Catalytic coating for the self-cleaning of ovens and ranges

Catalytic coating for self-cleaning ovens and stoves

The present invention relates to cooking, roasting, baking and grilling appliances and parts vondiesen devices, in particular ovens and stoves, which are contaminated during use with food residues and their surface is treated so that such contaminants can be addressed more effectively and the surface the device or the equipment part looks clean without mechanical cleaning.

Self-cleaning oven coatings are already known from the prior art. Thus, DE-A 28 28 613 describes a self-cleaning coating on objects that are heated during use. The coating consists of a porous layer of a base material of an inorganic binder with a matting agent on the surface of the articles distributed in this layer, particles of a catalyst (which tends as a result -seiner catalytic action of changing its color) and a non-coloring substance , The catalytic action is at least one of the metal oxides Mn₂θ₃, MnO₂ or CuO and / or a solid acid catalyst, such as zeolite, worried. A suitable group not discolouring substances are thus ferrites, for example, be as a matting agent Al₂O₃, SiO₂, TiO₂, ZrO₂, ZnO, CaO or MgO proposed. Common inorganic binders include frits of borosilicate glass, phosphate and lead frits and alkali metal silicates.

A further self-cleaning coating is described in DE-A 29 28 895th On the object to be self-cleaning, is a coating film formed from a composition containing an oxidation catalyst, a synthetic silicone resin as binder and an organic solvent (LM), formed by heating the mass to 300 to 400 ° C. The catalyst is preferably a noble metal or a metal oxide or a mixture of two or more metal oxides, in particular Pt, Pd, or MnO₂, A Cu oxide, a Fe oxide, an Ni oxide or a Cr oxide.

A polymerization inhibitor containing an inorganic coating to produce self-cleaning surfaces is described in DE-C 30 19 828th The necessary for the preparation of the coating coating composition is in a liquid binder comprising a contains oxidation catalyst (metal or metal oxide) and the above-mentioned polymerization inhibitor dispersed. Liquid binders are silicone resins dissolved in organic solvents in particular. Polymerization inhibitors are, in particular Al (OH)₃, Sb₂θ₃, Phosphate frit material or a mixture of at least two of these, and metal oxide / metals are oxides of V, Cr, Mo, Mn, Ni and Cu, specifically MnO₂, Ni₂O₃, CuO, or Pt or Pd, especially colloidal Pt or Pd on Al₂O₃,

An unpleasant smells and soiling avoided or surface-reducing coating is described in DE-A 199 15 378th The necessary for the preparation of the coating coating composition comprises (1) a polycondensate of (A) a silane, or an oligomer derived therefrom, and (b) optionally a compound of glass-forming elements and (2) particles of a transition metal oxide having a diameter of 10 nm to 20 microns. The silane is a compound of the general formula R_aSiX (._a) with a = 0, 1, 2, 3 or 4, wherein R is a non-hydrolysable radical, X is a hydrolyzable group on the other hand; Examples of transition metal oxides are the oxides of La, Ce, Ti, Zr, V, Cr, Mo, W, Mn, Fe, Co, Ni, Cu, Ag or Zn. In the preparation of the polycondensate or the coating material or thereafter, can the coating composition inorganic particles in the nanometer or micrometer scale, especially in the form of a sol, can be added. Preferred inorganic particles are Al₂O₃, SiO₂, SnO₂, Iron oxides and C (graphite, carbon black). Here are the proportions for the nano-scale particles so that dieTeilchengröße (diameter) is up to 300 nm. Preferred, however, are up to 50 or up to 100 nm.

