On the press development of a diazo based printing plate

According to the present invention there is provided a method for making a lithographic printing plate comprising the steps of: PA1 (1) image-wise exposing an imaging element having of a flexible film support in the order given (i) a uniform ink-repellant layer comprising a cross-linked hydrophilic binder and (ii) a photosensitive layer comprising a diazonium salt or a diazo resin; PA1 (2) and developing a thus obtained image-wise exposed imaging element by mounting it on a print cylinder of a printing press and wiping it with a disposable absorbing tissue moistened with water or fountain solution.

DESCRIPTION 
1. Field of the Invention 
This application is a non-provisional application of provisional 
application No. 60/011,906 filed Feb. 20, 1996. 
The present invention relates to a method for making a printing plate 
involving the use of a diazo based printing plate. In particular, the 
method of the present invention involves on press development of a diazo 
based printing plate without the production of wet waste. 
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. 
Several types of supports can be used for the manufacturing of a 
diazo-sensitized lithographic printing plate. Common supports are metal 
supports like Al or Zn and paper bases. These supports, if not sufficient 
hydrophilic by themselves, are first coated with a hydrophilic layer to 
form the hydrophilic background of the printing plate and a top layer 
containing the diazo compound is then applied (see for example 
DE-P-1900469, DE-P-2030634 and U.S. Pat. No. 3971660). 
It is known to use as hydrophilic layer in these systems a layer containing 
polyvinyl alcohol and hydrolyzed tetraethyl orthosilicate and preferably 
also silicium dioxide and/or titanium dioxide as described in e.g. EP-A 
601240, GB-P-1419512, FR-P-2300354, U.S. Pat. No. 3971660 and 4284705. 
This hydrophilic layer is overcoated with a light-sensitive layer 
containing a diazo resin or a diazonium salt in a polymeric binder. 
Upon image-wise exposure of the light-sensitive layer the exposed image 
areas become developer insoluble and the unexposed areas remain developer 
soluble. The plate is then developed with a developer to remove the 
diazonium salt or diazo resin in the unexposed areas. 
Most comercially available lithographic printing plates as described above 
use a special developer and are not developable in water or in a 
commercial dampening solution. When developing these printing plates wet 
waste is produced. 
Commercially available diazo based printing plates most commonly use an 
anodized and roughened aluminum as a support having a hydrophilic surface. 
However, commercial plates are also available that use a flexible support 
such as paper provided with a hydrophilic layer that offer a advantage of 
cost over aluminum based printing plates. For example, Lithocraft 10008 
FOTOPLATTE.TM. is a diazo based printing plate that comprises on a paper 
support a hydrophilic layer on top of which is provided a diazo based 
photosensitive layer. According to plate instructions of the supplier, a 
plate can be prepared by image-wise exposure of the lithographic printing 
plate precursor or imaging element, mounting the exposed imaging element 
on the press and wiping its surface with Lithocrafte 10008 Developer 
Desensitizer. The plate instructions also contemplate a method wherein no 
developer desensitizer is used. However, such method most often results in 
poor lithographic performance so that in practice a Developer Desensitizer 
is almost always needed. This problem is particularly apparent when the 
imaging element was stored for a long time before using it in making a 
printing plate. 
Hydroprint, a lithographic printing plate on polyester sold by 
Agfa-Gevaert, Belgium, can be developed with plain water with good 
lithographic results. This has the advantage that no chemistry is needed 
so that said development is safer and more convenient for the printer and 
that there is less organic pollution of the environment 
However in most cases said lithographic plate becomes dry or is exposed to 
daylight after the development step. The exposure to daylight or starting 
printing with a dry plate give lithographic problems such as fog, 
increased dot gain, fingerprints, etc. 
Development of such a water developable diazo based printing plate having a 
flexible support on a printing press thus potentially has the advantage 
that the plate does not become dry or is not exposed to daylight resulting 
in a further cost reduction and moreover more convenience for a printer. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to improve the lithographic 
performance of a diazo based printing plate comprising as a lithographic 
base a flexible support provided with an ink-repellant layer when used in 
on the press development and whereby no wet waste is produced. 
