Magnetic recording medium
FIELD OF THE INVENTION 
     The present invention relates to a magnetic recording medium, such as a magnetic disk or a magnetic sheet, and more particularly it relates to a magnetic recording medium having improved durability. 
     BACKGROUND OF THE INVENTION 
     For a magnetic recording medium, particularly a magnetic disk or a magnetic sheet which is suitable for high density recording and is required to have high durability, both surface smoothness and durability thereof are required to be improved in order to improve electromagnetic properties thereof. However, conventional methods have not yet been able to fully meet the above requirement. 
     That is, in order to realize high density recording of a magnetic disk, it is important that the surface of a magnetic layer is smooth and that ferromagnetic particles are uniformly dispersed in the magnetic layer. However, sufficient durability of a magnetic layer has not so far been obtained by the use of fine magnetic particles, by improvements of smoothing treatment such as calendering treatment and by techniques for dispersing magnetic coating composition. Further, the thickness of the magnetic layer is forced to become thinner and thinner as high density recording has become further developed. 
     Generally used magnetic layers contain carbon black having an average particle size of about 10 mμ as an antistatic agent. Surface electric resistance of a magnetic layer having from 3 to 8 mμ thickness is generally controlled so that it is about from 1×10 6  to 1×10 9  Ω. And when a magnetic layer is made thinner and the carbon black is used in the same content as before, the surface electric resistance becomes higher. In this case, in order to reduce the surface electric resistance, it is necessary that an amount of carbon black added are increased. It is known that since carbon black particles are finer than generally used pigments, carbon black is dispersed with difficulty, and further it deteriorates the dispersibility of magnetic particles and the surface properties of a coated surface of a magnetic recording medium. When carbon black is forcibly dispersed under improved dispersing conditions, the coated surface is made smoother, but durability of the magnetic layer decreases. 
     It is described in U.S. Pat. No. 3,630,910 that a fatty acid is used as the lubricating agent and that the total amount of the lubricating agent added is less than 7 wt%, with respect to the weight of the magnetic particles, in order to increase durability. 
     The amount of the lubricating agent is limited to less than 7 wt% because, when the lubricating agent is used in an amount of about 3 wt% or more, the magnetic layer of a video tape tends to become plasticized and experiences drop-outs, and when lubricating agents such as fatty acids and fatty acid esters are used, the magnetic layer tends to stick to the magnetic head in some cases. In the case of an audio tape, when a lubricating agent is used in an amount of about 6 wt% or more, the magnetic layer thereof tends to become plasticized, and tape squeal is easily generated by the influence of temperatures and humidities, μm value of the magnetic layer decreases, an edge of a tape which was wound to a reel disorderly waves by rubbing with a flange in running and white powers (i.e., scratched powders) generate, thereby tending to stain the magnetic head. A magnetic recording medium, particularly a magnetic disk which has high commercial value, must have a high durability such that the magnetic disk can continuously rotate more than 10,000,000 times. In such a case, satisfactory high durability has not yet been obtained when conventional binder composition is used and a lubricating agent is used in an amount of less than 7 wt% per ferromagnetic particles. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a magnetic recording medium such as a magnetic disk or a magnetic sheet wherein electric resistance on the surface of a magnetic layer is suitably controlled and both the surface properties and the durability are simultaneously improved. 
     The inventors of the present invention have conducted extensive research as to the combined use of carbon black having different particle sizes, kinds of lubricating agents, and the added amounts thereof, and as a result thereof, they have attained the present invention. 
     The present invention is directed to a magnetic recording medium comprising a non-magnetic support having provided thereon a magnetic layer containing ferromagnetic particles, a binder, non-magnetic particles, and lubricating agents, wherein said magnetic layer contains (i) carbon black (A) having an average particle size of 25 mμ or less and a BET specific surface area of from 150 to 250 m 2  /g and carbon black (B) having an average particle size of from 10 to 30 mμ and a BET specific surface area of 700 m 2  /g or more, and (ii) a fatty acid ester and a fatty acid amide in a total amount of 7 wt% or more based on the weight of the ferromagnetic particles. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is described in more detail below. 
     The two types of carbon black used in the present invention are: carbon black (A) having an average particle size of 25 mμ or less, and preferably from 16 to 20 mμ, and having a BET specific surface area of from 150 to 250 m 2  /g, and preferably from 180 to 230 m 2  /g; and carbon black (B) having an average particle size of from 10 to 30 mμ, and preferably from 20 to 30 mμ, and having a BET specific surace area of 700 m 2  /g, and preferably from 900 to 1,100 m 2  /g. Typical examples of carbon black (A) having an average particle size of 25 mμ or less and from 150 to 250 m 2  /g of BET specific surface area include &#34;RAUEN 2100&#34;, &#34;RAUEN 2000&#34;, &#34;RAUEN 1800&#34;, &#34;RAUEN 1500&#34;, &#34;CONDUCTEX 950&#34;, and &#34;CONDUCTEX SC&#34;, trademarks of products manufactured by Columbian Carbon Co., Ltd. and &#34;HCF #2200B&#34;, &#34;MCF #1000&#34;, &#34;MCF #900&#34;, &#34;MCF-88&#34; and &#34;MA 600&#34;, trademarks of the products manufactured by Mitsubishi Chemical Industries Ltd. Among these examples, &#34;CONDUCTEX SC&#34; is preferred. Typical examples of carbon black (B) having an average particle size of from 10 to 30 mμ and a BET specific surface area of 700 m 2  /g or more include &#34;ROYAL SPECTRA&#34;, &#34;NEO SPECTRA MARK I&#34;, &#34;NEO SPECTRA MARK II&#34;, &#34;NEO SPECTRA AG&#34;, &#34;SUPER SPECTRA&#34; and &#34;RAVEN 8000&#34;, trademarks of products manufactured by Columbian Carbon Co., Ltd., and &#34;Ketjenblack EC&#34;, a trademark of the product manufactured by Lion Akzo Co., Ltd. And among these examples, &#34;Ketjenblack EC&#34; is preferred. These two types of carbon black are used in a total amount of from 1 to 25 wt%, and preferably from 5 to 15 wt%, based on the weight of the ferromagnetic particles. The mixing ratio of the two types of carbon black is optional, but the mixing ratio of carbon black (A) to carbon black (B) is preferably from 1/5 to 20/1, and more preferably from 1/3 to 10/1. 
