METHOD FOR PRODUCING CONDUCTIVE FIBER

Provided is a method for producing a conductive fiber including a synthetic fiber and a conductive film that is formed on a surface of the synthetic fiber and includes single-wall carbon nanotubes, the method including: a conductive ink immersion step of immersing the synthetic fiber in a conductive ink having the single-wall carbon nanotubes dispersed in an aprotic solvent and causing the conductive ink to adhere to the synthetic fiber; and a solvent removal step that is performed after the conductive ink immersion step for drying the synthetic fiber to which the conductive ink adheres and removing the aprotic solvent.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is based on, and claims priority from the prior Japanese Patent Application No. 2023-120624, filed on Jul. 25, 2023, the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to a method for producing a conductive fiber.

BACKGROUND

Single-Wall Carbon Nanotubes (SWCNTs) have excellent mechanical, electrical, and thermal properties, and also high flexibility. Therefore, SWCNT films containing SWCNTs are expected to be used as next-generation flexible conductive materials, for example.

Investigations and research into laminating a SWCNT conductive film on a surface of a synthetic fiber to impart functionality as a conductive fabric have been made in many fields. JP 2010-059561 A discloses a technique for forming a SWCNT conductive film on a synthetic fiber using dispersion containing SWCNTs, binders, and surfactants as dispersants of SWCNTs.

SUMMARY OF THE INVENTION

However, the binder components and surfactant components added as dispersants remain in the formed SWCNT conductive film disclosed in JP 2010-059561 A. Since these components are non-conductive, there is a risk that electrical properties of the SWCNT conductive film, such as electric conductivity characteristics may deteriorate. In addition, compared with polyester fibers (PET fibers), which have high crystallinity and stable surface properties, the technique has problems in that the adhesion strength of an interface is weak, detachment occurs, and properties of SWCNTs are not fully utilized.

The present disclosure has been made in view of the problems of the related art. An object of the present disclosure is to provide a method for producing a conductive fiber having a carbon nanotube conductive film strongly adhered to a synthetic fiber.

A method for producing a conductive fiber according to the present embodiment is a method for producing a conductive fiber including a synthetic fiber and a conductive film that is formed on a surface of the synthetic fiber and includes single-wall carbon nanotubes, the method including: a conductive ink immersion step of immersing the synthetic fiber in a conductive ink having the single-wall carbon nanotubes dispersed in an aprotic solvent and causing the conductive ink to adhere to the synthetic fiber; and a solvent removal step that is performed after the conductive ink immersion step for drying the synthetic fiber to which the conductive ink adheres and removing the aprotic solvent.

According to the present disclosure, it is possible to provide a method for producing a conductive fiber having a carbon nanotube conductive film strongly adhered to a synthetic fiber.

DETAILED DESCRIPTION OF THE INVENTION

A method for producing a conductive fiber according to the present embodiment will be described in detail below with reference to the drawings. Note that dimensional ratios in the drawings are exaggerated for convenience of the description and may be different from actual ratios.

A conductive fiber1illustrated inFIG.1includes a synthetic fiber10, and a conductive film20which is formed on a surface of the synthetic fiber10and includes single-wall carbon nanotubes (hereinafter referred to as SWCNTs)25. By immersing the synthetic fiber10in a conductive ink24having the SWCNTs25dispersed in an aprotic solvent23and causing the ink to adhere to the synthetic fiber10, the conductive fiber1having the conductive film20strongly adhered to the synthetic fiber10is obtained.

The synthetic fiber10is not particularly limited, but the synthetic fiber10is preferably at least one kind of fiber selected from the group consisting of a polyester fiber, polyamide (nylon) fiber, polyolefin fiber, acrylic fiber, polyurethane fiber, and cellulose fiber. Among them, a polyethylene terephthalate fiber (hereinafter referred to as PET fiber) is a kind of polyester fiber. The conductive ink24having the SWCNTs25dispersed in the aprotic solvent23easily permeates the PET fiber, and the conductive film20easily adheres strongly to the PET fiber. Therefore, the synthetic fiber10is preferably the polyester fiber, and more preferably the PET fiber. A structure of the PET fiber is illustrated in chemical formula 1.