Further objects of the present invention will become clear from the 
description hereinafter. 
According to the present invention there is provided a method for making a 
lithographic printing plate comprising the steps of: 
(1) image-wise exposing an imaging element having on a flexible film 
support in the order given (i) a uniform ink-repellant layer comprising a 
cross-linked hydrophilic binder and (ii) a photosensitive layer comprising 
a diazonium salt or a diazo resin; 
(2) and developing a thus obtained image-wise exposed imaging element by 
mounting it on a print cylinder of a printing press and wiping it with a 
disposable absorbing tissue moistened with water or fountain solution. 
The present invention also provides a method for making multiple copies of 
an original comprising the steps of: 
(1) image-wise exposing an imaging element having on a flexible film 
support in the order given (i) a uniform ink-repellant layer comprising a 
cross-linked hydrophilic binder and (ii) a photosensitive layer comprising 
a diazonium salt or a diazo resin; 
(2) mounting a thus obtained image-wise exposed imaging element on a print 
cylinder of a printing press and wiping it with a disposable absorbing 
tissue moistened with water or fountain solution; 
(3) subsequently rotating said print cylinder while supplying an aqueous 
dampening liquid and/or supplying ink to said photosensitive layer of said 
imaging element and 
(4) transfering ink from said imaging element to a receiving element, which 
is generally a sheet of paper. 
DETAILED DESCRIPTION OF THE INVENTION 
It has been found that by development of an imaging element in accordance 
with the present invention on the printing press, excellent lithographic 
performance is obtained in case a flexible film support provided with a 
cross-linked hydrophilic binder layer as an ink-repellant layer is used. 
Moreover, no particular liquid such as a developer desensitizer needs to 
be wiped on the exposed imaging element to achieve this lithographic 
performance. It has further been found that only solid waste is produced 
during the development of said imaging element. 
Preferably, the disposable absorbing tissue in accordance with the present 
invention is an absorbing paper, more preferably with a swell capacity of 
at least 2. The swell capacity of a tissue is defined as : weight at 
maximum water absorption--dry weight/ dry weight. Suitable papers are e.g. 
Edetfort 350 and Kimtex Magnum tissue (trade name from Kimberley-Clark). 
After mounting an image-wise exposed imaging element on a print cylinder of 
a printing press and wiping it with a disposable absorbing tissue 
moistened with water or fountain solution the print cylinder is 
subsequently rotated while supplying an aqueous dampening liquid and/or 
supplying ink. Subsequently in this context means that drying of the plate 
or excessive exposure to daylight is not possible. The time between wiping 
the image-wise exposed imaging element and the start of the printing may 
depend on the climatic conditions in the printing room but is preferably 
less than 30 min., more preferably less than 20 min., most preferably less 
than 10 min. 
An exposed imaging element in accordance with the present invention is 
preferably mounted on a printing press and used to print shortly after the 
exposure. It is however possible to store an exposed imaging element for 
some time in the dark before using it on a printing press to print copies. 
The imaging element for use in accordance with the present invention 
comprises on a flexible film support in the order given (i) an 
ink-repellant layer comprising a cross-linked hydrophilic binder and (ii) 
a photosensitive layer comprising a diazonium salt or a diazo resin. 
As hydrophilic binder in the ink repellant layer 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. 
According to a highly preferred embodiment in connection with the present 
invention, a hydrolyzed tetraalkyl orthosilicate cross-linker is used to 
cross-link the hydrophilic binder. However, other cross-linkers can be 
used such as e.g. a formaldehyde, glyoxal, polyisocyanate etc. . . 
Examples of hydrolyzed tetraalkyl orthosilicate crosslinking agents are 
hydrolyzed tetraethyl orthosilicate and hydrolyzed tetramethyl 
orthosilicate. 
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. 
The ink repellant layer in the imaging element used 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 ink repellant 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 the ink repellant layer in the material of this invention 
may vary in the range of 0.2 to 25 .mu.m and is preferably 1 to 10 .mu.m. 
