Field of the invention
The present invention relates to tone modifiers for use in
thermographic recording materials.
Background of the invention.
M. Ikeda in 1980 in Photographic Science and Engineering, Volume
24, Number 6, pages 277-280, disclosed a thermodynamic and NMR study
on several silver salts of fatty acids (silver behenate, silver
stearate, silver palmitate, silver myristate and silver laurate).
The results of thermal analyses indicated that the salts exhibit
thermotropic liquid crystalline behaviour analogous to those of the
alkali metal soaps and that formation in silver behenate of the
mesophase, called "sub-waxy", was available for development in the
case of a commercial dry silver paper. M. Ikeda and Y. Iwata also
in 1980 in Photographic Science and Engineering, Volume 24, Number
6, pages 273-276, disclosed a study of the morphology and structure
of silver laurate and silver behenate. It was found that these
salts undergo phase transitions with increasing temperature. A
polarizing microscope was used to determine how the molecular
alignment in silver laurate changed with increasing temperature, a
super-curd phase (implying crystal phases having different crystal
structure) being observed at 109°C and a sub-waxy phase (a mesophase
inherent in liquid crystals) at 114°C. In the mesophase,
formed at a temperature higher than 120°C, which generally
corresponded to the heat-developable temperature for dry silver
paper, the arrangement of Ag atoms was unalterable, but the
orientation of the paraffinic chains was random. For silver behenate
the long spacing, the distance between Ag atom layers, was found to
be unalterable through sub-waxy.
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.
US 3,080,254 specifically discloses a substantially light-insensitive
thermographic recording material with a thermosensitive
element containing silver behenate, phthalazinone and polyvinyl
butyral.
US 3,951,660 discloses a photographic radiation sensitive
recording material having therein a radiation sensitive composition
and at least one layer containing dispersed in a binding agent a
substantially non-light sensitive silver salt, a reducing agent for
the non-light-sensitive salt, and a toner compound, the improvement
which comprises the toner being a heterocyclic toner compound of the
following formula:
in which X represents O or N-R
5; R
1, R
2, R
3 or R
4 represent hydrogen,
alkyl, cycloalkyl, alkoxy, alkylthio, hydroxy, dialkylamino or
halogen, in addition to which R
1 and R
2 or R
2 and R
3 or R
3 and R
4 can
represent the ring members required to complete an anullated
aromatic ring, and R
5 represents alkyl.
EP 599 369, EP 669 875, EP 669 876 and EP 726 852 disclose in
their invention examples substantially light-insensitive
thermographic recording materials with a thermosensitive element
consisting of silver behenate, 3,3,3',3'-tetramethyl-5,6,5',6'-tetrahydroxy-1'1'-spiro-bis-indane,
polyvinyl butyral,
benzo[e][1,3]oxazine-2,4-dione (compound 25 in US 3,951,660) and
silicone oil in which the weight ratio of silver behenate to
polyvinyl butyral varies between 2:1 and 1:1 and the molar ratio of
benzo[e][1,3]oxazine-2,4-dione to silver behenate is about 0.20.
EP-A 752 616 discloses a thermographic material comprising at
least one element and wherein the element(s) contain(s) therein a
substantially light-insensitive organic heavy metal salt and an
organic reducing agent therefor, the material being capable of
thermally producing an image from the organic heavy metal salt and
reducing agent, wherein the material contains a 1,3-benzoxazine-2,4-dione
toning agent having general formula (I):
wherein R
1 represents hydrogen, -CH
2OH, (C=O) -R, -CONHR, or M; R
2,
R
3, R
4 and R
5 each independently represents hydrogen, -O-(C=O)-OR or
-NH-(C=O)-OR and at least one of which is not hydrogen if R
1 is also
hydrogen; R represents an alkyl or aryl group either of which may be
substituted; and M represents a monovalent heavy metal ion.
EP-A 752 616 specifically discloses substantially light-insensitive
thermographic recording materials with a thermosensitive
element containing silver behenate and 5, 10 and 20 mol% 7-(ethylcarbonate)-benzo[e][1,3]oxazine-2,4-dione
(compound 1) with
respect to silver behenate.
In printing with thermographic materials for medical
applications and graphic arts applications it is desirable to
increase the throughput of thermographic materials. This requires
that thermal development take place over as short a period as
possible. In the case of substantially light-insensitive
thermographic recording materials in which thermal development is
obtained by image-wise heating, this requires that the heating time
per pixel be as short as possible. In the case of image-wise
heating with the resistance elements of a thermal head, this means
that the line time of the thermal head be as short as possible
without loss in image tone in continuous tone images. Image tone
can be assessed on the basis of the L*, a* and b* CIELAB-values,
which are determined by spectrophotometric measurements according to
ASTM Norm E179-90 in a R(45/0) geometry with evaluation according to
ASTM Norm E308-90.
Objects of the invention.
It is an object of the present invention to provide a novel tone
modifier for use in substantially light-insensitive thermographic
recording materials.
It is a further object of the present invention to provide a
means of obtaining clinically acceptable image tones in high
throughput substantially light-insensitive thermographic recording
material for printers with thermal head line times of 20 ms to 4.5
ms or less at a resolution of at least 118 dots per cm (= 300 dots
per inch).
Further objects and advantages of the invention will become
apparent from the description hereinafter.
Summary of the invention
Three crystalline phases of silver behenate have been identified
by X-ray diffraction measurements with a copper Kα1 X-ray source,
which have been designated: phase I, phase II and phase III. At
25°C pure silver behenate only exists in the well-known crystalline
phase designated as phase I [ICDD reference spectrum: 4.53°, 6.01°,
7.56°, 9.12°, 10.66°, 12.12° and 13.62°(National Institute of
Standards, Gaithersburg, MD-20899-0001, USA)] and as an amorphous
phase. Phase II silver behenate, a mesomorphous phase having an X-ray
diffraction spectrum upon irradiation with a copper Kα1 X-ray
source with Bragg angles 2Θ of 5.34-5.78°, 6.12-6.41°,7.68-7.79°,
8.30-8.59°, 9.36-9.84°, 10.6-10.96°, and phase III silver behenate,
a mesomorphous phase having an X-ray diffraction spectrum upon
irradiation with a copper Kα1 X-ray source with Bragg angles 2Θ of
4.76-4.81°, 5.9-6.3°, 6.76-7.35°, 8.27-8.44° and 9.06-9.43° are, in
the case of pure silver behenate, only stable between 130-140°C and
ca. 156°C; and between ca. 156°C and ca. 180°C respectively.
It has been surprisingly found that phase II and phase III are
identifiable on the basis of their X-ray diffraction spectra in
certain substantially light-insensitive thermographic recording
materials after thermographic printing. Furthermore, the presence
of these silver behenate phases in the printed thermographic
materials could surprisingly be correlated with improved image tone.
Therefore, such silver behenate phases, when stabilized at room
temperature, act as tone modifiers.
Surprisingly, it has been found in model experiments that phase
II and phase III silver behenate, which for pure silver behenate are
only stable at temperatures between 130-140 and ca. 156°C and ca.
156°C and ca. 180°C respectively, can be stabilized at 25°C to
different degrees by the presence of a compound selected from the
group consisting of: glutaric acid, benzo[e] [1,3]-oxazine-2,4-dione;
substituted benzo[e] [1,3]-oxazine-2,4-dione compounds such as 7-(ethylcarbonato)
-benzo[e][1,3]-oxazine-2,4-dione, 7-methyl-benzo[e][1,3]-oxazine-2,4-dione
and 7-methoxy-benzo[e][1,3]-oxazine-2,4-dione;
phthalazinone; and polyvinyl butyral, or combinations
thereof. The broadness of the XRD-peaks for these two phases leads
to significant XRD-peak overlap between the two phases, when present
at the same temperature. This stabilization of phase II and phase
III silver behenate is only observable in materials in which all the
residual silver behenate is not converted into another silver salt,
during the thermal development process.
