Patent Publication Number: US-2019169792-A1

Title: Composite Material and Reinforcing Fiber

Description:
TECHNICAL FIELD 
     The present invention relates to a composite material including a fiber with carbon nanotubes (hereinafter referred to as “CNTs”) adhered to a surface of the fiber, and to a reinforced fiber including the composite material. 
     BACKGROUND ART 
     It is desirable that CNTs uniformly adhere to the surface of a fiber to allow the above-described composite material to exhibit its functions. 
     Such a composite material is produced as follows: a fiber is immersed in a solution (which may be referred to as CNT nano dispersion in the following description), in which CNTs are dispersed at a nano level; after the CNTs adhered to the surface of the fiber, the fiber is taken out of the CNT nano dispersion and dried. 
     Van der Waals force causes irreversible aggregation of the CNTs in the CNT nano dispersion. To allow the CNTs to adhere uniformly to the surface of the fiber, it has been necessary to add a large amount of dispersing agent to the CNT nano dispersion to prevent the aggregation of the CNTs and disperse the CNTs. 
     Ultrasonic irradiation or stirring may be performed in an auxiliary manner (see Patent Literature 1) to disperse the CNTs in the CNT nano dispersion. The dispersing agent has been required to disperse the CNTs. Generally, the dispersing agent has been regarded as indispensable for the production process of the composite material. 
     In addition to the dispersing agent, an adhesive and other additives are added to the CNT nano dispersion to bond the CNTs to the surface of the fiber. 
     Owing to the CNTs adhered to the surface, the thus-produced composite material is expected to exhibit not only intrinsic functions of fiber but also improvements in electrical conductivity, thermal conductivity, mechanical strength, and the like, which are derived from the CNTs. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Patent Laid-Open Publication No. 2010-59561 
     SUMMARY OF INVENTION 
     Technical Problem 
     Despite the large amount of CNTs adhered to the fiber, electrical conductivity, thermal conductivity, mechanical strength, and the like of the composite material have not achieved levels required for practical use. This is because a dispersing agent, an adhesive, and other additives intervene between the fiber and the CNTs. 
     An object of the present invention is to provide a composite material and a reinforced fiber which exhibit both of intrinsic functions of a fiber and functions, such as electrical conductivity, thermal conductivity, and mechanical strength, derived from CNTs. 
     Solution to Problem 
     A composite material according to the present invention includes a fiber and a plurality of carbon nanotubes disposed on a surface of the fiber. The carbon nanotubes adhere directly to the surface of the fiber. 
     A reinforced fiber according to the present invention includes the composite material and a resin composition covering a surface of the composite material. 
     Advantageous Effects of Invention 
     According to the present invention, the CNTs adhere to the surface of the fiber, without any intervening material between the surface of the fiber and the CNTs. Hence, the CNTs are not likely to peel off from the surface of the composite material. Therefore, the composite material and the reinforced fiber exhibit both of the intrinsic functions of the fiber and the functions, such as electrical conductivity, thermal conductivity, and mechanical strength, derived from CNTs. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a conceptual diagram of a composite material according to a first embodiment. 
         FIG. 2  is a conceptual diagram of the composite material according to the first embodiment. 
         FIG. 3  is a production flow diagram of a reinforced fiber according to a second embodiment. 
         FIGS. 4A-4D  show SEM images of a composite material according to Examples:  FIG. 4A  shows an SEM image of a first site;  FIG. 4B  shows an SEM image of a second site; 
         FIG. 4C  shows an SEM image of a third site; and  FIG. 4D  shows an SEM image of a fourth site. 
         FIG. 5  is an SEM image of a fiber according to Comparative Examples. 
         FIGS. 6A-6D  show SEM images of a composite material with the CNTs entangled with each other to be in direct contact with each other or in direct connection with each other to form a network structure, without any intervening material:  FIG. 6A  shows an SEM image of a first site;  FIG. 6B  shows an SEM image of a second site;  FIG. 6C  shows an SEM image of a third site; and  FIG. 6D  shows an SEM image of a fourth site. 
         FIG. 7  is a schematic diagram showing a method of measuring interfacial adhesion strength. 
         FIGS. 8A-8D  show SEM images of the surfaces of samples after evaluation of interfacial properties of composite materials:  FIG. 8A  shows the SEM image of the Comparative Example; and  FIG. 8B  shows the SEM image of the Example. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, the embodiments of the present invention will be described in detail with reference to the drawings. 
     1. First Embodiment 
     (1) Configuration 
     As shown in  FIG. 1 , a composite material  1  includes a fiber  3  and CNTs  5  disposed on a surface of the fiber  3 . 
     The material of the fiber  3  is not particularly limited. The fiber  3  may include a carbon material, a resin material, a metallic material, a ceramic material, or the like. In this embodiment, the fiber  3  is a carbon fiber. Note that in the drawing, the fiber  3  is illustrated as a single fiber, but the fiber  3  may be a bundle of fibers. Examples of the fiber  3  include, but are not particularly limited to, a carbon fiber having a diameter of approximately 3 to 150 μm made from polyacrylonitrile, rayon, or pitch, which is produced by heat-treatment of organic fiber derived from petroleum, coal, or coal tar or by heat-treatment of organic fiber derived from wood or plant. 
