WO2019234597A1

WO2019234597A1 - Thermal transfer printing system, method and substrate printed therewith

Info

Publication number: WO2019234597A1
Application number: PCT/IB2019/054598
Authority: WO
Inventors: Benzion Landa
Original assignee: Landa Labs (2012) Ltd.
Priority date: 2018-06-06
Filing date: 2019-06-04
Publication date: 2019-12-12
Also published as: GB201809302D0; GB2574439A; GB2574439B

Abstract

A method and system for thermal transfer printing onto a printing substrate are disclosed, as well as a substrate printed therewith. A monolayer coating of thermoplastic particles is applied to at least a segment of an imaging surface of a moveable transfer member. Thermoplastic particles within one or more selected regions of the monolayer coating are heated to render such thermoplastic particles tacky. The imaging surface and a surface of the substrate are pressed against one another to cause the thermoplastic particles that have been rendered tacky to be transferred to the surface of the substrate, thereby forming thereon a receptive image. Embellishment particles are subsequently applied onto the surface of the substrate. The receptive image, or parts thereof, can be heated prior to, and/or during, application of the embellishment particles, in order to ensure tackiness of the receptive image and cause the embellishment particle to only adhere thereto.

Description

THERMAL TRANSFER PRINTING SYSTEM, METHOD

AND SUBSTRATE PRINTED THEREWITH

This application claims Paris Convention priority from GB 1809302.1, filed on 6 June 2018, the contents of which are incorporated by reference in their entirety as if fully set forth herein.

FIELD

The present disclosure relates to thermal transfer printing.

BACKGROUND

The present Applicant has previously disclosed a printing system for thermal transfer printing onto a surface of a substrate, the system comprising:

a) a movable transfer member having an imaging surface on the front side, b) a coating station at which a monolayer of particles made of, or coated with, a thermoplastic polymer is applied to the imaging surface or at least a segment thereof,

c) an imaging station at which energy in the form of electromagnetic (EM) radiation is applied to selected regions of the particles coated imaging surface to render the particles thereon tacky within the selected regions, and

d) a transfer station at which the imaging surface and the surface of the substrate, or respective segments thereof, are pressed against one another to cause transfer to the surface of the substrate of only the regions of the particle coating that have been rendered tacky.

In different previously disclosed embodiments, the EM radiation could be applied to the imaging surface from either the front side or the rear side of the transfer member. The former case is detailed in WO 2016/189512. In the latter case, as taught in WO 2018/100528, the rear side of the transfer member is formed of a body transparent to the EM radiation, and an EM radiation absorbing layer made of an elastomeric silicone is provided on the front side of the transfer member adjoining the transparent body, the imaging surface being formed on, or as part of, the radiation absorbing layer.

In order to obtain, in a single pass, a desired ink image on the printing substrate, the thermoplastic polymer particles included a coloring agent (e.g., a dye or a pigment). The polymer particles could either be formed wholly of a polymer or they could be particles that only have a polymer coating. Digital printing by selective irradiation of a monolayer of colored thermoplastic particles, as previously taught by the present Applicant, may require a relatively high amount of energy if the particles are to be sufficiently softened to transfer to the substrate at the nip of the transfer station to form an ink image. This high energy requirement is especially challenging if the particles are of relatively large diameter, as would be required, for instance, for high gloss images. Increased heating not only increases energy cost, but places high demands on the ability of the imaging station to dissipate heat while achieving good image resolution.

To overcome this problem, the Applicant has previously proposed applying only enough energy during the selective irradiation of the imaging surface to enable transfer to the printing substrate at the nip, the transferred films of thermoplastic polymer particles readily losing tackiness upon exit of the transfer station. The size of the particles was also minimized to reduce the amount of energy required to soften them, which made it difficult to produce an ink image having any significant depth. To achieve a desired surface finish, such as a gloss, it was proposed to pass the substrate with its ink image between heated fuser rollers that would re soften the polymer particles and flatten the ink image surface. However, the small thickness of the ink image placed limits on the surface texture that could be achieved.

OBJECT

The present invention seeks to provide inter alia an improved thermal transfer system and method which ultimately enables larger particles to be deposited on the printing substrate to allow various surface effects to be achieved.

SUMMARY

In one aspect, the present disclosure relates inter alia to a system for thermal transfer printing onto a printing substrate, which comprises: a) a movable transfer member having an imaging surface on a front side; b) a coating station for applying to at least a segment of the imaging surface a monolayer coating of thermoplastic particles that are formed of, or coated with, a thermoplastic polymer; c) an imaging station at which energy is applied to one or more selected regions of the particle coating on the imaging surface to heat thermoplastic particles within each selected region of the particle coating and render the thermoplastic particles within each selected region tacky; d) a transfer station at which the imaging surface and a surface of the substrate (or respective segments thereof) are pressed against one another to cause the thermoplastic particles that have been rendered tacky to transfer to the surface of the substrate, thereby forming a receptive image on the surface of the substrate; e) an embellishment station disposed downstream of the transfer station to apply embellishment particles onto the surface of the substrate; and f) a heating station disposed downstream of the transfer station for heating the receptive image prior to, and/or during, appl ication of the embellishment particles, in order to ensure tackiness of the receptive image and cause the embellishment particles to adhere only to the receptive image.

The section of the thermal transfer printing system which includes the stations above- detailed in a), b), c) and d) may be considered as a receptive image forming section.

In another aspect, the present disclosure relates inter alia to a method of thermal transfer printing onto a printing substrate, which comprises applying to a surface of a substrate a receptive image by: a) providing a movable transfer member having an imaging surface; b) applying to at least a segment of the imaging surface a monolayer coating of thermoplastic particles that are formed of, or coated with, a thermoplastic polymer: c) heating thermoplastic particles within one or more selected regions of the monolayer coating to render the thermoplastic particles within the one or more selected regions of the monolayer coating tacky; d) pressing the imaging surface and the surface of the substrate (or respective segments thereof) against one another to cause the thermoplastic particles that have been rendered tacky to transfer to the surface of the substrate, thereby forming a receptive image on the surface of the substrate; e) applying embellishment particles onto the surface of the substrate; and f) heating the receptive image prior to, and/or during, application of the embellishment particles, in order to ensure tackiness of the receptive image and cause the embellishment particles to adhere only to the receptive image.

In accordance with a further additional aspect, the invention provides a substrate bearing printed image applied by thermal transfer, wherein the printed image comprises a first layer formed of thermoplastic first particles in contact with, and adhering to, the substrate and a second layer overlying the first layer formed of second particles, adhering to and/or at least partly' embedded within the first layer, wherein the second particles (and/or the second layer formed thereby) differ from the first particles (and/or the first layer formed thereby) physically and/or chemically. In some embodiments, the substrate bearing the printed image was obtained in any of the various embodiments of the printing system as herein disclosed. In some embodiments, the substrate bearing the printed image was obtained by implementing any of the various embodiments of the printing method as herein disclosed.

As shall be further detailed herein-below, the energy being selectively applied at the imaging station results in thermal energy being perceived by the thermoplastic particles coating the regions targeted therewith. It can be achieved by using a thermal print head in thermal contact with the rear side of the transfer member (the rear side being opposite the front side) and operative to apply energy to particles on the imaging surface by heat conduction through the transfer member, such thermoplastic particles therefore being rendered tacky by thermal conduction. Additionally or alternatively, thermoplastic particles can be rendered tacky by applying EM radiation, either from the rear side of a transfer member transparent to such radiations in its rear side, or from the front side of any transfer member including near or at the imaging surface a layer able to absorb such radiations, such thermoplastic particles therefore being rendered tacky by exposure to the EM radiation. Additionally or alternatively, convection can be used to apply energy to thermoplastic particles.

In accordance with an additional aspect, the invention provides a system for thermal transfer printing onto a printing substrate, which comprises A- a receptive image forming section that includes: a) a movable transfer member having opposite front and rear sides with an imaging surface on the front side, b) a coating station at which a monolayer of particles made of, or coated with, a thermoplastic polymer is applied to the imaging surface, c) an imaging station at which energy is applied to selected regions of the particles coated imaging surface to render the particles thereon tacky within the selected regions, and d) a transfer station at which the imaging surface and the substrate surface are pressed against one another to cause transfer to the surface of the substrate of only the regions of the particle coating that have been rendered tacky; and B- an embellishment section disposed downstream of the receptive image forming section for applying embellishment particles to the surface of the substrate, the embellishment particles adhering only to regions of the substrate coated with the transferred thermoplastic particles.

In accordance with a further aspect of the invention, there is provided a system for thermal transfer printing onto a printing substrate, which comprises a receptive image forming section that includes:

a) a movable transfer member having opposite front and rear sides with an imaging surface on a front side,

b) a coating station at which a monolayer coating of thermoplastic particles that are formed of, or coated with, a thermoplastic polymer is applied to the imaging surface or to at least a segment thereof, c) an imaging station at which energy is applied via the rear side of the transfer member to one or more selected regions of the particle coating on the imaging surface to render the particles within each selected region tacky, the imaging station comprising a thermal print head in thermal contact with the rear side of the transfer member and operative to apply energy to the particles on the imaging surface by heat conduction through the transfer member, and d) a transfer station at which the imaging surface and a surface of the substrate (or respective segments thereof) are pressed against one another to cause the thermoplastic particles that have been rendered tacky to transfer to the surface of the substrate, thereby forming a receptive image on the surface of the substrate,

the system further comprising:

e) an embellishment station disposed downstream of the transfer station of the receptive image forming section for applying embellishment particles to the surface of the substrate, the embellishment particles adhering only to regions of the substrate coated with the receptive image.

In some embodiments, the printing system wherein the imaging station of the receptive image forming section comprises a thermal print head in thermal contact with the rear side of the transfer member further comprises a heating station disposed downstream of the transfer station for heating the receptive image prior to, and/or during, application of the embellishment particles. The presence of a heating station ensures tackiness of the receptive image, facilitating (allowing or increasing) the adhesion of the embellishment particles thereto.

In accordance with another aspect of the invention, there is provided a system for thermal transfer printing onto a printing substrate, which comprises a receptive image forming section that includes:

a) a movable transfer member having an imaging surface on a front side;

b) a coating station for applying to at least a segment of the imaging surface a monolayer coating of thermoplastic particles that are formed of, or coated with, a thermoplastic polymer;

c) an imaging station at which electromagnetic (EM) radiation is applied to one or more selected regions of the particle coating on the imaging surface to render the thermoplastic particles within each of the selected regions tacky, and d) a transfer station at which the imaging surface (including the particle coated segment! s)) and a surface of the substrate are pressed against one another to cause only the regions of the particle coating that have been rendered tacky to transfer to the surface of the substrate, thereby forming a receptive image on the surface of the substrate,

the system further comprising;

e) an embellishment station disposed downstream of the transfer station of the receptive image forming section for applying embellishment particles to the surface of the substrate, the embellishment particles adhering only to regions of the substrate coated with the receptive image (formed by the transferred thermoplastic particles).

