2 FIELD OF THE INVENTION

3 The present invention pertains generally to imaging

4 and specifically to enhancement of the optical smoothness f 5 and reflective qualities of the image surface.

6 BACKGROUND OF THE INVENTION

* 7 Use of unpigmented material to enhance the reflection

8 of luminous flux (gloss) from the surface of a pigmented

9 image, is well known in the art. Typically, a colored image

10 is treated to a lamination step in which the image is

11 covered completely with what is intended to be a

12 homogeneous, optically clear coating or film which enhances

13 specular reflection from the image surface.

14 U.S. Patent 4,885,603 discloses an image recording

15 apparatus for producing a glossy surface on an image. The

16 image recording medium is a photo-pressure sensitive paper

17 (so-called transfer type paper) coated by microcapsules

18 containing photo-curable resin, colorless chromogenic

19 material and polymerization initiator. The apparatus

20 includes an image recording section, in which a desired

21 image is recorded on an image medium, and a glossing

22 section, in which the medium is furnished with glosses. The

23 glossing section includes a heating device which heats the

24 medium, and a feeding device which, together with the

25 heating device with the medium therebetween, presses and

26 feeds the medium.

27 U.S. patent 4,908,669 discloses a device for smoothing

28 the surface of a fixed toner image produced in an

29 electrophotographic copying machine by application of an

30 auxiliary transparent resin to the image on the final

31 substrate from a coating roller. A heater is provided

32 within a porous elastic layer on the roller for melting the

33 transparent resin.

34 SUMMARY OF THE INVENTION

35 The present invention improves the glossiness of a

36 pigmented toner image on a final substrate by a process of

37 developing an electrostatic latent image with unpigmented

38 toner to form a glossy layer over all or part of the

pigmented image, as desired. The image on the final substrate thus exhibits a high surface luster, in selectably glossy regions, as a result of treatments carried out during the image development and post- development process, without requiring a separate lamination or coating step or a separate drying process. BRIEF DESCRIPTION OF THE DRAWING The present invention will be understood and appreciated from the following detailed description, taken in conjunction with the drawing in which: Fig. 1 is a schematic diagram illustrating electrographic imaging apparatus in accordance with a preferred embodiment of the invention. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS A multicolor electrophotographic imaging process using liquid toners, in accordance with a preferred embodiment of the present invention, is described with reference to Fig. 1. There is provided a photoconductive image bearing surface 9 of selenium or any other suitable photoconductor known in the art typically embodied in a drum 10, which rotates in the sense indicated by arrow 19. Operatively associated with photoconductive drum 10 is photoconductor charging apparatus 11 and writing head 12, for providing a desired latent image on drum 10. In a preferred embodiment of the present invention, writing head 12, when so instructed by digital input information from a computer 13 to which it is connected, exposes photoconductive surface 9 to laser light, creating thereon a latent image in the form of an array of discrete latent image elements or "pixels", the size of which is typically on the order of 50 microns. The latent image normally includes image areas at a first electrical potential and background areas at another electrical potential. As drum 10 rotates, photoconductive surface 9, bearing the electrostatic latent image, comes into propinquity with a roller developer electrode 17 forming a development region 93 therebetween. The latent image is developed at development region 93 by sequentially applying thereat a

plurality of liquid toners, each comprising charged toner particles dispersed in an insulating, nonpolar carrier liquid. The liquid toners are selectively dispensed by liquid toner dispensing assembly 14 in response to commands from computer 13. In accordance with a preferred embodiment of the present invention, dispensing assembly 14. sequentially supplies at least one pigmented toner and at least one unpigmented toner to development region 93. As illustrated in Fig. 1 it is seen that liquid toner dispensing assembly 14 receives separate supplies of pigmented toner via four different supply lines, referenced by letters Y, C, M and K, indicating the usual Yellow, Cyan, Magenta, and Black toners, from a toner dispensing system 15. Color images are thus composed of a plurality of single color separations, as is known in the art. Computer 13 is operative to control writing head 12 and dispensing system 15 to sequentially produce the single color separation images which comprise a full color image. In a preferred embodiment of the invention, dispensing assembly 14 also receives unpigmented toner via a separate supply line reserved therefor, referenced by letter U in Fig. 1. The color content in the vicinity of a particular image element or pixel is determined by computer 13, which controls the sequential formation of the latent images and the sequential dispensing of the desired toner colors to each region of the image to be developed, based on image data which it receives. In the same way, formation of a latent image and its subsequent development with unpigmented toner, resulting in the presence or absence of gloss in the vicinity of a given image element, is computer controlled. As will be more fully described below, the unpigmented layer formed by this development overlays the colored areas, thereby improving their gloss. In an especially preferred embodiment of the invention, substantially only the colored areas are overlaid, with optionally a small amount of additional coverage to compensate for imperfect registration with the separations.

