Field of Invention This invention relates to toners and methods of use of such toners that are capable of being directly deposited on x-y-plane-conductive dielectric recording media (e. g., vinyl-containing films) by an electrostatic process.

Background of Invention The term"electrostatic"recording process refers to a type of electrographic process in which a recording head is utilized to impose an electrostatic pattern upon a recording medium, and in which a toner material is subsequently attracted to, and affixed to the electrostatic pattern. Processes of this type are employed for preparing engineering graphics, artwork for advertisements, displays, and the like.

In a typical electrostatic imaging process, a recording head which includes a linear array of a plurality of separately chargeable electrodes, generally referred to as"nibs,"is scanned across a recording medium, and the nibs are selectively energized to impose an electrostatic pattern upon the medium. The charged surface of the medium, which is typically negatively charged, is contacted with a toner from a toner fountain (i. e., toner station). Color printers typically have four toner stations, one for each of the colored toners (black (K), magenta (M), cyan (C), and yellow (Y)), and an optional fifth station for a spot color or clearcoat.

The toner is typically a liquid containing colorant particles that are typically positively charged. Excess toner is removed from the medium, leaving toner only on the charged areas. The toner is subsequently dried or otherwise fixed to produce a permanent image. The toner system chosen must be compatible with the recording medium such that it is accepted and retained. The process can be utilized for single color or multiple color graphics and can be completed in a single pass across the medium or in multiple passes across the medium.

Electrostatic printing of media conventionally requires the printing of electrostatic images on volume-conductive or through-conductive dielectric recording media (i. e., recording media that allows electrical conductivity through the media, i. e., along the z-axis), such as a dielectric paper construction. This image can then be transferred to a polymer film. Such conventional electrostatic imaging is disclosed in U. S. Pat. No. 5,114,520 (Wang et al.). The dielectric paper construction typically comprises a paper or paper-like substrate (or bulk conductive material), a conductive layer coated on a major surface of the substrate, a dielectric layer coated over the conductive layer, and an optional release layer coated above, beneath, or with the dielectric layer to assure that the image received above the dielectric layer can be optionally transferred to the final substrate upon application of heat and pressure.

Alternatively, electrostatic printing can occur directly onto non- through-conductive or x-y-plane-conductive dielectric recording media (i. e., recording media that allows electrical conductivity in the x-y plane, i. e., the major plane, of the recording media, but not along the z axis), such as polymer films.

This is disclosed, for example, in U. S. Pat. No. 5,385,771 (Willetts et al.), Japanese Kokai Publication No. 3-69960, and U. S. Pat. No. 5,736,228 (Morris et al.). Such polymer films include polyester-containing films and vinyl-containing films, for example, and are referred to as direct-print films. Typically, a direct print film includes a polymeric substrate having a solvent-coated conductive layer disposed thereon and a dielectric layer coated on the conductive layer.

With either through-conductive or x-y-plane-conductive dielectric recording media, it is necessary to allow for negative charge to flow out of the conductive layer of the dielectric media to ground during the charge writing process. This grounding can occur via a path to ground through the back-side of through-conductive dielectric recording media. Alternatively, it can occur through the edges of x-y-plane-conductive (also referred to herein as x-y-conductive) dielectric recording media. This path to ground via the edges of the x-y-conductive dielectric recording media is generally sufficient for printing with low writing currents (e. g., for line drawings) or at very slow speeds. It is not sufficient,

however, for high fill images (e. g., at least about 50% fill), especially in the center of a wide film medium used for large format graphics, because the path to edge ground is much longer. When the path to edge ground is insufficient, excess negative charge in the conductive layer underneath the dielectric layer appears over the toner fountain before it can be effectively grounded out to the edges. When the excess negative charge appears over the toner fountain either by the charge spreading out in the conductive layer or in the relative movement of the media to the printing apparatus, the positively charged toner particles are attracted to the excess negative charges in the conductive layer. An image artifact known as "fogging"or"film ghosting"results, which produces poor image quality.

Another artifact that results from incomplete neutralizing of the charge on recording media is known as"cover over"or"overplating."This results when the toner does not neutralize all the charge on the sheet on which it is deposited and a subsequently applied toner deposits in these areas even though no additional charge was applied. This unwanted deposition of subsequent toner (s) produces contaminated colors and poor image quality.

Typically, for toners used with through-conductive dielectric recording media there is a tradeoff between achieving high print density (also referred to as reflective optical density), which typically requires low conductivity toners (often below about 500 picomhos/centimeter at 2% solids) with a low charge to mass ratio, and low cover over, which typically requires higher conductivity toners. Typically, toner formulations along with printer design and printing conditions have been optimized for through-conductive dielectric recording media to achieve desired reflective optical densities with little or no cover over. Unfortunately, however, such toners may not be suitable for use with certain techniques that reduce or eliminate fogging and/or cover over on x-y- conductive dielectric recording media, as they can result in poor image density.

For example, both fogging and cover over can be eliminated, or at least reduced, on x-y-conductive dielectric recording media by applying a bias voltage to the media or to least one station (e. g., a toning station, also known as a developing station or toner fountain) in an electrostatic printer (see, U. S. Pat. No.

5,815,188 (Speckhard et al.) and EP 0869 402 (Xerox Corp.)). However, when a bias voltage is applied, the print density of toners designed for printing on through- conductive dielectric recording media is typically reduced. Thus, when using such toners (those designed for printing on through-conductive media) to print on x-y- conductive dielectric recording media, the print density is decreased and thus the image quality is reduced.

Some other approches that have been tried to get good print density on x-y-conductive dielectric recording media (e. g., direct print polymeric films) without significant fogging or cover over include using a lower dielectric coating weight to increase apparent surface voltage of deposited charge; however, this coating weight control is difficult to implement. Also, the fogging field is increased and thus more bias may be needed to reduce the fogging with a lower dielectric coating weight, which may negate any increase in density. Another approach has involved printing at a slower speed (e. g., at no greater than about 0.5 inch (1.3 centimeter) per second); however, this is unacceptable from a productivity perspective. Yet another approach involves applying a positive bias to the conductive layer, as described in U. S. Pat. No. 5,055,862 (Hansen et al.); however, this also reduces the print density and may not reduce cover over if toner conductivity is decreased. Still another approach to get good print density on x-y- conductive dielectric recording media without significant fogging or cover over involves increasing the conductivity of the conductive layer. This helps to reduce the fogging field by better grounding; however, for multiplexed printers this also reduces print density due to collapse of the field from the plate electrodes. Another possible approach is to separate physcially the writing head and the toning station by a large distance to allow more time for the fogging field to dissipate. In a multipass printer a similar effect could be achieved by writing charge in a first pass and then toning the charge in a second pass. However, these approaches adversely affect productivity and/or the size and cost of operating the printer.

Summarv of Invention

Thus, what is needed are liquid toners that can be used with x-y- conductive dielectric recording media (e. g., direct print polymeric films) in printing systems and methods that apply a bias, preferably, a high negative bias field, at one or more stations, such as a toning station, of an electrostatic printer. The methods of the present invention preferably use a single-pass printer (i. e., printer that applies all toners in a single pass of the media) to produce a multicolor image (i. e., an image formed from two or more different colored toners).

