LOW-VAPOR-PRESSURE FERROFLUIDS AND METHOD OF MAKING SAME

FERROFLUIDES A VAPEUR BASSE PRESSION, ET METHODE DE FABRICATION CONNEXE

English Abstract

ABSTRACT OF THE DISCLOSURE
A stable ferrofluid composition is described which
comprises a colloidal dispersion of finely-divided magnetic
particles in a liquid, low-pressure, polyphenyl ether carrier and
a dispersing amount of a surfactant, which surfactant comprises a
polyphenyl ether surfactant containing a functional group, such
as an acidic group, and an aromatic tail group which is soluble
in the polyphenyl ether carrier, to provide a stable ferrofluid
composition.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A ferrofluid composition which comprises a colloidal
dispersion of finely-divided magnetic particles in a liquid
polyphenyl ether carrier and a dispersing amount of a surfactant,
which surfactant comprises a surfactant containing a functional
polar group which forms a chemical bond with the surface of the
magnetic particles and a tail group containing phenyl, benzyl or
phenoxy groups which are soluble in the carrier, and a linking
group to separate the polar and tail groups, to provide a stable
ferrofluid composition.
2. The ferrofluid composition of claim 1 wherein the
surfactant comprises from 1:2 to 1:20 by volume of the magnetic
particles.
3. The ferrofluid composition of claim 1 wherein the
magnetic particles are activated magnetite particles.
4. The ferrofluid composition of claim 1 wherein the
carrier liquid has a vapor pressure of about 10 4 torr or less
at 20°C.
5. The ferrofluid composition of claim 1 wherein the
functional polar group of the surfactant comprises a carboxylic,
amine, mercaptan, aldehyde or alcohol group reactive with the
surface of the magnetic particles.
6. The ferrofluid composition of claim 1 wherein the
magnetic particles comprise from about 1% to 20% by volume of the
13

ferrofluid composition.
7. The ferrofluid composition of claim 1 wherein the
magnetic particles have a particle size of from about 20 to 500
Angstroms.
8. The ferrofluid composition of claim 1 wherein the
carrier liquid has the structural formula:
<IMG>
where n is a whole integer of from 1 to 6.
9. The ferrofluid composition of claim 1 wherein the
linking group of the surfactant is an aliphatic group.
10. The ferrofluid composition of claim 1 wherein the
surfactant is a phenoxy-, benzyl- or phenyl-terminated aliphatic
or aromatic carboxylic acid or alcohol.
11. The ferrofluid composition of claim 1 wherein the
surfactant is a phenoxy-terminated long-chain fatty acid.
12. The ferrofluid composition of claim 1 wherein the
surfactant is selected from the group consisting of phenoxy
benzoic acid, phenoxy undecanoic acid and phenoxy benzyl alcohol.
13. The ferrofluid composition of claim 1 wherein the
surfactant comprises a phenoxy-terminated undecanoic acid and the
14

liquid polyphenyl ether carrier comprises a four-ring polyphenyl ether carrier.
14. The ferrofluid composition of claim 1 wherein the surfactant
comprises a mono or di phenoxy long-chain C6-C16 aliphatic carboxylic acid.
15. A ferrofluid composition which comprises a colloidal dispersion
of
(a) finely-divided, activated, magnetite particles, comprising
from about 1% to 20% by volume of the ferrofluid composition,
in (b) a liquid polyphenyl ether carrier, the carrier liquid having
the structural formula
<IMG>
where n is a whole integer of from l to 6, and
(c) a dispersing amount of a phenoxy-, benzyl- or phenyl-terminated
aliphatic or aromatic carboxylic acid or alcohol surfactant comprising from
1:2 to 1:20 by volume of the magnetite particles, which surfactant comprises
a surfactant containing a functional polar group which forms a chemical bond
with the surface of the magnetite particles and a tail group containing phenyl,
benzyl or phenoxy groups which are soluble in the carrier and a linking
group to separate the polar and tail groups, to provide a stable ferrofluid
composition.
16. The ferrofluid composition of claim 15 wherein the

