Note: Descriptions are shown in the official language in which they were submitted.
~ ~94/10694 2~ . Pcr/uss3/10~8s 1~`
De~cri~on
~AGNETO~HEOLOGICAL MATEE~LALS UTILI~G
SURFACE-MODIFIED PARTICLES
Technical Field
.~ The present invention relates to certain fluid materials which
exhibit substantial increases in flow resistance when exposed to
magnetic fields. More specifically, the present invention relates to
m2gneto-rheological materials that utilize a surface-modified particle
component in order to enhance yield strength.
Background Art
Fluid compositions which uIldergo a change in apparent
viscosity in the presence of a magnetic field are commonly referTed to
as Bingham magnetic fluids or magnetorheological materials.
Magnetorheological materials normally are comprised of ferro-
15 magnetic or paramagnetic particles, typically greater than 0.1micrometers in diameter, dispersed within a carrier fluid and in the
presence of a magnetic field, the particles become polarized and are
thereby organized into chains of particles within the fluid. The chains
of particles act to increase the apparent viscosity or flow resistance of
2~ the overall material and in the absence of a magnetic field7 the
particles return to an unorganized or free state and the apparent
viscosity or flow resistance of the overall material is correspondingly
reduced. These Bingham magnetic fluid compositions exhibit
controllable behavior similar to that commonly obser~ed for electro-
25 rheolog~cal materials, which are responsive to an electr~c field insteadof a magnetic field.
Both electrorheological and magnetorheological materials are
useful in providing varying damping forces within devices, such as
dampers, shock absorbers and elastomeric mounts, as well as in
30 controlling torque and or pressure levels in various clutch, brake and
valve devices. Magnetorheological mateF~als inherently offer several
advantages over electrorheological materials in these applications.
2~ sn~l ~
WO 94t10694 PCI/US93/1028'`
Magnetorheological fluids exhibit higher yield strengths than
electrorheological materials and are, therefore, capable of generating
greater damping forces. Furthermore, magnetorheological materials
are activated by magnetic fields which are eas;ly produced by simple,
5 lovr voltage electromagnetic coils as compared to the expensive high
voltage power supplies required to effectively operate electrorheological
materials. A more specific description of the type of devices in which
magnetorheologic~l materials can be effectively utilized is provided in
co-pending U.S. Patent Application Serial Nos. 07/900,571 and
10 07/900,567 entitled "Magnetorheological Fluid Dampers" and
"Magnetorheological Fluid Devices," respectively, both filed on June
18, 1992, the entire contents of which are incorporated herein by
reference.
Magnetorheological or Bingham magnetic fluids are distin-
15 guishable from colloidal magnetic fluids or ferrofluids. In colloidalmagnetic fluids the particles are typically 5 to 10 nanometers in
diameter. Upon the application of a magnetic field, a colloidal
ferrofluid does not e~hibit particle structuring or the development of a
resistance to flow. Instead, colloidal magnetic fluids experience a
2~ body force on the entire material that is proportional to the magnetic
field gradient. This force causes the entire colloidal ferrofluid to be
attracted to regions of high magnetic field strength.
Magnetorheological fluids and corresponding devices have
been discussed in various patents and publications. For e~cample, U.S.
25 Pat. No. 2,575,360 provides a description of an electromechanically
controllable torque-applying device that uses a magnetorheological
material to provide a drive connection between two independently
rotating components, such as those found in clutches and brakes. A
fluid composition satisfactory for this application is stated to consist of
30 50% by volume of a soft iron dust, commonly referred to as "carbonyl
iron powder", dispersed in a suitable liquid medium such as a light
lubricating oil.
Another apparatus capable of controlling the slippage between
moving parts through the use of magnetic or electric fields is disclosed
35 in U.S. Pat. No. 2,661,825. The space between the moveable parts is
2 1 ~ 8 1 ~: ~ o 94/10694 PCI~/US~3/10285
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
-5 mineral oil.
U.S. Pat. No. 2,886,151 describes force transm~tting devices,
such as clutches and brakes, that utilize a fluid film coupling
responsive to either electric or magnetic fields. An e~ample of a
magnetic field responsive fluid is disclosed to contain reduced iron
10 oxide powder and a lubricant grade oil having a viscosity of from 2 to
20 centipoises at 25~C.
The construction of valves useful for controlling the fllow of
magnetorheological fluids is described in U.S. Pat. Nos. 2,670,749 and
3,010,471. The magnetic fluids applicable for utilization in the
15 disclosed valve designs include ferromagnetic, paramagnetic and
diamagnetic materials. A specific magnetic fluid composition
specified in U.S. Pat. No. 3,010,471 consists of a suspension of carbonyl
iron in a light weight hydrocarbon oil. Magnetic fluid mi~tures useful
in U.S. Pat. No. 2,670,749 are described to consist of a carbonyl iron
2D powder dispersed in either a silicone oil or a chlorinated or fluorinated
suspension fluid.
Various magnetorheological material mixtures are disclosed
in U.S. Patent No. 2,667,237. The mixture is defined as a dispersion of
small paramagnetic or ferromagnetic particles in either a liquid,
25 coolant, antio2~idant gas or a semi-solid grease. A preferred com-
position for a magnetorheological material consists of iron powder and
light machine oil. A specificàlly preferred magnetic powder is stated
to be carbonyl iron powder with an average particle size of 8
micrometers. Other possible carrier components include kerosene,
30 grease, and silicone oil.
U.S. Pat. No. 4,992,190 discloses a rheological material that is
responsive to a magnetic field. The composition of this material is
disclosed to be magnetizable particles and silica gel dispersed in a
liquid carrier ~ehicle. The magnetizable particles can be powdered
2l4x~nl ~
WO94~10~4 PC~tUS93/102~
magnetite or carbonyl iron powders with insulated reduced carbonyl
iron powder, such as that manufactured by GAF Corporation, being
specifically prefelTed. The liquid carrier vehicle is describéd as
having a viscosity in the range of 1 to 1000 centipoises at 100F
5 Specific examples of suitable vehicles include Conoco LVT oil,
kerosene, light paraffin oil, mineral oil, and silicone oil. A preferred
ca~Tier vehicle is silicone oil having a v~scosity in the range of about 10
to 1000 cen~poise at 100F.
In many demanding applications for magnetorheological
10 materials, such as darnpers or brakes for automobiles or trucks, it is
desirable for the magnetorheological material to e~:hibit a high yield
stress so as to be capable of tolerating the large forces experienced in
these types of applications. It has been found that only a noxninal
increase in y~eld stress of a given magnetorheological material can be
15 obtained by selecting among the different iron particles traditionally
utilized in magnetorheological materials. In order to increase the
yield stress of a given magnetorheological material, it is typically
~ecessary to increase the volume fraction of magnetorheological
particles or to increase the strength of the applied magnetic field.
20 Neither of these techniques is desirable since a high volume fraction of
the particle component can add significant weight to a magneto-
rheological device, as well as increase the overall off-state viscosity of
the material, thereby restricting the size and geometry of a
magnetorheological device capable of utilizing that material, and high
25 magnetic fields significantly increase the power requirements of a
magnetorheological device.
A nsed therefore e~ists for a magnetorheological particle
component that will independently increase the yield stress of a
magnetorheological material without the need for an increased
30 particle volume fraction or increased magnetic field.
Disclosure of ~nvention
The present invention is a magnetorheological material
comprising a carrier fluid and a magnetically active particle wherein
the particle has been modified so that the surface of the particle is
.... . .. .. ... . . . .
