METHOD OF MANUFACTURING AN OPTICAL RECORDING MEDIUM

Abstract: An optical recording medium has a space therein and is prepared such that a pair of substrates (1a, 1b) are partially adhered by an adhesive obtained by dispersing granular or filler type spacing members therein. At least one of the substrates (1a, 1b) has an optical recording layer (2a, 2b) thereon, the recording layer (2a, 2b) faces inside and a specific gravity of the spacing member falls within a range of 0.9 to 1.3 times that of the adhesive. When applying the adhesive to part of at least one of the substrates, a mixture of the spacing members and an adhesive is stored in a tank and is supplied in a small amount in each application cycle.

Description: The present invention relates to a method for the manufacture of an optical recording medium. In recent years, extensive studies and developments have been made for information recording media for optically recording/reproducing information particularly to develop high-density, high-capacity recording, random access, and a low cost per bit. A sectional structure of a conventional optical recording medium (two-sided recording type) having a hollow structure is illustrated in Fig. 1. Disk-like substrates 1a and 1b have central holes, are transparent to laser light, and are made of glass, plastics or the like. Optical recording layers 2a and 2b are respectively formed on the substrates 1a and 1b. The recording layers 2a and 2b are made of a material subjected to a shape change, a phase change, a magneto-optical change, a chemical decomposition, or the like upon laser radiation. An auxiliary layer such as a very thin reflecting layer or a protective layer may be formed on each recording layer. The substrates 1a and 1b respectively having the recording layers 2a and 2b thereon are adhered by ring-like adhesive layers 3 and 4 respectively defining the inner and outer peripheral portions of the substrates 1a and 1b, so that adhesive layers 3 and 4 serve as spacers surrounding a hollow space 5 between the substrates 1a and 1b. In the conventional optical recording medium described above, the thicknesses of the adhesive layers serving as the spacers must be uniform. If variations in distance between the substrates 1a and 1b occur, vertical displacements of the surface of the recording disk occur upon its rotation on a turntable. It is therefore difficult to perform focusing control and tracking control. In order to set the distance between the substrates 1a and 1b uniform, Japanese Kokai No. 226046/1985 describes an optical recording medium, the sectional structure of which is illustrated in Fig. 2. The medium shown in Fig. 2 does not have a hollow structure. Substrates 1a and 1b having recording layers 2a and 2b thereon are adhered by an adhesive 6 prepared by dispersing spacing members 7 therein, so that the exposed inner surfaces of the substrates 1a and 1b and recording layers 2a and 2b are in contact with the adhesive 6. With the above structure, the distance between the substrates 1a and 1b is kept uniform depending on the thickness of the spacing members 7. Each spacing member 7 is made of glass or a resin material such as polymethyl methacrylate (PMMA). An ultraviolet curing adhesive is preferably used as the adhesive 6. Japanese Kokai No. 168947/1984 also discloses an example wherein the above technique is applied to an optical recording medium having a hollow structure. This example will be described with reference to the sectional view of Fig. 3. Referring to Fig. 3, the inner and outer peripheral portions of disk-like substrates 1a and 1b having recording layers 2a and 2b thereon are adhered by ring-like adhesive layers 13 and 14 serving as spacers, respectively. The adhesive layers 13 and 14 are made of an adhesive 15 in which granular or filler type spacing members 16 are dispersed. The size of a space 6 between the substrates 1a and 1b is kept uniform by the spacing members 16. Application of the above-mentioned technique to the production of quantity optical recording medium however has proved a problem. It has been found by the present inventors that the yield of quantity media is unacceptable, the more so for large quantity production. Following a study of this problem it has been found that many adhesive mixtures have a tendency to deteriorate significantly within a time scale that is comparable to the duration of a production run. An excessive concentration of the spacing members in the dispersed mixture or a substantial depletion thereof is found to occur and the resultant yield is unpredictable and/or unsatisfactory. The present invention is intended as a solution to the above problem. According to the present invention there is provided a method for the manufacture of an optical recording medium, which method includes: applying to an exposed surface of a first substrate a quantity of adhesive mixture, which mixture comprises an adhesive substance and a multiplicity of granular or filler-type spacing members dispersed