WO1994009097A1

WO1994009097A1 - Modified electrorheological materials having minimum conductivity

Info

Publication number: WO1994009097A1
Application number: PCT/US1993/009499
Authority: WO
Inventors: Beth C. Munoz; Stephen R. Wasserman; J. David Carlson; Keith D. Weiss
Original assignee: Lord Corporation
Priority date: 1992-10-16
Filing date: 1993-10-05
Publication date: 1994-04-28

Abstract

Electrorheological materials containing a particle component and a carrier fluid that has been modified to minimize conductivity. The carrier fluid is modified by extensive purification or by the formation of a miscible solution with a low conductivity carrier fluid. The modification techniques allow previously unacceptable carrier fluids to be utilized in an electrorheological material which exhibits significant electrorheological activity over a broad temperature range.

Description

MODIFIED ELECTRORHEOLOGICAL MATERIALS HAVING MINIMUM CONDUCTIVITY

Technical Field

The present invention relates to certain fluid materials which exhibit substantial increases in flow resistance when exposed to electric fields. More specifically, the present invention relates to high strength electrorheological materials that utilize carrier fluids which have been modified to minimize conductivity.

Background Art

Fluid compositions which undergo a change in apparent viscosity in the presence of an el .trie field are commonly referred to as electrorheological fluids or materials. Electrorheological materials normally are comprised of particles dispersed within a carrier fluid and in the presence of an electric field, the particles become polarized and are thereby organized into chains of particles within the fluid. The chains of particles act to increase the apparent viscosity or flow resistance of the overall fluid and, in the absence of an electric field, the particles return to an unorganized or free state and the apparent viscosity or flow resistance of the overall material is correspondingly reduced.

An electrorheological fluid composed of a non-conductive solid dispersed within an oleaginous oil vehicle is described in U.S. Patent No. 3,047,507. The compositions contain a minimum amount of water and a minimum amount of a surface active dispersing agent, a non- conductive solid consisting of finely divided particles having an average diameter of from about 0.1 to about 5 microns, and an oleaginous oil vehicle having a viscosity not greater than that of lubricating oil and a dielectric constant between 2.0 to 5.5. In order to achieve the best performance, it is preferred that the dielectric constant of the oil component be as close to 2.0 as possible. Specific examples given for the oil component include mineral oils, kerosene, polyoxyalkylene glycols, aliphatic esters, fluorinated hydrocarbons, and silicone oil.

U.S. Patent No. 3,367,872 and U.S. Patent No. 3,397,147 disclose electrorheological materials consisting of alumina or silica-alumina particles, a surface active agent and a high resistivity oleaginous vehicle having a dielectric constant less than 10, preferably between 2.0 and 5.5. Specific examples of the oleaginous vehicles disclosed include paraffin, olefin and aromatic hydrocarbons.

An electrorheological material that utilizes as the particle component a substantially anhydrous electronic conductor such as an organic semiconductor comprised of unsaturated, fused polycyclic systems containing conjugated π-bonds is disclosed in U.S. Patent No. 4,687,589. Specific examples of particle components include phthalocyanine-type compounds such as copper phthalocyanine, violanthrone B, porphin or azaporphin systems, poly(acene-quinone) polymers, and polymeric SchifFs Bases. Halogenated aromatic liquids are specified as the preferred continuous phase of the electrorheological material.

Electrorheological materials exhibiting low viscosity, low electrical conductivity, low toxicity and low freezing points are described in U.S. Patent No. 4,502,973. The composition of these materials include suspensions of a finely divided hydrophilic solid, such as solid polyhydric alcohols or cross-linked lithium polymethacrylate polymer salts, in a diaryl derivative as a hydrophobic liquid.

The utilization of electrorheological materials consisting of hydrophilic solids dispersed in a hydrophobic liquid is described in U.S. Patent No. 4,812,251. In this case the preferred hydrophobic liquid component comprises a mixture of a fluorosilicone whose average molecular weight is in the range of 200-700 A.M.U. (atomic mass unit) and polychlorotrifluorethylene (i.e., Fluorolube FS-5, Hooker Chemical Co.), a high molecular weight fluorosilicone or a halogenated aromatic hydrocarbon. The addition of a low molecular weight fluorosilicone additive to another polymer is found to effectively reduce the viscosity of the overall fluid mixture.

Variations in the magnitude of the electrorheological effect have been observed for materials comprised of different particle and carrier components, such as those described above. In order for a material to polarize and respond as an electrorheological material, the particle and the carrier fluid must have different complex permittivities. The polarizability, β, of a particle in a fluid medium is given by: [82-εJ β = (Eq. l)

[£2+2ει]

where £ι and ^~ι are the complex permittivities of the carrier fluid and particle, respectively. By definition the complex permittivity is dependent upon both the dielectric constant (relative permittivity) and conductivity of the material. Any situation in which the polarizability of the particle is altered will inherently effect the observed electrorheological activity of the material. An explanation for the observed differences in electrorheological activity is also disclosed in U.S. Patent No. 5,075,021, which is incorporated herein by reference.

In general, the electrorheological activity of an electrorheological material has been found to increase proportionately with the dielectric constant of the carrier fluid, given a particle component having a fixed dielectric constant. However, when the dielectric constant of the carrier fluid becomes too high, the conductivity of the carrier fluid can reach unacceptably high levels so as to substantially interfere with the polarizability of the particle component and the overall electrorheological activity of the material. An ideal carrier fluid for electrorheological materials therefore possesses a sufficiently high dielectric constant without an unacceptably high level of conductivity. It has been found that carrier fluids having dielectric constants greater than about 7.5 typically have conductivities that exceed about 1 x 10"^ S/m. Carrier fluids having conductivities that exceed about 1 x 10"? S/m have been found to be unacceptable for use in electrorheological materials.