In the case of the particles in the micrometer scale, a structuring of the coating and the generation of voids is achieved. The particle diameter is in this case 1 to 500 microns. Even with these particles are Al₂O₃ and SiO preferred, particularly kieselguhr, aluminum oxide 90, silica gel 40 or 60th

When these inorganic particles are used, they are impregnated with the transition metal oxide, which leads to a reduction of the pore volume of the particles. In the above described impregnation means brine inner and outer surfaces are coated, which for the removal of unpleasant odors due to the large catalytically active surface is advantageous (because gases can, in contrast to oily substances penetrate into small pore). This however does not apply to the prevention / removal of dirt. This until now described and commercially available options of self- cleaning coatings distinguish themselves in such a way that its self-cleaning ability significantly decreases relatively quickly. There is therefore a need for better and longer lasting capable of self-cleaning coatings in or on cooking, roasting, baking and grilling devices or their parts, especially in ovens and stoves. Determine can be the quality and durability of the groups capable of self-cleaning coatings / layers of cooking, roasting, baking and grilling devices with reference to DIN ISO 8291 at a thickness of at least 150 microns. The well-known in the prior art coatings may 5-10 cycles of dripping of soybean oil and then heating to withstand 250 + 10 ° C before a laking the surface (assessment according to DIN ISO 8291 by the appearance of gloss) occurs. The number of cycles to be increased in the defined conditions, consequently, is the main aim, which have provided to achieve the present inventors.

The present inventors have searched for a way out of this technically unsatisfactory situation and set himself the task, the ability to create, in part or in cooking, frying, baking and grilling devices, especially in ovens and stoves, which in its use are frequently contaminated with food residues to treat compared to the prior art changed so as to 8291 are at least 10-15 cycles according to DIN ISO self-cleaning and remain, ie that the pollution caused by food residues and during cooking, frying, baking grilling occurs in eg oven / stove on the inside walls of the muffle, over a longer period (at the same and equal to frequent use) without mechanical action disappears and the oven muffle looks cleaner than conventional coatings nowadays the case.

Starting from the prior art which essentially describes qualitatively different coating compositions or coatings, the inventors have asked the question whether it might not be possible to achieve an improvement in some other way. In particular, the considerations by the inventors were directed to improve the structure of the coating to the effect that there is permanently a sufficient contact between food residue (hereinafter referred to merely as a "contaminant") is on the one hand and oxygen (eg oxide) solid phase (three-phase boundary) on the other. This can be made possible by that very special cavities are provided, of which at least one type for spreading / absorbing the mobile food residues and at least one other variety acts as an oxygen reservoir, in which the food waste but can not penetrate. In this manner, the surface on which takes place the (thermal or oxidative) degradation maximized as in the layer itself permanently oxygen is present and the degradation is not dependent solely on supply of oxygen from the surface and the sides. A sealing the surface by large amounts of food therefore does not lead to blockage of degradation throughout the layer.

Of further importance is that the surface of the cooking, roasting, baking or grilling device or its parts remains mechanically loaded, ie, the adhesion of the coating to its substrate, their impact resistance, chemical resistance, their resistance to temperature changes and its scratch resistance must be satisfactory, which is achieved according to the invention.

According to the present invention, the term impurity or food group is a food residue, which is fluid enough at the operating temperatures of the cooking, roasting, baking and grilling unit to penetrate into the structure of the self-cleaning coating. It may be liquid fats / oils, aqueous phases (eg salt solutions) and with these mixed carbohydrates act / proteins at the operating temperatures.

The object, the inventors of the present invention achieved in that they have developed coatings with thermally (durable to 500 ° C) and chemically stable, porous oxides, especially metal oxides, but also such stable carbides and nitrides.

Obviously, the thermal decomposition and thus the self-cleaning power of today usual coatings of parts on or in cooking, frying, baking and grilling devices limited because the food waste does not come to the solid phase with sufficient oxygen in contact, which for the degradation of contaminants to (ideally) CO₂ and H₂O is necessary. The inventors have therefore provided layers with structures available in the contaminants from entering where they spreaded and ideally completely CO₂ and H₂O can be dismantled.

