Optical information medium

Abstract: An optical information medium comprises a pair of discs (1 and 5) which are bonded to each other, an optical recording layer (12) provided on at least one of the discs (1 and 5), a reflecting layer (13) formed on an upper side of the optical recording layer (12). The optical information medium further includes an adhesive (11) for bonding the pair of discs (1 and 5) provided on the reflecting layer (13), wherein the adhesive (11) has a thickness ranging from 10 to 80 µm. A protecting layer (14) is provided on the upper side of the reflecting layer (13) wherein the pair of discs (1 and 5) may be bonded by the protecting layer (14). The optical recording layer (12), reflecting layer (13) and protecting layer (14) may be directly sequentially provided on the disc (1) but another substrate layer may be inserted between these layers. There is employed, as the adhesive (11), that including a reactive curing resin or a hot melt material, and it is preferable that the shrinkage rate of the adhesive (11) be less than 15%.

Description: The present invention relates to an optical information medium comprising a pair of discs which are bonded to each other, an optical recording layer provided on at least one of the discs, a reflecting layer formed on an upper side of the optical recording layer, an adhesive provided on the reflecting layer and bonding said pair of discs, wherein the adhesive has a thickness ranging from 10 to 80 µm, characterized in that the optical information medium further includes a non-bonding area to which the adhesive is not partially bonded which are provided at peripheries of center holes of the discs, wherein a boundary of the non-bonding area is not constant or fixed in distance from centers of the discs. A digital video disc (DVD) capable of recording and reproducing information with high-density has been now put into practical use associated with the recent development and practical utilization of a laser having a short wavelength. The optical information medium of this type can adopt a structure for bonding two pieces of discs. A read-only optical information medium having such a structure for bonding two surfaces of discs has a center hole at the center thereof and a clamping area at the outside thereof. A pair of discs which are bonded to each other include at least one transparent substrate comprising at one surface thereof, an information recording area formed outside the clamping area, pits serving as an information recording means formed on the information recording area, and a reflecting layer formed of a metal film and provided on the pit. Further, as an optical information medium capable of recording information, a tracking guide is formed on the information recording area of at least one substrate, and the substrate is coated with organic dye, etc. by means of spin-coating process, to form an optical recording layer, and a reflecting layer formed of a metal film is formed on the optical recording layer. Still further, the entire surfaces of a pair of discs are coated with reactive curing resin as an adhesive by means of a spin-coating process or a screen printing, and the coated surfaces are laid on top of the other to oppose each other, and two discs are bonded to each other by the adhesive set forth above. In a WORM (write once, read many times) optical information medium, there is employed a system for coating the transparent substrate with dye material such as an organic dye with a given thickness to form an optical recording layer, and forming a reflecting film such as a metal film on the optical recording layer. In such a WORM optical information medium, if the optical recording layer of the disc is coated with an adhesive such as a reactive curing resin by a spin coating process or a screen printing, etc., and two discs are bonded to each other by this adhesive, residual stress is generated in the jointing boundary between the different materials, whereby the discs are liable to warp and the adhesive surface is liable to flake owing to the warpage of the discs. Further, there occurs a problem that when a shock is applied to the discs, the discs are liable to flake, thereby deteriorate a shock resistance. Particularly, the flaking is liable to occur at the edges of two discs, which causes a problem of brittleness against the shock on the edges. Still further, since the adhesion between two discs is deteriorated, the moisture in the air is liable to enter the discs, which causes a problem of deterioration of the layer with moisture. If a reaction-sensitive resin is employed as an adhesive layer, the shrinkage involved in the curing occurs, and the stress generated thereby influences the recording layer, thereby distorting the recording layer. As a result, a variation of tracking signal and/or reflectance occurs'when the discs are reproduced with laser beam. The variation rate exceeds 30%, thereby obstructing the record and reproduction of the information. When the discs are bonded by the adhesive, one surface of one disc is coated with an adhesive, and another surface of another disc is overlaid on one disc, thereafter they are irradiated with light so as to cure the adhesive. When two discs are laid on top of the other, bubbles are liable to enter between two discs, and hence the stress is liable to occur by bubbles at the time of bonding the discs, and the magnitude of the stress increases. Accordingly, the influence of the stress upon the recording layer also increases, and there is a possibility that the optical disc is deformed to the extent to be recognized visually. A vacuum degassing apparatus is employed to prevent bubbles from entering the adhesive layer when two discs are bonded to each other, which makes the apparatus large-scaled, and hence increasing the operating process, which causes a problem of deterioration of productivity. EP-A-0330197 discloses an optical information medium according to the preamble of claim 1. With such an optical information medium it is possible that the adhesive is bond to the peripheral surfaces of the center holes as well as that the adhesive passes through the center holes or moves around the recording surfaces opposite to the bonding surfaces of the discs. It is the object of the invention to avoid above mentioned possibilities and oscillations of the discs of the time of high revolution. This object is achieved by the features according to the