therein; and assembling said optical recording medium by placing a second substrate over said first substrate and applied adhesive mixture, and curing said adhesive substance; this method being characterised in that: said steps of applying and assembling are performed repeatedly to manufacture optical recording media in quantity; said adhesive mixture is stored in and supplied from a storage tank for repeated application; and said spacing members and adhesive substance have specific gravities in ratio selected such that an adequate dispersion is maintained whilst the adhesive mixture is applied repeatedly. The present inventors have found that the deterioration in the distribution of dispersed spacing members occurs as a result of the spacing members either floating or sinking during storage, and that this can be remedied by choosing an adhesive mixture of appropriate composition - i.e. a mixture in which the specific gravities of spacing members and adhesive substance lie within a limited ratio range. For practical purposes - e.g. the production of one hundred or so media in a period of the order of thirty minutes or so, a composition in which the ratio aforesaid is in the range 0.9 to 1.3 inclusive proves to be satisfactory. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1 to 3 are sectional views showing structures of conventional optical recording media, respectively; Fig. 4 is a sectional view showing a structure of an optical recording medium manufactured according to the present invention; Fig. 5 is a schematic perspective view of a constant delivery apparatus used for manufacturing the medium according to the present invention; Figs. 6A to 6C and Figs. 7A to 7C are schematic perspective views of methods each showing alignment of a pair of substrates constituting the medium manufactured according to the present invention; Fig. 8 is a sectional view showing a curing step of an adhesive during the fabrication of the medium manufactured according to the present invention; Fig. 9 is a schematic perspective view for explaining a method of applying an adhesive to a substrate by a roller; Fig. 10 is a sectional view for explaining the method of applying an adhesive to the substrate by a roller; Fig. 11 is a sectional view for explaining a method of applying an adhesive to the substrate by rollers; Fig. 12 is a schematic perspective view showing a substrate applied with an adhesive by screen printing; Figs. 13A and 13B are sectional views showing voids formed when the adhesive is not uniformly applied to the substrate; and Fig. 14 is a sectional view showing a method of moving the pair of substrates relative to each other after the substrates are aligned during the fabrication of the medium. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The present invention will be described in detail with reference to the accompanying drawings. Fig. 4 is a sectional view showing an optical recording medium manufactured according to an embodiment of the present invention. Disk-like substrates 1a and 1b have central holes, are transparent to laser light, and are made of glass, plastics or the like. Optical recording layers 2a and 2b are respectively formed on the substrates 1a and 1b. The recording layers 2a and 2b are made of a material subjected to a shape change, a phase change, a magnetooptical change, a chemical decomposition, or the like upon laser radiation. Examples of such a material are a material represented by a low melting metal (e.g.,Te and Bi) and an organic dye (e.g., a cyanin dye and a methine dye) and subjected to pit formation, a material represented by As-Te-Ge and TeO x and subjected to a phase change, and a material represented by GdCo, TbFe, TbFeCo, and GdTbFeCo and subjected to magnetic inversion. Auxiliary layers such as very thin reflecting layers and protective layers may be formed on upper and lower surfaces of the recording medium, respectively. The substrates 1a and 1b respectively having the recording layers 2a and 2b thereon are adhered by ring-like adhesive layers 3 and 4 respectivley defining the inner and outer peripheral portions of the substrates 1a and 1b, so that adhesive layers 23 and 24 serve as spacers surrounding a hollow space 5. The adhesive layers 23 and 24 are made of an adhesive 25 prepared by dispersing granular or filler type spacing members 26 therein. The specific gravity of the spacing member 26 falls within the range of 0.9 to 1.3 times that of the adhesive 25. With the above arrangement, the distance between the substrates 1a and 1b is kept uniform according to the size (diameter if the member is spherical) of the spacing member 26. A method of manufacturing the above optical recording medium will be described below. The recording layers 2a and 2b are respectively formed on the substrates 1a and 1b. At this time, if the recording layer is made of an organic dye or the like, spin coating is generally employed. However, if the recording layer is made of a metal or a metal compound, the layer is deposited on