In some embodiments, the printing system wherein electromagnetic (EM) radiation is applied at the imaging station, either via the front side of the transfer member or via the rear side (the rear side being transparent to the EM radiation), further comprises a heating station disposed downstream of the transfer station for heating the receptive image prior to, and/or during, application of the embellishment particles (for reasons already described).

The region(s) on the surface of the substrate selectively coated with the thermoplastic particles selectively rendered tacky in corresponding region! s) at the imaging station (i.e the transferred thermoplastic particles) can be referred to as a receptive image. As a result of the pressure applied to respective sections of the imaging surface and the substrate at the transfer station, the tacky particles actually transferred to the printing substrate are generally flattened as compared to their original shape. Depending on the desired resolution of the receptive image, the transferred thermoplastic particles may form individual or contiguous films and the receptive image formed therefrom can alternatively be termed a receptive film or a receptive layer.

The invention differs from the teaching of the Applicant’s prior proposals that are mentioned above in that hitherto the thermoplastic particles transferred to the substrate in step d) formed the desired visible ink image. By contrast, in the present invention, the steps a) to d) serve only to apply image-wise to the substrate a receptive layer of thermoplastic particles which need not necessarily be visible as it is the embellishment particles that form the visible ink image. The embellishment particles do not need to be rendered tacky in order to adhere to the thermoplastic particles of the receptive layer as the latter can be tacky and thereby ensure that the embellishment particles will only adhere to regions of the substrate that have been coated with a receptive layer. As the embellishment particles need not be rendered tacky in selective regions, their particle size is no longer limited by the amount of energy that can be applied to them. Thus, the layer of embellishment particles can be thick enough to acquire a high gloss or other desired surface finish. Furthermore, the embellishment particles need not necessarily be made of a thermoplastic material but may alternatively or additionally be made of a thermoset elastomer, glass, or metal, thereby allowing a range of special effects to be achieved, for example metallization.

It is necessary for the thermoplastic particles of the receptive layer, or the film formed therefrom, to be tacky on reaching the embellishment section. In some embodiments of the invention, this can be achieved by appropriate selection of the thermoplastic polymer. Different polymers have different“open-times”, i.e. they remain tacky for different lengths of time after energy was directly or indirectly applied to them in step c), via the front side or the rear side of the transfer member, at the imaging station of the receptive image forming section, in manners herein detailed. For a given energy exposure in step c), polymers with lower melting points and lower specific heats will remain tacky for longer and it is possible by selecting a polymer with a longer open-time to ensure that the particles (and the receptive image formed thereby) will remain tacky on reaching the subsequent embellishment station or section of the system.

Alternatively or additionally, it is possible to heat the receptive layer after exiting the transfer station of feature or step d). This may be performed, for instance, by exposure to EM radiation, by convection (e.g., blowing a stream of hot gas (e.g., air) over the receptive layer or any other suitable radiant heating) and/or by means of thermal conduction (e.g., by means of a hot plate, a hot cylinder, and/or any other suitable element(s) in thermal contact, for instance, with the underside of the substrate and/or by means of any suitable element(s) in direct physical contact, for instance, with the receptive image). In the case of non-contact heating, heating the receptive image may enhance its adhesion to the printing substrate, its abrasion resistance, its chemical resistance and the like. In the case of heating means which contact the image, such as silicone-coated fuser rolls or belts, in addition to the benefits of non-contact heating, the image film may also acquire higher gloss and scratch resistance.

Heating the receptive layer after exiting the transfer station and prior to, and/or during, application of the embellishment particles may allow the use at the coating station of step b) of thermoplastic particles having an open-time sufficient for transfer to the substrate but insufficient for the subsequent adhesion of embellishment particles, were the receptive layer not heated once on the surface of the printing substrate. To the extent that heating is applied to the receptive image following exit of the transfer station and prior to entry into the embellishment section, such heating need not necessarily render tacky the entire receptive layer. For instance, a part of the receptive image may be selectively heated to regain sufficient tackiness to enable the adhesion of subsequently applied embellishment particles, while the part of the receptive image not further heated (unable to retain the embellishment particles) may provide for a different effect (e.g., gloss, texture, etc.) on the printed substrate.

Alternatively, a printing system as herein disclosed may include A) a receptive image forming section (according to any of the various embodiments herein disclosed); B) a first embellishment station and a first heating station disposed downstream of the transfer station for heating a first part of the receptive image prior to application of first embell ishment particles at the first embellishment station; and C) a second embellishment station and a second heating station disposed downstream of the first embell ishment station for heating a second part of the receptive image prior to application of second embellishment particles at the second embellishment station. As in the previous alternative, the first part of the receptive image rendered tacky by the first heating station and the second part rendered tacky by the second heating station need not jointly constitute the entire receptive layer formed at the transfer station. In other words, there may be a third part not rendered tacky by further heating, the third part providing any desired effect other than the attachment of embellishment particles. Furthermore, a skilled person may readily appreciate that while illustrated with two pairs of heating and embellishment stations, the same principles can be applied with any other desired number of pairs of such stations. Such embodiments are schematically illustrated in Figure 3

Regardless of the number of pairs of stations selectively heating parts of the receptive image which may he implemented in some embodiments of the printing system, the stations of the different pairs need not be the same. For instance, the heating of a first heating station downstream of the transfer station may be by a first healing mean/apparatus, while the heating of a second heating station downstream of a first embellishment station may be by a second heating mean/apparatus, different from the first. Similarly, the embellishing particles applied at a first embellishment station may be applied to the receptive image by a first device/method, while embellishing particles applied at a second embellishment station may be applied by a second device/method, different from the first.

The embellishment section of the printing system (alternatively referred to as embellishment station) may be implemented in a variety of ways. In one embodiment, a gas stream carrying embellishment particles may be blown onto the substrate surface bearing the receptive image by means of a nozzle that is surrounded by a cowling connected to a suction source. In this case, no contact is made with the surface of the substrate and the embellishment section can be termed contact-less. Embellishment particles are merely sprayed onto the substrate surface and adhere only to regions of the surface that bear a tacky receptive layer (e.g., made of thermoplastic particles having a long enough open-time and/or heated prior to or during application of the particles). A limited amount of the embellishment particles, nominally a monolayer, will adhere to the receptive layer and the remaining particles will be sucked up in the low pressure area created by the cowling.

In an alternative embodiment of the invention, the embellishment section or station can be constructed in a manner similar to the coating station for applying to the imaging surface a monolayer coating of thermoplastic particles in the receptive image forming section. Hence, this section can also be termed a contacting embellishment section. In such a case, the embellishment section may comprise another (i.e. second) transfer member, serving as a donor surface, another (i.e. second) coating station for coating the donor surface with embell shment particles and another (i.e. second) transfer station at which the donor surface is brought into contact with the substrate surface bearing a tacky receptive layer. However, the second coating station need not necessarily apply a monolayer coating of the embellishment particles to the donor surface, superfluous particles (if any) being subsequently eliminated. Embellishment particles from the donor surface will only adhere to the tacky receptive layer on the surface of the substrate, and particles not well adhered to the surface of the substrate (or not well adhered to the surface of the tacky receptive layer, if the donor surface was not a monolayer particle coating) can be sucked away A soft brush may be used in conjunction with suction to help dislodge any embellishment parts that are not adhered to the tacky receptive layer.

The donor surface can be the outer surface of a cylinder, or a belt, or any other suitable mechanical support for enabling the embellishment particles to contact the receptive layer on a facing printing substrate. The donor surface should be able to retain embellishment particles and to release them to the receptive layer at the second transfer station of the embellishment section. The functions served by the first and second transfer members are not necessarify the same and they need not therefore be constructed in the same manner.

Moreover, while the donor surface and the coating station of the embellishment section may provide for the formation of a monolayer of embellishment particles on the donor surface, as explained, this is not essential, the amount of embellishment particles able to transfer to the printing substrate being dictated by the receptive image.

After exiting the embellishment station, whether of the contact-less or contacting type, the substrate may optionally enter one or more finishing sections or stations. Finishing sections can apply one or more diverse treatments to the layer of embellishment particles disposed on or within the receptive image. Such finishing sections may promote the fusing of suitable embellishment particles, the curing of the receptive layer, the levelling or patterning of the embellishment particles on or within the receptive layer, the overcoating of the substrate (or parts thereof) with a varnish, and any such step enabling a desired printing effect. Finishing can be contact-less, such as exposing to UV-radiation a UV-curable receptive layer, or can involve contacting of the embellishment particles disposed on the receptive layer attached to selective areas of the printing substrate (which for simplicity can also be termed an“embellished substrate”). By way of non-limiting examples, the embellished substrate can be passed between rollers, that are optionally heated, to impart a high gloss or a desired surface texture to the embellishment particles, or if the embellishment particles are, for instance, metal flakes, the embellished substrate may be passed through a burnishing section.

In accordance with an additional aspect of the invention, there is provided a method of thermal transfer printing onto a printing substrate, which comprises applying to a surface of a substrate a receptive image by:

a) providing a movable transfer member having opposite front and rear sides with an imaging surface on the front side,

b) applying to at least a segment of the imaging surface a monolayer coating of thermoplastic particles that are formed of, or coated with, a thermoplastic polymer,

c) heating one or more selected regions of the monolayer particle coating to render the thermoplastic particles within each selected region tacky by means of heat conduction through the transfer member, and

d) pressing the imaging surface (including the particle coating) and a surface of the substrate against one another to cause only the regions of the particle coating that have been rendered tacky to transfer to the surface of the substrate, thereby forming a receptive image on the surface of the substrate,

characterized by: e) applying embellishment particles to the surface of the substrate, the embellishment particles adhering only to regions of the substrate coated with the receptive image (i.e., the transferred thermoplastic particles).

In some embodiments, the method wherein the monolayer coating of thermoplastic particles is selectively heated by means of heat conduction through the transfer member, further comprises heating the receptive image prior to, and/or during, application of the embellishment particles. The inclusion of an additional heating step following transfer (by pressing) of the receptive image to the substrate, enhances the tackiness of the receptive image, facilitating (allowing or increasing) the adhesion of the embellishment particles thereto.

