FIELD OF THE INVENTION
[0001] The present invention concerns a thermal head printer and process for printing substantially
light-insensitive recording materials.
BACKGROUND OF THE INVENTION
[0002] Thermography is an image-forming process including a heating step and hence includes
photothermography in which the image-forming process includes image-wise exposure
and direct thermal processes in which the image-forming process includes an image-wise
heating step. In direct thermal printing a visible image pattern is produced by image-wise
heating of a recording material e.g. image signals can be converted into electric
pulses and then via a driver circuit selectively transferred to a thermal head, which
consists of microscopic heat resistor elements, thereby converting the electrical
energy into heat via the Joule effect. This heat brings about image formation in the
substantially light-insensitive thermographic material. In thermal heads, only those
regions which produce heat higher than a certain value are effective for printing,
and the regions capable of generating sufficient heat for the printing spread in proportion
to voltage applied to the heating resistors. If, therefore, higher voltage is applied
to the heating resistors, the size of the printing dots increases in proportion.
[0003] US 5,825,395 discloses a printing system, comprising: a thermal head, color thermal recording
paper having a surface with a plurality of different color developing layers disposed
thereon, said plurality of different color developing layers corresponding to a plurality
of different colors, and means for feeding said color thermal recording paper in a
feed direction at a feed pitch; said thermal head producing printed dots in a desired
color developing layer over said surface of said color thermal coloring paper by selectively
and directly heating said color thermal recording paper; said thermal head comprising
an array of a predetermined number of heating elements, each of said heating elements
having a length, in said feed direction of said color thermal recording paper, and
being controllable to radiate a selected level of thermal energy, said array being
operationally disposed with respect to said color thermal recording paper so that
said selected level of thermal energy radiated by said each heating element produces
one of said dots without damaging said color thermal recording paper; and said length
of each heating element having a value of from 2 to 3.5 said feed pitch. The feed
pitch is defined in
US 5,825,395 as the distance between adjacent image density peaks in the feed direction as can
be seen from Figure 4 and heating element lengths in the feed direction of m, 310µm,
360µ 360µm, 460µm and 560µm are disclosed therein, which according to the specification
are longer than those in the case of conventional thermal heads. No information is
provided with respect to distance between adjacent heating elements.
EP-A 0 500 334 discloses a thermal recording device for forming an image with a dot matrix by applying
a thermal head consisting of heat emitting elements arranged in a single row at a
first pitch along a primary scanning direction onto a surface of a thermal recording
material and moving said thermal recording material relative to said thermal head
in a secondary direction perpendicular to said primary scanning direction, and selectively
heating said heat emitting elements for each successive movement of said thermal recording
material at a second pitch in said scanning direction, wherein: a ratio of a length
of said heating elements of said thermal head in said primary scanning direction to
said first pitch is 30 to 70%, and a ratio of a length of each of said heat emitting
elements of said thermal head in said secondary scanning direction to said second
pitch is 60 to 95%. Embodiments 1, 4 and 7 and Examples 1 and 4 disclose aspect ratios,
a/b, where a and b are the lengths of each heat emitting element in the primary and
secondary directions, respectively, the primary direction corresponding to the lateral
direction of the paper or direction of the row of heat emitting elements as shown
in Figure 2, of 60/25 (= 2.4), 60/35 (= 1.71), 60/44 (= 1.36), 60/53 (= 1.13) and
85/44 (= 1.93) respectively. Figure 2 also shows that the pitch in the primary direction,
P
a, is the distance between the centre of one heat-emitting element and the centre of
the next heat-emitting element in the primary direction. Embodiments 1, 4 and 7 and
Examples 1 and 4 disclose b/ P
a ratios of 60/63.5(= 0.94), 60/63.5(= 0.94), 60/63.5(= 0.94), 60/63.5(= 0.94) and
85/63.5 (= 1.34) respectively. P
b is the dot pitch of the matrix in the secondary direction.
[0004] US 5,559,546 discloses a method of perforating a heat sensitive stencil, the method comprising
the steps of: bringing the heat sensitive stencil into contact with a thermal head
having a plurality of heating resistors arranged in a row; moving the heat sensitive
stencil in a sub-scanning direction which is orthogonal to a main-scanning direction
in which the plurality of heating resistors are arranged; and perforating the heat
sensitive stencil in a dot matrix shape with selectively heated heating resistors
of said plurality of heating resistors; the method further comprising the further
steps of: making the heat sensitive stencil of substantially only a thermoplastic
resin film; and making a width of a space between two adjacent heat resistors of said
plurality of heating resistors in the main scanning direction 30% or more of a pitch
between the two adjacent heating resistors in the main scanning direction so as to
regulate a width of a non-perforated portion of the heat sensitive stencil between
two adjacent perforations in the main scanning direction to be 20% or more of the
pitch between the two adjacent heating resistors in the main-scanning direction. The
ratio of b, the length of the heating resistor in the sub-scanning direction, to the
pitch Pa between two adjacent resistors in the main scanning direction in the Example
according to Embodiment 1 is 40/63.5 (= 0.63) and those of Comparative Examples A
and B are both 60/63.5 (= 0.94).
[0005] JP 58-089385A discloses the obtaining of a constant print density all the time even when thermal
heads are exchanged by a method in which the same resistor elements as heating elements
are provided to the thermal head having plural resistor heating elements, and on the
basis of the resistance values of the elements, applied voltage is controlled. A resistor
element 5 made of the same material as that of a heating resistor element 2 is provided
to part of a thermal head in which heating resistor elements 2-1-2-7 are provided
on a ceramic base plate 1, and also the resistor element 5 is connected to the terminals
Ra and Rb of a constant-voltage regulation circuit through a lead pattern. When connecting
a thermal head in which the resistance value of the heating element is lower than
that of standard heating element, the base voltage of a transistor Tr2 rises because
the resistance value of the resistor element 5 is also lower than standard, and therefore,
collector current is increased and the voltage drop of the resistor R1 is increased.
By this, the impedance between the collector bases of the transistor Tr1 is increased
and output voltage Vout becomes lower than standard. When the resistance value of
the heating element 2 is high, the output voltage also becomes higher.
