Optical disc

Abstract: An optical disc having a transparent substrate 1, being recordable with a signal(s) by irradiating recording light at the side of the transparent substrate 1 thereof, comprises a cover sheet 6 being attached on the transparent substrate 1 at a surface side for incidence of recording light and reproducing light, in detachable manner, wherein the stretching rate of the cover sheet 6 is selected to be equal or less than 200%. Further, the thermal contraction rate of the cover sheet 6 is selected to be equal or less than 1.5%, and the bonding force thereof onto a surface of the transparent substrate 1 per 25 mm width to be from 10 g/25 mm width to 300 g/25 mm width. Thereby, the curve on the optical disc can be suppressed, if it is attached with the cover sheet 6 on the surface of the transparent substrate 1 thereof.

Description: 1. BACKGROUND OF THE INVENTION FIELD OF THE INVENTION The present invention relates to a recordable optical disc on to which signals can be recorded by irradiating recording light from the transparent substrate side thereof, and in particular to an optical disc, on the surface of which is adhered with a cover sheet, at the transparent substrate side thereof, i.e., a light irradiation or incident surface. 2. DESCRIPTION OF PRIOR ART Optical discs, such as a compact disc (i.e., CD) used as a medium for use in music and a CD-ROM used as a medium mainly for use in a computer and so on, both of which are available on a market, are media of read only (i.e., it is only possible to read out the signal or information from them). On such the read only optical discs, ordinary, signals are recorded by forming so-called pits of concave and/or convex shapes on a surface of the above transparent substrate, with use of a pattern or mold which can be obtained through an electric casting process or press process, etc., when forming the transparent substrate from polycarbonate or the like. With such the optical disc, however it is impossible for a user to record information by him/herself. On the contrary to this, from the end of 1980's to the beginning of 1990's, a so-called a CD of Write-once type (i.e., CD-R) was developed, which is recordable and compatible with the above CD. With this CD-R, ordinary, it is shipped without recording the signals thereon, and the user can record an arbitrary information on it at his/her desire only once. Thereby, it is possible for the user to obtain a copy or reproduce of the CD or CD-ROM on which the arbitrary information is recorded, and which is compatible with such the CD or CD-ROM, with a relatively low cost. Ordinary, such the CD-R is formed with a disc-like transparent substrate of a transparent synthetic resin, of such as polycarbonate or the like, on which is formed a recording layer of coloring matter or pigment, such as cyanine pigment. And further thereon, there is attached a metal reflection layer of such as Au film, on which is further provided a protection layer of ultraviolet curing resin. On the side surface of the above transparent substrate on which the pigment recording layer is formed, there is provided a tracking guide of a spiral shape, having a pitch of 1.6 µ m for example. When recording the signal onto such the CD-R, a spot of recording light ray being pulse-modulated is irradiated onto the pigment recording layer, while tracking the spot of the recording light ray onto the tracking guide, which is irradiated from an optical pickup through the transparent substrate onto the pigment recording layer, thereby writing the pulse signal as the pits onto it. In this instance, the pigment material of the pigment recording layer being heated to be melt and partially decomposed corresponding to the pulse signal interacts with the resin material of the transparent substrate being heated and melt, mutually, thereby forming the pits on the transparent substrate. With this, desired information signal which can be read out in the same manner as in the CD and the CD-ROM can be written onto it. With this CD-ROM, it is possible to obtain an optical disc, on which the information being completely compatible with the CD and the CD-ROM can be recorded with ease on a desk-top environment, without large-scaled manufacturing facilities nor complicated manufacturing processes which are used in the manufacturing processes of such as the CD and the CD-ROM for use of read only. The transparent substrate of the optical disc is easily attached with dusts on the light incident surface due to the static electricity generated thereon, since it is made of the synthetic resin, such as polycarbonate or the like, and is easily scratched when receiving friction with hard things or shocks on it. As mentioned previously, the dusts and/or the scratches attached upon the light incident surface of the transparent substrate disturbs the incidence of the laser light, thereby greatly deteriorating the characteristics, in particular in the wiring characteristic