HEAT TRANSFER SHEET

Abstract: A heat transfer sheet comprising a base film and a hot melt ink layer formed on one surface of the base film, said hot melt ink layer comprising one or more components which impart fill­ ing to the printed areas of a transferable paper during transfer­ ring. Another type of a heat transfer sheet comprising a base film, a hot melt ink layer laminated on one surface of the base film, and a filling layer laminated on the hot melt ink layer, said filling layer comprising one or more components which impart filling to the printed areas of a transferable paper during trans­ fering. According to the heat transfer sheets, high quality print­ ing can be attained even at a high speed without any staining.

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. A hot melt ink layer comprising a colourant, a wax, atactic polypropylene and a rosin resin, having a melt viscosity of 50-15000 cps is known from JP-A-59-24691. 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, aiming 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 an ink layer comprising specific materials on the transferable paper-contacting surface of a heat transfer sheet is extremely effective. That is to say, a heat transfer sheet according to the present invention is characterized in that one surface of a base film is provided with a hot melt ink layer having an action of effecting filling of printed areas of a transferable paper during transfer. Specifically, this hot melt ink layer comprises an ink composition having a melt viscosity of from 10 cps to 60 cps at 100°C, and incorporates an ethylene-vinyl acetate copolymer (EVA). In another embodiment of the present invention, the thermal head-contacting surface may be provided with an antisticking layer. In another embodiment of the present invention, a base film may have a mat layer on its surface to which an ink layer is applied; or the base film surface to which ink layer is applied may be mat processed. In a further embodiment of the present invention, a releasable layer may be interposed between a base film and an ink layer. In a still further embodiment of the present invention, each layer of a heat transfer sheet, particularly, an antisticking layer and/or filling layer may contain an antistatic agent. 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 thickness 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, and comprises EVA copolymer. 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. 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 incorporated in the vehicle. 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 resins which can be used in addition to ethylene-vinyl acetate copolymer (EVA), include 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, polyisobutylene 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. 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-vinylacetate 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 raw materials were blended in proportions (% by weight) shown in Table 1 to prepare a hot melt ink composition having a filling effect. The melt viscosity of the resulting ink composition at 100°C (represented by "Vis.") was measured by means of a B-type viscometer. The results obtained are also shown in Table 1. The hot melt ink composition was coated onto a polyester base film (6 µm) to a thickness of about 5 µm to form a heat transfer ribbon. This ribbon was used in a commercially available heat transfer printer, and common papers having various smoothnesses were used as 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 2. When the percent area dot is 80% or more, it can be said that this is fully high quality printing visually. Comparative Example 1-1 corresponds to a prior art heat transfer ribbon. If it is a wood-free paper having a smoothness of at least 50 seconds, good printing can be carried out. However, in the case of papers having a low smoothness, the printing quality becomes inferior. As can be seen from Table 2, this example using the ink composition having a low melt viscosity can provide high quality printing even in the case of papers having a considerably low smoothness. Further, above examples were repeated except that an antisticking layer was formed using the following composition. High quality printing is attained even at a low temperature (0°C). The antisticking layer was coated in an amount of 0.5 g/m² (on a dry basis; the coating weight is similarly described on a dry basis) by a gravure coating process. EXAMPLE 2 Example 1 was repeated except that an ink composition for mat layer having the following formulation was prepared before coating a hot melt ink composition having a filling effect onto a base film. A 50% butyl acetate solution of isocyanate "Takenate D-204" (manufactured by Takeda Seiyaku Kogyo, Japan) was incorporated into the ink composition at a weight ratio of the mat composition to isocyanate solution of 20:3, and thereafter the mixture was coated onto a base film. The amount is 1 g/m². A heat transfer ribbon was prepared and the transferability was measured in the same manner as described in Example 1. The resulting heat transfer ribbon exhibited similar performance, and provided mat readable printing. EXAMPLE 3 A polyethylene terrepthalate film having a thickness of 3.5µm was used as a base film to prepare a transfer sheet wherein a releasable layer and an ink layer having the following composition were formed on the base film. The releasable layer was coated in an amount of 0.1 g/m² by a gravure coating process. 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. This transfer sheet was evaluated for printing on several papers (i.e. wood-free paper having a high smoothness and medium paper having a low smoothness). A commercially available thermal head was used to carry out heat transfer printing. At energy of the thermal head of 0.7 mJ/dot, high speed printing of 40 words/second was carried out. This transfer sheet exhibited good transfer performance against all transferable papers without any release noise. EXAMPLE 4 The same base film as that of Example 3 was used to prepare a transfer sheet wherein a primer layer, a releasable layer and an ink layer having the following composition were formed on the base film. The primer layer was coated in an amount of 0.5 g/m² by a gravure coating process. The releasable layer was coated in an amount of 1 g/m² by a gravure coating process. Hot Melt Ink Layer: The same as that of Example 3 When a releasable layer is formed from materials which are not readily adhered to a PET base film and readily released from the hot melt ink layer, such as PVA, it is preferable to provide a primer layer to obtain adhesion between the base film and the releasable layer, as shown in this example. Other processes for improving adhesion include those processes wherein the surface of the base film is subjected to corona and plasma treatments by a conventional method. This transfer sheet was evaluated for printing in the same manner as described in Example 3. This transfer sheet exhibited good transfer performance without any release noise. EXAMPLE 5 A PET film having a thickness of 6 µm was used as a base film to prepare a transfer sheet wherein an antisticking layer and an ink layer having the following composition were formed on the base film. The antisticking layer was coated in an amount of 0.5 g/m² by a gravure coating process. The ink composition had melt viscosity of 45 cps at 100°C. 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 4 g/m² by a hot melt roll coating process. The obtained heat transfer sheet was evaluated for antistatic property by using a static honest meter (Shishido Shokai, Japan). Comparative example was prepared in the same manner as described above except that an antistatic agent (Elenon 19M) was excluded from an antisticking layer. Example 5 exhibited high antistatic property as follows. EXAMPLE 6 The same base film as that of Example 5 was used, and the same ink layer as that of Example 5 was formed. Further, an antistatic agent layer having the following composition was formed onto the base film surface opposite to the ink layer. Coating was carried out by a gravure coating process using a 150 line/inch cylinder having a plate depth of 40 µm. The obtained transfer sheet was evaluated for printing in the same manner as described in Example 5. This transfer sheet exhibited high antistatic property as follows. EXAMPLE 7 A PET containing an antistatic agent was used as a base film, and the same ink layer as that of Example 5 was formed to prepare a transfer sheet. The obtained transfer sheet was evaluated for printing in the same manner as described in Example 5. This transfer sheet exhibited high antistatic property as follows. 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 spewed heat transfer printing and high speed heat transfer printing. (c) When the thermal head-contacting surface of the base film is provided with the antisticking layer, so-called "sticking phenomenon" (i.e., the base film may heat bond with the thermal head) can be effectively prevented. (d) When the layer or layers constituting the heat transfer sheet contains the antistatic agent, various drawbacks due to static electricity can be overcome. (e) When the releasable layer is interposed between the base film and the ink layer, the release of both layers can be readily carried out, transfer efficiency is improved, and release noise is also reduced. Further, in the case where the ink layer is transferred together with the releasable layer or the releasable layer is divided into two separate layers during the transferring operation, the wear resistance of the printed area improves. (f) When the base film has a mat layer on its surface to which the ink layer is applied, or the base film surface to which the ink layer is mat processed, the gloss of the printed areas can be removed to obtain readily readable printing.

