JP2007250025A - Information recording medium - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an information recording medium which has satisfactory coloring properties, glossiness and ink absorptivity, wherein an error caused by warpage of a substrate generated when information is recorded and reproduced is suppressed in the information recording medium having at least an optical information recording layer, an underlayer and a printable label layer. <P>SOLUTION: The information recording layer is characterized in that the printable label layer contains amorphous synthetic silica dispersed or pulverized in water so as to have ≤500 nm average secondary particle diameter, a UV cured resin and at least one water soluble metal compound, and the amorphous synthetic silica content is ≥50 mass% of the total solid of the printable label layer. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an information recording medium having at least an optical information recording layer, an underlayer and a printable label layer on a substrate, and in particular, has good color developability, gloss, coating layer strength and ink absorbability, and is caused by warpage of the substrate. The present invention relates to an information recording medium in which occurrence of errors during information recording and reproduction is suppressed.

  Currently, information recording materials having an optical information recording layer capable of optically recording and reading information, which are widely accepted in the market, include CD (compact disc) and DVD (digital versatile disc). Both CD and DVD are read-only CD-ROM, DVD-ROM, write-once type CD-R, DVD-R, DVD + R capable of recording information only once, rewritable type capable of rewriting information recording any number of times CD-RW, DVD-RW, DVD + RW, and DVD-RAM.

  In order to facilitate classification and storage of these information recording materials, or to make it easy to confirm the data contents, the contents of the recorded data are described in an index in the case. Label printing is performed on the disc itself by a printing method such as printing.

  However, label printing using the existing printing method is not a problem in mass production, but in small-volume production, it is necessary to produce a printing plate, which increases costs or changes in printing design. Each time there is a problem that workability deteriorates, such as the need to change the printing plate and perform color matching.

  In response to these problems, an information recording medium has been developed in which a printable label layer is coated on an information recording layer so that printing can be performed with an ink jet printer or a sublimation thermal transfer printer on the side opposite to the surface on which information is recorded and read by light. And are commercially available. In addition to such a printable information recording medium, for inkjet printers, as disclosed in Japanese Patent Application Laid-Open No. 5-182411, a method of printing on a label and then affixing to the information recording medium is also disclosed. However, label alignment is very complicated. In recent years, many inexpensive models that can print directly on information recording media have been released and are very easy to create. Information recording media that can perform such inkjet recording not only for business use but also for home use It is becoming widespread.

  As the printable label layer of the information recording medium capable of such ink jet recording, for example, Japanese Patent Application Laid-Open No. 2004-234764, Japanese Patent Application Laid-Open No. 2004-30716, Japanese Patent Application Laid-Open No. 2004-30769, Japanese Patent Application Laid-Open No. 2004-503610, and Japanese Patent Application Laid-Open No. 2004-503610. As described in Japanese Patent Application Laid-Open No. 2001-6225, Japanese Patent Application Laid-Open No. 8-279179, Japanese Patent Application Laid-Open No. 9-245380, etc., UV curable resins or mixed prescriptions using UV curable resins and ink swellable resins are used as main components. However, these printable label layers mainly absorb the ink by utilizing the swelling of the ink-swelling resin (referred to as swelling type), so that the ink is absorbed by the capillary action of the recent fine voids (referred to as the void type). Compared to photographic printable inkjet recording materials, the image and fingers are soiled when the print part is touched with a finger after printing, blurring of the printed image occurs, and stored in the case together with the index after printing. In some cases, it may become unusable due to sticking to the index, or in a commercial duplication system, several tens to several hundreds of sheets may be printed continuously. Ink absorption of so-called inks that block each other was insufficient.

  In order to avoid these problems, for example, JP-A-2000-57635 (Patent Document 1) and JP-A-2004-30716 (Patent Document 2) describe examples in which pigment particles are contained in a printable label layer. Yes. However, it is difficult to disperse the pigment particles in the UV curable resin, causing problems such as a decrease in coating layer strength, a decrease in adhesion to the substrate, and a decrease in color development and gloss. there were. In addition, the storage stability of images formed by inkjet recording, such as low dye fixing properties, has also been a problem. Furthermore, if the application amount of the printable label layer is increased in order to increase the ink absorbability, the substrate may be warped and an error may be generated during optical information recording and reproduction. It was insufficient as an information recording medium to be used in the above.

  Japanese Patent Application Laid-Open No. 2004-262236 (Patent Document 3) and Japanese Patent Application Laid-Open No. 2005-225156 (Patent Document 4) propose an ink jet recording sheet having an ink receiving layer made of inorganic fine particles and an ultraviolet curable resin. Even if it is intended to apply these ink receiving layers as a printable label layer, an information recording medium that has high ink absorption, particularly ink absorption speed and gloss, and can suppress drying shrinkage that may cause warping of the substrate. Could not be provided.

On the other hand, in order to avoid the problems of such a printable label layer made of an ultraviolet curable resin, JP 2004-276298 A, JP 2004-276299 A, JP 2005-96258 A, and JP 2005-116072 A1. Proposes a printable label layer containing at least fine particles, polyvinyl alcohol, a boron compound and a mordant. However, the applicability by screen printing or spin coating, which is widely used as a highly productive coating method for the printable label layer, is low and the practicality is low.
JP 2000-57635 A JP 2004-30716 A JP 2004-262236 A JP 2005-225156 A

  Accordingly, an object of the present invention is to provide an information recording medium that has good color developability, gloss, coating layer strength, and ink absorbability, and is less likely to cause errors during information recording and reproduction due to warping of the substrate. is there.

The above problems have been solved by the following means of the present invention.
1. An information recording medium having at least an optical information recording layer, an underlayer and a printable label layer on a substrate, wherein the printable label layer is dispersed or pulverized in water to an average secondary particle diameter of 500 nm or less, An information recording medium comprising an ultraviolet curable resin and at least one water-soluble metal compound, and the content of the amorphous synthetic silica is 50% by mass or more of the total solid content of the printable label layer .
2. 2. The information recording medium according to 1 above, wherein the amorphous synthetic silica is gas phase method silica.
3. 3. The information recording medium according to 1 or 2, wherein the ultraviolet curable resin is a urethane acrylate resin.
4). 4. The information recording medium according to 1, 2, or 3, wherein the water-soluble metal compound is a basic polyaluminum hydroxide compound or zirconium acetate.
5). 5. The information recording medium according to any one of 1 to 4, wherein the printable label layer is provided on a base layer by silk clean printing or spin coating.
6). 6. The information recording medium according to any one of 1 to 5, wherein the ISO whiteness of the underlayer is 80% or more and the surface roughness Ra is 0.2 μm or less.

