Imaging element and a method for producing a lithographic plate therewith

According to the present invention there is provided an imaging element for making a lithographic printing plate comprising on a support having a hydrophilic surface a photosensitive layer and a thermosensitive layer, said thermosensitive layer being opaque to light for which said photosensitive layer has spectral sensitivity and said thermosensitive layer being capable of rendered transparent upon exposure to laser light characterised in that said thermosensitive layer is soluble or swellable in an aqueous medium.

DESCRIPTION 
1. Field of the Invention 
The present invention relates to an imaging element for making a 
lithographic printing plate wherein that imaging element comprises a 
thermosensitive mask on a photosensitive coating. 
2. Background of the Invention 
Lithography is the process of printing from specially prepared surfaces, 
some areas of which are capable of accepting lithographic ink, whereas 
other areas, when moistened with water, will not accept the ink. The areas 
which accept ink form the printing image areas and the ink-rejecting areas 
form the background areas. 
In the art of photolithography, a photographic material is made imagewise 
receptive to oily or greasy inks in the photo-exposed (negative-working) 
or in the non-exposed areas (positive-working) on a hydrophilic 
background. 
In the production of common lithographic printing plates, also called 
surface litho plates or planographic printing plates, a support that has 
affinity to water or obtains such affinity by chemical treatment is coated 
with a thin layer of a photosensitive composition. Coatings for that 
purpose include light-sensitive polymer layers containing diazo compounds, 
dichromate-sensitized hydrophilic colloids and a large variety of 
synthetic photopolymers. Particularly diazo-sensitized systems are widely 
used. 
Upon image-wise exposure of the light-sensitive layer the exposed image 
areas become insoluble and the unexposed areas remain soluble. The plate 
is then developed with a suitable liquid to remove the diazonium salt or 
diazo resin in the unexposed areas. 
Alternatively, printing plates are known that include a photosensitive 
coating that upon image-wise exposure is rendered soluble at the exposed 
areas. Subsequent development then removes the exposed areas. A typical 
example of such photosensitive coating is a quinone-diazide based coating. 
Typically, the above described photographic materials from which the 
printing plates are made are camera-exposed through a photographic film 
that contains the image that is to be reproduced in a lithographic 
printing process. Such method of working is cumbersome and labor 
intensive. However, on the other hand, the printing plates thus obtained 
are of superior lithographic quality. 
Attempts have thus been made to eliminate the need for a photographic film 
in the above process and in particular to obtain a printing plate directly 
from computer data representing the image to be reproduced. In particular 
it has been proposed to coat a silver halide layer on top of the 
photosensitive coating. The silver halide can then directly be exposed by 
means of a laser under the control of a computer. Subsequently, the silver 
halide layer is developed leaving a silver image on top of the 
photosensitive coating. That silver image then serves as a mask in an 
overall exposure of the photosensitive coating. After the overall exposure 
the silver image is removed and the photosensitive coating is developed. 
Such method is disclosed in for example JP-A 60-61752 but has the 
disadvantage that a complex development and associated developing liquids 
are needed. 
GB 1.492.070 discloses a method wherein a metal layer or a layer containing 
carbon black is provided on a photosensitive coating. This metal layer is 
then ablated by means of a laser so that a image mask on the 
photosensitive layer is obtained. The photosensitive layer is then overall 
exposed by UV-light through the image mask. After removal of the image 
mask, the photosensitive layer is developed to obtain a printing plate. 
This method however still has the disadvantage that the image mask has to 
be removed prior to development of the photosensitive layer by a 
cumbersome processing,. 
U.S. Pat. No. 5,262,275 discloses a photosensitive printing element having 
a photosensitive layer and an IR ablatable mask layer on a polymeric 
support. This material has however the disadvantage that the mask is 
image-wise removed by ablation so that the ablated particles can soil the 
IR exposure soil. 
WO96/02021 discloses a process for preparing a lithographic plate by using 
an original form comprising a substrate, a photosensitive layer and a 
light-intercepting layer which is image-wise removable by laser beams. 
This method however still has the disadvantage that the image-wise exposed 
mask has to be removed prior to overall exposure of the photosensitive 
layer. 
Systems are also known where a mask is image-wise formed on a 
photosensitive layer by transfer of a masking substance to the 
photosensitive coating, e.g. by means of laser transfer or xerography as 
disclosed in EP-A 1138. However, such method is generally slow and may not 
meet the required image resolutions for obtaining high quality prints. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a method for making 
printing plates of high quality in a convenient way and wherein certain 
disadvantages of the prior art are removed. 
In accordance with the present invention there is provided an imaging 
element for making a lithographic printing plate comprising on a support 
having a hydrophilic surface a photosensitive layer and a thermosensitive 
layer, said thermosensitive layer being opaque to light for which said 
photosensitive layer has spectral sensitivity and said thermosensitive 
layer being capable of rendered transparent upon exposure to laser light 
characterised in that said thermosensitive layer is soluble or swellable 
in an aqueous medium. 
The present invention further discloses a method for obtaining a 
lithographic printing plate using an imaging element as defined above. 
