Abstract:
Disclosed are a lubricant whose oxidation degradation is inhibited, and a dynamic pressure bearing device which has a long lifespan by prolonging the period of time of depression of a lubricating function of the lubricant occurring as a result of oxidation degradation. Ester-based lubricating oil serving as the lubricant is brought into continuous or intermittent contact with an ionic compound or an ionic compound solution obtained by dissolving the ionic compound in a solvent. In the dynamic pressure bearing device, the ionic compound is provided in a portion of a bearing or a shaft member and is thus brought into contact with the lubricating oil, in which the ionic compound or ionic compound solution is substantially insoluble in the ester-based lubricating oil.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a method of preventing degradation of a lubricant which is mainly used to lubricate a bearing device, to a lubricant, and to a dynamic pressure bearing device using the lubricant. 
       BACKGROUND OF THE INVENTION 
       [0002]    A bearing mechanism is typically lubricated using a lubricant. The lubricant functions to lubricate components of the bearing mechanism but is gradually degraded due to oxidation or the like. In particular, because a dynamic pressure bearing mechanism used for a spindle motor for a hard disk drive is heated to about 60° C. upon use and also is used for a long period of time, the degradation of the lubricant is considered to be a major problem. 
         [0003]    As a method of preventing the degradation of the lubricant, in particular, the oxidation degradation thereof, there have been proposed, for example, a lubricant (Japanese Patent Laid-open Publication No. Hei. 1-188592) using trimethylolpropane fatty acid triester as base oil and containing a hindered phenol-based oxidation inhibitor and a benzotriazole derivative, a lubricant (Japanese Patent Laid-open Publication No. Hei. 1-225697) containing a hindered phenol-based oxidation inhibitor and an aromatic amine-based oxidation inhibitor at a specific ratio, a lubricant (Japanese Patent Laid-open Publication No. Hei. 8-34987) using carbonate ester as base oil and containing a sulfur-containing phenol-based oxidation inhibitor and a zinc-based extreme pressure additive, and a lubricant (Japanese Patent Laid-open Publication No. Hei. 10-183159) containing base oil composed mainly of carbonate ester and a phenol-based oxidation inhibitor. 
         [0004]    A lubricant containing a conventionally known amine-based oxidation inhibitor or phenol-based oxidation inhibitor is inhibited from degradation compared to lubricants to which neither of them is added. However, in the case where such a lubricant is used under high-temperature conditions for a long period of time, oxidation degradation is not sufficiently inhibited. 
         [0005]    Although the amount of inhibitor added may be increased to prolong the degradation inhibition period, it is difficult to drastically improve the inhibitory effect. The degradation inhibition period may be prolonged to some degree by an increase in the amount of the inhibitor. However, in the case where the amount thereof exceeds a critical level, even when the amount is further increased, the inhibition period is not prolonged any further. 
       SUMMARY OF THE INVENTION 
       [0006]    In view of the above, the present invention provides a lubricant which is used to lubricate a bearing device or the like, thus enabling the inhibition of degradation thereof, in particular, oxidation degradation thereof, for a long period of time. 
         [0007]    In the present invention, a degradation inhibitor selected from among ionic compounds substantially insoluble in ester-based lubricating oil is prepared, and is thus brought into contact with the ester-based lubricating oil. Using the ester-based lubricating oil in this state, a target is lubricated. 
         [0008]    In this case, there is no need to bring the lubricating oil into continuous contact with the degradation inhibitor. Typically, lubricating oil for lubricating the bearing device or the like circulates inside the bearing device through sliding of the components of the device, and, during the circulation thereof, it may come into contact with the degradation inhibitor. 
         [0009]    In the case where the device to be lubricated includes a vessel for reserving the lubricating oil, the granular degradation inhibitor may be introduced into the vessel. In the present invention, because the degradation inhibitor substantially insoluble in the lubricating oil is used, even though such a degradation inhibitor is added, separation or precipitation occurs. Nevertheless, because the lubricating oil is brought into contact with the degradation inhibitor, the degradation of the lubricating oil may be prevented. 
         [0010]    Also in the present invention, the ionic compound is a material made up of a cation and an anion bonded to each other mainly through an ionic bond to thus form a molecule or crystal. This material is not typically dissolvable in oil. 
         [0011]    The degradation inhibitor which is not dissolvable in the lubricating oil or is very slightly soluble therein is used in a state of being brought into contact with the lubricating oil, and thereby the following effects are obtained. During use, surrounding impurities dissoluble in the lubricating oil are absorbed on the degradation inhibitor, thereby preventing the change in properties of the lubricating oil. Further, because the degradation inhibitor which is dissolvable in only a very small amount in the lubricating oil may always be supplied, a condition in which the lubricating oil contains a very small amount of the degradation inhibitor may be maintained for a long period of time. Furthermore, the degradation inhibitor is barely dissolvable in the lubricating oil, and thus the properties of the lubricating oil, including viscosity, are not changed even in the presence of the degradation inhibitor. 
         [0012]    The useful degradation inhibitor may be selected from among salts in which the atom of an acid molecule, in particular, the hydrogen atom emitted as a hydrogen cation upon electrolytic dissociation, is substituted with a metal ion. This material mainly has a low solubility in the ester-based lubricating oil. 
