Note: Descriptions are shown in the official language in which they were submitted.
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APPARATUS FOR CLEANING A SURFACE
[D88CRIPTION]
FIELD OF THE INVENTION
s The present invention relates to an apparatus for cleaning a
surface, especially for removing the coating from a lithographic
printing plate so that the lithographic substrate can be recycled
and used again for applying a coating thereto.
so BACKGROUND OF THE INVENTION
Lithographic printing presses use a so-called printing master
such as a printing plate which is mounted on a cylinder of the
printing press. The master carries a lithographic image on its
ss surface and a print is obtained by applying ink to said image and
then transferring the ink from the master onto a receiver material,
which is typically paper.. In conventional lithographic printing, ink
as well as an aqueous fountain solution (also called dampening
liquid) are supplied to the lithographic image which consists of
ao oleophilic (or hydrophobic, i.e. ink-accepting; water-repelling)
areas as well as hydrophilic (or oleophobic, i.e. water-accepting,
ink-repelling) areas. In so-called driographic printing, the
lithographic image consists of ink-accepting and ink-abhesive (ink-
repelling) areas and during driographic printing, only ink is
as supplied to the master.
Printing masters are generally obtained by the so-called
computer-to-film method wherein various pre-press steps such_as
typeface selection, scanning, color separation, screening, trapping,
layout and imposition are accomplished digitally and each color
3o selection is transferred to graphic arts film using an image-setter.
After processing, the film can be used as a mask for the exposure of
an imaging material called plate precursor and after plate
processing, a printing plate is obtained which can be used as a
master.
35 In recent years the so-called computer-to-plate method has
gained a lot of interest. This method, also called direct-to-plate
method, bypasses the-.creation of-film because the digital document
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is transferred directly to a plate precursor by means of a so-called
plate-setter. Especially thermal plates, which are sensitive to heat
or infrared light are widely used in computer-to-plate methods,
because of their daylight stability. Such thermal materials
s preferably comprise a compound that converts absorbed light into
heat. The heat, which is generated on image-wise exposure, triggers
a (physico-)chemical process, such as ablation, polymerization,
insolubilization by cross-linking of a polymer, decomposition, or
particle coagulation of a thermoplastic polymer latex, and after
20 optional processing, a lithographic image is obtained.
Plates which do not require wet processing are particularly
suited for on-press imaging in a so-called digital press, i.e. a
printing press comprising an integrated exposure device on every
color station. Such digital presses allow computer-to-press
Zs workflows wherein the unexposed plate precursor is first mounted on
the print cylinder of each color station and the exposure is cared
out while the plate is clamped on the cylinder,. resulting in a goad
registration without the need of readjustment of the plate
positions.
ao Whereas a plate precursor normally consists of a sheet-like
support and one or more functional coatings, computer-to-press
methods have been described, e.g. in GB1546532, wherein a
composition, which is capable to form a lithographic surface upon
image-wise exposure and optional processing, is provided directly on
2s the surface of a plate cylinder of the press. EP-A 101 266 describes
the coating of a hydrophobic layer directly on the hydrophilic
surface of a plate cylinder. After removal of the non-printing areas
by ablation, a master is obtained. Such on-press coating methods are
also described in US-P 5,713,287 and EP-A 802 457. After the press-
3o run, the coating can be removed from the plate cylinder by an on-
press cleaning step using a cleaning liquid and optionally
mechanical rubbing, so that the cleaned substrate can be re-used in
a next cycle of coating, exposure, printing and cleaning.
