Note: Descriptions are shown in the official language in which they were submitted.
~ 10.1,79' '1 RHD 78006
.
Method of manufacturing an electrophotographic recording
material.
The in-vention relates to a method of manufac~
turing an electrophotographic recordi.ng material having
a porous layer of photoconductor-binder which is provided
between an electrically conductive layer and a dielectric
foil, by providing a dispersion of photoconductor crystal
grains an~l a binder in an organic solvent on the elec-
trically conductive layer, drying the dispersion and
pressing the foi.l. The invention also relates to an
electrophotographic recording foil thus formed.
In electrophoto~raphy a number of recording
methods have become known in which a multi-layer re-
cording material i~s used. In contrast with xerography
in which the picture-producing layer must simultaneous-
ly store the charge image, these functions can be divid-
ed in several layers and each layer can be optimalized.
In a simple multi-layer system a dielectric foil is
present on a photoconductive layer, for e~ample, a
: layer of photoconductor-binder, which foil can capa-
citively store the chàrge carriers generated in the
20 photoconductor by pictorial irradiation with an applied ~'
electric field. As a result of this, very sensitive
substances with larger dark conductivity may also be
used as photoconductors. Because the development of the
charge image rnay taL~e place on the upper. side of the
d;.electric .oil~ it is also possible to use porous
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10.1.79 2 P~ 78006
layers of photoconductor-binder. Upon uniting the porous
layers with the foils~ however, problems present them-
selves.
German Auslegeschrift 21 45 l12 states th~t
eleetrophotographic recording materials are disclosed
in German Offenlegungsschrift l5 72 344 whieh contain
an electrically insulating and a photoconductive layer
which on the one hand is bonded as one assembly to the
strongly insulating layer and on the other hand to an
eleetrically conductive layer. Latent images are pro-
duced at the surface of the electrically insulating
layer. The problem of the porosity of the photoconduc-
tive layer upon providing the electrically insulating
layer is not described in German Auslegeschrift 21 45
~15 112.
According to German Off`enlegungsschrift
15 72 3l~l~, the eleetrophotographic recording materials
are manufactured in that orthorhombic lead monoxide as
a photoconductor in the form of partieles havlng a par-
- 20 tiele size in the range between 0.25 and 10/um is dis-
persed in a solution of an insulating binder. The sus-
pension is provided on an electrically conductive layer
as a substratum. The solve~lt is evaporated in air,
that is to say it is dried and a layer of photoconduc-
~5 tive Material remains on the substratum. The weight
! ratio of lead monoxide with respect to resin as a binder
may vary ln the photoconductive layer between 1 : 1 and
16 : 1. A hard smooth material, fcr example a wax, is
provided on the surface of the photoconductive layer
as an electrieally insulating layer. As a result of` this,
the recording rnaterial may be used again up to 100 times.
After the manufacture of the hard layer the recording
material must bs subjec-ted -to a thermal treatment if
photosensitivities necessary for the medieal X-ray ra-
- 35 diation must be aehieved. German Off`enlegungsschrift
15 72 344 does not ~tate either that the provision of
the hard, smooth layer on the photoconductive layer
~_ presents problems if the photoconductive layer is porous.
...... ...... . .. . . , .. .. .. ~.. .. ... . ., .. . .. .... ....... .... . .. ., . .. .......... . . ,, .. , .. ., _ .. ........ . .. .... .
. 7 ~_ ~ _ .. _ _,, .. , .. , _ _ _ . _. _ .. _ ., _ . _ _ _ _ ,., .. , ., _ ,_ ... . _ ,.. .. ... .
10.1.~9- 3 PHD 78006
;,
....... , These problems, however, are the subject matter
of German Aus.~egeschrift 19 56 166 which discloses a
method of manufacturing an electrophotographic recording
mater:ial having a layer support, a photoconductive layer
~5 and an insulating coveri.ng layer, the insulating cover-
ing layer oeing pressed OIl the photoconductive layer.
Several m~thods are mentioned of manufacturing photo-
. sensitive elements for electrophotography by providingan i.nsulating covering layer and an insulating layer,
respectively, on a photoconductive layer and a photo-
resistive layer, respectively. For example, the insulat-
ing layer may simply be provided on the surface of the
. photoresistive layer. So only two layers are provided
one on top of the other so that the formation of an un-
even surface and also bubbles and pleats are possible.
