Heat transfer sheet

Abstract: A heat transfer sheet for heat-sensitive printing by means of thermal heads, comprises a base film, a hot melt ink layer formed on the surface of said base film, and a filling layer formed on said hot melt ink layer. The filling layer comprises a fine particle layer of wax, which can be formed by coating the hot melt ink layer with a wax emulsion.

Description: BACKGROUND OF THE INVENTION This invention relates to improvement of a heat transfer sheet (a heat-sensitive transfer sheet), and more particularly, to a heat transfer sheet capable of providing high quality printing even in the case of a transferable paper (i.e., a paper to be transferred) having a low surface smoothness and further capable of preventing any staining (e.g. scumming or smudging) caused by a hot melt ink composition. When the output print of computers and word processors is printed by heat transfer systems, a heat transfer sheet comprising a heat melt ink layer provided on one surface of a film, as well as at least one thermal head are used. Prior art heat transfer sheets are those which are produced by using, as a base film, papers such as condenser paper and paraffin paper having a thickness of from 10 to 20 µm, or films of plastics such as polyester and cellophane having a thickness of from 3 to 20 µm, and coating on the base film described above a hot melt ink layer wherein pigments are incorporated into waxes. The heat transfer sheet is used in the form of a film or in rolled form in most cases. In general, however, when heat transfer printing is carried out, a hot melt ink layer of a heat transfer sheet directly contacts with the surface of a transferable paper, and a time lag between the moving velocity of the heat transfer sheet and that of the transferable paper is liable to occure at the time of initiating and stopping the printing or moving to a new line. This is because staining occurs. Particularly, in high-speed printing the staining is liable to occure. While the heat transfer system can be used to print to common papers, distinct printing is not necessarily carried out in all the common papers. It is possible to carry out maximum printing if the transferable papers are calendered wood-free papers or coated papers which exhibit a value of at least 100 seconds when the smoothness of the transferable papers is expressed in terms of Beckmann smoothness. Even in the case of wood-free papers having a value of the order of 50 seconds, sufficient printing quality can be obtained. However, when transferable papers having a low smoothness i.e., less than 50 seconds are used, the distinctiveness of printing is reduced. This is because in the case of papers having very uneven surfaces, an ink composition cannot entirely come into contact with papers under a thermal head-urging pressure and the uncontacted portions exhibit inferior transfer. Further, the heat transfer system is slower in printing speed as compared with an impact system, and improvement is required. In order to carry out printing at a higher speed, the level of heat energy which is given to a thermal head must be increased. However, this tends to lead to bleeding of printing and to make the staining described above worse. We have carried out studies in order to overcome the drawbacks and disadvantages described above, looking to provide a heat transfer sheet wherein no staining is generated even if high speed heat transfer is carried out and wherein distinct printing can be obtained even in the case of transferable papers having a low surface smoothness. SUMMARY OF THE INVENTION We have manufactured and tested heat transfer sheets comprising various elements. As a result, we have now found that the provision of layer comprising specific materials on the transferable paper-contacting surface of a heat transfer sheet is extremely effective. According to the present invention there is provided a heat transfer sheet for heat-sensitive printing by means of thermal heads, comprising a base film, a hot melt ink layer formed on the surface of said base film, and a filling layer formed on said hot melt ink layer; said filling layer being obtainable by coating the surface of the hot melt ink layer with a wax emulsion. DETAILED DESCRIPTION OF THE INVENTION Each material, etc. of a heat transfer sheet of the present invention will be described in detail hereinafter. Base Film A conventional base film can be used as it is, as a base film used in the present invention. Other films can be used. The base film of the present invention is not particularly restricted. Examples of the base film materials include plastics such as polyester, polypropylene, cellophane, polycarbonate, cellulose acetate, polyethylene, polyvinyl chloride, polystyrene, nylon, polyimide, polyvinylidene chloride, polyvinyl alcohol, fluorine resins, rubber hydrochloride, and ionomers; papers such as condenser paper, and paraffin paper; and nonwoven fabrics. Composite films thereof may be also used. The thickness of this base film can suitably vary depending upon materials in order to obtain appropriate strength and thermal conductivity. The thickless of the base film is, for example, from 1 to 25 