Patent Application
20250043504
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.
The present disclosure relates to a method for producing a conductive fiber.
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.
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.
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 fiber 1 illustrated in
The synthetic fiber 10 is not particularly limited, but the synthetic fiber 10 is 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 ink 24 having the SWCNTs 25 dispersed in the aprotic solvent 23 easily permeates the PET fiber, and the conductive film 20 easily adheres strongly to the PET fiber. Therefore, the synthetic fiber 10 is preferably the polyester fiber, and more preferably the PET fiber. A structure of the PET fiber is illustrated in chemical formula 1.
The conductive film 20 includes the SWCNTs. A center diameter of each SWCNT 25 included in the conductive film 20 is 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 SWCNT 25 included in the conductive film 20 is in a range from 1 μm to several 10 μm, for example. Still further, a thickness of the conductive film 20 is 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 fiber 10, for example.
The synthetic fiber 10 is immersed in the conductive ink 24 having the SWCNTs 25 dispersed in the aprotic solvent 23, the ink adheres to the synthetic fiber 10, further the synthetic fiber 10 is dried, and the aprotic solvent 23 is removed. Accordingly, the conductive film 20 having the SWCNTs is strongly adhered on the surface of the synthetic fiber 10.
The conductive fiber 1 is produced by performing the following method.
A method for producing a conductive fiber is a method for producing the conductive fiber 1 including the synthetic fiber 10 and the conductive film 20 which is formed on the surface of the synthetic fiber 10 and has the SWCNTs 25. The method for producing the conductive fiber 1 includes a conductive ink immersion step and a solvent removal step.
As illustrated in
The aprotic solvent 23 is used as a dispersing solvent of the conductive ink 24. The aprotic solvent 23 is 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 solvent 23 and the SWCNTs 25 have high polarities. Even if a dispersant such as a surfactant is not added, the SWCNTs 25 are uniformly dispersed in the aprotic solvent 23, and a favorable conductive ink of the SWCNTs 25 alone can be obtained. When the synthetic fiber 10 is 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 SWCNTs 25 have 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 ink 24 having the SWCNTs 25 uniformly 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 cm3 of 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 in
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 ink 24, 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 ink 24 not including a surfactant, it is possible to fabricate the conductive fiber 1 having higher electrical properties than those in the past.
A method for preparing the conductive ink 24 having the SWCNTs 25 dispersed in the aprotic solvent 23 is not particularly limited. In order to enhance the dispersibility of the SWCNTs in the aprotic solvent 23, the SWCNTs 25 can 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 solvent 23. Both the aprotic solvent 23 and the SWCNTs 25 have high polarities, and even if a dispersant such as a surfactant is not added, the SWCNTs 25 can be uniformly dispersed in the aprotic solvent 23. The wettability of the SWCNTs 25 is enhanced in the aprotic solvent 23, and the SWCNTs 25 can be finely loosened.
A method for immersing the synthetic fiber 10 in the conductive ink 24 and 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 ink 24, the aprotic solvent 23 easily permeates into the synthetic fiber 10. The attack property of the synthetic fiber 10 becomes strong, and thus may cause dissolution of the synthetic fiber 10, change in a crystal structure, and deterioration in physical properties.
Therefore, as illustrated in
The concentration of the SWCNTs 25 in the conductive ink 24, relative to 100% by mass of the conductive ink 24, 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 SWCNTs 25 is within the above range, the uniform conductive film 20 is 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 a Soletellina 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 fiber 10 to which the conductive ink 24 is adhered is dried, and the aprotic solvent 23 is 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 fiber 10 becomes 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 fiber 10.
From the viewpoint of the adhesion of the conductive film 20 to the synthetic fiber 10, it is preferable to use a vacuum drying method for the solvent removal step. If DMF is used for the aprotic solvent 23 and a PET fiber is used for the synthetic fiber 10, 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 fiber 1 according to the present embodiment is the method for producing the conductive fiber 1 including the synthetic fiber 10, and the conductive film 20 which is formed on the surface of the synthetic fiber 10 and has the SWCNTs 25. The method for producing the conductive fiber 1 includes the conductive ink immersion step of immersing the synthetic fiber 10 in the conductive ink 24 having the SWCNTs 25 dispersed in the aprotic solvent 23 and 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 fiber 10 to which the conductive ink 24 adheres is dried, and the aprotic solvent 23 is removed. In accordance with the method for producing the conductive fiber 1 according to the present embodiment, it is possible to provide the method for producing the conductive fiber 1 having a carbon nanotube conductive film strongly adhered to the synthetic fiber 10.
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.
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.
As illustrated in
Meanwhile, in
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.
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 in
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 in
Electromagnetic wave shielding properties of a conductive fiber were evaluated. Specifically, as illustrated in
SE=20×log10(E4/E1) (3)
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 in
While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2023-120624 | Jul 2023 | JP | national |