According to ano-ther method, the insulating layer may be
provided in intimate contact with the photoresistive
layer and may be adhered by means o~ an adhesive resin
, binder obtained in the photoresistive layer. The physi-
~ 20 cal properties of the photoresistive layers are con-
: , siderably influenced by the choice of the binder. Especial-
ly when a smaller quantity of binder is used, many dis-
. . advantages occur. For example, the photo-resistive layer
has a porous surface so that upon bonding an insulating
'25 material to said surface 9 bubbles are easily enclosed
between the covering layer and the photoresistive layer.
In order to remove these difficulties in po-
rous photoresistive layers it is not sufficient to press
the insulating covering layer by means of a de~ormable
,3~ synthetic resin containing no solvent on the photocon-
ductive layer, as is disolo,sed in German Auslegeschrift
19 56 166. Although this e~tra layer of synthetic resin,
not counting extra costs of material and labour, involves
i a reduction in the sensiti~ity o.f the electrophotographic
recording material, it is necessa.ry in porous photocon-
ductive layers to provide 'an insulati.ng intermediate
layer the insula-tirlg materials of which penetrate for
: -,,,. ,,the greater part into the pores of the photoconductive
' .. .... .. ............ .... . .... . .... .... . ..... ..... ....... . j
i
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~26~9~
layer. The disadvantage hereof is that the porosity of the photoconductive
layer is reduced.
It is the object of the invention to provide a method of manufactur-
ing an electrophotographic recording material in which the great porosity of
the layer of photoconductor-b.inder is maintained on which inter alia a high
sensitivity to X-rays is based. Simultaneously, the formation of an uneven
surface and bubbles and pleats must be prevented.
According to the invention, this object is achieved with a method
of the kind mentioned in the opening paragraph in which the dispersion to be
provided comprises a quantity of binder of frcm 0.1 to 5% by weight with res-
pect to the ultimate layer, the foil is pressed as long as the layer of photo-
conductor-binder is still moist and hence deformable, and drying is discon-
tinued only after pressing.
The photoconductor crystal grains consist, for example, of in-
organic material, in particukar cadmium sulphide cadmium selenide, metallic
selenium, zinc oxide, zinc sulphide selenium telluride, ti-tanium dioxide,
lead monoxide or sulphur. The crystal grains preferably consist of lead
monoxide. This is reccmmended especially for electroradiographic applica-
tions of the recording material manufactured according to the inventio~. The
photoconductor crystal grains prefercibly have a diameter between 1 and 50 ~m,
in particular between 11 and 20 ~m.
me photoconductor crystal grains are dispersed in a solution of
the binder, for example, by stirr.ing or grinding. Suitable as binders for
the manufacture of the dispersiGn are, for example, polyacrylic acid ester,
pol-methacrylic acid ester, vinylpolymers, for e~ample, pol~styrene, poly-
vinyl chloride, polyvinyl acetate and oopolymers of ~hese materials, poly-
esters, aIkyde resins
i~
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10.1.79 5 PHD 78006
,,,,,, and polyphenylene oxides. Alkyde resins are preferably
used, in particular commercially available product~
which may be used for the manufacture of lacquer and are
~ commercially available, ~or example, by the trade names
i~ 5 ~P~ral~c~ l,ioptali~ Synolac~ Alkyda~ furthermore, poly-
esters, for examp1e, T 203 (Chemische 1~erke Witten).
Suitab:Le as organic solvents for the binder are, for
example, toluene~ alcohols and phthalic acid esters
liquid at room temperature, methyl ethyl ketone and
~l buta~one. The solvent toluene is preferably used for
alkyde resins.
The ratios in the quantities in the disper-
sion are chosen to be ,so that the finished photoconduc-
tive layer has a quantity of binder of from 0.1 to 5%
by weight. This comparatively small quantity of binder
contributes to a great porosity of the layer.