In a preferred embodiment, the present invention provides a method of producing a multicolor electrostatic image including: providing an electrostatic printer having at least one toner station; applying a bias voltage to at least one toner station comprising a liquid toner; providing an x-y-plane-conductive dielectric recording medium (preferably, a vinyl-containing film); applying an imagewise distribution of charge on a first major surface of the x-y-plane- conductive dielectric recording medium; and intimately contacting the x-y-plane- conductive dielectric recording medium with a liquid toner from the biased toner station, wherein the liquid toner comprises a carrier liquid (preferably, a hydrocarbon carrier liquid) and toner particles, thereby depositing the toner particles in a pattern corresponding to the surface charge on the x-y-plane- conductive dielectric recording medium to form an imaged surface.

Advantageously, in this method the image is applied at a speed of at least about 0.5 inch per second (1.3 centimeters per second) with at least about 50% of the area of the imaged surface of the x-y-plane-conductive dielectric recording medium imaged by at least one toner that is subsequently contacted by another toner.

Significantly, the imaged surface has fog of no greater than about 1.5 and cover over of no greater than about 5, both of which are expressed as Delta E* units. The toner particles of at least one toner that is subsequently contacted by another toner are deposited in a thickness (i. e., amount per unit area) sufficient to otherwise provide a 100%-filled image Reflective Optical Density value selected from the group of about 1.2 to about 1.6 for magenta, about 0.8 to about 1.1 for yellow, about 1.2 to about 1.6 for cyan, about 1.2 to about 1.6 for black, and combinations thereof. By this it is meant that the image includes at least two toners, at least one

of which is in the desired range of 100%-filled image Reflective Optical Density.

Preferably, the toner that is in the desired range of 100%-filled image Reflective Optical Density is subsequently contacted by another toner (i. e., it is not the last toner deposited). Furthermore, preferably, toner particles from at least two of the toners, more preferably, at least three of the toners, and most preferably, at least four of the toners, are within the desired 100%-filled image Reflective Optical Density.

The methods of the present invention are significant because they can provide a relatively high reflective optical density by direct printing on an x-y- conductive dielectric recording medium at a relatively fast rate of image formation with little or no fog or cover over. This is particularly significant for vinyl- containing films or other x-y-conductive dielectric recording media of relatively large size, such as those of at least about 24 inches (61 centimeters) in width, preferably, at least about 36 inches (91 centimeters) in width, and more preferably, at least about 54 inches (137 centimeters) in width.

The rate at which the imagewise distribution of charge is applied is typically the same as the rate at which the toner particles are deposited, although the individual rates of each of these steps could be different. For the methods of the present invention, both rates are involved in determining the overall rate of image formation, although the step of applying a charge is typically the rate- controlling step. The overall rate of applying an image is preferably at least about 0.5 inch (1.3 centimeter) per second, more preferably, at least about 0.75 inch (1.9 centimeter) per second, and most preferably, at least about 1.0 inch (2.5 centimeter) per second.

The present invention also provides a direct print film having a multicolor electrostatic image disposed on a first major surface of an x-y-plane- conductive dielectric recording medium and an adhesive layer disposed on a second major surface of the recording medium. At least about 50% of the area of the imaged surface (i. e., the first major surface) of the x-y-plane-conductive dielectric recording medium is imaged by toner particles from at least one toner that is subsequently contacted by toner particles from at least one other toner. By

this it is meant that the toner particles that form an at least about 50%-filled image is not the last toner used in the process. Significantly, the imaged first major surface has fog of no greater than about 1.5 and cover over of no greater than about 5, both of which are expressed as Delta E* units. The image includes toner particles of at least one toner that is subsequently contacted by another toner, which are deposited in a thickness sufficient to otherwise provide a 100%-filled image Reflective Optical Density value selected from the group of about 1.35 to about 1.45 for magenta, about 0.90 to about 1.00 for yellow, about 1.30 to about 1.40 for cyan, about 1.40 to about 1.50 for black, and combinations thereof.

Preferably, the image includes all color toners within their target densities.

The present invention also provides a method of improving the print density of an electrostatic image at a fixed bias voltage, fixed print speed, and fixed writing voltage, the method comprising: providing an electrostatic printer having at least one toner station; applying a bias voltage to at least one toner station comprising a first liquid toner; providing an x-y-plane-conductive dielectric recording medium; applying an imagewise distribution of charge on a first major surface of the x-y-plane-conductive dielectric recording medium; intimately contacting the x-y-plane-conducti çe dielectric recording medium with a first liquid toner from the biased toner station, wherein the liquid toner comprises a carrier liquid and toner particles, thereby depositing the toner particles in a pattern corresponding to the surface charge on the dielectric medium to form an imaged surface having fog of no greater than about 1.5 and cover over of no greater than about 5, both of which are expressed as Delta E* units; and replacing the first liquid toner with a second liquid toner having a lower conductivity than the first liquid toner, and repeating the steps of applying a bias voltage, applying an imagewise distribution of charge, and intimately contacting the x-y-plane conductive dielectric recording medium with the second liquid toner to form an imaged surface having fog of no greater than about 1.5 and cover over of no greater than about 5, both of which are expressed as Delta E* units, wherein the imaged surface comprises toner particles of at least one toner that is subsequently contacted by another toner, which are deposited in a thickness sufficient to

otherwise provide a 100%-filled image Reflective Optical Density value selected from the group of about 1.35 to about 1.45 for magenta, about 0.90 to about 1.00 for yellow, about 1.30 to about 1.40 for cyan, about 1.40 to about 1.50 for black, and combinations thereof. Preferably, the first liquid toner has a conductivity sufficient to provide the same 100%-filled image Reflective Optical Density values when no bias voltage is applied to the toner station.

The invention also provides a liquid toner. The toner includes a carrier liquid and toner particles, wherein the toner particles are present in an amount of less than about 5 wt-% and capable of being electrostatically deposited in a pattern corresponding to the surface charge on an x-y-plane-conductive dielectric recording medium to form an imaged surface at a speed of at least about 0.5 inch per second. The toner particles are capable of being deposited in a thickness sufficient to otherwise provide a 100%-filled image Reflective Optical Density value selected from the group of about 1.2 to about 1.6 for magenta, about 0.8 to about 1.1 for yellow, about 1.2 to about 1.6 for cyan, about 1.2 to about 1.6 for black, and combinations thereof. Furthermore, the toner particles are capable of forming an imaged surface wherein at least about 50% of the area of the imaged surface of the x-y-plane-conductive dielectric recording medium is imaged by at least one toner that is subsequently contacted by another toner, and the imaged surface has fog of no greater than about 1.5 and cover over of no greater than about 5, both of which are expressed as Delta E* units.

As used herein,"fog" (or sometimes referred to as film ghosting or leading edge fog) is a term used to describe faint but unacceptable toning of areas on an electrostatic graphic due to a background field in the conductive layer.

