Patent Description:
A peculiar early abnormal flaking involving formation of white etching area occurring on the rolling surface of a rolling bearing has been a problem since the mid-<NUM> because it reduces the fatigue life of the rolling bearing. Such flaking is called white flaking, white band flaking, brittle flaking, hydrogen brittle flaking, or hydrogen embrittlement flaking.

Although the mechanism of how such flaking takes place has not yet been elucidated, Patent Literature <NUM> introduces a hydrogen hypothesis, for example. Specifically, the hypothesis is as follows: when grease is used under a high load, the grease decomposes to generate hydrogen; the hydrogen penetrates into the steel material of the rolling bearing and reacts with carbide at the grain boundaries; and as a result, the steel material becomes brittle. Patent Literature <NUM> reports that, when a grease composition contains a specific compound containing at least one sulfur atom such as a thiazole derivative, a sulfurized oil and/or fat, or a sulfurized olefin, it is possible to deal with the problem of white band flaking, that is, the intrusion of hydrogen generated by decomposition of the lubricant into the metal.

The mechanism of how flaking takes place is also explained from the viewpoint of the formation of a new metal surface. Specifically, the mechanism is as follows: when the metal transfer surface wears, a new surface is easily formed by the wear; the newly formed surface brings about catalysis to chemically decompose the grease; and as a result, a large amount of hydrogen is generated, and the generated hydrogen penetrates into the steel to finally produce cracks on the metal surface. Patent Literature <NUM> reports an additive which is a passivating oxidizer such as a nitrite, where the additive is added to the grease to oxidize the metal surface and suppress the catalytic activity of the surface, thereby suppressing the generation of hydrogen due to the decomposition of the lubricant. Patent Literature <NUM> reports a technique of combining a passivating oxidizer with an organic sulfonate. Patent Literature <NUM> reports a technique of allowing grease to contain a specific amount of an azo compound. Patent Literature <NUM> reports a technique that suppresses the generation of hydrogen from grease by using a phenyl ether-based synthetic oil as the base oil of the grease.

The following patent literature also refers to grease and/or lubricant compositions. <CIT>; <CIT>; <CIT>; <CIT>; <CIT>; <CIT>; <CIT>; <CIT>; <CIT>; <CIT>; <CIT>; and <CIT>.

Meanwhile, it is known that plasma is generated in a minute range of several µm to several mm on the friction surface (<NPL>)). Such plasma is called "triboplasma. " Discharge luminescence and electric corrosion also take place on an elastohydrodynamic lubrication (EHL) thin film of grease formed on rolling bearings. From these facts, there is a report suggesting that discharge plasma is generated on an EHL thin film (<NPL>).

The present inventors considered that the suppression of triboplasma generation could prevent white band flaking of e.g. a rolling bearing.

In view of the above, an object of the present invention is to provide an anti-flaking agent capable of suppressing white band flaking of e.g. a rolling bearing, and a lubricant composition containing the anti-flaking agent.

The present inventors measured the amount of hydrogen generated using a candidate compound for a base oil accounting for a large percentage of the lubricant composition or the grease composition, and have found that a compound having a specific volume resistivity of <NUM> × <NUM><NUM> Ω·cm or less can effectively suppress hydrogen generation. Based on this knowledge, the present inventors have completed an invention which can effectively prevent white band flaking of e.g. a rolling bearing.

Specifically, the present invention provides the following use.

The present invention also provides the following lubricant composition.

The anti-flaking agent and lubricant composition of the present invention can prevent white band flaking effectively (<NUM>% or less as compared with n-hexadecane).

[<FIG> is a schematic view of an apparatus, used in Examples, for generating hydrogen gas by triboplasma.

In the present specification, the specific volume resistivity represents a ratio between a DC electric field (V/m) applied to the sample at <NUM> and a current per unit cross-sectional area applied to the sample at that time, and is equal to the resistance between opposing faces of a cubic sample with one side being <NUM>. The specific volume resistivity can be measured based on the testing methods of electrical insulating oils specified in JIS C2101.

In the present specification, the dielectric constant ε is a coefficient representing the relationship between the electric charge in the substance and the force given thereby. The dielectric constant ε was measured at <NUM> with E4991B Impedance Analyzer (Keysight Technologies).

