DE3884877T2 - Dye receiving layer for thermal dye transfer printing. - Google Patents

Description

BACKGROUND OF THE INVENTION Field of the invention

The invention relates to an improvement for image-receiving sheets for thermal transfer printing adapted for printing images thereon by transferring a sublimable dye from a thermal transfer sheet by application of heat from a heating head according to image signals.

Description of the state of the art

Thermal printing systems, in which printed images are obtained by receiving input signals, consist of a relatively simple device and are inexpensive and low-noise, so that they are increasingly used in various fields, such as in picture telegraphy or teletext, terminal printers for electronic computers, printers for measuring instruments, video printers and similar devices.

The recording medium generally used in these thermal printing systems is a so-called heat-sensitive recording paper for color development, which comprises a recording layer that undergoes a physical or chemical change by application of heat to form color. However, the color development type recording paper has the disadvantage that it undesirably develops color during the manufacturing process or during storage. Furthermore, the image printed on the paper has poor storage stability and fades in contact with organic solvents or chemicals. To avoid this, To avoid this, a thermal transfer printing system has been proposed in which a recording medium using a dye or colorant is used instead of the color-developing heat-sensitive recording paper.

In this thermal transfer printing system, a dye or a colored material melts, evaporates or sublimates by application of heat and is transferred to the recording paper, and a recorded image is formed by adhesion, adsorption or reception of the dye on the recording paper. For example, a mechanism as shown in Figure 2 has been proposed. According to this mechanism, a thermal transfer sheet 201 having a dye layer on the substrate and a thermal transfer image-receiving sheet 203 mounted on a platen roller 202 are superposed and heated from the non-opposite side of the thermal transfer sheet 201 by means of a heater 204 such as a heating head. The heating head 204 is controlled by electric signals corresponding to the image information. As a result, the dye of the thermal transfer sheet is transferred to an image-receiving layer.

For full color printing, a thermal transfer sheet is used which is a sheet coated or printed as shown in Figure 3 and comprising a start mark 301 and dyes including yellow 302, magenta 303 and cyan 304 and optionally a black dye. This sheet is placed on the image receiving sheet in thermal transfer and the yellow, magenta and cyan dyes are successively applied according to the respective image signals whose color is separated to form an image in which the colors are superimposed, heated. This creates a photographic image with natural color.

A known thermal transfer image-receiving sheet used in this type of printing system is one comprising an image-receiving layer obtained by coating a paper substrate such as a high-quality paper with a layer of a dispersion of finely powdered silica or calcium carbonate in a binder such as a thermoplastic polyester resin.

However, when the image receiving layer is formed on the substrate such as woodfree paper whose smoothness is low, it is difficult to obtain a satisfactory printed image. In particular, transfer parts are missing in the halftone to low tone parts when an image having a low degree of gradation is transferred. Although images such as symbols, letters and numbers are substantially filled in and so the missing transfer parts are not conspicuous, the missing transfer parts will be undesirably noticeable in photographs or colored filled images, causing substantial defects. There is therefore a great demand for the elimination of such defects.

In response to this need, various improvements have been proposed, including, for example, (1) a method proposed in Japanese Patent Application Laid-Open No. 61-172795 in which an intermediate layer is formed below the image-receiving layer to improve the smoothness on the surface of the image-receiving layer, and (2) a method proposed in U.S. Patent No. 4,720,480, in which an intermediate layer consisting mainly of a flexible resin is formed.

However, these improvements are accompanied by other difficulties and therefore satisfactory results cannot always be obtained. For example, the method (1) has the problem that productivity is lowered since it includes the step of bringing the intermediate layer, which has been kept soft or plastic, into close contact with the mirror-like chrome-plated metal drum for heating and drying. The method (2) has the problem that a soft resin such as MBR, polyurethane, polybutadiene, SBR or a similar resin, which has been formulated to improve adhesion, easily sticks when winding is carried out after coating and drying. In addition, the resin dissolves in the organic solvent of the coating solution with which the image-receiving layer is formed, so that a uniform image-receiving layer is not formed, thereby causing deterioration of the optical density of the printed image.

SUMMARY OF THE INVENTION

The invention is based on the object of providing an improved image-receiving sheet for thermal transfer printing, in which the previously mentioned difficulties that occur with the known image-receiving sheets do not occur and with which beautifully printed images with high optical density can be formed without the known problems associated with a missing transfer part occurring.

It is an object of the present invention to provide an improved image-receiving sheet capable of forming images of high quality and good gradation and free from the known problems associated with missing transfer parts, even when it comprises various types of paper made mainly from pulp fibers as a base sheet.

The present inventors have made extensive studies to solve the above problem and, as a result, have found that when a base sheet comprising an intermediate layer consisting mainly of a resin insoluble in an organic solvent is first formed and then an image-receiving layer consisting mainly of a resin soluble in organic solvents is formed, the smoothness and anti-blocking properties are significantly improved. Thus, beautifully printed images are obtained without any part missing by transfer and printed images having excellent optical density are obtained.

The invention is briefly explained using the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic cross-section of an image-receiving sheet for thermal transfer printing according to the invention. 101 denotes the base sheet or substrate, 102 denotes an intermediate layer and 103 denotes the image-receiving layer;

Figure 2 is a schematic diagram of a printing system of the thermal transfer system. 201 is a thermal transfer sheet, 202 is a platen roller, 203 is a sheet for receiving the image by thermal transfer, and 204 is a heating print head;

Figure 3 is a schematic view of a full-color thermal transfer sheet, in which 301 represents the start mark, 302 represents a yellow dye, 303 represents a magenta dye, and 304 represents a cyan dye.

DETAILED DESCRIPTION OF THE INVENTION

The invention relates to an image-receiving sheet for thermal transfer printing, which is characterized by a substrate, an intermediate layer which is formed on the substrate and comprises a resin which is insoluble in an organic solvent, and an image-receiving layer which is formed on the intermediate layer and contains, as a main component, a resin which is soluble in an organic solvent.

Figure 1 is a schematic cross-sectional view showing an image-receiving sheet for thermal transfer printing according to the invention. In the figure, 101 denotes a substrate, 102 an intermediate layer and 103 an image-receiving layer.

These elements of the image-receiving sheet according to the invention are explained in more detail below.

intermediate layer 102;

First, the characteristic properties of the intermediate layer, which is one of the essential features of the invention, will be described in detail.

The intermediate layer of the image-receiving sheet of the present invention contains, as a main component, resins insoluble in organic solvents. It has been found that when a copolymer latex having a specific type of monomer composition and a certain gel content (content of toluene-insoluble component) is used as the resin insoluble in an organic solvent, the organic solvent resistance and blocking properties of the intermediate layer are greatly improved without affecting the smoothness of the image-receiving layer. This results in effective production of an image-receiving sheet which gives printed images which are free from missing transfer parts, have an optical density and are very beautiful.

The monomer composition and gel content of the copolymer latex are given below.

Monomer composition:

(A) Aliphatic conjugated diolefinic monomer 20 - 70 wt.%

(B) Unsaturated acid monomers 0.5 - 15 wt.%

(C) Other olefinic monomer 15 - 79.5 wt.%

Gel content: 85 - 100 wt.%

The aliphatic conjugated diolefinic monomer (A) is generally selected from those containing 4 to 10 carbon atoms. Specific examples include 1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, halogen-substituted butadienes, substituted linear conjugated pentadienes, linear or branched conjugated hexadienes and the like. The aliphatic conjugated diolefinic monomer should be present in an amount of 20 to 70 wt% based on the copolymer latex. If the content is less than 20 wt%, the resulting resin becomes hard and brittle and exhibits poor film-forming properties and poor adhesion strength. If it is over 70 wt%, the resulting resin becomes too soft even if the gel content is over 85 wt%, and sticking to the rollers during coating or blocking occurs.

