Xeroprinting process

Abstract: Disclosed is a xeroprinting process which comprises (1) providing a xeroprinting master comprising (a) a substrate (22); and (b) a layer of softenable material (25), a charge-transport material (27) capable of transporting charges of one polarity, and migration marking material (26) situated contiguous to the surface of the softenable layer spaced from the substrate, wherein a portion of the migration marking material has migrated through the softenable layer toward the substrate in imagewise fashion; (2) uniformly charging the xeroprinting master to a polarity opposite to the polarity of the charges that the charge transport material in the softenable layer is capable of transporting; (3) uniformly exposing the charged master to activating radiation (31), thereby discharging those areas of the master wherein the migration marking material has migrated toward the substrate and forming an electrostatic latent image; (4) developing the electrostatic latent image, and (5) transferring the developed image to a receiver sheet. The process results in greatly-enhanced contrast potentials or contrast voltages between the charged and uncharged areas of the master subsequent to exposure to activating radiation, and the charged master can be developed with either liquid developers or dry developers.

Description: The present invention is directed to a xeroprinting process. More specifically, the present invention is directed to a xeroprinting process employing a migration imaging member and resulting in improved contrast potential. One embodiment of the present invention is directed to a xeroprinting process which comprises (1) providing a xeroprinting master comprising (a) a substrate and (b) a layer of softenable material, a material capable of transporting charges of one polarity, and migration marking material situated contiguous to the surface of the softenable layer spaced from the substrate, wherein a portion of the migration marking material has migrated through the softenable layer toward the substrate in imagewise fashion; (2) uniformly charging the xeroprinting master to a polarity opposite to the polarity that the charge transport material in the softenable layer is capable of transporting; (3) uniformly exposing the charged master to activating radiation, thereby discharging those areas of the master wherein the migration marking material has migrated toward the substrate and formed an electrostatic latent image; (4) developing the electrostatic latent image; and (5) transferring the developed image to a receiver sheet. Several advantages are associated with the process of the present invention, including increased contrast potential and suitability for development utilizing either dry toners or liquid toners. In the art of printing/duplicating, various techniques have been developed for preparing masters for subsequent use in printing processes. For example, lithographic or offset printing is a well known printing process. In general, lithography is a method of printing from a printing plate which depends upon different properties of the imaged and non-imaged areas for printability. In conventional lithography, a lithographic intermediate is first prepared on silver halide film from the original; the printing plate is then contact exposed by intense ultra-violet radiation (UV light) through the intermediate. UV exposure causes the exposed area of the printing plate to become hydrophobic; the non-exposed area is washed away by chemical treatment and becomes hydrophilic. Printing ink is then applied to the printing plate, and the ink image is transferred to an offset roller where the actual printing takes place. Although lithographic printing provides high quality prints and high printing speed, the processes require the use of expensive intermediate films and printing plates. Additionally, considerable cost and time are consumed in their preparation, often requiring highly skilled labor and strict control measures. A further disadvantage is the time consuming process and difficulty in setting up the printing press to achieve the proper water to ink balance required to produce the desired results during the printing process. This results in further increased cost and delay time in obtaining the first acceptable print. The above mentioned problems become especially severe in the manufacture of high quality color prints when several color separation images must be superimposed on the same receiving medium. Because of the high cost and complexities associated with the preparations of expensive printing plates and press runs, color proofing is employed to form representative interim prints (called proofs) from color separation components to allow the end user to determine whether the finished prints faithfully reproduce the desired results. As is often the case, the separation components can require repeated alteration to satisfy the end user. Only when the end user is satisfied with the results, a printing plate associated with each separation component is prepared and ultimately employed in the press run. An example of a color proofing system is the CROMALIN system, introduced by E.I. duPont de Nemours & Company in 1972 and widely used in the printing industry, and consisting of a light- sensitive tacky photopolymer layer laminated to paper. The photopolymer layer is contact exposed through a color separation component under a UV source. The exposed areas polymerize and lose their tackiness, while the non-exposed areas remain tacky. Toners are then applied and adhere to the tacky areas. Since very different processes are employed in proofing and press runs, the proofs at best can only simulate the press sheets. Additionally, preparation of the color proofs is a time consuming process, and can require about 30 minutes per proof. Xerographic printing is another well known printing technique. In conventional xerographic printing, an electrostatic image is first produced, either by lens coupled exposure to visible light or by laser scanning, on a conventional photoreceptor; the electrostatic image is then toned, followed by transfer of the toner image to a receiving medium. While this printing process offers the advantages of ease of operation and printing stability and requires less skilled involvement and labor cost, the combined requirements of high quality and high printing speed needed in commercial printing cannot be met easily at reasonable cost because, to provide high quality and avoid certain artifacts, very high-picture-element density is also required. If a new image were to be written, for example, on the photoreceptor for each print, these requirements for high speed and high density would imply electronic bandwidths and (if laser scanning were used) modulation rates and prismatic mirror rotation speeds which are very unlikely to be available at reasonable cost in the foreseeable future. In addition, the difficulties associated with conventional xerographic duplicating and printing include the necessity to repeat the imagewise exposure step continually at high speed. Xeroprinting is another xerographic printing method. Conceptually, xeroprinting overcomes the above problems in a very simple way. Xeroprinting is an electrostatic printing process for printing multiple copies from a master plate or cylinder. The master plate can comprise a metal sheet upon which is imprinted an image in the form of a thin electrically insulating coating. The master plate can be made by photomechanical methods or by xerographic techniques. From the original, a single xeroprinting "master" can, for example, first be made slowly, in, for example, 30 to 60 seconds. This imaged material is typically an electrical conductor with an imagewise pattern of insulating areas made by photomechanical or xerographic techniques; it has different charge acceptance in the imaged and non-imaged areas. Thus, generally, the imaging surface of the master plate comprises an electrically insulating pattern corresponding to the desired image shape and electrically conductive areas corresponding to the background. The xeroprinting master is then uniformly charged; the charge remains trapped only on the insulating areas, and this electrostatic image can then be toned. After toner transfer to paper and possibly cleaning, the charge-tone-transfer-clean process is repeated at high speed. In principle, then, it is possible with a xeroprinting process to retain much of the simplicity, stability and quality of the xerographic process without the need for repeated imagewise exposure. As an additional bonus, it may not be necessary to employ a cleaning step, since the same area is repeatedly toned. Moreover, conventional toners can be used, avoiding the problem of lack of color saturation which is encountered with comparable schemes employing magnetography. High contrast potential and high resolution of the electrostatic latent image are important characteristics that determine print qualities of documents prepared by xeroprinting. However, these known xeroprinting techniques can produce prints of inferior quality because an insulation pattern on a metal conductor cannot be fully and uniformly charged near its boundaries. As contrast potential builds up along the boundaries of the insulation pattern, fringing electric fields from the insulating image areas repel incoming ions from the charging device, which is usually a corona-charging device, to the adjacent electrically conductive background areas. This results not only in low contrast potential but also in poor print resolution. Additionally, some xeroprinting processes require numerous processing steps and complex equipment to prepare the master and/or final xeroprinted product. Some xeroprinting techniques also require messy photochemical processing and removal of materials in either the image or non-image areas of the master. In US-A-3,574,614, a xeroprinting process is disclosed in which the xeroprinting master is formed by applying an electric field to a layer of photoelectrophoretic imaging suspension between a blocking electrode and an injecting electrode, one of which is transparent, the suspension comprising a plurality of photoelectrophoretic particles in an insulating carrier liquid, imagewise exposing the suspension to electromagnetic radiation through the transparent electrode to form complementary images on the surfaces of the electrodes (the light-exposed particles migrating from the injecting electrode to the blocking electrode), transferring one of the images to a conductive substrate, uniformly applying to the image-bearing substrate an organic insulating binder such that the binder thickness, both within the image formed and the non-image areas, ranges from 1 to 20 µm. The xeroprinting process consists of applying a uniform charge to the surface of the image-bearing substrate in the presence of electromagnetic radiation to form an electrostatic residual charge pattern corresponding to the non-image areas (areas void of photoelectrophoretic particles), developing the residual charge pattern, transferring the developer from the residual charge pattern to a copy sheet, and repeating the charging, developing and transferring steps. Alternatively, the insulating binder can be intimately blended with the dispersion of the photoelectrophoretic particles prior to insertion of the liquid mixture between the electrodes. The areas from which photoelectrophoretic particles have migrated become insulating and capable of supporting an electrostatic charge. A major problem, however, is that insulating images supported directly on a conducting substrate cannot be charged close to the edges, because fringe fields drive incoming ions to the grounded substrate. Another disadvantage of these processes is that they require the use of a liquid photoelectrophoretic imaging suspension to prepare the master. Additionally, the master making processes are extremely complicated, entailing the removal of one of the electrodes, transfer of one of the complementary images to a conductive substrate, and application of an organic insulating binder to the conductive substrate. Such complicated master making processes are inconvenient to the user and can adversely affect the print quality. They also require additional time to dry the image prior to use as a xeroprinting master. Unlike the liquid photoelectrophoretic imaging suspension system described in US-A-3,574,614, solid imaging members have been prepared for dry migration systems. Dry migration imaging members are well known, and are described in detail in, for example, US-A-3,975,195 US-A-3,909,262, US-A-4,536,457, US-A-4,536,458, US-A-4,013,462, and "Migration Imaging Mechanisms, Exploitation, and Future Prospects of Unique Photographic Technologies, XDM and AMEN", P.S. Vincett, G.J. Kovacs, M.C. Tam, A.L. Pundsack, and P.H. Soden, Journal of Imaging Science 30 (4) July/August, pp. 183-191 (1986). Migration imaging members containing charge transport materials in the softenable layer are also known, and are disclosed, for example, in US-A-4,536,457 and 4,536,458. In a typical embodiment of these migration imaging systems, a migration imaging member comprising a substrate, a layer of softenable material, and photosensitive marking material is imaged by first forming a latent image by electrically charging the member and exposing the charged member to a pattern of activating electromagnetic radiation such as light. Where the photosensitive marking material is originally in the form of a fracturable layer contiguous with the upper surface of the softenable layer, the marking particles in the exposed area of the member migrate in depth toward the substrate when the member is developed by softening the softenable layer. The expression "softenable" as used herein is intended to mean any material which can be rendered more permeable, thereby enabling particles to migrate through its bulk. Conventionally, changing the permeability of such material or reducing its resistance to migration of migration marking material is accomplished by dissolving, swelling, melting, or softening, by techniques, for example, such as contacting with heat, vapors, partial solvents, solvent vapors, solvents, and combinations thereof, or by otherwise reducing the viscosity of the softenable material by any suitable means. The expression "fracturable" layer or material as used herein means any layer or material which is capable of breaking up during development, thereby permitting portions of the layer to migrate toward the substrate or to be otherwise removed. The fracturable layer is preferably particulate in the various embodiments of the migration imaging members. Such fracturable layers of marking material are typically contiguous to the surface of the softenable layer spaced apart from the substrate, and such fracturable layers can be substantially or wholly embedded in the softenable layer in various embodiments of the imaging members. The expression "contiguous" as used herein is intended to mean in actual contact, touching, also, near, though not in contact, and adjoining, and is intended to describe generically the relationship of the fracturable layer of marking material in the softenable layer with the surface of the softenable layer spaced from the substrate. The expression "optically sign-retained" as used herein is intended to mean that the dark (higher optical density) and light (lower optical density) areas of the visible image formed on the migration imaging member correspond to the dark and light areas of the illuminating electromagnetic radiation pattern. The expression "optically sign-reversed " as used herein is intended to mean that the dark areas of the image formed on the migration imaging member correspond to the light areas of the illuminating electromagnetic radiation pattern, and the light areas of the image formed on the migration imaging member correspond to the dark areas of the illuminating electromagnetic radiation pattern. The expression "optical contrast density" as used herein is intended to mean the difference between maximum optical density (D max ) and minimum