Stock material or miscellaneous articles – Circular sheet or circular blank
Reexamination Certificate
2000-01-28
2002-09-03
Evans, Elizabeth (Department: 1774)
Stock material or miscellaneous articles
Circular sheet or circular blank
C428S064400, C428S065200
Reexamination Certificate
active
06444285
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an adhesive used when bonding two discs, at least one of which has an information recording layer, to each other, and more particularly relates to an ultraviolet curable composition.
The present application is based on Japanese Patent Application No. Hei 10-147000 and Japanese Patent Application No. Hei 10-147001, the contents of which are incorporated herein by reference.
BACKGROUND ART
Optical discs having a bonded structure which have hitherto been well known include, for example, magneto optical discs capable of recording and reproducing information. A magneto optical disc has a transparent substrate of polycarbonate, etc., on which an information recording material composed of, for example, a TbFeCo alloy, etc., and provided on both sides thereof with a protective layer composed of silicon nitride, etc., is laminated. An information recording layer is formed by further forming on the outermost layer a thin film mainly composed of aluminum as a reflecting layer for a laser beam for reading information. If needed, a protective layer composed of an ultraviolet curable resin is further formed on the thin film composed mainly of aluminum. In one case, this is used as it is as a single plate, and in another case, the single plates are bonded via an adhesive such that the information recording layers oppose each other, before they are used.
As the adhesive, a hot-melt type adhesive and an adhesive composed of an ultraviolet curable resin are used.
Thus, an optical disc having a conventional bonded structure in which both of the two discs have an information recording layer is of such a structure that “thin films of the same kinds of metal” composed mainly of aluminum oppose each other via an adhesive. Japanese Patent Publications Nos. 2553171 and 2555178 disclose technologies relative to ultraviolet curable adhesives suitable for the optical discs.
A digital video disc or digital versatile disc (DVD), a new type of optical disc having a bonded structure, is produced by a method of bonding two discs of which at least one disc has an information recording layer, and use of an ultraviolet curable composition as an adhesive is being studied.
In the case of read-only DVDs, the structure of the bonded two substrates may be one of the following types or the like, which can be selected properly depending on the application: a type in which polycarbonate substrates are used which are each provided on one surface thereof with irregularities called pits which correspond to information to be recorded and further, as a reflecting film for a laser beam for reading information there, an aluminum layer, for example, is formed, to provide an information recording layer (DVD-10); a type in which one of such substrates is formed with a semitransparent film of gold, silicon nitride or the like to provide an information recording layer (DVD-9); a type in which as one of the substrates a transparent polycarbonate substrate having no information recording layer is used (DVD-5); and a type in which each substrate has two information recording layers composed of a layer of aluminum, etc., and a semitransparent film of gold, silicon nitride or the like, and two such substrates are bonded (DVD-17).
Of these, a DVD-9 disc is of a structure in which “thin films of different kinds of material” such as aluminum and gold or silicon nitride are opposed through an adhesive. When the ultraviolet curable compositions as described in Japanese Patent Publications Nos. 2553171 and 1555178 are used as an adhesive, the durability of the discs is insufficient, and they are very weak in adhesive force to the gold or silicon nitride in the semitransparent film, so that the compositions described in the above patents are poor in serviceability as an adhesive for DVDs.
When gold is used as a semitransparent film for DVD-9, the structure becomes such that “different types of metal” are opposed to each other through an adhesive.
Different metals, when immersed in aqueous solutions, have intrinsic potentials, and the contact of or the electrical connection of metals having different potentials results in formation of a battery, and the metal having a lower potential is corroded. This is well known and the details thereof are described in, for example, “Denki Kagaku Hou (Electrochemical Method) (published by Kodansha).”
The standard electrode potentials of various compounds in aqueous solutions are described in detail in, for example, Denki Kagaku Binran (Manual of Electrochemistry) (ed. Denki Kagaku Kyokai (Electrochemical Association), published by Maruzen Co., Ltd.); for the Al
3+
/Al reaction on an aluminum electrode, the potential is −1.68 V, and for the Au
3+
/Au reaction on an aluminum electrode, the potential is +1.50V.
Therefore, for example, when a bonded disc of the DVD-9 type having thin films of aluminum and of gold is placed in a high temperature and high humidity environment, under the condition in which these metal thin films are electrically connected to each other, the corrosion of the aluminum, which has a lower potential, is promoted. This is specific to DVD-9 discs, which have metal thin films of different types. Accordingly, the prior art adhesive for optical discs directed to discs having thin films of the same kinds of metal composed mainly of aluminum has not been designed taking such electrochemical corrosion mechanisms into consideration, so that there has been the problem of insufficient durability when the DVD-9 discs are used without modification.
That is, a problem to be solved by the present invention is to provide an ultraviolet curable composition having excellent durability in DVD-9 discs by making a design capable of inhibiting the phenomenon of corrosion specific to not only the case where a semitransparent film composed of an inorganic compound such as silicon nitride is used but also to the case where a semitransparent film of a metal such as gold is used and to provide an ultraviolet curable composition used as an adhesive for optical discs which can be used advantageously for all the formats including DVD-5, DVD-10, and DVD-17.
DISCLOSURE OF THE INVENTION
The present inventors have carried out intensive study to solve the above problem, and, as a result, have found that when the specific conductivity of pure water in which a cured coating film of an ultraviolet curable composition has been immersed is equivalent to or below a specified value, the durability of the bonded disc increases, and that this effect is particularly remarkable in DVD-9 discs having thin films of different kinds of metal, and thus the present inventors achieved the present invention.
That is, the present invention provides an ultraviolet curable composition for optical discs, the composition containing an ultraviolet curable compound and a photopolymerization initiator and being used when bonding two substrates which have information recording layers, respectively, on each outermost layer of which a thin film of different type of material is formed, wherein the ultraviolet curable composition is characterized in that water in which a cured coating film has been immersed has a specific conductivity of 100 &mgr;S/cm or less.
BEST MODE FOR CARRYING OUT THE INVENTION
In optical discs of the present invention, the specific conductivity of water in which a cured coating film, which corresponds to a cured adhesive, has been immersed is defined as follows.
First, about 1 g of a cured coating film is peeled and is weighed exactly to four decimal places.
Then, 100 g of pure water is put in a 250 ml brown polymer bottle, and further the cured coating film weighed as above is added. As the pure water, pure water which is initially adjusted to 18 M&OHgr;•cm is used.
Further, the polymer bottle containing this sample is placed in an oven at 80° C. and taken out after 96 hours, and the specific conductivity (A) of water in which the cured coating film has been immersed is measured at 25±1° C.
The calculations are carried out according to the follow
Armstrong Westerman & Hattori, LLP
Dainippon Ink and Chemicals Inc.
Evans Elizabeth
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