Photoelectrographic elements and imaging method

A photoelectrographic element comprising a conductive layer in electrical contact with an acid photogenerating layer which (a) is free of photopolymerizable materials and (b) comprises an electrically insulating binder and an acid photogenerator is disclosed. A method for forming images with the element is also disclosed.

FIELD OF THE INVENTION 
This invention relates to new photoelectrographic elements, an imaging 
method using such elements and novel acid photogenerators. 
BACKGROUND OF THE INVENTION 
Acid photogenerators are known for use in photoresist imaging elements. In 
such imaging processes, the acid photogenerator is coated on a support and 
imagewise exposed to actinic radiation. The layer bearing the acid 
photogenerator is then contacted with a photopolymerizable or curable 
composition such as epoxy and epoxy-containing resins. In the exposed 
areas, the acid photogenerator generates a proton which catalyzes the 
polymerization or curing of the photopolymerizable composition. Acid 
photogenerators are disclosed, for example, in U.S. Pat. Nos. 4,081,276; 
4,058,401; 4,026,705; 2,807,648; 4,069,055; and 4,529,490. 
Electrophotographic compositions and imaging processes are also known. In 
these processes an electrophotographic element bearing a layer containing 
a photoconductor is electrostatically charged and then imagewise exposed 
to form a latent electrostatic image. The latent electrostatic image is 
subsequently developed with a toner composition. Electrophotographic 
elements and processes are disclosed, for example, in U.S. Pat. Nos. 
3,141,770, 3,615,414 and all of the patents cited therein. The problem is 
that with any electrophotographic element, it is always necessary to 
electrostatically charge the element prior to imagewise exposure. 
SUMMARY OF THE INVENTION 
The present invention provides a photoelectrographic element comprising a 
conductive layer in electrical contact with an acid photogenerating layer 
which (a) is free of photopolymerizable materials and (b) comprises an 
electrically insulating binder and an acid photogenerator. The elements of 
this invention can be imagewise exposed and electrostastically charged in 
any order. 
The present invention also provides a photoelectrographic imaging method 
comprising the steps of: 
(a) providing a photoelectrographic element comprising a conductive layer 
in electrical contact with an acid photogenerating layer which (i) is free 
of photopolymerizable materials and (ii) comprises an electrically 
insulating binder and an acid photogenerator; 
(b) carrying out the following steps (b) (i) and (b) (ii) concurrently or 
separately in any order, to form an electrostatic latent image, 
(i) imagewise exposing the acid photogenerating layer to actinic radiation, 
(ii) electrostatically charging the acid photogenerating layer, and 
(c) developing the electrostatic latent image with charged toner particles. 
The present invention also provides a polymer comprising appended anionic 
groups having aromatic onium salt photogenerators as the counter ion. 
The imaging method and elements of this invention use acid photogenerators 
in thin layers coated over a conductive layer to form images. This imaging 
technique or method takes advantage of our discovery that exposure of the 
acid generator significantly increases the dark decay of electrostatic 
charges in the exposed area of the layer. Imagewise radiation of the acid 
photogenerator layer creates differential dark decay between exposed and 
unexposed areas. In the method exposure can occur before, after or 
cotemperaneously with the charging step. This is different from 
electrophotographic imaging techniques where the electrophotographic 
element must always be charged electrostatically prior to exposure. 
The photoelectrographic elements of the invention are also advantageous in 
that the imagewise differential dark decay of electrostatic charges are 
erasable with heat. Moreover, the imagewise conductivity differential 
created by the exposure is permanent unless the element is subjected to 
heat. Thus, multiple copies of a document can be made from a single 
exposure. 
PREFERRED EMBODIMENTS 
Especially useful photoelectrographic elements of this invention utilizes 
acid photogenerators selected from the group consisting of aromatic onium 
salts including triarylselenonium salts and aryldiazonium salts, and 
6-substituted-2,4-bis(trichloromethyl)-5-triazines. Particularly useful 
acid photogenerators are arylhalonium salts and triarylsulfonium salts. 
DETAILS OF THE INVENTION 
In preparing acid photogenerating layers the acid photogenerator is 
dissolved in a suitable solvent in the presence of an electrically 
insulating binder. Then a sensitizer, if desired, is dissolved in the 
resulting solution prior to coating on a conducting support. 
Solvents of choice for preparing coating compositions of the acid 
photogenerators include a number of solvents such as aromatic hydrocarbons 
such as benzene and toluene; acetone, 2-butanone; chlorinated hydrocarbons 
such as ethylene dichloride, trichloroethane and dichloromethane, ethers 
such as tetrahydrofuran; or mixtures of these solvents. 
