Grease composition

Abstract

The present invention relates to a grease composition that contains a base oil, a thickener, and additives, the base oil being at least one type of base oil selected from the group that consists of polyoxyalkylenes, ether derivatives of polyoxyalkylenes, and mixtures of polyoxyalkylenes and ether derivatives of polyoxyalkylenes, and the additives including a first solid lubricant that is a polytetrafluoroethylene and a second solid lubricant that is at least one type of solid lubricant selected from the group that consists of calcium carbonate, calcium oxide, tricalcium phosphate, and calcium salts of fatty acids, the second solid lubricant content being at least 0.5 mass% of the total mass of the composition.

Description

Grease composition

[0001] The present invention relates to a grease composition that can be suitably used for machine parts that require resistance to peeling, such as reducers and ball screws.

[0002] In recent years, mechanical components have been required to be smaller and have higher output in order to reduce their weight. Therefore, greases used in lubricating parts of such mechanical components are also required to be able to withstand more severe operating environments than before, such as higher speeds, higher surface pressures, and an expanded temperature range. In particular, extending the life of greases used in components under high temperatures and high surface pressures is an important technical challenge. Longer grease life can be achieved by suppressing peeling of mechanical components. It has been known that the inclusion of reactive anti-peeling additives, such as amine phosphates and zinc dialkyldithiophosphates, in greases can extend the peeling life of greases (Patent Document 1). Reactive anti-peeling additives function by reacting with metal surfaces to form a protective film. Meanwhile, nitrile rubber (NBR) is widely used as a sealing material for mechanical components due to its oil resistance, abrasion resistance, heat resistance, processability, and low cost. However, sealing materials can deteriorate due to low winter temperatures and heat damage in tropical climates. Deterioration of sealing materials can also occur when the base oil contained in the grease that comes into contact with the sealing material causes the sealing material to swell. Therefore, long-term use of mechanical parts equipped with sealing materials may allow foreign matter to enter from the outside, causing poor lubrication and shortening the life of the mechanical parts. To address this issue, approaches have been taken from two perspectives: the selection of the sealing material and the selection of the grease base oil. Specifically, the use of ethylene propylene rubber (EPDM), which has superior heat resistance, cold resistance, weather resistance, and water resistance to NBR, as a sealing material can extend the life of mechanical parts. For mechanical parts in which EPDM or natural rubber is used as a sealing material for lubricated parts or peripheral components, swelling of the EPDM or natural rubber can be suppressed by using a polyoxyalkylene or a polyoxyalkylene derivative as the grease base oil (Patent Document 2).

[0003] Patent No. 6268642 Patent No. 2960561

[0004] Under these circumstances, there is a need for a grease composition containing a polyoxyalkylene or polyoxyalkylene derivative as a base oil that has excellent rubber compatibility and can be used in mechanical parts that use EPDM or natural rubber sealing members, thereby suppressing the occurrence of peeling of mechanical parts even at high temperatures or high surface pressures, thereby extending the life of the mechanical parts. By incorporating a reactive anti-exfoliation additive into a grease based on a polyoxyalkylene or polyoxyalkylene derivative, swelling of EPDM or natural rubber and peeling of mechanical parts can be suppressed. However, when this grease is used at high temperatures, there is a problem in that the acid components generated when the reactive anti-exfoliation additive forms a reaction film on the metal surface accelerates decomposition of the polyoxyalkylene or polyoxyalkylene derivative. Furthermore, reactive anti-exfoliation additives effectively suppress peeling in non-polar base oils such as polyalphaolefins. However, when a polar compound such as polyoxyalkylene or a polyoxyalkylene derivative is used, it disperses in the base oil and is less likely to adsorb to the lubrication field, making it difficult to achieve anti-flaking effects. Using a solid lubricant such as polytetrafluoroethylene (PTFE) or melamine cyanurate, i.e., a non-reactive anti-flaking additive, instead of a reactive anti-flaking additive can suppress base oil decomposition. Non-reactive anti-flaking additives (solid lubricants) function by adhering to metal surfaces and preventing metal-to-metal contact, but their anti-flaking properties are significantly inferior to those of reactive anti-flaking additives, making the resulting greases less versatile. In light of this technological trend, the problem that the present invention aims to solve is to improve the anti-flaking properties of grease compositions containing polyoxyalkylene or a polyoxyalkylene derivative as a base oil, even at high temperatures or under high surface pressure, without using a reactive anti-flaking additive, and to suppress thermal degradation of the base oil.

