Method for Producing Magnetic Recording Medium

This invention relates to a magnetic recording medium. More particularly, this invention relates to a magnetic recording medium containing non-alkali beads in the magnetic layer thereof to minimize the head abrasiveness of magnetic recording apparatus and a method for producing a magnetic recording medium.

Hitherto, various methods for controlling the head abrasiveness of magnetic recording apparatus have been proposed. Typical examples of such methods include a method comprising incorporating an abrasive into the magnetic layer of magnetic recording medium as disclosed in U.S. Pat. Nos. 3,007,807, 3,041,196 and 3,293,066; a method for improving the surface of magnetic layer as disclosed in U.S. Pat. Nos. 2,688,567, 2,998,325 and 3,783,023; a method for reducing the coefficient of friction as disclosed in U.S. Pat. Nos. 2,654,681, 3,470,021 and 3,492,235, etc.

However, these conventional methods are still unsatisfactory since they are liable to be affected by temperature changes and moisture conditions and also exhibit somewhat poor contact with tape transportation systems and, therefore, further improvements of such magnetic recording medium have been desired.

A primary object of the present invention is to provide a magnetic recording medium having an improved low head abrasiveness.

Another object of the present invention is to provide a magnetic recording medium having a low head abrasiveness which is not influenced by temperature changes and moisture conditions.

A further object of the present invention is to provide a method for producing a megnetic recording medium having the above-described improved properties.

The above objects of the present invention can be achieved by incorporating non-alkali beads into the magnetic recording layer of the magnetic recording medium comprising a non-magnetic support and a magnetic recording layer provided on the support.

The above objects of the present invention can also be achieved by a method for producing a megnetic recording medium which comprises preparing a dispersion containing ferromagnetic powder and a binder in the presence of non-alkali beads and coating the dispersion on a non--magnetic support.

The term "non-alkali beads" as used herein means glass beads which are substantially free from alkali metal elements such as lithium, sodium and potassium as glass components (the content of alkali metal elements at a level not more than 0.1% by weight based on the total weight of glass beads is acceptable). More specifically, the non-alkali beads means glass beads composed of at least one of SiO.sub.2, Al.sub.2 O.sub.3, MgO, CaO, TiO.sub.2, ZrO.sub.2, etc. The compositions of such non-alkali glass beads are disclosed, for example, in Japanese Patent Publication No. 38851/77, Japanese Patent Application OPI No. 102325/78, etc. (The term "OPI" as used herein refers to a "published unexamined patent application".)

The particle size of non-alkali beads used in the present invention differs depending upon the utility of the magnetic recording medium, but can be any size so long as they have a diameter less than about 10 mm in dispersing with a ferromagnetic powder. The non-alkali beads are pulverized into fine powder during the dispersion operation and are finally in the particle size less than about 20.mu. in the magnetic layer coated on the support. Generally, the non-alkali beads can be used in a proportion less than about 0.1 wt % based on the amount of the ferromagnetic powder in the magnetic layer.

The non-alkali glass beads can be incorporated in preparing a magnetic coating solution, in particular, in dispersing a ferromagnetic powder using a sand grinder, ball mill and the like. The magnetic coating solution and the preparation thereof as well as the production steps thereof are disclosed in detail in U.S. Pat. No. 4,135,016.

The magnetic recording medium according to the present invention makes it possible to minimize the head abrasiveness of magnetic recording apparatus and such effects are not adversely influenced by temperature changes and moisture conditions.

The present invention is further illustrated in greater detail by the following Examples. Unless otherwise indicated, all parts, percents and ratios and the like are by weight.

The composition A described below was charged into a 2-liter sand grinder and then 900 g of Glass Beads A or B was added thereto, followed by dispersing the mixture for 6 hours. To the resulting dispersion A was added 4.8 g of 75 wt% solution of a polyisocyanate (a reaction product of 3 mols of a 2,4-tolylenediisocyanate compound and 1 mol of trimethylolpropane) in ethyl acetate to prepare Coating Solution A. Then, Coating Solution A was coated on a polyethylene terephthalate film having a thickness of 14.5.mu. in a coating thickness of 4.5.mu. (on dry basis) and dried, and the resulting magnetic recording medium was subjected to calendering (temperature of metal roll surface, 100.degree. C.; calender speed, 100 m/min.; nip pressure, 450 kg/cm; number of nips, 3; number of repeating, 6) to obtain Magnetic Recording Medium A.

The above-obtained Magnetic Recording Medium A was slit into 1/2 inch tapes (for 2 hour-recording tapes) and mounted on VHS (Video Home System) type video recorder. Th tape was run in 50 passes (the total running time: 100 hours) and the head abrasiveness was determined before and after the tape running. The results obtained are shown in Table 1 below.

The procedure as described in Example 1 was followed except that Composition B was used instead of Composition A and the 75% polyisocyanate solution was used in an amount of 7.6 g instead of 4.8 g. The results obtained are shown in Table below.

As is apparent from the results shown in Table above, the magnetic recording medium containing non-alkali beads in the magnetic layer exhibit a markedly low head abrasiveness as compared with the magnetic recording medium containing alkali beads in the magnetic layer. This tendency is remarkable when the relative humidity increases and then the binder used is a vinyl chloride-vinyl acetate copolymer as compared with nitrocellulose.

While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and the scope thereof.