Sheet material for heat transfer printing

Abstract: A heat transfer sheet (dye doner sheet) comprises a base sheet, and a heat transfer layer formed on the base sheet. The heat transfer layer comprises a binder and a dye having the following chemical formula: (Foron Brilliant Yellow S - 6 GL) Second and third heat transfer layers, formed adjacent to said heat transfer layer, may comprise a binder and respectively the dye C.I. Disperse Red 60 and the dye C.I. Solvent Blue 63.

Description: This invention relates to a sheet material, more particularly to a heat transfer sheet for carrying out heat printing in accordance with image information by means of thermal heads or the like. Heretofore, a heat-sensitive color-producing paper has been primarily used to obtain an image in accordance with image information by means of the contact type dot-shaped heating means such as thermal heads or the like. In this heat-sensitive color-producing paper, a leuco dye which is colorless or pale-coloured at room temperature and a developer provided on a base paper are contacted by the application of heat to obtain a developed color image. Phenolic compounds, derivatives of zinc salicylate, rosins and the like are generally used as such a developer. However, the heat-sensitive color-producing paper as described above has a serious drawback in that its color disappears when the resulting developed color image is stored for a long period of time. Further, color printing is restricted to two colors, and thus it is impossible to obtain a color image having a continuous gradation. On the other hand, a heat-sensitive transfer sheet wherein a heat-fusing wax layer having a pigment dispersed therein is provided on a base paper has been recently used. When this heat-sensitive transfer sheet is laminated with a paper to be heat transfer printed, and then heat printing is carried out from the back of the heat-sensitive transfer sheet, the wax layer containing the pigment is transferred onto the heat transferable paper to produce an image. According to this printing process, an image having durability can be obtained, and a multi-color image can be obtained by using a heat-sensitive transfer paper each containing three primary color pigments and printing it many times. However, it is impossible to obtain an image having an essentially continuous gradation as in a photograph. In recent years, there has been a growing demand for obtaining an image like a color photograph directly from an electrical signal, and a variety of attempts have been made to meet this demand. One of such attempts provides a process wherein an image is projected onto a cathode-ray tube (CRT), and a photograph is taken with a silver salt film. However, when the silver salt film is an instant film, the running cost is disadvantageously high. When the silver salt film is a 35 mm film, the image cannot be instantly obtained because it is necessary to carry out a development treatment after the photographing. An impact ribbon process and an ink jet process have been proposed as further processes. In the former, the quality of the image is inferior. In the latter, it is difficult to simply obtain an image like photograph because an image processing is required. In order to overcome such drawbacks, there has been proposed a process wherein a heat transfer sheet provided with a layer of sublimable disperse dyes having heat transferability is used in combination with a heat transferable sheet, and wherein the sublimable disperse dye is transferred onto the heat transferable sheet while it is controlled to form an image having a gradation as in a photograph. (Bulletin of Image Electron Society of Japan, Vol. 12, No. 1 (1983)). According to this process, an image having continuous gradation can be obtained from a television signal by a simple treatment. Moreover, the apparatus used in the process is not complicated and therefore is attracting much attention. One example of prior art technology close to this process is a process for dry transfer calico printing polyester fibers. In this dry transfer calico printing process, dyes such as sublimable dispersed dyes are dispersed or dissolved in a solution of synthetic resin to form a coating composition, which is applied onto tissue paper or the like in the form of a pattern and dried to form a heat transfer sheet, which is laminated with polyester fibers constituting sheets to be heat transferred thereby to form a laminated structure, which is then heated to cause the disperse dye to be transferred onto the polyester fibers, whereby an image is obtained. However, even if the heat transfer sheet heretofore used in the dry transfer calico printing process for the polyester fibers is used as it is and subjected to heat printing by means of thermal heads or the like, it is difficult to obtain a developed color image of a high density. While improvement of the image quality due to printing density and heat sensitivity is an important task in the prior art technology as described above, another important point which is the problem in the practical process of forming a heat transferred image is the operability in the printing step. To describe about this operability, the following problems have been involved in the sheet for heat transference of the prior art. (a) In the heat transfer sheet of the prior art, when the sheet is conveyed by means of a printing conveying means, the sheet may be sometimes adhered to the roll within the means, whereby running performance of the heat transfer sheet becomes worse. (b) In the heat transfer sheet of the prior art, the so-called sticking phenomenon occurs, in which the base sheet itself is fused to the thermal heads, whereby running of the heat transfer sheet may become impossible or, in an extreme case, the sheet may be broken from the sticked portion. (c) In the sheet of the prior art, dust may be inhaled through the electrostatic attracting force created by running or friction of the sheet, whereby disadvantages such as dislocation of recording (partial failure of recording), damages of the dot-shaped heat printing means such as thermal heads or the like, bad running performance such as sagging of respective sheets, etc., caused by attachment of dust between the heat transfer sheet and the heat transferable