DE69119935T2 - Recording sheet for an electrostatic imaging process using a toner - Google Patents

Description

Field of the invention

The invention relates to a method for producing large format, full-color images by electrographic processes according to claim 1 and in particular to a receptor film for use in a multi-color electrographic process using a one-pass printer followed by image transfer to a receptor surface according to claim 10.

initial situation

A general discussion of color electrophotography can be found in "Electrophotography" by R.M. Schaffert, Focal Press, London & New York, 1975, pp. 178 ... 190.

Full color reproductions using electrophotography were disclosed by C.F. Carlson in his first patents (US-P-2,297,691), but no detailed mechanisms were described. Another early patent (US-P-2,752,833) by C.W. Jacob discloses a process based on a single transparent drum coated with a photoconductor around which a receptor paper web is fed. Electrostatic images are formed on the drum and by induction on the receptor paper by means of three color scanning exposures from inside the drum using a cathode ray tube. Each of these scanning positions is preceded by charging stations and followed by toner stations, with appropriate time delays between scans. The finished three-color image is assembled directly on the imaging paper. In US-A-4,033,688 (Agfa-Gevaert) a single photoconductive drum is exposed to three different color beams reflected from a color original. The incident reflections appear at points around its circumference, with the necessary charging and toning stations provided for each point. Mechanical time delays enable the recording of the three color images, which are then transferred to a receptor film. Other similar Systems have been disclosed in US-A-4,403,848 and 4,467,334. All of these systems use optical exposure as the method of addressing the imaging surface, thereby avoiding mechanical contact with the surface. The use of a sequence of exposure/toning stations in immediate succession, as opposed to the multiple drum rotations found in other methods (eg US-A-4,728,983), provides higher production speeds for color prints.

Many patents (e.g., US-P-2,986,466, 3,690,757, and 4,370,047) use three or four different photoconductor drums or ribbons for different colors and assemble the toner images in register on a receptor film.

Exposure by conventional optical scanning is disclosed in numerous patents, e.g., US-P-3,690,756, 4,033,688, 4,234,250. CRT scanning is disclosed in US-P-2,752,833 and laser scanning alone or in combination with conventional exposures in patents, e.g., US-P-4,234,250, 4,236,809, 4,336,994, 4,348,100, 4,370,047, 4,403,848 and 4,467,334.

The use of electrographic processes is also well represented in this field, in contrast to the electrophotographic processes described above. In these processes, the electrostatic latent image is formed by "spraying" charge directly onto the receiving dielectric surface, pixel by pixel. Often, pens are used to produce these charge patterns and are arranged in groups of lines across the width of the moving dielectric surface. These processes and the apparatus required are disclosed, for example, in U.S. Patent Nos. 4,007,489, 4,569,584, 4,731,542 and 4,808,832. In U.S. Patent No. 4,569,584, only a single pen arrangement is used and the receiving surface web is moved back and forth transversely to produce the successive images, with the toner stations on either side of the single charging station. In the other three patents mentioned above, the printer comprises three or more printer stations arranged in series, each of which comprises both loading areas and toner stations. In all of these, the multi-color toner image is assembled on the receiving surface and fixed there as a carrier for reproduction on that surface. In none of these patents is the transfer of the assembled image to a receptor surface disclosed or discussed.

The toners disclosed by C.F. Carlson (US-P-2,297,691) are dry powder. Staughan (US-P-2,899,335) and Metcalfe & Wright (US-P-2,907,674) pointed out that dry toners have many limitations related to image quality, especially when used with superimposed color images. These authors recommend the use of liquid toners for this task. These toners comprise a carrier liquid of high resistivity, e.g. 10¹⁰ ohm cm or more, and have both colorant particles dispersed in the liquid and, preferably, an additive designed to enhance the charge carried by the colorant particles. Matkan (US-P-3,337,340) discloses that an initially deposited toner can be sufficiently conductive to interfere with a subsequent charging step. He claims the use of insulating resins (with a resistivity greater than 10¹⁰ ohm cm) with low dielectric constants (less than 3.5) to cover each colorant particle.

In US-P-4,155,862, the charge per unit mass of toner is related to difficulties encountered in the art in superimposing multiple layers of different color toners. The latter problem was solved in a different way in US-P-4,275,136, where the adhesion of one toner layer to another is enhanced by an aluminum or zinc hydroxide additive on the surface of the toner particles.

In US-P-4 480 022 and 4 507 377 liquid Toners are disclosed which provide developed images with rapid self-fixation to a smooth surface at room temperature after removal of the carrier liquid. These toner images are said to have higher adhesion to the substrate and are less susceptible to cracking. There is no disclosure of their use in arrays of multi-color images.

A number of methods are disclosed in the patent literature to achieve high quality liquid toner image transfer.

The use of silicones and silicone-containing polymers for mold release coatings and flow control agents as additives to impart release properties to coatings is well known.

In the field of electrophotography, photoconductive layers with silicone layers as a top layer have been disclosed: US-P-3,185,777, 3,476,659, 3,607,258, 3,652,319, 3,716,360, 3,839,032, 3,847,642, 3,851,964, 3,939,085, 4,134,763, 4,216,283 and JP-A-81699/65.

US-P-3,652,319 discloses readily liquefiable solids, such as silicone waxes with melting points between 20º and 90ºC, which are continuously applied to the photoconductor surface during use under repeated cycling conditions. The temperature is slightly increased at the application site to melt the wax and allow it to spread. In a later cycle, the wax solidifies into a layer prior to exposure. In each cycle, the wax layer is renewed by further applications. The thickness of the wax layer appears to be in the range of 50...1500 nm with an optimal range of 200...800 nm.

US-P-3 839 032 and its two divisional applications US-P-3 851 964 and 3 939 085 deal with liquid toner development and toner image transfer from photoconductors to receptors in which the toner image is temporarily adhesive and exhibits greater adhesion to the receptor surface than to the surface of the Photoconductor. Novel formulations of liquid toners having these properties are disclosed. Weak adhesion to the surface of the photoconductor can be obtained by methods involving the application of a silicone layer to the surface. The examples disclose formulations for these layers but give no idea of the thickness. Two dependent claims state: "... reducing the affinity of the photoconductive layer for the tacky image...". In the introductory discussion it is pointed out that the invention (column 2, lines 1...16) solves the problems of incomplete transfer of liquid toner images and loss of resolution encountered in prior art designs.

US-P-3,850,829 is a more recent patent and refers to the results of US-P-3,839,032 with still a loss of resolution. This patent discloses that the inclusion of a silicone in a tacky liquid toner leads to better results than the silicone layer on the photoconductor.

In US-P-3,847,642 a transfer film between 2 microns and 25 microns (preferably about 5 microns) is applied to the photoconductor surface during the imaging cycle. The material must have a low and sharp melting point so that after the toner is applied (also referred to as "toning"), the application of heat melts this material and during image transfer a portion of the layer is transferred with the toner and resolidifies. Among these materials, low melting point silicone waxes are suggested.

In US-P-4 216 283 one of the embodiments (column 8, lines 63...68 and column 9, lines 1...30) describes a thin release layer, which may be of the type of Syl-OFF materials, and is applied to a photoconductor layer of zinc oxide (or other materials which obviously involve organic photoconductors) to ensure transfer of the liquid toner image. It gives no indication of the thickness of the Syl-OFF layer or its connection with the effectiveness of toner release. The main claims and their embodiments relate to the use of intermediate transfer films coated with an anti-adhesive layer (eg Syl-OFF) for use in the xerography system.

In addition to the patents dealing with silicone release layers, there are also patents describing the use of silicones in other ways. U.S. Patent Nos. 3,476,659, 3,594,161, 3,851,964, 3,935,154 and 4,078,927 all disclose the use of silicones as additives to the photoconductor layer itself to impart release properties to both the toners and the inks (electrographic printing plates). The patents also deal with intermediate transfer films, belts, rolls and blankets for transferring the toner image from the photoconductor to the receptor, suggesting silicone treatment of the intermediate. Exemplary patents are: US-P-3,554,836, 3,993,825, 4,007,041, 4,066,802 and 4,259,422.

Receptor films for the transfer of deposited liquid toner images are also known in the art. For example, US-P-4,337,303 discloses receptor layers that encapsulate the toner under elevated temperature from an imaging surface pressed against the receptor. The physical properties required for the receptor surface are disclosed, particularly in relation to the complex dynamic viscosity.

US-P-3,854,942 describes transparency films for electrostatic multicolor copying. A transparent thermoplastic film having strength and heat resistance to allow repeated projections (e.g. polysulfones or polycarbonates) is coated with a mixed polymeric composition such as a poly(vinyl chloride/vinyl acetate) copolymer and an acrylic resin.

US-P-4 074 000 describes the drawing of films with a pressure-sensitive adhesive layer for use in electrostatographic copiers. The film comprises a substrate having a matte coating of plasticizable thermoplastic material on one surface and a pressure-sensitive adhesive on the other surface of the substrate. Any film material can be used as the substrate, including poly(vinyl chloride), polyolefins and polyesters.

US-P-4,510,225 shows an electrophotographic process for producing an opaque print. An opaque surface has a transparent thin layer of plasticizable polymer. The opaque substrate includes plain or coated paper, metal, stone, and stretchable or inflatable media.

