Note: Descriptions are shown in the official language in which they were submitted.
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ELECTRICALLY CONDUCTIVE COATINGS
CONTAINING GRAPHENIC CARBON PARTICLES
FIELD OF THE INVENTION
[0002] The present invention relates to electrically conductive coatings
containing graphenic carbon particles.
BACKGROUND OF TI4E INVENTION
[0003] Many different types of coatings are subjected to environments where
electrical conductivity is desired. For example, improved conductivity
properties may
be advantageous for various types of clear coatings, colored coatings, primer
coatings,
static dissipative coatings and printed electronics, batteries, capacitors,
electric traces,
antennas, electrical heating coatings and the like.
SUMMARY OF '[HE INVENTION
[0004] An aspect or the invention provides an electrically conductive
coating
composition comprising a film-forming resin and thermally produced graphenic
carbon particles. When the coating composition is cured it has an electrical
conductivity greater than an electrical conductivity of the same coating
composition
without the thermally produced graphenic carbon particles.
[0005] Another aspect of the invention provides an electrically conductive
coating comprising a polymeric resin film and thermally produced graphenic
carbon
particles dispersed in the polymeric resin film.
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[0005] Another aspect of the invention provides an electrically conductive
coating comprising a polymeric resin film and thermally produced graphenic
carbon
particles dispersed in the polymeric resin film.
[0006] A further aspect of the invention provides a method of making an
electrically conductive coating composition comprising mixing thermally
produced
graphenic carbon particles with a film-forming resin.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] Fig. 1 is a graph illustrating electrical conductivity properties of
various coatings containing thermally-produced graphenic carbon particles in
accordance with embodiments of the present invention in comparison with
coatings
containing other types of commercial graphene particles.
[0008] Fig. 2 is a graph illustrating electrical conductivity properties of
various coatings containing one type of commercially available graphenic
carbon
particles in combination with either thermally-produced graphenic carbon
particles of
the present invention or other types of commercially available graphenic
carbon
particles.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0009] In accordance with embodiments of the present invention, graphenic
carbon particles are added to coating compositions to provide desirable
properties
such as increased electrical conductivity. As used herein, the term
"electrically
conductive", when referring to a coating containing graphenic carbon
particles, means
that the coating has an electrical conductivity of at least 0.001 S/m. For
example, the
coating may have a conductivity of at least 0.01, or at least 10 S/m.
Typically the
conductivity may be from 100 to 100,000 S/m, or higher. In certain
embodiments, the
conductivity may be at least 1,000 S/m or at least 10,000 S/m. For example,
the
conductivity may be at least 20,000 S/m, or at least 30,000 S/m, or at least
40,000
S/m.
[0010] In accordance with certain embodiments, the coatings do not exhibit
significant electrical conductivity absent the addition of graphenic carbon
particles.
For example, a conventional refinish clearcoat may have a conductivity that is
not
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measureable, while coatings of the present invention including graphenic
carbon
particles may exhibit conductivities as noted above. In certain embodiments,
the
addition of graphenic carbon particles increases conductivity of the coatings
by greater
than a factor of 10, typically greater than a factor of 1,000 or 100,000 or
higher.
[0011] In certain embodiments, the graphenic carbon particles may be added
to film-forming resins in amounts of from 0.1 to 95 weight percent based on
the total
coating solids. For example, the graphenic carbon particles may comprise from
1 to
90 weight percent, or from 5 to 85 weight percent. In certain embodiments, the
amount of graphenic carbon particles contained in the coatings may be
relatively
large, such as from 40 or 50 weight percent up to 90 or 95 weight percent. For
example, the graphenic carbon particles may comprise from 60 to 85 weight
percent,
or from 70 to 80 weight percent. In certain embodiments, conductivity
properties of
the coatings may be significantly increased with relatively minor additions of
the
graphenic carbon particles, for example, less than 50 weight percent, or less
than 30
weight percent. In certain embodiments, the present coatings have sufficiently
high
electrical conductivities at relatively low loadings of the graphenic carbon
particles.
