Thermal transfer printing dye sheet

Details Thermal transfer printing dye sheet Thermal transfer printing dye sheet

1996-07-16

1997-11-25

Hess, Bruce H.

Record receiver having plural interactive leaves or a colorless

Having plural interactive leaves

356138, 427146, 428195, 428913, 428914, B41M 5035, B41M 538

Patent

active

056912730

DESCRIPTION:

BRIEF SUMMARY
This invention relates to a thermal transfer printing (TTP) dye sleet.
Thermal transfer printing is a printing process in which a dye is caused, by thermal stimuli, to transfer from a dye sleet to a receiver street. In such a process, the dye sheet and the receiver sheet are placed in intimate contact, the thermal stimuli are applied to the dye sheet to cause aye transfer and the dye sheet and the 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 sheet to form the desired image. The thermal stimuli may be provided by a programmable print head which is in contact with the dye sheet or by a laser in a light induced thermal transfer process (LITT).
Dye sheets conventionally comprise a substrate having on one surface a dye coat, the essential components of which are a binder resin and, dispersed therein a thermally transferable dye. A back coat may be provided on the other surface to impart desireable properties, for example, good handling and thermal characteristics. Further a primer or subbing layer may be employed between the substrate and the dye coat and/or the substrate and the back coat to improve adhesion.
The dye coat is normally applied by coating a homogeneous solution of the dye and the polymer on to the substrate and then rapidly evaporating the solvent. Depending on the coating conditions, the distribution of the low molecular weight dye in the high molecular weight polymer can vary.
During the TTP process, application of a thermal stimulus to an area of the dye sheet heats that area to a temperature typically in excess of 100.degree. C. causing dye from a corresponding area of the dye coat to be transferred to the receiver sheet. However, the whole area of the dye coat is in contact with the receiver sheet and under certain conditions, for example high ambient temperature and/or prolonged use of a printer, the temperature can be sufficiently high to cause unwanted and uncontrolled transfer of dye. This problem, known as low temperature thermal transfer (LT3), is likely to more acute if there is a high concentration of dye at or near the surface of the dye coat, ie within the upper 0.5 .mu.m.
A further problem resulting from a high surface concentration of dye is that control of dye transfer at low levels, ie when reproducing pale shades, is more difficult.
Hence, it would be advantageous if the concentration of dye in the binder could be controlled so that the distribution was more homogeneous.
However, for such control to be possible, it is necessary that a measure of the homogeneity of the distribution of the dye in the polymer can be established.
It has now been found that this can be achieved by using the technique of Attenuated Total Reflection Spectroscopy(ATRS), otherwise known as Internal Reflection Spectroscopy(IRS).
ATRS is an inra-red technique which utilises a material of high refractive index as a guide for a beam of infra-red radiation. At angles above the critical angle, the beam is totally internally reflected within the guide. However, at each point of reflection, an exponentially decaying wave (the evanescent wave) extends for a small distance beyond the confines of the guide and can penetrate and interact with an IR absorbing sample placed against the reflecting surface of the guide and be absorbed at specific wavelengths and absorption spectra produced as in conventional infra-red spectroscopy. The propagating beam within the guide is thus attenuated and the degree of attenuation, which is dependent on the material of the sample, can be measured.
The penetration depth d.sub.p, ie the extent to which the evanescent wave penetrates the sample, normally defined as being the depth at which the evanescent wave has decreased to 1/e of its initial value at the interface, is given by the equation. ##EQU1## where .lambda. is the wavelength of the IR radiation, n.sub.2 and n.sub.1 are the refractive indices of the guide and the sample, and .phi. is the angle of incidencef the r

REFERENCES:
patent: 5240900 (1993-08-01), Burberry

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