Note: Descriptions are shown in the official language in which they were submitted.
21396~s~
--1
Image Formation
This invention relates to the formation of images
directly from electronically composed digital sources
and is concerned with the formation of images in this
way in dry planography in which there is used a
planographic printing plate having background or non-
image surface areas which, although not moistened by
water or other liquid, will not accept ink.
For many years it has been a long term aim in the
printing industry to form images directly from an
electronically composed digital database, i.e. by a so-
called "computer-to-plate" system. The advantages of
such a system over the traditional methods of making
printing plates are:
(i) the elimination of costly intermediate
silver film and processing chemicals,
(ii) a saving of time, and
(iii) the ability to automate the system with
consequent reduction in labour costs.
The introduction of laser technology provided the
first opportunity to form an image directly on a
printing plate precursor by scanning a laser beam
across the surface of the precursor and modulating the
beam so as to effectively turn it on and off. In this
way, radiation sensitive plates comprising a high
sensitivity polymer coating have been exposed to laser
beams produced by water cooled W argon-ion lasers and
electrophotographic plates having sensitivities
stretching into the visible spectral region have been
successfully exposed using low-powered air-cooled
argon-ion, helium-neon and semiconductor laser devices.
Imaging systems are also available which involve a
sandwich structure which, on exposure to a heat
generating infra-red laser beam, undergoes selective
(imagewise) delamination and subsequent transfer of
materials. Such so-called peel-apart~systems are
213~64~
--2--
generally used as replacements for silver halide films.
It is also known to overcome difficulties inherent
with conventional lithography - the best known form of
planography - via the use of planographic printing
plates which do not require a dampening of the printing
plate with an aqueous fountain solution to effectively
wet the non-image areas. Such so-called "driographic"
plates are typically derived from radiation sensitive
plates comprising a substrate coated with a
photosensitive layer, said layer being overlaid with a
coating of a low surface energy material, which is
repellant to printing ink. Alternatively, the ink-
repellant layer may be applied directly to the
substrate, with the photosensitive layer being coated
over the ink-repellant layer. As a further
alternative, a single photosensitive, ink-repellant
layer containing, for example, a photosensitive
silicone polymer, may be coated on a suitable
substrate. Several such driographic systems are
described, for example, in British Patent Nos.
1,522,228 and 2,034,911 and US Patent Nos. 3,511,178,
3,677,178, 3,894,873, 4,225,663 and 4,724,195.
Radiation sensitive plates for driography as
described in the prior art are imaged, in general, by
selective removal of areas of the ink-repellant coating
to reveal an oleophilic surface which readily accepts
printing ink. This may be accomplished in the case of
two layer systems, for example, by photochemically
changing the adhesion of the ink-repellant layer by
exposing the plate to ultraviolet radiation though a
photographic positive or negative transparency; the
photosensitive layer may be either positive-working, in
which case the adhesion of the ink-repellant layer will
be strengthened, or negative-working, in which case
adhesion will be weakened. In either case, the most
weakly adhered coating may subsequently be removed by
213964~
development with a processing solution. Alternatively,
plates having a single photosensitive, ink-repellant
layer may be imagewise exposed to harden the image
areas and then be developed with a processing solution
to remove unhardened coating.
Such plates may be used to produce high quality
images, but the required imagewise exposure via a
photographic transparency detracts from the efficiency
of the process. Similar disadvantages are associated
with the attendant development step using a processing
solution.
A digital imaging t~chn;que has been described in
US Patent No. 4,911,075 whereby a driographic plate is
produced by means of a spark discharge. In this case,
a plate precursor comprising an ink-repellant coating
cont~i ni ng electrically conductive particles coated on
a conductive substrate is used and the coating is
ablatively removed from the substrate. Unfortunately,
however, the ablative spark discharge provides images
having relatively poor resolution.
It is known to improve this feature by the use of
lasers to obtain high resolution ablation as described,
for example, by P.E. Dyer in "Laser Ablation of
Polymers" (Chapter 14 of "Photochemical Processing of
Electronic Materials", Academic Press, 1992. p 359-
385). Until recently, imaging via this method
generally involved the use of high power carbon dioxide
or excimer lasers. Unfortunately, such lasers are not
well-suited to printing applications because of their
high power consumption and excessive cost, and the
requirement for high pressure gas-handling systems.
