JPH0676981B2 - Moisture sensitive material and moisture sensitive element - Google Patents

Description

The present invention relates to a moisture-sensitive material and a moisture-sensitive element. More specifically, the present invention relates to a humidity-sensitive material and a humidity-sensitive element that can detect a change in relative humidity sharply and that is suitable for a sensor in controlling the humidity of air.

(B) Conventional Technology A moisture-sensitive material capable of detecting a change in physical properties that changes according to a change in humidity of air to detect a change in humidity in the air has been known for a long time.

Among such moisture-sensitive materials, its electrical resistance fluctuates according to the change of the humidity of the air, and as the one used for the humidity-sensitive element capable of detecting the change of the humidity of the air, the hydroxyl Apatite and certain ceramics are known.

As the ceramics, there is a porous titania sintered body obtained by firing a kneaded material of titanium oxide (TiO ₂ ) powder and a resin to remove the resin component, and a temperature sensor using this is known. is there.

(C) Problems to be Solved by the Invention However, the porous titania sintered body, which is the above-mentioned known moisture-sensitive material, has a large amount of resin in the kneaded product of titania powder and resin before sintering (called a green body). Since a non-uniform state occurs between a portion and a small portion, many have non-uniform pores. When a moisture-sensitive material having a non-uniform pore size is used, its resistance value is likely to fluctuate, and it is difficult to obtain a resistance value characteristic as a straight line. This is not preferable in the design of the humidity sensor, and the obtained humidity sensor has low accuracy.

The present invention has been made in view of such circumstances, and an object thereof is to provide a moisture-sensitive material and a moisture-sensitive element made of a porous titania sintered body having a uniform pore diameter.

(D) Means for Solving the Problems Thus, according to the present invention, a moisture-sensitive material made of a porous titania sintered body obtained by collecting and coating resin-coated titania spheres, and between a pair of electrodes. There is provided a moisture sensitive element in which the moisture sensitive material is electrically interposed.

The moisture-sensitive material of this invention is made of a porous titania sintered body. The sintered body is composed of porous titania spheres. A porous titania sintered body having uniform porosity can be obtained by adjusting the size of the titania spheres and assembling and sintering the same size.

As the above-mentioned porous titania sphere, for example, a resin-coated porous titania sphere obtained by subjecting titania powder to suspension polymerization or the like is preferable. According to the above method, the size of the obtained sphere can be adjusted and prepared. The following are mentioned as a preferable example of the said preparation method. That is,
A mixture of titania powder surface-treated with a lipophilic agent and a polymerizable vinyl-based monomer is prepared, and in some cases, an organic solvent that is compatible with the monomer and is substantially incompatible with water is used. Addition to the mixture, suppressing the increase in viscosity with the increase of titania powder in the mixture, and expanding the addition amount range of the titania powder, in the oily dispersion of the mixture in an aqueous system, individual oil droplets This is a method in which it is possible to maintain the particles dispersed in a spherical shape, and the titania powder (primary particles) dispersed in each oil droplet is aggregated into a spherical shape to form a titania sphere.

In the above method, the surface-treated titania powder is
It means that the titania powder only has to be surface-treated before the polymerization of the polymerizable vinyl-based monomer used. Therefore, the titania powder may be surface-treated in advance before being added to the polymerizable vinyl-based monomer or the mixture of the monomer and the predetermined organic solvent, and the polymerizable vinyl-based monomer may be added. Alternatively, titania powder may be added to a mixture of a predetermined organic solvent and a lipophilic agent used for surface treatment for surface treatment. The surface treatment is a treatment in which titania powder is brought into contact with the lipophilic agent to adsorb or bond the lipophilic agent on the surface of the powder.
The treatment is achieved by a usual mechanical method or the like. That is, a mixture composed of a lipophilic agent and titania powder, or a mixture composed of a lipophilic agent, titania powder and a polymerizable vinyl-based monomer, at room temperature or under cooling, for example, a propeller blade or a homogenizer at high speed. It is achieved by stirring.

The titania powder used in the above method may be one that allows the finally obtained sintered body to be suitably used as a moisture-sensitive material. During the turbid polymerization, the titania powder is easily desorbed from the dispersed oil droplets into the aqueous phase, and it becomes difficult for the resin-coated porous titania particles to maintain a spherical shape. On the other hand, in the case of using particles having an ultrafine particle diameter, even if a large amount of an organic solvent is added as a viscosity reducing agent, the viscosity is lowered until it can be dispersed in water as spherical oil droplets during aqueous suspension. Therefore, the titania powder preferably has a particle size of 0.01 to 2.0 μm, more preferably 0.1 to 1.5 μm.

