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 dye
is 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] 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.
[0009] 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 the 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.
[0010] In the past several recording materials were developed for direct thermal imaging,
e.g. heat-sensitive copying papers including a recording layer having a substantially
light-insensitive organic silver salt and organic reducing agent in a thermoplastic
binder such as polyvinyl acetate and polyvinyl butyral (ref. Re 30,107 being reissue
of US-P 3,996,397). Such recording materials are less suited for use in thermographic
recording operating with thermal printheads since these recording layers may stick
thereto. Moreover, organic ingredients of the thermosensitive recording layer may
exude by heat and may soil the thermal printhead at an operating temperature in the
range of 300 to 400 °C which are temperatures common in using thermal printheads (ref.the
above mentioned book "Handbook of Imaging Materials", p. 502). The undesirable transfer
of said ingredients may be promoted by the pressure contact of the thermal printhead
with the recording material. Pressures may be 200-500 g/cm² to ensure a good transfer
of heating. The heating time per picture element (pixel) may be less than 1.0 ms.
3. Objects and Summary of the Invention
[0011] It is an object of the present invention to provide a thermosensitive recording material
suited for use in direct thermal imaging, wherein the thermosensitive imaging layer
of said material is effectively protected without substantial loss in imaging properties
such as sensitivity and image resolution.
[0012] It is a particular object of the present invention to provide a thermosensitive recording
material suited for use in direct thermal imaging, wherein the thermosensitive imaging
layer of said material is coated with a protective layer that when contacted with
an imagewise energized heating element does not stick thereto and prevents soiling
of the heating element.
[0013] Other objects and advantages of the present invention will appear from the further
description.
[0014] According to the present invention a thermosensitive recording material suited for
use in direct thermal imaging by means of an information-wise energized heating element
is provided, which recording material comprises on the same side of a support, called
the heat-sensitive side, one or more layers containing in thermal working relationship
one or more substances yielding an optical density change by heat, characterized in
that one of said layers is coated with a protective transparent resin layer essentially
consisting of a cellulose nitrate having a substitution degree (DS) in the range of
2.2 to 2.32 which corresponds with a nitrogen content of 11.8 to 12.2 % by weight.
[0015] The present invention includes also the use of said recording material in a direct
thermal imaging method wherein said method comprises the step of heating the thermosensitive
recording layer of said thermosensitive recording material via said protective layer.
[0016] By "thermal working relationship" is meant here that said substances may be present
in a same layer or different layers wherefrom by heat they can come into reactive
contact with each other, e.g. by diffusion or mixing in the melt.
[0017] The layer in which the optical density change takes place is called the recording
layer.
4. Detailed Description of the Invention
[0018] The cellulose nitrate for forming a protective layer according to the present invention
may be applied from methanol, ethanol, ethyl, butyl and amyl acetates, acetone and
methyl ethyl ketone or mixtures thereof. The cellulose nitrate coating is preferably
applied from a solution in methanol which is a non-solvent for polyvinyl butyral being
a preferred binder for organic reducible silver salts.
[0019] The structure of a cellulose nitrate suitable for use as protective surface coating
is the following :
The value of n is related to the viscosity in a given solvent which for a cellulose
nitrate preferred for use according to the present invention is at least 50 mPa.s
when measured at 20 °C in methanol at a concentration of 7 g/100 ml.
[0020] Cellulose nitrate having a value of n in the range of 500 to 600 is used advantageously
for the purpose of the present invention and has been described for use as automotive
lacquer [ref. The Chemistry of Organic Film Formers by D. H. Solomon - John Wiley
& Sons, Inc. New York (1967), p. 151].
[0021] Cellulose nitrate may be mixed with polymer(s) providing a mixture having a glass
transition temperature (Tg) above that of the applied cellulose nitrate, e.g. is mixed
with poly(methacrylates) which are compatible therewith.
[0022] The cellulose nitrate protective layer used according to the present invention may
contain additives provided such materials do not inhibit its anti-sticking properties
and provided that such materials do not scratch, erode, contaminate, or otherwise
damage the thermal printing head or harm image quality.
[0023] Examples of suitable additives are e.g. plasticizers to improve flexibility. Preferred
for that purpose are the simple or polymeric esters of aliphatic or aromatic acids,
e.g. of sebacates and adipates. Drying-oil alkyd resins impart greater film strength
and resistance to embrittlement. Antioxidants must be added to the drying oil alkyds
to prevent cross-linking and partial solubility in the solvents for coating the cellulose
nitrate.
