Abstract:
A magnetic recording medium is disclosed which comprises a non-magnetic support having provided thereon a magnetic recording layer consisting ferromagnetic powder and containing graphite and a fatty acid ester.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a magnetic recording medium, and more particularly it relates to a magnetic recording medium having good still image like and providing reduced magnetic head wear. 
     BACKGROUND OF THE INVENTION 
     Almost all video tape recording decks for recording and playback of video images (VTR) developed recently are equipped with still mode settings for replaying still images. The magnetic recording medium used in such a VTR is required to have good still life, i.e., to be durable in a still mode operation for a long period of time. It is disclosed, for example, in U.S. Pat. No. 368,725 that various abrasives can be incorporated in the recording medium to improve the still life. However, when a large quantity of abrasives is added to improve still life as described in U.S. Pat. No. 4,135,016, wear of the magnetic head is greatly increased and the life of the head is correspondingly reduced. In known magnetic recording media, it has been very difficult to satisfy both requirements of good still life and reduced head wear. 
     SUMMARY OF THE INVENTION 
     A first object of the present invention is to provide a magnetic recording medium having good still life. 
     A second object of the present invention is to provide a magnetic recording medium having reduced head wear. 
     As a result of thorough investigations, it has been found that these and other objects of the present invention can be attained by a magnetic recording medium comprising a non-magnetic support having coated thereon a magnetic layer, said magnetic layer containing graphite and an ester of a fatty acid. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The graphite which is used in the present invention is not unduly limited, and can be natural graphite or artificial graphite in particular having a flake shape, a granular shape, or a lump (plumbago) shape. Among these, flake shaped graphite is superior because it provides greater reduction of head wear. The particle size of the graphite used can be selected as desired, it preferably has an average primary particle size of about 0.01 to 3μ, more preferably about 0.05 to 1μ, and most preferably about 0.1 to 0.5μ. If the particle size is larger than about 3μ, the electromagnetic properties of the resulting magnetic medium are adversely affected, and it is smaller than about 0.01μ, satisfactory lubricating effect cannot be obtained. 
     The amount of graphite added is from about 0.1 to 10 parts by weight and preferably from about 0.5 to 5 parts by weight per 100 parts by weight of magnetic particles. If the amount added is smaller, still life is reduced and dropouts increase. If the amount is larger, electromagnetic properties are adversely affected. 
     The fatty acid ester used in the present invention is an ester of a fatty acid having 2 to 22 carbon atoms and an alcohol having 2 to 27 carbon atoms, preferably an ester of a fatty acid having 6 to 18 carbon atoms and an alcohol having 2 to 14 carbon atoms. Both saturated and unsaturated fatty acids can be used. Preferred alcohols are monohydric alcohols, but dihydric and trihydric alcohols can be also used. 
     The amount of fatty acid ester used in the present invention is from about 0.05 to 10 parts by weight, preferably from about 0.1 to 5 parts by weight per 100 parts by weight of magnetic particles. A fatty acid ester can be used alone or a mixture of at least two kinds of fatty acid esters can be used. 
     Examples of suitable fatty acid esters include amyl stearate, butyl stearate, ethyl stearate, butyl myristate, oleyl oleate, butyl palmitate, etc. Of these, amyl stearate, butyl stearate and ethyl stearate are preferred with amyl stearate and butyl stearate being most preferred. Still life can be further increased by the addition of abrasives having Mohs hardness of 6 or more, preferably 6 to 9 such as Cr 2  O 3 , alumina, garnet, α--Fe 2  O 3  or SiC, to the above described graphite and fatty acid ester. The amount of the abrasive added is from about 0.1 to 30 parts by weight, preferably from about 1 to 20 parts by weight per 100 parts by weight of magnetic particles. The average particle size of the abrasive is from about 0.05 to 5μ, preferably from 0.1 to 2μ. 
     The electromagnetic properties of the magnetic recording medium can be further improved by smoothing the nonmagnetic support to a center line average roughness (Ra) of about 0.035μ (cut off 0.08 mm) or less, preferably about 0.025 or less. However, a support surface thus smoothed may adversely affect the running properties of the magnetic recording medium on a VTR. In addition, dropouts can be caused by the scraped tips of the support putting on the magnetic layer. Both of these problems can be avoided by (1) using a support having different surface smoothnesses on opposite surfaces, (2) providing a backing layer or (3) providing a layer containing a lubricant on the surface of the support opposite to the magnetic layer. Among these methods, the third is most effective because each layer has a separate function. If a backing layer is used carbon black and graphite particles are preferably incorporated into the backing layer to prevent electrostatic charging. 
     It is preferred that carbon black is additionally included in the magnetic layer. Because of its antistatic effect, which is useful for preventing dropout of signals caused by attached dust. 
     The carbon black which can be used in the present invention has an average particle size of about 30 mμ or less, preferably about 10 to 30 mμ and more preferably about 15 to 25 mμ. 
     Carbon black is added to the magnetic recording layer in an amount of from about 0.1 to 10 parts by weight and preferably from about 0.5 to 5 parts by weight per 100 parts by weight of magnetic particles. Various additives, supports and the methods for preparing the magnetic recording tapes are described in U.S. Pat. No. 4,135,016. 
    