A possible explanation for the tone modifying effect of phase II
and phase III silver behenate is that these phases promote the
formation of metallic silver nuclei clusters with a size at which
light scattering produces a blue-black neutral image tone [≥ 160 nm
according to Bird in Photographic Science and Engineering, volume
15, page 356 (1971)].
The above-mentioned objects are realized by providing a
substantially light-insensitive thermographic recording material
comprising a support and a thermosensitive element, the
thermosensitive element containing silver behenate including phase I
silver behenate having an X-ray diffraction spectrum upon
irradiation with a copper Kα1 X-ray source with Bragg angles 2Θ of
4.53°, 5.96-6.05°, 7.46-7.56°, 8.90-9.12°, 10.45-10.66°, 12.02-12.12°,
13.53-13.62°, a reducing agent therefor in thermal working
relationship therewith and a binder, wherein the thermographic
recording material is capable upon thermal development of containing
1% of phase II silver behenate, having an X-ray diffraction spectrum
upon irradiation with a copper Kα1 X-ray source with Bragg angles 2Θ
of 5.34-5.78°, 6.12-6.41°,7.68-7.79°, 8.30-8.59°, 9.36-9.84°, 10.6-10.96°,
which is stable at 25°C, and/or phase III silver behenate
phase, having an X-ray diffraction spectrum upon irradiation with a
copper Kα1 X-ray source with Bragg angles 2Θ of 4.76-4.81°, 5.9-6.3°,
6.76-7.35°, 8.27-8.44° and 9.06-9.43°, which is stable at
25°C, with respect to the quantity of the phase I silver behenate in
the thermographic recording material before the thermal development.
A recording process is also provided by the present invention for
a thermographic recording material, the thermographic recording
material comprising a thermosensitive element, the thermosensitive
element comprising silver behenate including the above-defined phase
I silver behenate, an organic reducing agent therefor in thermal
working relationship therewith and a binder, comprising: (i)
converting the silver behenate into the above-defined phase II
silver behenate and/or the above-defined phase III silver behenate;
and (ii) cooling the thermographic recording material to 25°C,
characterized in that at least 1% of the phase II silver behenate
and/or the phase III silver behenate, with respect to the quantity
of the phase I silver behenate in the thermographic recording
material before the recording process, is present in the cooled
thermally developed thermographic recording material at 25°C as
stable phases.
A use is also provided by the present invention of the above-defined
phase II silver behenate stabilized at 25°C and/or the
above-defined phase III silver behenate defined in claim 1
stabilized at 25°C as a tone modifier in thermographic recording
materials.
Preferred embodiments of the present invention are disclosed in
the dependent claims.
Detailed description of the invention.
The invention is described hereinafter by way of reference to
the accompanying figure wherein:
- Fig. 1
- is an image of silver behenate at 150°C taken with a video
camera mounted on a polarization microscope with cross
polarizers and a hot stage and a phase II liquid
crystalline phase (smectic A) of silver behenate with
clearly visible orientation.
- Fig. 2
- represents the relative concentrations of phase I, phase II
and phase III silver behenate as a function of temperature
upon heating up pure silver behenate.
In a preferred thermographic recording material according to the
present invention said thermographic recording material upon thermal
development contains at least 2% of the phase II silver behenate,
which is stable at 25°C, and/or the phase III silver behenate, which
is stable at 25°C, with respect to the quantity of the phase I
silver behenate in the thermographic recording material before the
thermal development. In a particularly preferred thermographic
recording material according to the present invention upon thermal
development said thermographic recording material contains at least
5% of the phase II silver behenate, which is stable at 25°C, and/or
the phase III silver behenate with respect to the phase I silver
behenate, which is stable at 25°C, with respect to the quantity of
the phase I silver behenate in the thermographic recording material
before the thermal development.
In a further preferred thermographic recording material of the
present invention, the reducing agent is 3,4-dihydroxybenzonitrile.
In a still further preferred thermographic recording material of
the present invention, phase II silver behenate and/or phase III
silver behenate is stabilized by the presence of a compound selected
from the group consisting of: glutaric acid, benzo[e][1,3]-oxazine-2,4-dione,
substituted benzo[e][1,3] -oxazine-2,4-dione compounds,
phthalazinone and polyvinyl butyral.
In another preferred thermographic recording material, according
to the present invention, the thermographic recording material is a
black and white thermographic recording material.
Definitions
By substantially light-insensitive is meant not intentionally
light sensitive.
The term thermographic recording material as used in the present
specification includes both substantially light-insensitive
thermographic recording materials and photothermographic recording
materials.
Mesomorphous means in a mesomorphic state, which is a state of
matter intermediate between a crystalline solid and a normal
isotropic liquid, in which long rod-shaped organic molecules contain
dipolar and polarizable groups.
Liquid crystalline means a liquid state in which the liquid is
not isotropic and as a result of the orientation of molecules
parallel to one another in large clusters is birefringent and
exhibits interference in polarized light.
A black and white thermographic recording material is a
thermographic recording material producing a monotone blue-black
image.
Heating in a substantially water-free condition as used herein,
means heating at a temperature of 80 to 250°C. The term
"substantially water-free condition" means that the reaction system
is approximately in equilibrium with water in the air, and water for
inducing or promoting the reaction is not particularly or positively
supplied from the exterior to the element. Such a condition is
described in T.H. James, "The Theory of the Photographic Process",
Fourth Edition, Macmillan 1977, page 374.
Silver behenate
Three crystalline phases of silver behenate have been identified
by X-ray diffraction measurements with a copper Kα1 X-ray source. At
25°C silver behenate exists in the well-known crystalline phase
designated as phase I [ICDD reference spectrum: 4.53°, 6.01°, 7.56°,
9.12°, 10.66°, 12.12° and 13.62°(National Institute of Standards,
Gaithersburg, MD-20899-0001, USA)] and as an amorphous phase. Upon
heating solid silver behenate, phase I silver behenate exists up to
a temperature of 120 to 130°C in which temperature range amorphous
silver behenate begins to be formed. This birefringent (doubly
refractive) silver behenate phase, which will be designated as phase
II, is observed at temperatures between 130-145°C, depending upon
the sample, and ca. 156°C. It exhibits a clear anisotropy i.e. it is
a liquid crystalline (mesomorphous) smectic A phase (as confirmed by
polarization microscopy, see Figure 1). From a temperature of 156°C
further changes are observed in the lattice spacings of the silver
behenate, indicating a second phase transition to the mesomorphous
phase III silver behenate. Phase III silver behenate is observed at
temperatures between ca. 156°C and ca. 180°C. Figure 2 summarizes
these phase transitions. The precise process by which the phase I
silver behenate or the amorphous silver behenate in the
substantially light-insensitive thermographic recording material is
converted during printing into phase II silver behenate and phase
III silver behenate is not important to the present invention, since
only the fact that such a conversion has taken place and that these
phases are stabilized at 25°C is important to the present invention.
No difference was be observed between phase II silver behenate
and phase III silver behenate under a polarization microscope, both
phases being mesomorphous. However, clear differences are observed
in XRD and DSC behaviour, the XRD spectrum of phase III silver
behenate being shifted to lower Bragg 2Θ angles with respect to
phase II silver behenate, indicating reduced d-values i.e. reduced
separation between polarizable groups in the mesomorphous phase of
the silver behenate. The X-ray diffaction peaks observed with phase
II and phase III silver behenate are significantly broader than
those observed for phase I silver behenate.