In accordance with a further aspect of the invention, there is provided a method of thermal transfer printing onto a printing substrate, which comprises applying to a surface of a substrate a receptive image by:

a) providing a movable transfer member having an imaging surface,

b) applying to at least a segment of the imaging surface a monolayer coating of thermoplastic particles that are formed of, or coated with, a thermoplastic polymer;

c) heating one or more selected regions of the monolayer particle coating to render the thermoplastic particles within each selected region tacky by exposure to EM radiation, and d) pressing the imaging surface (including the particle coating) and a surface of the substrate against one another to cause only the regions of the particle coating that have been rendered tacky to transfer to the surface of the substrate, thereby forming a receptive image on the surface of the substrate,

characterized by:

e) applying embellishment particles to the surface of the substrate, the embellishment particles adhering only to regions of the substrate coated with the transferred thermoplastic particles (i.e., the receptive image).

In some embodiments, the method wherein the monolayer coating of thermoplastic particles is selectively heated by means of electromagnetic (EM) radiation, either via the front side of the transfer member or via the rear side (the rear side being transparent to the EM radiation), further comprises heating the receptive image prior to, and/or during, application of the embellishment particles (for reasons already described). As explained in the context of the printing system, the heating of the receptive layer transferred to the substrate need not to necessarily affect the entire receptive image and may be selectively applied to render tacky only parts of the receptive image and not others. The parts rendered tacky would attach embellishment particles subsequently applied, while the parts of the receptive image not selectively heated would serve any other desired purpose, as previously detailed.

BRIEF DESCRIPTION OF THE DRAWINGS.

Some embodiments of the invention will now be described further, by way of example, with reference to the accompanying drawings, in which like reference numerals or characters indicate corresponding or like components.

In the Figures:

Figure 1 is a schematic representation of a thermal transfer printing system of the invention according to some embodiments where energy is applied from a rear side of a transfer member.

Figure 2 is a schematic representation of a thermal transfer printing system of the invention according to some alternative embodiments where energy is applied from a front side of a transfer member.

Figure 3 is a schematic representation of a thermal transfer printing system of the invention according to some additional embodiments where a receptive layer can be rendered tacky in selected regions thereof to enable application of embellishing particles at more than one embellishment station.

The description, together with the figures, makes apparent to a person having ordinary skill in the pertinent 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 may not be drawn to scale. DETAILED DESCRIPTION

Overview of an exemplary printing system

Figure 1 shows schematically a thermal transfer printing system of an embodiment of the invention which comprises four sections, two being optional, through which substrate sheets 20 supported on an endless transport belt 18 progress from right to left in the drawing.

The first section 10 is a receptive image forming section which can be constructed in the manner described in WO 2018/100528 by reference to Figure 3a. In this section 10, an endlessly recirculating transfer member 101, passes through a coating station 102 that applies to a hydrophobic outer surface of the transfer member a monolayer coating of particles that are formed of, or coated with a polymeric thermoplastic material. The surface of the transfer member upon which particles are applied can also be referred to as a first side or a front side of the transfer member, which in the present drawing is on the outer side of the looped member. The transfer member 101 then moves to an imaging and impression station 103 where one or more selected regions of the inner side of the transfer member (also called the rear side, opposite the front side) are irradiated at and/or adjacent the nip with laser beams which can be emitted by an array of VCSEL (Vertical Cavity Surface Emitting Laser) elements 104 and focused by a lens system which can be formed of GRIN (Gradient Index) rod elements 105 onto the inner side of the transfer member 101. The inner (rear) side of the transfer member is transparent to the laser radiation but the outer (front) side includes a layer that absorbs the radiation and allows heating the polymeric thermoplastic particles disposed thereon to soften them. The transfer member 101, substantially at the same time as being irradiated, is pressed between a transparent support 106 and an impression cylinder 107. As a result, the polymeric particles that have been selectively heated to a temperature higher than their glass transition temperature (Tg) separate from the hydrophobic outer surface of the transfer member 101 and adhere to a substrate 20 that passes between the transfer member 101 and the impression cylinder 107. In this way, the first section 10 forms on the substrate a receptive image of a polymeric material that may, but need not, be invisible to the naked eye.

Once the substrate leaves the receptive image forming section 10 of the printing system, the regions on the imaging surface corresponding to the selected tacky areas transferred to the substrate consequently become exposed, being depleted by the transfer of particles. The transfer member 101 can then complete its cycle, by returning to the coating station 102 where a fresh monolayer particle coating is applied to the exposed regions of the transfer member 101 from which the previously applied particles were transferred to the substrate in the impression station 103 This step can be viewed as a replenishment of the particle coating A suitable coating station 102 has been described in WO 2016/189512 and WO 2016/189513 to the same Applicant and will not therefore be detailed herein.

While Figure 1 relates to an imaging station wherein the energy is applied to the thermoplastic particles in the form of EM radiation, this should not be construed as limiting. The imaging station may alternatively include a thermal print head in thermal contact with the rear side of the transfer member and operative to apply energy to the particles on the imaging surface by heat conduction through the transfer member, thus the imaging and impression station 103 may' also schematically depict the positioning of a thermal print head. Suitable thermal print heads are known to persons skilled in thermal printing, an exemplary imaging station based on such heat conduction being described in WO 2018/100530 to the same Applicant. Therefore, whi le in the following description of the drawing reference may be made to an EM imaging device, or to compatibility with radiation, the same considerations apply to an imaging device based on heat conduction, or to compatibility with heat transfer, as can readily be appreciated by^r skilled persons.

The substrate 20, also termed printing substrate, may be made of various materials (eg., paper, cardboard, plastics, fabrics etc.), some optionally existing in coated and uncoatcd form depending on desired characteristics, and it can he supplied to the impression station in different forms (e.g., as sheets or continuous webs) by any suitable stack or roll feeder (not shown in the drawing).

On leaving the first section 10 of the printing system, depending on the open-time of the polymer, the receptive image may or may not be tacky. If it is tacky, and remains so during the period of time the receptive layer tra vels the distance to an embellishment section 14, then the substrate can pass directly to section 14. If, on the contrary, the receptive image is not tacky upon leaving the first section 10 and/or upon entering a third section 14, then a heating section 12 is provided to reheat the polymeric receptive image, e.g., to above the glass transition temperature of the polymer. Heating of the receptive image in the heating section 12 may be carried out by conduction, convection and/or EM radiation indeed, any desired form of heating may be used, regardless of whether or not it contacts the receptive image or the substrate upon which it is disposed, so long as the polymer is rendered tacky. In some embodiments, it ay also be desirable that the polymer not be disturbed (e.g., the receptive image substantially retaining its intended contour). While the heating station is schematically represented in Figure 1 as a second heating section 12 located upstream of a third (embellishment) section 14, the heating being performed prior to application of the embellishment particles, this is not necessarily solely the case and in some embodiments the heating station may additionally or alternatively include a heating section disposed within the embellishment section 14 and heating of the receptive image can be additionally or alternatively performed at the embellishment section 14, prior to and/or during application of the embellishment particles. For example, a first portion of the receptive image can be heated in a first section of the heating station disposed upstream of the embellishment section 14, and a second portion of the receptive image can be heated in a second section of the heating station that is disposed within the embellishment section, the first portion and second portion completely overlapping, partly overlapping, or having no overlap, depending on the implementation. When heating is applied both prior to and during application of the embellishment particles, the first section of the heating station disposed upstream of the embellishment section may in some embodiments heat the receptive layer to a first temperature, whereas the second heating section disposed within the embellishment section may heat the receptive layer to a second temperature or ensure that the temperature upon entry in the nip of the embellishment station remains within a desired range from the first temperature.

Regarding the heating means that may serve in the heating station(s) or sections(s) of the printing system, or for the corresponding heating step of the method, any and all devices presently existing or later developed, which are adapted to apply sufficient heat to the receptive image for the present systems and methods, are contemplated. Examples of convection may include blowing a stream of hot gas (e.g., air) over the receptive layer or any other suitable radiant heating. Examples of thermal conduction may include hot plate(s), cylinder(s), and/or any other suitable element(s) in thermal contact, for instance, with the underside of the substrate; and/or may include silicone-coated fuser roll(s), belt(s) and/or any other suitable element(s) in direct physical contact, for instance, with the receptive image. The impression cylinder 147 to be discussed in the following, for example, can be internally heated to restore the tackiness of the receptive layer at the nip.

In some embodiments, the heating may be applied selectively in order that the receptive image (or elected part thereof) becomes tacky. For example, thermal conduction that is applied to the underside of the substrate, may include thermal contact with the underside only directly below the receptive image or at least part thereof. Thermal conduction involving direct physical contact may include physical contact with the receptive image (or a part thereof), but not with any point on the surface of the substrate that does not bear the receptive image. Application of radiation and/or a stream of hot gas, may include directing of the radiation and/or hot gas toward the receptive image only. However, in some other embodiments, at least part of the heating that is applied may not necessarily be applied selectively, neither to parts of the receptive image, nor to the sole receptive image. For example, thermal conduction may include thermal contact with point/ s) on the underside that are not directly below the receptive image, and/or direct physical contact with point(s) on the surface of the substrate not bearing the receptive image. EM radiation and/or a stream of hot gas may also reach point(s) on the surface of the substrate not bearing the receptive image.

The third (embellishment) section 14 brings the receptive image into contact with visible embellishment particles that adhere only to the receptive image (or parts thereof) that has been rendered tacky at the receptive image forming section and/or at a heating section, but not to the background substrate. In the embodiment illustrated in Figure 1, the embellishment section 14 comprises an endlessly recirculating transfer member 141 of which the outer surface serves as a donor surface. In a coating station 142, a layer of embellishment particles is applied to the donor surface of the transfer member 141 and at an impression station 143, the substrate carrying the tacky receptive image is pressed between the transfer member 141 and an impression cylinder 147. The embellishment particles adhere only to the tacky receptive image and any particle adhering to the background substrate (or to parts of the receptive image not being tacky), if any, can be brushed or blown aw ay.

The embellishment particles can be polymeric particles, in which case they may be larger than the particles used in the first section 10, where the size of the particles is limited by the available laser beam power. However, as the embellishment particles need not be themselves tacky, they may be made of non-thermoplastic materials (e.g., thermoset plastic polymers) or lion-polymeric materials, such as metals, alloys, glasses or ceramics. The embellishment particles can have any desired shape, including by way of non-limiting examples, a globular shape, a rod-like shape, a flake-like shape or any irregular shape that may suitably adhere to the tacky receptive layer.

It wall be appreciated that the embellishment section may be constructed differently and need not include a transfer member. For example, it is possible simply to spray the receptive image with a gas stream containing a suspension of the embellishment particles and to remove excess particles that do not adhere to the tacky receptive image by means of a brush or of a vacuum suction. After being coated with the embellishment particles, the substrate may pass through an optional finishing section 16 of which the function would depend on the nature of the embellishment particles and the desired visual effect. Thus, the one or more finishing stations of such section may serve, by way of non-limiting example, to create a gloss or matt finish, to provide an overcoating, to imprint an embossing pattern, to create a texture or to burnish the embellishment layer. Such finishing stations and their construction are known to the persons skilled in the field of printing and need not he further detailed herein.

