WO2000075253A1

WO2000075253A1 - Adhesive composition and optical disc using the same

Info

Publication number: WO2000075253A1
Application number: PCT/US2000/011422
Authority: WO
Inventors: Sana Fujii
Original assignee: 3M Innovative Properties Company
Priority date: 1999-06-07
Filing date: 2000-04-27
Publication date: 2000-12-14
Also published as: JP2000345111A; EP1183316A1

Abstract

The invention aims to provide an adhesive composition which is useful for direct bonding of optical disc substrates, and can control corrosion of a metallic film, particularly an aluminum film, in a durability test of a product optical disc under elevated temperature and high humidity conditions, and also to provide an optical disc having controlled corrosion under elevated temperature and high humidity. The adhesive composition of the invention is an adhesive composition comprising (1) a urethane acrylate having a poly (tetramethylene glycol) skeleton, (2) an acrylic ester having a hydroxyl group in its molecule, and (3) a photoinitiator, the composition comprising 40% by weight or more of the urethane acrylate (1) based on the sum of weights of the urethane acrylate (1) and the acrylic ester (2).

Description

ADHESIVE COMPOSITION AND OPTICAL DISC USING THE SAME

The present invention relates to an adhesive composition for an optical disc, and more particularly to an adhesive composition useful in the manufacture of an optical disc used as an optical storage medium obtained by laminating transparent substrates and provided with at least one reflecting metallic film, as in a DND (digital video disc or digital versatile disc). CDs (compact discs), DNDs, and other optical discs are used extensively as recording media in a wide variety of fields, from audio and video recordings to computer- related storage. With such optical discs, digital signals recorded on the disc are sensed as variations in the reflection intensity of semiconductor laser light emilted by a read/write optical head, and the recorded data is read. Such discs can be read-only, recordable/nonerasable, or both recordable and erasable.

These optical discs are manufactured by a process in which an aluminum or other very thin (several tens of nanometers) metallic film layer is formed a reflecting layer by vapor deposition or sputtering on the surface (signal-recording surface) of a polycarbonate or other transparent substrates provided with pits that correspond to the recorded digital signal. In the case of a CD, the signal surface is further provided with a protective layer.

In a DND structure, on the other hand, two components, each of which is half the thickness of the CD substrate, are used as such transparent substrates, and these are laminated such that their signal-recording surfaces face inward. With such a DVD, the signal can be recorded on only one of the two laminated substrates, or it can be recorded on both substrates. Another feature of DNDs is that two recording layers can be provided to a single substrate because recorded signals from two signal-recording surfaces can be read from one side by moving the object lens of the optical head for reading recorded signals in the focusing direction (in the direction of propagation of laser light), in which case the reflecting film on the recording surface of the first layer (layer on which the readout laser light impinges first) is formed from gold or another semitransparent material.

Consequently, DNDs employ the following systems for reading recording signals: those in which a single layer is read on one side, those in which two layers are read from each side, those in which two layers are read from one side, and those in which each of the two layers are read from each side.

Aluminum or other metallic films formed on the signal surface of an optical disc substrate are affected by the oxygen and moisture contained in the air, and the characteristics of the films tend to deteriorate markedly. Specifically, an aluminum film affected by oxygen or contained moisture is gradually corroded and rendered transparent beginning at the disc edge, ultimately losing its reflecting characteristics. Thermoplastic resins or resins based on acrylate resins or epoxy resins have therefore been conventionally used as protective coating agents for metallic films. Epoxy ultraviolet- curing resin compositions used for this purpose are described in Japanese Patent Laid- open 2-107630, and acrylate compositions are described in Japanese Patent Laid-open 4-264166. Using such protective coating agents complicates the manufacturing process because an adhesive is applied and substrates are laminated after a protective coating has been formed on the signal surface of an optical disc substrate. Another drawback is that conventional discs are difficult to adapt to the need for higher density and minimal pit width.

Adhesives that allow substrates provided with signal surfaces to be directly bonded without the use of such protective coatings and that allow optical discs with minimal aluminum deterioration to be obtained are described, for example, in Japanese Patents Laid-open 59-71317, 63-167442, 9-31416, 10-8018, 10-102007, 10-67977, and

10-130602.

