JPH07103392B2 - Electrorheological fluid - Google Patents

Description

TECHNICAL FIELD The present invention relates to an electrorheological fluid whose viscosity can be changed by voltage control, particularly a fluid in which solid particles containing no water are dispersed in an electrically insulating oily medium. , Actuators for clutches, valves, shock absorbers, etc.

[Prior Art] When an external electric field is applied to a fluid in which water-containing solid fine particles such as silica, starch, and cellulose are dispersed in an electrically insulating oily medium such as transformer oil, spindle oil, or chlorinated paraffin, the viscosity of the fluid is increased. There is a phenomenon in which is significantly increased. This phenomenon has long been known as the Winslow effect, and its application to clutches, valves, vibration elements, etc.
It has been considered since the generation.

Many proposals have been made so far as methods for enhancing the Winslow effect. For example, one obtained by dispersing water-containing fine particles of a water-containing strongly acidic or strongly basic ion exchange resin in a higher alkyl ester of an aromatic carboxylic acid (Japanese Patent Laid-Open No. 50-92278), a halogenated diaryl compound or It is known that a dispersion of hydrophilic solid particles containing water in a silicone oil (JP-A-58-501178 or JP-A-61-44998) exhibits an excellent electrorheological effect.

[Problems to be Solved by the Invention] Most of the electrorheological fluids that have been conventionally proposed are those in which hydrophilic solid fine particles are made to contain water and dispersed in an insulating oily medium. The electric double layer formed on the surface of the particles due to the presence of water causes the movement of free ions by the application of a high voltage from the outside to cause polarization. This polarization charge causes cross-linking between particles in the direction of the electric field due to electrostatic attraction, which resists shearing force in the direction perpendicular to the cross-linking, increasing viscosity.
It is based on the so-called electric double layer theory.

By the way, the conventional electrorheological fluid using such water-containing fine particles lacks stability due to migration of water in and out of the particles.
Insufficient durability such as melting of electrode metal due to high voltage application, further problem of low temperature characteristics such as ionization is promoted when temperature rises, current increases and temperature further rises.
Many problems attributed to the presence of water have hampered its commercialization despite its promise of major applications.

On the other hand, in response to the presence of such water, there have been few proposals to use particles of a ferroelectric substance or a semiconductor instead of the water-containing particles. For example, a method of dispersing particles of calcium titanate, which is a ferroelectric substance, in naphthenic insulating oil [J.Appl.Physics, 38 (1) 67 (1967)], or organic semiconductors such as poly (acene-quinone) A method of dispersing particles in insulating oil (Japanese Patent Laid-Open No. 61-216202) and the like are all problematic for practical use, such as low electrorheological effect and too high current amount.

By the way, Japanese Patent Application No. 61-241929 discloses an electrothermoviscous fluid in which solid particles containing no water are dispersed in an electrically insulating oily medium. In this electrorheological fluid, the dielectric particles are particles having a multi-layered structure in which a conductive thin film layer and then an electrically insulating thin film layer are formed on the surface of the organic solid particles as the center. Therefore, it is possible to solve problems such as poor long-term stability due to the presence of water, elution of electrodes, and further decrease in temperature dependency of electrorheological characteristics. However, at least two laminating steps are required in order to coat the organic solid particles with the conductive thin film layer to form the conductive particles, and to further stack the electrically insulating thin film layer thereon. Further, as a background that requires such a complicated laminating step, when using particles obtained by covering the surface of metal particles that are conductors with an insulating coating, it is possible to ensure the uniformity of the conductive thin film layer. There are problems such as deterioration prevention.

The present invention provides a new practical electrorheological fluid that solves the above problems.

[Means for Solving Problems] If the Winslow effect is based on polarization of water on the surface of fine particles due to an external electric field, there is another method for causing polarization on the surface of fine particles without the presence of water. There is potential for new electrorheological fluids that do not have many problems due to the presence of water such as.

