1. Field of the Invention
[0001] The present invention relates to a recording material suited for use in direct thermal
imaging.
2. Background of the Invention
[0002] Thermal imaging or thermography is a recording process wherein images are generated
by the use of imagewise modulated thermal energy.
[0003] In thermography two approaches are known :
1. Direct thermal formation of a visible image pattern by imagewise heating of a recording
material containing matter that by chemical or physical process changes colour or
optical density.
2. Thermal dye transfer printing wherein a visible image pattern is formed by transfer
of a coloured species from an imagewise heated donor element onto a receptor element.
[0004] Thermal dye transfer printing is a recording method wherein a dye-donor element is
used that is provided with a dye layer wherefrom dyed portions or incorporated dyes
are transferred onto a contacting receiver element by the application of heat in a
pattern normally controlled by electronic information signals.
[0005] A survey of "direct thermal" imaging methods is given e.g. in the book "Imaging Systems"
by Kurt I. Jacobson-Ralph E. Jacobson, The Focal Press - London and New York (1976),
Chapter VII under the heading "7.1 Thermography". Thermography is concerned with materials
which are substantially not photosensitive, but are sensitive to heat or thermosensitive.
Imagewise applied heat is sufficient to bring about a visible change in a thermosensitive
imaging material.
[0006] Most of the "direct" thermographic recording materials are of the chemical type.
On heating to a certain conversion temperature, an irreversible chemical reaction
takes place and a coloured image is produced.
[0007] A wide variety of chemical systems has been suggested some examples of which have
been given on page 138 of the above mentioned book of Kurt I. Jacobson et al., describing
the production of a silver metal image by means of a thermally induced oxidation-reduction
reaction of a silver soap with a reducing agent.
[0008] As described in "Handbook of Imaging Materials", edited by Arthur S. Diamond - Diamond
Research Corporation - Ventura, Calfornia, printed by Marcel Dekker, Inc. 270 Madison
Avenue, New York, New York 10016 (1991), p. 498-499 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 electric
pulses thus converted into thermal signals manifest themselves as heat transferred
to the surface of the thermal paper wherein the chemical reaction resulting in colour
development takes place.
[0009] According to US-P 3,080,254 a typical heat-sensitive copy paper includes in the heat-sensitive
layer a thermoplastic binder, e.g ethyl cellulose, a water-insoluble silver salt,
e.g. silver stearate and an appropriate organic reducing agent, of which 4-methoxy-1-hydroxy-dihydronaphthalene
is a representative. Localized heating of the sheet in the thermographic reproduction
process, or for test purposes by momentary contact with a metal test bar heated to
a suitable conversion temperature in the range of about 90-150 °C, causes a visible
change to occur in the heat-sensitive layer. The initially white or lightly coloured
layer is darkened to a brownish appearance at the heated area. In order to obtain
a more neutral colour tone a heterocyclic organic toning agent such as phthalazinone
is added to the composition of the heat-sensitive layer. Thermo-sensitive copying
paper is used in "front-printing" or "back-printing" using infra-red radiation absorbed
and transformed into heat in contacting infra-red light absorbing image areas of an
original as illustrated in Figures 1 and 2 of US-P 3,074,809.
[0010] US-P 3,241,997 concerns a heat-sensitive copying material having two separate layers
located one on top of the other and carried by a supporting material, the layers having
different melting points and being soluble in different solvents, the solvent of one
layer being incapable of dissolving the other layer, the layers containing at least
two different chemical reagents capable of reacting with each other when at least
one of them is in molten form to produce colour, the colour-producing reagents being
located in two different layers, whereby each layer contains at least one colour-producing
reagent and whereby no one layer contains all of the colour-producing reagents, at
least one of the layers and at least one of the colour-producing reagents melting
at most at 150
oC.
[0011] US P 3,795,532 concerns sheet material containing metal soap reactants and adapted
for producing a copy of an original in a heat-activated copying process when associated
with a coreactant source to form a couple. The coreactant may be provided as a separate
coating directly overlying and bonded to the soap layer.
[0012] A heat-sensitive recording material containing silver behenate and 4-methoxy-1-naphthol
as reducing agent in adjacent water-insoluble polymeric binder layers is also described
in Example 1 of US-P 3,094,417.
[0013] Further, the separate application in a thermosensitive recording material of an organic
silver salt and hydroxylamine type reductor in thermal working relationship in adjacent
layers containing a thermoplastic water-insoluble binder such as ethyl cellulose and
after-chlorinated polyvinyl chloride is described already in US-P 4,082,901.
[0014] In a special embodiment of direct thermal imaging a heat-sensitive recording material
is used in the form of an electrically resistive ribbon having a multilayered structure
in which a carbon-loaded polycarbonate is coated with a thin aluminium film (ref.
Progress in Basic Principles of Imaging Systems - Proceedings of the International
Congress of Photographic Science Köln (Cologne), 1986 ed. by Friedrich Granzer and
Erik Moisar - Friedr. Vieweg & Sohn - Braunschweig/Wiesbaden, Figure 6. p. 622). Current
is injected into the resistive ribbon by electrically addressing a print head electrode
contacting the carbon-loaded substrate, thus resulting in highly localized heating
of the ribbon beneath the energized electrode.
[0015] The fact that in using a resistive ribbon recording material heat is generated directly
in the resistive ribbon and only the travelling ribbon gets hot (not the print heads)
an inherent advantage in printing speed is obtained. In applying the thermal printing
head technology the various elements of the thermal printing head get hot and must
cool down before the head can print without cross-talk in a next position.