sheet or between the dot-shaped heat printing means and the heat transfer sheet remain as problems to be solved. (d) In the heat transferable sheet of the prior art, running performance of the sheet is bad depending on the base sheet employed and, further, the strain created by the heat during image formation disadvantageously remains on the sheet to cause curling of the sheet. (e) For formation of a color image by heat-sensitive transfer printing, a heat-sensitive transfer sheet in which transfer layers are provided by coating in different areas of a plurality of colors has been invented. However, even such layers may be provided by coating in different areas, there is no guarantee that the area of a desired color can be heat printed and therefore it is necessary to confirm the transfer layer every time of heat printing. Also, in the case of a monochromatic heat-sensitive transfer sheet, it has been inconveniently impossible to confirm the residual amount, the direction, back or front, grade, etc. of the heat-sensitive transfer sheet. (f) The heat transferable sheet of the prior art is ordinarily a merely white sheet in appearance and therefore, even a paint prepared from various resins, optionally with addition of additives, may be applied in one layer or multiple layers, it is difficult to discriminate one from another with naked eyes. Not only distinction from papers for other recording systems such as electrostatic copying paper or heat-sensitive recording paper or the like, as a matter of course, but also distinction between several kinds of heat transferable sheets depending on adaptability for recording devices or heat transfer sheets or uses are greatly required. However, in the prior art, once this kind of heat transferable sheet is unwrapped from a package, distinction by appearance is hardly possible and yet no measure for distinction has been taken. FR-A2510042 discloses a thermal transfer system involving dye image receiving sheets comprising a dye-receptive layer, and dye-donor sheets comprising sublimable dyes in a binder. EP-A-0119275 discloses an ink ribbon for use in sublimation transfer process hard copying. The ribbon carries transferable ink portions in a predetermined arrangement and also marks for detecting the positions of the ink portions. SUMMARY OF THE INVENTION The present invention provides a heat transfer sheet having a heat transfer layer on one surface of a base sheet, said heat transfer layer being formed of a material containing a dye substantially dissolved in a binder with a weight ratio of the dye to the binder (dye/binder ratio) or 0.3 or more, and said base sheet having a heat-resistant slipping layer provided on the surface on which the above heat transfer layer is not provided said dye having the following chemical formula: which is sold by Sandoz KK under the trade name "Foron Brilliant Yellow S - 6 GL". BRIEF DESCRIPTION OF THE DRAWINGS Figures 1 to 3 are sectional views of sheets used for heat transfer printing, respectively; Figures 4 and 5 are perspective views of sheets used for heat transfer printing, respectively. DETAILED DESCRIPTION OF THE INVENTION The present invention is described in detail below by referring to the drawings. As shown in Figure 1, when carrying out generally heat printing by heat transfer, a donor sheet 1 (hereinafter called heat transfer sheet) comprising a heat transfer layer 3 formed on a base sheet 2 is laminated with a receptor sheet 6 (hereinafter called heat transferable sheet) having a receptive layer 5 formed on a base sheet 4, and the dye in the heat transfer layer is caused to be migrated into the receptive layer by supplying heat energy corresponding to the image information to the interface between the heat transfer layer 3 and the receptive layer 5 thereby to form an image. As the heat source for supplying heat energy, the contact type dot-shaped heating means such as thermal head 7 may be preferably employed . In this case, the supplied heat energy can be continuously or stepwise varied by modulating the voltage or the pulse width applied to the thermal head. [A] Heat transfer sheet As shown in Fig. 2, the heat transfer sheet 1 of the present invention comprises basically a heat transfer layer 3 made of a specific material on one surface of a base sheet 2 and a heat-resistant slipping layer 8 on the other surface. FIG. 3 is a sectional view of the heat transfer sheet according to another embodiment of the present invention, having further a heat-resistant layer 9 between the base sheet 2 and the heat-resistant slipping layer 8, and also an antistatic layer 10 is formed on the surface of the heat-resistant layer 9. The materials, functions and others of these respective layers are to be described in detail below. Heat transfer layer The heat transfer layer 3 comprises a heat sublimable dye and a binder. One specific feature of the heat transfer sheet of the present invention resides in that it comprises a material containing a dye dissolved in a binder with a weight ratio of the dye to the binder (dye/binder ratio) of 0.3 or more. With the above conditions, excellent printing density and heat sensitivity can be obtained to improve image quality. On the other hand, if the dye/binder ratio is greater than 2.3, the storage stability of the sheet will be lowered. Accordingly, the dye/binder ratio may preferably be within the range of from 0.3 to 2.3, more preferably from 0.55 to 1.5. Base sheet Papers or films such as condenser paper, aramide (aromatic polyamide) film, polyester film, polystyrene film, polysulfone film, polyimide film, polyvinyl alcohol film and cellophane can be used as the base sheet 2. The thickness of the base sheet is from 2 to 50 µm, preferably from 2 to 15 µm. Of these papers or films, if cost and heat resistance in an untreated state are regarded as being imporatant, condenser paper is used. If resistance to rupturing (the substrate sheet has mechanical strength and does not rupture during handling in the preparation of a heat transfer printing sheet or during running in a thermal printer) and smooth surface are regarded as being important, an aramide (aromatic polyamide) film, a polyester film is preferably used. (a) Dye Additional dyes may be contained in further heat transfer layer: preferably a heat