GREASE COMPOSITION

Abstract

A grease composition includes a base oil, a thickener, and an additive. The base oil is at least one selected from the group consisting of polyoxyalkylene, ether derivatives of polyoxyalkylene, and mixtures thereof. The additive includes polytetrafluoroethylene as a first solid lubricant and at least one selected from the group consisting of calcium carbonate, calcium oxide, tricalcium phosphate, and calcium salts of fatty acids as a second solid lubricant. The content of the second solid lubricant is 0.5% by mass or more based on the total mass of the composition.

Claims

1. A grease composition comprising a base oil, a thickener, and an additive, wherein the base oil is at least one selected from the group consisting of polyoxyalkylene, ether derivatives of polyoxyalkylene, and mixtures thereof, the additive comprises polytetrafluoroethylene as a first solid lubricant and at least one selected from the group consisting of calcium carbonate, calcium oxide, tricalcium phosphate, and calcium salts of fatty acids as a second solid lubricant, and a content of the second solid lubricant is 0.5% by mass or more based on a total mass of the composition.

2. The grease composition according to claim 1, wherein a content of the first solid lubricant is 0.5 to 20% by mass based on the total mass of the composition.

3. The grease composition according to claim 1, wherein a content of the second solid lubricant is 0.5 to 10% by mass based on the total mass of the composition.

4. The grease composition according to claim 1, wherein the second solid lubricant is calcium carbonate.

5. The grease composition according to claim 1, wherein the first and second solid lubricants are contained in a ratio of 3 to 5% by mass of the second solid lubricant to 10% by mass of the first solid lubricant.

6. A mechanical part to which the grease composition according to claim 1 is applied.

Description

DESCRIPTION OF EMBODIMENTS

<Base Oil>

[0023] A base oil used in a grease composition of the present invention is polyoxyalkylene and/or an ether derivative of polyoxyalkylene. The polyoxyalkylene and/or the ether derivative of polyoxyalkylene have little adverse effect on rubber, which is used as a sealing material. The polyoxyalkylene and/or the ether derivative of polyoxyalkylene is expressed by Formula (1) below.

##STR00001##

[0024] The polyoxyalkylene or its ether derivative is a compound in which R.sub.1 and R.sub.3 in Formula (1) are each independently hydrogen or an alkyl group having 1 to 6 carbon atoms, such as a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, an isobutyl group, a pentyl group, or a hexyl group, R.sub.2 is hydrogen or an alkyl group having 1 to 2 carbon atoms, and n is a number of 5 to 55.

[0025] The polyoxyalkylene is a diol obtained by ring-opening polymerization of alkylene oxides such as ethylene oxides and propylene oxides. Its ether derivative is a monoether in which one of R.sub.1 and R.sub.3 is an alkyl group having 1 or more carbon atoms, or a diether in which both of R.sub.1 and R.sub.3 are alkyl groups having 1 or more carbon atoms.

[0026] Specific examples of polyoxyalkylene diols include polyoxyethylene, polyoxypropylene, poly(oxypropyleneoxyethylene), poly(oxybutyleneoxyethylene), poly(oxybutyleneoxypropylene), poly(oxypentyleneoxyethylene), and poly(oxypentyleneoxypropylene).

[0027] Specific examples of ether derivatives of polyoxyalkylene include polyoxypropylene monopropyl ether, polyoxypropylene monobutyl ether, polyoxybutylene monobutyl ether, polyoxyethyleneoxypropylene monopropyl ether, polyoxyethyleneoxypropylene monobutyl ether, and polyoxyethyleneoxypropylene monopentyl ether.

[0028] Among them, polyoxyethylene, poly(oxypropyleneoxyethylene), and ether derivatives thereof are water-soluble. Therefore, greases using them as base oils have poor water resistance. For this reason, a suitable base oil for the present invention is polyoxyalkylene or an ether derivative thereof in which R.sub.2 is an alkyl group having one or more carbon atoms, preferably polyoxypropylene monobutyl ether, particularly preferably polyoxypropylene monobutyl ether in which n is 10 to 25, and even more particularly preferably polyoxypropylene monobutyl ether in which n is 10 to 22.

[0029] The base oil of the present invention may be a so-called biomass oil, which is produced from biological resources derived from an animal, a plant, or the like.

[0030] The base oil of the present invention preferably has a kinetic viscosity at 100 C. of 2 to 100 mm.sup.2/s. This provides excellent low temperature properties. The kinematic viscosity at 100 C. is preferably 2 to 50 mm.sup.2/s, more preferably 2 to 20 mm.sup.2/s, and further preferably 6 to 19 mm.sup.2/s.