         [0013]    In the case where the salt, more particularly, an alkali metal carbonate, alkali metal bicarbonate or alkali metal carboxylate, all of which are alkaline when provided in the form of an aqueous solution, is used, a material exhibiting superior degradation inhibitory effects is found. 
         [0014]    Also, the degradation inhibitor need not be a solid. The ionic compound may be used in the form of a solution by dissolving it in a solvent such as water or the like. In either case, an interface is present between the degradation inhibitor and the lubricating oil which are in contact with each other. When two materials, for example, water and oil, which are not mixed with each other, come into contact, the interface is a boundary therebetween. These materials are not limited to liquids, and the boundary between a solid and a liquid is also referred to as an interface. 
         [0015]    In the case where the lubricating oil of the present invention is applied to the dynamic pressure bearing device, the degradation inhibitor is retained in a portion of any one of a shaft and a bearing, and the surface thereof is brought into contact with the lubricating fluid. An example of the portion in which the degradation inhibitor is retained includes a dent, a groove and a hole. Alternatively, the hole of a sintered member such as a metal sintered body or the like may be filled with the degradation inhibitor. 
         [0016]    A conventional degradation inhibitor which is dissolved in the lubricating oil may be used along with the degradation inhibitor which is not dissolved in the lubricating oil, thereby further inhibiting the degradation. 
         [0017]    In accordance with the present invention, lubricating oil having good properties and which is usable for a long period of time can be provided. Also, by the use of the lubricating oil, a dynamic pressure bearing device exhibiting stable performance and high reliability for a long period of time can be provided. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  is a longitudinal cross-sectional view showing a storage disk drive apparatus. 
           [0019]      FIG. 2  is a longitudinal cross-sectional view showing a spindle motor including a dynamic pressure bearing device in accordance with the present invention. 
           [0020]      FIG. 3  is a graph showing the oxidation degradation rates of base oil to which each alkali metal carbonate was added. 
           [0021]      FIG. 4  is a graph showing the relationship between the amount of sodium carbonate added to base oil and an oxidation degradation rate thereof. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     (1) Lubricating Oil Containing Solid Ionic Compound 
     (1-1) Base Oil 
       [0022]    Base oil used for the lubricating oil of the present invention is an ester-based oil, and specific examples thereof include monoester, diester, polyolester (trimethylol propane, pentaerythritol, dipentaerythritol, neopentyldiol ester, complex ester), polyglycol ester, glycerin ester, and aromatic ester. 
         [0023]    Further, the above ester-based lubricating oil may be added with ether oil, such as alkylated diphenyl ether, alkylated triphenyl ether, alkylated tetraphenyl ether and alkylated polyphenyl ether, various poly-α-olefins, various silicone oil species, and various fluorinated oil species. 
         [0024]    Also, examples of the monoester include monoesters composed of any one organic acid selected from among caprylic acid, capric acid, lauric acid, myristic acid, palmitic acid, palmitoleic acid, stearic acid, oleic acid, ricinoleic acid, linoleic acid, linolenic acid, arachidonic acid, eicosa pentaenoic acid, erucic acid, docosa hexaenoic acid and lignoceric acid, and any one monovalent alcohol selected from among methanol, ethanol, propanol, butanol, pentanol, hexanol, heptanol, octanol, nonanol, decanol, undecanol, dodecanol, tridecanol, tetradecanol and pentadecanol. 
         [0025]    Examples of the diester include diesters composed of any one organic acid having two carboxylic groups selected from among malonic acid, methyl malonic acid, succinic acid, methyl succinic acid, dimethyl malonic acid, ethyl malonic acid, glutaric acid, adipic acid, dimethyl succinic acid, pimelic acid, tetramethyl succinic acid, suberic acid, azelaic acid, sebacic acid and brassylic acid, and a same type of or different types of two monovalent alcohol molecules selected from among methanol, ethanol, propanol, butanol, pentanol, hexanol, heptanol, octanol, nonanol, decanol, undecanol, dodecanol, tridecanol, tetradecanol and pentadecanol. 
         [0026]    Examples of the polyolester include polyolesters composed of any one selected from among trimethylol ethane, trimethylol propane and pentaerythritol, and any one selected from among caprylic acid, capric acid, lauric acid, myristic acid, palmitic acid, palmitoleic acid, stearic acid, oleic acid, ricinoleic acid, linoleic acid, linolenic acid, arachidonic acid, eicosa pentaenoic acid, erucic acid, docosa hexaenoic acid and lignoceric acid. 
         [0027]    Examples of the polyglycol ester include glycol esters composed of polyglycol and any one selected from among caprylic acid, capric acid, lauric acid, myristic acid, palmitic acid, palmitoleic acid, stearic acid, oleic acid, ricinoleic acid, linoleic acid, linolenic acid, arachidonic acid, eicosa pentaenoic acid, erucic acid, docosa hexaenoic acid and lignoceric acid. 
         [0028]    Examples of the glycerin ester include monofatty acid glycerin ester, difatty acid glycerin ester, and trifatty acid glycerin ester. The fatty acid linked to glycerin includes one or more selected from among caprylic acid, capric acid, lauric acid, myristic acid, palmitic acid, palmitoleic acid, stearic acid, oleic acid, ricinoleic acid, linoleic acid, linolenic acid, arachidonic acid, eicosa pentaenoic acid, erucic acid, docosa hexaenoic acid and lignoceric acid. 
         [0029]    The polyphenyl ether may have no alkyl group, or may have a linear or branched alkyl group. Specific examples of the alkyl group include methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, tert-butyl, n-pentyl, isopentyl, neopentyl, tert-pentyl, 2-methylbutyl, n-hexyl, isohexyl, 3-methylpentyl, ethylbutyl, n-heptyl, 2-methylhexyl, n-octyl, 2-ethylhexyl, 3-methylheptyl, n-nonyl, methyloctyl, ethylpentyl, n-decyl, n-undecyl, n-dodecyl and n-tetradecyl. 
