Note: Descriptions are shown in the official language in which they were submitted.
1046~6 K-22 2 7
PROCESS FOR THE PREPARATION OF PRINTING FORMS
This invention relates to the preparation of printing forms or
metallic etchings from electrophotographic or electrographic reproduc-
tion material composed of a suitable support with a photoconductive
or high-ohmic layer thereon, by charging and image-wise exposure
of the materia.l, or by image-wise charging of the material, followed
by development of the electrostatic image with a fine.ly-divided toner,
fixing, remova.l of the layer in the image-free areas by means of a
decoating solution, and, optionally, etching of the bared areas of
the support.
It is known to use electrophotographic reproduction materials
ior printing purposes. German Offenlegungsschrift No. 1,522,497,
for example, describes a process for the preparation of printing forms,
according to which a copying material composed of a support and an
electrophotographic layer comprising a polymerizable organic photo- -
conductor ls electrostatically charged, image-wise exposed, devel-
oped, and the developed image is finally heated to 50 to 300C.
The developer used contains or consists of a substance which forms
radicals under the influence of heat, so that polymerization of the
photoconductor layer occurs in the developed image areas during
heating. In this manner, the solubility of the photoconductor layer
ls reduced and a printing form may be prepared by dissolving away
the image-free areas.
Further, it is known from German Offenlegungsschrift No.
1,572,312, to initiate polymerization by replacing the radical-forming
substance disclosed in the aforementioned publication by a substance
which effects ionic polymerization under the influence of heat.
~ `
1046866 K-2227
,
Further, it is known from French Patent No. 1,299,869, to
prepare a printing plate by electrophotographic means from a material
in which the ~inc oxide used as the photoconductor is finely distribut-
ed in a cross-linkable binder within the layer, development of the
latent electrostatic image being effected by applying a finely divided
solid substance which catalyzes the cross-linking reaction of the
binder, so that the image areas become more difficultly soluble, or
even lnsoluble, in certain solvents when the developer is burned in.
German Patent No. 974,162, describes a process for the prep-
aration of lithographic printing plates from coated papers by electro- -
photographic means, wherein the latent electrostatic image is
developed with a powder which accepts greasy printing ink, and the
non-printing areas of the developed printing plate are rendered
hydrophilic .
Further, it is known to prepare printing plates by electro-
photographic means with the aid of a dry developer which contains a
resin comprising vinyl radicals or chlorine radicals, the photoconduc-
tlve layer of the materlal being impregnated with a zinc salt. After
development, the material is heated so that the zinc salt and the
resin react with each other to form a black substance (U.S. Patent
No . 2, 735, 785) .
The described processes have the disadvantage that, after
electrostatic charging, exposure and development, the copying ma-
terials to be converted into printing plates must be heated to tempera-
tures up to 300C. in a separate process step, or that pressure or
solvents must be applied for fixation, which processes not only
require time, but also a substantial expenditure on equipment.
Thus, it is the object of the present invention to provide a
process for the preparation of printing forms which may be performed
1046866
at a relatively low temperature, if possible at room temperature, and
which requires no additional fixing step.
A dry imaging process already has been suggested for the
electrophotographic fièld (German Auslegeschrift No. 1,057,449) wherein
development and fixing are substantially performed by a chemical reaction
between a component of the toner and a component present in the photo-
conductor layer, but this process has not been used in practice. Moreover,
the process cannot be used for the preparation of printing forms because
the proposed reactions are dyestuff reactions which do not cause a
differentiation between the solubility of the image areas and the solubility
of the non-image areas during the subsequent decoating step. Further, it
is also suggested in this patent to improve the process by the application
of heat for fixation.
According to the invention there is provided in a process for
the preparation of printing forms or metallic etchings from electrophoto-
graphic or electrographic reproduction materials composed of a support with
a photoconductive organic or high-ohmic organic layer thereon~ by charging
and imagewise exposure, or by imagewise charging, development of the
electrostatic image with a finely divided toner, fixing, and removal of
the layer in the image-free areas by means of a decoating solution, the
improvement which comprises developing the electrostatic image with a
i,~
i developer which chemically reacts at leasts~perficially with the image areas
,~
of the photoconductive organic or high-ohmic organic layer at room
temperature, thereby simultaneously effecting development and resistance,
to the decoating solution~ or developing the electrostatic image with a
developer which chemically reacts at room temperature with the decoating
solution and thereby deactivates it in the image areas.