Hereinafter, the structure of the coating according to the present invention will be described in detail. Here are used interchangeably, the terms "particles" and "particles". When talking about the size of the particles, their diameter is meant. The coating to be generated with the capacity for self-cleaning system consists of at least one kind of particles A with open porosity. The size of the particles is 5 to 100 μ. Preferred sizes are from 10 to 80 microns, from 20 to 60 microns and 30 to 50 microns. The pores in a particle A are either in the order that the impurities can not penetrate, experience has shown that less than 1 .mu.m, preferably 0.1 to 0.6 microns. In the case of larger pores with a> lμm the particles A are coated with a porous membrane (pore c). This membrane prevents the ingress of contaminants into the porous particles A. However, also constitutes the so-called binder phase, so for the binding of the particles A with each other and to the substrate (to the surface). For the case that the pores are sufficiently small a (a <1 micron), the membrane, the pure binding function occurs only, ie, the membrane need not necessarily be porous in such a case. In such a case (the small enough pores a), since non-porous binder, such as glass, can be used, the particles A are not completely enclosed, but preferably provided only at the contact points between adjacent particles A with the binder. This ensures that as many of the pores a remain accessible for air.

Chemically speaking, the particles A, as mentioned above, thermally and chemically stable, porous oxides, especially metal oxides, carbides or nitrides. Exemplary representatives of the substances listed for particle B below. Particularly preferred particles A but are γ-Al₂O₃ and SiO₂,

The interstices between the particles A (hereinafter referred to as pores b) are responsible for the penetration and spreading of the liquid food residues in the layer. The goal is the best possible distribution / spreading of the impurities in the layer to maximize the surface for the thermal degradation. The size distribution of pores b is essentially determined by the size of the particles A and the volume fraction of their binder phase. The amount of used binder can be readily determined by routine tests, if it is considered that the erfϊndungsgemäß most advantageous amount is so large that one hand the spreading in the pores b and on the other hand a sufficient quality of the mechanical properties (scratch resistance, adhesion to the substrate) of the coating is ensured. The volume fraction of the binder phase according to the invention is 5-40%, preferably 20-30% or 15-25%. The pores between the particles b A are substantially larger than the pores of a, so that contaminants from entering the structure and can spread well. In this way it is ensured that always (enough) is oxygen for the thermal degradation in contact with the contaminants to be removed, especially since the pores regenerate a repeatedly, ie, can absorb air. Figure 1 shows the structure of the particles, coating and pore described herein.

All pores systems according to the invention referred to, ie the pores a and b, as well as the pores of the binder, if present, open-pore are.

The coating composition, which constitutes the basis for the invention to be produced coating, but it can contain additional types of particles which (a) the reduction of the roughness of the coating and improve the feel, (b) the improvement of the bond on the one hand between the particles A and on the other hand are used between coating and substrate, (c) the adjustment of color or (d) the improvement of the thermal decomposition or the spreading ability (to name a few examples).

Examples of particles that fall under (a), but also (b), nanoscale particles (particles B '), particles in the micrometer range are (particles B) and color body particles (eg spinels), an example of (c) are spinels, and finally are oxides of transition metals (La, Ce, Ti, V, Cr, Mo, W, Mn, Fe, Co, Ni, Cu, Ag or Zn), an example to (d).

In the case of the particles B and B^ς whose chemical composition is not important. All that matters is that the particles are thermally and chemically stable as described above. They may, however, need not be porous. Suitable substances as nanoscale or micron particles are oxides, especially metal oxides, carbides and nitrides, for example, SiO₂, TiO₂, Al₂O₃, ZrO₂, SiC, Si₃N₄ and B₂O₃Especially Al₂O₃, The choice of material composition of the particle B, and B 'is not dependent on the chemical composition of the particles A. That is, when the particles A according to the invention preferably Al₂O₃ are, the particles B and B 'also Al₂O₃, As well as well as SiO₂, TiO₂, ZrO₂, SiC, Si₃N₄, B₂be O or a spinel (or other stable component).

Importantly, however, the magnitude of the particle B andB '. The nanoscale particles B 'have a diameter of up to 100 nm, are advantageously but only 20 to 60 nm. The particles in the micrometer range instead have a diameter of 0.5 to 10 microns, with the additional restriction applies to those particles B to be at least five times, but not more than twenty times smaller than the particles A.

