JP7570194B2 - Exfoliated graphite and resin composition - Google Patents

Description

The present invention relates to exfoliated graphite of a specific shape and a resin composition containing the same.

Exfoliated graphite such as graphene obtained by exfoliating graphite, which is a laminated material, is used as a conductive assistant for electrodes in secondary batteries (see, for example, Patent Document 1), conductive ink (see, for example, Patent Document 2), filler for resins and elastomers (see, for example, Patent Documents 3 and 4), gas barrier material (see, for example, Patent Documents 5 and 6), etc. Exfoliated graphite becomes thinner as it is exfoliated, and the fewer the number of layers, the more likely it is to cause aggregation, so it is difficult to disperse in the matrix, and in some cases sufficient physical properties cannot be obtained.

JP 2016-060887 A International Publication No. 2016/002254 Special Publication No. 2013-517200 Special Publication No. 2015-517583 International Publication No. 2013/062246 Special table 2018-503541 publication

The objective of the present invention is to provide exfoliated graphite that has good dispersibility and a large effect of improving physical properties when blended with resins, paints, etc.

As a result of intensive research into the above-mentioned problems, the inventors discovered that exfoliated graphite having an average thickness, specific surface area and bulk density within specific ranges can solve the above-mentioned problems, and thus completed the present invention.
That is, the present invention provides exfoliated graphite having an average thickness of 10 nm to 200 nm, a specific surface area of 10 m ² /g to 40 m ² /g, and a bulk density of 0.05 g/cm ³ to 0.3 g/cm ³ .

The present invention also relates to a resin composition containing the above-mentioned exfoliated graphite and a synthetic resin.

The present invention makes it possible to provide exfoliated graphite that has good dispersibility when blended into resins, paints, etc., and that has a significant effect of improving physical properties such as toughness, elasticity, and impact resistance.

[Exfoliated graphite]
Exfoliated graphite is a material in which graphite has been exfoliated, and has a layered structure in which one to several thousand graphite unit layers are laminated. Graphite is a layered compound having unit layers made of carbon. In addition to graphite, graphite includes expanded graphite in which the spaces between graphite layers are expanded, and graphite oxide in which graphite is oxidized with an oxidizing agent.

The average thickness of the exfoliated graphite of the present invention is 10 nm to 200 nm. If the average thickness is less than 10 nm or more than 200 nm, dispersibility in resins, etc. may decrease. The average thickness of the exfoliated graphite is preferably 20 nm to 100 nm, more preferably 25 nm to 50 nm, and most preferably 30 nm to 40 nm.

In the present invention, the thickness of exfoliated graphite is the thickness in the direction perpendicular to the lamination plane of the exfoliated graphite, and the average thickness of exfoliated graphite is the average value of the thicknesses of any 30 or more pieces of exfoliated graphite. The thickness of exfoliated graphite can be measured, for example, using a SEM image of the exfoliated graphite taken with a scanning electron microscope (SEM). Note that exfoliated graphite consisting of one unit layer is called graphene, and its theoretical thickness is approximately 0.335 nm.

The specific surface area of the exfoliated graphite of the present invention is 10 m ² /g to 40 m ² /g. In the present invention, the specific surface area of the exfoliated graphite is a value measured by the BET method, and is measured in accordance with JIS Z8830 (Method for measuring specific surface area of powder (solid) by gas adsorption). When the specific surface area of the exfoliated graphite is smaller than 10 m ² /g or larger than 40 m ² /g, dispersibility in resins and the like may decrease. The specific surface area of the exfoliated graphite is preferably 12 m ² /g to 30 m ² /g, and more preferably 15 m ² /g to 20 m ² /g.

The bulk density of the exfoliated graphite of the present invention is 0.05 g/cm ³ to 0.3 g/cm ^3. In the present invention, the bulk density of the exfoliated graphite is measured in accordance with JIS K1469 (acetylene black for batteries). When the bulk density of the exfoliated graphite is less than 0.05 g/cm ³ or more than 0.3 g/cm ³ , the dispersibility in resins and the like may decrease. The bulk density of the exfoliated graphite is preferably 0.055 g/cm ³ to 0.2 g/cm ³ , and more preferably 0.06 g/cm ³ to 0.1 g/cm ³ .