Based on such an idea, the present invention has explored a new non-hydrous particle type electrorheological fluid and conducted many experiments, and as a result, uses a dielectric fine particle whose surface is coated with an electrically insulating thin film. As a result, they have found that they exhibit an excellent electrorheological effect without the presence of water, and have reached the present invention. That is, the present invention is an electrorheological fluid in which dielectric particles are dispersed in an oily medium having excellent electrical insulation, wherein the dielectric fine particles are homogeneous materials having an electrical resistance of 10 ⁴ Ω · cm or less. The surface of the conductive particles consisting of is coated with an insulating thin film with a thickness of 1 μm or less, and the dielectric particles are 10 ⁸ Ω · cm.
An electrorheological fluid having the above electrical resistance value and having a water content of 1% by weight or less in the dielectric fine particles. Here, the homogeneous material means that it does not have at least a layered structure, and includes a complex of a conductive substance and a non-conductive substance as described later.

Examples of the conductor particles used in the present invention include metals or alloys such as aluminum, nickel, copper, tin, silicon, duralumin, and silmine (aluminum-silicon alloy), conductive carbon black, carbon allotropes such as graphite,
Organic conductive polymers based on polythiophene, polyacetylene, polypyrrole, etc., copper sulfide, indium oxide, ferric oxide, titanium boride, zinc boride, tungsten carbide, conductive compounds such as zinc carbide, lithium perchlorate / Ethylene carbonate / polyacrylonitrile, solid electrolyte such as solid sulfuric acid, etc. Also, a mixture of these substances or a non-conductive substance such as a complex with an insulating synthetic polymer or cellulose and having conductivity can be used. In any case, any substance having an electric resistance of 10 ⁴ or less, preferably 10 ² or less [unit: Ω · cm or Ω] can be basically used.

The shape of these particles is preferably spherical or elliptical with a rounded shape as much as possible, rather than having sharp corners, and spherical is most preferable. The particle size is preferably 1 μm to several tens of μm,
In particular, a smaller particle size is preferable from the viewpoint of preventing particle settling and sliding friction. However, if it is less than 1 μm, cross-linking between particles is weak, which is not preferable. Regarding the particle size distribution, a monodisperse particle as close as possible tends to exhibit stable electrorheological characteristics.

When using a metal-based material as conductive particles in the present invention,
The particles tend to settle due to the difference in specific gravity from the oil medium. Examples of methods for preventing this include a method of using an oily medium having a high specific gravity and a method of forming hollow, porous or closed cell-containing particles. A hollow shape is particularly preferable. Also, a few μ
The use of particles having a diameter of about m is also effective for preventing sedimentation.

Next, the electrically insulating thin film is a thin film made of an organic and inorganic electrically insulating material formed on the surface of the conductive particles, and is placed under an electric field by forming this thin film on the surface of the conductive particles. The polarization charge generated on the surface of conductive particles can easily cause charge neutralization by contact between particles, or can form a conductive path between electrodes to cause dielectric breakdown with sparks. Can be prevented.

Organic and inorganic insulating substances that can be used for such purposes include polyvinylidene fluoride, polyimide, polyamide, polyacrylonitrile and other organic synthetic polymers,
Typical examples are organic natural polymers such as wax, asphalt and varnish, and inorganic compounds such as silica, alumina, aluminum hydroxide and barium titanate. Generally, the volume or surface electric resistance is 10 ⁸ or more, preferably 10 ¹⁰ or more (unit [Ω · cm] or [Ω]),
It is preferable that the dielectric breakdown strength and the dielectric constant are as high as possible. The thickness of the insulating film is preferably as thin as possible, and is generally less than 1 μm, unless dielectric breakdown or charge neutralization occurs. The Coulomb force between particles is extremely small at 1 μm or more, and conversely tends to become large at 0.5 μm or less.
If it is 0.5 μm or less, an excellent electrorheological effect can be obtained.

