Electroconductive grease, electroconductive rolling bearing, and additive for electroconductive grease

Abstract

Provided is an electroconductive grease that can suppress or prevent electrolytic corrosion for an extended period of time. The electroconductive grease according to the present disclosure comprises a compound having an aromatic ring, wherein the compound having an aromatic ring is contained at an amount of 0.2-8 mass% relative to the total amount of the electroconductive grease.

Description

Conductive Grease, Conductive Rolling Bearing and Additive for Conductive Grease

[0001] The present invention relates to a conductive grease, a conductive rolling bearing and an additive for conductive grease.

[0002] In recent years, the electrification of automobiles has progressed, and electric vehicles driven by motors have come into widespread use. In addition, many industrial machines have conventionally been operated by motor drive. Motors for automotive auxiliary machines and industrial machinery are increasingly required to be smaller, have higher performance and higher output year by year, and their operating conditions have become severer. In these motors for automotive auxiliary machines and industrial machinery, rolling bearings are generally used to support rotating shafts.

[0003] When current from a circuit flows into the inside of a rolling bearing incorporated in a motor, sparking may occur through the oil film on the rolling contact surface between the rolling elements and the raceway surface (hereinafter referred to as the "rolling raceway surface"). This sparking causes "electric corrosion", in which the rolling raceway surface is locally melted, which may deteriorate the bearing function. In order to prevent such problems, so-called "conductive bearings", in which the bearing itself is provided with conductivity, are sometimes used.

[0004] Examples of conductive bearings include bearings in which conductive grease is sealed. For example, in order to reduce the volume resistivity of the grease itself (increase conductivity) and prevent electric corrosion, a bearing in which conductive grease containing carbon black particles or the like as a conductive substance and thickener is sealed is known (see Patent Document 1). A conductive grease obtained by blending graphite particles as a thickener into fluorine oil, which is a base oil, is also known (see Patent Document 2). Furthermore, a rolling bearing using an ionic liquid as the base oil of the grease and carbon black as the thickener has also been proposed (see Patent Document 3). A grease composition containing tungsten disulfide powder has also been proposed (see Patent Document 4).

[0005] Japanese Unexamined Patent Publication No. 2002-53890, Japanese Unexamined Patent Publication No. 2004-162909, Japanese Unexamined Patent Publication No. 2006-250323, Japanese Unexamined Patent Publication No. 2013-112711

[0006] However, in the technologies described in Patent Document 1 or Patent Document 2, the conductive material may gradually separate from the base oil or aggregate due to prolonged use of the bearing. As a result, even if the bearing's resistance is low initially, it may increase over time, making it more susceptible to electrolytic corrosion. Furthermore, in the technology described in Patent Document 3, the combination of base oil and thickener is limited in order to reduce the volume resistivity of the grease to be sealed, which may make it difficult to ensure a long bearing life for a wide range of bearing types. In addition, there was room for further improvement in the technology described in Patent Document 4 from the perspective of extending the bearing life.

[0007] This invention was made to address these problems, and aims to provide a conductive grease that suppresses the occurrence of galvanic corrosion in bearings and contributes to extending the bearing life, as well as a conductive rolling bearing containing this conductive grease.

[0008] To solve the above problems, a conductive grease according to one aspect of the present invention is a conductive grease containing a compound having an aromatic ring, wherein the compound having an aromatic ring is contained in an amount of 0.2 to 8% by mass relative to the total amount of the conductive grease.

[0009] Another aspect of the present invention relates to a conductive grease comprising a compound having an aromatic ring, wherein the ratio of the molecular weight of the aromatic ring portion to the total molecular weight of the compound having the aromatic ring is 0.5 or more, and the aromatic ring portion of the compound having the aromatic ring is present in an amount of 0.1 to 10% by mass relative to the total amount of the conductive grease.

[0010] According to one aspect of the present invention, a conductive grease can be provided that can suppress or prevent electrolytic corrosion in bearings and the like for a long period of time.

