Note: Descriptions are shown in the official language in which they were submitted.
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TITLE: ENHANCING THERMAL CONDUCTIVITY OF FLUIDS WITH GRAPHITE
NANOPARTICLES AND CARBON NANOTUBE
BACKGROUND OF THE INVENTION
This application is part of a government project, Contract
No. V~1031-109-ENG-38 by the Department of Energy. The Government
has certain rights in this invention.
Technical Field
Fluids of enhanced thermal conductivity are prepared by
dispersing carbon nanomaterials of a selected thermal
conductivity into the fluid serving as the liquid medium.
Dispersion is achieved by physical and chemical treatments.
Methods are described and fluid compositions are identified which
exhibit enhanced thermal conductivity due to the dispersion of
carbon nanomaterials in aqueous and/or petroleum liquid medium
utilizing selected dispersants and mixing methods to form stable
carbon nanomaterial dispersions.
Description of the Prior Art
Lubricants and coolants of various types are used in
equipment and in manufacturing processes to remove waste heat,
among other functions. Traditionally, water is most preferred
for heat removal, however, to expand it's working range, freeze
depressants such as ethylene glycol and/or propylene glycol are
sometimes added, typically at levels above 10o concentration by
volume, for example, automotive coolant is typically a mixture
of 50-70o ethylene glycol, the remainder water. The thermal
Conductivity of the freeze depressed fluid is then about 2/3 as
good as water alone. In many processes and applications, water
Can not be used for various reasons, and then a type of oil, e.g.
mineral oil, polyalpha olefin oil, ester synthetic oil, ethylene
oxide/propylene oxide synthetic oil, polyalkylene glycol
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synthetic oil, etc. are used. The thermal conductivity of these
oils, is typically 0.1 to 0.17 W/m-K at room temperature, and
thus they are inferior to water, with comparable thermal
Conductivity of 0.61 W/m-K, as heat transfer agents. Usually
these oils have many other important functions, and they are
carefully formulated to perform to exacting specifications for
example for friction, wear performance, low temperature
performance, etc. Often designers will desire a fluid with
higher thermal conductivity than the conventional oil, but are
restricted to oil due to the many other parameters the fluid must
meet.
The use of graphite solids in fluids such as lubricants is
well known. The graphite is added as a friction reducing agent,
which also carries some of the load imposed on the working fluid,
and therefore helps to reduce surface damage to working parts;
however, the thermal conductivity property of the graphite is not
an important consideration in conventional applications. While
there have been various patents filed on lubricants containing
graphite, e.g. U. S. Patent 6,169,059, there are none which
specifically rely on graphite to improve the thermal conductivity
of the fluid.
While graphite-containing automotive engine oil was once
commercialized (ARCOTM graphite), the potential to use graphite
as a heat transfer improving material in this oil was not
realized. The particle size of graphite used was larger (on the
order of one to several microns) than for the instant invention.
As a result, the graphite incorporated in the aforementioned
automotive engine oil had strong settling tendency in the fluid.
Graphite of this size also significantly effected the friction
and wear properties of the fluid, and heretofore has been used to
reduce friction and improve wear performance of the fluid, e.g.
in metalworking fluids. The use of graphite in lubricants for
recirculating systems was made unpopular, partly due to the
publication by NASA that graphite could "pile up" in restricted
flow areas in concentrated contacts, thereby leading to lubricant
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starvation. No recognition on the effect of graphite particle
size on this phenomena was ever established. Furthermore, none
of the prior art references teach the use of utilizing nano-sized
graphite particles with mean particle size less than 500nm to
enhance thermal conductivity in fluids.
Carbon nanotubes are a new form of the nanomaterial formed
by elemental carbon, which possesses different properties than
the other forms of the carbon materials. It has unique atomic
structure, very high aspect ratio, and extraordinary mechanical
properties (strength and flexibility), making them ideal
reinforcing fibers in composites and other structural materials.
Carbon nanotubes are characterized as generally to rigid
porous carbon three dimensional structures comprising carbon
nanofibers and having high surface area and porosity, low bulk
density, low amount of micropores and increased crush strength
and to methods of preparing and using such structures. The
instant process is applicable to nanotubes with or without
amorphous carbon.
The term "nanofiber" refers to elongated structures having
a cross section (e. g., angular fibers having edges) or diameter
(e. g., rounded) less than 1 micron. The structure may be either
hollow or solid. Accordingly, the term includes "bucky tubes"
and "nanotubes". The term nanofibers also refers to various
fibers, particularly carbon fibers, having very small diameters
including fibrils, whiskers, nanotubes, buckytubes, etc. Such
structures provide significant surface area when incorporated
into a structure because of their size and shape. Moreover, such
fibers can be made with high purity and uniformity. Preferably,
the nanofiber used in the present invention has a diameter less
than 1 micron, preferably less than about 0.5 micron, and even
more preferably less than 0.1 micron and most preferably less
than 0.05 micron. Carbon nanotubes are typically hollow graphite
tubules having a diameter of generally several to several tens
nanometers which exist in many forms either as discrete fibers or
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aggregate particles of nanofibers
The term "internal structure" refers to the internal
structure of an assemblage including the relative orientation of
the fibers, the diversity of and overall average of fiber
orientations, the proximity of the fibers to one another, the
void space or pores created by the interstices and spaces between
the fibers and size, shape, number and orientation of the flow
channels or paths formed by the connection of the void spaces
and/or pores. The structure may also include characteristics
relating to the size, spacing and orientation of aggregate
particles that form the assemblage. The term "relative
orientation" refers to the orientation of an individual fiber or
aggregate with respect to the others (i.e., aligned versus non-
aligned) . The "diversity of" and "overall average" of fiber or
aggregate orientations refers to the range of fiber orientations
within the structure (alignment and orientation with respect to
the external surface of the structure).
