THERMAL TRANSFER SHEET

Abstract: This invention relates to a thermal transfer sheet composed of a base sheet and a dye carrier layer formed on one side of the base sheet, wherein the dye contained in the dye carrier layer is one represented by following general formula (I) or (II). This sheet gives rise to an image which is excellent in fastness and particularly excellent in stain resistance.

Description: TECHNICAL FIELD The present invention relates to a heat transfer sheet and, more particularly, to a heat transfer sheet capable of easily providing a recording image excelling in various fastness properties. BACKGROUND ART Heretofore, various heat transfer techniques have been known in the art, including sublimation type transfer systems wherein a sublimable dye is carried on a substrate sheet such as paper to make a heat transfer sheet, which is then overlaid on an imageable material (image-receiving material), for instance, a woven fabric made of polyester to apply heat energy in the form of a pattern from the back side of the heat transfer sheet, thereby transferring the sublimable dye into the imageable material. With a sublimation textile printing system of the above sublimation transfer systems, in which the imageable material used is made of, e.g., a polyester textile, relatively satisfactory dye transfer is achieved, since heat energy is applied over a relatively extended period of time so that the imageable material per se can be heated by that heat energy. With an advance in recording techniques, however, fine characters or figures or photographic images could have been formed at high speeds on imageable materials made of, e.g., polyester or paper sheets having thereon a dye-receiving layer by means of a thermal head, etc. In this case, the application of thermal energy is required to be achieved within a time as short as fractions of a second. However, no image of sufficient density can be obtained, since the sublimable dyes and imageable materials are not well heated within such a short time. In order to cope with such high-speed recording, sublimable dyes excelling in sublimability have thus been developed. However, problems with such dyes of excellent sublimability are that after transfer, they transfer into the imageable materials or bleed onto their surfaces with time, generally because of their low molecular weight. In consequence, the images, once formed, become out of order or blurred, or otherwise contaminate surrounding articles. In the art of heat transfer using sublimable dyes, there is thus still strong demand for the development of a heat transfer sheet which provides a clear image of sufficient density by the application of thermal energy within such short a time as mentioned above and imparts improved fastness properties to the formed image. It is therefore an object of the present invention to satisfy the above demand. DISCLOSURE OF THE INVENTION Thus, the present invention provides a heat transfer sheet for heat transfer recording comprising a substrate sheet and a dye carrying layer formed on its major side, characterized in that a dye included in said dye carrying layer is expressed by the following general formula (II): wherein: A stands for a cyano, alkoxycarbonyl, aryloxycarbonyl, carbamoyl, alkylcarbamoyl, arylcarbamoyl, alkylcarbonyl, arylcarbonyl, alkylsulfonyl, alkylsulfonylamino, arylsulfonyl or aryl group, R₁ denotes a substituted or unsubstituted alkyl, aralkyl or aryl group, or an atom or atomic group which form a five- or six-membered ring together with Z, R₂ indicates a substituted or unsubstituted alkyl, aralkyl or aryl group, said R₁ and R₂ may form a five- or six-membered ring which may include an oxygen or nitrogen atom, R₃ stands for a hydrogen or halogen atom or an alkyl, alkoxy or acylamino group which may include a substituent or alkylsulfonylamino, R₄ denotes a hydrogen or halogen atom or an alkyl, alkoxy, nitro, cyano, acylamino or aryl group which may include a substituent, Z indicates a hydrogen atom or an atom or atomic group which forms a five- or six-membered ring together with R₁, and n and m each are 1 or 2. Studies of the present inventors have revealed that it is an essential condition for conventional sublimation textile printing techniques using textiles or woven fabrics of polyester, etc. that the dye used is sublimable or vaporizable (i.e., has the property of being capable of transferring through a space present between a heat transfer sheet and a woven fabric), since the heat transfer sheet is unlikely to come into close contact with the woven fabric that is an imageable material due to the latter's surface being lacking in smoothness. In the case of using as an imageable material a polyester sheet