WO2024176859A1 - Fluorine rubber composition and seal for electric vehicle - Google Patents

Abstract

The purpose of the present invention is to provide a fluorine rubber composition that exhibits excellent roll machining properties and electrical conductivity.　The fluorine rubber composition contains a peroxide-vulcanized fluorine rubber and 16-26 mass% of a conductive agent.

Description

Fluororubber composition and seal for electric vehicle

The present invention relates to a fluororubber composition.

Fluororubber compositions have excellent resistance to oil and fuel, and are used as a material for sealing materials such as oil seals, O-rings, and packings in a wide range of fields, including automobiles and industrial machinery. In recent years, electric vehicles (hereinafter sometimes referred to as "EVs") have begun to become popular, and as a result, the prime mover has shifted from engines to motors. In EVs, a seal is installed between the motor, which acts as the prime mover, and the reduction gear.

Patent Document 1 (Japanese Patent No. 6288398) discloses Tecnoflon BR9151 and BR9171 as rubbers that do not deteriorate due to reactions with additives in oil. Patent Document 2 (International Publication No. 2014/175079) discloses a fluororubber made of vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene-ethylene-perfluoro(methyl vinyl ether) 5-component copolymer, and discloses Tecnoflon BR9151 and BR9171 as specific examples of such fluororubber.

Here, in technical fields such as automobiles equipped with conventional engines, insulating fluororubber compositions are generally used as materials for seals. However, unlike automobiles equipped with conventional engines, in new technical fields such as EVs, if the seals are made of insulating fluororubber compositions, the seals will become charged and a potential difference will occur, raising concerns about the occurrence of electrolytic corrosion in the vehicle interior unit. In addition, there is a possibility that radio noise will be generated due to electromagnetic waves generated by the charging of the seals. In response to this, in conventional EVs, conductive parts such as carbon earth brushes and knuckle earths are connected to the reducer and body earth to prevent the charging and electrolytic corrosion of the vehicle interior unit. However, taking such measures increases the number of parts in the EV, requiring new space for the parts in the EV, and there is a concern that problems such as increased costs will occur. On the other hand, there is a concern that if the composition of the fluororubber composition is changed from the conventional one in order to impart the desired conductivity to the fluororubber composition, the roll processability of the fluororubber composition will decrease.

As such, conventional fluororubber compositions are desired to have excellent roll processability as well as excellent electrical conductivity from the viewpoint of preventing static electricity and electrolytic corrosion.

Patent No. 6288398 International Publication No. 2014/175079

The inventors have discovered that excellent roll processability and electrical conductivity can be achieved by forming a fluororubber composition containing a specific type of fluororubber and a specified content of a conductive agent, and have completed the present invention. In other words, the present invention provides a fluororubber composition that has excellent roll processability and electrical conductivity.

The gist and configuration of the present invention are as follows.
[1] A fluororubber composition containing a peroxide-vulcanized fluororubber and 16 to 26 mass % of a conductive agent.
[2] The fluororubber composition according to the above [1], having a volume resistivity of 1.0 Ω cm or less.
[3] A seal for an electric vehicle, comprising the fluororubber composition according to the above [1] or [2].

It is possible to provide a fluororubber composition with excellent roll processability and electrical conductivity.

FIG. 1 is a diagram illustrating a sealing device having a conductive part that includes a vulcanized fluororubber composition according to one embodiment as a seal.

The vulcanized fluororubber composition of the present invention contains a peroxide vulcanization-based fluororubber and 16 to 26% by mass of a conductive agent. By using a peroxide vulcanization-based fluororubber as the fluororubber, the elongation of the fluororubber composition can be improved, and the roll processability of the fluororubber composition can be improved. By using a conductive agent content of 16% by mass or more in the fluororubber composition, the fluororubber composition can have excellent electrical conductivity. In addition, by using a conductive agent content of 26% by mass or less in the fluororubber composition, the hardness and viscosity of the fluororubber composition become appropriate, and the roll processability of the fluororubber composition can be improved. For example, when a shaft seal, packing seal, etc. are made from the fluororubber composition of the present invention vulcanized in an EV, the seal itself has excellent electrical conductivity, so that extra parts such as earth brushes and knuckle earths are not required as in conventional EVs, and space can be saved in the EV. In addition, rotating shafts such as shafts generate heat due to their high rotation speed, but seals made from the fluororubber composition of the present invention have excellent heat resistance, so that they can maintain excellent electrical conductivity and sealing properties even when used as a seal for a rotating shaft or the like at high temperatures. Furthermore, in the fluororubber composition of the present invention, it is not necessary to increase the content of processing aids in order to improve roll processability. The fluororubber composition is preferably a fluororubber composition for shaft seals or packing seals, and more preferably a fluororubber composition for seals such as shafts and packings of electric vehicles.

