Method for producing lithographic plates with imaging elements comprising a photosensitive acid precursor

The present invention provides a method for obtaining a lithographic printing plate comprising the steps of: (a) image-wise or information-wise exposing a photosensitive imaging element comprising on a hydrophilic surface of a support a hydrophobic photosensitive layer contiguous to said hydrophilic surface, comprising (i) a photosensitive acid precursor and (ii) a hydrophobic polymeric binder and (b) developing said exposed imaging element by the steps of: (i) laminating before or after said exposure an uppermost layer of said imaging element to a receptor layer and (ii) peeling away the receptor layer from the hydrophilic surface of the support thus transferring said hydrophobic photosensitive composition patternwise to the receptor layer, characterized in that said hydrophobic photosensitive layer comprises as photosensitive acid precursor a non-ionic photosensitive precursor of a sulphonic acid or a non-ionic photosensitive precursor of an optionally partially esterified phosphonic acid or a non-ionic photosensitive precursor of an optionally partially esterified phosphoric acid, the latter phosphonic or phosphoric acid precursors in the absence of an acid-sensitive composition of (a) low molecular weight compound(s).

DESCRIPTION 
1. Field of the Invention. 
The present invention relates to a photosensitive imaging element 
comprising a photosensitive acid precursor and a hydrophobic polymeric 
binder and to a method for the formation of lithographic printing plates 
therewith. 
2. Background of the Invention. 
The use of photosensitive imaging elements comprising a photosensitive acid 
precursor and an acid-sensitive composition for the production of images 
by information-wise exposure thereof to actinic radiation is well known. 
Almost all these materials are based on the principle of introducing a 
differentiation in solubility between the exposed and non-exposed parts of 
the acid-sensitive composition. The thus produced difference in solubility 
may be subsequently employed in a developing step to produce a visible 
image. 
A difference in solubility between the exposed and non-exposed parts of the 
acid-sensitive composition is often used for the production of 
lithographic printing plates where a hydrophilic base is coated with the 
acid-sensitive composition, subsequently exposed and developed using a 
solvent, in general an aqueous solution to remove pattern-wise the 
acid-sensitive composition. When the photogenerated acid enhances the 
solubility of the acid-sensitive composition in the solvent used. The 
sufficiently exposed areas of the imaging element are removed during the 
development and the process is positive working. The process can be 
negative working as well when the photogenerated acid diminishes the 
solubility of the acid-sensitive composition in the solvent used, the 
non-exposed or insufficiently exposed parts of the imaging element then 
being removed during the development. Such a process is for example 
described in "Unconventional imaging processes" by E. Brinckman, G. 
Delzenne, A. Poot and J. Willems, Focal Press London-New York, first 
edition 1978, pages 88 to 91. 
The use of a difference in tackiness to obtain an image is described in 
e.g. U.S. Pat. No. 4,810,601. According to the method disclosed in this US 
patent the image-wise exposed acid-sensitive composition becomes, after 
heating, tacky in the exposed parts while the non-exposed parts remains 
non-tacky. The exposed parts can therefore be colored with absorbing dyes 
to make the image visible. 
The development of photosensitive imaging elements comprising a 
photosensitive acid precursor and an acid-sensitive composition and based 
on the principle of introducing a differentiation in solubility between 
the exposed and non-exposed parts of the acid-sensitive composition 
requires in most cases the use of an aqueous developer. This is a 
cumbersome and inherently dangerous operation for the user of said imaging 
elements due to the possible contact with said hazardous aqueous 
developer. 
Moreover, the developed imaged elements need a further rinsing after the 
development step so that a sizeable amount of diluted aqueous waste is 
produced during the processing of said imaging elements, being an 
ecological and economical disadvantage for said process. 
GB-A 2,009,436 and 2,009,955 however disclose a process of forming an 
image, which comprises (a) imagewise exposing a photosensitive 
image-forming material composed of a support and a layer of a 
thermoplastic photosensitive composition, said photosensitive composition 
comprising specified photosensitive acid precursors, (b) placing the 
exposed photosensitive material with said layer in intimate contact with a 
peeling development carrier sheet, at least one surface of which is 
composed of a thermoplastic material, and heating the assembly to a 
temperature above the softening temperature of at least one of the 
photosensitive composition layer and the peeling development carrier 
sheet, and then (c) peeling the development carrier sheet from the 
photosensitive image forming material at a temperature which is the same 
as or less than said heating temperature, thereby to leave the exposed or 
unexposed area of the photosensitive composition layer on the peeling 
development carrier sheet, and the corresponding unexposed or exposed area 
on the support. 
However, the specified photosensitive acid precursors are thermally 
unstable (diazo compounds) or ecologically harmful (organic polyhalogen 
compounds) compounds. 