The unsaturated acid monomer (B) includes, for example, acrylic acid, methacrylic acid, maleic acid, fumaric acid, itaconic acid, dicarboxylic acid anhydrides, dicarboxylic acid monoalkyl esters and the like. The unsaturated acid monomer is present in an amount of 0.5 to 15% by weight based on the copolymer latex. If the content is less than 0.5% by weight, the mechanical stability of the copolymer latex deteriorates, whereas if it is more than 15% by weight, the latex becomes undesirably too viscous.

The olefinic monomer (C) other than the aliphatic conjugated diolefinic monomer (A) and the unsaturated acidic monomer (B) includes, for example, aromatic vinyl monomers such as styrene, alpha-methylstyrene, dimethylstyrenevinyltoluene and the like, acrylate monomers such as methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, lauryl acrylate, 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate and the like, and methacrylate monomers such as methyl methacrylate, ethyl methacrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate and the like, unsaturated nitrile monomers such as Acrylonitrile, methacrylonitrile and similar and acrylamide, N-methylolacrylamide, glycidyl acrylate, glycidyl methacrylate and similar.

The gel content of the copolymer latex, which is an essential feature of the present invention, should be controlled in the range of 85 to 100 wt%. Although the gel content can be influenced depending on the monomer composition, the yield of emulsion polymerization, the polymerization temperature and the like, it should be controlled above 85 wt% by controlling the amount of a chain transfer agent such as carbon tetrachloride, dodecyl mercaptan, octyl mercaptan or the like or by using a suitable crosslinking agent such as methylenebisacrylamide, divinylbenzene, diallyl phthalate or the like. If the gel content is below 85 wt%, there is a risk that blocking will occur, making it impossible to wind the final product after coating.

It should be noted that the gel content as used herein is a value determined according to the following method. A latex is allowed to stand on a glass plate at room temperature (in air-dried state) for 24 hours. Thereafter, it is dried in vacuum for 2 to 3 hours to obtain a latex film about 1 mm thick. About 0.5 g of the film thus obtained is accurately weighed and placed in 50 ml of toluene at room temperature for 24 hours. Thereafter, the toluene solution is passed through a 200 mesh stainless steel sieve to separate the insoluble matter. Then, it is dried at 135°C for 3 hours and the weight is determined. Then, the ratio (wt%) to the weight is calculated. calculated as gel content before immersion in the toluene solution.

In carrying out the present invention, the above specific type of copolymer latex should preferably be contained in the intermediate layer in an amount of not less than 60% by weight, more preferably not less than 70% by weight, based on the total solid of the intermediate layer, in order to obtain the desired high optical density for the images upon printing.

The intermediate layer may further contain various additives including pigments such as silica, alumina, clay, calcium carbonate and plastic pigments, lubricants, fluorescent dyes and other adhesives in amounts that do not affect the effects of the resin which is insoluble in an organic solvent.

When large amounts of fine particles of inorganic pigments are added to the intermediate layer, the heat insulating properties deteriorate and the damping properties of the intermediate layer lower, whereby the optical density becomes lower as the number of missing transfer parts increases. To avoid this, a larger amount of fine particles of organic pigments having higher heat insulating properties than the inorganic pigments are used. However, since the image receiving layer is generally formed using organic solvents, such organic pigments should have high resistance to organic solvents.

Solvent-resistant organic pigments in fine particle form include, for example, those of polyolefin resins, phenolic resins, urea resins, melamine resins, Allyl resins, epoxy resins, polyimide resins, benzoguanamide resins and the like. However, the resins except the polyolefin resins are all thermosetting resins and thus lower the cushioning properties of the intermediate layer. Therefore, studies were made with fine particles of various polyolefin resins. As a result, it was found that certain types of polyolefin resins in fine particle form significantly improve the anti-blocking properties and eliminate the known problems related to the occurrence of missing transfer parts. However, they lower the optical density for the printed images. Further studies have shown that the above results are greatly influenced by the thermal properties of the fine particles of polyolefin resins. That is, it was found that when fine particles of polyolefin resins having certain ranges for the melting point and heat absorption themselves are used together with conventional latexes as an adhesive, good anti-blocking properties are obtained with good smoothness of the image receiving layer. This results in the formation of an image-receiving sheet which is free from the difficulty associated with missing transfer parts and which enables a high optical density of the images to be printed.

The fine particles of a certain type of polyolefin resin should preferably have a melting point of not lower than 70°C, more preferably not lower than 100°C and a heat absorption accompanied by softening and phase transfer at the time of heating of not higher than 50 cal/g. If the melting point is lower than 70°C, the fine particles melt during coating for the intermediate layer, so that they stick to each other by melting or penetrate into the substrate. The smoothness of the resulting intermediate layer deteriorates. If the If heat absorption exceeds 50 cal/g, heat is consumed by the heating head in the form of latent heat, which leads to a reduction in the transfer density.

The fine particles of the polyolefin resin include those of polyethylene, polypropylene, polybutene-1, polyisobutene, polypentene-1, polyhexene-1, poly-3-methylbutene-1, polymethylpentene-1, poly-5-methylhexene-1, and copolymers of these olefins. For the preparation of the image-receiving sheet for thermal transfer printing of the present invention, fine particles of the above polyolefin resins having defined ranges for melting point and heat absorption are used with an organic solvent-insoluble resin in the intermediate layer. Polyethylene, polypropylene, and copolymers of ethylene and propylene are preferably used as the polyolefin resin in the form of fine particles.

It should be noted that the melting point as used herein means a temperature at which an endothermic peak appears when measured by the differential thermal analysis. The heat absorption accompanying softening or phase transfer means absorption of heat in the vicinity of the melting point as determined by the differential thermal analysis.

If the size of the fine particles of polyolefin resins is smaller than 0.1 µm, a satisfactory high smoothness cannot be obtained. If it is over 20 µm, the smoothness also deteriorates. Due to these facts, the particle size is preferably in the range of 0.1 to 20 µm.

The desired effects are obtained when the fine particles are present in an amount of not less than 10% by weight, preferably from 60 to 90 wt.%, as total solids content in the intermediate layer.

If the content is less than 10 wt%, satisfactory effects in reducing the blocking tendency and improving the smoothness cannot be expected. If it is more than 90 wt%, the strength of the intermediate layer deteriorates, with the risk that the intermediate layer may peel off from the substrate during printing, or the coating composition for the image-receiving layer may penetrate into the intermediate layer, or a satisfactory image-receiving layer may not be formed on the substrate when it is coated.

As a binder for the fine particles of polyolefin resins, a specific type of copolymer latex as previously stated can be used with very good results. However, when fine particles are used in amounts of not less than 60% by weight in the intermediate layer, the resins commonly used as binders or adhesives, for example, SBR, polyurethane, polybutadiene, MBR, vinyl acetate-ethylene copolymers can be used as they are.

The present inventors have made extensive studies on the resins resistant to an organic solvent. As a result, it has been found that when hollow particles are contained in the resin, the heat insulating properties of the intermediate layer can be greatly improved and that a concomitant remarkable improvement in the optical density of the images to be printed is obtained.

