Abstract:
To provide a grease composition for HDD which is excellent in low outgas generation and acoustic properties and a process for production thereof. In a grease composition used in the bearing for a hard disk drive, a basal oil comprising a paraffin-base hydrocarbon and a polyurea as a thickener are contained.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to grease compositions used for a hard disk drive and a process for production thereof.  
           [0003]    2. Description of the Related Art  
           [0004]    [0004]FIG. 1 shows a structure of cross section of the hitherto conventional hard disk drive (also referred to as an HDD). As seen from the accompanying figure, magnetic disk  1  is fixed on the axis  4  of rotation in a freely rotatable state through the rotor  3  of DC motor  2 , wherein axis  4  is hold by ball bearing  5 . Pivot assembly  10  that is attached to magnetic disk  1  to read and record the magnetic data is equipped with swing arm  12  which has a magnetic head  11  at the tip. Swing arm  12  is fixed on shaft  14  in a freely swinging state through ball bearing  13 .  
           [0005]    For such ball bearings  5  and  13 , grease is used, for which is usually required a lubricating property, particularly a long-term acoustic property. In recent years, however, also required is reduction of the level of outgas generated under operating conditions. That is, it becomes the most important problem to reduce the outgas increasing with the use of a large-capacity HDD or small-size HDD, increase of reliability and accuracy, etc. Accordingly, the reduction of outgas generation is highly necessitated, so as to be at a much lower gas level than in the conventional grease.  
           [0006]    As the conventionally used grease, thickened grease prepared by adding an isocyanate compound and an amine to a basal oil for a reaction to yield a diurea is known (e.g., Japanese Patent Publication No. (herein after referred to as JP-B) 39-3114, JP-B 53-9243). Such grease, however, cannot be used for HDD because of the high level of outgas.  
           [0007]    On the other hand, Japanese Unexamined Patent Publication No.11-325086/99 discloses low outgas grease prepared by adding a lithium soap-type thickener to a basal oil comprising an aromatic ester alone or an oily mixture of an aromatic ester and a polyol ester. This official gazette also discloses the diurea-thickened grease added to the basal oil as an example for comparison. This, however, seems inappropriate to use for HDD in view of its acoustic property and outgas.  
           [0008]    However, it was found that such low outgas grease has no performance required of grease for recent high performance HDD.  
           [0009]    In view of the above circumstances, the object of the present invention is to provide a grease composition for BDD, which is excellent in low outgas generation and acoustic property, and a method for production thereof.  
         SUMMARY OF THE INVENTION  
         [0010]    The first embodiment of the invention for solving the above-mentioned problems is to provide a grease composition for a hard disk drive which comprises a basal oil comprising a paraffin-base hydrocarbon and a polyurea as a thickener, which is used in the bearing for the hard disk drive.  
           [0011]    The second embodiment of the invention is to provide a grease composition for a hard disk drive as described in the first embodiment, wherein the paraffin hydrocarbon comprises a polyolefin represented by the following general formula: 
             H—[R   1  ] n   —H   
           [0012]    (wherein R 1  is a saturated hydrocarbon group of which the carbon number is 8-12; and n is a real number of 3-5) alone or as a mixture of the olefins as a main component  
           [0013]    The third embodiment of the invention is to provide a grease composition for a hard disk drive as described in the first and second embodiments, wherein the paraffin-base hydrocarbon comprises components of which the carbon number is 30 or more as main components.  
           [0014]    The fourth embodiment of the invention is to provide a grease composition for a hard disk drive as described in the third embodiments, wherein the paraffin-base hydrocarbon comprises components from which volatile components of which the carbon number is less than 30 have been eliminated in advance.  
           [0015]    The fifth embodiment of the invention is to provide a grease composition for a hard disk drive as described in the third and fourth embodiments, where in the content of the volatile components of which the carbon number is less than 30 is 30 ppm or less.  