For the formation of insulating thin film, solution or powder coating,
Known coating methods such as surface polymerization, vapor deposition and surface reaction can be applied. The insulating thin film preferably coats the entire surface of the particles with a uniform thickness. During coating, it is also important to prevent the generation of secondary particles as much as possible. As an example of such a coating method, industrial library 25 "microcapsules"
(Asahi Kondo, Nikkan Kogyo Shimbun) or various chemical methods such as selective oxidation or nitriding of the surface of the conductive thin film are preferable.

In practice, those having these insulating thin films formed on the surface of the conductive particles require a considerable adhesive force at their interfaces. Physical treatment, coupling agent,
In many cases, the treatment of a binding aid such as an anchor coating agent is effective. When metal particles such as aluminum particles or silicon particles are used as conductive particles, the method of directly forming an insulating film on the surface of the particles by a chemical treatment such as oxidation or nitriding does not have a problem of interfacial adhesion and is uniform. It is also relatively easy and preferable to form such a film to a desired thickness.

The substantially water-free particles referred to in the present invention are those that are substituted with acetone, alcohol, or the like, or do not contain attached water that is removed by high-temperature vacuum drying, specifically, 200 Moisture content is 1% or less when measured by Karl Fischer moisture by heating at ℃, generally 0.5%
In the case of the conventional water-containing particle-based electrorheological fluid, the particle water content of this level is in a range in which almost no electrorheological effect is exhibited.

The oily medium used in the present invention has been used in conventional electrorheological fluids. Diphenyl chloride, butyl sebacate, aromatic polycarboxylic acid higher alcohol ester, halophenyl alkyl ether, trans oil, chlorinated paraffin,
Of course, fluorine-based oils, silicone-based oils, etc., as long as they have high electrical insulation properties and electrical breakdown strength, are chemically stable, and have a not so large difference in specific gravity from dispersed fine particles,
Basically usable.

The mixing volume ratio of the dielectric particles of the present invention with the oil medium is 1
It is selected in the range of 99:50 to 50:50, preferably 5:95 to 40:60. Additives can be used in the mixed electrorheological fluid for the purpose of stabilizing dispersion, preventing rust, and preventing oxidation, as long as the electrical insulating property is not significantly deteriorated.

By the way, the electrorheological fluid of the present invention has the following characteristics which are completely different from the conventional water-containing particle-based electrorheological fluid. That is, the latter has an electrorheological effect on the application of AC and DC voltages, while the former shows AC but almost no DC. Although the mechanism of manifestation of the electrorheological effect of the fluid of the present invention is not clear, it is not due to the formation of the electric double layer on the particle surface proposed in the conventional hydrous particle system, but is related to the dielectric dispersion under alternating current. It is considered to be a thing.

[Operation] The electrorheological fluid of the present invention solves the problems of long-term stability due to the presence of water, electrode corrosion, and large change in electrorheological effect due to temperature, which are the greatest drawbacks of the conventional ones. However, it makes it possible to realize various electromechanical actuators such as compact, easily electrically controlled valves, clutches, and shock absorbers.

[Examples] Hereinafter, the present invention will be described in more detail with reference to Examples. The electrorheological characteristics in this example are 40 mm inside diameter with the same central axis.
Is evaluated by the method of measuring the shear stress and current generated when a voltage is applied between the cylinder and the sample fluid enclosed in the gap (1.0 mm) between the outer diameter 38 mm rotor and the sample fluid. is there.

Example 1 Spherical aluminum particles having an average particle diameter of 20 μm (US Valimet
Company H-3) was placed in a 1% aqueous solution of calcium hydroxide and stirred slowly at room temperature for 8 hours. After thoroughly washing the particles with water, the particles were transferred to methanol to replace the water almost completely, and then 120
It was vacuum dried at ℃ for 48 hours. This particle is 200 ℃ and 600 ℃
From the results of heating Karl Fischer moisture measurement and thermogravimetric analysis of heating up to 700 ° C., it was found that there was no free moisture and an aluminum hydroxide layer was formed on the surface to a thickness of about 0.25 μm. Immediately without absorbing the particles in a dry nitrogen atmosphere, the particles were immediately mixed and dispersed in dry tri-2-ethylhexyl trimellitate (Kao Soap Co., Ltd., Trimex T-08) at a particle concentration of 20 Vol% and sheared. At a speed of 200 sec ^-1 , the electro-viscous characteristics were evaluated when AC and DC voltages were applied.

