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Description  |
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BACKGROUND OF THE INVENTION
This invention relates to electrostatic printing and, more particularly, to
an improved electrostatic printing master adapted for the use of
conventional silver halide photographic techniques during preparation of
the master for printing.
Electrostatic printing is well-known in the art and has been proposed as an
alternative to other printing techniques. In one method of electrostatic
printing, one first prepares a "master" that is capable of selectively
holding electrostatic charges to form the desired image. The master is
exposed to a corona discharge that forms a latent electrostatic image, and
contacted with dry or liquid toner of the opposite electrostatic charge to
develop the image. The toned image is then transferred to a substrate,
typically paper, where the toner is fused to fix the image, and the master
is returned for the next printing cycle.
It has been suggested in U.S. Pat. No. 4,069,759 that an improved
electrostatic printing master can be fabricated by dispersing a
conventional silver halide photographic salt in an insulating polymer
(e.g., gelatin), and coating the dispersion on a conducting substrate. The
coating is exposed imagewise, and is developed to cause the exposed silver
halide to be reduced to metallic silver. The unexposed silver halide is
then dissolved and removed from the coating to fix the image. While the
master suggested in U.S. Pat. No. 4,069,759 offers many advantages, and
permits the use of conventional aqueous silver halide photographic
chemistry when gelatin is selected as the insulating polymer, it has been
found that gelatin is too highly sensitive to humidity to have practical
application in a typical workplace. Gelatin rapidly absorbs mositure from
the air and at moderate to high humidities no longer functions as an
insulating medium, but provides a conductive path that grounds surfaces
charges imposed on the master during the electrostatic printing process.
Thus, there is a need for an improved electrostatic printing master that
will offer the advantages of being based on conventional aqueous silver
halide photographic chemistry and provide superior insulating properties
under relative humidity conditions commonly encountered during printing.
SUMMARY OF THE INVENTION
This invention provides a photosensitive composition adapted for use in
preparing an electrostatic printing master, the composition consisting
essentially of a silver halide photographic salt dispersed in an
insulating polymeric binder that is swellable in aqueous photographic
processing solutions having a pH higher than approximately 81/2, and
retains significant insulating properties under relative humidity
conditions normally encountered during the printing process. The
composition has an insulation value such that it will support an apparent
macroscopic electric field of at least five (5) volts/micron, as measured
by an electrostatic surface voltage probe two (2) seconds following full
charging of its surface that has been allowed to equilibrate at 50%
relative humidity at 20.degree. C. for an hour. Common photographic
gelatin, practically the only medium conventionally used for wet
processing, holds approximately one (1) volt/micron or less after
equilibration under these test conditions. Since the binder is swellable
under pH conditions higher than approximately 81/2, conventional aqueous
silver halide developing solutions can be used to process the master for
use in electrostatic printing. Copolymers of acrylic or methacrylic acid
having acid numbers in the range of 70 to 160 are a preferred binder that
may be selected in practicing the invention. The silver halide/binder
composition is typically coated onto a conducting substrate, which may be
mounted on a flexible support, for use as an electrostatic master. After
the master is imaged with actinic light, the master is developed to
contain a silver image using conventional aqueous silver halide developing
and fixing chemistry.
In a second embodiment, a diffusion transfer film is prepared by coating
the polymeric binder which contains development nuclei onto a conductive
support, and overcoating the binder with a conventional silver halide
photographic emulsion. The photosensitive element is exposed and then
developed using conventional diffusion transfer techniques to provide an
imaged electrostatic master.
It now has been found that polyfunctional epoxide and aziridine
crosslinking agents unexpectedly improve optical density, D.sub.max, of
the conducting silver image contained in the developed electrostatic
master, improving its utility as a phototool.
As used herein, the term "electrostatic master" refers to the film element
that will be used for electrostatic printing, whether the film element
contains silver particles in the form of the desired image, and thus is
ready for the printing process, or contains silver halide particles that
yet have to be exposed and/or developed.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic sectional view of an electrostatic printing master in
which a silver halide photographic salt is dispersed in the insulating
binder to form photosensitive layer 1.
FIG. 2 shows the master of FIG. 1 in which a latent image has been formed
and developed.
