U.S. patent application number 12/480225 was filed with the patent office on 2009-12-10 for polymers containing hexagonal boron nitride particles coated with turbostratic carbon and process for preparing the same.
This patent application is currently assigned to E.I. DU PONT DE NEMOURS AND COMPANY. Invention is credited to SALAH BOUSSAAD.
Application Number | 20090304922 12/480225 |
Document ID | / |
Family ID | 40937515 |
Filed Date | 2009-12-10 |
United States Patent
Application |
20090304922 |
Kind Code |
A1 |
BOUSSAAD; SALAH |
December 10, 2009 |
POLYMERS CONTAINING HEXAGONAL BORON NITRIDE PARTICLES COATED WITH
TURBOSTRATIC CARBON AND PROCESS FOR PREPARING THE SAME
Abstract
The present invention describes polymer compositions containing
boron nitride particles that are encapsulated in layers of
turbostratic carbon. The polymers so prepared exhibit enhanced
thermal conductivity.
Inventors: |
BOUSSAAD; SALAH;
(Wilmington, DE) |
Correspondence
Address: |
E I DU PONT DE NEMOURS AND COMPANY;LEGAL PATENT RECORDS CENTER
BARLEY MILL PLAZA 25/1122B, 4417 LANCASTER PIKE
WILMINGTON
DE
19805
US
|
Assignee: |
E.I. DU PONT DE NEMOURS AND
COMPANY
Wilmington
DE
|
Family ID: |
40937515 |
Appl. No.: |
12/480225 |
Filed: |
June 8, 2009 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
61131196 |
Jun 6, 2008 |
|
|
|
Current U.S.
Class: |
427/215 |
Current CPC
Class: |
C08K 9/10 20130101; C08J
2379/08 20130101; Y10T 428/2991 20150115; C08J 3/212 20130101; C08J
5/18 20130101; C08K 3/38 20130101 |
Class at
Publication: |
427/215 |
International
Class: |
B05D 7/24 20060101
B05D007/24 |
Claims
1. A process for making a polymeric composite composition
comprising a polyimide having dispersed therein a loading of
particles of hexagonal boron nitride having a coating of
turbostratic carbon, the process comprising dispersing in a
solution of a polyamic acid in an organic solvent, particles of
hexagonal boron nitride having a coating of turbostratic carbon,
casting a film therefrom, extracting said organic solvent, and
imidizing said polyamic acid in said film.
2. The process of claim 1 wherein the hexagonal boron nitride
particles are encapsulated by the coating of turbostratic
carbon.
3. The process of claim 1 wherein the coating is characterized by a
plurality of layers.
4. The process of claim 1 wherein the coating ranges in thickness
from 5 nm to 5000 nm.
5. The process of claim 1 wherein the loading is 10% to 60% by
weight.
6. The process of claim 1 wherein the hexagonal boron nitride is in
the form of platelets characterized by a thickness of 0.1 to 5
micrometers and maximum in-plane dimension of 1 to 200 micrometers.
Description
FIELD OF THE INVENTION
[0001] The present invention is directed to novel polymer
compositions containing boron nitride particles that are
encapsulated in layers of turbostratic carbon. The polymers so
prepared exhibit enhanced thermal conductivity.
BACKGROUND OF THE INVENTION
[0002] Sugiyama et al., JP2887874, discloses coating boron nitride
(BN) particles 100 micrometers (.mu.m) in diameter with colloidal
graphite particles of a size less than 5 .mu.m in diameter by
immersing the BN into an aqueous colloidal suspension of the
graphite, followed by drying at up to 300.degree. C.
[0003] Kenji et al., JP Hei 2[1990]-169192, discloses applying a
graphite coating to a sintered body of cubic BN. A dispersion of
1-3 .mu.m graphite powders in ethanol is sprayed onto the surface
of the sintered body. The thus coated surface is said to be more
absorbing of laser radiation, facilitating cutting of the sintered
body.
[0004] Turbostratic carbon is a form of carbon that, like graphite,
forms platelet structures and is highly ordered in the plane of the
platelet, but, unlike graphite, shows no organization between
platelets--that is in the direction normal to the plane of the
platelet. Turbostratic carbon and methods for the formation thereof
are described in detail in Graphite Fibers and Filaments, M. S.
Dresselhaus et al., Springer-Verlag (1988), pp. 42-48 and
51-55.