Devices for performing the cleaning step have been described in
ss JP63-4947; US 5,713;287; US 5,644,986 and US 5,603,775. Tn the
latter patent, a device is-described as depicted in Fig. 1, which
has been reproduced fr~m'US 5,603;775. The nozzle head includes a
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housing 25 consisting of a casing with circular cross section, and a
jet nozzle 24 arranged in the casing 25 in the immediate vicinity of
the center line of the casing. The casing 25, which is preferably
cylindrical, itself forms an elongate suction nozzle 26 which
s terminates in an orifice edge 29 surrounding an opening 39 free from
mechanical parts. The suction nozzle 26 contains a chamber 32
comprising said opening 39 and is arranged spaced from the surface
to be cleaned to forma circumferential gap 34 between the shell
surface 22 and orifice edge 29. The jet nozzle 24 is arranged in the
a.o chamber 32 of the casing, spaced axially from the opening 39, to
emit a jet 40 of cleaning liquid producing a predetermined treatment
area 41 on the surface. A holder 27 carries the jet nozzle 24, the
orifice 28 of which is located centrally in the casing 25. The front
end of the suction nozzle 26 is shaped with a contour to fit the
s5 curvature of the surface to be cleaned to produce said gap 34. The
holder 27 comprises a supply channel 30 communicating with the jet
nozzle 24. The holder 27 is also provided with a plurality of
peripheral, axial through-holes 31 through which an evacuation pipe
communicates openly with the chamber 32 of the suction nozzle 26.
ao A problem associated with the on-press coating, exposure and
cleaning methods is that the wet coating and cleaning steps involve
a risk of damaging or contaminating the optics and electronics of
the integrated image-setter. Often, the known cleaning methods also
fail because no suitable compromise can be found between the
2s chemical reactivity of the cleaning liquid versus the ink-accepting
areas which have to be removed on the one hand and the required
inertness of said cleaning liquid versus the fragile lithographic
surface on the other hand. A typical lithographic surface is
mechanically as well as chemically quite vulnerable. A lithographic
3o surface consists generally of a micro-pore structure in order to
obtain a good differentiation between the spreading properties of
the ink and the fountain. Anodized aluminum plates comprise a
lithographic surface containing one or more metal oxides on which
absorption phenomena can take place. These metal oxides are very
3s susceptible to chemical conversion into forms that are no longer
lithographically active. The above mentioned micro-porosity of a
lithographic surface-is -also highly susceptible to mechanical
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damage. The presence of solid particles in cleaning liquids, which
is often required for efficient mechanical cleaning of the
lithographic surface, results inevitably in a disturbance of the
micro-structure of said surface. Because ink and the coated imaging
s layer penetrate in the micro-pore structure, it is necessary to
carry out a vigorous cleaning so as to avoid ghost images in the
subsequent printing cycles, which are due to an incomplete removal
of the previous image.
2o SUMMARY OF THE INVENTION
It is an object of the present invention to provide an
apparatus for effectively cleaning a surface, in particular for
removing the coating from a lithographic printing plate without
Zs damaging the lithographic quality of the substrate, and which
reduces the risk of contaminating the environment, e.g. the optics
and electronics of a nearby image-setter. It is also an object to
provide a small apparatus which can easily be integrated in a
printing press. These objects are realized by the apparatus defined
ao in claim 1. The apparatus of claim 1 is essentially the same as the
one depicted in Fig. 1, with the additional feature that, upon
operation of the apparatus, rotating rubbing means 8 (Fig. 2 and 3),
e.g. brush hairs, form a barrier between, on the one hand, the area
of the surface 13 that is treated by the nozzle 4, i.e. the area
z5 which corresponds to the portion 5 of the opening 3, and, on the
other hand, the suction chamber 9 so that the cleaning liquid and
any material removed from the surface 13 first pass the rotating
rubbing means before being drawn into the suction chamber.
Specific features for preferred embodiments of the invention
so are set out in the dependent claims. Further advantages and
embodiments of the present invention will become apparent from the
following description and drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a perspective view of an apparatus known in the prior art.
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Fig. 2 is a lateral view of a preferred embodiment of an apparatus
according to the invention:
Fig. 3 is an end view of the apparatus shown in Fig. 2.