According to a particularly preferred embodiment, the pores a are free of solid or liquid secondary phases. This applies to the period of use of the cooking, roasting, baking and grilling equipment, but also for the unused equipment. In other words, it is particularly preferred that the pores of a metal oxide not, no color bodies, no particles absorb or having B or B 'and no impurities. Accordingly, the pores are a accordance with the invention either sufficiently small, so that impurities, in use of the device on the one hand and metal oxide pigments, particles B or B 'etc. in the production of the coating on the other hand can not penetrate into the pores of a, or by means of the porous described above membrane (pore c) are prevented from penetrating into the pores a. By this structure is available for the thermal degradation of the impurities in addition to the oxygen in the air in the pores b particular the oxygen in the pores a.

The above already said porous membrane is a porous adhesive (hereinafter also referred to as a porous binder) or consists of such a material. This is an inorganic temperature-resistant binder, preferably an inorganic polymer (eg silicone resins and polymeric phosphates) or an inorganic sol, both on the basis of, for example Al₂O₃, SiO₂, TiO₂, ZrO₂, P₂O₅, SiC, Si N₄or B₂O₃ or mixtures of these.

Further examples of the adhesive / binder are glass (eg, open-cell, but also dense glass) which only up to its softening point (T_e) Or to a temperature slightly below T_e has been heated, and silicates, such as clays or water glass.

The size of the binder particles is in the case of a sol known to be 100 nm or less, wherein the diameter of the particles 5 to 100 nm, preferably be 20 to 80 nm. In the case that a binder based on clay or glass etc. is used, diameter of 0.5 to 10 microns, especially 1 to 5 microns are preferred in the invention.

According to a preferred embodiment, the coating or coating composition on a conventional metal or metal oxide catalyst (oxidation catalyst and / or polymerization inhibitor). According to the present invention are generally all catalysts described in the prior art for the self-cleaning of ovens suitable. In this context, reference is made to the publications cited in the introduction. According to the invention is the use of oxides of La, Ce, Ti, V, Cr, Mo, W, Mn, Fe, Co, Ni, Cu, Ag or Zn, especially MnO₂, Ni₂O₃, CuO or Cr₂O₃, But also of Pt and Pd. Also mixtures of said oxides useful in this invention as an oxidation catalyst / polymerization.

The metal oxide or the mixture of the metal oxides has particle sizes of below 100 nm up to 0.5, 1 or even 2 microns. According to a preferred embodiment of the present invention, the catalyst has particle diameter of well below 1 micron. The particle diameter is preferably 500 nm or less, 300 nm or less, 200 nm or less and more preferably 100 nm or less.

A further preferred embodiment of the present invention is the catalyst to add one or more substances that inhibit the temperature at which the catalyst has a good conversion rate, lowers to temperatures of 200 to 250 ° C / decrease. Substances for the purposes of this preferred embodiment, elements of the 1st and 2nd main group of the PTE, ie for example, sodium, potassium, magnesium, calcium or strontium.

The coating composition of the invention comprises according to a further preferred embodiment, an inorganic pigment or a not-staining material of / bears the ensure that any possible color changes are covered in consequence of a value change of the metal of the catalyst.

Suitable inorganic pigments are especially spinels such MgAl₂O₄, MgFe₂O₄, MnFe₂O₄, FeAl₂O₄, NiAl₂O₄ or MgCr₂O₄, Other suitable not discoloring materials are SiC and graphite. The particle diameter of the color bodies optionally used is 0.2 to 5 microns, preferably 0.5 to 3 microns.

According to a further preferred embodiment, the coating on optical promise touched upon or Granitierungen to mask any visual pollution. In the former case, only one color component, in the case of Granitierung at least two color components are used. Suitable color components are eg spinels, but also Al₂O₃, TiO₂, ZrO₂ and mixtures of these. The diameter of these components is from 50 to 2000 .mu.m.

According to another preferred embodiment, the particles A SiO₂When particles B α-Al₂O₃, A nanoscale sol-gel binder on the basis of TiO₂ or TiO_xWhen pigment a spinel (MnFe₂O₄) With a diameter of 1-3 microns and as mottling TiO₂ a size of 500-1000 .mu.m used. The particles A have a size of 30-50 microns and in an amount of 50-80, in particular 60-70 wt .-%. The particles B on the other hand have a size of 0.7-1 microns and in an amount of 10-20 wt .-%, especially 15-20 wt .-%, are used. The sol-gel binder makes 7-15 wt .-%, especially 10 wt .-% of the pigment and the mottling each account for 1-3 percent by .-%, with 2 and 1 wt .-% particularly advantageous values are.

