US3598582A

US3598582A - Photoconductive element exhibiting photoconductive dichroism and process of using same

Info

Publication number: US3598582A
Application number: US668697A
Authority: US
Inventors: Clifford E Herrick Jr; Meredith David Shattuck
Original assignee: International Business Machines Corp
Current assignee: International Business Machines Corp
Priority date: 1967-09-18
Filing date: 1967-09-18
Publication date: 1971-08-10
Anticipated expiration: 1988-08-10
Also published as: BE720358A; SE339911B; DE1797306B2; GB1244765A; NL158624B; NL6812934A; CH496264A; JPS4926907B1; ES357985A1; DE1797306A1; FR1577855A

Abstract

AN ELECTROPHOTOGRAPHIC PROCESS FOR THE PRODUCTION OF REFLEX COPIES IN WHICH A DOCUMENT IS POSITIONED ADJACENT A PHOTOCONDUCTIVE ELEMENT WHICH EXHIBITS PHOTOCONDUCTIVE DICHROISM AND HAS A PREFERRED ABSORPTION AXIS, AND IN WHICH THE PHOTOCONDUCTIVE ELEMENT IS UNIFORMLY EXPOSED THROUGH THE PHOTOCONDUCTIVE ELEMENT WITH POLARIZED LIGHT WHOSE VECTOR, RELATIVE TO THE ABSORPTION AXIS, IS SUCH THAT THE LIGHT IS NOT ABSORBED. THE POLARIZED LIGHT IN STRIKING THE DOCUMENT IS ABSORBED IN SOME AREAS, NORMALLY THE DARK IMAGE AREAS, AND DEPOLARIZED AND REFLECTED IN OTHERS, NORMALLY THE LIGHT BACKGROUND AREAS. THE LIGHT FROM THE REFLECTED AREA, BEING DEPOLARIZED, CONTAINS LIGHT WITH AN ELECTRIC VECTOR WHICH WILL BE ABSORBED BY THE PHOTOCONDUCTIVE ELEMENT AND THE ELEMENT IS THUS EXPOSED TO A PATTERN CORRESPONDING TO THE PATTERN OF THE DOCUMENT. THIS RENDERS THE PHOTOCONDUCTIVE ELEMENT CONDUCTIVE AND CAPABLE OF TRANSPORTING AN ELECTROSTATIC CHARGE AND, HENCE, PERMITS THE FORMATION OF AN ELECTROSTATIC CHARGE PATTERN CORRESPONDING TO THE DOCUMENT.

Description

United States Patent Olfice 3,598,582 Patented Aug. 10, 1971 US. Cl. 96-1.5 24 Claims ABSTRACT OF THE DISCLOSURE An electrophotographic process for the production of reflex copies in which a document is positioned adjacent a photoconductive element which exhibits photoconductive dichroism and has a preferred absorption axis, and in which the photoconductive element is uniformly exposed through the photoconductive element with polarized light whose vector, relative to the absorption axis, is such that the light is not absorbed. The polarized light in striking the document is absorbed in some areas, normally the dark image areas, and depolarized and reflected in others, normally the light background areas. The light from the reflected areas, being depolarized, contains light with an electric vector which will be absorbed by the photoconductive element and the element is thus exposed to a pattern corresponding to the pattern of the document. This renders the photoconductive element conductive and capable of transporting an electrostatic charge and, hence, permits the formation of an electrostatic charge pattern corresponding to the document.

BACKGROUND OF THE INVENTION Field of invention This invention relates in general to contact reflex reproduction and in particular to electrophotography with a contact reflex exposure of a photoconductive element.

Description of prior art In electrophotographic processes, of which so-called xerography is a single example, an element comprising a photoconductive insulator is uniformly electrostatically charged and exposed to a pattern of light to cause the formation of electrostatic charge patterns on the photoconductive element. The thus-formed electrostatic pattern is then developed with an electroscopic powder either while it still is on the photoconductive element or, for example, after it has been transferred to another surface sheet. In the former (xerographic) process, the developed electrostatic charge pattern is subsequently transferred to paper and the photoconductive element is cleaned to remove any non-transferred powder.

In general, in the above described exposure step, graphic information has normally been transferred from original to photoconductive element by employing lenses or similar optical systems with the result that the photoconductive element is exposed to a pattern of ligh and dark, corresponding o the graphic information on the original. With optical graphic information transfer, a substantial part of the cost of the electrophotographic apparatus is attributable to the optical system. Moreover, a relatively large and bulky housing must be provided to support not only the optical system but to provide, as well, predetermined distances between the optical system and the original depending on the focal length of the optical system. Further, optical systems are very ineflicient in their utilization of available light and, therefore, some photoconductors would require too high an exposure time in an optical system employing a conventional relatively inexpensive light source so that more powerful and more expensive light sources must be employed.

In lieu of an optical information transfer, there is an exposure technique known as contact reflex. With this type of graphic information transfer, the original to be copied is brought into contact with the photosensitive element; the sandwich thus formed is exposed to light which passes first through the back of the photosensitive element. The light passing through the photoconductive element and striking the original is absorbed by the dark areas of the original and reflected back from the white or light areas of the original, forming an exposure pattern on the photosensitive element.

There are numerous reasons why this type of exposure has not achieved commercial acceptance in electrophotography. It is clear that for useful degrees of light modulation to occur, the element must freely pass light to the surface of the original. Commercially used photoconductors, such as selenium, are essentially opaque to the exposing radiation. In order to secure adequate transmission of light to the original through the element, it has been necessary to construct photoconductive elements having elaborate and expensive screen type configurations. In addition, when these commercially used photoconductors are employed in thin layers to render them transparent, the resulting voltage difference between the areas of photoconductor exposed by the reflected radiation relative to the areas of the photoconductor only exposed by the uniform exposure is not suflicient to achieve high quality copies of the original.