Particular examples of suitable ink-repellant 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. 3971660, U.S. Pat. No. 4284705 and EP-A 
514490. 
Examples of low-molecular weight diazonium salt for use in the present 
invention include: benzidine tetrazoniumchloride, 3,3'-dimethylbenzidine 
tetrazoniumchloride, 3,3'-dimethoxybenzidine tetrazoniumchloride, 
4,4'-diaminodiphenylamine tetrazoniumchloride, 3,3'-diethylbenzidine 
tetrazoniumsulfate, 4-aminodiphenylamine diazoniumsulfate, 
4-aminodiphenylamine diazoniumchloride, 4-piperidino aniline 
diazoniumsulfate, 4-diethylamino aniline diazoniumsulfate and oligomeric 
condensation products of diazodiphenylamine and formaldehyde. 
Examples of diazo resins useful in the present invention include 
condensation products of an aromatic diazonium salt as the light-sensitive 
substance. Such condensation products are known and are described, for 
example, in German Pat. no. 1214086. They are in general prepared by 
condensation of a polynuclear aromatic diazonium compound, preferably of 
substituted or unsubstituted diphenylamine-4-diazonium salts, with active 
carbonyl compounds, preferably formaldehyde, in a strongly acidic medium. 
Preferred diazonium salts or resins are diazonium salts or resins of 
p-aminodiphenylamine containing as substituent an alkyl or alkoxy group 
and the weight percentage of said diazonium salt(s) and/or diazo resin(s) 
containing or being a diazonium salt of p-aminodiphenylamine containing as 
substituent an alkyl or alkoxy group versus the total amount of diazo 
resin and/or diazonium salt ranges between 22 and 40%. More preferably 
said substituent contains from 1 to 4 carbon atoms, still more preferably 
1 or 2 carbon atoms. Even more preferably said substituent is an alkoxy 
group. Particularly preferably said diazo resin or diazonium salt 
containing or being a diazonium salt of p-aminodiphenylamine containing as 
substituent an alkyl or alkoxy group is a 
3-methoxydiphenylamine-4-diazonium salt. Most preferably said diazo resin 
or diazonium salt containing or being a diazonium salt of 
p-aminodiphenylamine containing as substituent an alkyl or alkoxy group is 
a salt of the condensation product of formaldehyde with a 
3-methoxy-diphenylamine-4-diazonium salt. 
The light sensitive layer of an imaging element in connection with the 
present invention preferably also contains a hydrophilic binder. A 
suitable hydrophilic binder is for example pullulan. 
Pullulan is a polysacharide that is produced by micro-organisms of the 
Aureobasidium pullulans type (Pullularia pulluians) and that contains 
maltotriose repeating units connected by a .alpha.-1,6 glycosidic bond. 
Pullulan is generally produced on industrial scale by fermentation of 
partially hydrolysed starch or by bacterial fermentation of sucrose. 
Pullulan is commmercially available from e.g. Shodex, Pharmacosmos. 
Alternatively the light sensitive layer may contain polyvinylacetate 
hydrolysed to an extent of at least 95% by weight as a binder, more 
preferably to an extent of at least 97% by weight. 
Preferably the light sensitive layer further includes a cationic fluor 
containing surfactant, preferably a perfluorinated surfactant and more 
preferably a perfluorinated ammonium surfactant. Typical examples of 
perfluorinated ammonium surfactants are: n.C.sub.8 F.sub.17 SO.sub.2 
NH--(CH.sub.2).sub.3 N.sup.+ (CH.sub.3).sub.3 I.sup.- (Fluorad FC 135 from 
3M) n.C.sub.9 F.sub.19 SO.sub.2 NH--(CH.sub.2).sub.4 N.sup.+ 
(CH.sub.3).sub.3 Br.sup.- n.c.sub.7 F.sub.15 CONH--(CH.sub.2).sub.3 
N.sup.+ (CH.sub.3).sub.3 I.sup.- (n.C.sub.8 F.sub.17 
COO--(CH.sub.2).sub.4).sub.2 N.sup.+ (CH.sub.3).sub.2 I.sup.- 
The light sensitive layer according to the present invention preferably 
also contains dispersed water-insoluble polymers. Said aqueous dispersion 
of water insoluble polymer is preferably cationic or nonionic either e.g. 
as a result of an emulsifier or by having the cationic or nonionic group 
linked to the polymer. The water insoluble polymer is preferably a solid 
particulate having a size in the range of about 100 Angstroms to 1 micron 
in diameter and does not form a film below 30.degree. C. In general, any 
polymer which carries a cationic or nonionic group or which can be 
formulated into an emulsion using a cationic or nonionic emulsifier can be 
employed in the present invention. Suitable polymers include homopolymers 
and copolymers of styrene, methylacrylate, ethylacrylate, butylacrylate, 
methylmethacrylate, ethylmethacrylate, butyl methacrylate, vinyl acetate, 
vinyl chloride, vinylidene chloride, butadiene, methyl styrene, vinyl 
toluene, dimethylaminoethyl acrylate, acrylic acid, methacrylic acid, 
isoprene, chloroprene, maleic anhydride, ethylene glycol acrylates such as 
polyethylene glycol acrylate, halogenated vinyl aromatics such as 
chlorostyrene and bromostyrene, methylvinyl ether, vinyl pyrrolidone, 
polyurethane and the like. 
Among the cationic and nonionic emulsifiers which can be used in the 
present invention are: ammonium salts of substituted amines containing 
alkyl and/or aryl groups attached to the nitrogen, alkyl or aryl sulfonium 
salts, alkyl and alkyl-aryl polyethers, cationic or nonionic 
fluorosurfactants and polyoles. 
The thickness of the photosensitive layer in the material of this invention 
may vary in the range of 0.1 to 10 .mu.m and is preferably between 0.5 and 
2.5 .mu.m. 
The photosensitivity of an imaging element in connection with the present 
invention is preferably such that an exposure to daylight to an extent of 
not more than 250 000 lux.s does not substantially result in changes in 
the lithographic behaviour of the printing plate. This will allow 
sufficient convenience in handling and mounting of an image-wise exposed 
imaging element. The photosensitivity of the imaging element may be easily 
adapted by appropriate choice of a particular diazo resin or diazonium 
salt, the amount thereof and the thickness of the photosensitive layer. 
The imaging element in connection with the present invention advantageously 
contains water-soluble dyes such as rhodamines, sudan blue, methylen blue, 
eosin or trifenylmethane dyes such as crystal violet, victoria pure blue, 
malachite green, methylviolet and fuchsin or dye pigments. These colorants 
may be incorporated in the photosensitive layer and/or ink-repellant 
layer. 
Suitable supports that can be used in an imaging element in accordance with 
the present invention are e.g. photographic film bases e.g. substrated 
polyethylene terephthalate film, substrated polyethylene 
naphthalenedicarboxylate film, cellulose acetate film, plastics having a 
metal layer or deposit thereon, and polyolefin (e.g. polyethylene) coated 
paper, the polyolefin surface of which may have been subjected to a corona 
discharge to improve the adherence of a hydrophilic layer. 
One or more subbing layers may be coated between the support and the 
hydrophilic layer for use in accordance with the present invention in 
order to get an improved adhesion between these two layers. 
A preferred subbing layer for use in connection with the present invention 
is a subbing layer comprising a hydrophilic binder and silica. 
As hydrophilic binder in said subbing layer usually a protein, preferably 
gelatin may be used. Gelatin can, however, be replaced in part or 
integrally by synthetic, semi-synthetic, or natural polymers. Synthetic 
substitutes for gelatin are e.g. polyvinyl alcohol, poly-N-vinyl 
pyrrolidone, polyvinyl imidazole, polyvinyl pyrazole, polyacrylamide, 
polyacrylic acid, and derivatives thereof, in particular copolymers 
thereof. Natural substitutes for gelatin are e.g. other proteins such as 
zein, albumin and casein, cellulose, saccharides, starch, and alginates. 
In general, the semi-synthetic substitutes for gelatin are modified 
natural products e.g. gelatin derivatives obtained by conversion of 
gelatin with alkylating or acylating agents or by grafting of 
polymerizable monomers on gelatin, and cellulose derivatives such as 
hydroxyalkyl cellulose, carboxymethyl cellulose, phthaloyl cellulose, and 
cellulose sulphates. 
A preferred silica in said subbing layer is a siliciumdioxide of the 
anionic type. The colloidal silica preferably has a surface area of at 
least 100 m.sup.2 per gram, more preferably a surface area of at least 300 
m.sup.2 per gram. 
The surface area of the colloidal silica is determined according to the 
BET-value method described by S. Brunauer, P. H. Emmett and E. Teller, 
J.Amer. Chem. Soc. 60, 309-312 (1938). 
The silica dispersion may also contains other substances, e.g. aluminium 
salts, stabilising agents, biocides etc. 
Such types of silica are sold under the name KIESELSOL 100, KIESELSOL 300 
and KIESELSOL 500 (KIESELSOL is a registered trade name of Farbenfabriken 
Bayer AG, Leverkusen, West-Germany whereby the number indicates the 
surface area in m.sup.2 per gram). 
The weight ratio of the hydrophilic binder to silica in the subbing layer 
is preferably less than 1. The lower limit is not very important but is 
preferably at least 0.2. The weight ratio of the hydrophilic binder to 
silica is more preferably between 0.25 and 0.5. 
The coverage of said subbing layer is preferably more than 200 mg per 
m.sup.2 but less than 750 mg per m.sup.2, more preferably between 250 mg 
per m.sup.2 and 500 mg per m.sup.2. 
The coating of the above defined subbing layer composition preferably 
proceeds from an aqueous colloidal dispersion optionally in the presence 
of a surface-active agent. 
According to a preferred embodiment in connection with the present 
invention there is provided an intermediate layer of an organic compound 
having cationic groups between the hydrophilic layer and the light 
sensitive layer as described in EP-A 601240. As a consequence the 
development by plain water of such a diazo based imaging element is 
improved. 
Organic compounds having cationic groups for use in an intermediate layer 
are preferably hydrophilic and may be low moleculair weight compounds but 
are preferably polymers. Preferred compounds are those having one or more 
ammonium groups or amino groups that can be converted to ammonium groups 
in an acidic medium. An especially preferred type of cationic compounds 
are polycharides modified with one or more groups containing an ammonium 
or amino group. 
Most preferred organic compounds having cationic groups are dextrans or 
pullulan wherein at least some of the hydroxy groups of the dextran or 
pullulan has been modified into one or more of the following groups: 
EQU --O--R.sup.1 
EQU --O--CO--R.sup.2 
wherein R.sup.1 represents an organic residue containing an amino or 
ammonium group, e.g. an amine substituted alkyl, an amine substituted 
alkylaryl etc. 
R.sup.2 has one of the significances given for R.sup.1 or stands for 
--OR.sup.3 or --N(R.sup.4)R.sup.5, wherein R.sup.3 has one of the 
significances gives for R.sup.1 and each of R.sup.4 and R.sup.5 which may 
be the same or different have one of the significances given for R.sup.1. 
Examples of dextrans or pullulan that can be used in accordance with the 
present invention are dextrans or pullulan wherein some of the hydroxyl 
groups have been modified in one of the groups shown in table 1. 
TABLE 1 
______________________________________ 
no. modified group 
______________________________________ 
1 --O--CH.sub.2 --CH.sub.2 --NH.sub.2 
2 --O--CO--NH--CH.sub.2 --CH.sub.2 --NH.sub.2 
3 --O--CO--NH--CH.sub.2 --CH.sub.2 --N 
(CH.sub.2 --CH.sub.2 --NH.sub.2).sub.2 
4 --O--CH.sub.2 --CH.sub.2 --NH--CH.sub.2 --CH.sub.2 --NH.sub.2 
5 --O--CH.sub.2 --CH.sub.2 --NH--CH.sub.2 --CHOH--CH.sub.2 --N.sup.+ 
(CH.sub.3).sub.3 Cl.sup.- 
6 --O--(CH.sub.2 --CH.sub.2 --O).sub.n --CH.sub.2 --CH.sub.2 --NH.sub.2 
3 
wherein n represents an integer from 1 to 50 
7 --O--CO--NH--CH.sub.2 --CH.sub.2 --NH--CH.sub.2 --CHOH--CH.sub.2 
--N.sup.+ 
(CH.sub.3).sub.3 Cl.sup.- 
8 --O--CH.sub.2 --CH.sub.2 --N(CH.sub.2 --CH.sub.3).sub.2 .HCl 
9 --O--CH.sub.2 --CH.sub.2 --N(CH.sub.2 --CH.sub.2 --NH.sub.2).sub.2 
10 --O--CONH--CH.sub.2 --CH.sub.2 --N(CH.sub.2 --CH.sub.2 --NH.sub.2).su 
b.2 
11 --O--CONH--(CH.sub.2 --CH.sub.2 --O).sub.n --CH.sub.2 --CH.sub.2 
--NH.sub.2 
______________________________________ 
The modified dextrans or pullulan can be prepared by a reaction of a 
dextran with e.g. alkylating agents, chloroformates, acid halides, 
carboxylic acids etc. . . 
An intermediate layer containing the organic compound having one or more 
cationic groups is preferably provided in an amount of 5 to 500 mg/m.sup.2 
and more preferably in an amount of 10 to 200 mg/m.sup.2. 
The exposure of the imaging element used in the present invention 
advantageously proceeds with ultraviolet light optionally in combination 
with blue light in the wavelength range of 250 to 500 nm. Useful exposure 
sources are high or medium pressure halogen mercury vapour lamps, e.g. of 
1000 W. Since most lithography is done by the offset process, the imaging 
element is exposed in such a way that the image obtained thereon is right 
reading. The exposure may be an exposure using optics or a contact 
exposure. 
Subsequent to image-wise exposure, the image-wise exposed imaging element 
is mounted on a print cylinder of a printing press with the backside of 
the imaging element (side of the support opposite to the side having the 
photosensitive layer). According to the invention a thus mounted 
image-wise exposed imaging element is wiped with a disposable absorbing 
tissue moistened with water or fountain solution. In 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 resolutions 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 liquids 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.

EXAMPLE 1 
Preparation of a lithographic base. 
To 440 g of a dispersion containing 21.5 of TiO.sub.2 (average particle 
size 0.3 to 0.4 .mu.m) and 2.5% of polyvinyl alcohol in deionized water 
were subsequently added, while stirring, 250 g of a 5% of polyvinyl 
alcohol solution in water, 105 g of a hydrolyzed 22% 
tetramethylorthosilicate emulsion in water and 12 g of a 10% solution of a 
wetting agent. 
To this mixture was added 193 g of deionized water and the pH was adjusted 
to pH=4. 
The obtained dispersion was coated on a polyethyleneterephthalate film 
support (coated with a hydrophilic adhesion layer) to a wet coating 
thickness of 50 g/m.sup.2, dried at 30.degree. C., and subsequently 
hardened by subjecting it to a temperature of 57.degree. C. for 1 week. 
To this base was further provided an aqueous solution (pH=5) of Dormacid (a 
dextran modified with a diethylaminoethyl group available from Pfeifer & 
Langen) and a cationic wetting agent to a dry coating thickness of 30 mg 
Dormacid per m.sup.2. 
The obtained element was then heated for 1 week at 57.degree. C. 
Preparation of the imaging elements. 
An imaging element according to the invention was produced by preparing the 
following light-sensitive composition and coating it to the above 
described lithographic base in an amount of 35 g/m.sup.2 (wet coating 
amount) and drying it at 30.degree. C. 
Preparation of the light-sensitive coating. 
To 63 g of a 20% dispersion of polymethylmethacrylate (particle diameter of 
40 nm) stabilized with cetyltrimethylammoniumbromide in deionized water 
was subsequently added, while stirring, 120 g of a 5% solution of a 98% 
hydrolized polyvinylacetate, having a weight average molecular weight of 
200 000 g/mol (MOWIOL 56-96 available from Hoechst) in water and 15 g of a 
dispersion containing 10% of Heliogen Blue D 7565 (available from BASF) 
and 5% of polyvinylalcohol (MOWIOL 56-98) in water. 46 g of a 15% solution 
of the condensation product of diphenylamine diazonium salt and 
formaldehyde (NEGALUX N18 available from PCAS) and 20 g of a 15% solution 
of the condensation product of methox-diphenylamine diazonium salt and 
formaldehyde (DIAZO No. 8 available from Fairmount) in water were then 
slowly added. Finally 30 g of a 1.6% solution of a cationic fluor 
containing surfactant (Fluorad FC135 available from 3M) in water, and 726 
ml of water were added. 
Preparation of a printing plate and making copies of the original 
The above described imaging element was exposed through an original (mask) 
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 mounted on an AB Dick 360 offsetpress with 
VARN dampening system equipped with Van Son RB 2329 ink and fountain 
solution G 671c (3% in water, commercially available from AGFA-GEVAERT). 
Subsequently water is applied to the plate surface using a disposable 
absorbing tissue wetted with water, wiping the entire plate surface and 
hereby removing the non-image parts. Subsequently the press is started by 
first dropping the dampener rollers on the wet plate for 5 revolutions and 
subsequent dropping the ink rollers for 5 revolutions. 
Printing was started on a 80 g paper and a good printing quality was 
obtained without any ink uptake in the non-image parts and a screen 
rendering 2-80% on paper. 
As absorbing paper was used: Edetfort 350. 
No liquid waste has to be drained and the amount of solid waste could be 
limited to max. 25 g. 
EXAMPLE 2 
(Comparative Example) 
The imaging element as described above was exposed through an original 
(mask) to a high pressure haloge n mercury vapour lamp of 1000 W at a 
distance of 70 cm for 90 s, and hereafter the unexposed parts were 
washed-off manually: 
herefore the plate was put on a flat and solid surface, 
Water was applied to the plate surface, allowing the unexposed parts to 
dissolve, 
while rinsing the entire plate surface was wiped with a soft sponge using a 
firm pressure onto the plate surface, this was done until all non-image 
parts were removed, 
the plate surface was rinsed with water, 
the two latter steps were repeated briefly, 
excess water was removed and the plate was buffed dry with a soft paper. 
The plate was further allowed to air-dry in a yellow light room (20.degree. 
C. at 50% relative humidity) avoiding exposure to UTV-light during 2 
hours, and hereafter mounted on an AB Dick 360 offsetpress with VARN 
dampening system containing the same ink and fountain as mentioned above. 
Subsequently the plate is prewetted and the press is started by first 
dropping the dampener rollers on the plate for 5 revolutions and 
subsequent dropping the ink rollers for 5 revolutions. Printing on 80 g 
paper was started and a good printing quality was obtained without any ink 
uptake in the non-image parts and a screen rendering 2-70/80% on paper. 
The obtained screen rendering is lower than was obtained using the method 
according to example 1 (method according to the invention) and some 
fingerprint contamination was noticed. 
EXAMPLE 3 
(Comparative Example) 
The imaging element as described above was exposed through an original 
(mask) to a high pressure halogen mercury vapour lamp of 1000 W at a 
distance of 70 cm for 90 s, and hereafter the imaging element was mounted 
on an AB Dick 360 offsetpress with VARN dampening system equipped with Van 
Son RB 2329 ink and fountain solution G 671c (3% in water, commercially 
available from AGFA-GEVAERT). 
Subsequently water is applied to the plate surface using a (reusable) 
sponge wetted with water, wiping the entire plate surface and hereby 
removing the non-image parts. 
Subsequently the press is started by first dropping the dampener rollers on 
the wet plate for 5 revolutions and subsequent dropping the ink rollers 
for 5 revolutions. 
Printing was started on a 80 g paper and a good printing quality was 
obtained without any ink uptake in the non-image parts and a screen 
rendering 2-80% on paper. 
The contaminated sponge has to be properly cleaned for reuse and this 
requires at least 100 ml water (plate 381.times.254 mm) that has to be 
collected or may be drained.