         [0030]    Although a diester-based oil is used as the base oil of the lubricating oil in the present embodiment, the aforementioned base oil species may be used in various mixture forms. The mixing of two or more oil species may be carried out through a known mixing process. 
       (1-2) Degradation Inhibitor 
       [0031]    In the present invention, to the base oil of the lubricating oil, a degradation inhibitor selected from among ionic compounds is added. In this regard, the ionic compound indicates a material made up of a cation and an anion bonded to each other mainly through an ionic bond to thus constitute a molecule or crystal. The ionic compound typically has a low solubility in oil. In the present invention, a material which is barely dissolvable in the base oil is used as the degradation inhibitor. 
         [0032]    In order to prevent oxidation degradation, particularly useful is an alkali metal carbonate, an alkali metal bicarboante or an alkali metal carboxylate. Among them, however, lithium salts exhibit insignificant oxidation degradation inhibitory effects. The metal carbonates may be used alone or in combinations of two or more. In the case where an alkali metal carbonate, an alkali metal bicarboante or an alkali metal carboxylate is used in an aqueous solution form, the solution thereof is alkaline. The acid dissociation constants pKa of these materials fall in the range of about 9 to 11. 
         [0033]    The carboxylic acid used for the metal carboxylate may vary, and examples thereof include aliphatic saturated monocarboxylic acid, aliphatic unsaturated carboxylic acid, aliphatic dicarboxylic acid, and aromatic carboxylic acid. Examples of the aliphatic saturated monocarboxylic acid include linear saturated acids such as formic acid, acetic acid, propionic acid, butyric acid, valeric acid, caproic acid, caprylic acid, capric acid, laurylic acid, myristic acid, palmitic acid, stearic acid, arachic acid, cerotic acid and laccelic acid, and branched fatty acids such as isopropionic acid, isobutanoic acid, isopentanoic acid, 2-methyl pentanoic acid, 2-methyl butanoic acid, 2,2-dimethyl butanoic acid, 2-methyl hexanoic acid, 5-methyl hexanoic acid, 2,2-dimethyl heptanoic acid, 2-ethyl-2-methyl butanoic acid, 2-ethyl hexanoic acid, dimethyl hexanoic acid, 2-n-propyl pentanoic acid, 3,5,5-trimethyl hexanoic acid, dimethyl octanoic acid, isotridecanoic acid, isomyristic acid, isostearic acid, isoarachic acid and isohexanoic acid. Examples of an unsaturated carboxylic acid include palmitoleic acid, oleic acid, elaidic acid, linoleic acid and linolenic acid, and unsaturated hydroxylic acids such as ricinoleic acid. Examples of the aliphatic dicarboxylic acid include adipic acid, azelaic acid and sebacic acid, and examples of the aromatic carboxylic acid include benzoic acid, phthalic acid, trimelitic acid and pyromellitic acid. Also, an alicyclic fatty acid such as naphthenic acid may be used. The carboxylic acids may be used in combinations of two or more. 
         [0034]    The metal element associated per carboxylic acid may be not only one type but also may be of two or more types. Also, metal carbonates and metal carboxylates each may be used alone or in combinations of two or more. 
         [0035]    In addition to the ionic compound, a conventional inhibitor such as a phenol-based oxidation inhibitor or an amine-based oxidation inhibitor may be used together, thereby more effectively preventing oxidation degradation. 
         [0036]    Examples of the phenol-based oxidation inhibitor include 4,4′-methylenebis(2,6-di-tert-butylphenol), 4,4′-bis(2,6-di-tert-butylphenol), 4,4′-bis(2-methyl-6-tert-butylphenol), 2,2′-methylenebis(4-ethyl-6-tert-butylphenol), 2,2′-methylenebis(4-methyl-6-tert-butylphenol), 4,4′-butylidenebis(3-methyl-6-tert-butylphenol), 4,4′-isopropylidenebis(2,6-di-tert-butylphenol), 2,2′-methylenebis(4-methyl-6-nonylphenol), 2,2′-isobutylidenebis (4,6-dimethylphenol), 2,2′-methylenebis(4-methyl-6-cyclohexylphenol), 2,6-di-tert-butyl-4-methylphenol, 2,6-di-tert-butyl-4-ethylphenol, 2,4-dimethyl-6-tert-butylphenol, 2,6-di-tert-α-dimethylamino-p-cresol, 2,6-di-tert-butyl-4(N,N′-dimethylaminomethylphenol), 4,4′-thiobis(2-methyl-6-tert-butylphenol), 4,4′-thiobis(3-methyl-6-tert-butylphenol), 2,2′-thiobis(4-methyl-6-tert-butylphenol), bis(3-methyl-4-hydroxy-5-tert-butylbenzyl)sulfide, bis(3,5-di-tert-butyl-4-hydroxybenzyl)sulfide, 2,2′-thio-diethylenebis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], tridecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, pentaerythrityl-tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, and 3-methyl-5-tert-butyl-4-hydroxyphenyl-substituted fatty acid esters. Mixtures of two or more thereof may be used as the degradation inhibitor. 
         [0037]    Examples of the amine-based oxidation inhibitor include monoalkyl diphenylamines such as monooctyl diphenylamine and monononyl diphenylamine, dialkyl diphenylamines such as 4,4′-dibutyl diphenylamine, 4,4′-dipentyl diphenylamine, 4,4′-dihexyl diphenylamine, 4,4′-diheptyl diphenylamine, 4,4′-dioctyl diphenylamine and 4,4′-dinonyl diphenylamine, polyalkyl diphenylamines such as tetrabutyl diphenylamine, tetrahexyl diphenylamine, tetraoctyl diphenylamine and tetranonyl diphenylamine, and naphthylamines such as α-naphthylamine, phenyl-α-naphthylamine, butylphenyl-α-naphthylamine, pentylphenyl-α-naphthylamine, hexylphenyl-α-naphthylamine, heptylphenyl-α-naphthylamine, octylphenyl-α-naphthylamine and nonylphenyl-α-naphthylamine. Mixtures of two or more of the amino acid-based oxidation inhibitors may be used as the degradation inhibitor. 
         [0038]    A combination of the phenol-based oxidation inhibitor and the amine-based oxidation inhibitor may also be used. 
         [0039]    In the case where the phenol-based oxidation inhibitor or the amine-based oxidation inhibitor is contained in the lubricating oil for use in the dynamic pressure bearing device of the present invention, the amount thereof should be set to 5.0 wt % or less, preferably 3.0 wt % or less, and more preferably 1.0 wt % or less, based on the total amount of the lubricant. When the amount thereof exceeds 5.0 wt %, an adequate oxidation inhibitory effect in comparison to the amount added is not obtained. In order to attain a desired oxidation degradation inhibitory effect, the oxidation inhibitor should be used in an amount of at least 0.1 wt % based on the total amount of the lubricant. 