The adhesive described in Japanese Patent Laid-open 59-71317 is a photosetting adhesive composition containing a prescribed hydrocarbon compound that has two or more acryl or methacryl groups. This composition affords better adhesive power or the like between the substrates of an optical disc.

The adhesive described in Japanese Patent Laid-open 63-167442 is a composition consisting of hydrocarbon polyol-based polyurethane. Imparting flexibility to the composition per se yields an adhesive that can change its shape in conformity with deformation (warping) of the optical disc substrates. The adhesive described in Japanese Patent Laid-open 9-31416 is a photosetting adhesive composition containing as its essential components an ultraviolet-curing compound and a photopolymerization initiator whose molar absorption coefficient at wavelengths of 360-450 nm is 400 or higher.

The adhesive described in Japanese Patent Laid-open 10-8018 is an adhesive composition containing an ultraviolet-curing resin and a photopolymerization initiator obtained by combining a photopolymerization initiator whose maximum molar absorption coefficient at wavelengths of 360 nm or lower is 5000 or higher, and a photopolymerization initiator whose maximum molar absorption coefficient at wavelengths of 360 nm or higher is 500 or higher. The resulting cured products have adequate adhesive power and the like. The adhesive described in Japanese Patent Laid-open 10-102007 is a composition having at least a prescribed bisacylphosphine oxide compound and a prescribed acylphosphine oxide compound.

A common feature of the adhesives disclosed above is that they are directly applied to optical disc substrates and are used to bond the substrates together. There is, however, no particular provision concerning their corrosion of metallic films.

The adhesive described in Japanese Patent Laid-open 10-67977 is disadvantageous in that a very expensive fluorene-skeleton acrylate is used as the starting material, that the resulting cured product has low impact resistance at room temperature because of the extremely high glass transition temperature (Tg) thereof, and the like. In addition, the durability evaluation conditions for the adhesive described in this official publication are comparatively mild (70 °C, 50% relative humidity), and the time period is short (96 hours), rendering such a durability evaluation inadequate.

The adhesive composition described in Japanese Patent Laid-open 10-130602 contains a mixture of compounds containing unsaturated groups (A), an organic peroxide (B), and a curing promoter (C) as essential components, and a photopolymerization initiator (D) as an optional component. It is reported that no visible pinholes or other defects form in vapor-deposited aluminum films when the aforementioned adhesive composition is allowed to stand for 100 hours at 80 °C and 85% relative humidity. In this case, however, the evaluation time is too short (100 hours) to yield a reliable durability evaluation, and the adhesive is difficult to use in the sense that low productivity results, the adhesive layer cannot maintain its thickness, and other problems occur because the curing rate tends to vary depending on the particular combination of organic peroxide and curing promoter, and because curing tends to proceed rapidly during mixing or application when the curing rate is too high, and a considerable post-lamination standing time is needed when the curing rate is low.

An adhesive used for bonding optical disc substrates is generally required to have adequate adhesion characteristics, physical characteristics (particularly transparency, impact characteristics, and applicability to substrates), and curing characteristics during bonding. The adhesive is also required to have the quality of not producing, or resisting production of, corrosion of a metallic film during an environmental resistance test (durability test) when the adhesive is used for direct application to a recording surface provided with an aluminum or other metallic film. Specifically, an adhesive for bonding optical disc substrates must have the following characteristics:

(1) Adhesion Characteristics

Prescribed adhesive power must be achieved with respect to the surface of polycarbonate, aluminum, gold, or another material constituting an optical disc with a cured film thickness of 1-100 μm, and preferably about 50 μm.

(2) Impact Characteristics

Disc substrates must not separate from each other in a drop test.

(3) Adhesive Curing Characteristics

A photosetting adhesive must be able to bond substrates under exposure to ultraviolet light lasting no more than 1 minute, and preferably 1-30 seconds.

(4) Transparency

Adequate transparency to laser light (for example, red semiconductor laser light with a wavelength of 650-780 nm) must be afforded.

(5) Corrosion of Metallic Films It is inadmissible for aluminum films with a thickness of approximately several tens of nanometers (for example, about 50 nm) to be corroded. In particular, no corrosion is permissible in an aluminum film over a period of 500 hours or longer during a durability test in an 80 °C environment with 90% relative humidity.

(6) Applicability to Disc Substrates Must have a viscosity of 500-10,000 mPa • s, and preferably 1000-

5000 mPa ^■ s, at room temperature in the absence of a solvent, and form a cured film of about 1-100 μm when directly applied by spin coating to the surface of a disc substrate provided with a metal layer.

Adhesives for bonding optical disc substrates that would have all these required characteristics have yet to be developed, and conventional adhesives still have problems in terms of corrosion of metallic films, impact characteristics, curing treatment conditions during bonding, or the like.

For example, the adhesives described in Japanese Patents Laid-open 10-67977 and 10-130602 are tested only briefly in respect of corrosion of aluminum films in durability tests (and, in the case of Japanese Patent Laid-open 10-67977, the testing conditions per se are extremely mild), and are not adequate for durability evaluation purposes. The adhesive described in Japanese Patent Laid-open 10-67977 has poor impact resistance at room temperature because of the extremely high glass transition temperature (T_g) of the cured product, and the adhesive described in Japanese Patent Laid-open 10-130602 has poor adjustability of the curing rate. In particular, none of the optical disc adhesives currently available on the market can be regarded as adhesives that remain non-corrosive toward aluminum films over a period of 500 hours or longer during an environmental resistance test in an 80 °C environment with 90% relative humidity.

Thus, none of the conventional adhesives used for direct bonding of optical disc substrates devoid of protective coating layers for the reflecting metallic film has all the required characteristics. In particular, improvements can still be made concerning corrosivity in relation to reflecting metallic films such as aluminum films.

With the foregoing in view, it is an object of the present invention to provide an adhesive composition which is useful for direct bonding of optical disc substrates whose reflecting metallic films are devoid of protective coating layers, and which can control corrosion of a metallic film, particularly an aluminum film, in an extended durability test of a product optical disc under elevated temperature and high humidity conditions. It is also an object of the present invention to provide an optical disc having controlled corrosion of a metallic film under elevated temperature and high humidity, obtained by laminating substrates with the aid of such an adhesive composition.

The adhesive composition of the present invention comprises (1) a urethane acrylate having a poly (tetramethylene glycol) skeleton, (2) an acrylic ester having a hydroxyl group in its molecule, and (3) a photoinitiator, characterized in that the composition comprises 40% by weight or more of the urethane acrylate (1) based on the sum of weights of the urethane acrylate (1) and the acrylic ester (2).

The optical disc of the present invention is an optical disc comprising disc substrates laminated using an adhesive, and at least one reflecting metallic film, wherein the adhesive is an adhesive composition comprising (1) a urethane acrylate having a poly (tetramethylene glycol) skeleton, (2) an acrylic ester having a hydroxyl group in its molecule, and (3) a photoinitiator, characterized by being obtained by the curing of an adhesive composition comprising 40% by weight or more of the urethane acrylate (1) based on the sum of weights of the urethane acrylate (1) and the acrylic ester (2).

Preferably, the urethane acrylate ( 1 ) is an oligomer made up of, in addition to the poly (tetramethylene glycol) skeleton moiety, at least moieties derived from tetramethylxylene diisocyanate, hydroxyethyl acrylate, and ethylene glycol.

It is also preferable for the acrylic ester (2) to be one or more of hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxybutyl acrylate, and hydroxyphenoxypropyl acrylate.

Figure 1 illustrates an optical disc pertaining to one aspect of the present invention.

Figure 2 illustrates an optical disc pertaining to another aspect of the present invention. Key to Numbers

10, 20 optical discs

11, 11', 21, 21'. disc substrates

12, 24 reflecting metallic films

13, 25 adhesive layers 22 semitransparent film

The principal component of the adhesive composition of the present invention is a urethane acrylate having a poly (tetramethylene glycol) skeleton. An oligomer prepared at least from tetramethylxylene diisocyanate, hydroxyethyl acrylate, and ethylene glycol components in addition to the poly (tetramethylene glycol) skeleton moiety should preferably be used as the urethane acrylate contained as the principal component in adhesive compositions of the present invention useful for controlling the corrosion of a reflecting metallic film, and an aluminum film in particular, in a durability test performed under elevated temperature and high humidity conditions for applications involving lamination of optical disc substrates. Oligomers whose weight-average molecular weight is about 4000-7000 and the mixing molar ratio of the poly (tetramethylene glycol), tetramethylxylene diisocyanate, hydroxyethyl acrylate, and ethylene glycol is about 62: 15: 14:9 may be cited as examples of the urethane acrylates suitable for use in the present invention. Such urethane acrylate oligomers are commercially available from Nihon Gosei Kagaku under the trade name UN-6100B, and can be expressed by the following general formula:

The molecular weight of such a urethane acrylate oligomer and the mixing ratios of the constituent components of the oligomer may be varied broadly as long as the above- described characteristics required of the optical disc are attained.

In the adhesive composition of the present invention, an acrylic ester having a hydroxyl group in its molecule and acting as a reactive diluent is used in addition to the principal component (urethane acrylate having a poly (tetramethylene glycol) skeleton). Examples of appropriate acrylic esters include hydroxyethyl acrylate, 2-hydroxypropyl acrylate, 4-hydroxybutyl acrylate, and 2-hydroxy-3-phenoxypropyl acrylate. Hydroxyethyl acrylate, 2-hydroxypropyl acrylate, and 4-hydroxybutyl acrylate are even more preferred as acrylic esters. The reason that these are preferred is that they yield uniform compositions during mixing when the adhesive composition of the present invention is prepared, and that the viscosity of the composition can be readily adjusted.

In the adhesive composition of the present invention, the principal component (urethane acrylate having a poly (tetramethylene glycol) skeleton) must be contained in an amount of 40% by weight or more, based on the sum of weights of the urethane acrylate and the acrylic ester having a hydroxyl group in its molecule. At less than 40% by weight, the product cannot endure for 500 hours or longer in a durability test at 80 °C and 90% relative humidity. It is unsuitable for the urethane acrylate having a poly (tetramethylene glycol) skeleton to be present in an amount of more than 90% by weight, based on the sum of weights of the urethane acrylate and the acrylic ester having a hydroxyl group in its molecule, because in this case excessively high viscosity results, application is impaired, and adhesion to metallic films is compromised. A more preferred content of the urethane acrylate having a poly (tetramethylene glycol) skeleton is 50-80% by weight, based on the sum of weights of the urethane acrylate and the acrylic ester having a hydroxyl group in its molecule.

The adhesive composition of the present invention can be cured and formed into an adhesive layer on an optical disc by performing photopolymerization with the aid of a photopolymerization initiator or by performing thermopolymerization with the aid of a thermopolymerization initiator. Unlike thermopolymerization, photopolymerization- induced curing makes it possible to prevent disc substrates from developing deformation

(mainly warping) and other heat-induced defects during curing. Consequently, it is preferable to perform curing based on photopolymerization. Commercially available products may commonly be used as photopolymerization initiators. Darocur 1173 manufactured by Ciba-Geigy can be cited as a typical example of such a commercially available photopolymerization initiator.

An aspect may also be considered in which the adhesive composition of the present invention is used as a second-generation acrylic adhesive (SGA), in which case organic peroxides and curing promoters are used. In an SGA, a mixture of urethane acrylate (1) and acrylic ester (2) are divided into two components, one a component (referred to hereinbelow as "liquid A") containing an organic peroxide, and the other a component

(designated as "liquid B") containing a curing promoter, which two are mixed together for use as an adhesive. Alternatively, either of the aforementioned components (for example, liquid A) may be applied to one of the disc substrates, the other component (for example, liquid B) applied to the other disc substrate, and the disc substrates then laminated together.

The adhesive composition of the present invention may also contain optional thickeners, plasticizers, dispersants, flame retardants, fillers, colorants, antioxidants, thermal aging inhibitors, UN absorbers, silane coupling agents, and various other additives commonly used in adhesive compositions.

The adhesive composition of the present invention can be easily prepared by uniformly mixing the following mandatory components with the aid of a common mixing means: a urethane acrylate having a poly (tetramethylene glycol) skeleton, an acrylic ester having a hydroxyl group in its molecule, and a photopolymerization initiator, as well as any other optional components, added as needed.

The adhesive composition should preferably be applied to an optical disc substrate by spin coating. To use spin coating, the viscosity of the coating solution should be adjusted to about 500-10,000 mPa • s, and preferably about 1000-5000 mPa • s. When the aforementioned UN-6100B available from The Nippon Synthetic Chemical Ind. Co., Ltd. or an equivalent product is used as the urethane acrylate (1) having a poly (tetramethylene glycol) skeleton, using hydroxyethyl acrylate, 2-hydroxypropyl acrylate, 4-hydroxybutyl acrylate, or a mixture thereof as the acrylic ester (2) having a hydroxyl group in its molecule can make it easier to obtain a viscosity that falls within the appropriate range.

Application of the adhesive composition is not limited to spin coating and may be accomplished using other methods. The thickness of the cured adhesive layer formed from the adhesive composition should preferably be 1-100 μm. For DND 9, for example, the thickness of the adhesive layer is set to 55 + 15 μm in accordance with the standards described in the DND Book. The disc substrates coated with the adhesive composition should preferably be laminated under vacuum degassing. This approach is effective for rapid gas removal from the applied adhesive composition and for preventing gas bubbles from remaining between the substrates of the finished optical disc.

The optical disc of the present invention comprises disc substrates laminated by means of an adhesive, and at least one reflecting metallic film. As schematically shown in Figure 1, in its simplest aspect, the optical disc 10 of the present invention comprises two disc substrates 11, 11' of transparent plastic and a reflecting metallic film 12 made of a metal such as aluminum, for example, and formed on the signal-recording surface 11R of one of the disc substrates 11 , yielding a structure in which the disc substrates 11, 11 ' are laminated through the agency of an adhesive layer 13 formed from the adhesive composition of the present invention, with the reflecting metallic film 12 disposed on the inside. Minute pits (not shown) formed in correspondence to recorded digital signals are present on the surface (signal-recording surface 11R) of the substrate 11 in contact with the reflecting metallic film 12, and the presence or absence of pits corresponding to recorded digital signals is read out by an object lens (not shown) by sensing the presence or absence of reflected light from the reflecting metallic film 12 as it is illuminated with readout laser light traveling in the direction of arrow A in the drawing. An optical disc of this aspect corresponds to the previously described type of disc in which data is read from a single layer on one side.

It will be apparent that the previously described type of optical disc in which data is read from a single layer on each side can be obtained by modifying the aspect in

Figure 1 , providing to substrate 11 ' pits and a reflecting metallic layer analogous to those provided to substrate 11 , and illuminating substrate 11 ' with laser light directed from the direction opposite that indicated by A in the drawing.

Figure 2 depicts another aspect of the optical disc. The optical disc 20 pertaining to this aspect comprises two disc substrates 21, 21'; a semitransparent film 22 made of gold or the like and provided to one of the disc substrates 21; and an aluminum reflecting metallic film 24 on the other disc substrate 21'. The two disc substrates 21 and 21' are laminated through the agency of an adhesive layer 25 formed from the adhesive composition of the present invention, with the semitransparent film 22 and the reflecting metallic film 24 disposed on the inside. In this aspect, pits (not shown) corresponding to recorded digital signals are formed on a signal-recording surface 21R in contact with the semitransparent film 22 of the substrate 21 , and on the surface (signal-recording surface 25R) in contact with the reflecting metallic film 24 of the adhesive layer 25. Moving the object lens (not shown) of an optical head for reading recorded signals in the focusing direction (in the direction of propagation of laser light, indicated by A in the drawing) allows signals recorded on (signal-recording surface 21R) to be read by sensing light reflected from the semitransparent film 22, and signals recorded on (signal-recording surface 25R) to be read by sensing light reflected from the reflecting metallic film 24 following passage through the semitransparent film 22. Thus, this aspect allows signals on the two recording layers formed on the optical disc 20 to be read from one side. Specifically, the optical disc pertaining to this aspect corresponds to the previously described type of disc in which two layers are read from one side.