  According to the present invention, it is possible to provide an information recording medium that has good color developability, gloss, coating layer strength, and ink absorptivity, and has few errors during information recording and reproduction due to warping of the substrate.

  Hereinafter, the configuration of the information recording medium of the present invention will be described with reference to the drawings. FIG. 1 is a configuration diagram of an information recording medium of the present invention. The information recording medium of the present invention includes a substrate 10 and an optical information recording layer 11 for recording data on the substrate, a light reflecting layer 12 for reflecting a laser made of metal, a protective layer 13 for protecting the light reflecting layer, It has a structure in which a base layer 14 for improving adhesion to the printable label layer and improving image coloration and gloss, and a printable label layer 15 is laminated on the outermost layer.

  The laser irradiated during recording and reproduction is incident from the direction of the substrate opposite to the printable label layer. In addition, characters and images are recorded on the printable label layer by an inkjet printer having an inkjet recording head.

  The printable label layer of the present invention contains amorphous synthetic silica dispersed or pulverized in water with an average secondary particle size of 500 nm or less.

  Amorphous synthetic silica can be roughly classified into wet method silica, gas phase method silica, and others depending on the production method. Wet method silica is further classified into precipitation method silica, gel method silica, and sol method silica according to the production method. Precipitated silica is produced by reacting sodium silicate and sulfuric acid under alkaline conditions, and the silica particles that have grown are agglomerated and settled, and are then commercialized through the steps of filtration, washing, drying, pulverization and classification. Precipitated silica is commercially available, for example, from Tosoh Silica Co., Ltd. as a nip seal and from Tokuyama Co., Ltd. as a Toxeal. Gel silica is produced by reacting sodium silicate and sulfuric acid under acidic conditions. During aging, the microparticles dissolve and reprecipitate so as to bind the other primary particles, so that the distinct primary particles disappear and form relatively hard aggregated particles having an internal void structure. For example, it is commercially available as nip gel from Tosoh Silica Co., Ltd., and as syloid and silo jet from Grace Japan Co., Ltd. The sol method silica is also called colloidal silica, and is obtained by heating and aging a silica sol obtained through metathesis of sodium silicate acid or the like through an ion exchange resin layer. For example, it is commercially available as Snowtex from Nissan Chemical Industries, Ltd. Yes.

  In the present invention, precipitation method silica and gas phase method silica are preferably used among wet method silicas, and gas phase method silica is particularly preferably used.

  Vapor phase silica is also called a dry method as opposed to a wet method, and is generally made by a flame hydrolysis method. Specifically, a method of making silicon tetrachloride by burning with hydrogen and oxygen is generally known, but silanes such as methyltrichlorosilane and trichlorosilane can be used alone or silicon tetrachloride instead of silicon tetrachloride. Can be used in a mixed state. Vapor phase silica is commercially available as Aerosil from Nippon Aerosil Co., Ltd. and QS type from Tokuyama Co., Ltd.

The average primary particle diameter of the vapor phase silica used in the present invention is preferably 30 nm or less, and preferably 15 nm or less in order to obtain higher gloss. More preferably, an average primary particle diameter of 3 to 15 nm (particularly 3 to 10 nm) and a specific surface area by the BET method of 200 m ² / g or more (preferably 250 to 500 m ² / g) are used. The average primary particle diameter as used in the present invention refers to the average particle diameter obtained by observing fine particles with an electron microscope and taking the diameter of a circle equal to the projected area of each of 100 primary particles present within a certain area as the particle diameter of the particles. The BET method referred to in the present invention is one of the powder surface area measurement methods by the vapor phase adsorption method, and is a method for obtaining the total surface area, that is, the specific surface area of a 1 g sample from the adsorption isotherm. is there. Usually, nitrogen gas is often used as the adsorbed gas, and the most frequently used method is to measure the amount of adsorption from the change in pressure or volume of the gas to be adsorbed. The most prominent expression for expressing the isotherm of multimolecular adsorption is the Brunauer, Emmett, and Teller formula, called the BET formula, which is widely used for determining the surface area. The adsorption amount is obtained based on the BET equation, and the surface area is obtained by multiplying the area occupied by one adsorbed molecule on the surface.

  The vapor-phase process silica used in the present invention is dispersed or pulverized in water so that the average secondary particle diameter is 500 nm or less, preferably 400 nm or less, more preferably 250 nm or less. As a dispersion method, normal propeller stirring, turbine-type stirring, homomixer-type stirring, etc. are preferably premixed with vapor phase silica and water as a dispersion medium, preferably in the presence of a cationic compound, and then ball mill, It is preferable to perform dispersion using a media mill such as a bead mill or a sand grinder, a pressure disperser such as a high pressure homogenizer, an ultra high pressure homogenizer, an ultrasonic disperser, a thin film swirl disperser, or the like. The average secondary particle size referred to in the present invention was obtained by observing the printable label layer of the obtained information recording medium with an electron microscope, thereby obtaining the average value of the particle size of the dispersed aggregated particles to be observed. Is.

  Moreover, when using wet process silica in this invention, the average primary particle diameter becomes like this. Preferably it is 3-40 nm, and it is preferable that an average aggregated particle diameter is 5-50 micrometers. Wet silica particles are preferably dispersed or pulverized to an average secondary particle size of 500 nm or less, preferably 400 nm or less, more preferably 250 nm or less in the presence of a cationic compound.

  Since the wet process silica produced by a normal method has an average aggregate particle diameter of 1 μm or more, it is used after being finely pulverized. As a pulverization method, a wet dispersion method in which silica dispersed in an aqueous medium is mechanically pulverized can be preferably used. At this time, the increase in the initial viscosity of the dispersion is suppressed, high concentration dispersion is possible, and the pulverization / dispersion efficiency is increased so that the particles can be further pulverized. Therefore, the oil absorption is 210 ml / 100 g or less, the average aggregated particle diameter It is preferable to use precipitated silica of 5 μm or more. By using a high concentration dispersion, the productivity of the information recording medium is also improved. The oil absorption is measured based on the description of JIS K-5101.

