JP2668945B2 - optical disk - Google Patents

Description

The present invention relates to an optical disc, and more particularly, to a substrate composed of a polyvinylcyclohexane-based resin having a specific molecular weight and a softening point, which is excellent in thermal processing stability and the like. The present invention relates to an optical disk having the same.

[Prior Art] Optical recording using a laser is capable of high-density information recording, storage, and reproduction, and has been actively developed in recent years. An example of such optical storage is an optical disk. Generally, an optical disc is basically composed of a transparent substrate and various recording media coated thereon.

In many cases, a transparent substrate of an optical disc is made of a colorless and transparent synthetic resin, and a typical example thereof is polycarbonate (hereinafter, abbreviated as “PC”) or polymethyl methacrylate (hereinafter, abbreviated as “PMMA”). ). Although these resins excel in colorless transparency, they also have excellent properties unique to each,
Not all of the requirements for optical materials, especially optical disk substrates, are met, and there are problems to be solved. For example, PC has a problem of birefringence caused by the aromatic ring, and also has a problem of water absorption or water permeability. On the other hand, problems with heat resistance, water absorption, and toughness have been pointed out for PMMA.

As described above, each of these resins is used with inherent problems inherent therein. However, in practice, the relationship with a recording medium coated on a transparent substrate made of these resins is further increased. , A new problem as described below has arisen.

On the other hand, a wide variety of recording media have been conventionally developed according to the use of optical disks. For example, a write-once type dedicated to recording and reproduction is a drilling type, and an erasable type for recording-reproduction-erasing-re-recording is a phase transition type utilizing a crystal transition phenomenon. And a magneto-optical type utilizing the magneto-optical effect are known. Materials for these recording media include tellurium or its oxides in the write-once type, alloy compounds, etc., and GdFe in the erasable type,
Inorganic materials such as rare earth-transition metal amorphous alloy compounds such as TbFe, GdFeCo, and TbFeCo are mainly used, and are generally formed by forming a film on a transparent substrate by a dry process such as sputtering under high vacuum. Have been.

By the way, the hygroscopicity and water permeability of PC and PMMA cause, on the one hand, a problem of warpage due to expansion of the substrate itself when absorbing moisture, but on the other hand, the permeation of moisture through the substrate causes corrosion of the recording medium, and the optical disk This is a cause of shortening the life of the substrate.

Further, if further mentioning the heat resistance of the substrate resin,
There are the following problems. That is, in an optical disk, particularly in a write-once type, an erasable type or the like, the temperature of the recording medium at the time of writing and erasing the recording is 200 ° C.
That's all. For this reason, even if this heat is not directly applied to the disk substrate, it is expected that the substrate will become extremely hot at the time of writing or erasing the recording. Problems can occur.

On the other hand, in the optical disk production process, a heat treatment process is often adopted for the purpose of preventing the substrate or the recording medium from changing over time. However, in order to improve the productivity, the process is performed by processing at the highest possible processing temperature. It is desired to reduce the time. From such a viewpoint, if the heat resistance of the resin is low, a high processing temperature cannot be adopted, and there is a problem that productivity cannot be increased.

For these reasons, PMMA with low heat resistance is not enough to withstand the high temperatures of the optical disc production process or usage conditions. Conventionally, PCs with higher heat resistance are exclusively considered as transparent substrate materials. Have been. However, PCs are not always evaluated as having sufficient heat resistance, and the emergence of resin materials having higher heat resistance is desired.

To compensate for the drawbacks of conventional resins such as PC and PMMA, Japanese Patent Application Laid-Open No. 63-43910 and Japanese Patent Application No. 62-157324 propose a method using a polyvinylcyclohexane-based resin as an optical disk substrate.

[Problems to be Solved by the Invention] However, even when a polyvinylcyclohexane-based resin is used, a high-performance optical disc substrate is not always obtained, and a satisfactory result in terms of heat resistance and the like is not obtained. Is the current situation.

The present inventors have studied in detail the problems in the production of an optical disk using a polyvinylcyclohexane-based resin as a substrate material and the like proposed in the above-mentioned patent application, and have obtained the following knowledge.

The polyvinyl cyclohexane resin is a resin generally obtained by aromatic hydrogenation of a vinyl aromatic polymer and is generally known as a brittle polymer like polystyrene.

As described above, optical disks of the write-once type, erasable type and the like receive a considerable heat history in their use and production process, and therefore require high heat resistance to the resin substrate. In addition, the disk substrate-like resin material is required to have at least a toughness that does not cause damage during mold release during injection molding.