2 ~ n, n ,l ~
94/ l Q694 PCI / US93/ 1028
substantially free of contamination products such as corrosion
products. The formation of corrosion products on the sllrface of a
magnetically active particle results from both chemical and
electrochemical reactions of ~he particle's surface with water and
5 atmospheric ga~es, as well as with electrolytes and particulates or
contaminants that are either present in the atmosphere or left as a
residue during particle preparation or proce~sing. Corrosion
products can either be compact and strongly adherent to the surface of
the metal or loosely bound to the surface of the metal and can be in the
10 form of a powder, film, flake or scale. The most common types of
corrosion products include various forms of a metallic o2:ide layer,
which are sometimes referred to as rust, scale or mill scale.
It has pre~ently been discovered that the yield stress exhibited
by a magnetorheological material c~n be significantly enhanced by the
15 removal of contam~nation products from the ~urface of the magneti-
cally active particles. Contamination products can be efficiently
removed from the surface of metallic particles through abrader
proces~ing, chemical treatment or a combination thereof. In order to
be effective, these techniques must be employed during the preparation
of the magnetorheological material (ig ~itu) or immediately prior to
either the preparation of the magnetorheological material or the
application of a protective coating. Abrader processing involves the
physical or mechanical removal of the contamination products by
impacting the surface of the magnetically active particles at a high
25 velocity with an abrasive media. This abrasive media can either be an
abrasive additive to the magnetorheological material or a form of
grinding media used as a processing aid.
.
Chemical treatment methods or techniques applicable to the
removal of contamination products during the preparation of the
30 magnetorheological material include acid etching, cleaning or
pickling; alkaline cleaning; electrolytic cleaning; ultrasonic cleaning
and combinations thereof. Additional chemical treatment methods
applicable to the removal of the contamination products prior to
preparing the magnetorheological material include metal reduction
2 ~ n n l j ,
WO 94/10694 PCI/US93/1028
or reactive gas processes, plasma cleaning, ion etching, sputter
cleaning and combinations thereof.
The types of barrier coatings that are effective in protecting the
surface of the particles can be comprised of nonmagnetic metals,
5 ceramics, high performance thermoplastics, thermosetting polymers
and combinations thereof. In order to effectively protect the sur~ace of
the particle from recontamination by a contamination product, it is
necessary that this coating or layer substantially encase or encap-
sulate the particle.
Brief ~esc~ption of the Drawillg
Figure 1 is a plot of magnetorheological effect at 25C as a
function of magnetic field strength for magnetorheological materials
prepared in accordance with Example 12 and comparative Example
13.
Best Mode for Carrying Out the Invention
The present invention relates to a magnetorheological
material comprising a carrier fluid and a particle component wherein
the particle component has been modified so that the surface of the
particle component is substantially free of contamination products.
ao The contamination products can essentially be any foreign
material present on the surface of the particle and the contamination
products are typically corrosion products. As stated above, the
formation of corrosion products on the surface of a magnetically active
particle results from both chemical and electrochemical reactions of
25 the particle's surface with water and atmospheric gases, as well as
with electrolytes and particulates or contaminants that are either
present in the atmosphere or left as a residue during particle
preparation or processing. Examples of atmospheric gases commonly
involved in this surface degradation process include 2. S02, H2S,
30 NH3, N02, NO, CS2, CH3SCH3, and COS. Although a metal may
resist attack by one or more of these atmospheric gases, the su~face of
a metal is typically reactive towards several of these gases. Examples
of chemical elements contaminating the surface of metal particles
-21~"8n.n ~ ~
i:. 3 94/10694 PCr/l~S93/102~5 .
resulting from known powder processing techniques and methods
include carbon, sulfur, oxygen, phosphorous, silicon and manganese.
Examples of atmospheric particulates or contaminants involved in the
formation of corrosion products on various metals include dust, water i-
5 or moisture, dirt, carbon and carbon compounds or .soot, metal ox~des7
(NH4)S04, various salt~ (i.e., NaCl, etc.) and corrosive acids, such as
hydrochloric acid, sulfuric acid, nitric acid and chromic acid. It is
normal that metallic corrosion takes place in the presence of a
combination of 3everal of these atmospheric gases and contaminants.
10 The presence of solid particulates, such as dust, dirt or ~oot on the
surface of a metal increases the rate of degradation because of their
ability to retain corrosive reactants, such as moisture, salts and acids.
A more detailed discussion of the atmospheric corrosion of iron and
other metals is provided by H. Uhlig and R. Rene in "Corrosion and
~5 Corrosion Control," (John Wiley & Sons, New York, 1985), the entire
content of which is incorporated herein by reference.
The inherent degradation of the surface of a metal e2~posed to
the atmosphere typically continues until either the corrosion product
completely encompasses or encapsulates the particle or the entire bulk
20 of the particle has reacted with the contaminants~ Corrosion products
can either be compact and stro~gly adherent to the surface of the
metal or loosely bound to the surface of the metal as a powder, film,
flake or s~ale. The most common types of corrosion products include
various forms of a metallic o~ide layer, which are sometimes referred
25 to as rust, scale or mill scale.
The pre~ent invention is based on the discovery that the
remcval of contamination products from the`surface of a magnetically
polarizable particle causes the particle to be particularly effective in
creating a magnetorheological material which is capable of exhibiting
30 high yield stresses. Contamination products can be efficiently
removed from the surface of metallic particles through abrader
processing, chemical treatment or a combination thereo In order to
be effective, these techniques must be employed during the preparation
of the magnetorheological material (in situ) or immediately prior to
2l~s~n~
WO 94/10694 PCI/US93/102
either the preparation of the magnetorheological material or the
application of a particle barrier layer or coating.
Abrader processing involves the physical or mechanical
removal of the contamination products resulting ~om impacting the
5 surface of the magnetically active particles at a high velocity with an
abrasive media. This abrasive media can either be an abrasive
additive to the magnetorheological material or a form of grinding
media used only as a processing aid.
The abrasive additive of the invention must be a material
10 capable of sufficiently abrading a magnetorheological particle so as to
substantially remove the contamination products from the surface of
the particle. The abra3ive additive must, therefore, possess a
substantial degree of hardness or roughness so as to effectively abrade
the surface of the magnetorheological particle. Various types of
15 abrasive materials capable of removing contamination products from
the surface of a metal are well known to those skilled in the art of
tribology or superabrasives and can be utilized as abrasive additives in
the invention. The abrasive additives of the invention are typically in
the form of a powder and can be comprised of various materials such
20 as the 02cides of aluminum, chromium, zirconium, hafnium,
titanium, silicon, and magnesium; the carbides, nitrides and borides
of aluminum, silicon and boron; and cermets, such as WC-Co and Ni-
Cr-A1203, as well as combinations thereof. Specific e2~amples of
abrasive additives include diamond dust, garnet, corundum,
25 alumina, black mineral slag, Cr203, HfO2, TiO2, MgO, glass, sand,
silica, aluminum silicates, pumice, rouge, emery, feldspar, SiC, B4C,
BN, Si3N4, AlN, cerium o~ide, and fused alumina, as well as other
refractory or ceramic abrasives.
Iron oxides have also been found to be effective as abrasive
30 additives for purposes of the present invention. Specifically, it has
been found that the relatively hard iron o~ides can be utilized in
combination with relatively soft iron powders such that the
contamination products are removed from the surface of the iron by
the iron oxides. It should be noted that although used in relatively
35 minor amounts in the overall magnetorheological material, the iron
94/10694 2 l 4 8 n n 1 PCT/US93/10285
o2~ides are magnetically active and also function as an additional
magnetorheological particle in combination with the iron. The iron
02nde includes all known pure iron oxides, such as Fe203 and Fe~304,
as well as those containing small amounts of other elements, such as '.
5 manganese, zinc or barium. Specific examples of iron o~ide include
ferrites and magnetites with fe~Tites being preferred.