DETAILED DESCRIPTION OF THE INVENTION
The apparatus of the present invention comprises an elongated
housing 1, which is preferably cylindrical, having at an end thereof
io an edge 2 which surrounds an opening 3. The edge 2 preferably has a
circular cross section and may be flat or concave, sows to comply
with flat or curved surfaces to be cleaned. In a preferred
embodiment, the edge 2 is inwardly curved and its curvature is the
same as the curvature of the roll surface which is to be cleaned.
zs The housing 1, edge 2 and opening 3 form a suction nozzle for
removing the cleaning liquid and impurities from the surface 13. The
term "impurities" refers to all material that, during operation of
the apparatus, adheres to surface 13, in particular a lithographic
coating on the surface of a lithographic substrate, together with
ao ink, paper fibers, and any other material that is present on said
coating and said surface.
The apparatus comprises one or more jet br spray nozzles 4,
spaced from opening 3 by a certain distance, so that a cleaning
liquid can be projected onto a treatment zone of surface 13. The
as terms "jet" and "spray" refer to a stream of a liquid phase, which
is projected from nozzle 4 to surface 13. A °spray" is typically an
aerosol of droplets of the cleaning liquid in pressurized air or
another propellant. Preferred values of spray parameters have been
defined in EP99203064, filed on 15.09.1999. A "jet" is normally
30 obtained without a propellant, preferably at a liquid supply
pressure of between 50 and 150 bar and a supply rate of 20 to 60
ml/sec, more preferably 30 to 40 m1/sec.
The cleaning liquid is supplied to nozzle 4 via a supply
channel 10, which is coupled to the known devices for feeding a
35 liquid to a nozzle, such as a pump, hoses, filters, etc. In another
embodiment, the apparatus also comprises a propellant supply channel
(not shown in the figu.res~) which'forms a spray together with the
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cleaning liquid supplied via channel 10. The spray can be formed in
nozzle 4 by the known methods, e.g. by passing a flow of propellant
along the cleaning liquid as used in carburetors of combustion
engines. Nozzle 4 may comprise a mixing chamber (now shown) wherein
s the propellant and the cleaning liquid are mixed.
The jetted or sprayed liquid forms a body which may have any
shape, that is referred to hereafter as a "cone", regardless of the
specific form of said body. The treatment zone of each nozzle 4 has
an area which depends on the diverging angle of the jet or spray
so cone 14, the above mentioned distance between nozzles) 4 and
surface 13 and the angle between the axis of cone 14 and surface 13.
It should be stressed that the specific values of these angles and
said distance depend on many parameters such as the nature of the
impurities to be removed from surface 13, the composition of the
Zs cleaning liquid, and the composition and morphology of surface 13.
Without limiting the scope of the invention thereto, typical values
of the diverging angle of cone 14 may be between 5° and 50°, the
distance between nozzle 4 and opening 3 is preferably between 60 and
100 mm, and the angle between the center axis of cone 14 and surface
zo 13 may typically be between 45° and 90°. The latter angle is
determined by the angle between the center axis of housing l and
surface 13 as well as by the angle between the center axis of nozzle
4 and the center axis of housing 1. In a preferred embodiment, both
the latter angles are about 90° as shown in Fig. 2. In a preferred
25 embodiment, said diverging angle of cone 14, said distance between
nozzle 4 and opening 3 and said angle between the center axis of
nozzle 4 and the center axis of housing 1 is adjustable by the
operator or by the manufacturer of the apparatus.
The distance between nozzle 4 and surface 13 is the sum of the
3o distance between nozzle 4 and opening 3 on the one hand and the gap
which is left during operation of the apparatus between opening 3
and surface 13 on the other hand. Preferably, said gap is
essentially zero, i.e. during operation of the apparatus the edge 2
preferably touches surface 13 or is put very close to surface 13.
3s Otherwise, a substantial gap may be left between the apparatus and
.. surface 13, e.g. a few millimeters wide, so that air may flow from
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outside the apparatus into the suction chamber 9, thereby forming a
barrier against liquid or impurities leaving the apparatus.