The removal of the layer in the image-free areas by means of
the decoating solution can be followed by etching of the bared surface
^ B ~ 3 -
.
~046866
of the support.
In a preferred form of the invention, the reaction in the image
areas results from using a developer which reacts at least partially with ::
the resin binder
- 3a -
~ 1046866
contained in the photoconductive or high-ohmic layer, to form complex
compounds, chelate compounds, or salts, or by using a developer which
reacts at least partially with the photoconductor contained in the layer
to form a sparingly soluble donor-acceptor complex. Alternatively, the
reaction may be effected by using a developer which reacts at least
partially with a component of the decoating solution to form a salt, thus
preventing the decoating solution from acting upon the reproduction layer
in the areas covered by the developer.
Consequently, the developer material to be used depends on the
composition of the photoconductive or high-ohmic layer and/or on the de-
coating solution employed.
It is achieved by the process of the present invention that the
additional fixing step performed, for example, by heating or burning-in
within a substantially constant and controllable temperature range, using
expensive apparatus, can be completely omitted, providing instead a process
for the preparation of printing plates which can be easily and economically
performed, using even such supporting materials as paper and metallized
plastic films. If metallized films are used, for example, decoating and
etching of the metal layer in the non-image areas produces transparent
images whose image areas are distinguished by a very high optical density.
' The images produced on the film bases are true to scale, because fixing
of the toner image by heat action - which always involves the risk of a
possible change in dimensions - is not necessary. Altogether, the inventive
process provides a considerable saving in time and apparatus, which is of
decisive importance with a view to the automation of the entire process.
By the process of the present invention, printing forms for
planographic printing, gravure printing, letterpress printing, and
:
--4--
K-2227
1046866
screen printing, as well as printed circuits, may be produced by
electrophotographic or electrographic methods.
The electrophotographic printing form used as the starting
material for the process of the invention is known in principle. The
use of photoconductive organic substances in electrophotographic
layers and their conversion into printing forms are known, e.g., from
the publications mentioned above for defining the present state of
teehnology. Photoconductors containing one or more dialkyl amino
groups may be used with particular advantage in the present invention. -
-- 10 Partieular mention is to be made of heteroeyclic compounds,
such as the oxadiazole derivatives which are known from German
Patent No. 1,058,836, especially the 2,5-bis-(4'-diethylaminophenyl)-
oxadiazole-1,3,~. Other suitable photoconductors are, for example,
trlphenylamine derivatives, highly condensed aromatic compounds,
such as anthracene, benzo-condensed heterocyclic compounds, and
pyrazoline or imidazole derivatives. The tria~ole and oxazole deriva-
tlves dlsclosed in German Patents Nos. 1,060,260, and 1,120,875,
also may be used. Further, vinyl-aromatic polymers, such as poly-
vlnyl anthracene, polyacenaphthylene, and poly-N-vinyl-earbazole,
and the copolymers of these eompounds, are also suitable, provided
they produce a differentiation in the solubility eharacteristies, if
neeessary in eombination with a resin binder. Other suitable eom-
pounds are polyeondensates of aromatie amines and aldehydes, sueh
as those known from German Auslegesehrift No. 1,197,325. In addi-
tion to the compounds just mentioned, which preponderantly are of
p-eonduetive nature, n-conduetive eompounds may also be used.
These so-called electron acceptors are known, for example, from
German Patent No. 1,127,218.
1046866 K-222 7
As regards film-forming characteristics and adhesion, both
natural and synthetic resins are suitable as resin binders. In addi-
tion to their film-forming and electrical characteristics and their
adhesion to the support, the solubility characteristics of the resins
are of particular importance in their selection. Resin binders which
are soluble in aqueous or alcoholic solvent systems, to which, if de-
sired, an acid or an alkaline substance may be added, are particularly
suitable for practical purposes. For physiologica1 and safety reasons,
readily flammable aromatic or aliphatic solvents are ruled out. There-
fore, suitable resin binders are high molecular weight substances
containing groups which render them alkali-soluble, such as acid
anhydride groups, carboxyl groups, phenol groups, sulfonic acid groups,
sulfonamide or sulfonimide groups. Resin binders having high acid
numbers are preferred because they are particularly easily soluble in
alkaline-aqueous-alcoholic solvent systems. Copolymers containing
anhydride groups may be used with particular advantage because, due
to the absence of free acid groups, the conductivity of the electro-
photographic layer in the dark is low, in spite of its good solubility
in alkaline solutions.
