Porous reusable ZnO electrophotographic element

A reusable electrophotographic element comprising a photoconductive layer containing sensitized zinc oxide particles and first and second binding agents that are incompatible is produced by employing as the first binding agent a macromolecular compound that has a higher affinity to zinc oxide than the second binding agent, is largely deposited on the zinc oxide, has an average molecular weight of at least 12,000 and is present in the photoconductive layer in an amount of 1.5 to 9% by weight calculated on the zinc oxide, with the second binding agent present in substantially larger amount. The photoconductive layer is formed of agglomerates of zinc oxide particles substantially enveloped in the first binding agent, which agglomerates have a diameter of between 2.5 and 6 .mu.m and are stuck together by portions of the second binding agent, thus providing a substantially porous photoconductive layer having a negative charge density of at most 1 m Coulomb per m.sup.2. The photoconductive layer is produced by mixing together the zinc oxide, any desired dye sensitizer, and solutions of the binding agents in one or more volatilizable solvents, applying a layer of the resulting dispersion to a substrate suited for electrophotography and drying the applied layer. The electrophotographic element has a very high resistance to both electrical and mechanical influences, thus being suited for long service life in an electrophotographic copying machine.

The present invention relates to a reusable electrophotographic element and 
to a process for producing such an electrophotographic element. 
Reusable electrophotographic elements are employed particularly in indirect 
electrophotographic copying machines which produce copies by a succession 
of steps that include charging the electrophotographic element, exposing 
it imagewise and developing it with a developer powder and transferring 
the resulting powder image to a receiving material and fixing it thereon. 
After transferring the powder image, the electrophotographic element is 
cleaned and can be reused for copying. Reusable electrophotographic 
elements are also employed in copying machines in which a charge pattern 
obtained by charging and exposing the element is transferred to a 
receiving material and developed thereon. 
For the use in indirect electrophotographic copying machines there is a 
continuing need to increase the number of times the electrophotographic 
element can be used to make satisfactory copies. This is especially 
important to high-volume copying machines, because an element having a 
short useful life has to be renewed too often. A short useful life is a 
drawback particularly in the case of electrophotographic elements having a 
photoconductive layer based on zinc oxide dispersed in a binding agent. 
Although the frequency of necessary renewal of such an electrophotographic 
element has been reduced by using the element in the form of a long 
endless belt, so that another part of the belt is continually used for the 
image forming process, this technique has disadvantages in that a large 
part of the space in the copying machine is occupied by the belt and in 
that renewal of the belt is a rather cumbersome task requiring special 
care, because a long photoconductive belt is not easy to handle. 
The useful life of an electrophotographic element based on zinc oxide 
dispersed in a binder is limited by various electrical and mechanical 
influences, such as the following. With repeated charging of the 
photoconductive layer the dyes used for sensitizing the zinc oxide tend to 
be decomposed. This is a result, it is believed, of the formation of 
oxidizing substances such as ozone, nitrogen oxides, and ions, by the 
charging process. The charging also causes the formation on the surface of 
the zinc oxide-binder layer of hygroscopic substances, probably comprising 
oxidized organic binder, which disturb the image forming process and do so 
particularly at high relative humidities because in that case they render 
the surface of the photoconductive layer electrically conductive. 
Furthermore, conductive spots resulting from electrical breakdown will 
appear locally on the photoconductive layer. 
Among the mechanical influences which limit the useful life of the 
electrophotographic element are wear of the photoconductive layer, as 
caused by its moving contact with other materials or objects in the 
developing, transfer and cleaning stations of the copying machine, and 
tensile and pressure loads which result from the driving, bending and 
bending back of the endless electrophotographic belt element as it is 
passed over various rollers. 
A more particular form of mechanical load on the photoconductive element is 
involved in the use of a transfer system in which the developed image is 
transferred first onto an intermediate having a silicone rubber surface 
and then from that intermediate to the receiving material. Such a transfer 
system is often used with development by use of a one-component developer 
powder, as a result of which this developer and the use of an intermediate 
in the transfer process reduce the degradation of the photoconductive 
layer as compared with other developing and transfer systems. Even in this 
practice, however, an increased temperature and pressure involved in 
transferring the images onto the intermediate will cause a certain degree 
of plastic deformation of the photoconductive layer to set in at its 
surface. 
All the mechanical loads mentioned above cause changes in the structure of 
the photoconductive layer and reduction of the adherence of the zinc oxide 
particles to the binding agent of the layer, thus changing the 
electrophotographic properties of the element, for the most part, 
unfavorably. 