           [0016]    The sixth embodiment of the invention is to provide a grease composition for a hard disk drive as described in any of the first to fifth embodiments, wherein the polyurea is formed by reacting a polyisocyanate readily soluble in the paraffin-base hydrocarbon with a primary or secondary (poly)amine in the paraffin-base hydrocarbon.  
           [0017]    The seventh embodiment of the invention is to provide a grease composition for a hard disk drive as described in the sixth embodiment, wherein the polyisocyante is an aliphatic isocyanate.  
           [0018]    The eighth embodiment of the invention is to provide a grease composition for a hard disk drive as described in the sixth or seventh embodiment, wherein the (poly)amine has a molecular weight of 400 or less.  
           [0019]    The ninth embodiment of the invention is to provide a grease composition for a hard disk drive as described in any of the sixth to eighth embodiment, wherein the residual amount of the starting material contained in the polyurea is 30 ppm or less.  
           [0020]    The tenth embodiment of the invention is to provide a grease composition for a hard disk drive as described in any of the first to eighth embodiment, wherein the polyurea is contained as particles of 1 or less in average size.  
           [0021]    The 11th embodiment of the invention is to provide a process for producing a grease composition used in the bearing for a hard disk drive, which comprises a step for producing a polyurea as a thickener by adding a polyisocyanate readily soluble in a paraffin-base hydrocarbon and a primary or secondary (poly)amine into a basal oil comprising the paraffin-base hydrocarbon.  
           [0022]    The 12th embodiment of the invention is to provide a process for producing a grease composition for a hard disk drive as described in the 11th embodiment, wherein the paraffin-base hydrocarbon comprises a polyolefin of the above-described general formula alone or a mixture of the polyolefins as main components.  
           [0023]    The 13th embodiment of the invention is to provide a process for producing a grease composition for a hard disk drive as described in the 11th or 12th embodiment, wherein the polyisocyanate is an aliphatic isocyanate.  
           [0024]    The 14th embodiment of the invention is to provide a process for producing a grease composition for a hard disk drive as described in any of the 11th to 13th embodiment, wherein the (poly)amine has a molecular weight of 400 or less.  
           [0025]    The 15th embodiment of the invention is to provide a process for producing a grease composition for a hard disk drive as described in any of the 11th to 14th embodiment, which comprises a step for treating at least the basal oil under reduced pressure.  
           [0026]    The 16th embodiment of the invention is to provide a process for producing a grease composition for a hard disk drive as described in any of the 11th to 15th embodiment, which comprises a step of adding an excess amount of the (poly)amine to the polyisocyanate and eliminating the residual (poly)amine after the reaction.  
           [0027]    The 17th embodiment of the invention is to provide a process for producing a grease composition for a hard disk drive as described in any of the 11th to 16th embodiment, wherein the residual amount of the starting material is 30 ppm or less.  
           [0028]    According to claim the invention, a grease composition for EDD of which the outgas level is very low can be realized. Moreover, the polyurea contained in the basal oil exists as very fine particles and is very effective in an increase of an acoustic lifetime.  
           [0029]    The grease composition for a hard disk drive of the invention contains a basal oil comprising a paraffin-base hydrocarbon and a polyurea as a thickener.  
           [0030]    The paraffin-base hydrocarbon is used as basal oil because it is more stable and less in a volatile component than, for example, ester-type basal oils. In view of this point, it is preferred to use a synthetic paraffin-base hydrocarbon, in which the main component is preferably those of which the carbon number is 30 or more and contains substantially no volatile component of which the carbon number is less than 30. The volatile component of which the carbon number is less than 30 may preferably be contained at a content of 30 ppm or less. This volatile component may be eliminated in advance or in the course of the production.  
           [0031]    In a preferred embodiment, it is preferable to use a polyolefin represented by the above-mentioned formula  1  alone or as a mixture. Such a polyolefin is available as SHINFIELD PAO (trade name) (made by Nippon Steel Chemical Co. Ltd.) or PAO SH series (trade name)(made by Exxon-Mobil Corp.).  