While in Figure 1 the imaging and impression station substantially target the same segment of the transfer member 101, this is not essential to the present printing system. An imaging station comprising individually controllable laser beam emitting elements 104 and an appropriate lens system 105 can alternatively be positioned upstream of the impression station 103, which can also be referred to as a transfer station. An upstream imaging station can be positioned externally with respect to the endless transfer member 101 , so as to target its radially outer side, i.e., the imaging surface.

This alternative embodiment is schematically depicted in Figure 2, where for simplicity the imaging station is represented by arrow 123. imaging station 123 (which may include the previously described individually controllable laser beam emiting elements 104 and lens system 105) is positioned upstream of a nip formed between a transfer roller 116 and impression cylinder 107, impression station 133 being in a segment of the transfer member downstream of the one targeted by imaging station 123. In such case the transfer member need not be transparent to EM radiation on its rear side and may include within its body a compressible layer. Any positioning of the imaging station upstream of the nip and downstream of the coating station is permissible, as long as the thermoplastic particles selectively irradiated at such upstream station can remain tacky upon entering the transfer station 103. Again, while the description of Figure 2 refers to an imaging station wherein EM radiation is selectively applied to thermoplastic particles, so as to render them tacky, and form therewith, following transfer, a receptive image, the imaging station schematically depicted as arrow 123 can alternatively represent a thermal print head or any other means capable of rendering the particles sufficiently tacky when facing the front side of the transfer member upon which the particles are disposed.

The terms“tacky” and“sufficiently tacky” as used herein are not intended to mean that the particle coating selectively exposed to energy while on the imaging surface or forming the receptive image while on the substrate is necessarily tacky to the touch, but only that it is softened sufficiently to enable its adhesion to the surface of a printing substrate when pressed against it in the transfer station of the receptive image forming section, or to enable the adhesion of embellishment particles to the surface of a transferred receptive film at a subsequent embellishment section. The tacky particles, e.g., within selected regions of the particle coating on the imaging surface are believed to form individual films or contiguous films which following their transfer to a printing substrate may optionally yield thinner films, as a result of the pressure being applied upon contacting of the imaging surface (or a segment thereof) to the substrate (or a corresponding segment thereof).

The intended meaning of the term“monolayer” and different ways in which a monolayer can be achieved are disclosed in WO 2016/189512 and WO 2016/189513 which provide details of the particle size, polymer film thickness as well as the design and construction of an imaging station for emitting laser radiation. Particular imaging devices that may serve in such imaging stations are further detailed in WO 2016/189510 and WO 2016/189511 to the same Applicant.

Briefly, in order to facilitate replenishment of the particle coating on the imaging surface after each transfer, thermoplastic particles that adhere to the imaging surface more strongly than they do to one another are utilized. This results in an applied layer that is substantially a monolayer of individual particles, with little, if any, overlap, the thickness of the monolayer being therefore commensurate (e.g., l-3-times) with the thickness of the particles. Stated differently, the layer is only one particle thick over a maj or proportion of the area of the imaging surface and most, if not all, of the particles have at least some direct contact with the imaging surface.

Advantageously, a monolayer of particles facilitates the targeted delivery of radiation. This may ease the control of the imaging station, as the selectively irradiated particles reside on a single defined layer, which may facilitate focusing the laser radiation to form upon transfer to a substrate a dot of approximately even thickness and/or relatively defined contour.

Another advantage of having a monolayer of thermoplastic particles is that it can provide for good thermal coupling between the particles and the imaging surface on which the particles are coated.

To permit the transfer on the substrate of patterns of receptive images corresponding to the selected regions exposed to radiation, the affinity of the heated tacky particles needs to be greater to the substrate than to the imaging surface. Moreover, this relatively higher affinity of the tacky particles to the substrate in the selected regions shall also be greater than the affinity of the bare substrate to the particles not rendered tacky. In the present context, a substrate is termed“bare”, or background substrate, if lacking any desired image pattern to be printed by the present method or system. Though the bare substrate should for most purposes have substantially no affinity to the thermoplastic particles, to enable the selective affinity of the tacky ones, some residual affinity can be tolerated (e.g., if not visually detectable). Undesired transfer of particles to areas of the bare substrate is also termed parasite or parasitic transfer.

Such gradient of affinities between the thermoplastic particles (before and after heating), the fluid carrying the native particles for coating or replenishing of the transfer member, the imaging surface, the printing substrate, any such element of the system or method, can be modulated by selection of suitable materials or characteristics, such as hardness, smoothness, hydrophobicity, hydrophilicity, charge, polarity and any such properties known to affect interaction between any two elements.

For assisting in the transfer of the tacky film of particles from the imaging surface to the substrate, the imaging surface may be hydrophobic.

In some embodiments, the thermoplastic particles may themselves be hydrophobic. In such case, the relative affinity between the particles in their different states and the imaging surface can be based, at least partially, on hydrophobic -hydrophobic interactions. In some embodiments, attachment of the monolayer of particles to the imaging surface is assisted by the relatively low hardness of the imaging surface. A relatively soft imaging surface may assist in forming an intimate contact with each individual particle, such intimate contact manifesting itself in a relatively large contact area between the imaging surface and the particle, in contrast to the discrete contact formed between the particle and a relatively hard surface. Such intimate contact may thus further intensify effects of any short-range attraction forces between the imaging surface and the thermoplastic particles, such as, e.g., hydrophobic -hydrophobic interactions or Van der Waals forces.

The present printing system comprising the herein detailed receptive image forming section and embellishment section may be an offline, stand-alone machine, or may be in-line with a printing press and/or other finishing units as above-exemplified. For instance, the printing system according to the present disclosure can serve as one station or module in offset, flexographic, gravure, serigraphic and digital printing presses.

For instance, the present printing system may include, upstream to the receptive image forming section, a station for applying a background image, the receptive layer being subsequently applied thereupon to form (following impression) a foreground image on the previously applied background. Conversely, the receptive layer and the embellishment particles attached thereto can form a background image whereas a foreground image is thereafter applied at a station downstream of the embellishment section. The foreground and background images may form distinct parts of the image to be printed, but may also overlap. Each of the foreground and background images, if both are desired for a particular image to be printed, can be applied by any printing system.

For instance, a background image can be applied at a first station for flexographic printing of a colored surrounding, and a receptive layer can be subsequently applied according to the present disclosure in a manner that may either at least partially overlap with the background image or in a separate non-overlapping region of the substrate, later attaching the embellishment particles accordingly.

The term “thermoplastic particles” is used to refer to all particles comprising a thermoplastic polymer, whether coating the particle or forming substantially all of the particle, including any intermediate range of presence of the polymer allowing the thermoplastic particles to serve their intended purposes. In the latter cases, wherein the thermoplastic polymer! s) can be homogeneously present in the entire particle, not being particularly restricted to an external coating, the particles ma also be said to be made of a thermoplastic polymer. The polymer, or the thermoplastic particles, need to be compatible with the EM or heat conduction imaging device.

For instance, if a laser is used to emit light of a particular wavelength then the thermoplastic material or particles formed therewith should be able to convert the incoming radiation into thermal energy. If necessary, agents able to achieve or facilitate such conversion may be included in the thermoplastic particles. Non-limiting examples of radiation absorbing agents include dyes, fillers, organic or inorganic pigments which can also be colourless. Alternatively, or additionally, the radiatio absorbing agents may be present in the imaging surface, as further detailed herein-below.

Thermoplastic polymers are plastic materials formed of repeating units (monomers), the polymer chains associating with one another through intermolecular forces which weaken with increased temperature. Above their glass transition temperature (Tg), the thermoplastic polymers become sufficiently softened and flexible to be shaped, by a variety of processing techniques, solidifying upon cooling. Depending on their morphology below their respective Tg, thermoplastic polymers are classified into amorphous, semi-amorphous (or semi crystalline) and crystalline plastics. Amorphous and crystalline thermoplastic polymers, having typically less than 30% or more than 70% crystalline components respectively, are believed to typically have shorter open- time than semi-amorphous thermoplastic polymers. The degree of amorphism/crystallinity of a specific thermoplastic polymer may depend on its chemical family, the degree of branching, the extent of cross-linking, the number and type of monomers present (affecting also the average molecular weight of the polymer, whether it is a homopolymer or a copolymer, its affinity towards other constituents of the system, and like factors readily appreciated by a person skilled in polymer chemistry).

For copolymers, the ratio between monomers of different chemical families and/or their distribution along the polymer chain (random or block copolymers) may also play a role on the properties of the polymers formed therefrom, including inter alia on their open-time such as within a printing system and method as herein disclosed. Furthermore, the monomers can have functionalized moieties also affecting the open-time of the polymer or copolymer functionalized therewith. Moreover, as thermoplastic polymers or copolymers can each display a variety of open-times, a particular value can be obtained by mixing two or more thermoplastic materials in respective amounts allowing the tailoring of a desired open-time.

Thermoplastic polymers can, for instance, be selected from polyacrylate compounds (PAN), polyamides (PA), polycarbonates (PC), polyesters (e.g., PET), polyethylenes (PE), polypropylenes (PP), polystyrenes (PS), polyurethanes (PUR), and polyvinyl chlorides (PVC), to name a few. Copolymers based on such chemistry may also provide suitable thermoplastic polymers. Functional groups that may be used to modulate the afore-said polymers and/or copolymers (whether random or block), and which in turn can modify the open-time of the thermoplastic particles, include amine groups, epoxy groups, acidic groups, such as carboxylic groups or acrylic groups, hydroxyl groups and salts.

Particles can be formed from aforesaid suitable thermoplastic polymers by any appropriate method known to the skilled person. For instance, the thermoplastic polymer particles can be prepared by a first plastic compounding step (e.g., by mixing, kneading, extruding, and like procedures, typically under elevated temperatures suitably softening the polymer) and a second size reduction step (e.g. , by milling, attrition, sonication, shear mixing, micro-emulsification, etc.). For simplicity, the thermoplastic particles that can be used in a receptive image forming section can also be termed first particles, while the embellishment particles that can be used at an embellishment section can also be referred to as second particles. When second particles also include thermoplastic polymers, they need not be the same as the first particles (e.g. , having a different composition, shape, size, color, etc.).

The above-listed exemplary methods may generate particles of various sizes and shapes. The first particles can have approximately a globular / spherical shape, but can also have a flake-like platelet shape or any intermediate non-spherical form. In other words, the dimensionless aspect ratio between the smallest dimension of the particle and its longest dimension in the largest plane orthogonal to the smallest dimension can vary from approximately 1 : 1 for particles having an almost spherical shape, to at least 1 :5 (e.g., bean shaped particles) or at least 1 : 10 for non-spherical shapes (e.g. , rod-like particles), some flake like particles having an aspect ratio of at least 1 : 15, of at least 1 :20, at least 1:40, at least 1:60, or even of at least 1: 100.

