In thermal dye transfer printing, an image is formed on a receptor sheet by selectively transferring an image forming material to the receptor sheet from a dye donor sheet. Material to be transferred from the dye donor sheet is selected by a thermal printhead, which consists of small, electrically heated elements. These elements transfer image-forming material from the dye donor sheet to areas of the dye receptor sheet in an imagewise manner.
There are three broad classes of thermal transfer systems that are known, (1) chemical reaction systems, (2) thermal mass transfer systems, and (3) thermal dye transfer systems.
In chemical reaction systems, the image is formed upon the receptor as a result of the imagewise transfer of some chemical reactant from the donor sheet. An example is the transfer of a mobile molecule, such as phenol, to the receptor sheet, which bears a leuco compound thereon. The phenol is transferred by being volatilized by the heat from the thermal print head, and, upon reaching the receptor sheet, reacts with the leuco compound to convert it from the colorless to the colored form. Alternately, the phenol can be on the receptor sheet and the leuco compound can be on the donor sheet.
In thermal mass systems, no color forming chemical reaction takes place. Instead, the image is formed simply by the transfer of a mass of material containing colorant therein, such as pigment-filled polymer coatings.
In thermal dye transfer systems, a dye donor sheet is used in combination with a dye receptor sheet wherein, with the application of heat, a dye is transferred onto the receptor sheet at a controlled amount to obtain a dye image having gradation like in a photograph.
Each system has its own advantages and disadvantages for the particular application of thermal printing. Various problems have been encountered with each proposed system. For thermal dye transfer, dye release layers have been proposed to enable efficient transfer of the dye layer from the dye donor sheet. Also, various dye-permeable release layers on the dye receptor layer have been proposed. The dye-permeable release layer is coated over the dye receptor layer, and is formulated to prevent sticking between the donor layer and the receptor layer during the transfer of the dye across the binder membranes. The release layer must also be formulated to allow effective transfer of the dye through the release layer. In general, many of these problems have been related to a specific resin used in the composition of the dye donor or dye receptor layers.
Selection of the functional resin systems for the dye donor and the receptor sheet layers has been the topic of concern for many proposed dye systems. In consideration of the above mentioned requirements, efficient dye transfer and sticking between the dye donor layer and the dye receptor layer during transfer, a good, functional resin system to eliminate some of these problems is needed. Interestingly, a unique resin system has been found that provides an efficient working dye donor sheet and dye receptor sheet. The resin system comprises a chlorinated polyvinyl chloride (CPVC).
U.S. Pat. No. 3,584,576 describes a heat sensitive stencil sheet comprising a film adhered to a porous thin fibrous sheet. The stencil sheet is perforated by exposure to infrared rays. The film consists essentially of at least 75% by weight of a chlorinated polyvinyl chloride resin, the balance being a polyvinyl chloride resin. A colorant may be present in the film. Upon being heated by infrared radiation, the film melts and forms perforations. The pores in the remaining fibrous sheet enable stencilling to be done through the perforations and the sheet.
There are noticable difference between the above mentioned prior art use of CPVC in a thermally sensitive stencil applications and in the present invention. The prior art uses CPVC merely as a resinous binder, with or without other resinous binders. It is, in particular, used not as a receptor layer for a thermal dye transfer sheet, but as the thermoplastic binder for a thermal stencil sheet. The novel use of the CPVC resin in the thermal dye transfer printing of the present invention has been found to give surprisingly new and unique properties for use as the primary resinous thermal plastic binder in both a dye donor sheet, and a dye receptor sheet. Typically, commercially available dye donor sheets and dye receptor sheets are comprised of chemically different binders with different functionals.