The silica useful as an abrasive additive in the invention must
be hydrophobic. In other words, the surface of the silica must be
treated so as to contain a miI~imal amount of hydro~yl funtionality and
10 the silica must be relatively free of adsorbed moisture. It is important
that the surface of the silica be chemically treated to be hydrophobic
~ince it has been found that con~rentional drying of otherwise
hydrophilic silica (e.g., silica gel such as that supplied by PP(::
Industries under the trade name HI-SIL 233) is not sufficient to
15 render the silica hydrophobic for purposes of the invention. Although
not completely understood, it is believed that an excess of adsorbed
moisture and/or hydroxyl functionality prevents the hydrophilic silica
from sufficiently abrading the su~face of the particle component.
-The hydrophobic silica of the invention can be prepared by
2~ reacting the hydroxyl groups on the surface of the silica with various
organofunctional monomeric silanes or silane coupling agents, such
as hydro2:ysilanes, acylo~ysilanes, epoxy~ilanes, o~imesilanes, alk-
oxysilanes, chloro ilanes and aminosilanes as is known in the art.
The hydroxyl groups on the surface of the silica may also be reacted
2~i with polymeric compounds such as silicone oils, mineral oils and
paraffin oils. The modification of the surface of silica with various
materials to render the silica hydrophobic is described by W. Noll in
"Chemistry and Technology of Silicones," Academic Press, Inc., New
York, 1968 and by E. P. Plueddemann in "Silane Coupling Agents," i ~ :~
30 Plenum Press, New York, 1982, both the entire disclosures of which i::
are incorporated herein by re~erence. Specific ea~amples of
hydrophobic silicas include those commercially obtainable under the
trade names AEROSIL and CABOSIL from Degussa Corporation and
Cabot Corporation, respectively.
2 1 ~
wo 94~106~4 Pcr/us93~1028~
The preferred abrasive additives of the present invention
include hydrophobic silica, iron o~ndes, and alumina because of their
potential to contribute to the formation of a thixotropic networ~ as
described hereinafter. Iron oxides are specifically preferred due to
5 their magnetorhe31Ogical activity and relatively high specific gravity.
The diameter of the abrasive additives utilized herein can
range from about 0.001 to 50.0 ~lm, preferably from about 0.001 to 20.0
~m with about 0.001 to ~.0 ~m being specifically preferred. The~e
abrasive additives are typically utilized in an amount ranging from
10 about 0.05 to about 10.0, preferably from about 0.1 to about 5.0, with
about 0.2 to about 3.0 percent by volume of the total magnetorheological
material being especially preferred.
In order to be effectiv~, the abrasive additive must be caused to
impact the surface of a magnetorheological particle with a kinetic
15 energy high enough to efficiently remove contamination products from
the sllrface of the particle. This can be carried out during the
preparation of the magnetorheological material (~ ~3a), immediately
prior to the preparation of the magnetorheological material or
immediately prior to the application of a protective coating to the
2D particle. If carried out during the preparation of the magneto-
rheological material, the abrasive additive is combined with the
magnetorheological particle component, carrier fluid and any optional
ingredients, and the resulting combination of ingredients is initially
mi~ed by hand with a spatula or the like and then more thoroughly
25 mi~ed with a homogenizer, mechanical mi2cer, mechanical shaker, or
an appropriate milling device such as a ball mill, sand mill, attritor
mill, colloid mill, paint mill, pebble mill, shot mill, vibration mill, roll
mill, horizontal small media mill, or the like (all hereinaftèr
collectively referred to as "mi~ring devices").
It is the mass of the abrasive additive, as well as the velocity
achieved by this additive during the mi~,ring or dispersing process that
determines the magnitude of kinetic energy available for the removal
of contamination products from the magnetorheological particles.
The velocity of the abrasive additive is dependent upon the viscosity of
35 the magnetorheological material and the speed at which the miYing
2 ~ Ll ~ n g~
: :~ ;)94/10694 PCr/uS93/10285
device i9 operated. For a typical magnetorheological material v~rith a
viscosity less than about 1000 centipoi~e at 25C, sufflcient velocity is
achieved by the abrasive additive to ef~ectively remove contamination
products from the magnetorheological particles when the mixing
5 device is operated with a minimum tip speed of about 50 fVm~n. The
ingredients mu~t be mixed together or dispersed for a sufficient length
of time to substantially remove contamination products from the
surface of the magentorheological particle. An increase in the velocity
of the abrasive additive will usually result in a decrease in the
10 required mixing or dispersion time. In general, $he ingredients
should be mi~ced for a period of time typically ranging from about 1
minute to 24 hours, preferably ranging from about 5 minutes to 18
hours. A certain amount of experimentation may be required to
determine the optimum parameters that will allow for efficient
~5 removal of contamination products from a particular magnetorheo-
logical particle.
Confirmation of the substantial removal of contamination
products from the surface of a magnetorheological particle may be
obtained by utilizing various material characterization techniques
20 known to those skilled in the art of analytical chemistry and surface
analysis. Examples of several known techniques for the quantita-
tive/qualitative detection of atomic and/or molecular species include
neutron activation analysis; scanning ion mass spectrometry (SIMS);
~c-ray methods, ~uch as x-ray powder diffraction, x-ray fluorescence
25 sp~ctro~copy (XRF), x-ray photoelectron spectroscopy (XPS) and
electron spectroscopy for chemical analysis (ESCA); and microscopy
methods, such as scanning tunneling micro~copy (STM), scanning
electron microscopy (SEM), scanning auger microanalysis (SAM),
and electron probe microanalysis (EPMA). Microscopy of powder
30 samples are typically performed using an ultramicrotomy procedure
well known to those s~illed in the art.
If the contamination products are to be removed from the
surface of the magnetorheological particle immediately prior to either
the preparation of the magnetorheological mate~ial or the application
35 of a protective coating, the above mi2 ing procedure is followed except
216~00~ ~
WO 94~10694 PCl/US93/102~"'' ' 1 '`
12 '.
that only the magnetorheological particle and abrasive additive are
utilized. After the mixing procedure, the abraded particle may be
immediately combined with the other ingredients to prepare a
magnetorheological material or immediately coated with a protective
5 coating to prevent the reformation of corrosion products. By
"immediately," it is typically meant that the abraded particle is com-
bined with the other ingredients of the magnetorheological material or
coated wit;h a protective coating within no more than about 60 minutes,
preferably within no more than about 30 minutes, after completion of
10 the mi~ing procedure, unless the particles are stored ~or a longer time
period ILnder a contaminant-free atmosphere.
Contamination products can also be removed from the particle
component through abrader processing using various grinding media
as a processing aid. This form of abrader processing can also be
15 performed during the preparation of the magnetorheological material
or immediately prior to either preparing the magnetorheological
material or applying a protective coating to the particles. The type of
grinding media and the nature of the corresponding equipment -
needed to perform this abrading process are described as those capable
20 of reducing or changing the diameter or size of the particle
component. Specific types of approp~iate media and equipment are
well known to those skilled in the art of manufacturing paints and
coatings. Devices such as homogenizers, mechanical mi~ers and
shakers that do not utilize a milling-type process, and are therefore
25 not capable of reducing particle size, provide inadequate removal of
contamination products from the surface of magnetorheological
particles for purposes of the present invention, unless an abrasive
additive as previously described is utilized in combination with the
device.
