Patent Publication Number: US-2021178422-A1

Title: Method for coating a surface with a transferable layer of thermoplastic particles and related apparatus

Description:
RELATED APPLICATIONS 
     This application is a Continuation-In-Part of U.S. patent application Ser. No. 15/363,615, filed on Nov. 29, 2016, which is a Continuation-In-Part of International Patent Application No. PCT/IB2016/053140, filed on May 27, 2016, which claims priority from GB Patent Application No. 1514618.6, filed on Aug. 17, 2015, and from GB Patent Application No. 1509080.6, filed on May 27, 2015. The entire disclosures of all of the aforementioned applications are incorporated herein by reference for all purposes as if fully set forth herein. 
    
    
     FIELD 
     The present disclosure relates to a method and an apparatus for coating a surface with a layer of thermoplastic particles, in particular with a single layer. 
     BACKGROUND 
     In certain types of printing, a film supported by a carrier is transferred to a substrate (e.g., paper, cardboard, plastic films etc.) by application of pressure and/or heat in a desired pattern. One example of this is found in thermal transfer typewriters, where a ribbon carries an ink film that is transferred to paper by the application of heat. 
     A problem in using a conventional film coated carrier, be it a sheet, a web or a ribbon, is that the process is wasteful, and therefore expensive. This is because, at the time that it has to be discarded, only a small proportion of the film coating will have been used (e.g., for printing) and most of the film coating will remain on the carrier. 
     OBJECT 
     An aspect of the present invention seeks inter alia to provide a method for applying to a surface, hereinafter termed a donor surface, a coating of individual particles that is transferable to a substrate, in which the parts of the surface from which the coating has been removed in an operating cycle (also termed “exposed regions”) can be recoated without substantially increasing the coating thickness on the remaining parts of the surface, so as to enable the entire surface to be re-used time and again. 
     SUMMARY 
     In accordance with an aspect of the present disclosure there is proposed a method of coating a donor surface with a layer of thermoplastic particles, the method comprising providing a supply of the thermoplastic particles suspended in a fluid, the fluid being a liquid that does not wet the donor surface or being a gas, the surface energies of the thermoplastic particles and of the donor surface being selected such that the particles have a higher tendency to adhere to the donor surface than to one another; applying the fluid to the donor surface, by an applying system, in a manner to cause the particles suspended in the fluid to adhere to the donor surface so as to form a substantially continuous particle coating on the donor surface as the donor surface is moved relative to the applying system; and causing fluid flow within an interior plenum of a housing and over a portion of the donor surface partially disposed within the plenum, the fluid flow being of sufficient magnitude to entrain particles that are not in direct contact with the donor surface and insufficient to entrain particles that are in direct contact with the donor surface; and, extracting fluid and particles which are not in not direct contact with the donor surface from the plenum, so as to leave adhering to the donor surface a particle coating that is substantially only a single particle deep. 
     In accordance with another aspect of the present disclosure, there is proposed a coating apparatus for coating with a layer of thermoplastic particles a donor surface movable relative to the apparatus, the apparatus comprising:
         a) a supply system of thermoplastic particles suspended in a fluid that does not wet the donor surface, the particles adhering more strongly to the donor surface than to one another,   b) an application device for applying the fluid to the donor surface in a manner to cause the particles suspended in the fluid to adhere to the donor surface, so as to form a substantially continuous particle coating on the surface, and   c) a surplus extraction system operative to extract fluid and remove surplus particles that are not in direct contact with the surface, so as to leave adhering to the donor surface a coating that is substantially only a single particle deep.       

     The coating apparatus is one embodiment capable of performing the method aspect proposed by the present disclosure. 
     The term “supply system” should be construed as a conduit for supplying the required particles, and may be implemented internally to the coating device or to a housing thereof, such as a tank or container, or to an external supply system which transports appropriate particles to the application device. The element(s) capable of delivering the suspended particles at the terminus of the supply system within the application device can be referred to as “supply system outlet(s)”, a spray head being a non-limiting example of such an outlet and delivery method. 
     In the present disclosure, a non-limiting use of the coating method and apparatus is illustrated as part of a printing system and particles from the donor surface will in use be transferred to a printing substrate. The donor surface may be looked upon as equivalent to a ribbon as used in a thermal transfer printer, but with the advantage that it can be restored to its original state after use. 
     As the coating applied to the donor surface is made up of a mosaic of individual particles, the proportion of the donor surface covered with particles will be less than 100% on account of the interstices between the individual particles. Depending on the use that is subsequently made of the applied particle coating, the proportion of the donor surface that is covered may need only be 95%, or 90%, or 85%, or 80%, or 75%, or 70% or even 65% or less. The acceptable proportion in a printing system would depend, for example, on the color of the particles and the color of the substrate. If printing with light colored particles on a white substrate, acceptable quality may be achieved with only 65% coverage, or possibly even less (e.g., about 40%), whereas printing with white particles on a black substrate would benefit from greater coverage to avoid the color of the substrate showing through and giving the print a grey appearance. 
     After the particles in selected areas of the donor surface are transferred to a printing substrate, those areas will be left depleted of particles and the donor surface will be exposed. If the donor surface is now again passed through the coating apparatus, a fresh coating of particles will be applied onto the depleted areas. However, under a disclosed method, the surplus extraction system will remove substantially any particles that are not in direct contact with the donor surface, so that there will be no build up of coatings on areas from which particles are not transferred to the printing substrate. 
     In some embodiments, particles remaining on the donor surface after an impression has been made on a printing substrate can be removed from the donor surface upstream of the coating apparatus (on the entry side, after the impression on the substrate) so that the entire donor surface may be recoated with fresh particles. 
     In the present disclosure, the term “suspended in” and its variations is to be understood as “carried by” and like terms, not referring to any particular type of mixture of materials of same or different phase. The fluid which may be a gas or a liquid, can optionally be maintained at a desired controlled temperature. 
     When the particles are applied in a liquid fluid, the coating apparatus may further comprise, if needed, a dryer enabling the particle coating to be substantially dry by the time it reaches a subsequent station where such particle layer can be used or subjected to further steps. A dryer can be a blower, a heater, a liquid extractor or any other device suitable to remove surplus liquid. The dryer, if present, is advantageously compatible with the particle layer, and for instance does not negatively affect the particles and/or the integrity of the coating formed of the particles. 
     The coating apparatus embodying the method may comprise separate housings enclosing different sub-systems, each housing independently defining an interior plenum or chamber. For example, a first housing of the coating apparatus may comprise a particle supply and an application device; a second housing may contain a first surplus extraction system operative to remove any particles that are not in direct contact with the donor surface; and a second surplus extraction system operative to extract surplus fluid that may be contained in a third housing. Additional combinations of sub-systems and options of housing are apparent to the skilled person, and for instance, each housing, if more than one, may have its own surplus fluid extraction system having a suitable suction source. Each housing may have a rim adjacent to the surface that is configured to prevent egress of particles from a sealing gap defined between the rim of the housing and the surface being coated. 
     In the present disclosure, because the particles adhere to the donor surface more strongly than they do to one another, the applied particle coating is substantially a monolayer, i.e. only one particle deep. While some overlap may occur between particles, the layer may be only one particle deep over a major proportion of the area of the surface and most, if not all, of the particles will have at least some direct contact with the surface. The creation of a monolayer occurs for the same reason that an adhesive tape, when used to pick up a powder from a surface, will only pick up one layer of powder particles. When the adhesive tape is fresh, the powder will stick to the adhesive until it covers the entire tape surface. However, once the adhesive has been covered with powder, the tape cannot be used to pick up any more powder because the powder particles will not stick strongly to one another and can simply be brushed off or blown away from the tape. Similarly, the monolayer herein is formed from the particles in sufficient contact with the donor surface and is therefore typically a single particle deep. Contact is considered sufficient when it allows the particle to remain attached to the donor surface at the exit of the coating station, e.g., following surplus extraction, burnishing, or any other like steps, which are described in more detail herein. 
     Taking, for example, a platelet shaped particle contacting the donor surface over most of its planar face (e.g., being substantially parallel), the resulting thickness of the monolayer (in the direction perpendicular to the surface) would approximately correspond to the thickness of the particle, hence the average thickness of the particle coating can be approximated by the average thickness of the individual particles forming it. However, as there could be partial overlaps between adjacent particles, the thickness of the monolayer can also amount, in some places, to a low multiple of the dimension of the constituting particles, depending on the type of overlap, for instance on the relative angles the particles may form with one another and/or with the donor surface and/or the extent of the overlap. A monolayer particle coating may therefore have a maximum thickness corresponding to about one-fold, and only in some regions, of about two-fold, or about three-fold, or any intermediate value, of a thinnest dimension characteristic to the particles involved (e.g., the thickness of the particles for flake shaped ones or essentially the particle diameter for spherical ones). In the present disclosure, such a particle coating is said to be substantially only a single particle deep and is also referred to as a monolayer. 