For example, the above-noted electrical conductivities may be achieved at
graphenic
carbon particle loadings of less than 20 or 15 weight percent. In certain
embodiments,
the particle loadings may be less than 10 or 8 weight percent, or less than 6
or 5
weight percent. For example, for coatings comprising film-forming polymers or
resins that by themselves are non-conductive, the addition of from 3 to 5
weight
percent of thermally produced graphenic carbon particles may provide an
electrical
conductivity of at least 0.1 S/m, e.g., or at least 10 S/m.
[0012] The coating compositions can comprise any of a variety of
thermoplastic and/or thermosetting compositions known in the art. For example,
the
coating compositions can comprise film-forming resins selected from epoxy
resins,
acrylic polymers, polyester polymers, polyurethane polymers, polyamide
polymers,
polyether polymers, bisphenol A based epoxy polymers, polysiloxane polymers,
styrenes, ethylenes, butylenes, copolymers thereof, and mixtures thereof.
Generally,
these polymers can be any polymers of these types made by any method known to
those skilled in the art. Such polymers may be solvent borne, water soluble or
water
dispersible, emulsifiable, or of limited water solubility. Furthermore, the
polymers
may be provided in sol gel systems, may be provided in core-shell polymer
systems,
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dispersions in a continuous phase comprising water and/or organic solvent, for
example emulSion polymers or non-aqueous dispersions.
[0013] Thermosetting or curable coating compositions typically comprise film
forming polymers or resins having functional groups that are reactive with
either
themselves or a crosslinking agent. The functional groups on the film-forming
resin
may be selected from any of a variety of reactive functional groups including,
for
example, carboxylic acid groups, amine groups, epoxide groups, hydroxyl
groups,
thiol groups, carbamate groups, amide groups, urea groups, isocyanatc groups
(including blocked isocyanatc groups and tris-alkylcarbamoyltriazine)
mercaptan
groups, styrenic groups, anhydride groups, acetoacetate acrylates, uretidione
and
combinations thereof.
[0014] Thermosetting
coating compositions typically comprise a crosslinking
agent that may be selected from, for example, aminoplasts, polyisocyanates
including
blocked isocyanates, polyepoxides, beta-hydroxyalkylamides, polyacids,
anhydrides,
organometallic acid-functional materials, polyamines, polyamides, and mixtures
of
any o f the foregoing. Suitable polyisocyanates include multifunctional
isocyanates.
Examples of multifunctional polyisocyanates include aliphatic diisocyanates
like
hexamethylene diisocyanate and isophorone diisocyanate, and aromatic
diisocyanates
like toluene diisocyanate and 4,4'-diphenylmethane diisocyanate. The
polyisocyanates
can be blocked or unblocked. Examples of other suitable polyisocyanates
include
isocyanurate trimers, allophanates, and uretdiones of diisocyanates. Examples
of
commercially available polyisocyanates include DESMODUR N3390, which is sold
by Bayer Corporation, and TOLONATE HDT90, which is sold by Rhodia Inc.
Suitable aminoplasts include condensates of amines and or amides with
aldehyde. For
example, the condensate of melamine with formaldehyde is a suitable
aminoplast.
Suitable aminoplasts are well known in the art. A suitable aminoplast is
disclosed, for
example, in U.S. Pat. No. 6,316,119 at column 5, lines 45-55. In certain
embodiments,.the resin can be self crosslinking. Sclf crosslinking means that
the
resin contains functional groups that are capable of reacting with themselves,
such as
alkoxysilane groups, or that the reaction product contains functional groups
that are
coreactive, for example hydroxyl groups and blocked isocyanate groups.
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[0015] The dry film thickness of the cured coatings may typically range from
less than 0.5 microns to 100 microns or more, for example, from 1 to 50
microns. As
a particular example, the cured coating thickness may range from 1 to 15
microns.
[0016] In accordance with certain embodiments, when the coating
compositions are cured, the resultant coatings comprise a continuous matrix of
the
cured resin with graphenic carbon particles dispersed therein. The graphenic
carbon
particles may be dispersed uniformly throughout the thickness of the coating.
Alternatively, the graphenic carbon particles may be distributed non-
uniformly, e.g.,
with a particle distribution gradient through the thickness of the coating
and/or across
the coating.
