FIELD OF THE INVENTION

The invention relates to magnetorheological fluid compositions that have reduced friction and improved low-temperature viscosity performance. More specifically, the present invention relates to magnetorheological fluid compositions containing an amine oleate salt additive, and optional molybdenum disulfide particles, which work in compliment with traditional magnetorheological fluid additives.

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/cm3) and the metal particles (about 7.86 g/cm3 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. Unfortunately, additives that lower friction in MR fluids tend to make settling worse, often resulting in a “hard-pack” of settled particles on the bottom of the chamber. It would be advantageous to have magnetorheological fluids that provide a better balance of low-friction characteristics and anti-settling properties. This would reduce wear when the fluid is moving contact with surfaces of various parts, and allow for easier re-dispersing with small shear forces after the magnetic-responsive particles settle. The present invention provides such fluids.

SUMMARY OF THE INVENTION

In a first embodiment of the present invention, a magnetorheological fluid is provided comprising magnetically responsive particles, a carrier fluid, and a friction reducing agent comprising an amine oleate salt. The amine oleate salt preferably comprises an amine salt of a carboxylic acid.

In another embodiment of the present invention, the amine portion of the amine oleate salt comprises the structure:
CR3—(CR2)x—CR3
preferably where at least two of the R groups are selected from NH2and NHR1, and most preferably R1comprises a tallow alkyl. In a further embodiment of the present invention, the carboxylic acid comprises one carboxylic acid functionality and comprises about 10 to about 24 carbon atoms, and preferably about 16-18 carbon atoms. In a preferred embodiment of the present invention, the amine oleate salt comprises at least one of a diamine dioleate or a triamine dioleate, and most preferably wherein the amine oleate salt comprises N-(tallowalkyl)-1,3,-propanediamine dioleate. In another preferred embodiment of the present invention, the amine oleate salt is present in an amount from about 0.4 to 0.6 percent by weight based upon the total weight of the MR fluid.

In a further embodiment of the present invention, the fluid further comprises at least one of an organomolybdenum, a phosphorous-containing additive, or a sulfur-containing additive, preferably an organomolybdenum and a thiophosphorous compound. In a still further embodiment of the present invention, the carrier fluid comprises at least one of mineral oil, paraffin oil, cycloparrafin oil, and synthetic hydrocarbon oil, preferably a poly-α-olefin.

In another embodiment of the present invention, the fluid further comrises molybdenum disulfide with an average particle size of less than 2.0 microns, preferably about 1.5 microns or less.

In an additional embodiment of the present invention, the inventive fluid at −20° C. the viscosity is at least 75% less than the viscosity of an otherwise identical fluid without the amine oleate salt additive, wherein the viscosity is measured on an ARES-G2 rheomoeter with a 25 mm top plate and 40 mm bottom cup. Further, the coefficient of friction of this fluid is at least 50% of the coefficient of friction measured in an otherwise identical fluid without the amine oleate salt additive, wherein the coefficient of friction is measured on an ARES-G2 rheomoeter with a “ball on three ball” configuration.

Thus, there has been outlined, rather broadly, the more important features of the invention in order that the detailed description that follows may be better understood and in order that the present contribution to the art may be better appreciated. There are, obviously, additional features of the invention that will be described hereinafter and which will form the subject matter of the claims appended hereto. In this respect, before explaining several embodiments of the invention in detail, it is to be understood that the invention is not limited in its application to the details and construction and to the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways.

It is also to be understood that the phraseology and terminology herein are for the purposes of description and should not be regarded as limiting in any respect. Those skilled in the art will appreciate the concepts upon which this disclosure is based and that it may readily be utilized as the basis for designating other structures, methods and systems for carrying out the several purposes of this development. It is important that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention.

So that the manner in which the above-recited features, advantages and objects of the invention, as well as others which will become more apparent, are obtained and can be understood in detail, a more particular description of the invention briefly summarized above may be had by reference to the embodiment thereof which is illustrated in the appended drawings, which drawings form a part of the specification and wherein like characters of reference designate like parts throughout the several views. It is to be noted, however, that the appended drawings illustrate only preferred and alternative embodiments of the invention and are, therefore, not to be considered limiting of its scope, as the invention may admit to additional equally effective embodiments.

