Grease composition

Abstract

To provide a grease composition having excellent acoustic property and water resistance.SOLUTION: The grease composition contains a base oil, a thickening agent, a metal detergent, and a zinc fatty acid salt, and the volume-based median particle diameter of the solid particles contained in the grease composition is 0.060 μm or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to a grease composition. [Background technology]

[0002] Grease compositions that focus on water resistance have been proposed. For example, Patent Document 1 proposes "a grease composition containing a lubricating base oil, at least one compound selected from the group consisting of lithium soaps and lithium complex soaps as a thickener, and a specified fatty acid zinc salt as a metal detergent."

[0003] Grease compositions that focus on acoustic performance have been proposed. For example, Patent Document 2 proposes "a grease composition comprising a synthetic ester oil as a base oil, a Li soap-based thickener, and calcium sulfonate, wherein the calcium sulfonate has a calcium content of 0.2 to 20 wt %, and the calcium sulfonate is blended in at a ratio of 0.5 to 10 parts by weight per 100 parts by weight of the total amount of the base oil and thickener." [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2023-026889 [Patent Document 2] Japanese Patent Application Laid-Open No. 2003-147378 Summary of the Invention [Problem to be solved by the invention]

[0005] Since the objects to which grease compositions are used, such as steelmaking machinery, construction machinery, and automobiles, are sometimes used in environments exposed to water, water resistance is one of the important properties required of grease compositions. On the other hand, in recent years, the miniaturization of bearings has progressed along with the miniaturization of devices in which grease compositions are used, and it is expected that the rotational speed of bearings will increase in order to maintain high output. For this reason, further improvement in acoustic properties (specifically, quietness) is also desired for grease compositions from the viewpoint of suppressing noise generation.

[0006] An object of one embodiment of the present disclosure is to provide a grease composition that has excellent acoustic properties and water resistance. [Means for solving the problem]

[0007] The grease composition according to the present disclosure includes the following embodiments.

[0008] <1> A grease composition comprising a base oil, a thickener, a metal detergent, and a fatty acid zinc salt, wherein the arithmetic mean particle size of solid particles contained in the grease composition is a median size of 0.060 μm or less, based on volume. <2> The thickener contains at least one selected from lithium soap and lithium complex soap. <1> The grease composition according to claim 1. <3> The metal detergent comprises an overbased calcium sulfonate. <1> or <2> The grease composition according to claim 1. <4> The fatty acid zinc salt contains a fatty acid zinc salt represented by the following formula (1): <1> ~ <3> 1. The grease composition according to claim 1 .

[0009] [ka]

[0010] In formula (1), R1 and R2 each independently represent an aliphatic hydrocarbon group having 8 to 18 carbon atoms.

[0011] <5> The fatty acid zinc salt represented by formula (1) includes a fatty acid zinc salt in which at least one of R1 and R2 is an isostearyl group. <4> The grease composition according to claim 1. [Effects of the Invention]

[0012] According to one embodiment of the present disclosure, a grease composition having excellent acoustic properties and water resistance can be provided. DETAILED DESCRIPTION OF THE INVENTION

[0013] Specific embodiments of the grease composition according to the present disclosure will be described in detail below, however, the grease composition according to the present disclosure is not limited to the following embodiments and can be implemented with appropriate modifications within the scope of the object of the present disclosure.

[0014] In the present disclosure, a numerical range indicated using "to" means a range that includes the numerical values ​​before and after "to" as the minimum and maximum values, respectively. In the present disclosure, the upper or lower limit of a numerical range described in stages may be replaced with the upper or lower limit of another numerical range described in stages. Furthermore, in the present disclosure, the upper or lower limit of a numerical range described in stages may be replaced with a value shown in the examples. In the present disclosure, a combination of two or more preferred embodiments is a more preferred embodiment. In the present disclosure, "mass %" and "weight %" are synonymous. In the present disclosure, when there are multiple substances corresponding to each component, the amount of each component means the total amount of the multiple substances unless otherwise specified. In this disclosure, "JIS" is used as an abbreviation for Japanese Industrial Standards.

[0015] [Grease composition] The grease composition according to the present disclosure is a grease composition containing a base oil, a thickener, a metal detergent, and a fatty acid zinc salt, and the arithmetic mean particle size (hereinafter simply referred to as "particle size") of the solid particles contained in the grease composition on a volume basis is a median size of 0.060 μm or less. By having the above configuration, the grease composition according to the present disclosure has excellent acoustic properties and water resistance.

