U.S. patent application number 14/911486 was filed with the patent office on 2016-06-30 for heat-dissipating film, and its production method and apparatus.
The applicant listed for this patent is Seiji KAGAWA. Invention is credited to Seiji KAGAWA.
Application Number | 20160185074 14/911486 |
Document ID | / |
Family ID | 52468334 |
Filed Date | 2016-06-30 |
United States Patent
Application |
20160185074 |
Kind Code |
A1 |
KAGAWA; Seiji |
June 30, 2016 |
HEAT-DISSIPATING FILM, AND ITS PRODUCTION METHOD AND APPARATUS
Abstract
A heat-dissipating film comprising a heat-conductive layer
obtained by burning a mixture layer of flaky carbon and a binder
resin to carbonize or burn off the binder resin, and plastic films
covering the heat-conductive layer, the heat-conductive layer
having a density of 1.9 g/cm.sup.3 or more and thermal conductivity
of 450 W/mk or more, is produced by (1) sandwiching a mixture layer
of flaky carbon and a binder resin with a pair of first plastic
films to form a laminated film; (2) heat-pressing the laminated
film to densify the mixture layer; (3) burning the mixture layer to
carbonize the binder resin in the mixture layer; (4) pressing the
resultant burnt layer to form the heat-conductive layer; and (5)
sealing the heat-conductive layer with second plastic films.
Inventors: |
KAGAWA; Seiji;
(Koshigaya-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
KAGAWA; Seiji |
Koshigaya-shi |
|
JP |
|
|
Family ID: |
52468334 |
Appl. No.: |
14/911486 |
Filed: |
August 11, 2014 |
PCT Filed: |
August 11, 2014 |
PCT NO: |
PCT/JP2014/071241 |
371 Date: |
February 11, 2016 |
Current U.S.
Class: |
428/341 ;
156/247; 156/583.1; 428/408 |
Current CPC
Class: |
B32B 9/048 20130101;
B32B 9/045 20130101; B32B 27/365 20130101; B32B 27/06 20130101;
B32B 2457/00 20130101; B32B 2307/302 20130101; B32B 2307/72
20130101; B32B 27/306 20130101; C01B 32/05 20170801; B29D 7/01
20130101; B32B 27/36 20130101; B32B 27/304 20130101; B32B 37/10
20130101; B32B 27/34 20130101; B32B 2260/046 20130101; B32B 27/308
20130101; B32B 27/288 20130101; B32B 9/007 20130101; B32B 2264/00
20130101; B32B 5/16 20130101; B32B 27/32 20130101; B32B 2309/02
20130101; B32B 27/285 20130101; B32B 37/06 20130101; B32B 37/22
20130101; B32B 38/10 20130101; B32B 27/14 20130101; B32B 5/30
20130101; B32B 27/302 20130101; H01L 2924/0002 20130101; B32B 27/18
20130101; B32B 3/04 20130101; B32B 27/281 20130101; B32B 27/286
20130101; B32B 2307/416 20130101; H01L 2924/0002 20130101; B32B
2307/30 20130101; C09K 5/14 20130101; B32B 2264/108 20130101; B32B
37/02 20130101; B32B 2037/243 20130101; B32B 2250/40 20130101; B32B
2260/025 20130101; H01L 23/3737 20130101; H01L 2924/00
20130101 |
International
Class: |
B32B 5/16 20060101
B32B005/16; B32B 37/10 20060101 B32B037/10; B32B 37/06 20060101
B32B037/06; B32B 27/32 20060101 B32B027/32 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 12, 2013 |
JP |
2013-167925 |
Claims
1. A heat-dissipating film comprising a heat-conductive layer
obtained by burning a mixture layer of flaky carbon and a binder
resin to carbonize or burn off said binder resin, and plastic films
covering said heat-conductive layer; said flaky carbon in said
heat-conductive layer being aligned substantially in parallel; and
said heat-conductive layer having a density of 1.9 g/cm.sup.3 or
more and thermal conductivity of 450 W/mk or more.
2. The heat-dissipating film according to claim 1, wherein said
flaky carbon is graphite or graphene.
3. The heat-dissipating film according to claim 1, wherein said
heat-conductive layer has a density of 2 g/cm.sup.3 or more and
thermal conductivity of 600 W/mk or more.
4. The heat-dissipating film according to claim 1, wherein said
heat-conductive layer has a thickness of 50-250 g/m.sup.2
(expressed by the weight of flaky carbon per 1 m.sup.2).
5. (canceled)
6. A method for producing a heat-dissipating film comprising the
steps of (1) sandwiching a mixture layer of flaky carbon and a
binder resin with a pair of first plastic films to form a laminated
film; (2) heat-pressing said laminated film to densify said mixture
layer; (3) burning said mixture layer exposed by peeling said first
plastic films to carbonize or burn off said binder resin in said
mixture layer; (4) pressing the resultant burnt layer to form a
densified heat-conductive layer; and (5) sealing said
heat-conductive layer with second plastic films.
7. The method for producing a heat-dissipating film according to
claim 6, wherein a step of applying a dispersion comprising 5-25%
by mass of flaky carbon and 0.05-2.5% by mass of a binder resin in
an organic solvent, a mass ratio of said binder resin to said flaky
carbon being 0.01-0.1, to a surface of each first plastic film, and
then drying said dispersion is repeated plural times, to form said
mixture layer.
8. The method for producing a heat-dissipating film according to
claim 6, wherein the amount of said dispersion applied by one
operation is 5-30 g/m.sup.2 (expressed by the weight of flaky
carbon per 1 m.sup.2).
9. The method for producing a heat-dissipating film according to
claim 6, wherein said binder resin is an acrylic resin, a
polystyrene resin or polyvinyl alcohol.
10. The method for producing a heat-dissipating film according to
claim 6, wherein said laminated film is heat-pressed by passing
through at least a pair of heating rolls.
11. The method for producing a heat-dissipating film according to
claim 10, wherein the heat-pressing temperature is 150-250.degree.
C., and the heat-pressing pressure is 20 MPa or more.
12. The method for producing a heat-dissipating film according to
claim 6, wherein to carbonize said binder resin in said mixture
layer, said mixture layer is burned by exposing both surfaces
thereof to a flame, or exposing a die containing said mixture layer
to a high temperature of 500.degree. C. or higher.
13. The method for producing a heat-dissipating film according to
claim 6, wherein said burnt layer is pressed in a die constraining
at least a pair of opposing sides of said burnt layer.
14. The method for producing a heat-dissipating film according to
claim 6, wherein a lower die part having a cavity and an upper die
part having a projection received in said cavity are used wherein
after said burnt layer is put in the cavity of said lower die part,
said upper die part is combined with said lower die part such that
said projection enters said cavity; and wherein the combined upper
and lower die parts are caused to pass through a gap between a pair
of pressing rolls plural times, to densify said burnt layer.
15. The method for producing a heat-dissipating film according to
claim 13, wherein said burnt layer is pressed while vibrating a die
containing said burnt layer.
