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 a colorant-receptive layer, the substrate comprising a plastic film having microvoids, the fractal dimension of the microvoids being not less than 1.45. There is yet 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. Another object of the present invention is to provide a thermal transfer image-receiving sheet having a white opaque layer, which is excellent in adhesion between the substrate and the white opaque layer and has excellent appearance. The present inventors have found that the use of a substrate as defined in Claim 1 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 and a colorant-receptive layer, said substrate comprising a plastic film having microvoids, the fractal dimension of said microvoids being not less than 1.45; as defined below. 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 first aspect of 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 microvoids of particular fractal dimension> The thermal transfer image-receiving sheet according to the present invention comprises a substrate and a colorant-receptive layer, said substrate comprising a plastic film having microvoids, the fractal dimension of said microvoids being not less than 1.45. Substrate The substrate comprises a plastic film having microvoids and an optional layer described below. The plastic film may be prepared by the following two methods. In the first method, a resin is mixed with inorganic fine particles and the resulting mixture (compound) is extruded into a film, whereupon a suitable biaxial stretching is conducted on the film. In this stretching, the inorganic fine particles serve as a nucleus to form voids in the film. Examples of the resin used include various polyolefin resins, such as polypropylene, and polyester resins. Among the polyester resins, polyethylene terephthalate is particularly preferred. Examples of the inorganic fine particles to be mixed with the above resin include titanium oxide, calcium carbonate, barium carbonate, barium sulfate, zinc oxide, and other known white pigments. The amount of inorganic fine particles may be 1 to 10 parts by weight based on 100 parts by weight of the resin. In the second method for preparing the plastic film having microvoids, a resin as a main component is mixed with a polymer immiscible with the resin, and the resulting mixture is extruded into a film, whereupon a suitable biaxial stretching is conducted on the film. The microscopic observation of the mixture reveals that the resin and the polymer together constitute a fine islands-sea structure. The formation of a film from the mixture followed by stretching of the film causes cleavage at the interface of the islands-sea structure or large deformation of the resin constituting the islands, resulting in the formation of microvoids. The resin as a main component for constituting the plastic film may be the above resin, that is, a polyolefin or a polyester. Examples of the polymer immiscible with the resin include rubbers such as polyisoprene, acrylic resins such as polymethyl methacrylate, and resins such as polymethylpentene and polystyrene. The amount of the polymer used may be 2 to 10 parts by weight based on 100 parts by weight of the above resin. It is particularly preferred to use as the main resin polypropylene in combination with polymethyl methacrylate, polystyrene, polyisoprene, or a mixture thereof as the immiscible polymer. Polymethyl methacrylate is particularly preferred as the immiscible polymer. In the present invention, it is important that the microvoids in the plastic film formed by the above methods have a fractal dimension of 1.45 or more. The significance of this particular parameter will now be described. For the sensitivity in printing of a dye sublimation transfer image-receiving sheet, the heat insulating property of the substrate is particularly important. The present inventors have found that a main factor governing the heat insulating property of the substrate is not the percentage void or density of the substrate but the shape or morphology of voids. Specifically, when two substrates have the same percentage void and density but are different from each other in the morphology of voids, i.e., one of which has relatively uniform and large voids in a smaller number with the other having relatively ununiform smaller voids in a larger number, the latter provides higher sensitivity in printing and has better heat resistance. When the state or morphology of voids existing in the substrate is expressed in terms of the appearance of the voids in the section of the substrate, it can be said that more complicated shape or figure provides better results. The complexity of such a shape or figure of microvoids can be best defined in terms of "fractal index". The "fractal dimension" is known as an index for expressing the complexity of the shape and distribution of an object. There are many known definitions of the fractal dimension. The definition, which is the most common and adopted in the present invention, is as follows. The required minimum number of circles of r in radius entirely covering the microvoids