Grease composition for resin lubrication

Abstract

To provide a grease composition for resin lubrication that is capable of suppressing slippage of a lubricating area of a reduction gear of, e.g., a power steering for automobiles at the time when it is not assisted by an electric motor, the slippage of the gear being suppressed by increasing the static friction coefficient of the lubricated area when the gear is not assisted by the motor, while preventing the increase of the dynamic friction coefficient of the lubricating area and maintaining the dynamic friction coefficient at around the same level without being dragged by the increase of the static friction coefficient.SOLUTION: A grease composition for use in lubrication between resin members or between a resin member and a member made of a material other than resin contains, with respect to the total mass of the composition, more than 0.5 mass% and less than 7.5 mass% of a solid organic molybdenum compound.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a grease composition for resin lubrication, and more particularly to a grease composition for resin lubrication suitable for use in lubricating between resin members or between a resin member and a member made of another material, such as a metal member. [Background technology]

[0002] In the past, various metal parts were widely used in automobile parts, but in recent years, resin parts have been increasingly used instead of metal parts in order to reduce weight. Therefore, both resin and metal parts are currently used. For example, resin (polyamide) worm wheel gears and steel worm gears are used in the reduction gears of automobile electric power steering. As a grease composition used for lubrication between these resin members and between resin members and metal members, Patent Document 1 describes a lubricating grease composition containing polyolefin wax. When this grease composition is used in a resin-lubricated part, it can realize low friction and can meet the demand for improving the efficiency of parts having resin-lubricated parts. Patent Document 2 describes a grease composition for resin lubrication, which is characterized by containing montan wax in a grease containing a thickener and a base oil. This grease composition is excellent in that it can reduce the static friction coefficient of the lubricated part and extend the durability life of the lubricated part. However, when these grease compositions are applied to the reducer of an electric power steering system, the static friction coefficient is low, and therefore when the steering wheel is turned slightly while not being assisted by the motor, for example when driving on a highway, the lubricated part of the reducer may slip, causing the steering wheel to go off-center. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 9-194867 [Patent Document 2] JP 2002-371290 A Summary of the Invention [Problem to be solved by the invention]

[0004] Therefore, an object of the present invention is to provide a grease composition for resin lubrication that increases the static friction coefficient of the lubricated parts, suppresses an increase in the kinetic friction coefficient caused by the increased static friction coefficient, and maintains the kinetic friction coefficient at a relatively low level, thereby suppressing slippage in the lubricated parts of a reducer when not assisted by a motor, and maintaining lubrication efficiency when assisted. [Means for solving the problem]

[0005] The present invention relates to a grease composition for resin lubrication, which is characterized by containing a solid organic molybdenum compound. That is, the present invention provides the following grease composition. [1] A grease composition for use in lubricating resin members and resin members or resin members and members made of other materials, The grease composition contains more than 0.5% and less than 7.5% by weight of a solid organo-molybdenum compound, based on the total weight of the composition. [2] The grease composition according to the above [1], wherein the content of the solid organic molybdenum compound is 1 to 5 mass %. [3] The grease composition according to [1] or [2], wherein the thickener is a lithium soap-based thickener. [4] The grease composition according to any one of the above [1] to [3], wherein the base oil contains at least one compound selected from the group consisting of synthetic hydrocarbon oils and mineral oils. [5] The grease composition according to any one of the above [1] to [4], wherein the resin member to be lubricated is a polyamide. [6] The grease composition according to any one of the above [1] to [5], wherein the member made of a material other than resin to be lubricated is a metal member. [7] A reduction gear for an electric power steering, filled with the grease composition according to any one of [1] to [6] above. Effect of the Invention

