WO1993003124A1

WO1993003124A1 - Electroviscous fluid

Info

Publication number: WO1993003124A1
Application number: PCT/JP1992/000973
Authority: WO
Inventors: Tetsuo Miyamoto; Hiroshi Nakanishi; Hirotaka Tomizawa
Original assignee: Tonen Corporation
Priority date: 1991-07-31
Filing date: 1992-07-31
Publication date: 1993-02-18
Also published as: EP0555487A4; EP0555487A1

Abstract

An electroviscous fluid prepared by dispersing fine solid particles in an electrically insulating fluid. The fine solid particles have a specific gravity equal to, or greater than, that of the elctrically insulating fluid, and have a hydroxyl group on the surface thereof. The hydroxyl group is covalently bonded to a nonionic surfactant having a hydroxyl group and/or an -NH group as a hydrophilic group through a silane coupling agent, or polystyrene is covalently bonded thereto. The fluid is excellent in response, reproducibility, durability and thickening effect, particularly in dispersibility and shelf life stability, and can be utilized for electrical control of vibration control machines such as variable damping dampers, engine mounts, bearing dampers, clutches, valves, shock absorbers, precision equipment, acoustic instruments, etc., and for display devices.

Description

Specification

Electrorheological fluid

Technical field

The present invention relates to an electrorheological fluid whose viscosity can be controlled by applying a voltage, to vibration of a variable damper, an engine mount, a bearing damper, a clutch, a valve, a shock absorber, a precision machine, an acoustic machine, and the like. The present invention relates to an electrorheological fluid that can be used for electrical control of a control mechanical device or a display element, and particularly to an electrorheological fluid having improved dispersibility.

Background technology

Electro-rheological fluids (Ble ctro-Rheological Fluids Blectroviscous Fluid する), whose viscosity changes when voltage is applied, have been known for a long time (Duff, A. tt. Physical Review Vol. 4, 4, No. 1 (1896) twenty three) . Initial research on electrorheological fluids focused on liquid-only systems, and their effects were inadequate.After that, they moved on to research on solid-dispersion electrorheological fluids. Can now be obtained. One

For example, Klass describes the mechanism of the onset of the thickening effect (ER effect) in an electrorheological fluid. Each particle, which is a dispersoid in an electrorheological fluid, has a two-layer dielectric polarization in the electric field. ), Which is considered to be the main cause (Klass, D 丄, et al., J. of Applied Physics, Vol. 38, No. 1 (1967) 67). Explaining this from the electric double layer, the ion adsorbed around the dispersoid (silica gel, etc.) is uniformly arranged on the outer surface of the dispersoid when E (electric field) = 0. However, when E (electric field) = finite value, the ion distribution is biased, and each particle mutually acts electrostatically in the electric field. In this way, each particle forms a bridge (crosslink) between the electrodes and develops a shear resistance against stress, that is, an ER effect is exhibited.

Wins low proposed paraffin and silica gel powder, and an electrorheological fluid using water to make the system slightly conductive (Winslow, WM, J. of Applied Physics, Vol. 20 (1949)). 1137). According to this Winslow study, the electrorheological effect of the electrorheological fluid is called the Winslow effect.

As the electrically insulating fluid in such an electrorheological fluid of a face-body dispersion system, although the specific gravity of the base oil is usually 0.78 to 92 (15 t :), face particles, for example, The specific gravity of silica gel powder is about 2.2, and the difference in specific gravity is large.Even if a dispersant is used, the sedimentation is high, so that the electrorheological effect fluctuates during operation or deteriorates with time. Cause unfavorable ER effect

0

An object of the present invention is to provide an electrorheological fluid having more excellent dispersibility by imparting dispersibility to silica fine particles as a dispersoid.

Disclosure of the invention

The electrorheological fluid of the present invention is obtained by dispersing solid fine particles in an electric insulating fluid, the solid fine particles having a specific gravity equal to or higher than that of the electric insulating fluid, and having a hydroxyl group on the surface thereof, The glacial group is converted to a chlorophilic group and Z or 1N as a permanent group via a silane coupling agent. It is characterized by being covalently bonded to a nonionic surfactant having an H group.