E~:amples of common grinding media appropriate for use as a i`-
processing aid include balls, beads, pellets, pebbles, grit or shot
comprised of ~aFious materials including stainless steel, ceramic,
porcelain, flint, high carbon steel, high manganese steel, cast nickel
alloy, low carbon forged steel, tungsten carbide, glass, zirconium
35 silicate, zirconium oxide, and alumin un oxide. Examples of common
94/10694 2 ~ A~ ~? n n ~ Pcr/USg3/10285
rnedia milling devices or mills that utilize these types of grinding
media include sand mills, ball mills, attritor mills, pebble mills, shot
mills, vibration mills and horizontal small media mills. In addition,
the grinding media may be in the form of a wheel, disc or blade, such
5 as that typically used in roll mills. A more complete description of
media mills is provided by G. Tank and T. Patten in "Industrial Paint
Finishing Techniques and Proce~ses" (Ellis Horwood Limited, West
Sussex, England, 1991) and "Paint Flow and Pigment Dispersion" (2nd
edition, John Wiley & Sons, New York, 1979), respectively, the entire
10 contents of which are incorporated herein by reference.
As is the case with the abrasive additive described above, the
grinding media must be caused to impact the surface of a
magnetorheological particle with a kinetic energy high enough to
sufficiently remove contamination products from the surface of the
15 particle. It is the mass of the grinding media, as well as the velocity
achieved by ~is media during the milling process that determines the
magnitude of kinetic energy available for the removal of contami-
nation products from the magnetorheological particles. The velocity of
the grinding media is dependent upon the viscosity of the magneto-
20 rheological material and the speed at which the milling device isoperated. For a typical magnetorheological material with a viscosity
less than about 1000 centipoise at 26C, sufficient velocity is achieved by
the grinding media to effectively remove contamination products from
the magnetorheological particles when the milling device is operated
25 with a minimum tip speed of about 300 f~lmin. The ingredients must
be mixed together or dispersed for a su~ficient length of time to
substantially remove contamination products from the surface of the
magentorheological particle An increase in the yelocity of the
grinding media will usually result in a decrease in the required
30 milling time. In general, the ingredients should be mixed for a period
of time typically ranging from about 1 hour to 48 hours, preferably
ranging from about 2 hours to 24 hours. A certain amount of experi-
mentation may be required to determine the optimum parameters that
will allow for efficient removal of contamination products from a
3~ particular magnetorheological particle~
2 ~ n .~
WO 94/10694 PCr/US~3/102"`..~
14
It should be noted that abrasive additives can be utilized in
combination with grinding media and, in this ca~e, the efficiency of
the corresponding milling device may be increased resulting in a
lesser amount of both time and speed of the milling device needed to
- S remove the contamination products from the surface of the magneto- rheological particle.
Removal of contamination products from the surface of the
magnetorheological particle can also be accomplished through
chemical treatment techniques. The chemical treatment can be
10 carried out during the preparation of the magnetorheological mate~al
(in ~a), immediately prior to the preparation of the magnetorheo-
log~cal material or immediately prior to the application of a protective
coating to the particle. Chemical treatment methods or techniques
applicable to the removal of contamination products during the
15 preparation of the magnetorheological material include acid cleaning,
alkaline cleaning, electrolytic cleaning, ultra~onic cleaning and
combinations thereof, such as the combination of electrolytic cleaning
- and alkaline cleaning commonly utilized in the electroplating
industry. Examples of alkaline cleaners useful in the invention
ao include alkali metal orthophosphates, condensed phosphates,
hydro~ides, carbonates, bicarbonates, silicates and borates. Alkaline
cleaners are typically utilized in combination with a surfactant as is
known in the art.
E2~amples of common acid cleaners u~eful in the invention
25 include organic acids, such as citric, tartaric, acetic, o~alic and
gluconic acid, acid salts, such as sodium phosphate, ammonium
persulfate, sodium acid sulfate and bifluoride salts, and inorganic
acids, such as sulfuric acid, phosphoric acid and hydrochloric acid.
Acid and alkaline cleaning during the preparation of the
30 magnetorheological material can be carried out by adding an acid or
alkaline cleaner to the ingredients utilized to prepare the magneto~
rheological material and then thoroughly mixing the ingredients first
by hand with a spatula or the like and then with a mechanical mixing
device. The acid or alkaline cleaner is typically utilized in an amount
t~., 94/10694 ~ ` PCI/US93/10~85
1'
ranging from 0.1 to 5.0, preferably from about 0.5 to 3.0, percent by
weight of the particle component.
Electrolyl~ic cleaning or electroclear~ing during the preparation
of the magnetorheological material is typically carried out by applying
5 an electric current to the material in order to produce vigorous
gassing on the surface of the particles and promote the release of
contaminants. ElectroclearLing can be either cathodic or anodic in
nature. This technique is generally used in conjunction with acid or
alkaline cleaning as previously described.
Ultrasonic cleaning during the preparation of the magneto-
rheological material is typically carried out by passing sound waves at
high frequencies through the material. These ultrasonic waves create
tiny gas bubbles throughout the carrier component, which vigorously
cleans the surface of the particles. This technique is often used in
15 conjunction with acid or alkaline cleaning as previously described.
The chemical treatment methods that are applicable to the
removal of contamination products immediately prior to either
preparing the magnetorheological material or applying a protective
coating include the techniques described above for in ~ treatment,
2~) as well as metal reduction; plasma cleaning; ion etching; sputter
cleaning and combinations thereof. Metal reduction typically involves
- the reduction of the metal particle's surface through a reaction with a
-gaseous molecule, such as hydrogen, at elevated temperatures. Due to
their ability to provide ~ery clean surfaces and control contaminant
25 removal, the preferred methods of removing contamination products
from the surface of the particles prior to preparing a magneto-
rheological material dr applying a protective coating are metal
reduction and plasma cleaning.
A thorough description of the chemical treatment techniques
30 described above is provided by B. Bhushan and B. Gupta in "Handbook ~A
of Tribology," McGraw-Hill, Inc., New York, 1991 (hereinafter
referred to as Bkushan), the entire contents of which are incorporated
herein by reference.
2lAs~ni ~
WO 94/10694 PCr/US93/102~
As stated above, immediately after removing the contami-
nation products from the particle's surface through either abrader
processing, chemical treatment or a combination thereof, a protec~tive
coating can be applied to the surface of the particle. In order to
5 effectively protect the surface of the particle from recontamination by a
contamination product, it is necessary that the protective coating
substantially, preferably entirely, encase or encapsulate the particle.
Protective coatings that substantially encapsulate the particle are to be
distinguished from insulation coatings, such as those presently found
10 on carbonyl iron powder such as the insulated reduced carbonyl iron
powder supplied by GAF Corporation under the designations "GQ-4"
and "GS-6."
The insulation coatings found on insulated reduced carbonyl
iron are intended to prevent particle-to-particle contact and are simply
15 formed by dusting the particles with silica gel particulates. Insulation
coatings therefore do not substantially encapsulate the particle so as to
prevent the formation of contamination products. The sporadic
coverage of a particle's surface by an insulation coating can be seen in
the scanning electron micrographs presented in the article by J.
ao Japka entitled "Iron Powder for Metal Injection Molding"
(International Journal of Powde~ MetallurFy, 27(2), 107-114), the entire
contents of which are incorporated herein by reference. Incomplete
coverage of the particle's surface by a coating typically will result in
the accelerated formation of contamination products through the
25 process described above for solid atmospheric particles, such as dust
and soot. Iron o~de, previously described in the literature as being
useful as an insulation coating, cannot be used as a protective coating
for purposes of the present invention because iron o~ide itself is a
corrosion product.
