Abstract:
A magnetic recording medium comprised of a nonmagnetic support base having a magnetic recording layer on one surface thereof and a backing layer on the other surface thereof is disclosed. The backing layer is comprised of (1) carbon black having a particle size in the range of 10 to 150 mμ, (2) graphite having a particle size of 3μ or less, and (3) a curing agent. By utilizing this particular backing layer the resulting recording medium has improved durability with respect to the backing layer and shows a decreased in the number of dropouts and scratches during use.

Description:
This is a continuation of application Ser. No. 586,390, filed Mar. 5, 1984, now abandoned. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a magnetic recording medium comprising a support having provided thereon a backing layer to improve durability of the magnetic recording medium. 
     BACKGROUND OF THE INVENTION 
     Durability of magnetic recording media is required because the media are usually wound at high speed. 
     Hitherto, a backing layer has been provided to the support to improve the durability. Graphite particles and carbon black may be incorporated into the backing layer as disclosed in Japanese Patent Publication No. 10241/74. In the conventional backing layer, carbon black having a chain structure is incorporated to elastically maintain graphite particles in order to prevent the graphite particles from coming off the backing layer. However, carbon black does not sufficiently prevent the dropping off of graphite particles from the backing layer and is not effective for decreasing drop out and scratches on the backing layer. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a magnetic recording medium having an improved durability of the coated backing layer as well as having decreased drop out and scratches on the backing layer. 
     The above object of the present invention can be attained by a magnetic recording medium comprising a nonmagnetic support having a magnetic recording layer on one surface thereof and a backing layer on the opposite surface thereof, the backing layer containing (1) carbon black, (2) graphite, and (3) a curing agent. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Carbon black which can be used in this invention is not particularly limited, but the carbon black preferably has an average particle size of about 10 to about 150 mμ, more preferably 50 to 100 mμ. If the particle size is smaller than about 10 mμ, the durability of the resulting backing layer tends to decrease. If the particle size is larger than about 150 mμ, surface unevenness of the resulting backing layer increases and is transferred to the surface of a magnetic recording layer when the magnetic recording medium is wound thereby deteriorating electgromagnetic properties of the medium. 
     Graphite which can be used in this invention preferably has an average particle size of about 0.01 μm to 3 μm, more preferably 0.01 μm to 1 μm, most preferably 0.01 μm to 0.5 μm. 
     If the particle size of the graphite is larger than about 0.5 μm, graphite particles tend to drop off from the backing layer and make it difficult to filter the coating composition for a backing layer. 
     Graphite can be of various shapes such as flakes, granules, lumps or soils, both natural and artificial graphite can be used. The mixing ratio of the graphite and carbon black is preferably about 10:1 to about 1:10, more preferably 5:1 to 1:5 and most preferably 3:1 to 1:3 by weight. 
     If the mixing ratio of graphite to carbon black is too large, the backing layer tends to be easily scraped off. The powders which are scraped off tend to stain guiding systems of video tape recorders (VTR), causing drop out and scratches and deteriorates running properties. On the other hand, if the ratio of graphite to carbon black is too small, the graphite does not provide sufficient lubricity, deteriorating running properties and causing scratches and drop out. 
     In order to obtain the effect of this invention, graphite should preferably be used in such an amount that the total amount of carbon black and graphite is 100 to 175 wt%, more preferbly 110 to 160 wt%, and most preferably 120 to 150 wt%, based on the total amount of the binder used in a backing layer. 
     Any conventional polyisocyanates as described in, for example, U.S. Pat. No. 4,439,486 filed Mar. 12, 1982 can be used as a curing agent in the present invention. 
     Examples of suitable curing agent include ethanediisocyanate, decanediisocyanate, 1-methylbenzole-2,4,6-triisocyanate, triphenylmethane-4,4&#34;4&#34;-triisocyanate, an adduct of 1 mole of trimethylolpropane and 3 moles of toluylenediisocyanate, etc. 
     Further, polyamides can also be used as a curing agent in the present invention. 
     The curing agent can be used in an amount of 5 to 80 parts by weight, preferably 10 to 50 parts by weight, per 100 parts by weight of the binder. 
     Resins as described in U.S. Pat. No. 4,135,016 can be used as a binder for the backing layer in the magnetic recording medium of the present invention. The resins can be used alone or in combination. Also, other additives can be added to the resins. 
     Additives for a backing layer include inorganic pigments such as magnesium silicate, calcium carbonate, aluminum silicate, barium sulfate or clay; organic particles such as benzoguanamine; abrasives, surfactants and lubricants which are used in the magnetic recording layer hereinafter described, if necessary. 
     The inorganic pigments and organic particles are added in order to control unevenness of the surface of the coated backing layer, improve resistance to scratches of the backing layer, adjust the viscosity of a coating liquid and adjust the color of the backing layer. These materials are described in, for example, U.S. Pat. No. 4,273,797. 
     A ferromagnetic particle used in this invention, a method for preparing a magnetic coating composition, a binder used in a magnetic recording layer and a backing layer, additives used for a magnetic recording layer (e.g., a dispersing agent, a lubricant, an abrasive, a surface activec agent, an antistatic agent and the like), a method for preparing a support and a magnetic recording medium are disclosed in U.S. Pat. No. 4,135,016. 
    
