In recent years, thermal transfer systems have been developed to obtain prints from pictures that 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 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 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. Pat. No. 4,621,271, the disclosure of which is hereby incorporated by reference.
Thermal prints are susceptible to retransfer of dyes to adjacent surfaces and to discoloration by fingerprints. This is due to dye being at the surface of the dye-receiving layer of the print. These dyes can be driven further into the dye-receiving layer by thermally fusing the print with either hot rollers or a thermal head. This will help to reduce dye retransfer and fingerprint susceptibility, but does not eliminate these problems. However, the application of a protective overcoat will practically eliminate these problems. This protective overcoat is applied to the receiver element by heating in a likewise manner after the dyes have been transferred. The protective overcoat will improve the stability of the image to light fade and oil from fingerprints.
In a thermal dye transfer printing process, it is desirable for the finished prints to compare favorably with color photographic prints in terms of image quality. The look of the final print is very dependent on the surface texture and gloss. Typically, color photographic prints are available in surface finishes ranging from very smooth, high gloss to rough, low gloss matte.
If a matte finish is desired on a thermal print, it has been previously accomplished by using matte sprays or by matte surface applications through post printing processors. However, both of these solutions are costly and add a degree of complexity to the process.
U.S. Pat. No. 6,346,502 and JP 09/323482 relate to the use of expandable microspheres in a transferable protection layer area of a dye-donor element. However, there is a problem with these microspheres in that they will not provide a defect-free print with a desired gloss at a low print head temperature.
The transferable protection layer of the dye donor is manufactured by a gravure coating process between the temperatures of 12° C. and 49° C. (55° F. and 120° F.), preferably between 18° C. and 38° C. (65° F. and 100° F.). A coating melt or solution is prepared from a solvent soluble polymer and thermally expandable microspheres or beads and is transferred in the liquid state from the etching of the gravure cylinder to the dye donor support. The unengraved area of the cylinder must be kept free of any accumulation of liquid coating melt such that unwanted transfer of liquid to the dye donor support is avoided. Such transfer leads to undesirable contamination of the dye donor support when subsequent patches of dye are coated.
Inorganic particles such as colloidal silica is added to the surface of the expandable beads during manufacture to prevent coalescence of the oil phase droplets during manufacture and agglomeration of the dry microspheres during storage. The dispersed microspheres typically bear on the surface of the microspheres inorganic particles in an amount of at least 1.8% by weight of the microspheres. The colloidal silica progressively forms a scum on the surface of the gravure cylinder. The scum builds up with time to a point where the coating machine must eventually be shut down and the scummed cylinder replaced with a clean cylinder because of the unwanted transfer of liquid coating melt to the donor web described above.
Materials constituting the coating composition useful for creating a matte finish protective overcoat layer for a thermal dye transfer image are described in U.S. Pat. No. 6,184,181 B1, by Lum et al, and subsequently by Simpson et al. in published GB 2,348,509. The materials are combined in a multiple-solvent coating composition, to provide the overcoat layer as a repeating patch in the multicolor dye-donor element containing patches of cyan, magenta and yellow.
A multi-station gravure-coating machine is used to coat the multicolor dye-donor element as well as this matte-finish protective overcoat in sequentially registered patches. Contamination of any of the patches from one color to the next is not desirable for product quality. Any contamination from the protective overcoat layer coating cylinder to an area in the donor element where either the cyan, magenta or yellow dye is to be subsequently coated causes a failure in the making of the thermal dye transfer image. The contamination on the gravure coating process was seen to form fairly rapidly hindering the length of a successful production before interruption for cleaning.
Altering various process conditions is somewhat effective in extending the time between cleanings, but a further and more reliable method for extending the period is a problem to be solved.