Summary

In the practice of the present invention, the term "electrography" refers to a process for creating images by addressing an imaging surface, usually a dielectric material, with static electrical charges (e.g., from a pen) to create a latent image which is subsequently developed with a suitable toner. The term is distinct from "electrophotography," in which a latent electrostatic charge image is created by addressing a photoconductive surface with light. The term "electrostatic printing," etc., is used generally in the literature to encompass both electrography and electrophotography.

The present invention provides a receptor film used to produce stable, high quality and large format full color images, especially for outdoor displays. A conformable substrate made of poly(vinyl chloride) or polyurethane is provided with a receptor coating on at least one surface to obtain the receptor film. In a preferred embodiment, an adhesive is present on the other substrate surface. The substrate itself must have properties which enable it to mask or cover some of the visible attributes of the surface to which the imaged article is applied in use. To achieve this, it is important for the substrate of the imageable film to have a haze value (expressed as a percentage) of greater than 30%. Preferably, the haze value is 50% or more, and most preferably 75% or more (eg 100% would be particularly acceptable).

Haze is usually determined according to ASTM Standard D 1003-61 (revised 1977) called "Standard Test Method for Haze and Light Transmission of Transparent Plastics". These tests can be carried out using commercially available devices, such as the "Gardner Model US10 Hazemeter" turbidity meter and a "Gardner PG 5500 Digital Photometric Unit" recording spectrophotometer.

Percent haze readings greater than 30% represent scattering or translucent substrates and exclude transparent substrates. This is because haze refers to the percentage of transmitted light that passes through the sample (substrate) and deviates from the incident light by forward scattering.

The haze is a more reliable and important indicator of optical properties in the practical application of the present invention than the optical densities in specular transmission and the optical densities in diffuse transmission, which can be at least 1.0 and at least 0.5, respectively.

Electrostatic printers suitable for the process of the present invention (such as those manufactured by Synergy Computer Graphics) may have a series of printing stations having the following configuration, which sequentially contact the imaging surface:

(a) a pen or electrostatic imaging Rod by means of which an electrostatic image is created on the dielectric surface of the imaging film when it is moved behind the station;

(b) a liquid toner development device, which normally comprises an applicator roller rotating at a different speed to the advancing dielectric surface or even rotating in the opposite direction relative to that surface;

(c) a vacuum scraper for removing excess toner and then a drying system to remove the solvent present in the pixel-deposited toner.

The mechanical assemblies in (a), (b) and especially (c) come into mechanical contact with the imaging surface and are excessively stressful to the surface compared to non-contact processes such as those using light-addressed electrophotographic materials. These printers were previously used in a mode of operation in which the toner image was permanently fixed to the dielectric imaging film surface. They have proven particularly useful in the art for producing large format prints and have been manufactured as imaging surface webs of 1 to 1.3 m (3 to 4 ft) width and of essentially unlimited length. This is in contrast to the largely limited size of prints produced by the various electrophotographic methods.

When large format prints are required, especially for outdoor displays, the properties of the dielectric imaging film are often not sufficient for the ready-to-use image carrier. Typical paper substrates lack the water and UV resistance required for outdoor signage, and more durable substrates such as Plexiglas, 3M Panaflex, 3M Scotchcaltm and polyester films cannot be used in direct contact due to their mechanical or electrical properties. way to be printed.

Transferring the image from the imaging film to a separate receptor film allows the selection of the required properties of the latter for ready-to-use printing. In this case, however, the imaging film must have a lower adhesion to each of the various toners than the receptor film has to the toners during the thermal transfer/print transfer process. This can be easily achieved except that there is a conflicting requirement that the image toners deposited on the imaging film must adhere sufficiently strongly to the receptor and to each other to ensure that they are not removed or damaged during passage through the one-pass printer. In practice, the combination of properties to meet these requirements has proven difficult to obtain. The adhesion generated during the printing sequence for the respective toner on the imaging surface must not only be sufficiently low to be detached during the subsequent transfer, but the adhesion of a toner to each of the other toners must be sufficiently high to prevent separation and, in addition, the cohesion and strength of the toner layers generated during the printing process must be large enough to prevent damage.

Show it:

Fig. 1 shows one of the printing stations usable in the present invention in schematic detail;

Fig. 2 is a graphical representation of the relationship between the complex dynamic viscosity of the surface coating on a receptor film and the CIELAB color difference value, AE, for the toner remaining on the imaging film after transfer of the image to the receptor film.

Detailed description of the drawings

Fig. 1 is a schematic representation of one of the printing stations, which is a necessary for the practical application of the The intermediate photoconductive receptor 1 comprises a paper substrate having a first dielectric layer 4 and then a release coating 6 on at least one of the surfaces. This surface of the intermediate receptor 1 passes through the station in a direction 8 so that the coated surface of the paper first passes a rod-like writing head 10 which deposits a charge 12 pixel by pixel so as to leave spaces 14 on the surface which are not charged. After the intermediate receptor 1 has passed the writing head 10, it passes to a toner station consisting of a toner applicator 16 which contacts a liquid toner bath 18 in a container. The liquid toner 22 is received on the toner applicator 16 so that it is deposited pixel by pixel on the intermediate receptor 1 to create toned areas 24 and non-toned areas 26. The toner-coated areas of the intermediate receptor 1 then pass under a vacuum scraper 28, where the excess toner is removed.

Fig. 2 is a graph of viscosity (x 10-5) in poise (1 cP = 1 mPa s) at 110 ºC for five different materials as a function of (E). The five materials are Elvacite 2044 (A), a 1:1 blend of clear and white (TiO2-filled) blends of 4:1 vinyl chloride and acrylic resin (B), a 4:1 blend of PVC and acrylic resin pigmented with TiO2 (C), Elvacite 2010 (D) and clear cast polyvinyl chloride resin (E).

In Fig. 1, a typical electrographic printing station for carrying out the method of the invention is shown in schematic form. At each of these printing stations a separate image is deposited, usually one of the four differential colors black, cyan, magenta and yellow. One of the printing stations is illustrated in Fig. 1 where the web 1 passes over and in contact with a charging bar 10, then to a liquid developer roller 16 and then to a vacuum stripper 28 and finally dried by a stream of air from the vacuum dryer or vacuum scraper 28 (or fans, not shown). In order to obtain complete development of the latent electrostatic image by the toner in the shortest possible time, the developer roller 16 rotates at a speed greater than the web speed and is typically nipped to facilitate the delivery of toner to the surface having the dielectric coating 4. The properties of the toner must be such that its adhesion to the imaging surface and to any underlying toner is sufficient to ensure that the toner of the image is not removed again during its own or subsequent development. This development with a nipped roller in contact with the image is in contrast to the applied field induced electrophoretic development normally used in electrophotographic systems in which no mechanical part contacts the image.

Such printers are well known in the art and can be obtained, for example, from Synergy Computer Graphics. The finished image is displayed on the imaging material with a dielectric surface.

For the reasons stated above, the process in the present invention is completed by the transfer of the final toner image from the dielectric surface to a receptor film. The imaged surface of the web bearing the composite toner image is pressed against the receptor surface and heat is applied for a short period of time. This can be done in many ways known in the art, such as passing the films together through heated feed rollers or by placing them on a hot plate in a vacuum extraction enclosure. The latter is the preferred method in the present invention.

If the final image on the receptor film is to be of high quality and color fidelity, the transfer ((Transfer)) must be complete and without distortion of the various color images. Under the conditions of the transfer process, the toner image must therefore be easily detached from the imaging surface and adhere to the receptor surface.

Detailed description of the invention

From the above described background, it can be seen that release layers used in toner transfer steps are common in electrophotographic systems. However, in the present invention, it is surprising that a release layer can be used without removal or partial removal of the deposited image toner under the stress of continued development with the corrugated rollers; serious damage to the image can result in some cases and in many cases objectionable damage, depending on the particular combination of toners, release layer and receptor surface limited in the present invention. Furthermore, it has been found that the combination of toners currently used on printers such as the Synergy machine is unsuitable for electrography in the present invention.

The disclosure of the present invention teaches how to make the correct choice of release surface, toners and especially the receptor surface so that no image damage occurs and yet a complete transfer is achieved.

We have found that surface energy values can be measured for the components used in the coatings and that the required abhesive/adhesive properties can be specified in terms of these values, namely:

(a) the dielectric imaging surface must have a surface energy between 14 erg/cm2 and 20 erg/cm2 (1 erg = 10⁻⁷ J), with the polar component not exceeding 5%;

(b) the adhesion work between two moving overlapping toners deposited on the imaging surface must be greater than the work of adhesion between the imaging surface and the toner deposited on it, but this is not the case for the toner developed last in the process since the toner has no further toner deposited above it;

(c) no deposited toner shall have a surface energy greater than 50 erg/cm², and

(d) the receptor surface must have a surface energy greater than that of the imaging surface.

Preferably, the differences in (b) and (d) should be at least 5 erg/cm2 and more preferably at least 10 erg/cm2. Polar components in the surface energy values contribute strongly to the adhesion values, with the limit of polar content in the imaging surface in (a) required for it to be a release surface being based on this property.