  As a specific method for obtaining wet method silica fine particles having an average secondary particle diameter of 500 nm or less of the present invention, first, silica particles and a cationic compound are mixed in water (whichever comes first or at the same time). And each dispersion or aqueous solution may be mixed, and at least one dispersion device such as a sawtooth blade type dispersion device, a propeller blade type dispersion device, or a rotor stator type dispersion device is used. To obtain a preliminary dispersion. If necessary, an appropriate low boiling point solvent may be added. The higher the solid content concentration of the silica pre-dispersion, the higher the concentration. However, since the dispersion becomes impossible when the concentration is too high, the preferred range is 15 to 40% by mass, more preferably 20 to 35% by mass. Next, by applying a stronger mechanical means, a wet process silica fine particle dispersion having an average secondary particle diameter of 500 nm or less is obtained. As the mechanical means, a known method can be adopted, for example, a media mill such as a ball mill, a bead mill, a sand grinder, a high pressure homogenizer, a pressure disperser such as an ultra high pressure homogenizer, an ultrasonic disperser, and a thin film swirl disperser. Can be used.

  The printable label layer of the present invention contains at least one water-soluble metal compound for the purpose of improving color developability and water resistance, but as a cationic compound used for dispersion and pulverization of the amorphous synthetic silica, The water-soluble metal compound may be used, and a cationic polymer may be used.

  Examples of the water-soluble metal compound include a water-soluble polyvalent metal salt, and among them, a compound made of aluminum or a Group 4A metal (for example, zirconium or titanium) of the periodic table is preferable. In the present invention, when used for dispersing and pulverizing amorphous synthetic silica, a water-soluble aluminum compound is particularly preferable. As a water-soluble aluminum compound, for example, as an inorganic salt, aluminum chloride or a hydrate thereof, aluminum sulfate or a hydrate thereof, ammonium alum and the like are known. Furthermore, a basic polyaluminum hydroxide compound which is an inorganic aluminum-containing cationic polymer is known and preferably used.

The basic polyaluminum hydroxide compound has a main component represented by the following general formula 1, 2, or 3, for example, [Al ₆ (OH) ₁₅ ] ³⁺ , [Al ₈ (OH) ₂₀ ] ⁴⁺ _{, [Al 13 (OH) 34} ] 5+, with _{[Al 21 (OH) 60]} 3+, basic water-soluble that contain polynuclear condensed ions of a polymer stable at of polyaluminum hydroxide as an equal is there.

[Al ₂ (OH) _n Cl _6-n ] _m General formula 1
[Al (OH) ₃ ] _n AlCl ₃ general formula 2
Al _n (OH) _m Cl _(3n-m) 0 <m <3n General formula 3

  These are water treatment agents from Taki Chemical Co., Ltd. under the name of polyaluminum chloride (PAC), from Asada Chemical Co., Ltd. under the name of polyaluminum hydroxide (Paho), and from Riken Green Co., Ltd. It is marketed under the name of Purachem WT and from other manufacturers for the same purpose, and various grades can be easily obtained.

  As the water-soluble compound containing the Group 4A element of the periodic table used in the present invention, a water-soluble compound containing titanium or zirconium is more preferable. Examples of the water-soluble compound containing titanium include titanium chloride and titanium sulfate. Water-soluble compounds containing zirconium include zirconium acetate, zirconium chloride, zirconium oxychloride, hydroxy zirconium chloride, zirconium nitrate, basic zirconium carbonate, zirconium hydroxide, zirconium lactate, zirconium carbonate / ammonium, zirconium carbonate / potassium, zirconium sulfate. Zirconium fluoride compound and the like, preferably zirconium acetate showing excellent bleeding resistance is used. In the present invention, the term “water-soluble” means that 1% by mass or more dissolves in water at room temperature and normal pressure.

  The total addition amount of the water-soluble metal compound is preferably 10% by mass or less, more preferably 0.5 to 8% by mass in terms of solid content with respect to the amorphous synthetic silica.

  As the cationic polymer, polyethyleneimine, polydiallylamine, polyallylamine, alkylamine polymer, JP-A-59-20696, JP-A-59-33176, JP-A-59-33177, JP-A-59-155088, Sho 60-11389, Sho 60-49990, Sho 60-83882, Sho 60-109894, Sho 62-198493, Sho 63-49478, Sho 63-115780, Shosho 63-280681, No. 1-40371, No. 6-234268, No. 7-125411, No. 10-193976, etc. The polymer having is preferably used. In particular, diallylamine derivatives are preferably used as the cationic polymer. In terms of dispersibility and dispersion viscosity, the molecular weight of these cationic polymers is preferably about 2,000 to 100,000, and particularly preferably about 2,000 to 30,000.

  The amorphous synthetic silica having an average secondary particle diameter of 500 nm or less contained in the printable label layer of the present invention is preferably a vapor phase silica, so that excellent ink absorbability in terms of quality and concentration in terms of production. Further, since the viscosity can be adjusted relatively easily, there is an advantage that it is easy to produce a highly productive paint.

  The content of the amorphous synthetic silica fine particles in the present invention is 50% by mass or more, preferably 60% by mass or more, particularly preferably 70% by mass or more, based on the total solid content of the printable label layer. is there. By setting it within the above range, good ink absorbability can be obtained.

  In the present invention, an ultraviolet curable resin is used as a binder for the printable label layer. A cured coating film is obtained by irradiating a coating liquid of a printable label layer containing an ultraviolet curable resin and amorphous synthetic silica, which is applied to the upper layer of the underlayer, with ultraviolet rays.

  By using UV curable resin for the printable label layer, it is easy to obtain compatibility with the base layer, the layer strength of the formed coating layer is good, and there are advantages in terms of quality that it is difficult to block even in a wet environment. It is done. Furthermore, there are also advantages in terms of production such that silk screen printing and spin coating, which are widely used as production methods for high productivity such as CD and DVD, are easy.

  Examples of the ultraviolet curable resin used in the present invention include compounds having an ethylenically unsaturated bond, and specific examples include the following compounds.

