Stock material or miscellaneous articles – Structurally defined web or sheet – Including components having same physical characteristic in...
Utility Patent
1999-03-25
2001-01-02
Resan, Stevan A. (Department: 1773)
Stock material or miscellaneous articles
Structurally defined web or sheet
Including components having same physical characteristic in...
C428S425900, C428S522000, C428S690000, C428S690000, C428S690000, C428S900000
Utility Patent
active
06168856
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a magnetic recording medium, and particularly to a magnetic recording medium improved in electromagnetic characteristics and durability.
BACKGROUND OF THE INVENTION
In recent years, for avoiding deterioration of signals caused by repeated copies, digitization of magnetic recording media has proceeded. Further, the recording density thereof has also been desired to be increased, because of an increase in the amount of recording data.
As magnetic powders, therefore, finer magnetic metal powders or hexagonal tabular barium ferrite having high saturation magnetic flux density have been used.
On the other hand, for increasing the recording density, it is necessary to consider the thickness loss and the self-demagnetization loss of the media, and from such a viewpoint, it has been desired to make magnetic layers thinner.
However, when the magnetic layers are made thinner, the surface properties of supports are reflected in surfaces of the magnetic layers to deteriorate the electromagnetic characteristics in some cases. It has therefore been proposed that non-magnetic layers, for example, using thermosetting resins as undercoats, are formed on the surfaces of the support, and the magnetic layers are formed thereon. However, such non-magnetic layers have the problem of insufficient durability.
Further, when the non-magnetic layers and the magnetic layers are coated, the non-magnetic layers are once formed by coating and drying, and then, the magnetic layers are formed. Accordingly, depending on the resins used, the problem arises that the surface properties of the magnetic layers deteriorates.
From such circumstances, JP-A-63-191315 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) and JP-A-63-191318 disclose a method of coating a non-magnetic lower layer in which, for example, a non-magnetic powder is dispersed in a plastic resin binder, and applying coating solutions for the lower layer and the magnetic layer in multiple layers in a wet state, thereby improving the electromagnetic characteristics, the durability and the head wear resistance.
Further, for improving the surface properties, in JP-A-6-259751, granular &agr;-iron oxide/carbon black are used as inorganic pigments to form a non-magnetic lower layer, and coating is conducted by the above-mentioned production method. The use of granular &agr;-iron oxide provides the effect of improving the surface properties compared with needle-like &agr;-iron oxide and materials such as titanium oxide even if they are granular.
On the other hand, with respect to binders used for preparing paints for non-magnetic layers, when a-iron oxide/carbon black pigments are used, JP-A-6-259751 discloses examples in which sulfonic acid group-containing resins are used, JP-A-8-17035 discloses examples in which vinyl chloride resins having amine polar groups alone and urethane resins having sulfonic acid polar groups are used, and JP-A-9-265623 discloses examples of urethane resins having metal salts of sulfamic acid as polar groups, in their examples and claims.
However, in magnetic recording media such as video tapes and floppy disks of the digital recording system, it is increasingly required that the thickness of magnetic layers is made thinner, resulting from an increase in recording density. Accordingly, sufficient dispersion cannot be obtained by the use of only the pigments and binders disclosed in JP-A-6-259751, JP-A-8-17035 and JP-A-9-265623, resulting in insufficient surface properties.
For example, it has hitherto been known that binders having sulfonic acid polar groups well dispersing a-iron oxide and aluminum oxide are poor in dispersibility of carbon black, whereas binders having amine polar groups well dispersing carbon black are poor in dispersibility of iron oxide. Even if vinyl chloride resins having amine polar groups alone and urethane resins having sulfonic acid polar groups are used as described in JP-A-8-17035, mutual defects appear to results in failure to obtain a satisfactory dispersed state. Then, for obtaining desired surface properties, a further improvement in a dispersion level has been required.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a magnetic recording medium excellent in surface properties, good in electromagnetic characteristics and excellent in durability by improving the dispersibility of all pigments different in properties from one another such as &agr;-iron oxide, carbon black and aluminum oxide.
As a result of intensive investigation for attaining the above-mentioned object, the present inventors have discovered that a vinyl chloride resin having an amine polar group and a carboxylic acid polar group in the same molecule, and a polyurethane resin having phosphobetaine and/or phosphamine as a polar group can improve the dispersibility of all of &agr;-iron oxide, aluminum oxide and carbon black contained therein, and that the combination of these two resins can further provide a magnetic recording medium having smooth surface properties, excellent durability and electromagnetic characteristics which have hitherto been very difficult to be achieved, thus completing the present invention.
That is to say, the present invention provides:
(1) A magnetic recording medium comprising a non-magnetic support, a non-magnetic layer formed on the support and a magnetic layer formed thereon, in which said non-magnetic layer contains, as binders, a vinyl chloride resin having carboxylic acid and amine polar groups, and a polyurethane resin having phosphobetaine and/or phosphamine as a polar group;
(2) The magnetic recording medium described in the above (1), in which the non-magnetic layer contains granular &agr;-Fe
2
O
3
and carbon black having a BET of 60 m
2
/g to 250 m
2
/g and a DBP oil absorption of 40 ml/100 g to 150 ml/100 g as pigments;
(3) The magnetic recording medium described in the above (1), in which the non-magnetic layer contains granular &agr;-Fe
2
O
3
, &agr;-Al
2
O
3
and carbon black having a BET of 60 m
2
/g to 250 m
2
/g and a DBP oil absorption of 40 ml/100 g to 150 ml/100 g as pigments;
(4) The magnetic recording medium described in the above (1), in which a magnetic powder contained in the magnetic layer is a magnetic metal powder or a hexagonal ferromagnetic powder;
(5) The magnetic recording medium described in the above (1), in which the average thickness of the magnetic layer is 1.0 &mgr;m or less, and the average thickness of the non-magnetic layer is 1 &mgr;m to 3 &mgr;m;
(6) The magnetic recording medium described in the above (1) or (5), in which the non-magnetic layer is formed, and the magnetic layer is formed thereon in multiple layers simultaneously or successively while the non-magnetic layer is still in a wet state;
(7) The magnetic recording medium described in the above (1), in which the amount of the carboxylic acid polar group contained in the vinyl chloride resin is 0.1% to 2.0% by weight;
(8) The magnetic recording medium described in the above (1), in which the amount of the amine polar group contained in the vinyl chloride resin is 50 ppm or more;
(9) The magnetic recording medium described in the above (1), in which the average polymerization degree ranges from 100 to 700;
(10) The magnetic recording medium described in the above (1), in which the number average molecular weight of the polyurethane resin ranges from 5,000 to 200,000;
(11) The magnetic recording medium described in the above (1), in which the weight ratio of the vinyl chloride resin to the polyurethane resin is from 9/1 to 5/5;
(12) The magnetic recording medium described in the above (2) or (3), in which the particle size of granular &agr;-Fe
2
O
3
is 100 nm or less;
(13) The magnetic recording medium described in the above (2) or (3), in which the amount of carbon black added is from 20 parts to 80 parts by weight based on 100 parts by weight of granular &agr;-Fe
2
O
3
; and
(14) The magnetic recording medium described in the above (3), in which the amount of &ag
Miyamori Shigeyo
Nagashima Naoyuki
Sasaki Hideki
Uchida Masatoshi
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Resan Stevan A.
TDK Corporation
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