Heat transfer process and heat transfer ink sheet for use in the process

Abstract: The present invention provides a heat transfer printer, a heat transfer process and a heat transfer ink sheet (1; 11), where a letter or image or both is recorded by selectively heating a heat transfer ink sheet (1, 11) having ink layer regions each containing a sublimable or vaporizable dye by heating by a thermal head controlled by a con­ trolling means, thereby selectively heat transferring the sublimated or vaporized dyes on a recording sheet (17), and is characterized by heat transferring a precoating layer (13) for the dyes from the ink sheet (1,11) onto the recording sheet (17) before the heat transfer recording, and then laying the ink sheet (1,11) on the precoating layer (13) transfered on the recording sheet (17) by the thermal head, thereby sublimating or vaporizing the dyes from the respective ink layers onto the precoating layer (13) and forming the letter or image or both on the recording sheet (17).

Description: BACKGROUND OF THE INVENTION This invention relates to heat transfer recording, where a desired letter or an image or both is to be recorded on a recording sheet by heating, and more particularly to a heat transfer process capable of recording on an ordinary, versatile paper sheet a heat-sublimable or vaporizable dye without using any specific recording sheet, and to a heat transfer ink sheet. The heat transfer ink sheet so far used in the heat transfer printers is such a heat transfer ink sheet that a dye is fixed on a substrate sheet by a binder as an ink layer, where recording is carried out by heating the heat transfer ink sheet, as laid on a recording sheet, by a heating means, thereby selectively transferring the dye as a sublimable or vaporizable dye or a meltable ink in the ink layer onto the recording sheet in accordance with a letter or an image or both. However, in the case of the meltable ink disclosed e.g. by DE-A-3415827, it is hard to obtain a tonal graduation. In recording a full color image having a tonal graduation, generally an ink sheet containing a sublimable or vaporizable dye is widely used. One example each of a conventional heat transfer printer and a heat transfer ink sheet will be described below according to the disclosure of JP-A-57-169370. A heat transfer ink sheet comprises a substrate sheet and an ink layer laid thereon. The substrate sheet (base sheet) is composed of polyethylene terephthalate, etc., and the ink layer is a layer comprising a sublimable or vaporizable dye fixed by a binder resin, etc. A heat transfer printer comprises a platen rubber roller and a thermal head. The heat transfer ink sheet is laid on a recording sheet so that the ink layer of the ink sheet may be in contact with the recording sheet, and passed between the platen rubber roller and the thermal head under a pressing load. At the same time, the contact part is heated by the thermal head, while controlling the heating temperature and the heating time, thereby transferring the dye to the recording sheet from the ink layer. The recording sheet for use in the heat transfer is limited, and a specific sheet is used. That is, a polyester sheet, a polyester-treated sheet, a cross-linked polyester-treated sheet, a sheet treated with a mixture of polyester with polyvinylpyrrolidone, etc. have been used as the recording sheet. In other words, a specific sheet comprising a sheet and a polyester-based resin, etc. readily dyeable with a sublimable or vaporizable dye, as applied thereto, has been used as such a recording sheet. When the commercially available, ordinary, versatile paper sheet is used as recording paper in place of a specific recording sheet together with a heat transfer ink sheet comprising a sublimable or vaporizable dye, the letter or image or both transferred onto the recording sheet by the sublimable or vaporizable dye from the heat transfer ink sheet has or have such problems as being weak and less discriminate, and has or have poor recording preservability. A specific recording sheet has no such problems, but is expensive and is not easily available owing to the limited versatility. JP-A-61-139490 discloses a thermal transfer sheet comprising a sheeted base material, and a thermal transfer layer provided on one surface of the sheeted base material, characterized in that the thermal transfer layer comprises a colorant layer containing a sublimable colorant, and a fixing agent layer containing a synthetic resin capable of causing fixing reactions with the sublimable colorant, and that the colorant layer and the fixing agent layer are juxtaposed on the sheeted base material without overlapping each other. As a fixing agent a polyester may be used. EP-A-192435 discloses a thermal transfer dyesheet having a printing surface against which a receptor substrate may be held to receive a thermally transferable dye in response to thermal stimuli applied to the dyesheet, the dyesheet comprising a substrate supporting a dyecoat comprising the thermally transferable dye dispersed as solid particles throughout a polymeric binder, characterised in that the polymeric binder comprises a thermoset silicone composition as a continuous layer overlying the dyecoat. SUMMARY OF THE INVENTION A first object of the present invention is to provide a process for clearly recording a letter or image or both on an ordinary, versatile paper sheet as a recording sheet in the heat transfer recording, using a sublimable or vaporizable dye. A second object of the present invention is to provide an ink sheet for clearly recording a letter or image or both on an ordinary, versatile paper sheet as a recording sheet in the heat transfer recording, using a sublimable or vaporizable dye, where the heat transfer ink sheet is less adhesive to the recording sheet during the heat transfer. Other