Thermal transfer sheet

Abstract: A thermal transfer sheet which is capable of preventing ground staining, trailing, etc., at the time of printing and has been improved in image density, resolution, etc., is provided by forming a sensitizing layer (3) having a lower melt viscosity than an ink layer (4) between the ink layer (4) and a substrate film (2), even when such a thermal transfer sheet is used for an n -fold recording mode. Further, a thermal transfer sheet suitable for an n -fold recording mode which is capable of uniformly transferring an ink layer, is capable of preventing white dropout, ground staining, trailing, etc., at the time of printing, and has been improved in image density, resolution, etc., is provided by forming a surface layer on the surface of the ink layer, and forming the ink layer and surface layer so that the ink layer has a melt viscosity of 1000-5000 cps at 100°C and the surface layer has a melt viscosity of 2000-10000 cps at 150°C.

Description: The present invention relates to a thermal transfer sheet, and, more specifically, to a thermal transfer sheet capable of preventing ground staining or trailing at the time of printing and of providing printed letters improved in image density and resolution, when used in a thermal transfer method wherein the moving speed of a transfer-receiving material is higher than that of the thermal transfer material (hereinafter, such a recording mode simply referred to as "n-fold recording mode"). Hitherto, in a case where output from a computer or word processor is printed by a thermal transfer system, there has been used a thermal transfer sheet comprising a substrate film and a heat-fusible ink layer disposed on one surface side thereof. Such a conventional thermal transfer sheet comprises a substrate film comprising a paper having a thickness of 10 to 20 µm such as capacitor paper and paraffin paper, or comprising a plastic film having a thickness of 3 to 20 µm such as polyester film and cellophane film. The above-mentioned thermal transfer sheet has been prepared by coating the substrate film with a heat-fusible ink comprising a wax and a colorant such as dye or pigment mixed therein, to form a recording material layer on the substrate film. One of the problems encountered in the above-mentioned conventional thermal transfer sheet is an economic problem such that a portion of the conventional thermal transfer sheet is only capable of conducting a single printing operation and therefore the thermal transfer sheet is consumed in a length which is the same that of the resultant printed letters. As the method of solving such a problem, there has been known a method using a thermal transfer sheet for multiple use which is capable of conducting plural printing operations by using the same portion thereof. In this method, however, the resultant image density is decreased as the number of printing operations becomes large, whereby it is difficult to provide printed letters having uniform image densities. As another method of solving the above-mentioned problem, there has been proposed an n-fold recording method wherein printing is effected so that the moving speed of a transfer-receiving material is higher than that of a thermal transfer sheet used in combination therewith ( the moving directions of the thermal transfer sheet and the transfer-receiving material may be the same or reverse to each other). In this method, when the moving speed of the transfer-receiving material is represented by N , the moving speed of the thermal transfer material is represented by N ', and N > N', the length of the printed portion is N , but the length of the consumed thermal transfer sheet is N '. Accordingly, for example, it is supposed that N = 5 and N' = 1, the length of the consumed thermal transfer sheet is 1/5 times that in the prior art. As a result, such a method is fairly economical. In this method, however, since the transfer-receiving material and the termal transfer sheet are moved so that they are rubbed with each other, ground staining and printed letter trailing are liable to occur, whereby it is difficult to obtain clear printed letters having a high resolution. In order to solve the problem of ground staining, Japanese Laid-Open Patent Publication (JP-A, KOKAI) No. 178088/1985 has proposed a method wherein a colorless wax layer is formed on the surface of an ink layer. However, since such a surface layer is removed by the above-mentioned rubbing, the problem is not sufficiently solved. On the other hand, in order to solve the trailing, there has been proposed a method wherein an ink layer is formed by using a wax having a relatively high melting point. In this method, however, the ink layer cannot provide a good wetting property with respect to the transfer-receiving material. Accordingly, in the case of a transfer-receiving material such as paper having a rough surface, void (or white dropout) is liable to occur, whereby it is difficult to obtain printed letters having high image density and high resolution. Further, Japanese Laid-Open Patent Publication No. 11381/1988 proposes a thermal transfer sheet wherein a layer predominantly comprising a wax is disposed between a substrate film and an ink layer predominantly comprising a vehicle of heat-fusible synthetic resin, so that the transferability of the ink to a transfer-receiving material is improved. In the n-fold recording method, however, such simple provision of the wax layer cannot effectively prevent the occurrence of void, whereby it is difficult to provide printed letters