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, California, 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 the
formation of a visible image takes place.
[0010] According to the last mentioned book (ref. p. 499-551) in the past several systems
were developed for direct thermal imaging of which the leuco dye system has found
commercial use. Optical density obtained with embodiments of said system is usually
not higher than 2 and requires mixtures of leuco dye compounds to produce black.
[0011] The 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 are less suited for use in
thermographic recording operating with thermal printheads since these recording layers
may stick to said printheads. Moreover, organic ingredients may leave the thermosensitive
recording layer on heating and 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. Possible contact-pressures may be 200-500 g/cm²
to ensure a good transfer of heat. The heating time per picture element (pixel) may
be less than 1.0 ms.
3. Objects and Summary of the Invention
[0012] 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.
[0013] 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 deform and does not stick thereto
and that prevents soiling of the heating element.
[0014] Other objects and advantages of the present invention will appear from the further
description.
[0015] 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 recording layers is coated with a protective transparent resin layer
essentially consisting of a polycarbonate or copolycarbonate derived from one or more
bisphenols, wherein at least 25 mole % of said bisphenols consists of a bis-(hydroxyphenyl)-cycloalkane
corresponding to following general formula (I):
wherein:
each of R¹, R² , R³, and R⁴ (same or different) represents hydrogen, halogen, a
C₁-C₈ alkyl group including a substituted C₁-C₈ alkyl group, a C₅-C₆ cycloalkyl group
including a substituted C₅-C₆ cycloalkyl group, a C₆-C₁₀ aryl group including a substituted
C₆-C₁₀ aryl group or a C₇-C₁₂ aralkyl group including a substituted C₇-C₁₂ aralkyl
group; and
X represents the atoms necessary to complete a 5- to 8-membered alicyclic ring,
which either carries at least one C₁-C₆ alkyl group or at least one 5- or 6-membered
cycloalkyl group, or carries a fused-on 5- or 6-membered cycloalkyl group.
[0016] The present invention includes also the use of said recording material in direct
thermal imaging.
[0017] 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. The layer in which
the optical density change takes place is called the recording layer.
4. Detailed Description of the Invention
[0018] Homopolycarbonates derived from bis-(hydroxyphenyl)-cycloalkanes corresponding to
general formula (I) can have a glass transition temperature of about 260°C, which
is substantially higher than the Tg of homopolycarbonates derived from bisphenol A
that have a Tg of only about 150°C.
[0019] By copolycondensation with other bisphenols than the bisphenols according to said
general formula (I) the Tg can be varied e.g. between 180 and 240 °C.
[0020] Protective layers containing (co)polycarbonates derived from bis-(hydroxyphenyl)-cycloalkanes
corresponding to general formula (I) show better heat-stability, e.g. less deformation,
than heat-resistant layers containing conventional polymeric thermoplasts.
[0021] The homopolycarbonates derived from bis-(hydroxyphenyl)-cycloalkanes corresponding
to said general formula (I) are soluble in ecologically acceptable solvents such as
ethyl methyl ketone and ethyl acetate which is not the case with polycarbonates derived
from bisphenol A.
[0022] For increasing the solubility in said solvents preferably one or two carbon atoms
of said alicyclic ring in said general formula (I) carry a C₁-C₆ alkyl group, e.g.
a methyl group.
[0023] Bisphenol compounds according to said general formula (I) are preferred wherein said
alicyclic ring is substituted with two C₁-C₆ alkyl groups in β-position to its diphenyl-substituted
carbon atom.
[0024] Bis-(hydroxyphenyl)-cycloalkanes corresponding to said general formula (I), which
are preferably used for preparing polycarbonates applied according to the present
invention correspond to one of the following structural formulae (II) to (IV) :
A particularly preferred bis-(hydroxyphenyl)-cycloalkane for use in the preparation
of a polycarbonate intended for coating a protective layer on a direct thermal recording
layer is 1,1-bis-(4-hydroxyphenyl)-3,3,5-trimethylcyclohexane [see formula (II)].
[0025] The bis-(hydroxyphenyl)-cycloalkanes corresponding to general formula (I) can be
prepared according to a known method by condensation of phenols corresponding to general
formula (V) and ketones corresponding to general formula (VI) :
wherein R¹, R², and X have the same significances as given to them in general formula
(I).