The lipophilic agent used in the above method has a functional group capable of strongly adsorbing or binding to the titania powder, and a hydrocarbon having a high affinity for the polymerizable vinyl-based monomer, or a functional group capable of binding to the monomer. A substance having a group can be used. Examples thereof include higher fatty acids such as oleic acid, stearic acid and palmitic acid, unsaturated carboxylic acids such as acrylic acid and methacrylic acid, and polar groups such as aminoethyl acrylate, hydroxyethyl acrylate and cyanoethyl acrylate. Acrylic ester, a titanate coupling agent, a coupling agent such as a silane coupling agent and the like can be mentioned. Among these, those having a functional group that strongly binds to the titania powder, such as a titanate coupling agent and a silane coupling agent, are preferable. Examples include pyrophosphate-type titanate coupling agents such as isopropyl tris (dioctylpyrophosphate), bis (dioctylpyrophosphate) titanate, and phosphate-type titanate coupling agents tetraoctylbis (ditridecylphosphate) titanate. Examples of the silane coupling agent include vinyltrichlorosilane, vinyltrimethoxysilane, and γ-methacryloxypropyltrimethoxysilane having a radically polymerizable functional group.

As the polymerizable vinyl-based monomer used in the method,
Any known monomer can be used as it is, as long as it can exist as spherical oil droplets in a state of being dispersed in an aqueous system and can form a polymer under the polymerization conditions. The monomers may be used alone or in combination of two or more, and may be used in combination with a known crosslinking agent. Preferable examples of the monomer include acrylic acid esters such as methyl acrylate and ethyl acrylate, methacrylic acid esters such as methyl methacrylate and ethyl methacrylate, and aromatic vinyl compounds such as styrene. Suitable examples of the cross-linking agent include methacrylic acid esters of polyhydric alcohols such as ethylene glycol dimethacrylate, tetraethylene glycol dimethacrylate and trimethylolpropane trimethacrylate, and divinylbenzene. Further, the polymerization initiator may be any that is soluble in the polymerizable vinyl-based monomer used, for example, benzoyl peroxide, lauroyl peroxide, diacetyl peroxide and other commonly used peroxides and azobis. Examples thereof include azo compounds such as isobutyronitrile and azobismethylvaleronitrile.

The organic solvent used in the above method, in order to make the resin-coated porous titania spheres once obtained excellent in spherical shape,
Used as needed. As the organic solvent, a solvent that is compatible with the polymerizable vinyl-based monomer and substantially not compatible with water is used. The phrase “substantially incompatible with water” means one that is insoluble or slightly soluble in water. This organic solvent is used to adjust the viscosity of the mixture of titania powder and polymerizable vinyl-based monomer described below, and is therefore 3.0 centipoise (cP) at room temperature.
Those having the following viscosities are preferable, and those having an affinity for the polymerizable vinyl-based monomer used are more preferable. Examples of such organic solvents include acetic acid esters such as methyl acetate and ethyl acetate, hydrocarbons such as hexane and heptane, ketones such as methyl ethyl ketone and methyl isobutyl ketone, and aromatic hydrocarbons such as benzene and toluene.

A slurry-like mixture is prepared from the titania powder surface-treated with the lipophilic agent, the polymerizable vinyl monomer, the organic solvent added as necessary, and the like. In the preparation of this slurry, the titania powder is formed in the resin-coated porous titania spheres described below to form an aggregate by aggregation of the titania powders, and as a result, a resin-coated porous titania spheres can be obtained in an amount sufficient. Used. This amount is set according to the specific gravity and properties of the titania powder used. For example, it is used in an amount of 150 parts by weight or more with respect to 100 parts by weight of the polymerizable vinyl-based monomer, and it is preferably used in the range of 230 to 1000 parts by weight in terms of porosity and sphericity of the resulting titania particles. The lipophilic agent is determined by the minimum coating area of the lipophilic agent and the specific surface area of the titania particles to be used, but it is usually used in the range of 0.2 to 0.3 parts by weight based on the titania powder. Further, the organic solvent is used in an amount necessary to adjust the slurry mixture to a viscosity capable of maintaining spherical oil droplets when dispersed in an aqueous system described below. The amount is suitably 1 to 200 parts by weight, preferably 1 to 150 parts by weight, based on 100 parts by weight of the polymerizable vinyl monomer. When the amount is 200 parts by weight or more, it is difficult to obtain the substantial strength of the polymer that binds the titania powders together, which is not preferable. The polymer initiator is usually used in the range of 0.1 to 2.0% by weight based on the polymerizable vinyl monomer used. The slurry-like mixture prepared above is prepared into a uniform slurry by the same mechanical method as the above-mentioned surface treatment.

