Electroviscous fluid

Abstract: An electroviscous fluid comprising an electrical insulating liquid and particles dispersed therein, said particles being composed of semiconducting particles having a weight average particle size of 1 to 100 µm and dielectric particles having a weight average particle size of 1 to 3 µm, the weight average particle size of the dielectric particles being not more than 30% of that of the semiconducting particles, the amount of the dielectric particles being 5 to.40 vol% based on the total particles, and the amount of the particles in the whole fluid being 10 to 60 vol%.

Description: The present invention relates to an electroviscous fluid and, more particularly, to an electroviscous fluid which has an excellent insulating property, which is usable in a high electric field and which generates an excellent electroviscous effect. The electroviscous fluid is useful for a vibration isolator and a power transmission which are electrically controllable. An electroviscous fluid is a fluid whose apparent viscosity rapidly and reversibly changes depending on the ON and OFF states (change) of an electric field applied. An electroviscous fluid is generally produced by dispersing polarizable particles in an electrical insulating liquid. It is considered that an electroviscous effect is produced as follows: when an electric field is applied to the electroviscous fluid, the particles dispersed in the liquid are polarized and agglomerate in the form of a chain by electrostatic attraction based on the polarization; as a result an electroviscous effect is displayed. Many kind of electroviscous fluids containing dispersed particles which have adsorbed a polarizable solvent such as water are known. As a fluid containing dispersed particles having no adsorbent, a fluid containing semiconducting particles such as polyacene quinone (JP-A-216202/1986), a fluid containing electrical conductive particles such as particles of aluminium covered with an electrical insulating film (T. Sasada et al: Proc. 17th Japan Cong. Mater. Res., 228 (1974)), a fluid containing composite particles having a three-layered structure with an electrical conductive layer such as a metal layer disposed as an intermediate layer (JP-A-97694/1988), and a fluid containing dielectric particles such as barium titanate (JP-A-17585/1978), for example, are known. A fluid containing dispersed particles having different particle sizes in order to enhance the redispersibility of the deposited particles has also been proposed (JP-A-)160094/1991). However, a fluid containing particles which have adsorbed a polarizable solvent such as water is disadvantageous in that when it is exposed to a high temperature for a long time, the adsorbent evaporates and the electroviscous effect is lowered. In a fluid containing particles having no adsorbent, in the case of using the fluid containing composite particles, it is difficult to cover the particles with a uniform thin film, and hence a sufficient electroviscous effect is not always obtained. In a fluid containing only dielectric particles, since the specific gravity of the particles is generally high, the particles may disadvantageously sediment. In a fluid containing only semiconducting particles, the insulating property is insufficient, so that application of an electric field sometimes causes dielectric breakdown, and as a result it is difficult to obtain a sufficient electroviscous effect. In a fluid containing particles having different particle sizes, the insulating property of the particles as a whole is sometimes lowered depending on the kinds of the particles, so that dielectric breakdown is caused in a low electric field. As a result it is difficult to obtain a sufficient electroviscous effect. As a result of the inventors' studies to solve the above described problems, it has been found that by dispersing semi-conducting particles having a comparatively large average particle size and dielectric particles having a smaller average particle size than that of the semi-conducting particles in an electrical insulating liquid, the electroviscous fluid obtained is not deteriorated at a high temperature, has an excellent insulating property, is usable in a high electric field and shows an excellent electroviscous effect. The present invention has been achieved on the basis of this finding. EP-A-455,362 discloses an electrorheological fluid containing, dispersed in an oily medium having electrical insulating property, a powder comprising composite particles each having minute particulates uniformly dispersed in a matrix phase, said matrix phase having an electrical conductivity of 10 -10 to 10 2 S.cm -1 , and said particulates having an electrical conductivity of up to 1/10 of that of said matrix phase. The present invention