Liquid developing material layer charging

Abstract

 An electrostatic latent image development method and apparatus, wherein a thin layer (58) of liquid developing material (54) is brought into a process nip (59) formed by operative engagement of first (10) and second (40) movable members for positioning the thin layer (58) of liquid developing material in pressure contact with the electrostatic latent image. An ion source (48) is provided for selectively charging the layer (58) of liquid developing material prior to entry into the process nip (59). By applying a charge to the layer (58) of liquid developing material, image separation in the nip (59) is optimized and the use of liquid developing materials (54) having toner particles with poor or neutral charge thereon, as well as toner particles having reverse charge polarity thereon is enabled.

Description

[0001] This invention relates generally to liquid developing material based electrostatographic printing, and, more particularly, concerns a system for development of an electrostatic latent image, wherein a substantially uniform layer of charged liquid developing material is brought into pressure contact with a latent image bearing surface for causing selective image-wise separation of the liquid developing material layer to produce a desired output image corresponding to the latent image.

[0002] Generally, processes for electrostatographic copying and printing are initiated by selectively charging and/or discharging a charge receptive imaging member in accordance with an original input document or an imaging signal in order to generate an electrostatic latent image on the imaging member. The electrostatic latent image is subsequently developed into a visible image by a process in which charged toner particles are brought into the vicinity of the latent image and caused to migrate to image areas thereof. Typically, the developing material may be comprised of carrier granules having marking or toner particles adhering triboelectrically thereto, wherein the toner particles are electrostatically drawn away from the carrier granules and attracted to the latent image areas to create a powder toner image on the imaging member. Alternatively, the developing material may comprise a liquid developing material comprising a carrier liquid having charged pigmented marking particles (or so-called toner solids) immersed therein, wherein the charge on the marking particles is created by a soluble ionic surfactant, or so-called charge director material dispersed and/or dissolved in the liquid carrier/marking particle composition to create an electrochemical reaction which results in the exchange of ionic species between the marking particles and micelles formed by the charge director. While this electrochemical reaction creates a charge on the toner particles, it also generates mobile charges in the liquid developing material in the form of co-ions, counter-ions and other similar mobile charge species.

[0003] Regardless of the type of developing material employed, the toner or marking particles of the developing material are typically uniformly charged, either triboelectrically or via charge directors, and electrostatically attracted to the latent image via electrostatic fields, forming a visible developed image corresponding to the latent image on the imaging member in the case of traditional liquid developing material based development processes, the liquid developing material is generally applied to the surface of a latent image bearing member, with the charged marking particles being caused to electrophoretically precipitate from the liquid developing material dispersion so as to migrate to and be deposited upon the image areas of the latent image to form a developed liquid image. The developed image is subsequently transferred, either directly or indirectly, from the imaging member to a copy substrate, such as paper or the like, to produce a "hard copy" output document. In a final step, the imaging member is cleaned to remove any residual developing material and/or charge therefrom in preparation for a subsequent image forming cycle.

[0004] As described hereinabove, the typical electrostatographic printing process includes a development step whereby developing material including toner or marking particles is physically transported into the vicinity of a latent image bearing imaging member, with the toner or marking particles being caused to migrate via electrical attraction of toner or marking particles to the image areas of the latent image so as to selectively adhere to the imaging member in an image-wise configuration. The development process is most effectively accomplished when the particles carry electrical charges opposite in polarity to the latent image charges, with the amount of toner or marking particles attracted to the latent image being proportional to the electrical field associated with the image areas.

[0005] Numerous and various alternative methods of developing a latent image have been described in the art of electrophotographic printing and copying systems. Of particular interest with respect to the present invention is the concept of forming on a surface a thin layer of liquid developing material having a high concentration of charged marking particles, with the layer being brought into contact with an electrostatic latent image on another surface, wherein development of the latent image occurs upon separation of the first and second surfaces, as a function of the electric field strength generated by the latent image. In this process, toner particle migration or electrophoresis is replaced by direct surface-to-surface transfer of a toner layer induced by image-wise fields. For the purposes of the present description, the concept for latent image development via direct surface-to-surface transfer of a toner layer via image-wise fields will be identified generally as Contact Electrostatic Printing (CEP). Exemplary patents which may describe certain general aspects of CEP, as well as specific apparatus therefor, may be found in U.S. Patent No. 5,436,706 and 5,596,396.

[0006] Image quality in electrostatographic printing applications, including CEP, may vary significantly due to numerous conditions affecting image development, including, but certainly not limited to charge levels and charge density, both in the latent image, as well as in the developing material itself. It is these charges that generate the electric forces necessary to cause toner migration, electrophoretic precipitation or, in the case of CEP, selective imagewise toner layer separation for producing the desired output image. Moreover, in the specific case of CEP, wherein a concentrated layer of liquid developing material is subjected to contact pressure in addition to electrical fields to yield selective separation of the liquid developing material layer in an imagewise manner, it has been found that it is beneficial to selectively modify the charge magnitude and charge density in the liquid developing material layer. Thus, in accordance with the present invention, a charge modification device is provided in a CEP system, for adding or removing charge from the layer to enhance image separation and associated development efficiency of the imagewise separated liquid developing material layer and increasing the operational latitude of the contact electrostatic printing process so as to optimize output image quality.