DETAILED DESCRIPTION OF THE INVENTION 
Thanks to the use of a thermosensitive layer that is soluble or swellable 
in an aqueous alkaline medium a lithographic printing plate can be 
obtained from an imaging element according to the invention by using only 
one development step with common developing solutions. The present 
invention will now be described in more detail without the intention to 
limit the invention to any of the embodiments described hereinafter. 
a. Imaging Element 
a.1. Thermosensitive Layer 
A thermosensitive layer for use in connection with the present invention 
comprises an infrared pigment dispersed in a binder. A particularly 
desirable infrared pigment is carbon black. However, other pigments can be 
used such as e.g. a conductive polymer particle, metal carbides, borides, 
nitrides, carbonitrides, bronze-structured oxides and oxides structurally 
related to the bronze family but lacking the A component e.g. WO.sub.2.9. 
As a binder for the thermosensitive layer, any binder that is aqueous 
soluble or swellable can be used. However, polymeric binders are 
especially preferred. Preferred polymeric binders include hydrophilic 
binders particular those that are swellable or soluble in an alkaline 
aqueous medium. Examples of suitable binders are e.g. polyvinyl alcohol, 
polyvinylpyrrolidon, polyethyleneoxide, celluloses, sacharides, gelatin, 
carboxyl containing polymers such as e.g. homo- or copolymers of 
(meth)acrylic acid, maleinic acid anhydride based polymers, polymers 
containing phenolic hydroxy groups e.g. polyvinylphenols etc. and the 
alkali soluble binders mentioned in the composition of the photosensitive 
layer. 
It is further particularly desirable that a thermosensitive layer in this 
invention further includes a thermodegradable polymer in particular one 
that decomposes exothermally. In the latter case, sensitivity of the 
thermosensitive layer is improved. A particular example of a 
thermodegradable polymer that decomposes exothermally is a nitrocellulose. 
A thermosensitive layer in connection with the present invention may 
further be cross-linked to make the imaging element less prone to damage 
caused during handling of the imaging element. 
A thermosensitive layer in the present invention should also be opaque to 
light for which the photosensitive layer has spectral sensitivity. 
Generally used photosensitive layers and in particular those preferred in 
this invention are UV sensitive. A thermosensitive layer for use therewith 
should therefore be opaque to UV-light. According to a preferred 
embodiment, this may be accomplished by using an infrared pigment that 
also shows a substantial absorption for UV-light. A particularly preferred 
compound in this respect is carbon black. However, in accordance with an 
alternative embodiment, the thermosensitive layer may include a 
UV-absorbing compound in addition to a compound A capable of converting 
laser light into heat. Said compound A can also be present in a layer 
adjacent to said thermosensitive layer. Examples of such UV-absorbing 
compounds are UV absorbing dyes or pigments. Suitable UV absorbing masking 
dyes have a sufficient high extinction coefficient in the wavelength range 
from 300 to 450 nm so that with a reasonable amount of masking dye, the 
amount of UV light penetrating through the thermosensitive layer is not 
more than 10% and more preferably not more than 3% and most preferably not 
more than 1%. Examples of suitable UV-masking dyes are UVINUL D49, UVINUL 
E50 and UVINUL N539 from BASF, TINUVIN P from Ciba-Geigy, INTRAWITE OB and 
INTRAWITE YELLOW 2GLN from Crompton and Knowles Ltd., 
4-dimethylaminobenzophenone, 4-phenylazophenol etc. The compound A is 
preferably an infrared absorbing compound and the laser light is 
preferably infrared laser light. More preferably said infrared absorbing 
compound is an infrared absorbing dye or a conductive polymer. 
The thickness of the thermosensitive layer in connection with the present 
invention is preferably chosen such that sufficient opaqueness of the 
layer is obtained (preferably 90% or more of the light to which the 
photosensitive layer is responsive is absorbed by the thermosensitive 
layer, more preferably 99%, most preferably 99.9%) while maintaining a 
suitable sensitivity. Typically, the thickness of the thermosensitive 
layer ranges for example from 0.1 .mu.m to about 4 .mu.m and more 
preferably between 0.5 .mu.m and 1.5 .mu.m 
A thermosensitive layer in connection with the present invention may 
further include additional other components to realise various other 
desired functionalities such as e.g. visual inspection that may be 
realised by including a colored dye in the thermosensitive layer. 
a.2. Photosensitive Layer 
A photosensitive layer in accordance with the present invention may 
comprise any suitable light-sensitive composition from which an ink 
accepting image on the hydrophilic surface of the support of the imaging 
element can be obtained. Examples of such light-sensitive compositions 
used herein are those comprising diazo compounds; those comprising azide 
compounds as disclosed in U.K. Patent Nos. 1,235,281 and 1,495,861; those 
containing photo-crosslinkable photopolymers as disclosed in U.S. Pat. 
Nos. 4,072,528 and 4,072,527 and in particular those described in more 
detail below. 
Among these light-sensitive compositions, preferably used are those 
comprising diazo compounds since they are synthetically excellent in 
various properties such as storage properties of the photosensitive layer; 
developability, for instance, development latitude; image properties, e.g. 
quality of images; printing properties, e.g., ink receptivity and wear 
resistance; and low possibility of causing environmental pollution of 
developers used. 
The light-sensitive compositions containing diazo compounds can roughly be 
divided in two groups, i.e. negative-working type and positive-working 
type ones. 