Claims: A heat transfer sheet comprising a base film and a hot melt ink layer formed on one surface of the base film, said hot melt ink layer comprising a binder resin containing a dye which is to be transferred to a sheet to be heat transferred upon being heated, said binder comprising an ethylene-vinyl acetate copolymer, said hot melt ink layer having a melt viscosity of from 10 cps to 60 cps at 100°C thereby effecting filling of the printed areas of the sheet to be transferred during heat transfer printing. A heat transfer sheet according to Claim 1, wherein the thermal head-contacting surface is provided with an antisticking layer. A heat transfer sheet according to Claim 2, wherein said antisticking layer contains an antistatic agent. A heat transfer sheet according to any one of claims 1 to 3, wherein said hot melt ink layer contains an antistatic agent. A heat transfer sheet according to any one of Claims 1 to 4 wherein an antistatic agent is coated on the surface of the base film. A heat transfer sheet according to any one of Claims 1 to 5, wherein a mat layer is interposed between the base film and the ink layer. A heat transfer sheet according to any one of Claims 1 to 6, wherein the base film surface to which the ink layer is applied is mat processed. A heat transfer sheet according to any one of Claims 1 to 7, wherein a releasable layer is interposed between the base film and the hot melt ink layer.

Inventors: HIDA, YOSHIAKI, MIZOBUCHI, AKIRA, TAKAHASHI, KYOHEI, UMISE, SHIGEKI C/O DAI NIPPON INSATSU K.K., YAMAMOTO, KYOICHI
Applicants: DAI NIPPON INSATSU KABUSHIKI KAISHA
Application year: 1985
Main classification: B41M5
IPC codes: B41M5