The protective coatings of the invention that are effective in
prevent~g the formation of contamination products on the surface of
magnetorheological particles can be derived from a variety of
materials including nonmagnetic metals, ceramics, high perfor-
mance thermoplastics, thermosetting polymers and combinations
35 thereof. E~amples of thermosetting polymers useful for forming a
i 21h.n~s'
) 94/10694 PCI /lJS93/102~5 t
17
protective coating include polyesters, polyimides, phenolics, epo~ies,
urethanes, rubbers and silicones, while thermoplastic polymeric
materials include acrylics, cellulosics, polyphenylene sulfides,
polyquinoxilies, polyetherimides and polybenzimidazoles. Typical
5 nonferrous metals useful for ~orming a protective coating include
refractory transition metals, such as titanium, zirconi~n, hafni~
vanadium, niobium, tantulum, chromium, molybdenum, tungsten,
copper, silver, gold, and lead, tin, zinc, cadmium, cobalt-based
intermetallic alloys, such as Co-Cr-W-C and Co-Cr-Mo-Si, and nickel-
10 based intermetallic alloys, such as Ni-Cu, Ni-Al, Ni-Cr, Ni-Mo-C, Ni-
Cr-Mo-C, Ni-Cr-B-Si-C, and Ni-Mo-Cr-Si. E2~amples of ceramic
materials useful for forming a protective coating include the carbides,
nitrides, borides, and silicides of the refractory transition metals
described above; nonmetallic o~ides, such as A1203, Cr~03, ZrO3,
15 HfO2, TiO~, SiO2, BeOs MgO, and ThO2; nonmetallic nonoxides, such
as B4C, SiC, BN, Si3N4, AlN, and diamond; and various cermets.
A thorough description of the various materials typically
utilized to protect metal surfaces from the growth of corrosion
products is provided by C. Munger in "Corrosion Prevention by
ao Protective Coatings" (National Association of Corrosion Engineers,
Houston, Texas, 1984), the entire content of which is incorporated
herein by reference. A commercially available iron powder that is
encapsulated with a polyetherimide coating i~ manufactured under
the trade name ANCC)R by Hoeganaes.
The protective coatings of the invention can be applied by
techniques or methods well known to those skilled in the art of
tribology. E~:amples of common coating techniques include both
physical deposition and chemical vapor deposition methods. Physical
deposition techniques include both physical vapor deposition and
30 liquid or wetting methods. Physical vapor deposition methodology l `
includes direct, reactive, activated reactive and ion-beam assisted
evaporation; DC/RF diode, alternating, triode, hollow cathode dis-
charge, sDutter ion, and cathodic arc glow discharge ion plating;
direct, cluster ion and ion beam plating; DC/RF diode, triode and
35 magnetron glow discharge sputtering; and single and dual ion beam
21~'3~ i ~
wo 94/10694 Pcr/uS93/102
18
sputtering. Common physical liquid or wetting methodology includes
air/airless spray, dip, spin-on, electrostatic spray, spray pyrolysis,
spray fusion, fluidized bed, electrochemical deposition, chemical
deposition such as chemical conversion (e.g., phosphating, chrom-
5 ating, metalliding, etc.), electroless deposition and chemical reduc-
tion; intermetallic compounding, and colloidal dispersion or sol-gel
coating application techniques. Chemical vapor deposition methodo-
logy includes conventional, low pressure, laser-induced, electron-
assisted, plasma-enhanced and reactive-pulsed chemical vapor
10 deposition, as well as chemical vapor polymerization. A thorough
discussion of these various coating processes is provided in Bhushan.
Due to the additional production costs associated with
removing the corrosion products from the surface of the particles prior
to preparing a magnetorheological material, the preferred abrader
- 1~ processing and chemical treatment methods of the invention include
those performed during the preparation of the magnetorheological
material. In this regard, abrader processing is generally preferable
over chemical treatment.
The magnetically active particle component to be modified
20 according to the present invention can be comprised of essentially any
~olid which is known to e~hibit magnetorheological activity and which
can inherently form a contamination product on its surface. Typical
particle components usefill in the present invention are comprised of,
for example, paramagnetic, superparamagnetic, or ferromagnetic
25 compolmds. Specific e~amples of particle components useful in the
present inv~ntion include particles comprised of materials such as
iron, iron nitride, iron carbide, carbonyl iron, chromium dioxide, low
carbon steel, silicon steel, nickel, cobalt, and mixtlLres thereof. In
addition, the particle component can be comprised of any of the known
30 alloys of iron, such as those containing aluminum, silicon, cobalt,
nickel, vanadium, molybdenum, chromium~ tungsten, manganese
and/or copper. The particle component can also be comprised of the
~pecific iron-cobalt and iron-nickel alloys described in the U.S. patent
application entitled "Magnetorheological Materials Based on Alloy
35 Particles" filed concllrrently herewith by Applicants J. D. Carlson and
2 ~ 4 ~
~) 94/1~694 Pcr/vss3/lo285
K. D. Weiss and also assigned to the present assignee, the entire
disclosure of which is incorporated herein by reference.
The particle component is typically in the ~orm of a metal
powder which can be prepared by processes well known to those
5 skilled in the art. Typical methods for the preparation of metal
powders include the reduction of metal oxides, grinding or attrition,
electrolytic deposition, metal carbonyl decomposition, rapid solidifi-
cation, or smelt processing. Various metal powders that are commer-
cially available include straight iron powders, reduced iron powclers,
10 insulated reduced iron powders, and cobalt powders. The diameter of
the particles utilized herein can range from about 0.1 to 500 !lm and
preferably range fi~om about 1.0 to 50 llm.
The preferred particles of the present invention are straight
iron powders, reduced iron powders, iron-cobalt alloy powders and
~5 iron-nickel alloy powders~
The particle component typically comprises from about S to 50,
preferably about 15 to 40, percent by volume of the total composition
depending on the desired magnetic activ~ty and viscosity of the overall
material.
2~) The carrier fluid of the magnetorheological material of the
present invention can be any camer fluid or vehicle previously
disclosed for use in magnetorheological materials, such as the
mineral oils, silicone oils and paraffin oils described in the patents set
forth above. Additional carrier fluids appropriate to the invention
25 include silicone copolymers white oils, hydraulic oils, chlorinated
hydrocarbons, transformer oils, halogenated aromatic liquids,
halogenated paraffilns, diesters, polyoxyalkylenes? perfluorinated
polyethers, fluorinated hydrocarbons, fluorinated silicones, and
mixtures thereof. As known to those familiar with such compounds,
30 transformer oils refer to those liquids having characteristic properties
of both electrical and thermal insulation. Naturally occurring
transformer oils include refined mineral oils that have low viscosity
and high chemical stability. S~nthetic transformer oils generally
comprise chlorinated aromatics (chlorinated biphenyls and
21~n~
wo 94/10694 PCr/~S~3/102
trichloroben~ene~, which are known collectively as "askarels", silicone
oils~ and esteric liquids such as dibutyl sebacates.
Additional carrier fluids suitable for u~e in the present
invention include the silicone copolymers, hindered ester compounds
5 and cyanoalkylsilo~ane homopolymers disclosed in co-pending U.S.
Patent Application Serial No. 07/942,549 filed September 9, 1992, and
entitled "High Strength, Ilow Conductivity Electrorheological
Materials," the entire disclosure of which is incorporated herein by
reference. The carrier fluid of the invention may also be a modified
10 ca~Tier fluid which has been modified by extensive purification or by
the formation of a miscible solution with a low conductivity carrier
fluid so as to cause the modified carrier fluid to have a conductivity
less than about 1 x 10-7 S/m. A detailed description of these modified
carrier fluids can be found in the U.S. Patent Application entitled
15 "Modified Electrorheological Materials Having Minimum
Conductivity," filed October 16, 1992, by Applicants B. C. Munoz,
S. R. Wasserman, J. D. Carlson, and K. D. Weiss, and also assigned to
the present assignee, the entire disclosure of which is incorporated
herein by reference.
