JPH0244202Y2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a moisture-sensitive element using a microporous body composed of hydrophilic inorganic powder and a hydrophobic synthetic resin. More specifically, it relates to a humidity sensing element for detecting changes in electrical characteristics due to changes in humidity, and is used for automatic control in response to changes in humidity. be.

There are various types of humidity sensing elements for detecting changes in humidity based on changes in electrical characteristics associated with changes in humidity. For the reason,
Moisture-sensitive elements made of synthetic resin are attracting attention.

BACKGROUND ART Conventionally, as a moisture sensing element made of synthetic resin, one using a synthetic resin microporous body is known. For example, JP-A No. 47-31191 discloses a method using a polyvinyl chloride thin film with an average pore diameter of 0.5 μm. This moisture sensing element utilizes the fact that the amount of moisture adsorbed within the pores of a microporous material changes in response to changes in humidity, and changes the electrical resistance of the microporous material based on changes in the amount of adsorbed moisture. The aim is to detect changes in humidity.

However, according to the findings of the present inventor, the above-mentioned conventional moisture-sensing element has a microporous material that is not very good at desorbing moisture, so it is good when there is a relatively gradual humidity change, but when there is a sudden and large change in humidity. The disadvantage is that the response speed to humidity changes is slow.

The inventor of the present invention conjectures the reason for this as follows. That is, the microporous material used in the conventional moisture-sensitive element is simply a synthetic resin membrane having a large number of fine holes, and the holes themselves have narrowed parts that allow moisture to be sensitively desorbed. It is thought that the number of overflows is small, and even if there are overflows, they are only sparse.
Therefore, changes in electrical characteristics will not occur unless a large amount of moisture is desorbed throughout the hole, and especially when there is a sudden and large change in humidity, the desorption of moisture cannot keep up with the humidity change, resulting in a slow response speed. It is considered to be stored away.

This invention speeds up the response speed of humidity sensing elements especially to sudden and large changes in humidity, and expands the applications of humidity sensing elements using synthetic resin microporous materials, which are excellent in mass production, ease of handling, and durability. The purpose is to

The structural feature of this invention is critical surface tension
It consists of a hydrophobic synthetic resin with a surface tension of 40 dyn/cm or less and a hydrophilic and water-insoluble inorganic powder with a surface tension of a water contact angle of 10° or less, and the inorganic powder is continuous between at least the front and back surfaces. A microporous body with an average pore diameter of 0.01 to 1μ, a porosity of 30 to 80%, and a thickness of 10μ or more is used and is held in a synthetic resin, and an electrode is attached to this microporous body.

The above-mentioned present invention was made based on the following knowledge of the present inventor. In other words, moisture is desorbed from the microporous material mainly at the narrow overflows within the pores, and the formation of many adjacent overflows allows moisture to be absorbed by sensitively responding to humidity. You will have to remove it. According to the moisture sensing element according to the present invention, a large number of overflows are formed adjacently between the inorganic powder particles that are filled in the through holes of the microporous body, This narrowing portion sensitively desorbs water and changes the electrical properties of the microporous material. The moisture sensing element of the present invention is not a microporous body that desorbs a large amount of moisture throughout the body as described above, but a microporous body that desorbs moisture in many narrow areas that sensitively react to humidity. Because of this, the response speed is extremely fast even to sudden and large changes in humidity.

Hereinafter, the present invention will be described in more detail with reference to the drawings showing embodiments of the present invention.

FIG. 1 is a perspective view showing an embodiment of a moisture sensing element according to the present invention.

In the figure, reference numeral 1 indicates a microporous body, which is shaped like a plate, a sheet, or a film. A set of electrodes 2a and 2b are provided on both sides of this microporous body 1.
is installed. Moreover, electric cords 3a, 3b for connecting to a measuring device (not shown) are connected to both electrodes 2a, 2b.

The moisture sensing element according to the present invention detects moisture in the microporous body 1 due to changes in humidity by applying a constant voltage or flowing a constant current between the electrodes 2a and 2b from a measuring device. This is used to detect changes in electrical characteristics due to attachment and detachment as changes in current amount and voltage, and detect changes in humidity.

Figure 2 shows the electrode 2 of the humidity sensing element shown in Figure 1.
FIG. 3 is an enlarged longitudinal sectional view showing the surface condition of the microporous body 1. FIG.

