JP2633572B2 - Magneto-optical recording medium - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magneto-optical recording body made of a plastic substrate, which can maintain stable magneto-optical recording characteristics for a long period of time.

[Conventional technology]

A magneto-optical recording medium in which a magneto-optical recording layer is provided on a substrate via a protective layer is known. That is, JP-A-60-201546 describes a magneto-optical recording medium in which a recording layer is provided on a substrate via a protective layer made of cadmium sulfide (CdS). JP-A-61-202351 describes a magneto-optical recording medium in which a recording layer is provided on a substrate via a protective layer made of a selenium compound such as zinc selenide (ZnSe).

[Problems to be solved by the invention]

Among the magneto-optical recording media having a protective layer represented by these known documents, the conventional magneto-optical recording media using a transparent plastic as a substrate has a lower oxidation degradation of the recording layer than the magneto-optical recording media having no protective layer. As a result, the magneto-optical recording characteristics can be maintained for a relatively long time. However, for the purpose of the magneto-optical recording medium, it is better that the recording characteristic retention period is as long as possible. There is still room for improvement.

In fact, Japanese Patent Application Laid-Open No. 60-201546 generally states that the CdS layer prevents oxidative deterioration of the magnetic layer and can maintain stable magneto-optical recording properties over a long period of time, but a plastic substrate was used. However, there is no time-dependent data on the characteristics of the magneto-optical recording medium. Japanese Patent Application Laid-Open No. 61-202351 also has time-lapse data of magneto-optical characteristics using a glass substrate, but does not include data using a plastic substrate.

As described above, the magneto-optical characteristics of the conventional plastic substrate cannot be stably maintained over a long period of time because the conventionally used plastic substrate is polymethyl methacrylate (PMMA) or polycarbonate (PC). This is probably because the water absorption of the plastic itself is high, and the oxidation rate of the recording layer is increased.

The present invention is intended to improve long-term magneto-optical characteristics of a magneto-optical recording medium using a plastic substrate,
Moreover, the present invention relates to a plastic magneto-optical recording medium having good adhesion between a substrate and a protective layer.

(Means for Solving the Problems) That is, the present invention relates to a magneto-optical recording medium in which a transparent plastic substrate is laminated with a magneto-optical recording layer via at least a protective layer via a protective layer. The constituent monomer is a copolymer comprising at least ethylene and a cyclic olefin represented by the formula (1), and the monomer unit of the formula (1) in the copolymer has a structure having the structure represented by the formula (2) This is a magneto-optical recording medium characterized by being a substantially amorphous copolymer, wherein the protective layer is made of Si ₃ N ₄ , AIN, ZnSe, ZnS, Si or CdS.

(Wherein, R ^{1 to} R ¹² are hydrogen, an alkyl group, or halogen, which may be the same or different, and further, R ⁹ or R ¹⁰ and R
¹¹ or R ¹² may form a ring with each other. n is 0 or a positive number of 1 or more, and when R ^{5 to} R ⁸ are repeated a plurality of times, they may be the same or different. [Operation] The transparent plastic constituting the substrate is a copolymer of at least ethylene and the monomer of the formula (1) as described above. There are various types of monomers represented by the formula (1). However, from the viewpoints of heat resistance to laser beam heating during magneto-optical recording, ease of synthesis of monomer components, availability, and economy, etc. In equation (1), n
= 1 is preferred, and those in which R ^{1 to} R ¹² are hydrogen are preferred.

The ethylene content is about 50 to 80 when the total number of moles of ethylene and the monomer of the formula (1) is 100.
Those having an ethylene content of from 55 to 70 mol% are preferably used.

To produce the above-mentioned monomer component, for example, US Pat. No. 3,557,072 (Japanese Patent Publication No. 46-14910) or
No. 154133 can be applied.