    
     The present invention is illustrated in more detail by the following Examples, but the present invention is not to be construed as being limited thereto. Materials, quantities and the sequence of operation can be changed by those skilled in the art so long as the gist of the present invention is not essentially changed. 
     In the Examples and Comparative Examples, all parts percents and ratios are by weight unless otherwise indicated. 
     EXAMPLE 1 
     
         ______________________________________Co-containing γ-Fe.sub.2 O.sub.3                    100    partsGraphite (Flake Shaped, Average                    5      partsPrimary Particle Size: 0.3μ)Fatty Acid Ester (Butyl Stearate)                    1      partCopolymer of Vinyl Chloride and Vinyl                    20     partsAcetate Containing a Carboxylic Group(Mol Ratio: 83:9:8; Molecular Weight:30,000)Polyurethane Resin (Molecular Weight:                    20     parts50,000)Polyisocyanate (Molecular Weight: 656)                    20     partsAbrasive (Cr.sub.2 O.sub.3 ; Average Particle Size:                    6      parts0.6μ)Methyl Ethyl Ketone      150    partsButyl Acetate            150    parts______________________________________ 
    
     The above composition was milled in a ball mill for 20 hours, and was adjusted to a viscosity of 40 poise by adding a mixed solvent of methyl ethyl ketone and butyl acetate (1:1 by weight) and agitating the mixture in an agitater and then coated on a polyester support having a thickness of 15μ and a surface smoothness of Ra 0.035μ in a thickness of 5μ (the same thickness used in the following Examples) to prepare Sample No. 1. 
     COMPARATIVE EXAMPLE 1 
     Comparative Sample No. 2 was prepared in the same manner as in Example 1 except that graphite and the fatty acid ester were omitted from the composition. 
     EXAMPLE 2 
     
         ______________________________________Co-containing γ-Fe.sub.2 O.sub.3                  100      partsGraphite (Flake Shaped, Average                  5        partsPrimary Particle Size: 0.3μ)Fatty Acid Ester (Butyl Stearate)                  1        partsVinyl Chloride-Vinyl Acetate Copolymer                  20       partsContaining a Carboxylic Group (MolRatio: 83:9:8; Molecular Weight: 30,000)Polyurethane Resin (Molecular Weight:                  20       parts50,000)Polyisocyanate (Molecular Weight: 656)                  20       partsAbrasive (Cr.sub.2 O.sub.3 ; Average Particle Size:                  6        parts0.6μ)Carbon Black (Average Primary Particle                  5        partsSize: 30 mμ)Methyl Ethyl Ketone    150      partsButyl Acetate          150      parts______________________________________ 
    
     Sample No. 3 having the above composition was prepared in the same manner as in Example 1 except that carbon black was added to the composition. 
     Sample Nos. 4, 5 and 6 were prepared in the same manner as above except that graphite having a particle size of 0.5μ, 1μ and 3μ, respectively, was used. 
     COMPARATIVE EXAMPLE 2 
     Comparative Sample No. 7 was prepared in the same manner as Example 2 except that graphite and fatty acid ester were omitted from the composition of Example 2. Comparative Sample No. 8 was prepared in the same manner as Example 2 but only graphite was omitted therefrom and Comparative Sample No. 9 was prepared in the same manner but the fatty acid ester was omitted. 
     EXAMPLE 3 
     Sample No. 10 was prepared by coating the same magnetic recording layer as in Example 2 on one surface of the same support as in Example 2, and further coating a backing layer having the following formulation in a thickness of 5μ (the same as used in the following Examples) on the opposite surface thereof. 
     