16. An apparatus for producing a heat-dissipating film comprising
(a) a means for conveying a pair of first plastic films; (b) at
least one dispersion-applying means disposed for each first plastic
film, such that a dispersion comprising flaky carbon and a binder
resin is applied to said first plastic film plural times; (c) a
means for drying each applied dispersion; (d), a means for
laminating a pair of said first plastic films each having the
resultant mixture layer comprising said flaky carbon and said
binder resin, with the said mixture layer inside; (e) a means for
heat-pressing the resultant laminated film; (f) a means for peeling
said first plastic films from said laminated film; (g) a means for
burning said mixture layer, such that the exposed binder resin in
said mixture layer is carbonized; (h) a pressing die means for
densifying the resultant burnt layer to form a heat-conductive
layer; and (i) a means for sealing said heat-conductive layer with
second plastic films.
17. The apparatus for producing a heat-dissipating film according
to claim 16, which comprises pluralities of dispersion-applying
means arranged with predetermined intervals along the movement
direction of each first plastic film.
18. The apparatus for producing a heat-dissipating film according
to claim 16, wherein a pair of dispersion-applying means and
laminating rolls are disposed in said chamber; and wherein said
chamber comprises first openings through which the first plastic
films enter, a pair of hot air inlets disposed near said first
openings, a gas outlet, and a second opening through which said
laminated film exits.
19. The apparatus for producing a heat-dissipating film according
to claim 16, wherein said means for burning said mixture layer is a
burner ejecting a flame onto each surface of said mixture layer, or
a die containing said mixture layer and exposed to a high
temperature of 500.degree. C. or higher.
20. The apparatus for producing a heat-dissipating film according
to claim 16, wherein said pressing die means for densifying the
burnt layer comprises (a) a die comprising a lower die part having
a cavity, and an upper die part having a projection engageable with
said cavity; (b) a pair of pressing rolls for pressing said die
from above and below; (c) guide plates extending upstream and
downstream of a gap of said rolls; and (d) a means for
reciprocating said die along said guide plates, such that it passes
through a gap between a pair of said pressing rolls.
21. (canceled)
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a heat-dissipating film for
efficiently dissipating heat generated from electronic devices,
etc. in small electronic appliances such as note-type personal
computers, smartphones, mobile phones, etc., and its production
method and apparatus.
BACKGROUND OF THE INVENTION
[0002] In small electronic appliances such as note-type personal
computers, smartphones, mobile phones, etc., which have been
provided with increasingly higher performance and more functions,
electronic devices such as microprocessors, imaging chips,
memories, etc. should be mounted densely. Accordingly, to prevent
malfunction due to heat, the dissipation of heat generated from
such electronic devices has become increasingly important.
[0003] As a heat-dissipating sheet composed of flaky carbon such as
graphite for electronic devices, a graphite sheet obtained by
heat-treating polyimide at 3000.degree. C. in an oxygen-free
atmosphere to remove hydrogen, oxygen and nitrogen, and annealing
the remaining carbon for crystallization, is used. The graphite
sheet has as high thermal conductivity as 800 W/mk in an in-plane
direction and 15 W/mk in a thickness direction. However, because
expensive polyimide is heat-treated at a high temperature, the
graphite sheet is extremely expensive.
[0004] As an inexpensive heat-dissipating sheet of flaky carbon, JP
2006-306068 A discloses a heat-conductive sheet comprising at least
a graphite film and an adhesive resin composition, which is a
reaction-curable vinyl polymer. This graphite film is (a) expanded
graphite formed by an expanding method, or (b) obtained by
heat-treating a polyimide film, etc., at a temperature of
2400.degree. C. or higher. The expanded graphite film is obtained
by immersing graphite in acid such as sulfuric acid, etc. to form a
graphite interlayer compound, heat-treating the graphite interlayer
compound to foam it, thereby separating graphite layers, washing
the resultant graphite powder to remove acid, and rolling the
resultant thin-film graphite powder. However, the expanded graphite
film has insufficient strength. Also, the graphite film obtained by
the heat treatment of a polyimide film, etc. is disadvantageously
expensive despite high heat dissipation.
[0005] JP 2012-211259 A discloses a heat-conductive sheet
comprising graphite pieces, which comprise pluralities of first
graphite pieces obtained by thinly cutting a thermally decomposed
graphite sheet, and second graphite pieces smaller than the widths
of the first graphite pieces, at least the first graphite pieces
connecting both surfaces of the heat-conductive sheet. This
heat-conductive sheet is obtained, for example, by blending the
first and second graphite pieces with a mixture of an acrylic
polymer and a solvent, and extruding the resultant blend. However,
the extruded heat-conductive sheet does not have sufficient heat
dissipation, because of a high volume fraction of the resin.
[0006] JP 2006-86271 A discloses a heat-dissipating sheet as thick
as 50-150 .mu.m comprising graphite bonded by a binder resin having
a glass transition temperature of -50.degree. C. to +50.degree. C.,
such as an amorphous saturated copolyester, a mass ratio of
graphite/binder resin being 66.7/33.3 to 95/5. This
heat-dissipating sheet is produced by applying a slurry of graphite
and a binder resin in an organic solvent to a parting-agent-coated
film on the side of a parting layer, drying the slurry by hot air
to remove the organic solvent, and then pressing it, for example,
at 30 kg/cm.sup.2. JP 2006-86271 A describes that the pressing of a
graphite/binder resin sheet improves its thermal conductivity.
Though a slurry of graphite, a binder resin and an organic solvent
is coated by one step in JP 2006-86271 A, it has been found that
one-step coating provides a non-uniform graphite distribution. In
addition, because a mass ratio of graphite to a binder resin is not
so high in Examples (80/20 in Example 1, and 89/11 in Example 2),
inherently high thermal conductivity of graphite cannot be fully
used.
[0007] JP 11-1621 A discloses a high-thermal-conductivity, solid
composite material for a heat dissipater comprising highly oriented
graphite flakes and a binder polymer polymerized under pressure.
This solid composite material is produced by mixing graphite flakes
with a thermosetting monomer such as an epoxy resin to prepare a
composition comprising at least 40% by volume of graphite, and
polymerizing the monomer while compressing the composition under
sufficient pressure to align graphite substantially in parallel. JP
11-1621 A describes that the volume fraction of graphite in the
composite material can be 40-95%, and is preferably 55-85%.
However, graphite flakes are unevenly distributed in an epoxy resin
containing graphite flakes at as high a concentration as 95%.
Accordingly, JP 11-1621 A describes only experimental results when
the volume fraction of graphite flakes is 60%.
[0008] As described above, conventional heat-dissipating sheets
containing graphite blended with binder resins cannot sufficiently
use high thermal conductivity of graphite because of low thermal
conductivity of binder resins. In addition, when the distribution
of graphite is non-uniform, the heat-dissipating film exhibits
further reduced heat dissipation, and provides non-uniform graphite
distribution in a heat-dissipating film cut to a predetermined
shape and size for being disposed in a small electronic appliance,
resulting in unevenness performance.
OBJECT OF THE INVENTION
[0009] Accordingly, the first object of the present invention is to
provide an inexpensive heat-dissipating film capable of exhibiting
excellent heat dissipation when disposed in small electronic
appliances, because flaky carbon is densely and uniformly
distributed.
[0010] The second object of the present invention is to provide a
method and an apparatus for producing such a heat-dissipating film
at low cost.