The conductive film20includes the SWCNTs. A center diameter of each SWCNT25included in the conductive film20is in a range from 0.5 nm to 5 nm, and preferably in a range from 1 nm to 3 nm, from the viewpoint of conductivity, for example. Further, a length of each SWCNT25included in the conductive film20is in a range from 1 μm to several 10 μm, for example. Still further, a thickness of the conductive film20is in a range from 50 nm to 500 nm, and preferably in a range from 100 nm to 200 nm, from the viewpoint of the adhesion of an interface with the synthetic fiber10, for example.

The synthetic fiber10is immersed in the conductive ink24having the SWCNTs25dispersed in the aprotic solvent23, the ink adheres to the synthetic fiber10, further the synthetic fiber10is dried, and the aprotic solvent23is removed. Accordingly, the conductive film20having the SWCNTs is strongly adhered on the surface of the synthetic fiber10.

The conductive fiber1is produced by performing the following method.

A method for producing a conductive fiber is a method for producing the conductive fiber1including the synthetic fiber10and the conductive film20which is formed on the surface of the synthetic fiber10and has the SWCNTs25. The method for producing the conductive fiber1includes a conductive ink immersion step and a solvent removal step.

As illustrated inFIG.3, the conductive ink immersion step is a step of immersing the synthetic fiber10in the conductive ink24having the SWCNTs25dispersed in the aprotic solvent23, and causing the ink to adhere to the synthetic fiber10.

The aprotic solvent23is used as a dispersing solvent of the conductive ink24. The aprotic solvent23is preferably at least one solvent selected from the group consisting of N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), N-methylacetamide (DMA), and acetonitrile. Both the aprotic solvent23and the SWCNTs25have high polarities. Even if a dispersant such as a surfactant is not added, the SWCNTs25are uniformly dispersed in the aprotic solvent23, and a favorable conductive ink of the SWCNTs25alone can be obtained. When the synthetic fiber10is a PET fiber, DMF is more preferable from the viewpoint of easy permeation into the PET fiber.

DMF is an amide formed by condensation of formic acid and dimethylamine. An amide bond is relatively stable and is used as an organic solvent because it is less likely to react with a nucleophile and electrophile. DMF is polarized and has a high polarity because DMF has a carbonyl group, and therefore DMF is referred to as an aprotic polar solvent. Meanwhile, the SWCNTs25have a high polarity and a high stereoregularity, and exhibit characteristics in which molecules easily attract and aggregate. Since a DMF molecule has an amide bond, and the amide bond and carbon are easily adsorbed, even if a dispersant such as a surfactant is not added, the conductive ink24having the SWCNTs25uniformly dispersed in a DMF solvent can be obtained.

A solubility parameter (SP value) of DMF is 12.0, and this approximates 10.7 as a solubility parameter of the PET fiber. Therefore, DMF easily permeates into an amorphous portion inside a crystalline polymer with a stable surface such as the PET fiber, and causes swelling and softening effects on the PET fiber surface, modifying the PET fiber surface. The change in the PET fiber surface state due to DMF greatly affects the adhesion of an interface.

An SP value is an index of the magnitude of the intermolecular force (cohesive energy density) of a substance, and is expressed as a square root of the heat required for 1 cm3of liquid to evaporate. Substances with close SP values (a) tend to mix easily and a solubility becomes high. An SP value ((cal/cm3)1/2) can be calculated from the following mathematical formula (1).

In the above mathematical formula (1), the latent heat of evaporation of a compound is expressed as AH (cal/mol), the gas constant is expressed as R (cal/mol), the absolute temperature is expressed as T (K), the molar volume is expressed as V (cm3/mol), the density is expressed as d (g/cm3), the gram molecular weight is expressed as (g/mol), and the cohesive energy is expressed as CE (cal/mol).

