Method of preparing lithoplates and plate

A process for preparing a lithoplate by electrophotographic means is provided wherein the toned image is fused, the photoconductive layer in the nonimage area is decoated, and finally, the fused toner is selectively removed leaving oleophilic photoconductive material as the image portion.

DESCRIPTION 
BACKGROUND OF THE INVENTION 
This invention relates to an improved electrophotographic reproduction 
process using a photoconductive insulating layer on a conductive support 
to make a lithographic printing plate. Particularly, it relates to a 
process for removal of unwanted fused toner as a last step prior to 
printing and to the product thereby produced. 
In the practice of lithography using bimetallic plates, such as copper 
laminated to aluminum, it is necessary to use a photoresist layer to 
prepare the image. The photoresist is coated over the surface of the 
copper, imaged under a mask and developed. After development, the residual 
photoresist is present in imagewise configuration leaving a pattern of 
exposed copper corresponding to the nonimage. At this point, the plate is 
etched with, for example, ferric chloride solution to dissolve the 
unprotected copper. After rinsing, the residual photoresist is stripped 
with a suitable solvent, leaving an oleophilic copper image on a 
hydrophilic aluminum support. The copper may also be treated with dilute 
acids to enhance its oleophilicity. 
Lithographic offset plates have been prepared by electrophotographic 
methods. Such plates are normally composed of a photoconductive material 
such as zinc oxide, cadmium sulfide or certain organic compounds dispersed 
in an ink-repelling binder and coated on a suitable base material such as 
paper, metal or a film. These plates are imaged by the normal 
electrophotographic process involving forming an electrostatic charge on 
the surface of the plate, exposing the charged plate on an electrically 
conductive support to an image pattern of electromagnetic radiation to 
leak away the charge on the areas struck by light, developing the 
resulting electrostatic image pattern by contact with an electroscopic 
liquid or solid developer, and fixing the developed image by drying or 
heating. The resultant imaged plate may be then used as a master for 
offset lithographic printing. 
Following the aforementioned fixing step, the fused or fixed toner lies in 
imagewise configuration upon a thin adherent, continuous layer of 
photoconductive material which in turn lies upon the conductive support. 
In those cases where the non-toned photoconductor surface is not 
hydrophilic, the toner covered image may be treated with a decoating 
solution, which removes photoconductive coating without removing that part 
of the coating masked by fixed toner to reveal the nonimage areas. At this 
point, the plate is put on the press whereupon the oleophilic toner over 
the image areas attracts ink and transmit ink to the blanket in offset 
printing (or directly in direct lithography) while the decoated support 
areas attract water and repel ink as in normal lithography. 
Plates may be charged, exposed, developed and decoated in automatic 
machinery using incoherent light as in U.S. Pat. No. 3,999,511 or in fully 
automatic laser exposing machinery as described in U.S. Pat. No. 
4,149,798. 
It has been found that sporadically, and without explanation, the printed 
image may become mottled in large black areas at the outset of a printing 
run until about three thousand impressions have been made. This phenomenon 
may be present with either dry toned or liquid toned plates, but persists 
somewhat longer, when it occurs, with liquid toned plates. In any case, 
the mottled prints are unsightly and must be discarded, causing an 
economic loss. 
On some occasions yellow or other light colored inks may slowly dissolve 
toner from the image on the plate during printing with consequent 
degradation of purity of color on the printed sheet. 
In critical printing applications, dot gain is to be avoided. It is 
believed in some quarters that the toned image contributes undesirably to 
dot gain in these cases. 
Accordingly, it is an object of this invention to provide a product and a 
process which eliminates the problem of mottled prints on the press. 
It is another object to provide purer colors in color printing. Still 
another object is to minimize dot gain in critical printing. 
BRIEF SUMMARY OF THE INVENTION 
This and other objects of the invention are achieved by removing the fixed 
toner either manually or in automatic processing equipment by employing, 
after the decoating step, a step of selective removal of toner. 
A number of specific solvents are capable of removing toner without 
attacking or dissolving the photoconductive material underlying the toner. 
They include certain aromatic, aliphatic, naphthenic hydrocarbons and 
their mixtures.