[0016] In another embodiment of direct thermal imaging the recording material is image-wise
or pattern-wise heated by means of a modulated laser beam. For example, image-wise
modulated infra-red laser light is absorbed in the recording layer in infra-red light
absorbing substances converting infra-red radiation into the necessary heat for the
imaging reaction.
[0017] The image-wise applied laser light has not necessarily to be infrared light since
the power of a laser in the visible light range and even in the ultraviolet region
can be thus high that sufficient heat is generated on absorption of the laser light
in the recording material. There is no limitation on the kind of laser used which
may be a gas laser, gas ion laser, e.g. argon ion laser, solid state laser, e.g. Nd:YAG
laser, dye laser or semi-conductor laser.
[0018] The use of an infrared light emitting laser and a dye-donor element containing an
infrared light absorbing material is described e.g. in US-P 4,912,083. Suitable infra-red
light absorbing dyes for laser-induced thermal dye transfer are described e.g. in
US-P 4,948,777, which US-P documents for said dyes and lasers applied in direct thermal
imaging have to be read in conjunction herewith.
[0019] The image signals for modulating the laser beam or current in the micro-resistors
of a thermal printhead are obtained directly e.g. from opto-electronic scanning devices
or from an intermediary storage means, e.g. magnetic disc or tape or optical disc
storage medium, optionally linked to a digital image work station wherein the image
information can be processed to satisfy particular needs.
[0020] When used in thermographic recording operating with thermal printheads said recording
materials will not be suited for reproducing images with fairly large number of grey
levels as is required for continuous tone reproduction.
[0021] According to EP-A 622 217 relating to a method for making an image using a direct
thermal imaging element, improvements in continuous tone reproduction are obtained
by heating the thermal recording element by means of a thermal head having a plurality
of heating elements, characterized in that the activation of the heating elements
is executed line by line with a duty cycle Δ representing the ratio of activation
time to total line time in such a way that the following equation is satisfied :
wherein P
max is the maximal value over all the heating elements of the time averaged power density
P (expressed in W/mm
2) dissipated by a heating element during a line time.
[0022] Although by controlling the heating of the heating elements of a thermal head in
the way as described in said EP-A already a substantial improvement in continuous
tone reproduction is obtained, from the side of the composition of the thermal recording
element further improvements to lower the image gradation are still desirable.
3. Objects and Summary of the Invention
[0023] It is an object of the present invention to provide a heat-sensitive recording material
suited for use in direct thermal imaging, wherein said material is capable of yielding
images with maximum density preferably higher than 2.5 and with a gradation sufficiently
low for continuous tone reproduction as is needed e.g. in a portrait for an identification
document and in the medical diagnostic field wherein images are produced by e.g. radiography,
ultrasound or nuclear magnetic resonance (NMR) signals.
[0024] A further object of the present invention is a reduction in the use of ecologically
suspect organic solvents in the coating of such heat-sensitive recording materials.
[0025] Other objects and advantages of the present invention will appear from the further
description.
[0026] According to the present invention a heat-sensitive recording material suited for
use in direct thermal imaging is provided, said recording material comprising :
(i) a layer (1) containing uniformly distributed in a film-forming water-insoluble
resin binder a substantially light-insensitive organic metal salt, preferably a silver
salt, and
(ii) a layer (2) in direct contact with said layer (1) or in thermal working relationship
therewith through the intermediary of a spacer layer (3), characterized in that said
layer (2) contains uniformly distributed in a film-forming water-soluble hydrophilic
binder at least one organic reducing agent, that is capable of diffusing out of said
layer (2) into said layer (1) on heating said recording material.
[0027] The present invention also provides a method for producing a heat-sensitive recording
material, as defined above, characterized in that said layer (1) is coated from a
non-aqueous medium and said layer (2) is coated from an aqueous medium.
[0028] The present invention further includes a recording process wherein a heat-sensitive
recording material, as defined above, is image-wise heated.
[0029] In particular the present invention provides a thermographic recording method with
improved continuous tone reproduction wherein a heat-sensitive recording material,
as defined above, is image-wise heated by means of a thermal head containing a plurality
of image-wise electrically energized heating elements.
[0030] By "thermal working relationship" is meant here that the reagents responsible for
the image formation can come into reactive contact with each other by diffusion of
at least one of the reagents under the influence of heat, e.g. by diffusion through
said spacer layer.
[0031] The layer wherein image-wise a metal image is formed on image-wise reducing the substantially
light-insensitive metal salt is called furtheron the recording layer or imaging layer.
4. Brief Description of the Drawings
[0032] Fig. 1, 2 and 3 represent characteristic sensitometric curves of prints obtained
with heat-sensitive "non-invention" and "invention" recording materials. Said characteristic
curves were obtained by plotting optical density (D) (logarithmic values) in the ordinate
and linearly increasing amounts of exposure heat (relative values) (rel. H) in the
abscissa.
5. Detailed Description of the Invention
[0033] The term "gradation" refers to the slope of a characteristic curve representing the
relationship of optical density (D) plotted in the ordinate versus linearly increasing
amounts of heat plotted in the abscissa, said different amounts of heat being applied
to the thermographic material in neighbouring area analogously to the production of
a stepwedge.
[0034] The linear increase of heat is obtained e.g. by linearly increasing the heating time
at different areas of the recording material while keeping constant the heat input
(J) per time unit (s). Alternatively the heating time can be kept constant and the
amount of input-heat is increased linearly.