[0006] US 4,841,120 discloses a thermal head for recording on a recording medium comprising: a substrate
having a thick portion and a thin portion having a first flat surface and a second
surface formed on an opposite side of said substrate from said first flat surface,
said second surface being adapted to contact said recording medium, a plurality of
heat resistor elements formed on said first flat surface of said thin portion of said
substrate, wiring circuit means for said heat resistor elements formed on said first
flat surface of said substrate and driving means for driving said heat resistor elements
formed by said first flat surface of said substrate, said thin portion being made
by grinding said second surface of said substrate. Figures 7 and 8 illustrate split
resistors.
[0007] JP 61-086271A discloses the elimination of the need to provide an interpolation line even at the
time of high-speed recording, by a construction wherein the first and the second heating
dot group are provided on the same substrate, and the two systems of the heating dot
group are selectively used in accordance with recording condition, i.e., high resolution
recording or high-speed recording. The first and third feeder lines 3, 19 and the
second and fourth feeder lines 4,20 are so provided as to clamp respectively both
side parts of a heating resistor 2 therebetween, and two heating dots 17, 18 differing
in length are provided on the same substrate. In high-resolution recording, the first
heating dot 17 is selectively operated for recording by using the first and second
feeder lines 3,4 whereas in high-speed recording, the second heating dot is selectively
operated for recording by using the third and fourth feeder lines 19,20.
[0008] US 5,485,193 discloses a line-type thermal head for half-tone printing which expresses various
densities by utilizing printing dots of various sizes, the thermal head having a main
scanning axis and comprising: a substrate, a plurality of heating elements arranged
on a substrate along the main scanning axis, each of the heating elements including
at least one non-rectangular parallelogramatic resistor for generating heat; and means
for supplying electric energy, an amount of which corresponds to a size of a printing
dot to be recorded, to each of the heating elements to make the resistor generate
heat, wherein the resistor has a region which generates sufficient heat for recording
the printing dot, and a size of the region is changed in response to the amount of
electric energy applied to the resistor so that printing dots having various sizes
are produced by each of the resistors, the supply means including lead electrodes
connected electrically to one pair of opposite sides of the resistor, each of the
lead electrodes having a width not less than a length of one side of the one pair
of opposite sides of the resistor; wherein a ratio of the length of one side of the
one pair of opposite sides of the resistor to that of one side of another pair of
opposite sides of the resistor is not greater than 1.5 and an acute angle formed by
two sides of the one and another pair of opposite sides of the resistor is no more
than 45°C.
[0009] US 5,483,274 discloses a thermal head for a thermal recording apparatus, comprising: means for
recording a continuous line of elliptic dots on a recording sheet, said means including
a plurality of heat generation resistors arranged one-dimensionally along a given
direction, each of the heat generation resistors formed to have a parallelogram shape
including four sides and two diagonal lines and configured such that the four sides
of the parallelogram shape have directions crossing the given direction and such that
two diagonal lines extending between opposing corners of the parallelogram have directions
crossing the given direction; and a plurality of drive electrodes respectively connected
to said heat generation resistors; wherein an ink film and the recording sheet for
thermal recording, which are stacked on one another, are brought into contact with
said heat generation resistors and moved in a direction orthogonal to the given direction
along which said heat generation resistors are arranged and, during this movement,
ink coated on the ink film is melted by said heat generation resistors to allow and
image to be transferred to the recording sheet.
[0010] US 4,970,530 discloses a thermal head arranged in a printing device so as to be opposite to a
printed object fed in a constant direction, the thermal head comprising: a base member,
a plurality of heating resistors arranged in said base member; electrodes disposed
in said base member corresponding to said heating resistors; and a slit formed on
a surface of said heating resistors and having a shape in which the width of the slit
in a feeding direction of said printed object is less than that in an arranging direction
of said heating resistors perpendicular to said feeding direction and the width of
the slit in the arranging direction of the heating resistors is approximately equal
to a half length of a pitch of said resistors in said arranging direction thereof.
[0011] Conventional thermal heads, such as used in the thermal head printers manufactured
by AGFA-GEVAERT N.V. e.g. DRYSTAR
™ 2000, DRYSTAR
™ 3000 and DRYSTAR
™ 4500, have a ratio of heating element length in the transport direction, L, to the
pitch, P, between adjacent heating elements, the so-called aspect ratio L/P, of between
1.5 and 1.80. The thermal head disclosed in
EP-A 1 006 000,
EP-A 1 006 403 and
EP-A 1 006 404 had heating elements with dimensions 85 µm x 85 µm i.e. an aspect ratio L/P of 1.0.
[0012] There is a need for using a given substantially light-insensitive thermographic material
developed for high throughput with a conventional thermal head for printing at lower
throughputs without a significant change in image tone and other imaging properties.
ASPECTS OF THE INVENTION
[0013] It is therefore an aspect of the present invention to provide a thermal head printer
for printing a substantially light-insensitive thermographic material with a particular
image tone with printing configurations with different printing speeds.
[0014] It is therefore a further aspect of the present invention to provide a process for
printing a substantially light-insensitive thermographic material at different printing
speeds with a thermal head comprising heating elements without significant variation
in image tone.
[0015] Further aspects and advantages of the invention will become apparent from the description
hereinafter.
SUMMARY OF THE INVENTION
[0016] It has been surprisingly found that variation in the length of the resistive elements
of a thermal head in the transport direction of a substantially light-insensitive
thermographic material can be used to realize a particular image tone as characterized
by CIELAB a* and b* values at different printing speeds with the same substantially
light-insensitive thermographic material. The L*, a* and b* CIELAB-values were determined
by spectrophotometric measurements according to ASTM Norm E179-90 in a R(45/0) geometry
with evaluation according to ASTM Norm E308-90.
[0017] The above-mentioned aspects are realised by a thermal head printer for printing a
substantially light-insensitive thermographic material, the thermal printer comprising:
a transport system having a transport direction, n thermal heads, where n is an integer,
each of the thermal heads comprising an array of substantially rectangular energizable
heating elements, the heating elements having a length L
n in the transport direction and a pitch P
n between adjacent heating elements, and a means for supplying electrical energy to
each of the substantially rectangular energizable heating elements in at least one
of the thermal heads, wherein the transport system being capable of transporting the
light-insensitive thermographic material in contact or proximity with at least one
of the thermal heads, wherein at least one of the thermal heads comprises heating
elements for which L
n/P
n is between 0.25 and 0.88, wherein at least one of the thermal heads comprises heating
elements with a width of the space between adjacent resistors along a line in the
plane of said heating elements which bisects all the heating elements of 20% or less
of P
n, wherein said printer is capable of image-wise printing said substantially light-insensitive
thermographic material, and wherein said substantially light-insensitive thermographic
material is not perforated by the printer.