FIG. 3 shows the master of FIG. 2 after the image has been fixed.
FIG. 4 shows the master of FIG. 3 after being charged.
FIG. 5 illustrates the master of FIG. 4 in which toner particles have been
attracted to the charged surface.
FIG. 6 is a schematic sectional view of a second embodiment in which the
photosensitive layer 8 is a diffusion transfer film.
FIG. 7 shows the embodiment of FIG. 6 in which the diffusion transfer film
has been imaged and development has commenced.
FIG. 8 shows the embodiment of FIG. 7 after development is complete.
FIG. 9 shows the embodiment of FIG. 8 after the photosensitive layer 8 has
been removed, at which time it is ready to be used as an electrostatic
master.
DETAILED DESCRIPTION OF THE INVENTION
The use of conventional aqueous silver halide photographic chemistry
ideally serves the requirements for the preparation of electrostatic
printing masters, particularly when high resolution is required for
high-quality half-tone or continuous-tone applications. Sharp image
resolution can be obtained due to the fine grain size of silver that may
be obtained when using aqueous photographic chemistry well known in the
art.
Insulating binders that may be selected in practicing the invention are
"swellable" in aqueous solutions having a pH higher than approximately
81/2, typically in the range of 9 to 14, that are common to conventional
aqueous developing solutions used in silver halide photography. By
"swellable" it is meant that the binder readily takes up water, and indeed
swells in this pH range similar to gelatin. When using preferred polymers
described hereinafter, swelling is accomplished by ionizing acidic groups
(usually carboxylic acid groups that are chemically bonded to the
insulating binder) by basic solutions at a pH of approximately 8.5 or
higher. This characteristic permits the aqueous developer (reducing)
solution to come into intimate contact with the silver halide. When
negative working silver halide emulsions are used, the exposed silver
halide is reduced by developer solutions to metallic silver and complexing
agents dissolve the unexposed silver halide salt. When positive working
silver halide emulsions are used (e.g. those prepared by such well-known
techniques as solarization or chemical fogging) the unexposed silver
halide is reduced to metallic silver and the exposed silver halide is
removed.
In the embodiment described in greater detail hereinafter in which negative
working silver halide is dispersed in the insulating binders provided by
the invention, developer above approximately pH 8.5 swells the binder and
reduces exposed silver halide to metallic silver and complexing agents,
usually in a fixer solution, remove unexposed silver halide. In the
diffusion transfer embodiment where negative working photosensitive silver
halide is in an emulsion layer (usually gelatin) that is separate from the
insulating binder containing a fine dispersion of development nuclei,
developer solution having a pH above approximately 8.5 swells both the
emulsion layer and insulating binder layer provided by this invention,
thereby developing the exposed silver halide to metallic silver in the
emulsion layer and dissolving the unexposed silver halide with complexing
agents ("silver solvents"). The complexed unexposed silver halide then
diffuses into the swollen binder layer wherein the silver ions are
selectively reduced to silver metal on the development nuclei.
Although the insulating binders are swellable in the developing solution,
the insulating properties do not drastically deteriorate as those of
gelatin do under typical humidity conditions encountered in the workplace.
As a consequence, the binders will retain an applied charge during
electrostatic printing and it is not necessary to provide special humidity
controls or dry the master before each printing cycle, as would be
necessary using a gelatin binder.
The binders generally are characterized as being capable of supporting an
apparent macroscopic electric field of at least 5 volts per micron, and
preferably at least 30 volts per micron, as measured by an electrostatic
surface voltage probe two (2) seconds following full charging of the
surface after the surface has been allowed to equilibrate, and thus absorb
moisture, at 50% relative humidity and 20.degree. C. Equilibration for
testing purposes will normally occur within approximately 60 minutes. In
contrast, gelatin is significantly inferior and exhibits an apparent
macroscopic electric field in the order of approximately one (1) volt per
micron or less under this test procedure.