Alternatively, air or another gas may be supplied to the gap from a
distributor (not shown) around edge 2, as described in US 5,603,775.
s If a gap is left between edge 2 and surface 13, then the rubbing
means 8 should protrude beyond edge 2 sows to maintain mechanical
contact between rubbing means 8 and surface 13
In the embodiment wherein the apparatus comprises a single jet
or spray nozzle 4, that nozzle is preferably positioned near the
so center axis of the housing 1. In another embodiment comprising a
plurality of nozzles 4, these nozzles 4 may be positioned around the
center axis of housing 1 and the axis of each nozzle 4 may be
inclined versus the axis of housing 1 so that the treatment areas of
all nozzles 4 overlap or are identical. According to still another
zs embodiment, the treatment areas of each nozzle 4 do not overlap or
overlap only to a minor extent so that the combined treatment area
of all nozzles 4 is substantially larger that the treatment area of
a single nozzle 4.
The jet or spray cone 14 of nozzle 4 intersects opening 3 to
ao form portion 5 of said opening 3. Said portion 5 has a perimeter 6
which may have any form, but preferably i.s oblong, oval or circular
(the latter is shown in Fig. 3). The treatment area has essentially
the same shape as portion 5. The treatment area coincides with
portion 5 when no gap is left between the apparatus and surface 13.
25 In the embodiment wherein the treatment area has not a circular
shape, nozzle 4 may be rotatably mounted in housing 1 so that a
circular treatment area is produced by a single revolution along the
center axis of nozzle 4.
The section of opening 3 which does not coincide with portion 5
3o forms a suction orifice that is coupled to a suction chamber 9,
which is surrounded by housing 1. Said suction orifice preferably
surrounds portion 5. Suction chamber 9 is coupled to an evacuation
channel 11 which is coupled to means for maintaining a sub-pressure
in suction chamber 9 at a level which produces the necessary suction
3s force to evacuate effectively the cleaning liquid and impurities
backwards from the suction orifice. Such means are generally known
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to the skilled person and may comprise a vacuum source such as a
pump, hoses, filters, etc.
The rubbing means 8 in the apparatus of the present invention
form a barrier between the jet or spray cone 14 and the suction
s chamber 9. More particularly, the rubbing means 8 extend from within
the apparatus towards opening 3 and are positioned along perimeter 6
of the portion 5 of opening 3,,wherein cleaning liquid is jetted or
sprayed; the rubbing means thereby form a boundary between said
portion 5 of opening 3 on the one hand and the suction chamber 9 on
so the other hand. The wording "along perimeter 6" shall be understood
as meaning that the rubbing means 8 are positioned on or nearby
perimeter 6. The rubbing means 8 may form a complete boundary or an
incomplete boundary, i.e. the rubbing means 8 may be provided along
the whole perimeter 6 or along a section or sections of perimeter 6
is (3 sections shown in Figure 3). The term °boundary" shall not be
understood as a closed physical barrier for the cleaning liquid and
impurities contained therein, since the advantageous effect of the
present invention is produced by the effect that, upon operation of
the apparatus, the jetted or sprayed cleaning liquid, which hits
2o surface 13, passes through or between rubbing means 8 before being
drawn into suction chamber 9 and evacuated. The rubbing means 8 can
be e.g. composed of a fabric or cloth, which is permeable for the
cleaning liquid, or of small rubbing bodies consisting of, covered
with or coated with a material that is capable of effecting friction
2s on surface 13, e.g. rubber, cotton, or plastic. In a preferred
embodiment, rubbing means 8 comprise brush hairs between which the
cleaning liquid can pass and enter into the suction chamber 9. An
incomplete boundary, wherein the rubbing means 8 along perimeter 6
are spaced apart, may be advantageous for a better evacuation of
3o cleaning liquid and impurities present therein.
The rubbing means 8 are rotatably mounted in the apparatus,
e.g. on a shaft 12, so that the rubbing means 8 are capable of
rotating, thereby exerting a friction on surface 13. The rotational
movement of the rubbing means 8 can be driven by the known means,
3s such as a motor. In a preferred embodiment, the rotation is driven
by the pressure of the media that are supplied to nozzle 4, such as
the cleaning liquid~o_r propellant, e.g. by providing shaft 12 with
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one or more fins or other known means such as those used in drills
driven by pressurized air.