[0005] Polymers, including polymers filled with inorganic,
non-electrically conductive particulate matter, have found
widespread commercial use as dielectric or insulating materials
such as in wire and cable, printed circuits, including flexible
printed circuits, and including multi-layer printed circuits. In
applications where power levels generate significant heat,
improvement in thermal management can be derived from polymeric
materials exhibiting increased thermal conductivity.
SUMMARY OF THE INVENTION
[0006] The present invention provides a polymeric composite
composition comprising a polymer having dispersed therein a loading
of particles of hexagonal boron nitride having a coating of
turbostratic carbon.
[0007] The present invention further provides a process for making
a composition comprising a polyimide having dispersed therein a
loading of particles of hexagonal boron nitride having a coating of
turbostratic carbon, the process comprising dispersing in a
solution of a polyamic acid in an organic solvent particles of
hexagonal boron nitride having a coating of turbostratic carbon,
casting a film therefrom, extracting said organic solvent, and
imidizing said polyamic acid in said film.
BRIEF DESCRIPTION OF THE DRAWING
[0008] The Drawing consists of six figures.
[0009] FIG. 1 is a photograph of turbostratic carbon-coated hBN
particles lying in a layer in a quartz boat.
[0010] FIG. 2 is a scanning electron micrograph of a turbostratic
carbon-coated hBN particle showing complete coverage of a particle
surface with the carbon coating.
[0011] FIG. 3 is a transmission electron micrograph (TEM) of a
cross section of a coated hBN particle, showing the distinctive
layers of turbostratic carbon making up the coating as prepared
according to Example 1.
[0012] FIG. 4 is a transmission electron micrograph (TEM) of a
cross section of a coated hBN particle, showing the distinctive
layers of turbostratic carbon making up the coating as prepared
according to Example 2.
[0013] FIG. 5 is a transmission electron micrograph (TEM) of a
cross section of a coated hBN particle, showing the distinctive
layers of turbostratic carbon making up the coating as prepared
according to Example 3.
[0014] FIG. 6 is a transmission electron micrograph (TEM) of a
cross section of a coated hBN particle, showing the distinctive
layers of turbostratic carbon making up the coating as prepared
according to Example 4.
DETAILED DESCRIPTION
[0015] Hexagonal BN (hBN) is known in the art to exist in the form
of platelets. A particle of hBN is formed by the layering of a
plurality of hBN platelets thereby forming a sheet-like structure
having a characteristic planar morphology and a thickness normal to
the plane, and wherein the in-plane dimensions exceed the normal
dimensions by ca. 10.times. or greater. For the purposes of the
present invention, hBN particles suitable for use are characterized
by the thickness of the particle and the maximum dimension in the
plane thereof. Suitable for use in the present invention are
hexagonal (hBN) particles ranging in thickness from 0.1 to 5
micrometers (.mu.m), and ranging in maximum in-plane dimension from
1 to 200 .mu.m. These dimensions are estimates based upon
examination of scanning electron micrographs of the particles
employed. The operability of the present invention does not depend
critically upon the particular average dimensions of the hBN
particles. While not expressly determined, there is no reason why
hBN particles outside the stated range of particle sizes would not
also be operable herein.
[0016] For the purposes of the present invention, the term
"turbostratic carbon" shall be understood to refer to a crystalline
carbon forming a lamellar structure whereof the basal planes have
slipped sideways to one another, causing the spacing of the planes
be greater than would be obtained in a graphitic structure.
[0017] The turbostratic carbon layers formed according to the
process described below are ca. 1 nm in thickness. The
encapsulating coating disposed upon the hBN typically comprises a
plurality of said turbostratic carbon layers. The thicknesses of
coatings useful in the practice of the present invention range from
5 to 5000 nm. The coating thickness can be made to increase with
increasing time of exposure to a reactant gas composition, or by
increasing the concentration of hydrogen during formation of the
coating. The encapsulated hBN disclosed herein is further
characterized by an encapsulating sheet about 5 nanometers (nm) to
about 5000 nm in thickness, comprising a plurality of layers of
turbostratic carbon. The sheet adheres to the surface of the hBN,
and closely follows the topography of the hBN particle.
[0018] The encapsulated hBN particles suitable for use in the
present invention are useful as fillers for polymeric materials
when an increase in polymeric thermal conductivity is desired.
