1. Field of the Invention
This invention relates to a magnetic recording substance and more particularly, it is concerned with a magnetic recording medium in which the surface electric resistance of a magnetic layer is lowered without deteriorating the properties of the magnetic recording layer.
2. Description of the Prior Art
Generally, unless the surface electric resistance of a magnetic recording substance is lowered, static electricity is generated and accumulated by friction between tapes or a tape and pole in a tape travelling system, which absorbs dusts in the air and, in extreme cases, causes electric discharge and noise in sound or in a monitor picture.
Up to the present time, there have been proposed various methods for lowering the electric resistance of magnetic recording media, such as (1) by adding hygroscopic surfactants and (2) by adding carbon black. However, the first method (1) using surfactants has disadvantages that a magnetic layer tends to soften and to creak under a high humidity and the second method (2) comprising adding carbon black has also disadvantages that it is necessary to add carbon black in an amount of 7% by weight or more to a magnetic material and there thus takes place lowering of the filling degree of a magnetic material per unit volume of a magnetic recording layer and deterioration of the surface smoothness of a magnetic recording layer, resulting in decrease of the signal output and increase of the noise level. In the second method, moreover, dispersion is excessively carried out in a ball mill and, consequently, the surface electric resistance is rather increased.
We, the inventors have made various studies to improve the above described second method comprising adding carbon black and consequently have reached the present invention.