Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Including a second component containing structurally defined...
Reexamination Certificate
2000-04-24
2002-11-05
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...
C428S328000, C428S330000, C428S690000, C428S900000
Reexamination Certificate
active
06475613
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a magnetic recording medium having a back-coat layer and, particularly, to a magnetic recording medium which is reduced in the occurrence of a dropout and has excellent durability.
2. Disclosure of the Related Art
In many magnetic recording media having a magnetic layer on one surface of a non-magnetic support, a back-coat layer is formed on the surface opposite to the magnetic layer of the support for the purpose of improving running stability and preventing the electrification of the magnetic layer.
As shown in Japanese Laid-open Patent Application Nos. 3-141021 (1991) and 9-115134 (1997) , it is known that as carbon black, one having a large particle size and one having a small particle size are used in combination with calcium carbonate for a back-coat layer. It is insisted that such a structure resulted in the production of a magnetic recording medium which has low friction, is reduced in the falling of carbon black from the back-coat layer and has high reliability.
However, if carbon black having an average particle size of as large as 230 to 300 nm is used for the back-coat layer as described in Japanese Laid-open Patent Application No. 9-115134, there is a tendency that a dropout occurs and the error rate is adversely affected due to damages and concaves of a magnetic surface which are caused by the transfer of the carbon black to the magnetic surface since carbon black having large particle size has poor dispersibility. Poor dispersibility of carbon black also causes carbon black having large particle size to be fallen away from the back-coat layer with ease and hence there is a tendency that the durable friction characteristics is deteriorated and damages to the magnetic layer and adhesion of the carbon black to a head are caused.
In Japanese Laid-open Patent Application No. 3-141021, it is disclosed concretely in the examples that carbon black having an average particle size of as small as 27 nm, carbon black having an average particle size of as large as 72 nm and calcium carbonate with a Mohs hardness of 3 to 4 are used for the back-coat layer.
However, when, as the carbon black having small particle size, one having a particle size exceeding 25 nm is used, the resulting recording medium increases in dropout after it is stored in a high temperature and high humidity condition under such a severe evaluation criterion as 2 &mgr;sec-12 dB as shown in the examples of the present specification.
In the above JP-A-3-141021, also the proportion of carbon black having large particle size is as large as 17 to 40% by weight based on the total amount of the aforementioned two types of carbon black and inorganic powder. This increases the irregularities of the surface of the back-coat and deteriorates, a dropout and the durable friction characteristics. A dropout increases particularly after storage in a high temperature and high humidity condition.
Moreover, in the above JP-A-3-141021, CaCO
3
and BaSO
4
having Mohs hardness of 3 to 6 are described as an inorganic powder. However, the particle size of the inorganic powder is nowhere described and the use of these compounds often increases a dropout and deteriorates the durable friction characteristics.
SUMMARY OF THE INVENTION
In view of the above situation, it is an object of the present invention to solve the above problem and to provide a magnetic recording medium which is reduced in the occurrence of a dropout and has excellent durability.
The present inventors have made earnest studies and, as a result, found that the above problem is solved by using three types of non-magnetic powder each having a specific particle size as essential components for a back-coat layer to attain a magnetic recording medium according to the present invention.
Namely, the present invention relates to a magnetic recording medium comprising a magnetic layer on one surface of a non-magnetic support and a back-coat layer containing mainly a non-magnetic powder and a binder on the other surface, wherein the back-coat layer includes (a) a carbon black having an average particle size of 15 to 25 nm, (b) a carbon black having an average particle size of 60 to 80 nm and (c) CaCO
3
and/or BaSO
4
having an average particle size of 30 to 75 nm as the non-magnetic powder.
In the present invention, it is preferable that said carbon black (a) having an average particle size of 15 to 25 nm has an oil absorption ranging from 50 to 110 cc/100 g and a specific surface area (BET value) ranging from 100 to 300 m
2
/g measured by the BET method.
In the present invention, it is preferable that said carbon black (b) having an average particle size of 60 to 80 nm has an oil absorption ranging from 20 to 80 cc/100 g and a specific surface area (BET value) ranging from 10 to 50 m
2
/g measured by the BET method.
In the present invention, the back-coat layer preferably includes said carbon black (a), said carbon black (b) and said CaCO
3
and/or BaSO
4
(c) in a ratio of (a)/(b)/(c)=60-80/3-15/15-30 by weight.
According to the present invention, a magnetic recording medium which is reduced in the occurrence of a dropout and has excellent durability is provided. The present invention can be used in various applications like magnetic tapes such as video tapes and audio tapes and magnetic disks.
DETAILED DESCRIPTION OF THE INVENTION
The magnetic recording medium of the present invention is provided with a magnetic layer on one surface of a non-magnetic support and a back-coat layer containing mainly of a non-magnetic powder and a binder on the other surface. In the magnetic recording medium of the present invention, a lubricant coating layer or various coating layers protecting the magnetic layer may be formed as required on the magnetic layer. Also, on the surface of the non-magnetic support, on which the magnetic layer is to be formed, an undercoat layer (adhesion facilitating layer) can be formed to improve the adhesion between the coating film and the non-magnetic support. Further, when a magnetic layer having a thickness of 0.5 &mgr;m or lower is formed, there are many cases where absolute film strength and the quantity of a lubricant are unsatisfied. Therefore, a non-magnetic under-layer having a thickness of 0.5 &mgr;m or more and preferably 0.8 to 1.5 &mgr;m may be formed between the magnetic layer and the non-magnetic support.
In the present invention, at least two kinds of carbon black which differ in particle size are used as the essential components of the back-coat layer.
The first carbon black (a) is used to form a basic surface of the back-coat layer and is a primary pigment component for forming the back-coat layer. The average particle size of the first carbon black (a) is 15 to 25 nm, preferably 15 to 20 nm and more preferably 15 nm or larger and less than 20 nm. When the average particle size of the carbon black (a) is less than the lower limit of the above range, the dispersibility is impaired and hence the irregularities of the surface of the back-coat later increase, resulting in a dropout increase after storage in a high temperature and high humidity condition. On the other hand, when the average particle size of the carbon black (a) exceeds the upper limit of the above range, the increase in irregularities of the surface of the back-coat layer is caused by the deterioration of the dispersibility and the influence of the average particle size, resulting in, particularly increased dropouts after storage in a high temperature and high humidity condition. Also, since the irregularities of the layer surface are increased, the effect of the second carbon black (b) on the friction is lost.
Preferably the aforementioned carbon black (a) has an oil absorption ranging from 50 to 110 cc/100 g and a BET value ranging from 100 to 300 m
2
/g. When the oil absorption and/or BET value of the carbon black (a) exceed each upper limit of the above ranges, the dispersibility decreases and as a consequence, it is feared that the layer surface becomes rough. Also, powder fal
Ide Tsutomu
Saitoh Akira
Rader & Fishman & Grauer, PLLC
Resan Stevan A.
TDK Corporation
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