Thermal transfer image-receiving sheet

Abstract: There is provided a thermal transfer image-receiving sheet comprising a substrate sheet and a colorant-receptive layer, the substrate sheet having microvoids and having been formed by extruding a compound comprising a polyester resin and a polyolefin resin and biaxially stretching the resultant extrudate, the number of microvoids in the section of the substrate sheet being 3.7 x 10 4 to 2.2 x 10 5 /mm 2 . There is further provided a thermal transfer image-receiving sheet comprising a substrate and, provided thereon in the following order, an adhesive layer composed mainly of a hydrophilic resin, a white opaque layer and a colorant-receptive layer.

Description: The present invention relates to a thermal transfer image-receiving sheet and more particularly to a thermal transfer image-receiving sheet for use in a thermal transfer recording system wherein a sublimable dye is used as a colorant. Various thermal transfer recording systems are known in the art, and one of them is a dye sublimation transfer recording system in which a sublimable.dye as a colorant is transferred from a thermal transfer sheet to an image-receiving sheet by means of a thermal head capable of generating heat in response to recording signals, thereby forming an image. In this recording system, since a dye is used as the colorant and the gradation of the density is possible, a very sharp image can be formed and, at the same time, the color reproduction and tone reproduction of half tone are excellent, making it possible to form an image having a quality comparable to that formed by the silver salt photography. By virtue of the above excellent performance and the development of various hardwares and softwares associated with multi-media, the dye sublimation transfer recording system has rapidly increased the market in a full-color hard copy system for computer graphics, static images through satellite communication, digital images represented by CD-ROM, and analog images such as video. Specific applications of the image-receiving sheet in the dye sublimation transfer recording system are various, and representative examples thereof include proof printing, output of an image, output of a design, such as CAD/CAM, output applications for various medical instruments for analysis, such as CT scan, output applications for measuring equipment, alternatives for instant photography, output of photograph of a face to identification (ID) cards, credit cards, and other cards, and applications in composite photographs and pictures for keepsake in amusement facilities, such as pleasure grounds, museums, aquariums, and the like. The thermal transfer image-receiving sheet for dye sublimation transfer used in the above various applications (hereinafter referred to simply as "thermal transfer image-receiving sheet" or "image-receiving sheet") generally comprises a substrate (referred to also as a "support") and a color-receptive layer formed thereon. What is first required of this image-receiving sheet is high sensitivity in printing and heat resistance. When the heat resistance is poor, heating at the time of printing causes curling or traces of a thermal head on the surface of the image-receiving sheet, deteriorating the image quality. Regarding the sensitivity in printing, an increase in a dye sublimation transfer recording speed in recent years has led to a strong demand for an image-receiving sheet having high sensitivity in printing. The properties of the color-receptive layer are, of course, important to the sensitivity of the image-receiving sheet in printing. In addition, the properties of the substrate are also very important. Various substrates have hitherto been proposed for the purpose of improving the sensitivity in printing and the heat resistance of the image-receiving sheet. For example, Japanese Patent Laid-Open No. 136783/1989 teaches that a substrate which uses, as part or entirety thereof, a film having in its interior microvoids, prepared by extruding and biaxially stretching a resin composition comprising a mixture of polyethylene terephthalate with an inorganic pigment and an olefin, and which has a particular degree of cushioning, possesses high sensitivity in printing and thus can provide a sharp image. Japanese Patent Laid-Open No. 168493/1989 teaches that good results can be obtained when a substrate prepared in the same manner as the substrate described in Japanese Patent Laid-Open No. 136783/1989 has in its interior closed cells and a particular specific gravity. Japanese Patent Laid-Open No. 207694/1991 specifies the density of the substrate. Japanese Patent Laid-Open