CA1303275C - Pressure responsive variable electrical resistive rubber material - Google Patents

Abstract

Abstract of the Disclosure A pressure-responsive variable electrical resistive rubber material including at least one inorganic filler selected from a group consisting of conductive carbon black, short fibers of nonmetallic inorganic material, powder, and whiskers. The filler is dispersed in a matrix formed of an electrically insulative rubber. The rubber material exhibits a gradually reducing resistance value as a gradually increased pressing force is applied thereto. The rubber material provides a wide range of relationship between the applied force and the electrical resistance there-of. The rubber material is adapted for use as a pressure sensor having excellent fatigue creep resistance.

Description

13~32 ~S MBL R-104 PRESSURE RES~ONSI~E ~ARIABLE ELEC~RICAL
RESISTIVE RUBBER MATERIAL

BACKGROUND OF THE INVENTION
This invention relates to a pressure-sensitive conductive rubber material and, more particularly~ to such rubber material having an excellent maintained elasticity.
The material includes an inorganic filler in a rubber matrix and exhibits a decrease in the electrical resistance thereof when pressurized ~rom a nonpressurized 6tate. The resistance value varies sensitively with changes in the pressurizing force.
It i~ known, for example in Japanese Patent Laid-open Nos. 58504/1980, 147547/1980 and 5840/19~31 official gazettes, and U.S. Patents Nos. 2,951,817 and 3,758,213, that electrically insulative rubber containing carbon black or metallic particles reacts to deformation under pressure to provide a variable electric resistance.
~ t i6 also known, for example in Japanese Patent Laid-open No. 152033/1983 official gazette, that a di6persion of conductiv~ magnetic metallic particle~ in an clastic electrically insulating polymer may ~e molded while applying a magnetic field in a predetermined directionj before or during the cFosslinklng to produce a pressure-~L3~3~S
M~L R-104 sensitive conductive rubber in which the metallic particles are arranged along the ~agnetic field in a predetermined direction.
When a pressure-sensiti~e conductive ru~ber sheet of this type is distorted by a pressing force, there is a substantial probability of the conductive particles contacting each other and thereby reduce the resistance value of the material.
Another form o~ pressure ~ensitive ~heet material comprises a rubber sheet having a cellular structure on the surface of the sheet. A conductive material, such as metallic powder, i8 provided therein to improve the 8en8itivity of the rubber sheet. Such a sheet i8 disclosed, for example, in Japanese Patent ~aid-open No. 20981/1983 official gazette.
Various conductive rubber sheets in which metallic fibers are provided extending in the thicknesswise direction of the sheet (e.g., Japanese Patent Laid-open ~o.
2203D7~1983 official gazette) are also known. When ~uch a conductive rubber ~heet i6 preesed by lectrode plates at opposite 6ides of the ~heet, the surface ~f the rubber sheet contacting the ~lectrode plates gradually increases with the result that its resistance v~lu- decreases.

~3~3~ 75 MBL R-104 However, the known pressure-sensitive conductive rubber materials have a serious disadvantage ln that the probability of contact between the conductive particlec varies with the temperature of the matrix. Further, the S conductive particles ~ay ~eparate due to the compressive deformation of the rubber sheet thereby making it ~if~icult ~ .
to obtain a stahle resistance value.
When such known conductive rubber material is subjected to large etrains repeatedly for a long period of time, the surface portion hardens from fatigue and the rubber loses creeping resistance. As a result, the electrode plates contact the conductive materials in the surface portion and the eleotric resi~tance value prior to force being appliad to the plate contacting the ~urface is decreased o that the desired variation in the electric resistanc~ value proportional to the pressurizing face is not attained. It i~ therefore i~possible to maintain desira~le pressure-eeneitive performanre over a long period of time.
In addition, even though the resistance value of the known pressure-~ensit~Ye conductive rubbor decrease proportional to the increase in pressing force, when the pressuring force arrives at a predet~rmined value, the ~ 3~J3 ~}~ MBL R-104 resistance value d2creases rapidly resu~ting in poor pressure ~ensitive performance.
The relationship between the pressing ~orce and the resistance value of Xnown pressure-sensitive conductive rubber is not a predetermined prop~rty thereof. Since the variation of the resistance value in a gi~en range of the pressing force is ~mall resulting in poor pressure-sensitive performance, these known rubbers cannot be used in appli-cations ac pre66ure-sensitive sensors.
Further, other kno~wn conductive rubber sheets have complicated surfaae structures which cause metallic particles or flber therein ko sink from the 6ur~ace of the rubber matrix causing recesses on the surface. In the absence of a pressing force, the rubber sheet~ may inhibit current flow since the ~letallic filler does not directly contact the olectrode plates. The electrode plates will, however, contact the metallic filler due to the deformation of the rubber matrix when the rubber ~heet is subject to a pressurizing force which reduces its eloctrlc resistance 2a value. However~ in the latter instance, the resi~tance value does not vary until the rubber matrix is deformed ~ufficiently so that the electrode plates are in contact with the metall~c fillers. Additionally, the resistance of 13~32~$ MBL R-104 these known rubber ~heet has a tendency to abruptly decrease even with only a mall change in pressurizing force due to ~mall ~oreign materials ~uch a~ dust ~eing interposed within fine recesses on the ~ur~ace of the rubber matrix, thereby reducing the electric resistance value when 6ub~ect to the pressurizing ~orce, causing the sensitivity to be decreased.

SUMMARY OF THE INVEN~ION

~ ccordingly, a primary ob;ect of this invention is to provide a pressure-sensitive aonductive rubber material which eliminate~ the disadvantages of known conductive rubber material~ as described above, which improves the properties of the rubber material, including an insulating rubber matrix, in which a conductive member i6 dispersed to gradually reduce its resistanae value as a pressing force increases, which provides a u~8~ul relationship between the pressing ~orce and the resistance value over a large range of resistance values, and which i seful as ~ pre sure sensor having excellent ~atigue creeping reslstances~
An~ther object of this invention is to provide a ~0 pressure-sensitive conductive rubber material which may ~3~3~:~S

function in an ON-OFF manner by abruptly reducing its resistance value at a predetermined pressing force.
According to one feature of this invention, there is provided a pressure-sensitive conductive rubber material in which an inorganic filler of at least one type selected from a con-ductive carbon black, a short fiber made of inorganic nonmetallic material, powder or whisker is dispersed in an electrically insulating rubber matrix to form a composite material and part of the inorganic filler is exposed on the surface of the rubber matrix on the surface of the composite material.
According to another feature of this invention, there is provided a pressure-sensitive conductive xubber material in which an inorganic filler of at least one type selected from a conductive carbon black, a short fiber made of inorganic non-metallic material, powder or whisker is uniformly di.spersed inan electrically insulating rubber matrix to form a sheet-shaped composite material and to disperse electrically insulating powder on at least one surface of the composite material.
According to still another feature the invention con-templates an electrically conductive rubber material havingpressure-responsive variable electxical resistance and that material comprises a matrix formed of an electrically insulative rubber, carbon black dispersed in the matrix, and a Eiller of semiconductive acicular ceramic whiskers dispersed in the rubber material selected from the group consisting of alpha-silicon carbide, beta-silicon carbide, alpha-alumina, titanium oxide, tin oxide, graphite, Fe, and Ni having a diameter of 0.05 to 3 microns, and a length of 5 to 500 microns, 30 to 70 parts by weight of carbon black and 10 to 60 parts by weight of whiskers being provided in 100 parts by welght of the rubber and the total amount of the carbon black and the whiskers being 40 to 90 parts by weight in 100 parts of the rubber.