FIG. 1 illustrates the photoconductive insulating layer on a conductive 
support with an image portion or dot composed of toner just after 
development lying upon the photoconductive layer. FIG. 2 illustrates the 
toner dot after fusing. FIG. 3 shows the decoating or removal of 
photoconductive material where it has not been protected by the fused 
image. FIG. 4 shows an image dot composed of photoconductive material left 
on the support after the removal of toner. 
DETAILED DESCRIPTION 
The suitable solvents for removal of fused toner from the image are certain 
aromatic, aliphatic and naphthenic hydrocarbons. The principle 
requirements for suitable solvents are high solvency for fused toner and 
little or no attack on photoconductors. These two requirements can be 
called solubility differentiation. Additionally, there should be no 
evidence of attraction of ink to the nonimage, failure of the image to 
attract ink and transfer it to either blanket or paper or image loss when 
printing. It is further desirable that solvents have as high flash points 
as possible to minimize flammability. 
Suitable solvents are m,p-diethylbenzene, methylcyclohexane, mesitylene, 
chlorinated hydrocarbons, tetralin, methyl decanoate, decalin and 
commercial mixtures of hydrocarbons approximately (by weight) 29%-95% 
aromatic, 0%-28% naphthenic, 5%-43% paraffins, and mixtures of these. Of 
these, the aforementioned m,p diethylbenzene, mesitylene, 
tetrachloroethylene, methylcyclohexane and the mixtures of aromatic, 
naphthenic and paraffinic hydrocarbons are preferred. 
Toners may be either liquid or dry as are well known in the practice of 
electrophotography. Dry toners are finely powdered pigmented 
thermoplastics which are charged oppositely to the charge of the image. 
They are thus attracted to it during development. Liquid toners are 
pigment particles suspended in an insulating liquid. When the charged 
image is sprayed with or briefly immersed in liquid toner, the charged 
particles deposit on it. The surplus liquid is removed. This is the 
development step. These standard processes are described in Jacobson and 
Jacobson, "Imaging Systems," John Wiley & Sons, New York, 1976, p. 269. 
After the development step, the now visible image is fixed or fused. Heat 
or solvent vapors may be used for this purpose. 
Following fixation, the plate is decoated to remove the nonimage 
photoconductor which is normally as oleophilic as the image. This permits 
the support which is hydrophilic to operate in concert with the oleophilic 
image to produce lithographic printing. Typical solutions known as 
decoaters are described in British Pat. No. 944,126. These are strongly 
alkaline solutions containing alkaline phosphates and silicates augmented 
by organic solvents such as alcohols, glycols or glycol ethers. 
Preferred photoconductors, which the decoater removes and which should not 
be attacked or dissolved by the solvents for fused toner, include organics 
such as the various oxazole compounds disclosed in U.S. Pat. No. 
3,257,203, including 4,5 diphenyloxazoles, triphenylamine derivatives, 
higher condensed aromatic compounds such as anthracene, benzocondensed 
heterocyclic compounds, pyrazoline and imidazole derivatives, triazole and 
oxadiazole derivatives disclosed in U.S. Pat. No. 3,189,447, especially 
2,5-bis(p-aminophenyl)-1,3,4 oxadiazoles, and vinyl aromatic polymers such 
as polyvinyl anthracene, polyacenaphthylene, poly-N-vinylcarbazole, as 
well as copolymers thereof. The photoconductive insulating layer may also 
contain a resinous binder if desired, and a sensitizer which selectively 
sensitizes the photoconductive material to light, for example 400 to 500 
nm. Where the nonimage areas of the photoconductive insulating layer are 
to be removed for offsetting printing, the photoconductive compound and 
binder, if present, should be suitable for solubility differentiation with 
respect to the toner covered image areas such that the nonimage areas of 
the photoconductive insulating layer may be removed by decoater solutions 
without affecting the toned image areas. Especially suitable printing 
plates for processing in accordance with the present invention are 
marketed under the trademark ELFASOL.RTM. by the Kalle Division of 
Hoechst, AG, of Wiesbaden, West Germany, and by the Azoplate Division of 
American Hoechst Corporation, of Murray Hill, New Jersey. 
The support sheet should be relatively conductive. Metal, such as aluminum, 
zinc, magnesium or copper plates, and plates of cellulosic origin such as 
specially treated papers, cellulose hydrate, cellulose acetate or 
cellulose butyrate films may be used. Some plastic materials, for example 
polyamides in film form or metal vaporized films, may also be used as 
supports. 