     The CNTs  5  are dispersed on substantially the entire surface of the fiber  3  and adhere directly to the surface of the fiber  3 , without any intervening material between the surface of the fiber  3  and the CNTs ( FIG. 2 ). That is, the CNTs  5  adhere directly to the surface of the fiber  3 , without a dispersing agent such as a surfactant, an adhesive, or the like between the surface of the fiber  3  and the CNTs. Here, the term adhesion (or adhere to) refers to bonding by Van der Waals force. 
     The CNTs  5  are locally and unevenly distributed on the surface of the fiber  3 . Due to the uneven distribution, the surface of the fiber  3  is partly exposed through the CNTs  5 . The term “unevenly distributed” means that the CNTs  5  do not uniformly cover the entire surface of the fiber  3 , and does not include evaluation of the dispersibility of the CNTs  5 . 
     It is preferred that the CNTs  5  are entangled with each other, so that the CNTs  5  are directly in contact with each other or directly connected to each other to form a network structure, without the presence of the intervening material. Here, the term “connection (connect, connected)” includes physical connection (mere contact). Further, the terms “direct contact (directly in contact with) and direct connection (directly connected to)” include a state where the CNTs  5  are simply in contact with each other and a state where the CNTs  5  are connected together, without the presence of the intervening material, and are not to be taken in a limited sense. 
     The length of the CNT  5  is preferably 0.1 to 50 μm. In a case where the length of the CNT  5  is 0.1 μm or more, the CNTs  5  are entangled with each other and directly connected to each other. In a case where the length of the CNT  5  is 50 μm or less, the CNTs are likely to be dispersed uniformly. On the other hand, in a case where the length of the CNT  5  is less than 0.1 μm, the CNTs  5  are not likely to be entangled with each other. In a case where the length of the CNT  5  is more than 50 μm, the CNTs  5  are likely to aggregate. 
     It is preferred that the average diameter of the CNTs  5  is approximately 30 nm or less. In a case where the CNT  5  has a diameter of 30 nm or less, it is flexible enough to bend (deform) in accordance with the curvature of the surface of the fiber  3 . Therefore, the CNTs  5  more securely adhere to the surface of the fiber  3 . On the other hand, in a case where the CNT  5  has a diameter of more than 30 nm, the CNT  5  is not flexible and does not bend along the surface of the fiber  3 . Therefore, the CNTs  5  are not likely to adhere to the surface of the fiber  3 . Note that the diameter of the CNT  5  is defined as an average diameter measured from a transmission electron microscope (TEM) image. It is more preferred that an average diameter of the CNTs  5  is approximately 20 nm or less. 
     It is preferred that the concentration of the CNTs  5  adhered to the surface of the fiber  3  is in a range of 0.001 to 20 wt % with respect to the fiber  3 . When the concentration of the CNTs  5  is in the above range, some portions of the surface of the fiber  3  are not covered with the CNTs  5 . The portion of the surface of the fiber  3  not covered with the CNTs  5  is not covered with an adhesive or the like, i.e. the portion of the surface of the fiber  3  is exposed. Hence, the functions of the fiber  3  are not impaired by the CNTs  5 . The concentration of the CNTs  5  is more preferably in a range of 0.01 to 10 wt % with respect to the fiber  3 . 
     (2) Production Method 
     Next, a method for producing the composite material  1  will be described. The composite material  1  is produced by preparing the CNTs  5 , then preparing a liquid dispersion containing the CNTs  5 , and allowing the CNTs  5  to adhere to the surface of the fiber  3  in the liquid dispersion. Hereinafter, each step will be described in sequence. 
     (Preparation of CNT) 
     The CNTs  5  are produced as follows: a catalyst film including aluminum and iron is formed on a silicon substrate by a thermal CVD method described in, for example, Japanese Patent Laid-Open Publication No. 2007-126311; a catalytic metal for growth of the CNTs  5  is atomized into fine particles; and a hydrocarbon gas is brought into contact with the catalytic metal in a heated atmosphere. Thus, the CNTs  5  are produced. It is also possible to use the CNTs  5  produced by another production method such as an arc discharge method or a laser vaporization method. It is preferred to use the CNTs  5  with no or minimum possible amount of impurities. The impurities may be removed by high temperature annealing in an inert gas after the CNTs  5  are produced. The CNT  5  produced in this example has a high aspect ratio of the length in a range of several hundred micrometers (μm) to several millimeters (mm) to the diameter of 30 nm or less, and linearity. The CNT  5  may be a single-walled or multi-walled carbon nanotube, but is preferably a multi-walled carbon nanotube. 
     (Preparation of Liquid Dispersion) 
     Next, the liquid dispersion is prepared. In the liquid dispersion, the above-produced CNTs  5  are nano-dispersed. Nano dispersion (or nano-dispersed) refers to a state in which the CNTs  5  dispersed in a solution are physically separated from and not entangled with one another and the proportion of aggregation, in which two or more CNTs  5  aggregate in bundles, is 10% or less. 