having high image density and high resolution. EP0173532 (DAI NIPPON) discloses a thermal transfer sheet comprising a substrate film, a releasable layer formed on one surface side of the substrate film and an ink layer formed on the surface of the releasable layer. The ink layer and the releasable layer are heat-fusible. The releasable layer improves the releasability between the base film and the ink layer so that transfer efficiency is improved and release sound reduced. DE-A-3634049 (KONISHIROKU PHOTO) discloses a thermal transfer sheet comprising a substrate film, a sensitizing layer formed on one surface side of the substrate film, and an ink layer formed on the surface of the sensitizing layer, wherein the sensitizing layer and the ink layer are heat-fusible. An object of the present invention is to solve at least some of the above-mentioned problems encountered in the prior art and in its most preferred embodiments to provide a thermal transfer sheet which is capable of preventing ground staining or trailing at the time of printing and is capable of providing printed letters improved in image density, resolution, etc.. The present invention provides a thermal transfer sheet comprising a substrate film, an ink layer formed on one surface side of the substrate film, and a surface layer formed on the surface of the ink layer, wherein the ink layer has a melt viscosity of 1000-5000 mPa.s (cps) at 100°C measured using a Rotovisco ® PK-100 viscometer, and the surface layer has a melt viscosity of 2000-10000 mPa.s (cps) at 150°C measured using a Rotovisco ® PK-100 viscometer. The above-mentioned thermal transfer sheet is one for an n-fold recording mode capable in preferred embodiments of providing images improved in image density and resolution as a consequence of prevention of causing ground staining, trailing and void at the time of printing. The invention will be further described and illustrated with reference to the accompanying drawings, in which: Figure 1 is a schematic sectional view showing an embodiment of a thermal transfer sheet according to the present invention but including an optional sensitising layer (3). Figure 2 is a schematic sectional view showing another embodiment of a thermal transfer sheet according to the present invention but including an optional sensitizing layer (13). Figure 1 is a schematic sectional view showing an embodiment of the thermal transfer sheet according to the present invention. Referring to Figure 1, the thermal transfer sheet (1) comprises a substrate film (2), an ink layer (4), and a surface layer (5) formed on one other surface side of the substrate film (2). The above-mentioned substrate film (2) is one capable of contacting a thermal head. Optionally there is provided a sensitizing layer (3). The substrate film (2) to be used in the present invention may be one selected from those used in the conventional thermal transfer sheet. However, the above-mentioned substrate film (2) is not restricted thereto and can be any of other films. Preferred examples of the substrate film (2) may include: plastic films such as those comprising polyester, polypropylene, cellophane, polycarbonate, cellulose acetate, polyethylene, polyvinyl chloride, polystyrene, nylon, polyimide, polyvinylidene chloride, polyvinyl alcohol, fluorine-containing resin, chlorinated rubber, and ionomer resin; papers such as capacitor paper and paraffin paper; non-woven fabric; etc.. The substrate film (2) can also comprise a combination or laminate of the above-mentioned films. The substrate film (2) may preferably have a thickness of 2 to 25µm, while the thickness can appropriately be changed corresponding to the materials thereof so as to provide suitable strength and heat conductivity. The ink layer preferably comprises a colorant and a vehicle. The ink layer can also contain an optional additive selected from various species thereof, as desired. The colorant may preferably be one having a good recording property as a recording material, which is selected from organic or inorganic dyes or pigments. For example, the colorant may preferably be one having a sufficient colorant density (or colouring power) and is not substantially faded due to light, heat, temperature, etc.. As a matter of course, the colorant may generally have a black colour, but may also have another colour such as cyan, magenta and yellow. In the present invention, since n-fold printing is effected by using an ink layer having a relatively small area, it is necessary to set a relatively high colorant concentration in the ink layer. The concentration can also vary depending on the thickness of the ink layer, but may preferably be 20-70 wt.%, more preferably 30-50 wt.% when the ink layer has a thickness in a preferred range of 3-20µm. If the concentration is too low, the image density may be insufficient. If the concentration is too high, the wettability of the ink to paper is poor, and void is undesirably liable to occur. When a black ink layer is formed as the ink layer, the ink layer comprises carbon black and a vehicle, and can also contain various additives, as desired. The carbon black is required preferably to have a specific surface area of 100 m²/g or above, (preferably 120 to 300 m²/g), and oil absorption of 130 cm³/100g or below (preferably 50 to 130 cm³/100g). When the specific surface area is below 100 m²/g, the colouring power of the carbon black is insufficient and it is difficult to obtain printed letters having a high image density. On the other hand, when the oil absorption exceeds 130 cm³/100 g, the melt viscosity of the ink layer becomes too high and the resolution of the resultant image is lowered. Commercially available examples of carbon black to be used in the present invention having the above-mentioned properties may include: MA-600, MA100, MA 7, MA8, #40, #44, #900, #950 mfd. by Mitsubishi Kasei K.K., Morgal ® L, Morgal ® BPL, mfd. by Cabot Co., Printex ® 80, Printex ® 85, Printex ® 90 mfd. by Degusa Co., #8200, #8500, #7550 and #7700 mfd. by Tokai Carbon K.K. The concentration of the carbon black in the ink layer may preferably be in the range of 20 to 30 wt.