objects of the present invention will be comprehensible from the disclosure of the specification. The first object is achieved, according to the present invention, by a printing method for forming a letter and/or image on a recording sheet, as claimed in claim 1. The second object is achieved, according to the present invention, by an ink sheet for use in said printing method, as claimed in claim 2. Heretofor, no clear letter or image has been obtained on an ordinary, versatile paper sheet when using a sublimable or vaporizable dye. The present invention is to improve this disadvantage by forming a precoating layer for receiving a dye on the site to be printed on the recording sheet just before being introduced into the heat transfer printer. Furthermore, the formation of the precoating layer can be carried out in a known heat transfer printer without using any special device, but by improving the ink sheet. In the present invention, an ink sheet has an ink layer region and a precoating layer region provided discretely one after another on the same base sheet, and at first the precoating layer is heat transferred from the ink sheet and deposited onto the site to be printed on an ordinary, versatile paper recording sheet. Then, the ink sheet is moved to place the ink layer region on the precoating layer deposited on the recording sheet and is subjected to heat transfer. That is, the ink sheet has the precoating layer regions and the ink layer regions provided alternately one after another on the base sheet. In the case of multicolored printing, ink layer regions of different colors are arranged in a given succession with respect to the precoating layer regions, and it is desirable that the succession is repeated. The precoating layer formed on the base sheet is composed of an organic material having a heat transfer temperature of 50 to 250° C onto the ordinary, versatile paper sheet. The organic material preferably has at least one of an ester bond and an amide bond. For example, an organic material composed of at least one of polyester resin, polyamide resin, higher fatty acid ester and higher fatty acid amides is used. Polyester resin having a number average molecular weight of 2,000 to 10,000 is particularly preferable. The polyester resin has a good susceptibility to dying and is less adhesive to the ink layer during the image transfer recording. To prevent the adhesion of the ink sheet to the recording sheet during the image transfer recording, an adhesion-preventing layer is formed on the ink layer. As the adhesion-preventing layer, a film comprising a silicone resin is preferable. The desirable film thickness is not more than 1 µm. The adhesion or fusion prevention can be more improved between the ink sheet and the recording sheet thereby. By further forming the silicone resin layer between the precoating layer and the base sheet, transfer and deposition of the precoating layer to the ordinary, versatile paper sheet can be facilitated and also unwanted adhesion to the ink sheet can be prevented during the image transfer recording. As the silicone resin, heat or photo curable type silicone resin is preferable. The base sheet for the ink sheet to be used in the present invention includes, for example, a thin leaf sheet such as condenser paper, etc., and films of polyimide, polycarbonate, acetyl cellulose, polyethylene terephthalate, etc. The base sheet may sometimes undergo sticking due to the heat from a thermal head. To prevent the sticking, a thin film layer of heat-resistant resin such as silicone resin, melamine resin, etc. may be formed on the contact surface side (opposite side to the ink layer-formed side) with the thermal head. Formation of the precoating layer on the recording sheet is carried out by laying the precoating layer region of the ink sheet on the recording sheet under a pressing load and heating the site of the recording sheet at which the precoating layer is to be formed, thereby transferring and depositing the precoating layer onto the recording sheet. It is preferable to make selective heating by means of the thermal head for recording a letter or image or both, thereby forming a precoating layer at the necessary site of the recording sheet for recording the letter or image or both. The ink sheet includes a tape form, a ribbon form, a film form, etc. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a plan view of a heat transfer ink sheet according to one embodiment of the present invention. Fig. 2 is a cross-sectional view along the line II-II of Fig. 1. Fig. 3 is a cross-sectional view along the line III-III of Fig. 1. Figs. 4, 8 and 9 are diagrams showing relationships between the turn-on time and the optical density each for primary colors when the heat transfer ink sheet of the present invention is combined with an ordinary, versatile paper recording sheet. Fig. 5 is a plan view of a conventional heat transfer ink sheet. Fig. 6 is a diagram showing relationship between the turn-on time and the optical density for primary colors when a conventional heat transfer ink sheet is combined with an ordinary, versatile paper recording sheet. Fig. 7 is a diagram showing a relationship between the turn-on time and the optical density for primary colors when a conventional heat transfer ink sheet is combined with a specific recording sheet. Fig. 10 is a schematic view of a heat transfer printer. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The present heat transfer ink sheet, without adhesion-preventing layers being shown, will be described in detail below, referring to Figs. 1 to 3, where Fig. 1 shows a plan view of one embodiment of the present heat transfer ink sheet 1 , Fig. 2 is a cross-sectional view along the line II-II of Fig. 1, and Fig. 3 is a cross-sectional view along the line III-III of Fig. 1. Base sheet 3 is a sheet of polyethylene terephthalate, etc., and precoating layer regions 4 are provided on the base sheet in parallel to one another and in positions adjacent to ink layer regions 2 each containing a sublimable or vaporizable dye of individual primary color. According to the present embodiment, primary colors of Yellow, Magenta and Cyan are used as dyes, and thus Yellow ink layer regions 2Y , Magenta ink layer regions 2M and Cyan ink layer regions 2C are laid discretely on the base sheet 3 . One heat-transferable precoating layer region 4 is provided between a Yellow ink layer region 2Y and a Cyan ink layer region 2C . In this manner, precoating layer regions 4 , Yellow ink layer regions 2Y , Magenta ink layer regions 2M and Cyan ink layer regions 2C are arranged in this succession and in a repeated manner. Fig. 2 is a cross-sectional view along the line II-II of the precoating layer region 4 in Fig. 1, where the precoating layer 4 is laid on the base sheet 3 , and Fig. 3 is a cross-sectional view along the line III-III of Yellow ink layer region 2Y in Fig. 1, where the ink layer 2 containing a heat-sublimable or vaporizable dye and a binder resin is laid on the base sheet 3 . The Magenta ink layer regions 2M and the Cyan ink layer regions 2C have the same structure as shown in Fig. 3. A process for transferring to a recording sheet with the said heat transfer ink sheet will be described below with reference to Fig.10. A heat transfer ink sheet 11 is laid on a recording sheet 17 so that the precoating layer 13 of the heat transfer ink sheet 11 can be in contact with the recording sheet 17 , and then supplied to a heat transfer printer. By heat from a thermal head 18 , or the like as a heating means of the printer, the precoating layer 13 is at first transferred from the heat transfer ink sheet 11 on the recording sheet. Then, the ink sheet 11 is moved forward, and the successive Yellow ink layer region 14 is laid on the transferred precoating layer on the recording sheet 17 , and the sublimable or vaporizable Yellow dye is heat transferred into the transferred precoating layer only in an amount according to an image signal. Then, the sublimable or vaporizable Magenta dye and the sublimable or vaporizable Cyan dye are likewise heat transferred thereto in this succession, and ultimately a desired full color image can be obtained after the dyes of primary colors have been heat transferred into the transferred precoating layer on the recording sheet 17 according to the respective image optical densities. In the foregoing embodiment, a case of recording a full color image by the transfer of Yellow, Magenta and Cyan in this succession has been described, but the order of dyes of primary colors to be transferred is not limited, as long as the precoating layer 13 is at first transferred to a recording sheet 17 . Furthermore, the present invention is not limited to color recording, but can be applied to recording with only sublimable or vaporizable black dye. As described above, the precoating layer 13 is first transferred to a recording sheet 17 , and then sublimable or vaporizable dyes are transferred and received by the transferred precoating layer in the present invention, and thus an ordinary, versatile paper sheet can be used in the present invention as a recording sheet 17 . As the dye for the ink layer 2 or 14 , 15 and 16 , those which can be sublimated or vaporized by heat from a thermal head 18 , etc. can be used and include, for example, C.I. (Color Index) Disperse yellow 16, C.I. Solvent Red 146, C.I. Solvent Blue 36 and TS Blue 603 (products made by Sumitomo Kagaku Kogyo K.K., Japan), Kayalon Fast Orange, Kayalon Fast Blue and Kayalon Fast Green (products made by Nihon Kayaku K.K., Japan), Dianix Fast Yellow (a product made by Mitsubishi Kasei Kogyo K.K., Japan), Aizen QZ Red (a product made by Hodogaya Kagaku Kogyo K.K., Japan), etc. Furthermore, quinazoline dye, disperse monoazo dye, disperse anthraquinone dye, disperse nitrodiphenylamine dye, smoke dye, etc. can be also used. As a binder for fixing these dyes to the base 3 or 12, a resin having a high adhesiveness to the base sheet 3 or 12 is used, and particularly polyester resin, polyamide resin, cellulose resin, etc. are preferable. Materials for the precoating layer 4 or 13 are preferably those having a softening temperature or melting temperature of 50° to 250° C. The heat transfer ink sheet 1 or 11 is preserved usually in a rolled state, and thus if the softening temperature or the melting temperature of the precoating layer 4 or 13 is below 50° C, there are such problems that the precoating layer 4 or 13 may be transferred to the back of the base sheet 3 or 12 or may become adhesive during the preservation or transportation, whereas if it exceeds 250° C, it will be difficult to heat transfer the precoating layer 3 or 13 from the heat transfer ink sheet 1 or 11 to the recording sheet 17 by the same amount of heat as that used when an image is recorded. Needless to say, it is possible to solve this problem by increasing the amount of heat from a thermal head 18 , but such measures are not preferable, because of increased power consumption of the heat transfer printer, decreased life of the thermal head 18 , occurrence of sticking phenomena, etc. Specific materials for the precoating layer 4 or 13 preferably comprise compounds having at least one of an ester bond and an amide bond in the molecule as the main constituent, whereby the precoating layer 4 or 13 is readily dyed with a sublimable or vaporizable dye. The precoating layer 4 or 13 can contain appropriate amounts of an antistatic agent, an ultraviolet absorbing agent, metal powder capable of increasing the heat conductivity, a separation-facilitating agent capable of increasing transferability, and other additives to meet other requirements. In the heat transfer printer, a means