In the present specification, the "Hansen solubility parameters" are each an index indicating the solubility of a certain solute in a certain solvent, and include three components: the dispersion term (δD), the polar term (δP), and the hydrogen bond term (δH). The dispersion term (δD) represents the effect due to the dispersion force, the polar term (δP) represents the effect due to the force between dipoles, and the hydrogen bond term (δH) represents the effect due to the hydrogen bond force. Details of the definitions and calculation methods for the Hansen solubility parameters are described in the following literature: <NPL>.

In the present specification, "white band flaking" refers to a peculiar early abnormal flaking involving formation of white etching area. In the present specification, the term "white band flaking" is synonymous with a term called e.g. white flaking, white band flaking, brittle flaking, hydrogen brittle flaking, or hydrogen embrittlement flaking in the art. Normally, for rolling fatigue, the life can be estimated based on the life calculation formula defined in the standards (ISO281, JIS B-<NUM>). However, in the case where white band flaking takes place, the lifetime is reached in a shorter time than the calculated lifetime. In the actual market, it has been reported that the life is reached at about <NUM>/<NUM> to <NUM>/<NUM> of the calculated life. White band flaking is one type of internal origin damage, and shows a specific phenomenon in which a white band is observed when the metal structure after the occurrence is etched with a nital solution.

The compound used in the present invention is a compound having a specific volume resistivity of <NUM> × <NUM><NUM> Ω·cm or less, and is a diester as defined above. The present inventors have found that a compound having such a physical property can suppress hydrogen generation by plasma. Although the experimental methods and results are described in detail in the Examples section, the present inventors systematically examined the influence of the carbon chain length of ester on the amount of hydrogen generated. Then, as regards the length of the carbon chain derived from the dibasic fatty acid constituting the ester with methanol (R<NUM>OOC-R<NUM>-COOR<NUM>) (that is, R<NUM>), no hydrogen was generated when the number of carbon atoms was <NUM> or less. Conversely, when the number of carbon atoms was <NUM> (that is, when the dibasic acid was sebacic acid), hydrogen was generated. However, the amount generated was only <NUM>% compared with n-hexadecane used as a standard substance. It was considered that triboplasma was generated in the case of using dimethyl sebacate. Thus, the specific volume resistivity was measured and found to be <NUM> × <NUM><NUM> Ω·cm. Moreover, when the specific volume resistivity was measured while changing the number of carbon atoms of R<NUM>, it was found that the specific volume resistivity increased as the number of carbon atoms of R<NUM> increased.

On the other hand, the present inventors examined the influence of the length of the carbon chain derived from the alcohol constituting the ester with sebacic acid (that is, R<NUM>) on the amount of hydrogen generated. Then, it was found that the specific volume resistivity increased as the number of carbon atoms of R<NUM> increased. This tendency was also observed in the case of monoesters.

The present inventors have also found that a specific aromatic compound can effectively suppress hydrogen generation even when the compound has a specific volume resistivity exceeding <NUM> × <NUM><NUM> Ω·cm.

Therefore, the compound of the present invention is at least one selected from the group consisting of (A) a compound having a specific volume resistivity of <NUM> × <NUM><NUM> Ω·cm or less, wherein the compound (A) is a diester selected from the group consisting of dimethyl phthalate, dimethyl maleate, diethyl malonate, dibutyl malonate, and dihexyl malonate.

The compound (A) preferably has a specific volume resistivity of <NUM> × <NUM><NUM> Ω·cm or less.

In addition, the compound (A) is preferably liquid at <NUM>.

In addition, the compound (A) preferably has a dielectric constant ε of <NUM> or more at <NUM> (<NUM>) and <NUM> (<NUM>).

In addition, the compound (A) preferably has a Hansen solubility parameter polar term δp of <NUM> or more.

The term δp is expressed by the following formula, and δp increases as the dielectric constant ε increases. Generally, it is said that the dielectric constant ε of oil affects electron wave absorption, and it is said that the larger the diel ectric constant ε and the larger the dielectric loss tangent, the more effectively electron waves can be absorbed, which can be a countermeasure against electron wave noise.

It has been found that a compound having a specific volume resistivity of <NUM> × <NUM><NUM> Ω·cm has a δp of <NUM> or more. Therefore, it is considered that, when δp is <NUM> or more, hydrogen generation can be prevented and white band flaking can be prevented. The term δp is preferably <NUM> or more. The Hansen solubility parameter polar term δp is preferably <NUM> or less.

