Compositions – Magnetic – Flaw detection or magnetic clutch
Reexamination Certificate
2001-09-21
2003-10-28
Koslow, C. Melissa (Department: 1755)
Compositions
Magnetic
Flaw detection or magnetic clutch
Reexamination Certificate
active
06638443
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to magnetorheological fluids and, in particular, to base liquids suitable for magnetorheological fluid formulations.
BACKGROUND OF THE INVENTION
Magnetorheological (MR) fluids are substances that exhibit an ability to change their flow characteristics by several orders of magnitude and in response times on the order of milliseconds under the influence of an applied magnetic field. An analogous class of fluids are electrorheological (ER) fluids which exhibit a like ability to change their flow or rheological characteristics under the influence of an applied electric field. In both instances, these induced rheological changes are completely reversible. The utility of these materials is that suitably configured electromechanical actuators which use magnetorheological or electrorheological fluids can act as a rapidly responding active interface between computer-based sensing or controls and a desired mechanical output. With respect to automotive applications, such materials are seen as a useful working media in shock absorbers, for controllable suspension systems, vibration dampers in controllable powertrain and engine mounts, and in numerous electronically controlled force/torque transfer devices, such as clutches and brakes.
MR fluids are noncolloidal suspensions of finely divided (typically one to 100 microns in diameter) low coercivity, magnetizable particles of a material such as iron, nickel, cobalt, and their magnetic alloys dispersed in a base liquid or liquid vehicle such as a mineral oil, synthetic hydrocarbon, water, silicone oil, esterified fatty acid or other suitable organic liquid. MR fluids have an acceptably low viscosity in the absence of a magnetic field but display large increases in their dynamic yield stress when they are subjected to a magnetic field of, for example, about one Tesla. At the present state of development, MR fluids appear to offer significant advantages over ER fluids, particularly for automotive applications, because the MR fluids are less sensitive to common contaminants found in such environments, and they display greater differences in Theological properties in the presence of a modest applied field. Examples of magnetorheological fluids are illustrated, for example, in U.S. Pat. No. 4,992,190 issued Feb. 12, 1991, entitled “Fluid Responsive to a Magnetic Field”; U.S. Pat. No. 5,167,850 issued Dec. 1, 1992, entitled “Fluid Responsive to a Magnetic Field”; U.S. Pat. No. 5,354,488 issued Oct. 11, 1994, entitled “Fluid Responsive to a Magnetic Field”; U.S. Pat. No. 5,382,373 issued Jan. 17, 1995, entitled “Magnetorheological Particles Based on Alloy Particles”; and U.S. Pat. No. 5,667,715 issued Sep. 16, 1997, entitled “Magnetorheological Fluids.”
As suggested in the above patents and elsewhere, the viscosity of a typical MR fluid, in the absence of a magnetic field, is a function of variables such as base liquid composition, particle composition, particle size, the particle loading, temperature, and the like. However, in the presence of an applied magnetic field, the suspended magnetizable particles agglomerate to thicken or gel the MR fluid and drastically increase its effective viscosity. In the absence of a magnetic field, the base liquid must have an acceptable viscosity over a range of continuous operating temperatures. The viscosity of the MR fluid is acceptable if the base liquid is flowable at all temperatures within the range of continuous operating temperatures. For example, a suitable base liquid should have a viscosity in the range of about 13 centipoise (cp) to about 16 cp at a continuous operating temperature of about 20° C., and a viscosity in the range of about 90 cp to about 120 cp at a continuous operating temperature of about −20° C.
The base liquid must also exhibit compatibility with any elastomeric seals which the MR fluid wets in the MR device and which maintain the MR device liquid-tight. Furthermore, the base liquid must have a depressed volatility so that significant amounts of MR fluid are not vaporized or volatized. The elastomeric seals in MR devices are neither designed nor intended to provide a gas-tight fit. As a result, volatized base liquid can escape from the MR device by permeating between the elastomeric seals and their respective sealing surfaces. Finally, the base liquid must have a pour point that is less than the minimum continuous operating temperature. The pour point of the base liquid represents the lowest ambient temperature at which the MR device can operate.
MR devices utilized in certain automotive applications subject the MR fluid to continuous operating temperatures ranging between about −40° C. and about 100° C. Synthetic hydrocarbon base liquids currently used for such MR fluids typically contain a mixture of synthetic hydrocarbons known as polyalphaolefins or PAOs that are derived from the C
10
monomer 1-decene, H
2
C:CH(CH
2
)
7
CH
3
. Dimer 1-decene polyalphaolefin has a 20-atom carbon chain length and is oligomerized from the monomer. Dimer 1-decene polyalphaolefin has an acceptable viscosity over the operating temperature range of such MR device applications. However, dimer 1-decene polyalphaolefin has an unacceptably high volatility if heated to a temperature near the upper end of the operating temperature range of the aforementioned MR devices. Trimer 1-decene polyalphaolefin is a 30-atom carbon chain length molecule formed by a oligomerization reaction from the monomer. Trimer 1-decene polyalphaolefin has a negligible volatility when heated to a temperature near the upper end of the operating temperature range but has an unacceptably high viscosity over the operating temperature range of such MR devices. To provide a base liquid with an acceptable volatility and viscosity and a volatility considered suitable for use in fluid formulations used in such MR devices, trimer 1-decene polyalphaolefin and dimer 1-decene polyalphaolefin are blended.
The viscosity and the volatility of these mixtures of dimer 1-decene polyalphaolefin and trimer 1-decene polyalphaolefin, such as a typical 50:50 blend of a mixture by volume, are superior in these MR device applications to a base liquid comprising either one of the 1-decene polyalphaolefins alone. The addition of dimer 1-decene polyalphaolefin to the blend lowers the effective viscosity of the mixture to an acceptable value. However, the significant volatility of the dimer 1-decene polyalphaolefin at temperatures less than about 100° C. contributes to an increasingly significant loss of the base liquid of the MR fluid. Thus, an MR device containing an MR fluid formulated with a base liquid consisting of a 50:50 mixture of dimer 1-decene polyalphaolefin and trimer 1-decene polyalphaolefin would exhibit a significant loss of base liquid about its elastomeric seals when the MR device is operating at a continuous operating temperature near 100° C.
In certain MR devices used in automotive applications for vibration damping, annular elastomeric seals are utilized to provide a dynamic seal for a piston rod attached to a piston which reciprocates in response to the applied vibrations. The inner and outer diameters of such annular elastomeric seals can be dimensioned so as to provide liquid-tight seals, respectively, with the exterior of the moving piston rod and with the gland or sealing groove in which the seal is captured. However, such sizing would result in a high friction between the piston rod and the elastomeric seal if the seal experiences a volumetric expansion when exposed to the MR fluid.
Seal swell is the swelling of elastomeric gaskets or seals as a result of exposure to petroleum, synthetic lubricants, or other hydraulic fluids. Elastomeric seal materials vary widely in their resistance to the effect of such fluids. To take advantage of the volumetric expansion due to seal swelling, the elastomeric seals in MR devices are intentionally undersized to minimize the friction between the piston rod and the seal and to provide a moderate amount of swelling which is relied upon to improv
Foister Robert T.
Iyengar Vardarajan R.
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