µm, preferably from 3 to 25 µm. Hot Melt Ink Layer having a Filling Effect In a heat transfer sheet according to a first embodiment of the present invention, an ink layer comprises a hot melt ink composition having a melt viscosity of from 10 cps to 60 cps at 100°C. A hot melt ink composition of a prior art heat transfer sheet has a melt viscosity of from about 100 to about 150 cps at 100°C, and therefore the hot melt ink composition used in the first embodiment of the present invention has a low viscosity which has not been heretofore used. Due to the low viscosity of the hot melt ink composition, the wetting of the heated molten ink composition (by thermal heads) to a transferable as well as a filling effect of printed areas are improved. The low viscosity of the hot melt ink composition facilitates the migration of the ink composition to areas wherein the contact of the transfer sheet with paper is incomplete. Thus, high printing quality can be obtained. If the melt viscosity at 100°C of hot melt ink composition is higher than 60 cps, the expected effect cannot be obtained. If the melt viscosity is lower than 10 cps, bleeding may occur and thus printing quality is deteriorated. A hot melt ink layer comprises a coloring agent and a vehicle, and may contain various additives, as needed. The coloring agents include organic or inorganic pigments or dyes. Preferred of these are pigments or dyes having good characteristics as recording materials, for example, those pigments or dyes having a sufficient color density and exhibiting no discoloration or fading under conditions such as light, heat and humidity. The coloring agents may be materials wherein while they are colorless when they are not heated, they form color on heating. Che coloring agents may be such materials that they form color by contacting it with a material contained in a transferable sheet. In addition to the coloring agents which form cyan, magenta, yellow and black, coloring agents having other various colors can be used. That is to say, the hot melt ink composition contains, as coloring agents, carbon black or various dyes or pigments selected depending upon color which is desired to provide to the ink composition. Waxes, drying oils, resins, mineral oils, celluloses and rubber derivatives and the like, and mixtures thereof can be used as such vehicles. Preferred examples of waxes are microcrystalline wax, carnauba wax and paraffin wax. In addition, representative examples of waxes which can be used include various eaxes such as Fischer-Tropsch wax, various low molecular weight polyethylene and partially modified waxes, fatty acid esters, amides, Japan wax, bees wax, whale wax, insect wax, wool wax, shellac wax, candelilla wax, and petrolatum. Examples of the resins which can be used include ethylene-vinyl acetate copolymer (EVA), ethylene-ethyl acrylate copolymer (EEA), polyethylene, polystyrene, polypropylene, polybutene, petroleum resins, vinyl chloride resins, polyvinyl alcohol, vinylidene chloride resins, methacrylic resins, polyamide, polycarbonate, fluorine resins, polyvinyl formal, polyvinyl butyral, acetyl cellulose, nitrocellulose, vinyl acetate resins, polyisobtylene and polyacetal. In order to impact good thermal conductivity and melt transferability to the ink layer, a thermal conductive material can be incorporated into the ink composition. Such materials include carbonaceous materials such as carbon black, and metallic powders such as aluminum, copper, tin oxide and molybdenum disulfide. The hot melt ink layer can be directly or indirectly coated onto the base film by hot melt coating, ordinary printing or coating methods such as hot lacquar coating, gravure coating, gravure reverse coating, roll coating, gravure printing and bar coating, or many other means. The thickness of the hot melt ink layer should be determined such that the balance between the density of necessary printing and heat sensitivity is obtained. The thickness is in the range of from 1 to 30 µm, and preferably from 1 to 20 µm. Hot Melt Ink Layer A hot melt ink layer used in the second embodiment of the present invention comprises a coloring agent and a vehicle, and may contain various additives, as needed. The coloring agents include organic or inorganic pigments or dyes. Preferrred of these are pigments or dyes having good characteristics as recording materials, for example, those pigments or dyes having a sufficient color density and exhibiting no discoloration or fading under conditions such as light, heat and humidity. The coloring agents may be materials wherein while they are colorless when they are not heated, they form color on heating. The coloring agents may be such materials that they form color by contacting it with a material contained in a transferable sheet. In addition to the coloring agents which form cyan, magenta, yellow and black, coloring agents having other various colors can be used. That is to say, the hot melt ink composition contains, as coloring agents, carbon black or various dyes or pigments selected depending upon color which is desired to provide to the ink composition. Waxes, drying oils, resins, mineral oils, celluloses and rubber derivatives