Process for reheating dye-receiving element containing stabilizer

Abstract: A process is described for reheating a dye image-receiving element containing a thermally-­ transferred dye image and stabilizer. Stratifica­ tion of the transferred dye image in the receiving element is thereby reduced, resulting in an increase in stability to light of the transferred dyes.

Description: This invention relates to a process for forming a stable dye transfer image. In recent years, thermal transfer systems have been developed to obtain prints from pictures which have been generated electronically from a color video camera. According to one way of obtaining such prints, an electronic picture is first subjected to color separation by color filters. The respective color-separated images are then converted into electrical signals. These signals are then operated on to produce cyan, magenta and yellow electrical signals. These signals are then transmitted to a thermal printer. To obtain the print, a cyan, magenta or yellow dye-donor element is placed face-to-face with a dye-receiving element. The two are then inserted between a thermal printing head and a platen roller. A line-type thermal printing head is used to apply heat from the back of the dye-donor sheet. The thermal printing head has many heating elements and is heated up sequentially in response to the cyan, magenta and yellow signals. The process is then repeated for the other two colors. A color hard copy is thus obtained which corresponds to the original picture viewed on a screen. The thermal transfer system described above utilizes differentially applied heating power for image discrimination. This means that low density image areas are heated less than high density areas in order to transfer less dye from the dye-donor element to the dye-receiving element. Since the time of heating is very short (generally less than 5 msec), thermal equilibrium is usually not attained. Thus a thermal gradient exists, the lower depths of the dye-receiving layer being less heated than near the exterior surface. These inherent factors of thermal dye transfer printing can lead to various problems. One problem that has developed with the above-described thermal transfer system is that dye stratifies at the exterior surface of the dye-receiving layer. This is especially evident in lower density areas where the dye appears to be primarily near the surface of the dye-receiving layer. This dye stratification accentuates light stability problems and the possibility of "retransferring" the dye to another undesired surface. Extreme stratification can also lead to changes in the covering power of the dye and may even give the dye an undesirable appearance of a metallic, golden sheen. EP-A- 0 097 493 describes a method for transferring a dye precursor (electron donor) imagewise to a receiver containing an electron acceptor to form a dye. A reheating step is used to facilitate the colored complex formation. Japanese patent publication J59/182785 and EP-A- 0 147 747 describe various stabilizers, including a variety of multialkoxy derivatives, useful in thermal dye transfer systems. While these stabilizers have been found useful for their intended purpose, it is an object of this invention to find a way to further increase the stability of the transferred images. These and other objects are achieved in accordance with this invention which comprises a process of forming a stable dye transfer image comprising: a) imagewise-heating a dye-donor element comprising a support having thereon a dye layer, b) transferring a dye image to a dye-receiving element to form a dye transfer image, said dye-receiving element comprising a support having thereon a dye image-receiving layer containing a stabilizer compound to increase the stability to light of said transferred dye image, and c) heating said dye-receiving element containing said transferred dye image, so that stratification of said transferred dye image in said dye-receiving element is reduced, thereby further increasing the stability to light of said transferred dye image; said stabilizer compound having the following formula : wherein each R independently is an alkyl or substituted alkyl group of from 1 to 20 carbon atoms, or two adjacent R groups may be joined together to form methylene or ethylene; and x is at least 2; or the following formula : wherein each R¹ and R² is independently an alkyl or substituted alkyl group of from 1 to 20 carbon atoms, or two adjacent R¹ groups may be joined together to form methylene or ethylene, or By use of the invention, a synergistic effect is obtained which is greater than