Grease composition

Abstract

The present invention pertains to a grease composition containing a base oil, a thickener, and additives. The base oil is at least one selected from the group consisting of polyoxyalkylenes, polyoxyalkylene ether derivatives, and mixtures thereof. The additives include polytetrafluoroethylene as a first solid lubricant, and at least one selected from the group consisting of melamine cyanurate, tricalcium phosphate, and sodium sebacate as a second solid lubricant. The content of the second solid lubricant is at least 0.5 mass% relative to 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 load resistance, 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 wider temperature ranges. In particular, extending the life of greases used in components under high temperatures and high surface pressures is an important technical challenge. Extending the life of greases can be achieved by suppressing seizure of mechanical components. It has been known that the seizure life of greases can be extended by incorporating so-called reactive load-bearing additives, such as molybdenum disulfide, zinc dialkyldithiophosphates, and molybdenum dialkyldithiocarbamates, or solid lubricants into greases (Non-Patent Document 1). Reactive load-bearing additives function by reacting with metal surfaces to form a protective film. On the other hand, solid lubricants function by adhering to metal surfaces and preventing metal-to-metal contact. In other words, solid lubricants can also be considered non-reactive load-bearing additives. Nitrile rubber (NBR) is widely used as a sealing material for machine parts due to its oil resistance, abrasion resistance, heat resistance, processability, and low cost. However, seals can deteriorate due to low winter temperatures and heat damage in tropical climates. Deterioration of seals can also occur when the base oil contained in the grease that comes into contact with the seals causes the seals to swell. Therefore, long-term use of machine parts equipped with seals can allow foreign matter to enter from the outside, causing poor lubrication and shortening the life of the machine 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, using ethylene propylene rubber (EPDM), which has better heat resistance, cold resistance, weather resistance, and water resistance than NBR, as a sealing material can extend the life of machine parts. For mechanical parts in which EPDM or natural rubber is used as a sealing material for lubricated parts or surrounding components, swelling of the EPDM or natural rubber can be suppressed by using polyoxyalkylene or a polyoxyalkylene derivative as the base oil of the grease (Patent Document 1).

[0003] Patent No. 2960561

[0004] Toshio Sakurai, "Physical Chemistry of Lubrication", Saiwai Shobo, 1974, pp. 216-232

[0005] 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 extending the life of the mechanical parts by suppressing seizure of the mechanical parts even at high temperatures or under high surface pressure. By incorporating a reactive load-bearing additive into a grease based on a polyoxyalkylene or polyoxyalkylene derivative, swelling of EPDM or natural rubber and seizure of the 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 load-bearing additive forms a reaction film on the metal surface accelerate the decomposition of the polyoxyalkylene or polyoxyalkylene derivative. Furthermore, the reactive load-bearing additive effectively suppresses seizure in non-polar base oils such as polyalphaolefins. However, when a polyoxyalkylene or polyoxyalkylene derivative has polarity, it disperses in the base oil and is difficult to adsorb to the lubrication field, making it difficult to achieve seizure suppression. The decomposition of the base oil can be suppressed by using a solid lubricant such as polytetrafluoroethylene (PTFE) or melamine cyanurate, i.e., a non-reactive load-bearing additive, instead of a reactive load-bearing additive. However, the load-bearing capacity of non-reactive load-bearing additives is significantly inferior to that of reactive load-bearing additives, and the resulting grease lacks versatility. In light of this technological trend, the problem to be solved by the present invention is to improve the load-bearing capacity of a grease composition using a polyoxyalkylene or polyoxyalkylene derivative as a base oil, even at high temperatures or under high surface pressure, without using a reactive load-bearing additive, and to suppress thermal degradation of the base oil.

[0006] The present inventors have solved the problem of improving load-carrying capacity 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 melamine cyanurate, tricalcium phosphate, and sodium sebacate 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 melamine cyanurate. 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 3 to 5 mass% of the second solid lubricant to 10 mass% of the first solid lubricant. 6. A mechanical part to which the grease composition according to any one of the above items 1 to 5 is applied.

[0007] According to the present invention, the load-bearing property and heat resistance of a grease composition can be improved even at high temperatures or under high surface pressures, without using a reactive load-bearing additive, and without promoting decomposition of polyoxyalkylene and / or ether derivatives of polyoxyalkylene.