The term δp is preferably <NUM> or more, and the reason is as follows. Such a value makes it possible to achieve a conductivity to an extent sufficient to prevent charging and a high dielectric constant, and it is therefore considered that white band flaking can be suppressed through suppression of triboplasma generation.

The flash point of the compound of the present invention is preferably <NUM> or lower because there is a risk of ignition by plasma generated due to friction of the lubrication portion. The flash point can be measured based on JIS K2265.

Since being liquid at room temperature, the compound can be used alone as a lubricant composition, can also be used as a lubricant or a base oil of a grease, or can be mixed with a conventional base oil as a lubricant or a base oil of a grease to form a lubricant composition.

As the conventional base oil, one having a specific volume resistivity exceeding <NUM> × <NUM><NUM> Ω·cm can be used. One containing a saturated or unsaturated hydrocarbon group having <NUM> or more carbon atoms in total is preferable, and specific examples thereof include mineral oils and synthetic oils. As the mineral oil, it is possible to use a paraffinic mineral oil, a naphthenic mineral oil, or a mixture thereof. It is preferable to contain a highly refined mineral oil (that is, a mineral oil which has been subjected to dewaxing treatment to reduce wax component precipitation at low temperature, thereby lowering its pour point as compared with the pour point of unrefined mineral oils (-<NUM> to -<NUM>, measured according to JIS K <NUM>)). Examples of synthetic oils include synthetic hydrocarbons, ester oils, ether oils, glycol oils, silicone oils, and fluorinated oils. Examples of synthetic hydrocarbon oils include poly alpha olefins ("PAOs") and polybutene. Among these, poly alpha olefins are preferable. Examples of ester oils include diesters, trimellitate esters, and polyol esters. Examples of ether oils include alkyl diphenyl ethers ("ADEs"), dialkyl diphenyl ethers, and polypropylene glycol. Examples of glycol oils include polypropylene glycol and polypropylene alkyl ethers.

In the case of use in combination with a mineral oil or synthetic hydrocarbon (especially poly alpha olefin), hydrogen generation can be effectively suppressed even when the compound is in a small amount, for example more than <NUM>% by mass, preferably more than <NUM>% by mass, more preferably <NUM>% by mass or more, and further preferably <NUM>% by mass or more based on the total mass of the lubricant composition. The content of the compound in the lubricant composition of the present invention can be, for example, <NUM>% by mass or less, <NUM>% by mass or less, <NUM>% by mass or less, <NUM>% by mass or less, or <NUM>% by mass or less. Considering the compatibility with the compounds described above, preferable conventional oils are ester oils such as diesters and polyol esters, ether oils such as alkyl phenyl ether oils, glycol oils such as water-insoluble polyalkylene glycols, silicone oils, fluorinated oils, and the like. From the viewpoints of resin resistance and heat resistance, mineral oils, synthetic oils, hydrocarbon oils, phenyl ether oils, and alkyl phenyl ether oils are preferable.

The kinematic viscosity at <NUM> of the base oil in the lubricant composition of the present invention (that is, the compound (A) and/or (B) alone, or a mixture oil with the conventional oil) is preferably <NUM> to <NUM><NUM>/s. When the kinematic viscosity at <NUM> of the base oil is less than <NUM><NUM>/s, it may be impossible to achieve a sufficient oil film at low speed or high temperature. Meanwhile, when the kinematic viscosity at <NUM> of the base oil exceeds <NUM><NUM>/s, there is a risk that the torque may rise at high speed or low temperature. For the same reasons, the range is more preferably <NUM> to <NUM><NUM>/s and further preferably <NUM> to <NUM><NUM>/s. Note that the kinematic viscosity of the base oil can be measured based on JIS K2283.

The content of the base oil in the lubricant composition of the present invention is preferably <NUM> to <NUM> parts by mass, more preferably <NUM> to <NUM> parts by mass, and further preferably <NUM> to <NUM> parts by mass relative to <NUM> parts by mass in total of the base oil and the anti-flaking agent. The content of the base oil is preferably in such ranges because of excellence in lubricity and low volatility.

The lubricant composition of the present invention may further contain a general-purpose additive as necessary. For example, a rust inhibitor, a load-bearing additive, an antioxidant, and the like can be contained as necessary. The content of these optional additives is usually <NUM> to <NUM>% by mass based on the total mass of the lubricant composition of the present invention.