[0016] The reason why the grease composition according to the present disclosure has excellent acoustic properties and water resistance is presumed to be as follows. That is, the fact that the particle size of the solid particles in the grease composition according to the present disclosure is controlled to a median size of 0.060 μm or less is correlated with the grease composition being homogenized by a homogenization treatment (e.g., roller treatment), and it is presumed that the homogenization treatment and the inclusion of a metal detergent and aliphatic zinc cause changes in the orientation and structure of the thickener, resulting in the formation of an appropriate structure in the grease composition that has excellent acoustic properties and water resistance. However, the above presumption is not intended to limit the grease composition according to the present disclosure, but is merely an example.

[0017] On the other hand, Patent Documents 1 and 2 contain a base oil, a thickener, a metal detergent, and a fatty acid zinc salt in a grease composition, and do not focus on setting the median particle size of the volume-based particle size of the solid particles contained in the grease composition within a predetermined range.

[0018] The grease composition according to the present disclosure will be described in detail below.

[0019] <Solid particle diameter (median diameter)> The solid particles contained in the grease composition according to the present disclosure have a median particle size (ie, 50% particle size) on a volume basis of 0.060 μm or less.

[0020] In the present disclosure, the solid particles contained in the grease composition include all solid particles that have a particulate shape among all components contained in the grease composition, including thickeners such as lithium soaps and lithium complex soaps, solid additives, aggregates of various components, and foreign matter such as dust.

[0021] The median particle size of the solid particles contained in the grease composition according to the present disclosure is 0.060 μm or less, and from the viewpoint of acoustic properties, it is preferably 0.057 μm or less, and more preferably 0.055 μm or less. The lower limit of the median diameter is, for example, 0.007 μm.

[0022] The particle size of the solid particles contained in the grease composition according to the present disclosure is the arithmetic mean particle size on a volume basis, and is measured by a laser diffraction / scattering method in accordance with JIS Z 8825:2013 (corresponding international standard: ISO13320). Specifically, a measurement sample is prepared from the grease composition to be measured. Next, the prepared sample is subjected to measurement using a laser diffraction particle size distribution analyzer to measure the volume distribution of the solid particles contained in the sample. As the laser diffraction particle size distribution measuring device, SALD-7500nano manufactured by Shimadzu Corporation can be used. The details of the measurement method will be explained in the examples below. Based on the obtained measurement values ​​(volume distribution), the median diameter (50% particle diameter) of the solid particles contained in the sample can be determined.

[0023] The median diameter of the solid particles can be controlled, for example, by subjecting the grease composition to a roller treatment, which shifts the particle distribution (volume distribution) of the solid particles contained in the grease composition toward smaller diameters, thereby improving the acoustic performance of the grease composition.

[0024] The roller treatment can be carried out by kneading the grease composition using, for example, a roll mill. Examples of the roll mill include hydraulic and non-hydraulic three-roll mills. Details of the device and treatment conditions used for the roller treatment will be explained in the examples below.

[0025] <Base oil> The grease composition according to the present disclosure contains a base oil. The base oil is not particularly limited and may be, for example, a mineral oil, a synthetic oil, or a mixture thereof. The base oil may be a single type or a combination of two or more types.

[0026] Examples of synthetic oils include ether-based synthetic oils such as alkyldiphenyl ether, ester-based synthetic oils such as diesters, polyol esters, and complex-type polyol esters, synthetic hydrocarbon oils such as polyalphaolefins, and alkylnaphthalene-based synthetic oils.

[0027] Examples of mineral oils include those obtained by refining lubricating oil fractions of crude oil using an appropriate combination of refining methods such as solvent refining, hydrorefining, hydrocracking refining, hydrodewaxing, etc. Examples of mineral oils include paraffinic mineral oils that are highly refined by subjecting hydrorefined oils, catalytic isomerized oils, etc. to treatments such as solvent dewaxing or hydrodewaxing.

[0028] The base oil may contain mineral oil from the viewpoint of balancing the desired performance with raw material costs.

[0029] The kinematic viscosity of the base oil at 100°C is 3.000mm 2 / s~30.00mm 2 / s is preferred, 3.000mm 2 / s~25.00mm 2 / s is more preferable, 5.000mm 2 / s~20.00mm 2 / s is even more preferable. When the base oil is a mixture of two or more base oils, the kinematic viscosity of the mixture is within the above range. The kinematic viscosity of the base oil at 100°C is the value measured in accordance with JIS K 2283:2000 "Kinematic Viscosity Testing Method."