In a particular embodiment the first particles have approximately a globular / spherical shape with a dimensionless aspect ratio between a smallest dimension and a longest orthogonal dimension not exceeding 1 : 10, being typically no more than 1 :5, 1 :4, or 1 :3, near spherical particles having an aspect ratio of less than 1 :2, less than 1: 1.5, and approximately 1 : 1. Depending on the shape of the particles, the characterizing dimensions of a particle can be at least one of the longest dimension, the smallest dimension and the diameter, such dimensions being typically provided as average values for a population of representing particles.

The average longest dimension or diameter of the first particles (i.e. made of thermoplastic polymers) generally does not exceed 10 micrometers (pm), being of at most 5pm, at most 4pm, at most 3pm, or at most 2pm. In some embodiments, the average longest dimension or diameter of the thermoplastic particles does not exceed l,500nm, being of at most l,000nm, or at most 750nm. The average smallest dimension or diameter of the first particles is typically of at least lOOnm, 200nm, or 300nm. In particular embodiments, the average diameter of the first particles is between about lOOnm and about 4pm, or between about 300nm and about 2pm, or between about 500nm and about l,500nm.

The average size of particles can be assessed by any known technique, such as microscopy or Dynamic Light scattering (DLS), the latter being particularly suitable for particles having a near spherical shape. In one embodiment, the size of the particles is assessed on a sample of the population of particles suspended in a suitable liquid (e.g., water optionally supplemented with a dispersant), in which case the average diameter of the particles is estimated by Dv50 (maximum particle hydrodynamic diameter below which 50% of the sample volume exists) as measured by DLS, DylO and Dv90 providing the range within which a predominant portion of the population of particles exists. In a particular embodiment, the average size of the first particles (or their average diameter when referring to spherical ones) is relatively uniform. Such relative size uniformity is believed to increase the correlation between a particular level of particles’ irradiation and the outcome of resulting thermal transformation on similarly sized particles, facilitating, in other words, the reproducibility of the printing system. Size uniformity is however not essential as some variations may assist in achieving a better packing of the particles on the imaging surface, smaller ones being able to fill voids in between larger ones, hence resulting in an increased coverage of the transfer member.

In some embodiments, the polymer film resulting from the conversion of the monolayer of particles by exposure to radiation (e.g. , the receptive image formed on the printing substrate) has a thickness of 5pm or less, 2pm or less, or ofless than lpm, or of less than 800nm. In some embodiments, the thickness of the polymer film is of lOOnm or more, or of more than 200nm, or of more than 300nm, or of more than 400nm, or of more than 500nm. The thickness of the receptive image may be in the range of 500nm-5pm, or 200nm-l,200nm, or of 300nm-l,000nm, or of 400nm-800nm.

Depending on the substrate and/or the desired effect and/or the embellishing particles to be subsequently applied, the receptive layer can be relatively thin (e.g., within the range of 3 OOnm- 1 ,000nm) or relatively thick (e. g. , within the range of 1 pm-5 pm) . Thin receptive images are suitable when the printing substrate is smooth and/or the printing effect includes following the roughness of the printing substrate and/or when the embellishing particles may form a relatively large contact area with the receptive layer (e.g., having a flake shape). Taking for illustrative example, an embellishment particle being a metallic flake, in order to enable burnishing of the transferred flakes, it is preferred that the receptive image be thinner than the flake thickness allowing it to“float” on the receptive layer exposing its upward surface to burnishing.

Thick receptive images are suitable when the printing substrate is rough and the printing effect includes masking such roughness (“smoothening” the substrate surface with the receptive layer) or when the embellishing particles would otherwise form an insufficient contact area with the receptive layer. Taking for illustrative example, an embellishment particle having a spherical shape, in order to facilitate anchoring of the particle on and partly within the receptive image, this layer could be at least as thick as the radius of the second particle. If the embellishment particles are to be embedded in the receptive image, its thickness can be tailored according to the size of the particles to be embedded.

If effects other than visual (e.g. , following the later application of embellishment particles) are to be achieved, the thermoplastic particles may further contain agents selected and adapted to provide the desired additional effect(s). By way of non-limiting example, the thermoplastic particles may contain agents providing for a desired thermal and/or electrical conductivity. Carbon black particles are an illustrative example for such agents.

Embellishment particles

Embellishment particles (second particles) which may be applied at an embellishment section on a receptive image previously formed by thermoplastic particles transferred to the printing substrate can be of any material suitable to achieve the desired printing effect. If the effect to be achieved is similar to foil imaging, such as used for instance for metal printing, then the particles may be grains or flakes of metals, such as aluminum, copper, iron, zinc, nickel, tin, titanium, gold or silver, or alloys, such as steel, bronze or brass, and like metallic compounds primarily including metals. In addition to being made of real metals or alloys, suitable particles can be made of compounds providing for a similar visual effect (e.g. , made of a polymeric or 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. By way of example, particles manufactured using the PVD (physical vapor deposition) method, wherein a polymer foil is vapor coated in vacuum with the metal of interest (including chrome, magnesium and the above-mentioned exemplary metals) and thereafter crushed to form individual flakes, may form metal-like particles if the polymer backbone is retained and can be deemed“metallic” if the polymer is eliminated following the deposition process.

If the effect to be achieved includes a glittering and/or a pearlescent and/or a nacreous effect, synthetic high polymers (including for example multi-layered structures of polyacrylates), magnesium fluoride, muscovite, aragonite, rutile or anatase titanium dioxide, mica compounds (typically coated with metal oxides) and the like can be used for the particles. All of the foregoing exemplary particles, including the genuinely metallic particles though collectively termed for simplicity“metal -looking” or“metal -like” particles (i.e., providing a visual effect similar to a metallic compound), may be coated or uncoated.

The coating of such metal-like embellishment particles, which can be applied by physical but more typically chemical means, can, among other things, reduce or prevent the particles sticking to one another (e.g., as achievable with anti -caking agents and the like), increase the repulsion between the particles (e.g., as achievable by increasing the charge of the particles), protect the particles from undesired chemical modification (e.g., reduce, prevent or delay the oxidation of metals and alloys or any other deleterious aging of the metal-looking particles) or further increase the affinity of the particles to the donor surface or to the selected regions of the substrate (e.g., to a receptive layer disposed thereon), as desired (e.g., modify the hydrophobicity of the coats/surfaces).

Embellishment particles suitable for a printing system and method according to the present teachings may for example be coated by one or more of i) an unmodified or modified carboxylic acid or fatty acid, the carboxylic acid selected from the group comprising, but not limited to, stearic acid, palmitic acid, behenic acid, benzoic acid, and oleic acid; ii) an oily substance selected from the group comprising, but not limited to, vegetal oils, such as linseed oil, sunflower oil, palm oil, soya oil, and coconut oil; mineral oils and synthetic oils; and iii) an oxide which may be of same or different material as the core particle being coated. For instance, aluminum particles may be coated with an aluminum oxide or a silicon dioxide, and mica particles may be coated with titanium dioxide and iron oxide, for example. The particle coating may optionally modify the coloring effect of the core particle, this can be achieved for instance with some metal oxides or with pigmented polymers (e.g., a polyacrylate containing inorganic or organic absorption pigments). Such coloring effect can also result from the choice of the core particle, or from a partial oxidation of the same.

For non-metallic appearance, the embellishment particles can be pigments, or particles made of thermoset or thermoplastic polymers containing a desired coloring agent or made of minerals, ceramics, glasses or any other solid materials providing a visually detectable effect.

Whether made of coloring agents, colored polymers, colored minerals, colored ceramics, colored glasses or metal-looking materials, the particles may provide, once transferred to the printing substrate, for a glossy or matte image, and for any other type of desired effect in accordance with the selected second particles. Depending on the shape of the second particles, the thickness of the receptive image may also contribute to the printing effect. Depending of the materials being considered, numerous methods may be used to produce embellishment particles of various sizes and shapes. The second particles can have approximately a globular / spherical shape, but can also have a flake-like platelet shape or any intermediate non-spherical form. In other words, the dimensionless aspect ratio between the smallest dimension of the particle and its longest dimension in the largest plane orthogonal to the smallest dimension can vary from approximately 1 : 1 for particles having an almost spherical shape, to at least 1 :5 (e.g., bean-shaped particles) or at least 1: 10 for non-spherical shapes (e.g., rod-like particles), some flake-like particles having an aspect ratio of at least 1: 15, of at least 1 :20, at least 1 :40, at least 1 :60, or even of at least 1 : 100.

When the embellishment particles are approximately spherical, their average diameter generally does not exceed lmm, their average diameter being of at most 200pm or at most lOOpm. The second particles typically have an average diameter of at most 40pm, at most lOpm, or at most 5pm. The average diameter of the second particles is typically of at least lOOnm, 250nm, or lpm. In particular embodiments, the average diameter of the embellishment particles is between about lOOnm and about 50pm, or between about 500nm and about lOpm, or between about lpm and about 5pm. While relatively large spherical embellishment particles could have a rather small contacting area with the receptive image insufficient for durable adhesion, this can be resolved by subsequently applying an over-coat at a suitable finishing station, provide that such contacting area is sufficient to retain the particles till over-coating.

When the embellishment particles have a flake-like shape, they can be characterized by their thickness and longest dimension in a plane orthogonal to the thickness of the flake. In some embodiments, the average thickness of these embellishment particles does not exceed l,200nm, the second particles typically having an average thickness of at most l,000nm, at most 800nm, at most 600nm, at most 500nm, at most 400nm, at most 350nm, at most 300nm, at most 250nm, at most 200nm, at most l75nm, at most l50nm, at most l25nm, or at most lOOnm. The average thickness of second particles having a flake-like shape is typically of at least lnm, at least 5nm, at least lOnm, at least l5nm, at least 20nm, at least 25nm, at least 30nm, at least 35nm, at least 40nm, or at least 50nm. In particular embodiments, the average thickness of the embellishment particles is between about lOnm and about 400nm, or between about 20nm and about l50nm, or between about 30nm and about lOOnm.