Figure 1 shows the measurement results of applied voltage, generated shear stress and current. In the figure, curves A and a are plots of shear stress and current generated corresponding to the applied voltage when an alternating current having a frequency of 60 Hz is applied. Curves B and b are plots of shear stress and current generated corresponding to the applied voltage when a direct current is applied. The electric resistance of the particles of this example is 8 × 10 ¹¹ Ω · cm.
Met.

From the results shown in FIG. 1, it can be seen that the electrorheological fluid of the present invention produces little shear stress and current at DC, but exhibits sufficient electrorheological effect at AC.

Example 2 Aluminum particles surface-treated and dried in the same manner as in Example 1 were further heated to 650 ° C. in nitrogen gas to change the surface aluminum hydroxide layer to an aluminum oxide layer. It was mixed with insulating oil (Trimex T-08) and the electroviscous properties were evaluated.

As a conventional electrorheological fluid for comparison, hydrated cellulose particles (Avicel Asahi Kasei Corporation, water content 6.2w)
A similar mixed fluid was prepared per t%) to evaluate the electrorheological effect.

FIG. 2 shows the shear rates of the electrorheological fluid of this example and the conventional electrorheological fluid using cellulose particles.
The electrorheological characteristics at 200 sec ^-1 when an AC voltage is applied are shown.
In the figure, curves C and c are plots of shear stress and current corresponding to the applied voltage for the electrorheological fluid of this example, respectively. Curves D and d are plots of shear stress and current corresponding to the applied voltage for the electrorheological fluid of the conventional example, respectively. In addition,
The electric resistance of the particles of this example is 3 × 10 ¹² Ω · cm.

From the results of FIG. 2, it can be seen that, as in the case of FIG. 1, the electrorheological fluid of this example has a large shear stress in the alternating current and exhibits a sufficient electrorheological effect. In the conventional example, almost no shear stress is generated and the electrorheological effect is not sufficiently exhibited. In this example as well, almost no generation of shear stress and current was observed with DC as in Example 1.

Example 3 With respect to the electrorheological fluid of Example 2 and the electrorheological fluid of the conventional example (cellulose), the temperature dependence of the electrorheological characteristics was measured from room temperature to 100 ° C., and the results are shown in FIG. In the figure, curves E and e show changes in shear stress and current with temperature in the electrorheological fluid of this example, and curves F and f show changes in shear stress and current with temperature in the conventional example. The measurement was carried out at a shear rate of 200 sec ^-1 , while applying an AC voltage (60 Hz) of 3.0 KV / mm while raising the temperature at a rate of 2 ° C / min.

It can be seen from FIG. 3 that the electrorheological fluid of Example 2 exhibits extremely stable characteristics even with temperature changes.

Example 4 After the surface of granular aluminum particles having an average particle size of 3 μm was treated with a silane coupling agent, the mixture was placed in heptane at 65 ° C. and stirred at high speed, acrylonitrile containing 1 wt% of dibenzoyl peroxide was gradually added dropwise to the aluminum. A film of polyacrylonitrile was formed on the surface of the particles. From the results of microscopic observation and thermogravimetric analysis, it was estimated that the aluminum particles were almost uniformly coated with the polyacrylonitrile layer, and the coating thickness was about 0.12 μm.

Fluorine-based oil (KRYTOX 143AY from Dupont)
The particles were dispersed so as to have a particle concentration of 20 Vol%, the electrorheological characteristics were evaluated, and the obtained results are shown in FIG. In the figure, curves G and g are plots of shear stress and current corresponding to the applied voltage for the electrorheological fluid of this example, respectively. The electrical resistance of the particles is 2 × 10 ¹² Ω ・
It was cm.

