JP7686812B2 - Grease composition for resin lubrication - Google Patents

Description

The present invention relates to a grease composition for resin lubrication. In particular, the present invention relates to a grease composition for resin lubrication that is suitable for use in lubricating between resin components, or between a resin component and a component made of another material, such as a metal component.

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 electric power steering of automobiles.
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.

Japanese Patent Application Publication No. 9-194867 JP 2002-371290 A

The object of the present invention is therefore to provide a grease composition for resin lubrication that increases the static friction coefficient of the lubricated parts, suppresses the increase in the dynamic friction coefficient caused by the increased static friction coefficient, and maintains the dynamic friction coefficient at a relatively low level, thereby suppressing slippage in the lubricated parts of the reducer when not assisted by the motor, and maintaining lubrication efficiency when assisted.

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 a resin member with another resin member or a resin member with a member made of another material,
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 [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 [1] to [3] above, 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 [1] to [4], wherein the resin member to be lubricated is a polyamide.
[6] The grease composition according to any one of [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 [1] to [6] above.

The present invention provides a grease composition for resin lubrication that increases the static friction coefficient of the lubricated parts and maintains a relatively low dynamic friction coefficient, thereby suppressing slippage in the lubricated parts of the reducer when not assisted by a motor and maintaining lubrication efficiency when assisted.

[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.

[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, poly-α-olefins ("PAO") and co-oligomers of ethylene and α-olefins, synthetic hydrocarbon oils such as polybutene, ether-based synthetic oils such as alkyl diphenyl 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 to include PAO in an amount of 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 to use 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.

From the viewpoint of low-temperature operability and durability, the kinetic viscosity of the base oil at 40° C. is preferably 20 to 80 mm ² /s, and more preferably 30 to 70 mm ² /s.
It is particularly preferred that the base oil contains 70 to 100 mass % of PAO having a kinetic viscosity of 15 to 420 ^mm2 /s at 40°C based on the total amount of the base oil. This provides a grease composition that is particularly excellent in low temperature operability. In this case, it is preferred that the kinetic viscosity of the entire base oil at 40°C is 30 to 70 ^mm2 /s.
The content of the base oil in the grease composition of the present invention is an amount normally used for producing a 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 %.

[Additives]
Generally, organomolybdenum compounds used as grease additives are classified into two types: solid and liquid at room temperature and pressure. Solid organomolybdenum compounds maintain their solid state even when mixed into grease, and therefore 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 mass% and less than 7.5 mass%, preferably 1 to 5 mass%, more preferably 2 to 3 mass%. When it exceeds 0.5 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 mass%, the low-temperature operability and lubrication efficiency of the lubricated member can be maintained good.

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.
The rust inhibitor may be zinc-based, carboxylic acid-based, carboxylate-based, amine-based, or sulfonate-based, and sulfonate-based, particularly Ca sulfonate, is preferred.Ca sulfonate may be calcium salt of petroleum sulfonic acid obtained by sulfonating aromatic hydrocarbon components in lubricating oil fraction, calcium salt of synthetic sulfonic acid such as dinonylnaphthalene sulfonic acid or alkylbenzene sulfonic acid, overbased calcium salt of petroleum sulfonic acid, or overbased calcium salt of alkyl aromatic sulfonic acid, and particularly overbased Ca sulfonate is 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. 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, N,N-bis(2-ethylhexyl)-5-methyl-1H-benzotriazole-1-methylamine, and the like are included, with 1-[N,N-bis(2-ethylhexyl)aminomethyl]-4-methylbenzotriazole being particularly 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.

The 60-time worked penetration of the grease composition of the present invention is preferably 190 to 415, 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.

<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., and then cooled to 100° C. or less while stirring, to prepare a base grease (total amount of base grease: 3000 g, amount of thickener: 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.

Test grease compositions in which the thickener is lithium stearate
957.2 g of stearic acid and 151.3 g of lithium hydroxide monohydrate were added to 1891.5 g 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.

[Test grease compositions in which the thickener is a lithium complex soap]
445.9 g of 12-hydroxystearic acid was added to 1776.0 g of base oil, and heated to a temperature (80-90°C) at which it became a completely transparent liquid state. 64.8 g of lithium hydroxide monohydrate was added to 363.5 g 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 vigorously stirring to form the lithium salt of 12-hydroxystearic acid. Next, 507.7 g of base oil and 139.4 g of azelaic acid were added, and the mixture was continued to be stirred until it became homogeneous. 64.8 g of lithium hydroxide monohydrate was added to 363.5 g of water and dissolved by heating, and the saponification reaction of azelaic acid was carried out while vigorously stirring. Next, the mixture entered a 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 a 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.

The ingredients used to prepare the test grease compositions were as follows:
<Base oil>
PAO: poly-alpha olefin, kinetic viscosity at 40° C.: 56 mm ² /s
Mineral oil: naphthenic mineral oil, kinematic viscosity at 40°C: 115 ^mm2 /s
The kinematic viscosity was measured in accordance with JIS K 2220 23.
<Additives>
・Solid organic Mo: MoDTC (product name: Molyvan A R.T. 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.)

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, and was measured in accordance with JIS K 2220 7.

<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: 1 mm/s
Stroke: 10mm
Load: 19.6N
Temperature: 25℃
Amount of grease applied: 0.2 mm thick

Criteria: Coefficient of static friction
0.155 or more = ○ (pass), less than 0.155 = × (fail)
Criteria: Dynamic friction coefficient
0.150 or less = ○ (pass), over 0.150 = × (fail)

Claims (6)

A reducer for an electric power steering filled with a grease composition for use in lubricating resin members and resin members or resin members and members made of other materials,
The reducer, wherein the grease composition contains more than 0.5 mass % and less than 7.5 mass % of a solid organic molybdenum compound, based on the total mass of the composition. 2. The reducer according to claim 1, wherein the content of the solid organic molybdenum compound is 1 to 5 mass %. 3. The reducer according to claim 1, wherein the thickener is a lithium soap-based thickener. 4. The reducer according to claim 1, wherein the base oil contains at least one compound selected from the group consisting of synthetic hydrocarbon oils and mineral oils. The reducer according to any one of claims 1 to 4, wherein the resin member to be lubricated is polyamide. 6. The reducer according to claim 1, wherein the member made of a material other than resin to be lubricated is a metal member.