JP5114790B2 - Pressure sensitive conductive material - Google Patents

Description

  The present invention relates to a pressure-sensitive conductive material suitable for use as a pressure-sensitive element.

  Conventionally, as pressure-sensitive conductive materials used when manufacturing pressure-sensitive elements such as pressure sensors, metal-based or carbon-based particles and short fibers are used as a base material for synthetic rubber and thermoplastic elastomer. What is blended as a conductive filler is known.

For example, Patent Document 1 below discloses a pressure-sensitive conductive silicone rubber composition obtained by blending metal plating particles in a base material using silicone rubber as a base material. Further, such a pressure-sensitive conductive silicone rubber composition exhibits a favorable change in resistance value against pressure and compression deformation, and is preferably used as a pressure sensor, a pressure contact switch, a connector and the like.
JP 2004-336552 A

  However, conventional pressure-sensitive conductive materials tend to reveal other problems when trying to improve one of a plurality of problems as described later. It was not easy.

  Specifically, conventional pressure-sensitive conductive materials are (1) difficult to use at low voltage due to excessively high resistance, (2) lack of flexibility and buffering, (3) pressure comparison Even if it fluctuates greatly, there are many drawbacks such that the fluctuation range of the resistance value is narrow. (4) Conversely, even if the pressure fluctuates slightly, the resistance value fluctuates excessively.

  For example, when it is desired to improve the defect (2) with respect to such a problem, a countermeasure such as “foaming a resin material” can be considered as a general technique. However, when the pressure-sensitive conductive material is foamed, the conductive path formed by the conductive filler is cut off by the bubbles, so that the resistance value increases and the above-mentioned defect (1) becomes obvious.

  Further, when it is desired to improve the defect (1), as a general technique, a measure such as “increasing the blending ratio of the conductive filler” can be considered. However, if such measures are taken, the material tends to become hard, and the above-mentioned defect (2) becomes obvious. In particular, in order to obtain electrical conductivity with graphite powder or the like, it is necessary to increase the amount of blending, so that there is a strong tendency for flexibility to be lost or brittleness to become excessively high.

  Alternatively, when it is desired to improve the above-mentioned defect (1), it is one idea to use a silver-based filler having a lower electrical resistance. However, the metal-based filler has a problem of low corrosion resistance. There is a problem in that it is inferior in reliability.

  Furthermore, when it is desired to detect how much pressure is acting in an analog manner, a device having the characteristic (3) can detect a change in pressure with a slight change in resistance value. However, there is a problem that a detection error tends to increase due to a slight noise.

However, even if it has the characteristics as described in (4) above, there is no problem in using it as a switch for viewing only ON-OFF, but it is not preferable for detecting a wide change in pressure.
In other words, the pressure-sensitive conductive material of the prior art still has room for improvement, and it has been extremely difficult to eliminate all of these drawbacks.

  The present invention has been made to solve the above-mentioned problems, and its purpose is that it can be used at a relatively low voltage, has excellent flexibility and buffering properties, and has a resistance value corresponding to the pressure fluctuation range. It is an object of the present invention to provide a pressure-sensitive conductive material that can improve pressure detection accuracy by changing extremely greatly.

In order to achieve the above object, the present invention employs the following configuration.
That is, the pressure-sensitive conductive material of the present invention is a conductive resin composition in which 5 to 25 parts by weight of vapor grown carbon fiber is blended in a weight ratio with respect to 100 parts by weight of the resin material in a resin material as a base material. By adding 1 to 20 parts by weight of a foaming agent capable of forming closed cells in the resin material in a weight ratio with respect to 100 parts by weight of the resin material , the conductive resin composition is 1.5 to It is characterized by being foamed 4 times .

  According to the pressure-sensitive conductive resin material of the present invention configured as described above, it can be used at a relatively low voltage, has excellent flexibility and buffering property, and has a very large resistance value according to the pressure fluctuation range. Therefore, the pressure detection accuracy can be improved.

