EP0547233A1

EP0547233A1 - Thermal transfer ink sheet for forming color image

Info

Publication number: EP0547233A1
Application number: EP92914001A
Authority: EP
Inventors: Hitomi Tech. Ctr. Of Fujicopian Co. Ltd Kawabata
Original assignee: Fuji Kagakushi Kogyo Co Ltd; Fujicopian Co Ltd
Current assignee: Fujicopian Co Ltd
Priority date: 1991-07-06
Filing date: 1992-07-06
Publication date: 1993-06-23
Anticipated expiration: 2012-07-06
Also published as: WO1993001058A1; EP0547233A4; JPH058566A; DE69223751D1; EP0547233B1; DE69223751T2

Abstract

A thermal transfer ink sheet for forming a color image, which can form a clear image even on coarse paper, does not generate reverse transfer during superposition transfer, can make suitable superposition of ink dots and can form a color image with high color reproducibility. To accomplish these objects, the ink sheet of the invention includes: A) a mold release layer (2) consisting of waxes, B) a color ink layer (3) containing a colorant mixed in a vehicle consisting principally of a film-formable thermoplastic resin, and C) a reverse transfer prevention layer (4) consisting of waxes, wherein each of the layers (1 to 3) is disposed on the surface of a subsate (1) in order named. The ink sheet is used suitably for the formation of a color image in indirect thermal transfer.

Description

TECHNICAL FIELD

The present invention relates to a thermal transfer ink sheet for color image formation. More specifically, it relates to a thermal transfer ink sheet for use in formation of color images wherein a yellow ink layer, a magenta ink layer or a cyan ink layer thereof is heated with a plurality of heating elements under selective heating control which are provided in a thermal head, transferred on a minute dot basis to form a yellow image, a magenta image or a cyan image, two or more of these images being superimposed one on the other (hereinafter referred to as "superimposing transfer") to form a color image on a receptor.