The hollow particles suitable for this purpose are, for example, those described below.

(1) Thermally expandable plastic materials

Hollow particles with capsule walls made of plastic material such as vinylidene chloride-acrylonitrile copolymers and with a volatile propellant such as propane, n-butane, isobutane or similar inside the individual particles.

Commercially available products: Matsumoto Microsphere F-30, manufactured by Matsumoto Yushi-Seiyaku Co., Ltd.; and Expancel 551, 642, manufactured by KemaNobel Company

(2) Hollow polymers in microcapsule form

Hard resins such as acrylic-styrene copolymers are used as a shell that contains water and flows away during drying, forming hollow polymer particles.

Commercially available product: Ropaque OP-84J, manufactured by Rohm & Haas Japan Kabushiki Kaisha.

The above hollow particles generally have a size of 0.1 to 200 µm. Preferably, hollow particles having a size of 0.1 to 20 µm are used.

If the size is less than 0.1 µm, satisfactory heat insulation effects cannot be expected. If it is more than 20 µm, the smoothness of the image receiving layer deteriorates. However, the hollow particles formed by thermally expanding thermally expandable Plastic materials have a damping effect and are flexible, so that they can be used when the size is not larger than 100 µm.

The above-mentioned hollow particles are all soluble in organic solvents such as methyl ethyl ketone and should be used after dispersion in a resin resistant to organic solvents and used as a binder. Alternatively, the intermediate layer may consist of a layer of hollow particles and a protective layer of a resin resistant to organic solvents formed on the layer of hollow particles.

Preferred organic solvent-resistant resins are copolymer latexes having a specific monomer composition and gel content. Independently of these latexes, resins having good film-forming properties and preventing the organic solvents from permeating may be used, including, for example, hydrophilic polymers such as polyvinyl alcohol, casein, starch and the like, acrylic esters, ethylene-vinyl acetate copolymers, carboxyl group-containing polyethylene and the like. Of these, the copolymer latexes and the hydrophilic polymers including polyvinyl alcohol, casein, starch and the like are preferred because they give good synergistic effects with the hollow particles. The hollow particles are preferably used in an amount of not less than 50% by weight based on the total solid in the intermediate layer.

The thickness of the intermediate layer can be appropriately controlled depending on the type of organic solvent resistant resin, and is generally in the range of 10 to 50 µm, preferably from 15 to 30 µm. When hollow particles made from a thermally expandable plastic material are used, the dry coating of the intermediate layer is in the range from 1 to 10 g/m², and preferably from 2 to 6 g/m². For non-thermally expandable hollow particles, the thickness is in the range from 5 to 50 µm, preferably from 10 to 30 µm.

The intermediate layer may further contain microcapsules containing a core substance having a plasticizing effect on the image-receiving layer made mainly of a resin soluble in organic solvents, for example, esters such as phosphoric acid esters, (tere)phthalic acid esters, adipic acid esters and the like, polyesters in the form of oligomers of dibasic acids and glycols or glycerin, epoxy fatty acid esters and the like. The thermal transfer susceptibility of the resulting image-receiving sheet for sublimable dyes is improved without missing any transfer part and a high optical density is obtained for the printed images. The walls of the capsule used above should allow thermal passage of the core substance through the capsule wall when heated at the time of printing. For this reason, the capsule wall is preferably made of polyurethane or polyurea.

Substrate 101:

As the substrate 101, for example, synthetic paper, wood-free paper, a coated paper No. 1 quality, a coated paper, a cast-coated paper, a polymer film and composite sheets or films made of these papers and films can be used.

There are a variety of paper sheets mainly comprising pulp fibers. It is generally known that such a paper sheet is significantly inferior to a synthetic film in the occurrence of missing transfer parts and the optical density of the image to be printed when used as a substrate for an image-receiving sheet for thermal transfer printing. For this reason, a paper sheet is not always considered as a suitable substrate. However, in the present invention, such paper sheets mainly comprising pulp fibers can also be used as the substrate 101. Even in the case where such a paper sheet is used as the substrate 101, the image-receiving sheet of the present invention, which is prepared by forming a specific intermediate layer and a special image-receiving layer thereon, can effectively achieve the object of the present invention.

As a result of the present inventors' intensive studies on a paper sheet to be used as the substrate 101, it has been found that for the formation of an intermediate layer having good cushioning and heat insulating properties and high smoothness, not only the formation of the intermediate layer but also the physical properties of the surface of the paper sheet substrate or support are very important factors. It has also been found that if the paper sheet substrate is appropriately defined in terms of the surface physical properties, an intermediate layer having the desired properties can be obtained very effectively. In particular, if the physical properties of the paper sheet substrate on the surface contacting the intermediate layer satisfy the following requirements, very good properties are imparted to the resulting intermediate layer. This results in the formation of an image-receiving sheet for thermal transfer printing which exhibits a high degree of gradation and is substantially free from the known problems associated with the occurrence of missing transfer portions during printing.

[Requirements for the physical properties of the surface of the paper sheet]

(1) Not less than 6% smoothness measured with a smoothness tester by specular reflection at a pressure of 20 kg/cm2.

(2) Water absorption: a 5-second value of 1 to 30 g/m2 determined according to the test method for water absorption of paper and paperboard described in JIS P-8140 (Copp method).

In general, the smoothness of the paper sheet is determined by means of a measuring instrument such as a mirror reflection smoothness tester, a Bekk smoothness tester, a Parker Print Surf or a similar device. According to the applicant's extensive investigations, the measurement by a Bekk air permeability smoothness tester or the Parker Print Surf (HE Messmer Ltd.) device is greatly influenced by the air permeability of the substrate sheet. It has been found that it is difficult to establish a correct relationship between a measurement and the actual smoothness for different substrate sheets whose properties vary greatly from each other. In particular, when intimate contact with a heating head or a thermal transfer sheet is an important factor, as in the image-receiving sheet for thermal transfer printing, it has been found that the measurement of smoothness under the conditions in which the Sample is pressed against a glass surface at a given pressure, as in the case of the mirror reflection smoothness tester, is most suitable in view of this interrelationship.

The smoothness of the paper substrate sheet used for carrying out the present invention should therefore be determined by specular reflection using a smoothness tester. When a paper substrate sheet having not less than 6%, preferably not less than 7%, measured at a pressure of 20 kg/cm² (a larger value results in better smoothness) is suitably used, an intermediate layer having the desired properties can be formed very effectively.

It takes only a few seconds from applying the coating for the interlayer to the substrate to drying the coating. However, if the sizing degree of the paper substrate sheet is too low, the coating solution for the interlayer penetrates into the sheet during coating, making it difficult to produce a smooth interlayer. Accordingly, the substrate sheet is required to have an appropriate sizing degree on the surface that comes into contact with the interlayer. This is why a paper substrate sheet having a 5-second value of 1 to 30 g/m², preferably 3 to 20 g/m², determined according to the above-mentioned test method for water absorption of paper and paperboard is selectively used.

If the water absorption is less than 1 g/m², the wetting between the paper substrate sheet and the coating solution for the intermediate layer becomes poor. Then, the problems arise that the aqueous coating solution is repelled from the substrate or that the Adhesion of the resulting layer is impaired. Above 30 g/m², the penetration of the coating solution into the paper substrate becomes too strong and it is not possible to obtain a smooth intermediate layer.