and the like, and mixtures thereof can be used as such vehicles. Preferred examples of waxes are microcrystalline wax, carnauba wax and paraffin wax. In addition, representative examples of waxes which can be used include various waxes such as Fischer-Tropsch wax, various low molecular weight polyethylene and partially modified waxes, fatty acid esters, amides, Japan wax, bees wax, whale wax, insect wax, wool wax, shellac wax, candelilla wax, and petrolatum. Examples of the resins which can be used include EVA, EEA, polyethylene, polystyrene, polypropylene, polybutene, petroleum resins, vinyl chloride resins, polyvinyl alcohol, vinylidene chloride resins, methacrylic resins, polyamide, polycarbonate, fluorine resins, polyvinyl formal, polyvinyl butyral, acetyl cellulose, nitrocellulose, vinyl acetate resins, polyisobutylene and polyacetal. In order to impart good thermal conductivity and melt transferability to the ink layer, a thermal conductive material can be incorporated into the ink composition. Such materials include carbonaceous materials such as carbon black, and metallic powders such as aluminum, copper, tin oxide and molybdenum disulfide. The hot melt ink layer can be directly or indirectly coated onto the base film by hot melt coating, ordinary printing or coating methods such as hot lacquer coating, gravure coating, gravure reverse coating, roll coating, gravure printing and bar coating, or many other means. The thickness of the hot melt ink layer should be determined such that the balance between the density of necessary printing and heat sensitivity is obtained. The thickness is in the range of from 1 to 30 µm, and preferably from 1 to 20 µm. Filling Layer In the present invention, a filling layer has both an action of effecting filling of printed areas of a transferable paper during transferring and a function of preventing staining of the printed areas. That is to say, in printing, a conventional heat transfer sheet is liable to generate staining of the transferable paper due to rubbing between the heat transfer sheet and the transferable paper. On the contrary, the present heat transfer sheet having the filling layer does not incur staining even if rubbing occurs because the surface portion of the filling layer only adheres to the transferable paper and the filling layer prevents the ink layer from directly contacting with the transferable paper. Further, when the hardness of the coating film of the filling layer is high (for example, carnauba wax, candelilla wax and the like), the degree of adhesion of the filling layer to the transferable paper is more reduced, little staining may occur. The term "filling" as used herein includes both (a) a case wherein the surface concave of the transferable paper is packed with a filler to exhibit filling, and (b) another case wherein a filler migrates onto the transferable paper while keeping the film state to come into contact with the surface convex to secure it, thus the concave is clogged in the form like a bridge, and consequently the surface of printed areas becomes smooth. In the present invention, the filling layer comprises waxes, and may contain extender pigments, as needed. The melting point of the filling layer can be selected depending upon the temperature of a thermal head used. It is preferred that the melting point of the filling layer be in the range of from 40° to 150°C. Examples of preferred waxes are microcrystalline wax, carnauba wax, and paraffin wax. In addition to such waxes, representative examples of waxes which can be used include various waxes such as Fischer-Tropsch wax, various low molecular weight polyethylenes and partially modified waxes, fatty acid esters and amides, Japan wax, bees wax, whale wax, insect wax, wool wax, shellac wax, candelilla wax, petrolatum, and vinyl ether waxes such as octadecyl vinyl ether. The wax used in the filling layer and the wax used in the hot melt ink layer described above are the same or different. In a preferred embodiment of the present invention, both waxes can be different as follows: the filling layer is provided on the hot melt ink layer; vehicles such as relatively low melting wax are used in both layers; and the hot melt ink composition having a lower melting point as compared with the filling layer, for example, from 40 to 80°C is used. Thereby, the heat sensitivity of the ink composition is increased, and high speed heat transfer becomes possible. By forming the filling layer which comes into contact with the transferable paper from the materials having a higher melting point as compared with the hot melt ink layer, for example, from 50° to 100°C, little bleeding of printing occurs in heat transfer at a high energy level. Accordingly, appropriate combinations can be determined such that the above melting point range and difference in melting point, for example 10 - 60°C are met. It is recommended that an appropriate amount of an extender pigment be incorporated into the filling layer because the bleeding and tailing of printing can be prevented. It is unsuitable that the particle diameter of the extender pigment be too large. Examples of extender pigment suitable for use herein include