optical density (D min ) of an image. Optical density is measured for the purpose of this invention by diffuse densitometers with a blue Wratten No. 94 filter. The expression "optical density" as used herein is intended to mean "transmission optical density" and is represented by the formula: D = log₁₀ [l o /l] where l is the transmitted light intensity and l o is the incident light intensity. For the purpose of this invention, all values of transmission optical density given in this invention include the substrate density of about 0.2 which is the typical density of a metallized polyester substrate. There are various other systems for forming such images, wherein non-photosensitive or inert marking materials are arranged in the aforementioned fracturable layers, or dispersed throughout the softenable layer, as described in the aforementioned patents, which also discloses a variety of methods which can be used to form latent images upon migration imaging members. Various means for developing the latent images can be used for migration imaging systems. These development methods include solvent wash away, solvent vapor softening, heat softening, and combinations of these methods, as well as any other method which changes the resistance of the softenable material to the migration of particulate marking material through the softenable layer to allow imagewise migration of the particles in depth toward the substrate. In the solvent wash away or meniscus development method, the migration marking material in the light-struck region migrates toward the substrate through the softenable layer, which is softened and dissolved, and repacks into a more or less monolayer configuration. In migration imaging films supported by transparent substrates alone, this region exhibits a maximum optical density which can be as high as the initial optical density of the unprocessed film. On the other hand, the migration marking material in the unexposed region is substantially washed away, and this region exhibits a minimum optical density which is essentially the optical density of the substrate alone. Therefore, the image sense of the developed image is optically sign reversed. Various methods and materials and combinations thereof have previously been used to fix such unfixed migration images. In the heat or vapor softening developing modes, the migration marking material in the light-struck region disperses in the depth of the softenable layer after development, and this region exhibits D min which is typically in the range of 0.6 to 0.7. This relatively high D min is a direct consequence of the depthwise dispersion of the otherwise unchanged migration marking material. On the other hand, the migration marking material in the unexposed region does not migrate and substantially remains in the original configuration, i.e. a monolayer. In migration imaging films supported by transparent substrates, this region exhibits a maximum optical density (D max ) of about 1.8 to 1.9. Therefore, the image sense of the heat or vapor developed images is optically sign-retained. Techniques have been devised to permit optically sign-reversed imaging with vapor development, but these techniques are generally complex and require critically controlled processing conditions. An example of such techniques can be found in US-A-3,795,512. For many imaging applications, it is desirable to produce negative images from a positive original, or positive images from a negative original (optically sign-reversing imaging), preferably with low minimum optical density. Although the meniscus or solvent wash away development method produces optically sign-reversed images with low minimum optical density, it entails removal of materials from the migration imaging member, leaving the migration image largely or totally unprotected from abrasion. Although various methods and materials have previously been used to overcoat such unfixed migration images, the post-development overcoating step can be impractically costly and inconvenient for the end users. Additionally, disposal of the effluents washed from the migration imaging member during development can also be very costly. The background portions of an imaged member can sometimes be made transparent by means of an agglomeration and coalescence effect. In this system, an imaging member comprising a softenable layer containing a fracturable layer of electrically photosensitive migration marking material is imaged in one process mode by electrostatically charging the member, exposing the member to an imagewise pattern of activating electromagnetic radiation, and softening the softenable layer by exposure for a few seconds to a solvent vapor, thereby causing a selective migration in depth of the migration material in the softenable layer in the areas which were previously exposed to the activating radiation. The vapor-developed image is then subjected to heat. Since the exposed particles gain a substantial net charge (typically 85 to 90 percent of the deposited surface charge) as a result of light exposure, they migrate substantially in depth in the softenable layer towards the substrate when exposed to a solvent vapor, thus causing a drastic reduction in optical density. The optical density in this region is typically in the region of 0.7 to 0.9 (including the substrate density of about 0.2) after vapor exposure, compared with an initial value of 1.8 to 1.9 (including the substrate density of about 0.2). In the unexposed region, the surface charge becomes discharged because of vapor exposure. The subsequent heating step causes the unmigrated, uncharged migration material in unexposed areas to agglomerate or flocculate, often accompanied by coalescence of the marking material particles, thereby resulting in a migration image of very low minimum optical density (in the unexposed areas) in the 0.25 to 0.35 range. Thus, the contrast density of the final image is typically in the range of 0.35 to 0.65. Alternatively, the migration image can be formed by heat, followed by exposure to solvent vapors and a second heating step, which also results in a migration image with very low minimum optical density. In this imaging system, as well as in the previously-described heat or vapor development techniques, the softenable layer remains substantially intact after development, with the image being self-fixed because the marking material particles are trapped within the softenable layer. The word "agglomeration" as used herein is defined as the coming together and adhering of previously substantially separate particles, without the loss of identity of the particles. The word "coalescence" as used herein is defined as the fusing together of such particles into larger units, usually accompanied by a change of shape of the coalesced particles towards a shape of lower energy, such as a sphere. Generally, the softenable layer of migration imaging members is characterized by sensitivity to abrasion and foreign contaminants. Since a fracturable layer is located at or close to the surface of the softenable layer, abrasion can readily remove some of the fracturable layer during either manufacturing or use of the imaging member, and adversely affect the final image. Foreign contamination, such as finger prints, can also cause defects to appear in any final image. Moreover, the softenable layer tends to cause blocking of migration imaging members when multiple members are stacked or when the migration imaging material is wound into rolls for storage or transportation. Blocking is the adhesion of adjacent objects to each other. Blocking usually results in damage to the objects when they are separated. The sensitivity to abrasion and foreign contaminants can be reduced by forming an overcoating, such as the overcoatings described in US-A-3,909,262. However, because the migration imaging mechanisms for each development method are different, and because they depend critically on the electrical properties of the surface of the softenable layer and on the complex interplay of the various electrical processes involving charge injection from the surface, charge transport through the softenable layer, charge capture by the photosensitive particles and charge ejection from the photosensitive particles, and the like, application of an overcoat to the softenable layer can cause changes in the delicate balance of these processes and result in degraded photographic characteristics compared with the non-overcoated migration imaging member. Notably, the photographic contrast density can become degraded. Recently, improvements in migration imaging members, and processes for forming images on these migration imaging members, have been achieved. These improved migration imaging members and processes are described in US-A-4,536,458 and US-A-4,536,457. US-A-3,574,614 discloses a process in which a layer of photoelectrophoretic imaging suspension is subjected to an applied electric field between a blocking electrode and an injecting electrode, one of which is transparent, the suspension comprising a plurality of photoelectrophoretic particles in an insulating carrier liquid, imagewise exposing the suspension to electromagnetic radiation through the transparent electrode to form complementary images on the surfaces of the electrodes (the light-exposed particles migrating from the injecting electrode to the blocking electrode), transferring one of the images to a conductive substrate, uniformly applying to the image-bearing substrate an organic insulating binder such that the binder thickness both within the image formed and the non-image areas ranges from 1 to 20 micrometers, applying a uniform charge to the surface of the image-bearing substrate in the presence of electromagnetic radiation to form an electrostatic residual charge pattern corresponding to the non-image areas (areas devoid of photoelectrophoretic particles), developing the residual charge pattern, transferring the developer from the residual charge pattern to a copy sheet and repeating the charging, developing and transferring steps. Alternatively, the insulating binder can be intimately blended with the dispersion of the photoelectrophoretic particles prior to insertion of the liquid mixture between the electrodes. The areas from which photoelectrophoretic particles have migrated become insulating and capable of supporting an electrostatic charge. US-A-4,536,458 discloses a migration imaging member comprising a substrate and an electrically insulating softenable layer on the substrate, the softenable layer comprising migration marking material located at least at or near the surface of the softenable layer spaced from the substrate, and a charge transport molecule. The migration imaging member is electrostatically charged, exposed to activating radiation in an imagewise pattern, and developed by decreasing the resistance to migration, by exposure either to solvent vapor or heat, of marking material in depth in the softenable layer at least sufficient to allow migration of marking material, whereby marking material migrates toward the substrate in image configuration. The preferred thickness of the softenable layer is about 0.7 to 2.5 micrometers, although thinner and thicker layers can also be utilized. US-A-4,536,457 discloses a process in which a migration imaging member, comprising a substrate and an electrically insulating softenable layer on the substrate, the softenable layer comprising migration marking material located at least at or near the surface of the softenable layer spaced from the substrate, and a charge transport molecule (e.g. the imaging member described in US-A-4,536,458), is uniformly charged and exposed to activating radiation in an imagewise pattern. The resistance to migration of marking material in the softenable layer is thereafter decreased sufficiently by the application of solvent vapor to allow the light-exposed particles to retain a slight net charge to prevent agglomeration and coalescence, and to allow slight migration in depth of marking material towards the substrate in image configuration, and the resistance to migration of marking material in the softenable layer is further decreased sufficiently by heating to allow non-exposed marking material to agglomerate and coalesce. The preferred thickness is about 0.5 to 2.5 micrometers, although thinner and thicker layers can be utilized. US-A-2,576,047 discloses a xeroprinting device and process in which, for example, an insulating pattern in image configuration coated on a metal drum is electrostatically charged and thereafter developed with developer powder. The resulting powder image on the insulating pattern is electrostatically transferred to a receiving member, and the insulating pattern is then cleaned and recycled. US-A-3,967,818 discloses a duplicating system for producing collated copy sets for precollated information. A xeroprinting master can be utilized as a master scroll that can move in reverse directions. The master is electrostatically charged and developed, and the resulting toner image is transferred to a receiving member. US-A-3,765,330 discloses a xeroprinting system which utilizes a printing member comprising a conductive substrate having raised and recessed areas of the same material, and a layer of electrically resistive material contacting the relief areas and spanning without touching the recessed areas. A uniform charge is applied to the printing member to form discharged areas where the resistive material contacts the relief areas and charged areas, where the resistive material spans the recessed areas. The printing member is then developed and the developed image is electrostatically transferred to a transfer sheet. US-A-3,820,984 and 3,648,607 disclose a migration imaging system having a migration imaging member with a binder layer of softenable material wherein a mixture of electrically photosensitive and inert fusible particles is dispersed and an imaging process wherein the fusible particles are fused, thereby fixing the migrated image of the two types of particles. The imaged member is used as a lithographic printing master. US-A-4,101,321 discloses an imaging system wherein an imaged migration imaging member is provided and either the background or the image areas of the image are selectively reduced to a more transparent condition. The imaged member comprises a softenable layer containing agglomerable materials in both image and complementary background configurations. The imaged member is contacted with solvent vapors capable of softening the softenable layer and heated, thereby causing the agglomerable material to agglomerate selectively in one of either the background or image areas. US-A-4,883,731 discloses a xeroprinting process wherein the xeroprinting master is a developed migration imaging member wherein a charge transport material is present in the softenable layer. According to the teachings of this patent, the xeroprinting process entails uniformly charging the master to a polarity the same as the polarity of charges which the charge transport material is capable of transporting, followed by flood exposure of the master to form a latent image, development of the latent image with a toner, and transfer of the developed image to a receiving member. The contrast voltage of the electrostatic latent image obtainable from this process generally initially increases with increasing flood exposure light intensity, typically reaches a maximum value of about 45 to 50 percent of the initially applied voltage and then decreases with further increase in flood exposure light intensity. The light intensity for the flood exposure step thus generally must be well controlled to maximize the contrast potential. US-A-4,880,715 discloses a xeroprinting process wherein the xeroprinting master is a developed migration imaging member wherein a charge transport material is present in the softenable layer and non-exposed marking material in the softenable layer is caused to agglomerate and coalesce. According to the teachings of this patent, the xeroprinting process entails uniformly charging the master to a polarity the same as the polarity of