[0024] The protective layer of the direct thermal recording material according to the present
invention may in addition to the inorganic silicate particles comprise or is coated
with minor amounts of such other agents like liquid lubricants.
[0025] Examples of suitable lubricating materials are surface active agents with or without
a polymeric binder. A surface active agent is an amphiphilic molecule containing an
apolar group in conjunction with (a) polar group(s) such as carboxylate, sulfonate,
phosphates, aliphatic amine salt, aliphatic quaternary ammonium salt groups, polyoxyethylene
alkyl ethers, polyethylene glycol fatty acid esters, and fluoroalkyl C₂-C₂₀ aliphatic
acids. Examples of liquid lubricants include silicone oils, synthetic oils, saturated
hydrocarbons and glycols.
[0026] Siloxane compounds may be applied as a topcoat on the protective layer, preferably
are coated in the form of a solution in a non-solvent for the cellulose nitrate e.g.
from isopropanol or a C₆-C₁₁ alkane.
[0027] The protective layer has preferably a thickness of about 0.1 to 3 µm, more preferably
of 0.3 to 1.5 µm, and may be coated on the thermosensitive recording layer by a coating
technique known as a gravure printing.
[0028] Optionally the protective layer according to the present invention is coated with
an outermost slipping layer (i.e. anti-sticking layer) compositions of which are described
in e.g. EP 138483, EP 227090, US-P 4,567,113, 4,572,860 and 4,717,711 and in published
European patent application 311841.
[0029] In an example a suitable slipping layer comprises as binder a styrene-acrylonitrile
copolymer or a styrene-acrylonitrile-butadiene copolymer or a mixture hereof and as
lubricant in an amount of 0.1 to 10 % by weight of the binder(s) a polysiloxane-polyether
copolymer or polytetrafluoroethylene or a mixture hereof.
[0030] Another suitable 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.
[0031] Other suitable slipping coatings are described e.g. in published European patent
applications (EP-A) 0 501 072 and 0 492 411.
[0032] A slipping layer may have a thickness of about 0.2 to 5.0 µm, preferably in the range
of 0.4 to 2.0 µm.
[0033] The thermographic recording material for direct thermal recording having a recording
layer protected with said cellulose nitrate-containing layer as described herein may
be of any type known in the art.
[0034] For obtaining optical densities above 2 preferably recording materials comprising
a thermosensitive recording layer containing substantially light-insensitive organic
silver salts in admixture with a reducing agent therefor in a water-insoluble resin
binder are used.
[0035] The reducing agent present may be of the type used in known thermographic recording
materials for producing a silver image by thermally initiated reduction of substantially
light-insensitive silver salts such as silver behenate. Examples of such reducing
agents are described in US-P 3,887,378 and prior art mentioned therein and also in
Re. 30,107 being reissue of US patent 3,996,397.
[0036] Sterically hindered phenols or bis-phenols (ref. US-P 3,547,648) may be used as auxiliary
reducing agents that become on heating reactive partners in the reduction of a light-insensitive
silver salt such as silver behenate.
[0037] Substantially light-insensitive organic silver salts particularly suited for use
in direct thermal recording materials according to the present invention are silver
salts of aliphatic carboxylic acids known as fatty acids, wherein the aliphatic carbon
chain has at least 12 C-atoms, e.g. silver palmitate, silver stearate and silver behenate,
but modified aliphatic carboxylic acids with thioether group as described e.g. in
GB-P 1,111,492 may be used likewise to produce a thermally developable silver image.
[0038] The silver image density depends on the coverage of the 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 3 can be obtained. Preferably at least 0.10 mole of reducing agent(s)
per mole of organic silver salt is used. In particular combinations the fatty acid
silver salts are present in combination with the free fatty acids.
[0039] The ratio by weight of the resin binder to organic silver salt is e.g. in the range
of 0.2 to 6, and the thickness of the recording layer is preferably in the range of
3 to 30 µm, more preferably in the range of 8 to 16 µm.
[0040] According to a particular embodiment the thermosensitive recording material contains
in one layer a substantially light-insensitive silver salt and in another layer in
thermal working relationship with said silver salt one or more reducing agents therefor.
[0041] A heat-sensitive recording material containing silver behenate and 4-methoxy-1-naphthol
as reducing agent in adjacent binder layers is described in Example 1 of US-P 3,094,417.