We have also found that even if the surface energy requirements are met, image damage will occur if the toners deposited during the process are too soft or not mechanically strong enough. This condition is found to be well defined by a subsequent scratch test. We have found that the toner strengths in the scratch test are only valid criteria if they relate to the conditions in the process itself. They must be carried out on toner samples immediately after deposition and preferably after no more than 8 minutes and more preferably no more than 2 minutes after the start of drying after deposition. It has been found that toner samples left for several hours after deposition give misleading values. To function properly in the invention, toner strengths in the scratch test indicated by compression or cracking of the surface in this test must be at least 40 g ((here to be understood as 1 gf = 9.806 N)) if the measurement is not taken more than 8 minutes after the start of drying.

Similarly, good transfer is not ensured by simply meeting the surface energy conditions of the receptor surface.

In addition, the Tg of the surface should be in the range of 10ºC to a value of about 5ºC below the temperature used in the transfer process (at an elevated temperature, i.e. above 30ºC, typically about 50ºC 150ºC and preferably about 90ºC...130ºC, e.g. 110ºC) and the complex dynamic viscosity of the surface material should be below about 2 x 10⁵ poise at the temperature of the transfer. These additional requirements improve adhesion and conformance to the imaging surface at the elevated temperature of the transfer.

The respective components of the materials in the process are discussed in more detail below.

Imaging slides

Imaging films comprise a flexible substrate on a surface on which the dielectric layer is located. The substrate itself must be electrically conductive or must have a conductive layer on the surface beneath the dielectric layer.

Substrates can be selected from a wide variety of materials, including paper, plastics, etc. If a separate electrically conductive layer is required, this can be made of a thin metal such as aluminum, or of tin oxide or other materials well known in the art to be stable at room temperatures and at the elevated temperatures of the transfer process.

Dielectric layers on a substrate for use in electrostatic printing are well known in the art, see for example "Neblette's Handbook of Photography and Reprography" by CB Neblette, edited by John Strang, 7th edition, published by Van Nostrand Reinhold, 1977. These layers typically comprise polymers selected from polyvinyl acetate, polyvinyl chloride, polyvinyl butyral and polymethyl methacrylate. Other ingredients may be selected from waxes, polyethylene, alkyd resins, nitrocellulose, ethyl cellulose, cellulose acetate, shellac, epoxy resins, styrene/butadiene copolymers, chlorinated rubbers and polyacrylates. The performance criteria are set out in the aforementioned work by Neblette. Such layers have also been described in U.S. Patent Nos. 3,075,859, 3,920,880, 4,201,701 and 4,208,467. The layers should have a thickness in the range of 1 to 20 micrometers and preferably in the range of 5 to 15 micrometers. The surface of such dielectric layers is advantageously rough to ensure good transfer of the charge during passage under the charging rod. This roughness can be obtained by including in the layer particles of sufficient size to impart surface irregularities to the layer. Particles in a diameter range of 1 to 5 micrometers are suitable. The particle composition is selected to obtain the dielectric constant required for the layer. These requirements for the dielectric properties of the layer are well known in the art (see, for example, US Pat. Nos. 3,920,880 and 4,201,701).

The required surface energy characteristics of the imaging film can be achieved either by applying a release layer to the free surface of the dielectric or by modifying the dielectric material. Release layers are usually used in materials for pressure-sensitive adhesive tapes, such as polyurethane, and have too high an adhesion, while the widely known release materials, such as dimethylsiloxane, prove to be too abhesive. Among the available materials, polymers in which dimethylsiloxane units are incorporated in a small and controlled number have proven to be particularly good.

The release coatings useful in the present invention should have the following properties:

1. no impairment of the electrographic imaging characteristics of the dielectric;

2. the transfer efficiencies of the toners in the last stage in the process should be high and preferably carry 95...100%. Preferably no appreciable amount of the release coating should be transferred with the toners as this could interfere with the protective coatings that may optionally be applied to the transferred image;

3. In order to survive the rest of the process, the deposited toners should anchor themselves sufficiently on the release surface;

4. No part of the release coating should dissolve into the liquid hydrocarbon carrier of the liquid toner and cause poisoning of the toner (the release coating should not be easily soluble from a film or dispersible into the carrier liquid, especially in a period of less than 2 minutes).

The image release layers tested were Dow Corning Syl-off 7610 (referred to as "premium release") and Syl-off 7610 "controlled release". Free silicone in the "controlled release" formulation leached into the liquid toner and interfered with the deposition of the toner. On the other hand, although nothing appeared to leach in the "premium release" formulation, their use resulted in significant abrasive damage to the toner image during the process.

It was concluded that a suitable release layer should have controlled release properties by incorporating small amounts of moieties (herein referred to as "unsubstituted groups") such as silicones, which However, silicones should not be firmly anchored to a polymer soluble in the toner carrier liquid. The presence of mobile silicones on the surface of the release layer has been found to be unacceptable, resulting in toner images that are susceptible to damage during processing. The non-silicone portion of the release layer material must have a high softening point. An example of such a polymer is a silicone/urea block polymer containing between 1% and 10% by weight polydimethylsiloxane (PDMS), described below with reference to the examples. The polymer was prepared in isopropanol and diluted to 3% solids with additional isopropanol for application to the dielectric surface. Percentages of PDMS above 20% were found to be less preferred, since increases in transfer efficiency are offset by decreases in the density of the developed image when the amount of PDMS exceeds 20%. However, under less severe processing conditions, the silicone content can be much higher, even up to 65% or more.

Further compositions for a controlled release layer can be obtained using monomers that can form condensation products with silicone moieties via their amine or hydroxy end groups, the monomer moieties being polymerized either during or after condensation. Examples of such compositions are urethane, epoxy and acrylic resins in combination with silicone moieties such as PDMS.

Dielectric layers with built-in release properties have the additional advantages of eliminating an additional application procedure and any electrical influences on the thickness of a separate release layer. There are known successful polymer compositions for this task, which are described below with reference to the examples.

These are copolymers of methyl methacrylate (MMA) with PDMS or terpolymers of MMA, polystyrene and PDMS. Usable concentrations of PDMS are in the range of 10% ... 30% by weight of the total polymer, with values in the range of 15 ... 30% resulting in transfer efficiencies of over 90%, but the optical density of the deposited toner will fall at higher percentages. An optimal value for these polymers was in the range of 10 ... 20%.

Physical requirements for a sedarate release layer

The functional surface of the imaging film, in addition to having a predetermined abhesiveness, must have a controlled roughness to facilitate charging as described above for the dielectric layer itself. If the release properties are provided by a separate layer deposited over the dielectric layer, the release layer must provide the required roughness by following the topography of the original dielectric surface.

When using a separate release layer, its density must be carefully controlled, since too little thickness will result in incomplete transfer, while too much thickness may impair the electrostatic image-receiving properties of the surface. A suitable range is obviously 0.05...2 micrometers. The preferred range of thickness is 0.08...0.3 micrometers. No significant change was observed between the characteristics of an uncoated dielectric surface and the characteristics of the same dielectric surface after coating with a release layer according to the invention in terms of the roughness of the surface after coating compared to that before coating.

The following example illustrates the relationship between the basis weight (and thus the dry thickness) of the release layer on the imaging film, the surface charge (measured as surface potential) deposited by the charging rod, as well as the developed image density and the efficiency of image transfer.

Silicone solutions of Syloff 23 ("Premium Released") in heptane were coated onto 2089-type dielectric paper (manufactured by James River Graphics Corp.) in such a way that only a portion of the 22" wide paper received the coating. The purpose of the partial coating was to be able to image both coated and uncoated portions of the paper simultaneously. Different solution concentrations and different sizes of spiral blades (Meyer blades) were used to produce coatings of different thicknesses. In these tests, the basis weight of the release layer was calculated from the solution concentration and the coating bar size using published wet film thicknesses that result from Meyer blades of different sizes, i.e. a solution with a higher concentration or a larger blade number (#) produces a thicker release layer.

The coated imaging film was loaded and developed using a Benson 9322 printer. The surface potential on the imaging film was measured with an electrostatic voltmeter probe installed between the loading stations and the liquid developer stations in the printer using Benson T3 black liquid toner for image development.

After measuring the optical density (OD) in the background and image areas of the developed imaging film, the toner image was transferred to a commercially available receptor paper that was coated with a thermoplastic material (Schoeller 67-33-1, which had a surface coating of a polymerized ethylene acrylic acid, commercially available as Primacor EAA) using heat and pressure. After the transfer, the optical density (OD) in the background and on the image areas was measured again. residual optical density remaining on the imaging surface was measured. The image transfer efficiency was calculated using the following formula:

Transfer efficiency (%) = 1 - {ODr - ODBr/OD - ODB} x 100 where OD is the optical density of the image on the imaging film before transfer, ODR is the residual optical density in the image area after image transfer, ODB is the optical density in the background area before transfer and ODBr is the residual optical density in the background area after transfer.

