Magnetorheological fluids and methods of making thereof

A magnetorheological fluid composition comprising magnetosolid particles, magnetosoft particles, a stabilizer, and a carrying fluid comprising an aromatic alcohol, a vinyl ester, and an organic solvent or diluent carrier such as kerosene, in proportions sufficient to provide substantially no agglomeration or sedimentation of magnetic particles over temperatures of from about -50.degree. to 120.degree. C. The composition can be made by preparing a carrying fluid comprising a vinyl ester, an aromatic alcohol and kerosene; preparing a first carrying fluid composition comprising magnetosoft particles, a stabilizer and a first sample of the carrying fluid; preparing a second carrying fluid composition comprising magnetosolid particles and a second sample of the carrying fluid; and admixing the first carrying fluid composition and the second carrying fluid composition.

FIELD OF THE INVENTION 
This invention relates to magnetorheological fluids, and more particularly 
to fluids containing a suspension of material which will change the fluid 
properties when acted on by a magnetic field, and methods for making such 
fluids. 
BACKGROUND OF THE INVENTION 
Fluids containing magnetic material are known in the art. Such fluids are 
designed to change viscosity or other fluid properties upon application of 
a magnetic field to the fluid. Typical uses of known magnetic fluid 
compositions have included shock absorbers, clutches, and actuating 
modules. However, prior art fluids have suffered from several 
disadvantages. Prior art fluids generally are not useful over a wide range 
of temperature. Known magnetic fluids also have suffered from instability 
of the magnetic particles in suspension. Such instability can include 
settling of the particles over time due to gravitational forces and/or 
agglomeration of the particles in the fluid suspension. 
Shtarkman, U.S. Pat. No. 4,992,190, describes a fluid responsive to a 
magnetic field comprising magnetizable particulate, silica gel as a 
dispersant and a vehicle. Shtarkman discloses a fluid composition 
comprising 20% by weight of silicone oil and 80% by weight of a mixture of 
carboxyl iron (99% by weight) and pre-dried silica gel (1% by weight). 
Shtarkman discloses that such a fluid is useful as the dampening fluid in 
a shock absorber. Shtarkman discloses that reduced magnetic particles can 
have an insulation coating (such as iron oxide) to prevent 
particle-to-particle contact, eddy currents or dielectric leakage. 
Fluids such as those described by Shtarkman have limited commercial 
applicability. The silicone oil vehicle is a poor lubricant, particularly 
on steel surfaces, and must be combined with lubricants and mineral oils 
to overcome this disadvantage. Moreover, the high compressibility of 
silicone oils is undesirable since it increases the time for system 
response to a magnetic field. Additionally, the silicone oils do not 
dissolve surfactants easily, precluding the use of nonorganic stabilizers. 
Chagnon, U.S. Pat. No. 4,356,098, describes a ferrofluid composition 
comprising a colloidal dispersion of finely-divided particles in a liquid 
silicone-oil carrier and a dispersing amount of a surfactant which 
comprises a silicone-oil surfactant containing a functional group which 
forms a chemical bond with the surface of the particles and a tail group 
which is soluble in the silicone-oil carrier. Fluids such as those 
disclosed by Chagnon suffer from an inability to viscosity to a sufficient 
degree upon application of a magnetic field. Such fluids generally change 
in viscosity by a factor of about two, which is considered unacceptable 
for many applications. 
OBJECTS AND SUMMARY OF THE INVENTION 
In light of the foregoing, it is an object of the invention to provide a 
stable magnetorheological fluid. It is a further object of the invention 
to provide a magnetorheological fluid which is stable over a range of 
temperature. 
It is a further object of the invention to provide a magnetorheological 
fluid in which the magnetic particles do not settle or agglomerate over 
time. 
It is a further object of the invention to provide a magnetorheological 
fluid which responds quickly to application of a magnetic field. 
