U.S. patent application number 10/151618 was filed with the patent office on 2003-04-03 for carbon fiber powder, a method of making the same, and thermally conductive composition.
Invention is credited to Shimoyama, Naoyuki, Tateda, Shinya, Tobita, Masayuki.
Application Number | 20030064017 10/151618 |
Document ID | / |
Family ID | 18997187 |
Filed Date | 2003-04-03 |
United States Patent
Application |
20030064017 |
Kind Code |
A1 |
Tobita, Masayuki ; et
al. |
April 3, 2003 |
Carbon fiber powder, a method of making the same, and thermally
conductive composition
Abstract
A carbon fiber powder is produced by graphitizing a polymeric
fiber having an aromatic ring on its main chain by heating and then
pulverizing or cutting it. The polymeric fiber is selected from the
group consisting of polybenzazole fiber, aromatic polyamide fiber,
aromatic polyimide fiber, polyphenylene sulfide fiber, and wholly
aromatic polyester fiber. The carbon fiber is preferably
graphitized by heating the polymeric fiber at least 2500 degree C.
under vacuum or in an inert gas. Thus, the carbon fiber powder that
has excellent thermal conductivity and filling capability, a method
of making the carbon fiber powder, and a thermally conductive
composition including the carbon fiber powder are provided.
Inventors: |
Tobita, Masayuki; (Tokyo,
JP) ; Shimoyama, Naoyuki; (Tokyo, JP) ;
Tateda, Shinya; (Tokyo, JP) |
Correspondence
Address: |
Y. ROCKY TSAO
Fish & Richardson P.C.
225 Franklin Street
Boston
MA
02110-2804
US
|
Family ID: |
18997187 |
Appl. No.: |
10/151618 |
Filed: |
May 20, 2002 |
Current U.S.
Class: |
423/447.2 |
Current CPC
Class: |
C01B 32/20 20170801;
C01B 32/205 20170801; C01B 32/21 20170801; D01F 9/32 20130101; D01F
9/24 20130101 |
Class at
Publication: |
423/447.2 |
International
Class: |
D01F 009/12 |
Foreign Application Data
Date |
Code |
Application Number |
May 22, 2001 |
JP |
2001-1525538 |
Claims
1. A carbon fiber produced by graphitizing a polymeric fiber having
an aromatic ring on its main chain by heating.
2. The carbon fiber according to claim 1, wherein the polymeric
fiber is selected from the group consisting of polybenzazole fiber,
aromatic polyamide fiber, aromatic polyimide fiber, polyphenylene
sulfide fiber, and wholly aromatic polyester fiber.
3. The carbon fiber according to claim 1, wherein the carbon fiber
has an interplanar spacing (d002) of graphite planes of less than
0.3370 nm by X-ray diffractometry.
4. A method of making a carbon fiber comprising: graphitizing a
polymeric fiber having an aromatic ring on its main chain by
heating the polymeric fiber at least 2500 degree C. under vacuum or
in an inert gas.
5. The method according to claim 4, wherein the polymeric fiber is
selected from the group consisting of polybenzazole fiber, aromatic
polyamide fiber, aromatic polyimide fiber, polyphenylene sulfide
fiber, and wholly aromatic polyester fiber.
6. A carbon fiber powder produced by graphitizing a polymeric fiber
having an aromatic ring on its main chain by heating.
7. The carbon fiber powder according to claim 6, wherein the
polymeric fiber is selected from the group consisting of
polybenzazole fiber, aromatic polyamide fiber, aromatic polyimide
fiber, polyphenylene sulfide fiber, and wholly aromatic polyester
fiber.
8. The carbon fiber powder according to claim 6, wherein the carbon
fiber powder has an interplanar spacing (d002) of graphite planes
of less than 0.3370 nm by X-ray diffractometry.
9. The carbon fiber powder according to claim 6, wherein the fiber
powder has a diameter of from 5 to 20 .mu.m and an average particle
size of from 5 to 500 .mu.m.
10. A method of making a carbon fiber powder comprising:
graphitizing a polymeric fiber having an aromatic ring on its main
chain by heating the polymeric fiber at least 2500 degree C. under
vacuum or in an inert gas.
11. The method according to claim 10 further comprising:
pulverizing or cutting the polymeric fiber after the graphitizing
step.
12. The method according to claim 10, wherein the polymeric fiber
is selected from the group consisting of polybenzazole fiber,
aromatic polyamide fiber, aromatic polyimide fiber, polyphenylene
sulfide fiber, and wholly aromatic polyester fiber.
13. A thermally conductive composition comprising: a matrix; and
carbon fiber powders mixed in the matrix, wherein the carbon fiber
powder is produced by graphitizing a polymeric fiber having an
aromatic ring on its main chain by heating.
14. The composition according to claim 13, wherein the matrix has
at least one polymeric material selected from the group consisting
of vulcanized rubber, thermoplastic elastomer, thermoplastic resin,
and thermosetting resin.
15. The composition according to claim 13, wherein the content of
the carbon fiber powders is from 1 to 800 parts by weight relative
to 100 parts by weight of the matrix.
16. The composition according to claim 13, wherein the composition
is a thermally conductive molded article molded into a
predetermined shape.
17. The composition according to claim 13, wherein the composition
is a thermally conductive sheet.
18. The composition according to claim 13, wherein the composition
is a thermally conductive adhesive.
19. The composition according to claim 13, wherein the composition
is a thermally conductive grease.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates to carbon fiber powder that has
excellent thermal conductivity and filling capability, a method of
making the same, and a thermally conductive composition containing
the powder.
[0002] With recent advancements, miniaturization, and lightening of
electronic hardware, semiconductor packages have become more
compact and more highly integrated and operated at higher speed.
Therefore, the heat generated by the electronic hardware is a very
important issue. Generally, to dissipate the heat from
heat-generating components to outside, a molded article and a
liquid composition, such as a sheet material made of a thermally
conductive polymer composition, polymer grease and adhesive, are
placed between a radiator and one of the followings: a printed
circuit board; a semiconductor package; and a heat radiator. The
heat radiator is, for example, a radiation plate or a heat
sink.
[0003] Such thermally conductive compositions include a matrix,
such as resin and rubber, and a filler that has high thermal
conductivity in the matrix. Possible fillers include metal oxide,
metal nitride, metal carbide, and metal hydroxide. Examples of such
possible fillers include aluminum oxide, boron nitride, silicon
nitride, magnesium oxide, zinc oxide, silicon carbide, quartz, and
aluminum hydroxide. However, such compositions do not necessarily
have sufficient thermal conductivity.