[0011] This figure shows the voltage waveform during discharge between electrodes in oil using pentaphenyl ether (5P4E) and poly-α-olefin (PAO) in a reference experiment. This figure shows an optical microscope image of the product from 5P4E in a reference experiment. This figure shows the Raman spectra of the products from 5P4E and PAO in a reference experiment. This figure shows the change in bearing voltage over time of a bearing using conductive grease with added 5P4E in an example. This figure shows the relationship between the amount of 5P4E added in an example and the number of times the bearing voltage exceeded 5V. This figure shows the relationship between the amount of alkylated diphenyl ether (ADE) added in an example and the number of times the bearing voltage exceeded 4.5V.

[0012] One embodiment of the present invention will be described in detail below. Unless otherwise specified in this specification, "A to B" representing a numerical range means "A or greater, and B or less."

[0013] [1. Conductive Grease] A conductive grease according to one embodiment of the present invention is a conductive grease containing a compound having an aromatic ring, wherein the compound having an aromatic ring is contained in an amount of 0.2 to 8% by mass relative to the total amount of the conductive grease.

[0014] Another embodiment of the present invention is a conductive grease comprising a compound having an aromatic ring, wherein the ratio of the molecular weight of the aromatic ring portion to the total molecular weight of the compound having the aromatic ring is 0.5 or more, and the aromatic ring portion of the compound having the aromatic ring is present in an amount of 0.1 to 10% by mass relative to the total amount of the conductive grease.

[0015] The ratio of the aromatic ring portion to the total amount of conductive grease can be calculated from the following formula (1): Ratio of the aromatic ring portion of the compound having an aromatic ring to the total amount of conductive grease [mass %] = 100 × (mass of the compound having an aromatic ring contained in the conductive grease / mass of the conductive grease) × (molecular weight of the aromatic ring portion of the compound having an aromatic ring / total molecular weight of the compound having an aromatic ring) (1) In order to solve the above-mentioned problems, the inventors have diligently conducted research and, as a result, have surprisingly found that by blending a compound having an aromatic ring into conductive grease in a specific ratio, the occurrence of electrolytic corrosion in rolling bearings can be suppressed or prevented.

[0016] As shown in the examples described later, the aromatic ring portion can become a graphite structure upon discharge. After hydrogen is removed from the aromatic ring during discharge, conductivity is exhibited when the aromatic ring polymerizes as amorphous carbon. Since compounds having aromatic rings have a high affinity for the base oil, it is unlikely that the compounds having aromatic rings will separate from the base oil or aggregate in the base oil. This is presumed to suppress or prevent electrolytic corrosion for a long period of time. The optimal blending ratio of compounds having aromatic rings to conductive grease is thought to vary depending on the combination with the base oil and the application of the grease. Furthermore, as described later, the optimal range for the addition ratio of the compounds having aromatic rings themselves is thought to differ depending on the number of aromatic rings. Therefore, it may be more preferable to specify the ratio of the aromatic ring portion of the compounds having aromatic rings to the total amount of conductive grease.

[0017] The proportion of the compound having an aromatic ring is preferably 0.2% by mass or more and 8% by mass or less, based on the total amount of conductive grease, taking viscosity into consideration. More preferably, the proportion of the compound having an aromatic ring is 0.5% by mass or more and even more preferably 1% by mass or more, based on the total amount of conductive grease. Furthermore, the proportion of the compound having an aromatic ring is preferably 6% by mass or less and more preferably 5% by mass or less, based on the total amount of conductive grease. In one embodiment, the proportion of the compound having an aromatic ring may be 15% by mass or less or 10% by mass or less.

[0018] When the ratio of the molecular weight of the aromatic ring portion to the total molecular weight of the compound having an aromatic ring is 0.5 or more, the ratio of the aromatic ring portion to the total amount of conductive grease is 0.1% by mass or more, preferably 0.2% by mass or more, more preferably 0.4% by mass or more, even more preferably 0.5% by mass or more, and particularly preferably 1% by mass or more. If the ratio of the aromatic ring portion is 0.1% by mass or more, the effect of suppressing or preventing the above-mentioned electrolytic corrosion can be sufficiently obtained. On the other hand, the upper limit of the blending ratio of the compound having an aromatic ring is not particularly limited as it is determined by the combination with the base oil and the application of the grease, but it is presumed that if the blending ratio of the compound having an aromatic ring is too high, the viscosity of the conductive grease will increase and it may affect fluctuations in bearing voltage. From this viewpoint, the upper limit of the ratio of the aromatic ring portion to the total amount of conductive grease is not particularly limited, but it is preferably 10% by mass or less, more preferably 8% by mass or less, and even more preferably 6% by mass or less or 5% by mass or less.

