JPH01172496A - Improved electroviscous fluid - Google Patents

Abstract

PURPOSE:To control the precipitation of fine particles to thereby realize the long-term stability of an electroviscous fluid having the viscosity of which can be changed by controlling voltage, which comprises a dispersion of fine dielectric particles in an oily medium having excellent electrical insulating properties, by using fine dielectric particles comprising a hollow material into the cavity of which the medium cannot penetrate. CONSTITUTION:Spherical hollow silica particles obtained through interfacial polymerization by the sol-gel process are dipped in an aqueous solution of polyacrylic acid to infiltrate this solution into the pores of the hollow particle wall and are then oven-dried to fill the pores with the polymer, thus giving fine dielectric particles comprising a hollow material into the cavity of which an oily medium cannot penetrate. The fine dielectric particles thus obtained are dispersed into an oily medium having excellent electrical insulating properties (e.g., a silicone oil or transformer oil) to produce an electroviscous fluid. This technique enables the use of particles of metal (e.g., Al or Ni) as well as particles of silica, alumina, etc. for the fine dielectric particles.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to an electrorheological fluid whose viscosity can be changed by voltage control, particularly an electrorheological fluid which suppresses sedimentation of fine particles and has excellent long-term stability. Used in actuators such as valves, shock absorbers, etc.

[Conventional technology]

Water-containing fine particles such as silica, starch, ion exchange resin, etc.
The viscosity of a stream dispersed in an electrically insulating oily medium such as transformer oil, chlorinated paraffin, silicone oil, etc. changes significantly instantaneously and reversibly upon application of a voltage. This phenomenon is explained by Winslow
This effect has been known for a long time, and its application to clutches, valves, vibration elements, etc. is being considered.

The mainstream of electrorheological fluids is those that apply this Winslow effect, and as a method to enhance this effect, many proposals have been made so far, mainly using fine particles. Particles containing an aqueous solution of metal ions and polar substances (Japanese Patent Publication No. 49-5117), strongly acidic or strongly basic water-containing ion exchange resin particles (Japanese Patent Application Laid-open No. 50-92278), polyacrylic acid Highly absorbent resin particles with acidic groups such as
3-93186), a method using water-containing fine particles,
In addition, ferroelectric particles such as calcium titanate (J,
Appl, Physics, 3B(1)67 (1
967)), organic semiconductor particles such as poly(acene-quinone) and aniline black (Japanese Patent Application Laid-Open No. 61-21620)
Examples include a method using non-water-containing particles such as Publication No. 2).

[Problem that the invention seeks to solve]

Each of these electrorheological fluids using microparticles exhibits characteristic electrorheological properties, but since all of them are solid particles dispersed in an insulating oily medium, not only inorganic particles but also organic particles can be used. However, due to the difference in density with the oily medium, particle sedimentation tends to occur, which is one of the major practical problems. As a result of examining prevention methods, it was confirmed that the present invention is the most effective.

[Means to solve the problem]

The sedimentation of particles is greatly influenced by the particle size, the difference in density between the oily medium and the particles, and the viscosity of the oily medium. In electrorheological fluids, there is a limit to the particle size due to the Winslow effect, and it cannot be made very small. The viscosity cannot be made very high in many cases, and it is also highly temperature dependent.

Therefore, the present inventors focused on the density difference between the particles and the medium, and conducted experiments based on many ideas for ways to reduce this difference, and found that the present invention can obtain the highest effect! Approved. That is, the present invention provides an electrorheological fluid in which dielectric fine particles are dispersed in an oily medium with excellent electrical insulation.
The purpose is to use hollow bodies that do not allow the medium to penetrate into the cavities of the dielectric particles.

The dielectric fine particles referred to in the present invention refer to particles whose surfaces themselves or processed surfaces are polarized by application of an external electric field to form bridges between the particles due to electrostatic attraction, thereby exhibiting the Winslow effect, such as: Metal oxides such as silica, alumina, silica-alumina, spinel, zirconia, titanium oxide, vanadium oxide, aluminum, silicon,
Metals such as nickel, copper, and graphite, or carbon (all of which are used with surface insulation), calcium titanate,
Examples include ferroelectric substances such as strontium titanate, synthetic polymers such as polyvinylidene fluoride, polyamide, and ion exchange resins (all of which are used after being modified or surface-treated as necessary). The shape of these particles is preferably as rounded as possible, spherical or ellipsoidal.
In particular, the zero particle size, which is preferably truly spherical, is preferably from 1 μm to 100 μm, but if it is smaller than 1 μm, it is difficult to exhibit the electrorheological effect, and if it is larger than 100 μm, the particles are likely to wear out or break.

In the present invention, the hollow particle that does not allow the medium to penetrate into the cavity is a particle that has one or more cavities, and this cavity is an independent cavity surrounded by a dense wall that does not allow the oily medium to penetrate. There needs to be.

The appropriate ratio of the cavity to the particles depends on the material, density and diameter of the particles, the density of the oily medium, the affinity between the particles and the oily medium, and is usually 20 to 80% by volume. If this ratio is too high, the mechanical strength of the particles will be poor, and if this ratio is too low, the effect of cavities will be small. For typical manufacturing methods of hollow particles, especially microballoons, see the chemical technology magazine MOL,
19 (6), 21 (1981) or Industrial Technology Library 25''Microcapsules'' (Nikkan Kogyo Shimbun). Also,
A method is also used in which a foaming agent or a cavity-forming nucleating agent is mixed in advance and a plurality of cavities or pores are formed during or after particle molding. Among these methods, there are methods in which the walls of the particles are made porous and the oily medium penetrates inside the hollow body, but in the case of hollow particles with such porous walls, the pores cannot be sealed by surface melting or surface coating. It is necessary to do so.