sublimable disperse dye, oil-soluble dye, basic dye, having a molecular weight of the order of about 150 to 800, preferably 350 to 700. The dye can be selected by considering heat sublimation temperature, hue, weatherability, ability to dissolve the dye ink compositions or binder resins, and other factors. Examples of such dyes are as follows: C.I. (Chemical Index) Yellow 51, 3, 54, 79, 60, 23, 7, 141 C.I. Disperse Blue 24, 56, 14, 301, 334, 165, 19, 72, 87, 287, 154, 26 C.I. Disperse Red 135, 146, 59, 1, 73, 60, 167 C.I. Disperse Violet 4, 13, 36, 56, 31 C.I. Solvent Violet 13, C.I. Solvent Black 3, C.I. solvent Green 3 C.I. Solvent Yellow 56, 14, 16, 29 C.I. Solvent Blue 70, 35, 63, 36, 50, 49, 111, 105, 97, 11 C.I. Solvent Red 135, 81, 18, 25, 19, 23, 24, 143, 146, 182 (b) Binder According to the studies by the present inventors, in the heat transfer sheet heretofore generally used, the disperse dye is dispersed in the binder in the form of particles. In order to heat the dye molecules in such a state to sublimate them, the dye molecules must be subjected to heat energy which breaks the interaction in the crystals and overcomes the interaction with the binder, thereby sublimating them to transfer to the heat transferable sheet. Accordingly, high energy is required. When the dye is contained in a high proportion in the binder resin in order to obtain a developed color image having a high density, an image having a relatively high density can be obtained. However, its bond strength in the heat transfer layer of the heat transfer sheet becomes low. Accordingly, when the heat transfer sheet and the heat transferable sheet are peeled off after they are laminated and subjected to printing by thermal heads or the like, the dye tends to transfer to the heat transferable sheet with the resin. Further, the dye is expensive and the use of excessive dye is economically disadvantageous from the standpoint of office automation (OA) instruments and home uses. On the other hand, if the dye can be retained in the binder in the form of molecules rather than particles, there will be no interaction in the crystals which occurs in the case where the dye is dispersed in the form of particles, and therefore an improvement in heat sensitivity can be expected. However, even if such a state is accomplished, a transfer paper having practicality cannot be obtained. This is because the molecular weight of the heat sublimable dye molecules is of the order of 150 to 800 and these molecules are liable to move in the binder. Accordingly, when a binder having a low glass transition temperature (Tg) is used in a heat transfer layer, the dye agglomerates with elapse of time to be deposited. Eventually, the dye may be in the same state as the case where the dye is dispersed in the form of particles as described above. Alternatively, bleeding of the dye may occur at the surface of the heat transfer layer. Accordingly, the dye may be caused to adhere to portions other than the heated portions by the pressure between a thermal head and a platen during recording. Thus, staining may occur to significantly lower the quality of the image. Further, even if the glass transition temperature (Tg) of the binder in the heat transfer layer is high, the dye molecules cannot be retained in the heat transfer printing layer unless the molecular weight of the binder is considerably high. Furthermore, even if the dye is dissolved in the form of molecules in a binder having a high glass transition temperature and a considerably high molecular weight, affinity between the dye molecules and the binder is required in order to achieve the state of storage stability. In view of the standpoints as described above, a polyvinyl butyral resin is preferably used as the binder resin. Its molecular weight is 60,000 or more for giving rise to a bond strength as the binder, and not more than 200,000 for making the viscosity during coating adequate. Further, in order to prevent agglomeration or deposition of the dye in the heat transfer layer 3, the glass transition temperature (Tg) of the binder resin must be at least 60°C, more preferably at least 70°C, and no more than 110°C from the standpoint of facilitating the sublimation of the dye. Further, the content of vinyl alcohol which exhibits good affinity for the dye due to a hydrogen bond and the like is from 10% to 40%, preferably from 15% to 30%, by weight of the polyvinyl butyral resin. If the vinyl alcohol content is less than 10%, the storage stability of the heat transfer layer will be insufficient, and agglomeration or deposition of the dye and the bleeding of the dye onto the surface will occur. If the vinyl alcohol content is more than 40%, the portions exhibiting affinity will be too large, and therefore the dye will not be released from the heat transfer printing layer during printing by means of thermal heads or the like, whereby the printing density becomes low. In order to improve the drying characteristics in applying/forming the heat transfer layer, cellulose resins can be incorporated into the binder resin in a quantity of up to 10% by weight of the binder resin. Examples of suitable cellulose resins are ethyl cellulose, hydroxyethyl cellulose, ethylhydroxy cellulose, ethylhydroxyethyl cellulose, hydroxypropyl cellulose, and nitrocellulose. As the binder resin, in addition to the above specific polyvinyl butyral resins, it is also possible to use cellulose resins such as ethyl cellulose, hydroxyethyl cellulose, ethylhydroxyethyl cellulose, hydroxypropyl cellulose, methyl cellulose, cellulose acetate, cellulose acetate butyrate and the like, vinyl resins such as polyvinyl alcohol, conventional polyvinyl butyral, polyvinyl pyrrolidone, polyester, polyvinyl acetate, polyacrylamide and the like. In order to provide the heat transfer layer 3 on the base sheet 2, the dye and the binder resin may be dissolved in a solvent to form an ink composition for a heat transfer layer. This ink composition may be provided on the base sheet 2 by a suitable printing process or application process. Optional additives may be admixed in the ink composition for the heat transfer layer as needed. A typical example of a preferable additive is a polyethylene wax, and this can improve the properties of the ink composition without any trouble in