The paper-based substrates are not critical as to type, provided that the above surface requirements are met. Examples of paper substrates include coated papers of the glossy, dull and matte types, such as No. 1 grade coated papers, coated papers, light coated papers, fine coated papers, cast coated papers and the like, and uncoated papers such as wood-free papers, medium papers, supercalendered papers and the like.

Image receiving layer 103:

Finally, the image-receiving layer is described. As described above, the image-receiving layer of the image-receiving sheet for thermal transfer printing according to the present invention is mainly formed of resins soluble in organic solvents. Examples of resins soluble in organic solvents are given below.

(a) Resins with ester bonds:

Polyester resins, polyacrylate resins, polycarbonate resins, polyvinyl acetate resins, styrene-acrylate resins, vinyltoluenecarboxylate resins and similar.

(b) Resins with urethane bonds:

Polyurethane resins and similar.

(c) Resins with amide bonds:

Polyamide resins (nylons) and similar.

(d) Resins with urea bonds:

Urea resins and similar.

(e) Vinyl chloride-(meth)acrylic ester copolymers.

The (meth)acrylic acid monomers include: acrylic acid; acrylic acid salts such as calcium acrylate, zinc acrylate, magnesium acrylate, aluminum acrylate and the like; acrylates such as methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, 2-ethoxyethyl acrylate, 2-hydroxyethyl acrylate, n-stearyl acrylate, tetrahydrofurfuryl acrylate, trimethylolpropane triacrylate and the like; methacrylic acid; and methacrylates such as methyl methacrylate, ethyl methacrylate, t-butyl methacrylate, tridecyl methacrylate, cyclohexyl methacrylate, triethylene glycol dimethacrylate, 1,3-butylene dimethacrylate, trimethylolpropane methacrylate and the like.

The ratio of vinyl chloride and (meth)acrylic acid monomers for the copolymerization is preferably vinyl chloride/acrylic acid monomer = 50 to 90%/50 to 10%. The molecular weight is generally in the range of 5,000 to 40,000, preferably 10,000 to 30,000.

Independently of the above copolymers, copolymers of monomers such as acrylonitrile, vinylpyrrolidone, N-substituted maleimides, maleic acid and the like, such as vinyl chloride and a (meth)acrylic acid monomer are preferably used. In this case, the ratio of the other monomer is preferably from 0.1 to 30%.

Of course, the copolymers can be mixed with other resins.

(f) Other resins with high polarity bonds:

Polycaprolactone resins, styrene-maleic acid resins, polyvinyl chloride resins, polyacrylonitrile resins and similar.

These resins have good absorption capacity for sublimable dyes and good storage properties and are therefore preferably used in the practice of the present invention. Of these, polyester resins and vinyl chloride-(meth)acrylic acid ester copolymers are preferred because they have good transferability of dyes, especially paper dyes, and their lightfastness of the transferred dyes is good.

Besides these resins, various additives may optionally be added to the image-receiving layer. Examples of such additives include mineral pigments such as titanium dioxide, zinc oxide, aluminum hydroxide, calcium carbonate, finely powdered silica and the like, and organic white pigments such as plastic pigments to improve whiteness and thermal fusion properties, modified silicone resin, solid waxes, polyethylene waxes, amide waxes, calcium silicate and the like. Furthermore, stabilizers for printed images, UV absorbers and light stabilizers may be added in amounts not affecting the effects of the present invention.

The overcoat for the image-receiving layer is applied to the substrate to a dry thickness of 1 to 15 µm, preferably 2 to 10 µm, using any known coating device such as a bar coater, a gravure coater or the like, and then dried.

PREFERRED EMBODIMENT OF THE INVENTION

The following examples and comparative examples illustrate the invention without limiting it.

EXAMPLE 1

A paper base comprising 30 parts of bleached NKP and 70 parts of bleached LKP, to which 0.4 parts of rosin size, 10 parts of talc and 2.3 parts of aluminum sulfate were added, was prepared and paper was made therefrom by means of a Fourdrinier paper machine at a speed of 600 m/min in such a manner that the dry weight was 101 g/m². This was followed by coating with oxidized starch solution having a concentration of 6 wt% using a sizing press in an amount of 50 ml/m², then drying and winding. The resulting base paper was subjected to supercalendering treatment under the following conditions.

Supercalendering conditions:

Linear pressure: 200 kg/cm

Surface temperature of the chrome-plated metal roller: 25ºC

Paper running speed: 250 m/min

Number of nips for paper travel: 4

After supercalendering, the base paper had a Copp water absorption (5-second value) of 10.4 g/m² and a smoothness of 13%, determined by the mirror reflection smoothness tester at a pressure of 20 kg/cm².

An intermediate layer was formed on the surface of the base paper in the following manner.

A coating composition for forming the intermediate layer was prepared by adding 10 parts of precipitated calcium carbonate (trade name: Brilliant S-15, manufactured by Shiraishi Calcium Kaisha, Ltd.) to 90 parts of modified styrene-butadiene methyl methacrylate copolymer latex (trade name: JSR-0530, manufactured by Japan Synthetic Rubber Co., Ltd.) in a mixer with stirring. Then, water was added to adjust the concentration to 50%. Thereafter, 3 parts (solid content) of oxidized starch and 1 part of calcium stearate (trade name: Nopcote C-104, manufactured by SAN NOPCO Co., Ltd.) were added to the dispersion, then water was added to make the solid content 47%. The resulting coating composition was applied to the surface of the base paper by means of a wire rod to a dry thickness of 25 µm, and then dried to form the intermediate layer.

An overcoat for the image-receiving layer was prepared as follows.

100 parts of polyester resin (trade name: Vylon 200, manufactured by Toyobo Co., Ltd.), 0.5 part of amino-modified silicone (trade name: KF-393, manufactured by The Shinetsu Chemical Co., Ltd.) and 0.5 part of epoxy-modified silicone (trade name: X-22-343, manufactured by Shinetsu Chem. Ind. Co., Ltd.) were added to methyl ethyl ketone/toluene (weight ratio 1:1) with stirring in a mixer to obtain the coating composition for the image-receiving layer having a solid content of 15%. The coating composition was coated on the surface of the intermediate layer to a dry thickness of 7 µm. by means of a wire bar and dried, and then pressed under the following supercalendering conditions to form an image-receiving sheet for thermal transfer printing.

Supercalendering conditions:

Linear pressure: 90 kg/cm

Surface temperature of the chrome-plated metal roller: 70ºC

Paper running speed: 5 m/min

Number of nips for paper travel: 2

On the other hand, a thermal transfer sheet was prepared in the following manner.

40 parts of a sublimable disperse dye (trade name: Kayaset Blue 714, manufactured by Nippon Kayaku Co., Ltd.) and 60 parts by weight of polyvinyl butyral were added to methyl ethyl ketone/toluene (weight ratio 1:1) with stirring in a mixer to obtain a dye composition for forming the thermal transfer layer having a solid content of 10%. The composition was applied to a 4.5 µm thick PET film, the underside of which had been subjected to a heat resistance treatment, in a dry weight of 1 g/m² by means of a wire bar, and then dried to form a thermal transfer sheet.

The thermal transfer sheet thus obtained was placed on the image-receiving sheet for thermal transfer printing, and then under the following conditions, namely, an output power of 1 W/dot, a pulse width of 0.3 to 4.5 ms and a dot density of 3 dots/mm, for testing. The results are shown in Table 1.

EXAMPLE 2

A base paper sheet was prepared in the same manner as described in Example 1 and provided with a coating prepared as follows.