charges which the charge transport material is capable of transporting, followed by flood exposure of the master to form a latent image, development of the latent image with a toner, and transfer of the developed image to a receiving member. The contrast voltage of the electrostatic latent image obtainable from this process generally initially increases with increasing flood exposure light intensity, typically reaches a maximum value of about 60 percent of the initally applied voltage, and then decreases with further increase in flood exposure light intensity. The light intensity for the flood exposure step thus generally must be well controlled to maximize the contrast potential. US-A-4,853,307 discloses a migration imaging member containing a copolymer of styrene and ethyl acrylate in at least one layer adjacent to the substrate.When developed, the imaging member can be used as a xeroprinting master. According to the teachings of this patent, the xeroprinting process entails uniformly charging the master to a polarity the same as the polarity of charges which the charge transport material is capable of transporting, followed by flood exposure of the master to form a latent image, development of the latent image with a toner, and transfer of the developed image to a receiving member. US-A-4,407,918 discloses electrophotographic processes and apparatus for preparing plural copies from a single image. A photosensitive member is described which includes an electrically conductive substrate, a first photoconductive layer applied on the substrate, a charge-retentive insulating layer applied on the first photoconductive layer, and a second conductive layer applied on the charge-retentive layer. The photosensitive member is uniformly charged to a negative polarity and exposed to visible light. An image of a document to be copied is projected while the photosensitive member is positively charged. The photosensitive member is then exposed to visible and ultraviolet light, thereby trapping latent charged images across the charge-retentive layer. US-A-4,518,668 discloses a method for preparing a lithographic printing plate. A light-sensitive material comprising a light-sensitive layer and a photoconductive insulating layer is imagewise exposed and processed to form an electrostatic latent image on the photoconductive insulating layer. The image is then developed by charged opaque developer particles. This developed image is used for contact exposure of the underlying light-sensitive lithographic master layer. US-A-4,520,089 discloses an electrophotographic offset master comprising a base paper, one side of which is provided with a layer made of sericite. Another side of the base paper is provided with a layer of a photoconductor and an adhesive. The master is prepared by imagewise exposure of the photoconductor followed by subsequent development and fixation thereof. US-A-4,533,611 discloses a process for preparing a planographic printing plate in which a charged image is produced on a photoconductive layer and dielectric film applied thereon. The image is then developed and transferred to the printing plate. Many disadvantages can be associated with some of these known techniques. For example, some known xeroprinting techniques produce poor quality prints because of their poor resolution capabilities caused by fringing electric fields as explained above. Some xeroprinting processes require numerous processing steps and complex equipment to prepare the master and/or final xeroprinted product. Messy photochemical processing, and removal of materials in either the image or non-image areas of the master, are also required for some xeroprinting techniques. In some approaches an insulating image is formed on a " leaky" dielectric; that is, a substrate that will accept and retain charge for a time longer than the time charges are applied to each particular spot, but that discharges over a relaxation time shorter than the time between charging and developing the latent image. The fundamental problem with this approach is that most resistive ("leaky") dielectric films are sensitive to relative humidity, and sometimes to age and temperature as well. The relaxation time varies beyond acceptable tolerance limits, over the normally encountered range of relative humidity, temperature, and product life. These shortcomings are particularly detrimental for color printing/duplicating applications which require high quality, high resolution, and high speed. In recent years, the use of computer technology has become increasingly widespread in the commercial printing industry. While this has resulted in greatly increased efficiency and productivity of the printing process, the benefits of computer technology have mostly been confined to the prepress operations such as text editing, composition, pagination and the like. In order to provide the high quality, high resolution and high printing speed, the dominant printing process is still offset lithography, which is not compatible with computer technology because of the very low photosensitivity of conventional printing plates. Other printing technologies, such as laser xerography, thermal printing, ionography, ink jet printing, magnetography, and the like, are compatible with computer technology, but they do not satisfy the combined requirements of high quality, high resolution, and high throughput speed. Therefore, there continues to be a need for improved imaging members and improved processes of xeroprinting. In addition, a need remains for xeroprinting processes wherein the masters exhibit improved contrast voltage or contrast potential. The terms 'contrast voltage' and 'contrast potential' generally refer to the difference in voltage between the image areas and non-image areas (or between the D max areas and D min areas) of the master subsequent to formation of the master, uniform charging of the master, and exposure of the charged master to light. Generally, the contrast potential of an imaging member is a function of the thickness of the softenable layer, or of the combined thickness of the softenable layer and the charge-transport layer, adhesive layer, and/or charge-blocking layer, if present. Higher contrast potentials are particularly desirable when the image is to be toned with a dry toner, since dry toners typically require higher contrast potentials than liquid developers for acceptable development of images, especially at high printing speeds of 375 mm or more per second. Although contrast potential can be increased by increasing the thickness of the softenable layer, the thickness of the imaging member generally is limited by the mechanical requirements of the imaging member, such as flexibility, adhesion of the softenable layer to the substrate, and the like, as well as cost and ease of manufacture. Furthermore, it is desirable that during the exposure of the master to uniform light during xeroprinting, a wide range of light energies or intensities can be employed while maintaining maximum contrast potential. Thus, there is a need for an improved xeroprinting process which overcomes the above noted disadvantages and also provides enhanced contrast potential and a broader range of acceptable light energies or intensities for the uniform light exposure step. It is an object of the present invention to provide an imaging system which overcomes the above-noted disadvantages. It is another object of the present invention to provide an improved imaging system which has the combined advantages of producing high quality, high resolution prints at high throughput speed, is compatible with computer technology in that the xeroprinting master precursor possesses high photosensitivity and is capable of being digitally imaged to result in a xeroprinting master, and to provide a system suitable for both color proofing and printing/duplicating applications. These and other objects of the present invention are achieved by providing a xeroprinting process which comprises (1) providing a xeroprinting master comprising (a) a substrate; and (b) a layer of softenable material, a charge transport material capable of transporting charges of one polarity, and migration marking material situated contiguous to the surface of the softenable layer spaced from the substrate, wherein a portion of the migration marking material has migrated through the softenable layer toward the substrate in imagewise fashion; (2) uniformly charging the xeroprinting master to a polarity opposite to the polarity of the charges that the charge transport material in the softenable layer is capable of transporting; (3) uniformly exposing the charged master to activating radiation, thereby discharging those areas of the master wherein the migration marking material has migrated toward the substrate and forming an electrostatic latent image; (4) developing the electrostatic latent image, and (5) transferring the developed image to a receiver sheet. The present invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 illustrates schematically an imaging member suitable as a precursor to the xeroprinting master employed in the process of the present invention; Figures 2, 3, and 4 illustrate schematically a process for preparing a xeroprinting master for use in the process of the present invention; Figures 5, 6, 7, and 8 illustrate schematically a xeroprinting process according to the present invention; Figure 9 illustrates the photodischarge characteristics of the D max and D min areas and the resulting electrostatic contrast voltage efficiency of a xeroprinting master which is uniformly charged to a polarity the same as the polarity that the charge transport material in the softenable layer is capable of transporting and then uniformly exposing the charged master to activating radiation, in accordance with known processes, and Figure 10 illustrates schematically the photodischarge characteristics of the D max and D min areas and the contrast voltage efficiency of the electrostatic latent image of a xeroprinting master prepared and used in accordance with the process of the present invention, which entails uniformly charging the xeroprinting master to a polarity opposite to the polarity that the charge transport material in the softenable layer is capable of transporting and then uniformly exposing the charged master to activating radiation. The process of the present invention entails the use of a xeroprinting master comprising a substrate and a layer of softenable material containing migration marking material and a charge transport material, wherein a portion of the migration marking material has migrated toward the substrate. This xeroprinting master is prepared from a master precursor comprising a substrate and a layer of softenable material containing migration marking material and a charge transport material. Optional layers can also be present. An example of a migration imaging member suitable as a master precursor is illustrated schematically in Figure 1. As shown in Figure 1, a migration imaging member 1 comprises a substrate 3, an optional adhesive layer 5 situated on the substrate, an optional charge-blocking layer 7 situated on adhesive layer 5, an optional charge-transport layer 9 situated on charge-blocking layer 7, and a softenable layer 10 situated on charge-transport layer 9, the softenable layer 10 comprising softenable material 11, migration marking material 12 situated at or near the surface of the layer spaced from the substrate, and charge-transport material 13 dispersed throughout the softenable material 11. An optional coating layer 15 is situated on the surface of softenable layer 10 spaced from the substrate 3. Any or all of the optional layers can be absent from the imaging member. In addition, any of the optional layers present need not be in the order shown, but can be in any suitable arrangement. The migration imaging member can be in any suitable configuration, such as a web, a foil, a laminate, a strip, a sheet, a coil, a cylinder, a drum, an endless belt, an endless mobius strip, a circular disc, or any other suitable form. The substrate can be either electrically conductive or electrically insulating. When conductive, the substrate can be opaque, translucent, semitransparent, or transparent, and can be of any suitable conductive material, including copper, brass, nickel, zinc, chromium, stainless steel, conductive plastics and rubbers, aluminum, semitransparent aluminum, steel, cadmium, silver, gold, paper rendered conductive by the inclusion of a suitable material therein or through conditioning in a humid atmosphere to ensure the presence of sufficient water content to render the material conductive, indium, tin, metal oxides, including tin oxide and indium tin oxide, and the like. When insulative, the substrate can be opaque, translucent, semitransparent, or transparent, and can be of any suitable insulative material, such as paper, glass, plastic, polyesters such as 'Mylar' (trademark) or 'Melinex 442', (trademark), and the like. In addition, the substrate can comprise an insulative layer with a conductive coating, such as vacuum-deposited metallized plastic, such as titanized or aluminized 'Mylar' polyester, wherein the metallized surface is in contact with the softenable layer or any other layer situated between the substrate and the softenable layer. The substrate has an effective thickness, generally from 6 to 250 micrometers, and preferably from about 50 to 200 micrometers. The softenable layer can comprise one or more layers of softenable materials, which can be any suitable material, typically a plastics or thermoplastic material which is soluble in a solvent or softenable, for example, in a solvent liquid, solvent vapor, heat, or any combination thereof. When the softenable layer is to be softened or dissolved, either during or after imaging, it should be soluble in a solvent that does not attack the migration marking material. By 'softenable' is meant-any material that can be rendered by a development step as described herein permeable to migration material migrating through its bulk-. This permeability typically is achieved by a development step entailing dissolving, melting, or softening by contact with heat, vapors, partial solvents, as well as combinations thereof. Examples of suitable softenable materials include styrene-acrylic copolymers, such as styrene-hexylmethacrylate copolymers, styrene acrylate copolymers, styrene butylmethacrylate copolymers, styrene butylacrylate ethylacrylate copolymers, styrene ethylacrylate acrylic acid copolymers, and the like, polystyrenes, including polyalphamethyl styrene, alkyd substituted polystyrenes, styrene-olefin copolymers, styrene-vinyltoluene copolymers, polyesters, polyurethanes, polycarbonates, polyterpenes, silicone elastomers, mixtures thereof, copolymers thereof, and the like, as well as any other suitable materials as disclosed, for example, in US-A-3,975,195 directed to migration imaging members. The softenable layer can be of any effective thickness, generally from about 1 to 30 micrometers, and preferably from about 2 to 25 micrometers. The softenable layer can be applied to the conductive layer by any suitable coating process. Typical coating processes include draw bar coating, spray coating, extrusion, dip coating, gravure roll coating, wire-wound rod coating, air knife coating and the like. The softenable layer also contains migration marking material. The migration marking material can be electrically photosensitive, photoconductive, or of any other suitable combination of materials, or possess any other desired physical property and still be suitable for use in the migration imaging members of the present invention. The migration marking materials preferably are particulate, wherein the particles are closely spaced from each other. Preferred migration marking materials generally are spherical in shape and submicron in size. The migration marking material generally is capable of substantial photodischarge upon electrostatic charging and exposure to activating radiation and is substantially absorbing and opaque to activating radiation in the spectral region where