Optical Disc

Abstract: An optical disc comprises a light transmissive antistatic film (4,14,24) on the surface of a plastics substrate (1,11,21) opposite that bearing a recording film (2,12,22). The antistatic film includes tin oxide and phosphorus as electroconductive filler in a synthetic resin.

Description: The present invention relates to an optical disc for recording, erasing or reproducing information using a laser beam and more particularly, to a protective coat for the substrate of an optical disc which is made of plastics. As a substrate for an optical disc, thermoplastic resin such as PC (polycarbonate) or PMMA (poly methyl- methacrylate) are widely used. Since such plastic substrates are dielectric, they attract static electricity, so that dust or the like are easily attached onto the substrate surface, causing a laser beam to scatter. So, recording and reproducing do not occur normally. In order to solve this problem, there is provided a transparent antistatic resin film on the side of the substrate where the optical beam is applied, as disclosed in Japanese Patent Publication No. 158643/1989. The transparent antistatic resin film, used comprises an antistatic resin film mixed with a surface active agent, a siloxane, an inorganic transparent electro-conductive compound , an electro-conductive filler or the like. As the ratio of the surface-active agent to the antistatic resin is increased, the antistatic effect is heightened but surface hardness is lowered. Conversely, as this ratio is decreased, the surface hardness is improved but the anti-static effect is lowered, which means that maximal antistatic effect and maximal surface hardness cannot be achieved simmultaneously . In addition, when it is exposed to high temperature and humidity conditions, for example 80°C and 90%RH, the surface-active agent comes out on the surface of the film and attracts water which forms droplets on the surface and affects long-term stability. Although the antistatic resin film mixed with a siloxane, exhibits high antistactic effect and the film is hard the film thickness is, below 0.2µm, so sufficient surface hardness cannot be obtained on a soft plastics substrate. Also when an optical disc comprising the antistatic resin film mixed with siloxane is subjected to a low temperature and high temperature cycle test, surface cracks may be generated because the adhesive property between the film and the substrate is poor. When it is exposed to high temperature conditions, for example 80°C, the antistatic effect is considerably lowered and the long-term stability is insufficient. Antistatic resin film mixed with an inorganic transparent electro-conductive compound such as ITO (In 2 O 3 : Sn) or tin oxide (SnO 2 ), exhibits similar problems the adhesive property with plastic substrates is poor and the film thickness is thin, so that sufficient surface hardness cannot be obtained. Antistatic resin film mixed with an electroconductive filler, produces a high antistatic effect . However, when the antistatic resin film is mixed with the electro-conductive filler, adequate stability and light transmission properties cannot be achieved at the same time. More specifically, when the electro-conductive filler comprises only tin oxide, stability, shelf life and reproducibility need to be improved. Tin oxide doped with antimony improves stability but lowers light transmission properties. In addition, since the surface of the antistatic resin film mixed with the electro-conductive filler is not a flat surface, when it is used in the optical disc substrate, the incident beam is scattered, causing the light transmission properties to be lowered and recording and reproducing of signals to deteriorate. The present invention provides an optical disc comprising an antistatic film which has light transmission properties and which is formed of a synthetic resin, it includes tin oxide with phosphorous as an electro-conductive filler and is provided on the side of a plastics substrate of the optical disc where an optical beam is applied. A doping amount of phosphorous in the tin oxide in the electro-conductive filler is 3 to 7wt%. The content of the electro-conductive filler in the antistatic film is 25 to 45wt% the average particle diameter thereof is 0.15µm or less. The invention is described below in greater detail by way of example only with reference to the accompanying drawings, in which: Fig. 1 is a graph showing the relationship between the content of an electro-conductive filler to be employed in the invention and surface flatness; Fig. 2 is a graph showing the relationship between the content of the electro-conductive filler to be employed in the invention and surface resistivity; Fig. 3 is a graph showing the relationship between an average particle diameter of the electro-conductive filler to be employed in the invention and surface flatness; Fig. 4 is a graph showing the relationship between the average particle diameter of the electro-conductive filler to be employed in the invention and the surface resistivity; Fig. 5 is a view showing the structure of an optical disc according to an embodiment of the invention; Fig. 6 is a view showing the structure of an optical disc according to another embodiment of the invention ; and Fig. 7 is a view showing the structure of an optical disc according to still another embodiment