Optical recording medium

Abstract: In an optical recording medium, a light absorbent can be incorporated into a color developing layer without separately constituting a light absorbing layer by hold­ ing the light absorbent in a dispersed or dissolved state in a solvent soluble or heat fusible material. Thus, an optical recording medium which exhibits satisfactory image density with a small amount of the light absorbent can be provided.

Description: The present invention relates to a recording medium which uses light as a recording source and performs visually readable recording utilizing a material capable of emitting heat after thermal conversion of the absorbed light. In a direct recording system which does not require development and fixing steps, heat-sensitive recording papers which use basic dyes and organic developers as couplers are excellent in operation and maintenance. On this basis, heat-sensitive recording papers are widely used for facsimiles and printers. In this system, however, heat sensitive recording is conducted by bringing a thermal head or an exothermic IC pen into direct contact with the heat-sensitive recording paper. Consequently, colored substances in a fused state adhere to the thermal head or the exothermic IC pen and cause problems such as residue adhesion and sticking, and lead to problems of record obstruction and impairment of record quality. In particular, when a continuous line is drawn (as in the case of a plotter printer), it has been impossible to carry out continuous printing without residue adhesion. Further, in a recording system using a thermal head, it has been considered difficult to increase image resoltuion to 8 dots/mm or more. Consequently, non-contact recording systems using light have been proposed to eliminate the problems of residue adhesion and sticking and further improving image resolution. Japanese Patent Laid-Open Publication No. 4142/1979 describes a heat-sensitive recording medium prepared by coating a substrate with a heat-sensitive recording layer essentially consisting of a leuco dye, which comprises using a metallic compound having a lattice defect. The metallic compound absorbs in the visible and infrared regions, performs thermal conversion and enables heat-sensitive recording. Japanese Patent Laid-Open Publication No. 209594/1983 describes an optical recording medium prepared by lamianting on a substrate at least one layer consisting of a near infrared absorbent having an absorption wave length of 0.8 to 2 µm and a heat-sensitive color-developing layer. Japanese Patent Laid-Open publication No. 94494/1983 describes a recording medium obtained by coating a substrate with one or more heat-sensitive color-developing materials and one or more near infrared absorbents composed of compounds having a maximum absorption wave length of 0.7 to 3 µm. It is disclosed that recording on these recording media can be carried out by use of a hot plate or laser beam having a wave length in the vicinity of the near infrared region. Japanese Patent Laid-Open Publication Nos. 94494/1983 and 209594/1983 describe a method for obtaining a light-absorbing heat-sensitive color-developing layer by applying and drying a coating compound prepared by direct addition of a near infrared absorbent to a hest-sensitive color-developing material. These methods, however, exert a desensitizing effect or coloring effect on the heat-sensitive color-developing material. Most of these methods impair color development and the ground colors, and hence it is difficult and impractical to select a useful near infrared absorbing material. As a countermeasure to this problem, a method has been proposed to alternatively lamiante a layer containing the near infrared absorbent and another layer containing the heat-sensitive color-developing material. However, such an increase in the number of laminations is disadvantageous in manufacturing the recording material. The near infrared absorbents described in these publications are cyanine dyes, thiol nickel complexes and squalium dyes. Other near infrared absorbents which are known are nitroso compounds and metal complexes thereof, polymethine dyestuffs (cyanine dyestuffs), cobalt and palladium complexes of thiol, phthalocyanine dyestuffs, triallyl methane dyestuffs, immonium or diimmonium dyestuffs and naphthoquinone dyestuffs. These near infrared absorbents are described in the paper entitled "Near Infrared Absorption Dyestuffs" [Chemical Industry (Japan), 43 , May 1986]. All of the above conventional near infrared absorbents are remarkably colored and provide poor ground color for the optical recording medium containing the near infrared absorbents as such. In addition, when these near infrared absorbents are directly contained in the heat-sensitive color-developing materials, these absorbents exhibit a desensitizing action and hence color density is poor. When the optical recording