JPH0129517B2 - - Google Patents

Description

[Detailed description of the invention]

<Technical Field of the Invention> The present invention relates to a reversible temperature indicating material, which is a type of temperature control material that has the property of changing hue depending on temperature changes. <Technical background of the invention and its problems> Conventionally, there is a temperature control material called thermopaint (registered trademark of NOF Corporation), which is a temperature indicating material that changes color at a certain temperature or higher. These temperature control materials are available in irreversible, semi-irreversible, and reversible types with a wide range of discoloration temperatures, and are very widely used. These materials are mainly organic substances, and the binders and the like are also mainly organic substances. For this reason, the heat-resistant limit temperature of the temperature control material is generally around 250°C, even if it is high. As the heat-resistant limit temperature rises, applications will expand to include areas around various heat treatment devices that operate at relatively high temperatures, so there is a need for materials that can withstand high temperatures. Furthermore, there is a demand for adding a reversible color-changing temperature indicating function to various home appliances, especially cooking utensils that involve heating, such as heating plates of electromagnetic cookers and hot plates. The purpose of this addition is not to measure temperature, but to warn the user with a change in color tone to prevent the user from inadvertently touching the heated part and getting burnt. However, it is said that the heating part of an electromagnetic cooker can reach a temperature of approximately 450℃.
It must be able to withstand thermal cycling from room temperature to at least this temperature. Furthermore, in this case, it is extremely important that the device be harmless to the human body from the perspective of a device used on a daily basis in a general household. Considering other factors such as lifespan, stability, price, and workability, it can be said that a material that fully satisfies the conditions required for this use has not yet been obtained. <Object of the Invention> The present invention was made in view of the above-mentioned problems, and an object of the present invention is to provide a chemically stable and inexpensive temperature-indicating material, which can be used in a wider range of applications. <Example> First, the progress up to the present invention will be briefly explained. Conventionally, the color tone of reversible temperature-indicating materials has overwhelmingly changed within similar colors, such as yellow←→orange, while changes within dissimilar colors, such as blue←→red, where the change is easily visible, have traditionally been predominantly changed. It was quite rewarding. this is only a few
This is because it is difficult to produce a phenomenon in which a material's visible wavelength reflection spectrum shifts significantly due to a temperature change of 10 degrees Celsius or several hundred degrees Celsius. In addition, there were almost no materials whose colors ranged from white, which is considered to have very good visibility of color tone changes. The inventor is
Various attempts have been made to use white materials, but in rare cases some materials change color from white, but sufficient contrast has not been achieved. Therefore, by combining two types of coloring materials with different reflection spectral ranges,
There are no systems that exhibit an almost white color at room temperature, and as the temperature rises, the reflection spectral range of one type shifts and overlaps with the other, leaving only one type of reflection spectrum and no color change. Something?
I investigated various compounds. the result
It was confirmed that a somewhat similar phenomenon was observed in CoWO ₄ and tungsten oxide compounds. We tried to finely change the composition ratio of this compound system, but it was difficult to obtain a system with a white color at room temperature and a _broad reflection spectrum over the entire visible range. It is light gray. However, it was superior in visibility compared to conventional changes within similar colors. Furthermore, although the contrast after discoloration is slightly lower, it has been confirmed that by devising a processing method, it can take on an almost white tone. Next, a manufacturing method of an embodiment of the reversible temperature indicating material according to the present invention will be explained. CoO as a cobalt oxide compound as a starting material,
Co ₃ O ₄ as tungsten oxide compound WO ₃ ,
H ₂ WO ₄ and WO ₂ are used. Regardless of which method is used, the final products are CoWO ₄ (monoclinic system) and WO ₃ (tetragonal system), which have basically the same composition. In the composition range according to the present invention, (1-n)WO ₃
A produced phase having a composition of +nCoWO ₄ (n=1.00 to 0.03) is obtained. There is no particular need to change the calcination and calcination temperatures depending on the starting materials. The calcination temperature is about 750℃,
A firing temperature of about 900°C is appropriate. The color tone of this compound system largely depends on the composition ratio of Co and W. Table 1 shows each composition ratio (n value)
Shows the color tone at room temperature, color tone after temperature-induced discoloration, and lower limit temperature of discoloration.

