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

Abstract

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, applying the fluid to the donor surface, in a manner to cause the particles suspended in the fluid to form a substantially continuous particle coating on the donor surface, causing fluid flow within an interior plenum of a housing over a portion of the donor surface partially disposed therein, 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 therewith; and extracting from the plenum fluid and particles which are not in not direct contact with the donor surface, so as to leave adhering to the donor surface a particle coating that is substantially only a single particle deep.

Claims

1. A method of coating a donor surface with a layer of thermoplastic particles, the method comprising: a) 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; b) 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; c) 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, d) 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; the plenum having a rim adjacent to the donor surface, the rim being configured to prevent egress of particles and/or fluid from a sealing gap defined between the rim of the housing and the donor surface.

2. A coating method as claimed in claim 1, wherein at least selected regions of the particle coating are transferable from the donor surface to a substrate at an impression station.

3. A coating method as claimed in claim 2, wherein the donor surface is an endless donor surface cyclically movable past the applying system and the impression station, each passage serving to coat the donor surface with a fresh layer of thermoplastic particles that is substantially only a single particle deep, the fresh coating replenishing the at least selected regions transferred to the substrate at a prior cycle.

4. A coating method as claimed in claim 3, further comprising the step of cooling the donor surface prior to applying the fluid and the particles suspended therein to the donor surface.

5. A coating method as claimed in claim 3, further comprising the step of heating the donor surface after applying the fluid and the particles suspended therein to the donor surface, prior to moving the donor surface to the impression station.

6. A coating method as claimed in claim 1, wherein the fluid and the particles suspended therein are directly applied onto the donor surface by one or more spray heads.

7. A coating method as claimed in claim 1, wherein the fluid and the particles suspended therein are applied onto the donor surface by one or more rotatable applicators.

8. A coating method as claimed in claim 7, wherein at least one of the rotatable applicator a) is a cylindrical sponge; or b) includes a plurality of flexible strips or bristles extending radially from a rotatable axle.

9. A coating method as claimed in claim 8, wherein the sponge and/or the flexible strips or bristles are formed of a closed-cell foam.

10. A coating method as claimed in claim 1, wherein the step of extracting from the housing surplus particles that are not in direct contact with the donor surface and their suspending fluid includes connecting the housing to a suction source.

11. A coating method as claimed in claim 3, further comprising the step of recirculating in a subsequent cycle at least a portion of surplus particles and fluid extracted in a prior cycle.

12. A coating method as claimed in claim 1, wherein the fluid within which the thermoplastic particles are suspended is a gas.

13. A coating method as claimed in claim 1, wherein the fluid within which the thermoplastic particles are suspended is a liquid.

14. A coating method as claimed in claim 13, further comprising the step of removing the liquid from the applied particle coating, so as to at least partially dry or substantially dry the particle coating adhering to the donor surface.

15. A coating method as claimed in claim 1, wherein the thermoplastic particles comprise a thermoplastic polymer and optionally a coloring agent.

16. A coating method as claimed in claim 1, wherein the particles are substantially spherical.

17. A coating method as claimed in claim 1, wherein the particles have the form of flat platelets.

18. A coating method as claimed in claim 1, further comprising homogenizing at least one property of the particle coating.

19. A coating method as claimed in claim 1, wherein a fluid flow passage is disposed at the rim of the housing to enable fluid to be drawn from, or introduced into, at least regions of the sealing gap.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) 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.

(2) Embodiments of the disclosure will now be described, by way of example, with reference to the accompanying drawings, in which:

(3) FIG. 1 depicts schematically an embodiment of a printing system incorporating a coating apparatus of the present disclosure;

(4) FIG. 2 is a view similar to that of FIG. 1 showing an embodiment having an alternative application device;

(5) FIG. 3 shows an embodiment in which particle application, cleaning and drying are carried out in three separate housings;

(6) FIG. 4 is a view similar to that of FIGS. 1 and 2 but showing an alternative embodiment of the coating apparatus;

(7) FIG. 5 is a schematic section through a donor surface coated with platelet-like particles;

(8) FIG. 6 is a schematic illustration of a cross section view of an embodiment of an applicator; and

(9) 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

(10) 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.

(11) Overall Description of an Exemplary Printing System

(12) 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.

(13) 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.

(14) 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.

(15) 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.

(16) 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.

(17) 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.

(18) 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.

(19) 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.

(20) 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.

(21) 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.

(22) The Coating Apparatus

(23) 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.

(24) 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.

(25) 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.

(26) 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.

(27) 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.

(28) 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.

(29) 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.

(30) 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.

(31) 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.

(32) 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).

(33) 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.

(34) 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.

(35) 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.

(36) 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.

(37) 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.

(38) 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.

(39) 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 1420a to 1420f but there may be fewer or more. In FIG. 4, each of the applicators 1420a to 1420f 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.

(40) 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.

(41) 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.

(42) 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.

(43) 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.

(44) The Particles

(45) 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.

(46) 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.

(47) 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.

(48) 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 D50 (50% of the population, e.g., by number or volume of particles) is about the intended size; whereas a population of particles wherein the D90 (e.g., D.sub.N90, D.sub.V90) is about the intended size implies a vast majority of particles (90% of the population) satisfy the same.

(49) 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.

(50) 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.

(51) 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.

(52) 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.

(53) 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: (D90-D10)≤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: (D90-D10)/D50≤2.0 and so on.
D10, D50 and D90 can be assessed by number of particles in the population, in which case they may be provided as D.sub.N10, D.sub.N50 and D.sub.N90, or by volume of particles, in which case they may be provided as D.sub.N10, D.sub.V50 and D.sub.V90.

(54) 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.

(55) 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.

(56) 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.

(57) 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.

(58) 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.

(59) 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.

(60) 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.

(61) 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.

(62) Alternative Configuration of Coating Stations

(63) 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).

(64) 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.

(65) 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.

(66) 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.

(67) 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.

(68) The Particle Carrier

(69) 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).

(70) 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.

(71) 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.

(72) The Donor Surface

(73) 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.

(74) The hydrophobicity is to enable the tacky film created by exposing the particles to radiation or the designated areas of the particles' monolayer contacted with the selectively tacky adhesive bearing substrate to transfer cleanly to the substrate without splitting.

(75) 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.sub.0, which is comprised between and can be calculated from the receding (minimal) contact angle Θ.sub.R and the advancing (maximal) contact angle Θ.sub.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).

(76) 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.sub.A of the above-described donor surface was found to be 101.7°±0.8° and the receding contact angle Θ.sub.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.

(77) 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.

(78) 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.

(79) 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.

(80) 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.

(81) 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).

(82) 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.

(83) 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.

(84) 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.

(85) 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.

(86) The Imaging Station

(87) 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.

(88) 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.

(89) 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.

(90) 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).

(91) The Substrate

(92) 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.

(93) The Impression Station

(94) 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.

(95) 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.

(96) 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.

(97) 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.

(98) 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.

(99) 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.

(100) 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.