thereof. For dissolving the above-mentioned problem, in accordance with the conventional optical disc, a detachable cover sheet is attached upon the surface of the transparent substrate, on which the recording light and the reproducing light are incident, so as to protect the surface of the transparent surface. For example, with the disc on which the cover sheet of transparent is attached, the recording and the reproducing are possible with attaching the cover sheet upon the surface of the transparent substrate. While in a case where it is attached with a colored sheet, which has an effect of cutting off the light of the light sensitive wavelength region of the pigment recording layer or which is opaque or non-transparent, it has also an effect of protecting from the deterioration of the pigment recording layer due to the natural light, etc. Further, with the cover sheet containing anti-static agent therein, it is possible to protect from adhesion of the duct due to the static electricity. In this manner, with attaching the cover sheet upon the surface of the transparent substrate of the recordable optical disc such as the CD-R or the like, there can be obtained various advantages. While, with attaching the cover sheet upon the surface of the transparent substrate, however, there also can be caused disadvantages. A representative example of the disadvantages is a curve or warp of the optical disc. For instance, according to a so-called Orange Book which describes a standard for the CD-R, an allowable maximum angle of curve for the disc is determined to be equal or less than 0.6 deg. If the optical disc is curved with attaching the cover sheet on it, and if the curved angle exceeds that of the standard, it may causes recording error and/or reproducing error. The document EP-A-0 599 695 discloses an optical disc having a transparent substrate being recordable with a signal by irradiating recording light at the transparent side thereof. The disk comprises a cover sheet being provided on the transparent substrate at a surface side thereof for incidence of recording light and reproduction light in a detachable manner. The document US 5 205 974 discloses a process for the manufacture of filaments, tapes and films from a plastic material This plastic material is stretched at a temperature between the melting temperature and a critical temperature and subsequently rapidly cooled the stretched material to a continuous object. SUMMARY OF THE INVENTION An object is, according to the present invention, for dissolving the drawbacks in the conventional optical disc attached with the cover sheet as mentioned above, to provide an optical disc attached with a cover sheet on the surface of a transparent substrate thereof, but being hardly occurred the curve thereon. For achieving the above-mentioned object, as a result of investigation by the inventors of the present invention, as one of the causes of occurring the curve in the optical disc can be listed the shrinkage or contraction of the sheet material due to change with the passage of time. Accordingly, it is necessary to use the cover sheet made of the material showing as small change as possible with the passage of time, for making the curve of the optical disc small. As the result of investigating this respect in more detail, it is noted and found that, the smaller in a tensile elongation or stretch rate of the sheet material, the smaller in the curve of the optical disc due to the change with the passage of time. Then, the stretch rate of the cover sheet 6 which is attached onto the transparent substrate 1 of the optical disc is selected to be equal or less than 200%. Further, it is also noticed that, the smaller in heat or thermal contraction rate of the sheet material, the smaller in the curve of the optical disc due to the change with the passage of time. Then, the thermal contraction rate of the cover sheet 6 which is attached onto the transparent substrate 1 of the optical disc is selected to be equal or less than 1.5%. Furthermore, it is also noticed that, the smaller in bonding force of the cover sheet 6 onto the transparent substrate 1, the smaller in the curve of the optical disc due to the change of the material of the cover sheet 6 with the passage of time. Then, the bonding force of the cover sheet 6 onto the surface of the transparent substrate 1 per a width of 25 mm is selected to be from 10 g/25mm width up to 300 g/25mm width. An optical disc, according to the present invention, being recordable with signals by irradiation of recording light from a side of transparent substrate thereof, comprises a cover sheet being provided on the transparent substrate 1 at a surface side thereof for incidence of recording light and reproducing light of, in detachable manner, and wherein a stretch rate of the cover sheet 6 is equal to or less than 200%. The smaller in the stretch rate of the cover sheet 6, the better, however, there is no material which shows the stretch rate of zero (0). Therefore, it is impossible to determine the lower limit of the stretch rate particularly or definitively. Also, the above-mentioned upper limit in the stretch rate of the cover sheet 6 is so selected that the curve of the optical disc does not exceed 0.6 deg. being determined for the CD-R, when it is attached with the cover sheet 6 and is conducted by an environment test under a predetermined condition which will be mentioned later. Further, here the thermal contraction rate of the cover sheet 6 is selected to be equal or less than 1.5%. Also, the smaller in the thermal contraction rate of the cover sheet 6, the better, however, there is no material which shows the thermal contraction rate of zero (0). Therefore, it is impossible to determine the lower limit of the thermal contraction rate particularly or definitively. And also, the above-mentioned upper limit in the thermal contraction rate of the cover sheet 6 is so selected that the curve of the optical disc does not exceed 0.6 deg. being determined for the CD-R, when the optical disc is attached with the cover sheet 6 and is conducted by the environment test under the predetermined condition which will be mentioned later. Further, the bonding force of the cover sheet 6 onto the surface of the transparent substrate 1 is selected to be from 10 g/25mm width to 300 g/25mm width. If the bonding force of the cover sheet 6 onto the transparent substrate 1 is too strong, the transparent substrate 1 is bent or curved due to the shrinkage of the cover sheet 6 with the passage of time, thereby causing the curve. The upper limit in the above-mentioned bonding force of the cover sheet 6 is so selected that the curve of the optical disc does not exceed 0.6 deg. being determined for the CD-R, when the optical disc is attached with the cover sheet 6 and is conducted by the environment test under the predetermined condition which will be mentioned later. Moreover, if the bonding force of the cover sheet 6 is too strong, the adhesive is easily remained from the surface of the transparent substrate 1, and it dims the surface of the transparent substrate 1. Such the dim induces errors in the recording or reproducing operations. The upper limit of the above bonding force is almost coincident with the upper limit, at which the dim does not occurs. On the contrary, if the adhesive is too weak, the cover sheet 6 is automatically removed by itself from the surface of the transparent substrate 1, thereby performing no function being inherent as the cover sheet 6. Accordingly, the lower limit of the bonding force is so determined that the cover sheet 6 can perform the inherent function thereof at the minimum. It is also effective to form the fine wrinkles of convex and/or concave shape on the cover sheet 6 and attach it onto the surface of the transparent substrate 1. The wrinkles reduce the bonding force onto the surface of the transparent substrate 1, as well as absorb the extension or shrinkage of the cover sheet 6 due to the change with the passage of time therewith, so as to prevent it from reaching up to the transparent substrate 1. Therefore, no stress occurs in the transparent substrate 1 following the change of the cover sheet 6 with the passage of time, thereby preventing the transparent substrate 1 from the curve due to the bending thereof. However, the thickness of the cover sheet 6 is selected to be from 10 µm up to 1,000 µm. This is because, if the cover sheet is too thin, the strength of the sheet itself is weaken, thereby making the handling of it difficult, i.e., crinkle or crumple is easily occurs when it is adhered onto the transparent substrate 1. Wile, if it is too thick, it becomes a cause of increasing the stress occurring in the transparent substrate due to the deformation which follows the thermal contraction of the cover sheet 6 and so on. As is mentioned in the above, according to the present invention, it is possible to prevent from occurring the curve in the optical disc, following the change of the cover sheet 6 with the passage of time, thereby enabling protection of the optical disc in advance from the curve thereof when it is attached with the cover sheet on the surface of the transparent substrate 1 thereof. BRIEF DESCRIPTION OF DRAWAINGS Fig. 1 shows a perspective view of a transparent substrate of the optical disc according to the present invention, removing a cover sheet partially from the surface thereof and seeing from the side of the substrate; Fig. 2 shows an enlarged perspective view of the transparent substrate including the section views thereof, removing the cover sheet and seeing from a protection layer side thereof; Fig. 3 shows a vertical cross section view of the optical disc according to the present invention; Fig. 4 shows a perspective view from the transparent substrate side of the optical disc according to the present invention, for showing a method of measuring bonding force of the cover sheet per a width of 25 mm thereof; and Fig. 5 shows a perspective view of an another embodiment of the optical disc according to the present invention, seeing from the transparent substrate side thereof. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Hereinafter, embodiments according to the present invention will be fully explained by referring to the attached drawings. As a transparent substrate 1 is used a disc-like synthetic resin substrate, such as a polycarbonate substrate formed with an injection mold method. The transparent substrate 1 is a disc of a shape of donuts, having sizes, for example, a thickness 1.2 mm, an outer diameter 120 mm  an inner diameter 15 mm . The bore of the diameter of 15 mm  formed in the center of the transparent substrate 1 is a so-called a clamping bore 5 for positioning it onto a spindle for rotating the optical disc in a recording and/or reproducing apparatus for the optical disc. The area or region from 26 mm  up to 33 mm  at the outside of the clamping bore 5 is a so-called clamping area at which the spindle for rotating the optical disc clamps it. The transparent substrate 1 has a tracking guide on one side surface thereof. For instance, the area or region from 46 mm  up to 117 mm  in the diameter on the one side surface of the transparent substrate 1 is a signal recording area, in which is formed a pre-groove of the spiral shape, having a width 0.8 µm, a depth 0.08 µm, a pitch 1.6 µm, as shown in Figs. 2 and 3. Dissolving a coloring matter, such as cyanine pigment etc., into an organic solvent for example, it is coated onto the area from 45 mm  up to 118 mm  in the diameter on the surface of the above-mentioned transparent substrate 1, by means of a spin coat method or the like, and is hardened so as to form the pigment recording layer 2 as shown in Figs. 2 and 3. For example, 0.65 g of 1,1 dibutyl 3,3'3' tetramethyl 4,5,4'5' dibenzoid-dicarbocyanine-parchlorate (Nippon-Kannkousikiso Co., Ltd., Product No. NK3219) is dissolved into 10 ml of diacetone alcohol solvent. This is coated onto the surface of the above transparent substrate 1 also with the spin coat method, thereby forming the pigment recording layer 2 of the thickness 130 nm. Next, upon a whole surface of the area from 45 mm  up to 118 mm  in the diameter of the disc is formed such as Au film layer of thickness of 70 nm with a vacuum evaporation method, thereby forming a reflection layer 3 as shown in Figs. 2 and 3. On the reflection layer 3, ultraviolet curing resin is coated with the spin coat method, and then it is cured by irradiation of the ultraviolet ray on it, thereby forming a protection layer 4 of the thickness of 10 µm. Further, a thin cover sheet 6 of polyprene is prepared by coating with adhesive on one side surface thereof, and it is attached onto the surface of the above transparent substrate 1 at the adhesive surface on which the adhesive is coated. As the material of the cover sheet 6 is listed acrylic polymers, which can be obtained by using at least one or more kinds of momomers being selected from, such as, ionomer resin, ethylene-vinyl chloride copolymer resin, ethylene vinyl acetate copolymer, ethylene vinyl acetate-vinyl chloride copolymer graft copolymer resin, vinyliden chloride resin, vinyl chloride resin, vinyl acetate resin, phenoxy resin, butadiene resin, polyacetal, polyamid (nylon), polyamidoimide, polyarylate, thermoplastic polyimide, polyetherimide, polyethylene, polyethylene terephthalate (PET), polycarbonate, polystyrene, polyvinyl alcohol (PVA), polyphenylene ether, polybutylene terephthalate, polymethylpentene, ethylenevinyl alcohol, etc., or further using polymer in appropriate. The polymer can be formed in a film, or also can be coated on the transparent substrate 1 after forming acrylic adhesive layer thereon, so as to form the cover sheet 6 by drying it. As adhesive for attaching the cover sheet 6 on the surface of the transparent substrate 1, acrylic adhesive is used, which is considerably weak in bonding force comparing to general adhesives. From a view point that the cover sheet 6 can be exfoliated or removed from the surface of the transparent substrate 1 without causing a phenomenon of remaining the adhesive on it, as well as for preventing from floating thereof, the bonding force should be from 1 g/25 mm to 100 g/25 mm, and more preferably to be from 1 g/25 mm to 30 g/25 mm. And, the thickness of the adhesive layer should be from 1 µm to 50 µm, and more preferably to be from 5 µm to 20 µm. As the acrylic adhesives can be listed one, which is made of composition mainly contains a main monomer as a component to soften by decreasing glass transition point (Tg is preferable to be equal or less than 100 °C), a small amount of comonomer as a component to improve adhesiveness and cohesive force by increasing the glass transition point (Tg is preferable to be within a range from 150 °C to 250 °C), and further acrylic copolymer having monomer containing functional group in copolymerization component for improving on bridge and adhesiveness, and it can be made of a synthetic such as acrylic acid ester. As the above-mentioned main monomer can be ethylacrylate (Tg: -22 °C), butylacrylate (Tg: -55 °C), 2-ethylhexylacrylate (Tg: -70 °C), etc. As the above-mentioned comonomer, there can be listed up vinyl acetate, acrylonitrile (Tg: 97 °C), acrylamide (Tg: 165 °C), styrene (Tg: 80 °C), methylmethacrylate (Tg: 105 °C), methylacrylate (Tg: 8 °C), etc. As the above-mentioned functional monomer can be listed up unsaturated monobasic acid, such as methacrylic acid (Tg: 228 °C), acrylic acid (Tg: 106 °C), etc., unsaturated dibasic acid such as itaconic acid (bridging base point), hydroxyethyl methacrylate (Tg: 55 °C, 86 °C), hydroxypropyl methacrylate (Tg: 76 °C), dimethylaminoethyl methacrylate (Tg: 13 °C), acrylamide (Tg: 165 °C), meteororacrylamide (self-bridging), glycidyl methacrylate (bridging base point), maleic anhydride (adhesiveness, bridging base point), etc. The Tg indicates the glass transition point of these polymers. With the acrylic copolymer obtained from those monomers, it is preferable that the main monomer is contained to be equal or less than 35 wt% and that the monomor containing the functional group therein to be from 15 wt% up to 3 wt%, and for polymerization, a radical polymerization starter is added to those, such as benzoyl peroxide, azobisisobutyroonitrile, etc. More preferably, to those is further added polyamine, polyisocyanate, polyol, etc., to bridge. For the acrylic copolymer, it is preferable to be from 600,000 to 900,000 in average molecular weight before the bridging thereof, and is also preferable that the polymerization component with the main monomer occupies at least 50 % therein. The average molecular weight range and the composition mentioned above are preferable to obtain a desired property with the adhesive. Polymerization ratio of the main monomer can be calculated from the composition rate and polymerization ratio of the monomer. In more detail, a composition of copolymer, obtained by mixing 50 to 30 weight portion of alkyl acrylate ester or alkyl methacrylate ester (carbon number 4 to 12), 50 to 20 weight portion of vinyl acetate or short-chained alkyl acrylate ester or alkyl methacrylate ester (carbon number 1 to 4), and 1 to 6 weight portion of maleic acid, is polymerized under nitrogen atmosphere in a single solvent of toruene, benzene, or ethyle acetate, or in a mixture thereof (from 80 to 60 portion to the total copolymer from 20 to 40 portion), containing therein the radical starter, such as benzoyl peroxide, azobisisobutyroonitrile, under a temperature from 60 °C to 70 °C. After polymerization, an equivalent from 0.01 to 3.0 of polyol, polyamine or polyisocianate with respect to the maleic anhydride contained in the copolymer is added into the solvent of adhesive, for the bridging. Thereby is obtained adhesive which is superior in heat resistance. The cover sheet 6 is adhered with the bonding force of the adhesive surface onto the surface of the transparent substrate, i.e., onto a light incident surface, thereby covering the surface. However, as shown in Fig. 1, by drawing up the cover sheet 6 at the edges thereof from the light incident surface 5 of the transparent substrate 1, it is easily possible to remove or detach it. The cover sheet 6 shown in the figure has a rip-like tub 7 which is formed extending outside the transparent substrate 1, therefore it can be easily removed or detached from the surface of the transparent substrate 1 by picking up the tub 7. The cover sheet 6 removed or detached from the surface of the transparent substrate 1 can be disposed, so as to be renewed or re-covered with an another new cover sheet 6 to be attached thereof. However, in Fig. 3, a reference numeral 9 indicates the adhesive layer of the cover sheet 6. Here, a stretch or tensile elongation rate of the cover sheet 6 which is attached on the surface of the transparent substrate 1 is selected to be equal or less than 200%. More preferably, the stretch rate of the cover sheet 6 should be kept to be equal or less than 120%. The stretch rate of the cover sheet 6, of course depending upon the material which forms the cover sheet, however, it is not determined evenly nor uniformly depending only on the material thereof. Since the stretch rate of the cover sheet 6 also depends on the degree of polymerization and the density of the synthetic resin forming the cover sheet 6, it differs for each of the cover sheets 6 even when they are formed from the same material. Accordingly, the stretch rate of the cover sheet 6 must be confirmed or checked by measuring it on test pieces which are produced under the same condition as the actual cover sheet 6. The stretch rate of the cover sheet 6 is measured in accordance with JIS K 7127 (tensile test). Using a test piece of the length 150 mm and the width 15 mm, the test piece is