~3~

-6a-In a further inventive aspect the invention provides an electrically conductive rubber material having pressure-responsive variable electrical resistance, and that material comprises a matrix formed of an electrically insulative rubber, carbon black distributed in the matrix, and an inorganic filler distributed in the matrix consisting of semiconductive acicular ceramic whiskers selected from the group consisting of alpha-silicon carbide, beta-silicon carbide, alpha-alumina, titanium oxide, tin oxide, graphite, Fe, and Ni having a diameter of O.OS
to 3 microns, and a length of 5 to 500 microns, 30 to 70 parts by weight of carbon black and 10 to 60 parts by weight of whiskers being provided in 100 parts by weight of the rubber, and the total amount of the carbon black and the whiskers being 40 to 90 parts by weight in 100 parts of the rubber.
In yet another aspect the invention contemplates an electrically conductive rubber material having pressure-responsive variable electrical resistance and that material comprises a matrix formed of an electrically insulative rubber, carbon black distributed in the matrix, an inorganic filler distributed in the matrix consisting of semiconductive acicular ceramic whiskers selected from the group consisting of alpha-silicon carbide, beta-silicon carbide, alpha-alumina, titanium oxide, tin oxide, graphite, Fe, and Ni having a diameter of 0.05 to 3 microns, and a length of 5 to 500 microns, 30 to 70 parts by weight of carbon black and 10 to 60 parts by weight of whiskers being pro-vided in 100 parts by weight of the rubber, and the total amount of the carbon black and the whiskers being 40 to 90 parts by weight in 100 parts of the rubber, and an elec~rically insulating powder having a particle size of 0.1 to 100 microns in at least one surface portion of the matrix with the powder being present in the amount of O.OS to 5% by weight of the material at the-surface portion.

~3~3~7~i The above and other objects and features of the invention will be apparent from a readlng of the following description of the disclosure found in the accompanying drawings and the novelty thereof pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS
. _ . .. _ ... .

Fig. 1 is a longitudinal sectional view of an embodiment of a pressure-sensi-tive conductive rubber sheet according to the present invention;

Fig. 2 is a longitudinal sectional view of an alternate embodlment of a pressure-sensitive conductive rubber sheet according to the present invention;

Fig. 3 i.s a graphical diagram showing the relationship between the pressing force and the resistance value of the conductive rubber material of example 5 in which the whisker is varied in content;

Fig. 4 is a graphical diagram showing the relationship between the pressing Eo=ce and thc resistance ~3~3Z~

value of the conducti~e rubber material of example 6 in which carbon black i8 varied in content;
Fig. 5 is a graphical diagr~m showing the relationship between the compression distortion and the voltage of the example 7 of the invention and comparison example.
Fig. 6 is a graphical diagram showing the relationship between the resistance value retention when sub;ect to repetitive pressing ~orce, example 7 of the $nvention, and the comparison example;
Fig. 7 is a graphical diagram showing creep characteristic o~ the co~ductive rubber material of the example 7 of the invention and the comparison example;
~ ig. 8 15 a graphical diagram showing ~irst to third varying ourves obtained by plotting the variation of electric resistance value upon pressing of the rubber sheet o~ the example 8 of the lnvention and the comparison example, Fig. 9 is a graph showing resistance values plotted with respect to the pressing force of the rubber material of example 11 of the invention and comparison examples;
Fig. 10 is a graph showing electric capacity plottad with respect to a pressing foroe of the rubber material of example 11 of the invention and comparison example.
., ~3~3Z 75 MBL R-104 Fig. 11 is a graph showing the degrees sf variations of varying curves of electric reslstance values with respect to the first and third pressings obtained by plotting the variations o~ the electric resistance value with respect to the pressing force of the pressure-sensitive conductive rubbex material of the example 11 of the invention and the comparison example.

DESCRIPTION OF THE_PREFERRED EMBODIMENTS

This invention will be described in detail with respect to the accompanying drawings.
According to the invention, an electrically insulating rubber may be, for example, natural rubber, polybutadiene rubber, polyisoprene rubber, styrene~butadiene copolymer rubber, nitrile rubber, butyl rubber, chloroprene rubber, acrylonitrile-butadiene copolymer xubbert ethylene-propylene copolymer rubber or ~ilicone rubber. The rubber may contain two or more of the absve rubber types.
The rubber may use crosslinking rubber of eulfur, sulfide or peroxide so as to ~mpro~e t~e mechanical s~rength ~nd head resistanca, and may be used after crosslinking.

13~

--10-- .

Conductive carbon black used in this invention may be, for example, furnaceblacks, acetylene blacks, thermal blacks, or channel blacks ordinarily used and known per se.
The amount of the carbon black used i8 2.0 to loo part~ by S weight with respect to 100 parts by weight of the rubber. A
preferable range is 10 to 80 parts by weight of carbon black. A most pre~erable range is 30 to 70 parts by weight of carbon black.
If the aontent of the car~on black is less than 2 parts by weight, the resistance value of the rubber material i8 always high and the rubber material does not adequately function as a pressure-sensitive conductive rubber.
Conversely, if the content exceeds 100 parts by weight, the rubber is hardened to a point which reduces the variation in the resistance value caused by a pressing force.
I~ the carbon black content is 30 to 70 parts by weight, the resistance value gradually varies as the :
~pressing force varies over a large varying range of the reæistance value, An inorganic filler used in the invention may be, for example, short fiber of nonmetallic inorganic material, powder or whisker. The short fiber may~be, for ~ample, ceramics or~sil~con carbide (SiC~, glass, 8ilicon ni~ride , ~ :

13~32~5 (Si3N4) lOO micron to 100 ~m in length and 3 to 30 micr~n in diameter. Powders, called "ceramic powder'l being 0.05 to 100 ~icron in diameter, may be, for example, carbides such as ~ilicon carbide (SiC~, titanium carbide (TiC), boron ~ar~ide (~4C), or tungsten carbide (WC), nitrides ~uch as silicon nitride (SigN4), aluminum nitride (AlN), boron nitride (BN) or titanium nitride (TiC) a~d oxides such as alumina ~Al2O3)~ zirconia (ZrO2) or beryllia ~eO), and most pre~erably silicon carbide or silicon nitride.
Further, the whisXer may be, for example, alpha-silicon carbide (alpha-SiC), beta-silicon carbide ~beta-SiC), 6ilicon nitride (SigN4), alpha-alumina (Al2O3), titanium oxide, zinc oxide, tin oxide, graphite, Fe, Cu, or Ni, and ha~ an aaicular crystal having a 6ize 0.05 to 3 micron in diameter and 5 to 500 micron in length. When the inorganic fill~r i5 added to the rubber, the rubber is treated with a silane coupling agent or a titanium coupling agent. Alternately, when the filler i6 mixed with the rubber, the ~ilane coupling agent or titanium coupling agent may be ad~ed. ~hus, reinforcing e~f~ct of the rubber is enhanced to improve the dinpersion of the filler into the rubber.

