Electrographically produced imaged article

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BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to electrographic processes for making color images. More particularly, this invention relates to electrographic processes and the elements used therein for the production of large size, full color images.

2. Description of Related Art

The use of electrographic processes in the manufacture of multicolor images is well known in the art. In such processes, an electrostatic latent image is produced directly by imagewise depositing charge onto an accepting dielectric surface. Typically, styli are used to create these charge patterns and are arranged in linear arrays across the width of the moving dielectric surface. Such processes and the required apparatus are disclosed, for example, in U.S. Pat. Nos. 4,007,489; 4,731,542; and 4,569,584. In U.S. Pat. No. 4,569,584, only one stylus array is used and the accepting surface web is traversed to-and-fro to make successive images, the toning stations being disposed on either side of the single charging station. In the other designated U.S. patents, the electrographic printer consists of three or more printing stations in sequence, each containing charging arrays and toning stations. In all of the disclosed processes, a multicolor toner image is assembled on an accepting surface of a support and fixed there for display.

In a number of applications the electrographically produced imaged is transferred onto a receiving substrate. A method for transferring such as an electrographically produced toner image, from an initial substrate to a final substrate is disclosed in U.S. Pat. No. 4,983,487. The disclosed method employs an adhesive-coated film to lift the image from its initial substrate and to secure it to the final substrate. The film remains in place after the transfer is completed and serves to encapsulate and protect the image. The initial substrate remains intact and may be reused.

Another transfer process is disclosed in U.S. Pat. No. 5,102,768 for providing a non-electrostatically transferred toned image. In this process, an electrostatic latent image is conventionally formed on the surface of an element and that element is conventionally developed into a visible image by applying toner powder. The toned image is then thermally transferred from the surface of an element by contact to the face of a thermoplastic film that is strippably laminated to a paper or like backing. The film is then positioned against a receiver with the toner image therebetween, and the composite is subjected to two successive stages of compressive heating. It is disclosed that the process produces high resolution images from very small particle size toner powder on rough paper.

An offset transfer process of electrographically produced toner images is disclosed in U.S. Pat. No. 5,108,865. In the disclosed process, a liquid toned image is generated on the surface of an electrographic element. The image is adhered to the adhesive surface of a temporary receptor sheet which comprises a carrier layer, releasable release layer, and a transferable adhesive layer secured to the release layer. The temporary receptor sheet with the image adhered thereto is removed from the electrographic element, and then the image surface of the temporary receptor sheet is contacted with a final receptor surface. The adhesive layer secures the toner image, adhesive layer and release layer to the final receiving layer and the carrier layer is removed from the release layer to generate the final image wherein the release layer now is a top protective layer.

A toner developed electrostatic imaging process for outdoor signs is disclosed in European Patent Publication No. 0437073 A2 (E.P. Application No. 90313976.4). This publication describes an electrographic imaging process (as contrasted to an electrophotographic process), in which electrostatic images are toned in sequence to form an intermediate image on a temporary dielectric receptor. The intermediate image is then transferred from the temporary dielectric receptor to a permanent receptor. In the disclosed process certain relative properties of the toner and the intermediate image, such as surface energy, Tg, work of adhesion, and complex dynamic viscosity, were identified as being important to the production of good final images.

Each of the electrographic processes disclosed in the patent publications discussed supra, employ a transfer of the toned image from an electrographic element to the final substrate using an intermediate transfer element. Although advances have been made in retaining the integrity of the toned image, such transfer steps remain prone to image degradation by abrasion or chemical interaction unless added laminating or coating steps are used. There continues to be a need for a simplified process to provide protected, distortion-free, full-color images, particularly, for use on large format posters, billboards and the like.

SUMMARY OF THE INVENTION

These needs are met by the electrographic imaging process of this invention in which a specially developed electrographic imaging element is used to produce an image on a receptor surface. Such an element comprises:

a) a conductive base having a front and a back side,

b) a release layer coated on the front side, and

c) a single combined dielectric and adhesive layer overlaying the release layer, whose adhesive properties are activated at a pressure and a temperature which is above ambient pressure and temperature of the electrographic element.

