JPH0786492B2 - Humidity sensor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial application] The present invention relates to a humidity sensor.

Specifically, a substrate, a solvent-soluble polyimide resin layer deposited on the surface of the substrate, a pair of electrodes formed on the surface of the substrate in proximity to each other, and a moisture-sensitive resin formed by connecting the electrodes. The present invention relates to a humidity sensor composed of layers.

[Conventional technology and its problems]

As a device for measuring humidity (humidity sensor), hair hygrometers, dry and wet bulb hygrometers, and lithium chloride hygrometers, which have long been known, and ceramic-based polymer humidity sensors that have developed rapidly in recent years, There are many types.

When these are separated from the detection principle, they are roughly classified into the following.

(A) Mechanical sensor: A sensor that draws a chart through a link mechanism or converts it into pressure and takes it out as an electrical signal by utilizing the fact that hair, cellophane, nylon, etc. swell due to moisture absorption.

(B) Wet and dry bulb sensor: A thermometer called a dry bulb, which exposes the temperature sensitive part, and a thermometer called a wet bulb, which wraps the temperature sensitive part with a case that is always wet with water, are paired. Relative humidity is calculated from the temperature difference between the two and the dry-bulb temperature.

(C) Electromagnetic sensor: Since electromagnetic waves such as infrared rays and microwaves have a specific wavelength band that is easily absorbed by water, humidity is obtained by measuring the amount of electromagnetic wave attenuation in this wavelength band.

(D) Impedance change type sensor: A sensor in which electrical characteristics, especially impedance remarkably changes due to adsorption of water, is used as a humidity sensitive element, and the change is measured to obtain humidity.

Among these, the impedance change type sensor is drawing attention, and there are a resistance change type sensor and a capacitance change type sensor. Although the resistance change type sensor has a high degree of freedom in assembling a drive circuit, it has a drawback that it has a high resistance in a low humidity region and measurement becomes difficult. The capacitance change type sensor has a wide measurement range and can obtain a linear output with respect to humidity.

Further, the impedance change type sensors are roughly classified into ceramic type sensors and polymer type sensors in terms of the material of the moisture sensitive material. Wetting agents for ceramic type sensors include Ti,
A mixture of oxides such as Sn, Zr, Mg, Cr, Si, and V is used, and it has high heat resistance and a wide operating temperature range, but its moisture-sensitive property is derived from the porous structure of the ceramic. However, it has a problem of poor reproducibility and inevitable change of adsorbed water with time due to capillary condensation. To avoid aging and performance change due to contamination,
Although there is a type that employs a heating cleaning method that recovers the characteristics by heating to several hundreds of degrees Celsius, it consumes a large amount of power and is not necessarily satisfactory. Examples of the moisture sensitive material of the polymer sensor include polyacrylic acid, polystyrene sulfonic acid, polymethylaminoethyl methacrylate salt, and acid or salt of styrene-divinylbenzene copolymer. The moisture sensitivity of these polymer-based sensors is good because they are derived from the hygroscopicity and ionic conductivity of the prepared polymer itself, and they do not require high-temperature firing during production, so they can be mass-produced at low cost. It is also possible. It is also highly resistant to pollution and is not easily affected by other gas species. However, there is a problem that the heat resistance is low. Some polymer-based sensors use polyimide as a moisture-sensitive material, but they are of the type that polycondensates by heating after being applied on the substrate in the form of polyamic acid, and from the viewpoint of productivity. Not an advantage.

On the other hand, the substrate for the humidity sensor needs to be an electrically insulating substrate, and examples of the inorganic material include ceramics, and examples of the organic material include polyimide film. However, if it is a ceramic type, it is expensive in itself, and high temperature firing is required when a pair of electrodes formed on the surface is made of a conductive paste, which is not advantageous in terms of production cost. In addition, organic materials such as polyimide films cannot avoid dimensional changes due to moisture absorption, and are not said to be advantageous. Further, there is also a method of forming a polyimide thin film by applying polyamic acid on a substrate and heating it to cause polycondensation, but it takes a long time for the heat treatment, which is not advantageous in manufacturing.

