Subbing layers for use with thermographic materials

Details Subbing layers for use with thermographic materials Subbing layers for use with thermographic materials

1998-08-20

2001-10-02

Chea, Thorl (Department: 1752)

Radiation imagery chemistry: process, composition, or product th

Radiation sensitive product

Silver compound sensitizer containing

C430S530000, C430S531000, C430S533000, C427S148000, C503S210000, C503S214000

Reexamination Certificate

active

06296999

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a substantially light-insensitive thermographic material comprising a subbing layer with a low concentration of leachable non-fluoro-halide ions.
BACKGROUND OF THE INVENTION
Thermal imaging or thermography is a recording process wherein images are generated by the use of thermal energy.
In thermography three approaches are known:
1. Direct thermal formation of a visible image pattern by image-wise heating of a recording material containing matter that by chemical or physical process changes colour or optical density.
2. Image-wise transfer of an ingredient necessary for the chemical or physical process bringing about changes in colour or optical density to a receptor element.
3. Thermal dye transfer printing wherein a visible image pattern is formed by transfer of a coloured species from an image-wise heated donor element onto a receptor element.
Most of the “direct” thermographic materials are of the chemical type. On heating to a certain conversion temperature, an irreversible chemical reaction takes place and a coloured image is produced. A wide variety of chemical systems has been suggested some examples of which have been given on page 138 of “Imaging Systems” by Kurt I. Jacobson-Ralph E. Jacobson, The Focal Press—London and New York (1976), describing the production of a silver metal image by means of a thermally induced oxidation-reduction reaction of a silver soap with a reducing agent.
JP 57-69,095 discloses a support coated with a subbing layer containing an ionomer, and then coated with a heat-sensitive recording layer whose solubility in water decreases upon heating to give a thermographic sheet. EP-A 496 017 discloses a heat-sensitive recording medium, comprising: (a) a synthetic-resin support; (b) an ionomer-resin layer on said support; and (c) a heat-sensitive recording layer on said ionomer resin layer.
GB 2 114 767 discloses a thermosensitive recording sheet comprising: a support material; a primer layer formed on the support material and comprising a filler and a binder agent; a thermosensitive colouring layer formed on the primer layer and comprising a colourless light-coloured leuco dye, and an acidic material which colours the leuco dye upon application of heat hereto; and a protective layer formed on the thermosensitive colouring layer and comprising a water-soluble agent and a filler.
U.S. Pat. No. 5,006,451 discloses in the specification that “polymeric subbing layers used to promote the adhesion of coating compositions to polyester film supports are very well known in the photographic art. Useful compositions for this purpose include interpolymers of vinylidene chloride such as vinylidene chloride/acrylonitrile/acrylic acid-terpolymers or vinylidene chloride/methyl acrylate/itaconic acid-terpolymers”.
However, the inventors of the present invention found that the light-stability and archivability of substantially light-insensitive thermographic materials coated from aqueous media were surprisingly, considering their thinness relative to the thickness of the thermosensitive element thereof, dependent upon the choice of subbing layer and that, moreover, polymer subbing layers used to promote adhesion between polyester support and conventional photographic emulsion layers were in the main unusable (see COMPARATIVE EXAMPLE 2) due to prohibitive light-sensitivity and poor archivability. There is therefore a need for subbing layers for use on the supports of thermographic materials, which have no adverse effect upon the light-stability and archivability of thermographic materials.
OBJECT OF THE INVENTION
It is therefore an object of the present invention to provide thermographic materials coatable from aqueous media which exhibit improved archivability and/or improved light stability, while maintaining high maximum density and low minimum density levels upon printing.
Further objects and advantages of the invention will become apparent from the description hereinafter.
SUMMARY OF THE INVENTION
Thermographic materials adhere poorly to unsubbed polyethylene terephthalate, as is shown in COMPARATIVE EXAMPLE 1. In the photographic art adhesion of the gelatinous silver halide emulsion layers to a polyethylene terephthalate support is attained by the use of a subbing layer consisting of two sub-layers, one to render the polyethylene terephthalate support sufficiently hydrophilic to adhere to a gelatinous layer to which gelatinous silver halide emulsion layers would adhere. COMPARATIVE EXAMPLE 2 shows that such a subbing layer indeed provides good adhesion between the polyethylene terephthalate support and gelatinous thermographic materials, but at the expense of poorer archivability and lower light stability. It is surprising that the composition of a subbing layer for the polymeric or polymer-coated support on the same side of the support as the thermosensitive element has a considerable influence on the light-stability and archivability of a substantially light-insensitive black and white thermographic material despite the thinness of a subbing layer compared to that of the thermosensitive element. In particular certain ingredients such as silica, whether colloidal or non-colloidal, have, above a concentration of 20% by weight, and the presence of non-fluoro-halide ions leachable into water at a concentration of 0.6 mg/m
2
or more have been found to have a prohibitive effect on the light-stability and archivability of thermographic materials coated from aqueous media.
The above-mentioned objects are realized with a substantially light-insensitive black and white thermographic material comprising a polymeric or polymer-coated support, a subbing layer on the support and on the same side of the support as the subbing layer a thermosensitive element containing a substantially light-insensitive organic silver salt, a reducing agent therefor in thermal working relationship therewith and a binder, wherein the subbing layer contains a binder, less than 20% by weight of silica and covalently bonded acid groups in said binder, if present, are either substantially present as free acid or substantially present as acid salts and has a leachable non-fluoro-halide ion content into water at room temperature over a period of 120 minutes of less than 0.6 mg/m
2
.
A process is also provided for producing the above-referred to substantially light-insensitive thermographic material comprising the steps of: coating the support with a subbing-layer composition thereby forming the subbing layer; producing one or more aqueous coating compositions together containing the substantially light-insensitive organic silver salt, the reducing agent and the binder;
and applying the one or more aqueous coating compositions to the same side of the support as the subbing layer thereby forming after drying the thermosensitive element.
Preferred embodiments of the present invention are disclosed in the detailed description of the invention.
DETAILED DESCRIPTION OF THE INVENTION
Subbing Layer
The subbing layer used in accordance with the present invention contains a binder, has less than 20% by weight of silica, a leachable non-fluoro-halide ion content into water at room temperature of less than 0.6 mg/m
2
over a period of 120 minutes, and when the binder has covalently bonded acid groups the acid groups are either substantially present as free acid or substantially present as acid salts. The subbing layer used in accordance with the present invention may consist of one or more sub-layers. The leachable non-fluoro-halide ion is preferably a chloride ion.
By the term ionic group is meant an ionized group, for example carboxylate, sulfinate, sulfonate, quaternary ammonium, quaternary phosphonium, ternary sulfonium and phosphate groups. Suitable binders include any natural, modified natural or synthetic resins, polysilicic acid, hydrolyzed polyalkoxysilanes etc. or mixtures thereof.
The preferred leachable non-fluoro-halide ion content of the subbing layer used in accordance with the present invention is less than 0.5 mg/m
2
, with less than 0.4

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