The method for producing exfoliated graphite of the present invention is not particularly limited, and can be performed by applying shear force, ultrasonic vibration, cavitation, microwaves, etc. to graphite using a known device to exfoliate it until the average thickness, specific surface area, and bulk density are within the above-mentioned range. Examples of devices used for exfoliating graphite include media-agitating mills such as sand mills, attritors, and bead mills; container-driven mills using balls or rods as media such as rotary mills, vibration mills, and planetary mills; jet mills, roll mills, hammer mills, pin mills, high-pressure emulsifiers, ultrasonic emulsifiers, and microwave ovens. Examples of high-pressure emulsifiers include penetration-type high-pressure emulsifiers and collision-type high-pressure emulsifiers. Examples of the penetration type of the penetration-type high-pressure emulsifier include a single nozzle type and a slit nozzle type. Examples of the collision type of the collision-type high-pressure emulsifier include a type in which a liquid containing raw materials is collided with a flat surface such as a valve or a spherical surface such as a ball, and a type in which liquids containing raw materials are collided with each other.

Graphite can be exfoliated using either a wet exfoliation method that uses a solvent, or a dry exfoliation method that does not use a solvent, and the method can be selected according to the exfoliation method of each device.

As solvents to be used in the wet flaking method, since they are less likely to become statically charged, preferred are alcohol-based solvents such as methanol, ethanol, isopropanol, ethylene glycol, propylene glycol, and methoxyethanol; ketone-based solvents such as acetone and methyl ethyl ketone; heterocyclic solvents such as pyridine, piperidine, morpholine, tetrahydrofuran, and dioxane; ionic liquids such as 1-ethyl-3-methylimidazolium tetrafluoroborate and 1-butyl-3-methylimidazolium hexafluorophosphate, dimethylformamide, N-methylpyrrolidone, dimethylsulfoxide, and water.

When graphite is exfoliated by the wet exfoliation method, a surfactant may be used in combination. The surfactant disperses the graphite in a solvent, facilitating exfoliation, and also facilitates the penetration of the solvent between the layers of the graphite, facilitating exfoliation. After exfoliation, the surfactant may be removed by washing with a solvent as necessary. Preferred surfactants include nonionic surfactants such as glycerin fatty acid esters, sorbitan fatty acid esters, polyoxyethylene alkyl ethers, polyoxyethylene alkyl phenyl ethers, and alkyl glycosides.

When graphite is exfoliated, a water-soluble salt may be used in combination. In the exfoliation process, the solid water-soluble salt functions as a medium to promote exfoliation, and the water-soluble salt dissolved in a solvent acts between the layers of the graphite to promote exfoliation. After exfoliation, the water-soluble salt can be easily removed by washing with water. Preferred water-soluble salts include sodium chloride, potassium chloride, magnesium chloride, sodium sulfate, potassium sulfate, calcium sulfate, and sodium acetate.

Exfoliated graphite having the above average thickness, specific surface area and bulk density is less likely to aggregate in the substrate and has significantly improved dispersibility. This improves the physical properties of exfoliated graphite, such as electrical conductivity, heat dissipation and mechanical properties (impact resistance, bending strength, compressive strength, etc.). The exfoliated graphite of the present invention can be suitably used as an additive to resins such as synthetic resins, elastomers, paints, inks, etc.; as a conductive additive for battery electrodes, etc.

[Resin composition]
The resin composition of the present invention contains the exfoliated graphite of the present invention and a synthetic resin. Examples of synthetic resins that can be preferably used in the resin composition of the present invention include epoxy resins, phenolic resins, melamine resins, urea resins, alkyd resins, polyethylene terephthalate (PET) resins, polybutylene terephthalate (PBT) resins, polycarbonate resins, polyacetal resins, modified polyphenylene ether resins, polyurethanes, polyimides, polyimide amides, polyetherimides, polyethylene, polypropylene, polyvinyl chloride, polystyrene, polyvinyl acetate, fluorine-based resins, ABS resins, AS resins, acrylic resins, and silicones. Among these synthetic resins, epoxy resins are preferred because they are less likely to aggregate and can greatly improve physical properties even when a large amount of the exfoliated graphite of the present invention is blended.