Although the decision of what is considered unacceptable is generally subjective and varies with the type of image being printed, the actual amount of fog can be measured as a color difference from the normal base color (e. g., white) background using a colorimeter or spectrophotometer and expressed in"Delta E*"units, which is described in greater detail below. Fogging is proportional to the charge per unit area per unit time. Thus, it is proportional to the print speed (amount of fogging increases as print speed increases) and the percent fill or percent coverage for an

individual color (toner) in the image. Percent fill or percent coverage for a single color toner is the area of the media that is written with charge and then toned with a single color toner to produce an image compared to the total area of the imaged surface of the medium. Thus, if an entire surface of the medium is covered with a single toner the percent fill or percent coverage is 100% (the image is referred to as a 100%-filled image), whereas if only half the imaged surface is covered with a single toner the percent fill or percent coverage is 50%.

As used herein,"cover over" (sometimes referred to as"plating"or "overplating") is a term that describes where one color gets"covered over"by a second color in areas where the second color is not supposed to deposit (no additional charge written). It happens when the first toner does not satisfy essentially all of the charge deposited by the writehead on the dielectric surface.

Then when the surface is subsequently contacted by the second toner it may deposit to some extent thereby contaminating the desired color of the first toner.

Cover over can be evaluated visually or by a color shift measurement. In the color shift measurement approach the change in color of a solid area of a single toner (primary color) such as cyan is measured before and after exposure to a subsequent toner using standard colorimeteric values as described in greater detail below. The change in color before an after the second toner exposure can be summarized in terms of Delta E* values.

As used herein, a 100%-filled image Reflective Optical Density is the reflective optical density measured according to the test procedure outlined in the Examples Section. This test refers to an image formed in a 100%-filled image test pattern prepared prior to carrying out the imaging process described herein.

Typically, this involves routine eyperimentation in selecting the processing conditions (e. g., imaging speed, bias voltage, writing voltage) to provide the desired target ranges of reflective optical density for each color toner. Once the processing conditions are obtained, the process of the present invention is carried out to form a multicolor image.

Brief Description of Drawings Fig. 1 is a cross-sectional view of a film on which the toners of the present invention can be deposited.

Embodiments of Invention This invention describes liquid toners that achieve better image quality on x-y-conductive dielectric recording media, such as direct print vinyl films. These toners have relatively low conductivity, yet result in acceptable print density with reduced fogging and cover over when printed with a bias voltage.

Toners Liquid toners are well known in the imaging arts. See, for example, Schmidt, S. P.; Larson, J. R.; Bhattacharya, R. in Handbook of Imaging Materials, Diamond, A. S., Ed.: Marcel Dekker, New York, 1991, pp. 227-252 or Lehmbeck, D. R. in Neblette's Handbook of Photography and Reprography, Sturge, J., Ed.: Van Nostrand Reinhold, New York, 1977, Chapter 13, pp. 331-387. A liquid toner is a dispersion of colloidal size particles, hereinafter referred to as"toner particles" in a carrier liquid (or dispersing medium) which has a low dielectric constant. The toner particles typically include a colorant and a film-forming resin and carry an electrostatic charge. The toner particles in the dispersion are capable of migrating under the influence of an electric field and being deposited on a surface bearing an opposite charge, thereby forming an image.

Toner particles typically contain a colorant, which may be in the form of pigments, which may be fluorescent, dyes, or other materials that provide additional properties when exposed to ultraviolet light, in addition to charge directors (i. e., charge control agents) and film-forming resins. The toners of the present invention preferably include pigments. Another advantage of pigments as colorants for toners is the durability of the color and high optical densities of the image that can be achieved.

A wide variety of pigments known in the art for use in toners may be used in the toner particles of this invention. Such pigments include, but are not

limited to, phthalocyanines, such as copper phthalocyanine; carbon black; nigrosine dye; Aniline Blue; Chrome Yellow; DuPont Oil Red from DuPont; Monoline Yellow; Sunfast Blue, Sunfast Magenta, Sun Yellow, Sun Red and other pigments available from Sun Chemical Co.; Fanchon Fast Yellow, Quindo Magenta and other pigments from Bayer; Harmon Quindo Red; Regal 300; Fluorol Yellow 088, Fluorol Green Gold 084, Lumogen Yellow S 0790, Ultramarine Blue, Ultramarine Violet, Ferric Ferrocyanide, and other pigments available friom BASF; Malachite Green Oxalate; lamp black; Rose Bengal; Malastral Red; magnetic pigments such as magnetite, ferrites, such as barium ferrite and manganese ferrite, hematite, etc.

Charge directors are used to improve the dispersion stability of the film-forming resin. Examples include polyvalent metal ions, which are typically introduced in the form of a metal salt. Examples of metal salts include metal ions and organic anions as the counterion. Examples of metal ions are Ba (II), Ca (II), Mn (II), Zn (II), Zr (IV), Cu (II), AI (III), Cr (III), Fe (II), Fe (III), Sb (III), Bi (III), Co (II), La (III), Pb (II), Mg (II), Mo (III), Ni (II), Ag (I), Sr (II), Sn (IV), V (V), Y (III), and Ti (IV). Examples of organic anions are carboxylates or sulfonates derived from aliphatic or aromatic carboxylic or sulfonic acids. Examples of aliphatic fatty acids include stearic acid, behenic acid, neodecanoic acid, diisopropylsalicylic acid, octanoic acid, abietic acid, n phthenic acid, octanoic acid, lauric acid, tallic acid, and the like. Other charge directors (i. e., charge control agents) are disclosed in U. S. Pat. No. 5,744,269 (Bhattacharya et al.).

The toner particles of this invention may be dispersed in a carrier liquid to form a liquid toner. Any of a wide variety of carrier liquids may be used.

Typically, the carrier liquid is a hydrocarbon that has a low dielectric constant (e. g., less than about 3) and a vapor pressure sufficiently high to ensure rapid evaporation of solvent following deposition of the toner onto recording media.

Rapid evaporation is particularly important for cases in which multiple colors are sequentially deposited and/or transferred to form a single image, particularly when a single-pass printer is used.

Preferred hydrocarbon solvents have a resistivity of at least about 10"ohm-centimeter (Q-cm) and preferably at least about 10l3 Q-cm, a dielectric constant less than about 3.5, and a boiling point in a range of about 140°C to about 220°C. This includes aliphatic hydrocarbons and aromatic hydrocarbons.

Hydrocarbon solvents that are useful in this invention include higher molecular weight hydrocarbons such as mineral spirits, D-Limonene, kerosene, and those commercial hydrocarbon solvents available under the trade designations Isopar, Norpar'M, and ExxsolTM.

The toner particles may be stabilized in the carrier liquid electrostatically or sterically, or both. Electrostatic stabilization involves the incorporation of a charged species, such as Zr4+, into the particle, which repels like- charged particles, thereby preventing agglomeration. Steric stabilization involves the use of a solvent-swollen polymeric shell insolubilized at the surface but having soluble polymeric chains extending into the solution. These soluble chains provide for volume exclusion, thereby preventing the approach of another particle and aggregation of particles in dilute toner solutions.

In addition to the colorant, the toner particles include a film-forming resin. The resin stabilizes the dispersion of the toner particles in the carrier liquid by means of the solvated floating chains of the resin as well as by electrostatic charges, which could be imparted to the resin either by metal ion complexation or the chemical structure of the resin. The resin assists in film formation of toner deposited on a receptor by the coalescence of the toner particles and the fixing of the pigment particles. In addition, a resin which provides the toner film with differential adhesion properties with respect to the substrate surfaces is usually desired. Finally, the glass transition temperature and the morphology of the resin control other toner film properties such as scratch resistance and overprintability of different colors.