         [0040]    If necessary, various additives which are conventionally known, such as a viscosity enhancer, a pour point depressant, a metal inactivator, a surfactant, a rust-proof agent, and an anticorrosive agent may be added while exhibiting the effects of the present invention. 
       (1-3) Description of Lubricant 
       [0041]    Below, the compositions of 12 types of lubricants as examples of the present invention and 7 types of lubricants of the comparative examples will be described. The base oil used for the following examples is diester. 
       (1-3-1) Compositions of Exemplary Lubricants of the Present Invention 
     Example 1 
       [0042]    1 wt % of sodium carbonate was added to 100 parts by weight of the base oil. 
       Example 2 
       [0043]    1 wt % of sodium carbonate and 0.2 wt % of a mixture of 2,6-di-tert-butyl-4-ethylphenol and 4,4′-dibutyl diphenylamine were added to 100 parts by weight of the base oil. 
       Example 3 
       [0044]    1 wt % of sodium bicarbonate and 0.2 wt % of a mixture of 2,6-di-tert-butyl-4-ethylphenol and 4,4′-dibutyl diphenylamine were added to 100 parts by weight of the base oil. 
       Example 4 
       [0045]    1 wt % of lithium carbonate and 0.2 wt % of a mixture of 2,6-di-tert-butyl-4-ethylphenol and 4,4′-dibutyl diphenylamine were added to 100 parts by weight of the base oil. 
       Example 5 
       [0046]    1 wt % of potassium carbonate and 0.2 wt % of a mixture of 2,6-di-tert-butyl-4-ethylphenol and 4,4′-dibutyl diphenylamine were added to 100 parts by weight of the base oil. 
       Example 6 
       [0047]    1 wt % of rubidium carbonate and 0.2 wt % of a mixture of 2,6-di-tert-butyl-4-ethylphenol and 4,4′-dibutyl diphenylamine were added to 100 parts by weight of the base oil. 
       Example 7 
       [0048]    1 wt % of cesium carbonate and 0.2 wt % of a mixture of 2,6-di-tert-butyl-4-ethylphenol and 4,4′-dibutyl diphenylamine were added to 100 parts by weight of the base oil. 
       Example 8 
       [0049]    1 wt % of sodium formate and 0.2 wt % of a mixture of 2,6-di-tert-butyl-4-ethylphenol and 4,4′-dibutyl diphenylamine were added to 100 parts by weight of the base oil. 
       Example 9 
       [0050]    1 wt % of sodium acetate and 0.2 wt % of a mixture of 2,6-di-tert-butyl-4-ethylphenol and 4,4′-dibutyl diphenylamine were added to 100 parts by weight of the base oil. 
       Example 10 
       [0051]    1 wt % of ethylenediamine tetraacetate-tetrasodium (EDTA-4Na) and 0.2 wt % of a mixture of 2,6-di-tert-butyl-4-ethylphenol and 4,4′-dibutyl diphenylamine were added to 100 parts by weight of the base oil. 
       Example 11 
       [0052]    0.5 wt % of sodium carbonate and 0.2 wt % of a mixture of 2,6-di-tert-butyl-4-ethylphenol and 4,4′-dibutyl diphenylamine were added to 100 parts by weight of the base oil. 
       Example 12 
       [0053]    0.25 wt % of sodium carbonate and 0.2 wt % of a mixture of 2,6-di-tert-butyl-4-ethylphenol and 4,4′-dibutyl diphenylamine were added to 100 parts by weight of the base oil. 
         [0054]    The lubricants of Examples 1 to 12 were formulated by adding the additives to the base oil and then performing a stirring process. Because of the stirring process, the additives except for the salt were dissolved in the base oil, but most of the added salt did not dissolve but was precipitated. 
       (1-3-2) Comparative Examples 
     Comparative Example 1 
       [0055]    The base oil was used alone. 
       Comparative Example 2 
       [0056]    0.2 wt % of an oxidation inhibitor composed of a mixture of 2,6-di-tert-butyl-4-ethylphenol and 4,4′-dibutyl diphenylamine was added to 100 parts by weight of the base oil. 
       Comparative Example 3 
       [0057]    1 wt % of calcium carbonate and 0.2 wt % of a mixture of 2,6-di-tert-butyl-4-ethylphenol and 4,4′-dibutyl diphenylamine were added to 100 parts by weight of the base oil. 
       Comparative Example 4 
       [0058]    1 wt % of barium carbonate and 0.2 wt % of a mixture of 2,6-di-tert-butyl-4-ethylphenol and 4,4′-dibutyl diphenylamine were added to 100 parts by weight of the base oil. 
       Comparative Example 5 
       [0059]    1 wt % of diethyl carbonate and 0.2 wt % of a mixture of 2,6-di-tert-butyl-4-ethylphenol and 4,4′-dibutyl diphenylamine were added to 100 parts by weight of the base oil. 
       Comparative Example 6 
       [0060]    1 wt % of sodium sulfate and 0.2 wt % of a mixture of 2,6-di-tert-butyl-4-ethylphenol and 4,4′-dibutyl diphenylamine were added to 100 parts by weight of the base oil. 
       Comparative Example 7 
       [0061]    0.3 wt % of sodium hydroxide and 0.2 wt % of a mixture of 2,6-di-tert-butyl-4-ethylphenol and 4,4′-dibutyl diphenylamine were added to 100 parts by weight of the base oil. 
         [0062]    The lubricants of Comparative Examples 1 to 7 were formulated by adding the additives to the base oil and then performing a stirring process. Because of the stirring process, the additives except for the salt and sodium hydroxide were dissolved in the base oil. 
       (1-4) High-Pressure Oxygen Test 
       [0063]    For the lubricants of Examples 1 to 10 and Comparative Examples 1 to 7, the following high-pressure oxygen test was conducted, and the oxidation degradation of the lubricating oil was evaluated. 
       (1-4-1) Test Conditions 
       [0064]    Each of the lubricants was sealed with oxygen (0.9 MPa) and allowed to stand in a thermostatic bath at 150° C. for 60 hours (but 8 hours in Example 1 and Comparative Example 1), after which the degradation rate of the lubricating oil was measured. 
         [0065]    The degradation rate was determined using liquid chromatography. The peak of the base oil and the peak of the polymer or monomer subjected to oxidation degradation were detected. The proportion (%) of the peak area of each of the polymer and monomer subjected to oxidation degradation per the total peak area was calculated, and thus determined to be a polymer degradation rate and a monomer degradation rate. The results are shown in Table 1 below. 
       (1-4-2) Test Results 
       [0066]      
         [0000]    
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 Addition of 
                 Polymer 
                 Monomer 
                 (Polymer + 
               
