane, or of which the membrane 
life is short, and when these are used a calcining temperature of about 
250.degree.-370.degree. C. is sufficient, which is advantageous as even 
with a .gamma.-MnO.sub.2 the activity is not reduced. 
As explained hereinabove, when the self-cleaning plate of the present 
invention in which in a metal porous body, covering its framework and 
filling the porosities thereof are oxidation catalyst particles uniformly 
contained and dispersed in the entirety of a binder, is applied to the 
inner walls of a cooking appliance chamber, and as members for use during 
cooking, not only is it possible for the dirt spattered from food to be 
automatically cleaned with higher efficiency than with prior art types, at 
relatively low temperatures, but also it is of extremely high practical 
value as peeling and cracking of the binder do not readily occur and it is 
tough and has a long life.