3. Summary of the Invention. 
It is an object of the present invention to provide a method for obtaining 
a lithographic printing plate of a high quality using a photosensitive 
imaging element comprising a photosensitive acid precursor and a 
hydrophobic polymeric binder by a dry-developing step wherein said 
photosensitive acid precursor is a thermally stable and ecologically 
acceptable compound. 
It is another object of the present invention to provide a method for 
obtaining a lithographic printing plate using said photosensitive imaging 
element in a convenient way, offering economical and ecological 
advantages. 
Further objects of the present invention will become clear from the 
description hereinafter. 
According to the present invention there is provided a method for obtaining 
a lithographic printing plate comprising the steps of: (a) image-wise or 
information-wise exposing a photosensitive imaging element comprising on a 
hydrophilic surface of a support a hydrophobic photosensitive layer 
contiguous to said hydrophilic surface, comprising (i) a photosensitive 
acid precursor and (ii) a hydrophobic polymeric binder and (b) developing 
said exposed imaging element by the steps of: (i) laminating before or 
after said exposure an uppermost layer of said imaging element to a 
receptor layer and (ii) peeling away the receptor layer from the 
hydrophilic surface of the support thus transferring said hydrophobic 
photosensitive composition patternwise to the receptor layer, 
characterized in that said hydrophobic photosensitive layer comprises as 
photosensitive acid precursor a non-ionic photosensitive precursor of a 
sulphonic acid or a non-ionic photosensitive precursor of an optionally 
partially esterified phosphonic acid or a non-ionic photosensitive 
precursor of an optionally partially esterified phosphoric acid, the 
latter phosphonic or phosphoric acid precursors in the absence of an 
acid-sensitive composition of (a) low molecular weight compound(s). 
4. Detailed Description of the Invention. 
It has been found that lithographic printing plates can be obtained 
according to the method of the present invention using an imaging element 
comprising on a hydrophilic base a hydrophobic photosensitive layer 
containing a hydrophobic polymeric binder and a photosensitive acid 
precursor as described above by using a dry-developing step. More 
precisely it has been found that said printing plates are of high quality, 
contains no thermally unstable or ecologically harmful compounds and are 
provided in a convenient way, thereby offering economical and ecological 
advantages. 
Preferred non-ionic photosensitive precursors of a sulphonic acid are 
represented by iminosulfonate as described in Polymer Preprints Japan, by 
M. Tunooka et al, 35 (8), by disulfon compounds described in JP-Pi 
61-166544, by .alpha.-sulphonyloxy ketones, by 
.alpha.-hydroxymethylbenzoine sulphonates, by o-nitrobenzyl sulphonates, 
by .alpha.-sulphonyl acetophenones and by sulphonyl imides, the 
preparation of these compounds being well known in the literature. 
Preferred non-ionic photosensitive precursors of optionally partially 
esterified phosphonic acids or of optionally partially esterified 
phosphoric acids are derivatives of optionally partially esterified 
phosphonic acid or optionally partially esterified phosphoric acid 
containing as photosensitive group a o-nitrobenzylgroup, a 
1-(2-nitrophenyl)ethylgroup, a benzoinegroup, a 
3,5-dimethoxybenzoinegroup, phosphonic acid esters or phosphoric acid 
esters of pyrenemethanol, iminophosphonates or iminophosphates and 
imidophosphonates or imidophosphates, the preparation of these compounds 
being well known in the literature. 
Acid-sensitive compositions of (a) low molecular weight compounds in 
accordance with the invention are monomers capable of undergoing cationic 
polymerization which are well known to one skilled in the art, a compound 
with at least two hydroxyl groups and a reagent which is capable of 
crosslinking under the influence of an acid said compound with at least 
two hydroxyl groups and a compound comprising at least two latent or 
masked electrophilic groups that are transformed into electrophilic groups 
upon reaction with acid and a compound containing an aromatic moiety that 
is susceptible to electrophilic aromatic substitution. 
Most preferably, the photosensitive acid precursors used in connection with 
the present invention are o-nitrobenzyl phosphonates, o-nitrobenzyl 
phosphates and o-nitrobenzyl sulphonates. 
The support of the imaging element according to the present invention has a 
hydrophilic surface and should be stable at the processing conditions. 
Said support with a hydrophilic surface may be a metallic support e.g. a 
grained and anodized aluminium support. 
More preferably, said support with a hydrophilic surface comprises a 
hardened hydrophilic layer, containing a hydrophilic binder and a 
hardening agent coated on a flexible support. 