         ______________________________________Carbon Black (Average Primary Particle                    100    partsSize: 90 mμ)Nitrocellulose (Molecular Weight:                    30     parts27,000; Degree of Nitration: 12.0)Polyurethane Resin (Molecular Weight:                    5      parts25,000)Copolymer of Vinylidene Chloride-                    15     partsAcrylonitrile (Mol Ratio: 80:20;Molecular Weight: 30,000)Polyisocyanate (Molecular Weight: 656)                    25     partsCalcium Carbonate        50     partsα-Fe.sub.2 O.sub.3 (Average Particle Size: 1.0μ)                    2      partsMethyl Ethyl Ketone      1,000  partsButyl Acetate            500    parts______________________________________ 
    
     COMPARATIVE EXAMPLE 3 
     Comparative Sample Nos. 11, 12 and 13 were prepared in the same manner as in Example 2 except that instead of fatty acid ester of Example 2, a fatty acid (oleic acid), silicone (dimethyl polysiloxane) and liquid paraffin, were used, respectively. 
     Comparative Sample No. 14 was prepared by substituting a fatty acid ester for a fatty acid (oleic acid) and by increasing the amount of the abrasive to 30 parts. 
     EXAMPLE 4 
     
         ______________________________________Magnetic Recording LayerCo-containing FeOx (x = 1.4) (Average                    100    partsParticle Size: 0.3μ × 0.03μ;Co/Fe = 3/97)Graphite (Flake Shaped, Average                    7      partsPrimary Particle Size: 0.4μ)Fatty Acid Ester (Amyl Stearate)                    1      partFatty Acid Ester (Ethyl Stearate)                    1      partFatty Acid (Myristic Acid)                    0.5    partSilicone Oil (Dimethyl Polysiloxane)                    0.1    partCopolymer of Vinyl Chloride-Vinyl                    25     partsAcetate-Vinyl Alcohol (Mol Ratio:90:3:7; Molecular Weight: 22,000)Polyurethane Resin (Molecular Weight:                    15     parts50,000)Polyisocyanate (Molecular Weight: 656)                    25     partsAbrasive (α-Al.sub.2 O.sub.3 ; Average Particle                    5      partsSize: 0.4μ)Carbon Black (Average Primary                    10     partsParticle Size: 20 mμ)Toluene                  150    partsButyl Acetate            150    partsBacking LayerCarbon Black (Average Primary                    100    partsParticle Size: 30 mμ)Graphite (Flake Shaped, Average                    100    partsPrimary Particle Size: 0.4μ)Copolymer of Vinyl Chloride and                    75     partsVinyl Acetate Containing aCarboxylic Group) (Mol Ratio:83:9:8; Molecular Weight: 30,000)Polyurethane Resin (Molecular                    35     partsWeight: 30,000)Polyisocyanate (Molecular Weight: 656)                    50     partsTalc Powder (Average Particle Size:                    50     parts1.5μ)Calcium Carbonate        50     partsGoethite (Average Particle Size:                    50     parts1.2μ × 0.2μ (Acicular))Methyl Ethyl Ketone      1,000  partsCyclohexanone            300    parts______________________________________ 
    
     The above described composition for the magnetic layer was milled in a three roll mill (30 passes) and then a sand grinder (5 kg/min, 10 passes) and the above described composition for the backing layer was also milled and each composition was coated on a polyester support having a thickness of 20 μm and a surface smoothness Ra: 0.020μ, respectively, on opposite surfaces to prepare Sample No. 15. 
     EXAMPLE 5 
     The same procedure as in Example 3 was repeated except that the amount of the abrasive (Cr 2  O 3 ) was changed to 0.05 part, 0.1 part, 1 part, 10 parts, 20 parts, 30 parts and 40 parts, respectively to prepare Sample Nos. 16, 17, 18, 19, 20, 21 and 22. 
     The thus-prepared magnetic tape samples were evaluated using a VHS type VTR. Still life was determined by measuring the time (min.) required for recorded images of the samples replayed in the still mode of the VTR to disappear. Head wear is shown by the degree of wear (μ) resulting when the samples were run on the VTR for 100 hours. Video S/N is shown by the relative value (dB) to the standard magnetic tape prepared by Fuji Photo Film Co., Ltd. The dropout measurement equals the dropouts (number/min.) which were observed on a TV monitor over a period of at least 12 minutes. The results are shown in the following Table. 
     