Recent developments have, however, led to the
availability of more suitable infra-red diode lasers,
which are compact, highly efficient and very economical
solid state devices. High power versions of such
lasers which are capable of delivering up to 3000
213964~
mJ/cm2 are now commercially available.
Coatings which may be imaged by means of ablation
with infra-red radiation have previously been proposed.
Thus, for example, a proofing film in which an image is
formed by imagewise ablation of a coloured layer on to
a receiver sheet is described in PCT Application No.
90/12342. This system is, however, disadvantageous in
requiring a physical transfer of material in the
imaging step, and such methods tend to give rise to
inferior image resolution.
It is an object of the present invention to
provide a driographic printing plate precursor which
does not require any wet processing and which can be
imaged digitally by means of an infra-red diode layer
or a YAG laser to provide an image having high
resolution.
It is a further object of the present invention to
provide a driographic printing plate precursor on which
the image is formed directly through the elimination of
unwanted material, rather than by means of transfer
from another substrate, and which may be imaged
directly on a printing press.
According to one aspect of the present invention,
there is provided a driographic plate precursor
comprising a base substrate carrying an infra-red
radiation ablatable ink repellant coating, the coating
being covered with a transparent cover sheet.
According to another aspect of the present
invention there is provided a method of image formation
which comprises
(a) providing a driographic plate precursor
comprising:
a base substrate carrying an infra-red
radiation ablatable ink repellant coating
covered with a transparent cover sheet,
(b) image-wise exposing the precursor by
2139~4~
--5
directing the beam from an infra-red laser at
sequential areas of the coating so that the
coating ablates and loses its ink repellancy in
those areas to form an image,
(c) removing the cover sheet and ablation
products, and
(d) inking the image.
The infra-red radiation ablatable ink repellant
coating may be in the form of a single layer including
an infra-red radiation ablatable moiety and an ink
repellant moiety. Generally, however, the coating is
in the form of at least two layers, viz one or more
layers providing the coating with its ablatable
properties and a further outer layer providing the
coating with its ink repellant properties.
The ablatable properties may be provided by a
layer including an ablatable polymer which also absorbs
infra-red radiation. Alternatively, an ablatable
polymer which does not significantly absorb infra-red
radiation can be used in combination with a material
which absorbs infra-red radiation. In this case, the
infra-red radiation absorbing material may be present
in the same layer as the ablatable polymer or in
another layer adjacent thereto. Typically, up to 60
of infra-red absorbing material may be present and
typical coating weights for the ablatable layer(s) are
from 0.1 to 5.0 g/m2. At least one of the base
substrate and the ablatable layer or layers should have
ink-receptive properties so that the image produced an
exposure is ink-receptive. Thus, the coating may be a
simple mixture of an ink repellant component and an
infra-red radiation ablatable component or an
interpenetrating polymeric network formed in situ on
the substrate. Another alternative comprises a random
block on graft copolymer of an ink repellant monomer
units and ablatable monomer units containing infra-red
213~
--6
absorbing material.
The plate precursor may contain one or more
additional layers so as to increase adhesion of the
coating to the substrate or to increase adhesion
between layers, improve resistance to abrasion, or
enhance the performance of the system in other
respects.
The base substrate employed in the present
invention can be any substrate suitable for the
preparation of printing plates. Examples of such
substrates are paper, plastics materials, metals, or
plastics materials or paper coated with metal by
lamination or by any other suitable method. It is
advantageous to employ substrates with good rigidity to
preserve dimensional stability. Preferred substrates
are metals, or plastics, or metallised plastics where
there is a need for high reflectivity for infra-red
radiation so as to 'trap' the radiation in the
overlying coating.
Suitable polymers for the ablatable layer(s) are
those which thermally decompose to products which are
volatile at ambient temperature. A wide variety of
polymers has been found to function acceptably.
