JP6945840B2 - Electromagnetic wave shielding carbon nanotube coating liquid and electromagnetic wave shielding material - Google Patents

Description

The present invention relates to an electromagnetic wave shielding carbon nanotube coating liquid, an electromagnetic wave shielding material, and a method for producing an electromagnetic wave shielding carbon nanotube coating liquid.

Electromagnetic wave shielding materials that protect an object from various electromagnetic waves are widely used in many fields in order to avoid various adverse effects of electromagnetic waves on the human body and for stable operation of devices and the like. Therefore, various electromagnetic wave shielding materials have been manufactured mainly on metals having an electromagnetic wave shielding ability. However, the electromagnetic wave shielding material using metal has various problems such as being difficult to thin film, being inferior in flexibility, inferior in adhesiveness, inferior in coatability, and inferior in heat resistance. As a method for solving these problems, a method has been proposed in which carbon nanotubes (hereinafter, also referred to as CNTs), which are nanomaterials having high electrical conductivity and high electromagnetic wave shielding ability, are composited with rubber or a polymer.

For example, Patent Document 1 states that "at least one selected from the group consisting of (A) a nanoscale carbon tube containing a ferromagnetic substance in a space inside the tube and an amorphous nanoscale carbon tube, (B) a resin and (C). ) Paint for absorbing electromagnetic waves containing an organic solvent ”is described.

Patent Document 2 states that "in a conductive film having a substrate and a conductive film formed on the substrate, the conductive film has an average diameter (Av) and a standard deviation (σ) of the diameter. The conductive film obtained by using carbon nanotubes satisfying the relational expression and an acidic aqueous dispersion containing a polymer compound having an acidic group in the side chain is conductive in addition to excellent conductivity and transparency. Excellent adhesion between the film and the base material "and" Carbon nanotubes (A) in which the average diameter (Av) and the standard deviation (σ) of the diameter satisfy the relational expression: 0.60> 3σ / Av> 0.20. ), And a carbon nanotube-containing acidic aqueous dispersion containing a polymer compound (B) having an acidic group in the side chain ”is described.

Further, Patent Document 3 describes "an electromagnetic wave shielding material composed of a thermoplastic resin composition containing 0.5 to 20% by weight of carbon nanotubes and 5 to 50% by weight of conductive fibers".

Japanese Unexamined Patent Publication No. 2004-162052 Japanese Unexamined Patent Publication No. 2014-152296 Japanese Unexamined Patent Publication No. 2016-111341

The electromagnetic wave shielding material that has been put into practical use and produced so far has an electromagnetic wave shielding property of about 10 dB / 10 μm at 5.8 GHz, and considering practical use, it has a shielding effect of 99% or more (20 dB / 10 μm or more). Materials are required, and further improvement is desired. Further, an electromagnetic wave shielding carbon nanotube coating liquid having other high characteristics is desired.

The present invention provides a method for producing an electromagnetic wave shielding carbon nanotube coating solution, an electromagnetic wave shielding material, and an electromagnetic wave shielding carbon nanotube coating solution, which are excellent in electromagnetic wave shielding property and heat resistance, by a method other than compounding with rubber or a polymer.

According to one embodiment of the present invention, an electromagnetic wave shielding carbon nanotube coating liquid containing carbon nanotubes and an organic acid solution is provided.

The molecular weight of the organic acid may be 500 or more and 1,000,000 or less.

The solvent of the solution may be water or an organic substance.

The concentration of the carbon nanotubes in the electromagnetic wave shielding carbon nanotube coating liquid may be 1% by weight or less with respect to the coating liquid.

The concentration of the organic acid with respect to the coating liquid may be 0.1 times or more and 100 times or less the concentration of the carbon nanotubes with respect to the coating liquid. The concentration of the organic acid with respect to the coating liquid may be 1 time or more and 50 times or less the concentration of the carbon nanotubes with respect to the coating liquid. The concentration of the organic acid with respect to the coating liquid may be 2 times or more and 5 times or less the concentration of the carbon nanotubes with respect to the coating liquid.

The organic acid may be polyacrylic acid or paraphenylsulfonic acid.

According to one embodiment of the present invention, an electromagnetic wave shielding material formed by using an electromagnetic wave shielding carbon nanotube coating liquid is provided.

According to one embodiment of the present invention, there is provided a method for producing an electromagnetic wave shielding carbon nanotube coating liquid in which a continuous network structure by physical contact with deflated carbon nanotubes is constructed in an organic acid solution.

The molecular weight of the organic acid may be 500 or more and 1,000,000 or less.

The solvent of the solution may be water or an organic substance.

The concentration of the carbon nanotubes in the electromagnetic wave shielding carbon nanotube coating liquid may be 1% by weight or less with respect to the coating liquid.

The concentration of the organic acid with respect to the coating liquid may be 2 times or more and 5 times or less the concentration of the carbon nanotubes with respect to the coating liquid.

The organic acid is not particularly limited, and may be polyacrylic acid, paraphenylsulfonic acid, or the like.

According to one embodiment of the present invention, there is provided a method for producing an electromagnetic wave shielding material in which the electromagnetic wave shielding carbon nanotube coating liquid is applied by a bar coater, a spray coater, or a dip coater.