image formation. Although an extender pigment can also improve the properties of the ink composition, the quality of the printed image is impaired thereby. Heat-resistant slipping layer Heat-resistant slipping layer imparts an appropriate lubricating property (slippability) to the sheet surface and also prevents heat fusion between the thermal heads and the heat transfer sheet (sticking phenomenon), thus playing very important roles in improvement of the running performance of the sheet. The heat-resistant slipping layer 8, in a first embodiment, consists mainly of (a) a reaction product between polyvinyl butyral and an isocyanate, (b) an alkali metal salt or an alkaline earth metal salt of a phosphoric acid ester and (c) a filler. In a second embodiment, the heat-resistant slipping layer 8 consists of a layer containing further (e) a phosphoric acid ester not in the form of a salt in addition to the above components (a), (b) and (c). Polyvinyl butyral can react with isocyanates to form a resin having good heat resistance. As the polyvinyl butyral, it is preferred to employ one having a molecular weight as high as possible and containing much -OH groups which are the reaction sites with isocyanates. Particularly preferred of polyvinyl butyral are those having molecular weights of 60,000 to 200,000, glass transition temperatures of 60 to 110°C, with the content of vinyl alcohol moiety being 15 to 40% by weight. Examples of isocyanates to be used in forming the above slipping layer are polyisocyanates such as diisocyanates, triisocyanates or the like, which may be used either singly or as a mixture. Specifically, the following compounds may be employed: p-phenylenediisocyanate, 1-chloro-2,4-phenylenediisocyanate, 2-chloro-1,4-phenylenediisocyanate, 2,4-toluenediisocyanate, 2,6-toluenediisocyanate, hexamethylenediisocyanate, 4,4'-biphenylenediisocyanate, triphenylmethanetriisocyanate, 4,4',4''-trimethyl-3,3',2'-triisocyanate-2,4-6-triphenylcyanurate; adduct of toluenediisocyanate and trimethylolpropane (e.g. Coronate L produced by Nippon Polyurethane Co.); or the like. Isocyanates are used generally in an amount generally of 1 to 100%, preferably 5 to 60%, by weight of polyvinyl butyral. The alkali metal salt or alkaline earth metal salt of a phosphoric acid ester has the function of imparting lubricating property to the heat-resistant slipping layer, and GAFAC RD 720 (Sodium Polyoxyethylene alkyl ether phosphate) produced by Toho Kagaku and others may be employed. The alkali metal salt or alkaline earth metal salt of the phosphoric acid ester is used in an amount of 1 to 50%, preferably 10 to 40%, by weight of polyvinyl butyral. The alkali metal salt or alkaline earth metal salt of a phosphoric acid ester, which is added as the lubricating material in the state dissolved in molecules in the binder, has the advantage of being free from occurrence of roughness at the printed portion, as compared with the case when a solid lubricating material such as mica or talc is added. Sodium salts of phosphoric acid esters are particularly preferred as the alkali metal salt or alkaline earth metal of phosphoric acid ester, and examples thereof are represented by the formulae shown below: (wherein R is an alkyl or alkylphenyl having 8 to 30 carbon atoms, and n is an average number of moles of ethylene oxide added). When the alkali metal salt or alkaline earth metal salt of a phosphoric acid ester is compared with its corresponding phosphoric acid ester (not in the form of a salt), it is lower in acidity than the corresponding phosphoric acid ester, as can be seen from the fact that the former exhibits pH 5 to 7 when dissolved in water, while the latter exhibits pH 2.5 or less. Whereas, as described above, polyvinyl butyral reacts with isocyanates to form a base for the heat-resistant slipping layer, and this reaction can proceed with difficulty under strongly acidic region, whereby a long reaction time is required and the crosslinking degree itself is lowered. Accordingly, when a phosphoric acid ester (not in the form of a salt) is added into the reaction system of polyvinyl butyral and isocyanates, long time is needed for the reaction therebetween and yet the crosslinking degree of the product obtained will become necessarily low. In contrast, when an alkali metal salt or alkaline metal salt of a phosphoric acid ester is added to the reaction of polyvinyl butyral with isocyanates, the reaction between both can proceed rapidly and yet a product with great crosslinking degree can be obtained. For this reason, it may be considered that a heat transfer sheet having a heat-resistant slipping layer obtained by addition of an alkali metal salt or alkaline earth metal salt of a phosphoric acid ester to the reaction system of polyvinyl butyral and isocyanates can be wound up and stored without migration of the dye in the heat transfer layer into the heat-resistant slipping layer. Further, by use of an alkali metal salt or alkaline earth metal salt of a phosphoric acid ester as the agent for imparting lubricating property in the heat-resistant slipping layer, there is an additional advantage that the alkali metal salt or alkaline earth metal salt of the phosphoric acid ester will not be migrated into the heat transfer layer at all, even if the heat transfer layer and the heat-resistant slipping layer may contact closely each other, whereby no staining of the heat transfer layer is recognized. Examples of filler which can be used are inorganic or organic fillers having heat resistance such as clay, talc, zeolite, aluminosilicate, calcium carbonate, Teflon powder, zinc oxide, titanium oxide, magnesium oxide, silica, carbon, condensates of benzoguanamine and formalin, and others. The filler should desirably have a mean particle size of 3 µm or less, preferably from 0.1 to 2 µm. The filler is used in an amount of 0.1 to 25%, preferably 1.0 to 10%, by weight of polyvinyl butyral. By use of such a filler in the heat-resistant slipping layer, fusion between thermal heads and the heat transfer occurs less frequently, whereby no sticking phenomenon is observed at all. For provision of the heat-resistant slipping layer 8 on the base sheet 2, the above components may be dissolved in an appropriate solvent to prepare an ink composition for formation of