The copolymer of the monomer component of the formula (1) and ethylene may be copolymerized with another monomer component as long as its properties are not impaired. Specific examples include α-olefins and cyclic olefins, and specific examples thereof include α-olefins having 3 to 20 carbon atoms, preferably 3 to 10 carbon atoms,
For example, propylene, 1-butene, 3-methyl-1-butene, 1-pentene, 3-methyl-1-pentene, 4-
Methyl-1-pentene, 1-hexane, 1-octane,
Examples thereof include 1-decene, 1-dodecene, 1-tetradecene, 1-hexadecane, and 1-eicosene. Cyclic olefins include non-crosslinked cycloolefins and styrenes, such as cyclopentene, cyclohexene,
Examples thereof include 3,4-dimethylcyclopentene, 3-methylcyclohexene, 2- (2-methylbutyl) -1-cyclohexene, styrene, and α-methylstyrene.
Polyenes such as dicyclopentadiene, ethylidene norbornene and vinyl norbornene can be copolymerized as well. Unsaturated carboxylic acids such as (anhydride) maleic acid can also be copolymerized.

The copolymer is produced by polymerization using a well-known Ziegler catalyst, particularly a vanadium-based Ziegler catalyst. More specifically, it is disclosed in a prior patent application filed by the applicant (for example, JP-A-60-168708). A characteristic of the copolymer is that the monomer component of the formula (1) has a structure mainly represented by the formula (2) in the copolymer, whereby the iodine value of the copolymer is usually 5 or less, Many are less than one. In addition, this configuration is equivalent to ¹³ C-N
Supported by MR.

By adopting this configuration, the copolymer is chemically stable, excellent in water resistance and chemical resistance such as alkali and acid, and also excellent in solvent resistance, heat resistance and weather resistance. Also, it has an extremely low water content.

In addition, the copolymer is substantially amorphous, and thus has excellent transparency and dimensional stability. Further, the birefringence in plastics is as small as that of PMMA, and the birefringence value is hardly changed due to the change in the incident angle of light rays, which is almost constant.

Compounds used as protective layers include Si ₃ N ₄ , AIN, ZnS
e, ZnS, a compound selected from Si and CdS, which are described in JP-A-60-201546 and JP-A-61-2023 described above.
As shown in JP-A-51-51, it can be formed on a substrate by a known means such as sputtering or vapor deposition.

When the specific copolymer as described above is used as the substrate as in the present invention, and these compounds are formed on the substrate as the protective layer, it is extremely different from the case where the compound is formed on a conventional PMMA or PC substrate. It exhibits high adhesiveness, and does not partially peel off the substrate and the protective layer even when left for a long time under high temperature and high humidity.

In forming a protective layer having a refractive index of n, the thickness d of the protective layer is determined by the following formula:
= Λ / 4n, the enhancement effect occurs, the Kerr rotation angle increases, the read signal intensity increases, and the S / N ratio can be greatly improved.

Normally, the protective layer is formed with a thickness of 100 mm or more from the viewpoint of uniform smoothness.

The magneto-optical recording layer formed on the protective layer may be a magnetic layer that produces a magneto-optical effect, for example, a film surface made of a transition metal such as Fe or Co and a rare earth element such as Gd, Tb, or Dy. An amorphous perpendicular magnetization film having an easy axis of magnetization perpendicular to the film can be exemplified. Also, a noble metal component such as Pt or Pd, or a hardly corrosive component such as Ti may be substituted with a part of a transition metal or a rare earth element. The formation of these magneto-optical recording layers can also be achieved by a known method, for example, by sputtering or the like.

In the present invention, a protective layer may be further formed on the magneto-optical recording layer.

〔effect〕

The present invention selects a specific copolymer as described above as a plastic constituting a substrate, and also selects a specific six kinds of compounds as described above for a protective layer, and combines both to form a plastic substrate. As a magneto-optical recording medium, remarkable long-term stability of magnetic recording characteristics can be achieved.

〔Example〕

Hereinafter, the content of the present invention will be described with a preferred example,
The present invention is not limited to these Examples unless otherwise specified.

The test methods performed in the following examples are as follows.

(1) Measurement of photoelastic constant For a test piece of 10 × 10 × 0.5mm formed by press molding, at no load or 50g, 100g, 200g
The birefringence of the test piece in a state where stress was applied by suspending the weight of each was measured using a transmission ellipsometer of a helium-neon laser light source with a wavelength of 632.8 mm,
The photoelastic constant was determined from the relationship between the birefringence and the stress.

(2) Retension (double pass) It was determined by (birefringence under no load × thickness of test piece) × 2.