[0004] In order to improve the thermal conductivity, various
compositions have been proposed that include highly thermally
conductive graphite powders or carbon fibers as filler in the
matrix.
[0005] For example, Japanese Laid-Open Patent Publication
No.62-131033 discloses a molded body made of thermally conductive
resin in which the resin is filled with graphite powders. Japanese
Laid-Open Patent Publication No.4-246456 discloses a composition of
polyester resin containing carbon black or graphite. Japanese
Laid-Open Patent Publication No.5-17593 discloses a thermally
conductive molded body of great mechanical strength in which the
carbon fibers are arranged in a certain direction and are
impregnated with graphite powder and thermosetting resin. Japanese
Laid-Open Patent Publication No.5-222620 discloses a thermally
conductive material using pitch-based carbon fibers that have a
specific cross section. Japanese Laid-Open Patent Publication
No.5-247268 discloses a rubber composition in which is mixed
synthetic graphite having a particle size of 1 to 20 .mu.m.
Japanese Laid-Open Patent Publication No.9-283955 discloses a
thermally conductive sheet in which the graphitized carbon fibers
of specific aspect ratio are dispersed in polymer, such as silicone
rubber. Japanese Laid-Open Patent Publication No.10-298433
discloses a composition and a radiation sheet in which silicone
rubber has, mixed within it, spherical graphite powders having an
interplanar spacing of crystals from 0. 330 to 0. 340 nm. Japanese
Laid-Open Patent Publication No.2-242919 and Japanese Laid-Open
Patent Publication No.7-48181 disclose certain pitch-based carbon
fibers as highly thermally conductive carbon fibers.
[0006] Thus, pitch-based carbon fiber has been known for highly
thermally conductive carbon fiber used as thermally conductive
filler. More particularly, isotropic pitch-based carbon fiber and
mesophase pitch-based carbon fiber have been known for that
purpose. Such pitch-based carbon fiber is obtained by spinning raw
pitch, infusibilizing the resulting pitch fiber, and graphitizing
it by heating. On the other hand, highly thermally conductive fiber
cannot be obtained from PAN-type carbon fibers and rayon-type
carbon fibers produced by graphitizing polyacrylonitrile fiber and
rayon fiber by heating, since the graphitization upon heating is
hard to achieve. To this end, a pitch-based carbon fiber is
advantageous.
[0007] Meanwhile, Japanese Laid-Open Patent Publication No.9-324127
discloses highly thermally conductive powdery graphite, die bond
adhesive for a semiconductor element, and a semiconductor device in
which the powdery graphite is blended in thermosetting adhesive
resin. The powdery graphite is obtained by graphitizing a polymeric
film by heating and pulverizing or cutting the resultant
graphitized film.
[0008] However, for recent high performance electronic parts, due
to increase in amount of heat generation, the need for greater
thermal conductivity has increased. Therefore, the thermal
conductivity is still insufficient for the above-mentioned
compositions that include graphite powder or carbon fibers as
thermally conductive filler.
[0009] Further, as for pitch-based carbon fibers, pitch fiber used
as a raw material has poor heat resistance. Therefore, to prevent
pitch fiber from melting at high temperature, infusibilizing for
several hours below 400 degree C. with oxygen-containing gas such
as air, placing in oxidative water solution such as nitric acid and
chromic acid, or polymerizing with light or gamma ray, are required
before the graphitization process by heating. This leads to low
productivity of the fiber.
[0010] The powdery graphite proposed in Japanese Laid-Open Patent
Publication No.9-324127 has an advantage that the infusibilization
process is eliminated. However, since a planar polymeric film is
used as a raw material of the powdery graphite, cumbersome
pulverization or cutting of the film is needed after heat
treatment. In addition, the resultant pulverized products or cut
products have non-uniform shape or size. In other words, fine
powders and coarse powders are present in the products. Thus, it is
difficult to contain such powdery graphite filler in a matrix at
high concentration.
[0011] The objective of the present invention is to provide carbon
fiber powder that has excellent thermal conductivity, as well as is
filled in a matrix at high concentration; a simple method of making
the carbon fiber powder; and a thermally conductive composition
including the carbon fiber powder.
BRIEF SUMMARY OF THE INVENTION
[0012] According to the present invention, a carbon fiber is
produced by graphitizing a polymeric fiber having an aromatic ring
on its main chain by heating. The carbon fiber may be in the powder
form.
[0013] Other aspects and advantages of the invention will become
apparent from the following description, taken in conjunction with
the accompanying drawings, illustrating by way of example the
principles of the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0014] Embodiments of the present invention are described in detail
below.
[0015] 1. Carbon Fiber Powder
[0016] The carbon fiber powder is a pulverized or cut product of
the carbon fiber that is obtained by graphitizing by heating a
polymeric fiber having an aromatic ring on its main chain.
[0017] 1.A. Polymeric Fiber as a Raw Material
[0018] Firstly, a polymeric fiber of the present invention used as
a raw material is described. The polymeric fiber of the present
invention has an aromatic ring on its main chain. As used herein,
an aromatic ring generally refers to a ring that belongs to an
aromatic group and means a group of organic compounds including
aromatic hydrocarbons, such as benzene ring, naphthalene ring, and
anthracene ring, and derivatives thereof.
[0019] The polymeric fiber of the present invention is not
particularly limited to, but includes, polybenzazole fiber,
aromatic polyamide fiber, aromatic polyimide fiber, polyphenylene
sulfide fiber, wholly aromatic polyester fiber, polyphenylene
benzoimidazole fiber, polyparaphenylene fiber, and
polyparaphenylene vinylene fiber.
[0020] One of the reasons for using the polymeric fiber having an
aromatic ring on its main chain as a raw material is that such a
polymeric fiber is easily graphitized upon heating and thus
produces highly thermally conductive carbon fiber or carbon fiber
powder, in which graphite structure is highly developed. On the
other hand, when a conventional polymeric fiber having no aromatic
ring on its main chain is used as a raw material, for example,
polyacrylonitrile fiber, rayon fiber, or phenol fiber, the
graphitization upon heating is difficult. Thus, highly thermally
conductive carbon fibers or carbon fiber powder can not be
obtained.