[0035] By definition all gradients or slopes of said characteristic curve create together
the gradation of the thermographic image. A gradient corresponds with the slope at
a single point on the characteristic curve. The gamma (γ) is the maximum gradient
of said characteristic curve, which is normally the gradient between the end of the
toe and the beginning of the shoulder of the characteristic curve.
[0036] The heat-sensitive recording material according to the present invention yields a
gradation which is much lower than can be obtained without using the above mentioned
layer assemblage.
[0037] The speed of diffusion the organic reducing agent(s) out of the hydrophilic binder
layer (2) is controlled by the intensity of the heating and the adsorption affinity
of said agents to the hydrophilic binder giving rise to a larger amount of visually
recognizable "grey-levels" in the imaging layer that contains the reducible substantially
light-insensitive metal salt.
[0038] The coating of a relatively thick (dry coating thickness in the range of 2 to 20
µm) hydrophilic binder layer containing the diffusible reducing agent in a binder/reducing
agent weight ratio in the range of 1/1 to 10/1 is particularly favourable for lowering
the gradation.
[0039] The hydrophilic water-soluble binder of the layer containing the therein diffusible
organic reducing agent may be any hydrophilic polymeric binder used in the preparation
of photographic silver halide emulsion layers, preferably is a protein-type binding
agent such as gelatin, casein, collagen, albumin, or gelatin derivative, e.g. acetylated
gelatin. Further suitable water-soluble binding agents are : polyvinyl alcohol, dextran,
gum arabic, zein, agar-agar, arrowroot, pectin, carboxymethyl cellulose, hydroxyethyl
cellulose, poly(acrylic acid), and polyvinylpyrrolidone.
[0040] The optionally present spacer layer (3) is a thin (preferably 0.1 µm to 10 µm thick)
polymeric layer allowing the passage by diffusion of said reducing agent(s).
[0041] The spacer layer (3) may be a hydrophilic polymeric layer, e.g. made of modified
cellulose, e.g. cellulose diacetate, that has been coated free from reducing agent(s)
or is a polymeric layer coated from a latex that on drying has some micro-pores allowing
the passage of diffusing reducing agent(s). According to an other embodiment said
spacer layer (3) contains a polymeric binder applied from an organic solvent which
binder is permeable for the organic reducing agent(s) in molten state. When such spacer
layer is present preference is given to the use of reducing agents the melting point
of which is in the range of 60 to 120 °C (melting point of catechol = 105 °C) or reducing
agents are used that are characterized by sublimation, e.g. 2-bromo-1,4-benzenediol
(bromohydroquinone).
[0042] In said layer (2) the use of polyhydroxy-benzene reducing agents having at least
two hydroxy groups in ortho- or para-position is preferred for obtaining continuous
tone images. Examples of such reducing agents are catecol (pyrocatechol), hydroquinone,
2-methyl hydroquinone, 2-chloro-hydroquinone, 3-(3,4-dihydroxyphenyl) propionic acid,
1,2-dihydroxybenzoic acid, methyl gallate, ethyl gallate, propyl gallate and tannic
acid. Another diffusible (very water-soluble) reducing agent is ascorbic acid belonging
to the class of the reductones.
[0043] Diffusible reducing agents that are useful in the recording material of the present
invention, and more particularly the polyhydroxy benzene reducing agents having at
least two hydroxy groups in ortho or para position and melting point near 100 °C,
may be used in combination with other reducing agents operating as auxiliary reducing
agents, e.g. sterically hindered phenols, that on heating become reactive partners
in the reduction of the substantially light-insensitive silver salt such as silver
behenate, or are used in combination with bisphenols as described in US-P 3,547,648.
The auxiliary reducing agents may be present in the imaging layer (1) and/or in the
hydrophilic polymeric binder layer (2).
[0044] The film-forming water-insoluble resin binder of the recording layer containing the
substantially light-insensitive organic metal salt is preferably a thermoplastic resin
or mixture of such resins, wherein the silver salt can be dispersed homogeneously.
For that purpose all kinds of natural, modified natural or synthetic water-insoluble
resins may be used, e.g. cellulose derivatives such as ethylcellulose, cellulose esters,
e.g. cellulose nitrate, 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 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 and polyethylene or mixtures thereof.
[0045] A particularly suitable polyvinyl butyral containing a minor amount of vinyl alcohol
units is marketed under the trade name BUTVAR B79 of Monsanto USA and provides a good
adherence to hydrophilic water-soluble polymeric layers.
[0046] The above mentioned polymers or mixtures thereof forming the water-insoluble binder
may be used in conjunction with waxes or "heat solvents" also called "thermal solvents"
or "thermosolvents" improving the reaction speed of the redox-reaction at elevated
temperature.
[0047] By the term "heat solvent" in this invention is meant a non-hydrolyzable organic
material which is in solid state in the recording layer at temperatures below 50 °C
but becomes a plasticizer for the recording layer in the heated region and/or liquid
solvent for at least one of the redox-reactants, e.g. the reducing agent for the organic
silver salt, at a temperature above 60 °C. Useful for that purpose are a polyethylene
glycol having a mean molecular weight in the range of 1,500 to 20,000 described in
US-P 3,347,675. Further are mentioned compounds such as urea, methyl sulfonamide and
ethylene carbonate being heat solvents described in US-P 3,667,959, and compounds
such as tetrahydrothiophene-1,1-dioxide, methyl anisate and 1,10-decanediol being
described as heat solvents in Research Disclosure, December 1976, (item 15027) pages
26-28. Still other examples of heat solvents have been described in US-P 3,438,776,
and 4,740,446, and in published EP-A 0 119 615 and 0 122 512 and DE-A 3 339 810.