for heating the present heat transfer ink sheet 1 or 11 in contact with the recording sheet 17 is not limited to a thermal head 18 , and a laser device, a thermal pen, a thermal printing type, etc. can also be used. The heat transfer ink sheet 1 or 11 and the recording sheet 17 are heated while controlling the heating temperature and the heating time of the heating means by a controller 26 according to the colors and optical densities of the original image. In the present invention, a clear full color image can be recorded on an ordinary, versatile recording paper sheet 17 . For the heat-transferable precoating layer 4 or 13 , polyester resin having a number average molecular weight of 2,000 to 10,000 and a softening temperature of 80° to 150° C is preferable. Below 80° C, the heat transfer ink sheet 1 or 11 having such a precoating layer 4 or 13 has a poor preservability, whereas above 150° C the heat transferability of the precoating layer 4 or 13 is deteriorated. When the polyester resin is used as a precoating layer 4 or 13 to be transferred to an ordinary, versatile paper sheet 17 , a very good heat transferability to the sheet 17 can be obtained. Furthermore, the precoating layer 4 or 13 of the polyester resin can be readily dyed with a sublimable or vaporizable dye, and thus a high quality image equivalent to that obtained on a specific recording sheet provided with a dye-receiving layer on the base sheet in advance can be obtained. An adhesion-preventing layer (not shown) has to be provided on the ink layer 2. The thickness of the adhesion-preventing layer is preferably not more than 1 µm. Above 1 µm, sublimation or vaporization and diffusion of sublimable or vaporizable dye from the ink layer 2 to the transferred precoating layer on the recording sheet 17 become difficult during the heat transfer recording, resulting in unclear printing of low optical density. If no adhesion-preventing layer is provided on the ink layer 2 , trouble may occur during the heat transfer recording of a sublimable or vaporizable dye from the ink layer 2 into the transferred precoating layer on the recording sheet 17 . That is, the precoating layer 4 or 13 is composed of a heat-transferable material, i.e. a heat-melting or softening material, and thus may be melted or softened by the heat for heat transferring the sublimable or vaporizable dye, resulting in adhesion of the transferred precoating layer to the ink layer 2 . The adhesion force depends upon the species of the binder contained in the ink layer 2 , the species of a material as a constituent for the precoating layer 4 or 13 , heat transfer conditions for sublimable or vaporizable dye, etc., and too high a adhesion force makes it difficult to separate the ink sheet 1 or 11 from the recording sheet 17 , resulting in peeling of the ink layer 2 from the base sheet 3 or 12 (as called "abnormal transfer"), or peeling of the transferred precoating layer from the recording sheet 17 , or failure of ink sheet 1 or 11 travelling or breakage of the ink sheet 1 or 11 . When the adhesion-preventing layer is provided on the ink layer 2 , the transferred precoating layer on the recording sheet 17 will have no stronger adhesion force because of the non-adhesion action of the adhesion-preventing layer, even if the transferred precoating layer is melted or softened. Likewise, the transferability of a precoating layer 4 or 13 depends upon the species of a material as a constituent for the precoating layer 4 or 13 , the species of the recording sheet 17 , conditions for heat transfer, etc., when the precoating layer is heat transferred from the base sheet 3 or 12 on the recording sheet 17 . Thus, in some cases the transfer of the precoating layer 4 or 13 is incompletely carried out, depending upon these conditions. Furthermore, when the precoating layer 4 or 13 is less separable from the base sheet 3 or 12 or when the adhesion is strong between the recording sheet 17 and the precoating layer 4 or 13 , separation of the ink sheet 1 or 11 from the recording sheet 17 becomes difficult after the transfer of the precoating layer 4 or 13 , resulting in failure of ink sheet travelling or breakage of the ink sheet 1 or 11. To solve these problems, a separation-facilitating layer (not shown) is also provided between the base sheet 3 or 12 and the precoating layer 4 or 13, whereby the precoating layer 4 or 13 can be always stably and uniformly heat transferred on the recording sheet 17, and adhesion of the heat transfer ink sheet 1 or 11 to the recording sheet 17 can be prevented. Thus, the travelling trouble of the ink sheet 1 or 11 can be also overcome thereby. Among the following Examples 1 to 8, only the Examples 7 and 8 form part of the invention. Examples 1-6 are reference examples not falling within the scope of the claims. Example 1 Figs. 1, 2, 3 and 10 are schematic views showing one embodiment without adhesion-preventing layers being shown, where numeral 1 or 11 is a heat transfer ink sheet, which comprises a substrate 3 or 12 in a sheet form, composed of polyethylene terephthalate and heat-transferable precoating layer regions 4 or 13 , Yellow ink layer regions 2Y or 14 , Magenta ink layer regions 2M or 15 and Cyan ink layer regions 2C or 16 provided alternately in succession on the surface of the substrate 3 or 12 . The precoating layer 4 or 13 was composed of polyamide resin having a softening temperature of about 110°C (Versamid 930, a product made by Henkel Japan K.K., Japan). The Yellow ink layer 2Y or 14 was composed of a mixture of one part by weight of C.I. (color index) Disperse Yellow 16 as a yellow sublimable dye and 2 parts by weight of polyester resin (Vylon 290, a product made by Toyobo K.K., Japan) as a binder resin. The Magenta ink layer 2M or 15 had the same composition as that of the Yellow ink layer 2Y or 14 , except that one