Table 1 shows the progressive reduction in developed optical density OD with increasing thickness of the release layer on the surface of the imaging film. Table 2 shows the effect of the release layer on the surface potential and image transfer efficiency. Increasing the thickness of the release layer resulted in increased image transfer efficiency, but there was a decrease in surface potential and hence in the resulting image density. Table 1: Decrease in image OD caused by a release layer with release composition coated areas Table 2: Influence of the thickness of the release layer on the surface potential and the transfer efficiency

The toners

Liquid toners for use in the present invention can be selected from those well known in the art. These toners comprise a stable dispersion of toner particles in an insulating carrier liquid, which is usually a hydrocarbon. The toner particles carry a charge and comprise a polymer or resin and a color pigment. Preferably, however, they should meet the following general requirements in addition to the interfacial energy and scratch test strength requirements already set forth in the present disclosure. These general requirements have been discussed in some detail in copending patent application U.S. Serial No. SN 279,424 (filed December 2, 1988).

These requirements include:

(a) a quotient of less than 0.6, preferably less than 0.4 and most preferably less than 0.3, of the conductivity of the carrier liquid present in the liquid toner and the conductivity of the liquid toner itself, and

(b) Toner particles with zeta potentials in a narrow range centered between +60 mV and +200 mV.

The liquid toner should preferably also meet the following requirements:

(c) deposited toner particles have a Tg less than 100ºC and greater than -20ºC, and more preferably less than 70ºC and greater than -10ºC;

(d) largely monodisperse toner particle sizes with an average diameter in the range of 0.1 ... 0.7 micrometers;

(e) a conductivity in the range of 0.1 x 10⊃min;¹¹ S/cm ((1 S/cm = 1 Ohm/cm))⊃min;¹ and 20 x 10⊃min;¹¹ Siem with a solids concentration in the liquid toner in the range of 0.5% ... 3.0% by weight and preferably 1.0% ... 2.0 weight percent

The insulating carrier fluid in these liquid toners has been shown in our work to be particularly important in relation to the robustness of the deposited toner layers during the process as predicted by scratch test strengths. There is an extensive range of hydrocarbon carrier fluids (e.g. the Isopar series) with a range of boiling points. Isopar C, E, G, H, K, L, M and V fluids have boiling points of 98ºC, 116ºC, 156ºC, 174ºC, 177ºC, 188ºC, 206ºC and 255ºC respectively. Mixtures of different members of such series are often used in liquid toner formulations. It was found that the presence of high-boiling representatives resulted in a lower robustness and lower strength in the scratch test. In particular, we found that, in contrast to Isopar G, high amounts of Isopar L appeared to be harmful.

To save on transport costs and storage space, toners are usually produced in a concentrated form. To use the toner in the printer, this concentrate is diluted with additional carrier fluid to obtain a so-called liquid toner in working concentration.

The toners can be deposited on the imaging film surface in any order, but for colorimetric reasons the inversion that occurs during transfer must be taken into account, so that the deposition of the images in the order black, cyan, magenta and yellow is preferred. The printers used so far (including the Synergy printers) also deposited the toners with black first, but since no transfer was applied, the final image had black as the background of the image composition. Since the lighter and usually more scattering color toners can appear above the black, the appearance of the resulting image color showed desaturation. In our setup, black appeared as the top toner and delivered colors at full color depth.

Production of toner

1. Production of stabilizers with a proportion of complex-forming groups and grafts

In a 500 mL 3-neck round bottom flask equipped with a stirrer, thermometer and a condenser connected to a nitrogen source was added a mixture of 69 g of lauryl methacrylate (LMA), 4.5 g of 5-methacryloxymethyl-8-hydroxyquinoline (HQ), 1.5 g of 2-hydroxyethyl methacrylate (HEMA) and 175 g of Isopar H. The mixture was purged with nitrogen and heated to 70 ºC with stirring until the quinoline monomer was dissolved.

Then, 1.5 g of 2,2'-azobisisobutyronitrile initiator (AIBN) was added to the solution and the mixture was polymerized for about 20 hours at 70 ºC. The conversion was quantitative.

After heating at 90 ºC for 1 hour to destroy any residual AIBN, the mixture was cooled to room temperature, the nitrogen source was replaced by a drying tube and equal molar amounts, i.e. 1.8 g of 2-isocyanatoethyl methacrylate (IEM) and 0.36 g of dibutyltin dilaurate were added to the flask. The mixture was then stirred at room temperature for 24...48 hours. The conversion was quantitative and the resulting solution of the stabilizer could be used to prepare the organosol.

The product is a copolymer of LMA, HQ and HEMA and contains side chains of IEM. It was designated as LMA/HQ/HEMA-IEM.

2. Preparation of organosol with a poly(vinyltoluene) core

Case A

A reaction flask equipped as described in Example 1 was charged with 110 g of LMA/HQ/HEMA-IEM stabilizer, 33 g of vinyltoluene, 457 g of Isopar H and 0.66 g of AIBN and the resulting mixture was polymerized for 21 hours at 70 ºC. The conversion rate was 56%. After stripping off residual monomer under vacuum, the product was ready for use as a binder in liquid toner formulations.

Case B

In the setup described in Example 1, the flask was charged with a mixture of 110 g of stabilizer (LMA/HQ/HEMA-IEM = 92.8/2.9/2.0-2.3 at 30% solids), 33 g of vinyl toluene (VT), 457 g of Isopar H and 0.5 g of tert-butyl peroxide. The resulting solution was vented with nitrogen for 10 minutes and then polymerized at 130 ºC for 8.5 hours. The conversion yield was 95.3% and the dispersion contained 10.74% solids.

The product was a poly(vinyltoluene) organosol containing long grafts of LMA, HQ and HEMA copolymer. It was designated LMA/HQ/HEMA-IEM//VT.

3. Preparation of a black toner for use with silicon-coated dielectric paper

A 15% solids toner concentrate was prepared by mixing BK-8200 carbon black pigment and LMA/HQ/HEMA-IEM//VT Organosol (feed composition: 45.90/1.95/0.98-1.17//50.0) in a 1:1 ratio with Isopar H and ball milling the dispersion to reduce the mean particle size to 367 + 114 nm. The charging agent (hereinafter referred to as "charging agent") Zr neodecanoate was then added to the dispersion at a concentration of 0.238%. The concentrate was diluted with Isopar G and additional Organosol and Zr neodecanoate added to bring the toner to Working concentration with the following properties:

Organosol/BK-8200 in a weight ratio of 2.0;

Solids concentration: 2.0%;

Zr neodecanoate: 0.147 ... 0.2%;

specific conductivity:

12.4 x 10-11 ... 15.9 x 10-11/Ohm cm.

Using the Synergy Colorwriter 400 printer, good image adhesion to silicone-coated dielectric paper was obtained. The reflected optical density (ROD) of the image was 1.14 or greater.

4. Black liquid toner for use with Use of urea-coated dielectric paper

The toner concentrate was prepared by dispersing Regal 300R carbon black pigment in LMA/HQ/HEMA-IEM//VT (feed composition: 44.92/2.93/0.98-1.17//50) organosol using a ball mill to produce a mean particle size of approximately 306 nm. The weight ratio of organosol to carbon black was 1, and the solids concentration was 15%.

A toner was prepared at a working concentration of 1.08% by diluting the concentrate with Isopar G and adding Cr neodecanoate and additional organosol to increase the organosol/pigment ratio to 2.0. The concentration of Zr neodecanoate in the toner was 0.13%.

The toner had a specific conductivity of 7.98 x 10-1/Ohm cm and produced images on urea/silicone coated dielectric paper with a ROD of 1.41.

5. Cyan liquid toner for use with Urea/silicone coated dielectric paper

A concentrate with a 15% solids concentration was prepared by ball milling a 1:1 mixture of LMA/HQ/HEMA-IEM//VT (45.85/0.97/1.45-.741/50.0 as feed composition) organosol and Sunf ast 248-3750 cyan pigment in Isopar H.

The concentrate was diluted with Isopar G and Zr neodecanoate and additional organosol was added to produce a toner at a working concentration of 1% solids.

The toner had the following properties:

Organosol/pigment weight ratio = 2.0;

Zr neodecanoate: 0.028%;

Particle size: 337 + 91 nm;

specific conductivity: 7.02 x 10⊃min;¹¹/Ohm cm;

Image ROD: 1.18.

6. Magenta liquid toner for use with urea/silicone coated dielectric paper

A 15% toner concentrate was prepared by dispersing Monastral 796D magenta pigment in LMA/HQ/HEMA-IEM//VT (45.85/0.97/1.45-1.74//50.0 as feed composition) organosol using a ball mill.

The toner concentrate was diluted with Isopar G and Zr neodecanoate and additional organosol was added to produce a toner at a working concentration of 1.15% with the following properties:

Organosol/pigment weight ratio = 2.0;

Zr neodecanoate: 0.028%;

Particle size: 456 + 158 nm;

specific conductivity: 3.90 x 10⊃min;¹¹/Ohm cm;

Image ROD: 1.13.

7. Yellow liquid toner for use with Urea/silicone coated dielectric paper

The toner concentrate was prepared as described in Example 6 using the following pigment and organosol:

Pigment: Sun's 274-1744 AAOT yellow;

Organosol:LMA/HQ/HEMA-IEM//VT (45.85/0.97/1.45-1.74//50 as feed composition).

The concentrate was diluted with Isopar G and Zr neodecanoate and further organosol was added to obtain a toner at a working concentration of 1.0% with the following properties to produce:

Organosol/pigment weight ratio = 2.0;

Zr neodecanoate: 0.014%;

Particle size: 388 + 87 nm;

specific conductivity: 1.31 x 10⊃min;¹¹/Ohm cm;

Image ROD: 1.09.