medium is a photodisc, reading is carried out mechanically and hence a small optical contrast at the reading wave length serves for reading purposes. However, a readable contrast between image density and ground color is required for direct reading of recorded information with the naked eye. The above methods which use near infrared absorbents cannot provide sufficient contrast. Additionally, GB-A-2 112 160 describes a heat-sensitive recording material for use with an infrared laser (especially a CO 2 gas laser emitting at 10.6µm), containing (a) a color forming material, (b) a color developing material, and (c) a light absorbing material selected from natural and synthetic silicate compounds and products obtained by baking a zinc compound and a clay mineral at at least 500°C. In one example, the recording material is prepared using an aqueous dispersion of pva and actinolite as the light absorbing material. The present invention seeks to provide an optical recording medium having a high image density, good ground color and sufficient contrast for direct reading with the naked eye by utilizing a light absorbent capable of absorbing light other than in the visible region, specially that from a low output laser having a dominant wave length in the vicinity of the near infrared, thereby emitting heat, and a material capable of developing color by heat. Such an optical recording medium can be obtained by mounting on a substrate a near infrared absorbent which has been homogeneously dispersed or dissolved in a solvent-soluble or heat-fusible material as described in claim 1 and a material which thermally develops color by itself or by reaction with other substances. According to the present invention, there is provided an optical recording medium comprising a substrate bearing a color-developing layer comprising a near infrared absorbent which is capable of emitting heat by absorption of recording light, the near infrared absorbent having been previously homogeneously dispersed or dissolved in a molten heat-fusible material, or in a solution comprising a solvent-soluble material, the solvent being then removed, the near infrared absorbent and heat-fusible material/solvent soluble material mixture subsequently pulverized to a particle size of 0.1 to 3µm, and a material capable of developing color by itself or by thermal reaction with other substances present in the color-developing layer. Figure 1 of the accompanying drawings is referred to in detail below and illustrates a block diagram of a laser optical recording device for use in developing an optical printing by irradiating laser beams on an optical recording medium. The near infrared absorbent which is a component of the optical recording medium of this invention is a material for absorbing light from the recording source, converting the absorbed light into heat and emitting the heat out of the material. Preferred near infrared absorbents can absorb a broad range of light from the recording source and convert as much as possible into beat. The maximum absorption wave length of the near infrared absorbent preferably coincides with or is approximated by the dominant wave length of the recording beam in views of heat conversion efficiency and evolved quantity of heat. In order to obtain a record suitable for reading with the naked eye, the maximum wave length of the near infrared absorbent and the dominant wave length of the recording beam is preferably outside the visible region. The near infrared absorbent having the maximum absorption wave length in the visible region has a characteristic hue and strongly colored. Consequently, such light absorbent can only be used for specific applications. No particular limitation is imposed upon the light source for recording. It is, however, preferred that a small and simple device can develop sufficient intensity of light and the near infrared absorbent can evolve enough heat to develop full coloration of the thermally color-developing material. It is also preferred that the light beam can be readily condensed and is safe. In consideration of these points, the best light source at present is a laser beam having a dominant wave length in the near infrared region. Most near infrared absorbents correspond to the material having a dominant wave length in the near infrared and absorbing light to carry out thermal conversion. The near infrared absorbent may have a characteristic absorption in the near infrared of 0.7 to 3 mµ and has been described in Japanese Patent Laid-Open Publication Nos. 4142/1979, 209594/1983 and 94494/1983. Exemplary absorbents which have been conventionally known include cyanine dyes, nickel complexes of thiol, squalium dyes; nitroso compounds and their metal complexes, polymethine dyes (cyanine dyes), cobalt or palladium complexes of thiol, phthalocyanine dyes, triallyl methane dyes, immonium or diimmonium dyestuffs and naphthoquinone