[0042] In order to obtain a neutral black image tone in the higher densities and neutral
grey in the lower densities the recording layer contains in admixture with said organic
silver salt and reducing agent(s) a so-called toning agent known from thermography
or photo-thermography.
[0043] Suitable toning agents are the phthalimides and phthalazinones within the scope of
the general formulae described in the already mentioned Re. 30,107. Further reference
is made to the toning agents described in US-P 3,074,809, 3,446,648 and 3,844,797.
Other useful toning agents are benzoxazine dione compounds, e.g. 3,4-dihydro-2,4-dioxo-1,3,2H-benzoxazine
described in US-P 3,951,660.
[0044] In addition to said ingredients the recording layer may contain 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, ultraviolet light absorbing compounds, white
light reflecting and/or ultraviolet radiation reflecting pigments, colloidal silica,
and/or optical brightening agents.
[0045] As binding agent for said ingredients preferably thermoplastic resins are used wherein
the ingredients can be dispersed homogeneously or form therewith a solid-state solution.
For that purpose all kinds of natural, modified natural or synthetic resins may be
used, e.g. cellulose derivatives such as ethylcellulose, cellulose esters, carboxymethylcellulose,
starch ethers, galactomannan, polymers derived from α,β-ethylenically unsaturated
compounds such as polyvinyl chloride, copolymers of vinyl chloride and vinyl acetate,
polyvinyl acetate and partially hydrolyzed polyvinyl acetate, polyvinyl alcohol, polyvinyl
acetals, e.g. polyvinyl butyral, copolymers of acrylonitrile and acrylamide, polyacrylic
acid esters, polymethacrylic acid esters and polyethylene. A particularly suitable
ecologically interesting (halogen-free) binder is polyvinyl butyral.
[0046] The above mentioned polymers or mixtures thereof forming the binder in the thermographic
recording layer may be used in conjunction with waxes or "heat solvents" also called
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 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 tetrahydro-thiophene-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 support for the heat-sensitive recording material 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 may be subbed to improve its adherence to the thereon coated thermo-sensitive
recording layer.
[0049] The coating of the recording layer composition may proceed by any coating technique
known in the art using a solvent or solvent mixture for the coating ingredients. Common
coating techniques are described e.g. 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.
[0050] Suitable coating techniques are screen-printing, gravure, forward and reverse roll
coating. Screen-printing, spray coating and gravure coating are used as a precision
method for applying very thin coatings with more accuracy than can be achieved with
other techniques.
[0051] The direct thermal recording material according to the present invention is particularly
suited for use in electrically energized thermal printhead recording.
[0052] During recording the thermal printhead makes contact with the protective coating
of the direct thermal recording material. The thermal printhead contains tiny selectively
electrically energized resistors that may not be soiled and have to be protected against
wear.
[0053] A survey of printhead requirements is given in the already mentioned Handbook of
Imaging Materials, Chapter 11, p. 510-514. Commercially available thermal printheads
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.
[0054] The information-wise heating may proceed likewise by means of a resistive ribbon
wherein an electrical current is injected through tiny printhead electrodes (styli)
into a resistive layer (surface resistivity in the range of 500 to 900 ohms/square)
coated at the side opposite said electrodes with a continuous electrode, e.g. in the
form of vacuum-deposited aluminium layer. A large ground plate electrode aside said
printhead electrodes and in contact with the resistive layer ensures that Joule heating
is minimized as the current flows to ground (see the already mentioned book "Progress
in Basic Principles of Imaging Systems -Proceedings of the International Congress
of Photographic Science Köln (Cologne)", (1986) Figure 6 on page 622 dealing with
an embodiment of resistive ribbon printing technology).
[0055] The fact that in using a resistive ribbon heat is generated directly in the resistive
ribbon and only the travelling ribbon gets hot (not the printheads) 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.
[0056] The composition and production of a polycarbonate ribbon for non-impact printing
(resistive ribbon) is described e.g. in US-P 4,103,066.
[0057] The image signals for modulating the electrical energy to be converted into thermal
energy in said thermal printhead or resistive ribbon is 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.
[0058] According to still other thermal recording embodiments the present recording material
is used in conjunction with an information-wise modulated laser beam or ultrasonic
pixel printer as described e.g. in US-P 4,908,631.
[0059] Direct thermal imaging can be used for both the production of transparencies and
reflection type prints having an opaque white light reflecting background.