[0006] The present invention provides a grease composition for resin lubrication that can increase the static friction coefficient of the lubricated parts and maintain a relatively low dynamic friction coefficient, thereby suppressing slippage of the lubricated parts of a reducer when not assisted by a motor and maintaining lubrication efficiency when assisted. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0007] [Thickener] The thickener used in the grease composition of the present invention is not particularly limited. For example, soap-based thickeners such as Li soap and complex Li soap, urea-based thickeners such as diurea, inorganic thickeners such as organo-clay and silica, and organic thickeners such as PTFE are listed. Among these, soap-based thickeners are preferred. Since soap-based thickeners themselves have a low dynamic friction coefficient, the use of soap-based thickeners provides excellent lubrication efficiency. Lithium 12-hydroxystearate, lithium stearate, and complex Li soap (e.g., complex Li soap formed from 12-hydroxystearic acid and azelaic acid) are more preferred, and lithium 12-hydroxystearate and complex Li soap are particularly preferred. The use of lithium 12-hydroxystearate and complex Li soap provides excellent low-temperature operability because the thickener can be thickened with a small amount of thickener. The amount of thickener in the grease composition of the present invention is preferably 3 to 20 mass%, more preferably 4 to 17 mass%, and particularly preferably 5 to 15 mass%, based on the total mass of the composition. By making it 3 mass% or more, a sufficient thickener effect for forming a grease can be exhibited, and leakage from the lubricated parts can be suppressed. By making it 20 mass% or less, the grease composition can have an appropriate hardness for flowing into the lubricated parts.

[0008] [Base oil] The base oil used in the grease composition of the present invention is not particularly limited. For example, mineral oil, ester-based synthetic oils such as diesters and polyol esters, polyalphaolefins ("PAO") and ethylene-alpha-olefin co-oligomers, synthetic hydrocarbon oils such as polybutene, ether-based synthetic oils such as alkyldiphenyl ethers and polypropylene glycols, silicone oils, fluorinated oils, etc. may be used. Two or more of these may be used in combination. However, since the output of an electric power steering is smaller than that of a hydraulic power steering, the operating torque at low temperatures is large, which may cause malfunction. When the grease composition of the present invention is used in the lubricated parts of an electric power steering, such as the reducer part, the operating torque at low temperatures can be reduced by including a synthetic oil as the base oil. In particular, it is preferable to include a synthetic hydrocarbon oil, and it is more preferable to include a poly-alpha-olefin. The proportion of the synthetic oil in the base oil is not particularly limited, but in order to improve low-temperature operability, it is preferable that the PAO is preferably 50 to 100 mass%, more preferably 60 to 100 mass%, and even more preferably 70 to 100 mass%. At this time, the type of the remainder of the base oil does not matter, but from the viewpoint of cost, it is preferable that it is a mineral oil. Therefore, when the grease composition of the present invention is used in the lubricated parts of an electric power steering, such as the reducer part, the proportion of the mineral oil in the base oil is preferably 0 to 50 mass%, more preferably 0 to 40 mass% (for example, 20 to 40 mass%), and even more preferably 0 to 30 mass%. Any mineral oil can be used as the mineral oil. Examples include refined mineral oil, high viscosity index oil, dewaxed mineral oil, etc. From the viewpoint of low temperature operability, dewaxed mineral oil is particularly preferred.

[0009] From the viewpoint of low-temperature operability and durability, the kinetic viscosity of the base oil at 40°C is 20 to 80 mm 2 / s is preferable, and 30 to 70 mm 2 / s is more preferable. The base oil has a kinetic viscosity of 15 to 420 mm at 40°C. 2It is particularly preferable that the PAO having a viscosity of 70 to 100 mass % based on the total amount of the base oil is contained. This makes it possible to obtain a grease composition having particularly excellent low-temperature operability. In this case, the kinematic viscosity of the entire base oil at 40°C is 30 to 70 mm 2 / s is preferred. The content of the base oil in the grease composition of the present invention is an amount normally used for producing grease, for example, 75 to 95 mass%, and from the viewpoint of low-temperature operability, it is preferably 83 to 96 mass%, and more preferably 85 to 95 mass%.

[0010] [Additives] Generally, organomolybdenum compounds used as grease additives are divided into those that are solid and those that are liquid at room temperature and pressure. Because solid organomolybdenum compounds maintain their solid state even when mixed into grease, they can exhibit different performance than liquid organomolybdenum compounds. The solid organomolybdenum compound used in the present invention is not particularly limited as long as it is solid at ambient temperature (for example, 25° C.). The amount of the solid organic molybdenum compound in the composition of the present invention is more than 0.5% by mass and less than 7.5% by mass, preferably 1-5% by mass, more preferably 2-3% by mass. When it exceeds 0.5% by mass, the static friction coefficient can be significantly increased, and slippage of the lubricated member can be suppressed. When it is less than 7.5% by mass, the low-temperature operability and lubrication efficiency of the lubricated member can be maintained good.

