Note: Descriptions are shown in the official language in which they were submitted.
CA 02393320 2002-07-15
METHOD FOR MAKING PRINTING PLATE BY INKJET
DEPOSITION ON POSITIYE~WORKING MEDIA
Field of the invention
The invenfron pertains to the field of lithographic printing and, in
particular, to
the making of offset printing plates using inkjet technology
~o Background of the invention
In the art of lithographic printing it is generally required that one or more
lithographic printing dates (masters) be mounted on a printing press. In the
case of wet offset lithographic printing, the lithographic printing (master)
is
~s characterized by having on its printing surface oleophilic-ink receiving
areas in
the form of the image to be printed, and hydrophilic water-receiving areas
corresponding to the other, non-printing areas of the surface. Because of the
immiscibility of oil-based lithographic inks and water, on a well-prepared
printing master, ink will fully coat the oleophilic areas of the printing
surface of
2o the plate and not adhere to the hydrophilic areas. The operating press
brings
the inked master surface into intimate contact with an impression cylinder or
elastic transfer blanket that transfers the ink image to the media to be
printed.
Lithographic printing masters generally have images that are planographic,
zs i.e., substantially flat. However, other printing plates with similar
photosensitive coatings may have raised images for relief printing or intaglio
images for gravure printing. Lithographic printing processes may use water as
described above, or they may use a waterless printing technique. If a
waterless technique is used, the discrimination between the inked and non-
~o inked areas of the plate surface is based on having different surface
energies
CA 02393320 2002-07-15
in the imaged and unimaged areas, leading to differences in oleophilicity.
Plates based on a silicone formulation, being one of a very few practical
materials that are inherently oieophobic, are typical examples.
Traditionally, a lithographic plate is photographically imaged. The plate
substrate is most commonly aluminum, treated so that the printing surface is
hydrophilic, although treated or untreated plastic or paper substrates are
also
used. The hydrophilic substrate is then coated with one or more layers of
materials which function as the imageable layer of the plate. The deposited
to coatings vary considerably and much effort has been expended by many
parties in the industry to develop coatings of increased sensitivity and
durability. Coating layers are commonly about 1 to 3 microns in thickness.
At least one of the layers of the plate coating is sensitive to light of some
is wavelength or another. Ultra-violet, visible and infrared light-sensitive
coating
compositions for lithographic printing plates are well known in the art. Many
conventional plates are ultraviolet-sensitive in the 325 nm to 400 nm range,
being based on diazo materials, and lithographic printing plates suitable for
offset printing are typically produced from these plates via processes similar
2o to a photographic process.
To prepare a lithographic precursor for use as a wet offset printing plate,
commonly referred to as a lithographic master, in order to differentiate
between the blank plate (the precursor) and the processed plate, (the master)
:!s the plate is first exposed to light in the pattern to be printed using a
photographic film negative. The exposed plate is then washed in a developing
solution. In one group of plate products, known as negative-working plates,
the exposed areas of the plate coating are insoluble and the development
process quantitatively removes the unexposed areas of the coating from the
io hydrophilic aluminum surface of the plate substrate. By convention, such a
2
CA 02393320 2002-07-15
preparation process is referred to as a negative-working process because the
unexposed coating is removed. Diazonium salt-based plates, as a specific
group, represent a typical example of conventional ultra-violet-sensitive
negative-working plates.
Conversely, in a positive-working process, the pattern to be printed is masked
and the photosensitive exposed coating is rendered soluble in a developer.
Until after the development step, the printing artisan or press operator
generally endeavors not to allow incidental exposure of the plate to typical
to white light or sunlight. Undeveloped plates are typically handled only in
low
light or "yellow light" rooms or conditions.
Traditionally, lithographic precursors have been imaged by photographic
transfer from original artwork. Unfortunately, this process is labor-intensive
is and costly. Hence, with the advent of the computer engendering a revolution
in the graphics design process preparatory to printing, there have been
extensive efforts to directly pattern printing plates, in particular
lithographic
printing plates, using a computer-controlled apparatus such as a platesetter
which is supplied with digital data corresponding to the image to be printed.
A
;!o typical platesetter has the capability to supply an image-forming agent,
typically light energy or one or more chemicals, to a precursor according to
various images as defined by digital data, i.e., to imagewise apply an image-
forming agent. The term "computer-to-plate" has been generally used to
describe such machines that are capable of directly imaging printing
2s pprecursors from computer data.
