Compositions – Magnetic – Iron-oxygen compound containing
Reexamination Certificate
2000-05-03
2002-11-05
Koslow, C. Melissa (Department: 1755)
Compositions
Magnetic
Iron-oxygen compound containing
C252S062530, C252S062540, C252S062550
Reexamination Certificate
active
06475404
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to mixtures of dry powders that form functional magnetorheological fluids when a carrier fluid is added to the mixtures. These magnetorheological fluids 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.
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.
A limitation of these magnetorheological fluids is that they are prepared with organic carrier fluids, such as oils, which can become polymerized, degrade, promote growth of bacteria and be flammable. In addition, organic carrier fluids can be incompatible with components of the device in which it is used.
To avoid the disadvantages of oil-based magnetorheological fluids, some attempts have been made to prepare magnetorheological fluids that do not include organic carrier fluids or which only include water-miscible organic solvents. Prior attempts at preparing water-based magnetorheological fluids used various thickening agents, such as xanthan gum and carboxymethyl cellulose as described in U.S. Pat. No. 5,670,077. However, these formulations can be difficult to mix, tend to exhibit clumping problems and tend to settle over time. Moreover, both oil-based and water-based magnetorheological fluids have the disadvantage of being bulky in transit, causing high transportation costs.
Thus, it would be advantageous to have a magnetorheological fluid which avoids the problems of the prior art, is easy to transport and is stable both during transport and after the magnetorheological fluid is put to use. It further would be advantageous to have a magnetorheological fluid which can be easily mixed and will not settle over time. It also would be advantageous to have a product which can be easily shipped to the end user. The present invention provides such a product.
SUMMARY OF THE INVENTION
The present invention provides a mixture of components that will form a magnetorheological fluid when a carrier fluid is added. The carrier fluid may be an aqueous or an organic fluid. The mixture of components utilized is substantially dry and comprises magnetic-responsive powder and a thixotropic agent. This mixture provides an instant magnetorheological fluid mix or concentrate which can be shipped and stored in powder form until the user is ready to form a magnetorheological fluid. A magnetorheological fluid of the present invention is comprised of the magnetorheological fluid concentrate and an aqueous fluid or an organic fluid. In a preferred embodiment, the magnetorheological fluid concentrate comprises about 90% to about 99.9% by weight iron powder, about 0.1% to about 10% by weight rust inhibiting agent and about 0.1% to about 5% by weight, water-soluble thixotropic agent.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is directed to a magnetorheological fluid concentrate or instant magnetorheological fluid mix which can be used to form a magnetorheological fluid. The concentrate is a mixture of substantially dry powders that will form a magnetorheological fluid upon the addition of a carrier fluid such as water or oil. In particular, the magnetorheological fluid concentrate of the invention comprises a substantially dry mixture of a magnetic-responsive powder and a thixotropic agent. The term “substantially dry” means that the powders generally will have less than about 2% by weight water or moisture. In a preferred embodiment, the powders will have less than about 0.5% by weight moisture. Thus, the concentrate or instant mix of the invention does not contain a significant liquid component and is prepared without addition of a fluid or liquid. The substantial absence of liquid in the concentrate of the invention allows for easy transport of the concentrate to the end user.
The instant magnetorheological fluid mix is prepared by mixing or dispersing the dry thixotropic agent in the magnetic-responsive powder prior to transporting the magnetorheological fluid concentrate to the location of the end user. The combination of dry ingredients can then be easily mixed in a carrier fluid such as water or oil for use as a magnetorheological fluid. In an alternative embodiment, the magnetorheological fluid concentrate may contain the magnetic-responsive particles only, with the thixotropic agent added later by the end user prior to forming the magnetorheological fluid. In a further alternative embodiment, the concentrate may contain all the components of a magnetorheological fluid such as the thixotropic agent, except the magnetic-responsive powder, which powder is added at the time the magnetorheological fluid is formed.
The magnetic-responsive powder useful in the present invention may be any powder known to exhibit magnetorheological activity. In a preferred embodiment, the magnetic-responsive powder is iron powder. The iron powder may be any form of powdered iron, particularly carbonyl iron, reduced carbonyl iron, potato iron, crushed iron, milled iron, melt-sprayed iron, iron alloys or mixtures of any of the foregoing. In the most preferred embodiment, the iron will be reduced carbonyl iron.
The particle size of the magnetic-responsive powder should be selected so that it exhibits multi-domain characteristics when subjected to a magnetic field. Average particle diameter sizes for the magnetic-responsive powder are generally between about 0.1 micron and about 100 microns, preferably between about 0.5 micron and about 50 microns. In the most preferred embodiment, the average particle diameter size of the magnetic-responsive powder is about 1 micron to about 10 microns.
The magnetic-responsive powder is present in the magnetorheological fluid concentrate in an amount of about 90% to about 99.9% by weight, preferably in an amount of about 95% to about 99% by weight.
The thixotropic agent is any agent which can mixed or dispersed in a substantially dry form with the magnetic-responsive powder and which provides thixotropic rheology when hydrated. The thixotropic agent is selected based on the desired carrier fluid. For example, the thixotropic agent should be water-soluble if the magnetorheological fluid will be formed by adding an aqueous fluid to the substantially dry mix. Magnetorheological fluids formed from the magnetorheological fluid concentrates of the invention which utilize water-soluble thixotropic agents are easily mixed and avoid problems with clumping, as is sometimes seen with aqueous magne
Dearth Miles B.
Koslow C. Melissa
Lord Corporation
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