[0031] The base oil of the present invention preferably has a pour point of 10 C. or lower. This provides excellent low temperature properties. The pour point is more preferably 20 C. or lower, further preferably 30 C. or lower, and particularly preferably 35 C. or lower.

[0032] As the base oil of the present invention, polyoxypropylene monobutyl ether is most preferred in which a kinematic viscosity at 100 C. is 6 to 19 mm.sup.2/s, a pour point is 35 C. or lower, and n in Formula (1) is 10 to 22.

[0033] The content of the base oil in the grease composition of the present invention is, for example, preferably 60 to 90% by mass and more preferably 60 to 80% by mass.

<Solid Lubricant>

[0034] The first solid lubricant in the present invention is polytetrafluoroethylene.

[0035] The content of the first solid lubricant is preferably 0.5% by mass or more based on the total mass of the composition. This provides excellent spalling preventive properties. The content of the first solid lubricant is more preferably 1% by mass or more. From the viewpoint of the inflow ability of the grease, the upper limit is preferably 20% by mass or less and more preferably 15% by mass or less.

[0036] The second solid lubricant in the present invention is at least one selected from the group consisting of calcium carbonate, calcium oxide, tricalcium phosphate, and calcium salts of fatty acids. Among them, calcium carbonate is preferred from the viewpoint of spalling preventive properties. As a fatty acid constituting the calcium salts of fatty acids, a fatty acid having 1 to 22 carbon atoms is preferred and a fatty acid having 16 to 20 carbon atoms is more preferred.

[0037] From the viewpoint of the effect of improving the spalling preventive properties when the second solid lubricant is used in combination with the first solid lubricant, the second solid lubricant preferably has a larger particle size than that of PTFE, which is the first solid lubricant.

[0038] The content of the second solid lubricant is 0.5% by mass or more based on the total mass of the composition. This provides excellent spalling preventive properties. The content of the second solid lubricant does not have to be equal to the content of the first solid lubricant. The content of the second solid lubricant is more preferably 1% by mass or more. From the viewpoint of the inflow ability of the grease, the upper limit is preferably 10% by mass or less, and more preferably 5% by mass or less.

[0039] Regardless of a type of the second solid lubricant, the spalling preventive properties of the grease composition are particularly excellent when the second solid lubricant is contained in an amount of 1 to 3 parts by mass per 10 parts by mass of PTFE. Therefore, from the viewpoint of spalling preventive properties, it is particularly preferable that calcium carbonate, which has a larger particle size than that of PTFE, be contained in the ratio of 1 to 3 parts by mass to 10 parts by mass of PTFE.

[0040] The total mass of the first solid lubricant and the second solid lubricant in the grease composition of the present invention is 1 to 20% by mass preferably and 5 to 15% by mass more preferably. The total mass of the first solid lubricant and the second solid lubricant within the above range is preferred because the inflow ability of the grease has little influence on the performance.

[0041] Since both the first and second solid lubricants in the present invention are non-polar, the first and second solid lubricants, even when contained in polyoxyalkylene and/or an ether derivative of polyoxyalkylene, can improve the spalling preventive properties without being affected by the base oil.

[0042] Without wishing to be bound to any theory, it is believed that the existence of polytetrafluoroethylene, which has a smaller particle size than that of the second solid lubricant, in a lubrication field not only provides excellent spalling preventive properties, but also enables the second solid lubricant to be supplied stably to the lubrication field, thereby significantly improving the spalling preventive properties.

<Thickener>

[0043] As a thickener for the grease of the present invention, any thickener may be used without particular limitation. Specifically, these include soap-based thickeners represented by Li soap and Li complex soap, urea-based thickeners represented by diurea, inorganic thickeners represented by organo-bentonite and silica, and organic thickeners represented by sodium terephthalate.

[0044] Among them, a Li soap and a diurea compound are preferred. These are practical thickeners because they have few defects and are inexpensive.

[0045] The Li soap is preferably lithium 12-hydroxystearate (Li-(12OH)St) or lithium stearate (Li-St). These have excellent lubricity.

[0046] The Li complex soap is a complex of a lithium salt of aliphatic carboxylic acid such as stearic acid or 12-hydroxystearic acid with a lithium salt of dibasic acid, or the like. As the dibasic acid, there are a succinic acid, a malonic acid, an adipic acid, a pimelic acid, an azelaic acid, a sebacic acid, and the like. The azelaic acid and sebacic acid are preferred. In particular, a Li complex soap which is a mixture of a salt of azelaic acid and lithium hydroxide and a salt of 12-hydroxystearic acid and lithium hydroxide is preferred.