From such a point of view, as a result of studying the case of manufacturing an optical disc substrate using the polyvinylcyclohexane-based resin described in the above-mentioned conventional application, the polymers disclosed in these conventional applications have the above-mentioned required characteristics. Turned out to be unsatisfactory. That is, a polymer having a high nuclear hydrogenation rate and a high heat resistance has a low molecular weight and is likely to be damaged during injection molding. Therefore, a satisfactory disk substrate cannot be obtained. On the other hand, a polymer with a small decrease in molecular weight had a low nuclear hydrogenation rate and insufficient heat resistance.

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above-mentioned conventional problems and to provide an optical disc having a substrate made of polyvinyl cyclohexane resin having excellent heat resistance and good moldability.

[Means and Actions for Solving the Problems] The optical disc of the present invention has a vinyl aromatic hydrocarbon polymer segment content of 80% by weight or more and a nuclear hydrogenation rate of 90% or more,
Moreover, the number average molecular weight is more than 50,000 and the softening point is 15
It is characterized by having a substrate made of polyvinyl cyclohexane-based resin at 0 ° C. or higher.

That is, the present inventors have conducted intensive studies on the improvement of heat resistance and moldability of the above-mentioned polyvinylcyclohexane-based resin, and as a result, a styrene-based polymer obtained by nuclear hydrogenation at a specific nucleus hydrogenation rate is obtained. By using a polyvinylcyclohexane-based resin having a molecular weight and a softening point, the present inventors have found that an optical disk that can be used effectively under severe thermal conditions such as a write-once type and an erasable type can be obtained with high productivity. The invention has been reached.

When the content of the vinyl aromatic hydrocarbon polymer segment of the polyvinylcyclohexane-based resin used in the present invention is less than 80% by weight, the heat resistance of the resin obtained after nucleus hydrogenation decreases, and the resin is unsuitable as an optical disk substrate. It will be. Further, if the nuclear hydrogenation rate is less than 90%, there are problems such as a decrease in heat resistance of the obtained resin and an increase in birefringence, which is not preferable.
Further, when the number average molecular weight is 50,000 or less, the toughness of the resin is poor, and the substrate is damaged during mold release during injection molding. If the softening point is lower than 150 ° C., there arises a problem that the substrate is warped or a groove is deformed by heat at the time of writing or erasing a record in an optical disk such as a write-once type or an erasable type. In addition, the heat treatment temperature in the optical disc production process must be lowered, and the productivity will be lower than that of the PC that has been conventionally studied.

 Hereinafter, the present invention will be described in detail.

The polyvinyl cyclohexane resin used in the present invention is obtained by nuclear hydrogenating a styrene polymer.

Examples of the raw material styrene-based polymer include a vinyl aromatic hydrocarbon polymer and a vinyl aromatic hydrocarbon block copolymer. As the latter vinyl aromatic hydrocarbon block copolymer, a vinyl aromatic hydrocarbon polymer segment (hereinafter abbreviated as "A segment") at least one or more conjugated diene polymer segment 7 (hereinafter "B segment").
Abbreviated as "segment". ).

Examples of the vinyl aromatic hydrocarbon used as a monomer include styrene, p-methylstyrene, α-methylstyrene, and the like, and styrene is particularly typical.

Examples of the vinyl aromatic hydrocarbon polymer include a homopolymer or a copolymer of two or more of these vinyl aromatic hydrocarbons. In particular, when adhesiveness or the like is required, the vinyl aromatic hydrocarbon and an unsaturated monomer having a polar group copolymerizable therewith lose the properties of the vinyl aromatic hydrocarbon polymer. It is preferable to use a copolymer obtained by copolymerization within a range not to be restricted.

Next, A in the vinyl aromatic hydrocarbon block copolymer
Examples of the segment include those similar to the above-mentioned vinyl aromatic hydrocarbon polymer. Further, as the conjugated diene of the B segment in the block copolymer, 1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene, 1,
Examples thereof include 3-pentadiene and 1,3-hexadiene. In particular, 1,3-butadiene and isoprene are common. A
The block copolymer comprising a segment and a B segment can be easily prepared by a known method called so-called living anionic polymerization, for example, a method in which an organic lithium compound is used as an initiator, and a method of polymerizing in a hydrocarbon solvent such as hexane and heptane. Obtainable. Incidentally, the content of the A segment in such a block copolymer is 80% by weight or more,
It is preferably at least 90% by weight, more preferably at least 93% by weight. When the content of the A segment is less than 80% by weight, the heat resistance of the resin obtained after hydrogenation is lowered, and it becomes unsuitable as an optical disk substrate.