BACKGROUND ART

There has hitherto been known a thermal transfer ink sheet wherein on the surface of a film-like foundation is provided a colored ink layer in which a colorant is admixed with a vehicle mainly composed of a wax.
With a thermal transfer ink sheet of this type, however, there has been a problem that although clear images can be obtained on a sheet of paper having a smooth surface, unclear images are likely to be produced on a receptor having a rough surface such as coarse paper because the ink of the ink sheet is hard to reach recessed portions of the uneven surface thereof and the ink present in the portions corresponding to such recessed portions is likely to be untransferred, resulting in the ink dots transferred involving voids, dropout portions or the like.
Attempts have been made to overcome such a problem by using a thermal transfer ink sheet wherein a vehicle mainly composed of a film-formable thermoplastic resin is used for the colored ink layer instead of the vehicle mainly composed of a wax as in the above conventional ink sheet. By thus improving the film-formability of the colored ink layer, transfer of the ink onto a sheet of rough-surface paper has been accomplished with the recessed portions thereof bridged over by the colored ink layer. As a result, clear images with ink dots free from voids or dropout portions have been obtained even on a sheet of rough-surface paper as on a smooth-surface receptor.
Although clear images have come to be obtained, even if the receptor is a sheet of rough-surface paper, by using a vehicle mainly composed of the film-formable thermoplastic resin in the colored ink layer, there has arisen another problem that the colored ink layer will not be readily released from the foundation because of a high affinity between the film-formable thermoplastic resin and the resin of the foundation, resulting in unclear images. This phenomenon is conspicuous when the ink sheet is used in a high-speed printer especially.
This problem has been solved by providing a release layer composed of a wax or the like between the foundation and the colored ink layer for causing the colored ink layer to be readily released from the foundation.
However, attempts by the present inventor to form color images by the superimposing transfer using a thermal transfer ink sheet of the above constitution have revealed frequent occurrences of a phenomenon to be referred to as "countertransfer" hereinafter. The countertranfer is such that when on the ink dots in a certain color (hereinafter referred to as "first color ink") which have been transferred on a receptor are superimposed ink dots in another color (hereinafter referred to as "second color ink"), the color ink of the precedingly transferred first color ink dots is transferred back onto the second color ink layer. According to the examination by the present inventor, the countertransfer phenomenon is presumed to occur in the following manner.
Figs. 5 and 6 are each an explanatory view schematically illustrating the countertransfer phenomenon. Referring to Fig. 5, numeral 20 denotes a thermal transfer ink sheet wherein on a foundation 21 is provided a release layer 22 on which a colored ink layer 23 is provided which is composed of a mixture of a colorant and a vehicle mainly composed of a film-formable thermoplastic resin. Numeral 24 denotes a receptor on which a first color ink dot 25 is transferred. In transferring a second color ink dot onto the first color ink dot 25, supply of excessive heat energy from a heating element 26 of a thermal head to the ink sheet 20 (which would occur in some cases due to heat accumulation in the heating element) would melt or soften the first color ink dot 25 while, at the same time, somewhat softening the color ink layer 23 adjacent the second color ink dot to be transferred. Since the release layer 22 cannot be made so thick for ensuring transfer sensitivity, the release layer 22 transferred together with the first color ink dot 25 is not substantially thick. For this reason there occurs a phenomenon such that the softened, colored ink layer 23 of the first color ink dot 25 adheres directly to the softened, colored ink layer 23 adjacent the second color ink dot. In such a condition, exfoliating the ink sheet 20 from the receptor 24 causes a portion 25' of the colored ink layer 23 of the first color ink dot 25 to be removed therefrom and adhere to the second color ink layer 23 adjacent the location thereof from which the second color ink dot 27 has been removed, as shown in Fig. 6. This phenomenon is the coutertransfer.
If the countertransfer phenomenon occurs, there cannot be obtained an ink dot of a predetermined density because the colored ink is partially removed from the first color ink dot, thereby rendering the color reproducibility poor.
The present inventor has further attempted to form color images in accordance with an indirect transfer method, and found that the countertransfer phenomenon occurs conspicuously.
The aforesaid indirect tranfer method is an image formation method using a device as shown in Fig. 7. Referring to Fig. 7, numeral 30 denotes a rotatable transfer drum of which the surface is formed of an elastic material of good releasing property such as silicone rubber or fluorine-containing rubber. Numeral 31 denotes a recording part which is arranged so that a thermal head 33 can press a thermal transfer ink sheet 32 against the transfer drum 30. The ink sheet 12 is moved in the direction indicated by an arrow as the transfer drum 30 rotates for recording. Numeral 34 denotes a transfer part which is arranged so that a receptor 35 can be pressed against the transfer drum 30 by means of a pressing roller 36. The receptor 35 is fed in the direction indicated by an arrow for the transfer operation.
The thermal head 33 heats the thermal transfer ink sheet 32 so as to soften or melt the ink thereof, which is then transferred onto the surface of the transfer drum 30. While the transfer drum 30 and the ink sheet 32 are thus moved in the directions indicated by the arrows, respectively, the ink is transferred onto the transfer drum 30 so as to form an ink image 37 thereon. The ink image 37 is moved to the transfer part 34 as the transfer drum 30 rotates, pressed against the receptor 35 there, and transferred onto the receptor 35 to form a final ink image 38.
To form color images in accordance with the aforesaid indirect transfer method, ink images different in color need to be superimposed one on the other on the transfer drum. In this case, the transfer drum is always heat at such a temperature as to soften the ink of the ink image thereon for facilitating transfer of the ink image onto the receptor. This causes frequent occurrence of the countertransfer phenomenon. In more detail, when a second color ink dot is to be transferred onto a first color ink dot on the transfer drum, the first color ink dot is already in the softened state. Therefore, the countertransfer phenomenon is likely to occur even when heat energy supplied from the thermal head is not excessive.
In view of the foregoing, it is an object of the present invention to provide a thermal transfer ink sheet for color image formation which is capable of forming clear images on rough-surface paper as well as on smooth-surface paper, preventing the countertransfer to occur upon superimposing transfer, and further, assuring proper superimposing of ink dots, while in addition it can be suitably applied even to the indirect transfer method.