~IL3~3~'S

The amount of the inorganic filler is 1 to 80 parts by weight and preferably 5 to 40 parts by weight with respect to 100 parts by weight of the rubber. If the amount of the filler is less than 1 part by weight, the exposing effect of the filler on the surface of the rubber sheet decreases to reduce the resistance value when the rubber is not subject to a pressurizing force, and the electric resistance value is high with a low pressuxiæing ~orce of 0.5 kg/cm2 ~o that the xesistance value varies significantly with respect to slight prsssure changes. I~ the amount of the filler exceeds 80 parts by weight, the resistanGe varies little with pressure changes due to the hardening o~ the rubber.
A method used to mix the respective ingrodients or components is not limited, and includes kneading and pressurizing by a suitable known means or process using, for example, a Banbury mixer, a kneader or rolls.
Accordlng to the lnvention, a eoftenlng agent, an age preventing agent, a processlng aid, a~vulcanization accelerator, and/or crosslinking agent ordlnarily used ~or rubber ma~;be added as:would be obvlous to one skilled in the ar~. ..

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~3~3~ ~ MBL R-104 -~3-The pressure-sensitive conductive rubber sheet thus obtained is a composite material in which the above-mentioned inoxganic ~iller is dlspersed in the rubber matrix, and is p~rtly expo~ed on the surface of the composite material. Thus, the resietance value of the pressure-sensitive conductive rubber sheet depends on the extent to which the filler exposed on the surface contacts with the electrode plates. With no pressure applied, the resistance value may be so high as to be effectly in an insulating ~tate with no conducting path being formed.
As the inorganic filler exposed on the ~urface is pressed on the rubber matrix with relatively low pre~sing forces, it grAdually contacts the electrode plates with the pressure-aensitive conduative rubber described above and lS simultaneously applies a local distortion to the rubber matrix, carbon black, and the inorganic filler. The carbon black and the inorganic fillers approach each other to readily form a link, and the resistance value starts gradually decreasing. Further, if the pressing force increases, the entire rubber matrix uni~ormly deforms~
Thus, the condu~tive members contact therebetween to increase the probability of for~ing the link, thereby improvlng the conductivity.

~3~3~

Since the w~i~ker o~ ~norganic fil~er used ~n the invention has an acicular crystal and very rigid property, the whi~ker is dispersed and exposed on the rubber ~atrix in several directional angles due to it~ specific properties.
~he wAisker dispersed in the rubber matrix feasibly approaches the c~rbon black and the exposed whisker when force is applied to readily form a link, and its resistance sensitively varies by the small pressing force. Further~
since the whisker is acicular with a very large aspect ratio, while the carbon black is substantially granular, the conductive members of both are di~persed in entirely di~erent 6tates. Thus, it is presumed that different members suah as the acicular member and the granular member, or the acicular member and the acicular member can readily approach each other as compared with the manner that the granular members approach each other whsn ~orce is applied.
The whisker i~ buried in the rubber matrix on the qurface o~ the composite material. However, the whi~ker may be partly expoeed in ~uch a manner that the pro;ecting amount is 0.05 to ~00 micron and more preferably 0.1 to ~00 micron.
In the inven~ion, powder or ~hort fiber of the same material as the whisker may be u6ed in ~ddi~1on ~o the ~3V`32 ~ 5 ~BL R-104 whisker as described above. In this case, when the powder or the short fiber ls di~persed near in the ~urface of the unvulcanized sheet extruded from rolls and vulcanized so as to be exposed on the ~ur~ace of the rubber matrlx like the whisker, the short fiber or the powder is partly exposed on the surface of the rubber matrix.
On the other hand, in the pressure-sensitive conduotive rubber material of the invention, when the powder having electric insulation i5 buried in the surface layer, a rubber material ha~ing good creep and fatigue r~sistances with good pressure ~ensitive per~ormance together with the above characteri~tics can be provided.
More particularly, Fig. 1 shows a longitudinal sectional view of an embodiment of a pressure-sensitive conductive rubber sheet according to this invention. Fig. ~
shows a longitudinal ~ectional view of another embodiment of a pressure-sensitive conductive rubber sheet according to this invention. A pressure-sen~itive conductive rubber sheet 1 i~ formed of ~ composite material 2 mixed with at least one type ~elected from conductive c rbon black, ~hort ~i~er made of nonmetallic inorganic material, powder and whisker in an electrically insulating rubber matrix. An -~L3~3Z~

-~6-electrically insulating powder 4 is buried in the composite material 2 on both side surface layers 3 of the rubber sheet 2.
Alternately, the rubber sheet 1 shown in Fig. 2 contains powder 4 buried in only one surface layer 3 of a composlte material 2.
The thickness of the surface layer 3 shown in Figs. 1 and 2 is pre.ferably 0.1 to 20% of the entire thickness of the composite material 2.
The electrically insulating powder 4 used in the invention may be, for example, inorganic materials such as glass, calcium carbonate, alay, talc or organic materials such as phenolic resin, epoxy resin, urea resin or ebonite.
The powder 4 may be of ~hapes ~u¢h as powder, granular or short length rod shape, being 0.1 to 100 micron in diameter.
The amount of the powder 4 is 0.05 to 5% by weight. If less than 0005% by weight is used, the results of this invention ~annot be obtained, while i~ the powder exceeds 5~ by weight, the hardness of the surfaoe layer increases. Thus, since the 6urface layer 3 of the rubber sheet 1 is mixed and dispersed with the electrically insulating powd r 4, no link :
is formed between the composite material 2 and ~he electrode plates contacting with the ~urface layer when no ~orce is , ,;., ~ .....
.~ ," ,.. ..

~3~,3~ MBL R-104 present. When force is applied, the possibility of contacting the conductive member dispersed in the surface layer 3 and the inner layer to form a conductive link increases.
Further, when the powder ~ is buried in the surfacP layer 3 of the sheet 1, ~rictional cQefficient decreases with the reduction of the adhesiveness of ~he surface, and the electrode plate can be readily separated from the surface of the rubber sheet 1.
Thus, it is apparent that the pressure~sensitive conduative rubber sheet of the present invention shows less variation in the varying range of electrical resistance over period~ of time due to the further mechanical reinforcement provided by the powder 4, and also 1e6S fatigue decreases commonly caused by repeated pressing.
The rubber sheet 1 is produced by adding conductive carbon black and inorganic filler to electrically insulating rubber and, a~ re~uired, ~oftening agent, age . preventlng agent, vulcanizing agent, and/o~ crosslinking agent used ordlnarily in rubber, forming a sheet-~haped ~ateriaI from a kneaded m~xture of the above ingredients by, for example, a Banbury mixer, or kneader roll~, wiping the electrically insulating powder 4 on the surface of the ., . . ~ . . , ~3~)3~ MBL R-104 6heet-shaped rubber material, then vulcanizing it, or alternately, forming in advance a ~heet-shaped ~urface layer 3 mixed with the electrically insulating powder 4, laminating it on the ~heet-shaped material and then vulcanizing it.
When the whisker is used as the inorganic filler in the pressure-sensitive conductive rubber material of the invention, the desirable properties described in paragraphs (1) to ~3), below, can be obtained.
(1) The resistance v~lue of the pressure-sensitive conductive rubber material of the present invention, when certain amounts of conductive materials such as conductive carbon black and whisker are used, gradually varies as pressing force increases, thereby providing a pressure-sensitive conductive rubber material ha~ing a large resistance varying range. In this case, the amount of the conductive carbon black is 30 to 70 parts by weight and preferably 40 to 60 parts by weight with respect to 100 p~rts by weight of the rubber. I~ less than 30 parts by weight of carbon black i5 used, its resistance value does not adequately decrease as the pressing force increases even i~ the predetermined amount of whisker is added. If ~ore than 70 parts by weight uf carbon ~lack i~ used, the 1 3~13 2 ~ 5 ~BL R-104 resi~tance value abruptly decreases as the pressing ~orce increases, and at a predetermined ~orce, the remaining resi~tance varying range becomes so ~mall that the pressure sensing performance is inadequate.
The amount of the whisker added should be 10 to 60 parts by weight and preferably lO to 40 parts by wPight with respect to lO0 parts by weight of the rubber. If less than lO parts by weight i8 used, the relationship between the pressing force (log P) and the resistance value (log R) becomes a curve being convex in the upward direction.
Subsequently, when th~ forae i~ repeatedly applied, the irregularity of the resistance value increases, and its creep resistance decreases. on the other hand, i~ the whisker exceeds 60 parts by weight, its resi~tance increases, and the e~fect of adding the whisker is eliminated. When the amoun~ of the whi6ker i6 10 to ~0 parts by weight, the relation~hip between the pressing force (log P) and the resi~tance value (log R~ becomes linear.
Th~ total amount of the above conductive members such as carbon black and the whisker is 40 to 90 parts by weight and pre~erabIy 50 ~o ~0 parts by weight with respect to lO0 parts by weight of the electrically in~ulatlng rubber~ I~ less than 40 parts by weight are added/ the ~3~3~'7S