[0047] The diurea compound is generally expressed by Formula (2) below.

R.sub.4NHCONHR.sub.5NHCONHR.sub.6(2),

where R.sub.4 and R.sub.6 may be the same or different and each represent a C6-30 alkyl group, a C5-8 cycloalkyl group, or a C6-10 aryl group, and R.sub.5 represents a C6-15 divalent aromatic hydrocarbon group.

[0048] The diurea compound is preferably aliphatic diurea in which R.sub.4 and R.sub.6 are C6-30 alkyl groups which may be the same or different, alicyclic aliphatic diurea in which one of R.sub.4 and R.sub.6 is a C5-8 cycloalkyl group and the other is a C6-30 alkyl group, or aromatic diurea in which R.sub.4 and R.sub.6 are C6-10 aryl groups which may be the same or different.

[0049] As the aliphatic diurea, aliphatic diurea in which both R.sub.4 and R.sub.6 are C8 alkyl groups, aliphatic diurea in which both R.sub.4 and R.sub.6 are C18 alkyl groups, and aliphatic diurea in which one of R.sub.4 and R.sub.6 is a C8 alkyl group and the other is a C18 alkyl group are more preferred. Aliphatic diurea in which one of R.sub.4 and R.sub.6 is a C8 alkyl group and the other is a C18 alkyl group is particularly preferred. Aliphatic diurea in which the ratio of the number of moles of C8 alkyl groups to the total number of moles of C8 alkyl groups and C18 alkyl groups is 30 to 70 mol % is even more particularly preferred.

[0050] As the alicyclic aliphatic diurea, alicyclic aliphatic diurea in which one of R.sub.4 and R.sub.6 is a cyclohexyl group and the other is a C18 alkyl group is more preferred. Alicyclic aliphatic diurea in which the ratio of the number of moles of cyclohexyl groups to the total number of moles of cyclohexyl groups and C18 alkyl groups is 30 to 90 mol % is particularly preferred.

[0051] As the aromatic diurea, aromatic diurea in which both R.sub.4 and R.sub.6 are p-toluyl groups is particularly preferred.

[0052] The content of the thickener in the grease composition of the present invention is, for example, preferably 4 to 25% by mass and more preferably 5 to 20% by mass. The content of the thickener within the above range is preferred because the grease has appropriate hardness and is prevented from leaking from lubrication sites.

<Other Additives>

[0053] The grease composition of the present invention may contain, as needed, any additives that are generally used in grease compositions. Examples of the additives include antioxidants, rust inhibitors, corrosion inhibitors, oiliness agents, viscosity index improvers, and so on. It is preferable that the grease composition contain an antioxidant and/or a rust inhibitor. On the other hand, it is preferable that the grease composition be free of any reactive additive (i.e., an additive that reacts on lubricated surfaces to produce a component to degrade the base oil, such as molybdenum disulfide, amine phosphate, zinc dialkyldithiophosphate, or molybdenum dialkyldithiocarbamate).

[0054] The antioxidants include amine-based, phenol-based, quinoline-based, and sulfur-based antioxidants, with the amine-based and quinoline-based antioxidants being preferred.

[0055] The rust inhibitors include zinc-based, carboxylic acid-based, carboxylate-based, succinic acid-based, amine-based, sulfonate-based, and naphthenic acid-based rust inhibitors. Amine-based and naphthenic acid-based rust inhibitors are preferred. A mixture of these is more preferred.

[0056] The corrosion inhibitors include thiadiazole-based, benzimidazole-based, and benzotriazole-based corrosion inhibitors.

[0057] The oiliness agents include fatty acids, fatty acid esters, and phosphate esters.

[0058] When the grease composition of the present invention contains other additives, the content thereof is usually 0.5 to 10% by mass and preferably 0.5 to 5% by mass based on the total mass of the grease composition.

[Consistency]

[0059] The consistency of the grease composition in the present invention is adjusted according to a use purpose, and is preferably 235 to 370. It is possible to obtain a grease composition having good low temperature properties with the consistency set at 235 or higher, and to obtain a grease composition having excellent adhesion to mechanical parts with the consistency set at 370 or lower. In the present specification, the term consistency refers to a 60-stoke worked penetration. The consistency may be measured in accordance with JIS K2220 7.