Examples of the rust inhibitor include inorganic rust inhibitors and organic rust inhibitors. Examples of the inorganic rust inhibitors include inorganic metal salts such as sodium silicate, lithium carbonate, potassium carbonate, and zinc oxide. Examples of the organic rust inhibitors include benzoates such as sodium benzoate and lithium benzoate, sulfonates such as calcium sulfonate and zinc sulfonate, carboxylates such as zinc naphthenate and sodium sebacate, succinic acid derivatives such as succinic acid, succinic anhydride, and succinic acid half ester, sorbitan esters such as sorbitan monooleate and sorbitan trioleate, and fatty acid amine salts.

Examples of the load-bearing additive include phosphorus-containing ones such as phosphate esters, sulfur-based ones such as polysulfide and sulfurized oils and/or fats, phosphorus-sulfur-based ones such as phosphorothioates, thiocarbamates, thiophosphates, and organic phosphate esters.

The antioxidant is known to suppress oxidative degradation of grease, and examples thereof include phenol-based antioxidants and amine-based antioxidants.

Examples of the phenol-based antioxidants include <NUM>,<NUM>-di-tert-butyl-p-cresol (BHT), <NUM>,<NUM>'-methylenebis(<NUM>-methyl-<NUM>-tert-butylphenol), <NUM>,<NUM>'-butylidenebis(<NUM>-methyl-<NUM>-tert-butylphenol), <NUM>,<NUM>-di-tert-butyl-phenol, <NUM>,<NUM>-dimethyl-<NUM>-tert-butylphenol, tert-butylhydroxyanisole (BHA), <NUM>,<NUM>'-butylidenebis(<NUM>-methyl-<NUM>-tert-butylphenol), <NUM>,<NUM>'-methylenebis(<NUM>,<NUM>-di-tert-butylphenol), <NUM>,<NUM>'-thiobis(<NUM>-methyl-<NUM>-tert-butylphenol), and octadecyl-<NUM>-(<NUM>,<NUM>-di-tert-butyl-<NUM>-hydroxyphenyl)propionate. Among these, octadecyl-<NUM>-(<NUM>,<NUM>-di-tert-butyl-<NUM>-hydroxyphenyl)propionate is preferable.

Examples of the amine-based antioxidants include N-n-butyl-p-aminophenol, <NUM>,<NUM>'-tetramethyl-di-aminodiphenylmethane, α-naphthylamine, N-phenyl-α-naphthylamine, phenothiazine, and alkyl diphenylamines. Among these, alkyl diphenylamines are preferable.

The lubricant composition of the present invention can be used as lubricating oil, conductive oil, dynamic pressure oil, and the like. The lubricant composition of the present invention is effective in preventing flaking wear.

The lubricant composition of the present invention may further contain a thickener to form a grease composition.

For the same reasons as described for the lubricant composition, the content of the compound (A) and/or (B) is preferably more than <NUM>% by mass, more preferably more than <NUM>% by mass, further preferably <NUM>% by mass or more, and particularly preferably <NUM>% by mass or more based on the total mass of the grease composition of the present invention, and the upper limit can be, for example, <NUM>% by mass or less, <NUM>% by mass or less, <NUM>% by mass or less, <NUM>% by mass or less, or <NUM>% by mass or less.

Examples of the thickener which can be used in the grease composition of the present invention include urea-based thickeners typified by diurea, lithium soap-based thickeners typified by lithium soap and lithium complex soap, and solid thickeners such as bentonite and silica gel. Urea-based thickeners and lithium soap-based thickeners are preferable.

The grease composition of the present invention may further contain a general-purpose additive as necessary. Examples of additives which can be used include ones described for the lubricant composition. The content of the optional additive is usually <NUM> to <NUM>% by mass based on the total mass of the grease composition of the present invention.

The worked penetration of the grease composition of the present invention is preferably <NUM> to <NUM> and more preferably <NUM> to <NUM>. When the worked penetration exceeds <NUM>, leakage due to high-speed rotation increases, which may result in failure to satisfy a sufficient lubrication life. Meanwhile, when the worked penetration is less than <NUM>, the fluidity of the grease is deteriorated, which may result in failure to satisfy a sufficient lubrication life. Note that, in the present specification, the term "penetration" refers to a <NUM>-stroke worked penetration. The penetration can be measured according to JIS K2220-<NUM>.