(1) Aliphatic, alicyclic, aromatic, araliphatic polyhydric alcohol and poly (meth) acrylate of polyalkylene glycol (2) Aliphatic, alicyclic, aromatic, araliphatic polyhydric alcohol (3) Polyester poly (meth) acrylate (4) Polyurethane poly (meth) acrylate (5) Epoxy poly (meth) acrylate (6) Polyamide poly (meta) ) Acrylate (7) Poly (meth) acryloyloxyalkyl phosphate ester (8) Vinyl-based or diene-based compound having (meth) acryloyloxy group at the side chain or terminal (9) Monofunctional (meth) acrylate, vinylpyrrolidone , (Meth) acryloyl compounds (10) have ethylenically unsaturated bonds Cyano compounds (11) Mono- or polycarboxylic acids having an ethylenically unsaturated bond, and alkali metal salts, ammonium salts, amine salts thereof, etc. (12) Ethylenically unsaturated (meth) acrylamide or alkyl-substituted (meth) acrylamide and (13) Vinyl lactam and polyvinyl lactam compounds (14) Polyethers having ethylenically unsaturated bonds and esters thereof (15) Esters of alcohols having ethylenically unsaturated bonds (16) Having ethylenically unsaturated bonds Polyalcohol and its ester (17) Aromatic compound having one or more ethylenically unsaturated bonds such as styrene and divinylbenzene (18) Polyorganosiloxane compound having (meth) acryloyloxy group at side chain or terminal 19) silicone compounds having an ethylenically unsaturated bond (20) above (1) to (19) multimeric or oligoester (meth) acrylate modified compounds of the compounds described

  These resins can be used alone or in combination with other resins. Moreover, it can also apply | coat without a solvent, can also be diluted with a solvent, can apply | coat, and can also apply | coat, dry and harden and use it in an emulsion state. Among them, the urethane acrylate resin shown in the above (4) is suitable in the present invention because it has a relatively large molecular weight, and thus has a small shrinkage on drying and is less likely to cause warpage of the substrate.

  In the present invention, it is preferable to use a water-soluble ultraviolet curable resin in order to obtain affinity with amorphous synthetic silica dispersed and pulverized in water. In recent years, solvent-free water-based types have been marketed by various manufacturers in consideration of the work environment. For example, Arakawa Chemical Industry Co., Ltd. has a beam set series and Shin Nakamura Chemical Industry Co., Ltd. has an NK oligo series.

  The content of the ultraviolet curable resin in the present invention is 5 to 100% by mass, preferably 8 to 80% by mass, and particularly preferably 10% by solid content with respect to the content of amorphous synthetic silica. It is the range of -70 mass%. By setting it within the above range, good ink absorbability and strong coating layer strength can be obtained.

  In the present invention, the ultraviolet curable resin is preferably cured by irradiating with ultraviolet rays after a photopolymerization initiator is contained in the printable label layer. Photopolymerization initiators include acetophenones such as di- and trichloroacetophenone, benzophenone, Michler ketone, benzyl, benzoin, benzoin alkyl ether, benzyl dimethyl ketal, tetramethylthiuram monosulfide, thioxanthones, azo compounds, various silver salts, etc. The amount of the photopolymerization initiator used is usually in the range of 0.1 to 10% in terms of solid content with respect to the ultraviolet curable resin. In addition, a storage stabilizer such as hydroquinone may be used as the photopolymerization initiator.

  Also, the ultraviolet curable resin can be cured by irradiation with an electron beam. In that case, the electron beam irradiation has an acceleration voltage of 100 to 1000 kV in terms of transmission power and curing power, and more preferably, an electron beam accelerator of 100 to 300 kV is used, and a one-pass absorbed dose is 0.5 to 20 Mrad. It is preferable to do so. If the accelerating voltage or the electron beam irradiation amount is lower than this range, the electron beam transmission power is too low to perform sufficient curing, and if it is larger than this range, not only the energy efficiency is deteriorated, but also the resin, Unfavorable effects on quality such as decomposition of additives and reduction of strength of recording material appear. As an electron beam accelerator, any of an electro curtain system, a scanning type, a double scanning type, etc. may be sufficient, for example. In the electron beam irradiation, since the curing of the electron beam curable resin is hindered when the oxygen concentration is high, substitution with an inert gas such as nitrogen, helium or carbon dioxide is performed, and the oxygen concentration is 600 ppm or less, preferably 400 ppm or less. Irradiation is preferably performed in an atmosphere that is suppressed to a low level.

  As described above, the printable label layer of the present invention contains amorphous synthetic silica dispersed in or pulverized in water with an average secondary particle diameter of 500 nm or less, an ultraviolet curable resin, and at least one water-soluble metal compound. In addition, when the content of the amorphous synthetic silica is 50% by mass or more of the total solid content of the printable label layer, excellent ink absorbability, coating layer strength, and water resistance of the formed image are obtained in terms of quality. be able to. Further, it is a printable label layer suitable for an information recording medium that has excellent gloss and color developability, and has a small drying shrinkage of the coating layer, so that errors during information recording and reproduction due to warping of the substrate are small.

  The printable label layer of the present invention may contain a cationic fixing agent in order to improve the fixability of the formed image by ink jet recording. As the cationic fixing agent, the various water-soluble metal compounds and cationic polymers described above as the cationic compound used for dispersing and pulverizing amorphous synthetic silica can be used.

  The printable label layer of the present invention may contain a binder component other than the ultraviolet curable resin as long as the effects of the present invention are not impaired. Examples of the polymer latex include styrene-butadiene copolymer latex, polyacrylate ester latex, polymethacrylate ester latex, vinyl acetate latex, ethylene-vinyl acetate latex, and water-soluble resin. Polyvinyl alcohol compounds, polyethylene glycol compounds, starches, dextrin, carboxymethyl cellulose, and derivatives thereof can be used.

  Especially, it is preferable to contain 1 or more types of polymer latex whose average particle diameter is 100 nm or less and whose glass transition temperature Tg is 40 degrees C or less. By containing these, while maintaining the color developability, gloss and ink absorbability of the printable label layer, it is possible to further suppress the shrinkage fluctuation of the coating layer due to the drying shrinkage rate and temperature / humidity fluctuation of the printable label layer. Errors during recording and playback can be avoided.

  The polymer latex is preferably used in an amount of 5 to 30% by mass, more preferably 5 to 20% by mass, based on the inorganic fine particles contained in the printable label layer. By being in the above-mentioned range, it is possible to achieve both the effect of suppressing the shrinkage fluctuation of the coating layer and the ink absorbability.

