COMPOSITE MATERIAL AND METHOD FOR PRODUCING SAME

A composite material includes a fiber and a structure including a plurality of carbon nanotubes and having a network structure in which the carbon nanotubes are in direct contact with each other and in which the carbon nanotubes directly adhere to a surface of the fiber. The carbon nanotubes have a bent shape including a bent portion.

TECHNICAL FIELD

The present invention relates to a composite material and a method for manufacturing the composite material.

BACKGROUND ART

There is suggested a composite material including a structure that includes a fiber and a plurality of carbon nanotubes (hereinafter, referred to as “CNTs”) adhered to a surface of the fiber (for example, PTL 1). In the structure of the composite material, the plurality of CNTs have a network structure in which the CNTs are connected to each other, and adhere to the surface of the fiber. A fiber-reinforced molded article in which a resin is reinforced by the composite material as a reinforcement fiber includes the fiber, and thus higher strength and rigidity are obtained in comparison to a resin alone, and has improved electrical conductivity, thermal conductivity, and a mechanical property which are derived from the CNTs.

CITATION LIST

Patent Literature

SUMMARY OF INVENTION

Technical Problem

The use of the above-described fiber-reinforced molded article is expanded to various fields, and a request for the fiber-reinforced molded article further increases. On the other hand, in a fiber having a small diameter, an effect of improving mechanical properties and the like derived from the CNTs is likely to be obtained, but in a fiber having a large diameter, the effect of improving the mechanical properties and the like derived from the CNTs is less likely to be obtained, and thus it is desired to further enhance properties derived from the CNTs.

The invention has been made in consideration of the circumstances, and an object thereof is to provide a composite material capable of enhancing properties derived from CNTs adhered to a fiber, and a method for manufacturing the composite material.

Solution to Problem

According to an aspect of the invention, there is provided a composite material including: a fiber; and a structure which includes a plurality of carbon nanotubes and has a network structure in which the carbon nanotubes are in direct contact with each other, and in which the carbon nanotubes directly adhere to a surface of the fiber. The carbon nanotubes have a bent shape including a bent portion.

According to another aspect of the invention, there is provided a method for manufacturing a composite material. The method includes: an ultrasonic process of applying ultrasonic vibration to a dispersion in which a plurality of carbon nanotubes having a bent shape including a bent portion are dispersed; and an adhesion process of immersing a continuous fiber in the dispersion to which the ultrasonic vibration is applied, and adhering the plurality of carbon nanotubes to the fiber to form a structure on a surface of the fiber.

Advantageous Effects of Invention

According to the invention, since the carbon nanotubes adhered to the fiber have a bent shape including a bent portion, the number of the carbon nanotubes adhered to the fiber increases, and properties derived from the carbon nanotubes can be enhanced.

According to the invention, since the composite material is manufactured by causing the carbon nanotubes having a bent shape including a bent portion to adhere to the surface of the fiber in a structure knitted like a nonwoven fabric fiber, the number of the carbon nanotubes adhered to the fiber can be increased, and it is possible to manufacture a composite material in which properties derived from the carbon nanotubes are further enhanced.

DESCRIPTION OF EMBODIMENTS

First Embodiment

InFIG.1, a composite material10includes a fiber11and a structure12that is formed on a surface of the fiber11. In the structure12, a plurality of carbon nanotubes (hereinafter, referred to as “CNTs”)14are entangled. For example, the composite material10is set as a fiber in which a resin or the like is impregnated in the structure12(hereinafter, referred to as “resin-impregnated fiber”), or is used as a single yarn that constitutes a multifilament or a reinforcement fiber of a fiber-reinforced molded article. The composite material10, and the resin-impregnated fiber manufactured by using the composite material10, the multifilament, the fiber-reinforced molded article, or the like (hereinafter, referred to as “secondary product”) includes the structure12on a surface of the fiber11, and thus mechanical properties and the like are improved. That is, the mechanical properties and the like are improved due to the CNTs14adhered to the surface of the fiber11.

As in an example of a multifilament15using the composite material10as illustrated inFIG.2, the multifilament15includes a plurality of the composite materials10and a matrix resin16. InFIG.2, the multifilament15including six pieces of the composite materials10is illustrated, but the number of the composite materials10is not particularly limited, and the multifilament15may be constituted by several thousands of composite materials10or several hundred thousands of composite materials10. In addition, one multifilament15can be formed by twisting a plurality of the composite material10.

As the matrix resin16, for example, a resin such as polyurethane, or an elastomer such as a synthetic rubber can be used. The matrix resin16is interposed between the composite materials10to couple the composite materials10to each other. The matrix resin is impregnated up to the structure12of the composite material10and is cured.

The fiber11is not particularly limited, and examples thereof include a resin fiber such as nylon, polyester, vinylon, and acryl, a glass fiber, and a mineral fiber. In addition, a diameter of the fiber11is also not particularly limited, and a fiber having a diameter within a range of 5 to 100 μm can be preferably used, and a fiber having a diameter within a range of 5 to 30 μm can be more preferably used. As the fiber11, a long fiber is used, and the length is preferably 50 m or longer, more preferably within a range of 100 to 100,000 m, and still more preferably within a range of 100 to 10,000 m. Note that, for example, the fiber11may be cut to a short size after forming the structure12.

The CNTs14which constitute the structure12are evenly dispersed and entangled over approximately the entirety of a surface of the fiber11and form a network structure in which a plurality of the CNTs14are connected in a state of being entangled to each other. Connection stated here includes physical connection (simple contact) and chemical connection. The CNTs14come into direct contact with each other without a dispersing agent such as a surfactant or an inclusion such as adhesive between the CNTs14.

As schematically illustrated inFIG.3, some CNTs14which constitute the structure12directly adhere to the surface of the fiber11and are fixed to the surface. According to this, the structure12directly adheres to the surface of the fiber11. Description of “the CNTs14directly adhere to the surface of the fiber11” represents that the CNTs14directly adhere to the fiber11without inclusions such as a dispersing agent including a surfactant, and adhesive between the CNTs14and the surface, and the adhesion (fixing) is obtained due to coupling by a Vander Waals force. Since some CNTs14which constitute the structure12directly adhere to the surface of the fiber11, it enters a direct contact state in which the structure12comes into direct contact with the surface of the fiber11without inclusions such as the dispersing agent and the adhesive.