DISCLOSURE OF THE INVENTION
[0011] As a result of intensive research in view of the above
objects, the inventor has found that when a composite film
comprising flaky carbon having excellent thermal conductivity and a
small amount of a binder resin is burned and pressed, the binder
resin is carbonized or burned off, resulting in a heat-dissipating
film having a heat-conductive layer composed of densely bonded
flaky carbon. The present invention has been completed based on
such finding.
[0012] Thus, the heat-dissipating film of the present invention
comprises a heat-conductive layer obtained by burning a mixture
layer of flaky carbon and a binder resin to carbonize or burn off
the binder resin, and plastic films covering the heat-conductive
layer; the flaky carbon being aligned substantially in parallel in
the heat-conductive layer; and the heat-conductive layer having a
density of 1.9 g/cm.sup.3 or more and thermal conductivity of 450
W/mk or more.
[0013] The flaky carbon is preferably graphite or graphene.
[0014] The heat-conductive layer preferably has a density of 2
g/cm.sup.3 or more. Also, the heat-conductive layer preferably has
thermal conductivity of 600 W/mK or more.
[0015] The heat-conductive layer preferably has a thickness
(expressed by the weight of flaky carbon per 1 m.sup.2) of 50-250
g/m.sup.2.
[0016] The heat-dissipating film preferably has surface resistivity
of 20 .OMEGA./square or less. The heat-dissipating film preferably
has an electromagnetic wave-shielding ratio (reflection ratio) of
90% or more.
[0017] The method of the present invention for producing the above
heat-dissipating film comprises the steps of (1) sandwiching a
mixture layer of flaky carbon and a binder resin with a pair of
first plastic films to form a laminated film; (2) heat-pressing the
laminated film to densify the mixture layer; (3) burning the
mixture layer exposed by peeling the first plastic films to
carbonize or burn off the binder resin in the mixture layer; (4)
pressing the resultant burnt layer to form a densified
heat-conductive layer; and (5) sealing the heat-conductive layer
with second plastic films.
[0018] The mixture layer of the flaky carbon and the binder resin
preferably comprises 5-25% by mass of flaky carbon and 0.05-2.5% by
mass of a binder resin, and is formed preferably by repeating a
step of applying an organic solvent dispersion, in which a mass
ratio of the binder resin to the flaky carbon is 0.01-0.1, to a
surface of each first plastic film and then drying the dispersion
plural times.
[0019] The amount of the dispersion coated by one application is
preferably 5-30 g/m.sup.2 (expressed by the weight of flaky carbon
per 1 m.sup.2).
[0020] The binder resin is preferably an acrylic resin, a
polystyrene resin or polyvinyl alcohol.
[0021] The organic solvent is preferably at least one selected from
the group consisting of ketones, aromatic hydrocarbons and
alcohols.
[0022] The application of the dispersion is preferably carried out
by a spraying method.
[0023] The first plastic film preferably has a parting layer on a
surface to be coated with the dispersion. The parting layer is
preferably a deposited aluminum layer.
[0024] The drying step is carried out preferably at 30-100.degree.
C.
[0025] The heat pressing of the laminated film is carried out
preferably by passing the laminated film between at least a pair of
heating rolls. The heat-pressing temperature is preferably
150-250.degree. C. The heat-pressing pressure is preferably 20 MPa
or more.
[0026] The mixture layer is burned preferably by applying both
surfaces thereof to flame, or exposing a die containing the mixture
layer to a high temperature of 500.degree. C. or higher, so that
the binder resin in the mixture layer is carbonized. The mixture
layer can be burned in air or an inert gas, or in vacuum.
[0027] The burnt layer is preferably pressed in a die constraining
at least a pair of opposing sides of the burnt layer.
[0028] It is preferable that after the burnt layer is put in a
cavity of the lower die part, the upper die part is combined with
the lower die part such that the projection enters the cavity, and
the combined upper and lower die parts are caused to pass through a
gap between a pair of pressing rolls plural times, thereby
densifying the burnt layer.
[0029] The burnt layer in the die is preferably pressed under
vibration.
[0030] The second plastic film is preferably thinner than the first
plastic film. The heat-conductive layer is preferably heat-sealed
by the second plastic films. The second heat-sealing plastic film
preferably has a sealant layer on a side attached to the
heat-conductive layer.
[0031] The heat-conductive layer is preferably cut to a desired
shape, and then sealed with the second plastic films.
[0032] The apparatus of the present invention for producing the
above heat-dissipating film comprises (a) means for conveying a
pair of first plastic films; (b) at least one dispersion-applying
means disposed for each first plastic film for applying a
dispersion comprising flaky carbon and a binder resin to the first
plastic film plural times; (c) a means for drying the dispersion
after each application; (d) a means for laminating a pair of the
first plastic films each having the resultant mixture layer of the
flaky carbon and the binder resin, with the mixture layer inside;
(e) a means for heat-pressing the resultant laminated film; (f) a
means for peeling the first plastic films from the laminated film;
(g) a means for burning the mixture layer to carbonize or burn off
the binder resin in the exposed mixture layer; (h) a pressing die
means for densifying the resultant burnt layer to form a
heat-conductive layer; and (i) a means for sealing the
heat-conductive layer with the second plastic films.
[0033] The apparatus of the present invention preferably comprises
pluralities of dispersion-applying means arranged with
predetermined intervals along the moving direction of each first
plastic film.
[0034] It is preferable that a pair of dispersion-applying means
and the laminating rolls are disposed in the chamber, and that the
chamber comprises first openings through which the first plastic
films enter, a pair of hot air inlets disposed near the first
openings, a gas outlet, and a second opening through which the
laminated film exits. In the case of comprising plural pairs of
dispersion-applying means, all dispersion-applying means are
preferably disposed in the chamber.
[0035] Each first plastic film is preferably conveyed horizontally
on both sides of the laminating rolls by the means (a).
[0036] The dispersion-applying means is preferably a spraying
nozzle.
[0037] Both of the laminating means and the heat pressing means are
preferably heat rolls.
[0038] The means for burning the mixture layer is preferably a
burner ejecting a flame to both surfaces of the mixture layer, or a
die containing the mixture layer and exposed to a high temperature
of 500.degree. C. or higher.
[0039] The pressing die means for densifying the burnt layer
preferably comprises (a) a die comprising a lower die part having a
cavity, and an upper die part having a projection engageable with
the cavity; (b) a pair of rolls for pressing the die from above and
below; (c) guide plates extending upstream and downstream of a roll
gap; and (d) a means for reciprocating the die along the guide
plates, such that it passes through a gap between a pair of the
pressing rolls.
[0040] The pressing die means preferably further comprises a means
for giving vibration to one of the pressing rolls, thereby
densifying the burnt layer by pressing under vibration
BRIEF DESCRIPTION OF THE DRAWINGS
[0041] FIG. 1 is a cross-sectional view showing the
heat-dissipating film of the present invention.
[0042] FIG. 2 is a schematic view showing a method for determining
the particle size of flaky carbon.
[0043] FIG. 3 is a cross-sectional view showing a state where flaky
carbon is agglomerated as a result of thickly applying a flaky
carbon/binder resin dispersion on a plastic film.
