JP3996570B2 - Method for evaluating the conductivity of ultrafine conductive fibers - Google Patents

Description

  The present invention relates to a method for evaluating the conductivity of ultrafine conductive fibers and a quality control method for ultrafine conductive fibers using the same, and more particularly to a method for evaluating the conductivity of carbon nanotubes in which fibers are entangled with each other and the quality control of carbon nanotubes. Regarding the method.

  Conventionally, as a method for measuring the conductivity of ultrafine conductive fibers such as carbon nanotubes, there is a method in which ultrafine conductive fibers are directly compression-molded with a press or the like and the conductivity of the compression-molded one is measured. Are known. In this method, when the conductivity inherent to the material is significantly different, such as graphitic carbon (graphite) and amorphous carbon, there is a significant difference in the measured value of conductivity, so The relative conductivity difference of the fine conductive fibers can be known. However, in the case of ultra-fine carbon of the same type and the same shape with different particle sizes or slight amounts of impurities, there is almost no significant difference, and the conductivity of the ultra-fine conductive fiber to be measured is evaluated. There was a problem that I could not. Furthermore, even after compression molding, compression unevenness has occurred, so the measured conductivity is coupled with the fact that there is a difference in the partial bulk density depending on the aggregate state of the original ultrafine conductive fibers. There is a problem that variations occur. However, if the compression ratio is increased in order to uniformly compress the mass of ultrafine conductive fibers, the ultrafine conductive fibers that resist compression break or buckle, and the original fine conductive fibers There also arises a problem that representative measured values cannot be obtained. In short, with respect to the compression molded product, reliable conductive data inherent to the ultrafine conductive fiber cannot be obtained.

  On the other hand, the method of measuring the conductivity as a mixture with the resin by kneading the target ultrafine conductive fiber into the resin etc. is used in the production of the conductive composite material, but it is necessary to knead into the resin etc. For this reason, there is a problem that work is troublesome and at the same time a large amount of ultrafine conductive fiber sample is required for measurement. In addition, depending on the conditions when kneading into a resin or the like, for example, the type of resin or the kneading method, the dispersion state is different, so the measurement results are often not reproducible. Furthermore, in the case of ultrafine conductive fibers, particularly fine fibrous materials such as carbon nanotubes, which are the subject of the present invention, the primary structure may be broken when kneaded into a resin or the like. However, reliable conductivity data of the ultrafine conductive fiber or the composite material containing the ultrafine conductive fiber itself could not be obtained.

  Because of these factors, the conductivity of the same type of intertwined ultrafine fibrous conductive fibers that differ in particle size or contain a small amount of impurities etc. cannot be measured accurately, and intertwined ultrafine It was impossible to accurately and stably control the quality of the fibrous conductive fibers.

  In particular, so-called ultra-fine carbon nanotubes with an average diameter of 80 nm or less, which have been actively developed in recent years, are often produced by vapor phase epitaxy, and are extremely entangled with each other. In addition to impurities such as soot and catalyst residues, it is possible to accurately and easily measure the conductivity of the obtained carbon nanotubes, or to develop a quality control method for the obtained carbon nanotubes. Was demanded.

  When evaluating the conductivity of a resin composite material obtained by combining the carbon nanotube and the resin in order to impart conductivity to a resin or the like, the present inventors have determined the conductivity of the slurry containing the carbon nanotube. It was found that the evaluation has good correspondence with the product characteristics and is suitable for lot management. In the case of producing a resin composite material, there is no step of making the carbon nanotubes into a slurry, but in the case of producing a resin composite material, a small amount of sample from the carbon nanotube lot obtained by the production of carbon nanotubes is used. The sample can be prepared into a slurry, and the conductivity of the slurry can be measured, and if there is a correlation between the conductivity of the slurry and the conductivity of the resin composite, In order to manage the manufacturing process of the resin composite so that the conductivity becomes a desired value, it is only necessary to measure the conductivity of the slurry.