[0007] US-A-5,436,706 discloses an imaging apparatus including a first member having a first surface having formed thereon a latent electrostatic image, wherein the latent electrostatic image includes image regions at a first voltage and background regions at a second voltage. A second member charged to a third voltage intermediate the first and second voltages is also provided, having a second surface adapted for resilient engagement with the first surface. A third member is provided, adapted for resilient contact with the second surface in a transfer region. The imaging apparatus also includes an apparatus for supplying liquid toner to the transfer region thereby forming on the second surface a thin layer of liquid toner containing a relatively high concentration of charged toner particles, as well as an apparatus for developing the latent image by selectively transferring portions of the layer of liquid toner from the second surface to the first surface.

[0008] US-A-5,596,396 discloses an imaging apparatus including: a first member having a first surface having a latent electrostatic image formed thereon, wherein the latent electrostatic image includes image regions at a first voltage and background regions at a second voltage; a second member charged to a third voltage intermediate the first and second voltages and having a second surface adapted for resilient engagement with the first surface; and a third member adapted for resilient contact with the second surface in a transfer region. The imaging apparatus also includes an apparatus for supplying liquid toner to the transfer region to form on the second surface a thin layer of liquid toner containing a relatively high concentration of charged toner particles, as well as an apparatus for developing the latent image by the selective transfer of portions of the layer of liquid toner from the second surface to the first surface.

[0009] In accordance with one aspect of the present invention, there is provided an imaging apparatus, comprising: a first movable member for having an electrostatic latent image formed thereon including image areas defined by a first voltage potential and non-image areas defined by a second voltage potential; a second movable member for operative engagement with the first movable member, forming a process nip therebetween; a liquid developing material supply apparatus adapted for providing a layer of liquid developing material to a surface associated with one of the first or second movable members so as to transport the layer of liquid developing material into the process nip; and a charge modification device for adjusting electrical charge in the layer of liquid developing material prior to transport thereof into the process nip.

[0010] In accordance with another aspect of the present invention, an electrostatographic imaging process is disclosed, comprising the steps of: providing a first movable member for having an electrostatic latent image formed thereon including image areas defined by a first voltage potential and non-image areas defined by a second voltage potential; providing a second movable member for operative engagement with the first movable member to form a process nip therebetween; providing a layer of liquid developing material to a surface associated with one of the first or second movable members; applying a net charge to the layer of liquid developing material via an external charging source; and transporting the layer of liquid developing material into the process nip, wherein the electrostatic latent image on the first member generates imagewise electric fields across the layer of liquid developing material in the process nip. The charge modification device may be operative to remove or partially remove mobile charge species from the liquid developing material layer.

[0011] Particular embodiments of the present invention will now be described with reference to the accompanying drawings; in which:-

[0012] These and other aspects of the present invention will become apparent from the following description in conjunction with the accompanying drawings in which:

FIG. 1 is a schematic elevational view depicting a contact electrostatic printing (CEP) apparatus of the type used for development of an electrostatic latent image by placing a layer of concentrated liquid developing material in pressure contact with a latent image bearing surface, including a charge modification device for selectively varying the charge in the layer of liquid developing material prior to being placed in pressure contact with the latent image bearing surface in accordance with the present invention; and

FIG. 2 is another schematic elevational view depicting an alternative embodiment of a CEP apparatus in accordance with the present invention, incorporating alternative embodiments for various subsystems therein relative to the embodiment of FIG. 1.

[0013] Reference is now made to FIG. 1 which illustrates an imaging apparatus constructed and operative in accordance with one possible embodiment of the present invention. The apparatus of FIG. 1 comprises a first movable member in the form of an imaging member 10 including an imaging surface of any type capable of having an electrostatic latent image formed thereon. An exemplary imaging member 10 may include a typical photoconductor or other photoreceptive component of the type known to those of skill in the art of electrophotography, wherein a surface layer 14 having photoconductive properties is supported on a conductive support substrate 16. Although the following description will describe, by example, a system and process in accordance with the present invention incorporating a photosensitive imaging member, it will be understood that the present invention contemplates the use of various alternative imaging members as are well known in the art of electrostatographic printing, including, for example, but not limited to, non-photosensitive imaging members such as a dielectric charge retaining member of the type used in ionographic printing machines, or electroded substructures capable of generating charged latent images.

[0014] Imaging member 10 is rotated, as indicated by arrow 12, so as to transport the surface thereof in a process direction for implementing a series of image forming steps in a manner similar to typical electrostatographic printing processes. It will be understood that, while imaging member 10 is shown and described herein in the form of a drum, the imaging member may alternatively be provided in the form of a continuous flexible belt which is entrained about a series of rollers, and is movable in the same direction as shown.

