Patent Application
20250230378
The present disclosure relates to a grease composition suitably usable for mechanical parts that are required to achieve spalling preventive properties, such as reducers and ball screws.
In recent years, there has been a demand that mechanical parts be smaller and achieve higher output for weight reduction. Accordingly, greases to be used in lubrication sites of these mechanical parts have been required to cope with harsher operating environments than before, such as higher speeds, higher surface pressures, and a range of use temperature expanded to a higher temperature side. In particular, one of the most important technological issues is to extend the service life of a grease used for parts under high temperature and high pressure.
The extension of the service life of the grease can be achieved by suppressing spalling of mechanical parts. Heretofore, it has been known that the service life of a grease until spalling can be extended when the grease contains a so-called reactive anti-spalling additive such as amine phosphate and zinc dialkyldithiophosphate (Patent Literature 1). The reactive anti-spalling additive works by causing a reaction on a metal surface and forming a protective film thereon.
Meanwhile, as a sealing member to be used for mechanical parts, nitrile rubber (NBR) has been widely used from the reasons such as oil resistance, wear resistance, heat resistance, workability, and low cost. However, the sealing member may deteriorate due to a low temperature in winter, a heat damage caused by tropical climates, and so on. The deterioration of the sealing member may also occur because a base oil contained in a grease, which comes into contact with the sealing member, causes the sealing member to swell. For this reason, if a mechanical part including a sealing material is used for a long period of time, the sealing member may allow the entry of foreign matters from the outside, thereby inducing poor lubrication and shortening the service life of the mechanical part. Countermeasures to this problem have been approached from two perspectives, that is, the selection of a sealing material and the selection of a grease base oil. More specifically, use of ethylene propylene rubber (EPDM) superior to NBR in terms of heat resistance, cold resistance, weather resistance, and water resistance makes it possible to extend the service lives of mechanical parts. For mechanical parts in which EPDM or natural rubber is used as a sealing material for a lubrication site or its surrounding member, the swelling of the EPDM or natural rubber can be suppressed by using polyoxyalkylene or a polyoxyalkylene derivative as a base oil of a grease (Patent Literature 2).
Under these circumstances, there is a demand for a grease composition containing polyoxyalkylene or a polyoxyalkylene derivative as a base oil that has excellent compatibility with rubber and therefore is also applicable to a mechanical part in which EPDM or natural rubber is used as a sealing member, the grease composition being capable of extending the service life of a mechanical part by suppressing the occurrence of spalling of the machine part even under high temperature or high surface pressure.
When the grease containing polyoxyalkylene or a polyoxyalkylene derivative as a base oil contains a reactive anti-spalling additive, it is possible to suppress swelling of EPDM or natural rubber and spalling of a mechanical part. However, when this grease is used under high temperature, there is a problem in that the decomposition of the polyoxyalkylene or polyoxyalkylene derivative is accelerated by acid components produced when the reactive anti-spalling additive forms a reaction film on the metal surface.
In addition, the reactive anti-spalling additive effectively exhibits a spalling prevention effect in a non-polar base oil such as polyalphaolefin. However, when the base oil has polarity as in polyoxyalkylene or a polyoxyalkylene derivative, the reactive anti-spalling additive disperses in the base oil and is less likely to adsorb to the lubrication field, which makes it difficult to obtain the spalling prevention effect.
When a solid lubricant such as polytetrafluoroethylene (PTFE) or melamine cyanurate, in other words, a non-reactive anti-spalling additive, is used in place of the reactive anti-spalling additive, the decomposition of the base oil may be suppressed. The non-reactive anti-spalling additive works by adhering to metal surfaces and preventing metal-to-metal contact, but is significantly inferior to the reactive anti-spalling additive in spalling preventive properties, and the resulting grease lacks versatility.
In this technological trend, an object of the present invention is to improve the spalling preventive properties of a grease composition containing polyoxyalkylene or a polyoxyalkylene derivative as a base oil, even under high temperature or high surface pressure, without using a reactive anti-spalling additive, and to suppress thermal degradation of the base oil.
