JPS6157872B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a self-purifying coating layer that has the effect of using a catalyst to decompose and purify oil that scatters and becomes dirt on the inner wall of a cooking appliance during cooking. In recent years, with the spread of various types of cooking appliances, appliances having a coating layer with a self-cleaning effect have become commercially available. These coating layers can be broadly classified into enamel coatings using glass frit as a binder, and inorganic paint coatings using silicate as a binder.
A typical example of the former method, as disclosed in Japanese Patent Publication No. 49-33088, is one in which the catalyst is supported on a metal substrate using a molten mixture with a glassy substance as a component of the frit. On the other hand, in the latter method, as disclosed in Japanese Patent Publication No. 49-28120, a catalyst is dispersed and supported on a metal substrate using sodium silicate as a binder. Both of these have their own merits and demerits, but from the viewpoint of the manufacturing method or the scope of application to objects, those using silicate as a binder are superior. The reasons for this are that the baking temperature is approximately 300°C and that the type, thickness, and shape of the substrate metal does not matter. On the other hand, since the firing temperature for enamel is 800°C or higher, the type, shape, and thickness of the base material are naturally limited. It would therefore be desirable, if possible, to use a self-cleaning coating layer with conventional silicates as binders, but this also has its problems.
One of the reasons for this is that the bonding force is weak and the material peels off from the substrate due to mechanical impact. Yet another serious drawback is that silicates dissolve in water vapor. When cooking in a cooker, a considerable amount of water vapor is always generated.
Particularly in Japan, ovens are used for foods that generate a large amount of steam, such as chawanmushi, rather than for foods that generate relatively little steam, such as grilled meat, unlike in Western countries. Therefore, in Japan, self-purifying coating layers using silicate as a binder, which are not so problematic in Western countries, are subject to peeling, elution, corrosion, etc. due to aging. The present invention improves upon the drawbacks of the conventional silicate-based self-cleaning coatings mentioned above.
That is, the present invention is characterized by comprising a water-soluble silicate, a catalyst, phosphoric acid or a phosphate, and at least one member selected from the group consisting of alumina and magnesium oxide. The water-soluble silicate referred to in the present invention has a general formula of
It is represented by _{M2O.xSiO2.yH2O} (M is a metal belonging to Group 1A _{of the} periodic table), and includes sodium silicate, potassium silicate, lithium silicate, etc. In addition, there are tertiary amine silicates, quaternary ammonium silicates, guanidine silicates, etc., but these are not very common and are expensive, so they are not preferred in the present invention. These silicates may be used alone or in combination of two or more. The value of x in the general formula is usually about 1 to 5, and the value of y is not particularly limited as it provides appropriate viscosity to the final composition. Taking sodium silicate as an example, the curing mechanism of alkali silicates is based on silicate anions (Si
(OH) ^2-6 forms aggregates, _and Na ⁺ ,
As heating progresses further from a state containing OH ^- and H ₂ O,
Si-O ^- groups are formed, condensation occurs to form Si-O-Si bonds, and siloxane bonds are formed to solidify. Na ⁺ present in the system becomes trapped and immobilized in a three-dimensional cage created by siloxane bonds. However, if this is exposed to water or steam for a long time, this Na ⁺ will be liberated in the water, and the siloxane bonds will gradually collapse. These are the reasons why adhesive strength is weak and water resistance is poor when silicate is used as a binder. The present invention improves the adhesion strength of silicate by adding phosphoric acid or phosphate.
This is intended to improve water resistance. The phosphates used in the present invention include, for example, primary aluminum phosphate, secondary aluminum phosphate, tertiary aluminum phosphate, primary phosphate, secondary phosphate, and tertiary phosphate of group A metals. Contains salt. These phosphoric acids and phosphates are present in silicate, and Na ⁺ of silicate,
It seems that K ⁺ and Li ⁺ are fixed as insoluble salts. M ⁺ +H ₂ PO ₄ ^- +nSi(OH) ₄ →MH ₂ PO ₄・nH ₄ SiO ₄ (1) 3M ⁺ +PO ₄ ^3- →M ₃ PO ₄ (2) Here, M represents Na, Li, and K. ing. Thus, phosphoric acid or phosphate compounds are
It is very good for insolubilizing silicates, but
If you mix this directly with the binder silicate,
Since the activity of the phosphoric acid compound is too strong, curing begins immediately, which poses a practical problem. Therefore,