dyestuffs, i.e., dyestuffs reported in the paper entitled "Near Infrared Absorption Dyestuffs", Chemical Industry (Japan), 43 , May, 1986; products obtained by mixing and heating treatment of thiourea derivatives such as 1,3-diphenylthiourea and 1,3-dibenzylthiourea with organic acid salts, alcoholates or hydroxides of metals having an atomic weight of 40 or more except metals in the groups Ia and IIa of the periodic table, i.e., compounds which have been found by the present inventors; and dispersible near infrared absorbents such as copper sulfide and graphite which are related to optical recording medium using dispersible near infrared absorbents. The component of the optical recording medium of this invention which thermally develops color by itself or by reaction with other substances is, for example, an electron donating colorless dye or electron accepting organic developer. These materials include compounds which have been known as components of heat-sensitive recording papers. Typical examples of the electron donating colorless dyes are leuco dyes including triphenylmethane dyes such as crystal violet lactone, fluoran dyes such as 3-diethylamino-6-methyl-7-anilinofluoran, azaphthalide dyes such as 3-(4-diethylamino-2-ethoxyphenyl)-3-(1-ethyl-2-methylindol-3-yl)-4-azaphthalide and fluorene dyes such as 3,6,6'-tris(dimethylamino)spiro[fluorene-9,3-phthalide]. Exemplary electron accepting organic developers include bisphenol-A's, 4-hydroxybenzoic acid ester, 4-hydroxyphthalic acid diesters, phthalic acid monoesters, bis(hydroxyphenyl)phenyl sulfides, 4-hydroxyphenylaryl sulfones, 4-hydroxyphenylarylsulfonates, 1,3-di[2-hydroxyphenyl)-2-propyl]-benzenes, 4-hydroxybenzoyloxybenzoic acid ester and bisphenol sulfones. These organic compounds are fully described in Japanese Patent Laid-open Publication No. 187082 (1987). The heat-sensitive color-developing layer can be prepared using the material capable of developing color by itself or by reaction with other substances as used in the manufacture of conventional heat-sensitive recording papers. That is, a coating compound is usually obtained by adding quality regulators to the material capable of developing color by itself or by reaction with other substances, and applied on the substrate to prepare the color-developing layer. The quality regulators include binder, white pigment, sensitizer, image stabilizer and other agents for suitably adjusting properties such as slip of the recording layer. Practical examples of the quality regulators which may be used are described in Japanese Patent Laid-Open No. 187082/1987 and other various known literature on heat-sensitive recording paper. In the present invention, the near infrared absorbent which absorbs the recording beam to carry out thermal conversion is not added directly to the heat-sensitive color -developing material. The present invention is characterized in that the above near infrared absorbent is previously dispersed or dissolved in a solvent soluble or molten heat fusible material, followed by pulverizing as described in claim 1, and then added to the heat-sensitive color-developing material to be used. No particular restriction is placed upon the heat fusible material for use in this invention so long as said material has a melting point equivalent to or lower than that of the thermally color-devleoping material by itself or by reaction with other substances. That is, any quality regulators including dye, color developer, sensitizer, image stabilizer and other additive which are presently used in the field of heat-sensitive recording paper can be used so long as these regulators satisfy the above condition. The sensitizer is particularly suitable among these quality regulators. The compounds illustrated below are preferred in particular, becasue these compounds are neutrals, contain fewer functional groups, have a melting point lower than that of the thermally color-developing material in a fused state by itself or by reaction with other materials and can dissolve both the color developing material and the near infrared absorbent. The heat fusible materials other than those illustrated above included, for example, fatty acid amides such as stearic acid amide and palmitic acid amide, ethylenebisamide, montan wax, polyethylene wax, dibenzyl terephthalate, benzyl p-benzyloxybenzoate, di-p-tricarbonate, p-benzylbiphenyl, phenyl-α-naphthylcarbonate, 1,4-dethoxynaphthalene, 1-hydroxy-2-naphthoic acid phenyl ester, and 1,2-di(3-methylphenoxy)ethylene. These heat fusible materials can be used singly or in combiantion. The heat fusible materials containing the near infrared absorbent can be prepared by dissolving or dispersing the near infrared absorbent in the heat fusible material in molten state, or by previously mixing the heat fusible material with the light absrobent and then heat-fusing the resulting mixture. The solvent soluble material for use