     Lubricating agents used in the present invention include a fatty acid ester and a fatty acid amide and other lubricating agents can be added thereto, if desired. 
     The fatty acid ester includes an ester such as ethyl stearate, butyl stearate, amyl stearate, butyl palmitate, hexyl laurate, butyl laurate, or butyl myristate, and an alkoxy ester such as butoxy ethyl stearate, butoxy butyl stearate, ethoxy ethyl stearate, ethoxy butyl stearate, ethoxy butyl palmitate, or butoxy ethyl palmitate. Of these fatty acid esters, preferred are butyl stearate and butyl palmitate. A fatty acid amide includes stearyl amide, oleyl amide, erucyl amide, palmityl amide, and myristyl amide. Further, a fatty acid amide which is on the market includes Alflo P-10, Alflo E-10, and Alflo S-10, produced by Nippon Oil &amp; Fats Co., Ltd., and Armoslip E, Armoslip O, and Armoslip Cp, produced by Lion-Akzo Co. Ltd. Of these fatty acid amides, preferred are oleyl amide, stearyl amide, and palmityl amide. These fatty acid ester and fatty acid amide must be contained in a total amount of 7 wt% or more based on the weight of the ferromagnetic particles. 
     Lubricating agents additionally added to the above lubricating agents include saturated or unsaturated fatty acid, fatty acid-modified silicones, α-olefin oxide, or saturated hydrocarbon. Non-magnetic particles having a Mohs&#39; hardness of 6 or more are preferably added. 
     Non-magnetic particles having a Mohs&#39; hardness of 6 or more are effective for the durability of a magnetic layer and these particles which are generally referred to as abrasive agents, such as, for example, Cr 2  O 3 , alumina, SiO, MgO, SiC, and TiC can be used. Non-magnetic particles having a Mohs&#39; hardness of 6 or less are not effective for improving durability, and rather deteriorate it. The particle size of non-magnetic particles having a Mohs&#39; hardness of 6 or more is preferably 1 μm or less, and more preferably from 0.2 to 0.7 μm. The amount of non-magnetic particles added is preferably 3 wt% or more, and particularly preferably from 6 to 15 wt% based on the weight of the ferromagnetic particles. 
     Non-magnetic supports used in the present invention are those that are conventionally used for a magentic recording media, such as polyethylene terephthalate, triacetyl cellulose, polyethylene naphthalate, polyamide, or polyimide. A polyester film vapour-deposited with Al can also be used. 
     In order to improve the surface properties of magnetic recording medium, the carbon black can be previously mixed with magnetic particles, binders, and organic solvents. In such a case, the whole amount or a part of the carbon black can be previously mixed. Like carbon black, amounts of magnetic particles and binders which are previously mixed can be adjusted. It is more effective that a whole amount of ferromagnetic particles are previously mixed. However, the previous mixing condition must be determined by the amounts and kinds of binders which are previously mixed. When carbon black is added only to improve surface smoothness of a magnetic layer, it is preferred that a whole amount of carbon black is introduced and previously mixed. When surface electric resistance of a magnetic layer is required to have a value lower than a certain predetermined value (i.e., 5×10 9  Ω), it is preferred that a part of carbon black is previously mixed and the rest of the carbon black is introduced at a later step. 
     A three-roll mill, a pressure kneader, an open kneader, and Werner kneader are preferably used for previous mixing, although any devices which are generally used for kneading and mixing pigments can also be used in the present invention. 
     Solvents used for previous mixing include acetic acid esters such as ethyl acetate or butyl acetate, ketones such as methyl ethyl ketone, methyl isobutyl ketone, or cyclohexanone and solvents generally used for a magnetic tape. 
     Ferromagnetic particles used in the present invention include oxides such as γ-Fe 2  O 3 , Fe 3  O 4 , CrO 2 , cobalt-containing γ-Fe 2  O 3  or cobalt-containing FeO x  (x=1.33 to 1.5) and metal particles such as Fe-Ni, Fe-Ni-Co, Fe, or Fe-Co. These ferromagnetic particles have a mean length of from 0.1 to 1.0 μm and an acicular ratio (long axis/short axis) of from 2/1 to 15/1. 
     Binders used in the present invention include those that are conventionally used for a magnetic recording medium such as polyvinyl chloride, a copolymer of vinyl chloride, and vinyl acetate, a copolymer of vinyl chloride and vinyl acetate containing a carboxylic group, a copolymer of vinyl chloride and vinyl acetate containing vinyl alcohol, polyvinylidene chloride, a copolymer of vinylidene chloride, and acrylonitrile, nitrocellulose, and various cellulose derivatives, acrylonitrile and butadiene rubber, styrene and butadiene rubber, polyester, an epoxy resin, polyisocyanate polyurethane, or polyamide. The amount of binder used is generally from 15 to 50 wt% based on the weight of the ferromagnetic particles. 
     If desired, a plasticizer such as DBP (dibutyl phthalate) or TPP (triphenylphosphate) and a dispersing agent such as lecithin can also be mixed into a magnetic layer. 
     The thus-obtained magnetic recording medium such as a magnetic disk and a magnetic sheet has excellent surface smoothness and excellent durability of a magnetic layer. 
    