[0018] The above-mentioned aspects are also realized by a first process for printing a substantially
light-insensitive thermographic material with the above-described thermal head printer
comprising the steps of: choosing a thermal head, providing the substantially light-insensitive
thermographic material, transporting the substantially light-insensitive thermographic
material past the thermal head, and image-wise heating of the substantially light-insensitive
thermographic material by supplying electrical energy to the heating elements.
[0019] Aspects of the present invention are also realized by a second process for printing
a substantially light-insensitive thermographic material at different printing speeds
with a different thermal head at each printing speed without significant variation
in image tone, wherein each of the different thermal heads comprises heating elements
with a different length in the transport direction of the substantially light-insensitive
thermographic material and the length of the heating elements in the transport direction
of the substantially light-insensitive thermographic material decreases with decreasing
printing speed and the different thermal heads are each on different substrates.
[0020] Preferred embodiments of the present invention are disclosed in the detailed description
of the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0021] The present invention will be described in greater detail in the following with reference
to the accompanying drawings, wherein:
Figure 1 is a schematic representation of an array of conventional substantially rectangular
heating elements with a length, L, in the transport direction, T, and a pitch, P.
Figure 2 is a schematic representation of an array of substantially rectangular split
resistor heating elements with a length, L, in the transport direction, T, and a pitch,
P.
Definitions
[0022] The L*, a* and b* CIELAB-values are defined in ASTM Norm E179-90 in a R(45/0) geometry
with evaluation according to ASTM Norm E308-90.
[0023] A heating element as used in disclosing the present invention is a resistor, which
becomes hot upon being energized.
[0024] A split resistor, see Figure 2, is a U-shaped heating element with the arms of the
U parallel to the transport direction of the substantially light-insensitive recording
material, which enables contacts to be made at the same side of the thermal head.
[0025] The spatial resolution of a thermal head, or thermal head resolution, is the number
of lines that can be distinguished in an image on a thermographic material expressed
in lines or dots per unit length e.g. in lines/mm or dots/mm, or in dots per inch
(dpi).
[0026] The thermal head pitch, P, is the distance between the geometric middle of one heating
element and the geometric middle of an adjacent heating element along a line in the
plane of the heating elements which bisects all the heating elements (see Figures
1 and 2). This line is lateral to the transport direction of the substantially light-insensitive
recording material. In the case of a split resistor, the geometric middle may be in
the gap between the two arms of the split resistor (see Figure 2).
[0027] The heating element aspect ratio is the length of the heating element in the transport
direction, T, of the substantially light-insensitive recording material L (see Figures
1 and 2) divided by the thermal head pitch P (see Figures 1 and 2).
[0028] The line time is the time taken to print one line lateral to the transport direction
of the substantially light-insensitive recording material i.e. at an angle to the
transport direction of 90° ± 20°.
[0029] Transport speed, i.e. the speed of the substantially light-insensitive thermographic
material, is the distance between adjacent lines of image dots in the transport direction
divided by the line time.
[0030] A transport system can consist of a moving belt, motor-driven drums, capstans etc.
[0031] Substantially rectangular means having angles which deviate from 90° by no more than
20°.
[0032] Substantially light-insensitive means not intentionally light sensitive.
[0033] The descriptor aqueous in the term aqueous medium for the purposes of the present
invention includes mixtures of water-miscible organic solvents such as alcohols e.g.
methanol, ethanol, 2-propanol, butanol, iso-amyl alcohol etc.; glycols e.g. ethylene
glycol; glycerine; N-methyl pyrrolidone; methoxypropanol; and ketones e.g. 2-propanone
and 2-butanone etc. with water in which water constitutes more than 50% by weight
of the aqueous medium with 65% by weight of the aqueous medium being preferred and
80% by weight of the aqueous being particularly preferred.
[0034] A leuco-dye is a colourless or weakly coloured compound derived from a dye. Colourless
or light coloured dye precursor leuco-dye systems include leuco triarylmethane, indolyl
phthalide, diphenylmethane, 2-anilinofluoran, 7-anilinofluoran, xanthene and spiro
compounds such as disclosed in
EP-A 754 564.
Thermal head printer
[0035] Aspects of the present invention are realised by a thermal head printer for printing
a substantially light-insensitive thermographic material, the thermal printer comprising:
a transport system having a transport direction, n thermal heads, where n is an integer,
each of the thermal heads comprising an array of substantially rectangular energizable
heating elements, the heating elements having a length L
n in the transport direction and a pitch P
n between adjacent heating elements, and a means for supplying electrical energy to
each of the substantially rectangular energizable heating elements in at least one
of the thermal heads, wherein the transport system being capable of transporting the
light-insensitive thermographic material in contact or proximity with at least one
of the thermal heads, wherein at least one of the thermal heads comprises heating
elements for which L
n/P
n is between 0.25 and 0.88 wherein at least one of the thermal heads comprises heating
elements with a width of the space between adjacent resistors along a line in the
plane of said heating elements which bisects all the heating elements of 20% or less
of P
n, wherein the printer is capable of image-wise printing the substantially light-insensitive
thermographic material, and wherein the substantially light-insensitive thermographic
material is not perforated by the printer. The thermal head can be associated with
one or more further thermal heads each with an array of heating elements having a
length L
n in the transport direction and a pitch P
n between adjacent heating elements. These thermal heads may be staggered or butted.
[0036] According to a first embodiment of the thermal head printer, according to the present
invention, the thermal head printer comprises a replaceable thermal head or set of
thermal heads.
[0037] According to a second embodiment of the thermal head printer, according to the present
invention, the thermal head printer comprises at least two thermal heads, configured
such that a first thermal head can be replaced by an nth thermal head while being
capable of maintaining a comparable image tone with said substantially light-insensitive
thermographic material.
[0038] According to a third embodiment of the thermal head printer, according to the present
invention, the substantially rectangular heating element is a split resistor.
[0039] According to a fourth embodiment of the thermal head printer, according to the present
invention, the heating elements are exclusive of a slit formed on a surface of the
heating elements and having a shape in which the width of the slit in a feeding direction
of the printed object is less than that in an arranging direction of the heating elements
perpendicular to the feeding direction and the width of the slit in the arranging
direction of the heating elements is approximately equal to a half length of a pitch
of the heating elements in the arranging direction thereof.