the effect which can be obtained by using the reheating technique alone or by using a stabilizer alone. This effect will be demonstrated by the examples hereinafter. Reheating of a dye-receiving element which contains a stabilizer not only drives the transferred dye from the surface of the dye image-receiving layer deeper into the layer but also promotes more intimate and effective contact of the dye with the stabilizer. Any reheating technique or device can be employed in the invention as long as it will provide useful results. There can be employed, for example, a separate heating device as disclosed in Japanese patent publication J60/125697, a pair of heated rollers as disclosed in European patent application 97,493, or use of the thermal head itself. The stabilizer nay be present at any concentration which is effective for the intended purpose. In general, good results have been obtained when the stabilizer is present at a concentration of from 5 to 20% by weight of the dye image-receiving layer. Specific stabilizers useful in the invention include the following: 1) 2) 3) 4) The support for the dye-receiving element employed in the invention may be a transparent film such as a poly(ether sulfone), a polyimide, a cellulose ester such as cellulose acetate, a poly(vinyl alcohol-co-acetal) or a poly(ethylene terephthalate), The support for the dye-receiving element may also be reflective such as baryta-coated paper, polyethylene-coated paper, white polyester (polyester with white pigment incorporated therein), an ivory paper, a condenser paper or a synthetic paper such as duPont Tyvek®. In a preferred embodiment, polyester with a white pigment incorporated therein is employed. The dye image-receiving layer may comprise, for example, a polycarbonate, a polyurethane, a polyester, polyvinyl chloride, poly(styrene- co -acrylonitrile), poly(caprolactone), or mixtures thereof. The dye image-receiving layer may be present in any amount which is effective for the intended purpose. In general, good results have been obtained at a coverage of from 1 to 10 g/m² of dye image-receiving layer. A dye-donor element that is used with the dye-receiving element employed in the process of the invention comprises a support having thereon a dye layer. Any dye can be used in such a layer provided it is transferable to the dye image-receiving layer of the dye-receiving element of the invention by the action of heat. Especially good results have been obtained with sublimable dyes such as those disclosed `in U.S. patent 4,541,830. The above dyes may be employed singly or in combination to obtain a monochrome. The dyes may be used at a coverage of from 0.05 to 1 g/m² and are preferably hydrophobic. The dye in the dye-donor element is dispersed in a polymeric binder such as a cellulose derivative, e.g., cellulose acetate hydrogen phthalate, cellulose acetate, cellulose acetate propionate, cellulose acetate butyrate, cellulose triacetate; a polycarbonate; poly(styrene-co-acrylonitrile), a poly(sulfone) or a poly(phenylene oxide). The binder may be used at a coverage of from 0.1 to 5 g/m². The dye layer of the dye-donor element may be coated on the support or printed thereon by a printing technique such as a gravure process. Any material can be used as the support for the dye-donor element provided it is dimensionally stable and can withstand the heat of the thermal printing heads. Such materials include polyesters; polyamides; polycarbonates; glassine paper; condenser paper; cellulose esters; fluorine polymers; polyethers; polyacetals; polyolefins; and polyimides. The support generally has a thickness of from 2 to 30 µm. It may also be coated with a subbing layer, if desired. A dye-barrier layer comprising a hydrophilic polymer may also be employed in the dye-donor element between its support and the dye layer which provides improved dye transfer densities. The reverse side of the dye-donor element may be coated with a slipping layer to prevent the printing head from sticking to the dye-donor element. Such a slipping layer would comprise a lubricating material such as a surface active agent, a liquid lubricant, a solid lubricant or mixtures thereof, with or without a polymeric binder. The dye-donor element employed in certain embodiments of the invention may be used in sheet form or in a continuous roll or ribbon. If a continuous roll or ribbon is employed, it may have only one dye thereon or may have alternating areas of different dyes, such as cyan, magenta, yellow, black, etc., as disclosed in U.S. Patent 4,451,830. In a preferred embodiment of the invention, a dye-donor element is employed which