The acid photogenerating layers are coated on a conducting support in any 
well-known manner such as doctor-blade coating, swirling, dip-coating, and 
the like. 
The acid photogenerating materials should be chosen so that at certain 
concentrations in the layer, the layer has a relatively small dark decay 
before irradiation, but the dark decay level should increase by radiation 
exposure. In preparing the coating composition, useful results were 
obtained where the acid photogenerator was present in an amount equal to 
at least about 1 weight percent of the coated layer. The upper limit of 
the amount of acid photogenerator is not critical as long as no 
deleterious effect on the initial dark decay of the film is encountered. A 
preferred weight range for the acid photogenerator in the coated and dried 
composition is from 10 weight percent to about 60 weight percent. 
Coating thicknesses of the acid photogenerator can vary widely. Normally a 
wet coating thickness in the range from about 0.1 .mu.m to about 50 .mu.m 
are useful. Coating thicknesses outside these ranges will also be useful. 
The photoelectrographic elements of the present invention are employed in 
the photoelectrographic process described hereinbefore. In this process, 
the element is given a blanket electrostatic charge by placing the same 
under a corona discharge which serves to give a uniform charge to the 
surface of the acid photogenerator layer. The layer is then exposed 
imagewise. Exposure and charging can be carried out in any order or at the 
same time. The charge is dissipated by the layer in exposed areas. Thus, 
the combination of the charging and imagewise exposure steps create an 
electrostatic latent image of the type produced in electrophotographic 
processes. 
The electrostatic latent image is then developed or transferred to another 
sheet and developed by treatment with a medium comprising 
electrostatically attractable particles. Such particles are used 
extensively in developing electrophotographic images. The particles are 
generically referred to as toners. The toners in the form of a dust, 
powder, a pigment in a resinous carrier, or in a liquid developer in which 
the toner particles are carried in an electrically insulating liquid 
carrier. Methods of development of this type are widely known and have 
been described in the electrophotographic patent literature in such 
patents, for example, as U.S. Pat. No. 2,296,691 and in Australian Pat. 
No. 212,315. 
The charged toner may have the same sign as the electrographic latent image 
or the opposite sign. In the former case, a negative image is developed. 
In the latter case, a positive image is developed. 
Any compound which generates an acid upon exposure will be useful. Useful 
aromatic onium salt acid photogenerators are disclosed in U.S. Pat. Nos. 
4,081,276; 4,529,490; 4,216,288; 4,058,401; 4,069,055; 3,981,897; and 
2,807,648. Such aromatic onium salts include Group Va, Group VIa and Group 
VIIa elements. The ability of triarylselenonium salts, aryldiazonium salts 
and triarylsulfonium salts to produce protons upon exposure to light is 
described in detail in "UV Curing, Science and Technology", Technology 
Marketing Corporation, Publishing Division, 1978. 
A representative portion of the useful aryl iodonium salts are the 
following: 
##STR1## 
A representative portion of useful Group Va onium salts are: 
##STR2## 
A representative portion of useful Group VIa onium salts, including 
sulfonium salts, are: 
##STR3## 
Other salts from which acid photogenerators may be selected are: 
1. Triarylselenonium salts, such as disclosed in Belgian Pat. Nos. 828,670 
and 833,472. 
The following salts are representative: 
##STR4## 
2. Aryldiazonium salts such as disclosed in U.S. Pat. Nos. 3,205,157; 
3,711,396; 3,816,281; 3,817,840 and 3,829,369. The following salts are 
representative: 
##STR5## 
3. 6-Substituted-2,4-bis(trichloromethyl)-5-triazines such as disclosed in 
British Pat. No. 1,388,492. The following compounds are representative: 
______________________________________ 
R 
______________________________________ 
##STR6## 
##STR7## 
##STR8## 
##STR9## 
##STR10## 
______________________________________ 
Another especially useful group of acid photogenerators include polymers 
comprising appended anionic groups having an aromatic onium acid 
photogenerator as the positive counter ion. Examples of useful polymers 
include 
##STR11## 
The polymers of this invention are made by simply exchanging ions between a 
commercially purchased or other anionic polymer salt and a simple 
nonpolymeric onium salt in aqueous solution. For example, a polymeric 
sulfonate salt will readily exchange anions in water with a diaryliodonium 
hydrogen sulfate. The reaction is driven to completion by precipitation of 
the new diaryliodonium polymeric sulfonate salt. 