[0060] In the hard copy field recording materials on white opaque base, e.g. paper base
are used. Said base and/or a layer between the recording layer may contain white light
reflecting pigments.
[0061] Black-and-white transparencies are widely used in the medical diagnostic field in
inspection techniques operating with a light box.
[0062] The following example illustrates the present invention without however limiting
it thereto. All mentioned percentages and ratios are by weight unless otherwise indicated.
EXAMPLE
[0063] A subbed polyethylene terephthalate support having a thickness of 100 µm was doctor
blade-coated so as to obtain thereon after drying the following recording layer including
:
silver behenate |
5.28 g/m² |
polyvinyl butyral |
5.34 g/m² |
behenic acid |
0.53 g/m² |
reducing agent S as defined hereinafter |
2.0 g/m² |
3,4-dihydro-2,4-dioxo-1,3,2H-benzoxazine |
0.39 g/m² |
Reducing agent S is a polyhydroxy spiro-bis-indane, viz. 3,3,3',3'-tetramethyl-5,6,5',6'-tetrahydroxy-spiro-bis-indane.
[0064] After drying said recording layer was dip-coated at a wet coating thickness of 75
g/m² with the following coating composition for forming an outermost protective layer.
methanol |
92.2 g |
cellulose nitrate (DS : 2.25) |
7 g |
TEGOGLIDE 410 (tradename) lubricant |
0.53 g |
A solution of 7 g of said cellulose nitrate in 100 ml of methanol has a viscosity
of 58 mPa.s at 20 °C.
[0065] The coated layer was dried for 10 minutes at 70 °C.
[0066] A first sample of the thus obtained recording material was used immediately after
said drying in direct thermal recording with thermal printer MITSUBISHI CP100 (tradename);
a second sample was used after 3 days conditioning at 57 °C and relative humidity
of 34 %, and a third sample was used after 7 days conditioning at 45 °C and relative
humidity of 70 %. During printing the printhead was kept in contact with the protective
coating.
[0067] The adherence of the protective layer to the thermosensitive layer proved to be excellent
for the three samples. Marks (stripes) resulting from sticking to the thermal printhead
material surface are practically nil, whereas without said protective layer stripes
are clearly visible and thus deteriorating the image.
[0068] The optical densities of the protected imaged and non-imaged areas were measured
in transmission with densitometer MACBETH TD 904 (tradename) provided with an ortho
filter (maximal transmission at about 500 nm). The measured minimum optical density
(D
min) was 0.05 and the maximum optical density (D
max) was for all three samples above 3.
1. A thermosensitive recording material suited for use in direct thermal imaging by means
of an information-wise energized heating element, which recording material comprises
on the same side of a support, called the heat-sensitive side, one or more layers
containing in thermal working relationship one or more substances yielding an optical
density change by heat, characterized in that one of said layers is coated with a
protective transparent layer essentially consisting of a cellulose nitrate having
a substitution degree (DS) in the range of 2.2 to 2.32.
2. Thermosensitive recording material according to claim 1, wherein said cellulose nitrate
when dissolved in methanol at a concentration of 7 g/100 ml yields at 20 °C a viscosity
of at least 50 mPa.s.
3. Thermosensitive recording material according to claim 1 or 2, wherein said protective
coating contains a polymer increasing the glass transition temperature above the glass
temperature of said cellulose nitrate.
4. Thermosensitive recording material according to any of claims 1 to 3, wherein said
protective coating contains a plasticizer.
5. Thermosensitive recording material according to any of claims 1 to 4, wherein said
protective coating contains a liquid lubricant.
6. Thermosensitive recording material according to any of claims 1 to 5, wherein said
material comprises a thermosensitive recording layer containing a substantially light-insensitive
organic silver salt in admixture with a reducing agent therefor in a water-insoluble
resin binder.
7. Thermosensitive recording material according to any of the claims 1 to 5, wherein
one of said layers contains a substantially light-insensitive organic silver salt
and another layer contains in thermal working relationship with said silver salt one
or more reducing agents therefor.
8. Thermosensitive recording material according to claim 6 or 7, wherein said silver
salt is a silver salt of an aliphatic carboxylic acid the aliphatic carbon chain of
which contains at least 12 C-atoms.
9. A direct thermal imaging method wherein in conjunction with an information-wise energized
heating element a thermosensitive recording material according to any of the claims
1 to 8 is used, characterized in that said method comprises the step of heating the
thermosensitive recording layer of said material via said protective layer contacting
said heating element.