[Means for solving problems]

The inventors of the present invention provided a layer of a solvent-soluble polyimide resin represented by a specific formula described below on a substrate and further formed a pair of electrodes on the surface thereof in close proximity to each other to further improve various types of electrodes. By forming a thin film of molecular moisture sensitive agent, humidity can be measured according to the characteristics of each moisture sensitive material, and a humidity sensor with excellent heat resistance, chemical resistance, and dimensional stability can be manufactured at low cost. They have found what they can do and have reached the present invention.

The present invention will be described in detail below.

In the present invention, the substrate is an inorganic or organic material, and has heat resistance sufficient to withstand the heat treatment temperature of the polyimide resin adhered to the surface, and also has resistance to the solvent of the polyimide resin, Further, the surface may be flat and have sufficient mechanical strength. For example, various metal plates,
Inorganic materials such as a glass plate and a silicon substrate are preferred, and a metal plate is particularly preferred.

The metal plate used in the present invention is preferably a stainless plate, an aluminum plate, an aluminum alloy plate, a copper plate, a copper plywood, an iron plate, a molybdenum plate, a chrome plate, a zinc plate, etc.
Particularly, a stainless plate and an aluminum plate are preferable. When an aluminum plate is used, the surface can be anodized to improve the adhesion with the solvent-soluble polyimide resin deposited on the surface.

All known stainless steel plates are used.
For example, JIS standards SUS201, 302, 304, 305, 310S, 31
6, 329, 420, 430, 434, 436, 444, 630, 631 and the like.

The shape of the metal plate is not particularly limited, but a flat surface is preferable so that the solvent-soluble polyimide resin can be applied to the surface, and the surface of the polyimide resin should be as thin as possible so that pinholes cannot be formed in the thin film. It should have a low roughness. For example, if it is a stainless steel plate, Rmax
It is preferably 1 μm or less, preferably 0.5 μm or less. The thickness of the metal plate is not particularly limited as long as it does not affect the electrical characteristics of the sensor, but is usually 0.05 to 10 mm, preferably 0 in terms of ease of handling.
1 to 1 mm is used.

The hardness of the metal plate is also related to the above-mentioned plate thickness, but if it is too hard, the workability is poor, and if it is too soft, edge deformation (burrs) is likely to occur during press punching or the like. For example, a stainless steel plate preferably has a Vickers hardness of 100 to 300, preferably 150 to 2
50 ones are used.

In the present invention, the solvent-soluble type polyimide resin is a solvent-soluble type and refers to copolyimide or copolyamideimide represented by the following specific formula.

(A) 10 to 30 mol% of the repeating unit has the formula With a structure represented by A copolyimide having a structure represented by: or (B) 70 to 90 mol% of the repeating unit is represented by the formula And has a structure represented by A copolyamideimide having a structure represented by:

The solvent-soluble polyimide resin also includes a mixture of a solvent-soluble polymer such as polysulfone or polyether sulfone as the second component.

The above copolyimides include, for example, 3,3 ', 4,4'-benzophenone tetracarboxylic acid dianhydride (BTDA) and two aromatic diisocyanates, namely diphenylmethane-4,
It can be synthesized by copolymerizing 4'-diisocyanate (MDI) and tolylene diisocyanate (TDI).

Further, the above copolyamideimide can be synthesized by copolymerizing two kinds of aromatic polycarboxylic acids, that is, trimellitic anhydride and isophthalic acid, with MDI.

These solvent-soluble polyimide resins are excellent in heat resistance, and particularly the copolyamideimide is also excellent in flexibility, and thus can be suitably used when bending is required.

The logarithmic viscosity (ηinh) of the copolyimide and copolyamideimide used in the present invention is selected from the range of 0.1 to 10 dl / g (0.5% in N-methylpyrrolidone, measured at 30 ° C).