[0026] Examples of suitable phenols corresponding to general formula (V) are i.a. phenol,
o-cresol, m-cresol, 2,6-dimethylphenol, 2-chlorophenol, 3-chlorophenol, 2,6-dichlorophenol,
2-cyclohexylphenol, diphenylphenol, and o- or p-benzylphenol.
[0027] Most of these phenols are commercially available and can be prepared according to
known methods (see e.g. for the preparation of cresols and xylenols "Ullmanns Encyklopädie
der technischen Chemie" 4. neubearbeitete und erweiterte Auflage, Band 15, pages 61
to 77, Verlag Chemie-Weinheim-New York 1978. For the preparation of chlorophenols
see "Ullmanns Encyklopädie der technischen Chemie" 4. Auflage, Band 9, pages 573 to
582, Verlag Chemie 1975. For the preparation of alkylphenols see "Ullmanns Encyklopädie
der technischen Chemie" 4. Auflage, Band 18, pages 191 to 214, Verlag Chemie 1979).
[0028] Ketones corresponding to general formula (VI) are prepared e.g. according to methods
described in Beilsteins Handbuch der Organischen Chemie, 7. Band, 4. Auflage, Springer-Verlag,
Berlin, 1925 and corresponding Ergänzungsbände 1-4; Journal of American Chemical Society,
Vol. 79 (1957), pages 1488, 1490 and 1491; US 2,692,289; Journal of Chemical Society,
1954, pages 2186 and 2191; Journal of Organic Chemistry, Vol. 38, No. 26, 1973, page
4431; Journal of American Chemical Society, Vol. 87, 1965, page 1353 (especially page
1355). A general method for preparing ketones within said formula (VI) is given in
Organikum, 15. Auflage, 1977, VEB-Deutscher Verlag der Wissenschaften, Berlin, page
698.
[0029] Examples of suitable ketones corresponding to general formula (VI) are: 3,3-dimethylcyclopentanone,
2,2-dimethylcyclohexanone, 3,3-dimethylcyclohexanone, 4,4-dimethylcyclohexanone, 3-ethyl-3-methylcyclopentanone,
2,3,3-trimethylcyclopentanone, 2,4,4-trimethylcyclopentanone, 3,3,4-trimethylcyclopentanone,
3,3-dimethylcycloheptanone, 4,4-dimethylcycloheptanone, 3-ethyl-3-methylcyclohexanone,
4-ethyl-4-methylcyclohexanone, 2,3,3-trimethylcyclohexanone, 2,4,4-trimethylcyclohexanone,
3,3,4-trimethylcyclohexanone, 2,5,5-trimethylcyclohexanone, 3,3,5-trimethylcyclohexanone,
3,4,4-trimethylcyclohexanone, 2,3,3,4-tetramethylcyclopentanone, 2,3,4,4-tetramethylcyclopentanone,
3,3,4,4-tetramethylcyclopentanone, 2,2,5-trimethylcycloheptanone, 2,2,6-trimethylcycloheptanone,
2,6,6-trimethylcycloheptanone, 3,3,5-trimethylcycloheptanone, 3,5,5-trimethylcycloheptanone,
5-ethyl-2,5-dimethylcycloheptanone, 2,3,3,5-tetramethylcycloheptanone, 2,3,5,5-tetramethylcycloheptanone,
3,3,5,5-tetramethylcycloheptanone, 4-ethyl-2,3,4-trimethylcyclopentanone, 2-isopropyl-4,4-dimethylcyclopentanone,
4-isopropyl-2,4-dimethylcyclopentanone, 2-ethyl-3,5,5-trimethylcyclohexanone, 3-ethyl-3,5,5-trimethylcyclohexanone,
3-ethyl-4-isopropyl-3-methyl-cyclopentanone, 4-s-butyl-3,3-dimethylcyclopentanone,
2-isopropyl-3,3,4-trimethylcyclopentanone, 3-ethyl-4-isopropyl-3-methyl-cyclohexanone,
4-ethyl-3-isopropyl-4-methylcyclohexanone, 3-s-butyl-4,4-dimethylcyclohexanone, 3-isopropyl-3,5,5-trimethylcyclohexanone,
4-isopropyl-3,5,5-trimethylcyclohexanone, 3,3,5-trimethyl-5-propylcyclohexanone, 3,5,5-trimethyl-5-propyl-cyclohexanone,
2-butyl-3,3,4-trimethylcyclopentanone, 2-butyl-3,3,4-trimethylcyclohexanone, 4-butyl-3,3,5-trimethylcyclohexanone,
3-isohexyl-3-methylcyclohexanone, 5-ethyl-2,4-diisopropyl-5-methylcyclohexanone, 2,2-dimethylcyclooctanone,
and 3,3,8-trimethylcyclo-octanone.