part by weight of C.I Solvent Red 146 was used as a Magenta sublimable dye in place of the yellow dye. The Cyan ink layer 2C or 16 had the same composition as that of the Yellow ink layer 2Y or 14 except that one part by weight of C.I Solvent Blue 36 was used as a Cyan sublimable dye in place of the yellow dye. The heat transfer ink sheet 1 or 11 was subjected to recording together with a Xerox 4024 paper (Bekk smoothness: about 50 seconds), which was a most popular recording sheet 17 for electrophotographic copying, by applying a voltage of 6.5 V to a thermal head 18 having many heatgenerating resistor elements (electrical resistance: 210 Ω; element density: 6 elements/mm). The whole precoating layer region 4 or 13 could be transferred from the heat transfer ink sheet 1 or 11 on the recording sheet 17 for 20 m sec., time of passing a current to the thermal head 18 (turn-on time), per line of the thermal head 18. From the Yellow ink layer region 2Y or 14 , Magenta ink layer region 2M or 15 , and the Cyan ink layer region 2C or 16 could be transferred Yellow, Magenta and Cyan, respectively, in this succession for a turn-on time of less than 30 m sec. according to the respective image signals, whereby a clear full color image could be ultimately recorded on the ordinary, versatile paper sheet 17 . Relationships between the turn-on time for heating and the optical density of recorded images when the precoating layers 4 or 13 were transferred from the heat transfer ink sheet 1 or 11 onto the said recording sheet 17 and Yellow, Magenta and Cyan dyes were transferred thereinto singly are shown in Fig. 4, where curve Y shows Yellow, curve M Magenta and curve C Cyan. Each color had a high tonal graduation, and clear, high quality recordings could be obtained in this Example. Comparative Example 1 A conventional heat transfer ink sheet 5 having no precoating layer was used, and its structure is shown in Fig. 5 by way of a plan view, where a heat transfer ink sheet 5 comprises a base sheet and Yellow ink layer regions 2Y , Magenta ink layer regions 2M , and Cyan ink layer regions 2C provided alternately in succession on the surface of the base sheet. The heat transfer ink sheet 5 was subjected to recording in the same manner as in Example 1 together with the same Xerox 4024 paper as used in Example 1 as a recording sheet. Relationships between the turn-on time for heating and the optical density of recorded images for each of Yellow, Magenta and Cyan dyes are shown in Fig. 6, where curve Y shows Yellow, curve M Magenta and Curve C Cyan. It can be seen from comparison with the results of the present heat transfer ink sheet 1 or 11 shown in Fig. 4 that each of the colors had a lower optical density and no practical recording could be obtained without using the precoating layer 4 or 13 of the present invention. Furthermore, a color image obtained by transferring primary colors one upon another had a low optical density and only an obscure recording could be obtained without the precoating layer 4 or 13 . With a specific sheet for sublimation type heat transfer as a recording sheet 17 (polypropylene-based synthetic sheet coated with polyester resin) and with the ink sheet 5 as shown in Fig. 5, relationships between the turn-on time for heating and the optical density of recorded images for each of the primary colors were investigated, and the results are shown in Fig. 7. It is obvious from Fig. 7 that the corresponding curves of Fig. 7 and Fig. 4 are substantially identical with each other, and thus the present invention can attain an effect equivalent to that obtained with the specific recording sheet. Example 2 With the same heat transfer ink sheet 1 or 11 as used in Example 1 except that the mixture of 6 parts by weight of polyamide resin having a softening temperature of about 120° C (Versamid 756, a product made by Henkel Japan K.K., Japan) and 4 parts by weight of glycerol monostearate ester having a melting temperature of about 70°C (Excel T-95, a product of Kao Sekken K.K., Japan) was used as the material for the precoating layer 4 or 13 on the heat transfer ink sheet 1 or 11 of Example 1, and with an ordinary paper recording sheet 17 widely used for melting type heat transfer (TKP-13, a product made by Kanzaki Seishi K.K., Japan; Bekk smoothness: about 170 seconds), recording was carried out in the same manner as in Example 1. Relationships between the turn-on time for heating and the optical density of recorded images for each of the primary colors are shown in Fig. 8. It can be seen therefrom that a substiantially equivalent recording to that of Fig. 7 could be obtained. In Fig. 8, curve Y shows Yellow, curve M Magenta and curve C Cyan. A clear color image could be obtained by recording of the primary colors one upon another. Example 3 With the same heat transfer ink sheet 1 or 11 as used in Example 1 except that ester wax having a melting temperature of about 100° C (Kao Wax 230-2, a product of Kao Sekken K.K., Japan) was used as a material for the precoating layer 4 or 13 of the heat transfer ink sheet 1 or 11 , and with an official postal card (Bekk smoothness: about 30 seconds) as a paper recording sheet 17 , recording was carried out in the same manner as in Example 1. Relationships between the turn-on time for heating and the optical density of recorded images for each of the primary colors are shown in Fig. 9 where curve Y shows Yellow, curve M Magenta and curve C Cyan. It can be seen therefrom that substantially the same results as in Fig. 7 could be obtained. Example 4 An ink sheet 1 or 11 was prepared by providing precoating layer regions 4 or 13 and ink layer regions 2 or 14 , 15 and 16 on one side of a polyethylene terephthalate sheet as a base sheet 3 or 12 , 6 µm thick, provided with a heat cured silicone resin layer (KS-722, a product made by Shinetsu Kagaku