%. If the concentration is below the range, the resultant image density may be insufficient. If the concentration exceeds the above range, the melt viscosity of the ink layer may become too high. Further, the ink layer may preferably have a thickness of 3-20µm. If the thickness is below the range, the resultant image density may become insufficient. If the thickness exceeds the range, the printing sensitivity may become lowered. When a black dye such as nigrosine dye is used in a concentration of several wt.% based on the weight of the ink layer in combination with the carbon black, the resultant image density is not lowered and printed letter of jet-black colour can be obtained, even when the carbon black concentration is lowered. The vehicle may predominantly comprise a wax or may comprise a mixture of a wax and another component such as drying oil, resin, mineral oil, and derivatives of cellulose and rubber. In the present invention, a lubricating agent or lubricant can be added to the ink layer. Specific examples thereof may include lubricants having a lubricating property, such as wax, silicone wax, fluorine-containing resin, silicone resin, higher fatty acid amide, higher fatty acid ester, and surfactant. It is preferred to add such a lubricant in an amount of 0.2-5 wt. parts per 100 wt. parts of the ink layer. If the addition amount is below the above-mentioned range, the slip property between the substrate film and a transfer-receiving member due to heat accumulation at the time of printing may be insufficient. If the addition amount is too large, the adhesion property between the ink layer and substrate film may be undesirably decreased. In the present invention, it is also possible to use a diurethane compound and a resin compatible with the diurethane compound, as a vehicle. The diurethane compound used herein is one represented by the following general formula: wherein R denotes an alkyl group having 1-5 carbon atoms, and n denotes an integer of 2-10. In the present invention, it is particularly preferred to use a diurethane compound having a melting point of 70-90 °C, wherein R is methyl, ethyl or propyl group, and n is 6. It is known that the above mentioned compound is used as a binder of an ink layer, as disclosed in Japanese Laid-Open Patent Publication No. 82853/1982. The compatible resin is a resin having a compatibility with the diurethane resin in the coating liquid for forming the ink layer in the presence of a solvent, or in a heat-melted state thereof at the time of coating operation in the absence of a solvent. Specific examples of the compatible resin may include: cellulose derivatives such as nitrocellulose, acetylcellulose, cellulose propionate, cellulose butyrate, cellulose acetate butyrate, and benzyl cellulose. In addition, it is also possible to use many resins used as a binder for known gravure ink, such as polyurethane resin, vinyl chloride/vinyl acetate copolymer, polyamide resin, polyester resin, and polyvinyl butyral resin. According to our investigation, we have found that when the above-mentioned compatible resin is added to the above-mentioned diurethane compound and the resultant mixture is used for formation of an ink layer, these two components are compatible with each other at the time of ink layer formation so that a homogeneous or uniform ink layer is formed; and these components provide a dispersion state of a sort of island-sea structure in the ink layer after the formation of the ink layer, whereby the releasability of the ink layer comprising the diurethane compound is well controlled. It is preferred to use the compatible resin in an amount of 40-250 wt. parts per 100 wt. parts of the diurethane compound. If the amount of the compatible resin is below the above range, it is difficult to control the release amount of the ink layer. If the amount exceeds the above range, the transfer of the ink layer per se becomes difficult. In the present invention, it is also possible to add a small amount of a thermoplastic resin such as polyvinyl butyral resin and polyester resin to the above-mentioned binder so that transfer control property of the ink layer to a transfer-receiving material is improved. In addition, it is possible to add inorganic or organic filler such as silica, alumina, clay, and plastic pigment to the ink layer so that ground staining of a transfer receiving material may be prevented at the time of printing. The ink comprising the colorant and the vehicle as described above may preferably be so constituted that the melt viscosity at 100 °C 15 1000 mPa.s (cps) or higher. In a case where a melt viscosity of 1000 mPa.s (cps) or higher cannot be obtained by using a wax alone, it is possible to use various thermoplastic resins such as vinyl-type resin in combination to enhance the cohesion thereof, so that the melt viscosity is improved. The melt viscosity is 1000 - 5000 mPa.s (cps) at 100°C. If the melt viscosity is too high, void is liable to occur. The