provides an electroviscous fluid comprising an electrical insulating liquid, semi-conducting particles dispersed in said liquid and dielectric particles dispersed in said liquid, wherein: the semi-conducting particles have a weight average particle size of 1 to 100 µm, the dielectric particles have a weight average particle size of 0.1 to 3 µm, the weight average particle size of the dielectric particles is not more than 30% of that of the semi-conducting particles, the amount of the dielectric particles is 5 to 40 vol% based on the total particles, and the amount of the particles in the whole fluid is 10 to 60 vol%. Fig. 1 is a graph showing a relation between the electroviscous effect of the electroviscous fluid in Example 1 to the voltage applied. The electrical insulating liquid used in the present invention is a liquid having an insulating property, corresponding to a resistivity of usually not less than 10 9 Ωcm, preferably not less than 10 10 Ωcm. Examples of the electrical insulating liquid are silicone oils, ester oils and mineral oils. More specifically, dimethyl polysiloxane, dioctyl phthalate, dibutyl phthalate, diisononyl phthalate, trioctyl trimellitate, triisodecyl trimellitate, dibutyl adipate, butyl stearate, paraffin-based mineral oil and naphthene-based mineral oil may be exemplified. The semiconducting particles used in the present invention are composed of a material having an electrical conductivity of usually 10 -2 to 10 -10 S·cm -1 , preferably 10 -4 to 10 -9 S·cm -1 . If the electrical conductivity is too low, it may be difficult to obtain an electroviscous effect. On the other hand, if the electrical conductivity is too high, application of an electric field may cause dielectric breakdown. The preferred specific gravity of the semiconducting particles depends upon the particle size and the amount of particles added, but semiconducting particles having a specific gravity of not more than 3, more preferably 0.7 to 2.5, are preferred in order to suppress the sedimentation of the particles. Examples of the semiconducting particles used in the present invention are particles composed of a carbonaceous material such as coke, having the above-mentioned electric conductivity and obtained by heating a hydrocarbon at a temperature of not higher than 1000°C, copper phthalocyanine or polyacene quinone. The semiconducting particles have a weight average particle size of 1 to 100 µm. If the weight average particle size is less than 1 µm, since the specific surface area of the particles increases, the viscosity of the electroviscous fluid when no electric field is applied may be so high that the ratio of the viscosity when an electric field is applied to the viscosity when no electric field is applied is reduced. On the other hand, if the weight average particle size is more than 100 µm, the particles may easily sediment, and the power exerted on each particle when an electric field is applied becomes large, resulting in breakage or wear of the particle . The preferred weight average particle size of the semiconducting particles is 5 to 50 µm. The particle size distribution of the semiconducting particles is not specified, but particles uniform in size are preferred. The dielectric particles used in the present invention are composed of an insulating material having a large dielectric constant. To produce a large electroviscous effect, a larger dielectric constant of the dielectric particles is preferable. Dielectric particles having a dielectric constant of not less than 100 are preferred. For example, particles of an inorganic dielectric material such as BaTiO 3 , (Ba, Sr, Ca)TiO 3 , (Ba, Ca)(Zr, Ti)O 3 , Pb(Zn, Nb)O 3 , Pb(Fe, Nb)O 3 , Pb(Mg, Nb)O 3 and Pb(Fe, W)O 3 are usable. The weight average particle size of the dielectric particles is 0.1 to 3 µm, for example 1 to 3 µm. If the weight average particle size is less than 0.1 µm, since the specific surface area of the particles increases, the viscosity of the electroviscous fluid when no electric field is applied may so high that the ratio of the viscosity when an electric field is applied to the viscosity when no electric field is applied is reduced. On the other hand, if the weight average particle size is more than 3 µm, the particles may easily sediment. The preferred weight average particle size of the dielectric particles is 0.3 to 2 µm. The particle size distribution of the dielectric particles is not specified, but particles uniform in size are preferred. The weight average particle size of the dielectric particles not only satisfies the above described requirements but also is not more than 30%, preferably 0.1 to 20%, of that of the weight average particle size of the semiconducting particles. It is considered that by using dielectric particles having a smaller