  That is, in the case of the present invention, since the conductive resin composition is foamed, the flexibility and buffering properties are relatively excellent as compared with the conventional products. Moreover, in the present invention, since vapor-grown carbon fibers are used as the conductive filler, the resistance value can be kept relatively low despite the foaming of the conductive resin composition. Use with voltage becomes possible.

  In this regard, in the case of conventional products that use other general carbon fibers, carbon-based particles, metal plating particles, etc. as conductive fillers, the conductive path formed by the conductive fillers is broken by bubbles when foamed, It is difficult to reduce the resistance value as much as the present invention.

  In the case of the present invention, since vapor-grown carbon fiber is used as the conductive filler, not only the resistance value is lowered, but also the fluctuation range of the resistance value is widened. Can be improved.

  In this regard, the reason why the fluctuation range of the resistance value is remarkably widened when the vapor growth carbon fiber is used has not been clearly clarified. However, vapor-grown carbon fibers are thinner and shorter than common carbon fibers, so even when the same weight of conductive filler is blended, the total number of fibers is much larger, and there are only a few continuous parts between the bubbles. However, it is presumed that a conductive path is easily formed, and this may be one of the factors that develop good characteristics.

  Incidentally, mixing the vapor-grown carbon fiber in the resin material itself is a technique that has already been implemented, and the technique itself for foaming the resin material is also a widely known technique. However, as long as the present inventor knows, when the vapor-grown carbon fiber is blended in the foamed resin material, excellent characteristics are expressed as a pressure-sensitive conductive resin material. is there.

  In particular, in this type of pressure-sensitive conductive material, foaming of the base material is not necessarily preferable. For example, it is suggested that foaming is regarded as a problem even when the description in paragraph [0023] of Patent Document 1 is seen. Has been. However, the present invention employs a unique means of deliberately foaming the conductive resin composition containing the vapor-grown carbon fiber without being bound by such technical common sense, and as a result, excellent as described above. It expresses the characteristics.

  If the pressure-sensitive conductive resin material of the present invention having such excellent characteristics is used, it can be expected that the performance of the pressure-sensitive element can be greatly improved over conventional products. The performance of the pressure sensor for detecting the degree of pressure in an analog manner, such as a touch pad, a controller of a game machine, or the like, can be improved.

  In addition, as for the resin material used as a base material, various synthetic rubbers and thermoplastic elastomers can be arbitrarily used as long as the necessary performance is exhibited. Among them, when silicone rubber is used as a base material, This is suitable as a pressure-sensitive conductive resin material.

  Furthermore, the physical properties of the vapor-grown carbon fiber are not particularly limited as long as the required performance is expressed. For example, the diameter is 0.01 μm to 0.2 μm, the fiber length is 1 μm to 500 μm, It is preferable to use one having an aspect ratio of 10 to 500.

Moreover, what can form a closed cell is utilized about a foaming agent . By adopting such a configuration , the hygroscopicity can be lowered, so that a pressure-sensitive conductive resin material in which a characteristic change due to moisture absorption hardly occurs can be obtained.

Next, an embodiment of the present invention will be described with an example.
First, the conductive fillers were blended in the silicone rubber as a base material at a blending ratio as shown in Table 1 below, and a foaming agent was blended for some of the fillers.

In Table 1, the silicone rubber includes a two-part silicone gel composition (product name: CY52-276, manufactured by Toray Dow Corning Silicone Co., Ltd.) and a curing accelerator (RD-1, Toray Dow Corning). -Silicone Co., Ltd. was used. The vapor grown carbon fiber is a commercial product (product name: VGCF (registered trademark) -H, average fiber diameter 150 nm, fiber length 10 to 20 μm) manufactured by Showa Denko KK A commercially available product (product name: Dieform H850) manufactured by Chemical Industries, Ltd. was used.