[0017] As used herein, the term "graphenic carbon particles" means carbon
particles having structures comprising one or more layers of one-atom-thick
planar
sheets of sp2-bonded carbon atoms that are densely packed in a honeycomb
crystal
lattice. The average number of stacked layers may be less than 100, for
example, less
than 50. In certain embodiments, the average number of stacked layers is 30 or
less,
such as 20 or less, 10 or less, or, in some cases, 5 or less. The graphenic
carbon
particles may be substantially flat, however, at least a portion of the planar
sheets may
be substantially curved, curled, creased or buckled. The particles typically
do not
have a spheroidal or equiaxed morphology.
[0018] In certain embodiments, the graphenic carbon particles present in the
coating compositions of the present invention have a thickness, measured in a
direction perpendicular to the carbon atom layers, of no more than 10
nanometers, no
more than 5 nanometers, or, in certain embodiments, no more than 4 or 3 or 2
or 1
nanometers, such as no more than 3.6 nanometers. In certain embodiments, the
graphenic carbon particles may be from 1 atom layer up to 3, 6, 9, 12, 20 or
30 atom
layers thick, or more. In certain embodiments, the graphenic carbon particles
present
in the compositions of the present invention have average particle sizes,
i.e., widths
and lengths, measured in a direction parallel to the carbon atoms layers, of
at least 10
or 30 nanometers, such as more than 50 nanometers, in some cases more than 100
nanometers up to 1,000 nanometers. For example, the average particle size of
the
graphenic carbon particles may be from 200 to 800 nm, or from 250 to 750 nm.
The
graphenic carbon particles may be provided in the form of ultrathin flakes,
platelets or
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sheets having relatively high average aspect ratios (aspect ratio being
defined as the
ratio of the longest dimension of a particle to the shortest dimension of the
particle) of
greater than 3:1, such as greater than 10:1 up to 2000:1. For example, the
aspect
ratios may be greater than 15:1, or greater than 25:1, or greater than 100:1,
or greater
than 500:1.
[0019] In certain embodiments, the graphenic carbon particles used in the
coating compositions of the present invention have relatively low oxygen
content. For
example, the graphenic carbon particles used in certain embodiments of the
compositions of the present invention may, even when having a thickness of no
more
than 5 or no more than 2 nanometers, have an oxygen content of no more than 2
atomic weight percent, such as no more than 1.5 or 1 atomic weight percent, or
no
more than 0.6 atomic weight, such as about 0.5 atomic weight percent. The
oxygen
content of the graphenic carbon particles can be determined using X-ray
Photoelectron
Spectroscopy, such as is described in D. R. Dreyer et al., Chem. Soc. Rev. 39,
228-
240 (2010).
[0020] In certain embodiments, the graphenic carbon particles used in the
coating compositions of the present invention have a B.E.T. specific surface
area of at
least 50 square meters per gram, such as at least 70 square meters per gram,
or, in
some cases, at least 100 square meters per grams. For example, the surface
area may
be from 100 or 150 to 500 or 1,000 square meters per gram, or from 150 to 300
or 400
square meters per gram. In certain embodiments, the surface area is less than
300
square meters per gram, for example, less than 250 square meters per gram. As
used
herein, the term "B.E.T. specific surface area" refers to a specific surface
area
determined by nitrogen adsorption according to the ASTMD 3663-78 standard
based
on the Brunauer-Emmett-Teller method described in the periodical "The Journal
of
the American Chemical Society", 60, 309 (1938).
[0021] In certain embodiments, the graphenic carbon particles used in the
coating compositions of the present invention have a Raman spectroscopy 2D/G
peak
ratio of at least 1.1, for example, at least 1.2 or 1.3. As used herein, the
term "2D/G
peak ratio" refers to the ratio of the intensity of the 2D peak at 2692 cm-1
to the
intensity of the G peak at 1,580 cm-1.