Typical computer to-plate systems variously use ablative themnal plates,
where the image is imparted to the plate by ablating away the areas that are
not to be printed (inherently positive-working), and, more recently, thermal
ao plates that are imaged with lower power laser beams that induce by various
CA 02393320 2002-07-15
mechanisms a change in the hydrophilicity or oieophilicity of the imaged area.
Typically, but not exclusively, comparatively lower cost near-infra-red diode
lasers are employed and light-to-heat converter materials are added to the
coating on the precursor to adapt the precursor to the wavelength of the
laser.
Both positive- and negative-working variants of such media have been
developed.
A special type of a computer-to-plate process involves the exposure of a
precursor while it is mounted on a plate cylinder of a printing press. This is
io done by means of an plate-setter that is integrated in the press. This
method
may be called "computer-to-press" and printing presses with an integrated
plate-setter are sometimes called digital presses. A review of digital presses
is
given in the Proceedings of the Imaging Science 8~ Technology's 1997
International Conference on Digital Printing Technologies (Non-Impact
us Printing 13). Computer-to-press (CTP) methods have been widely described
and are well known to those schooled in the art of commercial printing.
Typical plate materials used in computer-to-press methods are based on
ablation. A problem associated with ablative plates is the generation of
debris,
which is difficult to remove and may disturb the printing process or may
2o contaminate the exposure optics of the integrated image-setter. Other
methods require wet processing with chemicals. Such processes may
damage or contaminate the electronics and optics of the integrated image-
setter and other devices of the press.
zs Whereas a precursor normally consists of a sheet-like support and one or
more functional coatings, computer-to-press methods have been described
wherein a composition, capable of forming a lithographic surface upon image-
wise exposure and optional processing, is provided directly on the surface of
a plate cylinder of the press. Techniques have also been described in which a
CA 02393320 2002-07-15
coating of a hydrophobic layer is applied directly on the hydrophilic surface
of
a plate cylinder. After removal of the non-printing areas by ablation, a
master
is obtained. However, ablation should be avoided in computer-to-press
methods, as discussed above. In U.S. Patent No. 5,713,287 (Gelbart), a
computer-to-press method is described wherein an imageable medium is
applied directly on the surface of a plate cylinder. The imageable medium is
converted from a first water-sensitive or oil-sensitive property to an
opposite
water sensitive or oil-sensitive property by image-wise exposure.
io Most of the computer-to-press methods referred to above use so-called
thermal or heat-mode materials, i.e. precursors or on-press coatable
compositions, which 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
~s by cross-linking of a polymer, decomposition, or particle coagulation of a
thermoplastic polymer latex, and after optional processing, a lithographic
image is obtained.
A computer to-press method has also been disGosed in which an oleophilic
zo substance is image-wise transferred from a foil to a rotary press cylinder
by
melting said substance locally with a laser beam. The strip-shaped transfer
foil has a narrow width compared to the cylinder and is translated along a
path
which is parallel to the axis of the cylinder while being held in close
contact
with the surface of the cylinder so as to build up a complete image on that
2s surface gradually. As a result, this system is rather slow and requires a
long
downtime of the printing press, thereby reducing its productivity.
An on-press coating method has been described wherein an aqueous liquid,
comprising a hydrophilic binder, a compound capable of converting light to
CA 02393320 2002-07-15
heat and hydrophobic thermoplastic polymer particles, is coated on the plate
cylinder so as to form a uniform, continuous layer thereon. Upon image-wise
exposure, areas of the coated layer are converted into a hydrophobic phase,
thereby defining the printing areas of the printing master. Such methods of on-
press coating, on-press exposure and on-press cleaning of the master are
commercially attractive because, contrary to conventional lithographic
printing, they can be carried out without specialized training or experience.
Such presses require less human intervention than conventional presses.
io Coating of plate masters off-press has existed since the 1960's as hand-
wiped plates. This process, due to poor coating quality associated with hand
coating, has fallen out of favour given increased demand for quality printing
and has in general been replaced by pre-coated masters. In the case of
hand-coated masters, however, the substrates were not reused. There is
is value in reusing the lithographic substrate as the materials and production
of
such substrate can be costly. This becomes even more feasible for shorter
print runs where the mechanical properties of the substrate do not degrade
significantly. There is thus interest in the process of reusing lithographic
substrates by removing the lithographic master from the press, and installing
it
2o in a separate device whereby the printing surface is removed, the substrate
is
recoated, and optionally imaged for reuse in printing.