[0021] Another reason for using the polymeric fiber of the present
invention as a raw material is that such polymeric fiber has so
high heat resistance that it is hard to melt but often keeps its
shape upon heating, and thus the above-mentioned infusibilization
process is not necessarily needed. Thus, productivity at a
manufacturing step is improved.
[0022] A further reason for using the polymeric fiber of the
present invention as a raw material is that pulverization or
cutting is easier when using such fiber rather than using the
planar graphite film as a raw material. Besides, fine and uniform
carbon fiber powders having a limited bulk can be easily
obtained.
[0023] Among them, the polymeric fiber of the present invention is
more preferably at least one fiber selected from the group
consisting of polybenzazole fiber, aromatic polyamide fiber,
aromatic polyimide fiber, polyphenylene sulfide fiber, and wholly
aromatic polyester fiber. Most preferably, the polymeric fiber is
at least one fiber selected from the group consisting of
polybenzazole fiber, aromatic polyimide fiber, and aromatic
polyamide fiber. The polymeric fiber having an aromatic ring on its
main chain tends to be graphitized easier upon heating as it has
more aromatic rings. Thus, carbon fiber or carbon fiber powder that
has extremely excellent thermal conductivity is obtained by using
such preferred polymeric fibers. The diameter, profile of cross
section, and length of the polymeric fiber of the present invention
is not limited.
[0024] As used herein, polybenzazole fiber refers to a polymeric
fiber that is formed of polybenzazole polymer. The polybenzazole
fiber is generally excellent in strength, modulus of elasticity,
heat resistance, flame resistance, and electric insulation. The
polybenzazole polymer (PBZ) refers to polybenzooxazole homopolymer
(PBO), polybenzothiazole homopolymer (PBT); or random copolymer,
sequential copolymer, block copolymer, or graft copolymer of PBO
and PBT. PBZ may be synthesized by a known method. An example of
commercially available PBZ is ZYLON.TM. from Toyobo Co., Ltd.
[0025] The aromatic polyamide fiber includes poly(paraphenylene
isophthalic amide) and poly(metaphenylene isophthalic amide).
Examples of commercially available aromatic polyamide fiber are,
for poly(paraphenylene isophthalic amide), KEVLAR.TM. from Du
Pont-Toray Co., Ltd., Technora.TM. from Teijin Limited, and
Twaron.TM. from Akzo; and for poly(metaphenylene isophthalic
amide), NOMEX.TM. from Du Pont-Toray Co., Ltd., CONEX.TM. from
Teijin Limited, and Apyeil.TM. from Unitika Ltd. The aromatic ring
of these products may include a substituent such as halogen group,
alkyl group, cyano group, acetyl group, and nitro group.
[0026] In addition, aromatic polyimide fiber (from Inspec Fibres
GmbH), polyphenylene sulfide fiber (PROCON.TM. from Toyobo Co.,
Ltd.), wholly aromatic polyester fiber (VECTRAN.TM. from Unitika
Ltd. And ECONOL.TM. from Sumitomo Chemical Company Limited),
polybenzoimidazole fiber(from Hoechst Celanse) are easily
available.
[0027] 1.B. A Method of making Carbon Fiber Powders
[0028] Next, a method of making carbon fiber powders is described.
The carbon fiber powders are produced by graphitizing the
above-mentioned polymeric fiber of the present invention by
heating, and pulverizing or cutting the resultant carbon fiber.
[0029] The heating temperature should be at least 2500 degree C.
When the temperature is lower than 2500 degree C., graphitization
of the fiber becomes insufficient and carbon fiber that has high
thermal conductivity can not be obtained. Preferably, the heating
is conducted under vacuum or in an inert gas, such as argon gas or
nitrogen gas. When the fiber is not heated under vacuum or in an
inert gas, the polymeric fiber may be undesirably degenerated by
oxidation. In practice, the fiber is preferably heated for a given
time at a high temperature from 2800 to 3200 degree C. in argon
gas. This actively promotes graphitization of the fiber to produce
highly thermally conductive carbon fiber in which graphite
structure highly develops. The process is not particularly limited
to specific rates of heating temperature or to a specific treating
period.
[0030] In heating the polymeric fiber, polymeric fibers are
preferably bundled. Such bundle of the fibers allows a large amount
of the fibers to be placed in a certain volume of the heating
reactor. Thus, the heating treatment is conducted efficiently and
the productivity is improved. Further, by pressurizing the bundled
polymeric fibers, an even larger amount of the fibers can be placed
in a certain volume of the heating reactor, thereby improving
productivity.
[0031] To pulverize or cut fibers, pulverizing machines are
available, such as a Victory mill, a jet mill, and a high-speed
rotation mill or cutters for chopping fibers. To make the
pulverization or cutting effective, it is advantageous if a rotor
of each machine that has blades that rotate at high speed to cut
the fibers in a direction perpendicular to the fibers. The average
length of the pulverized or cut fibers is changed by adjusting the
rotation number of the rotor or an angle of the blades. Grinding
machines such as a ball mill could be used for pulverizing the
fibers. However, such machines are undesirable in that they apply
perpendicular pressure to fibers, which generates cracks in the
axial direction of the fibers. This pulverization or cutting
process may be conducted either before or during the heating of the
fiber.
[0032] 1.C. Properties of the Carbon Fiber Powders
[0033] The properties of resultant carbon fiber powders are
described.
[0034] The graphitized carbon fiber powders specifically take the
form of fiber (including a pulverized product or a cut product that
keeps the fibrous form), a scale, a whisker, a micro coil, or a
nanotube. However, other forms are also applicable.
[0035] The diameter of the graphitized carbon fiber powders is not
particularly limited but is preferably 5-20 .mu.m. The fiber
powders that have a diameter of 5-20 .mu.m are easily produced
industrially. The fiber diameters smaller than 5 .mu.m or larger
than 20 .mu.m decrease the productivity of the fiber powders.
[0036] The average particle size or length of the graphitized
carbon fiber powders is not particularly limited but is preferably
5-500 .mu.m. When the average particle size of each fiber powder is
smaller than 5 .mu.m, the contact of the fiber powders in the
matrix is reduced and a heat transfer becomes insufficient. This
reduces the thermal conductivity of the resultant thermally
conductive composition. When the average particle size is larger
than 500 .mu.m, the fiber powders are too bulky to be mixed in the
matrix at a high concentration. The average particle size can be
calculated from the particle size distribution by laser
diffractometry model.