of the invention. The present invention provides an optical disc of plastic substrate which is covered by an antistatic resin film mixed with an electro-conductive filler in which stability and light transmission properties are high, the surface is flat and antistatic function is effective. The plastics substrate may be formed of a material which has light transmission properties and is not likely to be deformed. Typically, a polycarbonate is used. Besides, an acrylic resin, an epoxy resin or the like may be used for the substrate. In addition, the thickness of the substrate is normally 1.15 to 1.25mm. The optical disc according to the present invention has a recording film and a protective film for protecting the recording film on at least one side of the plastics substrate. In addition, the antistatic film is provided on the other side (where the optical beam is applied) of the plastics substrate normally through a film for preventing moisture transmission. The optical disc according to the present invention is not limited to the above, and it may be any one of a photomagnetic disc, a compact disc, a write once type disc, a photochromic type disc which are all well known, or in any case, where a specific antistatic resin film has to be provided on a substrate on a side where an optical beam is applied. The recording film, the protective film, the film for preventing moisture transmission, and the antistatic resin film used in the optical disc will be described hereinafter. The structure of the recording film is well-known in the prior art. Typically, they are a four-layer structure such as AlN/GdTbFe/AlN/Al or AlN/DyFeCo/AlN/Al or a three-layer structure such as SiN/TbFeCo/SiN or SiAlON/TbFeCo/SiAlON. The film thickness is normally 150 to 300nm. Although the recording film is normally formed on the whole surface of the plastics substrate, it may be formed partially thereon. The protective film mainly protects the recording film, it may be formed of acrylic urethane UV curing resin, it may be formed of acrylic urethane UV curing resin, polyvinylidene chloride resin, poly-trifluoride polyethylene chloride resin or the like. The film is formed by applying those. The film thickness is normally 2 to 20µm. The film for preventing moisture transmission may be formed of an inorganic material such as AlN, SiN, ZnS, Al 2 O 2 , SiO 2 , SiAlOH or an organic material such as polyvinylidene chloride resin or poly-trifluoride ethylene resin. Among them, AlN is especially preferable. The thickness of the film varies with the material used. Generally, the thickness of the film formed of the inorganic material ranges from 1 to 300nm, preferable 1 to 200nm. Alternatively, the thickness of the film formed of the organic material ranges from 2 to 20µm, preferably 2 to 15µm. For example, in the case of AlN, it preferably ranges from 1 to 20nm. Within the above range of film thickness, transient warp can be prevented and also an interference band of the optical disc can be prevented. The antistatic resin film mixed with the electro-conductive filler containing the transparent electro-conductive filler prevents dust from being attached to the surface of the optical disc and also prevents damage it is preferable that its hardness is high and its surface resistivity is relatively low. The hardness is preferably pencil hardness of HB or more. In addition, the surface resistivity is preferably approximately 10 13 Ω or less. The antistatic resin film mixed with the electro-conductive filler can be formed by mixing a synthetic resin or its raw material with the transparent electro-conductive filler in a predetermined ratio and, if necessary, adding an inorganic filler to improve the hardness, before they are applied to the substrate. As the synthetic resin, for example, acrylic urethane UV curing resin, acrylic UV curing resin or the like is used. The film is formed onto the substrate by, for example a spin coating method, a roll coating method, a dip coating method or the like and cured by means such as UV ray irradiation, heating or cooling, depending on the synthetic resin to be used. The film thickness is preferably 1 to 20µm. The transparent electro-conductive filler comprises tin oxide doped with phosphorous because acceptable light transmission properties and stability can be provided at the same time. The addition, of the doping phosphorous, such as phosphoric acid or sodium phosphate, to the tin oxide is performed by coprecipitation and sintering from a liquid phase. The doping amount is 3 to 7wt% of the whole amount of the electro-conductive filler. The content of the electro-conductive filler in the antistatic resin film is 25 to 45wt% the average particle diameter thereof is 0.15µm or less. Although the optical disc according to the present invention is typically a photomagnetic disc, it may also be a compact disc, a write once type disc or a photochromic type disc using a photochromic material as the recording film. The following Examples illustrate the invention. Example 1 Fig. 5 is a view showing the structure of an optical disc according to an example 1 of the present invention. Referring to Fig. 5, reference numeral 1 designates a plastics substrate, reference numeral 2 designates a recording film, reference numeral 3 designates a protective film, reference numeral 4 designates an antistatic resin film mixed with