One of the key drawbacks then to contact reflex methods is the fact that contrast is degraded owing to the neces sary pre-exposure of the entire surface to non-information bearing light, which pre-exposure is intrinsic to all reflex methods.

A non-electrophotographic contact reflex exposure process has been described in which this drawback is overcome because the photoprinting material contains an oriented dichroic light sensitive diazo compound. For the exposure step, the original to be copied is brought into contact with the photoprinting material and the sandwich thus formed is uniformly exposed through the photoprinting material. The uniform incoming light, however, is plane polarized with its electric vector normal to the principal transition moment of the dichroic light sensitive diazo compound. Decomposition of the diazo compound is produced only by absorbed light. Since the uniform incoming light is not absorbed due to the lack of parallelism between the electric vector of the plane polarized light and the absorption axis of the dichroic diazo compound, no photo decomposition occurs. However, upon reflection by the surface of the original, the polarized light becomes substantially depolarized (e.g., no preferred electric vector direction exists) in being reflected from the white areas of the original back to the photoprinting material, and since a component of electric vector now exists in the direction of the transition moment of the dichroic diazo it is now substantially absorbed by the diazo compound thereby decomposing the diazo compound to form a developable image corresponding to the original image. Such substantial depolarization as described always results when plane polarized light is reflected from the diffusely reflecting surface, such as paper.

While this non-electrophotographic process teaches an important step forward in the contact reflex exposure art, no commercial use appears to have been made of the process most probably because the photoprinting material can only be used a single time. The cost of manufacturing an oriented photoprinting material useful for making a copy of only a single, particular original is too great to make it practically useful.

3 SUMMARY OF THE INVENTION Accordingly, it is an object of the invention to provide a novel reproduction process which is relatively inexpensive to implement into a compact reproduction device and capable, because of the high light-utilizing efficiency of the contact copying mode, of yielding high quality copies at a very high rate of speed.

It is a more detailed object of the present invention to provide a contact reflex exposure process using polarized light in which the photosensitive element is capable of being reused repeatedly in the reproduction of copies, thereby spreading the cost burden of the element over thousands of copies.

Still another object of the invention is to provide a novel reusable photoconductive element which is capable of use in electrophotographic reproduction processes with contact reflex exposure using polarized light and which produces high quality copies in such reproduction processes.

A further object of the present invention is to provide a novel photoconductive element which is well adapted to copying half tone images of extended area with high quality, even with differential voltage development methods such as the so-called cascade developing method.

In general, the foregoing objects are achieved by a novel reusable photoconductive element comprising an oriented dichroic material which is capable of transmitting essentially all of the polarized light having a direction of electric vibration such that the photoconductive element does not absorb the light and hence does not become conductive when exposed to such polarized light. Thus, the photoconductive element can be brought into contact with a document and uniformly exposed to such polarized light and the light is efficiently transmitted to the surface of the document without the photoconductive element becoming photoconductive as the light initially passes through the element. However, a substantial portion of the light reflected back from the light reflecting and depolarizing areas of the document is absorbed, thereby resulting in a high contrast latent image in the photoconductive element corresponding to the image of the document. This latent image is the basis for an electrostatic image which can be developed in any of the conventional ways to provide a copy of the document. Because the element is a photoconductor, it can be used repeatedly to provide copies of the same or different documents.

It is not necessary that the reflected light be completely depolarized, but only that, following reflection, it contains components of electric vibration which will be absorbed by the photoconductive element; for example, components of electric vibration normal to the polarized light transmitted through the element.

Other and further objects and features of the invention will be apparent in the following more particular description of the preferred embodiment of the invention.

Absorption or emission of light in a single molecule or a cooperating array of molecules is qualitatively ascribable to a motion of electric charge within the molecule or cooperating molecular array. Light generated by the motion of electric charge within a molecule is characterized by an electric field disturbance (the electric vector) whose direction coincides precisely with the direction of motion of the electric charge in the molecule giving rise to the light. Conversely, light absorption requires that the incident light possess a component of electric field disturbance in the direction of the electric moment of the molecule. The magnitude and direction of the electric charge motion within the molecule is measured by the socalled transition moment of the molecule and governs the intensity of the absorption and emission of light. In general, a molecule is characterized by three mutually perpendicular transition moments which relate directly to the capability of the molecule to interact with light. The three transition moments, however, may vary considerably in magnitude and, in particular, one moment may be very large relative to the remaining two. If a large number of such molecules are aligned (oriented) into an (uniaxially crystalline) structure so that the major transition moments are all parallel for all the molecules, the components of electric field of natural light incident on the array which are parallel to the aligned electric moments of the array are preferentially absorbed with the result that the remaining light is plan polarized, i.e.light is produced whose electric vector vibrates in a single well-defined direction. The foregoing is the basis for the so-called dichroic polarizer as exemplified by a Polaroid sheet. It will be understood that this explanation as to molecules is equally applicable to crystals.

A material then is dichroic or exhibits dichroism if the absorption of linearly polarized light varies in accordance with the direction of the electric vector. Stated another way, a strongly dichroic material is one which will transmit essentially all of the linearly polarized light having an electric vector normal to its absorption axis while strongly absorbing the linearly polarized light whose electric vector is parallel to its absorption axis. It should be evident that, if linearly polarized light encounters an oriented dichroic array whose principal transition moment or absorption axis is normal to the direction of the electric vector of the light, no light can be absorbed by the oriented dichroic array, e.g.such light will be without substantial effect of any kind on the oriented dichroic array.

