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The present invention relates to an electroviscous fluid which comprises the dispersion of wet-method silica particles in an electrically insulating fluid.
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Fluids whose viscosity can be varied by the application of an external voltage have received attention in the last several years because they exhibit such functionalities as drive power transmission, impact absorption, valve-like behavior, and so forth. Such fluids whose viscosity is increased by means of an electric field are generally called "electroviscous fluids". However, in order to be able to withstand the severe service in, for example, a clutch, engine mount, or shock absorber, a fluid is required which undergoes a substantial increase in yield value at low voltages.
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Various types of these fluids have already been proposed, and they are typified by, for example, dispersions of porous inorganic particles (e. g., silica, alumina, talc) in an electrically insulating fluid. In each case, through the formation of an electrical double layer by means of water adsorbed on the particle surfaces, the particles become oriented in response to an external electric field and the viscosity increases (more specifically, the fluid is converted into a Bingham fluid, which exhibits a yield value). This effect is called the "Winslow effect". The following disadvantages have been associated with silica- based electroviscous fluids: they have limited application temperatures (approximately 10°C to 80 ° C), they abrade the surrounding machinery, and the particles form a sediment. Still, since silica is easily obtained on an industrial basis and is highly susceptible to improvement and manipulation, it has been considered potentially useful for certain areas of application, for example, machinery which would be used in the vicinity of room temperature and which would undergo little abrading motion. Silica-based electroviscous fluids are disclosed in United States Patent Number 3,047,507 and in Japanese Patent Application Laid Open [Kokai or Unexamined] Number 61-44998 [44,998/86], but in each case these exhibit an impractically weak Winslow effect. Also, Japanese Patent Application Laid Open Number 01-284595 [284,595/89] discloses an electroviscous fluid in the form of a dispersion in an electrically insulating fluid of wet-method silica whose surface adsorbed water has been replaced by polyvalent alcohol. Based on the formation of an electrical double layer by the polyvalent alcohol, this electroviscous fluid exhibits an electroviscous behavior more or less equal to that of the dispersion of the unmodified silica, but also retains its characteristics at higher temperatures (90 ° C). However, even in this case, the intensity of the Winslow effect is still basically equal to that of the prior wet-method silica-based systems. Moreover, because the dielectric constant of the polyvalent alcohol declines with increasing temperature, the Winslow effect still declines at higher temperatures.
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As a consequence, all of these heretofore proposed electroviscous fluids remain unsatisfactory from a practical standpoint.
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The present invention introduces a silica dispersion- type electroviscous fluid which develops a Winslow effect sufficient to satisfy industrial applications. The present inventor carried out extensive investigations with a view to solving the aforementioned problems, and discovered as a result that the aforementioned problems are substantially reduced by using the disperse phase of silica which is advantageously prepared by replacing the water adsorbed on the surface of the wet-method silica with a particular type of compound. The present invention was developed based on this discovery.
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It is an object of the present invention to introduce an electroviscous fluid which develops an excellent Winslow effect. It is also an object of the present invention to provide an electroviscous fluid which comprises a dispersion in an electrically insulating fluid, wherein there is 0.1 to 50 weight% of wet-method silica whose surface adsorbed water has been replaced by a nitrile group-containing organic compound. A further object of this invention is to provide an electroviscous fluid exhibits a substantial increase in yield value at low voltages and an excellent shear stability.
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The present invention relates to an electroviscous fluid which comprises a dispersion of 0.1 to 50 weight% wet-method silica whose surface adsorbed water has been replaced by a nitrile group-containing organic compound in an electrically insulating fluid.
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To explain the preceeding in greater detail, the wet-method silica particles employed by the present invention are prepared by the production of silica by the addition of acid under wet conditions to a water glass starting material. These wet-method silica particles are an ideal disperse phase for electroviscous fluids because their surfaces possess a layer of adsorbed water, which is ideal for the development of the Winslow effect, and because they have optimal particle sizes. The type of wet-method silica employed by tile present invention is not specifically restricted.
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According to the present invention, the water adsorbed on the surface of this wet-method silica is replaced by a nitrile-containing organic compound. Therefore the surface of wet-method silica is normally covered with a layer of adsorbed water. While the particular weight proportion for this adsorbed water in the total silica weight will vary with the particular type of wet-method silica, in general it will fall within the range of 5% to 10%. Since this layer of adsorbed water is merely hydrogen bonded to a layer of structural water which resides immediately inward, it can be almost completely eliminated by heating to around 100°C. However, as discussed hereinabove, this adsorbed water layer plays a significant role in the development of the Winslow effect. While not limiting the present invention with any particular theory, it is believed that the adsorbed water layer causes a Winslow effect due to the high dielectric constant of the water (approximately 80 at room temperature). However, its ease of elimination by heating extinguishes the Winslow effect. In the present invention, this adsorbed water layer on the surface of wet-method silica is replaced with a nitrile-group containing organic compound. The nitrile group-containing organic compound as specified herein is exemplified by aliphatic nitriles such as acetonitrile, propionitrile, n-capronitrile, succinonitrile, etc., and by aromatic nitriles such as benzonitrile, alpha-tolunitrile, and so forth. Various methods can be devised for the replacement procedure, but the following method has proven to be simple and straightforward. First, the wet-method silica is placed under a nitrogen current at 150°C in order to remove the surface adsorbed water. After cooling to room temperature under the nitrogen current, the nitrile compound is then added in a quantity corresponding to the weight loss due to the desorbed water with mixing to physical homogeneity in, for example, a mixer. After such a treatment, the surface of the wet-method silica will be covered by a layer of the nitrile compound. Due to the high dielectric constant of the treated water, a Winslow effect can be developed which is at least equivalent to that for the adsorbed water. Moreover, because in this case the dielectric constant is only slightly temperature dependent, the decline in the Winslow effect at higher temperatures is suppressed.