As illustrated inFIG.2, a crystal portion11and an amorphous portion12of the synthetic fiber10(PET fiber) form a microphase-separated structure. When the aprotic solvent23(DMF) acts thereon, DMF penetrates into the amorphous portion12of the PET fiber, causing softening and swelling effects. In the action, if a temperature becomes high, the movement of molecules becomes active, and DMF permeates further into the amorphous portion12. Since a glass transition temperature of the PET fiber is 70° C., if the temperature becomes 70° C. or higher, the movement of molecules in the crystal portion11becomes active, DMF also permeates into the crystal portion11, collapsing a crystal structure. When recrystallization is performed, the dispersion of crystal grains becomes unstable, and physical properties may deteriorate. Therefore, the action of DMF needs to be performed at a temperature of 70° C. or lower to maintain physical properties of the PET fiber.

In addition, there is an interface between the PET fiber and a SWCNT, and both of them are intertwined at a surface layer due to the softening and swelling effects of the PET fiber. When DMF is removed, a state of the PET fiber is restored to an original state, the adhesion strength of the interface can be enhanced, and therefore a strong conductive film of the SWCNT alone can be formed.

As described above, a dispersant is not necessary for the conductive ink24, and the ink may not contain a surfactant. As in the past, if a surfactant component added as a dispersant remains in a conductive film, electrical properties such as electric conductivity characteristics of the conductive film may deteriorate, because the surfactant component is non-conductive. Therefore, due to the conductive ink24not including a surfactant, it is possible to fabricate the conductive fiber1having higher electrical properties than those in the past.

A method for preparing the conductive ink24having the SWCNTs25dispersed in the aprotic solvent23is not particularly limited. In order to enhance the dispersibility of the SWCNTs in the aprotic solvent23, the SWCNTs25can be milled using a mill such as a ball mill, a rotor speed mill, a cutting mill, a homogenizer, a vibration mill, or an attritor and dispersed in the aprotic solvent23. Both the aprotic solvent23and the SWCNTs25have high polarities, and even if a dispersant such as a surfactant is not added, the SWCNTs25can be uniformly dispersed in the aprotic solvent23. The wettability of the SWCNTs25is enhanced in the aprotic solvent23, and the SWCNTs25can be finely loosened.

A method for immersing the synthetic fiber10in the conductive ink24and causing the ink to adhere to the fiber, is not particularly limited, and a general impregnation treatment method such as a method for applying micro vibration can be used. However, when micro vibration is applied, with an increasing temperature of the conductive ink24, the aprotic solvent23easily permeates into the synthetic fiber10. The attack property of the synthetic fiber10becomes strong, and thus may cause dissolution of the synthetic fiber10, change in a crystal structure, and deterioration in physical properties.

Therefore, as illustrated inFIG.3, it is preferable to use a ball mill50for performing the method for immersing the synthetic fiber10in the conductive ink24and causing the ink to adhere to the fiber. In order to form a uniform conductive film20on the synthetic fiber10while maintaining the dispersibility of the SWCNTs25in the aprotic solvent23, the synthetic fiber10and the conductive ink24are put into the ball mill50, and the conductive ink24is dispersed on the synthetic fiber10while stirring the mixture. A certain amount of balls51are put into a cylindrical container of the ball mill50as dispersion media. When the ball mill50is rotated about a horizontal axis thereof, along with the rotation of the cylindrical container, the balls51are lifted to a certain height along an inner wall and are circulated in the cylindrical container in a certain direction by sliding along the inner wall or rolling down. During the circulation movement of the balls51, the uniform conductive film20can be formed on the synthetic fiber10. In the method using this kind of ball mill50, there is little temperature rise, permeation of the aprotic solvent23into the synthetic fiber10is suppressed, and the conductive film20can be laminated on the synthetic fiber10while suppressing deterioration in physical properties of the synthetic fiber10.