As may be seen from the foregoing, the technology of on-press imaging and
on-site platemaking has made major strides and represents a major benefit to
2s industry. However, a need remains associated with coating substrate
materials, both on-press and in dedicated off-press coating and imaging
equipment, in that operators of such facilities wish to have costs reduced as
far as possible. This has led to the need for re-usable lithogrpahic masters.
In
the case of fully on-press platemaking, the lithographic support may be the
:~o cylinder of the press itself. As this is an expensive piece of high
precision
6
CA 02393320 2002-07-15
equipment, the platernaking process employed needs to allow for the
repeated re-use of this cylinder.
Various attempts have been made to address this issue by creating cylinders
that have permanent oxide or ceramic coatings that may be switched between
various states of hydrophilicity by incident imagewise-applied radiation. The
inherent problem with all of these switchable drum technologies is that
inadequate lithographic latitude is obtained in that the variation in
hydrophilicity induced in the permanent oxide or ceramic layer is simply
io inadequate to produce a reliable industrial result outside the laboratory
under
practical pressroom conditions.
Heretofore, many of the new CTP systems have been relatively large,
complex, and expensive, being characterized by having sophisticated servo-
~s mechanics and optics in order to both manage the light from laser arrays
and
provide the required resolution on the plate over large areas. They are often
used by larger printing companies as a means to streamline the prepress
process of their printing operations, and to take advantage of the rapid
exchange and response to the digital information of graphic designs provided
2o by their customers. There remains a strong need for a lower gist economical
and efficient CTP system for the many smaller printers who utilize
lithographic
printing.
In recent years, inkjet printers have replaced laser printers as the most
2s popular hare) copy output printers for computers. Inkjet printers have
several
competitive advantages over laser printers. One advantage is that, as a result
of semiconductor processing technological advances, it is possible to
manufacture arrays of hundreds of inkjet nozzles spaced very accurately and
closely together in a single inexpensive printhead. This nozzle array
~o manufacturing capability enables fast printing inkjet devices to be
CA 02393320 2002-07-15
manufactun3d at a much lower gist than laser printers requiring arrays of
lasers. The precision with which such a nozzle array can be manufactured,
combined with the jetting reliability of the incorporated nozzles, allow these
arrays to be used to print high quality images comparable to photo or laser
imaging techniques. Inkjet printers are increasingly being used for prepress
proofing and other graphic arts applications requiring very high quality hard
copy output. In spite of the large and rapidly-growing installed base of
inkjet
printers for hard copy output, inkjet printing technology is not commonly used
in CTP systems.
to
There are many challenging technical requirements facing the practitioner
who would design such an inkjet-based CTP system, as can be seen in the
prior art. A first requirement is that the inkjet ink used to image the
printing
plate be jettable, able to form ink drops of repeatable volume and in an
is unvarying direction. Further, for practical comrrtercial application, the
ink must
have a long shelf life, in excess of one year or more.
US 5,970,873 (DeBoer et al.) describes the jetting of a mixture of a sol
precursor in a liquid to a suitably-prepared printing substrate. However, any
ao ink constituents of limited solubility will render unlikely the practical
formulation
of a jettable, shelf stable ink. Similar problems exist in US 5,820,932
(Hallman et al.), in which complex organic resins are jetted, and US 5,738,013
(Kellet) in which marginally-stable transition metal complexes are jetted.
:!s Another requirement is that, to be of wide utility, the inkjet-based CTP
system
must be able to prepare printing plates with small printing dots,
approximately
75 microns in diameter or smaller, so that high resolution images can be
printed. Inkjet printers can produce such small dots, but of those having
substantial commercial acceptance, only inkjet printers employing aqueous-
3o based inks and other low viscosity carriers or solvents are practically
capable
CA 02393320 2002-07-15
of printing such small dots. Thus, the systems described in US 4,003,312
(Gunther), US 5,495,803 (Gerber), US 6,104,931 (Fromson et al.), and US
6,019,045 (Kato), which use high viscosity hot melt inks, will not allow the
preparation of the high resolution printing plates necessary for printed
images
of high quality.
It is also required that the prepared printing plates be rugged, capable of
sustaining press runs of many thousands of impressions. The waxes used in
the hot melt inks described in US 6,019,045 (Kato) and US 4,833,486 (Zerillo)
to would wear out in such a long press run.
