Thermal transfer recording medium

Abstract: A thermally transferred sheet used in combination with a thermally transferable sheet, such as an ink ribbon, in which a laminated layer of the thermally transferrable sheet can be positively transferred, with the thermally transferred sheet exhibiting superior running performance and resistance to blocking to enable a picture of high quality and high resolution to be produced. To this end, there is provided a thermally transferred sheet made up of a substrate and a dye reception layer formed on the substrate. The dye reception layer contains a copolymer of a compound having a pre-set chemical formula and another monomer.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a thermally transferred sheet used in combination with a thermally transferable sheet, such as an ink ribbon, and on to which the dye from the thermally transferable sheet is transferred on application of a heat quantity. Description of Related Art For developing the picture information inputted to a video apparatus onto a thermally transferred sheet, such as a printing paper sheet, there is used a method employing a sublimable dye or a thermally fusible dye. In this thermal transfer method, a thermally transferrable sheet (ink ribbon) having formed thereon a dye layer containing a sublimable dye or a thermally fusible dye, and a thermally transferred sheet (printing paper sheet), having formed thereon a reception layer for receiving the dye, are superimposed on one another, so that the dye layer will face the reception layer, and heat is applied by e.g., a thermal head in a dot pattern responsive to picture signals. This causes the dye in the dye layer to be sublimed or fused and transferred to the reception layer of the printing paper sheet to manifest the picture on the printing paper sheet. This thermally transferred sheet is of a dual layer structure comprised of a sheet-like substrate and the reception layer formed thereon. This reception layer 2 is a layer for receiving a picture of a dye transferred from the ink ribbon, for example, a picture of a sublimable disperse dye, and for maintaining the picture formed on reception, and is routinely formed of a resin exhibiting dyeing properties, such as polyester, polycarbonate or polyvinyl chloride. For improving heat resistance, polyisocyanate, as a hardener, is sometimes added to the reception layer. Also, for improving transfer sensitivity and light-fastness, that is resistance to light, plasticizers are sometimes added to the reception layer. In addition, silicone oil, as a release agent, is sometimes added to the reception layer for improving peeling of the thermally transferred sheet from the dye layer surface. On the other hand, the thermally transferrable sheet is routinely comprised of a substrate of, for example, polyester, and ink layers of respective colors, namely yellow, magenta, cyan and, if necessary, black, formed surface-sequentially thereon. In addition to the respective ink layers, a laminate layer may be provided, which is transferred as a protective layer on the reception layer after forming the picture on the thermally transferred sheet. That is, the thermally transferrable sheet, carrying the laminated layer, can form a protective layer on the reception layer of the thermally transferred sheet. If, in the above-described thermally transferred sheet, the transfer properties of the laminate layer are to be improved, it may be contemplated to decrease the amount of addition of a hardener, such as pooyisocyanate, which is added to the reception layer, or to to add a plasticizer to the reception layer to lower the glass transition temperature of the resin to soften the reception layer. However, in such case, the ink surface of the thermally transferrable sheet is fused to the reception layer of the thermally transferred sheet to detract from the quality of the formed picture or to cause running troubles. Also, the reception layer is softened in this case, so that, if plural thermally transferred sheets are stacked together for storage under high temperature conditions, the so-called blocking, in which the reception layer 2 tends to be stuck to the back surface of the thermally transferred sheet, is likely to be produced. On the other hand, if, in the above-described thermally transferred sheet, the running performance or resistance to blocking under high temperature conditions is to be improved, it may be contemplated to increase the amount of addition of the hardener, such as polyisocyanate, to harden the reception layer, or to increase the amount of addition of silicone oil to improve release properties between the thermally transferrable sheet and the thermally