The d~spersion consisting of photoconductor
crystal grains and binder can be provided on the e~ec~
trically conduc-tive layer in known manner, for e~ample,
by sedimentacion, dipping~ spraying, while using a spa-
tula or a di,pping roller, When the dispersion contains
tetragonal lead monoxide, the sedimentation method is
to be preferred. The electrically conductive layer con-
sists, for example, of steel, aluminium, copper, brass,
also with noble metal coatings, or tin dioxide or indium
; oxide layers on glass. Aluminium and alloys thereof are
preferably used as materials for 1;he electrically con-
' ductive layer.
The provided dispersion is then dried so as
to remove a part of the solvent, for e~ample by storagein air. It is of importance that after this the layer
of photoconductor-binder stil] contains solvents, pre-
ferably in quantities of from 0.1 to 50% by weight,
, with respect tO the reduction in weight of the photo-
conductor, At any rate the solvent must still be clear-
ly observable by its smell.
l`he dielectric foil is now pressed on the
",,,,,,layer of photoconrluctor-binder thus manufactl1red which
., .., ~ , . , ...,, . _ ., , . . , , .,, .,, , , , .. . ,, . ..., .,, . , , . . . , ., , .. .,, , . ~ ,. .. . . . . . . . . .. ... .. . .
~ T~D~
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~26~
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10.1.79 6 PHD 78006
is still moist and hence deformable. Foils suitable
for this purpose consi~;tJ for e~ample, of polycarbonate,
polyethyleneterephthalate, polystyrene or cellulo~e
acetate having resistivities of approximately 101 to
Ohm.cm. Foi:Ls of polyethylene terephthalate are
preferably used. The pressing of the foil is carried
out with only a slight pressure, for example, between
0.01 and 1 bar. ~or e~ample, a roller is used for this
purpose. The dielectric foil preferably has a thickness
between 3 and 50/um, in particular between 8 and 20/um.
The subsequent drying is carried out, for
example, by storage at room temperature for 12 hours
or by heating at 30 to 80C for 5 hours. During drying,
the solvent diffuses through the foil. Foil and solvent
are therefore matched to each other preferabl~T in such
manner that the solvent can diffuse through the foil.
If desired, the recording material ls then
subjected to the thermal treatment which is conventional
for sensitive layers of photoconductor-binder, for exam--
~20 ple, at 150 to 200C for ~4 hours. Such a thermal treat-
ment is necessary in layers of lead monoxide-binder be-
cause otherwise the sensitivity is considerably less.
This does not apply to the same extent to other photo-
! conductor materials.
;25 As a result of the choice according to the
invention of a comparatively small quantity of binder
and the use preferably of comparatively large photo-
conductor particles and crystal grains, respectively,
so that a high porosity of appro~imately 60 to 70/~ is
obtained, the advantage is obtained of a high sensiti--
tivity, in particular to X-rays. The conductivity
mechanism in the layers of photoconductor-binder may
probably be imagined so that a large part of the charge
carrlers gencrated by the light rays or X-rays wanders
through the volume of the photoconductor crystal grain
in the applied electric field. The grain limits are a
barrier for these charge carriers which is to be over- !
! . _ come. Many grain li;nits (hence fine powder) would con- ~
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, 10.1.79 7 PHD 78006
i
sequently result in losses and hence in a smaller sensi-
tivity. Filling the pores between the crystal grains with
a hardening binder would deteriorate the electric contact
between t~e crystal grains. This would also result in a
decrease of the sensitivity.
The advantage of the very high sensitivity,
especially to X-rays, remains present by adhering the
dielectric foil to the porous layer of photocondL1ctor-
binder without extra adhesive. Because the dielectric
~10 foil is provided Oll the layer of photoconductor-binder
which is still moist and hence deformable, surface un-
evennesses can be removed by a slight pressure.
The invention will be described in greater
detail with reference to a drawing and a few examples.
The sole figure of the drawing shows a mul-ti-
layer recording material for electrophotography. ~n the
figure, 1 denotes an electrically conductive substratum,
for exaMple of aluminium, noble steel or glass with con-
ductive layer. A layer 2 of photoconductor-binder, for
~0 example, with lead monoxide~ is provided on the sub-
stratum 1 as a photoconductor. On top of this a dielec-
, tric foil 3 is provided, for example, of polyethyleneterepththalate.
, , Example '1.