characterizing portion of claim 1. The optical information medium comprises a pair of discs 1 and 5 which are to be bonded to each other, an optical recording layer 12 which is formed on at least one of the discs 1 and 5 at the surface where the discs 1 and 5 are bonded and a reflecting layer 13 formed over the optical recording layer 12. The optical information medium according to the present invention has an adhesive 11 for bonding a pair of discs 1 and 5 by way of the optical recording layer 12 and reflecting layer 13 in the aforementioned optical information medium, and a thickness of the adhesive 11 ranges from 10 to 80 µm. A protecting layer 14 may be formed over the reflecting layer 13, and a pair of discs 1 and 5 are bonded to each other by way of the protecting layer 14. The optical recording layer 12, the reflecting layer 13 and the protecting layer 14 to be formed on the disc 1 are sequentially directly formed in this order, but another layer may be inserted between these layers. The reactive curing resin, or a hot melt material, or the like is used as the adhesive 11, particularly it is preferable that the shrinkage rate of the adhesive 11 be less than 15% when it cured by irradiation of UV ray. Since the thickness of the adhesive 11 ranges 10 to 80 µm, the coating layer of the UV curing resin adhesive for instance shows a sufficient viscosity, and even if bubbles are produced when the main surface of other disc contacts the coating layer, the viscose adhesive of the coating layer flows to fill up the bubble holes. Even if the viscose adhesive of the coating layer is cured by the irradiation of the UV rays while bubbles remain therein, the stress generated based on these bubbles can be reduced by the deformation caused by the plasticity and elasticity of the resin, thereby reducing the influence upon the recording layer. If the thickness of the adhesive 11 is less than 10 µm, the bubbles can not be filled up by the deformation of the adhesive 11. On the other hand, if the thickness of the adhesive 11 exceeds 80 µm, the curing speed of the adhesive 11 reduces so that the adhesive 11 is liable to be non-curable. As mentioned above, in the optical information medium of the present invention can reduce the stress even if the stress is generated owing to the bubbles produced in the adhesive layer. Further, the shrinkage of the adhesive 11 when it cures is small, the variation rate of the reflection and the variation of push-pull signal do not reach the critical level owing to the shrinkage of the adhesive 11 when it cures. The deformation of the adhesive when it cures is not recognized from the external appearance, and further the adhesive can be selected from an ordinary material without using the specially prepared material, and the large-scaled degassing apparatus is not required, so that the bonding of the discs by the adhesive can be performed in the ordinary bonding manner. Grooves 6 and 6 are defined in at least one of the discs 1 and 5 at the outer periphery of the adhesive surface thereof, and the outer peripheries of the discs 1 and 5 may be bonded to each other by the adhesive 11 filled in these grooves 6 and 6. In such an optical information medium, the adhesive 11 is filled in the grooves 6 and 6 to increase the bonding surface area by the adhesive 11 compared with a case where only the flat main surfaces of the discs 1 and 5 alone are bonded to each other by the adhesive 11, so that the resistivity of friction at the outer peripheries of the discs 1 and 5 increases by a so-called anchor effect. The grooves 6 and 6 may be defined in the outer peripheries of the discs 1 and 5 or may be defined in the center side from the outer peripheries of the discs 1 and 5. In the latter case, since the grooves 6 and 6 do not appear on the outer peripheral surfaces of the discs 1 and 5, they present the same outer peripheral surfaces in external appearance as an optical information medium having no grooves 6 and 6. A part 11a of the adhesive 11 bonding the discs 1 and 5 may be fixed to an outer peripheral surfaces of at least one of the discs 1 and 5. Even in such a optical information medium, the fixing surface area of the adhesive 11 increases because the adhesive 11 is fixed to the outer peripheral surfaces of the discs 1 and 5 compared with the case where the adhesive 11 is fixed to only the flat main surfaces of the discs 1 and 5, the resistivity of friction at the outer peripheries of the discs 1 and 5 increases by a so-called anchor effect. In this case, inclination surfaces 15 and 15 which are obtuse or acute relative to the bonding surfaces of the discs 1 and 5 may be formed on the outer peripheral surfaces of the discs 1 and 5 to which the part 11a of the adhesive 11 is fixed. If the inclination surfaces 15 and 15 which are obtuse relative to the bonding surfaces of the discs 1 and 5 are formed, the adhesive 11 is not necessary to protrude largely from the outer peripheral edges of the discs 1 and 5 so that adhesive 11 does not influence largely on the outer peripheral shape of the optical information medium. On the other hand, if the inclination surfaces 15 and 15 are acute relative to the bonding surfaces of the discs 1 and 5 are formed, the outer periphery of the adhesive 11 moves around the back side of the bonding surfaces of the discs 1 and 5 and it is bonded to the back side, enhances the anchor effect to obtain high resistivity of friction. According to an embodiment being not part of the present invention the discs 1 and 5 are bonded together by the adhesive 11 not only at an information recording area r on the bonding surfaces of the discs 1 and 5 but also at an inner information non-recording area i formed at the inner peripheries of the discs 1 and 5. That is, the entire surfaces of the discs 1 and 5 are bonded together by the adhesive 11 to extend from the center holes 4 to the outer most peripheries thereof. As a result, the entire surfaces of the two discs 1 and 5 including the information non-recording area i formed at the inner peripheries of the discs 1 and 5 are bonded to each other so that the resistivity of friction of the discs 1 and 5 increases and the adhesion between the discs 1 and 5 is enhanced. Particularly, since the adhesive 11 reaches the center holes 4 of the discs 1 and 5 and the edges of the center holes 4 of the discs 1 and 5 are bonded by the adhesive 11, the discs 1 and 5 hardly flake at the inner peripheries of the center