JPS61141714A - Optical disk - Google Patents

Abstract

PURPOSE:To decrease markedly the double refraction of an optical disk and at the same time to lower its hygroscopicity, by copolymerizing methyl methacrylate, a methacrylate ester and styrene as essential materials at a specified ratio and using the produced polymer as a material for the body of a disk. CONSTITUTION:A copolymer formed by copolymerizing essential materials consisting of 30-60wt% methyl methacrylate, 10-40wt% methacrylate ester in which a 3-8C hydrocarbon group is bonded with the acid group and 0-30wt% styrene so that the obtained copolymer may have a photoelastic constant of -4.0X10<-13>-+4.0X10<-13>cm<2>/dyn. Examples of the hydrocarbon groups in the methacrylate ester include linear and cyclic hydrocarbon groups, such as n-propyl, n-butyl, 2-ethylhexyl and cyclohexyl.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field] The present invention relates to an optical information recording disk (hereinafter abbreviated as "optical disk") in which recorded signals are read out by reflection or transmission of a laser beam.

[Background technology]

The optical information recording/reproducing method, in which a spot beam of a laser beam is applied to a disk and the signals recorded by minute pits on the disk are read out by detecting the amount of reflected or transmitted light from the laser beam, can significantly increase recording density. In addition, since the images and sound quality reproduced from it have excellent characteristics, it is expected to be widely used in recording and reproducing images and sounds, and recording and reproducing large amounts of information. In addition to being transparent so that laser beams can pass through the disc body, the discs that are manufactured must also have strong optical homogeneity to reduce reading errors. Birefringence occurs when the laser beam passes through the disk body due to residual stress due to thermal stress, molecular orientation, volume change near the softening point, etc. generated during the flow process. Optical non-uniformity due to this birefringence A large value is a fatal flaw for an optical disc.

In addition, dimensional changes and warpage of the optical disc (molded product) due to moisture absorption can also increase reading errors, so the moisture absorption value needs to be much lower than that of conventional methacrylic resins. [Purpose of the Invention] This invention was made in view of these circumstances,
The object of the present invention is to provide an optical disc with extremely low birefringence and low hygroscopicity.

fDisclosure of the Invention] The birefringence index is expressed as the product of the photoelastic coefficient and the residual stress, as shown in equation (A) below.

(n,-n2)=C(ty,-az)・”(A
)(n+nz): Birefringence (σ1-σ2): Residual stress C: Photoelastic coefficient (A) As can be seen from the formula, birefringence can be reduced by reducing either the photoelastic coefficient or the residual stress. The rate can be reduced. Therefore,.

The inventors first focused on reducing residual stress. However, the residual stress changes depending on the molding conditions during disk molding, and it has been found that it is difficult to reduce the residual stress to near zero even when the optimal molding conditions are selected.

Next, the inventors focused on reducing the photoelastic coefficient. The photoelastic coefficient is an index of how easily birefringence occurs in response to stress. This is a value specific to the material, such that when stress is applied, the refractive index in the same direction as the stress direction is thicker than the refractive index in the direction perpendicular to the stress direction, and the photoelastic coefficient is positive. There are resins with negative photoelastic coefficients, such as resins with a large refractive index in the direction perpendicular to the stress direction, and resins with negative photoelastic coefficients. We thought that it would be possible to obtain a copolymer (resin) with a very small photoelastic coefficient by copolymerizing the two types of resins by appropriately combining them.As a result, we found that both types of resins A copolymer with a photoelastic coefficient of zero or a value close to zero can be obtained by copolymerizing a predetermined amount of these monomers, and this copolymer also has a small birefringence. It has been found that an optical disk with a small birefringence can be obtained by using the material as the disk body material.
The photoelastic coefficient of the copolymer is -4, OX 10-13d/
dyne~+4. If it is in the range of OX 10-" cj/dyne, it is possible to mold the birefringence to be extremely small. Here, as a monomer that becomes a resin with a positive photoelastic coefficient, styrene is used.

Examples include rose methyl styrene, cyclohexyl methacrylate (cyclohexyl methacrylate), phenyl methacrylate (phenyl methacrylate), and examples of monomers that become negative resins include methyl methacrylate and α-methylstyrene. However, even if monomers that can make both types of resin are copolymerized in appropriate combinations so that the photoelastic coefficient is extremely small, it is not always possible to obtain a copolymer with low hygroscopicity. That's what I found out. Therefore, the inventors conducted research in an attempt to obtain a copolymer with low hygroscopicity, and found that a methacrylic acid ester in which a hydrocarbon group having 3 to 8 carbon atoms is bonded to an acid group was used as a comonomer.
It has been found that the hygroscopicity of methyl methacrylate can be improved by always using it as a methyl methacrylate and also using styrene as necessary. Furthermore, in addition to optical homogeneity and hygroscopicity, we also considered transparency, thermal decomposition resistance, thermal deformation resistance, etc., and conducted repeated research in an effort to obtain a copolymer that was fully satisfactory in these areas. Ta. As a result, by using methyl methacrylate, a methacrylic ester in which a hydrocarbon group having 3 to 8 carbon atoms is bonded to an acid group, and styrene within a certain range, the photoelastic coefficient was made to be extremely small within a certain range. They discovered that it is sufficient to copolymerize these materials, and have now completed this invention.

Therefore, this invention provides methyl methacrylate 30-6
0% by weight (hereinafter abbreviated as %), 10% to 40% of methacrylic acid ester in which a hydrocarbon group having 3 to 8 carbon atoms is bonded to an acid group (hereinafter simply referred to as "methacrylic ester"), and 0 to 40% of styrene. The main component is a copolymer copolymerized with 30% as an essential raw material and a photoelastic coefficient of -4. The gist is an optical disk whose main body material is a resin composition.