Various proposals have been made for extending the useful life of 
electrophotographic elements having a photoconductive layer formed of a 
dispersion of zinc oxide in a binder. For example, it has been proposed 
simply to wash off the electrophotographic element at regular intervals, 
which in itself seems to be a simple procedure but actually is not 
practicable in a high-volume copying machine because then the 
electrophotographic element has to be removed frequently from the machine, 
as often even as once or twice a day, to wash it off with an appropriate 
liquid and re-dry it carefully. 
It has also been proposed, many times, to provide the zinc oxide-binder 
layer with a top layer of a polymer, but in practice this too does not 
work satisfactorily. If the top layer is very thin it has little effect, 
and if the top layer is thick enough to have a significant effect then a 
residual voltage too high to be removed by prolonged exposure will be left 
on the background after the charging and imagewise exposure steps. Due to 
the fact that, generally, the surface of a zinc oxide-binder layer is not 
smooth, a top layer applied thereon will have a varying thickness, which 
results in an unequal charge distribution. This is particularly 
detrimental in the background and the light gray tones of an image. 
A third proposal for extending the useful life of electrophotographic 
elements having a photoconductive layer based on zinc oxide is described 
in U.K. Patent application No. 2 015 764. This relates to pretreating zinc 
oxide with a solution containing a sensitizing dye and a first binding 
agent in the form of a hydrophilic resin, such as polyvinyl alcohol, 
polyvinyl pyrrolidone or polyvinyl butyral, in a solvent. After being 
dried, the zinc oxide is covered with the dye and with a quantity of the 
resin which, calculated on the zinc oxide, is smaller than 1% by weight. 
The resulting product is then dispersed in a second binding agent having 
an acid value of from about 10 to 15, which has been dissolved in a 
solvent that does not dissolve the hydrophylic resin. Using this 
dispersion, a layer having a dry thickness of 15 to 20 .mu.m is coated on 
a metal plate, such as aluminum. 
According to Examples 1 and 2 of the U.K. patent application, the resulting 
product can be charged and discharged 7,000 to 10,000 times without its 
photosensitivity being deteriorated too seriously. Repeated charging and 
discharging, however, gives only an impression of the resistance to 
electrical load. As is evidenced by Example 8 of the U.K. patent 
application, the useful life is low when copies are made in a copying 
machine where the mechanical load also plays a role; that example mentions 
making 500 copies under moist conditions. According to the U.K. 
application, the useful life in a copying machine can be extended by 
measures such as washing off at regular intervals and/or applying a 
silicone resin top layer, and also by handling the electrophotographic 
element under dry conditions. Although dry conditions can be achieved in a 
humid environment by the use of heating elements, this not only is 
energy-consuming but also causes discomfort in the season in which a high 
relative humidity prevails in copying rooms. 
Another process for pretreating zinc oxide is described in German Patent 
application No. 29 52 664. In that process a binding agent is precipitated 
on zinc oxide by dispersing the zinc oxide in a solution of the binder and 
adding a liquid in which the binder does not dissolve, or by dispersing 
the zinc oxide in a solution of the binder in a solvent and a non-solvent 
and subsequently evaporating the solvent. The zinc oxide thus obtained is 
filtered off and dried and then is processed with a second binding agent 
to form a photoconductive layer. A product resulting from that process is 
described as being usable as many as 10,000 times in a specified copying 
machine. However, the useful life is considerably lower if a 
photoconductive element having such a photoconductive layer is used in a 
copying machine provided with a magnetic brush developing device employing 
one-component developer powder, and with a transfer system employing a 
heated intermediate. Moreover, the process has the drawback of being 
time-wasting, because its various steps require a dispersing time of some 
hours, and of requiring heating for even a longer time to dry the product 
after precipitation of the binding agent on the zinc oxide. 
The object of the present invention is to provide an electrophotographic 
element which can be prepared in a simple way and can be reused frequently 
in a copying machine without employing additional expedients, such as 
washing off at regular intervals, keeping dry, and top layers, with the 
associated disadvantages, and which moreover can be used in a copying 
machine provided with a heated intermediate transfer member over a much 
longer time than the photoconductive elements already known. 
According to this invention, a reusable electrophotographic element is 
provided which is similar to certain known elements in that it comprises a 
substrate suited for use in electrophotography and a photoconductive layer 
containing sensitized zinc oxide particles and first and second binding 
agents that are incompatible, the first binding agent having a higher 
affinity to zinc oxide than the second binding agent and being largely 
deposited on the zinc oxide. The electrophotographic element of this 
invention, however, is characterized in that the first binding agent is a 
macromolecular compound having an average molecular weight of at least 
12,000 and is present in the photoconductive layer in an amount of 1.5 to 
9% by weight calculated on the zinc oxide, and in that the amount of the 
second binding agent contained in the photoconductive layer is larger than 
that of the first agent; and the photoconductive layer is composed of 
agglomerates of zinc oxide particles substantially enveloped in the first 
binding agent, which agglomerates have a diameter between 2.5 and 6 .mu.m 
and are stuck together by the second binding agent to form a porous layer 
having a negative charge density of at most 1 m Coulomb per m.sup.2. 