resistance value does not adequately vary due to the variation in the pressing force, causiny the rubber to provide inadequate pressure sensiti~e-conductivity. On the other hand, if more than 90 parts by weight are u~ed, the s conductivity i~cxeases and the variation in the resistance value with respect to the Yariation in the pressing ~orce is small, and the pressure sensitivity decreases.
The pressure-sensitive conductive rubbar material described above gradually varies its resistance value as the pressing force increases by mixing and dispersing the carbon black and the whisker ln limited amounts, and increases the variation range of the resi~tance value. Thus, when the pressing force ls repeatedly applied, only a ~mall variation in the resistance value is evident, and further the rubber material also has a property of varying the voltage by the amount of compressing deformation.

(2) ~ccording to the present invention, if the thickness of the pressure ~ensitive conductive rubber material having only the whisker disper6ed and ~ixed as the conductive member in the electrically insulating rubber is less than approximately 2Q0 micron, the rubber material functions in an ON-OFF ~anner. The rubber materlal abruptly decrea es in resistance value if the pres ing force of a ~3~3~ MBL R-104 predetermined valua is applied. In this ca~e, the amount of the whi~ker is 2 to 400 parts by weight and preferably 5 to 200 parts by weight with respect to 1~0 part~ by weight of the rubber. If the amount of th~- whisker is lesa than 2 parts by weight, the pressure 6ensitive conductivity decreAses, while if more than 400 parts by weight o~ whisker is used, mixture and dispersion of the whisker into the rubber becomes di~ficult preventing the rubber fxom performing the objects of this invention.
Since the whisker used in this invention has acicular crystals, it is exposed on the surface of the eheet-shaped composite material while the whisker i~
uniformly dispersed in the sheet. Thus, when the composite material is pressed by both electrode plates, the possi-bility of contactlng the whisker pro~ected Prom the sur~ace of the ~heet and the whisker di~persed in the interior increases to perform the pressure-sensitive conductivity.
However, if the thickness of the sheet- haped composite material of this invention exceeds 200 micron, even if the material i6 pressed, it acts in an insulating state, and ~t does not exhibit pressure-conductive conductivi~y. Thus, the thickne~s of the composite ~a~erial ~3~3~ MBL R-104 ~s necessarily llmlted to 200 micron or less and preferably 20 to 60 micron.

(3) The rubber matexial in which only the whisker is disp~rsed and mixed as the conductive member becomes pressure-sensitive conducti~e for ~ensitively detecting the variation in a resistance value or an electric capacity with respect to the variation in the pressurization. The thickness of the pressure-sensitive conductive rubber material is not limi.ted.
~xamples of the pressure-sensitive conductive rubber material of this invention will be described.
However, this invention is not limited to the particular Examples described herein.

Example 1 After the rubber mixture~ based on Table 1 were kneaded by a Banbury mixer, the mixtures were extruded by rolls into 2mm thick sheets. The sheets were ~eguentially ~engaged in a mold, and vulcanized at 150- for ~0 minutes.
The vulcanized sheets were cut to 30x33 mm as test pieces, and electric resistance values of the test piecPs were measured in both nonpressurized and pressurized Gtates.
Several 6econds after a load of 0.5 kg/cm2 was mounted on ..

~3~3~ MBL R-104 the test piece, the load was removedO This operation was repeated ten times, and the electric resistance values of the test pieces were measured in both the nonpressurized state and with a force o~ 0.5 kg/cm2. ~he resistance value retentivity (%) of the test pieces after ten operations were determined by dividing the resistance value in the pressurized and in the that at nonpressurized states. The results axe indiaated on Table 1.
The electric resi6tance value was determined by first holding the test piece between Te~lo~ plates of approx. 200 g., mounting a copper plate of approx. 0.3 mm of thickness between the test piece and the Teflon*plate at that time, and obtaining the resi~tance value by a digital multimeter with a pair of oopper plates as electrode plates.
The pressing was applied by mounting a weight on the Te~lon plate.
As lndicated in Table 1, the pressure-sensitive conductive rubber sheet mixed with the carbon black and the whisker exhibits a trend that~:the resistanGe value decreases 6ensitively as the load increases as compared with the rubber sheet mixed only with the carbon black as the conductive ~ember, and the resistance value retPntivity increases, and the resistance value decreases 6igni~1cantly.
* Teflon is a registered trade mark.

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~3~3Z~7~ MBL R-104 ;
: -24-~: Table l (Unit: parts by weight) _ _ . . . v . .. _ _ . .. .
; Example 1 Comparison example 1 2 3 4 1 2 ~ 3 4 5 Chloroprene rubber100100 lOO lOO lOO 100 lOO lOO 100 ; ¦ Stearic acid 2 2 2 2 2 ' 2 2 2 2 : I ~agnesium oxide 5 5 5 5 5 5 5 5 5 Age preventing agent Process oil 4 4 4 4 4 4 4 4 4 ~n 5 5 5 5 5 5 5 5 5 Ethylene thiourea0.50.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 ; Carbon black *l 36 36 36 36 0 18 36 54 . _ . .. . . .. _ _. _ _. . . .. .
Carbon black ~2 0 0 36 : Whisker ~3 3.6 9.2 19 40 0 0 0 0 0 ._. .. _._ _ _. ._ Initial electric resi~tance value (ohm) No load 1.2M 880k l.lM 1.7M 1400M 1.2M 640R 37K 17K
O.5 kg/cm2 8k 9k 9k 14k 640k 64k 11k 7k 4k l.O 4.5k 4k 4k llk 610k 35k 7k 14Gk 460 ; 1.5 ~ 3k 3k 3k lOk - l9k 4 90 270 ~epetition (ohm) ~ ~
O ~ 1.2M 600k 900k 140k 1400M l~ 500k 20k 4k :: :
~ ~ : 0.5 ky/cm2 : lOk lOk 8k 18k 600M 200k 50k lk 800 i~ :
, ~esistance value retentivity:(%) 0.9 1~7 0.9 13 42.9 20 10 5 20 *1: Sees~ 116:(~urnace type) *2: N220 (Furnace type) *3: SlC : ~ ~ :
: * Trade mark : : ~ :
~ ~ ' 13~32~ MBL R-104 :. ' Exampie 2 SiC, Si3N4, or BN were alternately used as ceramic powder of the inorganic filler and SiC or glass fiber were alternately used as glass or ~hort fiber. The mixtures and the results are listed in Table 2. The variations in the electric resistance values at no load time and after : pressing forces were applied axe less desirable when as comp~red w1th that ~btained when using the whi~Xe~.