Thermal conductivity in polymer films in a direction normal to the
plane of the film is well-known in the art to be quite low, in many
cases more than an order of magnitude lower than in the plane of
the film, making polymers quite useful as thermal insulators.
However, that same insulating property can be detrimental to the
utility of a polymer film where thermal insulation is undesirable.
Very significant enhancement has been found in thermal conductivity
normal to the plane of polymer films containing loadings ranging
from ca. 10 weight-% to ca. 60 weight-% of the encapsulated hBN
suitable for use herein.
[0019] The turbostratic carbon-encapsulated hBN suitable for use
herein may be prepared by exposing hBN having a platelet morphology
characterized by a thickness in the range of 0.1 to 5 micrometers
and a maximum in-plane dimension of 1 to 200 micrometers to a gas
phase mixture of hydrogen, at least one alkane, and at least one
alkene at a temperature in the range of 850-2000.degree. C. for ca.
60 min to ca. 6 sec. In one embodiment, the temperature can range
from 900-1200.degree. C., and the exposure time can range from 6
min to 30 sec. At temperatures below 850.degree. C. the coatings
are not uniform.
[0020] The turbostratic carbon coating provides the same benefits
as graphite as a coating for hBN, but can be produced under
conditions of lower temperatures and shorter times than can
graphite.
[0021] In one embodiment, the alkane is a fluid at room
temperature. In one embodiment, the alkane is a gas at room
temperature. In a further embodiment, the alkane is methane.
[0022] In another embodiment, the alkene is a fluid at room
temperature. In a further embodiment, the alkene is a gas at room
temperature. In a further embodiment, the alkene is ethylene.
[0023] In one embodiment of the process for preparing the
turbostratic carbon-coated hexagonal boron nitride suitable for use
herein, hydrogen, methane, and ethylene are introduced as a
reactant gas composition into a nitrogen purged furnace that has
been pre-heated to 900.degree. C. and containing a layer.ltoreq.ca.
1 cm thick of hBN particles for a period of 1-6 minutes, after
which the reaction gas flow is stopped and the nitrogen purge
resumed as the furnace is cooled. It is found in the practice of
the invention that when the reactant gas composition comprises all
three reaction gases, the uniformity of the coating and control of
coating thickness are improved. It is further found in the practice
of the process hereof that the volumetric flow rate ratio of
H.sub.2 to CH.sub.4 should be in the range of 1:1 to 2:1. At ratios
below 1:1, the carbon coating may be amorphous. At ratios above 2:1
the carbon coating may not form a uniform coating. It is further
found in the practice of the process hereof that the volumetric
flow ratio of CH.sub.4 to C.sub.2H.sub.4 should be at least 10:1,
preferably at least 20:1. At ratios of less than 10:1 safety may be
compromised by dangerous and uncontrolled pressure increases.
CH.sub.4 serves as an important moderator of the reaction from both
a safety viewpoint and from a product uniformity viewpoint.
[0024] After cooling, the thus treated hBN particles are found to
have changed from white to black, indicating the successful coating
operation. The particles thus prepared can then be incorporated
into polymers for the purpose of preparing a composite having a
thermal conductivity higher than that of the corresponding neat
polymer (that is, the polymer not containing the particles). In
particular, it is found that the thermal conductivity normal to the
plane of a polymeric film is improved several fold over that of the
polymer itself.
[0025] The particular filler loading and method for forming the
filled polymer will depend upon the specific properties of the
polymer, and the requirements of the specific end use. It is
well-known in the art of filled polymers that inorganic particulate
fillers often cause some degradation in the properties of the neat
polymer. For example, filled polymers are typically stiffer, more
brittle, and less tough than the corresponding neat polymer. The
turbostratic carbon-encapsulated hBN suitable for use herein may
conveniently be melt blended with a thermoplastic polymer and then
cast into films. Melt blending may be effected using milling, high
intensity mixers, or twin screw extruders. It is often desirable to
first create the filled polymer in pellet form and then cast films
from the pellet form. Alternatively, the films may be cast directly
from the blending operation without an intermediate pelletization
step.
