Thermal transfer printing dye sheet

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428195, 428500, 428913, 428914, B41M 5035, B41M 538

Patent

active

055959563

DESCRIPTION:

BRIEF SUMMARY
This invention relates to a thermal transfer printing (TTP) dye sheet, in particular to a dye sheet having an improved dye binder.
Thermal transfer printing is a printing process in which a dye is caused, by thermal stimuli, to transfer from a dye sheet to a receiver sheet thereby to form an image on the receiver sheet. In such a process, the dye sheet and receiver sheet are placed in intimate contact, the thermal stimuli are applied to the dye sheet to cause dye transfer and the dye sheet and receiver sheet are then separated. By applying the thermal stimuli to pre-determined areas in the dye sheet, the dye is selectively transferred to the receiver to form the desired image. The thermal stimuli my be provided by a programmable print head which is in contact with the dye sheet or by for example, a laser in a light-induced thermal transfer process (LITT).
Dye-sheets conventionally comprise a substrate having on one surface thereof a dye coat which typically comprises a thermally transferable dye dispersed or dissolved in a binder. Dye-sheets may also comprise a back coat to impart desirable properties for example, good handling and thermal characteristics to the dye sheet. Further, a primer or subbing layer my be employed between the substrate and the dye coat and/or the substrate and the back coat for example to improve the adhesion of the coat to the substrate.
Many materials have been suggested for use as the binder including, polyvinylbutyral as disclosed in EP-A-141678, cellulosic polymers as disclosed in J03264393, epoxy resins and phenolic resins.
During the TTP process, application of a thermal stimulus to an area of the dye coat on the the dye sheet heats that area of the sheet to a temperature typically in excess of 100.degree. C. as a result of which the dye in that area of the dye sheet is transferred to the receiver sheet. On removal of the thermal stimulus the temperature of the heated area then decreases to the ambient operating temperature in the process.
However, once the thermal stimulus is removed and the temperature of the heated area of the dye sheet is decreasing, there may still be sufficient heat to cause unwanted, uncontrolled residual transfer of dye to the receiver sheet which may cause a reduction in image quality. This problem is referred to herein as low temperature thermal transfer.
We have now found that by employing a dye sheet binder having a particular mixture of polymer components, problems due to low temperature thermal transfer may be reduced or avoided and other significant advantages may also be secured.
Accordingly, a first aspect of the invention provides a thermal transfer printing dye sheet which comprises a substrate having on one side, a dye coat comprising a binder and a thermally transferable dye wherein the binder comprises a mixture of a polyvinyl butyral polymer and a second polymer having, as at least 10% of the monomeric units of the polymer, a monomeric unit of formula (I); cyano group or an ester group of formula --CO.sub.2 Z wherein Z is an optionally substituted C.sub.1 to C.sub.6 hydrocarbyl group with the proviso that where Y is an unsubstituted phenyl group the second polymer also has a substituted styrene and/or a non-styrene monomeric unit.
Dye-sheets according to the present invention have been found to exhibit a particularly advantageous combination of characteristics. In particular, good resistance to low temperature thermal transfer may be achieved which provides for improved image quality and images may be produced which have excellent optical density.
Also, we have found that when the dye sheet is to be used to produce multi-colour images by using a dye sheet comprising a plurality of panels of different uniform colours, usually magenta, yellow and cyan, improved colour balance between the dyes by matching the three colours may be achieved. A practical benefit of this is that an excess of one or more of the colours leading to a colour imbalance and hence a tinge of the image may be reduced or avoided, such imbalance being particularly unde

REFERENCES:
patent: 4946826 (1990-08-01), Kubo et al.
Database WPI Week 8903 Derwent Publications Ltd., London, GB; AN 89-019534 & JP A 63 295292 (Matsushita Elec. Ind. KK) See Abstract.

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