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~3~3z7~ MBL R-104 ~able 2 (~nit: parts by weight) ! Example 2-1 2-2 "2-3 2-4 . 2-5 2-6 Chlor~prene rubber100100 100 100 100 100 Stearic acid 2 . 2 2 2 2 2 Magnesium oxide 5 5 5 5 5 5 Age preventing agent Process oil 4 4 4 4 4 4 Zn 5 5 5 5 5 5 Ethylene thiourea0.5 0.5 0.5 0.5 0.5 0.5 Carbon black *1 36 36 36 36 36 36 __ .......... . __ . . . _ . _ Powder SiC 32 Si3N4 40 n ~n 2B.5 " glass 32 Short fiber SiC 32 ~ glass 32 Initial electric resistance value lohm) No load 8M 1.8~ 5M 1.8M . 3M }2M
0.5 kg/cm2 19Ok 60k 240k 130k30k 36k 1.0 160k 45k 180k 48k 12k 20k 1.5 lSOk 30k 130k 25k .5k lOk ---- - _ _. , _~__ Repetition (ohm) : O 5M 5~ 5M 1.5M 3M 12M
0.5 kg/cm2 200k lOO]c 300k 90k 30M 120k Re~istance value retentivity (~ 4 2 6 6 ; 1 ' 1 ; 13~3Z~7~ MBL R-104 ~27-,':
; Example 3 Rubber sheets were made having acetylene black as carbon black, chloroprsne rubber, nitrile rubber (NBR), or a blend of natural rubber (NR~ and butyl rubber (BR) as electrically insulating rubber. The mixture and the results of the electric resistance values are listed in Table 3.
Since the ~cetylene black was mixed, the resistance value at no load time was small and the resistance value retentivity after pressurizing was small.
The acetylene black i6 very effeative and the blend of the natural rubber and the butyl rubber as the xubber exhibits good result.
The electric resistance value i6 low with good conductivity at no load time with only the aaetylene black used. However, the resistan4e value retentivity o~ 20% is high and thus the desirable pre~sure-eensitive conductive rubber sheot cannot be obtaln-d, ~' ~
';`' ~: :

13~32 ~ MBL R-104 .
2 8 ~

T~ble 3 (Unit: parts by weiyht) ¦ Examp~e C. Example ~ - r I , I
: 1 3 1 1 3-2 3-3 3-4 3-5' 3-6 3-7 3-8 l3_9 . ___ j _ ... . .... , . _ ., ,, . . _ . . . .

NR/BR *3 i 100 100 100 100 100 Zn 5 5 5 5 4 4 4 4 4 Stearic acid 1 2 2 0,5 0.5 Process oil ~ 4 4 4 4 4 4 4 4 4 Accelerator CM ' 0.5 0.5 Sulfur 0 0 2 2 Acetylene black ' 54 54 54 54 54 54 54 54 50 Powder SiC ¦ 5 5 5 40 0 Whisker SiC I 5 5 5 40 0 _ .~ .. _ .. _.. , .. . , , . _ . , .. ,, . . .. . . , . .. _ Repetition (ohm) 0 3.5M 8k25k50k 15k20k 35k 15k 250 0-5 kg/cm2200k 200 500 300 . 80 , 60 110 6D 50 resistance value i~ ' retentivity (%) 5.7 ,2.5 2.0 ~.6 Q~5 0~3 0.3 0.4 ~0 ~3: N~/BR-6~4 ::

: ' :

,` ~ :
~.

~:

13tf~3~ MBL R-104 : -29-Example 4 Pres~ure-sensitive conductive rubber sheet was produced by ~electing the rubber, the carbon black and inorganic filler to be most preferable as apparent from the above-mentioned Example 3, and the resistance values at no load time after repetitive operations and the resistance value at pressures of 0.5 Xg/cm2 were obtained. The results axe listed in Table 4.
From the results, even if the materials were the same as the inorganic ~iller, the whisker was effective in that th~ variation in the electric r~istance value when pressurized was large. Thus, a pressure-sensitive conduc-tive rubber sheet having large variation in the electric resistance value when pressurized and high response ko the sensitive variation in the resistance can be provided.
The inorganic filler o~ the above-mentioned whisker is partly exposed o~ the sur~ace of the rubber matrix, and dispersed and buried ln the rubber matrix therein. Thus, the ~iller exposed on the 6urface at no load ; 20 time is in contact with the electrode plates. Since the electrode plate do not aome in complete contact with the rubber matrix and the electrio resi6tanre value o~ the rubber sheet inoreases Wh-- th- preseur--seneitlve .

13~3~7S MBL R-104 ; -30-conductive rubber Gheet is pressed, the ~iller exposed on the surface on the rubber matrix gradually comes in contact with the electrode plates. When the pxesslng force increases, the probability of forming a lin~ further increases due to the contact with the conductive member, thereby reducing the re~istance value.
As ~escribed above, the pressure-sensitive conductive rubber sheet of this invention mixes inorganic ; filler selected from short fiber, powder and whisker in the rubber matrix together with the carbon black, and partly expose~ the ~iller on the surface of the rubber matrix, thereby greatly reducing the resistance value when force is applied, and improving the 6ensitivity o~ the varying resistance value in response to the variation in the lS pressln~ ~orce.

:
.

~3~3z~ BL R-104 Table 4 (Unit: parts by weight) ¦ Example 4-l 4-2 4-3 4-4 .~
NR/3R 100 lO0 100 lO0 Zn 4 4 4 4 . Stearic acid , Process oil 4 4 4 4 : Accelerator CM
Sulfur Acetylene black54 54 54 54 Powder SiC 5 40 : ~ Si3N4 40 Whiskex SiC 5 40 Repetition lohm) ¦ 0 5k 70klk 1.2k 0.5 kg/cm2 60 6020 1.2 Resistance value retentivity (~) 0.12Ø085 2.0 1.0 ' ~ MBL R-104 Example 5 (Effect of ~arying amount of whisker~
After rubber mixture was kneaded in a ~anbary mixer according to the mixture shown in ~able 5, the mixture was extruded by rolls into 2 mm thick sheet The sheets were engaged in a mold, and vulcanized by pressing at 150 for 20 minutes. The sheets thus obtained were cut to approx. 10 cm2 as test pieces. A predetermined load was applied to the pieces to measure the relationship between the pressing force and the resistance value of the sheets.
~he results are shown in Fig. 3.
The measurement~ of the electric resistance values were f~rst aonducted by engaging the test piece between 0.3 mm thick stainless steel plates and 100 g electrode plates, and then applying 6 V of constant voltage to obtain the resist~nce value by a digital multimeter.
; When a pressing force of 10 g/cm2 or larger was applied, a Teflon plate and a weight were placed on the electrode plates.
Fig. 3 shows the relationship between the pressing Porce and the resistance value by the variable amounts of the whisker con6tantly with respect to 30 parts by weight of carbon blaok. I~ thc whisker was not ~ded, the di~erenoe , ~3~3~7~ MBL R-104 of the resistance value between the pressing forces 1 g/cm2 ;: and 1000 g/cm2 was 6mall, and if the repetit~on nu~ber of ~, .
pressing was increased to 5 times, the resistance value decreases to exhibit a resistance value curve having a smoo~h ~lope. On the other hand, when using 80 parts by weight, a larger resi6tance value i~ obtained than when ~ u~ing 40 part6 by weight, and the adding ef~ect of the ;~ whi~ker i~ obviated, Howevert if the whi~ker i5 added, even i~ the pressing was repeated five times, the xesistance value ls 8table. The rubber material with the whisker added exhibits the results after five repetitions.
From t~e above results, the adding amount of the whisker i5 preferably 10 t~ 40 part~ by weight.