    
     The present invention is illustrated in more detail by the following Examples and Comparative Examples. In Examples and Comparative Examples, all parts are by weight. 
     EXAMPLE 1 
     
         ______________________________________                                    
Co-containing γ-Fe.sub.2 O.sub.3                                    
                          100 parts                                       
(particle size: 0.4 μm × 0.04 μm                              
Copolymer of vinyl chloride and                                           
                          10 parts                                        
vinyl acetate                                                             
(&#34;VMCH&#34;, trademark of a product                                           
manufactured by Union Carbide                                             
Co., Ltd.)                                                                
Polyurethane              4 parts                                         
(&#34;N-2304&#34;, trademark of a product                                         
manufactured by Nippon Polyurethane                                       
Co., Ltd.)                                                                
Polyisocyanate            8 parts                                         
(&#34;Urecoat&#34; B for &#34;Mylar #1 Clear&#34;,                                        
trademark of a product manufactured                                       
by Higashi Nippon Toryo Co., Ltd.)                                        
Carbon black                                                              
(1) &#34;CONDUCTEX SC&#34; trademark of a                                         
                          8 parts                                         
product manufactured by Columbian                                         
Carbon Co., Ltd., average                                                 
particle size of 17 mμ, BET                                            
specific surface area 200 m.sup.2 /g                                      
(2) &#34;Ketjenblack EC&#34;, trademark of a                                      
                          3 parts                                         
product manufactured by Lion Akzo                                         
Co., Ltd., average particle size                                          
of from 20 to 30 mμ, BET specific                                      
surface area of 1000 m.sup.2 /g                                           
Cr.sub.2 O.sub.3          5 parts                                         
(manufactured by Nippon Chemical                                          
Industrial Co., Ltd., average                                             
particle size of 0.5 μm, Mohs&#39;                                         
hardness of from 8 to 9)                                                  
Ethoxybutyl stearate      1 part.sup.                                     
Ethoxy butyl palmitate    4 parts                                         
Butyl stearate            3 parts                                         
Erucyl amide              3 parts                                         
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     Solvent: methyl ethyl ketone/toluene/methyl isobutyl ketone (mixing ratio: 2/2/1) 
     The above composition except polyisocyanate was roughly dispersed by a stirring device, and further dispersed and finely ground in a ball mill, and then polyisocyanate was added to the dispersing agent to adjust the viscosity so that the dispersing agent had a viscosity of 70 poise. After filtration, the magnetic composition was coated on a polyethylene terephthalate film having a thickness of 75 μm by a reverse roll coating method, which was then, dried and subjected to surface smoothing treatment using a super calender, which was processed to prepare a flexible disk of 5.25 inches. This sample was identified as Sample No. 1. 
     EXAMPLE 2 
     The same procedure as in Example 1 was repeated to prepare Sample Nos. 2 to 4, except that the kinds of carbon black were changed as shown in Table 1. 
     