In general, the photoconductive element of this invention can be embodied in a number of ways, the preferred of which is by employing one of the novel photoconductors of US. Pats. 3,489,558 and 3,501,293, which exhibits dichroism when uniaxially oriented. Such photoconductors may be used either alone as the photoconductive material or in association with another material which is capable of transporting charge carriers and which is transparent to the wavelengths of polarized light employed. This material may be either insulating or semiconducting so long as its resistivity is sufiiciently high so that the composite photoconductive element will retain a charge on its surface in the dark. Preferably, the resistivity of the material is 10 ohm-cm. or greater. Thus, this second material is inactive until light is absorbed by the dichroic photoconductor.

Another embodiment for the photoconductive element of the invention is to use a photoconductor, again which is transparent to the wavelengths of polarized light, in conjunction with an oriented dichroic activator or sensitizer, which coacts with a photoconductor in some manner such as through the formation of a charge transfer complex. With this embodiment, the photoconductor is photoconductively inactvie until the dichroic activator or sensitizer absorbs light and transfers the absorbed energy to the photoconductor.

Regardless of the embodiment of the photoconductive element employed, the dichroism of the element is either due to the crystalline form or the molecular form of the dichroic material. In either case from a practical standpoint of manufacturing a dichroic material of a large area or sheet size, essentially all of the crystals or molecules must be aligned so that they act as a single dichroic unit, i.e., have essentially the same preferred absorption axis. With crystals, this can be accomplished by dispersing the crystals in a stretchable sheet such as a sheet of polyvinyl alcohol, and stretching the sheet in an unidirection. Of course, the stretchable sheet must be essentially transparent to the Wave lengths of light which the dichroic material absorbs. In addition, the crystals should be microcrystalline in size to minimize light scattering.

Dichroic molecules can be aligned in a number of ways, such as-(a) stretching as previously described, (b) attaching the molecules chemically within. a homogeneous material that already has a high degree of orientation in an unidirection, (c) coating the molecules onto the surface of a sheet which already has a preferred direction of orientation, (d) coating them on a surface by undirectional rubbing which may be followed by stretching, and extruding.

Normally, the extent that a dichroic sheet prepared by one of the above methods exhibits dichroism is measured by its optical dichroic ratio, R This ratio, R is defined as d /d wherein d is the optical density obtained when the incident light is linearly polarized with the direction of the electric vector parallel to the transition moment axis for maximum absorption and wherein d is the optical density obtained when the incident light is linearly polarized with the direction of the electric vector perpendicular to the axis for minimum absorption. It will be apparent that dichroic sheets with high dichroic ratios, especially over a wide bandwidth, are desired.

While this optical dichroic ratio is an indication of the usefulness of a particular dichroic material in the photoconductive element of the present invention, a dichroic photodecay ratio is more accurate and more important in determining the suitability of a particular material. This ratio, R is defined as 12 /11 wherein p is the decay rate of an electrostatic charge on the photoconductive element when the incident light is linearly polarized having the electric vector for maximum absorption and wherein p, is the decay rate obtained when the incident polarized light has the electric vector for a minimum absorption. These two rates may be based either on the initial decay rate or the decay at T (the exposure time required to reach one-half of the original electrostatic potential). This ratio is a measure of the usefulness of the oriented dichroic material of the present invention.

In general, for producing good quality copies of the original, the dichroic photodecay ratio should be at least greater than 2 and preferably should be greater than 5. It should be recognized that the dichroic photodecay ratio can be varied by changing the concentration of the dichroic material and will vary depending upon the method of fabrication of the element and its final configuration. The ratio, R can be increased by activating or sensitizing a dichroic photoconductive material so as to increase 1ts photoconductivity through the formation of a charge transfer complex or some other mechanism. Likewise, in the embodiment of the photoconductive element employing a charge transporting material insensitive to the wave lengths of polarized light, it too can be activated or sensitized as long as it is not rendered sensitive to the wave lengths of polarized light.

With the above general description of the photoconductive element of the present invention, the following describes the use of such an element in the electrophotographic process of the present invention. The photoconductive element is uniformly electrostatically charged, following which a document to be reproduced is brought into contact with the charged surface of the photoconductive element. Next, the free surface of the photoconductive element is exposed to polarized light having its electric vector so oriented with respect to the absorption axis of the ele ment that there is maximum transmittance of the light through the dichroic photoconductive element. Upon striking the original, the polarized light is essentially absorbed in the dark or black areas, usually the print areas, and depolarized in the light or white areas, normally background. Such depolarized light is reflected back to the photoconductive element and a sufficient portion of it has an electric vector normal to the vector of the originally transmitted polarized light and parallel to the absorption axis of the dichroic material that suflicient light is adsorbed by the photoconductive element to cause selective dissipation of electrostatic charge and the formation of a charge pattern corresponding to the pattern of the document. Now, the electrostatic charge pattern can be developed with toner in one of the known electrophotographic ways, such as cascade, magnetic brush or fur brush, and the developed pattern transferred to paper to provide 6 a high quality copy of the document. Alternatively, the electrostatic charge pattern can be transferred to a dielectric surface and developed thereon. After being cleaned, if the electrostatic charge pattern is developed on the photoconductive element, the photoconductive element is ready for additional cycles from which result additional high quality reproductions of the same or different documents.

-It will be understood that document includes not only those having areas which without further intervention depolarize and reflect the light back into the photoconductive element but also includes printed transparencies or translucencies which have been backed by a depolarizing and reflecting element.