     The application device may comprise a spray head, or any other suitable supply system outlet, for spraying the fluid and suspended particles directly onto the surface, or a rotatable applicator operative to wipe the fluid and suspended particles onto the surface. The applicator may for example be a cylindrical sponge or may comprise a plurality of flexible strips extending radially from a rotatable axle. The sponge or the flexible strips may be formed of a closed-cell foam. The fluid comprising the suspended particles may be supplied externally to such an applicator (e.g., the fluid is sprayed on a portion of the applicator typically facing away from the surface) or may be supplied internally (e.g., the fluid is provided from a supply duct or spray positioned within the applicator, for instance, in parallel to the rotatable axis, and diffuse along the material towards the external surface of the applicator). 
     In some embodiments, the applicator may at least partially remove any particles that are not in direct contact with the surface and optionally at least partially burnish the particles coated on the surface as a monolayer. As used herein, the term “burnish” is to be understood broadly to encompass any flattening action on the particles that may further homogenize at least one property of the particle coating, be it the thickness of the layer, the orientation of the particles, their distribution on the surface, their size, their shine or any other like characteristic. 
     In some embodiments, the surplus extraction system, that serves to remove any particles that are not in direct contact with the surface, is configured similarly to the applicator, namely as a roller contacting the donor surface or particles thereupon. In such case, the fluid being externally or internally supplied to the applicator-like element to serve as an excess particles remover, would not have any particles suspended in it. The fluid of the surplus extraction system may be the same as, or different from, the fluid in which the particles are suspended for the application device. For instance, particles may be applied while suspended in water or any other aqueous medium, and excess thereof may be removed by the same aqueous medium or by a different fluid, such as by an air stream. 
     In some embodiments, the applicator-like element of the surplus extraction system removes substantially all particles that are not in direct contact with the surface and optionally at least partially burnishes the particles coated on the surface as a monolayer. Substantial removal may mean that in any monolayer of particles, the proportion of particles of a coating not in direct contact with the donor surface is at most 35%, at most 30%, at most 25%, at most 20%, at most 15%, at most 10%, at most 7%, at most 5%, at most 3%, or at most 2%, by number, of said particles. 
     In some embodiments, the application device is contained within an interior plenum of a housing that has a rim adjacent the donor surface, the rim being configured to prevent egress of particles from a sealing gap defined between the rim of the housing and the surface. 
     There are various ways of preventing egress of particles from the housing and of removing surplus particles from the surface to leave only a monolayer. In some embodiments of the apparatus, a wiper member may be provided at least on the upstream side of the housing to prevent fluid escape through the sealing gap during operation of the apparatus. 
     In some embodiments, a fluid flow passage may be provided at the rim of the housing to enable a fluid to be drawn from, or introduced into, at least regions of the sealing gap disposed downstream of the housing and/or of the coating device. The fluid which may serve to “seal” the apparatus, or any sub-housing, is being introduced or removed from the apparatus by a suction source, and as used herein this term relates to both positive and negative supply of the fluid of relevance. 
     Optionally, the fluid flow passage may be coupled to the same suction source of a surplus extraction system, or to a second suction source, so as to draw from the gap any fluid that would otherwise escape from the interior plenum through the gap. 
     As an alternative, the fluid flow passage may be connected to a supply of a gas devoid of suspended particles at above ambient pressure, so that fluid within which the particles are suspended is prevented from escaping from the interior plenum through the gap owing to the pressure in the gap being higher than the pressure in the interior plenum. Such type of confinement can be achieved with an air knife. 
     The fluid within which the particles are suspended may be a gas, preferably air, and in such embodiments the particles may be entrained into the gas stream by a Venturi, such as an optional Venturi tube  1411  depicted in  FIG. 1 . 
     Alternatively, the fluid within which the particles are suspended may be a liquid (e.g., water). In such an embodiment, the liquid may be sucked from the surface, so as to leave the particle layer at least partially dry or substantially dry on exiting the apparatus. 
     In the interest of economy, particles sucked from the interior of the chamber of a housing may be recycled to the supply and/or application device. 
     The particles may be substantially spherical in shape and may comprise a thermoplastic polymer and optionally a coloring agent (e.g., a pigment or a dye). In addition to the afore-mentioned more conventional coloring agents, the coloration of the particles may also be provided by metallic compounds or ceramic compounds being enveloped by the thermoplastic polymer. Such possible coloring agents can be made of a metal such as aluminum, copper, iron, zinc, nickel, tin, titanium, gold and silver, or of an alloy, such as steel, brass and bronze, and like compounds predominantly including metals. Additionally these coloring agents can be made of compounds providing for a similar visual effect (e.g., made of a ceramic material having a metallic appearance). Such “metal-like” materials are typically predominantly non-metallic, a metal coat optionally serving to provide the light reflectivity that may be perceived as metallic, mica compounds (typically coated with a metal oxide) can be further embedded into a thermoplastic polymer for the preparation of suitable particles. As used hereinafter, the terms coloring agent, colorant, colored, or like variations, refer to all above-described agents and thermoplastic polymers including the same, respectively. The thermoplastic particles can thus be said to be formed of, or coated with, a thermoplastic polymer. 
     Such globular (e.g., colored) polymers, may further include any agent facilitating the processing of the monolayer. For example, if the layer of particles is to be selectively exposed to laser radiation in order to render the irradiated particles sufficiently tacky to perform a subsequent step, then the polymer may further include an IR absorbing agent tuned to the wavelength of the laser, hence facilitating the softening of the particle. 
     In certain cases, the particles may have a tendency to adhere to the donor surface not only on account of the interaction between two different hydrophobic surfaces but also as a result of a charge based interaction. Optionally in such embodiments, subjecting the donor surface for a conditioning treatment, such as exposure to a corona discharge, or any other method otherwise affecting a charge thereto, or application of a chemical treatment solution enhances the affinity between the particles and the donor surface. 
     In order to achieve an even surface, if so desired, it is possible for spray head(s) to spray particles onto the carrier member with sufficient force to cause the applied particle coating to be burnished. Alternatively, the particles may be applied by an intermediate applicator, following which they may optionally be further burnished by an optional burnishing device. 
     The application device may, in some embodiments, be formed by one or more spray heads aimed directly at the surface to be coated. In such embodiments, the force of the spray will cause a layer of particles to contact the surface, the particles&#39; adhesion thereto being furthered by their respective properties and interactions therebetween. The force of the spray will subsequently dislodge and/or entrain any further particles and prevent them from adhering to the coating layer in direct contact with the surface. This is as a result of the fact that the particles adhere more strongly to the donor surface than they do to one another. The force of the spray may also act to flatten the first layer of the particles against the surface, effectively burnishing the particles, at least partially. 
     The coating apparatus may further include a dryer to dry the particles coating on leaving the coating apparatus. Such a dryer may be a radiant heater, a hot air or gas blower or a roller that acts as a mop to absorb surplus liquid. Such a roller may, for example, comprise a sponge, made of a closed-cell or open-cell foam, which is squeezed by a pressure bar or roller as it rotates to extract from it liquid mopped from the donor surface. 
     When the donor surface is used in a printing system in which particles on its surface are rendered tacky by exposure to radiation, it is desirable to control the temperature of the particles at different phases of the operating cycle. For example, it is desirable for the particles to be at a temperature near their softening point on leaving the coating apparatus to reduce the amount of radiant energy required to render the particles tacky. This may be achieved by controlling the temperature of the fluid in which the particles are suspended and/or by the provision of a heating device such as, but not limited to, a hot air blower, at the exit end of the coating apparatus. Such a heating device may additionally serve as a drier for drying the particle coating. If the donor surface is that of a drum or a continuous belt, the heating device may be located either within or outside its perimeter. Thus, by way of example for a donor surface in the form of a drum or an endless belt, a heating element may be disposed to heat the drum or belt from the side opposite the donor surface, i.e. within the perimeter bounded by such donor surfaces. Additionally and alternatively, the heating element may be “facing” the donor surface, i.e. disposed to heat the donor surface from outside the perimeter of the drum or belt per se. 