[0030] Examples of preferred ketones are :
The synthesis of some bis-(hydroxyphenyl)-cycloalkanes corresponding said general
formula (I) is described in German patent 3 832 396.
[0031] In the preparation of high molecular weight, thermoplastic, aromatic copolycarbonates
for use according to the present invention the bis-(hydroxyphenyl)-cycloalkanes corresponding
to general formula (I) can also advantageously be used in combination with other bisphenol
compounds corresponding to the following general formula :
HO-Z-OH (VII)
wherein Z stands for a bivalent organic group in which the HO-groups of said general
formula are linked to the same aromatic nucleus or to different aromatic nuclei, in
the latter case said nuclei may be linked directly or through a bivalent radical or
group different from the group CX mentioned in general formula (I), e.g. Z represents
-O-, -S-, carbonyl, sulfoxyl, sulfonyl or a bivalent straight chain or branched cain
aliphatic group or bivalent further unsubstituted cycloaliphatic group.
[0032] Examples of compounds corresponding to said general formula (VII) are i.a. hydroquinone,
resorcinol, dihydroxydiphenyl, bis-(hydroxy-phenyl)-alkanes, bis-(hydroxyphenyl)-cyclohexane,
bis-(hydroxy-phenyl)-sulfide, bis-(hydroxyphenyl)-ether, bis-(hydroxyphenyl)-ketone,
bis-(hydroxyphenyl)-sulfone, bis-(hydroxyphenyl)-sulfoxide, α,α'-bis-(hydroxyphenyl)-diisopropylbenzene,
and such compounds carrying at least one alkyl and/or halogen substituent on the aromatic
nucleus.
[0033] These and other suitable compounds corresponding to general formula (VII) are described
in e.g. US 3,028,365, US 2,999,835, US 3,148,172, US 3,275,601, US 2,991,273, US 3,271,367,
US 3,062,781, US 2,970,131, US 2,999,846, DE 1,570,703, DE 2,063,050, DE 2,063,052,
DE 2,211,956, FR 1,561,518, and in "Chemistry and Physics of Polycarbonates", Interscience
Publishers, New York, 1964.
[0034] Preferred compounds corresponding to said general formula (VII) are i.a. 4,4'-dihydroxydiphenyl,
2,2-bis-(4-hydroxyphenyl)-propane, 2,4-bis-(4-hydroxyphenyl)-2-methylbutane, 1,1-bis-(4-hydroxyphenyl)-cyclohexane,
α,α'-bis-(4-hydroxyphenyl)-p-diisopropyl-benzene, 2,2-bis-(3-methyl-4-hydroxyphenyl)-propane,
2,2-bis-(3-chloro-4-hydroxyphenyl)-propane, bis-(3,5-dimethyl-4-hydroxyphenyl)-methane,
2,2-bis-(3,5-dimethyl-4-hydroxyphenyl)-propane, bis-(3,5-dimethyl-4-hydroxyphenyl)-sulfone,
2,4-bis-(3,5-dimethyl-4-hydroxy-phenyl)-2-methylbutane, 1,1-bis-(3,5-dimethyl-4-hydroxyphenyl)-cyclohexane,
α,α'-bis-(3,5-dimethyl-4-hydroxyphenyl)-p-diisopropylbenzene, 2,2-bis-(3,5-dichloro-4-hydroxyphenyl)-propane,
2,2-bis-(3,5-dibromo-4-hydroxyphenyl)-propane, 2,2-bis-(4-hydroxyphenyl)-propane,
2,2-bis-(3,5-dimethyl-4-hydroxyphenyl)-propane, 2,2-bis-(3,5-dichloro-4-hydroxy-phenyl)-propane,
2,2-bis-(3,5-dibromo-4-hydroxyphenyl)-propane, and 1,1-bis-(4-hydroxyphenyl)-cyclohexane.