The black, cyan, magenta and yellow toners described in 4...7 above were used in the Synergy Colorwriter 400 printer to print test patterns for each individual color and the color combinations were superimposed on release-coated dielectric paper (silicone/urea composition as release layer). A high quality image was obtained, i.e. there were no scratches and the toner had good overprinting ability to produce composite colors. The image was transferred by thermal transfer to a modified Scotchcal image receptor (a 30 micron thick overcoat of butyl methacrylate was applied to the surface of the Scotchcal polyvinyl chloride overcoat) without leaving a residue on the release surface of the imaging film.

Receptor foils

These films comprise a substrate comprising a poly(vinyl chloride) or polyurethane film having an optical density in transmission greater than 1.0, usually with specific requirements for its properties, and a coated layer on one of the surfaces of the substrate which provides the required value of surface energy together with a value for the Tg of 10 ºC ... 105 ºC. In order to ensure adequate conformability with the surface of the imaging film, this layer should also have an appropriate complex dynamic viscosity. The surface coating of the receptor film can be selected from a wide range of thermoplastic polymers which satisfy the requirements described above. Examples of Such materials are: acrylates and especially methacrylates, such as methyl acrylates, butyl methacrylates, methyl methacrylate, methyl methacrylate copolymers with other acrylates, ethyl methacrylates, isobutyl methacrylates; low molecular weight vinyl acetate/vinyl chloride copolymers and aliphatic polyesters. Examples of materials that do not allow satisfactory transfer are high molecular weight polymethyl methacrylates. The complex viscosity of polymers is known to be a function of their molecular weight (see page 69 in "Polymer Rheology" by LE Nielsen, published by Marcel Dekker, 1977). At low molecular weights, say below 40,000, the complex viscosity is directly proportional to the molecular weight. At higher molecular weights, the viscosity is a power function of the molecular weight with an exponent of about 3.4. Therefore, high molecular weight polymers are most likely not suitable for receptor coatings of the present invention.

The receptor coating of the invention need not have a matte surface as is the case with many prior art imaging materials. A relatively smooth, non-matte surface is preferred. Less than 0.25 weight percent of the total coating layer should be in the form of particles having an average thickness of at least 10% of the thickness of the layer in order not to have a matte surface. More preferably, less than 0.1 weight percent of the receptor coating layer should be in the form of particles having an average diameter greater than 110% of the thickness of the coating layer. Most preferably, less than 0.1 weight percent should be particles having average diameters smaller than 90% of the layer thickness. This is defined and referred to as a smooth surface or a non-matte surface. Some particulate substances, such as pigments, may be present in the layer, whereby However, it is not desirable for these pigments to produce a matte surface.

The receptor film substrate must be flexible poly(vinyl chloride) or polyurethane with a thickness between 30 and 500 micrometers. The receptor coating must have a Tg between 10 ºC and 105 ºC, a dynamic complex viscosity of less than 2.0 x 10⁻⁷ mPa s (2.0 x 10⁵ poise) ((1 poise = 10² mPa s)) and a thickness between 3 and 50 micrometers. The receptor film must have the percentage haze values already described. Preferably, the flexible substrate of the film itself has an optical density in directional (regular) transmission of at least 1.0. This optical density is preferably achieved by pigmentation and/or bubbles in the film. The most preferred color is in optical density white, beige or ivory, although other colors (specially tinted white colors) may be used.

The rheological evaluation of receptor materials was carried out using a rheometer "Rheometrics Mechanical Spectrometer, Model RMS-605". The instrument was calibrated with polydimethylsiloxane (GE# 5E30) to obtain rheological functions in accordance with those described on pages 6...10 of the Rheometrics Inc. Rheometics Mechanical Spectrometer Operations Manualvi, Issue 0381, 5. 6.. .10. The complex viscosities were obtained from parallel vibrating plate measurements made at a strain of 2% at an angular frequency of 10 rad/s at a temperature of 110 ºC. The samples of films used were either taken from commercially produced material (e.g., standard cast white vinyl film from 3M) or were cast from solution, air dried, and then further dried for 3 to 5 days in a vacuum oven at temperatures chosen approximately equal to or less than the Tg of the material. Test samples of these receptor materials were prepared consisting of film layers cured at 110 ºC. between selected parallel plates with a diameter of 25 mm to give a gap thickness in the range of 0.5 mm to about 2.0 mm.

The image transfer efficiency of a series of receptor films was determined by measuring the amount of toner left on the imaging film after the transfer process after drying at 110ºC and 1 atmosphere pressure for 5 minutes in a vacuum fume hood. Since the residual toner on the imaging film after transfer causes the color of the surface to appear different from the background, i.e. in the areas that do not contain any image, the measurement of the "color difference CIELAB", usually referred to as ΔE, provided a good estimate of the image transfer efficiency, with small values indicating good transfer. (For a description of ΔE, see page 68 of "Measuring Color" by R.W.G. Hunt, published by John Wiley & Sons, New York 1987). In addition, the imaging films and their corresponding receptor films were visually evaluated to determine acceptability and ranking.

The color difference ΔE was measured using a Macbeth "Color Eye" spectrophotometer on areas of the imaging film surface from which toner images had been transferred. The measurement window was 7 mm x 7 mm. Areas from which black patterns had been transferred were used in these measurements.

In Table 3, the values of complex dynamic viscosity and shear modulus are given for various receptor coating materials and these values are related to the transfer properties obtained in the present invention by measurement on the ΔE scale. Table 3: Rheological evaluation of receptor coating materials

The ΔE range was correlated with the visual evaluation and it was found that a value of 4 indicated transmitted images that were already unacceptable. For the present invention, it was therefore determined that the ΔE value should be less than 4. From the values in In Table 3, a plot of the complex dynamic viscosity versus the ΔE values in Fig. 2 was made. A second order regression line was drawn through the measurement points and shown in Fig. 2. Using the visually determined upper limit of 4 for ΔE in Fig. 2, it can be seen that the value of the complex dynamic viscosity should be less than about 2.4 x mPa s (2.5 x 10⁵ poise) for a good transfer with a vacuum hood giving a pressure of about 1 atmosphere. Preferably, the value should be less than about 2.0 x 10⁷ poise. These values were obtained at 110ºC and the corresponding transfers were carried out at this temperature. As long as this value is obtained at this temperature, the same limit for the complex dynamic viscosity is expected to apply at other transfer temperatures. Our experiments have shown that receptor surfaces that are unacceptable to the limit give better results at increased transfer temperatures. This was expected from the published literature that the complex dynamic viscosity gradually decreases with increasing temperature (see the above-mentioned reference by LE Nielsen).

The substrate should preferably be conformable to the microscopic waves of surface roughness of the imaging surface. Materials such as PVC and polyurethane conform well to the imaging surface, whereas materials such as polycarbonate do not and consequently give poorer transfer of the toner image. Other materials which do not work as well as the preferred substrate composition in the practice of the present invention, but which are still broadly usable, are acrylic resins, polyolefins, polyethylene/acrylic acid copolymers and polyvinyl acetals (e.g. butyrals). Commercially available composites such as Scotchcal and Panaflex are also suitable substrates. On substrates such as PVC, the applied layer thickness can be as low as 3 micrometers. whereas on Scotchlite retroreflective material an applied layer thickness of 30 microns may be required.

The poly(vinyl chloride) and polyurethane substrates can be homopolymeric compositions or copolymeric compositions (including terpolymers and tetrapolymers). In the practice of the present invention, vinyl chloride copolymers with vinyl acetate, vinylidene chloride, styrene, butadiene, and the like can be readily used. Graft and block copolymers of the vinyl chlorides and urethanes can also be used. Preferably, the substrates comprise at least 50 weight percent units derived from vinyl chloride.

Transfer conditions

The preferred transfer device in the present invention is the vacuum draw housing. Typical pressures and temperatures in such a device when used in the present invention are 1 atmosphere and 110°C. The pressure is set by normal ambient air pressure, but measures to increase the local external pressure could provide higher transfer pressures in the vacuum draw apparatus. Temperatures in the range of at least 90°C...130°C can be used by selecting the receptor layer material according to the requirements given above. This method is preferred because there is no image distortion during transfer either by flow of the receptor film coating or by stripping of the receptor substrate. With the nip roller method of transfer, distortion is very likely to occur due to the higher pressures involved. On the other hand, complete transfer can be achieved more easily and the requirements for the properties of the receptor coating are less stringent. Because of the lack of distortion of the final image, the vacuum deposition method is preferred in the present invention, but the receptor properties must be carefully controlled.

Protective covers

To protect against mechanical damage and/or actinic damage to the image, coating of the transferred image may optionally be used. These coatings are compositions well known in the art and typically comprise a clear film-forming polymer dissolved or suspended in a volatile solvent. An ultraviolet light absorber may optionally be added to the coating solution. Laminating protective coatings to the image surface is well known in the art and may be used in the present invention.

Surface energy measurements

(a) Preparation of the sample

Release coatings

"Films" were deposited on clean glass plates (24 mm x 60 mm x 1 mm) using dip coating solutions (3 ... 5 % solids) of the test materials. In some cases, the coatings had to be dried at 40 ºC and in a low relative humidity (40 %) environment to obtain clear "films".