                       TABLE 1                                                     
______________________________________                                    
                                      BET                                 
                                      specific                            
Sam-                    A-            surface                             
ple                     mounts  Size  area                                
No.  Kinds of carbon black                                                
                        (parts) (mμ)                                   
                                      (m.sup.2 /g)                        
______________________________________                                    
2    (1) CONDUCTEX 950  8       21    245                                 
     (2) NEO SPECTRA MARK I                                               
                        3       11    1065                                
3    (1) RAVEN 1800     8       18    214                                 
     (2) SUPER SPECTRA  3       13    742                                 
4    (1) MCF 88         8       18    200                                 
     (2) Ketjenblack EC 3       20-30 1000                                
______________________________________                                    
 
    
     Comparative Example 1 
     The same procedure as in Example 1 was repeated to prepare Sample Nos. 5 to 8 except that the kinds of carbon black were changed as shown in Table 2. 
     
                       TABLE 2                                                     
______________________________________                                    
                                    BET specific                          
Sample               Amounts  Size  surface area                          
No.   Kinds of carbon black                                               
                     (parts)  (mμ)                                     
                                    (m.sup.2 /g)                          
______________________________________                                    
5     CONDUCTEX SC   8        17    200                                   
6     Ketjenblack EC 5        20-30 1000                                  
7     (1) Vulcan XC-72*                                                   
                     8        30    254                                   
      (2) Ketjenblack EC                                                  
                     3        20-30 1000                                  
8     (1) CONDUCTEX SC                                                    
                     8        17    200                                   
      (2) Asahi #60* 3        50     58                                   
______________________________________                                    
 *Notes-                                                                  
 Vulcan XC72: trademark of a product manufactured by Cabot U.S.A.         
 Asahi #60: trademark of a product manufactured by Asahi Carbon Co., Ltd. 
 
    
     EXAMPLE 3 
     The same procedure as in Example 1 was repeated to prepare Sample Nos. 9 to 11 except that the kinds of lubricating agents were changed as shown in Table 3. 
     
                       TABLE 3                                                     
______________________________________                                    
Sample No.  Lubricating agents                                            
                          Amounts (part)                                  
______________________________________                                    
 9          Amyl stearate 3                                               
            Butyl myristate                                               
                          2                                               
            Oleyl amide   4                                               
10          Butyl palmitate                                               
                          4                                               
            Myristyl amide                                                
                          5                                               
11          Butyl stearate                                                
                          4                                               
            Palmityl amide                                                
                          3                                               
            Oleic acid    2                                               
______________________________________                                    
 
    
     Comparative Example 2 
     The same procedure as in Example 1 was repeated to prepare Sample Nos. 12 and 13 except that the added amounts of ethoxy butyl stearate, ethoxy butyl palmitate, butyl stearate, and erucyl amide were changed as shown in Table 4. 
     
                       TABLE 4                                                     
______________________________________                                    
         Ethoxy   Ethoxy                                                  
Sample   butyl    butyl       Butyl  Erucyl                               
No.      stearate stearate    stearate                                    
                                     amide                                
______________________________________                                    
12       --       2           2      2                                    
13       0.5      1           1      1                                    
______________________________________                                    
 
    
     Regarding Sample Nos. 1 to 13 prepared in Examples and Comparative Examples, durability and surface electric resistance thereof were measured by the method shown below using 5.25-inch floppy disk drive and the results are shown in Table 5. 
     
                       TABLE 5                                                     
______________________________________                                    
        Durability   Surface                                              
        (number of   electric Example or                                  
Sample  rotations,   resistance                                           
                              Comparative                                 
No.     × 10,000)                                                   
                     (Ω)                                            
                              Example                                     
______________________________________                                    
1       2420         3 × 10.sup.6                                   
                              Example                                     
2       2200         5 × 10.sup.7                                   
                              &#34;                                           
3       2280         1 × 10.sup.8                                   
                              &#34;                                           
4       2360         8 × 10.sup.6                                   
                              &#34;                                           
5       1520         9 × 10.sup.9                                   
                              Comparative                                 
                              Example                                     
6        760         1 × 10.sup.4                                   
                              Comparative                                 
                              Example                                     
7       1080         8 × 10.sup.5                                   
                              Comparative                                 
                              Example                                     
8       1200         .sup. 2 × 10.sup.10                            
                              Comparative                                 
                              Example                                     
9       2350         3 × 10.sup.6                                   
                              Example                                     
10      2030         4 × 10.sup.6                                   
                              &#34;                                           
11      2350         2 × 10.sup.6                                   
                              &#34;                                           
12       980         2 × 10.sup.6                                   
                              Comparative                                 
                              Example                                     
13       420         1 × 10.sup.6                                   
                              Comparative                                 
                              Example                                     
______________________________________                                    
 
    
     The evaluations for the results shown in Table 5, were carried out in the following manner. 
     (1) Durability: 
     Durbility was indicated in terms of the number of rotations of each sample which had been rotating continuously using a floppy disk drive at 23° C. and 60% RH under 10 g/15 mm 2  pad pressure between a floppy disk drive and a floppy disk, until the output of a floppy disk decreased to 80% or less of an initial output. It is desirable that a commercially valuable magnetic disk has the number of rotation times of 10,000,000 or more. 
     (2) Surface electric resistance: 
     Surface electric resistance was measured using a digital super insulation resistance meter &#34;TR-8611A&#34;, a trademark of the product manufactured by Takeda Riken. It is commercially desirable that a value of surface electric resistance is from 5×10 5  to 5×10 9  Ω. 
     It is clearly seen from the above results that Sample Nos. 1 to 4 and 9 to 11 exhibit excellent durability of 20,000,000 of rotations, as well as a desired surface electric resistance value of from 1×10 8  to 2×10 6  Ω. Durability of Sample Nos. 5 to 8 is inferior to Samples of the present invention. It is apparent from Comparative Examples that durability of Comparative Examples in which only one kind of carbon black &#34;CONDUCTEX SC&#34; or &#34;Ketjenblack EC&#34; was used inferior to that of Examples of the present invention. A remarkable improvement of durability was obtained when the above two types of carbon black were used together, as is apparent from Sample No. 1 of Example 1. The reason was not clear, but it appears that good results can be obtained when a magnetic layer contained lubricating agents in a total amount of 7 wt% or more, and a mixture of the first carbon black having 25 mμ or less of an average particle size and having from 150 to 250 m 2  /g of specific surface area, and the second carbon black having an average particle size of from 20 to 30 mμ and having a BET specific surface area of 700 m 2  /g or more, and further an abrasive agent in combination. 
     It appears that when lubricating agents were used in an amount of less than 7 wt%, as in the case of Sample Nos. 12 and 13, durability was extremely decreased. 
     A magnetic recording medium prepared in accordance with the present invention has an desired surface electric resistance on a magnetic layer and has superior durability. 
     While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.