surfactant is selected from the group consisting of phenoxy
benzoic acid, phenoxy undecanoic acid and phenoxy benzyl alcohol.
17. In a method of preparing a stable ferrofluid composition,
which method comprises dispersing finely-divided magnetic particles
in a liquid with a dispersing amount of a surfactant to form a
colloidal dispersion of a ferrofluid composition, the improvement
which comprises employing as the liquid carrier a liquid polyphenyl
ether and as the surfactant a surfactant having a functional polar
group reactive with the surface of the magnetic particles during
the dispersion, to form a chemical bond with the magnetic particles
and a tail group containing phenyl, benzyl or phenoxy groups which
are soluble in the carrier, and a linking group to separate the
polar and tail groups, to provide a stable ferrofluid composition.
18. The method of claim 17 wherein the surfactant comprises
from 1:2 to 1:20 by volume of the magnetic particles.
19. The method of claim 17 wherein the magnetic particles
are activated magnetite particles.
20. The method of claim 17 wherein the carrier liquid has
a vapor pressure of about 10-7 torr or less at 20°C.
21. The method of claim 17 wherein the carrier liquid has
the structural formula:
<IMG>
16

where n is a whole integer of from 1 to 6.
22. The method of claim 17 wherein the surfactant is a
phenoxy-, benzyl- or phenyl-terminated aliphatic or aromatic
carboxylic acid or alcohol.
23. The method of claim 17 wherein the surfactant is
selected from the group consisting of phenoxy benzoic acid,
phenoxy undecanoic acid and phenoxy benzyl alcohol.
24. The method of claim 17 wherein the surfactant comprises
a phenoxy-terminated undecanoic acid and the liquid polyphenyl
ether carrier comprises a four-ring polyphenyl ether carrier.
25. The method of claim 17 wherein the surfactant comprises
a mono or di phenoxy long-chain C6-C16 aliphatic carboxylic acid.
26. The method of claim 17 wherein the functional polar
group of the surfactant comprises a carboxylic, amine, mercaptan,
aldehyde or alcohol group reactive with the surface of the
magnetic particles.
27. The method of claim 17 which includes:
a) grinding the magnetite in a ball mill to disperse
the magnetic particles in a solution of the surfactant and a
solvent, to form a colloidal suspension of the magnetic particles,
surfactant and solvent;
b) separating the surfactant-coated magnetic particles
from the solvent, and
c) dispersing the surfactant-coated magnetic particles
in the carrier liquid.
17

28. The method of claim 27 which includes separating the
magnetic particles from the solvent by evaporating the solvent.
29. The method of claim 28 which includes separating the
magnetic particles from the solvent by flocculating with the
addition of a polar solvent.
18

Description

~4~43
Ferromagnetic liquids commonly are referre~ to as
ferrofluids and typically comprise a colloidal dispersion of
finely-divided magnetic particles, such as iron, ~ Fe203 ~hematite),
magnetite and combinations thereof, of subdomain size, such as, for
example, 10 to 800 Angstroms, and more particularly 50 to 500
Angstroms, dispersed in a liquid through the use of a surfactant-
type material. Typically ferrofluids are remarkably unaffected by
the presence of applled magnetic fields or by other force fields,
and the magnetic particles remain uniformly dispersed throughout
the liquid carrier. Ferrofluid compositions are widely known, and
typical ferrofluid compositions are described, for example, in
United States Patent 3,700,595, issued October 24, 1972, and
United States Patent 3,764,540, issued October 9, 1973, while a
particular process for preparing such ferrofluid compositions is
described more particularly in United States Patent 3,917,538,
issued November 4, 1975, which describes a grinding or ball-mill
technique for preparing ferrofluid compositions, and United States
Patent 4,019,994, issued April 26, 1977, describing more
particularly a precipitation technique for preparing ferrofluid
compositions.
Ferrofluids have been suggested to be prepared using a
wide variety of liquid carriers. However, current state-of-the-art
ferrofluids typically employ a hydrocarbon carrier or, for
example, a diester liquid, such as ethyl-hexyl a~ilate. Liquid
ferrofluids typically comprise a dispersion of colloidal magnetite
stabilized by an aliphatic surfactant in a hydrocarbon-liquid
carrier, such as, for example, the use of an oleic-acid-type
surfactant. These diester ferrofluids have found use in audio-
- 1 - ~,~