[0030] The content of the base oil is not particularly limited, and can be adjusted appropriately within the range of, for example, 65% by mass to 96.5% by mass relative to the total mass of the grease composition.

[0031] <Thickener> The grease composition according to the present disclosure contains a thickener. There are no limitations on the thickener as long as it is applicable to grease compositions, and it can be appropriately selected taking into consideration the desired physical properties of the grease composition, the environment in which the grease composition will be used, etc. Examples of thickeners include lithium soap, lithium complex soap, calcium soap, calcium complex soap, and diurea compounds.

[0032] The grease composition according to the present disclosure preferably contains at least one compound selected from the group consisting of lithium soaps and lithium complex soaps.

[0033] Lithium soap is a lithium salt of a higher fatty acid. Lithium complex soap is a complex soap that combines a lithium salt of a higher fatty acid and a lithium salt of a dicarboxylic acid.

[0034] From the viewpoint of heat resistance, the lithium soap is preferably a lithium salt of a monocarboxylic acid having 12 to 24 carbon atoms. From the viewpoint of heat resistance, the lithium complex soap is preferably a complex of a lithium salt of a monocarboxylic acid having 12 to 24 carbon atoms and a lithium salt of a dicarboxylic acid having 2 to 12 carbon atoms. Suitable monocarboxylic acids include stearic acid, 12-hydroxystearic acid, beef tallow, hydrogenated castor oil, etc. Suitable dicarboxylic acids include succinic acid, malonic acid, adipic acid, pimelic acid, azelaic acid, sebacic acid, etc.

[0035] As the thickener, lithium soap is more preferred in terms of availability and ease of handling.

[0036] The grease composition according to the present disclosure may contain only one type of thickener, or may contain two or more types of thickener.

[0037] In the grease composition according to the present disclosure, the content of the thickener is preferably 5% by mass to 20% by mass, more preferably 7% by mass to 18% by mass, and even more preferably 8% by mass to 15% by mass, relative to the total amount of the grease composition, from the viewpoint of acoustic properties.

[0038] <Metal cleaning agents> The grease composition according to the present disclosure contains a metal detergent. The metal detergent is preferably a basic metal detergent, such as alkali metal sulfonate, alkali metal phenate, alkali metal salicylate, alkali metal phosphonate, alkali metal carboxylate, alkaline earth metal sulfonate, alkaline earth metal phenate, alkaline earth metal salicylate, alkaline earth metal phosphonate, or alkaline earth metal carboxylate.

[0039] As the metal detergent, from the viewpoint of water resistance, alkaline earth metal sulfonates are preferred, and overbased calcium sulfonates are more preferred.

[0040] The grease composition according to the present disclosure may contain only one type of metal detergent, or may contain two or more types.

[0041] In the grease composition according to the present disclosure, the content of the metal detergent is preferably 0.05% by mass to 0.60% by mass, more preferably 0.1% by mass to 0.55% by mass, and even more preferably 0.15% by mass to 0.50% by mass, relative to the total amount of the grease composition, from the viewpoint of water resistance.

[0042] <Zinc fatty acid> The grease composition according to the present disclosure contains a fatty acid zinc salt. The fatty acid zinc salt preferably contains a fatty acid zinc salt represented by the following formula (1) (hereinafter also referred to as "specific fatty acid zinc salt").

[0043] [ka]

[0044] In formula (1), R1 and R2 each independently represent an aliphatic hydrocarbon group having 8 to 18 carbon atoms.

[0045] The aliphatic hydrocarbon group having 8 to 18 carbon atoms represented by R1 or R2 is a saturated or unsaturated aliphatic hydrocarbon group having 8 to 18 carbon atoms, and may be any of a straight-chain, branched-chain, and cyclic aliphatic hydrocarbon group. The aliphatic hydrocarbon group having 8 to 18 carbon atoms represented by R1 or R2 is preferably a branched-chain aliphatic hydrocarbon group having 8 to 18 carbon atoms. The branched-chain aliphatic hydrocarbon group may be a saturated group or an unsaturated group, but is preferably a saturated group.

[0046] The aliphatic hydrocarbon group having 8 to 18 carbon atoms represented by R1 or R2 is preferably a saturated branched aliphatic hydrocarbon group having 8 to 18 carbon atoms, and examples thereof include a 2-ethylhexyl group, a neodecyl group, and an isostearyl group.