The negative-working light-sensitive compositions containing diazo 
compounds comprise light-sensitive diazo compounds and preferably 
polymeric compounds. As the light-sensitive diazo compounds there may be 
used any ones conventionally known and preferred examples thereof are 
salts of organic solvent-soluble diazo resins such as salts of condensates 
of p-diazodiphenylamine and formaldehyde or acetaldehyde with 
hexafluorophosphates and salts thereof with 
2-hydroxy-4-methoxybenzophenone-5-sulfonic acid salts. 
On the other hand, examples of the foregoing polymeric compounds preferably 
used are copolymers of acrylic acid or methacrylic acid; crotonic acid 
copolymers; itaconic acid copolymers, maleic acid copolymers, cellulose 
derivatives having carboxyl groups on the side chains thereof, polyvinyl 
alcohol derivatives having carboxyl groups on the side chains thereof, 
copolymers of hydroxyalkyl (meth)acrylate having carboxyl groups on the 
side chains thereof, and unsaturated polyester resins having carboxyl 
groups. 
As the diazo compounds used in a positive-working light-sensitive 
composition, any compounds conventionally known may be utilized and 
typical examples thereof are o-quinonediazides and preferably 
o-naphthoquinonediazide compounds. Particularly preferred are 
o-naphthoquinonediazidosulfonic acid esters or o-naphthoquinone 
diazidocarboxylic acid esters of various hydroxyl compounds; and 
o-naphthoquinonediazidosulfonic acid amides or 
o-naphthoquinonediazidocarboxylic acid amides of various aromatic amine 
compounds. Examples of such phenols include phenol, cresol, resorcin and 
pyrogallol; examples of such carbonyl group-containing compounds are 
formaldehyde, benzaldehyde and acetone. Examples of preferred hydroxyl 
compounds include phenol-formaldehyde resin, cresol-formaldehyde resin, 
pyrogallol-acetone resin and resorcin-benzaldehyde resin. 
Typical examples of o-quinonediazide compounds include esters of 
benzoquinone-(1,2)-diazidosulfonic acid or 
napthoquinone-(1,2)-diazidosulfonic acid and phenol-formaldehyde resin or 
cresol-formaldehyde resin; ester of 
naphthoquinone-(1,2)-diazido-(2)-5-sulfonic acid and pyrogallol-acetone 
resin as disclosed in U.S. Pat. No. 3,635,709; and ester of 
naphthoquinone-(1,2)-diazido-(2)-5-sulfonic acid and 
resorcin-pyrogallol-acetone copolycondensates as disclosed in J.P. KOKAI 
No. Sho 55-76346. 
Examples of other useful o-quinonediazide compounds are polyesters having 
hydroxyl groups at their termini esterified with 
o-napthoquinonediazidesulfonyl chloride as disclosed in J.P. KOKAI No. Sho 
50-117503; homopolymers of p-hydroxystyrene or copolymers thereof with 
other copolymerizable monomers esterified with 
o-naphtoquinonediazidosulfonyl chloride as disclosed in J.P. KOKAI No. Sho 
50-113305; ester of bisphenol-formaldehyde resin and 
o-quinonediazidosulfonic acid as disclosed in J.P. KOKAI No. Sho 54-29922; 
condensates of alkyl acrylate-acryloyoxyalkyl carbonatehydroxyalkyl 
acrylate copolymers with o-naphthoquinonediaziosulfonyl chloride as 
disclosed in U.S. Pat. No. 3,859,099; reaction products of copolymerized 
products of styrene and phenol derivatives with o-quinonediazidosulfonic 
acid as disclosed in J.P. KOKOKU No. Sho 49-17481; amides of copolymers of 
p-aminostyrene and monomers copolymerizable therewith and 
o-naphthoquinonediazidosulfonic acid or o-naphthoquinonediazidocarboxylic 
acid as disclosed in U.S. Pat. No. 3,759,711; as well as ester compounds 
of polyhydroxybenzophenone and o-naphthoquinonediazidosulfonyl chloride. 
These o-quinonediazide compounds may be used alone, but are preferably used 
as a mixture with an alkalki-soluble resin to form a light-sensitive 
layer. 
Preferred alkali-soluble resins include novolak type phenol and typical 
examples thereof are phenolformaldehyde, cresol-formaldehyde resin, and 
phenol-cresol-formaldehyde copolycondensed resins as disclosed in J.P. 
KOKAI No. Sho 55-57841. More preferably, the foregoing phenol resins are 
simultaneously used with a condensate of phenol or cresol substituted with 
an alkyl group having 3 to 8 carbon atoms and formaldehyde such as 
t-butylphenol-formaldehyde, as described in J.P. KOKAI No. Sho 50-125806. 
Moreover, it is also possible to optionally incorporate, into the 
light-sensitive composition, alkali-soluble polymers other than the above 
listed alkali-soluble novolak phenolic resins. Examples of such polymers 
are styrene-acrylic acid copolymer, methyl methacrylate-methacrylic acid 
copolymer, alkali-soluble polyurethane resin, and alkali-soluble vinylic 
resins and alkali-soluble polybutyral resins as disclosed in J.P. KOKOKU 
No. Sho 52-28401. 