Most preferred is 2,2-bis-(4-hydroxyphenyl)-propane (bisphenol A).
[0035] Incorporation of bisphenol A in the polycarbonate for use according to the present
invention reduces the brittleness of the polycarbonate but such at the expense of
a lower Tg. A lower brittleness makes the protective layer less scratchable. A compromise
may be found between scratchability and deformability by heat.
[0036] When in the preparation of the above mentioned copolycarbonates the bis-(hydroxyphenyl)-cycloalkanes
corresponding to general formula (I) are used together with at least one bisphenol
compound corresponding to general formula (VII); the amount of bis-(hydroxyphenyl)-cycloalkanes
corresponding to said general formula (I) is preferably at least 25 mole %, more preferably
at least 50 mole % with regard to the totality of bisphenols.
[0037] The bis-(hydroxyphenyl)-cycloalkane units and the units resulting from the compounds
corresponding to general formula (VII) can be present in the copolycarbonates in different
blocks or the different units can be distributed randomly.
[0038] In the preparation of (co)polycarbonates for use according to the present invention
a branching agent may be used to still further increase the Tg and mechanical resistance.
Small amounts, preferably from 0.05 to 2.0 mol % (in respect of the bis-(hydroxyphenyl)-cycloalkane)
of tri- or higher functional compounds, in particular compounds having three or more
phenolic groups, can be added to obtain branched (co)polycarbonates. Useful branching
agents having three or more phenolic groups are i.a. phloroglucinol, 4,6-dimethyl-2,4,6-tri-(4-hydroxyphenyl)-heptene-2,
4,6-dimethyl-2,4,6-tri-(4-hydroxyphenyl)-heptane, 1,3,5-tri-(4-hydroxyphenyl)-benzene,
1,1,1-tri-(4-hydroxyphenyl)-ethane, tri-(4-hydroxyphenyl)-phenylmethane, 2,2-bis-(4,4-bis-(4-hydroxyphenyl)-cyclohexyl)-propane,
2,4-bis-(4-hydroxyphenyl-isopropyl)-phenol, 2,6-bis-(2-hydroxy-5'-methyl-benzyl)-4-methylphenol,
2-(4-hydroxyphenyl)-2-(2,4-dihydroxyphenyl)-propane, orthoterephthalic acid hexa-(4-(4-hydroxyphenyl)-isopropyl)-phenyl)
ester, tetra-(4-hydroxyphenyl)-methane, tetra-(4-(4-hydroxyphenyl-isopropyl)-phenoxy)-methane,
and 1,4-bis-((4'-4''-dihydroxytriphenyl)-methyl)-benzene.
[0039] Examples of other trifunctional compounds are i.a. 2,4-dihydroxy-benzoic acid, trimesic
acid, cyanuric chloride, and 3,3-bis-(3-methyl-4-hydroxyphenyl)-2-oxo-2,3-dihydroindole.
[0040] For terminating the chain elongation and controlling the molecular weight of the
(co)polycarbonates use can be made of monofunctional compounds known in the art.
Suitable compounds for said purpose are e.g. phenol, t-butylphenol, and other C₁-C₇-alkyl-substituted
phenols. Particularly small amounts of phenols corresponding to the following general
formula (VIII) are useful in this respect :
wherein :
- R
- represents at least one substituent chosen from branched C₈-alkyl groups and branched
C₉-alkyl groups, and
- n
- is 1, 2, 3, 4, or 5; in case n is 2 to 5 the R groups may have a same or different
significance.
[0041] Preferably the contribution of CH₃-protons in the alkyl group(s) R is between 47
and 89 % and the contribution of CH- and CH₂-protons is between 53 and 11 %. Preferably,
the alkyl group(s) R is (are) situated in o- and/or p-position with respect to the
OH-group, and in particular the ortho substitution amounts to at the most 20 %. The
compounds used to terminate the chain elongation are in general used in concentrations
of 0.5 to 10, preferably 1.5 to 8 mol % with respect to the content of the bis-(hydroxyphenyl)-cycloalkanes
corresponding to general formula (I).
[0042] The (co)polycarbonates for use according to the present invention can be prepared
according to the interfacial polycondensation method as known in the art (see e.g.
H. Schnell, "Chemistry and Physics of Polycarbonates", Polymer Reviews, Vol. IX, page
33, Interscience Publ., 1964). According to this method the bisphenols are dissolved
in aqueous alkaline phase.