[0044] FIG. 4 is a cross-sectional view schematically showing a
state where flaky carbon is uniformly dispersed as a result of
thinly applying a dispersion on a plastic film.
[0045] FIG. 5 is a cross-sectional view showing a state where a
flaky carbon dispersion is thinly applied after the dispersion
applied to the plastic film is dried.
[0046] FIG. 6 is a cross-sectional view showing the apparatus of
the present invention for producing a heat-dissipating film.
[0047] FIG. 7 is a plan view showing a cutting line along which a
margin of a mixture layer is cut off
[0048] FIG. 8(a) is a schematic cross-sectional view showing an
example of burning methods of a mixture layer.
[0049] FIG. 8(b) is a schematic cross-sectional view showing
another example of burning methods of a mixture layer.
[0050] FIG. 9(a) is a plan view showing a lower die part in a burnt
layer-pressing apparatus.
[0051] FIG. 9(b) is an exploded cross-sectional view showing upper
and lower die parts in a burnt layer-pressing apparatus.
[0052] FIG. 10 is a cross-sectional view showing the pressing of a
burnt layer by a die comprising upper and lower die parts.
[0053] FIG. 11(a) is a partial cross-sectional view showing the
pressing of the die of FIG. 10 by a pair of pressing rolls.
[0054] FIG. 11(b) is a partial cross-sectional view showing the
repeated pressing of the die of FIG. 10 by a pair of pressing
rolls.
[0055] FIG. 12(a) is a plan view showing a burnt layer-pressing
die.
[0056] FIG. 12(b) is a cross-sectional view showing a burnt
layer-pressing die.
[0057] FIG. 13 is a plan view showing a cutting line along which a
margin of the pressed heat-conductive layer is cut off to provide
the pressed heat-conductive layer with a predetermined size.
[0058] FIG. 14 is a plan view showing cutting lines along which the
pressed heat-conductive layer is divided to those having a final
shape.
[0059] FIG. 15 is a cross-sectional view showing a method of
laminating a second plastic film of FIG. 13 having heat-conductive
layers to another second plastic film.
[0060] FIG. 16(a) is a plan view showing heat-conductive layers of
a final shape attached to a second plastic film.
[0061] FIG. 16(b) is a plan view showing a laminate comprising a
second plastic film to which heat-conductive layers of a final
shape are attached, and another second plastic film.
[0062] FIG. 17 is a cross-sectional view showing a method of
laminating a second plastic film having heat-conductive layers of a
final shape to another second plastic film.
[0063] FIG. 18 is a plan view showing cutting lines for dividing a
laminate having heat-conductive layers of a final shape to
individual heat-dissipating films.
[0064] FIG. 19 is a cross-sectional view showing a heat-dissipating
film comprising a heat-conductive layer sealed by a pair of second
plastic films.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0065] The embodiments of the present invention will be explained
in detail below referring to the attached drawings without
intention of restriction, and various modifications may be made
within the scope of the present invention. Unless otherwise
mentioned, explanations of one embodiment are applicable to other
embodiments.
[0066] [1] Heat-Dissipating Film
[0067] As shown in FIG. 1, the heat-dissipating film 1 of the
present invention comprises a heat-conductive layer 10, and a pair
of plastic films 2, 2 adhered to both surfaces of the
heat-conductive layer 10 for sealing.
[0068] (1) Heat-Conductive Layer
[0069] The heat-conductive layer 10 obtained by burning a mixture
layer of flaky carbon and a binder resin is mainly composed of
flaky carbon. The flaky carbon is preferably graphite or graphene.
Unless otherwise mentioned, they are explained as flaky carbon
below.
[0070] (a) Flaky Carbon
[0071] Flaky graphite and flaky graphene have a planar multi-layer
structure constituted by two-dimensionally connected benzene rings.
Because both of them have a hexagonal lattice structure, each
carbon atom is bonded to three carbon atoms, one of four peripheral
electrons used for chemical bonding being in a free state (free
electron). Because free electrons can move along the crystal
lattice, flaky graphite has high thermal conductivity.
[0072] Because both of them have a plate-like shape, their sizes
are represented by the diameters of their planar surfaces. Because
flaky graphite (graphene) 31 has a planar contour of an irregular
shape as shown in FIG. 2, the size (diameter) of each flaky
graphite (graphene) 31 is defined as a diameter d of a circle
having the same area S. Because the size of each flaky graphite
(graphene) 31 is expressed by a diameter d and a thickness t, the
average diameter of flaky graphite (graphene) 31 used is expressed
by (Ed)/n, wherein n represents the number of flaky graphite
(graphene) 31 measured, and the average thickness of flaky graphite
(graphene) 31 is expressed by (Et)/n. The diameter d and thickness
t of flaky graphite (graphene) 31 can be determined by the image
treatment of a photomicrograph of flaky graphite (graphene) 31.
[0073] The average diameter of flaky graphite may be in a range of
5-100 .mu.m. When the average diameter of flaky graphite is less
than 5 .mu.m, bonded carbon atoms are not sufficiently long,
providing the heat-conductive layer 10 with too small thermal
conductivity. On the other hand, flaky graphite having an average
diameter of more than 100 .mu.m would make spray coating difficult.
The average diameter of flaky graphite is preferably 5-50 .mu.m,
more preferably 10-30 .mu.m. The average thickness of flaky
graphite is 200 nm or more, preferably 200 nm to 10 .mu.m, more
preferably 200 nm to 5 .mu.m.
[0074] The average diameter of flaky graphene may be in a range of
5-100 .mu.m. When the average diameter of flaky graphene is less
than 5 .mu.m, bonded carbon atoms are not sufficiently long,
providing the heat-conductive layer 10 with too small thermal
conductivity. On the other hand, flaky graphene having an average
diameter of more than 100 .mu.m would make spray coating difficult.
The average diameter of flaky graphene is preferably 5-50 .mu.m,
more preferably 10-30 .mu.m. The average thickness of flaky
graphene may be in a range of 5-50 nm. When the average thickness
of flaky graphene is less than 5 nm, the heat-conductive layer 10
obtained by burning the flaky graphene/binder resin mixture has
large resistance. On the other hand, when the average thickness of
flaky graphene is more than 50 nm, flaky graphene is easily broken
when it is uniformly dispersed in a solvent. The average thickness
of flaky graphene is preferably 5-30 nm, more preferably 10-25
nm.
[0075] (b) Binder Resin
[0076] The binder resin is not particularly restricted, as long as
it can be dissolved in an organic solvent and can uniformly
disperse flaky carbon. It may be acrylic resins such as
polymethylacrylate and polymethylmethacrylate, polystyrenes,
polycarbonates, polyvinyl chloride, ABS resins, low-stereospecific
polypropylene, atactic polypropylene, etc. Among them,
polymethylmethacrylate, polystyrenes and low-stereospecific
polypropylene are preferable.
[0077] (c) Comprising Ratio
[0078] A smaller mass ratio of the binder resin to flaky carbon can
provide the heat-conductive layer 10 with higher density and
thermal conductivity. However, too low a percentage of the binder
resin provides insufficient strength of adhering flaky carbon in
the mixture layer, so that the mixture layer is easily broken. To
have high thermal conductivity and strength, the binder resin/flaky
carbon mass ratio is preferably 0.01-0.1. The upper limit of the
binder resin/flaky carbon mass ratio is more preferably 0.08, most
preferably 0.06. Though the lower limit of the binder resin/flaky
carbon mass ratio is preferably as low as possible as long as flaky
carbon is securely bonded, it is 0.01 as a technical limit, and
practically 0.03.