The preferred embodiment of the photoconductive element of the present invention comprises a transparent conductive substrate, such as a layer of cellulose triacetate having an aluminized surface, carrying an oriented dichroic photoconductor, such as 2,6-bis-[p-dimethylaminocinnamyldeneamino] benzo 1,2-d 4,5 -d' bisthiazole described in US. Pat. 3,501,293. Herein, the dichroic photoconductor is rubbed on in dry powder form in an unidirection which establishes a preferred axis for maximum absorption when the electric vector of the polarized light is parallel to the axis and a minimum absorption or maximum transmission of the light when its electric vector is perpendicular to the axis. On top of the oriented dichroic photoconductor is a layer of a transparent normally insulating material which is essentially insensitive to the wave lengths of polarized light, but capable of transporting charges generated by the dichroic photoconductor. Herein, the layer comprises poly-N-vinylcarbazole which is essentially insensitive to the visible light to be used for the exposure of the photoconductive element.

With the surface of the tarnsparent poly-N-vinylcarbazole layer carrying a uniform electrostatic charge, the photoconductive element is exposed to visible polarized light from, for example, an unpolarized incandescent light source fitted with a light polarizing sheet, such as a H- sheet polarizer (manufactured by Polaroid Corporation). The photoconductor element is oriented relative to the "polarizer such that the electrical vector of the polarized light is prependicular to the absorption axis of the dichroic photoconductor. Thus, there is maximum transmittance of the polarized light through the oriented dichroic photoconductor and, hence, through the photoconductive ele ment as the light strikes the substrate side of the element.

With a document in contact with the charged surface of the photoconductive element during this exposure, the polarized light is essentially depolarized in the light colored or White areas of the document and is reflected back to the photoconductive element. This time, however, the electric vibrations of up to one-half of the depolarized light are parallel with the absorption axis of the dichroic photoconductor so that up to one-half of the reflected light may be absorbed. The areas of the dichroic photoconductor absorbing the light become conductive and, it is believed, generate charge carriers which are transported by the normally insulating layer. Regardless of the theory, the result is that the charge on the transparent photoconductor is dissipated so that an electrostatic charge pattern is formed corresponding to the pattern of the document. Of course, the polarized light striking the dark or black areas of the document is essentially absorbed and not reflected so that charge remains in such areas. The electrostatic charge pattern can be developed by one of the conventional techniques.

Another embodiment of the photoconductive element of the present invention is one in which the positions of the transparent photoconductive layer and the dichroic photoconductive layer are reversed so that the transparent layer is adjacent the aluminized substrate. Because the dichroic photoconductive layer now forms the top layer, it is desirable in those cases in which this layer is easily abraded, to overcoat the dichroic layer with a protective layer, such as a transparent insulating material of, for example, cellulose acetate, or any other transparent material which will be sufliciently insulating to hold an electrostatic charge and will not be easily abraded.

A third embodiment employing a dichroic photoconductor comprises a single layer carried on a transparent conductive substrate. Herein, in this embodiment, the dichroic photoconductor, such as one of those described in US. Pat. 3,489,558, is dispersed in an insulating transparent matrix, such as polyvinyl formal. To orient the dichroic photoconductor, the matrix is stretched in a unidirection prior to applying it, for example, by lamination, to the conductive substrate, such as aluminized cellulose triacetate. If desired, the insulating film may also be photoconductive as long as the film is essentially insensitive to, or not rendered photoconductive by virtue of transmitting, the wave lengths of polarized light used for exposure.

To increase the photoconductivity of one of the abovedescribed photoconductive elements, there is incorporated in the element either a dye sensitizer or an activator which is also known as an electron acceptor or, in some cases, when the photoconductor is an electron acceptor, an electron donor. Examples of such dye sensitizers and activators are set forth in U.S. Pats. 3,037,861, 3,169,060 and 3,287,113. In addition, if it is desired to have the photoconductive element exhibit persistent conductivity, the dye sensitizer and activator combinations described in U.S. application Ser. No. 474,977, filed July 26, 1965, may be used in the preparation of such photoconductive elements.

If the embodiment of the photoconductive element is one in which a charge transfer complex is formed, either with a dichroic photoconductor and an activator, or a photoconductor and a dichroic activator, and the complex acts as a dichroic entity, then the absorption spectrum of the complex should be within or essentially match the wave lengths of polarized light employed. If a complex is formed but is not a dichroic entity, the non-dichroic activator or non-dichroic photoconductor must absorb essentially outside the wave lengths of the polarized light employed. The same is true for a non-dichroic dye sensitizer. When the photoconductive element comprises one of the embodiments which includes a charge transporting layer, the dye sensitizer or activator added to this layer must absorb essentially outside the wave lengths of polarized light being employed.

Except for the charge transfer embodiment, all of the previous embodiments described photoconductive elements in which the dichroic material is in itself a photoconductor. The following embodiments of the present invention are directed to a photoconductive element in which the dichroic material serves as a sensitizer or activator for the photoconductor which is essentially insensitive or non-photoconductive in the wave lengths of polarized light employed for exposure.

The first of these embodiments comprises a transparent conductive substrate on which is applied a polymeric layer having a direction of orientation due to, for example, unidirectional stretching. This polymeric layer is stained with a material such as iodine which causes the polymer to exhibit dichroism and also serves as an activator for a photoconductive layer applied over the polymeric layer. This photoconductive layer is essentially transparent to the wave lengths of polarized light to be employed. In addition, the activator must be sufliciently abundant at the interface of the polymeric layer and the photoconductive layer to activate or render the latter layer conductive when light is absorbed by the polymeric layer.