the photosensitive migration marking particles photogenerate charges. The migration marking material is generally present as a thin layer or monolayer of particles situated at or near the surface of the softenable layer spaced from the conductive layer. When present as particles, the particles of migration marking material preferably have an average diameter of up to 2 micrometers, and more preferably of from 0.1 to 1 micrometers. The layer of migration marking particles is situated at or near that surface of the softenable layer spaced from or most distant from the conductive layer. Preferably, the particles are situated at a distance of from 0.01 to 0.1 micrometer from the layer surface, and more preferably from 0.02 to 0.08 micrometer from the layer surface. Preferably, the particles are situated at a distance of from 0.005 to 0.2 micrometer from each other, and more preferably at a distance of from 0.05 to 0.1 micrometer from each other, the distance being measured between the closest edges of the particles, i.e. from outer diameter to outer diameter. The migration marking material contiguous to the outer surface of the softenable layer is present in an effective amount, preferably from 5 to 25 percent by total weight of the softenable layer, and more preferably from 10 to 20 percent by total weight of the softenable layer. Examples of suitable migration marking materials include selenium, alloys of selenium with alloying components such as tellurium, arsenic, mixtures thereof, and the like, phthalocyanines, and any other suitable materials as disclosed, for example, in US-A-3,975,195. The migration marking particles can be included in the imaging members by any suitable technique. For example, a layer of migration marking particles can be placed at or just below the surface of the softenable layer by solution coating the first conductive layer with the softenable layer material, followed by heating the softenable material in a vacuum chamber to soften it, while at the same time thermally evaporating the migration marking material onto the softenable material in a vacuum chamber. Other techniques for preparing monolayers include cascade and electrophoretic deposition. An example of a suitable process for depositing migration marking material in the softenable layer is disclosed in US-A-4,482,622. The migration imaging members contain a charge transport material. The charge transport material contained in the softenable layer can be any suitable charge transport material either capable of acting as a softenable layer material or capable of being dissolved or dispersed on a molecular scale in the softenable layer material. When a charge transport material is also contained in another layer in the imaging member, preferably there is continuous transport of charge through the entire film structure. The charge transport material is defined as a material which is capable of improving the charge injection process for one sign of charge from the migration marking material into the softenable layer and also of transporting that charge through the softenable layer. The charge transport material can be either a hole transport material (transports positive charges) or an electron transport material (transports negative charges). The charge used to sensitize the migration imaging member during preparation of the master can be of either polarity. Charge transporting materials are well known in the art. Typical charge transporting materials include the following: Diamine transport molecules of the type described in 4,304,829; 4,233,384; 4,115,116; 4,299,897 and 4,081,274. Typical diamine transport molecules include N,N′-diphenyl-N,N′-bis(3″-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine, N,N′-diphenyl-N,N′-bis(4-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine, N,N′-diphenyl-N,N′-bis(2-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine, N,N′-diphenyl-N,N′-bis(3-ethylphenyl)-(1,1′-biphenyl)-4,4′-diamine, N,N′-diphenyl-N,N′-bis(4-ethylphenyl)-(1,1′-biphenyl)-4,4′-diamine, N,N′-diphenyl-N,N′-bis(4-n-butylphenyl)-(1,1′-biphenyl)-4,4′-diamine, N,N′-diphenyl-N,N′-bis(3-chlorophenyl)-[1,1′-biphenyl]-4,4′-diamine, N,N′-diphenyl-N,N′-bis(4-chlorophenyl)-[1,1′-biphenyl]-4,4′-diamine, N,N′-diphenyl-N,N′-bis(phenylmethyl)-[1,1′-biphenyl]-4,4′-diamine, N,N,N′,N′-tetraphenyl-[2,2′-dimethyl-1,1′-biphenyl]-4,4′-diamine, N,N,N′,N′-tetra-(4-methylphenyl)-[2,2′-dimethyl-1,1′-biphenyl]-4,4′-diamine, N,N′-diphenyl-N,N′-bis(4-methylphenyl)-[2,2′-dimethyl-1,1′-biphenyl]-4,4′-diamine, N,N′-diphenyl-N,N′-bis(2-methylphenyl)-[2,2′-dimethyl-1,1′-biphenyl]-4,4′-diamine, N,N′-diphenyl-N,N′-bis(3-methylphenyl)-[2,2′-dimethyl-1,1′-biphenyl]-4,4′-diamine, N,N′-diphenyl-N,N′-bis(3-methylphenyl)-pyrenyl-1,6-diamine, and the like. Pyrazoline transport molecules as disclosed in US-A-4,315,982; 4,278,746 and 3,837,851. Typical pyrazoline transport molecules include 1-[lepidyl-(2)]-3-(p-diethylaminophenyl)-5-(p-diethylaminophenyl)pyrazoline, 1-[quinolyl-(2)]-3-(p-diethylaminophenyl)-5-(p-diethylaminophenyl)pyrazoline, 1-[pyridyl-(2)]-3-(p-diethylaminostyryl)-5-(p-diethylaminophenyl)pyrazoline, 1-[6-methoxypyridyl-(2)]-3-(p-diethylaminostyryl)-5-(p-diethylaminophenyl) pyrazoline, 1-phenyl-3-[p-dimethylaminostyryl]-5(p-dimethylaminostyryl)pyrazoline, 1-phenyl-3-[p-diethylaminostyryl]-5-(p-diethylaminostyryl)pyrazoline, and the like. Substituted fluorene charge transport molecules as described in US-A-4,245,021. Typical fluorene charge transport molecules include 9-(4′-dimethylaminobenzylidene)fluorene, 9-(4′-methoxybenzylidene)fluorene, 9-(2′,4′-dimethoxybenzylidene)fluorene, 2-nitro-9-benzylidene-fluorene, 2-nitro-9-(4′-diethylaminobenzylidene)fluorene, and the like. Oxadiazole transport molecules such as 2,5-bis(4-diethylaminophenyl)-1,3,4-oxadiazole, pyrazoline, imidazole, triazole, and the like. Other typical oxadiazole transport molecules are described, for example, in DE-B-1,058,836; 1,060,260 and 1,120,875. Hydrazone transport molecules, such as p-diethylamino benzaldehyde-(diphenylhydrazone), o-ethoxy-p-diethylaminobenzaldehyde-(diphenylhydrazone), o-methyl-p-diethylaminobenzaldehyde-(diphenylhydrazone), o-methyl-p-dimethylaminobenzaldehyde-(diphenylhydrazone), 1-naphthalenecarbaldehyde 1-methyl-1-phenylhydrazone, 1-naphthalenecarbaldehyde 1,1-phenylhydrazone, 4-methoxynaphthlene-1-carbaldeyde 1-methyl-1-phenylhydrazone, and the like. Other typical hydrazone transport molecules are described, for example in US-A-4,150,987; 4,385,106; 4,338,388 and 4,387,147. Carbazole phenyl hydrazone transport molecules such as 9-methylcarbazole-3-carbaldehyde-1,1-diphenylhydrazone, 9-ethylcarbazole-3-carbaldehyde-1-methyl-1-phenylhydrazone, 9-ethylcarbazole-3-carbaldehyde-1-ethyl-1-phenylhydrazone, 9-ethylcarbazole-3-carbaldehyde-1-ethyl-1-benzyl-1-phenylhydrazone, 9-ethylcarbazole-3-carbaldehyde-1,1-diphenylhydrazone, and the like. Other typical carbazole phenyl hydrazone transport molecules are described, for example, in US-A-4,256,821 and 4,297,426. Vinyl-aromatic polymers such as polyvinyl anthracene, polyacenaphthylene; formaldehyde condensation products with various aromatics such as condensates of formaldehyde and 3-bromopyrene; 2,4,7-trinitrofluorenone, and 3,6-dinitro-N-t-butylnaphthalimide as described, for example, in US-A-3,972,717. Oxadiazole derivatives such as 2,5-bis-(p-diethylaminophenyl)-oxadiazole-1,3,4 described in US-A-3,895,944. Tri-substituted methanes such as alkyl-bis(N,N-dialkylaminoaryl)methane, cycloalkyl-bis(N,N-dialkylaminoaryl)methane, and cycloalkenyl-bis(N,N-dialkylaminoaryl)methane as described in US-A-3,820,989. 9-Fluorenylidene methane derivatives having the formula wherein X and Y are cyano groups or alkoxycarbonyl groups; A, B, and W are electron-withdrawing groups independently selected from the group consisting of acyl, alkoxycarbonyl, nitro, alkylaminocarbonyl, and derivatives thereof; m is a number of from 0 to 2; and n is the number 0 or 1, as described in US-A-4,474,865. Typical 9-fluorenylidene methane derivatives encompassed by the above formula include (4-n-butoxycarbonyl-9-fluorenylidene)malononitrile, (4-phenethoxycarbonyl-9-fluorenylidene)malononitrile, (4-carbitoxy-9-fluorenylidene)malononitrile, (4-n-butoxycarbonyl-2,7-dinitro-9-fluorenylidene)malonate, and the like. Other charge transport materials include poly-1-vinylpyrene, poly-9-vinylanthracene, poly-9-(4-pentenyl)-carbazole, poly-9-(5-hexyl)-carbazole, polymethylene pyrene, poly-1-(pyrenyl)-butadiene, polymers such as alkyl, nitro, amino, halogen, and hydroxy substitute polymers such as poly-3-amino carbazole, 1,3-dibromo-poly-N-vinyl carbazole, 3,6-dibromo-poly-N-vinyl carbazole, and numerous other transparent organic polymeric or non-polymeric transport materials as described in US-A-3,870,516. Also suitable as charge transport materials are phthalic anhydride, tetrachlorophthalic anhydride, benzil, mellitic anhydride, S-tricyanobenzene, picryl chloride, 2,4-dinitrochlorobenzene, 2,4-dinitrobromobenzene, 4-nitrobiphenyl, 4,4-dinitrophenyl, 2,4,6-trinitroanisole, trichlorotrinitrobenzene, trinitro-O-toluene, 4,6-dichloro-1,3-dinitrobenzene, 4,6-dibromo-1,3-dinitrobenzene, P-dinitrobenzene, chloranil, bromanil, and mixtures thereof, 2,4,7-trinitro-9-fluorenone, 2,4,5,7-tetranitrofluorenone, trinitroanthracene, dinitroacridene, tetracyanopyrene, dinitroanthraquinone, polymers having aromatic or heterocyclic groups with more than one strongly electron withdrawing substituent such as nitro, sulfonate, carboxyl, cyano, or the like, including polyesters, polysiloxanes, polyamides, polyurethanes, and epoxies, as well as block, graft, or random copolymers containing the aromatic moiety, and the like, as well as mixtures thereof, as described in US-A-4,081,274. When the charge transport molecules are combined with an insulating binder to form the softenable layer, the amount of charge transport molecule which is used can vary depending upon the particular charge transport material and its compatibility (e.g. solubility) in the continuous insulating film forming binder phase of the softenable matrix layer and the like. Satisfactory results have been obtained using between 5 to 50 percent by weight charge transport molecule based on the total weight of the softenable layer. A particularly preferred charge transport molecule is one having the general formula wherein X, Y and Z are selected from the group consisting of hydrogen, an alkyl group having from 1 to about 20 carbon atoms and chlorine, and at least one of X, Y and Z is independently selected to be an alkyl group having from 1 to 20 carbon atoms or chlorine. If Y and Z are hydrogen, the compound can be named N,N′-diphenyl-N,N′-bis(alkylphenyl)-[1,1′-biphenyl)-4,4′-diamine wherein the alkyl is, for example, methyl, ethyl, propyl, n-butyl, or the like, or the compound can be N,N′-diphenyl-N,N′-bis(chlorophenyl)-[1,1′-biphenyl]-4,4′-diamine. Excellent results can be obtained when the softenable layer contains between 8 to 40 percent by weight of these diamine compounds based on the total weight of the softenable layer. Optimum results are achieved when the softenable layer contains between about 16 percent to about 32 percent by weight of N,N′-diphenyl-N,N′-bis(3″-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine based on the total weight of the softenable layer. The charge transport material is present in the softenable material in an effective amount, generally from 5 to 50 percent by weight, and preferably from 8 to 40 percent by weight. Alternatively, the softenable layer can employ the charge transport material as the softenable material if the charge transport material possesses the necessary film-forming characteristics and otherwise functions as a softenable material. The charge transport material can be incorporated into the softenable layer by any suitable technique. For example, it can be mixed with the softenable layer components by dissolution in a common solvent. If desired, a mixture of solvents for the charge transport material and the softenable layer material can be employed to facilitate mixing and coating. The charge transport molecule and softenable layer mixture can be applied to the substrate by any conventional coating process. Typical coating processes include draw bar coating, spray coating, extrusion, dip coating, gravure roll coating, wire-wound rod coating, air knife coating, and the like. The optional adhesive layer can include any suitable adhesive material. Typical adhesive materials include copolymers of styrene and an acrylate, polyester resin such as DuPont 49000 (available from E.I. duPont & de Nemours Company), copolymer of acrylonitrile and vinylidene chloride, polyvinyl acetate, polyvinyl butyral and the like and mixtures thereof. The adhesive layer can have a thickness of from 0.05 to 1 micrometer. When an adhesive layer is employed, it preferably forms a uniform and continuous layer having a thickness of about 0.5 micrometer or less to ensure satisfactory discharge during the xeroprinting process. It can also optionally include charge transport molecules. The optional charge transport layer can comprise any suitable film forming binder material. Typical film forming binder materials include styrene acrylate copolymers, polycarbonates, co-polycarbonates, polyesters, co-polyesters, polyurethanes, polyvinyl acetate, polyvinyl butyral, polystyrenes, alkyd substituted polystyrenes, styrene-olefin copolymers, styrene-co-n-hexylmethacrylate, a custom synthesized 80/20 mole percent copolymer of styrene and hexylmethacrylate having an intrinsic viscosity of 0.179 dl/gm; other copolymers of styrene and hexylmethacrylate, styrene-vinyltoluene copolymers, polyalpha-methylstyrene, mixtures thereof, and copolymers thereof. The above group of materials is not intended to be limiting, but merely illustrative of materials suitable as film forming binder materials in the optional charge transport layer. The film forming binder material typically is substantially electrically insulating and does not adversely chemically react during the xeroprinting master making and xeroprinting steps of the present invention. Although the optional charge transport layer has been described as coated on a substrate, in some embodiments, the charge transport layer itself can have sufficient strength and integrity to be substantially self supporting and can, if desired, be brought into contact with a suitable conductive substrate during the imaging process. As is well known, a uniform deposit of electrostatic charge of suitable polarity can be substituted for a conductive layer. Alternatively, a uniform deposit of electrostatic charge of suitable polarity on the exposed surface of the charge transport spacing layer can be substituted for a conductive layer to facilitate the application of electrical migration forces to the migration layer. This technique of "double charging" is well known. The charge transport layer is of an effective thickness, generally from 1 to 25 micrometers, and preferably from 2 to 20 micrometers. Charge transport molecules suitable for the charge transport layer are described in detail herein. The specific charge transport molecule utilized in the charge transport layer of any given master can be identical to or different from the charge transport molecule employed in the adjacent softenable layer. Similarly, the concentration of the charge transport molecule utilized in the charge transport spacing layer of any given master can be identical to or different from the concentration of charge transport molecule employed in the adjacent softenable layer. When the charge transport material and film forming binder are combined to form the charge transport spacing layer, the amount of charge transport material used can vary depending upon the particular charge transport material and its compatibility (e.g. solubility) in the continuous insulating film forming binder. Satisfactory results have been obtained using between 5 and 50 percent based on the total weight of the optional charge transport spacing layer, although the amount can be outside this range. The charge transport material can be incorporated