In the figure, 4 is a synthetic resin layer, and 5 is an inorganic powder. As is clear from the figure, the inorganic powder 5 is held in the synthetic resin layer 4 in a complicated manner vertically and horizontally. In particular, in this embodiment, the synthetic resin layer 4 has a three-dimensional network structure with through holes filled with inorganic powder. Here, the three-dimensional network structure refers to a structure in which the through holes do not simply extend linearly in the thickness direction, but have a complex spread shape in which the through holes also extend in the planar direction.

The synthetic resin constituting the synthetic resin layer 4 of the microporous body 1 in the present invention has a critical surface tension.
It is necessary to use a hydrophobic synthetic resin of 40 dyn/cm or less. If the synthetic resin layer 4 is hydrophilic, the amount of water absorbed by the microporous body 1 will be large as a whole,
Moisture cannot be quickly attached and detached, and the response speed becomes slow. Specific synthetic resins include polyolefin-based synthetic resins, fluorine-based synthetic resins, and the like. More specifically, polyethylene, polypropylene, polybutene and mixtures thereof, double or higher polymers of ethylene, propylene, butene, hexene, copolymers of ethylene and tetrafluoroethylene, polyvinylidene fluoride, polytetrafluoroethylene, etc. Fluorinated ethylene, etc.

The inorganic powder 5 needs to be a hydrophilic inorganic powder having a surface tension of 10° or less at a water contact angle and insoluble in water. If the material is hydrophobic, no matter how many overflows are formed in the pores of the microporous body 1, water cannot be stored in these overflows, and eventually the microporous body 1 becomes unable to store water in the overflows, resulting in poor resistance to humidity changes. Reactions become sluggish. In addition, if the inorganic powder 5 is soluble in water, the adsorbed water will dissolve the inorganic powder 5 and the carefully formed overflow will disappear, maintaining performance. become unable to do so.

Specific examples of the inorganic powder 5 include silica, titanium oxide, tin oxide, etc., with particles as small as possible;
In addition, it is preferable that the surface area is large in order to facilitate moisture adsorption. The average particle size of the inorganic powder 5 is
It is preferable that it has a specific surface area of 50 to 500 m ² /g.

The microporous body 1 according to the present invention has an average pore size as a whole in which the above-mentioned inorganic powder 5 is held in a synthetic resin layer 4 in a continuous state between at least the front and back surfaces.
It is necessary to have a thickness of 0.01 to 1μ, 30 to 80%, and a thickness of 10μ or more. If the inorganic powder 5 is held in a state where it is not continuous between the front and back surfaces, changes in electrical characteristics due to moisture desorption cannot be obtained. In addition, even when the inorganic powder 5 is held continuously between the front and back surfaces, if it is held in a three-dimensional network structure like the one shown in the figure, the held inorganic powder 5 can be Good communication makes it easy to obtain uniform moisture desorption throughout the microporous body 1. If the average pore diameter is too small, the penetration of moisture is hindered and its desorption is inhibited, whereas if it is too large, the number of narrow overflows is proportionally reduced, slowing down the response speed. Furthermore, when the same amount of the same inorganic powder 5 is held, if the porosity is too small, the amount of moisture adsorption and desorption will be small, making it impossible to obtain a sufficient change in electrical characteristics. On the other hand, if the porosity is too large under the same conditions, the narrowed portion will decrease accordingly, the amount of change in electrical characteristics will be small, and the sensitivity will become dull. The thickness of the microporous body 1 needs to be 10 μm or more in order to obtain the necessary strength. Further, in order to enable uniform and rapid desorption of moisture between the vicinity of the surface and the deep part, the thickness is preferably 1000 μm or less.

In the present invention, the average pore diameter and porosity refer to values determined as follows.

(1) Porosity = water content - dry weight / volume of microporous material 1 x 100 (2) Average pore diameter is the long diameter of 200 openings when the surface of microporous material 1 is observed with a scanning microscope. Calculated by weighted average of the short axis.