inorganic bulking agents such as silica, talc, calcium carbonate, precipitated barium sulfate, alumina, titanium white, clay, magnesium carbonate and tin oxide. If the amount of the extender pigment used is too small, the effect obtained is poor. If the amount is more than 60%, dispersibility is reduced, thus it is difficult to prepare an ink composition and the coating obtained is liable to peel off from the base film. Accordingly, it is desirable that the extender pigment be added in an amount of from 0.1 to 60% by weight. As described above, the filling layer may contain a coloring agent (e.g. pigments or dyestuffs) if necessary or may not contain any coloring agent. If the coloring agent is used, the combination of the coloring agent of the filling layer with the coloring agent of the ink layer provides recording having a sufficient density. If only a colorless vehicle is used, it is possible to prevent such a situation that the transferable paper and the ink layer are directly contacted to rub to cause staining. Further, a coloring agent having a masking effect, such as titanium white, is advantageously used, for example, to sharply develop the color of the transferred ink by virtue of the effect of masking the color of the surface of the transferable paper. The filling layer can be also coated by various techniques. It is suitable that the thickness of this layer be from about 0.1 to 30 µm. Antisticking Layer If the material from which a base film is produced has a low degree of heat tolerance, it is preferable that the thermal head-contacting surface be provided with a layer for preventing sticking to the thermal head since high energy and heat are transmitted by the thermal head when printing is carried out under a low temperature atmosphere or at a high speed. The following compositions can be used for preparing the antisticking layer. (a) Compositions containing (i) a thermoplastic resin having an OH or COOH group, such as acrylpolyol, urethane having an OH group, and vinylchloride-vinyl-acetate copolymer, polyesterpolyol, (ii) a compound having at least 2 amino groups, diisocyanate or triisocyanate, (iii) a thermoplastic resin, and (iv) a material which acts as a heat relasing agent or lubricant. (b) Compositions containing (i) a resin such as silicon-modified acrylic resin, silicone-modified polyester resin, acrylic resin, polyester resin, vinylidene fluoride resin, vinylidene fluoride-ethylene tetrafluoride copolymer resin, polyvinyl fluoride resin, and acrylonitrile-styrene copolymer resin, and (ii) a heat releasing agent or lubricant. Examples of the heat releasing agents or lubricants are materials which melt on heating to exhibit their action, such as for example waxes and amides, esters or salts of higher fatty acids; and materials which are useful in the form of solid per se , such as for example fluorine resins and inorganic material powders. The provision of such an antisticking layer makes it possible to carry out thermal printing without occurring sticking even in a heat transfer sheet wherein a heat unstable plastic film is used as a substrate. The merits of plastic films such as good resistance to cutting and good processability can be put to practical use. Mat Layer and Mat Processing While heat transfer generally provides glossy and beautiful printing, it is difficult to read the printed documents in some cases. Accordingly, mat printing may be desirable. In this case, a heat transfer sheet which provides mat printing can be produced by coating a dispersion of inorganic pigments such as silica and calcium carbonate in a resin dissolved in a suitable solvent, onto a base film to form a mat layer, and coating a hot melt ink composition onto the mat layer. Alternatively, a base film per se may be mat processed to use the mat processed base film. Of course, the present invention can be applied to a heat transfer sheet for color printing, and therefore a multicolor heat transfer sheet is also included in the scope of the present invention. Releasable Layer A releasable layer is provided in order to improve the releasability between the base film and the ink layer. Thus, transfer efficiency is improved and release sound is reduced. When the releasable layer remains on the surface of the ink layer after releasing the releasable layer also functions as a protective layer for the printed areas, and contributes to improvement of abrasion resistance of the printed image. The following can be preferably used as materials from which the releasable layer is produced. (a) Resins (i) Silicone resins. (ii) Mixture of a silicone resin and a thermoplastic or thermosetting resin which is compatible therewith. (iii) Silicone-modified resins such as silicone-modified acryl and silicone-modified polyesters. (iv) PVA, protein, amino acid resins, gelatin, vinylidene fluoride, chlorinated polyethylene, NC, CAP, CAB, NC/isocyanate, CAP/isocyanate, CAB/isocyanate, polyamide, polycaprolactone and the like. (b) Thermoplastic Resin + Releasing Agent (i) Releasing Agent Waxes such as silicone-modified wax, polyethylene, paraffin and microcrystalline wax; higher fatty acid, higher fatty acid amides, higher fatty acid esters, and