The concentration of the SWCNTs25in the conductive ink24, relative to 100% by mass of the conductive ink24, is preferably in a range from 0.01 to 0.5% by mass, more preferably in a range from 0.05 to 0.2% by mass, and even more preferably in a range from 0.08 to 0.12% by mass. When the concentration of the SWCNTs25is within the above range, the uniform conductive film20is easily formed.

There may be a pretreatment step of immersing a synthetic fiber in polydopamine (hereinafter referred to as PDA) prior to the conductive ink immersion step. PDA mimics a protein which is referred to as a byssus of aSoletellina diphos(species of clam), and is a catechol-based polymer which contains a catechol group, an amino group, and a benzene ring, and binds to a variety of materials. By changing surface properties of hydrophilic cotton fibers, hydrophobic PET fibers, and the like, it is possible to enhance adhesion to a conductive film containing SWCNTs. PDA is a polymer which is obtained from self-polymerized dopamine while oxidizing under alkaline conditions in an oxygen atmosphere. Dopamine shown on the left side of chemical formula 3 becomes an intermediate, and the intermediate becomes PDA shown on the right side.

By performing the pretreatment step of immersion in PDA, the surface modification of a hydrophilic or hydrophobic fiber is possible by interaction with a dopamine derivative (hydrogen bonding, π-π interaction, and the like). In particular, it is preferable to perform the pretreatment step of immersion in PDA because a cotton fiber, as a kind of cellulosic fiber having hydroxyl groups as illustrated in chemical formula 2, has high water absorption properties. By performing the pretreatment of immersion in PDA, hydrogen bonding between the hydroxyl groups of the cotton fiber is made, the surface can be modified to be hydrophobic, and adhesion to hydrophobic SWCNTs can be enhanced.

When a pretreatment step of immersing a PET fiber in PDA is performed, since the PET fiber has some water absorption properties, a π-π interaction acts with a benzene ring existing in dopamine, and this can enhance hydrophobicity and adhesion of an interface.

The solvent removal step is performed after the conductive ink immersion step. In the solvent removal step, the synthetic fiber10to which the conductive ink24is adhered is dried, and the aprotic solvent23is removed. As described above, if the temperature is equal to or higher than the glass transition temperature, the movement of molecules in the crystal portion of the synthetic fiber10becomes active, the solvent also permeates into the crystal portion, and a crystal structure collapses. When recrystallization is performed, the dispersion of crystal grains becomes unstable, and physical properties may deteriorate. Therefore, the temperature of the solvent removal step is preferably equal to or lower than the glass transition temperature of the synthetic fiber10.

From the viewpoint of the adhesion of the conductive film20to the synthetic fiber10, it is preferable to use a vacuum drying method for the solvent removal step. If DMF is used for the aprotic solvent23and a PET fiber is used for the synthetic fiber10, since the boiling point of DMF is 153° C., a drying condition of 150° C. or higher is required to remove DMF from the PET fiber, for example. However, if the temperature is raised to be 70° C. or higher, which is the glass transition temperature of the PET fiber, DMF penetrates into a crystalline structure of the PET fiber, and physical properties of the PET fiber deteriorate. Therefore, it is preferable to use vacuum drying and perform solvent removal at a temperature condition of 70° C. or lower. When drying, the PET fiber swollen by DMF shrinks to be an original state, and the adhesion of an interface can be strengthened in this process.