Such hydrophilic binders are disclosed in e.g. EP-A 450,199, which therefor 
is incorporated herein by reference. Preferred hardened hydrophilic layers 
comprise partially modified dextrans or pullulan hardened with an aldehyde 
as disclosed in e.g. EP-A 514,990. More preferred hydrophilic layers are 
layers of polyvinyl alcohol hardened with a tetraalkyl orthosilicate and 
preferably containing SiO.sub.2 and/or TiO.sub.2 wherein the weight ratio 
between said polyvinylalcohol and said tetraalkyl orthosilicate is between 
0.5 and 5 as disclosed in e.g. GB-P 1,419,512, FR-P 2,300,354, U.S. Pat. 
No. 3,971,660, U.S. Pat. No. 4,284,705, EP-A 405,016 and EP-A 450,199. 
A particularly suitable hydrophilic layer is a layer of polyvinyl alcohol 
hardened with tetramethylorthosilicate or tetraethylorthosilicate 
containing TiO.sub.2, wherein the weight ratio between said 
polyvinylalcohol and said tetramethylorthosilicate or 
tetraethylorthosilicate is between 0.8 and 2 and wherein the weight ratio 
between said polyvinylalcohol and said titaniumdioxide is preferably not 
higher than 1. 
The above mentioned flexible supports may be opaque or transparent, e.g. a 
paper support or resin support. When a paper support is used preference is 
given to one coated at one or both sides with an Alpha-olefin polymer, 
e.g. a polyethylene layer which optionally contains an anti-halation dye 
or pigment. It is also possible to use an organic resin support e.g. 
cellulose esters such as cellulose acetate, cellulose propionate and 
cellulose butyrate; polyesters such as poly(ethylene terephthalate); 
polyvinyl acetals, polystyrene, polycarbonates; polyvinylchloride or 
poly-Alpha-olefins such as polyethylene or polypropylene. 
One or more subbing layers may be coated between the flexible hydrophobic 
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 which is contiguous to the hydrophilic layer and 
contains gelatin and silica. 
According to the present invention the imaging element comprises on the 
hydrophilic surface of the support and contiguous thereto a hydrophobic 
photosensitive layer comprising at least one photosensitive acid precursor 
and at least one hydrophobic polymeric binder. 
According to a preferred mode of the present invention the photosensitive 
acid precursors are comprised in a hydrophobic thermoplastic layer 
comprising a hydrophobic thermoplastic polymer. 
Said thermoplastic layer is preferably solid at temperatures below 
40.degree. C. and is after image-wise exposure pattern-wise transferable 
at temperatures between 40.degree. C. and 250.degree. C. 
Suitable hydrophobic thermoplastic polymers for use in accordance with the 
present invention include: 
(A) Copolyesters, e.g. those prepared from the reaction product of an 
alkylene glycol e.g. polymethylene glycol of the formula 
HO(CH.sub.2).sub.v OH, wherein v is a whole number 2 to 10 inclusive, and 
(1) hexahydroterephthalic, sebacic and terephthalic acids, (2) 
terephthalic, isophthalic and sebacic acids, (3) terephthalic and sebacic 
acids, (4) terephthalic and isophthalic acids, and (5) mixtures of 
copolyesters prepared from said glycols and (i) terephthalic, isophthalic 
and sebacic acids and (ii) terephthalic, isophthalic, sebacic and adipic 
acids. 
(B) Nylons or polyamides, e.g. N-methoxymethyl polyhexamethylene adipamide; 
(C) Vinylidene chloride copolymers, e.g. vinylidene chloride/acrylonitrile; 
vinylidene chloride/methylacrylate and vinylidene chloride/vinylacetate 
copolymers; 
(D) Ethylene/vinyl acetate copolymer; 
(E) Cellulosic ethers, e.g. methyl cellulose, ethyl cellulose and benzyl 
cellulose; 
(F) Polyethylene; 
(G) Synthetic rubbers, e.g. butadiene/acrylonitrile copolymers, and 
chloro-2-butadiene-1,3 polymers; 
(H) Cellulose esters, e.g. cellulose acetate, cellulose acetate succinate 
and cellulose acetate butyrate, cellulose nitrate; 
(I) Polyvinyl esters, e.g. polyvinyl acetate/acrylate, polyvinyl 
acetate/methacrylate and polyvinyl acetate; 
(J) Polyacrylate and alpha-alkyl polyacrylate esters, e.g. polymethyl 
methacrylate and polyvinyl acetate; 
(K) High molecular weight polyethylene oxides of polyglycols having average 
molecular weights from about 4,000 to 1,000.000; 
(L) Polyvinyl chloride and copolymers, e.g. polyvinyl chloride/acetate, 
polyvinylchloride/acetate/alkohol; 
(M) Polyvinyl acetal, e.g. polyvinyl butyral, polyvinyl formal; 
(N) Polyformaldehydes; 
(O) Polyurethanes and copolymers; 
(P) Polycarbonate and copolymers; 
(Q) Polystyrenes and copolymers e.g. polystyrene/acrylonitrile, 
polystyrene/acrylonitrile/butadiene. 