In some embodiments, the average longest dimension of the flake plane of such embellishment particles is of at most lmm, at most 800pm, at most 600pm, at most 400pm, at most 250pm, at most l50pm, at most lOOpm, at most 80pm, at most 60pm, at most 40pm, at most 25mih, at most 20pm, at most 15mih, at most 12mm, at most IOmih, at most 8mih, at most 6mm, at most 4mih, at most 3(mi, at most 2mih, at most 1.5mih, at most 1.2mm, at most I .Omih, at most 0.8mm, at most 0.7mih, at most 0.65mih, or at most 0.6mm. The average longest dimension of flake-shaped second particles is typically of at least 40nm, at least 50nm, at least 60nm, at least 80nm, at least lOOnm, at least l20nm, at least l50nm, or at least 200nm. In particular embodiments, the average longest dimension of the flake-like embellishment particles is between about lOOnm and about 250pm, or between about 500nm and about lOOpm, or between about lpm and about 10 pm.

The above value regarding flakes are relevant if they are transferred via a transfer member. However, if the particles are sprayed blown then they may be larger. If this would result in a weak attachment to the receptive layer, then the substrate could be over-coated to gain the desired adhesion.

If effects other than visual are to be achieved, the second particles may alternatively or additionally be selected and adapted to provide the desired additional effect(s). By way of non limiting example, the second particles may contain agents or be made of a material providing for a desired thermal and/or electrical conductivity.

Imaging surface

The first transfer member 101, used in the receptive image forming section of the illustrated embodiment of the invention, can have the form of an endless belt (as illustrated) or of a drum. When rear side imaging is employed, it comprises a body transparent to electromagnetic (EM) radiation lying within a predetermined range of frequencies disposed on a rear side of the transfer member 101, a radiation absorbing elastomeric silicone layer opaque to the EM radiation adjoining the transparent body and disposed on or adjacent a front side of the transfer member, and a hydrophobic release layer formed on the front side of the transfer member, the release layer being in thermal contact with, or formed as part of, the radiation absorbing layer. The imaging surface (on the front side of the transfer member) is preferably continuous (e.g., the belt being a seamless belt or the drum being coated by a sleeve) and substantially uniform over its entire surface.

In some embodiments, the radiation absorbing layer may have an absorbance per pm of at least 0. l/pm, the absorption being measured at a wavelength of said EM radiation or within a proximal range thereof. In some embodiments, the absorption may be at least 0.2/pm, or at least 0.3/pm. The imaging surface should be compatible with the radiant energy intermittently applied by the imaging station to heat desired selected areas By compatible, it is meant for instance, that the imaging surface is relatively resilient and/or inert to the radiation at the irradiated frequency/wavelength range, for example, the transfer member and the imaging surface maintain mechanical characteristics such as strength and flexibility under such radiation. Also, the imaging surface may be able to prevent or minimize heat loss to the transfer member and promote effective heating to the thermoplastic particles. Additionally or alternatively, the imaging surface may be able to conduct heat that is generated by the radiation, such conduction being advantageously restricted to the thin layer adjoining the imaging surface

Particular transfer members and imaging surfaces that may serve in thermal transfer printing systems and methods as disclosed herein are further detailed in WO 2018/100528 and WO 2018/100541 , to the same Applicant.

In some embodiments the imaging surface is a hydrophobic surface, made typicall of an elastomer that can be tailored to have properties as herein disclosed, generally prepared from a silicone-based (release-prone) material . The hydrophobicity of the imaging surface enables the tacky film created by exposing the thermoplastic particles to radiation to transfer cleanly to the substrate without splitting. The silicone-based matrix may have any thickness and/or hardness suitable to bond the intended thermoplastic particles. The suitable hardness is to provide a strong bond to the particles when they are applied to the imaging surface of transfer member 101 in the coating station 102, the bond being stronger than the tendency'^· of the particles to adhere to one another. It is believed that for relatively thin imaging surfaces (e.g., 100 mhi or less), the silicone-based material may have a medium to low hardness; whereas for relatively thick imaging surfaces (e.g., up to about 1 mm), the silicone-based material may have a relatively'· high hardness. In some embodiments, a relatively high hardness between about 60 Shore A and about 80 Shore A is suitable for the imaging 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 imaging surface has a hardness of about 30-40 Shore A, a lower hardness believed to be preferable for spherical particles. The hardness is of at least 5 Shore A.

Advantageously, an imaging surface suitable for use with a printing system herein disclosed can be flexible enough to be mounted on a drum, appropriately extendible or inextensible if to be mounted as a belt, have sufficient abrasion resistance and/or resilience, be inert to the particles and/or fluids being employed, and/or be resistant to any operating condition of relevance (e.g. , irradiation, pressure, heat, tension, and the like). The imaging surface can absorb EM radiation at the wavelength of the iaser emitting elements. For instance, if the radiation is emitted in any portion of the near infrared (NIR) range within about 800-2, OOOnm, then the imaging surface should absorb over at least such portion of the NIR spectrum. In this way, the heating up of the imaging surface assists in the softening of the particles disposed thereupon, sufficient heating rendering the particles suitably tacky so as to transfer to a printing substrate.

Advantageously, the EM radiation absorbing material is such that it may absorb over a relatively wide range of laser wavelengths, compatible with different types of particles, each eventually having a different sub-range, even minute ones, of laser absorbance. Carbon black (CB), which has a broad absorption and is a strong absorber in the NIR region, can be used to provide desired corresponding properties to the energy absorbing layer of the imaging surface. Incorporation of carbon black into silicone -based layers may also contribute to the thermal conductivity of the imaging surface and allow it to be modulated, if and as desired. Silicone- based elastomers comprising CB particles and methods of preparing the same are detailed in the following sections.

The transfer member, whether formed as a sleeve over a drum or a belt over guide rollers, may comprise in addition to the imaging surface, on the side opposite the release layer, a body. As the transfer member in the present illustration of the disclosure is irradiated from its rear side (i.e. the side opposite that carrying the monolayer of thermoplastic polymer particles), the body needs to be transparent to the radiation, so that the radiation may reach the energy absorbing layer which is next to, or which incorporates the imaging surface.

The body of the transfer member may comprise different layers each providing to the overall transfer member one or more desired property selected, for instance, from mechanical resistance, thermal conductivity, compressibility (e.g., to improve “macroscopic” contact between the imaging surface and the impression cylinder), conformability (e.g., to improve “microscopic” contact between the imaging surface and the printing substrate on the impression cylinder) and any such characteristic readily understood by persons ski fled in the art of printing transfer members.

An imaging surface may be made of any material capable of providing sufficient adhesion to native (non-tacky) particles and enough release to particles softened by irradiation to selectively transfer. High release elastomers provide a variety of suitable candidates, including but not limited to liquid silicone resins (LSR), room temperature vulcanization (RTV) silicones, polydialkyl siloxanes (PDAS), including for instance polydimethyl siloxanes (PDMS) silicones, which can be, if needed, further functionalized by desired reactive groups (e.g. , amine groups, vinyl groups, silane or silanol groups, alkoxy groups, amide groups, acrylate groups etc., and combinations thereof, as known in the art of silicones) to produce functionalized silicones. Such silicon-based materials can be cured by any suitable curing method in presence of appropriate curing agents, if necessary. The imaging surface may comprise two distinct layers, the outermost release layer and an underneath radiation absorbing layer, but in some embodiments the release layer is simply the outermost surface of the radiation absorbing layer. The release layer may have, in some embodiments, a thickness of 3pm or less, of 2pm or less, or between 0.5pm and l .5pm. A radiation absorbing layer can have, in some embodiments, a thickness of 25 pm or less, or between 200nm and lpm, or between 500nm and 2pm, or between 2pm and 20pm, or between 2pm and lOpm. The radiation absorbing layer can be made of the same silicon-based elastomers as described for the release layer, such elastomers being supplemented with a material suitably absorbing radiation, allowing the heat generated by the application of radiation by the imaging device to dissipate rapidly enough for the heating of the thermoplastic particles to be time and/or spot specific (e.g., enabling the formation of a desired pixel).

Carbon black (CB) particles are preferred as radiation absorbing material for transfer members suitable for receptive image forming sections according to the present disclosure.

EXAMPLES

Example 1 : Thermoplastic particles having a relatively short open-time - First manufacturing method

In order to facilitate the visualization of transfer of the particles selectively exposed to radiation from an imaging surface to a printing substrate, pigment was incorporated in the thermoplastic polymer. One weight percent (wt.%) of cyan pigment (Heliogen^® Blue D 7079, BASF) was gradually added to 99wt.% of an amorphous linear polyester-polyol copolymer with primary hydroxyl functionality and medium molecular weight (Dynacoll^® 7150, Evonik), this thermoplastic polymer being known for the preparation of short open-time adhesives. The polymer was poured into a spinning tree-roll mill compounding machine (Model JRS230, manufactured by Changzhou Fongxin Machinery Co. Ftd., China), and operated for about 5 minutes, at a circulating oil temperature of l60°C sufficient to melt the thermoplastic polymer. The pigment was then gradually added to the melted polymer and the compounding process was continued for four additional cycles at same temperature, until the pigment was well dispersed within the homogeneous polymeric mass. The temperature was controlled by circulation of heated oil within the kneading rolls, oil heated to have a temperature of l60°C typically yielded a temperature lower by 20-30°C on the surface of the rolls (i.e.. l30-l40°C). The speed of the kneading rolls was adapted to the viscosity of the paste as the compounding process proceeded, the speed typically not exceeding 100-500 rpm.

The polymer mass (compounded and colored with the pigment) was then allowed to cool down till room temperature (RT; about 23°C). The cooled mass was then grinded in a coffee- bean grinder (KG40 from De'Longhi Appliances, Italy), until a powder of thermoplastic particles having an approximate longest dimension of up to a few millimeters was obtained. The fine powder was then fed at a steady rate to a jet mill (Alpine 100 AS, Hosokawa Micron Powder Systems, Japan) at a feed air pressure of 5 atm and a circulating air pressure of 4 atm. The comminuted particles were collected not from the main outlet, but from the exhaust outlet using a filter sleeve made of a woven fabric having a pore size of about lpm. Particle size was determined by suspending a small amount of the particles so obtained in an aqueous solution including about 0. lwt.% of either Triton^® X-100 or Triton^® BG-10 (Dow Chemical Company). The aqueous sample was subjected to DLS analysis (Malvern Mastersizer AW A 2003, Malvern Instruments Ltd., United Kingdom) and the average particle size Dv50 was found to be of about 2.7pm, with a Dv 10 of about l .6pm and a Dy90 of about 4.5pm, the particles having an irregular shape as confirmed by light microscope at a magnification of X20.