in the section of a film is assumed to be N(r). In this case, when the size of r, i.e., the area of the circle S(r) 2 is varied, the N(r) value too is, of course, varied. This means that the object or shape in question is formed of circles, in a number of N(r). Therefore, the fractal dimension can be determined from the gradient of log-log plotting of the area S(r ) of the circle and N(r). That is, LogN(r) = a x LogS(r) + C(constant) D = 1 - a wherein D represents the fractal dimension. A substrate having high sensitivity in printing and excellent heat resistance can be obtained when the fractal dimension of microvoids in the plastic film is made 1.45 or more. When the fractal dimension is less than 1.45, the substrate is poor essentially in the sensitivity in printing. Regarding the upper limit of the fractal dimension, when the fractal dimension is 2.0 or more, the microvoids should theoretically cover the whole section, which is actually impossible. According to studies by the present inventors, the upper limit of the fractal dimension is about 1.85 from the practical point of view. The fractal dimension value in the above range can be attained by properly setting, depending upon the kind of the resin used, film forming conditions in the production of the plastic film, such as the degree of kneading of the compound and film stretching ratio. When the above two methods for forming the plastic film are compared, the latter is more suitable for providing fractal dimension ≧ 1.45. In the latter method, the islands-sea structure in the mixture can be made very fine simply by an adequate kneading of the resins, whereby relatively ununiform small microvoids having a complicated shape can be obtained more easily. In the present invention, the above plastic film having microvoids whose fractal dimension is 1.45 or more is essentially used as the substrate. If desired, a plastic layer not having any microvoid and/or a plastic layer having microvoids whose fractal dimension is less than 1.45 may be laminated onto the above plastic film. This additional layer can be provided, for example, by coextruding the material for forming this layer at the time of formation of the plastic film. The material for the additional layer can be the same as that for the layer having microvoids with a fractal dimension of 1.45 or more. For example, a plastic film having a multilayer structure comprising a layer of a resin, such as polypropylene, as a core layer and, formed on the both sides thereof, layers of the plastic film having microvoids whose fractal dimension is 1.45 or more, may be used as the substrate. As such a plastic film having a multilayer structure, a commercially available synthetic paper can be employed. Further, it is also possible to use as the substrate a laminate comprising as a core layer the plastic film having microvoids, whose fractal dimension is 1.45 or more, and, laminated on the both sides of the core layer, opaque layers containing an inorganic pigment. These opaque layers can be formed by co-extrusion with the core layer. Further, depending upon applications, a layer not having any microvoids may be provided on the layer having microvoids of the above synthetic paper or plastic film having a multilayer structure to form a laminate having a five-layer structure so as to obtain high gloss and surface smoothness. The thickness of the layer not having any microvoid is preferably 1 to 10 µm. A thickness of less than 1 µm is insufficient for imparting the gloss and smoothness. On the other hand, when the thickness exceeds 10 µm, the sensitivity in printing is lowered. Furthermore, it is also possible to use as the substrate a laminate comprising the plastic film having microvoids whose fractal dimension is 1.45 or more and, laminated thereon, paper, a plastic film, or the like. In this case, the lamination is preferably conducted so as to provide a symmetric structure, i.e., by laminating plastic films having microvoids whose fractal dimension is 1.45 or more onto the both sides of paper or PET as a core layer. 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 on 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 on both sides of the core. Furthermore, a lubricious back surface layer may also be provided on the side of the image-receiving sheet remote from the colorant-receptive layer, according to an image-receiving sheet carrying system of a printer used. The back surface layer is preferably provided by coating a dispersion of an inorganic or organic filler in a resin at a coverage of 0.3 to 3 g/m 2 . The resin to be used for the lubricious layer may be any known resin. A lubricant, such as silicone, or a release agent may be added to the back surface layer. 