Suitable examples include self-oxidising binders such
as nitrocellulose and modified nitrocellulose as
described in, for example, Cellulose and its
Derivatives, by Ister and Flegien; non-self-oxidising
binders, for example ethylcellulose, (meth)acrylic
polymers and copolymers, such as poly(methyl
methacrylate), poly(hydroxyethyl methacrylate), poly(n-
butyl acrylate), poly(lauryl acrylate-co-methacrylic
acid), etc., many of which are available commercially
as Elvacite~ resins from Du Pont or Neocryl~ resins
from ICI; styrenic resins such as polystyrene and
poly(alpha methyl)styrene; rubbers based on isoprene;
poly(vinylbutyrates) and poly(hydroxy butyric acid);
~ 2139~14
--7--
beta lactones; polylactic acid, and its copolymers with
glycolic acid and valeric acid, and other polyesters;
vinyl chloride-vinyl acetate copolymers; polyurethane;
polycarbonates and carbonate copolymers; polysulphones
and sulphone copolymers. In general acrylic polymers
and self-oxidising polymers are most preferred. Also
preferred are resins which decompose according to so-
called "chemical amplification" schemes described by
Frechet et al (J. Imaging. Sci., 30(2), (1986), 59-64);
Ito and Willson ("Polymers in Electronics", ACS
Symposium Series, 242, T. Davidson, Ed., ACS,
Washington, DC, (1984) p. 11); E. Reichmanis and L.F.
Thompson (Microelectronic Engineering, 13, (1991), 3-
10); and others. In these systems, acid-catalysed
degradation of specific ester, ether, or carbonate
moieties leads to generation of carbon dioxide and
other gases, and regeneration of the catalyst acid,
providing amplification. Specific polymers conforming
to chemical catalysis structural criteria are
poly(propylene carbonate) (supplied as QPA0~ by Air
Products, Allentown, Pa.), poly(4-t-
butoxycarbonyloxystyrene sulphone), poly(p-tert-
butoxycarbonyloxy-alpha-methylstrene), and various
polyurethanes which are described in PCT Application
No. 90/12342. Systems have also been described by Y.
Jiang and J.M.J. Frechet (Macromolecules, 24 (12),
1991, 3528-32) and C.G. Wilson et al (Proc. IUPAC,
Macromol, Symp., 28th, 1982, 448) in which a polymer
with a ceiling temperature lower than the ambient is
end-capped, conferring ambient stability. Removing the
protecting group or otherwise initiating degradation
drops the ceiling temperature and the polymer
spontaneously decomposes. Specific examples of such
polymers are acetyl-capped poly(phthalaldehyde) and
poly(vinyl-tert-butyl carbonate sulphone).
In the case where it is necessary to include a
~ 1 ~ 9 6 4 4
material capable of sensitising the ablatable polymer
to infra-red radiation, this may be any suitable
substance which absorbs infra-red radiation such as an
infra-red absorbing sensitizing dye or, preferably,
carbon black. Commercial predispersions of carbon
black in suitable binders are especially preferred.
Suitable commercial materials are, for example,
Dispercel~ CBJ-A (Pigment Black 7 in nitrocellulose)
and Magnacryl (Pigment Black 7 in acrylic resin), both
available from Tennant-KVK Ltd, London, England;
Microlith~ CWA (Pigment Black 7 in acrylic resin),
Microlith~ CK (Pigment Black 7 in vinyl chloride-vinyl
acetate), and Microlith~ CA (Pigment Black 7 in ethyl
cellulose), available from Ciba-Geigy Pigments
(Manchester, England). Suitable sensitising dyes are
cyanines, or variations of cyanines in which the
central conjugation system contains squarylium,
croconium, cyclopentenyl, or other structures capable
of inducing appropriate bathochromic shifts, as
described in, for example, Infrared Sensitizing Dyes,
M. Matsuoka, Ed., Plenum Press, New York, NY (1990).
Infra-red sensitising dyes are available from Sumitomo
Chemical (Japan), Eastman Chemicals (Rochester, New
York, USA), and other suppliers. For sensitisation to
830 nm diode output, certain squarane dyes are
preferred as described in US Patent No. 5,019,549. For
YAG laser exposure at 1064 nm, Cyasorb IR 165 from
American Cyanamid is preferred. Infra-red sensitising
dyes can also be mixed with carbon black.
Optional additives to the ablatable layer include
materials which thermally degrade to gaseous products
(blowing agents), for example, azodicarbonamide,
sulphonyl hydrazide, and dinitrosopentamethylene
tetramine (Porofor~ products available from Bayer, UK).
