Compositions – Magnetic – Flaw detection or magnetic clutch
Reexamination Certificate
1999-07-01
2001-03-20
Koslow, C. Melissa (Department: 1755)
Compositions
Magnetic
Flaw detection or magnetic clutch
28, 28
Reexamination Certificate
active
06203717
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed to fluid materials that exhibit substantial increases in flow resistance when exposed to magnetic fields.
BACKGROUND OF THE INVENTION
Magnetorheological fluids are fluid compositions that undergo a change in apparent viscosity in the presence of a magnetic field. The fluids typically include ferromagnetic or paramagnetic particles dispersed in a carrier fluid. The particles become polarized in the presence of an applied magnetic field, and become organized into chains of particles within the fluid. The particle chains increase the apparent viscosity (flow resistance) of the fluid. The particles return to an unorganized state when the magnetic field is removed, which lowers the viscosity of the fluid.
Magnetorheological fluids have been proposed for controlling damping in various devices, such as dampers, shock absorbers, and elastomeric mounts. They have also been proposed for use in controlling pressure and/or torque in brakes, clutches, and valves. Magnetorheological fluids are considered superior to electrorheological fluids in many applications because they exhibit higher yield strengths and can create greater damping forces.
Magnetorheological fluids are distinguishable from colloidal magnetic fluids or ferrofluids. In colloidal magnetic fluids, the particle size is generally between 5 and 10 nanometers, whereas the particle size in magnetorheological fluids is typically greater than 0.1 micrometers, usually greater than 1.0 micrometers. Colloidal magnetic fluids tend not to develop particle structuring in the presence of a magnetic field, but rather, the fluid tends to flow toward the applied field.
Some of the first magnetorheological fluids, described, for example, in U.S. Pat. Nos. 2,575,360, 2,661,825, and 2,886,151, included reduced iron oxide powders and low viscosity oils. These mixtures tend to settle as a function of time, with the settling rate generally increasing as the temperature increases. One of the reasons why the particles tend to settle is the large difference in density between the oils (about 0.7-0.95 g/cm
3
) and the metal particles (about 7.86 g/cm
3
for iron particles). The settling interferes with the magnetorheological activity of the material due to non-uniform particle distribution. Often, it requires a relatively high shear force to re-suspend the particles.
Various surfactants and suspension agents have been added to the fluids to keep the particles suspended in the carrier. Conventional surfactants include metallic soap-type surfactants such as lithium stearate and aluminum distearate. These surfactants typically include a small amount of water, which can limit the useful temperature range of the materials.
In addition to particle settling, another limitation of the fluids is that the particles tend to cause wear when they are in moving contact with the surfaces of various parts. It would be advantageous to have magnetorheological fluids that do not cause significant wear when they are in moving contact with surfaces of various parts. It would also be advantageous to have magnetorheological fluids that are capable of being re-dispersed with small shear forces after the magnetic-responsive particles settle out. The present invention provides such fluids.
SUMMARY OF THE INVENTION
Magnetorheological fluid compositions, devices including the compositions, and methods of preparation and use thereof are disclosed. The compositions include a carrier fluid, magnetic-responsive particles, and a hydrophobic organoclay. The fluids typically develop structure when exposed to a magnetic field in as little as a few milliseconds. The fluids can be used in devices such as clutches, brakes, exercise equipment, composite structures and structural elements, dampers, shock absorbers haptic devices, electric switches, prosthetic devices, including rapidly setting casts, and elastomeric mounts.
The hydrophobic organoclay is present as an anti-settling agent, which provides for a soft sediment once the magnetic particles settle out. The soft sediment provides for ease of re-dispersion. The hydrophobic organoclay is also substantially thermally, mechanically and chemically stable and typically has a hardness less than that of conventionally used anti-settling agents such as silica or silicon dioxide. In addition, it has been unexpectedly found that hydrophilic clays do not provide the soft sedimentation exhibited by the hydrophobic organoclays. The fluids of the invention typically shear thin at shear rates less than 100/sec
−1
, and typically recover their structure after shear thinning in less than five minutes.
DETAILED DESCRIPTION OF THE INVENTION
The compositions form a thixotropic network that is effective at minimizing particle settling and also in lowering the shear forces required to re-suspend the particles once they settle. The compositions described herein have a relatively low viscosity, do not settle hard, and can be easier to re-disperse than conventional magnetorheological fluids, including those which contain conventional anti-settling agents such as silicon dioxide or silica.
Thixotropic networks are suspensions of colloidal or magnetically active particles that, at low shear rates, form a loose network or structure (for example, clusters or flocculates). The three dimensional structure supports the particles, thus minimizing particle settling. When a shear force is applied to the material, the structure is disrupted or dispersed. The structure reforms when the shear force is removed.
The compositions typically have at least ten percent less sediment hardness than comparative fluids that include silica rather than the hydrophobic organoclay, where the test involves repeated heating and cooling cycles over a two week period. The compositions also typically cause at least ten percent less device wear than comparative fluids that include silica rather than the hydrophobic organoclay.
I. Magnetorheological Fluid Composition
A. Magnetic-Responsive Particles
Any solid that is known to exhibit magnetorheological activity can be used, specifically including paramagnetic, superparamagnetic and ferromagnetic elements and compounds. Examples of suitable magnetizable particles include iron, iron alloys (such as those including aluminum, silicon, cobalt, nickel, vanadium, molybdenum, chromium, tungsten, manganese and/or copper), iron oxides (including Fe
2
O
3
and Fe
3
O
4
), iron nitride, iron carbide, carbonyl iron, nickel, cobalt, chromium dioxide, stainless steel and silicon steel. Examples of suitable particles include straight iron powders, reduced iron powders, iron oxide powder/straight iron powder mixtures and iron oxide powder/reduced iron powder mixtures. A preferred magnetic-responsive particulate is carbonyl iron, preferably, reduced carbonyl iron.
The particle size should be selected so that it exhibits multi-domain characteristics when subjected to a magnetic field. Average particle diameter sizes for the magnetic-responsive particles are generally between 0.1 and 1000 &mgr;m, preferably between about 0.1 and 500 &mgr;m, and more preferably between about 1.0 and 10 &mgr;m, and are preferably present in an amount between about 5 and 50 percent by volume of the total composition.
B. Carrier fluids
The carrier fluids can be any organic fluid, preferably a non-polar organic fluid, including those previously used by those of skill in the art for preparing magnetorheological fluids as described, for example. The carrier fluid forms the continuous phase of the magnetorheological fluid. Examples of suitable fluids include silicone oils, mineral oils, paraffin oils, silicone copolymers, white oils, hydraulic oils, transformer oils, halogenated organic liquids (such as chlorinated hydrocarbons, halogenated paraffins, perfluorinated polyethers and fluorinated hydrocarbons) diesters, polyoxyalkylenes, fluorinated silicones, cyanoalkyl siloxanes, glycols, and synthetic hydrocarbon oils (including both unsaturated and saturated). A mixture of these fluids may be used as the carrier component o
Adams Gary W.
Kitchin John R.
Munoz Beth C.
Ngo Van Trang
Koslow C. Melissa
Lord Corporation
Rupert Wayne W.
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