DISCLOSURE OF THE INVENTION

The present invention relates to a thermal transfer ink sheet for color image formation comprising a release layer (A) comprising a wax, a colored ink layer (B) comprising a mixture of a colorant and a vehicle mainly composed of a film-formable thermoplastic resin, and a countertransfer-prevention layer (C) comprising a wax, the layers (A), (B) and (C) being stacked in this order on a foundation.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic sectional view showing an embodiment of the thermal transfer ink sheet for color image formation according to the present invention.
Fig. 2 is a plan view illustrating an example of an arrangement of ink layers of respective colors in the thermal transfer ink sheet according to the present invention.
Fig. 3 is an explanatory view illustrating how superimposing transfer is carried out using the thermal transfer ink sheet according to the present invention.
Fig. 4 is a schematic sectional view showing another embodiment of the thermal transfer ink sheet according to the present invention.
Fig. 5 is an explanatory view for illustrating a countertransfer phenomenon which occurs upon the superimposing transfer using a conventional thermal transfer ink sheet, and showing a state wherein a second color ink layer is superimposed on a first color ink dot which has been precedingly transferred onto a receptor.
Fig. 6 is, like Fig. 5, an explanatory view for illustrating the countertransfer phenomenon, and which shows a state wherein the ink sheet is exfoliated from the receptor.
Fig. 7 is an explanatory view of a device for use in the indirect transfer method.
The thermal trasfer ink sheet for color image formation according to the present invention will be described with reference to the drawings.
Fig. 1 is a schematic sectional view showing an embodiment of the thermal transfer ink sheet according to the present invention. In Fig. 1, numeral 1 denotes a foundation, on one side of which are formed sequentially from the foundation side a release layer 2 composed of a wax, a colored ink layer 3 in which a colorant is mixed with a vehicle mainly composed of a film-formable thermoplastic resin, and a countertransfer-prevention layer 4 composed of a wax.
The colored ink layer 3 usually comprises a yellow ink layer, a magenta ink layer and a cyan ink layer. These color ink layers may be provided on separate foundations, respectively, or on a single foundation of a strip-like shape. Fig. 2 shows an example of the thermal transfer ink sheet wherein ink layers are arranged in such a manner. In Fig. 2 there are arranged on a strip-like foundation 1 a yellow ink layer Y, magenta ink layer M and cyan ink layer C in the longitudinal direction of the foundation 1, which layers Y, M and C are repeatedly disposed in units of U. The yellow ink layer Y is herein meant to include the release layer 2, colored ink layer 3 and countertransfer-prevention layer 4 which are shown in Fig. 1. The same holds true for both the magenta ink layer M and the cyan ink layer C. The order of arrangement of these three color ink layers can be selected as desired. The color ink layers may be disposed in a mutual abutment relation or mutually spaced apart relation, or in a mutually slightly overlapped relation within a range such as not to cause hindrance in practical use. Further, there may be provided a margin in one end or either end portion along the longitudinal direction of the foundation 1 and a marker for controlling the feed of the ink sheet in the margin. In addition the repeating unit of U may incorporate a black ink layer.
Color image formation with use of the above thermal ink sheet is achieved by selectively transferring the yellow ink layer Y, magenta ink layer M or cyan ink layer C onto a receptor to form a separation image in yellow, magenta or cyan and superimposing separation images in respective colors one on the other on the receptor. In this color image formation, intermediate colors other than yellow, magenta and cyan are obtained by subtractive color mixture wherein two or more kinds of ink dots in yellow, magenta and cyan are superimposed one on the other. It should be noted that the order of superimposing the above separation images in respective colors one on the other can be selected as desired.
The thermal transfer ink sheet of the above constitution enjoys the following effects.
Even if the receptor is a rough-surface paper, ink dots can be transferred thereonto as bridging over recessed portions of the uneven surface thereof because of the film-formability of the colored ink layer. Thus, a first color ink dot is directly transferred onto the rough-surface paper with the shape of a heating element completely reproduced. Subsequently a second color ink dot is transferred and superimposed on the flat and smooth first color ink dot. As a matter of course, satisfactory transfer is realized.