Another requirement of a successful inkjet-based CTP system is that a mature
plate technology is to be preferred. There are many tradeoffs in the
manufacture of commercially-practical lithographic precursors. They must be
us highly sensitive to the imaging process and yet thermally stable, stable in
high
humidity storage environments and yellow light, resistant to fingerprints, of
minimal toxicity and environmentally benign, easily developed in that small
dots are quantitatively resolved without dot blooming using developers that
are of minimal toxicity and environmentally benign, able to sustain long press
?o runs, manufacturable at a low cost per square foot, and many other
practical
requirements.
US 5,695,908 (Furukawa) describes a process for preparing a printing plate
comprising a new plate coating containing a water-soluble polymer that
:~s becomes water-insoluble in contact with a metal ion in a solution jetted
imagewise. But such a new plate coating is unlikely to meet the wide array of
constraints on a successful plate technology. US 6,025,022 (Matzinger)
describes a new plate coating on a glass substrate that would be unlikely to
find wide acceptance.
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While there is a considerable body ofi art on the subject of platemaking via
inkjet, those that address the making of non-relief lithographic ( that is,
non-
gravure and non-flexographic) plates very often focus on depositing the
material that is to form oleophilic ink-bearing areas. Some processes are also
specifically directed to the making of waterless plates in this way. In a more
limited number of cases there is some form of chemical reaction, either
between different inkjetted materials, or between inkjetted materials and
materials pre-coated on the plate, to cn3ate, via this reaction, a third
material
composition which is either removed by development (positive-working) or
to which creates the areas to be inked (negative-working). Some inventions
employ special additives to the ink or special chemicals on the plate surface
to trigger, enhance or stimulate this process.
In US 6,315,916 (Deutsch,et al.) describes an example of a reactive process
is for preparing wet offset lithographic plates by inkjet imaging of
presensitized
plates comprising °diazo" compounds. According to this process, an
alkaline
or chemically basic ink comprising one or more suitable pH-elevating
chemicals is imagewise jetted onto a lithographic plate having a coating
comprising "diazo" compounds. The latent image on the plate is cured by
,o heating, and next developed by washing with a conventional chemical
development solution.
Brief Summary of the Invention
zs The invention provides a method for making a negative-working lithographic
master using a positive-working radiation-imageable medium on a hydrophilic
base. The method comprises imagewise inkjet deposition of droplets of
masking fluid on the surface of the posi#ive-working radiation-imageable
medium, forming a mask that is substantially resistant to a developer. The
3o exposed areas of the medium, not coven3d by the mask, are then exposed to
io
CA 02393320 2002-07-15
a developer capable of removing the exposed medium, such as a quaternary
nitrogen compound in an aqueous carrier. As a result, those areas that are
written with the masking fluid will, at the end of plate processing, be
hydrophobic and will render an image during wet lithographic offset printing.
s Normally, when imaged with actinic radiation in the absence of a mask, the
radiation-imageable lithographic medium is positive-working. By the method of
the present invention, the same material, however, allows a negative-working
lithographic master to be made. The combination of positive-working
radiation-imageable medium and masking fluid of this invention may be used
io in the fully on-press fabrication of a negative-working lithographic
master,
which may optionally also be made on a re-usable base.
In an alternative embodiment of the invention, the masked medium is flood
illuminated, for example with UV radiation, and the unmasked areas of the
is medium are then exposed to a developer capable removing such irradiated
medium, for example an aqueous alkaline solution.
Detailed Description of the Preferred Embodiments
zo The method of the present invention comprises the use of a positive-working
offset lithographic precursor, in the form of a hydrophilic lithographic base
coated with positive-working radiation-imageable medium, to make a
negative-working lithographic master. A latent image is created in the coating
of the lithographic precursor by imagewise depositing of a chemical forming a
:as mask that is substantially resistant to a developer, the mask being in the
form
of the image to be printed. This causes the masked area of the coating to be
largely unaffected by the developer. The unmasked area of the coating is
removable by the developer, without the need for irradiation with actinic
radiation. It is simplest and prefer-ed to formulate a masking fluid
comprising
n
CA 02393320 2002-07-15
the masking chemical, and to use an inkjet printer for the imagewise
application of the masking fluid.
In this specification, the phrase " the mask being in the form of the image to
be printed" is used to describe a situation where the mask is specifically the
positive image of the image to be printed. The mask only needs to resist the
developer to such a degree that, during the period required to remove
positive-working radiation-imageable medium that is exposed to the
developer, the positive-working radiation-imageable medium coated with the
io mask is not sign~cantly removed.