It has been found that the photoconductive layer of an electrophotographic 
element according to the present invention has a very high resistance both 
to electrical influences and to mechanical influences, including the 
influences caused by pressure and increased temperature in a transfer 
system employing an intermediate transfer member. As a consequence, by the 
use of an electrophotographic element according to the invention a very 
large number of copies can be made on the same portion of the 
photoconductive layer without suffering serious deterioration of its 
electrophotographic properties. 
These properties and results of the new element are considered 
attributable, on the one hand, to the zinc oxide particles being fully 
covered with the first binding agent, resulting in an effective protection 
of the sensitizing dyes, and on the other hand to a high pore volume 
resulting in the layer having a remarkably low negative charge density, 
which per m.sup.2 is not higher than 1 m Coulomb and is in the range of 
from 0.4 to 0.7 m Coulomb for the most suitable photoconductive layers. In 
contrast, a charge density of 1.5 or higher is measured with 
photoconductive layers obtained according to the above-mentioned UK and 
German applications, and with other zinc oxide-binder layers known for 
indirect electrophotographic application, which contain only one binding 
agent or contain mixtures of compatible binders. The low charge density 
has as a consequence that, at a certain potential, less charge is 
deposited to the photoconductive layer, fewer oxidation products thus 
being formed on the surface. The greatly improved mechanical properties 
are believed to be partially caused by the high volume of open pores. 
Bending of the photoconductive layer could indeed result, for example, in 
the formation of small tears; but due to the large open pores, bending 
will occur more readily and result less fast in the zinc oxide particles 
being torn off the binding agent. For the same reason, the squeezing 
effect of a heated transfer medium could result far less rapidly in the 
zinc oxide particles being torn off the binding agent. Moreover, a 
considerably longer time will be required before the volume of the pores 
can become so filled up with degraded material that the properties of the 
layer are changed substantially. 
A photoconductive layer made with two incompatible binding agents and open 
pores has been described in U.S. Pat. No. 3,857,708, which does not relate 
otherwise to electrophotographic elements suitable for repeated use. In 
the layers described in that patent the zinc oxide particles are not 
enveloped in the first binding agent, with the result that free contact 
with the ambient atmosphere is possible. Also the typical structure of 
more or less spherical agglomerates is missing; as shown in FIG. 5 of the 
patent, the zinc oxide particles are distributed at random and particles 
are found at the walls of the pores. The structure of such a layer, when 
used repeatedly, causes the sensitizing dyes to be bleached out rapidly, 
and after being used several times for image formation the 
electrophotographic element will soon be rendered unusable. This is most 
likely a consequence of the method used to produce the photoconductive 
layer, as it is to be obtained by dispersing the zinc oxide in an 
admixture of liquids in which the two binding agents remain dissolved, 
then slowly drying the mixture at a relatively low temperature to 
evaporate one of the solvents and gradually precipitate one of the binding 
agents, and then precipitating the second binding agent in a subsequent 
drying stage at a higher temperature. 
The photoconductive element according to the present invention can be 
produced by mixing together the zinc oxide, the first and second binding 
agent, one or more solvents for dissolving these agents and, if desired, 
one or more sensitizing dyes, and then applying a layer of the resulting 
mixture to a substrate that is suitable for electrophotographic purposes 
and drying the applied layer; the binding agents and the solvent or 
solvents for dissolving them having been preselected in a combination that 
produces two inmiscible liquid phases during the mixing. The zinc oxide 
can be presensitized by treating it with a dyestuff solution, but the dye 
or dyes can also be added to the dispersion in the form of a solution, 
e.g. of 0.5 to 1% by weight, in methanol. The zinc oxide has such a strong 
affinity to sensitizing dyes that these are nearly quantitatively adsorbed 
to the zinc oxide. Use can also be made of so-called pink zinc oxide, such 
as that obtained by treating zinc oxide with ammonia and carbon dioxide 
followed by heating, as described in U.K. Patent Specification No. 