[0079] (d) Uniform Distribution of Flaky Carbon
[0080] If flaky carbon were not uniformly distributed in the
mixture layer, (a) flaky carbon would be agglomerated, generating
regions with insufficient fine flaky carbon, and thus failing to
provide the heat-dissipating film with desired thermal
conductivity; and (b) the resultant heat-dissipating film would
have non-uniform thermal conductivity distribution, providing
insufficient thermal conductivity when divided to pieces for
individual electronic appliances or parts. To obtain a
heat-conductive layer with uniform thermal conductivity
distribution, a mixture layer of uniformly distributed flaky carbon
should be formed in each application step.
[0081] When flaky carbon is agglomerated in the mixture layer, the
mixture layer has regions in which flaky carbon is agglomerated,
and regions free of or scarcely containing flaky carbon. Because
the existence of regions free of or scarcely containing flaky
carbon provides the heat-conductive layer with low thermal
conductivity as a whole, flaky carbon should be dispersed as
uniformly as possible.
[0082] (e) Surface Resistivity
[0083] The heat-dissipating film of the present invention can also
function as an electromagnetic wave-shielding film. To exhibit a
sufficient electromagnetic wave-shielding function, the surface
resistivity of the heat-conductive layer 10 is preferably 20
.OMEGA./square or less, more preferably 10 .OMEGA./square or less.
The surface resistivity is measured by a DC two-terminal method on
a square specimen of 10 cm.times.10 cm cut out of the
heat-conductive layer 10.
[0084] (f) Thickness
[0085] The thermal conductivity of the heat-conductive layer 10
depends on the thickness of the heat-conductive layer 10. Because
what largely contributes to thermal conductivity is flaky carbon in
the heat-conductive layer 10 composed of flaky carbon and a binder
resin at least partially carbonized by burning, the thickness of
the heat-conductive layer 10 is preferably expressed by the amount
of flaky carbon per a unit area. The thickness of the
heat-conductive layer 10, which is expressed by the amount of flaky
carbon per a unit area, is preferably 50-250 g/m.sup.2, more
preferably 70-220 g/m.sup.2, most preferably 80-200 g/m.sup.2.
[0086] (2) Plastic Film
[0087] Resins forming the plastic film are not particularly
restricted, as long as they have sufficient strength, flexibility
and formability in addition to insulation. They may be, for
example, polyesters (polyethylene terephthalate, etc.), polyarylene
sulfide (polyphenylene sulfide, etc.), polyether sulfone, polyether
ether ketone, polyamides, polyimides, polyolefins (polypropylene,
etc.), etc. The thickness of the plastic film may be about 5-20
.mu.m.
[0088] (3) Cutting of Heat-Dissipating Sheet
[0089] Because a relatively large heat-dissipating sheet can be
formed in the present invention, it may be cut to a proper size, so
that it is attached to small electronic appliances. In such a case,
the heat-conductive layer 10 is preferably cut with a cutter having
flat portions on both sides of each blade, while fusing a cut
portion of the plastic film 2 by heating or ultrasonic waves, to
avoid the cut cross section of the heat-conductive layer 10 from
being exposed. Alternatively, as described below, the
heat-conductive layer 10 may be first cut to a predetermined size,
and laminated with plastic films 2, which are then cut.
[0090] [2] Apparatus and Method for Producing Heat-Dissipating
Film
[0091] FIG. 6 schematically shows an apparatus 100 for producing a
heat-dissipating film. In the depicted example, the apparatus
comprises a pair of dispersion-applying means laterally arranged.
The production apparatus 100 comprises (a) a chamber 4 comprising a
pair of dispersion-applying regions 14a, 14b each having an inlet
41a, 41b of each first plastic film 12a, 12b and a hot air inlet
42a, 42b, and a gas outlet 43 disposed at a position at which both
dispersion-applying regions 14a, 14b are merged; (b) a pair of
nozzles 45a, 45b mounted to a ceiling of the chamber 4 in the
dispersion-applying regions 14a, 14b for spraying a flaky
carbon/binder resin dispersion to form mixture layers 11a, 11b on
the first plastic films 12a, 12b; (c) a pair of rolls 46a, 46b for
laminating the first plastic films 12a, 12b having the mixture
layers 11a, 11b with the mixture layers 11 a, 11 b inside; (d) at
least a pair of heating rolls (two pairs of heating rolls 47a, 47b,
48a, 48b in the depicted example) for heat-pressing the resultant
laminated film 1'; (e) a guide roll 49 for conveying the laminated
film 1'; (f) a pair of rolls 101a, 101b for peeling the first
plastic films 12a, 12b from the laminated film 1'; (g) a means (not
shown) for burning the exposed mixture layer 11; (h) a means 120
for pressing the resultant burnt layer 110; (i) a pair of rolls
102a, 102b for sealing the resultant heat-conductive layer 10 with
a pair of second plastic films 13a, 13b; and (j) a reel 60 for
winding up the resultant heat-dissipating film 1. The first plastic
films 12a, 12b wound off from reels 70a, 70b are sent to the
openings 41 a, 41 b laterally arranged in the chamber 4 via
pluralities of guide rolls. Though each dispersion-applying region
14a, 14b has one dispersion-spraying nozzle 45a, 45b in the
depicted example, of course, each dispersion-applying region 14a,
14b may have pluralities of nozzles.
[0092] To withstand the laminating step and the heat-pressing step,
the first plastic films 12a, 12b should have sufficient mechanical
strength and heat resistance. Accordingly, the first plastic films
12a, 12b are preferably relatively thick films made of
heat-resistant resins. As the heat-resistant resins, polyethylene
terephthalate, polyimides, etc. are preferable. The first plastic
films 12a, 12b are preferably as thick as 20-60 .mu.m. The first
plastic film 12 can be used again after peeling.
[0093] However, the use of thick plastic films having low thermal
conductivity provides a heat-dissipating sheet 1 having a structure
comprising a heat-conductive layer 10 covered with plastic films on
both surfaces with low thermal conductivity. Accordingly, the
plastic films covering both surfaces of the heat-conductive layer
10 should be as thin as possible. Thus, the second plastic films
13a, 13b are preferably as thick as 5-15 .mu.m. Each second plastic
film 13a, 13b preferably has a sealant layer, such that it is
strongly fused to the heat-conductive layer 10 by a heat lamination
method, etc. Though materials for the second plastic films 13a, 13b
may be the same as those for the first plastic films 12a, 12b, it
is practically preferable to use commercially available extremely
thin polyethylene terephthalate films.
[0094] The chamber 4 is provided with a pair of opened vertical
walls 4a, 4b on both lateral sides of the gas outlet 43, and a
region partitioned by each opened vertical wall 4a, 4b is a
dispersion-applying region 14a, 14b having each nozzle 45a, 45b.
There are horizontal plates 44a, 44b supporting the first plastic
films 12a, 12b on both lateral sides of the laminating rolls 46a,
46b, and each first plastic film 12a, 12b is horizontally conveyed
on each horizontal plate 44a, 44b.