According to the present invention, it is possible to provide an electromagnetic wave shielding carbon nanotube coating liquid, an electromagnetic wave shielding material, a method for producing an electromagnetic wave shielding carbon nanotube coating liquid, and a method for producing an electromagnetic wave shielding material, which are excellent in electromagnetic wave shielding property and heat resistance.

It is a schematic diagram of the electromagnetic wave shielding film formed by applying the electromagnetic wave shielding carbon nanotube coating liquid which concerns on one Embodiment of this invention. It is a schematic diagram which shows the continuous network which expanded a part of the carbon nanotube structure in the electromagnetic wave shielding film formed by applying the electromagnetic wave shielding carbon nanotube coating liquid which concerns on one Embodiment of this invention. It is a micrograph (magnification 100 times) of the shielding film of the silver-based paint which concerns on a comparative example. In the figure, arrows are attached to the places where cracks occur. It is a micrograph (magnification 100 times) of the electromagnetic wave shielding film which concerns on one Example of this invention.

Hereinafter, a method for producing an electromagnetic wave shielding carbon nanotube coating liquid, an electromagnetic wave shielding material, and an electromagnetic wave shielding carbon nanotube coating liquid according to the present invention will be described with reference to the drawings. The method for producing the electromagnetic wave shielding carbon nanotube coating liquid, the electromagnetic wave shielding material, and the electromagnetic wave shielding carbon nanotube coating liquid of the present invention is not construed as being limited to the contents of the embodiments and examples shown below. .. In the drawings referred to in the present embodiment and the examples described later, the same parts or parts having the same functions are designated by the same reference numerals, and the repeated description thereof will be omitted.

The electromagnetic wave shielding carbon nanotube coating liquid (hereinafter, also referred to as electromagnetic wave shielding carbon nanotube coating liquid) according to the present invention is a fluid (liquid) containing an organic acid and carbon nanotubes in water, an organic solvent, or the like.

Further, the electromagnetic wave shielding material according to the present invention is obtained by applying an electromagnetic wave shielding carbon nanotube coating liquid to a base material and drying it. In such an electromagnetic wave shielding material, the carbon nanotubes having an electromagnetic wave shielding ability are materials having a diameter on the order of nm. Therefore, even if the electromagnetic wave shielding material produced by thinly applying the electromagnetic wave shielding carbon nanotube coating liquid according to the present invention is thinned. The electromagnetic wave shielding ability per unit film thickness does not decrease, and it functions as an electromagnetic wave shielding material.

The electromagnetic wave shielding carbon nanotube coating liquid according to the present invention is formed by dispersing an organic acid and carbon nanotubes in water, an organic solvent, or the like. Then, in the present invention, a remarkable effect is obtained by setting the destination for dispersing the carbon nanotubes as an "organic acid solution". That is, in the present invention, the organic acid plays a role as a doping agent for improving conductivity by dispersing in an organic acid solution, a role as a binder (adhesive) for connecting carbon nanotubes to each other, and in water. It can play a role in increasing the dispersibility of.

The coating liquid means any fluid to be applied on the object to be coated.

The organic acid preferably has a molecular weight of 500 or more and 1,000,000 or less.

Examples of the organic acid solution include an aqueous solution of an organic acid and a solution in which an organic acid is dissolved in an organic solvent, but the solution is not particularly limited.

Examples of organic acids include polyacrylic acid, paraphenylsulfonic acid, sulfanic acid, 5-sulfoisophthalic acid, N-ethylenediaminetetra (methylenephosphonic acid, amidosulfate, p-phenolsulfonic acid, poly (2-acrylamide-2-). Examples thereof include, but are not limited to, methyl-1-propanesulfonic acid and poly (p-styrenesulfonic acid, etc.).

Further, the organic acid used is preferably a high molecular weight one from the viewpoint of the strength of the shielding coating film to be formed, and preferably a low molecular weight from the viewpoint of the effect of doping. Therefore, the molecular weight is preferably 500 or more and 1,000,000 or less. More preferably, it is 3000 or more and 50,000 or less. Most preferably, it is 1000 or more and 30,000 or less.

In addition to carbon nanotubes, organic acids, and solvents, reinforcing materials, modifiers, flame retardants, fillers, colorants, and the like may be added to the coating liquid.

Examples of the reinforcing material include glass fiber, glass cloth, paper base material, and glass non-woven fabric.

Examples of the modifier include natural rubber, styrene-butadiene copolymer (SBR), nitrile rubber (NBR), styrene-butadiene-styrene block copolymer (SBS), styrene-isoprene-styrene copolymer (SIS), and the like. Examples thereof include ethylene-propylene-dienter polymer (EPDM), ethylene-vinyl acetate copolymer (EVA), and elastomers such as hydrides thereof.

Examples of the antioxidant include various antioxidants for plastics and rubbers such as hindered phenols, phosphorus and amines. These antioxidants may be used alone, but it is preferable to use two or more in combination.

Examples of the flame retardant include phosphorus-based flame retardants, nitrogen-based flame retardants, halogen-based flame retardants, metal hydroxide-based flame retardants such as aluminum hydroxide, and antimony compounds such as antimony trioxide. The flame retardant may be used alone, but it is preferable to use two or more in combination.

Examples of fillers other than carbon nanotubes include glass powder, ceramic powder, and silica. Two or more kinds of these fillers may be used in combination. Further, as the filler, one surface-treated with a silane coupling agent or the like can also be used. The amount of the filler is preferably 0 parts by weight to 600 parts by weight, more preferably 0 parts by weight to 300 parts by weight, and particularly preferably 0 parts by weight to 100 parts by weight with respect to 100 parts by weight of the cycloolefin monomer.