One such film-forming resin includes an amphipathic, non-gel copolymer as disclosed in U. S. Pat. No. 5,744,269 (Bhattacharya et al.). This copolymer has at least one polymer segment that is soluble in the liquid carrier and at least one polymer segment that is insoluble in the liquid carrier. The soluble

polymer segment includes at least one alkyl acrylate or alkyl methacrylate monomer optionally copolymerized with at least one vinyl aromatic monomer.

The insoluble polymer segment includes at least one vinyl ester monomer optionally copolymerized with at least one acrylic acid or methacrylic acid monomer.

Another such film-forming resin includes an organosol as all or part of the resin component. Organosols usually comprise a thermoplastic resinous portion, which is not highly soluble in the carrier liquid, chemically anchored to steric stabilizing polymers or copolymers which are soluble in the carrier liquid.

When dispersed in the carrier liquid, the solubility differences of these two portions of the organosol cause it to have a core-shell or"micelle"type of structure, where the substantially insoluble thermoplastic resin forms a"core"which is surrounded by the"shell"of soluble polymer or copolymers. When the carrier liquid evaporates, either the core or shell properties can dominate the physical properties of the resulting film. One such ccre-shell resin is disclosed in U. S. Pat. No.

5,753,763 (Rao et al.). Toner particles are typically made by a process having the steps of polymerizing the resin or binder (this step typically occurs in a solvent), milling the resin with pigment particles, drying the milled mixture, grinding the dried mixture to form colloidal sized particles and suspending the particles which are a composite of pigment and resin in the carrier liquid.

The liquid toners of this invention are preferably prepared in a concentrated form to conserve storage space and reduce transportation costs. In order to use the toners in the printer, the concentrate is typically diluted with additional carrier liquid to give what is termed the working strength liquid toner.

An example of how working strength liquid toners are used is disclosed in U. S.

Pat. No. 5,832,834 (Corn et al.). Toners are typically prepared as dispersions that contain from about 10% to about 30% by weight solids. Those that are actually used in imaging processes can be prepared by diluting the foregoing dispersions to concentrations of from about 1% to about 5% by weight solids. Preferably, the liquid toners used in the present invention have conductance (i. e., conductivity)

values of about 50 picomhos/centimeterto about 500 picomhos/centimeter for a dispersion containing 2% by weight solids.

In conventional electrostatic printing processes, the life of a starting liquid toner (also known as a working strength toner or developer or premix) is prolonged by replenishing the system with a supplemental toner (also known as a concentrate, replenisher, or intensifier), which has a higher pigment (percent solids) level, at a point in time prior to the starting liquid toner demonstrating diminished image quality. Supplemental toners typically have at least about 20% lower conductance values (when compared at the same percent solids) than the working strength toners. They are used to replenish the system with pigment/resin and charge carriers to increase the toner life. They are not normally used as the starting toners because their low conductivity leads to cover over and/or higher than desired print densities which can lead to other artifacts. Such a method, although for electrostatic photography rather than electrostatic printing, is disclosed in U. S. Pat. No. 4,886,730 (OtA et al.) and GB 1,411,287.

Surprisingly, to increase the optical density of the image, the present invention can use typical supplemental toners (i. e., those having at least about 20% lower conductance values than the working strength toners) as the starting toner.

Surprisingly, this is done without the need for increasing the bias voltage (described below) to reduce cover over and/or fogging. This is significant in that lower conductivity toners typically provide problems with increased cover over and have been believed to provide problems with increased fogging

Recording Medium Referring to Fig. 1, a typical construction of an x-y-plane conductive dielectric recording medium in the form of a film 10, to which the toners of the present invention can be applied, comprises a substrate film 12 having on a major surface thereof, a conductive layer 14 and a dielectric layer 16. On the opposite major surface of film substrate 12 resides optional pressure sensitive adhesive 18 protected by a release liner 20. Such films are disclosed in U. S. Pat.

No. 5,736,228 (Morris et al.).

Substrates Substrates are preferably made of a durable material that resists swelling or other loss of continuity when coated with the conductive layer and also resists deleterious effects of exterior signing environments including large ambient temperature ranges of from about-60°C to about +107°C, as well as direct exposure to sun. Such materials are preferably conformable for fixing to exterior surfaces wherein the film may be adhered over surfaces with some compound curvature or non-uniformity, e. g., walls or surfaces with screw heads or rivets slightly proud of the surface without easily ripping the material or"tenting. "However, in some aspects of the invention, the substrate need not be limited to these durable, conformable substrates. A less durable plastic is useful for interior signing applications.

Substrates can be clear, translucent, or opaque depending on the application of the invention. Opaque substrates are useful for viewing an image from the image side of the printed sheet in lighting conditions such as artificial lighting or sunlight. Translucent substrates are particularly useful for backlit usages, for example, a luminous sign. Preferred substrates are of a relatively large size, such as those of at least about 11 inches (28 centimeters) in width, preferably, at least about 36 inches (91 centimeters) in width, and more preferably, at least about 54 inches (137 centimers) in width.

Substrates useful in the practice of the present invention are commercially available and many are designed to be exterior durable, which is

preferred. Nonlimiting examples of such substrates include Scotchcal Marking Films and Scotchcal Series 9000 Short-Term Removable (STR) Film available from 3M Company, AveryTM GLTM Series Long Life Films, AveryTM XLTM Series Long Life Films, Avery SXrm Series Long Life Films, suitable films from the FasCalTm or FasFlex range of films or any other suitable marking, graphic or promotional films available from Fasson, Avery, or Meyercord. However, other manufacturers of suitable materials exist and the invention shall not be limited to the above. Almost any material composed of a plastic sheet could be used depending on the use of the final image, for example, whether outdoor durability is required, and providing that the conductive layer can adhere to the film surface sufficiently well.

Useful substrates can have a variety of surface finishes such a matte finish as provided with Scotchcal Series 9000 Short-Term Removable (STR) Film or glossy finish as provided with ScotchcaF 3650 Marking Film. Plastic films can be extruded, calendared or cast different plastic materials may be used, such as those exemplified by the Scotchcal plasticized poly (vinyl chloride) or Surlyn, an ionomer. Any suitable plastic material can be employed. Nonlimiting examples include polyester materials exemplified by MylarTM available from E. I. Du Pont de Nemours & Company, Melinex available from Imperial Chemicals, Inc., and Celanafrm available from Celanese Corporation. Preferred materials for substrates can include those that are plasticized poly (vinyl chloride) s or ionomers although the invention is not limited to these. Preferred materials are white opaque or translucent materials but transparent materials and colored opaque, translucent or transparent materials could be useful in special applications.

Typical thicknesses of the substrate are in the range of about 0.05 millimeters (mm) to about 0.75 mm. However, the thickness can be outside this range and almost any thickness can be useful provided the film resists tearing or splitting during the printing and application process. Given all considerations, any thickness is useful provided the substrate is not too thick to feed into an electrostatic printer of choice.