the heat-resistant slipping layer, which is formed on the base sheet 2 according to a suitable printing process or application process, followed by drying simultaneously with causing the reaction to occur between polyvinyl butyral and isocyanates by heating to a temperature from 30 to 80°C, thereby to form a heat-resistant slipping layer. During this operation, it is preferred to prepare a filler-kneaded dispersed composition by previously kneading a filler with the alkali metal salt of alkaline earth metal salt of the phosphoric acid ester. The heat-resistant slipping layer 8 should preferably have a film thickness of 0.5 to 5 µm, more preferably 1 to 1 µm. If the film thickness is thinner than 0.5 µm, the effect as the heat-resistant slipping layer is not satisfactory, while a thickness over 5 µm will result in poor heat transmission from the thermal heads to the sublimable transfer layer, whereby the printing density is disadvantageously lowered. As described above, a heat-resistant slipping layer having satisfactorily excellent performance can be obtained by forming the heat-resistant slipping layer from (a) a reaction product of polyvinyl butyral and isocyanates, (b) an alkali metal salt or alkaline earth metal salt of a phosphoric acid ester and (c) a filler. However, in some cases, when a heat transfer sheet having such a heat-resistant slipping layer is conveyed internally of, for example, a printing conveying device, a problem with respect to conveying characteristic of the heat transfer sheet may occur depending on the tension applied on the heat transfer sheet or the printing pressure of the thermal heads. In such a case, it is preferred to add (e) a phosphoric acid ester not in the form of a salt in addition to the above components (a), (b) and (c) in the heat-resistant slipping layer. The phosphoric acid esters not in the form of salts as shown in the alkali metal salts or alkaline earth metal salts of phosphoric acid esters as described above may be used. Specifically, Plysurf 208S (Polyoxyethylene alkyl ether phosphoric acid) produced by Daiichi Kogyo Seiyaku, GAFAC RS710 produced by Toho Kagaku and the like can be used. Such a phosphoric acid ester not in the form of a salt is used in an amount of 1 to 50%, preferably 1 to 30%, by weight of polyvinyl butyral. At a level in excess of 50% by weight, the dye or the pigment, particularly the dye in the heat transfer layer will undesirably be migrated into the heat resistant slipping layer when stored under piled or wound-up state. The order in which the heat transfer layer 3 and the heat-resistant slipping layer 8 are provided should preferably be as follows. While it is preferable to apply heating for promoting the reaction between polyvinyl butyral and isocyanates, in order for the heat transfer layer to be unaffected by the heat during this heating, it is preferable to provide first the heat-resistant slipping layer on the base sheet 2 and then the heat transfer layer 3. By provision of the above heat-resistant slipping layer, the following effects can be obtained. (a) Even when heated to a considerably high temperature by thermal heads, no sticking phenomonon will occur. (b) No unclearness occurs at the printed portion. (c) Even when the heat transfer sheet is stored under wound-up state, the dye in the heat transfer layer will not be migrated into the heat-resistant slipping layer. Thus, storage stability is excellent. (d) When the heat transfer sheet is conveyed by a printing conveying means, no adhesion of the heat transfer sheet to rolls occurs, whereby conveying performance can be excellent. Heat-resistant layer It is preferable to provide a heat-resistant layer 9 separately from the above heat-resistant slipping layer for improvement of heat resistance. Many kinds of combinations can be used as the synthetic resin curable by heating and its curing agent constituting the heat resistant layer. Typical examples are polyvinyl butyral and polyvalent isocyanate, acrylic polyol and polyvalent isocyanate, cellulose acetate and titanium chelating agent, and polyester and organic titanium compound. Including those, the names of the products readily available in the market and their amounts to be formulated (parts by weight) are shown in the following Table. It is sometimes preferable to add an extender pigment to the above synthetic resin. Examples of the extender pigment suited for this purpose are magnesium carbonate, calcium carbonate, silica, clay, talc, titanium oxide and zinc oxide. The amount formulated may generally be suitably 5 to 40% by weight of the resin. Addition and mixing may be conducted desirably so as to effect satisfactory dispersion by means of a three-roll mill or a sand mill. If adhesive force of the heat-resistant layer to the base film is lacking, corona discharging treatment may be applied or a suitable primer may be used. Generally speaking, a component for imparting lubricating characteristic (slippability) to the sheet surface and a component for imparting heat resistance tend to cancel each other. For example, in the above heat-resistant slipping layer 8, heat resistance is lowered by increase of the lubricating component. Accordingly, for obtaining good heat resistance, the thickness of the heat-resistant slipping layer must be made thick. In order to circumvent this problem, it is preferable to provide the above heat-resistant layer 9 laminated with the heat-resistant slipping layer 8. With such a constitution, (1) both of lubricity and heat resistance can be improved at the same time, and (2) the film thickness can consequently be made thinner. Antistatic layer The antistatic layer 10 has the action of preventing various troubles caused by static electricity, for example, adhesion of dust, generation of wrinkles by attracting force and others. The antistatic layer 10 makes it easy for charges generated on a heat transfer sheet by charging during handling of the heat transfer sheet to be escaped, and it may be formed by use of a material having semiconductivity. For example, by use of a metal foil as the base sheet 2, the inconveniences caused by charging can be cancelled. Alternatively, even when the base sheet 2 itself may be a plastic film, a metal foil or a metal vapor deposited film can be laminated therewith to exhibit