The volume resistivity of the fluororubber composition after vulcanization is preferably 1.0 Ω·cm or less, more preferably 0.5 Ω·cm or less, and even more preferably 0.3 Ω·cm or less. To measure the volume resistivity of the fluororubber composition, a sample made of the vulcanized fluororubber composition with dimensions of 140 mm x 100 mm x 2 mm is used. This sample is prepared by preparing the fluororubber composition, performing primary vulcanization at 160-200°C for 3-30 minutes in a molding press to obtain dimensions of 140 mm x 100 mm x 2 mm, and then performing secondary vulcanization at 150-250°C for 0.5-24 hours in a thermostatic bath. It has been confirmed through prior testing that the volume resistivity of the fluororubber composition does not change within the above temperature and vulcanization time ranges. The volume resistivity of the fluororubber composition is measured according to the four-probe method in accordance with JIS K7194:1994, using a Hioki Milliohm High Tester 3540 (product name) measuring device, and is measured at room temperature using the parallel terminal electrode method.

Each component constituting the fluororubber composition of the present invention will be described in detail below.
(Fluorine rubber)
The fluororubber is not particularly limited as long as it is a peroxide vulcanized fluororubber after vulcanization, but one or more homopolymers or copolymers of fluorine-containing olefins can be used. Examples of fluorine-containing olefins include vinylidene fluoride, hexafluoropropylene, pentafluoropropylene, trifluoroethylene, trifluorochloroethylene, tetrafluoroethylene, vinyl fluoride, perfluoroacrylic acid ester, perfluoroalkyl acrylate, perfluoromethyl vinyl ether, perfluoroethyl vinyl ether, and perfluoropropyl vinyl ether. These fluorine-containing olefins can be used alone or in combination of two or more. Examples of fluorine-containing rubbers include vinylidene fluoride-hexafluoropropylene binary copolymers, vinylidene fluoride-tetrafluoroethylene-hexafluoropropylene ternary copolymers, and vinylidene fluoride-hexafluoropropylene-perfluoroalkyl vinyl ether ternary copolymers.

Fluororubber may be obtained by solution polymerization, suspension polymerization, or emulsion polymerization, or may be obtained as a commercially available product. Examples of commercially available fluororubber include Chemours' products "VITON (registered trademark) GBL-600S" and "VITON (registered trademark) GLT-600S," and Daikin Industries' products "DAI-EL (registered trademark) GBR-6002" and "DAI-EL (registered trademark) G-901." Fluororubber is a peroxide-vulcanized fluororubber that is vulcanized with peroxide. Peroxide-vulcanized fluororubber is a polymer that introduces cure sites such as Br or I to the molecular chain end at the time of polymerization and uses this to undergo a vulcanization reaction. Peroxide-vulcanized fluororubber can be confirmed by, for example, (a) detecting Br or I in the fluororubber by elemental analysis using XRF (X-ray fluorescence analysis), or (b) not detecting elements derived from an acid acceptor such as Mg or Ca in the molded fluororubber composition by elemental analysis. Peroxide-vulcanized fluororubber does not require an acid acceptor compared to polyol-vulcanized fluororubber, so the fluororubber composition can achieve excellent roll processability. For this reason, in a preferred embodiment of the present invention, the fluororubber composition does not contain an acid acceptor.

(Conductive agent)
The fluororubber composition after vulcanization contains 16 to 26% by mass of the conductive agent. The fluororubber composition can have excellent electrical conductivity and roll processability by containing 16 to 26% by mass of the conductive agent. The conductive agent is not particularly limited as long as it can impart electrical conductivity to the fluororubber composition, but the conductive agent is preferably at least one material selected from the group consisting of Ketjen black, graphite, carbon nanotubes, and carbon fibers. The content of the conductive agent in the fluororubber composition is preferably 18 to 26% by mass, more preferably 20 to 26% by mass, and even more preferably 22 to 26% by mass. By keeping the content of the conductive agent in the fluororubber composition within the above range, it is possible to maintain excellent electrical conductivity and roll processability while keeping other physical properties within the desired range. The conductive agent in the fluororubber composition may be one type or two or more types.