As the CNTs14, CNTs having a bent shape are used. According to this, some of the CNTs14which constitute the structure12may be entangled with other CNTs14to be fixed to the fiber11without direct contact with the surface of the fiber11. In addition, some of the CNTs14directly adhere to the surface of the fiber11and are entangled with other CNTs14to be fixed to the fiber11. Hereinafter, description will be made while fixing of the CNTs14to the fiber11is collectively referred to as adhesion to the fiber11. Note that, a state in which the CNTs14are entangled or intertwined includes a state in which some of the CNTs14are pressed against other CNTs14.

As described above, in addition to direct contact with the surface of the fiber11, some of the CNTs14which constitute the structure12, which are not in direct contact with the surface of the fiber11are fixed to the fiber11by entanglement with other CNTs14, or the like. Accordingly, the structure12of this example includes more CNTs14than the CNTs which directly adhere to the surface of the fiber as in the structure of the composite material of the related art. That is, the number of the CNTs14which adhere to the fiber11further increases and the thickness of the structure12is larger in comparison to the related art.

As described above, since the plurality of CNTs14are connected to each other without inclusions between surfaces thereof and constitute the structure12, the composite material10exhibits the performance of electrical conductivity and thermal conductivity derived from the CNTs. In addition, since the CNTs14adhere to the surface of the fiber11without inclusions, the CNTs14which constitute the structure12are less likely to be peeled off from the surface of the fiber11, and mechanical strength of the composite material10and secondary products using the composite material10is improved. In addition, since the thickness of the structure12is large, the mechanical strength is further improved. Accordingly, even in the fiber11having a large diameter, the composite material10and the secondary products having improved mechanical strength are obtained. Note that, hereinafter, description will be given of a case of a fiber-reinforced molded article as the secondary products, but this is also true of a case of a resin-impregnated fiber or a multifilament.

For example, in the fiber-reinforced molded article, a matrix resin is impregnated into a fiber bundle including a plurality of the composite materials10provided with the structure12and is cured therein. The matrix resin of the fiber-reinforced molded article is impregnated into the structure12and is cured, and thus the structure12of the composite material10is fixed to the surface of the fiber11and the matrix resin. According to this, a state in which each of a plurality of the fibers11is strongly bonded to the matrix resin is obtained, and peeling strength between the composite materials10and the matrix resin is improved. In addition, bonding with the matrix resin occurs over the entirety of the composite materials10, and thus a fiber reinforcing effect is obtained over the entirety of the fiber-reinforced molded article.

As described above, a region formed when the matrix resin is impregnated into the CNTs14constituting the structure12and is cured (hereinafter, referred to as “composite region”) exists at the periphery of each of the fibers11in the fiber-reinforced molded article. Since the structure12includes a CNT14that is floating from a surface, a concentration (density) of the CNTs14in the composite region becomes lower as being spaced apart from the composite material10. In the composite region, the CNTs14and the matrix resin are composited, and thus high strength and flexibility derived from the CNTs14are provided. In addition, an effect of mitigating stress concentration, a constraining effect of suppressing displacement of the composite material10, an effect of efficiently absorbing mechanical energy from the outside, and the like are obtained due to the composite region.

For example, in a case where energy such as vibration propagates between the fibers11, energy of the propagating vibration is absorbed and damped by friction of the composite region at the periphery of each of the fibers11. As a result, for example, vibration damping properties (damping properties) of the fiber-reinforced molded article are improved. In addition, when an external force is applied to the fiber-reinforced molded article and displacement occurs at the inside of the fiber-reinforced molded article, displacement occurs in the fiber11inside the fiber-reinforced molded article. Due to the displacement of the fiber11, the structure12in the composite region is stretched, and a constraining effect is obtained due to a network structure of the CNTs14. According to this, properties of the CNTs14are exhibited, and thus an elastic modulus of the fiber-reinforced molded article can be raised.

The properties derived from the CNTs14of the fiber-reinforced molded article are exhibited due to the properties of the composite region as described above, the effect by the composite region, and the like. As described above, since the structure12has a structure in which the number of the CNTs14adhered to the fiber11is increased, and the CNTs14are knitted like a non-woven fabric fiber, the properties of the fiber-reinforced molded article which are derived from the CNTs are higher in comparison to a composite material in which a structure like the structure12is not formed.

A plurality of the structures12formed in a plurality of the composite materials10have an independent structure, and the structure12of one composite material10and the structure12of another composite material10do not share the same CNT14. That is, CNTs14included in the structure12provided in one fiber11are not included in the structure12provided in another fiber11.

For example, a sizing agent (not illustrated) is fixed to surfaces of the CNTs14which constitute the structure12. The sizing agent is formed from a cured article or an uncured article of a reactive curable resin, a thermosetting resin, or a thermoplastic resin. The sizing agent is formed by performing a sizing treatment.

The sizing agent covers surfaces of the CNTs14, and at a contact portion where the CNTs14are in contact with each other, the sizing agent forms an inclusion portion that wraps and covers the contact portion. Due to the inclusion portion, a state in which the CNTs14are in contact with each other is made to be stronger, and the structure12is less likely to collapse. At the inclusion portion, since the sizing agent is fixed to the CNTs14in a state in which the sizing agent does not enter between the CNTs14which are in contact with each other, the CNTs14are in direct contact with each other at the contact portion.

In addition, in the structure12, a void portion (mesh) that is surrounded by a plurality of the CNTs14is formed due to the CNTs14, but it is preferable that the sizing agent does not close the void portion so that impregnation of the matrix resin into the structure12is not hindered. In order for the void portion not to be closed, a volume of the sizing agent is preferably set to be 30% or less of a volume of the CNTs14of the structure12. Note that, the sizing agent is formed on the surfaces of the CNTs14, and is different from the following fixing resin part that enters the inside of the structure12and fixes the CNTs14to the fiber11. In addition, the sizing agent may not be applied to the structure12.

As described above, the CNTs14adhered to the fiber11have a bent shape. The bent shape of the CNTs14is obtained because a bent portion is provided due to existence of a five-membered ring, a seven-membered ring, and the like in a graphite structure of the CNTs14, and the bent shape is a shape from which the CNTs14can be evaluated to be curved, to be bent, or the like from observation with a SEM. For example, the bent shape of the CNTs14represents that the bent portion exists at least atone site per an average length of a use range of the CNTs14to be described later. Even in a case where the bent shape is long, the CNTs14having the bent shape adhere to the surface of the fiber11which is a curved surface in various postures. In addition, the CNTs14having the bent shape are likely to form a space (gap) between the CNTs14and the surface of the fiber11to which the CNTs14adhere, or between the adhered CNTs14, and another CNT14enters the space. According to this, when using the CNTs14having the bent shape, the number of the CNTs14adhered to the fiber11(the number of the CNTs14forming the structure12) further increases in comparison to the case of using CNTs having a shape with high linearity.