Carbon fibrils can be used to form a rigid assemblage or be
made having diameters in the range of 3.5 to 70 nanometers. The
fibrils, buckytubes, nanotubes and whiskers that are referred to
in this application are distinguishable from continuous carbon
fibers commercially available as reinforcement materials. In
contrast to nanofibers, which have desirably large, but
unavoidably finite aspect ratios, continuous carbon fibers have
aspect ratios (L/D) of at least 10~ and often 106 or more. The
diameter of continuous fibers is also far larger than that of
fibrils, being always > 1.0 microns and.typically 5 to 7
microns. Continuous carbon fibers are made by the pyrolysis
of organic precursor fibers, usually rayon, polyacrylonitrile
(PAN) and pitch. Thus, they may include heteroatoms within their
structure. The graphitic nature of "as made" continuous carbon
fibers varies, but they may be subjected to a subsequent
graphitization step. Differences in degree of graphitization,
orientation and crystallinity of graphite planes, if they are
present, the potential presence of heteroatoms and even the
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absolute difference in substrate diameter make experience with
continuous fibers poor predictors of nanofiber chemistry.
Carbon nanofibrils are vermicular carbon deposits having
diameters less than 1.0 micron, preferably less than 0.5 micron
even more preferably less than 0.2 micron and most preferably
less than 0.05 micron. They exist in a variety of forms and have
been prepared through the catalytic decomposition of various
carbon-containing gases at metal surfaces.
Carbon nanotubes are typically hollow graphite tubules
having a diameter of generally several to several tens
nanometers. Carbon nanotubes exist in many forms. The
nanofibers can be in the form of discrete fibers or aggregate
particles of nanofibers. The former results in a structure having
fairly uniform properties. The latter results in a structure
having two-tiered architecture comprising an overall
macrostructure comprising aggregate particles of nanofibers
bonded together to form the porous mass and a microstructure of
intertwined nanofibers within the individual aggregate particles.
For instance, one form of carbon fibrils are characterized by a
substantially constant diameter, length greater than about 5
times the diameter, an ordered outer region of catalytically
grown, multiple, substantially continuous layers of ordered
carbon atoms having an outside diameter between about 3.5 and.70
nanometers, and a distinct inner core region. Each of the layers
and the core are disposed substantially concentrically about the
cylindrical axis of the fibril. The fibrils are substantially
free of pyrolytically deposited thermal carbon with the diameter
of the fibrils being equal to the outside diameter of the ordered
outer region.
Moreover, a carbon fibril suitable for use with the instant
process defines a cylindrical carbon fibril characterized by a
substantially constant diameter between 3.5 and about 70
nanometers, a length greater than about 5 times the diameter, an
outer region of multiple layers of ordered carbon atoms and a
distinct inner core region, each of the layers and the core being
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disposed concentrically about the cylindrical axis of the fibril.
Preferably the entire fibril is substantially free of thermal
carbon overcoat. The term "cylindrical" is used herein in the
broad geometrical sense, i.e., the surface traced by a straight
line moving parallel to a fixed straight line and intersecting a
curve. A circle or an ellipse are but two of the many possible
curves of the cylinder. The inner core region of the fibril may
be hollow, or may comprise carbon atoms which are less ordered
than the ordered carbon atoms of the outer region. "Ordered
carbon atoms," as the phrase is used herein means graphitic
domains having their c-axes substantially perpendicular to the
cylindrical axis of the fibril. In one embodiment, the length
of the fibril is greater than about 20 times the diameter of the
fibril. In another embodiment, the fibril diameter is between
about 7 and about 25 nanometers. In another embodiment the inner
core region has a diameter greater than about 2 nanometers.
Dispersing the nanotubes into organic and aqueous medium has
been a serious challenge. The nanotubes tend to aggregate, form
agglomerates, and separate from the dispersion.
Some industrial applications require a method of preparing
a stable dispersion of a selected carbon nanomaterials in a
liquid medium. For instance, U.S. Patent 5,523,006 by Strumban
teaches the user of a surfactant and an oil medium; however, the
particles are Cu-Ni-Sn-Zn alloy particles with the size from
0.01~.m and the suspension is stable for a limited period of time
of approximately 30 days. Moreover, the surfactants do not
include the dispersants typically utilized in the lubricant
industry.
U.S. Patent 5,560,898 by Uchida et al. teaches that a liquid
medium is an aqueous medium containing a surfactant; however, the
stability of the suspension is of little consequence in that the
liquid is centrifuged upon suspension.
U.S. Patent 5,853,877 by Shibuta teaches dispersing
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disentangled nanotubes in a polar solvent and forming a coating
composition with additives such as dispersing agents; however, a
method of obtaining a stable dispersion is not taught.
U.S. Patent 6,099,965 by Tennent et al. utilizes a kneader
for mixing a dispersant with other reactants in a liquid medium,
yet sustaining the stability of the dispersion is not taught.