Copolymers of styrene and maleic anhydride, such as those
known by the name of "Lytronl' ~ (marketed by Monsanto Chemical
Company, St. Louis, Mo., USA), and phenol resins, such as those
known by the name of "Alnovol"~ (marketed byChemischeWerkeAlbert,
Wiesbaden-8iebrich, Germany) have proved to be particularly suitable.
Further, the copying layer of the electrophotographic printing
form used as the starting material may contain known sensitizers.
Only small quantities of sensitizer are added to the copying layer,
- i.e. about 0.001% to about 1% by weight, calculated on the weight
6 -
K-2227
~046866
of the photoconductor component. Suitable sensitizers, most of them
dyestuffs, are known, for example, from Belgian Patent No. 558,078.
If electrographic layers are used, the same resin binders as
described in connection with the electrophotographic layers may be
employed. The layers must meet the same requirements as the photo-
conductive layers, but the photoconductor component is omitted.
Any of the known materials suitable for this purpose, e.g.
aluminum, zinc, magnesium, chromium, or copper plates, may be used
as the support of the electrophotographic or electrographic printing
plate used as the starting material, and their surfaces may be pre-
treated, if desired. Cellulose products, such as cellulose hydrate,
cellulose acetate, or cellulose butyrate films, or paper that has been
superficially hydrophilized and made electrically conductive, as also
plastic films and compound materials composed of paper or plastic film
and metal Iayers, also may be used. Supports composed of layers of
different metals also are suitable.
For the preparation of metallic etchings, metallized plastic
materlals in the form of films or plates are used as supports, the metal
layer being applied by vapor deposition, lamination, or by chemical or
galvanic metal deposition.
Suitable developers are solid inorganic or organic compounds
which are capable of reacting at room temperature with a component of
the layer, to form either a complex compound, a chelate compound, or
a salt, which are resistant to the decoating solution, or, alternatively,
a donor-acceptor complex resistant to the decoating solution. The
developer substances must have similar solubility characteristics to
those of the copying layer. They must be capable of being superfici-
ally dissolved at the beginning o~ the decoating process for the
reaction to proceed.
.
1046866 K-22 2 7
In the case of the particularly preferred copolymer of styrene
and maleic anhydride, the salts of multivalent metals, such as chrom-
ium, manganese, iron or copper, or the salts of the metals of the IInd,
IIIrd, or IVth main groups of the Periodic System, e.g. magnesium,
aluminum, or tin, have proved to be particularly advantageous. On
the other hand, if the above-mentioned phenol resins are used,
excellent results are obtained when using either developer substances
of the triphenyl methane dyestuff type, e.g. "Crystal Violet" (Schultz'
Farbstofftabellen, Vol. I, 7th Ed. 1931, No. 785, page 329) or aro-
matic diazonium salts. Developer substances which at least partially
react with the photoconductor component are compounds such as tetra-
phenyl cyclopentadienone, benzoquinone, dicyanochlorobenzoquinone,
benzo-anthraquinone, tetrachloroquinone, dibromo-succinic acid,
tetrachlorophthalic anhydride, dinitronaphthalic anhydride, tetranitro-
naphthalene, and the like. These compounds produce excellent results,
especlally when used with oxadiazoles and oxazoles as the photocon-
ductlve substances.
Suitable developer substances are also those compounds which
react at least partially with a component of the decoating solution,
w1th salt formation, thus rendering the decoating solution ineffective
in the areas covered by the developer. The chemlcal characteristics
of the developer depend on the decoating method. If an aqueous or
alcoholic-alkaline medium is used for decoating, inorganic or organic
acids are used as developers. Boric acid and toluene sulfonic acid
are particularly suitable for this purpose. On the other hand, if de-
coating is effected in an acid environment, the developer substances
must have acid-binding characteristics. If an aqueous phosphoric
acid solution is used as the decoating solution, calcium oxide has
; proved to be particuLarly suitable as the developer substance.
-B-
1046866
The above-mentioned substances may be used either in a dry
development process, or they may be applied as developer dispersions in
a liquid developing process. A liquid developing process is preferred,
using a developer composed of a high-ohmic liquid phase with a finely-
divided solid phase dispersed therein. Liquid development is preferred
because the developing process is clean and free from dust and the copies
produced are distinguished by a very good photographic dissolution.