Further, the electrorheological fluid of the present invention is obtained by dispersing solid fine particles in an electric insulating fluid, and the solid fine particles have a specific gravity equal to or higher than that of the electric insulating fluid, and have a hydroxyl group on a surface thereof. It is characterized in that polystyrene having a degree of polymerization of 5 to 32 and poly or polyisoprene are covalently bonded to the hydroxyl group.

In an electrorheological fluid, for example, when silica fine particles are dispersed in an electrically insulating fluid, a dispersant is usually used. In such an electrorheological fluid, it is presumed that the dispersant physically adsorbs to the silica fine particles and the repellent action of the adsorbed dispersants causes a dispersion effect, but the dispersion stability is poor. , Which causes the electrorheological results to decrease due to the sedimentation of silica particles.o

The dispersoid in the present invention has a specific gravity equal to or higher than that of the electrically insulating fluid, and has a hydroxyl group and a Z or mono NH group as hydrophilic groups via a silane coupling agent on solid fine particles having a hydroxyl group on the surface. Or a polystyrene covalently bonded to a non-ionic surfactant having the formula (1), whereby a stable dispersibility can be obtained in an electrically insulating fluid. BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a diagram showing the dispersion stability of the electrorheological fluid of the present invention. FIG. 2 is a diagram showing the dispersion stability of the electrorheological fluid of the present invention.o Best Mode for Carrying Out the Invention The electrically insulating fluid is not particularly limited, and examples thereof include mineral oil and synthetic lubricating oil. Yes, specifically paraffinic mineral oil, naphthenic mineral oil, poly-α-olefin, polyalkylene glycol, silicone, ester, diester, polyol ester

, Phosphoric acid esters, silicon compounds, fluorine oils, oils such as alkylbenzenes, alkylbenzenes, alkenylbiphenyls, alkylnaphthalenes, polyfuunyl ethers, and synthetic hydrocarbons.The viscosity range is 4 O: In this case, those with 5 to 300 cSt can be used.

The solid fine particles have a specific gravity equal to or higher than that of the electrically insulating fluid and have a hydroxyl group on the surface thereof, and specific examples thereof include silica gel and zeolite. For example, silica gel fine particles having a specific gravity of 2.2 (15 t :), a particle size of 0.01 to 200 jum, a surface area of 100 to 70 OrnVg (BET method), and a silanol group density of 1 to it can be used as the 1 0 SiOH / 100 a ^2.

First, a description will be given of the planar fine particles in which a nonionic surfactant is bonded to the surface of the planar fine particles.

The nonionic surfactant that is covalently bonded to the surface of the solid fine particles needs to have a glacial group and a Z or 1 NH group as a permanent group, and specifically, a polyalkylenylsuccinic acid imidate, Succinate, Polyethylene ethylene ether, Polio Examples thereof include xylethylene alkyl ether, and fatty acid esters of polyhydric alcohols having a hydroxyl group.

As the polyalkenyl succinic acid imid, a mono-form succinic acid imid represented by the following general formula (1) is used.

General formula (1)

(In the formula, R, represents a group of carbon atoms having 30 or more carbon atoms, R 2 represents a C ₂ -C ₄ alkylene group, and m represents an integer of 1-10.)

This polyalkenyl succinic acid imide has a structure in which the polyolefin polymer represented by R, has a carbon number of 30 £ 1 or more, preferably 40 to 400, and an average molecular weight of 500 to 5, As long as it is 0 0 0, the off-line used for the production may be, for example, carbon such as ethylene, propylene, 1-butene, isobutylene, 1-hexene, 2-methylpentene 1-1, 1-octene, etc. Numbers 2 to 8 can be used. The polyolefin polymer is preferably polypropylene or polybutylene.

As the polyalkylenepolyamine, a polyalkylenepolyamine having a number of repeating units m of 1 to 10 in each of the above formulas is preferably used. Examples of the polyalkylenepolyamine include polyethylenepolyamine, polypropylenepolyamine, Lithylene polyamine and the like, and polyethylene polyamine is particularly preferable. As a method for preparing this dispersoid, for example, silica fine particles and a silane coupling agent are added in an amount of 50 to 400 parts by weight, preferably 80 to 100 parts by weight, based on 100 parts by weight of the silica fine particles. In a solvent at a ratio of 200 parts by weight, the silane coupling agent is bonded to the silanol group in the silica fine particles, and the above reaction product and polyalkenyl succinic acid imide are mixed under reflux conditions. Silane coupling by reacting 100 parts by weight to 100 parts by weight, preferably 200 parts by weight to 800 parts by weight with respect to 0 parts by weight of silica fine particles 1 Q The polyalkenyl succinic acid imide can be bound to the fine particles of silylation via an agent. At this time, when an alcohol such as T1 butanol phenol is added as a catalyst, the binding amount of the polyalkylenylsuccinic acid imid can be increased. These catalysts may be added in an amount of from 20 to 300 parts by weight, preferably from 50 to 200 parts by weight, based on 100 parts by weight of polyalkyl succinic acid imide.