    
     The present invention is further illustrated in more detail by the following Examples, but the present invention is not limited thereto. In the examples, all parts are by weight. 
     EXAMPLE 1 
     
         ______________________________________Co--added γ-Fe.sub.2 O.sub.3                     300    parts(particle size, 0.35μ, acicular ratio, 1:8;coercive force, 650 Oe)Vinyl chloride-vinyl acetate copolymer                     30     parts(weight ratio, 87:13; degree ofpolymerization, 420)Polyester polyurethane    30     parts(synthesized from butylene adipate and4,4&#39;-diphenylmethane diisocyanate; styreneequivalent molecular weight, about 130,000)Cr.sub.2 O.sub.3          12     partsLecithin                  6      partsDimethyl polysiloxane     3      partsButyl acetate             600    parts______________________________________ 
    
     The above composition was dispersed in a ball mill for 48 hours. To the dispersion was added 40 parts (solid content: 30 parts) of &#34;Desmodule L-75&#34; manufactured by Bayer Co., Ltd. (a 75 wt% ethyl acetate solution of an adduct of 1 mole of trimethylolpropane and 3 moles of toluylenediisocyanate) and the mixture was dispersed for 30 minutes. Then, the dispersion was filtrated by a filter having an average pore size of 1 to prepare a magnetic coating composition. 
     A polyethylene terephthalate film having a thickness of 14μ which had been coated with a coating composition for a backing layer having the following composition was coated and the above with the magnetic coating composition on the surface opposite the backing layer by a doctor coating method. The magnetic layer was subjected to magnetic orientation in a machine direction and to calendering treatment. The resulting magnetic recording medium was then slit into a width of 1/2 inch to prepare a video tape for VHS (Sample No. 1). 
     
         ______________________________________Carbon black            300    parts(&#34;furnace black&#34;, average particlesize, 60 mμ)Graphite                300    parts(average particle size, 1μ)Curing agent            133    parts(&#34;Desmodule L-75&#34;)Nitrocellulose          300    partsMethyl ethyl ketone     2000   partsMethyl isobutyl ketone  2000   parts______________________________________ 
    
     The above composition for the backing layer (except for the hardening agent) was dispersed in a ball mill for 40 hours. To the dispersion was then added the curing agent and, after adjusting the viscosity of the mixture, it was filtrated by a filter having a pore size of 5μ and coated. 
     The dry thickness of the magnetic recording layer was 5μ and that of the backing layer was 1.5μ. 
     EXAMPLE 2 
     The same procedure as described in Example 1 was repeated except that the following coating composition for a backing layer was used to prepare Sample No. 2. 
     
         ______________________________________Carbon black              300    parts(&#34;furnace black&#34;, 95 mμ)Graphite (0.5μ)        150    partsCuring agent (&#34;Desmodule L-75&#34;)                     100    partsTalc powder (3μ)       150    partsred iron oxide (0.5μ)  50     partsVinyl chloride-vinyl acetate-vinyl                     300    partsalcohol copolymer (degree ofpolymerization, 400; weight ratio of vinylchloride:vinyl acetate:vinyl alcohol = 85:10:5)Cyclohexanone             500    partsMethyl ethyl ketone       3000   parts______________________________________ 
    