     The liquid dispersion is prepared by adding the above-produced CNTs  5  into a solvent. The CNTs  5  in the solvent are dispersed uniformly by application of mechanical energy such as compressive force, shearing force, impulse force, or frictional force. The mechanical energy may be applied, for example, with a stirrer, a ball mill, a jet mill, a homogenizer, an ultrasonic dispersion machine, or the like. 
     Examples of the solvent include 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, and ethyl acetate. The liquid dispersion does not contain a dispersing agent, a surfactant, an adhesive, an additive, and the like. 
     (Adhesion of CNT) 
     The mechanical energy is applied to the above-prepared liquid dispersion in a manner similar to the above while the fiber  3  is immersed in the liquid dispersion. Thereby the CNTs  5  adhere to the surface of the fiber  3 . 
     The mechanical energy applied to the liquid dispersion causes a state (reversible reaction state) of reversible reaction, in which dispersion and aggregation of the CNTs  5  occur constantly. 
     When the fiber  3  is immersed in the liquid dispersion in the reversible reaction state, the reversible reaction, in which the CNTs  5  disperse and aggregate in a reversible manner, occurs also on the surface of the fiber  3 . The CNTs  5  adhere to the surface of the fiber  3  during transition from the state of dispersion to the state of aggregation. 
     The Van der Waals force acts between the CNTs  5  when the CNTs  5  aggregate. Due to the Van der Waals force, the CNTs  5  adhere to the surface of the fiber  3 . Then, the fiber  3  is taken out of the liquid dispersion and dried. Thereby, the composite material  1  with the CNTs  5  adhered to the surface of the fiber  3  is produced. 
     (3) Action and Effect 
     In the present embodiment, since the CNTs  5  adhere to the surface of the fiber  3  without any intervening material, the CNTs  5  are not likely to be peeled off from the surface of the fiber  3  of the composite material  1 . Therefore, the composite material  1  exhibits both of the intrinsic functions of the fiber and the functions, such as electrical conductivity, thermal conductivity, and mechanical strength, derived from the CNTs. 
     2. Second Embodiment 
     Next, a tow prepreg, which is a reinforced fiber, according to a second embodiment will be described. The tow prepreg includes a composite material bundle and the resin composition covering a surface of the composite material bundle. The composite material bundle is a bundle of the plurality of, for example, several thousands to several ten thousands of composite materials  1 . 
     The tow prepreg according to the present embodiment is prepared as follows: the resin composition is removed from a commercially available tow prepreg (with no CNTs adhered thereto), to obtain a bundle of fibers; the composite material bundle is prepared by allowing the CNTs to adhere to a surface of the bundle of fibers; and the composite material bundle is covered with the resin composition. Hereinafter, each step will be described with reference to  FIG. 3 . 
     A tow prepreg production apparatus  10  shown in  FIG. 3  includes a resin removal tank  11 , a CNT adhesion tank  14 , and a resin coating tank  20 . The resin removal tank  11  contains an organic solvent such as methyl ethyl ketone as a resin remover  12 . The CNT adhesion tank  14  contains the liquid dispersion (a reference numeral  16  in this drawing) described in the above first embodiment. The resin coating tank  20  contains a resin composition  22  melted by heating. 
     First, a commercially available tow prepreg  100  is wound around a guide roller disposed in the resin removal tank  11 . The tow prepreg  100  travels at a predetermined travelling speed. Thereby, the commercially available tow prepreg  100  is immersed in the resin remover  12 , so that the resin composition is removed. It is preferred to apply the above-described mechanical energy to the resin remover  12  while the prepreg  100  is immersed therein. The tow prepreg  100  is loosened by the application of the mechanical energy, so that the resin composition is efficiently removed. The resin composition is thus removed to obtain the bundle of fibers (hereinafter referred to as “fiber tow”)  102 , the surface of which is exposed. 
     The fiber tow  102  is wound around a guide roller disposed in the CNT adhesion tank  14  and travels in the CNT adhesion tank  14  at a predetermined travelling speed. Thereby, the fiber tow  102  is immersed in the liquid dispersion  16 . The CNTs are efficiently adhered to the surface of the fiber by applying the mechanical energy to the liquid dispersion  16  while the fiber tow  102  is immersed in the liquid dispersion  16 . Thus, a composite material bundle  18  is obtained. 
     Next, the composite material bundle  18  is wound around a guide roller disposed in the resin coating tank  20  and travels in the resin coating tank  20  at a predetermined travelling speed. Thereby, the composite material bundle  18  is immersed in the molten resin composition  22 . Examples of the resin composition include thermosetting resins such as an epoxy resin, a phenol resin, a melamine resin, a urea resin, an unsaturated polyester resin, an alkyd resin, and a thermosetting polyimide, thermoplastic resins such as polyethylene, polypropylene, polystyrene, an acrylonitrile/styrene resin, an acrylonitrile/butadiene/styrene resin, a methacrylic resin, and vinyl chloride, engineering plastics such as polyamide, polyacetal, polyethylene terephthalate, ultra-high molecular weight polyethylene, and polycarbonate, and super engineering plastics such as polyphenylene sulfide, polyether ether ketone, a liquid crystal polymer, polytetrafluoroethylene, polyetherimide, polyarylate, and polyimide. Thus, a tow prepreg  24  including the composite material bundle  18  and the resin composition is produced. The surface of the composite material bundle  18  is covered with the resin composition. 