Polysiloxanes and per~luorinated polyethers hav~ng a viscosity
between about 3 and 200 centipoise at 25C are also appropriate for
utilization in the magnetorheological material of the present
invention. A detailed description of these low viscosity polysilo2~:anes
and perfluorinated polyethers is given in the U.S. patent application
25 entitled "Low Viscosity Magnetorheological Materials," filed con-
currently herewith by Applicants K. D. Weiss, J. D. Carlson, and
T. G. Duclos, and also assigned to the present assignee, the entire
disclosure of which is incorporated herein by reference. The preferred
carrier fluids of the present invention include mineral oils, paraffin
30 oils, silicone oils, silicone copolymers and perfluorinated polyethers,
with silicone oils and mineral oils being especially preferred.
The carrier fluid of the magnetorheological material of the
present invention should have a viscosity at 25C that is between about
2 and 1000 centipoise, preferably between about 3 and 200 centipoise,
35 with a viscosity between about 5 and 100 centipoise being especially
` 21~nnl ~.;
) 94/1~694 - PC~/US93/1028
21
preferred. The carrier fluid of the present invention is typically
utilized in an amount ranging from about 50 to 95, preferably from
about 60 to 85, percent by volume of the total magnetorheological il
material. '
Particle settling may be minimized in the magnetorheological
materials of the invention by forming a thixotropic network. A
thixotropic network is defined as a suspension of particles that at low
shear ~ates form a loose network or structure, sometimes re~erred to
as clusters or flocculates. The presence of this three-dimensional
10 structure imparts a small degree of rigidity to the magnetorheololgical
material, thereby, reducing particle settling. However, when a
shearing force is applied through mild agitation, this structure is
easily disrupted or dispersed. When the shearing force is removed,
this loose network is reformed over a period of time.
~5 A thi~:otropic network or structure is formed through the
utilization a hydrogen-bonding thixotropic agent andlor a polymer-
modified metal oxide. Colloidal additives may also be utilized to assist
in the formation of the thixotropic network. The formation of a
thixotropic network utilizing hydrogen-bonding thixotropic agents,
ao polymer-modified metal oxides and colloidal additives is further
described in the U.S. Patent Application entitled "Thixotropic
Magnetorheological Materials," filed concurrently herewith by
applicants K. D. Weiss, D. A. Nixon, J. D. Carlson and A. J. Margida
and also assigned to the pre~ent assignee, the entire disclosure of
25 whicn is incorporated herein by reference.
The formation of a thixotropic network in the invention can be
assisted by ;the addition of low molecular weight hydrogen-bonding
molecules, such as water and other molecules containing hydroxyl,
carbo2cyl or amine functionality. Typical low molecular weight ~-
30 hydrogen-bonding molecules other than water include methyl, ethyl,
propyl, isopropyl, butyl and hexyl alcohols; ethylene glycol; diethylene }
glycol; propylene glycol; glycerol; aliphatic, aromatic and heterocyclic `.-
amines, including primary, secondary and tertiary amino alcohols
and amino esters that have from 1-16 atom~ of carbon in the molecule;
35 methyl, butyl, octyl, dodecyl, hexadecyl, diethyl, diisopropyl and
Z 1 ~ 8 ~
wo 94/10694 PCr/US93/102~
2Z
dibutyl amines; ethanolamine; propanolamine; etho~yethylamine;
dioctylamine; triethylamine; trimethylamine; tributylamine; ethylene-
diamine; propylene-diamine; triethanolamine; tnethylenetetramine;
pyridine; morpholi~e; imidazole; and mixtures thereof. The low
5 molecular weight hydrogen-bonding molecules, if utilized, are
typically employed in an amount ranging from about 0.1 to 10.0,
preferably from about 0.5 to 5.0, percent by weight relative to the weight
of the particle component.
An additional surfactant to more adequately disperse the
10 particle component may be optionally utilized in the present invention.
Such surfactants include known surfactants or dispersing agents
such as ferrous oleate and naphthenate, sulfonates, phosphate esters,
glycerol monooleate, sorbitan s~squioleate, stearates, laurates, fatty
acids, fatty alcohols, and the other surface active agents discussed in
15 U.S. Pat. No. 3,047,507 (incorporated herein by reference). Alkaline
soaps, such as lithium stearate and ~odium stearate, and metallic
soaps, such as aluminum tristearate and aluminum distearate can
also be presently utilized as a surfactant. In addition, the optional
surfactants may be comprised of steric stabilizing molecules,
2() including fluoroaliphatic polymeric esters, such as FC-430 (3M
Corporation), and titanate, aluminate or zirconate coupling agents,
such as KEN-REACT(~ (Kenrich Petrochemicals, Inc.) coupling
agents. Finally, a precipitated silica gel, such as that disclo~ed in U.S.
Patent No. 4,992,190 (incorporated herein by reference), can be u~ed to
25 disperse the particle component. In order to reduce the presence of
moisture in the magnetorheological mate~ial, it is preferred that the
precipitated silica gel, if utilized, be dried in a convection oven at a
temperature of ~rom about 110C to 150C for a period of time from
about 3 to 24 hours.
The surfactant, if utilized, is preferably a "dried" precipitated
silica gel, a fluoroaliphatic polymeric ester, a phosphate ester, or a
coupling agent. The optional surfactant may be employed in an
amount ranging from about 0.1 to 20 percent by weight relative to the
weight of the par~cle component.
-- 214~n~ ~
... . .
-~ 94/106g4 Pcr/uss3/lo285
æ
The preparation of magnetorheological materials according to
the invention where contamination products are removed from the
surface of the magnetorheological particle in ~ has previuusly been
described. If contamination products are removed from the particle
5 immediately prior to either the preparation of the magnetorheological
material or the application of a protective coating, the magnetorheo-
logical materials of the present invention can be prepared by simply
mixing together the carrier fluid, the pre-treated particle component,
and any optional ingredients.
The ingredients of the magnetorheological materials may be
initially mixed together by hand with a spatula or the like and then
subse~uently more thoroughly mixed with a homogenizer, mechan-
ical mi~er, mechanical shaker, or an appropriate milling dence such
as a ball mill, sand mill, attritor mill, colloid mill, paint mill, pebble
16 mill, shot mill, vibration mill, roll mill, horizontal small media mill
or the like, in order to create a more stable suspension. The mixing
conditions for the preparation of a magnetorheological material
utilizing a magnetorheological particle that has had contamination
products prenously removed can be somewhat less rigorous than the
~0 conditions required for the preparation and in situ removal of con-
tamination products.
Evaluation of the mechanical properties and characteristics of
the magnetorheological materials of the present invention, as well as
other magnetorheological materials, can be obtained through the use
25 of parallel plate and~or concentric cylinder couette rheometry. The
theories which provide the basis for these techniques are adequately
described by S. Oka in Rheology, Theory and Applica~ions (volume 3,
F. R. Eirich, ed., Academic Press: New York, 1960) the entire contents
of which are incorporated herein by reference. The information that
30 can be obtained from a rheometer includes data relating mechanical
shear stress as a function of shear strain rate. For magneto-
rheological materials, the shear stress versus shear strain rate data
can be modeled after a Bingham plastic in order to determine the
dynamic yield stress and viscosity. Within the confines of this model
35 the dynamic yield stress for the magnetorheological material corres-
2 1 ~L 8
WO 94/106~4 Pcr/uss3/l02i^~
ponds to the zero-rate intercept of a linear re~ression curve fit to the
measured data. The magnetorheological effect at a particular
magnetic field can be further defined as the difference between the
dynamic yield stress measured at that magnetic field and the dynamic
5 yield StL ess measured when no magnetic field is present. The
viscosity for the magnetorheological material corresponds to the slope
of a linear regression curve fit to the measured data.