It has been found that synthetic polymers having an acid number of
approximately 70 to 160 are particularly useful in practicing the
invention. A preferred class of polymers contains 10 to 25% by weight of
acrylic or methacrylic acid to impart swellability. The polymer typically
will also contain styrene, or other aromatic monomers, that are not
compatible with water, and thus render the polymer less hydrophilic to
moisture in the air. Generally, the polymer will also contain monomers,
such as appropriate acrylic or methacrylic esters, that contribute to film
clarity, flexibility, toughness, processibility, etc. Other comonomers,
such as alkenes having 2 to 12 carbon atoms, haloolefins, vinyl acetate,
vinyl ethers having 3 to 12 carbon atoms, methacrylamide, and the like can
be similarly useful.
Preferred polymers are copolymers containing styrene and acrylic or
methacrylic acid monomers, and preferably also an acrylic or methacrylic
ester monomer. Polymers containing 25 to 35% by weight styrene, 10 to 25%
by weight acrylic or methacrylic acid, with the remainder comprising
acrylic or methacrylic esters, are particularly preferred. The molecular
weight of the preferred copolymers will typically be in the range of
25,000 to 150,000. These polymers are compatible with silver halide
dispersions, will form reasonably durable films that have clarity, and are
readily available from commercial sources, or can be made using
conventional techniques such as free radical polymerization in suspension
or emulsion. Equivalent polymers that will be useful in practicing the
invention will be readily apparent to those skilled in the art. These
polymers include acrylic acid and methacrylic acid polymers and
copolymers, and include commercially available polymers such as
Carboset.RTM. 525 and Carboset.RTM. 526 manufactured by B. F. Goodrich
Company, and Joncryl.RTM. 67 manufactured by Johnson & Johnson.
A preferred class of polymers constitutes terpolymers and tetrapolymers of
(1) a styrene-type monomer, (2) an acrylate-type monomer, and (3) an
unsaturated carboxyl-containing monomer. The first component lends
hardness and moisture resistance to the polymer; the second, flexibility
and plasticity to the polymer backbone; and the third,
alkali-swellability. The styrene-type monomer will typically be styrene,
an alpha-substituted styrene having a 1 to 6 carbon alkyl group, and those
wherein the benzene ring has functional substituted groups such as nitro,
alkoxy, acyl, carboxy, sulpho, or halo, with simple compounds such as
styrene, alphamethyl styrene, para-methyl styrene and para-t-butyl styrene
being preferred. The acrylate-type component includes alkyl and
hydroxyalkyl acrylates and methacrylates wherein the alkyl group has from
1 to 12, preferably from 1 to 6 carbon atoms such as methyl methacrylate,
ethylmethacrylate, ethyl acrylate, hydroxypropyl methacrylate,
hydroxyethyl methacrylate and hydroxyethyl acrylate, and mixtures thereof.
The unsaturated carboxyl-containing monomer will typically be a monomer
having from 3 to 15 carbon atoms, preferably 3 to 6, and includes cinnamic
acid, crotonic acid, sorbic acid, itaconic acid, maleic acid, fumaric
acid, or more preferably acrylic or methacrylic acid, their corresponding
half ester or the corresponding anhydride.
When this class of polymer is selected in practicing the invention, the
ratio of the three monomer components is selected such that the conductive
film element has the following properties: the silver halide, when
incorporated into the conductive film element, is processible by
conventional aqueous photographic techniques; the electrostatic master
made therefrom retains applied charges in the nonsilver areas under
ambient relative humidity conditions; and the electrostatic master is
flexible and durable, but not tacky. Typical proportions used to achieve
these results are shown in Table 1:
TABLE 1
______________________________________
Broad Range
Preferred Range
Binder Component
(weight %) (weight %)
______________________________________
Styrene-type 10-50 25-35
Acrylate-type 0-85 40-65
Carboxylic 5-50 10-25
acid-type
______________________________________
Polymers within this class also generally offer the advantage of being
insensitive to Isopar.RTM., the commonly used carrier employed in liquid
toning systems.
Insulating polymeric binders described above are made by conventional
free-radical polymerization techniques, as illustrated in the examples.
These polymers are soluble in basic solutions and can be coated from
aqueous solutions of triethylamine, ammonia, or potassium hydroxide, and
the like. These polymers are compatible with silver halide dispersions and
will form reasonably durable films that have clarity. It may be desired to
modify the binder (crosslink, harden, plasticize, adjust acidity, etc.)
prior to aqueous photographic processing, and thereby control swelling or
improve durability. Various modifying agents may be added for these
purposes. Typical modifying agents include aldehydes, multifunctional
aziridines, and epoxides. The diglycidyl ether of 1,4-butanediol is a
preferred modifying agent for this class of polymers in practicing the
invention.