[0037] From X-ray diffractometry, it is preferred that the
graphitized carbon fiber powders have an interplanar spacing (d002)
of graphite planes of less than 0.3370 nm. When the interplanar
spacing (d002) is less than 0.3370 nm, carbon fiber powders and a
thermally conductive composition that have higher thermal
conductivity can be achieved. When the interplanar spacing (d002)
is 0.3370 nm or greater, the thermal conductivity is inadequate.
Accordingly, a composition that has high thermal conductivity can
not be obtained by using such carbon fiber powders as thermally
conductive filler. The lower limit of the interplanar spacing
(d002) is a theoretical value of 0.3354 nm.
[0038] In the X-ray diffractometry, a diffractometry pattern of the
carbon fibers is measured by using CuK alpha as a X-ray source and
highly purified silicon as a standard material. The interplanar
spacing (d002) is calculated from the peak position and half-value
width of the (002) diffractometry pattern. This calculation is
based on a method pursuant to Japan Society for the Promotion of
Science.
[0039] The thermal conductivity is not particularly limited but
preferably at least 400W/(m.multidot.K), more preferably at least
800W/(m.multidot.K), and most preferably at least
1000W/(m.multidot.K).
[0040] 2. Thermally Conductive Composition
[0041] A thermally conductive composition includes the
above-mentioned carbon fiber powders in a matrix as thermally
conductive filler.
[0042] 2.A. Matrix
[0043] The matrix is preferably selected according to required
characteristics or applications such as a shape, hardness,
mechanical properties, thermal properties, electrical properties,
durability, and reliability of the resultant sheet. Matrix is
preferably a polymeric material if molding capability is taken into
consideration.
[0044] The polymeric material is preferably selected from
thermoplastic resin, thermoplastic elastomer, thermosetting resin,
and vulcanized rubber according to required characteristics. For
example, preferred thermally conductive adhesives include adhesive
polymeric materials such as epoxy resin, polyimide, and acrylic
resin. Preferred molding materials include thermoplastic resin,
thermoplastic elastomer, thermosetting resin, and vulcanized
rubber.
[0045] The thermoplastic resin includes polyethylene,
polypropylene, ethylene-.alpha.-olefin copolymer such as
ethylene-propylene copolymer, polymethylpentene, polyvinyl
chloride, polyvinylidene chloride, polyvinyl acetate, ethylene
vinyl acetate copolymer, polyvinyl alcohol, polyacetal,
fluororesins such as polyvinylidene fluoride and
polytetrafluoroethylene, polyethylene terephthalate, polybutylene
terephthalate, polyethylene naphthalate, polystyrene,
polyacrylonitrile, styrene acrylonitrile copolymer, ABS resin,
polyphenylene ether (PPE) resin and modified PPE resin, aliphatic
and aromatic polyamides, polyimide, polyamide imide,
polymethacrylic acid and polymethacrylates such as polymethyl
methacrylate, polyacrylic acids, polycarbonate, polyphenylene
sulfide, polysulfone, polyether sulfone, polyether nitrile,
polyether ketone, polyketone, liquid crystal polymer, silicone
resin, and ionomer.
[0046] The thermoplastic elastomer includes repeatedly moldable and
recyclable thermoplastic elastomers such as styrene-butadiene or
styrene-isoprene block copolymers and hydrogenated polymer thereof,
styrenic thermoplastic elastomer, olefinic thermoplastic elastomer,
vinyl chloride thermoplastic elastomer, polyester thermoplastic
elastomer, polyurethane thermoplastic elastomer, and polyamide
thermoplastic elastomer.
[0047] The thermosetting resin includes epoxy resin, polyimide,
bis-maleimide resin, benzocyclobutene, phenol resin, unsaturated
polyester, diallyl phthalate, silicone resin, polyurethane,
polyimide silicone, thermosetting polyphenylene ether resin and
modified PPE resin.
[0048] The vulcanized rubber and analogues thereof include natural
rubber, butadiene rubber, isoprene rubber, styrene-butadiene
copolymer rubber, nitrile rubber, hydrogenated nitrile rubber,
chloroprene rubber, ethylene-propylene rubber, chlorinated
polyethylene, chlorosulfonated polyethylene, butyl rubber and
halide butyl rubber, fluorine rubber, urethane rubber, and silicone
rubber.
[0049] In terms of temperature characteristics such as thermal
resistance and electric reliability, the matrix polymer is
preferably at least one material selected from the group consisting
of silicone rubber, epoxy resin, polyurethane, unsaturated
polyester, polyimide, bis-maleimide, benzocyclobutene, fluororesin,
polyphenylene ether resin and thermoplastic elastomer. More
preferably, the matrix polymer is at least one material selected
from the group consisting of silicone rubber, epoxy resin,
unsaturated polyester resin, polyimide, polyurethane and
thermoplastic elastomer.
[0050] In an application for a wiring board where permittivity and
dielectric loss tangent are small and frequency characteristic is
required, fluororesin, thermosetting polyphenylene ether resin,
modified PPE resin, and polyolefin resin are desired. To obtain a
flexible thermally conductive polymer sheet, a polymer matrix such
as low-hardness vulcanized rubber and low-hardness thermoplastic
elastomer may be used.
[0051] One or more of the above polymers can be used as
appropriate. Further, a polymer alloy formed of a plurality of
these polymeric materials may be used. The methods of cross-linking
thermosetting resin or vulcanized rubber are not limited to
thermosetting but include known methods such as light setting and
moisture setting.
[0052] 2.B. Properties of Thermally Conductive Composition
[0053] The intended thermally conductive composition is obtained by
mixing the above-mentioned carbon fiber powders with the matrix and
degassing it if necessary. In mixing, known mixing machines such as
a blender, a mixer, a roller, an extruder may be used.
[0054] The content of the carbon fiber powders is preferably 1 to
800 parts by weight, more preferably 5 to 500 parts by weight, and
most preferably 20 to 300 parts by weight relative to 100 parts by
weight of the matrix. When the content is less than 1 part by
weight, the thermal conductivity of the resultant composition is
lowered and radiating property is decreased. When the content is
more than 800 parts by weight, the viscosity of the composition is
increased, which makes it difficult to disperse the carbon fiber
powders in the matrix uniformly. Also, gas bubbles are inevitably
included in the matrix.
[0055] For the purpose of improvement of the surface of the carbon
fiber powders, the surface of the powders may be previously
oxidized by electrolytic oxidation or treated with a known coupling
agent or a known sizing agent.