In order to control the molecular weight compounds terminating the chain elongation
may be added (e.g. compounds corresponding to the general formula VIII). The condensation
reaction takes place in the presence of an inert organic phase containing phosgene.
Preferably as inert organic phase a water-immiscible solvent is used which is a solvent
for the (co)polycarbonate formed.
The reaction temperature is preferably between 0°C and 40°C.
[0043] If branching agents are used, they can be added in an amount of 0.05 to 2 mol % to
the aqueous alkaline phase together with the bis-(hydroxyphenyl)-cycloalkanes and
other diphenols or they can be added to the organic phase before phosgenation takes
place.
[0044] In addition to the bis-(hydroxyphenyl)-cycloalkanes and other diphenols also their
mono- and/or bis-chlorocarbonate esters can be used, added in the form of a solution
in an organic solvent. The amount of chain-terminating agent and branching agent is
then levelled against the amount of bis-(hydroxyphenyl)-cycloalkane and other diphenol
structural units. When chlorocarbonate esters are used, the amount of phosgene can
be reduced as known in the art.
[0045] Suitable organic solvents for dissolving the chain-terminating agent, the branching
agent, and the chlorocarbonate ester are e.g. methylene chloride, chlorobenzene, acetone,
acetonitrile, and mixtures of these solvents, in particular mixtures of methylene
chloride and chlorobenzene. Optionally, the chain-terminating agent and the branching
agent are dissolved in the same solvent.
[0046] As organic phase for the interfacial condensation are used e.g. methylene chloride,
chlorobenzene and mixtures of methylene chloride and chlorobenzene.
[0047] As aqueous alkaline phase are used e.g. aqueous sodium hydroxide solutions.
[0048] The preparation of polycarbonates according to the interfacial polycondensation method
can be catalyzed as known in the art by adding catalysts such as tertiary amines,
in particular tertiary aliphatic amines such as tributylamine or triethylamine; the
catalysts are used in amounts of from 0.05 to 10 mol % in respect of the content of
bis-(hydroxyphenyl)-cycloalkanes and other diphenols. The catalysts can be added before
the start of the phosgenation, during the phosgenation, or after the phosgenation.
[0049] The isolation of the (co)polycarbonates is performed as known in the art.
[0050] The (co)polycarbonates for use according to the present invention can also be prepared
in homogeneous phase according to a known method (the so-called pyridine method) or
according to the known melt ester-interchange process by using e.g. diphenyl carbonate
instead of phosgene. In this case as well, the (co)polycarbonates are isolated according
to methods known in the art.
[0051] Preferably, the molecular weight of the (co)polycarbonates is at least 8000, preferably
from 8000 to 200,000 and more preferably from 10,000 to 80,000.
[0052] Examples of polycarbonates that can be used advantageously in accordance with the
present invention are i.a. :
- PC1
- Homopolycarbonate having the following structure :
wherein n has a value giving the polymer a viscosity ratio of 1.295 as determined
for a 0.5 %wt solution of the polymer in dichloromethane. 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.
- PC2
- Homopolycarbonate having the same structure as PC1 but giving a viscosity ratio of
2.2.
- PC3
- Copolycarbonate having the following structure :
wherein x=55 mol% and y=45 mol% ; giving a viscosity ratio of 1.295.
[0053] A mixture of two or more of different (co)polycarbonates may be used in the heat-resistant
layer.
[0054] The protective layer of the direct thermal recording material according to the present
invention may in addition to said (co)polycarbonates contain one or more of the thermoplastic
binders commonly used for heat-resistant layers such as e.g. poly(styrene-co-acrylonitrile),
poly(vinyl alcohol-co-butyral), poly(vinyl alcohol-co-acetal), poly(vinyl alcohol-co-benzal),
polystyrene, poly(vinyl acetate), cellulose nitrate, cellulose acetate propionate,
cellulose acetate hydrogen phthalate, cellulose acetate, cellulose acetate butyrate,
cellulose triacetate, ethyl cellulose, poly(methyl methacrylate), and copolymers of
methyl methacrylate. The addition of poly(styrene-co-acrylo-nitrile) is preferred.
[0055] The protective layer applied 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.