1,489,793. Although the sensitization of pink zinc oxide with dye 
sensitizer is preferred, this zinc oxide can be used without such 
sensitizers because it already possesses a reasonable sensitivity to 
visible light. Any dye commonly used to sensitize well-known zinc 
oxide-binder layers can be applied as sensitizing dye fcr the 
photoconductive layers according to the invention, such as for instance 
triphenylmethane dyes, bromophenol blue, chlorobromophenol blue, Rose 
Bengal, erythrosin, eosin or fluorescein or admixtures of such dyes. The 
amount of dye is customary as well. Very suitable amounts range between 
0.1 and 1% by weight, calculated on the zinc oxide. 
The sequence of adding the various ingredients for the mixing can be chosen 
at will, because, due to their high affinity to zinc oxide, the 
sensitizing dyes and the first binding agent land on the surface of the 
zinc oxide particles. However, the dispersing time should be sufficiently 
long to effect binding of these ingredients to the surface of the zinc 
oxide. A short dispersing time of about 10 or 15 minutes will suffice if a 
solution of the second binding agent is added to a dispersion of 
sensitized zinc oxide in a solution of the first binding agent. Because of 
this short dispersing time, the procedure in which the solution of the 
second binding agent is added last, is preferred. In addition, by 
employing this preferred procedure a photoconductive layer having 
remarkably accurately reproducible properties is obtained. 
Mixing of the solutions of the first and second binding agents results in 
separation of the mixture into two liquid phases. When zinc oxide is 
already present in the system or is added, a heterogeneous phase will be 
formed which consists of small spheres containing the zinc oxide particles 
in a concentrated solution of the first binding agent, with any added 
sensitizing dye fully adsorbed to the surface of the zinc oxide particles. 
The homogeneous phase of the system contains practically all of the second 
binding agent and the remainder of the solvent or solvents, although small 
amounts of the second binding agent may be incorporated in the 
heterogeneous phase, while also a small percentage of the first binding 
agent may be left in the homogeneous phase. 
It is remarkable that upon employing various binding agents, the spheres 
formed in the mixed layer composition always have the same diameter of 
approximately 8 .mu.m if, calculated on the zinc oxide, approximately 1.5 
to 6% by weight of the first binding agent is used. By reducing the amount 
of the first binding agent to below 1.5% by weight, the size of the 
spheres decreases quickly, and the useful life of the final product 
prepared under those conditions also decreases as a result of the zinc 
oxide particles being no longer effectively enveloped in the first binding 
agent. Upon increase of the amount of first binding agent from about 6 to 
8% by weight, the size of the spheres increases and also the favorable 
properties of the photoconductive layer formed. When the amount of the 
first binding agent is raised above 8% by weight, the useful life of the 
final product will soon be shortened, but at amounts up to about 9% by 
weight it is maintained at a high level. When percentages of first binding 
agent exceeding 8% are used, it is very likely that during the formation 
of the photoconductive layer the structure of the spheres is disturbed 
increasingly, or a less uniform layer is formed increasingly, because the 
spheres become too big for a normal thickness of the layer or because the 
dispersion shows too great a tendency to deposit. For this reason it is 
necessary for the formation of a product according to the invention that 
the amount of the first binding agent be in the range of between 1.5 and 
9% by weight. For obtaining an optimal result a percentage of the first 
binding agent in the range of between 4 and 8% by weight, calculated on 
the zinc oxide, is preferred. 
The amount of the second binding agent is not critical so long as it is 
larger than that of the first binding agent. Even an amount of second 
binding agent eight times that of the first binding agent can be used. An 
amount sufficient to bring the total quantity of binding agent to a zinc 
oxide to binder weight ratio of between 3:1 and 8:1, as is customary for 
well known zinc oxide-binder layers, will generally be sufficient for 
forming the aforesaid spheres and forming a proper photoconductive layer. 
According to the invention, however, the ratio of zinc oxide to total 
binder can be set at considerably lower values, for instance at 2:1, at 
which ratio the known zinc oxide-binder layers made with one binder no 
longer produce a usable product. The most favorable results are achieved 
with photoconductive elements according to the invention in which the 
photoconductive layer contains an amount of second binding agent 
approximately 3 to 6 times larger than the amount of first binding agent. 
Accordingly, the ratio of zinc oxide to total binder is preferably set at 
a value between 2.5:1 and 5:1. 
After a photoconductive layer according to the invention is applied on a 
substrate and dried, the heterogeneous structure of the dispersion from 
which the layer has been formed will remain recognizable. Due to 
evaporation of the solvent or solvents, the spheres of approximately 8 
.mu.m will shrink to form agglomerates having a diameter of between 2.5 
and 3.5 .mu.m, and spheres having a diameter, for example, of about 12 
.mu.m will shrink to form agglomerates of approximately 5 .mu.m in 
diameter. The second binding agent in the homogeneous phase of the 
dispersion does not remain homogeneous but, on the one hand, forms a thin 
film on the agglomerates of the zinc oxide particles already enveloped in 
the first binding agent and, on the other hand, sticks the agglomerates 
together to form a very porous layer of which the air content is more than 
1.5 times that of layers obtained from a dispersion of zinc oxide, or of 
zinc oxide previously enveloped with resin, in a single binding agent. 