The present invention provides a conductivity evaluation method capable of eliminating the conventional problems and evaluating the conductivity of a resin composite material containing ultrafine conductive fibers with a small amount of sample simply and accurately. it is a child.

In order to solve the above-described problems, the method for evaluating the conductivity of a resin composite material including ultrafine conductive fibers according to the present invention includes a dispersion step of preparing a slurry obtained by dispersing ultrafine conductive fibers in a solvent, and the slurry A measurement process for measuring the electrical resistance of the resin, and an evaluation process for evaluating the conductivity of the resin composite material including the ultrafine conductive fiber based on the measured value of the electrical resistance .

In the method for evaluating conductivity of a resin composite material including ultrafine conductive fibers according to the present invention, the ultrafine conductive fibers are preferably carbon nanotubes having a fiber diameter of 0.5 to 80 nm and a fiber length of 0.1 to 1000 μm.

In the method for evaluating conductivity of a resin composite material including ultrafine conductive fibers according to the present invention, the dispersion step includes an ultrasonic vibrator that performs dispersion in a minute region, and a stirring device that makes the entire slurry uniform. It is preferable to include a step of combining and dispersing the ultrafine conductive fibers in a solvent.

In the conductivity evaluation method for a resin composite material including ultrafine conductive fibers according to the present invention, the slurry preferably has a concentration of the ultrafine conductive fibers of 0.001 to 3.0% by weight.

In the method for evaluating conductivity of a resin composite material including ultrafine conductive fibers according to the present invention, the electrical resistance measurement step of the slurry is used for passing a constant current between a container for maintaining the slurry in a predetermined shape and a predetermined position. A four-terminal method having a voltage measurement terminal for measuring a potential difference between the current terminal and a predetermined position is preferable.

  In the present invention, as a result of measuring the electrical resistance of a slurry obtained by dispersing ultrafine conductive fibers in a solvent, if the slurry has good conductivity, the ultrafine conductive fibers and the resin are formed. It can be determined that the conductivity of the resin composite is good. Therefore, as in the case where the conductivity is measured after creating a resin composite material, the creation work of the resin composite material, the creation of a large amount of resin composite material necessary for measurement, and the conditions at the time of resin composite material creation change. Therefore, the conductivity of the resin composite material can be evaluated without inconvenience such as variations in the measured conductivity value. The conductivity evaluation method in the present invention is based on the discovery of the fact that the conductivity of the resin composite and the conductivity of the ultrafine conductive fiber-containing slurry are highly correlated.

  The manufactured ultrafine conductive fiber is mixed with the resin as a lump containing carbon nanotubes having various diameters and aspect ratios, and soot as impurities. The presence of soot in the mass and carbon nanotubes having a large fiber diameter lower the conductivity of the whole mass. Therefore, evaluating the magnitude of conductivity by the conductivity evaluation method of the present invention evaluates the amount of soot and carbon nanotubes having a large fiber diameter contained in the lump. Therefore, according to the conductivity evaluation method of the present invention, the quality of the manufactured ultrafine conductive fiber mass can be evaluated, and as a result, the quality of the manufacturing process of the ultrafine conductive fiber can also be evaluated.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 shows a flowchart of a method for evaluating the conductivity of ultrafine conductive fibers of the present invention. As shown in FIG. 1, the conductivity evaluation method of the present invention includes a dispersion step of preparing a slurry in which ultrafine conductive fibers to be measured are dispersed in a solvent, and a measurement of measuring the electrical resistance of the slurry. Process.

[Dispersion process]
First, a dispersion | distribution process is a process of preparing the slurry formed by disperse | distributing the ultrafine conductive fiber used as a measuring object in a solvent.

  Here, the ultrafine conductive fiber is not particularly limited, and may be any fine fiber that has conductivity. The fineness of the ultrafine electrical fiber is usually such that the ultrafine fibrous conductive fiber has a fiber diameter of 0.5 to 80 nm and a fiber length of 0.1 to 1000 μm.