It is also desirable that the continuous component or carrier fluid of an electrorheological material exhibit several other basic characteristics. These characteristics include: (a) chemical compatibility with both the particle component of the fluid and device materials; (b) low viscosity; (c) high dielectric breakdown strength; (d) relatively low cost; and (e) high density. Electrorheological materials should also be non-hazardous to the surrounding environment and, more importantly, be capable of functioning over a broad temperature range. Most of the carrier fluid components that are traditionally used in electrorheological materials cannot adequately meet all of these requirements. A need therefore exists for the development of new carrier fluids from which electrorheological materials can be prepared.

Disclosure of Invention

The present invention is an electrorheological material which exhibits a significant level of electroactivity over a broad temperature range. More specifically, the present invention relates to electrorheological materials that utilize carrier fluids which have been modified to reduce conductivity. Prior to being subjected to the modification techniques of the present invention, the unmodified carrier fluids have high dielectric constants, and more significantly, unacceptably high conductivities. The modification techniques of the invention enable the unmodified carrier fluids to be converted to carrier fluids having conductivities which are acceptable for use in electrorheological materials. Prior to modification, the carrier fluids utilized in the present invention have conductivities greater than about 1.0 x 10" ^ S/m. Subsequent to modification, however, the carrier fluids have conductivities less than about 1.0 x 10" ? S/m and are therefore suitable for use in an electorheological material.

The modified carrier fluids of the invention can be prepared by extensively purifying a high dielectric constant carrier fluid having a conductivity greater than about 1.0 x 10"7 S/m or by forming a miscible solution of the high dielectric constant, high conductivity carrier fluid with a second fluid exhibiting a low conductivity. The electro¬ rheological material of the invention therefore comprises a modified carrier fluid and a particle component wherein the carrier fluid has been modified by extensive purification or by the formation of a miscible solution with a low conductivity carrier fluid so as to cause the modified carrier fluid to have a conductivity less than about 1.0 x 10-7 s/m.

Brief Description of the Drawings

Figure 1 is a plot of shear stress data as a function of shear rate measured for an electrorheological material containing the purified chlorinated paraffin carrier fluid of Example 2 at various A.C. electric field strengths (frequency = 1.0 kHz) and ambient temperature (25°C). The dynamic yield stress for the electrorheo- logical material (Bingham plastic model) corresponds to the y-inter- cept of the exhibited data.

Figure 2 is a plot of shear stress data as a function of shear rate measured for an electrorheological material containing the unpurified or "as received" chlorinated paraffin carrier fluid of comparative Example 3 at various A.C. electric field strengths (frequency = 1.0 kHz) and ambient temperature (25°C). The dynamic yield stress for the electrorheological material (Bingham plastic model) corresponds to the y-intercept of the exhibited data.

Best Mode for Carrying Out the Invention

The electrorheological materials of the invention comprise a carrier fluid and a particle component wherein the carrier fluid has been modified through extensive purification or by forming a miscible solution with a low conductivity carrier fluid in order to lower the exhibited conductivity.

With respect to the carrier fluids modified by extensive purification, it has been found that the conductivity exhibited by high dielectric constant, high conductivity carrier fluids can be significantly reduced through the efficient removal of low molecular weight complexes (i.e., water) and ionic impurities (i.e., salts). The removal of these contaminants can be accomplished through methods well known to those skilled in the art of manufacturing high purity materials for use in, for example, the electronics and medical industries. Examples of these methods include chemical purification, distillation, adsorptive filtration, electrostatic separation and combinations thereof.

Chemical purification typically involves the addition of a compound or reagent to the carrier fluid that will either react with or adsorb/absorb the existing impurities. Examples of dehydrating agents that react with water include sodium metal, potassium metal, triacetyl borate, calcium hydride, lithium aluminum hydride, barium oxide, calcium oxide and phosphorous pentoxide. Examples of common adsorbing/absorbing agents include calcium sulfate, magnesium sulfate, sodium sulfate, sulfuric acid, potassium carbonate, sodium hydroxide, potassium hydroxide, alumina, silica gel and calcium chloride, as well as aluminosilicates, zeolites or molecular sieves. A more complete description of reactive reagents that can be used in the purification of liquids is provided by A. Gordon and R. Ford in "The Chemist's Companion", John Wiley & Sons, New York, 1972, the entire content of which is incorporated herein by reference.

Distillation is a separation process that typically involves the evaporation or vaporization of the carrier component followed by its subsequent condensation. Various distillation processes are well known to those skilled in the art of manufacturing chemical products. Examples of these processes include destructive distillation, batch distillation, extractive distillation, flash distillation, continous distillation, simple distillation, fractional distillation, azeotropic distillation, vacuum or reduced pressure distillation, molecular distillation and hydrodistillation. A more complete description of basic distillation methodology is provided by J. Landgrebe in "Theory and Practice in the Organic Laboratory" (D. C. Heath and Company, Inc., Lexington, Mass., 1973), hereinafter referred to as Landgrebe. and K. B. Wiberg in "Laboratory Techniques in Organic Chemistry" (McGraw-Hill Book Company, Inc., New York, , 1960), hereinafter referred to as Wiberg. the entire contents of which are incorporated herein by reference.