[0040] According to a fifth embodiment of the thermal head printer, according to the present
invention, said heating elements of at least one thermal head have a length, L
n, in the transport direction of less than 88 µm.
[0041] According to a sixth embodiment of the thermal head printer, according to the present
invention, said heating elements of at least one thermal head have a pitch, P
n, of less than 100 µm.
[0042] According to a seventh embodiment of the thermal head printer, according to the present
invention, at least one of the thermal heads comprises heating elements for which
L
n/P
n is between 0.40 and 0.75.
[0043] According to an eighth embodiment of the thermal head printer, according to the present
invention, at least one of the thermal heads comprises heating elements with a width
of a space between adjacent resistors along a line in the plane of the heating elements
which bisects all the heating elements which is 15% or less of the pitch, P
n, between two adjacent heating elements.
[0044] The line time has been defined above as the time taken to print one line lateral
to the transport direction of the substantially light-insensitive recording material
i.e. at an angle to the transport direction of 90° ± 20°. It should be pointed out
that for a particular transport speed and for heating elements with a particular dimension,
the image tone attained for printed pixels with a length in the transport direction
no larger than the length of the heating element in the transport direction does not
depend upon the line time, since varying the line-time simply results in a variation
in the length of the printed pixel in the transport direction, the length of the printed
pixel being proportional to the line-time.
[0045] According to a ninth embodiment of the thermal head printer, according to the present
invention, said heating elements are thin film heating elements.
[0046] According to a tenth embodiment of the thermal head printer, according to the present
invention, said heating elements are connected to the means of supplying electrical
energy on the same side of the heating elements.
Processes for printing a substantially light-insensitive thermographic material
[0047] Aspects of the present invention are realized by a first process for printing a substantially
light-insensitive thermographic material with the above-described thermal head printer
comprising the steps of: choosing a thermal head, providing the substantially light-insensitive
thermographic material, transporting the substantially light-insensitive thermographic
material past the thermal head, and image-wise heating of the substantially light-insensitive
thermographic material by supplying electrical energy to the heating elements.
[0048] Aspects of the present invention are also realized by a second process for printing
a substantially light-insensitive thermographic material at different printing speeds
with a different thermal head at each printing speed without significant variation
in image tone, wherein in that each of said different thermal heads comprises heating
elements with a different length in the transport direction of the substantially light-insensitive
thermographic material and said length of the heating elements in the transport direction
of the substantially light-insensitive thermographic material decreases with decreasing
printing speed and the different thermal heads are each on different substrates.
[0049] It has been surprisingly found that with a particular substantially light-insensitive
thermographic material a constant image tone can be realized at different transport
speeds of the substantially light-insensitive thermographic material by changing the
length of the heating element in the transport direction e.g. if the transport speed
is reduced the same image tone can be realized by reducing the length of the heating
element in the transport direction.
[0050] The operating temperature of common thermal heads is in the range of 300 to 400°C
and the pressure contact of the thermal printhead with the recording material to ensure
a good transfer of heat being e.g. 200-1000g/linear cm i.e. with a contact zone (nip)
of 200 to 300 µm a pressure of 5000 to 50,000 g/cm
2. Activation of the heating elements can be power-modulated or pulse-length modulated
at constant power.
[0051] According to a first embodiment of the first process, the first embodiment of the
second process and a first embodiment of the second process, according to the present
invention, at least one thermal head has a line-time of less than 20 ms.
Substantially light-insensitive thermographic material
[0052] The term substantially light-insensitive thermographic material includes all materials
which produce a change in optical density upon the application of heat.
[0053] According to a second embodiment of the processes, according to the present invention,
the substantially light-insensitive thermographic material is a black and white material.
[0054] According to a third embodiment of the processes, according to the present invention,
the substantially light-insensitive thermographic material is a two sheet material
in which an ingredient necessary for the image-forming process is transferred upon
image-wise application of heat from one sheet to the other where it reacts with one
or more further ingredients to produce an image.
[0055] According to a fourth embodiment of the processes, according to the present invention,
the substantially light-insensitive thermographic material is a monosheet material.
[0056] According to a fifth embodiment of the processes, according to the present invention,
the substantially light-insensitive thermographic material contains a thermosensitive
element comprising one or more layer, the one or more layers containing an image-forming
system.
[0057] Suitable image-forming systems include monosheet substantially light-insensitive
thermographic materials such as colourless or light coloured dye precursor leuco-dye
systems, as disclosed in
US-P 4,370,370,
EP-A 479 578 and
EP-A 754 564, diazo systems, as disclosed in
JP 60-01077A, or two-sheet thermal dye transfer systems, such as disclosed in
EP-A 656 264 and
US-P 4,943,555. Alternatively the image-forming systems may comprise at least one substantially
light-insensitive organic silver salt and at least one organic reducing agent therefor
either in a two-sheet material in which upon image-wise application of heat at least
one organic reducing agent is image-wise transferred to a sheet containing the at
least one substantially light-insensitive organic silver salt whereupon the image-forming
reaction takes place or in a monosheet material in which the at least one substantially
light-insensitive organic silver salt is in thermal working relationship with the
at least one organic reducing agent therefor.
[0058] In a sixth embodiment of the processes, according to the present invention, the substantially
light-insensitive thermographic material is a monosheet material comprising a thermosensitive
element and a support, the thermosensitive element comprising at least one substantially
light-insensitive organic silver salt, at least one organic reducing agent therefor
in thermal working relationship therewith, i.e. during the thermal development process
the organic reducing agent must be present in such a way that it is able to diffuse
to the substantially light-insensitive organic silver salt particles so that reduction
of the substantially light-insensitive organic silver salt can take place, and a binder.
Such materials include the possibility of one or more substantially light-insensitive
organic silver salts and/or one of more organic reducing agents therefor being encapsulated
in heat-responsive microcapsules, such as disclosed in
EP-A 0 736 799 herein incorporated by reference.
Organic silver salts
[0059] Preferred substantially light-insensitive organic silver salts for use in the thermosensitive
element of the substantially light-insensitive elongated imaging material used in
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, which
silver salts are also called silver soaps.