Table 1 gives the Bragg 2Θ angles of the principal XRD-peaks of
phases I, II and III silver behenate. It should be noted that
neither phase II nor phase III silver behenate is stable at 25°C in
the absence of stabilizing compounds.
Silver behenate phase | Temperature range in which pure silver behenate is stable [°C] | Bragg angles 2Θ of silver behenate phase upon irradiation with a copper Kα1 X-ray source |
Phase I | below ca. 135°C | 4.53°, 5.96-6.05°, 7.46-7.56°, 8.90-9.12°, 10.45-10.66°, 12.02-12.12°, 13.53-13.62° |
Phase II | 130-140 to ca. 156°C | 5.34-5.78°, 6.12-6.41°,7.68-7.79°, 8.30-8.59°, 9.36-9.84°, 10.6-10.96° |
Phase III | ca. 156 to ca. 180°C | 4.76-4.81°, 5.9-6.3°, 6.76-7.35°, 8.27-8.44°, 9.06-9.43° |
Thermosensitive element
The thermosensitive element, according to the present invention,
contains silver behenate including phase I silver behenate, at least
one 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 silver behenate is in reactive association with the
organic reducing agent i.e. during the thermal development process
the organic reducing agent must be present in such a way that it is
able to diffuse to the particles of silver behenate or other
substantially light-insensitive organic silver salt so that
reduction to silver can occur. Addition of photosensitive silver
halide to the thermosensitive element in catalytic association with
the silver behenate renders the thermosensitive element photo-addressable.
Organic reducing agents
Suitable organic reducing agents for the reduction of silver
behenate are organic compounds containing at least one active
hydrogen atom linked to O, N or C, such as is the case with,
aromatic di- and tri-hydroxy compounds. 1,2-dihydroxybenzene
derivatives, 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. In particularly
preferred substantially light-insensitive thermographic materials
according to the present invention the at least one organic reducing
agent is described in EP-B 692 733 e.g. ethyl 3,4-dihydroxybenzoate,
n-butyl 3,4-dihydroxybenzoate and/or EP-A 903 625 e.g. 3,4-dihydroxybenzonitrile,
3,4-dihydroxyacetophenone and 3,4-dihydroxybenzophenone.
In an especially preferred embodiment of the
present invention the at least one organic reducing agent comprises
3,4-dihydroxybenzonitrile in a concentration of at least 30 mol%
with respect to the substantially light-insensitive organic silver
salt.
Combinations of organic 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. Combinations of ethyl 3,4-dihydroxybenzoate with 3,4-dihydroxybenzonitrile
and 3,4-dihydroxybenzophenone with 3,4-dihydroxybenzonitrile
are particularly preferred.
Aliphatic polycarboxylic acids and anhydrides thereof
According to a preferred embodiment of the thermographic
recording material according to the present invention, the
thermosensitive element further contains at least one polycarboxylic
acid and/or anhydride thereof in a molar percentage of at least 20
with respect to the substantially light-insensitive organic silver
salt comprising phase I silver behenate and in thermal working
relationship therewith. The aliphatic polycarboxylic acid may be
saturated, unsaturated or cycloaliphatic as disclosed in US-P
5,527,758, e.g. an α,ω-alkyldicarboxylic acid, and may be used in
anhydride form, in particular an intramolecular form, or partially
esterified on the condition that at least two free carboxylic acid
groups remain or are available in the thermal development step.
Glutaric acid, an α,ω-alkyldicarboxylic acid, is a particularly
strong stabilizer of phase II and phase III silver behenate at room
temperature (25°C).
Binder of the thermosensitive element
The film-forming binder of the thermosensitive element 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. Polyvinyl butyral is a stabilizer of phase II and
phase III silver behenate at room temperature (25°C).
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 gelatine, modified
gelatines such as phthaloyl gelatine, 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
gelatine.
Binders are preferred which do not contain additives, such as
certain antioxidants (e.g. 2,6-di-tert-butyl-4-methylphenol), or
impurities which adversely affect the thermographic properties of
the thermographic recording materials in which they are used.
Additional toning agents
In a preferred embodiment of the substantially light insensitive
thermographic recording material of the present invention, the
thermosensitive element may further contain one or more additional
toning agents known from thermography in order to obtain a neutral
black image tone in the higher densities and neutral grey in the
lower densities.
Suitable toning agents are the phthalimides and phthalazinones
within the scope of the general formulae described in US 4,082,901.
Further reference is made to the toning agents described in
US 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 1,439,478, US 3,951,660 and US 5,599,647 and in particular
benzo[e][1,3]oxazine-2,4-dione, 7-methyl-benzo[e] [1,3]oxazine-2,4-dione,
7-methoxy-benzo[e] [1,3]oxazine-2,4-dione, 7-(ethylcarbonato)-benzo[e]
[1,3]oxazine-2,4-dione and phthalazinone.
Antifoggants
Antifoggants may be incorporated into the thermographic
recording materials of the present invention in order to obtain
improved shelf-life and reduced fogging.
Preferred antifoggants are benzotriazole, substituted
benzotriazoles, tetrazoles, mercaptotetrazoles and aromatic
polycarboxylic acid such as ortho-phthalic acid, 3-nitro-phthalic
acid, tetrachlorophthalic acid, mellitic acid, pyromellitic acid and
trimellitic acid and anhydrides thereof.
Surfactants and dispersion agents
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
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 F3C(CF2)6CONH(CH2CH2O)-H, silicone oil, e.g. BAYSILON™
MA (from BAYER AG, GERMANY).
Support
The support for the thermosensitive element according to the
present invention may be opaque, transparent or translucent and is a
thin flexible carrier made of transparent resin film, e.g. made of a
cellulose ester, cellulose triacetate, polypropylene, polycarbonate
or polyester, e.g. polyethylene terephthalate.
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. It may be pigmented with a blue pigment as
so-called blue-base. One or more backing layers may be provided to
control physical properties such as curl and static.
Protective layer
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.
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.
The protective layer according to the present invention may be
crosslinked. Crosslinking can be achieved by using crosslinking
agents such as those described in WO 95/12495.
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.
The protective layer of the thermographic recording material
according to the present invention may comprise a matting agent.
Preferred matting agents are described in WO 94/11198, e.g. talc
particles, and optionally protrude from the protective layer.
Coating
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
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
substantially light-insensitive thermographic material preferably
containing an infra-red absorbing compound, or by direct thermal
imaging with a thermal head.
In a preferred embodiment of the recording process of the
present invention, the recording process further comprises thermal
development at a line time of less than 20 ms and at an image
resolution of at least 118 dots per cm (= 300 dots per inch), with a
line time of 7 ms or less with an image resolution of at least 118
dots per cm being preferred and a line time of 4.5 ms or less with
an image resolution of at least 118 dots per cm being particularly
preferred.
In the recording process for a thermographic recording,
according to the present invention, silver behenate including phase
I silver behenate is converted into phase II and/or phase III silver
behenate and cooling the thermographic recording material to 25°C,
whereby at least 1% of phase II and/or phase III silver behenate
with respect to the phase I silver behenate in the thermographic
recording material before thermal development is present in the
cooled thermographic recording material as stable phases. This
conversion process preferably takes place by means of heat and
requires the presence of compounds which stabilize phase II and/or
phase III silver behenate at 25°C.
In a further 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.
In a still further preferred embodiment of the recording
process, according to the present invention, the thermal development
takes place under substantially water-free conditions.