The electrodes 2a and 2b are made of a metal with excellent conductivity, and silver or gold is preferable from the viewpoint of oxidation resistance. In addition, electrode 2
a and 2b may be formed by adhering metal foil to the microporous body 1 as appropriate, but if they are formed by vapor deposition, the synthetic resin layer 4 will be removed unless vapor deposition is performed for a long time.
This is optimal because it can be formed without blocking the openings on the surface. This is because since the openings on the surface of the synthetic resin layer 4 are maintained, there is no fear that the sensitive reaction of the microporous body 1 between the electrodes 2a and 2b to changes in humidity will be hindered. Electrode 2
a, 2b are in contact with the inorganic powder 5 held there through the opening of the microporous body 1, and both electrodes 2
Electricity is maintained between a and 2b.

FIG. 4 is a perspective view showing another embodiment of the moisture sensing element according to the present invention, in which electrodes 2a and 2b are arranged separately on both sides of one side of the microporous body 1.

Next, an example of a method for manufacturing a microporous body used in the present invention will be explained. However, the present invention is not limited to the use of microporous bodies manufactured by the following method.

First, a polyolefin with a weight average molecular weight of less than 300,000 but a number average molecular weight of 15,000 or more, and an organic liquid (plasticizer) with a solubility parameter (SP value) of 8.4 to 9.9.
and, based on the total volume of inorganic powder, 6 to 35% by volume of inorganic powder and 30 to 75% by volume of organic liquid are mixed,
Adsorb organic liquid onto the surface of inorganic powder. then
Polyolefin powder is added and mixed in an amount of 10 to 60% by volume and 3/8 to 9 times the amount (weight) of the inorganic powder.
A microporous material that can be used in the present invention can be obtained by extracting an organic liquid from the film obtained by melt-kneading and melt-molding this three-component mixture at a temperature below the melting point of the polyolefin using a solvent. It will be done. The microporous material obtained by this method holds inorganic powder in the aforementioned three-dimensional network structure.

The organic liquid used in the above method includes phthalic esters, phosphoric esters, adipic esters, glycols, hydrocarbons, etc.
It is liquid during melt molding and easily soluble in general organic solvents or water.

The advantages of the present invention are listed below.

(1) A quick response can be obtained even to sudden and large changes in humidity.

(2) Basically, a synthetic resin membrane is used, so
It is easy to manufacture, can be made into any shape, and is inexpensive.

(3) Since it is composed of stable components, there is little deterioration over time and high performance can be obtained over a long period of time.

(4) There is little loss of precision due to surface contamination, etc., and the fact that it is based on a synthetic resin film, which is a material that is difficult to break, provides excellent handling.

Experimental examples of the moisture-sensitive element according to the present invention are shown below.

Experimental Example A microporous body was manufactured by the method described above using polyethylene as the synthetic resin and silica as the inorganic powder. The average pore diameter of this microporous material is
It was 0.05μ, the aperture ratio was 55%, and the thickness was 100μ.

Gold was vapor-deposited as electrodes on the front and back surfaces of the microporous body to form a humidity sensing element, and the response speed of this humidity sensing element was investigated. The results are shown in Figure 5.

As is clear from FIG. 5, the humidity sensing element according to the present invention completely responds to changes in humidity between 26% and 77% in about 15 seconds, indicating an extremely fast response speed. Incidentally, R in FIG. 5 is the resistance value of the present moisture-sensitive element.

[Brief explanation of drawings]

FIG. 1 is a perspective view showing an embodiment of the moisture sensing element according to the present invention, FIG. 2 is an enlarged sectional view on the electrode,
FIG. 3 is an enlarged view showing the surface condition of the microporous body, FIG. 4 is a perspective view showing another example of the moisture-sensitive element according to the present invention, and FIG. 5 is a graph showing the results of an experimental example. . 1: Microporous body, 2a, 2b: Electrode, 3a, 3
b: Electric cord, 4: Synthetic resin layer, 5: Inorganic powder.

Claims (1)

[Scope of utility model registration request] Consisting of a hydrophobic synthetic resin with a critical surface tension of 40 dyn/cm or less and a hydrophilic and water-insoluble inorganic powder with a surface tension of 10° or less in water contact angle, the inorganic powder is continuous between at least the front and back surfaces. 1. A moisture-sensing element characterized in that an electrode is attached to a microporous body having an average pore diameter of 0.01 to 1 μ, a porosity of 30 to 80%, and a thickness of 10 μ or more, which is held in the synthetic resin in a state in which the electrodes are attached.