As described above, the method for producing the conductive fiber1according to the present embodiment is the method for producing the conductive fiber1including the synthetic fiber10, and the conductive film20which is formed on the surface of the synthetic fiber10and has the SWCNTs25. The method for producing the conductive fiber1includes the conductive ink immersion step of immersing the synthetic fiber10in the conductive ink24having the SWCNTs25dispersed in the aprotic solvent23and causing the ink to adhere to the fiber, and the solvent removal step which is performed after the conductive ink immersion step. In the solvent removal step, the synthetic fiber10to which the conductive ink24adheres is dried, and the aprotic solvent23is removed. In accordance with the method for producing the conductive fiber1according to the present embodiment, it is possible to provide the method for producing the conductive fiber1having a carbon nanotube conductive film strongly adhered to the synthetic fiber10.

The present disclosure will be described below in further detail based on an example, but the present disclosure is not limited to only the example.

EXAMPLE

For synthetic fibers, four types of PET fibers, CALCULO (registered trademark), NANOFRONT (registered trademark), Octa (registered trademark), and WAVERON (registered trademark), all of which are PET fibers manufactured by TEIJIN FRONTIER CO., LTD., were used. For the purpose of imparting sweat-absorbent and quick drying properties, unlike a conventional PET fiber with a round cross-section, all of the PET fibers have the following characteristics in their cross-sectional shape and fiber diameter. CALCULO has a cross-section of a random cross-sectional shape with deep grooves. NANOFRONT is an ultra-fine fiber with a diameter of 700 nm and has a surface area which is several tens of times greater than that of an ordinary fiber. Octa has an octopod cross-section in which eight protrusions are radially arranged from a hollow fiber with a hole. WAVERON has a cross-section of four flat peaks.

As raw materials for a conductive ink, eDIPS EC1.5 (center diameter: 1 to 3 nm), manufactured by Meijo Nano Carbon. Co., Ltd., was prepared as a SWCNT, and DMF, which does not contain any dispersant, was prepared as an aprotic solvent. Using an ultrasonic homogenizer, a conductive ink was prepared by dispersing SWCNTs in DMF such that the amount of the SWCNTs was 0.1% by mass. Conditions of the ultrasonic homogenizer were pulse cycle of five seconds and amplitude control of 200 W, and a treatment was performed for one hour.

The PET fibers were immersed in the conductive ink, and the ink was caused to adhere to the fibers using a ball mill. Conditions of the ball mill were a speed of 350 rpm and a time of 30 minutes.

A PET fiber to which a conductive ink was adhered was dried at about 26° C. for about 6 hours by means of a vacuum drying method and DMF was removed. Accordingly, a conductive fiber was obtained which includes a conductive film that is formed on a surface of the PET fiber and includes SWCNTs.

FIG.4Aillustrates a SEM photograph before a conductive film formation when CALCULO is used for the PET fiber,FIG.4Billustrates a SEM photograph after a conductive film formation when CALCULO is used,FIG.5Aillustrates a SEM photograph before a conductive film formation when NANOFRONT is used for the PET fiber, andFIG.5Billustrates a SEM photograph after a conductive film formation when NANOFRONT is used.

As illustrated inFIG.4B, a state was confirmed in which SWCNTs were entangled and adhered to each PET fiber. Since the SWCNTs were directly entangled with each PET fiber, the adhesion of an interface became stronger. It is considered that since each PET fiber used had deep grooves in a surface thereof and had a random cross-sectional shape, and individual yarns had random cross-sectional shapes in a direction of a fiber axis, a gap between fibers was large and each PET fiber was easily entangled with the SWCNTs. In addition, since there was a gap between PET fibers, DMF easily permeated to the inside of PET fibers, a conductive film was able to cover the entire PET fibers, and favorable adhesion was obtained.

Meanwhile, inFIG.5B, it was confirmed that the conductive film was densely laminated to cover aggregated PET fibers. It was presumed that the PET fibers used were ultra-fine fibers with a diameter of 700 nm, the surface area of the fibers was large, the surface characteristics had high adsorption and gripping properties (high friction force), and the adhesion of an interface was enhanced by means of an anchoring effect due to the fine irregularity of the fiber surface.

