Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Including a second component containing structurally defined...
Reexamination Certificate
2000-04-17
2001-03-27
Resan, Steven A. (Department: 1773)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Including a second component containing structurally defined...
C428S328000, C428S329000, C428S690000, C428S900000
Reexamination Certificate
active
06207267
ABSTRACT:
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The present invention relates to a magnetic recording medium having a non-magnetic primer layer containing a binder and non-magnetic particles, on a flexible non-magnetic support, and having a magnetic layer containing a binder and a ferromagnetic acicular metal powder, thereon. Particularly, it relates to a magnetic recording medium which is excellent in durability and suitable for high density recording.
DISCUSSION OF BACKGROUND
In recent years, it has strongly been demanded to adapt magnetic recording media to high density recording with a high frequency. In view of such a demand, a study has been made on a magnetic recording medium employing a thin metal film for the magnetic layer. However, a coating type magnetic recording medium is superior from the viewpoint of the productivity, durability, corrosion resistance, etc., and improvements of a coating type magnetic recording medium have been studied from various aspects. In high frequency recording, problems in the overwriting characteristics and self-demagnetization loss during recording, are substantial, and to avoid such problems, it is necessary to make the magnetic layer thin. However, if the magnetic layer is simply made thin, the durability and the surface property deteriorate. Therefore, a magnetic recording medium of a double layer structure has heretofore been proposed wherein a non-magnetic layer composed mainly of a non-magnetic powder and a binder, is formed beneath the magnetic layer (JP-A-62-154225, JP-A-62-159338). However, with such a double layer structure only, it has been difficult to obtain a magnetic recording medium which is excellent in durability and in electromagnetic conversion characteristics and thus is suitable for high density recording.
JP-A-7-287833 proposes to improve the surface property of the magnetic layer by coating a coating material for a non-magnetic primer layer containing a non-magnetic powder treated with antimony or tin for coating and coating a coating material for a magnetic layer containing a magnetic powder while the coating material for the primer layer is still in a wet state. However, if the upper layer is coated while the lower layer is still in a wet state, the interface will be roughened, and the electromagnetic conversion characteristics tend to deteriorate, when the thickness of the upper layer is thin. Further, the roughening of the interface influences the surface, whereby no satisfactory surface property can be obtained.
It is an object of the present invention to solve such problems and to provide a magnetic recording medium having a magnetic layer and a non-magnetic layer laminated, wherein roughening of the interface between the two layers is little, whereby the electromagnetic conversion characteristics are excellent, and a process for its production.
SUMMARY OF THE INVENTION
The present invention provides a magnetic recording medium comprising a flexible non-magnetic support, a non-magnetic primer layer containing a binder and non-magnetic particles, formed on the support, and a magnetic layer containing a binder and a ferromagnetic acicular metal powder or a hexagonal ferromagnetic powder (hereinafter sometimes referred to generally as the ferromagnetic metal powder), formed on the primer layer, wherein the ferromagnetic acicular metal powder is one having an average long axis length (L) of at most 0.2 and an aspect ratio (K) of at most 15, and the hexagonal ferromagnetic powder is one having a plate length (L′) of at most 0.07 &mgr;m and a plate ratio (K′) of at most 10, and the thickness of the magnetic layer is at most 0.5 &mgr;m, and wherein the variation (D) of the interface between the magnetic layer and the non-magnetic primer layer is represented by D≦2L/K or D≦2L′/K′, whereby it is possible to obtain a magnetic recording medium which is excellent in durability and in the electromagnetic conversion characteristics, particularly the output and overwriting characteristics, and which is suitable for high density recording.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The magnetic recording medium of the present invention is one having a non-magnetic primer layer formed on a flexible non-magnetic support and having a magnetic layer further laminated thereon. If desired, another layer may be formed between the support and the non-magnetic primer layer, or on the magnetic layer.
The flexible non-magnetic support may be made of any optional material which is commonly used in such applications. Typical examples include polyesters such as polyethylene terephthalate and polyethylene-2,6-naphthalene, polyolefins such as polypropylene, cellulose derivatives such as cellulose triacetate and cellulose diacetate, and plastics such as aramide and polycarbonate. The shape of the flexible non-magnetic support is usually a film-form or a tape-form. Further, in order to improve the adhesion between this flexible non-magnetic support and the non-magnetic primer layer, corona discharge treatment or surface treatment by a surface modifier such as an aqueous amine solution, trichloroacetic acid or a phenol, may be applied to the flexible non-magnetic support.
The non-magnetic primer layer is composed mainly of non-magnetic particles and a binder. As the non-magnetic particles, it is preferred to employ carbon black. However, as other materials, titanium oxide, &agr;-iron oxide, &agr;-alumina, silicon carbide, chromium oxide, cerium oxide, geothite, corundum, silicon nitride, silicon dioxide, tin oxide, magnesium oxide, zirconium dioxide, calcium carbonate, calcium sulfate, barium sulfate and molybdenum disulfide may, for example, be employed.
When carbon black is used as the non-magnetic particles, its volatile content is usually at least 3%, preferably from 3 to 10%. The present inventors have found that the amount of the volatile content relates to the dispersibility of carbon black. Namely, up to a volatile content of about 10%, the dispersibility of carbon black is usually improved as the volatile content increases. If the volatile content is less than 3%, such carbon black is poor in the dispersibility and tends to deteriorate the durability of the magnetic recording medium. Here, the volatile content is meant for the volatile loss according to JIS K-6221-1982 when carbon black is put in porcelain or platinum crucible with a lid and heated at 950° C. for 7 minutes. The carbon black preferably constitutes from 30 to 98 wt %, particularly preferably from 35 to 85 wt %, of the non-magnetic primer layer.
Further, the specific surface area of the carbon black is preferably at least 100 m
2
/g, more preferably from 100 to 150 m
2
/g. In the non-magnetic primer layer and in the magnetic layer thereon, an organic lubricant such as an aliphatic carboxylic acid or its ester, is contained in many cases. If carbon black having an extremely small specific surface area, is employed, the action of the non-magnetic primer layer to maintain such an organic lubricant, tends to be weak, whereby the durability of the magnetic recording medium is likely to deteriorate.
Further, the average particle size of primary particles of carbon black is preferably from 20 to 100 nm.
As the binder, it is preferred to employ one which is excellent in the abrasion resistance and adhesion to the support and which has a glass transition point of from −100 to 150° C. and a number average molecular weight of from about 1,000 to 150,000. Commonly employed binders include, for example, a polyurethane resin, a polyester resin, a cellulose derivative such as cellulose acetate butyrate, cellulose diacetate or nitrocellulose, a vinyl chloride type resin such as a vinyl chloride-vinyl acetate copolymer, a vinyl chloride-vinylidene chloride copolymer or a vinyl chloride-acrylic copolymer, various synthetic rubbers such as a styrene-butadiene copolymer, an epoxy resin and a phenoxy resin. These binders may be used alone or in combination as a mixture of two or more of them. It is preferred to use the binder so that its cont
Kinoshita Yoshihiro
Ochi Makoto
Shimomura Osamu
Mitsubishi Chemical Corporation
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Resan Steven A.
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