  Moreover, in order to suppress the shrinkage fluctuation of the coating layer due to the drying shrinkage rate and temperature / humidity fluctuation, the printable label layer of the present invention may contain one or more alkanediols having 3 to 5 carbon atoms. The alkanediol having 3 to 5 carbon atoms is liquid at room temperature, has high hygroscopicity, low volatility, and is water-soluble. The compound can suppress the curling fluctuation of the substrate by suppressing the shrinkage fluctuation of the coating layer due to the temperature and humidity fluctuations without causing a decrease in color developability and cracking, and stable information recording and reproducing performance of the information recording medium. Is obtained.

  Examples of the alkanediol having 3 to 5 carbon atoms include propanediol, butanediol, and pentanediol, and all of these isomers are included. Also included are branched butanediols such as 3-methyl 1,3-butanediol. Among these, propanediol and butanediol having 3 to 4 carbon atoms are preferable, and propanediol is particularly preferable. Propanediol includes propylene glycol (1,2-propanediol) and trimethylene glycol (1,3-propanediol), with propylene glycol being particularly preferred. The amount of the compound added is preferably 0.1 to 50% by mass, more preferably 1 to 20% by mass, based on the total solid content of the printable label layer. By being in the above range, the effect of suppressing the shrinkage fluctuation of the coating layer, the ink absorbency, and the water resistance of the coating layer can be satisfied.

  The printable label layer of the present invention, as long as it does not impair the ink absorbency and coating strength, known various inorganic pigments such as alumina, hydrated alumina, calcium carbonate, magnesium carbonate, titanium dioxide, organic pigments, A dispersant, an image preservative, an antifoaming agent, a plasticizer, a crosslinking agent, a colorant, and other additives may be contained.

The dry coating amount of the printable label layer in the present invention is 12 to 28 g / m ² , preferably 12 to 25 g / m ² , and more preferably 14 to 23 g / m ² . If the amount of dry coating is small, the ink absorbability is lowered, and if the amount of coating is large, the weight of the printable label layer and drying shrinkage tend to warp the substrate, and errors in information recording and reproduction are likely to occur. .

  The coating method of the printable label layer of the present invention is not particularly limited, and a known coating method can be used. For example, slide lip method, curtain method, extrusion method, air knife method, roll coating method, rod bar coating method, spin coating method, screen printing method, etc. can be used as appropriate, because high productivity can be obtained. It is preferably formed by a screen printing method or a spin coating method. In addition, when forming ultraviolet curable resin by screen printing, in order to improve the smoothness of an information recording medium, the time for leveling the resin after printing is required suitably.

  After coating the printable label layer, ultraviolet rays are irradiated from above the printable label layer with an ultraviolet irradiation lamp to cure the ultraviolet curable resin in the coating layer. Further, in order to eliminate the thickness unevenness of the protective layer to be formed, a treatment such as leaving it for a certain period of time before curing the resin may be appropriately performed.

  Examples of the ultraviolet irradiation device include a low-pressure mercury lamp, a medium-pressure mercury lamp, a high-pressure mercury lamp, a metal halide lamp, and the like, and an ozone-less type that generates less ozone. Generally, a plurality of lamps having an output of 30 w / cm or more are used in parallel.

  In the present invention, after the printable label layer is cured with ultraviolet rays, the remaining solvent such as water and solvent is evaporated by means such as natural drying, warm air drying, infrared / far infrared drying, high frequency drying, oven drying and the like. The drying temperature in that case becomes like this. Preferably it is 60 degrees C or less, More preferably, it is 50 degrees C or less. If the temperature exceeds 60 ° C., the quality of the optical information recording layer having low heat resistance is deteriorated, and errors during information recording and reproduction frequently occur.

  The information recording medium of the present invention may have an overcoat layer on the printable label layer. With the overcoat layer, the surface strength can be further improved, and the storage stability of the image can be improved. The overcoat layer needs to have the property of accepting ink or allowing it to permeate quickly, and is a void-type coating layer comprising inorganic fine particles such as amorphous synthetic silica and alumina and a water-soluble resin such as polyvinyl alcohol. Is preferred. The layer thickness of the overcoat layer is preferably 0.01 to 1000 μm, and more preferably 0.1 to 100 μm. The coating method is preferably formed by a screen printing method or a spin coating method.

  In the present invention, the base layer 14 is provided under the printable label layer 15 for the purpose of improving the adhesiveness with the printable label layer and improving the color development and gloss of the image. The undercoat layer of the present invention preferably has an ISO whiteness of 80% or more and a surface roughness Ra of 0.2 μm or less, so that it is possible to obtain high colorability and glossiness as in photographic paper. . Further, it is more preferable that the ISO whiteness is 90% or more and the surface roughness Ra is 0.15 μm or less. In order to increase the whiteness, it is preferable to add a white pigment having a high hiding property or a milky white dye in the underlayer.

  The ISO whiteness in the present invention is the whiteness defined by ISO2470, and the surface roughness Ra is Ra (center surface average roughness) defined in JIS-B0601: 2001. Ra is a measured value of a stylus type surface roughness meter, and is measured with a reference length of 8 mm. When determining Ra, the reference length is extracted from a portion where there is no extraordinary high mountain or deep valley considered as a scratch. When the surface shape has directionality, the measurement is performed in the direction in which Ra is the largest.

  As the substrate 10, various materials used as substrate materials for conventional optical recording media that are transparent to the wavelengths of the recording light and the reproducing light can be arbitrarily selected and used. Specific examples include acrylic resins such as polycarbonate and polymethyl methacrylate, vinyl chloride resins such as polyvinyl chloride and vinyl chloride copolymers, epoxy resins, amorphous polyolefin, polyester, metals such as aluminum, and the like. If desired, these may be used in combination. Among the above materials, amorphous polyolefin and polycarbonate are preferable, and polycarbonate is particularly preferable from the viewpoint of chemical resistance, dimensional stability, light transmittance, and low cost. Further, the thickness of the substrate is preferably 0.5 to 1.2 mm, and more preferably 0.6 to 1.1 mm. Further, the substrate is provided with unevenness (pregroove) representing information such as a tracking guide groove or an address signal. The pitch, depth (groove depth), and half width of the pre-group can be arbitrarily set depending on the type of information recording medium.

  In the information recording medium of the present invention, as described above, the optical information recording layer, the light reflecting layer, and the protective layer are provided between the substrate and the base layer. In the present invention, the optical information recording layer and the light reflecting layer are indispensable as an information recording medium. However, regarding the protective layer, if there is a base layer having both functions of the protective layer and the base layer, it is not always necessary to provide it. Absent.

