Magnetic recording medium

Abstract

A magnetic recording medium comprises a non-magnetic support having provided thereon a lower magnetic layer having a thickness of 2.5 μm or higher and an upper magnetic layer having a thickness of 2 μm or lower in this order, wherein both the upper and lower magnetic layers contain ferromagnetic particles, and the lower magnetic layer contains carbon black having an average primary particle diameter of less than 20 μm in the amount of from 1.0 to 20 parts by weight per 100 parts by weight of the ferromagnetic particles contained in the lower magnetic layer, and the upper magnetic layer contains carbon black having an average primary particle diameter of at least 40 μm but less than 80 μm in an amount of from 0.1 to 10.0 parts by weight per 100 parts by weight of the ferromagnetic particles contained in the upper magnetic layer, but less than the amount of carbon black used in the lower magnetic layer.

What is claimed is: 
     
       1. A magnetic recording medium comprising a non-magnetic support having provided thereon a lower magnetic layer . .having a thickness of 2.5 μm or higher.!. and an upper magnetic layer having a thickness of 2 μm or lower . .in this order.!., wherein both the upper and lower magnetic layers contain . .ferromagnetic.!. .Iadd.magnetic .Iaddend.particles .Iadd.and a binder.Iaddend., and the lower magnetic layer contains carbon black having an average primary particle diameter of less than 20 mμ in an amount of from 1.0 to 20 parts by weight per 100 parts by weight of the . .ferromagnetic.!. .Iadd.magnetic .Iaddend.particles present in the lower magnetic layer, and the upper magnetic layer contains carbon black having an average primary particle diameter of at least 40 mμ but less than 80 mμ in an amount of from 0.1 to 10.0 parts by weight per 100 parts by weight of the . .ferromagnetic.!. .Iadd.magnetic .Iaddend.particles present in the upper magnetic .Iadd.layer.Iaddend.. ., but less than the amount of carbon black used in the lower magnetic layer.!.. 
     
     
       2. . .A.!. .Iadd.The .Iaddend.magnetic recording medium as in claim 1, wherein the lower magnetic layer contains .Iadd.said .Iaddend.carbon black having an average primary particle diameter of less than 20 mμ in an amount of from 3 to 15 parts by weight per 100 parts by weight of the . .ferromagnetic.!. .Iadd.magnetic .Iaddend.particles present in the lower magnetic layer. 
     
     
       3. . .A.!. .Iadd.The .Iaddend.magnetic recording medium as in claim 1, wherein the upper magnetic layer contains an abrasive agent in an amount of from 0.1 to 12 parts by weight per 100 parts by weight of the . .ferromagnetic.!..Iadd.magnetic .Iaddend.particles present in the upper magnetic layer. 
     
     
       4. . .A.!. .Iadd.The .Iaddend.magnetic recording medium as in claim 1, wherein . .the lower magnetic layer has a thickness of 3.0 μm or higher and.!. the upper magnetic layer has a thickness of .Iadd.from 0.1 to .Iaddend.1.5 μm . .or lower.!.. 
     
     
       5. . .A.!. .Iadd.The .Iaddend.magnetic recording medium as in claim . .4.!. .Iadd.13.Iaddend., wherein the lower magnetic layer has a thickness of from 3.0 μm to 6.0 μm. 
     
     
       6. . .A.!. .Iadd.The .Iaddend.magnetic recording medium as in claim 1, wherein the specific surface area measured by the BET method of the . .ferromagnetic.!. .Iadd.magnetic .Iaddend.particles present in the lower magnetic layer is 40 m 2  /g or lower, and the specific surface .Iadd.area .Iaddend.as measured by the BET method of the . .ferromagnetic.!. .Iadd.magnetic .Iaddend.particles present in the upper magnetic layer is 30 m 2  /g or higher. 
     
     
       7. . .A.!. .Iadd.The .Iaddend.magnetic recording medium as in claim 1, wherein the . .ferromagnetic.!. .Iadd.magnetic .Iaddend.particles present in the upper magnetic layer have a coercive force of 1,000 Oe or lower, .Iadd.and.Iaddend. the . .ferromagnetic.!. .Iadd.magnetic .Iaddend.particles present in the lower magnetic layer have a coercive force of 600 Oe or higher. ., and the ferromagnetic particles present in the lower and the upper magnetic layers are cobalt modified γ-Fe 2  O 3  .!.. 
     