In practice, adsorptive filtration is a type of column chromatography which involves the elution of a carrier component through a column or bed of a solid absorbent. The elution or flow of the carrier fluid through the packed absorbent can be achieved through the use of high pressure, reduced pressure and gravitational forces. Common absorbents used in adsorptive filtration or column chromatography include aluminum oxide, silica gel, and magnesium silicates. A more complete description of the basic principles involved in column chromatography is provided in Landgrebe and Wiberg.

Electrostatic separation utilizes an electric field to purify liquids, clean gases and separate particular matter and is well known to those skilled in the art of electrostatics. For example, the use of electrostatic techniques to efficiently remove various salts from crude oil is commercially practiced in many refineries. The basis for electrostatic separation resides in the electrophoretic or dielectro- phoretic movement of polar particles, such as the low molecular weight contaminants (i. e., water, etc.) and ionic impurities present in a carrier fluid, upon the application of an electric field. A carrier fluid is allowed to flow past or through a region subjected to an electric field. Due to an interaction with the electric field the polar contaminants remain within the region subjected to the electric field. The carrier component purified by this technique exhibits a lower conductivity than the original fluid prior to purification. The utilization of electrostatic separation in conjunction with the previously described adsorptive filtration technique is often referred to as electrofiltration. A more complete description of electrostatic separation and electro- filtration is provided by H. A. Pohl in "Dielectrophoresis" (Cambridge University Press, London, 1978), the entire content of which is incorporated herein by reference.

After purification, it is imperative that the carrier fluids be stored under appropriate conditions to insure that they are not recontaminated by the absorption of water or ionic impurities. It is recommended that the carrier fluids be stored after purification under a dry atmosphere and in a plastic container comprised of, for example, polyethylene.

It has been discovered that carrier fluids with a dielectric constant greater than or equal to about 50 cannot be adequately purified to exhibit a conductivity less than about 1.0 x 10"? S/m. Carrier fluids which have a dielectric constant between about 7.5 and 50 and a conductivity greater than about 1.0 x 10"? S/m are, however, eligible for purification according to the present invention. These carrier fluids are further defined to be aprotic solvents, liquids, monomeric oils, oligomeric oils or polymeric oils that possess a net permanent dipole. The carrier fluids eligible for purification according to the present invention can be any known polar aromatic or nonaromatic, halogenated or nonhalogenated straight chain, branched, or cyclic hydrocarbon, ester, ketone, ether, or aldehyde compounds or oils. In addition, these carrier fluids may contain an organo-silicon, -sulfur, -nitrogen or -phosphorous substituent or moiety. Examples of commercially available carrier fluids eligible for purification according to the present invention include chlorinated paraffin oils, brominated paraffin oils, poly(2,2,2-trifluoromethyl propylene oxide), benzaldehyde, benzonitrile, benzoyl chloride, 3- bromoaniline, butyric anhydride, γ-butyrolactone, epichlorohydrin, cyclohexanone, o-dichlorobenzene, diethyl maleate, diethyl malonate, diethyl oxalate, diethyl sulfate, N,N-dimethylacetamide, N,N- dimethylaniline, N,N-dimethylformamide, dimethyl o-phthalate, dimethyl sulfate, dimethyl sulfoxide, 2-ethoxyethyl acetate, ethyl acetoacetate, ethyl cyanoacetate, hexamethylphosphoramide, hexanenitrile, isoquinoline, methyl cyanoacetate, nitrobenzene, heptyl cyanide, 2,4-pentanedione, benzyl cyanide, proprionic anhydride, sulfolane, 1,1,2,2-tetrachlorethane, 1,1,3,3-tetramethylurea, tributyl phosphate, γ-decanolactone, δ-decanolactone, halogenated polyvinyl ethers, and oligomers of acrylonitrile, vinyl chloride, methyl methacrylate, or vinyl acetate, with halogenated paraffin oils and poly(2,2,2-trifluoromethyl propylene oxide) being preferred. In general, it is preferred that the carrier fluid to be modified have a dielectric constant between about 7.5 and 20. Electrorheological materials prepared using the purified carrier fluids of the present invention exhibit lower levels of conductivity than those prepared using unpurified carrier fluids. Thus, these electrorheological materials can be exposed to higher electric fields without exhibiting detrimental electrical properties. In addition, these purified carrier fluids will not interfere with the polarizability of the particle component, thereby allowing for a maximum electrorheological effect to be observed.

A carrier fluid having a dielectric constant greater than about 7.5 and a conductivity greater than about 1.0 x lO"? S/m can also be modified according to the present invention by forming a miscible solution with another carrier fluid or blend of fluids having a conductivity less than about 1.0 x 10"7 S/m. "Low conductivity carrier fluid" herein refers to a carrier fluid having a dielectric constant less than about 50 and a conductivity less than about 1.0 x 10"? S/m, while "high conductivity carrier fluid" herein refers to a carrier fluid having a dielectric constant greater than about 7.5 and a conductivity greater than about 1 x 10" ? S m.

The high conductivity carrier fluids to be modified by the formation of a miscible solution with a low conductivity carrier fluid can essentially be the same carrier fluids as those described above as being eligible for purification according to the invention, except that the high conductivity carrier fluids may also include carrier fluids having a dielectric constant greater than about 50.