In accordance with one aspect of the present invention, it has been found
that use of polyfunctional epoxide or aziridine crosslinking agents with
the above described copolymers of an aromatic monomer and a carboxylic
acid, when used in quantities of approximately 1 to 30% by weight (based
on the copolymer weight), preferably 2 to 12% by weight, will unexpectedly
improve optical density (D.sub.max) of the conducting silver image
contained in the developed electrostatic master, in addition to usually
improving adhesion, durability, and processibility of the copolymer. When
the crosslinking agent is used, it is conveniently added to the binder
solution, which is then coated and air dried. The coated composition may
be heated to expedite crosslinking, if so desired.
A variety of polyfunctional epoxide and aziridine crosslinking agents are
known in the art. See, for example, Handbook of Epoxy Resins, H. Lee and
K. Neville, McGraw Hill, N.Y., 1967; Encyclopedia of Chemical Technology,
3d. Ed., Wiley-Interscience, N.Y., 1980, Vol. 9, Epoxy Resin Chemistry, I
and II, ACS Symposium Series Nos. 114 and 221, American Chemical Society,
Washington, DC, 1979 & 1983. These crosslinking agents may contain
aliphatic, cycloaliphatic, aromatic or heterocyclic backbones derived from
polyols or low molecular weight condensation polymers. Representative
polyfunctional epoxides include: butadiene dioxide; dimethylpentane
dioxide; diglycidyl ether; vinylcyclohexene dioxide; limonene dioxide;
bis-(2,3-epoxycyclopentyl)ether; divinyl benzene dioxide; 1,4-butanediol
diglycidyl ether; trimethylolpropane triglycidyl ether; ethoxylated
trimethylolpropane triglycidyl ether; neopentyl glycol diglycidyl ether;
cyclohexanedimethanol diglycidyl ether; glycerine triglycidyl ether;
3,4-epoxy-6-methylcyclohexane carboxylate; the diglycidyl ether of
resorcinol; the diglycidyl ethers of bis-phenol F and bis-phenol A; epoxy
phenol novolac resins; methylene dianiline derived epoxy resins;
p-aminophenol derived epoxy resins; triazine based epoxy resins; and
hydantoin epoxy resins. Preferred are: 1,4-butanediol diglycidyl ether;
ethoxylated trimethylolpropane triglycidyl ether; (3,4-epoxycyclohexyl)
methyl 3,4-epoxy-cyclohexane carboxylate; diglycidyl ethers of bis-phenol
F and bis-phenol A; methylene dianiline derived epoxy resins;
p-aminophenol derived epoxy resins; and triazine based epoxy resins.
Representative polyfunctional aziridines include: pentaerythritol
tri-beta-azirindinylpropionate;
N,N'-hexamethylene-1,6-bis(1-aziridinecarboxyamide); and
N,N'-diphenylmethane- 4,4'-bis(1-aziridinecarboxyamide).
The light sensitive silver halide selected for dispersion in the binder can
be any of the well-known salts used in photographic applications.
Representative useful salts include silver chloride, silver bromide,
silver iodide, silver chlorobromide, silver iodobromide, and silver
chloroiodobromide, either singly or in mixtures. Precipitation of the
halide is carried out in conventional manner in gelatin. The amount of
gelatin present should be limited, or subsequently reduced by rinsing, to
avoid defeating purposes of the invention. Generally, levels of gelatin as
high as 3 to 15 grams per mole of silver can be tolerated in the
electrostatic printing masters, without adverse effect.
Grain size distribution and sensitization of the silver halide can be
controlled to adapt the silver halides for the selected class of
photographic process, including general continuous tone, X-ray,
lithographic, microphotographic, direct positive, and the like.
Ordinarily, the silver salt dispersions will be sensitized with
conventional compounds such as sulfur, gold, rhodium, selenium and the
like, or with organic sensitizing dyes such as cyanine,
1,1'-diethyl-4,4'-cyanine iodide, methine and polymethine cyanine dyes,
kryptocyanines, merocyanines, and the like. Other additives commonly
employed in silver halide photographic compositions, may also be present
if desired.