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. 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/cm2
to ensure a good transfer of heat.
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.
Activation of the heating elements can be power-modulated or
pulse-length modulated at constant power. 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.
During thermal development of substantially light-insensitive
thermographic materials according to the present invention the
silver behenate is converted into an amorphous phase only part of
which is converted into elemental silver particles. After thermal
development the non-converted silver behenate may be present in one
or more of the following states: an amorphous state, in the same
crystalline state as that prior to thermal development and in one or
more new crystalline states. Such new crystalline states may
include one or more liquid crystalline states as the organic silver
salt is heated up or cooled down.
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.
Photothermographic printing
Photothermographic recording materials, according to the present
invention, may be exposed with radiation of wavelength between an X-ray
wavelength and a 5 microns wavelength with the image either
being obtained by pixel-wise exposure with a finely focused light
source, such as a CRT light source; a UV, visible or IR wavelength
laser, such as a He/Ne-laser or an IR-laser diode, e.g. emitting at
780nm, 830nm or 850nm; or a light emitting diode, for example one
emitting at 659nm; or by direct exposure to the object itself or an
image therefrom with appropriate illumination e.g. with UV, visible
or IR light. For the thermal development of image-wise exposed
photothermographic recording materials, according to the present
invention, any sort of heat source can be used that enables the
recording materials to be uniformly heated to the development
temperature in a time acceptable for the application concerned e.g.
contact heating, radiative heating, microwave heating etc.
Industrial application
Thermographic imaging can be used for the production of
reflection type prints and transparencies, in particular for use in
the medical diagnostic field in which black-imaged transparencies
are widely used in inspection techniques operating with a light box.
The invention is illustrated hereinafter by way of COMPARATIVE
EXAMPLES 1 to 3 and INVENTION EXAMPLES 1 to 17. The percentages and
ratios given in these examples are by weight unless otherwise
indicated. The ingredients used in the invention and comparative
examples, are:
- organic silver salt:
AgB = silver behenate; - organic reducing agent:
- R01 = n-butyl 3,4-dihydroxybenzoate;
- R02 = ethyl 3,4-dihydroxybenzoate;
- R03 = 3,4-dihydroxybenzonitrile;
- R04 = 3,4-dihydroxyacetophenone;
- R05 = 3,4-dihydroxybenzophenone;
- non-stabilizing dicarboxylic acid:
D01 = adipic acid; - antifoggants:
- S01 = tetrachlorophthalic acid anhydride;
- S02 = benzotriazole;
- toning agent:
TA01 = phthalazine - phase II and phase III silver behenate-stabilizing compounds:
- P01 = glutaric acid;
- P02 = benzo[e] [1,3]oxazine-2,4-dione;
- P03 = 7-(ethylcarbonato)-benzo[e][1,3]oxazine-2,4-dione;
- P04 = phthalazinone;
- P05 = S-LEC BL5-HPZ, a polyvinyl butyral binder from SEKISUI
Chemical Co. Ltd;
- P06 = BUTVAR™ B79, a polyvinyl butyral binder from SOLUTIA;
- silicone oil:
Oil = BAYSILON™ MA, a polydimethylsiloxane from BAYER; - surfactant:
S01 = MARLON™ A-396, sodium dodecyl sulfonate from HÜLS.
INVENTION EXAMPLES 1 and 2 and COMPARATIVE EXAMPLE 1
The substantially light-insensitive thermographic recording
materials of INVENTION EXAMPLES 1 and 2 and COMPARATIVE EXAMPLE 1
were produced by coating a subbed 175µm thick blue-pigmented
polyethylene terephthalate support (a* = -6.86; b* = -14.46; Dvis =
0.181) with a composition containing 2-butanone as
solvent/dispersing medium, so as to obtain thereon, after drying,
the thermosensitive elements of INVENTION EXAMPLES 1 and 2 and
COMPARATIVE EXAMPLE 1 with the compositions given in Table 2:
Invention Example nr. | AgB [g/m2] | Phase II and III AgB stabilizers | Oil mg/m2 | Reducing agent | S01 g/m2 | S02 g/m2 | D01 g/m2 |
| | P01 [g/m2] | P02 [g/m2] | P03 [g/m2] | P05 [g/m2] | | g/m2 | type |
1 | 3.72 | 0.289 | 0.203 | 0.105 | 14.87 | 0.033 | 0.750 | R02 | 0.117 | 0.097 | - |
2 | 3.72 | 0.266 | 0.203 | 0.105 | 14.87 | 0.033 | 0.750 | R02 | 0.117 | 0.097 | - |
Comparative example nr. |
1 | 4.105 | - | 0.223 | 0.115 | 12.315 | 0.036 | 0.827 | R02 | 0.130 | 0.108 | 0.293 |
The thermosensitive element was then provided with a protective
layer by coating with an aqueous composition with the following
composition expressed as weight percentages of ingredients present:
- polyvinylalcohol (Polyviol™ WX 48/20 from Wacker Chemie): 2.5%
- Ultravon™ W (dispersion agent from Ciba Geigy) converted into
acid form by passing through an ion exchange column: 0.09%
- talc (type P3 from Nippon Talc): 0.05%
- colloidal silica (Levasil™ VP AC 4055 from Bayer AG, a 15%
aqueous dispersion of colloidal silica): 1.2%
- silica (Syloid™ 72 from Grace): 0.10%
- mono[isotridecyl polyglycolether (3 EO)] phosphate
(Servoxyl™ VPDZ 3/100 from Servo Delden B.V.): 0.09%
- mixture of monolauryl and dilauryl phosphate (Servoxyl™
VPAZ 100 from Servo Delden B.V.): 0.09%
- glycerine monotallow acid ester (Rilanit™ GMS from Henkel AG): 0.18%
- tetramethylorthosilicate hydrolyzed in the presence of
methanesulfonic acid: 2.1%
The pH of the coating composition was adjusted to a pH of 3.8 by
adding 1N nitric acid. Those lubricants in these compositions which
were insoluble in water, were dispersed in a ball mill with, if
necessary, the aid of a dispersion agent. The compositions were
coated to a wet layer thickness of 85 µm and were then dried at 40°C
for 15 minutes and hardened at 45°C for 7 days thereby producing a
protective layer.
Thermographic printing
The printing was carried out with a DRYSTAR® 3000 printer from
AGFA-GEVAERT equipped with a thin film thermal head with a
resolution of 300 dpi and was operated with line times (the line
time being the time needed for printing one line) of 11.8 ms and 4.5
ms respectively (corresponding to 63mW/pixel and 96mW/pixel
respectively). During this line time the print head received
constant power. The thermal head resistors were time-modulated to
produce different image densities.
The substantially light-insensitive thermographic recording
materials of INVENTION EXAMPLES 1 and 2 and COMPARATIVE EXAMPLE 1
were printed at line times of 11.8 ms and 4.5 ms in such a manner
that a step wedge was produced with 8 density steps from 0 to 7
corresponding to equal increments of heating energy, step 0
corresponding to a heat energy ca. 25% of that of step 7
corresponded to the minimum image density, Dmin, and step 7
corresponded to the maximum image density, Dmax, respectively.
Evaluation of the density steps by X-ray Diffraction Spectroscopy
The X-ray diffraction spectra were determined in a Philips X'Pert
XRD apparatus with a CuKα X-ray source for the density steps of the
step wedges obtained with the substantially light-insensitive
thermographic recording materials of INVENTION EXAMPLES 1 and 2 and
COMPARATIVE EXAMPLE 1 at line times of 11.8 and 4.5 ms respectively.