in this invention includes a material which is selected from the above heat fusible material and soluble in solvent, material capable of developing color by itself or by react on with other substances, and the above heat-sensitive color-developing material, i.e., material having thermochromism which thermally develops color by itself, electron donating colorless dye and electron accepting developer. The solvent soluble material may also be selected from the image stabilizer and other quality regulators. The preferred image stabilizer includes, for examples, metal salts of phthalic acid monoester, metal salts of p-tert-butylbenzoic acid and metal salts of p-nitrobenzoic acid. Other quality regulators which may be preferably used included for examples sticking inhibitors such as metal salts of fatty acids, pressure color development inhibitors such as fatty acid amides, ethylene bis-amide, montan wax, and polyethylene wax; dispersants such as sodium dioctylsulfosuccinate, sodium dodecylbenzenesulfonate, sodium laurate, sodium salt of laurylalcoholsulfate ester and alginic acid; ultraviolet absorbers such as benzophenone derivatives and triazole derivatives; known antifoaming agents which can be used for dyestuff base heat-sensitive recording papers; fluorescence whitening dyes; and waterproofing agents. In the practice of this inventions, the near infrared absorbent dissolved or dispersed in the solvent soluble material or heat fusible material is prepared as follows: The near infrared absorbent is added to the solution containing the above solvent soluble material or added to the solvent together with the solvent soluble material, thoroughly mixed to dissolve or disperse in the solvent and the solvent is removed. When the near infrared absorbent is insoluble or difficulty soluble in the solvent, the near infrared absorbent is pulverized prior to use as such or in combination with a suitable dispersing agent by using a grinding machine such as a sand grinder or a dispersing machine. The particle size obtained after pulverization is the finer, the better, preferably 3 µm or less, more preferably 1 µm or less. Next, the above obtained near infrared absorbent which is dispersed or dissolved in the solvent soluble material or in the heat fusible material is wet ground as in the case of usual heat-sensitive materials by using a grinding machine or a dispersing machine such as an attritor, sand grinder or a ball mill. The near infrared absorbent thus obtained has a particle size of 0.1 to 3 µm and is uniformly mixed with other necessary components to obtain a coating liquid. For example, when the solvent soluble material or the heat fusible material is a dye which thermally develops color by itself, the near infrared absorbent is dissolved in the solvent together with the dye, dried to remove the solvent and pulverized. The mixture thus obtained is mixed with a binder, white pigment, sensitizer, and quality regulators to obtain the coating liquid. When the solvent soluble or heat fusible material has a melting point lower than that of one or more of the heat-sensitive color-developing material i.e., when the material is a so-called sensitizer, the light absorbent is uniformly mixed with the molten heat-fusible material, cooled and then pulverized. The resultant sensitizer containing the light absorbent is further mixed with the heat-sensitive material which develops color by itself or by reaction with other substances, binder, white pigment and quality regulators to prepare the coating liquid. In this case, the same sensitizer as that containing the near infrared absorbent or other kind of sensitizer may be further added. When the molten sensitizer containing the near infrared absorbent cannot sufficiently dissolve the heat-sensitive color-developing material, simultaneous use of other light absorbent which dissolve, in the molten state, the heat-sensitive color-developing material is a favorable method. When the solvent soluble material or the heat fusible material is one of the materials which thermally develops color by reaction with other substances, for example, the electron accepting organic developer, the electron accepting organic developer and the light absorbent are dissolved in a solvent, mixed and dried to remove the solvent. Alternatively, the developer and the absorbent are fused by heating, mixed and cooled. The resultant electron accepting organic developer containing the light abosrbent is pulverized and uniformly mixed with the electron donating colorless dye, binder, white pigment, sensitizer and quality regulators to obtain the coating liquid. The preferred binders for use in the coating liquid include, for examples, modified polyvinyl alcohols such as completely saponified polyvinyl alcohol having a polymerization degree of 200 to 1900 and amide modified polyvinyl alcohol, hydroxyethyl cellulose and styrene butadiene copolymer. The