Although the colored ink layer has film-formability, interposition of the release layer between the foundation and the colored ink layer allows the colored ink layer to exfoliate well from the foundation and to be transferred.
Further, the aforesaid countertransfer phenomenon can be prevented. This will be explained with reference to Fig. 3. Fig. 3 is an explanatory view illustrating a state where a thermal transfer ink sheet 10 is superimposed on a receptor 11 for transfer, on which a first color ink dot 12 has been transferred.
In this case, even if supply of excessive energy from a heating element 13 to the ink sheet 10 has somewhat softened the colored ink layer 3 in a peripheral portion 14 which is adjacent the second color ink dot to be transferred, as well as the colored ink layer 3 of the first color ink dot 12, the presence of the countertransfer-prevention layer 4 on the colored ink layer 3 in the peripheral portion 14 prevents the colored ink layer 3 of the first color ink dot 12 from directly adhering to the aforesaid colored ink layer 3 in the peripheral portion 14. While the colored ink layer 3 of the first color ink dot 12 adheres to the countertransfer-prevention layer 4 in the aforesaid peripheral portion 14, the adhesive strength between them is weak relative to that between the colored ink layers and, hence, it is unlikely that the colored ink layer 3 of the first color ink dot 12 is partially transferred back onto the ink sheet 10 while adhering to the countertransfer-prevention layer 4 in the peripheral portion 14. Thus, the countertransfer is prevented.
The same mechanism as above works to prevent the countertransfer in the case where color images are formed in accordance with the aforesaid indirect transfer method with use of the thermal transfer ink sheet of the present invention.
Further, when the thermal transfer ink sheet of the present invention is used, the countertransfer-prevention layer composed of a wax provided therein exhibits good adhesiveness to the receptor such as a sheet of paper when heated. Furthermore, when a second color ink dot is transferred onto a first color ink dot, the countertransfer-prevention layer of the second color ink dot which layer is composed of a wax adheres to the release layer of the first color ink dot which layer is composed of a wax, thereby ensuring adhesion between ink dots. As a result, color image formation by the superimposing transfer is achieved satisfactorily.
Next, the present invention will be described specifically.
The aforesaid release layer is composed of a wax. Examples of the wax are natural waxes such as whale wax, bees wax, lanolin, carnauba wax, candelilla wax, montan wax and ceresine wax; petroleum waxes such as paraffin wax and microcrystalline wax; synthetic waxes such as oxidized wax, ester wax, low molecular weight polyethylene and Fischer-Tropsch wax; higher fatty acids such as lauric acid, myristic acid, palmitic acid, stearic acid and behenic acid; higher aliphatic alcohols such as stearyl alcohol and docosanol; esters such as higher fatty acid monoglycerides, sucrose fatty acid esters and sorbitan fatty acid esters; amides such as oleic amide, and bisamides. These waxes may be used singly or in combination of two or more species thereof.
The melting point of the release layer, which is measured with DSC at a heating rate of 10°C /min, is suitably in the range of about 50° to about 100°C, especially about 60° to about 80°C. The melting point below that range results in an ink sheet of poor storage stability, whereas that above the range makes the heat transferability of the colored ink layer degrade.
The release layer is formed by applying, with an appropriate applying means such as bar coater or gravure coater, a solution prepared by dissolving the wax in an organic solvent such as methyl ethyl ketone, toluene or tetrahydrofuran, a dispersion prepared by dispersing the wax in a solvent which does not dissolve the wax, such as methanol or isopropyl alcohol, or an aqueous emulsion prepared by adding an emulsifier to the wax, followed by drying. The coating amount thereof is preferably within the range of about 0.2 to about 2.0 g/m² in terms of solid content.
The aforesaid color ink layer is composed of a mixture of a colorant and a vehicle mainly composed of a film-formable thermoplastic resin. From the viewpoint of ensuring the transfer-sensitivity of the colored ink layer, a desired amount of the ink transferred and a desired degree of the bridging property, the softening point of the vehicle is preferably within the range of about 50° to about 200°C, especially about 80° to about 120°C, and the viscosity of the colored ink layer (a value at a temperature 5° to 20°C above the softening point of the vehicle and which is measured with a rheometer manufactured by Rheology Co., Ltd; hereinafter the same) is preferably within the range of about 10³ to about 10⁸ cP, especially about 10⁴ to about 10⁶ cP. The softening point below the above range degrades the storage stability of the ink sheet, while on the other hand that above the range makes the transfer-sensitivity thereof poor; hence, either of the cases is not preferable. The viscosity lower than the above range degrades the bridging property or causes excessive transfer of ink, with the result that there is likely a blurred or collapsed image or an image with tail in which an ink stain follows the image like a tail. On the other side, the viscosity higher than the range causes uneven transfer, with the result that an image with a void or dropout portion is likely to be produced. Therefore, either of the cases is not preferable.