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[0022] In certain embodiments, the graphenic carbon particles used in the
coating compositions of the present invention have a relatively low bulk
density. For
example, the graphenic carbon particles used in certain embodiments of the
present
invention are characterized by having a bulk density (tap density) of less
than 0.2
g/cm3, such as no more than 0.1 g/cm3. For the purposes of the present
invention, the
bulk density of the graphenic carbon particles is determined by placing 0.4
grams of
the graphenic carbon particles in a glass measuring cylinder having a readable
scale.
The cylinder is raised approximately one-inch and tapped 100 times, by
striking the
base of the cylinder onto a hard surface, to allow the graphenic carbon
particles to
settle within the cylinder. The volume of the particles is then measured, and
the bulk
density is calculated by dividing 0.4 grams by the measured volume, wherein
the bulk
density is expressed in terms of g/cm3.
[0023] In certain embodiments, the graphenic carbon particles used in the
coating compositions of the present invention have a compressed density and a
percent densification that is less than the compressed density and percent
densification
of graphite powder and certain types of substantially flat graphenic carbon
particles.
Lower compressed density and lower percent densification are each currently
believed
to contribute to better dispersion and/or rheological properties than
graphenic carbon
particles exhibiting higher compressed density and higher percent
densification. In
certain embodiments, the compressed density of the graphenic carbon particles
is 0.9
or less, such as less than 0.8, less than 0.7, such as from 0.6 to 0.7. In
certain
embodiments, the percent densification of the graphenic carbon particles is
less than
40 percent, such as less than 30 percent, such as from 25 to 30 percent.
[0024] For purposes of the present invention, the compressed density of
graphenic carbon particles is calculated from a measured thickness of a given
mass of
the particles after compression. Specifically, the measured thickness is
determined by
subjecting 0.1 grams of the graphenic carbon particles to cold press under
15,000
pound of force in a 1.3 centimeter die for 45 minutes, wherein the contact
pressure is
500 MPa. The compressed density of the graphenic carbon particles is then
calculated
from this measured thickness according to the following equation:
Compressed Density (g/cm3) = 0.1 grams
n*(1.3cm/2)2*(measured thickness in cm)
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[0025] The percent densification of the graphenic carbon particles is then
determined as the ratio of the calculated compressed density of the graphenic
carbon
particles, as determined above, to 2.2 g/cm3, which is the density of
graphite.
[0026] In certain embodiments, the graphenic carbon particles have a
measured bulk liquid conductivity of at least 10 microSiemens, such as at
least 30
microSiemens, such as at least 100 microSiemens immediately after mixing and
at
later points in time, such as at 10 minutes, or 20 minutes, or 30 minutes, or
40
minutes. For the purposes of the present invention, the bulk liquid
conductivity of the
graphenic carbon particles is determined as follows. First, a sample
comprising a
0.5% solution of graphenic carbon particles in butyl cellosolve is sonicated
for 30
minutes with a bath sonicator. Immediately following sonication, the sample is
placed
in a standard calibrated electrolytic conductivity cell (K=1). A Fisher
Scientific AB
30 conductivity meter is introduced to the sample to measure the conductivity
of the
sample. The conductivity is plotted over the course of about 40 minutes.
[0027] In accordance with certain embodiments, percolation, defined as long
range interconnectivity, occurs between the conductive graphenic carbon
particles.
Such percolation may reduce the resistivity of the coating compositions. The
conductive graphenic particles may occupy a minimum volume within the coating
such that the particles form a continuous, or nearly continuous, network. In
such a
case, the aspect ratios of the graphenic carbon particles may affect the
minimum
volume required for percolation. Furthermore, the surface energy of the
graphenic
carbon particles may be the same or similar to the surface energy of the
elastomeric
rubber. Otherwise, the particles may tend to flocculate or demix as they are
processed.
[0028] The thermally produced graphenic carbon particles utilized in the
coating compositions of the present invention are made by thermal processes.
In
accordance with embodiments of the invention, thermally-produced graphenic
carbon
particles are made from carbon-containing precursor materials that are heated
to high
temperatures in a thermal zone such as a plasma. As more fully described
below, the
carbon-containing precursor materials are heated to a sufficiently high
temperature,
e.g., above 3,500 C, to produce graphenic carbon particles having
characteristics as
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described above. The carbon-containing precursor, such as a hydrocarbon
provided in
gaseous or liquid form, is heated in the thermal zone to produce the graphenic
carbon
particles in the thermal zone or downstream therefrom. For example, thermally-
produced graphenic carbon particles may be made by the systems and methods
disclosed in U.S. Patent Nos. 8,486,363 and 8,486,364.