white pigments which may be added are inorganic or organic fillers which are usually used in the field of paper processing and include, for example, clay, calcium carbonated, and plastic pigment. In particular, a hollow plastic pigment is preferred because of excellent reflection of light and good retention of heat. As to various materials for use in the heat-sensitive color-developing layer, the basic colorless dye, the organic developer, the binder, the sensitizer, the filler and the quality regulators which have been described by the present inventors in Japanese Patent Application No. 272702/1988 may be similarly used in the present invention. Types and amounts of the electron donating colorless dye, electron accepting acid material, binder, sensitizer, filler and other various components in the color developing layer used in the present invention are dependent upon a required performance and recording properties, and not affected by particular restriction. However, it is usually desirable that the electron accepting acid material is 3 to 12 parts, the sensitizer is 3 to 12 parts and the filler is 1 to 20 aprts based on 1 part of the electron donating colorless dye and the binder is 10 to 25 parts in the total solids of the color developing layer. The coating liquid thus obtained is applied on the substrate with a coating machine such as a bar coater, air knife coater or blade coater which is generally used for paper coating to obtain the otpical recording medium. No particular limitation is imposed upon the substate. However, a base paper obtained by mixing pulp with fiber, if necessary, is most generally used in view of readiness in coating with a water-based coating compound. It is preferred to provide a protective layer for diminishing or preventing environmental contamination such as moisture, gases, water, solvents and oils. The preferable protective layer is permeable to visible light and recording light and does not have an adverse effect on the heat-sensitive color-developing layer. Usually, one or more binders are selected from those used for the heat-sensitive color-developing layer and employed for the protective layer. Alternatively, the protective layer may also contain a small amount of the light absorbent which is soluble in water or solvent and transmits visible light as much as possible. Furthermore, the underside of the heat-sensitive color-developing layer may also be equipped with a light absorbing layer containing the light absorbent which is dissolved or dispersed in the binder. As mentioned above, when the near infrared absorbent is used according to the present invention in the form of a dispersion or a solution in the solvent soluble or heat fusible material, an optical recording medium having high image density, good ground color and sufficient contrast for direct reading with the naked eye can be obtained. The reason is not clear but is thought to depend upon the following facts: (1) When the near infrared absorbent is contained in the heat fusible material (sensitizer) having a melting point lower than that of the heat-sensitive color-developing material, the evolved heat from the near infrared absorbent by absorbing the recording light immediately fuses the sensitizer around the near infrared absorbent. The fused sensitizer dissolves and mixes with the surrounding heat-sensitive color-developing component to emerge as a color developing phenomenon. That is, application of the sensitizer having light absorbing function accelerates the fusing rate of the sensitizer which is the rate-determined step and thus enhances the initiating step of color development. (2) Since the near infrared absorbent is located very close to the heat-sensitive color-developing component, energy converted from light into heat is rapidly and effectively transmitted. The process is restricted to the portion where light is absorbed and rapidly starts the initiation step of color development, and results in high image density and good resolution. (3) The near infrared absorbent does not directly contact the heat-sensitive color-developing component and is isolated in other colorless material. Consequently, characteristic coloring of the near infrared absorbent is masked. In addition, the coating liquid obtained by mixing with other components such as the heat-sensitive color-developing components has a low level of coloring. Even after application of the coating liquid, disadvantages such as coloring and desensitization due to mutual reaction are difficult to occur. As a result, the ground color of the optical recording medium is improved and visual contrast of the image is enhanced. The present invention will be illustrated with reference to the following examples. In the examples, parts and % are parts by weight and % by weight. The optical recording media obtained in the following examples and comparative examples were evaluated by the below