The supply channel and the evacuation channel are preferably
connected to a supply pipe and an evacuation pipe respectively,
s which may consist of a hose. The supply pipe for fresh cleansing
liquid preferably extends inside the evacuation pipe for spent
liquid and impurities. The supply pipe and/or the evacuation pipe
may be connected to a service unit which preferably includes a tank
for fresh cleaning liquid, equipment for treating the used liquid
so containing impurities, a vacuum pump connected to the evacuation
pipe, an optional high-pressure pump connected to the supply pipe,
filters, and the necessary electronics and mechanics for driving the
service unit.
The apparatus of the present invention preferably cleans
2s surface 13 scanwise. When used for cleaning a cylindrical surface
such as a print cylinder of a printing press, the apparatus is
preferably guided along a line parallel to the axis of the cylinder
while the cylinder itself rotates and the edge of the apparatus is
held at a constant distance close to the surface. The center axis of
zo the housing 1 is preferably held perpendicular to the surface,
although other configurations are also possible. During the cleaning
operation, the axial translation of the apparatus and the revolution
speed of the cylinder are preferably driven by a control unit which
may also be coupled to the service unit that controls the rate of
as feed to the nozzle head and the rate of evacuation from the suction
chamber. Both supply and evacuation pressures are preferably
adjusted so as to obtain efficient cleaning without any liquid or
impurities penetrating out the apparatus through the gap, and
preferably also to obtain an essentially dry surface immediately
3o after the passage of the apparatus.
According to a first method of the present invention, the
cleaning of surface l3 is carried out by jetting or spraying a
cleaning liquid with an apparatus as defined above and evacuating
the cleaning liquid together with the impurities via the suction
3s chamber. The cleaning maybe achieved by chemical as well as
mechanical effects. Suitable cleaning liquids comprise solvents
wherein the impurities are dispersed or solubilized. The impact of
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droplets of the cleaning liquid may further produce a mechanical
impact on surface 13, which may be enhanced by the addition of solid
particles in the cleaning liquid, by ultrasonic treatment, etc.
However, the presence of rubbing means 8 in the apparatus of the
s present invention also enables an effective cleaning without
substantially pressurizing the cleaning liquid or the propellant. It
is normally sufficient to supply the cleaning liquid at about
atmospheric pressure to surface 13, since the action of rubbing
means 8 effects the mechanical cleaning thereof.
so In another method according to the present invention, a film of
a first cleaning liquid is applied on surface 13, e.g. by using the
apparatus of the present invention as a coating apparatus; i.e.
without engaging the vacuum suction. After a suitable period of
time, during which the cleaning liquid attacks the impurities, the
ss apparatus is used for spraying or jetting a second cleaning liquid
onto the surface and removing impurities from the surface by
engaging the vacuum suction. The second cleaning liquid can be the
same as the first cleaning liquid. Preferably, the second cleaning
liquid does not chemically attack the impurities but is only used as
ao a carrier for withdrawing the impurities from the surface, e.g.
plain water.
According to still another method of the present invention, a
film of a cleaning liquid is applied on surface 13 as described
above and, after a while, said film is removed together with
zs impurities by engaging only the vacuum suction of the apparatus,
i.e: without supplying a second cleaning liquid..
As mentioned above, the apparatus of the present invention is
particularly suited for removing ink-accepting areas from a
lithographic substrate so as to recycle said substrate which then
3o can be provided with a fresh image-recording layer. The cleaning
step can be performed on-press, i.e. while the printing master is
mounted in a printing press, or off-press, e.g. in a dedicated
cleaning device which comprises an apparatus according to the
present invention. Such a cleaning device can be mechanically
ss coupled to the printing press, i.e. the printing master can be
automatically removed from the press and conveyed to the cleaning
device by mechanica3 means so that the printing masters) can be
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exchanged without human intervention. According to a preferred
embodiment, the apparatus of the present invention is present in a
digital press, which also comprises an integrated plate-setter.