the substrate by vacuum deposition, sputtering or the like. Furthermore, if a one-sided recording medium is to be fabricated, the recording layer is formed on only one of the substrates. An adhesive is applied to at least one substrate by dispenser coating using a constant delivery apparatus shown in the schematic perspective view of Fig. 5. Referring to Fig. 5, the substrate 1a is placed on a turntable 17 and is then rotated. A syringe 18 contains a mixture 23a prepared by mixing the spacing members 26 and the adhesive 25. The mixture 23a is deliverd from the syringe 18 to the surface of the rotating substrate 1a in a small amount while the delivery amount is controlled by a dispenser controller 19, thereby forming the adhesive layer 23. If the specific gravity of the spacing member falls within a predetermined range with respect to the specific gravity of the adhesive, the spacing members do not float or sink in the syringe 18. Therefore, the delivered adhesive contains a predetermined amount of spacing members, thereby preventing a nonuniform distribution of the spacing members on the substrate. For example, hollow glass beads (diameter: 320 to 400 µm) having various specific gravities were dispersed in an ultraviolet curing type acrylic adhesive having a specific gravity of 1.16, and the resultant adhesives were continously applied to substrates by using a dispenser. Results are summarized in Table 1. As is apparent from Table 1, if the specific gravity of the spacing member falls within the range of 0.9 to 1.3 times that of the adhesive, stable, uniform coating can be continued in practice. As a result, a recording medium obtained by adhering the substrates has a uniform thickness, i.e., a uniform distance between the substrates. Sample No. Average Specific Gravity of Hollow Glass Beads Specific Gravity of Hollow Beads/Specific Gravity of Adhesive Condition after 30-min. Continuous Coating 1 0.93 0.8 Beads float in syringe and beads are not dispensed. 2 1.05 0.9 Uniform coating is possible. 3 1.10 1.0 Uniform coating is possible. 4 1.34 1.15 Uniform coating is possible. 5 1.51 1.3 Uniform coating is possible. 6 1.63 1.4 Beads sink in syringe and are concentrated in adhesive. 7 2.2 1.9 Beads sink in syringe and are concentrated in adhesive. Adhesive: ultraviolet curing type acrylic adhesive OP-6040 available from Denki Kagaku Kogyo K.K. Viscosity: 4,700 cps (25°C) Granules: hollow glass beams having a diameter of 320 to 400 µm Coating Condition: delivery rate of 8 g/min The adhesive must have a viscosity suitable for coating or printing and proper thixotropic properties (thixotropy). A material of the spacing member used in the present invention may be any material if it has a specific gravity falling within the range of 0.9 to 1.3 times that of the adhesive. Examples of such a material are a porous or nonporous body or a hollow body of an inorganic compound such as glass, silica, or a metal oxide (e.g., MgO and Al₂O₃), or a plastic such as polystyrene,polyethylene, polypropylene, polyester, polyacryl, nylon, and silicone resin. The material of the spacing member should not be dissolved or swelled by an adhesive. Of the granular materials, a polystyrene-or acrylic-crosslinked plastic is effective to decrease solubility in the adhesive. The shape of the spacing member is not limited to the spherical shape, but can be replaced with a hexagonal or tetragonal shape. However, the spacing member preferably has a uniform size so as to assure a uniform distance between the substrates. The size of the spacing member is set substantially equal to the distance between the substrates. Normally, the distance between the substrates falls within the range of 10 µm to 1,000 µm. If a 400-µm diameter spacer member is used, the distance between the substrates is set to substantially 400 µm. The adhesive used in the present invention may be selected from an ultraviolet curing type adhesive, a thermosetting adhesive, a twin pack adhesive, a solvent volatile type adhesive, a humidity curing adhesive, and a hot melt adhesive. Of these adhesives, the ultraviolet curing type adhesive and the twin pack adhesive are very advantageous in operability and productivity. The substrate 1a applied with the adhesive described above is aligned with and adhered to the other substrate 1b (an adhesive may also be applied to this substrate) such that the surface with the adhesive faces inside. In this case, as shown in Fig. 6A, a ring-like jig 8 having an inner diameter substantially equal to the outer diameter of each of the substrates 1a and 1b is placed on a flat plate, and the substrates 1a and 1b are stacked in the jig 8. Then, the substrates 1a and 1b can be adhered to each other without misalignment of their centres. An adhesion jig 9 (Fig. 6B) obtained by fixing the same ring as in Fig. 6A on the flat base, or an adhesion jig 10 (Fig. 6C) as a combination of a plurality of fins but not as a ring-like jig may be used. Since the standard value of the diameter of the disk is determined