described methods on the coloring density of the recorded spots and the ground color. The results are illustrated in Table 1. Density of the recorded image (colored spot): Density was measured with a microdensitometer (made by Konishiroku Photo Industry Co., Ltd.; Model PDM-5). Measured value was converted to Macbeth density. Ground color of the optical recording paper: Ground color was measured with a Macbeth densitometer. Example 1 A dispersion of an electron donating colorless dye (Liquid A), a dispersion of an electron accepting acid material (Liquid B) and a dispersion of a light absorbing sensitizer (Liquid C) having the following compositions, respectively, were separately wet ground for an hour with a sand grinder. Then, to a mixture 7.3 parts of Liquid (A), 30 parts of Liquid (B) and 20 parts of Liquid (C), 25 parts of 25% aqueous dispersion of white pigment silica (Mizukasil P-527; made by Mizusawa Kagaku Co., Ltd.) and 10 parts of 10% aqueous PVA solution were added and mixed to obtain a heat-sensitive color-developing coating liquid which contains the near infrared absorbent. The coating liquid thus obtained was applied on a wood free paper having a basic weight of 60 g/m 2 in an amount of 5 g/m 2 using a meyer bar, followed by drying to obtain an optical recording medium. Liquid (A): Electron donating colorless dye dispersion Black color developing dye: ODB 2.0 parts 10% PVA aqueous solution 3.4 parts Water 1.9 parts Total 7.3 parts ODB: 3-Diethylamino-6-methyl-7-anilinofluoran Liquid (B) : Electron accepting acid material dispersion Developer: BPA 6.0 parts 10% PVA aqueous solution 15.0 parts Water 9.0 parts Total 30.0 parts BPA: Bisphenol A Liquid (C): Light absorbent containing material dispersion p-Benzylbiphenyl (PBB) which is a sensitizer of the heat-sensitive color-developing recording paper was used as a heat fusible material. A near infrared absorbent NK-125 (made by Nippon Kankoshikiso Kenkyusho Co., Ltd.: Maximum absorption wave length, 741 mµ) was used as a near infrared absorbent. To 49 parts of PBB, 1 part of NK-125 was added, heated to 100-150°C and mixed in the molten state. The resulting mixture was cooled and pulverized to an average particle diameter of 1 .5 µm by using a sand grinder to give a light absorbent containing material. Liquid (C) has the following composition: The above light absorbent containing material 4.0 parts 10% PVA aqueous solution 10.0 parts Water 6.0 parts Total 20.0 parts The optical recording medium thus prepared was irradiated with laser beams by the use of a device shown in Figure 1 in which a semiconductor laser head (laser diode collimeter head LDC-8330-CINC: made by Applied Optic Co., Ltd., central wave elngth 830 nm, output 30 mW) was combined with a condenser (made by Olympus Optical Co., Ltd., MDPLAN 5, 0.1) in order to carry out optical printing. In this case, irradiation was carried out for 1/500 second. In Figure 1, 1 is the laser diode collimeter head, 2 is the shutter, 3 is the condenser group, 4 is the optical recording paper, and 5 is the power source. Example 2 An optical recording medium was prepared by following the procedures described in Example 1 except that the light absorbent containing material of Example 1 was prepared using 0.5 part of the near infrared absorbent NK-125 and 49.5 parts of sensitizer p-benzyl biphenyl. Example 3 An optical recording medium was prepared by following the procedures described in Example 1 except that 0.5 parts of the near infrared absorbent NK-125 was used and p-benzyl biphenyl was replaced by 49.5 parts of sensitizer MPE having the formula: Example 4 The light absorbent containing material described in Example 1 was prepared by adding 1 part of the near infrared absorbent NK-125 and 9 parts of p-benzyl biphenyl to 300 parts of acetone, stirring to give a homogeneous solution, removing acetone by distillation under reduced pressure, and pulverizing with a ball mill to a particle size of about 1 µm. Using 4 parts of the light absorblent containing material thus obtained, a dispersin of the material was prepared by the procedures described in Liquid (C) of Example 1. An optical recording medium was obtained by following the procedures described in Example 1. Examples 5-9 Optical recording media were prepared by following the procedures described in Example 1 except that types and amounts of the near infrared absorbents employed were changed as shown in Table 1. In Table 1, abbreviations are as follows: Dye: Electron donating colorless dye Developer: Electron accepting acid material Sensitizer: Heat fusible material Dye ODB-2: Dibutylamino-6-methyl-7-anilinofluoran S-205: 3-(N-ethyl-N-isoamyl)-6-methyl-7-anilinofluoran Developer POB: Benzyl p-hydroxybenzoate Sensitizer The symbol (') indicates that the agent was used in the form containing the light absorbent. Near infrared absorbent NK-123: Near infrared absorbent made by Nippon Kanko shikiso