[0026] In one embodiment, the treated hBN suitable for use herein
is dispersed in a solution of polyamic acid, the resulting
solution/dispersion formed into a film, and the film imidized to
form a hBN filled polyimide film. The polyimide employed maybe a
homopolymer or a copolymer. In another embodiment, the polyimide so
prepared is not in the shape of a film. For example, the polyimide
filled with turbostratic carbon-coated hBN according to the
invention hereof, can be in the form of stock shapes, such as, but
not limited to, cylinders and prisms.
[0027] It is found that desirable improvements in thermal
conductivity in the direction normal to the plane of a polymeric
film is obtained at loadings ranging from ca. 10% to ca. 60% by
weight. At loadings higher than 60% by weight, degradation of the
properties of the polymer matrix can become excessive, while at
loadings less than 10% by weight little improvement is seen in
thermal conductivity.
[0028] The polymeric composite composition of the present invention
is particularly useful as a dielectric or insulating material in
electronic devices and wire and cable applications requiring good
thermal conductivity in order to provide good cooling.
EXAMPLE 1
[0029] 1 g of white hexagonal boron nitride (hBN) platelet
particles with an average particle size in the range of 0.1 to 5
.mu.m in thickness and 1-200 .mu.m in the maximum in-plane
dimension, as estimated by examination of scanning electron
micrographs of the particles, (PT620, Momentive Performance
Materials) was spread by hand using a stainless steel glazed finish
micro spoon from Bel-Art products onto the rectangular surface of a
quartz boat to a depth of approximately 8 mm. The home-made quartz
boat was inserted into an EasyTube computer-controlled tube furnace
(FirstNano, Ronkonkoma, N.Y.), which was sealed with a PTFE thread
sealant tape from Plastomer Technologies. The thus sealed tube
furnace was purged at room temperature with nitrogen gas (N.sub.2,
scientific grade; GTS-Welco, Allentown, Pa.) for 25 min at a flow
rate of 1 L/min measured using a model 2179A calibrated electronic
flow meter (MKS Instruments, Andover, Mass.). At the end of the
purge, the amount of oxygen (O.sub.2) in the tube furnace was
determined to be 0.1% or less. The partial pressure of oxygen
inside the tube furnace was monitored using an RGA (SRS-100) from
Stanford Research Systems. The temperature of the tube furnace was
ramped to 900.degree. C. in 14 min under N.sub.2 at a flow rate of
1 L/min. When 900.degree. C. was reached, the flow of N.sub.2 gas
was stopped, and hydrogen, methane, and ethylene gases were
introduced at flow rates of 1 L/min, 0.500 L/min, and 0.030 L/min,
respectively. The sample was soaked in the gas mixture for a period
of 2 min, after which the gas flows were stopped, the furnace
turned off, and allowed to cool down to room temperature under
N.sub.2 at a flow rate of 1 L/min for 120 min. The thus produced
carbon coated hBN particles are shown in the quartz boat in FIG. 1.
The hBN had changed from the white color of the neat hBN to black,
indicating that the hBN particles were encapsulated with a
turbostratic carbon coating.
[0030] A scanning electron micrograph (SEM) of the coated hBN
particles so produced showed that the platelet structure of the hBN
was retained (FIG. 2). A transmission electron micrograph (TEM) of
a cross-section of the coated particles showed that the multi-layer
turbostratic carbon coating conformed to the shape of and
encapsulated the hBN platelet particle. The number of carbon layers
which formed the turbostratic structure on the platelets of hBN
ranged from 6 to 10 depending upon the specific location
examined.
[0031] A polyamic acid was prepared from reaction of 100 parts of a
diamine known in the art as RODA having the structure:
##STR00001##
with 80 parts of a dianhydride known in the art as ODPA having the
structure
##STR00002##
and 20 parts of pyromellitic dianhydride, having the structure
##STR00003##
according to Example 4 of U.S. Pat. No. 5,298,331.
[0032] 2.6 g of the thus synthesized polyamic acid was dissolved in
2.2 g of N,N-dimethylacetamide (DMAC) to form a solution in a glass
vial. 0.5 g of the turbostratic carbon coated hBN prepared as
described above was dispersed into the solution so prepared by
stirring overnight with a Color Squid magnetic stirrer from IKA
Works under house vacuum (25 inches of Hg) to form a dispersion.