           [0032]    The content of the polyurea in the grease composition for HDD of the invention is usually in 2-50 weight parts for 100 weight parts of basal oil, at which content the basal oil can be thickened.  
           [0033]    The polyurea may be prepared by adding a starting poly-isocyanate and a primary or secondary amine or polyamine (herein the amine or polyamine is referred to as (poly)amine) to a basal oil, in which reaction it is preferable to use a polyisocyanate readily soluble in the paraffin-base hydrocarbon. The reason is that when the polyisocyanate exists as large particles incompatible with the basal oil the generated polyurea is not of fine particles and poor in acoustic properties.  
           [0034]    In this invention, the polyurea is preferably contained in an average particle size of 1 im or less, because particles larger than this size results in an insufficient acoustic property.  
           [0035]    In this connection, it is preferable to use an aliphatic polyisocyanate as readily soluble in the paraffin-base hydrocarbon, including, for example, 1, 6-hexane diisocyanater isophorone diisocyanate, hydrogenated diphenylmethane diisocyanate, and the like. In addition, metaxylylene diisocyanate that is readily soluble in paraffin-base hydrocarbons may preferably be used though it is aromatic. Thus, the polyisocyanate can be dissolved compatibly as fine particles in a paraffin-base hydrocarbon, in which the (poly) amine added is allowed to react to yield the polyurea as fine particles. In this operation, it is necessary to select a stirring condition in order to give a compatible solubility as fine particles to the polyisocyanate.  
           [0036]    In the reaction of the starting polyisocyanate and the primary or secondary (poly)amine added to the basal oil to yield the polyurea, it is preferable to add an excess amount of the (poly)amine to the polyisocyanate since the (poly)amine can more easily be removed among the starting materials remaining unchanged. In this respect, the (poly) amine is preferably used in a molecular weight as small as possible, preferably, of 400 or less.  
           [0037]    Accordingly, it is preferable to add the polyisocyanate and the (poly) amine to the basal oil to yield the polyurea and then treat the latter under reduced pressure.  
           [0038]    Thus, the starting materials remaining unchange regarded as the cause of outgas can be removed so that the content of the starting materials is preferably 30 ppm or less.  
           [0039]    In this operation, the treatment under reduced pressure may preferably be carried out in such a condition as at least the starting materials for the polyurea can be removed, for example, at a degree of 300 mmHg or lower, preferably, 10 mmHg or lower. The treatment under reduced pressure may be performed at a temperature of 50-200° C., preferably 100-150° C., for a period of 30 minutes or longer, preferably 2 hours or longer.  
           [0040]    The treatment under reduced pressure may be achieved with a vacuum pump or water-jet pump, as well as with molecular distillation, azeotropic distillation, etc.  
           [0041]    By such treatment, the starting materials remaining unchanged and the volatile components in the basal oil are removed and very low outgas-level grease can be realized.  
           [0042]    In this connection, the treatment under reduced pressure may be effected not only after the reaction completion but also on the basal oil alone. Thus, low molecular weight components can be removed even in the basal oil containing low molecular components. Accordingly, the components of which the carbon number is less than 30 are preferably reduced, for example, to a content of 30 ppm or less, more preferably so that they contain substantially no low molecular weight component.  
           [0043]    The grease compositions of the invention may contain further additives for improving other grease properties as far as they afford decisive effect on the generation of outgas. Such additives include antioxidants, lubrication enhancers, rust preventives, thickeners, surfactants, and the like. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0044]    [0044]FIG. 1 depicts a diagrammatic section view of a hard disk drive. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0045]    The invention is illustrated in more detail according to claim the following examples.  