In this specification, the term "negative-working radiation-imageable medium"
is used to describe a medium which, when coated as a layer on a lithographic
base, will be hydrophilic and will become hydrophobic, or be removable by a
~s developer to reveal an underlying hydrophilic surface, when irradiated with
that particular radiation. Conversely, the term "positive-working radiation-
imageable medium" is used to describe a medium which, when coated as a
layer on a lithographic base, will be hydrophobic and will either become
hydrophilic, or be removable by a developer to reveal an underlying
zo hydrophilic surface, when irradiated with that particular radiation. It
will be
understood that, in a preferred method of the present invention, the positive-
working radiation-imageable medium is not irradiated with radiation to
produce an image; rather, the positive-working radiation-imageable medium
(more precisely, the unmasked part of it) is removed by a developer to
zs produce the desired image.
In this specification, the term "negative-working lithographic master" is used
to
describe a lithographic master on which, during the process of transferring
printing ink from the master to a printing medium for receiving printing ink,
the
:3o printing ink adheres to those areas that were irradiated or written to in
any
12
CA 02393320 2002-07-15
way whatsoever by an imaging head and, conversely, on which printing ink
does not adhere to those areas that were not irradiated or written to in any
way by that imaging head. Whether the master is referred to as negative-
working or positive-working is therefore not determined by the means of
creating ink-bearing and non-ink-bearing areas on the master, but rather on
whether the positive image to be created on the printing medium for receiving
the printing ink, or the negative of it, is transferred to the master from the
imaging head. In brief, on a "negative-working lithographic master", those
areas that are written by the imaging head will carry printing ink.
to
In accordance with the preferred embodiment of the invention, a computer-to-
plate system comprising an inkjet printer and a conventional developing
processor machine is used. In the most preferred embodiment, the inkjet
printer used is a commercially-available drop-on-demand printer capable of
is printing small ink drops, such as, for example, the EPSON Stylus Color 3000
inkjet printer available from Epson America, Inc., Long Beach, Calif. In an
alternative embodiment a continuous inkjet printer head can be used such as
those supplied by Iris Graphics of Billerica, Massachusetts. However, the
great flexibility available to the practitioner in formulating a masking fluid
zo according to the invention means that a well-performing jettable masking
fluid
can be formulated such that the printhead of almost any inkjet punter will be
able to form regular drops with good reliability.
The imagewise-deposited masking fluid applied to the positive-working offset
:is lithographic precursor imagewise coats the layer of positive-working
radiation-
imageable medium. In the affected areas of the coating, a layer is created
which is substantially impermeable to the developer. The mask is preferably
non-transmissive to UV radiation. The printed areas may exhibit a slight color
change, which may be the result of an indicator dye added to the masking
3o solution for the purpose of enabling inspection of the imaged plate or
before
13
CA 02393320 2002-07-15
development. When the precursor is processed or washed with the developer,
the unprinted areas of the coating are quantitatively removed, leaving bare
the
hydrophilic aluminum base of the positive-working plate, while the previously
masked areas of the coating are hydrophobic and can carry printing ink. The
lithographic master so produced is therefore a negative-working lithographic
master, suitable for wet lithographic offset printing.
The masking fluid comprises a liquid vehicle, typically water or a mixture of
water and another solvent or alternatively solvents such as methyl ethyl
~o ketone, ethyl acetate, dimethyl formamide, acetone, simple alcohols, and
other like chemicals, or mixtures of such chemicals, provided the solvent or
mixture of solvents does not substantially remove the imageable coating of
the lithographic plate. The vehicle also contains an active masking
ingredient.
is
A class of compounds useful in a masking fluid are nitrogen-containing
compounds wherein at least one nitr~en atom is either quartemised, or
incorporated in a heterocyclic ring, or both quartemised and incorporated in a
heterocyclic ring.
Examples of useful quarternised nitrogen-containing compounds are triaryl
methane dyes such as Crystal Violet (CI basic violet 3) and Ethyl Violet,
Basic
Blue 7 and tetraalkyl ammonium compounds such as Cetrtmide and
benzotrimethyl ammonium salts. Examples of suitable nitrogen-containing
zs heterocyclic compounds are quinoline and triazoles, such as 1,2,4-triazole,
imidazoles, and indoles. Examples of suitable quartemised heterocyclic
compounds are imidazoline compounds, such as Monazoline C, Monazoline
O and Monazoline T, which are manufactured by Mona Industries,
quinolinium compounds, such as 1-ethyl-2-methyl quinolinium iodide and 1-
3o ethyl-4-methyl quinolinium iodide, and benzothiazolium compounds, such as
14
CA 02393320 2002-07-15
3-ethyl-2-methyl benzothiazolium iodide, and pyridinium compounds, such as
cetyl pyridinium bromide, ethyl viologen dibromide and fluoropyridinium
tetrafluoroborate. Usefully, the quinolinium or benzothiazolium compounds
are cationic cyanine dyes, such as Dye A, Quinoldine Blue and 3-ethyl-2-[3-
(3-ethyl-2(3H)-benzothiazoylidene)-2-methyl-1-propenyl]benzothiazolium
iodide.