holes 4, thereby enhancing the hermeticity between the discs 1 and 5. A groove 7 is defined between the center holes 4 of the discs 1 and 5 and the information non-recording area i at the bonding surfaces, and the adhesive 11 is filled in the groove 7, thereby increasing the fixing surface area of the adhesive 11, and hence the resistivity of friction increases at the inner peripheries of the discs 1 and 5 by a so-called anchor effect. If a rough surface 8 is formed between the center holes 4 of at least one of the discs 1 and 5 and the information non-recording area i at the bonding surfaces thereof, the fixing surface area of the adhesive 11 increases by the rough surface 8 so that the resistivity of friction at the inner peripheries of the discs 1 and 5 increases. Such a rough surface 8 may be character information which is engraved on the surfaces of the discs 1 and 5, for example a lot number or a product number. The discs 1 and 5 are bonded in principal at the entire surfaces which oppose each other. In this case, it is preferable that the surface including an area having no optical recording layer 12 is bonded to another disc 5 without forming the optical recording layer 12 on at least a part of the surface except the information recording area r of the disc 1. In other words, a marginal part where no optical recording layer 12 is formed at the outer periphery and/or inner periphery of the information recording area r in which the signal of the disc 1 is recorded, and the surface including this marginal part where the optical recording layer 12 of the disc 1 is provided may be bonded to another disc 5. The width of the marginal part of the disc 1 is more than 0.1 mm. It is more preferable that the width of the marginal part exceed 1.5 mm. In such an optical information medium, the optical recording layer 12 is not formed on the outside of the information recording area r of the disc 1, namely, on the marginal part outside the area whole information recording, and the marginal part is bonded directly by the adhesive 11 without interposing the optical recording layer 12. Accordingly, the discs 1 and 5 are bonded to each other with high resistivity of friction at the marginal part. Since the marginal part locates outside the information recording area r, the resistivity of friction adjacent to the edges of the discs 1 and 5 is enhanced, thereby protecting the information recording area r inside the edges. According to the present invention a non-bonding area m is formed at the peripheries of the center holes 4 of the discs 1 and 5 where the part of the adhesive 11 is not bonded without bonding the entire surfaces of the discs 1 and 5. In such an optical information medium, the non-bonding area m is formed on the peripheries of the center holes 4 and the adhesive 11 is not bonded onto the non-bonding area m so that the adhesive 11 is not bond to the peripheral surfaces of the center holes 4. When the discs 1 and 5 are coated with the adhesive 11, the adhesive 11 neither passes through the center holes 4 nor move around the recording surfaces opposite to the bonding surfaces of the discs 1 and 5. In such an optical information medium, the discs 1 and 5 are generally coated with the adhesive 11 and bonded thereby at the outer side thereof by a spin coating process. In this case, if the grooves 7 and 7 are defined in at least one of the bonding surfaces of the discs 1 and 5 at the area outside the non-bonding area m, when the discs 1 and 5 are coated with the adhesive 11, the adhesive 11 can stay in the grooves 7 and 7. As a result, it is possible to surely prevent the adhesive 11 from bonding to the non-bonding area m. Meanwhile, if a center hole 4' of the disc 5 is greater than the diameter of the center hole 4 of the disc 1, and it is also greater than the diameter of the non-bonding area m, the adhesive 11 does not enter the non-bonding are m when the discs 1 and 5 are coated with the adhesive 11 at the bonding surfaces thereof and they are laid on top of the other. Accordingly, it is also surely possible to prevent the adhesive 11 from bonding to the non-bonding area m. In this case, if the part of the adhesive 11 moves around and bonds to the peripheral surface of the center hole 4' of the disc 5 having a large diameter, the resistivity of friction is reinforced at this portion. The boundary of the non-bonding area m is not disposed at a fixed range from the centers of the discs 1 and 5 but indefinite. That is, the boundary of the non-bonding area m may be eccentric relative to the centers of the discs 1 and 5 or the boundary of the non-bonding area m may be of different shape. In such a manner, the maximum amplitude Q of a mechanical resonance frequency f 0 may be reduced, so that the oscillation of the optical information medium can be reduced at the high speed revolution thereof. Fig. 1(a) to 1(d) are longitudinal sectional views of a part of an optical information medium, which are cut partially, according to a first embodiment of the invention; Fig. 2(a) to 2(e) are longitudinal sectional views of a part of an optical information medium, which are cut partially, according to a second embodiment of the invention; Fig. 3 is a longitudinal partial sectional view of the optical information medium, which is cut partially, according to a further embodiment; Fig. 4 is a longitudinal partial sectional view of the optical information medium, which is cut partially, according to a further embodiment; and Figs. 5(a) and 5(b) are plan views showing an example of the non-bonding area of the optical information medium of the present invention. The optical information medium according to various embodiments will now be described in detail with reference to the attached drawings. One example of the optical information medium of the invention is of a WORM type having one surface record/reproduction structure formed by bonding two surfaces of discs. A material for the disc 1 includes, for example, glass and plastics such as epoxy resins, methacrylate resins, polycarbonate resins, polyester resins, polyvinyl chloride resins and the like. As a material for the disc 5 to be laminated thereto in the case of one-side recording type, there may be used, other than the above mentioned material, thermoplastic resins such as polyolefin, polyamide, polyvinyl alcohol, polyacetal, fluoroplastics and the like, as well as phenol resins, polyurethane resins, epoxy resins, amino resins, unsaturated polyester resins, alkyd resins, silicone resins and the like. An information recording area r is set or formed on one surface of the transparent disc 1 outside an inner peripheral side information non-recording area i. A tracking guide 3 comprising spiral grooves is formed on the information recording area r. The tracking guide 3 has a pitch which normally ranges from 0.74 to 0.8 µm. Further, an information recording layer 2 is provided on the main surface of the disc 1 on which the information recording area r is formed. For example, the disc 1 is coated with organic dye, etc. to provide an optical recording layer 12 by means of a spin coating process, etc., then a reflecting layer 13 made of a metal film is provided on the optical recording layer 12. The optical recording layer 12 and reflecting layer 13 form the information recording layer 2, and a portion where the optical recording layer 12 and reflecting layer 13 are provided is the information recording area r. A protecting layer 14 is provided on the entire surface of the main surface of the disc 1 so as to cover the reflecting layer 13. Further, an information non-recording area i is set or formed between the inner side of the information recording area r to a center hole of the disc 1. The optical recording layer 12 may be formed, for example, by spin coating a solution of cyanine dyestuff, azo dyestuff, and other organic dyestuff followed by drying. The optical recording layer 12 may further contain another compound such as a single state oxygen quencher, and a photo absorbing agent and the like. As a solvent for dissolving the above mentioned dyestuff, there may be used chloroform, dichloroethane, fluorinated solvent including fluorinated alcohol, etc., methyl ethyl ketone, dimethylformamide, methanol, toluene, cyclohexanone, acetyl acetone, diacetone alcohol, celloslves including methyl cellosolve, etc., dioxane and the like. In this case, an amount of cyanine dye to be mixed is preferably 1 to 10 % by weight. As the reflecting layer 13, it is exemplified a metal film such as gold, aluminum, silver and copper formed by evaporation, sputtering, and an alloy metal film of the combination thereof. As the protecting layer 14, it is exemplified a radioactive curing resin solvent such as a UV curing resin which is coated by a spin coating method and the resin is cured with UV light. The WORM optical information medium has the following concrete dimensions. The diameter of the center hole 4 of the disc 1, namely, the inner diameter of the disc 1 is 15 mm, and the outer diameter and the thickness of the disc 1 are respectively 120 mm and 0.6 mm. The tracking guide 3 and information recording layer 2 are provided on the disc 1 at the area ranging from 48 mm to 116 mm in diameter thereof to form the information recording area r. Further, another disc 5 is prepared in addition to the disc 1. Although another disc 5 is made of the same material as the disc 1 and has the same size as the disc 1, the tracking guide 3 and information recording layer 2 are not provided on the disc 5 although they are provided on the disc 1. It is needless to say it is possible that the information recording area r having the tracking guide 3 and information recording layer 2 are formed on another disc 5 like the disc 1. Thereafter, two discs 1 and 5 are bonded to each other. For example, one main surface of at least one of the discs 1 and 5 is entirely coated with reactive curing resin as an adhesive 11 by a normally employed coating means such as a spin coating process or screen printing process, gravure printing process, dipping process, a spray coating process, then the discs 1 and 5 are laid on top of the other while they oppose each other, thereafter the aforementioned reactive curing resin is cured. As a result, then main surfaces of the discs 1 and 5 are bonded to each other by the adhesive 11 which is formed when the reactive curing resin cures. In this case, the surface of the disc 1 on which the information recording layer 2 and the protecting layer 14 are provided is bonded to the main surface of another disc 5. As the adhesive layer, there are exemplified an adhesive made of curing resin or that made of hot melt material. If the UV curing resin is used as the curing resin adhesive, the protecting layer 14 of the disc 1 is coated thereof by a spin coating process, a screen printing, etc. then the disc 5 is laid on the disc 1, thereafter the UV curing resin is irradiated with the UV rays from the side of the disc 1 or disc 5. As a result, the adhesive is cured. It is also possible to use an electron curing resin or other radioactive curing resin other than the UV curing resin. In case of the adhesive layer formed of hot melt material, the protecting layer 14 of the disc 1 is coated with the melted adhesive by a roll coater, then the disc 5 is laid on the disc 1, and the adhesive layer is cooled to cure in this state. The adhesive 11 is shrunk when it cures and the shrinkage rate is less than 15%. As the means for making the shrinkage rate less than 15% is the selection of a resin material. For example, in the case of a radiation curing resin such as an UV curing resin, a reactive diluent, a photo initiator and the like may be added to a resin component. More precisely, a monomer and an oligomer of a polymerizable organic compound may be coated to the disc 1 followed by forming an adhesive layer through a crosslinking reaction. When the adhesive layer of an organic polymer is formed by the crosslinking reaction, it is convenient from a standpoint of workability to add a small amount of a reaction initiator and a reaction catalyst to a mixture of a monomer and an oligomer of an organic compound which has one or more than two reactive acrylonitrile radicals in a molecule, coat the liquid mixture on the disc and subject to a crosslinking reaction by ultraviolet or electronic radiation. However, a crosslinking process is not restricted to the above mentioned method and may include thermal crosslinking which is applied to an epoxy resin or an urethane resin. In the case of a hot-melt material, it is preferable that the melting temperature is, for example, 130 °C and the melting viscosity is, for example, 170,000 centipoise at 160 °C from a standpoint of coating workability. A practical material includes MELTORON 3S49 available from DIAPOND INDUSTRY Co., Ltd. Such a hot-melt material is quite easily used by simply cooling the material without dipping a surface thereof to be coated in a solvent or applying light irradiation for curing. It is necessary that the shrinkage rate of the adhesive is less than 15%. If the shrinkage rate exceeds 15%, the influence caused by the stress thereof affects the optical recording layer, thereby generating distortion whereby the reflectance of the laser beam is deteriorated to deform the optical information medium. The thickness of the adhesive 11 needs to range from 10 to 80 µm. If the adhesive 11 has such a thickness, the curing resin such as UV curing resin shows a viscosity at the coating layer like the UV curing resin. Even if bubbles are generated in the main surface of the coating layer when the main surface of another disc contacts the coating layer, the viscose adhesive flows to fill up the holes of the bubbles and also reduces the stress generated based on the bubbles by the plasticity and elasticity of the resin even if the viscose adhesive is solidified by the irradiation of UV rays while the bubbles remain therein, thereby reducing the influence of the stress against the recording layer. If the thickness of the adhesive is less than 10 µm, such a reduction cannot be performed while if the thickness of the adhesive exceeds 80 µm, the curing speed of the adhesive layer decreases so that the adhesive is liable not to cure. The thickness of the adhesive 11 preferably ranges from 30 to 70 µm. When the adhesive 11 is spread outward from the inner peripheral surfaces of the center holes 4 so as to move around the inner peripheral surfaces of the discs 1 and 5 to fix thereto, thereby enhancing shock absorbance at the center holes 4. For example, at least one surface of the discs 1 and 5 is turned upward, and UV curing resin which does not cure on the surface as the adhesive 11 drops on the surface so as to level the resin while the discs 1 and 5 rotate. The adhesive 11 drops at the portion adjacent to the center holes 4. Thereafter, the given surfaces of the discs 1 and 5 to be bonded to each other are laid on top of the other. As a result, the adhesive 11 spreads between the discs 1 and 5 owing to the pressure which the discs 1 and 5 receive or capillary phenomenon. At the time when the adhesive spreads between the discs 1 and 5 at the entire surface of the information non-recording area i, the discs 1 and 5 are rotated at high speed to wipe off surplus UV curing resin. Then the UV curing resin is irradiated with UV rays from one surface side of the transparent discs 1 and 5, to cure the adhesive so as to form the adhesive layer, whereby the discs 1 and 5 are brought into contact with each other and fixed by the adhesive 11. Considering the shock absorbance of the optical information medium, the hardness of the adhesive after curing is preferable to be higher than that of the substrate. Fig. 1 shows a part of an optical information medium according to a first embodiment of the invention. In Fig. 1(a), the groove 7 is defined in only the outer peripheral edge of the information recording layer 2, and the groove 6 is not defined in the disc 1. In Figs. 1(b) to (d), the grooves 6 and 6 are defined in the main surfaces of the discs 1 and 5 at the position close to the inner peripheral surface rather than the outer peripheral surface, and they do not appear on the outer peripheral surfaces of the discs 1 and 5. Accordingly, in the optical information medium shown in Figs. 1(b) to (d), it appears as if it had no outer peripheral surfaces like the optical information medium having no grooves 6 and 6 in external appearance. The grooves 6 and 6 of the discs 1 and 5 shown in Fig. 1(b) are rectangular. The grooves 6 and 6 of the discs 1 and 5 shown in Fig. 1(c) are semi-circular. The grooves 6 and 6 of the discs 1 and 5 shown in Fig. 1(d) have trapezoidal shape each having an inclination at one surface close to the centers of the discs 1 and 5. Particularly, in the optical information medium shown in Fig. 1(c) and (d), the adhesive is liable to flow into the grooves 6 and 6, and hence it is adapted for introducing the adhesive having high viscosity in the narrow grooves 6 and 6. Fig. 2 shows a part of an optical information medium according to a second embodiment of the invention. In Figs. 2(a) and (b), the part 11 a of the adhesive 11 moves around the outer peripheral surface 9 of the disc 1 and is fixed to the outer peripheral surface 9 but the adhesive 11 is bonded only to the outer peripheral surface 9 of the disc 1 where the information recording layer 2 is provided. Particularly, in Fig. 2(a), although the part 11a of the adhesive 11 is bonded to a part of the outer peripheral surface 9, but in Fig. 2(b) a part 11b of the adhesive 11 is bonded to the entire surface of the outer peripheral surface 9 of the disc. In Figs. (c) to (e), the inclination surfaces 15 and 15 are defined in the outer peripheral edges of the discs 1 and 5, and the part 11a of the adhesive 11 is bonded to the inclination surfaces 15 and 15. Particularly in Figs. 2(c) and (d), the inclination surfaces 15 and 15 of the outer peripheries of the discs 1 and 5 are defined in the bonding surface sides. That is, the inclination surfaces 15 and 15 are obtuse relative to the bonding surfaces of the discs 1 and 5. In Fig. 2(c), the part 11a of the adhesive 11 bonded to the edges of the inclination surfaces 15 and 15 protrude toward the outer peripheral sides while in Fig. 2(d), the part 11a of the adhesive 11 bonded to the edges of the inclination surfaces 15 and 15 form recesses at the outer peripheral sides. Meanwhile, in Fig. 2(e), the inclination surfaces 15 and 15 of the outer peripheries of the discs 1 and 5 are formed on the surfaces opposite to the bonding surfaces. That is, the inclination surfaces 15 and 15 have inclinations which are acute relative to the bonding surfaces of the discs 1 and 5. The part 11a of the adhesive 11 is bonded to both inclination surfaces 15 and 15. In an optical information medium shown in Figs. 3 according to another embodiment, no groove is defined in the discs 1 and 5 but the center hole 4' of-another disc 5 is larger than the center hole 4 of the disc 1. The diameter of the center hole 4' is substantially the same or greater than the outer diameter of the non-bonding area m. The portion corresponding to the outside portion of the center hole 4' alone of the bonding surfaces of the discs 1 and 5 is coated with the adhesive 11. The part of the adhesive 11 moves around the inner peripheral surface of the center hole 4'of the disc 5 and is fixed thereto, thereby reinforcing the resistivity of friction at the inner peripheral sides of the discs 1 and 5. In an optical information medium shown in Fig. 4 according to a further embodiment, the center hole 4' of another disc 5 is greater than the center hole 4 of the disc 1 like the embodiment of Fig. 3. The diameter of the center hole 4' of the large diameter of the disc 5 is substantially the same or greater than the outer diameter of the non-bonding area m. The groove 7 is defined in the outside of the non-bonding area m of the disc 1 having the center hole 4 of the small diameter. Only the portion corresponding to the outside of the groove 7 of the bonding surfaces of the discs 1 and 5 is coated with the adhesive 11. Since the adhesive 11 enters the groove 7 and stays therein, it hardly moves around the peripheral surface of the center hole 4' but the resistivity of friction is reinforced at the inner peripheral sides of the discs 1 and 5 owing to the adhesive 11 which entered the groove 7. Fig. 5 shows the shape of the non-bonding area m set forth above, wherein denoted by dotted lines show a boundary of the non-bonding area m which is hidden behind the disc 1 or disc 5. In the example of Fig. 5(a), the boundary of the non-bonding area m is eccentric relative to the centers of the discs 1 and 5. In Fig. 5(b), the boundary of the non-bonding area m is not circular, elliptical and polygonal, but ungeometrical irregular shape. In any case of Figs. 5(a) and (b), the boundary of the non-bonding area m is not constant or fixed in distance from the centers of the discs 1 and 5. Accordingly, the maximum amplitude Q of the mechanical resonance frequency F 0 becomes small, thereby decreasing the oscillation the discs at the time of high revolution of the optical information medium. The concrete examples of the invention will be now described with reference to concrete numeric values. (1 st Example) A carbocyanine dye having a trimethylene chain (NK 4321; available from Nippon Kankoshikiso Kenkyusho Co., Ltd.), represented by the following formula, was dissolved in 3 ml of diacetone alcohol to form a solution of 40 mg/ml concentration, and a solution of the cyanine dye thus prepared was coated on a polycarbonate substrate of 0,6 mm in thickness by spin coating process to form an optical recording layer comprising a layer of 100 mm thick of the optical absorption dye. A reflecting layer formed of gold is provided on the optical recording layer by a sputtering process, then the reflecting layer is coated with the UV curing resin (made of Dainippon Ink & Chemicals under the product name of SD211) by a spin coating process, and the UV curing resin is cured to form a protecting layer (5 µm) formed of the UV curing film, whereby a pair of discs are fabricated. The protecting layer of one disc is coated with an adhesive made of the UV curing resin (made of Dainippon Ink & Chemicals under the product name of SD318) by a spin coating process, and the protecting layer of another disc is permitted to contact this adhesive, then the adhesive is irradiated with UV rays through the contacted another disc, thereby curing the adhesive. The curing coating film of the UV curing resin employed by the adhesive layer has a pencil hardness (hand writing process of JISK5400 is used) of 2H at the temperature of 25°C, and the curing shrinkage rate is 8.5%, and the curing film of the adhesive layer has substantially the same hardness and shrinkage. In such a manner, the both surface recording type optical disc is obtained by bonding a pair of opposing substrates each having a main surface on which the optical absorbing layer, reflecting layer and protecting layer are provided, and the protecting layers of both substrates are bonded to each other by the adhesive layer. Upon visual inspection of the external appearance of the cured adhesive layer, bubbles are not mingled in the adhesive layer and no deformation of the optical disc is recognized. The variation rate of reflectance and the variation of push-pull signals (tracking error signal variation, hereinafter referred to as PP) of the optical disc are checked with an optical pick up (NA: 0.6, λ :638nm). As a result, the variation rate is less than 5% and the PP is less than 15%. The structure of the optical disc is shown in Table 1 and the result of test is shown in Table 3. The indication of "None" of "Mixing of Bubbles" in Table 3 shows that visible bubbles are not recognized. This is applied to the following examples. (2nd Example) The optical disc is fabricated in the same manner as the 1st Example except the thickness of the adhesive layer is 37 µm, and the test is carried out like the 1st Example. The result of test is shown in Table 3 and the structure of 2nd Example is shown in Table 1. It is possible to make the adhesive layer thick by any of a method of coating resin fluid in concentrated form, a method of decreasing the speed of rotation of a spin coating apparatus, or a method of increasing the number of times of coating. (3rd and 4th Examples) The optical disc of the 3rd and 4th Examples is fabricated in the same manner as the 1st Example except bubbles are mixed in the adhesive layer and the thicknesses of the adhesive layers are 37 µm and 35 µm, and the test is carried out like the 1st Example. The result of test is shown in Table 3 and the structures of the 3rd and 4th Examples are shown in Table 1. The indication of "Bubbles Each Having Diameter of Exceeding Several 100 µm Are Not Mingled" means that bubbles each having the diameter of less than several 100 µm may be mixed in the adhesive layer. This is applied to the following examples. Either the bubbles are mingled in the adhesive layer or not may be carried out by subjecting the coating layer to degassing or not under a vacuum when coating. (5th and 6th Examples) The optical disc of the 5th and 6th Examples is fabricated in the same manner as the 1st and 4th Examples except the protecting layer is not provided and the thicknesses of the adhesive layers are 65 µm and 67 µm, and the test is carried out like the 1st Example. The result of test is shown in Table 3 and the structures of the 5th and 6th Examples are shown in Table 1. (7th and 8th Examples) The optical disc of the 7th and 8th Examples is fabricated in the same manner as the 5th and 6th Examples except the thicknesses of the adhesive