Nos. 16539/1993 and 169865/1993 describe substrates having a particular percentage void, and Japanese Patent Laid-Open No. 246153/1993 describes a substrate comprising a particular material and having particular density and voids. Further, Japanese Patent Laid-Open Nos. 115687/1989, 263691/1990, and 290790/1988 disclose substrates wherein the sensitivity in printing is improved by improving the cushioning and insulating properties. According to the studies by the present inventors, however, all the above substrates are still unsatisfactory in at least one of the sensitivity in printing and the heat resistance. Regarding properties required of the thermal transfer image-receiving sheet, in addition to the above described high sensitivity in printing and heat resistance, there is also an ever-increasing demand in the market in recent years for sufficient whiteness, opacity, and uniform appearance (uniform surface independently of whether the surface is glossy or matte), according to intended uses of image-receiving sheets. Further, with a recent increase in recording speed (line speed) in the dye sublimation transfer system, the temperature of the thermal head of a printer is becoming higher. With an increase in the temperature of the thermal head, delamination between the substrate of the thermal transfer image-receiving sheet and the layers overlying the substrate is more likely to occur. Especially in the case of an image-receiving sheet provided with a white opaque layer between the substrate and the colorant-receptive layer, since a white inorganic pigment is present in the white opaque layer, the adhesion between the substrate and the white opaque layer is likely to be poor, which is likely to cause delamination between the substrate and the white opaque layer during printing, making it impossible to provide a high-quality image. Further, the delamination gives rise to carrying error in a printer. Various attempts have been made to enhance the adhesion between the substrate of the image-receiving sheet and a layer overlying the substrate. For example, Japanese Patent Laid-Open No. 211089/1991 teaches a surface modification of a polyester film as a substrate by a corona or plasma treatment. However, the adhesive property imparted by the corona or plasma treatment is unstable and it decreases with the elapse of time. Furthermore, Japanese Patent Laid-Open No. 211089/1991 describes an alternative method wherein a resin, such as an acrylic resin, having good adhesion both to the colorant-receptive layer and to the substrate is applied. However, the use as an adhesive layer of such resins as an acrylic resin, which are soluble in organic solvents, has the following problem. When a coating solution for a colorant-receptive layer, in which an organic solvent is generally used, is coated on the adhesive resin layer, the adhesive layer is attacked by the organic solvent contained in the coating solution, which remarkably deteriorates the appearance of the image-receiving sheet to lower the commercial value of the product. Accordingly, an object of the present invention is to provide a thermal transfer image-receiving sheet having high sensitivity in printing and heat resistance. The present inventors have found that the use of a substrate composed of a specific resin and having a specific number of microvoids can provide a thermal transfer image-receiving sheet having high sensitivity in printing and high heat resistance. Thus, according to the present invention, there is provided a thermal transfer image-receiving sheet comprising a substrate sheet and a colorant-receptive layer formed on the substrate sheet, said substrate sheet having microvoids and having been formed by extruding a compound comprising a polyester resin and a polyolefin resin and biaxially stretching the resultant extrudate, the number of microvoids in the section through said substrate sheet being 3.7 x 10 4 to 2.2 x 10 5 /mm 2 , wherein the number of microvoids is the average value of the number of microvoids in the section in the longitudinal direction and the number of microvoids in the section in the transverse direction of the substrate sheet, said microvoids having a major axis of 1 to 20 microns and a minor axis of 0.5 to 4 microns and a ratio of the minor axis to major being 0.01 to 0.50. The thermal transfer image-receiving sheet according to the present invention has high sensitivity in printing and, at the same time, excellent heat resistance. Therefore, these image-receiving sheets effectively prevent the occurrence of curling due to heat upon printing, exhibit no traces of a thermal head on an image face and can produce