When the synthetic resin contained in the resin composition of the present invention is a thermoplastic resin, the exfoliated graphite of the present invention may be blended in the process of kneading the synthetic resin and additives, as with other additives. When the synthetic resin contained in the resin composition of the present invention is a thermosetting resin, the exfoliated graphite of the present invention may be blended in the uncured thermosetting resin, and the resin composition may be cured as necessary.

The amount of exfoliated graphite in the resin composition of the present invention varies depending on the type of synthetic resin used and the desired physical properties, but it is preferable that the amount of exfoliated graphite of the present invention is 1 to 150 parts by mass, and more preferably 2 to 100 parts by mass, per 100 parts by mass of synthetic resin.

The present invention will be described in more detail below with reference to examples and comparative examples. However, the present invention is not limited in any way by the following examples. In the examples, "parts" and "%" are by weight unless otherwise specified.

[Production Example 1]
Exfoliated graphite A1 was prepared from natural graphite in accordance with Experimental Example 1 of WO 2016/148252. That is, 74 parts of 1-butyl-3-methylimidazolium hexafluorophosphate and 26 parts of polyethylene glycol (manufactured by Fujifilm Wako Pure Chemical Industries, product name: polyethylene glycol 20000) were mixed and dissolved by heating, and 10 parts of natural graphite (manufactured by Fujifilm Wako Pure Chemical Industries) were dispersed. 0.6 g of this dispersion was collected in a 0.5 cm ³ vial and capped, and then the dispersion was irradiated with 2450 MHz microwaves at 170 ° C. for 30 minutes using a microwave synthesis device (Initiator + manufactured by Biotage Japan). Thereafter, the dispersion was washed with acetone, filtered, and then heated and dried in an oven to obtain exfoliated graphite A1.

[Production Example 2]
Except for changing the microwave irradiation conditions from 170° C. for 30 minutes to 175° C. for 30 minutes, the same operation as in Production Example 1 was performed to obtain exfoliated graphite A2.

[Production Example 3]
Except for changing the microwave irradiation conditions from 30 minutes at 170° C. to 60 minutes at 180° C., the same operation as in Production Example 1 was carried out to obtain exfoliated graphite A3.

[Production Example 4]
15 parts by mass of exfoliated graphite A3 was added to 100 parts by mass of distilled water, and the mixture was circulated three times at a liquid temperature of 25° C. using a bead mill (manufactured by Kotobuki Industries, product name: UAM-015). The beads used were 450 parts by mass of zirconia beads with a diameter of 0.1 mm. The beads were removed from the resulting dispersion, filtered, and then heated and dried in an oven to obtain exfoliated graphite A4.

[Production Example 5]
Except for using expanded graphite (manufactured by Ito Graphite Industries Co., Ltd., product name: EC1500) instead of natural graphite, the same operation as in Production Example 1 was performed to obtain exfoliated graphite A5 derived from expanded graphite.

[Production Example 6]
Except for changing the microwave irradiation conditions from 170° C. for 30 minutes to 175° C. for 30 minutes, the same operation as in Production Example 5 was performed to obtain exfoliated graphite A6.

[Production Example 7]
Except for changing the microwave irradiation conditions from 30 minutes at 170° C. to 60 minutes at 180° C., the same operation as in Production Example 5 was performed to obtain exfoliated graphite A7.

[Production Example 8]
15 parts by mass of natural graphite (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was added to 100 parts by mass of distilled water, and the mixture was circulated three times at a liquid temperature of 25° C. using a bead mill (manufactured by Kotobuki Industries, product name: UAM-015). As the beads, 450 parts by mass of zirconia beads having a diameter of 0.1 mm were used. The beads were removed from the obtained dispersion, filtered, and then heated and dried in an oven to obtain coarsely pulverized graphite. Except for using this coarsely pulverized graphite instead of natural graphite, the same operation as in Production Example 1 was performed to obtain exfoliated graphite A8 derived from expanded graphite.

[Production Example 9]
Except for changing the microwave irradiation conditions from 170° C. for 30 minutes to 175° C. for 30 minutes, the same operation as in Production Example 8 was performed to obtain exfoliated graphite A9.

[Production Example 10]
Except for changing the microwave irradiation conditions from 30 minutes at 170° C. to 60 minutes at 180° C., the same operation as in Production Example 8 was performed to obtain exfoliated graphite A10.