     
       8. . .A.!. .Iadd.The .Iaddend.magnetic recording medium as in claim 1, wherein the weight ratio of magnetic particles/binder in the upper magnetic layer and in the lower magnetic layer is from 2.5 to 10.0. 
     
     
       9. . .A.!. .Iadd.The .Iaddend.magnetic recording medium as in claim 1, wherein the . .ferromagnetic.!. .Iadd.magnetic .Iaddend.particles in the lower magnetic layer have a specific surface area by .Iadd.the .Iaddend.BET method of 50 m 2  /g or lower. 
     
     
       10. . .A.!. .Iadd.The .Iaddend.magnetic recording medium as in claim 3, wherein the abrasive . .agents.!. .Iadd.agent .Iaddend.in the upper magnetic layer . .have.!. .Iadd.has .Iaddend.an average particle diameter of from 0.05 to 1.0 μm. 
     
     
       11. . .A.!. .Iadd.The .Iaddend.magnetic recording medium as in claim 10, wherein the abrasive . .agents.!. .Iadd.agent .Iaddend.in the upper magnetic layer . .have.!. .Iadd.has .Iaddend.an average particle diameter of from 0.10 to 0.50 μm. 
     
     
       12. . .A.!. .Iadd.The .Iaddend.magnetic recording medium as in claim 1, wherein the lower magnetic layer contains no abrasive agents. .Iadd.13. The magnetic recording medium as in claim 1, wherein the lower magnetic 
     
     
        layer has a thickness of 2.5 μm or more..Iaddend..Iadd.14.  The magnetic recording medium as in claim 1, wherein the upper magnetic layer contains said carbon black in an amount less than the amount of carbon black present in the lower magnetic layer..Iaddend..Iadd.15. The magnetic recording medium as in claim 1, wherein the average primary particle diameter of carbon black present in the upper magnetic layer is from 40 to mμ to 75 mμ..Iaddend..Iadd.16. The magnetic recording medium as in claim 15, wherein the average primary particle diameter of said carbon black is from 40 mμ to 70 mμ..Iaddend..Iadd.17. The magnetic recording medium as in claim 1, wherein the average primary particle diameter of carbon black present in the lower magnetic layer is from 10 
     
     
        mμ to less than 20 mμ..Iaddend..Iadd.18.  The magnetic recording medium as in claim 1, wherein the upper magnetic layer contains said carbon black in an amount of from 0.1 to 6.0 parts by weight per 100 parts by weight of the magnetic particles present in the upper magnetic layer..Iaddend..Iadd.19. The magnetic recording medium as in claim 1, wherein the magnetic particles present in the lower and upper magnetic layers are cobalt-modified γ-Fe 2  O 3 ..Iaddend..Iadd.20. The magnetic recording medium as in claim 19, wherein the magnetic particles present in the lower magnetic layer have a specific surface area measured by the BET method of 40 m 2  /g or lower and a coercive force of 600 Oe or higher, and the magnetic particles present in the upper magnetic layer have a specific surface area measured by the BET method of 30 m 2  /g or higher and a coercive force of 1,000 Oe or lower..Iaddend..Iadd.21. The magnetic recording medium as in claim 1, wherein the upper magnetic layer is coated on the lower magnetic layer while the lower magnetic layer is in 
     
     
        a wet condition..Iaddend..Iadd.22.  The magnetic recording medium as in claim 14, wherein the lower magnetic layer has a thickness of 2.5 μm or more..Iaddend..Iadd.23. The magnetic recording medium as in claim 1, wherein the upper magnetic layer is coated on the lower magnetic layer 
     
     
        while the lower magnetic layer is in a wet condition..Iaddend..Iadd.24. The magnetic recording medium as in claim 22, wherein the upper magnetic layer is coated on the lower magnetic layer while the lower magnetic layer 
     
     
        is in a wet condition..Iaddend..Iadd.25.  The magnetic recording medium as in claim 24, wherein the magnetic particles in the upper and lower layers are ferromagnetic particles..Iaddend..Iadd.26. A magnetic recording medium comprising a non-magnetic support and provided thereon a plurality of layers comprising an upper magnetic layer, which is the outermost magnetic layer, containing at least magnetic particles and a binder, and a lower magnetic layer containing at least magnetic particles and a binder, wherein said upper magnetic layer contains carbon black having an average primary particle diameter ranging from 40 mμ to less than 80 mμ, and said lower magnetic layer contains carbon black having an average primary particle diameter of less than 20 mμ; and said upper layer is coated on said lower layer while the lower layer is in a wet condition; and wherein said carbon black is contained in the lower magnetic layer in a content of from 1.0 to 20 weight parts per 100 weight parts of the magnetic particles present in the lower magnetic layer, and said carbon black is contained in the upper magnetic layer in a content of from 0.1 to 10.0 weight parts per 100 weight parts of the magnetic particles present in the upper layer, said upper magnetic layer having a thickness of 2 μm or less..Iaddend..Iadd.27. The recording medium of claim 26, wherein said average primary particle diameter of carbon black contained in the upper layer ranges from 40 mμ to 70 mμ..Iaddend..Iadd.28. The recording medium of claim 26, wherein said average primary particle diameter of carbon black contained in the lower layer ranges from 10 mμ to less 
     