  In addition, the comparative carbon fiber used as the conductive filler in the prototype cited as a comparative example is a commercial product (product name: DIALEAD (registered trademark) K223SE, manufactured by Mitsubishi Chemical Corporation) having a fiber diameter of 10 μm. In addition, as a silver plating whisker, a commercial product manufactured by Otsuka Chemical Co., Ltd. (product name: Super Dentor (registered trademark) SD100) is used, and as artificial graphite, a commercial product manufactured by Showa Denko Co., Ltd. (product name: UF-G30). )It was used.

  Each composition as described above was kneaded by the following procedure and processed into a sheet-like molded product. That is, first, each raw material weighed in a predetermined amount was stirred and mixed at room temperature for 10 minutes using a defoaming mixer (ACM-5 LVT type manufactured by Kodaira Seisakusho). Next, the stirring mixture is poured into a mold so as to have a height of 2 mm, and the whole mold is heated in a thermostat at 110 ° C. for 5 minutes to accelerate the curing of the silicone rubber.

  Next, the rubber-cured mixture is taken out of the mold, transferred to a constant temperature bath at 170 ° C., and heated for 10 minutes. By this heating, foaming of the foaming agent is started and a foamed material is obtained. In addition, in the comparative example which does not add a foaming agent, the height poured into a formwork was 3 mm, and it heated on the same conditions after that.

  A test piece of 50 × 50 × 3 mm was cut out from each sheet-like material manufactured by the above procedure, and the relationship between pressure and resistivity was determined for each test piece using a tensile and compression tester (manufactured by Minebea Co., Ltd .; TCM-50) was combined with a material testing apparatus as shown in FIG.

  The material testing apparatus shown in FIG. 1 has a structure in which a test piece S placed on an insulating lower compression plate 1 is sandwiched between insulating upper compression plates 3 and can be compressed vertically by a tensile compression tester. The compression amount can be arbitrarily changed by a tensile compression tester, and the pressure acting on the test piece S at that time can be measured. In this test example, the crosshead speed was 20 mm / min.

  In addition, a pair of electrodes 5 was disposed between the test piece S and the lower compression platen 1 so that the resistance value between the electrodes 5 could be measured with a multimeter 7. The contact surface of each electrode 5 with respect to the test piece S was a circle having a diameter of 10 mm, and the distance between the centers of the electrodes was 20 mm. The measurement results are shown in FIGS.

  As can be seen from FIG. 2, in Example A1, the resistance value fluctuated greatly according to the pressure fluctuation range, and the characteristics as a pressure-sensitive conductive resin material were excellent. On the other hand, as shown in FIG. 3, Comparative Example a1 uses the same conductive filler as Example A1, but does not contain a foaming agent, so that the resistance value does not vary as much as Example A1. It became.

  Such a tendency is the same even if the type of carbon fiber is changed. For example, Comparative Example a2 has a smaller fluctuation range of the resistance value. Moreover, since the comparative example a1 and the comparative example a2 were not made to foam, the density was high and it was difficult to make a softness | flexibility and buffer property high like Example A1.

  Furthermore, in Example A1, the characteristic that the resistance value varies greatly as described above is not a characteristic that is caused only by foaming the base material. This is apparent when the comparative examples a3 and a4 shown in FIG.

  That is, Comparative Example a3 and Comparative Example a4 are both foamed by adding a foaming agent in the same manner as Example A1. However, since a material other than vapor-grown carbon fiber is used as the conductive filler, the resistance value does not fluctuate as much as in Example A1, as is clear when FIG. 2 and FIG. 4 are compared. .

  In particular, Comparative Example a4 is considered to have a configuration close to Example A1 from the viewpoint of using a carbon-based conductive filler, but is in a region where the resistance value is relatively high and is used at a low voltage. Was a difficult characteristic.

  On the other hand, Comparative Example a3 is in a region where the resistance value is relatively low, and thus it is easy to use at a low voltage. Judging from the above results is not as easy as in Example A1.

  As is clear from the above description, the pressure-sensitive conductive material mentioned as Example A1 can be used at a lower voltage than Comparative Example a4 and the like, and can be more flexible than Comparative Examples a1 and a2. Excellent in shock-absorbing properties, and the resistance value fluctuates greatly depending on the pressure fluctuation range as compared with Comparative Examples a1 to a4. This makes it possible to improve pressure detection accuracy. Become a material.