The electrorheological fluid of this example has no particles settling for a long time,
It also showed excellent stability against sliding wear.

Example 5 A homogenous mixture of 84 wt% styrene, 5 wt% divinylbenzene, 10 wt% carbon black and 1 wt% dibenzoyl peroxide was polymerized at 80 ° C. in a 5 wd% polyvinyl alcohol aqueous solution with vigorous stirring to give an average particle size of 16 μm, Electric resistance 1 ×
Polymer microbeads of 10 ² Ω · cm were obtained.

On the surface of this polymer bead, polyvinylidene fluoride dissolved in dimethylacetamide was measured by a fluidized bed method to give an average film thickness of 0.
Coated to a thickness of 15 μm. The electrical resistance of the polyvinylidene fluoride used and the resulting coated beads are, respectively,
It was 4 × 10 ¹³ and 6 × 10 ¹² Ω · cm. The beads were added to the fluorine-based oil used in Example 4 at a bead concentration of 30 Vol%.
And dispersed so that the electrorheological characteristics were evaluated. As a result, the shear stress was 5.3g / cm ² and the current was 2m at the applied AC voltage of 3KV / mm.
Got A. Therefore, it can be seen that the excellent electrorheological effect is exhibited. The coating thickness of polyvinylidene fluoride is 0.6 μm.
And the shear stress of 1.1 μm was 4.1 g /
It was cm ² and 0.4 g / cm ² . The coating thickness is 1.1 μm
Is a comparative example.

Example 6 The potassium hydroxide treatment time used in Example 1 was changed, samples were respectively synthesized, and the electrorheological characteristics thereof were applied with an alternating voltage (60 Hz) of 3.0 KV / mm. Shown in the table.

From the results shown in Table 1, it can be seen that the treatment time, that is, the insulating film thickness is largely related to the generation of the shear stress, and that the shear stress obtained becomes remarkably low when it becomes 1 μm or more. The sample having a treatment time of 72 hr is a comparative example, and in the water containing state of several wt%, shear stress was observed when a DC voltage was applied.

[Effects of the Invention] As described above, in an electrorheological fluid in which dielectric particles are dispersed in an oil medium having excellent electrical insulation, the dielectric particles have an electrical resistance of 10 ⁴ Ω · cm or less. The surface of the conductive particles made of a homogeneous material having a thickness of 1 μm or less is covered with an insulating thin film, and the dielectric fine particles have an electric resistance value of 10 ⁸ Ω · cm or more. The content is configured to be 1% by weight or less. Therefore, compared to the previous application in which a plurality of layers are laminated on organic solid particles, it is easier to manufacture, and poor long-term stability due to the presence of water, electrode elution, and further decrease in temperature dependence of electrorheological characteristics. To solve the problems such as
Problems such as instability of fluid performance due to poor uniformity and deterioration of the conductive thin film layer can be solved. Therefore, it is extremely practical in actuators such as valves, clutches, vibration elements, and vibration absorption elements.

[Brief description of drawings]

1, 2 and 4 are diagrams showing the relationship between applied voltage and shear stress and current, and FIG. 3 is a diagram showing the relationship between measured temperature and shear stress and current.

Continuation of the front page (51) Int.Cl. ⁶ Identification code Office reference number FI technical display location C10M 103: 04 103: 06) A (C10M 107/00 107: 24 107: 38) (C10M 111/04 103: 04 107: 42) C10N 10:06 20:00 Z 20:06 A 40:14

Claims (1)

[Claims] 1. An electrorheological fluid in which dielectric particles are dispersed in an oil medium having excellent electrical insulation, wherein the dielectric particles are made of a homogeneous material having an electric resistance of 10 ⁴ Ω · cm or less. The surface of the conductive particles is coated with an insulating thin film having a thickness of 1 μm or less, the dielectric particles have an electric resistance value of 10 ⁸ Ω · cm or more, and the water content of the dielectric particles is 1% by weight. % Or less, an electrorheological fluid.