For purposes of this invention, the low conductivity carrier fluid can be either a purified carrier fluid as described above or a carrier fluid inherently exhibiting a dielectric constant less than about 7.5. Examples of low conductivity carrier fluids with a dielectric constant less than about 7.5 are well known to those skilled in the art of electrorheological materials. These fluids can be either mineral oils, silicone oils, white oils, paraffin oils, hydraulic oils, transformer oils, diesters, polyoxyalkylenes, perfluorinated polyethers, fluorinated hydrocarbons, fluorinated silicones, or hindered ester compounds, as well as mixtures or blends thereof, with silicone oils, mineral oils and paraffin oils being preferred. As known to those familiar with such compounds, transformer oils refer to those liquids having characteristic properties of both electrical and thermal insulation. Naturally occurring transformer oils include refined mineral oils that have low viscosity and high chemical stability. Synthetic transformer oils generally comprise chlorinated aromatics (chlorinated biphenyls and trichlorobenzene), which are known collectively as "askarels," silicone oils, and esteric liquids such as dibutyl sebacates. Additional low conductivity carrier fluids appropriate for use in the present invention include the silicone copolymers, hindered ester compounds, and cyanoalkylsiloxane homopolymers described in co-pending U.S. patent application Serial No. 07/942,549 filed September 9, 1992, entitled "High Strength, Low Conductivity Electrorheological Materials," the disclosure of which is incorporated by reference.

Miscible solutions of high conductivity carrier fluids and low conductivity carrier fluids are typically prepared by simply mixing the fluids together by hand with a spatula or the like and then subsequently more thoroughly mixing with a mechanical mixer or shaker. The amount of each fluid used to form the miscible mixture is dependent entirely upon the conductivity exhibited by both the high conductivity and low conductivity carrier fluids. Thus, a carrier fluid having a very high conductivity will require a greater amount of low conductivity carrier fluid to form an acceptable miscible solution as compared to a carrier fluid having a lower conductivity. Similarly, a carrier fluid having a very low conductivity can be used in a smaller amount than a carrier fluid having a slightly higher conductivity in order to form an acceptable miscible solution with a high conductivity carrier fluid. A slight degree of experimentation may be required to determine the exact amounts of high and low conductivity carrier fluids needed to form a miscible solution having an overall conductivity less than about 1 x 10"? S/m. One example of a miscible solution in accordance with the invention is a 1:1 weight ratio mixture of unpurified chlorinated paraffin oil and mineral oil.

The high and low conductivity carrier fluids must be miscible with one another and the miscibility of various fluids can easily be determined through simple standard solubility tests well known to those skilled in the art of organic chemistry and synthesis.

Electrorheological materials prepared using the carrier fluid mixtures of the invention exhibit synergistically improved electro- 5 rheological properties as compared to materials prepared entirely with low conductivity carrier fluids, as well as lower levels of conductivity as compared to materials prepared entirely with high conductivity carrier fluids.

The carrier fluids of the present invention should have a 10 viscosity that is between about 0.5 and 1000 mPa-s, preferably between about 5 and 150 mPa-s. The carrier fluid of the present invention is typically utilized in an amount ranging from about 50 to 95, preferably from about 60 to 85, percent by volume of the total electrorheological material. This corresponds to approximately 19 to 82, preferably 26 to 15 57, percent by weight when the carrier fluid and particle of the electrorheological material have a specific gravity of about 1.0 and 4.3, respectively.

The particle component can essentially be any solid which is known to exhibit electrorheological activity. Typical particle

20 components useful in the present invention include amorphous silicas, synthetic silicas, precipitated silicas, fumed silicas, silicates, aluminum silicates, ion exchange resins and other inorganic particles known in the art such as those composed of titanium dioxide, barium titanate, lithium hydrazinium sulfate and insulated metallic

25 particulates. Other typical particle components useful in the present invention include polyvinyl alcohols, polyhydric alcohols, silicone ionomer reaction products, monosaccharides, porphin systems, metallo-porphin systems, poly(acene-quinone) polymers, polymeric Schiff bases, anionic surfactants, polyelectrolytes, carbonaceous

30 particulates, and other organic and polymeric particles known in the art such as those composed of polymethacrylic acid salts and copolymers of phenol, aldehydes, olefins, ethers and/or acids. The particle component may also be an ionic or non-ionic dye such as described in U.S. Patent Application Serial Nos. 07/806,981 and

35 07/852,586 entitled "Ionic Dye-Based Electrorheological Materials" and "Colorant-Containing Electrorheological Materials," respectively, the entire contents of which are incorporated herein by reference. The preferred particle components of the present invention include insulated metallic particles, as well as atomically polarizable particles such as those described in U.S. Patent Application Serial No. 07/829,137 entitled "Atomically Polarizable Electrorheological Materials," the disclosure of which is incorporated herein by reference.

The diameter of the particles utilized herein can range from about 0.1 to 500 μm and preferably range from about 1.0 to 50 μm. The particle component typically comprises from about 5 to 50, preferably from about 15 to 40, percent by volume of the total composition depending on the desired electroactivity and viscosity of the overall material. This corresponds to approximately 18 to 81, preferably 43 to about 74, percent by weight when the carrier fluid and particle of the electrorheological material have a specific gravity of about 1.0 and 4.3, respectively.