K.K., Japan) on the other side. That is, the precoating layer 4 or 13 was formed by applying a solution of 3 parts by weight of polyester resin having a number average molecular weight of 6,000 and a softening temperature of 102° C (ATR-2005, a product made by Kao Sekken K.K., Japan) in 7 parts by weight of tetrahydrofuran to the polyethylene terephthalate sheet 3 or 12 , followed by drying. The ink layers 2 or 14 , 15 and 16 were formed by applying a solution of 1 part by weight of a sublimable dye and 2 parts by weight of polyester resin (Vylon 290, a product made by Toyobo K.K., Japan) in 27 parts by weight of tetrahydrofuran to the polyethylene terephthalate sheet 3 or 12 , followed by drying. The sublimable dye was C.I. Disperse Yellow 3 (Kayaset Yellow 937, a product made by Nihon Kayaku K.K., Japan) for Yellow ink, C.I. Disperse Violet 17 (Kayaset Red 130, a product made by Nihon Kayaku K.K., Japan) for Magenta ink, and C.I. Solvent Blue 36 (Kayaset Blue 136, a product made by Nihon Kayaku K.K., Japan) for Cyan ink. With the thus obtained heat transfer ink sheet 1 or 11 and with the same Xerox 4024 paper as used in Example 1 as a recording sheet 1 7, recording was carried out with a heat transfer printer having a thermal head 18 (element density: 6 elements/mm). Transfer of the precoating layer 4 or 13 from the ink sheet 17 to the recording sheet was carried out by applying a voltage of 7.0 V to the thermal head 18 for a turn-on time of 20 m sec. per line of the thermal head 18 . The precoating layer 4 or 13 could be uniformly transferred from the base sheet 3 or 12 of the heat transfer ink sheet 1 or 11 on the recording sheet 17 . Then, Yellow, Magenta and Cyan dyes were heat transferred from the ink sheet 1 or 11 into the transferred precoating layer on the recording sheet 17 in this sequence according to the respective image signals, whereby a clear full color image could be obtained on the recording sheet 17 . Relationships between the turn-on time for heating and the optical density of recorded image when the individual dyes were heat transferred singly from the respective Yellow, Magenta and Cyan ink layers 2Y , 2M and 2C or 14 , 15 and 16 into the respective precoating layers transferred on the recording sheet 17 according to the present Example are given in Table 1. Dye Turn-on time (m sec.) Optical density Example 4 Conventional Example Comparative Example Yellow 10 0.25 0.28 0.04 15 0.43 0.45 0.08 20 0.68 0.71 0.14 Magenta 10 0.41 0.40 0.05 15 0.93 0.92 0.10 20 1.28 1.29 0.19 Cyan 10 0.46 0.45 0.06 15 0.94 0.97 0.13 20 1.40 1.41 0.21 (Voltage applied: 7.0 V) It can be seen from Table 1 that the recorded images according to the present Example had substantially equivalent optical densities to those obtained with the conventional specific sheet (Conventional Example) and have considerably higher optical densities than those of Comparative Example wherein no precoating layer is used, and Xerox 4024 paper as the recording sheet is used. Thus, a clear recording of high density could be obtained by heat transferring a precoating layer 4 or 13 onto the recording sheet 17 in the present invention even if an ordinary, versatile paper sheet was used as a recording sheet 17 . Example 5 Heat transfer recording was carried out in the same manner as in Example 4, except that polyester resin having a number average molecular weight of 3,400 and a softening temperature of 105°C (ATR-2009, a product made by Kao Sekken K.K., Japan) was used as a material for the precoating layer 4 or 13 and (TKP-13 paper (a product made by Kanzaki Seishi K.K., Japan; Bekk smoothness: about 280 seconds) was used as a recording sheet 17 in place of the material for the precoating layer 4 or 13 and the recording sheet 17 of Example 4, respectively. The precoating layer 4 or 13 could be uniformly heat transferred on the recording sheet 17 , and Yellow, Magenta and Cyan dyes could be also transferred into the transferred precoating layer on the recording sheet 17 according to the respective image signals, whereby a clear image could be recorded. Separately, when Yellow, Magenta and Cyan dyes were heat transferred singly into the respective precoating layers transferred on the recording sheet 17 , images of equal or a little higher optical density than that of Example 4 could be obtained. Comparative Example 2 Heat transfer recording was carried out in the same manner as in Example 5 except that polyester resin having a number average molecular weight of 20,000 and a softening temperature of 158° C (Vylon 103, a product made by Toyobo K.K., Japan) was used as a material for the precoating layer 4 or 13 . About half of the precoating layer region 4 or 13 (total area) could not be heat transferred on the recording sheet 17 , and remained on the base sheet 3 or 12 of the heat transfer ink sheet 1 or 11 . That is, the precoating layer 4 or 13 could not be uniformly heat transferred onto the recording sheet 17 . Example 6 Heat transfer recording was carried out in the same manner as in Example 4 except that polyester resin having a number average molecular weight of 7,400 and a softening temperature of 136° C (ATR-2010, a product made by Kao Sekken K.K., Japan) was used as a material for the precoating layer 4 or 13 and a bond paper having a low surface smoothness (Bekk smoothness: about 5 seconds) (13C, a product made by Southworth Co., USA) was used as a recording sheet 17 . The precoating layer 4 or 13 could be uniformly heat transferred on the recording sheet 17 , and then Yellow, Magenta and Cyan dyes could be heat transferred into the transferred precoating layer on the recording sheet 17 according to the respective image signals, whereby a clear image could be recorded. Separately, when Yellow. Magenta and Cyan dyes were heat transferred singly into the respective precoating layers transferred on the