Process for the manufacture of a lithographic printing form by electrophotography

Disclosed is a process for manufacturing a lithographic printing form by electrophotographic means, comprising the steps of uniformly charging an oleophilic photoconductive layer which is present on an electrically conductive hydrophilic layer support and which contains a photoconductor and a binder; image-wise exposing the charged photoconductive layer to produce a charge image; developing the charge image with toner to produce a toner image comprised of imaged areas and non-image areas; fixing the toner image; decoating the non-image areas of the photoconductive layer to bare the electrically conductive layer support; coating the bared areas of the layer support with a solution comprising a water-soluble, but hydrocarbon-insoluble film-forming polymer and a vinyl phosphonic acid compound; drying the coating solution to form a uniform coating; and removing the toner image by rinsing with a hydrocarbon composition containing aromatic portions in an amount of at least about 20% by weight. Also disclosed is an improved printing form made by this process.

BACKGROUND OF THE INVENTION 
The present invention relates to a process for the manufacture of a 
lithographic printing form by electrophotographic means. 
Processes of this kind are known and are, e.g., described in German Pat. 
No. 11 17 391, German Offenlegungsschrift No. 23 22 047, and German 
Offenlegungsschrift No. 25 26 720. In conventional embodiments of the 
process, a photoconductive layer which has been applied to an electrically 
conductive layer support is charged, exposed image-wise, and developed 
with liquid or dry toner to give an image. The toner image is then fixed 
by heating, and finally the printing plate is decoated by removing the 
unprotected background areas of the photoconductive layer. The offset 
printing form thus obtained is ink-receptive in the toner image areas and 
water-receptive in the bared areas of the support surface. 
It has shown that, especially in larger continuous full shade areas, 
printing forms prepared in this way have a tendency to exhibit voids where 
no ink is transferred. When examining the causes, it was found that the 
toner, which usually contains a resin which is soluble in organic 
solvents, most probably swells and gets sticky when it remains in contact 
with ink for a longer period of time. Then, the toner is able to hold dirt 
particles or to form an uneven, wrinkled surface. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide an improved 
process for the manufacture of lithographic printing forms. 
A further object of the present invention resides in the provision of a 
process for manufacturing lithographic printing forms by an 
electrophotographic method which produces a printing form which does not 
possess the disadvantages referred to above. 
It is also an object of the present invention to provide an improved 
lithographic printing form. 
In accomplishing the foregoing objects, there has been provided in 
accordance with the present invention a process for the manufacture of a 
lithographic printing form by electrophotographic means, comprising the 
steps of uniformly charging an oleophilic photoconductive layer which is 
present on an electrically conductive hydrophilic layer support and which 
contains a photoconductor and a binder; image-wise exposing the charged 
photoconductive layer to produce a charge image; developing the charge 
image with toner to produce a toner image comprised of imaged areas and 
non-image areas; fixing the toner image; decoating the non-image areas of 
the photoconductive layer to bare the electrically conductive layer 
support; coating the bared areas of the layer support with a solution 
comprising a water-soluble, but hydrocarbon-insoluble film-forming polymer 
and a vinyl phosphonic acid compound; drying the coating solution to form 
a uniform coating; and removing the toner image by rinsing with a 
hydrocarbon composition containing aromatic portions in an amount of at 
least about 20% by weight. The process optionally includes the step of 
decoating the non-image areas of the layer support by rinsing with water. 
In accordance with another aspect of the present invention, there has also 
been provided a lithographic printing form produced by the process defined 
above. 
Further objects, features and advantages of the present invention will 
become apparent from the detailed description of preferred embodiments 
which follows. 
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
The present invention is based on a process for the preparation of 
lithographic printing forms by electrophotographic means, wherein an 
oleophilic photoconductive layer, which has been applied to an 
electrically conductive hydrophilic layer support and comprises a 
photoconductor and a binder, is uniformly charged, exposed image-wise, 
treated with toner and fixed, and thereafter the non-image areas of the 
photoconductive layer are removed by means of a decoating composition. 
The process according to the present invention is characterized in that, 
after decoating, the bared areas of the layer support are coated with a 
solution of a water-soluble, but hydrocarbon-insoluble film-forming 
polymer and a vinyl phosphonic acid compound. This solution is dried to 
form a uniform coating, and thereafter the toner image is removed by 
rinsing with a hydrocarbon composition containing aromatic portions in an 
amount of at least about 20% by weight, and then the background areas of 
the layer support are optionally bared anew by rinsing with water. 