As the colorant, dyes, pigments and the like are used. There are various types of dyes, and known dyes may be appropriately selected and used.

The concentration of CNT in the electromagnetic wave shielding carbon nanotube coating liquid is preferably 1% by weight or less with respect to the coating liquid.

The concentration of the organic acid with respect to the coating liquid is 0.1 times or more and 100 times or less, preferably 1 time or more and 50 times or less, and more preferably 1.5 times or more and 10 times or less the CNT concentration with respect to the coating liquid.

By the way, FIG. 1 is a schematic view of an electromagnetic wave shielding film formed by applying the electromagnetic wave shielding carbon nanotube coating liquid according to the embodiment of the present invention. The electromagnetic wave shielding film formed by applying the electromagnetic wave shielding carbon nanotube coating liquid contains carbon nanotubes 10 and an organic acid, and has a continuous network in which the carbon nanotubes 10 are highly defibrated and formed in contact with each other. ..

See FIG. The carbon nanotubes 10 contained in the electromagnetic wave shielding film formed by applying the electromagnetic wave shielding carbon nanotube coating liquid according to the embodiment of the present invention have a structure in which the carbon nanotubes 10 are defibrated from a bundle of the carbon nanotubes 10. Have. In the electromagnetic wave shielding film formed by applying the electromagnetic wave shielding carbon nanotube coating liquid, the carbon nanotubes 10 are physically intertwined with each other to form a highly developed continuous network.

The mechanism by which the material does not transmit the incident electromagnetic wave and shields it is the reflection loss due to the generation of eddy current on the surface and the absorption loss that converts the energy of the electromagnetic wave into Joule heat in the material. In the case of a shielding material filled with a conductive filler (carbon nanotube) having a thickness shorter than the wavelength of the electromagnetic wave, reflection loss is the main electromagnetic shielding mechanism. In order to improve the reflection loss against electromagnetic waves perpendicularly incident on the sample, that is, to improve the shielding ability, it is important to increase the in-plane conductivity. In order to increase the conductivity, it is important that the carbon nanotubes form a three-dimensional conductive network.

Further, in order to improve the absorption loss of electromagnetic waves, it is necessary for the filler to absorb the electromagnetic waves and convert them into heat energy. For that purpose, carbon nanotubes need to interact with electromagnetic waves. The point where the carbon nanotube and the electromagnetic wave interact most is the point where the amplitude of the electric field of the electromagnetic wave is the largest. Since the electromagnetic wave to be absorbed is a white electromagnetic wave having various wavelengths and phases, the point where the amplitude of the electromagnetic wave is maximized is various. Since the three-dimensional network structure of dense carbon nanotubes constructed in the sheet efficiently interacts with white electromagnetic waves, it is considered that the absorption loss of electromagnetic waves is also large.

In addition, the coating liquid (electromagnetic wave shielding carbon nanotube coating liquid) capable of constructing a continuous network of such carbon nanotubes 10 is mechanically robust, chemically stable, and has excellent characteristics such as high tearing characteristics. show.

The carbon nanotubes 10 are not bundled but are defibrated separately, so that they can be easily physically contacted and a conductive path can be easily formed. Since the carbon nanotubes 10 are in physical contact with each other or are very close to each other, the physical contact points 15 between the carbon nanotubes 10 suppress the shrinkage of the carbon nanotubes 10 and maintain the form as a structure. Can be done. The distance between the physical contact points 15 of the carbon nanotubes 10 observed by the scanning electron microscope (SEM) is about 1 μm or more and 100 μm or less. As a method of measuring the distance between the physical contact points 15 of the carbon nanotubes 10, for example, there is a dynamic mechanical property measuring device (DMA). When the frequency of the electromagnetic wave shielding film formed by applying the electromagnetic wave shielding carbon nanotube coating liquid according to the present invention is changed from 0.001 Hz to 100 Hz at room temperature, a frequency-independent elastic modulus region appears (plateau). region). The storage elastic modulus of the electromagnetic wave shielding film formed by applying the electromagnetic wave shielding carbon nanotube coating solution in the plateau region is 10 ³ Pa or more and 10 ⁶ Pa or less. In the electromagnetic wave shielding film formed by applying the electromagnetic wave shielding carbon nanotube coating liquid according to the present invention, the distance between the physical contact points 15 can be estimated from the elastic modulus. Generally, in a well-developed carbon nanotube network, the estimated distance between the contact points 15 is in the range of 100 nm to 100 μm.

(Pore distribution of carbon nanotube structure)
The electromagnetic wave shielding film formed by applying the electromagnetic wave shielding carbon nanotube coating liquid has a pore distribution of the remaining carbon nanotube structure 50 of 0.5 μm to 10 μm when held at 500 ° C. in a nitrogen atmosphere for 6 hours or more. In the range of, the value of dV / dlog (d) is 0.6 or less. Further, the pore diameter has one or more peaks in the range of 1 nm or more and 100 μm or less, preferably 1 nm or more and 20 μm or less, and more preferably 1 nm or more and 10 μm or less.