[0019] In other embodiments of the present invention, when the compound having an aromatic ring is at least one selected from the group consisting of diphenyl ethers and their alkylates described later, and / or when the ratio of the molecular weight of the aromatic ring portion to the total molecular weight of the compound having an aromatic ring is less than 0.5, particularly when, for example, the compound having an aromatic ring is an alkylated diphenyl ether and the ratio of the molecular weight of the aromatic ring portion to the total molecular weight of the compound having an aromatic ring is less than 0.5, the ratio of the aromatic ring portion to the total amount of conductive grease is 0.05% by mass or more, preferably 0.1% by mass or more, more preferably 0.2% by mass or more, even more preferably 0.4% by mass or more, and particularly preferably 0.5% by mass or more. If the ratio of the aromatic ring portion is 0.05% by mass or more, the effect of suppressing or preventing the above-mentioned electrolytic corrosion can be sufficiently obtained. The upper limit of the blending ratio of the compound having an aromatic ring is not particularly limited as it is determined by the combination with the base oil and the application of the grease, but it is presumed that if the blending ratio of the compound having an aromatic ring is too high, the viscosity of the conductive grease will increase and may affect fluctuations in bearing voltage. From this viewpoint, the proportion of the aromatic ring portion to the total amount of conductive grease is preferably 2.8% by mass or less, more preferably 2% by mass or less, and even more preferably 1.5% by mass or less. Even when the ratio of the molecular weight of the aromatic ring portion to the total molecular weight of the compound having an aromatic ring is 0.5 or more, as described above, the proportion of the aromatic ring portion to the total amount of conductive grease is in the range of 0.05% by mass to 2.8% by mass and similarly good properties are obtained. The proportion of the aromatic ring portion to the total amount of conductive grease is preferably 0.1% by mass or more, more preferably 0.2% by mass or more, even more preferably 0.4% by mass or more, and particularly preferably 0.5% by mass or more.

[0020] The compound having the aromatic ring is not particularly limited, but is preferably at least one selected from the group consisting of phenyl ethers and their alkylates, phenyl thioethers and their alkylates, biphenyls, aromatic esters, alkylbenzenes, and alkylnaphthalenes. The compound having the aromatic ring is more preferably a compound having two or more aromatic rings, even more preferably a compound having a phenyl ether structure, and particularly preferably a polyphenyl ether. In this specification, polyphenyl ether means a phenyl ether having three or more aromatic rings.

[0021] Examples of phenyl ethers and their alkylated products include pentaphenyl ether, tetraphenyl ether, diphenoxybenzene, diphenyl ether, and their alkylated products (e.g., phenyl ethers having C8-28 alkyl groups). Examples of pentaphenyl ether include bis(phenoxyphenoxy)benzene. Examples of tetraphenyl ether include (phenoxyphenoxy)biphenyl. It is preferable that the phenyl ether includes 1,3-bis(3-phenoxyphenoxy)benzene, 3-(3-phenoxyphenoxy)-1,1'-biphenyl, 2-(3-phenoxyphenoxy)-1,1'-biphenyl, diphenyl ether, and their alkylated products. Here, based on the number of aromatic rings and ether bonds, bis(phenoxyphenoxy)benzene is denoted as 5P4E and (phenoxyphenoxy)biphenyl as 4P2E.

[0022] Examples of phenylthioethers and their alkylated products include diphenylthioethers, bis(phenylthio)benzene, and their alkylated products (for example, phenylthioethers having C8-28 alkyl groups).