By the process according to the present invention, the toner image is 
completely removed in image-wise configuration from the photoconductive 
layer which lies underneath it without there being deposited on the 
background areas of the support oleophilic substances which would cause 
toning of the printing form. A faultless reproduction of the full shades 
by the printing form is thus maintained. A further advantage of the 
process according to the present invention is an improved tone rendering, 
especially in the case of screened images, since the image elements which 
have been slightly enlarged by the fused toner are reduced to their 
original size by removing the toner. 
Suitable hydrocarbon compositions, in which the fused toner image is 
soluble, include any commercial compositions for technical use containing 
at least 20%, but preferably 70-100% of aromatic portions. Especially 
preferred are higher-boiling benzene homologues, i.e., alkyl-substituted 
benzenes having about 8 to 15, preferably about 9 to 13 carbon atoms. The 
amount of component parts which are detrimental to health, such as 
trimethyl benzene and propyl benzene, should be kept as low as possible, 
and the flashpoint of the composition should be higher than about 
40.degree. C. As the non-aromatic component parts, other higher-boiling 
hydrocarbons, especially naphthenes, are suitable. 
The density of the hydrocarbon composition preferably lies between about 
0.85 and 0.95 g/cm.sup.3 at 12.degree. C., measured according to DIN 51 
757, and its boiling range preferably lies between about 115.degree. C. 
and 280.degree. C. The kauributanol value according to ASTM D-1133, in 
general, ranges between about 70 and 100. 
Mixtures of this kind can be employed in conventional commercial processing 
equipment, and it is not necessary to take any special protective measures 
against explosion. 
By coating the support surface, which has been bared during the decoating 
step, with the aqueous solution of a film-forming polymer and a vinyl 
phosphonic acid compound, the hydrophilic support surface is protected 
from contaminations caused by the organic toner solution. 
In principle, any of those substances can be used as the water-soluble 
film-forming polymers, which are conventionally employed as constituents 
of preserving solutions for lithographic printing plates, such as, e.g., 
gum arabic, dextrin and polyvinyl alcohol. The concentration of said 
polymer, in general, varies between about 4 and 30%, and preferably 
between about 5 and 15% by weight. 
Suitable vinyl phosphonic acid compounds are, e.g., vinyl phosphonic acid, 
polyvinyl phosphonic acid and vinyl phosphonic acid monomethyl ester. 
Further suitable compounds are aromatic, substituted aromatic, substituted 
and non-substituted saturated and unsaturated cyclic, aliphatic, 
heterocyclic phosphonic acids and also polymers and interpolymers of 
unsaturated phosphonic acids with each other or with other vinyl 
compounds. Polyvinyl phosphonic acid is preferably used. The vinyl 
phosphonic acid compound can be used in concentrations between about 0.05 
and 10% by weight, preferably between about 0.2 and 4.0% by weight. 
After being treated with the hydrocarbon solvent, the plate surface is 
usually rinsed with water, during which the water-soluble protective 
coating is removed together with still adhering toner residues, and the 
printing form gets a clean outward appearance. If the printing form is not 
used for printing immediately, it is coated with a conventional preserving 
solution and dried. It is also possible, especially if the printing form 
which has been freed from the toner as proposed by this invention is to be 
stored for only a short period, to leave the protective layer of 
water-soluble polymers and vinyl phosphonic acid compounds on the 
background areas and to rinse it off with water just before printing is 
started. 
The photoconductors employed are preferably organic photoconductors which 
are, e.g., of the types described in German Pat. No. 11 20 875 or in 
German Auslegeschrift No. 25 26 720. The binders contained in the 
photoconductive layer are preferably compositions which are insoluble in 
water and soluble in aqueous-alkaline solutions. Special preference is 
hereby given to copolymers containing carboxyl groups. The manufacture of 
suitable electrophotographic materials and their processing into printing 
forms are conventional and are described in the above publications, the 
disclosure of which is hereby incorporated by reference.

By way of the following examples, preferred embodiments of the process 
according to this invention are described. If not otherwise stated, 
percentages and ratios mean parts by weight. 
EXAMPLE 1 
A solution of 
40 parts by weight of 2-vinyl-4-(2-chlorophenyl)-5-(4-diethylamino 
phenyl)-oxazol, 
47 parts by weight of a copolymer of styrene and maleic acid anhydride 
having a decomposition temperature ranging from 200.degree. to 240.degree. 