Dispensing assembly 14 and roller 17 form part of developer assembly 16 which is operative and constructed for use in the exemplary process. Preferably associated with developer roller 17 are a plurality of toner specific toner cleaning assemblies 92, each of which is brought selectably into operative association with the developer roller when pigmented toner of the color corresponding thereto is supplied to development region 93 by liquid toner dispensing assembly 14. Similarly, cleaning assembly 92A comes into operative association with the developer roller when unpigmented toner of a composition corresponding thereto is supplied to development region 93. In this way contamination by mixing of the various toner colors and/or compositions is prevented. Each of cleaning assemblies 92 comprises a resilient blade member 94 which, when activated, is urged against the cylindrical surface of the rotating developer roller thereby removing pigmented toner particles from the surface into pigmented toner collection apparatus 104 for recycling to liquid toner dispensing assembly 14. In an analogous manner, cleaning assembly 92A, comprising a resilient blade member 94A, removes unpigmented toner particles from the surface into unpigmented toner collection apparatus 104A. Embodiments of developer assembly 16 are described more fully in PCT Publication WO 90/14619, the disclosure of which is incorporated herein by reference. The insulating, nonpolar liquid used as toner carrier liquid preferably has a resistivity in excess of about 10 ⁹ ohm-cm (to prevent dissipation of the electrostatic image) , a dielectric constant below about 3.0 and a low vapor pressure (less than about 10 torr at 25 C) . Suitable are liquids such as hydrocarbons, preferably aliphatic and more preferably, iso erized aliphatic hydrocarbons such as those marketed by Exxon Corporation under the ISOPAR trademark, e.g. ISOPAR-G, ISOPAR-H, ISOPAR-L and ISOPAR-M. In addition to meeting the preferred resistivity and dielectric constant requirements, these isoparaffinic solvents are colorless, are available in high purity, have low chemical

reactivity and low odor. The Kauri-Butanol number of all four above mentioned ISOPARS is 27 or 28. Other high purity paraffinic carrier liquids having the preferred resistivity and dielectric requirements are Exxon Corporation's NORPAR 12 and NORPAR 13, having Kauri-Butanol numbers 23 and 22 respectively. Alternatively, light mineral oils such as those manufactured by the Humble Oil and Refining Company under the trademarks MARCOL 52 and MARCOL 62 may be used. Charged toner particles, suspended in the carrier liquid, pass by electrophoresis to the electrostatic latent image at development region 93. When the liquid toner particles are pigmented, the developed latent image is rendered visible. Polymers meeting the requirements of binders for pigments may be thermoplastic. One group of preferred polymers is marketed by E. I. du Pont de Nemours & Company under the trademark ELVAX II. These polymers are ethylene copolymers combining carboxylic acid functionally, high molecular weight and thermal stability. Another group of preferred polymers is marketed by E. I. du Pont under the trade name SURLYN. Other polymers known in the art may also be used. When pigmented toner particles of selectable colors are used, the image may be developed in a plurality of single color separations to yield a multicolored image. In the present invention the pigmented portions of the images have been realized using pigmented liquid toner compositions and particle morphologies according to U.S. Patent 4,794,651, the disclosure of which is fully incorporated herein by reference. The toner particles forming the subject of U.S. Patent 4,794,651 have pigment dispersed therein. Generally, the pigment may be present in the amount of 10 to 35 percent by weight in respect of the weight of the polymer, if the pigment is Cabot Mogul L (black pigment) . If the particles are colored pigment may be present in the amount of between 3 and 25 percent by weight in respect of the weight of the polymer. The liquid toner system further comprises so-called charge directors, i.e. compounds capable of imparting to

the toner particles an electrical charge of the desired polarity so that the particles may be electrophoretically deposited on the photoconductive surface to form a toner image. The preferred morphology of the toner particles is one in which fibers, tendrils, ligaments, elongations or the like extend from the body of an irregular -thermoplastic polymer particle. These fibers may interdigitate or interlink physically in an image developed with a liquid toner having particles possessing this particular morphology. Other toners known in the art may also be used in the practice of the invention. The present invention uses unpigmented toner in addition to the pigmented toner used in the development of the visible image, thereby allowing efficient attainment of certain surface textures and optical qualities of the image. Specifically, in accordance with a preferred embodiment of the present invention, the electrostatic image is developed with a plurality of liquid toners at least one of which is an unpigmented toner containing unpigmented toner particles, prepared, for example, according to the procedure disclosed in U.S. Patent 4,794,651. The preferred unpigmented polymers are the same as those for the pigmented toner, cited earlier. In the final image the unpigmented toner forms an optically clear, reflection enhancing covering over the pigmented toner particles, thereby endowing a glossy quality to the image. Generally, the present inventors have found that gloss enhancement is optimal when the unpigmented toner layer has a thickness of about 1.5 microns. However, under certain conditions, the gloss or image surface quality may be improved still further by laying down the unpigmented covering in a plurality of superimposed layers. In the present invention, computerized control of the formation of the electrostatic image, as an array of discrete image elements or pixels, affords considerable flexibility in implementing gloss enhancement steps. Thus, in a preferred embodiment of the present invention.