When an acid-catalysed degradable polymer is used in
the ablatable layer, materials which thermally degrade
213964~
g
to produce acid can be added to the layer. Such
materials include iodonium, sulphonium, and phosphonium
salts, organic esters such as 2,6-dinitrobenzyl
tosylate, oxime sulphonates, dicarboximide sulphonates,
and triazines. These materials are available from
Ciba-Geigy (Manchester, England),~Akzo (Longjumeau,
France), and Eastman Kodak (Rochester, New York, USA).
Additives which improve coating quality may be
incorporated, and these include fluorinated surfactants
(such as Zonyl~ surfactants from Du Pont and Fluorad~
surfactants from 3M), silicone-based materials (Sil-
Wet~ products from Dow-Corning), and other well-known
classes of surfactants. Additives which improve
adhesion to either the substrate or the overlying ink-
repellant layer may be incorporated. Such materialsinclude chlorosiliane or methoxysilane bonding agents
from Dow-Corning (Reading, England).
The ink-repellant properties of the coating are
provided by a suitable ink-repellant material providing
the required degree of toughness, impermeability to
gases, adhesion to its underlayer and ink repellency.
Such materials include fluropolymers and silicone
polymers, for example poly(dimethylsiloxane). The
preferred materials for producing ink-repellant
coatings with the required characteristics are silicone
oligomers which cure by an addition mechanism.
Suitable silicone oligomers for this purpose are
available, for example, from Dow Corning SA (Seneffe,
Belgium) under the trade name Syl-Off~, and from Thomas
Goldschmidt AG (Essen, Germany) under the trade name
TEGO~ RC. For example Dow-Corning Syl-Off~ 7046 (30%
reactive siloxane polymer, thought to be vinylsiloxane)
may be combined with Dow-Corning Syl-Off~ 7048 (>95%
polymethylhydrogensiloxane, thought to contain a
platinum catalyst) to produce a coatable mixture which
can be heat cured to produce an ink-repellant film.
2139644
-10-
Additives are available from Dow-Corning which will
improve the adhesion and toughness of the silicone
film, such as Syl-Off~ 297 (anchorage additive, a
mixture of acetoxysilane and epoxy functional silane),
and Syl-Off~ 7210 (controlled release additive, 60%
silicone resin solution in xylene). Fluropolymer
oligomers are available, from example, from Du Pont
(Wilmington, Delaware, USA) under the trade name
Zonyl~.
In the case where the ink repellant material is
present as a separate layer overcoated onto an
ablatable layer, the coating weight of ink repellant
material may be from 0.1 to 5 0 g/m2 and the ablatable
material and the ink-repellant material may be
successively coated on the substrate by means of
coating techniques such as spin coating, bar coating,
dip coating, reverse roll coating, gravure coating,
knife coating and vacuum or plasma deposition
processes. The layers may be cured by baking at 50-
180C for between 30 seconds and 10 minutes, or byexposure to ultra-violet radiation in the case where
the layers are photocrosslinkable.
In the alternative case wherein the ablatable
material and the ink-repellant material are combined in
a single layer, said single layer comprises (i) either
(a) a suitable ablatable ink-repellant material, such
as a fluropolymer or silicone polymer, for example a
polysiloxane, or (b) a combination of an ablatable
material and an ink-repellant material and, where
necessary, (ii) a material capable of sensitising said
ablatable material to infra-red radiation. Such
sensitising material may be any suitable substance
which absorbs infra-red radiation, such as an infra-red
absorbing sensitising dye or, preferably, carbon black.
The single layer may also contain a sensitivity
enhancing agent such as a blowing agent or a non-
21396~
--11--
absorbing, thermally degradable acid-release compound,
in addition to additives which improve coating quality
and adhesion. The layer may be coated on the substrate
by means of the coating techniques previously
described, and cured.
The driographic plate precursor includes a
transparent cover sheet overlaid on the surface of the
coating. Thus, in the case where the ink repellant
material and the ablatable material are in separate
layers, the sheet is overlaid on the ink-repellant
layer. Optionally, an adhesive layer may be present
between the coating and the sheet. Said cover sheet
enables the loosely bound debris which is produced in
the image areas on exposure to be trapped and, thus, be
prevented from being released to the atmosphere. The
debris may then be removed from the exposed precursor
simply by removal of the sheet prior to inking. The
sheet may be comprised of, for example, polypropylene
or poly(ethylene terephthalate), or other suitable film
material which is transparent to infra-red radiation.
Thus, for example, the cover sheet may be formed from a
masking film having a structure and composition as
described in EP-A-323880. The thickness of the sheet
may be within the range of from 8~m to lOO,um.
The driographic plate precursor may also contain
an anti-reflective layer coated on the cover sheet.
Suitable anti-reflective coatings are described in US
Patent Nos. 3,793,022 and 4,769,306 and have a
refractive index of from 1.0 to 1.6, preferably about
1.3. Suitable materials for inclusion in the anti-
reflective layer include, for example, fluorinated
polymers available from Du Pont under the trade name
Zonyl~.
In use, the driographic plate precursor is imaged
by a beam of radiation from a laser operating in the
infra-red region of the spectrum. Particularly
2139~4
preferred are YAG lasers and diode lasers, for example
the Sanyo SDL-7032-101 100 mW diode laser, set to
deliver energy of up to 3000 mJ/cm2 to the coating.
Exposure to the beam of radiation causes ablation of
the coating, which in turn drives away the repellant
material. Loosely bound debris on the exposed
precursor may then simply be peeled away together with
the cover sheet to reveal the underlying ink-receptive
surface. The 1~ ~ining, unimaged, areas do not accept
ink. The images produced show a high degree of
resolution.
The following Examples are illustrative of the
invention:
Example 1
This example demonstrates that the precursor of
the present invention can be exposed with an infra-red
diode laser to impart an image which accepts
driographic printing ink.
An infra-red absorbing ablatable composition was
20 prepared by mixing the following ingredients:
Ciba-Geigy Microlith~ CWA30.0 g
Ethanol, 95~ in water 18.0 g
! Distilled water 50.0 g
Ammonia 2.0 g
25 to form a uniform mixture. (The binder of the
Microlith CWA constituted the ablatable polymer). The
viscosity of the mixture was reduced by adding 100.0 g
of distilled water, and the mixture was coated onto
Howcolo~ polyester film with a wire-wound bar. The
30 coating was dried in an oven for 90 seconds at 100C.
The coating weight after drying was 1.02 g/m2. Both
the coating and the substrate accepted driographic ink
A silicone coating composition was prepared by
mixing the following ingredients:
Dow-Corning Syl-Off~ 7046 5.7 g
Isopar~ H 59.3 g
21396~
-13-
Dow-Corning Syl-Off~ 7048 0.1 g
to form a uniform mixture (Isopar H is a proprietary
hydrocarbon liquid). The silicone mixture was spin-
coated at 100 RPM onto the ablatable layer, forming a
silicone overcoat with a coating weight of 0.78 g/m2.
The overcoat was cured at 140C for 5 minutes. The
overcoated film repelled driographic ink.
The resultant precursor was mounted on a motor-
driven drum and imaged on the coated side with a
scanning diode laser (Sharp LT015MDO, 40mW, 828 nm)
configured to deliver 30 mW continuous wave to a 15
micron spot.
A plurality of precursors was imaged in this way
using different energies. This was effected by
changing the rotation speed of the drum which alters
the exposure time per spot i.e. the so-called "dwell-
time". The dwell time was varied from 10~sec (200
mJ/cm2) to 30,usec (600 mJ/cm2).
Upon exposure to the infra-red radiation, the
ablatable layer ablated from the substrate in the image
areas thereby removing the overlying silicone layer in
those areas to reveal the underlying ink-accepting
material. The surface of each exposed precursor was
rubbed lightly with Isopar~ H-dipped cotton cloth to
removed loose debris and was then allowed to dry. The
dried precursors were then inked with DaiNippon
Dricolor~ QS magenta ink using a roller applicator.
The images which had been produced with greater than
about 500 mJ/cm2 accepted ink, whereas the areas which
were unexposed, or which were exposed with less than
about 500 mJ/cm2, remained clean.
Example 2
This example demonstrates that the thickness of
the ink-repellant overlayer can be varied by a factor
of three without affecting sensitivity or resolution.
An infra-red absorbing ablatable composition was
--- 21396~
-14-
prepared and coated onto Howcolon~ polyester substrate
as described in Example 1. The coating weight after
drying was 3.1 g/m2. Both the coating and the
substrate accepted driographic ink.
A silicone coating composition was prepared by
mixing the following ingredients:
Dow-Corning Syl-Off~ 7046 20.0 g
Isopar~ H 119.8 g
Dow-Corning Syl-Off~ 7048 0.2 g
to form a uniform mixture. The silicone mixture was
spin-coated at 100 RPM onto the ablatable coating to
form a silicone overcoat with a coating weight of 2.1
g/m2. The experiment was repeated with the silicone
bath diluted 1:1 with an equal weight of Isopar~ H to
form a silicone overcoat with a coating weight of 0.73
g/m2. In each case, the overcoat was cured at 140C
for 5 minutes and repelled driographic ink.
A plurality of the resultant precursors were
mounted on a motor-driven drum and imaged on the coated
side with a scanning diode laser (Sanyo SDL-7032-101,
100 mW, 8302 nm) configured to deliver 55 mW continuous
wave to a 25 micron spot. The energies delivered to
the precursors were varied by changing the rotation
speed of the drum, which altered the dwell time. The
dwell times varied from 23,usec (400 mJ/cm2) to 124 ~sec
(2300 mJ/cm2).
Upon exposure, the infra-red absorbing layer
ablated from the substrate, removing the overlying
silicone layer and revealing ink-accepting material.
The total amount of silicone and ablatable polymer
debris which remained in the exposed areas in each case
was estimated by examining the coatings at 100 times
magnification and is given in the table below.
To remove debris, the exposed precursors were
rubbed lightly with Isopar~ H-dipped cotton cloth and
allowed to dry. The resultant printing plates were
21~9644
then inked with DaiNippon Dricolor magenta ink using a
roller applicator. The imaged areas accepted ink with
no discernible difference between the two layers of
different coating weight. Inked resolution was
estimated by examining the plates at 100 times
magnification. As the table shows, features imaged at
the shortest dwell times had resolution narrower than
10 microns while at the longest dwell time, the feature
size increased to about 20 microns.
Percentage Rema2ning
Debris 3.1 g/m
Ablatable Layer
Dwell Energ~ Feature 0.73 g/m2 2.1 g/m
Time (mJ/cm ) SizeSilicone Silicone
(,usec) (,um)
207 2320 20 10 10
15127 1428 20 20 20
93 1040 15 20 20
72 803 15 30 50
63 702 15 30 60
53 596 12 30 50
2046 519 10 40 50
42 465 10 40 50
449 9 50 50
38 422 8 50 50
Example 3
This example demonstrates a composition which
ablates from a conventional aluminium printing plate
substrate when exposed with an infra-red diode laser.
Infra-red absorbing ablatable compositions were
prepared by mixing the following ingredients:
A. 10% Sensitiser
Polymethylmethacylate
(Medium Molecular Weight, Aldrich)10.8 g
Ciba-Geigy Microlith~ CA 1.2 g
Methyl Ethyl Ketone (MEK) 178.0 g
Dow-Corning 1248 wetting agent3.0 ml
21396~
.
-16-
B. 18% Sensitiser
Polymethylmethacylate
(Medium Molecular Weight, Aldrich) 10.8 g
Ciba-Geigy Microlith~ CA 2.4 g
Methyl Ethyl Ketone (MEK)178.0 g
Dow-Corning 1248 wetting agent3.0 ml
to form uniform mixtures. The mixtures were coated
onto 30 gauge anodised aluminum sheets by whirling at
100 RPM, and the coatings were then dried at 60C for
30 seconds. Each mixture was then diluted by adding
MEK in an amount equal to the mass of the mixture and
the experiment was repeated to obtain coated sheets of
reduced coating weight. Each mixture was then diluted
once more with an equal mass of MEK, and used to obtain
further coated sheets of even further reduced coating
weight. The coated sheets were mounted on a motor-
driven drum and imaged on the coated side with a
scanning diode laser (Sharp LT015MD, 40 mW, 828 nm)
configured to deliver 30 mW continuous wave to a 15
micron spot. The dwell times were varied from lO,usec
(200 mJ/cm2) to 30,usec (600 mJ/cm2) by changing the
rotation rate of the drum. In certain cases i.e. where
low energy exposures or low sensitivity levels, or a
combination of both variants, were employed, ablation
was incomplete, and a roughened surface was observed
when the coating was examined under magnification.