[0008] <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):

[0009]

[0010] 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.

[0011] 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.

[0012] 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 %.

[0013] <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 results in excellent load-bearing properties. It is more preferably 1% by mass or more. From the viewpoint of the inflowability of the grease, the upper limit is preferably 20% by mass or less, more preferably 15% by mass or less.

[0014] The second solid lubricant in the present invention is at least one selected from the group consisting of melamine cyanurate, tricalcium phosphate, and sodium sebacate. Among these, melamine cyanurate is preferred from the viewpoint of wear resistance. The second solid lubricant preferably has a particle size larger than that of the first solid lubricant, PTFE, from the viewpoint of the effect of improving load resistance when used in combination.

[0015] 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 load-bearing properties. 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 the inflowability of the grease, 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 load-bearing properties of the grease composition are particularly excellent when the second solid lubricant is contained in an amount of 3 to 5 parts by mass per 10 parts by mass of PTFE. Therefore, from the viewpoint of load-bearing properties, it is particularly preferable to contain melamine cyanurate, which has a particle size larger than PTFE, in a ratio of 3 to 5 parts by mass per 10 parts by mass of PTFE.

[0016] 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.

[0017] 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 load-bearing capacity 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 load-bearing capacity while enabling the second solid lubricant to be stably supplied to the lubrication field, thereby significantly improving load-bearing capacity.

[0018] <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.

[0019] 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.

[0020] 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.

[0021] 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.

[0022] 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.

[0023] <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, 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 inhibitors. 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 %, preferably 0.5 to 5 mass %, based on the total amount of the grease composition.

[0024] [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.

[0025] 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 load 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.

[0026] 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) Melamine cyanurate (solid) Tricalcium phosphate (solid) Sodium sebacate (solid) MoDTC: molybdenum dithiocarbamate (liquid)・MoS2: Molybdenum disulfide (solid) ・ZnDTP: Zinc dithiophosphate (liquid) Note that MoDTC, MoS2 and ZnDTP are load-bearing additives for the reaction system for comparison. <Other additives> ・Antioxidant: 2,2,4-trimethyl-1,2-dihydroquinoline polymer ・Rust inhibitor

[0027] <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.

[0028] 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.

[0029] <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)

[0030] - Evaluation of wear resistance and load resistance by high-speed four-ball load-bearing performance test This was carried out in accordance with ASTM D 2596, and the L.W.I. (Load Wear Index) and W.P. (Weld Point) were determined. [Evaluation criteria] L.W.I. 551 or more... ◎ (Pass) L.W.I. 451 to 550... ○ (Pass) L.W.I. 351 to 450... △ (Fail) L.W.I. 350 or less... × (Fail) [Evaluation criteria] W.P. 3923N or more... ◎ (Pass) W.P. 3089N... ○ (Pass) W.P. 1961N to 2451N... △ (Fail) W.P. 1569N or less...× (failure) The results are shown in Tables 1 and 2.

[0031]

[0032]

[0033] Examples 1 to 12, which use a first solid lubricant, polytetrafluoroethylene, and a second solid lubricant, at least one selected from melamine cyanurate, tricalcium phosphate, and sodium sebacate, as additives, have superior load-bearing properties compared to Comparative Examples 1 to 5. Examples 1 to 12 also have superior heat resistance compared to Comparative Examples 6 to 8. While the heat resistance of greases generally varies depending on the type of thickener, the improvement in heat resistance in the examples was observed for both the 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 load-bearing properties and heat resistance of greases even at high temperatures or high surface pressures, without using a reactive load-bearing additive or regardless of the thickener used.

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 polyoxyalkylenes, ether derivatives of polyoxyalkylenes, and mixtures thereof; the additives comprise polytetrafluoroethylene as a first solid lubricant, and at least one selected from the group consisting of melamine cyanurate, tricalcium phosphate, and sodium sebacate as a second solid lubricant, and the content of the second solid lubricant is 0.5 mass % or more based on the total mass of the composition.

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

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

4. 2. The grease composition according to claim 1, wherein the second solid lubricant is melamine cyanurate.

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

6. A machine part to which the grease composition according to any one of claims 1 to 5 is applied.