transferred sheet. However, in this case, the laminate layer of the thermally transferred sheet is worsened in transfer characteristics such that the laminate layer cannot be transferred or transferred only incompletely. Thus, in the conventional thermally transferred sheet, there is a relation of trade-offbetween the transfer characteristics of the laminate layer on one hand and the running performance and resistance against blocking under high temperature conditions, such that the two requirements cannot be met simultaneously. The prior art in this technical field is as follows: The teaching of EP 0 283 048 describes an image-receiving sheet comprising a base sheet and a receiving layer. The base sheet has a porous structure or a foam structure. The receiving layer consists of a dyeable resin which preferably is a copolymer of vinylchloride and an acrylic acid type monomer. A preferred acrylic acid type monomer is 2-hydroxyethylacrylate. The molecular weight of the copolymer is 5,000 to 40,000, preferably 10,000 to 30,000. The copolymer contained in the receiving layer of the image-receiving sheet is not cross-linked. The disclosure of US 5,786,300 concerns a thermal dye transfer assemblage comprising a dye-donor element and a dye-receiving element The dye-receiving element is formed of a support having thereon a polymeric dye image-receiving layer comprising a polymer having a Tg of less than about 9°C and an acidic metal salt. The copolymer contained in the dye-receiving element is not cross-linked. EP 0 368 320 describes a heat transfer image-receiving sheet having a dye-receiving layer provided on the surface of a substrate sheet The dye-receiving layer comprises a graft copolymer having at least one releasing segment selected from the group consisting of polysiloxane segments, fluorinated carbon segments and long-chain alkyl segments graft-bonded to the main chain in the graft polymer. As the polymer for the main chain any polymer having a reactive functional group known in the art may be used. The polymer preferably has a molecular weight in a range of about 5,000 to 40,000. In Reference Example Al, a copolymer of methylmethacrylate and hydroxyethylmethacrylate is reacted with a polysiloxane isocyanate to introduce polysiloxane side chains into the polymer. The graft polymer is then used to prepare a dye-receiving layer. Prior art document US-A-5 472 789 discloses an ink-receptive transparent sheet comprising a transparent polymeric film coated with a layer comprising a semi-interpenetrating network (SIPN). Such SIPN comprises a cross-linked polymeric matrix component and an uncross-linked polymeric liquid-absorbent component, whereby a polyfunctional aziridine is used as crosslinking agent. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a thermally transferred sheet onto which the laminate layer of the thermally transferrable sheet can be positively transferred and which exhibits superior running performance and resistance to blocking to enable a picture of high quality and high resolution to be produced. The present invention provides a thermally transferred sheet made up of a substrate and a dye reception layer formed thereon, wherein said dye reception layer contains a copolymer having a weight average molecular weight of 100,000 to 1,000,000, said copolymer being obtained by polymerization of a compound having the following formula (1) wherein R is H or CH 3 , with another monomer, wherein in said copolymer the proportion of said compound of formula (1) is 5 to 25 wt.-% and said copolymer is crosslinked by a hardener added to said dye reception layer and wherein the hardener is an epoxy-based hardener or an isocyanate-based hardener. With the above-described thermally transferred sheet according to the present invention, in which a copolymer of the compound shown by the chemical formula (1) and another monomer is used as a material for the thermally transferred sheet, and in which the proportion in the copolymer of the compound shown by the chemical formula (1) is prescribed to a pre-set range, the dye reception layer can be set to a desired state of flexibility. Specifically, the proportion of the compound having the chemical formula (1) is set to 5 to 25 wt%. If the proportion of the compound having the chemical formula (1) is set to less than 5 wt%, the dye reception layer is lowered in strength, whereas, if the proportion of the compound having the chemical formula (1) is set to larger than 25 wt%, the glass transition temperature of the copolymer becomes excessively low to soften the dye reception layer excessively. BRIEF DESCRIPTION OF THE DRAWINGS Fig.1 is a cross-sectional view showing essential portions of a thermally transferred sheet according to the present invention. Fig.2 is a plan view showing essential portions of a thermally transferrable sheet. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawings, a preferred embodiment of a thermally transferred sheet according to the present invention will be explained in detail. Fig.1 shows an example of a thermally transferred sheet according to the present invention. This thermally transferred sheet is used as a so-called printing paper sheet in combination with a thermally transferrable sheet, such as an ink ribbon, having a dye layer containing a dye, to form a desired picture by the dye transferred from the thermally transferrable sheet. This thermally transferred sheet includes a sheet-like substrate 1 and a dye reception layer 2 formed on the substrate 1 for receiving the dye. With the present thermally transferred sheet, the dye is migrated from the thermally transferrable sheet to the dye reception layer 2, responsive to e.g., video signals, to form an image by selective heat application by a thermal head. Referring to Fig.2, the thermally transferrable sheet is made up of a substrate of the thermally transferrable sheet 5, an ink layer 9 having a yellow dye layer 6, a magenta dye layer 7 and a cyan dye layer 8, and which is arranged on a major surface 5a of the substrate 5, and a laminate layer 10 arranged on the major surface 5a of the thermally transferrable sheet 5 adjacent to the ink layer 9. The thermally transferred sheet is combined with the thermally transferrable sheet, with the dye reception layer 2 facing the ink layer 9, in order to form an image. In the thermally transferred sheet, the substrate 1 may be a routinely used substrate, such as paper, inclusive of a high-quality paper sheet and a coated paper sheet, or a variety of plastics sheets, or compound sheets of the paper and plastic sheets. In addition, in the thermally transferred sheet, the dye reception layer 2 contains a copolymer comprised of a compound represented by the chemical formula (1): wherein R is H or CH 3 , and other monomers. In the present thermally transferred sheet, the above-mentioned other monomers may be enumerated by acrylic or methacrylic acid esters, such as phenoxy polyethylene glycol methacrylate, phenoxyethylmethacrylate, phenoxyethoxyethylmethacrylate, methyl methacrylate, ethyl methacrylate, phenyl methacrylate, cyclohexyl methacrylate, isoboronyl methacrylate or amino ethyl methacrylate, vinyl monomers, such as styrene, chlorostyrene or vinyl phenol, and vinyl aromatic carboxylic acid esters, such as vinyl benzoate or vinyl chlorobenzoate. These may be used alone or in combination. The copolymer may be manufactured by any techniques, there being no limitation as to the sort of the manufacturing method. Specifically, the copolymer may be manufactured by a suspension polymerization method, block polymerization method, solution polymerization method or by an emulsion polymerization method. In particular, in the present thermally transferred sheet, the proportions of the ingredients of the copolymer made up of the compound of the chemical formula (1) and other monomers are prescribed so that the compound of the chemical formula (1) accounts for 5 to 25 wt%. If the proportion of the chemical formula (1) in the copolymer is less than 5 wt%, the amount of hydroxy groups reacted with isocyanates added as a hardener is not sufficient, with the result that the strength of the dye reception layer 2 by the hardener is lowered. In particular, if the thermally transferrable sheet has the aforementioned laminate layer 10, since the hydroxy groups in the chemical formula (1) are polar groups, there is produced a bond between the resin constituting the laminate layer 10 and the hydroxy group. If, in such case, the proportion of the compound of the chemical formula (1) in the copolymer of the dye reception layer 2 is less than 5 wt%, the hydroxy groups for forming the bond with the resin of the laminate layer 10 fall in shortage. Thus, if the proportion of the compound of the chemical formula (1) in the copolymer of the dye reception layer 2 is less than 5 wt%, it becomes impossible to transfer the laminate layer 10 positively to the dye reception layer 2. Also, if the proportion of the chemical formula (1) exceeds 25 wt%, the glass transition temperature of the copolymer is too low, such that the dye reception layer 2 is softened excessively to lower the running performance at elevated temperatures, thus producing the blocking. Moreover, the dye reception layer 2 is worsened in surface luster, thus tending to detract from the quality of the produced image. The weight average molecular weight of the copolymer is 100,000 to 