When using a phosphoric acid compound, it is necessary to add either alumina or magnesium oxide to slow down the reactivity. When such a compound is present, the reaction between phosphate and silicate is slow and storage stability is excellent. Next, the catalyst constituting the self-purifying coating layer will be described. Any catalyst can be used as long as it oxidizes and purifies the oil scattered during cooking. The most common are transition metal oxides such as Cu, Mn, Co, Ni, Fe, and Cr, but anything else that contributes to the decomposition of fatty acids can be used. For example, zeolite, lime aluminate, calcium silicate, titanium oxide, zirconium oxide, and other alkaline earth metal compounds are also effective in decomposing fatty acids. In addition to the catalyst, various refractory fillers may optionally be added for the purpose of increasing porosity. Refractory fillers include SiO ₂ , Al ₂ O ₃ , MgO, CaO
In addition to the above, minerals containing these as one component can also be used. (1) Base agent Example 1 Sodium silicate (x=2) 30% by weight aqueous solution
100 parts by weight Catalyst - γ-MnO ₂ 50 Catalyst - CuO 20 Alumina 100 The above raw materials are mixed in a ball mill. Example 2 Potassium silicate (x=3) 30% by weight aqueous solution
100 parts by weight catalyst - Ferrite (Fe-Mn-Zn system)
50 parts by weight catalyst - lime aluminate 40 〃 MgO 100 〃 The above raw materials are mixed in a ball mill. (2) Hardening agent Example 1 Phosphoric acid (89%) 170 parts by weight Alumina 30 Zinc oxide 27 Magnesium chloride 40 Zinc titanate 40 Water 100 Mix the above raw materials in a kneader and heat at 150℃.
After baking for an hour, 20 parts by weight of sodium silicate and 10 parts by weight of titanium oxide were added to 100 parts by weight of the pulverized product, and the mixture was pulverized and mixed in a ball mill for 13 hours to obtain a curing agent in the form of a slurry. Example 2 Phosphoric acid (89%) 10 parts by weight Monoaluminum phosphate 8 Zinc oxide 10 parts by weight Calcium chloride 7 Calcium silicate 5 Water 18 The above raw materials were mixed in a kneader and heated to 300°C.
After baking for an hour, 10 parts by weight of potassium silicate was added to 100 parts by weight of the pulverized product, and the mixture was pulverized and mixed in a ball mill for 18 hours. Example 1 Main ingredient (main ingredient in Example 1) 100 parts by weight Hardening agent (hardening agent in Example 1) 100 〃 Water 70 〃 The above materials were mixed, thickness 0.8 mm, size 10×
It was applied onto a 10 cm aluminized steel plate using a spray gun and baked at 250°C for 30 minutes. 100 mg of lard was attached to this test piece, and 200 mg of lard was attached to it.
℃, 250℃, and 300℃ for 30 minutes each, the percentage of weight loss expressed as a purification rate is 25%, 75%, and 98%, respectively.
It was %. In addition, to check the adhesion, the test piece was bent at 180° with the coating layer on the outside. As a result, no peeling between the substrate and the coating layer was observed. To examine water resistance, this test piece was boiled in boiling water for 10 hours, and the weight loss was 0.8 mg/cm ² . When the surface hardness was scratched with a piece of copper, the copper was scraped off. As a reference example, when only the base agent was applied and baked, it was dissolved and peeled off in boiling water for 3 hours. Example 2 Base resin (base resin in Example 2) 100 parts by weight Hardener (hardener in Example 2) 100 Water 70 Example 3 Base resin (base resin in Example 1) 100 parts by weight Hardener (hardener in Example 2) 100 〃 Water 70 〃 Example 4 Base ingredient (Main ingredient of Example 2) 100 parts by weight Hardening agent (Hardening agent of Example 1) 100 〃 Water 70 〃 Example 5 The following raw materials were pulverized and mixed in a ball mill for 1 hour. Sodium silicate (x=2) 30% by weight aqueous solution
100 parts by weight catalyst γ-manganese dioxide 50 〃 Zeolite 30 〃 Alumina 100 〃 Aluminum phosphate 5 〃 Calcium silicate (CaSiO ₃ ) 3 〃 Using the coating layer forming materials having the compositions of Examples 2 to 5 above, Example 1 and Test pieces were prepared in the same manner and performance evaluation was performed. The purification rate of lard and its weight loss in boiling water are shown in the following table. When bent at 180°, no peeling of the coating layer was observed, and the surface hardness was sufficient.

[Table] Although an aluminized steel plate was used as the base material in the above examples, the base material to which the coating layer of the present invention can be applied is not limited to this. Furthermore, a material coated with enamel can also be used as a corrosion-resistant layer. INDUSTRIAL APPLICATION As mentioned above, this invention improves especially the adhesiveness and water resistance of the self-cleaning type coating layer which uses a silicate as a binder.

Claims (1)

[Claims] 1. A self-purifying coating comprising a water-soluble silicate, a catalyst, phosphoric acid or a phosphate, and at least one member selected from the group consisting of alumina and magnesium oxide. layer. 2. The self-cleaning coating layer according to claim 1, wherein the water-soluble silicate is at least one selected from the group consisting of sodium silicate, lithium silicate, and potassium silicate.