In a concentric cylinder cell configuration, the magnetorheo-
logical material is placed in the annular gap formed between an inner
10 cylinder of radius Rl and an outer cylinder of radius R2, while in a
simple parallel plate configuration the material is placed in the
planar gap formed between upper and lower plates both with a radius,
R3. In these techniques either one of the plates or cylinders is then
rotated with an angular velocity cl~ while the other plate or cylinder is
~5 held motionless. A magnetic field can be applied to these cell configu-
rations across the fluid-filled gap, either radially for the concentric
cylinder configuration, or a~ially for the parallel plate configuration.
The relationship between the shear stress and the shear strain rate is
then derived from this angular velocity and the torque, T, applied to
20 maintain or resist it.
The following e2~amples are gi~en to illustrate the invention
and should not be construed to limit the scope of the invention.
E~amples 1 and 2
In E~cample 1, a magnetorheological material is prepared by
25 slowly adding a total of 117.9 g of carbonyl iron powder (Sigma
Chemical Company) to a~ mixture of 3.54 g of an 11 M phosphoric acid
solution, which is prepared using phosphoric acid (99%, Aldrich
Chemical Company) and distilled water, and 28.29 g of 20 cstk mineral
oil (DRAKEOL 10, Pennzoil Products Company). The temperature of
30 the magnetorheological material is maintained during this initial
mi~nng procedure within the temperature range of about 30 to 45C.
The fluid is initially mixed by hand with a spatula (low shear) and
then more thoroughly dispersed into a homogeneous mia~ture through
the use of a high speed disperserator (high shear) equipped with a 16-
s n ~
- j 94/10694 Pcr/uss3/lo285
tooth rotary head. The weight amount of the chemically treated iron
particles in the magnetorheological material is equivalent to a volume
fraction of about 0.30. The magnetorheological material is stored in a
polyethylene container.
In E2~ample 2, a magnetorheological material is prepared
accarding to the procedure described in E~ample 1. However, in this
e~ample the phosphoric acid solution is replaced with 3.54 g of an 11 N
sulfuric acid solution, which is prepared using sulfuric acid (95-98%,
Aldrich Chemical Company) and distilled water. The amount of
10 mineral oil is adjusted to maintain the particle volume fraction in the
magnetorheological material at 0.30. The magnetorheolo~ical
material is stored in a polyethylene container.
Comp~tive Example 3
In Example 3, a magnetorheological material is prepared
15 according to the procedure described in Example 1. However, in this
example a total of 117.9 g of carbonyl iron powder (~igma Chemical
Company), 2.35 g of stearic acid (Aldrich Chemical Company) as a
dispersant and 28.67 g of 20 cstk mineral oil (DRAKEOL 10, Pennzoil
Products Company) are mi~ed together. The weight amount of
ao untreated iron particles in the magnetorheological material is
equivalent to a volume fraction of about 0.30. The conventional
magnetorheological material is stored in a polyethylene container.
Ma~etorheologicalA~tintyfor~mples1~
The magnetorheological materials prepared in Examples 1, 2
25 and 3 are evaluated through the use of parallel plate rheometry. A
summary of the magnetorheological effect observed for these
magnetorheological materials at various magr~etic field strengths and
25C is provided in Table 1. A significantly higher magnetorheological
effect is observed for the magnetorheological materials utilizing
30 particles wherein contamination products have been removed by
chemical treatment (E~camples 1 and 2) as compared to a magneto-
rheological material containing conventional untreated particles
(E~ample 3). At a magnetic field strength of ~000 Oersted the mag-
2~0~) l
WO 94/10694 PCI/US93/102'~
~ .-. .
netorheological effect exhibited by the magnetorheological materials
containing the chemically treated ptarticles is about 71% greater than t
that e~hibited by a conventional magnetorheological material. ~ t
Table 1
~0..~ ~ .~ .~ . .
~32.gl46.2172.~71 100.71
~ 10.0 38.0 65.4 80.3 94.5
Example 4
A magnetorheological rnaterial is prepared by m~xing together
a total of 123.2 g of carbonyl iron powder (Sigma Chemical Company),
2.46 g of stearic acid (Aldrich Chemical Company) as a dispersant and
- 34.20 g of 200 cstk silicone oil (Dow Corning Corporation). This weight
10 amount of iron particles is equivalent to a volume fraction in the
magnetorheological material of about 0.30. The fluid is made into a
homogeneous mi~ture using an Union Process 01HD attritor mill
equipped with a 110 cm3 tank. The grinding media used in this
attritor mill is in the form of stain~less steel balls. This mill has the
15 capability to reduce the mean size and distribution of the particle
component when the impacting grinding media has high kinetic
energy. This grinding media imparts sufficient kinetic energy to
remove the contamination products from the particle component when
the agitator shaft and arms of this mill are rotated at tip speed of about
20 300 ft./min. The ma~imum tip speed of this mill is measured to be
about 600 ft./min. The magnetorheological material is aggressively
milled in this abrader process over a 48-hour period with a tip speed of
`;`- 21Q8~nl
94/10694 PCr/US93/10~85
- Z7 ,,
about 445 ft./min. The magnetorheological material is separated from
the grinding media and stored in a polyethylene container. I
~. ,
Comparative E2cample 5
A magnetorheological material is prepared according to the
5 procedure described in Example 4. However, in this e2~ample the mill
is operated with a tip speed of about 250 ft./min. o~er a 96-hour period.
The rotation of the agitator shaft and arms at this angular speed does
not impart sufficient kinetic energy to the stainless steel glinding
media to remove the contamination products from the surface of
10 particle component. The conventional magnetorheological material is
separated from the grinding media and stored in a polyethylene
container.
Magnetor~eological ~vit~ for E~amples 4 & B
The magnetorheological materials prepared in Examples 4 and
15 5 are evaluated through the use of parallel plate rheometry. A
summary of the magnetorheological effect for these magnetorheo-
logical materials at various magnetic field strengths and 25C is
provided in Table 2. A significantly higher magnetorheological effect
is observed for the magnetorheological material utilizing particles
20 wherein contamination products have been removed by stainless steel
grinding media in an abrader process ~Example 4) as compared to a
magnetorheological material containing conventional particles
(Example 5). At a magnetic field strength of 3000 Oersted the mag-
netorheological effect e~chibited by the magnetorheological material
25 containing the abrader process modified particles is about 69% greater
than that exhibited by a conventional magnetorheological mateIial.
2l~s~n.~ . ~,
WO 94/106~4 PCI/US93/102
28
Table 2
.... ~ ~
~3 ~ 19.1 430 843
~ ~ .. ~..
.~
8.0 319 50.0
. ~ ~ .
Example 6
A magnetorheological material is prepared by adding together a
total of 117.9 g of reduced iron powder (ATOMET 95G, Quebec Metal
5 Powders Limited), 8.75 g of Mn/Zn ferrite powder (#73302-0, D. M.
Steward Manufacturing Company) as an abrasive additive, 2.53 g poly-
oxyethylene/silicone graft copolymer (SILWET L7500, Union Carbide
Chemicals and Plastics Company, Inc.) as a thixotropic agent and
29.13 g of 10 cstk silicone oil (L-45, Union Carbide Chemicals and
10 Plastics Company, Inc.). The fluid is initially mixed by hand with a
spatula (low shear) and then more thoroughly dispersed into a homo-
geneous mi2~ture through the use of a high speed disperserator (high
shear) equipped with a 16-tooth rotary head and operated at a tip speed
of about 400 ft~min. for about 5 minutes. The weight amount of the
15 iron particles in the magnetorheological material is equivalent to a
volume fraction of about 0.30. The presence of the abrasive ferrite
powder in this magnetorheological material efflciently removes the
contamination products from the surface of the iron particles. The
magnetorheological material whose particle component has been
2~ modified by abrader processing is stored in a polyethylene container.