[0060] The grease composition of the present invention may be used for any purpose. In other words, the grease composition may be applied to any types of mechanical parts. Examples thereof include rolling bearings, ball screws, linear guide bearings, reducers, injection molding machines, linear guides, machine tools, various gears, cams, constant velocity joints, journal bearings (sliding bearings), pistons, screws, ropes, chains, and so on. Among them, reducers, ball screws, and the like are required to achieve strict levels of heat resistance and spalling preventive properties, but the grease composition of the present invention can meet such high requirements.

[0061] A type of a sealing member included in a mechanical part is not particularly limited, and examples thereof include NBR, EPDM, natural rubber, and so on.

EXAMPLES

[0062] Grease compositions in Examples and Comparative Examples were prepared by using the components specified below.

<Base Oil>

[0063] PPG: Polyoxypropylene monobutyl ether (product name UNILUBE MB-7, manufactured by NOF Corporation, the number of moles of propylene oxide added of 12, the average molecular weight of 700, the kinematic viscosity at 40 C.: 32.8 mm.sup.2/s, the kinematic viscosity at 100 C.: 6.7 mm.sup.2/s, and the pour point: 47.5 C.)

<Thickener>

[0064] Lithium soap: Lithium 12-hydroxystearate [0065] Aliphatic diurea: Reaction product of diphenylmethane diisocyanate with octylamine and stearylamine (a molar ratio of octylamine to stearylamine is 5:5) [0066] Alicyclic aliphatic diurea: Reaction product of diphenylmethane diisocyanate with cyclohexylamine and stearylamine (a molar ratio of cyclohexene to stearylamine is 7:1) [0067] Aromatic diurea: Reaction product of diphenylmethane diisocyanate with p-toluidine

<First and Second Solid Lubricants>

[0068] PTFE: Polytetrafluoroethylene (solid) [0069] Calcium carbonate (solid) Calcium oxide (solid) [0070] Tricalcium phosphate (solid) [0071] Calcium stearate (solid) [0072] MoDTC: Molybdenum dithiocarbamate (liquid) [0073] MoS.sub.2: Molybdenum disulfide (solid) [0074] ZnDTP: Zinc dithiophosphate (liquid) [0075] Amine phosphate (liquid)

[0076] Note that MoDTC, MoS.sub.2, and ZnDTP are reactive anti-spalling additives for comparison.

<Other Additives>

[0077] Antioxidant: 2,2,4-trimethyl-1,2-dihydroquinoline polymer [0078] Rust Inhibitor

Test Grease

Preparation Example 1: Test Grease Compositions Each Containing a Diurea Compound as a Thickener

[0079] In the base oil, 4.4-diphenylmethane diisocyanate and a specified amine in the ratio of 1 mole to 2 moles were reacted with each other and then cooled to make a base grease.

[0080] The additives in the ratio specified in Table 1 were mixed with the above base grease, and the additional base oil was added so that the amount of the thickener became the ratio specified in Table 1, followed by dispersion by a three-roll mill to prepare each test grease composition. The consistency of the test grease compositions was 280.

Preparation Example 2: Test Grease Compositions Containing a Lithium Soup as a Thickener

[0081] In the base oil, lithium 12-hydroxystearate was added and stirred, and then was heated to 230 C. After that, the resultant mixture was cooled to 100 C. or lower with stirring to obtain a base grease.

[0082] The additives in the ratio specified in Tables 1 and 2 were mixed with the above base grease, and the additional base oil was added so that the amount of the thickener became the ratio specified in Tables 1 and 2, followed by dispersion by the three-roll mill to prepare each test grease composition. The consistency of the test grease compositions was 280.

[0083] A percent by mass of each component in the test grease compositions is as specified in Tables 1 and 2.

[0084] The kinematic viscosity at 100 C. of the base oil was measured in accordance with JIS K2220 23. The pour point of the base oil was measured in accordance with JIS K2269. The consistency of the grease composition was measured in accordance with JIS K2220 7.

[0085] The grease compositions thus obtained were tested and evaluated in the following methods.

<Test Method>

Evaluation of Heat Resistance by High-Temperature Thin Film Test

[0086] Each grease was applied to a steel plate specified below and left to stand in a thermostatic chamber at specified temperature for a specified period of time. After that, gel permeation chromatography analysis was performed to check whether decomposition of the base oil occurred.