stretched or pulled at the both ends thereof at a velocity of 200 mm/min. to be broken, under the condition of the temperature 23 ±2 °C and the humidity 50 ±5 %RH. In this instance, comparing the distances between standard lines which are previously marked on the test piece, before and after the stretch or tensile test, and the difference between them is the stretch rate. Namely, assuming that the distance between the standard lines at the maximum load is L and the distance between the standard lines before the stretching is L0, the stretch rate is equal to (L-L 0 )/L 0 (%) (= (L-L 0 )/L 0 (%)). Further, a heat or thermal contraction or shrinkage rate of the cover sheet 6 which is attached onto the surface of the transparent substrate 1 is selected to be equal or less than 1.5%. The thermal contraction rate of the cover sheet 6 also depends on the material forming it, but it is not determined evenly nor uniformly only depending thereon. Since the thermal contraction rate of the cover sheet 6 also depends on the degree of polymerization and the density of the synthetic resin forming the cover sheet 6, it differs for each of the cover sheets 6 even when they are formed from the same material. Accordingly, the thermal contraction ratio of the cover sheet 6 also must be confirmed or checked by measuring it on test pieces which are produced under the same condition as the actual cover sheet 6. The thermal contraction ratio of the cover sheet 6 is measured in accordance with JIS K 6734. Using a test piece of sizes 120mm×120mm, standard points are set or marked at a distance 100mm vertically and horizontally in the central portion thereof. The test piece is positioned horizontally, it is heated at the temperature 80 ±2 °C for ten (10) minutes, and thereafter it is returned into a normal temperature (i.e., a room temperature). Comparing the distances between standard points on the test piece, before and after the heating, the difference between them is the thermal contraction rate. Namely, assuming that the distance between the standard points after the heating is 1 and that the distance between the standard points before the heating is 1 0 =100mm, the thermal contraction rate is equal to (I-I 0 )/I 0 (%) (= (I-I 0 )/I 0 (%)). Further, bonding force of the above cover sheet 6 onto the transparent substrate 1 per the width of 25 mm is selected to be from 10 g/25 mm width up to 300g/25 mm width. The reason of suppressing the bonding force of the cover sheet 6 being equal or less than 300 g/25 mm width is for the purpose of buffering the stress transmission onto the transparent substrate 1 due to the contraction or shrinkage of the cover sheet following a change with the passage of time. However, if the bonding force of the cover sheet 6 is too small, it is easily exfoliated or detached, then it looses the function as the cover sheet 6. Therefore, the bonding force of the cover sheet 6 onto the transparent substrate 1 per the width of 25 mm must be equal or greater than 10 g/25 mm width. The bonding force of the cover sheet 6 onto the transparent substrate 1 per the width of 25 mm is measured as shown in Fig. 4. The cover sheet 6 is cut with the width 25 mm, and the cut portion of the width 25 mm is risen up and turned at an angle 180( with respect to the surface of the transparent substrate 1, and it is exfoliated at the angle of 180 ° ( with respect to the surface of the transparent substrate 1 as shown by an arrow. The bonding force per the width of 25 mm is measured by the exfoliating load applied thereto in that instance. The bonding force, since it depends on not only the characteristics of the adhesive of the cover sheet 6 but also on surface condition of the transparent substrate 1, it must be confirmed or checked by a result of measuring on actual samples thereof. Further, the cover sheet 6 is selected to be from 10 µm up to 1,000 µm in the thickness thereof. More preferably, it should be around from 50 µm up to 200 µm in the thickness thereof. If the cover sheet 6 is too thin, the change with the passage of time come to be large, while if it is too thick, it comes to be a cause of generating a large stress in the transparent substrate following the thermal contraction of the cover sheet 6. In the manufacturing process of the CD-R, normally, such the cover sheet 6 is attached on the surface of the transparent substrate 1 in the following manner. First, the optical disc as mentioned in the above is positioned on a stage turning the side surface of the transparent substrate 1 upside, and the sheet cover 6 is also positioned on a separate stage. On each of the stages, there are provided a plurality suction holes, therefore by making the suction holes at negative pressure (vacuum), the optical disc and the cover sheet 6 are held on the stages, respectively. The stage holding the cover sheet 6 is turned over and is faced with the transparent substrate 1 of the above optical disc which is held on the separate