60 parts of ground calcium carbonate (trade name: Sofuton 1800, manufactured by Bihoku Funka Kogyo Kabushiki Kaisha), precipitated calcium carbonate (trade name: Brilliant S-15, manufactured by Shiraishi Calcium Kaisha, Ltd.) and 0.2 part of sodium polyacrylate were dispersed in water by means of a Cowles dissolver (manufactured by Morehouse Industries, Inc.) to obtain a pigment slurry having a solid content of 55%. To the slurry, 3 parts (as solid) of oxidized starch and 12 parts (as solid) of modified styrene-butadiene copolymer latex (trademark: JSR-0696, manufactured by Japan Synthetic Rubber Co., Ltd.) are added, and then water is added until a coating composition having a solid content of 50% is obtained.

The coating composition thus obtained is coated on a base paper obtained in the same manner as described in Example 1 to a dry thickness of 25 µm and then dried. The coated paper is pressed under the following supercalender conditions.

Supercalendering conditions:

Linear pressure: 60 kg/cm

Surface temperature of the chrome-plated metal roller: 25ºC

Paper running speed: 5 m/min

Number of nips for paper travel: 2

The coated paper obtained after supercalendering treatment has a Copp water absorption (5-second value) of 12.3 g/m² and a smoothness (at a pressure of 20 kg/cm²) of 16%, determined by the smoothness tester by specular reflection.

Thereafter, the procedure of Example 1 for the interlayer formation and the subsequent procedure for preparing the image-receiving sheet for thermal transfer printing were repeated. Thereafter, the sheet was evaluated in the same manner as described in Example 1. The results are shown in Table 1.

EXAMPLE 3

0.2 parts of rosin size, 10 parts of talc and 0.7 parts of aluminum sulfate were added to a pulp mixture of 60 parts of bleached NKP and 40 parts of bleached LKP to obtain a paper base. Paper was then made using a papermaking machine. After drying, a base paper with a basis weight of 101 g/m² was obtained. The base paper was coated with 2% oxidized starch solution in an amount of 10 ml/m² using a sizing press, dried and subjected to a supercalendering treatment under the following conditions.

Supercalendering conditions:

Linear pressure: 100 kg/cm

Surface temperature of the chrome-plated metal roller: 70ºC

Paper running speed: 5 m/min

Number of nips for paper travel: 4

The base paper had a Copp water absorption (5-second value) of 41.2 ml/m² and a smoothness (at a pressure of 20 kg/cm²) of 23%, determined by the mirror reflection smoothness tester.

Thereafter, the procedures of Example 1 for interlayer formation and subsequent procedures to prepare a thermal transfer sheet were repeated. The sheet was evaluated in the same manner as described in Example 1. The results are shown in Table 1.

EXAMPLE 4

A base paper was prepared in the same manner as described in Example 1, and thereon was applied a coating prepared as follows.

50 parts of ground calcium carbonate (trade name: BF-100, Bihoku Funka Kogyo Kabushiki Kaisha), 50 parts of ground calcium carbonate (trade name: Sofuton 1800, manufactured by Bihoku Funka Kogyo Kabushiki Kaisha) and 0.2 part of sodium polyacrylate were dispersed in water by means of a carbon disperser to form a pigment slurry having a solid content of 55%. 2 parts (solid content) of oxidized starch and 10 parts (solid content) of modified styrene-butadiene copolymer latex (trademark: JSR-0696, manufactured by Japan Synthetic Rubber Co., Ltd.) were added to the slurry. Thereafter, water was further added until a coating composition having a solid content of 50% was obtained. The coating composition thus obtained was coated on the base paper prepared in the same manner as in Example 1. described, was applied to a dry thickness of 25 µm and then dried. The coated paper was not subjected to supercalendering.

The coated paper showed a Copp water absorption (5-second value) of 11.5 g/m² and a smoothness (under pressure of 20 kg/cm²) of 5%, determined by a mirror reflection smoothness tester.

Thereafter, the procedure of Example 1 was repeated for the subsequent processes including the formation of the intermediate layer and the preparation of an image-receiving sheet for thermal transfer printing. Thereafter, testing was carried out in the same manner as described in Example 1. The results are shown in Table 1.

EXAMPLE 5

A base paper obtained in the same manner as described in Example 1 was coated with a coating prepared as follows.

60 parts of kaolin (trade name: Ultra White 90, manufactured by EMC) and 40 parts of precipitated calcium carbonate (trade name: Brilliant S-15, manufactured by Shiraishi Calcium Kaisha, Ltd.) were dispersed in water by means of a Cowles dissolver disperser to obtain a pigment slurry having a solid content of 58%. To the slurry were added 10 parts (solid content) of casein, 16 parts (as solid) of modified styrene-butadiene copolymer latex (trademark: JSR-0696, manufactured by Japan Synthetic Rubber Co., Ltd.) and 3% (as solid) of zinc sulfate heptahydrate. Thereafter, water was further added to prepare a coating composition having a solid content of 50%. The coating composition thus obtained was coated on the base paper obtained in the same manner as described in Example 1 to a dry thickness of 25 µm and then dried. The coated paper was molded under the following conditions while rewetting.

Deformation conditions during re-wetting:

Drum temperature: 105ºC

Running pressure: 100 kg/cm

Rewetting solution: aqueous solution of 0.5% polyethylene emulsion

The coated paper obtained after deformation and rewetting had a Copp water absorption (5-second value) of 15.1 g/m² and a smoothness (under pressure of 20 kg/cm²) of 85%, determined by a mirror reflection smoothness tester.

The procedures of Example 1 were then repeated with respect to the interlayer formation and subsequent processes to obtain an image-receiving sheet for thermal transfer printing. Thereafter, the sheet was evaluated in the same manner as described in Example 1. The results are shown in Table 1.

EXAMPLE 6

An intermediate layer was formed on the surface of a substrate sheet prepared in the same manner as described in Example 1 in the following manner. 5 parts of a polypropylene resin of fine particles A (test sample from SAN NOPCO Co., Ltd.) and 95 parts of modified styrene-butadiene-methyl methacrylate copolymer latex (trademark: JSR-0530, manufactured by Japan Synthetic Rubber Co., Ltd.) were stirred in a mixer, and then water was added to make a 45% slurry. 10 parts (as solid) of oxidized starch and 1 part of calcium stearate (trademark: Nopcote C-104, manufactured by SAN NOPCO Co., Ltd.) were added, and then water was added to prepare a 40% coating composition.

The resulting coating composition was applied to the substrate sheet, obtained as above, in a dry thickness of 25 µm by means of a wire rod and dried to form an intermediate layer.

Thereafter, the procedure of Example 1 was repeated with respect to image-receiving layer formation and subsequent processes, and an image-receiving sheet for thermal transfer printing was prepared. The test was conducted in the same manner as described in Example 1. The results are shown in Table 1.

EXAMPLE 7

An intermediate layer was formed on the surface of a substrate sheet prepared in the same manner as described in Example 2 in the following manner.

15 parts of a polypropylene resin of fine particles A (test sample from SAN NOPCO Co., Ltd.) and 85 parts of modified styrene-butadiene-methyl methacrylate copolymer latex (trademark: JSR-0530, manufactured by Japan Synthetic Rubber Co., Ltd.) were stirred in a mixer, then water was added to prepare a 45% slurry. 10 parts (as solid) of oxidized starch and 1 part of calcium stearate (trademark: Nopcote C-104, manufactured by SAN NOPCO Co., Ltd.) were added, followed by water to prepare a 40% coating composition.