The binding agents for the electrophotographic element according to the 
invention can be selected from a large group of polymers so long as a 
suitable solvent or solvent mixture can be found in which the polymers 
will separate into two liquid phases. It cannot be predicted in advance 
which system of incompatible binding agents will result in a separation of 
liquid phases, and which in a separation of a solid phase. The suitable 
combinations can be determined experimentally, by mixing the binding 
agents with solvents and visual observation of the mixture. Moreover, the 
first binding agent must form the spheres referred to above in the 
presence of zinc oxide and the second binder solution. These conditions 
can be satisfied, if the first binding agent has an average molecular 
weight of at least 12,000 and contains polar groups that are at least as 
strong as those of the second binding agent. In such cases, the first 
binding agent separates from the mixture in the form of a concentrated 
solution having a higher affinity to zinc oxide than the diluted solution 
of the second binding agent. If the molecular weight of the first binding 
agent is lower than 12,000, no spheres will be formed in the dispersion 
and the photoconductive layer made of it will have a considerably lower 
useful life. The cause of this is not known. 
Photoconductive elements having optimal properties are obtained when the 
second binding agent is a binder that also produces optimal properties 
when used in prior art photoconductive layers containing zinc oxide and 
one binding agent. Such binding agents, as most used in practice, all have 
a relatively weakly polar character and in most cases are selected from 
among the polyvinyl esters, such as polyvinyl acetate, acrylate resins 
such as copolymers of ethyl acrylate and styrene, alkyd resins, or 
mixtures of such polymers. These polymers dissolve in solvents that form 
no hydrogen bridges, or practically none, such as aromatic hydrocarbons 
having a boiling point between 110.degree. and 150.degree. C.; for 
instance, toluene, the xylenes and ethyl benzene. When selecting this 
weakly polar type of polymers for use as second binding agents in 
combination with such solvents, polymers which are very suitable for use 
as the first binding agent are, inter alia, phenoxy resins, linearly 
saturated polyesters, polyvinyl acetals such as polyvinyl formal or 
polyvinyl butyral, and cellulose derivatives including ethyl cellulose and 
cellulose esters such as cellulose acetate butyrate. Of these binding 
agents, a phenoxy resin is preferred for use in combination with a styrene 
acryate copolymer as second binder. The polymers mentioned as first 
binding agents are more difficultly soluble in solvents that form no 
hydrogen bridges, or practically none, such as toluene. In some cases, a 
solvent that forms hydrogen bridges will then be necessary to dissolve the 
first binding agent, in which event a solvent is preferred which is 
individually miscible with, and has a lower boiling point than, the 
non-hydrogen-bridge forming solvent. Such a miscible, lower boiling 
solvent may be selected from the ketones, esters, alcohols, or cyclic 
ethers such as tetrahydrofuran. The lower boiling point is desirable since 
the structure of the layer formed may be disturbed if the solvent for the 
first binding agent is the last to evaporate upon drying. 
Weakly polar polyvinyl esters or acrylate resins can also be used as first 
binding agents, in which case the second binding agent should be selected 
from the polymers having no or nearly no polar character, such as 
polystyrene or polyvinyl carbazole. Such combinations yield a product 
having reasonably good but not optimal properties. This was unexpected, as 
an entirely useless product is obtained when polystyrene and polyvinyl 
carbazole are used as the only binding agent in zinc oxide-binder layers. 
A similar situation occurs if the binders are selected from strongly polar 
polymers, such as partially or substantially entirely saponified polyvinyl 
acetate, for which a strongly polar solvent containing water is required. 
In this case also no more than a reasonably good result is achieved, which 
may be due to small amounts of strongly polar solvent being left in the 
layer formed, in spite of intensive drying, and possibly also to a 
displacement of sensitizing dyes by the strongly polar solvent with 
resulting less intimate adherence of the dye to the zinc oxide particles. 
The substrate may be any substrate that is suitable for electrophotographic 
purposes, such as metal or an electrically insulating material coated with 
a conductive layer of metal, or a conductive plastic layer such as a 
dispersion of carbon in cellulose acetate butyrate or in a vinylchloride 
vinylacetate-maleic acid anhydride terpolymer hardened by means of a 
melamine-formaldehyde precondensate. If desired, an intermediate layer 
such as a thin binding layer or barrier layer may be applied between the 
substrate and the photoconductive layer. In principle, paper also is 
usable, but preferably it is not used as such because ordinary paper 
substrates will wear out before the photoconductive layer will show signs 
of degradation. Paper that is reinforced in one way or another, for 
instance by having each side provided with a plastic layer, can of course 
be used without difficulty.