Example 2: Thermoplastic particles having a relatively long open-time - Second manufacturing method

In order to facilitate the visualization of transfer of the particles selectively exposed to radiation from an imaging surface to a printing substrate, pigment was incorporated in the thermoplastic polymers. One weight percent of cyan pigment (Heliogen^® Blue D 7079, BASF) was mixed with a blend of thermoplastic polyester polymers combining 59wt.% Diacron FC- 1588 (an amorphous polyester resin by Mitsubishi Rayon) and 40wt.% Uvecoat^® 9010 (a semi crystalline unsaturated polyester resin by Allnex), this thermoplastic polymer blend being expected to enable a relatively long open-time hotmelt adhesive. The compounding was done in a stepwise manner as follows. First, the beads of Diacron FC-1588 were poured into a spinning tree-roll mill compounding machine, the circulating oil being at a temperature of l40°C (i.e.. the surface of the kneading rolls being at about H0-l20°C). Once this first thermoplastic polymer was melted, the pigment was gradually added and the compounding process was continued until a first homogeneous polymeric mass was obtained. Last, Uvecoat^® 9010 was added and the compounding process continued until a homogeneous final mass including the blend of melted polymers was obtained. The entire process took about 4 compounding cycles at a kneading speed not exceeding 100-500 rpm.

While a polymeric mass providing for the preparation of particles having a relatively long open-time within the printing system of the disclosure could be comminuted as previously described, alternative methods more suited for the thermo-rheological properties of the polymeric materials can be preferred. Such methods are known to the person skilled in the art of size reducing thermoplastic polymers to particles, a non-limiting exemplary process being provided in the following

14g of Carbowax™ PEG 400 (a low-molecular-weight grade of polyethylene glycol, Dow Chemical Company, CAS No. 25322-68-3) and 0.6g of Tetronic^® 908 (a polyoxy- propylene-polyoxyethylene polymer surfactant, BASF, CAS No. 11111-34-5) were melted in pre-heated stainless double jacket vessel (heater set at 200°C). 6g of the previously compounded blend of polymeric materials was then added to the mix and the resulting melted mixture was sheared using an IKA mixer (S 50 N - G 45 F Dispersing element) at 5,000-10,000 rpm. The size of the micro-emulsified particles resulting from the shearing process was monitored over time by optical microscope (Olympus BX51). Once the particle size reached the target size (e.g., a Dv50 of about I pm), the heater was shut-down and the sheared mixture was allowed to cool down to RT under continuing low shear (500 rpm). Deionized water was then added in excess to the sheared mixture at RT to form an aqueous dispersion of the sheared particles, so as to obtain a solid content of about 10wt.%. The dispersion was then centrifuged (Hermle Z383, rotor 22G.30 Vo2, Hermle LaborTechnik, Germany) for 25 minutes at 6,000 rpm. The supernatant as then removed, fresh water w'as added to the sediment so as to resuspend it using an ultrasonic tip (disintegrator Misonix sonicator S-4000-010, Misonix, USA) at 10-20% amplitude for 10-2Qsec, and the newly dispersed particles were again centrifuged as previously described. This procedure was repeated for a total of seven cycles, following which the product of the final centrifugation and resuspension cycle was frozen using liquid nitrogen. The frozen dispersion of particles was then freeze-dried in a lyophilizer (Labconco freezone 2 5, Labconco, USA) at a vacuu of O.OOBmbar and a temperature of -85°C and the ready-to-use particles were collected.

Particle size was determined by suspending a small amount of the particles so obtained in deionized water, the aqueous sample being then subjected to DLS analysis (in Malvern Zetasizer Nano S) under constant circulation to maintain a measurable dispersion. The average particle size Dv50 was found to be of about 866nm, with a DvlO of about 300nm and a Dv90 of about l2l0nm, the particles having a spherical shape as confirmed by light microscope at a magnification of X20.

Example 3 : Imaging Surface for Thermoplastic Particles

An imaging surface suitable for a first station of a printing system or method according to the present teachings was prepared as follows. A powder of Carbon Black (CB; Colour Black FW 182, Orion Engineered Carbons, CAS No. 1333-86-4, 20wt.% volatile matter, pH 2.5, 550 m²/g BET Surface) having a primary particle size (PPS) of l5nm and a secondary particle size (SPS) distribution displaying a DvlO of about 2.9pm, a Dv50 of about 4.5pm, and a Dv90 of about 6. lpm, as measured by DLS (Malvern Zetasizer Nano S) was dried for at least 2 hrs at l20°C. 50g of the dried CB powder were mixed with 50g of amino-silicone dispersant (BYK LPX 21879, having an amine number of about 36, BYK Additives & Instruments) in 200g of xylene AR (having a boiling point of about l38.4°C, CAS No. 1330-20-7, Bio-Lab Ltd.). The dispersion was carried out in an attritor bead mill (Attritor HD-01, Union Process^®) with stainless steel beads of about 4.76 mm (SS 302 3/16 inch beads, Glen Mills Inc.) at 700 rpm until the CB particles reached an average SPS ( e.g . , as assessed by Dv50) of less than lOOnm, generally of about 70nm, which typically required about 1.5-2.5 hrs, depending on the batch size. The size reduction was performed under controlled temperature of 50°C. The size distribution was then assessed by DLS (Malvern Zetasizer Nano S) on a sample comprising about 0. lwt.% of CB and the CB particles co-milled with the dispersant were found to be predominantly in the nano-range (having a DvlO of about 48nm, a Dv50 of about 74nm, and a Dv90 of about l39nm).

The CB dispersion was added to a two-part LSR silicone fluid, so as yield l5wt.% CB per weight of the final matrix (i.e. excluding the volatile solvent). The according weight of CB dispersions was added to 20g of Silopren^® LSR 2540 (Part A), gently hand mixed, then poured into 20g of Silopren^® LSR 2540 (Part B), by Momentive Performance Materials Inc. It is noted that adding the CB materials to a pre-mix of Part A and Part B of the LSR was also found to be satisfactory. The resulting CB silicone fluid was further mixed for about three minutes in a planetary centrifugal mixer (Thinky ARE-250, Thinky Corporation) operated at 2,000 rpm at ambient temperature and allowed to defoam under sole same centrifugal conditions for another three minutes. A sample was cured at l40°C for about 2 hrs. The pattern of dispersion of the CB particles in the silicone matrix was assessed by light microscopy (Olympus^® BX61 U- LH100-3) and found stable over the curing period of the LSR components.

To facilitate the application of the afore-mentioned CB dispersed LSR silicone fluid, the stock was diluted in excess volatile solvent, xylene in the present case, typically at a weight per weight ratio of at least 1 :4, for instance at 1 :9 wt./wt. The CB particles in the diluted silicone matrix appeared to remain stably dispersed for a period of time corresponding at least to the duration of casting, as assessed by light microscopy.

The diluted CB - LSR - xylene suspension was applied to a smooth releasable support (e.g., non-treated PET sheet) by spray coating using an air pressure brush. Alternative application methods are possible (e.g., rod coating and the like for flat supports or spin casting for sleeve-like ones). While partial curing of the silicone matrix may proceed at relatively low temperature of l00-l20°C (taking at most 2 hrs, but generally about 0.5-1 hr, depending on layer thickness), such step can be accelerated by raising the temperature (e.g. , reducing curing duration to about 20 minutes if cured at l40°C). A clear silicone layer (due to serve as a conformational layer if relatively thin and additionally as support if sufficiently thick) was then cast on top of such a partially cured radiation absorbing layer / imaging layer. One such silicone overcoat was a two-component clear liquid silicone, QSil 213, commercially available from Quantum Silicones. The resulting PET-supported layers were further partially cured at about l00°C for approximately 1-2 hrs. The PET support was then peeled away and the two layers inverted so as to have the CB-loaded radiation absorbing layer facing up or outward and the clear conformational layer optionally serving as support facing down or inward if mounted on a cylinder. The latter layer if relatively thin (e.g., 200-600pm) can be attached to any desired support (e.g., a transparent support, such as PET) by any suitable method (e.g., using a compatible thermal silicone adhesive) so as to form a thick enough combined body (e.g., at least 750pm, at least l,000pm, or at least 1, 500pm). Alternatively, and in particular for the sake of present examples, the conformational layer can be thick enough to provide by itself sufficient mechanical support to the radiation absorbing layer.

The above-detailed procedure allows the preparation of a silicone matrix having a very smooth release surface and a relatively high load of carbon black particles (l5wt.%), such particles having the advantage of being in the sub-micron range and even predominantly in the nano-range. An imaging surface having a radiation absorbing layer with a thickness of 2-3pm, and a conformational layer of 1 -2,000pm, thus also serving as support was prepared by such planar coating and spin-casting methods.

Example 4: Transfer Test - First Screening

Thermoplastic polymers and blends thereof suitable for the preparation of particles which can be transferred from an imaging surface to a printing substrate in a printing system or method according to the present teachings were evaluated as follows. Thermoplastic particles having an average particle size (Dv50) of no more than 5 pm and generally no more than 3 pm were prepared by any suitable method typically involving a first plastic compounding step (e.g. , by mixing, kneading, extruding, etc.) and a second size reduction step (e.g. , by milling, attrition, sonication, shear mixing, micro-emulsification etc.). Two non-limiting exemplary methods are provided in Examples 1 and 2.

A monolayer of the polymer particles under study was gently applied on the release surface of the transfer member with a soft brush. A non-limiting example of an imaging surface is provided in Example 3. The formation of a monolayer was confirmed by microscope analysis (confocal laser scanning microscope; LEXT OLS4000 3D of Olympus Corporation). When satisfactorily applied, the thickness of a cross section of the coated sleeve corresponded to the thickness of the bare sleeve and approximately the average size of the particles (and typically no more than 2-fold the Dv50 of the applied particles). The image surface segment coated with the monolayer of particles under study was then placed on a hot plate (Fried Electric Ltd., Israel) and heated so as to obtain on the outer release surface of the transfer member any desired temperature. This elevated temperature was generally selected within the range of 60-l50°C depending on the chemical identity of the thermoplastic polymer and on the heat sensitivity of the printing substrate. A printing substrate of interest was then placed on top of the heated particles, the substrates including a sample of coated paper, uncoated paper, and plastic foil (untreated PET sheet having a thickness of 250pm, PGCL250, Jolybar, Israel). Pressure was then applied on the substrate with a silicone rubber coated roller, to ensure intimate contact with the heated particles. The sample assembly of imaging surface/particles/substrate was then removed from the heating source and allowed to cool down to RT.

The printing substrate was peeled away from the imaging surface release layer and the behavior of the intermediate layer resulting from the melting of the thermoplastic particles was visually monitored. Particles of thermoplastic polymers compatible with the operating conditions of the experimental set-up (e.g. , with the printing substrate, and/or at the temperature tested, and/or under the transfer pressure applied, etc.) essentially fully transferred from the imaging surface to the printing substrate. Such suitable particles are reported as positive (+) in the following table. Unsuitable particles and/or experimental conditions resulted in any of the following outcomes: a) no transfer, the layer of particles substantially remaining on the imaging surface; b) partial transfer, part(s) of the layer of particles remaining on the imaging surface, while other part(s) transferred to the substrate in non-overlapping / complementing areas; c) split transfer, part of the layer of particles remaining on the imaging surface, while another part of a same overlapping area transferred to the substrate; and d) combination of partial and split transfer. Such unsuitable particles or conditions are reported as negative (-) in the following table, independently of the type of failure displayed.