Preferably, the hydrophobic thermoplastic polymers used in connection with 
the present invention are copolymers of styrene, more preferably 
copolymers of styrene and (meth)acrylates, most preferably copolymers of 
styrene and butyl methacrylate. 
The photosensitive layer used in accordance with the present invention is 
preferably coated in a range from 0.30 g/m.sup.2 to 5.00 g/m.sup.2, more 
preferably in a range from 1.00 g/m.sup.2 to 3.50 g/m.sup.2. The weight 
ratio of the photosensitive acid precursor to the polymeric binder in said 
layer preferably ranges from 5% to 40%, more preferably from 8% to 30%. 
In general, the photosensitive layer used in accordance with the present 
invention also comprises at least one spectral sensitizer in order to 
sensitize said photosensitive layer to a wavelength region longer than far 
ultraviolet in which a photosensitive acid precursor used has no 
absorption. 
Preferred spectral sensitizers include benzophenone, 
p,p'-tetraethylmethyldiaminobenzophenone, 
p,p'-tetraethylethylaminobenzophenone, thio-xanthone, 
2-chlorothio-xanthone, anthrone, 9-ethoxyanthracene. anthracene, pyrene, 
perylene, phenothiazine, benzil, acridine orange, benzoflavin, 
Setoflavin-T, 9,10-diphenylanthracene, 9-fluorenone, actophenone, 
phenathrene, 2-nitrofluorene, 5-nitroacenaphthene, benzoquinone, 
2chloro-4-nitro-aniline, N-acyl-p-nitroaniline, p-nitroaniline, 
N-acyl-4-nitro-1-naphthylamine, picramide, anthraquinone, 
2-ethylanthraquinone, 2-tert-butylanthraquinone, 1,2-benzanthraquinone, 
3-methyl-1,3-diaza-1,9-benzanthrone, dibenzalacetone, 1,2-naphthoquinone, 
3,3'-carbonyl-bis-(5,7-dimethoxycarbonylcoumarin) and coronene. 
To the hydrophobic photosensitive layer there can also be added 
non-thermoplastic polymeric compounds to give certain desirable 
characteristics, e.g. to improve adhesion to said hydrophilic surface of 
the support used in accordance with the present invention, wear 
properties, chemical inertness, etc. Suitable non-thermoplastic polymeric 
compounds include cellulose, phenolic resins, melamine-formaldehyde 
resins, etc. If desired, the hydrophobic photosensitive layer can also 
contain immiscible polymeric or non-polymeric organic or inorganic fillers 
or reinforcing agents which are essentially transparent at the 
wave-lengths used for the exposure of the photosensitive imaging element, 
e.g. organophilic silicas, bentonites, silica, powdered glass, colloidal 
carbon, as well as various types of dyes and pigments in amounts varying 
with the desired properties of the hydrophobic photosensitive layer. The 
fillers are useful in improving the strength of the composition, reducing 
tack and in addition, as coloring agents. 
Agents to improve the wetting and/or adjust the adhesion of the hydrophobic 
photosensitive layer may be added. Suitable agents are e.g. silicons, 
silicon containing polymers e.g. a poly(dimethylsiloxane)-polyether 
copolymer, poly(dimethylsiloxane) polyester, silicon containing 
surfactants, fluor containing copolymers and fluor containing surfactants 
etc. 
Various dyes, pigments, thermographic compounds, UV-absorbers, 
anti-oxidants and color forming components as disclosed in EP-A 522,616 
can be added to the photosensitive layer to give a variety of images after 
the processing. These additive materials should however preferably not 
absorb excessive amounts of light at the exposure wavelength or inhibit 
the acid generating reaction. 
The imaging element may comprise a temporary protective layer on top of the 
photopolymerizable composition, preferably a cellulose triacetate or 
polyethylene terephtalate film base coated with a hydrophilic release 
layer e.g. polyvinyl alcohol. Said temporary layer should be removed 
before or after the photo-exposure step but before the processing steps. 
Preferably, the photosensitive imaging element comprises on top of the 
photosensitive layer one or more protective layers selected from the group 
consisting of (i) a transfer layer which is capable of adhering to a 
receptor layer and (ii) a receptor layer which is capable of adhering to 
the underlying contiguous layer and which is upperlying said transfer 
layer if the latter is present. 
Said receptor layer is preferably stable at the processing conditions. The 
particular layer used is dependent on the nature of the composition of the 
imaging element. Suitable receptor layers include paper; cardboard; metal 
sheets; foils and meshes e.g. aluminium, copper, steel, bronze etc.; 
transparent organic resins e.g. cellulose esters such as cellulose 
acetate, cellulose propionate and cellulose butyrate, polyvinyl acetals, 
polystyrene, polycarbonate or polyvinylchloride; opaque foamed or 
pigmented polyester; silk; cotton and viscose rayon fabrics or screens. 