  The optical information recording layer 11 is provided on the substrate 10 and is formed of a material capable of recording digital information by irradiation with laser light. Usually, it is formed as an optical information recording layer made of an organic dye as represented by CD-R or DVD-R, or as an optical information recording layer made of an inorganic material as represented by CD-RW or DVD-RW. The

  The optical information recording layer made of an organic dye is provided using a dry thin film forming method such as a vacuum deposition method or a sputtering method, or a wet thin film forming method such as a cast method, a spin coating method, or an immersion method. Of these, the spin coating method is particularly preferred because of advantages such as mass productivity and cost. As a specific method, an organic dye as a recording material is dissolved in a suitable solvent together with a binder or the like to prepare a coating solution, and this coating solution is then applied to a surface on which a substrate pregroove is formed by spin coating. After coating to form a coating film, it is formed by drying. The temperature at the time of applying the spin coating method is preferably 25 to 60 ° C., more preferably 30 to 50 ° C. from the viewpoint of productivity and heat resistance of the dye.

  Examples of the organic dyes include cyanine dyes, oxonol dyes, metal complex dyes, azo dyes, and phthalocyanine dyes. Among these, phthalocyanine dyes are preferable because of excellent light resistance and durability. JP-A-4-74690, JP-A-8-127174, 11-53758, 11-334204, 11-334205, 11-334206, 11-334207 The dyes described in JP-A-2000-43423, JP-A-2000-108513, JP-A-2000-158818, and the like are also preferably used.

  Solvents for the coating solution of the optical information recording layer made of organic dyes include butyl acetate, ethyl lactate, 2-methoxyethyl acetate, methyl ethyl ketone, cyclohexanone, methyl isobutyl ketone, dichloromethane, 1,2-dichloroethane, chloroform, dimethylformamide, methylcyclohexane. , Ethers such as tetrahydrofuran, ethyl ether, dioxane, alcohols such as ethanol, n-propanol, isopropanol, n-butanol diacetone alcohol, 2,2,3,3-tetrafluoropropanol, ethylene glycol monomethyl ether, ethylene glycol monoethyl Examples include ether and propylene glycol monomethyl ether. The above solvents can be used alone or in combination of two or more in consideration of the solubility of the recording material used. Various additives such as a binder, an antioxidant, a UV absorber, a plasticizer, and a lubricant may be further added to the coating solution depending on the purpose.

  Examples of the binder include gelatin, cellulose derivative, dextran, rosin, rubber, polyethylene resin, polypropylene resin, polystyrene resin, polyisobutylene resin, polyvinyl chloride resin, polyvinylidene chloride resin, polyvinyl chloride / polyvinyl acetate copolymer Examples include coalesced resins, polymethyl acrylate resins, polymethyl methacrylate resins, polyvinyl alcohol, chlorinated polyethylene resins, epoxy resins, butyral resins, rubber derivatives, and phenol / formaldehyde resins. When a binder is used in combination as a material for the optical information recording layer, the amount of the binder used is generally in the range of 0.01 to 50 mass times in solid content with respect to the organic dye, preferably 0.1 to 5 mass. Double range. Thus, the density | concentration of the organic pigment | dye in the coating liquid prepared is generally the range of 0.01-10 mass% by solid content, Preferably it is the range of 0.1-5 mass%.

  The optical information recording layer can contain various anti-fading agents represented by singlet oxygen quenchers in order to improve the light resistance of the optical information recording layer. As the antifading agent, a singlet oxygen quencher is generally used. The amount of the anti-fading agent such as a singlet oxygen quencher used is usually in the range of 0.1 to 50% by mass, preferably in the range of 0.5 to 45% by mass with respect to the amount of the organic dye. More preferably, it is the range of 3-40 mass%.

  The optical information recording layer made of an inorganic substance is formed using a vapor deposition method, an ion plating method, a sputtering method, or the like. Among these, the sputtering method is particularly preferable from the viewpoint of mass productivity and cost.

  The inorganic substance is preferably a phase change type optical recording material composed of at least Ag, Al, Te, and Sb that can take at least two states of a crystalline state and an amorphous state. A known dielectric layer is formed on the optical information recording layer as necessary.

  The optical information recording layer in the present invention may be a single layer or a multilayer, and the layer thickness varies depending on the type of the optical information recording layer, but is generally in the range of 20 to 500 nm, preferably in the range of 30 to 300 nm, more preferably The range is 50 to 100 nm.

  The light reflecting layer 12 is provided on the optical information recording layer 11 and is provided for reflecting the laser light normally irradiated from the substrate 10 side to the substrate side. After forming the optical information recording layer, a light reflecting material is deposited on the optical information recording layer by vapor deposition, sputtering or ion plating to form a light reflecting layer. In forming the light reflection layer, a mask is usually used, and the formation region of the light reflection layer can be adjusted thereby.

  For the light reflecting layer, a light reflecting material having a sufficiently high reflectivity with respect to laser light is used. The reflectance is preferably 70% or more. As a light reflective material having a high reflectance, Mg, Se, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Re, Fe, Co, Ni, Ru, Rh, Pd , Ir, Pt, Cu, Ag, Au, Zn, Cd, Al, Ga, In, Si, Ge, Te, Pb, Po, Sn, Bi, and other metals and semi-metals or stainless steel. These light reflecting materials may be used alone, or may be used in combination of two or more kinds or as an alloy. Among these, Cr, Ni, Pt, Cu, Ag, Au, Al, and stainless steel are preferable. From the viewpoint of cost and reflectance, Au, Ag, Al, or an alloy thereof is particularly preferable, and Au, Ag, or an alloy thereof is most preferable.

  The thickness of the light reflecting layer is generally in the range of 10 to 300 nm, and preferably in the range of 50 to 200 nm.

  The protective layer 13 is provided on the light reflecting layer 12 for the purpose of preventing moisture from entering and scratching. The thickness is preferably in the range of 1 to 200 μm, more preferably in the range of 50 to 150 μm. The material constituting the protective layer is preferably a radiation curable resin, a visible light curable resin, a thermosetting resin, silicon dioxide, or the like, and more preferably a radiation curable resin. As the radiation curable resin, for example, various ultraviolet curable resins such as “SD-640” manufactured by Dainippon Ink and Chemicals, Inc. can be used.