Here, the pore distribution can be measured with a mercury press-fitting porosimeter. The peak is a point where the differential pore volume becomes 0 and the differential pore volume changes from negative to positive. The pore diameter corresponds to the distance between the carbon nanotube 10 and the carbon nanotube 10, and the electromagnetic wave shielding film formed by applying the electromagnetic wave shielding carbon nanotube coating liquid having such a peak has a small pore diameter, and the carbon nanotube 10 Since the continuous network of the above is formed, the network structure of the carbon nanotube 10 is maintained even if the thickness is thinned, and the state where the reflection loss of the electromagnetic wave is large is maintained, so that the shielding ability does not decrease. Further, since the carbon nanotube spacing is close, there is almost no electromagnetic wave transmitted through the material, and it is possible to produce a good and highly reliable electromagnetic wave shielding material.

(Electromagnetic wave shielding ability)
In the present specification, the electromagnetic wave shielding ability shall be evaluated by ASTM-D4935. The electromagnetic wave shielding film formed by applying the electromagnetic wave shielding carbon nanotube coating liquid is 100 MHz (coaxial line jig) when the single-layer carbon nanotubes previously reported by the present inventors described later are used as the carbon nanotube 10. And the electromagnetic wave shielding ability at 1 GHz (coaxial line jig) and 10 GHz (X-band waveguide jig) is 0.1 × 10 ^-1 dB / μm or more, preferably 0.2 × 10 ^-1 dB / μm. Above, more preferably 0.5 × 10 ^-1 dB / μm or more. However, the thickness of the electromagnetic wave shielding CNT polymer composite 100 to 0.1μm over 10 ⁵ [mu] m or less. Generally, the electromagnetic wave shielding ability decreases in proportion to the film thickness, but the electromagnetic wave shielding ability per unit thickness decreases in a thin film of 100 μm or less. This is because the structure required for shielding electromagnetic waves is thinned and broken.

On the other hand, in the present invention, the electromagnetic wave shielding ability standardized at a film thickness of 10 μm at 5.8 GHz is 10 dB / 10 μm or more, preferably 20 dB / 10 μm or more, and more preferably 30 dB / 10 μm or more.

In the present invention, since the carbon nanotube 10 has a fine structure, the structure required for electromagnetic wave shielding, that is, the continuous network structure can be maintained even if the thickness is reduced to 10 μm, which is 100 μm or less. The electromagnetic wave shielding ability can be maintained. Due to this feature, the electromagnetic wave shielding film formed by applying the electromagnetic wave shielding carbon nanotube coating liquid according to the present invention can be used as a thin film electromagnetic wave shielding material such as a coating material.

(Specific surface area of carbon nanotubes)
The specific surface area of the carbon nanotubes 10 contained in the electromagnetic wave shielding film formed by applying the electromagnetic wave shielding carbon nanotube coating liquid is not particularly limited, but is usually 100 m ² / g or more, preferably 300 m ² / g or more, more preferably. It is 600 m ² / g or more and 2000 m ² / g or less. The carbon nanotubes 10 having such a large specific surface area are suitable because the number of contact points between carbon nanotubes required for forming a continuous network increases, so that a continuous network can be easily formed. Further, the carbon nanotube 10 having a large specific surface area is suitable because the reflection loss becomes large with respect to the irradiation of the electromagnetic wave, so that the electromagnetic wave can be easily reflected and shielded.

(Diameter of carbon nanotube)
As shown in FIG. 1, in the carbon nanotubes 10 contained in the electromagnetic wave shielding film formed by applying the electromagnetic wave shielding carbon nanotube coating liquid, the carbon nanotubes 10 intersect with the plurality of carbon nanotubes 10 and are subjected to van der Waals force. It has a network structure connected at points. The diameter of the carbon nanotube 10 is not particularly limited, but is 10,000 nm or less, preferably 1000 nm or less, more preferably 100 nm or less, still more preferably 10 nm or less, and 1 nm or more. Since the carbon nanotube 10 having such a small diameter has a large specific surface area, the number of contact points between carbon nanotubes required for forming a continuous network increases, so that a continuous network can be easily formed, which is suitable.

Further, as the diameter of the carbon nanotube 10 becomes smaller, the specific surface area of the carbon nanotube 10 tends to increase, and it becomes easier to absorb electromagnetic waves. Further, in the carbon nanotube 10 having a diameter of 1.0 nm or less, the metal type and the semiconductor type carbon nanotube are clearly separated, and the semiconductor type carbon nanotube does not contribute to the reflection of electromagnetic waves. Therefore, the carbon nanotube of 1.0 nm or more Is desirable.

(Number of carbon nanotube layers)
Further, the carbon nanotube 10 may be a multi-walled carbon nanotube or a single-walled carbon nanotube. The number of carbon nanotube layers is 10 or less, preferably 5 or less, more preferably 2 or less, and most preferably a single layer. Here, the number of carbon nanotube layers is the average of the number of layers of 100 carbon nanotubes observed by a transmission electron microscope (TEM), and the double-walled carbon nanotubes are carbon nanotubes in which more than half of the whole are two-walled carbon nanotubes. The single-walled carbon nanotubes are those in which more than half of the whole is single-walled carbon nanotubes. The smaller the number of layers, the more flexible the carbon nanotubes 10 are and the easier it is to build a continuous network. Therefore, the smaller the number of layers, the more contact points between carbon nanotubes required to form a continuous network. Easy and suitable.