a high-density, high-quality image. Fig. 1 is a conceptual diagram showing the shape and distribution of microvoids contained in the substrate sheet of the thermal transfer image-receiving sheet according to the present invention; and Fig. 2 is a conceptual diagram, to be compared with Fig. 1, showing the state of microvoids in the case where the number of microvoids in the substrate sheet is outside the scope of the present invention (smaller than the number of microvoids specified in the present invention). <Image-receiving sheet having specific number of microvoids> The thermal transfer image-receiving sheet according to the present invention comprises a substrate sheet and a colorant-receptive layer, said substrate sheet having microvoids and having been formed by extruding a compound comprising a polyester resin and a polyolefin resin and biaxially stretching the resultant extrudate, the number of microvoids in the section of said substrate sheet being 3.7 x 10 4 to 2.2 x 10 5 / mm 2 . Substrate sheet Examples of the polyester resin to be used for the substrate sheet include polyethylene terephthalate and polybutylene terephthalate. Polyethylene terephthalate is most preferred. The polyester resin, by virtue of its excellent heat resistance, can prevent the occurrence of curling due to heat upon printing and the development of traces of a thermal head on an image face. The use of the polyester resin alone, however, causes lack of flexibility as the substrate sheet, and, for this reason, a polyolefin resin is added to the polyester resin to impart plasticity. Examples of the polyolefin resin usable for this purpose include polyethylene, polypropylene, ethylene/vinyl acetate copolymer, polymethylpentene, ethylene/acrylic acid copolymer, ethylene/acrylic ester copolymer, and α-alkyl olefin-modified olefin resins. Among them, polypropylene and polymethylpentene are preferred. The amount of the polyolefin resin used is preferably 5 to 30 parts by weight based on 100 parts by weight of the polyester resin from the viewpoint of a balance between the heat resistance and the flexibility of the substrate sheet. If necessary, other polymers including rubbers, such as polyisoprene, acrylic resins, such as polymethyl methacrylate, and polystyrene resin may be used in an amount up to 10% by weight based on the total amount of the polyester and the polyolefin. The substrate sheet may, if necessary, contain inorganic fine particles as a filler and additives such as a brightening agent. The inorganic fine particles used as a filler include white pigments or extender pigments commonly used in the art, such as titanium oxide, calcium carbonate, talc, aluminum hydroxide, and silica. The addition of these fine particles can impart opacity and whiteness to the resulting image-receiving sheet. The amount of these fine particles added is preferably 1.5 to 4.0 parts by weight based on 100 parts by weight of the above resins. The substrate sheet has microvoids in the particular number specified above. The microvoids can be formed by conducting proper biaxial stretching in the preparation of the substrate sheet by mixing the above polyester and polyolefin resins and optionally the above polymer, filler or additives, a surfactant, a foaming agent, etc., extruding the resulting compound through a die to form into a sheet. The mechanism by which the microvoids are formed is as follows. When the above compound contains as a filler the above inorganic fine particles, the inorganic fine particles, during biaxial stretching, serve as a nucleus to form microvoids. Even when the compound does not contain inorganic fine particles, the microvoids are formed through another mechanism. Thus, in the mixture of a polyester with a polyolefin, the polyester and the polyolefin are compatible with each other but not miscible with each other. That is, the mixture has an islands(polyolefin)-sea(polyester) structure as viewed microscopically. Stretching of the mixture having an islands-sea structure causes cleavage at the interface of sea and islands or deformation of the polyolefin constituting the islands, thereby forming microvoids. When the compound contains inorganic fine particles, the microvoids are formed through the above two mechanisms with the contribution of the latter mechanism to the formation of microvoids being larger. In the present invention, stretching conditions, such as stretch ratio, are set so that the number of microvoids observed in the section of the substrate sheet is 3.7 x 10 4 to 2.2 x 10 5 /mm 2 . The above number of microvoids is the average value of the number of microvoids in the section in the longitudinal