In recent years, thermal transfer systems have been developed to obtain prints from pictures which have been generated from a camera or scanning device. 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 one of the cyan, magenta or 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.
Dye receiving elements used in thermal dye transfer generally include a support (transparent or reflective) bearing on one side thereof a dye image-receiving layer, and optionally additional layers. The dye image-receiving layer conventionally comprises a polymeric material chosen from a wide assortment of compositions for its compatibility and receptivity for the dyes to be transferred from the dye donor element. Dye must migrate rapidly in the layer during the dye transfer step and become immobile and stable in the viewing environment. Care must be taken to provide a receiving layer which does not stick to the hot donor as the dye moves from the surface of the receiving layer and into the bulk of the receiver. An overcoat layer can be used to improve the performance of the receiver by specifically addressing these latter problems. An additional step, referred to as fusing, may be used to drive the dye deeper into the receiver.
In sum, the receiving layer must act as a medium for dye diffusion at elevated temperatures, yet the transferred image dye must not be allowed to migrate from the final print. Retransfer is potentially observed when another surface comes into contact with a final print. Such surfaces may include paper, plastics, binders, backside of (stacked) prints, and some album materials.
Polycarbonates (the term “polycarbonate” as used herein means a polyester of carbonic acid and a diol or diphenol) and polyesters have both been used in image-receiving layers. For example, polycarbonates have been found to be desirable image-receiving layer polymers because of their effective dye compatibility and receptivity. As set forth in U.S. Pat. No. 4,695,286, bisphenol-A polycarbonates of number average molecular weights of at least about 25,000 have been found to be especially desirable in that they also minimize surface deformation which may occur during thermal printing. These polycarbonates, however, do not always achieve dye transfer densities as high as may be desired, and their stability to light fading may be inadequate. U.S. Pat. No. 4,927,803 discloses that modified bisphenol-A polycarbonates obtained by co-polymerizing bisphenol-A units with linear aliphatic diols may provide increased stability to light fading compared to ummodified polycarbonates. Such modified polycarbonates, however, are relatively expensive to manufacture compared to the readily available bisphenol-A polycarbonates, and they are generally made in solution from hazardous materials (e.g. phosgene and chloroformates) and isolated by precipitation into another solvent. The recovery and disposal of solvents coupled with the dangers of handling phosgene make the preparation of specialty polycarbonates a high cost operation.
Polyesters, on the other hand, can be readily synthesized and processed by melt condensation using no solvents and relatively innocuous chemical starting materials. Polyesters formed from aromatic diesters (such as disclosed in U.S. Pat. No. 4,897,377) generally have good dye up-take properties when used for thermal dye transfer; however, they exhibit severe fade when the dye images are subjected to high intensity daylight illumination. Polyesters formed from alicyclic diesters are disclosed in U.S. Pat. No. 5,387,571 of Daly, the disclosure of which is incorporated by reference. These alicyclic polyesters also generally have good dye up-take properties, but their manufacture requires the use of specialty monomers which add to the cost of the receiver element. Polyesters formed from aliphatic diesters generally have relatively low glass transition temperatures, which frequently results in receiver-to-donor sticking at temperatures commonly used for thermal dye transfer. When the donor and receiver are pulled apart after imaging, one or the other fails and tears and the resulting images are unacceptable.
U.S. Pat. No. 5,302,574 to Lawrence et al. discloses a dye-receiving element for thermal dye transfer comprising a support having on one side thereof a dye image-receiving layer, wherein the dye image-receiving layer comprises a miscible blend of an unmodified bisphenol-A polycarbonate having a number molecular weight of at least about 25,000 and a polyester comprising recurring dibasic acid derived units and diol derived units, at least 50 mole % of the dibasic acid derived units comprising dicarboxylic acid derived units containing an alicyclic ring within two carbon atoms of each carboxyl group of the corresponding dicarboxylic acid, and at least 30 mole % of the diol derived units containing an aromatic ring not immediately adjacent to each hydroxyl group of the corresponding diol or an alicyclic ring. Thus, the alicyclic polyesters were found to be compatible with high molecular weight polycarbonates.
The above-cited patents to Daly and Lawrence et al. disclose that resistance to sticking during thermal printing may be enhanced by the addition of release agents to the dye receiving layer or to an overcoat layer, such as silicone based compounds, as is conventional in the art. Also, in the Examples, a fluorosurfactant Flurorad FC-431 is used as a release agent.
Such release agents in the prior art have been found to exhibit decreased effectiveness when added to a composition for a dye-receiving layer that is extruded. Consequently, problems related to image quality during dye transfer can occur. Also, it has been found that release agents of the prior art can cause cross-linking and degradation of the extruded polymer mixture at the high temperatures used during extrusion.
Accordingly, it would be highly desirable to provide a receiver element for thermal dye transfer processes with a dye image receiving layer comprising a release agent that provides excellent thermal stability during extrusion, excellent dye uptake and retransfer resistance and which can be effectively printed in a thermal printer.