Coating composition for ir radiation heating

Abstract: A coating composition for IR radiation heating which comprises an oxide of a transition metal or a mixed oxide of transition metals, ZrO₂ and polyborosiloxane, a coating film prepared from which has improved IR radiation efficiency.

Description: The present invention relates to a coating composition for IR radiation heating which is coated on a heating element such as a cooking utensil and a heater which utilizes IR radiation heating. Many coating film having various compositions are known and prepared as IR radiation heating coating film. When the coating composition for IR radiation heating is applied and dried to form a film on a hard surface, particularly a metal surface, it can radiate IR at high efficiency. One example of such coating composition for IR radiation heating comprises an inorganic binder consisting of alumina or silica and an oxide of a transition metal such as iron, cobalt, nickel, copper and titanium. The coating composition is applied on a surface of a heating element dried and then sintered to form a coating having a thickness of several ten to several hundred µm on the heater surface. Another example of the coating composition for IR radiation heating comprises water glass and an oxide, carbide or nitride of a metal such as zirconium, silicon, aluminum, iron, chromium, nickel, cobalt, titanium or manganese. This composition is coated on the surface of the heating element, dried at a temperature at which the water glass hardens and sintered to form a coating film having a thickness of several ten to several hundred µm. The heating element having the coating film for IR radiation heating on its surface is widely used as heating means for industrial or domestic use. The reason why the ceramic type coating film comprising the oxide, carbide or nitride of the metal is used for IR radiation heating is that its IR radiation characteristics is superior to the metal. Fig. 1 shows radiation characteristics of three kinds of surfaces, in which the curves 1 to 3 represent spectroradiation characteristic curves for the surfaces of following materials, respectively: 1: Stainless steel 2: Coating film comprising ZrO₂, SiO₂ and Al₂O₃ 3: Coating film comprising a transition metal oxide (e.g. Fe₂O₃, Mn₂O₃ and Cr₂O₃), ZrO₂, SiO₂ and Al₂O₃ As understood from these characteristic curves, The stainless steel 1 has lower emissivity than the coating films 2 and 3 at all the wavelengths specified in Fig. 1. The coating 2 has higher emissivity in a longer wavelength range. The coating 2 has high emissivity at all wavelengths. From these results, it is apparent that the coatings 2 and 3 can give a more amount of radiation heat than the stainless steel when the same amount of heat is input to the heating element from a heat source. However, the conventional ceramic type coating films for IR radiation heating still have some drawbacks. The first drawback is that design of the coating film is difficult since little is known about a relationship between the radiation characteristics and coating conditions such as a formulation of the composition and a thickness of the coating film. The second one is that since the coating film comprises the ceramics, it has poor resistance against mechanical shock so that it tends to be easily cracked or peeled off from the surface of the heating element when external stress such as flexure is applied thereon. Further, the ceramic coating tends to be cracked or peeled off by thermal shock if the substrate deforms thereby. The third one is that when the coating composition is applied and dried to form the film on the heating element, the coating composition comprising the inorganic binder has poor wettability with the substrate and is repelled so that it is difficult to form a uniform coating film. Thus, the surface of the heating element should be pretreated in some steps, which makes the coating conditions more complicate. In addition, the organic binder should be heat resistant to be used as a part of the heating element. The fourth one is that, when the heating element is used at a high temperature, for example, when it is red heated at a temperature of 800 to 1,000°C, the coating components, particularly the inorganic binder is thermally deteriorated and the adhesion of the coating film to the substrate is decreased. Chemical Abstracts, vol. 104, No. 26, 30th June 1986, page 286, abstract No. 229394v, Columbus, Ohio, US, discloses a coating composition comprising borosiloxane, Ti0₂-coated Zr0₂ particles and other oxide particles useful for cooking or room heating gas burners, respectively. SUMMARY OF THE INVENTION It has been found out that Ti0₂-coated Zr0₂ particles only provide for good IR radiation properties when the particle content is small. The present invention is intended to make clear a relationship between the coating composition and radiation characteristics so as to provide a coating composition for IR radiation heating which can form a coating film on a heating element having improved IR radiation characteristics and good film properties such as adhesivity and heat resistance. According to the present invention, there is provided a coating composition for IR radiation heating which comprises an oxide of a transition metal, ZrO₂ and polyborosiloxane. Polyborosiloxane is a binder having good adhesivity even at a high temperature. According to the present invention, film properties such as hardness of the film and corrosion resistance of not only the coating but also the substrate can be controlled by properly selecting mixing conditions of the composition, sintering temperature and the thickness of the coating film. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a graph showing IR emissivity of three kinds of surfaces, Fig. 2 is an enlarged schematic cross section of one embodiment of the coating film according to the present invention, Fig. 3 is a graph showing IR emissivity of the coating film shown in Fig. 2, Fig. 4 is a graph showing weight loss of a solution of polyborosiloxane by thermogravimetric analysis, Fig. 5 is a graph showing IR emissivity of various coating films, Fig. 6 is a graph showing a relationship between the average emissivity and the thickness of the coating film, Fig. 7 is a graph showing influence of the wavelength of light on emissivity, Fig. 8 is a graph showing a relationship between the viscosity of the coating composition and the hardness of the coating film prepared therefrom, Fig. 9 is a graph showing a relationship between the viscosity and the mixing time of the coating composition, Fig. 10 schematically shows an apparatus which is used in a heating test of the coating film for IR radiation, Fig. 11 is a graph showing the temperature increase curve for various heating surfaces. DETAILED DESCRIPTION OF THE DRAWINGS Fig. 2 shows an enlarged schematic cross section of one embodiment of the coating film formed from the coating composition of the present invention on a substrate 4. The coating film comprises polyborosiloxane 5 as a binder, ZrO₂ 6, Fe₂O₃·Mn₂O₃·CuO 7, Al₂O₃ 8, MgO 9 and mica 10, and has a thickness of 10 to 20 µm. When the substrate is heated externally or internally, the temperature of the coating film is raised and the coating film emits radiation heat. In the coating of Fig. 2, Al₂O₃ 8 is used for filling the spaces among other components, MgO 9 is used for neutralizing foreign materials to improve the corrosion resistance, and mica 10 is of flake form and used to physically prevent penetration of the foreign materials. These components, namely Al₂O₃, MgO and mica are optional ones. The embodiment of Fig. 2 is explained in connection with a practical application. For example, the substrate 4 can be a metal plate constituting an inner wall of an electric oven. An electric heater is installed outside this metal plate, and the coating film for IR radiation heating is formed on the inner surface of the metal plate. When electric current is passed through the electric heater and the metal plate is heated, radiation of the generate heat is facilitated by the coating film for IR radiation heating and emitted from the surface of the coating film into an interior of the electric oven. By the radiated heat, food is thermally cooked. Fig. 3 is a graph showing radiation characteristics of the coating shown in Fig. 2. The characteristic curve is similar to the curve 2 in Fig. 1 for the coating 2. However, the thickness of the coating according to the present invention is smaller than that of the coating 2. The conventional coating cannot provide high emissivity when its thickness is as small as 10 to 20 µm. Polyborosiloxane to be used as a binder according to the present invention may be represented by the formulas: Polyborosiloxane (I) or (II) has a backbone chain comprising silicon, boron and oxygen atoms and phenyl groups as side chains bonded to the silicon atoms in the backbone chain. A molecular weight of polyborosiloxane has not been precisely measured. Since polyborosiloxane has phenyl groups and is soluble in a strongly polar solvent such as NMP and DMF, it is used in the form of a solution in such solvent. The solution form of polyborosiloxane makes easy the preparation of the coating composition and application thereof. Therefore, polyborosiloxane has far better handling properties than the conventionally used inorganic binder. Fig. 4 shows results of thermogravimetric analysis of a solution of polyborosiloxane in NMP. Weight loss up to about 200°C is due to evaporation of NMP. In a temperature range higher than 200°C, the weight loss is attributed to the loss of the phenyl groups due to scission of the bonding between the phenyl