direction and the number of microvoids in the section in the transverse direction of the substrate sheet. By bringing the number of microvoids to 3.7 x 10 4 /mm 2 or more, the cushioning property and the heat-insulating property of the substrate sheet can be improved and, at the same time, the sensitivity of the image-receiving sheet in printing can be improved. However, when the number of microvoids exceeds 2.2 x 10 5 /mm 2 , the percentage void of the whole sheet is increased, raising problems of deterioration in heat resistance, heat curling, and traces of a thermal head of the substrate sheet. This results in lowered overall performance and commercial value of the image-receiving sheet. Fig. 1 is a conceptual diagram showing the shape and distribution of microvoids in the substrate sheet, having microvoids the number of which is in the above specified range, according to the present invention, and Fig. 2 is a conceptual diagram showing the shape and distribution of microvoids in a substrate sheet, as prepared in comparative examples described below, having microvoids the number of which is smaller than the lower limit of the above specified range. As is apparent from the both drawings, the microvoids shown in Fig. 2 are flatter than those shown in Fig. 1. Further, it is apparent that, for the size of individual microvoids, the microvoids shown in Fig. 1 are, on the average, smaller than those shown in Fig. 2. For the microvoids, as shown in Fig. 1, falling within the particular range specified above in terms of the number of microvoids, the major axis is 1 to 20 µm, and the minor axis is 0.5 to 4 µm with the minor axis to major axis ratio being 0.01 to 0.50. Colorant-receptive layer The resin usable for the colorant-receptive layer may be any resin conventionally used for dye sublimation thermal transfer image-receiving sheets. Specific examples of the resin include polyolefin resins, such as polypropylene; halogenated resins, such as polyvinyl chloride and polyvinylidene chloride; vinyl resins, such as polyvinyl acetate and polyacrylic ester, and copolymers thereof; polyester resins, such as polyethylene terephthalate and polybutylene terephthalate; polystyrene resins; polyamide resins; copolymers of olefins, such as ethylene or propylene, with other vinyl monomers; ionomers; and cellulose derivatives. These resins may be used alone or as a mixture of two or more. Of these resins, polyester resins and vinyl resins are preferred. The colorant-receptive layer may contain a release agent for the purpose of preventing heat fusing between the colorant-receptive layer and a thermal transfer sheet during the formation of an image. Silicone oil, phosphate plasticizers, and fluorine compounds may be used as the release agent. Among them, silicone oil is preferred. The amount of the release agent added is preferably 0.2 to 30 parts by weight based on the resin for forming the receptive layer. The colorant-receptive layer may be coated on the substrate sheet by conventional methods, such as roll coating, bar coating, gravure coating, and gravure reverse coating. The coverage thereof is preferably 0.5 to 10 g/m 2 (on a solid basis). Additional layer The thermal transfer image-receiving sheet of the present invention may consist of the above substrate sheet and the above colorant-receptive layer alone. If necessary, however, additional layers may be provided. For example, in order to impart high whiteness and opacity to the image-receiving sheet, a white opaque layer may be provided between the substrate sheet and the colorant-receptive layer. The white opaque layer may comprise a mixture of a known white inorganic pigment, such as titanium oxide or calcium carbonate, with a binder. The binder may be one of or a blend of known resins such as polyurethane, polyester, polyolefin, modified polyolefin, and acrylic resins. Further, in order to improve the resistance of the image-receiving sheet to curling associated with printing or curling associated with environment, various plastic films or various types of paper may be laminated on the image-receiving sheet. More specifically, coated paper, art paper, wood-free paper, glassine paper, resin EC paper, a polyester, polypropylene, or the like may be laminated onto the substrate sheet on its side remote from the receptive layer. Further, if necessary, the substrate may have a sandwich structure comprising a core formed of one of the above various types of paper or plastic films and substrate sheets laminated onto the both sides of the core. The following examples further illustrate the present invention but are not intended to limit it. In the following