     The application device may further include temperature controlling elements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Some embodiments of the coating apparatus are described herein with reference to the accompanying drawings. The description, together with the figures, makes apparent to a person having ordinary skill in the art how the teachings of the disclosure may be practiced, by way of non-limiting examples. The figures are for the purpose of illustrative discussion and no attempt is made to show structural details of an embodiment in more detail than is necessary for a fundamental and enabling understanding of the disclosure. For the sake of clarity and simplicity, some objects depicted in the figures are not to scale. 
       Embodiments of the disclosure will now be described, by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  depicts schematically an embodiment of a printing system incorporating a coating apparatus of the present disclosure; 
         FIG. 2  is a view similar to that of  FIG. 1  showing an embodiment having an alternative application device; 
         FIG. 3  shows an embodiment in which particle application, cleaning and drying are carried out in three separate housings; 
         FIG. 4  is a view similar to that of  FIGS. 1 and 2  but showing an alternative embodiment of the coating apparatus; 
         FIG. 5  is a schematic section through a donor surface coated with platelet-like particles; 
         FIG. 6  is a schematic illustration of a cross section view of an embodiment of an applicator; and 
         FIG. 7  depicts schematically a process diagram embodying both common, as well as optional, steps for performing the methods according to various embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     The method and the coating device will be described herein mainly by reference to its application in digital printing systems however its use is not limited to this application, and different aspects of the invention may be implemented to provide formation of monolayers of particles for any desired purpose. 
     Overall Description of an Exemplary Printing System 
     The particle coating apparatus according to the present disclosure may be used in numerous industrial applications wherein a monolayer of particles is desired for the sought method, use or product. In the following, the particle coating apparatus is described in the context of a printing system, but this needs not be construed as limiting.  FIG. 1  shows a drum  10  having an outer surface  12  that serves as a particle carrier member, also termed hereinafter donor surface  12 . As the drum rotates clockwise, as represented by an arrow, it passes beneath a coating apparatus  14  where it acquires a monolayer coating of fine particles. After exiting the coating apparatus  14 , the donor surface  12  passes beneath an imaging station  16  where, in one embodiment, selected regions of the donor surface  12  are exposed to laser radiation, which renders the particle coating on the selected regions of the surface  12  tacky (as defined below). Next, the surface passes through an impression station  18  where a printing substrate  20  is compressed between the drum  10  and an impression cylinder  22 . The pressure applied at the impression station  18  causes the selected regions of the particle coating on the donor surface  12  that have been rendered tacky by exposure to laser radiation in the imaging station  16  to transfer from the donor surface  12  to the substrate  20 , or from at least a portion of the donor surface being in such a contact with at least a corresponding portion of the substrate. The regions on the donor surface corresponding to the selected tacky areas transferred to the substrate  20  consequently become exposed, being depleted by the transfer of particles. The donor surface  12  can then complete its cycle by returning to the coating apparatus  14  where a fresh monolayer particle coating is applied only to the exposed regions from which the previously applied particles were transferred to the substrate  20  in the impression station  18 . 
     The terms “tacky” and “sufficiently tacky” as used herein are not intended to mean that the particle coating is necessarily tacky to the touch but only that it is softened sufficiently to be able to adhere to the surface of a substrate when pressed against it in the impression station  18 . Similarly, when used in connection with the substrate, the term more broadly relates to the higher affinity of any “tacky” region of the substrate towards the particles, than the bare substrate, said affinity being higher than the affinity of the particles towards the donor surface and sufficient to allow the particles to transfer from the donor surface to such regions during impression. 
     In the printing system shown in  FIG. 1 , heat generated by exposure to laser radiation is relied upon to select the regions of the particle coating that are to be transferred to the substrate. When incorporated in a system, the location of the coating apparatus may also be referred to as a coating station  14 . Laser radiation is given only as an example. Instead of relying on heat, the transfer of the coating to the substrate may take place at the impression station owing to the selective application of pressure, as in foil blocking. Thus, the impression cylinder  22  may have an embossed surface or it may carry a stamp or a die. 
     As a still further alternative, the substrate  20  may, as will be described below by reference to  FIG. 2 , have selected regions pre-coated with an adhesive so that particles are only transferred at the impression station to regions of the substrate having an active adhesive coating. The adhesive pre-coating may take place in-line (e.g., an adhesive is selectively deposited in the desired image pattern upstream of the impression station, for instance by printing plates, silk screens or ink jetting) or off-line (e.g., the substrate is fed to the printing system already pre-coated by any of the previously mentioned exemplary methods). The coating apparatus may even be used in a system where transfer takes place over the entire surface of the substrate  20  not just selected regions, in which case pressure may be relied upon to effect the transfer and no embossing need be present on the surface of the impression cylinders  22  or  32 , nor any particular pattern on blanket cylinder  30 . 
       FIG. 7  depicts schematically a process diagram embodying both common, as well as optional, steps for performing the methods according to various embodiments of the invention. Dashed lines and frames are utilized to indicate optional steps. 
     The standard process begins  700  when particles from the particle supply  701  are suspended in a fluid and are applied to the donor surface  710 . The fluid is caused to flow over the donor surface at the same time as the time the particles are applied. 
     As described, after the particles contact the donor surface, the respective surface energies of the particles and the donor surface cause the particles nearest the donor surface to adhere thereto rather than to one another. Excess particles which are not in direct contact with the donor surface are carried away by the fluid  720  and the fluid and particles entrained therein, also referred as surplus, are then extracted  730 . Optionally at that point, as indicated by path  734 , the particles can be recycled back to the particle supply  701 . 
     Optionally, prior to removal of the excess particles from the donor surface at step  720 , the coating formed by the particles on the surface can be homogenized  765 . By way of example, the thickness of the layer of particles may be rendered more even, the orientation of the particles or their distribution on the surface may be rendered more uniform. 
     Optionally, after the excess particles are removed and the donor surface carries thereupon a layer of particles that is substantially a single particle deep, selected regions of the layer may be transferred  745  to a substrate. After such transfer, or any other step depleting the donor surface from particles coated thereon, the donor surface may return, as indicated by path  747 , for another cycle through the process, wherein a fresh coating of particles would be applied at least to the regions depleted of particles in a prior cycle. A coating method including this recycling of the donor surface can be embodied when the donor surface is an endless donor surface cyclically movable between a station at which the particles are applied thereon and a station at which at least selected regions can be transferred to a substrate. 
     Further optionally, one or more steps may be exercised, such as, by way of example, if the fluid is a liquid, it may be dried  732  from the particle layer on the donor surface, the donor surface or portions thereof may optionally be heated  735 , and in certain optional embodiment the donor surface may be cooled  755 , prior to particles being applied thereto  710  and/or after the selected regions have been transferred at step  745 . 
     The Coating Apparatus 
     The coating apparatus  14  in the embodiment of  FIG. 1  comprises a plurality of spray heads  1401  that are aligned with each other along the axis of the drum  10  and only one is therefore seen in the section of the drawing. The sprays  1402  of the spray heads  1401  are confined within a bell housing  1403 , of which the lower rim  1404  is shaped to conform closely to the donor surface  12  leaving only a narrow gap between the bell housing  1403  and the drum  10 . The spray heads  1401  are connected to a common supply rail  1405  which supplies to the spray heads  1401  a pressurized carrier fluid (gaseous or liquid) having suspended within it the fine particles to be used in coating the donor surface  12 . If needed the suspended particles may be regularly or constantly mixed, in particular before their supply to the spray head(s)  1401 . The particles may be circulated in the coating apparatus  14  at any suitable flow rate, generally not exceeding 50 liter/min, and by way of example within a flow rate range of 0.1 to 10 liter/minute, or in the range of 0.3 to 3 liter/min. The surplus fluid and particles from the sprays heads  1401 , which are confined within a plenum  1406  formed by the inner space of the housing  1403 , are extracted through an outlet pipe  1407 , which is connected to a suitable suction source represented by an arrow, and can be recycled back to the spray heads  1401 , as presented schematically by an optional recycling system  1415 . Though herein referred to as spray heads, any other type of supply system outlet (e.g., nozzle or orifice) along the common supply pipe or conduit of the supply system allowing applying the fluid suspended particles, are encompassed. 
     It is important to be able to achieve an effective seal between the housing  1403  and the donor surface  12 , in order to prevent the particle carrying fluid and the fine particles from escaping through the narrow gap that remains between the housing  1403  and the donor surface  12  of the drum  10 . Different ways of achieving such a seal are shown schematically in the drawing. 