These and other objects of the invention are achieved by a 
magnetorheological fluid composition comprising magnetosolid particles, 
magnetosoft particles, a stabilizer, and a carrying fluid comprising an 
aromatic alcohol, a vinyl ether, and an organic solvent or diluent carrier 
such as kerosene, in proportions sufficient to provide substantially no 
agglomeration or sedimentation of magnetic particles over temperatures of 
from about -50.degree. to 120.degree. C. The invention further comprises a 
method for making a magnetorheological fluid composition comprising a 
method of making a stable magnetorheological fluid composition comprising 
preparing a carrying fluid comprising a vinyl ether, an aromatic alcohol 
and an organic solvent or diluent carrier such as kerosene; preparing a 
first carrying fluid composition comprising magnetosoft particles, a 
stabilizer and a first sample of the carrying fluid; preparing a second 
carrying fluid composition comprising magnetosolid particles and a second 
sample of the carrying fluid; and admixing the first carrying fluid 
composition and the second carrying fluid composition. 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The magnetorheological fluid composition of the present invention comprises 
a non-colloidal ferromagnetic powder suspended in a carrying fluid which 
contains a stabilizer. 
The ferromagnetic particles of the invention are a mixture of coarse 
magnetosoft particles and fine magnetosolid particles. The magnetosoft 
particles preferably are made from carbonyl iron. The magnetosoft 
particles are generally spherical in shape. A preferred particle size 
range is about 1 to about 10 .mu.m, though broader ranges are suitable. It 
is more important that the magnetosoft particles be proportionately larger 
than the magnetosolid particles. Preferably, the magnetosoft particles are 
at least about ten times larger than the magnetosolid particles. 
The magnetosolid particles preferably are made from iron oxide or chromium 
dioxide. The magnetosolid particles are anisodiametric in shape. A 
preferred particle size range is about 0.1 to about 1.0 .mu.m, though 
relative size to the magnetosoft particles is considered more important to 
achieving the properties of the invention. 
Magnetosoft carbonyl iron particles are produced by thermal decomposition 
of pentacarbonyl iron (Fe(CO).sub.5). Preferred carbonyl iron particles 
are commercially marketed powders used in conjunction with 
radioengineering equipment, such as those sold under Russian trademarks 
P-10, P-20, P-100, or those marketed by GDS BASF under the trademarks SF, 
TH, E. Iron oxide needle-like magnetosolid particles can be produced by 
oxidation of a magnetite such as Fe.sub.3 O.sub.4. Chromium dioxide 
particles preferably are formed by the decomposition of chromium angidrid 
(CrO.sub.3) under high pressure in the presence of oxygen. 
The magnetosolid particles preferably are adsorbed onto the surface of the 
magnetosoft particles, imparting to the magnetic particles a brush-like 
effect. The magnetosolid particles are preferably small, needle-like 
magnets which attach at one end to the more coarse magnetosoft particles. 
Adsorption of magnetosolid particles onto magnetosoft particles has been 
shown to give the resulting fluid composition higher stability and greater 
relative viscosity change upon application of a magnetic field. 
Preferably, the magnetosoft particles are multidomain, that is, they are 
randomly distributed in a volume of liquid, and have no residual 
magnetization. The magnetosolid particles are preferred to have a 
needle-like shape and have their own magnetic moments, in order to provide 
the brush-like effect described above with the magnetosoft particles. 
The carrying fluid of the invention is made from an organic solvent or 
diluent carrier, an aromatic alcohol, and a vinyl ether. A preferred 
organic solvent is a liquid hydrocarbon such as kerosene. The organic 
solvent preferably has low volatility, good anti-corrosion properties, low 
toxicity, and high flash temperature and temperature of self-ignition. A 
preferred aromatic alcohol is .alpha.-naphthol (C.sub.10 H.sub.7 OH). A 
preferred vinyl ether is polyvinyl-n-butyl ether (CH.sub.2 .dbd.CHOC.sub.4 
H.sub.9).sub.n. The aromatic alcohol and vinyl ether preferably contain 
one or more of the following properties: solubility in the organic 
solvent; low freezing temperature (preferably below about 100.degree. C.); 
ability to thicken the organic solvent; and resistance to mechanical 
loading (preferably up to about 10.sup.6 Pascals shear stress under flow). 
The aromatic alcohol and the vinyl ether are dissolved in the organic 
solvent to form the carrying fluid. 
Other components can also be added to the carrying fluid, such as 
antifoaming agents, such as polysiloxane compounds, antiwear agents, such 
as tricresylphosphate ((CH.sub.3 C.sub.6 H.sub.4 O).sub.3 PO). 