examples, "parts" are by weight, and the coverage of the colorant-receptive layer is on a dry basis. Example A1 Compound 1 having the following composition was extruded, and the extrudate was biaxially stretched to prepare a 125 µm-thick substrate sheet. The number of microvoids in the section of the substrate sheet was 7.84 x 10 4 /mm 2 . [Compound 1] Polyester (FR-PET, manufactured by Teijin Chemicals Ltd.) 100 parts Polymethylpentene (TPX, manufactured by Mitsui Petrochemical Industries, Ltd.) 10 parts Titanium oxide (average particle diameter: 2 µm, anataze type) 2 parts The substrate sheet was coated with a coating solution, for a receptive layer, having the following composition by gravure reverse coating at a coverage of 4.0 g/m 2 to prepare a thermal transfer image-receiving sheet. [Coating solution for receptive layer] Vinyl chloride/vinyl acetate copolymer (#1000A, manufactured by Denki kagaku Kogyo K.K.) 7.2 parts Styrene/methyl methacrylate copolymer (#400A, manufactured by Denki kagaku Kogyo K.K.) 1.6 parts Polyester (Vylon 600®, manufactured by Toyobo Co., Ltd.) 11.2 parts Vinyl-modified silicone (X-62-1212, manufactured by Shin-Etsu Chemical Co., Ltd.) 2 parts Methyl ethyl ketone 39 parts Toluene 39 parts Example A2 Compound 2 having the following composition was extruded, and the extrudate was biaxially stretched to prepare a 125 µm-thick substrate sheet. The number of microvoids in the section of the substrate sheet was 5.91 x 10 4 /mm 2 . [Compound 2] Polyester (as used in Example A1) 100 parts Polypropylene (MA2, manufactured by Mitsubishi Petrochemical Co., Ltd.) 10 parts Calcium carbonate (average particle diameter: 3.5 µm) 2 parts The substrate sheet was coated with the same coating solution for a receptive layer as in Example A1 in the same manner as in Example A1, thereby preparing a thermal transfer image-receiving sheet. Example A3 Compound 1 as used in Example A1 was extruded, and the extrudate was biaxially stretched to prepare a 75 µm-thick sheet. This sheet was laminated onto the both sides of OK Coat (basis weight: 72.3 g/m 2 , manufactured by New Oji Paper Co., Ltd.). The resultant laminate on its one surface was coated with a coating solution, for a white opaque layer, having the following composition, thereby forming a white opaque layer which was then coated with the same coating solution, for a receptive layer, as used in Example A1, thereby preparing an image-receiving sheet. [Coating solution for white opaque layer] Binder (N-2303, manufactured by Nippon Polyurethane Industry Co., Ltd.) 10 parts White pigment (TiO 2 , average particle diameter 0.5 µm) 15 parts Organic solvent 60 parts Example A4 Compound 3 having the following composition was extruded, and the extrudate was biaxially stretched to prepare a 35 µm-thick substrate sheet. The number of microvoids in the section of the substrate sheet was 8.52 x 10 4 /mm 2 . [Compound 3] Polypropylene (as used in Example A2) 100 parts Polyethylene terephthalate (as used in Example A1) 10 parts Polyethylene (Mirason 16P, manufactured by Mitsui Nisseki Polymers Co., Ltd.) 2 parts The substrate sheet was laminated onto the both sides of OK Coat (basis weight: 157 g/m 2 , manufactured by New Oji Paper Co., Ltd.) by dry lamination. The laminate on its one side was coated with the coating solution for a receptive layer as used in Example A1 in the same manner as in Example A1, thereby preparing a thermal transfer image-receiving sheet. Example A5 Compound 4 having the following composition was extruded, and the extrudate was biaxially stretched to prepare a 35 µm-thick substrate sheet. The number of microvoids in the section of the substrate sheet was 6.72 x 10 4 /mm 2 . [Compound 4] Polypropylene (as used in Example A2) 100 parts Polyethylene terephthalate (as used in Example A1) 8 parts Polyisoprene (JSR-Butyl No. 268, manufactured by Japan Synthetic Rubber Co., Ltd.) 3 parts The substrate sheet was laminated onto the both sides of OK Coat (basis weight: 157 g/m 2 , manufactured by New Oji Paper Co., Ltd.) by dry lamination. The laminate on its one side was coated with the coating solution for a receptive layer as used in Example A1 in the same manner as in Example A1, thereby preparing a thermal transfer image-receiving sheet. Comparative Example A1 A 125 µm-thick substrate sheet was prepared using the compound as used in Example A1 in the same manner as in Example A1, except that the sheet forming temperature and the stretch ratio were lower than those used in Example A1. The number of microvoids in the section of the substrate sheet thus obtained was 3.4 x 10 4 /mm 2 . Thereafter, the substrate sheet was coated with the coating