  The protective layer is formed by a casting method, a spin coating method, a screen printing method, or the like. In particular, by applying the spin coating method, damage to the optical information recording layer (dissolution of the dye, chemistry of the dye and the protective layer material) The protective layer can be formed without giving a reaction or the like. In the case where an ultraviolet curable resin is used for the protective layer, after the protective layer is formed, the ultraviolet curable resin is cured by irradiating ultraviolet rays from above the protective layer with an ultraviolet irradiation lamp (metal halide lamp). Further, in order to eliminate the thickness unevenness of the protective layer to be formed, a treatment such as leaving it for a certain period of time before curing the resin may be appropriately performed.

  In the present invention, the base layer 14 as described above is provided between the protective layer and the printable label layer. Any known method may be used for forming the underlayer, but from the viewpoint of productivity, it is preferable to contain an ultraviolet curable resin and to form it by a spin coating method. As a film thickness of a base layer, 0.1-100 micrometers is preferable, 1-50 micrometers is more preferable, and 3-20 micrometers is the most preferable.

  As described above, an information recording medium comprising a laminate in which an optical information recording layer, a light reflecting layer, a protective layer, a base layer, and a printable label layer are sequentially provided on a substrate is produced.

  The information recording medium of the present invention is used because the track pitch of the pregroove formed on the substrate and the material constituting the optical information recording layer are appropriately set, so that the track pitch is narrower than that of a conventional DVD or the like. The present invention can also be applied to an information medium in which information can be recorded / reproduced with a laser beam having a wavelength smaller than the laser beam.

  Hereinafter, examples will be shown to describe the present invention more specifically, but the present invention is not limited to these examples, and various applications can be made within the scope of the claims. In the description, “parts” and “%” are solid parts by mass and solids mass%.

<Preparation of information recording medium before application of printable label layer>
A polycarbonate resin substrate having a thickness of 1.2 mm and a groove having a width of 0.45 μm and a depth of 155 nm was produced by injection molding. On this substrate, an alcohol solution of a metal-containing azo dye was applied by spin coating and dried to form an optical information recording layer having a thickness of 70 nm. Subsequently, silver was sputtered on the optical information recording layer to form a light reflecting layer having a thickness of 60 nm. Subsequently, an ultraviolet curable resin (trade name: SD318, manufactured by Dainippon Ink & Chemicals, Inc.) is applied onto the light reflecting layer by spin coating, and cured by irradiating with ultraviolet rays using a high-pressure mercury lamp. A protective layer having a thickness of 5 μm was formed. Thereafter, ultraviolet curable ink (trade name: SSD F27, manufactured by Dainippon Ink & Chemicals, Inc.) was printed by screen printing, and after 20 seconds, cured by irradiating with ultraviolet light using a high-pressure mercury lamp. A white underlayer was formed. The information recording medium 1 before coating of the printable label layer comprising the substrate, the optical information recording layer, the light reflecting layer, the protective layer, and the base layer was produced by the above steps. The ISO whiteness of the information recording medium 1 before application of the printable label layer was 92%, and the surface roughness Ra was 0.12.

<Coating of printable label layer>
Add 4 parts of dimethyldiallylammonium chloride homopolymer (molecular weight 9,000) and 100 parts of vapor phase silica (average primary particle diameter 12 nm) to water, and use a sawtooth blade type disperser (blade peripheral speed 30 m / sec). Used to prepare a pre-dispersion. Next, the obtained preliminary dispersion was treated with a high-pressure homogenizer to prepare a gas phase method silica dispersion having a solid content concentration of 20%. The average secondary particle diameter of the vapor phase method silica was 130 nm as observed with an electron microscope.
In this dispersion, 60 parts of urethane acrylate resin (NK Oligo UA-7100, manufactured by Shin-Nakamura Chemical Co., Ltd.) and zirconium acetate (first rare element chemical industry) (100 parts by vapor phase method silica) Co., Ltd. Zircozol ZA-20) 2 parts, photopolymerization initiator (2-hydroxy-2-methyl-1-phenyl-propan-1-one) 1 part was added sequentially, and finally the concentration became 40% by mass. Thus, water was added to obtain a printable label layer coating solution. The printable label layer coating solution was applied to the information recording medium 1 before coating the printable label layer by a screen printing method so that the solid coating amount was 15 g / m ² , and 9 m under an ultraviolet lamp with an irradiation energy of 80 W / cm. After being cured by passing at a conveyance speed of / min, air of 50 ° C. was blown and dried to produce the information recording medium of Example 1.

  The same procedure as in Example 1 was carried out except that the UV curable resin used in the printable label layer of Example 1 was changed to an epoxy acrylate resin (Arakawa Chemical Industries, Ltd. Beam Set AQ-9). An information recording medium was produced.

  The same procedure as in Example 1 was conducted except that the vapor phase silica used in the printable label layer of Example 1 was changed to wet method silica (oil absorption 200 ml / 100 g, average primary particle size 16 nm, average aggregated particle size 9 μm). Then, an information recording medium of Example 3 was produced. The average secondary particle diameter after dispersion and pulverization of the wet process silica was 170 nm.

  An information recording medium of Example 4 was produced in the same manner as in Example 1 except that the number of UV curable resins contained in the printable label layer of Example 1 was changed to 90 parts.

  The information recording medium of Example 5 was the same as Example 1 except that the zirconium acetate used in the printable label layer of Example 1 was changed to basic polyaluminum hydroxide (Purechem WT manufactured by Riken Green Co., Ltd.). Produced.

  The information recording medium 2 before the printable label layer coating is the same as the information recording medium 1 before the printing of the printable label layer, except that the base layer of the information recording medium 1 before the printing of the printable label layer is formed to have a thickness of 5 μm. Was made. Subsequently, an information recording medium of Example 6 was produced in the same manner as in Example 1 except that the pre-printable label layer information recording medium 2 was used. Note that the ISO whiteness of the information recording medium 2 before application of the printable label layer was 78%, and the surface roughness Ra was 0.14.

  The printable label layer is the same as the information recording medium 1 before application of the printable label layer except that the time from the screen printing of the underlayer of the information recording medium 1 before application of the printable label layer to the irradiation of ultraviolet rays is set to 5 seconds. A pre-coating information recording medium 3 was produced. Subsequently, an information recording medium of Example 7 was produced in the same manner as in Example 1 except that the information recording medium 3 before coating the printable label layer was used. The ISO whiteness of the information recording medium 3 before the application of the printable label layer was 92%, and the surface roughness Ra was 0.22.