Alternatively, the ion exchange could be performed on an anionic monomer 
and the monomer, with any desirable comonomers, polymerized by 
conventional polymerization technniques. 
A specific preparation follows for Polyonium 1. 
In a one liter beaker, 3.27 gm (0.00690 mole) of 
di-(4-t-butylphenyl)iodonium hydrogen sulfate was dissolved in about 300 
ml of water. To the stirred solution in the beaker, was added dropwise 
1.09 gm (0.00575 mole) of preformed poly(sodium p-styrenesulfonate) 
dissolved in about 200 ml of water. A precipitate of polyonium 1 started 
to form on mixing. After complete addition, the precipitate was filtered, 
redissolved in dichloromethane, washed twice with water and reprecipitated 
into a large volume of heptane. The polymer was then filtered and dried at 
100.degree. C. for ten minutes. 
Such polymers should comprise sufficient cationic acid photogenerator 
groups to achieve the differential dark decay for imaging purposes. In 
general, such polymers comprise from 1 to 100 mole percent of acid 
generating groups. Ionic polymers from which the polyoniums of the present 
invention can be made are disclosed in U.S. Pat. Nos. 3,042,221; 
3,506,707; 3,547,899; 3,411,911; 3,062,674 and 3,220,544. 
Useful electrically insulating binders for the acid photogenerating layers 
include polycarbonates, polyesters, polyolefins, phenolic resins and the 
like. Desirably, the binders are film forming. Mixtures of such polymers 
can also be utilized. To be useful, such polymers should be capable of 
supporting an electric field in excess of 6.times.10.sup.5 V/cm and 
exhibit a low dark decay of electrical charge. 
Preferred binders comprise styrene-butadiene copolymers; silicone resins; 
styrene-alkyd resins; soya-alkyd resins; poly(vinyl chloride); 
poly(vinylidene chloride); vinylidene chloride, acrylonitrile copolymers; 
poly(vinyl acetate); vinyl acetate, vinyl chloride copolmyers; poly(vinyl 
acetals), such as poly(vinyl butyral); polyacrylic and methacrylic esters, 
such as poly(methyl methacrylate), poly(n-butyl methacrylate), 
poly(isobutyl methacrylate), etc; polystyrene; nitrated polystyrene; 
poly(vinylphenol)polymethylstyrene; isobutylene polymers; polyesters, such 
as phenolformaldehyde resins; ketone resins; polyamide; polycarbonates; 
etc. Methods of making resins of this type have been described in the 
prior art, for example, styrene-alkyd resins can be prepared according to 
the method described in U.S. Pat. Nos. 2,361,019 and 2,258,423. Suitable 
resins of the type contemplated for use in the photoconductive layers of 
this invention are sold under such tradenames as Vitel PE 101-X, Cymac, 
Piccopale 100, and Saran F-220. Other types of binders which can be used 
include such materials as paraffin, mineral waxes, etc. 
The amount of spectral or speed enhancing sensitizer which can be added to 
a particular acid generating composition to give optimum sensitization 
varies widely. The optimum amount will, of course, vary with the acid 
photogenerator used and the thickness of the coating as well as with the 
particular sensitizer. In general, substantial speed gains and wavelength 
adjustments can be obtained where an appropriate sensitizer is added at a 
concentration up to about 30 percent by weight based on the weight of the 
acid generating composition. 
The iodonium salt acid photogenerators may be sensitized using ketones such 
as xanthones, indandiones, indanones, thioxanthones, acetophenones, 
benzophenones or other aromatic compounds such as anthracenes, 
diethoxyanthracenes, perylenes, phenothiazines, etc. 
Triarylsulfonium salt acid generators may be sensitized by aromatic 
hydrocarbons, anthracenes, perylenes, pyrenes and phenothiazines. 
Useful conducting layers include any of the electrically conducting layers 
and supports used in electrophotography. These include, for example, paper 
(at a relative humidity about 20 percent); aluminum-paper laminates; metal 
foils, such as aluminum foil, zinc foil, etc.; metal plates, such as 
aluminum, copper, zinc, brass, and galvanized plates; regenerated 
cellulose and cellulose derivatives; certain polyesters, especially 
polyesters having a thin electroconductive layer (e.g. cuprous iodide) 
coated thereon; etc.

The following examples further clarify how to make and use the invention of 
this application. 