The length of the CNTs14is preferably within a range of 0.1 to 10 μm. When the length is 0.1 μm or longer, the CNTs14can more reliably form the structure12in which the CNTs14are entangled and come into direct contact with each other or are directly connected to each other, and it is possible to more reliably form the space which another CNT14enters as described above. In addition, when the length of the CNTs14is 10 μm or less, the CNTs14do not adhere between a plurality of the fibers11. That is, as described above, the CNT14that is contained in the structure12provided in one fiber11is not contained in the structure12provided in another fiber11.

The length of the CNTs14is more preferably within a range of 0.2 to 5 μm. When the length of the CNTs14is 0.2 μm or longer, the number of the CNTs14adhered increases and the structure12can be made thick. When the length is 5 μm or less, when causing the CNTs14to adhere to the fiber11, the CNTs14are less likely to aggregate, and the CNTs14are likely to be more evenly dispersed. As a result, the CNTs14more evenly adhere to the fiber11.

Note that, with regard to the CNTs adhered to the fiber11, mixing-in of CNTs with high linearity or mixing-in of CNTs having a length out of the above-described range are not excluded. For example, even in a case where mixing-in occurs, since the CNTs with high linearity enter a space formed by the CNTs14, it is possible to increase the number of the CNTs adhered to the fiber11.

It is preferable that an average diameter of the CNTs14is within a range of 1 nm to 15 nm, and more preferably a range of 3 nm to 10 nm. When the diameter is 15 nm or less, the CNTs14are very flexible and are likely to adhere to the fiber11along a surface thereof, and are likely to be fixed to the fiber11in a state of being entangled with other CNTs14. In addition to this, formation of the structure12becomes more reliable. In addition, when the diameter is 10 nm or less, coupling between the CNTs14constituting the structure12becomes strong. Note that, the diameter of the CNTs14is set as a value measured by using a transmission electron microscope (TEM) photograph. The CNTs14may be a single-layer structure or a multi-layer structure, but the multi-layer structure is preferable.

As described above, when the CNTs14are set to have the bent shape, it is possible to further increase the number of the CNTs14adhered to the fiber11in comparison to the case of using CNTs with high linearity, and it is possible to increase the thickness of the structure12. In addition, the structure12in which the CNTs14are knitted like a non-woven fabric fiber is formed. As a result, the mechanical strength is raised, and in a case where an external force is applied to a secondary product and the fiber11is displaced, a constraining effect due to the structure12is large, and thus the elastic modulus can be further raised. In addition, a mechanical energy absorbing effect due to the composite region at the periphery of the fiber11also increases, and the vibration damping property of secondary products can be further enhanced.

As an example of the improved mechanical strength of the secondary products, an improvement in durability against repetitive bending can be exemplified. As described above, in the fiber-reinforced molded article using the composite material10in which the CNTs14adhere to the surface of the fiber11, it is considered that the durability against the repetitive bending can be enhanced by a peeling strength improving effect due to inclusion of the structure12, and the mechanical energy absorbing effect due to the composite region. The peeling strength improving effect and the mechanical energy absorbing effect can be further enhanced in proportion to an increase in the number of the CNTs14adhered to the surface of the fiber11, and thus the durability against the repetitive bending becomes high. The composite material10having the above-described properties is suitable as a spring material of a coil spring or a leaf spring, or the like to which a load is repetitively applied, and thus the secondary product containing the composite material10is applicable to various springs such as the coil spring and the leaf spring.

In the secondary products using the composite material10, for example, in the fiber-reinforced molded article, composite regions in which the matrix resin is impregnated into the structure12and is cured are fixed to each other, and thus a cross-linking structure that cross-links the fibers11occurs. Each of the composite regions in which the impregnated matrix resin is cured has higher hardness in comparison to a cured matrix resin alone, and a large elastic limit, that is, high elasticity. In addition, the composite region has higher wear resistance in comparison to the matrix resin. Due to mutual coupling of a plurality of the composite regions, coupling between the composite materials10becomes strong, and resistance against repetitive bending of the fiber-reinforced molded article using the composite material10is improved. Since the cross-linking structure is formed in a case where a distance between the composite materials10is short to a certain extent in which a plurality of the structures12come into contact with each other, the larger the thickness of the structure12, the more advantageous because the more cross-links occur. In addition, in a case where the composite material10is set to have a fabric shape, or in a case where the composite material10is bundled like a multifilament, a cross-linking portion where the composite regions are fixed to each other increases, and the effect due to the cross-linking structure increases.

For example, the thickness of each portion of the structure12(a length in a diameter direction of the fiber11) can be acquired as follows. Specifically, a part of the structure12on the surface of the fiber11is bonded to a cellophane tape or the like and is peeled off, and a cross-section of the structure12remaining on the surface of the fiber11is measured with a SEM or the like to acquire the thickness. In order to almost uniformly cover a measurement range of a predetermined length along a fiber axis direction of the fiber11, the thickness of the structure12is measured at ten sites in the measurement range, and an average thereof is set as the thickness of the structure12. For example, the length of the measurement range is set to a length that is five times an upper limit of a range of the length of the CNTs14. The number of CNTs14adhered to the fiber11can be evaluated with the thickness of the structure12.

The thickness (average) of the structure12which is obtained as described above is within a range of 10 nm to 300 nm, preferably within a range of 15 nm to 200 nm, and more preferably 50 nm to 200 nm. When the thickness of the structure 12 is 200 nm or less, an impregnation property with a resin between the fibers11is more satisfactory.

[Method for Manufacturing Composite Material]

Next, a method for manufacturing the composite material10will be described. In this example, description will be given of a case of manufacturing the composite material10by using a fiber bundle20(refer toFIG.4) including a plurality of the fibers11, but the composite material10can be manufactured in a similar manner by using the fiber11alone instead of the fiber bundle20.

The fibers11which constitute the fiber bundle20are substantially not entangled, and a fiber axis direction of each of the fibers11is aligned. The fiber axis direction is an axial direction (extension direction) of the fibers11. In this example, the fiber bundle20includes a plurality of fibers11. The number of the fibers11constituting the fiber bundle20is not particularly limited, and can be set, for example, within a range of 100 to 10,000,000.

