JPH0128714B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field The invention of this application relates to an optical recording medium. More specifically, the present invention relates to a heat mode optical recording medium in which recorded information can be erased and rewritten. Prior Art Optical recording media have the advantage that the recording medium does not deteriorate due to wear since the medium and the writing or reading head are not in contact with each other, and for this reason, research and development of various optical recording media are being carried out. Among such optical recording media, heat mode optical recording media are being actively developed because they do not require image processing in a darkroom. This heat mode optical recording medium is an optical recording medium that uses recording light as heat, and a part of the medium is melted or removed using a laser beam to form a small hole called a pit. It records information. However, with conventional heat mode optical recording media,
There are disadvantages in that information recorded as pits cannot be erased, and written information cannot be corrected or rewritten, or it is difficult. To explain this situation more specifically, one of the conventionally known heat mode optical recording media has a recording layer made of nitrocellulose and a light absorbent. For example, a laser beam of 1 μm is applied to such a medium as a recording light.
When irradiated as a minute spot of about φ, the irradiated area becomes extremely hot in a short time, the nitrocellulose ignites and disappears, and 1-bit information is recorded as a minute hole. However, recorded information cannot be erased from such media. On the other hand, heat mode optical recording media having a recording layer made of tellurium or tellurium-selenium-arsenic are also known. However, in this case as well, since the pits are formed by melting a metalloid with a high melting point, it is extremely difficult to restore the recorded pits. Further, JP-A-55-161690 describes a heat mode optical recording medium having a recording layer made of a light-absorbing dye and a thermoplastic resin on a reflective substrate. In this medium, the recording layer is thinned to about 0.01 to 0.2 μm, and the resin in the irradiated area is melted and moved by laser light irradiation, or the light-absorbing dye in the irradiated area is moved laterally. Then, the reflective substrate is exposed and pits are formed. However, even in this case, it is difficult to return the light-absorbing dye that has once moved or to backfill and flatten the small holes that have once reached the bottom of the layer. Therefore, the publication does not disclose or suggest that this medium is capable of erasing and rewriting recorded information. In contrast, using a medium in which thermoplastic is coated on a conductor, a charge is uniformly applied to the thermoplastic layer, a part of the thermoplastic is melted by laser light irradiation, and the change in volume generates electricity. A recording method is known in which a change in the attraction force is generated to obtain pits due to surface irregularities corresponding to laser beam irradiation. In this method, by reheating the medium, the surface unevenness returns to a flat surface and the recorded information can be erased, but it requires a corona discharger, etc., complicates the mechanism of the writing device, and consumes the device. There are disadvantages such as increased power consumption. Purpose of the Invention The invention of this application was made in view of the above circumstances. The first object of the invention of this application is to provide a heat mode optical recording medium that can be erased and rewritten. The second purpose is that in such an erasable optical recording medium, a clear threshold value appears in the optical energy or temperature required to form pits in the recording layer, and pits are always formed with good reproducibility above a predetermined input energy. Pits are not formed with energy below a predetermined value, and the range of input light energy or temperature that causes variations in the reproducibility of pit formation is narrowed. There is little deterioration in the S/N ratio of the information signal written in the pit, and the surface of the pit or its surrounding area is not deformed by the readout light, so the S/N ratio of the written information signal does not deteriorate. In addition, the writing sensitivity is high, and the reading S/N is also high.
