Technical Field
[0001] The present invention relates to a method for producing a composite material in which
carbon nanotubes (hereinbelow, the carbon nanotubes are referred to as CNTs) are attached
to the surface of a plurality of continuous carbon fiber constituting a carbon fiber
bundle, and a composite material.
Background Art
[0002] Fiber-reinforced molded articles including reinforcing fibers dispersed in a resin,
the base material, have excellent mechanical properties and dimensional stability,
and thus find applications in a wide range of fields. CNT/carbon fiber composite materials
having structures in which a plurality of CNTs is entangled to form a CNT network
thin film on the surface of carbon fibers have been proposed as reinforcing fibers
(for example, Patent Literature 1).
[0003] Carbon fiber bundles, formed by bundling continuous carbon fibers into each unit
of thousands to tens of thousands, have excellent properties such as low density,
high specific strength, and a high specific modulus of elasticity. Prepregs, obtained
by impregnating such carbon fiber bundles with a resin, have promise for applications
having stricter requirements on the performance (such as aeronautical and aerospace
applications).
Citation List
Patent Literature
[0004]
Patent Literature 1 Japanese Petent Laid-Open No. 2013-76198
Patent Literature 2 International Patent Application WO 201/175319 A1
Summary of Invention
Technical Problem
[0005] In Patent Literature 1, a CNT network is formed on a carbon fiber surface by immersing
carbon fibers in a dispersion containing CNTs and imparting energies such as vibrations,
light irradiations, and thermal to the dispersion. It is mentioned that a fiber-reinforced
molded article that takes advantage of the characteristics of a base material as well
as includes the base material and carbon fibers strongly bonded to each other can
be provided when the base material is impregnated with the composite material of Patent
Literature 1.
In Patent Literature 2, a CNT network is formed on a carbon fiber surface by immersing
carbon fibers in a dispersion containing CNTs while giving ultrasonic waves to the
dispersion.
[0006] In a carbon fiber bundle including a plurality of continuous carbon fibers, in the
case in which CNTs are attached to the surface of each of the carbon fibers, more
excellent reinforcing fibers also having CNT-derived properties (composite material)
can be obtained. Such composite materials are required for producing high-strength
prepregs.
[0007] It is thus an object of the present invention to provide a method for producing a
composite material from which a high-strength prepreg having CNT-derived properties
fully exerted is obtained and such a composite material.
Solution to Problem
[0008] The method for producing a composite material according to the present invention
is characterized by comprising a step of immersing a carbon fiber bundle including
a plurality of continuous carbon fibers in a carbon-nanotubes-isolated dispersion
containing a plurality of isolatedly-dispersed carbon nanotubes and applying ultrasonic
vibrations at a frequency of 130 kHz or more and 180 kHz or less to form a structure
comprising a plurality of carbon nanotubes on a surface of each of the plurality of
carbon fibers, wherein the structure is directly attached to the surface of each of
the plurality of carbon fibers and has a network structure in which the carbon nanotubes
are directly connected to one another, and, in the composite material, the plurality
of carbon fibers align in one direction while maintaining the linearity substantially
without entangling with one another.
[0009] The composite material produced by the method aforementioned is disclosed.
Advantageous Effect of Invention
[0010] According to the method for producing a composite material of the present invention,
by immersing a carbon fiber bundle including a plurality of continuous carbon fibers
in a CNT-isolated dispersion and applying ultrasonic vibrations at a frequency of
more than 40 kHz and 180 kHz or less to the dispersion, CNTs are allowed to be attached
to the surface of each carbon fibers in the carbon fiber bundle. Since the frequency
exceeds 40 kHz, a possibility that the linearity of the carbon fibers in the carbon
fiber bundle is disturbed is reduced. In the composite material to be obtained, there
is substantially no entanglement among carbon fibers. Each carbon fiber in the carbon
fiber bundle can contribute to the strength, and the strength inherent in the carbon
fiber bundle is exerted. Furthermore, since the frequency is specified to be 180 kHz
or less, CNTs can be allowed to be satisfactorily attached to the surface of each
carbon fiber. In this manner, there is obtained a composite material that may sufficiently
exert CNT-derived properties.