The average thickness, specific surface area and bulk density were measured for exfoliated graphites A1 to A10. Since exfoliated graphites A2 , A8 and A9 satisfy the average thickness, specific surface area and bulk density of the present invention, they were designated as Examples 1 , 4 and 5 , and exfoliated graphites A1, A4, A6 and A7 were designated as Comparative Examples 1 to 4. Exfoliated graphites A3, A5 and A10 were designated as Reference Examples. The results are shown in Table 1.

[Preparation of resin sheet]
100 parts by mass of bisphenol A type epoxy resin (manufactured by ADEKA Corporation, product name: ADEKA RESIN EP4100E), 35 parts by mass of exfoliated graphite, and 0.5 parts by mass of an imidazole-based catalyst (1-benzyl-2-methylimidazole) were mixed using a planetary stirring and degassing device. This mixture was heated at a temperature of 160°C and a pressure of 5 MPa for 1 hour, and cured by heat press to produce a resin sheet with a thickness of 3 mm. Using this resin sheet, dispersibility, mechanical properties (bending properties), electrical properties, and thermal properties were evaluated. The results are shown in Table 2.

[Evaluation of Dispersibility]
The prepared resin sheet was cut using a microtome, the center of the cross section was photographed using a microscope, and the number of particles ^per 100 μm2 and the aggregate ratio were measured using image analysis software. The aggregate ratio is the ratio (%) of the total area of aggregates to the total area of particles when particles with an area of 40 μm2 or ^more are considered as aggregates. The greater the number of particles, the better the dispersibility, and the greater the aggregate ratio, the greater the proportion of aggregates.

[Evaluation of mechanical properties]
The resin sheet was processed into a strip of 25 mm width, and the bending stress, bending strain, and bending modulus were measured according to JIS K7171 (Plastics - Determination of bending properties) Method A. Higher bending stress, bending strain, and bending modulus indicate better mechanical properties related to bending.

[Evaluation of Electrical Characteristics]
The surface resistivity was measured by the four-probe method in accordance with JIS K7194 (resistivity test method for conductive plastics by the four-probe method). The lower the surface resistivity, the more improved the conductivity and the more excellent the electrical properties.

[Evaluation of thermal properties]
Thermal conductivity was measured by the hot wire method in accordance with ASTM D7984-16. Higher thermal conductivity indicates improved heat dissipation and superior thermal properties.

The resin sheet containing the exfoliated graphite of the Example has a larger particle number and a smaller aggregate ratio than the resin sheet containing the exfoliated graphite of the Comparative Example, which shows that the exfoliated graphite of the Example is more dispersible in epoxy resin than the exfoliated graphite of the Comparative Example. Furthermore, the resin sheet containing the exfoliated graphite of the Example has improved bending properties (bending stress, bending strain, and bending modulus) and lower surface resistivity and thermal conductivity than the resin sheet containing the exfoliated graphite of the Comparative Example. This is believed to be due to the improved dispersibility of the exfoliated graphite in the epoxy resin sheet.

[Preparation of resin strands]
100 parts by mass of polypropylene resin (homopolymer, melt flow rate: 8) and 2 parts by mass of exfoliated graphite were mixed and extruded at 230° C. using a twin-screw mixer to produce a resin strand having a thickness of 4 mm.

[Evaluation of Dispersibility]
The obtained resin strand was cut using a microtome, the center of the cross section was photographed using a microscope, and the number of particles ^per 100 μm2 and the aggregate ratio were measured using the same method as in the evaluation of the dispersibility of the resin sheet. The results are shown in Table 3.

The resin strands containing the exfoliated graphite of the examples have a larger particle number and a smaller aggregate ratio than the resin strands containing the exfoliated graphite of the comparative examples, which shows that the exfoliated graphite of the examples is more dispersible in polypropylene resin than the exfoliated graphite of the comparative examples.

Claims (3)

Exfoliated graphite having an average thickness of 10 nm to 100 nm, a specific surface area of 10 m ² /g to 30 m ² /g, and a bulk density of 0.108 g/cm ³ to 0.2 g/cm ³ . A resin composition containing the exfoliated graphite according to claim 1 and a synthetic resin. The resin composition according to claim 2, wherein the synthetic resin is an epoxy resin.