higher fatty acid salts; higher alcohols; and phosphoric esters such as lecithin. (ii) Thermoplastic Resins Acrylic resins, polyester resins, vinylidene fluoride resins, maleic acid resins, polyamides, polycaprolactone, vinylidene fluoridetetrafluoroethylene copolymer resins, polyvinyl fluoride resins, acrylonitrile-styrene copolymer resins, acryl-vinyl chloride copolymer resins, nitrile rubbers, nylon, polyvinylcarbazole, rubber chloride, cyclized rubbers, polyvinyl acetate resins, polyvinyl chloride resins, vinyl chloride-vinyl acetate copolymer resins and the like. (c) Waxes (i) All waxes such as paraffin wax, microcrystalline wax, carnauba wax and montan wax. (ii) Silicone-modified waxes. (iii) Higher alcohols. (iv) Higher fatty acids, higher fatty acid amides, higher fatty acid esters and higher fatty acid salts. (v) Phosphoric esters such as lecithin. Antistatic Agents In order to overcome drawbacks due to static electricity, it is recommended that at least one layer of the heat transfer sheet contains an antistatic agent. The antistatic agent can be incorporated into any of the base film, the ink layer, the filling layer and the antisticking layer. Particularly, it is preferable that the antistatic agent be incorporated into the antisticking layer and/or the filling layer. Antistatic agents used in the present invention include any known antistatic agent. Examples of antistatic agents include a variety of surfactant-type antistatic agents such as various cationic antistatic agents having cationic groups such as quaternary ammonium salt, pyridinium salt and primary, secondary or tertiary amino groups; anionic antistatic agents having anionic groups such as sulfonate, sulfate, phosphate and phosphonate; amphoteric antistatic agents of amino acid type, aminosulfate type or the like; and nonionic antistatic agents of amino-alcohol type, glycerin type, polyethylene glycol type or the like. Further antistatic agents include polymeric antistatic agents obtained by polymerizing the antistatic agents as described above. Other antistatic agents which can be used include polymerizable antistatic agents such as radiation polymerizable monomers and oligomers having tertiary amino or quaternary ammonium groups, such as N,N-dialkylaminoalkyl(meth)acrylate monomers and quaternarized products thereof. Particularly, the use of such polymerizable antistatic agents can provide stable antistatic properties for a long period of time because these antistatic agents integrate with the formed resin layer. In order to indicate more fully the nature and utility of this invention, the following examples are set forth, it being understood that these examples are presented as illustrative only and are not intended to limit the scope of the invention. All parts used herein are by weight unless otherwise specified. Example 1 The following hot melt ink and filler compositions were prepared. Composition of a Hot Melt Ink Layer: Carbon black (CB) "Diablack G" (manufactured by Mitsubishi, Kasei, Japan) 15 parts Ethylene-vinyl acetate (EVA) copolymer (manufactured by Mitsui Polychemical, Japan) 8 parts Paraffin wax (PW) "Paraffin 150°F" (manufactured by Nippon Seiro, Japan) 47 parts Carnauba wax (CW) 30 parts Composition of a Filler Layer: Natural wax emulsion "Diejet T-10" (a melting point of 80°C; 30% solid; manufactured by Gooh Kagaku, Japan) 57 parts Paraffin wax emulsion "Diejet EK" (a melting point of 55°C; 33% (solid); manufactured by Gooh Kagaku, Japan) 43 parts The hot melt ink and filler compositions described above were coated onto a oolyester base film (6 µm) to a thickness of 3 µm and 2 µm, respectively, thereby forming a heat transfer ribbon. This ribbon was used in a commercially available heat transfer printer, and a common papers having various smoothnesses were used as a transferable papers to examine transferability. A degree of the ink composition applied was measured by means of a dot analyzer "Alliadack 1500" (manufactured by Konishiroku Shashin Kogyo, Japan), and the transferability was represented in terms of a percent area dot. The results are shown in Table 1. When the percent area dot is 80% or more, it can be said that this is fully high quality printing visually. For comparison, a transfer ribbon having only a hot melt ink layer having a thickness of 5 µm without providing any filling layer was used. The results are also shown in Table 1. Smoothness 4.6 sec 10.1 sec. 33.1 sec. 52.1 sec. Example 1-1 80.6% 82.3% 86.1% 92.1% Comparative Example 1-1 47.9 54.1 66.6 81.6 In the case of Comparative Example which corresponds to a prior art heat transfer ribbon, if it is a wood-free paper having a smoothness of 50 seconds or more, good printing can be carried out. However, in the case of papers having a low smoothness, the printing quality becomes inferior. On the contrary, in this example using the transfer sheet having the filling layer, high quality printing can be obtained even in the case of papers having a considerably low smoothness. In this example, a transfer sheet was then formed wherein the thermal heat-contacting surface was provided with an antisticking layer having the following composition. Antisticking Layer: Vinylidene