into the charge transport layer by similar techniques to those employed for the softenable layer. The optional charge blocking layer can be of various suitable materials, including aluminum oxide, polyvinyl butyral, silane and the like, as well as mixtures thereof. This layer, which is generally applied by known coating techniques, is of an effective thickness, generally from 0.05 to 0.5 micrometer, and preferably from 0.05 to 0.1 micrometer. Typical coating processes include draw bar coating, spray coating, extrusion, dip coating, gravure roll coating, wire-wound rod coating, air knife coating and the like. The optional coating layer can be substantially electrically insulating, or have any other suitable properties. The coating preferably is substantially transparent, at least in the spectral region where electromagnetic radiation is used for imagewise exposure step in the master making process and for the uniform exposure step in the xeroprinting process. The coating layer is continuous and preferably of a thickness up to about 1 to 2 micrometers. More preferably, the coating has a thickness of between about 0.1 micrometer and about 0.5 micrometer to minimize residual charge buildup. Coating layers greater than about 1 to 2 micrometers thick can also be used. Typical coating materials include acrylic-styrene copolymers, methacrylate polymers, methacrylate copolymers, styrene-butylmethacrylate copolymers, butylmethacrylate resins, vinylchloride copolymers, fluorinated homo or copolymers, high molecular weight polyvinyl acetate, organosilicon polymers and copolymers, polyesters, polycarbonates, polyamides, polyvinyl toluene and the like. The coating layer generally protects the softenable layer to provide greater resistance to the adverse effects of abrasion during handling, master making, and xeroprinting. The coating layer preferably adheres strongly to the softenable layer to minimize damage. The coating layer can also have abhesive properties at its outer surface which provide improved resistance to toner filming during toning, transfer, and/or cleaning. The abhesive properties can be inherent in the overcoating layer or can be imparted to the coating layer by incorporation of another layer or component of abhesive material. These abhesive materials should not degrade the film-forming components of the coating and preferably have a surface energy of less than about 20 ergs/cm². Typical abhesive materials include fatty acids, salts and esters, fluorocarbons, silicones, and the like. The coatings can be applied by any suitable technique such as draw bar, spray, dip, melt, extrusion or gravure coating. It will be appreciated that these overcoating layers protect the xeroprinting master before imaging, during imaging, after the members have been imaged, and during xeroprinting. If an optional coating layer is used on top of the softenable layer to improve abrasion resistance, and if solvent softening is employed to effect migration of the migration marking material through the softenable material, the coating layer should be permeable to the vapor of the solvent used and additional vapor treatment time should be allowed so that the solvent vapor can soften the softenable layer sufficiently to allow the light-exposed migration marking material to migrate towards the substrate in image configuration. Solvent permeability is unnecessary for a coating layer if heat is employed to soften the softenable layer sufficiently to allow the exposed migration marking material to migrate towards the substrate in image configuration. Further information concerning the structure, materials, and preparation of migration imaging members is disclosed in US-A-3,975,195; 3,909,262; 4,536,457; 4,536,458; 4,013,462; 4,883,731; 4,853,307; 4,880,715; and P.S. Vincett, G.J. Kovacs, M.C. Tam, A.L. Pundsack, and P.H Soden, Migration Imaging Mechanisms, Exploitation, and Future Prospects of Unique Photographic Technologies, XDM and AMEN , Journal of Imaging Science 30 (4) July/August, pp. 183-191 (1986). The migration imaging member is then imaged and developed to prepare a xeroprinting master for use in the process of the present invention. The process of preparing the master is illustrated schematically in Figures 2 through 4, and the process of xeroprinting with the master is illustrated schematically in Figures 5 through 8. Figures 2 through 8 illustrate schematically a migration imaging member comprising a conductive substrate 22 that is connected to a reference potential such as ground, a softenable layer 24 comprising softenable material 25, migration marking material 26, and charge transport material 27. To prepare a xeroprinting master, as shown in Figure 2, the member is uniformly charged in the dark to either polarity (negative charging is illustrated in Figure 2) by a charging means 29 such as a corona charging apparatus. Alternatively, the member can comprise an electrically insulating substrate instead of a conductive substrate, and can be charged by electrostatically charging both sides of the member to surface potentials of opposite polarities. Subsequently, as illustrated schematically in Figure 3, the charged member is exposed imagewise to activating radiation 31, such as light, prior to substantial dark decay of the uniform charge on the member surface, thereby forming an electrostatic latent image thereon. Preferably, exposure to activating radiation is prior to the time when the uniform charge has undergone dark decay to a value of less than 50 percent of the initial charge, although exposure can be subsequent to this time. As illustrated schematically in Figure 4, subsequent to imagewise exposure to form a latent image, the imaging member is developed by causing the softenable material to soften by any suitable means (in Figure 4, by uniform application of heat energy 33 to the member). The heat development temperature and time depend upon factors such as how the heat energy is applied (e.g. conduction, radiation, convection, and the like), the melt viscosity of the softenable layer, thickness of the softenable layer, the amount of heat energy, and the like. For example, at a temperature of 110°C to about 130°C, heat need be applied for only a few seconds. For lower temperatures, more heating time can be required. When the heat is applied, the softenable material 25 decreases in viscosity, thereby decreasing its resistance to migration of the marking material 26 through the softenable layer 24. In the exposed areas 35 of the imaging member, the migration marking material 26 gains a substantial net charge which, upon softening of the softenable material 25, causes the exposed marking material to migrate in image configuration towards the substrate 22 and disperse in the softenable layer 24, resulting in a D min area. The unexposed migration marking particles 26 in the unexposed areas 37 of the imaging member remain essentially neutral and uncharged. Thus, in the absence of migration force, the unexposed migration marking particles remain substantially in their original position in softenable layer 24, resulting in a D max area. Thus, as illustrated in Figure 4, the developed image is an optically sign-retaining visible image of an original (if a conventional light-lens exposure system is utilized). Exposure can also be by means other than light-lens systems, including raster output scanning devices such as laser writers. The developed imaging member can then be employed as a xeroprinting master. If desired, solvent vapor development can be substituted for heat development. Vapor development of migration imaging members is well known. Generally, if solvent vapor softening is utilized, the solvent vapor exposure time depends upon factors such as the solubility of softenable layer in the solvent, the type of solvent vapor, the ambient temperature, the concentration of the solvent vapors, and the like. The application of either heat, or solvent vapors, or combinations thereof, or any other suitable means should be sufficient to decrease the resistance of the softenable material of softenable layer 24 to allow migration of the migration marking material 26 through softenable layer 24 in imagewise configuration. With heat development, satisfactory results can be achieved by heating the imaging member to a temperature of 100°C to 130°C for only a few seconds when the uncoated softenable layer contains a custom synthesized 80/20 mole percent copolymer of styrene and hexylmethacrylate having an intrinsic viscosity of 0.179 dl/gm and N,N′-diphenyl-N,N′-bis(3″-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine. The test for a satisfactory combination of time and temperature is to maximize optical contrast density and electrostatic contrast potential for xeroprinting. With vapor development, satisfactory results can be achieved by exposing the imaging member to the vapor of toluene for between 4 and 60 seconds at a solvent vapor partial pressure of between 650 Pa to 4000 Pa (5 and 30 millimeters of mercury) when the uncoated softenable layer contains a custom synthesized 80/20 mole percent copolymer of styrene and hexylmethacrylate having an intrinsic viscosity of 0.179 dl/gm and N,N′-diphenyl-N,N′-bis(3″-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine. The imaged xeroprinting master shown in Figure 4 is transmitting to visible light in the exposed region because of the depthwise migration and dispersion of the migration marking material in the exposed region. The D min obtained in the exposed region generally is slightly higher than the optical density of transparent substrates underlying the softenable layer. The D max in the unexposed region generally is essentially the same as the original unprocessed imaging member because the positions of migration marking particles in the unexposed regions remain essentially unchanged. Thus, optically sign-retained visible images with high optical contrast density in the region of 0.9 to 1.2 can be achieved for xeroprinting masters. In addition, exceptional resolution, such as 228 line pairs per millimeter, can be achieved on the xeroprinting masters. The imaging member illustrated in Figures 2 through 8 is shown without any optional layers such as those illustrated in Figure 1. If desired, alternative imaging member embodiments, such as those employing any or all of the optional layers illustrated in Figure 1, can also be employed. The prepared xeroprinting master as illustrated in Figure 4 can thereafter be utilized in a xeroprinting process. The use of the xeroprinting master in a xeroprinting process is illustrated schematically in Figures 5 through 8. As illustrated schematically in Figure 5, the xeroprinting master is uniformly charged by a charging means 39 such as a corona charging device. Charging is to an effective magnitude; generally, positive or negative voltages of from 50 to 1200 volts are suitable for the process of the present invention, although other values can be employed. The polarity of the charge applied depends on the nature of the charge transport material present in the master, and is opposite in polarity to the type of charge of which the charge transport material is capable of transporting; thus, when the charge transport material in the softenable layer is capable of transporting holes (positive charges), the master is charged negatively, and when the charge transport material in the softenable layer is capable of transporting electrons (negative charges), the master is charged positively. As illustrated in Figure 5, charge transport material 27 is capable of transporting holes; accordingly, the master is uniformly negatively charged. The charged xeroprinting master is then uniformly flash exposed to activating radiation 41 such as light energy as illustrated schematically in Figure 6 to form an electrostatic latent image. The activating electromagnetic radiation used for the uniform exposure step should be in the spectral region where the migration marking particles photogenerate charge carriers. Light in the spectral region of 300 to 800 nanometers is generally suitable for the process of the present invention, although the wavelength of the light employed for exposure can be outside of this range, and is selected according to the spectral response of the specific migration marking particles selected. The exposure energy should be such that the desired and/or optimal electrostatic contrast potential is obtained, and preferably is from 10 to 100,000 ergs per square centimeter, and more preferably at least 100 ergs per square centimeter. Because of the differences in the relative positions (or particle distribution) of the migration marking material in the D max and D min areas of the softenable layer 24, the D max and D min areas exhibit different photodischarge characteristics and optical absorption characteristics. Preferably, the potential difference between the migrated areas of the master and the unmigrated areas of the master is from 50 to 1200 volts, although this value can be outside of the specified range. Contrast potential efficiency, determined by dividing the potential difference between the migrated areas of the master and the unmigrated areas of the master by the initial voltage to which the master was charged prior to flood exposure and multiplying by 100 to obtain a percentage figure, can range from 20 to 99 percent for the process of the present invention, preferably is from 50 to 99 percent, more preferably from 60 to 99 percent, and even more preferably is from 90 to 99 percent. In known xeroprinting systems, such as those disclosed in US-A-4,853,307, 4,880,715, and 4,883,731, a xeroprinting master is prepared as set forth in Figures 1 through 4 and then is charged to a polarity the same as the polarity of the type of charge of which the charge transport material is capable of transporting. ln contrast, the process of the present invention entails charging the xeroprinting master to a polarity opposite to the polarity of the type of charge of which the charge transport material is capable of transporting. It has been found that charging the master to a polarity opposite to the polarity of the type of charge of which the charge transport material is capable of transporting greatly enhances the contrast potential or contrast voltage obtained when the charged xeroprinting master is flood exposed to form a latent image. For example, a xeroprinting master with a softenable layer of about 6 micrometers thick generally can attain a maximum contrast potential or contrast voltage of from about 45 to about 50 percent of the initial surface voltage applied to the master when it has been charged to a polarity the same as the polarity of the type of charge of which the charge transport material is capable of transporting. In contrast, the same xeroprinting master, when charged to a polarity opposite to the polarity of the type of charge of which the charge transport material is capable of transporting, generally can attain a contrast potential or contrast voltage in excess of 90 percent of the initial surface voltage applied to the master. The electrostatic contrast potential needed for good quality prints depends on the specific developer used and the development speed required for a particular application. Generally, while a contrast potential in the range of from 50 to 600 volts is often adequate for a liquid development system, a contrast potential in the range of from 200 to 800 volts is frequently desired for a dry toner development system. The electrostatic contrast potential of the electrostatic image of the present invention is also affected by the combined thickness of the imaging softenable layer and any optional charge transport layer present. For a given xeroprinting master, however, contrast potential can be greatly enhanced by charging the master according to the process of the present invention. For example, a xeroprinting master having a thickness of about 9 micrometers for the softenable layer and charged to an initial surface voltage of about 900 volts, generally can attain a contrast voltage of about 400 volts when charged to a polarity the same as the