If desired, the two above-described layers may be reversed so that the polymeric layer is uppermost. However, inasmuch as the polymeric layer must be capable of holding an electrostatic charge in the dark on its surface, it must have a dark resistivity of at least 10 ohm-cm. and preferably should be in the range of 10 to 10 ohm-cm. Thus, some polymeric materials, such as polyvinyl alcohol, may not be suited for this embodiment, or may have to be overcoated with an insulating material to achieve the proper resistivity.

Another activator or sensitizer embodiment of the present invention comprises a single layer of an oriented polymeric photoconductor, such as a terpolymer of N- pentenylcarbazole, N-hexenylcarbazole, and pentene-l (mole ratio of 40:40:20, respectively) which has been stained by, for example, 2,4,7-trinitro-9-fluorenone which activates the polymeric photoconductor as well as serves to render it dichroic.

Other useful substrates than those previously mentioned are metallized (i.e.aluminum, and copper) polycarbonate and glass. Also NESA glass may be used. Normally, it is preferred to use a transparent material which will neither depolarize nor change direction of the electric vector of polarized light. However, if desired, the substrate may serve to polarize incoming unpolarized light or change incoming polarized light so that its electric vector is the proper direction for transmission through the photoconductor.

A further modification of the present invention concerns a process in which a semi-transparent mirror is inserted in back of the photoconductive element. That is, the exposure step now comprises what can be termed mirror dichroic reflex. With a semi-transparent mirror disposed behind the photoconductive element during exposure, the incoming polarized light passes through the semi-transparent mirror and the photoconductive element, but the depolarized light reflected from the document which is not absorbed by the photoconductive element is reflected back into the photoconductor by the highly reflective surface of the mirror. Thus, the reflected depolarized light not initially absorbed by the photoconductive element will be reflected between the mirror and the document, with additional light being absorbed by the photoconductive element during each such reflection. This greatly increases the absorption efficiency of the photoconductive element with a consequent gain in the ultimate contrast between the print and background of the copy. Accordingly, this makes the performance of the process of the present invention relatively insensitive to the depolarization factor of the document being copied. This gain is similar to that described in U.S. Pat. application Ser. No. 593,051 filed Nov. 9, 1966. Preferably, if the metal forming the conductive surface of the substrate is made highly reflective, it may also serve as the semi-transparent mirror.

Filters may be used with the exposing light source if it is necessary to eliminate wavelengths of light which would be absorbed by other than the dichroic entity or to confine the exposure light to those spectral regions which the element exhibits adequate dichroism.

The invention now will be further illustrated by the following examples, but it is to be understood that the invention is not restricted thereto.

EXAMPLE I A glass substrate was metallized with aluminum to a thickness such that the optical transmission density was 1.0 (i.e.10% transmitting, the aluminum serving as a conductive electrode as well as a highly reflective mirror. A dichroic photoconductor of 2-p-N,N-dimethylaminobenzylideneamino) 6 (pnitrobenzylideneamino [1,2-d:5,4-d']bisthiazole (described in US. Pat. 3,489,- 558) in dry powder form was lightly rubbed with an unidirectional motion to form a thin coating having an optical density in the region of 0.20.6 with polarized light oriented for absorption. A 10% by weight poly-N- vinyl carbazole in tetrahydrofuran was coated over the dichroic photoconductive coating with a doctor blade set at a 5 mil wet gap, the resulting dry thickness being about 8-l0 microns.

The thus prepared photoconductive element was electrostatically charged with a Xerox Model D processor set at a negative --7000 volts to form a uniform negative electrostatic charge. The charged element was brought into face-to-face contact with a document (black print on a white background). Next, the document and charged photoconductive element was exposed to polarized light from a 375 watt photo EBR flood lamp passing through a light polarizing film (supplied by Bausch and Lomb, Catalog No. 31-52-62-26) through the back of the eleconductor (see table below) was rubbed in a unidirection on the coating. Next, i /2% solution of poly-n-vinylcarbazole in benzene was coated on the meniscus coater with a sufiicient number of passes to form an approximately micron layer.

Following the reproduction procedure of Example I, except for the dichroic photoconductors and exposure conditions set forth in the following table:

Distance from source, Time, Photoconductor Light source inches sec.

III 2,6-bis-[p-dimethylaminobenzylideneamino]-benzo[1,2-d,4,5-d]bi hiazo1e 375 watt photofiood (Example I) 12 0, 4 IV 2,6-bis-[p-dimethylamino einnamylideneaminolbenzo[1,2-d,4,5-d]b1sthiaz0le 40 watt incandescent 13 0. 3 V 2,6-bis-[p-dimethylarninocinnamylideneaminokbenzo[l,2-d,5, 1,d]b1sthiazole 376 watt photofiood (Example I) 12 0. 4 VI 2,a-bis-[5-( -dimethylaminopheny1)penta-2,4-dl n yhd n ammol-benzo[1,2-d,5,4-d] 40 watt incandescent 13 4 bisthiazole. t

ment with the dichroic photoconductor aligned for low absorption relative to the electric vector of the polarized light so that the light was essentially transmitted through the element. In striking the white background of the document, it was depolarized, reflected back, and absorbed. The exposure was for 0.4 seconds at a distance of 12 inches. Next, the document was separated from the photoconductive element and the remaining electrostatic charges in the unexposed (print) areas was de- 'veloped by cascading positively charged toner particles which were attracted to the negative electrostatic charge pattern. The developed pattern was transferred to a copy sheet to yield a copy of the document which had high print density, excellent contrast and only a faint background.

