Field of the invention
[0001] The present invention relates to thermographic recording materials whose prints have
improved image tone.
Background of the invention.
[0002] Thermal imaging or thermography is a recording process wherein images are generated
by the use of thermal energy. In direct thermal thermography a visible image pattern
is formed by image-wise heating of a recording material containing matter that by
chemical or physical process changes colour or optical density. Such recording materials
become photothermographic upon incorporating a photosensitive agent which after exposure
to UV, visible or IR light is capable of catalyzing or participating in a thermographic
process bringing about changes in colour or optical density.
[0003] Research Disclosure number 17029, published in June 1978, gives a survey of different
methods of preparing organic heavy metal salts in section II. The invention examples
of US-P 5,380,635 and US-P 5,434,043 describe the production of organic silver salts
using fatty acids of the type HUMKO Type 9718 & Type 9022 from WITCO Co., which contain
according to the manufacturer's catalogue a mixture of different fatty acids, in connection
with their use in photothermographic recording materials. DE-OS 27 21 828 discloses
a thermally developable light-sensitive material, consisting of a support, which contains
thereon or in one or more layers at least (a) an organic silver salt, (b) a photocatalyst
and (c) a reducing agent, wherein the organic silver salt (a) contains at least a
silver salt with an uneven number of 21 or more carbon atoms; and examples with mixtures
of two and three organic silver salts of monocarboxylic acids precipitated together,
but all with 20 are more carbon atoms.
[0004] US-P 5,677,121 discloses a heat-developable silver halide infrared ray-sensitive
material comprising a support having on one side of the support an emulsion layer
containing a binder, a nonsensitive silver salt, a reducing agent for silver ion and
silver halide grains spectrally sensitized at a wavelength within the region of from
750 to 1400nm, wherein the nonsensitive silver salt comprises a mixture of silver
salts of at least three organic carboxylic acids, one of the acids is behenic acid,
and the content of the behenic acid in the acids is from not less than 35 to less
than 90 mol %.
[0005] However, technology from photothermographic materials on the basis of an organic
silver salt, silver halide and a reducing agent is not readily extrapolatable to substantially
light-insensitive thermographic recording materials on the basis of an organic silver
salt and a reducing agent, since thermographic recording materials are subjected to
image-wise heating whereas photothermographic materials are subjected to image-wise
exposure and overall heating and much stronger reducing agents are used in thermographic
recording materials than in photothermographic recording materials. Furthermore, thermographic
recording materials are heated for much shorter times, typically 10 to 20 ms, during
thermal development in thermographic printing than photothermographic recording materials,
for which 10s is an average heating time. Such shorter heating times make it difficult
to obtain neutral image tones.
[0006] EP-A 730 196 discloses a heat-sensitive recording material suited for use in direct
thermal imaging and having image-stabilization properties which material contains
in a binder on a support (i) a substantially light-insensitive organic silver salt
capable of thermally activated reduction to silver in thermal working relationship
with (ii) at least one reducing agent capable of reducing the substantially light-insensitive
organic silver salt when thermally activated, characterized in that the recording
material contains in admixture with the reducing agent(s) at least one colourless
photo-oxidizing substance that on exposure to ultraviolet radiation yields free radicals
capable of inactivating the reducing agent(s) by oxidation, thereby rendering the
reducing agent(s) incapable of reducing the organic silver salt to silver. Furthermore,
in sub-claims the organic silver salt is silver palmitate, silver stearate or silver
behenate or mixtures thereof. However, the efficacy of such physical mixtures is not
exemplified. Physical mixtures in which each component forms a separate phase cannot
be equated with mixed crystals in which the components together form a single phase.
[0007] Prior art substantially light-insensitive black and white thermographic recording
materials exhibit an insufficiently neutral image colour. This is particularly important
for thermographic recording materials for medical diagnostic applications for which
image tone requirements are particularly severe, particularly at low optical densities.
Prior art thermographic recording materials coated from solvent exhibit image tone
closer to these requirements than those coated from aqueous media, although the latter
are producible using much more environmentally friendly coating processes.
Objects of the invention.
[0008] It is therefore an object of the present invention to provide substantially light-insensitive
black and white thermographic recording materials coated from solvent media whose
prints exhibit a more neutral image tone.
[0009] It is therefore another object of the present invention to provide substantially
light-insensitive black and white thermographic recording materials coated from aqueous
media whose prints exhibit a more neutral image tone.
[0010] Further objects and advantages of the invention will become apparent from the description
hereinafter.
Summary of the invention
[0011] Surprisingly it has been found that substantially light-insensitive black and white
thermographic recording materials coated from solvent or aqueous media and comprising
mixed crystals of substantially light insensitive organic silver salts exhibit a more
neutral image tone than physical mixtures thereof.
[0012] The above mentioned objects are realized by a substantially light-insensitive black
and white thermographic recording material having a support and a thermosensitive
element containing a substantially light-insensitive organic silver salt, an organic
reducing agent therefor in thermal working relationship therewith and a binder, wherein
the thermosensitive element contains substantially light-insensitive mixed crystals
of two or more silver salts of organic carboxylic acids with one or more carboxylic
acid groups.
[0013] A recording process is further provided according to the present invention comprising
the steps of: (i) bringing an outermost layer of the above-mentioned thermographic
recording material in proximity with a heat source; and (ii) applying heat from the
heat source imagewise to the recording material while maintaining proximity to the
heat source to produce an image; and (iii) removing the recording material from the
heat source.
[0014] Preferred embodiments of the invention are disclosed in the dependent claims.
Detailed description of the invention.
[0015] In a preferred embodiment of the recording process, according to the present invention,
the heat source is a thermal head with a thin film thermal head being particularly
preferred.
Substantially
[0016] By substantially light-insensitive is meant not intentionally light sensitive. By
substantially solvent-free aqueous medium is meant that solvent, if present, is present
in amounts below 10% by volume of the aqueous medium.