ground staining or trailing can also be suppressed by incorporating a lubricant as described hereinafter into the ink layer. The melt viscosity of the ink used in such an embodiment is regulated by a value thereof measured by means of a viscometer (Rotovisco ® PK-100, mfd. by Haake Co.) using a sensor PK5-0.5° (cone plate) and a shear rate of 512 (1/s). Accordingly, such a measurement means is different from the measurement means (Rotovisco ® M-500) for measuring the melt viscosity of the sensitising layer described hereinafter. The melt viscosity of 1000 mPa.s (cps) according to Rotovisco ® PK-100 corresponds to a melt viscosity of 300 mPa.s (cps) according to Rotovisco ® M-500. In order to form the above-mentioned ink layer, there may be used various methods for applying a coating liquid such as hot-melt coating, hot-lacquer coating, gravure coating, gravure reverse coating, and roller coating. The surface layer formed on the surface of the ink layer may be formed by using the above-mentioned wax, or vehicle (or medium) for the ink layer. In addition, it is preferred to form the surface layer by using a lubricant such as lubricating wax, silicone wax, fluorine-containing resin, silicone-type resin, higher fatty acid amide or ester, and surfactant or by using a thermoplastic resin in combination with such a lubricant so that the film strength thereof may be improved. Further, the surface layer can have a two-layer structure comprising a lubricant layer and a resin layer. In the above-mentioned surface layer, it is preferred to form a minute linear unevenness shape having an angle of, e.g., 15-60° with respect to the moving direction of the thermal transfer sheet. The minute unevenness shape may easily be formed by using a gravure plate having oblique grooves at the time of the surface layer formation. Particularly, when the surface layer is formed by applying an aqueous dispersion comprising a lubricant and vehicle and drying the resultant coating at a low temperature, a surface with minute unevenness shape retaining particulate shapes may be provided. In such an embodiment, the sticking of the thermal transfer sheet to a transfer-receiving material is prevented at the time of printing, and the thermal transfer sheet can be caused to have a further improved resistance to ground staining. The surface layer has a melt viscosity of 2000 - 10000 mPa.s (cps) at 150°C. If the melt viscosity is below 2000 mPa.s (cps), it may easily be removed due to friction with a transfer-receiving material so that ground staining of the transfer-receiving material is liable to occur. If the melt viscosity exceeds 10000 mPa.s (cps), the transferability of the ink layer becomes insufficient and white dropouts are liable to occur. Such a melt viscosity may easily be controlled by changing the mixing ratio between the wax and thermoplastic resin. The melt viscosity of the surface layer may be regulated on the basis of a value thereof measured by means of a viscometer (Rotovisco ® PK-100, mfd. by Haake Co.) using a sensor PK 5-0.5° (cone plate) and a shear rate of 512 (1/s), in the same manner as in the case of the above-mentioned melt viscosity of the ink layer. The surface layer may be formed by using various techniques in the same manner as in the formation of the ink layer. The surface layer may preferably have a thickness of 0.1-5µm so that the sensitivity does not become insufficient even when printing energy is decreased as in the case of a high-speed-type printer. The thermal transfer sheet may comprise a sensitizing layer between the heat-fusible ink layer and the substrate. The sensitizing layer may predominantly comprise a wax. Representative examples of the wax may include microcrystalline wax, carnauba wax, paraffin wax, etc.. In addition, specific examples of the wax may include: various species thereof such as Fischer-tropsch wax, various low-molecular weight polyethylene, Japan wax, beeswax, whale wax, insect wax, lanolin, shellac wax, candelilla wax, petrolactam, partially modified wax, fatty acid ester, and fatty acid amide. Among these, it is preferred to use those having a melt viscosity of 100 mPa.s (cps) or lower, more preferably 50 mPa.s (cps) or lower. If the melt viscosity is too high, it becomes similar to that of the ink layer and sensitizing function thereof becomes insufficient, whereby void is liable to occur. Such a sensitising layer may preferably have a thickness of 0.1-2µm, more preferably 0.5-1.5µm. If the sensitizing layer is too thin, the sensitizing effect thereof becomes insufficient. If the sensitizing layer is too thick, the sensitivity is decreased. The above-mentioned melt viscosity is regulated by a value measured by means of a viscometer (Rotovisco ® M-550, mfd. by Haake Co.) using a sensor MV-1 and a shear rate of 256 (1/s). In a case where the above-mentioned sensitizing layer is formed by a hot-melt coating method etc., in the same manner as in the prior art, it is difficult to form a layer having a uniform thickness since the layer is extremely thin. Accordingly, in the present invention, the sensitizing layer is preferably formed by an emulsion method using an aqueous dispersion containing a wax. The sensitizing layer may preferably be formed by applying an aqueous dispersion of a wax on to a substrate film and drying the resultant coating at a temperature which is not higher than or not lower than the melting point of the wax. The above-mentioned aqueous medium to be used in combination with the wax is suitably water or a mixture comprising water and a water-soluble organic solvent such as methanol, ethanol and isopropanol. When such a water-soluble organic solvent is used in an amount of 5-400 wt. parts per 100 wt. parts of water, the wettability of the aqueous wax dispersion to the substrate film is enhanced. The above-mentioned aqueous wax dispersion can further contain a small amount of a known additive such as emulsifying agent (surfactant) and levelling agent. The solid content of such a dispersion may be about 10-50 wt.