[0023] Aromatic esters include phthalate esters (for example, compounds in which a C6-12 alkyl group is esterified to phthalic acid), trimellitic acid esters (for example, compounds in which a C6-12 alkyl group is esterified to trimellitic acid), and pyromellitic acid esters (for example, compounds in which a C6-12 alkyl group is esterified to pyromellitic acid).

[0024] Examples of alkylbenzenes include alkylbenzenes having C8 to C28 alkyl groups. Examples of alkylnaphthalenes include alkylnaphthalenes having C8 to C28 alkyl groups.

[0025] The number of alkyl groups (alkyl additions) in compounds having these aromatic rings is not particularly limited, but it may be, for example, 1 to 4, 1 to 3, or 1 to 2.

[0026] The ratio of the molecular weight of the aromatic ring portion to the total molecular weight of the compound having an aromatic ring is preferably 0.2 or more, more preferably 0.3 or more, and even more preferably 0.5 or more. The molecular weight of the compound having an aromatic ring is preferably 150 to 800, more preferably 150 to 500, and even more preferably 170 to 500. Examples of the structure and molecular weight of compounds having an aromatic ring are shown in Tables 1 and 2. The molecular weight was calculated using atomic weights of C = 12.01, H = 1.01, and O = 16. In the table, R represents an alkyl group and m represents the number of substitutions.

[0027] Conductive grease typically contains a base oil, a thickener, and / or a conductive compound.

[0028] As the base oil, ordinary base oils used in greases can be used. Examples of base oils include mineral oils refined from crude oil by appropriately combining processes such as vacuum distillation, solvent delamination, solvent extraction, hydrocracking, solvent dewaxing, sulfuric acid washing, clay refining, and hydrorefining; diester synthetic oils such as dibutyl sebacate, di-2-ethylhexyl sebacate, dioctyl adipate, diisodecyl adipate, ditridecyl adipate, ditridecyl glutarate, and methylacetyl ricinolate; polyol ester synthetic oils such as trimethylolpropane caprylate, trimethylolpropane pelargonate, pentaerythritol-2-ethylhexanoate, and pentaerythritol pelargonate; and polyhydric alcohols, dibasic acids, and monosalts. Examples of synthetic oils include: ester-based synthetic oils such as complex ester oils, which are oligoesters of a mixed fatty acid with a base acid; polyglycol-based synthetic oils such as polyethylene glycol, polypropylene glycol, polyethylene glycol monoether, and polypropylene glycol monoether; synthetic hydrocarbon-based oils such as poly-α-olefins or their hydrogenated products, such as normal paraffin, isoparaffin, polybutene, polyisobutylene, 1-decene oligomer, and co-oligomer of 1-decene and ethylene; silicone-based synthetic oils such as dimethylpolysiloxane and alkyl-modified polysiloxane; and fluorine-based synthetic oils such as perfluoropolyether. One or more of these can be used. Preferably, ester-based synthetic oils, polyglycol-based synthetic oils, or synthetic hydrocarbon-based oils are used as the base oil, and more preferably poly-α-olefins are used. Furthermore, it is preferable that the base oil is a compound that does not have an aromatic ring. The base oil is usually present in an amount of 50% by mass or more, and preferably 80% by mass or more, based on the total amount of conductive grease.

[0029] As thickeners, urea compounds, metal soaps, sodium terephthalate, fluorine, organic bentonite, silica gel, etc., can be used. Examples of metal soaps include lithium soap, calcium soap, sodium soap, and aluminum soap. Among urea compounds, diurea compounds are preferred because they offer excellent high-temperature, high-speed, long-life performance and water resistance. Typically, the thickener may be included in the total amount of conductive grease at 10 to 40% by mass, depending on the desired consistency.

[0030] The method for imparting conductivity to grease is not particularly limited; for example, conductivity can be imparted by adding a conductive substance to the grease. Examples of conductive substances include carbon black, metal nanoparticles, and conductive polymers, with carbon black being preferred. Typically, the conductive substance may be present in an amount of 10 to 20% by mass relative to the total amount of conductive grease. Alternatively, conductivity can also be imparted to grease by adding an ionic liquid.