When higher energy exposures or higher sensitivity
levels, or a combination of both variants, were
employed, the coating ablated completely and ablated
line widths could be determined. Typical results are
summarised in the following table:
2139644
-17-
Coating Sensitiser Width of Ablated Line (,um)
Weig2ht Percentage 600
(g/m ) 200 mJ/cm2 mJ/cm2
0.4 18 5 10
1.0 18(incomplete ablation) 5
2.1 18(incomplete ablation) 5
0.4 10(incomplete ablation) 14
1.1 10(incomplete ablation) 7
2.6 10(incomplete ablation) 5
Example 4
This example demonstrates that metallised
polyester coated sequentially with a primer layer, an
ablatable layer, and an ink-repellant overlayer can be
imaged by infra-red diode laser and inked with
driographic ink to form a high resolution image.
A primer layer was prepared by m; xi ng the
following ingredients:
Epikote~ 1004 5g
Methyl Ethyl Ketone 100 ml
The primer layer was coated onto Mylar~ pre-coated with
a 50 nm thick layer of smooth aluminum using a wire-
wound bar and oven dried at 140C for 30 seconds,
reaching a dry coating weight of 2.69 g/m2.
An infra-red absorbing ablatable composition was
prepared by mixing the following ingredients:
Polymethylmethacylate
(Low Molecular Weight, Aldrich)20 g
Microlith~ Black CK 5 g
Methyl Ethyl Ketone 100 ml
This mixture was coated using a wire-wound bar onto the
primer layer and oven dried at 140C for 30 seconds,
reaching a dry coating weight of 3.17 g/m2.
An ink-repellant layer was prepared by mixing the
following ingredients:
Dow-Corning Syl-Off~ 7046 20.0 g
Isopaff~ H 40.0 g
213964~
-18-
Dow-Corning Syl-Off~ 7048 0.5 g
Dow-Corning Syl-Off~ 297 0.2 g
until a uniform mixture was achieved. The mixture was
coated onto the primed ablatable layer with a wire-
wound bar and then oven cured at 140C for 2 minutes.The dry coating weight of the ink-repellant layer was
0.83 g/m2. The coating rejected driographic ink.
The resultant precursor was exposed with an infra-
red laser according to Example 1. On exposure at 600
mJ/cm2, the coating ablated. When viewed at 100 times
magnification, the ablated areas could be seen as
clearly defined lines, less than lO~m wide. The plate
was wiped with an Isopar~ H-dipped cloth and allowed to
dry at room temperature. When driographic ink was
applied according to Example 1, the image areas
accepted ink, while the unexposed areas rejected it.
Example 5
This example demonstrates the use of an ablatable
layer with improved sensitivity, together with a
driographic overlayer.
An infra-red absorbing ablatable composition was
prepared comprising the following composition:
Microlith~ Black CK 5g
Methyl Ethyl Ketone 100 ml
The mixture was coated using a wire wound bar onto
aluminium coated Mylar~ as used in Example 4. The
coated film was oven dried at 140C for 30 seconds.
The coating weight was 0.17 g/m2.
The coated film was then overcoated with an ink-
repellant layer according to Example 4, and the
resultant precursor exposed to an infra-red laser
according to Example 1. On exposure at 200 mJ/cm2, the
infra-red absorbing layer ablated, removing the ink-
repellant overcoat. When viewed with a microscope the
ablated areas could be seen as clearly defined lines,
less than lO,um wide.
2139644
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When driographic ink was applied according to
Example 1, the ablated areas accepted ink whereas the
unexposed areas reject it.
Example 6
An infra-red absorbing ablatable composition was
prepared by blending the following ingredients:
Dispercel~ CBJ-A 2.0 g
MEK 8.0 g
to form a uniform mixture. The mixture was coated on
Howcolon~ polyester using a No. 5 gauge wire-wound bar
and allowed to air dry for five minutes to form a
coating with a dry weight of around 1.0 g/m2.
