Patent Description:
A carbon fiber bundle in which continuous carbon fibers on the order of several thousands to several tens of thousands are bound excellent characteristics such as low density, high specific degree of strength, high specific degree of elasticity. A prepreg obtained by impregnating such a carbon fiber bundle with resin is expected to meet demands for use in applications requiring high performance (aerospace applications, etc.).

A CNT/carbon fiber composite material has been proposed as reinforced fiber, which has a structure in which a CNT network thin film is formed by entanglement of a plurality of carbon nanotubes (hereinafter also referred to as CNT) upon the surface of carbon fibers (for example, Patent Literature <NUM>). Such a composite material is useful as a base material of a carbon-fiber-reinforced molded body such as a carbon fiber reinforced plastic (hereinafter also referred to as CFRP). For example, Patent Literature <NUM> discloses a method for producing a suitable reinforced thermosetting polymer composite, comprising preparing a fiber; producing a coating comprising carbon nanotubes and a polymeric binder; applying the coating to the fiber; obtaining a coated fiber; impregnating the coated fiber with a thermoset polymer precursor and transferring a portion of the carbon nanotubes from the coating into the thermoset polymer precursor; and curing the precursor including the coated fiber and the transferred carbon nanotubes to achieve a reinforced thermoset polymer composite. Patent Literature <NUM> discloses a composition including a carbon nanotube (CNT) infused carbon fiber material including a carbon fiber material of spoolable dimensions and carbon nanotubes (CNTs) infused to the carbon fiber material. Patent Literature <NUM> discloses a method for manufacturing a substrate, including (<NUM>) a process for forming a dispersion wherein metal-based catalytic fine particles are dispersed in a solvent, (<NUM>) a process for covering a surface of a single crystal substrate with a modifying substance, and (<NUM>) a process for fixing the metal-based catalytic fine particles on the single crystal substrate by bringing the single crystal substrate whose surface is covered into contact with the dispersion. Patent Literature <NUM> discloses a fiber-reinforced polymer composition comprising fibers and an adhesive composition, wherein the adhesive composition comprises at least a thermosetting resin and a curing agent. Patent Literature <NUM> discloses a carbon fiber-reinforced molded article comprising a base material (<NUM>) and a composite material (<NUM>) dispersed in the base material (<NUM>), characterized by: the composite material (<NUM>) including carbon fibers (<NUM>) and a structure that is formed on the surface of the carbon fibers (<NUM>) and includes a plurality of carbon nanotubes (18a); the plurality of carbon nanotubes (18a) forming a network structure in which same are directly connected to each other; and the plurality of carbon nanotubes (18a) being directly attached to the surface of the carbon fibers (<NUM>), using part of the surface as an attachment section (<NUM>), and being physically joined to said surface by a binding member (<NUM>) provided in at least part of the surface other than the attachment section (<NUM>).

In Patent Literature <NUM>, carbon fibers are immersed in a dispersion containing CNTs, which are subjected to application of energy such as vibrations, optical radiation, heat, etc., and thereby a CNT network is formed on the surface of the carbon fibers. When the effects of the CNTs can be fully exhibited, it is made possible to obtain a composite material having superior characteristics. The most part of the mode of the destruction of the structural member using CFRP is interlayer peeling. As a result, CFRP is required to have a large resistance to progression of interlayer peeling crack.

In view of this, an object of the present invention is to provide a composite material capable of fully exhibiting the effects of carbon nanotubes, a prepreg using the same, a carbon-fiber-reinforced molded article having larger resistance to progression of interlayer peeling crack, and a method for manufacturing the composite material.

In order to solve the problem described above, a composite material as defined in claims <NUM> and <NUM> is provided.

A prepreg as defined in claim <NUM> in accordance with the present invention includes the above-described composite material and a matrix resin impregnated with the composite material.

A carbon-fiber-reinforced molded article as defined in claim <NUM> in accordance with the present invention is made of a cured material of the above-described prepreg.

The present invention further relates to a method for manufacturing a composite material as defined in claim <NUM>.

The composite material of the present invention includes a bundle of carbon fibers to the surface of which CNTs adhere. Since the fixation of the CNTs on the surface of the carbon fibers is partial, at locations which are not fixed, the CNTs are allowed to be detached from the surface of the carbon fiber and raised. By virtue of the fact that there is CNTs in a free state where they are raised above the surface of the carbon fibers, the effects of the CNTs can be fully exhibited.

In the prepreg in which the composite material of the present invention is impregnated with the matrix resin, the CNTs raised from the carbon fibers are brought into direct contact with the matrix resin. In the carbon-fiber-reinforced molded article obtained by curing of such a prepreg, a CNT composite resin layer is formed in which the CNTs and the matrix resin are combined in the form of a composite element. By virtue of the fact that the CNT composite resin layer is provided, the resistance to progression of interlayer peeling crack of the carbon-fiber-reinforced molded article of the present invention can be enhanced.