Conductive Layer For electrostatic imaging on film 10, a conductive coating layer 14 is provided from an organic solvent-based conductive coating solution on the upper major surface of film substrate 12. The organic solvent-based conductive coating solution does not swell conformable substrates used in the present invention.

Furthermore, conductive coating solutions employing organic solvents are used to assure that the conductive layer has good ply adhesion with the conformable substrate surface. Also, use of organic solvents in the conductive coating solutions permit the conformable substrate to avoid any priming of its upper major surface to receive the conductive layer. Better wettability can be achieved on an unprimed substrate, to avoid foaming caused by aqueous based coating solutions.

The conductive coating layer can be electronically conductive or ionically conductive. Electronically conductive layers employ a plurality of particles of a transparent, electrically conductive material such as antimony doped tin oxide or the like, disposed in a polymeric matrix.

Attributes of conductive layer 14 include adhesion to film substrate 12, deposition using a suitable solvent system, and moisture insensitivity after the layer 14 is dried on substrate 12. When an electrically conductive layer is desired, conductive layer 14 is prepared from a solution of a conductive formulation that generally comprises a binder, conductive pigments, dispersant, and organic-based solvent, the latter of which is removed during the manufacturing process. The weight percent of solids to organic solvent in the conductive formulation can range from about 10 weight percent (wt-%) to about 40 wt-%, with about 25 wt-% being presently preferred for ease of application to film substrate 12.

After coating of a conductive formulation on film substrate 12 and evaporation or other removal of organic solvent, the thickness or caliper of the conductive layer 14 can range from about 2 micrometers (pm) to about 5 um with about 3 am being presently preferred. As stated above, the conductive layer 14 should have a surface resistance ranging from about 0.2 megaohms per square to about 3 megaohms per square.

Nonlimiting examples of binders include acrylics, polyester, and vinyl binders. Specific examples of suitable binders are disclosed in U. S. Pat. No.

5,736,228 (Morris et al.).

Conductive pigments can include antimony-containing tin oxide pigments or other pigments such as indium doped tin oxide, cadmium stannate, zinc oxides, and the like. Specific examples of suitable conductive pigments are disclosed in U. S. Pat. No. 5,736,228 (Morris et al.). Particle size of the conductive pigments in the conductive layer 14 can range from about 0.02, um to about 10 pLm.

Preferably, the average particle size can range from about 0.5 gm to about 4 llm, with particles of about 1 um being most preferred. The bulk powder resistivity of the conductive pigments can range from about 2 Ohm-centimeter (Q-cm) to about 15 Q-cm with about 2 Q-cm to about 10 Q-cm being preferred and about 6 Q-cm to about 7 Ohm-cm being presently preferred.

A variety of surfactant materials can be employed as dispersants for the conductive layer 14 in the present invention, including nonionic and anionic dispersants. In general, anionic dispersants are most preferred, although the invention is not limited thereto. Specific examples of suitable surfactants are disclosed in U. S. Pat. No. 5,736,228 (Morris et al.).

Nonlimiting examples of solvents for the conductive formulation include ethyl acetate and ethanol.

Formulations of the conductive layer 14 require a weight ratio from about 5: 1 to about 1: 1 of pigment: binder with a preference of a weight ratio of about 3: 1 pigment: binder. When the pigment to binder ratio falls below about 1: 1, there is inadequate bulk conductivity of layer 14. When the weight ratio of pigment: binder exceeds about 5: 1, there is insufficient cohesive strength of the layer 14 on film substrate 12.

Dielectric Laver Dielectric layer 16 can be coated on conductive layer 14 to provide the electrostatic capacitance required for electrostatic imaging. The dielectric layer 16 is of relatively high electrical resistivity and contributes to the performance of film 10 for direct printing of images electrostatically. In addition to providing the interface of film 10 with the recording head and toner, dielectric layer 16 covers and protects conductive layer 14 and provides the top surface for film 10.

Dielectric layer 16 is coated on layer 14 from a dielectric formulation that comprises particulate matter of both spacer particles and abrasive particles, preferably in particular ratios dispersed in a binder.

Both the spacer particles and the abrasive particles should be selected with consideration to the refractive index thereof, so as to provide index matching to the remainder of dielectric layer 16 and film 10. In this manner, film 10 has a uniform appearance. This is especially so when transparent products are desired. In the case of opaque products, a uniform appearance would not be critical.

The spacer particles can be fabricated from a material having sufficient rigidity to withstand coating and handling, but need not be highly abrasive. Nonlimiting examples of materials useful as spacer particles include relatively soft materials such as a polymer or a mineral such calcium carbonate or relatively hard materials such as silica or glass, provided that such relatively hard materials have a relatively rounded configuration. More particularly, useful spacer particles can be made from synthetic silicas, glass micro beads, natural minerals (e. g., calcium carbonate), polymeric materials such as polypropylene, polycarbonate, fluorocarbons or the like. Typically spacer particles have an average size ranging from about 1 um to about 15 um, and preferably below about 10 pm. In general, spacer particles will be present in a distribution of sizes, although it is most preferred that the particles remain in a size range of about 3 gm to about 10 um.

Abrasive particles useful for dielectric layer 16 of the present invention are provided to assure that the performance of spacer particles and abrasive are effectively decoupled so as to provide an optimized dielectric medium.

The abrasive particles will generally be harder than the spacer particle material chosen and will usually have a more irregular configuration or texture than the spacer particle material. Among some of the preferred abrasive materials are silica materials such as microcrystalline silica and other mined or processed silicas, as well as other abrasives such as carbides and the like. The abrasive particles generally have the same size range as the spacer particles, typically in the range of about 1 um to about 15 um, and preferably less than about 10 um.

The proportion of spacer particles to abrasive particles are such that the spacer particles are present in a larger amount. Preferably, the ratios of spacer to abrasive particles fall within the range of about 1.5: 1 to about 5: 1. Most preferably, the ratio of spacer to abrasive particles is approximately 3: 1.

The spacer particles and abrasive particles are disposed is a binder which generally comprises a polymeric resin. The resin should be of fairly high electrical resistivity, and should be compatible with both types of particles and the toner. The resin should have sufficient durability and flexibility to permit it to function in the electrostatic imaging process and should be stable in ambient atmospheric conditions. There are large number of resins that meet these criteria, such as acrylic copolymers of the type commercially available from Rohm and Haas of Philadelphia, PA under the brand"Desograph-E342-R".

A coating mixture to prepare dielectric layer 16 can employ solvents such as toluene into which the binder, spacer particles, and abrasive particles can be added as solids. The range of total solids in the coating mixture can be from about 10 wt-% to about 35 wt-%, and preferably about 15 wt-% to about 25 wt-% of the total coating mixture. Of the total solids, the binder solids can comprise from about 93 wt-% to about 78 vit-%, and preferably 82 wt-%. Of the total solids, the particles solids (preferably in a 3: 1 spacer: abrasive mixture) can comprise from about 7 wt-% to about 22 wt-%, and preferably aboutl8 wt-%.

The particle solids for the coating mixture can be blended by ball milling for approximately two hours at room temperature. Under these conditions, there is no significant reduction in particle morphology, and the ball milling process only serves to mix and disperse the particles. Other processes could be employed.