EXAMPLE II A glass substrate was coated with polyvinylidene chloride to form a thermoplastic coating. Next, a dichroic photoconductor of 2,4 bis(p-N,N dimethylaminobenzyhdeneamino)-benzo[1,2-d:4,5-d]-bisthiazole was rubbed in an unidirection on a film of poly-N-vinylcarbazole activated with 2% by weight of tetrachlorophthalic anhydride and carried on a temporary polyethylene terephthalate substrate. On the poly-N-vinyl carbazole film was an aqueous solution of a methylvinylether-maleic anhydride copolymer and quaternary ammonium salt in equal parts by weight using a doctor blade set at a 1 /2 mil wet gap. Upon drying, the thickness of this conductive coating was about 2-3 microns. To complete the photoconductive element, the polyvinylidene chloride coating on the glass substrate was brought into contrast with the conductive coating on the poly-N-vinylcarbazole and laminated thereto by heating to about 100 C. After cooling, the polyethylene terephthalate temporary substrate was separated to leave the finished photoconductive element.

This photoconductive element was used for reproducing a copy of a document in the same manner as Example 1 except that the light source was a 40 watt incandescent lamp and the element was 12 inches from the lamp when exposed for 1 second. The copy had high print density and good contrast with fair background.

When the electric vector of the incoming polarized light was rotated 90 so that initially the light was uniformly adsorbed by the photoconductive element, and the rest of the procedure remained the same, the copy produced had very poor contrast and high background, and was totally unsatisfactory.

EXAMPLES III-VI A film of cellulose triacetate metallized with aluminum having an optical density of 0.8 was coated with an aqueous solution of a methylvinylether-maleic anhydride copolymer and quartenary ammonium salt in equal parts by weight using a meniscus coater. Upon drying, the coating was about 2-3 microns thick. A dichroic photo- The copies reproduced had high print density and good contrast with only faint background.

When the electric vector of the incoming polarized light was rotated so that initially the light was uniformly absorbed by the photoconductive elements and the rest of the procedure remained the same, the copies produced had very poor contrast, high background and were totally unsatisfactory.

EXAMPLE VII A glass substrate was metallized with aluminum to a thickness such that the optical density was 1.0 10% transmitting). A solution of 10% by weight of poly-N- vinyl-carbazole in tetrahydrofuran was coated with a doctor blade set at a 5 mil wet gap, the resulting dry thickness being about 810 microns. Next, a dichroic photoconductor of 2,6-bis-[p-dimethylaminobenzylideneamino]- benzo[l,2-d,5,4-d]bisthiazole in dry powder form was lightly rubbed with an unidirectional motion or in the long direction of the substrate to form a thin coating having an optical density in the region of 0.2-0.6 with polarized light oriented for absorption.

Following the reproduction procedure of Example I, except that the Xerox Model D processor was set at a positive +7000 volts and that the light source was a 40 watt incandescent lamp spaced at a distance of 12 inches from the above prepared photoconductive element, and the exposure was 1 second, a copy of a document was reproduced and it had high print density, good contrast, and only a faint background.

When the electric vector of the incoming polarized light was rotated 90 so that initially the light was uniformly absorbed by the photoconductive element, and the rest of the procedure remained the same, the copy produced had very poor contrast and high background, and was totally unsatisfactory.

EXAMPLE VIII For determining the dichroic photodecay ratio of photoconductive elements and, in addition, the measuring difference in surface potential of the photoconductive elements is exposed to strongly absonbed polarized light and weakly absorbed polarized light, the following described electrometer was used and serves as a simulated reproduction process. The exposures are over a period of time and this gives a measure of the latitude of the photoconductive elements.

The electrometer comprises an electrostatic corona discharge charging unit set at a potential of 6000 volts and a Model 566 Charge Amplifier (manufactured by Kistler) having a transparent NESA glass probe. A rotatable photoconductive element holder on a pivoted arm capable of moving the photoconductive element holder from the charging unit to in front of the transparent glass probe. For exposure of the photoconductive element, the electrometer has a watt tungsten lamp with a filter holder disposed adjacent the lamp. In optical contact with the 1 1 holder is a light tube which, in turn, is attached to the transparent glass probe. On the back side of the glass probe is an H-sheet polarizer (manufactured by Polaroid). For observing the measurements of the electrometer, the charge amplifier is connected to a Type 535 Oscilloscope with a 53/ 54C Plug-In unit.

In operation, a photoconductive element sample is placed in the holder with the photoconductive surface facing out of the holder and aligned, relative to the electric vector of the polarized light, to only weakly absorb the light. The arm is moved to place the sample in front of the charging unit and is given an uniform electrostatic charge. Next, the charged sample is moved in front of the transparent probe and exposed to polarized light from the lamp passing through the polarized light. The exposure is observed on the oscilloscope as a trace moving from left to right. Depending on the dissipation of the electrostatic charge on the photoconductive element, the trace also curves downwardly. The exposure is retained until the trace reaches the right side of the oscilloscope. This is a measure of the photoconductive elements response to polarized light, which it only weakly absorbed. Now, the element is rotated 90 and the arm is moved back to the charging unit for uniform charging of the element. After charging, the arm is again moved to the transparent probe and again exposed to polarized light, only this time the element is aligned to strongly absorb the light. During exposure, another trace moves from left to right across the oscilloscope. This trace will curve downward substantially and the former curve will have very little downward movement if the element is a good dichroic photoconductor. The face of the oscilloscope has a scale which permits calibrating the curves which are a plot of surface potential vs. exposure time. A Polaroid land camera is mounted on the oscilloscope and double exposure pictures or oscillograms of the traces are taken of each dichroic photodecay ratio. These curves also yield the photoconductive speed T /2 of each element.