  Examples of such fibrous ultrafine conductive fibers include carbon nanotubes, and more preferably carbon nanotubes produced by a vapor deposition method.

  The amount of the fibrous ultrafine conductive fibers added to the solvent, that is, the slurry concentration is not particularly limited, but as described below, as the dispersion proceeds, the slurry viscosity increases. To 0.001 to 3.0% by weight. More preferably, it is in the range of 0.01 to 1.0% by weight.

  If the concentration of the fibrous ultrafine conductive fiber is less than 0.001% by weight, it may be difficult to measure the electrical resistance value of the slurry. Further, if the concentration of the fibrous ultrafine conductive fiber is 3.0% by weight or more, the viscosity of the slurry is too high, and it may be difficult to uniformly disperse the fibrous ultrafine conductive fiber in the solvent. is there.

  The solvent is not particularly limited as long as it can sufficiently wet the ultrafine conductive fiber and can be sufficiently dispersed. If the solvent is solid at room temperature, it can be heated to the melting point or higher and used in a liquid state, but a preferred solvent is liquid at room temperature and has a boiling point of 50 ° C. or higher. Among the preferred solvents, nonpolar solvents are particularly preferred. This is because, when electrolytes are mixed as impurities in ultrafine conductive fibers, these electrolytes, which are factors that make measured electrical resistance values inaccurate, are not ionized.

  Examples of such solvents include aliphatic hydrocarbon solvents, alicyclic hydrocarbon solvents, aromatic hydrocarbon solvents, halogenated hydrocarbon solvents, ester solvents, ether solvents, ketone solvents, 1 to 3 Valent alcohol solvents and the like. Among these various solvents, glycols such as ethylene glycol are preferable from the viewpoint of liquidity, boiling point, and safety.

  The method for dispersing the ultrafine conductive fiber in the solvent is not particularly limited as long as it can be sufficiently dispersed. For example, a fine method such as a bead mill in which a high shear force is applied to the ultrafine conductive fiber fine particles in the solvent. Examples thereof include a pulverizer / disperser and a method using an ultrasonic vibrator such as an ultrasonic homogenizer that does not apply a shearing force. Among the above, since there is a concern that the primary structure of the ultrafine conductive fiber is broken by a high shearing force, as shown in FIG. 2, a mechanical stirrer having a weak shearing force and the ultrasonic vibrator are provided. It is preferred to combine and use these simultaneously to disperse. Examples of the mechanical stirrer having a weak shearing force include a stirrer in which a change in the viscosity of the slurry is 30% or less after 1 hour has elapsed since the start of stirring with respect to the viscosity of the slurry before starting stirring. An example of such a stirrer is a stirrer that rotates a magnetic stirrer having a diameter of 6 mm and a length of 30 mm at 300 rpm.

  According to this, the aggregate of the ultrafine conductive fibers that have been aggregated is separated into individual ultrafine conductive fibers by an ultrasonic vibrator, and the separated ultrafine conductive fibers are superseded by a mechanical stirrer. The fine conductive fibers can be uniformly dispersed in the solvent without breaking.

  Examples of such a mechanical stirrer having a low shearing force that enables dispersion include a throwing stirrer equipped with a stirring blade, a magnetic stirrer, and the like.

  In the dispersion step, the viscosity of the slurry increases as the ultrafine conductive fibers are gradually dispersed in the solvent. When the load energy by the ultrasonic vibrator is sufficiently large, the dispersion proceeds, the temperature of the slurry rises, the viscosity of the slurry increases rapidly, and the equilibrium viscosity is very thixotropic in a short time. The ultrafine conductive fiber spreads in the solvent, and it becomes a solution structure in which the solvent is incorporated into the uniform network of ultrafine conductive fibers. Fiber does not settle and separate.