The addition of an aromatic alcohol and a vinyl ether to the organic 
solvent creates a carrying fluid having a higher viscosity, greater 
lubricant properties and greater protection against breakdown of the 
organic solvent than the organic solvent alone. Preferably, the carrying 
fluid contains 90 to 95 parts by weight organic solvent, 0.01 to 0.10 
parts aromatic alcohol, and 4.9 to 9.99 parts vinyl ether. A particularly 
preferred carrying fluid composition comprises 92.75 weight percent 
kerosene, 0.05 weight percent .alpha.-naphthol, and 7.2 weight percent 
polyvinyl-n-butyl ether. 
In most preferred embodiment of the invention, a stabilizer is used in 
addition to the carrying fluid to provide added stability to the fluid 
composition. Preferred stabilizers include unhydrated, inorganic silicone 
compounds. A particularly preferred stabilizer is AEROSIL (SiO.sub.2). 
The stabilizer particles preferably are approximately 0.005-0.015 .mu.m in 
diameter and are preferred to be about one-tenth to two-tenths the size of 
the magnetosolid particles. The relatively small diameter of the 
stabilizer particles results in the particles having a relatively large 
surface area. A stabilizer particles surface area of about 350 to 400 
m.sup.2 /g is preferred. 
The stabilizer particles can be spherical in shape and preferably are 
non-porous. The stabilizer particles are designed so that in a shear flow, 
the structure formed by the particles are reversibly deformed. Preferably, 
the stabilizer is present in an amount of about 4 to 9 weight percent of 
the carrying fluid. 
The magnetorheological fluid composition of the invention preferably is 
made using a multi-step process comprising admixing the carrying fluid 
ingredients, adding a stabilizer and magnetosoft particles to a first 
admixture of carrying fluid, adding magnetosolid particles to a second 
admixture of carrying fluid, and combining the two magnetic 
particle-containing carrying fluid compositions. The carrying fluid 
preferably is formed by dissolving the vinyl ether and aromatic alcohol in 
kerosene at ambient conditions. 
The first carrying fluid admixture contains 5 to 25 parts by weight of 
magnetosoft particles to 10 parts of carrying fluid, and formed under 
continuous mixing. The stabilizer preferably is injected into the first 
carrying fluid admixture by use of a pulverizer. 
A sufficient amount of stabilizer is added until a gelatinous composition 
is obtained, typically about 5 to 15 weight percent of the first carrying 
fluid admixture. Then the magnetosoft particles are added to the 
composition, which is homogenized, such was with a ball mill. Ball milling 
will minimize agglomeration of the magnetosoft particles which may occur 
upon addition to the composition. 
The magnetosolid particles are added to the second admixture of carrying 
fluid and homogenized, such as by agitation. It is preferred that about 1 
to 15 parts by weight magnetosolid particles per 10 parts by weight 
carrying fluid be present. Preferably, a surfactant is employed in this 
stage of the process to facilitate complete dispersion of the magnetosolid 
particles. The surfactant preferably is a fatty acid, with oleic acid 
being particularly preferred. The surfactant can minimize coagulation of 
the dispersed magnetosolid particles, and to aid in stably dispersing the 
particles in suspension. Preferably, less than 5 weight percent surfactant 
is employed in the second carrying fluid admixture, with less than one 
percent particularly preferred. 
The two particle-containing carrying fluid mixtures are combined and 
homogenized. A ball mill is suitable for this purpose. Preferably, 
approximately 5 to 10 parts by weight of the first carrying fluid mixture, 
containing the magnetosoft particles, is added per 100 parts by weight of 
the second carrying fluid mixture. The resultant suspension is stable and 
responsive to application of a magnetic field. 
Magnetorheological fluids of the present invention can be used in a variety 
of applications, such as polishing, seals, casting technology, controlled 
heat carriers, drives, clutches, hydraulic systems, and vibration systems 
(such as shock absorbers), including in conventional applications already 
known in the art. The fluids can be used in a variety of polishing 
applications such as optical lens polishing, and polishing of ceramics, 
the inner surfaces of tubes and pipes, and semiconductor materials. The 
fluids are particularly suitable for polishing objects having irregular 
shapes. The fluid can be used in heat carrier applications such as heat 
exchangers and audio speakers. Typical drive systems which can employ the 
fluid of the invention include robotics and actuating modules. Other 
applications for magnetorheological fluids known in the art may also take 
advantage of this novel composition. 