[0048] The layer containing the organic metal salt is commonly coated from an organic solvent
containing the binder in dissolved form.
[0049] The binder to organic metal salt weight ratio is preferably in the range of 0.2 to
6, and the thickness of the recording layer is preferably in the range of 5 to 20
µm.
[0050] Substantially light-insensitive organic silver salts particularly suited for use
in a heat-sensitive recording material according to the present invention are silver
salts of aliphatic carboxylic acids known as fatty acids, wherein the aliphatic carbon
chain has preferably at least 12 C-atoms, e.g. silver laurate, silver palmitate, silver
stearate, silver hydroxystearate, silver oleate and silver behenate, which silver
salts are also called "silver soaps". Modified aliphatic carboxylic acids with thioether
group as described e.g. in GB-P 1,111,492 and other organic silver salts as described
in GB-P 1,439,478, e.g. silver benzoate and silver phthalazinone, may be used likewise
to produce a thermally developable silver image. Further are mentioned silver imidazolates
and the substantially light-insensitive inorganic or organic silver salt complexes
described in US-P 4,260,677.
[0051] The silver image density depends on the coverage of the above defined reducing agent(s)
and organic silver salt(s) and has to be preferably such that, on heating above 100
°C, an optical density of at least 2.5 can be obtained.
[0052] Useful substantially light-insensitive organic metal salts other than silver salts
are e.g. iron salts of an organic acid, e.g. the iron salts described in published
European patent application 0 520 404, more particularly iron o-benzoylbenzoate.
[0053] In order to obtain a neutral black image tone in the higher densities and neutral
grey in the lower densities the recording layer contains preferably in admixture with
said organic silver salts and reducing agents a so-called toning agent known from
thermography or photo-thermography.
[0054] 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 within the scope of following general formula :
in which :
X represents O or N-alkyl;
each of R
1, R
2, R
3 and R
4 (same or different) represents hydrogen, alkyl, e.g. C1-C20 alkyl, preferably C1-C4
alkyl, cycloalkyl, e.g. cyclopentyl or cyclohexyl, alkoxy, preferably methoxy or ethoxy,
alkylthio with preferably up to 2 carbon atoms, hydroxy, dialkylamino of which the
alkyl groups have preferably up to 2 carbon atoms or halogen, preferably chlorine
or bromine; or R
1 and R
2 or R
2 and R
3 represent the ring members required to complete a fused aromatic ring, preferably
a benzene ring, or R
3 and R
4 represent the ring members required to complete a fused aromatic aromatic or cyclohexane
ring. Toners within the scope of said general formula are described in GB-P 1,439,478
and US-P 3,951,660.
[0055] A toner compound particularly suited for use in combination with polyhydroxy benzene
reducing agents is 3,4-dihydro-2,4-dioxo-1,3,2H-benzoxazine described in US-P 3,951,660.
[0056] In addition to said ingredients the recording layer may contain other additives such
as free fatty acids, surface-active agents, 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 A (tradename of BAYER AG - GERMANY), ultraviolet
light absorbing compounds, white light reflecting and/or ultraviolet radiation reflecting
pigments, colloidal silica, and/or optical brightening agents.
[0057] The support for the heat-sensitive recording material according to the present invention
is preferably a thin flexible carrier made e.g. from paper, polyethylene coated paper
or transparent resin film, e.g. made of a cellulose ester, e.g. 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 heat-sensitive recording layer.
[0058] The coating of the layers (1) and (2) may proceed by any coating technique e.g. 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.
[0059] Direct thermal imaging can be used for both the production of transparencies and
reflection type prints. Such means that the support may be transparent or opaque,
e.g. the support has a white light reflecting aspect. For example, a paper base is
used which may contain white light reflecting pigments, optionally also applied in
an interlayer between the recording layer and said base. In case a transparent base
is used, said base may be colourless or coloured, e.g. has a blue colour.
[0060] In the hard copy field recording materials on white opaque base are used, whereas
in the medical diagnostic field black-imaged transparencies find wide application
in inspection techniques operating with a light box.
[0061] The recording materials of the present invention are particularly suited for use
in thermographic recording techniques operating with thermal print-heads. Suitable
thermal printing heads are e.g. a Fujitsu Thermal Head (FTP-040 MCS001), a TDK Thermal
Head F415 HH7-1089, and a Rohm Thermal Head KE 2008-F3.
[0062] In a particular embodiment the hydrophilic binder layer (2) containing the reducing
agent(s) is covered with the recording layer (1). Said embodiment is suited for use
in by laser induced heat-recording wherein image-wise modulated laser light is directed
through a transparent support and heats said layer (2) internally by absorption of
the laser light therein. When using infrared laser light said layer (2) contains preferably
infra-red light absorbing substances, examples of which are given in US-P 4,912,083,
5,219,703 and 5,256,620.
[0063] In a special embodiment in order to avoid direct contact of the printheads with the
hydrophilic polymeric binder layer (2) being outermost layer (not provided with a
protective layer), the imagewise heating of the recording material with said printheads
proceeds through a contacting but removable resin sheet or web wherefrom during said
heating no transfer of imaging material can take place.
[0064] The layer (2) containing at least one reducing agent when being the outermost layer
(the imaging layer may be the outermost layer) may contain hydrophilic finely divided
(colloidal) optically transparent inert inorganic pigments such as transparent colloidal
silica not masking the lateron formed silver image.