[0005] The present inventors have solved the problem of improving anti-flaking properties by using a non-reactive solid lubricant in combination. That is, the present invention provides the following grease compositions: 1. A grease composition containing a base oil, a thickener, and an additive, wherein the base oil is at least one selected from the group consisting of polyoxyalkylenes, ether derivatives of polyoxyalkylenes, and mixtures thereof, and 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 being 0.5% by mass or more based on the total mass of the composition. 2. The grease composition according to item 1 above, wherein the 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 item 1 or 2 above, wherein the content of the second solid lubricant is 0.5 to 10% by mass based on the total mass of the composition. 4. 4. The grease composition according to any one of the above items 1 to 3, wherein the second solid lubricant is calcium carbonate. 5. The grease composition according to any one of the above items 1 to 4, comprising the first and second solid lubricants in a ratio of 1 to 3 mass% of the second solid lubricant to 10 mass% of the first solid lubricant. 6. A machine part to which the grease composition according to any one of the above items 1 to 5 is applied.

[0006] According to the present invention, the anti-flaking properties and heat resistance of a grease composition can be improved even at high temperatures or under high surface pressures, without using a reactive anti-flaking additive, and without promoting the decomposition of polyoxyalkylene and / or ether derivatives of polyoxyalkylene.

[0007] <Base Oil> The base oil used in the grease composition of the present invention is a polyoxyalkylene and / or a polyoxyalkylene ether derivative. The polyoxyalkylene and / or the polyoxyalkylene ether derivative has little adverse effect on the rubber used as a sealing material. The polyoxyalkylene and / or the polyoxyalkylene ether derivative is represented by the following formula (1):

[0008]

[0009] Polyoxyalkylene or its ether derivative is a compound in which R1 and R3 in formula (1) are each independently hydrogen or an alkyl group having 1 to 6 carbon atoms, such as methyl, ethyl, propyl, isopropyl, butyl, isobutyl, pentyl, or hexyl; R2 is hydrogen or an alkyl group having 1 to 2 carbon atoms; and n is a number between 5 and 55. Polyoxyalkylene is a diol obtained by ring-opening polymerization of an alkylene oxide, such as ethylene oxide or propylene oxide. The ether derivative is either a monoether in which either R1 or R3 is an alkyl group having 1 or more carbon atoms, or a diether in which both R1 and R3 are alkyl groups having 1 or more carbon atoms.

[0010] Specific examples of polyoxyalkylene diols include polyoxyethylene, polyoxypropylene, poly(oxypropyleneoxyethylene), poly(oxybutyleneoxyethylene), poly(oxybutyleneoxypropylene), poly(oxypentyleneoxyethylene), and poly(oxypentyleneoxypropylene). Specific examples of polyoxyalkylene ether derivatives include polyoxypropylene monopropyl ether, polyoxypropylene monobutyl ether, polyoxybutylene monobutyl ether, polyoxyethyleneoxypropylene monopropyl ether, polyoxyethyleneoxypropylene monobutyl ether, and polyoxyethyleneoxypropylene monopentyl ether. Of these, polyoxyethylene, poly(oxypropyleneoxyethylene), and their ether derivatives are water-soluble, so greases using these as base oils have poor water resistance. For this reason, the base oil of the present invention is preferably a polyoxyalkylene or an ether derivative thereof in which R2 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. The base oil of the present invention may also be a so-called biomass oil produced using biological resources derived from animals and plants as raw materials.

[0011] The base oil of the present invention has a kinematic viscosity at 100°C of 2 to 100 mm 2 This provides excellent low temperature properties. The kinematic viscosity at 100°C is preferably 2 to 50 mm 2 / s is more preferable, and 2 to 20 mm 2 / s is more preferable, and 6 to 19 mm 2 It is particularly preferred that the base oil of the present invention has a pour point of -10°C or lower. This results in excellent low-temperature properties. The pour point is more preferably -20°C or lower, even more preferably -30°C or lower, and particularly preferably -35°C or lower. The base oil of the present invention has a kinematic viscosity at 100°C of 6 to 19 mm 2 Most preferred is a polyoxypropylene monobutyl ether in which, in formula (1), n ​​is 10 to 22, and which has a viscosity of 1 / s and a pour point of −35° C. or lower. The content of the base oil in the grease composition of the present invention is, for example, preferably 60 to 90 mass %, more preferably 60 to 80 mass %.