[0011] Various additives can be added to the grease composition of the present invention as necessary. Examples of such additives include antioxidants, rust inhibitors, metal corrosion inhibitors, oiliness agents, antiwear agents, extreme pressure agents, solid lubricants, etc. Specifically, antioxidants include amine-based, phenol-based, quinoline-based, sulfur-based, zinc dithiophosphate, etc., with amine-based being preferred. Examples of amine-based include phenyl α-naphthylamine, alkylphenyl α-naphthylamine, alkyldiphenylamine, etc., with alkyldiphenylamine being particularly preferred. Rust inhibitors include zinc-based, carboxylic acid-based, carboxylate-based, amine-based, and sulfonate-based agents, with sulfonate-based agents being particularly preferred, particularly Ca sulfonate.Ca sulfonates include calcium salts of petroleum sulfonic acid obtained by sulfonating aromatic hydrocarbon components in lubricating oil fractions, calcium salts of synthetic sulfonic acids such as dinonylnaphthalene sulfonic acid and alkylbenzene sulfonic acid, overbased calcium salts of petroleum sulfonic acid, and overbased calcium salts of alkyl aromatic sulfonic acids, with overbased Ca sulfonate being particularly preferred. Metal corrosion inhibitors include thiadiazoles, benzimidazoles, and benzotriazoles, with benzotriazoles being preferred. Benzotriazoles include 1,2,3-benzotriazole, 1,H-benzotriazole, 4-methyl-1,H-benzotriazole, 4-carboxyl-1,H-benzotriazole, sodium tolyltriazole, 5-methyl-1,H-benzotriazole, benzotriazole butyl ether, silver benzotriazole, 5-chloro-1,H-benzotriazole, 1-chloro-benzotriazole, 1-di(C8H17)aminomethyl-benzotriazole, 2,3-dihydroxypropyl-benzotriazole, and the like. Examples of such amines include azole, 1,2-dicarboxyethyl-benzotriazole, (C8H17)aminomethyl-benzotriazole, bis(benzotriazol-1-yl-methyl)(C8H17)amine, N,N-bis(2-ethylhexyl)-4-methyl-1H-benzotriazole-1-methylamine, and N,N-bis(2-ethylhexyl)-5-methyl-1H-benzotriazole-1-methylamine, and particularly 1-[N,N-bis(2-ethylhexyl)aminomethyl]-4-methylbenzotriazole is preferred. The oily agent includes fatty acids, fatty acid esters, and phosphoric acid esters. Antiwear agents and extreme pressure agents include phosphorus-based, sulfur-based, and organometallic agents. Solid lubricants include metal oxide salts, molybdenum disulfide, polytetrafluoroethylene, melamine cyanurate, and graphite. The content of these optional additives is, for example, 0.1 to 10 mass %, preferably 0.1 to 5 mass %, and more preferably 0.5 to 3 mass %, based on the total mass of the composition of the present invention.

[0012] The 60-time worked penetration of the grease composition of the present invention is preferably 190 to 415, and more preferably 235 to 370, from the viewpoints of preventing oil separation and leakage and of low-temperature operability. The grease composition of the present invention can be easily produced by mixing the above-mentioned components and other additives in the desired mixing ratio according to a conventional method. The grease composition of the present invention is used for lubrication between resin members, or between a resin member and a member made of another material, for example, a metal member. Specifically, it can be suitably used in rolling bearings, ball screws, reducers of electric power steering devices, support yokes, etc. The resin constituting the resin member is not particularly limited, but is preferably polyamide from the viewpoint of strength and hardness. EXAMPLES

[0013] <Test grease> [Test grease compositions in which the thickener is lithium 12-hydroxystearate] 570.0 g of 12-hydroxystearic acid was completely dissolved in 2352.3 g of base oil at 90° C. In a separate container, 77.7 g of lithium hydroxide was completely dissolved in 388.5 g of pure water at 90° C., and the two were mixed and heated to 220° C. Then, the mixture was cooled to 100° C. or less while stirring, and this was used as the base grease (the total amount of base grease was 3000 g, and the amount of thickener was 19%). A solid organic molybdenum compound was blended with the above base grease in the ratio shown in Table 1, and additional base oil was added so as to obtain the thickener amount shown in Table 1. The mixture was dispersed using a three-roll mill to prepare a test grease composition. The consistency of the test grease composition was 325.

