JPH0129716B2 - - 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 are also known in which the recording layer is a layer made of tellurium or tellurium-selenium-arsenic. 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. On the other hand, 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 causes an electric charge to be applied to the thermoplastic layer. 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 on the memory, and the surface of the pit or its surrounding area is not deformed by the readout light, and the S/N ratio of the written information signal is not deteriorated. Furthermore, it is an object of the present invention to provide an optical recording medium with high writing sensitivity and an extremely high reading S/N 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 polyester,
The inventors have discovered that an erasable and rewritable medium can be realized and the above objects can be achieved, and the invention of this application has been made. That is, the invention of this application forms a recording layer containing a thermoplastic resin and a light-absorbing dye or pigment on a substrate, 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 has a number average molecular weight of
20,000 or less, and the recording layer contains 0.002 to 10 parts by weight of a light-absorbing dye or pigment per 1 part by weight of the polyester. 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 polyester. As a thermoplastic resin, this polyester softens or melts and deforms as the temperature rises in the area irradiated with recording light.
Recording pits are formed on the surface. In this case, the polyester is oxalic acid,
Aliphatic dibasic acids such as succinic acid, maleic acid, adipic acid, sebastenic acid, or isophthalic acid,
Condensates and co-condensates of various dibasic acids such as aromatic dibasic acids such as terephthalic acid and glycols such as ethylene glycol, tetramethylene glycol and hexamethylene glycol are suitable. Among these, particularly preferred are condensates of aliphatic dibasic acids and glycols, and cocondensates of glycols, aliphatic dibasic acids, and aromatic dibasic acids. Furthermore, for example, a modified gliptal resin obtained by esterifying and modifying glyptal resin, which is a condensation product of phthalic anhydride and glycerin, with a fatty acid, a natural resin, etc., is also suitably used. The number average molecular weight of such polyester is 20,000 or less as long as it is obtained as a solid. This is because when the value is 20,000 or less, both the writing sensitivity and the reading S/N ratio become higher. Such a polyester 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, the recording layer contains a light-absorbing dye or pigment along with such polyester. This light-absorbing dye or pigment exhibits a large light-absorbing dye with respect to the recording light, and contributes to the temperature rise in the irradiated area. Therefore, depending on the wavelength of the recording light, it absorbs light with a wavelength of 400 to 800 nm.
Various known dyes, carbon black, ultrafine metal powder, and various known inorganic or organic pigments such as lake pigments can be used. On the other hand, polyester contained in the recording layer,
The content ratio of the light-absorbing dye or pigment to 1 part by weight of polyester can generally be selected within a wide range of about 0.002 to 10 parts by weight. 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 due to 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 polyester and light-absorbing dye or pigment. Examples of such additives include various oligomers and polymers. In this case, the polymer or oligomer is generally
It can be contained in a range of 30% by weight or less to improve adhesion to the support, improve coatability, and change the softening temperature. In addition, various plasticizers, surfactants, antistatic agents, lubricants, flame retardants, ultraviolet absorbers, antioxidants,
Stabilizers, dispersants, etc. can be included. On the other hand, there are no particular limitations on the basics for forming and supporting such a recording layer, and various materials can be used for the material. However, in terms of thermal conductivity, various types of glass, various ceramics, polymethacrylic resin, polyacrylic resin, polycarbonate resin,
It is preferable to use various resins such as phenol resin, epoxy resin, diallyl phthalate resin, unsaturated polyester resin, and polyimide resin. 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. Writing and erasing information using the optical recording medium of this application configured as described above may be performed as follows. First, recording light is irradiated. The recording light is produced by condensing various lasers such as He--Ne, He--Cd, Ar, and semi-dynamic lasers having a wavelength of about 400 to 850 nm, and various outputs can be used. Furthermore, the scanning conditions, pulse width, focusing conditions, etc. of the laser beam can be varied widely; for example,
Under the normal irradiation conditions of 1 to 20 mW of power on the disk surface 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 the recording light with such a laser, the polyester 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 at the irradiated portions. In this case, under normal recording light irradiation conditions, the recording pits do not reach the bottom of the recording layer, and a layer containing polyester and a light-absorbing dye or pigment remains at the bottom of the pits. 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 a lower power than the recording beam is used, which is focused and scanned to generate 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 was once recorded and turned into uneven pits, remelts and returns to a flat surface. As mentioned above, this is
This is because polyester and light-absorbing dye or pigment remain at the bottom of the pit during recording. That is,
Reheating during erasing melts and softens the polyester, returning the medium surface to a flat surface. When heating is carried out by light irradiation, the light-absorbing dye or pigment makes it possible to raise the temperature and melt and soften the polyester. 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 S/N deterioration due to 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, the number average molecular weight of the polyester is
When using 20,000 or less, 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 As a polyester, condensation of succinic acid and tetramethylene glycol was performed, molecular weight fractionation was performed, and
An aliphatic polyester resin with a number average molecular weight of 10,000 was obtained. This polyester 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 the polyester 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 pulse 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. In addition, when we conducted an experiment by lowering the ambient temperature by 20℃ and evaluated the broadening of the input energy threshold, we found that at the above threshold energy, pits were not formed or formed in any of the comparison media. In contrast, the media of the invention of this application did not form pits. 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 polyester 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 the polyester was 1000 and 1000, respectively.
3000, 10000, and 100000, and the organic pigments (referred to as OP) were replaced with copper-phthalocyanine dye oleosol first blue EL (manufactured by Sumitomo Chemical Co., Ltd., referred to as D) and 13 μm carbon black (referred to as D), respectively. Experiments were conducted in the same manner as in Experimental Example 1, using various media instead of CB (referred to as CB) and nickel ultrafine powder with an average diameter of 10 nm (manufactured by Shinku Yakini Co., Ltd., referred to as Ni). The results are shown in Table 2.

【table】

[Table] Regarding heat resistance, -5% for all media.
It showed only the following deterioration. In addition, regarding noise generation due to readout light, at a readout light frequency of 300Hz for all media,
No pitting was performed. From the results in Table 2, the number average molecular weight of polyamide is
It can be seen that more favorable results can be obtained when the number is 20,000 or less. Experimental Example 3 Various media as shown in Table 3 below were prepared, and the results shown in the table were obtained. In this case, the co-condensed polyester contains oxalic acid and terephthalic acid (oxalic acid/terephthalic acid =
3) and hexamethylene glycol were co-condensed together, and the modified glyptal resin was obtained from phthalic anhydride, glycerin, and oleic acid, and the molecular weights were fractionated and used.

【table】

[Table] From the results shown in Table 3, it can be seen that all the media of this application exhibit excellent properties. 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, the surface of the recording layer is flattened again. The above-mentioned thermoplastic resin is a polyester having a number average molecular weight of 20,000 or less, and the above-mentioned recording layer contains a light-absorbing dye or a light-absorbing dye per 1 part by weight of the polyester. An optical recording medium containing 0.002 to 10 parts by weight of a pigment.