layers are 68 µm and 69 µm and the reflecting layer is provided by sputtering aluminum (Al) instead of that of Au, and the test is carried out like the 1st Example. The result of test is shown in Table 3 and the structures of the 7th and 8th Examples are shown in Table 1. (9th Example) The optical disc of the 9th Example is fabricated in the same manner as the 1st Example except the thickness of the adhesive layer is 35 µm and the reflecting layer is provided by sputtering aluminum (Al) instead of that of Au, and the test is carried out like the 1st Example. The result of test is shown in Table 3 and the structure of the 9th Example is shown in Table 1. (10th Example) The optical disc of the 10th Example is fabricated in the same manner as the 9th Example except the optical recording layer is coated with a silane solution (made of The Shin-Etsu Chemical Co., Ltd. under the product name of KR220) by a spin coating process to form a film, then an optical enhance layer having a thickness of 80 nm is formed with the thickness of the adhesive layer being 36 µm, and the test is carried out like the 1st Example. The result of test is shown in Table 3 and the structure of the 10th Example is shown in Table 1. (11th Example) The optical disc of the 11th Example is fabricated in the same manner as the 1st Example except solution of 20% of azo dyestuff is employed instead of cyanine dyestuff, and the thickness of the adhesive layer is 25 µm, and the test is carried out like the 1st Example. The result of test is shown in Table 3 and the structure of the 11th Example is shown in Table 1. (12th Example) The optical disc of the 12th Example is fabricated in the same manner as the 1st Example except the resin of the adhesive layer is changed from the UV curing resin (made of Dainippon Ink & Chemicals under the product name of SD318) to the UV curing resin (made of Dainippon Ink & Chemicals under the product name of SD211), and the thickness of the adhesive layer is 25 µm, and the test is carried out like the 1st Example. The result of test is shown in Table 3 and the structure of the 12th Example is shown in Table 1. The UV curing resin (made of Dainippon Ink & Chemicals under the product name of SD211) employed by the adhesive layer has a pencil hardness of 2H and the curing shrinkage rate is 10.3% which are respectively measured in the same manner as the curing coating film set forth above, and the curing film of the adhesive layer has substantially the same hardness and shrinkage rate. (13th Example) The optical disc of the 13th Example is fabricated in the same manner as the 1st Example except the kind of resin of the adhesive layer is changed from the UV curing resin to a hot melt material, and the hot melt material is coated by a roll coating method, and the protecting layer of another disc is brought into contact with the coating layer of molten viscous hot melt material to cool the protecting layer, then both discs are bonded to each other with the thickness of the adhesive layer being 75 µm, and the test is carried out like the 1st Example. The result of test is shown in Table 3 and the structure of the 13th Example is shown in Table 1. The hot melt material employed by the adhesive layer is that made of Daiya Bond Industry Co. and having melting temperature of 125°C, and the curing shrinkage rate at the normal temperature is measured from the molten state thereof using Archimedes' process (Archimedes' principle), which reveals that the shrinkage rate is 6%, and the curing film of the adhesive layer has substantially the same shrinkage rate. (14th Example) The optical disc of the 14th Example is fabricated in the same manner as the 1st Example except the resin of the adhesive layer is changed from the UV curing resin (made of Dainippon Ink & Chemicals under the product name of SD318) to the UV curing resin (made of Nippon Steel Corporation under the product name of V-2152), and the thickness of the adhesive layer is 72 µm, and the resin of protecting layer is changed from the UV curing resin (made of Dainippon Ink & Chemicals under the product name of SD211) to the UV curing resin (made of Dainippon Ink & Chemicals under the product name of SD17, and the thickness of the protecting layer is 4 µm, and the test is carried out like the 1st Example. The result of test is shown in Table 3 and the structure of the 14th Example is shown in Table 1. The UV curing resin (made of Nippon Steel Corporation under the product name of V-2152) employed by the adhesive layer has a pencil hardness of 2H and the curing shrinkage rate is 8.8% which are respectively measured in the same manner as the curing coating film set forth above, and the curing film of the adhesive layer has substantially the same hardness and shrinkage rate. (15th Example) The optical disc of the 15th Example is fabricated in the same manner as the 14th Example except the thickness of the adhesive layer is 41 µm, and the thickness of the protecting layer is 6 µm, and the test is carried out like the 1st Example. The result of test is shown in Table 3 and the structure of the 15th Example is shown in Table 1. (1st Comparative Example) The optical disc is fabricated in the same manner as the 5th Example except the thickness of the adhesive layer is 6 µm, and the test is carried out like the 1st Example. The result of test is shown in Table 3 and the structure of the 1st Comparative Example is shown in Table 2. (2nd Comparative Example) The optical disc is fabricated in the same manner as the 1st Example except the thickness of the adhesive layer is 5 µm, and the test is carried out like the 1st Example. The result of test is shown in Table 3 and the structure of the 2nd Comparative Example is shown in Table 2. (3rd Comparative Example) The optical disc is fabricated in the same manner as the 1st Example except the thickness of the adhesive layer is 85 µm, and the test is carried out like the 1st Example. The result of test is shown in Table 3 and the structure of the 2nd Comparative Example is shown in Table 2. (4th Comparative Example) The optical disc is fabricated in the same manner as the 1st Example except the kind of resin of the adhesive layer is changed to the UV curing acrylic resin having the pencil hardness of H and the curing shrinkage rate of 18% which are respectively measured in the manner set forth above, and the thickness of the adhesive layer is 10 µm, and the test is carried out like the 1st Example. The