As depicted in <FIG>, a composite material <NUM> of this embodiment includes a carbon fiber bundle <NUM> in which a plurality of continuous carbon fibers 12a are arranged. The diameter of the carbon fibers 12a is <NUM> to <NUM> micrometers (µm). The carbon fibers 12a can be obtained by baking of organic fibers derived from fossil fuels and organic fibers derived from wood and plant fibers. While the figure depicts only <NUM> carbon fibers 12a for the sake of explanation, the carbon fiber bundle <NUM> in this embodiment can include <NUM>,<NUM> to <NUM>,<NUM> carbon fibers 12a.

CNTs 14a adhere to the surfaces of the respective carbon fibers 12a. The CNTs 14a are dispersed on the surface of the carbon fibers 12a and entangled with one another and are capable of thereby forming a network structure by being brought into direct contact with or directly connected to each other. It is preferable that there is no intervening material between the CNTs 14a such as dispersing agents such as surfactants, adhesives, etc..

The CNTs 14a adhere directly to the surface of the carbon fibers 12a. Connection in this context may include physical connection (simple contact). Also, adhesion in this context refers to bonding by Van Der Waals forces. Further, "direct contact or direct connection" may include a state where a plurality of CNTs are in simple contact with one another, in addition to which it may include a state where a plurality of CNTs are interconnected in an integral manner.

The CNTs 14a uniformly adhere to the surface of the carbon fibers 12a. Specifically, as will be described later with reference to the actual measurement examples, the uniformity can be evaluated using the following procedure. First, <NUM> frames of <NUM> square are set at equal intervals in a region of <NUM> in the length direction of the carbon fibers 12a. Subsequently, for each of these frames, the number of the carbon nanotubes intersecting any one side of the four sides of each frame is counted. Finally, the standard deviation based on the above counting result is determined. In the invention, the standard deviation of the above-mentioned number is <NUM> or less.

It is preferable that the CNTs 14a have a length of <NUM> or more. The length of the CNTs 14a is the length of the carbon nanotubes observed when the above-mentioned uniformity was measured. Also, the length of the CNTs 14a may be determined based on an optical photomicrograph. When the length of the CNTs 14a is equal to or longer than <NUM>, then it will be easier for them to adhere uniformly to the individual surfaces of the carbon fibers 12a. The CNTs 14a may have a length of <NUM> or more. The state of adhesion of the CNTs 14a can be evaluated by observation by an SEM and visual inspection of the obtained image. At least <NUM>% of the measured CNTs 14a have a length of <NUM> or more. It is preferable that the ratio of the CNTs 14a having the length of <NUM> or more is <NUM>% or more, and it is most preferable that the ratio is <NUM>% or more.

It is preferable that the CNTs 14a has an average diameter of <NUM> or less. CNTs 14a having an average diameter of <NUM> or less are very flexible, and capable of creating a network structure on the surfaces of the carbon fibers 12a. The diameter of the CNTs 14a is an average diameter measured using a transmission electron microscope (TEM) photograph. It is more preferable that the CNTs 14a have an average diameter of <NUM> or less.

The plurality of CNTs 14a are partly fixed to the surface of the carbon fibers 12a by the plurality of fixing resin parts 16a. <FIG> provides an SEM photograph of the surface of the carbon fibers 12a in the composite material <NUM>. A plurality of CNTs 14a adhere to the surface of the carbon fibers 12a. The black regions present in a scattered manner in the photograph correspond to the fixing resin parts 16a. The fixing resin part 16a is made of cured material obtained from reactive curing resin, thermosetting resin, or thermoplastic resin. As will be described later in detail, the fixing resin parts 16a is formed by performing sizing treatment with a sizing agent containing resin in the form of droplets with a particle size on the order of <NUM> to <NUM> emulsion type.

<FIG> schematically depicts part of the surface of the carbon fibers 12a illustrated in <FIG>. The fixing resin parts 16a cover <NUM>% or more and <NUM>% or less of the surface of the carbon fibers 12a to which the CNTs 14a adhere. In the invention, the fixing resin parts 16a are present at a rate of <NUM> to <NUM> pieces per <NUM> square in the surface of the carbon fibers 12a to which the CNTs 14a adhere. All of the CNTs 14a adhering to the surfaces of the carbon fibers 12a are fixed to the carbon fibers 12a by the fixing resin parts 16a at a certain location in their individual lengths.