[0011] In the production method of the present invention, the carbon fiber bundle is immersed
in a CNT-isolated dispersion in which CNTs are isolated dispersed. In the CNT-isolated
dispersion, CNTs are dispersed in a state in which the CNTs are each physically separated
and not entangled with one another in a dispersion medium. That such a CNT-isolated
dispersion is used is one of the reasons why CNTs can be satisfactorily attached to
the surface of each carbon fiber.
[0012] Since the composite material of the present invention is produced by the method of
the present invention, there is substantially no entanglement of carbon fibers with
one another, and the CNTs are satisfactorily attached to the surface of each of the
carbon fibers. For this reason, it is possible to obtain a high-strength prepreg by
impregnating the composite material of the present invention with a resin.
Brief Description of Drawings
[0013]
FIG. 1 is a partial schematic view illustrating the configuration of the composite
material according to the present embodiment.
FIG. 2 is a schematic diagram showing the relation between the frequency of ultrasonic
vibrations applied to a dispersion and the cavitation occurring in the dispersion.
FIG. 3 is an SEM micrograph of the composite material of Example 1, in which FIG.
3A shows a portion of the surface of a carbon fiber in a carbon fiber bundle and FIG.
3B is an enlarged photo of the carbon fiber surface.
FIG. 4 is an SEM micrograph of the composite material of Example 2, in which FIG.
4A shows a portion of the surface of a carbon fiber in a carbon fiber bundle and FIG.
4B is an enlarged photo of the carbon fiber surface.
FIG. 5 is an SEM micrograph of the composite material of Comparative Example 1, in
which FIG. 5A shows a portion of the surface of a carbon fiber in a carbon fiber bundle
and FIG. 5B is an enlarged photo of the carbon fiber surface.
FIG. 6 is an SEM micrograph of the composite material of Comparative Example 2, in
which FIG. 6A shows a portion of the surface of a carbon fiber in a carbon fiber bundle
and FIG. 6B is an enlarged photo of the carbon fiber surface.
FIG. 7 is an SEM micrograph of the composite material of Comparative Example 3, in
which FIG. 7A shows a portion of the surface of a carbon fiber in a carbon fiber bundle
and FIG. 7B is an enlarged photo of the carbon fiber surface.
FIG. 8 is a schematic view illustrating a carbon fiber to which CNTs are attached
by a conventional method, in which FIG. 8A is a schematic view illustrating a state
in which CNT aggregates are attached and FIG. 8B is a schematic view illustrating
a state in which attachment is insufficient.
Description of Embodiment
[0014] The present inventors, with regard to producing a composite material by forming a
structure containing CNTs on the surface of each carbon fiber, have established an
approach to attach CNTs to the surface of carbon fibers by immersing the carbon fibers
in a CNT dispersion and applying ultrasonics. Using this approach enables to form
a structure in which a plurality of CNTs is directly connected to one another to form
a network structure as well as are directly attached to the carbon fiber surface.
Then, when this approach was applied to a carbon fiber bundle including a plurality
of continuous carbon fibers to attach CNTs to the surface of the carbon fibers, entanglement
of the carbon fiber with one another was observed. When entanglement of the carbon
fibers with one another in the carbon fiber bundle is caused, the number of carbon
fibers capable of contributing to the strength becomes lesser than the original number,
and thus, the strength inherent in the carbon fiber bundle is not exerted. Even with
a carbon fiber bundle that comprises carbon fibers having CNTs attached to their surface,
it is difficult to obtain a high-strength prepreg by impregnating the bundle with
a resin when entanglement of the carbon fibers with one another is caused.
[0015] Entanglement of the carbon fibers with one another in the carbon fiber bundle is
responsible for disturbance in the linearity of the carbon fibers due to cavitation
occurred in the dispersion by application of ultrasonics. The present inventors, with
attention paid on this point, have enabled CNTs to attach to the surface of each of
the carbon fibers while entanglement of the carbon fibers with one another in the
carbon fiber bundle is avoided.
[0016] Hereinbelow, embodiments of the present invention will be described in detail with
reference to the drawings.
1. Entire configuration
[0017] As shown in FIG. 1, a composite material 10 of the present embodiment comprises a
carbon fiber bundle 12 that includes a plurality of continuous carbon fibers 12a.
In the figure, only 10 carbon fibers 12a are illustrated for description, but the
carbon fiber bundle 12 in the present embodiment is constituted by 10,000 to 30,000
carbon fibers 12a. The carbon fibers 12a constituting the carbon fiber bundle 12 align
in one direction while maintaining the linearity substantially without entangling
with one another.