This element is used in a process for forming an electrographic image on a receptor substrate comprising the steps:

A) electrographically producing on the surface of an electrographic element a toned image layer to form an imaged electrographic element, wherein the electrographic element comprises in the order given;

a) a conductive base,

b) a release layer, and

c) a single, combined, substantially tack-free, dielectric and adhesive layer which is activated at a pressure and a temperature which is above ambient pressure and temperature of the electrographic element;

wherein the toned image layer is adhered to the surface of the combined dielectric and adhesive layer; and

B) pressure laminating the receptor substrate to the toned image layer at a temperature which is above the ambient temperature, to form a laminated image element.

In a preferred mode, the process includes an additional step (C) wherein the conductive base and release layer are removed from the combined dielectric and adhesive layer of the laminated image element formed following the lamination step (B).

BRIEF DESCRIPTION OF THE DRAWINGS

The invention can be more fully understood from the following description thereof in connection with the accompanying drawings described as follows:

FIG. 1, is a schematic cross-sectional representation of an element in accordance with the present invention.

FIG. 2, is a schematic cross-sectional representation of an alternate element in accordance with the present invention.

FIG. 3, is a schematic cross-sectional representation of yet another element in accordance with the present invention.

FIG. 4, schematically illustrates a toned imaged electrographic element used in the process of producing an image according to the present invention.

FIG. 5, schematically illustrates the step of laminating the element of FIG. 4 on a receptor surface for producing an image according to the present invention.

FIG. 6, schematically illustrates the step of peeling of the base and release layer from the laminated composite created in the step illustrated in FIG. 5.

DETAILED DESCRIPTION OF THE INVENTION

The electrographic process of this invention will now be described by reference to the accompanying drawings. Throughout the following description, similar reference characters refer to similar elements in all FIGURES of the drawings. Referring to FIG. 1, an electrographic element (10) is shown which comprises a conductive base (11) having a front side (17) and a back side (13). The front side (17) is covered by a release layer (15) which in turn is overlaid by a combined dielectric and adhesive layer (16).

In an alternate embodiment illustrated in FIG. 2, the base back side (13) of base (11) is covered by a conductive layer (19). And in yet another embodiment, illustrated in FIG. 3, the element (10) comprises a base that includes a carrier layer (12) and a conductive layer (14) over the carrier layer. An optional second conductive layer (19) shown in phantom lines in FIG. 3, may again be provided on the back side (13) of the base.

In all of the above elements, the base (11) functions as a support to the superposed layers and may be any web or sheet material possessing suitable flexibility, dimensional stability and adherence properties to the release layer (15). Typically, the base will have an electrical resistivity of about 1 to 30 meg-ohm per .quadrature..

Suitable web or sheet materials for the base are flexible polymeric films, e.g., such as polyethylene terephthalate film and the like, or a foraminous material, e.g., such as a paper sheet and the like, treated to be electrically conductive or semi-conductive. Other suitable materials are for instance, metal foils, metallized polymeric films such as polyethylene terephthalate films having a metallic coating thereon, conductive paper sheeting and the like. The web or sheet may also be surface treated or coated with a material to enhance desired surface characteristics.

As illustrated in FIG. 3, the base may comprise a combination of carrier layer (12) and a conductive layer (14). In this embodiment the carrier layer (12) is again a flexible web or sheet material, which may again be a flexible polymeric film, e.g., such as polyethylene terephthalate film and the like, or a foraminous material, e.g., such as a paper sheet and the like. A conductive layer (14) is coated over the carrier layer (12) which carrier layer may or may not be itself conductive.