[0056] The protective layer of the direct thermal recording material according to the present
invention may comprise or is coated with minor amounts of such other agents like liquid
lubricants.
[0057] Examples of suitable lubricating materials are surface active agents with or without
a polymeric binder. The surface active agents may be any agents known in the art having
a hydrophobic molecule part 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.
[0058] Preferably, these silicone compounds for forming a topcoat are coated in the form
of a solution in a non-solvent for the polycarbonate of the heat-resistant layer e.g.
in isopropanol or a C₆-C₁₁ alkane.
[0059] The protective layer has preferably a thickness of about 0.1 to 15 µm, more preferably
of 0.5 to 5.0 µm and may be coated on the thermosensitive recording layer means of
a known coating technique for thin-layer coating.
[0060] 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.
[0061] 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.
[0062] 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.
[0063] Other suitable slipping coatings are described e.g. in published European patent
applications (EP-A) 0 501 072 and 0 492 411.
[0064] 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.
[0065] The thermographic recording material for direct thermal recording and having a recording
layer protected with said (co)polycarbonate containing layer as described herein may
be of any type known in the art.
[0066] For obtaining optical densities above 2 preferably recording materials on the basis
of substantially light-insensitive organic silver salts in admixture with a reducing
agent therefor in a water-insoluble resin binder are used.
[0067] 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.
[0068] 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.
[0069] 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.
[0070] 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.
[0071] 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.
[0072] According to a particular embodiment the thermosensitive recording material contains
in one layer a substantially light-insensitive organic silver salt and in another
layer in thermal working relationship with said silver salt one or more reducing agents
therefor.
[0073] 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.
[0074] 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.
[0075] 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.
[0076] 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.
[0077] 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.
[0078] 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.
[0079] 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.
[0080] 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.
[0081] 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.
[0082] 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.
[0083] The direct thermal recording material according to the present invention is particularly
suited for use in conjunction with an electrically energized thermal printhead.
[0084] 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.
[0085] 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.
[0086] 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).
[0087] 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.
[0088] The composition and production of a polycarbonate ribbon for non-impact printing
(resistive ribbon) is described e.g. in US-P 4,103,066.
[0089] 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.
[0090] 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.
[0091] Direct thermal imaging can be used for both the production of transparencies and
reflection type prints having an opaque white light reflecting background.
[0092] 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.
[0093] Black-and-white transparencies on colourless or blue base are widely used in the
medical diagnostic field in inspection techniques operating with a light box.
[0094] The following examples illustrate the present invention without however limiting
it thereto. All mentioned percentages and ratios are by weight unless otherwise indicated.
EXAMPLE 1
[0095] 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 |
4.47 g/m² |
polyvinyl butyral |
2.24 g/m² |
reducing agent S as defined hereinafter |
0.85 g/m² |
3,4-dihydro-2,4-dioxo-1,3,2H-benzoxazine |
0.32 g/m² |
silicone oil |
0.02 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.
[0096] After drying said recording layer was coated at 22 °C at a wet coating thickness
of 30 µm with the following coating composition for forming a protective layer.
methyl ethyl ketone |
90 g |
polycarbonate derived from 45 mole % of bisphenol (II) and 55 mole % of bisphenol
A |
10 g |
[0097] The thus coated layer was dried for 10 minutes in an air current at 50 °C whereby
a protective scratch-resistant layer was obtained.
[0098] Onto said protective layer the following coating composition for forming a slipping
layer was coated at 22 °C at a wet coating thickness of 30 µm.
TEGOGLIDE 410 (tradename) lubricant |
99.5 g |
isopropanol |
0.50 g |
[0099] Said layer was dried as described for the protective layer.
[0100] The obtained direct thermal imaging material was used in thermal printing with a
thermal printer MITSUBISHI CP100 (tradename). During printing the printhead was kept
in contact with the slipping layer and no signs of surface deformation were detected.
[0101] The optical densities of the 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 0.08 and the maximum optical density (D
max) was 3.3 .
EXAMPLE 2
[0102] Example 1 was repeated with the difference however that in the protective layer composition
the polycarbonate derived from 45 mole % of bisphenol (II) and 55 mole % of bisphenol
A was replaced by a polycarbonate derived from 100 mole % of bisphenol (II).
[0103] The thus obtained recording material was used in a thermal printer as described in
Example 1 and provided same good results.