Abstract: A method of preparing a magnetic coating composition comprises a first process for kneading ferromagnetic grains, a binder and a binder solution Y1 by use of a two-shaft continuous kneading and mixing machine to obtain a kneaded mixture K1 having a solid content within the range of 65 wt % to 95 wt %, a second process for kneading a binder solution Y2 and the kneaded mixture K1 by use of the two-shaft continuous kneading and mixing machine to obtain a primary diluted mixture K2 having a solid content within the range of 50 wt % to 70 wt %, a third process for diluting and dispersing the primary diluted mixture K2 in an organic solvent to obtain a secondary diluted mixture K3 having a solid content within the range of 25 wt % to 50 wt %, and a fourth process for finely dispersing the secondary diluted mixture K3 to obtain a dispersion mixture K4 having a solid content within the range of 25 wt % to 45 wt %.

Abstract: A method for making a magnetic recording medium comprises the steps of mixing ferromagnetic grains and a surface treating agent, grinding the resulting mixture, and thus obtaining a ground mixture. A binder and a solvent are added to the ground mixture, which results from the mixing and grinding process, the resulting mixture is kneaded and diluted, and a kneaded and diluted mixture is thus obtained. An additive and a solvent are then added to the kneaded and diluted mixture, which results from the kneading and diluting process, a dispersion process is carried out on the resulting mixture, and a dispersion mixture is thereby obtained. Thereafter the dispersion mixture, which results from the dispersion process, is applied to a non-magnetic substrate.

Abstract: A method of preparing a kneaded mixture for a magnetic coating material comprises a kneading start process and a continuous kneading process. In the kneading start process, a low-viscosity composition, which is composed of magnetic layer forming materials containing ferromagnetic grains and a binder, and a solvent, is introduced into a kneading vessel of a two-shaft continuous kneading and mixing machine, and kneading is started. In the continuous kneading process, after the start of the kneading, a high-viscosity composition, in which a solvent amount is smaller by at least 7 wt % than the solvent amount in the low-viscosity composition used in the kneading start process, is continously introduced into the kneading vessel, and the kneading is continued while high shearing force is being given to the high-viscosity composition.

Abstract: A magnetic recording medium which comprises a non-magnetic support having thereon a magnetic recording layer containing a ferromagnetic fine powder and a binder as main components is disclosed. The magnetic recording layer containing as the binder (1) a vinyl chloride-vinyl acetate-maleic acid copolymer which has a mean polymerization degree of about 300 to about 400 and a maleic acid fraction of about 0.6 to about 1.5 wt %, (2) a polyurethane resin which has a number average molecular weight of 40,000 to 100,000 and is prepared from (i) a mixture of polyneopentyladipate having a mean molecular weight of 500 to 2,500 and polybutyleneadipate having a mean molecular weight of 500 to 2,500 with a mixing ratio of the former to the latter of from 2/8 to 9/1 by weight and (ii) diphenylmethane diisocyanate, and optionally (3) a low molecular weight polyisocyanate compound.