               
                   
                 Added 
                 Oxidation 
                 Oxidation 
                 Oxidation 
                 Monomer) Oxidation 
               
               
                   
                 Sample 
                 Inhibitor 
                 Degradation (%) 
                 Degradation (%) 
                 Degradation (%) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Ex. 1 
                 Na 2 CO 3   
                 No 
                 3.69 
                 13.32 
                 17.25 
               
               
                 Ex. 2 
                 Na 2 CO 3   
                 Yes 
                 1.41 
                 5.56 
                 6.97 
               
               
                 Ex. 3 
                 NaHCO 3   
                 Yes 
                 3.30 
                 7.67 
                 10.97 
               
               
                 Ex. 4 
                 Li 2 CO 3   
                 Yes 
                 11 
                 12.11 
                 23.46 
               
               
                 Ex. 5 
                 K 2 CO 3   
                 Yes 
                 0.00 
                 2.97 
                 2.97 
               
               
                 Ex. 6 
                 Rb 2 CO 3   
                 Yes 
                 0.75 
                 5.93 
                 6.68 
               
               
                 Ex. 7 
                 Cs 2 CO 3   
                 Yes 
                 1.82 
                 8.55 
                 10.37 
               
               
                 Ex. 8 
                 HCOONa 
                 Yes 
                 4.28 
                 7.99 
                 12.27 
               
               
                 Ex. 9 
                 CH 3 COONa 
                 Yes 
                 8.05 
                 9.75 
                 17.80 
               
               
                 Ex. 10 
                 EDTA-4Na 
                 Yes 
                 6.90 
                 7.71 
                 14.61 
               
               
                 C. Ex. 1 
                 None 
                 No 
                 13.62 
                 15.07 
                 28.69 
               
               
                 C. Ex. 2 
                 None 
                 Yes 
                 13.73 
                 10.14 
                 23.87 
               
               
                 C. Ex. 3 
                 Ca 2 CO 3   
                 Yes 
                 10.28 
                 14.19 
                 24.47 
               
               
                 C. Ex. 4 
                 Ba 2 CO 3   
                 Yes 
                 14.19 
                 13.61 
                 27.80 
               
               
                 C. Ex. 5 
                 Et 2 CO 3   
                 Yes 
                 13.52 
                 9.29 
                 22.81 
               
               
                 C. Ex. 6 
                 Na 2 SO 4   
                 Yes 
                 23.02 
                 15.60 
                 38.62 
               
               
                 C. Ex. 7 
                 NaOH 
                 Yes 
                 12.06 
                 10.17 
                 22.23 
               
               
                   
               
               
                 * C. Ex. stands for Comparative Example. 
               
             
          
         
       
     
         [0067]    As is apparent from the results of Example 1 and Comparative Example 1 in Table 1, the oxidation degradation rate of polymer in Example 1 is considerably low. That is, sodium carbonate can be seen to remarkably inhibit oxidation degradation of polymer. 
         [0068]    As is apparent from the results of Example 2 and Comparative Example 2, the oxidation degradation rate polymer in Example 2 is further improved compared to that of Example 1, and also the oxidation degradation rate of monomer is drastically improved. That is, sodium carbonate can be seen to further inhibit both the polymer oxidation degradation and the monomer oxidation degradation in the presence of the conventional oxidation degradation inhibitor. 
         [0069]    Table 2 below shows the results of Examples 4, 2, 5, 6 and 7 among the examples of Table 1, in order to exhibit the effects of carbonates of alkali metals from lithium to cesium in the periodic table. These results are graphed in  FIG. 3 . 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
                 Polymer 
                 Monomer 
                 {Polymer + 
               