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The electroviscous fluid according to the present invention comprises the dispersion of wet-method silica particles as specified hereinbefore in an electrically insulating fluid. The electrically insulating fluid itself is not particularly limited as long as it is a liquid at room temperature and is electrically insulating. Such electrically insulating fluids are exemplified by mineral oils, dibutyl sebacate, chlorinated paraffins, fluorine oils, and silicone oils. Among the preceding, silicone oils are preferred for their strong electrical insulation, low temperature-dependent viscosity variation, and so forth. The silicone oils are concretely exemplified by the fluid diorganopolysiloxanes with the following chemical structure:
wherein each R denotes a monovalent hydrocarbon group as exemplified by alkyl groups such as methyl, ethyl, and propyl, and aryl groups such as phenyl and naphthyl. It is preferred that at least 30% of the R groups are methyl groups. Moreover, while the degree of polymerization n is not particularly limited, it preferable that n does not exceed 1,000 in order to achieve a practical viscosity range. Values not exceeding 100 are even more preferred. Silicone oils with this structure are available in the form of a large number of commercial products, for example, SH200 from Toray Dow Corning Silicone Company, Limited.
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Furthermore, among the silicone oils, fluoroalkyl- containing diorganopolysiloxanes are particularly preferred because they enhance the Winslow effect and inhibit the particle sedimentation caused by specific gravity differences. These are concretely expressed by the following structural formula:
wherein R is defined as above, R
2 is a fluoroalkyl group having 10 or fewer carbons, and m and p are integers with values not exceeding 1,000.
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The structure of the aforementioned C<10 fluoroalkyl group is not particularly specified, but the 3,3,3-trifluoropropyl group is preferred from the standpoint of ease of synthesis. In order to obtain a substantial enhancement of the Winslow effect, it will be preferable for each molecule to contain at least 30 mole% fluoroalkyl group. Moreover, while the degree of polymerization m is again not particularly limited, it preferably does not exceed 1,000 in order to achieve a practical viscosity range. Values not exceeding 100 are even more preferred. The mechanism by which the fluoroalkyl group enhances the Winslow effect is not clear. While not limiting the present invention to any particular theory it is believed that a strong intramolecular dipole is generated by the simultaneous presence in the molecule of the electronegative fluorine atom and electropositive silicon atom separated by a suitable distance. Polarization of the double layer is then promoted by contact between this dipole and the electrical double layer on the wet-method silica particle. Otherwise, fluorine-containing fluids tend to have larger specific gravities, which results in an accompanying inhibition of silica particle sedimentation.
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These fluoroalkyl-containing diorganopolysiloxanes are commercially available, for example, as FS1265 from Toray Dow Corning Silicone Company, Limited.
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The electroviscous fluid according to the present invention comprises the dispersion of wet-method silica particles as described hereinbefore in an electrically insulating fluid as described hereinbefore. The quantity dispersed should fall within the range of 0.1 to 50 wt% and preferably is in the range of 10 to 40 wt%. A satisfactory thickening effect is not obtained at less than 0.1 wt%. At values exceeding 50 wt%, the viscosity of the system is so substantially increased as to be impractical.
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The electroviscous fluid according to the present invention as described hereinabove is useful as the working oil or functional oil in particular types of machinery, for example, machinery which will be employed in the vicinity of room temperature and where there will be little abrading motion.
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The present invention will be explained in greater detail below through the use of illustrative and comparison examples. In the examples, cs = centistokes and the viscosity is the value at 25 °C. The electroviscous behavior was measured as follows. The test fluid was placed in an aluminum cup (interior diameter = 42 mm) into which an aluminum rotor (diameter = 40 mm, length = 60 mm) was subsequently inserted. The resulting cylindrical cell was set up vertically, and the cup was linearly accelerated from a shear rate (D) of zero to 330 s-1 over 40 seconds. During this period, the torque applied to the rotor was measured with a torque sensor, and this was converted into the shear stress (S) and the D-versus-S curve was drawn on an X-Y recorder. In addition, the rotor was electrically grounded and D-versus-S curves were also recorded while applying a direct-current voltage to the cup. The intersection of the extrapolation of the linear segment with the S-axis was designated as the yield value at the particular field strength. The thermal and shear stress stability and the sedimentability of the wet-method silica particles were also examined. The electroviscosity test was also set up in such a manner that the cell temperature could be varied.
