Nippon Zeon Company, Ltd.

Kenji Akimoto

Katsuya Nakamura

Masahiro Yamakawa

Takahiro Takasaki

Fumio Takano

Tamotsu Yamamoto

A binder for coating materials containing dispersed inorganic powder comprises a polyurethane resin containing (a) 2.2 to 7.0 terminal epoxy groups in one molecule in average and (b) 0.1 to 5.0% by weight as SO.sub.3 -- of a sulfonic acid salt group, 0.05 to 3.0% by weight as COO-- of a carboxylic acid salt group, or 0.02 to 1.0% by weight as N.sup.+ of a quaternary ammonium salt group in the molecule, and having a number-average molecular weight of 2,000 to 10,000. A coating material containing dispersed inorganic powder and a magnetic recording medium both use this binder. In a recording medium comprising a magnetic layer placed on a non-magnetic substrate, the magnetic layer comprises polyurethane resin (A) as a binder, vinyl chloride polymer (B) containing 0.1 to 20% by weight of epoxy group and having an average degree of polymerization of 100 to 500, as another binder, (C) an alicylic acid anhydride as a crosslinking agent, and (E) a tertiary amine as a catalyst, and at least one of the binders is crosslinked between the epoxy groups therein by the reaction in which components (C) and (E) take part. The coating material containing fine inorganic powder dispersed with stability and having a low viscosity can be obtained by using, as the binder, the polyurethane resin having terminal epoxy groups and one of a sulfonic acid salt group, a carboxylic acid salt group, and a quaternary ammonium salt group. The magnetic recording material prepared by using magnetic powder as the inorganic powder and crosslinking the binder is excellent in dispersion of the magnetic powder and smoothness of the surface. Therefore, the magnetic recording medium is excellent in magnetic properties, durability, and running properties.

Magnetic recording medium having a specified binder system

The present invention provides a magnetic recording medium comprising a non-magnetic substrate and a magnetic layer formed on a substrate, comprising (a) a binder containing a vinyl chloride resin having sulfoxide groups and (b) a magnetic powder dispersed in the binder (a); and a binder used in said medium for dispersion of magnetic powder, containing a vinyl chloride resin containing sulfoxide groups. The present invention further provides a process for producing said vinyl chloride resin (a binder resin) having sulfoxide groups. The use of the binder resin of the present invention makes it possible to obtain a magnetic coating in which a fine magnetic powder is dispersed stably and which has good coatability. The magnetic recording medium obtained using the magnetic coating has good surface smoothness and good surface durability and is excellent in travelling, magnetic property and electromagnetic conversion property.

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Kawasaki, Japan

Kenji Akimoto