Substantially light-insensitive mixed crystals of two or more organic silver salts
[0017] The substantially light-insensitive mixed crystals of two or more silver salts of
organic carboxylic acids with one or more carboxylic acid groups of the present invention
are produced by slow addition, preferably metered, of a soluble silver salt to a solution
or dispersion of a mixture of acids, or their salts, whose silver salts are capable
of forming mixed crystals.
[0018] Mixed crystals of the present invention have the X-ray diffraction pattern of the
organic silver salt which is present in the greatest quantity, although the peaks
may be slightly shifted compared with the X-ray diffraction spectrum of pure crystals
of the organic silver salt present in the greatest quantity.
[0019] It is preferred that the silver salts of organic carboxylic acids with one or more
carboxylic acid groups are present in the mixed crystals of the mixed crystals used
in the thermographic recording material of the present invention in molar concentrations
of at least 5 mole% and particularly preferably with molar concentrations of at least
8 mole%.
[0020] It is also preferred that at least one of the two or more organic silver salts in
the mixed crystals used in the thermographic recording material of the present invention
is a silver salt of an aliphatic monocarboxylic acid with at least 12 carbon atoms,
e.g. silver laurate, silver palmitate, silver stearate, silver hydroxystearate, silver
behenate, silver arichidate and silver salts of modified aliphatic carboxylic acids
with thioether group as described e.g. in GB-P 1,111,492. Particularly preferred silver
salts of aliphatic carboxylic acids are selected from the group consisting silver
stearate, silver arichidate and silver behenate.
[0021] It is further preferred that the total molar concentration of silver salts of aliphatic
monocarboxylic acids in the mixed crystals used in the thermographic recording materials
of the present invention is at least 40 per cent and particularly preferably at least
51% per cent.
[0022] In a particular embodiment of the present invention, the mixed crystals consist of
two or more silver salts of aliphatic monocarboxylic acids with at least 12 carbon
atoms, with three or more silver salts of aliphatic monocarboxylic acids with at least
12 carbon atoms being preferred and mixed crystals consisting of a mixture of silver
stearate, silver behenate and silver arichidate being particularly preferred. The
mixed crystals consisting of two or more silver salts of aliphatic monocarboxylic
acids with at least 12 carbon atoms may be used on their own or in admixture with
one or more organic silver salt in the thermosensitive element.
[0023] In a further embodiment of the present invention, one of the two or more organic
silver salts is a silver salt of an aliphatic dicarboxylic acid. Preferred aliphatic
dicarboxylic acids are selected from the group consisting of silver adipate, silver
pimelate, silver suberate, silver azealate, silver sebacate, silver nonane-dicarboxylate,
silver decane-dicarboxylate and silver undecane-dicarboxylate. It is preferred that
the molar concentration of the silver salt of an aliphatic dicarboxylic acid in the
mixed crystals of two or more organic silver salts is at least 15 per cent. Furthermore
it is preferred that the molar concentration of silver salt of an aliphatic dicarboxylic
acid in the mixed crystals of two or more organic silver salts is less than 50 per
cent. The mixed crystals of two or more organic carboxylic acid may be used on their
own or in admixture with one or more organic silver salt in the thermosensitive element.
[0024] Any organic silver salt may be used in admixture with the mixed crystals of the present
invention. Preferred organic silver salts are silver salts of aliphatic monocarboxylic
acids, known as fatty acids, wherein the aliphatic carbon chain has preferably at
least 12 C-atoms.
Substantially light-insensitive organic silver salt dispersions
[0025] Mixed crystals of two or more organic silver salts may be dispersed by standard dispersion
techniques e.g. using ball mills, bead mills, microfluidizers, ultrasonic apparatuses,
rotor stator mixers etc. have been found to be useful in this regard.
Thermosensitive element
[0026] The thermosensitive element, according to the present invention, comprises substantially
light-insensitive mixed crystals of two or more organic silver salts, an organic reducing
agent therefor in thermal working relationship therewith and a binder. The element
may comprise a layer system in which the ingredients may be dispersed in different
layers, with the proviso that the two ingredients are in reactive association with
one another i.e. during the thermal development process the reducing agent must be
present in such a way that it is able to diffuse to the mixed crystal of two or more
organic silver salts and any organic silver salt present so that reduction to silver
can occur.
Reducing agents
[0027] Suitable organic reducing agents for the reduction of mixed crystals of two or more
organic silver salts are organic compounds containing at least one active hydrogen
atom linked to O, N or C.
[0028] Catechol-type reducing agents, i.e. reducing agents containing at least one benzene
nucleus with two hydroxy groups (-OH) in ortho-position, such as catechol, 3-(3,4-dihydroxyphenyl)
propionic acid, 1,2-dihydroxybenzoic acid, gallic acid and esters e.g. methyl gallate,
ethyl gallate, propyl gallate, tannic acid, and 3,4-dihydroxy-benzoic acid esters
are preferred, with those described in EP-B 692 733 and EP-A 903 625.
[0029] Combinations of reducing agents may also be used that on heating become reactive
partners in the reduction of the substantially light-insensitive organic silver salt
containing mixed crystals of two or more organic silver salts. For example, combinations
of sterically hindered phenols with sulfonyl hydrazide reducing agents such as disclosed
in US-P 5,464,738; trityl hydrazides and formyl-phenyl-hydrazides such as disclosed
in US-P 5,496,695; trityl hydrazides and formyl-phenyl-hydrazides with diverse auxiliary
reducing agents such as disclosed in US-P 5,545,505, US-P 5.545.507 and US-P 5,558,983;
acrylonitrile compounds as disclosed in US-P 5,545,515 and US-P 5,635,339; and 2-substituted
malonodialdehyde compounds as disclosed in US-P 5,654,130.