The resulting coating composition was applied to the substrate obtained as described above to a dry thickness of 25 µm using a wire bar and then dried to form an intermediate layer.

Thereafter, the procedures of Example 1 for the formation of the image-receiving layer and the subsequent procedures were repeated, and an image-receiving sheet for thermal transfer printing was prepared. Evaluation was carried out in the same manner as described in Example 1. The results are shown in Table 1.

EXAMPLE 8

An intermediate layer was formed on the surface of a substrate sheet obtained in the same manner as described in Example 2 in the following manner.

30 parts of polypropylene resin in the form of fine particles A (test sample from SAN NOPCO Co., Ltd.) and 70 parts of modified styrene-butadiene-methyl methacrylate copolymer latex (trademark: JSR-0530, manufactured by Japan Synthetic Rubber Co., Ltd.) were stirred in a mixer, then water was added to prepare a 45% slurry. 10 parts (as solid) of oxidized starch and 1 part of calcium stearate (trademark: Nopcote C-104, manufactured by SAN NOPCO Co., Ltd.) were added and Water was then added to produce a 40% coating mass.

The resulting coating composition was applied to the substrate sheet obtained as described above in a dry thickness of 25 µm by means of a wire rod and dried to form an intermediate layer.

Thereafter, the procedures of Example 1 regarding the formation of the image-receiving layer and the subsequent procedures were repeated, and an image-receiving sheet for thermal transfer printing was prepared. The evaluation was carried out in the same manner as described in Example 1. The results are shown in Table 1.

EXAMPLE 9

An intermediate layer was formed on the surface of a substrate sheet obtained in the same manner as described in Example 2 in the following manner.

70 parts of polypropylene resin in the form of fine particles A (test sample from SAN NOPCO Co., Ltd.) and 30 parts of a modified styrene-butadiene copolymer latex (trademark: JSR-0696, manufactured by Japan Synthetic Rubber Co., Ltd.) were stirred in a mixer, then water was added to prepare a 43% slurry. 10 parts (as solid) of oxidized starch and 1 part of calcium stearate (trademark: Nopcote C-104, manufactured by SAN NOPCO Co., Ltd.) were added, then water was added to prepare a 40% coating mass.

The resulting coating composition was applied to a substrate sheet, obtained as above, to a dry thickness of 25 µm using a wire rod and dried to form an intermediate layer.

Thereafter, the procedures of Example 1 were repeated with respect to image-receiving layer formation and subsequent processes, and an image-receiving sheet for thermal transfer printing was prepared. The evaluation was carried out in the same manner as described in Example 1. The results are shown in Table 1.

EXAMPLE 10

An intermediate layer was formed in the following manner on the surface of a substrate sheet obtained in the same manner as described in Example 2.

130 parts of polyethylene resin in the form of fine particles B (test sample from SAN NOPCO Co., Ltd.) and 70 parts of modified styrene-butadiene-methyl methacrylate copolymer latex (trademark, JSR-0530, manufactured by Japan Synthetic Rubber Co., Ltd.) were stirred in a mixer, then water was added to prepare a 45% slurry. 10 parts (as solid) of oxidized starch and 1 part of calcium stearate (trademark: Nopcote C-104, manufactured by SAN NOPCO Co., Ltd.) were added, then water was added to prepare a 40% coating composition.

The resulting coating composition was applied to a substrate sheet, prepared as above, in a dry thickness of 25 µm using a wire rod and dried to form an intermediate layer.

Thereafter, the procedures of Example 1 were repeated with respect to image-receiving layer formation and subsequent processes, and an image-receiving sheet for thermal transfer printing was prepared. The evaluation was carried out in the same manner as described in Example 1. The results are shown in Table 1.

EXAMPLE 11

An intermediate layer was formed on the surface of a substrate sheet obtained in the same manner as described in Example 2 in the following manner.

30 parts of ethylene-propylene copolymer resin in the form of fine particles C (a test sample from SAN NOPCO Co., Ltd.) and 70 parts of modified styrene-butadiene-methyl methacrylate copolymer latex (trademark: JSR-0530, manufactured by Japan Synthetic Rubber Co., Ltd.) were stirred in a mixer, then water was added to prepare a 45% slurry. 10 parts (as solid) of oxidized starch and 1 part of calcium stearate (trademark: Nopcote C-104, manufactured by SAN NOPCO Co., Ltd.) were added, then water was added to prepare a 40% coating composition.

The resulting coating composition was applied to the substrate sheet obtained as above to a dry thickness of 25 µm by means of a wire bar and then dried to form the intermediate layer.

Thereafter, the procedures of Example 1 were repeated with respect to the formation of the image-receiving layer and the subsequent procedures, and an image-receiving sheet for thermal transfer printing was prepared. The evaluation was carried out in the same manner as described in Example 1. The results are given in Table 1.

EXAMPLE 12

An intermediate layer was formed on the surface of a substrate sheet obtained in the same manner as described in Example 2 in the following manner.

30 parts of polyethylene resin in the form of fine particles D (test sample from Toho Chem. Co., Ltd.) and 70 parts of modified styrene-butadiene-methyl methacrylate copolymer latex (trademark: JSR-0530, manufactured by Japan Synthetic Rubber Co., Ltd.) were stirred in a mixer, then water was added to prepare a 45% slurry. 10 parts (as solid) of oxidized starch and 1 part of calcium stearate (trademark: Nopcote C-104, manufactured by SAN NOPCO Co., Ltd.) were added, then water was added to prepare a 40% coating mass.

The resulting coating composition was applied to the substrate sheet obtained as described above in a dry thickness of 25 µm by means of a wire bar and then dried to form the intermediate layer.

Thereafter, the procedures of Example 1 for the formation of the image-receiving layer and the subsequent procedures were repeated, and an image-receiving sheet for thermal transfer printing was prepared. Evaluation was carried out in the same manner as described in Example 1. The results are shown in Table 1.

EXAMPLE 13

An intermediate layer was formed in the following manner on the surface of a substrate sheet obtained in the same manner as described in Example 2.

30 parts of fine particles of polyethylene resin (trademark: Chemipearl W-300, manufactured by Mitsui Petrochemical Industries Co., Ltd.) and 70 parts of modified styrene-butadiene-methyl methacrylate copolymer latex (trademark: JSR-0530, manufactured by Japan Synthetic Rubber Co., Ltd.) were stirred in a mixer, then water was added to prepare a 45% slurry. 10 parts (as solid) of oxidized starch and 1 part of calcium stearate (trademark: Nopcote C-104, manufactured by SAN NOPCO Co., Ltd.) were added, then water was added to prepare a 40% coating composition.

The resulting coating composition was applied to the substrate sheet obtained as described above in a dry thickness of 25 µm by means of a wire rod and then dried to form an intermediate layer.

Thereafter, the procedures of Example 1 with respect to the formation of the image-receiving layer and the subsequent processes were repeated, and an image-receiving sheet for thermal transfer printing was prepared. The evaluation was carried out in the same manner as described in Example 1. The results are shown in Table 1.

EXAMPLE 14

An intermediate layer was prepared in the following manner on the surface of a substrate sheet which was coated on the same manner as described in Example 2.

