PROCESS FOR INCREASING THE DENSITY OF IMAGES OBTAINED BY THERMAL DYE TRANSFER

Abstract: A process for increasing the density of a thermal dye transfer image comprising image­ wise-heating a dye-donor element comprising a support having thereon a dye layer and transferring a dye image to a dye-receiving element comprising a transparent support having thereon a dye image-receiving layer to form an image having a certain density, characterized in that another portion of the dye-donor element or another dye-donor element is imagewise heated at least one more time to transfer a second dye image, which is of the same hue as the first dye image and is in register with the first dye image, to the dye-receiving element to increase the density of the transferred image.

Description: This invention relates to a process for increasing the density of images obtained by a thermal dye transfer process, which is used for transparencies. 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. Further details of this process and an apparatus for carrying it out are contained in U.S. Patent No. 4,621,271 by Brownstein entitled "Apparatus and Method for Controlling A Thermal Printer Apparatus," issued November 4, 1986. The process described above can be used to obtain reflection prints which have a transferred reflection density of about 1.6-2.0. In applications such as transparencies, however, much higher transmission densities on the order of at least about 2.5 must be obtained. One of the ways to increase the density of a transferred image is to merely increase the amount of dye in the dye-donor element and also to increase the amount of power used to transfer the dye. However, this is costly in terms of power supply requirements. In addition, it is harder to coat higher amounts of dye in the dye-binder layer and increasing the power to the thermal head (duration and time) creates problems of receiver deformation. Another way to increase the density of a transferred image would be to lower the amount of binder in the dye-donor element, thereby lowering the path length for dye diffusion and increasing the dye transfer efficiency. There is a problem in doing that, however, since a higher amount of dye in the dye layer generally creates a tendency for the dye to crystallize on keeping. In addition, there would also be a higher amount of sticking of the donor to the receiver during the printing operation. Other ways to increase the density of the transferred image is to either find new dyes which have higher thermal dye efficiency or find materials which could be added to the dye layer to increase the transfer efficiency. This would mean, however, in the case of reflection prints and transparencies, that different dye-donor elements would be required, resulting in increased manufacturing costs and inconvenience to the user. Another problem that was found in obtaining a transparency by a thermal process was nonuniform density areas. When printing a D-max area, most of the dye in the dye-donor is supposed to be transferred uniformly. In practice, however, this does not always happen with the resultant transferred image being nonuniform or having a mottled appearance. In the case of a reflection print, slight density variations are not noticeable. However, in the case of a transparency, a variation in dye density is very noticeable since the image is magnified and projected. In EPA 0 181 981, a printing method is described wherein solid ink layers are transferred to a paper support a number of times in an overlapping manner in order to achieve a gradation of picture elements. There is a problem with this method in that the paper has to be repositioned each time in alignment with another portion of the donor sheet. Such a process is slow and requires a great amount of paper handling and positioning to achieve any amount of gradation of picture elements. It is an object of this invention to provide a way to reduce or eliminate nonuniform density areas in the transferred dye image of a transparency. It is another object of this invention to provide a way to increase the density of transferred images in thermal dye transfer processes used to obtain a transparency. It is still another object of this invention to find a way to use the same dye-donor element for a reflection print as for a transparency, without increasing the power requirements to obtain the transparency. These and other objects are achieved in accordance with this invention which comprises a process for increasing the density of a thermal dye transfer image comprising imagewise-heating a dye-donor element comprising a support having thereon a dye layer comprising a sublimable dye in a polymeric binder and transferring a dye image to a dye-receiving element comprising a transparent support having thereon a dye image-receiving layer to form an image having a certain density, characterized in that another portion of the dye-donor element or another dye-donor element is imagewise heated at least one more time to transfer a second dye image, which is of the same hue as the first dye image and is in register with the first dye image, to the dye-receiving element to increase the density of the transferred image. The above process can be repeated two or more times in order to increase the density to the desired level. Thus, in a preferred