[0031] Furthermore, various additives such as antioxidants, extreme pressure agents, wear inhibitors, dyes, color stabilizers, thickeners, structural stabilizers, metal deactivators, viscosity index improvers, and rust inhibitors may be added to the conductive grease in appropriate amounts, as long as they do not impair the effects of the present invention. Typically, the additives may be present in an amount of 1 to 20% by mass relative to the total amount of conductive grease.

[0032] One embodiment of the present invention also includes an additive for conductive grease containing a compound having a phenyl ether structure. This additive imparts conductivity to the grease and may be a conductivity enhancer for conductive grease, or an additive for suppressing or preventing electrolytic corrosion. Examples of compounds having a phenyl ether structure include the phenyl ether and its alkylates mentioned above. The additive for conductive grease may be added such that the proportion of the aromatic ring portion of the compound having an aromatic ring to the total amount of conductive grease after addition is 0.1 to 10% by mass. Alternatively, the compound having an aromatic ring may be added so that the blending ratio to the conductive grease is 0.2 to 8% by mass. In one embodiment, the additive for conductive grease may be added such that the proportion of the aromatic ring portion of the compound having an aromatic ring to the total amount of conductive grease after addition is 0.4 to 10% by mass. Alternatively, the compound having an aromatic ring may be added so that the blending ratio to the conductive grease is 0.5 to 15% by mass.

[0033] [2. Conductive Rolling Bearing] A conductive rolling bearing according to one embodiment of the present invention includes the conductive grease described above. The structure of the conductive rolling bearing is not particularly limited and may include, for example, an inner ring, an outer ring, and rolling elements disposed between them. The conductive rolling bearing may also have a cage disposed between the inner ring and the outer ring, and the rolling elements may be held by the cage. The rolling elements may be balls or rollers. Examples of places where the conductive grease is sealed include between the inner ring and the outer ring and the rolling elements, and between the cage and the rolling elements.

[0034] Patent Document 4 shows phenyl ether-based synthetic oils such as monoalkyltriphenyl ether, alkyldiphenyl ether, dialkyldiphenyl ether, pentaphenyl ether, tetraphenyl ether, monoalkyltetraphenyl ether, and dialkyltetraphenyl ether as examples of base oils. In contrast, the present inventors have for the first time discovered that conductive amorphous graphite particles are generated from compounds having aromatic rings contained in conductive grease, and that by adjusting the ratio of the aromatic ring portion to the total amount of conductive grease to 0.1 to 10% by mass, it is possible to suppress or prevent long-term electrolytic corrosion in rolling bearings sealed with conductive grease.

[0035] In other words, the conductive rolling bearing according to one embodiment of the present invention is a bearing that has a long lifespan even in harsh environments because it is sealed with conductive grease. Therefore, the conductive rolling bearing can be used in automotive electrical components and engine accessories such as alternators, electromagnetic clutches for car air conditioners, intermediate pulleys, and electric fan motors, which are used in extremely harsh environments such as high temperature, high speed, high load, and vibration. The conductive rolling bearing is not limited to these, but can also be used in motors for home appliances, fan motors for clean rooms, motors for ventilation fans, motors for water heaters, etc. The amount of conductive grease sealed in the conductive rolling bearing can be the amount known conventionally and can be appropriately changed according to the type and dimensions of the place of use.

[0036] According to one embodiment of the present invention, it is possible to extend the lifespan of bearings, which is expected to lead to resource conservation, waste reduction, and energy conservation. Such effects may contribute to achieving, for example, Goal 12 of the United Nations' Sustainable Development Goals (SDGs), "Ensure sustainable consumption and production patterns."

[0037] The present invention is not limited to the embodiments described above, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present invention.