  Next, in order to investigate the relationship between the blending ratio of the foaming agent and the characteristics, the foaming agent was blended at a blending ratio as shown in Table 2 below. The mixing ratio of the base material (silicone rubber) and the conductive filler (vapor-grown carbon fiber) was fixed.

Each of these compositions was formed into a sheet in the same manner as in the above experimental example, and the test as described above was performed on the test piece cut out from the sheet. The measurement results are shown in FIG.

  As is clear from FIG. 5, in each of Examples B1 to B4, the resistance value changed greatly according to the pressure fluctuation range, and the fluctuation range greatly exceeded that of Comparative Example b1.

  In Example B1 with the lowest expansion ratio, the expansion ratio is about 1.5 times, and in Example B4 with the highest expansion ratio, the expansion ratio is about 4 times. From this fact, the expansion ratio is set to about 1.5 to 4 times Then, it has been found that the fluctuation range of the resistance value is increased, and the characteristics as the pressure-sensitive conductive material can be improved.

  Although the expansion ratio can be further increased, as is clear from Example B4, as the expansion ratio increases, the resistance tends to increase overall. Therefore, even if the foaming ratio is increased unnecessarily, the resistance value increases for a range in which the fluctuation range of the resistance value does not increase. Therefore, the problem of the increase in resistance value and the pressure-sensitive conductive material described above can be obtained. Considering both the expansion ratio that can improve the characteristics of the above, it is considered that the expansion ratio is preferably 4 times or less.

  By the way, for conductive fillers other than vapor-grown carbon fibers (the above-mentioned comparative carbon fibers, silver-plated whiskers and artificial graphite), an experiment was conducted to increase the blending ratio of the foaming agent. It became too insulated and almost insulative.

  In addition, in order to reduce the resistance value that has become too high due to the increase in the amount of foaming agent as described above, when the blending ratio of the conductive filler is further increased, the brittleness becomes too high after mixing and it can be cured to a certain shape. I'm gone.

  Therefore, with conductive fillers other than vapor grown carbon fibers (the above-mentioned comparative carbon fibers, silver-plated whiskers, and artificial graphite), it is difficult to improve the performance with the foaming agent. It has been found that the characteristics as a pressure-sensitive conductive material can be improved for the first time by combining with a foaming agent.

As mentioned above, although embodiment of this invention was described, this invention is not limited to said specific one Embodiment, In addition, it can implement with a various form.
For example, in the above-described embodiment, an example in which silicone rubber is used as a base material has been shown. However, as long as the base material does not impair the characteristics as a pressure-sensitive conductive material, various synthetic rubbers, natural rubbers, and thermoplastics An elastomer or the like can be used.

Explanatory drawing which shows schematic structure of the apparatus used for the performance test. The graph which shows the characteristic of Example A1. The graph which shows the characteristic of the comparative example a1 and the comparative example a2. The graph which shows the characteristic of the comparative example a3 and the comparative example a4. The graph which shows the characteristic of Example B1- Example B4 and the comparative example b1.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Lower compression board, 3 ... Upper compression board, 5 ... Electrode, 7 ... Multimeter.

Claims (3)

With respect to the conductive resin composition in which 5 to 25 parts by weight of vapor-grown carbon fiber is blended in a weight ratio with respect to 100 parts by weight of the resin material in the resin material as a base material, closed cells are formed in the resin material. The conductive resin composition is foamed 1.5 to 4 times by adding 1 to 20 parts by weight of a foaming agent that can be formed in a weight ratio with respect to 100 parts by weight of the resin material. Pressure sensitive conductive material. The pressure-sensitive conductive material according to claim 1, wherein the resin material is silicone rubber. The vapor-grown carbon fiber has a diameter of 0.01 to 0.2 [mu] m, fiber length 1Myuemu～500myuemu, aspect ratio according to claim 1 or claim 2, characterized in that 10 to 500 Pressure sensitive conductive material.