or surface-processed paper having a plain surface, etc., however, it has now been found that as the heat transfer sheet is brought into full contact with the imageable material at the time of heat transfer, what is of crucial importance is not only the sublimability and vaporizability but also the property of the dye that it can travel thermally across the interface between the two brought into close contact with each other, and that such a property is greatly affected by the chemical structure of the dye used, what substituent it has, or where that substituent is located. This leads to another findings that with selected dyes of suitable molecular structures, even if they have a molecular weight so high that they are considered unusable from a common sense standpoint, good thermal transfer is achievable. It has thus been found that by using a heat transfer sheet carrying such a dye, it is possible to make a record of an image of high density and improved fastness properties even with the application of thermal energy within a very short time, since the dye used transfers easily into an imageable material. BEST MODE FOR CARRYING OUT THE INVENTION The present invention will now be explained in more detail with reference to the preferred embodiments. The present invention will then be explained. The dyes used according to the present invention are well-known in themselves, and may be easily prepared by the condensation-with-dehydration of dihydrobenzothiophene-1,1-dioxide derivatives expressed by the following general formula (III) with nitroso compounds expressed by the following general formula (IV) in the presence of solvents. wherein R₁-R₅, X and Z have the same meanings as defined above. In the present invention, preference is given to dyes which have a molecular weight of 400 or more, and in which the substituent Z is a hydrogen atom, R₁ and/or R₂ are an ethyl group which may have a hydroxyl group, R₃ is an alkyl or alkoxy group located at the ortho-position with respect to the azomethyne group or alkylsulfonylamino, R₄ is a hydrogen atom, A is a cyano or alkoxycarbonyl group and m and n are both 1. Preference is also given to dyes in which at least one of the substituents R₁-R₄ and A includes a polar group such as hydroxyl, amino, alkylamino, acylamino, sulfonylamino, aminocarbonyl, aminosulfonyl, alkoxycarbonyl, alkoxysulfonyl, cyano, alkoxy, phenyl, cycloalkyl or nitro group or a halogen atom. Illustrative dyes suitable for the present invention will now be summarized in Table B1 showing illustrative examples of the substituents R₁-R₄, A and Z, m and n in the general formula (II) as well as their molecular weight. No. Z R₁ R₂ m R₃ n R₄ A Molecular Weight 1 H C₂H₅ C₂H₅ 1 OC₂H₅ 1 H CN 434 2 H C₂H₅ C₂H₄OH 1 CH₃ 1 H CN 420 3 H C₂H₅ C₂H₅ 2 OC₂H₅ 1 H CN 478 4 H CH₃ Ph 1 Cl 1 NO₂ CN 503.5 5 H C₂H₅ C₂H₄Cl 1 CH₃ 1 NHCOCH₃ CN 495.5 6 CHCH₃ CH₂C(CH₃)₂ C₂H₅ 1 CH₃ 1 H CN 458 7 H C₆H₁₃ C₆H₁₃ 1 NHSO₂CH₃ 1 H CN 596 8 H *2 c₂H₅ 1 H 1 OC₂H₅ CN 515 9 H C₂H₅ C₂H₄OH 1 OC₂H₅ 1 H CN 450 10 H C₂H₅ C₂H₅ 2 CH₃ 1 CN CN 444 11 H C₂H₅ CH₂Ph 1 H 1 H CN 452 12 H C₂H₅ C₂H₄CN 1 NHCOCH₃ 1 Cl CN 506.5 13 H CH₃ C₂H₅ 1 CH₃ 1 H CN 404 14 H (CH₂)₅ 1 CH₃ 1 H COOCH₃ 463 15 H C₂H₅ C₂H₅ 1 CH₃ 1 H COOCH₃ 437 16 H C₂H₅ C₂H₅ 1 CH₃ 2 CN COOCH₃ 487 17 H C₂H₅ C₂H₄OH 1 C₄H₉ 1 CH₃ COOC₂H₅ 524 18 H C₂H₅ C₂H₅ 1 OC₂H₅ 1 Cl COOC₂H₅ 480 19 H C₂H₅ C₂H₅ 1 CH₃ 1 H COPh 483 20 H C₂H₄OH C₂H₅ 1 CH 1 H CONHC₃H₇ 480 21 H C₂H₄OH C₂H₅ 1 CH₃ 1 H CONHPh 514 22 H C₂H₅ C₂H₅ 1 CH₃ 1 H NHSO₂CH₃ 472 23 H C₂H₄OH C₂H₅ 1 CH₃ 1 H Ph 471 The heat transfer sheets according to the present invention are characterized in the such specific dyes as mentioned above are used, and may otherwise be identical with conventional known heat transfer sheets. As the substrate sheets used for the heat transfer sheets containing the above dyes according to the present invention, use may be made of any known material having some heat resistance and strength. By way of example alone, use may be made of paper sheets, various-processed paper sheets, polyester films, polystyrene films, polypropylene films, polysulfone films, polycarbonate films, aramide films, polyvinyl alcohol films, cellophane and so on, all having a thickness of about 0.5 to 50 µm, preferably about 3 to 10 µm. Particularly preference is given to polyester films. The dye carrying layers formed on the surfaces of such substrate sheets as mentioned above may be obtained by carrying the dyes of the general formula (II) on any suitable binder resin. As the binder resins to carry thereon the above dyes, use may be made of any known resin. Preferable to this end are cellulosic resins such as ethyl cellulose, hydroxyethyl cellulose, ethylhydroxy cellulose, hydroxypropyl cellulose, methyl