     
        than 20 mμ..Iaddend..Iadd.29.  The recording medium of claim 26, wherein said content of said carbon black contained in the upper layer is 0.1 to 6.0 weight parts per 100 weight parts of the magnetic particles..Iaddend..Iadd.30. The recording medium of claim 29, wherein said content of said carbon black contained in the upper layer is 0.1 to 2 weight parts per 100 weight parts of the magnetic particles..Iaddend..Iadd.31. The recording medium of claim 26, wherein said content of said carbon black contained in the lower layer is 1.0 to 10 weight parts per 100 weights parts of the magnetic particles..Iaddend..Iadd.32. The recording medium of claim 40, wherein said content of said carbon black contained in the lower layer is 1.0 to 8 weight parts per 100 weight parts of the magnetic particles..Iaddend..Iadd.33. The recording medium of claim 26, wherein said layer thickness of the upper layer is 0.1 to 1.5 
     
     
        μm..Iaddend..Iadd.4.  The recording medium of claim 33, wherein said layer thickness of the upper layer is 0.5 to 1.5 μm..Iaddend..Iadd.35. The recording medium of claim 31, wherein a layer thickness of the lower layer is 2.5 μm or higher..Iaddend..Iadd.36. The recording medium of claim 35, wherein said layer thickness of the lower layer is 2.5 to 6.0 μm..Iaddend..Iadd.37. The recording medium of claim 36, wherein said layer thickness of the lower layer is 2.5 to 3.5 μm..Iaddend..Iadd.38. The recording medium of claim 26, having a coefficient of dynamic friction of 0.28 or less..Iaddend..Iadd.39. The recording medium of claim 38, having a coefficient of dynamic friction of 0.25 or less..Iaddend..Iadd.40. The recording medium of claim 39, having a coefficient of dynamic friction of 0.21..Iaddend..Iadd.41. The recording medium of claim 26, having a surface electric resistance of 
     
     
        1×10 7  to 8×10 7  Ω/sq..Iaddend..Iadd.42.  A magnetic recording medium comprising a non-magnetic support and provided thereon a plurality of layers comprising an upper magnetic layer, which is the outermost magnetic layer, containing at least magnetic particles and a binder, and a lower magnetic layer containing at least magnetic particles and a binder, wherein said upper magnetic layer contains carbon black having an average primary particle diameter ranging from 40 mμ to less than 80 mμ, and said lower magnetic layer contains carbon black having an average primary particle diameter of less than 20 mμ and said upper layer is coated on said lower layer while the lower layer is in a wet condition; and wherein said carbon black is contained in the lower magnetic layer in a content of from 1.0 to 20 weight parts per 100 weight parts of the magnetic particles present in the lower magnetic layer, said carbon black is contained in the upper magnetic layer in a content of from 0.1 to 10.0 weight parts per 100 weight parts of the magnetic particles present in the upper layer, said upper magnetic layer has a thickness of 2 μm or less, said lower magnetic layer has a thickness of 2.5 μm or higher, and the content of carbon black contained in the upper layer is less than the content of carbon black contained in the lower magnetic layer..Iaddend..Iadd.43. The recording medium of claim 42, wherein said lower layer thickness is 2.5 to 6.0 μm..Iaddend..Iadd.44. The recording medium of claim 43, wherein said lower layer thickness is 2.5 to 3.5 μm..Iaddend..Iadd.45. The recording medium of claim 42, having a 
     
     
        coefficient of dynamic friction of 0.28 or less..Iaddend..Iadd.46.  The recording medium of claim 42, having a coefficient of dynamic friction of 0.25 or less..Iaddend..Iadd.47. The recording medium of claim 42, having a coefficient of dynamic friction of 0.21..Iaddend..Iadd.48. The recording medium of claim 42, having a surface electric resistance of 1×10 7  to 8×10 7  Ω/sq..Iaddend.