The presence of phase I silver behenate was detected in the step
wedges produced upon printing the substantially light-insensitive
thermographic recording materials of INVENTION EXAMPLES 1 and 2 and
COMPARATIVE EXAMPLE 1 was established by the presence of strong
peaks at characteristic Bragg angle 2Θ at 4.53°, 5.96-6.05°, 7.46-7.56°,
8.90-9.12°, 10.45-10.66°, 12.02-12.12°, 13.53-13.62° and the
presence of phase II and phase III silver behenate phases in prints
produced with the substantially light-insensitive thermographic
recording materials of INVENTION EXAMPLES 1 and 2 by the presence of
strong peaks at characteristic Bragg angles 2Θ of 4.76-4.81° and
6.76-7.35°, which are the principal phase III silver behenate peaks
boosted by overlap with the phase II silver behenate present in
lower concentrations than the phase III silver behenate.
The concentrations of phase I silver behenate and of the
combined phase II and phase III silver behenate in the density steps
of the step wedges in the prints produced with the substantially
light-insensitive thermographic recording materials of INVENTION
EXAMPLES 1 and 2 and COMPARATIVE EXAMPLE 1 at line times of 11.8 ms
and 4.5 ms were determined by adding up the peak heights of the
above-mentioned peaks for the respective phases or phase
combinations. Relative concentration of phase I silver behenate and
of the combined phase II and phase III silver behenate with respect
to the concentration of phase I silver behenate in step 0
(corresponding to to Dmin), arbitrarily set at 100%, are summarized in
Table 3 below for line times of 11.8 ms and 4.5 ms.
However, this normalization procedure normalizes to the
crystallization condition of silver behenate in step 0 applying
after printing and not to the original concentration of phase I
silver behenate. Exposure to ca. 25% of the heat energy for step 7
during the printing process is known to reduce the crystallinity,
i.e. the quantity of phase I silver behenate, by up to a factor of
two. Therefore, this procedure will underestimate the quantity of
phase I silver behenate before printing by up to a factor of two.
As a result the relative percentages reported are a considerable
overestimate, but notwithstanding this deficiency serve to
illustrate the present invention.
Only phase I silver behenate was observed in prints produced
with the substantially light-insensitive thermographic recording
material of COMPARATIVE EXAMPLE 1 at line times of 11.8 ms and 4.5
ms.
On the other hand, the relative percentages of phase I silver
behenate phase and the combination of the phase II and phase III
silver behenate for prints produced with the substantially light-insensitive
thermographic recording materials of INVENTION EXAMPLES
1 and 2 clearly show that phase II and phase III silver behenate are
present in the density steps produced at line times of 11.8 ms and
4.5 ms. As regards interpretation of these results, it is important
to note that it is in steps 1, 2 and 3 of the step wedge
(approximately corresponding to densities of 0.5, 1.0 and 1.5
respectively) that the strongest effects on image tone are observed.
| | line time = 11.8 ms | line time = 4.5 ms |
Example nr | Step nr | phase I AgB phase [%] | Phase II & phase III AgB [%] | Total of AgB phases [%] | phase I AgB phase [%] | Phase II & phase III AgB [%] | Total of AgB phases [%] |
Comparative 1 | 0 | 100 | 0.0 | 100 | 100 | 0.0 | 100 |
1 | 7.6 | 0.0 | 7.6 | 24.8 | 0.0 | 24.8 |
| 2 | 5.8 | 0.0 | 5.8 | 12.7 | 0.0 | 12.7 |
| 3 | 3.0 | 0.0 | 3.0 | 7.5 | 0.0 | 7.5 |
| 4 | 2.7 | 0.0 | 2.7 | 4.9 | 0.0 | 4.9 |
| 5 | 2.1 | 0.0 | 2.1 | 4.6 | 0.0 | 4.6 |
| 6 | 0.0 | 0.0 | 0.0 | 3.9 | 0.0 | 3.9 |
| 7 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 |
Invention 1 | 0 | 100 | 0.0 | 100 | 100 | 0.0 | 100 |
1 | 24.9 | 19.3 | 44.2 | 37.7 | 16.1 | 53.8 |
| 2 | 19.6 | 22.2 | 41.8 | 22.6 | 14.7 | 37.3 |
| 3 | 12.6 | 16.8 | 29.4 | 13.0 | 12.0 | 25.0 |
| 4 | 5.6 | 10.5 | 16.1 | 9.9 | 12.0 | 21.9 |
| 5 | 3.8 | 7.6 | 11.4 | 6.2 | 9.2 | 15.4 |
| 6 | 2.9 | 6.4 | 9.4 | 5.1 | 7.5 | 12.7 |
| 7 | 1.5 | 4.4 | 5.8 | 1.7 | 6.2 | 7.9 |
Invention 2 | 0 | 100 | 0.0 | 100 | 95.3 | 4.7 | 100 |
1 | 18.6 | 14.5 | 33.1 | 36.0 | 15.8 | 51.8 |
| 2 | 11.8 | 14.8 | 26.6 | 19.0 | 12.3 | 31.2 |
| 3 | 5.9 | 12.7 | 18.6 | 14.6 | 18.2 | 32.8 |
| 4 | 3.3 | 9.2 | 12.4 | 7.1 | 12.3 | 19.4 |
| 5 | 3.0 | 5.6 | 8.6 | 3.2 | 9.5 | 12.6 |
| 6 | 0.6 | 3.0 | 3.6 | 1.2 | 6.3 | 7.5 |
| 7 | 0.3 | 2.4 | 2.7 | 0.0 | 4.0 | 4.0 |
Image evaluation
The image tone of fresh prints made with the substantially
light-insensitive thermographic recording materials of INVENTION
EXAMPLES 1 and 2 was assessed on the basis of the L*, a* and b*
CIELAB-values as described above. The a* and b* CIELAB-values 24
hours after printing of the substantially light-insensitive
thermographic recording materials of INVENTION EXAMPLES 1 and 2 at
an optical density, D, of 1.0, approximating to step 2, are given in
Table 4 for line times of 11.8 ms and 4.5 ms.
In evaluating these image tone data, a useful reference as
regards image tone is the clinically well received SCOPIX™ LT2B film
with a* and b* CIELAB values at a density D = 1.0 of -4.5 and -8.8
respectively. As regards clinical perception, the blue tone, as
represented by the negative b* value, is more important than the a*
value.
Invention example nr. | CIELAB values for D = 1.0 (line time = 11.8 ms) | CIELAB values for D = 1.0 (line time = 4.5 ms) |
| a* | b* | a* | b* |
1 | -5.12 | -7.88 | -0.15 | -12.44 |
2 | -3.90 | -8.05 | 2.14 | -11.00 |
Comparative example nr. |
1 | -6.13 | -4.28 | -4.59 | -6.27 |
It can be seen that the CIELAB values obtained with prints
produced with the substantially light-insensitive thermographic
recording materials of INVENTION EXAMPLES 1 and 2, that the b*
values are substantially more negative than those obtained with
prints produced with the substantially light-insensitive
thermographic recording materials of COMPARATIVE EXAMPLES 1. Thus,
the additional presence of phase II and III silver behenate can be
seen as having a tone modifying influence on the thermographic
image.