According to a most preferred embodiment, the printing press also
s comprises an on-press coating unit which applies a lithographic
coating on the substrate, which may be a plate mounted around the
plate cylinder of the press or the plate cylinder itself: After
coating, an integrated plate-setter exposes the coating image-wise
to heat or light; and after optional processing, the printing pres s
so is started. After the press-run, the ink-accepting areas, are removed
with an apparatus according to the present invention, and the
recycled substrated can then be reused in a next cycle of coating,
exposure, printing and cleaning. A.11 these steps are preferably
performed on-press, i.e. while the lithographic substrate is mounted
2s in a printing press.
The lithographic substrate used in the methods of the present
invention may be a sheet-like material such as a plate or it may be
a cylindrical element such as a sleeve which can be slid around a
print cylinder of a printing press. Alternatively, the substrate can
2o also be the. print cylinder itself. In the latter option, the image-
recording layer is provided on the print cylinder, e.g. by on-press
spraying or jetting of a coating liquid. The lithographic substrate
may be a hydrophilic support or a support which is provided with a
hydrophilic layer. Preferably, the support is a metal support such
2s as aluminum or stainless steel.
A particularly preferred lithographic substrate is an
electrochemically grained and anodized aluminum support. The
anodized aluminum support may be treated to improve the hydrophilic
properties of its surface. For example, the aluminum support may be
so silicated by treating its surface with a sodium silicate solution at
elevated temperature, e.g. 95°C. Alternatively, a phosphate
treatment may be applied which involves treating the aluminum oxide
surface with a phosphate solution that may further contain an
inorganic fluoride. Further, the aluminum oxide surface may be
ss rinsed with a citric acid or citrate solution. This treatment may
be carried out at room temperature or may be carried out at a
slightly elevated temperature of-about 30 to 50°C. A further
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interesting treatment involves rinsing the aluminum oxide surface
with a bicarbonate solution. Still further, the aluminum oxide
surface may be treated with polyvinylphosphonic acid,
polyvinylmethylphosphonic acid, phosphoric acid esters of polyvinyl
s alcohol, polyvinylsulfonic acid, polyvinylbenzenesulfonic acid,
sulfuric acid esters of polyvinyl alcohol, and acetals of polyvinyl
alcohols formed by reaction with a sulfonated aliphatic aldehyde It
is further evident that one or more of these post treatments may be
carried out alone or in combination. More detailed descriptions of
2o these treatments are given in G8-A- 1 084 070, DE-A- 4 423 140,
DE-A- 4 417 907, EP-A- 659 909, EP-A- 537 633, DE-A- 4 001 466,
EP-A- 292 801, EP-A- 291 760 and US-P- 4 458 005.
According to another embodiment, the substrate can also be a
flexible support, which is provided with a hydrophilic layer,
zs hereinafter called 'base layer'. The flexible support is e.g. paper,
plastic film or aluminum. Preferred examples of plastic film are
polyethylene terephthalate film, polyethylene naphthalate film,
cellulose acetate film, polystyrene film, polycarbonate film, etc.
The plastic film support may be opaque or transparent.
zo The base layer is preferably a cross-linked hydrophilic layer
obtained from a hydrophilic binder cross-linked with a hardening
agent such as formaldehyde, glyoxal, polyisocyanate or a hydrolyzed
tetra-alkylorthosilicate as disclosed in EP-A- 601 240,
G8-P- 1 419 512, FR-P- 2 300 354, US-P- 3 971 660, and
2s US-P- 4 284 705. It is particularly preferred to use a film support
to which an adhesion improving layer, also called subbing layer, has
been provided. Particularly suitable adhesion improving layers for
use in accordance with the present invention comprise a hydrophilic
binder and colloidal silica as disclosed in EP-A- 619 524,
so EP-A- 620 502 and EP-A- 619 525. Preferably, the amount of silica
in the adhesion improving layer is between 200 mg/m2 and 750 mg/m2.