Film-forming binders of the thermosensitive element
[0030] The film-forming binder of the thermosensitive element containing mixed crystals
of two or more organic silver salts may be all kinds of natural, modified natural
or synthetic resins or mixtures of such resins, in which the mixed crystals of two
or more organic silver salts can be dispersed homogeneously either in aqueous or solvent
media: e.g. cellulose derivatives such as ethylcellulose, cellulose esters, e.g. cellulose
nitrate, carboxymethylcellulose, starch ethers, galactomannan, polymers derived from
α,β-ethylenically unsaturated compounds such as polyvinyl chloride, after-chlorinated
polyvinyl chloride, copolymers of vinyl chloride and vinylidene chloride, copolymers
of vinyl chloride and vinyl acetate, polyvinyl acetate and partially hydrolyzed polyvinyl
acetate, polyvinyl alcohol, polyvinyl acetals that are made from polyvinyl alcohol
as starting material in which only a part of the repeating vinyl alcohol units may
have reacted with an aldehyde, preferably polyvinyl butyral, copolymers of acrylonitrile
and acrylamide, polyacrylic acid esters, polymethacrylic acid esters, polystyrene
and polyethylene or mixtures thereof.
[0031] Suitable water-soluble film-forming binders for use in thermographic recording materials
according to the present invention are: polyvinyl alcohol, polyacrylamide, polymethacrylamide,
polyacrylic acid, polymethacrylic acid, polyvinylpyrrolidone, polyethyleneglycol,
proteinaceous binders such as gelatin, modified gelatines such as phthaloyl gelatin,
polysaccharides, such as starch, gum arabic and dextran and water-soluble cellulose
derivatives. A preferred water-soluble binder for use in the thermographic recording
materials of the present invention is gelatin.
[0032] Preferred water-dispersible binders for use according to the present invention are
water-dispersible film-forming polymers with covalently bonded ionic groups selected
from the group consisting of sulfonate, sulfinate, carboxylate, phosphate, quaternary
ammonium, tertiary sulfonium and quaternary phosphonium groups. Further preferred
water-dispersible binders for use according to the present invention are water-dispersible
film-forming polymers with covalently bonded moieties with one or more acid groups.
Water-dispersible binders with crosslinkable groups, e.g. epoxy groups, aceto-acetoxy
groups and crosslinkable double bonds are also preferred. Particularly preferred water-dispersible
binders for use in the thermographic recording materials of the present invention
are polymer latexes.
Toning agent
[0033] In order to obtain a neutral black image tone in the higher densities and neutral
grey in the lower densities, the thermosensitive element preferably further contains
a so-called toning agent known from thermography or photothermography.
[0034] Suitable toning agents are the phthalimides and phthalazinones within the scope of
the general formulae described in US-P 4,082,901. Further reference is made to the
toning agents described in US-P 3,074,809, 3,446,648 and 3,844,797. Other particularly
useful toning agents are the heterocyclic toner compounds of the benzoxazine dione
or naphthoxazine dione type as disclosed in GB-P 1,439,478, US-P 3,951,660 and US-P
5,599,647.
Stabilisers and antifoggants
[0035] In order to obtain improved shelf-life and reduced fogging, stabilizers and antifoggants
may be incorporated into the thermographic recording materials of the present invention.
Polycarboxylic acids and anhydrides thereof
[0036] According to the recording material of the present invention the thermosensitive
element preferably further contains at least one polycarboxylic acid and/or anhydride
thereof in a molar percentage of at least 10 with respect to all the organic silver
salt(s) present and in thermal working relationship therewith, with a molar percentage
of at least 15 with respect to all the organic silver salt(s) being particularly preferred.
The polycarboxylic acid may be aliphatic (saturated as well as unsaturated aliphatic
and also cycloaliphatic) or an aromatic polycarboxylic acid. These acids may be substituted
e.g. with alkyl, hydroxyl, nitro or halogen. They may be used in anhydride form or
partially esterified on the condition that at least two free carboxylic acids remain
or are available in the heat recording step.
Surfactants and dispersion agents
[0037] Surfactants and dispersants aid the dispersion of ingredients or reactants which
are insoluble in the particular dispersion medium. The thermographic recording materials
of the present invention may contain one or more surfactants, which may be anionic,
non-ionic or cationic surfactants and/or one or more dispersants.
Other additives
[0038] The recording material may contain in addition to the ingredients mentioned above
other additives such as antistatic agents, e.g. non-ionic antistatic agents including
a fluorocarbon group as e.g. in F
3C(CF
2)
6CONH(CH
2CH
2O)-H, silicone oil, e.g. BAYSILONE™ Öl MA (from BAYER AG, GERMANY), ultraviolet light
absorbing compounds, white light reflecting and/or ultraviolet radiation reflecting
pigments and/or optical brightening agents.
Support
[0039] The support for the thermosensitive element according to the present invention may
be transparent, translucent or opaque, e.g. having a white light reflecting aspect
and is preferably a thin flexible carrier made e.g. from polypropylene, polycarbonate
or polyester, e.g. polyethylene terephthalate.
[0040] The support may be in sheet, ribbon or web form and subbed if need be to improve
the adherence to the thereon coated thermosensitive element. The support may be made
of an opacified resin composition. Should a transparent base be used, the base may
be colourless or coloured, e.g. having a blue colour. One or more backing layers may
be provided to control physical properties such as curl and static.
Outermost layer
[0041] The outermost layer of the recording material may in different embodiments of the
present invention be the outermost layer of the thermosensitive element, a protective
layer applied to the thermosensitive element or a layer on the opposite side of the
support to the thermosensitive element.
Protective layer
[0042] According to a preferred embodiment of the recording material, according to the present
invention, the thermosensitive element is provided with a protective layer to avoid
local deformation of the thermosensitive element and to improve resistance against
abrasion.
[0043] The protective layer preferably comprises a binder, which may be solvent-soluble,
solvent-dispersible, water-soluble or water-dispersible. Among the solvent-soluble
binders polycarbonates as described in EP-A 614 769 are particularly preferred. However,
water-soluble or water-dispersible binders are preferred for the protective layer,
as coating can be performed from an aqueous composition and mixing of the protective
layer with the immediate underlayer can be avoided by using a solvent-soluble or solvent-dispersible
binder in the immediate underlayer.
[0044] A protective layer according to the present invention may comprise in addition a
thermomeltable particle optionally with a lubricant present on top of the protective
layer as described in WO 94/11199. In a preferred embodiment at least one solid lubricant
having a melting point below 150°C and at least one liquid lubricant in a binder is
present, wherein at least one of the lubricants is a phosphoric acid derivative.