[0065] According to a special embodiment the recording material still being capable of obtaining
an increase in optical density in the area that were not heated imagewise is stabilized
against further density-increasing heat by removing the reductor source, viz. the
layer (2) (in case it is a top layer). The removal of said layer may proceed by wash-off
with an aqueous liquid, optionally while applying mild rubbing, or by stripping, e.g.
using an adhesive sheet or web material firstly laminated to said layer and thereupon
peeling apart (stripping off) therewith said layer (2) from the recording layer or
spacer layer.
[0066] In an other embodiment in order to improve resistance against abrasion which may
occur by frictional contact with the printheads, the layers (1) or (2) when being
a top layer are coated with a protective coating or substances having anti-sticking
properties through the presence of (a) lubricating agent(s). Thus, the outermost layer
of the heat-sensitive recording material according to the present invention may comprise
a dissolved lubricating material and/or a dispersed particulate lubricating material,
e.g. talc particles, optionally protruding from the outermost layer. Examples of suitable
lubricating materials are a surface active agent, a liquid lubricant, a solid lubricant
or mixtures thereof.
[0067] The surface active agents may be any agents known in the art such as carboxylates,
sulfonates, phosphates, aliphatic amine salts, aliphatic quaternary ammonium salts,
polyoxyethylene alkyl ethers, polyethylene glycol fatty acid esters, fluoroalkyl C
2-C
20 aliphatic acids. Examples of liquid lubricants include silicone oils, synthetic oils,
saturated hydrocarbons and glycols. Examples of solid organic lubricants include various
higher alcohols such as stearyl alcohol, fatty acids and fatty acid esters.
[0068] As examples of outermost slipping layers are mentioned layers made from a styrene-acrylonitrile
copolymer or a styrene-acrylonitrile-butadiene copolymer or binder mixture hereof
containing as lubricant in an amount of 0.1 to 10 % by weight with respect to said
binder(s) a polysiloxane-polyether copolymer or polytetrafluoroethylene or a mixture
hereof.
[0069] Another suitable outermost slipping layer may be obtained by coating a solution of
at least one silicon compound and a substance capable of forming during the coating
procedure a polymer having an inorganic backbone which is an oxide of a group IVa
or IVb element as described in published European patent application 0554576.
[0070] Other suitable protective layer compositions that may be applied as slipping (anti-stick)
coating are described e.g. in published European patent applications (EP-A) 0 501
072 and 0 492 411.
[0071] The following comparative and invention examples illustrate the present invention.
The percentages and ratios are by weight unless otherwise indicated.
COMPARATIVE EXAMPLE 1
[0072] A subbed polyethylene terephthalate support having a thickness of 175 µm was doctor
blade-coated from a coating composition containing methyl ethyl ketone as a solvent
and the following ingredients so as to obtain thereon after drying a layer containing
per m
2 :
silver behenate |
4.42 g/m2 |
polyvinyl butyral (BUTVAR B79-tradename) |
4.42 g/m2 |
3,4-dihydro-2,4-dioxo-1,3,2H-benzoxazine |
0.34 g/m2 |
[0073] Onto said dried first layer a second layer was coated from a coating composition
containing methyl ethyl ketone as a solvent and the following ingredients so as to
obtain thereon after drying a layer containing per m
2 :
catechol |
2.5 g/m2 |
copolycarbonate PC3 as identified furtheron |
10.0 g/m2 |
TEGO-GLIDE 410 (trademark) as identified furtheron |
0.5 g/m2 |
[0074] Copolycarbonate PC3 has the following structure :
wherein x=55 mol% and y=45 mol% ; having a viscosity ratio of 1.295.
[0075] The viscosity ratio is by definition the quotient of the viscosity of the polymer
solution and of the pure solvent measured at the same temperature, here 20 °C.
[0076] The synthesis of said copolymer is described in unpublished European patent application
93200653.9 filed March 3, 1993.
[0077] TEGO-GLIDE 410 (trademark) is a lubricant of the polysiloxane/polyether type and
commercially available from TEGO-CHEMIE.
COMPARATIVE EXAMPLE 2
- Thermosensitive recording material A1
[0078] A subbed polyethylene terephthalate support having a thickness of 100 µm was doctor
blade-coated from a coating composition containing methyl ethyl ketone as a solvent
and the following ingredients so as to obtain thereon after drying a layer containing
per m
2 :
silver behenate |
7.46 g/m2 |
polyvinyl butyral (BUTVAR B79-tradename) |
3.73 g/m2 |
3,4-dihydro-2,4-dioxo-1,3,2H-benzoxazine |
0.85 g/m2 |
hydroquinone |
0.84 g/m 2 |
INVENTION EXAMPLE 1
- Thermosensitive recording material B1
[0079] A subbed polyethylene terephthalate support having a thickness of 100 µm was doctor
blade-coated from a coating composition containing methyl ethyl ketone as a solvent
and the following ingredients so as to obtain thereon after drying a first layer containing
per m
2 :
silver behenate |
7.46 g/m2 |
polyvinyl butyral (BUTVAR B79-tradename) |
3.73 g/m2 |
3,4-dihydro-2,4-dioxo-1,3,2H-benzoxazine |
0.85 g/m2 |
[0080] Onto said dried first layer a second layer was coated from aqueous medium so as to
obtain thereon after drying a hydrophilic polymeric binder layer containing :
hydroquinone |
0.84 g/m2 |
gelatin |
1.00 g/m2 |
- Thermographic printing
[0081] The heat-sensitive recording materials A1, B1 and that of comparative example 1 were
exposed to a pattern of linearly increasing amounts of heat in a thermal head printer
built for thermosensitometric purposes, using a separable polyethylene terephthalate
ribbon of 6 µm thickness between the thermal print head and the outermost non-support
layer of the heat-sensitive recording materials.
