Magnetic recording medium having a magnetic layer with specified limits of both RA and R10Z surface roughness

Abstract

Disclosed are a magnetic recording medium which comprises a non-magnetic support, one or more coating layers provided on the non-magnetic support and an upper layer of the coating layers being a magnetic layer containing ferromagnetic metallic powder with the total thickness of the coating layers being 0.5 to 2.5 μm, wherein the magnetic layer has a surface roughness Ra of 1.0 to 4.6 nm and a surface roughness R10z of 8.0 to 35.0 nm, which satisfy the relations of: Ra≦-0.47t+0.5/t+3.8 and R10z≦-1.50t+6.0/t+23.0 where t is the total thickness of the coating layers in terms of μm, and an aliphatic acid ester is contained in the surface portion of the magnetic layer in an amount of 5 to 50 mg/m 2 , and a process for preparing the same.

We claim: 
     
       1. A magnetic recording medium comprising a non-magnetic support having provided thereon two or more coating layers, . .an.!. .Iadd.wherein the .Iaddend.uppermost . .of said.!. coating . .layers being.!. .Iadd.layer is .Iaddend.a magnetic layer . .containing.!. .Iadd.comprising .Iaddend.a ferromagnetic metal powder, . .a lower of said.!. .Iadd.the lowest .Iaddend.coating . .layers being.!. .Iadd.layer is .Iaddend.a non-magnetic layer . .containing.!. .Iadd.comprising .Iaddend.a non-magnetic powder, . .a total thickness of said.!. .Iadd.wherein the .Iaddend.coating layers . .being.!. .Iadd.have a thickness from .Iaddend.0.5 to 2.5 microns, . .said.!. .Iadd.the .Iaddend.magnetic layer having a surface roughness Ra of 1.0 to 4.6 nm, and a surface roughness R10z of 8.0 to 35.0 nm, Ra≦-0.70t+0.5/t+3.8, and R10z≦-3.50t+6.0/t+23.0, wherein t is the total thickness of the coating layers in microns, Ra . .and R10z.!. being as defined in JIS B0601 (1976), .Iadd.and R10z is a distance, in a direction perpendicular to a plane of the magnetic recording medium, between a first line and a second line parallel thereto, the first line passing through a 10th highest peak measured from the non-magnetic support and parallel thereto, the second line passing through a 10th lowest ravine from the non-magnetic support and parallel thereto, the peak and ravine being determined at a vertical longitudinal section through the magnetic recording medium within 2.0 mm of a center line laterally midway between edges of the non-magnetic medium and over a longitudinal distance of 0.5 mm; and .Iaddend.a surface amount of aliphatic acid ester of said magnetic layer being .Iadd.about .Iaddend.5 to 50 mg/m 2 .   
     
     
       2. The medium of claim 1 wherein the medium has a total thickness of 7.5 μm or less. 
     
     
       3. The medium of claim 1 wherein the non-magnetic powder is at least one selected from the group consisting of carbon black, CaCO 3 , TiO 2 , barium sulfate, α-Al 2  O 3 , α-Fe 2  O 3 , α-FeOOH and Cr 2  O 3 . 
     
     
       4. A process for preparing a magnetic recording medium comprising; .Iadd.a) .Iaddend.forming a non-magnetic support,   . .using a kneading device to form.!.   .Iadd.b) forming .Iaddend.a first predispersed coating material. ., and coating said first predispersed coating material on said non-magnetic support to form at least one non-magnetic layer.!. containing .Iadd.a .Iaddend.non-magnetic powder .Iadd.using a kneading device.Iaddend.,   . .using a kneading device to predisperse a second coating material to form a second predispersed coating material, and coating said second predispersed coating material on said at least one non-magnetic layer to form at least one magnetic layer containing a ferromagnetic metallic powder, a total combined thickness of said at least one non-magnetic layer and said at least one magnetic layer being 0.5 to 2.5 μm,   said kneading and coating conditions being adjusted whereby said at least one magnetic layer has a surface roughness Ra of 1.0 to 4.6 nm, and a surface roughness of R10z of 8.0 to 35.0 nm, Ra≦-0.70t+0.5/t+3.8, and R10z≦-3.5t+6.0/t+23.0, wherein t is a total thickness of the coating layers in microns, and Ra and R10z being as defined in JIS B0601 (1976), a surface amount of aliphatic acid ester on said magnetic layer being 5 to 50 mg/m 2  .!.   .Iadd.c) coating the non-magnetic support with the first predispersed coating material to form at least one non-magnetic layer;   d) forming a second predispersed coating material containing a ferrometallic powder using a kneading device; and   e) coating the top non-magnetic layer with a second predispersed coating material to form at least one magnetic layer, wherein the non-magnetic layer and the magnetic layer have a combined thickness from about 0.5 μm to about 2.5 μm, and   wherein forming and coating conditions are adjusted to provide a surface roughness Ra of the magnetic layer from about 1.0 nm to about 4.6 nm and a surface roughness R10z from about 8.0 nm to about 35.0 nm, Ra≦-0.7t+0.5/t+3.8 and R10z≦-3.5t+6.0/t+23.0, wherein t is the thickness of the coating layers and Ra is defined in JIS B0601 (1976), R10z is a distance, in a direction perpendicular to a plane of the magnetic recording medium, between a first line and a second line parallel thereto, the first line passing through a 10th highest peak measured from the non-magnetic support and parallel thereto, the second line passing through a 10th lowest ravine from the non-magnetic support and parallel thereto, the peak and ravine being determined at a vertical longitudinal section through the magnetic recording medium within 2.0 mm of a center line laterally midway between edges of the non-magnetic medium and over a longitudinal distance of 0.5 mm.Iaddend..     
     
     
       5. The process of claim 4, wherein said kneading device is a pressure kneader or a continuous kneader.