A further useful class of compounds suitable for use in a masking fluid are
carbonyl functional group-containing compounds. Examples of suitable
~o carbonyl-containing compounds are flavones, such as 7,8-benzoflavone,
trihydroxyflavone, and naphthaflavone; flavanones or isoflavanones, for
example hydroxy-dimethoxyflavanone; coumarins; chromones; indeneones;
chalcones; xanthones; thioxanthones; benzophenones; phthalimides; and
phenanthrenequinones.
is
Other suitable classes of materials are potysubstituted siloxanes such as
polyphenylsiloxane, substituted pyrans and the like and perfluorinated
compounds.
2o Another useful com~und for use in a masking fluid is acridine orange base
(CI solvent orange 15).
A preferred ink comprises a vehicle with 0.5 to 5.0 weight percent of active
masking ingredient.
zs
Various additions may be rr~de to the masking fluid employed in the present
invention in order to improve its functioning. For reliable jetting, and so
that
during idle periods the masking fluid does not dry out in the inkjet nozzle
causing it to clog, a humidifying co-solvent may be added to the masking
fluid.
3o The co-solvent can be polyhydric alcohols such as glycerin, ethoxylated
is
CA 02393320 2002-07-15
glycerin, ethylene glycol, diethylene glycol, triethylene glycol, propylene
glycol,
dipropylene glycol, or trimethylol propane, other high boiling point liquids
such
as pyrrolidone, methylpyrrolidone, or methanol amine, other simple alcohols
such as isopropyl alcohol or tertiary butyl alcohol, or mixtures of such
s solvents. When used, the co-solvent would typically comprise 1 to 40 percent
weight of the masking fluid.
A dye compatible with the vehicle may also be added at a level of a few
percent to enhance the visibility of the latent image. This is useful in
to facilitating the inspection of the imaged plate, before it is subjected to
the
developer.
The masking fluid may also optionally contain one or more surfactants or
wetting agents to control the surface tension of the masking fluid, enhancing
is jettability, and to control the spread of the drop on the coated plate. The
surfactants and wetting agents may include Iconol DA, Iconol NP, Iconol OP,
Iconol TDA, Surfionyl TDA, Surfonyl TG-E, Strodex, Cal-Fax, Tergitol TMN,
Tergitol X, Tergitol 15-S, IPA, )so-butanol, and similar chemicals or mixtures
of similar chemicals. When used, surfactants and wetting agents typically
zo comprise 0.001 to 10 percent of the masking fluid.
The masking fluid may additionally contain biocides to prolong the shelf life
of
the ink. Suitable biocides include for Kathon PFM, CanGuard 409, Sumquat
6020, and similar chemicals or mixtures of such chemicals. When used, the
:~s biocide would typically comprise 0.1 to 3 percent of the masking fluid.
A preferred formulation for a masking fluid comprises:
Water 67% : Co-solvent 25%: mask 3% : dye 2% : surfactant 2% : Biocide
1%
:30
16
CA 02393320 2002-07-15
Regarding the lithographic plates employed in the present invention, and,
more particularly, the coatings of positive-working radiation-imageable
medium on them, 1,2-naphthoquinone-2-diazide-4- or -5-sulfonyl derivatives
are preferably used as the quinonediazide. The esters are particularly
preferred. Suitable naphthoquinonediazides are known from US 3,106,465,
US 3,180,733 and US 4,266,001. The quantity of the naphthoquinonediazide
compounds in the positive-working radiation-imageable medium is generally
between about 3 and 50, and preferably between about 8 and 25, percent by
weight, relative to the content of the nonvolatile constituents.
uo
The novolak polycondensates, proven in many positive copying materials
based on 1,2-quinonediazides, have here again proved to be advantageous
as binders in the positive-working radiation-imageable media compositions.