Entangling of the fibers11in the fiber bundle20can be evaluated with the degree of disturbance of the fibers11. For example, the fiber bundle20is observed with a scanning electron microscope (SEM) at a constant magnification, and lengths of a predetermined number of (for example, 10) fibers11in an observation range (a predetermined length range of the fiber bundle20) are measured. The degree of disturbance of the fibers11can be evaluated on the basis of a variation, a difference between a maximum value and a minimum value, and a standard deviation of the lengths which are obtained from the measurement results and relate to the predetermined number of fibers11. In addition, it can be determined that the fibers11are not substantially entangled by measuring the degree of entanglement, for example, in conformity to a method of measuring the degree of entanglement in JIS L1013:2010 “Testing methods for man-made filament yarns”. The smaller the measured degree of entanglement is, the less the fibers11are entangled with each other in the fiber bundle20.

In the fiber bundle20in which the fibers11are not substantially entangled with each other, or are less entangled with each other, the fibers11are likely to be uniformly opened. According to this, it is easy to cause the CNT14to uniformly adhere to each of the fibers11, a resin is uniformly impregnated into the fiber bundle20, and each of the composite materials10contributes to the strength.

In order to form the structure12by causing the CNTs14to adhere to each of the fibers11in the fiber bundle20, the fiber bundle20is immersed in a CNT isolated dispersion (hereinafter, simply referred to as “dispersion”) in which the CNTs14are isolated and dispersed, and mechanical energy is applied to the dispersion. The term “isolated and dispersed” represents a state in which the CNTs14are physically separated one by one and are dispersed in a dispersion medium without entanglement, and a state in which a ratio of an aggregate in which two or more CNTs14are aggregated in a bundle form is 10% or less. Here, when the ratio of the aggregate is 10% or more, aggregation of the CNTs14in the dispersion medium is promoted, and adhesion of the CNTs14to the fibers11is inhibited.

As illustrated inFIG.4, as an example, an adhesion device21includes a CNT adhesion tank22, guide rollers23to26, an ultrasonic wave generator27, a travelling mechanism (not illustrated) that causes the fiber bundle20to travel at a constant speed, and the like. A dispersion28is stored in the CNT adhesion tank22. The ultrasonic wave generator27irradiates the dispersion28in the CNT adhesion tank22with ultrasonic waves from a lower side of the CNT adhesion tank22.

The fiber bundle20having a long length (for example, approximately 100 m) in which the structure12is not formed is continuously supplied to the adhesion device21. The fiber bundle20that is supplied is wound around the guide rollers23to26in this order, and travels at a constant speed by the travelling mechanism. The fiber bundle20in which the sizing agent does not adhere to the fiber11is supplied to the adhesion device21. Note that, the sizing agent stated here represents an object adhered to the surfaces of the fibers11to prevent entanglement of the fibers11, and the like, and is different from the sizing agent and the fixing resin part described above.

The fiber bundle20is wound around the guide rollers23to26in an opened state. Appropriate tension acts on the fiber bundle20wound around the guide rollers23to26, and thus the fibers11are less likely to be entangled with each other. It is preferable that the winding of the fiber bundle20around the guide rollers24to26is set to a smaller winding angle (90° or less).

Any of the guide rollers23to26is a flat roller. As illustrated inFIG.5, a roller length (a length in an axial direction) L1of the guide roller23is set to be sufficiently larger than a width WL of the fiber bundle20that is opened. With regard to the guide rollers24to26, as in the guide roller23, the roller length is set to be sufficiently larger than the width WL of the opened fiber bundle20. For example, the guide rollers23to26have the same size. In the opened fiber bundle20, a plurality of the fibers11are aligned in the thickness direction (a diameter direction of the guide rollers).

Among the guide rollers23to26, the guide rollers24and25are disposed in the CNT adhesion tank22. According to this, the fiber bundle20linearly travels between the guide rollers24and25in the dispersion28at a constant depth.

A travelling speed of the fiber bundle20is preferably set within a range of 0.5 to 10,000 m/minute. The higher the travelling speed of the fiber bundle20is, the further productivity is improved. The lower the travelling speed is, the more effective for uniform adhesion of the CNTs14, and the more effective for suppression of entanglement of the fibers11. In addition, the less entanglement between the fibers11is, the further uniformity of adhesion of the CNTs14to the fibers11is raised. When the travelling speed of the fiber bundle20is 100 m/minute or less, entanglement between the fibers11is more effectively suppressed, and adhesion uniformity of the CNTs14can be further raised. In addition, the travelling speed of the fiber bundle20is more preferably set within a range of 5 to 50 m/minute.

The ultrasonic wave generator27applies ultrasonic vibration as mechanical energy to the dispersion28. According to this, in the dispersion28, a reversible reaction state in which a dispersion state in which the CNTs14are dispersed and an aggregation state in which the CNTs14are aggregated vary alternately is formed. When the fiber bundle20is caused to pass through the dispersion28that is in the reversible reaction state, at the time of transitioning from the dispersion state to the aggregation state, the CNTs14adhere to the fibers11due to Van der Walls force. The mass of the fibers11is as large as 100,000 or more times the mass of the CNTs14, energy necessary for detachment of the adhered CNTs14is more than energy due to the ultrasonic vibration. According to this, the CNTs14adhered once to the fibers11are not peeled off from the fibers11by the ultrasonic vibration after adhesion. Note that, since the mass is very small, the dispersion state and the aggregation state alternately vary between the CNTs14due to the ultrasonic vibration.

When transition from the dispersion state to the aggregation state is repetitively performed, a plurality of CNTs14adhere to each of the fibers11, and the structure12is formed. As described above, when using the CNTs14having a bent shape, other CNTs14enter a space formed between the CNTs14and the surfaces of the fibers11to which the CNTs adhere, between the adhered CNTs14, or the like, and thus more CNTs14adhere to the fibers11and the structure12is formed.

A frequency of the ultrasonic vibration applied to the dispersion28is preferably 40 to 950 kHz. When the frequency is 40 kHz or higher, entanglement between the fibers11in the fiber bundle20is suppressed. In addition, when the frequency is 950 kHz or lower, the CNTs14adhere to the fibers11in a satisfactory manner. In order to further reduce entanglement of the fibers11, the frequency of the ultrasonic vibration is preferably 100 kHz or higher, and more preferably 130 kHz or higher. In addition, the frequency of the ultrasonic vibration is more preferably 430 kHz or lower.

In addition, the present inventors found that the number of the CNTs14adhered to the fibers11becomes almost the maximum while securing uniformity of adhesion of the CNTs14to the fibers11when the number of times of transition from the dispersion state to the aggregation state in the CNTs14reaches 65,000. Note that, the maximum value of the number of the CNTs14adhered varies in accordance with a CNT concentration of the dispersion28, and increases as the CNT concentration of the dispersion28is higher. However, when the CNT concentration of the dispersion28becomes a high concentration at which the CNTs17cannot take a dispersion state when applying the ultrasonic vibration, adhesion of the CNTs17to the fibers11cannot be performed.