[0029] In certain embodiments, the thermally produced graphenic carbon
particles may be made by using the apparatus and method described in U.S.
Patent
No. 8,486,363 at [0022] to [0048] in which (i) one or more hydrocarbon
precursor
materials capable of forming a two-carbon fragment species (such as n-
propanol,
ethane, ethylene, acetylene, vinyl chloride, 1,2-dichloroethane, allyl
alcohol,
propionaldehyde, and/or vinyl bromide) is introduced into a thermal zone (such
as a
plasma), and (ii) the hydrocarbon is heated in the thermal zone to a
temperature of at
least 1,000 C to form the graphenic carbon particles. In other embodiments,
the
thermally produced graphenic carbon particles may be made by using the
apparatus
and method described in U.S. Patent No. 8,486,364 at [0015] to [0042] in which
(i) a
methane precursor material (such as a material comprising at least 50 percent
methane, or, in some cases, gaseous or liquid methane of at least 95 or 99
percent
purity or higher) is introduced into a thermal zone (such as a plasma), and
(ii) the
methane precursor is heated in the thermal zone to form the graphenic carbon
particles. Such methods can produce graphenic carbon particles having at least
some,
in some cases all, of the characteristics described above.
[0030] During production of the graphenic carbon particles by the thermal
production methods described above, a carbon-containing precursor is provided
as a
feed material that may be contacted with an inert carrier gas. The carbon-
containing
precursor material may be heated in a thermal zone, for example, by a plasma
system.
In certain embodiments, the precursor material is heated to a temperature of
at least
3,500 C, for example, from a temperature of greater than 3,500 C or 4,000 C up
to
10,000 C or 20,000 C. Although the thermal zone may be generated by a plasma
system, it is to be understood that any other suitable heating system may be
used to
create the thermal zone, such as various types of furnaces including
electrically heated
tube furnaces and the like.
[0031] The gaseous stream may be contacted with one or more quench streams
that are injected into the plasma chamber through at least one quench stream
injection
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port. The quench stream may cool the gaseous stream to facilitate the
formation or
control the particle size or morphology of the graphenic carbon particles. In
certain
embodiments of the invention, after contacting the gaseous product stream with
the
quench streams, the ultrafine particles may be passed through a converging
member.
After the graphenic carbon particles exit the plasma system, they may be
collected.
Any suitable means may be used to separate the graphenic carbon particles from
the
gas flow, such as, for example, a bag filter, cyclone separator or deposition
on a
substrate.
[0032] Without being bound by any theory, it is currently believed that the
foregoing methods of manufacturing thermally produced graphenic carbon
particles
are particularly suitable for producing graphenic carbon particles having
relatively low
thickness and relatively high aspect ratio in combination with relatively low
oxygen
content, as described above. Moreover, such methods are currently believed to
produce a substantial amount of graphenic carbon particles having a
substantially
curved, curled, creased or buckled morphology (referred to herein as a "3D"
morphology), as opposed to producing predominantly particles having a
substantially
two-dimensional (or flat) morphology. This characteristic is believed to be
reflected
in the previously described compressed density characteristics and is believed
to be
beneficial in the present invention because, it is currently believed, when a
significant
portion of the graphenic carbon particles have a 3D morphology, "edge to edge"
and
"edge-to-face" contact between graphenic carbon particles within the
composition
may be promoted. This is thought to be because particles having a 3D
morphology
are less likely to be aggregated in the composition (due to lower Van der
Waals
forces) than particles having a two-dimensional morphology. Moreover, it is
currently
believed that even in the case of "face to face" contact between the particles
having a
3D morphology, since the particles may have more than one facial plane, the
entire
particle surface is not engaged in a single "face to face" interaction with
another single
particle, but instead can participate in interactions with other particles,
including other
"face to face" interactions, in other planes. As a result, graphenic carbon
particles
having a 3D morphology are currently thought to provide the best conductive
pathway
in the present compositions and are currently thought to be useful for
obtaining
electrical conductivity characteristics sought by embodiments of the present
invention,
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particularly when the graphenic carbon particles are present in the
composition in
relatively low amounts.