Solvent-soluble polyimide resin can control the thickness to be applied on the substrate according to the ratio of dilution with a solvent, and the heat treatment of the coating film is also easy, after applying a conventional polyamic acid,
It is more advantageous in manufacturing than the case of using a polyimide of a type which is heated and polycondensed.

The film thickness of the solvent-soluble polyimide resin layer is 0.01-100
μm, preferably 0.1 to 10 μm. The film thickness is
When the thickness is smaller than this range, defects such as pinholes are likely to occur, and when the thickness is larger than this range, it takes a long time for drying and heat treatment of the coating film, both of which are not preferable.

The pair of electrodes formed in proximity to each other of the present invention,
A conductive metal thin film or organic conductive material thin film, A
l, Cr, Ni, Cu, etc., as well as Au, Ag, Ru, Rh, Pd, Os, Ir, P
A noble metal such as t or an organic conductive material such as polyacetylene or polypyrrole is used. Particularly, a metal such as Au or Ag that is not easily oxidized is preferable, and a metal oxide such as RuO ₂ is also preferable. The thickness of the electrode is not particularly limited as long as it has sufficient conductivity, and is usually about 0.01 to 1 μm.

The shape is not particularly limited as long as two electrodes that are electrically separated are formed close to each other on the surface of the polyimide resin layer. For example, the two comb-shaped electrodes may be formed so as to be close to each other without contacting the comb-shaped portions. The distance to be brought close is usually 0.01 to 1 mm, preferably 0.05 to
It is about 0.5 mm. The smaller the distance and the longer the sides of the adjacent portions, the larger the amount of change in resistance and the easier it is to detect humidity, which is preferable. Where the distance is 0.01
When the thickness is less than mm, it is not preferable because the presence of foreign matter causes conduction and patterning itself becomes difficult.

A moisture-sensitive resin layer is formed on the pair of electrodes so as to connect and cover the electrodes. Moisture-sensitive resins that can be used include polyacrylic acid, polystyrene sulfonic acid,
Polymethylaminoethylmethacrylate salt, acid or salt of styrene-divinylbenzene copolymer, polyvinyl alcohol, cellulose acetate, cellulose acetate butyrate, polyamide, polyimide, polysulfone, organic polymers such as perfluorocarbon, and inorganic such as polyphosphazene Known polymers such as polymers can be used.

The temperature sensor of the present invention applies a voltage between a pair of electrodes formed close to each other on the surface of a solvent-soluble polyimide layer. To apply a voltage between both electrodes, a pair of electrodes is used. May be directly connected to the terminals of the driving circuit, or may be connected to the terminals of the driving circuit after attaching a lead wire to each of the pair of electrodes with a conductive adhesive or the like.

Next, a method for manufacturing the humidity sensor of the present invention will be described.

Examples of the method of applying the solvent-soluble polyimide resin on the substrate include roll coating, spin coating, flow coating, doctor blade coating, bar coating, and other known methods, but the type and concentration of the solvent solution of the resin, and the viscosity. It may be appropriately selected depending on the film thickness to be applied. For example, in the case of roll coating, it can be applied to a wet film thickness of about 3 to 300 μm, depending on the type of roll, the viscosity of the resin solution, the concentration, etc., so that the dry film thickness can be a predetermined value. The above operating conditions may be adjusted appropriately.

When coating is completed, heat treatment is performed. Depending on the type of solvent used for the resin solution to be applied, it may absorb moisture and coagulate and precipitate the resin when left in the air after application.In this case, depending on the humidity of the atmosphere, it takes a few seconds to a few minutes. It is necessary to immediately heat the mixture within that time so that the residual solvent concentration becomes a sufficiently low value. As a method of heat treatment, 100-400 ℃
Heat for about 5 to 30 minutes. Specifically, it may be appropriately determined depending on the type of solvent and the degree of humidity sensitivity.