Organic reducing agents
[0060] Suitable organic reducing agents for the reduction of the substantially light-insensitive
organic silver salts are organic compounds containing at least one active hydrogen
atom linked to O, N or C. The choice of reducing agent influences the thermal sensitivity
of the imaging material and the gradation of the image. Imaging materials using gallates,
for example, have a high gradation. In a preferred embodiment of the present invention
the thermosensitive element contains a 3,4-dihydroxyphenyl compound with ethyl 3,4-dihydroxybenzoate,
n-butyl 3,4-dihydroxybenzoate, 3,4-dihydroxy-benzophenone and 3,4-dihydroxy-benzonitrile
being particularly preferred.
Binder
[0061] The thermosensitive element of the substantially light-insensitive elongated imaging
material used in the present invention may be coated onto a support in sheet- or web-form
from an organic solvent containing the binder dissolved therein or may be applied
from an aqueous medium using water-soluble or water-dispersible binders.
[0062] Suitable binders for coating from an organic solvent are all kinds of natural, modified
natural or synthetic resins or mixtures of such resins, wherein the organic heavy
metal salt can be dispersed homogeneously or mixtures thereof.
[0063] Suitable water-soluble film-forming binders include: polyvinyl alcohol, polyacrylamide,
polymethacrylamide, polyacrylic acid, polymethacrylic acid, polyethyleneglycol, polyvinylpyrrolidone,
proteinaceous binders such as gelatin and modified gelatins, such as phthaloyl gelatin,
polysaccharides, such as starch, gum arabic and dextrin, and water-soluble cellulose
derivatives. Suitable water-dispersible binders are any water-insoluble polymers.
Poly(vinylbutyral) is the preferred binder.
[0064] In the case of substantially light-insensitive thermographic recording materials
containing substantially light-insensitive organic silver salts, the binder to organic
silver salt weight ratio decreases the gradation of the image increasing. Binder to
organic silver salt weight ratios of 0.2 to 6 are preferred with weight ratios between
0.5 and 3 being particularly preferred.
[0065] The above mentioned binders or mixtures thereof may be used in conjunction with waxes
or "heat solvents" to improve the reaction speed of the image-forming reaction at
elevated temperatures.
Toning agents
[0066] In order to obtain a neutral black image tone in the higher densities and neutral
grey in the lower densities, the substantially light-insensitive thermographic material
used in the present invention may contain one or more toning agents. In the case of
substantially light-insensitive thermographic recording materials containing substantially
light-insensitive organic silver salts, the toning agents should be in thermal working
relationship with the substantially light-insensitive organic silver salt and reducing
agents during thermal processing.
[0067] Suitable toning agents are described in
US 3,074,809,
US 3,446,648 and
US 3,844,797 and
US 4,082,901. Other particularly useful toning agents are the heterocyclic toning compounds of
the benzoxazine dione or naphthoxazine dione type as disclosed in
GB 1,439,478,
US 3,951,660 and
US 5,599,647.
[0068] According to an seventh embodiment of the process, according to the present invention,
the substantially light-insensitive thermographic material contains a thermosensitive
element, the thermosensitive element containing one or more toning agents selected
from the group consisting of phthalazinone, benzo[e][1,3]oxazine-2,4-dione, 7-methyl-benzo[e][1,3]oxazine-2,4-dione,
7-methoxy-benzo[e][1,3]oxazine-2,4-dione and 7-(ethylcarbonato)-benzo[e][1,3]oxazine-2,4-dione.
Stabilizers and antifoggants
[0069] In order to obtain improved shelf-life, archivability and reduced fogging, stabilizers
and antifoggants may be incorporated into the substantially light-insensitive thermographic
material used in the present invention. Suitable stabilizers compounds for use in
the substantially light-insensitive thermographic material used in the present invention
include benzotriazole, tetrachlorophthalic acid anhydride and those compounds represented
by general formula I:
where Q are the necessary atoms to form a 5- or 6-membered aromatic heterocyclic ring,
A is selected from hydrogen, a counterion to compensate the negative charge of the
thiolate group or a group forming a symmetrical or an asymmetrical disulfide.
Surfactants and dispersants
[0070] Surfactants and dispersants aid the dispersion of ingredients which are insoluble
in the particular dispersion medium. The substantially light-insensitive thermographic
material used in the present invention may contain one or more surfactants, which
may be anionic, non-ionic or cationic surfactants and/or one or more dispersants.
Suitable dispersants are natural polymeric substances, synthetic polymeric substances
and finely divided powders, e.g. finely divided non-metallic inorganic powders such
as silica.
Support
[0071] According to a eighth embodiment of the processes, according to the present invention,
the substantially light-insensitive thermographic material has a transparent or translucent
support and is preferably a thin flexible carrier made 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 needs be to improve the adherence to the thereon coated
thermosensitive element. The support may be dyed or pigmented to provide a transparent
coloured background for the image.
Protective layer
[0072] In a preferred embodiment of the present invention a protective layer is provided
for the thermosensitive element. In general this protects the thermosensitive element
from atmospheric humidity and from surface damage by scratching etc. and prevents
direct contact of printheads or other heat sources with the recording layers. Protective
layers for thermosensitive elements which come into contact with and have to be transported
past a heat source under pressure, have to exhibit resistance to local deformation
and good slipping characteristics during transport past the heat source during heating.
A slipping layer, being the outermost layer, may comprise a dissolved lubricating
material and/or particulate 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, with or without
a polymeric binder.
Coating techniques
[0074] The following examples and comparative examples illustrate the present invention.
The percentages and ratios used in the examples are by weight unless otherwise indicated.