The liquid developer is composed of a dispersing medium, the
reactive component, and additives which promote the dispersion of the
components and influence their electrical charge.
The preparation and use of suitable additives are described in
German Auslegeschrift No. 1,047,616.
The developer dispersions may further contain additives which
do not participate in the reaction, but serve to increase the hydrophobic
properties or improve the coverage of the image areas. Bitumen and wax
or resin-like substances in finely-dispersed form have proved to be
particularly suitable for this purpose. Suitable dispersing media are
those liquids which do not dissolve the solid phase, e.g. halogenated
hydrocarbons, and, above all, liquid aliphatic hydrocarbons, for example
isoparaffins with a boiling range between 185 and 210C. The polarity of
the charge of the dispersed phase depends upon the properties of the dis-
persed substance itself as well as upon the selected dispersion medium.
The developer may be applied in known manner. In the case of
a liquid developer, the developer may be applied by immersion or by roller
application, the latter method being preferred because it results in better
uniformity.
_g_
K-2227
1046866
After development of the latent electrostatic image, the devel-
oper substance adheres to the copying layer in the form of the image.
Application of the decoating solution follows.
The decoating solution is distributed over the layer, for example
by wiplng with a cotton pad. Alternatively, the plates may be immersed
ln the decoating solution. As a preferred method, the decoating solu-
tlon is sprayed upon the layer. Suitably constructed apparatuses also
may be used for decoating, e.g., appllcator roll systems. In this
manner, the differentlation between hydrophilic and oleophllic areas
~10 necessary for offset printing ls produced, the image areas representlng
the oleophllic portions and the bared surface of the support represent-
lng the hydrophilic portions of the printing plate.
The composition of suitable decoating solutions is known, for -
example, from German Patent No. 1,117,391. Decoating solutions
which have proved to be particularly suitable for the inventlve process
are mlxtures of alcohols containing sodium metasllicate or ethanol-
amine as the alkaline component. A 5% by weight aqueous solution of
phosphoric acid has proved to be particularly advantageous as an acid
decoatlng solution.
After treatment wlth the decoating solution, the printing plate
ls advantageously rinsed with water; lf desired, its hydrophobic prop-
erties may be increased by wiping it over with dilute phosphoric acld
solution. After inking with greasy ink, printing may be performed in
known manner in a planographic printing machine (offset printing).
Alternatively, printing forms for letterpress or, if desired, for
gravure printing may be produced by the subsequent dissolution of the
bared supporting material. Dissolution may be performed in known
one-bite or multi-stage etching machlnes used for this purpose. If a
.
~ . - 10-
K-2227
1046866
multi-metal material is used as the support, etching is performed by
conventional photoengraving methods. The printing plates obtained
by the present process are capable of very long runs.
By the process of the present invention, printing forms and
printed circuits may be produced; further, it may be used in the X-ray
and microfilm field.
The invention will be further illustrated by reference to the
following examples:
Exam~le 1
A solution containing 10 g of 2,5-bis-(4'-diethylaminophenyl)-
1,3,4-oxadiazole, 10 g of a styrene/maleic anhydride copolymer with
a softening point of 210C, and 20 mg of "Rhodamine FB" in 300 ml of
glycol monomethyl ether is applied to a superficially roughened, about
l O 0~ thick, aluminum foil . The photoconductive layer produced after
evaporation of the solvent is charged in the dark to a negative potential
of about 400 volts by means of a corona. The charged plate is exposed
in a camera and then developed, using a toner liquid prepared by finely
dispersing 3.0 g of magnesium sulfate in a solution of 7.5 g of penta-
erythritol resin ester in 1,200 ml of an lsoparaffin having a boiling
range between 185 and 210C. After removal of excess toner liquid,
the plate is immersed for 60 seconds in a solution containing 35 g of
sodium metasilicate hydrate in a mixture of 140 ml of glycerol, S50
ml of ethylene glycol, and 140 ml of ethanol. The plate is then rinsed
down with a strong water ~et, whereby the areas of the photoconductive
layer not covered by the toner are removed.
The resulting plate may be used for printing in the conventional
manner in a printing machine. Very long runs are obtainable.