The conductive layer (14) preferably comprises a film-forming material which may be an organic material, e.g., such as a cation type styrene-methacrylate copolymer having an electrical resistivity of about 1 to 30 meg-ohm per .quadrature.. Other suitable film-forming, organic materials include polymeric quaternary ammonium compounds, polystyrene sulfonic acid, polymeric matrices capable of ionizing inorganic electrolytes contained therein, and the like. The film-forming, organic material may be used alone or with conductive, inorganic materials and/or metals dispersed therein, e.g., such as tin oxide, aluminum and the like.

The release layer which is adhered to the front surface (17) of the base (11) or the conductive layer (14) as shown in FIGS. 1, 2 and 3, typically comprises a film forming silicone polymer, or a film forming fluoropolymer. The release layer may also be heat cured, U.V. radiation cured, or electron beam cured. The release layer may itself be conductive or contain conductive agents such as a quaternary ammonium polymer, and may further include a surfactant. Good release performance has been obtained when the surface energy of the release layer is between 20 and 40 dynes/cm and preferably between 25 and 35 dynes/cm.

The combined dielectric and adhesive layer 16 may be any conventional film-forming material having a dielectric constant of about 2 to about 5. This layer typically has a thickness in the range of about 1 .mu.m to about 20 .mu.m and preferably in the range of about 5 .mu.m to about 15 .mu.m.

This combined dielectric and adhesive layer (16) typically comprises one or more polyesters; polyurethanes; polyamides; polyolefins; polycarbonates; polystyrenes; and/or polymers or copolymers of acrylic or methacrylic acids, esters, amides, or the like (such as polymethylmethacrylate), styrenes, acrylonitriles, vinyl esters, alkyd substituted vinyl esters, vinyl alcohol, vinyl acetals (e.g., polyvinyl butyral), vinyl chloride, vinyl fluoride, vinylidene chloride, 1,4-dienes (e.g., butadiene, isoprene and the like); ethylene/vinyl alcohol copolymers; copolymers of styrene with acrylic and methacrylic monomers; modified cellulosic resins such as cellulose acetate and cellulose acetate butyrate; block copolymer thermoplastic rubbers (e.g., styrene/ethylene/ butylene/styrene block copolymer); and blends of the above.

The combined dielectric and adhesive layer (16) in addition to its dielectric properties, is activated at a pressure and a temperature that is above the normal ambient pressure and temperature of the electrographic element prior to use to exhibit its adhesive properties. Thus the combined dielectric and adhesive layer is non tacky prior to activation.

The surface of the combined dielectric and adhesive layer (16) may be rough to ensure good transfer of charge during passage of the element under the stylus bar during imaging. This roughness can be obtained by including in the layer particles sufficiently large to give surface irregularities to the layer. Particles of diameter in the range of about 1 .mu.m to about 15 .mu.m are suitable. Particle composition and size are chosen to give the required dielectric constant to the layer as well as the appropriate surface topography and abrasive properties to the layer.

The combined dielectric and adhesive layer (16) is visually transparent in at least one region within the visible spectral region and preferably is transparent throughout the visible spectral region. By transparent, it is meant that the layer allows the visual observation of a toned image placed on one side of said layer through the layer. In describing this invention the term transparent is used to designate both commonly referred to as translucent and transparent layers.

This layer may contain components which strongly absorb ultraviolet radiation thereby reducing damage to underlying images by ambient ultraviolet light, e.g., such as 2-hydroxybenzophenones; oxalanilides; aryl esters and the like; hindered amine light stabilizers, such as bis(2,2,6,6-tetramethyl-4-piperidinyl) sebacate and the like; and combinations thereof. This layer serves as a protective layer to the transferred toned image after the image has been transferred onto a receptor substrate as will be described bellow. In such case the layer typically will withstand scribing with the point of a 4H pencil without breakthrough.

At times it is desired to provide a range of surface finishes to the finished image. This is done by controlling the surface of the imaged layer on the receptor. This surface is the combined dielectric and adhesive layer surface and its nature will depend on the nature of the surface of the release layer in contact therewith. Thus if the release layer on