stage. Thereafter, the stage holding the cover sheet 6 is descended down so as to position the cover sheet 6 onto the transparent substrate 1 of the optical disc, and then the suction by means of the suction holes is stopped. Further, releasing the cover sheet 6 from the stage while stopping it down by means of a roller, and thereafter it is pressed as a whole. With this, the air lying between the cover sheet 6 and the surface of the transparent substrate 1 is removed, and at the same time the cover sheet 6 is attached onto the surface of the transparent substrate 1. Thereby, the optical disc as shown in Fig. 1 is completed Next, explaining an example shown in Fig. 5, the cover sheet 6 is treated with the crimp process so as to form fine wrinkles or lines in convex and/or concave shapes on it, in this example. With this, the cover sheet 6 can be prohibited from being totally adhered onto the surface of the transparent substrate 1 of the optical disc as a whole. By doing so, in particular with the wrinkles, they reduce the bonding force of the cover sheet 6 onto the surface of the transparent substrate 1, and they also absorb the contraction or expansion of the cover sheet 6 due to the change with the passage of time not to reach the transparent substrate 1. Accordingly, the stresses hardly occur in the transparent substrate 1, following the change with the passage of time on the cover sheet 6, thereby it is possible to protect the transparent substrate 1 from being curved by bending thereof. Next, the more detailed embodiments of the present invention will be explained by referring concrete values thereof. <Embodiment 1> As the transparent substrate 1 is prepared a polycarbonate disc in the shape of donuts, having sizes, the thickness 1.2 mm, the outer diameter 120 mm , the inner diameter 15 mm , and it includes the spiral pre-groove of the width 0.8 µm, the depth 0.08 µm and the pitch 1.6 µm, which is formed on the one surface thereof in the area from 46 mm up to 117 mm in the diameter. Dissolving 0.65 g of the 1,1 dibutyl 3,3'3' tetramethyl 4,5,4'5' dibenzoid-dicarbocyanine-parchlorate (Nippon-Kannkousikiso Co., Ltd., Product No. NK3219) into 10 ml of the diacetone alcohol solvent, and it is coated onto the surface of the above transparent substrate 1, in the area where the above pre-groove is formed, with the spin coat method, thereby forming the pigment recording layer 2 of the thickness 130 nm as shown in Figs. 2 and 3. Next, upon the whole surface of the area from 45 mm  up to 118 mm  in the diameter of the disc is formed the Au film layer of thickness of 70 nm with the vacuum evaporation method, thereby forming the reflection layer 3 as shown in Figs. 2 and 3. On the reflection layer 3, the ultraviolet curing resin is also coated with the spin coat method, and then it is cured by irradiation of the ultraviolet ray on it, thereby forming the protection layer 4 of the thickness of 10 µ m. Preparing a polypropylane film having gas permeability, of thickness of 100 µ m, the acrylic adhesive of bridged acrylic acid copolymer is coated on a one side of the polypropylane film to be dried, thereby forming the adhesive layer of thickness of 13 µm thereon. This obtained polypropylane film is punched into a shape of a ring of inner diameter 40 mm  and outer diameter 119 mm , and it is attached upon the surface of the above polycarbonate substrate 1 at the above adhesive layer with a method of pressing and moving a roller. In this instance, three kinds of the cover sheets 6 of 100%, 200% and 300% in the stretch rate are prepared to be attached onto the surface of the transparent substrate 1, and those cover sheets 6 are attached onto them so as to prepare a hundred (100) pieces of the optical discs for each. The stretch rate is adjusted with degree of polymerization of the resin from which the cover sheet 6 is formed. However, the thermal contraction rate is 1.0% and the bonding force onto the surface of the transparent substrate 1 per 25 mm width is 200 g/25 mm width, for every one of those cover sheets 6. Those three (3) kinds of the optical discs are introduced into an environment of temperature 70 °C and of humidity 85 %RH with attaching the cover sheet thereon, and are disposed for 100 hours therein. Thereafter, the optical discs are taken out therefrom, the angle of curve is measured for each of them, in accordance with the standard for CD-R, i.e., the Orange Book. As the result of this, the angle of curve for the optical disc on which is attached the cover sheet 6 of the stretching rate of 100 % is 0.3 deg., for the optical disc on which is attached the cover sheet 6 of the stretching rate of 200 % is 0.6 deg., and for the optical disc on which is attached the cover sheet 6 of the stretching rate of 280 % is 0.8 deg. According to the standard for CD-R, i.e., the Orange Book, the angle of curve for the CD-R is determined to be equal or less than 0.6 deg. <Embodiment 2> With the optical discs produced