In the case where the area percentage of the fixing resin part 16a in the surface of the carbon fibers 12a is less than <NUM>%, the CNTs 14a cannot be sufficiently fixed to the surface of the carbon fibers 12a, so that the CNTs 14a are peeled off from the surface of the carbon fibers 12a during producing of the prepreg. If the area percentage of the fixing resin part 16a exceeds <NUM>%, the entirety of the CNTs 14a will be fixed to the carbon fibers 12a. Also, in the case where the area percentage of the fixing resin part 16a is <NUM>% or more and <NUM>% or less, when the number of pieces per <NUM> square is deviated from the above-mentioned range, the desired effect cannot be obtained. If the number of pieces is less than <NUM>, then the fixing resin parts 16a having a large area will be sparsely arranged, which causes creation of CNTs 14a that are not at all fixed to the carbon fibers 12a. If the number of pieces exceeds <NUM>, the area of the fixing resin part 16a per piece is too small, which makes it impossible for the CNTs 14a to be sufficiently fixed to the carbon fibers 12a. Any of the above-described cases will lead to decrease in the strength of the obtained carbon-fiber-reinforced molded article.

The substantial area per piece of the fixing resin parts 16a is on the order of <NUM> to <NUM><NUM>. If the individual areas of the fixing resin parts 16a are lower than the lower threshold, the strength of adhesion is weak, making it impossible for the CNTs 14a to be sufficiently fixed to the surfaces of the carbon fibers 12a. In this case as well, in the same manner as described above, decrease in the strength of the obtained carbon-fiber-reinforced molded article will result. If the particle size of the resin in the form of droplets is <NUM> or less, then it is possible to avoid a situation where the CNTs 14a sink in the fixing resin parts 16a. If the particle size of the resin in the form of droplets e size is <NUM> or more, then the CNTs 14a can be fixed to the surface of the carbon fibers 12a. It is preferable that the particle size of the resin in the form of droplets is on the order of <NUM> to <NUM>. In some cases, multiple resins in the form of droplets may be made into one piece on the surface of the carbon fiber, so that the upper threshold of the area of the fixing resin part 16a should be as discussed above.

If the length of the CNTs 14a is <NUM> or more, the CNTs 14a are present in such a manner that they extend to the outside of the fixing resin part 16a without being completely covered by the fixing resin parts 16a. For example, the total area of the fixing resin parts 16a in <NUM> square of the surfaces of the carbon fibers 12a is on the order of <NUM> to <NUM><NUM>.

The state of the CNTs 14a in the surface of the carbon fibers 12a will be described with reference to <FIG> is a schematic diagram of regions P1, P2, and P3 in the cross section taken along the line L in <FIG>. At the region P1, two CNTs 14a adhering onto the carbon fibers 12a are covered by the fixing resin part 16a. At the region P2, one CNT 14a adhering onto the carbon fibers 12a is covered by the fixing resin part 16a. At the regions P1 and P2, the CNTs 14a are fixed in this manner to the carbon fibers 12a.

In the region P3, no fixing resin parts 16a as in the cases of the regions P1, P2 are present. The CNTs 14a in the region P3 are not fixed to the carbon fibers 12a and only adhere to the surface of the carbon fibers 12a by Van der Waals forces. As a result, the CNTs 14a in the region P3 can be detached from the surface of the carbon fibers 12a and raised. In this case as well, the CNTs 14a are fixed at a certain location in their lengths to the surfaces of the carbon fibers 12a by the fixing resin part 16a.

As mentioned above, the CNTs 14a adhere to the surface of the carbon fibers 12a contained in the composite material <NUM> and, further, the fixing resin parts 16a are provided at a predetermined area percentage. <FIG> provides a longitudinal cross-sectional view of the prepreg of this embodiment including this composite material <NUM> viewed in its length direction. The prepreg <NUM> includes the composite material <NUM> of this embodiment and a matrix resin layer <NUM>.

With regard to the matrix resin used in the matrix resin layer <NUM>, thermosetting resin or thermoplastic resin can be used. With regard to the thermosetting resin, for example, epoxy resin, unsaturated polyester resin, vinyl ester resin, phenol resin, melamine resin, urea resin, cyanate ester resin, bismaleimide resin may be mentioned. With regard to the thermoplastic resin, for example, polyolefin resin, polyamide resin, polycarbonate resin, polyphenylene sulfide resin may be mentioned.

<FIG> is a perspective view of the carbon-fiber-reinforced molded article of this embodiment. Since the carbon-fiber-reinforced molded article <NUM> is a cured material of the prepreg <NUM>, it includes a cured resin layer <NUM> resulting from the curing of the matrix resin layer <NUM> and a composite material <NUM>. The carbon-fiber-reinforced molded article <NUM> is of carbon fiber reinforced plastic (CFRP). The carbon-fiber-reinforced molded article <NUM> may have any suitable dimensions according to the JIS standards relevant to the test being conducted.