[0015] Initially, in the exemplary embodiment of Fig. 1, the photoconductive surface 14 of imaging member 10 passes through a charging station, which may include a corona generating device 20 for applying a substantially uniform electrostatic charge to the surface of the imaging member 10. The corona generating device 20 is provided for charging the photoconductive surface 14 of imaging member 10 to a relatively high, substantially uniform electrical charge potential. It will be understood that various charging devices, such as charge rollers, charge brushes and the like, as well as inductive and semiconductive charge devices, among other devices which are well known in the art, may be utilized at the charging station for applying a substantially uniform charge potential to the photosensitive surface 14 of the imaging member 10.

[0016] After the imaging member 10 is brought to a substantially uniform charge potential, the charged surface thereof is advanced to an image exposure station, identified generally by reference numeral 30. The image exposure station projects onto the charged photoconductive surface a light image corresponding to the input image, wherein the light image selectively dissipates the charge on the photoconductive surface 14 for recording an electrostatic latent image on the imaging member 10. The electrostatic latent image generally comprises, in image configuration corresponding to the input image information, image areas defined by a first charge voltage potential and non-image areas defined by a second charge voltage potential. It will be understood that the image exposure station 30 may incorporate various optical image projection and formation components as are known in the art, and may include various well known light lens or digital scanning systems for forming and projecting an image from an original input document onto the imaging member 10. Alternatively, various other electronic devices available in the art may be utilized for generating electronic information to create the electrostatic latent image on the imaging member 10. It will also be understood that the electrostatic latent image may be comprised of image and non-image areas that are defined by regions having opposite charge polarities or by regions having distinguishable first and second voltage potentials which are of the same charge polarity.

[0017] As previously noted hereinabove, in a typical electrostatographic printing process, after the electrostatic latent image is generated on the surface of an imaging member, the latent image is developed into a visible image by transporting developing material into the vicinity of the latent image bearing imaging member, wherein the differential voltage potentials associated with the image and non-image areas of the latent image induce the selective attraction of individual toner particles in accordance with the desired image being generated. In the case of many liquid developing material based systems, the developing material deposited on the latent image is typically a low solids content liquid composition having a relatively low concentration of charged toner particles dispersed in a liquid carrier agent, wherein image development occurs due to electrophoretic precipitation of the charged toner particles from the liquid dispersion. By contrast, in accordance with the so-called contact electrostatic printing process to which the present invention is directed, a thin layer of relatively high toner solids content liquid developing material having a relatively high concentration of charged toner particles dispersed in the liquid carrier agent is brought into pressure contact with the entire surface of the latent image bearing imaging member 10, whereby the developed image is created by separating and selectively transferring portions of the liquid developing material layer in correspondence with the image and non-image regions of the latent image.

[0018] Thus, in accordance with known CEP processes, after the liquid developing material layer or toner cake 58 is formed on the surface of the liquid developing material layer applicator 40, the toner cake 58 is transported directly to process nip 59 formed by the operative engagement of the layer applicator member 40 and the imaging member 10. In the process nip, the liquid developing material layer is separated into image and non-image regions with the latent image forming imagewise electric fields in the process nip 59, for causing imagewise separation of the layer. It will be understood that the presence of the latent image on the imaging member 10 may generate some fringe fields in areas of interface between image and non-image areas of the latent image. However, imagewise separation and development occurs as a function of surface to surface transfer of an assemblage or aggregate of particles making up a particular section of the toner cake associated with an image or non-image area of the latent image, as opposed to electrostatic attraction or precipitation of individual toner particles dispersed in the carrier liquid.

[0019] The layer of high solids content liquid developing material used in CEP is generally characterized as having a solids content on the order of approximately 20% or greater, wherein the solids content of the liquid developing material is made up of charged marking or toner particles. Thus, a liquid developing material delivery system is provided for transporting a layer of high solids content liquid developing material 58, or a so-called "toner cake", into pressure contact with the latent image in the process nip 59. It will be understood that the toner cake may be formed on either the surface of the applicator 40 or on the surface of the imaging member 10 for being transported into the process nip.

[0020] The toner cake having a solids content on the order of approximately 20% or greater can be created in various ways. In accordance with the exemplary embodiment of Fig. 1, a movable member in the form of a liquid developing material layer applicator 40 is provided in combination with a liquid developing material supply apparatus 50, including a reservoir 52 adapted to accommodate a supply of liquid developing material 54, generally made up of a low concentration level of toner particles immersed in a liquid carrier material. This supply of liquid developing material 54 also typically includes a charge director for providing a mechanism for producing an electro-chemical reaction in the liquid developing material composition which generates the desired electrical charge on the toner particles. While the charge director generates a charge on the toner particles, the charge director also produces mobile charge species in the bulk liquid developing material, such that the liquid developing material layer as a whole has a substantially neutral net charge. The present invention permits the avoidance of the use such charge directors if desired, and also permits modification of the bulk charge in the liquid developing material layer for generating a net charge therein, as will be discussed.