polarity of the type of charge of which the charge transport material is capable of transporting. In contrast, a xeroprinting master having the same thickness of about 9 micrometers for the softenable layer and charged to the same initial surface voltage of about 900 volts generally can attain a contrast voltage of about 800 volts when charged to a polarity opposite to the polarity of the type of charge of which the charge transport material is capable of transporting. A contrast potential in this range is suitable for both dry and liquid development processes, even at high printing speeds of at least 375 mm per second. While not being limited by theory, it is believed that with the known xeroprinting processes, wherein the xeroprinting master was charged to a polarity the same as the polarity of the type of charge of which the charge transport material was capable of transporting, the D max areas (areas where the migration marking material has not migrated toward the substrate) of the master photodischarge rapidly and nearly completely upon flood exposure of the charged master. It is believed that this is a result of the charge transport material being capable of transporting efficently the photogenerated charge carriers to the conductive substrate when the master is charged to a polarity the same as the polarity of the type of charge of which the charge transport material was capable of transporting. The D min areas (areas where the migration marking material has migrated toward the substrate) also photodischarge upon flood exposure of the charged master, but at a much lower rate. It is believed that this is because the migration and dispersion of the migration marking material in D min areas has degraded the photosensitivity in the D min areas of the master, compared with the photosensitivity of the D max areas where the migration marking material remains substantially in its initial configuration. It it believed that particle to particle hopping transport causes photodischarge in the D min areas. Thus, uniform charging and subsequent uniform illumination of the xeroprinting master charged to the same polarity as the polarity of the type of charge of which the charge transport material is capable of transporting causes photodischarge to occur predominantly in the D max region of the image. Charge is substantially retained in the regions containing the migrated marking particles and is substantially dissipated in the regions containing the unmigrated particles. The contrast voltage of the electrostatic image is the difference between the photodischarged voltage of the D max areas and the photodischarged voltage of the D min areas. As the flood exposure energy increases, the contrast voltage initially increases, reaches a maximum, and then decreases in this situation. In contrast, in the process of the present invention wherein the xeromaster is charged to a polarity opposite to the polarity of the type of charge of which the charge transport material is capable of transporting, it is believed that the D max areas of the master behave almost like an electrical insulator, exhibiting very little photodischarge even when the intensity of the flood exposure light is greatly increased. This is because the photogenerated charge carriers cannot be transported to the conductive substrate when the master is charged to a polarity opposite to the polarity of the type of charge of which the charge transport material is capable of transporting. As a result, the photogenerated charge carriers become trapped in the unmigrated marking particles. The D min areas behave as a "spoiled" photoreceptor which exhibits low photosensitivity, but which can still be photodischarged almost completely if intense light is employed for flood exposure. It is believed that in the D min areas, particle to particle hopping transport allows full discharge if intense light is employed for flood exposure. Subsequently, as illustrated in Figure 7, the electrostatic latent image formed by flood exposing the charged master to light is then developed with toner particles 43 to form a toner image corresponding to the electrostatic latent image overlying the D max area. In Figure 7, the toner particles 43 carry a positive electrostatic charge and are attracted to the oppositely charged portions overlying the D max area (unmigrated particles). However, if desired, the toner can be deposited in the discharged areas by employing toner particles having the same polarity as the charged areas (negative in the embodiment shown in Figure 7). The toner particles 43 will then be repelled by the charges overlying the D max area and deposit in the discharged areas (D min area). Electrically-biased development electrodes can also be employed, if desired, to direct toner particles to either the charged or discharged areas of the imaging surface. The developing (toning) step is identical to that conventionally used in electrophotographic imaging. Any suitable conventional electrophotographic dry or liquid developer containing electrostatically attractable marking particles can be employed to develop the electrostatic latent image on the xeroprinting master.Typical dry toners have a particle size of between 6 and 20 µm. Typical liquid toners have a particle size of between 0.1 and 6 µm. The size of toner particles generally affects the resolution of prints. For applications demanding very high resolution, such as in color proofing and printing, liquid toners are generally preferred because their much smaller toner particle size gives better resolution of fine half-tone dots and produce four color images without undue thickness in densely toned areas. Conventional electrophotographic development techniques can be utilized to deposit the toner particles on the imaging surface of the xeroprinting master. This invention is suitable for development with dry two-component developers. Two-component developers comprise toner particles and carrier particles. Typical toner particles can be of any composition suitable for development of electrostatic latent images, such as those comprising a resin and a colorant. Typical toner resins include polyesters, polyamides, epoxies, polyurethanes, diolefins, vinyl resins and polymeric esterification products of a dicarboxylic acid and a diol comprising a diphenol. Examples of vinyl monomers include styrene, p-chlorostyrene, vinyl naphthalene, unsaturated mono-olefins such as ethylene, propylene, butylene, isobutylene and the like; vinyl halides such as vinyl chloride, vinyl bromide, vinyl fluoride, vinyl acetate, vinyl propionate, vinyl benzoate, and vinyl butyrate; vinyl esters such as esters of monocarboxylic acids, including methyl acrylate, ethyl acrylate, n-butyl acrylate, isobutyl acrylate, dodecyl acrylate, n-octyl acrylate, 2-chloroethyl acrylate, phenyl acrylate, methylalpha-chloroacrylate, methyl methacrylate, ethyl methacrylate, butyl methacrylate, and the like; acrylonitrile, methacrylonitrile, acrylamide, vinyl ethers, including vinyl methyl ether, vinyl isobutyl ether, and vinyl ethyl ether; vinyl ketones such as vinyl methyl ketone, vinyl hexyl ketone, and methyl isopropenyl ketone; N-vinyl indole and N-vinyl pyrrolidene; styrene butadienes; mixtures of these monomers; and the like. The resins are generally present in an amount of from about 30 to about 99 percent by weight of the toner composition, although they can be present in greater or lesser amounts. Any suitable pigments or dyes or mixture thereof can be employed in the toner particles. Typical pigments or dyes include carbon black, nigrosine dye, aniline blue, magnetites, and mixtures thereof, with carbon black being a preferred colorant. The pigment is preferably present in an amount sufficient to render the toner composition highly colored to permit the formation of a clearly visible image on a record member. Generally, the pigment particles are present in amounts of from 1 to 20 percent by weight based on the total weight of the toner composition; however, lesser or greater amounts of pigment particles can be present. Other colored toner pigments include red, green, blue, brown, magenta, cyan, and yellow particles, as well as mixtures thereof. Illustrative examples of suitable magenta pigments include 2,9-dimethyl-substituted quinacridone and anthraquinone dye, identified in the Color Index as CI 60710, CI Dispersed Red 15, a diazo dye identified in the Color Index as CI 26050, CI Solvent Red 19, and the like. Illustrative examples of suitable cyan pigments include copper tetra-4-(octadecyl sulfonamido) phthalocyanine, X-copper phthalocyanine pigment, listed in the color index as CI 74160, CI Pigment Blue, and Anthradanthrene Blue, identified in the Color Index as CI 69810, Special Blue X-2137, and the like. Illustrative examples of yellow pigments that can be selected include diarylide yellow 3,3-dichlorobenzidene acetoacetanilides, a monoazo pigment identified in the Color Index as CI 12700, CI Solvent Yellow 16, a nitrophenyl amine sulfonamide identified in the Color Index as Foron Yellow SE/GLN, CI Dispersed Yellow 33, 2,5-dimethoxy-4-sulfonanilide phenylazo-4′-chloro-2,5-dimethoxy aceto-acetanilide, Permanent Yellow FGL, and the like. These color pigments are generally present in an amount of from about 15 weight percent to about 20.5 weight percent based on the weight of the toner resin particles, although lesser or greater amounts can be present. When the pigment particles are magnetites, which comprise a mixture of iron oxides (Fe₃O₄) such as those commercially available as Mapico Black, these pigments are present in the toner composition in an amount of from 10 to 70 percent by weight, and preferably in an amount of from 20 to 50 percent by weight, although they can be present in greater or lesser amounts. The toner compositions can be prepared by any suitable method. For example, the components of the dry toner particles can be mixed in a ball mill, to which steel beads for agitation are added in an amount of approximately five times the weight of the toner. The ball mill can be operated at about 37 m per minute for about 30 minutes, after which time the steel beads are removed. Dry toner particles for two-component developers generally have an average particle size between 6 and 20 micrometers. Any suitable external additives can also be utilized with the dry toner particles. The amounts of external additives are measured in terms of percentage by weight of the toner composition, but are not themselves included when calculating the percentage composition of the toner. For example, a toner composition containing a resin, a pigment, and an external additive can comprise 80 percent by weight resin and 20 percent by weight pigment; the amount of external additive present is reported in terms of its percent by weight of the combined resin and pigment. External additives can include any additives suitable for use in electrostatographic toners, including straight silica, colloidal silica (e.g. Aerosil R972®, available from Degussa, Inc.), ferric oxide, unilin, polypropylene waxes, polymethylmethacrylate, zinc stearate, chromium oxide, aluminum oxide, stearic acid, polyvinylidene flouride (e.g. Kynar®, available from Pennwalt Chemicals Corporation), and the like. External additives can be present in any suitable amount. Any suitable carrier particles can be employed with the toner particles. Typical carrier particles include granular zircon, steel, nickel, iron ferrites, and the like. Other typical carrier particles include nickel berry carriers as disclosed in US-A-3,847,604. These carriers comprise nodular carrier beads of nickel characterized by surfaces of recurring recesses and protrusions that provide the particles with a relatively large external area. The diameters of the carrier particles can vary, but are generally from 50 to 1,000 µm, thus allowing the particles to possess sufficient density and inertia to avoid adherence to the electrostatic images during the development process. Carrier particles can have coated surfaces. Typical coating materials include polymers and terpolymers, including, for example, fluoropolymers such as polyvinylidene fluorides as disclosed in US-A-3,526,533; 3,849,186 and 3,942,979. The toner may be present, for example, in the two-component developer in an amount equal to 1 to 5 percent by weight of the carrier, and preferably is equal to about 3 percent by weight of the carrier. Typical dry toners are disclosed, for example, in US-A-2,788,288; 3,079,342 and Reissue 25,136. If desired, development can be effected with liquid developers. Liquid developers are disclosed, for example, in US-A-2,890,174 and 2,899,335. Liquid developers can comprise aqueous-based or oil-based inks, and include both inks containing a water-or oil-soluble dye substance and pigmented inks. Typical dye substances are Methylene Blue, commercially available from Eastman Kodak Company, Brilliant Yellow, commercially available from the Harlaco Chemical Company, potassium permanganate, ferric chloride and Methylene Violet, Rose Bengal and Quinoline Yellow, the latter three available from Allied Chemical Company, and the like. Typical pigments are carbon black, graphite, lamp black, bone black, charcoal, titanium dioxide, white lead, zinc oxide, zinc sulfide, iron oxide, chromium oxide, lead chromate, zinc chromate, cadmium yellow, cadmium red, red lead, antimony dioxide, magnesium silicate, calcium carbonate, calcium silicate, phthalocyanines, benzidines, naphthols, toluidines, and the like. The liquid developer composition can comprise a finely divided opaque powder, a high resistance liquid, and an ingredient to prevent agglomeration. Typical high resistance liquids include such organic dielectric liquids as paraffinic hydrocarbons such as the Isopar® and Norpar® family, carbon tetrachloride, kerosene, benzene, trichloroethylene, and the like. Other liquid developer components or additives include vinyl resins, such as carboxy vinyl polymers, polyvinylpyrrolidones, methylvinylether maleic anhydride interpolymers, polyvinyl alcohols, cellulosics such as sodium carboxy-ethylcellulose, hydroxypropylmethyl cellulose, hydroxyethyl cellulose, methyl cellulose, cellulose derivatives such as esters and ethers thereof, alkali soluble proteins, casein, gelatin, and acrylate salts such as ammonium polyacrylate, sodium polyacrylate, and the like. Any suitable conventional electrophotographic development technique can be utilized to deposit toner particles on the electrostatic latent image on the imaging surface of the xeroprinting master. Well known electrophotographic development techniques include magnetic brush development, cascade development, powder cloud development, electrophoretic development, and the like. Magnetic brush development is more fully described, for example, in US-A-2,791,949; cascade development is more fully described, for example, in US-A-2,618,551 and 2,618,552; powder cloud development is more fully described, for example, in US-A-2,725,305; 2,918,910 and 3,015,305; and liquid development is more fully described, for example, in US-A-3,084,043. As illustrated schematically in Figure 8, the deposited toner image is subsequently transferred to a receiving member 45, such as paper, by applying an electrostatic charge to the rear surface of the receiving member by means of a charging means 47 such as a corona device. The transferred toner image is thereafter fused to the receiving member by conventional means (not shown) such as an oven fuser, a hot roll fuser, a cold pressure fuser, or the like. The deposited toner image can be transferred to a receiving member such as paper or transparency material by any suitable technique conventionally used in electrophotography, such as corona transfer, pressure transfer, adhesive transfer, bias roll transfer, and the like. Typical corona transfer entails contacting the deposited toner particles with a sheet of paper and applying an electrostatic charge on the side of the sheet opposite to the toner particles. A single wire corotron having applied thereto a potential of between about 5000 and about 8000 volts provides satisfactory transfer. After