       EXAMPLE 1  
       [0046]    In a 100 ml flask equipped with a stirrer was placed 50 g of a paraffin-base hydrocarbon as basal oil, PA0401 (trade name; Nippon Steel Chemical Co., Ltd.), to which were added 5 g of liquid polyisoprene rubber, LIR-290 (trade name; Kuraray Co., Ltd.), and 3.0 g of isophorone diisocyanate. After stirring, 2.0 g of n-butylamine was dropwise added, and the mixture was allowed to react at 30° C. for 1 hour to yield polyurea (polyurea content: 10 weight %). Thereafter, the mixture was kept under reduced pressure of 1 mmHg at 100° C. for 60 minutes to yield grease for HDD.  
       EXAMPLE 2  
       [0047]    In a 200 ml flask equipped with a stirrer was placed 100 g of a paraffin-base hydrocarbon, PA0401 (trade name; Nippon Steel Chemical Co., Ltd.) as basal oil, to which were added 4 g of a methacrylic polymer, KANELUBE 815 (trade name; Kanebo Chemical Industires, Ltd.), and 6.65 g of 1,6-hexane duisocyanate. After stirring, 13.35 g of n-dioctylamine was dropwise added, and the mixture was allowed to react at 50° C. for 2 hours to yield polyurea (polyurea content: 20 weight %) Thereafter, the mixture was kept under reduced pressure of 10 mmHg at 120° C. for 240 minutes to yield grease for HDD.  
       EXAMPLE 3  
       [0048]    In a 100 ml flask equipped with a stirrer was placed 50 g of a paraffin-base hydrocarbon as basal oil, PA0601 (trade name; Nippon Steel Chemical Co., Ltd.), to which were added 17 g of a polyolefin polyol, EPOL (trade name; Atofina Idemitsu Co., Ltd.), and 4.50 g of hydrogenated diphenylmethane diisocyanate. After stirring, 3.00 g of cyclohexylamine was dropwise added, and the mixture was allowed to react at 15° C. for 1 hour to yield polyurea (polyurea content: 15 weight %). Thereafter, the mixture was kept under reduced pressure of 160 mmHg at 150° C. for 480 minutes to yield grease for HDD.  
       EXAMPLE 4  
       [0049]    Decene tetramer as basal oil was provided by molecular distillation of a polyolefin, PAO801 (trade name; Nippon Steel Chemical Co., Ltd.). This decenetetramer (carbon number:40)(3 g) was placed in a glass petri dish and the outgas generated at 85° C. for 3hours was measured by means of GC-MS. The volatile components of which the carbon number was less than 30 was contained at a level of 15 ppm.  
         [0050]    In a 200 ml flask equipped with a stirrer was placed 100 g of this decene tetramer, to which were added 10 g of liquid polybutadiene, LIR-30 (tradename; Kuraray Co., Ltd.), and 7.95 g of 1,6-hexane diisocyanate. After stirring, 0.93 g of ethylene diamine and 6.12 g of laurylamine were dropwise added, and the mixture was allowed to react at 50° C. for 1 hour to yield polyurea (polyurea content: 15 weight %). Thereafter, the mixture was kept under reduced pressure of 50 mmHg at 80° C. for 120 minutes to yield grease for HDD.  
       Comparative Example 1  
       [0051]    To an alkyl-substituted diphenyl ether (ADE) (trade name; MORESCO-HILUBE LS150) as basal oil were added toluene diisocyanate and n-butylamine to yield diurea as grease (diurea content: 20 weight %).  
       Comparative Example 2  
       [0052]    To a paraffin-base hydrocarbon, PA0401 (trade name; Nippon Steel Chemical Co., Ltd.) as basal oil, was added 20 weight % of lithium soap (lithium stearate) as thickener to yield grease.  
       Comparative Example 3  
       [0053]    To an ester oil (medium fatty acid ester) as basal oil was added 20 weight % of lithium soap (lithium stearate) as thickener to yield grease.  