- -- 11 --
K-2227
~046866
Example 2
A solution containing 10 g of 2-vinyl-4-(2'-chlorophenyl)-5-
(4"-diethyl-aminophenyl)-oxazole, 10 g of a styrene/maleic anhydride
copolymer with a softening point of 210C, and 20 mg of "Rhodamine
FB" in a mixture of 235 ml of glycol monomethyl ether and 65 ml of
butyl acetate is applied to an about 100~ thick, superficially rough-
ened aluminum foil. After evaporation of the solvent, the resulting
photoconductor layer is charged by means of a corona to a positive
potential of about 400 volts and then exposed in a camera. A fine
dispersion of 3 g of potassium aluminum sulfate in a solution of 7.5
g of pentaerythritol resin ester in 1, 200 ml of an isoparaffin with a
boiling range between 185 and 210C is used for development.
The printing plate thus produced is converted into a printing
form by immersing it for 1 minute in a solution of 35 g of sodium
silicate hydrate in a mixture of 140 ml of glycerol, 550 ml of ethylene
glycol, and 140 ml of ethanol, and then rinsing it with a water ~et,
with gentle brushing.
ExamPle 3
A photoconductor layer is produced on a roughened aluminum
layer as described in Example 2. The layer is charged to a negative
potential of approximately 400 volts and exposed under a negative
transparency in a re-enlargement apparatus. At stop 8 and an enlarge-
ment ratio of 1: 5, the exposure time is 20 seconds when a 100 watt
incandescent lamp is used. Development and converslon into a print-
ing form are performed as described in Example 2. A positive image
is obtained from the negative transparency.
Exam~le 4
,:,
; A superficially roughened aluminum foil of about 100~ thick-
ness is coated with a solution containing 10 g of 2-(4'-diethylamino-
1046866 K- 2227
phenyl)-6-methyl-benzthiazole-(N, N-dimethylsulfonamide) (prepared
as described in German Patent No. 1,137,625), 10 g of a phenol-
formaldehyde resln with a softening range of 108 - 118C, 5 g of a
low viscosity chlorinated rubber, and 100 mg of "Rhodamine FB" in
100 ml of butanone. The eleckophotographic printing plate thus ob-
tained is charged in the dark to a negative potential of 200 volts by
means of a corona and is then exposed in a vacuum printing frame under
and in contact with a positive transparency. Using an incandescent
lamp of 100 watts at a distance of 65 cm, the exposure time is 7
seconds. The latent image thus produced is developed with a devel-
oper tiquid prepared by f~nely dispersing 0.3 g of 4-diazo-2,5-di-n-
butoxy-phenyl-morpholine-fluoborate in a solution of 0.4 g of a -
pentaerythritol resin ester in 1,000 ml of an isoparaffin with a boiling
range between 185 and 210 C. The developed plate is immersed for
15 seconds in a solution of 22 g of sodium metasilicate hydrate in a
mixture of lS0 ml of ethylene glycol, 85 ml of glycerol, 170 ml of
methanol, and 75 ml of water, and then rinsed with a strong water ~et -
with gentle brushing. The printing plate thus produced is capable of
printing long runs.
Exam~le 5
A solution of 15 g of a condensation product of N-ethylanlline
and formaldehyde, 0.41 g of dibromo-succinic acid, and 0.15 g of
"Rhodamine FB" in a mixture of 40 ml of toluene, 20 ml of trichloro-
ethylene, and 25 ml of methanol is coated upon a mechanically
s roughened aluminum foil. After evaporation of the solvents, a photo-
conductor layer is produced which is cha}ged in the dark to a negative
potential of 350 volts by means of a corona. The charged foil is then
, exposed for approximately 2 minutes in a camera and developed with
; '
-- 13 --
` -
1046866 K-22 2 7
a toner liquid which protects the image areas from the effect of the
decoating solution by undergoing a reaction with the latter. The toner
used ts a mixture of 5 g of finely ground calcium oxide and 100 g of
iron powder of a grain size between 75 and 150,~. A 5% phosphoric
acid solution containing 10% of ethanol is used for removal of the
photoconductor layer. The toner-covered plate is bathed in this solu-
tion for about 30 seconds and then rinsed with water.
