Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – From phenol – phenol ether – or inorganic phenolate
Reexamination Certificate
2003-01-21
2004-12-21
Boykin, Terressa (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
From phenol, phenol ether, or inorganic phenolate
C264S176100, C264S211240, C264S219000, C359S642000, C359S107000, C428S064200
Reexamination Certificate
active
06833427
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a molding material for optical use, for example, a resin molding material suitable for the manufacture of an optical recording medium for recording various information signals such as voice signals or image signals and to a substrate obtained therefrom.
DESCRIPTION OF THE PRIOR ART
Optical disks for recording and reproducing information through exposure to a laser beam, such as digital audio disks (so-called compact disks), optical video disks (so-called laser disks), recordable disks, opto-magnetic disks and phase-change disks have been implemented.
Out of these, the compact disks and the laser disks are read-only memory (ROM) type optical disks. Generally these optical disks have pits corresponding to information signals on a transparent resin substrate and an aluminum (Al) reflection layer as thick as 40 nm or more formed on the substrate. In these optical disks, a change in reflectance caused by an optical interference produced by a pit is detected to reproduce an information signal.
Meanwhile, the recordable optical disks are optical disks to which desired information can be written by a user and the opto-magnetic disks and the phase-change type disks are RAM (random access memory) type optical disks to which desired information can be written repeatedly.
That is, the recordable optical disks comprise a transparent resin substrate and a recordable recording layer thereon whose optical properties are changed irreversibly by exposure to a laser beam or whose surface becomes uneven by exposure to a laser beam. This recording layer is made from a cyanine-based, phthalocyanine-based or azo-based organic dye which is decomposed by heat from exposure to a laser beam to change its optical constant and causes the deformation of the substrate through its volume change.
The opto-magnetic disks are rewritable optical disks in which information can be written and erased by a user repeatedly and which comprise a vertically magnetized layer having a magneto-optical effect (for example, Kerr effect), such as a Tb—Fe—Co amorphous alloy thin film, formed on a transparent resin substrate. A recording pit is formed in this opto-magnetic disk by magnetizing a micro-area corresponding to an information signal of the vertically magnetized layer in an upward direction or a downward direction. 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 layer.
The phase-change disks are rewritable disks like the opto-magnetic disks and a Ge—Sb—Te phase-change material which is initially crystalline and becomes amorphous upon exposure to a laser beam is used therein, for example. In this recording layer, a recording pit is formed by changing the phase of a micro-area corresponding to an information signal and the difference in reflectance between an amorphous portion corresponding to the pit and other crystalline portion is detected to reproduce the information signal.
The above opto-magnetic disks and phase-change disks have a four-layer structure consisting of a recording layer, transparent dielectric layers sandwiching the recording layer and an aluminum (Al) reflection layer formed thereon in order to prevent the oxidation of the recording layer and increase the modulation degree of a signal by multiple interference in most cases. The dielectric layers are a silicon nitride layer or Zn—SiO
2
mixed film.
Studies are being made energetically to use the above optical disks for recording digital images. A digital versatile disk (DVD) has been developed as such an optical disk.
This DVD has the same diameter of 120 mm as CD and is designed to record image information equivalent to one movie and reproduce high-quality image information equivalent to that of the current TV.
A recording capacity 6 to 8 times greater than CD is required to record such image information on an optical disk. Therefore, the wavelength of a laser beam is reduced to 635 to 650 nm in DVD compared with 780 nm of CD and the numerical aperture NA of an objective lens is increased to 0.52 or 0.6 in DVD compared with 0.45 of CD in order to reduce the track pitch and the shortest recording mark length of pits, thereby raising recording density.
Out of these, an increase in the numeral aperture NA of the object lens reduces the warp tolerance of a disk substrate. The thickness of the substrate of DVD is reduced to 0.6 mm which is much smaller than 1.2 mm of CD in order to shorten the distance of a laser beam passing through the disk substrate so as to compensate for a reduction in the warp tolerance. To compensate for a reduction in the strength of the disk resulted by the reduced thickness of the substrate, as described in JP-A 6-274940 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”), a bonding-together structure that another substrate is bonded on the recording layer formed on the substrate is adopted. The recording layer of the laminated optical disk may be a ROM type recording layer, recordable type recording layer or RAM type recording layer which is used in the above single-substrate structure.
Further, the bonded optical disk is available in a one-side bonded optical disk in which only one side is used and both-side bonded optical disk in which both sides are used.
A polycarbonate resin which is excellent in moldability, strength, light transmission and humidity resistance is widely used in the above optical disk resin substrates.
Since an optical disk makes use of micro-irregularities formed on a resin substrate to record or reproduce information by a laser beam, when a defect existent in the resin substrate is larger than each irregularity, it has a great influence on the reliability of information recording or reproduction. Therefore, the production of a silver streak which is such a defect must be suppressed.
The causes of producing a silver streak include the hydrolysis of a resin pellet due to incomplete drying, thermal decomposition in a cylinder and the inclusion of air from a hopper side. As means of suppressing the production of a silver streak caused by the inclusion of air, it has been proposed to limit the length of a pellet (JP-A 7-52272) and to limit the length and long diameter of a pellet (JP-A 11-35692). However, no explanation is given of the bulk density of pellets in these proposals and the bulk density cannot be set to a suitable range simply by limiting the length and long diameter of each pellet in these proposals.
Problem to be Solved by the Invention
The present invention has been made in view of the above problems and intensive studies have been made on the problems. As a result, it has been found that the production of a silver streak is suppressed by setting the bulk density value of pellets to a certain range and that use of the pellets can reduce the metering time, can shorten the molding cycle, prevents a pellet feed trouble and smoothen molding operation. The present invention has been accomplished by these findings.
Means for Solving the Problems
According to the present invention, there are provided a polycarbonate resin molding material for optical use which is pellets formed from a polycarbonate resin and having (1) a bulk density of 0.72 kg/l (liter) or more, and an optical disk substrate formed from the material.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
In the present invention, a detailed description is given of pellets of an aromatic polycarbonate resin as a molding material suitable for molding a substrate for improving the reliability of information recording and reproduction as an optical disk substrate for digital versatile disks (DVD) typified by CD-R, CD-RW, MO, DVD-ROM, DVD-Audio, DVD-R and DVD-RAM, particularly a high-density optical disk substrate for DVD.
The polycarbonate resin used in the present invention is generally obtained by reacting a diphenol with a carbonate precursor by an interfacial
Izumi Fumi
Kawano Shinzi
Boykin Terressa
Teijin Chemicals Ltd.
Wenderoth Lind & Ponack LLP
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