[0082] From the prints obtained in said materials A1, B1 and that of comparative example
1 characteristic sensitometric curves A1, B1 and Comp. 1 were plotted respectively
in Fig. 1 with optical density (D) (logarithmic values) in the ordinate and linearly
increasing amounts of heat (relative values) (rel. H) in the abscissa.
[0083] The optical density was measured in transmission with Macbeth TD 904 densitometer
behind ortho-filter having its main transmission in the green part (500 nm to 600
nm) of the visible spectrum.
[0084] From the obtained curves can be learned that the slope of the linear part (between
toe and shoulder) of the sensitometric curve in the cases of A1 and Comp. 1 of the
non-invention materials A1 and comparative example 1 are particularly steep. The obtained
density is high in both cases (more than 3).
[0085] The slope of the linear part (between toe and shoulder) of sensitometric curve B1
corresponding with the invention material B1 is considerably less steep than the slope
of curve A1, and proves a soft gradation characteristic allowing the reproduction
of at least 40 increasing grey values, whereas the non-invention material A1 may yield
only 20 recognizable steps.
[0086] From the obtained sensitometric results it may be concluded that by coating from
an aqueous medium a proper combination of hydrophilic binding agent (gelatin) and
reducing agent in a hydrophilic binder layer adjacent to the imaging layer a soft
gradation together with a high optical density can be obtained, whereas by coating
from a non-aqueous medium a hydrophobic binding agent (PC3) and the same reducing
agent in a hydrophobic layer adjacent to the imaging layer an undesirably hard gradation
is obtained.
COMPARATIVE EXAMPLE 3
- Thermosensitive recording material A2
[0087] A subbed polyethylene terephthalate support having a thickness of 100 µm was doctor
blade-coated from a coating composition containing methyl ethyl ketone as a solvent
and the following ingredients so as to obtain thereon after drying a layer containing
per m
2 :
silver behenate |
7.46 g/m2 |
polyvinyl butyral (BUTVAR B79-tradename) |
3.73 g/m2 |
3,4-dihydro-2,4-dioxo-1,3,2H-benzoxazine |
0.85 g/m2 |
catechol |
0.84 g/m2 |
INVENTION EXAMPLE 2
- Thermosensitive recording material B2
[0088] A subbed polyethylene terephthalate support having a thickness of 100 µm was doctor
blade-coated from a coating composition containing methyl ethyl ketone as a solvent
and the following ingredients so as to obtain thereon after drying a first layer containing
per m
2 :
silver behenate |
7.46 g/m2 |
polyvinyl butyral (BUTVAR B79-tradename) |
3.73 g/m2 |
3,4-dihydro-2,4-dioxo-1,3,2H-benzoxazine |
0.85 g/m2 |
[0089] Onto said dried first layer a second layer was coated from aqueous medium so as to
obtain thereon after drying a hydrophilic polymeric binder layer containing :
catechol |
0.84 g/m2 |
gelatin |
2.10 g/m2 |
INVENTION EXAMPLE 3
- Thermosensitive recording material C2
[0090] The preparation of recording material C2 was the same as of recording material B2
with the difference however, that the coverage of catechol in the second layer was
1.26 g/m
2 and the coverage of gelatin 4.20 g/m
2.
INVENTION EXAMPLE 4
- Thermosensitive recording material D2
[0091] The preparation of recording material D2 was the same as of recording material C2
with the difference however, that the coverage of gelatin in the second layer was
doubled, viz. 8.40 g/m
2.
- Thermographic printing
[0092] The heat-sensitive recording materials A2, B2, C2 and D2 were exposed to a pattern
of linearly increasing amounts of heat in a thermal head printer built for thermosensitometric
purposes, using a separable polyethylene terephthalate ribbon of 6 µm thickness between
the thermal print head and the outermost non-support layer of the heat-sensitive recording
materials.
[0093] From the prints obtained in said materials A2, B2, C2 and D2 characteristic sensitometric
curves A2, B2, C2 and D2 were plotted respectively in Fig. 2 with optical density
(D) (logarithmic values) in the ordinate and linearly increasing amounts of heat (relative
values) (rel. H) in the abscissa.
[0094] The optical density was measured in transmission with Macbeth TD 904 densitometer
behind ortho-filter having its main transmission in the green part (500 nm to 600
nm) of the visible spectrum.
[0095] From the obtained curves can be learned that the slope of the linear part (between
toe and shoulder) of the sensitometric curve A2 of the non-invention material A2 is
particularly steep. The density obtained is high (more than 3).
[0096] The slope of the linear part (between toe and shoulder) of sensitometric curves B2,
C2 and D2 corresponding with the invention materials B2, C2 and D2 respectively is
much less steep than the slope of curve A2. From the obtained sensitometric results
it may be concluded that by coating from an aqueous medium a proper combination of
hydrophilic binding agent (gelatin) and reducing agent in a hydrophilic binder layer
adjacent to the imaging layer a soft gradation together with a high optical density
can be obtained.