%. The sensitizing layer comprising the above-mentioned wax may be formed by applying an ink composition containing the wax by a known coating method and then drying the resultant coating. When the drying is conducted at a temperature which is not lower than the melting point of the wax, there may be formed a sensitizing layer having surface smoothness. On the other hand, the drying is conducted at a temperature lower than the melting point of the wax, there may be formed a sensitizing layer having a surface with minute unevennesses wherein the particulate form of the dispersion is retained. The above-mentioned sensitising layer can further contain a pigment or dye having the same hue as that of an ink layer described hereinafter. In such an embodiment, the resultant image density (or printing density) is further improved. In order to form the above-mentioned sensitizing layer, there may be used various methods for applying a coating liquid such as hot-melt coating, hot-lacquer coating, gravure coating, gravure reverse coating, and roller coating. There may be a sealing layer on the above-mentioned surface layer. The sealing layer has a function of filling the surface unevenness of rough paper and is required to be easily transferred to the paper surface due to friction between the thermal transfer sheet and the paper in an n -fold printing method. The sealing layer having such a function may preferably be formed by using a relatively soft or brittle wax selected from those described hereinabove. For example, such a wax may preferably have a melt viscosity of 20-100 mPa.s (cps) at 100°C. If the melt viscosity is below the above range, it poses a problem in handling thereof such as blocking. If the melt viscosity exceeds the above range, the transferability of the sealing layer becomes insufficient. The sealing layer may preferably have a thickness of 1.0-6.0µm. If the sealing layer is too thin, the sealing effect thereof becomes insufficient. If the sealing layer is too thick, the printing sensitivity is undesirably lowered. The melt viscosity of the sealing layer is regulated on the basis of a value thereof measured by means of a viscometer (Rotovisco ® M-500, mfd. by Haake Co.) using a sensor MV-1 and a shear rate of 256 (1/s). In the embodiment shown in Figure 2, the thermal transfer sheet comprises a substrate film (12), an ink layer (14) and a surface layer (15), formed on one surface side of the substrate film (12). Optionally there is a sensitizing layer (13) between the substrate film (12) and the ink layer (14), and a back coating layer (16) formed on the other surface side of the substrate film (12). The back coating layer (16) has a function of preventing sticking of a thermal head. The above-mentioned optional back coating layer (16) may comprise a binder resin and an optional additive. Specific examples of the binder resin may include: cellulose resins such as ethylcellulose, hydroxyethyl cellulose, ethylhydroxy-ethylcellulose, hydroxypropyl cellulose, methylcellulose, cellulose acetate, cellulose acetate butyrate, and nitrocellulose; vinyl-type resins such as polyvinyl alcohol, polyvinyl acetate, polyvinyl butyral, polyvinyl acetal, polyvinyl pyrrolidone, acrylic resin, polyacrylamide, and acrylonitrile-styrene copolymer; polyester resin, poly-urethane resin, silicone-modified or fluorine-modified urethane resin, etc.. Among these, it is preferred to use a resin having a somewhat greater reactivity (e.g. one having hydroxyl group, carboxyl group, or epoxy group) in combination with a crosslinking agent such as polyisocyanate so as to provide a crosslinked resin layer. The back coating layer 16 may preferably comprise a binder resin predominantly comprising a styrene-acrylonitrile copolymer, and an optional additive. The above-mentioned styrene-acrylonitrile copolymer used in the present invention may be obtained by co-polymerizing styrene and acrylonitrile. Such a copolymer may easily be prepared in an ordinary manner. In addition, any of the commercially available products of various grades can be used in the present invention. Specific examples thereof may include those sold under the trade names of Sebian ® AD, Sebian ® LD, and Sebian ® NA (mfd. by Daiseru Kagaku K.K.). Among styrene-acrylonitrile copolymers of various grades, it is preferred to use one having a molecular weight of 10 x 10⁴ to 20 x 10⁴ (more preferably 15 x 10⁴ to 19 x 10⁴), and/or an acrylonitrile content of 20 to 40 mol % (more preferably 25 to 30 mol %). Such a copolymer may preferably have a softening temperature of 400°C or higher according to differential thermal analysis, in view of heat resistance and dissolution stability to an organic solvent. In a case where the substrate film (12) comprises a polyethylene terephthalate film, the adhesion property between the above-mentioned styrene-acrylonitrile copolymer and the substrate film (12) is not necessarily sufficient. Accordingly, in such a case, it is preferred to subject a monomer containing a small amount (e.g., several mol percent) of a functional group (such as methacrylic