The steps of charging, exposing to incoherent light or laser light, 
developing, fixing and decoating may be manually accomplished in separate 
operations or in tandem in automatic equipment. 
Turning now to the drawings, FIG. 1 shows a section of an 
electrophotographic plate just after development. The conductive support 
is 1. Directly adherent thereupon is the insulating photoconductor 2, 
while 3 represents a portion of the image composed of unfused toner. FIG. 
2 shows the toner image, 3a, now fused due to heat from a source 6. As an 
alternative to heat, solvent vapor may be employed. FIG. 3 shows the 
decoating step in which decoating solution 4 in a vessel falls upon the 
plate. It removes the photoconductor 2 in all areas unprotected by the 
fused toner 3a. FIG. 3 shows the resultant decoated plate, in which fused 
toner lies upon photoconductor with both in imagewise configuration. In 
previous practice, at this point, the plate is placed upon the press. 
Finally, in FIG. 4 is shown the result after solvent, 5, has removed fused 
toner from the photoconductor. This yields a press-ready 
electrophotographic printing plate comprising the support with a 
toner-free thin layer comprising a photoconductive insulating material 
adherent thereupon in imagewise configuration. This layer consists of only 
the photographic insulating material. In accordance with the invention, it 
is at this point that the plate is now placed on the press. A plate which 
has its toner removed in this manner does not sporadically cause mottling 
in large solid areas. 
Solvents were screened for effectiveness in removing toner by placing plate 
samples which were charged, toned and fixed, on the Gardner Straight Line 
Washability and Abrasion Machine. Thirty ml of the solvent being tested 
was poured on a fresh applicator pad. The machine was turned on and the 
plate was scrubbed with the wet pad until all the toner was removed. The 
number of scrubbing strokes necessary was recorded. 
To test the solvent attack on the plates, untoned plates were baked in a 
forced air oven at 180.degree. C. for 40 seconds. Using the abovementioned 
apparatus and technique these samples were scrubbed with 30 ml. of each 
solvent for ten times the number of strokes found necessary to remove the 
toner with that particular solvent. The amount of coating removed was 
determined from weight loss. 
The following examples illustrate the operation of the invention: 
EXAMPLE 1 
An automatic processor was filled with 12 liters of decoating solution in 
its first (decoating) station. The decoating solution contained 
ethoxyethoxyethanol, n-propanol, sodium metasilicate and tripotassium 
phosphate. In the second (rinsing) station there was 5 liters of a 
hydrocarbon solvent with a composition of 29% aromatics (all above C.sub.8 
level), 28% naphthenics and 43% paraffinics. The boiling range of the 
solvent was 300.degree.-400.degree. F. In the third station there was 5 
liters of a dilute solution containing phosphoric acid. 
An electrophotographic plate with an insulating photoconductor layer 
according to the teachings of U.S. Pat. No. 3,189,447 was used. After 
toning, fusing took place at 150.degree. C. 
The machine's operating parameters were: 
______________________________________ 
Decoating Temperature: 30.degree. C. 
Transport Speed: 1.7 in/sec. 
______________________________________ 
Thirty plates were processed as described above. Every tenth plate was 
inked with a standard rub-up ink. The toner was found to be almost 
completely removed with no evidence of scumming, blinding or image loss. 
EXAMPLES 2-8 
Using the Gardner Straight Line Washability and Abrasion Machine, the 
preferred solvents were tested to determine the number of strokes required 
to remove toner and the percent of photoconductor remaining after using 
ten times the number of strokes with each of the solvents. 
______________________________________ 
Toner Removal 
No. of Strokes 
Elfasol Coating 
Solvent Required (% Remaining) 
______________________________________ 
M,p - Diethylbenzene 
3 42% 
Mesitylene 3 59% 
Tetrachloroethylene 
3 44% 
Methylcyclohexane 
3 91% 
Amsco Super Hi-Flash 
3 50% 
Shell Super VM&P 
3 95% 
Amsco 46 Spirits 
5 91% 
______________________________________ 
The composition of the above trade named solvents as inferred from 
manufacturers data is as follows: 
______________________________________ 
% aromatic 
% naphthenic 
% aliphatic 
______________________________________ 
Amsco Super Hi-Flash 
95 0 5 
Shell Super VM&P 
66 57 28 
Amsco 46 Spirits 
29 28 43 
______________________________________