     The simplest form of seal is a wiper blade  1408 . Such a seal makes physical contact with the donor surface and could score the applied coating if used on the exit side of the housing  1403 , that is to say on the side downstream of the spray heads  1401 . For this reason, if such a seal is used, it is preferred that it would be located only upstream of the spray heads  1401  and/or at the axial ends of the housing  1403 . The terms “upstream” and “downstream” as used herein are referenced to points on the donor surface  12  as it passes or cycles through the coating apparatus. 
       FIG. 1  also shows how egress of the fluid within which the particles are suspended from the sealing gap between the housing  1403  and the drum  10  can be prevented without a member contacting the donor surface  12 . A gallery  1409  extending in the present illustration around the entire circumference of the housing  1403  is connected by a set of fine passages  1410  extending around the entire rim of the housing  1403  to establish fluid communication between the gallery  1409  and the sealing gap. 
     In a first embodiment, the gallery  1409  is connected to a suction source of a surplus extraction system, which may be the same suction source as is connected to the outlet  1407  or a different one. In this case, the gallery  1409  serves to extract fluid passing through the gap before it exits the housing  1403 . The low pressure also sucks off the drum  10  any particles that are not in direct contact with the donor surface  12  and, if the sprayed fluid is a liquid, it also sucks off surplus liquid to at least partially dry the coating before it leaves the coating apparatus  14 . 
     Surplus liquid can alternatively and additionally be removed by a liquid extracting roller positioned on the exit side of the coating apparatus. Such a roller is shown in the embodiment of  FIG. 4  in which it is designated  1440 . The outer surface  1442  of the roller  1440  has sponge-like liquid absorbing properties (e.g., closed-cell foam), and can be independently driven to rotate at a speed and/or in a direction differing from the speed and direction of drum  10 . The liquid extracting roller can contact the particles coated on the donor surface  12  and extract surplus liquid by drawing it within its fluid absorbing outer surface  1442 , advantageously sufficiently smooth and even, so as not to affect the layer of particles retained on the donor surface prior to their selective transfer to the substrate  20 . As the extracting roller  1440  continues to rotate following the absorption of the surplus liquid, it approaches a wiper  1444 , or any other suitable structure, positioned so as to squeeze the roller or otherwise release the extracted liquid out of its absorbing surface. A suction outlet  1446  can be positioned adjacent to such wiper or replace the wiper, so as to permit the immediate removal of the liquid so extracted from the particle coated donor surface and so forced out of the roller outer surface. Following such elimination of the removed liquid, the roller  1440  can complete its cycle, contacting again the donor surface and further extracting surplus liquid. Though illustrated in  FIG. 4  as being internal to a coating station  14 , a liquid extracting roller  1440 , if present, can alternatively be positioned downstream of the coating station, as long as it remains upstream of a station where the particle coating needs to be substantially dry. The liquid extracting element, if present, is advantageously compatible with the particle layer, and for instance does not negatively affect the particles and/or the integrity of the layer formed therefrom. 
     The printing system may further or alternatively comprise a dryer (e.g., hot or cold air blower) on the exit side of the coating apparatus  14 , or further downstream, so as to allow the particle coat to reach a subsequent station in substantially dry form. The drying element, if present, is advantageously compatible with the particle layer, and for instance does not negatively affect the particles and/or the integrity of the layer formed therefrom. 
     In an alternative embodiment, the gallery  1409  is connected to a source of gas at a pressure higher than the pressure in the plenum  1406 . Depending on the rate of fluid supply to the plenum through the spray heads  1401 , or other particle supply method, and the rate of extraction through the outlet  1407 , the plenum  1406  may be at a pressure either above or below the ambient atmospheric pressure. 
     If the plenum is maintained at sub-atmospheric pressure, then it suffices for the gallery  1409  to be at ambient atmospheric pressure, or the gallery may be omitted altogether. In this case, because the pressure within the sealing gap will exceed the pressure in the plenum  1406 , gas flow through the gap will be towards the interior of the housing with no risk of fluid egress. 
     If the plenum is at above ambient pressure, then the gallery  1409  may be connected to a gas supply, preferably air, that is pressurized at higher pressure than the plenum pressure. In this case, air will be forced into the sealing gap under pressure through the passages  1410  and will split into two streams. One stream will flow towards the plenum  1406  and will prevent egress of the fluid within which the particles are suspended. That stream will also dislodge and/or entrain particles not in direct contact with the donor surface and assist in at least partially drying the coating if the carrier fluid is a liquid. The second stream will escape from the coating apparatus without presenting a problem as it is only clean air without any suspended particles. The second gas stream may also assist in further drying of the particle coating on the donor surface  12  before it leaves the coating apparatus  14 . If desired, the gas stream can be heated to facilitate such drying, and/or to raise the temperature of the particle layer and the donor surface before it reaches a subsequent station (e.g., an imaging station  16 ). 
     In an alternative embodiment, the afore-mentioned gallery  1409  does not extend around the entire circumference of the housing, so as to seal the plenum  1406  on all sides. It can be a “partial” gallery or a combination of one or more air knives (with negative or positive flow) positioned either downstream or upstream of the spray head(s) and/or intermediate applicator(s) in parallel to the axis of the drum and/or on the lateral edges of the spray heads in a direction perpendicular to the axis of the drum. A “partial” gallery on the exit side may, in some embodiments, serve as gas blower (e.g., cold or hot air) additionally or alternatively facilitating the drying of the particles, in which case the passages  1410  may be dimensioned to provide sufficient flow rate. 
     Independently of the type of fluid carrying the suspended particles being applied to the donor surface, the coating apparatus  14  may include at its exit side, as shown in  FIG. 1 , and typically at an external downstream location, a heater  1424  allowing the temperature of the particle layer and the donor surface to be raised before it reaches a subsequent station (e.g., an imaging station  16 ). In the non-limiting example of a digital printing system wherein the particle coating can be selectively transferred to a substrate by targeted radiation, the temperature of the particles and the donor surface may in this way be raised from ambient temperature (e.g., above circa 23° C.), to above 30° C., or above 40° C. or even above 50° C., so as to reduce the amount of laser energy that is needed to render the particles tacky. However, the heating should not itself render the particles tacky and in most embodiments should not raise their temperature to above 80° C. or possibly to above 70° C. Such heating of the particles and the donor surface may be further facilitated by using a fluid carrier at a desired temperature. 
     Also as shown in  FIG. 1 , in some embodiments, there can be included on the entry side of the coating apparatus  14 , and typically at an external upstream location, a cooler  1422  allowing lowering the temperature of the donor surface before the particle layer is being replenished in the previously exposed regions. Taking again the example of the coating apparatus being integrated and used in the afore-mentioned printing system, it is believed that a donor surface at a temperature of less than 40° C., or less than 30° C., or even less than 20° C., but typically above 0° C., or even above 10° C., can reduce the temperature of the particles neighboring the exposed regions so that by the time the surface is being replenished, the so cooled particles may have no or reduced “residual tackiness”, that is to say a partial softening insufficient for a subsequent step (e.g., transfer to a printing substrate). The cooled coating of particles behaves in the same manner as the particles freshly deposited on the exposed regions of the donor surface. In this manner, only particles selectively targeted by a laser element would become sufficiently tacky for a subsequent transfer step. Such cooling of the particles and the donor surface may be further facilitated by using a fluid carrier at desired temperature. The particle characteristics and other considerations, such as ambient temperature, humidity, and the like, may require adjustments to the above-described temperatures. 
     In some embodiments, there can be included both a cooler  1422  on the entry side of the coating apparatus  14  and a heater  1424  on the exit side, each cooler and heater operating as above described. Additionally, the drum  10  can be temperature controlled by suitable coolers and/or heaters internal to the drum, such temperature controlling elements being operated, if present, in a manner to allow the outer surface of the donor surface to be maintained at any desired temperature. 