The carbon nanotube 10 having such a number of layers can move electrons excited by electromagnetic waves in a wider region, and can more effectively reflect or absorb the energy of electromagnetic waves. Further, as the number of layers is smaller, the carbon nanotube 10 has more interfaces with the organic acid solution, so that it becomes easier to excite electrons with respect to electromagnetic waves, and the energy of electromagnetic waves is efficiently reflected or absorbed. Is possible.

(Length of carbon nanotubes)
The length of the carbon nanotube 10 is preferably 1 μm or more, more preferably 5 μm or more, still more preferably 10 μm or more. Since such long carbon nanotubes 10 have many bonding points between carbon nanotubes, it is possible to form a network structure having excellent shape retention. In the present invention, any carbon nanotube may be contained as long as it contains such a long carbon nanotube, and the method for producing the same is not particularly limited. The single-walled carbon nanotubes having the above-mentioned physical characteristics can be produced by the method described in International Publication No. 2006/011655. Further, the multi-walled carbon nanotubes can be produced by the method disclosed in International Publication No. 2012/060454 and Japanese Patent Application Laid-Open No. 2004-526660.

(Content of carbon nanotubes)
In one embodiment, the electromagnetic wave shielding film formed by applying the electromagnetic wave shielding carbon nanotube coating liquid has 1 carbon nanotube with respect to the total weight of the electromagnetic wave shielding film formed by applying the electromagnetic wave shielding carbon nanotube coating liquid. It is preferable to include parts by weight or less. If the content of the carbon nanotubes is less than 0.1 parts by weight, a fully developed continuous network cannot be formed on the electromagnetic wave shielding film formed by applying the electromagnetic wave shielding carbon nanotube coating liquid.

(Electromagnetic wave shielding material)
The electromagnetic wave shielding film formed by applying the electromagnetic wave shielding carbon nanotube coating liquid according to the present invention is thin and can be suitably used as an electromagnetic wave shielding material having sufficient electromagnetic wave shielding ability. In the present invention, the electromagnetic wave shielding material has a certain degree of stability in electromagnetic wave shielding ability against deformation of the base material and the like. In addition, the continuous network structure of carbon nanotubes follows the deformation of the base material with respect to expansion and contraction, deformation, and expansion of the electromagnetic wave shielding material, and is not easily broken.

The electromagnetic wave shielding carbon nanotube coating liquid electromagnetic wave containing the carbon nanotube and the organic acid solution according to the present invention is excellent in shielding property and heat resistance.

In the present specification, the heat resistance is evaluated by visually confirming whether or not cracks have occurred after heat treatment under the condition of holding at 180 ° C. for 24 hours.

[Production method]
The method for producing the electromagnetic wave shielding carbon nanotube coating liquid according to the present invention described above will be described. The production method described below is an example, and the production method of the electromagnetic wave shielding carbon nanotube coating liquid according to the present invention is not limited to these.

[Summary of process]
The method for producing an electromagnetic wave shielding carbon nanotube coating liquid according to the present invention includes a step of manufacturing carbon nanotubes, a step of unraveling the fibers of the manufactured carbon nanotubes (“defibration” step), and a step of dispersing the carbon nanotubes in an organic acid solution. ..

[Manufacturing process of carbon nanotubes]
The carbon nanotubes used for producing the electromagnetic wave shielding carbon nanotube coating liquid according to the present invention include, for example, International Publication No. 2006/011655 (single-walled carbon nanotube), International Publication No. 2012/060454 (multi-walled carbon nanotube), and Special Table 2004. It can be produced by the method disclosed in Japanese Patent Publication No. -526660 (multi-walled carbon nanotubes), but is not limited thereto. The carbon nanotubes produced by such a production method have a very large specific surface area because they have a small diameter and a small number of layers. For this reason, the number of contact points between carbon nanotubes required to form a continuous network increases, so that a continuous network can be easily formed, and the electromagnetic wave shielding ability and tear strength of the electromagnetic wave shielding carbon nanotube coating liquid can be improved. , Suitable.

[Carbon nanotube drying process]
Carbon nanotubes are manufactured as an aggregate, but when water is adsorbed, the surface tension of water causes the carbon nanotubes to stick to each other, making it extremely difficult for the carbon nanotubes to unravel, resulting in good dispersion in organic acids. I can't get sex. The carbon nanotubes are heated to 180 ° C., preferably 200 ° C. or higher, and held at 10 Pa or lower, preferably 1 Pa or lower for 24 hours or longer, preferably 72 hours or longer to remove water adhering to the surface of the carbon nanotubes. By removing the water on the surface of the carbon nanotubes, the wettability with the solvent in the next step can be improved and the defibration can be facilitated. This facilitates the formation of a continuous network of carbon nanotubes.

[Classification process]
It is preferable that the size of the carbon nanotube aggregate is within a predetermined range to obtain a carbon nanotube aggregate of a uniform size. Carbon nanotube aggregates also include large chunks of synthetics. Since these large-sized massive carbon nanotube aggregates have different dispersibility, the dispersibility as a whole is lowered. Therefore, if only the carbon nanotube aggregates excluding the large massive carbon nanotube aggregates that have passed through the net, filter, mesh, etc. are used in the subsequent steps, the dispersibility of the carbon nanotubes in the electromagnetic wave shielding carbon nanotube coating liquid can be improved. Can be enhanced.