Release coatings were applied to dielectric paper by applying the solutions with a coating rod (#0 Meyer doctor blade). The test plates required for the contact angle measurements according to the Wilhelmy method (L. Wilhelmy, Ann. Physik, 119 (1863), 177) were then prepared by sticking a 24 mm wide polyester film carrier to both sides of the coated paper in such a way that after immersion only the release-coated surface could come into contact with the test liquid.

Receptor surfaces

Test plates of receptor materials were prepared by dip coating clean slides. However, if only an adhesive-supported If a film of the material was available, the test plate was prepared by removing the protective liner from the adhesive and by gluing two 24 mm wide strips (back to back) so that only the surface of interest was presented to the test liquid.

Liquid toner

Continuous, smooth "films" of liquid toner were prepared by electroplating toner particles from their dispersions in Isopar G carrier liquid onto anodized and silicate-coated aluminum plates. The particles were deposited using -150 volts applied to the aluminum substrate for deposition times ranging from 10 to 60 seconds, depending on the characteristics of the particular toner dispersion. After electroplating, the plates were rinsed by immersion in pure Isopar and dried in air at room temperature.

(b) Contact angle measurements

A measuring device "Cahn-322 Model Dynamic Contact Angle Analyzer" was used to measure the contact angles of the spreading and receding wetting liquid on the surface of the Wilhelmy plate. Contact angles during spreading were measured on 3 to 5 different areas of the surface of the Wilhelmy plate and reproducible values were obtained within an error limit of less than ± 1% in most cases and ± 2% in a few cases. At least 4 liquids with very different γd and γP were used as the wetting liquids for the respective test surface.

(c) Calculation of the surface energy using the data of the

Contact angle

From the measured contact angles during spreading of the test liquids with known γ₁d and γ1P on the solid surface, the surface energy was calculated using the following equation (HY Erbil and RA Meric, Colloids & Surfaces, 33, (.1988) 85-97, and the original references cited therein) is calculated:

where i denotes the liquid and j the solid and where i = 1, 2, ... n and n is the number of test liquids in a series with surface energy values published in the field covering a range of polarities.

The values of surface tension γtotal and the dispersion and polar components of surface tension γd and γP for various test fluids were taken from Kaelble et al., (D.H. Kaelble, P.J. Dynes and L. Maus, J. Adhesion, 6, (1974), 239-258) (see Table 1). The values for ethylene glycol were measured with the Wilhelmy balance using test solids with known properties.

(d) Adhesion work

The thermodynamic adhesion work (Wa) between the release layers and the toner films was calculated from the following equations:

γs ... surface energy of the release layer

γt ... surface energy of the toner film

The following equation was used to calculate the polar component of the adhesion work, Wa-polar:

(e) Interfacial tension between polymer layers

The interfacial tension between polymer layers 1 and 2 was calculated from the Fowkes equation (S. Ross and ID Morrison in "Colloidal Systems and Interfaces"(1988), John Wiley & Sons):

where the (-values refer to the surface tension and W₁₂ is the work of adhesion between surfaces 1 and 2 .

(f) Propagation coefficient (Girifalco-Good) Φ

in which are:

Φ = 1 for complete propagation,

Φ < 1 for lower spreading (poor adhesion) Release index = 1/Φ

The release capability is proportional to 1/Φ.

Surface energy measurements were made on a range of materials which were representative for use in the present invention. Table 4 contains the values for the silicone/urea release layers described above and Table 5 contains the values for a selected range of other possible surfaces. Table 4: Surface energy values of release layers Table 5: Surface energy values of dielectric paper and other release surfaces

The work of adhesion of toners to release surfaces can be calculated from the surface energy values using the equation given in the previous discussion (see "Work of Adhesion"). These values, Wa, are a measure of the relative abhesion/adhesion of two surfaces in a stacked arrangement of a toner or toners on a surface. Tables 6 and 7 give these values for the toner/release and toner/toner layers, respectively. Table 8 shows the values of Wa, between toner layers and uncoated dielectric paper (Type 6 paper in Table 5). As can be seen, these values are much higher than the values with an added release layer in Table 6, even with a PDMS concentration as low as 1%. On the other hand, the values in Table 8 are very similar to the adhesion values between toner layers according to Table 7. Table 6: Adhesion work (Wa) between toner and release layers Table 7: Adhesion work (Wa) between toner layers (double printing process) Table 8: Adhesion work between toner layers and non-coated dielectric paper (Type 6 - Paper) Toner layers

Implications of Surface Energy Measurements The effects of good and poor release properties in the imaging film surface can be influenced by a number of imaging toner deposition conditions that differ in terms of the type and number of the four toners involved. For a 10% PDMS release coating, all three toners will release together, while for a 0% PDMS release coating, the three will be split at the M/C interface. In the sixth image, the split would be at the Y/C interface, and for the second image, at the C/B interface. All remaining imaging conditions would be transferred by splitting at the interface with the dielectric layer surface, so that all toners are transferred. If the correct release layer is used, none of the imaging conditions are split within the toner composition, only at the release surface.

However, this analysis assumes that the adhesive film strength of the toner layers themselves is such that no splitting can occur within a toner layer. Adhesive film strengths are obtained as twice the surface energy of the toner layer (see the relationship of adhesion work to polar and dispersive components of the surface energy values of the two surfaces as outlined above, and remember that in the volume of a single material the two ranges of surface energy are identical.) These, like the adhesion work, must be greater than the adhesion work of the lower toner to the dielectric (release) surface.

For the imaging process to be successful, one final criterion must be met. During the process itself, the deposited toners must be sufficiently tough to withstand the abrasion to which they are subjected by the charging rods and development rollers. The scratch tests described in the following chapter offer one way to to determine whether the abrasion resistance of the toner is sufficient for this task.

The scratch tests

The procedures for liquid toner films are described below.

(a) Preparation of the sample

A 35 mm wide and 95 mm long strip of 76 micrometer thick polyester film is provided with a vapor-deposited aluminum layer on one side and placed in a cell filled with a liquid toner dispersion (1 ... 2% solids) in such a way that the aluminum side is 5 mm away from the counter electrode. After connecting the aluminum layer to the negative and the counter electrode to the positive terminal of a DC voltage source, a voltage of 150 volts is applied for 20 seconds to cause electrophoretic deposition of toner particles on the aluminum layer.

To remove excess liquid from the toner layer, the sample is rinsed after toner deposition by immersion in Isopar G and air-dried for 5 ... 7 minutes

The scratch test is carried out immediately after the liquid film has evaporated to examine the properties of the toner layer under conditions approximating those in the electrostatic printer when the image-bearing transfer medium of the first color has just arrived at the imaging station for the second color, where the first image has arrived with the charging head, the rotating development electrode and the edges of the vacuum opening.

(b) Execution of the scratch test

The scratch test consists of a rod held down with weights and dragged across the surface of the toner layer. The radius of curvature for the rod tip (ball guide) is approximately 0.75 mm, with the weights used to change the load on the surface of the Toner layer can be adjusted.

The marks created on the surface of the toner layer by the rod were examined under a microscope (190x magnification) and classified as follows (increasing in degree of damage):

(C) the toner layer is merely compressed;

(Scr) Compression of the layer plus fine scratch lines;

(Cr) toner layer is compressed and cracking;

(S) the rod "skims", partial removal of the layer;

(TR) Removal of the entire layer over a wide contact area.

A toner layer that shows cracks or "overreading" at lower rod load than another toner layer is interpreted as mechanically weaker.

The "force strength" is defined in the present invention as the load in grams required to produce damage up to the Cr level.

Examples

Dielectric paper type 2089, manufactured by James River Graphics Corporation, was coated with a 5% solution of silicone/urea copolymer in isopropanol (silicone content 10%) using a combination of Meyer 5 and 0 doctor blades. The estimated dry thickness of the silicone/urea layer was approximately 0.11 micrometers. The silicone-coated dielectric paper was used for imaging tests in the electrostatic Synergy printer used.

In a series of tests, various combinations of liquid toners were evaluated for their ability to produce high quality images in the Synergy Colorwriter 400 electrostatic printer by printing solid test patterns and 40% halftone patterns of all individual and all superimposed color combinations using black, cyan, magenta and yellow liquid toners. The toners were evaluated for sensitivity of the image to abrasion damage on the release-coated imaging surface and for their ability to produce uniform toner deposits over a previously formed image of a different color.

example 1

The release coated dielectric paper and liquid toners B-1 (black), C-1 (cyan), M-2 (magenta) and Y-1 (yellow) were loaded into the Synergy printer and the test image printed as described above at a paper speed of 3.18 mm/s (0.15 in/s). The image on the release surface was of high quality, i.e., there were no rub marks on any of the test patterns and the deposition of a second color over a first color image produced in a previous image station was uniform and of sufficient thickness to produce good second colors of green, blue and red (yellow over cyan, magenta over cyan and yellow over magenta).