(Processing aids)
The fluororubber composition may further contain a processing aid. The processing aid is not particularly limited, but is preferably at least one material selected from the group consisting of vegetable wax, polyester, and fluoropolyether derivatives. The content of the processing aid in the fluororubber composition is preferably 3 to 14 mass%, more preferably 6 to 10 mass%, and even more preferably 6 to 7 mass%.

(Other additives)
The fluororubber composition of the present invention contains a fluororubber and a conductive agent, and the fluororubber composition may further contain, as components other than the fluororubber and the conductive agent, an antioxidant; a thermoplastic resin; a plasticizer; a softener; a foaming agent; a foaming assistant; a colorant; a dispersant; a flame retardant; a tackifier; a release agent; various metal powders, and the like.

(Method for producing fluororubber composition)
The fluororubber composition can be obtained by kneading 16 to 26% by mass of a conductive agent, a peroxide-vulcanized fluororubber, and, if necessary, materials such as a processing aid, using kneading equipment (rolls, kneaders, etc.). The vulcanized fluororubber composition of the present invention can be obtained by adding a vulcanizing agent to the fluororubber composition obtained as described above, and then heating the composition in a molded state into a predetermined shape. When obtaining a desired molded product by molding and vulcanizing the fluororubber composition, a known manufacturing device can be used. For example, the fluororubber composition can be put into a cavity of a predetermined shape using an injection molding machine, a compression molding machine, or the like, and heated under appropriate conditions to obtain a vulcanized fluororubber composition. At this time, the vulcanization may be performed in multiple stages. For example, the fluororubber composition may be pressed into a predetermined shape and subjected to primary vulcanization, and the fluororubber composition after the primary vulcanization may be further subjected to secondary vulcanization.

(Molded product of fluororubber composition)
As described above, a molded product of a desired shape can be produced by molding and vulcanizing the fluororubber composition. The product containing the fluororubber composition is preferably a seal such as a shaft seal or a packing seal for an electric vehicle. The seal for an electric vehicle preferably has an overall electrical conductivity of 10Ω or less, more preferably 9Ω or less, and even more preferably 5Ω or less. The electrical conductivity of the entire seal can be evaluated as an electrical resistance value measured under the conditions of 500kHz/5V using an impedance analyzer (IM3570 (device name), manufactured by Hioki E.E.C. Co., Ltd.) after the seal is placed so as to be engaged between the shaft and the housing of a rotation tester and the shaft and the housing are in contact with a conductor.

FIG. 1 is a diagram showing one embodiment of a sealing device having a conductive part 120 containing the vulcanized fluororubber composition of the present invention as a seal, and shows a schematic cross-sectional view (schematic cross-sectional view) of the rotating shaft 20 and housing 38 of the sealing device 40, taken along a plane including the axis ω of the rotating shaft 20.

As shown in FIG. 1, the sealing device 40 is disposed in a gap 107 between the outer surface 20s of the rotating shaft 20 and the inner surface 38s that constitutes the shaft hole in the housing 38, which has a shaft hole into which the rotating shaft 20 is inserted.

As mentioned above, the rotating shaft 20 may be, for example, the motor shaft of a drive motor in an EV (Electric Vehicle) or HEV (Hybrid Electric Vehicle). Typically, the rotating shaft 20 is rod-shaped (cylindrical) and its cross section is typically circular. The housing 38 is typically grounded.

<Sheath tube part>
The sheath pipe portion 110 of the sealing device 40 shown in FIG. 1 will be described below. The sheath pipe portion 110 is an annular portion that is directly or indirectly fixed to the outer surface 20s of the rotating shaft 20. The sheath pipe portion 110 is directly fixed to the outer surface 20s of the rotating shaft 20. The means for fixing the sheath pipe portion 110 to the outer surface 20s of the rotating shaft 20 is not particularly limited. For example, the sheath pipe portion 110 and the outer surface 20s of the rotating shaft 20 may be fitted together by a press fit method to fix the sheath pipe portion 110 to the outer surface 20s of the rotating shaft 20. Alternatively, the sheath pipe portion 110 may be fixed to the outer surface 20s of the rotating shaft 20 using an adhesive or the like.