Examples of the aforesaid film-formable thermoplastic resin include ethylene polymers such as ethylene-vinyl acetate copolymer and ethylene-ethyl acrylate copolymer, diene polymers such as styrene-butadiene copolymer and polybutadiene, acrylic ester polymers such as polyacrylic ester and polymethacrylic ester, polyamide resins, polyester resins, polyurethane resins, and the like. These resins can be used singly or in combination of two or more species thereof. These resins each exhibit good film-formability even when solidified and, hence, have a feature of imparting images formed on a receptor with an excellent scratch or abrasion resistance. Preferably, the thermoplastic resin accounts for 30 to 80% by weight based on the total amount of the solid contents of the colored ink layer.
To enhance the adhesiveness of the colored ink layer with the release layer and countertransfer-prevention layer, the colored ink layer may be incorporated with a glass-like resin as a tackifier such as rosins, hydrogenated rosins, hydrogenated rosin esters, α -pinene resins, terpene resins, cumarone-indene resins, ketone resins, maleic acid resins and phenol resins. The amount of the tackifier to be added is preferably not more than 50% by weight, particularly about 10 to about 40% by weight, based on the total amount of the solid contents of the colored ink layer.
Preferably, the colorants for yellow, magenta and cyan as used in the colored ink layer are transparent ones.
Examples of transparent colorants for yellow include organic pigments such as Naphthol Yellow S, Hansa Yellow 5G, Hansa Yellow 3G, Hansa Yellow G, Hansa Yellow GR, Hansa Yellow A, Hansa Yellow RN, Hansa Yellow R, Benzidine Yellow, Benzidine Yellow G, Benzidine Yellow GR, Permanent Yellow NCG and Quinoline Yellow Lake, and dyes such as Auramine. These colorants may be used singly or in combination of two or more species thereof.
Examples of specific transparent colorants for magenta include organic pigments such as Permanent Red 4R, Brilliant Fast Scarlet, Brilliant Carmine BS, Permanent Carmine FB, Lithol Red, Permanent Red F5R, Brilliant Carmine 6B, Pigment Scarlet 3B, Rhodamine Lake B, Rhodamine Lake Y and Arizalin Lake, and dyes such as Rhodamine. These colorants may be used singly or in combination of two or more species thereof.
Examples of specific transparent colorants for cyan include organic pigments such as Victoria Blue Lake, metal-free Phthalocyanine Blue, Phthalocyanine Blue and Fast Sky Blue, and dyes such as Victoria Blue. These colorants may be used singly or in combination of two or more species thereof.
The term "transparent pigment" is herein meant by a pigment which gives a transparent ink when dispersed in a transparent vehicle.
If the superimposing of the three colors, yellow, magenta and cyan, can hardly give a clear black color, there may be provided a black color ink layer containing a colorant for black such as carbon black, Nigrosine Base or the like. The black color ink layer for this purpose is not adapted for the superimposing with other color ink layer and, hence, need not be necessarily transparent. Nevertheless, the black color ink layer is preferably transparent for the purpose of giving a desired color such as blue black by the superimposing with other color ink layer.
The amount of each colorant to be used is preferably about 2 to about 40% by weight, particuarly about 5 to about 30% by weight relative to the total amount of the solid contents of the colored ink layer.
There may be a case where printed images of poor sharpness are produced because the film-formability of the colored ink layer makes hard the cutting of edge portions of an ink dot upon transfer. In such a case it is preferable to incorporate into the colored ink layer fine powder of a material having poor affinity with the vehicle. Specifically, the fine powder has an average particle size of about 0.1 to about 2 µm and is of silica, calcium carbonate, magnesium carbonate, aluminum hydroxide, clay, talc or the like. Among these, preferable are highly transparent ones. The amount of the fine powder to be added is preferably about 3 to about 30% by weight based on the total amount of the solid contents of the colored ink layer.