Next, a method for manufacturing the composite material <NUM>, the prepreg <NUM>, and the carbon-fiber-reinforced molded article <NUM> in accordance with this embodiment will be explained.

The composite material <NUM> can be manufactured by immersing a carbon fiber bundle <NUM> containing a plurality of carbon fibers 12a in a CNT dispersion in which the CNTs 14a are isolated and dispersed (which may also be hereinafter referred to simply as a dispersion), to attach the CNTs 14a to the respective surfaces of the carbon fibers 12a, and then subjecting the carbon fiber bundle <NUM> to sizing treatment. Hereinafter, the respective steps will be explained in order.

In the preparation of the dispersion, it is possible to use CNTs 14a manufactured in the following manner. The CNTs 14a can be created, for example, by forming a catalyst film composed of aluminum and iron on a silicon substrate using a thermal CVD method as described in <CIT>, micronizing the catalyst metal for CNT growth, and bringing the catalyst metal into contact with hydrocarbon gas in a heating atmosphere.

CNTs created by other methods such as arc discharge method and laser evaporation method may be used as long as the CNTs contain as few impurities as possible. Impurities can be removed by annealing the manufactured CNTs at high temperature in inert gas. The CNTs thus created has a high aspect ratio and linearity as they have a diameter of <NUM> or less and a length of several hundred micrometers to several millimeters. The CNTs may have either a single layer or multiple layers but preferably have multiple layers.

A dispersion in which the CNTs 14a are isolated and dispersed is prepared using the CNTs 14a that have been prepared as described above. Isolated dispersion means a state in which the CNTs 14a are physically separated one by one and dispersed in the dispersion medium without being entangled and the proportion of aggregates in which two or more CNTs 14a are gathered in a shape of a bundle is <NUM>% or less.

Dispersion aims to make the dispersion of CNTs 14a uniform by using a homogenizer, shear force, an ultrasonic dispersion machine, etc. As the dispersion medium, it is possible to use water; alcohols such as ethanol, methanol, and isopropyl alcohol; and organic solvents such as toluene, acetone, tetrahydrofuran (THF), methyl ethyl ketone (MEK), hexane, normal hexane, ethyl ether, xylene, methyl acetate, ethyl acetate, etc..

For the preparation of the dispersion, additives such as dispersing agent and surfactant are not necessarily required, but such additives may be used within a range where they do not inhibit the functions of the carbon fibers 12a and the CNTs 14a.

The carbon fiber bundle <NUM> is immersed in the dispersion that has been prepared in the above-described manner, and then mechanical energy is applied to the dispersion to attach the CNTs 14a to the surfaces of the carbon fibers 12a. As the mechanical energy, vibrations, ultrasonic waves, fluctuation may be mentioned. By application of the mechanical energy, a reversible reaction state is created in the dispersion, where a state in which the CNTs 14a are dispersed and a state in which they are aggregated always occur.

When the carbon fiber bundle <NUM> containing the plurality of continuous carbon fibers 12a are immersed in the dispersion placed in the reversible reaction state, then the reversible reaction state of the dispersion state and the aggregation state of the CNT 14a also occurs on the surfaces of the carbon fibers 12a. The CNTs 14a adhere to the surfaces of the carbon fibers 12a when the CNTs 14a exit the dispersion state and enter the aggregation state.

During aggregation, van der Waals forces act on the CNTs 14a and the CNTs 14a adhere to the surfaces of the carbon fibers 12a by the van der Waals forces. In this manner, a carbon fiber bundle (CNT-adhered carbon fiber bundle) is obtained with the CNTs 14a adhering to the respective surfaces of the carbon fibers 12a in the carbon fiber bundle <NUM>. In this embodiment, during adhesion of the CNTs 14a, adjustment is performed such that the period of time of the equilibrium state becomes longer than in the conventional cases. By virtue of this, the uniformity of the adhesion of the CNTs 14a on the surfaces of the carbon fibers 12a can be increased.

In the sizing treatment, a sizing agent of emulsion type is used. The sizing agent of emulsion type is a sizing agent containing resin in the form of droplets with a particle size of <NUM> to <NUM>. The particle size can be determined by laser analysis. With regard to the resin, for example, a reactive resin may be mentioned. The reactive resin is a resin having a functional group having high reactivity with a carboxyl group, specifically, a resin having an oxazoline group. With regard to the reactive resin emulsion, for example, Epocross (TM) (manufactured by Nippon Shokubai Co. ) may be mentioned. The Epocross has a concentration of the reactive resin on the order of <NUM>% by mass.