According to this, it is preferable to determine the travelling speed of the fiber bundle20, a travelling distance of the fiber bundle20in the dispersion28(an interval between the guide rollers24and25), and the frequency of the ultrasonic vibration that is applied to the dispersion28so that the length of a period during which the fiber bundle20is travelling in the dispersion28, that is, time (hereinafter, referred to as “immersion time”) for which the fiber bundle20is travelling between the guide rollers24and25becomes 65,000 or more times a cycle of the ultrasonic vibration applied to the dispersion28. That is, it is preferable that when the frequency of the ultrasonic vibration is set as fs (Hz), and the immersion time is set as Ts (second), a relationship of “Ts≥65,000/fs” is satisfied. For example, when the frequency of the ultrasonic vibration is 130 kHz and the distance along which the fiber bundle20travels in the dispersion28is 0.1 m, the travelling speed of the fiber bundle20can be set to 12 m/minute or less. In addition, even in a case where the fiber bundle20is immersed in the dispersion28in a plurality of times in a division manner, when a total number of immersion times is set to 65,000 or more times the cycle of the ultrasonic vibration, the number of the CNTs14adhered can be almost the maximum.

As schematically illustrated inFIG.6, a standing wave in which a distribution of a sound pressure (amplitude) is determined is generated in the dispersion28inside the CNT adhesion tank22due to the ultrasonic vibration applied from the ultrasonic wave generator27. In the adhesion device21, positions of the guide rollers24and25in a depth direction are adjusted so that the fiber bundle20travels in the dispersion28at a depth at which a standing wave node of the ultrasonic vibration, that is, a sound pressure becomes the minimum. Accordingly, a depth from a liquid surface of the dispersion28at which the fiber bundle20travels in the dispersion28is set as D, a wavelength of a standing wave of ultrasonic vibration generated in the dispersion28is set as λ, and n is set as an integer of 1 or more, these values are determined to satisfy a relationship of “D=n·(λ/2)”. Note that, the wavelength λ of the standing wave can be obtained on the basis of a sound speed in the dispersion28and a frequency of the ultrasonic vibration applied from the ultrasonic wave generator27.

As described above, through adjustment of the depth of the fiber bundle20that travels in the dispersion28, vibration of the fibers11due to the sound pressure is suppressed, thread disorder due to thread sagging can be prevented, scraping between the fibers11or between the CNTs14adhered to surfaces of the fibers11can be suppressed, and the structure12having a large thickness can be formed. As the diameter of the fiber11is large, an influence of vibration due to the standing wave is large. Accordingly, when satisfying the condition of the depth at which the fiber bundle20travels in the dispersion28, the effect of suppressing vibration of the fibers11is significantly obtained. In addition, stretching in the fiber axis direction due to the sound pressure of the standing wave is less likely to occur in the fiber11, and plastic deformation of the fiber11in the fiber axis direction can be prevented. Note that, the depth at which the fiber bundle20travels in the dispersion28may slightly deviate from the standing wave node, and in this case, the depth is preferably set within range (n·λ/2−λ/8≤D≤n·λ/2+λ/8) that is equal to or larger than n·λ/2−λ/8 and equal to or less than n·λ/2+λ/8. According to this, it is possible to set the thread disorder of the fibers11due to thread sagging in a permissible range.

The fiber bundle20is taken out from the dispersion28and is dried. A sizing treatment and drying are sequentially performed with respect to the dried fiber bundle20, and thus the sizing agent is applied to the structure12. The sizing treatment can be performed by a typical method.

The sizing agent is not particularly limited, and various reactive curable resins, thermosetting resins, and thermoplastic resins can be used as described above. Examples of the thermosetting resins include an epoxy resin, a phenol resin, a melamine resin, a urea resin, unsaturated polyester, an alkyd resin, a thermosetting polyimide, a resin including a reactive group, and the like. In addition, Examples of the thermoplastic resin include general-purpose resins such as polyethylene, polypropylene, polystyrene, an acrylonitrile/styrene (AS) resin, an acrylonitrile/butadiene/styrene (ABS) resin, a methacrylic resin (PMMA or the like), and vinyl chloride, engineering plastics such as polyamide, polyacetal, polyethylene terephthalate, ultrahigh molecular weight polyethylene, and polycarbonate, and super engineering plastics such as polyphenylene sulfide, polyether ether ketone, liquid crystal polymer, polytetrafluoroethylene, polyetherimide, polyarylate, and polyimide. In the sizing treatment, it is preferable to use a solution in which a resin that becomes the sizing agent is dissolved, and for example, the solution is applied to the fiber bundle20to cause the sizing agent to adhere to the CNTs14of the structure12.

For example, the dispersion28that is used when causing the CNTs14to adhere to the composite material10is prepared as follows. Along CNT (hereinafter, referred to as “material CNT”) is added to a dispersion medium, the material CNT is cut by a homogenizer, a shearing force, an ultrasonic disperser, or the like to obtain the CNTs14having a desired length, and to realize dispersion uniformity of the CNTs14.

As the dispersion medium, 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, and a mixed solution containing these materials in arbitrary ratios can be used. The dispersion28does not contain a dispersing agent and adhesive.

A material CNT that becomes a source of the CNTs14having a bent shape as described above may also have a bent shape. In the material CNT, it is preferable that diameters of individual material CNTs are arranged. An example of the material CNT is shown with a SEM photograph inFIG.7. With regard to the material CNT, even when a length of each CNT generated from cutting, it is preferable that the CNT can be isolated and dispersed. According to this, the dispersion28in which the CNTs14satisfying the above-described length condition are isolated and dispersed is easily obtained.

In the composite material10in this example, as described above, since CNTs having the bent shape as the CNTs14are caused to adhere, other CNTs14enter a space formed between the CNTs14and the surfaces of the fibers11to which the CNTs14adhere, between the adhered CNTs14, or the like. According to this, more CNTs14adhere to the fiber11. In addition, the CNTs14strongly adhere to the fiber11and the structure12is formed, and thus the CNTs14are less likely to be peeled off from the fiber11. In addition, in the fiber-reinforced molded article manufactured by using the composite material10, the properties derived from the CNTs are further enhanced.

As described above, in the secondary products prepared by using the composite material10as described above, mechanical properties such as a vibration damping property (damping property) and a variation property of the elastic modulus are further improved in comparison to a secondary product using a composite material in the related art. With regard to the variation property of the elastic modulus, an increase in the elastic modulus of the fiber-reinforced molded article is suppressed with respect to an increase in a collision speed to the fiber-reinforced molded article.