C., 
10 parts by weight of a chlorinated rubber having a viscosity of 5 
mPa.multidot.s at 25.degree. C. in a 20% toluene solution, and 
0.2 part by weight of rhodamine FB (C.I. 45.170) in 
510 parts by weight of tetrahydrofuran, 
330 parts by weight of ethylene glycol monomethyl ether, and 
150 parts by weight of butyl acetate 
is applied onto a layer support of electrolytically roughened and 
anodically oxidized aluminum having a thickness of 300 .mu.m and dried to 
form a uniform photoconductive layer having a weight per unit area of 5 
g/m.sup.2. By means of a corona, the plate is charged to -450 V and during 
25 seconds exposed in a reproduction camera with 8 commercial lamps of 500 
W each. The charge image thus obtained is developed with a toner powder 
comprised of 
18 parts by weight of a copolymer of 35% of n-butylmethacrylate and 65% of 
styrene, and 
2 parts by weight of carbon black pigment. 
Then the toner image is fixed by heating it to about 
170.degree.-180.degree. C. for a short time. From the non-image areas, the 
photoconductive layer is removed by means of a solution of 
12 parts by weight of diethylene glycol monoethyl ether, 
10 parts by weight of n-propanol, 
1.4 parts by weight of sodium metasilicate nonahydrate, and 
76.6 parts by weight of water, 
and by rinsing with water. The plate is cleaned with a doctor blade and 
then uniformly coated with a solution of 
10 parts by weight of tapioka dextrin having a pH of 2-3 and an Oswald 
viscosity of 2-50 mPa.multidot.s, 
1 part by weight of glycerol, 
0.3 part by weight of phosphoric acid, 85%, 
2.5 parts by weight of sodium n-octyl sulfate, and 
2 parts by weight of polyvinyl phosphonic acid having a viscosity of 30 
mPa.multidot.s at 20.degree. C. in water in a concentration of 33% by 
weight, in 
84.2 parts by weight of water, and then dried. Thereafter, the toner is 
washed off by slightly rubbing with a mixture of benzene homologues, 
wherein the C.sub.9 portion is 8%, the C.sub.10 portion is 72%, and the 
C.sub.11 portion is 20%. The surplus solution is removed with a doctor 
blade, the plate is rinsed with water and conventionally preserved with a 
preserving solution. A printing form free from toning is obtained. 
The above test is repeated, whereby the dextrin solution is prepared 
without the addition of polyvinyl phosphonic acid. The printing form thus 
obtained is clearly susceptible to toning. 
EXAMPLE 2 
A layer support of mechanically roughened and anodically oxidized aluminum 
having its oxide layer treated with an aqueous 0.1% strength solution of 
polyvinyl phosphonic acid is coated with a solution of 
40 parts by weight of 2,5-bis-(4-diethylamino phenyl)-oxdiazole, 
40 parts by weight of a copolymer of styrene and maleic acid anhydride 
having a decomposition temperature ranging from 200.degree. to 240.degree. 
C., and 
2 parts by weight of Astrazonorange R (C.I. 48.040) in 
14 parts by weight of methanol, 
105 parts by weight of butyl acetate, 
400 parts by weight of tetrahydrofuran, and 
300 parts by weight of ethylene glycol monomethyl ether. 
After drying a photoconductive layer having a weight per unit area of 5 
g/m.sup.2 is obtained. 
As described in Example 1, the plate is charged, image-wise exposed, 
developed with toner, fixed, decoated, and then coated with a solution of 
7 parts by weight of gum arabic, 
0.3 part by weight of formaldehyde, and 
1 part by weight of polyvinyl phosphonic acid having a viscosity of 230 
mPa.multidot.s at 20.degree. C. in water in a concentration of 33% by 
weight in 
91.7 parts by weight of water and dried. Then the toner image is removed by 
rubbing with a mixture of 14% of naphthene hydrocarbons, 20% of C.sub.10 
benzene homologues, 22% of C.sub.11 benzene homologues, and 44% of 
C.sub.12-13 benzene homologues. The surplus solvent is removed with a 
doctor blade, the plate washed with water and preserved with a commercial 
preserving solution. The printing form thus obtained is free from toning. 
Similar results were obtained when the test was carried out by using a 
layer support consisting of electrolytically roughened and anodically 
oxidized aluminum.