Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – From phenol – phenol ether – or inorganic phenolate
Reexamination Certificate
2002-04-29
2003-11-11
Boykin, Terressa M. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
From phenol, phenol ether, or inorganic phenolate
C428S064200, C428S064100
Reexamination Certificate
active
06646098
ABSTRACT:
DESCRIPTION
1. Field of the Invention
The present invention relates to a polycarbonate resin molding material for optical use which volatilizes during the molding of an optical disk substrate and improves continuous productivity by reducing the amount of a substance (deposit) accumulated on a metal mold and a stamper and to an optical disk substrate. The present invention further relates to a polycarbonate resin molding material for optical use which volatilizes during the molding of an optical disk substrate, can improve continuous productivity by reducing the amount of a substance (deposit) accumulated on a metal mold and a stamper and retains high reliability for a long time and to an optical disk substrate made from the material.
2. Prior Art
Optical disks for recording and reproducing information by the irradiation of laser light, such as digital audio disks (so-called compact disks), optical video disks (so-called laser disks), various Write Once Optical disks, optomagnetic disks and phase change disks, have already been implemented.
Out of these, compact disks and laser disks are ROM (Read Only Memory) optical disks. In these optical disks, pits corresponding to information signals are formed in a transparent substrate as rugged shape and a 40 nm or more thick Al reflection layer is formed on the pits. In such optical disks, information signals are reproduced by detecting changes in reflectance caused by optical interferences generated by the pits.
Meanwhile, the Write Once Optical disks are recordable optical disks to which any information can be written by a user and optomagnetic disks and phase change disks are RAM (Random Access Memory) optical disks to which any information can be written many times.
That is, the recordable optical disk comprises a transparent substrate and a Write Once recording layer whose optical characteristics are changed irreversibly or rugged shapes are formed by the irradiation of laser light and which is formed on the transparent substrate. This recording layer is made from a cyanine-, phthallocyanine- or azo-based organic pigment which is decomposed by heating, for example, with the irradiation of laser light, whereby its optical constant is changed and its volume is also changed, thereby causing the deformation of the substrate.
The optomagnetic disk is a rewritable optical disk to which a user can write and erase information repeatedly and which comprises a vertically magnetized film having a magneto-optic effect (for example, Kerr Effect) such as a Tb—Fe—Co amorphous alloy thin film formed on a transparent substrate. In this optomagnetic disk, a recording pit is formed by magnetizing a micro area of the vertically magnetized film upward or downward according to an information signal. The information signal is reproduced making use of the fact that the rotation angle &thgr;k (Kerr rotation angle) of linear polarization of reflected light differs according to the magnetization direction of the vertically magnetized film.
The phase change disk is a rewritable disk like the optomagnetic disk and made from a Ge—Sb—Te phase change material which assumes a crystal state initially and changes from the crystal state to an amorphous state by the irradiation of laser light. In this recording layer, a recording pit is formed by changing the phase of a micro area according to an information signal and the information signal is reproduced by detecting a difference in reflectance between an amorphous portion corresponding to the pit and other crystal area.
The optomagnetic disk and phase change disk are in most cases a disk having a four-layer structure consisting of a recording layer, two transparent dielectric layers sandwiching the recording layer from both sides of the recording layer and an Al reflection layer formed on the dielectric layer in order to prevent the oxidation of the recording layer and increase the degree of signal modulation by multiple interferences. The dielectric layer is a silicon nitride film or Zn—SiO
2
composite film.
Studies have recently been made energetically on use of the above optical disks for recording digital images, and a digital versatile disk (DVD) has been developed as such an optical disk.
This DVD has the same diameter as CD (120 mm) but can record image information equivalent to one movie film and reproduce image information of quality as high as current TV images.
To record such image information on an optical disk, a recording capacity 6 to 8 times more than CD is necessary. Therefore, in DVD, the wavelength of laser light is reduced to 635 to 650 nm compared with 780 nm for CD and the numerical aperture (NA) of an objective lens is increased to 0.52 or 0.6 compared with 0.45 for CD, thereby reducing the track pitch or the shortest recording mark length of a pit to increase the recording density.
An increase in the numerical aperture (NA) of an objective lens results in a reduction in the tolerance of deflection of a disk substrate. Therefore, compared with 1.2 mm for CD, the thickness of a DVD substrate is reduced to 0.6 mm so as to shorten the distance of the disk substrate through which laser light passes, thereby compensating for the reduction in the tolerance of warpage (Nikkei Electronics No. 630 issued on Feb. 27, 1995). To compensate for a reduction in the strength of a disk caused by the reduction in the thickness of the substrate, a so-called laminate structure that another substrate is laminated on a recording layer formed on a substrate is employed as disclosed by JP-A 6-274940 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”). The recording layer of the laminated optical disk is a ROM recording layer, recordable recording layer or RAM recording layer used in the above-described single substrate structure.
Further, the laminated optical disk includes a one-side laminated optical disk using only one side thereof and a both side laminated optical disk using both sides thereof.
A polycarbonate resin which has excellent moldability, strength, light transmittance and humidity resistance is used in most of the above optical disk substrates.
However, when substrates are molded continuously, the characteristic properties such as transferability and optical and mechanical properties of the substrates deteriorate and dust is adhered to the substrates.
It has been found that volatile matter generated from the polycarbonate is adhered to a metal mold or a stamper as the cause of the above problems, thereby causing such a trouble as outgassing with the result that the optical and mechanical properties of the substrates deteriorate and the deposit comes off, falls on the stamper and is transferred to the substrate.
Therefore, a measure for reducing the amount of a low molecular weight polycarbonate compound contained in the raw materials as a causative agent is taken. For example, as described in JP-A 9-208684, the formation of a deposit is prevented by reducing the amount of a low molecular weight polycarbonate compound having a molecular weight of 1,000 or less contained in a polycarbonate. However, it has been confirmed that the agent of the deposit is not only the low molecular weight polycarbonate compound and that this effect is not sufficient.
The polycarbonate resin is readily hydrolyzed at a high temperature and a high humidity, resulting in reductions in molecular weight and impact strength. Further, when it is left at a high temperature and a high humidity for a long time, very small white points are formed on the substrate, thereby impairing long-term reliability.
Meanwhile, one of characteristic properties required of an optical disk substrate and an optical information recording medium comprising the same is the retention of high reliability for a long time. However, as the polycarbonate resin is easily hydrolyzed at a high temperature and a high humidity, it has been difficult to fully satisfy this requirement.
Problems to be solved by the Invention
Currently, the continuous molding of compact disk substrates which are generally used must be suspen
Boykin Terressa M.
Sherman & Shalloway
Teijen Chemicals
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