The content of the thickener is preferably <NUM> to <NUM>% by mass and more preferably <NUM> to <NUM>% by mass based on the total mass of the grease composition of the present invention. When the content is less than <NUM>% by mass, the grease is soft and may leak, which could result in failure to satisfy a sufficient lubrication life. Meanwhile, when the content is more than <NUM>% by mass, the fluidity is inferior and thus it becomes difficult for the grease to enter the lubrication portion, which could result in failure to satisfy a sufficient lubrication life.

The content of the base oil is preferably <NUM> to <NUM>% by mass and more preferably <NUM> to <NUM>% by mass based on the total mass of the grease composition of the present invention. The content of the base oil is preferably in such ranges because of excellence in lubricity and low volatility.

The grease composition of the present invention is used in various rolling bearings for industrial machines and automobiles. Examples for industrial machines include rolling bearings in various motors for industrial machines, reducers and hydraulic equipment of industrial robots, main shafts and reducers of wind power generators, and peripherals of elevator hoists. The use for automobiles is preferably a rolling bearing for automobile electrical equipment and auxiliaries. Examples of the automobile electrical equipment and auxiliaries include alternators, electromagnetic clutches for automobile air conditioners, intermediate pulleys, idler pulleys, and tension pulleys.

The amount of hydrogen generated was measured according to the method described in <NPL>.

Specifically, a triboplasma generator (<FIG>) was used capable of generating triboplasma between the needle and the flat plate electrode. The needle was the cathode and the flat plate was the anode. The material of the needle was SCM435 steel and the apex angle of the needle was <NUM>°. The needle was arranged perpendicular to the anode flat plate, and was fixed at a position where the distance between the tip of the needle and the upper surface of the anode was <NUM>. The distance between the needle and the flat plate electrode was controlled by a micrometer. The material of the anode flat plate was SPCC steel. The anode flat plate constituted the bottom portion inside the container. The container was charged with the anti-flaking agent and the like of Examples or Comparative Examples, and the needle was in contact with the anti-flaking agent and the like inside the container. The anode flat plate and the cathode needle were connected by a high voltage power source. The voltage and current when a voltage was applied was measurable by an oscilloscope. The container and the needle were surrounded by a larger casing (hereinafter referred to as the "atmosphere control chamber") so as to cover both. The top portion of the atmosphere control chamber had an opening provided therein, and the gas inside the atmosphere control chamber was collectable through a microsyringe. The upper side portion of the atmosphere control chamber also had an opening provided therein so as to introduce dry air therethrough. The gas inside the atmosphere control chamber was detectable by a semiconductor sensor.

Dry air was introduced for <NUM> seconds to replace the gas inside the atmosphere control chamber. After the gas inside the atmosphere control chamber was replaced with dry air, the atmosphere control chamber was subjected to discharging for <NUM> seconds while monitoring the current value and the voltage value with an oscilloscope, and then left for <NUM> seconds to collect the generated gas through a microsyringe. The collected gas was introduced into gas chromatography to measure the amount of hydrogen gas. Note that the gas chromatography was measured using a gas chromatograph GC-<NUM> (manufactured by Shimadzu Corporation), a column RT-Msieve φ0. <NUM> × <NUM>, and a detector TCD. The amount of hydrogen generated for each compound was calculated with the amount of hydrogen generated for n-hexadecane set to <NUM>%.

Tables <NUM> to <NUM> present the results. Examples <NUM> to <NUM> are examples of the anti-flaking agent, and Examples <NUM> to <NUM> are examples of the lubricating oil composition containing the anti-flaking agent. Example <NUM> is a mixture of <NUM>% by mass of dimethyl malonate of Example <NUM> and <NUM>% by mass of poly alpha olefin of Comparative Example <NUM>, and indicates that, even when the specific volume resistivity of the mixture exceeds <NUM> × <NUM><NUM> Ω·cm, the amount of hydrogen generated can be suppressed to <NUM>% if a predetermined amount of the anti-flaking agent of the present application having a specific volume resistivity of <NUM> × <NUM><NUM> Ω·cm or less is contained.

Examples <NUM> to <NUM>, <NUM> to <NUM>, <NUM> to <NUM> and <NUM> to <NUM> are comparative.

The suppliers and trade names of the compounds used in Examples and Comparative Examples are presented below.

Claim 1:
Use as an anti-flaking agent of at least one selected from the group consisting of
(A) a compound having a specific volume resistivity of <NUM> × <NUM><NUM> Ω·cm or less, wherein the compound (A) is a diester selected from the group consisting of dimethyl phthalate, dimethyl maleate, diethyl malonate, dibutyl malonate, and dihexyl malonate.