[0038] One embodiment of the present invention may include the following configurations: <1> A conductive grease containing a compound having an aromatic ring, wherein the compound having an aromatic ring is contained in an amount of 0.2 to 8% by mass relative to the total amount of the conductive grease. <2> The conductive grease according to <1>, wherein the compound having an aromatic ring is contained in an amount of 0.5 to 8% by mass relative to the total amount of the conductive grease. <3> A conductive grease containing a compound having an aromatic ring, wherein the compound having an aromatic ring has a molecular weight ratio of the aromatic ring portion to the total molecular weight of the compound having an aromatic ring of 0.5 or more, and the aromatic ring portion of the compound having an aromatic ring is contained in an amount of 0.1 to 10% by mass relative to the total amount of the conductive grease. <4> The conductive grease according to <3>, wherein the aromatic ring portion of the compound having an aromatic ring is contained in an amount of 0.4 to 10% by mass relative to the total amount of the conductive grease. <5> The conductive grease according to any one of <1> to <4>, wherein the compound having an aromatic ring is at least one selected from the group consisting of phenyl ether and its alkylate, phenyl thioether and its alkylate, biphenyl, aromatic ester, alkylbenzene, and alkylnaphthalene. <6> The conductive grease according to any one of <1> to <5>, wherein the conductive grease contains carbon black. <7> The conductive grease according to any one of <1> to <6>, wherein the compound having an aromatic ring is pentaphenyl ether. <8> The conductive grease according to <1>, wherein the compound having an aromatic ring is at least one selected from the group consisting of diphenyl ether and its alkylate, and the aromatic ring portion of the diphenyl ether and its alkylate is contained in 0.05 to 2.8% by mass relative to the total amount of the conductive grease. <9> A conductive rolling bearing comprising the conductive grease according to any one of <1> to <8>. <10> An additive for conductive grease comprising a compound having a phenyl ether structure.

[0039] One embodiment of the present invention is described below.

[0040] (Reference Experiment) First, pentaphenyl ether (5P4E, S-3105 manufactured by MORESCO), which is a compound having an aromatic ring, or poly-α-olefin (PAO, kinematic viscosity at 100°C of 10 mm 2 / s) was used to confirm decomposition products when electric discharge occurs in a bearing. Specifically, a pulse voltage of 10 kHz, 400 V (0-peak) was applied between electrodes disposed at an interval of about 10 µm from each other in the above compound or synthetic oil, with a 10 kΩ resistor connected in series. The voltage between the electrodes at that time was observed with an oscilloscope. After the discharge experiment, the compound or synthetic oil was filtered to recover a solid product. The product was observed with an optical microscope and analyzed by Raman spectroscopy.

[0041] FIG. 1 shows waveforms of the inter-electrode voltage in each compound or synthetic oil. In the case of 5P4E, when voltage is applied, the inter-electrode voltage fluctuates violently between -150 V and 400 V, whereas in the case of PAO, the voltage fluctuation is remarkably small. When voltage is applied to the electrodes, if no discharge (or current conduction) occurs, the applied voltage becomes the inter-electrode voltage. However, during discharge, the resistance between the electrodes decreases due to current conduction, causing the inter-electrode voltage to drop, and thus the voltage fluctuates. That is, since discharge repeatedly occurs in 5P4E, it is estimated that discharge or current conduction is more likely to occur in 5P4E than in PAO.

[0042] Next, FIG. 2 shows an optical micrograph of the product obtained from 5P4E. A Raman spectrometer was used to capture this micrograph. FIG. 3 shows Raman spectroscopy analysis results of products obtained from each compound or synthetic oil. XploRA manufactured by HORIBA was used as the Raman spectrometer, and laser light with a wavelength of 532 nm was used as excitation light.

[0043] Raman spectroscopic analysis of the product obtained from the compound after the discharge experiment revealed that in 5P4E, black fine particles with a diameter on the submicron order were generated, and the product exhibited a Raman spectrum indicating an amorphous carbon structure. From this result, it is estimated that the decomposition product of 5P4E is carbon particles having a conductive graphite structure. On the other hand, in the case of PAO, although black fine particles are slightly observed, the peak intensity in the Raman spectrum is low, so it is considered that the amount of amorphous carbon generated is extremely small. That is, since 5P4E contains an aromatic ring that serves as a base for the graphite structure, it is presumed that after hydrogen is eliminated from the aromatic ring during discharge, the aromatic rings polymerize as amorphous carbon to generate conductive carbon particles, and these carbon particles form a conductive path between the electrodes, thereby enhancing the conductivity between the electrodes.