[0047] One preferred embodiment of the specific fatty acid zinc is a fatty acid zinc in which at least one of R1 and R2 represents an isostearyl group (hereinafter also referred to as specific fatty acid zinc A). When at least one of R1 and R2 is an isostearyl group, only one of R1 and R2 may be an isostearyl group, or both R1 and R2 may be isostearyl groups.

[0048] Another preferred embodiment of the specific fatty acid zinc is a fatty acid zinc in which at least one of R1 and R2 represents a 2-ethylhexyl group (hereinafter also referred to as specific fatty acid zinc B). When at least one of R1 and R2 is a 2-ethylhexyl group, only one of R1 and R2 may be a 2-ethylhexyl group, or both R1 and R2 may be 2-ethylhexyl groups. Specific fatty acid zinc B may also be a fatty acid zinc corresponding to specific fatty acid zinc A (i.e., a fatty acid zinc in which one of R1 and R2 is an isostearyl group and the other is a 2-ethylhexyl group). Specific fatty acid zinc B may also be a fatty acid zinc having a different structure from specific fatty acid zinc A (i.e., a fatty acid zinc in which one of R1 and R2 is a 2-ethylhexyl group and the other is a branched-chain aliphatic hydrocarbon group other than an isostearyl group).

[0049] Another preferred embodiment of the specific fatty acid zinc is a fatty acid zinc in which at least one of R1 and R2 represents a neodecyl group (hereinafter also referred to as specific fatty acid zinc C). When at least one of R1 and R2 is a neodecyl group, only one of R1 and R2 may be a neodecyl group, or both R1 and R2 may be neodecyl groups. The specific fatty acid zinc C may be a fatty acid zinc corresponding to the specific fatty acid zinc A (i.e., a fatty acid zinc in which one of R1 and R2 is an isostearyl group and the other is a neodecyl group). The specific fatty acid zinc C may be a fatty acid zinc corresponding to the specific fatty acid zinc B (i.e., a fatty acid zinc in which one of R1 and R2 is a neodecyl group and the other is a 2-ethylhexyl group). The specific fatty acid zinc C may be a fatty acid zinc having a structure different from that of the specific fatty acid zinc A and the specific fatty acid zinc B (i.e., a fatty acid zinc in which one of R1 and R2 is a neodecyl group and the other is a branched-chain aliphatic hydrocarbon group other than an isostearyl group or a 2-ethylhexyl group).

[0050] When the specific fatty acid zinc salt contains two or more selected from specific fatty acid zinc salt A, specific fatty acid zinc salt B, and specific fatty acid zinc salt C, it is preferable that the specific fatty acid zinc salt corresponds to a mixture of two or more selected from zinc isostearate, zinc 2-ethylhexanoate, and zinc neodecanoate.

[0051] From the viewpoint of water resistance, the fatty acid zinc compound contained in the grease composition according to the present disclosure is preferably a specific fatty acid zinc compound A, that is, a fatty acid zinc compound in which at least one of R1 and R2 in formula (1) represents an isostearyl group.

[0052] The grease composition according to the present disclosure may contain only one type of fatty acid zinc salt, or may contain two or more types.

[0053] From the viewpoint of water resistance, the content of fatty acid zinc is preferably 0.5 to 4.0 mass %, more preferably 0.5 to 3.0 mass %, and even more preferably 0.5 to 2.5 mass %, relative to the total mass of the grease composition.

[0054] <Other additives> The grease composition according to the present disclosure may contain other additives. Other additives include dispersants such as alkenyl succinimide, boronated alkenyl succinimide, benzylamine, and alkylpolyamine; various anti-wear agents such as zinc dialkyldithiophosphate, phosphorus-based, sulfur-based, amine-based, ester-based, and rapeseed oil-based; various viscosity index improvers such as polymethacrylates, ethylene propylene copolymers, styrene-isoprene copolymers, hydrogenated styrene-isoprene copolymers, and polymers such as polyisobutylene; alkylphenols such as 2,6-di-tert-butyl-p-cresol; bisphenols such as 4,4'-methylenebis-(2,6-di-t-butylphenol); n-octadecyl-3-(4'-hydroxy- Various antioxidants, such as phenolic compounds such as 3',5'-di-tert-butylphenol) propionate, and aromatic amine compounds such as naphthylamines and dialkyldiphenylamines; extreme pressure agents, such as sulfurized olefins, sulfurized oils and fats, methyl trichlorostearate, chlorinated naphthalene, iodinated benzyl, fluoroalkylpolysiloxane, and polytetrafluoroethylene; various rust inhibitors, such as carboxylic acids, dicarboxylic acids, metal soaps, amine salts of carboxylic acids, metal salts of heavy sulfonic acids, partial esters of carboxylic acids of polyhydric alcohols, and phosphate esters; various corrosion inhibitors, such as benzotriazole, substituted triazoles, and benzimidazole; and various antifoaming agents, such as silicone oil. The other additives may be used singly or in combination of two or more.

