Optical disk substrate and molding material therefor

Details Optical disk substrate and molding material therefor Optical disk substrate and molding material therefor

2002-06-25

2004-04-20

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, C264S219000, C359S107000, C369S047360, C369S059110, C428S064200, C528S198000

Reexamination Certificate

active

06723824

ABSTRACT:

DETAILED DESCRIPTION OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical disk substrate and a molding material therefor. More specifically, it relates to an optical disk substrate that is formed of an aromatic polycarbonate resin and can maintain high reliability for a long period of time, and a molding material therefor.
2. Prior Art
For a transparent substrate of an optical information recording medium that is a recording medium for recording and/or reproducing information with a laser beam, such as an audio disk, a laser disk, an optical disk memory, a magneto-optical disk, that is, for an optical disk substrate, generally, a polycarbonate resin is used, which is excellent over other resins in moldability, mechanical strength, transparency, and the like. However, the polycarbonate resin having the above excellent properties has a defect that it is easily hydrolyzed at a high temperature under a high humidity to decrease its molecular weight and impact strength. Further, it has a defect that its reliability that should extend for a long period of time is impaired, since a substrate made thereof is caused to have white spots when left at high temperatures under high humidity for a long period of time. At present, further, in substrate materials for high-density optical disks typified by DVD-ROM, DVD-Video, DVD-Audio, DVD-R and DVD-RAM as digital versatile disks (DVD), it is being required to satisfy higher-degree longer-term reliability.
As a method for producing a polycarbonate resin, there is known an interfacial polycondensation method in which a dihydroxy compound and phosgene are directly reacted or a melt-polymerization method in which a dihydroxy compound and carbonate diester are allowed to undergo an ester interchange reaction under heat and under reduced pressure. Of these methods, the later melt-polymerization method has an advantage that a polycarbonate resin can be produced at a low cost as compared with the former interfacial polycondensation method.
Generally, a method for producing an aromatic polycarbonate according to a conventional melt-polymerization method uses, as a catalyst component, a metal catalyst such as an alkali metal compound or an alkaline earth metal compound. For example, JP-A-8-59975 includes a description concerning a method for producing an aromatic polycarbonate according to the melt-polymerization method.
Disk substrates for an optical disk, a laser disk, etc., are generally produced by injection molding, and a molding temperature is a high temperature of 300° C. or higher. Further, a continuous production is required, so that the polycarbonate resin is required to have high thermal stability. However, an aromatic polycarbonate resin obtained by a melting method in the presence of the above metal catalyst is sometimes partially pyrolyzed during melt-molding due to a residual metal catalyst, and the aromatic polycarbonate resin is poor in thermal stability. Further, a disk is caused to have white spots in its substrate when left at high temperatures under high humidity for a long period of time, and it has a defect that its reliability that should extend for a long period of time is impaired. In recent years, the disk substrates are increasingly required to have further improved performances including a solution to the above problem.
Meanwhile, for applying an aromatic polycarbonate resin to an optical disk substrate, it is proposed to decrease a gelled substance content in the resin to a specific range.
That is, it is described in JP-A-2-135222 that a gelled substance is present in an aromatic polycarbonate resin and that the content of the gelled substance is decreased to a specific range. In the above known technique, the gelled substance present in the resin causes a refractive index anomaly in an optical use (particularly, a use for an optical disk), so that the number of gelled substances is limited to 50 pieces or less per kg of the resin. The above gelled substance refers to a substance that remains on a filter having openings having a diameter of 20 &mgr;m each when a solution of the resin in methylene chloride is filtered. The above known technique is specifically intended for application to a resin obtained by a method in which an aromatic dihydroxy compound and phosgene are reacted in an organic solvent such as methylene chloride (generally referred to as “solution polymerization method”). That is, a resin according to the above solution polymerization method is obtained in the form of a powder, and when the powder is pelletized by extrusion with an extruder, the resin suffers a heat hysteresis in the extruder. The above known technique is intended for limiting the amount of gelled substances that occur during such an occasion to a specific range.
According to studies made by the present inventors, it has been found that, when a resin powder obtained by the above solution polymerization method is melt-pelletized and when pellets are molded into a disk substrate, the number of refractive index anomalies of the disk substrate to be formed is decreased by decreasing the number of gelled substances in the pellets. The present inventors have made further studies and have found that a disk substrate whose gelled substance content is decreased by the above known method shows a decrease in the number of refractive index anomalies immediately after its molding, but that when it is held for a long period of time, particularly, when it is held under high humidity at a high temperature for a long period of time, white spots occur in the substrate and impede reading and reproducing the recorded information. While the cause therefor is not clear, it is presumably caused by inherent impurities such as a catalyst (e.g., sodium, etc.) and an organic solvent (e.g., methylene chloride) used in the solution polymerization method and oligomers.
PROBLEMS TO BE SOLVED BY THE INVENTION
Meanwhile, the present inventors have studied application of an aromatic polycarbonate resin obtained by a reaction between an aromatic dihydroxy compound and a carbonate diester (also generally called “melt polymerization method”) to disk substrates.
In the above melt polymerization method, a resin suffers heat hysteresis at a high temperature for a long period of time beyond comparison in the process of the polymerization as compared with the above solution polymerization. Therefore, there occur a large amount of undissolved substances that are insoluble in methylene chloride solvent while they are not removable through a filter of an extruder. The present inventors have studied these undissolved substances and have found that the behavior thereof differs from that of the above gelled substance involved in the solution polymerization method. The types and numbers of the undissolved substances in a resin obtained by a melt polymerization method are larger than the types and number of those in a resin obtained by a solution polymerization method. Studies have been made with regard to influences of the types and number of the undissolved substances on refractive index anomalies and the formation of white spots found after holding for a long period of time.
As a result, it has been found that the number of luminous undissolved substances generated by irradiation with specific wavelength (wavelength of 380 nm), of the undissolved substances in a resin, has something to do with the number of white spots that occur after the holding for a long period of time and that the number of white spots to occur can be decreased to a tolerance limit or less by decreasing such specific undissolved substances to a specific number or less.
That is, according to the studies by the present inventors, the tolerance range of the number of the undissolved substances that emit light by irradiation with a wavelength of 380 nm, in a resin obtained by a melt polymerization method, is 100 pieces or less per kg of the resin. While this tolerance range is broader than the tolerance range (50 pieces or less) of gelled substances in the above known technique, it is

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