               
                   
                   
                 Oxidation 
                 Oxidation 
                 Monomer} 
               
               
                   
                 Alkali 
                 Degradation 
                 Degradation 
                 Oxidation 
               
               
                   
                 Metal 
                 (%) 
                 (%) 
                 Degradation (%) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Ex. 4 
                 Li 
                 11.35 
                 12.11 
                 23.46 
               
               
                 Ex. 2 
                 Na 
                 1.41 
                 5.56 
                 6.97 
               
               
                 Ex. 5 
                 K 
                 0.00 
                 2.97 
                 2.97 
               
               
                 Ex. 6 
                 Rb 
                 0.75 
                 5.93 
                 6.68 
               
               
                 Ex. 7 
                 Cs 
                 1.82 
                 8.55 
                 10.37 
               
               
                   
               
             
          
         
       
     
         [0070]    As is apparent from the results of Table 2 and  FIG. 3 , carbonates of alkali metals from sodium to cesium play a role in drastically improving both the polymer degradation rate and the monomer degradation rate. The greatest improvement effect is exhibited in the case of potassium. Sodium and rubidium exhibit improvement effects slightly inferior to those of potassium, and effects based on cesium become a lot more inferior. However, it is obvious that a carbonate of any metal selected from among these metals can inhibit the degradation. 
         [0071]    Among alkali metal carbonates, lithium carbonate has an insignificant oxidation degradation inhibitory effect. Among alkali metals, lithium is a slightly special element, and a carbonate thereof has properties different from those of the other alkali metal carbonates, which may result in a different improvement effect. 
         [0072]    Table 3 below shows the results of a high-temperature oxygen test of Examples 2, 11 and 12 and Comparative Example 2. These results are graphed in  FIG. 4 . 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                   
                 Polymer 
                 Monomer 
                 {Polymer + 
               
               
                   
                 Sodium 
                 Oxidation 
                 Oxidation 
                 Monomer} 
               
               
                   
                 Carbonate 
                 Degradation 
                 Degradation 
                 Oxidation 
               
               
                   
                 (wt %) 
                 (%) 
                 (%) 
                 Degradation (%) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Ex. 2 
                 1.00 
                 1.41 
                 5.56 
                 6.97 
               
               
                 Ex. 11 
                 0.50 
                 2.66 
                 6.62 
                 9.28 
               
               
                 Ex. 12 
                 0.25 
                 8.41 
                 9.31 
                 17.72 
               
               
                 C. Ex. 2 
                 0 
                 13.73 
                 10.14 
                 23.87 
               
               
                   
               
             
          
         
       
     
         [0073]    As is apparent from Table 3 and  FIG. 4 , as the amount of sodium carbonate added increases, the polymer oxidation degradation rate and the monomer oxidation degradation rate are improved. Even when 0.25 wt % of sodium carbonate is added, the inhibitory effect is exhibited but is not largely different from that of the comparative example. As depicted in  FIG. 4 , when the sum of the polymer oxidation degradation rate and the monomer oxidation degradation rate is determined to be 20% or less as the minimum level for improvement, sodium carbonate should be added in an amount of at least 0.1 wt %. 
       (2) Lubricating Oil Added with Aqueous Solution 
     (2-1) Composition of Additives 
       [0074]    50 g of base oil added with the mixture of 2,6-di-tert-butyl-4-ethylphenol and 4,4′-dibutyl diphenylamine was added with 1.2 ml of an aqueous metal carbonate solution, thus preparing a lubricant (Example 13). In addition, 50 g of base oil added with the mixture of 2,6-di-tert-butyl-4-ethylphenol and 4,4′-dibutyl diphenylamine was added with 1.2 ml of an aqueous EDTA-4Na solution, thus preparing a lubricant (Example 14). Thereafter, an oxidation stability test was conducted. 
       (2-2) Oxidation Stability Test (RBOT Test) 
       [0075]    The oxidation lifespan of the lubricating oil was measured using a RBOT method according to a JIS standard test (JIS K2514), and the RBOT value was calculated. That is, in a sealable vessel, water, a copper coil and the lubricating oil were introduced into the above aqueous solution and then pressurized to 620 kPa by oxygen, after which the sealed vessel was placed in a thermostatic bath at 150° C. and was then continuously rotated at 100 rpm while being kept inclined at an angle of 30°. When the inner pressure reached the maximum level, the period of time required to drop the pressure to 175 kPa was measured. The lubricating oil of Comparative Example 2 was subjected to the same test without the use of the above aqueous solution. In this case, the lubricating oil used for the test was in a state of being in contact with water in lieu of the aqueous solution. The results are shown in Table 4 below. 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                 Ex. 13 
                 Ex. 14 
                 C. Ex. 2 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 RBOT value [min] 
                 10394 
                 15202 
                 4828 
               
               
                   
                   
               
             
          
         
       