A concentration of the CNTs14in the dispersion28is preferably within a range of 0.003 to 3 wt %. The concentration of the CNTs14in the dispersion28is more preferably 0.005 to 0.5 wt %.

In the above-described embodiment, fixing of CNTs to a surface of a fiber is obtained by coupling between the fiber and the CNT due to a Vander Waals force, but in addition to this, a binding part configured to reinforce the fixing of the CNTs to the surface of the fiber may be formed. For example, the binding part is an epoxy resin that is cured in a state of entering gaps formed between the fiber and respective surfaces (peripheral surfaces) of the CNTs directly adhered (in contact) to the fiber. For example, the epoxy resin is dissolved in a solvent such as toluene, xylene, acetone, methyl ethyl ketone, methyl isobutyl ketone (MIBK), butanol, ethyl acetate, and butyl acetate to form a solution, a fiber bundle including fibers on which a structure is formed is immersed in the solution, and heating is performed. According to this, the epoxy resin that is not cured is caused to enter gaps formed between the fibers and the respective surfaces of the CNTs, and the epoxy resin is cured.

Note that, when forming the binding part, an epoxy resin solution that is a material of the binding part may be used in a state of an emulsion. For example, an emulsifier such as a nonionic emulsifier maybe added to the solution obtained by dissolving the epoxy resin in the solvent to obtain the emulsion. In addition to the epoxy resin, the binding part may be formed by, for example, a phenol resin, a polyurethane resin, a melamine resin, a urea resin, a polyimide resin, or the like. In addition, a silane coupling agent or inorganic adhesive can also be used as the binding part.

Second Embodiment

A composite material of a second embodiment includes a plurality of fixing resin parts which partially fix some of a plurality of CNTs which constitute a structure to a surface of a fiber. The composite material of the second embodiment is the same as the composite material of the first embodiment except that the fixing resin parts are provided instead of the sizing agent, and thus the same reference numeral will be given to substantially the same member and detailed description thereof will be omitted.

InFIG.8, a composite material10A is provided with a plurality of fixing resin parts38which partially fix some of a plurality of the CNTs14constituting the structure12to a surface of the fiber11. The composite material10A is set as a resin-impregnated fiber, or is used as a single yarn that constitutes a multifilament or a reinforcement fiber of a fiber-reinforced molded article.

Each of the fixing resin parts38is obtained when a resin is cured in a granular shape ranging from the surface of the structure12to the surface of the fiber11. The fixing resin part38is fixed to the surface of the fiber11to which the bottom of the fixing resin part38adheres, and is fixed to the CNTs14of the structure12that covers the fiber11on an upper portion in comparison to the bottom portion. In this manner, the fixing resin part38is fixed to both the surface of the fiber11and parts of the CNTs14to fix the CNTs14to the fiber11.

As described above, the fixing resin part38is configured to partially fix parts of a plurality of the CNTs14constituting the structure12to the surface of the fiber11, is provided to be scattered on the surface of the fiber11, and fixes the CNTs14of the structure12for every site. In the fixing resin part38that is provided to be scattered, an upper portion thereof is exposed to the surface of the structure12, and is observed in a state of being scattered on the surface of the structure12.

As schematically illustrated inFIG.9, in a diameter direction of the fiber11(thickness direction of the structure12), the fixing resin part38is formed in a range from the surface of the structure12to the surface of the fiber11, is fixed to the surface of the fiber11, and is fixed to a portion of the CNTs14that is included on an inner side.

In the CNTs14of the structure12, a portion adheres to the surface of the fiber11, and a portion that overlaps other CNTs14or a portion interposed between the other CNTs14is fixed to the fixing resin part38. In addition, in the CNTs14, an end or a central portion is fixed to the fixing resin part38. As described above, the CNTs14which are fixed to the fixing resin part38are strongly fixed to the fiber11due to the fixing resin part38. As described above, a portion of the CNTs14, which is not covered with the fixing resin part38, adheres to the surface of the fiber11by a Vander Waals force. Accordingly, the portion may be separated from the surface of the fiber11and float therefrom due to an operation of a force weaker in comparison to the portion that is fixed by the fixing resin part38, and may move on the surface of the fiber11. In addition, on the surface of the structure12, the portion of the CNTs14which is not fixed by the fixing resin part38enters a free state in which the portion is separated from the surface of the structure12and floats from the surface.

As described above, since the CNTs14of the structure12are fixed by the fixing resin part38, partial detachment of the structure12is further suppressed in comparison to a case where the CNTs14of the structure12are not fixed by the fixing resin part38. According to this, properties of the fiber-reinforced molded article which are derived from the CNTs can be further enhanced. The CNTs14of the fixing resin part38are constrained to the surface of the fiber11, and thus it is easy to cause a current to flow from the fiber11. As a result, the CNTs14can contribute to an improvement of conductivity of the fiber-reinforced molded article.

Note that, all of the CNTs14constituting the structure12may be fixed to the fiber11by the fixing resin part38, but it is sufficient if some of the CNTs14are fixed. That is, at least a part of the CNTs14constituting the structure12may be fixed by the fixing resin part38. The CNTs14form a membrane as the structure12having a non-woven fabric structure, and thus when at least a part of the CNTs14constituting the structure12is fixed, performance can be exhibited.

In the fixing resin part38, a number ratio N that is the number per 5 μm square on the surface (outer peripheral surface) of the structure12in a plan view is preferably within a range of 27 to 130. In addition, on the surface of the structure12in a plan view, an area ratio S of the fixing resin part38is preferably within a range of 6% to 45%, and more preferably within a range of 7% to 30%. On the surface of the structure12in a plan view, the area ratio S is a ratio of an area of the surface of the structure12which is covered by a plurality of the fixing resin parts38to a surface area of the structure12. When the surface area of the structure12in a predetermined range is set as S2, and the area of the surface of the structure12which is covered by the fixing resin parts38in a predetermined range is set as S1, the area ratio S is obtained as “S=S1/S2×100 (%)”. In the plan view, a peripheral surface of the structure12is observed in a planar manner from a direction orthogonal to the fiber axis direction of the fiber11.

Note that, since the thickness of the structure12is sufficiently smaller than a radius of the fiber11, the surface of the structure12can be regarded as the surface of the fiber11. In addition, with regard to the fixing resin parts38, the area of the surface of the structure12which is covered by the fixing resin parts38and the area of the surface of the fiber11which is covered by the fixing resin parts38can be regarded to be substantially the same as each other. Accordingly, the number ratio N and the area ratio S can be regarded as the number ratio that is the number of the fixing resin parts38per 5 μm square on the surface (outer peripheral surface) of the fiber11in a plan view, and a ratio of the area of the surface of the fiber11which is covered by the fixing resin parts38to the surface area of the fiber11on the surface of the fiber11in a plan view, respectively.