[0012] <Solid Lubricant> The first solid lubricant in the present invention is polytetrafluoroethylene. 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 peeling resistance. It is more preferably 1% by mass or more. From the viewpoint of grease inflow, the upper limit is preferably 20% by mass or less, more preferably 15% by mass or less.

[0013] 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. Of these, calcium carbonate is preferred from the viewpoint of peeling resistance. As the fatty acid constituting the calcium salt of fatty acid, a fatty acid having 1 to 22 carbon atoms is preferred, and a fatty acid having 16 to 20 carbon atoms is more preferred. From the viewpoint of the effect of improving peeling resistance when used in combination, the second solid lubricant preferably has a particle size larger than that of the PTFE, which is the first solid lubricant.

[0014] The content of the second solid lubricant is 0.5% by mass or more, based on the total mass of the composition. This results in excellent peeling resistance. It does not need to be the same as the content of the first solid lubricant. It is more preferably 1% by mass or more. From the viewpoint of grease inflowability, the upper limit is preferably 10% by mass or less, more preferably 5% by mass or less. Regardless of the type of second solid lubricant, the grease composition has particularly excellent peeling resistance 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 peeling resistance, it is particularly preferable to contain calcium carbonate, which has a particle size larger than that of PTFE, in a ratio of 1 to 3 parts by mass per 10 parts by mass of PTFE.

[0015] The total amount of the first solid lubricant and the second solid lubricant in the grease composition of the present invention is preferably 1 to 20 mass %, more preferably 5 to 15 mass %. When the total amount of the first solid lubricant and the second solid lubricant is in this range, it is preferable because the influence of the inflowability of the grease on the performance is small.

[0016] Since the first and second solid lubricants of the present invention have no polarity, even when they are contained in polyoxyalkylene and / or polyoxyalkylene ether derivatives, they can improve the anti-flaking property without being affected by the base oil. Without being bound by any theory, it is believed that the presence of polytetrafluoroethylene, which has a smaller particle size than the second solid lubricant, in the lubrication field provides excellent anti-flaking property while also enabling the second solid lubricant to be stably supplied to the lubrication field, thereby significantly improving the anti-flaking property.

[0017] <Thickener> The thickener for the grease of the present invention can be any thickener without particular limitations. Specific examples include soap-based thickeners such as Li soap and Li complex soap, urea-based thickeners such as diurea, inorganic thickeners such as organo-bentonite and silica, and organic thickeners such as sodium terephthalate. Of these, Li soap and diurea compounds are preferred because they have few drawbacks and are inexpensive, making them practical thickeners.

[0018] As the Li soap, lithium 12-hydroxystearate (Li-(12OH)St) or lithium stearate (Li-St) is preferred. These have excellent lubricity. Examples of Li complex soaps include complexes of lithium salts of dibasic acids with lithium salts of aliphatic carboxylic acids such as stearic acid and 12-hydroxystearic acid. Examples of dibasic acids include succinic acid, malonic acid, adipic acid, pimelic acid, azelaic acid, and sebacic acid. Azelaic acid and sebacic acid are preferred. In particular, Li complex soaps that are mixtures of a salt of azelaic acid and lithium hydroxide and a salt of 12-hydroxystearic acid and lithium hydroxide are preferred.

[0019] Diurea compounds are generally represented by the following formula (2): R4-NHCONH-R5-NHCONH-R6 (2) (wherein R4 and R6 may be the same or different and represent a C6-30 alkyl group, a C5-8 cycloalkyl group, or a C6-10 aryl group, and R5 represents a C6-15 divalent aromatic hydrocarbon group.) Preferred diurea compounds are aliphatic diureas in which R4 and R6 are the same or different C6-30 alkyl groups, alicyclic aliphatic diureas in which one of R4 and R6 is a C5-8 cycloalkyl group and the other is a C6-30 alkyl group, or aromatic diureas in which R4 and R6 are the same or different C6-10 aryl groups.

[0020] As the aliphatic diurea, aliphatic diureas in which both R4 and R6 are C8 alkyl groups, aliphatic diureas in which both R4 and R6 are C18 alkyl groups, and aliphatic diureas in which one of R4 and R6 is a C8 alkyl group and the other is a C18 alkyl group are more preferred. Aliphatic diureas in which one of R4 and R6 is a C8 alkyl group and the other is a C18 alkyl group are particularly preferred. Aliphatic diureas 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% are even more particularly preferred. As the alicyclic aliphatic diurea, a alicyclic aliphatic diurea in which one of R4 and R6 is a cyclohexyl group and the other is a C18 alkyl group is more preferred. Aliphatic diureas 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% are particularly preferred. As the aromatic diurea, an aromatic diurea in which both R4 and R6 are p-toluyl groups is particularly preferred.