The present invention will be explained in detail below.

Here, the hydrocarbon group of methacrylic acid ester is
Examples include chain hydrocarbon groups such as alkyl groups and cyclic hydrocarbon groups. Specific hydrocarbon groups include, for example, n
-propyl, isopropyl, n-butyl, isobutyl,
Examples include 5ec-butyl, isoamyl, t-amyl, neopentyl, 3,3-dimethylbutyl, 2-ethylhexyl, and cyclohexyl. The methacrylic acid ester may be used alone, or multiple types may be used simultaneously. The photoelastic coefficient of methyl methacrylate, etc. is negative, and the photoelastic coefficient of cyclohexyl methacrylate, styrene, etc. is positive.

The essential raw materials are 30-60% of methyl methacrylate, 10-40% of methacrylic acid ester, and 0-30% of styrene, preferably 10-25%, and the photoelastic coefficient is 4
．． OX I 0-I3cIa/dyne or higher, +4
．． These are copolymerized by adjusting the proportion used so as to obtain a copolymer within the range of OX 10-13c11/dyne or less. By doing so, a copolymer having very low hygroscopicity and sufficiently satisfactory heat decomposition resistance, heat deformation resistance, etc. can be obtained.

In addition, for the purpose of improving the performance of an optical disc, a polymerizable monomer may be used in combination with the above-mentioned monomer, if necessary, within a range that does not impede the purpose of the present invention.

The copolymer may be produced by any of the known polymer production methods, such as bulk polymerization, suspension polymerization, solution polymerization, and emulsion polymerization.

As a component of the disc body material, auxiliary agents such as silicone, wax, fatty acids, fatty acid esters, fatty acid metal salts, fatty alcohols, etc. are used for the purpose of improving mold releasability from the stamper, and for the purpose of preventing static electricity, for example, Auxiliary agents such as sulfonic acid salts of higher alcohols and quaternary ammonium salts may be used in combination with the above copolymer to the extent that they do not impair the achievement of the objects of the present invention.

The optical disc of the present invention is manufactured, for example, in the following manner. First, a resin composition (disk body material) containing a copolymer as a main component is molded into a disk body by injection molding or press molding, using a mold equipped with a stand bar to transfer pits serving as recording signals.

It is preferable that the molding be performed under conditions that minimize residual stress. Next, in the case of a duplicate disc, a reflective layer is generally formed on the pit transfer surface by a method such as vacuum deposition of metal, sputtering, or ion blasting, and then a protective coating for the reflective layer is applied as necessary. You can go and manufacture it. In the case of a memory disk, after the pits that serve as tracking signals are transferred in the same manner as described above, the user may write on the pit surface, such as amorphous rare metals or compounds that can be thermally decomposed by laser beams. A memory optical disk can be manufactured by depositing or coating a possible recording layer and, if necessary, forming a reflective layer or a protective coating as described above.

The optical disc of the present invention obtained as described above has a very low birefringence and a very low hygroscopicity.

Next, Examples and Comparative Examples will be explained.

(Examples 1 to 3 and Comparative Example) Optical discs of Examples 1 to 3 and Comparative Example were made as follows. First, a copolymer (methacrylic resin) was prepared by reacting each monomer mixture whose composition of monomers (essential raw materials) was as shown in Table 1. This copolymer was molded to create a disk body under molding conditions that minimized residual stress. Next, a reflective layer was formed on the disc body, and a protective coating was applied to the reflective layer to create an optical disc.

The photoelastic coefficient, water absorption, glass transition temperature, thermal decomposition temperature 2 bending strength, and light transmittance of the copolymers used in the optical discs of Examples 1 to 3 and Comparative Example were investigated. The results are shown in Table 2.

(Left below) Table 2 shows that the copolymers used in Examples 1 to 3 had a very low water absorption (two-thirds or less) and a photoelastic coefficient compared to those used in the comparative examples. It can be seen that the value is also very small (approximately two-thirds or less than that of the comparative example). Therefore, it was optically very homogeneous and had little warpage. It can also be seen that the thermal decomposition temperature 2 bending strength and light transmittance are sufficiently satisfactory. The optical discs of Examples 1 to 3 are as follows:
Since such a copolymer is used, it has almost no birefringence, is optically homogeneous, and does not warp due to moisture absorption.

〔Effect of the invention〕

The optical disc according to the present invention uses 30 to 60% of methyl methacrylate, 10 to 40% of a methacrylic acid ester in which a hydrocarbon group having 3 to 8 carbon atoms is bonded to an acid group, and 30% of styrene O as essential raw materials, The photoelastic coefficient is -4,
OX 10-” col/dyne or more, +4.O
Since the disc body material is made of a resin composition whose main component is a copolymer copolymerized so as to give X 10-13 cJ/dyne or less, the birefringence is very low and the hygroscopicity is also very low. Therefore, by using the optical disc of the present invention, reading errors can be reduced.

Claims (1)

[Claims] (1) Methyl methacrylate 30-60% by weight, carbon number 3
The essential raw materials are 10 to 40% by weight of methacrylic acid ester in which ~8 hydrocarbon groups are bonded to acid groups and 0 to 30% by weight of styrene, and the photoelastic coefficient is -4.0×10^-^1^3
cm^2/dyne or more, +4.0 x 10^-^1^3
An optical disc whose disc main body material is a resin composition whose main component is a copolymer copolymerized to have a particle diameter of cm^2/dyne or less.