recording sheet 17 , images of equal or a little lower optical density than that of Example 4 could be obtained. Comparative Example 3 Heat transfer recording was carried out in the same manner as in Example 6, except that polyester resin having a number average molecular weight of 2,300 and a softening temperature of 180°C (Vylon 200, a product made by Toyobo K.K. Japan) was used as a material for the precoating layer 4 or 13 . Almost all of the precoating layer 4 or 13 could not be heat transferred on the recording sheet 17 and remained on the base sheet 3 or 12 of the heat transfer ink sheet 1 or 11 . Example 7 A heat transfer ink sheet 1 or 11 , according to the invention, was prepared by providing separation-facilitating layer regions on one side of a polyethylene terephthalate sheet as a base sheet 3 or 12 , 6 µm thick, having an ultraviolet-cured silicone resin layer (X-62-7245, a product made by Shinetsu Kagaku K.K., Japan) on the other side, providing precoating layer regions 4 or 13 on the separation-facilitating layer regions and Yellow ink layer regions 2Y or 14 , Magenta ink layer regions 2M or 15 and Cyan ink layer regions 2C or 16 on the remaining parts of the polyethylene terephthalate sheet 3 or 12 without the separation-facilitating layer alternately in this sequence thereon, as shown in Fig. 1, and further providing adhesion-preventing layers only on the surfaces of the individual ink layer regions 2Y , 2M and 2C or 14 , 15 and 16 . That is, the separation-facilitating layers as the underlayer for the precoating layer regions 4 or 13 were formed by applying a mixture composed of 10 parts by weight of a solution containing 10% by weight of silicone (KS-772, a product made by Shinetsu Kagaku K.K., Japan) in toluene and 0.5 parts by weight of a solution containing 0.5% by weight of a curing catalyst (CL-3, a product made by Shinetsu Kagaku K.K., Japan) in n-hexane to the base sheet (polyethylene terephthalate sheet) 3 or 12 , followed by dying at room temperature and heating at a temperature of 80° to 100° C for 3 minutes. The precoating layers 4 or 13 were formed by applying a solution containing 30% by weight of polyester resin having a softening temperature of 155° C (Vylon 600, a product made by Toyobo K.K., Japan) in tetrahydrofuran to the separation-facilitating layers, followed by drying. The same inks as in Example 4 were used for the ink layers 2 or 14 , 15 and 16 , except that C.I. No. Disperse Red 59 (Kayaset Red 026, a product made by Nihon Kayaku KK., Japan) was used for the Magenta ink. The adhesion-preventing layers on the ink layer regions 2 or 14 , 15 and 16 of primary colors were formed by applying a solution containing 5% by weight of ultraviolet-curable type silicone (X-62-7223, a product made by Shinetsu Kagaku K.K., Japan) in n-hexane to the surfaces of the ink layer regions 2 or 14 , 15 and 16 , followed by drying and irradiation of ultraviolet rays from a 120-W mercury lamp for 10 seconds. The thus formed adhesion-preventing layers had a thickness of about 0.5 µm. With the thus prepared heat transfer ink sheet 1 or 11 and with a Xerox 4024 paper (Bekk smoothness: about 50 seconds) as a recording sheet 17 , heat transfer recording was carried out by means of a heat transfer printer. Under such conditions for heat transfering the precoating layer 4 or 13 on the recording sheet 17 that the voltage applied to the thermal head 18 was 7.5 V and the turn-on time per line of the thermal head 18 was 20 m sec., the precoating layer 4 or 13 could be uniformly heat transferred from the ink sheet 1 or 11 onto the recording sheet 17 . The ink sheet 1 or 11 could be smoothly separated from the recording sheet 17 after the heat transfer. Then, the ink layer regions 2 or 14 , 15 and 16 of the heat transfer ink sheet 1 or 11 were laid on the transferred precoating layer on the recording sheet 17 , and Yellow, Magenta and Cyan dyes were heat transferred and received in this succession thereinto according to the respective image signals under a voltage of 7.5 V applied to the thermal head 18 , whereby a clear full color image was obtained. The heat transfer ink sheet 1 or 11 could be smoothly separated from the recording sheet 17 after the heat transfer recording, and no such an abnormal transfer phenomenon that the ink layers 2 or 14 , 15 and 16 adhered to the recording sheet 17 and peeled from the base sheet 3 or 12 of the heat transfer ink sheet 1 or 11 and stayed on the recording sheet 17 was observed. Separately, the precoating layers 4 or 13 were heat transferred on the recording sheet 17 and then the sublimable dyes were heat transferred singly from the respective Yellow, Magenta and Cyan ink layers 2Y , 2M and 2C or 14 , 15 and 16 into the respective precoating layer transferred on the recording sheet 17 . the relationships between the turn-on time for heating per line of the thermal head 18 and the optical density of the transferred images thus obtained are given in the following Table 2. Dye Turn-on time (m sec.) Optical density Example 7 Yellow 10 0.29 15 0.49 20 0.76 Magenta 10 0.48 15 1.02 20 1.37 Cyan 10 0.51 15 1.04 20 1.50 (Voltage applied: 7.5 V) Comparative Example 4 A heat transfer ink sheet 1 or 11 was prepared in the same manner as in Example 7 except that no ahesion-preventing layers were provided on the surfaces of the ink layer regions 2 or 14 , 15 and 16 . The precoating layer 4 or 13 was heat transferred from the ink sheet 1 or 11 on the recording sheet 17 and then the sublimable dyes were transferred thereinto from the respective ink layers 2Y , 2M and 2C or 14 , 15 and 16 in the same manner as Example 7. The ink layers 2Y , 2M and 2C or 14 , 15 and 16 and the precoating layer adhered to each other owing to the absense of the adhesion-preventing layers on the surfaces of the ink layer regions 2Y , 2M and 2C or 14 , 15 and 16 , and