an electro-conductive filler. In Fig. 5, the plastics substrate 1 is formed of polycarbonate and has a thickness of 1.2mm and light transmission properties. The photomagnetic recording film 2 is formed on one surface of the plastics substrate 1. The recording film 2 has a four-layer structure of AlN/DyFeCo/AlN/Al. The protective film 3 is formed on the surface of the recording film 2. The protective film 3 is formed by applying an acrylic urethane UV curing resin at a thickness of approximately 10µm. The antistatic resin film mixed with an electro-conductive filler 4 is formed on the other surface of the plastics substrate 1. The antistatic resin film 4 according to the present invention is made by dispersing the electro-conductive filler in the UV curing resin. First, when the filler mixed resin is 100, phosphorous and tin oxide are dispersed and mixed in the ratio of approximately 2.25 and 42.75 with a solvent by roll milling. While they are mixed, the UV curing resin is added to be jellied. Then, a film having a thickness of approximately 2.5µm is formed from the above jellied material by a spinner. The average particle diameter of the filler is approximately 0.12µm. Stability and light transmission properties of the optical disc substrate formed of tin oxide doped with phosphorous are shown in the following table 1. The surface resistivity is employed as an index of stability. More specifically, the lower the surface resistivity is, the higher the stability is. Stability Light Transmission (%) Tin Oxide doped with Phosphorous 8x10 9 Ω 90.0 Comparative Examples Stability and light transmission properties of the optical disc substrates formed of tin oxide only and formed of tin oxide doped with antimony are shown in the following table 2. The method for forming the antistatic resin film is the same as in example 1 and the ratio of antimony and tin oxide to the resin is 2.25 and 42.75 when antimony is doped(shown in bottom column of Table 2). Stability Light Transmission (%) Tin Oxide 7x10 12 Ω 90.8 Tin Oxide doped with Antimony 4x10 8 Ω 87.0 As can be seen from the above tables, only the optical disc in which the electro-conductive filler formed of tin oxide doped with phosphorus is used in the antistatic resin film has both high stability and good light transmission properties. Example 2 If the ratio of tin oxide to phosphorous in the electro-conductive filler is changed to 95 to 5 (constant) and the content in the resin is studied, using, The same method for forming the optical disc as in example 1. Fig. 1 is a graph showing the relationship between the content of the electro-conductive filler and surface flatness. The axis of abscissa in the graph shows weight percentage of the electro-conductive filler to the antistatic resin film, and the axis of ordinate therein shows a value of surface roughness measured by a terrain step method. The higher the content of the electro-conductive filler is, the lower the surface flatness is. When the content is 45wt% or more, the surface roughness exceeds 0.08µm. As the surface roughness is increased, the light transmission properties are lowered because light scatters, causing recording and reproducing signals to be degraded. When the surface roughness is 0.08µm or more, the decrease in rate of reflectivity exceeds 5%, which degrades signal quality significantly. Therefore, in order to obtain sufficient light transmission properties in the optical disc, the content of the electro-conductive filler has to be 45wt% or less as shown in Fig. 1. Fig. 2 is a graph showing the relationship between the content of the electro-conductive filler in the antistatic resin film and the surface resistivity thereof. As the content of electro-conductive filler is lowered, the surface resistivity is increased and when the content is 25wt% or less, the surface resistivity exceeds 10 13 Ω. According to a dust attachment test, when the surface resistivity is 10 13 Ω or less, effective prevention of attachment of dust occurs. More specifically, when the surface resistivity is 10 13 Ω or less, sufficient antistatic performance of the optical disc can be provided. Therefore, as can be seen from Fig. 2, in order to obtain sufficient antistatic performance, the content of the electro-conductive filler has to be 25wt% or more. Thus, the effective content of the electro-conductive filler ranges from 25 to 45wt%. Example 3 If the ratio of tin oxide to phosphorous in the electro-conductive filler is changed to 95 to 5 (constant) and the average particle diameter in the resin is studied, using, The same method for forming the optical disc as in example 1. Fig. 3 is a graph showing the relationship between the average particle diameter of the electro-conductive filler and surface flatness. As the average particle diameter of the electro-conductive filler is increased, the surface roughness is increased. When the average particle diameter is 0.15µm or more, the surface roughness exceeds 0.03µm. As described in the example 2, in order to obtain sufficient light transmission properties, the average particle diameter of the electro-conductive filler has to be 0.15µm or less. Fig. 4 is a graph showing the relationship between the average particle diameter of the electro-conductive filler in the antistatic resin and the surface resistivity thereof. As the average particle diameter is decreased, the