The optical disc of the present invention sustains very little corrosion of the metallic layers (particularly aluminum layers) when evaluated by means of durability tests under elevated temperature and high humidity conditions (80 °C, 90% relative humidity). When an adhesive used in the lamination of disc substrates contains traces of impurity ions or the like derived from starting materials or the like, the metallic reflecting layers must be provided with a protective coating because these impurities cause corrosion in aluminum and other thin metallic films. The adhesive composition of the present invention is characterized in that such thin metallic films experience very little corrosive action even in the presence of considerable amounts of impurity ions or other components introduced during the handling or synthesis of starting materials. In addition, an adhesive layer formed from the adhesive composition of the present invention, unlike a layer formed from a composition used as a common protective coating agent, can provide optical discs that are resistant to impact-induced breakage because this adhesive layer not only exhibits high adhesive power to aluminum, gold, polycarbonates, and various other materials used in the manufacture of optical discs, but also has high shock absorbing capabilities in relation to falls and other types of impact sustained by an optical disc. The adhesive composition of the present invention also satisfies the previously described characteristic requirements for applications involving the bonding of substrates to optical discs. Examples

The present invention will now be described in further detail through examples. It is apparent, however, that the present invention is not limited to these examples.

In these examples, the following acrylate oligomers, acrylate monomers, and photopolymerization initiators were used.

Oligomers Oligomer A: Poly (tetramethylene glycol)-based urethane acrylate (UN-6100B, manufactured by The Nippon Synthetic Chemical Ind. Co., Ltd.)

Oligomer B: Polyester-based urethane acrylate (UV-3000B, manufactured by The Nippon Synthetic Chemical Ind. Co., Ltd.)

Oligomer C: Polycaprolactone-based urethane acrylate (UX-4101, manufactured by Nippon Kayaku Co., Ltd.) Oligomer D: Propylene oxide-modified bisphenol A diacrylate (LIGHT

ACRYLATE BP-4PA, manufactured by Kyoeisha Chemical Co. Ltd.)

Oligomer E: Polycarbonate-based urethane acrylate (UN-9200A, manufactured by Negami Chemical Industrial Co. Ltd.)

Monomers HPA: 2-Hydroxypropyl acrylate (LIGHT ESTER HOP-A, manufactured by

Kyoeisha Chemical Co. Ltd.)

PEA: Phenoxyethyl acrylate (BISUKOOTO [1] # 192, manufactured by Osaka Organic Chemical Ind.)

HBA: 4-Hydroxybutyl acrylate (4-hydroxybutyl acrylate manufactured by Mitsubishi Chemical Industries)

HPPA: 2-Hydroxy-3-phenoxypropyl acrylate (Epoxy Ester M-600A, manufactured by Kyoeisha Kagaku)

IB A: Isobornyl acrylate (LIGHT ACRYLATE IB-XA, manufactured by Kyoeisha Chemical Co., Ltd.) Photopolymerization Initiator

Dl 173: Darocur 1173, manufactured by Ciba-Geigy The adhesives in Examples 1-6 and Comparative Examples 1-9 formulated according to the compositions shown in Tables 1 and 2 (for example, the "oligomer A/HPA/D1173 = 80/20/1" in the composition of Example 1 shown in Table 1 refers to the action of mixing oligomer A, HPA, and Dl 173 in a weight ratio of 80:20: 1) were applied by spin coating to the sputter film side of DND 9 half discs (opaque discs obtained by sputtering polycarbonate substrates with aluminum, and semitransparent discs obtained by sputtering polycarbonate substrates with gold), the components were laminated using DLB-9900 (a DND bonding apparatus manufactured by Sumitomo 3M), and the products were illuminated, on the side of the half disc provided with a thin metallic film, with ultraviolet light (for 20 seconds at 360 nm; UN illuminance: about

30 mW/cm^) from a high-pressure mercury UN lamp to cure the adhesive.