[0018] The entanglement of the carbon fibers 12a in the carbon fiber bundle 12 can be evaluated
with respect to the degree of disturbance of the carbon fibers 12a. For example, the
carbon fiber bundle 12 is observed by means of a scanning electron microscope (SEM)
at a constant magnification to determine the length of a predetermined number (for
example, 10) of the carbon fibers 12a. The degree of disturbance of the carbon fibers
12a can be evaluated based on variations in the length, the difference between the
maximum value and the minimum value, and the standard deviation for the predetermined
number of the carbon fibers 12a. That the carbon fibers 12a are not substantially
entangled also can be determined by measuring the degree of entanglement, for example,
in accordance with the degree of intermingle measurement method of JIS L1013: 2010
"Testing methods for man-made fiber filament yarns". The smaller the degree of intermingle
measured, the lesser the entanglement of the carbon fibers 12a with one another in
the carbon fiber bundle 12. Accordingly, when a prepreg is produced, the carbon fibers
12a are homogeneously spread with ease, and each of the carbon fibers 12a can contribute
to the strength. On the surface of each of such carbon fibers 12a, a structure 14
is formed.
[0019] The carbon fibers 12a are fibers having a diameter of about 5 to 20 µm, obtained
by baking organic fibers derived from petroleum, coal, or coal tar such as polyacrylonitrile,
rayon, and pitch or organic fibers derived from wood or plant fibers.
[0020] The structure 14 on the surface of each of the carbon fibers 12a includes a plurality
of CNTs 14a. The CNTs 14a are homogeneously dispersed and entangled across substantially
the entire surface of the carbon fibers 12a, being in direct contact with or directly
connected to one another to form a network structure. Among each of the CNTs 14a,
there is preferably no intervening material, for example, a dispersing agent such
as a surfactant and an adhesive. Additionally, CNTs 14a are directly attached to the
surface of the carbon fibers 12a. Connection as referred to herein includes physical
connection (mere contact). Attachment as referred to herein refers to bonding due
to a van der Waals force. Furthermore, "direct contact or direct connection" includes
a state in which a plurality of CNTs is integrally connected in addition to a state
in which a plurality of CNTs is merely in contact with one another, and should not
be interpreted limitedly.
[0021] The length of the CNTs 14a forming the structure 14 is preferably 0.1 to 50 µm. When
the length of the CNTs 14a is 0.1 µm or more, the CNTs 14a are entangled with one
another to be directly connected. When the length of the CNTs 14a is 50 µm or less,
the CNTs 14a become likely to be dispersed homogeneously. In contrast, when the length
of the CNTs 14a is less than 0.1 µm, the CNTs 14a become difficult to entangle with
one another. When the length of the CNTs 14a exceeds 50 µm, the CNTs 14a become likely
to aggregate.
[0022] The CNTs 14a preferably have an average diameter of about 30 nm or less. When the
diameter of the CNTs 14a is 30 nm or less, the CNTs 14a are highly flexible and can
form a network structure on the surface of each of the carbon fibers 12a. In contrast,
when the diameter of the CNTs 14a is more than 30 nm, the CNTs 14a lose flexibility
and hardly form a network structure on the surface of the each of the carbon fibers
12a. Incidentally, the diameter of the CNTs 14a is an average diameter measured by
using a transmission electron microscope (TEM) micrograph. The CNTs 14a preferably
have an average diameter of about 20 nm or less.
[0023] A plurality of CNTs 14a are preferably attached homogeneously to the surface of each
of the carbon fibers 12a in the carbon fiber bundle 12. The attachment state of the
CNTs 14a on the surface of the carbon fibers 12a can be observed by means of an SEM
and evaluated by visually inspecting the image obtained.
[0024] Incidentally, in the case in which the CNTs are allowed to attach to the carbon fibers
by the conventional method, carbon fibers 32a that include CNT aggregates 34b, in
addition to CNTs 34a, attached to their surface may exist in the carbon fiber bundle,
as shown in FIG. 8A. Alternatively, as shown in FIG. 8B, carbon fibers 42a that have
no structure formed on their surface because of an insufficient amount of CNTs 44a
attached may be included in a carbon fiber bundle 42.
[0025] In contrast to this, in the present embodiment, carbon fibers that have CNT aggregates
attached to their surface are not substantially included in the carbon fiber bundle.