average particle size than that of the semiconducting particles, the dielectric particles enter gaps between the semiconducting particles when the semiconducting particles agglomerate in the form of a chain when an electric field is applied, thereby suppressing dielectric breakdown. Consequently, it is possible to apply a high electric field and to obtain a large electroviscous effect. By combining particles having different particle sizes, the viscosity of the electroviscous fluid when no electric field is applied is reduced. If the weight average particle size of the dielectric particles is more than 30% of that of the semiconducting particles, it is difficult to obtain the above described effect. The amount of dielectric particles is 5 to 40 vol%, preferably 10 to 35 vol%, based on the total particles. If it is less than 5 vol%, it may be difficult to obtain the above-mentioned effect. On the other hand, if it is more than 40 vol%, since the particles having a large specific gravity increase, sedimentation of the particles sometimes arises. In addition, since the amount of fine particles increases, the viscosity of the electroviscous fluid when no electric field is applied may be unfavorably increased. The total amount of the particles in the whole fluid is 10 to 60 vol%, preferably 20 to 50 vol%. By increasing the amount of the particles, the electroviscous effect is enhanced and sedimentation of the particles is suppressed. If the amount is less than 10 vol%, it tends to be difficult to obtain the desired electroviscous effect and the particles tend to sediment. If the amount is more than 60 vol%, the viscosity when no electrical field is applied tends to be too high and the fluidity of the electroviscous fluid tends to be poor. In order to stabilize the dispersed state of the particles, it is possible to add a dispersant to the electroviscous fluid of the present invention. As the dispersant, modified silicone oils and esters are usable. Examples are amino-modified silicone oils, epoxy-modified silicone oils, epoxy polyether-modified silicone oils, acrylic ester polyfunctional ester polymers and polyvalent amine activators. The electroviscous fluid of the present invention has excellent stability at a high temperature. For example, even if the electroviscous fluid is allowed to stand at room temperature for 3 days after being allowed to stand at a temperature of 160°C for 6 hours, no supernatant separates and no deterioration in the properties is observed. The electroviscous fluid of the present invention has an excellent insulating property, so that it is usable in a high electric field, and it produces an excellent electroviscous effect. Examples The present invention will be explained in more detail in the following Example and Comparative Examples. Example 1 Coal tar was heated to 500°C to produce coke. The true specific gravity of the coke was 1.4. The coke was pulverized by a pulverizer, thereby obtaining semiconducting particles having a weight average particle size of 17 µm. (When the semiconducting particles were pressed to form a pellet, the electrical conductivity of the pellet was 10 -7 S·cm -1 ) BaTiO 3 particles having a weight average particle size of 0.68 µm, a specific gravity of 6.012 and a dielectric constant of 1500 were used as the dielectric particles. 40.61 g of these particles were mixed with 38.70 g of dioctyl phthalate having a specific gravity of 0.986 and 1.91 g of a polyfunctional dispersant (SN4114, produced by SANNOPCO, LTD.). The resultant mixture was thoroughly dispersed by a paint shaker. 24.31 g of the semiconducting particles were mixed, with the thus-obtained slurry, and the resultant mixture was further dispersed by a paint shaker to obtain a sample. The total amount of particles in the whole fluid was 38.1 vol%, the weight average particle size of the semiconducting particles was 17 µm, the weight average particle size of the dielectric particles was 0.68 µm, and the amount of the dielectric particles was 28 vol% of the total particles. The shear stress of the obtained sample when an electric field was applied thereto was measured by using a coaxial-double cylinder rotational viscometer while applying a voltage between the outer and the inner cylinders. The shear rate was 365 s -1 , the distance between the electrodes was 1 mm, and the measuring temperature was 25°C. The results are shown in Fig. 1. After the sample was allowed to stand at a temperature of 160°C for 6 hours, it was further allowed to stand at room temperature for 3 days. No supernatant separated and no deterioration in the properties was observed. Comparative Example 1 The coke obtained in the same way as in Example 1 was pulverized to obtain semiconducting particles having a weight average particle size of 3.0 µm and semiconducting particles having a weight average particle size of 65 µm. 9.46 g of the semiconducting particles having a weight average particle size of 3.0 µm were mixed with 38.70 g of dioctyl phthalate and 1.91 g of the same dispersant as in Example 1, and the resultant mixture was thoroughly dispersed by a paint shaker. 