Further, the colored ink layer may incorporate a small amount of a wax, for example, about 5 to about 20% by weight based on the total amount of the solid contents thereof for the purpose of improving the transferability and the clearness of printed images. The waxes as used in the aforesaid release layer can be used for such a wax.
In addition, the colored ink layer may be incorporated with a dispersant, an antistatic agent or the like as required.
In the formation of the colored ink layer, a coating liquid is first prepared by dissolving or homogeneously dispersing in an organic solvent the aforesaid thermoplastic resin and colorant and, as required, the tackifier or other additives. In this case there may be employed a dispersant such as fluorine-containing surface active agent, higher alcohol-type surface active agent or phosphate-type surface active agent so as to promote the dispersing of the pigment and the like. The resulting coating liquid is applied on the aforesaid release layer with an appropriate applying means such as roll coater, gravure coater, reverse coater, bar coater or the like. Usually the amount of the coating liquid to be applied is adjusted in the range of 0.5 to 3.0 g/m² on solid basis.
The aforesaid countertransfer-prevention layer is a layer composed of a wax. Any of the waxes usable for the release layer can be used as the wax for the countertransfer-prevention layer. Among these waxes, particularly preferable are microcrystalline wax and polyethylene wax from the viewpoints of improving the countertransfer-prevention property and superimposing transfer property.
The melting point of the countertransfer-prevention layer (a value measured with DSC at a heating rate of 10°C /min) is suitably about 65° to about 100°C, especially about 70° to about 90°C. The melting point thereof below the suitable range results in a countertransfer-prevention layer with a degraded countertransfer-prevention property, whereas that above the range results in a degradation in the transferability onto the receptor and the superimposing recordability.
The countertransfer-prevention layer is formed by applying, with an appropriate applying means such as bar coater or gravure coater, a solution prepared by dissolving the wax in an organic solvent such as methyl ethyl ketone, toluene or tetrahydrofuran, a dispersion prepared by dispersing the wax in a solvent which does not dissolve the wax, such as methanol or isopropyl alcohol, or an aqueous emulsion prepared by adding an emulsifier to the wax, followed by drying. The coating amount thereof is preferably within the range of about 0.2 to about 2.0 g/m², especially about 0.2 to about 1.0 g/m² in terms of solid content. The coating amount less than that range results in a countertransfer-prevention layer with a degraded countertransfer-prevention effect, while on the other hand the amount larger than the range causes to degrade the transferability onto the receptor and the superimposing recordability.
As the foundation as used in the present invention, usable are polyester films, polyamide films, and other various plastic films generally used as a foundation film for ink sheets of this type. When such plastic films are used, it is desired to prevent the ink sheet from sticking to a thermal head by providing on the back side (the side in slide contact with the thermal head) of the foundation a conventionally known stick-preventing layer composed of silicone rubber, fluorine-containing rubber, nitrocellulose resin, any of various lubricative heat resistant resins modified with them, or any of the foregoing heat resistant resins admixed with a lubricant. The foundation and/or the stick-preventing layer may contain an antistatic agent. Further, the foundation may be a thin sheet of paper having a high density such as condenser paper. The thickness of the foundation is preferably about 1 to about 9 µm, especially about 2 to about 4.5 µm for assuring good heat conduction.
In the present invention, it is preferable to control the releasability of the colored ink layer by providing a release control layer between the release layer and the foundation. The release control layer (hereinafter referred to as "control layer") is a layer in which the particles of a powder are dispersed in a resin binder. At least some of the particles project through the release layer into the colored ink layer at their top portion. The particles serve as an anchor relative to the colored ink layer and operatively work in such a direction as to suppress the releasability of the colored ink layer. Consequently, the colored ink layer is imparted with a suitable releasability, thereby giving clear printed images on a sheet of rough-surface paper with use of a high-speed printer. Action for suppressing the releasability of the colored ink layer can be controlled depending on the number and size of the particles projecting into the colored ink layer.