The slurry-like mixture prepared as described above is dispersed in an aqueous system, and the polymerizable vinyl-based monomer in the mixture is subjected to the polymerization conditions. The above dispersion is achieved by the same mechanical method as described above. At this time, the dispersion conditions are set so that the resin-coated titania spheres having a particle size range described later are dispersed by the size of the oil droplets. The above-mentioned polymerization can be achieved by adjusting it according to the kind of the polymerizable vinyl-based monomer used, but the usual suspension polymerization conditions can be applied as it is.

An aggregate of titania powder coated with a resin composed of a polymer of a polymerizable vinyl monomer in an aqueous system by the above dispersion / polymerization, which is a spherical and porous resin having a particle size of 1 μm to 1 mm. A coated titania sphere is obtained. The titania spheres are filtered and dried by a conventional method. The particle size is appropriately selected depending on the porosity or density of the sintered product which is the final product described below.

In the moisture-sensitive material of the present invention, the porous titania spheres are assembled and sintered into a porous titania sintered body.
For example, when the resin-coated porous titania spheres obtained by the above method are used, the resin-coated titania spheres are assembled into a predetermined shape and pressure-molded, and the obtained molded product is heated to remove the coating resin and sinter. By doing so, a porous titania sintered body having a desired shape can be obtained. The above molding is preferably uniaxial molding. In this case, the uniaxial molding conditions include 0.5 to 1.0 t / cm ² .

In the case of the above-mentioned molding, it is possible to use an adhesive such as a binder in order to maintain a desired shape, but since it may remain in the sintered body due to carbonization during firing described later,
Not very good.

The aggregate of resin-coated titania spheres held in a desired shape by the above pressure molding is then subjected to heat treatment. This heat treatment is carried out at a temperature at which the resin-coated titania spheres in the aggregate can be sintered but cannot be fused. Here, the above-mentioned fusion means a state in which spheres are melted and become substantially non-porous. Therefore, the heat treatment is performed under the condition that the coating resin is incinerated and removed and the titania spheres can be sintered together. In this case, it is preferable that the porosity of each individual titania sphere is maintained. In the above heat treatment, on the one hand, the coating resin is not carbonized in the process of being incinerated, and on the other hand, the titania powder to be sintered is not excessively fused due to the crystal growth accompanying sintering so that the porosity is not impaired. In addition, the heating temperature, the heating time, and the temperature rising state to the heating temperature are selected. The heating temperature is set to about 950 to 1200 ° C. At 950 ° C or less, the strength after sintering is small and the resulting titania spheres break,
At temperatures above 1200 ° C, crystals grow too much during sintering and the porous state of the titania spheres obtained cannot be maintained. In this case, it is preferably 1000 to 1100 ° C. As for the temperature rising state to the heating temperature, it takes a long time (for example, 5
Time) is necessary in order to avoid carbonization of the resin.

By the heat treatment, the porous titania spheres are aggregated and sintered to obtain a porous titania sintered body, and the strength of the obtained sintered body can be adjusted by adjusting the conditions of the heat treatment. .

By the treatment as described above, a porous titania sintered body obtained by assembling and sintering porous titania spheres having a particle size of 1 to 50 μm as a constitutional unit can be obtained.

In the moisture-sensitive material of the present invention, the shape of the porous titania sintered body obtained as described above is appropriately selected to be suitable for the production of the moisture-sensitive element described later. Further, in the above moisture-sensitive material, the degree of porosity that affects its performance, sensitivity, etc.
It is determined by the porous structure of the porous titania sphere and the porous structure of the porous titania sintered body. The pore diameter in the porous structure of the porous titania sphere can be controlled by adjusting the content of the titania powder used. The particle size of the porous titania spheres can be controlled by adjusting the particle size of the resin-coated titania spheres. Therefore, the structure of the porous titania sintered body finally obtained (for example, pore size, density, etc.) can be adjusted by adjusting the particle size of the sphere.

The present invention also provides a moisture sensitive element in which the moisture sensitive material made of the above-mentioned porous titania sintered body is electrically interposed between a pair of electrodes.

In the above moisture sensitive element, the form of the moisture sensitive material used is preferably a thick film type. As a form of the element applied in this case, as shown in FIG. 1, a pair of electrodes (2) and (2) are formed on both front and back surfaces of a thick film moisture sensitive material (1), and this moisture sensitive material is formed. As shown in FIG. 2, the pair of electrodes (2) and (2) are used to detect the variation of the impedance (Z) of the material.
Is preferably connected to the AC power source (4) by the leads (3) (3). When the humidity sensitive element is used, by measuring the relationship between the phase temperature in the air and the impedance (Z) corresponding thereto in advance, the impedance of the impedance measured by the humidity sensitive element in the measurement target atmosphere is measured. From the fluctuation, the relative humidity (% RH) in the atmosphere can be obtained. Specifically, reference is made to the description of Examples below.