The amount of the o-quinonediazide compounds is preferably 5 to 80% by 
weight and more preferably 10 to 50% by weight based on the total weight 
of the solid contents of the light-sensitive composition. On the other 
hand, that of the alkali-soluble resins is preferably 30 to 90% by weight 
and more preferably 50 to 85% by weight based on the total weight of the 
solid contents of the light-sensitive composition. 
A photosensitive layer in connection with this invention may be applied in 
the form of a multilayered structure. Moreover, the light-sensitive 
composition in the photosensitive layer or multi-layer package may further 
comprise optional components such as dyes, plasticizers and components for 
imparting printing-out properties (ability of providing a visible image 
immediately after imagewise exposure). 
The coated amount of a photosensitive layer applied onto the hydrophilic 
surface of a support preferably ranges from 0.1 to 7 g/m.sup.2 and more 
preferably 0.5 to 4 g/m.sup.2. 
Preferred photo-crosslinking materials in the present invention are based 
on photo-crosslinking polymers having a maleimido group at their side 
chain. In order to elevate their photosensitivity sensitisers are added 
such as thioxanthones, benzophenone, Michler's ketone, anthraquinones, 
anthracene, chrysene, p-dinitrobenzene, 2-nitrofluorene, as well as 
sensitizers described in JP-A-62-294238, JP-A-2-173646, JP-A-2-236552, 
JP-A-3-54566 and JP-A-6-107718. (The term "JP-A" as used herein means an 
"unexamined published Japanese patent application".) 
A photo-crosslinking polymer having a maleimido group at its side chain(s) 
includes, for example polymers that are described in U.S. Pat. No. 
4,079,041 (corresponding to JP-A-52-988); West German Patent 2,626,769; 
European Patents 21,019 and 3,552; Die Angewandte Makromolekulare Chemie, 
115 (1983), pp. 163-181; JP-A-49-128991 to JP-A-49-128993, JP-A-50-5376 to 
JP-A-50-5380, JP-A-53-5298 to JP-A-53-5300, JP-A-50-50107, JP-A-51-47940, 
JP-A-52-13907, JP-A-50-45076, JP-A-52-121700, JP-A-50-10844, 
JP-A-50-45087, JP-A-58-43951; West German Patents 2,349,948 and 2,616,276. 
Of these polymers, those having, at their side chains, two or more 
maleimido groups on average in one molecule and having a mean molecular 
weight of 1000 or more are preferably used in the present invention. 
An imaging element containing any of these polymers is preferably developed 
with an aqueous alkaline developer substantially not containing an organic 
solvent, in view of the environmental safety. Therefore, it is preferable 
that these polymers are soluble in or swellable with aqueous alkalis. 
Accordingly, it is preferably that monomers having a maleimido group at 
its side chain are copolymerized with monomers having a alkali-soluble 
group to obtain these polymers. 
As the alkali-soluble group, preferred are acid groups having a pKa of 14 
or less. Specific examples of such monomers having an alkali-soluble group 
include vinyl monomers having a carboxyl group, such as acrylic acid, 
methacrylic acid, maleic acid, itaconic acid; vinyl monomers having a 
--CONHSO.sub.2 -group; vinyl monomers having an --SO.sub.2 NH-group; vinyl 
monomers having a phenolic hydroxyl group; vinyl monomers having a 
phosphoric acid group or a phosphonic acid group; as well as maleic 
anhydride, and itaconic anhydride. 
The alkali-soluble group having monomer and the maleido group having 
monomer are generally copolymerized at a ratio of from 10/90 to 70/30 by 
mol, preferably from 20/80 to 60/40 by mol, to easily give a 
photo-crosslinking polymer for use in the present invention. The polymer 
has preferably an acid value of from 30 to 500, especially preferably from 
50 to 300. 
Of such photo-crosslinking polymers, especially useful are copolymers 
composed of an N-[2-methacryloyloxy)alkyl]-2,3-dimethylmaleimide and 
methacrylic acid or acrylic acid, such as those described in Die 
Angewandte Makromolekulare Chemie, 128 (1984), pp. 71-91. In producing 
these copolymers, vinyl monomers may additionally copolymerized, as the 
third component, to easily give polynary copolymers as desired. For 
instance, alkyl methacrylates or alkyl acrylates of which homopolymers 
have a glass transition temperature not higher than room temperature can 
be copolymerized as the third component to give flexible copolymers. 
A photo-crosslinking polymer for use in the present invention has 
preferably a weight average molecular weight of 1000 or more, more 
preferably from 10,000 to 500,000, and even more preferably from 20,000 to 
300,000. 
As mentioned above, it is desirable to add a sensitizer to a photosensitive 
layer having a photocross-linkable polymer as described above. Preferred 
is a triplet sensitizer having an absorption peak for ensuring sufficient 
light absorption at 300 nm or more. 
Examples of such sensitizer include benzophenone derivatives, benzanthrone 
derivatives, quinones, aromatic nitro compounds, naphthothiazoline 
derivatives, benzothiazoline derivatives, thioxanthones, naphthothiazole 
derivatives, ketocoumarin compounds, benzothiazole derivatives, 
naphthofranone compounds, pyrylium salts, and thiapyrylium salts. 
Specific examples thereof include Michler's ketone, 
N,N'-diethylaminobenzophenone, benzanthrone, 
(3-methyl-1,3-diazo-1,9-benz)anthronepicramide, 5-nitroacenaphthene, 
2-chlorothioxanthone, 2-isopropylthioxanthone, dimethylthioxanthone, 
methylthioxanthone-1-ethyl carboxylate, 2-nitrofluorenone, 
2-dibenzoylmethylene-3-methylnaphthothiazoline, 
3,3-carbonyl-bis-(7-diethylaminocoumarin), 2,4,6-triphyenlthiapyrylium 
perchlorate, 2-(p-chlorobenzoy)naphthothiazole, as well as sensitizers 
described in JP-B-45-8832, JP-A-52-129791, JP-A-62-294238, JP-A-2-173646, 
JP-A-2-131236, European Patent 368,327, JP-A-2-236552, JP-A-3-54566 and 
JP-A-6-107718. 
Of these, preferred are the sensitizers described in JP-A-2-236552, 
JP-A-3-54566 and JP-A-6-107718; and especially preferred are the 
sensitizers having one or more of --COOH, --NHSO.sub.2 R.sup.20, 
--CONCHCOR.sup.20 and/or --CONHSO.sub.2 R.sup.20 (where R.sup.20 
represents an alkyl group, an aromatic group or an alkyl-aromatic group) 
as an alkali-soluble group in one molecule, described in JP-A-6-107718. 
The amount of sensitizer in the photosensitive layer is conveniently from 1 
to 20% by weight, preferably from 2 to 15% by weight, and more preferably 
from 3 to 10% by weight, of the total amount of the compositions in the 
photosensitive layer. 
In addition to the photocross-linkable polymers described above it may be 
desirable to add a diazo resin to the photosensitive layer. Examples of 
diazo resins include co-condensates composed of aromatic diazonium 
compounds and aldehydes. Specific examples thereof include the diazo 
resins described in JP-B-49-48001, JP-B-50-7481, JP-B-5-2227, JP-A-3-2864, 
JP-A-3-240061, and JP-A-4-274429. 
Of these, preferred are diazo resins having a carboxyl group in the 
molecule, for example, diazo resins obtained by co-condensation with 
aromatic compounds having at least one carboxyl group, such as those 
described in JP-A-3-240061, and diazo resins obtained by condensation with 
aldehydes having a caboxyl group, such as those described in JP-A-2864. 
The amount of the diazo resin in the photosensitive layer is preferably 
from 0.1 to 30% by weight, preferably from 0.5 to 10% by weight, and more 
preferably from 1 to 5% by weight of the total amount of the compositions 
in the photosensitive layer. 
Further, there may be added a polymer having one or more polymerisable 
groups such as the polymers disclosed in JP-B-3-63740, U.S. Pat. Nos. 
3,376,138, 3,556,793. 
The above-mentioned polymer having one or more polymerisable groups is 
preferably soluble in or swellable with aqueous alkaline developers, like 
the polymers having a maleimido group at the side chain. Therefore, this 
polymer is preferably a copolymer composed of one or more monomers having 
an alkali-soluble group, e.g. as mentioned above. 
a.4. Support 
In accordance with the present invention, an imaging element comprises a 
support having a hydrophilic surface. Suitable supports for use in this 
invention are e.g. metal supports in particular grained and anodised 
aluminium or supports comprising a substrate such as e.g. paper or plastic 
film provided with a hydrophilic layer, preferably a hydrophilic layer 
that is cross-linked. 
A particularly suitable cross-linked hydrophilic layer may be obtained from 
a hydrophilic binder cross-linked with a cross-linking agent such as 
formaldehyde, glyoxal, polyisocyanate or a hydrolysed 
tetra-alkylorthosilicate. The latter is particularly preferred. 
As hydrophilic binder there may be used hydrophilic (co)polymers such as 
for example, homopolymers and copolymers of vinyl alcohol, acrylamide, 
methylol acrylamide, methylol methacrylamide, acrylic acid, methacrylic 
acid, hydroxyethyl acrylate, hydroxyethyl methacrylate or maleic 
anhydride/vinylmethylether copolymers. The hydrophilicity of the 
(co)polymer or (co)polymer mixture used is preferably the same as or 
higher than the hydrophilicity of polyvinyl acetate hydrolyzed to at least 
an extent of 60 percent by weight, preferably 80 percent by weight. 
The amount of crosslinking agent, in particular of tetraalkyl 
orthosilicate, is preferably at least 0.2 parts by weight per part by 
weight of hydrophilic binder, preferably between 0.5 and 5 parts by 
weight, more preferably between 1.0 parts by weight and 3 parts by weight. 
A cross-linked hydrophilic layer in accordance with the present invention 
preferably also contains substances that increase the mechanical strength 
and the porosity of the layer. For this purpose colloidal silica may be 
used. The colloidal silica employed may be in the form of any commercially 
available water-dispersion of colloidal silica for example having an 
average particle size up to 40 nm, e.g. 20 nm. In addition inert particles 
of larger size than the colloidal silica can be added e.g. silica prepared 
according to Stober as described in J. Colloid and Interface Sci., Vol. 
26, 1968, pages 62 to 69 or alumina particles or particles having an 
average diameter of at least 100 nm which are particles of titanium 
dioxide or other heavy metal oxides. By incorporating these particles the 
surface of the cross-linked hydrophilic layer is given a uniform rough 
texture consisting of microscopic hills and valleys, which serve as 
storage places for water in background areas. The thickness of a 
cross-linked hydrophilic layer may vary in the range of 0.2 to 25 .mu.m 
and is preferably 1 to 10 .mu.m. 
Further particular examples of suitable cross-linked hydrophilic layers for 
use in accordance with the present invention are disclosed in EP-A 601240, 
GB-P-1419512, FR-P-2300354, U.S. Pat. No. 3,971,660, U.S. Pat. No. 
4,284,705 and EP-A 514490. 
As substrate on which the hydrophilic layer is provided it is particularly 
preferred to use a plastic film e.g. substrated polyethylene terephthalate 
film, cellulose acetate film, polystyrene film, polycarbonate film etc. 
The plastic film support may be opaque or transparent. 
It is particularly preferred to use a polyester film support to which an 
adhesion improving 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 619524, 
EP-A 620502 and EP-A 619525. Preferably, the amount of silica in the 
adhesion improving layer is 200 mg per m.sup.2 and 750 mg per m.sup.2. 
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 
m.sup.2 per gram, more preferably a surface area of 500 m.sup.2 per gram. 
b. Method for Making a Lithographic Printing Plate 
b.1 Image-wise Exposure 
Image-wise exposure in connection with the present invention involves the 
use of a laser emitting in the infrared (IR), i.e. emitting in the 
wavelength range above 700 nm, preferably 700-1500 nm. Particularly 
preferred for use in connection with the present invention are laser 
diodes emitting around 830 nm (gallium-arsenide laser diodes) or a 
NdYAG-laser emitting at 1060 nm. 
A preferred imaging apparatus suitable for image-wise exposure in 
accordance with the present invention preferably includes a laser output 
that can be provided directly to the imaging elements surface via lenses 
or other beam-guiding components, or transmitted to the surface of a blank 
imaging element from a remotely sited laser using a fiber-optic cable. A 
controller and associated positioning hardware maintains the beam output 
at a precise orientation with respect to the imaging elements surface, 
scans the output over the surface, and activates the laser at positions 
adjacent selected points or areas of the imaging element. The controller 
responds to incoming image signals corresponding to the original document 
and/or picture being copied onto the imaging element to produce a precise 
negative or positive image of that original. The image signals are stored 
as a bitmap data file on a computer. Such files may be generated by a 
raster image processor (RIP) or other suitable means. For example, a RIP 
can accept Input data in page-description language, which defines all of 
the features required to be transferred onto the imaging element, or as a 
combination of page-description language and one or more image data files. 
The bitmaps are constructed to define the hue of the color as well as 
screen frequencies and angles in case of amplitude modulation screening. 
However, the present invention is particularly suitable for use in 
combination with frequency modulation screening as disclosed in e.g. EP-A 
571010, EP-A 620677 and EP-A 620674. 
The imaging apparatus for use in the present invention is preferably 
configured as a flatbed recorder or a drum recorder with the imaging 
element mounted to exterior cylindrical surface of the drum. In a 
preferred drum configuration, the requisite relative motion between the 
laser beam and the imaging element is achieved by rotating the drum(and 
the imaging element mounted thereon) about its axis and moving the beam 
parallel to the rotation axis, thereby scanning the imaging element 
circumferentially so the image "grows" in the axial direction. 
Alternatively, the beam can move parallel to the drum axis and, after each 
pass across the imaging element, increment angularly so that the image on 
the imaging element "grows" circumferentially. In both cases, after a 
complete scan by the beam and development, an image corresponding to the 
original will have been applied to the surface of the imaging element. In 
the flatbed configuration, the beam is drawn across either axis of the 
imaging element, and is indexed along the other axis after each pass. Of 
course, the requisite relative motion between the beam and the imaging 
element may be produced by movement of the imaging element rather than (or 
in addition to) movement of the beam. 
Regardless of the manner in which the beam is scanned, it is generally 
preferable (for reasons of speed) to employ a plurality of lasers and 
guide their outputs to a single writing array. The writing array is then 
indexed, after completion of each pass across or along the imaging 
element, a distance determined by the number of beams emanating from the 
array, and by the desired resolution (i.e. the number of image points per 
unit length). 
b.2. Overall Exposure 
Overall exposure in connection with the present invention is carried by 
means of a light source that emits at least in the wavelength range for 
which the photosensitive layer of an imaging element in connection with 
the present invention has spectral sensitivity. In a practical embodiment 
in this invention, the photosensitive layer is UV-sensitive and/or 
optionally sensitive to the short wavelength part of the visible spectrum 
e.g. upto green. Overall exposure may for example be carried out by 
exposure sources such as high or medium pressure halogen mercury vapour 
lamps, e.g. of 1000 W. 
b.3. Development 
Development in connection with the present invention is carried out by a 
suitable liquid capable of removing either the exposed or non-exposed 
areas of the photosensitive layer. The appropriate composition of a 
developing liquid in the present invention will depend on the kind of 
photosensitive layer and preferably is such that during development, the 
thermosensitive layer and optional intermediate layer are removed at the 
same time. 
For ecological reasons, it is highly preferred that an aqueous based 
developing liquid is used preferably without additional organic solvents. 
A particularly preferred developing liquid is an aqueous alkaline liquid. 
Particularly suitable developing liquids for use with the preferred 
photosensitive coatings described above are as follows. 
A developer preferably used in the invention is an aqueous solutions mainly 
composed of alkali metal silicates and alkali metal hydroxides. As such 
alkali metal silicates, preferably used are, for instance, sodium 
silicate, potassium silicate, lithium silicate and sodium metasilicate. On 
the other hand, as such alkali metal hydroxides, preferred are sodium 
hydroxide, potassium hydroxide and lithium hydroxide. From the viewpoint 
of preventing the formation of insoluble precipitates, it is particularly 
desirable that the developer comprise at least 20 mole % of potassium with 
respect to the total amount of alkali metals in the aqueous developing 
solution. 
The developers used in the invention may simultaneously contain other 
alkaline agents. Examples of such other alkaline agents include such 
inorganic alkaline agents as ammonium hydroxide, sodium tertiary 
phosphate, sodium secondary phosphate, potassium tertiary phosphate, 
potassium secondary phosphate, ammonium tertiary phosphate, ammonium 
secondary phosphate, sodium bicarbonate, sodium carbonate, potassium 
carbonate and ammonium carbonate; and such organic alkaline agents as 
mono-, di- or triethanolamine, mono-, di- or trimethylamine, mono-, di- or 
triethylamine, mono-, di- or isopropylamine, n-butylamine, mono-, di- or 
triisopropanolamine, ethyleneimine, ethylenediimine and 
tetramethylammonium hydroxide. 
It is particularly desirable that the molar ratio of silicate [SiO.sub.2 ] 
to alkali metal oxide [M.sub.2 O] in a developer for use with this 
invention is from 0.6 to 1.5, preferably 0.7 to 1.3. In addition, the 
concentration of SiO.sub.2 in the replenisher preferably ranges from 2 to 
4% by weight. 
In a developer used in the present invention, it is possible to 
simultaneously use organic solvents having solubility in water at 
20.degree. C. of not more than 10% by weight according to need. Examples 
of such organic solvents are such carboxilic acid esters as ethyl acetate, 
propyl acetate, amyl acetate, benzyl acetate, ethylene glycol monobutyl 
acetate, butyl lactate and butyl levulinate; such ketones as ethyl butyl 
ketone, methyl isobutyl ketone and cyclohexanone; such alcohols as 
ethylene glycol monobutyl ether, ethylene glycol benzyl ether, ethylene 
glycol monophenyl ether, benzyl alcohol, methylphenylcarbinol, n-amyl 
alcohol and methylamyl alcohol; such alkyl-substituted aromatic 
hydrocarbons as xylene; and such halogenated hydrocarbons as methylene 
dichloride and monochlorobenze. These organic solvents may be used alone 
or in combination. Particularly preferred is benzyl alcohol in the 
invention. These organic solvents are added to the developer generally in 
an amount of not more than 5% by weight and preferably not more than 4% by 
weight. 
A developer used in the present invention may simultaneously contain a 
surfactant for the purpose of improving developing properties thereof. 
Examples of such surfactants include salts of higher alcohol (C.sub.8 
.about.C.sub.22) sulfuric acid esters such as sodium salt or lauryl 
alcohol sulfate, sodium salt of octyl alcohol sulfate, ammonium salt of 
lauryl alcohol sulfate, Teepol B-81 (trademark: available from Shell 
Chemicals Co., Ltd.) and disodium alkyl sulfates; salts of aliphatic 
alcohol phosphoric acid esters such as sodium salt of cetyl alcohol 
phosphate; alkyl aryl sulfonic acid salts such as sodium salt of 
dodecylbenzene sulfonate, sodium salt of isopropylnapthalene sulfonate, 
sodium salt of dinaphthalene disulfonate and sodium salt of metanitrobenze 
sulfonate; sulfonic acid salts of alkylamides; and sulfonic acid salts of 
dibasic aliphatic acid esters such as sodium dioctyl sulfosuccinate and 
sodium dihexyl sulfosuccinate. These surfactants may be used alone or in 
combination. Particularly preferred are sulfonic acid salts. These 
surfactants may be used in an amount of generally not more than 5% by 
weight and preferably not more than 3% by weight. 
In order to enhance developing stability of the developers the following 
compounds may simultaneously be used. 
Examples of such compounds are neutral salts such as NaCl and KBr as 
disclosed in J.P. KOKAI No. Sho 58-75152; chelating agents such as EDTA 
and NTA as disclosed in J.P. KOKAI No. Sho 58-190952 (U.S. Pat. No. 
4,469,776); complexes such as [Co(NH.sub.3).sub.6 ]Cl.sub.3 as disclosed 
in J.P. KOKAI No. Sho 59-121336 (U.S. Pat. No. 4,606,995); ionizable 
compounds of elements of the group IIa, IIIa or IIIb of the Periodic Table 
such as disclosed in J.P. KOKAI No. Sho 55-25100; anionic or amphoteric 
surfactants such as sodium alkyl naphthalene sulfonate and 
N-tetradecyl-N,N-dihydroxyethyl betaine as disclosed in J.P. KOKAI No. Sho 
50-51324; tetramethyldecyne diol as disclosed in U.S. Pat. No. 4,374,920, 
nonionic surfactants as disclosed in J. P. KOKAI No. Sho 60-213943; 
cationic polymers such as methyl chloride quaternary products of 
p-dimethylaminomethyl polystyrene as disclosed in J.P. KOKAI No. Sho 
55-95946; amphoteric polyelectrolytes such as copolymer of vinylbenzyl 
trimethylammonium chloride and sodium sulfites as disclosed in J.P. KOKAI 
No. Sho 56-142528; reducing inorganic salts such as sodium sulfite as 
disclosed in J.P. KOKAI No. Sho 57-192952 (U.S. Pat. No. 4,467,027) and 
alkaline-soluble mercapto compounds or thioether compounds such as 
thiosalicylic acid, cysteine and thioglycolic acid; inorganic lithium 
compounds such as lithium chloride as disclosed in J.P. KOKAI No. Sho 
58-95444; organic lithium compounds such as lithium benzoate as disclosed 
in Japanese Patent Publication for Opposition Purpose (hereinafter 
referred to as J.P. KOKOKU) No. Sho 50-34442; organicometallic surfactants 
cotaining Si, Ti or the like as disclosed in J.P. KOKAI No. Sho 59-75255; 
organoboron compounds as disclosed in J.P. KOKAI No. Sho 59-84241 (U.S. 
Pat. No. 4,500,625); quaternary ammonium salts such as tetraalkylammonium 
oxides as disclosed in European Patent No. 101,010; and bacterides such as 
sodium dehydroacetate. 
In accordance with a method of the present invention the imaging element is 
wiped with a cleaning means, preferably a dry cleaning means e.g. a cotton 
pad or a paper towel after the image-wise exposure and before the overall 
exposure is effected. This is particularly preferred when the 
thermosensitive layer comprises carbon black or a metallic pigment. 
In accordance with the present invention the imaging element can also be 
processed after mounting the image-wise and overall exposed imaging 
element on a print cylinder of a printing press. This is especially 
suitable for imaging elements which can be processed by water or a 
water-alcohol solution. Such imaging elements are described in EP-A 631189 
which therefor is incorporated herein by reference. According to a 
preferred embodiment, the printing press is then started and while the 
print cylinder with the imaging element mounted thereon rotates, the 
dampener rollers that supply dampening liquid are dropped on the imaging 
element and subsequent thereto the ink rollers are dropped. Generally 
after about 10 revolutions of the print cylinder the first clear and 
useful prints are obtained. 
According to an alternative method, the ink rollers and dampener rollers 
may be dropped simultaneously or the ink rollers may be dropped first. 
Suitable dampening liquids that can be used in connection with the present 
invention are aqueous solutions generally having an acidic pH and 
comprising an alcohol such as isopropanol. With regard to dampening 
liquids useful in the present invention, there is no particular limitation 
and commercially available dampening liquids, also known as fountain 
solutions, can be used. 
According to an alternative method, the imaging element is first mounted on 
the print cylinder of the printing press and then image-wise and overall 
exposed directly on the press. Subsequent to exposure, the imaging element 
can be developed as described above. Suitable imaging elements for an 
image-wise exposure on the press are said imaging elements which can be 
developed while mounted on the print cylinder of the printing press as 
described above.

The present invention will now be further illustrated by way of the 
following examples without any intention to limit the invention thereto. 
All parts are by weight unless stated otherwise. 
EXAMPLE 1 
Preparation of the Imaging Element 
On a Ozasol N61 plate was coated a 2.5 wt % aqueous solution of 
polyvinylalcohol (MOWIOL 56-98) in an amount of 40 g/m.sup.2 (wet coating 
amount) and then dried at 40.degree. C. 
On top of this polyvinylalcohol layer an IR-sensitive coating was applied. 
Preparation of the IR-sensitive Coating 
The IR-sensitive formulation contains the following ingredients inparts by 
weight, as indicated. 
______________________________________ 
Methyl-ethyl ketone 834.5 
Novolak MILEX XL225 (available form MITSUI) 12.25 
Nitrocellulose E620 (available from Wolff Walsrode) 12.25 
(UV-absorber) 25 
(IR-dye 1) 10 
(IR-dye 2) 50 
(Cyan-dye 1) 7.5 
(Cyan-dye 2) 14.5 
(Magenta-dye 1) 11 
(Magenta-dye 2) 6.5 
(Yellow-dye 1) 8 
(Yellow-dye 2) 7.5 
______________________________________ 
##STR1## 
The UV-layer was overcoated by means of a knife coater with the 
IR-sensitive formulation to a wet coating thickness of 20 .mu.m. 
Preparation of a Printing Plate and Making Copies of the Original 
An imaging element as described above was subjected to a scanning Nd YLF 
infrared laser emitting at 1050 nm (scan speed 1.1 m/s, spot size 15 .mu.m 
and 400 mW power on the surface of the imaging element). After this 
exposure the IR-sensitive mask had disappeared in areas exposed to the 
laser-beam. 
Subsequently, the imaging element was exposed to a high pressure halogen 
mercury vapour lamp of 1000 W at a distance of 70 cm for 90 s. 
Further the imaging element was subjected to a developing process with the 
aqueous alkaline developer Fuji DN-5, hereby removing the non-image parts. 
After development, the imaging element was mounted on a GTO 46 offsetpress. 
As an ink was used K+E 123 W and as a fountain solution Rotamatic. 
Printing was started and a good printing quality was obtained without any 
ink uptake in the non-image parts.