Actually, when the number ratio N is counted, and the area ratio S is obtained, for example, the structure12formed on the peripheral surface of the fiber11is observed in a planar manner by using a SEM photograph. In addition, an observation frame of 5 μm square is set on a planar observation image of the structure12in the SEM photograph, the number of the fixing resin parts38in the observation frame is counted, and the number is set as the number ratio N. Similarly, the area ratio S can be calculated by obtaining an area of each of the fixing resin parts38observed in the observation frame, and setting a total area of the fixing resin parts38as the area S1, and the area of the observation frame as the surface area S2. Note that, the observation frame G may be set by making the center of the observation frame G and the center of the fiber11in a diameter direction match each other.

When the number ratio N and the area ratio S are increased, the CNTs14can be reliably fixed to the surface of the fiber11, and partial detachment of the structure12can be reduced. In addition, when the number ratio N and the area ratio S are decreased, a portion of the CNTs14which is not fixed by the fixing resin parts38increases, and the degree of freedom of the CNTs14and the degree of freedom of the structure12can be raised.

When the above-described number ratio N is 27 or more, or the area ratio S is 6% or more, the CNTs14can be reliably fixed to the fiber11by the fixing resin parts38, the effect of reducing partial detachment of the structure12is reliably obtained, and properties of the fiber-reinforced molded article which are derived from the CNTs can be enhanced. In addition, when the number ratio N is 130 or less or the area ratio S is 45% or less, the CNTs14which are entirely covered with the fixing resin parts38can be sufficiently reduced. According to this, the properties of the fiber-reinforced molded article which are derived from the CNTs, particularly, the effect based on floating of some of the CNTs14from the surface of the structure12is reliably obtained. It is preferable that both the number ratio N and the area ratio S are set within the above-described ranges in combination.

As to be described later, under constant conditions, the area ratio S can be increased or decreased in approximately proportion to the number ratio N, and the number ratio N and the area ratio S can simultaneously satisfy the conditions. Note that, in a case where the area ratio S is satisfied, the total area of the fixing resin parts38in 5 μm square on the surface of the structure12in a plan view becomes within a range of 1.5 to 11.25 μm2.

On the surface of the structure12, a substantial area per one of the fixing resin parts38is preferably set within a range of 0.03 to 1.12 μm2. When an area of each of the fixing resin parts38is 0.03 μm2or more, a fixing strength of reliably fixing the CNTs14to the surface of the fiber11is obtained. Even in this case, the effect of reducing partial detachment of the structure12is reliably obtained. When the area of each of the fixing resin parts38is 1.12 μm2or less, the degree of freedom of the CNTs14is sufficiently obtained.

The area ratio S and the number ratio N of the fixing resin parts38in a plan view, or the substantial area of each of the fixing resin parts38can be obtained by using image analysis software (for example, Winroof2015 (manufactured by Mitani Corporation)).

As described above, in the fiber-reinforced molded article and the like manufactured from the composite material10A including the structure12constituted by the CNTs14having the bent shape, the properties derived from the CNTs14are further improved in comparison to the related art.

The elastic modulus of the fiber-reinforced molded article using the composite material10A can be raised by a constraining effect of suppressing displacement of the fiber due to the composite region. In addition, in the fiber-reinforced molded article, an increase in the elastic modulus of the fiber-reinforced molded article is suppressed with respect to an increase in a collision speed to the fiber-reinforced molded article by the constraining effect due to the composite region between the fibers. As a result, speed dependency of the elastic modulus decreases. In addition, resistance to propagation of interlayer peeling fracture can be increased due to the fixing resin parts38.

A process of manufacturing the composite material10A is the same as in the first embodiment except that a fixing resin application treatment for forming the fixing resin parts38is performed instead of the sizing treatment for causing the sizing agent to adhere to the surface of the structure12. That is, the fixing resin application treatment is performed with respect to the fiber bundle20that is dried after being drawn from the dispersion28(refer toFIG.2) , thereby forming the fixing resin parts38. The fixing resin application treatment can be set as a treatment corresponding to a material that becomes the fixing resin parts38or a shape thereof.

As a preferred method of the fixing resin application treatment, a method of using an emulsion type treatment liquid in which an uncured resin (polymer) that becomes the fixing resin parts38is dispersed in a dispersion medium in a liquid droplet shape is exemplified. In this method, an adhesion treatment in which the fiber bundle20in which the structure12is formed on each fiber11is opened and is brought into contact with the treatment liquid to cause the resin to adhere to the fiber bundle20, and a curing treatment in which the dispersion medium is evaporated after the adhesion treatment and the resin is cured to be the fixing resin parts38are sequentially performed.

As the resin in the treatment liquid, a resin having a curing property, that is, a curable resin is used. The curable resin may be any of a thermosetting resin, a reactive curable resin, and the like. Note that, a cured resin obtained through curing of the curable resin may be a thermoplastic resin. Specific examples of the thermoplastic resin include an epoxy resin, a urethane resin, a urea resin, a polyimide resin, vinyl acetate, an acrylic resin, an olefin resin, vinyl chloride, a phenol resin, a melamine resin, a rubber-based resin, a silicon-based resin, inorganic adhesive, and the like, but there is no limitation to the resins.

Note that, it is preferable that the fixing resin parts38have high affinity with the matrix resin. According to this, it is preferable that the fixing resin parts38and the matrix resin are, for example, a combination of polar resins, or a combination of non-polar resins.

Examples of the dispersion medium of the treatment liquid include water, ethanol, acetone, MEK, N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, toluene, xylene, and the like. The dispersion media can be used alone or in combination of two or more kinds. As the dispersion medium, water is preferable from the viewpoints of handleability and safety. The concentration of the resin in the treatment liquid after dilution is appropriately adjusted to be a target amount of the fixing resin parts38adhered (a ratio (wt %) of the mass of the fixing resin parts38to the composite material10A). The amount of the fixing resin parts38adhered to the surfaces of the fibers11after the drying treatment is preferably within a range of 0.1 to 5.0 wt %, and more preferably within a range of 0.3 to 3.0 wt %.