embodiment of the invention, another dye-donor is imagewise heated and a third dye image, the same as the other two images of the same dye, is transferred in register to the dye-receiving element to form an image having even more density. By employing the process of the invention to obtain a transparency, nonuniform density areas are eliminated or substantially reduced. This is achieved by the multiple printing which has the effect of evening out or cancelling the nonuniform areas, i.e., a nonuniform density area on top of another nonuniform density area tends to be uniform since the densities are additive. The dye image-receiving layer of the dye-receiver employed in the invention 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 concentration of from 1 to 5 g/m². In a preferred embodiment of the invention, the dye image-receiving layer is a polycarbonate. The term "polycarbonate" as used herein means a polyester of carbonic acid and glycol or a divalent phenol. Examples of such glycols or divalent phenols are p-xylylene glycol, 2,2-bis(4-oxyphenyl)propane, bis(4-oxyphenyl)methane, 1,1-bis(4-oxyphenyl)ethane, 1,1-bis(oxyphenyl)butane, 1,1-bis(oxyphenyl)cyclo-hexane, 2,2-bis(oxyphenyl)butane, etc. In another preferred embodiment of the invention, the polycarbonate dye image-receiving layer is a bisphenol-A polycarbonate having a number average molecular weight of at least 25,000. In still another preferred embodiment of the invention, the bisphenol-A polycarbonate comprises recurring units having the formula wherein n is from 100 to 500. Examples of such polycarbonates include General Electric Lexan® Polycarbonate Resin #ML-4735 (Number average molecular weight app. 36,000), and Bayer AG Makrolon #5705® (Number average molecular weight app. 58,000). The later material has a T g of 150°C. The support for the dye-receiving element employed in the invention is 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). In a preferred embodiment, poly(ethylene terephthalate) is employed. A dye-donor element that is used with the dye-receiving element employed in 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 or any of the dyes 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. A black-and-white or neutral-hue dye image could also be obtained using the invention by employing mixtures of cyan, magenta and yellow dyes, using a neutral-hue dye, or by using the process described above repeatedly for each colour without differentiating the color record being printed. 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 such as poly(ethylene terephthalate); 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. 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 4,541,830. 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 above process steps are sequentially performed for each colour at least two times 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. In another embodiment of the invention, lasers could be used to transfer dye from the donor to the receiver. This could be accomplished by incorporating an infrared absorbing dye in the dye donor element. The following example is provided to illustrate the invention. Example Dye receivers were prepared by coating the following layers in the order recited on a 100 µm thick transparent poly(ethylene terephthalate) film support (a) Subbing layer of poly(acrylonitrile- co -vinylidene chloride- co -acrylic acid) (14:80:6 wt ratio) (0.059 g/m²) coated from 2-butanone; (b) Polymeric intermediate layer of poly (butylacrylate-co-acrylic acid ) (50:50 wt. ratio)( 8.1 g/m²); (c) Polymeric intermediate layer of FC-430® surfactant (3M Company) (0.0046 g/m²) and the following partially sulfonated glycol-phthalate (0.44 g/m²): (d) Dye-receiving layer of Makrolon 5705® polycarbonate (Bayer AG) (2.9 g/m²), 1,4-didecoxy-2,5-dimethoxybenzene (0.38 g/m²), Tone-300® polycaptolactone (Union Carbide Corp.) (0.38 g/m²), and FC-431® surfactant (3M Corp.) (0.01 g/m²) coated from a dichloromethane and trichloroethylene solvent mixture; and (e) Overcoat layer of Tone-300® polycaptolactone (Union Carbide Corp). (0.11 g/m²) and 3M Corp. FC-431® surfactant (0.005 g/m²) coated from a dichloromethane and trichloroethylene solvent mixture. A cyan, magenta and yellow dye-donor element was prepared as follows. On one side of a 6 µm poly(ethylene terephthalate) support, a subbing layer of titanium n-butoxide (duPont Tyzor TBT®) (0.081 g/m²) was Gravure-printed from a n-propyl acetate and 1-butanol solvent mixture. On top of this layer were Gravure-printed repeating color patches of cyan, magenta and yellow dyes. The cyan coating contained the cyan dye illustrated above (0.28 g/m²) and cellulose acetate propionate (2.5% acetyl, 45% propionyl) binder (0.44 g/m²) from a toluene, methanol and cyclopentanone solvent mixture. The magenta coating contained the magenta dye illustrated above (0.15 g/m²) in the same binder as the cyan dye (0.32 g/m²). The yellow coating contained the yellow dye illustrated above (0.14 g/m²) in the same binder as the cyan dye (0.25 g/m²). On the reverse side of the dye-donor was coated a subbing layer of Bostik 7650® polyester (Emhart Corp.) (43. mg/m²) coated from a toluene and 3-pentanone solvent mixture and a slipping layer of PS-513® amino-terminated silicone (Polymer Sciences) (0.013 g/m²) and p-toluenesulfonic acid (0.043 g/m²) in a cellulose acetate propionate (2.5% acetyl, 45% propionyl) binder (0.40 g/m²) from a toluene, methanol and 3-pentanone solvent mixture. The dye-side of the dye-donor element strip 4 inches (10. cm) wide was placed in contact with the dye image-receiving layer of a dye-receiver element strip of the same width. The assemblage was fastened in a clamp on a rubber-roller of 2.23 in (56.7 mm) diameter driven by a stepper motor. A TDK 6-2Q23-2 Thermal Head was pressed at a force of 8 pounds (3.6 kg) against the dye-donor element side of the assemblage pushing it against the rubber roller. The imaging electronics were activated causing the device to draw the assemblage between the printing head and roller at 0.28 inches/sec (7 mm/sec). Coincidentally the resistive elements in the thermal print were heated using a supplied voltage of approximately 24v, representing approximately 1.2 watts/pixel (28 mjoules/pixel group). Eleven-step graduated density test images were generated on each dye-receiver using the individual yellow, magenta, or cyan dye-donors. Each imaged area on the dye-receiver was then "over-printed" in register using an unused area of the dye-donor of the same hue as used for the original printing. Images with a single 1X-printing, 2X-printing (one over-printing), and 3X-printing (two over-printings) were produced on separate receivers and the transferred Status A blue, green, or red transmission densities were obtained. Neutral images were also obtained by printing in sequence a superposed-tricolor stepped imate from the yellow, magenta, and cyan dye-donors and then overprinting in sequence from the three dye donors to provide 1X, 2X, and 3X printings. Status A densities of these neutral images were also obtained. The following results were obtained: Single Color Transfer Step Yellow Dye Magenta Dye Cyan Dye Blue Density Green Density Red Density 1X 2X 3X 1X 2X 3X 1X 2X 3X 1 0.03 0.03 0.03 0.02 0.02 0.02 0.02 0.02 0.02 5 0.09 0.12 0.14 0.08 0.12 0.15 0.07 0.10 0.13 8 0.49 0.82 1.11 0.40 0.66 0.93 0.43 0.74 1.09 9 0.77 1.31 1.78 0.63 1.08 1.51 0.69 1.22 1.74 10 1.15 1.90 2.64 0.96 1.64 2.33 1.03 1.79 2.52 11 1.61 2.65 3.52 1.40 2.44 3.36 1.37 2.45 3.25 Neutral Hue Transfer (Cyan + Magenta + Yellow Dye) Step Blue Density Green Density Red Density 1X 2X 3X 1X 2X 3X 1X 2X 3X 1 0.03 0.03 0.03 0.02 0.02 0.02 0.02 0.02 0.02 5 0.10 0.14 0.19 0.09 0.11 0.14 0.07 0.09 0.11 8 0.67 1.14 1.58 0.55 0.93 1.27 0.50 0.83 1.10 9 1.05 1.84 2.22 0.90 1.57 2.15 0.84 1.42 1.85 10 1.44 2.52 3.37 1.31 2.27 3.05 1.21 2.02 2.59 11 1.80 3.01 4.03 1.67 2.90 3.84 1.54 2.57 3.25 The above results show that multiple printings significantly increase the transmission densities at the higher steps without affecting the minimum density.

Claims: A process for increasing the density of a thermal dye transfer image comprising imagewise-heating a dye-donor element comprising a support having thereon a dye layer comprising a sublimable dye in a polymeric binder and transferring a dye image to a dye-receiving element comprising a transparent support having thereon a dye image-receiving layer to form an image having a certain density, characterized in that another portion of said dye-donor element or another dye-donor element is imagewise heated at least one more time to transfer a second dye image, which is of the same hue as said first dye image and is in register with said first dye image, to said dye-receiving element to increase the density of said transferred image. The process of Claim 1 characterized in that another dye-donor is imagewise heated and a third dye image, the same as the other two images of the same dye, is transferred in register to said dye-receiving element to form said image having even more density. The process of Claim 1 characterized in that said imagewise heating is done with a thermal print head. The process of Claim 1 characterized in that said imagewise heating is done with a laser. The process of Claim 1 characterized in that said support is poly(ethylene terephthalate). The process of Claim 1 characterized in that said support for the dye-donor element is coated with sequential repeating areas of cyan, magenta and yellow dye, and said process steps are sequentially performed for each color at least two times to obtain a three-color dye transfer image. The process of Claim 1 characterized in that said support for the dye-donor element is coated with sequential repeating areas of cyan, magenta and yellow dye, and said process steps are sequentially performed without differentiation of the color record in order to obtain a neutral-hue dye transfer image. The process of Claim 1 characterized in that said support for the dye-donor element is coated with sequential repeating areas of a neutral-hue dye, and said process steps are sequentially performed to obtain a neutral-hue dye transfer image. The process of Claim 1 characterized in that said dye image-receiving layer is a bisphenol-A polycarbonate having a number average molecular weight of at least 25,000. The process of Claim 9 characterized in that said bisphenol-A polycarbonate comprises recurring units having the formula wherein n is from 100 to 500.

Inventors: LUM, KIN KWONG
Applicants: EASTMAN KODAK COMPANY (A NEW JERSEY CORPORATION)
Application year: 1988
Main classification: B41M5
IPC codes: B41M5