Examples of such silane coupling agents include r-chloropropyl'trimethoxysilane, β- (3,4-epoxycyclohexyl) ethyl'methoxysilane, τ-glycidoxypropyltrimethoxysilane, and r-isopropane. Suitable are cyanate propyl triethoxysilane, r-glycidoxy π-pyrmethyljetoxysilane, and the like.

Next, a description will be given of planar fine particles in which polystyrene, Z, or polystyrene is bonded to the surface of the solid fine particles.

In order to covalently bond polystyrene and / or polysoprene having a degree of polymerization of 5 to 30 Reacts with thionyl chloride to chlorinate the silanol groups in the silica fine particles. On the other hand, it is prepared by anionic polymerization of n-butyllithium and styrene to prepare anionic polystyrene, and reacting the anionic polystyrene with the chlorinated silica fine particles prepared above in a solvent.

The polystyrene chain length can be easily controlled by adjusting the reaction temperature. In the present invention, the degree of polymerization of polystyrene is from 5 to 320, preferably from 130 to 280. If the degree of polymerization is 5 or less, the lipophilicity is low, and the silica fine particles aggregate. The proportion of polystyrene in the dispersoid depends on the amount of the silanol group, but is usually 14 to 16% by weight. Although the description has been given of the case where styrene is used, the same applies to isoprene, and the same impeachment as in the case of polystyrene can be obtained by covalently bonding polysoprene to fine particles of silicic acid. Further, a mixture of polystyrene and polyisoprene may be used.

The dispersoid (hedron fine particles) thus prepared is used in a ratio of 0.1% by weight to 50% by weight in the entire electroviscous fluid. To the electrorheological fluid of the present invention, for example, polyvalent alcohol or a partial derivative thereof may be added as a polarization promoter.

Polyhydric alcohols include dihydric alcohol, trihydric alcohol, for example, ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol, glycerin, and proha. Diol, butanediol , Pentanediol, hexanediol, etc. O

The partial derivative of the polyhydric alcohol is a partial derivative of a polyhydric alcohol having at least one hydroxyl group, and some of the terminal hydroxyl groups of the polyhydric alcohol are a methyl group or an ethyl group.

, A propyl group, a partial ether substituted by an alkyl-substituted phenyl group (the alkyl group substituted with a phenyl group has 1 to 25 carbon atoms), etc. Examples thereof include partial esters obtained by esterification with acetic acid, propionic acid, acetic acid, or the like.

These polyvalent alcohols or partial derivatives thereof are generally used in an amount of 1 to 100% by weight, particularly preferably 2 to 80% by weight, based on the dispersoid. If the addition amount is less than 1% by weight, the ER effect is small, and if it exceeds 100% by weight, the current easily flows, which is not preferable. Of course, water may be used together with the polyhydric alcohol to such an extent that the ER effect is not inhibited. Further, an acid, salt, or base component may be further added. Such acid components include sulfuric acid, hydrochloric acid, nitric acid, perchloric acid, chromic acid, phosphoric acid, boric acid, and other inorganic acids, or acetic acid, formic acid, propionic acid, severe acids, isoacetic acid, valeric acid, and oxalic acid. Organic acids such as acid and pi-acid are used.

谊 is a compound comprising a metal or a basic group (NH ₄ ⁺ , N ₂ H ₅ + etc.) and an acid group, and any of these can be used. Among them, those that dissolve in polyhydric alcohols or polyhydric alcohol partial derivatives and dissolve in the system, for example, (Iii) Those which form typical ion crystals such as halides of earth metals, or alkali metal salts of organic acids are preferred. As seeds salt This, L i C l, NaC l , KC l, MgC l 2, CaC l 2, B a C 12. L i B r, NaB r, KB r, Mg Br 2, L i I, _{N a KI, AgN0 3, C} a (N0 3) 2, NaN0 2, NH 4 N0 3, K 2 S0 4, N a 2 S0 4, N a HS 0 4, (NH 4) 2 S_〇 ₄ or formic acid There are metal salts such as acetic acid, oxalic acid and succinic acid.