cellulose, cellulose acetate and cellulose acetate butyrate; and vinylic resins such as polyvinyl alcohol, polyvinyl acetate, polyvinyl butyral, polyvinyl acetal, polyvinyl pyrrolidone and polyacrylic amide. Of these resins, particular preference is given to polyvinyl butyral and polyvinyl acetal in view of heat resistance and dye-transfer properties. The dye carrying layers of the heat transfer sheets according to the present invention are basically formed of the above materials and, if required, may include various additives such as those heretofore known in the art. Preferably, such a dye carrying layer may be formed on the above substrate sheet by dissolving or dispersing the above dye, binder resin and any other components in a suitable solvent to prepare a coating or ink liquid for the formation of the dye carrying layer and, then, coating it on the substrate, followed by drying. Suitably, the carrying layer formed in this manner has a thickness of about 0.2 to 5.0 µm, preferably about 0.4 to 2.0 µm and a dye content of 5 to 70% by weight, preferably 10 to 60% by weight based on the weight thereof. The heat transfer sheets of the present invention may be successfully used as such for the purpose of heat transfer. By the provision of an anti-tack layer, i.e., a release coat on the surface of the dye carrying layer, however, it is possible to prevent the heat transfer sheet from sticking to an imageable material at the time of heat transfer and hence use a much more increased heat transfer temperature, thereby forming an image of much more improved density. Some anti-tack effect may be obtained by using only anti-tack inorganic powders for that release layer. However, more preferable results are obtained by forming a release layer of 0.01 to 5 µm, preferably 0.05 to 2 µm in thickness from a resin having excellent releasability such as silicone polymers, acrylic polymers and fluorinated polymers. It is understood that such inorganic powders or releasable polymers as mentioned above produce a sufficient release effect, even if they are contained in the dye carrying layer. Further, such a heat transfer sheet may additionally be provided on its back side with a heat-resistant layer so as to prevent the heat of a thermal head from having an adverse influence thereon. The imageable material used for forming an image with such a heat transfer sheet as mentioned above may be any material having its recording surface capable of receiving the above dye. In the case of paper, metal, glass, synthetic resin or the like having the property of being incapable of receiving the dye, they may be provided on one of their major surfaces with a dye receiving layer. As the imageable materials which may not contain any dye receiving layer, use may be made of fibers, woven fabrics, films, sheets and formings formed of, for instance, polyolefinic resins such as polypropylene, halogenated polymers such as polyvinyl chloride and polyvinylidene chloride, vinylic polymers such as polyvinyl acetate and polyacrylic esters, polyester resins such as polyethylene terephthalate and polybutylene terephthalate, polystyrene resins, polyamide resins, copolymeric resins of olefins such as ethylene and propylene with other vinylic monomers, ionomers, cellulosic resins such as cellulose diacetate and polycarbonate. Particular preference is given to polyester sheets or films or processed paper having a polyester layer. Non-dyeable imageable materials such as paper, metal and glass may be formed into imageable materials by coating a solution or dispersion of such a dyeable resin as mentioned above on their recording surfaces, followed by drying, or laminating a film of such resins thereon. As is the case with the above paper, such a dyeable imageable material may additionally be formed on its surface with a dye receiving layer of a resin of much more improved dyeability. The dye receiving layer prepared in this manner may be formed of a single material or a plurality of materials. As a matter of course, it may contain various additives, provided that the desired object is achievable. Such a dye receiving layer may have any suitable thickness but may generally be 3 to 50 µm in thickness. Although the dye receiving layer should preferably be provided in the form of a continuous coat, it may be provided in the form of a discontinuous coat by using a resin emulsion or dispersion. The imageable material is basically as mentioned above and may successfully be used as such. However, this imageable material or its dye receiving layer may contain inorganic powders for anti-tack purposes. In this way, much more improved heat transfer is achievable, since the heat transfer sheet is prevented from sticking to the imageable material