- ~ Comparati~e E~mple 7
A magnetorheological material is prepared according to the
procedure described in ExPmple 6 with the exception that the abrasive
ferrite powder is excluded. The weight amount of the oil component is
Q ~ 1 ~
.) 94/10694 PCr/lJS93/l0285
29 ~''
modified to maintain an iron particle volume fraction in the magneto-
rheological material of 0.30. This conventional magnetorheological
material is stored in a polyethylene container.
Ma~etor~eolo~ical A~vity for E2~amples 6 & 7
The magnetorheological materials prepared in Examples 6 and
7 are evaluated through the use of parallel plate rheometry. A
summary of the magnetorheological effect observed for these magneto-
rheological materials at various magnetic field strengths and 25C is
provided in Table 3. A significantly higher magnetorheological effect
10 is observed for the magnetorheological rnaterial utilizing particles
wherein contamination products have been removed by the presence of
an abrasive additive in an abrader process (E~mple 6) as compared to
a magnetorheological material containing conventional particles
(E2~ample 7). At a magnetic field strength of 5000 Oersted the mag-
1~ netorheological effect e~hibited by the magnetorheological material
containing the abrader process modified particles is about 147%
greater than that exhibited by a conventional magnetorheological
material.
Table 3
11.2 31.
amples ~10
~.
In Example 8, a magIletorheological material is prepared by
adding together a total of 117.9 g of straight carbonyl iron powder
(MICROPOWDER S-1640, GAF Chemicals Corporation), 1.18 g of 9
Boron Carbide (99%, Johnson Matthey Company) as an abrasive
21~1Q~.
WO 94/10694 PCI/US93/102~
1-`
additive, 2.36 g organomodifed polydimethylsiloxane copolymer
(SILWET L7500, Union Carbide Chemicals and Plastics, Company,
Inc.) as a hydrogen-bonding thi~otropic agent and 27.~5 g of 10 cstk
silicone oil (L-45, Union Carbide Chemicals and Plastics Company, '.
5 Inc.). The fluid is initially mixed by hand with a spatula (low shear)
and then more thoroughly di~persed into a homogeneous mixture
through the use of a high speed disperserator (high shear) equipped
with a 16-tooth rotary head and operated at a tip speed of about 400
f~./min. for about ~ minutes. The weight amount of the iron particles
10 in the magnetorheological material is equivalent to a volume fraction
of about 0.32. The presence of` the abrasive additive in this
magnetorheological material efficiently removes the contamination
products from the surface of the iron particles. The magneto-
rheological material whose particle component has been modified by
15 abrader processing is stored in a polyethylene container.
Magnetorheological materials are prepared in Examples 9 and
10 according to the procedure described for Example 8. However, in
Elcample 9 the boron carbide powder is replaced with 1.51 g silicon
carbide powder (alpha, 99.8%, Johnson Matthey Company) as an
20 abrasive additive. In Example 10, the abrasive additive is replaced
with 2.43 g iron (II, III) oxide powder (97%, Johnson Matthey
Company). The weight amount of the iron particles in each of the
magnetorheological materials is equivàlent to a volume fraction of
about 0.32. The magnetorheological materials whose particle
25 component has been modified by abrader processing is stored in a
polyethylene container.
Comparative Example 11
A magnetorheological material is prepared according to the
procedure described in Example 8. However, in this case no abrasive
30 additive is incorporated into the magnetorheological m~terial. The
amount o~` the carrier oil component is appropriately increased to
insure that the volume fraction of iron particles in the magneto-
rheological material is about 0.32. The conventional magnetorheo-
logical material is stored in a polyethylene container.
~`.)94/l0694 2~ '`?1 PCI/US93/10~85
M~gnetorheolo~ical Activ~ty for Examples ~
f
The magnetorheological materials prepared in Example~ 8, 9,
10 and 11 are evaluated through the use of parallel plate rheometry. A
summary of the magnetorheological effect observed for these .
5 magnetorheological ma$erials at various magnetic field strengths and
25C is pronded in Table 4. A significantly higher magnetorheological
effect is observed for the magnetorheological materials utilizing
particles wherein contamination products have been removed by the
presence of an abrasive additive or iron oxide particles in an abrader
10 process (Examples 8-10) as compared to a magnetorheological
material containing conventional particles (Example 11). At a
magnetic field strength of 3000 Oersted the magnetorheological effect
e~hibited by the magnetorheological material contain~ng the abrader
process modified particles is about 74% greater than that exhibited by a
15 conventional magnetorheological material.
Table 4
~e~
~ ~ ~`'~
18.8 74.3 131.0
l0.8 1 62.4 1120.6~] ~`
~ ~ 1~
i
2 1 4 ~ ~ O :............................................................... ç
WO 94/1 06g4 PCr/US93/ l O~o . ~.
32 ~,
I
Example 12
A magnetorheological material is prepared by adding together a
total of 117.9 g of reduced carbonyl iron powder (MICROPOWDER R-
1430, GAF Chemicals Corporation~, 1.90 g of hydrophobic fumed silica
5 (CABOSIL TS-7~0, Cabot Corporation) and 29.95 g of 10 cstk silicone oil
(L-45, Union Carbide Chemicals and Plastics Company, Inc.). The
fluid is initially mi~Led by hand with a spatula (low shear) and then
more thoroughly dispersed into a homogeneous mi~{ture through the
use of a high speed disperserator (high shear) equipped w~th a 16-tooth
10 rotary head and operated at a tip speed of about 400 ft./min. for about 5
minutes. The weight amount of the iron particles in the magneto-
rheological material is equivalent to a volume fraction of about 0.32.
The presence of the abrasive hydrophobic silica powder in this
magnetorheological material efficiently removes the contamination
~5 products from the surface of the iron particles. The magneto-
rheological material whose particle component has been modified by
abrader processing is stored in a polyethylene container.
Comparative li~mple 13
A magnetorheological material is prepared according to the
ao procedure described in E~:ample 12 with the e2~ception that the hydro-
phobic silica powder is replaced with an identical amount of a hydro-
philic silica gel dispersant (HI-SIL 233, PPG Industries). This silica
gel dispersant, which has previously been disclosed as a dispersant in
U.S. Patent No. 4,g92,190, is dried in a convection oven at 130C for 24
25 hours prior to use. This magnetorheological material contains a
particle volume fraction of 0.32. This conventional magnetorheo-
logical material is stored in a polyethylene container.
MagnetorheologicalA~vity for E~amples 12 & 13
The magIletorheological materials prepared in E2~amples 12 ~ i
30 and 13 are evaluated through the use of parallel plate rheometry. A
summary of the magnetorheological effect observed for these
magnetorheological materials at various magnetic field strengths and
25C is provided in Figure 1. A sigIlificantly higher magnetorheo-
~` 2 1 ~
) 94/10694 PCI/US93/10285
33 ' ' '
logical e~ect is obtained for the magnetorheological material utilizingparticles wherein contamination products have been removed by the
presence of an abrasive hyrophobic silica additive in an abrader
process (Example 12) as compared to a magnetorheological material
5 containing conventional particles (E:~ample 13). Rt a magnetic field
strength of ~000 Oersted the magnetorheological effect e~hibited by the
magnetorheological material containing the abrader process modified
particles is about 167% greater than that e~hibited by a conventional
magnetorheological material.
10As can be seen from the above examples, magnetorheological
materials that contain a par~icle component that has been modifed by
the removal of inherent contamination products through chemical
treatment or abrader processing exhibit a significantly higher :
magnetorheological effect than conventional magnetorheological
15 materials.
,-
r~