Example 2

The experiment described in Example 1 was repeated using other black and magenta and a similar yellow toner in combination, i.e. B-2 (black), C-1 (cyan), M-1 (magenta) and Y-1 (yellow). The image quality obtained with this combination of toners was drastically different and unacceptable, as shown by the description of the individual test patterns below:

Black (B) moderate image abrasion

Cyan (C) no abrasion

Magenta (M) no abrasion

Yellow (Y) no abrasion

Green (C + Y) slight abrasion damage

Blue (C + M) slight abrasion damage

Red (M + Y) M with abrasion where overpressure occurred through Y

(C + M + Y) severe abrasion damage

(B + M + Y) Abrasion stronger than M + Y

(B + C + M) severe abrasion damage

(B + C + Y) severe abrasion damage

Example 3

In the toner combination used in Example 2, the M-1 magenta toner was replaced with a different formulation, M-2. With this set of toners, the abrasion damage was eliminated in the test samples containing the new magenta (except where the magenta toner was deposited over a black toner layer):

B moderate abrasion damage

C no abrasion

M no abrasion

Y no abrasion

(C + Y) moderate abrasion damage

(C + M) no abrasion

(M + Y) no abrasion

(B + M + Y) Abrasion damage

(B + C + M) Abrasion damage

Reference examples

The following block copolymer examples 1 to 6 show how the release coating polydimethylsiloxane polymers for use in the present invention can be prepared. A detailed description of these polymers is also provided.

The general structure of this release coating is:

where Silicone is PDMS, DIPIP is dipiperidylpropane, IPDI is isophorone diisocyanate and Jeffamine is a polypropylene oxide with terminal diamine groups.

In the present application, the amount of hard The results have shown that there must not be less than 75% hard segment if there is a soft segment without silicone. The Tg results seem to be the most direct indication of the 75% minimum.

It has been demonstrated that a good image can be obtained with less than 75% hard segment, but only if there is no soft segment and the silicone (PDMS) has a higher proportion, such as 30 ... 50%.

This is well explained by the samples listed in the table, where with the exception of the sample with "0" silicone (PDMS), all samples provided a good image.

The solvent was isopropanol.

Silicone ... (PDMS) Polydimethylsiloxane

hard segment ... (DIPIP) dipiperidylpropane/IPDI (isophorone diisocyanate)

soft segment ... (Jeffamine) DU-700

where c = 11.2.

Other segments with PDMS take on the role of release material, but have been shown to produce blurred images, such as:

hard segments ... (MPMD) Methylpentanemethylenediamine/IPDI or

(BISAPIP) Bisaminopropylpiperazine/IPDI soft segment ... (PPO) Polypropylene oxide The preferred organopolysiloxane/polyurea block polymers comprise a repeating unit of the formula:

in which are:

Z is a divalent radical selected from the group consisting of phenylene, alkylene, aralkylene and cycloalkylene;

Y is an alkylene radical having 1 to 10 carbon atoms;

R is at least 50% methyl with the remainder to 100% of all R radicals selected from the group consisting of a monovalent alkyl radical having 2 to 12 carbon atoms, a vinyl radical, a phenyl radical and a substituted phenyl radical;

D is selected from the group consisting of hydrogen and an alkyl radical having 1 to 10 carbon atoms;

B is selected from the group consisting of alkylene, aralkylene, cycloalkylene, azaalkylene, cycloazaalkylene, phenylene, polyalkylene oxides, polyethylene adipate, polycaprolactone, polybutadiene and mixtures thereof and a radical which closes a ring structure including A to produce a heterocycle;

A selected from the group consisting of

wherein G is selected from the group consisting of hydrogen, an alkyl radical having 1 to 10 carbon atoms, phenyl and a radical which closes a ring structure, including B to form a heterocycle;

n is a number that is 10 or greater (preferably 70) and

m is a number that can be from 0 to about 25.

In the preferred block polymer, z is selected from the group consisting of hexamethylene, methylene-bis-(phenylene), isophorone, tetramethylene, cyclohexene and methylenedicyclohexene, and R is methyl.

The organopolysiloxane/polyurea block copolymer useful in the present invention must be compatible in an organic, non-aqueous solvent. The term "compatible" as used herein means that the copolymer is soluble in an organic solvent (non-aqueous solvents only). The water-compatible polymers contain ionic groups in the polymer chain and are not satisfactory as a functional release material for toner coated on dielectric material. During drying, water is removed, the polar "non-silicone" segment (quaternary amine) remains on the surface and the silicone remains mostly completely submerged under the polar "non-silicone" layer, thus not leaving sufficient silicone on the contact surface with the toner(s) and thus no toner release capabilities when image transfer is attempted.

The block polymers usable in the present invention can be prepared by polymerizing the corresponding components under reaction conditions in an inert atmosphere.

The components include:

1. A diamine having a number average molecular weight (Mn) of at least 500 and a molecular structure represented by the following formula II:

wherein R, Y, D and n are as defined in formula I.

2. at least one diisocyanate having a molecular structure represented by the following formulas III:

OCN-Z-NCO

where Z is as defined in formula I.

3. up to 95% by weight of diamine or dihydroxy chain extender having a chain length represented by the following formula IV Molecular structure shown:

H - A - B - A - H

where A and B are as defined above.

The combined molar ratio of silicone diamine, diamine and/or dihydroxy chain extender to diisocyanate in the reaction is such that it is suitable for producing a block polymer having the desired properties. Preferably, the ratio is kept in the range of about 1:0.95 ... 1:1.05.

Specifically, solvent compatible block polymers useful in the invention can be prepared by mixing organopolysiloxane diamine, diamine and/or dihydroxy chain extenders, if used, and diisocyanate under reaction conditions to produce the block polymer having hard or soft segments derived from the diisocyanate and organopolysiloxane diamine. The reaction is usually carried out in a reaction solvent. The donor element of the present invention can be prepared by a variety of methods. While preparation of the donor element can be readily accomplished, the surface to be treated must first be cleaned of all dirt and grease. Accepted cleaning methods can be used. The surface is then contacted with the organopolysiloxane/polyurea polymer solution using any of a number of methods, such as brushing, bar coating, spraying, roll coating, curtain coating, doctor blade coating, etc., and then processed at a temperature for a time sufficient to cause the polymer to form a dry film on the surface. For image release coatings, a suitable dry film thickness is in the range of 0.05 to 2.0 micrometers, with a preferred thickness range of 0.08 to 0.3 micrometers, and with best results about 0.12 to 0.18 micrometers.

The non-aqueous polymer solutions, diluted in a solvent such as isopropanol, up to a appropriate solids concentration are then applied to dielectric material. Once dried, the coating thickness can be measured using a chemical indicator method if the correct indicator has been incorporated into the non-aqueous release material prior to application to the dielectric material.

The methods for thickness measurement, such as cutting and weighing methods, are not effective due to the ultra-thin coatings.

A colorless pH indicator, preferably thymolphthalein (not more than 5% of the solids concentration of the silicone/urea polymer), is added to the non-aqueous silicone/urea coating material. This colorless indicator turns blue by development of an alkaline solution prior to spectrophotometer absorbance readings and calculations.

One requirement of the release coating is that it must be a very thin coating so that a high density image can be developed between the toner(s) and the dielectric material. The function of the indicator is to monitor the submicron coating weight of the silicone/urea polymer layer. The coating weight of the polymer, which is proportional to the amount of indicator, is calculated from the color developed in the alkaline solution by absorbance measurements. The indicator inside the blocked polymer coating must not only be colorless, but must also remain in a stable colorless state when applied to the dielectric material at neutral pH conditions. In addition, this colorless indicator must not interfere with the printing and transfer of the image or age the transferred image.

Other indicators may function as well as the preferred indicator given in the previous section, for example m-nitrophenol, o-cresolphthalein, phenolphthalein, ethyl bis(2,4-dinitrophenyl) acetate. Although not included in the evaluations, other Classes of indicators that should work in the same way, those that respond to oxidation-reduction.

The preferred method of manufacture which gives the best results uses: 5 ... 10% silicone with 15 ... 20% soft segment and 75% hard segment and a content of 12.6% solids.

This non-aqueous release polymer is diluted to a 3...5% solution and coated onto James River Graphics #2089 dielectric paper using a Meyer blade # or #1, creating a release coating thickness of 0.12 microns for the Meyer blade #0 and 0.18 microns for the Meyer blade #1. An acceptable range of coating thickness is 0.08...0.3 microns, with a preferred coating thickness range of 0.1...0.2 microns.

Block polymer - Example 1

To a solution of 0.38 g of polydimethylsiloxane (PDMS) diamine with a number average molecular weight (Mn) of 5,000, 1.50 g of Jeffamine (Du-700) with a number average molecular weight of 800 and 2.52 g of dipiperidylpropane (DIPIP) in 242.50 g of isopropanol (IPA) at 25 ºC, 3.10 g of isophorone diisocyanate (IPDI) were slowly added over a period of 5 minutes. The viscosity increased rapidly towards the end of the addition and the viscous and still clear reaction mixture was stirred for an additional 15 minutes. This yielded a 3 wt.% solution of the block polymer in IPA. The block polymer had 5 wt% PDMS as soft segment and 75 wt% DIPIP/IPDI as hard segments and 20 wt% Jeffamine as soft segment.