1~i4~3
voice-coil-damping and inertia-damping apparatus and for use in
bearings and seals. However, the hydrocarbon-based, and
particularly the diester-based ferrofluids have been limited in
some applications, particularly in seals, because o a relatively
high vapor pressure of the carrier of greater than about 10 4 torr
at 20C.
Ferrofluids have been employed in single and multistage
seals (see, for example, United States Patent 3,620,584, issued
November 16, 1971), which seals are used in the vacuum industry.
Attempts to provide stable, low-vapor-pressure, ferrofluid
compositions particularly for use at 10-~ torr or more, particular-
~ re~
ly equal to or ~ than 10 7 torr, have not been commercially
successful. Accordingly, there exists a need for stable, low-
vapor-pressure ferrofluids for seals and other uses and for a
method of manufacturing such ferrofluid compositions.
Our invention relates to improved ferrofluld composi-
tions, the method of making the compositions and the use of such
improved ferrofluid compositions. In particular, our invention
concerns an improved stable, polyphenyl-ether-based, ferrofluid
composition. More particularly, our invention is directed to an
improved stable, low-vapor-pressure, ferrofluid composition,
particularly useful in seals under vacuum, and the method of
making said ferrofluids.
In one aspect, the present invention relates to a
ferrofluid composition which comprises a colloidal dispersion of
finely-divided magnetic particles in a liquid polyphenyl ether
carrier and a dispersing amount of a surfactant, which surfactant
-- 2 --

comprises a surfactant containing a functional polar group which
forms a chemical bond with the surface of ~he magnetic particles
and a tail group containing phenyll benzyl or phenoxy groups which
are soluble in the carrier, and a linking group to separate the
polar and tail groups, to provide a stable ferrofluid composition.
In a further aspect, the present invention relates to
an impro~ed method of preparing a stable ferrofluid composition,
which method comprises dispersing finely-divided magnetic particles
in a liquid with a dispersing amount of a surfactant to form a
colloidal dispersion of a ferrofluid composition, wherein the
improvement comprises employing as the liquid carrier a liquid
polyphenyl ether and as the surfactant a surfactant having a
functional polar group reactive with the sur~ace of the magnetic
particles during the d~.spersion, to form a chemical bond with the
magnetic particles and a tail group containing phenyl, benzyl or
phenoxy groups which are soluble in the carrier, and a linking
group to separate the polar and tail groups, to provide a stable
ferrofluid composition.
We have discovered that stable -ferrofluids may be
0 prepared employing polyphenyl ether as a carrier liquid, which
r~
ferrofluids are characterized by a low vapor pressure of ~than
10 4 torr; for example, 10 7 torr, at 20C, and, therefore, are
particularly suitable for use in seals or other uses where low
vapor pressure is desirable. Our stable ferrofluids are prepared
employing a surfactant, wherein the surfactant contains a
functional group which forms a chemical bond with the surface of
the subdomain magnetic particles employed ln the ferrofluid, such
-- 3 --