Preferred receptor layers are commercially available paper brands as 
disclosed in EP-A 93201858.3 which therefor is incorporated herein by 
reference and films or coatings of polyesters such as polyethylene 
terephthalate or of poly-Alpha-olefins such as polyethylene or 
polypropylene. 
Optionally, a receptor layer is part of a receptor element. A receptor 
element for use in accordance with the invention comprises at least a 
receptor layer as an outside layer. 
In one embodiment, said receptor element comprises as the receptor layer 
one of the receptor layers mentioned before and further a thin additional 
layer. Examples are supports provided with a thin metal layer e.g. 
polyester supports provided with a vapour deposited metal layer and most 
useful polyethylene coated paper. Said receptor element may also comprise 
(a) additional layer(s) such as (a) backing layer(s). 
A transfer layer, which is capable of adhering to a receptor layer may be a 
thermo-adhesive layer or a pressure-adhesive layer. 
Suitable thermo-adhesive layers (TALs) for use in the present invention may 
have a glas transition temperature T.sub.g between 10.degree. C. and 
100.degree. C. as measured with the 1090 THERMOANALYZER of Du Pont Co. 
During the lamination and delamination step a minimal thermal load should 
be imposed to the material in order to save energy and diminish the risk 
for material change or deformation. For these reasons the T.sub.g of the 
TAL is preferably below 60.degree. C. The T.sub.g value of the TAL can be 
determined by the T.sub.g value of the polymer(s) used and/or by the 
addition of polymeric or low-molecular plasticizers or thermosolvents. 
The adherance of the TAL to the receptor layer is also determined by the 
flow properties of the TAL while heating above the T.sub.g. A parameter 
for describing this property is the melt viscosity. Preferably a TAL for 
use in accordance with the present invention has a melt viscosity of more 
than 3000 Poise measured at 120.degree. C. with the VISCOELASTIC MELT 
TESTER of Rheometrics Co, Surrey, UK. 
In order to induce easy film formation without unwanted sticking of the TAL 
to the backside of the imaging medium or to other materials a TAL is 
preferably used with a T.sub.g value between 20.degree. C. and 45.degree. 
C., a melt viscosity greater than 7000 Poise and an elasticity 
corresponding to a tg .delta..sup.-1 value greater than 1.30 measured at 
120.degree. C. with the VISCOELASTIC MELT TESTER of Rheometrics Co, 
Surrey, UK. The tg .delta..sup.-1 value is a measure for the elasticity as 
described in "Polymer Chemistry: the Basic Concept" by P. C. Hiemenz, 
1984, edit. by M. Dekker Inc., New York. 
For ecological and practical reasons the TAL is preferably coated from an 
aqueous medium. Therefore the polymers are preferably incorporated as 
latices. 
Preferred latices are latices of styrene, styrene-butadiene, 
styrene-(meth)acrylate and 
n.butylacrylate-methylmethacrylate-acrylonitrile. These latices can 
contain other comonomers which improve the stablitity of the latex, such 
as acrylic acid, methacrylic acid and acrylamide. Other possible latices 
are disclosed in EP-A 92203188.3, which therefor is incorporated herein by 
reference. 
Particularly suitable polymers for use in the TAL layer are the BAYSTAL 
polymer types, marketed by Bayer AG, Germany, which are on the basis of 
styrene-butadiene copolymers with a weight ratio between 40/60 and 80/20. 
If desired a few weight % (up to about 10%) of acrylamide and/or acrylic 
acid can be included. Other useful polymers are the EUDERM polymers, also 
from Bayer AG, which are copolymers comprising n.-butylacrylate, 
methylmethacrylate, acrylonitrile and small amounts of methacrylic acid. 
Various additives can be present in the TAL to improve the layer formation 
or the layer properties, e.g. thickening agents, surfactants, levelling 
agents, thermal solvents and pigments. 
Apart from an upper thermo-adhesive layer which should preferably comply 
with the requirements described above the material can contain one or more 
supplementary thermo-adhesive layer(s) positioned between the upper TAL 
and the hydrophobic photosensitive layer e.g. to optimize the adherance to 
the hydrophobic photosensitive layer in view of obtaining a better image 
quality after the delamination process. This (these) other TAL(s) can have 
a composition and/or physical properties different from those imposed to 
the upper TAL. This (these) layer(s) can contain one polymer or a mixture 
of polymers, optionally in combination with low-molecular additives like 
plasticizers or thermosolvents. Other ingredients which can be 
incorporated include waxes, fillers, polymer beads, glass beads, silica 
etc. 