The sizing agent can be diluted with a solvent and used as sizing liquid. With regard to the solvent, for example, water, ethanol, acetone, MEK, N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, toluene, styrene, etc. may be mentioned. These solvents can be used alone or in combination of two or more of them. Resin concentration after the dilution is adjusted as appropriate such that the amount of adhesion of the sizing agent on the carbon fiber surface after drying becomes a predetermined amount.

With regard to the solvent, a water solvent is preferred in terms of handleability and safety. The concentration of the sizing agent in the sizing liquid should be timely changed according to the target amount of adhesion of the sizing agent. The amount of adhesion of the sizing agent on the carbon fiber surface after the drying is on the order of <NUM> to <NUM>% by mass, and preferably on the order of <NUM>% by mass.

The sizing treatment can be performed by applying the sizing liquid onto the CNT-adhered carbon fiber bundle and then performing drying to cure the resin. With regard to the method of application of the sizing liquid, for example, roller dipping and roller contact methods may be mentioned. The amount of adhesion of the sizing agent on the carbon fiber surface can be adjusted by sizing liquid concentration tuning and metering adjustment. With regard to the means for drying, for example, hot air, hot plate, heating roller, various infrared heaters, etc. may be mentioned.

The composite material <NUM> of this embodiment is obtained by subjecting the CNT-adhered carbon fiber bundle to the sizing treatment using the sizing agent of emulsion type. In the composite material <NUM>, <NUM>% or more and <NUM>% or less of the surface of the carbon fibers 12a to which the CNTs 14a adhere is covered by a plurality of fixing resin parts 16a.

In this embodiment, a sizing agent of emulsion type containing resin in droplets with a particle size of <NUM> to <NUM> is used. As discussed above, if the particle size of the resin in the form of droplets is <NUM> or less, then the resin is allowed to reside between the CNTs 14a adhering to the surface of the carbon fibers 12a and forming a network structure. In this manner, the fixing resin parts 16a each having an area on the order of <NUM> to <NUM><NUM> are formed.

The area of the fixing resin parts 16a varies depending on the particle size of the droplets in the emulsion. The sizing agent in use is diluted as appropriate. In the case the sizing liquid is used which has been adjusted such that the amount of adhesion of the sizing agent on the carbon fiber surfaces after the drying becomes <NUM> to <NUM>% by mass, the number of the fixing resin parts 16a per <NUM> square of the surfaces of the carbon fibers 12a will be <NUM> to <NUM>. The number of the fixing resin parts 16a per unit area varies, for example, depending on the concentration of the resin in the sizing liquid.

The prepreg <NUM> of this embodiment can be produced by impregnating the matrix resin with the composite material <NUM>. The prepreg <NUM> can be produced, for example, by a wet method. In the case of the wet method, the matrix resin is dissolved in a solvent such as MEK and methanol to prepare a matrix resin solvent with low viscosity, which is impregnated with the composite material <NUM>. After that, the composite material <NUM> is taken out of the matrix resin solvent, the solvent is made to evaporate by an oven or the like, and the prepreg <NUM> is thus obtained.

The prepreg <NUM> may be manufactured by a hot melt method. According to the hot melt method, the matrix resin is heated to lower its viscosity and impregnated with the composite material <NUM>. Specifically, a resin film is used which has been created by coating exfoliate paper or the like with the matrix resin. A resin film or films are place on both sides or either side of the composite material <NUM> to apply heat and pressure thereto, and the matrix resin is impregnated with the composite material <NUM>. According to the hot melt method, the prepreg <NUM> can be obtained without solvent residues.

In order to obtain the carbon-fiber-reinforced molded article <NUM> of this embodiment, for example, the prepreg <NUM> is cut to a predetermined length and lamination is performed as required and a laminated body is created. The carbon-fiber-reinforced molded article <NUM> can be produced by imparting pressure to this laminated body and heating and curing the matrix resin. The method of applying heat and pressure can be selected from press molding, autoclave molding, bagging molding, wrapping tape technique, an internal pressure molding.

The composite material <NUM> in accordance with this embodiment is constituted by a bundle of carbon fibers 12a (carbon fiber bundle <NUM>) having CNTs 14a adhering to its surface and further including a plurality of fixing resin parts 16a. <NUM>% or more and <NUM>% or less of the surface of the carbon fibers 12a to which the CNTs 14a adhere is covered by a plurality of fixing resin parts 16a. As depicted in <FIG> and <FIG> as the regions P1 and P2, the CNTs 14a are partly fixed to the surface of the carbon fibers 12a by means of the fixing resin parts 16a. As a result, the CNTs 14a do not peel off from the surfaces of the carbon fibers 12a.