The novolaks additionally can have been modified in a known manner by
~s reaction of a part of their hydroxyl groups with, for example, chloroacetic
acid,
isocyanates, epoxides or carboxylic acid anhydrides. Further alkali-soluble or
alkali-swellable binders are polyhydroxyphenyl resins that are prepared by
condensation from phenols and aldehydes or ketones, or polymers or
copolymers of styrene and malefic anhydride, or polyvinylphenols.
zo Advantageously, a polymer or copolymer of an acrylic or methacrylic acid
ester with a polyhydric phenol can be used. The nature and quantity of the
alkali-soluble resin can differ depending on the intended use; preferably,
proportions of between about 90 and 30, and especially between about 85
and 55, percent by weight of total solids are preferred. Additionally,
polymers
:~s made from the reaction between polyvinyl alcohol and hydroxyphenyl-
substituted aldehydes and other aldehydes such as butyraldehyde can be
used.
The binders used are preferably those that have a content of phenolic
3o hydroxyl groups in the range from about 1 to 15 mmol/g and a molecular
m
CA 02393320 2002-07-15
weight below about 100,000, especially in the range from about 5,000 to
100,000.
Numerous other resins can also be used in combination. The combination of a
s cresol/formaldehyde novolak and an unplasticized, preferably alkyl-
etherified
melamine/formaldehyde resin has proved to be particularly advantageous. In
addition, epoxy resins and vinyl copolymers of the monomers on which they
are based, and hydrogenated or partially hydrogenated colophony derivatives
can also be present as resins. The advantageous proportion of these resins
uo depends on the application requirements and on the effect on the
development conditions. The proportion is generally not more than about 40,
and preferably about 1 to 20, percent by weight, relative to alkali-soluble
binder. For special requirements, such as flexibility, adhesion, gloss and
coloration, the layer of positive-working radiation-imageable medium can also
is contain small quantities of substances such as polyglycols, cellulose
derivatives such as ethylcellulose, wetting agents, dyes, adhesion promoters
and finely disperse pigments and also, if required, UV-absorbers.
Small amounts of materials may be included for the color change during
zo exposure to UV radiation when the plates are used in their normal roles as
ultraviolet-sensitive plates. These materials are present but typically are
unaffected by the materials used to produce the plates made by the process
of the present invention. Examples of these materials are cationic
triarylmethane dyes and methine dyes. The radiation-sensitive components
zs are, for example, 1,2 -naphthoquinonediazide-4-sulfonic acid chloride,
chromophorically-substituted halogenomethyl-s-triazines or diazonium
compounds in the form of their salts with complex acids such as tetrafluoboric
acid or hexafluorophosphoric acid.
3o For coating a suitable lithographic base, i.e., for producing the positive-
is
CA 02393320 2002-07-15
working radiation-imageable medium in a form to be coated, the mixture is
generally dissolved in a solvent. The choice of solvent must be matched to the
intended coating method, to the layer thickness and to the drying conditions.
Suitable solvents are ketones such as methyl ethyl ketone, chlorinated
s hydrocarbons such as trichloroethylene and 1,1,1-trichloroethane, alcohols
such as n-propanol, ethers such as tetrahydrofuran, alcohol ethers such as
ethylene glycol monoalkyl ethers and propylene glycol monoalkyl ethers, and
esters such as butyl acetate or propylene glycol alkyl ether-acetate. Mixtures
can also be used that additionally, for special purposes, contain solvents
such
to as acetonitrlle, dioxane or dimethylformamide. In principle, all solvents
can be
used that do not react irreversibly with the layer components. Partial ethers
of
glycols, especially ethylene glycol monomethyl ether and propylene glycol
methyl ether, are particularly preferred.
is The lithographic bases used in most cases are metals. The following are
used
preferably for offset printing plates: bright-rolled, mechanically or
electrochemically roughened aluminum which may have been anodized and
which additionally can have been pretreated chemically, for example, with
polyvinylphosphonic acid, silicates, phosphates, hexafluorozirconates or with
zo hydrolyzed tetraethyl orthosilicate.
The coating of the layer support is effected in a known manner by spraying,
dipping, roller application, by means of slot dies, blade-application or
coater-
application.
:2s
The developer used is one capable of removing the unmasked areas of the
medium without the need for any irradiation of the medium, while not
removing the masking. A preferred class of compositions having such
capability is quaternary nitrogen compounds in a carrier such as water. A
3o preferred such composition is tetramethylammonium hydroxide in water. The
19
CA 02393320 2002-07-15
composition is preferably 5 - 25% tetramethylammonium hydroxide and more
preferably 10 - 20%, by volume.