group and the silicon atom. At a temperature higher than 600°C, no weight loss is found, and a residual material is a ceramic consisting of silicon, boron and oxygen atoms, which realizes the strong adhesion between the coating and the substrate. The various advantageous properties of the coating film for IR radiation heating according to the present invention is now illustrated. The formulation of the coating composition and the radiation characteristics is firstly described. Fig. 5 shows radiation characteristic curves for various coating compositions, formulations of which are shown in Table 1. From the results of Fig. 5, it is seen that Fe₂O₃·Mn₂O₃·CuO improves the emissivity in a shorter wavelength range. This is because Fe₂O₃·Mn₂O₃·CuO has good IR absorption. In addition, since ZrO₂, Al₂O₃, SiO₂ and TiO₂ have characteristic absorption in a longer wavelength range, they contribute to the high emissivity in the longer wavelength range. Al₂O₃, SiO₂ and TiO₂ have substantially the same emission characteristics in the mixed system with ZrO₂. The reason why ZrO₂ is used as one of the essential components according to the present invention is as follows: When one of Al₂O₃, SiO₂, TiO₂, V₂O₅, Fe₂O₃, NiO, Y₂O₃, ZrO₂ and La₂O₃ is used in combination with polyborosiloxane, the coating film comprising ZrO₂ has better radiation characteristics than coating films comprising other oxides. For example, the coating film comprising ZrO₂ is used in a grill, a slice of bread can be toasted faster than other coating films. Therefore, according to a preferred embodiment of the present invention, the coating composition for IR radiation heating comprises polyborosiloxane, ZrO₂ and Fe₂O₃·Mn₂O₃·CuO. In another preferred embodiment, the coating composition further comprises, as an auxiliary, Al₂O₃ which is thermally stable and cheap. The particle size of ZrO₂ is preferably from 0.1 to 2 µm, more preferably from 0.2 to 0.8 µm. To obtain a coating film having the thickness of 10 to 20 µm, ZrO₂ having a larger particle size is not suitable. When ZrO₂ having a particles sizes of 0.2 µm or 0.8 µm was used, there was no noticeable difference in the radiation characteristics in the wavelength range in which the test was carried out. Fig. 6 shows a relationship between film thickness of the coating No. 2 in Table 1 and its average emissivity which is measured by a heat conduction type emissivity meter. When the coating film is thin, the emissivity is low because of influence of IR reflection by the substrate. When the coating has a thickness of 15 to 20 µm or larger, the influence by the substrate is eliminated and the emissivity is stabilized. For the composition No. 3 in Table 1, the emissivity is stabilized when the film thickness is 10 µm or larger. The influence of the wavelength is shown in Fig. 7. The emissivity of the coating No. 2 in Table 1 is plotted for the wave number of 2, 2,000 cm⁻¹ and 1,400 cm⁻¹ against the film thickness. From Fig. 7, it is seen that the emissivity is not influenced by the film thickness in case of the wave number of 2,000 cm⁻¹, while the emissivity increases as the film thickness increases in case of the wave number of 1,400 cm⁻¹. This means that IR radiation at 1,400 cm⁻¹ penetrates deeper into the coating than that at 2,000 cm⁻¹ and therefore the former is more influenced by the substrate than the latter. Preferably, the ZrO₂ particle surfaces are coated with Al₂O₃, SiO₂ or TiO₂. When a refractive index of the ZrO₂ particle is "n", a reflectance R is defined by the following equation: Therefore, the reflectance increases and the emissivity decreases as the refractive index increases. Since ZrO₂, Al₂O₃ and SiO₂ have refractive indexes of 2.2, 1.5 and 1.6, respectively, their reflectance are about 0.14, 0.04 and 0.05, respectively according to the equation (III). Although the equation represents regular reflection, the coating of ZrO₂ particle surfaces with Al₂O₃, SiO₂ or TiO₂ can reduce the reflectance of the ZrO₂ particle. The Al₂O₃, SiO₂ or TiO₂ coating should not be so thick as to deteriorate the properties of ZrO₂. Its thickness is preferably about 10 Å or less. The content of the particles in the coating film and the refractive index of the particle have following relationship. When the content of the particle is large, the refractive indexes of both of the surface and the inner part of the coating are close to that of the particles since the particles are densely packed in the coating film. According to the equation (III), the emissivity is large, when the refractive index is small. On the contrary, when the content of the particles is small, the particles are scarcely present in the coating film, so that the refractive index of the coating is close to that of the binder. In the present invention, since the refractive index of polyborosiloxane is as small as 1.5, one may think that