  The solution structure of such a thixotropic and non-settling slurry is naturally dependent on the load energy of the ultrasonic vibrator applied to the slurry, but at the same time, the ultrafine conductivity to be measured is measured. It also depends greatly on the primary particle structure of the conductive fiber itself and the amount added in the solvent. For example, in the case of carbon nanotubes having a fiber diameter of 80 nm or less, the output of the ultrasonic vibrator should be 5 to 100 W in order to reach the equilibrium viscosity within a slurry concentration of 0.1% by weight, a slurry amount of 100 ml, and a dispersion time of 60 minutes or less. preferable.

  The dispersion time for dispersing the ultrafine conductive fibers in the solvent is not particularly limited as long as the thixotropic property is high and the viscosity of the slurry is sufficient to reach the equilibrium value, but a preferable dispersion time is set. For example, each sample in which a predetermined amount of ultrafine conductive fibers are dispersed in a solvent is observed with an electron microscope or the like at regular intervals, such as 5, 10, 15 minutes,. It is to obtain a suitable dispersion time from the observation result. If the dispersion time is less than 10 minutes, the ultrafine conductive fibers may not be sufficiently dispersed in the solvent. If the dispersion time exceeds 100 minutes, the dispersion time becomes longer and the measurement efficiency becomes worse.

  Further, the dispersion temperature when dispersing the ultrafine conductive fibers in the solvent is not particularly limited, and may be room temperature, for example, about 20 ° C., but the temperature rises as dispersion proceeds.

  Furthermore, in order to increase the dispersion efficiency of ultrafine conductive fibers in the solvent, addition of a surfactant or polymer having a very small amount of dispersion function to the extent that it does not affect the electrical resistance value of the slurry in the next step An agent may be added.

[Measurement process]
Next, the measurement step is a step of measuring the electrical resistance of a slurry in which a predetermined amount of ultrafine conductive fibers are uniformly dispersed in the solvent.

  As a method for measuring the electrical resistance of the slurry, any method that can accurately measure the electrical resistance value may be used, and examples thereof include a three-terminal method, a four-terminal method, a five-terminal method, and a four-terminal pair method. It is done. In the conductivity evaluation method of the present invention, it is preferable to measure the electrical resistance value of the dispersion by a four-terminal method.

When measuring the electrical resistance, for example, when measuring the electrical resistance of the slurry by the four-terminal method, the calculation method according to the following equation (1) is performed. The volume of slurry is defined as I (A) for the current flowing through the current terminal, L (cm) for the interval between the voltage terminals, S (cm ² ) for the cross-sectional area of the tank containing the slurry, and V (V) for the measured voltage value. The specific resistance ρ (Ω · cm) is expressed by the following equation (1).

        ρ = V / I × S / L (1)

[Evaluation]
As described above, when the electrical resistance value of the slurry containing the ultrafine conductive fiber is measured, based on this electrical resistance value, for example, the conductivity of the composite material composed of the ultrafine conductive fiber and the resin is measured. Can be evaluated as follows.

[Quality control method]
As described above, when the electrical resistance value of the slurry containing the ultrafine conductive fiber is measured, for example, the quality of the ultrafine conductive fiber to be manufactured can be managed based on the electrical resistance value. . For example, when the conductivity measured by the conductivity evaluation method of the present invention is a low value, a large amount of soot and ultrafine conductive fibers having a large fiber diameter are mixed in the mass of the produced ultrafine conductive fibers. Can be guessed. Based on the presumption, it is possible to review the setting of the manufacturing conditions for the ultrafine conductive fiber.

  Here, as quality control, for example, determining the shipping standard of ultra-fine conductive fibers to be a product, classifying this product, mixing high and low conductivity based on the classification Averaging, evaluating the manufacturing process, adjusting the manufacturing conditions so that this evaluation becomes a predetermined value, or setting the addition ratio of ultrafine conductive fibers to obtain a predetermined composite material And so on.