The electrorheological material of the present invention may contain a small amount of an activator adsorbed onto the particle component. However, in order to effectively operate over a broad temperature range, it is preferred that no activator be used in combination with the particle component of the present invention. Typical activators for optional use in the present invention include water and other molecules containing hydroxyl, carboxyl or amine functionality. Typical activators other than water include methyl, ethyl, propyl, isopropyl, butyl and hexyl alcohols, ethylene glycol, diethylene glycol, propylene glycol, glycerol; formic, acetic, sulfuric and lactic acids; aliphatic, aromatic and heterocyclic amines, including primary, secondary and tertiary ami no alcohols and amino esters that have from 1-16 atoms of carbon in the molecule; methyl, butyl, octyl, dodecyl, hexadecyl, diethyl, diisopropyl and dibutyl amines, ethanolamine, propanolamine, ethoxyethylamine, dioctylamine, triethylamine, trimethylamine, tributylamine, ethylene- diamine, propylene-diamine, triethanolamine, triethylenetetramine, pyridine, morpholine, imidazole, and mixtures thereof. Water is the preferred activator for optional adsorption onto the particle component of the present invention. When employed, the activator is utilized in an amount from about 0.1 to 10, preferably from about 0.5 to 5.0, percent by weight relative to the weight of the particle component.

A surfactant to disperse the particle component may also be utilized in the present invention. Such surfactants include known surfactants or dispersing agents such as glycerol monooleate, sorbitan sesquioleate, stearates, laurates, fatty acids, fatty alcohols, and the other surface active agents discussed in U.S. Patent No. 3,047,507 (incorporated herein by reference) but preferably comprise non-ionic surfactants such as the steric stabilizing amino-functional, hydroxy- functional, acetoxy-functional, or alkoxy-functional polysiloxanes such as those disclosed in U.S. Patent No. 4,645,614 (incorporated herein by reference). Other steric stabilizers such as graft and block copolymers may be utilized as a surfactant for the present invention and such other steric stabilizers as, for example, block copolymers of poly(ethylene oxide) and poly(propylene oxide) are disclosed in detail in U.S. Patent No. 4,772,407 (incorporated herein by reference) and in Napper, "Polymeric Stabilization of Colloidal Dispersions," Academic Press, London, 1983 (incorporated herein by reference). Still other steric stabilizers include hyperdispersants, such as HYPERMER® (ICI Americas, Inc.) and SOLSPERSE® (ICI Americas, Inc.) hyperdispersants, fluoroaliphatic polymeric esters, such as FC-430 (3M Corporation), and titanate, aluminate or zircona e coupling agents, such as KEN-REACT® (Kenrich Petrochemicals, Inc.) coupling agents.

The surfactant, if utilized, is preferably an amino-functional polydimethylsiloxane, a fluoroaliphatic polymeric ester, a hyperdispersant or a coupling agent. The optional surfactant may be employed in an amount ranging from about 0.1 to 20 percent by weight relative to the weight of the particle component.

The electrorheological materials of the present invention can be prepared by simply mixing together the carrier fluid, the particle component and surfactant. The electrorheological material is preferably prepared by drying the particle component in a convection oven at a temperature of from about 110°C to 150°C for a period of time from about 3 to 24 hours. At this time an appropriate amount of an activator can optionally be adsorbed onto the particle component. The ingredients of the electrorheological materials may be initially mixed together by hand with a spatula or the like and then subsequently more thoroughly mixed with a mechanical mixer or shaker or dispersed with an appropriate milling device such as a ball mill, sand mill, attritor mill, paint mill, or the like, in order to create smaller particles and a more stable suspension.

Evaluation of the mechanical/electrical properties and characteristics of the electrorheological materials of the present invention, as well as other electrorheological materials, can be obtained through the use of concentric cylinder couette rheometry. The theory which provides the basis for this technique is adequately described by S. Oka in Rheology, Theory and Applications (volume 3, F. R. Eirich, ed., Academic Press: New York (1960), pages 17-82) which is incorporated herein by reference. The information that can be obtained from a concentric cylinder rheometer includes data relating mechanical shear stress to shear strain, the static yield stress and the electrical current density as a function of shear rate. For electrorheological materials, the shear stress versus shear rate data can be modeled after a Bingham plastic in order to determine the dynamic yield stress and viscosity. Within the confines of this model, the dynamic yield stress, τy, and viscosity, η, for the electrorheological material corresponds to the zero-rate intercept and slope, respectively, of a linear regression curve fit to the measured data. A Bingham plastic model, as described by Equation 2, recognizes that the property of an electrorheological material generally observed to change with an increase in electric field is the yield stress defining the onset of flow.

τ_y = τ(E) + ηγ (Eq. 2)

The dynamic yield stress (Ty,d) in a Bingham plastic-modeled electrorheological material can be defined as the zero-rate intercept of the linear regression curve fit. The static yield stress (Ty,s) is defined as the stress necessary to initiate flow within the electrorheological material regardless of whether or not a Bingham model accurately describes the material's behavior. Many electrorheological materials exhibit a widely different static yield stress than dynamic yield stress. In designing a device, it is necessary to consider the possible occurrence of this phenomenon. It is advantageous for the design of devices if the electrorheological material exhibits a static yield stress approximately equivalent to the dynamic yield stress. Materials having a static yield stress that is much different than the dynamic yield stress are difficult to control in a smooth, continuous manner. When used in devices, they result in discontinuous output where performance at any given time depends on the prior shear history of the fluid.

The test geometry that is utilized by these rheometers for the characterization of ER materials is a simple concentric cylinder couette cell configuration. The material is placed in the annulus formed between an inner cylinder of radius Rl and an outer cylinder of radius R2- One of the cylinders is then rotated with an angular velocity GO while the other cylinder is held motionless. The relationship between the shear stress and the shear strain rate is then derived from this angular velocity and the torque, T, applied to maintain or resist it.