     In the embodiment illustrated in  FIG. 2 , instead of being carried in a fluid sprayed directly onto the donor surface  12 , the suspended particles are applied by spray heads  1401  to an intermediate applicator  1420 . The applicator  1420  may be for example a sponge-like roller, of which the axis is parallel to the axis of drum  10 . The fluid and suspended particles may be sprayed onto the applicator  1420  in the manner shown in  FIG. 2 , or if the applicator is porous, or constructed in manner similar to the “brushes” used in automatic car washes that have loose fabric strips extending radially from a central axle, then the fluid may be introduced via the axle hub and allowed to escape through holes in the axle (not shown). The material of the roller or the fabric strip is to be “relatively soft”, selected so as to wipe the particles on the donor surface  12 , without affecting the integrity of the coat thereupon formed, in other words without scratching the layer of particles. The surface of the applicator, or of its bristles or strips, may suitably comprise a closed-cell foam (such as such as closed-cell polyethylene, closed-cell PVA or closed-cell silicone); or a relatively soft open-cell foam (such as a polyurethane foam); or a fabric, such as cotton, silk or ultra high molecular weight polyethylene (UHMWPE) fabric. A schematic cross section through a roller applicator having a plurality of flexible strips  1425  extending radially from a rotatable axle  1427  is depicted in  FIG. 6 . 
     As the roller or brush  1420  rotates along its axis, it applies the particles upon contact with donor surface  12  of drum  10 . The outer surface of the applicator  1420  need not have the same linear velocity as the donor surface and it can, by way of example, be up to about ten-fold higher. It may rotate in the same direction as drum  10  or in counter-direction. The applicator may be independently driven by a motor (not shown in  FIG. 2 ) or driven by drum  10  by gears, belts, friction, and the like. 
       FIG. 4  shows an embodiment in which the particle coating apparatus  14  comprises more than one applicator  1420  of particles.  FIG. 4  shows six such applicators  1420   a  to  1420   f  but there may be fewer or more. In  FIG. 4 , each of the applicators  1420   a  to  1420   f  has its own supply of particles as applied by sprays provided by spray heads  1401 , the relevant fluid being delivered by a supply conduct. It is alternatively possible to have two or more applicators having a common supply system or set of supply system outlets. Such applicator(s) may optionally provide some burnishing or flattening of the particles on the donor surface, or such function, if desired, can be provided by a separate element, as described below. 
     As schematically illustrated in  FIGS. 3 and 4 , the coating apparatus can further comprise a cleaning roller  1430 . A cleaning roller can be similar in structure to an applicator roller  1420 , except that it would lack the supply of particles. The cleaning roller applies a liquid supplied by a spray head  1431  that can correspond to the fluid carrier of the particles, but devoid of particles, or to any other suitable fluid. 
     As shown in  FIG. 4 , the compartment of the housing of the coating apparatus  14  containing the cleaning roller  1430  is separated from the remainder of the housing by an air knife  1433  so that the fluid present in the cleaning compartment and containing no particles does not mix with the fluid in the remainder of the housing. Separate extraction points  1446  are provided so that the two fluids can also be separately processed and returned to their respective spray heads. A second air knife may be provided at the exit ends of the cleaning compartment. 
     As an alternative to incorporating a cleaning roller within the coating apparatus  14 , it is possible, as shown in  FIG. 3 , for it to be positioned externally to the housing of the particles applicator(s), optionally in a separate housing with a distinct fluid supply and system for elimination and/or recirculation. In  FIG. 3 , which is described in greater detail below, three separate housings  210 ,  212  and  214  contain a coating station, a cleaning station and a drying station, respectively. 
     A cleaning device, if present, can be continuously operated. For instance, a cleaning roller as above-exemplified may serve to remove particles not in direct contact with the donor surface during any cycle of the surface in the coating station during operation of the system in which an apparatus as herein disclosed can be integrated. Additionally, and alternatively, a cleaning device can be used periodically. Such a cleaning device may for instance be used for maintenance and can serve to remove all particles from the entire donor surface. Such complete regeneration of the donor surface to be free of particles can be done intermittently or periodically, for example in the context of a printing system at the end of a print job, or when changing the particles to be printed (e.g., to a new batch or to a new type), or once a day, or once a week, or any other desired frequency. Periodical cleaning devices, which may rely on chemical or physical treatment of the donor surface achieving full particle removal, can be located externally to the coating station. They can be operated for at least one cycle of the donor surface. For this reason, the embodiment of  FIG. 4  has separate motors  1450  and  1452  of driving the cleaning rollers(s) and the applicator roller(s), respectively. 
     The Particles 
     The particles may be made of any thermoplastic material and have any shapes and/or dimensions suitable to provide for sufficient contact area with the donor surface, at least over a time period the particle coating is desired. 
     The shape and composition of the coating particle will depend in practice on the intended use of the layer of particles, and in the context of a non-limiting example of a printing system, on the nature of the effect to be applied to the surface of the substrate  20 . In a printing system, the particles may conveniently be formed of a pigmented polymer. For printing of high quality, it is desirable for the particles to be as fine as possible to minimize the interstices between particles of the applied monolayer coating. The particle size is dependent upon the desired image resolution and for some applications a particle size (e.g., a diameter or maximum long dimension) of 10 μm (micrometers) or possibly even more (i.e. having a larger size) may prove adequate. Considering for example globular pigmented polymers, an average diameter between 100 nm and 4 μm, or even between 500 nm and 1.5 μm can be satisfactory. For irregular platelets, the longest dimension may even reach 100 μm on average. However, for improved image quality, it is preferred for the particle size to be a small fraction or a fraction of a micrometer and more preferably a few tens or hundreds of nanometers. Commercially available flakes may have a thickness of about 60-900 nm and a representative planar dimension (e.g., mean diameter for near round flakes or average “equivalent diameter” for platelets having less regular plane projection, also characterized by shortest/longest dimensions) of about 1-5 μm, but flakes can also be prepared with a thickness of as little as 15 nm, 20 nm, 25 nm, 30 nm, 40 nm, or 50 nm and a mean or equivalent diameter in the region of 100-1000 nm or 500-800 nm. 
     Thus, particle selection and ideal size determination, will depend upon the intended use of the particles, the effect sought (e.g., visual effect in the case of printing; conductive effect in the case of electronics, etc.), and the operating conditions of the relevant system in which a coating apparatus according to the present teachings is to be integrated. Optimization of the parameters, may be done empirically, by routine experimentation, by one of ordinary skill in the art. 
     Depending on their shape, which can be relatively regular or irregular, the particles may be characterized by their length, width, thickness, mean or equivalent diameter or any such representative measurement of their X-, Y- and Z-dimensions. Generally, the dimensions of the particles are assessed on planar projections of their shape (e.g., vertical and/or horizontal projections). Typically, such sizes are provided as average of the population of particles and can be determined by any technique known in the art, such as microscopy and Dynamic Light Scattering (DLS). In DLS techniques the particles are approximated to spheres of equivalent behavior and the size can be provided in terms of hydrodynamic diameter. DLS also allows assessing the size distribution of a population. As used herein, particles having a size of, for instance, 10 μm or less, have at least one dimension smaller than 10 μm, and possibly two or even three dimensions, depending on shape. The particles are said to fulfill on average any desired size preference, if the D 50  (50% of the population, e.g., by number or volume of particles) is about the intended size; whereas a population of particles wherein the D 90  (e.g., D N   90 , D V   90 ) is about the intended size implies a vast majority of particles (90% of the population) satisfy the same. 
     The particles may have any suitable aspect ratio, i.e., a dimensionless ratio between the smallest dimension of the particle and the equivalent diameter in the largest plane orthogonal to the smallest dimension. The equivalent diameter can be for instance the arithmetical average between the longest and shortest dimensions of that largest orthogonal plane. Such dimensions are generally provided by the suppliers of such particles and can be assessed on a number of representative particles by methods known in the art, such as microscopy, including in particular by scanning electron microscope SEM (preferably for the planar dimensions) and by focused ion beam FIB (preferably for the thickness and length (long) dimensions). Such characteristic dimensions can be quantitatively determined for each individual particle or for a group of particles, for instance the entire field of view of an image captured at relevant magnification. 
     Particles having an almost spherical shape are characterized by an individual aspect ratio (or an average aspect ratio if considering a population of particles) of approximately 1:1 and typically no more than 2:1. Depending on the technique used for the determination of the characteristic dimensions of a particle, the average for a group of particles may be volume-averaged, surface-area averaged, or number averaged. 
     For simplicity, individual and average aspect ratio are hereinafter referred to as “aspect ratio” the population size being clear from context. While ball-like particles have an aspect ratio of about 1:1, flake-like particles can have an aspect ratio of 100:1 or more. Though not limiting, the particles suitable for a coating apparatus according to the present teachings can have an aspect ratio of about 100:1 or less, of about 75:1 or less, of about 50:1 or less, of about 25:1 or less, of about 10:1 or less, or even of about 2:1 or less. In some embodiments, the particles suitable for the present teachings may have an aspect ratio of at least 2:1, at least 3:1, at least 5:1, at least 10:1, at least 25:1, at least 40:1, or at least 70:1. 