solution for a receptive layer as used in Example A1 in the same manner as in Example A1, thereby preparing a thermal transfer image-receiving sheet. Comparative Example A2 Crysper (thickness: 125 µm, manufactured by Toyobo Co., Ltd.), a polyester sheet not containing a polyolefin, on its one side was coated with the coating solution for a receptive layer as used in Example A1 by gravure reverse coating at a coverage of 3.5 g/m 2 , thereby preparing a thermal transfer image-receiving sheet. Comparative Example A3 Compound 5 having the following composition was extruded, and the extrudate was biaxially stretched to prepare a 125 µm-thick substrate sheet. The number of microvoids in the section of the substrate sheet was 3.0 x 10 4 /mm 2 . [Compound 5] Polyester (as used in Example A2) 100 parts Polypropylene (as used in Example A2) 32 parts Calcium carbonate (as used in Example A2) 2 parts The above substrate sheet was coated with the coating solution for a receptive layer as used in Example A2 in the same manner as in Example A2, thereby preparing a thermal transfer image-receiving sheet. Comparative Example A4 The procedure of Example A1 was repeated, except that stretching conditions, such as stretch ratio, were changed so that the number of microvoids of the substrate sheet formed was 3.1 x 10 4 / mm 2 . A test pattern was printed on the thermal transfer image-receiving sheets prepared in the above examples and comparative examples under the conditions of an applied voltage of 12 V and a printing speed of 16 msec/line, and the gloss, uniformity of print, sensitivity in printing, and curling as a measure of heat resistance were evaluated by the following methods. The results are given in Table A1. (Evaluation methods) Gloss, uniformity of print, and curling: They were evaluated by visual inspection. Sensitivity in printing: The reflection density was measured with a Macbeth densitometer, and the sensitivity in printing was evaluated based on the optical density 1.0 of the print in Example A1. The sensitivity in printing is a relative value of the density. In Table A1, the symbols denote the following. O : good Δ : somewhat poor, but no problem for practical use X : unacceptable The number of microvoids given in Table A1 is one determined by measuring the number of microvoids in the section of an image-receiving sheet under an electron microscope (SEM) and converting the measured value to a value per unit sectional area (mm 2 ) of the image-receiving sheet. Example No. Gloss Uniformity of print Curling Sensitivity in printing (evaluation) Number of microvoids (microvoids /mm 2 ) Ex. A1 ○ ○ ○ 1.00 (○) 7.84 x 10 4 Ex. A2 ○ ○ ○ 0.96 (○) 5.91 x 10 4 Ex. A3 ○ ○ ○ 0.98 (○) 7.84 x 10 4 Ex. A4 ○ Δ ○ 1.06 (○) 8.52 x 10 4 Ex. A5 ○ ○ ○ 0.98 (○) 6.72 x 10 4 Comp. Ex. A1 ○ ○ ○ 0.88 (Δ) 3.40 x 10 4 Comp. Ex. A2 ○ X ○ 0.66 (X) - Comp. Ex. A3 Δ ○ X 0.95 (○) 3.00 x 10 4 Comp. Ex. A4 Δ ○ X 0.84 (Δ) 3.10 x 10 4

Claims: A thermal transfer image-receiving sheet comprising a substrate sheet and a colorant-receptive layer formed on the substrate sheet, said substrate sheet having microvoids and having been formed by extruding a compound comprising a polyester resin and a polyolefin resin and biaxially stretching the resultant extrudate, the number of microvoids in the section through said substrate sheet being 3.7 x 10 4 to 2.2 x 10 5 /mm 2 , wherein the number of microvoids is the average value of the number of microvoids in the section in the longitudinal direction and the number of microvoids in the section in the transverse direction of the substrate sheet, said microvoids having a major axis of 1 to 20 microns and a minor axis of 0.5 to 4 microns and a ratio of the minor axis to major being 0.01 to 0.50. The thermal transfer image-receiving sheet according to claim 1, wherein said polyester resin is polyethylene terephthalate and said polyolefin resin is polypropylene or polymethylpentene. The thermal transfer image-receiving sheet according to claim 1 or 2, wherein said compound further comprises inorganic fine particles. The thermal transfer image-receiving sheet according to claim 1, 2 or 3, wherein said compound further comprises 10% by weight or less, based on the total amount of said polyester resin and said polyolefin resin, of a polymer selected from polyisoprene, polymethyl methacrylate, and polystyrene.

Inventors: IMOTO KAZUNOBU, SHIRAI KOICHI, IMOTO, KAZUNOBU, SHIRAI, KOICHI, Imoto, Kazunobu, Dai Nippon Printing Co., Ltd., Shirai, Koichi, Dai Nippon Printing Co., Ltd.
Applicants: DAINIPPON PRINTING CO LTD, DAI NIPPON PRINTING CO., LTD.
Application year: 1995
Main classification: B41M1
IPC codes: B41M5