Crosslinking agents for outermost layer
[0045] The outermost layer according to the present invention may be crosslinked. Crosslinking
can be achieved by using crosslinking agents such as described in WO 95/12495 for
protective layers, e.g. tetra-alkoxysilanes, polyisocyanates, zirconates, titanates,
melamine resins etc., with tetraalkoxysilanes such as tetramethylorthosilicate and
tetraethylorthosilicate being preferred.
Matting agents for outermost layer
[0046] The outermost layer of the recording material according to the present invention
may comprise a matting agent. Suitable matting agents are described in WO 94/11198
and include e.g. talc particles and optionally protrude from the outermost layer.
Lubricants for outermost layer
[0047] Solid or liquid lubricants or combinations thereof are suitable for improving the
slip characteristics of the thermographic recording materials according to the present
invention. Preferred solid lubricants are thermomeltable particles such as those described
in WO 94/11199.
Antihalation dyes
[0048] In addition to the ingredients, the thermographic recording materials used in the
present invention may also contain antihalation or acutance dyes which absorb infra-red
light, for absorption by a dye which converts the absorbed infra-red light into heat,
which has passed through the thermosensitive element thereby preventing its reflection.
Such dyes may be incorporated into the thermosensitive element or in any other layer
of the recording material of the present invention.
Coating
[0049] The coating of any layer of the recording material of the present invention may proceed
by any coating technique e.g. such as described in Modern Coating and Drying Technology,
edited by Edward D. Cohen and Edgar B. Gutoff, (1992) VCH Publishers Inc. 220 East
23rd Street, Suite 909 New York, NY 10010, U.S.A.
Thermographic processing
[0050] Thermographic imaging is carried out by the image-wise application of heat either
in analogue fashion by direct exposure through an image of by reflection from an image,
or in digital fashion pixel by pixel either by using an infra-red heat source, for
example with a Nd-YAG laser or other infra-red laser, with a thermographic material
preferably containing an infra-red absorbing compound, or by direct thermal imaging
with a thermal head.
[0051] In thermal printing image signals are converted into electric pulses and then through
a driver circuit selectively transferred to a thermal printhead. The thermal printhead
consists of microscopic heat resistor elements, which convert the electrical energy
into heat via Joule effect. Such thermal printing heads may be used in contact or
close proximity with the recording material. The operating temperature of common thermal
printheads is in the range of 300 to 400°C and the heating time per picture element
(pixel) may be less than 1.0ms, the pressure contact of the thermal printhead with
the recording material being e.g. 200-500g/cm
2 to ensure a good transfer of heat.
[0052] In order to avoid direct contact of the thermal printing heads with the outermost
layer on the same side of the support as the thermosensitive element when this outermost
layer is not a protective layer, the image-wise heating of the recording material
with the thermal printing heads may proceed through a contacting but removable resin
sheet or web wherefrom during the heating no transfer of recording material can take
place.
[0053] Activation of the heating elements can be power-modulated or pulse-length modulated
at constant power. The image-wise heating can be carried out such that heating elements
not required to produce an image pixel generate an amount of heat (H
e) in accordance with the following formula: 0.5 H
D < H
e < H
D wherein H
D represents the minimum amount of heat required to cause visible image formation in
the recording material.
[0054] EP-A 654 355 discloses a method for making an image by image-wise heating by means
of a thermal head having energizable heating elements, wherein the activation of the
heating elements is executed duty cycled pulsewise. EP-A 622 217 discloses a method
for making an image using a direct thermal imaging element producing improvements
in continuous tone reproduction.
[0055] Image-wise heating of the recording material can also be carried out using an electrically
resistive ribbon incorporated into the material. Image- or pattern-wise heating of
the recording material may also proceed by means of pixel-wise modulated ultra-sound.
Industrial application
[0056] Thermographic imaging can be used for the production of transparencies and reflection
type prints. Application of the present invention is envisaged in the fields of both
graphics images requiring high contrast images with a very steep dependence of print
density upon applied dot energy and continuous tone images requiring a weaker dependence
of print density upon applied dot energy, such as required in the medical diagnostic
field. In the hard copy field thermographic recording materials on a white opaque
base are used, whereas in the medical diagnostic field black-imaged transparencies
are widely used in inspection techniques operating with a light box.
[0057] The invention is illustrated hereinafter by way of invention examples and comparative
examples. The percentages and ratios given in these examples are by weight unless
otherwise indicated. The ingredients used in the invention and comparative examples,
other than those mentioned above, are:
- the aliphatic carboxylic acids:
HAr = arachidic acid;
HB = behenic acid;
HPa = palmitic acid
HSt = stearic acid;
HAd = adipic acid;
HSeb = sebacic acid;
HSuc = succinic acid;
- the silver salts of aliphatic carboxylic acids:
AgAr = silver arachidate
AgB = silver behenate;
AgSt = silver stearate;
AgPa = silver palmitate
AgAd = silver adipate;
AgSeb = silver sebacate;
AgSuc = silver succinate;
- the binders:
B79 = BUTVAR™ B79, a polyvinyl butyral from MONSANTO;
K7598 = type 7598, a calcium-free gelatin from AGFA-GEVAERT GELATINEFABRIEK vorm.
KOEPFF & SÖHNE;
K17881 = type 17881, a calcium-free gelatin from AGFA-GEVAERT GELATINEFABRIEK vorm.
KOEPFF & SÖHNE;
- the reducing agent:
R01 = ethyl 3,4-dihydroxybenzoate;
- the toning agents:
T01 = 7-(ethylcarbonato)-benzo[e][1,3]oxazine-2,4-dione;
T02 = benzo[e][1,3]oxazine-2,4-dione; and
- the silicone oil:
BAYSILON™ MA, a polydimethylsiloxane from BAYER;
- the surfactants:
Surfactant Nr. 1 = MARLON™ A-365, a 65% concentrate of a sodium alkyl-phenylsulfonate
from HÜLS;
PREPARATION OF ORGANIC SILVER SALT TYPES I TO XIII
[0058] The behenic acid and the second acid, where appropriate, (together 0.8mol of carboxylic
acid)(see table 1 for quantity) was added to ca. 800mL of 2-butanone in a 5L vessel
and the dispersion heated with stirring at 350rpm to 70°C giving a clear solution.