The objective is to provide a medium with an extremely high ratio. Other objects of the invention of this application will become clear from the following description. The present inventors conducted various studies for this purpose and found that when a recording layer is formed by incorporating a light-absorbing dye or pigment into polyvinyl alcohol or polyvinyl acetal resin, it is possible to erase and rewrite the recording layer. The present inventors have discovered that the medium can be realized and that the above-mentioned second objective can also be effectively achieved, leading to the invention of this application. That is, the invention of this application basically forms a recording layer containing a thermoplastic resin and a light-absorbing dye or pigment, and when irradiated with recording light, the recording layer melts and softens to form recording pits. A layer containing both a thermoplastic resin and a light-absorbing dye or pigment remains at the bottom of the recording pits formed in the recording layer, and by heating the recording layer in which the recording pits are formed, , a rewritable optical recording medium configured such that the surface of the recording layer becomes flat again, wherein the thermoplastic resin is polyvinyl alcohol or polyvinyl acetal resin with a number average molecular weight of 100,000 or less, and the recording layer The optical recording medium is characterized in that it contains 0.002 to 10 parts by weight of a light-absorbing dye or pigment per 1 part by weight of a thermoplastic resin. Specific Structure of the Invention Hereinafter, the specific structure of the invention of this application will be explained in detail. The optical recording medium in this application has a recording layer provided on a substrate. The recording layer contains polyvinyl alcohol or polyvinyl acetal resin. This polyvinyl alcohol or polyvinyl acetal resin is a thermoplastic resin that softens or melts and deforms as the temperature rises in the area irradiated with recording light, forming erasable recording pits on the surface. The thermoplastic resin used in the present invention is polyvinyl alcohol or polyvinyl acetal resin. That is, in addition to polyvinyl alcohol, any polyvinyl acetal resin such as polyvinyl formal, polyvinyl acetoacetal, and polyvinyl butyral obtained by acetalizing polyvinyl alcohol can be suitably used. In this case, the degree of acetalization of the polyvinyl acetal resin can be arbitrary. The number average molecular weight of the polyvinyl alcohol or polyvinyl acetal resin used is 100,000 or less as long as it is obtained as a solid. This is because when it is less than 100,000, the broadening of the above-mentioned threshold value is further reduced, and both the writing sensitivity and the reading S/N ratio are further increased. Such polyvinyl alcohol or polyvinyl acetal resin is produced by a conventionally known method, and is used after molecular weight fractionation or purification, if necessary. Alternatively, commercially available products may be used as they are, or after fractionation, purification, etc. On the other hand, in addition to such polyvinyl alcohol and polyvinyl acetal resin, the recording layer contains
Contains light-absorbing dyes or pigments. This light-absorbing dye or pigment exhibits a large light absorption rate with respect to the recording light and is used to enable a temperature rise in the irradiated area. Therefore, depending on the wavelength of the recording light, various known dyes, carbon black,
Various known inorganic or organic pigments such as ultrafine metal powder and lake pigments can be used. On the other hand, the content ratio of the polyvinyl alcohol or polyvinyl acetal resin contained in the recording layer to the light-absorbing dye or pigment is generally within a wide range of about 0.002 to 10 parts by weight per 1 part by weight of the resin. can be selected. If this amount is less than 0.002 parts by weight, recording sensitivity decreases. Furthermore, if it exceeds 10 parts by weight, the bottom of the recording pit may reach the bottom of the recording layer, making erasing difficult.
Characteristics deteriorate with repeated recording and erasing. Such a recording layer is coated on a substrate using various known methods such as a spinner or a coater.
The thickness is generally 0.05 μm to 1 mm. Note that such a recording layer may contain other additives in addition to the above-mentioned polyvinyl alcohol or polyvinyl acetal resin and light-absorbing dye or pigment. Examples of such additives include various oligomers and polymers. In this case, the polymer or oligomer is contained in an amount of approximately 30% by weight or less based on the polyvinyl alcohol or polyvinyl acetal resin to improve adhesion to the support, improve coatability, and lower the softening temperature. You can change it. In addition, various plasticizers, surfactants, antistatic agents, lubricants, flame retardants, ultraviolet absorbers, antioxidants,
Stabilizers, dispersants, etc. can be included. On the other hand, the substrate on which such a recording layer is provided and supported is not particularly limited, and various materials can be used for the substrate. However, in terms of thermal conductivity, various types of glasses, ceramics, polymetal acrylic resins, polyacrylic resins, polycarbonate resins, phenol resins, epoxy resins, diallyl phthalate resins, unsaturated polyester resins, polyimide resins, etc. are usually used. It is preferable to use resin or the like. No matter which of these is used, the recording pits will not reach the bottom of the recording layer, and the effects of the present invention will be achieved. Further, the shape and dimensions can be varied depending on the intended use, such as a disk, tape, belt, or drum. In this case, the medium of this application may have the above-described recording layer on one surface of such a substrate, or may have recording layers on both surfaces thereof. Also, two substrates with recording layers coated