the same effect. However, when easiness in handling of the heat transfer sheet, its cost and the usual practice of employing a plastic film such as polyester film as the base sheet 2 are taken into consideration, it is most suitable to form a semiconductive layer by application of a semiconductive coating material containing a semiconductive substance. The place where the semiconductor layer is formed may be at any desired position on the heat transfer sheet as a general rule, but preferably on the outermost surface layer on the front or back of the sheet for the reason of permitting charges accumulated to be readily escaped. The semiconductive substance to be incorporated into the semiconductive coating material is fine powder of a metal or fine powder of a metal oxide. Alternatively, organic compounds called "antistatic agents" can be used as the semiconductive substance, and these are excellent with respect to easiness in preparation of a conductive coating material, although they are lower in antistatic ability at low humidity as compared with the above-mentioned metal or metal oxide. Cationic surfactants (e.g. quaternary ammonium salts, polyamide derivatives), anionic surfactants (e.g. alkylphosphates), amphoteric surfactants (e.g. betaine type) or nonionic surfactants (e.g. fatty acid esters) can be used as "antistatic agent". Further, polysiloxanes can be also used. In connection with the above "antistatic agent", amphoteric or cationic water-soluble acrylic resins can be formed solely without a binder into a coating material, from which a coating with a coated amount on drying of about 0.1 to 2 g/m 2 can be formed to provide a conductive layer. On the other hand, fine powder of titanium oxide or zinc oxide subjected to doping (treatment by baking a mixture of titanium oxide or zinc oxide with an impurity, thereby disturbing the crystal lattices of titanium oxide or zinc oxide) or fine powder of tin oxide may be used as the electron conductive inorganic powder. The semiconducive coating material containing a semiconductive substance as described above can be prepared according to a conventional process, but preferably, an antistatic agent is used in the form of an alcoholic solution or an aqueous solution. The electron conductive inorganic fine powder is used in the form as such, and is prepared by dispersing it in a solution of a resin for the binder in an organic solvent. The resin for the binder in the semiconductive coating material is preferably a resin selected from (a) thermosetting resins such as thermosetting polyacrylate resin, polyurethane resin, or (b) thermoplastic resins such as polyvinyl chloride resin, polyvinyl butyral resin, polyester resin, or the like. The semiconductive coating material prepared is coated by conventional coating methods by, for example, blade coater, gravure coater or alternatively by spray coating. The antistatic layer has a thickness of 1 to 3 µm, or 1 to 5 µm in some cases, and the ratio of the binder to the conductive substance is determined so that the surface resistivity of the antistatic layer after coating and drying (sometimes after curing) may become 1 x 10 10 ohm·cm. The amphoteric or cationic water-soluble acrylic resin may also be formulated into a coating material of an alcoholic solution with addition of 5 to 30% by weight of the binder as the conductive substance. Others The heat transfer sheet according to the present invention has basically the constitution as described above, and it is also possible to apply additional treatments as described below thereon. First, in FIG. 2, between the transfer layer 3 and the base sheet 2 or between the heat-resistant slipping layer 8 and the base sheet 2, a primer layer may be provided for improvement of adhesive force between the respective layers. Known materials may be available for the primer layer. For example, by use of a primer layer of an acrylic resin, a polyester resin, a polyol and a diisocyanate, or the like, adhesion between both layers can be improved particularly when employing a polyester or an aramide (aromatic polyamide) as the base sheet 2. Corona discharging treatment may also be applied for the same purpose. Form of heat transfer sheet, etc. The heat transfer sheet may be in the form of sheets separately cut to desired dimensions, or alternatively in the continuous or wound-up sheet, or further in the form of a narrow tape. In providing the heat transfer layer 3 on the base sheet 2, a coating composition for heat transfer layer containing the same colorant may be applied over the entire surface of the base sheet, or in some cases, a plurality of ink compositions for heat transfer layer containing different colorants, respectively, may be formed at different areas on the surface of the substrate sheet, respectively. For example, it is possible to use a heat transfer sheet as shown in FIG. 4, in which a black heat transfer layer 3a and a red heat transfer layer 3b are laminated in parallel on the base sheet 2, or a heat transfer sheet as shown in FIG. 5, in which a yellow heat transfer layer 3c, a red heat transfer layer 3b, a blue heat transfer layer 3d and a black heat transfer layer 3e are provided repeatedly on the base sheet 2. By use of a heat transfer sheet having such plural heat transfer layers with different hues, there ensues the advantage of obtaining a multicolor image with one heat transfer sheet. The heat transfer sheet (or dye donor sheet) of this invention is used in heat transfer printing in conjunction with a heat transferable sheet (or dye receptive sheet). Printing using this combination is described in the published specification of our related application no. 86301428.8, from which the present text is divided. Some specific embodiments of this invention are illustrated in the following Examples Reference Example 1 Preparation was made first of an ink composition I for a heat-resistant layer having the following composition (part by weight), which was in turn applied on a 4.5-micron thick polyethylene terephthalate film used as a base film with the use of a Wire bar No. 8, followed by warm-air drying. Ink Composition I for Heat-Resistant Layer: Acryl Polyol "45% solution of Acrit 6416 MA manufactured by Taisei Kako, Japan" 41.2 wt. parts Toluene 26.3 wt. parts Methyl Ethyl