Magnetic recording medium

Details Magnetic recording medium Magnetic recording medium

2003-08-29

2004-12-28

Resan, Stevan A. (Department: 1773)

Stock material or miscellaneous articles

Structurally defined web or sheet

Including components having same physical characteristic in...

C428S403000, C428S328000, C428S690000, C428S690000, C428S690000

Reexamination Certificate

active

06835445

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a magnetic recording medium capable of high density recording and excellent in electromagnetic characteristics and running durability.
BACKGROUND OF THE INVENTION
Magnetic heads making electromagnetic induction as a principle of operation (an induction type magnetic head) have been conventionally used and prevailed. However, there are the limits to their abilities for further higher density recording and reproduction use, and a magnetic head making MR (magneto resistance) as a principle of operation has been proposed and is coming to be used in a hard disc and the like. An MR head can provide reproduction output of several times as large as that by an induction type magnetic head and, since no induction coil is used, equipment noise such as impedance noise is widely reduced, therefore, it has become possible to obtain a great S/N ratio by reducing the noise of a magnetic recording medium. A magnetic recording layer suitable for an MR head has also been discussed in conventionally used coating type magnetic recording media excellent in productivity and capable of manufacturing inexpensively. A magnetic recording medium comprising a nonmagnetic support having provided thereon in the order of a substantially nonmagnetic lower layer and a magnetic layer comprising ferromagnetic fine powder dispersed in a binder is broadly used in the coating type magnetic recording medium for high density recording of the day as a means of capable of realizing thinning of a magnetic layer while maintaining surface smoothness of the magnetic recording medium (refer to JP-B-4-71244 (the term “JP-B” as used herein means an “examined Japanese patent publication”)).
It is important to make the particle size of magnetic powder smaller in order to realize high density recording. However, a problem such that noise increases particularly when the particle size of magnetic powder becomes small has arisen during the development of a coating type magnetic recording medium having a real recording density of higher than 0.3 G bit/inch
2
. When the particle size of magnetic powder is made small, dispersion of the magnetic powder in a binder becomes difficult at the time of preparation of a magnetic layer-coating solution, so that the smoothness of the magnetic layer surface cannot be maintained, as a result it becomes difficult to obtain an objective low noise medium. For holding down the noise, it is necessary (1) to get rid of the agglomeration of magnetic particles, and (2) to smooth the magnetic layer surface.
Good running durability is also required of a magnetic recording tape in addition to excellent electromagnetic characteristics. When magnetic powder is atomized, running durability is liable to deteriorate even if surface smoothness is the same degree. Accordingly, it is effective to add nonmagnetic carbon black fine powder to a magnetic recording layer in order to ensure running stability. However, the addition of an excessive amount of carbon black not only degrades electromagnetic characteristics and impedes higher density recording but causes hindrance of thinning of a magnetic recording layer. At the same time, carbon black fine powder is difficult to disperse, since it has such characteristics that a specific surface area (S
BET
) is great and a wetting property by a solvent is low, so that it is difficult to obtain a magnetic recording medium having a smooth surface.
For example, it is proposed in JP-A-2001-143243 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) to improve running performance by adhering carbon black to magnetic powder. However, the surface roughness of a medium prepared in practice is rough and it is not sufficient as the magnetic recording medium loaded with an MR head. Further, the same patent does not specifically disclose the improvement of electromagnetic characteristics.
Thus, the prior art techniques have not provided yet a magnetic recording medium sufficiently excellent in both electromagnetic characteristics and running durability.
SUMMARY OF THE INVENTION
The objects of the present invention are to solve the problems of the prior art technique and provide a magnetic recording medium for an MR head which is low in noise, high in capacity and has good running durability.
As a result of eager investigation to achieve the above objects of the invention, the present inventors have attained the above objects by a magnetic recording medium comprising a support having provided thereon a magnetic layer mainly comprising ferromagnetic metal powder and a binder, wherein the ferromagnetic metal powder is acicular composite magnetic particle powder having an average long axis length of from 20 to 50 nm, and at least a part of the surface of the particle is adhered with carbon black. Further, it is preferred to provide a substantially nonmagnetic lower layer between the magnetic layer and the support of the magnetic recording medium.
DETAILED DESCRIPTION OF THE INVENTION
The acicular composite magnetic particle powder for use in the magnetic layer of the invention is acicular composite magnetic particle powder having an average long axis length of from 20 to 50 nm, preferably from 20 to 45 nm, and at least a part of the surface of ferromagnetic metal powder (also referred to as core particle powder) is adhered with carbon black.
By using acicular composite magnetic particle powder, the present invention can improve the C/N ratio, weather resistance and running performance of a magnetic recording medium.
The acicular composite magnetic particle powder is obtained by adhering carbon black on a part of the surfaces or entire surfaces of the core particles. Adhesion of carbon black in the present invention may be physical adsorption of carbon black onto the core particle surfaces, or may be bond of carbon black to the core particle surfaces by covalent bond, but it is preferred that the adhesion state is retained in the magnetic layer of a magnetic recording medium in each case. By using the acicular composite magnetic particle powder which is obtained by adhering carbon black to the surfaces of core particles in the magnetic layer, dispersibility is increased, by which a surface property is ensured and a C/N ratio is improved and, at the same time, the friction coefficient of the magnetic surface can be suppressed even under high temperature high humidity conditions, thereby running durability can better.
A method of adhering carbon black to core particle powder is not particularly restricted, but it is preferred that the surfaces of core particles are previously treated before adhering carbon black to the core particle powder. The pretreatment is not particularly restricted and any of well-known methods can be used. As the pretreatment, adhesion of resins, e.g., a silicone resin, an acrylate resin and an epoxy resin, and other compounds to core particle surfaces is exemplified, and the treatment of adhering a silane compound is preferred. The material to be adhered to core particle powder by the pretreatment is called a pretreatment material.
The adhesion amount by these pretreatment materials is preferably from 0.1 to 10 mass % (wight %), and more preferably from 0.3 to 8 mass %, based on the core particle powder. When the adhesion amount is less than 0.1 mass %, it is difficult to sufficiently adhere carbon black, and when it is more than 10 mass %, the effect is saturated and the addition has no means, although carbon black can be adhered sufficiently. The weather resistance of acicular composite magnetic particle powder, i.e., the weather resistance of a magnetic layer, can be improved by adhering a pretreatment material to core particle powder.
A method of pretreatment of core particle powder by using a silicone compound is described below, but this method is also applicable to other pretreatment materials.
The adhesion treatment of core particle powder with a silicone compound may be performed by mechanically mixing and stirring core particle powder and a silicone compound, or

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