     
         [0076]    As is apparent from Table 4, in Example 13 using the aqueous metal carbonate solution, the RBOT value is at least two times that of Comparative Example 2 without the use of the aqueous solution. Also, in Example 14 using the aqueous EDTA-4Na solution, the RBOT value is at least three times that of Comparative Example 2. In both cases, oxidation stability is remarkably improved. The reason why the oxidation stability is increased in the RBOT test under a condition in which water is added to the lubricating fluid is because hydrolysis of the base oil is inhibited. 
       (3) Dynamic Pressure Bearing Device, Spindle Motor &amp; Disk Drive Apparatus 
     (3-1) Disk Drive Apparatus 
       [0077]      FIG. 1  shows the internal construction of a disk drive apparatus  60  (in the present embodiment, a hard disk drive apparatus). A housing  61  of the disk drive apparatus  60  has a clean space in which an amount of dust or impurities is very small. The housing  61  include therein a spindle motor  1  which drives a disk and is provided with a disk-shaped storage medium  62  for storing information, and an access unit  63  for writing/reading information to/from the storage medium  62 . 
       (3-2) Spindle Motor 
       [0078]      FIG. 2  is a longitudinal cross-sectional view showing the construction of the spindle motor  1 . The spindle motor  1  includes a stationary member and a rotational member. By the dynamic pressure bearing device in accordance with the embodiment of the present invention, the rotational member is rotatably supported with respect to the stationary member via a rotary shaft  32 . 
       (3-2-1) Stationary Member of Spindle Motor 
       [0079]    A base  10  has a flat portion  11  provided at the center thereof and an annular boss portion  13  provided on the central region of the flat portion  11 . An annular recess is defined between the annular boss portion  13  and an annular stepped portion  14  provided on the outer periphery of the flat portion  11 . A stator  17  fixed to the flat portion  11  and a rotor magnet  34  attached to a hub  31  which will be described later are disposed in the above recess. The annular boss portion  13  is positioned near the outer periphery of a cylindrical support wall  15  protruding upwards, and the stator  17  is fixed to the above outer periphery. The stator  17  includes an annular stator core  17   a  formed by laminating a plurality of electromagnetic steel plates, and multi-phase (e.g., three-phase) coils  17   b  wound on respective teeth of the stator core  17   a . The stator core  17   a  of the stator  17  is fitted onto the cylindrical support wall  15  and is fixed through press fitting, adhesion or the like. Thus, the stator  17  is fixed to the cylindrical support wall  15 . The fixing process includes press fitting, adhesion or the like. 
         [0080]    A bearing stationary portion  20  made of stainless steel, constituting a part of the dynamic pressure bearing device, is fitted into the annular boss portion  13  and is fixed thereto. The bearing stationary portion  20  includes a substantially cylindrical sleeve  21 , and a counter plate  22  closing the lower-end opening of the sleeve  21 . The inner peripheral surface of the through hole of the sleeve  21  is divided into a small-diameter inner peripheral surface  21   a  extending over substantially the entire length of the sleeve  21  where a radial bearing portion is located, an intermediate-diameter inner peripheral surface  21   b  located at the lower portion of the sleeve  21  and formed to have a greater diameter than that of the small-diameter inner peripheral surface  21   a , and a large-diameter inner peripheral surface  21   c  located at the lowest end of the sleeve  21  and formed to have a greater diameter than that of the intermediate-diameter inner peripheral surface  21   b . The counter plate  22  is disposed in the space inside the large-diameter inner peripheral surface  21   c  and is fixed to the sleeve  21  through press fitting, caulking, welding, adhesion or the like. The lower half portion of the outer peripheral surface of the sleeve  21  is fixed to the inner peripheral surface of the annular boss portion  13  through press fitting, adhesion, welding or the like. The upper outer peripheral surface of the sleeve  21  is formed with a tapered surface  23  which forms an inner peripheral surface of a tapered sealing portion which will be mentioned later. As the tapered surface  23  extends upward in the drawing, it becomes more distant from the central axis of the bearing. 
       (3-2-2) Rotational Member &amp; Dynamic Pressure Bearing Device 
       [0081]    A rotor  30  includes an inverted cup-shaped hub  31  and the rotary shaft  32  disposed in the rotational center of the hub  31 . Because the rotary shaft  32  is supported by the bearing stationary portion  20 , the rotor  30  is rotatable with respect to the flat portion  11 . 
         [0082]    The hub  31  is made of a ferromagnetic material such as iron or stainless steel. Connected to the outer periphery of a disk-shaped portion  31   a  constituting a top plate is a cylindrical portion  31   b  extending downward in the drawing. Provided at the lower end of the cylindrical portion  31   b  is a flange  31   c  protruding radially outwardly. Inside the cylindrical portion  31   b , an annular wall  31   d  extending downward from the disk-shaped portion  31   a  is disposed. The annular wall  31   d  is disposed between the sleeve  21  and the cylindrical support wall  15  to surround the upper outer periphery of the sleeve  21 . Also, there is formed a labyrinth gap for defining a labyrinth seal between the annular wall  31   d  and the cylindrical support wall  15 . 
         [0083]    An attachment hole  31   e  is formed in the center of the disk-shaped portion  31   a , and the upper end of the rotary shaft  32  having a slightly smaller diameter is press fitted into the hole. Accordingly, the hub  31  and the rotary shaft  32  are integrated with each other. The rotary shaft  32  is hollow, and a female threaded portion  32   b  is formed over substantially the entire length of the inner peripheral surface thereof. The outer peripheral surface  32   a  of the rotary shaft  32  and the small-diameter inner peripheral surface  21   a  of the sleeve  21  are radially arranged with a slight gap therebetween. 
         [0084]    The leading end of the rotary shaft  32  passed through the sleeve  21  slightly protrudes downward from the small-diameter inner peripheral surface  21   a . A removal prevention member  33  has a male threaded portion  33   a  threadedly engaged with the female threaded portion  32   b  of the rotary shaft  32  and a circular plate  33   b . The circular plate  33   b  has an outer diameter larger than the outer diameter of the rotary shaft  32  and smaller than the inner diameter of the intermediate-diameter inner peripheral surface  21   b . A gap is defined between the circular plate  33   b  and the sleeve, and the rotary shaft  32  including the removal prevention member  33  is rotatable with respect to the sleeve  21 . In the case where force is applied in the direction in which the rotary shaft  32  is removed from the sleeve, the circular plate  32   b  comes into contact with the sleeve  21 , thereby preventing the removal of the rotary shaft  32 . 