In the present invention, the molecular weight of such a raw material styrene-based polymer is preferably 80,000 or more in terms of number average molecular weight. If the molecular weight of the styrene polymer is too low, the heat resistance and toughness of the resin obtained after hydrogenation will decrease. The decrease in the toughness accompanying the decrease in the molecular weight is more remarkable as the content of the diene polymer in the polymer is smaller. Therefore, it is preferable that the number average molecular weight of the polymer not containing the diene polymer is larger than 100,000. On the other hand, the upper limit of the molecular weight is not particularly limited, but is usually preferably 400,000 or less.

The polyvinylcyclohexane-based resin can be obtained by hydrogenating such a styrene-based polymer in the presence of a hydrogenation catalyst having an aromatic hydrogenation ability. As the hydrogenation catalyst used here, for example, nickel, cobalt,
Examples thereof include metals such as ruthenium, rhodium, platinum and palladium, or oxides, salts and complexes thereof, and those supported on carriers such as activated carbon, diatomaceous earth and alumina.
Among these, Raney nickel, Raney cobalt,
Stabilized nickel and ruthenium, rhodium or platinum carbon or alumina supported catalysts are preferred from the standpoint of reactivity.

The nuclear hydrogenation reaction uses a saturated hydrocarbon solvent such as cyclohexane, methylcyclohexane, n-octane, decalin, tetralin, or naphtha as a solvent at a pressure of 50 to 250 kg / cm ² and a temperature of 100 to 200 ° C. It is preferable to carry out.

Incidentally, usually, in the nuclear hydrogenation of the styrene polymer,
As a side reaction, the molecular chain of the polymer is cut. In order to prevent this molecular chain scission, it is preferable to carry out the hydrogenation at as high a hydrogen pressure as possible and in a short time.

The nuclear hydrogenation rate of aromatic nuclei by the nuclear hydrogenation reaction is 90% or more, preferably 95% or more. When the nuclear hydrogenation rate is low and less than 90%, there are problems such as a decrease in heat resistance of the obtained resin and an increase in birefringence, which is not preferable.

In the present invention, the molecular weight of the polyvinylcyclohexane-based resin thus obtained depends on the composition, molecular weight, hydrogenation (hydrogenation) conditions and the like of the styrene-based polymer as a raw material, and the appropriate range also depends on the composition. Different, but usually
The number average molecular weight is more than 50,000, preferably more than 60,000. From the aspect of toughness, the smaller the content of the diene-based polymer, the higher the molecular weight is required, and for those not containing the diene-based polymer, the molecular weight is 60,0.
It should be greater than 00, preferably greater than 70,000. The upper limit of the molecular weight is defined from the viewpoint of moldability, but is usually 300,000 or less in number average molecular weight. The softening point thereof is 150 ° C or higher, preferably 160 ° C or higher, as measured by a thermomechanical analyzer.

Since such a polyvinylcyclohexane-based resin according to the present invention is a hydrocarbon-based resin, moisture in the resin can be removed by a simple drying treatment, and the resin can be used for molding such as injection molding. Upon molding, additives such as a heat stabilizer and an antioxidant for preventing deterioration of the resin during molding can be added as necessary.

In the production of the substrate of the optical disk of the present invention, first, an additive such as a stabilizer is added to the polyvinyl cyclohexane-based resin as needed, and mixed with a ribbon blender, a tumbler blender, a Henschel mixer, etc., and then a Banbury mixer, The mixture is melt-kneaded with a single-screw extruder, a twin-screw extruder, or the like, and pelletized to mix them.
Then, by using the pellets thus obtained, by injection molding at a molding temperature of 270 ~ 330 ℃, transparency,
It is possible to obtain an optical disc substrate having excellent heat resistance and a small optical distortion.

When the optical disk of the present invention is manufactured using such an optical disk substrate, Te is formed on the surface of the substrate by metal vapor deposition or the like.
Or its oxides, alloy compounds, etc., or TbFe, TbFeCo,
A method of providing a film of a recording medium such as an amorphous alloy compound of a rare earth-transition metal such as TbCo or DyFeCo, and further covering these with a protective film can be adopted.

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to Examples and Comparative Examples. However, the present invention is not limited to the following Examples as long as the gist of the present invention is not exceeded.

In addition, various physical properties in the following Examples and Comparative Examples,
It was measured by the following method.

Number average molecular weight: gel permeation chromatography (GP
C), tetrahydrofuran (THF) was used as a solvent in the same manner as polystyrene, and the polystyrene-equivalent number average molecular weight was determined.

Nuclear hydrogenation rate (%): A polyvinylcyclohexane-based resin was dissolved in tetrahydrofuran (THF) and measured by UV absorption.

