Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Including a second component containing structurally defined...
Reexamination Certificate
2001-05-25
2003-09-23
Resan, Stevan A. (Department: 1773)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Including a second component containing structurally defined...
C428S329000, C428S336000, C428S500000, C428S690000, C428S690000, C428S690000, C428S690000
Reexamination Certificate
active
06623849
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a magnetic recording medium having excellent electromagnetic characteristics and running durability, which comprises a support having provided thereon an underlayer (i.e., lower layer) comprising a nonmagnetic powder and a binder and provided on the underlayer, a magnetic layer comprising a ferromagnetic powder and a binder, wherein the binder of the magnetic layer comprises a (meth)acrylate copolymer.
BACKGROUND OF THE INVENTION
As magnetic recording media comprising a support having provided thereon a magnetic layer containing as a binder a (meth)acrylate copolymer, there are known those described below.
In the present specification, the term “(meth)acrylate” means and includes both acrylate and methacrylate, the term “(meth)acrylamide” means and includes both acrylamide and methacrylamide, and the term “(meth) acrylic” means and includes both acrylic and methacrylic.
The magnetic recording medium disclosed in JP-A-4-176016 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) contains as a binder for the magnetic layer a combination of a polyurethane resin and an acrylic resin containing polar groups and having a molecular weight of 1,000 to 10,000. Examples of the polar groups recited therein include hydroxyl groups and sulfonic acid (or sulfonate) groups, and examples of monomers for forming the skeleton of the acrylic resin as recited therein include saturated aliphatic esters of (meth)acrylic acid, ester linkage-dominated oligoester acrylates, urethane acrylates having a polyurethane main chain, and epoxy acrylates having an epoxy resin skeleton. However, no specific description on the skeleton and composition of the resin is made. Although the effects of the molecular weight of the acrylic resin and the species and content of the polar groups upon the dispersibility of the resin have been considered, no account is taken into the influence of the resin skeleton on the dispersibility. Therefore, there is no other choice but to understand that such a resin cannot ensure satisfactory dispersibility of superfine magnetic powder used for latest magnetic recording media.
The magnetic recording medium disclosed in JP-A-6-111279 contains as a binder for its magnetic coating a mixture of a polymethacrylic resin and a polyurethane resin. There is the description that the polymethacrylic resin can be synthesized by copolymerizing a methacrylic acid ester, an unsaturated monomer having a —R—OH group, a monomer having a —(O)SO
3
X group and other copolymerizable unsaturated monomers. As examples of the methacrylic acid ester, esters formed from methacrylic acid and saturated aliphatic alcohol containing 1 to 5 carbon atoms are described. In addition, specific examples of the copolymerizable unsaturated monomers are recited, but methacrylic acid esters are not described therein. Polymethacrylic resins prepared by copolymerization of a methacrylic acid ester formed from methacrylic acid and a saturated aliphatic alcohol containing 1 to 5 carbon atoms, an unsaturated monomer having a —R—OH group, a monomer having a —(O)SO
3
X group and a copolymerizable unsaturated monomer other than methacrylic acid ester are insufficient for dispersibility of superfine magnetic particles used in latest magnetic recording media.
The magnetic recording medium disclosed in JP-A-7-220263 contains as a main binder for its magnetic layer an acrylic copolymer comprising a monomer containing ether bonds, a polar group-containing monomer and an acrylic monomer. In JP-A-7-220263, as examples of the acrylic monomer, various acrylic monomers containing a benzene ring or a nitrogen atom are recited. As to the combined use of such acrylic monomers, only the description that they may be used in combination is made, and there is neither example using them in combination nor description on any positive effect of the combined use thereof. More specifically, an acrylic acid ester is not an essential component in the acrylic copolymers of JP-A-7-220263, and beside, the acrylic copolymers used in the examples do not contain as their constituent monomers any benzene ring- and nitrogen-containing acrylic monomers. As the benzene ring-containing copolymerizable monomer, only styrene is utilized in the acrylic copolymers of the examples. However, effect brought about by styrene is insufficient.
The magnetic recording medium disclosed in JP-A-8-67855 contains as a binder for its magnetic layer a copolymer of an acid group-containing monomer, a styrene monomer and/or a (meth)acrylic acid ester. The (meth)acrylic acid esters exemplified in JP-A-8-67855 include those containing nitrogen atoms, but those containing benzene rings are not included therein. The dispersibility of such a copolymer is still insufficient.
The magnetic recording medium disclosed in JP-A-8-180366 contains a (meth)acrylic copolymer synthesized by copolymerizing a radical-polymerizable monomer having a heterocyclic group containing oxygen and nitrogen atoms in the ring thereof and a radical-polymerizable monomer having a hydroxyl group and a sulfonic acid group. As an example of the (meth)acrylic monomer usable therein, a monomer containing benzene ring and nitrogen atom is also recited. The magnetic powder used in the examples is cobalt-&ggr;-iron oxide. Although the copolymer containing as the essential component, styrene which has a benzene ring is described in JP-A-8-180366, it is insufficient for dispersing fine ferromagnetic metal powder having an average major axis of at most 0.1 &mgr;m or ferromagnetic hexagonal ferrite powder having an average tabular diameter of at most 40 nm used in very thin layer-coated magnetic recording media designed for the latest MR (magneto resistive) heads. As a result, the copolymer described cannot ensure the magnetic recording medium necessary electromagnetic characteristics and surface properties.
Further, the description of an average major axis of magnetic powder cannot be found in any literature except JP-A-8-67855. such being the case, the copolymers hitherto known as binders have insufficient effects on dispersibility of ferromagnetic metal powder having an average major axis of at most 0.1 &mgr;m or a ferromagnetic hexagonal ferrite powder having an average tabular diameter of at most 40 nm which is applicable to very thin layer-coated magnetic recording media designed for the latest MR heads, and so cannot ensure necessary electromagnetic characteristics and surface properties.
SUMMARY OF THE INVENTION
Therefore, an object of the invention is to ensure outstanding electromagnetic characteristics and running durability in a magnetic recording medium comprising a support provided thereon an underlayer containing a non-magnetic powder and a binder and further on the underlayer at least one magnetic layer containing a ferromagnetic powder and a binder.
Another object of the invention is to provide a magnetic recording medium capable of keeping high output and having a flat and smooth surface and a low error rate.
More specifically, the object of the invention is to provide the following magnetic recording media (1) to (4) by forming on a support an underlayer comprising a non-magnetic powder, further forming on the underlayer a magnetic layer having a thickness of 0.01 &mgr;m to 0.5 &mgr;m and comprising a ferromagnetic metal powder having an average major axis of 0.01 &mgr;m to 0.10 &mgr;m and a crystallite size of 80 to 180 Å or a ferromagnetic hexagonal ferrite powder having an average tabular diameter of 5 nm to 40 nm, and using as a binder for the magnetic layer a (meth)acrylate copolymer having hydrophilic polar groups and comprising benzene ring-containing (meth)acrylate units and nitrogen-containing radical polymerizable monomer units, and if necessary, alkyl (meth)acrylate units.
(1) A magnetic recording medium provided with a magnetic layer containing a ferromagnetic powder in a highly dispersed state and having excellent electromagnetic characteristics.
(2) A magnetic recording medium provided with a ma
Hashimoto Hiroshi
Meguro Katsuhiko
Resan Stevan A.
Stroock & Stroock & Lavan LLP
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