acid) to copolymerisation, at the time of production of the styrene-acrylonitrile copolymer. Alternatively, there may also be used a method of using a small amount of another adhesive resin in combination, or a method of preliminarily forming a primer layer on the substrate film by use of such an adhesive resin. The adhesive resin may preferably comprise an amorphous linear saturated polyester resin having a glass transition point of 50°C or higher. Examples of such a polyester resin may include those sold under the trade names of Bairon ® (mfd. by Toyobo K.K.), Eriter ® (mfd. by Unitika K.K.), Polyester (mfd. by Nihon Gosei Kagaku K.K.). The resins of various grades are commercially available, and any of these resins can be used in the present invention. Particularly preferred examples of such a resin may include Bairon ® RV 290 (mfd. by Toyobo K.K., product containing epoxy groups introduced thereinto, molecular weight = 2.0 x 10⁴ to 2.5 x 10⁴, Tg = 77°C, softening point = 180°C, hydroxyl value = 5 to 8). In a case where the above-mentioned polyester resin is used for forming a primer layer, it is preferred to form the primer layer having a thickness of about 0.05 to 0.5µm. If the thickness is too small, the resultant adhesive property may be insufficient. If the thickness is too large, sensitivity to a thermal head or heat resistance may undesirably be lowered. In a case where the adhesive resin (e.g., polyester resin) is used in a mixture with the above-mentioned styrene-acrylonitrile copolymer, the adhesive resin content may preferably be 1 to 30 wt. parts per 100 wt. parts of the styrene-acrylonitrile copolymer. If the adhesive resin content is too low, the resultant adhesive property may be insufficient. If the adhesive resin content is too high, the heat resistance of the back coating layer may be lowered, or sticking may be caused. It is also possible to use a small amount of a binder resin in combination, specific examples of the binder resin may include: cellulose resins such as ethylcellulose, hydroxyethyl cellulose, ethyl-hydroxy-ethylcellulose, hydroxypropyl cellulose, methylcellulose, cellulose acetate, cellulose acetate butyrate, and nitrocellulose; vinyl-type resins such as polyvinyl alcohol, polyvinyl acetate, polyvinyl butyral, polyvinyl acetal, polyvinyl pyrrolidone, acrylic resin, polyacrylamide, and acrylonitrile-styrene copolymer; polyester resin, polyurethane resin, silicone-modified or fluorine-modified urethane resin, etc.. When the back coating layer is formed by using the above-mentioned material, a thermal release agent or lubricating agent (or lubricant) may also be contained therein. Specific examples of such a release agent or lubricating agent may include wax, higher fatty acid amide, ester, surfactant, higher fatty acid metal salt, and alkylphosphate multi-valent metal salt. Preferred examples of the lubricant may include an alkylphosphate (or alkylphosphoric acid ester) multi-valent metal salt. The alkylphosphate multi-valent metal salt may be obtained by replacing the alkali metal of an alkylphosphate alkali metal salt with a multi-valent metal, and the alkylphosphate multi-valent metal salt per se is known as an additive for plastic in the art. Such multi-valent metal salts of various grades are commercially available, and any of these multi-valent metal salts can be used in the present invention. The alkylphosphate multi-valent metal salt may include those represented by the following formula: and/or wherein R denotes an alkyl group having 12 or more carbon atoms such as cetyl, lauryl, and stearyl (particularly, stearyl); M denotes an alkaline earth metal such as barium, calcium and magnesium, and zinc, aluminum, etc.; and n denotes the valence of M . It is preferred to use the above-mentioned alkylphosphate multi-valent metal salt in an amount of 10 to 150 wt. parts with respect to 100 wt. parts of the above-mentioned binder resin. If the amount of the multi-valent salt to be used is below the above range, sufficient slip property is difficult to obtain. On the other hand, if the amount of the multi-valent salt exceeds the above range, the physical strength of the back coating layer may undesirably be lowered. In order to improve the heat-resistance of the back coating layer, it is possible to incorporate a heat resistance-imparting agent thereinto. Specific examples of such an agent may include: Hydrotalcite ® DHT-4A (mfd. by Kyowa Kagaku Kogyo), Talcmicroace ® L-1 (mfd. by Nihon Talc), Taflon Rubron ® L-2 (mfd. by Daikin Kogyo), Fluorinated Graphite SCP-10 (mfd. by Sanpo Kagaku Kogyo), Graphite AT40S® (mfd. by Oriental Sangyo), and fine particles such as silica, calcium carbonate, precipitated barium sulfate, crosslinked urea resin powder, crosslinked melamine resin powder, crosslinked styrene-acrylic resin powder, crosslinked amino resin powder, silicone resin powder, wood meal, molybdenum disulfide, and boron nitride. Further, in order to impart an antistatic property to the back coating layer, it is possible to add thereto a conductivity-imparting agent such as carbon black. The back coating layer may be formed by dissolving or dispersing the above-mentioned material in an appropriate solvent such as acetone, methyl ethyl ketone, toluene and xylene to prepare a coating liquid; and applying the coating liquid by an ordinary coating means such as gravure coater, roll coater, and wire bar; and drying the resultant coating. The coating amount of the back coating layer, i.e., the thickness thereof, is also important. In the present invention, a back