COMPARATIVE EXAMPLE 4
- Thermosensitive recording material A3
[0097] A subbed polyethylene terephthalate support having a thickness of 100 µm was doctor
blade-coated from a coating composition containing methyl ethyl ketone as a solvent
and the following ingredients so as to obtain thereon after drying a layer containing
per m
2 :
silver behenate |
7.46 g/m2 |
polyvinyl butyral (BUTVAR B79-tradename) |
3.73 g/m2 |
3,4-dihydro-2,4-dioxo-1,3,2H-benzoxazine |
0.85 g/m2 |
catechol |
0.84 g/m2 |
INVENTION EXAMPLE 5
- Thermosensitive recording material B3
[0098] A subbed polyethylene terephthalate support having a thickness of 100 µm was doctor
blade-coated from a coating composition containing methyl ethyl ketone as a solvent
and the following ingredients so as to obtain thereon after drying a first layer containing
per m
2 :
silver behenate |
7.46 g/m2 |
polyvinyl butyral (BUTVAR B79-tradename) |
3.73 g/m2 |
3,4-dihydro-2,4-dioxo-1,3,2H-benzoxazine |
0.85 g/m2 |
[0099] Onto said dried first layer a second layer was coated from aqueous medium so as to
obtain thereon after drying a hydrophilic polymeric binder layer containing :
hydroquinone |
0.84 g/m2 |
gelatin |
2.10 g/m2 |
INVENTION EXAMPLE 6
- Thermosensitive recording material C3
[0100] A subbed polyethylene terephthalate support having a thickness of 100 µm was doctor
blade-coated from a coating composition containing methyl ethyl ketone as a solvent
and the following ingredients so as to obtain thereon after drying a first layer containing
per m
2 :
silver behenate |
7.46 g/m2 |
polyvinyl butyral (BUTVAR B79-tradename) |
3.73 g/m2 |
3,4-dihydro-2,4-dioxo-1,3,2H-benzoxazine |
0.85 g/m2 |
[0101] Onto said dried first layer a second layer serving as spacer layer was coated from
methyl ethyl ketone so as to obtain thereon after drying a layer containing :
copolycarbonate PC3 as identified above |
5.00 g/m2 |
[0102] Onto said dried spacer layer a hydrophilic polymeric binder/reductor composition
was coated from aqueous medium so as to obtain thereon after drying a hydrophilic
polymeric binder layer containing :
catechol |
0.84 g/m2 |
gelatin |
2.10 g/m2 |
- Thermographic printing
[0103] The heat-sensitive recording materials A3, B3 and C3 were exposed to a pattern of
linearly increasing amounts of heat in a thermal head printer built for thermosensitometric
purposes, using a separable polyethylene terephthalate ribbon of 6 µm thickness between
the thermal print head and the outermost non-support layer of the heat-sensitive recording
materials.
[0104] From the prints obtained in said materials A3, B3 and C3 characteristic sensitometric
curves A3, B3 and C3 were plotted respectively in Fig. 3 with optical density (D)
(logarithmic values) in the ordinate and linearly increasing amounts of heat (relative
values) (rel. H) in the abscissa.
[0105] The optical density was measured in transmission with Macbeth TD 904 densitometer
behind ortho-filter having its main transmission in the green part (500 nm to 600
nm) of the visible spectrum.
[0106] From the obtained curves can be learned that the slope of the linear part (between
toe and shoulder) of the sensitometric curve A3 of the non-invention material A3 is
particularly steep. The density obtained is high (more than 3).
[0107] The slope of the linear part (between toe and shoulder) of sensitometric curves B3
and C3 corresponding with the invention materials B3 and C3 respectively is considerably
less steep than the slope of curve A3, and proves a soft gradation characteristic
suited for continuous tone reproduction.
[0108] The C3 invention recording material allows the reproduction of at least 48 increasing
grey values, whereas the non-invention material A3 may yield only 20 recognizable
steps.
INVENTION EXAMPLE 7
- Thermosensitive recording material A4
[0109] A subbed polyethylene terephthalate support having a thickness of 100 µm was doctor
blade-coated from a coating composition containing methyl ethyl ketone as a solvent
and the following ingredients so as to obtain thereon after drying a layer containing
per m
2 :
silver behenate |
6.00 g/m2 |
polyvinyl butyral (BUTVAR B79-tradename) |
3.00 g/m2 |
3,4-dihydro-2,4-dioxo-1,3,2H-benzoxazine |
0.22 g/m2 |
behenic acid |
0.60 g/m2 |
BAYSILONE Öl A (tradename) |
17 mg/m2 |
[0110] Onto said dried first layer a second layer was coated from acetone so as to obtain
thereon after drying a spacer layer containing :
cellulose diacetate |
7.00 g/m2 |
[0111] Onto said dried second layer a third layer was coated from water so as to obtain
thereon after drying a layer containing :
catechol |
1.26 g/m2 |
gelatin |
4.20 g/m2 |
- Thermographic printing
[0112] The heat-sensitive recording material A4 was exposed to a pattern of linearly increasing
amounts of heat in a thermal head printer built for thermosensitometric purposes,
using a separable polyethylene terephthalate ribbon of 6 µm thickness between the
thermal print head and the outermost non-support layer of the heat-sensitive recording
material.
[0113] The optical density was measured in transmission with Macbeth TD 904 densitometer
behind ortho-filter having its main transmission in the green part (500 nm to 600
nm) of the visible spectrum. The measured optical density was more than 3 in the stronger
heat-exposed area. In the non-exposed area the optical density was 0.06.
