Magnetic recording medium

Details Magnetic recording medium Magnetic recording medium

2002-10-30

2004-08-10

Resan, Stevan A. (Department: 1773)

Stock material or miscellaneous articles

Structurally defined web or sheet

Continuous and nonuniform or irregular surface on layer or...

C428S216000, C428S336000, C428S690000, C428S690000, C428S690000

Reexamination Certificate

active

06773789

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a magnetic recording medium comprising at least a magnetic layer provided on a support which exhibits excellent electromagnetic characteristics and durability.
BACKGROUND OF THE INVENTION
As tape-like magnetic recording media (i.e., magnetic recording particulate media) for audio, video and computer and disc-like magnetic recording media such as floppy disc there are-used magnetic recording media comprising a magnetic layer having a ferromagnetic powder such as &ggr;-iron oxide, Co-containing iron oxide, chromium oxide and ferromagnetic metal powder dispersed in a binder. As the support to be used in these magnetic recording media there is normally used a polyethylene terephthalate, polyethylene naphthalate or the like. These support materials are stretched to have a high crystallinity that enhances the mechanical strength and an excellent solvent resistance thereof.
A magnetic layer obtained by applying a coating solution having a ferromagnetic powder dispersed in a binder to a support has a high packing degree of ferromagnetic powder and hence a small elongation at break and thus is brittle. Therefore, such a magnetic layer formed without interposition of undercoat layer can easily break and be peeled off from the support when given a mechanical force. In order to eliminate these troubles, it has been practiced to provide an undercoat layer on the support, allowing the magnetic layer to be adhered to the support firmly.
For example, it is known that a compound having a functional group which cures when irradiated with electron ray, i.e., radiation-curing compound is used to form an undercoat layer.
For example, Japanese Patent (Application) Laid-Open No. 133529/1985, Japanese Patent Laid-Open No. 133530/1985, Japanese Patent Laid-Open No. 150227/1985 and Japanese Patent Publication No. 57647/1993 disclose a magnetic recording medium comprising an undercoat layer formed by a bifunctional aliphatic compound as a radiation-curing compound. The hardened layer of such an aliphatic compound exhibits a glass transition temperature of about 40° C. at highest and thus is disadvantageous in that adhesion can occur at the coating step after the coating of the undercoat layer.
On the other hand, such an aliphatic radiation-curing compound can be provided with an increased number of (meth)acryloyl functional groups to prevent adhesion. However, the aliphatic radiation-curing compound having a polyfunctionality undergoes more hardening shrinkage, making it impossible to obtain a smooth coat layer. Such an aliphatic radiation-curing compound exhibits a deteriorated adhesion to a support material such as polyethylene terephthalate (PET) and polyethylene naphthalate (PEN), making it impossible to obtain sufficient running durability or electromagnetic characteristics.
Further, Japanese Patent (Application) Laid-Open No. 13430/1986 and Japanese Patent Laid-Open No. 146023/1983 disclose an examples of electron radiation-curing compounds having a cyclic structure. Japanese Patent Laid-Open No. 13430/1986 uses an electron radiation-curing compound made of phthalic acid and polyether polyol. Japanese Patent Laid-Open No. 146023/1983 uses a product of reaction of a diisocyanate compound with a compound having an electron radiation-curing functional group and a group reactive with isocyanate. A diisocyanate compound has an aromatic ring such as tolylene diisocyanate. Such a diisocyanate compound having an aromatic ring forms a hardened coat layer which can easily be brittle and thus exhibits an insufficient adhesion to the support, making it easy for the magnetic coat layer to fall off to disadvantage.
In recent years, a reproduction head employing MR (magneto resistivity) as an operational principle has been proposed. Such a reproduction head has been used in hard disc, etc. Japanese Patent Laid-Open No. 227517/1996 proposes the application of such a reproduction head to magnetic tape. Since such an MR head can give a reproduced output of several times higher than induction type magnetic head and requires no induction coil, device noises such as impedance noise can be drastically lowered, making it possible to lower the generation of noises in the magnetic recording medium and obtain a great SN ratio. In other words, when the noise of magnetic recording medium which has heretofore been concealed in device noise can be lowered, recording/reproduction can be fairly conducted, making it possible to drastically improve the high density recording characteristics of the magnetic recording medium.
However, the MR head generates noises (thermal noises) when affected by minute heat. In particular, when the MR head comes in contact with protrusions on the surface of the magnetic layer, the resulting effect causes sudden and continuous rise of noise. In the case of digital recording, this trouble is so drastic that error correction is made impossible. This problem of thermal noise is remarkable particularly with a magnetic recording medium for a system for reproducing a recorded signal having a recording density of not smaller than 0.5 Gbit/inch
2
.
In order to lessen such a thermal noise, it is important to control the surface conditions of the magnetic layer. Thus, a desirable means for this purpose has been desired.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a magnetic recording medium excellent in running durability as well as in coating layer smoothness and electromagnetic characteristics.
The present invention relates to a magnetic recording medium comprising an undercoat layer formed by a radiation-curing resin provided on a support and at least magnetic layer containing a ferromagnetic powder and a binder provided thereon, wherein the radiation-curing resin is a compound having an alicyclic structure and two or more radiation-curing functional groups per molecule, the undercoat layer is a radiation-cured layer, the magnetic layer has a thickness of from 0.05 &mgr;m to 2.0 &mgr;m and the number of protrusions having a height of from 10 nm to 20 nm as measured by an atomic force microscope (AFM) is from 5 to 1,000 per 100 &mgr;m
2
of the surface of the magnetic layer.
A preferred embodiment of the present invention is as follows.
1) The magnetic recording medium as defined above, wherein the alicyclic structure is one formed by a plurality of rings having atoms in common.
DETAILED DESCRIPTION OF THE INVENTION
The radiation-curing resin to be used in the present invention is a compound having an alicyclic structure and two or more radiation-curing functional groups per molecule and is incorporated in at least the undercoating solution for forming the undercoat layer.
When given a radiation energy such as electron ray and ultraviolet ray, the radiation-curing resin to be used herein undergoes polymerization or crosslinking to polymer that causes hardening thereof. The radiation-curing resin doesn't undergo reaction unless given such an energy. Therefore, the coating solution containing such a radiation-curing resin exhibits a stabilized viscosity so far as it is not irradiated with radiation and thus can provide a coating layer having a high smoothness. Further, since the coating solution containing such a radiation-curing resin undergoes is a moment reaction when given a high radiation energy, the resulting coating layer can be provided with a high strength.
This is because the radiation-curing resin has a viscosity as relatively low as few mPa·s to 200 mPa·s and thus can provide an undercoat layer that exerts a leveling effect of shielding protrusions on the support to form a smooth support. It can be thought that the application of a magnetic solution to the undercoat layer makes it possible to provide a magnetic layer having an excellent surface smoothness and hence a magnetic recording medium having excellent electromagnetic characteristics. This effect can be remarkably exerted particularly on a magnetic layer having a thickness as relatively small as from 0.05 &mgr;m to 2.0 &mgr;m, making it possible to i

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