70 parts of thermally expandable hollow particles (trademark: Matsumoto Microsphere F-30, manufactured by Matsumoto Yushi-Seiyaku Co., Ltd.) whose shell walls had a softening temperature of 80 to 85ºC and 30 parts of modified styrene-butadiene latex (trademark: JSR-0696, manufactured by Japan Synthetic Rubber Co., Ltd.) were stirred in a mixer. Water was added to prepare a 45% coating composition.

The resulting coating composition was applied to the surface of a substrate sheet obtained as described above by means of a wire rod in a dry weight of 3.5 g/m² and dried at 120°C for 1 minute. After drying, the thermally expandable hollow particles were individually expanded to 30 to 70 times their size.

A coating of polyvinyl alcohol (trademark: PVA-110, manufactured by Kuraray Co., Ltd.) was formed using a wire rod in a dry weight of 3.5 g/m2 as an upper layer on the intermediate layer and then was dried to form the intermediate layer.

Thereafter, the procedures of Example 1 were repeated with respect to image-receiving sheet preparation and subsequent processes, and an image-receiving sheet for thermal transfer printing was prepared. Thereafter, evaluation was carried out in the same manner as described in Example 1. The results are shown in Table 1.

EXAMPLE 15

An intermediate layer was formed on the surface of a substrate sheet obtained in the same manner as described in Example 1 in the following manner.

70 parts of thermally expandable hollow particles (trademark: Matsumoto Microsphere F-30, manufactured by Matsumoto Yushi-Seiyaku Co., Ltd.) and 30 parts of polyvinyl alcohol (trademark: PVA-110, manufactured by Kuraray Co., Ltd.) were stirred in a mixer and then water was added to prepare a 20% coating composition.

The resulting coating composition was applied to the surface of a substrate sheet obtained as described above in a dry weight of 3.5 g/m2 by means of a wire rod and then dried at 120°C for 1 minute.

Thereafter, the procedures of Example 1 were repeated with respect to the formation of the image-receiving layer and the subsequent procedures, and an image-receiving sheet for thermal transfer printing was prepared. The sheet was evaluated in the same manner as described in Example 1. The results are shown in Table 1.

EXAMPLE 16

The procedure of Example 15 was repeated to prepare an image-receiving sheet for thermal transfer printing. Thereafter, a supercalendering treatment was carried out under the following conditions:

Supercalendering conditions:

Linear pressure: 90 kg/cm

Surface temperature of the chrome-plated metal roller: 70ºC

Paper running speed: 5 m/min

Number of nips for paper travel: 2

By the treatment, the thickness of the image-receiving sheet was reduced from 165 µm to 110 µm. Thereafter, the sheet was evaluated in the same manner as described in Example 1. The results are shown in Table 1.

EXAMPLE 17

An intermediate layer was formed in the following manner on the surface of a substrate sheet obtained in the same manner as described in Example 1.

80 parts of hollow particles of acrylic-styrene copolymer (trademark: Ropaque OP-84J, manufactured by Rohm & Haas Japan Kabushiki Kaisha) and 20 parts of modified styrene-butadiene copolymer latex (trademark: JSR-0696, manufactured by Japan Synthetic Rubber Co., Ltd.) were stirred in a mixer. Then, water was added to prepare a 40% coating composition.

The resulting coating composition was applied to a substrate obtained as described above using a wire rod to a dry thickness of 20 µm and dried.

As the upper layer of the intermediate layer, polyvinyl alcohol (trademark: PVA-110, manufactured by Kuraray Co., Ltd.) was coated with a wire rod in a dry weight of 3.5 g/m² is applied and dried to form the intermediate layer.

Thereafter, the procedures of Example 1 were repeated with respect to the formation of the image-receiving layer and the subsequent procedures, and an image-receiving sheet for thermal transfer printing was prepared. The evaluation was carried out in the same manner as described in Example 1. The results are shown in Table 1.

COMPARISON EXAMPLE 1

An intermediate layer was formed as follows on the surface of a substrate sheet prepared in the same manner as described in Example 2.

While stirring, 10 parts of precipitated calcium carbonate (trademark: Brilliant S-15, manufactured by Shiraishi Calcium Kaisha Ltd.) were added to 90 parts of modified styrene-butadiene copolymer latex (trademark: JSR-0696, manufactured by Japan Synthetic Rubber Co., Ltd.) in a mixer. Then, water was added to prepare a 45% slurry. 3 parts (as solid) of oxidized starch and 1 part of calcium stearate (trademark: Nopcote C-104, manufactured by SAN NOPCO Co., Ltd.) were added, then water was added so that the solid content became 40%.

The resulting coating composition was applied to the surface of a substrate sheet prepared as described above by means of a wire rod to a dry thickness of 25 µm and then dried.

Thereafter, the procedures of Example 1 were repeated with respect to the formation of the image-receiving layer and the subsequent The procedure was repeated and an image-receiving sheet for thermal transfer printing was prepared. This sheet was found to have fine wrinkles on its surface and the solvent for the image-receiving layer penetrated into the substrate in patches, causing streaking all over. It was clear that the intermediate layer was swollen or dissolved by the organic solvent for the image-receiving layer. The sheet was tested in the same manner as described in Example 1. The results are shown in Table 1.

COMPARISON EXAMPLE 2

An intermediate layer was formed in the same manner as described in Comparative Example 1 on the surface of a substrate sheet obtained in the same manner as described in Example 5.

Thereafter, the procedures of Example 1 with respect to the formation of the image-receiving layer and the subsequent procedures were repeated, and an image-receiving sheet for thermal transfer printing was prepared. As a result, the sheet was found to have fine wrinkles on its surface, showing that the interlayer was swollen or dissolved by the organic solvent for the image-receiving layer.

The leaf was evaluated in the same manner as described in Example 1. The results are given in Table 1.

COMPARISON EXAMPLE 3

The procedures of Example 14 were repeated, except that the upper layer of the intermediate layer was not formed after the formation of the lower layer and an image-receiving layer was directly prepared. As a result, it was found that the solvent for the image-receiving layer penetrated into the substrate and caused streaking. Then, it was also found that the thickness of the sheet before and after the formation of the image-receiving layer was reduced from 160 µm to 135 µm. This shows that the intermediate layer was dissolved by the organic solvent for the image-receiving layer.

The leaf was evaluated in the same manner as described in Example 1. The results are given in Table 1.

COMPARISON EXAMPLE 4

The procedures of Example 17 were repeated, except that the upper layer of the intermediate layer was not formed after the formation of the lower layer. An image-receiving layer was thus directly prepared. The resulting sheet was found to have fine wrinkles on its surface, showing that the intermediate layer was swollen and dissolved by the organic solvent for the image-receiving layer.

The leaf was evaluated in the same manner as described in Example 1. The results are given in Table 1.

The evaluation procedures for the various tests listed in Table 1 are described below.

1. Resistance to an organic solvent

Durability was determined by visual observation and measurement of sheet thickness.

A ----- no problem

E ----- no practical usability, with a large number of errors

2. Anti-lock properties

The intermediate layer and a black drawing paper were placed on top of each other and passed twice through a supercalender at a linear pressure of 50 kg/cm. After that, the layer and the paper were separated and the different surfaces were observed.

A ----- Few black fibers of the drawing paper were transferred to the interlayer and no practical problem occurred

E ----- The interlayer and the drawing paper stuck together

3. Optical density

The optical density of the cyan ink transferred to the image-receiving layer was determined using a reflection densitometer (Macbeth RD914).