Examples of the base include alkoxides of alkaline metal or alkaline earth metal, carbonates of alkaline metal, amines, etc., polyhydric alcohols, partial derivatives of polyhydric alcohols, and polyhydric alcohols. Those which dissolve in a system of Z or a polyhydric alcohol partial derivative and water and dissociate are preferable. As this type of base, Na_〇_H, KOH, C a (OH) 2, N a 2 C_〇 _3, NaHCO _3, K ₃ P_〇 _{_4,} N a ₃ P0 _4, Aniri down, Arukiruami down, ethano Ruami down and so on. In addition, the salt and the base described above can be used in combination. Acids, salts, and bases can increase the polarization effect, but when used in combination with polyhydric alcohol and / or a partial derivative of a polyhydric alcohol, the polarization effect can be further increased. It is recommended that the electrorheological fluid be used at a rate of 0.01 to 0.5% by weight. If the content is less than 0.01% by weight, the ER effect is small, and if it exceeds 5% by weight, energization becomes weak and power consumption increases. When an acid, salt, or base component is added to the electrorheological fluid of the present invention, the partially esterified product of the polyhydric alcohol must not hydrolyze. is necessary.

If necessary, an antioxidant, a corrosion inhibitor, an antiwear agent, an extreme pressure agent, an antifoaming agent, etc. may be added to the electrorheological fluid of the present invention.

The purpose of the antioxidant is to prevent the oxidation of the electrically insulating liquid and the polyhydric alcohol and partial derivatives of the polyhydric alcohol that are polarization promoters.

As the antioxidant, a substance which is inert to a polarization accelerator, a dispersoid, or the like may be used, and a commonly used phenol-based or oxamine-based antioxidant can be used. As a system

2-6-di-t-butylparacresol, 4 * 4'-methylenebis (2.6-di-t-butylphenol), 2-6-di-t-butylphenol, etc., and amine-based dioctyldiphenylamine Phenyl-α-naphthylamine, alkyldiphenylamine, dinitrosodiphenylamine, etc., which can be used in an amount of 0.01 to 10% by weight, preferably 0.1% by weight, based on the whole electrorheological fluid. It can be used at a ratio of from 2.0% by weight to 2.0% by weight. If it is less than 0.01% by weight, there is no antioxidant effect. There are problems such as occurrence of viscousness and increase in viscosity.

In addition, a corrosion inhibitor may be added, but it is preferable to use an inert substance such as a polarization accelerator or a dispersoid. Specifically, for nitrogen compounds, penzotriazole and its derivatives, imidazoline, and pyrimidine are used. For compounds containing zeolite and nitrogen, such as derivatives, 1.3.4-dithiolpolysulfide and L3.4-thiadiazolyl-2.5-bi It is also possible to use sujirujiruchicarpa 'mate, 2- (alkyldithio) benzomidazol, etc., / 9- (0-carboxybenzylthio) propionnitrile or propionic acid, etc. It may be used in a proportion of 0.001% to 10% by weight, preferably 0.01% to 1.0% by weight, based on the whole electrorheological fluid. If the content is less than 0.001% by weight, there is no corrosion prevention effect. If the content is more than 10% by weight, there are problems such as deterioration of hue, generation of turbidity, generation of sludge, and increase in viscosity.

Hereinafter, the present invention will be described based on specific examples.

[Example 1]

(Preparation of dispersoid)

A mixture of 10 g of silica fine particles (particle size 4 βτη), 20 ml of water, and 200 ml of dioxane was mixed in a ball mill for 60 hours, and then r-glycidoxyprovir trimethoxysilane 10 m 1 Was added and mixed for another 12 hours.

After the silica fine particles were centrifuged (14000 rpm, 60 min), they were added to 300 ml of dioxane, mixed with a ball mill for 5 hours, and then 100 ml of dioxane was distilled off in 30 minutes.

To the residual system were added 60 g of polyalkenyl succinic acid imide and 100 ml of dioxane, and the mixture was refluxed for 10 hours.