transfer, the transferred toner image can be fixed to the receiving sheet. The fixing step can be also identical to that conventionally used in electrophotographic imaging. Typical, well known electrophotographic fusing techniques include heated roll fusing, flash fusing, oven fusing, laminating, adhesive spray fixing, and the like. After the toned image is transferred, the xeroprinting master can be cleaned, if desired, to remove any residual toner and then erased by an AC corotron, or by any other suitable means. The developing, transfer, fusing, cleaning and erasure steps can be identical to that conventionally used in xerographic imaging. Since the xeroprinting master produces identical successive images in precisely the same areas, it has not been found necessary to erase the electrostatic latent image between successive images. However, if desired, the master can optionally be erased by conventional AC corona erasing techniques, which entail exposing the imaging surface to AC corona discharge to neutralize any residual charge on the master. Typical potentials applied to the corona wire of an AC corona erasing device range from 3 to 10 kilovolts. If desired, the imaging surface of the xeroprinting master can be cleaned. Any suitable cleaning step that is conventionally used in electrophotographic imaging can be employed for cleaning the xeroprinting master of this invention. Typical well known electrophotographic cleaning techniques include brush cleaning, blade cleaning, web cleaning, and the like. After transfer of the deposited toner image from the master to a receiving member, the master can, with or without erase and cleaning steps, be cycled through additional uniform charging, uniform illumination, development and transfer steps to prepare additional image receiving members. Unlike some conventional xeroprinting masters, the master utilized in the xeroprinting system of this invention can be uniformly charged to its full potential because the entire imaging surface is generally insulating (i.e. no insulating patterns on a metal conductor where fringing fields from the insulating areas repel incoming corona ions to the adjacent conductive areas). This yields electrostatic images of high contrast potential and high resolution on the master. Thus high quality prints having high contrast density and high resolution are obtained. In addition, unlike many known electronic and/or xerographic printing techniques employing a conventional photoreceptor, such as conventional laser xerography in which the imagewise exposure step must be repeated for each print, the imagewise exposure step need only be performed once to produce the xeroprinting master for this invention from which multiple prints can be produced at high speed. Thus the xeroprinting system of this invention surmounts the fundamental electronic bandwidth problem which prevents a conventional xerographic approach to very high quality, high speed electronic black-and-white or color printing. Accordingly, the combined capabilities of high photosensitivity, high quality, and high printing speed at reasonable cost make the xeroprinting system of this invention suitable for both high quality color proofing and for printing/duplicating applications. Compared with offset printing, the xeroprinting system of this invention offers the advantages of lower master costs (no need for separate lithographic intermediate and printing plates). Intermediates are needed in offset printing because the printing plates are not photosensitive enough to be imaged directly; instead, the printing plates are contact exposed to the intermediate using strong UV light, and then chemically developed. Another advantage of the present invention is that it eliminates the need of using totally different printing technologies for color proofing and printing as required by known techniques, and the end users can be reliably assured of the desired print quality before a large number of prints is made. Therefore, the xeroprinting system of this invention is also less costly than other known systems. By separating the film structure into different layers, the imaging member of the present invention allows maximum flexibility in selecting appropriate materials to maximize its mechanical, chemical, electrical, imaging, and xeroprinting properties. The xeroprinting master employed for the present invention is formed as a result of permanent structural changes in the migration marking material in the softenable layer without removal and disposal of any components from the softenable layer. Thus, because of its unique imaging characteristics, the xeroprinting master used in the xeroprinting system of this invention offers the combined advantages of simple fabrication, lower costs, high photosensitivity (laser sensitivity), dry, fast, and simple master preparation with no effluents, high quality, high resolution, and high printing speed. Therefore, applications for this xeroprinting system include various types of printing systems such as high quality color printing and proofing. If heat development is used, the master-making process of the present invention is totally dry, exceedingly simple (merely corona charging, imagewise exposure and heat development), and can be accomplished in a matter of seconds. Thus it is possible to configure a master-maker to utilize this process which can function either as a stand-alone unit or which can easily be integrated into a xeroprinting press to form a self-contained fully automated printing system suitable for use even in office environments. Because the xeroprinting master precursor member exhibits high photosensitivity and high resolution, computer-driven electronic writing techniques such as laser scanning can be advantageously used to create high resolution image (line or pictorial) on the xeroprinting master for xeroprinting. Therefore, in conjunction with its capabilities of high quality, high resolution, and high printing speed, a xeroprinting system of the present invention can deliver the full advantages of computer technology from the digital file input (text editing, composition, pagination, image manipulations, and the like) directly to the printing process to produce prints having high quality and high resolution at high speed. Specific embodiments of the invention will now be described in detail. These examples are intended to be illustrative, and the invention is not limited to the materials, conditions, or process parameters set forth in these embodiments. All parts and percentages are by weight unless otherwise indicated EXAMPLE I A xeroprinting master precursor member was prepared by dissolving about 16.8 grams of a terpolymer of styrene/ethylacrylate/acrylic acid (obtained from Desoto Company as E-335), and about 3.2 grams of N,N′-diphenyl-N,N′-bis(3″-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine in about 80.0 grams of toluene. The N,N′-diphenyl-N,N′-bis(3″-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine is a charge transport material capable of transporting positive charges (holes). The resulting solution was coated by solvent extrusion techniques onto a 300 mm wide 100 micrometer thick Mylar® polyester film (available from E.I. Du Pont de Nemours & Company) having a thin, semi-transparent aluminum coating. The deposited softenable layer was allowed to dry at about 115°C for about 2 minutes. The thickness of the dried softenable layer was about 6 micrometers. The temperature of the softenable layer was then raised to about 115°C to lower the viscosity of the exposed surface of the softenable layer to about 5 x 10³ poises in preparation for the deposition of marking material. A thin layer of particulate vitreous selenium was then applied by vacuum deposition in a vacuum chamber maintained at a pressure of about 53 mPa (4 x 10⁻⁴ Torr). The imaging member was then rapidly chilled to room temperature. A reddish monolayer of selenium particles having an average diameter of about 0.3 micrometer embedded about 0.05 to 0.1 micrometer below the exposed surface of the copolymer was formed. The resulting xeroprinting master precursor member was then uniformly negatively charged to a surface potential of about -600 volts with a corona charging device and was subsequently exposed by placing a test pattern mask comprising a silver halide color separation image in contact with the imaging member and exposing the member to light through the mask. The exposed member was thereafter developed by subjecting it to a temperature of about 115°C for about 5 seconds using a hot plate in contact with the polyester. The resulting xeroprinting master exhibited excellent image quality, resolution in excess of 228 line pairs per millimeter, and an optical contrast density of about 1.2. The optical density of the D max area was about 1.8 and that of the D min area was about 0.60. The D min was because of substantial depthwise migration of the selenium particles toward the aluminum layer in the D min regions of the image. EXAMPLE II (Comparative) Three xeroprinting masters prepared as described in Example I were uniformly positively charged and then flood exposed to light at varying illumination intensities as follows. A first xeroprinting master prepared as described in Example I was uniformly positively charged with a corona charging device to a potential of about +600 volts, followed by a brief uniform flash exposure to 400-700 nanometer activating illumination of about 40 ergs/cm². The surface potential was about +60 volts in the D max (unmigrated) region of the image and about +330 volts in the D min (migrated) region, thereby yielding an electrostatic contrast voltage of about +270 volts and a contrast voltage efficiency of about 45% of the initially applied voltage. The surface potentials of the D max areas and D min areas of the master were monitored with electrostatic voltmeters. A second xeroprinting master prepared as described in Example I was uniformly positively charged with a corona charging device to a potential of about +600 volts, followed by a brief uniform flash exposure to 400-700 nanometer activating illumination of about 20 ergs/cm². The surface potential was about +180 volts in the D max (unmigrated) region of the image and about +372 volts in the D min (migrated) region, thereby yielding an electrostatic contrast voltage of about +192 volts and a contrast voltage efficiency of about 32% of the initially applied voltage. The surface potentials of the D max areas and D min areas of the master were monitored with electrostatic voltmeters. A third xeroprinting master prepared as described in Example I was uniformly positively charged with a corona charging device to a potential of about +600 volts, followed by a brief uniform flash exposure to 400-700 nanometer activating illumination of about 80 ergs/cm². The surface potential was about +12 volts in the D max (unmigrated) region of the image and about +180 volts in the D min (migrated) region, thereby yielding an electrostatic contrast voltage of about +168 volts and a contrast voltage efficiency of about 28% of the initially applied voltage. The surface potentials of the D max areas and D min areas of the master were monitored with electrostatic voltmeters. These three processes illustrate the illumination at varying intensities for flood exposure of the xeroprinting master that is charged to a polarity the same as that of which the charge transport material is capable of transporting, in accordance with known processes. As can be seen from these results, when the master is charged to the same polarity as that of the charge of which the charge transport material is capable of transporting, varying the illumination intensity over a relatively narrow range of 20 to 80 ergs per square centimeter results in fluctuation of the contrast voltage efficiency of from 28% to 45%, with the maximum efficiency being near the middle of the range (40 ergs per square centimeter). In addition, the contrast potential efficiencies obtained for these processes are significantly lower than those obtained for the process of the present invention as illustrated in Example III, wherein contrast potentials of over 90% were obtained over a wide range of illumination intensities. Illustrated in Figure 9 is a line graph representing the photodischarged surface voltage (normalized to its initial surface potential by dividing the photodischarged surface voltage of the D min and D max areas by the initial surface potential) as a function of the flood exposure energy in ergs per square centimeter for a xeroprinting master of Example I when the xeroprinting master is charged to a polarity the same as the polarity of the type of charge of which the charge transport material is capable of transporting (+600 volts). In Figure 9, curve (a) represents the photodischarge characteristics for the D max areas of the master and curve (b) represents the photodischarge characteristics for the D min areas of the master. The contrast voltage efficency, represented by curve (c), is given by the difference between curve (a) and curve (b). The contrast voltage of the electrostatic image is the difference between the photodischarged voltage of the D max areas and the photodischarged voltage of the D min areas. As can be seen from this graph, as the flood exposure energy increases, the contrast voltage efficiency initially increases, reaches a maximum of about 45 to 50 percent, and then decreases in this situation. EXAMPLE III Three xeroprinting masters prepared as described in Example I were uniformly negatively charged and then flood exposed to light at varying illumination intensities as follows. A first xeroprinting master prepared as described in Example I was uniformly negatively charged with a corona charging device to about -600 volts, followed by a brief uniform flash exposure to 400-700 nanometer activating illumination of about 400 ergs/cm².The surface potential was about -575 volts in the D max (unmigrated) region of the image and about -30 volts in the D min (migrated) region, thereby yielding an electrostatic contrast voltage of about -545 volts and a contrast voltage efficiency of over 90% of the initially applied voltage. The surface potentials of the D max areas and D min areas of the master were monitored with electrostatic voltmeters. A second xeroprinting master prepared as described in Example I was uniformly negatively charged with a corona charging device to about -600 volts, followed by a brief uniform flash exposure to 400-700 nanometer activating illumination of about 800 ergs/cm². The surface potential was about -576 volts in the D max (unmigrated) region of the image and about -18 volts in the D min (migrated) region, thereby yielding an electrostatic contrast voltage of about -558 volts and a contrast voltage efficiency of about 93% of the initially applied voltage. The surface potentials of the D max areas and D min areas of the master were monitored with electrostatic voltmeters. A third xeroprinting master prepared as described in Example I was uniformly negatively charged with a corona charging device to about -600 volts followed by a brief uniform flash exposure to 400-700 nanometer activating illumination of about 3000 ergs/cm². The surface potential was about -575 volts in the D max (unmigrated) region of the image and about -7 volts in the D min (migrated) region, thereby yielding an electrostatic contrast voltage of about -568 volts and a contrast voltage efficiency of over 94% of the initially applied voltage. The surface potentials of the D max areas and D min areas of the master were monitored with electrostatic voltmeters. These three processes illustrate the wide range of illumination intensities that can be employed for flood exposure of the xeroprinting master that is charged to a polarity opposite to that of which the charge transport material is capable of transporting, in accordance with the process of the present invention, without degrading contrast potential. In addition, the contrast voltage efficiencies obtained with the process of the present invention greatly exceeded those obtained when the master is