The CNTs 14a have locations that are not fixed to the surface of the carbon fibers 12a (for example, the region P3 in <FIG> and <FIG>). The CNTs 14a in the region P3 adhere to the surface of the carbon fibers 12a by Van der Waals forces without any fixing resin part 16a, so that they can be detached from the surface of the carbon fibers 12a and raised. Since the CNTs 14a in this case are in a free state, the effects which will be described below will be obtained.

Specifically, in the prepreg <NUM> in which the matrix resin is impregnated with the composite material <NUM>, the CNTs 14a that have raised from the surface of the carbon fibers 12a are brought into direct contact with the matrix resin layer <NUM>. In a molded article <NUM> made by curing of such a prepreg <NUM>, as illustrated in <FIG>, the CNT composite resin layer <NUM> is formed at the interface between the cured resin layer <NUM> and the carbon fibers 12a. The CNTs 14a in the CNT composite resin layer <NUM> are present at a high concentration on the side of the carbon fibers 12a. The concentration of the CNTs 14a gradually decreases according to increase in the distance from the carbon fibers 12a.

In the CNT composite resin layer <NUM>, the CNTs 14a and the matrix resin are combined in the form of a composite element, as a result of which the CNT composite resin layer <NUM> has enhanced strength and flexibility which derive from the CNTs 14a. The CNT composite resin layer <NUM> also has the effect of relieving stress concentration. In addition, since the CNTs 14a are partly fixed to the surface of the carbon fibers 12a by the fixing resin part 16a, the remaining portions of the CNTs 14a are combined with the matrix resin in the form of a composite element, so that the adhesive strength between the carbon fibers 12a and the cured resin layer <NUM> is improved.

This CNT composite resin layer <NUM> suppresses progression of interlayer peeling in the carbon-fiber-reinforced molded article <NUM>. By virtue of this, the carbon-fiber-reinforced molded article <NUM> in accordance with this embodiment will have a larger aperture mode interlaminar fracture toughness value (GIR) in the process of the crack progression.

As mentioned above, in the composite material <NUM> in accordance with this embodiment, the CNTs 14a uniformly adhere to the surface of the carbon fibers 12a. This fact also is a factor that increases the GIR. For example, if there is a region which has an extremely small amount of the adhered CNTs 14a, then there is a small amount of CNTs 14a which are partly fixed to the surface of the carbon fibers 12a, so that the effect of the CNT composite resin layer <NUM> will also become insufficient. In this embodiment, by virtue of the fact that the CNTs 14a uniformly adhere to the surface of the carbon fibers 12a, the effect of the CNT composite resin layer <NUM> can be more reliably obtained.

The present invention will be described with reference to examples, but the present invention is not limited to the following examples.

The composite material of the example was created by the procedure described in the above-described manufacturing method. As the CNTs 14a, multi-walled carbon nanotubes MW-CNT grown to a diameter of <NUM> to <NUM> and a length of <NUM> or more on a silicon substrate by thermal CVD were used.

The CNTs 14a were washed with a <NUM>:<NUM> mixed acid of sulfuric acid and nitric acid to remove catalyst residues and then filtered and dried. The CNTs 14a were added to an MEK as a dispersion medium and the dispersion was prepared. The CNTs 14a were crushed using an ultrasonic homogenizer and cut to a length of <NUM>. The concentration of CNTs 14a in the dispersion was set to <NUM> t% by weight. This dispersion does not contain a dispersing agent or an adhesive agent.

Subsequently, carbon fiber bundle <NUM> was input to the dispersion while ultrasonic vibrations of <NUM> and <NUM> were applied to the dispersion. As the carbon fiber bundle <NUM>, T700SC-<NUM> (manufactured by Toray Industries, Inc. ) was used. In the carbon fiber bundle <NUM>, <NUM>,<NUM> carbon fibers 12a are contained. The diameter of the carbon fibers 12a is <NUM> and the length is about <NUM>. The carbon fiber bundle <NUM> was kept in the dispersion for ten seconds. As described above, during the adhesion, adjustment was performed such that the period of time of the equilibrium state becomes longer than in conventional cases. In this manner, the CNT-adhered carbon fiber bundle was obtained.

<FIG> provides an SEM photograph of part of the surface of the carbon fibers contained in the CNT-adhered carbon fiber bundle. A plurality of CNTs 14a adhere to the surface of the carbon fibers 12a uniformly in the radial direction and the length direction. The result of evaluation of the uniformity of adhesion of the CNTs 14a is explained below. First, as illustrated in <FIG>, in a region of <NUM> in the length direction of the carbon fibers 12a, multiple frames of <NUM> square were set at equal intervals. The shortest distance between two adjacent frames was set to <NUM>.