coating layer having sufficient performances may preferably be formed by using a coating amount of 0.5 g/m² or below, more preferably 0.1 to 0.5 g/m², based on the solid content thereof. If the back coating layer is too thick, the thermal sensitivity at the time of transfer operation may undesirably be lowered. It is also effective to form a primer layer comprising a polyester resin or polyurethane resin, etc., on the substrate film, prior to the formation of the above-mentioned back coating layer. The thermal transfer sheet can be in the form of sheet or leafs, but may generally be in the form of a roll obtained by winding the thermal transfer sheet around an appropriate core such as a paper tube. In this case, when an end detection mark is imparted to the back surface of the thermal transfer sheet near the joint portion thereof with the core material, it is possible that a sensor of a printer detects the mark and the printer is automatically stopped. The detection mark may suitably comprise a highly reflective mark obtained by printing using a silver or white ink, aluminum vapor deposition, aluminum foil attachment, etc.. As a matter of course, the present invention is applicable to a thermal transfer sheet for color printing. Accordingly, a multi-color thermal transfer sheet comprising a substrate and at least two color ink coating disposed thereon is also within the scope of the present invention. The transfer-receiving material to be used in the present invention may comprise various papers, synthetic papers, plastic sheets, etc., but at least printing surface thereof is required to have a Bekk smoothness of 20-800 s. The Bekk smoothness may arbitrarily be regulated by calendering, embossing, application of a coating liquid for surface treatment. If the Bekk smoothness of the printing surface exceeds 800 s, the thermal transfer sheet slips on the transfer-receiving material at the time of printing and the peeling of the ink layer becomes difficult, whereby it is difficult to obtain an image having a high image density. If the Bekk smoothness is below 20 s, drop-out or lacking of printed letters is liable to occur, whereby the image quality is undesirably lowered. Hereinbelow, the thermal transfer sheet according to the present invention is described in more detail with reference to examples. In the description appearing hereinafter, "part(s)" and "%" are "part(s) by weight" and "wt. %", respectively, unless otherwise noted specifically. In the description appearing hereinafter, the melt viscosity of a sensitizing layer is measured by means of a viscometer (Rotovisco ® M-500, mfd. by Haake Co.) using a sensor MV-1 and a shear rate of 256 (1/s), and the melt viscosity of an ink layer and a surface layer is measured by means of a viacometer (Rotovisco ® PK-100, mfd. by Haake Co.) using a sensor PK 5-0.5° (cone plate) and a shear rate of 512 (1/s). EXAMPLES Example 1 An ink composition comprising the following components was heated up to 65°C and applied onto the surface of a substrate film by a hot-lacquer gravure coating method so as to provide a coating amount of 8 g/m², whereby a heat-transferable ink layer (melt viscosity = 3000 mPa.s (cps) at 100°C) was formed. The substrate film was a 6µm - thick polyester film (Lumirror ® F-53, mfd. by Toray K.K.). Ink composition for formation of a transferable ink layer Carnauba 20 parts Ethylene-vinyl acetate copolymer (Sumitate ® KA-10, mfd. by Sumitomo Kagaku K.K.) 13 parts 150°F paraffin wax 45 parts Carbon black 30 parts Nigrosine dye 9 parts Xylene 50 parts Isopropanol 10 parts Thereafter, the following composition was applied onto the above-mentioned ink layer so as to provide a coating amount of 1.0 g/m² (based on solid content) and then dried to form thereon a surface layer (melt viacosity = 2500 mPa.s (cps) at 150°C). Coating liquid composition for a surface layer Carnauba wax 10 parts Polyethylene wax 20 parts Nonionic surfactant 1 part Isopropanol 100 parts Water 30 parts Example 2 A coating liquid-having the following composition was heated up to 100°C and was applied onto the surface of the same substrate film as described in Example 1 by a hot-melt roller coating method so as to provide a coating amount of 8 g/m², whereby a heat-transferable ink layer (melt viscosity = 3000 mPa.s (cps) at 100°C) was formed. Ink composition for formation of a transferable ink layer Carnauba wax 20 parts Ethylene-vinyl acetate copolymer (Evaflex ®, KA-10 mfd. by Mitsui Polychemical K.K.) 13 parts 150°F paraffin wax 45 parts Carbon black 30 parts Nigrosine dye 9 parts Thereafter, the following composition was heated up to 60°C, and was applied onto the above-mentioned ink layer by a hot-lacquer gravure coating method so as to provide a coating amount of 2.0 g/m² and then dried to form thereon a surface layer. Coating liquid composition for a surface layer Ethylene-vinyl acetate copolymer (Evaflex ®, #460, mfd. by Mitsui Polychemical K.K.) 40 parts Carnauba wax 20 parts 150°F paraffin wax 50 parts Xylene 100 parts Isopropanol 10 parts Example 3 A thermal transfer sheet was prepared in the same manner as in Example 1 except that 5 parts of a synthetic wax was added to each of the compositions for the ink layer and surface layer, respectively. Each of the examples as prepared above was subjected to printing by using an evaluation machine of N -fold recording mode (N = 6). The recording paper used was TRW1 (mfd. by Jujo Seishi K.K.). Each of the samples exhibited good printing resolution and good resistance to ground staining, trailing and void formation.