In a lens polishing application, the composition, which can optionally 
include abrasive polishing particles, is contacted with a workpiece to be 
polished. Upon application of a magnetic field, the fluid viscosity 
changes and the fluid starts moving. In a preferred method of operation, 
the workpiece is immersed in the composition and the field is applied such 
that the fluid flows circularly around the workpiece. As the magnetic 
particles and/or the abrasive polishing particles contact the workpiece, 
the workpiece is polished. Using the composition of the invention, 
irregular-shaped objects and difficult to polish articles such as those 
made from crystal can be polished effectively.

EXAMPLE 
A magnetorheological fluid of the invention was made using the following 
process. First, a carrying fluid sample was formed by dissolving 7.2 parts 
of polyvinyl-n-butyl ether 0.05 parts of .alpha.-naphthol in 92.75 parts 
kerosene. 
A first carrying fluid admixture is prepared by injecting AEROSIL 
(SiO.sub.2) A-380, manufactured by Industrial Association Chlorvinyl, 
Kalysha City, Ukraine, into the carrying fluid prepared as described 
above. Injection took place over an hour until a homogenous gelatinous 
system was obtained. Then, iron carboxide powder was added to the 
admixture. The entire admixture was homogenized in a ball mill over a 
period of 4 to 5 hours. The proportion of ingredients was iron carboxide 
powder (50 weight %), aerosil (7.5 weight %), carrying fluid (42.5 weight 
%). 
Chromium dioxide powder, oleic acid and a second carrying fluid sample were 
mixed and homogenized for 4 to 5 hours in a universal agitator in the 
following proportions: 
Chromium dioxide power--36 weight % 
Oleic acid--0.36 weight % 
Carrying fluid--63.63 weight % 
Next, the two magnetic particle-containing carrying fluid admixtures were 
combined and mixed in a ball mill for an hour to arrive at a final 
composition. 100 grams of the iron carboxide-containing admixture were 
added to 7.5 grams of the chromium dioxide powder-containing admixture. 
The resulting product exhibited changed viscosity, plasticity, elasticity, 
thermoconductivity, and electroconductivity in response to application of 
a magnetic field. The fluid was stable at temperatures of -50.degree. to 
120.degree. C. The composition was tested in a cylindrical coaxial rotary 
viscometer supplied by a magnetic field inductor. The applied field 
intensity H was varied up to 80 kA/m, and the shear rate 7 was varied from 
1.02 to 444.5 seconds.sup.-1. The response of the fluid viscosity to the 
magnetic field intensity is given in Table I below. It can be seen from 
Table I that increasing field intensity results in increasing viscosity at 
a given shear rate. The data in Table I also indicate that increasing 
shear rate results in generally lower viscosity at a given field 
intensity. Highest viscosity was obtained at low shear rate and high field 
intensity. 
TABLE I 
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H, kA/m 
0 12.7 
24.2 
35.0 
43.6 
48.2 
62.0 
77.0 
84.0 
.gamma., s.sup.-1 
.eta., Pa.s 
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1.02 
0.81 
5.32 
31.94 
51.86 
87.76 
135.6 
438.8 
492.0 
585.1 
1.84 
0.54 
3.23 
36.85 
29.32 
56.44 
76.24 
249.2 
300.6 
329.9 
2.97 
0.39 
2.27 
11.79 
20.41 
38.10 
50.80 
158.8 
190.5 
208.7 
5.42 
0.33 
1.49 
6.99 
11.49 
23.48 
29.97 
89.91 
107.3 
117.4 
9.10 
0.29 
1.03 
4.56 
9.13 
14.72 
19.72 
63.27 
78.48 
85.35 
16.45 
0.27 
0.91 
2.63 
5.35 
8.56 
12.68 
39.53 
49.41 
50.23 
27.70 
0.24 
0.73 
1.71 
3.40 
5.44 
8.16 
25.76 
32.07 
34.51 
49.40 
0.22 
0.49 
1.08 
2.03 
3.19 
4.81 
15.66 
20.79 
22.14 
82.30 
0.18 
0.34 
0.71 
1.31 
1.99 
2.91 
10.37 
13.77 
15.06 
147.80 
0.17 
0.26 
0.48 
0.86 
1.24 
1.84 
6.64 
8.69 
9.64 
246.0 
0.14 
0.19 
0.32 
0.56 
0.77 
1.08 
4.05 
5.29 
5.78 
444.5 
0.12 
0.14 
0.20 
0.32 
0.44 
0.59 
2.21 
2.92 
3.13 
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