JPS5947843B2 - High sensitivity pressure sensitive resistor and its manufacturing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a pressure-sensitive resistor made by mixing conductive particles into an electrically insulating elastic material such as rubber plastic, and a method for manufacturing the same.

In particular, the present invention relates to a sensitive pressure-sensitive resistor whose resistance value is significantly reduced by a small pressure, and a method for manufacturing the same.
Conventionally, a method for increasing the sensitivity of this type of pressure-sensitive resistor and significantly reducing the resistance value with a small pressure was disclosed in Japanese Patent Publication No. 46-617.
9 is known, and it is also known that, for example, the interface between the surface of a conductive material such as a metal particle and an elastic material such as silicone rubber is made loose by incompletely curing, or the surface is covered with a semiconductor component. A method has been adopted (Japanese Unexamined Patent Publication No. 49-114798).

When external stress is applied to these conventional pressure-sensitive resistors, compressive energy is stored inside the matrix, accelerating the deterioration of elastic materials such as rubber, or causing metal particles to fall out of the matrix. This phenomenon occurs, and when used as a
It was said that it would deteriorate as soon as the ON10FF was used less than 10,000 times. The object of the present invention is to produce a foamed pressure-sensitive resistor with a predetermined expansion ratio and hardness and good pressure-sensitive characteristics by incorporating open cells into a system consisting of an elastic polymer such as rubber and metal particles. It is about providing.

As a result of various studies on foamed pressure-sensitive resistors, the present inventors found that
It has been found that the relationship between the diameter of metal particles and the diameter of bubbles has a large effect on the properties of pressure-sensitive resistors.

In other words, when the diameter of the metal particles is significantly smaller than the diameter of the bubbles (approximately 1/10 or less), the matrix formed by the rubber 1 and the metal particles 2 will be smaller than that of the unfoamed material as shown in Figure 1. It was found that the presence of the bubbles 3 only had the effect of reducing the density of the material, without much difference.
On the other hand, the short diameter of the bubble 3 is about the same as the diameter of the metal particle 2, or
It was found that when the open bubbles are made to be about 1/0 or more, the bubbles 3 condense around the metal particles 2, as shown in Figure 2 (h), and it becomes possible for the metal particles to come into contact with each other with a very small pressure. Ta. In addition, since the matrix contains open cells, its volume changes more easily than a single cell matrix.
It is possible to minimize the compressive energy stored inside the matrix. As a result, not only does the deterioration of the polymeric elastic material such as rubber not occur, but the load on the inside of the matrix is reduced, so there is almost no dropout of metal particles, even after 10,000 times or more of 0N/0FF. A product that was sufficiently usable was obtained. In this manner, by appropriately adjusting the foam diameter and foaming ratio with respect to the diameter of the metal particles, a pressure-sensitive resistor exhibiting desired pressure-sensitive characteristics can be obtained.

However, if the diameter of the conductive metal particles is large, the number of current-carrying paths will be much smaller than if the diameter of the conductive metal particles is small, which has the disadvantage that the withstand voltage and current of the matrix will be small.
If high withstand voltage or current is required, it is necessary to use particles with small particle size. However, with conventional techniques, it has been impossible to produce fine open cells of about 1 μm. Therefore, the inventors conducted various studies on the properties of composite materials, and as a result, they were able to manufacture a foam having fine open cells of about 1 μm.
5 Generally, when a binder and a pigment are mixed, it is known that when the concentration of the pigment is gradually increased, when a certain concentration is reached, the physical properties suddenly change as an interfacial phenomenon. This concentration is the critical pigment concentration (Critical Pigm
entVOlumeCOcentratiOn. }Hereinafter referred to as CPVC.

) is defined as In the present invention, a normal foaming agent can be used to foam the elastic polymer to form an open cell, but a more suitable method is to
After adding an insoluble salt (however, the particle size of the salt is the same as or smaller than the diameter of the metal particles) to a mixture mainly consisting of conductive metal particles and an elastomer polymer, the mixture is By crosslinking and then dissolving the salt in a poor solvent for the crosslinked product and a good solvent for the salt, the desired foaming ratio can be achieved.
Pressure-sensitive resistors with a bubble diameter and a pressure sensitivity of 5 degrees can be manufactured with much higher precision and in a wider range than when using foaming agents or the like. It is particularly suitable for producing open-cell bodies with very small bubbles. As is known, the conductive metal particles mentioned above include iron, copper, tetrachromium, titanium, tungsten, platinum, tin, stainless steel, brass, silver, gold, nickel, cobalt, aluminum, zinc, etc. In terms of durability and performance, iron, copper, stainless steel, nickel, aluminum, tin, etc. are preferable.