[0021] Generally, the liquid carrier medium is present in a large amount in the introductory supply of developing material 54. Initially, the liquid carrier medium is present in an amount of from about 85 to as much as 99.5 percent by weight, although the percentage amount may vary from this range provided that the objectives of the present invention are achieved. By way of example, the liquid carrier medium may be selected from a wide variety of materials, including, but not limited to, any of several hydrocarbon liquids conventionally employed for liquid development processes, including hydrocarbons, such as high purity alkanes having from about 6 to about 14 carbon atoms, such as Norpar® 12, Norpar® 13, and Norpar® 15, and including isoparaffinic hydrocarbons such as Isopar® G, H, L, and N, available from Exxon Corporation. Other examples of materials suitable for use as a liquid carrier include Amsco® 460 Solvent, Amsco® OMS, available from American Mineral Spirits Company, Soltrol®, available from Phillips Petroleum Company, Pagasol®, available from Mobil Oil Corporation, Shellsol®, available from Shell Oil Company, and the like. Isoparaffinic hydrocarbons provide a preferred liquid media, since they are colorless, environmentally safe. These particular hydrocarbons may also possess a sufficiently high vapor pressure so that a thin film of the liquid evaporates from the contacting surface within seconds at ambient temperatures.

[0022] The toner particles or so-called marking particles can comprise any particulate material that is compatible with the liquid carrier medium, such as those contained in the liquid developing materials disclosed in, for example, U.S. Patents 3,729,419; 3,841,893; 3,968,044; 4,476,210; 4,707,429; 4,762,764; 4,794,651; and 5,451,483, among others. Preferably, the toner particles should have an average particle diameter ranging from about 0.2 to about 10 microns, and most preferably between about 0.5 and about 2 microns. The toner particles may be present in amounts of from about 5 to about 20 percent by weight of the developer composition. The toner particles can consist solely of pigment particles, or may comprise a resin and a pigment; a resin and a dye; or a resin, a pigment, and a dye or resin alone.

[0023] Suitable resins include poly(ethyl acrylate-co-vinyl pyrrolidone), poly(N-vinyl-2-pyrrolidone), and the like, including, for example Elvax and/or Nucrel available from E.I. DuPont de Nemours & Co. of Wilmington, Delaware. Suitable dyes include Orasol Blue 2GLN, Red G, Yellow 2GLN, Blue GN, Blue BLN, Black CN, Brown CR, all available from Ciba-Geigy, Inc., Mississauga, Ontario, Morfast Blue 100, Red 101, Red 104, Yellow 102, Black 101, Black 108, all available from Morton Chemical Company, Ajax, Ontario, Bismark Brown R (Aldrich), Neolan Blue (Ciba-Geigy), Savinyl Yellow RLS, Black RLS, Red 3GLS, Pink GBLS, and the like, all available from Sandoz Company, Mississauga, Ontario, among other manufacturers; as well as the numerous pigments listed and illustrated in U.S. Patents 5,223,368; 5,484,670, the disclosures of which are totally incorporated herein by reference. Dyes generally are present in an amount of from about 5 to about 30 percent by weight of the toner particle, although other amounts may be present provided that the objectives of the present invention are achieved.

[0024] Suitable pigment materials include carbon blacks such as Microlith® CT, available from BASF, Printex® 140 V, available from Degussa, Raven® 5250 and Raven® 5720, available from Columbian Chemicals Company. Pigment materials may be colored, and may include magenta pigments such as Hostaperm Pink E (American Hoechst Corporation) and Lithol Scarlet (BASF), yellow pigments such as Diarylide Yellow (Dominion Color Company), cyan pigments such as Sudan Blue OS (BASF); as well as the numerous pigments listed and illustrated in U.S. Patents 5,223,368; 5,484,670, the disclosures of which have been previously indicated to be incorporated by reference. Generally, any pigment material is suitable provided that it consists of small particles that combine well with any polymeric material also included in the developer composition. Pigment particles are generally present in amounts of from about 5 to about 60 percent by weight of the toner particles, and preferably from about 10 to about 30 percent by weight.

[0025] As previously indicated, in addition to the liquid carrier vehicle and toner particles, typical liquid developer materials also include a charge director (sometimes referred to as a charge control additive) in the form of a soluable ionic surfactant which aggregates into micelles or inverse micelles for facilitating and maintaining a uniform charge on the marking particles in the operative solution of the liquid developing material. It is believed that the toner or marking particles obtain their charge by deprotonating in the presence of charge director micelles which form loosely attached counter-ions, co-ions and the like. In turn, the micelles provide readily reactive sites for the formation of toner charge, thereby imparting an electrical charge of selected polarity (positive or negative) to the marking particles while also producing mobile charge species in the bulk liquid developing material. However, the bulk liquid developing material is neutral in the absence of an electric field even though the toner particles are charged due to the effect of the mobile charge species. Examples of suitable charge director compounds include lecithin, available from Fisher Inc.; OLOA 1200, a polyisobutylene succinimide, available from Chevron Chemical Company; basic barium petronate, available from Witco Inc.; zirconium octoate, available from Nuodex; as well as various forms of aluminum stearate; salts of calcium, manganese, magnesium and zinc; heptanoic acid; salts of barium, aluminum, cobalt, manganese, zinc, cerium, and zirconium octoates and the like. The charge control additive may be present in an amount of from about 0.01 to about 3 percent by weight of solids, and preferably from about 0.02 to about 0.05 percent by weight of solids of the developer composition.

[0026] Returning now to a description of the liquid developing material supply apparatus 50, the apparatus of the exemplary embodiment of FIG. 1 includes a supply roller 56 which is rotated in a direction as indicated by arrow 57 for transporting liquid developing material onto the surface of the liquid developing material layer applicator 40 which is preferably provided in the form of a relatively thin, substantially uniform layer 58 made up of densely packed toner particles in a liquid carrier. Depending on the materials utilized in the liquid developing material composition 54, as well as other process parameters related to the printing system, such as process speed and the like, a layer of liquid developing material having sufficient thickness and solid content, preferably between 2 and 15 microns and more than 15% solids, and more preferably on the order of 5 microns or less, may be formed on the surface of the liquid developing material layer applicator 40 by merely providing adequate proximity and/or contact pressure between the supply roller 56 and the roll surface of layer applicator 40. Alternatively, or additionally, in the case where the liquid developing material includes charged toner particles, as in the case where the supply of liquid developing material 54 includes a charge director compound, an electrical biasing source 55 may be coupled to the supply roller 56 to assist in electrostatically moving the toner particles onto the surface of the layer applicator 40. Thus, in one exemplary embodiment, the supply roller 56 can be coupled to an electrical biasing source 55 for implementing a so-called forward biasing scheme, wherein the toner applicator 56 is provided with an electrical bias of sufficient magnitude and polarity for creating electrical fields extending from the supply roll 56 to the surface of the layer applicator 40. These electrical fields cause toner particles to be substantially uniformly transported to the surface of the liquid developing material layer applicator 50, for forming the layer of liquid developing material 58 having a concentrated and substantially uniform distribution of toner particles therein.

[0027] It will be understood that numerous other devices or apparatus may be utilized for applying toner layer 58 to the surface of the toner layer applicator 40, including various well known apparatus used in conventional lithographic printing applications as well as traditional liquid electrostatographic applications, such as, but not limited to, the various known systems directed toward the transportation of liquid developing material having toner particles immersed in a carrier liquid. For example, the liquid transport system can include a fountain-type device as disclosed generally in commonly assigned U.S. Patent No. 5,519,473 (incorporated by reference herein), as is illustrated generally in the alternative embodiment of Fig. 2. In this alternative embodiment, a liquid developing material applicator 152 is provided, wherein a housing defines an elongated aperture adapted for transporting liquid developing material into contact with the surface of a layer applicator 40. Preferably, the housing also includes a planar surface adjacent the elongated aperture for providing a liquid developing material application region in which the liquid developing material can flow freely in contact with the layer applicator 40. A reverse roll member 154, situated adjacent to and downstream from the liquid developing material applicator 152, is also provided. The function of this roll member 154 can be twofold: for metering a portion of the liquid carrier away from the liquid developing material as it is applied to the surface of the layer applicator 40 to increase the toner solids concentration therein; and/or for electrostatically pushing (via biasing source 155 coupled thereto) the liquid developing material toward the surface of the layer applicator 40.

[0028] In accordance with the exemplary apparatus of FIG. 1, an electrical biasing source 45 is coupled to the liquid developing material layer applicator 40 for applying an electrical bias thereto so as to generate electrostatic fields between the surface of layer applicator 40 and the image or non-image areas on the surface of the imaging member 10. These electrostatic fields are generated in opposite directions, either toward the surface of the imaging member 10 or towards the surface of the layer applicator 40 in accordance with image and non-image portions of the latent image. Moreover, these fields cause the separation of the image and non-image areas of the toner cake layer 58 upon separation of the imaging member 10 and the layer applicator 40 at the nip exit for simultaneously separating and developing the liquid developing material layer 58 into image and non-image portions on the opposed surfaces of the imaging member 10 and the layer applicator 40. The liquid developing material layer applicator 40 may be biased so as to repel toner segments in image areas, thereby producing a developed image made up of selectively separated and developed portions of the toner cake on the surface of the imaging member 10, while leaving background image byproduct on the surface of the toner layer applicator 40. The resultant image/background separation is illustrated in the system of FIG. 1. Alternatively, the layer applicator 40 may be provided with an electrical bias appropriate for attracting toner segments in image areas while repelling non-image areas toward the imaging member 10, thereby maintaining toner cake portions corresponding to image areas on the surface of the liquid developing material layer applicator 40, yielding a developed image thereon. The resultant image/background separation for this alternative is illustrated in the system of FIG. 2.

[0029] As previously indicated, image development in the process of the present invention occurs as a function of surface to surface transfer of an assemblage or aggregate of particles making up a particular section of the toner cake, wherein the toner cake is separated into image and non-image segments as opposed to electrostatic attraction of individual toner particles dispersed in a carrier liquid. As such, contrary to the case of normal electrophoretic development, transfer of the concentrated layer of toner particles from the first surface to the second surface is not dependent on the mobility of the toner particles in the liquid developing material. Instead, image quality is dependent on the ability of the toner cake or liquid developing material layer 58, and in particular, the toner particles therein, to maintain their integrity as an assemblage of toner particles, whereby the toner particles in the toner cake maintain their initial distribution and density levels as the liquid developing material layer enters the nip, and allowing the toner particles therein to sustain an image pattern as it passes through the nip. Likewise, at the nip exit, the liquid developing material layer segment associated with the image areas will be maintained on one surface while the liquid developing material layer segment associated with the background or non-image areas will be maintained on the other surface in accordance with the imagewise electrical fields in the nip. Image quality is dependent on the ability of the toner particles in the liquid developing material layer 58 to break sharply under the influence of electric fields along the image/background boundary, where the electrostatic force is substantially zero.

[0030] The image quality and process latitude of a CEP development process depends critically on electric field contrast in the development nip which is limited by the latent image being used. For example, there are certain known photoreceptive materials and members capable of delivering higher charge contrast. It is also well know that a conductive surface opposite to the latent image in the development nip will maximize the field contrast with a given latent image.

[0031] In addition to the latent image contrast, the charge characteristics of the toner layer plays a critical roll in the development process. Traditional liquid development processes use a chemically charged toner, which, in addition to producing charged marking particles, also generate substantial amounts of other mobile charge species (such as co-ions and counter ions) exist. When subjected to an external electric field, these mobile charge species will move under the influence of the electric field, and in turn, cause the rearrangement of the charge distribution which will reduce the electric field and may eventually cause the collapse of the field with sufficient charge supply and time. In CEP development, the presence of mobile charge species will certainly reduce the effective contrast of the development field on the toner particles, and thus affect the image quality and process latitude. It has been found that too much charge in the liquid developing material layer will cause the development field to collapse and severe image degradation will result. The ratio of the total charge per unit area in the liquid developing material layer to the charge contrast of the latent image should generally be substantially less than 1, preferably less than 1/2.

[0032] Since, in contact electrostatic printing processes, the liquid developing material layer in the form of a thin layer supported on a first surface is placed under pressure in the process nip 59, it may desirable to provide either the layer applicator member 40 or the imaging member 10 in the form of a conformable member for permitting the surface of one member to correspond in form or character to the opposing surface in the nip region. When the surface of the applicator member 40 bearing the toner cake is engaged with the latent image bearing surface of imaging member 10, the toner cake layer 58 is substantially uniformly distributed within the nip created therebetween such that toner particle motion and/or liquid flow is negligible with no distortion being present or induced amongst the toner particles in the toner cake 58.

[0033] After the developed image is created at the exit of nip, either on the surface of the imaging member 10 or on the surface of the toner layer applicator 40, the developed image may then be transferred to a copy substrate 70 via any means known in the art, which may include an electrostatic transfer apparatus including a corona generating device of the type previously described or a biased transfer roll. Alternatively, a pressure transfer system may be employed which may include a heating and/or chemical application device for assisting in the pressure transfer and fixing of the developed image on the output copy substrate 70. In yet another alternative, image transfer can be accomplished via surface energy differentials wherein the surface energy between the image and the member supporting the image prior to transfer is lower than the surface energy between the image and the substrate 70, inducing transfer thereto. In one embodiment, as shown in Fig. 1, the image is transferred to a copy substrate via a heated pressure roll, whereby pressure and heat are simultaneously applied to the image to simultaneously transfer and fuse the image to the copy substrate 70. It will be understood that separate transfer and fusing systems may be provided, wherein the fusing or so-called fixing system may operate using heat (by any means such as radiation, convection, conduction, induction, etc.), or other known fixation process which may include the introduction of a chemical fixing agent. Since the art of electrostatographic printing is well known, it is noted that several concepts for transfer and/or fusing which could be beneficially used in combination with the system of the present invention have been disclosed in the relevant patent literature.

[0034] In a final step in the process, the background image byproduct on either the imaging member 10 or toner layer applicator 40 is removed from the surface thereof in order to clean the surface in preparation for a subsequent imaging cycle. Fig. 1 illustrates a simple blade cleaning apparatus 90 for scraping the imaging member surface as is well known in the art. Alternative embodiments may include a brush or roller member for removing toner from the surface on which it resides. In a preferred embodiment, the removed toner associated with the background image is transported to a toner sump or other reclaim vessel so that the waste toner can be recycled and used again to generate a toner cake in subsequent imaging cycles. Once again, it is noted that several concepts for cleaning and toner reclaim which could be beneficially used in combination with the image-wise development system of the present invention have been disclosed in the relevant patent literature.

[0035] As noted hereinabove, conventional liquid developing material based systems, including known CEP-type processes, utilize charged toner particles in the liquid developing material supply 54 to form the liquid developing material layer 58. Toner particle charging in liquid developing material based systems is typically accomplished via charge directors or charge control additives which, while generating a charge on the toner particles, also generate mobile charge species in the bulk of the liquid developing material. These mobile charge species cause the bulk to have a net neutral charge. Thus the liquid developing material layer 58 formed on the surface of the liquid developing material layer applicator 40, has a net neutral charge made up in part by charged toner particles and mobile charge species. In prior art systems, this net neutral charge layer of liquid developing material is transported directly into the process nip 59 formed by the operative engagement of the layer applicator member 40 and the imaging member 10 for separating the toner layer into image and non-image regions. Thus, traditional liquid development systems supply to a development region liquid developing material which has substantially zero net charge and within which substantial amount of charges of opposite polarities exist. Even though the toner particles within the developer is strongly charged (with respect to their chemically active charge director, micelles and etc.), the liquid developing material appears neutral due to the balance of counter charges around the charged marking particles. The liquid developing material gains net charge when charge injection or extraction occurs to the system. Particularly, toner particles gain net charge when counterions are removed from their vicinity. One discovery leading to the present invention is that the removal of any mobile charged species (co-ions, coutnerion, etc.) other than the marking particles can greatly improve the development process. Thus, in accordance with the present invention, a liquid developing material layer with substantial net charge is utilized in the development region to enhance the performance. Preferably, the net charge of the liquid developing material layer is created substantially from the charge of the toner particles in the liquid developing material.

[0036] The present invention is particularly directed toward modifying charge in the bulk liquid developing material layer 58. More specifically, it has been found by the present invention that, due to the relatively high toner solids concentration of the toner cake 58, it is important to optimize the toner charge level (as measured in terms of microcoulombs per gram) in the toner cake 58 in order to optimize the imagewise separation of the liquid developing material layer 58 in the process nip 59 and the subsequent layer 58 division into image and non-image areas on the opposed surfaces of the imaging member 10 and the liquid developing material layer applicator 40 as the toner layer exits the process nip 59. In addition, the present invention attempts to exploit the relatively high toner solids nature of the layer 58 which permits charging from an external ion source which would be highly inefficient if used in a low concentration liquid developing material layer of the type used in liquid development type processes which depend on electrophoretic development. The present invention is also directed toward inducing toner charge in the liquid developing material layer

[0037] In accordance with the present invention, therefore, an ion source is introduced in the form of a corona generating device 48 as shown in FIG. 1 (or in the form of an electrically biased roll member 49 as shown in FIG. 2), positioned adjacent the liquid developing material layer applicator 40 for applying a predetermined or selective charge to the toner cake 58 on the surface thereof. The discovery leading to the present invention is that the mobile charge species in the net neutral charged layer reduce the useful electrical fields necessary for imager separation. It may be desirable to form the liquid developing material layer using uncharged or neutrally charged toner particles, wherein a charge is induced on the toner particles after the formation of the liquid developing material layer. Alternatively, it may be desirable to selectively vary or modify the charge of a liquid developing material layer formed with inappropriately charged toner particles prior to transporting the liquid developing material layer into the process nip 59. In the absence of a charge director, a charged liquid developing material layer can be generated with an ion source such that no significant counter ion or co ions exist in the liquid developing material. As such, in accordance with the present invention, an ion source is provided and situated adjacent the liquid developing material layer applicator 40 so as to provide an apparatus for selectively varying the charge level of the toner particles in the liquid developing material layer 58 prior to entry of the layer 58 into the process nip 59.

[0038] The ion source of the present invention may also be provided for enhancing or reversing charge in the toner cake 58 (as may be previously provided via charge director in the liquid developing material), or may even be provided to produce a charge in a neutrally charged toner cake 58. The invention permits the use of liquid developing materials that do not require charge director materials such that no conductive species exist in the toner cake except for the toner particles themselves. Alternatively, the concept of the present invention can be incorporated into a system that does use charge director materials, wherein the counter-ions inherently produced in the micelles produced by the charge director induced electro-chemical reaction can be minimized or eliminated. In turn, the elimination of superfluous conductive species such as counter-ions in the toner cake permits the use of lower voltage differentials for defining the image and background regions of the electrostatic latent image and also yields a broader operational speed range for the printing system as a whole. The elimination of superfluous conductive species also advantageously allows for the use of simplified developing material compositions and uncomplicated liquid developing material management systems. It has been found that the present invention provides much greater operational latitude in a CEP process by facilitating a relatively simple process for controlling charge density in the toner cake.

[0039] FIGS. 1 and 2 illustrate a CEP system including a liquid developing material layer toner charging device, generally identified by reference numeral 49, for injecting ions into the toner cake 58. In the embodiment of FIG. 1, a well known corona generating device is used to spray ions toward the liquid developing material layer 58, wherein the ions travel through the liquid developing material layer 58 and either generate a charge on the toner particles therein or neutralize counter ions which may be present in the micelles generated by charge directors in the liquid developing material composition. Similarly, in the embodiment of FIG. 2, a well known electrically biased roll member, producing a net charge on the toner particles by stripping off counter ions from the toner particles in the liquid developing material layer. Exemplary apparatus which may used for roll member 49 include a biased charge roller of the type known in the art and described, for example, in U.S. Patent Nos. 2,912,586 and 5,144,368 among numerous other patents and technical literature available to one of skill in the art. Alternatively, exemplary apparatus may include a biased squeegee roller of the type known in the art and described, for example, in U.S. Patent Nos. 4,286,039 and 5,028,964 among other patents. The foregoing patents are hereby incorporated by reference for the purposes of providing a detailed description of the present invention.

[0040] It will be understood that the apparatus and processes described hereinabove represent only a few of the numerous system variants that could be implemented in the practice of the present invention. One particular variant printing system incorporating the teaching of the present invention is shown in Fig. 2, wherein a toner supply apparatus 150 includes a fountain-type applicator 152 in combination with a metering roll 154. Metering roll 154 includes a peripheral surface situated in close proximity to the surface of imaging member 10, preferably rotated in a direction opposite to the direction of movement of the imaging member 10, providing a shear force against the toner layer deposited on the surface of the imaging member, for controlling the thickness of the toner layer thereon. Thus, the metering roll 154 meters a predetermined amount of developing material (which may include toner particles immersed in liquid carrier) . The excess material eventually falls away from the metering roll and may be transported to a sump (not shown)for reuse in the toner applicator 152.

[0041] The embodiment of Fig. 2 also illustrates that the image bearing member 10 can be used to remove image background areas from the liquid developing material layer 58. Thus, the liquid developing material layer applicator 40 is biased so as to permit the imaging member 10 to attract background areas, thereby maintaining toner segments corresponding to image areas on the surface of the liquid developing material layer applicator 40. Accordingly, the liquid developing material segments on the imaging member 10 are transported to a cleaning device 90, embodied as a roll member, while image areas remaining on the surface of the liquid developing material layer applicator 40 are transported to a transfer station in a manner similar to that previously described and as found in conventional electrostatographic printing machines. The liquid developing material layer segments making up the image are transferred to a copy substrate via any method which may be known in the art. The transferred image may thereafter be fused to the copy substrate at fusing station if necessary and transported to an output device for retrieval by a machine operator.

Claims

1. An imaging apparatus, comprising:

a first movable member (10) for having an electrostatic latent image formed thereon including image areas defined by a first voltage potential and non-image areas defined by a second voltage potential;

a second movable member (40) for operative engagement with said first movable member (10), forming a process nip (59) therebetween; and,

a liquid developing material supply apparatus (50) adapted for providing a layer (58) of liquid developing material (54) to a surface associated with one of said first or second movable members (10,40) so as to transport the layer (58) of liquid developing material (54) into said process nip (59);

   characterised in that the apparatus also includes a charge modification device (48,45,55) for applying a charge to the layer (58) of liquid developing material (54) prior to transport thereof into said process nip (59).

2. An imaging apparatus according to claim 1, wherein the layer of liquid developing material consists essentially of neutrally charged toner particles immersed in a liquid carrier or of weakly charged toner particles immersed in a liquid carrier.

3. An imaging apparatus according to claim 1, wherein the layer of liquid developing material consists essentially of charged toner particles immersed in a liquid carrier including charge directors wherein:

the charge directors generate counter ions associated with the charged toner particles; and further wherein

said charge modification device (48) is operative to strip the counter ions from the charged toner particles.

4. An imaging apparatus according to any one of the preceding claims, wherein said liquid developing material supply apparatus (50) is adapted to deposit the liquid developing material layer having a solids percentage by weight of at least approximately 20%.

5. An imaging apparatus according to any one of the preceding claims, wherein said liquid developing material supply apparatus (50) includes:

a housing (52) adapted to accommodate a supply of liquid developing material (54) therein; and

a rotatably mounted applicator roll member (56) for transporting liquid developing material from said housing (52) to the surface of said second member (40).

6. An imaging apparatus according to claim 5, wherein said liquid developing material supply apparatus (50) further includes an electrical biasing source (55) coupled to said applicator roll (56) for applying an electrical bias thereto to generate electrical fields between said applicator roll (56) and said imaging member (10) so as to electrostatically assist in forming the liquid developing material layer (58) on the surface associated with one of said first or second movable members (10,40).

7. An imaging apparatus according to any one of the preceding claims, wherein said charge modification device for applying a charge to the layer (58) of liquid developing material includes a corona generating device (48) and/or an electrically biased roll member (56).

8. An electrostatographic imaging process, comprising the steps of:

providing a first movable member (10) for having an electrostatic latent image formed thereon including image areas defined by a first voltage potential and non-image areas defined by a second voltage potential;

providing a second movable member (40) for operative engagement with said first movable member (10) to form a process nip (59) therebetween;

providing a layer (58) of liquid developing material to a surface associated with one of said first or second movable members (59);

applying a charge to the layer (58) of liquid developing material via an external charging source (48,55); and,

transporting the layer (58) of liquid developing material into said process nip (59), wherein the electrostatic latent image on said first member (10) generates imagewise electric fields across the layer of liquid developing material in said process nip (59).

Drawing