Compositions – Liquid crystal compositions
Reexamination Certificate
2000-09-21
2003-04-15
Koslow, C. Melissa (Department: 1755)
Compositions
Liquid crystal compositions
Reexamination Certificate
active
06547986
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to magnetorheological grease compositions which have increased resistance to settling, wear, corrosion and oxidation. More specifically, the present invention relates to magnetorheological grease compositions that utilize high concentrations of thickening agents.
BACKGROUND OF THE INVENTION
Magnetorheological compositions typically include micron-sized magnetic-responsive particles. In the presence of a magnetic field, the magnetic-responsive particles become polarized and are thereby organized into chains of particles or particle fibrils. The particle chains increase the apparent viscosity (flow resistance) of the fluid, resulting in the development of a solid mass having a yield stress that must be exceeded to induce onset of flow of the magnetorheological fluid. The particles return to an unorganized state when the magnetic field is removed, which lowers the viscosity of the magnetorheological composition.
Magnetorheological compositions are responsive to a magnetic field, and exhibit controllable behavior accordingly. However, many magnetorheological materials suffer from excessive gravitational particle settling which can interfere with the magnetorheological activity of the material due to non-uniform particle distribution. One cause of gravitational particle settling in magnetorheological materials is the large difference between the specific gravity of the magnetic particles and that of the carrier fluid which can cause rapid particle settling in a magnetorheological material.
U.S. Pat. No. 5,645,752 discloses a magnetorheological material which contains a thixotropic additive to provide stability against particle settling. An optional colloidal additive may be utilized in combination with the thixotropic agent in order to facilitate the formation of a thixotropic network.
U.S. Pat. No. 5,382,373 relates to a magnetorheological material that utilizes a particle component which is capable of independently increasing the yield stress of the overall magnetorheological material. A surfactant to disperse the particle component may be optionally utilized. The surfactant, if utilized, is preferably a hydrophobic fumed silica, a “dried” precipitated silica gel, a phosphate ester, a fluoroaliphatic polymeric ester, or a coupling agent which is used in an amount ranging from about 0.1 to 20 percent by weight relative to the weight of the particle component.
U.S. Pat. No. 4,992,190 discloses a magnetic fluid composition that is responsive to a magnetic field. The fluid composition comprises a vehicle, solid magnetizable particles suspended in the vehicle, and a silica gel dispersant. An additional surfactant may be employed to impart thixotropic properties.
U.S. Pat. No. 2,661,825 describes an apparatus capable of controlling the slippage between moving parts through the use of magnetic or electric fields. The space between the moveable parts is filled with a field responsive medium. The development of a magnetic or electric field flux through this medium results in control of resulting slippage. A fluid responsive to the application of a magnetic field is described to contain carbonyl iron powder and light weight mineral oil.
U.S. Pat. No. 3,385,793 relates to an electroviscous fluid which contains a conductive material. The fluid includes silica gel and silicone oil, which functions as a vehicle. The fluid can also contain iron particles disclosed to function as a conductive agent. The composition is not described as one responsive to an electromagnetic field.
U.S. Pat. No. 3,006,656 relates to a magnetic material for shock absorbers, which comprises carbonyl iron powder, and an additive such as oil, kerosene, benzene, graphite, chalk, mica, soapstone, a silicone, and glycerine.
U.S. Pat. No. 2,751,352 describes a magnetic fluid for a clutch or like apparatus, which may comprise iron powder, organic lubricants as a base carrier and an oleophobic-ferrophilic liquid dispersant.
Magnetorheological technology is useful in devices or systems for controlling vibration and/or noise. However, magnetorheological fluids commonly have such low viscosity that an o-ring or other type of fluid sealing mechanism is required to seal the magnetorheological fluids within the devices. Magnetorheological devices may require precisely toleranced components, expensive seals, expensive bearings, and relatively large volumes of magnetically controllable fluid. The costs associated with such devices may be prohibitive to their use in certain applications.
Moreover, the use of low viscosity magnetorheological fluids requires high efficiency during the mixing of the magnetorheological fluids so as to avoid the settling problems introduced by the high specific gravity of the metallic magnetic particles.
A need exists in the art, therefore, for a magnetorheological grease composition which has a high enough consistency to remain in the device without the aid of a sealing mechanism. There is further a need in the art for a magnetorheological grease composition with resistance to settling of the magnetic particles. There also is a need in the art for a magnetorheological composition that is resistant to wear and oxidation. This invention provides such a composition.
SUMMARY OF THE INVENTION
The magnetorheological compositions according to the present invention comprise magnetic-responsive particles, a carrier fluid and at least one thickening agent, wherein the thickening agent is employed in a substantially effective amount to form a grease-like consistency. Preferably, the thickening agent is present in an amount ranging from about 30 to 90 percent by volume relative to the total volume of the magnetorheological grease composition.
In one aspect, the magnetorheological grease compositions according to the present invention comprise magnetic-responsive particles, a carrier fluid and at least one thickening agent, wherein the grease composition has a NLGI consistency number between about 00 and about 4.
In another aspect, the magnetorheological grease composition of the invention comprises magnetic-responsive particles, a carrier fluid and at least one thickening agent wherein the thickening agent provides the composition with a consistency effective to allow the composition to maintain a position within a magnetorheological device.
The magnetorheological grease compositions of the invention exhibit excellent resistance to settling and wear because of a substantially increased concentration of the thickening agents in the magnetorheological grease compositions.
DETAILED DESCRIPTION OF THE INVENTION
“Force output” as used herein means the damping force, torque, braking force or similar force depending on the device. “Yield strength” is the force required to exceed the yield stress. The “yield stress” is the stress that must be exceeded to induce onset of flow of the magnetorheological composition when subject to the presence of a magnetic field or in the “on-state.” The absence of a magnetic field is referred to herein as the “off-state.” “On-state forces” as used herein are the resultant forces of a device as a result of applying a magnetic field. “Off-state forces” means the forces generated by a device when no magnetic field is applied.
The present invention relates to a magnetorheological grease composition which comprises magnetic-responsive particles, a carrier fluid and at least one thickening agent. The thickening agent may be provided in an amount effective to produce a gel-like structure for the grease composition. The gel structure provides sufficient consistency to enable the grease composition to maintain its position in a magnetorheological device during its use in such a magnetorheological device. The term “grease” as used herein means a semi-fluid to solid product of a dispersion of a thickener in a carrier fluid. The term “thickener” means a substance dispersed in a carrier fluid to form the product's structure. “Consistency” means the degree of resistance to movement under stress and can be measured according to ASTM D 217 which provides sta
Carlson J. David
Kintz K. Andrew
Munoz Beth C.
Sessoms Julia D.
Dearth Miles B.
Koslow C. Melissa
Lord Corporation
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