Block polymer - Example 2

To a solution of 1.13 g of polydimethylsiloxane (PDMS)diamine with a number average molecular weight (Mn) of 5,000, 1.50 g of Jeffamine (Du-700) with a number average molecular weight of 800, and 2.52 g of dipiperidylpropane (DIPIP) in 242.50 g of isopropanol (IPA) at 25 ºC, 3.02 g of isophorone diisocyanate (IPDI) was slowly added over a period of 5 minutes. The viscosity increased rapidly to towards the end of the addition and the viscous and still clear reaction mixture was stirred for a further 15 minutes. This yielded a 3 wt% solution of the block polymer in IPA. The block polymer had 15 wt% PDMS as the soft segment and 75 wt% DIPIP/IPDI as the hard segments and 10 wt% Jeffamine as the soft segment.

Block polymer - Example 3

To a solution of 1.50 g of polydimethylsiloxane (PDMS)diamine with a number average molecular weight (Mn) of 5,000, 0.38 g of Jeffamine (Du-700) with a number average molecular weight of 800, and 2.65 g of dipiperidylpropane (DIPIP) in 242.50 g of isopropanol (IPA) at 25 ºC, 2.97 g of isophorone diisocyanate (IPDI) were slowly added over a period of 5 minutes. The viscosity increased rapidly towards the end of the addition and the viscous and still clear reaction mixture was stirred for an additional 15 minutes. This yielded a 3 wt% solution of the block polymer in IPA. The block polymer had 20 wt% PDMS as soft segment and 75 wt% DIPIP/IPDI as hard segments and 5 wt% Jeffamine as soft segment.

Block polymer - Example 4

To a solution of 3.75 g of polydimethylsiloxane (PDMS)diamine with a number average molecular weight (Mn) of 5,000, 0 g of Jeffamine (Du-700) with a number average molecular weight of 800 and 1.74 g of dipiperidylpropane (DIPIP) in 242.50 g of isopropanol (IPA) at 25 ºC, 2.01 g of isophorone diisocyanate (IPDI) were slowly added over a period of 5 minutes. The viscosity increased rapidly towards the end of the addition and the viscous and still clear reaction mixture was stirred for an additional 15 minutes. This yielded a 3% by weight solution of the block polymer in IPA. The block polymer had 50 wt% PDMS as soft segment and 50 wt% DIPIP/IPDI as hard segments and 0 wt% Jeffamine as soft segment.

Examples 1 to 4 all showed very good function with clear images during transfer. They were all performed under nitrogen atmosphere.

Block polymer - Example 5

To a solution of 65 g of polydimethylsiloxane (PDMS) diamine with a number average molecular weight (Mn) of 5,000, 15.2 g of bisaminopropylpiperazine (BISAPIP) in 530 ml of isopropanol (IPA) at 25 ºC, 19.8 g of isophorone diisocyanate (IPDI) were slowly added over a period of 5 minutes. In order to keep the temperature at 15 ºC ... 25 ºC during the addition, the exothermic course of the reaction was controlled using an ice/water bath. The viscosity increased rapidly towards the end of the addition and the viscous and still clear reaction (mixture) was stirred for a further hour. This yielded a 20 wt.% solution of the block polymer in IPA. The block polymer had 65 wt% PDMS as soft segment and 35 wt% BISAPIP/IPDI as hard segments.

Block polymer - Example 6

A 250 mL three-neck flask was charged with 5 g of 5,000 (Mn) PDMS-diamine, 1.29 g of BISAPIP, 0.56 g of methylpentamethylenediamine (MPMD), and 40 g of isopropanol. The resulting solution was cooled to 20 ºC using an ice bath while 2.76 g of IPDI was added. This yielded the silicone/polyurea as a very viscous and still clear solution in IPA. The block polymer had 52 wt% PDMS as soft segments and 48 wt% hard segments (35 wt% bisAPIP/IPDI and 13 wt% MPMD).

The following Examples 7 and 8 relate to polymeric materials for use in self-release dielectric layers in any of the embodiments of the present invention.

Dielectric layer - Example 7

The production of copolymers and terpolymers of vinyl monomers with siloxane macromonomers was P-4 728 571. The application of this preparation, the selection of methyl methacrylate (MMA) or a mixture of MMA and styrene as the vinyl monomer and a further selection of polydimethylsiloxane as the siloxane macromonomer provides the route to the polymers used in the present invention for the electrical self-release layers

Dielectric layer - Example 8

The dielectric layers were prepared by coating solutions containing the copolymer or terpolymer onto a paper substrate. The coating solutions were prepared from the polymer solutions according to the following recipe, where percentages are by weight.

Polymer solution 50%

- 30% solids in 2:1 ethyl acetate/toluene

Sound, Translink 37 3.75%

Calcium carbonate 2.50%

Titanium dioxide 1.25%

Toluene 50%

These solutions were ball milled for 4 hours and coated onto James River Graphics conductive paper backing using a #14 Meyer blade to give a wet thickness of 30.5 microns. After drying, the coatings were conditioned at 50% relative humidity and 21ºC (70ºF) for 12 hours prior to use in imaging.

Claims (12)

1. An electrographic method for producing multi-coloured toner images in an electrostatic printer, comprising the steps: (a) providing a flexible imaging film having dielectric and toner release properties on at least one surface characterized by a surface energy between 14 10-7 and 20 10-7 J/cm2 (14 and 20 erg/cm2), the surface energy comprising no more than 5% polar component; (b) passing the imaging film through the printer at a substantially stationary speed; (c) forming on the surface of the imaging film a first electrostatic latent image corresponding to a first color by pixel-by-pixel deposition of charges; (d) developing the first latent image by means of a rotating applicator bar with a first toner corresponding to the first color to form a first toner image, which first toner image then has a scratch resistance of not less than 40 g and a surface energy of not more than 50 erg/cm2; (e) drying the first toner image; (f) repeating steps (c), (d) and (e) in sequence using toners corresponding to at least one or more colors to complete the multi-colored toner image, such that where a later developed toner overlays an earlier developed toner, the interface between the earlier and later toners has a work of adhesion value that is greater than the largest work of adhesion values of interfaces created between the toners and the surface of the imaging film; (g) contacting the sample on the surface of the imaging Film deposited multicolored toner image with the surface of a receptor film under pressure and elevated temperature so that the multicolored toner image is transferred to the surface of the receptor film without distortion, wherein the surface of the receptor film has a surface energy which is greater than the surface energy of the imaging surface of the receptor film, and wherein the receptor film has a Tg value between 10 ºC and 105 ºC and at least 5 ºC below the elevated temperature, wherein the receptor film has a flexible substrate film carrying said surface, which substrate film comprises a polymer selected from the group consisting of acrylic resins, polyolefins, polyvinyl acetals, polyvinyl chloride and polyurethane, and wherein the substrate film has a haze value greater than 30 percent and a thickness of 30...500 micrometers. 2. Electrographic process according to claim 1, in which the flexible imaging film comprises a conductive substrate coated on one of its two major surfaces with a dielectric layer and a separate cover layer with release properties. 3. Electrographic process according to claim 2, wherein the cover layer comprises a release material selected from the group consisting of silicone/urea block polymers with 1 .... 65 weight percent polydimethylsiloxane, urethane/silicone copolymers, epoxy/silicone copolymers and acrylic/silicone copolymers. 4. An electrographic process according to claim 1, wherein the polymer of the substrate film comprises polyvinyl chloride or polyurethane. 5. An electrographic process according to claim 2, wherein the dielectric layer comprises materials selected from the group consisting of terpolymers of polydimethylsiloxane, methyl methacrylate and polystyrene, and Copolymers of polydimethylsiloxane and methyl methacrylate, wherein the polydimethylsiloxane comprises between 10% and 30% by weight of the total polymer. 6. The electrographic process of claim 1, wherein the receptor film comprises a substrate film bearing a thermoplastic layer comprising a polymer selected from the group consisting of thermoplastic polymers having a complex dynamic viscosity value of less than about 2.5 10⁴ Pa s (2.5 10⁵ poise) at a temperature equal to said elevated temperature. 7. An electrographic process according to claim 6, wherein the thermoplastic layer comprises polymers selected from the group consisting of methacrylates, butyl methacrylates, methyl methacrylate copolymers with other acrylates, ethyl methacrylates, isobutyl methacrylates, low molecular weight vinyl acetate/vinyl chloride copolymers, polyurethane and aliphatic polyesters. 8. Electrographic process according to claim 1, in which the elevated temperature is between 50º and 150ºC. 9. Electrographic process according to one of the preceding claims, in which the multi-coloured toner image is produced in (only) one pass through the electrostatic printer. 10. An electrographic receptor film comprising a polymeric substrate of 30 to 500 micrometers comprising a resin film having an absorptivity of at least 1.0 from a resin selected from the group consisting of acrylic resins, polyolefins, polyvinyl acetals, polyvinyl chloride and polyurethane, said substrate having on one of its surfaces a receptor layer composition having a Tg of 10 to 105°C, a dynamic complex viscosity of less than 1.0.10⁴ Pa s (1.0 10⁵ poise) and a dry thickness of 3 to 50 micrometers. 11. The electrographic film of claim 10, wherein the substrate comprises polyvinyl chloride resin. 12. Electrographic film according to claim 10 or 11, wherein a second surface of the substrate has an adhesive coating thereon