INVENTION EXAMPLES 3 to 6
The thermographic recording materials of INVENTION EXAMPLES 3 to
6 were produced by coating a subbed 175µm thick blue-pigmented
polyethylene terephthalate support (a* = -6.86; b* = -14.46; Dvis =
0.181) with a composition containing 2-butanone as
solvent/dispersing medium, so as to obtain thereon, after drying,
the thermosensitive elements of INVENTION EXAMPLES 3 to 6 with the
compositions given in Table 5:
Invention example nr. | AgB g/m2 | Phase II and III AgB stabilizers | Oil mg/m2 | Reducing agent | S01 g/m2 | S02 g/m2 |
| | P01 [g/m2] | P02 [g/m2] | P03 [g/m2] | P05 [g/m2] | | type | g/m2 |
3 | 3.70 | 0.26 | 0.203 | 0.105 | 14.9 | 33 | R02 | 0.75 | 0.12 | 0.10 |
4 | 3.70 | 0.26 | 0.203 | 0.105 | 14.9 | 33 | R03 | 0.56 | 0.12 | 0.10 |
5 | 3.70 | 0.26 | 0.203 | 0.105 | 14.9 | 33 | R04 | 0.63 | 0.12 | 0.10 |
6 | 3.70 | 0.26 | 0.203 | 0.105 | 14.9 | 33 | R05 | 0.88 | 0.12 | 0.10 |
thermographic printing
During the thermographic printing of the substantially light-insensitive
thermographic recording materials of INVENTION EXAMPLES
3 to 6, 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.
Printing was carried out with a DRYSTAR® 2000 printer from AGFA-GEVAERT
equipped with a thin film thermal head with a resolution of
300 dpi and line times (the line time being the time needed for
printing one line) of 11.8 ms, 7.0 ms and 4.5 ms (corresponding to
90mw/pixel, 99mW/pixel and 108mW/pixel respectively). During this
line time the print head received constant power. The thermal head
resistors were time-modulated to produce different image densities.
The substantially light-insensitive thermographic recording
materials of INVENTION EXAMPLES 3 to 6 were printed at line times of
11.8 ms, 7.0 ms and 4.5 ms in such a manner that a step wedge was
produced with 8 steps from 0 to 7 corresponding to equal increments
of heating energy, step 0 corresponding to a heat energy ca. 25% of
that of step 7 and corresponding to the minimum and maximum
densities of the image, Dmin and Dmax respectively.
The densities of the images measured through a visible filter
with a MACBETH™ TR924 densitometer were determined for step 7,
corresponding to Dmax for prints obtained with the substantially
light-insensitive thermographic recording materials of INVENTION
EXAMPLES 3 to 6 at line times of 11.8 ms, 7 ms and 4.5 ms
respectively and the results are given in Table 6 and 7.
Image evaluation
Image evaluation of the substantially light-insensitive
thermographic recording materials of INVENTION EXAMPLES 3 to 6 was
carried out as described for the substantially light-insensitive
thermographic recording materials of INVENTION EXAMPLES 1 and 2 and
COMPARATIVE EXAMPLE 1 except that the a* and b* CIELAB-values of the
substantially light-insensitive thermographic recording materials of
INVENTION EXAMPLES 3 to 6 at an optical density, D, of 1.0 were
determined 5 minutes and 24 hours after printing. The results are
given in Tables 6 and 7 respectively.
Invention Example nr. | AgB g/m2 | Phase II and III AgB stabilizers | Dmax (vis) | CIELAB b*-values for D = 1.0 |
| | P01 g/m2 | P02 mol% vs AgB | P03 mol% vs AgB | P05 g/m2 | | t= 5 min | t=24 h | Δb* |
LINE TIME = 11.8 ms |
3 | 3.70 | 0.26 | 14.93 | 4.99 | 14.9 | 2.84 | -13.7 | -12.8 | +0.9 |
4 | 3.70 | 0.26 | 14.93 | 4.99 | 14.9 | 2.25 | - 8.1 | -7.2 | +0.9 |
5 | 3.70 | 0.26 | 14.93 | 4.99 | 14.9 | 2.93 | -12.7 | -12.1 | +0.6 |
6 | 3.70 | 0.26 | 14.93 | 4.99 | 14.9 | 2.97 | -12.2 | -11.7 | +0.5 |
LINE TIME = 7.0 ms | | | | |
3 | 3.70 | 0.26 | 14.93 | 4.99 | 14.9 | 2.71 | -13.7 | -13.9 | -0.2 |
4 | 3.70 | 0.26 | 14.93 | 4.99 | 14.9 | 2.12 | -9.3 | -8.6 | +0.7 |
5 | 3.70 | 0.26 | 14.93 | 4.99 | 14.9 | 2.89 | -14.1 | -13.6 | +0.5 |
6 | 3.70 | 0.26 | 14.93 | 4.99 | 14.9 | 2.83 | -13.2 | -13.0 | +0.2 |
LINE TIME = 4.5ms | | | | |
3 | 3.70 | 0.26 | 14.93 | 4.99 | 14.9 | 1.82 | -15.8 | -12.7 | +3.1 |
4 | 3.70 | 0.26 | 14.93 | 4.99 | 14.9 | 1.48 | -10.2 | -9.2 | +1.0 |
5 | 3.70 | 0.26 | 14.93 | 4.99 | 14.9 | 2.03 | -15.8 | -14.1 | +1.7 |
6 | 3.70 | 0.26 | 14.93 | 4.99 | 14.9 | 1.95 | -14.4 | -12.6 | +1.8 |
Table 6 shows that a shift in b* for D = 1.0 takes place between
5 minutes and 24 hours after printing. This shift is between +0.5
and +0.9 for a line time of 11.8ms; -0.2 and +0.7 for a line time of
7.0ms; and +1.0 to +3.1 for a line time of 4.5ms. The shifts of the
thermographic recording materials of INVENTION EXAMPLES 3, 4, 5 and
6 are acceptable for line times of 11.8 and 7.0ms, but it is
desirable to reduce the line time to 4.5ms so that the throughput
can be optimized. However, in the case of a 4.5ms line time the
shift in b* is only acceptable in the case of the thermographic
recording material of INVENTION EXAMPLE 4 containing the AgB phase
II and phase III stabilizer compounds P01, P02 and P03 together with
the reducing agent R03 (3,4-dihydroxybenzonitrile).
Table 7 shows that a shift in a* for D = 1.0 takes place between
5 minutes and 24 hours after printing. This shift is between 0.0
and +0.4 for a line time of 11.8ms; -0.7 and +0.5 for a line time of
7.0ms; and +0.7 to +2.4 for a line time of 4.5ms. The shifts of the
thermographic recording materials of INVENTION EXAMPLES 3, 4, 5 and
6 are acceptable for line times of 11.8 and 7.0ms, but it is
desirable to reduce the line time to 4.5ms so that the throughput
can be optimized. However, in the case of a 4.5ms line time the
shift in a* is only acceptable in the case of the thermographic
recording material of INVENTION EXAMPLE 4 containing P01, P02 and
P03 as the AgB phase II and phase III stabilizer compounds in
combination with the reducing agent R03 (3,4-dihydroxybenzonitrile.