Ca. 1.1L of 0.75M aqueous sodium hydroxide was added slowly until a pH of ca. 9.9
was attained then after 5 minutes further stirring ca. 1L of 0.8M aqueous silver nitrate
was added at a constant rate of ca. 260mL/h until a UAg (defined as the potential
difference between a silver electrode of ≥ 99.99% purity in the aqueous liquid and
a reference electrode consisting of a Ag/AgCl-electrode in 3M KCl solution at room
temperature connected with the aqueous liquid via a salt bridge consisting of a 10%
KNO
3 salt solution) of 315mV was attained, thereby producing a ca. 12% dispersion of organic
silver salt. The organic silver salt was then filtered off and washed four times with
deionized water with 2% of 2-propanol, after which it was dried for 72 hours at 45°C.
Table 1
Organic silver salt type |
quantity of HB |
quantity of other acid |
quantity of 2-butanone [mL] |
volume of 0.75M NaOH added [mL] |
0.8M AgNO3 |
|
mol |
type |
mol |
|
|
volume added [mL] |
addition time [min] |
I |
0.455 |
HSt/HAr |
0.09/0.455 |
800 |
1096 |
1000 |
240 |
II |
0.455 |
HSt/HAr |
0.09/0.455 |
800 |
1096 |
1000 |
240 |
III |
0.75 |
HAd |
0.05 |
800 |
1158# |
945 |
229 |
IV |
0.402 |
HAd |
0.198 |
773 |
1087# |
935 |
245 |
V |
0.75 |
HSeb |
0.05 |
800 |
1156# |
950 |
254 |
VI |
0.402 |
HSeb |
0.198 |
773 |
1083 |
875 |
219 |
VII |
0.402 |
HSuc |
0.198 |
773 |
1085 |
865 |
237 |
VIII |
0.675 |
HSt |
0.075 |
750 |
1015 |
938 |
238 |
IX |
0.525 |
HSt |
0.225 |
750 |
1019 |
938 |
255 |
X |
0.375 |
HSt |
0.375 |
750 |
1016 |
938 |
242 |
XI |
0.450 |
HSt/HPa |
0.15/0.15 |
750 |
1012 |
939 |
245 |
XII |
0.80 |
- |
- |
800 |
1096 |
1000 |
240 |
XIII |
- |
HSt |
0.75 |
750 |
992 |
926 |
66 |
[0059] X-ray diffraction spectra were then run on the dried organic silver salts of types
I to VII and X to XII with an X-ray diffractometer using a CuKα X-ray source at a
current of 30mA and an energy of 40kV in the Bragg angle 2Θ range 1.5 to 55° with
a step-size of 0.05° and a step-time of 1s. The XRD-spectra obtained all corresponded
to the reference spectrum of the Joint Committee on Powder Diffraction Standards (JCPDS)
Powder Diffraction File for AgB: 4-48, published by the International Centre for Diffraction
Data, 12 Campus Boulevard, Newtown Square, Pennsylvania 19073-3273 U.S.A. Qualifying
remarks for the different organic silver salt types are given in table 2 below. These
XRD-spectra clearly demonstrate the presence of mixed crystals in the case of organic
silver salt types I to VII and X to XII.
Table 2
Organic silver salt type |
Composition |
XRD-spectra in comparison with the reference spectrum for AgB(JCPDS# 4-48) |
|
AgB mol% |
2nd silver salt |
|
|
|
type |
mol% |
|
I |
45.5 |
AgSt/AgAr |
9/45.5 |
AgB-peaks shifted to slightly larger angles, no extra peaks |
II |
45.5 |
AgSt/AgAr |
9/45.5 |
AgB-peaks shifted to slightly larger angles, no extra peaks |
III |
93.34 |
AgAd |
6.66 |
AgB-peaks shifted to slightly larger angles, no extra peaks |
IV |
67 |
AgAd |
33 |
as for type III except AgB-peaks more strongly shifted, extra signals at 2Θ = 8.3°
& 29.03 |
V |
93.34 |
AgSeb |
6.66 |
AgB-peaks more strongly shifted to larger angles than for type III, no extra peaks |
VI |
67 |
AgSeb |
33 |
as for type IV except for weak extra peaks at 2Θ = 5.7° & 11.3° |
VII |
67 |
AgSuc |
33 |
AgB-peaks shifted to slightly larger angles, extra peaks at 2Θ = 26.4° (very weak),
29.37°, 30.22°, 39.15° |
VIII |
90 |
AgSt |
10 |
no XRD spectrum run |
IX |
70 |
AgSt |
30 |
no XRD spectrum run |
X |
50 |
AgSt |
50 |
in addition to AgB, a second phase with the same crystal structure was present shifted
to larger angles, no AgSt phase was identifiable |
XI |
60 |
AgSt/AgPa |
20/20 |
AgB-peaks shifted to slightly smaller angles. no extra peaks |
XII |
100 |
- |
- |
extra signals at 2Θ = 20.8° & 32.4° |
XIII |
- |
AgSt |
100 |
no XRD spectrum run |
# Joint Committee on Powder Diffraction Standards |
INVENTION EXAMPLES 1 to 10 and COMPARATIVE EXAMPLE 1
preparation of organic silver salt dispersions
[0060] 10g of organic silver salt (for composition see table 2) was mixed with 2.5g of a
30% solution of B79 in 2-butanone and 737.5g of 2-butanone for 72 hours in a ball
mill. 30.83g of a 30% solution of B79 in 2-butanone and 7.67g of 2-butanone were then
added to the resulting dispersion and the mixture ball milled for a further 60 minutes.