After refluxing, the mixture was dispersed in toluene, centrifuged to remove unreacted polyalkenyl succinic acid imide, and allowed to stand for 12 hours to remove sediment.The supernatant was concentrated under reduced pressure at 7, After drying for 2 hours, 12 g of solid fine particles to which polyalkylenylsuccinic acid imide was bound was obtained. Conversion is 76%. The silica fine particles were analyzed by IR, combustion TCD detection method and the like, and it was confirmed that polyalkenyl succinic acid imide was bound to the silica fine particles.

(Preparation of electrorheological fluid)

An electrorheological fluid having the following composition was prepared and used as sample oil 1 (viscosity 4 O cSt (40)).

(1) Alkylbenzene (viscosity 25 cSt (40), specific gravity 0.88) · · · · 92.5 parts by weight

(2) Dispersoid prepared above

(3) Triethylene glycol-2.0 parts by weight

[Example 2]

In the preparation of the dispersoid in Example 1, a similar dispersoid was prepared using the same amount of r-clovic provir trimethoxysilane in place of r-daricidoxyproviltrimethoxysilane. Sample oil 2) was prepared.

[Example 3]

(Preparation of dispersoid)

A mixture of 10 g of silica fine particles (particle diameter: 1.4 m), 20 g of water, 20 ml of dioxane, and 20 ml of dioxane was mixed in a ball mill for 60 hours, and then r-glycidoxyprovir trimethoxysilane was added to the mixture. l was added and mixed briefly for 12 hours.

After centrifugal separation (O.O.rpm, 60 min), the silica fine particles were added to 300 ml of dioxane, mixed for 5 hours by a ball mill, and then dioxane 10 Orn-l was distilled off in 30 minutes.

60 g of polyalkenyl succinic acid imid in the residual system, dioxa 100 ml of n-butanol was further added, and 15 O ml of n-butanol was further added as a catalyst, and the mixture was refluxed for 10 hours.

After refluxing, the residue was dispersed in toluene, centrifuged to remove unreacted polyalkylenylsuccinic acid imide, and left for 12 hours to remove sediment.The supernatant was concentrated under reduced pressure at 7 After drying for 2 hours, 12 g of solid fine particles to which polyalkylenylsuccinic acid imide was bound was obtained. Conversion is 76%.

The silica fine particles were analyzed by IR, combustion TCD detection method and the like, and it was confirmed that polyalkenyl succinic acid imide was bound to the silica fine particles.

Using the dispersoid thus prepared, an electrorheological fluid (sample oil 3) was prepared in the same manner as in Example 1.

[Comparative Example 1]

An electrorheological fluid [viscosity 4 ϋ cSt (4G :)] with the following composition was adjusted and used as comparative oil 1.

(1) Alkylbenzene (viscosity 25 cSt (40), specific gravity 0.88) * 89.0 parts by weight

(2) Silica gel (particle size 1.4 βτη) · ♦ 4.0 parts by weight

(3) Triethylene glycol

(4) Imidium succinate5.0 parts by weight After thoroughly mixing sample oil 1 and comparative oil 1, each with an inner diameter of 13 ram

Each 12.5 g of a test tube having a height of 1 O cm was put into the test tube, and the sedimentation amount of the porous solid particles was measured.

The results are shown in Fig. 1, where the horizontal axis is the number of days elapsed and the vertical axis is the height of the sediment (ram). As can be seen from the figure, sample oil 1 has an extremely low sedimentation amount and is excellent in dispersibility. In addition, when the settling amounts of the porous face particles were measured for sample oil 2 and sample oil 3 in the same manner as for sample oil 1, results equivalent to or higher than sample oil 1 were obtained. In particular, no precipitation was observed for sample oil 3 after about one year.

Next, for the sample oils 1 to 3 and the comparative oil 1 prepared above, the following items were measured at 40 t and 90 using a rotational viscometer capable of applying a voltage. Ο was evaluated as a viscous fluid.

• Responsiveness-Evaluated by how long the viscosity stabilizes when the AC electric field is changed from 0 (V / ra) to 2.0 X 10 ^B (V / m).

• reproducibility one alternating electric field to 0 (V / m) → 2 . 0 x lO 6 (V / m) → 0 when it returns manipulate the cycle of (V / m), the electric field 1. 4 x lO ⁶ (V / Evaluated by the rate of change in viscosity at m).