1,000,000. If the weight average molecular weight of the copolymer is less than 100,000, the dye reception layer 2 tends to become brittle to worsen the film forming characteristics at the time of formation of the dye reception layer 2. On the other hand, if the weight average molecular weight of the copolymer exceeds 1,000,000, the paint tends to be increased in viscosity to present coating difficulties in applying the paint containing the copolymer to form the dye reception layer 2. On the other hand, inorganic pigments, such as titanum oxide, calcium carbonate or zinc oxide, or fluorescent whitening agents, may be added to the dye reception layer 2 to improve the whiteness. Mold release agents may also be added to the dye reception layer 2. Examples of the mold release agents include silicone oils, such as methyl styrene modified silicone oil, olefin modified silicone oil, polyether modified silicone oil, fluorine modified silicone oil, epoxy modified silicone oil, carboxy modified silicone oil, amino modified silicone oil or carbinol modified silicone oil, and fluorine-based mold release agents. To the dye reception layer 2 is added a hardener for improving its film characteristics. The hardeners are epoxy based hardeners, or isocyanate-based hardeners, and in particular by a non-yellow-becoming type polyfunctional isocyanate compounds. These polyfunctional isocyanate compounds may be enumerated by, for example, hexamethylene diisocyanate (HDI), xylene diisocyanate (XDI), toluene diisocyanate (TDI) and biuret polyisocyanate. These hardeners may be used alone or in combination. For the dye reception layer 2, an anti-static agent is preferably used to prevent static charges from being produced when the sheet is run in a printer apparatus. The anti-static agent may be enumerated by, for example, cationic surfactants, such as quaternary ammonium salts or polyamine derivatives, anionic surfactants, such as alkylbenzene sulfonate or alkyl sulfate ester sodium salts, amphoteric ion surfactants, and nonionic surfactants. These anti-static agents may be added to the dye reception layer 2 or coated on the surface thereof The dye reception layer 2 may also be added to with plasticizers as necessary. The plasticizers may be enumerated by phthalic acid esters, adipic acid esters, trimellitic acid esters, pyromellitic acid esters and polyphenol esters. The dye reception layer 2 may also be added to with ultraviolet light absorbers or anti-oxidants to improve shell life. The ultraviolet light absorbers may be enumerated by benzophenone-based, diphenyl acrylate based or benzotriazole-based ultraviolet light absorbers, whilst the anti-oxidants may be enumerated by phenol-based, organic sulfur based, phosphite-based or phosphoric acid based agents. In the thermally transferred sheet, constructed as described above, since the dye reception layer 2 is formed to have desired hardness, it is possible to prevent the dye reception layer 2 from being fused to the ink layer 9 to assure excellent picture quality. Also, in the present thermally transferred sheet, since the dye reception layer 2 has desired hardness, there is no risk of occurrence of so-called blocking, in which the dye reception layer 2 becomes attached to the back surface of the neighboring substrate 1, even if plural sheets are stored in a stacked state under high temperature conditions. Thus, the present thermally transferred sheet exhibits superior running performance even under elevated temperature conditions. Moreover, the present thermally transferred sheet exhibits superior film characteristics if it is added to with a hardener, such as isocyanates. That is, in the thermally transferred sheet, isocyanate groups are reacted efficiently with hydroxy groups of the chemical formula (1), even if the thermally transferred sheet is added to with a hardener, such that the operation of the hardener, such as isocyanates, occurs reliably. Also, the present thermally transferred sheet is preferably added to with a mold release agent, such as silicone oil. With the present thermally transferred sheet, added to with the mold release agent, such as silicone oil, isocyanate groups are reacted efficiently with the hydroxy groups of the chemical formula (1) to suppress the reaction of the isocyanate group with the silicone oil. If the polymer contained in the dye reception layer 2 is not provided with the compound shown by the chemical formula (1), isocyanates are reacted with the hydroxy groups in the silicone oil to bleed on the surface of the dye reception layer 2 along with the silicone oil. If the isocyanates are bled on the surface of the dye reception layer 2, transfer characteristics of the laminate layer 10 are