(Comparative Example 1)
In the gas phase method silica dispersion used in Example 1, 4 parts of boric acid, 20 parts of polyvinyl alcohol (saponification degree 88%, average degree of polymerization 3500), and 2 parts of zirconium acetate are sequentially added to 100 parts of the gas phase process silica. Finally, water was added to a final concentration of 15% by mass to obtain a printable label layer coating solution.
Then, it applied to the information recording medium 1 before the printable label layer coating by a screen printing method so that the solid content coating amount was 17 g / m ^{2, and} was dried by blowing air at 50 ° C. An information recording medium was produced.

(Comparative Example 2)
Information of Comparative Example 2 is the same as in Example 1 except that the vapor phase silica used in the printable label layer of Example 1 is changed to pseudoboehmite (average primary particle size 14 nm, average secondary particle size 170 nm). A recording medium was produced.

(Comparative Example 3)
An information recording medium of Comparative Example 3 was produced in the same manner as in Example 1 except that the preliminary dispersion of vapor phase silica of Example 1 was used as it was in the formulation of the printable label layer. In addition, the average secondary particle diameter of the preliminary dispersion liquid of vapor phase method silica was 800 nm from observation with an electron microscope.

(Comparative Example 4)
An information recording medium of Comparative Example 4 was produced in the same manner as in Example 1 except that the zirconium acetate used in the printable label layer of Example 1 was not included.

(Comparative Example 5)
An information recording medium of Comparative Example 5 was produced in the same manner as in Example 1 except that the number of parts of the ultraviolet curable resin contained in the printable label layer of Example 1 was changed to 110 parts.

  Each of the obtained information recording media was evaluated as follows. The results are shown in Table 1.

<Color development>
Using a commercially available inkjet printer capable of optical disc label printing (manufactured by Seiko Epson Corporation, PMG800), print a person and a landscape image on the surface of the printable label layer, and compare with the silver salt photograph produced using the image, the following criteria The color developability was visually evaluated.
○: It has color development more than silver salt photograph.
Δ: The color developability is slightly inferior to that of silver salt photographs, but it can be used in practice.
X: The color developability is clearly inferior to that of a silver salt photograph.

<Glossiness>
Using a commercially available inkjet printer capable of optical disc label printing (manufactured by Seiko Epson Corporation, PMG800), print a person and a landscape image on the surface of the printable label layer, and compare with the silver salt photograph produced using the image, the following criteria The gloss was visually evaluated.
○: Glossiness more than silver salt photograph.
Δ: Slightly inferior in gloss compared to silver salt photograph, but practical use is possible.
X: The gloss is clearly inferior to that of a silver salt photograph.

<Ink absorbability>
Using a commercially available inkjet printer capable of optical disc label printing (Seiko Epson, PMG800), solid prints of red, blue, green, and black are made on the surface of the printable label layer. The amount of ink that was crimped and transferred to PPC paper was visually observed. Comprehensive evaluation was performed according to the following criteria.
○: Not transferred at all.
Δ: Slightly transferred but practically usable.
X: Most of the images are transferred and cannot be used.

<Water resistance of image>
Water was dropped on the solid print portion of red, blue, green, and black printed with <Ink Absorption>. The bleeding after 12 hours was observed and visually evaluated according to the following criteria.
○: There is almost no bleeding.
Δ: Slight bleeding occurs, but actual use is possible.
X: It is a level that causes significant bleeding and causes a problem.

<Error occurrence during electronic information recording>
Electronic information was recorded on the information recording medium that was label-printed with the <color development>.
○: Recordable.
X: Recording is impossible.

From the results shown in Table 1, the information recording medium of the present invention has good color developability, gloss, coating layer strength, and ink absorbability, and the occurrence of errors during information recording and reproduction due to substrate warpage was suppressed. Is. Further, from Examples 1 to 3, it is particularly preferable that the printable label layer contains a vapor phase silica and a urethane acrylate resin. Further, from Examples 6 to 7, the ISO whiteness of the underlayer is 80. % And the surface roughness Ra is 0.2 μm or less, it can be seen that the color developability and glossiness are further improved.
On the other hand, since the information recording medium of Comparative Example 1 did not use an ultraviolet curable resin as a resin binder, the printable label layer had a large drying shrinkage, the information recording medium was warped, and an error occurred during electronic information recording. . The information recording medium of Comparative Example 2 was inferior in ink absorbability because it did not use amorphous synthetic silica. Further, the information recording medium of Comparative Example 3 was inferior in glossiness and color developability because inorganic fine particles having an average secondary particle size exceeding 500 nm were used. Further, since the information recording medium of Comparative Example 4 does not use a water-soluble metal compound, the water resistance of the image cannot withstand actual use. Further, from the comparison between Example 1 and Comparative Example 4, it can be seen that the color developability and glossiness of the recorded image are improved by the inclusion of the water-soluble metal compound. Further, in the information recording medium of Comparative Example 5, since the content of amorphous synthetic silica was less than 50% by mass of the total solid content of the printable label layer, the ink overflowed and the ink absorbency was poor. .

It is explanatory drawing which showed the structure of the information recording medium.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Board | substrate 11 Optical information recording layer 12 Light reflection layer 13 Protective layer 14 Underlayer 15 Printable label layer

Claims (6)

  An information recording medium having at least an optical information recording layer, an underlayer and a printable label layer on a substrate, wherein the printable label layer is dispersed or pulverized in water to an average secondary particle diameter of 500 nm or less, An information recording medium comprising an ultraviolet curable resin and at least one water-soluble metal compound, and the content of the amorphous synthetic silica is 50% by mass or more of the total solid content of the printable label layer .   The information recording medium according to claim 1, wherein the amorphous synthetic silica is vapor phase method silica.   The information recording medium according to claim 1, wherein the ultraviolet curable resin is a urethane acrylate resin.   The information recording medium according to claim 1, wherein the water-soluble metal compound is a basic polyaluminum hydroxide compound or zirconium acetate.   The information recording medium according to claim 1, wherein the printable label layer is provided on the base layer by silk clean printing or spin coating.   The information recording medium according to any one of claims 1 to 5, wherein the ISO whiteness of the underlayer is 80% or more and the surface roughness Ra is 0.2 µm or less.

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