even at elevated heat transfer temperatures. By far the most preference is given to finely divided silica. In place of or in combination with such inorganic powders as the above silica, such resins of improved releasability as already indicated may be added. By far the most preference is given to cured silicone compounds, typically, cured products comprising epoxy modified silicone oil and amino modified silicone oil. Such a release agent may preferably account for about 0.5 to 30% by weight of the dye receiving layer. In addition, the imageable material used may be either deposited on the surface of its dye receiving layer with such inorganic powders as already indicated so as to better its anti-tack effect or provided thereon with a layer consisting of such a release agent of improved releasability as already indicated. At a thickness of about 0.01 to 5 µm, such a release layer produces an effect so sufficient that much more improvements can be introduced in dye acceptability, while preventing any sticking of the dye receiving layer of the heat transfer sheet to the imageable layer. As the thermal energy applying means used for carrying out heat transfer with such a heat transfer sheet of the present invention as already indicated and such an imageable material as already stated, any of conventional means hitherto known in the art may be used. For instance, the desired object is successfully achievable by the application of a heat energy of about 5 to 100 mJ/mm² for a controlled recording time with such recording equipment as a thermal printer (e.g., Video Printer VY-100 made by Hitachi Co., Ltd., Japan). According to the present invention as detailed above, although the dye used for the heat transfer sheet of the present invention is much higher in molecular weight than sublimable dyes used for conventional heat transfer sheets (having a molecular weight of about 150 to 250), yet it shows improved thermal transferability and excellent dyeability and color developability with respect to the imageable material due to its specific structure and its having a substituent at a specific position. Moreover, it is unlikely to transfer through, or bleed on, the heat transfer sheet after transferring. Thus, the image formed with the heat transfer sheet of the present invention is so high in its fastness properties and so particularly improved in its resistance to both transfer and contamination that it cannot possibly be blurred or contaminate other articles, thus making it possible to solve various problems of the prior art. Reference Example B1 Dissolved in toluene were 1.2 parts of the nitroso compound expressed by the following structural formula: Then, a solution of 1.0 part of the dihydrobenzothiophene-1,1-dioxide derivative expressed by the following structural formula: in toluene was added dropwise to the resulting solution under cooling conditions. After the completion of the dropwise addition, the reaction was continued for a further two hours. Afterwards, the precipitated crystals were filtrated for separation, and then recrystallized from ethyl acetate to obtain 0.7 parts of the dihydrobenzothiophene methyne dye expressed by the following structural formula: The maximum absorption wavelength (ethyl acetate) of said dye was found at 635 nm. Reference Examples B2 to B22 With the nitroso compounds and dihydrobenzophenone-1,1-dioxide derivatives corresponding to Nos. 2 to 23 set forth in Table B1, Reference Example B1 was repeated to obtain the azomethyne dyes set forth in Table B1. Example B Prepared was an ink composition for the formation of a dye carrying layer, composed of the following ingredients, which was then coated a 6-µm-thick polyethylene terephthalate film subjected to heat-resistant treatment on its back side in a quantity of 1.0 g/m² on dry basis. Subsequent drying gave the heat transfer sheets according to the present and comparative examples. Dyes set forth in Table B1 3 parts Polyvinyl butyral resin 4.5 parts Methyl ethyl ketone 46.25 parts Toluene 46.25 parts It is noted, however, that when the dyes were insoluble in the above composition, DMF, dioxane, chloroform, etc. were optionally used as the solvents. Next, a coating solution composed of the following ingredients was coated on one side of a substrate sheet formed of synthetic paper (Yupo FPG #150 made by Oji Yuka Co., Ltd.) in an amount of 10.0 g/m² on dry basis, which was then dried at 100°C for 30 minutes to obtain an imageable material. Polyester resin (Vylon 200 made by Toyobo Co., Ltd., Japan) 11.5 parts Vinyl chloride/vinyl acetate copolymer (VYHH made by UCC9 5.0 parts Amino modified silicone (KF-393 made by the Shin-Etsu Chemical Co., Ltd., Japan) 1.2 parts Epoxy modified silicone (X-22-343 made by the Shin-Etsu Chemical Co., Ltd., Japan) 1.2 parts Methyl ethyl ketone/toluene/cyclohexanone (4:4:2 in weight ratio) 102.0 parts Each of the above heat transfer sheets according to the present invention was overlaid on the above imageable material with the dye-carrying and - receiving layers located in opposition to each other. Then, recording was carried out from the back side of the heat transfer sheet with a thermal head under the following conditions: at a voltage of 10 V applied to the head for a printing time of 4.0 msec. The results are summarized in Table B2. Comparative Examples B1 to B4 Example B1 was repeated, provided however that the dyes set forth in the following Table B3 were used in place of the dyes used therein. The results are shown in Table B3. Comp. Ex. Density of Developed Color Fastness Hue B1 0.99 X Indigo B2 1.16 ▵ Indigo B3 2.07 X Indigo B4 1.12 ▵ Indigo B5 1.02 X Purple B1: Disperse blue 14 B2: Disperse blue 134 B3: Solvent blue 63 B4: Disperse blue 26 B5: Disperse violet 4 It is noted that the density of developed color as referred to above was measured with Densitometer RD-918 made by Macbeth Co., Ltd. U.S.A. Fastness was measured after the recorded images had been allowed to stand in an atmosphere of 50°C for an extended period of time, and was estimated as follows. Double circles indicate that the sharpness of the images underwent no change at all and that when the test pieces were rubbed with white paper, it was not altogether colored; circles that the images lost sharpness with slight coloration of white paper; triangles that the images lost sharpness with white paper being colored; and crosses that the images became blurred with noticeable coloration of white paper. Light resistance was measured according to JIS L 0842 and estimated as follows. Double circles indicate the test pieces having an initial fastness of 3 or more, as determined according to the secondary exposure method provided by JIS L 0841; circles the test pieces having a value something of the order of 3; and crosses the test pieces having a value below 3. INDUSTRIAL APPLICABILITY The heat transfer sheets of the present invention find wide applications as imaging materials in heat transfer systems with thermal heads.

Claims: A heat transfer sheet for heat transfer recording comprising a substrate sheet and a dye carrying layer formed on its major side, characterized in that a dye included in said dye carrying layer is expressed by the following general formula (II): wherein: A stands for a cyano, alkoxycarbonyl, aryloxycarbonyl, carbamoyl, alkylcarbamoyl, arylcarbamoyl, alkylcarbonyl, arylcarbonyl, alkylsulfonyl, alkylsulfonylamino, arylsulfonyl or aryl group, R₁ denotes a substituted or unsubstituted alkyl, aralkyl or aryl group, or an atom or atomic group which form a five- or six-membered ring together with Z, R₂ indicates a substituted or unsubstituted alkyl, aralkyl or aryl group, said R₁ and R₂ may form a five- or six-membered ring which may include an oxygen or nitrogen atom, R₃ stands for a hydrogen or halogen atom or an alkyl, alkoxy or acylamino group which may include a substituent or alkylsulfonylamino, R₄ denotes a hydrogen or halogen atom or an alkyl, alkoxy, nitro, cyano, acylamino or aryl group which may include a substituent, Z indicates a hydrogen atom or an atom or atomic group which forms a five- or six-membered ring together with R₁, and n and m each are 1 or 2. A heat transfer sheet as recited in Claim 1, wherein said dye has a molecular weight of at least 400. A heat transfer sheet as recited in Claim 1, wherein Z is a hydrogen atom, R₁ and/or R₂ are an ethyl group which may contain a hydroxyl group, R₃ is an alkyl or alkoxyl group located at the ortho-position with respect to the azomethyne group or alkylsulfonylamino, R₄ is a hydrogen atom, A is a cyano or alkoxycarbonyl group, and m and n each are 1. A heat transfer sheet as recited in Claim 1, wherein at least one of said substituents R₁-R₄ and A contains a polar group selected from the group consisting of a hydroxyl, amino, alkylamino, acylamino, sulfonylamino, aminocarbonyl, aminosulfonyl, alkoxycarbonyl , alkoxysulfonyl, cyano, alkoxy, phenyl, cycloalkyl, alkylsulfonylamino or nitro group or a halogen atom.

Inventors: KANTO JUNPEI, NAKAMURA MASAYUKI, SAITO HITOSHI, SATO YASUSHI, KANTO, JUNPEI, NAKAMURA, MASAYUKI, SAITO, HITOSHI, SATO, YASUSHI, KANTO, JUNPEI DAI NIPPON INSATSU KABUSHIKI KAISHA, NAKAMURA, MASAYUKI DAI NIPPON INSATSU K.K., SAITO, HITOSHI DAI NIPPON INSATSU K.K., SATO, YASUSHI DAI NIPPON INSATSU KABUSHIKI KAISHA
Applicants: DAINIPPON PRINTING CO LTD, DAI NIPPON INSATSU KABUSHIKI KAISHA
Application year: 1989
Main classification: B41M5
IPC codes: A61F2, B41M5