Example 6
A bimetal plate consisting of layers of brass and chromium is
coated with a solution of 10 g of 2-vinyl-4-(2'-chlorophenyl)-5-(4"-
diethylaminophenyl)-oxazole, 10 g of a styrene/maleic anhydride
copolymer with a softening point of 210C, and 20 mg of "Rhodamine
FB" in a mixture of 235 ml of glycol monomethyl ether and 65 ml of
butyl acetate. For the preparation of the printing form, the coated
plate is charged to a negative potential of about 350 volts, exposed
in contact with a negative for 3 seconds to the light of a 100 watt
lncandescent lamp at a distance of 65 cm, and developed with a toner
llquid. The toner liquid is prepared by dispersing 3 g of tetraphenyl
cyclopentadienone, 1.2 g of a high-vacuum bitumen, and 6.0 g of a
pentaerythritol ester resin in 1,000 ml of an isoparaffln having a boil-
ing range between 185 and 210C. After removal of excess toner
liquid, the photoconductor layer is removed in the areas not covered
by the toner. The sodium metasilicate hydrate solution described in
Example 1 is used for this purpose. In the areas of the plate freed
from the photoconductor layer, the chromium layer is then removed by
etching, using a commercial chromium etching solution. A printing
form is thus` obtained from which very long runs (more than 100, 000
copies) may be printed.
K-2227
1046~66
Example 7
A photoconductor layer is applied as described in Example 1 to
a 350~ thick aluminum plate with a 1~ thick chrome-plated layer
thereon. The plate is charged to a negative potential of 350 volts,
exposed, and developed with a toner liquid prepared by finely dispers-
ing 0.5 g of potassium dichromate in a solution of 1.0 g of pentaeryth- -
ritol resln ester in 1,000 ml of an isoparaffin with a boiling range
between 185 and 210C. The photoconductor layer is removed with a
solution of 35 g of sodium metasilicate hydrate in a mixture of 140 ml
of glycerol, 550 ml of ethylene glycol, 140 ml of ethanol, and 200 ml --
of water. The printing plate thus produced is distinguished by the
particularly good hydrophilic properties of its image-free areas.
ExamPle 8
A solution of 6 g of a styrene/maleic anhydride copolymer with
a softening point of 210C in a mixture of 74 ml of glycol monomethyl
ether and 20 ml of butyl acetate is mechanically coated onto a 100
thick, superficially roughened aluminum foil. After evaporation of the
solvent, the coated foLl is charged by means of a corona, using an
image-wise blanked-out aluminum foil as a stencil. The charged
image areas have a negative potential of 300 volts. The charge image
thus produced is then developed with a toner liquid prepared by ~is-
persing 0.3 g of copper-II-chloride in a solution of 0.4 g of a penta-
erythritol resin ester in 20 ml of an isoparaffin and diluting the
, resulting dispersion with 1,000 ml of an isoparaffin with a boiling
,~ .
range between 189 and 210C. The toner-covered foil is converted
into a printing form as described in Example 1.
Example 9
~- A photoconductive material consisting of a layer of substituted
vinyl oxazole and ~a binder on an aluminum support and produced as
'
-- 15 --
K-2227
1046866
described in Example 2 is charged in the dark by means of a corona to
a negative potential of 400 volts.
The charged foil is exposed in contact with a positive trans-
parency as the original, the exposure time being 2 seconds for a 100
watt incandescent lamp at a distance of 65 cm. The plate is then de-
veloped with a developer liquid in which the toner particles consist
of toluene-4-sulfonic acid. The developer liquid is produced by finely
dlspersing 0.3 g of toluene-4-sulfonic acid in a solution of 0.4 g of a
pentaerythritol resin ester in 1,000 ml of an isoparaffin having a boil-
ing range of 185 - 200 C. 3 ml of a 6% by weight solution of zirconyl
llnoleate are added to the developer as a charge control agent. The
photoconductor layer is removed from the areas not covered by the
toner in the manner described in Example 2.
ExamPle 10
The procedure described in Example 9 is repeated, except that
the charged photoconductor layeris exposed in a re-enlargement appa-
ratus, using a negative transparency as the original. If a 100 watt
incandescent lamp is used, the exposure time is 20 seconds at stop 8
and an enlargement scale of 1: 5. The latent image produced by
exposure is developed with a dispersion of 0 . 3 g of boric acid in a
solution of 0.4 g of pentaerythritol resin ester in 1,000 ml of an iso-
paraffin wlth a boiling range between 185 and 200C. A positive toner
image consisting of boric acid is thus produced. The toner-covered
foil is converted into a printing form as described in Example 2.
It will be obvious to those skilled in the art that many modifi-
cations may be made within the scope of the present invention without
; departing from the spirit thereof, and the invention includes all such
modifications .
-- 16 --
. ~