Ingredients in the adhesion layers, Ad-L01 and Ad-L02:
[0075]
- PEDOT/PSS
- = 1.2% aqueous dispersion of a latex consisting of poly(3,9-ethylenedioxythiophene):
poly(styrenesulphonate) in a weight ratio of 1:2.46
- HOSTAPON T
- = a 40% concentrate of a sodium salt of N-methyl-N-2-sulfoethyl-oleylamide by HOECHST;
- LATEX01
- = 30% aqueous dispersion of a copolymer of 88% vinylidene chloride, 10% methyl acrylate
and 2% itaconic acid stabilized and 0.75% of HOSTAPON T
- KIESELSOL™ 100F=
- a 30% aqueous dispersion of colloidal silica from BAYER;
- MERSOLAT™ H
- = a 76% aqueous paste of a sodium pentadecylsulfonate from BAYER;
Ingredients in the backing layer, Ba-L:
[0076]
- POVAL 103 =
- a 98% hydrolyzed poly(vinyl alcohol) powder from Kuraray;
- boric acid =
- H3BO3
- AKYPO™ OP80 =
- an 80% concentrate of an octyl-phenyl-oxypolyethyleneglycol(EO 8)acetic acid from
CHEMY;
- SNOWTEX™ O =
- a 20% aqueous dispersion of colloidal silica, from NISSAN CHEMICAL;
- SUNSPHERE™ H51 =
- a 8.63% dispersion of 5.7 µm silica particles from Asahi Glass;
Ingredients in the thermosensitive element, Th-El:
[0077]
S-LEC BL5HP |
= |
a polyvinyl butyral from SEKISUI; |
BAYSILON |
= |
a silicone oil from BAYER; |
DESMODUR VL |
= |
a 4,4'-diisocyanatodiphenylmethane from BAYER; |
Reducing agents:
[0078]
R01 |
= |
3,4-dihydroxybenzonitrile; |
R02 |
= |
3,4-dihydroxybenzophenone; |
Toning agent:
[0079]
T01 |
= |
7-methyl-benzo[e][1,3]oxazine-2,4-dione; |
Stabilizers:
[0080]
S01 |
= |
glutaric acid |
S02 |
= |
tetrachlorophthalic acid anhydride |
S03 |
= |
benzotriazole |
Ingredients in the protective layer, PRO-L:
[0081]
ERCOL™ 48 20 |
= |
a polyvinylalcohol from ACETEX EUROPE; |
LEVASIL™ VP AC 4055 |
= |
a 15% aqueous dispersion of colloidal silica with acid groups predominantly neutralized
with sodium ions and a specific surface are of 500 m2/g, from BAYER AG has been converted into the ammonium salt; |
ULTRAVON™ W |
= |
75-85% concentrate of a sodium arylsulfonate from Ciba Geigy converted into acid form
by passing through an ion exchange column; |
SYLOID™ 72 |
= |
a silica from Grace; |
SERVOXYL™ VPDZ 3/100 |
= |
a mono[isotridecyl polyglycolether (3 EO)] phosphate, from SERVO DELDEN B.V.; |
SERVOXYL™ VPAZ 100 |
= |
a mixture of monolauryl and dilauryl phosphate, from SERVO DELDEN B.V.; |
MICROACE TALC P3 |
= |
an Indian talc from NIPPON TALC; |
RILANIT™ GMS |
= |
a glycerine monotallow acid ester, from HENKEL AG |
TMOS |
= |
tetramethylorthosilicate hydrolyzed in the presence of methanesulfonic acid. |
COMPARATIVE EXPERIMENTS 1 & 2 and INVENTION EXAMPLE 1
Substantially light-insensitive thermographic material
[0082] The substantially light-insensitive thermographic material used in evaluating the
thermal head printer and process for printing a substantially light-insensitive recording
materials, according to the present invention, consisted of a blue-pigmented PET support
coated on one side with adhesion layer Ad-L01 and on the other side with adhesion
layer Ad-L02. The adhesion layer Ad-L01 was further coated with the thermosensitive
element TH-EL which itself was further coated with a protective layer PRO-L. The adhesion
layer Ad-L02 was further coated with backing layer Ba-L.
Coating support with adhesion layers Ad-L01 and Ad-L02:
[0083] A 168µm thick blue-pigmented polyethylene terephthalate film with CIELAB-L*, a* and
-b* values of 85.5, -8.5 and -18.9 respectively and a density measured in transmission
with a MacBeth
™ TD924 densitometer through a visible filter of 0.195 was coated on one side with
an aqueous dispersion with the following ingredients to produce an adhesion layer
Ad-L01 with the composition:
LATEX01 |
= |
151 mg/m2 |
KIESELSOL™ F |
= |
35 mg/m2 |
MERSOLAT™ H |
= |
0.75 mg/m2 |
and on the other side with an aqueous dispersion with the following ingredients to
produce an antistatic adhesion layer Ad-L02 with the composition:
PEDOT/PSS |
= |
2.58 mg/m2 |
LATEX01 |
= |
147.3 mg/m2 |
KIESELSOL™ F |
= |
16.4 mg/m2 |
sorbitol |
= |
24.7 mg/m2 (partially evaporated) |
MERSOLAT™ H |
= |
0.74 mg/m2 |
Preparation of backing layer Ba-L:
[0084] 264 g of POVAL 103 was added to 1736 g of cold water. The temperature was increased
to 95°C and held at that temperature for 30 minutes. The resulting solution with a
solids content of 13.2% by weight was cooled down to room temperature (25°C). 2000
g of this polyvinyl alcohol solution was then added with mixing to 1067.6 g of deionized
water and then 130.7 ml of a 5 % by weight of an aqueous solution of AKYPO
™ OP80 was added, followed by 1978.5 g of SNOWTEX
™ O, Nissan Chemical and 45.85 g of SUNSPHERE
™ H51. The resulting solution had a pH of 4.8. The pH of this solution was then adjusted
to a pH of 3.5 with 1N nitric acid and finally 156 mL of 4% boric acid was added with
stirring to produce the coating solution.