ti43
as, for example, a functional group that is capable of forming a
complex, such as a coordination complex, or by entering into a
chemical reaction with the surface of the magnetic particles. The
surfactant may be the same as or different from the polyphenyl
ether carrier of the ferrofluid carrier, but preferably is of the
same general structure, which results in a stable dispersion of
the coated magnetic particles in the ferrofluid, since the
functional head group provides for a bond with the magnetic
particles, while the remaining or tail portion of the surfactant
molecule is solubilized in the polyphenyl ether carrier liquid.
The stable ferrofluids of our invention are stable
during storage or when subjected to a magnetic-force field or
other force fields and, due to the nature of the liquid polyphenyl
ether used as a carrier, exhibit very low vapor pressure.
It has been found that poly aromatic polyethers,
particularly liquid polyphenyl ethers, are suitable for use as
carrier liquids in ferrofluid compositions. Polyphenyl ethers
have wide acceptance in the vacuum industry, due in part to very
low vapor pressures. Our attempts to synthesize polyphenyl ether
ferrofluid compositions with conventional surfactants, such as
oleic and undecanolc acid, have not been successful, since
unstable ferrofluids have resulted. It has been discovered that
stable polyphenyl ether ferrofluid compositions can be produced
employing certain surfactants.
Our invention comprises a stable ferrofluid composition,
and particularly those ferrofluid composltions having a carrier
liquid with a low vapor pressurel whlch ferrofluid comprises

finely-divided magnetic particles, typically, for example, less
than 800 Angstroms; for e~ample, 20 to 500 ~ngstroms, and more
particularly 50 to 150 Angstroms in particle size, colloidally
dispersed in a polyphenyl ether carrier liquid and a small, but
effective, amount of a surfactant, typically a surfactant wherein
the surfactant contains a functional group which ~orms a chemical
bond, such as forming a complex or reactant with the surface of
the magnetic particles, a tail group of sufficient length and
structure that is miscible or soluble in the polyphenyl ether
carrier llquid, with a linking group between the head and tail
groups that is of sufficient length and rigidity to direct the
head group toward the magnetic particles and to separate the head
and tail groups from one another. Our invention also is directed
to a method of preparing the improved stab]e ferrofluids, wherein
a polyphenyl ether is employed as the liquid carrier, either
through a grinding, preclpitating or other technique of preparing
ferrofluids, and a surfactant is employed containing a reactive
functional group and proper tail group in the preparation of the
ferrofluid composition.
In general, our invention relates to the use of
surfactants to pr~duce gravitationally and magnetically stable
ferrofluid compositions, wherein magnetic particles can be
dispersed in a low-pressure liquid carrier. The surfactants used
may have the general structural formula:
R" - R~ - R - YH
and contain a functional reactive polar group which has an active
hydrogen and which reacts or forms a chemical complex or bond with

~4~i~3
the surface of the magne~ic par~icles, such as with iron atoms o~
activated ~inely-divided magnetlte particles. The YH head or
polar group is separated by and linked to an R group (which is
optional) which is linked to an R' group. ~he R~ group, -typically
an organic group, is of sufficient length and rigidity to separate
the polar head group YH from~the R" tail portion of the surfactant.
R' and R may be the same or different groups. The R" group is
selected to be soluble in and generally is the same or similar in
chemical structure and/or properties as the carrier liquid, so
~hat the R" group will be a carrier-soluble tail with the polar
head group YH attached to the magnetic particles.
The surfactants of choice will be illustrated in use
for the preparation of polyphenyl ether, carrier-liquid ferrofluid
compositions; however, it is recognized that the surfactants of
the general type described may be employed with other matching
liquid carriers, to provide a wide variety of stable ferrofluid
compositions.
The liquid carriers of our ferrofluid composition are
those aromatic high-molecular-weight polyethers which exhibit a
low vapor pressure and include those low-pressure poly aromatic
polyethers typically used in the vacuum industry and preferably
polyphenyl ethers, such as those disclosed in United States Patent
3,451,061, issued June 17, 1969, and similar polyether liquids.
Useful polyphenyl ethers are typically liquids at 15C to 20C and
have a repetitive chemical structure.
Typical polyphenyl ethers useful as carrier liquids
have the structural formula:
-- 6 --