In this way, by means of an anchoring effect and a contact area expansion effect, which use a characteristic cross-sectional shape of the PET fibers, the adhesion of an interface was enhanced, the PET fibers and the SWCNTs were more easily entangled, and the conductive film was strongly adhered to the PET fibers.

FIG.6Ais an optical image illustrating a conductive fiber fabricated using PET fibers (WAVERON, Octa, CALCULO, and NANOFRONT). Meanwhile,FIG.6Bis an optical image illustrating a conductive fiber fabricated in the same manner as described above using a PET fiber (fiber diameter: 50 μm) with a round cross-section, which is generally used as a cover for deploying automobile air bags. Since the conductive fiber illustrated inFIG.6Ahas a smaller fiber diameter of the PET fibers and a larger surface area than the conductive fiber illustrated inFIG.6B, it was revealed that the conductive film was uniformly laminated on the entire surface of the PET fibers.

Test samples of the obtained conductive fibers were evaluated by means of the following method.

The sheet resistance of a conductive film of a conductive fiber was measured by means of the Van der Pauw method. As illustrated inFIG.7, resistances R1and R2were obtained from the magnitude of a current flowing when a current power supply is connected between two terminals, and from the potential difference between the other two terminals. A correction factor f was obtained from a numerical table according to a ratio between the resistances R1and R2(R1>R2). Then, the sheet resistance Rs (Ω) was calculated by solving the following mathematical formula (2).

Table 1 illustrates measurement results of the sheet resistance (average value of n=5) of a conductive fiber fabricated by immersing PET fibers (CALCULO, NANOFRONT, Octa and WAVERON) once in a conductive ink using a ball mill. When CALCULO was used for a PET fiber, the resistance had the lowest value of 10.8 Ω/sq. and it was revealed that conductivity was favorable. When CALCULO was used for a PET fiber, the first sheet resistance was 10.8 Ω/sq., the second sheet resistance was 11.8 Ω/sq., the third sheet resistance was 8.4 Ω/sq., the fourth sheet resistance was 11.7 Ω/sq., and the fifth sheet resistance was 5.2 Ω/sq. As shown above, the variation per sample was small and the values were stable. It is considered that since the SWCNTs were entangled and adhered to each PET fiber as illustrated inFIG.4B, electric conductivity characteristics of a surface were favorable and stable characteristics were obtained.

Electromagnetic wave shielding properties of a conductive fiber were evaluated. Specifically, as illustrated inFIG.8, a vector network analyzer (VNA)60and waveguides63were connected using a coaxial cable61, and a conductive fiber as a shielding material62was interposed between the waveguides63to measure a shielding effect (SE). The shielding effect SE (dB) could be calculated from the following mathematical formula (3). The effect was obtained from an intensity ratio between an input wave E1from a port1, and an output wave E4from a port2.

The sheet resistance and shielding effect SE were measured for a conductive fiber fabricated by repeating immersion of a PET fiber (CALCULO) in a conductive ink four times using a ball mill, (hereinafter referred to as four time immersion fiber).

The sheet resistance of the four time immersion fiber was 2.6 Ω/sq. The sheet resistance of the four time immersion fiber was further reduced compared to 10.8 Ω/sq., which was the sheet resistance of the conductive fiber (CALCULO) fabricated by immersing the PET fibers once in the conductive ink as illustrated in Table 1.

Meanwhile, as a result of measuring electromagnetic wave shielding properties of the four time immersion fiber, a shielding effect SE was −33 dB. Since a generally recommended shielding effect SE is −20 dB or less, the shielding effect of the four time immersion fiber was favorable. As illustrated inFIG.4B, the SWCNTs are entangled and adhered to each PET fiber of the conductive fiber. It is considered that by immersing the PET fiber in the conductive ink four times, the amount of SWCNTs adhered to each PET fiber was further increased, electric conductivity characteristics of a surface were further enhanced, and a conductive fiber having stable electromagnetic wave shielding effects was obtained.