       Comparative Example 4  
       [0054]    Commercially available NIGACE (trade name; made by Nippon Grease Co., Ltd.) was used as grease.  
       Comparative Example 5  
       [0055]    Commercially available ASONIC GHY72 (trade name; made by Kluber Lubrication Munchen KG.) was used as grease.  
       Test Example 1  
     Determination of Outgas  
       [0056]    Grease obtained respectively in Examples and Comparative Examples was put on the pivot bearings in an amount of 3.1 mg each, and the bearings were baked in an oven at 85° C. for 3 hours. The gas generated by this operation was measured by means of AGC-MS.  
         [0057]    As GC-MS, a gas chromatography mass selection detector was used.  
         [0058]    Table 1 shows the results. The amount of outgas (ng/pcs) indicates the gas weight (ng) per pivot bearing.  
                                     TABLE 1                                   Amount of Outgas (ng/pcs)                                        Example 1   1220           Example 2   830           Example 3   1610           Example 4   980           Comparative Example 1   34200           Comparative Example 2   29900           Comparative Example 3   44000           Comparative Example 4   26500           Comparative Example 5   15800                      
 
       Test Example 2  
     Acoustic Test (Anderon Test)  
       [0059]    Grease obtained respectively in Examples and Comparative Examples was put on the pivot bearings. The inner ring was rotated at 1800 rpm, and a probe was made touch to the outer ring. The resulting vibration was measured with a measuring apparatus, ANDERON, at the following 3 bands.  
         [0060]    LOW: 50-300Hz, MID.: 300-1800Hz, HIGH: 1800Hz-10kHz  
                                                     TABLE 2                                   LOW   MID   HIGH                                        Example 1   2.6   0.6   0.7           Example 2   2.7   0.8   0.6           Example 3   2.6   0.6   0.8           Example 4   2.6   0.7   0.9           Comparative Example 1   4.2   1.5   2.1           Comparative Example 2   4.5   1.4   2.5           Comparative Example 3   4.4   1.8   2.8           Comparative Example 4   2.8   0.8   0.9           Comparative Example 5   2.8   0.8   1.0                      
 
         [0061]    From the above results it was found that the grease in Examples 1-4 has the same characteristics as or better than the commercially available grease shown in Comparative Examples 4 and 5.  
       Test Example 3  
     Torque Test  
       [0062]    Grease obtained respectively in Examples and Comparative Examples was put on the pivot bearings. To two inner rings of the pivot bearings of size 614 was given 600 grf of preload, and the shaft (inner ring side) was rotated at 2 rpm. A gauge was applied to the sleeve (outer ring side) to measure rotatory torque. Table 3 shows the results.  
                                             TABLE 3                                   Running Torque   Peak to Peak           g · cm   g · cm                                        Example 1   1.0   0.3           Example 2   1.0   0.3           Example 3   1.0   0.3           Example 4   0.9   0.3           Comparative Example 1   1.2   0.4           Comparative Example 2   1.3   0.3           Comparative Example 3   1.2   0.4           Comparative Example 4   1.1   0.4           Comparative Example 5   1.0   0.3                      
 
         [0063]    As results, it was found that any of grease has approximately the same performance.  
       Test Example 4  
     Life Test  
       [0064]    Grease obtained respectively in Examples and Comparative Examples was put on the pivot bearings. These pivot bearings were subjected to the life test in the following condition, and it was confirmed that there was no difference between the torque checked before the test and that after the test.  
       Condition  
       [0065]    Speed: 15Hz  
         [0066]    oscillating angle: 300  
         [0067]    Time: 38,000,000 revolutions  
         [0068]    As results, it was found that any of grease maintained faultless performance in view of the lifetime.  
         [0069]    As described above, according to claim the invention, it is possible to provide the most appropriate grease for HDD of which outgas is very low, which is prepared by making a thickener polyurea contain in basal oil comprising a given paraffin-base hydrocarbon, followed by treatment under reduced pressure.