4t~43
wherein n ranges from l to 6; for example, 1 to 4, and preferably
is 3 or 4.
The polyphenyl ether may be substltuted or unsubsti-
tuted. The polyphenyl ether may be used alone or in various
polyphenyl ether combinations, or, lf desired, with other low-
vapor-pressure carrier liquids.
The magnetic particles employed in the ferrofluid may
be those typical magnetic particles, either prepared by grinding
or precipitation, but typically are finely-divided magnetizable
particles usually recognized as magnetite, such as magnetite gamma
iron oxide, chromium dioxide, ferrites and similar materials, and
which materials also may include varlous elements and metallic
alloys. The preferred materials are magnetite, gamma lron oxlde
(Fe304) and (~ Fe203), wherein the magnetic partlcles are present
usually in an amount of from about 1% to 20%; for example, 2% to
12%, by volume of the ferrofluld.
The surfactant or stabillzing agents employed comprise
those surfactants which include at least polar-reactive or
-functionlng groups, a linking group and a long-chain polyphenyl
ether soluble tail. Typically, the sur~actant may be present in
an amount sufficient to provide the desired c~lloidal dispersion
-- 7 --

4~i~3
and stability to the ferrofluid compoSition, and generally iS used
in a ratio of sur~actant-to magnetic-particles o~' from about 1:2
to 1:20 by volume; for example, 1:1 to 1:5 by volume. If desired
and applicable, the liquid carrier may be used alone or in
conjunction with other liquid carrier materials or other additive
materials. The surfactant employed may be used alone or in combin-
ation with other types of surfactants where necessary or required,
such as those carboxylic acids or other dispersants or surface-
active agents useful in the dispersin~ or stabilizing of magnetite
particles.
Representative polar groups YH of the surfactant
typically include carboxylic acids (-COOH), alcohols (-OH), amines
(-NH2), aldehydes (R-C~ 0I ) and mercaptans (-SH) among other
compounds with so-called activé hydrogen groups. The tail groups
R" include molecules with phenyl, benzyl or phenoxy moieties of
various lengths and degrees of isomerization. The linking groups
R~ may consist of numerous aliphatic or aromatic structures whose
isomeric structures and lengths may be varied in nature.
Some types of surfactant that result in stable ferro-
~luids in polyphenyl ethers include, but are not limited to:phenoxy-terminated aliphatic acids, such as ll-phenoxy undecanoic
acid; phenoxy-terminated alcohols, such as phenoxy, phenoxy phenol
and 0-phenoxy benzyl alcohol; and phenoxy-terminated aromatic
acids such as 0-phenoxy-benzoic acid. To stabilize the colloid
in higher-molecular-weight polyphenyl ethers, such as pentaphenyl
ether (Nye synthetic oil #438), the surfactant may require more
than a single phenoxy unit in its structure.
-- 8 --

~ e have found that the use of undecanoic acid as a
surfactant With a four-ring polyphenyl ether carrier liquid is
unsatisfactory; however, the use of the phenoxy-terminated
undecanoic acid provides stable ferrofluid compositions. The
preferred suxfactants comprise:
L ~ - ~ R - COOH
wherein n is 1 to 6 and preferably 1 or 2, and R is a phenyl or
benzyl or aliphatic radical; for example, a long-chain hydrocarbon,
such as a C6-C18-saturated or -unsaturated radical, or where R and
the carboxylic groups are a fatty-acid radical.
In the illustration given, the phenoxy group is the R"
tail group and the Cll chain is the R~ linking group and the COOH
is the polar YH group. It is recognized that mono, di or tri
polar groups, such as mono, di and tri carboxylic groups, may be
employed in the surfactant. The most preferred surfactants are
those mono or di phenoxy long-chain C6-C16 aliphatic; for example,
fatty carboxylic acid, surfactants. The surfactants may, if
desired, have one or more polar YH groups of the same or different
reactive structure, such as a surfactant with COOH and SH polar
groups.
Thus, in practice, the solubilizing or tail groups of
the surfactant permit a very stable dispersion by being solubilized