surface resistivity is gradually increased but its variation is small, that is, the surface resistivity is 10 12 Ω or less, which is within a range that produces sufficient antistatic performance in the optical disc obtained. Thus, the effective average particle diameter of the electro-conductive filler is 0.15µm or less. Thus the content of the electroconductive filler ranges from 25 to 45wt% and the average particle diameter thereof is 0.15µm or less. Example 4 In the examples 4, 5 and 6, the effect of tin oxide doped with phosphorous in the optical disc having different structures is disclosed. In example 4, an optical disc having the same structure shown in example 1 is used. Although the manufacturing method is the same, the antistatic resin film is approximately 2µm in thickness, the average particle diameter of the filler is approximately 0.10µm and the ratio of phosphorous to tin oxide is approximately 1.5 to 23.5 when it is assumed that the resin is 100. Example 5 Fig. 6 is a view showing the structure of an optical disc according to example 5 of the present invention. Referring to Fig. 6, reference numeral 11 designates a plastics substrate, reference numeral 12 designates a storing film, reference numeral 13 designates a protective film, reference numeral 14 designates an electrically-conductive filler mixed with resin antistatic film, reference numeral 15 designates a film for preventing moisture transmission, and reference numeral 16 designates a protective film. In Fig. 6, the plastics substrate, the storing film, the protective film, and the antistatic resin film mixed with an electro-conductive filler are the same as in the example 2, the film 15 for preventing moisture transmission is formed of AlN with a thickness of approximately 50nm by sputtering. In addition, the protective film 16 is the same as the protective film 13, which is formed by applying an acrylic urethane UV curing resin at a thickness of approximately 5 to 10µm. The antistatic resin film mixed with an electro-conductive filler 14 in example 5 is formed by the same method as in example 4. Example 6 Fig. 7 is a view showing a structure of the optical disc according to example 6 of the present invention. Referring to Fig. 7, reference numeral 21 designates a plastics substrate, reference numeral 22 designates a recording film, reference numeral 23 designates a protective film, reference numeral 24 designates an antistatic resin film mixed with an electro-conductive filler, reference numeral 25 designates a film for preventing moisture transmission, reference numeral 26 designates a protective film, and reference numeral 27 designates an antistatic resin film mixed with an electro-conductive filler. In Fig. 6, the plastics substrate, the recording film, the protective film, the antistatic resin film mixed with an electro-conductive filler are the same as in example 4, and the film 25 for preventing moisture transmission is formed of AlN to a thickness of approximately 50nm by sputtering. In addition, the protective film 26 has the same thickness as that of the protective film 23, which is formed by applying an acrylic urethane UV curing resin to a thickness of approximately 5 to 10µm. In addition, the antistatic resin film mixed with electro-conductive filler 24 and 27 have the same thickness (approximately 2µm). The antistatic resin films mixed an electro-conductive filler 24 and 27 in example 6 are formed by the same method as in example 4. It is found that stability, light transmission properties, surface roughness and surface resistivity of the optical discs formed according to examples 4, 5 and 6 each show preferable results. Furthermore, it is found that warp of each of the substrates in the optical discs according to examples 5 and 6 is better than that in example 4. As described above, according to the present invention, there can be provided an optical disc having preferable light transmission properties, surface flatness and antistatic performance, and also having preferable long-term stability and surface hardness by using the electro-conductive filler mixed with antistatic film resin and limiting the kind, the content and the average particle diameter of the electro-conductive filler therein. In addition, shelf life and reproducibility before the electro-conductive filler is applied are high.

Claims: An optical disc comprising a plastics substrate (1,11,21) bearing a film (2,12,22) of a recording material on one surface and bearing a light transmissive antistatic film (4,14,24) on its other surface, the antistatic film being of a synthetic resin and including as electroconductive filler tin oxide and phosphorus, characterised in that the electroconductive filler includes from 3 to 7wt% phosphorus based upon the total electroconductive filler, the electroconductive filler constitutes from 25 to 45wt% of the antistatic film and the electroconductive filler has an average particle diameter of not more than 0.15µm.

Inventors: MIEDA MICHINOBU, NAGAURA TOCHIKAZU, OHTA KENJI, TAKEMORI HIROTOSHI, YOSHIDOME HIROO, MIEDA, MICHINOBU, NAGAURA, TOCHIKAZU, OHTA, KENJI, TAKEMORI, HIROTOSHI, YOSHIDOME, HIROO
Applicants: CATALYSTS & CHEM IND CO, SHARP KK, CATALYSTS & CHEMICALS INDUSTRIES CO., LTD., SHARP KABUSHIKI KAISHA
Application year: 1992
Main classification: G11B7
IPC codes: C09D5, G11B7, C09D175, C09D127, G11B11