comprises a poly(ethylene terephthalate) support coated with sequential repeating areas of cyan, magenta and yellow dye, and the process steps described above are sequentially performed for each color to obtain a three-color dye transfer image. Of course, when the process is only performed for a single color, then a monochrome dye transfer image is obtained. Thermal printing heads which can be used to transfer dye from the dye-donor elements employed in the invention are available commercially. There can be employed, for example, a Fujitsu Thermal Head (FTP-040 MCS001), a TDK Thermal Head F415 HH7-1089 or a Rohm Thermal Head KE 2008-F3. To obtain a three-color transfer image, after the first dye is transferred, the elements are peeled apart. A second dye-donor element (or another area of the donor element with a different dye area) is then brought in register with the dye-receiving element and the process repeated. The third color is obtained in the same manner. The following examples are provided to illustrate the invention. Example 1 - Black Dye A) A neutral or black dye-donor element was prepared by coating the following layers in the order recited on a 6 µm poly(ethylene terephthalate) support: 1) Dye-barrier layer of polyacrylic acid coated from a methanol and water solvent mixture (0.17 g/m²), 2) Dye layer containing the following "black" dye (0.38 g/m²) in cellulose acetate butyrate (28% acetyl, 17% butyryl) (0.32 g/m²) coated from a 2-butanone and acetone solvent mixture: A slipping layer was coated on the back side of the dye-donor element which consisted of poly(vinyl stearate) (0.34 g/m²) in poly(vinylbutyryl) (0.52 g/m²) coated from a tetrahydrofuran mixture. B) A "blank" donor element was prepared similar to A), except that no dye layer was coated on top of the barrier layer. Dye-receiving elements were prepared by coating a solution of Bayer AG Makrolon 5705® Polycarbonate (2.9 g/m²) and the amount of stabilizer compound identified above indicated in Table 1, equivalent to 0.54 mmoles/m², from a methylene chloride and trichloroethylene solvent mixture on top of an ICI Melinex® 990 "White Polyester" reflective support. A control dye-receiving element was prepared, as above except that it had 2.9 g/m² of polycarbonate resin only. The dye side of each dye-donor element was placed in contact with the dye image-receiving layer of the dye-receiver element one inch wide. The assemblage was fastened in the jaws of a stepper motor driven pulling device. The assemblage was laid on top of a 0.55 in. (14 mm) diameter rubber roller and a TDK Thermal Head (No. L-133) and was pressed with a spring at a force of 36 N against the dye-donor element side of the assemblage pushing it against the rubber roller. The imaging electronics were activated causing the pulling device to draw the assemblage between the printing head and roller at 0.123 inches/sec (3.1 mm/sec). Coincidentally, the resistive elements in the thermal print head were pulse-heated at increments from 0 to 8.3 msec to generate a graduated density test pattern. The voltage supplied to the print head was approximately 22v representing approximately 1.5 watts/dot (12 mjoules/dot) for maximum power. The dye-receiver was then separated from each of the dye donors and the Status A red reflection density of each stepped image was read. The dye-receiver was then each placed in contact with the barrier layer side of the "blank" donor element. Uniform reheating of the entire stepped image on the receiver at the full-power setting (i.e., that used originally to provide maximum dye density) was performed in the manner as described above. Each image was then subjected to fading for 4 days, 5.4 kLux, 5400°K, 32°C, approximately 25% RH. The density was re-read and the percent density losses at selected steps were calculated. The following results were obtained: (Status A Red) Stabilizer (g/m²) Reheating Step 8 Step 5 Init. Dens. % Loss After Fade Init. Dens. % Loss After Fade none (control) No 2.3 25 0.9 46 Yes 2.2 20 0.9 32 Compound 1 (0.24) No 2.6 14 1.1 34 Yes 2.5 13 1.1 15 Compound 2 (0.20) No 2.5 21 1.1 37 Yes 2.5 14 1.1 18 Compound 3 (0.16) No 2.6 22 1.1 37 Yes 2.5 13 1.1 20 Compound 4 (0.32) No 2.5 17 1.1 38 Yes 2.4 14 1.0 18 The results indicate that reheating of a receiver which contains a stabilizer in accordance with the invention provides a substantial improvement in dye stability beyond that obtained by reheating of a receiver which did not contain a stabilizer, or by just using a receiver containing a stabilizer but no reheating. Example 2 - Yellow Dye A) A yellow dye-donor element was prepared by coating the following