preparation of coating dispersions
[0061] 40g of the organic silver salt dispersion was mixed with 29.86g of a 30% solution
of B79, 0.90g of a 2-butanone dispersion containing 4.13% of B79, 14.01% of T01 and
25.86%, 0.40g of BAYSILON™ MA and 0.323g of adipic acid and 15g of 2-butanone to a
homogeneous dispersion. 1.844g of R01, 0.288g of tetrachlorophthalic anhydride, 0.24g
of benzotriazole and 17.08g of 2-butanone were then mixed separately and 9.59g thereof
was added to the homogenized organic silver salt dispersion prior to doctor blade
coating with the blade adjusted to 170µm onto a 175µm thick subbed polyethylene terephthalate
support. The resulting layers were dried for 1 hour at 50°C to produce the thermosensitive
elements of the thermographic recording materials of INVENTION EXAMPLES 1 to 10 and
COMPARATIVE EXAMPLE 1 produced with type I to VII and IX to XI mixed crystals of organic
silver salts and the silver behenate reference, organic silver salt type XII, respectively.
After drying the thermographic recording materials of INVENTION EXAMPLES 1 to 7 and
COMPARATIVE EXAMPLE 1 were subjected to 7 days conditioning at 45°C and 70% relative
humidity to produce "fresh materials" for printing.
thermographic printing
[0062] During the thermographic printing of the thermographic recording materials of INVENTION
EXAMPLES 1 to 10 and COMPARATIVE EXAMPLE 1, the print head was separated from the
imaging layer by a thin intermediate material contacted with a slipping layer of a
separable 5µm thick polyethylene terephthalate ribbon coated successively with a subbing
layer, heat-resistant layer and the slipping layer (anti-friction layer) giving a
ribbon with a total thickness of 6µm.
[0063] The printer was equipped with a thin film thermal head with a resolution of 300 dpi
and was operated with a line time of 19ms (the line time being the time needed for
printing one line). During this line time the print head received constant power.
The average printing power, being the total amount of electrical input energy during
one line time divided by the line time and by the surface area of the heat-generating
resistors was 1.6 mJ/dot being sufficient to obtain maximum optical density in each
of the thermographic recording materials of INVENTION EXAMPLES 1 to 10 and COMPARATIVE
EXAMPLE 1.
[0064] The maximum density, D
max, of the prints given in table 3 were measured through a visible filter with a MACBETH™
TR924 densitometer in the grey scale step corresponding to data levels of 64 and 0
respectively and the D
max-values are given in table 3 for INVENTION EXAMPLE 1 to 10 and COMPARATIVE EXAMPLE
1 together with the change in D
max-values upon printing a fresh material subjected to 7 days conditioning at 45°C and
70% relative humidity (RH) compared with printing a fresh material also measured through
a visible filer.
Image evaluation
[0065] The image tone of fresh prints made with the thermographic recording materials of
INVENTION EXAMPLES 1 to 10 and COMPARATIVE EXAMPLE 1 were assessed on the basis of
the L*, a* and b* CIELAB-values. The L*, a* and b* CIELAB-values were determined by
spectro-photometric measurements according to ASTM Norm E179-90 in a R(45/0) geometry
with evaluation according to ASTM Norm E308-90. The a* and b* CIELAB-values of fresh
prints of the thermographic recording materials of INVENTION EXAMPLES 1 to 10 and
COMPARATIVE EXAMPLE 1 at optical densities, D, of 0.5 and 1.0 are summarized in table
3.
Table 3
Invention example number |
Organic silver salt type |
Ag coverage [g/m2] |
fresh material Dmaxvisible |
ΔDmax visible for printing after 7d at 45°C/70% RH |
CIELAB at D = 0.5 |
CIELAB at D = 1.0 |
|
|
|
|
|
a* |
b* |
a* |
b* |
1 |
I |
1.085 |
3.33 |
-0.33 |
-1.8 |
2.3 |
-2 |
0.8 |
2 |
II |
1.085 |
3.75 |
+0.10 |
-1.9 |
3.1 |
-2 |
2 |
3 |
III |
1.124 |
3.31 |
-0.06 |
-2.1 |
4.4 |
-2.3 |
3.2 |
4 |
IV |
1.480 |
1.78 |
+0.02 |
-0.9 |
4 |
-1.5 |
1.8 |
5 |
V |
1.118 |
3.70 |
-0.10 |
-2.2 |
4.7 |
-2.5 |
3.6 |
6 |
VI |
1.416 |
3.00 |
-0.25 |
-0.5 |
2.1 |
-0.3 |
1.6 |
7 |
VII |
1.511 |
2.21 |
-0.01 |
-0.6 |
5.4 |
-0.6 |
2.7 |
8 |
IX |
- |
- |
- |
-2.3 |
3.1 |
-2.6 |
2.4 |
9 |
X |
- |
- |
- |
-2.6 |
2.4 |
-2.6 |
1.4 |
10 |
XI |
- |
- |
- |
-2.4 |
2.4 |
-2.5 |
1.9 |
Comparative example nr |
|
|
|
|
|
|
|
|
1 |
XII |
1.041 |
3.52 |
+0.08 |
-2 |
4.5 |
-2.4 |
4 |
a* of blue base used = -8.34 |
b* of blue base used = -15.71 |
In terms of the visual perception of an image as a whole, the image tone of elements
of the image with a density of 1.0 have a stronger effect than the image tone of elements
with lower or higher optical. Furthermore, the image tone generally becomes more neutral
as the density increases. The CIELAB co-ordinates for an optical density of 1.0 are
therefore critical in assessing the perceived image tone of an image.
[0066] Colour neutrality on the basis of CIELAB-values corresponds to a* and b* values of
zero, with a negative a*-value indicating a greenish image-tone becoming greener as
a* becomes more negative, a positive a*-value indicating a reddish image-tone becoming
redder as a* becomes more positive, a negative b*-value indicating a bluish image-tone
becoming bluer as b* becomes more negative and a positive b*-value indicating a yellowish
image-tone becoming more yellow as b* becomes more positive.