[0055] ~Properties of grease composition~ <Worked penetration> The worked penetration of the grease composition according to the present disclosure is preferably 220 or more, more preferably 220-295, and even more preferably 240-280. The worked penetration shall be measured in accordance with JIS K2220:2013.

[0056] ~Acoustic characteristics of grease compositions~ The acoustic properties of the grease composition according to the present disclosure are evaluated by the Anderon value. The anderon value is evaluated by measuring the anderon value in the high band (1,800 to 10,000 Hz) using an anderon meter. In the present disclosure, the specific measurement method and conditions for measuring the anderon value are those described in the Examples below. The grease composition according to the present disclosure is evaluated as having excellent acoustic properties when the Anderon value measured under the above-mentioned measurement conditions and by the above-mentioned measurement method is 1.0 or less.

[0057] ~Water resistance of grease composition~ The water resistance of the grease composition according to the present disclosure is evaluated by water washing resistance (79°C) (based on JIS K 2220:2013).

[0058] The grease composition according to the present disclosure is evaluated as having excellent water resistance when the value of water resistance after washing (79°C) (i.e., the amount of loss of the sample) is 7% by mass or less, and 6% by mass or less is more preferable.

[0059] ~Method for producing grease composition~ The method for producing the grease composition according to the present disclosure is not particularly limited, but it is preferred that the grease composition be produced, for example, by appropriately mixing other additives as needed in addition to the base oil, thickener, metal detergent, and fatty acid zinc salt to prepare a grease composition, and then subjecting the prepared grease composition to a roller treatment.

[0060] ~Target of use of grease composition~ The grease composition according to the present disclosure can be applied to bearings, etc. Because the grease composition according to the present disclosure has excellent acoustic properties and water resistance, it can be suitably applied to bearings provided in, for example, steelmaking machinery, construction machinery, automobiles, industrial machinery, home appliances, and other general-purpose equipment. [Example]

[0061] The grease composition according to the present disclosure will be specifically described below using examples, but the grease composition according to the present disclosure is not limited to these examples.

[0062] [Example 1, Comparative Examples 1 to 5] 1. Preparation of Grease Composition Each of the grease compositions of the Examples and Comparative Examples was prepared by mixing the components constituting the grease composition so as to obtain the composition shown in the composition column of Table 1.

[0063] The thickeners, lithium soaps A and B, were obtained by mixing a thickener base material with a base oil and reacting the thickener base material in the base oil, as described in detail below. The grease composition was prepared so as to contain the respective components in predetermined amounts, and then subjected to a milling process to uniformly disperse the thickener in the grease composition to obtain the final composition.

[0064] Details of each component contained in the grease compositions of the Examples and Comparative Examples are as follows:

[0065] <Base oil> ·Base oil A: Mineral oil (100℃ kinematic viscosity: 10.93mm 2 / s) ·Base oil B: Mineral oil (100℃ kinematic viscosity: 10.61mm 2 / s) ·Base oil C: Mineral oil (100℃ kinematic viscosity: 8.798mm 2 / s) ·Base oil D: Mineral oil (100℃ kinematic viscosity: 30.68mm 2 / s)

[0066] <Thickener> Lithium soap A A lithium soap synthesized by mixing a thickener base material with a base oil and reacting the thickener base material in the base oil using the following manufacturing procedure.

[0067] ~Manufacturing procedure (lithium soap)~ Each base oil listed in the table was placed in a heat-resistant container, and beef tallow was added as a thickener base material and heated. Next, an aqueous solution of lithium hydroxide was added at around 90°C, and lithium soap was produced through a saponification reaction. This was further heated and cooled with the base oil, and then milled to optimize the lithium soap crystals and uniformly mix and disperse them in the base oil.