lowered. Conversely, with the above-described thermally transferred sheet, the hardener, such as isocyanates, is reacted efficiently with the compound of chemical formula (1), to suppress bleeding of the hardener to permit satisfactory transfer of the laminate layer 10. Thus, the present thermally transferred sheet is able to demonstrate an image of high quality and high resolution. Examples The present invention is now explained with reference to illustrative Examples of the thermally transferred sheet according to the present invention in comparison with Comparative Examples. Examples 1 to 31 In the Examples 1 to 31, synthetic paper sheets, 150 µm thickness, manufactured by OJI YUKA CO. LTD., under the trade name of YUPO FPG-150, were used as the sheet-like substrates. Also, the paint for forming the dye reception layer was fabricated from the components shown in Table 1: paint for forming a reception layer parts by weight resin components of the reception layer (copolymers of Tables 2 to 5) 100.0 silicone oil 5 isocyanate compounds 10 As the copolymer component in the paint for the dye reception layer, pre-set amounts of "hydroxy ethyl acrylate" and/or "hydroxy ethyl methacrylate" shown by the above chemical formula were contained in the paint composition, as shown in Tables 2 and 3. On the other hand, the paint for the dye reception layer was prepared by dissolving the sum total of the solid ingredients of Table 1 so as to be a 20% solution in a 1/1 weight ratio mixed solvent of methyl ethyl ketone/toluene, by stirring the solution in a dissolver and by passing the solution through a filter 50 µm in diameter. The paint for the dye reception layer, thus obtained, was coated on the surface of a sheet-like substrate, using a coil bar, so that the dry coating film will be 5 to 6 µm in thickness. After drying at 120°C for two minutes, the coated substrate was cured at 50°C for 48 hours to give a thermally transferred sheet of Examples 1 to 31. Comparative Examples 1 to 5 In the Comparative Examples 1 to 5, thermally transferred sheets were prepared in the same way as in Example 1 except mixing copolymer components as shown in Table 4: Evaluation of characteristics In the Examples 1 to 31 and the Comparative Examples 1 to 5, thus prepared, transfer characteristics of the laminate film, resistance to blocking, running performance under high temperature conditions and surface luster were evaluated as follows: <transfer characteristics of a laminate film> For the prepared thermally transferred sheet, a thermal transfer printer, manufactured by SONY CORPORATION under the trade name of UP-D8800, and an ink ribbon composed of dyes of yellow (Y), magenta (M) and cyan (C), and a laminate film (L), manufactured by SONY CORPORATION under the trade name of UPC-8840, were used. The laminate film L was cut and bonded on the yellow (Y) dye and 20-gradation printing was carried out with yellow signals. Measurements were then made of the transfer start gradation of the laminate film L to the thermally transferred sheet. Evaluation was made, depending on the values of the transfer gradation, as follows: o ○: transfer gradation ≤ ten gradations ○: ten gradations < transfer gradation ≤ 14 gradations Δ: 14 gradations < transfer gradation ≤ 18 gradations × : 18 gradations < transfer gradation <resistance against blocking> In the course of preparation of a thermally transferred sheet, the paint for the dye reception layer was coated on the substrate surface. After drying at 120°C for two minutes, the coated substrate was sliced to a size of 5 cm by 5 cm, and two sliced pieces of the thermally transferred sheet were stacked one on another so that the dye reception layer of one of the pieces of the thermally transferred sheet will be contacted with the back surface of the other piece of the thermally transferred sheet. A weight 5 kg, with its bottom surface measuring 5 cm by 5 cm, was placed on the stacked pieces of the thermally transferred sheet and the resulting test system was allowed to stand at 50°C for 48 hours. The paired stacked pieces of the thermally transferred sheet were then separated from each other on peeling and changes in the shape of the pieces of the thennally transferred sheet from the initial state were visually checked. The resistance against blocking was evaluated, depending on the degree of shape changes, as follows: A: no changes in shape from the initial state B: partial changes in shape C: total shape changes <running performance under high temperature conditions> For respective thermally transferred sheets, a thermal transfer printer, manufactured by SONY CORPORATION under the trade name of UP-D8800, and an ink ribbon composed of dyes of yellow (Y), magenta (M) and cyan (C) and a laminate film (L), manufactured by SONY CORPORATION under the trade name of UPC-8840, were used. Under a condition of 50°C and 50%, all black continuous printing was performed. At this time, surface conditions after the end of running, the peeling sound during running and the running performance of the thermally transferred sheet were visually checked. The running performance under high temperature conditions was evaluated, depending on the degree of the running performance, as follows: o ○: no sticking between the thermally transferred sheet and the ink ribbon, no running sound nor running troubles ○: slight running sound, but no sticking of the thermally transferred sheet and the ink ribbon nor running troubles Δ: considerable running sound but no sticking of the thermally transferred sheet and the ink ribbon nor running troubles ×: sticking of the thermally transferred sheet and the ink ribbon and running troubles <surface luster> The surface luster of the thermally transferred sheet was evaluated by visually observing the luster of the thermally transferred sheet surface. The surface luster was evaluated, depending on its extent, as follows: ○: good luster and high quality Δ: luster is slightly inferior, but not so significant as to detract from picture quality ×: no luster; loss of luster is so significant as to detract from picture quality The results of evaluation of the transfer characteristics, resistance against blocking and the running performance under high temperature conditions of these laminate films are shown in Table 5. Meanwhile, in Table 5, the transfer characteristics of the laminate film, resistance against blocking, running performance under the high temperature conditions and surface luster are stated as "evaluation 1", "evaluation 2", "evaluation 3", "evaluation 4", respectively. It is seen from this Table 5 that, in the case of Examples 1 to 31, containing 5 to 25 wt% of hydroxy ethyl acrylate and/or hydroxy ethyl methacrylate, shown in the above chemical formula 1, superior effects are displayed in transfer characteristics of the laminate film, resistance against blocking, running performance under elevated temperatures, and surface luster. That is, if, in the copolymer contained in the dye reception layer, the proportions of hydroxy ethyl acrylate and/or hydroxy ethyl methacrylate shown in the chemical formula 1 are prescribed to be within pre-set ranges, the laminate film can be positively transferred to the thermally transferred sheet for positively suppressing the blocking. Moreover, positive running may be assured under high temperature conditions, whilst superior surface luster is displayed.

Claims: A thermally transferred sheet made up of a substrate and a dye reception layer formed thereon, wherein said dye reception layer contains a copolymer having a weight average molecular weight of 100,000 to 1,000,000, said copolymer being obtained by polymerization of a compound haying the following formula (1) wherein R is H or CH 3 , with another monomer, wherein in said copolymer the proportion of said compound of formula (1) is 5 to 25 wt.-% and said copolymer is crosslinked by a hardener added to said dye reception layer and wherein the hardener is an epoxy-based hardener or an isocyanate-based hardener. The thermally transferred sheet according to claim 1, wherein said another monomer is a vinyl monomer, such as a (meth)acrylic ester or a vinyl aromatic carboxylic acid ester. The thermally transferred sheet according to claim 1, wherein said another monomer is at least one monomer selected from the group consisting of phenoxypolyethyleneglycolinethacrylate, phenoxyethylmethacrylate, phenoxyethoxyethylmethacrylate, methylmethacrylate, ethylmethacrylate, phenylmethacrylate, cyclohexylmethacrylate, isobornylmethacrylate, styrene, chlorostyrene, vinylphenol, vinylbenzoate and vinylchlorobenzoate. The thermally transferred sheet according to claim 1, wherein the isocyanate-based hardener is a polyfunctional isocyanate compound selected from the group consisting of heacamethylenediisocyanate, xylenediisocyanate, toluenediisocyanate and biuret polyisocyanate.

Inventors: HORII AKIHIRO, ISAGO MASAYOSHI, KOZUMI TETSUO, MIZOGUCHI MASATAKA, OKADA HIROKAZU, HORII, AKIHIRO, ISAGO, MASAYOSHI, KOZUMI, TETSUO, MIZOGUCHI, MASATAKA, OKADA, HIROKAZU, Horii, Akihiro, c/o Sony Corporation, Isago, Masayoshi, c/o Sony Corporation, Kozumi, Tetsuo, c/o Sony Corporation, Mizoguchi, Masataka, c/o Fujikura Kasei Co., Ltd., Okada, Hirokazu, c/o Fujikura Kasei Co., Ltd.
Applicants: FUJIKURA KASEI KK, SONY CORP, FUJIKURA KASEI CO., LTD., SONY CORPORATION
Application year: 2000
Main classification: B41M5
IPC codes: B41M5