A silicone coating composition was prepared by
mixing the following ingredients:
Dow-Corning Syl-Off~ 7046 20.0 g
Dow-Corning Syl-Off~ 7048 0.05 g
Dow-Corning Syl-Off~ 297 0.5 g
Dow-Corning Syl-Off~ 7210 5.0 g
Isopar~ G 40.5 g
to form a uniform mixture (Isopar G is a proprietary
liquid hydrocarbon). The silicone mixture was coated
onto the ablatable layer using a No. 8 gauge wire-wound
bar and cured at 130C for 3 minutes. The resultant
precursor was cut into two pieces and mounted on the
motor-driven drum of the write engine described in
Example 3. Onto one of the precursors, there was
applied a cover sheet in the form of a clear adhesive
overlay film. Onto the other precursor there was taped
an 8,um Mylar~ cover sheet bearing no adhesive. The
precursors were then exposed as described in Example 3.
When the adhesive cover sheet was removed, it bore the
image written by the laser, consisting of ablated
pigment, polymer residue, and silicone ablated from the
film. The non-adhesive cover sheet bore a much fainter
image. Without wiping away loose debris, the two
exposed precursors were inked as described earlier, and
213964~
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the inked images were contact-transferred to smooth
paper. The precursor which had been provided with the
adhesive cover sheet printed a clear image, whereas the
precursors which had been provided with the non-
adhesive cover sheet printed a faint, patchy image.Thus, the adhesive cover sheet was an excellent means
for not only trapping ablated material, but also for
removing loose debris thus obviating the need for a
cleaning step. The adhesive cover sheet also carried a
copy of the printed image, allowing verification of the
exposure.
Example 7
This example demonstrates that driographic plate
precursors of the present invention can be imaged by
continuous-wave infra-red diodes in a computer-driven
write engine.
An infra-red absorbing ablatable composition was
prepared by blenAin~ the following ingredients:
Dispercel~ CBJ-A 2.0 g
MEK 8.0 g
to form a uniform mixture. This mixture was coated on
Howcolon~ polyester using a No. 5 gauge wire-wound bar
and allowed to air dry for five minutes to form a
coating with a dry weight of around 1.0 g/m2.
A silicone coating composition was prepared by
mixing the following ingredients:
Syl-Off~ 7046 20.0 g
Syl-Off~ 7048 0.5 g
Syl-Off~ 297 0.2 g
Isopar~ G 40.0 g
to form a uniform mixture. The silicone mixture was
coated onto the ablatable layer using a No. 8 gauge
wire-wound bar and cured at 130C for 3 minutes. A
Mylar cover sheet was then applied. The resultant
precursor was exposed through the cover sheet on a
write engine supplied by CREO (Vancouver BC) using an
2139611
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-21-
array of 32 infra-red lasers emitting at 830 nm (Sanyo
Semiconductor, SDL-7032-102, Allendale, NJ), to produce
a 150 line screen text pattern, using 130 mJ/cm2 of
incident laser energy. The imaged precursor was
removed from the drum and loose debris was wiped away
with an Isopar~ H-dampened cloth. The resultant
printing plate was inked as described previously, and
the inked image was contact-transferred to smooth
paper. A recognisable image was printed.
Example 8
Example 6 was repeated using Automask~ material as
the adhesive overlay film. Automask~ is a laminated
sheet product supplied by Autotype International Ltd
for use as a masking film in the preparation of
lithographic plates and screen printing stencils.
- The red adhesive coated membrane was separated
from the base film and applied to the surface of the
ink repellant layer. The precursor was exposed as
described in Example 3 and the membrane was removed.
The resultant plate was found to be free of loose
debris and did not require a cleaning step prior to
printing.
The adhesive of the membrane was based on a
natural rubber, with the addition of an alpha terpene
resin to improve tack, and provided a peel resistance
of 37-70 gm/30 mm sample width measured at an angle of
180 when the membrane and the base film were
separated.
Example 9
Example 6 was repeated using a cover sheet
provided in situ. An adhesive sublayer was produced by
applying a solution of natural rubber (12g) and an
alpha terpene resin (100 g) in hydrocarbon solvent
(5000 ml) to the surface of the silicone layer of the
ablatable precursor assembly described in Example 6.
2139644
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When dry, a composition comprising a solution of an
aliphatic polyurethane resin and cellulose nitrate as
described in Example lB of European Patent No.323,880
was coated over the adhesive layer to a thickness of 30
microns.
The precursor was exposed as described in Example
3. The cover sheet was removed to obtain a printing
plate re~uiring no cleaning stage prior to printing.