fluoride-tetrafluoroethylene copolymer (Kainer 7201, manufactured by Pennwalt Corporation) 8 parts Polyester polyol (40% MEK solution of Kaserak XU-534 TV, manufactured by Takeda Yakuhin Kogyo, Japan) 40 parts Fluorocarbon (F-57, manufactured by Accell) 5 parts Benzoguanamine resin powder (Epostar-S, manufactured by Nippon Shokubai Kagaku, Japan) 3 parts Lecithin (manufactured by Azinomoto, Japan) 1 part MEK 35 parts Toluene 45 parts A mixture of the composition described above and isocyanate (Collonate L; 75% ethyl acetate solution; manufactured by Nippon Polyurethane, Japan) at a weight ratio of composition to lsocyanate of 45:3 was coated by a gravure printing (0.5 g/m²), and dried at a temperature of 100°C to form an antisticking layer. High quality printing is attained even at a low temperature (0°C). Example 2 Example 1 was repeated, except that a heat transfer ribbon was prepared wherein a mat layer having the following composition Polyester resin "Byron 200" (manufactured by Toyobo, Japan) 6 parts Vinyl chloride-vinyl acetate copolymer resin "Vinyllite VAGH" (manufactured by UCC) 7 parts Silica "Erozeal OK 412" (manufactured by Nippon Aerozyl, Japan) 3 parts Talc "Microace L-1" (manufactured by Nippon Talc, Japan) 1 part Methyl ethyl ketone 30 parts Toluene 30 parts was prepared before coating the hot melt ink composition. The transferability was similar to that of Example 1 and mat readable printing was obtained. Example 3 A polyethylene terephthalate film having a thickness of 3.5 µm was used as a base film and the following two layers were coated thereon. First layer having a melting point of 60°C and a thickness of 4 µm: Carnauba wax 20 parts Paraffin wax (Paraffin 145°F) (manufactured by Nippon Seiro, Japan) 60 parts Carbon black "Siest SO" (manufactured by Tokai Denkyoku, Japan) 15 parts Ethylene-vinyl acetate copolymer "Evaflex 310" (manufactured by Mitsui Polychemical, Japan) 8 parts The above components were kneaded for 6 hours at a temperature of 120°C using an attritor, and coated at a temperature of 120°C by a hot melt roll coating process. Second layer having a melting point of 74°C and a thickness of 0.5 µm: Carnauba non-aqueous emulsion "4U-1128B (isopropanol emulsion containing 25% solids, manufactured by Nippon Carbide Kogyo, Japan) 10 parts Candelilla wax (25% isopropanol dispersion) 5 parts Carbon black described above 0.5 part Polybutene "2000 HEM 75AS" 0.2 part Isopropanol 2 part The layers were coated by a gravure coating process. This heat transfer sheet also exhibited good transfer performance. Example 4 A polyethylene terephthalate film having a thickness of 3.5 µm was used as a base film. Hot melt ink and filler compositions comprising the following components were prepared, and coated onto one surface of the base film, respectively, in the same manner as described in Example 3. Composition of a Hot Melt Ink Layer: CB 15 parts EVA 8 parts PW 50 parts CW 25 parts Composition of a Filling layer: 155°F Paraffin wax emulsion "WE- 70" (40% solids aqueous emulsion manufactured by Bond Wax Co.) 10 parts 60% isopropanol aqueous solution 15 parts There was used the above heat transfer sheet wherein the thickness of the ink layer and filling layer was 3.5 µm and 0.8 µm, respectively. Several different papers (wood-free paper having a high smoothness and medium paper having a low smoothness) were used as transferable papers. A commercially available thermal head was used to carry out heat transfer printing. At energy of thermal head of 0.7 mJ/dot, high speed printing of 40 words per second could be carried out in the case of all transferable papers with high quality without any staining. Example 5 The same base film as that of Example 4 was used and two layers having the following composition were coated thereon. Composition of a Hot Melt Ink Layer: The same as that of Example 4 Composition of a Filling Layer: 155°F Paraffin wax emulsion "WE-70" (40% solids aqueous emulsion manufactured by Bond Wax Company) 70 parts Silane-treated silica emulsion "Bond Wax WE-3" (10% solids; manufactured by Bond Wax Company) 30 parts 50% Isopropanol aqueous solution 50 parts The filling layer was coated in an amount of 0.5 g/m² by a gravure coating process. This heat transfer sheet having the thickness of the ink layer and filling layer of 3.5 µm and 0.5 µm, respectively, exhibited transfer performance as well as that of Example 4. EXAMPLE 6 The same base film as that of Example 4 was used, and three layers having the following composition were coated thereon. Composition of an Antisticking Layer: 40% Xylene solution of Silicone-modified acrylic resin "KR 5208" (manufactured by Shinetsu Kagaku, Japan) 10 parts Fluorocarbon "F-57" (manufactured by Accell) 3 parts Toluene 40 parts Xylene 40 parts Butanol 15 parts The antisticking layer was coated in an amount of 0.1 g/m² by a gravure coating process. Composition of a Hot Melt Ink Layer: The same as that of Example 4 Composition of a Filling Layer: Carnauba emulsion "WE-90" (40% solids; manufactured by Bond Wax Company) 10 parts 70% Isopropanol aqueous solution 10 parts The filling layer was coated in an amount of 0.3 g/m² by a roll coating process. The transfer