Optical disc and process for producing optical disc

Details Optical disc and process for producing optical disc Optical disc and process for producing optical disc

2001-04-25

2003-09-30

Mulvaney, Elizabeth (Department: 1774)

Stock material or miscellaneous articles

Circular sheet or circular blank

C428S064400, C428S065200

Reexamination Certificate

active

06627287

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to techniques for bonding two discs together, at least one of which has an information recording layer. More specifically, the present invention relates to an ultraviolet curable composition which can be used as an adhesive agent, an optical disc in which the ultraviolet curable composition is used, and a process for producing optical discs.
2. Description of Related Art
A DVD (“digital video disc” or “digital versatile disc”) is manufactured by a method of bonding two discs together, at least one of which has an information recording layer. Use of an ultraviolet curable composition as an adhesive in this method has been considered.
In the case of read-only DVDs, the structure of the two bonded substrates may be one of the following types or the like, which can be properly selected depending on the application: a type in which two substrates are used which are each provided on one surface with irregularities called pits which correspond to information to be recorded, and further, as a reflecting film for a laser beam for reading information thereon, an aluminum layer, for example, is formed, so as to provide an information recording layer (DVD-10); a type in which, as one of the substrates, a transparent substrate having no information recording layer is used (DVD-5); a type in which one of the substrates is provided with a semitransparent reflecting layer made of gold or the like, and the other substrate is provided with an aluminum reflecting layer (DVD-9); and a type in which two substrates are bonded together, each of which has two information recording layers on one side (DVD-18).
A polycarbonate, a polymethylmethacrylate, an amorphous polyolefin, and the like are common as the material for the substrates of these optical discs.
At present, gold is mainly used for the semitransparent films for DVD-9 discs. However, since gold is a very expensive material, the use of silicon or silicon compounds has been considered for the purpose of reducing the cost (“silicon compounds” hereinafter refer to “silicon or silicon compounds” unless otherwise defined). Examples of silicon compounds which may be used for this purpose are silicon and silicon nitride. However, a problem was found that due to poor adhesion of a thin film of a silicon compound to an ultraviolet curable composition, which is used as an adhesive, in comparison with a metallic thin film such as gold, the substrate on which the film was formed and to which the other substrate was bonded using the ultraviolet curable composition may come off after being left for a long time under conditions of high temperature and high humidity. Therefore, an adhesive agent made of an ultraviolet curable composition which provides long-term stability of signals and which allows production of highly reliable bonded discs has been desired.
BRIEF SUMMARY OF THE INVENTION
The object of the present invention is to provide an ultraviolet curable composition which is most suitable for an adhesive agent for optical discs which,have a bonded structure. In particular, the object of the present invention is to provide an adhesive composition which can bond a substrate having a reflecting layer made of a silicon compound to which adhesion of conventional ultraviolet curable compositions is insufficient. Another object of the present invention is to provide an optical disc, as well as a production method therefor, in which the adhering surfaces have sufficient adhesion, and in particular, the adhering surface of a reflecting layer which is made of a silicon compound has strong adhesion.
As a result of the present inventors' diligent research aimed at attaining the above objects, the present inventors have found that the addition of a silane coupling agent containing an epoxy silane or a (meth)acrylsilane to the ultraviolet curable composition improves the adhesion of the bonded disc, particularly when the disc is a DVD-9 disc in which a silicon compound is used as a semitransparent film, and thus the present inventors achieved the present invention.
According to the present invention, an ultraviolet curable composition is provided which comprises a silane coupling agent containing an epoxysilane or a (meth)acrylsilane, the ultraviolet curable composition being useful as an adhesive agent in the production of optical discs in which two plastic substrates are bonded together, at least one of which is provided with a thin film of a reflecting layer as the outermost layer so that the surface of the reflecting layer provides an adhering surface. In addition, according to the present invention, an optical disc in which at least one of the substrates has a thin film of a reflecting layer and a production process for the optical disc are provided, wherein an ultraviolet curable composition which is used as an adhesive agent comprises a silane coupling agent containing an epoxysilane or a (meth)acrylsilane.
The “epoxysilane” and the “(meth)acrylsilane” in the present invention are silane coupling agents which have an epoxy group or a (meth)acryl group, which forms a chemical bond with an organic material, as well as a group, such as a methoxy group, an ethoxy group, and a silanol group, which forms a chemical bond with an inorganic material such as glass and metal.
In the ultraviolet curable composition according to the present invention, in order to obtain good adhesion, it is preferable that the amount of the silane coupling agent in 100 parts by weight of the ultraviolet curable composition be 0.1 to 5 parts by weight.
In general, some cases are known in which the addition of a silane coupling agent to an adhesive agent improves the adhesion thereof. However, a silane coupling agent containing an epoxysilane or a (meth)acrylsilane has never been used in an ultraviolet curable composition for bonding substrates for an optical disc. A silane coupling agent containing an epoxysilane or a (meth)acrylsilane, which is used in the present invention, not only improves adhesion for the above purpose, but also has an excellent durability under conditions of high temperature and high humidity. Such a silane coupling agent is suitable as an adhesive agent for a thin film of a silicon compound, on which water drops are readily formed under the above conditions due to its strong water repellency.
Accordingly, use of the ultraviolet curable composition of the present invention allows production of optical discs such as DVD-9 discs, which have a semitransparent reflecting layer made of a silicon compound on one of the disc substrates, having desirable properties such as excellent durability and reliability, which are evaluated by tests in a high temperature and high humidity.
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described in the following, on the basis of preferable modes.
In the preparation of the ultraviolet curable composition of the present invention, a silane coupling agent, an ultraviolet curable compound, and a photopolymerization initiator are used as essential components.
Preferable examples of silane coupling agents for the present invention are as follows: examples of epoxysilanes are &bgr;-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, &ggr;-glycidoxypropyltrimethoxysilane, and &ggr;-glycidoxypropylmethyldiethoxysilane; and an example of (meth)acrylsilane is &ggr;-methacryloxypropyltrimethoxysilane.
Such silane coupling agents are sold, for example, under the trade names of “KBE403” (&ggr;-glycidoxypropylmethyldiethoxysilane), “KBM403” (&ggr;-glycidoxypropyltrimethoxysilane), “KBM502” (&ggr;-methacryloxypropylmethyldimethoxysilane), and “KBM503” (&ggr;-methacryloxypropyltrimethoxysilane) by Shin-Etsu Silicones Co., Ltd, and can be obtained easily.
For the ultraviolet curable compound, monofunctional (meth)acrylate or polyfunctional (meth)acrylate can be used as a polymerizable monomer component. One type of ultraviolet curable compound alone or a combination of two or more types of ultraviolet curable compounds may be used. For

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