[0033] In certain embodiments, the thermally produced graphenic carbon
particles may be combined with other types of graphenic particles, such as
those
obtained from commercial sources, for example, from Angstron, XG Sciences and
other commercial sources. In such embodiments, the commercially available
graphenic carbon particles may comprise exfoliated graphite and have different
characteristics in comparison with the thermally produced graphenic carbon
particles,
such as different size distributions, thicknesses, aspect ratios, structural
morphologies,
oxygen content, and chemical functionality at the basal planes/edges.
[0034] When thermally produced graphenic carbon particles are combined
with commercially available graphenic carbon particles in accordance with
embodiments of the invention, a bi-modal distribution, tri-modal distribution,
etc. of
graphenic carbon particle characteristics may be achieved. For example, the
graphenic carbon particles contained in the coatings may have multi-modal
particle
size distributions, aspect ratio distributions, structural morphologies, edge
functionality differences, oxygen content, and the like. The following Table 1
lists
average particle sizes, thicknesses and aspect ratios for thermally produced
graphenic
carbon particles in comparison with certain commercially available graphenic
carbon
particles produced from exfoliated graphite.
[0035] In an embodiment of the present invention in which both thermally
produced graphenic carbon particles and commercially available graphenic
carbon
particles, e.g., from exfoliated graphite, are added to a coating composition
to produce
a bi-modal graphenic particle size distribution, the relative amounts of the
different
types of graphenic carbon particles are controlled to produce desired
conductivity
properties of the coatings. For example, the thermally produced graphenic
particles
may comprise from 1 to 50 weight percent, and the commercially available
graphenic
carbon particles may comprise from 50 to 99 weight percent, based on the total
weight
of the graphenic carbon particles. In certain embodiments, the thermally
produced
graphenic carbon particles may comprise from 2 to 20 weight percent, or from 5
to 10
or 12 weight percent.
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[0036] In addition to the resin and graphenic carbon particle components, the
coatings of the present invention may include additional components
conventionally
added to coating compositions, such as cross-linkers, pigments, tints, flow
aids,
defoamers, dispersants, solvents, UV absorbers, catalysts and surface active
agents.
[0037] In certain embodiments, the coating compositions are substantially free
of certain components such as polyalkyleneimines, graphite, or other
components.
For example, the term "substantially free of polyalkyleneimines" means that
polyalkyleneimines are not purposefully added, or are present as impurities or
in trace
amounts, e.g., less than 1 weight percent or less than 0.1 weight percent.
Coatings of
the present invention have been found to have good adhesion properties without
the
necessity of adding polyalkyleneimines. The term "substantially free of
graphite"
means that graphite is not purposefully added, or is present as an impurity or
in trace
amounts, e.g., less than 1 weight percent or less than 0.1 weight percent. In
certain
embodiments, graphite in minor amounts may be present in the coatings, e.g.,
less
than 5 weight percent or less than 1 weight percent of the coating. If
graphite is
present, it is typically in an amount less than the graphene, e.g., less than
30 weight
percent based on the combined weight of the graphite and graphene, for
example, less
than 20 or 10 weight percent.
[0038] The coating compositions of the present invention may be made by
various standard methods in which the graphenic carbon particles are mixed
with the
film-forming resins and other components of the coating compositions. For
example,
for two-part coating systems, the graphenic carbon particles may be dispersed
into part
A and/or part B. In certain embodiments, the graphenic carbon particles are
dispersed
into part A by various mixing techniques such as sonication, high speed
mixing,
media milling and the like. In certain embodiments, the graphenic carbon
particles
may be mixed into the coating compositions using high-energy and/or high-shear
techniques such as sonication, 3-roll milling, ball milling, attritor milling,
rotor/stator
mixers, and the like.