Claims: A thermal transfer sheet (1) comprising a substrate film (2), an ink layer (4) formed on one surface side of the substrate film (2), and a surface layer (5) formed on the surface of the ink layer (4), characterised in that the ink layer (4) has a melt viscosity of 1000-5000 mPa.s (cps) at 100°C measured using a Rotovisco ® PK-100 viscometer, and the surface layer (5) has a melt viscosity of 2000-10000 mPa.s (cps) at 150°C measured using a Rotovisco ® PK-100 viscometer. A thermal transfer sheet as claimed in Claim 1, which further comprises a sensitizing layer (3) formed between the substrate film (2) and the ink layer (4). A thermal transfer sheet as claimed in Claim 2, wherein the sensitizing layer (3) has a melt viscosity of 100 mPa.s or lower at 100°C measured using a Rotovisco ® M-500 viscometer. A thermal transfer sheet as claimed in Claim 2 or Claim 3, wherein the sensitising layer (3) has been formed by the application of a wax emulsion. A thermal transfer sheet as claimed in any one of Claims 2 to 4, wherein the sensitizing layer (3) is a coloured layer. A thermal transfer sheet as claimed in any preceding claim, wherein the ink layer (4) contains carbon black and a black dye. A thermal transfer sheet as claimed in any preceding claim, wherein the ink layer (4) has a pigment concentration of 20-70 wt.%. A thermal transfer sheet as claimed in any preceding claim, wherein the ink layer (4) has a thickness of 3-20µm. A thermal transfer sheet as claimed in any preceding claim, wherein the ink layer (4) comprises a wax and a thermoplastic resin as a vehicle. A thermal transfer sheet as claimed in any preceding claim, which has been wound around a core material and has an end detection mark on the back surface thereof disposed near to the binding part with the core material. A thermal transfer sheet as claimed in any preceding claim, which is to be used for an n -fold recording mode. A thermal transfer sheet as claimed in Claim 1 or Claim 2, which further comprises a colourless sealing layer formed on the surface layer (5) on the ink layer (4). A thermal transfer sheet as claimed in Claim 12,, wherein the surface layer (5) has a melt viscosity of 2000-10000 mPa.s (cps) at 150°C measured using a Rotovisco ® PK-100 viscometer, and the sealing layer has a melt viscosity of 20-100 mPa.s (cps) at 100°C measured using a Rotovisco ® M-500 viscometer. A thermal transfer sheet (11) as claimed in any preceding claim, which further comprises a back coating layer (16) disposed on the other surface of the substrate film (12); said back coating layer (16) comprising a binder predominantly comprising a styrene-acrylonitrile copolymer. A thermal transfer sheet as claimed in Claim 14, wherein the styrene-acrylonitrile copolymer has an acrylonitrile copolymerisation ratio of 20-40 mol. %. A thermal transfer sheet as claimed in Claim 14 or Claim 15, wherein the styrene-acrylonitrile copolymer has a molecular weight of 10 x 10⁴ to 20 x 10⁴. A thermal transfer sheet as claimed in any one of Claims 14 to 16, wherein a linear polyester resin has been mixed in the back coating layer (16) as an adhesive resin. A thermal transfer sheet as claimed in any one of Claims 14 to 17, which further comprises a primer layer comprising a linear polyester resin formed between the substrate film (12) and the back coating layer (16).

Inventors: IMAMURA HIROKATSU C O DAI NIPP, NAKAMURA KOICHI C O DAI NIPPON, OHNISHI JIRO C O DAI NIPPON IN, IMAMURA, HIROKATSU, C/O DAI NIPPON INSATSU KK, NAKAMURA, KOICHI, ,C/O DAI NIPPON INSATSU KK, OHNISHI, JIRO, ,C/O DAI NIPPON INSATSU KK
Applicants: DAINIPPON PRINTING CO LTD, DAI NIPPON INSATSU KABUSHIKI KAISHA
Application year: 1990
Main classification: B41M5
IPC codes: B41M5