The following table lists the properties of seven photoconductive elements which were prepared by coating a poly-N-vinyl carbazole in tetrahydrofuran solution on polyethylene tcrephthalate substrate carrying a conductive layer of a quaternary ammonium salt in polyvinyl alcohol. To the poly-N-vinylcarbazole layer, the respectiwe dichroic photoconductor was applied in powder form with an unidirectional wiping motion. The thus prepared elements were tested on the above-described electrometer with the traces on the oscilloscope being photographed. The dichroic ratio is also given and was measured on a Cary Model 14 spectrophotometer. The filters are Balzers #2 and #3 filters, having x max, respectively, of 4500 A and 4800 A.

tungsten and 22 with a #3 filter. The dichroic photodecay ratio of the photoconductors of Examples III, IV, V, as measured by the above testing procedure to full tungsten, was l8, l0, and 10, respectively.

EXAMPLE XV An aqueous solution of polyvinyl alcohol was coated on an aluminum by an unidirectional wiping to form a thin oriented coating. This coating was then stained with a 1% iodine solution in acetone again wiping in the same unidirection. Next, a 10% polyvinylcarbazole solution in tetrahydrofuran was coated on the stained polyvinylalcohol with a doctor blade set at a 5 mil wet gap. When evaluated with the above-described electrometer, it had a dichroic photodecay ratio of 3.0. Accordingly, iodine in an oriented configuration is useful as a dichroic sensitizer in the dichroic reflex process of the present invention.

EXAMPLE XVI Light source: watt incandescent Distance from source: 12 inches Time: 1.0 sec.

The above-described document was then copied on a Xerox 720 and a visual comparison made. The Xerox 720 had good contrast between the black and white areas but there was essentially no grades of half tone reproduction. Instead, the half tone areas reproduced as continuous light black areas.

Conversely, the copy reproduced by the dichroic reflex process of the present invention had excellent contrast between the black and white areas and good reproduction of the half tone areas.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that variations in form may be made therein without departing from the spirit and scope of the invention. For example, the dichroic reflex process and photoconductive element of the present invention may be employed in persistent Diehroic photodecay ratio Optical Full Per- #2 Ier- #3 Perdiehroic tungcent filcent filcent Dichrole photoconductor ratio sten eff. ter etf. ter e11.

VIIL... 2-(p-N,N-dimethylaminobenzylideneamino)-6-(p-nltrobenzylideneamino)-benzo[1,2- 11.9 5 42 6 4G 5. 5 57 d,5,4-d]bisthiazole. IX 2,G-l%is(p-N,N-dirnethylaminobenzylldeneamino)-4-methylbenzp[1,2-d,5,4-d]blsthia 6.7 3.5 52 3 45 4. 5 67 zo e. X 2,7-bis-(p-N,N-dirnethylamlnobenzylideneamino)-benzo[1,2,d,4,3d]bisth1azole 8.3 4 48 2 24 2. 5 30 XI 2,6diis-(p-N,N-dimethylaminobenzylideneamino)-4-ehlorobenzo[1,2d,5,4-d]blsthia- 8. 3 6. 5 78 4 48 6 72 zo e. XII-.. 2,7-bis(p-N,N-dimethylaminobenzylldeneamino)-4-chlorobenzo[1,2-d,3,4-d1bisthia- 1.4 1 1. 35 96 1. 35 96 z e. XIII.-." 2,6-kbls-(Er-N,N-dimethylamlnobenzylideneamino)4-methoxybenzol1,2-d,5,4-d]bls- 8. 9 5. 5 G2 5 G0 5 t iazo e. XIV 2,7-bis(p-N,N-dimethylaminobenzylideneamino)-4-methoxybenzo[1,2-d,3,4d']bis- 2. 58 2. 5 93 2. 5 93 tlnazole.

All of the above compounds are useful as dichroic photoconductors in photoconductive elements for the dichroic reflex process of the present invention.

As evidence that the photodecay ratio will vary depending upon the method of fabrication, the dichroic photoconductor of Example XIII was prepared according to the procedure of Examples III-VI. The element thus electrophotographic methods, such as that disclosed in US. Pat. 2,845,348 or any other method where the photoconductor is exposed before charging. Also, the dichroic reflex process and photoconductive element of the present invention can be used in conjunction with charge transfer techniques, such as disclosed in US. Pat. 2,825,814. As a further example, the photoconductive element can be prepared had a dichroic photodecay ratio of 19 with full fabricated with a non-conductive substrate and the charging of the element can be accomplished by dual corona, such as described in U.S. Pat. 2,922,883.

What is claimed is:

*1. A photoconductive element, suitable for use in electrophotographic reflex copying, comprising a dichroic material oriented in an unidirection to have preferred maximum absorption and transmission axes, and exhibiting photoconductive dichroism, said dichroic material being capable of causing conductivity in those areas of the photoconductive element which are exposed to light having an electric vector of suflicient magnitude and in a direction parallel to said absorption axis.

2. The photoconductive element of claim 1 wherein said element contains a semitransparent mirror on the side of the absorption axis opposite to the side of the element to be brought into contact with a document, the reflective surface of said mirror facing towards the side of the element to be brought into contact with the document.

3. The photoconductive element of claim 2 wherein said mirror is formed of a conductive metal and also serves as an electrode.