[0056] This improves wettability or filling capability of the
carbon fiber powders to the matrix or the peel strength from the
matrix at the interface.
[0057] The surface of the carbon fiber powders may also be coated
with metal or ceramics by various methods such as electroless
plating; electroplating; physical vapor evaporation such as vacuum
evaporation, sputtering and ion plating; chemical vapor deposition,
spraying; coating; immersion; and mechanochemical method in which
fine particles are mechanically fixed on the surface of the
fibers.
[0058] Other than the carbon fiber powders of the present
invention, the thermally conductive polymer composition may also
include other thermally conductive materials, an incombustible
agent, a softening agent, a colorant, and a stabilizer as required.
The other thermally conductive materials include the following:
[0059] metal and ceramic such as silver, copper, gold, aluminum
oxide, magnesium oxide, boron nitride, aluminum nitride, silicon
nitride, silicon carbide, and aluminum hydroxide
[0060] metal-coated resin
[0061] conventional graphitized carbon fiber
[0062] non-graphitized carbon fiber
[0063] natural graphite
[0064] synthetic graphite
[0065] meso-carbon micro-bead
[0066] The carbon fibers, graphites, and beads may be in the form
of, for example, spherical powder, powder, fiber, needle, a scale,
a whisker, a microcoil, single-wall, or multi-wall nanotube.
[0067] In an application where electric non-conductance is
particularly required for a thermally conductive polymer sheet, an
end product, the composition preferably includes at least one
electrical insulative thermally conductive filler selected from the
group consisting of aluminum oxide, magnesium oxide, boron nitride,
aluminum nitride, silicon nitride, silicon carbide and aluminum
hydroxide.
[0068] The thermally conductive composition may be processed by,
for example, compression molding, extrusion molding, injection
molding, casting molding, blow molding, blade molding, and
calendering molding. When the composition is liquid, it may be
processed by painting, printing, dispensing, and potting other than
the above methods. For example, a thermally conductive molded
article that has excellent thermally conductive property may be
produced by processing the composition into a predetermined shape
by the above molding methods. A thermally conductive sheet which
has excellent thermal conductivity may be obtained by using
vulcanized rubber or thermoplastic elastomer which has low hardness
as a matrix and molding the composition into a sheet by the above
molding methods.
[0069] Thus, the thermally conductive composition may be used as a
material for liquid articles such as thermally conductive grease
and thermally conductive adhesive, for both of which a high heat
radiation property is required. Alternatively, the composition may
be used as a material for molded articles such as printed circuit
boards, semiconductor packages, heat radiation members including
radiation plates and heat sink, a housing, and a thermally
conductive sheet. The conductive composition may be used to form
thermally conductive grease and thermally conductive adhesive
placed between the heating element and the heat transfer member or
to form heat radiation members, such as a radiator, a cooler, a
heat sink, a heat spreader, a die pad, a printed circuit board, a
cooling fan, a heat pipe, and a housing. Thus, a heat-dissipating
measure is possible.
[0070] The advantages of the above embodiments are described
below.
[0071] For the polymeric fiber of the present invention, the
polymeric fiber having an aromatic ring on its main chain is used,
preferably, at least one polymeric fiber selected from the group
consisting of polybenzazole fiber, aromatic polyamide fiber,
aromatic polyimide fiber, polyphenylene sulfide fiber, and wholly
aromatic polyester fiber. Therefore, a highly thermally conductive
carbon fiber can be produced in which graphite structure highly
develops.
[0072] By pulverizing or cutting such carbon fiber, fine and
uniform carbon fiber powders can be produced that have two
characters required for thermally conductive filler: high thermal
conductivity and high filling capability.
[0073] The use of the graphitized carbon fiber powders that have an
interplanar spacing (d002) of graphite planes of less than 0.3370
nm by X-ray diffractometry makes it possible to produce highly
thermally conductive carbon fiber(s) or carbon fiber powders.
[0074] In the polymeric fiber having an aromatic ring on its main
chain, linear polymers run principally in the direction of the
fiber axis. Thus, the carbon fiber or carbon fiber powder can be
produced in which graphite planes highly develop in the running
direction of the linear polymers or in the direction of the fiber
axis. Consequently, the carbon fiber or carbon fiber powder has
excellent thermal conductivity, particularly in the direction
parallel to the graphite planes.
[0075] To produce the carbon fiber powder, the polymeric fiber
having an aromatic ring on its main chain, preferably, at least one
polymeric fiber selected from the group consisting of polybenzazole
fiber, aromatic polyamide fiber, aromatic polyimide fiber,
polyphenylene sulfide fiber, and wholly aromatic polyester fiber,
is heated at least 2500 degree under vacuum or inert gas to
graphitize. This produces a highly thermally conductive carbon
fiber in which the graphite structure is highly developed. By
pulverizing or cutting this carbon fiber, fine and uniform carbon
fiber powders that have good filling capability can be produced
simply. In addition, conventional infusibilization process is not
necessary required. Moreover, by not pulverizing or cutting planar
base material (graphite film) but by using a linear base
material(carbon fiber), fine and uniform carbon fiber powders can
be produced easily. Thus, pulverization or cutting process is
simplified, which results in improvement in productivity of the
powders at a manufacturing step.
[0076] The carbon fiber powders are mixed in the matrix. Therefore,
a thermally conductive complex material can be obtained in which
the content of the carbon fiber powders is larger and thermal
conductivity is greater than conventional ones, or a thermally
conductive complex material can be obtained in which the content of
the carbon fiber powders is smaller but thermal conductivity is
similar. When the interplanar spacing (d002) of graphite planes by
X-ray diffractometry is less than 0.3370 nm, the thermal
conductivity of the thermally conductive composition is greatly
improved. Although the reason is still unclear why the thermal
conductivity is improved like this, it is supposed that it is
ascribable to the very strong correlation between the
microstructure of the carbon fibers and thermal passages when the
carbon fibers are dispersed in the matrix.
[0077] As a matrix of the thermally conductive composition, at
least one polymeric material is used selected from the group
consisting of vulcanized rubber, thermoplastic elastomer,
thermoplastic resin, and thermosetting resin that has molding
capability. Thus, a thermally conductive composition can be
obtained that is applicable depending on various applications or
required characteristics.
[0078] The content of the carbon fiber powders in the composition
is 1 to 800 parts by weight relative to 100 parts by weight matrix.