Kenkyusho Co., Ltd. Maximum absorption wave length 817 mµ. NK-2268: Near infrared absorbent made by Nippon Kanko Shikiso Kenkyusho Co., Ltd. Maximum absorption wave length 831 mµ. NK-2014: Near infrared absorbent made by Nippon Kanko shikiso Kenkyusho Co., Ltd. Maximum absorption wave length 778 mµ. ICIS 116510/2: Near infrared absorblent made by ICI in England. Maximum absorption wave length 770 mµ. It is clearly understood from Table 1 that the optical recording medium prepared using the near infrared absorbent which is previously dispersed in the heat fusible material (sensitizer) exhibits sufficient recording density with a very small amount of the near infrared absorbent. Comparative Examples 1 and 2 Optical recording media were prepared by following the procedures described in Example 1 except that the near infrared absorbent was omitted in the preparation of Liquid (C) and the near infrared absorbent containing material was replaced with the same amount of the heat fusible material (sensitizer). Results on quality evaluation are summarized in Table 1. These Comparative Examples indicate the cases for using conventional heat-sensitive recording papers as the optical recording media. As clearly shown in Table 1, the heat-sensitive recording papers containing no near infrared absorbent do not develop color at all by the irradiation of light. Comparative Example 3 The near infrared asorbent was not dispersed in the heat-fusible material in advance. The near infrared absorbent and the heat fusible material (sensitizer) were separately mixed as such with the heat-sensitive color-developing dispersion. Liquid (C-1) having the following composition was prepared using the near infrared absorbent ICIS 116510/2 (ICI) as the near infrared absorbent which was used in Examples 5 and 7. Liquid (C-2) having the following composition was prepared using PBB of Example 1 as the sensitizer. Liquid (C-1): Light absorbent dispersion Near infrared absorbent 1 part water 19 parts Total 20 parts Dispersion was obtained by violent stirring with a stirrer. Liquid (C-2): Sensitizer dispersion Sensitizer 4 parts 10% PVA aqueous solution 10 parts water 6 parts Total 20 parts Dispersion was obtained by violent stirring with a stirrer. An optical recording medium was prepared by following the procedures described in Example 1 except that 0.8 part of Liquid (C-1) and 20 parts of Liquid (C-2) were added in place of Liquid C of Example 1. The amount of the near infrared absorbent used was 1% of the sensitizer PBB. Results on recording evaluation are illustrated in Table 1. The amounts of the near infrared absorbent and the sensitizer used in the comparative example are almost the same as in Examples. However, as clearly shown in Table 1, both the ground color and the developed color density of the comparative example are inferior to those of the examples. Comparative Example 4 The near infrared absorbent was placed on the underside of the heat-sensitive color-developing layer. A light absorbent dispersion obtained using the near infrared absorbent NK-125 as in Example 1 was applied on the wood-free paper used in Example 1 in an amount of 0.1 g/m 2 (as solid) to provide a light absorbing layer. On the light absorbing layer thus obtained, a heat-sensitive color-developing layer was formed by applying an amount of 5 g/m 2 of the sensitizer dispersion which was the same as that used in Comparative Example 1. The optical recording medium thus obtained was evaluated by the same method as in Example 1. The amount of the near infrared absorbent applied was equivalent to 0.05 g/m 2 in this comparative exmaple and 0.02 g/m 2 in Example 1. That is, this comparative example used 2.5 times the amount of near infrared absorbent as compared with Example 1. However, as clearly understood from Table 1, the ground color and the density of developed color are much inferior to those of Example 1. Comparative Example 5 The near infrared absorbent was placed on the surface of the heat-sensitive color-developing layer. The light absorbent dispersion of Comparative Example 4 was applied to the surface of the optical recording medium prepared in Comparative Example 1 in an amount of 0.1 g/m 2 . The resulting optical recording medium having a light absorbing layer on the heat-sensitive color-developing layer was evaluated by the same method as Example 1. This comparative example used a greater amount of the near infrared absorbent than Example 1. However, the density of color development is much inferior to that of Example 1, as clearly shown in Table 1. Example 10 Developer POB (benzyl p-hydroxybenzoate) used in Example 7 was employed for the heat fusible material. To 49.75 parts of POB, 0.25 part of the near infrared absorbent NK-125 which was used in Example 1 was added, fused at about 120°C to give a homogeneous mixture, cooled and pulverized by an attritor into an average particle diameter of 0.9 