Suitable pressure-adhesive layers (s) for use in the present invention 
comprise one or more pressure sensitive adhesives. Said pressure sensitive 
adhesives are preferably tacky elastomers e.g. block copolymers of 
styrene/isoprene, styrene/butadiene rubbers, butyl rubbers, polymers of 
isobutylene and silicones. Particularly preferred are natural rubbers and 
acrylate copolymers as disclosed in U.S. Pat. No. 3,857,731. The used 
pressure sensitive adhesive preferably has a continuous-coat (100% 
coverage) peel adhesion value, when applied to untreated paper, between 1 
and 10 N/cm width, more preferably between 2 and 7 N/cm width. 
The pressure-adhesive layer comprising a pressure sensitive adhesive may 
contain a binder. Suitable binders for use in combination with the 
pressure sensitive adhesives are binders that are inert towards the 
pressure sensitive adhesives i.e. they do not chemically attack the 
pressure sensitive adhesives or act as a solvent for them. Examples of 
such binders are nitrocellulose, urethanes, gelatin, polyvinyl alcohol 
etc. 
The amount of binder should be chosen such that the pressure sensitive 
adhesives are effectively anchored to the hydrophobic photosensitive 
layer. Preferably the amount of binder is lower than 2.5 parts by weight 
with respect to the pressure sensitive adhesives and more preferably lower 
than 0.6. 
The pressure-adhesive layer comprising a pressure sensitive adhesive may 
also contain a tackyfier e.g. rosin soap or a terpene. 
The imaging element containing a pressure-adhesive layer comprises 
preferably also a receptor layer on top of said pressure-adhesive layer. 
In general said receptor layer is (are) (a) transparent layer(s) 
contiguous to said pressure-adhesive layer e.g. a transparent organic 
resin layer. 
The thickness of the thermo-adhesive or pressure-adhesive transfer layer is 
important for the adherence of said layers to the photosensitive layer. 
Preferably the thickness of said transfer layers lie between 0.1 and 30 
.mu.m, more preferably between 1 and 15 .mu.m. 
The imaging element may be prepared by coating the layers on each other 
and/or by laminating layers or packets of layers to each other. 
In a practical embodiment the imaging element is prepared by the following 
steps: 
coating on a support with a hydrophilic surface in accordance with the 
present invention (i) a hydrophobic photosensitive layer comprising a 
photosensitive acid precursor and a hydrophobic polymeric binder and (ii) 
a thermo-adhesive layer with optionally an underlying pressure-adhesive 
layer. 
In another practical embodiment the imaging element is prepared by 
laminating the above described imaging element with its thermo-adhesive 
layer onto a receptor layer or onto a pressure-adhesive layer coated on a 
receptor layer. 
In still another practical embodiment the imaging element is prepared by 
the following steps: 
coating on a hydrophilic surface of a support in accordance with the 
present invention a hydrophobic photosensitive layer comprising a 
photosensitive acid precursor and a hydrophobic polymeric binder and 
laminating the element thus formed with its hydrophobic photosensitive 
layer onto a receptor layer or onto a pressure-adhesive or thermo-adhesive 
layer coated on a receptor layer. 
According to the method of the present invention for obtaining an image an 
imaging element according to the present invention is image-wise or 
information-wise exposed to actinic radiation to change the adhesion 
properties of the hydrophobic photosensitive layer pattern-wise. The 
exposure can be a contact exposure using e.g ultraviolet radiation, a 
camera exposure, a scanning exposure, or a laser exposure. The radiation 
source used in carrying out the exposure step includes e.g. sunlight, 
incandescent lamps, mercury vapour lamps, halogen lamps, xenon lamps, 
fluorescent lamps, light-emitting diodes, lasers, electron rays, and 
X-rays. 
Said exposure can be made through the front side or the back side of the 
imaging element. It goes without saying that for an exposure through the 
back the support has to be transparent for the radiation used for the 
exposure of the photoposensitive imaging element where for a front side 
exposure any protective layer, if presents has to be transparent for said 
radiation. Preferably the imaging element is exposed through the front 
side. 
Subsequent to the image-wise or information-wise exposure an image is 
obtained by (i) laminating before or after said exposure an uppermost 
layer of said imaging element to a receptor layer and (ii) peeling away 
the receptor layer from the hydrophilic surface of the support thus 
transferring said hydrophobic photosensitive layer pattern-wise to the 
receptor layer and uncovering the image comprised of the hydrophilic 
surface of the support and the retained hydrophobic photosensitive layer. 
When the imaging element does not comprises a protective layer, the 
receptor element can comprise as receptor layer a thermo-adhesive layer or 
a pressure-adhesive layer. Also a subbing layer can serve as a receptor 
layer. 
When the imaging element or the receptor element does not comprise a 
pressure-adhesive layer said laminating is effected by means of a heating 
step, preferably at a temperature between 25.degree. C. and 180.degree. 
C., more preferably at a temperature between 65.degree. C. and 120.degree. 
C. Said heating may be applied to either or both the hydrophilic base and 
the receptor layer before, while or after bringing the receptor layer in 
contact with the uppermost layer of the imaging element. A higher 
temperature results in a higher sensitivity probably due to a better 
pattern-wise adherance of the receptor layer to the imaging element. 
With an imaging element according to the present invention comprising as a 
transfer layer in the imaging element a thermo-adhesive layer different 
kinds of cheap plain paper can be applied as receptor layer. A broad range 
of commercial papers with diverging physical properties can be used. 
When the imaging element or the receptor element comprises a 
pressure-adhesive layer, said laminating requires a pressure step. Said 
pressure is applied while the receptor layer is in contact with the 
uppermost layer of the imaging element. 
An imaging element and a receptor layer, optionally a receptor element may 
be brought in contact before exposure. In such embodiment it is required 
that either the back of the imaging element and/or preferably the receptor 
layer or the optional receptor element is transparent for the radiation 
used for the exposure of the photopolymerizable composition. 
It may be advantageous to overall expose the image to light and/or heat to 
enhance its stability. Such a procedure is especially preferred when the 
image is used as a printing master as described below to improve the 
scratch resistance of the image. 
Because the imaging element according to the present invention comprises a 
hydrophobic photosensitive layer on a hydrophilic base, the obtained image 
can be used as a lithographic printing plate. Pattern-wise transfer of the 
photosensitive layer to a receptor layer and peeling away the receptor 
layer from the hydrophilic surface of the support results in an image-wise 
differentiation between hydrophilic and hydrophobic parts that can be used 
to print with an oily or greasy ink. The hydrophobic parts will be capable 
of accepting lithographic ink, whereas the hydrophilic 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. 
The imaging element can be either positive working or negative working 
depending mainly on the weight ratio in the hydrophobic photosensitive 
layer between photosensitive acid precursor and hydrophobic 
(thermoplastic) polymer. Although said ratio is influenced by i.a. the 
nature of the photosensitive acid precursor and the hydrophobic 
(thermoplastic) polymer, the weight ratio of a given photosensitive acid 
precursor to a given hydrophobic polymeric binder in said layer is higher 
for a negative working system than for a positive working system. 
In a negative working imaging element (i.e. where a reversal of the 
original image occurs) according to the present invention the 
information-wise exposure to actinic radiation increases the adhesion of 
the hydrophobic photosensitive layer to the hydrophilic surface of the 
support pattern-wise in correspondence to the information-wise 
distribution of actinic radiation. In this case the unexposed or 
insufficiently exposed areas of the hydrophobic photosensitive layer 
adheres to the receptor layer when peeling away the receptor layer from 
the hydrophilic surface of the support, uncovering the lithographic 
printing plate comprised of the hydrophilic surface of the support and the 
retained sufficiently exposed areas of the hydrophobic photosensitive 
layer. 
In a positive working imaging element (i.e. where no reversal of the 
original image occurs) according to the present invention the 
information-wise exposure to actinic radiation decreases the adhesion of 
the hydrophobic photosensitive layer to the hydrophilic surface of the 
support pattern-wise in correspondence to the information-wise 
distribution of actinic radiation. In this case the sufficiently exposed 
areas of the hydrophobic photosensitive layer adheres to the receptor 
layer when peeling away the receptor layer from the hydrophilic surface of 
the support, uncovering the lithographic printing plate comprised of the 
hydrophilic surface of the support and the retained unexposed or 
insufficiently exposed areas of the hydrophobic photosensitive layer. 
The following examples illustrate the present invention without limiting it 
thereto. All percentages are by weight unless stated otherwise.

EXAMPLE 1 
(Example according to the invention) 
Preparation of a hydrophilic base 
To 440 g of a dispersion contg. 21.5% of TiO.sub.2 (average particle size 
0.3-0.5 .mu.m) and 2.5% of polyvinylalcohol in deionized water were 
subsequently added, while stirring, 250 g of a 5% 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 polyethylene terephthalate film 
support having a thickness of 175 .mu.m (having provided thereon a 
hydrophilic subbing layer) at a wet coating thickness of 50 g/m2, dried at 
30.degree. C. and subsequently hardened by subjecting it to a temperature 
of 57.degree. C. for 1 week. 
Preparation of the imaging element 
Onto the above obtained lithographic base was coated a photosensitive 
composition consisting of a solution in methylethylketone of 8.50% by 
weight of a copolymer styrene/butylmethacrylate (Toner Resin OT 5154 from 
Degussa), 0.10% by weight of a blue dye (C.I. 61551), 1.00% by weight of 
2,6-dinitrobenzyltosylaat and 0.50% by weight of thioxantone at a wet 
coating thickness of 37.5 .mu.m. 