The frames depicted in <FIG> are set such that they are arranged uniformly in the length direction of the carbon fibers 12a, but the mode of arrangement is not limited to this. The frames can be set according to any suitable mode of arrangement as long as they are uniformly arranged from the center line toward the both sides in the radial direction of the carbon fibers 12a. Here, although the number of the frames is <NUM>, any appropriate number of frames can be set if the above-described condition is met.

For each frame, the length and the number of pieces of the CNTs 14a intersecting any one of the four sides were measured and counted. <NUM>% of the measured CNTs 14a had a length of less than <NUM>. If the shortest distance between two adjacent frames is shorter than the length of the CNTs 14a, then it follows that some CNTs 14a may exist that intersect the sides of the frames at two or more locations. The number of the CNTs 14a intersecting one side out of the four sides of the frame is counted as one piece on a per-side basis, so that, in this case, the number of the CNTs 14a intersecting the side is <NUM>. Likewise, with regard to a CNT 14a intersecting two sides of one frame, it is counted as two CNTs. The number of pieces measured on each side is summarized in the following Table <NUM>.

Based on the measurement results, the average and the standard deviation of the number of the CNTs intersecting one side of each frame were calculated as <NUM> and <NUM>, respectively. If the standard deviation of the number of pieces determined in this manner is equal to or smaller than <NUM>, the CNTs adhere uniformly to the substantially entire surfaces of the carbon fibers. Since the CNTs 14a were attached to the surface of the carbon fibers 12a by the above-described manner, the uniformity of adhesion of the CNTs was enhanced.

The CNT-adhered carbon fiber bundle was subjected to the sizing treatment and dried on a hot plate of <NUM>. In this manner, the composite material <NUM> of the example was obtained. The sizing agent that was used is Epocross (TM) (manufactured by Nippon Shokubai Co. The sizing agent contains therein reactive resin in the form of droplets with the particle size on the order of <NUM> to <NUM>. The sizing agent was diluted by deionized water and then used such that the amount of adhesion on the carbon fiber surface after the drying is on the order of <NUM>% by mass.

The area percentage of the fixing resin parts 16a on the surface of the composite material <NUM> of the example will be described with reference to the SEM photograph of <FIG>. As illustrated in <FIG>, a region X of <NUM> square is defined in the surface of the carbon fibers 12a to which the CNTs 14a adhere, and the number of the fixing resin parts 16a residing within this region X and the area of each fixing resin part 16a are determined using Winroof2015 (manufactured by Mitani Corporation). Here, the number of the fixing resin parts 16a is <NUM>. The areas of the individual fixing resin parts 16a that were confirmed is <NUM> to <NUM><NUM>. With regard to the region X in the composite material <NUM>, <NUM>% of the area of <NUM><NUM> is covered by the fixing resin parts 16a.

In order to study the tendency of the number of the fixing resin parts and the area percentage, samples of five types differing from one another in the amount of adhesion of the sizing agent were created. Conditions other than the dilution rate of the sizing agent such as the CNT-adhered carbon fiber bundle and the treatment condition were the same as those that have been discussed in the foregoing. For the samples obtained, a region of <NUM> square similar to that of the case of <FIG> was defined, and the number of the fixing resin parts residing within each region and the areas of each fixing resin part were measured in the same manner.

The number of the fixing resin parts in each region and the area percentage of the fixing resin parts were plotted in the graph of <FIG>. It is preferable that <NUM>% or more and <NUM>% or less of a region within <NUM> square of the surfaces of the carbon fibers 12a to which the CNTs 14a adhere is accounted for by the fixing resin parts 16a. The number of the fixing resin parts 16a residing in a region within <NUM> square of the surfaces of the carbon fibers 12a to which the CNTs 14a adhere is <NUM> to <NUM>.

By using the composite material <NUM>, a prepreg <NUM> as illustrated in <FIG> was created. The composite materials <NUM> were pulled in one direction and aligned, and made into a carbon fiber sheet (fiber weight: <NUM>/m<NUM>). The epoxy resin as the matrix resin was applied on the exfoliate paper using a knife coater and thereby a resin film was created. The resin content was set to <NUM>% by mass. The aforementioned carbon fiber sheet was sandwiched by two resin films and heat and pressure were applied thereto by heat roll at <NUM> and <NUM> atm. The matrix resin is impregnated with the composite material <NUM> and thus a prepreg <NUM> of the example having a matrix resin layer <NUM> was obtained.

The obtained prepreg was cut and laminated such that a predetermined dimension is obtained. Heat and pressure were applied to the obtained laminated body by autoclave to cure the matrix resin and, thereby a carbon-fiber-reinforced molded article <NUM> of the example as illustrated in <FIG> was obtained. The carbon-fiber-reinforced molded article <NUM> was tuned to the size in accordance with the JIS standard.