Further, the ratio of silica to hydrophilic binder is preferably
more than 1 and the surface area of the colloidal silica is
preferably at least 300 m2/gram, more preferably at least 500 m2/.
35 gram.
The imaging material used in the methods of the present
invention comprises at_least one-image-recording layer provided on
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the lithographic substrate. Preferably, only a single layer is
provided on the substrate. The material may be light- or heat-
sensitive, the latter being preferred because of daylight-stability.
The image-recording layer of the material is preferably non-
s ablative. The term "non-ablative" shall be understood as meaning
that the image-recording layer is not substantially removed during
the exposure step. The imaging material is preferably processless,
i.e. a lithographic image is obtained inunediately after exposure
without wet processing, or it can be processed by the supply of
so dampening liquid and/or ink, i.e. simply by starting the pressrun.
The material can be positive-working, i.e. the exposed areas of
the image-recording layer are rendered removable with a processing
liquid, thereby revealing the hydrophilic surface of the
lithographic substrate which defines the non-printing areas of the
is master, whereas the non-exposed areas are not removable with a
processing liquid such as fountain and define the hydrophobic,
printing areas of the master. In a more preferred embodiment, the
material is negative-working, i.e. the unexposed areas of the image-
recording layer are removable with the processing liquid, thereby
ao revealing the hydrophilic surface of the lithographic substrate
which defines the non-printing areas of the master, whereas the .
exposed areas are not removable with the processing liquid and
define the hydrophobic, printing areas of the master. The term
'removable" indicates that the image-recording layer can be removed
2s from the lithographic substrate by the supply of processing liquid,
e.g. by dissolution of the layer in the liquid or by the formation
of a dispersion or emulsion of the layer in the liquid.
Two highly preferred embodiments of a highly preferred
negative-working image-recording layer will now be discussed.
3o In a first highly preferred embodiment, the working mechanism
of the imaging layer relies on the heat-induced coalescence of
hydrophobic thermoplastic polymer particles, preferably dispersed in
a hydrophilic binder, as described in e.g. EP 770 494; EP 770 495;
EP 770 497; EP 773 112; EP 774 364; and EP 849 090. The coalesced
ss polymer particles define a hydrophobic, printing area which is not
readily removable with dampening liquid and/or ink whereas the
unexposed layer defines a non-printing area which is readily
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removable with dampening liquid and/or ink. The thermal coalescence
can be induced by direct exposure to heat, e.g. by means of a
thermal head, or by the light absorption of one or more compounds
that are capable of converting light, more preferably infrared
s light, e.g. emitted by a solid state laser, into heat. Particularly
useful light-to-heat converting compounds are for example dyes,
pigments, carbon black, metal carbides, borides, nitrides,
carbonitrides, bronze-structured oxides, and conductive polymer
dispersions such as polypyrrole, polyaniline or polythiophene-based
zo conductive polymer dispersions. Infrared dyes and carbon black are
highly preferred.
The hydrophobic thermoplastic polymer particles preferably have
a coagulation temperature above 35°C and more preferably above
50°C.
Coagulation may result from softening or melting of the
25 thermoplastic polymer particles under the influence of heat. There
is no specific upper limit to the coagulation temperature of the
thermoplastic hydrophobic polymer particles, however the temperature
should be sufficiently below the decomposition of the polymer
particles. Preferably the coagulation temperature is at least 10°C
2o below the temperature at which the decomposition of the polymer
particles occurs. Specific examples of hydrophobic polymer particles
are a:g. polyethylene, polyvinyl chloride, polymethyl
(meth)acrylate, polyethyl (meth)acrylate,,polyvinylidene chloride,
polyacrylonitrile, polyvinyl carbazole, polystyrene or copolymers
2s thereof. Most preferably used is polystyrene. The weight average
molecular weight of the polymers may range from 5,000 to 1,000;000
g/mol. The hydrophobic particles may have a particle size from 0.01
~m to 50 um, more preferably between 0.05 ~zm and 10 ~zm and most
preferably between 0.05 dun and 2 um. The amount of hydrophobic
3o thermoplastic polymer particles contained in the image forming layer
is preferably between 20~ by weight and 65~ by weight and more
preferably between 25~ by weight and 55~ by weight and most
preferably between 30~ by weight and 45~ by weight.