     The CNTs directly adhere to the surface of the fiber of the tow prepreg  24  of the present embodiment. Hence, the same effects as those in the above first embodiment are obtained. 
     In the tow prepreg  24 , the resin composition covers the fiber and the CNTs  5  directly adhered to the surface of the fiber. Therefore the mechanical strength of the tow prepreg  24  is improved by reinforced the resin composition with the CNTs directly adhered to the surface of the fiber. 
     The present embodiment describes the procedures of producing the composite material bundle  18 . The procedures include the procedure of removing the resin composition from the commercially available tow prepreg (with no CNTs)  100  and the procedure of allowing the CNTs to adhere to the surface of the fibers  3 . The present invention is not limited to this. The step of removing the resin composition is omitted in a case where the fiber is not covered with the resin composition. 
     The present embodiment describes the reinforced fiber including the composite material bundle in which approximately several thousands to several ten thousands of composite materials are bundled. The present invention is not limited to this. The reinforced fiber may be composed of one composite material. 
     3. Examples 
     (Sample) 
     The composite materials according to Examples were prepared by following the procedures of the above-described production method. MW-CNTs (Multi-Walled Carbon Nanotubes) were used as the CNTs. The MW-CNT was grown to the diameter of 10 to 15 nm and the length of 100 μm or more on a silicon substrate by a thermal CVD method. A mixed acid prepared by mixing sulfuric acid and nitric acid at a ratio of 3:1 was used to remove catalyst residues from the CNTs. The CNTs were washed, filtered, and dried. 
     A tow prepreg (model: T-700, manufactured by Toray Industries, Inc.), which is a bundle (tow) of 12000 carbon fibers each having a diameter of 7 μm, was used. MEK was used as the resin remover. In the liquid dispersion, methyl ethyl ketone was used as the solvent. 
     The concentration of the CNTs  5  in the liquid dispersion was set to 0.01 wt %. A thermosetting epoxy resin was used as the resin composition. 
     First, the tow prepreg was immersed in the resin remover to remove the resin composition. Thereby the fiber tow is obtained. While being immersed, the tow prepreg traveled at the travelling speed of 1.5 m/min. In addition, the mechanical energy was applied to the resin remover with ultrasonic waves. 
     Next, the fiber tow was immersed in the liquid dispersion to obtain a composite material bundle with the CNTs adhered to the fibers. While being immersed, the fiber tow traveled at the travelling speed of 1.5 m/min. In addition, the mechanical energy was applied to the liquid dispersion with ultrasonic waves. SEM images of the composite material thus obtained are shown in  FIG. 4A  to  FIG. 4D .  FIG. 4A  to  FIG. 4D  show the images of four sites, which are different in the longitudinal direction, on one composite material. 
     The composite materials according to the Examples were prepared as described above. In Comparative Examples, the fibers prior to the adhesion of the CNTs were used.  FIG. 5  shows a SEM image of the fiber prior to the adhesion of the CNTs. 
     Apart from the Examples 1 to 10,  FIGS. 6A to 6D  show SEM images of a composite material, which is prepared in a manner similar to the Examples, with a greater amount of CNTs than those of the Examples 1 to 10. The CNTs of the composite material  1  shown in  FIGS. 6A to 6D  are entangled with each other, so that the CNTs are directly in contact with each other or directly connected to each other to form a network structure, without any intervening material. 
     Finally, the fiber tow was immersed in the molten resin composition to produce the tow prepreg. While being immersed, the fiber tow traveled at the travelling speed of 1.5 m/min. 
     (Evaluation) 
     The resulting tow prepreg was washed with MEK, and one reinforced fiber was taken out as a sample. Interfacial adhesion strength of the sample was measured. Equipment (HM410, manufactured by Tohei Sangyo Co., Ltd.) for evaluating interfacial property of composite material was used for measuring the interfacial adhesion strength. First, a micro-droplet was prepared with a resin composition in an atmosphere of 80° C. to at least partially surround the sample, and heated under the condition of 125° C.×1 hour. The micro-droplet includes the resin composition, in this case, a thermosetting epoxy resin or a thermoplastic resin (product number: nylon 6 (1013B), manufactured by Ube Industries, Ltd.). 
     A sample  30  was cooled to room temperature and then sandwiched with blades  32 , as shown in  FIG. 7 . Next, the sample  30  was pulled out in the longitudinal direction (arrow direction in the drawing) of the sample  30  at the speed of 0.12 mm/min from a micro-droplet  34 . A maximum load F at the time of pulling out the sample  30  was measured with a load cell (not shown). The measurement was performed five times for each sample at room temperature in atmospheric air. Interfacial shear strengths τ were calculated by a following formula (1) to evaluate the interfacial adhesion strengths of Examples and Comparative Examples. In the following formula (1), F represents the maximum load during pulling out; d represents the diameter of the fiber; and L represents the length of the micro-droplet in the direction of pulling out. 
       τ= F /( dπL )  (1)
 