* When the durability one alternating field 2. that has 0 x lO ⁶ is constant (V / ni), evaluated by the amount of change in viscosity over time (%) (measurement time 5 0 hours). (The decrease in viscosity occurs due to the large amount of sedimentation of the porous face particles.)

• Thickening effect-Compared to when the electric field is Q (V / ra), the evaluation is based on the magnification of the viscosity when the AC electric field is set to 1.4 x lO ^e (V / m).

The results are shown in Table 1 below. table 1

It can be seen that the electrorheological fluid of the present invention is not inferior to the comparative oil in response, reproducibility, durability and thickening effect.

[Example 4]

(Preparation of dispersoid)

After refluxing 10 g of silica fine particles (particle size: 1.4 m) and 30 O ml of thionyl chloride under a nitrogen stream for 48 hours, the excess thionyl chloride was distilled off under reduced pressure, and further 140 And dried under reduced pressure for 60 hours.

On the other hand, 50 g of styrene was added dropwise to a tetrahydrofuran solution containing 0.1 g of n-butyllithium while maintaining the reaction temperature at 178, and the anion having an average molecular weight of 260,644 was added. Polystyrene (250-mers) was obtained.

The chlorinated silica fine particles obtained above were mixed with the obtained anionic polystyrene and reacted for 18 hours. After the reaction, The amount of methanol was added to inactivate the macromonomer.

The resulting reaction dispersion was washed three times by centrifugation using benzene to remove unreacted substances, and the particles were dried under reduced pressure. The tilling rate was 88%.

Observation of the particle size with an electron microscope revealed that the particle size was the same before and after the reaction.

The silica fine particles were analyzed by IR, combustion TCD detection method, etc. to confirm that polystyrene was bonded to the silica fine particles.

(Preparation of electrorheological fluid)

An electrorheological fluid having the following composition was adjusted to sample oil 4 (viscosity 45 cSt (40 t :)).

(1) Alkylbenzene (viscosity 25 cSt (at 40), specific gravity 0.88) · · · · 91.5 parts by weight

(2) Dispersoid prepared above 6.5 parts by weight

(3) Triethylene glycol2.0 parts by weight

[Comparative Example 2]

An electrorheological fluid [viscosity 45 cSt (40 :)] having the following composition was adjusted to make Comparative Oil 2.

(1) Alkylbenzene (viscosity 25 cSt (40), specific gravity 0.88) · ♦ · · 86.5 parts by weight

(2) Particle size of silica gel 1.4 im) 5.5 parts by weight

(3) Triethylene glycol2.0 parts by weight

(4) Imidium succinate-6.0 parts by weight The sedimentation amount of the porous solid particles was measured in the same manner as in Example 1. The result The results are shown in FIG.

As can be seen from the figure, the electrorheological fluid of the present invention has an extremely low sedimentation amount and is excellent in dispersibility.

Sample oil 4 and comparative oil 2 were evaluated as electroviscous fluids in the same manner as in Example 1. The results are shown in Table 2.

Table 2

As can be seen from the table, the electrorheological fluid of the present invention is comparable to the comparative oil in terms of responsiveness, reproducibility, thickening effect, and durability.

Industrial applicability

The electrorheological fluid of the present invention is an electrorheological fluid with particularly improved dispersibility, and includes a variable damper, an engine mount, a bearing damper, a clutch, a valve, a shock absorber, a precision machine, and a sound machine. It can be used for electrical control of vibration control mechanical devices and display elements.

Claims

The scope of the claims

(1) Solid fine particles are dispersed in an electrically insulating fluid, the solid fine particles have a specific gravity equal to or higher than that of the electrically insulating fluid, and have a permanent group on the surface thereof, and the hydroxyl group is a silane. An electrorheological fluid that is covalently bonded to a non-ionic surfactant having a glacial group and a Z or 1 NH group as hydrophilic groups via a force coupling agent.

(2) Solid fine particles are dispersed in an electrically insulating fluid, and the solid particles have a specific gravity equal to or higher than that of the electric fringing fluid, and have a permanent acid group on the surface thereof. An electroviscous fluid in which polystyrene having a degree of polymerization of 5 to 320 and Z or polysoprene are covalently bonded to an acid group.