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In the following examples all amounts (parts and percentages) are by weight unless otherwise indicated.
Example 1
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Wet-method silica (Nipsil ER from Nippon Silica Kogyo Kabushiki Kaisha) with an average particle size of 11 micrometers and pH = 7.0 to 8.5 (4 wt% aqueous suspension) was dried for 2 hours under a nitrogen current at 150°C. Drying caused this wet-method silica to suffer a weight loss of approximately 6 weight percent. After the dried wet- method silica had been cooled to room temperature under a nitrogen current, acetonitrile was added in a quantity equal to the weight loss. Stirring in a mixer for about 1 hour afforded an acetonitrile-treated wet-method silica. 15 Parts of this acetonitrile-treated wet-method silica was stirred into 85 parts trimethylsiloxy-terminated polydimethylsiloxane (viscosity = 100 cs) to afford a suspension in which the acetonitrile-treated wet-method silica was homogeneously dispersed in the polydimethylsiloxane. The electroviscous behavior of this suspension was then measured at a cell temperature of 25 ° C, and the measurement results are reported in Table I below.
Example 2
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The electroviscous behavior of a suspension prepared as in Example 1 was measured at a cell temperature of 90 ° C, and these measurement results are reported in Table I below.
Example 3
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Electroviscous fluid in the form of the suspension prepared in Example 1 was heated for 1 week at 90 ° C in an open system under air, then removed and cooled. After this heat treatment, the electroviscous behavior of the resulting electroviscous fluid was measured at a cell temperature of 25 ° C, and these results are reported in Table I.
Example 4
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An electroviscous fluid was prepared as in Example 1, but in this case using n-caprylonitrile in place of the acetonitrile used in Example 1. The electroviscous behavior of this electroviscous fluid was measured at a cell temperature of 25 ° C, and these results are reported in Table I below.
Example 5
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Wet-method silica (Nipsil ER from Nippon Silica Kogyo Kabushiki Kaisha) with an average particle size of 11 micrometers and pH = 7.0 to 8.5 (4 wt% aqueous suspension) was dried for 2 hours under a nitrogen current at 150°C. Drying caused this wet-method silica to suffer a weight loss of approximately 6 weight percent. After the dried wet- method silica had been cooled to room temperature under a nitrogen current, acetonitrile was added in a quantity equal to the weight loss. Stirring in a mixer for about 1 hour afforded an acetonitrile-treated wet-method silica. 15 Parts of this acetonitrile-treated wet-method silica was stirred into 85 parts trimethylsiloxy-terminated polydimethylsiloxane (viscosity = 100 cs) to afford a suspension in which the acetonitrile-treated wet-method silica was homogeneously dispersed in the polydimethylsiloxane. The electroviscous behavior of this suspension was then measured at a cell temperature of 25 ° C, and the measurement results are reported in Table I below.
Example 6
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An electroviscous fluid in the form of a suspension was prepared as in Example 1, but in this case using a trimethylsiloxy-terminated poly(methyl-3,3,3-trifluoropropyl)siloxane with a viscosity of 300 cs in place of the polydimethylsiloxane with viscosity = 100 cs used in Example 1. Its electroviscous behavior was measured and these results are reported in Table I below.
Comparison Example 1
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An electroviscous fluid in the form of a suspension was prepared as in Example 1, but in this case using the wet- method silica prior to its acetonitrile treatment in place of the acetonitrile-treated wet-method silica employed in Example 1. Its electroviscous behavior was measured, and these results are reported in Table I below.
Comparison Example 2
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The electroviscous behavior of the electroviscous fluid of Comparison Example 1 was measured at a cell temperature of 90 ° C, and these measurement results are reported in Table I below.
Comparison Example 3
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Electroviscous fluid as prepared in Comparison Example 1 was heated for 1 week at 90 ° C in an open system under air, then removed and cooled. The electroviscous behavior of the obtained electroviscous fluid was measured at a cell temperature of 25 ° C, and these results are reported in Table I below.
Comparison Example 4
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An ethylene glycol-substitution-treated wet-method silica was prepared according to the procedure in Example 1 by using ethylene glycol in place of the acetonitrile. Continuing to operate as in Example 1, a suspension was prepared by suspending and dispersing this in trimethylsiloxy-terminated polydimethylsiloxane (viscosity = 100 cs). The electroviscous behavior of this suspension was measured, and these results are reported in Table I below.
Comparison Example 5
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The electroviscous behavior of a suspension as prepared in Comparison Example 4 was measured at a cell temperature of 90 ° C, and these measurement results are reported in Table I below.
Comparison Example 6
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A suspension as prepared in Comparison Example 4 was heated for 1 week at 90 ° C in an open system under air, then removed and cooled. The electroviscous behavior of the obtained electroviscous fluid was measured, and these results are reported in Table I below.
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As is shown in the Examples delineated hereinabove, the electroviscous fluid according to the present invention which comprises a dispersion, in an electrically insulating fluid, of 0.1 to 50 weight% wet-method silica whose surface adsorbed water has been replaced by a nitrile group-containing organic compound, is characterized by a substantial increase in yield value at low voltages and excellent shear stability.