Optional Pressure Sensitive Adhesive and Liner Any conventional pressure sensitive adhesive used for the construction of image graphic films can be used with the films 10 of the present invention. Pressure sensitive adhesive 18 can be coated on film substrate 12 prior to, or contemporaneous with, construction of the film 10 of the present invention.

Nonlimiting examples of pressure sensitive adhesives useful with the present invention include those adhesives described in U. S. Pat. No. 5,736,228 (Morris et al.). Thickness of adhesive 18 can range from about 0.012 mm to about 1 mm with a thickness of about 0.025 mm (1 mil) being preferred.

Liner 20 can be constructed from any conventional release liner known to those skilled in the art for image graphic media. Nonlimiting examples are described in U. S. Pat. No. 5,736,228 (Morris et al.), for example.

Conductive Groundinz Stripe Referring again to Fig. 1., a pair of electroconductive ground stripes 22 and 24 can be provided in order to aid in the prevention of"leading edge fog" by providing an avenue for residual charge to be eliminated from the ground plane.

These stripes 22 and 24 ranging from about 0.76 mm to about 2.54 mm wide are applied to dielectric layer 16 at opposing lateral edges of film 10.

Stripes 22 and 24 can be made from a conductive ink sold under the brand"Multifilm, Conductive Black Ink 9093E20J"from Raffi and Swanson of Wilmington, MA and are configured to permeate dielectric layer 16 at such lateral edges of film in order to provide an electrical ground to the conductive layer 12.

Thus, a film 10 of the present invention can have in sequential order, a release liner 20 comprising from about 0.07 mm to about 0.15 mm (about

3 mils to about 6 mils) thickness, a field of pressure sensitive adhesive 18 comprising about 0.03 mm (about 1 mil) thickness, a film substrate 12 comprising from about 0.05 mm to about 0.10 mm (about 2 to about 4 mils) thickness, a conductive coating layer 14 comprising from about 1 pm to about 5 pm (about 0.04 mils to about 0.2 mils), a dielectric layer 16 comprising from about 2 pm to about 4 gm (about 0.08 mils to about 0.16 mils) thickness, and a pair of electroconductive ground stripes 22 and 24 at lateral edges of film 10 that permeate layer 16 to layer 14.

Electrostatic Printer The toners of the present invention can be used with the x-y-plane conductive dielectric recording media described herein in a method and antifogging apparatus for an electrostatic printer as described in U. S. Pat. No.

5,815,188 (Speckhard et al.). This system includes a power supply, means for applying voltage from the power supply to at least one station in the electrostatic printer, and means for electrically isolating printing media from at least one toner station. The method involves applying voltage to at least one station in the electrostatic printer; and electrically isolating printing media from at least one toner station.

In this printer, the print medium is transported across consecutive printing stations, each printing station includes an electrostatic charging head and toner station. In a printer that applies the four basic colors (cyan, magenta, yellow, and black), there are four printing stations, each station applying one of the four colors to the print medium. Electrically connected to each toner station, upon which there are insulators, is an electrical wire or wires and a power console, which contains sufficient electrical or electronic circuitry to provide negative bias to each toner station as desired. The present invention may also be implemented in a multi- pass printing system or a printer with optional 5 or more stations.

Since the bias voltage should be set at the minimum needed value to reduce the amount of optical density loss, it is likely that in many cases the amount of voltage needed to significantly reduce fogging will vary significantly from one

color to the next not just because of different amounts of data being written in each color but because of differences in the voltage applied for each color (often different to obtain desired density) or because of differences in toner characteristics.

The range of negative biasing of a toner station can range from about 0 volt to about 50 volts and up to about 10 mAmps and is quite dependent on characteristics of the media, printer speed, the amount of data written, and the like.

Preferably, the range of negative biasing of a toner station can range from about 1 volt to about 30 volts and up to about 1 mAmp. In the methods of the present invention, preferably each toner station has a bias voltage applied thereto, which may or may not be different.

Methods of the Present Invention The present invention provides methods of forming an image.

Images formed by the present invention may be of a single color or a plurality of colors, preferably, however, images formed by the present invention are multicolor images (i. e., images formed using more than one different colored toner).

Multicolor images can be prepared by repetition of the charging and toner application steps for the various colors desired. In multicolor imaging, the toners may be applied to the surface of the image sheet in any order desired for the best results for the particular application.

Using the methods of the present invention, the following preferred print densities, reported as 100%-filled image Reflective Optical Density values, for the four colors can be achieved: magenta = about 1.2 to about 1.6; yellow = about 0.8 to about 1.1; cyan = about 1.2 to about 1.6; and black = about 1.2 to about 1.6. Using the method of the present invention, the following more preferred print densities for the four colors can be achieved: magenta = about 1.35 to about 1.45; yellow = about 0.90 to about 1.00; cyan = about 1.30 to about 1.40; and black = about 1.40 to about 1.50. At least one toner (preferably, at least two, more preferably, at least three, and most preferably, at least four toners) is in the desired range of 100%-filled image Reflective Optical Density.

The methods of the present invention advantageously use lower conductivity (i. e., conductance) toners than previously thought possible, particularly for preparation of images that cover at least 50% of the area of the media (i. e., 50% fill). Preferably, this can be accomplished without substantially increasing (preferably, by not more than about 10%) the fog or cover over under the same conditions of operation. That is, once the conditions for providing an image are determined using a first toner (e. g., bias voltage, print speed, writing voltage, etc.), the same conditions can be used without significantly increasing fog or cover over with a second toner having a lower conductance value (preferably, at least about 20% lower) than that of the first toner. This is significant in that it would normally be expected that the bias voltage would need to be increased to reduce the amount of cover over (and perhaps, even fog).

For example, a preferred method of improving the print density of an electrostatic image at a fixed bias voltage, which can vary for each toner, fixed print speed, and fixed writing voltage (i. e., the voltage applied to the writing head of the printer for applying an imagewise distribution of charge, which can be different for each toner), the method comprising: providing an electrostatic printer having at least one toner station; applying a bias voltage to at least one toner station comprising a first liquid toner; providing an x-y-plane-conductive dielectric recording medium; applying an imagewise distribution of charge on a first major surface of the x-y-plane-conductive dielectric recording medium; intimately contacting the x-y-plane-conductive dielectric recording medium with a first liquid toner from the biased toner station, wherein the liquid toner comprises a hydrocarbon carrier liquid and toner particles, thereby depositing the toner particles in a pattern corresponding to the surface charge on the dielectric medium to form an imaged surface having fog of no greater than about 1.5 and cover over of no greater than about 5, both of which are expressed as Delta E* units; and replacing the first liquid toner with a second liquid toner having a lower conductivity than the first liquid toner, and repeating the steps of applying a bias voltage, applying an imagewise distribution of charge, and intimately contacting the x-y-plane conductive dielectric recording medium with the second liquid toner to form an

imaged surface having fog of no greater than about 1.5 and cover over of no greater than about 5, both of which are expressed as Delta E* units, wherein the images surface comprises toner particles deposited in a thickness sufficient to otherwise provide a 100%-filled image Reflective Optical Density value selected from the group of about 1.35 to about 1.45 for magenta, about 0.90 to about 1.00 for yellow, about 1.30 to about 1.40 for cyan, about 1.40 to about 1.50 for black, and combinations thereof. Preferably, the first liquid toner has a conductivity sufficient to provide the same 100%-filled image Reflective Optical Density values when no bias voltage is applied to the tone-station. More preferably, the conductivity of the second liquid toner is at least about 20% lower than that of the first liquid toner.