Fig. 4 is a schematic sectional view showing another embodiment of the thermal transfer ink sheet according to the present invention which is provided with the aforesaid control layer. In this ink sheet a control layer 5 is interposed between the foundation 1 and the release layer 2. The control layer 5 is incorporated with powder particles 6, at least some of which are not completely covered with the release layer 2 and are projecting into the the colored ink layer 3 at their top. These particles act to control the releasability of the colored ink layer 3 by their anchoring effect relative to the colored ink layer 3.
The control layer in the present invention is a layer in which powder particles are dispersed in a binder mainly composed of a resin.
The material for the binder is not particularly limited, provided that it shows good adherence to the foundation and will not be melted by momentary heating with heating elements of the thermal head while exhibiting good heat conductivity. Nevertheless, a preferred one has a softening point of 120°C or above. Usually, there are preferably used polyamide resins obtained from dimer acid and various diamines, polyester resins or the like. In addition thereto, usable are styrene-butadiene copolymer, oxidized polyethylene, ketone resins, epoxy resins, polyacrylate resins and the like.
As the powder particles to be added to the control layer, there is required to use those containing at least some particles sized not to be covered with the release layer and to project into the colored ink layer. The number of particles projecting into the colored ink layer is preferably about 1 to about 100, especially about 10 to about 50 per 1 µm² of the control layer. The number thereof larger than that range results in poor releasability of the colored ink layer, with the result that an unclear printed image is likely. On the other hand, the number thereof smaller than the range causes the colored ink layer to be released too easily, with the result that the receptor is likely to be stained with ink. The powder particles may have a particle size distribution to a certain extent or a substantially uniform particle size. Usually, preferably used are those having an average particle size ranging 0.1 to 2.0 µm and at maximum about 4 µm or smaller. The amount of the powder particles to be used depends upon the particle size distribution and the like as stated above but is usually about 10 to about 50% by weight relative to the amount of the binder.
The material for the powder particles is not particularly limited, but examples of the material therefor include inorganic and organic materials such as calcium carbonate, calcium sulfate, magnesium carbonate, amorphous silica, ultra fine powder of an acrylic resin, powder of a melamine resin and the like.
The aforesaid control layer is formed by dissolving the binder in a solvent which will not substantially dissolve the foundation, mixing the powder particles thereto to be sufficiently dispersed therein, and applying the mixture to the foundation with an appropriate applying means such as bar coater or gravure coater. The coating amount of the mixture is about 0.2 to about 2.0 g/m², preferably about 0.4 to about 1.0 g/m² in terms of solid content.
With the thermal transfer ink sheet according to the present invention, clear color images can be formed on a sheet of rough-surface paper having a Bekk smoothness of about 5 to about 20 seconds according to the common thermal transfer method with use of even a high-speed printer. As a matter of course, clear color images can also be formed on a sheet of smooth-surface paper or a plastic film.
Further, the thermal transfer ink sheet according to the present invention can be advantageously used with an indirect transfer method for forming color images. When color images are to be formed according to the indirect transfer method, the aforesaid conventional indirect transfer method is effected as it is without particular modifications except that separation images in yellow, magenta and cyan are superimposed one on the other on the transfer drum and then transferred onto a receptor simultaneously.
The transfer drum is preferably heated at about 60° to about 80°C so as to facilitate transfer of an ink image formed thereon onto the receptor. With the thermal transfer ink sheet according to the present invention, even if the first color ink dot is thus heated on such a transfer drum, countertransfer onto the second color ink layer will not occur. It should be noted that the ink image may be transferred onto the receptor while being heated with a heating roller in the transfer part instead of heating the transfer drum.
When the thermal transfer ink sheet according to the present invention is used with the aforesaid indirect transfer method to form color images, there is an advantage that color images of a good quality can be formed on any receptor, for example, a cloth, regardless of the kind of the receptor.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will be described by way of Experimental Examples.