to be smaller by 0 to -0.3 mm than the nominal value thereof, the inner diameter of the adhesion jig is smaller than the nominal value by 0 to +0.2 mm and more preferably 0 to +0.1 mm. For example, if a disk has an outer diameter of 130 mm, the inner diameter of the adhesion jig is preferably 130.0 to 130.2 mm. The two substrates may be aligned with and adhered to each other by other methods using an adhesion jig 11 (Fig. 7A) having a columnar member of the same inner diameter of the disk and placed on a flat plate so as to adhere the substrates, an adhesion jig (Fig. 7B) prepared by fixing the same columnar member 20 as in Fig. 7A on a flat plate, or a jig 22 (Fig. 7C) as a combination of a plurality of fins but not as a columnar member. A jig having any shape may be used if it properly fits with the central hole of the disk. Since the standard value of the disk itself is larger than the nominal value by 0 to 0.1 mm, the diameter of the aligning member is smaller than the nominal value by 0 to 0.1 mm. For example, if a disk has an outer diameter of 130 mm, its inner diameter is 15 mm. In this case, the diameter of the aligning member preferably falls within the range of 15.0 to 14.8 mm. The substrates 1a and 1b stacked as described above are held between press plates 27 and 28, and a uniform load is applied thereto from the above, as shown in Fig. 8. The adhesive is cured, and an optical recording medium is therefore prepared. In this case, if the adhesive is an ultraviolet curing adhesive, the press plate 28 is removed from the resultant structure which is irradiated with ultraviolet rays, thereby curing the adhesive. The present invention will be described in detail by way of its examples. (Example 1) A polymethine dye having alight absorption peak at about a wavelength of 830 nm was spin-coated on a disk-like polycarbonate substrate having an outer diameter of 200 mm, an inner diameter of 35 mm, and a thickness of 1.2 mm to form a 1,000-Å thick recording layer. Crosslinked PMMA resin beads classified into particle sizes of 420 µm to 490 µm as spacing members were mixed with an ultraviolet curing adhesive OP-6040 (tradename) available from Denki Kagaku Kogyo K.K. at a 10% weight ratio. The specific gravity of the crosslinked PMMA resin was 1.20 while the specific gravity of the adhesive was 1.16, so that these specific gravities were substantially identical. When the adhesive dispersed with the beads was delivered from the dispenser onto another substrate having the same shape as that described above, thereby forming ring-like adhesive layers near the inner and outer peripheral portions of the substrate. Each adhesive layer had a width of about 2 mm and a height of about 1 mm. The adhesive layer had a substantially semicircular section. This substrate was aligned with the substrate with the recording layer such that the surface with the recording layer faced inside and the central holes of the substrates were aligned. A pressure of 5 kg was applied to the entire area of the substrates. The adhered substrates were irradiated with ultraviolet rays (30-sec. radiation at a power of 20 mW), thereby curing the spacer layers constituted by the adhesion layers. The above steps were repeated to 100 two-sided write type optical disks according to the present invention. The thickness of each optical disk, i.e., the distance between the substrates constituting the optical disk was as uniform as 500 µm. (Example 2) Following the same procedures as in Example 1, 100 optical disks using different types of beads were prepared except that various granular beads having a particle size of 420 to 490 µm shown in Table 2 were added to an ultraviolet curing acrylic adhesive OP-6040 available from Denki Kagaku Kogyo K.K. to prepare an adhesive containing spacing members. Results are summarized in Table 2. Spacing Member Specific Gravity Specific Gravity of Spacing Member/Specific Gravity of Adhesive Thickness Precision of Disk and Productivity Hollow Glass Beads 0.80 0.7 Poor Polypropylene Beads 0.93 0.8 Poor Polystyrene Beads 1.05 0.9 Good Nylon Beads 1.13 1.0 Good Acrylic Beads 1.18 1.0 Good Porous Glass 1.25 1.1 Good Silicone Beads 1.30 1.1 Good Silica Gel 1.51 1.3 Good Hollow Glass Beads 1.65 1.4 Poor Glass Beads 2.20 1.9 Poor Alumina 3.90 3.3 Poor Good: 100 disks can be prepared by continuous coating and adhesion thickness precision of disks is ±30 µm or less. Poor: Continuous coating of 100 disks cannot be performed and adhesion thickness precision is ±30 µm or more. (Example 3) Following the same procedures as in Example l, 100 optical disks were continuously prepared except that (A) divinylbenzene-crosslinked polystyrene beads were added to a twin pack epoxy resin EPICRON 850 (tradename) available from DAINIPPON INK & CHEMICALS, INC. at a 10% weight ratio, and the resultant adhesive was used as an adhesive containing spacing members, and (B) the adhesive was exposed at room temperature for 20 hours and was cured. The crosslinked polystyrene resin had a specific