This prevents an increase in the viscosity of the composition and
prevents inclusion of bubbles. A thermally conductive composition
is obtained in which the carbon fiber powders are uniformly
dispersed in the matrix and which has improved thermal
conductivity.
EXAMPLES
[0079] The above-mentioned embodiments are further described with
reference to Samples, Examples and Comparative examples, which are
not intended to limit the scope of the present invention in any
way.
Sample 1, Carbon Fiber Powders
[0080] As a polymeric fiber having an aromatic ring on its main
chain, polybenzazole fiber (Toyobo Co., Ltd., a tradename ZYLON.TM.
HM: polybenzooxazole fiber) was used. After being bundled, the
polymeric fibers were placed in a heating reactor and heated at
3000 degree C. in an argon gas for two hours to be graphitized to
produce carbon fibers. The resultant carbon fibers were pulverized
with a high-speed rotation mill to form carbon fiber powders
(Sample 1). The carbon fiber powders had a fiber diameter of 9
.mu.m, an average particle size of 50 .mu.m, and an interplanar
spacing (d002) between the graphite planes of 0.3360 nm by X-ray
diffractometry.
Sample 2, Carbon Fiber Powders
[0081] As a polymeric fiber having an aromatic ring on its main
chain, polybenzazole fiber (Toyobo Co., Ltd., a tradename ZYLON.TM.
HM: polybenzooxazole fiber) was used. After being bundled, the
polymeric fibers were cut to an average length of 5 mm, placed in a
heating reactor, and heated at 3200 degree C. in an argon gas for
two hours to be graphitized to produce carbon fibers. The resultant
carbon fibers were further pulverized with a high-speed rotation
mill to form carbon fiber powders (Sample 2). The carbon fiber
powders had a fiber diameter of 9 .mu.m, an average particle size
of 25 .mu.m, and an interplanar spacing (d002) between the graphite
planes of 0.3358 nm by X-ray diffractometry.
Sample 3, Carbon Fiber Powders
[0082] As a polymeric fiber having an aromatic ring on its main
chain, aromatic polyimide fiber (Inspec Fibres, a tradename P84)
was used. After being bundled, the polymeric fibers were placed in
a heating reactor and heated at 3000 degree C. in an argon gas for
two hours to be graphitized to produce carbon fibers. The resultant
carbon fibers were pulverized with a high-speed rotation mill to
form carbon fiber powders (Sample 3). The carbon fiber powders had
a fiber diameter of 9 .mu.m, an average particle size of 50 82 m,
and an interplanar spacing (d002) between the graphite planes of
0.3364 nm by X-ray diffractometry.
Sample 4, Carbon Powders
[0083] As a polymeric film, an aromatic polyimide film (Du
Pont-Toray Co., Ltd., a tradename KAPTON.TM. a thickness of 25
.mu.m) was used. The polymeric film was placed in a heating reactor
and heated at 3000 degree C. in an argon gas for 2 hours to be
graphitized to produce a graphite film. The resultant graphite film
was pulverized with a high-speed rotation mill to form carbon
powders (Sample 4). Although the carbon powders had an average
particle size of 45 .mu.m, the shape and size of each particle is
unequal and fine powders of the size of less than 5 .mu.m and
coarse powders of the size of more than 500 .mu.m were mixed
considerably. The interplanar spacing (d002) between the graphite
planes by X-ray diffractometry was 0.3368 nm.
[0084] The fiber diameter, the average particle size, and the
interplanar spacing (d002) between the graphite planes by X-ray
diffractometry of the carbon fiber powders and the carbon powders
of Samples 1 to 4 are shown in Table 1.
1 TABLE 1 Sample 1 Sample 2 Sample 3 Sample 4 raw material
polybenzazole polybenzazole aromatic aromatic fiber fiber polyimide
fiber polyimide film fiber diameter 9 .mu.m 9 .mu.m 9 .mu.m 9 .mu.m
average particle 50 .mu.m 25 .mu.m 50 .mu.m 45 .mu.m interplanar
spacing 0.3360 nm 0.3358 nm 0.3364 nm 0.3368 nm (d002)
[0085] As described below, Examples 1 to 7 and Comparative examples
1 and 2 are the examples of a soft thermally conductive sheet
formed of silicone rubber. Examples 8 to 12 and Comparative example
3 are the example of a thermally conductive sheet in which
recyclable thermoplastic elastomer is used. Examples 13 to 17 and
Comparative example 4 are example of a thermally conductive molded
article that is capable of injection molding. Examples 18 to 22 and
Comparative example 5 is a thermally conductive adhesive of epoxy
resin.
Example 1
[0086] 90 parts by weight of the carbon fiber powders of Sample 1
with their surfaces treated with a silane coupling agent as a
thermally conductive filler, 220 parts by weight of aluminum oxide
powder (SHOWA DENKO K. K.), and 80 parts by weight of aluminum
hydroxide powder (SHOWA DENKO K. K.) were added to and dispersed in
100 parts by weight of an addition-type liquid silicone rubber (Dow
Corning Toray Silicone Co., Ltd.) as a matrix to prepare a
thermally conductive composition. The resultant composition was
hot-pressed and molded to form a thermally conductive sheet of a
thickness of 2 mm. The resultant sheet had an Asker C hardness of
17. The thermal conductivity in the thickness direction of the
sheet was 2.9W/(m.multidot.K).
Example 2
[0087] A thermally conductive composition was prepared as in
Example 1, except that the carbon fiber powders of Sample 2 were
used as thermally conductive filler. A thermally conductive sheet
having a thickness of 2 mm was formed. The resultant sheet had an
Asker C hardness of 15. The thermal conductivity in the thickness
direction of the sheet was 3.0W/(m.multidot.K).
Example 3
[0088] A thermally conductive composition was prepared as in
Example 1, except that the carbon fiber powders of Sample 3 were
used as thermally conductive filler. A thermally conductive sheet
having a thickness of 2 mm was formed. The resultant sheet had an
Asker C hardness of 15. The thermal conductivity in the thickness
direction of the sheet was 2.7W/(m.multidot.K).
Comparative Example 1
[0089] A thermally conductive composition was prepared as in
Example 1, except that the powders of Sample 4 were used as
thermally conductive filler. A thermally conductive sheet having a
thickness of 2 mm was formed. As described previously, the carbon
powders of Sample 4 were unequal so that the carbon powders are
difficult to mix in the matrix and gas bubbles could not be
removed. The resultant sheet had an Asker C hardness of 13. The
thermal conductivity in the thickness direction of the sheet was
1.0W/(m.multidot.K).