     Though not essential, the particles may preferably be uniformly shaped and/or within a symmetrical distribution relative to a median value of the population and/or within a relatively narrow size distribution. 
     A particle size distribution is said to be relatively narrow if at least one of the two following conditions applies:
         A) the difference between the hydrodynamic diameter of 90% of the particles and the hydrodynamic diameter of 10% of the particles is equal to or less than 150 nm, or equal to or less than 100 nm, or equal to or less than 50 nm, which can be mathematically expressed by: (D 90 -D 10 )≤150 nm and so on; and/or   B) the ratio between a) the difference between the hydrodynamic diameter of 90% of the particles and the hydrodynamic diameter of 10% of the particles; and b) the hydrodynamic diameter of 50% of the particles; is no more than 2.0, or no more than 1.5, or no more than 1.0, which can be mathematically expressed by: (D 90 -D 10 )/D 50 ≤2.0 and so on.       

     D 10 , D 50  and D 90  can be assessed by number of particles in the population, in which case they may be provided as D N   10 , D N   50  and D N   90 , or by volume of particles, in which case they may be provided as D N   10 , D V   50  and D V   90 . 
     As mentioned, such relatively uniform distribution may not be necessary for certain applications. For instance, having a relatively heterogeneously sized population of particles may allow relatively smaller particles to reside in interstices formed by relatively larger particles. 
     Depending on their composition and/or on the processes they undergo (e.g., milling, recycling, burnishing, etc.), the particles can be hydrophobic with different degrees, if any, of hydrophilicity. As the balance between the hydrophobic and hydrophilic nature of the particles may shift with time, the coating process is expected to remain efficient if the hydrophobic nature of the particles predominates. Additionally, the particles may be made of materials intrinsically hydrophilic, in which case they can be rendered hydrophobic by application of a particle coating. Materials suitable for such a particle coating can have a hydrophilic end with affinity to the particle and a hydrophobic tail. In the present disclosure such particles, whether intrinsically hydrophobic or coated to become hydrophobic or more hydrophobic, are said to be substantially hydrophobic. 
     The particles can be carried by either a gaseous or a liquid fluid when they are sprayed onto the donor surface or upon the intermediate applicator(s). When the particles are suspended in a liquid, in order both to reduce cost and minimize environmental pollution, it is desirable for the liquid to be aqueous. In such a case, it is desirable for the polymer or material used to form or coat the particles to be hydrophobic. Hydrophobic particles more readily separate from an aqueous carrier, facilitating their tendency to attach to and coat the donor surface. Such preferential affinity of the particles towards the surface of the coating device, rather than towards their fluid carrier and towards one another, is deemed particularly advantageous. Blowing a gas stream over the particle coating (which as mentioned can preferably be formed by hydrophobic particles on a hydrophobic donor surface) will both serve to dislodge and/or entrain particles not in direct contact with the donor surface and to at least partially dry the particle coating on the donor surface. 
     While in the above coating step, the preferential affinity of the particles is to the donor surface, the particles need to be compatible with their subsequent transfer. Taking for instance a printing application in which the particles would be transferred from the donor surface to a printing substrate, then the relative affinity of the particles at an impression station would “shift” from the donor surface to the substrate. This can be viewed as a “gradient of affinities”, the particles having greater affinity to the donor surface than to one another, and the substrate having greater affinity to the particles, than the particles to the donor surface. Such gradient can be obtained as above-exemplified through hydrophobic properties of all interfaces involved, but can also be facilitated or further tailored by reliance on additional types of interactions. For instance, the particles, the donor surface, and the surface of relevance to any subsequent step, can each have a gradient of charges, instead or in addition to a gradient of hydrophobicity. 
     If desired, it is possible to burnish or polish the particle coating while it is still on the donor surface  12 . Thus, a burnishing roller or other wiping element may be positioned immediately downstream or as part of the coating apparatus  14 . 
     Burnishing may be carried out with a dry roller or with a wet roller (e.g., impregnated and/or washed with the fluid on which the particles are suspended, for instance water). In the event an intermediate applicator is used, it may, in addition to applying the particles to the surface, also act to burnish them partly. 
     It is believed that during burnishing the size of the particles is reduced as compared to their original size upon initial injection into the coating apparatus and application upon the donor surface, and that, alternatively and additionally, the burnished particles are oriented in a substantially parallel manner with respect to the donor surface of the drum and/or more evenly distributed on the surface. 
     A layer of particles  512  that may be obtained by the coating apparatus described hereinabove, is schematically illustrated in the cross-section along the x-y plane presented in  FIG. 5 . While particles  502 , having an outer surface  504 , are illustrated as having an elongated cross-sectional shape (e.g., corresponding to a platelet like particle), this should not be construed as limiting. Particles  502  are positioned on top of a donor surface  12 , itself forming the outer surface of drum  10  or of any other physical support allowing for the relative displacement of the donor surface  12  with respect to the coating apparatus  14 . As previously explained, the surfaces  504  of particles  502  can be hydrophobic. In  FIG. 5 , several particles are shown to be partially overlapping, see section A, such overlap yielding an overall particle layer thickness denoted as T. In section B, the particles are illustrated as being contiguous, whereas section C points to a gap between neighboring particles. In section D, a particle  506  is shown as having no contact with the donor surface, as appearing in the present x-y-cross section. However, such an overlapping particle may be positioned over the particles contacting the underneath layer such that it could conceivably contact the donor surface at another point (not shown) along the z-direction. In section E, a particle  508  is shown as being overlapped by more than one adjacent particle. 
     Alternative Configuration of Coating Stations 
       FIG. 3  shows very schematically an embodiment that has three stations  210 ,  212  and  214  spaced circumferentially around the drum  10 . Each of the stations  210  and  212  is constructed in substantially the same way as the coating apparatus  14  in  FIG. 2 . The station  210  applies particles to the surface of the drum  10  and can be referred to as an application station. The station  212  is a first surplus extraction station able to remove particles applied in excess at station  210 , namely particles which are not in direct contact with the donor surface and thus not strongly bond to it. At this station the liquid applied to the applicator does not have any particles suspended in it and the applicator, used mainly to remove loosely bond excess particles, may also serve, if desired, to at least partially burnish the particles applied in the station  210 . For simplicity, and to differentiate from subsequent station, station  212  may be referred to as the cleaning or burnishing station, even though this particular use can be optional. Lastly, the station  214  forms a second part of the surplus extraction system and acts to at least partially dry the surface of the drum  10  and to remove from it any residual excess particles that were not eliminated at station  212 . As mentioned, though the direction of the arrow illustrates removal by negative suction, a similar confinement of any surplus due to remain in the apparatus can be achieved by the supply of a positive air flow at the exit side (e.g., air knife). 
     Though each of the afore-mentioned stations is described by its predominant function in such a configuration of the coating apparatus, it is to be noted that they may fulfill additional function of the coating apparatus. For instance, though station  214  predominantly acts as part of the surplus extraction system, other stations  210  and  212  may also be capable of at least partially extracting surplus fluid and/or particles. 
     Though in the previously described sub-station configuration of the coating apparatus, each type of station is mentioned once, this need not necessarily be the case. For instance, there can be two burnishing stations, were such function be desirable for the intended particles and use of the coating apparatus. 
     Burnishing is of particular advantage when operating the spray head(s) of the coating apparatus at relative low pressure and/or when including an intermediate applicator. Though shown as forming part of a separate station in  FIG. 3 , a burnishing roller (not shown) may be incorporated into the housing of the coating apparatus as illustrated in  FIGS. 1 and 2 . Burnishing of the monolayer of particles is advantageously carried out, when desired, before the coating reaches the impression station, but this need not necessarily be the case as some printing systems may benefit from burnishing of the particles following their transfer to the substrate. Burnishing may be carried out with a dry roller or with a wet roller (e.g., impregnated and/or washed with the fluid in which the particles are suspended, for instance water). In the event an intermediate applicator is being used, it may itself in addition to applying the particles to the surface also act to partly burnish them. 
     The outer surface of the optional burnishing roller may rotate at a linear speed different than that of the donor surface of the drum and/or of the outer surface of an intermediate applicator, if present. It can rotate in the same or counter-direction relative to the drum. 