on one side are used, and the recording layers are placed facing each other with a predetermined gap between them, and they are sealed tightly to prevent dust and scratches. You can also avoid it. Note that the above-mentioned medium may be provided with a subbing layer such as a metal reflective layer or various resin layers, if necessary, and a recording layer may be provided on this subbing layer. Specific Effects of the Invention Information can be written and erased in the following manner using the optical recording medium of this application configured as described above. First, recording light is irradiated. The recording light is produced by focusing various lasers, such as He--Ne, He--Cd, Ar, semiconductor lasers, etc., each having a wavelength of about 400 to 850 nm, and various outputs can be used. In addition, the scanning conditions, pulse width, focusing conditions, etc. of the laser beam can be varied widely, for example,
Under normal irradiation conditions of a power on the disk surface of 1 to 20 mW and a normal pulse width of about 20 to 1000 nsec, the recording pits do not reach the bottom of the recording layer, and the effects of the present invention are realized. By irradiating recording light with such a laser, the polyvinyl alcohol or polyvinyl acetal resin in the recording layer is melted and softened, and minute recording pits corresponding to the irradiated light are formed on the surface of the recording layer in the irradiated area. . In this case, under normal recording light irradiation conditions, the recording pit does not reach the bottom of the recording layer, and a layer containing polyvinyl alcohol or polyvinyl acetal resin and a light-absorbing dye or pigment remains at the bottom of the pit. I will do it. As a result of forming pits in this manner, erasing, which will be described later, becomes possible. In the medium of this application, pits with excellent sensitivity and good shape can be obtained. Furthermore, the threshold broadening of the recording light energy required for pit formation is extremely small. Furthermore, even when stored at high temperatures, there is very little deterioration in the S/N ratio of the read light from the pit. On the other hand, in order to read the information written on the medium from the pits formed in this way, a readout laser beam with lower power than the recording beam is used, which is focused and scanned to detect transmitted or reflected light. Detect any output of light. At this time, as described above, the pits formed in the medium of this application have a good shape, and a high S/N ratio can be obtained during reading. In addition, the S/N of information recorded on the medium by the readout light is
There is no possibility that the ratio will deteriorate or that unnecessary information will be recorded in areas other than the pit portions. On the other hand, information recorded in this way can be erased by reheating the medium. At this time, the surface, which had once been recorded as uneven pits, melts again and returns to its normal state. As mentioned above, this is
This is because the thermoplastic resin and light-absorbing dye or pigment remain at the bottom of the pit during recording. That is,
Reheating during erasing melts and softens the thermoplastic resin, returning the medium surface to a flat surface. When heating is performed by light irradiation, the light-absorbing dye or pigment makes it possible to raise the temperature, causing the thermoplastic resin to melt and soften. As heating for erasing,
Any of laser light irradiation, heating with various heaters, infrared lamp irradiation, etc. may be used. When such erasing and writing are repeated, the writing sensitivity is always good, the pits always show a good shape, and reading is performed with a high S/N ratio. does not deteriorate, and the surface always returns to flatness after erasing.
Erasing and writing can always be performed reliably and satisfactorily even if the number of repetitions of erasing increases. Specific Effects of the Invention According to the optical recording medium of this application, once written information can be easily and reliably erased. Moreover, the broadening of the threshold value of optical energy or temperature required for forming pits in the recording layer is extremely small, and the range of input optical energy or temperature within which the reproducibility of pit formation varies is extremely narrow. Furthermore, it has high heat resistance, and even if it is stored at a high temperature of about 50 to 60 degrees Celsius or higher, the S/N ratio of the information signal recorded on the pit will hardly deteriorate. Further, there is very little change in the shape of the pit or the shape around the pit due to the readout light, and there is very little deterioration of the S/N ratio due to repeated irradiation with the readout light. In addition, the writing sensitivity is good, and a high S/N ratio can be obtained whether transmitted light or reflected light is used for reading. Furthermore, since erasing is always performed stably, information can be written in a sufficiently stable manner even if erasing and rewriting are repeated many times. In this case, when polyvinyl alcohol or polyvinyl acetal resin having a number average molecular weight of 100,000 or less is used, these effects become even more excellent. The present inventors conducted various experiments to confirm the effects of the present invention. One example is shown below. Experimental example 1 A methanol solution of polyvinyl acetate was subjected to alkali saponification, molecular weight fractionation was performed, and the number average molecular weight was 10,000.
of polyvinyl alcohol and lake pigment (CI
Pigment Blue 1, color index number
42595―Lake; Fanal Blue B manufactured by BASF
Supra) at a weight ratio of 3:1, dispersed with a sand grind mill, and then filtered with a filter.
Remove particles larger than 0.5 μm and bar coat.
On a Pyrex glass plate with a diameter of 150 mm and a thickness of 1.2 mm.