Ketone 26.3 wt. parts Diisocyanate "45% Ethyl Acetate Solution of Colonate L manufactured by Nippon Polyurethane) 6.2 wt. parts Prepared then was an ink composition I for a heat-resistant slipping layer having the follolwing composition, which was in turn applied on a coating of the ink composition I for a heat-resistant layer with the use of a Wire bar, followed by warm-air drying. Ink Composition I for Heat-Resistant Slipping Layer: Polyvinyl Butyral Resin "S-LEC BX-l" 5.7 wt. parts Toluene 43.1 wt. parts Methyl Ethyl Ketone 43.1 wt. parts Phosphate "Prisurf A-208S" (manufactured by Dai-ichi Kogyo Seiyaku, Japan) 1.3 wt. parts Sodium Salt of Phosphate "GAFAC RD 720" (manufactured by Toho Kagaku, Japan) 1.7 wt. parts Talc "Microace L-l" (manufactured by Nippon Talc, Japan) 1.2 wt. parts Amine-Base Catalyst "Desmorapid PP" (manufactured by Sumitomo Bayer Urethane, Japan) 0.1 wt. parts Diisocyanate "45% Ethyl Acetate Solution of Colonate L" (manufactured by Nippon Polyurethane, Japan) 3.8 wt. parts For curing, this film was further heated at 60°C for 12 hours in an oven. The dry weight of the ink coating was then about 1.2 g/m 2 (2.7 g/m 2 in all). Apart from this, an ink composition for the formation of a heat-sensitive sublimation transfer layer having the following composition was prepared, and was coated on the surface of the base film opposite to the heat-resistant layer by means of a Wire bar No. 10, followed by warm-air drying. The amount of the transfer coating layer applied was about 1.2 g/m 2 . Ink for the Formation of Heat-Sensitive Sublimation Transfer layer: Disperse Dye "Kayaset Blue 714" (manufactured by Nippon Kayaku, Japan) 4 wt. parts Polyvinyl Butyral Resin "S-LEC BX-l" 4.3 wt. parts Toluene 40 wt. parts Methyl Ethyl Ketone 40 wt. parts Isobutanol 10 wt. parts On the other hand, use was made of a base film consisting of a synthetic paper sheet having a thickness of 150 microns "YUPO-FPG" (manufactured by Ohji Yuka, Japan), on which an ink for the formation of a receptive layer, having the following composition, was applied to a dry basis weight of 4.0 g/m 2 with the use of a wire bar No. 36, thereby obtaining a heat transferable sheet. Ink for the Formation of Receptive Layer: Polyester Resin "Vylon 200" (manufactured by Toyobo, Japan) 10 wt. parts Amino-Modified Silicone Oil "KF-393" (manufactured by Shin-etsu Silicone, Japan) 0.125 wt. parts Epoxy-Modified Silicone Oil "X-22-343" (manufactured by Shin-etsu Silicone, Japan) 0.125 wt. parts Toluene 70 wt. parts Methyl Ethyl Ketone 30 wt. parts The heat-sensitive sublimation transfer sheet and heat transferable sheet, obtained as mentioned above, were superposed upon each other with the heat transfer layer coming into contact with the receptive layer. Recording was then carried out from the heat-resistant layer side. The recording conditions were an output of lW/dot, a pulse width of 0.3 to 4.5 milliseconds and a dot density of 3 dot/mm. The heat-sensitive transfer sheet could run smoothly without any sticking and wrinkling. The reflection density of a highly developed color density portion at a pulse width of 4.5 milliseconds was 1.65, and the reflection density of a portion at a pulse width of 0.3 millisecond was 0.16. Thus, a recording having gradation in accordance with applied energy was achieved (as measured by a Machbeth densitometer RD-918). Reference Example 2 Preparation of Heat Transfer Sheets An ink composition for the formation of a heat transfer layer having the following composition was applied on the back side of a 9-micron thick PET subjected to heat-resistant treatment to a dry basis weight of 1.0 g/m 2 , and was then dried to obtain a heat transfer sheet. Disperse Dye: KST-B-136 (manufactured by Nippon Kayaku, Japan) 0.4 wt. parts Ethylhydroxyethyl Cellulose N14 (manufactured by Hercules) 0.6 wt. parts Methyl Ethyl Ketone/Toluene (weight ratio of 1:1) 9.0 wt. parts Preparation of Heat Transferable Sheets The substrate used was synthetic paper (manufactured by Ohji Yuka, Japan, under the trade name of Yupo-FPG No. 150). Each of the following ink compositions (A)-(I) for the formation of intermediate layers was independently applied on that substrate to a dry basis weight of 10 g/m 2 , followed by drying. Thereafter, an ink composition for the formation of a receptive layer, having the following composition, was applied onto the resulting coating, and was dried at 100°C for 10 minutes to prepare a receptive layer having a dry basis weight of 4.5 g/m 2 . In this manner, a heat transferable sheet was obtained. Ink Composition for the Formation of Receptive Layer: Polyester Resin: Vylon 200 (manufactured by Toyobo, Japan, Tg = 67° C) 0.5 wt. parts Polyester Resin: Vylon 290 (manufactured by Toyobo, Japan, Tg = 77° C) 0.5 wt. parts Amino-Modified Silicone: KF 857 (manufactured by Shin-etsu Kagaku Kogyo) 0.04 wt. parts Epoxy-Modified Silicone: KF 103 (manufactured by Shin-etsu Kagaku Kogyo) 0.04 wt. parts Methyl Ethyl Ketone/Toluene/Cyclohexanone (weight ratio of 4:4:2) 9.0 wt. parts Ink Compositions for the Formation of Intermediate Layers: (A) Polyurethane Resin (manufactured by Nippon Polyurethane, Japan, under the trade name of Nippolan 2301) 10.0 wt. parts Solvent (DMF/MEK = 1:1) 90 wt. parts (B) Polyurethane Resin (Nippolan 2314) 10 wt. parts Solvent (the same as (A)) 90 wt. parts (C) Polyurethane (Nippolan 5109) 10 wt. parts Solvent (the same as (A)) 90 wt. parts (D) Polyester Resin (Vylon 200) 10 wt. parts Solvent (Toluene/MEK = 1:1) 90 wt. parts (E) Polyester Resin (Vylon 200) 8 wt. parts Polyester Resin (Vylon 600) 2 wt. parts Solvent (the same as (D)) 90 wt. parts (F) Ethylene/Vinyl Acetate Copolymer Resin (manufactured by Mitsui Polychemical, Japan, under the trade name of Elvaloy U-741P) 20 wt. parts Solvent (MEK/Toluene = 1:1) 80 wt. parts (G) Linear Polyurethane Resin (manufactured by Sumitomo Bayer Urethane, Japan under the trade name of Desmocol 530) 10 wt. parts Solvent (MEK) 90 wt. parts (H) Caprolacton-Base Polyurethane (manufactured by Daiseru Kagaku Kogyo, Japan, under the trade name of Purakuseru EA-1422) 10 wt. parts Solvent (MEK) 90 wt. parts (I) Thermopolastic Polyurethane Resin (manufactured by Dai-Nippon Ink Kagaku Kogyo, Japan, under the trade name of Pandex T-5260S-35MT) 8 wt. parts Titanium Dioxide 2 wt. parts Solvent (MEK) 90 wt. parts With various combinations of the heat transfer sheets with the heat transferable sheets, both obtained as mentioned above, printing was carried out by means of a thermal head under the conditions of an output of lw/dot, a pulse width of 0.3 to 4.5 milliseconds and a dot density of 3 dots/mm. The results are set forth in Table P-1 together with 100% modulus of the resin in the intermediate layers and the coating amounts of the intermediate layers. 