Invention Example Nr. | AgB g/m2 | Phase II and III AgB stabilizers | Dmax (vis) | CIELAB a*-values for D = 1.0 |
| | P01 g/m2 | P02 mol% vs AgB | P03 mol% vs AgB | | t= 5 min | t=24 h | Δa* |
LINE TIME = 11.8 ms | | | |
3 | 3.70 | 0.26 | 14.93 | 4.99 | 2.84 | -2.4 | -2.0 | +0.4 |
4 | 3.70 | 0.26 | 14.93 | 4.99 | 2.25 | -4.8 | -4.8 | 0.0 |
5 | 3.70 | 0.26 | 14.93 | 4.99 | 2.93 | -3.3 | -3.2 | +0.1 |
6 | 3.70 | 0.26 | 14.93 | 4.99 | 2.97 | -2.7 | -2.4 | +0.3 |
LINE TIME = 7.0 ms | | | |
3 | 3.70 | 0.26 | 14.93 | 4.99 | 2.71 | 0.6 | 1.1 | +0.5 |
4 | 3.70 | 0.26 | 14.93 | 4.99 | 2.12 | -3.9 | -4.2 | -0.3 |
5 | 3.70 | 0.26 | 14.93 | 4.99 | 2.89 | -1.0 | -0.9 | +0.1 |
6 | 3.70 | 0.26 | 14.93 | 4.99 | 2.83 | 0.6 | -0.1 | -0.7 |
LINE TIME = 4.5 ms | | | |
3 | 3.70 | 0.26 | 14.93 | 4.99 | 1.82 | 5.2 | 7.6 | +2.4 |
4 | 3.70 | 0.26 | 14.93 | 4.99 | 1.48 | -1.6 | -0.9 | +0.7 |
5 | 3.70 | 0.26 | 14.93 | 4.99 | 2.03 | 3.3 | 5.0 | +1.7 |
6 | 3.70 | 0.26 | 14.93 | 4.99 | 1.95 | 5.3 | 6.8 | +1.5 |
In conclusion the thermographic recording materials of INVENTION
EXAMPLES 3, 4, 5 and 6 are all suitable for use with printers with
line times of 11.8 and 7.0ms, but only the thermographic recording
material of INVENTION EXAMPLE 4 is suitable for use with a printer
with a line time of 4.5ms.
This shows that the tone modifying properties of phase II silver
behenate and phase III silver behenate, when used in combination
with 3,4-dihydroxybenzonitrile as reducing agent, produce
particularly favourable image tones at printer line times of 4.5 ms.
X-ray diffraction evaluation
X-ray diffraction measurements were carried out as described for
INVENTION EXAMPLES 1 and 2 and COMPARATIVE EXAMPLE 1 in real time on
the thermographic recording materials of INVENTION EXAMPLES 3 to 6
after they emerged from the
DRYSTAR® 2000 printer. The amount of
noise due to the rapid XRD-scans meant that only qualitative
information could be obtained from these measurements. This
information is summarized in Table 8.
Invention example nr. | Reducing agent | Printer line time |
| | 11.8 ms | 4.5 ms |
3 | R02 | conc. phase I « conc. phases II & III | conc. phase I = conc. phases II & III |
more crystallization for 11.8 ms line time versus 4.5 ms |
more crystallization versus R03, R04 & R04 for 11.8 ms |
4 | R03 | only phases II & III present | only phases II & III present |
less crystallization than with R02, R04 and R05 |
5 | R04 | | conc. phase I < conc. phases II & III |
6 | R05 | | conc. phase I < conc. phases II & III |
The change in image tone subsequent to thermal development is
accompanied by changes in the phase structure of the silver
behenate. In the case of the thermographic recording materials of
INVENTION EXAMPLES 3, 5 and 6 the concentrations of phase I, phase
II and phase III silver behenate increased in the first 15 minutes
after thermal development. However, in the case of the
thermographic recording material of INVENTION EXAMPLE 4 in which
glutaric acid is present together with R03 as the reducing agent
(3,4-dihydroxybenzonitrile) this effect was considerably reduced and
phase II and phase III silver behenate was principally observed with
no detectable phase I silver behenate.
INVENTION EXAMPLES 7 to 10
The thermographic recording materials of INVENTION EXAMPLES 7 to
10 were produced by coating a subbed 175µm thick blue-pigmented
polyethylene terephthalate support (a* = -6.86; b* = -14.46; Dvis =
0.181) with a composition containing 2-butanone as
solvent/dispersing medium, so as to obtain thereon, after drying,
the thermosensitive elements of INVENTION EXAMPLES 7 to 10 with the
compositions given in Table 9:
Example Nr. | | Comparative 2 | Invention 7 |
AgB [g/m2] | | 4.11 | 3.71 |
P01 [g/m2] | | - | 0.266 |
P02 [g/m2] | | 0.223 | 0.203 |
P03 [g/m2] | | 0.115 | 0.105 |
P06 [g/m2] | | 12.315 | 14.87 |
Oil [mg/m2] | | 0.036 | 0.033 |
Reducing agent | type [g/m2] | R02 0.827 | R02 0.750 |
S01 [g/m2] | | 0.13 | 0.117 |
S02 [g/m2] | | 0.108 | 0.097 |
D01 [g/m2] | | 0.293 | - |
The thermosensitive element was then provided with a protective
layer by coating with an aqueous composition with the following
composition expressed as weight percentages of ingredients present:
- polyvinylalcohol (Polyviol™ WX 48/20 from Wacker Chemie): 2.5%
- Ultravon™ W (dispersion agent from Ciba Geigy) converted
into acid form by passing through an ion exchange column: 0.09%
- talc (type P3 from Nippon Talc): 0.05%
- colloidal silica (Levasil™ VP AC 4055 from Bayer AG, a 15%
aqueous dispersion of colloidal silica): 1.2%
- silica (Syloid™ 72 from Grace): 0.10%
- mono[isotridecyl polyglycolether (3 EO)] phosphate
(Servoxyl™ VPDZ 3/100 from Servo Delden B.V.): 0.09%
- mixture of monolauryl and dilauryl phosphate (Servoxyl™
VPAZ 100 from Servo Delden B.V.): 0.09%
- glycerine monotallow acid ester (Rilanit™ GMS from
Henkel AG): 0.18%
- tetramethylorthosilicate hydrolyzed in the presence of
methanesulfonic acid: 2.1%
The pH of the coating composition was adjusted to a pH of 3.8 by
adding 1N nitric acid. Those lubricants in these compositions which
were insoluble in water, were dispersed in a ball mill with, if
necessary, the aid of a dispersion agent. The compositions were
coated to a wet layer thickness of 85 µm and were then dried at 40°C
for 15 minutes and hardened at 45°C for 7 days thereby producing a
protective layer.
thermographic printing
Printing was carried out with a DRYSTAR® 4500 printer from AGFA-GEVAERT
equipped with a thin film thermal head with a resolution of
508 dpi adapted to print at a line time (the line time being the
time needed for printing one line) of 12 ms (corresponding to
35mW/pixel). During this line time the print head received constant
power. The thermal head resistors were time-modulated to produce
different image densities.
The substantially light-insensitive thermographic recording
materials of COMPARATIVE EXAMPLE 2 and INVENTION EXAMPLE 7 were
printed in such a manner that a step wedge was produced with 8 steps
from 0 to 7 corresponding to equal increments of heating energy,
step 0 corresponding to a heat energy ca. 25% of that of step 7 and
corresponding to the minimum and maximum densities of the image, Dmin
and Dmax respectively.
The densities of the images measured through a visible filter
with a MACBETH™ TR924 densitometer were determined for all eight
steps for prints obtained with the substantially light-insensitive
thermographic recording materials of COMPARATIVE EXAMPLE 2 and
INVENTION EXAMPLE 7 and the values are summarized in Table 10.
Evaluation of the density steps by X-ray Diffraction Spectroscopy
The X-ray diffraction spectra were determined in a Philips X'Pert
XRD apparatus with a CuKα X-ray source for the density steps of the
step wedges obtained with the substantially light-insensitive
thermographic recording materials of COMPARATIVE EXAMPLE 2 and
INVENTION EXAMPLE 7.