result of test is shown in Table 3 and the structure of the 4th Comparative Example is shown in Table 2. From the results shown in Tables, it is understood that the variation rate of the reflectance is less than 5% which is not critical level in each of the Examples 1 to 15, and there is not recognized the deformation of the optical disc caused by the stress in each of the Examples 1 to 15, and hence the stress actually influencing the adhesive layer is not generated. Whereupon, it is understood that there is recognized the deformation of the optical disc in the Comparative Examples 1 to 4, and the warpage occurs owing to the deformation of the optical disc in the Comparative Examples 3 and 4, which disables the recording of information. The results in the Comparative Examples are caused by the thickness of the adhesive layer, namely, they depend on too thin or too thick thicknesses and the curing shrinkage rate of the adhesive layer is too large. Further, it is understood that since there is no distortion of the optical disc even if it is recognized that bubbles each having a diameter of exceeding several 100 µm are not mixed in the adhesive layer in the Examples, the influence of the stress even if it is generated in the adhesive layer is reduced by the thickness of the adhesive layer, thereby reducing the influence thereof. It is preferable that the pencil hardness be more than 2H (at least 2H) in the Examples. Since there is no deformation of the optical disc and the variation of the reflectance can be restrained less than 5% in the Examples, it may be limited to "the optical recording medium which is not deformed by controlling the shrinkage rate of the adhesive when the adhesive layer is cured and the variation of the Push-Pull signal (PP signal (Push-Pull signal) tracking signal)) is less than ±15% (not exceeding ±15%) and the variation rate of the reflectance is less than 5% (not exceeding 5%)". The limiting conditions may be one of them. Meanwhile, 5% of the variation rate of the reflectance of the reflecting layer 13 is extended to a level which is not critical in the actual case in each level of the Examples, and this can be controlled by the shrinkage rate, etc. when the adhesive layer is cured. Still further, the aforementioned limitation of the conditions maybe added. The features disclosed in the foregoing description, in the claims and/or in the accompanying drawings may, both separately and in any combination thereof, be material for realising the invention in diverse forms thereof.

Claims: An optical information medium comprising a pair of discs (1, 5) which are bonded to each other, an optical recording layer (12) provided on at least one of the discs (1, 5), a reflecting layer (13) formed on a upper side of the optical recording layer (12), an adhesive (11) provided on the reflecting layer (13) and bonding said pair of discs (1, 5), wherein the adhesive (11) has a thickness ranging from 10 to 80 µm, characterized in the optical information medium further includes a non-bonding area (m) to which the adhesive (11) is not partially bonded which are provided at peripheries of center holes (4) of the discs (1, 5), wherein a boundary of the non-bonding area (m) is not constant or fixed in distance from centers of the discs (1, 5). The optical information medium according to claim 1, further including a protecting layer (14) protecting layer provided on an upper side of the reflecting layer (13) wherein the pair of discs (1, 5) are bonded by the adhesive (11) provided on the protecting layer (14). The optical information medium according to claim 1 or 2, wherein a shrinkage rate of the adhesive layer (11) is less than 15%. The optical information medium according to any one of claims 1 to 3, wherein the adhesive layer (11) includes an reactive curing resin. The optical information medium according to any one of claims 1 to 4, wherein the adhesive layer (11) is made of a hot melt material. The optical information medium according to any one of claims 1 to 5, further including a groove (6) defined in an outer periphery of at least one of the discs (1, 5), wherein the pair of discs (1, 5) are bonded to each other by the adhesive (11) filled in the groove. The optical information medium according to claim 6, wherein the groove (6) is defined in outer peripheral edges of the discs (1, 5). The optical information medium according to claim 6, wherein the groove (6) is defined in an outer peripheral portion other than edges. The optical information medium according to any one of claims 1 to 8, wherein a part (11a) of the adhesive (11) which bonded bonding surfaces of the discs (1, 5) is fixed to an outer peripheral surface of at least one of the discs (1, 5). The optical information medium according to claim 9, further including inclination surfaces (15, 15) formed on an outer peripheral portion of the discs (1, 5) to which the part (11a) of the adhesive (11) is fixed, wherein said inclination surfaces (15, 15) are obtuse or acute relative to the bonding surfaces of the discs (1, 5). The optical information medium according to any one of claims 1 to 10, wherein another disc (5) to be bonded to one disc (1) has no optical recording layer (12). The optical information medium according to any one of claims 1 to 11, further including grooves (7, 7) in which the adhesive (11) stays which are defined in at least one main surface of the discs (1, 5) outside the non-bonding area (m). The optical information medium according to any one of claims 1 to 11, wherein a diameter of a center hole (4') of another disc (5) is larger than that of a center hole (4) of one disc (1) and is also larger than a diameter of the non-bonding area (m). The optical information medium according to claim 13, wherein a part of the adhesive (11) moves around and is fixed to a peripheral surface of the center hole (4') having the larger diameter. The optical information medium according to any one of claims 1 to 14, wherein the adhesive (11) is not bonded to peripheral surfaces of center holes (4) of the discs (1, 5). The optical information medium according to any one of claims 1 to 11, wherein the adhesive (11) is coated on outside of the discs (1, 5) by a spin coating process.

Inventors: FUJII TORU, HAMADA EMIKO, TOMIZAWA YUJI, FUJII, TORU, HAMADA, EMIKO, TOMIZAWA, YUJI
Applicants: TAIYO YUDEN KK, TAIYO YUDEN CO., LTD.
Application year: 1997
Main classification: G11B7
IPC codes: G11B7, G11B23