The potential of carbon nanotubes to convey thermal
conductivity in a material is mentioned in U.S. Patent 5,165,909;
however, actual measurement of the thermal conductivity of the
carbon fibrils they produced was not given in the patent, so the
inference of thermal conductivity is general and somewhat
speculative, based on graphitic structure. Bulk graphite with
high thermal conductivity is available from P~CO GRAPHITE as a
graphite foam having a thermal conductivity of greater than 100
W/m-K, and from Carbide having a high thermal conductivity as
well. These bulk materials must be reduced to a powder of
nanometer size by various methods for use in the instant
invention.
SUI~2ARY OF THE INVENTION
In this invention, fluids of enhanced thermal conductivity
are prepared by dispersing carbon nanomaterials of a selected
thermal conductivity measured in W/m-K into a selected neat fluid
which serve as a liquid solvent medium or carrier. Dispersion of
the nanomaterials throughout the selected liquid medium is
achieved by physical and chemical treatments to yield a fluid
composition having an enhanced thermal conductivity as compared
to the neat fluid alone.
Fluid compositions that have enhanced thermal conductivity,
up to 2500 greater than their conventional analogues, and methods
of preparation for these fluids are identified. The compositions
contain at a minimum, a fluid media such as oil or water, and a
selected effective amount of particles necessary to enhance the
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thermal conductivity of the fluid. The graphite is a high
thermal conductivity graphite, exceeding that of the neat fluid
to be dispersed therein in thermal conductivity, and ground,
milled, or naturally prepared with mean particle size less than
500 nm, and preferably less than 200nm, and most preferably less
than 100nm. The graphite is dispersed in the fluid by one or
more of various methods, including ultrasonication, milling, and
chemical dispersion. Carbon nanotube with graphitic structure is
another preferred source of carbon nanomaterials, although other
nanomaterials are acceptable. To confer long term stability, the
use of one or more chemical dispersants is preferred. The
thermal conductivity enhancement, compared to the fluid without
carbon nanomaterial, is somehow proportional to the amount of
carbon nanomaterial added.
The present invention provides a fluid containing up to 900
carbon nanomaterials. Very good results were obtained with
nanomaterial loadings in a range of up to 20 percent by weight
and more particularly from 0.001 to 10 percent by weight, and
more typically from 0.01 to 2.5 percent by weight. Well
dispersed stable nanotube/nanoparticle in oil suspensions with up
to 2.5 percent by weight carbon nanomaterials resulted in
surprising good enhancement of the thermal characteristics of the
fluids developed according to the present invention. Preferably,
a minimum of one or more chemical dispersing agents and/or
surfactants is also added to achieve long term stability. The
term "dispersant" in the instant invention refers to a surfactant
added to a medium to promote uniform suspension of extremely fine
solid particles, often of colloidal size. In the lubricant
industry the term "dispersant" is generally accepted to describe
the long chain oil soluble or dispersible compounds which
function to disperse the "cold sludge" formed in engines. The
term " surfactant" in the instant invention refers to any
chemical compound that reduces surface tension of a liquid when
dissolved into i.t, or reduces interfacial tension between two
liquids or between a liquid and a solid. It is usually, but not
exclusively, a long chain molecule comprised of two moieties; a
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hydrophilic moiety and a lipophilic moiety. The hydrophilic and
lipophilic moieties refer to the segment in the molecule with
affinity for water, and that with affinity for oil, respectively.
These two terms are mostly used interchangeably in the instant
invention. The particle-containing fluid of the instant
invention will have a thermal conductivity higher than the neat
fluid, in this case the term "neat" is defined as the fluid
before the particles are added. The fluid can have any other
chemical agents or other type particles added to it as well to
impart other desired properties, e.g. friction reducing agents,
antiwear or anti-corrosion agents, detergents, antioxidants, etc.
Furthermore, the term fluid in the instant invention is broadly
defined to include pastes, gels, greases, foam, and liquid
crystalline phases in either organic or aqueous media, emulsions
and microemulsions.
As set forth above, the preferred carbon nanomaterial is
restricted to any graphitic nanomaterials with bulk thermal
conductivity exceeding that of the neat fluid to be enhanced.
For instance, the thermal conductivity of oil is about 0.2W/m-K;
the thermal conductivity of antifreeze (water and alcohol and/or
glycol mixtures) is usually about 0.4W/m-K; and the thermal
conductivity of water is about 0.6W/m-K. For most applications,
a carbon nanomaterial in the form of a carbon nanotube or
graphite nanoparticle is chosen having a thermal conductivity
exceeding 80W/m-K. A preferred form of carbon nanomaterial is
carbon nanotubes.
The carbon nanomaterial containing dispersion may also
contain a large amount of one or more other chemical compounds,
such as polymers, antiwear agents, friction reducing agents,
anti-corrosion agents, detergents, metal passivating agents,
antioxidants, etc. that are not for the purpose of dispersing,
but to achieve thickening or other desired fluid characteristics .
Furthermore, the dispersed nanomaterial solution can be pre-
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sheared, in a turbulent flow such as a nozzle, or high pressure
fuel injector, or ultrasonic device, in order to achieve a stable
viscosity. This may be desirable in the case where carbon
nanotubes with high aspect ratio are used as the carbon
nanomaterial source, since they will thicken the fluid but loose
viscosity when exposed in turbulent flows such as engines.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The present invention provides a carbon nanomaterial
dispersion in fluid medium that gives a high thermal conductivity
compared to conventional fluids of the same medium.