According to this embodiment as described above, the following effects are obtained.
(1) In the embodiment of the present invention, in a dispersion step of dispersing ultrafine conductive fibers in a solvent, a very small amount of equipment can be obtained by using a very simple apparatus such as a stirrer having a weak shear force and a powerful ultrasonic vibrator. This sample can be uniformly dispersed in a very short time with good reproducibility. By using this apparatus, first, the aggregates of the ultrafine conductive fibers that are aggregated with each other are separated into individual ultrafine conductive fibers by a small and powerful ultrasonic vibrator, and then the shear force The separated ultrafine conductive fibers can be uniformly dispersed in the solvent by a weak stirrer.

(2) Next, in the measurement step of measuring the electrical resistance of the obtained ultrafine conductive fiber slurry, when the electrical resistance of the slurry is measured by the four-terminal method, a steady current is introduced into the slurry sample by two terminals. Then, the electric resistance R is obtained from V = RI by measuring the voltage drop of the remaining two terminals. In this four-terminal method, assuming that no current flows between the terminals for voltage measurement, contact resistance and the like do not contribute to the voltage drop and can be ignored. Therefore, since it is not necessary to consider these extra electric resistances, the electric resistance of the slurry can be accurately measured.

  Based on the measured electrical resistance value of the slurry, for example, the conductivity of the resin composite material composed of the ultrafine conductive fibers and the resin can be evaluated.

  Moreover, based on the measured electrical resistance value of the slurry, quality control in the production of ultrafine conductive fibers can be performed.

  It should be noted that the present invention is not limited to the above-described embodiments, and modifications and improvements within the scope that can achieve the object of the present invention are included in the present invention. The specific structure, shape, and the like when carrying out the present invention may be other structures as long as the object of the present invention can be achieved.

  Hereinafter, the present invention will be described more specifically with reference to examples and comparative examples. In addition, this invention is not limited to the content of an Example. 2 and 3 show a dispersion device 1 and a measurement device 6 used in the embodiment, respectively.

Example 1
The dispersion apparatus 1 is an apparatus that prepares a slurry by dispersing an ultrafine electric material to be measured in a solvent. As shown in FIG. 2, the dispersion apparatus 1 has a stirrer (magnetic stirrer) 2 and a stirrer 2. A beaker 3 having a volume of 100 ml, the contents of which are filled with a solvent, a rotor 4 disposed in the beaker 3, and an ultrasonic homogenizer 5 disposed in the vicinity of the liquid level of the solvent in the beaker 3. I was prepared.

  On the other hand, the measuring device 6 is a device for measuring the conductivity of the slurry. As shown in FIG. 3, the sample case 7 into which the measurement sample is put, the fixing base 8 for fixing the sample case 7, and the electric resistance. And a resistance measuring instrument 9 for measurement. Here, the dimensions of the sample case 7 were 6 cm in length, 1 cm in width, and 1 cm in depth. Moreover, the resistance measuring instrument 9 had a structure which can measure the electrical resistance of a sample solution by a four-terminal method.

[Dispersion process]
First, 99.9 g of ethylene glycol as a solvent and a sample A of carbon nanotubes as an ultrafine conductive fiber to be measured (manufactured by Nikkiso Co., Ltd., average fiber diameter 10 nm, average fiber length 7 μm by electron microscope observation) 0.1 g In a 100 ml beaker 3. Next, the ethylene glycol containing the sample A was stirred using the stirrer 2 and the rotor 4. Simultaneously with this stirring, ultrasonic vibration was applied to ethylene glycol by an ultrasonic homogenizer 5 having an output of 50 W, and sample A was well dispersed in ethylene glycol. At this time, the samples were sampled every 20 minutes, 40 minutes, and 60 minutes of dispersion time and used as the next sample for measuring electrical resistance.

[Measurement process]
6 cc of the slurry obtained by dispersing the sample A in ethylene glycol was transferred to the sample case 7. The sample case 7 was fixed to the fixed base 8. Thereafter, the electrode of the electrical resistance measuring device 9 was inserted into a predetermined position of the sample case 7, and the electrical resistance of the slurry transferred into the sample case 7 was measured.

