Stock material or miscellaneous articles – Circular sheet or circular blank
Reexamination Certificate
2001-08-06
2003-11-11
Mulvaney, Elizabeth (Department: 1774)
Stock material or miscellaneous articles
Circular sheet or circular blank
C428S065200
Reexamination Certificate
active
06645596
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a bonded optical disc, and more particularly, relates to a bonded optical disc containing silver or an alloy primarily composed of silver in an information recording layer, and to an ultraviolet curable composition which is suitable for production of a bonded optical disc.
BACKGROUND ART
DVDs (“digital video discs” or “digital versatile discs”) are produced by a method in which two discs, of which at least one disc has an information recording layer, are bonded together, and an ultraviolet curable composition is generally used as an adhesive.
In the case of read-only DVDs, the structure of the two bonded substrates may be of one of the following types or the like, which can be selected properly depending on the application: a type (DVD-10) in which polycarbonate substrates, are used which are each provided on one surface thereof with irregularities, called “pits”, which correspond to recorded information and further, as a reflecting film for a laser beam for reading information, an aluminum layer, for example, is formed, to provide an information recording layer; a type (DVD-5) in which, as one of the substrates, a transparent polycarbonate substrate having no information recording layer is used; a type (DVD-9) in which one of such substrates is formed with a semitransparent film of a metal composed primarily of gold, a silicon compound, or the like to provide an information recording layer; and a type (DVD-18) in which each substrate has two information recording layers on one surface, and two such substrates are bonded together.
The semitransparent film of DVD-9 is primarily composed of gold, a silicon compound or the like. However, gold has a drawback in that it is disadvantageous in cost because of its very high price, while the silicon compound has a drawback in that formation of a film is very difficult.
On the other hand, silver or an alloy composed primarily of silver is suitable for use as a semitransparent film for DVD-9 because it has a lower price compared to gold and can be easily formed into a film. However, since silver or an alloy composed primarily of silver is chemically unstable, when using a conventional ultraviolet curable adhesive for DVDs, it causes deterioration of the surface when left in high temperature and high humidity environments for a long time, thus leading to errors in reading recorded information, poor appearance or the like.
Japanese Patent Application, First Publication No. Hei 9-11626 discloses that a fluororesin coating having a low water absorption rate is formed as a protective layer for silver or an alloy composed primarily of silver. However, the fluororesin coating has poor adhesion and is not ultraviolet-curable, thus making it difficult to use it as it is in a current DVD production process using an ultraviolet curable adhesive. Therefore, the fluororesin coating is not suitable for practical use.
As described above, although silver or an alloy composed primarily of silver is quite suitable for use in a semitransparent film for DVDs, an ultraviolet curable composition having sufficient protective characteristics has not yet been developed, and therefore, DVDs using silver or an alloy composed primarily of silver have not yet been commercialized.
DISCLOSURE OF THE INVENTION
An object to be achieved by the present invention is to provide DVDs having durability equal to that of existing DVDs of the DVD-5 and DVD-10 types as well as those of the DVD-9 type using gold or a silicon based compound as a semitransparent film, by using the above-described disc wherein a thin film of silver or an alloy composed primarily of silver is formed, and to provide an adhesive composition which does not cause deterioration of the thin film of silver or an alloy composed primarily of silver.
The present inventors have carried out intensive research to solve the above problems, and as a result, have found that durability equal to that of a conventional optical disc using gold or a silicon based compound can be realized by observing the increase in the error rate after an environmental test in a bonded optical disc using a thin film of silver or an alloy composed primarily of silver as a semitransparent film in an outermost layer of an information recording layer, and setting the error rate in a specified range, and that a remarkable effect can be obtained by setting a water absorption rate and a water-vapor permeability of a cured coating, and thus the present inventors have achieved the present invention.
That is, the present invention provides an optical disc comprising two substrates having information recording layers, respectively, a thin film of silver or an alloy composed primarily of silver being formed on an outermost layer of the information recording layer of at least one substrate, said two substrates being bonded together by an ultraviolet curable composition, wherein the optical disc is characterized in that ratios of error rates before and after an environmental test at 80° C. and 85% RH for 96 hours are 10 or less.
As the substrate used in the optical disc of the present invention, those typically used as a substrate for optical disc can be used and, particularly preferably, a polycarbonate substrate can be used.
The “alloy composed primarily of silver” used in the optical disc of the present invention includes, for example, silver incorporated with several % of palladium, copper, or the like therein.
As used herein, ratios of error rates are defined as ratios of PI error rates before and after an environmental test (error rate after the environmental test/error rate before the environmental test). As used herein, the term “error rates” has the same meaning as that used by those skill in the art in the case of evaluating an optical disc such as a DVD or the like.
The environmental test is conducted by allowing an optical disc to remain in an environment of 80° C. and 85% RH for 96 hours. In this environmental test, an apparatus such as a thermo-hygrostat or the like can be used.
The optical disc of the present invention is preferably a DVD (“digital video disc” or “digital versatile disc”), for example, a read-only DVD such as one of the DVD-5, DVD-10, DVD-9 or DVD-18 type, a writable DVD such as a DVD-R, or a rewritable DVD such as a DVD-RAM, DVD-RW, or DVD+RW, and the optical disc is more preferably a DVD-9. The optical disc of the present invention is not limited to these DVDs and may be, for example, an optical disc in which a cover layer having a thickness of about 0.1 mm is formed by bonding a transparent film or coating an ultraviolet curable composition on a substrate having a thickness of about 1.1 mm, an optical disc having a structure comprising three substrates having a thickness of 0.6 mm or less bonded to each other or the like.
If there is no change in the error rate before and after the environmental test, PI error rate ratios before and after the environmental test are 1. In general, since the error rates are determined by characteristics of each disc substrate, the error rates vary considerably, thus making it difficult to set to a fixed value for a bonded type disc. Such a relative ratio makes possible discussion on a uniform basis.
Furthermore, the present invention provides an ultraviolet curable composition for an optical disc, which is used in the production of an optical disc comprising two substrates having information recording layers, respectively, a thin film of silver or an alloy composed primarily of silver being formed on an outermost layer of the information recording layer of at least one substrate, said two substrates being bonded together by an ultraviolet curable composition, wherein the ultraviolet curable composition is characterized in that a water absorption rate of the cured coating film is. 2.0 wt % or less and a water-vapor permeability at a film thickness of 50 &mgr;m is 70 g/m
2
·day or less. Preferably, the ultraviolet curable composition is composed primarily of an ultraviolet curable compound composed of a (meth)acrylate having a alicyc
Kitsunai Takashi
Murakami Kazuo
Dainippon Ink and Chemicals Inc.
Mulvaney Elizabeth
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