     The results are shown in Table 1. As shown in the table, it was verified that the interfacial shear strength of the composite materials according to Examples 1 to 5 was approximately 10% higher than that of the fibers according to the Comparative Examples 1 to 5. The interfacial shear strength of the composite materials according to the Examples 6 to 10 was approximately 30% higher than that of the fibers according to the Comparative Examples 6 to 10. It is considered that the interfacial shear strengths of the composite materials according to the Examples have been improved because the thermosetting epoxy resin covering the surface of the fiber is reinforced by the CNTs directly adhered to the surface of the fiber. 
     In each of the Examples 1 to 10, the interfacial shear strength is higher than that of the corresponding Comparative Example. Thus, the interfacial shear strengths of the composite materials according to the Examples are improved regardless of the type of the resin composition used. 
       FIGS. 8A and 8B  are SEM images taken after the evaluations of the interfacial properties of the composite materials. The SEM images respectively show portions of the samples from which the micro-droplets were peeled off.  FIG. 8A  represents the Comparative Examples 6 to 10.  FIG. 8B  represents the Examples 6 to 10. As shown in  FIG. 8B , the CNTs and the resin composition have been partially peeled off from the surface of the composite materials according to the Examples 6 to 10, so that the surface of the fiber is partially exposed from the surface of the composite material. Thus, in the Examples 6 to 10, the thermoplastic resin organically bonded to the CNTs partially remains on a portion  104  on the surface of the fiber. Based on this fact, it can be said that the CNTs were organically bonded to the micro-droplet, which was formed from the thermoplastic resin and provided on the surface of the fiber, so that the interfacial shear strength was improved. 
     On the other hand, in the Comparative Examples 6 to 10 represented by  FIG. 8A , the micro-droplet, which was formed from the thermoplastic resin and provided on the surface of the fiber, was peeled off at the interface between the fiber and the resin. Hence, it is considered that the micro-droplet formed from the thermoplastic resin was not organically bonded to the fiber. Therefore the interfacial shear strengths were lower than those in the Examples. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 Interfacial 
                   