The above obtained imaging element was overcoated with a solution 
consisting of a 20% by weight aqueous dispersion of Baystal KA 8522 (from 
Bayer AG, Germany) which is a copolymer containing styrene, butadiene and 
acrylic acid with a glass transition temperature of 34.degree. C. 
(measured with the "1090 THERMOANALYZER" of Dupont Co.), a melt viscosity 
of more than 13420 Poise and an elasticity corresponding to a tg 
.delta..sup.-1 value of 3.54, both last properties measured at 120.degree. 
C. (with the "VISCOELASTIC MELT TESTER" of Rheometrics Co, UK) at a wet 
coating thickness of 30 g/m2. 
Preparation and evaluation of the lithographic plate 
On top of this imaging element and of those described below there was 
placed a test target with a 60 lines per cm screen as well as fine 
positive and negative lines, and the imaging element was exposed 
therethrough to ultraviolet radiation. 
Each of the exposed imaging elements was then placed in face-to-face 
contact with the receptor element, being a subbed polyethylene 
terephthalte support (having an upper subbing layer contg. gelatine and 
silica). 
The contacting elements were conveyed through a roll laminator device at 
90.degree. C. and at a speed of 0.3 m/min and the elements were peeled 
apart. 
In this example the exposed parts of the photosensitive layer were removed 
and the unexposed areas remained on the lithographic base, thus being a 
positive working system. 
With this sample, a good image was obtained with a line reproduction for 
lines of 12 .mu.m. 
The obtained image on the hydrophilic base could be used to print on a 
conventional offset press using a commonly employed ink and fountain. Good 
copies, free of stain were obtained. 
EXAMPLE 2 
(Example according to the invention) 
An imaging element was prepared, exposed, laminated and peeled apart 
similar to the imaging element of example 1 except that the photosensitive 
layer was coated onto the hydrophilic base from a solution in 
methylethylketone of 8.00% by weight of a copolymer 
styrene/butylmethacrylate (Toner Resin OT 5154 from Degussa), 0.10% by 
weight of a blue dye (C.I. 61551), 1.50% by weight of 
2,6-dinitrobenzyltosylaat and 0.50% by weight of thioxantone at a wet 
coating thickness of 37.5 .mu.m. 
In this example, the non-exposed parts of the photosensitive layer are 
removed and the exposed areas remain on the lithographic base, thus being 
a negative working system. 
With this sample, a good image was obtained with a line reproduction for 
lines of 12 .mu.m. 
The obtained image on the hydrophilic base could be used to print on a 
conventional offset press using a commonly employed ink and fountain. Good 
copies, free of stain were obtained. 
EXAMPLE 3 
(Example according to the invention) 
An imaging element was prepared, exposed, laminated and peeled apart 
similar to the imaging element of example 1 except that the photosensitive 
layer was coated onto the hydrophilic base from a solution in 
methylethylketone of 7.50% by weight of a copolymer 
styrene/butylmethacrylate (Toner Resin OT 5154 from Degussa), 0.10% by 
weight of a blue dye (C.I. 61551), 2.00% by weight of 
2,6-dinitrobenzyltosylaat and 0.50% by weight of thioxantone at a wet 
coating thickness of 37.5 .mu.m. 
In this example, the non-exposed parts of the photosensitive layer are 
removed and the exposed areas remain on the lithographic base, thus being 
a negative working system. 
With this sample, a good image was obtained with a line reproduction for 
lines of 12 .mu.m. 
The obtained image on the hydrophilic base could be used to print on a 
conventional offset press using a commonly employed ink and fountain. Good 
copies, free of stain were obtained. 
EXAMPLE 4 
(Example according to the invention) 
An imaging element was prepared, exposed, laminated and peeled apart 
similar to the imaging element of example 1 except that the photosensitive 
layer was coated onto the hydrophilic base from a solution in 
methylethylketone of 8.50% by weight of a copolymer 
styrene/butylmethacrylate (Toner Resin OT 5154 from Degussa), 0.10% by 
weight of a blue dye (C.I. 61551), 1.00% by weight of 
di-(2-nitrobenzyl)-monomethoxyethyl phosphate and 0.50% by weight of 
thioxantone at a wet coating thickness of 37.5 .mu.m. 
In this example, the non-exposed parts of the photosensitive layer are 
removed and the exposed areas remain on the lithographic base, thus being 
a negative working system. 
With this sample, a good image was obtained with a line reproduction for 
lines of 12 .mu.m. 
The obtained image on the hydrophilic base could be used to print on a 
conventional offset press using a commonly employed ink and fountain. Good 
copies, free of stain were obtained.