The dielectric properties of electrorheological materials of the present invention, as well as other electrorheological materials, can be obtained through the use of impedance spectroscopy. The impedance parameters that are typically measured include capacitance and conductance. From these parameters, the dielectric constant, dielectric loss factor, loss tangent and conductivity of the electro¬ rheological material can be calculated. A more complete description of the applicability of this technique to the measurement of the dielectric spectra for electrorheological materials is provided in a paper by Weiss and Carlson in the "Proceedings of the Third International Conference on Electrorheological Fluids" (ed., R. Tao, World Scientific Publishing Co., London, 1992, pp. 264-279), the entire disclosure of which is incorporated herein by reference. The following examples are given to illustrate the invention and should not be construed to limit the scope of the invention.

Purification of Chlorinated Paraffin Oil

Adsorptive Filtration

A total of 53.0 g of very fine and ultra pure neutral alumina

(Woelm N-Super 1, ICN Biomedical Inc.) is placed in a convection oven at 175°C for 12 hours. A flame-dried 3-neck reaction flask is equipped with a gas adapter, a glass stopper and a chromatography column enclosed with a rubber septum. The chromatography column is filled with the dried alumina powder. A total of 497 g of a chlorinated paraffin oil (PAROIL®-10, Dover Chemical Corp.) is cannulated into the column. Under reduced pressure the oil is pulled through the column at a rate of about 0.25 mL per minute. This procedure is repeated using the same batch of oil and freshly dried alumina. This purified 48 cstk chlorinated paraffin oil is stored in a polyethylene bottle under nitrogen.

Chemical Purification

A batch of 800 mL of a chlorinated paraffin oil (PAROIL®-10, Dover Chemical Corp.) is washed ten times with 40 mL portions of a 50:50 mixture by volume of concentrated sulfuric acid and distilled deionized water. This is followed by five additional washings with 75 mL portions of distilled deionized water. The paraffin oil is then washed three times with 35 mL portions of 10 percent NaHC03. This is followed by washing five times with 75 mL portions of distilled deionized water. The oil is then placed in a stirred flask for 24 hours with magnesium sulfate and activated charcoal. The "dry" oil is then filtered to remove the solids and stored in a polyethylene bottle.

Fractional Distillation

A batch of 400 mL of a chlorinated paraffin oil (PAROIL®-10, Dover Chemical Corp.) is purified through the use of fractional distillation. This distillation technique involves the use of a silver- jacketted vigeraux column. A purified fraction of the chlorinated paraffin oil is obtained at reduced pressure (0.01 mm Hg) and a temperature of approximately 160 °C. The purified chlorinated paraffin oil is stored in a polyethylene bottle.

For comparison purposes, Table 1 below shows the dielectric constant and conductivity (as measured using impedance spectroscopy) of the unpurified chlorinated paraffin oil and each of the chlorinated paraffin oils purified by the above techniques.

Table 1

The data in Table 1 demonstrates that the purification techniques of the present invention can significantly reduce the conductivity of an unpurified carrier fluid without dramatically affecting the dielectric constant exhibited by the fluid.

Example 1

An electrorheological material is prepared by combining 50.16 g of titanium dioxide (Ti-Pure® R960, E. I. Du Pont de Nemours & Co.), 34.58 g of the chlorinated paraffin oil purified by the adsorptive filtration procedure described above and 1.0 g of isopropyltri(dioctyl)phosphato titanate (KEN-REACT® KR12, Kenrich Petrochemical Inc.). The resulting combination of ingredients is thoroughly dispersed using a high speed disperser equipped with a 16- tooth rotary head. Before use, the titanium dioxide particles are oven- dried in a convection oven for 16 hours at a temperature of 125°C. The use of these weight amounts of ingredients corresponds to an electrorheological material containing 25 volume percent titanium dioxide particles. The formulated ER material is stored in a polyethylene bottle until mechanical and electrical properties can be tested. Electrorheological Activity at 25°C and 100°C

The electrorheological properties of the electrorheological material prepared in Example 1 are measured at 25°C and 75°C using concentric cylinder couette cell rheometry at an A.C. electric field of 2.0 kV/mm and a frequency of 1000 Hz. As shown in Table 2 below, the electrorheological material based on purified chlorinated paraffin oil is observed to exhibit substantial and similar static and dynamic yield stresses at both temperatures. Due to the low conductivity exhibited by the purified carrier fluid, this electrorheological material exhibits acceptable electrical performance at elevated temperatures. In fact, the electrorheological activity exhibited by this material actually improves as the temperature is increased.

Table 2

Bingham Plastic Behavior

The dynamic electrorheological properties (25°C) of the ER material prepared in Example 1 are further evaluated using concentric cylinder couette cell rheometry at various A.C. electric fields with a frequency of 1000 Hz. As shown in Figure 1 this ER material exhibits the development of a substantial dynamic yield stress upon the application of a variety of electric fields ranging from 1.0 to 4.0 kV/mm. The measured value of the dynamic yield stress is approximately equivalent to the zero-rate intercept of the stress versus rate data shown at each electric field. The plotted lines in Figure 1 independently intersect the Y-axis at points which have increasing value corresponding to increasing electric field strength. The electrorheological material of Example 1 therefore easily conforms to a Bingham plastic model as previously defined. Comparative Example 2

An electrorheological material is prepared according to the procedure described in Example 1 by combining 50.06 g of titanium dioxide (Ti-Pure® R960, E. I. Du Pont de Nemours & Co.), 1.00 g of isopropyltri(dioctyl)phosphato titanate (KEN-REACT® KR12, Kenrich Petrochemical Inc.) and 34.49 g of "as received," unpurified chlorinated paraffin oil (Paroil® 10, Dover Chemical Corporation). The electrorheological properties of this material are measured in accordance with the procedure of Example 1 at 25°C and the material is found to have a static yield stress of 33 Pa and a dynamic yields stress of 103 Pa. Attempts to measure the electrorheological properties of this material at 75°C were unsuccessful due to highly erratic electrical properties.