[0039] In accordance with certain embodiments, the coatings of the present
invention possess desirable mechanical properties, increased IR absorption,
increased
"jetness", increased thermal conductivity, decreased permeability to small
molecules
like water and oxygen may also be advantageous to these same coatings.
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[0040] The following examples are intended to illustrate various aspects of
the
invention, and are not intended to limit the scope of the invention.
Example 1
[0041] The electrical conductivities of coatings containing thermally produced
graphenic carbon particles were compared with similar coatings containing
commercial graphene particles, and no such particles. The coating compositions
were
made with aqueous latex particles that are stable in N-methyl pyrrolidone
(NMP)
solvent. The acrylic latex particles are crosslinked and are epoxy
functionalized, but
need not be functionalized to work. The latex forms a film at elevated
temperatures
and serves as the binder to hold the film together. Thermally produced
graphenic
carbon particles, labeled PPG A and PPG B, were produced by the thermal plasma
production method utilizing methane as a precursor material disclosed in U.S.
Patent
No. 8,486,364. The thermally produced PPG A and PPG B graphenic carbon
particles
have a surface area of about 250 ¨ 280 m2/g and are about 100-200 nm in size.
The
commercially available graphenic carbon particles included: XG-M5 (from XG
Sciences having an average particle size of 5 microns, thickness of about 6
nm, and
BET surface area of from 120 to 150 square meters per gram); XG-C750 (from XG
Sciences having an average particle size of about 1.5 micron, thickness of
about 2 nm,
and BET surface area of 750 square meters per gram); and PDR (from Angstron
Materials having an average particle size of about 10 microns, thickness of
about 1
nm and BET surface area of from 400 to 800 square meters per gram). Prior to
the
addition of the graphenic carbon particles to the coating solution, samples
are diluted
to 0.25 ¨ 2.5 weight percent in NMP solvent and horn sonicated for 15 min. The
PPG
B sample was dispersed with double the sonication energy per unit graphene in
comparison with the PPG A sample. The final coating composition is then made
by
mixing latex, NMP solvent, and the pre-dispersed graphenic carbon particles.
Samples are then bath sonicated for 15 minutes. Following sonication, samples
are
passed through a 150 mesh filter and then drawn down on glass substrates at a
6 mil
wet film build. The wet films are flashed at room temperature for 15 minutes,
followed by an oven cure at 100 C for 30 minutes.
[0042] Fig. 1 graphically illustrates the conductivities of the coatings
containing the thermally produced PPG A and PPG B graphenic carbon particles
at
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14
various loadings, in comparison with the other commercial graphenic carbon
particles
and a control coating containing no such particles, which measured no
conductivity.
Although the PPG A and PPG B particles are about an order of magnitude smaller
than the M5 particles, they produce similar electrical conductivities. As the
particles
get smaller, the resistance should increase in the film at a similar loading,
i.e.,
comparing the M5 with PDR, which is a ¨ 10 micron average particle size vs. 5
micron and the C750 which is 1.5 microns. However, the thermally produced
graphene particles provide a lower resistance coating. This may be due to the
extremely low oxygen content of the thermally produced graphenic carbon
particles
and the fact that their edge functionality may be limited to C-H bonds vs. C-
0, C-N,
bonds observed in commercial graphene samples, producing a lower particle-
particle
contact resistance for the thermally produced graphene. The thermally produced
graphene may also be inherently more conductive because of its turbostratic
crystal
structure.
Example 2
[0043] Coatings comprising one type of commercially available graphenic
carbon particles alone, and in combination with other graphenic carbon
particles
(including thermally produced graphenic carbon particles), were produced and
measured for electrical conductivity. The coating compositions were made with
10
weight percent graphenic carbon particles: either xGnP C-300 (from XG Sciences
having an average particle size of 1.5 micron, thickness of about 2 nm, and
BET
surface area of 300 square meters per gram), xGnP C-750 (from XG Sciences as
described in Example 1), xGnP M-25 (from XG Sciences having an average
particle
size of 25 microns, thickness of 6-8 nm, and BET surface area of 120-150
square
meters per gram), or PPG thermally produced graphenic carbon particles, with
1.67
weight percent ethyl cellulose (Aqualon, Ashland), and with 88.33 weight
percent
deionized water. These coating compositions were dispersed by adding 70g of
each
into 8 ounce glass jars with 220g of SEPR Ermil 1.0-1.25mm milling media. The
samples in the jars were shaken for 4 hours using a Lau disperser (Model DAS
200,
Lau, GmbH). The milling media was then filtered off from the coating
compositions.