The optical disc samples thus obtained were allowed to stand at room temperature for half a day or longer, placed in a 80 °C oven at 90% relative humidity, taken out 100, 250, or 500 hours later, inspected, and graded by durability. The grading results were represented as follows:

O: No particularly visible changes in aluminum film

Δ: Slight roughness of aluminum film noted x: Considerable deterioration and penetration pitting of aluminum film noted The grading results are shown in Tables 1 and 2. Table 1

Table 2

The tables indicate that accelerated durability tests performed for 500 hours at 80 °C and 90% relative humidity produced no visible changes in the aluminum films of discs obtained by bonding substrates with the adhesive compositions of Examples 1-6, which comprised a poly (tetramethylene glycol)-based urethane acrylate (oligomer A) and an acrylic monomer (HPA, HBA, HPPA) having hydroxyl groups, and in which the content of oligomer A constituted at least 40% of the total weight of the oligomer and acrylic monomer. By contrast, 500-hour testing produced visible roughness in the aluminum films of discs obtained using the adhesive composition of Comparative Example 1, which comprised oligomer A and the acrylic monomer HPA and in which the content of oligomer A was less than 40% of the combined content of the oligomer and acrylic monomer. In addition, it took 250 hours or less for accelerated tests performed at 80 °C and 90% relative humidity to produce corrosion in the aluminum films of discs fabricated using adhesive compositions that contained poly (tetramethylene glycol)-based urethane acrylates (oligomers A) but were devoid of acrylic monomers having hydroxyl groups (as in Comparative Examples 2 and 3), compositions that contained acrylic monomers having hydroxyl groups but were devoid of polyether-based urethane acrylates (oligomers A) (as in Comparative Examples 4, 5, 6, and 8), or compositions that contained neither polyether- based urethane acrylates (oligomers A) nor acrylic monomers having hydroxy groups, as in Comparative Example 7. Furthermore, the disc of Comparative Example 9, which was fabricated using a typical optical disc substrate adhesive available on the market, and the disc of Comparative Example 10, which was a commercially available DND 9 product, were both found to have undergone corrosion over part of their aluminum films when visually inspected 250 hours after the start of the same accelerated durability test.

Thus, in accordance with the present invention, it is possible to use an adhesive composition that can ensure direct bonding of optical disc substrates and can control corrosion of a metallic film, particularly an aluminum film, in a durability test of a product optical disc performed for 500 hours under elevated temperature and high humidity conditions (80 °C and 90% relative humidity). Using this adhesive composition makes it possible to provide an optical disc having controlled corrosion of a metallic film under elevated temperature and high humidity.

The present invention is not limited to DNDs or other optical discs and can be adapted to magnetooptical (MO) discs and the like.

Claims

What is claimed is:

1. An adhesive composition comprising (1) a urethane acrylate having a poly(tetramethylene glycol) skeleton, (2) an acrylic ester having a hydroxyl group in its molecule, and (3) a photoinitiator, characterized in that the composition comprises 40% by weight or more of the urethane acrylate (1) based on the sum of weights of the urethane acrylate (1) and the acrylic ester (2).

2. The adhesive composition of claim 1, wherein the urethane acrylate (1) is an oligomer made up of, in addition to the poly (tetramethylene glycol) skeleton moiety, at least moieties derived from tetramethylxylene diisocyanate, hydroxyethyl acrylate, and ethylene glycol.

3. The adhesive composition of claim 1 or 2, wherein the acrylic ester (2) is one or more of hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxybutyl acrylate, and hydroxyphenoxypropyl acrylate.

4. An optical disc comprising disc substrate bonded by an adhesive and at least one reflecting metallic film, characterized in that the adhesive is obtained by cure of the adhesive composition of any one of claims 1 to 3.