A dispersion consisting of 2 g of tetragonal
lead monoxide powder (average grain diameter approxi-
mately 20/um)'and 30 ml of a 5% alkyde resin solution
in toluene (alkyde resin : commercial product Paralac
of ICI) is provided on an approximately 50 x 50 mm large
`30 plate by sedimentation in a layer height of approxi-
mately 0.2 mm. The excessive alkyde resin solution is
carefully sucked off at the edge by means of strips of
, filtering paper. The covered plate is left in normally
flowing ambient air for 1 minute. A 12/um thick foil
`35 of polyethylene tereptha]ate (commercial product Mylar)
- , which had previously been subjected to an initial dis-
solving treatmerlt for one minute with methylene chloride
,,, a,nd had then been rinsed with acetone is then pressed on
,, ,. . .. _ . ___.__ .. . .... . ~ . . .. , .. ,.. .......... ........ ... .. ....... .... , .. . , .. _ . ....... .. ... ..... .....
10.1.73 8 PHD 78006
the layer of photoconductor-bindar, as long as the snlell
of the toluene is still observable~ by means of a metal
roller with a pressure ~f 0.2 bar. The resulting record-
ing material is dried by leaving to stand overnight at
room temperature with the foil side lowermost. The dried
material is then subjected to a thermal treatment at
200C for 24 hours. Finally the edges of the dielectric
foil and the covering f`oil, respectively, are adhered to
the aluminium substrat~m by means of an epoxyde resin;
said adhering is carried out only to obtain an easy
handlability of the material and has nothing to do with
the bonding of the covering foil to the photoconductive
layer.
The electrophotographic recording malterial
thus manufactured has a plane surface with readily ad-
hering covering foil. The sensitivity of the resulting
- recording material was measured by means of a material
test tube of the type MOD 151 Be (tungsten anode, peak
voltage 140 kV, ~40/um Bi-filter, dose power 50 mR/s,
Manufacturer: C.H.F. MUller). Since in the density
measurement of a developed electrophotographic record
the sensitivity of the developer used decisively in-
fluences the measured value, the sensitivity of the
electrophotographic recording material according to
the invention is characterized by that charge density
which is generatad by absorption of an X-ray dose of
2.58 x 10 7 C/kg. A voltage of 1500 V is applied -to the
electrophotographic system via a liquid electrode.
Good halftone records can be manufactured
with the material manufactured in the above~-described
manner. The resolving power is 10 line pairs per mm
~ith a generated charge density of 2 x 10 5 C/m .
Example ~
In a manner analogous to that of example 1,
a layer of lea~l monoxide-binder is manufactured, with
the difference that immediately after sucking off the
excessive binder so3ution, a 3/um thick foil of poly-
_ ethylene terepth~late is pressed with 0.1 bar. The further
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,
~L~2
r
10. 1 .79 9 PrID 78006
. treatment of the layer is carried out in a manner ana-
logous to that of example 1. In spite of the tendency
to form fine haircracks in the photoconductive layer,
' good records can be manufactured. The generated charge
`5 density is lO 5C/m2.
E.Yan!F)-l-_~
.~ layer of lead monoxide-binder is manufactured
in a manner analogous to that of exa~ple 1 with the di.f-
ference that another alkyde resln, namely the commercial-
ly available product l.ioptal, is used as a binder and theexcessive binder solution is poured off after the sedi-
mentation. After drying for 1 minute the 12/um thick foil
of polyethylene terepthalate :is pressed without pretreat-
ment by means of a metal roller at a pressure of 0.5 bar
and dried in an air current of 30C for 5 hours with the
foil side lowermost. A thermal treatment at 150C is thel~
carried out for 24 hours. The surface of the electrophoto-
graphic recording material thus manufactured is smooth
and the foil readily adheres to the layer of binder.
Good records can be made. The generated charge density
is 2 x 10 5 C/m2.
i ~ 4
;
A layer of lead monoxide-binder as in example
1 was sedimented and after-treated with the difference
`25 that lead monoxide-crystal powder with the finest grains
(average diameter 1/um) was used. The mechanical proper-
ties were as good as in the layer of exarnple 1. However9
the resolving power could not be increased by the finer
grain. The generated charge density dropped to 10 7 C/m
due to the higher losses o~ grain limits in the photo-
conductor layer~
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