Comparative Example 2
[0090] A thermally conductive composition was prepared as in
Example 1, except that commercially available pitch-based carbon
fiber powder (Petoca materials Ltd., milled Melblon) was used as
thermally conductive filler. A thermally conductive sheet having a
thickness of 2 mm was formed. The resultant sheet had an Asker C
hardness of 15. The thermal conductivity in the thickness direction
of the sheet was 2.3W/(m.multidot.K).
[0091] The contents of the thermally conductive composition of
Examples 1 to 3 and Comparative examples 1 and 2 and the thermal
conductivity in the thickness direction of their respective
thermally conductive sheets are shown in Table 2.
2 TABLE 2 Ex. 1 Ex. 2 Ex. 3 Comp. 1 Comp. 2 thermally conductive
filler (type) Sample 1 Sample 2 Sample 3 Sample 4 Pitch-based
thermally conductive filler (pbw) 90 90 90 90 90 silicone
rubber(pbw) 100 100 100 100 100 aluminum oxide (pbw) 220 220 220
220 220 aluminum hydroxide (pbw) 80 80 80 80 80 thermal
conductivity W/(m .multidot. K) 2.9 3.0 2.7 1.0 2.3 *pbw = part by
weight
[0092] A thermally conductive composition was prepared as in
Example 1, except that the content of the carbon fiber powders was
varied to 5, 20, 300, and 500 parts by weight, respectively. A
thermally conductive sheet having a thickness of 2 mm was formed.
The thermal conductivity in the thickness of the resultant sheets
was 1.6W/(m.multidot.K), 199W/(m.multidot.K), 3.6W/(m.multidot.K),
and 4.2W/(m.multidot.K), respectively.
[0093] The contents of the thermally conductive composition of
Example 1, Examples 4 to 7, and Comparative example 1 and 2 and the
thermal conductivity in the thickness direction of their respective
thermally conductive sheets are shown in Table 3.
3 TABLE 3 Ex. 1 Ex. 4 Ex. 5 Ex. 6 Ex. 7 Comp. 1 Comp. 2 thermally
conductive filler (type) Sample 1 Sample 1 Sample 1 Sample 1 Sample
1 Sample 4 Pitch-based thermally conductive filler (pbw) 90 5 20
300 500 90 90 silicone rubber(pbw) 100 100 100 100 100 100 100
aluminum oxide (pbw) 220 220 220 220 220 220 220 aluminum hydroxide
(pbw) 80 80 80 80 80 80 80 thermal conductivity W/(m .multidot. K)
2.9 1.6 1.9 3.6 4.2 1.0 2.3 *pbw = part by weight
Example 8
[0094] 100 parts by weight of low-hardness styrenic thermoplastic
elastomer (RIKEN VINYL INDUSTRY CO., LTD.) as a matrix, 100 parts
by weight of the carbon fiber powders of Sample 1 as a thermally
conductive filler, 20 parts by weight of boron nitride powder
(DENKI KAGAKU KOGYO KK.), and 20 parts by weight of aluminum
hydroxide powder (SHOWA DENKO K.K.) were mixed with a two-axis
extruder to prepare a pellet-like thermally conductive composition.
The resultant composition was extrusion-molded to form a thermally
conductive sheet of a thickness of 3 mm. The resultant sheet had a
Shore A hardness of 73. The thermal conductivity in the thickness
direction of the sheet was 2.1W/(m.multidot.K).
Examples 9 to 12
[0095] A pellet-like thermally conductive composition was prepared
as in Example 8, except that the content of the carbon fiber
powders was varied to 5, 20, 300, 500 parts by weight,
respectively. A thermally conductive sheet having a thickness of 3
mm was formed. The thermal conductivity in the thickness of the
resultant sheets was 1.3W/(m.multidot.K), 1.6W/(m.multidot.K),
2.9W/(m.multidot.K), and 3.6W/(m.multidot.K), respectively.
Comparative Example 3
[0096] A pellet-like thermally conductive composition was prepared
as in Example 8, except that commercially available pitch-based
carbon fiber powder (Petoca materials Ltd., milled Melblon) was
used as thermally conductive filler. A thermally conductive sheet
having a thickness of 3 mm was formed. The resultant sheet had a
Shore A hardness of 68. The thermal conductivity in the thickness
of the sheet was 1.4W/(m.multidot.K).
[0097] The contents of the thermally conductive composition of
Examples 8 to 12 and Comparative example 3 and the thermal
conductivity in the thickness direction of their respective
thermally conductive sheets are shown in Table 4.
4 TABLE 4 Ex. 8 Ex. 9 Ex. 10 Ex. 11 Ex. 12 Comp. 3 thermally
conductive filler (type) Sample 1 Sample 1 Sample 1 Sample 1 Sample
1 Pitch-based thermally conductive filler (pbw) 100 5 20 300 500
100 thermoplastic elastomer 100 100 100 100 100 100 boron
nitride(pbw) 20 20 20 20 20 20 aluminum hydroxide (pbw) 20 20 20 20
20 20 thermal conductivity W/(m .multidot. K) 2.1 1.3 1.6 2.9 3.6
1.4 *pbw = part by weight
Example 13
[0098] 100 parts by weight of polyacetal resin (Asahi Kasei
Corporation) as a matrix, 80 parts by weight of the carbon fiber
powders of Sample 2 with their surfaces treated with a silane
coupling agent as a thermally conductive filler, and 50 parts by
weight of aluminum oxide powder (SHOWA DENKO K.K.) were mixed with
a two-axis extruder to prepare a pellet-like thermally conductive
composition. The resultant composition was injection-molded to form
a thermally conductive molded article of a thickness of 3 mm. The
thermal conductivity in the thickness direction of the resultant
molded article was 1.7W/(m.multidot.K).
Examples 14 to 17
[0099] A pellet-like thermally conductive composition was prepared
as in Example 13, except that the content of the carbon fiber
powders was varied to 5, 20, 300, 500 parts by weight,
respectively. A thermally conductive molded article was formed
having a thickness of 3 mm. The thermal conductivity in the
thickness of the resultant molded articles was 1.1W/(m.multidot.K),
1.3W/(m.multidot.K), 2.8W/(m.multidot.K), and 3.5W/(m.multidot.K),
respectively.