A particle size of the resin in the treatment liquid is preferably within a range of 0.05 to 1 μm, and more preferably within a range of 0.1 to 0.4 μm. The particle size of the resin in the treatment liquid can be obtained by a laser analysis method. When the particle size of the resin in the treatment liquid is 0.05 μm or more, the CNTs14can be reliably fixed to the surfaces of the fibers11, and when the particle size is 1 μm or less, the resin can reliably enter between the CNTs14constituting the structure12, and the CNTs14can be reliably prevented from being covered with the resin. In addition, when the particle size is 0.1 μm or more, it can be said that the particle size is a resin size sufficient for fixing the structure12having a nonwoven fabric shape, and when the particle size is 0.4 μm or less, partial fixing of the structure12becomes possible.

In a case of using the emulsion type treatment liquid containing the resin having a particle size in a range of 0.1 to 0.4 μm, it is possible to form the fixing resin parts38each having an area in a range of 0.03 to 1.12 μm2. In a case of using the treatment liquid in which the amount of the fixing resin parts38adhered after the curing treatment is adjusted to be 0.1% by mass to 5.0% by mass, the number ratio N can be set within a range of 27 to 130.

The adhesion treatment method using the treatment liquid is not particularly limited, and examples thereof include a roller immersion method, a roller contact method, a spray method, and the like. In addition, the curing treatment method is not particularly limited, and for example, hot wind, a hot plate, a heating roller, various infrared heaters, and the like can be used.

FIG.10illustrates a flat belt40as an example of products using the composite material10. For example, the flat belt40is used as a power transmission belt or the like. The flat belt40has a structure in which a reinforcement fabric42and a surface rubber layer43are laminated on both surfaces of an inner rubber layer41, and a cord core wire45is embedded in the inner rubber layer41.

The inner rubber layer41is formed from, for example, nitrile rubber, carboxylated nitrile rubber, hydrogenated nitrile rubber, chloroprene rubber, chlorosulphonated polyethylene, polybutadiene rubber, natural rubber, EPM, EPDM, urethane rubber, acrylic rubber, or the like. The reinforcement fabric42is provided to improve durability of the flat belt40, and for example, woven or knitted fabrics such as a polyester fiber, a nylon fiber, an aramid fiber, a glass fiber, a carbon fiber and cotton are used. The surface rubber layer43is provided to obtain a predetermined frictional force between the flat belt40and a conveying object or a power transmission device, and is formed from, for example, nitrile rubber, carboxylated nitrile rubber, hydrogenated nitrile rubber, chloroprene rubber, chlorosulphonated polyethylene, polybutadiene rubber, natural rubber, EPM, EPDM, urethane rubber, acrylic rubber, silicone rubber, or the like.

As the cord core wire45, the above-described multifilament15in which composite materials10are coupled to each other by the matrix resin16that is impregnated up to the structure12and is cured is used. In the cord core wire45, a fiber axial direction of the multifilament15and a longitudinal direction of the flat belt40(a traveling direction of the flat belt40, an arrow X direction) match each other. In addition, a plurality of the cord core wires45are arranged with a predetermined pitch in a width direction orthogonal to the longitudinal direction of the flat belt40. In a case of using a twisted multifilament as the cord core wire45, it is preferable to alternately arrange an S-twisted one and a Z-twisted one in order to suppress meandering of the belt. As described above, since the flat belt40includes the multifilament using the composite material10as the cord core wire45, elongation against tension or compression is small, and durability against repetitive bending is high.

Description has been given of an example of the flat belt, but the composite material can be used as a cord core wire of various belts, and particularly, the composite material is suitable for a cord core wire of a toothed belt represented by a timing belt, or a transmission belt such as a V-belt and a V-ribbed belt. An example illustrated inFIG.11uses the multifilament15as a cord core wire45of a V-belt50, and an example illustrated inFIG.12uses the multifilament15as a cord core wire45of a timing belt60. The V-belt50inFIG.11has a structure in which a lower fabric51, a bottom rubber layer52, an adhesive rubber layer53, a rear rubber layer54, and an upper fabric layer55are laminated, and the cord core wire45is embedded in the adhesive rubber layer53. In addition, the timing belt60inFIG.12has a structure which includes a rubber layer61including a belt main body portion61aand a plurality of tooth portions61b,and a tooth fabric62that cover a surface of the rubber layer61on the tooth portion61bside, and in which the cord core wire45is embedded in the belt main body portion61a.

FIG.13illustrates results obtained by evaluating an interface adhesive strength between a glass fiber and a matrix resin due to a difference in presence or absence of CNT adhesion. In the evaluation, a plurality test pieces A in which a CNT composite fiber using a glass fiber as the fiber11is embedded in a soft epoxy resin, and a plurality of test pieces B in which a glass fiber (raw yarn) is embedded in a soft epoxy resin were prepared, and evaluation was performed by a fragmentation method.

With respect to the test pieces A, in accordance with the above-described procedure, the glass fiber (raw yarn) was allowed to pass through the dispersion28irradiated with ultrasonic waves, and the CNTs14were caused to sufficiently and uniformly adhere to the glass fiber to obtain the fiber11on which the structure12is formed. Note that, the sizing agent or the binding part was not applied. The test pieces A were prepared by drawing one glass fiber on which the structure12is formed, and by embedding the glass fiber in a soft epoxy resin. A diameter of the glass fiber (raw yarn) was approximately 16 μm. A SEM photograph obtained by observing the structure12that is formed on a surface of the glass fiber used in the test pieces A is shown inFIG.14.

The test pieces B were prepared by embedding one glass fiber (raw yarn) to which CNTs do not adhere in a soft epoxy resin. Note that, preparation conditions of the test pieces B were set to be the same as in the test pieces A except that the CNTs do not adhere to the glass fiber.

With respect to the test pieces A and the test pieces B, a tensile load was applied until the glass fiber is not cut, a length of each cut piece of the glass fiber at a constant length in the test piece was measured for each test piece, and an average (cut fiber length) of the length of the cut pieces was obtained with respect to each test piece.

The cut fiber length of the test pieces A and B which was measured by the fragmentation method as described above is shown inFIG.13. It can be seen that in the test pieces A, that is, test pieces in which the CNTs14are caused to adhere to the glass fiber, the cut fiber length is shorter and the interface adhesive strength between the glass fiber and the matrix resin is higher in comparison to the test pieces B, that is, test pieces in which the CNTs are not caused to adhere to the glass fiber. In the test pieces A and B, since the glass fiber has a diameter of 10 μm or more, stress concentration at an interface becomes significant. However, in the test pieces A, since the CNTs14exist on the surface of the glass fiber, it is considered that a resin elastic modulus at the interface is improved, and stress concentration is mitigated, and thus the interface adhesive strength is improved.

REFERENCE SIGN LIST