[0095] (1) Preparation of Flaky Carbon Dispersion
[0096] A dispersion comprising flaky carbon, a binder resin and an
organic solvent is preferably prepared by mixing a dispersion of
flaky carbon in an organic solvent with a solution of a binder
resin in an organic solvent. This is due to the fact that because
flaky carbon is easily agglomerated, the simultaneous mixing of
flaky carbon and a binder resin with an organic solvent likely
results in the agglomeration of flaky carbon. In a flaky carbon
dispersion obtained by mixing both solutions, the concentration of
flaky carbon is preferably 5-25% by mass, more preferably 8-20% by
mass, most preferably 10-20% by mass. A mass ratio of the binder
resin to flaky carbon is 0.01-0.1. The mass ratio of the binder
resin to flaky carbon is kept in the mixture layer without
change.
[0097] Organic solvents used for the dispersion are preferably
those well dispersing flaky carbon, dissolving the binder resin,
and easily evaporating to shorten the drying time. Examples of such
organic solvents include ketones such as methyl ethyl ketone,
aliphatic hydrocarbons such as hexane, etc., aromatic hydrocarbons
such as xylene, etc., and alcohols such as isopropyl alcohol, etc.
Among them, methyl ethyl ketone, xylene, etc. are preferable. They
may be used alone or in combination.
[0098] (2) Application and Drying of Dispersion
[0099] It has been found that when a dispersion having a desired
concentration is applied to a plastic film by one operation, flaky
carbon 31 in the dispersion 3 is agglomerated in a drying process
as schematically shown in FIG. 3. In FIG. 3, 33 represents regions
in which the flaky carbon 31 is agglomerated. Intensive research
has revealed that when a dispersion is divided to as small amounts
as possible and applied plural times, the agglomeration of flaky
carbon 31 can be prevented. In the first application shown in FIG.
4, a dispersion layer 3a is in a small amount, and its thickness is
sufficiently small relative to an average diameter of flaky carbon
31, so that the dispersion of flaky carbon 31 is kept without
agglomeration when the dispersion layer 3a is dried. Accordingly,
flaky carbon 31 bonded with a trace amount of a binder resin is
substantially uniformly distributed in a mixture layer 3a' obtained
by drying the dispersion layer 3a.
[0100] The amount of a dispersion applied by one operation is
preferably 5-30 g/m.sup.2, more preferably 7-20 g/m.sup.2, as the
weight of flaky carbon per a unit area. When the amount of a
dispersion is less than 5 g/m.sup.2, it takes too much time to form
the mixture layer. When the amount of a dispersion applied by one
operation exceeds 30 g/m.sup.2, flaky carbon is likely
agglomerated. To apply such a small amount of a dispersion
uniformly, a spraying method is preferable.
[0101] After the dispersion layer 3a is dried, the next application
is carried out. Though the dispersion layer 3a may be spontaneously
dried, heating may be carried out to such an extent as to avoid the
deformation of the plastic film, to reduce the application time.
The heating temperature is determined depending on the boiling
point of an organic solvent used. For example, when methyl ethyl
ketone is used, the heating temperature is preferably
30-100.degree. C., more preferably 50-80.degree. C. Drying need not
be carried out until an organic solvent in the applied dispersion
layer 3a is completely evaporated, but may be carried out to such a
level that flaky carbon does not become free in the next
application.
[0102] When the second application of a dispersion is carried out
on the dried first mixture layer 3a', a new dispersion layer 3b is
formed substantially without dissolving the first mixture layer
3a', as schematically shown in FIG. 5. When the dispersion layer 3b
is dried, the second mixture layer 3b' is formed integrally with
the first mixture layer 3a'. Thus, by repeating a cycle of applying
and drying the dispersion plural times, a relatively thick,
integral mixture layer comprising flaky carbon aligned in parallel
is obtained. The number of cycles of applying and drying the
dispersion is determined depending on the thickness of a mixture
layer to be formed.
[0103] To carry out the above step of applying and drying the
dispersion by the apparatus 100 shown in FIG. 6, the first plastic
films 12a, 12b wound off from the reels 70a, 70b are first stopped
in the chamber 4. In this state, a portion of each first plastic
film 12a, 12b in each dispersion-applying region 14a, 14b is
uniformly sprayed with a dispersion ejected from each nozzle 45a,
45b. To achieve the uniform application of a dispersion, each
nozzle 45a, 45b is freely movable. A dispersion layer formed on
each of the first plastic films 12a, 12b is dried by hot air. By
repeating this step of applying and drying a dispersion plural
times, a mixture layer 11a, 11b having predetermined thickness is
formed on each of the first plastic films 12a, 12b.
[0104] (3) Lamination and Heat Pressing of Mixture Layer
[0105] When the first plastic films 12a, 12b, on which the mixture
layers 11a, 11b are formed, are laminated by a pair of rolls 46a,
46b with the mixture layers 11a, 11b inside, the mixture layers
11a, 11b are adhered to each other to form an integral mixture
layer 11.
[0106] The laminating rolls 46a, 46b are preferably heated to a
high temperature, such that the mixture layers 11a, 11b are
completely fused to each other by lamination. Though variable
depending on the type of the binder resin, the temperature of the
laminating rolls 46a, 46b is preferably 100-250.degree. C., more
preferably 150-200.degree. C. The pressing pressure of the
laminating rolls 46a, 46b may not be high, but may be, for example,
1-10 MPa.
[0107] To a new portion of each first plastic film 12a, 12b
reaching the dispersion-applying region 14a, 14b, a step of
applying and drying a dispersion is repeated plural times, to form
each mixture layer 11a, 11b having predetermined thickness. Thus,
after the step of applying and drying a dispersion is repeated
plural times, each first plastic film 12a, 12b is intermittently
conveyed to form a laminated film 1' intermittently having mixture
layers 11 between a pair of first plastic films 12a, 12b.
[0108] As shown in FIGS. 4 and 5, flaky carbon 31 is dispersed
substantially in parallel in the dispersion layer 3a, but it is not
completely in parallel. Accordingly, only an organic solvent among
the flaky carbon 31 is evaporated when the dispersion layer 3a is
dried, leaving voids among substantially parallel flaky carbon
particles 31 bonded with a trace amount of the binder resin.
Because the mixture layers 3a' having voids among flaky carbon
particles 31 are laminated, the mixture layers 11a, 11b to be
laminated are porous. Accordingly, the mere lamination of the first
plastic films 12a, 12b having the mixture layers 11a, 11b would not
form a dense mixture layer 11.
[0109] Accordingly, a laminated film 1' obtained by passing the
laminating roll pairs 46a, 46b should be heat-pressed by
single-stage or multi-stage heat-pressing roll pairs 47a, 47b, 48a,
48b disposed downstream. Variable depending on the type of a binder
resin, the heat-pressing conditions are preferably a temperature of
100-250.degree. C. and pressure of 20 MPa (about 200 kgf/cm.sup.2)
or more. When the heat-pressing temperature is lower than
100.degree. C., the heat-conductive layer 10 is not sufficient
densified. Also, even if the heat-pressing temperature is elevated
to higher than 250.degree. C., the binder resin is not provided
with higher fluidity, economically disadvantageous. The
heat-pressing temperature is preferably 120-200.degree. C., more
preferably 150-180.degree. C. When the heat-pressing pressure is
less than 20 MPa, the mixture layer 11 is not sufficiently
densified.