     The Particle Carrier 
     The particle carrier, that is to say the fluid within which the particles are suspended, may be either a liquid or a gas. If liquid, the carrier is preferably water based and if gaseous the carrier is preferably air. The particles may be lyophobic (i.e., having no affinity) with respect to their carrier, for instance may be hydrophobic, while the carrier is an aqueous liquid. Such may result in particles being partly dispersed in the liquid, and partly phase separated (all types of such mixtures of materials of same or different phases being herein encompassed by the term “suspended”). In addition to the particles, the carrier may comprise any additive known in the art of particle formulation, such as dispersants, surfactants, water-miscible solvents, co-solvents, stabilizers, preservatives, viscosity modifiers, pH modifiers, and the like. All such additives and their typical concentrations are known to persons skilled in the art of dispersions and need not be further detailed herein. Additives (or mixtures thereof) not affecting the hydrophobicity of the particles and of the donor surface are preferred. Such agents, in particular the dispersing agents, may assist in maintaining or increasing the stability of the suspended particles in the liquid (including in phase separated form, if desired). The liquid carrier may also comprise excess of unbound material serving as particle coat, if desired when applicable. Any such additive and mix thereof, preferably should not affect the overall inertness of the liquid carrier towards the donor surface (e.g., avoiding or reducing any deleterious swelling of the surface that would prevent proper coating by attachment of the particles). 
     A liquid carrier is said to be aqueous if it contains at least 80 wt. % water (i.e., 80% by weight of the total composition), or at least 85 wt. %, or at least 90 wt. %, or at least even 95 wt. % water. It is to be understood that though final work aqueous compositions comprising the particles may predominantly contain water, as previously mentioned, it is possible to prepare intermediate aqueous compositions containing a higher amount of solid particles (and additives if any) and lower amount of water. Such intermediate compositions may serve as concentrates, which can be diluted to desired working concentrations when needed, but stored and/or shipped in smaller volumes. A concentrate may for instance comprise as much as about 80 wt. % of solids and about 20 wt. % of a water miscible co-solvent, the water being added during dilution of the concentrate. 
     A liquid carrier does not wet a donor surface if the wetting angle it may form on such surface exceeds 90°, as further explained hereinbelow. 
     The Donor Surface 
     The donor surface  12  in some embodiments is a hydrophobic surface, made typically of an elastomer that can be tailored to have properties as herein disclosed, generally prepared from a silicone-based material. Poly(dimethylsiloxane) polymers, which are silicone-based, have been found suitable. In one embodiment, a fluid curable composition was formulated by combining three silicone-based polymers: a vinyl-terminated polydimethyl-siloxane 5000 cSt (DMS V35, Gelest®, CAS No. 68083-19-2) in an amount of about 44.8% by weight of the total composition (wt. %), a vinyl functional polydimethyl siloxane containing both terminal and pendant vinyl groups (Polymer XP RV 5000, Evonik® Hanse, CAS No. 68083-18-1) in an amount of about 19.2 wt. %, and a branched structure vinyl functional polydimethyl-siloxane (VQM Resin-146, Gelest®, CAS No. 68584-83-8) in an amount of about 25.6 wt. %. To the mixture of the vinyl functional polydimethyl siloxanes were added: a platinum catalyst, such as a platinum divinyltetramethyldisiloxane complex (SIP 6831.2, Gelest®, CAS No. 68478-92-2) in an amount of about 0.1 wt. %, an inhibitor to better control curing conditions, Inhibitor 600 of Evonik® Hanse, in an amount of about 2.6 wt. %, and finally a reactive cross-linker, such as a methyl-hydrosiloxane-dimethylsiloxane copolymer (HMS 301, Gelest®, CAS No. 68037-59-2) in an amount of about 7.7 wt. %, which initiates the addition curing. This addition curable composition was shortly thereafter applied with a smooth leveling knife upon the support of the donor surface (e.g., an epoxy sleeve mountable on drum  10 ), such support being optionally treated (e.g., by corona or with a priming substance) to further the adherence of the donor surface material to its support. The applied fluid was cured for two hours at 100-120° C. in a ventilated oven so as to form a donor surface. 
     The hydrophobicity is to enable the tacky film created by exposing the particles to radiation or the designated areas of the particles&#39; monolayer contacted with the selectively tacky adhesive bearing substrate to transfer cleanly to the substrate without splitting. 
     The donor surface is preferably hydrophobic, that is to say the wetting angle with the aqueous carrier of the particles exceeds 90°. The wetting angle is the angle formed by the meniscus at the liquid/air/solid interface and if it exceeds 90°, the water tends to bead and does not wet, and therefore adhere, to the surface. The wetting angle or equilibrium contact angle Θ 0 , which is comprised between and can be calculated from the receding (minimal) contact angle Θ R  and the advancing (maximal) contact angle Θ A , can be assessed at a given temperature and pressure of relevance to the operational conditions of the coating process. It is conventionally measured with a goniometer or a drop shape analyzer through a drop of liquid having a volume of 5 μl, where the liquid-vapor interface meets the donor surface, at ambient temperature (circa 23° C.) and pressure (circa 100 kPa). 
     Such measurements were performed with a Contact Angle analyzer—Krüss™ “Easy Drop” FM40Mk2 using distilled water as reference liquid on a sample of silicon-based donor surface prepared as above described, the sample having a size of 2 cm×2 cm. The results were analyzed using “Drop shape analysis” program, circle computer method, the advancing contact angle Θ A  of the above-described donor surface was found to be 101.7°±0.8° and the receding contact angle Θ R  was found to be 99.9°±3.1°. Typically, donor surfaces prepared by this method had contact angles in the range of about 95° to about 115°, generally not exceeding 110°, and any elastomer providing for such contact angles, hence hydrophobicity, are expected to be suitable, as long as compatible with the particles to be applied thereon. 
     This hydrophobicity may be an inherent property of the polymer forming the donor surface or may be enhanced by inclusion of hydrophobicity additives in the polymer composition. Additives that may promote the hydrophobicity of a polymeric composition may be, for example, oils (e.g., synthetic, natural, plant or mineral oils), waxes, plasticizers and silicone additives. Such hydrophobicity additives can be compatible with any polymeric material, as long as their respective chemical nature or amounts do not prevent proper formation of the donor surface, and for instance would not impair adequate curing of the polymeric material. 
     Alternatively, and optionally additionally to hydrophobic-hydrophobic interactions, the relative affinity of the particles to the donor surface can be facilitated by each having opposite charges. For instance, a silicone based elastomer can have negative charge while the particles can be positively charged. The donor surface can therefore have any charge that would be compatible with the intended particles. Advantageously any said charge, if tailored and not inherent to the materials forming the surface, is also suitable for the subsequent selective release and transfer of the particles to a substrate of relevance. As above-explained, a variety of such gradient of properties can be suitable and can be tailored by one of ordinary skill in the art of the intended use. 
     The roughness or finish of the donor surface will be replicated by the layer of particles, and can be adapted to the intended use of the coating apparatus. If for instance, the apparatus is used in a system for printing a pattern of irradiated softened particles, it is understood that the film they would form upon transfer to a substrate would have, if desired, a smoother face if the donor surface is itself smoother. 
     The donor surface  12  may have any Shore hardness suitable to provide a strong bond to the particles when they are applied using the coating apparatus  14 , the bond being stronger than the tendency of the particles to adhere to one another. The hardness of the silicone-based surface may vary and for instance depend on the thickness of the donor surface and/or the particles intended to be bond. It is believed that for relatively thin donor surfaces (e.g., 100 μm or less), the silicone-based material may have a medium to low hardness; whereas for relatively thick donor surfaces (e.g., up to about 1 mm), the silicone-based material may have a relatively high hardness. Additionally, larger particles may typically benefit from a donor surface having a lower hardness than necessary to accommodate relatively smaller particles. In some embodiments, a relatively high hardness between about 60 Shore A and about 80 Shore A is suitable for the donor surface. In other embodiments, a medium-low hardness of less than 60, 50, 40, 30, 20 or even 10 Shore A is satisfactory. In a particular embodiment, the donor surface has a hardness of about 30-40 Shore A. 