sheet of this example having the thickness of the ink layer and filling layer of 3.5 µm and 0.3 µm, respectively, also exhibited good transfer performance even in a low temperature (0°C) atmosphere without any sticking and without any staining. Example 7 A polyethylene terephthalate film having a thickness of 3.5 µm was used as a base film and four layers having the following composition were coated. Composition of the Antisticking Layer: 40% xylene solution of silicone-modified acrylic resin "KR 5208" (manufactured by Shinetsu Kagaku, Japan) 10 parts Fluorocarbon "F-57" (manufactured by Accell) 3 parts Antistatic agent "Arcard T50" (manufactured by Lion Agzo, Japan) 1.2 part Toluene 40 parts Xylene 40 parts Butanol 15 parts Composition of the Mat Layer: The same as that of Example 2 (coated in an amount of 0.4 g/m²) Composition of the Hot Melt Ink Layer: 15 parts of CB, 8 parts of EVA, 50 parts of PW and 25 parts of CW in Example 1. The above components were kneaded for 6 hours at a temperature of 120°C using an attritor. This was applied in an amount or 4 g/m² at a temperature of 120°C by a hot melt roll coating process. Composition of the Filling Layer: Carnauba emulsion "WE-90" 40% solids (manufactured by Bond Wax Company) 10 parts EVA "Polysol EVAAD-5"56% solids (manufactured by Showa Kobunshi, Japan) 5 parts 50% Isopropanol aqueous solution 10 parts The filling layer was coated in an amount of 1.0 g/m² by a gravure coating process. The transfer sheet of this example having the thickness of the ink layer and filling layer of 4 µm and 1 µm, respectively, also exhibited transfer performance even in a low temperature (0°) atmosphere without any staining. Example 8 The same base sheet as Example 7 was used to prepare a transfer sheet wherein a releasabie layer, an antisticking layer, an ink layer and a filling layer having the following composition were formed on the base sheet. Releasable Layer: 40% Xylene solution of silicone-modified resin (KR 5208 manufactured by Shinetsu Kagaku Kogyo, Japan) 10 parts Toluene 40 parts Xylene 40 parts Butanol 15 parts The releasable layer was coated in an amount of 0.1 g/m² by a gravure coating process. Hot Melt Ink Layer: Carbon black "Siest SO" (manufactured by Tokai Denkyoku, Japan) 15 parts Ethylene-vinyl acetate copolymer "Evaflex 310" (manufactured by Mitsui Polychemical, Japan) 10 parts Paraffin wax "Paraffin 15°F" 40 parts Carnauba wax 15 parts The above components were kneaded for 6 hours at a temperature of 120°C using an attritor. The kneaded mass was coated at a temperature of 120°C in an amount of 5 g/m² by a hot melt roll coating process. Filling layer: Carnauba emulsion "WE-90" (40% solids; manufactured by Bond Wax Company) 10 parts 75% IPA aqueous solution 10 parts The filling layer was coated in an amount of 1 g/m² by a gravure coating process. Antisticking Layer: Vinylidene fluoride-tetrafluoroethylene copolymer "Kainer K 7201" (manufactured by Pennwalt Corporation) 5 parts Polyester polyol "SP-1510 (manufactured by Hitachi Kase, Japan) 4 parts CAB "Sellit BP 700-25" (manufactured by Bayer Aktiengesellschaft) 1 part Polyethylene wax "FC 113" (manufactured by Adeka Argus Chemical Co. Ltd., Japan) 1 part Fluorocarbon "F-57" (manufactured by Accell) 0.5 part MEK 60 parts Toluene 30 parts The antisticking layer was coated in an amount of 0.3 g/m² . There was used the above heat transfer sheet wherein the thickness of the ink layer and filling layer was 4 µm and 2 µm, respectively. Several papers (i.e., wood-free paper having a high smoothness and medium paper having a low smoothness) were used as a transferable papers. A commercially available thermal head was used to carry out in the case of all transferable papers without any staining. Printing could be carried out without any release noise. Also, even in a low temperature atmosphere (0°C), high quality printing was obtained. EXAMPLE 9 The same base film as that of Example 7 was used to prepare a transfer sheet wherein a releasable layer, a filling layer and an ink layer having the following composition were formed on the base film. Releasable Layer: Montan wax 10 parts Xylene 50 parts Toluene 40 parts The releasable layer was coated in an amount of 0.7 g/m² by a gravure coating process while warming to 50°C. Hot Melt Ink Layer: Product obtained by reacting hexamethylene diisocyanate with Ethyl alcohol at an equivalent weight (80°C, 10 hours) 30 parts Vinyl acetate "Esneal C-50" 6 parts Carbon black "Siest SO" (manufactured by Tokai Denkyoku, Japan) 6 parts Ethyl alcohol 50 parts IPA 20 parts The ink layer was coated in an amount of 3 g/m² by a gravure coating process. Filling Layer: The same as that of Example 4 (coated in an amount of 1 g/m²) This transfer sheet exhibited good transfer performance against all transferable papers without any staining. Printing could be carried out without any release noise. In case of this example, the releasable layer also functions as a protective layer for the printed areas. Example 10 A PET film having a thickness of 6 µm was used as a base film and an ink layer containing a quaternary ammonium salt (cationic) antistatic agent having the following composition and a filling layer were