Thermographic imaging elements and processes for their use

Details Thermographic imaging elements and processes for their use Thermographic imaging elements and processes for their use

2002-08-01

2003-10-21

Hess, Bruce H. (Department: 1774)

Record receiver having plural interactive leaves or a colorless

Having a colorless color-former, developer therefor, or...

Method of use, kit, or combined with marking instrument or...

C503S200000, C503S215000, C503S226000

Reexamination Certificate

active

06635601

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to thermally imageable elements for use in direct thermal imaging systems. Imaging methods of the invention utilize thermally induced catalytic transformation of image-forming chemistry within the elements to provide an image without the need for photosensitivity (that is the incorporation of any photosensitive component).
BACKGROUND OF THE INVENTION
Thermal imaging is a process in which images are recorded by the use of imagewise modulated thermal energy. A review of thermal imaging is provided, for example, in
Imaging Systems
by Jacobson and Jacobson (Focal Press, 1976). In general, there are two types of thermal recording systems.
In one system the image is generated by thermally activated transfer of a heat absorbing material from a donor element to a receiver element, while the other general process involves thermal activation using chemical or physical modification of components of a single imaging element. Processes of the first type include thermal dye transfer systems in which a dye is thermally transferred from one element (the donor sheet) to a second layer (the receiver sheet) as described, for example in U.S. Pat. No. 4,621,271 (Brownstein) and U.S. Pat. No. 5,618,773 (Bailey et al). Such systems, while providing color images of high quality, suffer from the disadvantage of requiring two sheets and the associated printer hardware for such a physical transfer of dye between two sheets.
Systems of the second type are those in which the image is formed in the element that is imagewise exposed using heat. The discussion that follows relates to systems of the second type.
Thermal energy can be delivered in a number of ways, for example, by direct thermal contact or by absorption of electromagnetic radiation. Examples of useful radiant energy sources include infrared lasers, thermal print heads, and electron beam devices. Modulation of thermal energy can be by intensity or time or both. For example, a thermal print head comprising microscopic resistor elements is fed pulses of electrical energy that are converted into heat by the Joule heating effect. In a particularly useful embodiment, the pulses are of fixed voltage and duration and the thermal energy delivered is then controlled by the number of such pulses sent to the print head. Radiant energy can also be modulated directly by means of the energy source, for example the voltage applied to a solid state laser.
Direct imaging by thermally induced chemical change in a recording element usually involves an irreversible chemical reaction which takes place very rapidly at elevated temperatures (for example, above 100° C.). At room temperature the reaction rate is orders of magnitude slower such that, effectively, the material is stable at the latter temperature. A particularly useful “dry silver” direct thermal imaging element uses an organic silver salt in combination with a reducing agent. In this system the chemical change induced by the application of thermal energy is the reduction of the transparent silver salt to a metallic silver image by the reducing agent incorporated in the coating formulation. Such thermographic elements, after imagewise thermal exposure, provide a final image without the need for any post-exposure solution processing.
In addition to the dry silver imaging elements, non-silver dry photothermographic imaging systems are also known. For example, it is known to produce tellurium images by disproportionation of tellurium dihalides, as illustrated U.S. Pat. No. 3,700,448 (Hillson et al). The images are formed in the presence of a processing liquid that promotes the disproportionation amplification reaction in the presence of catalytic amounts of photogenerated elemental tellurium (Te
0
). The tellurium dihalides, however, are dark in color causing poor image discrimination. Further, the tellurium dihalides are typically unstable in air and undergo light induced decomposition only when moistened with an organic solvent. Accordingly, the tellurium dihalides do not satisfy the needs of dry processing.
It is also known that certain tellurium (IV) compounds wherein the tellurium is bonded directly to one or more carbon atoms can be used in photothermographic imaging. In GB-A-1,405,628 certain tellurium compounds, wherein the tellurium is bonded directly to a carbon atom, are described as useful image forming materials in thermally developed systems. The process using these organotellurium (IV) compounds to form a tellurium image is a unit quantum photoreduction, that is the Te
0
is formed in a stoichiometric reaction by reduction of the Te(IV) compound by the photogenerated organic reducing agent. This process lacks any amplification and is, therefore, inherently slow in speed and, as a result, limited in usefulness.
An amplification step is an important factor in imaging systems having high speed. In such processes and elements, typically a redox reaction is catalyzed by a material that is generated in the exposure step. In the highest imaging speed materials, conventional wet processed silver halide photographic materials, high speeds are attributable to the following amplification process: exposure of photographic silver halide to light results in formation of small silver nuclei on the silver halide grain surfaces that catalyze the further reduction of silver halide in these exposed grains in a subsequent solution development employing a developing agent (a reducing agent) to give elemental silver in a high gain catalytic reaction.
Imaging materials have been described wherein a substance capable of darkening when heated is employed in the presence of a catalyst, such as described in U.S. Pat. No. 1,939,232 (Sheppard et al). This imaging material employs a compound such as silver oxalate to form an image and a compound such as tellurium dichloride as a catalyst. Thus, this system is quite different from the conventional photothermographic systems described above that rely on silver or a non-silver material, such as Te
0
to provide image density after an imagewise light exposure to produce a developable latent image, and a subsequent uniform heating of the entire imaged element to produce the final visible image.
Materials are also known in the imaging art in which metal nuclei are used to initiate physical development processes. For example, processes in which such catalytic metal nuclei are generated by a light exposure step and subsequently amplified by solution physical are well know in the art, as illustrated in U.S. Pat. No. 3,719,490 (Yudelson et al).
Thermally processed non-silver photographic processes that incorporate redox amplification have also been described in the art. For example, imaging elements containing a photosensitive catalyst precursor, along with a physical development element comprising a Te(II) or Te(IV) compound, incorporated in a polymeric matrix with an organic reducing agent, are exposed to a suitable light source and then thermally developed to give a dense, black image of elemental tellurium. Such elements are referred to as “photothermographic” that is an initial exposure step produces nuclei which act as a catalyst for the chemical reduction of the Te(II) or Te(IV) compound to Te
0
by an organic reductant upon subsequent thermal development of the exposed element. Thus, a small amount of invisible photoproduct (the “latent image”) is converted into a high density image by utilizing its catalytic property to initiate a redox reaction with a high amplification factor. Thermally processed photothermographic elements of this type have been described in U.S. Pat. No. 4,097,281 (Gardner et al) and U.S. Pat. No. 4,152,155 (Lelental et al).
In contrast to the above imaging processes involving light exposures, there has been a continuing need to provide improved thermographic compositions and processes in which an element can be thermally addressed to give directly an image without the need for an initial light exposure step. The use of so-called dry silver elements for this purpose is well known in the art. Such e

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