(E) Operation According to the present invention, a porous titania sintered body obtained by assembling resin-coated titania spheres and then sintering it can be obtained with a uniform degree of porosity. Then, the adsorption and desorption of moisture in the sintered body having such a uniform porosity are performed with good reproducibility, and therefore the moisture-sensitive material composed of this will follow well the fluctuation of the humidity of the atmosphere to be measured. .

Further, when the moisture sensitive material is electrically interposed between the pair of electrodes, linearity can be obtained in the characteristic of the change in the resistance value between the electrodes due to the fluctuation of the humidity in the atmosphere to be measured.

(F) Examples Hereinafter, the present invention will be described with reference to examples, but the present invention is not limited thereto.

Example 1 Preparation of resin-coated titania spheres In a beaker 1 279 g of methyl methacrylate, silane coupling agent [γ-methacryloxypropyltrimethoxysilane, SZ6030 manufactured by Toray Silicone Co., Ltd.] 21.0
g, 0.60 g of azobisisobutyronitrile, and completely dissolved, then 700 g of titanium oxide (rutile type, particle size 0.2 μm, JR-600A manufactured by Teikoku Kako Co., Ltd.) was added, and a propeller blade was provided. The surface was treated with a stirrer at 2000 rpm for 30 minutes.

5 Metathesis magnesium pyrophosphate 60 in autoclave
2.6 g of water and 2.6 g of sodium dodecylbenzene sulfonate 2.6
After adding kg, the above slurry was added and suspended, and after nitrogen substitution, the stirring speed was set to 500 rpm and polymerization was carried out at 60 ° C. After the polymerization was completed, the mixture was cooled to room temperature, the dispersant was decomposed with hydrochloric acid, and then separated by filtration. When the obtained particles were observed with a scanning electron microscope (SEM), they were spherical particles with a central diameter of about 10 μm, and were polymethylmethacrylate (PMMA).
A porous structure was formed by aggregating a plurality of titanium oxides (titania) coated with a resin.

Preparation of test piece 1 g of the resin-coated titania spheres obtained above was put into a 11.8 mm diameter round diameter molding machine (depth 20 mm) and uniaxially molded at a pressure of 1 t / cm ² to obtain a molded product (density: 0.5 g / cm ³ ) was placed in an electric furnace and fired at 1200 ° C. for 1 hour. Both surfaces of the obtained sintered pellet were polished with 1000 # sand paper, and then the polished surface was coated with gold by sputtering to form an electrode, and a test piece was obtained.

Relative Humidity Detection Test Attach the test piece obtained above to a measurement holder, connect it to an AC power source impedance analyzer (4192A, made by YHP), and apply each frequency AC voltage (20 ° C) shown in Table 1. Originally, the impedance (Z) of the test piece was measured over a range of relative humidity of 37.0% RH to 90.0% RH.
The results are shown in FIG. The values of impedance (Z) at the specific relative humidity in the above measurement are also shown in the same table.

From the above results, the test piece of the example, the frequency 1KHz and 100H
It can be seen that z has frequency characteristics with good linearity.

(G) Effect of the Invention According to the present invention, it is possible to obtain a moisture-sensitive material having a uniform pore size. Since the fine pores have a double structure of fine pores in the titania sphere and fine pores in the titania crystal, the surface area can be increased and a wide range of humidity fluctuations can be dealt with. Since it is composed of uniform pores, resistance value characteristics with respect to relative humidity can be obtained with good linearity. Then, it is possible to detect the variation of the moisture content in the gas online over a wide range.

[Brief description of drawings]

FIG. 1 is a perspective view of an example of a moisture-sensitive element made of the moisture-sensitive material of the present invention, FIG. 2 is a structural explanatory view for explaining a usage state of the moisture-sensitive element of FIG. 1, and FIG. It is a graph which shows the frequency characteristic of a moisture sensitive material. 1 ... moisture sensitive material, 2 ... electrode, 3 ... lead, 4 ... AC power supply.

Claims (3)

[Claims] 1. A moisture-sensitive material comprising a porous titania sintered body obtained by collecting resin-coated titania spheres and then sintering them. 2. The moisture-sensitive material according to claim 1, wherein the porous titania sintered body is a sintered body of a molded body obtained by uniaxially molding resin-coated titania spheres. 3. A moisture sensitive element, wherein the moisture sensitive material of claim 1 is electrically interposed between a pair of electrodes.