To prepare the dispersion of silver halide in the insulating polymeric
binder, the binder is conveniently first dissolved in an aqueous solution
containing amines, such as ammonia or triethyl amine. If desired, an
alcohol, such as methanol, ethanol, or isopropanol, may be added to aid in
solubilizing the polymer. Ketones, such as methyl ethyl ketone, may be
used as a cosolvent. An aqueous dispersion of the silver halide salt is
then added to the dissolved binder in the desired quantities. The
respective portions of silver halide to binder will depend on details of
the application, but will generally be such that surface of the master
immediately above the developed silver will discharge significantly faster
than areas devoid of silver. Weight ratios of silver to polymeric binder
in the range of 0.5:1 to 3:1 will typically provide useful results. A
preferred range is 1.7:1 to 2.3:1.
The polymeric binder containing the silver halide is usually applied to a
conductive substrate as a solution or dispersion in a carrier solvent,
usually an aqueous solution containing basic amines or sodium or potassium
hydroxide as described above. The coating procedure may be any
conventional one including spraying, brushing, applying by a roller or an
immersion coater, flowing over the surface, picking up by immersion, spin
coating, air-knife coating, wire-bar coating or any other suitable means.
The film thickness can be adjusted accordingly and after drying is usually
about 0.02 to about 0.3 mils (0.5-7.5 microns), preferably about 0.04 to
about 0.20 mils (1.0-5.0 microns). Depending on the application, the
conductive support may be a metal plate, such as aluminum, copper, zinc,
silver or the like; a conductive polymeric film; a support such as paper,
glass, synthetic resin and the like which has been coated with a metal,
metal oxide, or metal halide by vapor deposition or chemical deposition; a
support which has been coated with a conductive polymer; or a support
which has been coated with a polymeric binder containing a metal, metal
oxide, metal halide, conductive polymer, carbon, or other conductive
fillers.
In addition to components described above, various conventional
photographic additives, e.g., developing agents, super additives,
antifoggants, coating aids such as saponin, alkylarylsulfonic acids or
sulfoalkylsuccinic acids; plasticizers such as glycerol or
1,5-pentanediol; antistatic agents; agents to prevent the formation of
spots; antihalation dyes; underlayers, subbing or backing layers; and the
like may be added to the master as appropriate. Positive images may be
obtained by reversal processing of the silver halide using either light
fogging or a chemical fogging agent; or by using silver halide emulsions
that give direct positive images using the prefogging technique. Direct
positive emulsions have been described in Leersmaker U.S. Pat. No.
2,184,013, Illingsworth U.S. Pat. No. 3,501,307 and elsewhere.
Referring now to the drawings, FIG. 1 depicts an electrostatic printing
master in which photosensitive layer 1 contains sensitized silver halide
dispersed in the insulating polymeric binder in accordance with the
invention. Layer 1 is generally between 0.5 and 7.5 microns in thickness,
but the thickness can be decreased or increased for the specific intended
application. A thin layer 2 of an adhesion promoter such as gelatin, which
is optional, aids adherence of the photosensitive layer to the conducting
substrate 3, which in turn is mounted on supporting substrate 4.
The master is exposed imagewise using any of the procedures commonly used
with silver halide photographic materials, such as by imaging with actinic
light, a cathode ray tube, or laser. For negative-working emulsions the
latent image 5 is then developed by reducing the exposed silver halide
particles to metallic silver using a conventional aqueous developing
solution, as illustrated in FIG. 2. A conventional aqueous fixing
solution, such as sodium thiosulfate, is then used to remove the unexposed
silver halide particles, as illustrated in FIG. 3. The developed master is
then ready for the electrostatic printing process.
FIG. 4 illustrates the master of FIG. 3 after it has been charged by a
corona discharge that deposited positive charges 6 on the master surface.
The area of the film that contains silver 5 provides a pathway for
overlying charges to pass to ground, thus forming a latent image of
charges that remain on the master surface. Alternatively, charging can be
accomplished with the use of a negative corona discharge, shielded
corotron, scorotron, radioactive source, contact electrodes such as
electrically biased semiconductive rubber rollers, and the like.