The dynamic electrorheological properties (25°C) of the electrorheological material are further evaluated using concentric cylinder couette cell rheometry at various A.C. electric fields with a frequency of 1000 Hz. As shown in Figure 2, the plotted lines intersect at approximately the same point on the Y-axis regardless of electric field strength and this electrorheological material therefore does not exhibit the expected Bingham plastic behavior upon the application of a variety of electric fields ranging from 1.0 to 3.0 kV/mm. Attempts to measure the electrorheological properties of this material at 4.0 kV/mm were unsuccessful due to highly erratic electrical properties.

In addition, the mechanical behavior of this material is unstable as exhibited by the erratic nature of the measured data.

Example 1 and comparative Example 2 demonstrate that a high dielectric constant, high conductivity carrier fluid which is unacceptable for use in electrorheological materials can be modified by the techniques of the present invention to produce a carrier fluid capable of functioning effectively in an electrorheological material.

Example 3

An electrorheological fluid is prepared in accordance with Example 1 utilizing 50.81 g of titanium dioxide (Ti-Pure® R960); 1.02 g of titanate coupling agent (KEN-REACT® KR12); and 32.56 g of 1:1 weight ratio mixture of mineral oil (Aldrich Chemical Co.) with a dielectric constant of 2.05 and a conductivity of 1.00 x 10"^ S/m and unpurified chlorinated paraffin oil (PAROIL® 10, Dover Chemical Corporation) with a dielectric constant of 7.90 and a conductivity of 1.88 x 10"7 S/m. The mineral oil chlorinated paraffin carrier fluid miscible mixture exhibits a dielectric constant of 4.59 and a conductivity of 9.00 x 10"9 S/m (as measured by impedance spectroscopy).

Electrorheological Activity at 25 C and 100°C

The electrorheological activity of the material of Example 3 is measured at 25°C and 100°C using concentric cylinder couette rheometry. The results obtained at an A.C. electric field of 2.0 kV/mm and a frequency of 1000 Hz are set forth below in Table 3.

Table 3

As can be seen from the above data, the modified carrier fluids of the present invention can be utilized to prepare stable electro¬ rheological materials which are capable of exhibiting substantial electrorheological activity over a relatively broad temperature range.

Claims

Claji sWhat is claimed is:

1. An electrorheological material comprising a modified carrier fluid and a particle component wherein the carrier fluid has been modified by extensive purification or by the formation of a miscible solution with a low conductivity carrier fluid so as to cause the modified carrier fluid to have a conductivity of less than about 1.0 x 10-⁷ S/m.

2. An electrorheological material according to Claim 1 wherein the carrier fluid has been modified by extensive purification and, prior to modification, the carrier fluid had a dielectric constant between about 7.5 and 50 and a conductivity greater than about 1.0 x 10" ⁷ S/m.

3. An electrorheological material according to Claim 2 wherein the dielectric constant is between about 7.5 and 20.

4. An electrorheological material according to Claim 2 wherein the carrier fluid is an aprotic solvent, liquid, monomeric oil, oligomeric oil or polym ric oil that possesses a net permanent dipole.

5. An electrorheological material according to Claim 4 wherein the carrier fluid is a polar aromatic or nonaromatic, halogenated or nonhalogenated straight chain, branched, or cyclic hydrocarbon, ester, ketone, ether or aldehyde compound or oil.

6. An electrorheological material according to Claim 2 wherein the carrier fluid is selected from the group consisting of chlorinated paraffin oils, brominated paraffin oils, poly(2,2,2- trifluoromethyl propylene oxide), benzaldehyde, benzonitrile, benzoyl chloride, 3-bromoaniline, butyric anhydride, γ-butyrolactone, epi- chlorohydrin, cyclohexanone, o-dichlorobenzene, diethyl maleate, diethyl malonate, diethyl oxalate, diethyl sulfate, N,N-dimethyl- acetamide, N,N-dimethylaniline, N,N-dimethylformamide, dimethyl o-phthalate, dimethyl sulfate, dimethyl sulfoxide, 2-ethoxyethyl acetate, ethyl acetoacetate, ethyl cyanoacetate, hexamethylphos- phoramide, hexanenitrile, isoquinoline, methyl cyanoacetate, nitro¬ benzene, heptyl cyanide, 2,4-pentanedione, benzyl cyanide, proprionic anhydride, sulfolane, 1,1,2,2-tetrachloroethane, 1,1,3,3-tetramethyl- urea, tributyl phosphate, γ-decanolactone, δ-decanolactone, halo¬ genated polyvinyl ethers, and oligomers of acrylonitrile, vinyl chloride, methyl methacrylate and vinyl acetate.

7. An electrorheological material according to Claim 6 wherein the carrier fluid is a halogenated paraffin oil or poly(2,2,2- trifluoromethyl propylene oxide).

8. An electrorheological material according to Claim 1 wherein the carrier fluid has been modified by the formation of a miscible solution with a low conductivity carrier fluid and, prior to modification, the carrier fluid was a high conductivity carrier fluid having a dielectric constant greater than about 7.5 and a conductivity greater than about 1.0 x 10^-7 S/m.