Carbon fibers that have no structure formed on their surface because of an insufficient
amount of CNTs attached do not either exist in the carbon fiber bundle.
[0026] In the composite material 10 of the present embodiment, the CNTs 14a are directly
attached to the surface of each of the carbon fibers 12a in the carbon fiber bundle
12. In other words, no dispersing agent such as a surfactant, an adhesive intervene
between the CNTs 14a and the surface of the carbon fibers, and the CNTs 14a are directly
attached to the surface of the carbon fibers 12a.
2. Production Method
[0027] Next, the method for producing the composite material 10 according to the present
embodiment will be described. The composite material 10 can be produced by immersing
the carbon fiber bundle 12 including the plurality of continuous carbon fibers 12a
in a CNT-isolated dispersion in which the CNTs 14a are isolatedly dispersed (hereinbelow,
the CNT-isolated dispersion is also merely referred to as the dispersion) and applying
ultrasonic vibrations at a predetermined frequency to the dispersion to form the structure
14 on the surface of each of the carbon fibers 12a. Hereinbelow, each step will be
described in order.
(Preparation of Dispersion)
[0028] For preparation of the dispersion, the CNTs 14a produced as follows can be used.
The CNTs 14a can be produced by depositing a catalytic film formed of aluminum or
iron onto a silicon substrate by using the thermal CVD method as described in Japanese
Patent Laid-Open No.
2007-126311, for example, microparticulating the catalytic metals for CNT growth, and bringing
a hydrocarbon gas into contact with the catalytic metals in a heating atmosphere.
CNTs obtained by other production methods such as arc discharge and laser evaporation
also can be used, but those containing impurities other than CNTs as little as possible
are preferably used. These impurities may be removed by high temperature annealing
in an inert gas after CNTs are produced. CNTs produced in this production example
are linearly-oriented long CNTs having a diameter of 30 nm or less and a length of
several hundred micrometers to several millimeters, thus having a high aspect ratio.
CNTs may be single-walled or multi-walled CNTs, and preferably multi-walled CNTs.
[0029] Subsequently, the CNTs 14a produced above are used to prepare a dispersion in which
the CNTs 14a are isolatedly dispersed. Isolated dispersion refers to a state in which
CNTs 14a are dispersed in a dispersion medium so that the CNTs 14a are each physically
separated and not entangled with one another and means a state in which aggregates
each formed by two or more CNTs 14a aggregated in a bundle form account for 10% or
less of all the CNTs.
[0030] The dispersion is prepared by adding CNTs 14a produced as above to a dispersion medium,
and a homogenizer, a shear disperser, an ultrasonic disperser is used to achieve homogeneous
dispersion of the CNTs 14a. Examples of the dispersion medium that can be used include
water, alcohols such as ethanol, methanol, and isopropyl alcohol, organic solvents
such as toluene, acetone, tetrahydrofuran (THF), methyl ethyl ketone (MEK), hexane,
normal hexane, ethyl ether, xylene, methyl acetate, and ethyl acetate. Additives such
as a dispersing agent and a surfactant are not necessarily required for preparing
the dispersion, but such additives may be used within a range in which the functions
of carbon fibers 12a and CNTs 14a are not limited.
(Formation of Structure)
[0031] Ultrasonic vibrations at a frequency of more than 40 kHz and 180 kHz or less are
applied to the dispersion prepared as described above, in which the carbon fiber bundle
12 including a plurality of continuous carbon fibers 12a is immersed. Application
of ultrasonic vibrations allows a plurality of CNTs 14a to be directly attached to
the surface of each of the carbon fibers 12a in the carbon fiber bundle 12. The CNTs
14a attached to the surface of each of the carbon fibers 12a are directly connected
to one another to form a network structure, forming a structure 14 on the surface
of each of the carbon fibers 12a.
[0032] When the frequency is more than 40 kHz, entanglement of the carbon fibers 12a in
the carbon fiber bundle 12 with one another is suppressed. When the frequency is 180
kHz or less, the CNTs 14a are satisfactorily attached to the surface of the carbon
fibers 12a. In contrast when the frequency is 40 kHz or less, the entanglement of
the carbon fibers 12a with one another becomes pronounced. When the frequency is more
than 180 kHz, the attachment state of the CNTs 14a on the surface of the carbon fibers
12a becomes defective, and thus the structure 14 cannot be formed. In order to further
reduce the entanglement of the carbon fibers 12a, the ultrasonic frequency is 130
kHz or more.