EXAMPLE 1 
A general formulation consisting of 13.5 gm of poly(methyl methacrylate), 1 
gm of 9,10-diethoxyanthracene sensitizer and 70 gm of dichloromethane was 
mixed for complete dissolution on a roller mill. Aliquots of this 
formulation (7 gm) were taken with 0.1 gm of the acid generator being 
tested. Each of these solutions were coated on a copper coated polyester 
support with a 0.0254 mm knife, and dried at 90.degree. C. in an oven for 
about 30 minutes. The coated films were cut into 50.times.50 mm samples 
for exposure to a high pressure Hg lamp for 40 seconds. After exposure 
through a density step tablet, the films were corona charged for 60 
seconds negatively, followed by toning with a positive toner for 60 
seconds. The results are tabulated in Table I. 
TABLE I 
__________________________________________________________________________ 
T14 Density Step 
Tablet Speed 
Acid Photogenerator Solid Steps 
Contrast 
__________________________________________________________________________ 
##STR12## 3 Very Good 
##STR13## 2 OK 
##STR14## 6 Very Good 
##STR15## 4-5 Very Good 
__________________________________________________________________________ 
EXAMPLE 2 
Use of Spectral Sensitizers in the Acid Photogenerating Layer 
A stock solution containing 9 gm of poly(methyl methacrylate), 6 gm of 
##STR16## 
and 70 gm of dichloromethane was prepared. Aliquots (8.5 gm) of this stock 
solution were mixed with 0.1 gm of sensitizers 1, 2, 3 and 4 below: 
##STR17## 
These solutions were hand coated on a copper coated polyester support using 
a 0.05 mm blade and baked at 90.degree. C. for about 15 minutes; 
50.times.50 mm samples of each coating were exposed with a 200 watt high 
pressure Hg lamp for 40 seconds through a step tablet (0.15 log E). The 
exposed samples were corona charged for 60 seconds and developed for 60 
seconds in a liquid toner. The results are tabulated in Table II. 
TABLE II 
______________________________________ 
Sensitizers Imaging Results 
______________________________________ 
Control 0 
Sensitizer 1 6 solid steps 
Sensitizer 4 5-6 steps 
Sensitizer 3 3-4 steps 
Sensitizer 2 3 steps 
______________________________________ 
This example illustrates how the use of sensitizers can improve the 
spectral performance of the elements of this invention. 
EXAMPLE 3 
Concentration Effect 
A series of formulations using di(4-t-butylphenyl)iodonium 
hexafluorophosphate and poly(methyl methacrylate) in various 
concentrations with 9,10-diethoxyanthracene (0.1 gm) as the sensitizer 
were coated and tested similar to Example 2. The results are shown in 
Table III. 
TABLE III 
______________________________________ 
Iodonium Salt 
Concentration (%) Imaging Results 
______________________________________ 
1 very faint image 
5 5 steps, fair image 
10 good image, 6 solid steps 
20 good image, 6 solid steps 
30 good image, 6 solid steps 
40 good image, 6 solid steps 
50 good image, 6 solid steps 
60 good image, 6 solid steps 
______________________________________ 
EXAMPLE 4 
The Use of Polymeric Onium Salts in Photoelectrographic Imaging 
Coatings were made from a general formulation comprising 0.68 gm of the 
polymeric salt being tested and 0.05 gm of 9,10-diethoxyanthracene 
dissolved in 7 gm of dichloromethane. Each of the formulations were coated 
on copperized polyester support with a 0.0254 mm coating knife and dried 
at 90.degree. C. for 30 minutes in an oven. The coated films were cut in 
2.times.2 samples and tested as defined in Example 1. The results are 
tabulated in Table IV. 
TABLE IV 
______________________________________ 
T14 Speed 
Solid Steps 
Contrast 
______________________________________ 
##STR18## 3 Very Good 
##STR19## 3-4 OK 
______________________________________ 
EXAMPLE 5 
The iodonium salt 
##STR20## 
poly(vinylphenol) (1.35 gm) and 0.1 g of 9,10-diethoxyanthracene in 
tetrahydrofuran were coated on a copperized support and baked at 
100.degree. C. for about 15 minutes. Samples were exposed for 1 second to 
a Hg lamp, charged for 60 seconds negatively and developed for 60 seconds 
in a positive liquid toner. The speed of this layer was excellent. Six 
solid steps were developed. 
The invention has been described in detail with particular reference to 
preferred embodiments thereof, but it will be understood that variations 
and modifications can be effected within the spirit and scope of the 
invention.