The invention also relates to a process for producing a lithographic master by
exposing the positive-working radiation-imageable medium to the mask fluid,
then developing with an aqueous-alkaline solution, and gumming, which
process comprises using a recording material according to the invention.
After application of the masking fluid, the plate may be optionally heated.
to Such optional heating may be to temperatures between 40 and 130 C. for
befinreen 10 seconds and 3 minutes, for the purposes of curing the masking
fluid to create the mask. After the application of the masking fluid and
optional
heating, the plate is developed either by hand or preferably with a
conventional developing processor using a conventional developing solution.
is The plate is then gummed in accordance with processes well known to those
skilled in the art of offset platemaking to produce a plate ready for
printing. If
desired, because of the printing application, the developed plate may be post-
baked, or gummed and post-baked to achieve an additional toughening of the
plate coating. These treatments are well known to practitioners of the art. An
2o example treatment is described in patent GB1,513,368.
Plates coated with solvent-home photosensitive polymer solutions not
containing photosensitizing quinonediazide resins can also be prepared
according to the invention if the jetted masking fluid forms a well-defined
dot
2s on the coating without spreading excessively and a differential in
solubility in
the developer is obtained.
To facilitate accurate imaging of the precursor, the paper-handling or
substrate-handling subsystem of the inkjet printer should have a short,
3o straight paper path. A printing precursor is generally stiffer and heavier
than
CA 02393320 2002-07-15
the paper or media typically used in commercially-available inkjet printers.
If
the precursor fed into the printer mechanism must bend before or after being
presented to the imaging printhead, the movement of the precursor through
the printer may not be as accurate as the media for which the printer was
designed. The most preferred EPSON Stylus Color 3000 has such a short,
straight paper path. A platen is preferably placed at the entrance to the
paper
feed mechanism. The platen supports the presursor as it is pulled into the
printer by the mechanism, facilitating the accurate transport of the plate
under
the imaging printhead.
to
The combination comprising the positive-working radiation-imageable medium
and masking fluid may optionally be used in an apparatus that combines the
making of the a positive-working offset lithographic precursor and the
imagewise deposition of the masking fluid. As described in US 5,713,287
is (Gelbart), a cylindrical hydrophilic lithographic base may be coated with a
positive-working offset radiation-imageable medium and the coated layer may
be cured to create the positive-working offset lithographic precursor. The
inkjet deposition of the masking fluid may then be performed. The hydrophilic
lithographic base may be a plate or a sleeve that fits on or over a cylinder
or
2o drum. The plate and sleeve may be re-usable. Instead of using a separate
plate or sleeve as hydrophilic lithographic base, a suitably hydrophilic drum
or
cylinder may be employed. The entire plate-making arrangement may be
incorporated on a press to create a fully on-press plate-making facility. The
hydrophilic lithographic base may also be re-usable upon removal of any
2s existing imaged areas of positive-working offset radiation-imageable
medium,
this being particularly useful in the case where the drum or cylinder itself
provides the hydrophilic lithographic base.
In an alternative embodiment, rather than removing the unmasked areas of
3o the radiation-imageable medium from the lithographic base by means of a
21
CA 02393320 2002-07-15
developer capable of doing so without irradiation of tile medium, the
radiation-
imageable medium is, after the masking step, flood illuminated with radiation
of a suitable wavelength, for example ultraviolet, and the unmasked areas are
then removed using a conventional developer of the type capable of removing
s the irradiated medium. Such developers are well known in the art and are
generally aqueous alkaline solutions of graded alkalinity, preferably having a
pH in the range of 10 - 14, which can also contain small quantities of organic
solvents, surfactants and sequestering agents. Any of various commercially-
available developers used for the development of positive-working radiation-
:to imageable media can be used, including GOLDSTAR PLUS PD (trademark)
manufactured by Eastman K~iak Company of Rochester, New York, EP26
(trademark) manufactured by Agfa-Gavaert of Mortsel, Belgium, and DP4 and
DP5 (trademarks) manufactured by Fuji Hunt Photographic Chemicals, Inc. of
Allendale, New Jersey.
is
There have thus been outlined the important features of the invention in order
that it may be better understood, and in order that the present contribution
to
the art may be better appreciated. Those skilled in the art will appreciate
that
the conception on which this disclosure is based may readily be utilized as a
2o basis for the design of other methods and apparatus for carrying out the
several purposes of the invention. It is most important, therefore, that this
disclosure be regarded as including such equivalent methods and apparatus
as do not depart from the spirit and scope of the invention.
22