         [0085]    Provided inside the cylindrical portion  31   b  of the hub  31  is an annular rotor magnet  34  including a plurality of magnetic poles arranged in a circumferential direction. The rotor magnet  34  is disposed to surround the outer periphery of the stator  17 . The hub  31  made of a ferromagnetic material also functions as a back yoke of the magnet  34 . 
         [0086]    Mounted on the flange  31   c  of the hub  31  is a single or a plurality of storage disks (hard disks) (not shown). The hard disk has a hole at the center thereof, and the periphery of the hole is in contact with the outer peripheral surface of the cylindrical wall  31   b . A clamp member is attached to the hub. The clamp member is brought into contact with the upper surface near the hole of the disk to hold the disk together with the flange  31   c  therebetween. 
         [0087]    The clamp member is fixed to the rotary shaft by means of a screw threadedly engaged with the female threaded portion  32   b  of the rotary shaft  32  from above. 
         [0088]    A fine gap is formed between the small-diameter inner peripheral surface  21   a  of the sleeve  21  and the outer peripheral surface  32   a  of the rotary shaft  32  and between the lower surface of the disk-shaped portion  31   a  of the hub  31  and the upper end surface of the sleeve  21 , and is filled with the lubricating oil  40 . The lubricating oil  40  contains a mixture of 2,6-di-tert-butyl-4-ethylphenol and 4,4′-dibutyl diphenylamine. 
         [0089]    The lubricating oil  40  is also filled in the space defined by the intermediate-diameter inner peripheral surface  21   b  of the sleeve  21 , the surface of the counter plate  22 , and the surface of the circular plate  33   b  of the removal prevention member  33 . The lubricating oil  40  is in contact with the air in the tapered sealing portion  41  defined by the inner peripheral surface  31   f  of the annular wall  31   d  of the hub  31  and the tapered surface  23  of the upper outer periphery of the sleeve  21 , and the level surface thereof has an arcuate section shape. The tapered sealing portion  41  has a tapered shape in which the gap is gradually reduced as it goes upward. 
         [0090]    In the small-diameter inner peripheral surface  21  of the sleeve  21 , herringbone-shaped dynamic pressure generating grooves are respectively formed at two positions separated from each other in the axial direction, corresponding to reference numerals  42  and  43  in the drawing. The dynamic pressure generating grooves create a bearing force for holding the rotary shaft  32  in a radius direction when the spindle motor is rotated in a specific direction. That is, a pair of radial dynamic pressure bearings is disposed in the positions  42  and  43 . Also, a spiral-shaped dynamic pressure generating groove is formed in the upper end surface of the sleeve  21  to constitute a thrust dynamic pressure bearing  44 . The spiral-shaped groove functions to increase the pressure of the lubricating oil inward compared to the region where the dynamic pressure generating groove is formed when the spindle motor is rotated in the specific direction, and also to create a force for lifting the hub  31  upward in the axial direction. 
         [0091]    The sleeve  21  has a communication hole  45 , which extends in the axial direction thereof and is filled with the lubricating oil  40 . The lower end of the communication hole  45  is opened toward the intermediate-diameter inner peripheral surface  21   b  and the upper end thereof is opened at an inside area of the thrust dynamic pressure bearing  44  in the thrust gap. The communication hole  45  is formed such that both ends of two radial dynamic pressure bearings  42 ,  43  communicate with each other, and enables the circulation of the lubricating oil  40  in the bearing device. 
         [0092]    A recessed portion  70  is provided in the outer periphery of the intermediate-diameter inner peripheral surface  21   b . Potassium carbonate is applied inside the recessed portion  70  and is always in contact with the lubricating oil  40 . Instead of potassium carbonate, an aqueous solution of potassium carbonate may be used. 
         [0093]    The sleeve  21  may be made of a porous sintering metal instead of stainless steel. In this case, pores of a portion of the sleeve may be filled with potassium carbonate or an aqueous solution thereof and then sealed, whereas pores of the other portion of the sleeve are filled with the lubricating oil. In this way, the lubricating oil and the potassium carbonate may be in contact with each other in the sintered body. 
         [0094]    Alternatively, there may be provided a construction in which potassium carbonate is disposed in a lower portion  71  of the wall surface of the tapered sealing portion  41 , so that the lubricating oil  40  comes in contact with potassium carbonate only when the lubricating oil  40  expands with an increased temperature and the interface thereof is moved downward. In this case, only at high temperatures at which degradation rapidly progresses, does the lubricating oil  40  come into contact with the potassium carbonate serving as the degradation inhibitor. While the contact between the potassium carbonate and the lubricating oil is held at a minimum, the degradation of the lubricating oil may be effectively prevented. 
         [0095]    As described hereinbefore, the lubricant, the method of preventing the degradation of the lubricating oil, and the dynamic pressure bearing device in accordance with the present invention are illustrated, but the present invention is not limited thereto and various modifications can be made without departing from the scope of the present invention. 
         [0096]    For example, in the embodiment of the present invention, the dynamic pressure bearing device includes two radial dynamic pressure bearings and one thrust dynamic pressure bearing, but the structure thereof is not limited thereto. Also, the positions of the dynamic pressure generating grooves are not limited to those in the above embodiment. 
         [0097]    Also, examples of the ionic compound which is brought into contact with the lubricating oil are not limited to those generating oxidation degradation inhibitory effects. For example, a material such as silica gel having hygroscopicity may be disposed in the recessed portion  70  of  FIG. 2 .