[Test Conditions]

[0087] Steel plate: SPCC-SD 80 mm60 mm1 mm [0088] Temperature: 120 C. [0089] Time: 1152 h [0090] Coating thickness: 2 mm [0091] GPC measurement solvent: Chloroform [0092] GPC detector: RI detector

[Evaluation Criteria]

[0093] No decomposition of base oil . . . (Pass) [0094] Decomposition of base oil . . . x (Fail)

Evaluation of Spalling Preventive Properties by Four-Ball Rolling Test

[0095] Three steel bearing balls with a diameter of 15 mm were prepared and placed in a cylindrical container with an inner diameter of 40 mm and a height of 14 mm, and the container was filled with approximately 20 g of each test grease. When a inch steel bearing ball was placed on top of these three steel balls and rotated at a predetermined rotation speed, the lower three steel balls revolved while rotating around their own axes. This was rotated continuously until spalling occurred on the surfaces of the steel balls. [0096] Spalling occurred between the bolls having the highest surface pressure. [0097] The service life was measured as the number of contacts of the upper steel ball with the lower steel balls when the spalling occurred. The spalling preventive properties were evaluated based on the service life.

[Test Conditions]

[0098] Test steel balls: inch, Ra 0.45 m bearing steel ball (rotating ball) [0099] 15 mm bearing steel balls (driven balls) [0100] Test load: 400 kgf (6.5 GPa) [0101] Rotation speed: 1200 rpm [0102] Evaluation criteria: 150,000 contacts or more . . . (Pass) [0103] 100,000 or more to less than 150,000 contacts . . . (Pass) [0104] 50,000 or more to less than 100,000 contacts . . . (Fail) [0105] Less than 50,000 contacts . . . x (Fail) [0106] Tables 1 and 2 show the results.

TABLE-US-00001 TABLE 1 Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 7 Ex. 8 Ex. 9 Ex. 10 Ex. 11 Ex. 12 Ex. 13 Ex. 14 Ex. 15 Base oil PPG 78.5 77.0 69.0 78.5 77.0 69.0 78.5 77.0 69.0 78.5 77.0 69.0 77.0 77.0 65.5 Thick- Lithium soap 8.5 8.5 8.5 8.5 8.5 8.5 8.5 8.5 8.5 8.5 8.5 8.5 ener Aliphatic diurea 8.5 Alicyclic aliphatic 8.5 diurea Aromatic diurea 20 Addi- PTFE 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 tives Calcium 0.5 2.0 10 2.0 2.0 2.0 carbonate Calcium oxide 0.5 2.0 10 Tricalcium 0.5 2.0 10 phosphate Calcium stearate 0.5 2.0 10 Other Antioxidant 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 Addi- Rust inhibitor 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 tives Worked consistency 280 280 280 280 280 280 280 280 280 280 280 280 280 280 280 Heat resistance Spalling Preventive Properties

TABLE-US-00002 TABLE 2 Comp. Comp. Comp. Comp. Comp. Comp. Comp. Comp. Comp. Comp. Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 7 Ex. 8 Ex. 9 Ex. 10 Base oil PPG 79.0 78.9 83.5 83.5 83.5 83.5 77.0 77.0 77.0 77.0 Thickener Lithium soap 8.5 8.5 12 12 12 12 8.5 8.5 8.5 8.5 Additives PTFE 10 10 10 10 10 10 Calcium carbonate 0.1 2.0 Calcium oxide 2.0 Tricalcium phosphate 2.0 Calcium stearate 2.0 MoDTC 2.0 MoS.sub.2 2.0 ZnDTP 2.0 Amine phosphate 2.0 Other Antioxidant 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 Additives Rust inhibitor 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 Worked consistency 280 280 280 280 280 280 280 280 280 280 Heat resistance X X X X Spalling Preventive Properties X X

[0107] In the spalling preventive properties, Examples 1 to 15 in each of which a combination of polytetrafluoroethylene as the first solid lubricant and at least one selected from calcium carbonate, calcium oxide, tricalcium phosphate, and calcium salts of fatty acids as the second solid lubricant was used as the additive are superior to Comparative Examples 1 to 6, 9, and 10. In the heat resistance, Examples 1 to 15 are superior to Comparative Examples 7 to 10. In general, the heat resistance of a grease varies depending on a type of a thickener. However, the improvement of the heat resistance in Examples were observed for both the cases using the lithium soap and the urea thickeners. Therefore, the combination use of the first solid additive and the second solid additive specified in the present application as an additive makes it possible to improve the spalling preventive properties and the heat resistance of a grease even under high temperature or high surface pressures without using a reactive anti-spalling additive irrespective of a type of a thickener.