µm. To 4 parts of the light absorbent containing material thus obtained, 10 parts of 10% PVA aqueous solution and 6 parts of water were added. The resulting mixture was wet ground for one hour with a sand grinder to obtain a light absorbent containing developer dispersion. To 20 parts of the light absorbent containing developer dispersion thus obtained, 7.3 parts of Liquid (A) from Example 1, 25 parts of 25% silica dispersion, and 10 parts of 10% PVA aqueous solution were added and mixed to obtain a heat-sensitive color-developing liquid which contains the near infrared absorbent. The coating liquid thus obtained was applied on a wood free paper having a basic weight of 60 g/m 2 in an amount of 5 g/m 2 with a meyer bar and dried to obtain an optical recording medium. The optical recording medium thus obtained was irradiated with laser beams using a device shown in Figure 1 in which a semicondcutor laser head (laser diode collimetor head LDC-8330-CINC; made by Applied Optic Co., Ltd., central wave length 830 nm, output 30 mW) was combined with a condenser having a numerical aperture of 0.28. Optical printing was carried out at a transfer rate of 1,5 mm/s using a rotary stage to obtain a black circular line record. As illustrated in Table 1, the optical recording medium was excellent in the density of the recorded image and the ground color and had a good contrast. Example 11 A 5% solution of a near infrared absorbent was obtained by dissolving 1 part of a water-soluble near infrared absorbent, ICI 116510 (made by ICI, maximum absorption wave length 765 mµ) in 19 parts of 10% PVA aqueous solution. The solution of near infrared absorbent thus obtained was applied to the surface of the optical recording medium prepared in Example 1 in an amount of 1 g/m 2 with a meyer bar to provide a light absorbing and protective layer. Thus an optical recording medium containing light absorbent NK-125 in the heat-sensitive color-developing layer and light absorbent ICI 116510 in the protective layer was obtained. The optical recording medium thus obtained was evaluated with a recording device used in Example 10. As illustrated in Table 1, the medium provided an image having an extremely high density. In addition, the medium had a faster rate of initiation in color development to recording light. Example Heat sensitive material Near infrared absorbing material Evaluation Dye Developer Sensitizer Kind Addition ratio Color density Ground color 1 ODB BPA PBB' NK-125' 2% 1.40 0.18 2 ODB BPA PBB' NK-125' 1% 1.38 0.12 3 ODB BPA MPE' NK-125' 1% 1.37 0.11 4 ODB BPA PBB' NK-125' 2% 1.36 0.16 5 ODB-2 BPA NOB' ICIS116510/2' 1% 1.35 0.14 6 S-205 BPA 2BPE' NK-2104' 1% 1.42 0.11 7 ODB POB 2BPE' ICIS116510/2' 1% 1.35 0.15 8 ODB BPA NOB' NK-123' 1% 1.35 0.13 9 S-205 BPA MPE' NK-123' 5% 1.42 0.19 10 ODB POB' - NK-125 0.5% 1.40 0.10 11 ODB BPA PBB' NK-125' (ICIS116510) 2% 1.43 0.20 Comparative Example 1 ODB BPA PBB - - 0.06 0.06 2 ODB BPA MPE - - 0.06 0.06 3 ODB BPA PBB (ICIS116510/2) - 0.89 0.36 4 ODB BPA PBB (NK-125) - 0.83 0.25 5 ODB BPA PBB (NK-125) - 0.73 0.23

Claims: An optical recording medium comprising a substrate bearing a color-developing layer comprising a near infrared absorbent which is capable of emitting heat by absorption of recording light, the near infrared absorbent having been previously homogeneously dispersed or dissolved in a molten heat-fusible material, or in a solution comprising a solvent-soluble material, the solvent being then removed, and the near infrared absorbent and heat-fusible material/solvent soluble material mixture subsequently pulverized to a particle size of 0.1 to 3µm, and a material capable of developing color by itself or by thermal reaction with other substances present in the color-developing layer. An optical recording medium according to claim 1, wherein the material capable of developing color by thermal reaction with other substances is in an electron-donating colorless dye and/or an electron-accepting acid material. An optical recording medium according to claim 2, wherein the electron accepting acid material is an electron accepting organic acid material. An optical recording medium according to any one of claims 1 to 3, wherein the heat-fusible material has a melting point lower than that of the material capable of developing color by thermal reaction with other substances.

Inventors: FUKUI HIROSHI, NAGAI TOMOAKI, SATAKE TOSHIMI, SEKINE AKIO, YOKOYAMA MIYUKI, FUKUI, HIROSHI,, NAGAI, TOMOAKI,, SATAKE, TOSHIMI,, SEKINE, AKIO,, YOKOYAMA, MIYUKI,, FUKUI, HIROSHI, C/O C. R. LAB. JUJO PAPER CO.LTD., NAGAI, TOMOAKI, C/O C. R. LAB. JUJO PAPER CO.LTD., SATAKE, TOSHIMI, C/O C. R. LAB. JUJO PAPER CO.LTD., SEKINE, AKIO, C/O C. R. LAB. JUJO PAPER CO.LTD, YOKOYAMA, MIYUKI, C/O C. R. LAB. JUJO PAPER CO.LTD
Applicants: JUJO PAPER CO LTD, NIPPON PAPER INDUSTRIES CO., LTD.
Application year: 1990
Main classification: B41M5
IPC codes: B41M5, G11B7