The methods of the present invention are significant because they can provide a relatively high reflective optical density by direct printing on an x-y- conductive dielectric recording medium at a relatively fast rate of image formation with little or no fog or cover over. This is particularly significant for vinyl- containing films or other x-y-conductive dielectric recording media of relatively large size, such as those of at least about 24 inches (61 centimeters) in width, preferably, at least about 36 inches (91 centimeters) in width, and more preferably, at least about 54 inches (137 centimeters) in width. The overall rate of applying an image is preferably at least about 0.5 inch (1.3 centimeter) per second, more preferably, at least about 0.75 inch (1.9 centimeter) per second, and most preferably, at least about 1.0 inch (2.5 centimeter) per second.

The level of fog is generally subjective and varies with the type of image being printed. The actual amount of fog can be measured as a color difference from the normal base color (e. g., white) background using a colorimeter or spectrophotometer such as a Gretag SPM 50 spectrophotometer (Gretag Imaging, Chicopee, MA) per ASTM 2244 and expressed in"Delta E*"units.

Delta E* = sq. root of [ (L*-L*,, f)' + (a*-a*"-b where: L* = L* value for the fog area on the film; a* = a* value for the fog area on the film; b* = b* value for the fog area on the film; L*ref= L* value for the unprinted film; a*ref= a* value for the unprinted film; and b*,, f = b* value for the unprinted film.

The following fog data was collected and subjectively rated for acceptability: Table 1

COLOR DELTA E* SUBJECTIVE RATING BLACK 0. 58 ACCEPTABLE BLACK 0. 70 ACCEPTABLE BLACK 0. 72 ACCEPTABLE BLACK 0. 78 ACCEPTABLE CYAN 0. 88 ACCEPTABLE BLACK 0. 98 ACCEPTABLE CYAN 1. 08 MARGINAL BLACK 1. 19 MARGINAL BLACK 1. 26 MARGINAL CYAN 1. 33 MARGINAL YELLOW 1. 53 MARGINAL CYAN 3. 05 UNACCEPTABLE BLACK 3. 25 UNACCEPTABLE MAGENTA 3. 65 UNACCEPTABLE CYAN 3. 93 UNACCEPTABLE MAGENTA 4. 06 UNACCEPTABLE CYAN 4. 12 UNACCEPTABLE MAGENTA 4. 23 UNACCEPTABLE YELLOW 5. 08 UNACCEPTABLE BLACK 6. 07 UNACCEPTABLE BLACK 8. 38 UNACCEPTABLE Based upon this, the upper limit for Delta E* is preferably at about 1.5. More preferably, an acceptable level of fog is no greater than about 1.0.

Fogging is proportional to the charge per unit area per unit time.

Thus, it is proportional to the print speed (amount of fogging increases as print speed increases) and the percent fill or percent coverage for an individual color (toner) in the image. Percent fill or percent coverage for a single color toner is the area of the media that is written with charge and then toned with a single color toner to produce an image compared to the total area of the imaged surface of the medium. The percent fill or percent coverage is often determined roughly by the percentage of the dots in the image that are"on,"which corresponds to the individual nibs being fired once in each scan line to produce each dot. However, because of dot overlap this will not correspond directly to the actual percent coverage or fogging (see, for example, U. S. Pat. No. 5,055,862 (Hansen et al.)).

Cover over can be evaluated visually or by a color shift measurement. In the color shift measurement approach the change in color of a solid area of a single toner (primary color) such as cyan is measured before and after exposure to a subsequent toner using standard colorimeteric values (such as L*, a* and b*-see ASTM test method D2244-93). The change in color before and after the second toner exposure can be summarized in terms of Delta E* values.

Samples of the color before and after the second toner exposure can be created on multipass printers by running the same print twice. On one of the prints the process is terminated before the second color is printed. For a single pass printer a long downweb solid block of the first color is printed and the print is terminated while it is still printing so that portions of the solid toner color can be measured before and after exposure to the second toner.

The range of acceptable Delta E* values for cover over will depend somewhat on the image and colors being printed, the intended viewing distance, application for the graphic, and many other factors. Generally, values of Delta E* no greater than about 2 are most preferred and values no greater than about 5 are preferred, while values up to about 15 or 20 may be acceptable in some applications. Probably the most sensitive color combination is yellow cover over by cyan. In this case a Delta E* value of no greater than about 1 is most preferred, no greater than about 2 is preferred, while no greater than about 5 is acceptable for

some applications. As expected, uniform cover over is more tolerable than situations where the cover over varies on a visible scale due to variations in the media or the uniformity of the writing/toning/drying processes.

For an appreciation of the scope of the present invention, the examples and testing methods follow.

Testing Methods Color Shift: ASTM D 2244-93 published by American Society for Testing and Materials.

Color Density:"Reflective Optical Density on a Status T Method" under the requirements of ANSI/ISO 5/3-1984, ANSI PH2.18-1985 published by the Graphic Communications Association of Arlington, Virginia.

Toner Conductivity: The liquid toner conductivity was determined at approximately 18 Hz using a Scientifica model 627 conductivity meter (Scientifica Instruments, Inc., Princeton, NJ).

Example 1 The following example makes use of the following standard (ST) liquid toners, which are available from Minnesota Mining and Manufacturing Co., St. Paul, MN, USA listing the alte rnating current (ac) conductivity in picomhos/cm in parentheses: Scotchprint Toner Premix Exterior Yellow 8730 (290); ScotchprintTM Toner Premix Exterior Magenta 8731 (70); Scotchprint Toner Premix Exterior Cyan 8732 (220); Scotchprint Toner Premix Exterior Black 8733 (160).

Special high print density/low conductivity (LC) toners were also prepared by making 2 wt-% solids premixes form the following extended life concentrates, which are available from Minnesota Mining and Manufacturing Co., St. Paul, MN, USA, (alternating conductivity in picomhos/cm listed in parentheses): Scotchprint Toner Concentrate Yellow 8835 (189); Scotchprint Toner Concentrate Magenta 8836 (32); Scotchprint Toner Concentrate Cyan 8837 (99); ScotchprintTM Toner Concentrate Black 8838 (45).

All printing was done on a Scotchprint 2000 Electrostatic Printer commercially available from Minnesota Mining and Manufacturing Company (3M) of St. Paul, MN, USA. The parameter set-up was as follows: striation settings = 3 for all colors, firing sequence = 1; backrest rollers on nibline; toner <BR> <BR> <BR> <BR> <BR> rpm = 400 for black (K), cyan (C), and Yellow (Y); 300 for Magenta (M) ; media = ScotchprintTM Transfer Media No. 8601 paper and ControltacTM DES-184-10 direct print vinyl film, both of which are commercially available from Minnesota Mining and Manufacturing Co., St. Paul, MN, USA. The toners are referred to below as either standard (ST) for comparison purposes or low conductivity (LC).