100% modulus of the resin Coating amounts of the intermediate layers Reproducibility of dots (A) 70 kg/cm 2 3 g/m 2 ○ (B) 19 kg/cm 2 3 g/m 2 ○ (C) 200 kg/cm 2 3 g/m 2 X (D) 110 kg/cm 2 3 g/m 2 ▵ (E) 100 kg/cm 2 3 g/m 2 ○ (F) 21 kg/cm 2 10 g/m 2 ○ (G) 65 kg/cm 2 3 g/m 2 ○ (H) 25 kg/cm 2 5 g/m 2 ○ (I) 50 kg/cm 2 3 g/m 2 ○ ○ : good ▵ : medium X : worst Example 1 Reference Example 1 was repeated. However, the compositions given in the following table were used for the ink for the formation of heat-sensitive sublimation transfer layers, and gravure printing was carried out in such a manner that three heat-sensitive sublimation transfer layers different in tint from one another were repeatedly arranged. In this manner, a heat-sensitive sublimation transfer sheet was obtained, wherein the amount of the transfer coating of each tint was as follows. Cyan 1.2 g/m 2 Magenta 1.0 g/m 2 Yellow 0.8 g/m 2 Cyan Magenta Yellow Dye Kayaset Blue 714 5.00 MS Red G 2.60 Foron Brilliant Yellow S-6GL 5.50 Dye Macrolex Red Violet 1.40 Polyvinyl Butyral 3.92 4.32 4.52 Solvent MEK 22.54 43.34 48.49 Solvent Toluene 50.18 43.34 41.49 Solvent MIBK 13.00 Solvent Xylene 5.00 Solvent n-Propanol 5.00 Total 100.00 100.00 100.00 (weight %) MEK = Methyl Ethyl Ketone MIBK = Methyl Isobutyl Ketone On the other hand, a composition for the formation of an intermediate layer, having the following composition, was applied on the same synthetic paper as used in Reference Example 1 to a dry basis weight of 10 g/m 2 to obtain an intermediate layer. Subsequently, a composition for a receptive layer, having the following composition, was applied on that intermediate layer to a dry basis weight of 5 g/m 2 to prepare a receptive layer. In this manner, a heat transferable sheet was obtained. Composition for Receptive Layer: Polyester Resin (Vylon 200, manufactured by Toyobo, Japan) 7 weight parts Vinyl Chloride/Vinyl Acetate Copolymer Resin (Vinylite VYHH, manufactured by Union Carbide) 3 weight parts Amino-Modified Silicone (KF-393, manufactured by Shin-etsu Kagaku Kogyo, Japan) 0.5 weight parts Epoxy-Modified Silicone (S-22-343, manufactured by Shin-etsu Kagaku Kogyo, Japan) 0.5 weight parts Solvent (Toluene/Methyl Ethyl Ketone (1:1) 89 weight parts Recording was carried out in accordance with Reference Example 1. As regards the printing density, the highest density was 1.6 for cyan, 1.4 for magenta and 1.5 for yellow. Furthermore, when the said heat-sensitive sublimation transfer sheet was prepared, the polyethylene terephthalate film was subjected to corona discharge treatment on both its sides, aid a polyester resin was applied thereon as 0.2 g/m 2 (dry basis) primers, thus resulting in improvements in adherence. Example 2 Reference Example 1 was repeated. However, the thickness of the polyethylene terephthalate film was changed to 6 microns, the compositions given in the following table were used as the ink for the formation of heat-sensitive sublimation transfer layers, and three heat-sensitive sublimation transfer layers different in tint from one another were repeatedly arranged. In this manner, a heat-sensitive sublimation transfer sheet was obtained, wherein the coating amount of each color was as follows. Cyan 1.2 g/m 2 Magenta 1.0 g/m 2 Yellow 0.8 g/m 2 Cyan Magenta Yellow Dye Kayaset Blue 714 4.80 MS Red G 2.86 Foron Brilliant Yellow S-6GL 6.00 Dye Foron Brilliant Blue S-R 1.00 Macrolex Red Violet 1.56 Polyvinyl Butyral 4.60 4.32 4.52 PVDC powder 0.40 0.40 0.40 Solvent MEK 44.80 43.34 43.99 Solvent Toluene 44.80 42.92 40.99 Solvent Cyclohexanone 5.00 4.50 Total 100.00 100.00 100.00 PVDC = Poly Vinylidene Chloride The heat transferable sheet provided included an intermediate layer obtained by using an ink composition for the formation of an intermediate layer having the composition (D) of Reference Example 2 (the dry basis weight of that intermediate layer was 5.0 g/m 2 ). Recording was carried out in accordance with Reference Example 1. As regards the printing density, the highest density was 1.70 for cyan, 1.50 for magenta and 1.60 for yellow.

Claims: A heat transfer sheet having a heat transfer layer on one surface of a base sheet, said heat transfer layer being formed of a material containing a dye substantially dissolved in a binder with a weight ratio of the dye to the binder (dye/binder ratio) or 0.3 or more, and said base sheet having a heat-resistant slipping layer provided on the surface on which the above heat transfer layer is not provided said dye having the following chemical formula: A heat transfer sheet according to claim 1, wherein said binder comprises cellulose resins or vinyl resins. A heat transfer sheet according to claim 1 or claim 2, further comprising a second heat transfer layer formed adjacent to said heat transfer layer, the second heat transfer layer comprising a binder and the dye C.I. Disperse Red 60. A heat transfer sheet according to claim 1 or claim 2, further comprising a second heat transfer layer formed adjacent to said heat transfer layer, the second heat transfer layer comprising a binder and the dye C.I. Solvent Blue 63. A heat transfer sheet according to claim 3, further comprising a third heat transfer layer formed adjacent to said heat transfer layer, the third heat transfer layer comprising a binder and the dye C.I. Solvent Blue 63.

Inventors: AKADA MASANORI, ARITA HITOSHI, ITO YOSHIKAZU, KUTSUKAKE MASAKI, SAITO MASANORI, TAKANO ATSUSHI, TAKEDA HIDEICHIRO, YAMAUCHI MINEO, AKADA, MASANORI, ARITA, HITOSHI, ITO, YOSHIKAZU, KUTSUKAKE, MASAKI, SAITO, MASANORI, TAKANO, ATSUSHI, TAKEDA, HIDEICHIRO, YAMAUCHI, MINEO, Arita, Hitoshi c/o Dai Nippon Insatsu K.K., Kutsukake, Masaki c/o Dai Nippon Insatsu K.K.
Applicants: DAINIPPON PRINTING CO LTD, DAI NIPPON INSATSU KABUSHIKI KAISHA
Application year: 1986
Main classification: B41M5
IPC codes: B41M5