Covers for thermal transfer prints

Details Covers for thermal transfer prints Covers for thermal transfer prints

1996-09-24

1999-03-02

Hess, Bruce H.

Record receiver having plural interactive leaves or a colorless

Having plural interactive leaves

428195, 428206, 428324, 428329, 428331, 428913, 428914, B41M 5035, B41M 538

Patent

active

058771115

DESCRIPTION:

BRIEF SUMMARY
The invention relates to thermal transfer printing, and especially to thermally transferable protective covers for thermally transferred images.
Thermal transfer printing is a process in which one or more thermally transferable dyes are caused to transfer from selected areas of a dyesheet to a receiver by thermal stimuli, thereby to form an image. Using a dyesheet comprising a thin substrate supporting a dyecoat containing one or more dyes uniformly spread over at least a print size area of the dyesheet, printing is effected by heating selected discrete areas of the dyesheet while the dyecoat is pressed against a dye-receptive surface of a receiver sheet, thereby causing dye to transfer to corresponding areas of the receiver. The shape of the image transferred is determined by the number and locations of the discrete areas which are subjected to heating. Full colour prints can be produced by printing with different coloured dyecoats sequentially in like manner, and the different coloured dyecoats are usually provided as discrete uniform print-size areas arranged in a repeated sequence along a ribbon-shaped dyesheet.
High resolution photograph-like prints can be produced by thermal transfer printing using appropriate printing equipment, such as a programmable thermal print head or laser printer, controlled by electronic signals derived from a video, computer, electronic still camera, or similar signal generating apparatus. A typical thermal print head has a row of tiny selectively energizable heaters, spaced to print six or more pixels per millimetre, often with two heaters per pixel. Laser printers require absorbers to convert the laser radiation to heat, usually in or under the dyecoat, and similarly produce the print by transferring dyes to the receiver pixel by pixel.
The transfer mechanism is believed to depend very much on the conditions under which printing is carried out. Thus for example, when using a thermal head, the dyesheet and receiver are pressed together between the head and a platten roller, giving conditions favouring diffusion of the dyes from the dyesheet directly into the receiver, virtually precluding any sublimation. Where a small gap is provided between the dyesheet and receiver, as favoured in some laser driven printers for example, the transfer mechanism appears to be exclusively sublimation. However, in both cases the dyes are mobile molecules which can diffuse into and out of the receiver when warmed, or in the presence of various lyophilic liquids. In particular, grease from a finger holding a print can lead to migration of the dye to the surface, making the print seem dirty or causing smearing of the dyes, and plasticisers in plastic pouches can cause havoc with unprotected thermal transfer images. Particularly bad in this respect is dioctylphthallate, commonly used as a plasticiser in polyvinyl choride.
For many years various protective covers have been proposed to protect thermal transfer prints against abrasion, loss of dyes migrating to the surface, and protection against UV-induced fading, for example. Very thin covers are generally preferred, typically 1-2 .mu.m, which are difficult to handle without some form of support, and in the past it has been proposed first to prepare a donor sheet comprising a temporary carrier base sheet having a surface coated with a layer of transparent thermally transferable cover material, then thermally transferring the coating onto the printed receiver and removing the carrier. The transfer can be effected simultaneously over the whole print, and the carrier is then removed after the transfer is complete. Alternatively, transfer may be progressive, e.g. using heated rollers or a thermal head to transfer the cover line by line, and it is then generally more convenient to remove the carrier progressively as it emerges from the rolls or thermal head.
It has been recognised that polymeric compositions having higher Tg values generally provide better protective coatings, but higher Tg values can lose some of the advantages of the lower Tg materi

REFERENCES:
patent: 5387573 (1995-02-01), Oldfield et al.
patent: 5538831 (1996-07-01), Oshima et al.

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