EXPERIMENTAL EXAMPLES 1 to 7

A release layer was formed on one side of a 4.5 µm-thick and 297mm-wide polyethylene terephthalate film formed at the other side thereof with a 0.1 µm-thick stick-preventing layer composed of a silicone-modified urethane resin by applying thereto an aqueous emulsion of a mixed wax composed of 80 parts by weight of paraffin wax (melting point: 74.5 °C) and 20 parts by weight of microcrystalline wax (melting point: 84°C ), followed by drying. The release layer thus formed had a coating amount of 1.0 g/m² after drying and a melting point of 76.5°C.

In Experimental Example 5 or 6, a control layer was formed on the foundation by applying a coating liquid for control layer of the following composition, followed by drying. The control layer thus formed had a coating amount of 0.4 g/m² after drying. The above release layer was formed on the control layer.

INGREDIENT	% BY WEIGHT
Polyamide resin (acid component: dimer acid) (softening point: 135° to 145°C)	14.0
Calcium carbonate (particle diameter range: 0.1 to 2.0 µm, average particle diameter: 0.8 µm)	6.0
Toluene	80.0

The number of particles to be projected into a colored ink layer was measured by taking an electron micrograph upon application of the release layer on the control layer and counting the number of particles projecting from the release layer in the predetermined area to find the number thereof per unit area. As a result, the number of particles to be projected into the colored ink layer was 31/µm².
Color ink layers for yellow, magenta, cyan and black were formed on the release layer by repeatedly applying each of coating liquids for yellow, magenta, cyan and black ink layers having the compositions shown in Table 1 so as to have each color ink layer of A4 size, followed by drying. The coating amount for each color ink layer was 2.0 g/m² after drying. The arrangement of the color ink layers was as shown in Fig. 1 except for the provision of the black ink layer between the cyan ink layer C and the yellow ink layer Y.

On the color ink layers was formed a countertransfer-prevention layer by applying thereto an aqueous emulsion of the wax shown in Table 2, followed by drying. The coating amount of the resulting countertransfer-prevention layer was 0.3 g/m² after drying. The countertransfer-prevention layer was not provided in Experimental Example 7.

Table 2

Experimental Example	Wax
1	Microcrystalline wax (melting point: 83.6°C)
2	Polyethylene wax (melting point: 86.1°C)
3	Paraffin wax (melting point: 78.0°C)
4	Ester wax (melting point: 75.0°C)
5	Microcrystalline wax (melting point: 83.6°C)
6	Polyethylene wax (melting point: 86.1°C)
7	-

The ink sheets obtained in Experimental Examples 1 to 7 were subjected to printing test using a thermal transfer printer (PC-PR 350, manufactured by NEC Corporation), wherein the printing speed was 50 cps; as the receptor were used paper sheets for thermal transfer (Bekk smoothness: 600 seconds), paper sheets for PPC (Bekk smoothness: 50 seconds) and bond paper sheets (Bekk smoothness: 10 seconds). Further, the ink sheets were also subjected to a test of printing onto a transfer drum of which the surface was composed of silicone rubber and which was heated at 65°C , according to the indirect transfer method.
On the aforesaid receptor or transfer drum was formed a yellow image as the first color ink image, on which a magenta image was then superimposed as the second color ink image. Whereupon, judgement on a visual observation basis was conducted on whether the countertransfer phenomenon occurred or not and on the superimposing transferability. Evaluation was made according to the following ratings and the results are shown in Table 3.

(A) Countertransfer

Visual observation was made on a portion of the ink sheet from which the ink thereof had been removed, and rating was made into the following four stages:

4 No countertransfer phenomenon was recognized;
3 Countertransfer phenomenon was recognized a little;
2 Countertransfer phenomenon was recognized appreciably;
1 Countertransfer phenomenon was recognized conspicuously.

(B) Superimposing transferability

Images of Kanji characters, alphabetic letters and solid portions were subjected to visual observation, and rating was made into the following four stages:

4 The second color ink covered the first color ink completely;
3 A little void was recognized in the second color ink;
2 An appreciable void was recognized in the second color ink;
1 Coverage rate of the second color ink over the first color ink was less than 50%.

(C) Clearness of printed image

Kanji characters and alphabetic letters of the first color ink were magnified 100 times by optical microscope and observed, and rating was made into the following four stages:

4 very clear
3 clear
2 clear to some extent
1 unclear

The same test was conducted while combination of colors for the first color ink layer and second color ink layer was changed variously, and consequently the same results as in Table 3 were given.
With the thermal transfer ink sheet for color image formation according to the present invention, clear images can be formed even on a sheet of rough-surface paper as well as on a sheet of smooth-surface paper. Further, proper superimposing of ink dots can be realized without occurrence of countertransfer, whereby color images can be formed with good color-reproducibility. In addition the thermal transfer ink sheet according to the present invention can also be applied to the indirect transfer method.

Claims

A thermal transfer ink sheet for color image formation comprising:
a release layer (A) comprising of a wax;
a colored ink layer (B) comprising of a mixture of a colorant and a vehicle mainly composed of a film-formable thermoplastic resin; and
a countertransfer-prevention layer (C) comprising of a wax,
the layers (A), (B) and (C) being stacked in this order on a foundation.
The ink sheet of Claim 1, wherein said wax of said countertransfer-prevention layer is a microcrystalline wax or a polyethylene wax.
The ink sheet of Claim 1 or Claim 2, wherein said colored ink layer comprises a yellow ink layer, a magenta ink layer and a cyan ink layer which are arranged in a side-by-side relation on a single foundation.