layers in the order recited on a 6 µm poly(ethylene terephthalate) support: 1) Dye-barrier layer of gelatin nitrate (gelatin, cellulose nitrate and salicylic acid in approximately 20:5:2 weight ratio in a solvent of acetone, methanol and water) (0.11 g/m²), and 2) Dye layers containing the following yellow dye (0.19 g/m²), poly(tetrafluoroethylene) micropowder (16 mg/m²) and cellulose acetate propionate (2.5% acetyl, 45% propionyl) (0.41 g/m²) coated from a 2-butanone and cyclopentanone solvent mixture: A typical slipping layer was coated on the back side of the dye-donor element. B) A "blank" donor element was prepared similar to A), except that no dye layer was coated on top of the barrier layer. Dye-receiving elements were prepared as in Example 1. Processing was performed as in Example 1 except that the dye fade conditions were for 7 days at 50 kLux. The following results were obtained: (Status A Blue) Stabilizer (g/m²) Reheating Step 8 Step 5 Init. Dens. % Loss After Fade Init. Dens. % Loss After Fade none No 1.8 18 0.8 50 (control) Yes 1.6 12 0.7 18 Compound 1 (0.24) No 2.1 9 1.1 26 Yes 1.9 5 1.0 5 Compound 2 (0.20) No 2.0 11 1.0 29 Yes 1.9 5 0.9 8 Compound 3 (0.16) No 2.0 11 1.0 33 Yes 2.0 6 0.9 7 Compound 4 (0.32) No 2.0 12 1.0 32 Yes 1.9 6 0.9 07 The results indicate that reheating of a dye-receiver containing a stabilizer in accordance with the invention provides a synergistic effect when a yellow dye is used. Example 3 - Magenta Dye A) A magenta dye-donor element was prepared similar to Example 2 except that the dye was the following (0.17 g/m²): B) A "blank" donor element was prepared similar to A), except that no dye layer was coated on top of the barrier layer. Dye-receiving elements were prepared as in Example 1. Processing was performed as in Example 2. The following results were obtained: (Status A Green) Stabilizer (g/m²) Reheating Step 8 Step 5 Init. Dens. % Loss After Fade Init. Dens. % Loss After Fade none (control) No 1.9 23 0.9 55 Yes 2.1 05 1.0 5 Compound 1 (0.24) No 2.3 17 1.1 36 Yes 2.4 4 1.2 4 Compound 2 (0.20) No 2.1 21 1.0 41 Yes 2.4 5 1.2 4 Compound 3 (0.16) No 2.1 13 1.0 36 Yes 2.4 4 1.1 3 Compound 4 (0.32) No 1.9 14 0.9 37 Yes 2.3 3 1.1 3 The above results again indicate that reheating of a dye-receiver containing a stabilizer in accordance with the invention provides a synergistic effect when a magenta dye is used.

Claims: A process of forming a stable dye transfer image comprising: a) imagewise-heating a dye-donor element comprising a support having thereon a dye layer, b) transferring a dye image to a dye-receiving element to form a dye transfer image, said dye-receiving element comprising a support having thereon a dye image-receiving layer containing a stabilizer compound to increase the stability to light of said transferred dye image, and c) heating said dye-receiving element containing said transferred dye image, so that stratification of said transferred dye image in said dye-receiving element is reduced, thereby further increasing the stability to light of said transferred dye image; said stabilizer compound having the following formula : wherein each R independently is an alkyl or substituted alkyl group of from 1 to 20 carbon atoms, or two adjacent R groups may be joined together to form methylene or ethylene; and x is at least 2; or the following formula : wherein each R¹ and R² is independently an alkyl or substituted alkyl group of from 1 to 20 carbon atoms, or two adjacent R¹ groups may be joined together to form methylene or ethylene, or The process of Claim 1 characterized in that said heating of said dye-receiving element is accomplished by using a thermal print head. The process of Claim 1 characterized in that said heating of said dye-receiving element is accomplished by using a separate heating device. The process of Claim 3 characterized in that said heating device is a heated roller. The process of Claim 1 characterized in that each R independently is an alkyl group of from 1 to 10 carbon atoms and x is 4. The process of Claim 1 characterized in that each R¹ and R² independently is an alkyl group of from 1 to 6 carbon atoms. The process of Claim 1 characterized in that the support for the dye-donor element comprises poly(ethylene terephthalate) which is coated with sequential repeating areas of cyan, magenta and yellow dye, and said process steps are sequentially performed for each color to obtain a three-color dye transfer image.

Inventors: BYERS GARY WAYNE, BYERS, GARY WAYNE
Applicants: EASTMAN KODAK CO, EASTMAN KODAK COMPANY (A NEW JERSEY CORPORATION)
Application year: 1987
Main classification: B41M5
IPC codes: B41M5