Reference Example 1 Ethylene having an ethylene content of 60 mol% and tetracyclododecene ( ₁₃ C-NMR reveals that tetracyclododecene in the copolymer is And the photoelastic constants of polymethyl methacrylate (T10-10, Kyowa Gas Chemical Industry Co., Ltd.) and polycarbonate (Teijin Kasei, AD-5503), which are the raw materials for conventional optical discs. It was measured.

Also, the laser irradiation angle was set to 0 degree in the vertical direction, and the retension (double pass) was measured at a distance of radius r mm from the center of the circularly shaped test piece when the angle was changed by 10 degrees diagonally. . The results are shown in Table 1.

As can be seen from the results, since the copolymer used in the present invention has a small photoelastic constant, the optical effect is not affected even if the substrate is molded under molding conditions that cause residual stress as in injection molding. It can be expected that it hardly shows. On the other hand, since polycarbonate has a large photoelastic constant, when a large-diameter substrate is formed by injection molding, optical distortion occurs due to residual stress, and it can be seen that polycarbonate cannot be used as an optical disk. Furthermore, it is also found that polymethyl methacrylate having almost the same photoelastic constant has a change in birefringence due to a change in the incident angle of the laser. On the other hand, the copolymer used in the present invention has a constant birefringence even when the incident angle changes. Therefore, it can be seen that polymethyl methacrylate is inferior in performance to the polymer used in the present invention in optical disc applications where the laser is narrowed down in the disc in order to perform writing and reading with the laser.

Reference Example 2 Films of various compounds including the compound used as the protective layer of the present invention were formed on the ethylene / tetracyclododecene copolymer of Reference Example 1 by sputtering (50 W, 1 minute), Table 2 shows the film forming speed.

As can be seen from these results, the film formation rates of ZnSe and CdS are remarkably high, and both compounds are particularly excellent in terms of productivity.

As can be seen from these results, the film formation rates of ZnSe and CdS are remarkably high, and both compounds are particularly excellent in terms of productivity.

<Examples 1 to 6 and Comparative Examples 1 to 10> A disk substrate having a diameter of 130 mm was formed using the ethylene / tetracyclododecene copolymer (PO), PMMA, and PC used in Reference Example 1, and this substrate was formed. A compound as shown in Table 3 above is formed as a protective layer (intermediate layer) by sputtering at a thickness of 500 °, and a magneto-optical recording layer made of TbFe is further formed thereon by sputtering at a thickness of 1000 °, and Then, a protective layer having a thickness of 500 mm was formed by sputtering.

The magneto-optical recording medium thus obtained was subjected to a temperature of 70 ° C. and 85%
Leave under RH condition for 7 days and -20 ℃ for 2 hours at + 60 ℃
Were alternately left for 2 hours, and after a heat cycle test was continued for 7 days, the respective coercive forces (Hc) were measured. The results are shown in Table 3.

As can be seen from the results, the magneto-optical recording medium of the present invention maintains the initial coercive force extremely stably, while those having a different protective layer are preserved even if the substrate is the same as the present invention. Magnetic force is greatly reduced. Also, it can be seen that even if the protective layer is the same as that of the present invention, the coercive force decreases when the substrate is PMMA or PC.

 ──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-61-292601 (JP, A) JP-A-61-243977 (JP, A) JP-A-61-243976 (JP, A) JP-A-61-43976 237243 (JP, A) JP-A-57-169996 (JP, A) JP-A-60-201546 (JP, A)

Claims (1)

(57) [Claims] In a magneto-optical recording medium comprising a transparent plastic substrate and a magneto-optical recording layer laminated on at least a protective layer via a protective layer, the transparent plastic forming the substrate is composed of at least ethylene and ethylene (1). And a monomer unit of the formula (1) is a substantially amorphous copolymer having a structure represented by the formula (2). And the protective layer is Si
_{3 N 4, AIN, ZnSe,} ZnS, a magneto-optical recording medium, characterized in that it consists of Si or CdS. (Wherein, R ^{1 to} R ¹² are hydrogen, an alkyl group, or halogen, which may be the same or different, and further, R ⁹ or R ¹⁰ and R
¹¹ or R ¹² may form a ring with each other. n is 0 or a positive number of 1 or more, and when R ^{5 to} R ⁸ are repeated a plurality of times, they may be the same or different. )