[0110] Though the heat-pressing rolls 47a, 47b, 48a, 48b may be
arranged in a single- or multi-stage, their multi-stage arrangement
is preferable to produce a laminated film 1' comprising a
sufficiently dense mixture layer 11. The number of stages of the
heat-pressing rolls 47a, 47b, 48a, 48b may be properly determined
depending on a compression rate.
[0111] (4) Burning of Mixture Layer
[0112] Because the heat-pressed mixture layer 11 has slightly
rugged or irregular edge portions due to the flowing of the binder
resin, it is preferably cut to a predetermined shape and size, for
example, along the broken line 111 as shown in FIG. 7. For example,
as shown in FIG. 8(a), the cut mixture layer 112 is placed on a
wire net 113, and burned by flame at about 900-1200.degree. C.,
which is ejected from a burner 114. Burning may be carried out only
on one surface of the mixture layer 112, but it is preferable to
burn both surfaces of the mixture layer 112. The binder resin is
carbonized or burned off in the mixture layer 112 exposed to flame
ejected from the burner 114. Amorphous carbon generated by
carbonizing the binder resin may remain among the flaky carbon.
Because of volume reduction by carbonizing or burning off the
binder resin, voids generated thereby are removed by pressing to
densify the flaky carbon, thereby providing a heat-conductive layer
10 having higher thermal conductivity.
[0113] FIG. 8(b) shows another method of burning the mixture layer
112. This method uses a die comprising a lower die part 121 having
a cavity and an upper die part 122 slightly larger than the cavity.
After the mixture layer 112 is put in the lower die part 121, the
upper die part 122 is placed on the mixture layer 112, and
sandwiched by a pair of heaters 123a, 123b to heat the mixture
layer 112. The temperature of the heaters 123a, 123b may be about
500-1000.degree. C., and the heating time is determined to avoid
the binder resin from being excessively burned off. The burning of
the mixture layer may be carried out in air or an inert gas, or in
vacuum.
[0114] The carbonization or burning-off of the binder resin by
burning the mixture layer includes not only when the binder resin
is 100% carbonized or burned off, but also when the binder resin is
partially carbonized or burned off. Though the binder resin is
carbonized or burned off preferably as much as possible to obtain
high thermal conductivity, at least 90% of the binder resin need
only be carbonized or burned off.
[0115] (5) Pressing of Burnt Layer
[0116] To densify the burnt layer 131, a die 140 comprising a lower
die part 141 having a cavity 141a, and an upper die part 142 having
a projection 142a received in the cavity 141a is used as shown in
FIGS. 9(a) and 9(b). In the depicted example, the cavity 141a
longitudinally extends in the lower die part 141, with the same
width as that of the burnt layer 131. As shown in FIG. 10, after
the burnt layer 131 is put in the cavity 141a, the upper die part
142 is placed on the lower die part 141, such that the projection
142a covers the burnt layer 131. In this case, because the
thickness of the burnt layer 131 is sufficiently smaller than the
depth of the cavity 141a, the projection 142a of the upper die part
142 enters the cavity 141a, thereby accurately positioning the
upper die part 142 relative to the lower die part 141.
[0117] As shown in FIGS. 6, 11(a) and 11(b), a pressing die means
for densifying the burnt layer 131 comprises (a) a die 140
comprising a lower die part 141 and an upper die part 142 combined
to sandwich the burnt layer 131, (b) a pair of rolls 103a, 103b for
pressing the die 140, (c) guide plates 143a, 143b extending
upstream and downstream of the gap of the rolls 103a, 103b, and (d)
a means (not shown) for reciprocating the die 140 along the guide
plates 143a, 143b such that the die 140 passes through the gap of a
pair of pressing rolls 103a, 103b.
[0118] Among a pair of pressing rolls 103a, 103b, the lower
pressing roll 103a is a driving roll, and the upper pressing roll
103b is a follower roll. With the follower roll 103b having a
slightly smaller diameter than that of the driving roll 103a, the
upper die part 142 is not bent by pressing. The reciprocation range
of the die 140 is sufficiently longer than the burnt layer 131.
Reciprocation may be conducted once or several times. A pressing
force applied to the die 140 may be increased as passing through
the gap of the pressing rolls 103a, 103b. The burnt layer 131 is
turned to a heat-conductive layer 10 by pressing the die 140.
[0119] With the lower pressing roll 103a vibrated by a vibration
motor (not shown), the burnt layer 131 is further densified. The
vibration frequency is preferably 50-500 Hz, more preferably
100-300 Hz.
[0120] FIGS. 12(a) and 12(b) show another example of dies used in
the pressing means for densifying the burnt layer. This die 150
comprises a die body 151 having a vertically penetrating cavity
151a, an upper punch 152a inserted into the cavity 151a from above,
and a lower punch 152b inserted into the cavity 151a from below.
After the lower punch 152b is inserted into the cavity 151a, a
burnt layer 131 is put on the lower punch 152b, and the upper punch
152a inserted into cavity 151a from above is moved downward to
press the burnt layer 131 for densification. With the same
vibration as above applied to the upper punch 152a, a larger
pressing effect is obtained.
[0121] (6) Sealing of heat-conductive layer
[0122] Because the pressed heat-conductive layer 10 has slight
irregularity in edge portions, (1) the edge portions are cut off
from the heat-conductive layer 10 along the broken line 161 to
obtain a heat-conductive layer 10a of a predetermined size as shown
in FIG. 13, or (2) the heat-conductive layer 10 is divided along
the broken lines 162 to heat-conductive layers 10b of a final size
as shown in FIG. 14. In the case shown in FIG. 14, the size, shape
and number of the heat-conductive layers 10b may be arbitrarily
determined.
[0123] In the case of a large heat-conductive layer 10a as shown in
FIG. 13, a second plastic film 13a to which the heat-conductive
layers 10a are attached with predetermined intervals is laminated
with another second plastic film 13b by a pair of rolls 102a, 102b,
and the laminated film is cut in every heat-conductive layer 10a to
obtain individual heat-dissipating films, as shown in FIG. 15. With
the second plastic film 13a having an adhesive layer, the attached
heat-conductive layers 10a are not displaced. The second plastic
films 13a, 13b are preferably heat-laminated via the
heat-conductive layers 10a.
[0124] When cut to heat-conductive layers 10b of a final size as
shown in FIG. 14, one second plastic film 13a, to which pluralities
of heat-conductive layers 10b are attached with predetermined
intervals as shown in FIG. 16(a), is first laminated with another
second plastic film 13b by a pair of rolls 102a, 102b as shown in
FIG. 17, to obtain a laminated film shown in FIG. 18. As shown in
FIG. 18, the laminated film is divided along the broken lines 163
to obtain individual heat-dissipating films.
[0125] As shown in FIG. 19, each heat-conductive layer 10a (10b) is
sandwiched by a pair of second plastic films 13a, 13b, whose edge
portions are heat-sealed.