Table 5 (Unit: parts by weight~
! Example .C.Example ;
, 5-1 5-2 5-3 5-4 5-5 i Natural rubber100 l100 100 100 100 Zn 4 4 4 4 4 , Stearic acid 1 ' 1 1 1 1 I
Accelerator CM 4 4 4 4 4 Sulfur 1 } 1 1 1 Acetylene black 30 30 30 30 ~30 Whisker SiC i~ 20 40 0 ,80 .. _ . =

~3~3z7~ MBL R-104 -34~

Example 6 (E~fect sf varying amount of carbon black) ~ s shown in Table 6, the amo~nt of the whisker was fixed to 20 parts by weightt ~nd the ~mounts of the carbon black were varied. Test pieces were produced in the ~ame manner as the previous examples, and the relationship between the pressing force ~nd the resistance value of the sheet was measured. The results are 6hown in Fig. 4.
From the above, the r21ationship between log P and log R with 30 to 70 parts by weight of the carbon black is linear, and as the pressing force i~creases, the reisistance value gradually increases, and the varying range o~ the resistance value becomes large.

Table 6 tUnit: parts by weight) Example ' C.Example 6-1 6-2 ,6-3 ~6-4 6-5 ,6-6 6-7 1- . _ __ i Natural rubber ¦ 100 100 100 l1oo100 llO0 100 Zn 4 44 , 4 4 ,1 4 Stearic acid . 1 11 , 1 , 1 , 1 1 Accelerator CM 4 4 4 ~ 4 1 4 1 4 4 Sulfur : ` 1 1 1 1 1 1 1 Acetylene black 30 40 50 60 70 20 80 Whisker SiC 2~ 20 20 20 20 20 20 _ .

.

~~3~3~ t S MBL R-104 :
Example 7 Test pieces having approximately 2 mm thick . ~were produced in the ~ame manner as the previous Examples by the mixture llsted in Table 7, the relationship between the compressing distortion and the voltage of the ~heets, and the ~atigue resistance and creep resi~tance were measured.
In this Example, a method of measuring the relationship between the compressing di~tortion and the voltage included holding the sheet by the electrode plates, : 10 flowing a current of 5 microamperes therein, then mounting it in a load cell type compression tester and the voltage value was measured at pressing speed of 0.5 mm/sea. by a digital multimeter.
A method of measuring the fatigue resistance due to repetitive pressing included repetitively applying pressure of 0.5 kg/cm2 100,000 to 8,000,000 times ~t a rate of 25 times per second to the sheet, measuring the electric resistance value, and dividing it by the electric resi~tance value under pressure of 0.5 kg/cm2 ~efore the repetitive pressln~.
: ~urther, a method of measuring the creep :resistance included ~pplying a load of 0.5 kg/cm2 to the 6heet, and obtainlng the value produced by dividing the ~3~3z~ MBL R-104 reslstance value at creep applying time by the resistance ~alue at creep tlme being 0. The results are 6hown in Figs.

5 to 7.
From the results, the pressure-~ensitive conductive rubber material of this invention linearly varies the voltage according to the magnltude of the compressing - distortion, and i6 substantially linearly displaced even when the load was removed. Thus, its hysteresis is small, the resistance value after repetitive deformation is small, and the creep resistance is excellent.
According to this invention, as described above, the pressure-sensitive conductive rubber material includes : ~he conductive members cf the carbon black and the whisker mixed in the amounts Qf predetermined r~nges. Thus, the pressing force and the r~sistan~e value exhibit linearly analog variation so that the resistance value gradually varies with respect to the pressing:~orce, and the relationship between the compressing distortion and the voltage slmilarly varies. Therefore, the characteristics can be not only quantitatively obtained, but also the hysteresis is small, the variation in the resistance value a~ter repetitlvc de~orm~tions i~ ~m~ll, a~d the cre}p .
, :

~3~32 7S MBL R-104 ¦ resistance i6 excellent, thereby pro~iding h~gh reliability as a pressure- ensitive ~ensor.
, ', Table 7 ¦ I Example I C.Example I ,, 7 1 l72 !7-3-l-7~
NR 100 . ¦100 NBR 1100 1 .100 Zn 1 4 4 1 4 1 4 Stearic acid 1 1 , 1 1 1 Accelerator CM ' 4 4 ¦ 4 1 4 Sulfur 1 2 . l j Acetylene black , 50 ` 50 1 40 ~ 50 Whisker SiC , 20 20 . 20 , 0 Powder SiC ¦ 0 0 ~ 20 ___ ..

~ .

~3~`~5 ~ -38-:`
Example 8 After xubber ~ixtures were kneaded by a Banbary mixer according to the mixture ~hown in ~able 8~ it was extruded to ~heets 2 mm thick, by rolls. A~ter glass powder having electric insulation shown in Table 5 was wiped on both side surfaces of the sh~et, the sheets wer~ engaged therebetween and the sheets were vulcanized by pressing at 150C ~or 20 minutes. The obtained sheets were cut to 30x33 mm to form test pieces. Then, the initial electric resistan~e values when nonpressurized ~however, a Teflon plate of 100 g was placed on the test piece) or when pressurized. After the sheets were pressed 10 times under the pressing conditions of 0.5 ~g/am2, the electric resistance values when nonpressurized and pressurized at 0.5 kg/cm2 were measured. The resistance value retentivity (%) at 10 times repetitive pressing times was the value determined by dividing the resistance value which pr~ssurized by that when nonpressurlzed.
In order to ~easure the degr~e of variations in the electric resistance value, a large load was gradually applied diecontinuously ~or 5 to lS ~ec. to the te~t pieces.
The electric re6i~tance values at that time were ~easursd to obtain the varying curve of the flr~t electric resistance ~ ~ MBL R-104 value with respect to the pressing, and further the load was then removed. Loads were applied again for of 5 to 15 sec., and second and thlrd electric resistance values were obtained. The results are Bhown in Fig. 8.
Further, in order to measure the creep resistance of the rubber ~heet, a load of 0.5 kg/cm2 was applied to the 6heet to obtain the variation in the resistance value at creep applying time. The results are shown in Table 9.
From the results, the conductive rubber ~heet in which the glass powder was buried on the surface of the rubber sheet mixed with the carbon black and the whisker exhibits high resistance value when nonpres~urized as compared with that in which the glass powder was not buried on the surface. The vaxiation in the resistance value when pressuri2ed was large with good sensitivity, the variation in the resistance value with xespect to the repetitive pressing became small, the hysteresis of the electric resistance value became small, and the creep resistance is excellent.
~o According to this invention as described above, the pressure-sensitive conductive rub~er sheet i~ produced by burying glass powder having electrlc insulation on both or one oide ourface ~f the she-t obtained by mixing the .