charged to a polarity the same as that of which the charge transport material is capable of transporting, as can be seen by comparing these results with those of Example II. Illustrated in Figure 10 is a line graph representing the photodischarged surface voltage (normalized to its initial surface potential by dividing the photodischarged surface voltage of the D min and D max areas by the initial surface potential) as a function of the flood exposure energy in ergs per square centimeter for the xeroprinting master of Example I when the xeroprinting master is charged to the same initial surface voltage but to a polarity opposite to the polarity of the type of charge of which the charge transport material is capable of transporting (-600 volts). In Figure 10, curve (a) represents the photodischarge characteristics for the D max areas of the master and curve (b) represents the photodischarge characteristics for the D min areas of the master. The contrast voltage efficency, represented by curve (c), is given by the difference between curve (a) and curve (b). Compared with Figure 9, it can be seen that when the xeroprinting master is uniformly charged to a polarity opposite to the polarity of the type of charge of which the charge transport material is capable of transporting, contrast voltage efficiency in excess of 90 percent of the initial surface voltage is achieved. Furthermore, much broader process latitude for the flood exposure step is obtained while maintaining optimal contrast voltage. EXAMPLE IV A xeroprinting master prepared as described in Example I was incorporated into the Xeroprinter® 100 available from Fuji Xerox Company, Ltd. by replacing the original zinc oxide photoreceptor in the machine with the xeroprinting master. In addition, the incandescent flood exposure lamp in the machine was replaced with an 8 watt green fluorescent photoreceptor erase lamp as the flood exposure light source. The master was uniformly negatively charged to a potential of about -600 volts and then flood exposed to form an electrostatic latent image on the master surface. Subsequently, the latent image was developed with the black dry toner supplied with the Xeroprinter® 100 machine and the developed image was transferred and fused to Xerox® 4024 plain paper 275 x 425 mm size). The process was repeated at a printing speed of 50 copies per minute (about 375 mm per second), and was also repeated with the cyan and magenta dry toners supplied with the Xeroprinter® 100. The images thus formed exhibited high image contrast, clear background, and an excellent halftone dot range of about 6 to about 95 percent. Over 5000 prints were generated with the master with no apparent damage to the master and no degradation of image quality. EXAMPLE V A xeroprinting master prepared as described in Example I was uniformly negatively charged with a corona charging device to about -600 volts, followed by a brief uniform flash exposure to form an electrostatic latent image on the master surface. Subsequently, the latent image was developed with a liquid developer to form a deposited toner image. The liquid developer contained about 2 percent by weight of carbon black pigmented polyethylene acrylic acid resin and about 98% by weight of Isopar® L (isoparaffinic hydrocarbon). The deposited toner image was transferred and fused to a sheet of paper to yield a very high quality xeroprint. EXAMPLE VI A xeroprinting master precursor member was prepared as described in Example I. The xeroprinting master precursor member was then uniformly negatively charged to a surface potential of about -600 volts with a corona charging device and was subsequently digitally exposed in a commercial color scanner (Crosfield Magnascan® 645M) which uses an argon ion laser as the exposure source. The exposed member was thereafter developed by subjecting it a temperature of about 115°C for about 5 seconds using a hot plate in contact with the polyester. The resulting xeroprinting master exhibited excellent image quality exhibiting an optical contrast density of about 1.2. Optical density of the D max area was about 1.8 and that of the D min area was about 0.60. The D min area was because of substantial depthwise migration of the selenium particles toward the aluminum layer in the D min regions of the image. The xeroprinting master was then uniformly negatively charged with a corona charging device to about -600 volts, followed by a brief uniform flash exposure to form an electrostatic latent image on the master surface. Subsequently, the latent image was developed with a liquid developer to form a deposited toner image. The liquid developer contained about 2 percent by weight of carbon black pigmented polyethylene acrylic acid resin and about 98% by weight of Isopar® L. The deposited toner image was transferred and fused to a sheet of paper to yield a very high quality xeroprint. EXAMPLE VII Additional xeroprinting master precursor members were prepared by dissolving about 15.2 grams of a custom-synthesized 80/20 mole percent copolymer of styrene and co-n-hexylmethacrylate and about 4.8 grams of N,N′-diphenyl-N,N′-bis(3"-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine in about 80 grams of toluene. The resulting solution was coated by solvent extrusion techniques onto a 300 mm wide 100 micrometer thick Mylar® polyester film (available from E I. DuPont de Nemours & Company) having a thin, semi-transparent aluminum coating. The deposited softenable layer was allowed to dry at about 115°C for about 2 minutes. The thickness of the dried softenable layer was about 9 micrometers. The temperature of the softenable layer was then raised to about 115°C to lower the viscosity of the exposed surface of the softenable layer to about 5 x 10³ poises in preparation for the deposition of marking material. A thin layer of particulate vitreous selenium was then applied by vacuum deposition in a vacuum chamber maintained at a pressure of about 53 mPa (4 x 10⁻⁴ Torr). The imaging member was then rapidly chilled to room temperature. A reddish monolayer of selenium particles having an average diameter of about 0.35 micrometer embedded about 0.05 to 0.1 micrometer below the exposed surface of the copolymer was formed. The resulting xeroprinting master precursor member was then uniformly negatively charged to a surface potential of about -900 volts with a corona charging device and was subsequently exposed by placing a test pattern mask (comprising a silver halide color separation image) in contact with the imaging member and exposing the member to light through the mask. The exposed member was thereafter developed by subjecting it a temperature of about 115°C for about 5 seconds using a hot plate in contact with the polyester. The resulting xeroprinting master exhibited excellent image quality, resolution in excess of 228 line pairs per millimeter, and an optical contrast density of about 1.2. Optical density of the D max area was about 1.8 and that of the D min area was about 0.60. The D min was because of substantial depthwise migration of the selenium particles toward the aluminum layer in the D min regions of the image. The prepared xeroprinting master was uniformly negatively charged with a corona charging device to about -900 volts followed by a brief uniform flash exposure to 400-700 nanometer activating illumination of about 3000 ergs/cm². The surface potential was about -865 volts in the D max (unmigrated) region of the image and about -10 volts in the D min (migrated) region, thereby yielding an electrostatic contrast voltage of about -855 volts and a contrast voltage efficiency of about 95% of the initially applied voltage. The surface potentials of the D max areas and Dmin areas of the master were monitored with electrostatic voltmeters. EXAMPLE VIII A xeroprinting master precursor member was prepared by dissolving about 16.8 grams of a terpolymer of styrene/ethylacrylate/acrylic acid (obtained from Desoto Company as E-335), and about 3.2 grams of (4-phenethoxycarbonyl-9-fluorenylidene)malonontrile in about 80.0 grams of toluene. The (4-phenethoxycarbonyl-9-fluorenylidene)malonontrile is a charge transport material capable of transporting negative charges (electrons). The resulting solution was coated by solvent extrusion techniques onto a 300 mm wide 100 micrometer thick Mylar® polyester film (available from E.I. DuPont de Nemours & Company) having a thin, semi-transparent aluminum coating. The deposited softenable layer is allowed to dry at about 115°C for about 2 minutes. The thickness of the dried softenable layer is about 6 micrometers. The temperature of the softenable layer is then raised to about 115°C to lower the viscosity of the exposed surface of the softenable layer to about 5 x 10³ poises in preparation for the deposition of marking material. A thin layer of particulate vitreous selenium is then applied by vacuum deposition in a vacuum chamber maintained at a pressure of about 53 mPa (4 x 10⁻⁴ Torr). The imaging member was then rapidly chilled to room temperature. A reddish monolayer of selenium particles having an average diameter of about 0.3 micrometer embedded about 0.05 to 0.1 micrometer below the exposed surface of the copolymer was thus formed. The resulting xeroprinting master precursor member was then uniformly negatively charged to a surface potential of about -600 volts with a corona charging device and subsequently exposed by placing a test pattern mask comprising a silver halide color separation image in contact with the imaging member and exposing the member to light through the mask. The exposed member was thereafter developed by subjecting it a temperature of about 115°C for about 5 seconds using a hot plate in contact with the polyester. It is believed that the resulting xeroprinting master will exhibit excellent image quality, resolution, and optical contrast density. The xeroprinting master was then uniformly positively charged with a corona charging device to a potential of about +600 volts, followed by a brief uniform flash exposure to 400-700 nanometer activating illumination of about 400 ergs/cm². It is believed that the contrast voltage observed between the D max and D min areas subsequent to flash exposure will be well in excess of 50 percent. This process was repeated by uniformly positively charging the master to about +600 volts and flash exposing at about 800 ergs/cm². It is believed that the contrast voltage observed between the D max and D min areas subsequent to flash exposure will also be well in excess of 50 percent at this exposure energy. The process is again repeated by uniformly positively charging the master to about +600 volts and flash exposing at about 3000 ergs/cm². It is believed that the contrast voltage observed between the D max and D min areas subsequent to flash exposure will also be well in excess of 50 percent at this exposure energy. EXAMPLE IX A xeroprinting master prepared as described in Example VIII is incorporated into the Xeroprinter® 100 available from Fuji Xerox Company, Ltd. by replacing the original zinc oxide photoreceptor in the machine with the xeroprinting master. In addition, the incandescent flood exposure lamp in the machine is replaced with an 8 watt green fluorescent photoreceptor erase lamp as the flood exposure light source. The master is uniformly positively charged to a potential of about +600 volts and then flood exposed to form an electrostatic latent image on the master surface. Subsequently, the latent image is developed with the black dry toner supplied with the Xeroprinter® 100 machine and the developed image is transferred and fused to Xerox® 4024 plain paper (11″ x 17″ size). The process is repeated at a printing speed of 50 copies per minute (about 375 mm per second), and is also repeated with the cyan and magenta dry toners supplied with the Xeroprinter® 100. It is believed that the images thus formed will exhibit high image contrast, clear background, and an excellent halftone dot range of about 6 to about 95 percent. It is also believed that over 5000 prints can be generated with the master with no apparent damage to the master and no degradation of image quality. EXAMPLE X A xeroprinting master prepared as described in Example VIII is uniformly positively charged with a corona charging device to about +600 volts, followed by a brief uniform flash exposure to form an electrostatic latent image on the master surface. Subsequently, the latent image is developed with a liquid developer to form a deposited toner image. The liquid developer contains about 2 percent by weight of carbon black pigmented polyethylene acrylic acid resin and about 98% by weight of Isopar® L (isoparaffinic hydrocarbon). The deposited toner image is transferred and fused to a sheet of paper. It is believed that the transferred image will be a very high quality xeroprint.

Claims: A xeroprinting process which comprises (1) providing a xeroprinting master (1) comprising (a) a substrate (22); and (b) a layer of softenable material, (25) comprising a charge transport material (27) capable of transporting charges of one polarity, and migration marking material (26) situated contiguous to the surface of the softenable layer spaced from the substrate, wherein a portion of the migration marking material has migrated through the softenable layer toward the substrate in imagewise fashion; (2) uniformly charging the xeroprinting master to a polarity opposite to the polarity of the charges that the charge transport material in the softenable layer is capable of transporting; (3) uniformly exposing the charged master to activating radiation (31), thereby discharging those areas of the master wherein the migration marking material has migrated toward the substrate and forming an electrostatic latent image; (4) developing the electrostatic latent image, and (5) transferring the developed image to a receiver sheet (45). A xeroprinting process according to claim 1, wherein the charge transport material is capable of transporting positive charges, and the xeroprinting master is charged negatively. A xeroprinting process according to claim 2, wherein the charge transport material is of diamine hole-transporting materials, pyrazoline hole-transporting materials, hydrazone hole-transporting materials, or mixtures thereof. A xeroprinting process according to claim 1, wherein the charge transport material is capable of transporting negative charges, and the xeroprinting master is charged positively. A xeroprinting process according to claim 4 wherein the charge transport material is of 9-fluorenylidene methane derivative electron-transporting materials; vinyl aromatic electron-transporting materials; electron-transporting polymers of polyesters, polysiloxanes, polyamides, polyurethanes, and epoxies and having aromatic or heterocyclic groups with more than one substituent selected from the group consisting of nitro, sulfonate, carboxyl, and cyano; or mixtures thereof. A xeroprinting process according to any preceding claim, wherein the xeroprinting master contains a charge-transport layer (9) situated between the substrate and the softenable layer. A xeroprinting process according to any preceding claim, wherein the xeroprinting master contains a coating layer (15), and the softenable layer is situated between the coating layer and the substrate. A xeroprinting process according to any preceding claim, wherein the xeroprinting master contains an adhesive layer (5) situated between the substrate and the softenable layer. A xeroprinting process according to any preceding claim, wherein the xeroprinting master contains a charge-blocking layer (7) situated between the substrate and the softenable layer. A xeroprinting process according to any preceding claim, wherein the migration marking material is of selenium, alloys of selenium and tellurium, alloys of selenium and arsenic, alloys of selenium, tellurium and arsenic, phthalocyanines, or mixtures thereof.

Inventors: KOVACS GREGORY J, LOUTFY RAFIK O, TAM MAN C, KOVACS, GREGORY J, LOUTFY, RAFIK O, TAM, MAN C
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1990
Main classification: G03G13
IPC codes: G03G13, G03G17, G03G5