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 B1 Compound 1 having the following composition was extruded, and the extrudate was biaxially stretched to prepare a 60 µm-thick film having microvoids. [Compound 1] Polypropylene 100 parts Polymethyl methacrylate 8 parts This film had a percentage void of 20.9% and a fractal dimension D of 1.63. This film was laminated on both sides of white PET (W-400, manufactured by Diafoil Co., Ltd.) to prepare a substrate. The substrate on its one surface was coated with a coating solution, for a colorant-receptive layer, having the following composition by gravure reverse coating at a coverage of 4.0 g/m 2 , thereby preparing a thermal transfer image-receiving sheet. [Coating solution for colorant-receptive layer] Ethylene/vinyl acetate copolymer (#1000A, manufactured by Denki kagaku Kogyo K.K.) 7.2 parts Styrene/methyl methacrylate copolymer (#400, 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.) 2.0 parts Methyl ethyl ketone 39.0 parts Toluene 39.0 parts Example B2 Compound 2 having the following composition was extruded, and the extrudate was biaxially stretched to prepare a 60 µm-thick film having microvoids. [Compound 2] Polypropylene 100 parts Polymethyl methacrylate 7 parts This film had a percentage void of 18.9% and a fractal dimension D of 1.48. The 60 µm-thick film was laminated on the following coated paper on its side remote from the polyethylene layer, and a coating solution, for a white opaque layer, having the following composition was coated on the side of the 60 µm-thick film in the same manner as in Example B1, thereby preparing a thermal transfer image-receiving sheet. [Coated paper] New Top (basis weight: 104.9 g/m 2 , manufactured by New Oji Paper Co., Ltd.) with a 45 µm-thick polyethylene layer being formed on one side thereof by extrusion. [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 B3 Compound 3 having the following composition was extruded, and the extrudate was biaxially stretched to prepare a 60 µm-thick film having microvoids. [Compound 3] Polypropylene 100 parts Polymethyl methacrylate 5 parts This film had a percentage void of 13.6% and a fractal dimension D of 1.59. Thereafter, the procedure of Example B1 was repeated to prepare a thermal transfer image-receiving sheet. Comparative Example B1 Compound 4 having the following composition was extruded, and the extrudate was biaxially stretched to prepare a 60 µm-thick film having voids. [Compound 4] Polypropylene 100 parts Calcium carbonate 10 parts This film had a percentage void of 15.6% and a fractal dimension D of 1.40. Thereafter, the procedure of Example B1 was repeated to prepare a thermal transfer image-receiving sheet. Comparative Example B2 Compound 5 having the following composition was extruded, and the extrudate was biaxially stretched to prepare a 60 µm-thick film having voids. [Compound 5] Polypropylene 100 parts Titanium oxide 5 parts This film had a percentage void of 16.5% and a fractal dimension D of 1.41. Thereafter, the procedure of Example B1 was repeated to prepare a thermal transfer image-receiving sheet. A gradation test pattern was printed on the thermal transfer image-receiving sheets prepared in the above examples and comparative examples under conditions of an applied voltage of 15.7 V and a printing speed of 5.5 msec/line. In order to evaluate the sensitivity in printing, the print density in the 9th gradation among 14 gradations was determined by measuring the reflection density with a Macbeth densitometer. The print density was evaluated based on the optical density 1.0. The evaluation criteria are as follows. O: good with 4% or more improvement over the reference value Δ: somewhat improved over the reference value X: lower than the reference value. The heat resistance was evaluated by visual inspection of the surface appearance of the print (with respect to the presence of trace of a thermal head). The evaluation criteria are as follows. O: good Δ: somewhat poor, but still acceptable X: unacceptable The results are shown in the following table. Example No. Percentage void (%) Fractal dimension Print density Heat resistance Ex. B1 20.9 1.63 1.10 (O) Δ Ex. B2 18.9 1.48 1.23 (O) O Ex. B3 13.6 1.59 1.09 (O) O Comp. Ex. B1 15.6 1.40 1.00 (X) X Comp. Ex. B2 16.5 1.41 0.92 (X) X

Claims: A thermal transfer image-receiving sheet comprising a substrate and a colorant-receptive layer, said substrate comprising a plastic film having microvoids, the fractal dimension of said microvoids, as defined by the following equations, being not less than 1.45; LogN(r) = a x LogS(r) + C(constant) D = 1 -a wherein N(r) represents the required minimum number of circles entirely covering the microvoids in the section of the film, said circles having a radius r, S(r) represents the area of the circle of radius r, and D represents the fractal dimension. The thermal transfer image-receiving sheet according to claim 1, wherein said plastic film having microvoids is formed by extruding a mixture containing a main resin and a polymer immiscible with said main resin and biaxially stretching the resultant extrudate. The thermal transfer image-receiving sheet according to claim 2, wherein said main resin is polypropylene and said polymer immiscible with said main resin is polymethyl methacrylate.

Inventors: IMOTO KAZUNOBU, SHIRAI KOICHI, IMOTO, KAZUNOBU, SHIRAI, KOICHI, IMOTO, KAZUNOBU, C/O DAI NIPPON, SHIRAI, KOICHI, C/O DAI NIPPON
Applicants: DAINIPPON PRINTING CO LTD, DAI NIPPON PRINTING CO., LTD.
Application year: 1995
Main classification: B41M1
IPC codes: B41M5