469~3
easily in the carrier liquld, while the opposite end o~ the
reacting group is secured or chemically bonded to the magnetic
particles. The sur~actant may be the same as or dif~erent frorn
the carrier fluid, either in molecular weight, viscosity, chain
length or isometric chemical characteristics.
Stable ferrofluid compositions may be prepared in poly-
phenyl ether liquid carriers by, for example, grinding magnetite
in a ball mill in a solution of the reactive surfactant and a
solvent, such as a volatile organic solvent, and reacting the
activated magnetite with the reactive groups of the surfactant in
the solvent slurry, such as, for example, where the solvent is
methylethyl ketone, acetone and a hydrocarbon like xylene, toluene
or other volatile solvents.
Typical formulations for a grinding operation are:
Fe30 10 - 300 gm
Surfactant 1 - 100 gm
Solvent 100 - 2000 ml
After grinding for sufficient time to disperse the magnetite, the
colloidal suspension of magnetite, surfactant and grinding solvent
is removed from the ball mill. Separation of the surfactant-
coated particles from the solvent may be achieved by evaporation
or flocculation with acetone or other polar solvent. The
sedimentated material is easlly solubilized in a carrier liquid.
A typical formulation for a precipltation operatlon is:
Activated magnetite 10 - 100 gm
Surfactant 10 - 100 gm
Acetone 100 ~ 1000 ml
-- 10 --

~64ti~3
The activated magnetite is slurried in acetone for 5 to 30 minutes.
The surfactant is slowly added to the slurry during constant
stirring. A material, that consists of surfactant-coated
particles, precipitates from the solution, which material is
readily soluble in a four-ring polyphenyl ether as a carrier
liquid.
For the purpose of illustration only, our invention will
be described in connection with certain embodiments; however, it
is recognized that those persons skilled in the art may make
various changes and modifications in the ferrofluid compositions
and the method of preparing same, all without departing from the
spirit and scope of our invention.
Example 1
. . _ . . .
~ ctivated magnetite (Fe304) particles were prepared as
follows: 260 grams FeSO4, 460 ml 46% FeC13 and 100 ml water were
mixed to dissolve the salts~
800 grams of ice were added to obtain a temperature of
-3C. Fe304 was precipitated from the solution by the slow
addition of a solution of 600 ml concentrated NH40H and 400 cc
water cooled to ~1C. The Fe304 was magnetically separated from
the salt/ammoniacal solution and was washed with 200-ml portions
of hot water, 100-ml portions of acetone and 200-ml portions of
xylene and was vacuum dried. The activated magnetite prepared in
this or a similar manner was then used in the preparation of poly-
phenyl ether ferrofluid compositionsO
Then 50 grams of the activated magnetite, 15.55 grams
of ll-phenoxy undecanolc acid as the surfactant and 525.6 ml of

~L~6~643
xylene were d~spersed for three days ln a ball mill. The material
was then flocculated with three 100-ml portions of heptane and
redispersed with 8 ml of polyphenyl ether (4-ring Nye synthetic oil
#433) as a carrier li~uid. The product was a ferrofluid composi-
tion with a magnetization of 450 gauss and 1000 cp in viscosity.
Example 2
10 grams activated magnetite, 10 grams O phenoxy
benzoic acid as a surfactant and 100 ml xylene were ground in a
ball mill for 10 days. After flocculation with heptane, the
material was redispersed in a 4-ring polyphenyl ether. The ferro-
fluid was about 100 yauss and had a viscosity of 500 cp.
Example 3
10 grams activated magnetite, 10 grams O phenoxy benzyl
alcohol as a surfactant and 100 ml xylene were ground for 10 days.
The material was only slightly magnetic with a gauss of about 50.
The stable, low-vapor-pressure ferrofluids so prepared
are useful in multistage, ferrofluid, seal assemblie$, particularly
in the vacuum industry or for sealiny or lubricating purposes,
wherein polyphenyl ethers are desirable or useful,
- ~2 -