[0044] Based on this finding, it was considered that if a compound having an aromatic ring is added to a conductive grease using a conductive substance as a thickener, even if the conductive substance gradually separates from the base oil or aggregates, conductive amorphous carbon is generated by electric discharge, and the conductivity can be maintained for a long time. Therefore, the following experiment was conducted.

[0045] (Example) Conductive grease obtained by adding 5P4E to a commercially available conductive grease (HI-LUBE CG-334 (synthetic oil grease), Harves Co., Ltd.) containing carbon black and using poly-α-olefin (PAO) as a base oil was sealed in a bearing, and the conductivity of the bearing was compared. A deep groove ball bearing (608VV), which is a rolling bearing, was used as the bearing. Conductive greases were prepared by varying the amount of 5P4E to 0, 0.5, 1, 2, 5, 8, and 10% by mass, with the total mass of the conductive grease containing 5P4E being set to 100% by mass. 1 g of each conductive grease was sealed in the bearing. A carbon brush was attached to the inner ring side rotating shaft of the bearing. A voltage of 10 V was applied between the carbon brush and the fixed outer ring side using a stabilized DC power supply. A 4.7Ω current-regulating resistor was connected in series with the carbon brush and the fixed outer ring, and the voltage across the resistor was measured to convert it to the voltage between the inner and outer rings of the bearing. The inner ring rotation speed was set to 2000 rpm, and the fluctuation of the voltage between the inner and outer rings of the bearing (bearing voltage) with respect to rotation time was monitored. As an example, Figure 4 shows the fluctuation of the bearing voltage when the amount of 5P4E is 0, 1, and 8 mass%.

[0046] Figure 4 shows that the behavior of the bearing voltage differs depending on the proportion of 5P4E added. When no 5P4E was added (0 mass%), the bearing voltage was initially 3-4V, but increased over time. In this case, it means that initially a current of about 1.5A flows between the inner and outer rings of the bearing, but as the conductivity deteriorates over time, the current changes from a steady current to a current accompanied by transient discharge. In contrast, when 8 mass% of 5P4E was added, the fluctuation of the bearing voltage was suppressed. Furthermore, when the amount of 5P4E added was 1 mass%, the bearing voltage remained at 3-4V for 500 hours, with almost no change observed.

[0047] Therefore, by measuring the number of times the bearing voltage exceeded 5V within 500 hours and determining the relationship with the amount of 5P4E added, the relationship shown in Figure 5 was obtained. In addition, a similar test was conducted on conductive grease using alkylated diphenyl ether (ADE) instead of 5P4E, with the total mass of the conductive grease containing ADE set to 100% by mass, and varying the ADE blending ratio to 0, 1, 2, 5, and 8% by mass. In this test, the number of times the bearing voltage exceeded 4.5V within 500 hours was measured, and the results are shown in Figure 6. From these results, it was found that adding 0.5 to 10% by mass, preferably 0.5 to 6% by mass, of 5P4E reduces fluctuations in bearing voltage. Furthermore, in the case of ADE, it was found that fluctuations in bearing voltage were reduced within the range of blending ratios used in the test. In particular, adding 1 to 5% by mass of ADE showed good results. This suggests that even if the dispersion state of carbon black changes due to the flow of current through the bearing, and the conductivity due to carbon black alone deteriorates, if an appropriate amount of aromatic ring-containing compounds is added, amorphous carbon particles are generated from the aromatic ring-containing compounds, and current flows through these particles, thus maintaining the conductivity of the bearing. However, it is suspected that if the amount of aromatic ring-containing compounds added increases too much, the viscosity of the base oil in the conductive grease will increase, and the oil film thickness between the rolling elements and the inner and outer rings will become too thick, which may lead to large fluctuations in bearing voltage or a decrease in lubricity.

[0048] The 5P4E used in the examples mainly has the structure shown below, and the ratio of the molecular weight of the aromatic ring portion (382.49) to the total molecular weight (446.49) (molecular weight of aromatic ring portion / total molecular weight) is 0.86.