Table 2 Sample Nib & Plate Speed Bias Reflective Optical Density Comments Voltage Voltage K Y C M 1.8601/ST 300 2. 0 0 1. 56 1. 01 1. 53 1. 49 No fog or cover over 2. DES/ST 300 1.0 0 1.47.90 1.43 1.43 Lots of fog 3. DES/ST 300 1.0-10 1.45.83 1.31 1.35 Some fog 4. DES/ST 300 1.0-20 1.40.69 1.18 1.33 Slight fog 5. DES/ST 300 1.0-30 1.36.57 1.05 1.23 Very slight fog 6. DES/LC 300 1.0-30 1.52 1.00 1.45 1.39 Slight fog, Y covered 7. DES/LC 300 1.0-40 K, C 1.50.88 1.34 1.41 Slight fog, Y -30 covered M, Y by Magenta 8. DES/LC 300 1.0-10 1.56 1.08 1.54 1.51 Some fog, Y covered 9. DES/LC 300 1.0-40 K 1.50.92 1.40 1. 34 Slight fog, No -35 M, cover on Y C -25 Y 10.300 2.0 0 1.56 N. D. 1.48 1.53 High density- 8601/LC Lots of cover over on Y and some on C 11.250 2.0 0 1.54 1.07 1.53 1.45 Less cover over 8601/ST on Y No cover on C 12.250 2.0-10 K, 1.56 1.01 1.53 1.08 No cover over 8601/ST-20 Y -30 C, -35 M N. D. = Not Determined

Sample 1 demonstrates typical high print densities achieved at maximum voltage with a standard toner at 2 inches per second (ips). Sample 2 shows that the same toners even at the reduced speed of 1 ips on direct print vinyl film give lower densities and lots of fog without bias. Samples 3-5 show that the fog can be reduced to acceptable levels (slight to very slight) but only with a significant reduction of the print density using standard high conductivity toners.

Sample 6 shows that the use of the LC toners give much greater densities at the same bias with a small increase in fogging and some cover over of the yellow. Further optimization of the low conductivity toners led to Sample 9, which had good density, no cover over, and slight fog. These results are clearly superior to Sample 4, which is the best case with the standard higher conductivity toners. Even better results are likely to be obtained by further optimization of the conductivity and percent solids of the LC toners.

Sample 10 shows high densities and significant cover over but no fogging using the LC toners on the through-conductive media (8601) with no bias.

Sample 11 shows that the cover over can be reduced by lowering the print voltage and then further in Sample 12 by using bias. Sample 12 has good density (except for magenta) and no cover over. The magenta density could be improved by increasing the magenta voltage. Thus, this example demonstrates that the LC toner could be used successfully even on through-conductive media that would normally not require biasing to eliminate fogging. This is important for customers who do not want to switch toners when changing media.

Example 2 The toners used were similar to those used in Example 1, but from different production batches and in the case of the 100% low conductivity yellow it is a slightly modified resin formulation, which is sold as Scotchprint Toner Concentrate 8835X from Minnesota Mining and Manufacturing Company. Also, the low conductivity toners were mixed to be at 3% solids.

The Standard Toners used were: 3M Scotchprintz Toner Premix Yellow 8730; 3M Scotchprintz Toner Premix Magenta 8731; 3M ScotchprintTM Toner Premix Cyan 8732; 3M Scotchprintz Toner Premix Black 8733. The conductivities of these toners in pmho/cm were: Yellow = 309 at 2% equivalent to 464 at 3%; Magenta = 67 at 2% equivalent to 101 at 3%; Cyan = 224 at 2% equivalent to 336 at 3%; and Black = 221 at 2% equivalent to 332 at 3%.

For lower conductivity toners, the premix was mixed to 3% solids from the following Scotchprintz concentrates (conductivities at 3% listed in parentheses): 3M ScotchprintTM Toner Concentrate Yellow 8835X (170); 3M ScotchprintTM Toner Concentrate Magenta 8836 (37); 3M Scotchprintz Toner Concentrate Cyan 8837 (181); and 3M ScotchprintTM Toner Concentrate Black 8833R (56). Subsequent testing showed low density with this cyan so the same cyan described in Example 1 (8832R, conducivity of 99 at 2% equivalent to 149 at 3%) was used.

Also, blends of the above concentrates were made with standard premixes. In this case the premixes were the same product number but different production batches. The blends were made at 3% solids and were such that half of the solids was from the standard premix and half from the low conductivity concentrate (50/50) blend. The conductivity of the premix (estimated at 3% solids from data at 2% solids) is shown along with the actual data of the blend and the concentrate at 3% solids.

Table 3 100% LC mixture premix at 3% K 56 166 299 Y 321 393 450 C 181 307 359 M 37 64 87

Printing parameters are same as those described in Example 1 except as follows: DES film 3684-10 was used instead of the 184-10 (different adhesive and liner); toner roller rpm = 300 except 250 on Magenta.

The table below in Samples 2-4 shows density data for different toner sets printing at conditions optimized for printing on x-y-conductive dielectric films with low conductivity toners to give target densities with acceptable levels of fog and cover over (Nib and plate voltages for black = 265, <BR> <BR> <BR> <BR> yellow 270, cyan and magenta = 285 with bias voltages for black = 15, yellow = 12, cyan = 23, and magenta = 18). For comparison, Sample 1 shows that for standard toners, the density is too low even at maximum bias voltage.

Table 4 Sample Media Toner Comments Densities K Y C M Standard toners demonstrating low density on DES when typical bias is applied even at max voltage 1 DES Standard Low Density-fog except 1.459 0.972 1.078 1.186 cyan Conditions optimized for best printing with 100% LC toners 2 DES 100% LC Good Printing-Good 1.524 0.985 1. 381 1.500 density 3 DES 50/50 LC Good Printing-low 1.379 0.903 1.067 1.328 density 4 DES Standard Good Printing-low 1.326 0.877 1.031 1.140 density Sample 2 shows density results for the toners made from low conductivity concentrate at the above conditions (referred to as 100% to indicate no blending with standard premix). Sample 4 shows the results at the same print conditions as Sample 2 but using the standard toners described above. Note the low density especially for cyan and magenta. Sample 3 shows results at the same print conditions as Samples 1 and 2 but with the toners that are a blend of standard toners and low conductivity toners (50/50) described above. As expected the densities are higher than the standard toners but still not as high as the densities for

the toners made form the low con iuctivity concentrate. The densities are also still lower than the desired target densities. Also, shown above is Sample 1, which demonstrates that with standard toners even at maximum voltage on the printer the densities are still below target and/or objectionable fogging is present (all colors except cyan). Note that the reason why more fogging is present in Sample 1 than in Sample 4 (despite use of same bias voltages) is because of the higher write voltages (nib and plate) used in Sample 1. This result is surprising because one might have expected that the high density, low conductivity toners would cause more fogging and thus require even more bias to compensate for it. However, these toners do not inherently seem to need more bias than standard toners.

Because they can often be used at lower writing voltages, they actually require less bias thereby further increasing the range of attainable print densities.

The foregoing is given merely to exemplify the invention and is not meant to limit the scope of the invention. The scope of the invention is indicated by the following claims.