~3~3~ ~ MBL R-104 inoxganic ~iller ~elected from the 6hort fiber, powder ~nd the whi6ker in the rubber ~atr~x together with the carbon black, thereby maintaining the in~ula~lng ~tate wikh the . electric resi~tance value when nonpres~urized, while largely decreasin~ the electric resistance value in one pressurized and pro~iding large variation in the resistance value to provide good 6ensitivity, and further providing 6mall variation in th~ electric resistance value even if applying the repetitive pressing to the cheet to provide good fatigue resistance and creep resi6tance with long lifetime.

Table 8 (Unit: parts by weight) . . __ Example C~Example 8-1 ~8-2 18-3 ,8-~
NR 100 100 100 llO0 Zn ¦ 4 4 ' 4 1 4 Stearic acid I 1 1 1 1 Process oil 1 4 4 4 4 Accelerator CM
Sulfur Acetylene black , 50 50 50 50 Whisker SiC *1 20 20 0 0 Glass powder *1 2.3 1.4 none ~one . . _ ~1: % by ~eight .

~L3~32~S

Ta~le 9 . .. ---- . I ;
- , Example 'C.Example 8 - 1 8 - 2 ~ - 3 8 - 4 10 times repetition (ohm) o 80k lOOk Bk 250 0.5 kg/cm2 . 0.15 0.14 1 20 Creep resistance *2 ' ' :1 hr(ohm) 0.95 0.9' 1 1.5 . 60 hrs , 0.9 0.9 4 8 200 hrs I 0.9 , 0.9 15: 150 1 _ _ . . .
~2: value obtàined by dividing the resistance value (Rf) at cree~ing time by resistance value (Ro) before creeping.

Example 9 After rubber mixture was kneaded by a Banbury mixer according to the mixture shown in Table 10, the kneaded rubber was dissolved in a solvent as rubbar paste.
The paste was then coated on a ~lat aluminum plate to form a film, then filled in an oven, and vulcanized at 150C for 20 : min. The thin film having:80 micron of thickness obtained in thi~ ~anner was cut to~30x33 ~m to ~orm test pi~ces. The test pieces were then measurcd ~or electrlc Fe i~tance 1,' ,,.,., , ~

~3~32~

values when nonpressurized (however, an electrode plate having 12 g was mounted on the test piece) ~r when pressurized. The results are also shown in Table 10.
From the results, the rubber sheet ~ixed with the whisker has excellent pressure sensitive conductivity as compared with that mixed with powder even if the same materials are used.

13~3Z~ MBL ~-104 .~ .
Table 10 (Unit: parts by weight) __ ~
ExampleC. Example 9-1 9-2 9_3 9-4 9_5 9-6 9-7 9-B 3-9 ~-10 I 9-11 ., _ . -- ._ _ __ __ _ .
.. ~R 100 100 100 00 CR 100 , ;: ~ Silicone rubber 100 100 SE~S 100 Zn 5 5 5 5 5 5 5 00 Stearic acid 4 1 24 4 4 1 Process oil Aecelerator CM 1 1 1 1 1 1 Aeeelerator 22 . 0,5 MgO 4 3 3 I.
AerylLe aeid est 3 l 3 Peroxide 1 1 Sulfur 2 2 2 2 2 2 Whisker SlC 80 80 80 4o 20 40 Powder SiC B0 80 _ _ __ _ _ _ _ Nonpr~95i~g time 10 g/cm2 40k ~ ~ 4M
500 7k OM 80 lM 50 30 1 kgicm2 2k lM 330 OOk 15 20 :

: ~ilm thiekness ~m ~ron _ . _ _ _ _ _ _ . __ . 80 140160 100 90100 B0 80 140 100 100 .

.~ .
:~
, 13~3~

Example 10 The thickness of the rubber sheets having the mixtures in the Examples 9 (Nos. 9-1, 3, 4, 5 and 6) were varied from approx. 20 micron to 400 micron~ The electric resistance values of the obtained rubber sheets wPre measured when nonpressurized and when pressurized.
The xesults are shown in Table 11. Thus, if the thickness o~ the rubber sheet exceeds 180 micron, the property of the pressure-sensitive conductivity is erased.
~ccording to this invention as described above, khe whisker of acicular crystal is mixed and dispersed in the rubber, and the thickness i5 200 micron or smaller.
Thus, since no conductive material such as carbon black is mixed as conventional, a substantially completely insulting state can be held when nonpressurized, while the electric resistance value can be gradually decreased when pressurized to perform the pressure-sensit~ve conductivity. Since the thickness is small, a conductive thin film can b~ directly formed on the electrode plate to produce a switch having small etroke.

' 3;~

Table 11(Electric resistance value) ._ ._._Example 9-1 (NR) Example 9-3 (CR) Film thickness(micron)20 60 100 150 200 20 100 160 220 400 _ _ _ Nonpressing time ~ lOk 50k : 10 g/cm lOM ~~ ~ ~ lk 4k 4k 0.5 60 50k lM 800k ~ 5 20 15 1 kg/cm2 30 35D :lOOk 500k ~ 5 15 10 ~ ~
.. .~ . , , _. . .... _ . _ . . _ ~ p.- (NR) Example 9-5 (SBS) IEXamP1e 9-6(Silicon) Film thickness20 100 150 170 90 250 300 40 60 100 120 1270 (micron) . .
. .
Nonpressing time 10 g/cm2 lOM ~ ~ ~ 4M ~ ~ 70k 70k ~ ~ ~
0.5 30 lM800k ~50 ~ ~ 30 10 30 25k 40k 1 kg/cm2 10 lOOk 500k ~ 15 ~ ~ 15 7 20 15k 5k .~ .. ~ .
'.

L~=l~ll ', ' A~ter the rubbnr mixture W~B Xneaded by a ~anbary miXer aocording t~o'the mlxture ln Table 12, the mixtura was extruded by roll~ to sheet~ 2 ~m thlcX. The sheets were held by ~ mold nnd vulcanized by pressing at 150-C and 20 min. The obt~ined shaato were cut into 30x33 mm to form -'" , ' .
.

. . .
~: ' ' ' ' .
.

, ' . .
.~ .

13V3Z~ MBL R-104 -46~

test pieces. The test pieces were pressed, and the electric resistance value and the electric capacity at that time were measured.
The electric resistance value (RAC) was measurad by holding the test piece between Teflon plates of approx.
; 100 g by mounting a D. 3 mm thick copper plate between the test piece and the Teflon plate. The resistance is measured with the pair of the copper plates as electrode plates by an LCR meter. The resistance value at that time was the value with AC (100 kHz). The pressing was conducted by mounting a weight on the Teflon plate disposed at its upper side.
The electric capacity (C) was measured by the same LCR meter as ths electric resistance value. These results are sliown in Figs. 9 anfl 10.
Thus, the rubber sheet which used the whisker as compared with the powder of the same silioon carbide exhibited larger variations in the electric resistance values and the electric capacity value with respect to pressing.