[0049] Therefore, when the conductive grease contains 0.5 to 10% by mass of 5P4E, the proportion of the aromatic ring portion to the total amount of conductive grease is 0.43 to 8.6% by mass. When the conductive grease contains 0.5 to 8% by mass or 0.5 to 6% by mass of 5P4E, the proportion of the aromatic ring portion to the total amount of grease is 0.43 to 6.88% by mass or 0.43 to 5.16% by mass, respectively.

[0050] The ADE used in the examples mainly has the following structure (R ｍ = [C １２ H ２５ ] ２、 Here, R (alkyl group) in the formula indicates substitution with a hydrogen atom on the benzene ring, and m (m=2) represents the number of substitutions.

[0051] The ADE used in the examples has a ratio of the molecular weight of the aromatic ring portion (154.22) to the total molecular weight (506.94) of 0.30 (molecular weight of aromatic ring portion / total molecular weight). Therefore, when the conductive grease contains 0.5 to 8% by mass of the ADE, the proportion of the aromatic ring portion to the total amount of conductive grease is 0.15 to 2.4% by mass. When the ADE is contained at 1 to 6% by mass, the proportion of the aromatic ring portion to the total amount of grease is 0.3 to 1.8% by mass.

[0052] In ADE, the alkyl group may occupy a large portion of the total molecular mass. As shown in the discharge experiment described above, 5P4E, which has many aromatic rings, has been found to generate many amorphous carbon particles, suggesting that the optimal addition ratio of the aromatic ring compound itself may differ depending on the number of aromatic rings.

[0053] However, in 5P4E (B / A = 0.86) and ADE (B / A = 0.30), where the ratio of the molecular weight of the structure constituting the aromatic ring to the total molecular weight of the compound containing the aromatic ring (B / A) differs significantly, fluctuations in bearing voltage were reduced at a similar blending ratio of the compound containing the aromatic ring to the total amount of grease. From this, it can be concluded that by blending the compound containing the aromatic ring to the total amount of conductive grease at a ratio of 0.2 to 8% by mass, a conductive grease and rolling bearings that are less susceptible to galvanic corrosion over long periods of time can be obtained.

[0054] One aspect of the present invention can be used in conductive grease and rolling bearings, etc.

Claims

1. A conductive grease containing a compound having an aromatic ring, wherein the compound having an aromatic ring is contained in an amount of 0.2 to 8% by mass relative to the total amount of the conductive grease.

2. The conductive grease according to claim 1, wherein the compound having the aromatic ring is contained in an amount of 0.5 to 8% by mass relative to the total amount of the conductive grease.

3. A conductive grease containing a compound having an aromatic ring, wherein the compound having the aromatic ring has a molecular weight ratio of the aromatic ring portion to the total molecular weight of the compound having the aromatic ring of 0.5 or more, and the aromatic ring portion of the compound having the aromatic ring is contained in an amount of 0.1 to 10% by mass relative to the total amount of the conductive grease.

4. The conductive grease according to claim 3, wherein the aromatic ring portion of the compound having the aromatic ring is contained in an amount of 0.4 to 10% by mass relative to the total amount of the conductive grease.

5. The conductive grease according to claim 1 or 3, wherein the compound having an aromatic ring is at least one selected from the group consisting of phenyl ether and its alkylates, phenyl thioether and its alkylates, biphenyl, aromatic esters, alkylbenzenes, and alkylnaphthalenes.

6. The conductive grease according to claim 1 or 3, wherein the conductive grease comprises carbon black.

7. The conductive grease according to claim 1 or 3, wherein the compound having the aromatic ring is pentaphenyl ether.

8. The conductive grease according to claim 1, wherein the compound having an aromatic ring is at least one selected from the group consisting of diphenyl ether and its alkylates, and the aromatic ring portion of the diphenyl ether and its alkylate is contained in an amount of 0.05 to 2.8% by mass relative to the total amount of the conductive grease.

9. A conductive rolling bearing comprising the conductive grease according to claim 1 or 3.

10. An additive for conductive grease containing a compound having a phenyl ether structure.

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