formed to prepare a transfer sheet. Hot Melt Ink Layer: Product obtained by reacting hexamethylene diisocyanate with ethyl alcohol at an equivalent weight (80°C, 10 hours) 30 parts Vinyl acetate "Esneal C-50" (manufactured by Sekisui Kagaku, Japan) 6 parts Carbon black "Siest SO" (manufactured by Tokai Denkyoku, Japan) 6 parts Stachside concentrated solution (manufactured by TDK, Japan) 3 parts Ethyl alcohol 50 parts IPA 70 parts The hot melt ink layer was coated in an amount of 3.0 g/m² by a gravure coating process. Filling Layer: The same as that of Example 4 except that stachside concentrated solution (TDK) was added in an amount of 0.05 part. The filling layer was coated in an amount of 0.8 g/m². This transfer sheet was evaluated for printing using a static honest meter (Shinshido Shokai, Japan). This transfer sheet exhibited high antistatic property as follows. Saturated Potential Half-life Period Example 10 -500V 10 seconds Example 11 The same base film (4.5 µm) as that of Example 10 was used, and an ink layer and a filling layer containing a quaternary ammonium salt antistatic agent which have the following composition were formed to prepare a transfer sheet. Hot Melt Ink Layer: Carbon Black "Siest SO" (manufactured by Tokai Denkyoku, Japan) 15 parts Ethylene-vinyl acetate copolymer "Evaflex 310" (manufactured by Mitsui Polychemical, Japan) 7 parts Paraffin wax "Paraffin 150°F" 40 parts Carnauba wax 15 parts The hot melt ink layer was coated at a temperature of 120°C in an amount of 3.5 g/m² by a hot melt roll coating process. Filling layer: Carnauba emulsion "WE-90" (manufactured by Bond Wax Company) (40% solids) 10 parts 70% IPA aqueous solution 30 parts Antistatic "Arcard T-50" (manufactured by Lion Agzo, Japan) 0.2 part The filling layer was coated in an amount of 0.5 g/m² by a gravure coating process. Antisticking Layer: Vinylidene fluoride-tetrafuoroethylene copolymer "Kainer K 7201" (manufactured by Pennwalt Corporation) 5 parts Polyester polyol "SP-1510" (manufactured by Hitachi Kasei, Japan) 4 parts CAB "Sellit BP700-25" (manufactured by Bayer Atkienzesellschaft) 1 part Polyethylene wax "FC113" (manufactured by Adeka ARgus Chemical Co, Ltd, Japan) 1 part Fluorocarbon "F-57" (manufactured by Accell) 0.5 part Antistatic agent "Elenon 19M" (manufactured by Daiichi Kogyo Seiyaku, Japan) 0.6 part MEK 60 parts Toluene 30 parts The antisticking layer was coated in an amount of 0.5 g/m² by a gravure coating process. This transfer sheet was evaluated for printing in the same manner as described in Example 10. This transfer sheet exhibited high antistatic property as follows: Saturated Potential Half-life Period Example 11 -300V 5 seconds As can be seen from Examples described above, the heat transfer sheet of the present invention has effects and advantages as described hereinafter. (a) High quality printing can be attained even under severe conditions such as high speed heat transfer and the use of rough papers having a low surface smoothness as transferable papers. (b) The present heat transfer sheet can effectively prevent the printed areas from occurring a void, collapse, bleeding and staining in both cases of low speed heat transfer printing and high speed heat transfer printing. (c) When the coloring agent is added to the filling layer to impart color to the filling layer, it is superposed on the coloring agent of the hot melt ink composition to compensate the color of the ink composition. Further, when the coloring agent having hiding (masking) properties is used as the coloring agent described above, it masks the color of the surface of the transferable paper. (d) When the filling layer is provided on the ink layer, the storage properties of the heat transfer sheet are improved. (When the filling layer is formed from the high melting materials, the storage properties are particularly good.)

Claims: A heat transfer sheet for heat-sensitive printing by means of thermal heads, comprising a base film, a hot melt ink layer formed on the surface of said base film, and a filling layer formed on said hot melt ink layer; said filling layer being obtainable by coating the surface of the hot melt ink layer with a wax emulsion. A heat transfer sheet according to claim 1, wherein said wax is selected from micro-crystalline wax, carnauba wax, paraffin wax, Fischer-Tropsch wax, low molecular weight polyethylene, Japan wax, bees wax, whale wax, insect wax, wool wax, shellac wax, candelilla wax, montan wax, petrolatum, fatty acid esters and amides. A heat transfer sheet according to claim 1 or 2, wherein said filling layer contains an extender pigment selected from silica, talc, calcium carbonate, precipitated barium sulfate, alumina, titanium white, clay, magnesium carbonate and tin oxide. A heat transfer sheet according to claim 1, 2 or 3, wherein said filling layer has a thickness of from 0.1 to 30µm.

Inventors: HIDA YOSHIAKI, MIZOBUCHI AKIRA, TAKAHASHI KYOHEI, UMISE SHIGEKI, YAMAMOTO KYOICHI, HIDA, YOSHIAKI, MIZOBUCHI, AKIRA, TAKAHASHI, KYOHEI, UMISE, SHIGEKI, YAMAMOTO, KYOICHI
Applicants: DAINIPPON PRINTING CO LTD, DAI NIPPON INSATSU KABUSHIKI KAISHA
Application year: 1985
Main classification: B41M5
IPC codes: B41M5