[0033] Application of ultrasonic vibrations at a frequency of more than 40 kHz and 180 kHz
or less to the dispersion creates a reversible reaction state in which a dispersion
state of the CNTs 14a and an aggregation state of the CNTs 14a are continuously occurred
in the dispersion.
[0034] In the dispersion in this reversible reaction state, the carbon fiber bundle 12 including
the plurality of continuous carbon fibers 12a is immersed. This causes a reversible
reaction state including a dispersion state and an aggregation state of the CNTs 14a
also on the surface of each of the carbon fibers 12a. On transfer from the dispersion
state to the aggregation state, CNTs 14a are attached to the surface of each of the
carbon fibers 12a.
[0035] When aggregating, CNTs 14a are under the action of a van der Waals force, which attaches
the CNTs 14a to the surface of the carbon fibers 12a. Thereafter, the carbon fiber
bundle 12 is withdrawn from the dispersion and dried, and thus, the composite material
10 in which a network structure is formed on the surface of each of the carbon fibers
12a in the carbon fiber bundle 12 can be obtained. Drying can be achieved by placing
the bundle 12 on a hot plate, for example.
[0036] The composite material 10 of the present embodiment can be formed into a prepreg
by opening the carbon fibers 12a in the carbon fiber bundle 12 and impregnating the
opened fibers with a resin. Examples of the resin for impregnation include, but not
particularly limited to, thermosetting resins such as epoxy resin and thermoplastic
resins such as phenoxy resin and nylon.
[0037] As described above, substantially no entanglement of the carbon fibers 12a with one
another in the carbon fiber bundle 12 exists in the composite material 10 of the present
embodiment. Thus, the plurality of carbon fibers 12a is homogeneously spread with
ease when a prepreg is produced. Furthermore, a structure 14 to which the CNTs 14a
are satisfactorily attached is formed on the surface of each of the carbon fibers
12a in the carbon fiber bundle 12. In a prepreg, obtained by impregnating the composite
material 10 like this with a resin, the possibility of reduction in the strength caused
by the entanglement of the carbon fibers 12a with one another is extremely low. Since
the CNTs 14a are satisfactorily attached to the surface of each of the carbon fibers
12a and form the structure 14, the prepreg to be obtained can sufficiently exert its
CNT-derived properties.
3. Action and Effects
[0038] In the method for producing a composite material according to the present embodiment,
a carbon fiber bundle including a plurality of continuous carbon fibers is immersed
in a CNT-isolated dispersion, and ultrasonic vibrations at a frequency of more than
40 kHz and 180 kHz or less are applied to the dispersion. The present inventors have
found that a frequency range of more than 40 kHz and 180 kHz or less is optimal in
the manner described below.
[0039] As shown in FIG. 2, the lower the frequency of the ultrasonic vibrations to be applied
on the dispersion, the higher the occurrence frequency of cavitation that occurs in
the dispersion. In contrast, the higher the frequency, the lower the occurrence frequency
of cavitation.
[0040] When the frequency of ultrasonic vibrations is low, the dispersibility of the CNTs
14a in the dispersion is also increased by the effect of the cavitation occurred.
When the carbon fiber bundle 12 including the plurality of continuous carbon fibers
12a is immersed in a dispersion in which cavitation is active, the CNTs 14a in the
dispersion are satisfactorily attached to the surface of each of the carbon fibers
12a. Cavitation is advantageous in respect of facilitating satisfactory attachment
of the CNTs 14a to the surface of the carbon fibers 12a. However, cavitation disturbs,
due to its large mechanical vibrations, the linearity of the carbon fibers 12a in
the carbon fiber bundle 12. Due to disturbance in the linearity of the carbon fibers
12a, entanglement of the carbon fibers 12a with one another is caused.
[0041] In contrast, when the ultrasonic frequency is increased, the occurrence frequency
of cavitation is reduced, and thus disturbance in the linearity of the carbon fibers
12a in the carbon fiber 12 is suppressed. Accordingly, entanglement of the carbon
fibers 12a with one another is scarce. However, in this case, since the occurrence
frequency of cavitation in the dispersion is reduced, the attachment state of the
CNTs 14a on the surface of the carbon fibers 12a also tends to be reduced.