[0068] Lithium soap B A lithium soap was obtained in the same manner as lithium soap A, except that 12-hydroxystearate was used as the thickener base instead of beef tallow.

[0069] <Metal cleaning agents> Overbased calcium sulfonate (LUBRIZOL 6477C, manufactured by Lubrizol Corporation)

[0070] <Zinc fatty acid> Fatty acid zinc salt A (DICNATE 705S-S, manufactured by DIC Corporation, fatty acid zinc salt containing the compound represented by formula (1))

[0071] 2. Roller processing The grease compositions prepared in Example 1 and Comparative Examples 1 to 3 were subjected to roller treatment under the following roller conditions: The grease composition prepared in Comparative Example 4 was not subjected to roller treatment.

[0072] =Roller condition= The input pressure of the three-roll mill was adjusted to about 1 MPa and the output pressure to 0.1 MPa, and the grease composition was roll-treated twice at these flow rates.

[0073] [Measurement and Evaluation] The resulting grease compositions were subjected to the following measurements and evaluations.

[0074] (1) Median diameter of solid particles The median diameter (50% particle diameter) of the solid particles contained in the grease composition of each example was measured by laser diffraction. The laser diffraction particle size distribution measuring device used was a nanoparticle size distribution measuring device (product name: SALD-7500nano) manufactured by Shimadzu Corporation. First, about 0.10 g of the grease composition of each example was taken and applied with a spatula to the center of the surface of a high-concentration cell provided in the measuring device. Next, the arithmetic mean particle size on a volume basis of the solid particles in each grease composition was measured using a laser diffraction particle size distribution measuring device. From the obtained distribution, the median diameter (50% particle diameter) and particle diameter were calculated.

[0075] (2) Acoustic characteristics: Anderon value measurement The acoustic properties of the grease composition of each example were evaluated by measuring the Anderon value in the high band (1,800 to 10,000 Hz) using an Anderon meter. Specifically, each grease composition was filled into a ball bearing (inner diameter 8 mm, outer diameter 22 mm, width 7 mm) to a volume of approximately 35% of the bearing volume. The inner ring of this ball bearing was set on the rotating shaft of a test motor, and a preload of 20 N was applied to the outer ring in the axial direction, after which the ball bearing was allowed to rotate. At room temperature (approximately 25°C), a velocity pickup was placed in radial contact with the outer periphery of the outer ring of the ball bearing, and the ball bearing was rotated at a rotational speed of 1,800 rpm (revolutions per minute) for 5 minutes. After that, the mechanical vibration transmitted to the outer ring was detected while the preload, temperature conditions, and rotational speed remained the same, and the Anderon value was calculated. -Evaluation criteria- When the calculated Anderon value was 1.0 or less, the grease composition was evaluated as having excellent acoustic properties.

[0076] (3) Water resistance: Washing water resistance (79℃) For each grease composition, the water wash resistance (79°C) was measured based on JIS K2220:2013. The measurement was carried out twice for each grease composition, and the results (sample The average value of the loss amount (mass%) of the water-washing resistance was evaluated according to the following criteria. -Evaluation criteria- When the loss amount of the sample was 7% by mass or less, the grease composition was evaluated as having excellent water resistance.

[0077] The results are shown in Table 1.

[0078] [Table 1]

[0079] As shown in Table 1, it can be seen that the grease composition of Example 1 is superior to the grease compositions of Comparative Examples 1 to 4 in acoustic properties and water resistance.

Claims

1. A grease composition comprising a base oil, a thickener, a metal detergent, and a fatty acid zinc salt, wherein the grease composition contains solid particles whose arithmetic mean particle size on a volume basis is a median size of 0.060 μm or less.

2. 2. The grease composition according to claim 1, wherein the thickener comprises at least one selected from the group consisting of lithium soaps and lithium complex soaps.

3. 10. The grease composition of claim 1, wherein the metal detergent comprises an overbased metal detergent.

4. 2. The grease composition according to claim 1, wherein the fatty acid zinc salt comprises a fatty acid zinc salt represented by the following formula (1): 【Chemical 1】 In formula (1), R １ and R ２ each independently represents an aliphatic hydrocarbon group having 8 to 18 carbon atoms.

5. The fatty acid zinc salt represented by the formula (1) is R １ and R ２ 5. The grease composition according to claim 4, wherein at least one of the fatty acid zinc salts is an isostearyl group.

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