Magnetic recording medium

Details Magnetic recording medium Magnetic recording medium

2002-10-08

2004-07-27

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, C428S328000, C428S329000, C428S425800, C428S425900, C428S690000, C428S690000, C428S690000

Reexamination Certificate

active

06767613

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a magnetic recording medium having excellent running durability and electromagnetic characteristics.
BACKGROUND OF THE INVENTION
The magnetic recording media are widely employed in recording tapes, video tapes, floppy disks, and the like. In magnetic recording media, a magnetic layer in which ferromagnetic powder is dispersed in binder is laminated on a nonmagnetic support. The magnetic recording media must have a variety of high-level characteristics, such as electromagnetic characteristics, running durability, and running performance. That is, in audio tapes used to record and reproduce music, a greater ability to reproduce sound sources is required. Further, good electromagnetic characteristics such as the ability to reproduce a source image are required of video tapes. In addition to possessing such good electromagnetic characteristics, magnetic recording media must also have good running durability. The binder also plays an important role in improving durability and electromagnetic characteristics. Conventionally employed vinyl chloride resins, cellulose resins, urethane resins, acrylic resins, and the like have problems in that they yield magnetic layers with poor resistance to abrasion and pollute running system members of the magnetic tape.
Pollution of the magnetic head causes deterioration of electromagnetic characteristics. In particular, in equipment employed in high-density recording, the rotational speed of the magnetic heads is high. In digital videotape recorders, since the rotational speed of the magnetic head reaches 9,600 rpm, which is much higher speed than the 1,800 rpms of home-use analog videotape records and the 5,000 rpms of industrial-use products, the magnetic recording medium slides against the magnetic head at higher speed. Further, compact magnetic heads such as thin-film heads are now being employed, and there is a need for reduction in the pollution of magnetic heads by substances generated by the magnetic recording medium.
One way of achieving improvement with regard to this problem is to employ a hard binder to increase the hardness of the magnetic layer. For example, Japanese Unexamined Patent Publication (KOKAI) Heisei No. 11-96539 describes a magnetic recording medium in which polyurethane resin containing a diol component in the form of a dimer diol is employed as resin. Further, Japanese Unexamined Patent Publication (KOKAI) No. 2001-134921 employs a binder in the form of polyurethane resin using a polyol with a weight average molecular weight (Mw) of 400 to 5,000 having a prescribed polar group as a long-chain polyol component and a diol compound having both a ring structure and a long alkyl chain. When a polyurethane resin with a high glass transition temperature (Tg) is employed as binder, the coating strength increases, particularly at elevated temperature, affording good running durability. However, the Tg is increased by increasing ring-structured and urethane binding components. When ring-structured and urethane binding components are increased, there are problems in that solvent solubility decreases, precluding adequate dispersion and reducing coating smoothness.
Accordingly, it is an object of the present invention is to provide a magnetic recording medium having excellent running durability, a high degree of coating smoothness, and excellent electromagnetic characteristics.
SUMMARY OF THE INVENTION
The present inventors conducted extensive research into achieving the above-stated object. As a result, they discovered that incorporating a binder component in the form of polyurethane resin, the glass transition temperature of which ranged from 100 to 200° C., achieved good coating strength, and that specifying that the number of minute protrusions present on the surface of the magnetic layer permitted the achievement of a magnetic recording medium with both good running durability and good magnetic characteristics; the present invention was devised on that basis.
That is, the object of the present invention is achieved by:
(1) A magnetic recording medium comprising a lower layer comprising a magnetic powder or nonmagnetic powder and a binder and one or more upper magnetic layers comprising a ferromagnetic powder and a binder in this order on a nonmagnetic support, wherein
at least said lower layer comprises a binder comprising polyurethane resin having a glass transition temperature ranging from 100 to 200° C., and
a magnetic layer positioned as the topmost layer among said upper magnetic layers comprises 5 to 1,000 surface protrusions of 10 to 20 nm in height per 100 &mgr;m
2
on the surface thereof, or
(2) A magnetic recording medium comprising a lower layer comprising a magnetic powder or nonmagnetic powder and a binder and one or more upper magnetic layers comprising a ferromagnetic powder and a binder in this order on a nonmagnetic support, wherein
at least a magnetic layer positioned as the topmost layer among said upper magnetic layers comprises a binder comprising polyurethane resin having a glass transition temperature ranging from 100 to 200° C., and
said magnetic layer positioned as the topmost layer among the upper magnetic layers comprises 5 to 1,000 surface protrusions of 10 to 20 nm in height per 100 &mgr;m
2
on the surface thereof.
Hereinafter, the magnetic recording medium described in (1) above will be referred to as the “first mode,” and the magnetic recording medium described in (2) above as the “second mode.”
The magnetic recording medium of the present invention is described in greater detail below.
The magnetic recording medium of the first mode of the present invention has a multilayered structure in which multilayered magnetic layers or nonmagnetic layer and a magnetic layer in this order are provided on a nonmagnetic support, and a binder comprising polyurethane with a glass transition temperature of 100 to 200° C. is employed in at least the lower layer (magnetic layer or nonmagnetic layer). The use of a binder comprising polyurethane with a high glass transition temperature in the present invention in at least the lower layer essentially causes the polyurethane-comprising binder of the present invention to also be incorporated into the upper magnetic layer by migration and the like during simultaneous multilayer-coating, inhibiting plastic flowing of the magnetic layer due to frictional heat generated by sliding of the recording and reproduction head against the magnetic layer surface, imparting high coating strength, and achieving good running durability. The effect is particularly marked when the upper magnetic layer is a thin layer.
The magnetic recording medium of the second mode of the present invention has a multilayered structure in which multilayered magnetic layers or nonmagnetic layer and a magnetic layer in this order are provided on a nonmagnetic support, and a binder comprising polyurethane with a glass transition temperature of 100 to 200° C. is employed in at least the magnetic layer positioned as the topmost layer. This polyurethane is also sometimes incorporated into the upper magnetic layer in other than the topmost layer, but is essentially not incorporated into the lower layer (magnetic layer or nonmagnetic layer). Incorporating a binder comprising polyurethane having a high glass transition temperature of the present invention into the magnetic layer positioned as the topmost layer imparts a high modulus of elasticity at high temperature to the magnetic layer, inhibits plastic flowing of the magnetic layer due to frictional heat generated by sliding of the recording and reproduction head against the magnetic layer surface, imparts high coating strength, and achieves good running durability. In particular, since the mechanical strength of the magnetic layer decreases when the magnetic layer positioned as the topmost layer is made thin, the effect of the polyurethane of the present invention is marked when the topmost layer magnetic layer is thin.
Electromagnetic characteristics are known to decrease when minute protrusions of 1

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