[0014] Test grease compositions in which the thickener is lithium stearate 957.2g of stearic acid and 151.3g of lithium hydroxide monohydrate were added to 1891.5g of base oil and stirred, and then heated to 230°C. The mixture was then cooled to 100°C or less while stirring to obtain a base grease. A solid organic molybdenum compound was blended with the base grease in the ratio shown in Table 1, and additional base oil was added so that the amount of thickener was the ratio shown in Table 1. The mixture was dispersed using a three-roll mill to prepare a test grease composition. The consistency of the test grease composition was 325.

[0015] [Test grease compositions in which the thickener is a lithium complex soap] 445.9g of 12-hydroxystearic acid was added to 1776.0g of base oil, and heated to a temperature (80-90°C) where it became a completely transparent liquid. 64.8g of lithium hydroxide monohydrate was added to 363.5g of water and dissolved by heating, and the mixture was added to the mixture, and the saponification reaction of 12-hydroxystearic acid was carried out while stirring vigorously to form the lithium salt of 12-hydroxystearic acid. Next, 507.7g of base oil and 139.4g of azelaic acid were added, and the mixture was stirred until it became homogeneous. 64.8g of lithium hydroxide monohydrate was added to 363.5g of water and dissolved by heating, and the saponification reaction of azelaic acid was carried out while stirring vigorously. Next, the mixture entered the heating process, and the contents were gradually heated to 200°C. After that, the mixture was cooled to 100°C or less while stirring to obtain the base grease. A solid organic molybdenum compound was blended with the above base grease in the ratio shown in Table 1, and additional base oil was added so as to obtain the thickener amount shown in Table 1. The mixture was dispersed using a three-roll mill to prepare a test grease composition. The consistency of the test grease composition was 325.

[0016] The ingredients used to prepare the test grease compositions were as follows: <Base oil> PAO: Polyalphaolefin, Dynamic viscosity at 40℃: 56mm 2 / s Mineral oil: naphthenic mineral oil, kinematic viscosity at 40°C: 115mm 2 / s The kinematic viscosity was measured in accordance with JIS K 2220 23. <Additives> Solid organic Mo: MoDTC (product name: Molyvan A RT Vanderbilt) Polyolefin wax: Polyethylene wax (product name: LICOWAX PE190 P, manufactured by Clariant Chemicals Co., Ltd.) Montan wax: Montan acid partially saponified ester wax (product name: LICOWAX OP FL, manufactured by Clariant Chemicals Co., Ltd.)

[0017] The mass % shown in Table 1 is a value based on the total mass of the grease composition, with the remainder being the base oil. The worked penetration of the test grease composition means the 60-stroke worked penetration, which was measured in accordance with JIS K 2220 7.

[0018] <Test Method> The grease compositions of the examples and comparative examples were subjected to the tests described below, and their properties were evaluated by taking the arithmetic mean of the friction coefficients after stabilization. The results are shown in Table 1. Static and dynamic friction coefficients (Bauden test) [Test conditions] Test piece: Resin pin (diameter 5.0 mm, length 15 mm, nylon GF 30% blend) Plate (2.0 x 15 x 160 mm, surface roughness Ra = 0.2 μm, S45C) Sliding speed: 1mm / s Stroke: 10mm Load: 19.6N Temperature: 25℃ Amount of grease applied: 0.2 mm thick

[0019] Criterion: Coefficient of static friction 0.155 or more = ○ (pass), less than 0.155 = × (fail) Criteria: Coefficient of dynamic friction 0.150 or less = ○ (pass), over 0.150 = × (fail)

[0020]

Table 1

Claims

1. A grease composition for use in lubricating resin members and resin members or resin members and members made of other materials, comprising: The grease composition contains more than 0.5% and less than 7.5% by weight of a solid organo-molybdenum compound, based on the total weight of the composition.

2. 2. The grease composition according to claim 1, wherein the content of the solid organic molybdenum compound is 1 to 5 mass %.

3. 3. The grease composition according to claim 1, wherein the thickener is a lithium soap-based thickener.

4. The grease composition according to any one of claims 1 to 3, wherein the base oil contains at least one compound selected from the group consisting of synthetic hydrocarbon oils and mineral oils.

5. The grease composition according to any one of claims 1 to 4, wherein the resin member to be lubricated is a polyamide.

6. The grease composition according to any one of claims 1 to 5, wherein the member made of a material other than resin to be lubricated is a metal member.

7. A reduction gear for electric power steering, filled with the grease composition according to any one of claims 1 to 6.