Depending on the type of solvent used, the temperature may be rapidly vaporized and the surface property may be deteriorated depending on the type of solvent used, so it is preferable to raise the temperature gradually. In this case, the temperature may be raised continuously or stepwise at intervals of several tens of degrees, and a heating pattern may be appropriately selected depending on the surface property and the degree of residual solvent concentration.

On the surface of the polyimide resin layer formed as described above, a pair of mutually adjacent electrode layers made of a metal thin film is formed. Vapor deposition method, sputtering method,
Electroless plating method, screen printing method, etc. can be mentioned,
For example, when forming a gold thin film by vapor deposition,
Ni or Ni to improve adhesion to the polyimide resin layer
You may perform the surface treatment by vapor deposition of Cr. The electrode layer can be formed into an arbitrary shape by a known method such as an etching method, a mask method or a printing method.

Then, a moisture-sensitive resin layer is formed so as to connect and cover the pair of electrodes formed on the surface of the polyimide resin layer. The moisture-sensitive resin may be appropriately selected from the centers of the above-mentioned organic polymer and inorganic polymer, and this is coated as a thin film. Examples of the method for forming the thin film include known methods such as spin coating, roll coating, doctor blade coating, bar coating, flow coating, screen printing, spraying, and dip coating. Solvent at the time of coating the polymer, concentration The viscosity, drying conditions, heat treatment conditions and the like can be selected as appropriate. Regarding the thickness of the thin film, the thinner the film, the better the responsiveness, but if the film is too thin, the coverage on the electrodes on the substrate is insufficient, which is not preferable. Usually, it is selected from the range of 0.1 to 100 μm, though it depends on the type of polymer.

However, at the time of forming the polymer thin film, at least a part of each of the pair of electrodes should not be covered in order to electrically connect with the driving circuit.

The humidity sensor thus formed may be formed into a large number on a large-sized substrate and then cut or punched into individual sensors.

Further, a lead wire may be attached to each of the pair of electrodes of the humidity sensor thus formed by using a conductive adhesive or the like.

Furthermore, by mounting peripheral equipment such as an AC voltage generator, signal processing circuit, output device, and power supply on this humidity sensor, it becomes possible to measure and display humidity. A control signal can be output.

By using the humidity sensor using the humidity sensor substrate of the present invention, it is possible to measure the amount of resistance change and the amount of capacitance change corresponding to changes in humidity, but rather it is preferable to measure the amount of resistance change. .

〔Example〕

Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited to the examples as long as the gist thereof is not exceeded.

Preparation Reference Example 1 Using the procedure described in Example 4 of US Pat. No. 3,708,458, 3,3 ′, 4,4′-benzophenonetetracarboxylic anhydride and 80 mol% tolylene diisocyanate (2,
A copolyimide was polymerized from a mixture containing about 80 mol% of 4-isomer and about 20 mol% of 2,6-isomer) and 20 mol% of diphenylmethane-4,4'-diisocyanate (hereinafter abbreviated as MDI). .

N, N-dimethylformamide was used as the polymerization solvent, and the resin concentration was 21% by weight.

Logarithmic viscosity (ηinh) of this copolyimide at 30 ℃
(0.5% in N, N-dimethylformamide) was 0.6 dl / g.

Preparation Reference Example 2 104.8 pre-dried reactors were charged with 614.82 g (3.20 mol) trimellitic anhydride and 132.90 g (0.80 mol) isophthalic acid. The reactor was equipped with a thermometer, condenser, stirrer and nitrogen inlet.

In a dry bottle of 5, weigh 1000.96 g (4.0 mol) of MDI, then 434 ml of N-methylpyrrolidone (hereinafter N
(Abbreviated as MP) was weighed to dissolve MDI. This MDI solution was added to the reactor, and then 3650 ml NMP was added to rinse the bottle from which the MDI was weighed.

This solution was stirred at 65 rpm and a nitrogen atmosphere for 3 times.
The temperature was heated from 53 ° C to 170 ° C over 40 minutes, and further from 1 hour 55 minutes to 169 ° C to 171 ° C. In this way about 80 mol% of the repeating units Having a structure of about 20 mol% of repeating units 25% by weight of random copolyamideimide having the structure
An NMP solution was obtained.