The following detailed examples further illustrate practices of the 
invention. 
EXAMPLE 1 
A solution was prepared by mixing 
6.6 g of phenoxy resin (Rutapox 0717 of Bakelite GmbH, Germany) having an 
average molecular weight between 25,000 and 30,000 in 
46.2 g of tetrahydrofuran and 
85.8 g of toluene. 
The following ingredients were added to the solution: 
100 g of pink zinc oxide obtained, according to U.K. Patent Specification 
No. 1,489,793, by treating an electrophotographic zinc oxide with ammonia 
and carbon dioxide gas followed by heating to a constant weight at a 
temperature of 175.degree. C.; 
0.4 g of bromochlorophenol blue; and 
20 g of toluene. 
The dispersion was shaken with glass beads in a holder for 15 minutes and 
then 
53.2 g of a 50% by weight solution of a styrene acrylic copolymer in 
toluene (E 048 obtainable from De Soto Inc., USA) were added. 
The dispersion was shaken for a further 15 minutes with glass beads in a 
holder and then was applied to form a layer having a dry weight of 20 g 
per m.sup.2 on a polyethylene terephthalate film provided on each side 
with a conductive layer composed of a dispersion of carbon in cellulose 
acetate butyrate. The applied layer was dried with hot air to a constant 
weight. 
The photoconductive element so produced could be charged up to 366 Volt. A 
light energy of 14 m Joule per m.sup.2 was required for discharging it to 
8 Volt, using a xenon flash lamp through a filter having a passage of 400 
to 750 nm. The negative charge density at maximum charging was 0.55 m 
Coulomb per m.sup.2. This was determined by first charging the layer fully 
with negative charges then neutralizing it with positive charges and 
measuring the quantity of supplied positive charge necessary for 
neutralization. The photoconductive element was mounted in a copying 
machine in which it was subjected repeatedly to the following processing 
steps: charging to 60% of the maximum potential by means of a scorotron, 
imagewise exposing, developing with a conductive one-component developer 
powder, transferring the powder image via an intermediate medium 
comprising a layer of silicone rubber on paper, and cleaning with a 
magnetic brush. After 40,000 copying operations, a 40% higher light input 
permitted copies of good quality still to be produced from the same 
element. 
Using the same method and composition but leaving out the zinc oxide, it 
was found that the binding agents together with the solvents produce a 
separation into two liquid phases. In the presence of zinc oxide, spheres 
having a diameter of approximately 10 .mu.m were measured in the 
dispersion, which spheres after drying of the layer formed were 
discernable as agglomerates having a diameter of approximately 4.5 .mu.m. 
EXAMPLE 2 
A solution was prepared by mixing 
4 g of linearly saturated polyester having an average molecular weight 
between 20,000 and 30,000 (Vitel PE 222 of Company Francaise Goodyear) in 
20 g of tetrahydrofuran and 
60 g of toluene. 
The following ingredients were added to the solution: 
100 g of pink zinc oxide (prepared according to the U.K. Patent 
Specification No. 1,489,793) and 
0.4 g of bromochlorophenol blue. 
The dispersion was shaken with glass beads in a holder for 15 minutes and 
then 
42 g of a 50% by weight solution of a styrene ethyl acrylate copolymer in 
toluene (E 048 obtainable from De Soto Inc., USA) and 
80 g of toluene were added. 
The dispersion was shaken for a further 15 minutes with glass beads in a 
holder and then was applied to form a layer having a dry weight of 20 g 
per m.sup.2 on a polyethylene terephthalate film provided on each side 
with a conductive layer composed of a dispersion of carbon in cellulose 
acetate butyrate. The applied layer was dried with hot air to a constant 
weight. 
The photoconductive element so produced could be charged up to 300 Volt, 
and the negative charge density at maximum charging was 0.64 m Coulomb per 
m.sup.2 . Discharging down to a residual voltage of 3 Volt required a 
light energy of 13.5 m Joule per m.sup.2 (using the light source mentioned 
in Example 1). 
In the same copying machine as was used in Example 1 a very high useful 
life was noted as well. In this case also the separation into liquid 
phases was demonstrated by use of the same formula but leaving out the 
zinc oxide. In the presence of zinc oxide, spheres having a diameter of 
approximately 8 .mu.m were measured in the dispersion, which spheres after 
drying of the layer formed were discernable as agglomerates of 
approximately 3 .mu.m in diameter. 