A ----- Maximum value not less than 1.4

B ----- Maximum value from 1.2 to 1.4

C ----- Maximum value of less than 1.2

4. Image quality

Assessed by visual observation of the missing transmission part(s)

AA ----- Particularly excellent image quality, no transmission part is missing or no transmission parts are missing

A ----- Few missing transmission parts with excellent image quality

B ----- There are minor transmission parts missing, good image quality is observed

C ----- Transmission parts are missing, but there are no practical problems with regard to image quality

D ----- Many transmission parts are missing, the image quality is poor

E ----- A large number of transmission parts are missing, the image quality is practically unacceptable. TABLE 1 Example Comparative Example Base Sheet Smoothness (%) Water Adsorption Rate (g/m²) Main Components of Interlayer Solvent Resistance Anti-Blocking Property Optical Density Image Quality Woodfree Paper JSR-0530(1) (manufactured by Japan Synthetic Rubber Co., Ltd.) Coated Paper Same as Example Cast Coated Paper JSR-0696(2) Same as Comparative Example

remark

(1): Composition: Styrene 43 wt.%, butadiene 40 wt.% Gel content: 98 wt.%

(2): Composition: Styrene 46 wt.%, butadiene 38 wt.% Gel content: 83 wt.% Table 1 (continued 1) Example Base sheet Smoothness (%) Water adsorption rate (g/m²) Main components of the interlayer Solvent resistance Anti-blocking property Optical density Image quality Coated paper Proportion:(a) JSR-0530(1) Parts by weight (b) Polypropylene Parts by weight Same as Example Polypropylene(3) JSR-0696(2)

remark

(3) Product of SAN NOPCO Ltd.: Mp 140ºC, endothermic quantity (eq) 40 cal/g particle diameter (pd) 2.5 um Table 1 (continued 2) Example Base sheet Smoothness (%) Water adsorption rate (g/m²) Main components of the interlayer Solvent resistance Anti-blocking property Optical density Image quality Same as example Proportion:(a) JSR-0530(1) Parts by weight (b) Polypropylene(4) Parts by weight Polyethylene-polypropylene copolymer (5) Polyethylene(6) Polyethylene (7)

Remark

(4): Product of SAN NOPCO Ltd.: f.p. 130ºC, e.q. 45 cal/g, p.d. 3.5 um

(5): Product of SAN NOPCO Ltd.: f.p. 135ºC, e.q. 40 cal/g, p.d. 3.0 um

(6): Product of Toho Chemical Industry Co., Ltd.: F.p. 90ºC, e.q. 25 cal/g, p.d. 0.2 um

(7): Product of Mitsui Petrochemical Industries, Ltd.: mp 130ºC, eq 70 cal/g, pd 4.0 um Table 1 (continued 3) Example Base sheet Smoothness (%) Water adsorption rate (g/m²) Interlayer: (i) Main component of lower layer (ii) Main component of upper layer Solvent resistance Anti-blocking property Optical density Image quality Woodfree paper Proportion:(a) JSR-0696(2) parts by weight (b) Microsphere(8) F-30 parts by weight Polyvinyl alcohol(9) Same as example None None(*) Ropaque OP-84J(10)

remark

(8): Trademark of a product of Matsumoto Yushi-Seiyaku Co., Ltd.

(9): Product with trademark PVA-110 of Kuraray Co., Ltd.

(10): Trademark of a product of Rohm & Haas Japan Kabushiki Kaisha

(*): Calendered after formation of the image receiving layer Table 1 (continued 4) Comparative example Base sheet Smoothness (%) Water adsorption rate (g/m²) Intermediate layer: (i) Main component of the lower layer (ii) Main component of the upper layer Solvent resistance Anti-blocking property Optical density Image quality same as in example none

Claims (13)

1. An image-receiving sheet for thermal transfer printing, characterized in that it comprises an intermediate layer and an image-receiving layer formed on a substrate in this order, the intermediate layer mainly containing a resin which is insoluble in an organic solvent, the image-receiving layer mainly containing a resin which is soluble in an organic solvent, the insoluble resin being made from a copolymer latex which satisfies the following requirements (a) and (b): (a) Composition of ingredients: aliphatic conjugated diolefinic Monomer 20-70 wt.% unsaturated acid monomer 0.5-15 wt.% other olefinic monomer 15-79.5 wt.% (b) Gel content: 85-100 wt.% 2. An image-receiving sheet for thermal transfer printing according to claim 1, characterized in that the copolymer latex is present in an amount of not less than 60% by weight based on the total solids in the intermediate layer. 3. Image-receiving sheet for thermal transfer printing according to claim 1, characterized in that the substrate is a paper sheet which has been made mainly from pulp fibers. 4. Image-receiving sheet for thermal transfer printing according to claim 3, characterized in that the paper sheet has a smoothness of not less than 6% and a water absorption of 1 to 30 g/m², determined by the following methods: Smoothness: a value determined using a special reflection smoothness tester at a pressure of 20 kg/cm², and Water absorption: a 5-second value determined according to "a test method for water absorption of paper and paperboard (Copp method)" described in JIS P-8140. 5. An image-receiving sheet for thermal transfer printing, characterized in that it comprises an intermediate layer and an image-receiving layer formed on a substrate in this order, the intermediate layer mainly containing resin insoluble in an organic solvent, the image-receiving layer mainly containing a resin soluble in an organic solvent, the insoluble resin being formed from fine particles of a polyolefin resin having a melting point of not lower than 70°C and a heat absorption accompanied by softening or phase transition upon heating of not more than 50 cal/g. 6. An image-receiving sheet for thermal transfer printing according to claim 5, characterized in that the fine particles are present on the polyolefin resin in an amount of not less than 10% by weight based on the total solids in the intermediate layer. 7. An image-receiving sheet for thermal transfer printing according to claim 6, characterized in that the fine particles are present on the polyolefin resin in an amount of not less than 60% by weight based on the total solids in the intermediate layer. 8. An image-receiving sheet for thermal transfer printing according to claim 5, characterized in that the substrate is a paper sheet made mainly from pulp fibers. 9. Image-receiving sheet for thermal transfer printing according to claim 8, characterized in that the paper sheet has a smoothness of not less than 6% and a water absorption of 1 to 30 g/m², determined by the following methods: Smoothness: a value determined using a special reflection smoothness tester at a pressure of 20 kg/cm², and Water absorption: a 5-second value determined according to a "Test method for water absorption of paper and paperboard (Copp method)" described in JIS P-8140. 10. An image-receiving sheet for thermal transfer printing, characterized in that it contains an intermediate layer and an image-receiving layer formed on a substrate in this order, the intermediate layer mainly containing a resin which is insoluble in an organic solvent, and the image-receiving sheet mainly containing a resin which is soluble in an organic solvent, the intermediate layer containing hollow particles in the insoluble resin. 11. An image-receiving sheet for thermal transfer printing according to claim 10, characterized in that the hollow particles are present in an amount of not less than 50% by weight based on the total solids in the intermediate layer. 12. An image-receiving sheet for thermal transfer printing according to claim 10, characterized in that the substrate is a paper sheet made mainly from pulp fibers. 13. Image-receiving sheet for thermal transfer printing according to claim 12, characterized in that the paper sheet has a smoothness of not less than 6% and a water absorption of 1 to 30 g/m², determined by the following methods: Smoothness: a value determined using a special reflection smoothness tester at a pressure of 20 kg/cm², and Water absorption: a 5-second value determined according to a "Test method for water absorption of paper and paperboard (Copp method)" described in JIS P-8140.