[Pre-dispersion process]
If the carbon nanotubes are put into the disperser as large agglomerates, they may cause clogging. Therefore, by adding an organic solvent to the dried carbon nanotubes and defibrating the carbon nanotubes to a bundle of about 10 μm or less, the yield in the dispersion process is achieved. Can be improved. The pre-dispersion step can be carried out, for example, by stirring about 0.1 parts by weight of carbon nanotubes added to the organic solvent with a crosshead stirrer at 500 rpm or more and 8 hours or more. As the organic solvent for dispersing the carbon nanotubes, for example, an organic acid solution can be used. By performing the pre-dispersion step, defibration can proceed more easily in the defibration step, which is the next step. As the defibration progresses, a continuous network can be constructed in the organic acid.

[Carbon nanotube defibration process]
In the electromagnetic wave shielding carbon nanotube coating liquid according to the present invention, it is important that the bundle of carbon nanotubes is unraveled by defibration. "Unraveling" means that the carbon nanotubes expose a measurable surface of the gas adsorption method from the bundle.

The carbon nanotubes are defibrated in an organic solvent such as an organic acid solution. Although existing dispersion methods can be adopted, especially in a device such as a jet mill that disperses by a turbulent shear force, damage to carbon nanotubes can be reduced and defibration can be performed. In particular, in a wet jet mill, a mixture in a solvent is pumped as a high-speed flow from a nozzle arranged in a pressure-resistant container in a closed state. Carbon nanotubes are dispersed in a pressure-resistant container by collisions between countercurrents, collisions with container walls, turbulence caused by high-speed flow, shear flow, and the like. When, for example, Nanojet Pal Co., Ltd. (JN10, JN100, JN1000) is used as the wet jet mill, the processing pressure in the dispersion step is preferably in the range of 10 MPa or more and 150 MPa or less. On the other hand, an ultrasonic homogenizer can also be used.

In both the jet mill method and the ultrasonic homogenizer method, it is preferable to suppress the temperature rise during the defibration treatment in order to suppress the thermal deterioration of the organic acid. The holding temperature during the defibration treatment is usually 200 ° C. or lower, preferably 100 ° C. or lower, more preferably 50 ° C. or lower, still more preferably 10 ° C. or lower. On the other hand, if the temperature is lowered too much, the solvent will solidify, so it is preferable to stir at a temperature equal to or higher than the melting point of the solvent.

When a shear force is applied at a higher pressure, the carbon nanotubes are cut in the fiber axial direction. This has been confirmed by Raman spectroscopy, which evaluates defects in carbon nanotubes. Further, at a pressure of 10 MPa or less, carbon nanotubes cannot be efficiently defibrated. That is, by applying a pressure of 10 MPa to 150 MPa, the carbon nanotubes are more defibrated than cut and have a higher aspect ratio. This high aspect ratio is necessary for carbon nanotubes to build a highly developed continuous network structure. Further, in the present embodiment, a jet mill (HJP-17007) manufactured by Sugino Machine Limited may be used for the dispersion step of the carbon nanotube aggregate.

By defibrating the bundle of carbon nanotubes to a thickness of about 100 nm or less, the area of the interface between the carbon nanotubes and the organic acid in the electromagnetic wave shielding carbon nanotube coating liquid can be increased. The larger the specific surface area, the more contact points between the carbon nanotubes required to form the continuous network, so that the continuous network can be easily formed and the electromagnetic wave shielding ability of the electromagnetic wave shielding carbon nanotube coating liquid is improved.

[Kneading process with organic acid]
An appropriate amount of an organic acid solution is added to the obtained carbon nanotubes to prepare a dispersion liquid. The kneading step may be performed, for example, by adding an organic acid solution to the carbon nanotubes and mixing them in a beaker using a conical magnet stirrer. In this case, it is desirable to knead the defibrated carbon nanotubes and the organic acid by mixing at room temperature at 100 rpm or more, preferably 500 rpm or more, more preferably 1000 rpm or more for 12 hours or more. After that, as shown above, it is preferable to perform the defibration treatment of the carbon nanotubes using a jet mill or an ultrasonic homogenizer. By using a solvent having a high affinity for carbon nanotubes and an organic acid (having a close solubility parameter) (including an organic solvent), the carbon nanotubes and the organic acid are evenly distributed. As a result, a continuous network can be easily formed, and the electromagnetic wave shielding ability of the electromagnetic wave shielding carbon nanotube coating liquid can be improved.

Further, in the present invention, it is important that the carbon nanotubes are not solidified in one place and are uniformly distributed in the organic acid. In order for the electrons excited by the electromagnetic wave to move in the electromagnetic wave shielding carbon nanotube coating liquid, it is necessary that the carbon nanotubes are uniformly distributed in the organic acid. Further, when the carbon nanotubes are physically in contact with each other, a continuous conductive network is formed, the reflection loss of the electromagnetic wave is increased, the electromagnetic wave is shielded, and the electromagnetic wave shielding ability is improved.

[About application]
The electromagnetic wave shielding carbon nanotube coating liquid according to the present invention can be used as an electromagnetic wave shielding material by forming a film by spraying it on a substrate. When an organic acid solution / carbon nanotube solution is formed by a bar coater, spray coater, dip coating, or other solution coating method, a continuous network structure of carbon nanotubes is constructed, conductivity is further developed, and excellent electromagnetic wave shielding properties are exhibited. Can be given.