[0126] The present invention will be explained in further detail by
Examples below, without intention of restricting the present
invention thereto.
EXAMPLE 1
[0127] A dispersion comprising 25% by mass of flaky graphite
(UP-35N available from Nippon Graphite Industries, Co., Ltd., ash:
less than 1.0%, and average particle size: 25 .mu.m), 1.25% by mass
of polymethylmethacrylate (PMMA), and 73.75% by mass of methyl
ethyl ketone was applied to each aluminum-deposited surface of two
aluminum-deposited polyethylene terephthalate (PET) films (first
plastic films) 12a, 12b as thick as 30 .mu.m, and dried at
40.degree. C. for 2 minutes to obtain each coating layer of flaky
graphite and PMMA having a thickness of 50 g/m.sup.2 (expressed by
the grams of flaky graphite per 1 m.sup.2). This procedure was
repeated 5 times in total to form a mixture layer 11a, 11b
(thickness: 100 g/m.sup.2) of flaky graphite and PMMA on each PET
film 12a, 12b.
[0128] As shown in FIG. 6, a pair of PET films 12a, 12b each having
a mixture layer 11a, 11b were laminated at 120.degree. C. by a pair
of rolls 46a, 46b, with the mixture layers 11a, 11b inside, and
then heat-pressed at 150.degree. C. under 20 MPa by plural pairs of
heating rolls 47a, 47b, 48a, 48b, to form a laminated film 1'
comprising the mixture layer 11.
[0129] After peeling both PET films 12a, 12b from the laminated
film 1', each surface of the mixture layer 11 was exposed to a
flame (temperature: about 1000.degree. C.) ejected from a gas
burner 114 shown in FIG. 8(a), to burn the mixture layer 11. The
burnt layer was placed in a cavity 141a of the die 140 shown in
FIG. 9, and repeatedly pressed under a load of 7 tons by a pair of
rolls 103a, 103b with vibration of 200 Hz, as shown in FIGS. 11(a)
and 11(b).
[0130] After the resultant heat-conductive layer 10 was cut to a
predetermined size, its density, specific heat, and heat
diffusivity (m.sup.2/s) in in-plane and thickness directions were
measured. The thermal conductivity (W/mK) was determined from the
product of the heat diffusivity and the heat capacity
(density.times.specific heat). It was thus found that the
heat-conductive layer 10 had a density of 1.97 g/cm.sup.3, and
thermal conductivity of 499 W/mk in an in-plane direction and 17.7
W/mk in a thickness direction.
[0131] As shown in FIG. 15, one PET film 13a, to which a
heat-conductive layer 10a was attached, was laminated with another
PET film 13b by a pair of rolls 102a, 102b. The laminated film 1'
was cut to individual heat-dissipating films.
EXAMPLE 2
[0132] A heat-conductive layer was formed in the same manner as in
Example 1 except for changing the binder resin to
low-stereospecific polypropylene, thereby producing a
heat-dissipating film. The same measurements as in Example 1
indicate that the heat-conductive layer had a density of 1.98
g/cm.sup.3, and thermal conductivity of 516 W/mK in an in-plane
direction and 22.9 W/mk in a thickness direction.
EXAMPLE 3
[0133] A dispersion comprising 6% by mass of flaky graphene (H-15
available from XG Sciences, average diameter: 15 .mu.m), 0.06% by
mass of polymethylmethacrylate (PMMA), and 93.94% by mass of methyl
ethyl ketone was applied to two PET films 12a, 12b as thick as 30
.mu.m, and dried at 40.degree. C. for 2 minutes to form coating
layers of flaky graphene and PMMA as thick as 5 g/m.sup.2
(expressed by the grams of flaky graphene per 1 m.sup.2). This
procedure was repeated 15 times in total to form a mixture layer
11a, 11b of flaky graphene and PMMA (containing 75 g/m.sup.2 of
flaky graphene per 1 m.sup.2) on each PET film 12.
[0134] As shown in FIG. 6, a pair of PET films 12a, 12b each having
a mixture layer 11a, 11b were laminated at 120.degree. C. by a pair
of rolls 46a, 46b, with the mixture layers 11a, 11b inside, and
then heat-pressed at 150.degree. C. under 20 MPa by plural pairs of
heating rolls 47a, 47b, 48a, 48b, to form a laminated film 1'
comprising a heat-conductive layer 11 as thick as 100 .mu.m
(containing 150 g/m.sup.2 of flaky graphene per 1 m.sup.2).
[0135] After peeling both PET films 12a, 12b from the laminated
film 1', each surface of the mixture layer 11 was exposed to a
flame (temperature: about 1000.degree. C.) ejected from a gas
burner 114 shown in FIG. 8(a), to burn the mixture layer 11. The
burnt layer was placed in a cavity 141a of the die 140 shown in
FIG. 9, and repeatedly pressed under a load of 7 tons by a pair of
rolls 103a, 103b with vibration of 200 Hz, as shown in FIGS. 11(a)
and 11(b).
[0136] The resultant heat-conductive layer 10 had a density of 1.98
g/cm.sup.3, and thermal conductivity of 514 W/mk in an in-plane
direction and 5.8 W/mk in a thickness direction.
EFFECTS OF THE INVENTION
[0137] Because a mixture layer of flaky carbon and a binder resin
is densified by heat pressing, burned to carbonize or burn off the
binder resin, and then pressed to form a dense heat-conductive
layer in the present invention, the heat-conductive layer has high
density and thermal conductivity without unevenness in performance.
Further, because the dispersion of flaky carbon and a binder resin
is applied to a plastic film to form a mixture layer, the
production cost can be reduced. Accordingly, the heat-dissipating
film of the present invention uniformly has high thermal
conductivity. The heat-dissipating film of the present invention
having such features is suitable for use in small electronic
appliances such as note-type personal computers, smartphones,
mobile phones, etc.
DESCRIPTION OF REFERENCE NUMERALS
[0138] 1: Heat-dissipating film
[0139] 10: Heat-conductive layer
[0140] 11, 11a, 11b: Mixture layer
[0141] 12a, 12b: First plastic film
[0142] 13a, 13b: Second plastic film
[0143] 2: Plastic film
[0144] 3: Dispersion
[0145] 3a: Dispersion layer
[0146] 3a': Dried binder resin/flaky carbon layer
[0147] 3b: Next dispersion layer
[0148] 31: Flaky carbon
[0149] 32: Organic solvent
[0150] 33: Region in which flaky carbon is agglomerated
[0151] 34: Region free of or scarcely containing flaky carbon
[0152] 4: Chamber
[0153] 41a, 41b: Inlet for first plastic film
[0154] 42a, 42b: Inlet for hot air
[0155] 43: Gas outlet
[0156] 45a, 45b: Dispersion-spraying nozzle
[0157] 46a, 46b: Laminating roll
[0158] 47a, 47b, 48a, 48b: Heat-pressing roll
[0159] 49: Guide roll
[0160] 60, 70a, 70b: Reel
[0161] 100: Apparatus for producing heat-dissipating film
[0162] 101a, 101b: A pair of rolls for peeling first plastic
film
[0163] 102a, 102b: A pair of rolls for adhering second plastic
film
[0164] 103a, 103b: A pair of rolls for pressing die
* * * * *