[0042] As a result of intensive studies, the present inventors have obtained the following
findings. In other words, a shock wave ascribed to cavitation occurs in the dispersion
and entanglement of the carbon fibers 12a is caused by large mechanical vibrations
in the case in which the frequency of ultrasonic vibrations is 40 kHz or less. The
attachment state of CNTs 14a to the surface of each of the carbon fibers 12a becomes
defective in the case in which the frequency is more than 180 kHz. With a frequency
of 180 kHz or less, even if cavitation has not occurred, only the effect of the ultrasonic
vibrations enables CNTs 14a to be satisfactorily attached to the surface of the carbon
fibers 12a.
[0043] In the present embodiment, setting the frequency of ultrasonic vibrations to be applied
to the dispersion in which the carbon fiber bundle 12 is immersed to more than 40
kHz and 180 kHz or less has enabled the CNTs 14a to be satisfactorily attached to
the surface of each of the carbon fibers 12a while disturbance in the linearity in
the carbon fibers 12a was suppressed to reduce entanglement of the carbon fibers 12a
with one another.
[0044] Furthermore, in the present embodiment, the carbon fiber bundle 12 is immersed in
a CNT-isolated dispersion in which the CNTs 14a are isolatedly dispersed. In the CNT-isolated
dispersion, the CNTs 14a are dispersed in a state in which the CNTs 14a are each physically
separated and not entangled with one another in a dispersion medium. Use of such a
dispersion also leads to satisfactory attachment of CNTs to the surface of each of
the carbon fibers 12a in the carbon fiber bundle 12.
4. Modified Example
[0045] The present invention is not limited to the embodiment described above and can be
varied within the spirit of the present invention as appropriate.
[0046] As the carbon fiber bundle 12, a so-called regular tow constituted by 10,000 to 30,000
carbon fibers 12a can be used. The diameter of the carbon fibers 12a can be set within
a range of 5 to 10 µm as appropriate.
[0047] When the carbon fiber 12a is dried to obtain the structure 14 on the surface, the
dispersion medium may be evaporated from the carbon fiber bundle 12 by placing the
bundle on a hot plate, or otherwise by using an evaporator.
5. Example
[0048] Hereinbelow, the present invention will be described in detail with reference to
examples, but the present invention is not intended to be limited only to the following
examples.
[0049] A composite material of Example 1 was produced in the procedure described in the
production method described above. CNTs 14a used were Multi-walled Carbon Nanotubes
(MW-CNTs) which were grown to have a diameter of 10 to 15 nm and a length of 100 µm
or more on a silicon substrate by the thermal CVD method. For removal of catalyst
residues from the CNTs 14a, a 3:1 mixed acid of sulfuric acid and nitric acid was
used, and after washing, the CNTs 14a were filtered and dried. To cut the CNTs 14a,
the CNTs 14a were ground in a dispersion medium by an ultrasonic homogenizer to a
length of 0.5 to 10 µm. Methyl ethyl ketone was used as the CNT dispersion medium
to prepare a dispersion. The concentration of the CNTs 14a in the dispersion was set
to 0.01 wt%. This dispersion contains no dispersing agent and adhesive.
[0050] Then, while ultrasonic vibrations at 130 kHz was applied to the dispersion, T700SC-12000
(manufactured by Toray Industries, Inc.) as a carbon fiber bundle 12 was placed in
the dispersion. The carbon fiber bundle 12 used herein include 12,000 carbon fibers
12a. The carbon fibers 12a each have a diameter of about 7 µm and a length of about
100 m. The carbon fiber bundle 12 was maintained in the dispersion for 10 seconds.
[0051] Thereafter, the carbon fiber bundle 12 was removed from the dispersion and dried
on a hot plate at about 80°C to form a structure 14 on the surface of each of the
carbon fibers 12a constituting the carbon fiber bundle 12. In this manner, the composite
material 10 of Example 1 was obtained.
[0052] Additionally, the composite material of Example 2 was produced in the same manner
as Example 1 except that the ultrasonic frequency was changed to 160 kHz. Furthermore,
the composite materials of Comparative Examples 1, 2, and 3 were produced in the same
manner as Example 1 except that the ultrasonic frequency was changed to 28 kHz, 38
kHz, and 200 kHz, respectively.