The sheath tube portion 110 comprises a fixed portion 110a extending in a direction parallel to the axis ω of the rotating shaft 20, and a support portion 110b connecting to the end of the fixed portion 110a and extending in a direction approaching the inner surface 38s of the housing 38. It is preferable that the fixed portion 110a and the support portion 110b are each plate-shaped. In other words, it is preferable that the fixed portion 110a is cylindrical, and the support portion 110b is disk-shaped with a through hole in the center. In addition, the fixed portion 110a and the support portion 110b are approximately perpendicular in a cross section obtained by cutting the rotating shaft 20 along a plane including the axis ω. Therefore, the sheath tube portion 110 has an approximately L-shaped cross section. Here, "approximately vertical" means that it does not have to be completely vertical. In other words, it is preferable that the fixed portion 110a and the support portion 110b form an angle of 90 degrees, but this angle may be 75 to 105 degrees (preferably 80 to 100 degrees, and more preferably 85 to 95 degrees).

The sheath tube portion 110 is preferably made of a metal, and more preferably a metal having electrical conductivity. Such metals include stainless steel, cold rolled steel (SPCC), brass, and aluminum. The sheath tube portion 110 does not have to be made of a metal. For example, it may be made of resin. The sheath tube portion 110 can be formed by pressing or forging.

<Conductive Part>
The conductive part 120 of the sealing device 40 shown in FIG. 1 will be described below. The conductive part 120 is ring-shaped, with the ring outer peripheral part 120a in contact with the inner surface 38s of the housing 38, and the main surface 120s of the conductive part 120 being fixed to the sleeve pipe part 110. The main surface 120s of the conductive part 120 is fixed to the support part 110b of the sleeve pipe part 110. The conductive part 120 functions as a seal containing the vulcanized fluororubber composition of the present invention, and is sheet-shaped or plate-shaped. The conductive part 120 in FIG. 1 is deformed so that the cross section is substantially L-shaped, and the main surface 120s of the conductive part 120 is in contact with the main surface of the plate-shaped support part 110b in the sleeve pipe part 110. Here, it is preferable that the main surface 120s of the conductive part 120 is in close contact with the main surface of the support part 110b.

The main surface 120s of the ring outer periphery 120a is in contact with the inner surface 38s of the housing 38. When the rotating shaft 20 rotates, centrifugal force is applied to the sealing device 40, and it is preferable that the main surface 120s of the ring outer periphery 120a is in close contact with the inner surface 38s of the housing 38. In the conductive part 120 deformed so that the cross section is approximately L-shaped, the ring outer periphery 120a is cylindrical, and the remaining part is disk-shaped with a through hole in the center. The centrifugal force F generated by the rotation of the rotating shaft 20 increases the force with which the conductive part 120 is pressed against the inner surface 38s of the housing 38 compared to before the rotating shaft 20 rotates.

When the rotating shaft 20 rotates, centrifugal force F is applied to the conductive part 120, and the ring outer periphery 120a of the conductive part 120 is pressed against the inner surface 38s of the housing 38. As a result, the ring outer periphery 120a of the conductive part 120 is stably attached to the inner surface 38s of the housing 38, ensuring electrical continuity between the rotating shaft 20 and the housing 38.

The method of fixing the main surface 120s of the conductive portion 120 to the main surface of the support portion 110b of the sheath tube portion 110 is not particularly limited, and for example, they can be fixed using an adhesive. However, if they are fixed using metal bolts and nuts without using an adhesive, it is easier to ensure electrical continuity between the conductive portion 120 and the sheath tube portion 110, which is preferable as it results in easier electrical continuity between the rotating shaft 20 and the housing 38.

Next, examples will be described to further clarify the effects of the present invention, but the present invention is not limited to these examples.

(Examples 1 to 5, Comparative Examples 1 to 10)
In each example, a fluororubber composition was produced by kneading materials in the proportions shown in the following Table 1. The names of the materials used in each example are shown below.
Type 1 (ASTM) fluororubber: VITON (registered trademark) A-500 (manufactured by Chemours)
Type 2 (ASTM) fluororubber: VITON (registered trademark) B-600 (manufactured by Chemours)
Type 3 (ASTM) fluororubber: VITON (registered trademark) GLT-600S (manufactured by Chemours)
Type 6 (ASTM) fluororubber: Daiel (registered trademark) GBR-6002 (manufactured by Daikin Industries, Ltd.)
Ketjenblack A: KETJENBLACK EC600JD (manufactured by Lion Specialty Chemicals)
Ketjen Black B: Denka Black (manufactured by Denka Co., Ltd.)
Graphite: A-0 (manufactured by Higashi Nippon Carbon Co., Ltd.)
Carbon fiber: S-241 (Osaka Gas Chemicals)
Carbon nanotubes: Lucan CP1001M (LG Chem)
Clay: NN Kaolin Clay (Takehara Chemical Co., Ltd.)
Silica: NIPSIL (registered trademark) ER (manufactured by Tosoh Silica Corporation)
Vegetable wax: VPA#2 (Chemours)
Polyester: SYNCROFLEX 3142 (manufactured by CRODA)

The fluororubber compositions produced as described above were evaluated for hardness (Durometer A hardness), hardness (IRHD (International Rubber Hardness Degree) hardness), volume resistivity, seal conductivity, and roll processability. The preparation and measurement methods for the samples for each evaluation are shown below.