     The donor surface  12  in the drawings is the outer surface of a drum  10  which can be either directly cast thereupon or mounted as a sleeve separately manufactured. This, however, is not essential as it may alternatively be the surface of an endless transfer member having the form of a belt guided over guide rollers and maintained under an appropriate tension at least while it is passing through the coating apparatus. Additional architectures may allow the donor surface  12  and the coating station  14  to be in relative movement one with the other. For instance, the donor surface may form a movable plan which can repeatedly pass beneath a static coating station, or form a static plan, the coating station repeatedly moving from one edge of the plan to the other so as to entirely cover the donor surface with particles. Conceivably, both the donor surface and the coating station may be moving with respect to one another and with respect to a static point in space so as to reduce the time it may take to achieve entire coating of the donor surface with the particles dispensed by the coating station. All such forms of donor surfaces can be said to be movable (e.g., rotatably, cyclically, endlessly, repeatedly movable or the like) with respect to the coating station where any such donor surface can be coated with particles (or replenished with particles in exposed regions). 
     The donor surface may additionally address practical or particular considerations resulting from the specific architecture of a system in which such a coating station can be integrated. For instance, the donor surface can be flexible enough to be mounted on a drum, have sufficient abrasion resistance, be inert to the particles and/or fluids being employed, and/or be resistant to any operating condition of relevance (e.g., radiation, pressure, heat, tension, etc.). Fulfilling any such property tends to favorably increase the life-span of the donor surface. 
     If the donor surface is to be subjected to radiation intermittently generated by an imaging station exposing desired selected areas, to renders the particles thereupon tacky, then to be compatible with such a use, the donor surface can, for instance, be relatively resistant and/or inert to the radiation, and/or able to absorb the radiation, and/or able to retain the heat generated by the radiation. 
     While in the above-description, the donor surface has been described as being suitable “as is” for the intended particles, further treatments may be applied to facilitate its coating. Such treatments can be broadly classified as chemical treatments (e.g., applying a chemical agent to the donor surface enhancing its affinity to the particles and/or their releasibility therefrom) and physical treatments (e.g., corona treatment the discharged plasma suitably modifying the properties of the donor surface). Were such treatments of the donor surface required, a coating apparatus according to the present teachings may further comprise a corresponding treatment station. 
     The donor surface, whether formed as a sleeve over a drum or a belt over guide rollers or sliders, may further comprise a body forming therewith a particle transfer member. The transfer member body may comprise different layers each providing to the overall transfer member one or more desired property selected, for instance, from mechanical resistivity, thermal resistivity, compressibility (e.g., to improve “macroscopic” contact between the donor surface and an impression cylinder), conformability (e.g., to improve “microscopic” contact between the donor surface and the topography of the outer surface of printing substrate on the impression cylinder), high or low friction depending on the system conveying the transfer member and any such characteristic readily understood by persons skilled in the art of printing transfer members. 
     The Imaging Station 
     The imaging station  16  provides one way of selecting the regions of the particle coating applied to the donor surface  12  that will transfer to the substrate  20  at the impression station  18 . As earlier mentioned, such an imaging station is required in the implementation of a digital printing system but other systems that do not comprise an imaging system may employ the above described coating apparatus  14 . For example, if the entire surface of the substrate  20  is to be coated, then no imaging system is required and the impression station may instead serve to apply the pressure and/or heat required to ensure an effective transfer of the particle coating from the donor surface  12  to the substrate  20 . Likewise, the substrate may reach the impression station having on its surface adhesive applied in a desired pattern, the adhesive stripping off the particles from the monolayer. 
     An exemplary imaging station  16 , shown in  FIG. 1 , may comprise a support  1601  carrying an array of chips  1602  each having an arrangement of individually controllable laser sources capable of emitting laser beams. The laser sources may by way of example be of VCSEL (Vertical Cavity Surface Emitting Laser) type, however other types may be utilized. The chips  1602  that are optionally arranged in pair(s) of rows in positions that are accurately predetermined relative to one another (e.g., in a staggered manner providing laser sources suitable to target points along the entire width of the donor surface). The support  1601  may be fluid cooled to cope with the significant heat that may be generated by the chips. Laser beams emitted by the chips  1602  are focused by lenses  1603  which can be constructed as a corresponding number of rows, by way of example, of GRIN (Gradient-Index) rod lenses (each chip  1602 , and all laser elements thereupon, being associated with a corresponding focusing lens  1603 ). Signals supplied to the chips for the activation of one or more laser element are synchronized with the rotation of the drum  10  so as to allow a high resolution image to be traced on the donor surface  12  by the emitted laser beams. The effect of the irradiation of each pixel by a laser beam is to convert the particle at that pixel into a tacky film so that particles coating the donor surface  12  may later transfer to the substrate  20  when it is pressed against it at the impression station. In other words, such selective irradiation of the donor surface modifies the affinity of the particles located in the selected areas, so that said irradiated particles have more affinity towards the substrate than the non-irradiated particles have with the donor surface, the activated areas thus being able to selectively detach particles from the donor surface  12 . 
     The digital printing system shown in the drawing can only print in one color but multicolor printing can be achieved by passing the same substrate successively through multiple towers that are synchronized with one another and each printing a different color. 
       FIG. 2  shows an alternative method of transferring selected regions of the coating to a substrate  20 . Instead of rendering selected regions of the coating of particles tacky by exposure to radiation, an adhesive  26  is applied to the substrate  20  by an offset adhesive coating station  36  comprised of an impression cylinder  32 , a blanket cylinder  30  and an applicator cylinder  34 . This station applies adhesive  26  in the same way as a conventional offset litho tower applies ink. On passage through the nip  18 , particles are transferred from the donor surface  12  only to the regions  26  of the substrate  20  coated with an adhesive. After passing through the nip  18 , the substrate is moved by transport rollers  40  and  42  to a stacking station or winding roller (not shown). 
     The Substrate 
     The printing system shown in the drawing as a non-limiting example of a system in which a coating apparatus according to present teachings can be integrated, is not restricted to any particular type of substrate. The substrate may be individual sheets of paper or card or it may have the form of a continuous web. The substrate can also be made of a fabric or of leather. Because of the manner in which is the particles are applied to the substrate, the layer of particles tends to reside on the surface of the substrate. This allows printing of high quality to be achieved on paper of indifferent quality. Furthermore, the material of the substrate need not be fibrous and may instead be any type of surface, for example a plastics film or a rigid board and generally assume a wide range of roughness, from very smooth plastic foils to relatively rough fibrous substrates. 
     The Impression Station 
     The illustrated impression station  18  comprises only a smooth impression cylinder  22  that is pressed against the drum  10  and its outer donor surface  12 . The impression cylinder  22  may form part of a substrate transport system, in which case it may be equipped with grippers for engaging the leading edge of individual substrate sheets. In other than digital printing systems, the impression cylinder may have an embossed surface to select the regions of the particle coating to be transferred to the substrate  20 . 
     In the description and claims of the present disclosure, each of the verbs, “comprise” “include” and “have”, and conjugates thereof, are used to indicate that the object or objects of the verb are not necessarily a complete listing of members, components, elements, steps or parts of the subject or subjects of the verb. 
     As used herein, the singular form “a”, “an” and “the” include plural references and mean “at least one” or “one or more” unless the context clearly dictates otherwise. 
     Positional or motional terms such as “upper”, “lower”, “right”, “left”, “bottom”, “below”, “lowered”, “low”, “top”, “above”, “elevated”, “high”, “vertical”, “horizontal”, “backward”, “forward”, “upstream” and “downstream”, as well as grammatical variations thereof, may be used herein for exemplary purposes only, to illustrate the relative positioning, placement or displacement of certain components, to indicate a first and a second component in present illustrations or to do both. Such terms do not necessarily indicate that, for example, a “bottom” component is below a “top” component, as such directions, components or both may be flipped, rotated, moved in space, placed in a diagonal orientation or position, placed horizontally or vertically, or similarly modified. 
     Unless otherwise stated, the use of the expression “and/or” between the last two members of a list of options for selection indicates that a selection of one or more of the listed options is appropriate and may be made. 
     In the disclosure, unless otherwise stated, adjectives such as “substantially” and “about” that modify a condition or relationship characteristic of a feature or features of an embodiment of the present technology, are to be understood to mean that the condition or characteristic is defined to within tolerances that are acceptable for operation of the embodiment for an application for which it is intended, or within variations expected from the measurement being performed and/or from the measuring instrument being used. When the term “about” precedes a numerical value, it is intended to indicate +/−15%, or +/−10%, or even only +/−5%, and in some instances the precise value. 
     While this disclosure has been described in terms of certain embodiments and generally associated methods, alterations and permutations of the embodiments and methods will be apparent to those skilled in the art. The disclosure of the invention is to be understood as not limited by the specific embodiments described herein.