Mixtures of these coating compositions were then prepared, such that of the
total 10
percent by weight of graphenic carbon in each of the mixtures, there were two
types of
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graphenic carbon particles in the following weight percentages: 92% xGnP M-25
and
8% PPG thermally produced graphenic carbon particles, 92% xGnP M-25 and 8%
xGnP C-300, and 92% xGnP M-25 and 8% xGnP C-750. Each of these mixtures as
well as the coating composition with only xGnP M-25 were applied as 1-2 mm
wide
lines in a serpentine circuit pattern to a 2 x 3 inch glass slide
(Fisherbrand, Plain,
Precleaned) using a dispensing jet (PICO valve, MV-100, Nordson, EFD) and a
desktop robot (2504N, Janome) and then dried in an oven at 212 F for 30
minutes.
Electrical conductivity of each coated sample was determined by first
measuring the
resistance of the serpentine circuit vs. the length of the circuit line. Then,
the cross-
sectional area of the serpentine lines was measured using a stylus
profilometer
(Dektak). Using the measured values for the cross sectional area (A) and the
resistance (R) for a given length (L) of the circuit, the resistivity (p) was
calculated
using the equation p = RA/L. Then the conductivity (a) was calculated by
taking the
reciprocal of the resistivity, a = 1/p.
[0044] The results are shown in Fig. 2. The addition of a small amount of
PPG thermally produced graphenic carbon particles significantly increases the
conductivity, by about 200%, above that of a coating composition containing
only a
large-platelet type graphenic carbon (xGnP M-25). Fig. 2 shows that small
additions
of other commercially available graphenic carbon particles did not increase
the
conductivity as significantly (only about 50% increase for xGnP C-300 and only
about
90% increase for xGnP C-750).
[0045] For purposes of this description, it is to be understood that the
invention may assume various alternative variations and step sequences, except
where
expressly specified to the contrary. Moreover, other than in any operating
examples,
or where otherwise indicated, all numbers expressing, for example, quantities
of
ingredients used in the specification and claims are to be understood as being
modified in all instances by the term "about". Accordingly, unless indicated
to the
contrary, the numerical parameters set forth in the following specification
and
attached claims are approximations that may vary depending upon the desired
properties to be obtained by the present invention. At the very least, and not
as an
attempt to limit the application of the doctrine of equivalents to the scope
of the
claims, each numerical parameter should at least be construed in light of the
number
of reported significant digits and by applying ordinary rounding techniques.
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[0046] Notwithstanding that the numerical ranges and parameters setting forth
the broad scope of the invention are approximations, the numerical values set
forth in
the specific examples are reported as precisely as possible. Any numerical
value,
however, inherently contains certain errors necessarily resulting from the
standard
variation found in their respective testing measurements.
[0047] Also, it should be understood that any numerical range recited herein
is
intended to include all sub-ranges subsumed therein. For example, a range of
"1 to
10" is intended to include all sub-ranges between (and including) the recited
minimum
value of 1 and the recited maximum value of 10, that is, having a minimum
value
equal to or greater than 1 and a maximum value of equal to or less than 10.
[0048] In this application, the use of the singular includes the plural and
plural
encompasses singular, unless specifically stated otherwise. In addition, in
this
application, the use of "or" means "and/or" unless specifically stated
otherwise, even
though "and/or" may be explicitly used in certain instances.
[0049] It will be readily appreciated by those skilled in the art that
modifications may be made to the invention without departing from the concepts
disclosed in the foregoing description. Such modifications are to be
considered as
included within the following claims unless the claims, by their language,
expressly
state otherwise. Accordingly, the particular embodiments described in detail
herein
are illustrative only and are not limiting to the scope of the invention which
is to be
given the full breadth of the appended claims and any and all equivalents
thereof
