Magnetic recording medium

Details Magnetic recording medium Magnetic recording medium

2002-03-29

2004-04-27

Resan, Stevan A. (Department: 1773)

Stock material or miscellaneous articles

Web or sheet containing structurally defined element or...

Physical dimension specified

C428S458000, C428S474400, C428S690000, C428S690000

Reexamination Certificate

active

06726988

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a magnetic recording medium including a ferromagnetic metal thin film. More particularly, the present invention relates to a vapor-deposition-type magnetic recording medium having excellent eletromagnetic conversion properties.
2. Description of the Related Art
Generally, vapor-deposition-type magnetic recording media require no binding agent to bind magnetic materials together, unlike coating-type magnetic recording media. Such a vapor-deposition-type magnetic recording medium has high bulk density, and is suitable for high density recording. Accordingly, the vapor-deposition-type magnetic recording medium has excellent electromagnetic conversion properties, and has been widely utilized. The vapor-deposition-type magnetic recording medium is produced by vacuum vapor deposition of a ferromagnetic metal material, such as Co or Co—Ni, on a non-magnetic substrate such as a biaxially oriented polyethylene terephthalate film, a biaxially oriented polyethylene naphthalate film, a polyimide film, and a polyamide film.
The magnetic materials for the metal thin film magnetic layer include the aforementioned metals or alloys thereof. In particular, the magnetic material including Co has good electromagnetic conversion properties, and is suitable for the high density recording medium. In recent years, there has been a demand for recording with much higher density, and recording media with thinner films have been studied. So, thinner non-magnetic substrate has been studied.
However, the smaller the film thickness of the non-magnetic substrate, the less the tape-shaped magnetic recording medium contacts with a magnetic head (hereinafter referred to as “head touch”). As a result, the output undesirably decreases, especially in a short wavelength area. Furthermore, the magnetic recording medium may have poor traveling durability.
To overcome such problems, an aromatic polyamide film has been used to improve head touch, without decreasing the output, and to improve the operating durability, even if the non-magnetic substrate has a smaller film thickness.
When the aromatic polyamide film is used as the non-magnetic substrate, a decrease in the output and the operating durability, which are caused by insufficient strength of the non-magnetic substrate, are improved. However, the output is still decreased by tape deformation, such as cupping, after the final product is provided.
SUMMARY OF THE INVENTION
The present invention is made to solve the above-mentioned problems. According to the present invention, it has been found that the above-mentioned problems can be solved by setting the Young's modulus in the transverse direction (TD) and the thermal contraction coefficient in the transverse direction within the predetermined ranges.
A magnetic recording medium comprises a non-magnetic substrate, and a metal thin film magnetic layer formed on the substrate by an oblique evaporating method, the non-magnetic substrate being an aromatic polyamide film. The film has a Young's modulus in the transverse direction of 16000 MPa to 18000 MPa, and has a thermal contraction coefficient (which means the contraction coefficient determined by the ratio of the length after heating at 180° C. for 30 minutes and the length before heating; and hereinafter referred to as “thermal contraction coefficient (180° C. for 30 minutes)” or simply “thermal contraction coefficient”) of 0.7% to 1.3%. Thermal deformation (cupping) after the thin film is formed can be prevented to improve the head touch and the electromagnetic conversion properties.
If the thermal contraction coefficient in the transverse direction exceeds 1.3%, cupping after thin film formation becomes excessively convex on the vapor deposition side. If the thermal contraction coefficient in the transverse direction is less than 0.7%, the cupping after thin film formation becomes convex on the back coat side. In either case, the head touch becomes worse, and the electromagnetic conversion properties deteriorate. Also, the cupping deteriorates the operating properties, resulting in the decreased durability. From the viewpoint of head touch, it is preferable that the cupping be appropriately convex (about −0.2 mm) on the vapor deposition side. However, if the cupping is excessively convex, it may adversely affect the properties of the magnetic recording medium.
The larger the Young's modulus in the transverse direction, the better the head touch and the electromagnetic conversion property. If the Young's modulus exceeds 18000 MPa, it becomes difficult to control the thermal contraction coefficient in the transverse direction to 1.3% or less. If the Young's modulus is less than 16000 MPa, the thermal contraction coefficient can be easily controlled, but absolute strength lacking, the head touch becomes worse, and the output is decreased.
Conventionally, the recording medium had a smaller thickness for high density recording. In this invention the non-magnetic substrate is also thin and has a thickness of 3.0 &mgr;m to 6.0 &mgr;m. A back coat layer is formed on the substrate at a side opposite to the magnetic layer (evaporated layer), and has a thickness of 0.3 &mgr;m to 0.7 &mgr;m. If the back coat layer has a thickness of less than 0.3 &mgr;m, the cupping is increased on the evaporated side (becomes convex on the back coat side), which leads to deterioration of the head touch. If the back coat layer has a thickness of more than 0.7 &mgr;m, the cupping is increased on the back coat side (becomes convex on the evaporated side), which also leads to deterioration of the head touch. It is therefore preferable that the thickness of the back coat layer be 0.3 &mgr;m to 0.7 &mgr;m, and more preferably 0.4 &mgr;m to 0.6 &mgr;m.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the present invention, the aromatic polyamide film is a film comprising an aromatic polyamide resin which is represented by formula (1) or (2) below, or a copolymer of (1) and (2).
where AR
1
, AR
2
, and AR
3
are represented by the following formula (3),
and X and Y are selected from —O—, —CH
2
—, —CO—, —SO
2
—, —S—, —C(CH
3
)
2
and the like, but are not limited thereto.
One or more hydrogen atom in these aromatic rings may be substituted with a halogen group such as fluorine, chlorine, and bromine, preferably chlorine; a nitro group; an alkyl group such as a methyl group, an ethyl group, and a propyl group, preferably a methyl group; or an alkoxy group such as a methoxy group, an ethoxy group, a propoxy group, and an isopropoxy group. One or more of the hydrogen atoms in an amide bond of the polymer may be substituted with another substitutional group.
The polymer has preferably 50% or more, more preferably 75% or more, of the aromatic rings in a para orientation (hereinafter referred to as “para orientation aromatic polyamide”) since the film comprising such a polymer has increased stiffness and excellent heat resistance. The para orientation herein means that the bond configuration is linear.
When the content of the aromatic rings in which one or more of the hydrogen atoms is substituted with a halogen group is 30% or more, preferably 50% or more, more preferably 70% or more, moisture resistance is improved, and a change in size due to moisture and a decrease in stiffness are prevented. In addition, when organics and/or inorganics are coated on the aromatic polyamide film, the adhesion between the organic and/or inorganics and the film is improved. Examples of the halogen include fluorine, chlorine, and bromine. Chlorine is especially preferable.
The aromatic polyamide film is produced as follows: an aromatic diamine and an aromatic dicarboxylic acid are polymerized in an organic polar solvent such as N-methylpyrolidone, dimethylacetoamide, and dimethylformamide to synthesize an aromatic polyamide resin. As a neutralizer of the hydrogen chloride produced, an inorganic salt such as calcium hydroxide and calcium carbonate is used. The aromatic polyamide resin is formed into a film

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