[0067] In table 2, at least one of the CIELAB-values corresponding to an optical density
of 1.0 for prints with the thermographic recording materials of INVENTION EXAMPLES
1 to 10 is lower than the corresponding value for the reference thermographic recording
material of COMPARATIVE EXAMPLE 1. Prints with the thermographic recording materials
of INVENTION EXAMPLES 1 and 2 with mixed crystals of organic silver salts prepared
with HYSTRENE™ 9022 from WITCO exhibit good tone neutrality as did the prints made
with mixed crystals of organic silver salts of types IV and VI consisting of 67mol%
of silver behenate and 33mol% of silver adipate and silver sebacate of INVENTION EXAMPLES
4 and 6 respectively.
INVENTION EXAMPLES 11 to 14 and COMPARATIVE EXAMPLE 2
preparation of organic silver salt dispersions
[0068] 100g of the respective organic silver salt was added to a mixture of 100g of 10%
solution of Surfactant Nr. 1 and 300g of deionized water and the mixture stirred for
30 minutes with an ULTRA TURRAX stirrer. The resulting dispersions were then passed
through a Type M110F high pressure homogenizer from MICROFLUIDICS™ Corporation at
a pressure of 350 bar to obtain a finely divided aqueous dispersion of the organic
silver salt. The final concentration of the different organic silver salts in the
resulting dispersions is given in table 4.
preparation of coating dispersions
[0069] 0.310g of boric acid was mixed with 12.287g of deionized water and 3.450g of K17881
and the K17881 was allowed to swell for 30 minutes before heating the swollen gelatin
up to 36°C. The following ingredients were then added with stirring: 4.865g of an
aqueous dispersion of 6.44% of K7598 and 18.88% of phthalazinone followed by 10 minutes
stirring, then 2g of the organic silver salt dispersion (for type see table 4) and
deionized water (for quantity see table 4) followed by 10 minutes stirring, then the
rest of the organic silver salt dispersion (for total quantity, type and concentration
of the organic silver salt see table 4), then 1g of deionized water at a temperature
of 50°C and 1g of R01 dissolved in 2g of ethanol, then 1g of an aqueous solution containing
19.2% of formaldehyde and 6.75% of methanol and finally 2g of deionized water. The
pH of the dispersion was adjusted to 5.3 just before coating.
Table 4
Invention example number |
organic silver salt type |
Organic silver salt dispersion |
deionized water added [g] |
|
|
conc. [%] |
quantity [g] |
|
11 |
IV |
19.40 |
23.730 |
8.358 |
12 |
IV |
19.40 |
23.730 |
14.020 |
13 |
VII |
19.14 |
23.526 |
8.562 |
14 |
VII |
19.14 |
23.526 |
13.742 |
Comparative example nr |
|
|
|
|
2 |
XII |
15.96 |
27.680 |
4.408 |
The resulting organic silver salt dispersions were then doctor blade-coated onto
a 175µm thick subbed polyethylene terephthalate support to produce after drying for
10 minutes at 50°C the coating weights of silver given in table 5.
thermographic evaluation
[0070] Thermographic printing of the thermographic recording materials of INVENTION EXAMPLES
11 to 14 and COMPARATIVE EXAMPLE 2 was carried out as described for the thermographic
recording materials of INVENTION EXAMPLES 1 to 10 and COMPARATIVE EXAMPLE 1. The maximum
densities, D
max, and minimum densities, D
min, of the prints produced with the thermographic recording materials of INVENTION EXAMPLES
11 to 14 and COMPARATIVE EXAMPLE 2 measured through a blue filter with a MACBETH™
TR924 densitometer in the grey scale step corresponding to data levels of 64 and 0
respectively are given in table 5.
image tone evaluation
[0071] Image tone evaluation was carried out as described above for INVENTION EXAMPLES 1
to 10 and COMPARATIVE EXAMPLE 1. The a* and b* CIELAB-values of fresh prints of the
thermographic recording materials of INVENTION EXAMPLES 11 to 14 and COMPARATIVE EXAMPLE
2 at optical densities, D, of 0.5 and 1.0 are summarized in table 5.
Table 5
Invention example number |
Ag coverage [g/m2] |
organic silver salt type |
fresh material |
CIELAB at D=0.5 |
CIELAB at D = 1.0 |
|
|
|
Dmax blue |
Dmin blue |
a* |
b* |
a* |
b* |
11 |
1.537 |
IV |
3.76 |
0.09 |
-0.2 |
+9.9 |
+0.4 |
>+10 |
12 |
1.226 |
IV |
2.86 |
0.10 |
-0.5 |
+9.6 |
+0.3 |
+10.0 |
13 |
1.549 |
VI |
4.11 |
0.11 |
+1.3 |
+7.4 |
+1.9 |
+9.1 |
14 |
1.200 |
VI |
3.11 |
0.10 |
+1.0 |
+7.5 |
+2.6 |
+7.2 |
Comparative example nr |
|
|
|
|
|
|
|
|
2 |
1.264 |
XII |
4.19 |
0.09 |
-1.7 |
>10 |
-1.4 |
>10 |
a* of blue base used = -8.34 |
b* of blue base used = -15.71 |
Prints with the thermographic recording materials of INVENTION EXAMPLES 11 and 12
with mixed crystals of organic silver salts of types IV and INVENTION EXAMPLES 13
and 14 with mixed crystals of organic silver salts of type VI consisting of 67mol%
of silver behenate and 33mol% of silver adipate and silver sebacate respectively exhibited
a significant improvement in image tone neutrality compared with the thermographic
recording material of COMPARATIVE EXAMPLE 2 with silver behenate.
INVENTION EXAMPLES 15 to 17 & COMPARATIVE EXAMPLES 3 to 5
preparation of coating dispersions
[0072] 40g of an organic silver salt dispersion prepared as described for INVENTION EXAMPLES
1 to 10 & COMPARATIVE EXAMPLE 1 was mixed with 29.86g of a 30% solution of B79, 0.90g
of a 2-butanone dispersion containing 4.13% of B79, 14.01% of T01 and 25.86%, 0.40g
of BAYSILON™ MA and 0.371g of adipic acid and 15g of 2-butanone to a homogeneous dispersion.