gravity of 1.05 while the specific gravity of the adhesive was 1.02 (i.e., 1.05/1.02 = 1.03). The thickness of each resultant disk, i.e., the distance between the substrates constituting the disk was as uniform as about 500 µm, and all disks had uniform quality. (Example 4) Following the same procedures as in Example 3, 100 optical disks were prepared using various beads shown in Table 3. Results are shown in Table 3. Spacing Member Specific Gravity Specific Gravity of Spacing Member/Specific Gravity of Adhesive Thickness Precision of Disk and Productivity Hollow Glass Beads 0.80 0.8 Poor Polypropylene Beads 0.93 0.9 Good Polystyrene Beads 1.05 1.0 Good Acrylic Beads 1.18 1.2 Good Silica Gel 1.51 1.5 Poor When the optical recording media manufactured according to the present invention, a method other than the method using a dispenser of Fig. 5 in the step of applying an adhesive to the substrate may be employed. As shown in Fig. 9, an adhesive may be transferred from a plate or blanket cylinder to inner and outer peripheral portions of the substrate, thereby forming ring-like adhesive layers. Referring to Fig. 9, ring-like adhesive layers 23 and 24 are respectively formed along the inner and outer peripheral portions of a doughnut-like substrate, i.e., a disk substrate 1a having the central hole. A plate or blanket cylinder 29 is used to transfer ring-like adhesive portions 30 and 31 to the substrate 1a. Direct transfer from the plate cylinder to the disk substrate is illustrated in Fig. 10. In this case, a plate cylinder 32 may be a letterpress, intaglio, or litho cylinder. The letterpress and intaglio cylinders are advantageous in transfer efficiency. In order to transfer an adhesive layer from a plate cylinder 36 to a blanket cylinder 35, as shown in Fig. 11, the adhesive is transferred from the plate cylinder 36 to the blanket cylinder 35, and the adhesive is then transferred from the blanket cylinder 35 to the substrate. In this case, the blanket cylinder may be a rubber or metal cylinder. However, since the disk substrate is relatively hard, the rubber blanket cylinder is preferable. Adhesives 33 in which spacing members are dispersed in Figs. 10 and 11 are transferred by doctor blades 34, respectively. Even in the fabrication of media using above methods, if the specific gravity of the spacing member falls within the predetermined range with respect to the specific gravity of the adhesive, the spacing members and the adhesive can be uniformly mixed by the doctor blade, thereby adhering the substrates while the distance between the substrates is kept uniform. As shown in the schematic perspective view of Fig. 12, ring-like patterns as adhesive layers 23 and 24 on a substrate 1a having a recording layer 2a thereon may be formed by screen printing. The adhesive can be applied to the substrate according to various methods. In this case, as shown in Fig. 13A, if the thickness of an adhesive layer 23 containing spacing members 26 and applied to a substrate 1a is not uniform and the substrate 1a is brought into tight contact with the other substrate 1b, voids 38 are formed therebetween upon curing, as shown in Fig. 13B. These voids allow permeation of moisture or dust into the hollow portion and adversely affect stability of the recording layer over time. In order to prevent such a drawback, a technique shown in Fig. 14 may be utilized. As shown in Fig. 14, a load is applied between a base 39 and a press member 40, and the press member 40 is rotated while the load is being applied thereto. Only the upper substrate 1b is rotated about its axis while the lower substrate 1a is kept stationary. The distance between the substrates 1a and 1b is determined by spacing members 26 while a space is assured therebetween. Furthermore, even if an adhesive layer 23 is uniformly applied, voids formed between the substrate 1b and the adhesive layer 23 can be completely eliminated by the above-mentioned rotation. The range of angles subjected to rotation of the substrate is sufficiently 10° to 360°. However, the substrate may be rotated several times if needed. After the above operations, the adhesive is finally cured to prepare an information recording medium having an air-tight hollow structure and a uniform distance between the substrates. Only the upper substrate is rotated, as shown in Fig. 14. However, only the lower substrate may be rotated, or both substrates may be rotated in opposite directions. In addition, motion is not limited to rotation if it occurs in a direction parallel to the adhesion surface. The load acting on the substrates need not be applied during rotation, but may be during curing of the adhesive. The present invention is not limited to the particular embodiments described above. Various changes and modifications may be made within the scope of the invention defined in the claims that follow hereinafter. For example, the shape of the substrate is not limited to a disk-like shape, but may be extended to any shape such as a card-like shape.