Suitable hydrophilic binders are for example synthetic homo- or
as copolymers such as a polyvinylalcohol, a poly(meth)acrylic acid, a
poly(meth)acrylamide, a polyhydroxyethyl(meth)acrylate, a
polyvinylmethylether-or natural binders such as gelatin, a
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polysacharide such as e.g. dextran, pullulan, cellulose, arabic gum,
alginic acid.
In the second highly preferred embodiment, the imaging layer
comprises an aryldiazosulfonate homo- or copolymer which is
s hydrophilic and removable in dampening liquid and/or ink before
exposure and rendered hydrophobic and less removable after such
exposure. The exposure can be done by the same means as discussed
above in connection with thermal coalescence of polymer particles.
Alternatively, the aryldiazosulfonate polymer can also be switched
so by exposure to UV light, e.g. by a W laser or a UV lamp.
Preferred examples pf such aryldiazosulfonate polymers are the
compounds which can be prepared by homo- or copolymerization of
aryldiazosulfonate monomers with other aryldiazosulfonate monomers
and/or with vinyl monomers such as (meth)acrylic acid or esters
1s thereof, (meth)acrylamide, acrylonitrile, vinylacetate,
vinylchloride, vinylidene chloride, styrer~e, a-methyl styrene etc.
Suitable aryldiazosulfonate polymers for use in the present
invention have the following formula:
RZ (L)n A-N=N SO~M
R1 R°
wherein R~,1,2 each independently represent hydrogen, an alkyl
group, a nitrite or a halogen, e.g. C1, L .represents a divalent
linking group, n represents 0 or 1, A represents an aryl group and M
represents a cation. L preferably represents divalent linking group
2s selected from the group consisting of -Xt-CONR3-, -Xt-C00-, -X- and -
Xt-CO-, wherein t represents 0 or 1, R3 represents hydrogen, an alkyl
group or an aryl group, X represents an alkylene group, an arylene
group, an alkylenoxy group, an arylenoxy group, an alkylenethio
group, an arylenethio group, an alkylenamino group, an arylenamino
so group, oxygen, sulfur or an aminogroup. A preferably represents an
unsubstituted aryl group, e.g. an unsubstituted phenyl group or an
aryl group, e.g. phenyl, substituted with one or more alkyl group,
CA 02370071 2002-02-O1
s,. a
- 17 -
aryl group,, alkoxy group, aryloxy group or amino group. M preferably
represents a cation such as NH4+ or a metal ion such as a cation of
A1, Cu, Zn, an alkaline earth metal or alkali metal.
Suitable aryldiazosulfonate monomers for preparing the above
s polymers are disclosed in EP-A 339393, EP-A 507008 and EP-A 771645.
The imaging material may also comprise other layers provided on
the lithographic substrate, in addition to the image-recording
layer. The light absorbing compound may be present in another layer
close to the layer which contains the other ingredients mentioned
2o above, such as the hydrophobic thermoplastic polymer particles and
the aryldiazosulfonate polymer. Or the imaging material may comprise
a protective top layer which is removable by the processing liquid,
dampening liquid and/or ink and which provides protection against
handling or mechanical damage. A suitable protective top layer
is comprises polyvinylalcohol.
Suitable cleaning liquids and cleaning methods, which are
particularly effective for treating the above imaging materials have
been described in the following EP-AJs . EP00200176 (filing date
18.01.2000), EP00200177 (id.), EP00200178 (id.), EP00203224
so (18.09.2000), EP00204090 (21.11.2000), EP00204093 (id.) and
EP00204376 (07.12.2000). Suitable off-press cleaning methods and
equipment therefor have been described in EP00203967 and EP00203968
(both filed on 14.11.2000).