1.844g of R01, 0.288g of tetrachlorophthalic anhydride, 0.24g of benzotriazole and
17.08g of 2-butanone were then mixed separately and 9.59g thereof was added to the
homogenized organic silver salt dispersion prior to doctor blade coating with the
blade adjusted to 160µm onto a 175µm thick subbed polyethylene terephthalate blue-base
support. The resulting layers were dried for 1 hour at 50°C to produce the thermosensitive
elements of the thermographic recording materials of INVENTION EXAMPLES 15 to 17 and
COMPARATIVE EXAMPLES 3 to 5 produced with type VIII to X mixed crystals and mixtures
of type XII and type XIII, respectively. The coating weights of silver are given in
table 6. After drying all thermographic recording materials were subjected to 7 days
conditioning at 45°C and 70% relative humidity before printing.
thermographic evaluation
[0073] Thermographic printing of the thermographic recording materials of INVENTION EXAMPLES
15 to 17 and COMPARATIVE EXAMPLE was carried out as described for the thermographic
recording materials of INVENTION EXAMPLES 1 to 10 and COMPARATIVE EXAMPLE 1. The maximum
densities, D
max, and minimum densities, D
min, of the prints produced with the thermographic recording materials of INVENTION EXAMPLES
15 to 17 and COMPARATIVE EXAMPLE 3 to 5 measured through a blue filter with a MACBETH™
TR924 densitometer in the grey scale step corresponding to data levels of 64 and 0
respectively are given in table 5.
image tone evaluation
[0074] Image tone evaluation was carried out as described above for INVENTION EXAMPLES 1
to 10 and COMPARATIVE EXAMPLE 1. The a* and b* CIELAB-values of prints produced with
thermographic recording materials of INVENTION EXAMPLES 15 to 17 and COMPARATIVE EXAMPLES
3 to 5 on the fresh material (i.e. after 7 days conditioning at 45°C and 70% relative
humidity) at optical densities, D, of 1.0 and 2.0 are summarized in table 6.
[0075] In interpreting the neutrality of images on blue base it is necessary to refer to
the a* and b* values of blue base as representing a neutral image, rather than a*
and b* values of zero as was the case with polyethylene support without blue pigmentation.
These reference values are:
a* of blue base used = -8.34
b* of blue base used = -15.71
[0076] Colour neutrality of images on blue base on the basis of CIELAB-values therefore
corresponds to an a* value of -8.34 and a b* value of -15.71, with a more negative
a*-value than -8.34 indicating a greenish image-tone becoming greener as a* becomes
more negative, a value of a* more positive (i.e. less negative) than -8.34 indicating
a reddish image-tone becoming redder as a* becomes more positive (i.e. less negative),
a more negative b*-value than -15.71 indicating a bluish image-tone becoming bluer
as b* becomes more negative and a more positive (i.e. less negative) b*-value than
- 15.71 indicating a yellowish image-tone becoming more yellow as b* becomes more
positive.
Table 6
Invention example nr |
organic silver salt type(s) |
fresh material |
CIELAB at D = 1.0 |
CIELAB at D = 2.0 |
|
|
Dmax blue |
Dmin blue |
a* |
b* |
a* |
b* |
15 |
VIII |
3.24 |
0.10 |
-5.93 |
-7.61 |
-2.85 |
-5.72 |
16 |
IX |
2.97 |
0.10 |
-4.94 |
-7.90 |
-2.25 |
-6.10 |
17 |
X |
2.79 |
0.10 |
-5.42 |
-7.68 |
-2.13 |
-6.17 |
NEUTRAL IMAGE TONE WITH BLUE BASE |
-8.34 |
-15.71 |
-8.34 |
-15.71 |
Comparative example nr |
|
|
|
|
|
|
|
3 |
XII(90mol%) |
2.91 |
0.10 |
-4.90 |
-7.78 |
-1.93 |
-6.36 |
XIII(10mol%) |
4 |
XII(70mol%) |
2.95 |
0.10 |
-5.05 |
-7.76 |
-1.94 |
-6.33 |
XIII (30mol%) |
5 |
XII(50mol%) |
3.21 |
0.10 |
-5.09 |
-7.47 |
-1.68 |
-5.76 |
XIII(50mol%) |
a* of blue base used = -8.34 |
b* of blue base used = -15.71 |
[0077] The image tone perceived by an observer with images on a light box is more strongly
influenced by the image tone of image densities at lower densities due to the lower
blackness. Hence, the image tone for densities of 1.0 is more important than that
for 2.0. When the CIELAB-values for the thermographic recording materials of INVENTION
EXAMPLE 15, 16 and 17 with mixed crystals of silver behenate and silver stearate with
90, 70 and 50 mole% silver behenate respectively are compared with the thermographic
recording materials of COMPARATIVE EXAMPLES 3, 4 and 5 with physical mixtures of silver
behenate and silver stearate with 90, 70 and 50 mole% silver behenate respectively,
the a*- and b*-values for the thermographic recording materials of INVENTION EXAMPLES
3, 4 and 5 are more neutral (i.e. closer to -8.34 and -15.71 respectively) than the
respective COMPARATIVE EXAMPLES with the same molar concentration of silver behenate
but as mixed crystals rather than physical mixtures, with the exception of the a*-value
for the thermographic recording material of INVENTION EXAMPLE 16 which is comparable
to that for COMPARATIVE EXAMPLE 4. It is therefore clear that the image tone of thermographic
recording materials with mixed crystals of silver salts of organic carboxylic acids
is surprisingly improved over that with thermographic recording materials with physical
mixtures of the same silver salts of organic carboxylic acids in the same molar concentrations
and therefore that the performance of mixed crystals of silver salts of organic carboxylic
acids cannot be equated with that of physical mixtures thereof.
[0078] Having described in detail preferred embodiments of the current invention, it will
now be apparent to those skilled in the art that numerous modifications can be made
therein without departing from the scope of the invention as defined in the following
claims.