the ink sheet 1 or 11 could not be separated from the recording sheet 17 in the heat transfer printer. When the ink sheet 1 or 11 was forcedly separated from the recording sheet 17 , parts of the ink layers 2Y , 2M and 2C or 14 , 15 and 16 were peeled off the base sheet 3 or 12 and remained on the recording sheet 17 . Example 8 As shown in Fig. 10, an ink sheet 11 having precoating layer regions 13 formed in the same manner as in Example 7 and rolled in a roller 21 where precoating layer regions 13 Yellow ink layer regions 14 , Magenta ink layer regions 15 and Cyan ink layer regions 16 were provided on a base sheet 12 alternately in this succession, was subjected to heat transfer in a heat transfer printer having a transfer mechanism as shown in Fig. 10. A platen roller 19 and an ink sheet winding roll 21 ' were driven by motors 20 and 22 , respectively, driven by external signals to lay the precoating layer region 13 of the ink sheet 11 on a recording sheet 17 composed of an ordinary, versatile paper sheet. Then, the ink sheet 11 is pressed by a thermal head 18 and the image recording region of the ink sheet 11 is selectively heated by the thermal head 18 controlled by an external signal. By the selective heating of the ink sheet 11 , the precoating layer 13 was peeled off the base sheet 12 and transferred onto the image recording region on the recording sheet 17 . Then, the motor 20 was driven to make one turn of the recording sheet 17 by the platen roller 19 , and at the same time the motor 22 was driven to move the ink sheet 11 in the arrow direction to lay the Yellow ink layer region 14 on the transferred precoating layer on the recording sheet 17 . The ink sheet 11 was pressed by the thermal head 18 and the Yellow ink layer region 14 of the ink sheet 11 was selectively heated while making the thermal head selectively heat by a controller 26 based on an external signal. The Yellow dye in the ink layer region 14 was sublimated thereby to form an image of the dye in the transferred precoating layer on the recording sheet 17 . Then, the recording sheet 17 was made one turn and at the same time the ink sheet 11 was moved in the arrow direction in the same manner as above to lay the Magenta ink layer region 15 on the Yellow image-recorded precoating layer on the recording sheet 17 . An image of Magenta dye was formed in the precoating layer thereby. Likewise, an image of Cyan dye was formed in the precoating layer. A clear full color image could be thus formed and recorded on the recording sheet 17 . The ink sheet 11 could be used to smoothly form successive images on the recording sheet 17 without sticking to the thermal head 18 or adhering the recording sheet 17 . A clear letter or image can be recorded on an ordinary versatile paper sheet as a recording sheet 11 , using the present heat transfer ink sheet 1 or 11 . Furthermore, precoating layers 4 or 13 can be formed only at the necessary parts on the recording sheet 17 , and thus other parts can remain as the ordinary, versatile paper sheet, and thus it is possible to make additional writing or ordinary printing or others on the remaining ordinary, versatile paper sheet parts. Thus, the present invention has many additional industrial values.

Claims: A printing method for forming a letter and/or image on a recording sheet, which comprises the steps of (1) bringing an ink sheet comprising a base sheet and an ink layer region containing at least one heat-sublimable or vaporizable dye formed on the base sheet into contact with a recording sheet, and (2) heating the ink layer region by a thermal head to effect transfer of the dye by sublimation or vaporization to the surface of the recording sheet, wherein prior to the transfer of the dye, a pre-coating layer region of a material for receiving the dye, which precoating layer region is formed on the surface of the base sheet, adjacent to or next to the ink layer region, is transferred by heat to the surface of the recording sheet, wherein the material of the precoating layer region is composed of at least one organic material selected from polyester resin, polyamide resin, higher fatty acid ester and higher fatty acid amide, separation of the precoating layer is facilitated by providing a second lubricant layer between the base sheet and the precoating layer, and after the transfer of the precoating layer region, the dye is transferred through a first lubricant layer region for preventing adhesion of the ink layer region to the transferred precoating layer. An ink sheet for use in the printing method according to claim 1, comprising a base sheet (12), an ink layer region (2C, 2M, 2Y, 14, 15, 16) containing at least one sublimable or vaporizable dye, which is formed on the surface of the base sheet (12) wherein a precoating layer region (4, 13) to be transferred to the recording sheet (17) and for then receiving the dye on the base sheet (12) and being formed on the base sheet (12) adjacent to or next to the ink layer region (2C, 2M, 2Y, 14, 15, 16), the precoating layer being composed of at least one organic material selected from polyester resin, polyamide resin, higher fatty acid ester and higher fatty acid amide, a first lubricant layer region for preventing adhesion of the transferred precoating layer (4, 13) to the ink layer region (2C, 2M, 2Y, 14, 15, 16) being formed on the ink layer region, and a second lubricant layer region for assisting the separation of the precoating layer region from the base sheet is formed between the precoating layer region and the base sheet.

Inventors: AKASAKA SHINIICHI, HATTORI SHINTAROO, OHARA SHUICHI, OKA HIROYUKI, OYAMA RIEKO, AKASAKA, SHINIICHI, HATTORI, SHINTAROO, OHARA, SHUICHI, OKA, HIROYUKI, OYAMA, RIEKO
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1987
Main classification: B41J2
IPC codes: B41M5, B41J2, B41J31, B41J17