A composite material of a comparative example was created by the same method as described above except that ARE-ST-<NUM> (manufactured by ADVANCED RESIN LABOLATORYS) was used as the sizing agent. The sizing agent used here is not of an emulsion type. The composite material of the comparative example was SEM-observed. It was confirmed that the surface of the carbon fibers to which CNTs adhere is covered uniformly by the resin-cured material.

A prepreg of a comparative example was created by the same method as that of the example and using the composite material of the comparative example. Further, by using the obtained prepreg, a carbon-fiber-reinforced molded article of the comparative example was created by the same method as that of the example.

The carbon-fiber-reinforced molded article of the example and the carbon-fiber-reinforced molded article of the comparative example were used as the test pieces of the interlaminar fracture toughness mode I and the interlaminar fracture toughness values were measured. The interlaminar fracture toughness test was conducted using AUTOGRAPH AGX-V Series Precision Universal Testing Machine AG5-5kNX (manufactured by Shimadzu Corporation) on two each of the test pieces in accordance with JIS K7086.

As a test method, a double cantilever beam interlaminar fracture toughness test method (DCB method) was used. First, a precrack (initial crack) was made to occur at a point <NUM> to <NUM> away from the tip of the test piece, and, after that, the crack was made to further develop. The test was terminated at the time point at which the length of the progression of the crack reached <NUM> from the tip of the precrack. The cross head speed of the testing machine was changed according to the level of the crack progression. Specifically, the cross head speed until the level of the crack progression reaches <NUM> was set to <NUM>/min. When the level of the crack progression exceeded <NUM>, the cross head speed was set to <NUM>/min. The length of the crack progression was measured from both end faces of the test piece using a microscope, and the load, and the crack aperture displacement were measured and thereby the interlaminar fracture toughness value (GIC) was calculated.

Change in the interlaminar fracture toughness value due to the progression of the crack is shown in the graph of <FIG>. The interlaminar fracture toughness value was determined from the load-COD (Crack Opening Displacement) curve. With regard to the carbon-fiber-reinforced molded articles of the example and the comparative example, the average of the interlaminar fracture toughness values in the level of crack progression of <NUM> to <NUM> was adopted as the GIR. The GIR of the carbon-fiber-reinforced molded article of the example is <NUM> kJ/m<NUM>. In the case of the carbon-fiber-reinforced molded article of the comparative example, the GIR is <NUM> kJ/m<NUM>.

In the carbon-fiber-reinforced molded article of the example, resistance to the progression of interlayer peeling crack is larger than the carbon-fiber-reinforced molded article of the comparative example, and the GIR is increased by <NUM>%. In the carbon-fiber-reinforced molded article of the example, a composite material is used in which CNTs are partly fixed to the surface of the carbon fibers by the fixing resin parts. It is presumed that, by using such a composite material, a CNT composite resin layer is formed in the molded article of the example, by virtue of which the progression of the interlayer peeling crack was suppressed.

The present invention is not limited to the above-described embodiment and can be modified as appropriate within the range of the purport of the present invention.

The resin contained in the sizing agent of emulsion type may be thermosetting resin or thermoplastic resin. With regard to the sizing agent of emulsion type containing such resin, for example, epoxy resin emulsion, acrylic resin emulsion, and urethane resin emulsion may be mentioned.

Claim 1:
A composite material (<NUM>) comprising:
a carbon fiber bundle (<NUM>) in which a plurality of continuous carbon fibers (12a) are arranged;
carbon nanotubes (14a) adhering to respective surfaces of the carbon fibers (12a); and
a plurality of fixing resin parts (16a) partly fixing the carbon nanotubes (14a) on the surfaces of the carbon fibers (12a), wherein
the carbon nanotubes (14a) are dispersed on the surface of the carbon fibers (12a) and entangled with one another and form a network structure by being brought into direct contact with or directly connected to each other,
when measured according to the method disclosed in the description, at least <NUM>% of the carbon nanotubes (14a) intersecting any one of four sides of a frame of <NUM> square in a region of <NUM> in a length direction of the carbon fiber (12a) has a length of <NUM> or more, and a standard deviation of the number of the carbon nanotubes (14a) intersecting any one of four sides of a frame of <NUM> square in a region of <NUM> is <NUM> or less, and
the fixing resin parts (16a) cover <NUM>% or more and <NUM>% or less of the surfaces of the carbon fibers (12a) to which the carbon nanotubes (14a) adhere, and are provided at a rate of <NUM> to <NUM> pieces per <NUM> square on the surfaces to which the carbon nanotubes (14a) adhere.