The presence of phase I silver behenate was detected in the step
wedges produced upon printing the substantially light-insensitive
thermographic recording materials of COMPARATIVE EXAMPLE 2 and
INVENTION EXAMPLE 7 was established by the presence of strong peaks
at characteristic Bragg angle 2Θ at 4.53°, 5.96-6.05°, 7.46-7.56°,
8.90-9.12°, 10.45-10.66°, 12.02-12.12°, 13.53-13.62° and the
presence of phase II and phase III silver behenate phases in prints
produced with the substantially light-insensitive thermographic
recording material of INVENTION EXAMPLE 7 by the presence of strong
peaks at characteristic Bragg angles 2Θ of 4.76-4.81° and 6.76-7.35°,
which are the principal phase III silver behenate peaks
boosted by overlap with the phase II silver behenate present in
lower concentrations than the phase III silver behenate.
The results obtained expressed as percentages with respect to
the quantity of phase I silver behenate present in the thermographic
recording materials before thermal development are summarized in
Table 10 below.
Example nr | Step nr | Density (vis filter) | phase I AgB phase [%] | phase II & phase III AgB [%] | total of AgB phases [%] |
Comparative 2 | 0 | 0.23 | 80.2 | 0.0 | 80.2 |
1 | 0.23 | 88.8 | 0.0 | 88.8 |
| 2 | 0.23 | 85.5 | 0.0 | 85.5 |
| 3 | 0.26 | 27.3 | 0.0 | 27.3 |
| 4 | 0.65 | 8.6 | 0.0 | 8.6 |
| 5 | 1.70 | 2.2 | 0.0 | 2.2 |
| 6 | 2.95 | 2.0 | 0.0 | 2.0 |
| 7 | 3.97 | 2.0 | 0.0 | 2.0 |
Invention 7 | 0 | 0.22 | 81.9 | 0.0 | 81.9 |
1 | 0.22 | 83.8 | 0.0 | 83.8 |
| 2 | 0.22 | 87.1 | 0.0 | 87.1 |
| 3 | 0.23 | 27.7 | 5.0 | 32.7 |
| 4 | 0.68 | 9.1 | 13.8 | 22.9 |
| 5 | 1.94 | 1.9 | 6.2 | 8.1 |
| 6 | 2.86 | 1.9 | 2.1 | 4.0 |
| 7 | 3.14 | 0.2 | 0.5 | 0.7 |
It can be seen that exposure to ca. 25% of the heat energy for step
7 during the printing process reduced the crystallinity, i.e. the
quantity of phase I silver behenate observed, by between 5 and 20
mol%.
In the case of the substantially light-insensitive thermographic
recording materials of COMPARATIVE EXAMPLE 2 and INVENTION EXAMPLE
7, the amount of phase I silver behenate decreased with increasing
heating energy and increasing optical density.
No phase II and III silver behenate was observed with the
substantially light-insensitive thermographic recording material of
COMPARATIVE EXAMPLE 2.
With the substantially light-insensitive thermographic recording
material of INVENTION EXAMPLE 7, phase II and phase III silver
behenate was observed from step 3 with the maximum quantity observed
in step 4.
As regards interpretation of these results, it is important to
note that it is in steps 3, 4 and 5 of the step wedge (corresponding
approximately to densities of 0.25, 0.65 and 1.8 respectively) that
the strongest effects on image tone are observed.
Image evaluation
The image tone of fresh prints made with the substantially
light-insensitive thermographic recording materials of COMPARATIVE
EXAMPLE 1 and INVENTION EXAMPLE 7 was assessed on the basis of the
L*, a* and b* CIELAB-values as described above. The a* and b*
CIELAB-values 24 hours after printing are given below in Table 11 for
the substantially light-insensitive thermographic recording
materials of COMPARATIVE EXAMPLE 1 and INVENTION EXAMPLE 7 at
optical densities, D, of 1.0 and 2.0.
| | | CIELAB results: D= 1.0 | CIELAB results: D=2.0 |
Comparative Example nr | Dmax (vis) | Dmin (vis) | a* | b* | a* | b* |
2 | 3.97 | 0.23 | -6.36 | -3.71 | -2.63 | -2.73 |
Invention Example nr |
7 | 3.14 | 0.22 | -4.29 | -8.48 | -0.52 | -5.22 |
The much higher CIELAB b*-values observed with the substantially
light-insensitive thermographic recording material of INVENTION
EXAMPLE 7 compared with the substantially light-insensitive
thermographic recording material of COMPARATIVE EXAMPLE 2,
demonstrate the favourable impact of the presence of phase II and
phase III silver behenate on the image tone of substantially light-insensitive
thermographic recording materials.
INVENTION EXAMPLES 8 to 19 and COMPARATIVE EXAMPLES 3 and 4
X-ray diffraction experiments with mixtures of
silver behenate with different phase II and phase III
silver behenate-stabilizing compounds
The materials of INVENTION EXAMPLES 8 to 19 and COMPARATIVE
EXAMPLES 3 and 4 consisted of mixtures of silver behenate with
different phase II and III silver behenate-stabilizing compounds
with the compositions given in Table 12.
The silver behenate for INVENTION EXAMPLES 8 to 13 and
COMPARATIVE EXAMPLES 3 and 4 was used as a 24% by weight solids
aqueous dispersion containing silver behenate particles stabilized
with 6.3% by weight of surfactant S01 and this was mixed with the
required molar ratio of phase II and phase III silver behenate-stabilizing
compound and then dried before heating the resulting
mixture in the sample holder of a Philips X'Pert XRD (X-ray
diffraction) apparatus.
The silver behenate for INVENTION EXAMPLES 14 to 19 was used as
a methylethylketone dispersion with 20.25% by weight of solids and
containing silver behenate particles stabilized with P05 in a weight
ratio of 0.8 : 1. This was mixed with the required molar ratio of
phase II and phase III silver behenate-stabilizing compound and then
dried before heating the resulting mixture in the sample holder of a
Philips X'Pert XRD (X-ray diffraction) apparatus.
X-ray diffraction evaluation
The materials of INVENTION EXAMPLES 8 to 19 and COMPARATIVE
EXAMPLES 3 and 4 were heated up in the Philips X'Pert XRD apparatus
with a CuKα X-ray source from 25°C to 100°C and then from 100°C to
200°C in 10°C intervals with an XRD-spectrum being taken in the 2-range:
5-50° in a continuous scan and finally from 200 to 25°C. The
evolution of the XRD-spectrum of silver behenate with increasing
temperature was similar, with phase changes being observed at 120°C
(part of AgB becoming amorphous), at 138°C (additional XRD-peaks,
new structure = second phase transition), at 156°C (additional XRD-peaks,
new structure = third phase transition). This new structure
was observed 10°C lower in the case of P03 than with the other phase
II and III silver behenate-stabilizing compounds. From 170°C silver
metal formation was observed, which was more pronounced in the
presence of the phase II and phase III silver behenate-stabilizing
compounds (INVENTION EXAMPLES 8 to 19) than in their absence
(COMPARATIVE EXAMPLES 3 and 4). The materials were then cooled to
25°C and a further XRD-spectrum taken. The quantity of silver
behenate which crystallized upon cooling to 25°C with respect to the
quantity of phase I silver behenate before heating was determined as
described for INVENTION EXAMPLES 1 and 2 and COMPARATIVE EXAMPLE 1.
The results are summarized in Table 12.
The results of Table 12 show that compounds P01 to P05 stabilize
phases II and III silver behenate at 25°C and that the highest
degree of stabilization was observed with compound P01, glutaric
acid. It should be noted that P01 and TA1 promoted the formation of
significantly more crystalline phase I silver behenate upon cooling
the melt to 25°C, than the other compounds investigated.
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.