[Industrialization / mass production]
Further, as described above, the electromagnetic wave shielding carbon nanotube coating liquid according to the present invention can be produced by a method suitable for continuous and mass production such as a twin-screw kneader, and can be formed by various solution film forming methods. Therefore, the area of the electromagnetic wave shielding material can be easily increased.

(Example 1)
Examples using single-walled carbon nanotubes (hereinafter, also referred to as SG-SWNT) produced by the method described in International Publication No. O2006 / 011655 and polyacrylic acid having a molecular weight of 5000 (manufactured by Wako Pure Chemical Industries, Ltd.) as an organic acid. The electromagnetic wave shielding carbon nanotube coating liquid of No. 1 was produced.

The single-walled carbon nanotubes used had a length of 100 μm, an average diameter of 4.0 nm, and a number of layers as observed by TEM. Further, a 50 mg mass was taken out, and the adsorption / desorption isotherm of liquid nitrogen was measured at 77 K using BELSORP-MINI (manufactured by Nippon Bell Co., Ltd.) (adsorption equilibrium time was set to 600 seconds). When the specific surface area was measured from this adsorption isotherm by the methods of Brunuer, Emmett, and Teller, ^{it was about 1000 m 2} / g.

For single-walled carbon nanotubes, a carbon nanotube aggregate is placed on one side of a net with a mesh size of 0.8 mm, and the carbon nanotube aggregate is sucked through the net with a vacuum cleaner. The lumpy carbon nanotube aggregate was removed and classified (classification step).

The carbon nanotubes are defibrated by the method described in the embodiment, and the carbon nanotubes are dispersed in water by the method described in the embodiment in a polyacrylic acid solution having a molecular weight of 5000, which is an organic acid, and an aqueous solution containing the carbon nanotubes and the polyacrylic acid. The paint was made. The concentration of CNT was 0.4% by weight, and the concentration of polyacrylic acid was 0.8% by mass.

The obtained dispersion was applied with a spray coater and dried to confirm the film thickness, electromagnetic wave shielding at 5.8 GHz, and heat resistance (whether cracks can be visually observed after heat treatment at 180 ° C. for 24 hours). ..

(Example 2)
In Example 2, the same SG-SWNT and polyacrylic acid as in Example 1 were used, but the coating method was changed to a bar coater. The rest is the same as in Example 1.

(Example 3)
In Example 3, the same SG-SWNT as in Example 1 was used, and the molecular weight of polyacrylic acid was set to 1800. As in Example 2, the coating was performed by a bar coater.

(Example 4)
In Example 4, the same SG-SWNT as in Example 1 was used, and the molecular weight of polyacrylic acid was set to 25,000. As in Example 2, the coating was performed by a bar coater.

(Example 5)
In Example 5, the same SG-SWNT as in Example 1 was used, and the molecular weight of polyacrylic acid was set to 100,000. As in Example 2, the coating was performed by a bar coater.

(Example 6)
In Example 6, the same SG-SWNT as in Example 1 was used, and paraphenol sulfonic acid having a molecular weight of 100,000 was used. As in Example 2, the coating was performed by a bar coater.

(Comparative Example 1)
As Comparative Example 1, a commercially available electromagnetic wave shielding paint containing carbon black as a main component (Bunnybite FCP-1005 manufactured by Nippon Graphite Trading Co., Ltd.) was applied and dried to prepare a film, which was evaluated.

(Comparative Example 2)
As Comparative Example 2, a film prepared from a commercially available electromagnetic wave shielding paint containing silver (XA9015 manufactured by Fujikura Kasei Co., Ltd.) was used.

(Comparative Example 3)
The CNT was coated with a bar coater without adding an organic acid to prepare an electromagnetic wave shielding carbon nanotube coating solution of Comparative Example 1.

(Measurement of carbon nanotube filling amount)
For the electromagnetic wave shielding carbon nanotube coating liquid of Example 1, the carbon nanotube filling amount was measured by the following method. The measurement was performed using a differential thermal weight simultaneous measuring device (TG / DTA, STA7000, Hitachi High-Tech). For the primary temperature rise, 200 ml / min of nitrogen was supplied, and the temperature was raised from room temperature to 800 ° C. at 1 ° C./min. In the primary temperature rise, only the elastomer is sublimated, and the residual component is carbon nanotubes. When carbon fillers other than carbon nanotubes were contained, the secondary temperature was raised. For the secondary temperature rise, pure air of 200 ml / min was supplied, and the temperature was raised from room temperature to 800 ° C. at 1 ° C./min. In pure air, carbon nanotubes and carbon fillers burned at known temperatures, resulting in weight loss. The carbon nanotube filling amount was calculated from the weight reduction.

(Volume measurement of carbon nanotube structure)
The volume of carbon nanotubes was measured by the following method for the electromagnetic wave shielding carbon nanotube coating liquids of Examples and Comparative Examples. The length (mm), width (W), and thickness (D) (mm) of the electromagnetic wave shielding carbon nanotube coating liquid before the heat treatment were measured with a micrometer. The matrix component is set in a tube furnace, heated to 30 ° C. to 400 ° C. or 600 ° C. at about 20 ° C./min under a nitrogen atmosphere, and heat-treated at 400 ° C. or higher, preferably 600 ° C. or higher for 6 hours. Was removed by thermal decomposition. The volume of the carbon nanotube structure is measured by measuring the length (H') length (mm), width (W') length (mm) and thickness (D') (mm) of the sample on the sheet with a micrometer. Then, the volume was calculated by multiplying this.