As a special example, it is also possible to use a so-called composite particle in which a substance that exhibits an electrorheological effect is laminated on the surface of the hollow particle.

On the other hand, the oily medium used in the present invention includes spindle oil,
Typical examples include transformer oil, (chlorinated) paraffin, naphthenic oil, aromatic carboxylic acid higher alkyl ester, silicone oil, fluorine oil, polyhalophenyl alkyl ether, etc., but basically they are electrically insulating. ,
Any material with excellent heat resistance and stability (non-reactivity) can be used, but in practical terms, materials with low viscosity, high boiling point, low vapor pressure, low freezing point, high density, and low viscosity temperature are required. Dependence,
It is preferable to have properties such as high hydrophobicity, low toxicity, and low cost.

For sedimentation of particles in an electrorheological fluid, which is the object of the present invention, the density difference between the hollow body particle and the oily medium is determined by the particle size,
Although it depends on the shape, surface condition, viscosity of the oily medium, affinity with particles, viscosity temperature characteristics, etc., it is 0.8 or less, preferably 0.8 or less.
It is necessary to adjust the void ratio of the particles or the density of the oily medium so that it is 4 or less.

The mixing volume ratio of particles and oily medium ranges from 3:95 to 50:5.
0, preferably in the range of 5:95 to 30:70. Additives may be used in the mixed electrorheological fluid for the purpose of preventing rust, preventing oxidation, and improving the electrorheological effect as long as the electrical insulation properties are not significantly reduced.

[Effects of the present invention]

The present invention is applicable not only to particles such as silica and alumina, for which sedimentation has been one of the major obstacles despite the most research, but also to particles of high density, which had even been given up on consideration due to their high density. This enables the use of metal and inorganic particles, greatly expanding the range of particle selection and paving the way for the development of new types of electrorheological fluids.

In particular, the present invention is highly useful because it allows many metal particles to be applied to electrorheological fluids using surface-insulated metal particles.

Example 1 Spherical silica hollow particles obtained by interfacial polymerization using the sol-gel method (average particle diameter 4 μm, true specific gravity 2.1, average void ratio 45)
% by volume) was immersed in a 10% aqueous solution of polyacrylic acid to impregnate the inside of the pores of the hollow particle wall with this solution, and then dried completely to seal the pores with the polymer. Next, after controlling the temperature of the sealed hollow particles in the air to a water absorption rate of 5% by weight, dimethyl silicone [50 centistokes (25°C
)] at a particle concentration of 30% by volume. After standing still for one day,
After separating the suspended and settled particles, the electroviscosity of this fluid was measured. As a result, the fluid showed a change in viscosity comparable to that of conventional water-containing ion-exchange resins, and the fluid remained stable even after being left for 3 days. It was stable, with no sedimentation of particles observed. As a comparison, most of the silica hollow particles that were not treated with polyacrylic acid settled out after being left standing for half a day due to the penetration of dimethyl silicone into the inside of the particles.

Example 2 Glass balloon mixed with a blowing agent and melted and atomized (average particle diameter 30 μm, particle density 0.8, average void ratio 65% by volume)
) was treated with a sensitizer (aqueous solution of stannous chloride) and washed with water, then treated with an activator (aqueous palladium chloride solution) and washed with water, and then treated with an electroless nickel plating solution (nickel chloride, sodium hypophosphite, A nickel layer with a thickness of about 0.3 μm was formed on the surface of the particles by immersing them in a mixed aqueous solution of sodium acetate and stirring gently at 90° C. for 30 minutes. Next, the particles were immersed in a toluene solution containing 1% by weight of isopropoxyaluminum to cause the solution to adhere to the particle surface.
Place it in a large amount of ethanol containing 0.5% by weight of water (
The mixture was stirred to form an aluminum oxide film on the particle surface. The particles were heat-treated at 300° C. for 30 minutes in a nitrogen atmosphere to improve the film strength. By repeating this series of steps for forming an aluminum oxide film three times, an aluminum oxide film with a thickness of about 0.3 μm was formed on the nickel film. The thus obtained 3N structured particles were dispersed in dimethyl silicone oil at a particle concentration of 20% by volume to obtain an electrorheological fluid. The apparent density of the particles is 0.98, and the difference from that of dimethyl silicone oil is less than 0.05.Although some floating is seen in a small part, the particles are uniform with almost no settling even after 3 days of standing. It showed a dispersed state. Note that this fluid exhibited good electrorheological effects not only at room temperature but also at 120°C. - For comparison, these three-layer structure particles were mixed with fluorine-based insulating oil (DuPont "KRYTOX" 143AY,
When the particles were dispersed at a density of 1.88), floating particles were observed after half a day.

Patent applicant: Asahi Kasei Industries, Ltd.

Claims (1)

[Claims] An electrorheological fluid in which dielectric fine particles are dispersed in an oily medium having excellent electrical insulation properties, wherein the dielectric fine particles are particles consisting of a hollow body in which the medium does not penetrate into cavities.