[Table] When observing the reflection spectrum of a spectrophotometer, n=
1.0 ( _CoWO4 ) compound from 470nm at room temperature
It has a steep peak at 480nm. The other compound, WO ₃ , exhibits complex behavior with respect to temperature changes, as shown in Figure 2, but at room temperature
It shows a gentle spectrum with a peak from 480nm to 500nm. The color tone at room temperature is obtained by superimposing these with the composition of the n value shown in Table 1. As a representative example of the composition, the reflection spectrum of n=0.15 is shown in FIG.
The relatively steep peak around 475 nm decreases as the temperature rises from room temperature, and a phenomenon is observed in which the short wavelength region gradually disappears. Visually, as shown in Table 1, it appears as if green has appeared from gray. The thermal stability and durability of this system will be described. Regarding heat resistance, heating was repeated up to 950°C, and all of the above-mentioned compositions were stable at least up to this temperature. Further, it does not affect the color change characteristics in any way. The same applies when rapidly cooling from 950°C. It is also insoluble in water, and 1N hydrochloric acid,
It is also almost insoluble in nitric acid. There is no sign of deterioration even after prolonged exposure to UV light. Because it has extremely high thermal and chemical stability, it can be processed in a wide variety of ways as described above. Next, a specific example will be explained according to the process diagram of FIG. All are reagent grade CoO, Co ₃ O ₄ , WO ₃ ,
The weights of H ₂ WO ₄ and WO ₂ shown in Table 2 were weighed. The n value in this case is as shown in the right column of Table 2.