[0053] The surface of the carbon fibers in the composite materials of Examples 1 and 2 and
Comparative Examples 1 to 3 was observed by an SEM, and the images obtained are shown
in FIGs. 3A and 3B to 7A and 7B.
[0054] FIG. 3A is an SEM micrograph showing a portion of the surface of the carbon fibers
12a in the carbon fiber bundle 12 in the composite material of Example 1, and FIG.
3B is an enlarged photo of FIG. 3A. A state in which a plurality of CNTs 14a are dispersed
homogeneously on and attached to the surface of the carbon fibers 12a by forming a
structure 14 is shown.
[0055] FIG. 4A is an SEM micrograph showing a portion of the surface of the carbon fibers
12a in the carbon fiber bundle 12 in the composite material of Example 2, and FIG.
4B is an enlarged photo of FIG. 4A. Similarly to Example 1, it can be seen that the
plurality of CNTs 14a are dispersed homogeneously on and attached to the surface of
the carbon fibers 12a by forming a structure 14 also in Example 2.
[0056] In the carbon fibers 12a of the composite materials of Example 1 and Example 2, it
was confirmed that substantially no entanglement of the carbon fibers 12a exists in
the carbon fiber bundle 12.
[0057] With regard to producing the composite materials, the frequency of ultrasonic vibrations
was 130 kHz in Example 1 and 160 kHz in Example 2. In the composite materials of Examples
1 and 2, the CNTs 14a are satisfactorily attached to the surface of each of the carbon
fibers 12a in the carbon fiber bundle 12 and form the structure 14 on the surface.
Furthermore, substantially no entanglement of the carbon fibers 12a with one another
exists in the carbon fiber bundle 12. It is assumed that prepregs obtained by impregnating
such composite materials of Example 1 and Example 2 with a resin are capable of sufficiently
exerting CNT-derived properties and have high strength.
[0058] FIG. 5A is an SEM micrograph showing a portion of the surface of carbon fibers 52a
in the carbon fiber bundle in the composite material of Comparative Example 1, and
FIG. 5B is an enlarged photo of FIG. 5A. FIG. 6A is an SEM micrograph showing a portion
of the surface of the carbon fibers 52a in the carbon fiber bundle in the composite
material of Comparative Example 2, and FIG. 6B is an enlarged photo of FIG. 6A. FIG.
5B and FIG. 6B shows that a plurality of CNTs 14a are dispersed to form the structure
14 on the surface of the carbon fibers 52a in the composite materials of Comparative
Examples 1 and 2.
[0059] In the composite materials of Comparative Examples 1 and 2, much entanglement of
the carbon fibers 52a with one another in the carbon fiber bundle has occurred. With
regard to producing the composite materials, the frequency of ultrasonic vibrations
was 28 kHz in Comparative Example 1 and 38 kHz in Comparative Example 2. Since entanglement
of the carbon fibers 52a with one another in the carbon fiber bundle has been occurred,
it is difficult to provide high-strength prepregs even if the composite materials
of Comparative Examples 1 and 2 are impregnated with a resin.
[0060] FIG. 7A is an SEM micrograph showing a portion of the surface of the carbon fibers
62a in the carbon fiber bundle in the composite material of Comparative Example 3,
and FIG. 7B is an enlarged photo of FIG. 7A. As shown in FIG. 7B, carbon fibers 62a
that have few CNTs 14a attached to their surface are included in the composite material
of Comparative Example 3. On the surface of these carbon fibers 62a, no structure
14 is formed. In Comparative Example 3, the frequency of ultrasonic vibrations was
set to 200 kHz with regard to producing the composite materials. Since no structure
14 constituted by a network structure of a plurality of CNTs 14a has been formed on
the surface of the carbon fibers 62a, a prepreg in which CNT-derived properties are
sufficiently exerted is not obtained even if the composite material of Comparative
Examples 3 is impregnated with a resin.
[0061] The carbon fiber bundle including a plurality of continuous carbon fiber can be used
to produce a composite material from which a high-strength prepreg having CNT-derived
properties fully exerted is obtained.
Reference Signs List
[0062]
- 10
- Composite material
- 12, 42
- Carbon fiber bundle
- 12a, 32a, 42a, 52a, 62a
- Carbon fiber
- 34b
- CNT aggregate
- 14
- Structure
- 14a, 34a, 44a
- Carbon nanotube (CNT)