Stock material or miscellaneous articles – Circular sheet or circular blank – Recording medium or carrier
Reexamination Certificate
1999-12-21
2002-05-21
Buttner, David J. (Department: 1712)
Stock material or miscellaneous articles
Circular sheet or circular blank
Recording medium or carrier
C524S486000
Reexamination Certificate
active
06391418
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to substrate for information recording media. More particularly, it relates to an aromatic polycarbonate resin composition which can be used advantageously for the manufacture of OA equipment, automobile parts, optical disc substrates, transparent films and other molded articles which are required to have high transparency, high rigidity, faithful transcription of the fine surface configuration of the stamper to the article surface (which may hereinafter be referred to simply as “transcription ratio” (of the composition)), high proofness against warping and shrinkage after molding and vertical deviation when high-speed rotation. The present invention also relates to a substrate for information recording media such as, typically, optical discs and near field recording media, which is injection molded from an aromatic polycarbonate resin composition, and which is capable of correctly transcribing the grooves and pits of the information signals of the submicron order, has excellent flatness and rigidity, and is highly proof against warp and also minimized in vertical deviation when vibrated.
Aromatic polycarbonate resins have been popularly used as base material for producing various types of fine molded articles such as housings of OA equipment, automobile parts and optical disc-related parts, because of their many excellent properties such as high transparency, impact resistance, heat resistance, etc., and good dimensional stability of their moldings. Also, because of excellent moldability of the aromatic polycarbonate resins, various molding methods known in the art of molding of the thermoplastic resins, such as injection molding, extrusion molding, blow molding, etc., can be used for molding the said articles from the aromatic polycarbonate resins.
Especially the aromatic polycarbonate resins have been prevalently used as starting material for producing the optical disc substrates by injection molding. Since the polycarbonate resins used for the optical discs have relatively low molecular weight, these resins exhibit excellent fluidity and transcription ratio in the manufacture of the optical discs and have been popularly used for molding the substrates for various types of optical discs such as CD (compact disc), CD-R (recordable compact disc), MD (mini-disc), MO (magneto-optical disc), laser disc and DVD.
In recent years, however, in order to increase the signal density, it has been tried to reduce the thickness of the substrate itself or to deepen the pits and grooves of the information signal or to further reduce the interval (pitch) between these pits or grooves and therefore this has made it difficult to faithfully transcribe the stamper signal by use of the conventional polycarbonate resins for optical discs. Further, there has been developed recently an optical disc of the new recording system called near field recording or “first surface” recording system. According to this system, information is recorded and read directly from the surface thereof without mediation of the substrate.
A substrate for near field recording media is injection molded from an aromatic polycarbonate resin. Such information recording media are produced by a method in which using an injection mold called stamper on which the information signals comprising pits and grooves of the depth and pitch of the submicron order have been engraved, a molten thermoplastic resin is injected into the mold cavity attaching the stamper and the information signals are transcribed to the surface of the molded article (substrate).
Aromatic polycarbonate resins have been predominantly used for producing the said substrates for the following reason. This type of resin has high transparency, heat resistance, dimensional stability and impact resistance as well as good moldability, so that when it is used for injection molding the substrate, the fine jagged pattern (information signal) engraved on the stamper is accurately transcribed to the substrate (molded article) surface (good transcription ratio), and a high-quality substrate for information recording media proof against warping and having excellent flatness can be obtained.
Since the laser light is not pass through the inside of disc in near field recording system, transparency of the optical disc substrate is not required. But since a flying head is used, it is required to minimize warp of the substrate taken place during molding thereof and vertical deviation during high-speed rotation for avoiding contact of the flying head with the substrate. Further, because of high signal density, in case where a conventional polycarbonate resin for optical disc is used as base material, it needs to further raise the temperature of the injection mold for improving transcription ratio of the outerside region of the substrate, but rise of the mold temperature enhances the warping tendency of the produced optical disc substrate.
Another problem encountered in molding an optical disc substrate by injection molding is shrinking at the outer peripheral end of the substrate. As the flying head moves at high speed with its distance from the substrate being kept constant, it is imperative to minimize shrinkage at the outer peripheral end of the substrate. If the mold temperature is raised for improving transcription ratio of the stamper signal, its gap from room temperature is widened to encourage shrinkage caused by molding shrinkage. Further, since the polycarbonate resins per se are devoid of vibration-damping properties, use of a polycarbonate resin alone as base material gives rise to the problem of enlarged vertical deviation during high-speed rotation.
Various means, such as addition of a fluidity improver to the polycarbonate resin used as base material, have been proposed for improving transcription ratio of the high-density optical disc substrate, but there is yet available no idea that can satisfy all of the requirements for high transcription ratio, small warping, small shrinkage and excellent vibration-damping. A known method of affording vibration-damping properties to the optical disc substrate is to increase rigidity of the substrate. A typical method of increasing rigidity of the optical disc substrate is to add a filler such as glass fiber or other inorganic filler to the polycarbonate resin used as base material. But this method has the problem that the added filler might come out to the substrate surface to obstruct correct transcription of the signal. Therefore, it is not suitable to try to improve rigidity by adding a filler.
It is well known to add a thermoplastic elastomer to the polycarbonate resin used as base material for improving the vibration-damping properties of the high-density optical disc substrate (Japanese Patent No. 2,835,596). Addition of a thermoplastic elastomer to the polycarbonate resin can indeed improve the vibration-damping properties of the produced optical disc substrate, but it is incapable of improving transcription ratio and resistance against warping and shrinkage as well.
Further, there has been, in recent years, an increasing tendency to elevate the information recording density by, for instance, reducing the thickness of the substrate itself or deepening the pits and grooves of the information signal or by further reducing the interval (pitch) between the pits and/or the grooves for the purpose of increasing signal density of the substrate, but such tendency has added to the difficulty in effecting correct transcription of signal to the injection molded substrate. Further, requirement for flatness of the substrate, especially in the near field recording media, has intensified as so have the requirements for minimized warp of the substrate and suppressed vertical deviation when the substrate is vibrated.
When the thickness of the substrate itself is reduced, in case where fluidity of the resin used as base material for injection molding is low, transcription ratio of information signal tends to deteriorate, and also rigidity of the substrate tends to lower as the thick
Maruyama Hiroyoshi
Ueda Masaya
Buttner David J.
Dike, Bronstein, Roberts & Cushman, Intellectual Property Practi
Mitsubishi Engineering-Plastics Corp.
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