The latent image is then developed with liquid or dry toner 7 of the
opposite polarity, as illustrated in FIG. 5. Cascade, magnetic brush,
powder cloud, liquid, magne-dry and wetting development techniques are
suitable. Representative dry toners that may be used include Kodak
Ektaprint K toner, Hitachi HI-Toner HMT-414, Canon NP-350F toner, and
Toshiba T-50P toner. Examples of suitable liquid toners are Savin 24
toner, Canon LBP toner and James River Graphics T1818 toner. The latent
image so developed ("toned") is transferred to the usual substrate,
typically paper, where it is fixed in conventional fashion.
FIGS. 6 through 9 illustrate a second embodiment wherein conventional
diffusion transfer techniques such as those described in U.S. Pat. Nos.
2,352,104 and 2,983,606, are used to prepare an imaged electrostatic
printing master of dispersed silver in the insulating synthetic binder
previously described. In this embodiment, the insulating synthetic binder
9, approximately 0.25 to 3 microns in thickness, contains dispersed
development nuclei, and a photosensitive layer 8 containing silver halide
salts dispersed in a hydrophilic colloid that overlays the binder, wherein
the ratio of silver to binder 9 is 1:1 to 5:1. A conductive layer 3 and
substrate 4 are employed as hereinbefore described. Suitable development
nuclei are well-known in the art, and typically will be (1) a metal, such
as silver, gold, and rhodium; (2) sulfides, selenides, tellurides,
polysulfides, or polyselenides of metals including silver, zinc, chromium,
gallium, iron, cadmium cobalt, nickel, manganese, lead, antimony, bismuth,
arsenic, copper, rhodium, palladium, platinum, lanthanum, and titanium;
(3) easily reducible silver salts which form silver nuclei during
processing, such as silver nitrate or silver citrate; (4) inorganic salts
which react with the incoming diffusing silver salts to form nuclei; and
(5) organic compounds which (a) contain a labile sulfur atom and which
therefore lead to the formation of sulfide nuclei during processing,
including mercaptans, xanthates, thioacetamide, dithiooxamide, and
dithiobiurate or (b) are reducing agents such hydrazine derivatives or
silanes and give rise to silver nuclei when evaporated onto a silicic
acids or barium sulfate. Likewise the hydrophilic colloid can be any of
the substances commonly used in diffusion transfer processes, such as
gelatin, phthalated gelatin, cellulose derivatives such as
carboxymethylcellulose and hydroxymethylcellulose, and other hydrophilic
high molecular weight colloidal substances such as dextrin, soluble
starch, polyvinyl alcohol, or polystyrenesulfonic acid.
Referring to FIG. 7, photosensitive layer 8 is imaged in conventional
fashion to form a latent image with the sensitized silver halide. For
negative-working emulsions the photosensitive layer is then treated with a
developing agent that reduces the exposed silver halide to metallic
silver, in area 10, and an aqueous solvent composition that converts
silver halide in the unexposed areas to form a soluble silver halide
complex that diffuses into the binder of layer 9 where it contacts the
development nuclei and is reduced to insoluble silver particles 11,
forming a silver image. Layer 8 is then removed as illustrated in FIG. 9,
resulting in an electrostatic master that is ready for printing in
conventional manner. Developing baths for the diffusion transfer process
are well known in the art and are described, for example, in Photographic
Silver Halide Diffusion Processes by Andre Rott and Edith Weyde (Focal
Press, 1972) and Modern Photographic Processing, Vol. 2 by Grant Haist
(Wiley, 1979).
Many additional embodiments will be evident to those skilled in the art.
For example, a positive-working silver halide emulsion can be used in
conjunction with the diffusion transfer coating 8 illustrated in FIGS. 6
through 9, and the exposed silver halide can be complexed in aqueous
solutions to diffuse into the insulating binder layer 9, where it is
reduced by the development nuclei to form the desired silver image.
Similarly, a separate photosensitive film can be employed in lieu of
coating 8, and brought into operative association with the insulating
binder 9 before or after imaging, as in photomechanical transfer. The
photosensitive silver halide emulsion layer or coating 8, and the
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