[Table] The amount of each sample from samples 1 to 8 was constant at 50 g. Note that H ₂ WO ₄ releases one water molecule at 70℃.
The result is WO ₃ , which is basically no different from the case where WO ₃ was used from the beginning, but the experiment was conducted and described using it as one of the starting materials. Each mixture sample was thoroughly ground in an automatic mortar and then stirred in a ball mill to obtain a uniform mixed fine powder. This was transferred to a porcelain crucible and calcined in air at 750°C for about 20 hours. After cooling, it was taken out and pulverized again in an automatic mortar, followed by final firing. Firing was carried out at 900°C for 36 hours, then slowly cooled to 500°C at a rate of 50°C/hour, and then allowed to cool to room temperature. The produced phase of the obtained sample was confirmed using an X-ray diffractometer. _WO3 ,
If a phase other than CoWO ₄ was observed (a), the firing was repeated again. This is not necessary in case (b) where no phase other than WO ₃ and CoWO ₄ is observed. Generally, when the firing temperature is high, CoO (NaCl
mold) tends to precipitate. Samples 1 and 2 in Table 2 use different starting materials, but the phases produced are exactly the same. Next, several grams of a portion of all the samples except sample 2 were taken and ground in an agate mortar using two sieves to obtain fine powder with a particle size of 25 μm to 37 μm. The reflection spectrum from 400 to 800 nm was measured using a spectrophotometer. Measurement temperature is room temperature and 70℃ to 350℃
There are a total of 6 points at 70°C intervals. In addition to these basic measurements, we also checked for the presence or absence of thermal history, but no phenomena that would pose a particular problem were observed. As for the color tone, the composition ratio that is closest to white at room temperature when visually recognized is the one with an n value of 0.05.
It is in the range of 0.15. In particular, 0.10 was preferable as it had a white-gray color tone at room temperature. Next, we took a portion of all the samples and conducted further tests on heat followability of color change, thermal stability, repeated life, solubility in various solvents, stability under ultraviolet light, etc. We carefully examined his suitability for the position, and the results were perfect. Next, since the color tones of each sample were very different, we investigated the most effective processing method suitable for each sample and tried to form a film. Here, sample number 1 in Table 2
We will discuss the material where the value of n is 0.10. This material can be processed in various ways, taking advantage of its white color.
Film formation was carried out. Next, three of these processing methods will be described. (1) Processing method using glass frit First, we will introduce glass frit (6μm) and its weight.
A 40% sample was uniformly mixed in a ball mill to obtain an almost white fine powder. This was thoroughly mixed with ethanol to form a viscous paste. This was applied uniformly with a spatula onto an unglazed ceramic board, a thin glass board with a satin finish, a Pyrex glass board, and an asbestos board. After sufficiently drying this in a vacuum dryer, it was transferred to a Matsufuru furnace and baked at about 490°C for 3 hours. The resulting film is in this case translucent and white, although the surface smoothness and gloss depend slightly on the baking temperature, glass frit and amount of sample. At about 120℃,
A change in translucent white and green color was visible. (2) A processing method that emphasizes the white background by adding alumina powder to glass frit. A mixture of glass frit, the above sample corresponding to 40% of its weight, and alumina abrasive powder (10 μm) also corresponding to 10% was baked onto a substrate in the same manner as in (1) above. In this case, since alumina powder is dispersed in addition to the sample as a white component, a uniform white film with good reproducibility can be obtained even if the baking temperature varies slightly. Moreover, by repeating this film formation several times, it became possible to obtain a film with a thickness close to the intended thickness. In this case, approximately
It is 100μm. A color change from white to pale green can still be seen at around 120°C, but since the background color is white, it is visually vivid and has good visibility. In addition, the intensity of the green color after discoloration could be adjusted to some extent by changing the amount of alumina added. However, when added in too large a quantity, the product became brittle in terms of strength. I tried using zirconium oxide, titanium oxide, and zinc oxide as other white additives for alumina, but the results were the same, and a reversible discoloration film with a white background that changed to a greenish color was obtained. (3) A processing method that emphasizes the white background using a ceramic dispersion coating agent. The ceramic dispersion coating agent used is one in which SiO ₂ and Al ₂ O ₃ are added and dispersed in water and isopropyl alcohol. Therefore, the ingredients are the same as the example in the previous section, but it is easy to apply, mass-produced processing methods such as spray, brush, dip, and roll methods can be used, and the SiO ₂ component is already dissolved, so it can be applied at around 150℃. Advantages include that it can be fixed by heating or air-drying, which greatly expands the range of adherends, and that the film can withstand a temperature of 800°C. In addition to those described in (1) and (2) above, the film forming substrate can also be made of polyimide film (with a heat resistance limit temperature of approx.
500℃) and an aluminum plate. In order to improve the wettability and adhesive strength of the polyimide film to some extent, numerous scratches were made on the surface of the polyimide film using sandpaper or the like. The surface of the aluminum plate was also treated in the same manner in order to increase the adhesive strength after fixing. Specifically, the ceramic dispersion coating agent was first stirred to ensure uniform dispersion, and then a powder sample corresponding to 10% of its weight was mixed in and a uniform mixed dispersion liquid was created using a magnetic stirrer.
This was applied to various substrates using a brush. Since a uniform film cannot be obtained with one application,
The drying operation was repeated a minimum of three times. Since polyimide film has extremely poor wettability, the concentration of the dispersion was increased to form a paste-like viscous liquid, and then applied in one go by rubbing with a spatula. These were heat-treated at 150° C. for 1 hour to form a hard film with a thickness of about 100 μm. The degree of discoloration of these deposited films showed excellent visibility as described in item (2) above. Incidentally, the temperature indicating material may be directly mixed into the substrate by mixing, dispersing and molding porous glass, ceramics, asbestos, cement, plastic, etc. The embodiments of the reversible temperature indicating material described above have the characteristics summarized below. (1) Matters related to color changing function (a) At room temperature, it exhibits blue, whitish-gray, and yellow-gray tones, and at approximately 170°C to 120°C, it changes to greenish-blue, green, and greenish-yellow, respectively. (b) By changing the composition ratio, any color tone can be obtained within the above range. (c) By forming a film by adding glass, alumina, etc., the composition of one part becomes almost white at room temperature, and a pale green color appears at about 120°C. (2) Matters related to stability, processability, and productivity (a) Contains no substances harmful to the human body. (b) It is insoluble in water and does not deteriorate even when exposed to ultraviolet light. (c) It is thermally stable and can withstand up to approximately 950℃. (d) Processing that involves heating is possible. (e) It is relatively inexpensive and the manufacturing method is simple. <Effects of the Invention> According to the present invention described above, a chemically stable and inexpensive reversible temperature indicating material can be obtained.

[Brief explanation of drawings]

FIG. 1 is a process diagram of the manufacturing process of one embodiment of the reversible temperature indicating material according to the present invention, and FIGS. 2 and 3 are graphs of the temperature dependence of the reflection spectrum measured by a spectrophotometer of substances related to the reversible temperature indicating material. It is a diagram.

Claims (1)

[Claims] 1 The atomic ratio of cobalt and tungsten is (Co: 1.0, W: 1.0) (Co: 0.03, W: 1.00)
A reversible temperature indicating material characterized in that the raw material is cobalt oxide-tungsten oxide polycrystalline powder obtained by firing a composition mixture having a composition within the range of .