. .

~3~3~

~BL R-104 Ta~le 12 (Unit: parts by weightJ
,Example C~Example ; Ill-l11~2 11-3 11-4 .....
CR , 100 lO0 100 Zn - 5 5 5 5 Stearic acid 2 2 2 2 Accelerator CM 0.5 1.5 0.5 0.5 Sulfur 0 2.0 0 0 Whisker SiC80 80 0 0 Powder SiC 0 0 0 80 . .

Further, after the test pleoe~was repetitively pressed under the pr~ssurizing oondition of 0.5 kg/cm2 at predetermined timec~ the value was determined by dividing ~the electric capacity Co.s when pressurized ~t 0.5 kg/cm2 by the electric capacity Co when nonpressurized (h~wever, the ., .

, ~3~3~ ~S MBL R-104 Teflon plate of 100 g was placed on the test piece). The result is ~hown in Table 13, and when the rubber ~atrix and the natural rubber were uæed, the ~table pressur~-sensitive ; rubber molding haviny small electric capacity increasing ratio was obtained~
Further, in order to measure the degree of variations in the electric resistanae, a large load was gradually applied discontinuously to the test piece, the electric resistance value at that time was measured, thereby obtaining the varying curve of the first electric resistance value with respeat to the pressing. Further, the load was then removed, various loads were again applied, and the varying curves of the seaond and third electria resistance value~ were obtained~ The varying curve of ~ourth electric resistance value bacame the stable ~tate substantially the same as the third one. Thus, the difference of the variation in the varying curve of the electric resistance value with respect to the first and third extrusions was evaluated by the area S.
The results are shown in Fig. 11. According to this, the pressure sensitive rubber molding mixed with the whi ker of thi6 invention reduces the area S as compared with the molding mixed with the powder or 801e rubber, and ;

~3~3Z~

also decreases the variation in the resistance ~alue with respect to the repetitive pressing, and the small hysteresis . of the electric resistance value was eventually obtained.
' Ta~le 13 (Electric capacity increase ratio:C0 5/Co) ¦ Example ll-l Example 11-2 j RepetitiOn 1 1 4.1 '' 2.4 2 3.1 2.3 3 2.g 2.3 4 1 2.5 2.3 1.5 2.3 _ _, ~xample l2 The amounts o~ the wh1sker o~ the mixture in the ~ample No. 11-l o~ the previous Example 4 were varied to 50, 80, lO0 and 20~ parts ~y weight, and test p1eces made of vulcanized 6heet 3 mm thick were produced in the ame method ~ that in the previous Example~. A large load was ~pplied gradually discontinuously to the test pieces to 0.5 kg/cm2 ~ax1=u=, the lo~d was then removed, ~nd ~urther the lDad o~

, 13rt32 75 ~BL R-104 ,.
O. 5 kg/cm2 was again applied. After this operation was repeated five times, the electric capacity increasing ratio of the capacity CoOs when pressurized at 0.5 kg/cm2 divided by the electric capacity Co at no load (however, the Teflon plate of loo g. was placed on the test piece) was obtained.
Fuxther, the electric resistance value, when pressurized, of the test piece was obtained.
The results are shown in Table 14. According to ; the table, the more the amount of the whisker increases, the better pre6sure sensitive rubber molding can be obtained.
According to this invention ~s described above, the pressure~sensltive rubber molding is ~ormed from a composition mixed with inorganic filler made of the whisker ln the rubber. Conductive member ~uch as carbon black is not mixed in the composition. The rubber can sensitively senses the variations in the ~leotric resistance value and the electric capacity with respect to the pressuring variation, and stable resi~tance values can be produced with respect to repetitive pressing and the pressure-~ensitive rubber exhibits ~mall hysteresis.

~3~3~ NBL R-104 Table 14 Amount of whisker (by weight) -----50 80 lO0 200 Electxic capacity in~reasing ratio ~; C0 5/Co 1.3 2.5 2.~ 5.2 Resistance value reduction ratio ; RACo 5/RACot~) 70.0 24.2 17.4 7.5 -- ---- _ _ _ _, . .

Claims (9)

1. An electrically conductive rubber material having pressure-responsive variable electrical resistance, said material comprising:
a matrix formed of an electrically insulative rubber;
carbon black dispersed in said matrix; and a filler of semiconductive acicular ceramic whiskers dispersed in said rubber material selected from the group consisting of alpha-silicon carbide, beta-silicon carbide, alpha-alumina, titanium oxide, tin oxide, graphite, Fe, and Ni having a diameter of 0.05 to 3 microns, and a length of 5 to 500 microns, 30 to 70 parts by weight of carbon black and 10 to 60 parts by weight of whiskers being provided in 100 parts by weight of said rubber and the total amount of the carbon black and the whiskers being 40 to 90 parts by weight in 100 parts of said rubber.

2. The variable resistance rubber material of Claim 1 wherein said material has a thickness of up to 200 microns.

3. The variable resistance rubber material of Claim 1 wherein an insulative surface portion is provided in said matrix.

4. The variable resistance rubber material of Claim 1 wherein an insulative surface portion is provided in said matrix, said insulative surface portion having insulative powder dis-persed therein.

5. The variable resistance rubber material of Claim 1 wherein an insulative surface portion is provided in said matrix, said insulative surface portion having up to 5% by weight of insulative powder dispersed therein.

6. An electrically conductive rubber material having pressure-responsive variable electrical resistance, said material comprising:
a matrix formed of an electrically insulative rubber;
carbon black distributed in said matrix; and an inorganic filler distributed in said matrix consisting of semiconductive acicular ceramic whiskers selected from the group consisting of alpha-silicon carbide, beta-silicon carbide, alpha-alumina, titanium oxide, tin oxide, graphite, Fe, and Ni having a diameter of 0.05 to 3 microns, and a length of 5 to 500 microns, 30 to 70 parts by weight of carbon black and 10 to 60 parts by weight of whiskers being provided in 100 parts by weight of said rubber, and the total amount of the carbon black and the whiskers being 40 to 90 parts by weight in 100 parts of said rubber.

7. The variable resistance rubber material of Claim 6 wherein said inorganic filler is exposed at a surface of said matrix.

8. An electrically conductive rubber material having pressure-responsive variable electrical resistance, said material comprising:
a matrix formed of an electrically insulative rubber;
carbon black distributed in said matrix;
an inorganic filler distributed in said matrix consisting of semiconductive acicular ceramic whiskers selected from the group consisting of alpha-silicon carbide, beta-silicon carbide, alpha-alumina, titanium oxide, tin oxide, graphite, Fe, and Ni having a diameter of 0.05 to 3 microns, and a length of 5 to 500 microns, 30 to 70 parts by weight of carbon black and 10 to 60 parts by weight of whiskers being provided in 100 parts by weight of said rubber, and the total amount of the carbon black and the whiskers being 40 to 90 parts by weight in 100 parts of said rubber; and electrically insulating powder having a particle size of 0.1 to 100 microns in at least one surface portion of said matrix, said powder being present in the amount of 0.05 to 5% by weight of said material at said surface portion.

9. The variable resistance rubber material of Claim 8 wherein said powder on said surface is one or more powders selected from the group consisting of glass, calcium carbonate, clay, talc, phenolic resin, epoxy resin, urea resin and ebonite.