  Using the equation (1), the volume resistance of the ultrafine fibrous conductive fiber slurry was calculated.

(Examples 2 to 8)
As shown in Table 1, within the range specified by the present invention, the dispersion time was 60 minutes except that the shape of carbon nanotubes (average fiber diameter and average fiber length by electron microscope observation), slurry concentration, and dispersion solvent were changed. The slurry was dispersed in the same manner as in Example 1, and the electrical resistance of the resulting slurry was measured.

(Examples 9 to 11)
Three types of slurries (Samples 1, 2 and 3) were prepared for a part of the aggregate of the three types of carbon nanotubes in the same manner as in Example 1, and the conductivity of the slurry was determined as in Example 1. Measured as in The conductivity is shown in Table 2.

(Reference Examples 1-3)
A 6 g portion of each of the three types of carbon nanotube agglomerates and 54 g of the polycarbonate resin were kneaded by a batch type biaxial kneader to prepare a resin composite. Volume specific resistance values were measured for molded bodies (samples 1, 2 and 3) formed from three types of resin composites. The values of samples 2 and 3 are relative to sample 1. The results are shown in Table 2.

  From the results in Table 2, it is clear that there is a correlation between the conductivity of the slurry containing ultrafine conductive fibers and the conductivity of the molded body.

  Moreover, the same operation as Example 1 was repeated 5 times about the samples 1 and 2, and Cv value (standard deviation / average value) was calculated | required. The results are shown in Table 3.

[Evaluation results]
As is clear from Tables 1 to 3, the method for evaluating the conductivity of the ultrafine conductive fiber according to the present invention is a simple method, but the conductivity of the ultrafine conductive fiber is extremely accurate and reproducible. It can be seen that it can be measured. Moreover, since a small sample amount is sufficient, it turns out that it is suitable for the quality control method of the ultrafine conductive fiber concerning this invention.

FIG. 1 shows a flowchart of the conductivity evaluation method of the present invention. FIG. 2 shows a conceptual diagram of a dispersion apparatus according to the conductivity evaluation method of the present invention. FIG. 3 shows a conceptual diagram of a measuring apparatus according to the conductivity evaluation method of the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Dispersing device 2 Stirrer 3 Beaker 4 Rotor 5 Ultrasonic homogenizer 6 Measuring device 7 Sample case 8 Fixing base 9 Resistance measuring device

Claims (5)

A dispersion step of preparing a slurry obtained by dispersing ultrafine conductive fibers in a solvent;
A measuring step for measuring the electrical resistance of the slurry ;
Based on the measurement value of the electrical resistance, an evaluation process for evaluating the conductivity of the resin composite material containing ultrafine conductive fibers;
A method for evaluating the conductivity of a resin composite material including ultrafine conductive fibers. The method for evaluating the conductivity of a resin composite material including ultrafine conductive fibers according to claim 1, wherein the ultrafine conductive fibers are carbon nanotubes having a fiber diameter of 0.5 to 80 nm and a fiber length of 0.1 to 1000 µm. The dispersion step includes a step of dispersing the ultrafine conductive fibers in a solvent by combining an ultrasonic vibrator that performs dispersion in a minute region and a stirring device for making the entire slurry uniform. The conductivity evaluation method of the resin composite material containing the ultrafine conductive fiber of Claim 1 or 2 to do. The conductivity of the resin composite material containing ultrafine conductive fibers according to any one of claims 1 to 3, wherein the slurry has a concentration of ultrafine conductive fibers of 0.001 to 3.0% by weight. Sex assessment method. The electrical resistance measurement step of the slurry has four terminals including a container for keeping the slurry in a predetermined shape, a current terminal for flowing a constant current between the predetermined positions, and a voltage measuring terminal for measuring a potential difference between the predetermined positions. 5. The method for evaluating conductivity of a resin composite material including ultrafine conductive fibers according to any one of claims 1 to 4, wherein the method is a method.

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