Softening point (℃): Using a thermomechanical analyzer manufactured by Dupont
The softening temperature was measured at a heating rate of 5 ° C./min. The load was 5 g and the thickness of the test piece was 3 mm.

Measurement of heat resistance of optical disk: The optical disk with a recording medium and a protective film was measured for the carrier level (C / N ratio) using a dynamic characteristic detector with a PIN photodiode differential detector (0 dB when an Al film was used). Do)
Was measured. The measurement was performed before and after heat treatment at 100 ° C. for 2 hours, and the results were compared.

 The recording and reproducing conditions were as follows.

Recording conditions: CAV (constant angular velocity) 1800 rpm Radius 30 mm position, recording on groove Recording frequency 0.5 MHz Duty 50% Reproduction conditions: CAV 1800 rpm Reproduction power 0.8 mW Example 1 Number average molecular weight 70,000, nucleus obtained by hydrogenation of styrene homopolymer As a heat stabilizer, tetrakis [methylene-3- (3,5-di-t-butyl-4-hydroxyphenyl)] propionate methane was added to 100 parts by weight of polyvinylcyclohexane having a hydrogenation rate of 99% and a softening point of 171 ° C. (Nippon Ciba-Geigy Irganox 1010) and tetrakis (2,4-di-
Each 0.2 parts by weight of t-butylphenyl-4,4'-biphenylenephosphonite (the same Irgaphos-PEPQ) was added, and the mixture was melt-kneaded at 260 ° C. using an extruder to form pellets.

Injection molding machine for these pellets ("M-140A" manufactured by each machine)
Using, a stamper with a groove was attached to the movable mold side to form a disk-shaped optical disc substrate having a resin temperature of 300 ° C. and a thickness of 1.2 mm and a diameter of 130 mm.

The obtained substrate was charged into a sputtering apparatus, and first, 8
The exhaust was evacuated to × 10 ^-7 torr or less, and reactive sputtering of a Ta target was performed using a mixed gas of Ar and O ₂ to form an interference layer (thickness 800 Å) made of Ta ₂ O ₅ . Next, a TbFeCo recording layer (thickness: 300 mm) was provided by binary simultaneous sputtering using Ar gas using a Tb target and an FeCo target. Further, an Al target on which a Ti chip was placed was sputtered in Ar gas to form a 300 Å thick reflective layer.

The carrier level (C /
The N ratio) was 60.8 dB, which was unchanged after the heat treatment. In addition, there was no change in the groove shape.

Example 2 Styrene-butadiene-synthesized by anionic polymerization
Styrene block copolymer (butadiene content 5% by weight)
Number average molecular weight 61,000 obtained by nuclear hydrogenation, nuclear hydrogenation rate 99
%, A polyvinyl cyclohexane- (ethylene-butene-1) -vinyl cyclohexane block copolymer having a softening point of 165 ° C. was used to form a substrate and form a recording layer in the same manner as in Example 1.

The carrier level (C /
The N ratio) was 60.5 dB, which was unchanged after the heat treatment. In addition, there was no change in the groove shape.

Comparative Example 1 Injection molding was performed in the same manner as in Example 1 using polyvinylcyclohexane having a number average molecular weight of 35,000, a nucleus hydrogenation rate of 97%, and a softening point of 156 ° C., which was obtained by nuclear hydrogenation of a styrene homopolymer. Was. As a result, the molecular weight is low and the resin is brittle,
A normal substrate could not be obtained because the substrate was cracked when the mold was released. Therefore, it cannot be evaluated as an optical disc.

Comparative Example 2 A vinylcyclohexane-styrene copolymer having a number-average molecular weight of 80,000, a nuclear hydrogenation rate of 82%, and a softening point of 125 ° C., obtained by subjecting a styrene homopolymer to nuclear hydrogenation, was the same as in Example 1. Then, a substrate was formed and a recording layer was formed.

When the obtained optical disk was subjected to heat treatment, the groove shape changed due to low heat resistance.

 Table 1 summarizes the above results.

As is clear from Table 1, according to the present invention, an optical disc having excellent heat resistance and magneto-optical recording characteristics is provided.

[Effects of the Invention] As described in detail above, according to the optical disc of the present invention, the optical disc is excellent in heat resistance, transparency, etc., has low moisture absorption, low birefringence, and has remarkably good optical characteristics. Moldability,
An optical disk with good productivity is provided.

Claims (1)

(57) [Claims] 1. Polyvinylcyclohexane having a vinyl aromatic hydrocarbon polymer segment content of 80% by weight or more, a nuclear hydrogenation rate of 90% or more, a number average molecular weight of more than 50,000, and a softening point of 150 ° C. or more. An optical disc having a substrate made of a resin.