1. Field of the Invention
This invention relates to a magnetic recording medium, such as magnetic tape. More particularly, it relates to an improvement in a binder contained in the magnetic layer which is formed on the non-magnetic base material of the magnetic recording medium.
2. Description of the Prior Art
Recently, in magnetic recording media such as those for video tape recorders (VTRs), a demand is raised for improving magnetic properties as well as electro-magnetic conversion properties thereof for obtaining a high playback output even on the occasion of the short wavelength recording. As a measure therefor, it has been tried to comminute magnetic powders while increasing the coercive force and so-called packing density of the magnetic powders in the magnetic layer.
On the other hand, binders such as vinyl chloride binders including vinyl chloride-vinyl acetate copolymer, vinyl chloride-propionic acid copolymer or vinyl chloride-vinyl acetate-vinyl alcohol copolymer, have so far been used as the binder for the magnetic recording medium. Above all, the vinyl chloride-vinyl acetate-vinyl alcohol copolymers have been extensively used since the hydroxyl group of the vinyl alcohol monomer contributes to dispersibility of the magnetic powders while the active hydrogen of the hydroxyl group reacts with an isocyanate compound to form a cross-linked structure to enhance the mechanical strength of the coating layer.
However, with the increase in cohesive force due to increased coercive force and the increase in the specific surface area due to communition of the magnetic particles as described above, it is not possible with the aforementioned binder to obtain satisfactory dispersibility or surface properties such that it is becoming difficult to increase the packing density of the magnetic powders. Hence the binder leaves much to be desired in durability, magnetic properties and electro-magnetic conversion properties. Although suitable surfactants may be used as the dispersant, the surfactants are of low molecular weight such that, when they are present in the magnetic layer, problems are presented in durability or mechanical strength such as blooming caused by temporal changes or powder debris.
Hence, a demand is raised for a binder capable of significantly improving the aforementioned properties of the magnetic recording medium. Thus there is proposed a technology in which a polymeric material comprised of vinyl chloride-vinyl acetate copolymer or vinyl chloride-vinyl acetate-vinyl alcohol copolymer into the side chains of which are introduced polar groups such as sulfonic acid or alkali metal salts thereof, acidic sulfuric acid esters or alkali metal salts thereof, or carboxylic acid or metal salts thereof, is used as the binder.
Although the binder according to the above described technology shows some limited effect in improving the dispersibility in comparison with the conventional binder in which polar groups are not introduced, it has only insufficient effect for magnetic powders showing a higher coercive force and ultra-comminuted magnetic powders.
There has not been evolved a binder showing a sufficient dispersibility for these ultra-comminuted magnetic powders and hence it has been difficult to procure predetermined durability, magnetic properties and electro-magnetic conversion properties of the magnetic recording medium that make use of these ultra-comminuted magnetic particles.