The preferred carbon nanomaterials are carbon nanotubes, the
nanotubes can be either single-walled, or multi-walled, having a
typical nanoscale diameter of 1 -500 nanometers. More typically
the diameter is around 10-30 nanometers. The length of the tube
can be in submicron and micron scale, usually from 500 nanometers
to 500 microns. More typical length is 1 micron to 100 microns.
The aspect ratio of the tube can be from hundreds to thousands,
more typical 500 to 5000. The surface of the nanotube can be
treated chemically to achieve certain level of hydrophilicity, or
left as is from the production. Other acceptable carbon
nanomaterials are available, e.g. POCOFOAMTM, available from
PocoGraphite, Inc., located in Decatur, Texas, POCOFOAMTM is a
high thermal conductivity foamed graphite, thermal conductivity
from 100 to 150 V~1/m-K. To prepare it for the instant invention,
it must be pulverized to a fine powder, dispersed chemically and
physically in the fluid of choice, and then ball milled or
otherwise size reduced until a particle size of less than 500 nm
mean size is attained. The finer the particle size attained upon
milling, the better. In general, any high thermal conductivity
graphite can be used, provided that pulverization, milling and
other described chemical and physical methods can be used to
reduce the size of the final graphite particles to below a mean
particle size of 500 nm.
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Oil Basestocks
The petroleum liquid medium can be any petroleum distillates
or synthetic petroleum oils, greases, gels, or oil-soluble
polymer composition. More typically, it is the mineral
basestocks or synthetic basestocks used in the lube industry,
e.g., Group I (solvent refined mineral oils), Group II
(hydrocracked mineral oils), Group III (severely hydrocracked
oils, sometimes described as synthetic or semi-synthetic oils),
Group IV (polyalphaolefins), and Group VI (esters, naphthenes,
and others). One preferred group includes the polyalphaolefins,
synthetic esters, and polyalkylglycols.
Synthetic lubricating oils include hydrocarbon oils and
halo-substituted hydrocarbon oils such as polymerized and
interpolymerized olefins (e. g., polybutylenes, polypropylenes,
propylene-isobutylene copolymers, chlorinated polybutylenes,
poly(1-octenes), poly(1-decenes), etc., and mixtures thereof;
alkylbenzenes (e. g., dodecylbenzenes, tetradecylbenzenes,
dinonylbenzenes, di-(2-ethylhexyl)benzenes, etc.); polyphenyls
(e. g., biphenyls, terphenyls, alkylated polyphenyls, etc.),
alkylated diphenyl, ethers and alkylated diphenyl sulfides and
the derivatives, analogs and homologs thereof and the like.
Alkylene oxide polymers and interpolymers and derivatives
thereof where the terminal hydroxyl groups have been modified by
esterification, etherification, etc. constitute another class of
known synthetic oils.
Another suitable class of synthetic oils comprises the
esters of dicarboxylic acids (e. g., phtalic acid, succinic acid,
alkyl succinic acids and alkenyl succinic acids, malefic acid,
azelaic acid, suberic acid, sebacic acid, fumaric acid, adipic
acid, alkenyl malonic acids, etc.) with a variety of alcohols
(e.g., butyl alcohol, hexyl alcohol, dodecyl alcohol, 2-
ethylhexyl alcohol, ethylene glycol diethylene glycol monoether,
propylene glycol, etc.). Specific examples of these esters
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include dibutyl adipate, di(2-ethylhexyl) sebacate, di-hexyl
fumarate, dioctyl sebacate, diisooctyl azelate, diisodecyl
azealate, dioctyl phthalate, didecyl phthalate, dicicosyl
sebacate, the 2-ethylhexyl diester of linoleic acid dimer, the
complex ester formed by reacting one mole of sebacic acid with
two moles of tetraethylene glycol and two moles of 2-
ethylhexanoic acid, and the like.
Esters useful as synthetic oils also include those made from
CS to C12 monocarboxylic acids and polyols and polyol ethers such
as neopentyl glycol, trimethylolpropane, pentaerythritol,
dipentaerythritol, tripentaerythritol, etc. Other synthetic oils
include liquid esters of phosphorus-containing acids (e. g.,
tricresyl phosphate, trioctyl phosphate, diethyl ester of
decylphosphonic acid, etc.), polymeric tetrahydrofurans and the
like.
Polyalphaolefins (PAO), useful in the present invention
include those sold by BP Amoco Corporation as DURASYN fluids,
those sold by Exxon-Mobil Chemical Company, (formerly Mobil
Chemical Company) as SHF fluids, and those sold by Ethyl
Corporation under the name ETHYLFLO, or ALBERMARLE. PAO's include
the ETHYL-FLOW series by Ethyl Corporation, "Albermarle
Corporation," including ETHYL-FLOW 162, 164, 166, 168, and 174,
having varying viscosity from about 2 to about 460 centistokes.
MOBIL SHF-42 from Exxon-Mobil Chemical Company, EMERY 3004
and 3006, and Quantum Chemical Company provide additional
polyalphaolefins basestocks. For instance, EMERY 3004
polyalphaolefin has a viscosity of 3.86 centistokes (cSt) at
212°F (100°C) and 16.75 cSt at 104°F (40°C). It
has a viscosity
index of 125 and a pour point of -98°F and it also has a flash
point of about 432°F and a fire point of about 478°F. Moreover,
EMERY 3006 polyalphaolefin has a viscosity of 5.88 cSt at +212°F
and 31.22 cSt at +104°F. It has a viscosity index of 135 and a
pour point of -87°F.
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Additional satisfactory polyalphaolefins are those sold by
Uniroyal Inc. under the brand SYNTON PAO-40, which is a 40
centistoke polyalphaolefin.
It is contemplated that Gulf Synfluid 4 cSt PAO,
commercially available from Gulf Oil Chemicals Company, a
subsidiary of Chevron -Texaco Corporation, which is similar in
many respects to EMERY 3004 may also be utilized herein. MOBIL
SHF-41 PAO, commercially available from Mobil Chemical
Corporation, is also similar in many respects to EMERY 3004.
Especially useful are the polyalphaolefins will have a
viscosity in the range of up to 100 centistoke at 100°C, with
viscosity of 2 and 10 centistoke being more preferred.
The most preferred synthetic based oil ester additives are
polyolesters and diesters such as di-aliphatic diesters of alkyl
carboxylic acids such as di-2-ethylhexylazelate, di-
isodecyladipate, and di-tridecyladipate, commercially available
under the brand name EMERY 2960 by Emery Chemicals, described in
U.S. Patent 4,859,352 to Waynick. Other suitable polyolesters
are manufactured by Mobil Oil. MOBIL polyolester P-43, NP343
containing two alcohols, and Hatco Corp. 2939 are particularly
preferred.
Diesters and other synthetic oils have been used as
replacements of mineral oil in fluid lubricants. Diesters have
outstanding extreme low temperature flow. properties and good
residence to oxidative breakdown.
The diester oil may include an aliphatic diester of a
dicarboxylic acid, or the diester oil can comprise a dialkyl
aliphatic diester of an alkyl dicarboxylic acid, such as di-2-
ethyl hexyl azelate, di-isodecyl azelate, di-tridecyl azelate,
di-isodecyl adipate, di-tridecyl adipate. For instance, Di-2-
ethylhexyl azelate is commercially available under the brand name
of EMERY 2958 by Emery Chemicals.
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Also useful are polyol esters such as EMERY 2935, 2936, and
2939 from Emery Group of Henkel Corporation and HATCO 2352, 2962,
2925, 2938, 2939, 2970, 3178, and 4322 polyol esters from Hatco
Corporation, described in U.S. 5,344,579 to Ohtani et al. and
MOBIL ESTER P 24 from Exxon-Mobil Chemical Company. Esters made
by reacting dicarboxylic acids, glycols, and either monobasic
acids or monohydric alcohols like EMERY 2936 synthetic-lubricant
basestocks from Quantum Chemical Corporation and MOBIL P 24 from
Exxon-Mobil Chemical Company can be used. Polyol esters have
good oxidation and hydrolytic stability. The polyol ester for
use herein preferably has a pour point of about -100°C or lower
to -40°C and a viscosity of about 2 to 100 centistoke at 100°C.
A hydrogenated oil is a mineral oil subjected to
hydrogennation or hydrocracking under special conditions to
remove undesirable chemical compositions and impurities resulting
in a base oil having synthetic oil component and properties.
Typically the hydrogenated oil is defined by the American
Petroleum Institute as a Group III base oil with a sulfur level
less than 0.03 with saturates greater than or equal to 90 and a
viscosity index of greater than or equal to 120. Most useful are
hydrogenated oils having a viscosity of from 2 to 60 CST at 100
degrees centigrade. The hydrogenated oil typically provides
superior performance to conventional motor oils with no other
synthetic oil base. The hydrogenated oil may be used as the sole
base oil component of the instant invention providing superior
performance to conventional mineral oil bases oils or used as a
blend with mineral oil and/or synthetic oil. An example of such
an oil is YUBASE-4.
When used in combination with another conventional synthetic
oil such as those containing polyalphaolefins or esters, or when
used in combination with a mineral oil, the hydrogenated oil may
be present in an amount of up to 99 percent by volume, more
preferably from about 10 to 80 percent by volume, more preferably
from 20 to 60 percent by volume and most preferably from 10 to 30
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percent by volume of the base oil composition.
A Group I or II mineral oil basestock may be incorporated in
the present invention as a portion of the concentrate or a
basestock to which the concentrate may be added. Preferred as
mineral oil basestocks are the ASHLAND 325 Neutral defined as a
solvent refined neutral having a SABOLT UNIVERSAL viscosity of
325 SUS @ 100°F and ASHLAND 100 Neutral defined as a solvent
refined neutral having a SABOLT UNIVERSAL viscosity of 100 SUS C
100°F, manufactured by the Marathon Ashland Petroleum.
Other acceptable petroleum-base fluid compositions include
white mineral, paraffinic and MVI naphthenic oils having the
viscosity range of about 20-400 centistoke.s. Preferred white
mineral oils include those available from Witco Corporation, Arco
Chemical Company, PSI and Penreco. Preferred paraffinic oils
include API Group I and II oils available from Exxon-Mobil
Chemical Company, HVI neutral oils available from Shell Chemical
Company, and Group II oils available from Arco Chemical Company.
Preferred MVI naphthenic oils include solvent extracted oils
available from Equilon Enterprises and San Joaquin Refining,
hydrotreated oils available from Equilon Enterprises and Ergon
Refining, and naphthenic oils sold under the names HYDROCAL and
CALSOL by Calumet, and described in U.S. Patent 5,348,668 to
Oldiges.
Finally, the thermal conductivity of vegetable oils may also
be enhanced and utilized as the liquid medium in the instant
invention.
Aqueous Medi»m
A selected aqueous medium is water, or it can be any water-
based solution including alcohol or its derivatives, such as
ethylene glycol, propylene glycol, or any water-soluble inorganic
salt, e.g. molybdate salts, nitrates, nitrites, methyl alcohol,
ethyl alcohol, propyl alcohol, isopropyl alcohol, and
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combinations thereof, or organic compounds, such as aromatic
and/or aliphatic carboxylate acids more, particularly short chain
mono- and di-carboxylic acids. Such solutions are typically
utilized as antifreeze constituents and may include other
corrosion resistant additives together with the carbon
nanomaterial dispersed therein providing enhance thermal
properties.
Dispersaats
Dispersants used in Lubricant Industry
Dispersants used in the lubricant industry are typically
used to disperse the "cold sludge" formed in gasoline and diesel
engines, which can be either "ashless dispersants", or containing
metal atoms. They can be used in the instant invention since they
have been found to be an excellent dispersing agent for soot, an
amorphous form of carbon particles generated in the engine
crankcase and incorporated with dirt and grease.
The ashless dispersants commonly used in the automotive
industry contain an lipophilic hydrocarbon group and a polar
functional hydrophilic group. The polar functional group can be
of the class of carboxylate, ester, amine, amide, imine, imide,
hydroxyl, ether, epoxide, phosphorus, ester carboxyl, anhydride,
or nitrile. The lipophilic group can be oligomeric or polymeric
in nature, usually from 70 to 200 carbon atoms to ensure oil
solubility. Hydrocarbon polymers treated with various reagents
to introduce polar functions include products prepared by
treating polyolefins such as polyisobutene first with malefic
anhydride, or phosphorus sulfide or chloride, or by thermal
treatment, and then with reagents such as polyamine, amine,
ethylene oxide, etc.
Of these ashless dispersants the ones typically used in the
petroleum industry include N-substituted polyisobutenyl
succinimides and succinates, allkyl methacrylate-vinyl
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pyrrolidinone copolymers, alkyl methacrylate-dialkylaminoethyl
methacrylate copolymers, alkylmethacrylate-polyethylene glycol
methacrylate copolymers, and polystearamides. Preferred oil-
based dispersants that are most important in the instant
application include dispersants from the chemical classes of
alkylsuccinimide, succinate esters, high molecular weight amines,
Mannich base and phosphoric acid derivatives. Some specific
examples are polyisobutenyl succinimide-polyethylenepolyamine,
polyisobutenyl succinic ester, polyisobutenyl hydroxybenzyl-
polyethylenepolyamine, bis-hydroxypropyl phosphorate. For
instance, bis-succinimide is a dispersant based on polybutene and
an amine which is suitable for oil based dispersions and is
commercially available under the tradenames of INFINEUM 09231,
INFINEUM 09232, and INFINEUM 09235 which is sold by Infineum,
USA, L.P. The 09231 is borated while the 09232 and 09235 are
not; however, all are bis-succinimides which differ due to their
amine to polymer ratio.
The dispersant may be combined with other additives used in
the lubricant industry to form a "dispersant-detergent (DI)"
additive package, e.g., LUBRIZOLTM 9802A and/or the concentrated
package ( LUBRIZOLTM 9802AC), which are mixed Dispersants having
a high molecular weight succinimide and ester-type dispersant as
the active ingredient, and which also contains from about 5 to
9.9 percent by weight of zinc alkyldithiophosphate, from 1 to 4.9
percent by weight of a substituted phenol, from 1 to 4.9 percent
of a calcium sulfonate, and from 0.1 to 0.9 percent by weight of
a diphenylamine; wherein the whole DI package can be used as
dispersing agent for the carbon nanomaterial dispersion.
Another preferred dispersant package is LUBRIZOLTM OS#154250
which contains from about 20 to 29.9 percent by weight of a
polyolefin amide alkeneamine, from 0.5 to 1.5 percent by weight
of an alkylphosphite, about 1.1 percent by weight of a phosphoric
acid, and. from 0.1 to 0.9 percent by weight of a diphenylamine,
with primary active ingredient believed to be polyisobutenyl
succinimides and succinates. Another preferred dispersant
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package is a high molecular weight succinimide DI package for
diesel engines LUBRIZOLTM 4999 which also contains from about 5 to
9.9 percent zinc alkyldithiophosphate by weight.
Other Types of Dispersants
Alternatively a surfactant or a mixture of surfactants with
low HLB value (typically less than or equal to 8), preferably
nonionic, or a mixture of nonionics and Tonics, may be used in
the instant invention.
The dispersant for the water based carbon nanomaterial
dispersion, more specifically carbon nanotube dispersion, should
be of high HLB value (typically less than or equal to 10),
preferable nonylphenoxypoly (ethyleneoxy) ethanol-type
surfactants are utilized.
In both the water and oil based cases, the dispersants
selected should be soluble or dispersible in the liquid medium.
The dispersant can be in a range of up from 0.001 to 30
percent, more preferably in a range of from between 0.5 percent
to 20 percent, more preferably in a range of from between 1.0 to
8.0 percent, and most preferably in a range of from between 2 to
6 weight percent.
The carbon nanotube or graphite nanoparticles can be of any
desired weight percentage in a range of from 0.0001 up to 50
percent by weight providing for an effective amount to obtain the
desired thermal enhancement of the selected fluid media. For
practical application an effective amount of carbon nanomaterials
is usually in a range of from between 0.01 percent to 20 percent,
and. more preferably in a range of from 0.02 to 10 percent, and
most preferably in a range of from between 0.05 percent to 5
percent. The remainder of the formula is the selected medium
comprising oil, water, or combinations thereof together with any
Chemical additives deemed necessary to provide lubricity,
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corrosion protection, viscosity, or the like.
It is believed that in the instant invention the dispersant
functions by adsorbing onto the surface of the carbon nanotube.
This dispersion may also contain a large amount of one or
more other chemical compounds, preferably polymers, not for the
purpose of dispersing, but to achieve thickening or other desired
fluid characteristics.
The viscosity improvers used in the lubricant industry can
be used in the instant invention for the oil medium, which
include olefin copolymers (OCP), polymethacrylates (PMA.),
hydrogenated styrene-dime (STD), and styrene-polyester (STPE)
polymers. Olefin copolymers are rubber-like materials prepared
from ethylene and propylene mixtures through vanadium-based
Ziegler-Natta catalysis. Styrene-diene polymers are produced by
anionic polymerization of styrene and butadiene or isoprene.
Polymethacrylates are produced by free radical polymerization of
alkyl methacrylates. Styrene-polyester polymers are prepared by
first co-polymerizing styrene and malefic anhydride and then
esterifying the intermediate using a mixture of alcohols.
Other compounds which can be used in the instant invention
in either the aqueous medium or the oil medium include: acrylic
polymers such as polyacrylic acid and sodium polyacrylate, high-
molecular-weight polymers of ethylene oxide such as Polyox~ WSR
from Union Carbide, cellulose compounds such as
carboxymethylcellulose, polyvinyl alcohol (PVA), polyvinyl
pyrrolidone (PVP), xanthan gums and guar gums, polysaccharides,
alkanolamides, amine salts of polyamide such as DISPARLON AQ
series from King Industries, hydrophobically modified ethylene
oxide urethane (e.g., ACRYSOL series from Rohmax), silicates, and
fillers such as mica, silicas, cellulose, wood flour, clays
(including organoclays) and nanoclays, and resin polymers such as
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polyvinyl butyral resins, polyuretha~.e resins, acrylic resins and
epoxy resins.
Other chemical additives used in lubricants such as pour
point depressant can also be used in the instant invention. Most
pour point depressants are organic polymers, although some
nonpolymeric substances have been shown to be effective.
Commercial pour point depressants include alkylnaphthalenes,
polymethacrylates, polyfumarates, styrene esters, oligomerized
alkylphenols, phthalic acid esters, ethylenevinyl acetate
copolymers, and other mixed hydrocarbon polymers. The treatment
level of these additives is usually low. In nearly all cases,
there is an optimum concentration above and below which pour
point depressants become less effective.
Acrylic copolymers such as manufactured by Supeleo Inc. in
Bellefonte, Pennsylvania as Acryloid 3008 is a pour point
depressant useful in the present invention.
Still other chemical additives used in lubricants, such as
rust and oxidation inhibitors, demulsifiers, foam inhibitors, and
seal-swelling agents can also be used in the instant invention.
Physical Agitation
The physical mixing includes high shear mixing, such as with
a high speed mixer, homogenizers, microfluidizers, a KADY mill,
a colloid mill, etc., high impact mixing, such as attritor, ball
and pebble mill, etc., and ultrasonication methods.
Ultrasonication is the most preferred physical method in the
instant invention since it is less destructive to the carbon
nanomaterial, more specifically, carbon nanotube, structure than
the other methods described. Ultrasonication can be done either
in the bath-type ultrasonicator, or by the tip-type
ultrasonicator. More typically, tip-type ultrasonication is
applied for higher energy output. Sonication at the medium-high
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instrumental intensity for up to 30 minutes, and usually in a
range of from 10 to 20 minutes is desired to achieve better
homogeneity.
The raw material mixture may be pulverized by any suitable
known dry or wet grinding method. One grinding method includes
pulverizing the raw material mixture in the fluid mixture of the
instant invention to obtain the concentrate, and the pulverized
product may then be dispersed further in a liquid medium with the
aid of the dispersants described above. However, pulverization or
milling reduces the carbon nanotube average aspect ratio.
The instant method of forming a stable suspension of carbon
nanomaterials in a solution consist of two steps. First select
the appropriate dispersant for the carbon nanomaterials, which
include carbon nanotube or graphite nanoparticles, and the
medium, and dissolve the dispersant into the liquid medium to
form a solution, and second add the carbon nanotube or graphite
nanoparticles into the dispersant-containing solution while
agitating, ball milling, or ultrasonicating the solution or any
combination of physical methods named.
EXAMPLES
Specific compositions, methods, or embodiments discussed are
intended to be only illustrative of the invention disclosed by
this specification. Variation on these compositions, methods, or
embodiments are readily apparent to a person of skill in the art
based upon the teachings of this specification and are therefore
intended to be included as part of the inventions disclosed
herein. Reference to documents made in the specification is
intended to result in such patents or literature cited are
expressly incorporated herein by reference, including any patents
or other literature references cited within such documents as if
fully set forth in this specification.
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Example 1
Components Description Weight
percentage
Carbon nanotube Surface untreated, aspect ratio 2.5
2000, diameter 25 nm, length 50 ~m
Dispersant High Mol. Wt. Polyammine DI package 4.88
ORONITE (OLOA 9061)
Liquid solvent Poly(a-olefin), 6 cSt
92.62
Sonication FISHER SCIENTIFIC 550 Sonic
Dismembrator,
15 minutes
As set forth in Example 1, the thermal conductivity of the
above dispersion was 0.380 Wm-K for the fluid (solution of the
dispersant and solvent) containing the thermally enhancing
nanotubes, as compared to a thermal conductivity of 0.146 W/m-K
for the fluid (solution of the dispersant and solvent) without
the thermally enhancing nanotubes.
Example 2
Components Description Weight
percentage
~arbon nanotube Surface untreated, aspect ratio 2000, 0.1
diameter 25 nm, length 50 ~m
Dispersant High mol. Wt. Succinimide DI package 4.8
for diesel engines LUBRIZOLTM 4999
Liquid solvent Poly(a-olefin), 6 cSt 95.1
Sonication FISHER SCIENTIFIC 550 Sonic
Dismembrator, 15 minutes
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Example 3
Components Description Weight percentage
Carbon nanotube Surface untreated, 0.1
aspect ratio 2000,
diameter 25 nm, length
50 microns
Dispersant Mixed Dispersant (high4.8
mol. Wt. Succinimide
and ester-type
dispersant) DI package
LUBRIZOLTM 9802A
Liquid solvent Poly(a-olefin), 6cST 95.1
Sonication FISHER SCIENTIFIC
550
Sonic Dismembrator,
15
minutes
_.
Example 4
Components Description Weight
percent
Carbon nanotube Surface untreated, aspect ratio 0.10
2000, diameter 25 nm, length 50 ~m
Dispersant Bis-succinimide dispersant 4.80
(INFINEUM C9231)
Liquid solvent Poly(a-olefin), 6 cSt 95.10
Sonication FISHER SCIENTIFIC 550 Sonic
Dismembrator,
15 minutes
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Example 5
Components Description Weight
percentage
:arbon nanotube Surface untreated, aspect ratio 0.10
2000, diameter 25 nm, length 50 ~m
Dispersant Bis-succinimide dispersant 4.80
INFINEUM C9232)
Liquid solvent Poly(a-olefin), 6 cSt 95.10
Sonication FISHER SCIENTIFIC 550 Sonic
Dismembrator,
15 minutes
Example 6
Components Description Weight
percentage
'.arbon nanotube Surface untreated, aspect ratio 0.10
2000, diameter 25 nm, length 50 ~m
Dispersant Bis-succinimide dispersant 4.80
(INFINEUM C9235)
Liquid solvent Poly(a-olefin), 6 cSt 95.10
Sonication FISHER SCIENTIFIC 550 Sonic
Dismembrator,
15 minutes
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Example 7
Components Description Weight
percentage
Carbon nanotube Surface treated 0.10
Dispersant nonylphenoxy poly(ethyleneoxy) 5.00
ethanol, branched
Liquid solvent Water 94.90
Sonication FISHER SCIENTIFIC 550 Sonic
Dismembrator,
15 minutes
The dispersions in examples 1-7 are very uniform, and have not
shown any sign of separation or aggregation for a year.
Example 8
Components Description Weight percentage
Graphite nanoparticles POCOFOAMTM after 2.0
milling
Dispersant LubrizolTM OS#154250 7.55
VI Improver and Polyalkylmethacrylate, ACRYLO~ 3008TM acrylic 10.9
Other Chemicals copolymer and red dye
Liquid solvent G r o a p I I I 79.55
Base oil
Sonification FISHER SCIENTIFIC 550 Sonic Dismembrator,
15 minutes
In Example 8, the graphite particles were obtained through
pulverizing and milling the high thermal conductivity graphite
foam (bulk thermal conductivity as 100 to 150 W/MK), known as
POCOFOAMTM, to the desired nanometer size range. It was first
ground into coarse particles, and then dispersed into an oil
solution with dispersants and other chemicals. The dispersion is
then milled in a horizontal mill. The final dispersion after the
milling is sonicated to achieve homogeneity.
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As set forth in Example 8, the thermal conductivity of the
above dispersion was 0.175 Wm-K for the fluid containing the
thermally enhancing graphite particles, as compared to a thermal
conductivity of 0.140 W/m-K for the base fluid (solution of the
dispersant, viscosity index improver, and solvent) without the
thermally enhancing graphite particles.
The foregoing detailed description is given primarily for
clearness of understanding and no unnecessary limitations are to
be understood therefrom, for modification will become obvious to
those skilled in the art upon reading this disclosure and may be
made upon departing from the spirit of the invention and scope of
the appended claims. Accordingly, this invention is not intended
to be limited by the specific exemplification presented herein
above. Rather, what is intended to be covered is within the
spirit and scope of the appended claims.