(Comparative example 4
[0100] A pellet-like thermally conductive composition was prepared
as in Example 13, except that the carbon powders of Sample 4 were
used as thermally conductive filler. A thermally conductive molded
article was formed having a thickness of 3 mm. As described
previously, the carbon powders of Sample 4 were unequal so that the
carbon powders are difficult to mix in the matrix and gas bubbles
could not be removed. The thermal conductivity in the thickness of
the resultant molded article was 0.8W/(m.multidot.K).
[0101] The contents of the thermally conductive composition of
Examples 13 to 17 and Comparative example 4 and the thermal
conductivity of the thermally conductive molded articles are shown
in Table 5.
5 TABLE 5 Ex. 13 Ex. 14 Ex. 15 Ex. 16 Ex. 17 Comp. 4 thermally
conductive filler (type) Sample 2 Sample 2 Sample 2 Sample 2 Sample
2 Sample 4 thermally conductive filler (pbw) 80 5 20 300 500 80
polyacetal resin (pbw) 100 100 100 100 100 100 aluminum oxide (pbw)
50 50 50 50 50 50 thermal conductivity W/(m .multidot. K) 1.7 1.1
1.3 2.8 0.8 0.8 *pbw = part by weight
Example 18
[0102] 150 parts by weight of the carbon fiber powders of Sample 1
with their surfaces treated with a silane coupling agent as a
thermally conductive filler and 30 parts by weight of aluminum
oxide powder (SHOWA DENKO K.K.) were added to and dispersed in 100
parts by weight of bis-phenol F-type epoxy resin (YUKA SHELL EPOXY
CO., LTD) containing amine-type hardener as an adhesion polymer
matrix to prepare a thermally conductive composition which is a
thermally conductive adhesive. The resultant composition was
thermally hardened to form a plate specimen of thickness of 1 mm.
The thermal conductivity of the resultant plate specimen was
2.9W/(m.multidot.K).
Examples 19 to 22
[0103] A thermally conductive composition, which is a thermally
conductive adhesive, was prepared as in Example 18, except that the
content of the carbon fiber powders was varied to 5, 20, 300, 500
parts by weight, respectively. A plate specimen of thickness of 1
mm was formed. The thermal conductivity in the thickness of the
resultant plate specimen 1.2W/(m.multidot.K), 1.8W/(m.multidot.K),
3.4W/(m.multidot.K), and 4.1W/(m.multidot.K), respectively.
Comparative example 5
[0104] A thermally conductive composition, which is a thermally
conductive adhesive, was prepared as in Example 18, except that
commercially available pitch-based carbon fiber powder (Petoca
materials Ltd., milled Melblon) was used as thermally conductive
filler. A plate specimen of thickness of 1 mm was formed. The
thermal conductivity of the resultant plate specimen was
2.3W/(m.multidot.K).
[0105] The contents of Examples 18 to 22 and Comparative example 5
and the thermal conductivity of the thermally conductive adhesives
are shown in Table 6.
6 TABLE 6 Ex. 18 Ex. 19 Ex. 20 Ex. 21 Ex. 22 Comp. 5 thermally
conductive filler (type) Sample 1 Sample 1 Sample 1 Sample 1 Sample
1 Pitch-based thermally conductive filler (pbw) 150 5 20 300 500
150 epoxy resin (pbw) 100 100 100 100 100 100 aluminum oxide (pbw)
30 30 30 30 30 30 thermal conductivity W/(m .multidot. K) 2.9 1.2
1.8 3.4 4.1 2.3 *pbw = part by weight
Discussion
[0106] As described above, all the thermally conductive
compositions of Examples 1 to 22 include, in the matrix, the carbon
fiber powders of the present invention (Sample 1 to 3) that are
obtained by graphitizing the polymeric fiber having an aromatic
ring on its main chain by heating.
[0107] On the other hand, the thermally conductive compositions of
Comparative example 1 and Comparative example 4 include, in the
matrix, the conventional carbon powders (Sample 4) that are
obtained by graphitizing the aromatic polyimide film by heating and
pulverizing it.
[0108] The thermally conductive compositions of Comparative example
2, Comparative example 3 and Comparative example 5 also include, in
the matrix, the conventional pitch-based graphitized carbon fiber
powders that are produced by treating raw pitch by several
processes such as spinning, infusibilization, graphitization, and
pulverization.
[0109] The compositions of Examples 1 to 3 had higher thermal
conductivity compared with those of Comparative example 1 and
Comparative example 2. The compositions of Example 8 had a higher
thermal conductivity than that of Comparative example 3. The
compositions of Example 13 had a higher thermal conductivity than
that of Comparative example 4. The composition of Example 18 had a
higher thermal conductivity than that of Comparative example 5.
Accordingly, is was confirmed that the carbon fiber powders of the
present invention of Samples 1 to 3 had a higher thermal
conductivity than that of the conventional carbon powders of Sample
4 or the conventional pitch-based carbon fiber powders.
[0110] As described previously, the carbon powders of Sample 4 were
unequal so that the carbon powders are difficult to mix in the
matrix and gas bubbles could not be removed (See Comparative
example 1 and Comparative example 4). As appreciated from this
fact, it was confirmed that the carbon powders of Sample 4 had poor
filling capability. In addition, the resultant thermally conductive
compositions had poor thermal conductivity (See Comparative example
1 and Comparative example). On the other hand, such tendency could
not be observed when the content of the carbon fiber powders of
Samples 1 to 3 was varied in a range from 5 to 500 parts by weight.
Instead, it was confirmed that the resultant thermally conductive
compositions had high thermal conductivity (See Examples 1 to 22).
Thus, it was confirmed that the carbon fiber powders of the present
invention of Samples 1 to 3 were also superior in filling
capability to the conventional carbon powders of Comparative
example 4.
[0111] In the method of making the carbon fiber powders (Samples 1
to 3), the fiber is heated, graphitized, and pulverized or cut.
This eliminates the need of an infusibilization process. In
addition, by using carbon fibers rather than a film, the
pulverization or cutting process is simplified. Therefore, in this
method, high-performance carbon fiber powders that have two
characters of high thermal conductivity and high filling capability
can be manufactured easily.
[0112] It should be apparent to those skilled in the art that the
present invention may be embodied in many other specific forms
without departing from the spirit or scope of the invention.
[0113] Therefore, the present examples and embodiments are to be
considered as illustrative and not restrictive and the invention is
not to be limited to the details given herein, but may be modified
within the scope and equivalence of the appended claims.
* * * * *