The dispersion so formed was spread on a clean glass plate
(7.times.7 inches) using a two-path wedge gap film applicator from
GARDCO having an overall width of 6 inches and a gap of 8 mils to
produce a film 50 to 75 .mu.m thick. The thus formed film was first
dried in a VWR vacuum oven connected to house vacuum (25 inches of
Hg) at 80.degree. C. for 1 hour and then imidized in a box furnace
(Barnstead model 6000 Thermolyne) for 30 min at 355.degree. C. in a
nitrogen (UHP grade from GTS-Welco) atmosphere. A half inch
diameter sample was cut from the thus imidized polymer film. The
thermal conductivity normal to the plane of the film was determined
using an LFA457 MicroFlash from NETZSCH Instruments. The thus
prepared film was found to have a thermal conductivity of 1.022
W/m-.degree. K. A polyimide film of the same composition, but
lacking the turbostratic carbon coated hBN, was found to exhibit a
thermal conductivity normal to the plane of the film of only 0.2
W/m-.degree. K.
COMPARATIVE EXAMPLE 1
[0033] The procedures recited in Example 1 for forming a composite
polyimide comprising hBN were followed, but the hBN was used as
received and not subject to the coating procedure described in
Example 1 prior to incorporation into the polyimide. The thermal
conductivity normal to the plane was found to be only 0.574
W/m-.degree. K.
COMPARATIVE EXAMPLE 2
[0034] The procedures recited in Example 1 for coating the hBN were
followed except that the temperature of the tube furnace was ramped
to 750.degree. C. in 11 min and the soak period at 750.degree. C.
was 6 min. The thus treated hBN particles were removed from the
tube furnace after cooling. No change of color from white to black,
as reported in Example 1, had occurred. The color of the powder
remained white showing no evidence of carbon deposition on hBN.
COMPARATIVE EXAMPLE 3
[0035] The procedures recited in Example 1 for coating the hBN were
followed except that (i) 1.5 g of hBN were employed, (ii) the
volumetric flow rates of H.sub.2, methane (CH.sub.4) and ethylene
(C.sub.2H.sub.4) were 1 L/min, 0.500 L/min and 0 L/min,
respectively and (iii) the soak period of 2 min at 900.degree. C.
was repeated 3 times. Between the 2 min soak periods nitrogen was
introduced at a rate of 1 L/min for 10 min. The thus treated hBN
particles were removed from the tube furnace after cooling. No
change of color from white to black, as reported in Example 1, had
occurred. The color of the powder remained white showing no
evidence of carbon deposition on hBN.
EXAMPLE 2
[0036] The procedures recited in Comparative Example 3 were
repeated except that the volumetric flow rates of hydrogen,
methane, and ethylene were 1 L/min, 0 L/min, and 0.030 L/min. The
thus treated hBN particles were removed from the tube furnace after
cooling. A change of color from white to black had occurred.
Thermo-gravimetric analysis (TGA) of the thus treated hBN revealed
a weight decrease of 0.05% at an onset temperature of ca.
600.degree. C., which is characteristic of turbostratic carbon.
FIG. 4 is a TEM image of the coated hBN showing the platelet coated
with turbostratic carbon.
EXAMPLE 3
[0037] The procedures recited in Example 2 were followed except the
volumetric flow rates of hydrogen, methane, and ethylene were 0
L/min, 0.500 L/min and 0.030 L/min, respectively. The thus treated
hBN particles were removed from the tube furnace after cooling. A
change of color from white to black had occurred. TGA of the thus
coated hBN revealed a decrease of 1.90% in weight at an onset
temperature of ca. 600.degree. C. This large weight loss was an
indication that the concentration of H.sub.2 can be used to control
the thickness of the carbon coating. FIG. 5 is a TEM image of the
turbostratic carbon-coated hBN produced in accordance with this
example. Comparing FIG. 5 with FIG. 3, it can be seen that the
coating layer shown in FIG. 5 is generally thicker than that shown
in FIG. 3.
EXAMPLE 4
[0038] The procedures recited in Example 2 were followed except
that the volumetric flow rates of hydrogen, methane, and ethylene
were 1 L/min, 0.500 L/min, and 0.030 L/min, respectively. The thus
treated hBN particles were removed from the tube furnace after
cooling. A change of color from white to black had occurred. TGA of
the thus coated hBN revealed a decrease of 0.74% in weight at an
onset temperature of ca. 600.degree. C. FIG. 6 is a TEM image of
the turbostratic carbon-coated hBN produced in accordance with this
example.
* * * * *