EXAMPLE 3 
A solution was prepared by mixing 
4.5 g of polyvinyl formal (Formvar 770 of Shawinigan Ltd., England) in 
28 g of tetrahydrofuran. 
The following ingredients were added successively: 
100 g of tetrahydrofuran, 
0.5 g of bromochlorophenol blue and 
100 g of zinc oxide (Electrox 3500 of Durham Chemicals Ltd., England). 
The mixture was shaken with glass beads in a holder for 15 minutes. Then 
50 g of a 50% by weight solution of a styrene-ethyl acrylate copolymer in 
toluene (Synolac 620 S of Crayvalley Products, England) and 
75 g of toluene 
were added. 
The dispersion was shaken for a further 15 minutes with glass beads, and 
subsequently a layer of this dispersion was applied to a polyethylene 
terephthalate foil coated on each side with a layer of aluminum. The 
applied layer was dried with hot air and had a dry weight of 21 g per 
m.sup.2. 
The resulting photoconductive element could be charged up to 357 Volt. 
Discharging it down to 10 Volt required a light energy of 25 m Joule per 
m.sup.2, using the light source described in Example 1. The negative 
charge density at maximum charging was 0.40 m Coulomb per m.sup.2. In the 
same copying machine as used in Example 1 a very large number of good 
copies was prepared again. The photoconductive element then showed wear in 
the aluminum layer only, at the rear side; the photoconductive layer was 
still in a well usable condition. 
Using the same method and composition but leaving out the zinc oxide, it 
was found that the binding agents together with the solvents produced a 
separation into two liquid phases. In the presence of zinc oxide, spheres 
having a diameter of approximately 8 .mu.m were measured in the 
dispersion, which spheres after drying of the layer formed were 
discernable as agglomerates having a diameter of approximately 3 .mu.m. 
EXAMPLE 4 
A solution was prepared by mixing 
4 g of polyvinyl butyral having a molecular weight of 30,000 (Pioloform BL 
18 of Wacker Chemie GmbH, Germany) and 
104 g of toluene. 
The following ingredients were added: 
100 g of pink zinc oxide (prepared according to the U.K. Patent 
Specification No. 1,489,793) and 
0.4 g of bromochlorophenol blue. 
The mixture was shaken with glass beads for 12 minutes and then a solution 
of 
21 g of vinyl acetate-vinyl laureate copolymer (Vinnapas B 100/VL 20 of 
Wacker Chemie GmbH, Germany) in 
80 g of toluene 
was added. 
The resulting dispersion was shaken for 15 minutes with glass beads and was 
applied to form a layer on a polyethylene terephthalate foil coated on 
each side with a dispersion of carbon in cellulose acetate butyrate. The 
applied layer was dried with hot air and had a dry weight of 20 g per 
m.sup.2. 
The photoconductive element so produced could be charged up to 356 Volt and 
had a negative charge density of 0.77 m Coulomb per m.sup.2. Using the 
light source described in Example 1, discharging it down to a residual 
voltage of 3 Volt required 25 m Joule per m.sup.2. In the same copying 
machine as employed in Example 1 the result was almost identical to that 
obtained with an electrophotographic element according to Example 2. 
In this case also the separation into liquid phases was demonstrated by 
using the same formula but leaving out the zinc oxide. In the presence of 
zinc oxide, spheres having a diameter of approximately 8 .mu.m were 
measured in the dispersion, which spheres after drying of the layer formed 
were discernable as agglomerates of approximately 3 .mu.m in diameter. 
EXAMPLE 5 
A solution was prepared by mixing 
4 g of ethyl cellulose (type N 4 of Hercules Powder Co.) in 
80 g of toluene. 
The following ingredients were added: 
100 g of zinc oxide (Electrox 2500 sold by Durham Chemicals Ltd., England) 
and 
0.4 g of bromochlorophenol blue. 
The mixture was dispersed for 12 minutes by shaking with glass beads and 
then a solution of 
26 g of vinyl acetate-vinyl laureate copolymer (Vinnapas B 100/VL 20 of 
Wacker Chemie GmbH, Germany) in 
60 g of toluene was added. 
The resulting mixture was dispersed by shaking it with glass beads for 15 
minutes and then was applied to form a layer on a polyethylene 
terephthalate foil coated on each side with a dispersion of carbon in a 
cellulose acetate butyrate copolymer. After drying with hot air the weight 
of the applied layer was 20 g per m.sup.2. 
The photoconductive element obtained could be charged up to 250 Volt and 
had a negative charge density of 0.46 m Coulomb per m.sup.2. Using the 
same light source as described in Example 1, discharging it down to a 
potential of 14 Volt required 30 m Joule per m.sup.2. 
In the same copying machine as employed in Example 1 a very high useful 
life was established. The separation into liquid phases was demonstrated 
by using the same formula but leaving out the zinc oxide. In the presence 
of zinc oxide, spheres having a diameter of approximately 9 .mu.m were 
measured in the dispersion, which spheres after drying of the layer formed 
were discernable as agglomerates having a diameter of approximately 3.5 
.mu.m. 
The photoconductive layer of this example was photographed with a scanning 
electron microscope at a thousand-fold scale of enlargement. 
FIG. 1 of the accompanying drawing is a reproduction of the 
micro-photograph, in which the more or less spherical agglomerates are 
clearly visible. 
FIG. 2 of the drawing is a reproduction of a micro-photograph made for 
comparison at the same scale of enlargment, but in this case showing the 
quite different structure of a photoconductive zinc oxide-binder layer 
containing only one binding agent. 
EXAMPLE 6 
To a mixture of 
8.75 g of a 50% by weight solution of a styrene-ethyl acrylate copolymer in 
toluene (E 048 of De Soto Inc., USA), 
100 g of toluene and 
100 g of monochlorobenzene, the following were added: 
100 g of pink zinc oxide (prepared according to the U.K. Patent 
Specification No. 1,489,793) and 
0.8 g of bromochlorophenol blue. 
The dispersion was shaken with glass beads for 15 minutes and then 
15 g of polyvinyl carbazole (Luvican M 170 of BASF) dissolved in 
100 g of monochlorobenzene were added. 
The resulting dispersion was shaken with glass beads for 15 minutes and 
then was applied to form a layer having a dry weight of 20 g per m.sup.2 
on an electrically conductive substrate. The layer was dried with hot air 
to constant weight. 
The photoconductive element so produced could be charged up to 265 Volt, 
and the negative charge density at maximum charging was 1 m Coulomb per 
m.sup.2. Discharging the element down to a residual voltage of 2 Volt 
required a light energy of 15 m Joule per m.sup.2, using the light source 
mentioned in Example 1. The element was used 10,000 times for copying 
operations, each time by charging, imagewise exposing, developing, and 
transfer of the powder image to paper via a heated intermediate medium. 
The copies obtained were of reasonably good quality, but the copying 
process in this case required rather critical adjustments because the 
layer showed a rather high rate of dark decay; a loss of 30 Volt after one 
second was measured. Yet in contrast, a similar photoconductive layer 
prepared from zinc oxide and polyvinyl carbazole but without styrene-ethyl 
acrylate resin was entirely unusable; it could be charged up only to 51 
Volt and it lost two thirds of this potential within one second. 
EXAMPLE 7 
A solution was prepared by mixing: 
5 g of cellulose acetate propionate (482/20 of Eastman Kodak USA), 
64 g of toluene and 
16 g of 2-methoxy-ethanol. 
The following were added to the solution: 
100 g of pink zinc oxide (prepared according to the U.K. Patent 
Specification No. 1,489,793) and 
0.375 g of bromochlorophenol blue. 
The resulting dispersion was shaken in a holder with glass beads for 12 
minutes and then a solution containing: 
25 g of polyvinyl butyral (Butvar B76 of Shawinigan Ltd England), 
8 g of 2-methoxy-ethanol and 
72 g of toluene was added. 
This dispersion was shaken for a further 12 minutes with glass beads, and 
then a layer of it was applied to a plastic foil coated with a thin layer 
of palladium. The dispersion layer was dried with hot air and had a dry 
weight of 24 g per m.sup.2. 
The resulting photoconductive element could be charged up to 322 Volt. It 
then was discharged down to 12 Volt by a light energy of 18 m Joule per 
m.sup.2, using the light source described in Example 1. The negative 
charge density at maximum charging was 0.40 m Coulomb per m.sup.2. The 
photoconductive layer was still in a well usable condition after the 
production of 5000 good copies with it in the same copying machine as used 
in Example 1. 
EXAMPLE 8 
A photoconductive element prepared in the same manner with the same 
constituents as described in Example 7, but without the addition of 
cellulose acetate propionate, showed a negative charge density of 1.15 m 
Coulomb per m.sup.2, and considerable damaging of the photoconductive 
surface was observed after the production of 2000 copies with this 
element. 
EXAMPLE 9 
A photoconductive element prepared in the same manner with the same 
constituents as described in Example 7, but in which the polyvinyl butyral 
was replaced by 50 g of a 50% by weight solution of a styrene acrylate 
resin (E 048 of De Soto Inc., USA), gave substantially the same results as 
the photoconductive element of Example 7.