[0021] The content of the thickener in the grease composition of the present invention is, for example, preferably 4 to 25 mass %, and more preferably 5 to 20 mass %. When the content of the thickener is in this range, the grease has an appropriate hardness and is preferable because it prevents leakage from the lubricated parts.

[0022] <Other Additives> The grease composition of the present invention can optionally contain any additive commonly used in grease compositions. Examples include antioxidants, rust inhibitors, corrosion inhibitors, oiliness agents, and viscosity index improvers. It is preferable to contain an antioxidant and / or rust inhibitor. However, it is preferable to not contain reactive additives (i.e., additives that react on lubricated surfaces to produce components that decompose the base oil, such as molybdenum disulfide, amine phosphate, zinc dialkyldithiophosphate, and molybdenum dialkyldithiocarbamate). Examples of antioxidants include amine-based, phenol-based, quinoline-based, and sulfur-based antioxidants, with amine- and quinoline-based antioxidants being preferred. Examples of rust inhibitors include zinc-based, carboxylic acid-based, carboxylate-based, succinic acid-based, amine-based, sulfonate-based, and naphthenic acid-based. Amine-based and naphthenic acid-based inhibitors are preferred. Mixtures of these are even more preferred. Examples of corrosion inhibitors include thiadiazole-based, benzimidazole-based, and benzotriazole-based. Examples of oiliness agents include fatty acids, fatty acid esters, and phosphate esters. When the grease composition of the present invention contains other additives, the content thereof is usually 0.5 to 10 mass %, and preferably 0.5 to 5 mass %, based on the total amount of the grease composition.

[0023] [Consistency] The consistency of the grease composition of the present invention is adjusted according to the intended use, but is preferably 235 to 370. By setting the consistency to 235 or more, a grease composition with excellent low-temperature properties can be obtained, and by setting the consistency to 370 or less, a grease composition with excellent adhesion to mechanical parts can be obtained. In this specification, the term "consistency" refers to 60-stroke worked consistency. The consistency can be measured in accordance with JIS K2220 7.

[0024] The use of the grease composition of the present invention, i.e., the type of mechanical part to which the grease composition is applied, is not limited. Examples 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, etc. Among these, reducers and ball screws require strict levels of heat resistance and peeling resistance, but the grease composition of the present invention can satisfy such high requirements. The type of sealing material provided on the mechanical part is not particularly limited, and examples include NBR, EPDM, natural rubber, etc.

[0025] The grease compositions of the examples and comparative examples were prepared using the following components: <Base oil> PPG: polyoxypropylene monobutyl ether (product name "Unilube MB-7", manufactured by NOF Corporation, propylene oxide addition mole number 12, average molecular weight 700, kinematic viscosity at 40°C: 32.8 mm 2 / s, kinematic viscosity at 100 ° C: 6.7 mm 2 / s, pour point: -47.5°C) <Thickeners> Lithium soap: lithium 12-hydroxystearate Aliphatic diurea: reaction product of diphenylmethane diisocyanate with octylamine and stearylamine (molar ratio of octylamine to stearylamine: 5:5) Alicyclic aliphatic diurea: reaction product of diphenylmethane diisocyanate with cyclohexylamine and stearylamine (molar ratio of cyclohexene to stearylamine: 7:1) Aromatic diurea: reaction product of diphenylmethane diisocyanate with p-toluidine <First and second solid lubricants> PTFE: polytetrafluoroethylene (solid) Calcium carbonate (solid) Calcium oxide (solid) Tricalcium phosphate (solid) Calcium stearate (solid) MoDTC: molybdenum dithiocarbamate (liquid)・MoS2: Molybdenum disulfide (solid) ・ZnDTP: Zinc dithiophosphate (liquid) ・Amine phosphate (liquid) Note that MoDTC, MoS2, ZnDTP and amine phosphate are reactive anti-flaking additives for comparison. <Other additives> ・Antioxidant: 2,2,4-trimethyl-1,2-dihydroquinoline polymer ・Rust inhibitor

[0026] <Test Greases> Preparation Example 1 Test grease composition in which the thickener is a diurea compound One mole of 4.4'-diphenylmethane diisocyanate was reacted with two moles of a specified amine in a base oil, and the mixture was cooled to prepare a base grease. Additives were blended into the base grease in the proportions shown in Table 1, and additional base oil was added to obtain the thickener amount shown in Table 1. The mixture was then dispersed using a three-roll mill to prepare a test grease composition. The consistency of the test grease composition was 280.