9. An electrorheological material according to Claim 8 wherein the low conductivity carrier fluid has a dielectric constant less than about 50 and a conductivity less than about 1.0 x 10"⁷ S/m.

10. An electrorheological material according to Claim 8 wherein the high conductivity carrier fluid is selected from the group consisting of chlorinated paraffin oils, brominated paraffin oils, poly(2,2,2-trifluoromethyl propylene oxide), benzaldehyde, benzo- nitrile, benzoyl chloride, 3-bromoaniline, butyric anhydride, γ-butyro- lactone, epichlorohydrin, cyclohexanone, o-dichlorobenzene, diethyl maleate, diethyl malonate, diethyl oxalate, diethyl sulfate, N,N- dimethylacetamide, N,N-dimethylaniline, N,N-dimethylformamide, dimethyl o-phthalate, dimethyl sulfate, dimethyl sulfoxide, 2-ethoxy- ethyl acetate, ethyl acetoacetate, ethyl cyanoacetate, hexamethyl- phosphoramide, hexanenitrile, isoquinoline, methyl cyanoacetate, nitrobenzene, heptyl cyanide, 2,4-pentanedione, benzyl cyanide, proprionic anhydride, sulfolane, 1,1,2,2-tetrachlorethane, 1,1,3,3-tetra¬ methylurea, tributyl phosphate, γ-decanolactone, δ-decanolactone, halogenated polyvinyl ethers, and oligomers of acrylonitrile, vinyl chloride, methyl methacrylate and vinyl acetate.

11. An electrorheological material according to Claim 10 wherein the carrier fluid is a halogenated paraffin oil or poly(2,2,2- trifluoromethyl propylene oxide).

12. An electrorheological material according to Claim 8 wherein the low conductivity carrier fluid is selected from the group consisting of mineral oils, silicone oils, white oils, paraffin oils, hydraulic oils, transformer oils, diesters, polyoxyalkylenes, per- fluorinated polyethers, fluorinated hydrocarbons, fluorinated silicones, hindered ester compounds and mixtures thereof.

13. An electrorheological material according to Claim 12 wherein the low conductivity carrier fluid is a silicone oil, a mineral oil or a paraffin oil.

14. An electrorheological material according to Claim 8 wherein the miscible solution comprises a 1:1 weight ratio mixture of mineral oil and unpurified chlorinated paraffin oil.

15. An electrorheological material according to Claim 1 wherein the particle component is selected from the group consisting of amorphous silicas; synthetic silicas; precipitated silicas; fumed silicas; silicates; aluminum silicates; ion exchange resins and other inorganic particles such as those composed of titanium dioxide, barium titanate, lithium hydrazinium sulfate; insulated metallic particulates; polyvinyl alcohols; polyhydric alcohols; silicone ionomer reaction products; monosaccharides; porphin systems; metallo- porphin systems; poly(acene-quinone) polymers; polymeric Schiff bases; anionic surfactants; polyelectrolytes; carbonaceous parti¬ culates; ionic and non-ionic dye compounds; atomically polarizable particles; and other organic and polymeric particles such as those composed of polymethacrylic acid salts and copolymers of phenol, aldehydes, olefins, ethers and/or acids.

16. An electrorheological material according to Claim 15 wherein the particle component is an insulated metallic particle or an atomically polarizable particle.

17. An electrorheological material according to Claim 1 further comprising an activator selected from the group consisting of water; methyl, ethyl, propyl, isopropyl, butyl and hexyl alcohols; ethylene glycol; diethylene glycol; propylene glycol; glycerol; formic, acetic, sulfuric and lactic acids; aliphatic, aromatic and heterocyclic amines, including primary, secondary and tertiary amino alcohols and amino esters that have from 1-16 atoms of carbon in the molecule; methyl, butyl, octyl, dodecyl, hexadecyl, diethyl, diisopropyl and dibutyl amines; ethanolamine; propanolamine; ethoxyethylamine; dioctylamine; triethylamine; trimethylamine; tributylamine; ethylene- diamine; propylene- diamine; triethanolamine; triethylenetetramine; pyridine; morpholine; imidazole; and mixtures thereof.

18. An electrorheological material according to Claim 17 wherein the activator is water.

19. An electrorheological material according to Claim 1 further comprising a surfactant selected from the group consisting of glycerol monooleate; sorbitan sesquioleate; stearates; laurates; fatty acids; fatty alcohols; steric stabilizing amino-functional, hydroxy- functional, acetoxy-functional, or alkoxy-functional polysiloxanes; block copolymers of poly(ethylene oxide) and poly(propylene oxide); hyperdispersants; fluoroaliphatic polymeric esters; and coupling agents such as titanate, aluminate or zirconate coupling agents.

20. An electrorheological material according to Claim 19 wherein the surfactant is selected from the group consisting of amino- functional polydimethylsiloxanes, fluoroaliphatic polymeric esters, hyperdispersants and coupling agents.

21. An electrorheological material according to Claim 1 wherein the carrier fluid is present in an amount ranging from about 50 to 95 percent by volume and the particle component is present in an amount ranging from about 5 to 50 percent by volume of the total electrorheological material.

22. An electrorheological material according to Claim 21 wherein the carrier fluid is present in an amount ranging from about 60 to 85 percent and the particle component is present in an amount ranging from about 15 to 40 percent.

23. An electrorheological material according to Claim 1 wherein the extensive purification is selected from the group consisting of chemical purification, distillation, adsorptive filtration, electrostatic separation and combinations thereof.