[0085] This coating solution was applied to a wet thickness of 40µm to antistatic adhesion
layer Ad-L02 on one side of the 168 µm thick blue-pigmented polyethylene terephthalate
film. The film was then dried by an air flow with a temperature of 140°C to produce
a layer Ba-L with the composition:
POVAL 103 |
= |
2125 mg/m2 |
boric acid |
= |
50 mg/m2 |
AKYPO OP80 |
= |
53 mg/m2 |
SNOWTEX™ O |
= |
3166 mg/m2 |
SUNSPHERE™ H51 |
= |
32 mg/m2 |
Preparation of thermosensitive element, Th-El:
[0086] The adhesion layer Ad-L01 on one side of the 168 µm thick blue-pigmented polyethylene
terephthalate film was coated with a composition containing 2-butanone as solvent/dispersion
medium to a wet layer thickness of 95 µm, so as to obtain thereon, after drying at
85°C for 5 minutes, thermosensitive layer Th-El with the following composition:
silver behenate |
= |
4.149 g/m2 |
S-LEC BL5HP |
= |
16.596 g/m2 |
T01 |
= |
0.246 g/m2 |
BAYSILON |
= |
0.037 g/m2 |
R01 |
= |
0.438 g/m2 |
R02 |
= |
0.894 g/m2 |
S01 |
= |
0.294 g/m2 |
S02 |
= |
0.130 g/m2 |
S03 |
= |
0.109 g/m2 |
Desmodur VL |
= |
0.185 g/m2 |
Preparation of protective layer, PRO-L:
[0087] The thermosensitive element, Th-El, was then coated with an aqueous composition with
the following ingredients, which was adjusted to a pH of 3.8 with 1N nitric acid,
to a wet layer thickness of 85 µm and then dried at 40°C for 15 minutes to produce
a protective layer PRO-L with the composition:
ERCOL™ 48 20 |
= |
2.1g/m2 |
LEVASIL™ VP AC 4055 |
= |
1.05g/m2 |
ULTRAVOM™ W |
= |
0.075g/m2 |
SYLOID™ 72 |
= |
0.09 g/m2 |
SERVOXYL™ VPDZ 3/100 |
= |
0.075g/m2 |
SERVOXYL™ VPAZ 100 |
= |
0.075g/m2 |
MICROACE TALC P3 |
= |
0.045g/m2 |
RILANIT™ GMS |
= |
0.15g/m2 |
TMOS |
= |
0.87g/m2 (assuming that the TMOS was |
|
|
completely converted to SiO2) |
[0088] After coating the protective layer was hardened by heating the substantially light-insensitive
thermographic material at 50°C for 7 days.
Printing experiments
[0089] The substantially light-insensitive thermographic material was then printed with
the printing conditions given in Table 1 for COMPARATIVE EXAMPLES 1 to 3 and INVENTION
EXAMPLE 1 respectively.
[0090] It should be noted that, although some of the reported experiments were carried out
with thermal heads giving a resolution of 320 dpi and others carried out with thermal
heads giving a resolution of 508 dpi, substantially similar shifts in image tone with
transport speed were observed regardless of the resolution at which the experiments
were carried out.
Table 1:
|
Comparative Example 1 |
Comparative Example 2 |
Comparative Example 3 |
Invention Example 1 |
printer |
modified DRYSTAR™ 3000 |
prototype high throughput |
prototype high throughput |
prototype low throughput |
resolution of thermal head |
320 dpi |
508 dpi |
508 dpi |
320 dpi |
heating elements L x P |
140µm x 79.4µm |
75µm x 50µm |
75µm x 50µm |
50µm x 79.4µm# |
aspect ratio of heating element |
1.76 |
1.50 |
1.50 |
0.63 |
transport speed [mm/s] |
9.53 (vs 6.64*) |
14.3 |
7.15 |
9.53 (vs 6.64*) |
line time |
11.95 ms |
3.5 ms |
7.0 ms |
8.33 ms |
distance travelled per line |
113.9µm |
50µm |
50µm |
79.4µm |
* standard transport speed for a DRYSTAR 3000 configuration
# split heating element with "arms" 24.7 µm wide and a gap of 15 µm |
Image evaluation
[0091] The density of the prints obtained in COMPARATIVE EXAMPLES 1 to 3 and INVENTION EXAMPLE
1, D
vis, was determined in transmission with a MacBeth
™ TD924 densitometer using a visible filter and the image tone was evaluated as a function
of the print density, D
vis, using CIELAB measurements. The L*, a* and b* CIELAB-values of the prints were determined
by spectrophotometric measurements according to ASTM Norm E179-90 in a R(45/0) geometry
with evaluation according to ASTM Norm E308-90.
[0092] In the CIELAB-system a negative CIELAB a*-value indicates a greenish image-tone becoming
greener as a* becomes more negative, a positive a*-value indicating a reddish image-tone
becoming redder as a* becomes more positive. A negative CIELAB b*-value indicates
a bluish tone which becomes increasingly bluer as b* becomes more negative and a positive
b*-value indicates a yellowish image-tone becoming more yellow as b* becomes more
positive. In terms of the visual perception of an image as a whole, the image tone
of elements of the image with a density of 1.0 have a stronger effect than the image
tone of elements with lower or higher optical density.
[0093] Table 2 gives the CIELAB a*- and b*-values for optical densities, D
vis, between 0.2 and 2.8 for an AGFA-GEVAERT SCOPIX
™ LT2B silver halide emulsion laser medical hardcopy film (reference hardcopy film)
and for the substantially light-insensitive thermographic material printed according
to COMPARATIVE EXPERIMENTS 1 and 2 and INVENTION EXPERIMENT 1.
Table 2:
Dvis |
CIELAB a*-value |
CIELAB b*-value |
SCOPIX™ LT2B medical hardcopy film |
Comparative experiment |
Invention experiment |
SCOPIX™ LT2B medical hardcopy film |
Comparative experiment |
Invention experiment |
1 |
2 |
1 |
1 |
2 |
1 |
0.2 |
-7.05 |
-8.50 |
-8.40 |
-8.45 |
-14.27 |
-16.70 |
-16.40 |
-16.75 |
0.4 |
-6.23 |
-7.45 |
-7.50 |
-7.70 |
-11.07 |
-10.20 |
-11.08 |
-12.65 |
0.6 |
-5.53 |
-6.43 |
-6.60 |
-6.40 |
-10.25 |
-7.45 |
-9.50 |
-10.50 |
0.8 |
-4.93 |
-5.67 |
-5.72 |
-5.20 |
-8.75 |
-6.05 |
-8.70 |
-9.45 |
1.0 |
-4.40 |
-5.10 |
-4.90 |
-4.35 |
-7.5 |
-5.10 |
-8.13 |
-8.83 |
1.2 |
-3.90 |
-4.40 |
-4.24 |
-3.58 |
-6.45 |
-4.50 |
-7.65 |
-8.45 |
1.4 |
-3.47 |
-4.00 |
-3.68 |
-3.07 |
-5.55 |
-3.83 |
-7.10 |
-8.10 |
1.6 |
-3.05 |
-3.55 |
-3.28 |
-2.77 |
-4.83 |
-3.40 |
-6.53 |
-7.65 |
1.8 |
-2.73 |
-3.20 |
-2.83 |
-2.50 |
-4.45 |
-2.95 |
-5.86 |
-6.90 |
2.0 |
-2.39 |
-2.85 |
-2.50 |
-2.25 |
-3.30 |
-2.50 |
-5.05 |
-5.90 |
2.2 |
-1.70 |
-2.10 |
-1.98 |
-1.90 |
-2.43 |
-1.85 |
-3.90 |
-4.55 |
2.4 |
-1.05 |
-1.40 |
-1.55 |
-1.40 |
-1.55 |
-1.05 |
-2.55 |
-2.87 |
2.6 |
-0.70 |
-0.95 |
-1.16 |
-1.00 |
-0.97 |
-0.57 |
-1.46 |
-1.70 |
2.8 |
-0.48 |
-0.70 |
-0.80 |
-0.70 |
-0.60 |
-0.25 |
-0.85 |
-0.93 |
[0094] In considering the image tone of the prints produced with COMPARATIVE EXPERIMENT
2 and INVENTION EXPERIMENT 1 with the same substantially light-insensitive thermographic
material, it should be borne in mind that if the substantially light-insensitive thermographic
material was to be printed at half the speed i.e. a transport speed of 7.15 mm/s (corresponding
to a line-time of 7 mm/s) using the same thermal head as used in COMPARATIVE EXPERIMENT
2 as in the case of COMPARATIVE EXPERIMENT 3, shifts of up to -0.70 were observed
in the CIELAB a*-values and up to +3.70 were observed in the CIELAB b*-values (see
Table 3) with respect to that observed in COMPARATIVE EXPERIMENT 2, the exact shifts
being dependent upon D
vis, resulting in an unacceptable yellowish tinge to the image.