The logarithmic viscosity (ηin
h) (0.5% in N-methylpyrrolidone) was 0.603 dl / g.

This solution was added to methanol to precipitate a polymer, which was then dried at 150 ° C. for 3 hours to obtain a copolyamideimide powder.

Example 1 SUS430 having a size of 300 mm × 200 mm and a plate thickness of 0.15 mm was used as a metal plate. The hardness of the metal plate was Vickers hardness 160. As the polyimide resin, the solution described in Production Reference Example 1 was adjusted to a solid content concentration of 17% by weight using dimethylformamide as a solvent, and a solution passed through a filter having a pore size of 1 μm was used.

The polyimide resin solution is applied to one surface of the metal plate with a doctor knife at room temperature and immediately opened at 80 ° C for 15 minutes.
After that, the temperature was raised continuously and the temperature was raised at 320 ° C. for 5 hours. The film thickness of the obtained coating film was about 10 μm.
This is cut into 80 mm x 50 mm, two sets of comb-shaped holes are formed on the surface of the polyimide layer, and metal masks are formed so that the comb teeth are combined with each other. A pair of electrodes made of a vapor-deposited layer of gold having the above shape was formed. The edges of the comb teeth of both electrodes are 0.2m from each other.
It was m away.

This substrate was excellent in heat resistance, chemical resistance, and dimensional stability.

A 15% by weight aqueous solution of polyvinyl alcohol was applied on the pair of electrodes thus manufactured and dried to a thickness of 5 μm.
m film was formed. Furthermore, a conductive adhesive "Dotite" (trade name; manufactured by Fujikura Kasei Co., Ltd.) was used to attach a lead wire to each of the pair of electrodes formed of a gold vapor deposition layer to manufacture a humidity sensor.

The humidity sensor thus manufactured is placed in an atmosphere of 40 ° C. and 60% RH, and two lead wires of the sensor have an alternating current of a sine wave having a maximum voltage of +0.5 V and a minimum voltage of −0.5 V. A voltage (frequency: 100 Hz) was applied, and the insulation resistance was measured and found to be 2.1 × 10 ⁶ Ω. Similarly, 40 ℃, 90%
When placed in an RH atmosphere, it was 3.7 × 10 ⁵ Ω.

Example 2 A substrate was produced in the same manner as in Example 1 except that the copolyamideimide produced in Production Reference Example 2 was used as the polyimide resin and that the treatment was performed at 300 ° C for 5 minutes. This substrate was excellent in heat resistance, chemical resistance, and dimensional stability. After that, a humidity sensor was manufactured in the same manner as in Example 1 using this substrate. Further, in the same manner as in Example 1, a specific temperature,
When the insulation resistance was measured in an atmosphere of humidity, it was 40
4.9 × 10 ⁶ Ω for ℃, 60% RH, for 40 ℃, 90% RH
It was 6.0 × 10 ⁵ Ω.

〔The invention's effect〕

Since the humidity sensor of the present invention is excellent in heat resistance, chemical resistance, and dimensional stability, it is possible to detect humidity accurately for a long period of time.

Claims (3)

[Claims] 1. A solvent-soluble polyimide resin layer represented by the following formula is provided on a substrate, and a pair of electrodes formed in close proximity to each other and the electrodes are formed on the surface of the solvent-soluble polyimide resin layer. A humidity sensor including a moisture-sensitive resin layer formed by applying a voltage between a pair of electrodes. 10 to 30 mol% of the repeating unit is the formula And has a structure represented by A copolyimide having a structure represented by, or 70 to 90 mol% of repeating units are represented by the formula And has a structure represented by A copolyamideimide having a structure represented by: 2. The humidity sensor according to claim 1, wherein the substrate is a metal plate. 3. The humidity sensor according to claim 2, wherein the metal plate is a stainless plate or an aluminum plate.