4. The photoconductive element of claim 1 wherein said element comprises a transparent substrate having disposed thereon a thin layer of a dichroic photoconductor and a thicker essentially transparent charge transport layer, said photoconductive layer and said charge transport layer being in contact with each other, the composite of said layers being of suflicient resistivity to support an electrostatic charge in the dark, said dichroic photoconductive layer being oriented in an unidirection to provide said preferred absorption axis, the light absorption spectra of said charge transport layer being essentially outside the light absorption spectra of said dichroic photoconductive layer.

5. The photoconductive element of claim 4 wherein said substrate of the element carries a semitransparent mirror with its reflective surface facing said preferred absorption axis.

6. The photoconductive element of claim 5 wherein said semitransparent mirror is formed of a conductive material and also serves as an electrode.

7. A photoconductive element of claim 1 wherein said element comprises a transparent substrate having disposed thereon a layer of a polymeric material oriented in an unidirection, a layer of an essentially transparent photoconductor, and a material at the interface of said oriented polymeric layer and said photoconductive layer capable of rendering the oriented layer dichroic to provide said preferred absorption axis and of serving as an activator for the photoconductor.

8. The element of claim 7 wherein said material at said interface being capable of forming a charge transfer complex with said photoconductor.

9. The photoconductive element of claim 7 wherein said substrate of the element carries a semitransparent mirror with the reflective surface facing said preferred absorption axis.

10. The photoconductive element of claim 9 wherein said semitransparent mirror is formed of a conductive material and also serves as an electrode.

11. An electrophotographic process for the production of reflex copies comprising:

positioning a document, having a pattern of light absorbing areas and light depolarizing and reflecting areas, adjacent a photoconductive element which exhibits photoconductive dichroism and has a preferred absorption axis;

uniformly exposing through the photoconductive element with polarized light whose electric vector is essentially normal to the absorption axis of said photoconductive element, said polarized light being depolarized by those depolarizing and reflecting areas of the document and, in being reflected back into said element, containing light with an electric vector 14 parallel to the absorption axis of said element so as to render those areas of the element conductive, said conductive areas of said element being capable of transporting an electrostatic charge to cause formation of an electrostatic charge pattern corresponding to the pattern of said document.

12. The process of claim 11 wherein the photoconductive element is uniformly electrostatically charged prior to said exposure to polarized light.

13. The process of claim 12. wherein after said exposure, said document is removed from photoconductive element and the electrostatic charge pattern thus formed is developed.

14. The process of claim 13 wherein the electrostatic charge pattern is developed on the photoconductive element and the developed pattern transferred to a copy sheet.

15. The process of claim 14 wherein the photoconductive element is cleaned to remove any residual of the developed pattern and is reused for the reproduction of another copy.

16. In an electrophotog-raphic process, the step comprising:

exposing a photoconductive element, exhibiting photoconductive dichroism and having a preferred absorption axis, to a light pattern resulting from reflection of light from a document in contact with the photoconductive element and in accordance with the pattern of the document, said light pattern containing light with an electric vector parallel to the absorption axis of said element so as to render conductive those areas of the element corresponding the light pattern, said light prior to reflection from the document having a transmission direction opposite to the reflected light pattern and being polarized light with an electric vector essentially normal to the absorption axis of said photoconductive element as it passes the axis so that said polarized light is essentially transmitted by the photoconductive element.

17. The process of claim 16 wherein the photoconductive element contains a semitransparent mirror on the side of the absorption axis opposite to the side of the element in contact with the document whereby said polarized light is transmitted through the mirror but each portion of the reflected light pattern not absorbed at the absorption axis is reflected back and forth between the mirror and the document thereby increasing the total light absorbed by the photoconductive element.

18. The process of claim 17 wherein said mirror is formed of a conductive metal and also serves as an electrode.

'19. The process of claim 16 wherein said photoconductor comprises a transparent substrate having disposed thereon a thin layer of a dichroic photoconductor and a thicker charge transport layer, said photoconductive layer and said charge transport layer being in contact with each other, the composite of said layers being of sutficient resistivity to support an electrostatic charge in the dark, said dichroic photoconductive layer being oriented in an unidirection to provide said preferred absorption axis, and wherein the wavelengths of said polarized light are essentially outside the wavelengths of absorption by said charge transport layer.

20. The process of claim 19 wherein said substrate of the photoconductive element carries a semitransparent mirror with its reflective surface facing said preferred absorption axis.

21. The process of claim 20 wherein said semitransparent mirror is formed of a conductive metal and also serves as an electrode.

22. The process of claim 16 wherein said photoconduc tive element comprises a transparent substrate having dis posed thereon a layer of a polymeric material oriented in an unidirection, a layer of a photoconductor absorbing essentially outside the wavelengths of the polarized light,

15 and a material at the interface of said oriented polymeric layer and said photoconduetor layer capable of rendering the oriented layer dichroic to provide a preferred absorption axis and serving as an activator for the photoconductor.

23. The process of claim 22 wherein said substrate of the photoconductive element carries a semitransparent mirror with its reflective surface facing said preferred absorption axis.

24. The process of claim 23 wherein said semitransparent mirror is formed of a conductive metal and also serves as an electrode.

References Cited UNITED STATES PATENTS 16 3,113,022 12/1963 Cassiers et a1. 961.5 3,165,405 1/1965 Hoesteney 961.7

3,271,145 9/1966 Robinson 96-1 3,272,626 9/1966 Shinn 96-1 3,278,302 10/1966 Gundlach 96-1 3,287,122 11/1966 Hoegl 96-1.5 3,312,547 4/1967 Levy 961.5

3,317,317 5/1967 Clark 961.4

CHARLES E. VAN HORN, Primary Examiner US. Cl. X.R.