24.31 g of the semiconducting particles having a weight average particle size of 65 µm were mixed with the thus-obtained slurry and the resultant mixture was further dispersed by a paint shaker to obtain a sample. The total amount of the particles in the whole fluid was 38.1 vol%, and the amount of the semiconducting particles having a weight average particle size of 3.0 µm was 28 vol% of the total particles. When the shear stress of the sample when an electric field was applied thereto was measured in the same way as in Example 1, dielectric breakdown occurred at 1.4 k·Vmm -1 . A sufficient electroviscous effect was not produced. Comparative Example 2 The coke obtained in the same way as in Example 1 was pulverized to obtain semiconducting particles having a weight average particle size of 17 µm. 33.77 g of the semiconducting particles were mixed with 38.70 g of dioctyl phthalate and 1.91 g of the same dispersant as in Example 1, and the resultant mixture was thoroughly dispersed by a paint shaker. The amount of semiconducting particles in the whole fluid was 38.1 vol%. When the shear stress of the sample when an electric field was applied thereto was measured in the same way as in Example 1, dielectric breakdown occurred at 1.4 kV·mm -1 . A sufficient electroviscous effect was not produced. Comparative Example 3 145.03 g of BaTiO 3 particles used in Example 1 were mixed with 38.70 g of dioctyl phthalate and 1.91 g of the same dispersant as in Example 1, and the resultant mixture was thoroughly dispersed by a paint shaker. The amount of dielectric particles in the whole fluid was 38.1 vol%. When the sample was heated and allowed to stand in the same way as in Example 1, sedimentation of the particles was remarkable.

Claims: An electroviscous fluid comprising an electrical insulating liquid, semiconducting particles dispersed in said liquid and dielectric particles dispersed in said liquid, wherein : the semiconducting particles have a weight average particle size of 1 to 100 µm, the dielectric particles have a weight average particle size of 0.1 to 3 µm, the weight average particle size of the dielectric particles is not more than 30% of that of the semiconducting particles, the amount of the dielectric particles is 5 to 40 vol% based on the total particles, and the amount of the particles in the whole fluid is 10 to 60 vol%. An electroviscous fluid according to claim 1, wherein said electrical insulating liquid has a resistivity of not less than 10 9 Ωcm. An electroviscous fluid according to claim 1 or 2, wherein said electrical insulating liquid is dimethyl polysiloxane, dioctyl phthalate, dibutyl phthalate, diisononyl phthalate, trioctyl trimellitate, triisodecyl trimellitate, dibutyl adipate, butyl stearate, paraffin-based mineral oil or naphthene-based mineral oil. An electroviscous fluid according to any one of the preceding claims, wherein said semiconducting particles are composed of a material having an electrical conductivity of 10 -2 to 10 -10 S·cm -1 . An electroviscous fluid according to any one of the preceding claims, wherein said semiconducting particles are particles of a carbonaceous material obtainable by heating a hydrocarbon at a temperature of not higher than 1000°C, copper phthalocyanine and polyacene quinone. An electroviscous fluid according to any one of the preceding claims, wherein said semiconducting particles have a weight average particle size of 5 to 50 µm. An electroviscous fluid according to any one of the preceding claims, wherein said semiconducting particles have a specific gravity of not more than 3. An electroviscous fluid according to any one of the preceding claims, wherein said dielectric particles have a dielectric constant of not less than 100. An electroviscous fluid according to any one of the preceding claims wherein said dielectric particles are particles of BaTiO 3 , (Ba, Sr, Ca)TiO 3 , (Ba, Ca) (Zr, Ti)O 3 , Pb(Zn, Nb)O 3 , Pb(Fe, Nb)O 3 , Pb(Mg, Nb)O 3 and Pb(Fe, W)O 3 . An electroviscous fluid according to any one of the preceding claims, wherein said dielectric particles have a weight average particle size of 0.3 to 2 µm.

Inventors: CHIDA YUKIO, SHIMA KOJI, WATANABE KENJI, YAMAMOTO IWAO, CHIDA, YUKIO, SHIMA, KOJI, WATANABE, KENJI, YAMAMOTO, IWAO
Applicants: MITSUBISHI CHEM CORP, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD., MITSUBISHI CHEMICAL CORPORATION
Application year: 1992
Main classification: C10M171
IPC codes: C10N40, B01J13, C10N20, C10N30, C10M169, C10M171