[0027] Preparation Example 2: Test Grease Compositions Using Lithium Soap as a Thickener Lithium 12-hydroxystearate was added to a base oil and stirred, then heated to 230°C. The mixture was then cooled to 100°C or below while stirring to prepare a base grease. Additives were blended into the base grease in the proportions shown in Tables 1 and 2, and additional base oil was added to obtain the thickener amount shown in Tables 1 and 2. The mixture was dispersed using a three-roll mill to prepare test grease compositions. The consistency of the test grease compositions was 280. The mass percentages of each component in each test grease composition are shown in Tables 1 and 2. The kinematic viscosity of the base oil at 100°C 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. The grease compositions obtained above were tested and evaluated using the methods described below.

[0028] <Test method> - Evaluation of heat resistance by high-temperature thin film test Grease is applied to the following steel plate and left to stand in a thermostatic chamber at a specified temperature for a specified time, after which gel permeation chromatography analysis is carried out to check for the occurrence of base oil decomposition. [Test conditions] Steel plate: SPCC-SD 80mm x 60mm x 1mm Temperature: 120°C Time: 1152h Coating thickness: 2mm GPC measurement solvent: chloroform GPC detector: RI detector [Evaluation criteria] No base oil decomposition...○ (pass) Base oil decomposition...× (fail)

[0029] - Evaluation of flaking resistance using a four-ball rolling test Three 15mm diameter steel bearing balls were prepared and placed in a cylindrical container with an inner diameter of 40mm and a height of 14mm, which was filled with approximately 20g of test grease. A 5 / 8 inch diameter steel bearing ball was placed on top of these three steel balls and rotated at a specified rotation speed, causing the three lower steel balls to revolve while rotating on their own axes. This was continued until flaking occurred on the steel ball surfaces. *Flaking occurred between the balls with the highest surface pressure. *The lifespan was measured as the number of contacts the upper steel ball had with the lower steel ball at the point at which flaking occurred. Flaking resistance was evaluated based on the lifespan. [Test conditions] Test steel ball: φ5 / 8 in, Ra0.45 μm bearing steel ball (rotor ball) φ15 mm bearing steel ball (driven ball) Test load: 400 kgf (6.5 GPa) Rotation speed: 1200 rpm Evaluation criteria: 150,000 times or more... ◎ (pass) 100,000 times or more but less than 150,000 times... ○ (pass) 50,000 times or more but less than 100,000 times... △ (fail) Less than 50,000 times... × (fail) The results are shown in Tables 1 and 2.

[0030]

[0031]

[0032] Examples 1 to 15, which use a first solid lubricant, polytetrafluoroethylene, in combination with a second solid lubricant, at least one selected from calcium carbonate, calcium oxide, tricalcium phosphate, and calcium salts of fatty acids, as additives, exhibit superior anti-flaking properties compared to Comparative Examples 1 to 6, 9, and 10. Examples 1 to 15 also exhibit superior heat resistance compared to Comparative Examples 7 to 10. While the heat resistance of greases generally varies depending on the type of thickener, improvements in heat resistance in the examples were observed with both lithium soap and urea-based thickeners. Therefore, by using the first and second solid additives specified in the present application in combination as additives, it is possible to improve the anti-flaking properties and heat resistance of greases even at high temperatures or high surface pressures, without using a reactive anti-flaking additive or selecting a thickener.

Claims

1. A grease composition containing 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 grease composition comprising, as an additive, 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, wherein the content of the second solid lubricant is 0.5% by mass or more based on the total mass of the composition.

2. The grease composition according to claim 1, wherein the 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 or 2, wherein the 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 or 2, wherein the second solid lubricant is calcium carbonate.

5. The grease composition according to claim 1 or 2, comprising the first and second solid lubricants in a ratio of 1 to 3% by mass of the first solid lubricant to 10% by mass of the first solid lubricant.

6. A machine part to which the grease composition according to claim 1 or 2 is applied.