(1) Hardness (Durometer A hardness): A fluororubber composition was vulcanized under the conditions of primary vulcanization at 180°C for 4 minutes and secondary vulcanization at 230°C for 9 hours to prepare a sample having a thickness of 2 mm. The durometer A hardness of this sample was then measured at room temperature using a constant pressure rubber hardness tester P1-A (device name) manufactured by Kobunshi Keiki Co., Ltd. in accordance with JIS K6253-2:2012.
(2) Hardness (IRHD hardness): The fluororubber composition was vulcanized under the conditions of primary vulcanization at 180°C for 4 minutes and secondary vulcanization at 230°C for 9 hours to prepare a sample having dimensions of 140 mm x 100 mm x 2 mm. The IRHD hardness of this sample was then measured under IRHD (micro) conditions using a Digitest II (device name) manufactured by Burleis Co., Ltd. in accordance with JIS K6253-2:2012.
(3) Volume resistivity: The fluororubber composition was vulcanized under the conditions of primary vulcanization at 180°C for 4 minutes and secondary vulcanization at 230°C for 9 hours to prepare a sample having dimensions of 140 mm x 100 mm x 2 mm. Next, the sample was measured according to the four-probe method in accordance with JIS K7194:1994, and the volume resistivity was measured at room temperature using a milliohm high tester 3540 (device name) manufactured by Hioki E.E.C., using the parallel terminal electrode method.

(4) Electrical conductivity of seal: The fluororubber composition was vulcanized under the conditions of primary vulcanization at 180°C for 4 minutes and secondary vulcanization at 230°C for 9 hours to mold an oil seal with an inner diameter of 85 mm, an outer diameter of 105 mm, and a width of 13 mm. Next, the oil seal was installed so as to be fitted between the shaft and housing of a rotation tester, and the electrical resistance value was measured under the conditions of 500 kHz/5 V using an impedance analyzer (IM3570 (device name), manufactured by Hioki E.E.C. Co., Ltd.) with the shaft and housing in contact with a conductor.
(5) Roll processability: The fluororubber composition before vulcanization was kneaded in a kneader to prepare a dough, and then a vulcanizing agent was added to the dough of the fluororubber composition using a roll, and the winding property of the fluororubber composition around the roll when the vulcanizing agent was added was evaluated. When evaluating the winding property of the fluororubber composition around the roll, two rolls were rotated, and the gap between the two rolls was adjusted to about 5 mm, so that the thickness of the dough made of the fluororubber composition was set to 5 mm. At this time, the evaluation was performed visually, and if the dough made of the fluororubber composition separated from the surfaces of the two rolls, it was marked as "x" because it was dangerous when cutting out the dough with a knife after the rolling work was completed, and if the dough made of the fluororubber composition did not separate from the surfaces of the two rolls, it was marked as "○".
The evaluation items measured as described above are shown in Table 1 above.

As shown in Examples 1 to 5 in Table 1, the fluororubber compositions of the present invention had a volume resistivity of 0.3 Ω·cm or less, a seal conductivity of 9 Ω or less, and roll processability was also rated "good." On the other hand, as shown in Comparative Examples 1 to 10 in Table 1, the fluororubber compositions of the comparative examples all had very high volume resistivities and seal conductivity values, or the fluororubber compositions could not be kneaded, or the roll processability was rated "poor." From the above, it was confirmed that the present invention can provide a fluororubber composition with excellent conductivity and roll processability.

20 Rotating shaft 20s Outer surface of rotating shaft 38 Housing 38s Inner surface of housing 40 Sealing device 107 Gap 110 Sleeve tube portion 110a Fixed portion 110b in sleeve tube portion Support portion 120 Conducting portion 120a Ring outer periphery portion 120s in conducting portion Main surface of conducting portion ω Axial center of rotating shaft

Claims (3)

A fluororubber composition containing peroxide-vulcanized fluororubber and 16 to 26 mass% of a conductive agent. The fluororubber composition according to claim 1, which has a volume resistivity of 1.0 Ω cm or less. A seal for an electric vehicle comprising the fluororubber composition according to claim 1 or 2.

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