computer 13, as indicated earlier, receives image information regarding the pigmented portions of the image and controls writing head 12 and dispensing system 15 to sequentially form pigmented image portions. The computer determines the areas of the image which are pigmented and commands writing head 12 and system 15 to form at least one unpigmented layer for covering, in accordance with a preferred embodiment of the invention, substantially only the pigmented portions of the image. Alternatively, larger portions or all of the substrate may be covered. Following removal of excess liquid toner from photoconductor 9 by a squeegee roller, referenced by numeral 18 in Fig. 1, an intermediate transfer assembly 50 is used to transfer the image to a carrier sheet 25. The intermediate transfer assembly 50 comprises an intermediate transfer member 20 in operative engagement with photoconductive surface 9. Various types of intermediate transfer members are known and are described, for example in U.S. Patent 4,684,238, PCT Publication WO 90/04216 and U.S. Patent 4,974,027, the disclosures of all of which are incorporated herein by reference. While a drum type intermediate transfer member is shown in Fig. 1, the intermediate transfer member may also be of the belt type. Alternatively, in a preferred embodiment of the invention, the intermediate transfer member may be omitted and the image may be directly transferred to a final substrate. Transfer of the image from the photoconductor 9 to the intermediate transfer member 20 may take place in accordance with any suitable technique known in the art, but preferably, in accordance with a preferred embodiment of the present invention, by electrophoretic transfer. Other suitable transfer techniques may include electrostatic transfer, heat transfer, pressure transfer and a combination of the foregoing. Following transfer of the image from photoconductor 9 to the intermediate transfer element 20, the image may be subjected to a conversion process in which individual

unpigmented toner particles in the image, in response to the action of heat, are converted to a unitary, optically clear, film. Simultaneous application of mechanical pressure during the application of heat is expected to increase the degree of deformation or "melt-flow" of the unpigmented toner particles at a given temperature, thereby augmenting the tendency to unitary film formation and permitting lower temperatures to be employed for this step. The separations are separately transferred to the intermediate transfer member, unpigmented layer first, and are then transferred together from the intermediate transfer member to a final substrate, whereby the uppermost separation thereon is unpigmented. An alternative preferred embodiment of the invention provides for sequential development and transfer of the individual pigmented separations to the final substrate, optionally via an intermediate transfer member, prior to development and transfer of the unpigmented separations to the final substrate via the intermediate transfer member. Irrespective of whether or not an intermediate transfer member is used in the process, and whichever form of separation transfer sequence is adopted, i.e. sequential, unitary, or combinations thereof, between the various machine elements on which the image is mounted, it is to be understood that the image on the final substrate 25, such as a sheet of paper, is orientated with the optically clear film uppermost. Transfer to the final substrate may be effected in accordance with any suitable transfer technique which is known in the art, including for example, electrostatic transfer, heat transfer, pressure transfer, or, a combination of heat and pressure transfer. In accordance with a preferred embodiment of the present invention, the transfer is pressure-assisted heat transfer. The toner image must be permanently bonded or fused to the final substrate and when heat, either alone or in conjunction with pressure, is the agency of fusion there will be a concomitant deformation and flowing of the unpigmented toner particles. As a result, a degree of

smoothing of the uppermost (unpigmented) layer takes place. Although not describable as true planarization, the smoothing associated with the fusion induced flow of the unpigmented toner material results in less lateral scattering of light from the edges of the particles thereby augmenting the glossy quality of the image. From the foregoing, it will be appreciated that a conversion step whereby the individual unpigmented toner particles undergo deformation accompanied by a melt-flow effect to form a unitary, optically clear film over the image bonded to the final substrate, although preferable, is not crucial to the present invention. However, If carried out, the conversion may be done on a suitable intermediate transfer member to which the image is transferred from the image forming surface, during the heat assisted transfer to the substrate, or on the final substrate during fusing. It is understood that in addition to providing a gloss improving coating for the pigmented toner, the unpigmented layer provides physical protection for the image, improving its abrasion resistance. If different (generally improved) physical properties are required for the unpigmented layer from those of the pigmented toner (as, for example, improved abrasion or chemical resistance) , the polymer materials used for the unpigmented toner can be chosen with these qualities in mind and need not be the same as those of the pigmented toner. It will be appreciated by persons skilled in the art that the present invention is not limited by what has been particularly shown and described hereinabove. Rather the scope of the present invention is defined only by the claims which follow: