Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
2001-11-16
2004-02-03
Le, H. Thi (Department: 1773)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S690000, C428S690000, C428S690000, C428S690000, C428S690000, C428S690000
Reexamination Certificate
active
06686073
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a magnetic recording medium having very excellent electromagnetic conversion characteristics (i.e., electromagnetic characteristics) and running durability.
BACKGROUND OF THE INVENTION
The magnetic recording medium commonly used is a recording medium obtained by providing on a nonmagnetic support a magnetic layer comprising a binder having dispersed therein ferromagnetic powder particles. Recently, in the field of magnetic recording, digital recording is practically taking the place of conventional analog recording because of its less deterioration of recording. In the digital recording, a large number of signals must be generally recorded and moreover, the recording/reproducing apparatus and the recording medium used therefor are demanded to ensure high image quality and high sound quality and at the same time, realize downsizing and space saving. To cope with these requirements, still higher density recording is being demanded.
For achieving higher density recording, recording signal with a short wavelength and recording locus in a narrow track are employed. As for the recording medium, more improvement in the electromagnetic conversion characteristics is demanded and for this purpose, various proposals have been proposed, such as improvement in the magnetic characteristics of ferromagnetic powder, formation of fine particles, high filling, or ultra-smoothing of the medium surface. These are, however, not satisfactory for the higher density recording demanded in recent years.
For reducing noises, the matter of importance is to reduce the size of magnetic powder and the magnetic powder recently used is a ferromagnetic powder of 0.1 &mgr;m or less or a ferromagnetic hexagonal ferrite fine powder having a tabular diameter of 40 nm or less. The dispersion of such ferromagnetic fine powder is difficult as compared with the dispersion of conventional ferromagnetic fine powder.
When the recording wavelength is shortened, if the magnetic layer thickness is large, the self demagnetization loss at the time of recording and the thickness loss at the time of reproduction come out to serious problems. In order to avoid the saturation of reproducing head, the magnetic layer is rendered thinner, however, if the thickness of the magnetic layer is reduced to about 0.5 &mgr;n or less, the effect of support is liable to appear on the magnetic layer surface and the electromagnetic characteristics and error rate are liable to increase. To overcome this problem, if a nonmagnetic lower layer is provided on the support surface and the magnetic layer is provided as the upper layer, the effect by the surface roughness of the support may be eliminated, however, since the thickness of the magnetic layer is very small of about 0.5 &mgr;m or less as compared with about 2 &mgr;m of the nonmagnetic lower layer, the surface property of the magnetic layer which comes into direct contact with the recording/reproducing head is greatly affected by the surface property of the nonmagnetic lower layer and the surface roughness of the magnetic upper layer cannot be get rid of the effect from the surface roughness of the nonmagnetic lower layer.
Use of an acrylic resin having a hydrophilic polar group as a binder in the magnetic layer has been proposed with an attempt to improve the dispersibility and this is certainly effective in the improvement of smoothness.
As for the magnetic recording medium comprising a support having provided thereon a magnetic layer and containing a (meth)acrylate-based copolymer as a binder of the magnetic layer, for example, the following techniques are known.
JP-A-4-176016 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) discloses a magnetic recording medium containing an acrylic acid resin having a polar group and a molecular weight of 1,000 to 10,000 and a polyurethane resin as resin binders for the magnetic layer, where a hydroxyl group and a sulfonic acid (base) are described as the polar group and an oligoester mainly comprising a saturated aliphatic ester or ether bond of (meth)acrylic acid and an ester having a polyurethane or epoxy resin skeleton are described as the monomer for forming the skeleton of acrylic acid resin. In JP-A-4-176016, examples of the skeleton-forming monomer are set forth but the specific skeleton/composition is not described. The dispersibility is examined there only by taking notice of the molecular weight and the kind and amount of the polar group but the effect of the skeleton is not taken account of. The attained dispersibility is insufficient for the ultrafine magnetic powder used in recent magnetic recording mediums.
JP-A-6-111279 discloses a magnetic recording medium containing a mixture of polymethacrylic acid resin with polyurethane resin as a binder for the magnetic coating film and states that the polymethacrylic acid resin can be synthesized by the copolymerization of a methacrylic acid ester, an unsaturated monomer having —R—OH group, a monomer having —(O)SO
3
X group and a copolymerizable unsaturated monomer, where the nitrogen-containing radical polymerizable unit described is only N-methylolacrylamide as an example of the unsaturated monomer having —R—OH group. As for the methacrylic acid ester, esters comprising a methacrylic acid and a saturated aliphatic alcohol having from 1 to 5 carbon atoms are described. Furthermore, specific examples of the copolymerizable unsaturated monomer are described but a methacrylic acid ester is not included therein. The polymethacrylic acid resin obtained by the copolymerization of a methacrylic acid ester comprising a methacrylic acid and a saturated aliphatic alcohol having from 1 to 5 carbon atoms with a monomer having —R—OH group or —(O)SO
3
X group is insufficient in the dispersibility for the lower layer powder recently used in magnetic recording mediums.
Polyester polyurethane resin and polycarbonate polyurethane each having a urethane group concentration of 2.5 mmol/g or more are described as preferred examples of the polyurethane resin and with respect to the chain extending agent, those having a branched chain of 2 or more carbon atoms are not described.
In JP-A-6-111279, a polymethacrylic acid resin obtained by the copolymerization of a methacrylic acid ester comprising a methacrylic acid and a saturated aliphatic alcohol having from 1 to 5 carbon atoms, an unsaturated monomer having —R—OH group, a monomer having —(O)SO
3
X group and a copolymerizable unsaturated monomer other than the methacrylic acid ester is used in combination with polyester polyurethane or polycarbonate polyurethane as the chain extending agent, not having a branched chain of 2 or more carbon atoms and having a urethane group concentration of 2.5 mmol/g, however, this technique cannot ensure the electromagnetic conversion characteristics necessary for the dispersibility of fine ferromagnetic metal powder having a long axis length of 0.1 &mgr;m or less or ferromagnetic hexagonal ferrite powder having a particle size of 40 nm or less which are recently used in ultra-thin layer magnetic recording particulate mediums capable of coping with MR head.
JP-A-7-220263 discloses a magnetic recording medium mainly using, as the binder for the magnetic layer, an acrylic copolymer comprising an ether compound-type monomer, a polar group-containing monomer and an acrylic monomer. An acrylic acid ester is not an essential component of the acrylic copolymer and moreover, an acrylic monomer containing an aromatic ring and nitrogen is not used in Examples. Only styrene is described as the aromatic ring-containing copolymerizable monomer but with styrene, a sufficiently high effect cannot be obtained.
JP-A-8-67855 discloses a magnetic recording medium containing, as the binder for the magnetic layer, a copolymer comprising a vinyl monomer having an acid group, a styrene-based monomer and/or a (meth)acrylic acid ester. In JP-A-8-67855, as examples of (meth)acrylic acid esters, those containing nitrogen are described but those containing an aroma
Hashimoto Hiroshi
Meguro Katsuhiko
Murayama Yuichiro
Fuji Photo Film Co. , Ltd.
Le H. Thi
Stroock & Stroock & Lavan LLP
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