The present inventors achieved the above object to improve the spalling preventive properties by combination use of non-reactive solid lubricants. Specifically, the present invention provides the following grease composition.
According to the present invention, without using a reactive anti-spalling additive, it is possible to improve spalling preventive properties and heat resistance of a grease composition while avoiding acceleration of decomposition of polyoxyalkylene and/or an ether derivative of polyoxyalkylene even under high temperature or high surface pressure.
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.
The 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 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, R2 is hydrogen or an alkyl group having 1 to 2 carbon atoms, and n is a number of 5 to 55.
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 R1 and R3 is an alkyl group having 1 or more carbon atoms, or a diether in which both of R1 and R3 are alkyl groups having 1 or more carbon atoms.
Specific examples of polyoxyalkylene diols include polyoxyethylene, polyoxypropylene, poly(oxypropyleneoxyethylene), poly(oxybutyleneoxyethylene), poly(oxybutyleneoxypropylene), poly(oxypentyleneoxyethylene), and poly(oxypentyleneoxypropylene).
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.
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 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 be a so-called biomass oil, which is produced from biological resources derived from an animal, a plant, or the like.
The base oil of the present invention preferably has a kinetic viscosity at 100° C. of 2 to 100 mm2/s. This provides excellent low temperature properties. The kinematic viscosity at 100° C. is preferably 2 to 50 mm2/s, more preferably 2 to 20 mm2/s, and further preferably 6 to 19 mm2/s.
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.
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 mm2/s, a pour point is −35° C. or lower, and n in Formula (1) is 10 to 22.
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.
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 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.
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.
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.
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.
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.
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.
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.
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.
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.
Among them, a Li soap and a diurea compound are preferred. These are practical thickeners because they have few defects and are inexpensive.
The Li soap is preferably lithium 12-hydroxystearate (Li-(12OH)St) or lithium stearate (Li-St). These have excellent lubricity.
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.
The diurea compound is generally expressed by Formula (2) below.
R4—NHCONH—R5—NHCONH—R6 (2),
where R4 and R6 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 R5 represents a C6-15 divalent aromatic hydrocarbon group.
The diurea compound is preferably aliphatic diurea in which R4 and R6 are C6-30 alkyl groups which may be the same or different, alicyclic aliphatic diurea in which one of R4 and R6 is a C5-8 cycloalkyl group and the other is a C6-30 alkyl group, or aromatic diurea in which R4 and R6 are C6-10 aryl groups which may be the same or different.
As the aliphatic diurea, aliphatic diurea in which both R4 and R6 are C8 alkyl groups, aliphatic diurea in which both R4 and R6 are C18 alkyl groups, and aliphatic diurea in which one of R4 and R6 is a C8 alkyl group and the other is a C18 alkyl group are more preferred. Aliphatic diurea in which one of R4 and R6 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.
As the alicyclic aliphatic diurea, 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. 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.
As the aromatic diurea, aromatic diurea in which both R4 and R6 are p-toluyl groups is particularly preferred.
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.
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).
The antioxidants include amine-based, phenol-based, quinoline-based, and sulfur-based antioxidants, with the amine-based and quinoline-based antioxidants being preferred.
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.
The corrosion inhibitors include thiadiazole-based, benzimidazole-based, and benzotriazole-based corrosion inhibitors.
The 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% by mass and preferably 0.5 to 5% by mass based on the total mass of the grease composition.
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.
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.
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.
Grease compositions in Examples and Comparative Examples were prepared by using the components specified below.
Note that MoDTC, MoS2, and ZnDTP are reactive anti-spalling additives for comparison.
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.
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.
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.
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.
A percent by mass of each component in the test grease compositions is as specified in Tables 1 and 2.
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.
The grease compositions thus obtained were tested and evaluated in the following methods.
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.
Three steel bearing balls with a diameter of φ15 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 φ⅝ 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.
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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2022-049292 | Mar 2022 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2023/012238 | 3/27/2023 | WO |