                                           TABLE__________________________________________________________________________                                       Head    Drop-Sam-   Example or  Amount      Size of      Still                                       wear                                           Video                                               outple   Comparative     Size of          of    Fatty acid                      carbon                          Back-                              Back life                                       (μ/                                           S/N (number/No.   Example     graphite          abrasive                ester black                          ing coat (min)                                       100 h)                                           (dB)                                               min)__________________________________________________________________________ 1 Example 1     0.3μ    Butyl None                          None     &gt;400                                       3.0 +2.3                                               35   bad                Stearate                            runability 2 Comparative     None       None  None                          None      &lt;1 &gt;30 -1.5                                               250  bad   Example 1                                             runability 3 Example 2     0.3μ    Butyl 20 mμ                          None     &gt;400                                       1.5 +2.3                                               13                Stearate 4 Example 2     0.5μ    Butyl 20 mμ                          None     &gt;400                                       1.8 +2.2                                               17                Stearate 5 Example 2     1.0μ    Butyl 20 mμ                          None     &gt;400                                       2.3 +1.6                                               20   slightly                Stearate                            bad filtra-                                                    tion of                                                    liquid (*) 6 Example 2     3.0μ    Butyl 20 mμ                          None     &gt;400                                       5.0 -0.5                                               35   bad filtra-                Stearate                            tion of                                                    liquid (*) 7 Comparative     None       None  20 mμ                          None      &lt;1 20  +0.5                                               75   Example 2 8 Comparative     None       Butyl 20 mμ                          None       25                                        8  +1.8                                               60   Example 2         Stearate 9 Comparative     0.3μ          6  parts                None  20 mμ                              None   3 15  +0.7                                               60   Example 210 Example 3     0.3μ          6  parts                Butyl 20 mμ                              Provided                                   &gt;400                                       1.3 +2.1                                                3                Stearate11 Comparative     0.3μ          6  parts                None  20 mμ                              None   10                                       1.3 +2.3                                               14   Example 3         (fatty acid)12 Comparative     0.3μ          6  parts                None  20 mμ                              None   2 0.9 +1.6                                               17   Example 3         (silicone)13 Comparative     0.3μ          6  parts                None  20 mμ                              None   7 1.5 +2.0                                               16   Example 3         (liquid                paraffin)14 Comparative     None 30 parts                None  20 mμ                              None  250                                       &gt;30 +1.9                                               90   Example 3         (fatty acid)15 Example 4     0.3μ          5  parts                Amyl  20 mμ                              Provided                                   &gt;400                                       0.9 +2.3                                                3          (smooth                Stearate/          surface                Ethyl          support)                Stearate16 Example 5     0.3μ          0.05             part                Butyl 20 mμ                              Provided                                    300                                       0.8 +1.0                                                4                Stearate17 Example 5     0.3μ          0.1             part                Butyl 20 mμ                              Provided                                   &gt;400                                       0.8 +2.2                                                3                Stearate18 Example 5     0.3μ          1  part                Butyl 20 mμ                              Provided                                   &gt;400                                       1.0 +2.2                                                4                Stearate19 Example 5     0.3μ          10 parts                Butyl 20 mμ                              Provided                                   &gt;400                                       1.5 +2.1                                                3                Stearate20 Example 5     0.3μ          20 parts                Butyl 20 mμ                              Provided                                   &gt;400                                       3.0 +2.0                                                3                Stearate21 Example 5     0.3μ          30 parts                Butyl 20 mμ                              Provided                                   &gt;400                                       7.0 +2.0                                                5                Stearate22 Example 5     0.3μ          40 parts                Butyl 20 mμ                              Provided                                   &gt;400                                       15  +1.7                                                4                Stearate__________________________________________________________________________ (*) Time required for filtering through a filter having a thickness of 3 μm at a pressure of 2 kg/cm.sup.2 was measured. 
    
     Comparison of Sample No. 3 (invention) with Sample Nos. 7, 8 and 9 (Comparative Samples) demonstrates that still life and head wear resistance are remarkably improved along with video S/N and dropout characteristics only when graphite and a fatty acid ester are present in a magnetic recording layer according to the invention. 
     If the fatty acid ester in Sample No. 3 is replaced by other lubricants, the surprising improvements obtained by using a fatty acid ester in combination with graphite were not observed (Sample Nos. 11, 12 and 13). 
     In addition the results obtained using Sample Nos. 1, 3 and 10 show that the addition of carbon black to the magnetic layer and the coating of a backing layer was preferable. Sample Nos. 3 to 6 demonstrate that the use of graphite having a particle size of not more than 0.5μ is preferable. 
     Sample Nos. 10, 16, 17, 18, 19, 20, 21 and 22 demonstrate that if the amount of abrasive (Cr 2  O 3 ) is reduced the S/N ratio is decreased and still life is reduced, and that if the amount of an abrasive is increased, head wear also increases. The amount of the abrasive is preferably 1 to 20 parts by weight based on 100 parts by weight of magnetic particles. 
     While the invention has been described in detail and with reference to specific embodiment 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.