(Distribution of pores in carbon nanotube structure)
For the electromagnetic wave shielding carbon nanotube coating liquids of Examples and Comparative Examples, the pore distribution of the carbon nanotube structure was measured by the following method. The sample was set in a tube furnace, the temperature was raised from 30 ° C. to 600 ° C. at 20 ° C./min under a nitrogen atmosphere, and heat treatment was performed at 600 ° C. for 1.5 hours to remove matrix components by thermal decomposition. The distribution of the pore size of the obtained carbon nanotube residue was measured with a mercury porosimeter (PoreMaster 60GT manufactured by Quantachrome). The measurement was based on the Washburn method, and the mercury pressure was changed from 1.6 kPa to 420 MPa.

(Electromagnetic wave shielding measurement)
Based on the ASTM standard (ASTM D4935-10), a sample of the electromagnetic wave shielding carbon nanotube coating liquid of the example or comparative example is inserted into the junction with the coaxial waveguides facing each other, and a vector network analyzer (VNA) is used. The transmission loss (S21 parameter) was obtained from the level difference between the time when the sample was inserted and the time when the sample was not inserted, and the shielding amount was calculated from S21. S21 when the sample is enclosed in the jig and S21 in the empty state where the sample is not enclosed are measured, and the shielding amount is defined from the difference in amplitude expressed in dB. Coaxial line jigs were used at 1 MHz to 4.5 GHz, and VNAs were measured using an E5071C manufactured by Agilent Technologies. At higher frequencies (upper limit frequency: 110 GHz), similar measurements were made using a waveguide line jig, and an Agilent Technologies N5222A frequency expansion module as the VNA. In calculating the shielding amount per unit thickness, the film thickness of the 5-point sample was measured using a micrometer, and the average value was taken as the “sample film thickness”. A sample having a film thickness of about 10 μm was used for Examples and Comparative Examples.

The heat resistance was evaluated by visually confirming whether or not cracks had occurred after heat treatment at 180 ° C. for 24 hours.

Tables 1 and 2 show the results of confirming the film thickness, electromagnetic wave shielding property at 5.8 GHz, and heat resistance (whether or not cracks can be visually observed after heat treatment) according to Examples 1 to 6 and Comparative Examples 1 to 3. Write.

Further, FIG. 3 is a micrograph (magnification of 100 times) of the shielding film of the silver-based coating material according to Comparative Example 2 after the heat resistance test, and FIG. 4 is a carbon nanotube-based shielding film in the electromagnetic wave shielding film according to Example 2. It is a micrograph (magnification 100 times) after the heat resistance test of.

See the result above. The film using CB (carbon black) of Comparative Example 1 has "no cracks" and is resistant to heat, but has a difficulty of shielding property of 7 dB / 10 μm. Although the conventional Ag-based shielding film of Comparative Example 2 is very excellent in shielding property, it has difficulty in heat resistance because it is cracked by heat as shown in FIG. Although the film using CNT of Comparative Example 3 is resistant to heat, it has a difficulty of shielding property of 12 dB / 10 μm. On the other hand, the film according to Examples 1 to 6, that is, the film according to the present invention containing carbon nanotubes and an organic acid, has no cracks even after the heat resistance test and is also resistant to heat, as shown in FIG. It can be seen that it is strong and has excellent shielding ability. From the above, it was found that the electromagnetic wave shielding carbon nanotube coating liquid electromagnetic wave containing the carbon nanotube and the organic acid solution is excellent in shielding property and heat resistance. In particular, from the viewpoint of shielding property and heat resistance, it has been found that at least when the molecular weight of the organic acid is 1800 or more and 1,000,000 or less, the results are preferable as compared with Comparative Examples 1 to 3. As a result, it has been found that the present invention can provide a method for producing an electromagnetic wave shielding carbon nanotube coating liquid, an electromagnetic wave shielding material, and an electromagnetic wave shielding carbon nanotube coating liquid having excellent shielding properties and heat resistance.

10: Carbon nanotube 15: Contact point 50: Carbon nanotube structure 100: Electromagnetic wave shielding carbon nanotube coating liquid

Claims (4)

It is an electromagnetic wave shielding material
The electromagnetic wave shielding material, viewed contains a carbon nanotube, a polyacrylic acid molecular weight of 500 to 1,000,000, and,
The content of the carbon nanotubes is 0.1 parts by weight or more and 1 part by weight or less with respect to the total weight of the electromagnetic wave shielding material.
The content of the polyacrylic acid is 1 time or more and 50 times or less the content of the carbon nanotubes.
The electromagnetic wave shielding material has an electromagnetic wave shielding ability standardized at a film thickness of 10 μm at 5.8 GHz and is 20 dB / 10 μm or more.
Electromagnetic wave shielding material. The distance between the physical contact points in the carbon nanotubes is 1 μm or more and 100 μm or less.
The electromagnetic wave shielding material according to claim 1. The carbon nanotubes form a structure in which a bundle of carbon nanotubes is defibrated.
The pore distribution of the structure has a dV / dlog (d) value of 0.6 or less in a pore diameter range of 0.5 μm to 10 μm.
The electromagnetic wave shielding material according to claim 1 or 2. The storage elastic modulus of the electromagnetic wave shielding material is 10. ³ Pa or more 10 ⁶ Pa or less,
The electromagnetic wave shielding material according to any one of claims 1 to 3.

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