In particular, according to the method of the present invention, oxidation resistance is increased, so relatively inexpensive conductive metals such as iron, copper, aluminum, and nickel can be used effectively. Invention 1 (The diameter of the metal particles used is 1
Preferably, the diameter is from μm to 200 μm.

If it is smaller than 1 μm, it is necessary to make the bubbles even smaller, making manufacturing extremely difficult.

When the particle size is larger than 200 μm, it is relatively easy to form a foam, but as the particle size increases, the metal particles tend to fall off from the matrix, which is not preferable. Further, the mixed amount of the conductive metal particles used in the present invention is preferably 10 to 50 volume fraction (%). Its volume fraction is 10
If it is smaller than 50%, it usually does not become a pressure sensitive resistor and tends to become an insulator or semiconductor, while if it is larger than 50%, it tends to become a conductor, and the volume fraction is 10 to 50% due to the ease of processing. % is preferable.The polymeric elastic material used in the present invention itself is a high resistance material, such as polybutadiene, natural rubber, polyisoprene, SBR, NBR.
, diene rubber such as neoprene, EPDM, EPM
, urethane rubber, polyester rubber, epichlorohydrin rubber, silicone rubber, etc. can be used.

Especially when heat resistance and weather resistance are required, silicone rubber gives preferable results. The salt used in one of the foaming methods of the present invention may include a metal or a positive basic group (NH4
+, etc.) and a negative acid group, or an addition compound of an organic base and an acid, which is insoluble in a mixture mainly consisting of conductive metal particles and an elastomer polymer, and is chemically compatible with this mixture. It is fine as long as it does not cause any reaction.

Particularly suitable are metal salts of inorganic acids such as sodium chloride, potassium bromide, titanium iodide, sodium sulfate, and calcium carbonate. Furthermore, the poor solvent for the polymeric elastomer used to elute the above-mentioned salt is appropriately selected depending on the type of salt used, but for example, the solubility parameter (Inter
Science Publishersl967, P
See page 341 of OlymerHandbOOk; hereinafter referred to as SP value. ) is 10.0 or more, particularly 13.0 or more is preferably used. For example, methanol (SP value 14.5), ethylene glycol (SP value 14.6),
Methylformamide (SP value 16.1), water (SP value 2
3.4) etc. are suitable. The present invention will be explained below using Examples, but the present invention is not limited to the Examples unless it goes beyond the gist of the present invention.

Example 1 33.3% by volume of carbonyl nickel with a particle size of 1 to 3 μm and polybutadiene rubber (JSRB manufactured by Japan Synthetic Rubber Co., Ltd.)
ROI) 66.7 volume fraction % was prepared in advance in a kneader.
After kneading for 0 minutes, 2 parts of dicumyl peroxide was mixed to 100 parts of rubber, and potassium bromide (KBr) with a particle size of 1 to 2 μm, which had been ground in a ball mill, was gradually added thereto while kneading at the highest torque in a kneader. When 66.7 volumes of KBr were added to the original mixture volume of 100, the torque of the kneader suddenly increased, and at this point it became C.
I confirmed that it was PVC.

Thereafter, the mixture was kneaded for 5 minutes, taken out and formed into a sheet with a thickness of 1 TrrrrL, and crosslinked at 160° C. for 30 minutes using a vulcanization press. The pressure sensitive properties of the crosslinked product were measured and it was found to be an insulator. When a 5 cm square piece of this crosslinked material was taken out and boiled in 1ι of water for 1 hour, the entire amount of KBr was eluted, but no nickel particles were observed to fall off. After dehydrating this small piece and thoroughly drying it, the pressure sensitive property was measured.
The resistance value of the resistor, which was heated to Ω, decreased to 5Ω by applying a pressure of 3 kV/Cd, and exhibited good pressure-sensitive characteristics. Comparative Example 1 The system of Example 1 except for potassium bromide was crosslinked under the same conditions as Example 1.

When the pressure sensitive properties of the cross-linked material were measured, the resistance value slightly decreased from 5 MΩ when no pressure was applied to 980 KΩ even when CrA was applied for 10 hours, but no significant change in resistance value was observed as in Example 1. I couldn't help it. Example 2 Copper powder 40% by volume by gas atomization method with a particle size of about 40 μm
and silicone rubber (KEL3OORTV manufactured by Shin-Etsu Chemical) 60
A volume fraction of % (plus a predetermined amount of catalyst) was prepared and kneaded for 15 minutes in a kneader, and then sodium chloride (NaCl) having a particle size of 10 ttm was gradually added thereto.

When the torque of the kneader was observed in the same manner as in Example 2,
33 volumes of NaCt per 100 volumes of the original mixture
．． The torque of the kneader 1 suddenly increased when 3 was added, and it was confirmed that CPVC was present at this point. After kneading for 5 minutes, the mixture was taken out, formed into a sheet with a thickness of 1 Tm, crosslinked at room temperature for 1 day, and then heat-treated at 130° C. for 1 hour. When a 5 cm x 5 cm piece of this crosslinked material was taken out and boiled in 1 ton of water for 1 hour, almost all of the NaCt was eluted, but no copper powder was observed to fall off. After this small piece was dehydrated and sufficiently dried, its pressure sensitive properties were measured. As a result, the temperature of 5MΩ when no pressure was applied was 3k9/CwL-:! ) 0.5Ω by pressurization
The resistance value decreased to 100%, and good pressure-sensitive characteristics were exhibited. Comparative example
2 The system of Example 2 except for sodium chloride was crosslinked under the same conditions as Example 2.

When we measured the pressure sensitive properties of the crosslinked material, it was 300Ω when no pressure was applied, but it became 0.1Ω when pressure was applied for 3 h/Cd.
Although the resistance value decreased slightly, no significant change in resistance value was observed, and the resistance value was in the range of conductive rubber compared to the initial resistance value.
The samples prepared in Examples 1 and 2 and Comparative Examples 1 and 2 were subjected to repeated compression tests with a force of 3k9/Cd.
The results after 50,000 repetitions were as follows.

As mentioned above, Examples 1 and 2 showed good pressure-sensitive characteristics without much difference from the initial resistance value, but unfoamed Comparative Example 1
, 2, it became clear that the resistance value gradually increased.

The high-sensitivity pressure-sensitive resistor obtained in this way has low hardness because it is a foam, and is soft enough to be used in touch switches, keyboard switches, micro switches, etc.
It was found that the N/OFF switch has a good feel and the stroke can be controlled to some extent, giving a good switch feel, and is industrially useful as a switch with excellent repeatability.

[Brief explanation of the drawing]

FIG. 1 is a microscopic enlarged view showing the arrangement of bubbles and metal particles in a known pressure-sensitive resistor, and FIG. 2 is a similar view to FIG. 1 of a high-sensitivity pressure-sensitive resistor according to the present invention. 1... Elastic body, 2... Metal particles, 3... Air bubbles.

Claims (1)

[Scope of Claims] 1. Mainly composed of conductive metal particles and an elastic polymer, and has air bubbles within the elastic polymer, and the short diameter of the air bubble is about the same as or 1/2 the diameter of the metal particle. A highly sensitive pressure-sensitive resistor characterized by having approximately 10 or more open cells. 2. The high-sensitivity pressure-sensitive resistor according to claim 1, wherein the metal particles have a diameter of 1 μm to 200 μm. 3. The high-sensitivity pressure-sensitive resistor according to claim 1, wherein the polymer elastic body is silicone rubber. 4. Adding an insoluble salt to the mixture mainly consisting of conductive metal particles and an elastomer polymer in an amount equal to or higher than its limit pigment concentration, crosslinking the mixture, and then dissolving the salt in a poor solvent for the crosslinked product. A method for manufacturing a high-sensitivity pressure-sensitive resistor, characterized by: