U.S. patent application number 11/361255 was filed with the patent office on 2006-07-06 for expanded graphite and products produced therefrom.
This patent application is currently assigned to Board of Trustees operating Michigan State University. Invention is credited to Lawrence T. Drzal, Hiroyuki Fukushima.
Application Number | 20060148965 11/361255 |
Document ID | / |
Family ID | 32659084 |
Filed Date | 2006-07-06 |
United States Patent
Application |
20060148965 |
Kind Code |
A1 |
Drzal; Lawrence T. ; et
al. |
July 6, 2006 |
Expanded graphite and products produced therefrom
Abstract
Graphite nanoplatelets of expanded graphite and composites and
products produced therefrom are described. The preferred method of
expanding the graphite is by microwaves or other radiofrequency
wave treatment of intercalated graphite. The expanded graphite is
preferably then crushed to nanometer (substantially all 200 microns
or less). The expanded graphite is used in polymer composites. The
expanded graphite is particularly useful for batteries, anodes and
fuel cells.
Inventors: |
Drzal; Lawrence T.; (Okemos,
MI) ; Fukushima; Hiroyuki; (Lansing, MI) |
Correspondence
Address: |
MCLEOD & MOYNE, P.C.
2190 COMMONS PARKWAY
OKEMOS
MI
48864
US
|
Assignee: |
Board of Trustees operating
Michigan State University
East Lansing
MI
|
Family ID: |
32659084 |
Appl. No.: |
11/361255 |
Filed: |
February 24, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10659577 |
Sep 10, 2003 |
|
|
|
11361255 |
Feb 24, 2006 |
|
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|
60410263 |
Sep 12, 2002 |
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Current U.S.
Class: |
524/496 ;
429/232 |
Current CPC
Class: |
C08K 3/04 20130101; Y10T
428/30 20150115; C08K 7/00 20130101 |
Class at
Publication: |
524/496 ;
429/232 |
International
Class: |
H01M 4/62 20060101
H01M004/62; C08K 3/04 20060101 C08K003/04 |
Claims
1. A composite material which comprises: (a) expanded graphite
wherein a precursor graphite containing a chemical has been
expanded by heating with microwaves or radiofrequency waves to
produce the expanded graphite and to at least in part vaporize the
chemical; and (b) a polymer having the expanded graphite dispersed
therein.
2. A composite material which comprises: (a) expanded graphite
wherein a precursor graphite containing a chemical has been
expanded by heating with microwaves or radiofrequency waves to
produce the expanded graphite and to at least in part vaporize the
chemical; and (b) a polymer having the expanded graphite particles
dispersed therein, wherein the composite material contains up to
about 50% by volume of the graphite.
3. The composite material of claim 2 wherein the graphite is
present in an amount so that composite material is conductive.
4. The composite material of any one of claims 1, 2 or 3 wherein
the polymer is a thermoplastic or thermoset polymer.
5. (canceled)
6. The composite material of any one of claims 1, 2 or 3 wherein
the expanded graphite has been formed in a radiofrequency wave
applicator by heating a graphite precursor with the radiofrequency
waves or microwaves.
7. The composite material of any one of claims 1, 2 or 3 wherein
the polymer and the expanded graphite have been heated together
with the radiofrequency waves or microwaves.
8. The composite material of any one of claims 1, 2, or 3 wherein
the polymer is an epoxy resin.
9. The composite material of any one of claims 1, 2 or 3 wherein
the precursor graphite has been treated with a fuming oxy acid and
heated to form the expanded graphite.
10. The composite material of any one of claims 1, 2 or 3 wherein
the polymer is thermoplastic and is selected from the group
consisting of polyamides, proteins, polyesters, polyethers,
polyurethanes, polysiloxanes, phenol-formaldehydes,
urea-formaldehydes, melamine-formaldehydes, celluloses,
polysulfides, polyacetals, polyethylene oxides, polycaprolactams,
polycaprolactons, polylactides, polyimides, and polyolefins.
11. The composite material of any one of claims 1, 2 or 3 which
contains less than about 8% by weight of the expanded graphite.
12. A method for preparing a shaped composite which comprises: (a)
providing a mixture of an expanded graphite wherein a precursor
graphite containing a chemical has been expanded by heating with
microwaves or radiofrequency waves to produce the expanded graphite
and to at least in part vaporize the chemical and a polymer with
the expanded graphite dispersed therein; and (b) forming the shaped
composite material from the mixture.
13. A method for preparing a shaped composite material which
comprises: (a) providing a mixture of an expanded graphite wherein
a precursor graphite containing a chemical has been expanded by
heating with microwaves or radiofrequency waves to produce the
expanded graphite and to vaporize the chemical and a polymer with
the expanded graphite dispersed therein, wherein the composite
material contains up to about 50% by volume of the expanded
graphite platelets; (b) forming the shaped composite material from
the mixture.
14. The method of claims 12 or 13 wherein the expanded graphite is
provided in the polymer in an amount sufficient to render the
shaped composite conductive.
15. The method of claims 12 or 13 wherein the polymer is a
thermoplastic or thermoset polymer.
16. (canceled)
17. The method of claims 12 or 13 wherein the expanded graphite is
formed by heating the graphite precursor with the microwaves.
18. The method of claims 12 or 13 wherein the graphite precursor is
treated with a fuming oxy acid and heated to provide the expanded
graphite.
19. The method of any one of claims 12 or 13 wherein the polymer is
a curable thermoset resin which is mixed with the expanded graphite
and cured.
20. The method of claims 12 or 13 wherein the shaped composite
material contains less than 8% by weight of the expanded
graphite.
21. In a battery containing ions the improvement in the anode which
comprises a finely divided microwave or RF expanded graphite
wherein a precursor graphite containing a chemical has been
expanded by heating with microwaves or radiofrequency waves to
produce the expanded graphite and to vaporize the chemical.
22. In a catalytic conversion of an organic compound to hydrogen
with a catalytic material deposited on a substrate the improvement
in the substrate which comprises a microwave or RF expanded
graphite wherein a precursor graphite has been expanded by heating
with microwaves or radiofrequency waves to produce the expanded
graphite and to vaporize the chemical.
23-28. (canceled)
29. An expanded graphite which has been expanded by heating a
graphite precursor containing a chemical with microwaves or
radiofrequency waves to produce the expanded graphite and to
vaporize the chemical.
30. The graphite of claim 29 as a pressed sheet.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims priority to Provisional Application
Ser. No. 60/410,263, filed Sep. 12, 2002.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] Not applicable.
REFERENCE TO A "COMPUTER LISTING APPENDIX SUBMITTED ON A COMPACT
DISC"
[0003] Not Applicable.
BACKGROUND OF THE INVENTION
[0004] (1) Field of the Invention
[0005] Expanded graphite is provided in the present invention. The
present invention relates in part to polymer-expanded graphite
composites. The graphite platelets are preferably reduced in size
to less than about 200 microns. The invention also relates to
expanded graphite used for fuel cells, for battery anodes and for
catalytic converters. The graphite is preferably expanded using
microwave or radiofrequency wave heating.
[0006] (2) Description of Related Art
[0007] Graphite is a well known material occurring in natural and
synthetic form and is well described in the literature.
Illustrative of this art is a monograph by Michel A. Boucher,
Canadian Minerals Yearbook 24.1-24.9(1994).
[0008] Nanocomposites composed of polymer matrices with
reinforcements of less than 100 nm in size, are being considered
for applications such as interior and exterior accessories for
automobiles, structural components for portable electronic devices,
and films for food packaging (Giannelis, E. P., Appl.
Organometallic Chem., Vol. 12, pp. 675 (1998); and Pinnavaia, T. J.
et al., Polymer Clay Nanocomposites. John Wiley & Sons,
Chichester, England (2000)). While most nanocomposite research has
focused on exfoliated clay platelets, the same nanoreinforcement
concept can be applied to another layered material, graphite, to
produce nanoplatelets and nanocomposites (Pan, Y. X., et al., J.
Polym. Sci., Part B: Polym. Phy., Vol. 38, pp. 1626 (2000); and
Chen, G. H., et al., J. Appl. Polym. Sci. Vol. 82, pp. 2506
(2001)). Graphite is the stiffest material found in nature (Young's
Modulus=1060 MPa), having a modulus several times that of clay, but
also with excellent electrical and thermal conductivity.
[0009] A useful form of graphite is expanded graphite which has
been known for years. The first patents related to this topic
appeared as early as 1910 (U.S. Pat. Nos. 1,137,373 and 1,191,383).
Since then, numerous patents related to the methods and resulting
expanded graphites have been issued. For example, many patents have
been issued related to the expansion process (U.S. Pat. Nos.
4,915,925 and 6,149,972), expanded graphite-polymer composites
(U.S. Pat. Nos. 4,530,949, 4,704,231, 4,946,892, 5,582,781,
4,091,083 and 5,846,459), flexible graphite sheet and its
fabrication process by compressing expanded graphite (U.S. Pat.
Nos. 3,404,061, 4,244,934, 4,888,242, 4,961,988, 5,149,518,
5,294,300, 5,582,811, 5,981,072 and 6,143,218), and flexible
graphite sheet for fuel cell elements (U.S. Pat. Nos. 5,885,728 and
6,060,189). Also there are patents relating to
grinding/pulverization methods for expanded graphite to produce
fine graphite flakes (U.S. Pat. Nos. 6,287,694, 5,330,680 and
5,186,919). All of these patents use a heat treatment, typically in
the range of 600.degree. C. to 1200.degree. C., as the expansion
method for graphite. The heating by direct application of heat
generally requires a significant amount of energy, especially in
the case of large-scale production. RF or microwave expansion
method can heat more material in less time at lower cost. U.S. Pat.
No. 6,306,264 discusses microwave as one of the expansion methods
for SO.sub.3 intercalated graphite.
[0010] U.S. Pat. Nos. 5,019,446 and 4,987,175 describe graphite
flake reinforced polymer composites and the fabrication method.
These patents did not specify the methods to produce thin, small
graphite flakes. The thickness (less than 100 nm) and aspect ratio
(more than 100) of the graphite reinforcement was described.
[0011] Many patents have been issued related to anode materials for
lithium-ion or lithium-polymer batteries (U.S. Pat. Nos. 5,344,726,
5,522,127, 5,591,547, 5,672,446, 5,756,062, and 6,136,474). Among
these materials, one of the most widely investigated and used is
graphite flakes with appropriate size, typically 2 to 50 .mu.m,
with less oxygen-containing functional groups at the edges. Most of
the patents described graphite flakes made by carbonization of
precursor material, such as petroleum coke or coal-tar pitch,
followed by graphitization process.
SUMMARY OF THE INVENTION
[0012] An important aspect of utilizing graphite as a platelet
nanoreinforcement is in the ability to expand this material. With
surface treatment of the expanded graphite, its dispersion in a
polymer matrix results in a composite with not only excellent
mechanical properties but electrical properties as well, opening up
many new structural applications as well as non-structural ones
where electromagnetic shielding and high thermal conductivity are
requirements. In addition, graphite nanoplatelets are .about.500
times less expensive than carbon nanotubes.
[0013] Thus the present invention relates in part to a composite
material which comprises:
[0014] (a) finely divided expanded graphite consisting essentially
of single platelets which are less than 200 microns in length;
and
[0015] (b) a polymer having the expanded graphite platelets
dispersed therein.
[0016] In particular, the present invention relates to a composite
material which comprises:
[0017] (a) finely divided expanded graphite having single platelets
with a length less than about 200 microns and a thickness of less
than about 0.1 microns; and
[0018] (b) a polymer having the expanded graphite particles
dispersed therein, wherein the composite material contains up to
50% by volume of the graphite platelets. Preferably the expanded
graphite platelets are present in an amount so that composite
material is conductive.
[0019] A graphite precursor containing a chemical which was
vaporized by heat to form the expanded graphite. In most cases, the
chemical should be removed, preferably by heating, from the
graphite by sufficient heating before mixing with polymers, since
the chemical can degradate polymers. Preferably the expanded
graphite has been formed in a radiofrequency wave applicator by
heating the graphite precursor with the radiofrequency waves.
Preferably a precursor graphite has been treated with a fuming oxy
acid and heated to form the expanded graphite particles. Good
results have been achieved with expanded graphite composites
surface treated with acrylamide or other surface modifying
treatments.
[0020] The invention applied to thermoset polymer systems, such as
epoxy, polyurethane, polyurea, polysiloxane and alkyds, where
polymer curing involves coupling or crosslinking reactions. The
invention is applied as well to thermoplastic polymers for instance
polyamides, proteins, polyesters, polyethers, polyurethanes,
polysiloxanes, phenol-formaldehydes, urea-formaldehydes,
melamine-formaldehydes, celluloses, polysulfides, polyacetals,
polyethylene oxides, polycaprolactams, polycaprolactons,
polylactides, polyimides, and polyolefins (vinyl-containing
thermoplastics). Specifically included are polypropylene, nylon and
polycarbonate. The polymer can be for instance an epoxy resin. The
epoxy resin cures when heated. The epoxy composite material
preferably contains less than about 8% by weight of the expanded
graphite platelets. Thermoplastic polymers are widely used in many
industries. The expanded graphite can also be incorporated into
ceramics and metals.
[0021] Further the present invention relates to a method for
preparing a shaped composite which comprises:
[0022] (a) providing a mixture of a finely divided expanded
graphite consisting essentially of single platelets which are
essentially less than 200 microns in length and with a polymer
precursor with the expanded platelets dispersed therein; and
[0023] (b) forming the shaped composite material from the
mixture.
[0024] In particular, the present invention relates to a method for
preparing a shaped composite material which comprises:
[0025] (a) providing a mixture of an expanded graphite having
single platelets with a length less than about 200 microns and a
thickness of less than about 0.1 microns with a polymer precursor
with the expanded graphite platelets dispersed therein, wherein the
composite material contains up to about 50% by volume of the
expanded graphite platelets;
[0026] (b) forming the shaped composite material from the
mixture.
[0027] Preferably the expanded graphite is provided in the polymer
in an amount sufficient to render the shaped composite conductive.
Preferably the expanded graphite has been expanded with expanding
chemical which can be evaporated upon application of heat.
Preferably the expanded graphite platelets are formed in a
radiofrequency wave applicator by heating the graphite precursor
with radiofrequency waves and then the expanding chemical is
removed to form the graphite precursor. Preferably a graphite
precursor is treated with a fuming oxy acid and heated to provide
the expanded graphite particles.
[0028] The present invention also relates to an improvement in a
battery containing ions in the anode which comprises a finely
divided microwave or RF expanded graphite having single platelets
with a length less than about 200 microns and a thickness of less
than about 0.1 microns.
[0029] The present invention also relates to an improvement in a
catalytic conversion of an organic compound to hydrogen with a
catalytic material deposited on a substrate the improvement in the
substrate which comprises a finely divided microwave or RF expanded
graphite having single particles with a length less than about 200
microns and a thickness of less than about 0.1 microns.
[0030] Finally the present invention relates to a process for
producing platelets of expanded graphite which comprises:
[0031] (a) expanding graphite intercalated with a chemical which
expands upon heating to produce expanded graphite platelets;
and
[0032] (b) reducing the expanded graphite platelets so that
essentially all of the individual platelets are less than 200
microns in length, 0.1 micron in thickness. Preferably the chemical
agent is an inorganic oxy acid. Preferably the expanding is by
microwave or RF heating. Preferably the graphite is surface
modified such as with acrylamide.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] FIG. 1 is a scanning electron microscope (SEM) of
intercalated graphite flakes.
[0034] FIG. 2 is a SEM image of expanded natural graphite flakes
wherein the flakes are expanded by microwave.
[0035] FIG. 3 is a graph of an x-ray diffraction pattern of
intercalated natural graphite of FIG. 1. Some order is seen.
[0036] FIG. 4 is a graph of an x-ray diffraction pattern of the
expanded natural graphite of FIG. 2. No order is seen.
[0037] FIG. 5 is a SEM of pulverized exfoliated (expanded) natural
graphite.
[0038] FIG. 6 is a graph showing the size distribution of the
particles of FIG. 5 after being pulverized.
[0039] FIGS. 7 and 8 are graphs showing the flexural modulus (FIG.
7) and strength (FIG. 8) of cured epoxy resins containing 3% by
volume of the pulverized graphite particles of FIGS. 5 and 6.
[0040] FIG. 9 is a graph of the resistivity of control and graphite
nanoplatelet reinforced composites of FIGS. 7 and 8 as a function
of volume percent exfoliated graphite (Gr).
[0041] FIGS. 10A and 10B are TEM images of graphite nanoplatelets
in the polymer matrix of FIGS. 7 and 8.
[0042] FIG. 11 is a graph showing flexural strength versus expanded
graphite content for acrylamide grafted graphite.
[0043] FIG. 12 is a graph showing flexural modulus versus
acrylamide grafted expanded graphite content for acrylamide grafted
graphite.
[0044] FIGS. 13 to 18 are graphs showing flexural strength and
modulus for acrylamide modified graphite and various carbon
materials. "MW" is microwave, and "AA" is acrylamide.
[0045] FIGS. 19 to 21 are SEM images of various carbon materials.
FIG. 19 is PAN based carbon fiber, FIG. 20 is carbon film and FIG.
21 is carbon black.
[0046] FIGS. 22 to 24 are SEM images showing graphite in various
forms.
[0047] FIGS. 25 and 26 are TEM images of graphite
nanoplatelets.
[0048] FIGS. 27 and 28 are graphs showing size distribution of
graphite microplates and graphite nanoplatelets.
[0049] FIGS. 29 and 30 are graphs comparing flexural strength and
modulus for various samples including graphite modified with
acrylamide.
[0050] FIGS. 31 and 32 are graphs of flexural strength and modulus
for various carbon containing materials versus acrylamide
grafting.
[0051] FIG. 33 is a graph showing coefficient of thermal expansion
(CTE) of various composites with 3% by volume reinforcements and
without reinforcement.
[0052] FIG. 34 is a graph showing T.sub.g for various composites
with 3% volume percent of reinforcements and without
reinforcements.
[0053] FIG. 35 is a graph showing electrical resistivity of the
components versus percentage of reinforcement by weight.
[0054] FIG. 36 is a graph showing electrical percolation threshold
for various composites as a function of weight percent.
[0055] FIG. 37 is a graph showing impact strength for various
composites.
[0056] FIG. 38 is a separated perspective view of the basic
structure of a polymer battery. Cathode and Anode: electrically
conducting polymer on substrate. Polymer gel electrolytes:
Ionically conducting polymer gel film.
[0057] FIG. 39 is a schematic view of the basic structure of a fuel
cell.
[0058] FIG. 40 is a schematic view of the basic structure of a
lithium ion-battery.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0059] Graphite is a layered material. Individual molecular layers
are held together with weak Van der Waals forces which are capable
of intercalation with organic or inorganic molecules and eventual
expansion. These nanosized expanded graphite platelet materials are
very large platelets having large diameters and are very thin in
thickness. The graphite structure is stiff in bending. Graphite is
a very good thermal and electrical conductor.
[0060] Expanded graphite provides superior mechanical properties
and in addition provides electrical properties if a sufficient
amount is present in a polymer matrix. Expanded graphite platelets
have interbasal plane surfaces which have reactive sites on the
edges of the platelets. Different chemical groups can be added to
the edges. The application of an electric field can be used to
orient the expanded graphite platelets in a preferred direction
creating materials which are electrically or thermally conductive
in one direction. Submicron conductive paths can be created to act
as nanosized wires.
[0061] As used in the present application an expanded graphite is
one which has been heated to separate individual platelets of
graphite. An exfoliated graphite is a form of expanded graphite
where the individual platelets are separated by heating with or
without an agent such as a polymer or polymer component. In the
present application the term "expanded graphite" is used. The
expanded graphite usually does not have any significant order as
evidenced by an x-ray diffraction pattern.
[0062] The use of microwave energy or RF induction heating provides
a fast and economical method to produce expanded graphite
nanoflakes, graphite nanosheets, or graphite nanoparticles. The
microwave or RF methods are especially useful in large-scale
production and are very cost-effective.
[0063] The combination of RF or microwave expansion and appropriate
grinding technique, such as planetary ball milling (and vibratory
ball milling), produces nanoplatelet graphite flakes with a high
aspect ratio efficiently. Microwave or RF expansion and
pulverization of the crystalline graphite to produce suitable
graphite flakes enables control of the size distribution of
graphite flakes more efficiently. By incorporating an appropriate
surface treatment, the process offers an economical method to
produce a surface treated expanded graphite.
[0064] Chemically intercalated graphite flakes are expanded by
application of the RF or microwave energy. The expansion occurs
rapidly. Heating for 3 to 5 minutes removes the expanding chemical.
The graphite absorbs the RF or microwave energy very quickly
without being limited by convection and conduction heat transfer
mechanisms. The intercalant heats up past the boiling point and
causes the graphite to expand to many times its original volume.
The process can be performed continuously by using a commercially
available induction or microwave system with conveyors.
[0065] Although a commercial microwave oven operating at 2.45 GHz
was used for the following experiments, radio frequency (induction
heating) or microwave frequency energy across a wide range can be
used for this purpose.
[0066] The expanded graphite is pulverized for instance by ball
milling, mechanical grinding, air milling, or ultrasonic wave to
produce graphite flakes (platelets) with high aspect ratio. These
flakes are used as reinforcements in various matrices including
polymers and metals. Also these flakes can be used, for instance,
as anode materials, or substrates for metal catalysts. The
exfoliated graphite flakes can be provided in a polymer matrix
composite to improve the mechanical, electrical and thermal
properties.
[0067] Specifically, intercalated graphite flakes are expanded by
application of microwave energy at 2.45 GHz. This process can be
done continuously by using a commercially available microwave
system with conveyors. After the expansion, the graphite material
is calendared, with or without binder resins, to form a flexible
graphite sheet. The resultant sheet is cut into various sizes and
shapes and used as gaskets, sealing material, electrode substrates,
and separators for fuel cells.
[0068] Applications for the expanded graphite include thermally,
electrically and structural nanoreinforcements for polymers and
metals, electrode substrates for batteries, separators for fuel
cells, anode material, or substrates for metal catalysts.
EXAMPLE 1
[0069] The graphite was expanded before the polymer is introduced.
Intercalated graphite flakes were expanded by exposure to microwave
energy, typically at 2.45 GHz frequency, for a few seconds to a few
minutes in an oven. This process can be done continuously by using
commercially available microwave systems with conveyors or
batch-style process using individual microwave ovens. An automated
continuous system is preferred from an economical point of view. In
this case, the intercalated graphite flakes are first dispersed on
a conveyor and introduced into the microwave oven, then processed
under controlled conditions. Before or during this process
additional chemicals/additives can be added to the intercalated
graphite flakes to enhance the exfoliation, and/or apply surface
treatments to the graphite flakes. After this process, washing and
drying processes are applied, if necessary.
[0070] Typical starting materials are natural graphite flakes
intercalated with oxidizing agents, but synthetic graphite, kish
graphite, or the like can also be used. A preferred intercalating
agent is a mixture of sulfuric acid or sulfuric acid/phosphoric
acid mixture and an oxidizing agent such as nitric acid, perchloric
acid, chromic acid, potassium chlorate potassium permanganate,
potassium dichromate, hydrogen peroxide, metal halides or the
like.
[0071] FIG. 1 shows a SEM image of intercalated natural graphite
flakes. The microwave process heated the graphite flake, thereby
heating the intercalated acid causing a rapid expansion of the
graphite flakes perpendicular to the basal planes. During the
process, the flakes expanded as much as 300 times or more, but
still many of the layers were attached together and form worm-like
shapes. FIG. 2 shows a SEM image of expanded graphite material.
FIGS. 3 and 4 show XRD data of intercalated natural graphite and
expanded graphite processed by the microwave process. As FIG. 4
shows, the x-ray diffraction peak due to the highly and closely
aligned graphite sheets was significantly reduced because of the
expansion of the intercalated graphite by the microwave process.
The expanded graphite can be pressed to form flexible graphite
sheet. The thickness of the sheet can be controllable, depending on
the application.
[0072] The expanded graphite was pulverized into the small
platelets which have been crushed. FIGS. 5 and 6 show a SEM image
and size distribution of expanded graphite platelets. The size of
most graphite particles is 1 um or less after milling.
[0073] After the expansion, the graphite material can then be
pressed into sheet or pulverized into small flakes. In the former
case, the expanded graphite flakes are pressed by calendar roll,
press machine, or any other press methods, with or without binder
resins, to form a flexible graphite sheet. The resulting sheet can
be cut into various sizes and shapes and can be used as gaskets,
sealing material, electrode substrates, separators in fuel cells or
many other applications. In the latter case, the expanded graphite
flakes are pulverized by ball milling, planetary milling,
mechanical grinding, air milling, ultrasonic processing or any
other milling methods to produce graphite flakes with a high aspect
ratio. These expanded flakes can also be given further surface
treatments and can be used as reinforcements in various matrices
including polymers, ceramics, and metals. Also these flakes and/or
sheets can be used as electrodes and/or other parts for batteries,
or electrodes, separators, and/or other parts materials for fuel
cells, or substrates for various catalysts in many
chemical/biological reactions.
[0074] The expanded graphite nanoplatelets can be incorporated into
various types of matrices, including thermoplastic and thermoset
polymers. Before mixing with the polymeric matrix, surface
treatments can be applied to the graphite nanoplatelets to enhance
the adhesion between graphite platelets and matrix and the
dispersion of the platelets in the polymer. An example of composite
fabrication and its properties is described below.
EXAMPLE 2
[0075] Graphite flake that has been treated in the sulfuric acid to
intercalate the graphite with sulfuric acid in between the layers
was used. A commercial source used in this invention is
GRAFGUARD.TM. which is produced by UCAR Carbon Company (Lakewood,
Ohio).
[0076] Samples of acidic, neutral or basic intercalated graphite
(GRAFGUARD.TM. 160-50N, 160-50A or 160-50B from UCAR Carbon
Company, Parma, Ohio) were mixed into pure epoxy resin such as
diglycidylether of bisphenol-A (DGEBA) Shell Epon 828 or
equivalent. The mixture was heated to temperatures of at least
200.degree. C. at which time approximately the graphite experiences
a 15% weight loss due to the release of the trapped sulfuric acid
compounds. At the same time, the epoxy molecule entered the space
between the graphite layers. A very large volume expansion was
encountered which results in sorption of the epoxy in between the
graphite layers. This expanded graphite was dry to the touch
indicating that all of the epoxy has been sucked into the galleries
between the platelets. After cooldown, further epoxy and a curing
agent were added to this mixture and a composite material was
fabricated. There are various other routes available to attain the
same end point of removal of the sulfuric acid and intercalation of
the epoxy or similar polymer monomer in-between the graphite
layers. One way is to remove the acid from the expanded graphite by
heating.
[0077] Samples were made and mechanical properties were measured to
show that the graphite has been intercalated and exfoliated
(expanded) by the polymer.
EXAMPLE 3
[0078] Composite samples were fabricated using the following steps.
First, 1, 2, or 3 vol % (1.9, 3.8 or 5.8 wt %) of the expanded
graphite nanoplatelets of Example 2 were added into the epoxy
systems. (Epoxide; Shell Chemicals, EPON.TM. 828 (DGEBA), Curing
Agent: Huntsman Corporation, JEFFAMINE.TM. T403. The weight ratio
of EPON.TM. 828 to JEFFAMINE.TM. T403 was 100 to 45.) Then the
mixtures were cured by heating at 85.degree. C. for 2 hours
followed by 150.degree. C. for 2 hours. The heating ramp rate was
3.degree. C. per min. At the same time, a reference system was made
that did not have expanded graphite platelets in it but was
composed of the same epoxy system from the same batch. The
mechanical properties of these samples were determined. These
samples were investigated by flexural test. Also, the AC
conductivity of these materials was measured.
[0079] FIGS. 7 and 8 show the results of the flexural test. The
composite materials with 3 vol % graphite showed about 28% of
improvement in modulus and 12% improvement in strength compared to
the matrix material. This is an excellent increase with respect to
the relatively small amount of platelets reinforcements added to
the system.
[0080] FIG. 9 shows the AC resistivity of the control epoxy and the
graphite nanoplatelet reinforced composites. With 2% weight of
graphite platelets, the composite began displaying some
conductivity, which means that percolation threshold of this
material exists around 2% weight percent (1% in value). With 3%
volume graphite platelets, the composite shows a reduction of about
10 orders of magnitude which is a low enough resistivity for
electrostatic dissipation or electrostatic painting
applications.
[0081] The microstructure of the composite was observed by
preparing microtomed samples and viewing them in the transmission
electron microscope (TEM). The images are shown in FIGS. 10A and
10B. According to these images, the thickness of these
nanoplatelets was estimated around 15 to 30 nm. Multiple treatments
by the microwave process can reduce the platelet thickness to much
smaller dimensions.
EXAMPLE 4
[0082] This Example shows acrylamide grafting on a microwaved and
milled graphite platelet. The objective was to demonstrate the
mechanical properties of composites reinforced with acrylamide
grafted graphite nanoplatelets.
[0083] The graphite sample was microwave-exfoliated and vibratory
milled. The vibratory milling was for 72 hrs. The average diameter
was about 1 um.
[0084] The conditions for the grafting process were as follows:
Factors
1. Solvent System (O2 Plasma treatment: 1 min. moderate reflux
condition)
[0085] Benzene
[0086] Acetone
[0087] Isopropyl alcohol
[0088] Benzene/Acetone=50/50
[0089] Benzene/Acetone=75/25
[0090] Benzene/Acetone=87.5/12.5
2. O2 Plasma Treatment Time (solvent: Benzene. Moderate reflux
condition)
[0091] 0 min
[0092] 0.5 min
[0093] 1 min
[0094] 3 min
3. Reflux condition solvent: Benzene. O2 Plasma treatment: 1
min)
[0095] Moderate reflux. Hot Plate Temperature=110.about.120.degree.
C.
[0096] Vigorous reflux. Hot Plate Temperature=140.about.150.degree.
C.
The reaction procedure was:
[0097] The graphite samples were first treated with O.sub.2 plasma.
(RF 50%); the sample was then dispersed in a 1M-Acrylamide solution
and refluxed for 5 hours; and the sample was filtered and washed
with acetone, then dried in a vacuum oven. TABLE-US-00001 Organic
Component 1. Solvent System Solvent Benzene 15.37 wt % Acetone 6.39
wt % Isopropyl Alcohol 2.16 wt % Benzene/Acetone = 50/50 21.84 wt %
Benzene/Acetone = 75/25 18.95 wt % Benzene/Acetone = 87.5/12.5
17.75 wt % 2. O2 Plasma Treatment Time Plasma Treatment Time 0 min
2.91 wt % 0.5 min 9.73 wt % 1 min 15.37 wt % 3 min 11.53 wt % 3.
Reflux Condition Reflux Condition Moderate Reflux 15.37 wt %
Vigorous Reflux 38.25 wt %
[0098] The mechanical properties of composites of acrylamide
grafted graphite are shown in FIGS. 11 and 12 for a graphite sample
with 38.25 wt % acrylamide.
[0099] The effect of acrylamide grafting in forming composites with
the epoxy resin of Example 3 is shown in FIGS. 13 to 18.
EXAMPLE 5
[0100] Composites reinforced with nanoscopic graphite platelets
were fabricated and their properties were investigated as a
practical alternative to carbon nanotubes. The x-ray Diffraction
(XRD) and Transmission Electron Microscopy (TEM) results indicated
that the graphite flakes were well-exfoliated to achieve platelets
with thicknesses of 20 nm or less. Flexural tests and Differential
Mechanical Thermal Analysis (DMTA) results show that nanocomposite
materials made with these nanographite platelets have higher
modulus than that of composites made with commercially available
carbon reinforcing materials (i.e., PAN based carbon fiber, Vapor
Grown Carbon Fiber [VGCF], and Nanoscopic High-structure Carbon
Black). With the proper surface treatment, the graphite
nanoplatelets in polymeric matrices also showed better flexural
strength than composites with other carbon materials. Impedance
measurements have shown that the exfoliated graphite plates
percolate at below 3 volume percent, which is better than carbon
fiber and comparable with other carbon materials, and exhibit a
.about.10 order of magnitude reduction in impedance at these
concentrations.
[0101] In this Example, a special thermal treatment was applied to
the graphite flakes to produce exfoliated graphite reinforcements.
The composite material was fabricated by combining the exfoliated
graphite flakes with an amine-epoxy resin. X-ray Diffraction (XRD)
and Transmission Electron Microscopy (TEM) were used to assess the
degree of exfoliation of the graphite platelets. The mechanical
properties of this composite were investigated by flexural testing.
The glass transition temperature (Tg) of composite samples was
determined by Differential Mechanical Thermal Analysis (DMTA). The
coefficient of thermal expansion was examined by Thermal Mechanical
Analysis (TMA). The electrical conductivity was investigated by
impedance measurements using the 2-probe method.
Experimental
Materials
[0102] Epoxy was used as the matrix material. Diglycidyl ether of
bisphenol A (Epon 828) was purchased from the Shell Chemical Co.
Jeffamine T403 from Huntsman Petrochemical was used as the curing
agent for this matrix system.
[0103] Graphite was obtained from UCAR International Inc. and were
intercalated by acids. PAN based carbon fiber (PANEX 33 MC Milled
Carbon Fibers, average length: 175 um, average diameter: 7.2 um,
specific gravity: 1.81 g/cm.sup.3, Zoltek Co.), VGCF (Pyrograf III,
PR-19 PS grade, Length: 50.about.100 um, Average diameter: 150 nm,
Specific gravity: 2.0 g.cm.sup.3, Pyrograf Products, Inc.), and
nanosize carbon black (KETJENBLACK EC-600 JD, Average diameter:
400.about.500 nm, Specific gravity: 1.8 g/cm.sup.3, Akzo Novel
Polymer Chemicals LLC) were used as comparison. The SEM images of
these materials are shown in FIGS. 19, 20 and 21.
[0104] The UCAR graphite was processed thermally. After the
treatment, these graphite flakes showed significant expansion due
to the vaporization of intercalated acid in the graphite galleries.
The expanded graphite flakes were pulverized by use of an
ultrasonic processor and mechanical milling. The average diameter
and thickness of the flakes pulverized only by ultrasonic processor
were determined as 13 um and 30 nm, respectively (Graphite
microplate). Those of the flakes after milling were determined as
1.1 um and 20 nm, respectively (Graphite nanoplatelet). The SEM and
TEM images of as-received, expanded, and pulverized graphite flakes
are shown in FIGS. 22 to 25. The size distribution of the graphite
microplate and nanoplatelets is shown in FIGS. 27 and 28.
Composite Fabrication
[0105] The calculated amount of reinforcements were added to DGEBA
and mixed with the aid of an ultrasonic homogenizer for 5 minutes.
Then stoichiometric amount of Jeffamine T403 were added and mixed
at room temperature. The ratio of DGEBA/Jeffamine is 100/45 by
weight. The system was outgassed to reduce the voids and cured at
85.degree. C. for 2 hours, followed by post curing at 150.degree.
C. for 2 hours. The density of graphite flakes was assumed as 2.0
g/cm.sup.3. The densities of other carbon materials were obtained
from manufactures. The density of the epoxy matrix was measured as
1.159 g/cm.sup.3. Using these values, the volume fraction of
graphite platelets in composite samples was calculated.
Surface Treatments of Graphite Nanoplatelets
[0106] Surface treatments that can introduce carboxyl and/or amine
group were applied to the graphite according to the following
procedures.
Nitric Acid Treatment
[0107] A graphite nanoplatelet sample was dispersed in 69% (weight)
of nitric acid and heated at 115.degree. C. for 2 hours. The sample
was then washed by distilled water and dried in a vacuum oven.
O.sub.2 Plasma Treatment
[0108] Graphite nanoplatelets were dispersed on an aluminum foil
and covered by a stainless steel mesh. Then the sample was treated
by O.sub.2 plasma at RF level of 50% (275W) for 1 min.
UV/Ozone Treatment
[0109] Graphite nanoplatelets were packed in a quartz tube (ID: 22
mm, OD: 25 mm, Transparent to UV light down to wave length of 150
nm). The tube was filled with ozone (Concentration: 2000 ppm, Flow
rate: 4.7 L/min) and rotated at 3 rpm. Then the samples were
exposed to UV light for 5 min.
Amine Grafting
[0110] Graphite nanoplatelets were treated by O.sub.2 plasma to
introduce carboxyl group. Then the sample was dispersed in
tetraethylenepentamine (TEPA) and heated at 190.degree. C. for 5
hours to graft TEPA by forming an amide linkage. The sample was
washed with distilled water and methanol, then dried in a vacuum
oven (Pattman, Jr., et al., Carbon, Vol. 35, No. 3, pp. 217
(1997)).
Acrylamide Grafting
[0111] Graphite nanoplatelets were treated by O.sub.2 plasma to
introduce peroxide. Then the sample was dispersed in 1M
acrylamide/benzene solution and heated at 80.degree. C. for 5 hours
to initiate radical polymerization of acrylamide. The sample was
washed with acetone and dried in a vacuum oven (Yamada, K., et al.,
J. Appl. Polym. Sci., Vol. 75, pp. 284 (2000)). TABLE-US-00002
TABLE 1 XPS Data of Surface Treated Graphite Nanoplatelets and
Other Carbon Materials C O N S Na Al Others O/C N/C Graphite 93.5
6.1 0.0 0.0 0.0 0.0 0.4 0.055 0.000 Nanoplatelet HNO.sub.3 92.2 7.5
0.0 0.0 0.0 0.0 0.3 0.075 0.000 Treatment O.sub.2 Plasma 91.0 8.8
0.0 0.0 0.0 0.0 0.2 0.093 0.000 Treatment UV/O.sub.3 94.5 4.9 0.0
0.0 0.0 0.0 0.5 0.042 0.000 Treatment Amine Grafted 89.2 6.8 3.3
0.0 0.0 0.0 0.7 0.061 0.037 Acrylamide 78.3 14.0 7.8 0.0 0.0 0.0
0.0 0.177 0.100 Grafted PAN based CF 88.9 9.3 1.6 0.0 0.3 0.0 0.0
0.105 0.018 VGCF 95.1 4.9 0.0 0.0 0.0 0.0 0.0 0.052 0.000 Nanosized
91.7 8.2 0.0 0.0 0.0 0.0 0.0 0.089 0.000 Carbon Black
Results and Discussion XPS
[0112] The effect of surface treatments was investigated by X-ray
Photoelectron Spectroscopy (XPS). The results are shown in Table 1.
From this data, the acrylamide grafting treatment showed the
highest O/C and N/C ratio, suggesting many acrylamide groups were
introduced. The amine grafting treatment also showed an increase in
N/C ratio, suggesting amine groups were introduced. O.sub.2 plasma
treatment showed an increased O/C ratio, suggesting carboxyl groups
were introduced. The other two treatments didn't show impressive
results.
Mechanical Properties
Effect of Surface Treatments on Mechanical Properties
[0113] Graphite nanoplatelets treated by O.sub.2 plasma, amine
grafting, and acrylamide grafting were prepared and used as
reinforcements to fabricate composites with 1.0, 2.0 and 3.0 vol %
of graphite flakes. The flexural strength and modulus of each
sample are summarized in FIGS. 29 and 30.
[0114] The results indicate that the acrylamide grafting was the
most effective surface treatment in terms of both strength and
modulus enhancements. This is supported by XPS data that showed
largest N/C ratio for acrylamide grafting. These data suggest that
the amine groups grafted on graphite nanoplatelets improve the
compatibility between the graphite nanoplatelets and the matrix and
form a bond with the epoxy matrix and improve mechanical
properties.
Comparison with Commercially Available Carbon Materials
[0115] Composites reinforced with PAN based carbon fibers, VGCFs,
and nanosize carbon blacks were fabricated. The flexural properties
of these composites were measured and compared with those of
composites with acrylamide-grafted nanographite. The results are
shown in FIGS. 31 and 32. Here acrylamide grafted nanographite
showed the best results in terms of both strength and modulus
enhancement. This implies that the acrylamide grafting treatment is
a very effective surface treatment for graphite nanoplatelets.
Coefficient of Thermal Expansion
[0116] Coefficient of thermal expansion (CTE) of composites with 3
vol % of acrylamide grafted nanographite, PAN based carbon fiber,
VGCF, or nanosize carbon black were determined by TMA. The results
are shown in FIG. 33. The acrylamide grafted nanographite showed
the lowest CTE, indicating good dispersion and strong bonding
between the nanoreinforcements and the matrix.
Tg
[0117] Tg of composites with 3 vol % of acrylamide-grafted
nanographite, PAN based carbon fiber, VGCF, or nanosize carbon
black were determined by DMTA. The results are shown in FIG. 34.
The acrylamide grafted nanographite showed the slightly higher Tg,
but the difference is negligible considering the error margin of
the results. Thus these reinforcements didn't affect Tg of epoxy
matrix.
Electrical Property
[0118] The electrical resistivity of the composites with various
reinforcement contents were determined. The reinforcements used
were PAN based carbon fiber, VGCF, nanosize carbon black, graphite
microplate (exfoliated and sonicated, but not milled), and graphite
nanoplatelet. The size of each composite sample was about
30.times.12.times.8 mm. Each sample was polished and gold was
deposited on the surface to insure good electrical contacts. The
results are summarized in FIG. 35. The VGCF, carbon black and
graphite microplate percolated at around 2 wt % (1 vol %) while
conventional carbon fiber and graphite nanoplatelet showed
percolation threshold of about 8 to 9 wt % (5 to 6 vol %). Among
the former three reinforcements, graphite microplatelets and carbon
blacks produced composites with the lowest resistivity, which
reached around 10.sup.-1.5 ohm*cm. Thus, the exfoliated graphite
sample also showed excellent electrical property as reinforcement
in polymer matrix.
[0119] As shown by this Example, a new nanoplatelet graphite
material was developed by expansion (exfoliation) of graphite. An
appropriate surface treatment was established for the new material,
which produced a nanographite that increased the mechanical
properties of an epoxy system better than some commercially
available carbon materials at the same volume percentage. In
addition, the expanded (exfoliated) graphite material has been
shown to percolate at only 1 volume percent. Measurement of the
impedance of this material indicates that it could be used to
produce polymer matrix composites for new applications such as
electrostatic dissipation and EMI shielding.
[0120] The present invention provides a fast and economical method
to produce expanded graphite particles, expanded by using RF or
microwave energy as the expansion method. It is especially useful
in large-scale production and could be a very cost-effective method
which would lead to increased use of the exfoliated graphite
material.
[0121] The expanded graphite can be compressed or calendared to
make sheets with or without resins and/or other additives. These
sheets can be used as insulating material. In furnaces or
gaskets/sealing materials for internal combustion engines. Also
these sheets can be used as electrodes substrates for polymer
batteries (FIG. 38) or separator (or fluid flow field plates) for
fuel cells (FIG. 39).
[0122] The expanded graphite can be pulverized into platelets with
an appropriate grinding method. Platelets with a high aspect ratio
can be used as reinforcements in composites, which have high
mechanical properties as well as good electrical and thermal
conductivity.
[0123] Expanded graphite with an appropriate platelet size can hold
and release metal atoms such as lithium, which is suitable as anode
material for lithium-ion or lithium-polymer batteries (FIG.
40).
[0124] It is intended that the foregoing description be only
illustrative of the present invention and that the present
invention be limited only by the hereinafter appended claims.
* * * * *