the reflection R could be small according to the equation (III), and in turn the radiation characteristics of the coating film would be good. However, this is not the case. Since polyborosiloxane is porous, it tends to be easily affected by reflection effects of the substrate. Thus, it is preferred that the particles have a larger refractive index when they are contained in the substrate in a smaller ratio. The reason for this is as follows: When the particles are contained in the coating film, IR light tends to be multiple scattered on the surfaces of the particles and during multiple scattering, it is absorbed by the coating film. In view of the above phenomena, since TiO₂ has a larger refractive index of 2.8, it is suitable for coating the surfaces of the ZrO₂ particles. The larger absorbance means the larger radiation. As explained in the above, the coating of the surfaces of the ZrO₂ particles with Al₂O₃, SiO and/or TiO₂ can play an important role in view of the relationship among the particle content in the coating film, the refractive index of the particles and the radiation characteristics of the coating film. MgO and mica are optionally mixed in the coating film according to the present invention to improve the corrosion resistance of not only the coating but also the substrate. Since polyborosiloxane is degassed during sintering, the coating film according to the present invention is porous. Therefore, in an corrosive atmosphere containing a corrosive gas or liquid, the surface of the metal substrate such as stainless steel is corroded, and the coating of the present invention is peeled off from the corroded part of the substrate. This is particularly true for the cooking utensils. On the surface of the cooking utensils, not only protons generated by seasonings such as sauce, oil such as salad oil, vinegar etc., but also halogen ions contribute to the corrosion of the substrate and the coating. The protons elute metal ions such as Fe³⁺ and Cu²⁺ from the metal oxide in the coating film and the eluted metal ions may act as oxidizing agents for corroding the metal surface, and the halogen ions directly corrode the metal surface. To prevent the elution of the metal ions from the coating film, basic MgO is added to suppress the attack by the protons. Since a mica particle has a plane form and is present in the coating in parallel with the surface of the coating, it can prevent the penetration of the corrosive material into the coating to some extent. The coating film according to the present invention with or without MgO and mica was subjected to a moisture proof test and a seasoning corrosion test (for 24 hours). The results are shown in Table 2. The preferred coating compositions which have satisfactory radiation characteristics and corrosion resistance are a composition Nos. 7 and 8 shown in Table 3. The coating films prepared from the coating composition Nos. 7 and 8 have substantially the same radiation characteristic curves as shown in Fig. 3. Now, preparation, coating and sintering conditions and preservation conditions will be explained. For this purpose, the coating composition No. 7 in Table 3 is used by way of example. The conditions and procedures explained in connection with the composition No. 7 can be applied to all other coating compositions according to the present invention with minor changes if required. Preparation of the coating composition Since the coating composition according to the present invention contains powdery components in a larger content, preferably the components are vigorously dispersed. When the dispersion is insufficient in the preparation of the coating composition No. 7, the coating film formed therefrom is not hard enough and easily flawed. To evaluate a degree of dispersion, the relationship between a viscosity of the prepared coating composition (at 25°C) and pencil hardness of the coating film was examined. The result is shown in Fig. 8. To obtain a sufficiently hard coating film, the coating composition is dispersed to have a viscosity of at least 50 cps, preferably at least 100 cps. Thereby, the coating film has pencil hardness of at least 5H. A relationship between a period of mixing time and the viscosity of the coating composition is shown in Fig. 9. Sintering conditions Preferably, a sintering temperature is not lower than 600°C in view of the properties of polyborosiloxane. When the sintering temperature is lower than 600°C, the phenyl groups remain in polyborosiloxane so that the coating film may be deteriorated by the organic solvent and have poor hardness. For example, if the coating film is sintered at 400°C or lower, its hardness is unsatisfactorily low. Coating conditions and preservation conditions Since the coating composition contains the organic solvent, it can be easily coated on the substrate as an organic solvent type paint