[0114] After said measurement the two upper layers (second and third layer) of the recording
material A4 were removed by stripping off using an adhesive tape. Hereupon the remaining
material including the imaging layer (first layer) was overall heated at a temperature
of 118 °C for 5 seconds.
[0115] The measurement of optical density was repeated and the recording material after
said stripping off and overall heating possessed an optical density of 0.04 in the
previously non-heated areas whereas the optical density remained practically the same
in the previously image-wise heated areas.
1. Ein zur Verwendung bei direkter thermischer Bilderzeugung geeignetes wärmeempfindliches
Aufzeichnungsmaterial, das folgendes enthält :
(i) eine Schicht (1), die gleichmäßig verteilt in einem filmbildenden wasserunlöslichen
Harzbindemittel ein wesentlich lichtunempfindliches organisches Metallsalz enthält,
und
(ii) eine Schicht (2) in direktem Kontakt mit Schicht (1) oder in thermisch wirksamer
Beziehung zu Schicht (1) über eine Abstandshalterschicht (3), dadurch gekennzeichnet,
daß Schicht (2) gleichmäßig verteilt in einem filmbildenden wasserlöslichen hydrophilen
Bindemittel wenigstens ein organisches Reduktionsmittel enthält, das bei Erhitzung
des Aufzeichnungsmaterials in der Lage ist, aus Schicht (2) in Schicht (1) überzudiffundieren.
2. Wärmeempfindliches Aufzeichnungsmaterial nach Anspruch 1, dadurch gekennzeichnet,
daß das organische Metallsalz ein Silbersalz ist.
3. Wärmeempfindliches Aufzeichnungsmaterial nach Anspruch 1 oder 2, dadurch gekennzeichnet,
daß das hydrophile wasserlösliche Bindemittel ein proteinartiges Bindemittel, Polyvinylalkohol,
Poly(acrylsäure) oder Polyvinylpyrrolidon ist.
4. Wärmeempfindliches Aufzeichnungsmaterial nach Anspruch 3, dadurch gekennzeichnet,
daß das hydrophile wasserlösliche Bindemittel Gelatine ist.
5. Wärmeempfindliches Aufzeichnungsmaterial nach irgendeinem der Ansprüche 1 bis 4, dadurch
gekennzeichnet, daß Schicht (2) das (die) Reduktionsmittel in einem Bindemittel/ Reduktionsmittel-Gewichtsverhältnis
zwischen 1/1 und 10/1 enthält.
6. Wärmeempfindliches Aufzeichnungsmaterial nach irgendeinem der Ansprüche 1 bis 5, dadurch
gekennzeichnet, daß Schicht (2) ein Polyhydroxybenzol-Reduktionsmittel enthält.
7. Wärmeempfindliches Aufzeichnungsmaterial nach Anspruch 6, dadurch gekennzeichnet,
daß das Polyhydroxybenzol-Reduktionsmittel Pyrocatechin oder Hydrochinon ist.
8. Wärmeempfindliches Aufzeichnungsmaterial nach irgendeinem der vorstehenden Ansprüche,
dadurch gekennzeichnet, daß entweder Schicht (2) oder Schicht (1) eine Außenschicht
ist.
9. Wärmeempfindliches Aufzeichnungsmaterial nach irgendeinem der Ansprüche 1 bis 7, dadurch
gekennzeichnet, daß Schicht (1) oder Schicht (2) mit einer Außenschutzschicht überzogen
ist, die dank der Anwesenheit einer Gleitsubstanz Antiklebeeigenschaften aufweist.
10. Wärmeempfindliches Aufzeichnungsmaterial nach irgendeinem der vorstehenden Ansprüche,
dadurch gekennzeichnet, daß das Aufzeichnungsmaterial einen lichtundurchlässigen oder
lichtdurchlässigen Träger enthält.
11. Wärmeempfindliches Aufzeichnungsmaterial nach irgendeinem der vorstehenden Ansprüche,
dadurch gekennzeichnet, daß Schicht (2) auf einen lichtdurchlässigen Träger vergossen
wird, mit Schicht (2) überzogen ist und Laserlicht absorbierende, absorbiertes Laserlicht
in Wärme umwandelnde Substanzen enthält.
12. Ein Verfahren zur Erzeugung des in irgendwelchem der obigen Ansprüche definierten
wärmeempfindlichen Aufzeichnungsmaterials, dadurch gekennzeichnet, daß Schicht (1)
aus einem nicht-wäßrigen Medium und Schicht (2) aus einem wäßrigen Medium aufgetragen
wird.
13. Ein Aufzeichnungsverfahren, bei dem ein in irgendwelchem der obigen Ansprüche 1 bis
11 definiertes Aufzeichnungsmaterial bildmäßig erhitzt wird.
14. Aufzeichnungsverfahren nach Anspruch 13, dadurch gekennzeichnet, daß das wärmeempfindliche
Aufzeichnungsmaterial mittels eines mit einer Vielzahl von bildmäßig elektrisch erregten
Heizelementen versehenen Thermodruckkopfes bildmäßig erhitzt wird.
15. Aufzeichnungsverfahren nach irgendeinem der Ansprüche 13 und 14, dadurch gekennzeichnet,
daß Schicht (2) nach der bildmäßigen Erhitzung entfernt wird.
16. Aufzeichnungsverfahren nach Anspruch 15, dadurch gekennzeichnet, daß Schicht (2) durch
Abwaschen mit einer wäßrigen Flüssigkeit oder durch Delaminierung entfernt wird.