in the same manner as in the embodiment 1 mentioned above, also three (3) kinds of the cover sheets 6, of 1.0%, 1.5% and 1.9% in the thermal contraction rate are prepared. The thermal contraction rate is adjusted with the degree of polymerization of the resin, from which the cover sheet 6 is formed. Those cover sheets 6 are attached onto the optical discs, thereby producing a hundred (100) pieces of the optical discs for each. However, the stretch rate for any one of the cover sheets 6 is 100%, and the bonding force for each is 200 g/25 mm onto the surface of the transparent substrate 1 width per the 25 mm width thereof. Those three (3) kinds of the optical discs are introduced into the environment of the temperature 70 °C and the humidity 85 %RH with attaching the cover sheet thereon, and are disposed for 100 hours therein. Thereafter, the optical discs are taken out therefrom, the angle of curve is measured for each of them, in accordance with the standard for CD-R, i.e., the Orange Book. As the result of this, the angle of curve for the optical disc on which is attached the cover sheet 6 of the thermal contraction rate of 1.0% is 0.4 deg., for the optical disc on which is attached the cover sheet 6 of the thermal contraction rate of 1.5% is 0.6 deg., and for the optical disc on which is attached the cover sheet 6 of the thermal contraction of 1.9% is 0.9 deg. <Embodiment 3> With the optical discs produced in the same manner as in the embodiment 1 mentioned above, by attaching the cover sheets 6 onto the surfaces of the transparent substrates thereof with the bonding forces 400 g/25 mm width, 200 g/25 mm width and 100 g/25 mm width, respectively, three (3) kinds of the optical discs are prepared, with a hundred (100) pieces for each. However, for any one of the cover sheets 6, the stretch rate is 100%, and the thermal contraction rate is 10%. Those three (3) kinds of the optical discs are also introduced into the environment of the temperature 70 °C and the humidity 85 %RH with attaching the cover sheets thereon, and are disposed for 100 hours therein. Thereafter, the optical discs are taken out therefrom, the angle of curve is measured for each of them, in accordance with the standard for CD-R, i.e., the Orange Book. As the result of this, the angle of curve for the optical disc on which is attached the cover sheet 6 with the bonding force of 400 g/25 mm width is 0.7 deg., for the optical disc on which is attached the cover sheet 6 with the bonding force of 200 g/25 mm width is 0.5 deg., and for the optical disc on which is attached the cover sheet 6 with the bonding force of 100 g/25 mm width is 0.4 deg. By forming the cover sheet 6 upon the base film having the fine concave and convex thereon in the same manner as in the embodiment 1 mentioned above, the wrinkles of concave and convex shape as shown in Fig. 5 are formed on the cover sheet 5, thereby it is also possible to control the bonding force of the sheet cover 5 onto the surface of the transparent substrate 1 with the degree of the concave and convex of those wrinkles. The features disclosed in the claims may be material for realising the invention in diverse forms thereof.

Claims: An optical disc having a transparent substrate (1), being recordable with a signal by irradiating recording light at the transparent substrate side thereof, comprises a cover sheet (6) being provided on the transparent substrate (1) at a surface side thereof for incidence of recording light and reproducing light, in detachable manner, characterized in that a stretch rate is defined according to (L - L 0 )/L 0 , where L is a distance between two standard lines on a test piece at a maximum load in a stretching operation, which is performed under a condition of a temperature 23°C ± 2°C and a humidity 50 ± 5 % RH, and L 0 is the distance between the standard lines before performing said stretching operation, that stretch rate of the cover sheet being equal or less than 200 %. An optical disc as defined in the claim 1, wherein a thermal contraction rate of said cover sheet (6) is equal or less than 1.5%. An optical disc as defined in the claim 2, wherein a bonding force of said cover sheet (6) onto a surface of said transparent substrate per 25 mm width is from 10 g/25 mm width to 300 g/25 mm width. An optical disc as defined in any one of the claims 1 to 3, wherein thickness of said cover sheet (6) is from 10 µm to 1,000 µm. An optical disc as defined in any one of the claims 1 to 4, wherein said cover sheet (6) is formed with fine wrinkles of convex and/or concave shape thereon and is attached onto a surface of said transparent substrate (1).

Inventors: ARAI YUJI, ITO HIDEAKI, SHIN YUAKI, ARAI, YUJI, ITO, HIDEAKI, SHIN, YUAKI, Arai, Yuji c/o Taiyo Yuden Co.,Ltd., Ito, Hideaki c/o Taiyo Yuden Co.,Ltd., Shin, Yuaki c/o Taiyo Yuden Co.,Ltd.
Applicants: TAIYO YUDEN KK, TAIYO YUDEN CO., LTD.
Application year: 1999
Main classification: G11B7
IPC codes: G11B23, G11B7