Table 3:
Dvis |
CIELAB a*-value |
CIELAB b*-value |
SCOPIX™ LT2B medical hard-copy film |
Comparative experiment |
SCOPIX™ LT2B medical hard-copy film |
Comparative experiment |
2 |
3 |
Δ3-2 |
2 |
3 |
Δ3-2 |
0.2 |
-7.05 |
-8.40 |
-9.10 |
-0.70 |
-14.27 |
-16.40 |
-13.7 |
+2.70 |
0.4 |
-6.23 |
-7.50 |
-7.67 |
-0.17 |
-11.07 |
-11.08 |
-10.10 |
+0.98 |
0.6 |
-5.53 |
-6.60 |
-6.68 |
-0.08 |
-10.25 |
-9.50 |
-7.90 |
+1.60 |
0.8 |
-4.93 |
-5.72 |
-5.95 |
-0.23 |
-8.75 |
-8.70 |
-6.50 |
+2.20 |
1.0 |
-4.40 |
-4.90 |
-5.30 |
-0.40 |
-7.5 |
-8.13 |
-5.43 |
+3.70 |
1.2 |
-3.90 |
-4.24 |
-4.70 |
-0.46 |
-6.45 |
-7.65 |
-4.60 |
+3.05 |
1.4 |
-3.47 |
-3.68 |
-4.20 |
-0.52 |
-5.55 |
-7.10 |
-4.13 |
+2.97 |
1.6 |
-3.05 |
-3.28 |
-3.77 |
-0.49 |
-4.83 |
-6.53 |
-3.64 |
+2.89 |
1.8 |
-2.73 |
-2.83 |
-3.23 |
-0.40 |
-4.45 |
-5.86 |
-3.03 |
+2.83 |
2.0 |
-2.39 |
-2.50 |
-2.65 |
-0.15 |
-3.30 |
-5.05 |
-2.30 |
+2.75 |
2.2 |
-1.70 |
-1.98 |
-1.95 |
+0.03 |
-2.43 |
-3.90 |
-1.53 |
+2.37 |
2.4 |
-1.05 |
-1.55 |
-1.35 |
+0.20 |
-1.55 |
-2.55 |
-0.85 |
+1.70 |
2.6 |
-0.70 |
-1.16 |
-0.93 |
+0.23 |
-0.97 |
-1.46 |
-0.33 |
+1.13 |
2.8 |
-0.48 |
-0.80 |
-0.68 |
+0.12 |
-0.60 |
-0.85 |
+0.05 |
+0.90 |
[0095] It was therefore surprising that prints produced in COMPARATIVE EXAMPLE 2 and INVENTION
EXAMPLE 1 with the same substantially light-insensitive thermographic material but
with different line times, 3.5 ms and 8.33 ms respectively, and different transport
speeds, 14.3 mm/s and 9.53 mm/s respectively, had substantially identical CIELAB a*-
and b*-values at all image densities. This has been surprisingly realized by changing
the heating element aspect ratio from 1.50 to an aspect ratio between 0.25 and 0.88
i.e. 0.63.
[0096] An identical transport speed was used for the prints produced in COMPARATIVE EXPERIMENT
1 and INVENTION EXPERIMENT 1. Therefore the printing parameters for the prints produced
with COMPARATIVE EXPERIMENT 1 and INVENTION EXPERIMENT 1 only differed in line time.
However, as pointed out above, the image tone of a print produced with a pixel length
in the transport direction no larger than the length of the heating element in the
transport direction has a pixel length which increases with increasing line-time,
but an image tone which is independent of the line time. Therefore, if image tones
are being compared the printing parameters can be regarded as being comparable. Moreover,
these prints were produced with thermal heads with heating elements with considerably
different lengths in the transport direction of 140 µm and 50 µm respectively and
considerably different image tones were exhibited. These experiments show that by
reducing the length of the heating elements in the transport direction, there is virtually
no effect on the CIELAB a*-values at all image densities, but that the CIELAB b*-values
were dramatically affected being shifted to considerably more negative values over
a large part of the D
vis range i.e. the image became markedly bluer, with particularly high shifts of -3.73
and - 3.40 being observed at optical densities critical to the perception of the observer
i.e. the image tone of the print obtained in INVENTION EXAMPLE 1 is considerably more
acceptable to image analysts than the image tone of the print obtained in COMPARATIVE
EXAMPLE 1. The only substantial difference between the printing conditions of COMPARATIVE
EXAMPLE 1 and INVENTION EXAMPLE 1 is the length of the heating elements in the transport
direction or in other words the aspect ratios of the heating elements changing from
1.76 to 0.63. This again demonstrates the benefit of the use of heating elements with
aspect ratios in the range of 0.25 to 0.88.
[0097] The present invention may include any feature or combination of features disclosed
herein either implicitly or explicitly or any generalisation. In view of the foregoing
description it will be evident to a person skilled in the art that various modifications
may be made within the scope of the invention.