According to the invention, the thickness of the coating is at least 50 .mu.m, preferably 100 to 500 .mu.m, particularly preferably, however, thicknesses are from 150 to 450 microns, especially from 200 to 400 microns or 250 to 350 microns. However, larger layer thicknesses are technically feasible and advisable for economic reasons less interesting. If the thickness of the coating, however, less than 50 microns, this does not provide a sufficiently large pore volume of pores a, b and if appropriate c for receiving the one hand, impurities (only in the pores b) and on the other hand, air (in pores of a, b, c) ,

Parts of or in cooking, frying, baking and grilling devices in the sense of the present invention are not only for hitching and insertion parts for oven and range, but also the insides of oven muffles, so the stove or oven cavity (oven), and Fryers, in principle, all metal, glass, metal coated or enamelled parts of appliances such as furnaces, stoves, grills, etc., which are directly or indirectly heated in use and not in direct contact with the food.

According to a preferred variant of the invention, wherein the part to be coated to enameled parts such as enamelled steel, which is a steel which is provided with an enamel layer having a thickness in the order of 100 microns, which is used for corrosion protection.

For the production of self-cleaning coating of the invention, in particular three variants offer. According to the invention it is thereby preferred that the pores a free stay of any solid phase and thus have maximum Aufhahmekapazität for air. This can be achieved in particular in that the particles used (color bodies, particles B and B ', metal oxide, etc.) are not significantly smaller than the pores a. A further possibility, to be kept free the pores a solid phase, consists of particulate systems to use. Due to their zeta potential the particles do not penetrate into the pores, although or even if the latter are significantly larger than the particles.

Below are the 3 different manufacturing methods will be described briefly.

In the first method, the particles A (as Al are₂O₃) With an aqueous or organic (alcoholic) dispersed binder (the binder must not be solved because the pores a particle A were filled with the solid phase in this case! The same goes for all optional used / usable components) and optionally with (a ) particles B and / or B 'and (b) inorganic pigments (such as a spinel) and / or (c) treated one and / or other additives to form a slurry, the dried applied to the surface to be coated and thence to a so-called biscuit becomes. Then a firing takes place at 500 to 800 ° C and a cooling step, to which, if necessary, a spraying of eg metal oxide catalyst (MeO or Me₂O) eg in the form of a (particulate) dispersion of metal oxide or metal or Metalloxyhydroxid (eg Me₂O (OH)₂) Followed. Removing the LMs and the optional conversion of the salts into the oxide form takes place thermally.

In a second variant of the preparation of the coating is mixed, if necessary, the first particles A with a particulate metal oxide, metal hydroxide or Metalloxyhydroxid or with particles B and / or B '. Subsequently, the mixture whereupon a powder which, if necessary, or MeO Me is dried and calcined,₂O, is obtained. This powder is then as in the first variant with an aqueous or organic (alcoholic) dispersed binder (it can not be a solution) and optionally an inorganic pigment (eg a spinel) and other additives to a slip, which is to be coated on the is coated surface and dried there for biscuit. Then, a firing is carried out at 500 to 800 ° C, wherein the layer is solidified and optionally volatile components of the binder phase are expelled.

The third variant, finally, corresponds to variant 2, with the proviso that the particles A are coated with an organic polymer (eg cellulose) or sealed, so that they can be subsequently contacted with a solution of a metal nitrate, without the pores a metal nitrate are filled (after firing with metal oxide) or other slip components. By heating not only the metal nitrates to metal oxide is converted, but also the cellulose is pyrolyzed. The other steps are identical to those of variant 2, ie, dispersed binder, a slurry is prepared.

In all three variants of the application of the slip can electrophoretically, by means of spraying processes or by immersion.

If the firing at temperatures well above 500 ° C and is it in the part to be coated as to enamelled steel, the enamel softens so that improved adhesion between substrate and coating is achieved by sinking the layer.