by, for example, spray coating and dip coating. The pretreatment of the substrate is simple and can be carried out only by degreasing. The coating composition according to the present invention does not require any complicate pretreatment of the substrate that is required for the conventional inorganic coating. Since the polar solvent for polyborosiloxane (e.g. NMP) is hygroscopic, the coating composition of the present invention is preferably preserved and applied on the substrate in an atmosphere with low humidity. The heating effect of the coating for IR radiation heating according to the present invention is now explained. Fig. 10 schematically shows an apparatus which is used in a heating test of the coating film for IR radiation heating. In this apparatus, a heater 17 heats a radiation surface 18 of the coating film, and generated radiation heat raises temperature of an interior of the apparatus. Temperature increase is measured at 27 points in the interior of the apparatus. The results for the stainless steel surface and the coating films according to the present invention (Coating composition Nos. 4 and 7) are shown in Fig. 11, which shows the results measured at the center part of the apparatus. In Fig. 11, the curves 19, 20 and 21 represent the results for the stainless steel, the composition No. 4 and the composition No. 7, respectively. As is apparent from the results, the presence of the coating film according to the present invention raises the temperature more quickly to higher temperature than the stainless steel surface. Since the radiation heating effect of the coating according to the present invention is superior to the uncoated stainless steel, the cooking test by using the apparatus of Fig. 10 is carried out. The cooking test is carried out with a sponge cake, gratin, etc. The cooking time was shortened by 30 to 40 %. The coating film according to the present invention is applied on an inner surface of a kiln for baking a paint coating and its efficiency is compared with a conventional ceramic type heater. The baking time is shown in Table 4. In Table 4, "thickness of coating" is that of a coating for IR radiation, and "baking time" is that required for baking an organic paint. As understood from these results, the paint coating can be baked by using the coating film according to the present invention in an about half of the baking time required for the conventional ceramic coating. Therefore, an amount of energy, a length of the kiln and a space for installing the kiln are reduced. In addition, the ceramic coating tends to be peeled off from the surface of the substrate of the heater and cannot be used for a long time. On the contrary, the coating of the present invention does not suffer from such peeling off. The heating effect described in the above is due to high IR radiation effect of the coating film according to the present invention. The adhesivity of the coating according to the present invention is demonstrated. The test procedures are as follows: Test No. 1: The coated substrate is kept at 700°C for 5 minutes and then immersed in water for 5 minutes. These procedures are repeated. Test No. 2: The coating film is cut with a knife to form chess board like slits with a width of 1 mm and an adhesive tape is adhered thereon. Then, the adhesive tape is peeled off. Test No. 3: The coated substrate is continuously kept at 700°C. The results are summarized in Table 5. In these tests, the coating composition No. 7 was used. The results show that the coating film according to the present invention has good adhesivity to the substrate. The coating according to the present invention can be used at a high temperature of 700 to 800°C for a long time unless it is not heated under a very corrosive condition. Although the above examples are illustrated with Fe₂O₃·Mn₂O₃·CuO as the transition metal oxide, oxides of other transition metals such as cobalt, chromium and the like can be used according to the present invention, and various coating composition having different IR radiation characteristics can be prepared.

Claims: A coating film for IR radiation heating consisting of a sintered mass of a mixture which comprises polyborosiloxane, ZrO₂ particles having a particle size from 0.1 to 2 µm the surfaces of which are coated with at least one oxide selected from the group consisting of Al₂O₃ and SiO₂ and an oxide or a mixed oxide of one metal selected from the group consisting of iron, manganese, copper, cobalt, nickel and chromium. The coating film for IR radiation heating, wherein said mixture further comprises a magnesium oxide and mica.

Inventors: FUKUDA AKIO, ISOGAI MAMORU, KANEKO YASUNORI, MAKI MASAO, FUKUDA, AKIO, ISOGAI, MAMORU, KANEKO, YASUNORI, MAKI, MASAO
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1987
Main classification: C09D185
IPC codes: C08K3, C09D183