Claims: A method for the manufacture of an optical recording medium, which method includes: applying to an exposed surface of a first substrate a quantity of adhesive mixture, which mixture comprises an adhesive substance and a multiplicity of granular or filler-type spacing members dispersed therein; and assembling said optical recording medium by placing a second substrate over said first substrate and applied adhesive mixture, and curing said adhesive substance; this method being characterised in that: said steps of applying and assembling are performed repeatedly to manufacture optical recording media in quantity; said adhesive mixture is stored in and supplied from a storage tank for repeated application; and said spacing members and adhesive substance have specific gravities in ratio selected such that an adequate dispersion is maintained whilst the adhesive mixture is applied repeatedly. A method as claimed in claim 1 wherein said spacing members have a specific gravity of between 0.9 and 1.3, inclusive, times that of the adhesive substance. A method as claimed in either of the preceding claims, wherein each of said substrates comprises a disk having a central hole, and the adhesive mixture is applied such that the adhesive mixture is applied to the inner and outer peripheral portions of said first substrate whilst the first substrate is rotated. A method as claimed in either of claims 1 or 2, wherein the adhesive mixture is applied to said first substrate by transferring the adhesive mixture from a roll-like plate or blanket cylinder. A method as claimed in either of claims 1 or 2, wherein the adhesive mixture is applied to said substrate by screen printing. A method as claimed in any one of the preceding claims, wherein each said substrate comprises a disk having a central hole, and the step of assembling is performed using a jig for aligning the central holes of the substrates. A method as claimed in claim 6, wherein said jig comprises a ring-like member having an inner diameter substantially equal to an outer diameter of each said substrate. A method as claimed in claim 6, wherein said jig comprises a columnar member having an outer diameter substantially equal to an inner diameter of the central hole of each said substrate. A method as claimed in claim 1 wherein during assembly the substrates are displaced relative to one another to suppress formation of voids in the adhesive mixture. A method as claimed in claim 9 wherein a relative rotation is provided between said substrates. A method as claimed in any one of the preceding claims wherein said spacing members are spherical and have a substantially common diameter in a range of between 10 and 1000 µm inclusive. A method as claimed in any one of the preceding claims wherein said adhesive mixture comprises an acrylic adhesive and spacing members formed of material selected from crosslinked polymethyl methacrylate, polystyrene, nylon, acrylic resin, porous glass, silicone resin or silica gel. A method as claimed in any one of the preceding claims wherein said adhesive substance is of an ultraviolet curing type and curing is performed in the presence of applied ultraviolet radiation. A method as claimed in any one of the preceding claims 1 to 11 wherein said adhesive mixture comprises an epoxy adhesive and spacing members formed of material selected from divinyl benzene crosslinked polystyrene, polypropylene, polystyrene or acrylic resin. A method as claimed in any one of the preceding claims 1 to 12 wherein said adhesive substance has a viscosity of between 1,500 and 60,000 cps, inclusive. A method as claimed in any one of the preceding claims when performed at room temperature, wherein said adhesive mixture is stored and applied repeatedly for at least a duration of 30 mins.

Inventors: FUKUI, TETSURO, MIYAZAKI, TAKESHI, SANTOH, TSUYOSHI
Applicants: CANON KABUSHIKI KAISHA
Application year: 1987
Main classification: G11B7
IPC codes: G11B7