     COMPARATIVE EXAMPLE 1 
     Sample No. 3 was prepared in the same manner as described in Example 1 except that nitrocellulose was used instead of the curing agent in the coating composition for the backing layer in the same amount (solid content) as the hardening agent. 
     Sample No. 4 was prepared in the same manner as in Example 1 except that graphite was used instead of carbon black. 
     Sample No. 5 was prepared in the same manner as described in Example 1 except that carbon black was used instead of graphite. 
     COMPARATIVE EXAMPLE 2 
     Sample No. 6 was prepared in the same manner as described in Example 1 except that nitrocellulose was used instead of the curing agent in the coating composition for the backing layer in the same solid amount, and that the amount of the binder was increased to a weight ratio of (carbon black and graphite) to (binder) of 60 wt%. 
     EXAMPLE 3 
     Example Nos. 7 to 10 were prepared in the same manner as described in Example 1 except that the weight ratio of (carbon black and graphite) to (binder) was adjusted to 80 wt%, 100 wt%, 175 wt% and 200 wt%, respectively. 
     EXAMPLE 4 
     Sample Nos. 11 and 12 were prepared in the same manner as described in Example 1 except that graphite in the backing layer had an average particle size of 0.2μ and 3μ, respectively. 
     EXAMPLE 5 
     Sample No. 13 was prepared in the same manner as described in Example 4 for preparing Sample No. 11 except that a copolymer of vinyl chloride and vinyl acetate in Example 2 was used instead of nitrocellulose in the backing layer. 
     Results of evalution of Sample Nos. 1 to 13 obtained by VHS type VTR are shown in Table 1. 
     It is apparent that Sample No. 1 of this invention has less drop out and less scratches and markedly improved durability as compard with Comparative Sample Nos. 3 to 6 prepared in a conventional manner. It is clear from the comparison between Sample No. 1 and Sample Nos. 3 to 5, that the effect of this invention can be obtained by the backing layer which simultaneously contains (1) carbon black, (2) graphite and (3) a curing agent. It is also clear from the comparison of Sample Nos. 9, 7, 1, 8 and 10 that better characteristics can be obtained when the weight ratio of the total amount of carbon black and graphite to the total amount of binder is 100 to 175 wt%. If the total amount of carbon black and graphite is higher, the binder cannot be completely maintained, causing increased drop out. If the total amount is lower, the amount of the binder is excessive, resulting in poor running properties and uneven winding. 
     It is apparent from the comparison of Sample Nos. 1, 11 and 12 that when graphite having a larger size is used, scratches tend to occur slightly thereby causing drop out. It is believed that such graphite particles are larger as compared with the thickness of a backing layer and therefore are easily scraped off. 
     As is apparent from comparison of Sample Nos. 11 and 13, nitrocellulose is the most effective binder for the backing layer. 
     
                                           TABLE__________________________________________________________________________                                           Number of                 Curing        Weight Drop out                                           scratches      Carbon Gra-                 agent         ratio of                                      5° C.                                           at 60° C.  Sample      Black  phite                 Desmodule                       Binder  [(A + B)/C]                                      number/                                           80% relativeExample  No. (A)    (B) L-75  (C)     (wt. %)                                      min. humidity__________________________________________________________________________Example 1  1   furnace black             1μ                 present                       Nitrocellulose                               150     5   1      60 mμExample 1  2   furnace black             0.5μ                 &#34;     Vinyl chloride/                               120     4   2      95 mμ         vinyl acetate                       copolymerComparative  3   furnace black             1μ                 absent                       Nitrocellulose                               150     95* 37*                                              *Flakes drop off.Example 1  60 mμComparative  4   absent 1μ                 present                       &#34;       &#34;      80   21*                                              *Squeal occurred fromExample 1                                          30 passes.Comparative  5   furnace black             absent                 &#34;     &#34;       &#34;      65   19*                                              *Tape squeal occurredExample 1  60 μ                                 from 90 passes.Comparative  6   furnace black             1μ                 absent                       &#34;        60    300* 50*                                              *Flakes dropped off.Example 2  60 mμ                             .sup.  **                                              **Tape squeal                                              occurred                                              from 15 passes.Example 3  7   furnace black             &#34;   present                       Nitrocellulose                               100     5   2      60 mμExample 3  8   furnace black             &#34;   &#34;     &#34;       175     5   2      60 mμExample 3  9   furnace black             &#34;   &#34;     &#34;        80    22   15*                                              *Tape squeal occurred      60 mμ                                from 120 passes.Example 3  10  furnace black             &#34;   &#34;     &#34;       200     18* 12 *Flakes drop off, a                                              little      60 mμExample 4  11  furnace black             0.2μ                 &#34;     &#34;       150     2   1      60 mμExample 4  12  furnace black             3μ                 &#34;     &#34;       &#34;       10* 5  *Flakes drop off, a                                              little      60 mμExample 5  13  furnace black             0.2μ                 &#34;     Vinyl chloride/                               &#34;       8   6      60 mμ         vinyl acetate                       copolyer__________________________________________________________________________ 
    
     The evaluations shown in the above Table were conducted in the following manner. 
     The drop out was evaluatd by allowing a VTR to stand in a dry thermostatic room at 5° C., repeatedly running a sample tape (200 passes) which had been mounted in a VTR cassette, and recording and reproducing a signal. The drop out was visually counted on a TV monitor and expressed in terms of the number of drop out per minute having a signal ratio more than 1/5 H (Horizon). The lower the number, the higher the durability. 
     The scratches were determined by running a sample tape (200 passes) in a VTR at 60° C. and 80% relative humidity and counting the number of scratches on the backing layer by a 50 magnification microscope. The scratches were expressed in terms of the number of scratches per 5 mm of tape width. The lower the number, the higher the 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.