The media of the invention of this application were obtained by coating layers at a thickness of 30 μm and a thickness of 1 μm. Separately, for comparison, three types of comparative media were obtained by replacing polyvinyl alcohol with polystyrene, polyethylene, and polypropylene each having a number average molecular weight of 100,000. Among these four types of media, the relationship between the deformation of the recording layer surface and the temperature was measured for one having a 30 μm thick recording layer. That is, the medium is placed in a constant temperature bath,
A needle with a diameter of 0.64 mm and a weight of 50 g was placed on the surface of the recording layer, and the relationship between the degree of penetration of the needle into the layer and the degree of penetration of the needle into the layer was measured while increasing the temperature at a rate of 5° C./min. Then, the temperature range from when the needle began to penetrate into the layer until it reached a certain penetration depth was measured, and the broadening of the pit formation threshold was evaluated. The results are shown in Table 1 below. Separately, the following experiments were conducted on four types of media each having a 1 μm thick recording layer. First, a 10 mW He-Ne laser was focused to 1 μm using a 40x objective lens with an AN (numerical aperture) of 0.55, and pulsed irradiation was performed. The pulse width was changed, and the pulse width at which pits were formed on the surface of the recording layer was measured, and was taken as the reciprocal of the writing sensitivity (μsec). The results are shown in Table 1. At that time, we lowered the ambient temperature by 20°C and irradiated a pulse with the previously obtained frequency to evaluate the broadening of the input energy threshold, and found that no pit formation occurred in any of the comparative media. However, no pit formation was performed in the medium of the invention of this application. Next, the pulse width of the above laser was fixed at 0.5 μsec and writing was performed, and then a 1 mW He-Ne laser was focused to 1 μmφ using the same optical system as above, and irradiated for 1 μsec at a repetition frequency of 10 Hz. ,
The reflected light is detected with a photodiode, and the S/N
The ratio was calculated. In this case, the amplifier system used was one with a 10 MHz band, and the RMS value (effective value) was used for noise. The results are shown in Table 1. In addition, each medium was stored at 70° C. for 100 hours, and the deterioration (%) of the S/N ratio thereafter was measured to evaluate heat resistance. The results are shown in Table 1. Furthermore, the pulse repetition frequency of the readout laser was changed, irradiation was performed for 10 seconds, and the readout light frequency at which pits were formed on the surface of the recording layer was measured. The results are shown in Table 1.

[Table] From the results shown in Table 1, it can be seen that polyvinyl alcohol has extremely superior properties compared to other resins. Experimental Example 2 In the medium of this application in Experimental Example 1, the number average molecular weight of polyvinyl alcohol was
5000, 10000, and 200000, and the organic pigments (referred to as OP) were replaced with copper-phthalocyanine dyes oleosol first blue EL (manufactured by Sumitomo Chemical Co., Ltd., referred to as D) and 13mμ carbon black ( CB) and 10nmNi
The same experiment as in Experimental Example 1 was conducted using various media instead of using ultrafine powder (manufactured by Shinku Yakin Co., Ltd., referred to as Ni). The results are shown in Table 2. Furthermore, formalin and hydrochloric acid were added to the polyvinyl alcohol aqueous solution, and acetalization was performed at 30°C to obtain polyvinyl formal, and the degree of acetalization was
Using polyvinyl acetal of 60% and number average molecular weight of 5,000, 10,000, and 200,000, various media with different light absorbers were prepared and similar experiments were conducted. The results are shown in Table 3.

【table】

【table】

[Table] The heat resistance of all media was 6% or less, and as for noise generation due to readout light, no pit formation occurred in all media at a frequency of 300 Hz. From the results in Tables 2 and 3, it can be seen that more favorable results are obtained when the number average molecular weight of the polyvinyl alcohol or polyvinyl acetal resin is 100,000 or less. It has been confirmed that such effects can be similarly achieved even when dyes or pigments used for semiconductor lasers are used.

Claims (1)

[Claims] 1 A recording layer containing a thermoplastic resin and a light-absorbing dye or pigment is formed on a substrate, and upon irradiation with recording light, the recording layer melts and softens to form recording pits, which are formed in the recording layer. A layer containing both a thermoplastic resin and a light-absorbing dye or pigment remains at the bottom of the recording pit, and by heating the recording layer on which the recording pit is formed,
A rewritable optical recording medium configured such that the surface of the recording layer becomes flat again, wherein the thermoplastic resin is polyvinyl alcohol or polyvinyl acetal resin with a number average molecular weight of 100,000 or less, and the thermoplastic resin in the recording layer is An optical recording medium containing 0.002 to 10 parts by weight of a light-absorbing dye or pigment per 1 part by weight of a thermoplastic resin.