               
               
                   
                   
                 shear 
                 Average 
               
               
                   
                   
                 strength 
                 value 
               
               
                   
                 Resin composition 
                 (MPa) 
                 (MPa) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Examples 
                 1 
                 Thermosetting epoxy resin 
                 80.2 
                 81.2 
               
               
                   
                 2 
                 Thermosetting epoxy resin 
                 85.6 
               
               
                   
                 3 
                 Thermosetting epoxy resin 
                 80.1 
               
               
                   
                 4 
                 Thermosetting epoxy resin 
                 79.8 
               
               
                   
                 5 
                 Thermosetting epoxy resin 
                 80.2 
               
               
                   
                 6 
                 Thermoplastic resin 
                 57.8 
                 57.8 
               
               
                   
                 7 
                 Thermoplastic resin 
                 59.7 
               
               
                   
                 8 
                 Thermoplastic resin 
                 57.4 
               
               
                   
                 9 
                 Thermoplastic resin 
                 56.5 
               
               
                   
                 10 
                 Thermoplastic resin 
                 57.6 
               
               
                 Comparative 
                 1 
                 Thermosetting epoxy resin 
                 73.4 
                 72.9 
               
               
                 Examples 
                 2 
                 Thermosetting epoxy resin 
                 73.7 
               
               
                   
                 3 
                 Thermosetting epoxy resin 
                 73.5 
               
               
                   
                 4 
                 Thermosetting epoxy resin 
                 70.1 
               
               
                   
                 5 
                 Thermosetting epoxy resin 
                 73.8 
               
               
                   
                 6 
                 Thermoplastic resin 
                 47.3 
                 44.2 
               
               
                   
                 7 
                 Thermoplastic resin 
                 46.7 
               
               
                   
                 8 
                 Thermoplastic resin 
                 37.2 
               
               
                   
                 9 
                 Thermoplastic resin 
                 41.3 
               
               
                   
                 10 
                 Thermoplastic resin 
                 48.5 
               
               
                   
               
            
           
         
       
     
     REFERENCE SIGNS LIST 
     
         
         
           
               1  Composite material 
               3  Fiber 
               5  Carbon nanotube (CNT) 
               18  Composite material bundle 
               24  Tow prepreg (reinforced fiber) 
               102  Fiber tow