Polymer particles found suitable in the above-described test“analogous” to a first station at which an adhesive receptive layer can be digitally applied to a substrate in a printing system as herein disclosed, were subjected to a second line of screening as follows. In this series of experiments, conditions mimicking a second station at which embellishment particles can be applied to an adhesive receptive layer were reproduced. Embellishment particles of interest can be applied on any suitable support. For instance, aluminum flakes (6150, Quanzhou Manfong Metal Powder Co., China) were gently rubbed on a sheet of silicone rubber (e.g., having a polydimethylsiloxane donor surface, such as prepared in Example 6) and excess of particles was washed away. While the aluminum flakes applied by such method were found to form a monolayer on the silicone rubber, this is not essential to the experimental set-up. A segment of silicone rubber sheet coated with the embellishment particles under study was placed on a hot plate to be heated to any desired temperature within the range of 60-150°C, this second temperature being generally the same or less than the first temperature applied in previous transfer testing. The printing substrate coated with the adhesive layer obtained in previous transfer testing was then placed on top of the embellishment particles, the adhesive layer facing the aluminum flakes. Pressure was applied on the printing substrate and the sample assembly was removed from the heating source and cooled down to RT, as above-described. The printing substrate was peeled away from the silicone rubber supporting the aluminum flakes and the transfer of these embellishment particles from their initial support to the adhesive layer on the printing substrate was visually monitored according to the same principles previously detailed. Briefly, an adhesive layer made of particles of thermoplastic polymers compatible with the operating conditions of the experimental set-up (e.g., with the printing substrate, and/or at the temperature tested, and/or under the transfer pressure applied, etc.) essentially fully detached the tested embellishment particles from their support, allowing their transfer to the printing substrate. In other words, suitable thermoplastic and embellishment particles, under appropriate experimental conditions, provide for a printing substrate coated with an adhesive layer, which in turns is coated (or at least partially embedded) with the embellishment particles.

The particles having a relatively^‘‘short open-time” (SOT), of Example 1 , and the particles having a relatively“long open-time” (LOT), of Example 2, were tested for transfer and/or embellishment under the following conditions, the results being reported in the below table.

Table 1

The thickness of the film of polymeric particles transferred to the printing substrate (before the application of embellishment particles) was measured by confocal laser microscopy. It was typically found to be of about 400nm to 800nm. As such“transferred thickness” generally corresponds to no less than half the Dv50 value of the applied population of thermoplastic particles, it can be deduced that the particles remaining attached to the imaging surface following their gentle brushing thereon represent only a subgroup of the originally produced population of particles in other words, the way particles are manually applied on the imaging surface in this experimental set-up not only forms a monolayer of particles but also classifies them by size, keeping only a portion of the original distribution (the extreme sizes and largest particles being eliminated). For reference, the original population of SOT particles had a Dv50 of about 2.7pm and the original population of LOT particles had a Dy5Q of about 0.9pm.

Example 5: Printing Jig

While Example 4 provides a preliminary screening for materials that can be suitable in the system or method herein disclosed, an experimental set-up more similar to an actual printing environment is believed to be more predictive. The present example provides such a printing jig which can be used for the final selection and/or classification of materials suitable for any set of desired operating conditions. Advantageously, it allows assessing the open-time afforded by particles under tested conditions in a time frame of iess than a second, as opposed to previous preliminary screening lacking such resolution. The printing jig includes two cylinders: a) an imaging cylinder upon which an imaging surface or sleeve can be mounted, the imaging cylinder having a radius R; and b) an impression cylinder upon which a printing substrate can be mounted, the substrate bearing cylinder having a radius r. The cylinders contact one another at a nip a where particles deposited on the release layer of the imaging surface suitably selectively heated can transfer to a printing substrate conveyed by the substrate impression cylinder. In the printing jig of the present example J?==35mm and r 30mm. Pressure ranging from 0.5 kg/crrf to 30 kg/cm² can be applied on the cylinders to facilitate transfer. Both cylinders have on their respective outer surfaces a linear velocity of v (which can be set to assume any value in the range of Q 2<v<10m/sec).

The thermoplastic particles can be selectively softened by heat directly targeting the particles (e.g., the particles including a radiation absorbing agent capable of converting EM waves into thermal energy). They can alternatively, or additionally, be indirectly heated by their underlying support (e.g., the EM vraves heating the imaging surface or a layer thereof, the thermal energy reaching the release layer softening the particles disposed thereon). A laser emitting element (and its associated optical system) are able to generate such thermal energy in a focused and time selective manner. A laser emitting element (and its associated optical system) can be positioned within the imaging cylinder, so as to target the rear side of the imaging surface and particles disposed thereon substantially at the nip (point a). This configuration is adapted to test materials having an extremely brief open-time, that may be referred to as having a“zero” open-time, even if such period is not null.

Alternatively, the laser emitting element can be positioned externally to the imaging cylinder, so as to target the outermost“front” side of the imaging surface and the particles disposed on the release layer thereof. Such external positioning allows targeting any location x which can be between points b and c, upstream of nip a and respectively closer and farther apart therefrom. Typically, when using the present printing jig with an external laser, the distance between the upstream point x targeted by the laser and the nip a is between a-A=20mm, for the closest point, and a-c= lOOmm, for the farthest laser targeted point.

Such a jig can be constructed with diverse laser emitting elements having a variety of powers and focus diameters depending on the elected optical systems, if any. In the present example, a multimode fiber-coupled diode laser (L4 manufactured by Lumentum) capable of outputing continuous wave (CW) radiation with a power of up to 10W at a mean wavelength of 975nm, with a fiber having a core diameter of 105mih and a numerical aperture (NA) equivalent to 0.15 NA, served as laser source. This fiber-coupled diode laser, pulsable up to lMHz, was used in combination with an imaging system formed of two aligned focusing lenses (027-0250, FL=60mm and 027-0220, FL=30mm, manufactured by Optosigma), giving at a working distance 30mm of the imaging cylinder a magnification of M=-0.5, resulting on the imaging surface in a laser spot size having a diameter of 50pm.

At initial time point (to= 0), the laser heats up the particles at a position x upstream of nip a and at t_x= x/v the heated particles reach the nip, where they are pressed against the substrate. If the particles remain tacky during the period lasting from their initial exposure to laser radiation till the nip, the particles will transfer to the substrate. In other words, the“open-time” of the particles being exposed to the laser emitting element during which they remain sufficiently tacky so as to transfer to the printing substrate is at least equal to t_x. At a predetermined speed v, by modifying the position of the laser from x=xl to x2. x3 and so on, all being upstream points in the range of b-c where the laser beam output can be physically located, it is possible to accordingly measure various periods of time during which the heated particles may remain tacky or not. This printing jig, with the previously detailed configuration, allows measuring open-time in the millisecond (ms) ranges from t=20mm/l0m/s=2msec to t= 100mm/0.2m/s=500msec .

This printing jig also enables testing the tackiness of the particles having transferred to the substrate, and forming a film thereon, towards pristine and unheated particles still covering the imaging cylinder. As the radius R of the imaging cylinder is greater than the radius r of the impression cylinder bearing the printing substrate, particles having selectively transferred to the substrate will complete a full cycle before the imaging cylinder does. Thus transferred particles will reach the nip over again in an area of the imaging cylinder still covered with original untransferred thermoplastic particles. If the film of transferred particles is still tacky (i.e. have an open-time greater than \x+2nr\!v). a second layer of particles will transfer to the substrate area already selectively covered by the previous first layer of tacky film. If the film of particles selectively transferred at the beginning of a first cycle is no longer tacky by the time it reaches the nip for a second cycle of the substrate cylinder, then the open-time is less than \x+2nr\!v.

As readily appreciated from the above descriptions, experiments scanning through series of values for parameters x (distance between the point exposed to radiation and the nip) and/or v (cylinders’ linear velocity) allow determining the open-time for selective transfer from imaging cylinder to printing substrate and the open-time for secondary transfer of pristine particles to the film of tacky particles already on the substrate.

Moreover, the second cycle testing the ability of the film of transferred thermoplastic particles to detach pristine ones mimics the situation occurring at an embellishment section in a printing system according to the present teachings, where the particles being attached to the tacky film are the embellishment particles.

Example 6: Donor Surface for Embellishment Particles

The type of donor surface suitable for printing systems as herein disclosed where embellishment particles can be transferred from a physical support to the receptive layer previously deposited on the printing substrate can vary, for instance as a function of the type of embellishment particles to be transiently retained thereon. Donor surfaces suitable for the retention and subsequent release of particles made of metals or of materials providing a similar metallic appearance have been described, inter alia in WO 2016/189515 to the same inventor.

Polydimethylsiloxane (PDMS) polymers, which are silicone-based and provide for a hydrophobic outer surface, have been found suitable for various types of embellishment particles providing for a metallic appearance. The preparation of an exemplary donor surface follows. A fluid curable PDMS composition was formulated by combining three silicone-based polymers: a vinyl-terminated polydimethylsiloxane 5000 cSt (DMS V35, Gelest^®, CAS No. 68083-19-2) in an amount of about 44.8wt.%, 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 l9.2wt.%, and a branched structure vinyl functional polydimethyl siloxane (VQM Resin-l46, Gelest^®, CAS No. 68584-83-8) in an amount of about 25.6wt.%. 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. lwt.%, an inhibitor to better control curing conditions, Inhibitor 600 of Evonik Hanse, in an amount of about 2.6wt.%, 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 7.7wt.%, which initiates the addition curing of the PDMS compositions. The composition was thoroughly mixed in a planetary centrifugal mixer following the addition of each ingredient (amounting in total to about 3 minutes at 2,000 rpm), then degassed for about 2 minutes.

This addition curable PDMS composition was shortly thereafter applied with a smooth leveling knife upon the body of the donor surface, so as to form a release layer having any desired thickness. In the present example, a typically multilayered body (e.g., as described in the afore-said PCT publication) was replaced by a sheet of PET and the thickness of the applied PDMS was of about 200pm. The PET sheet (lOOpm thick, untreated, PGCL100, Jolybar, Israel) was corona treated (BD-20 ACV Corona Treater, Electro Technic Products, USA ) for about two minutes, then coated with a priming substance to further the adherence of the donor surface material (the curable PDMS composition) to this support. The priming substance was prepared by mixing in 87.5g of methanol (anhydrous, CAS No. 67-56-1, Sigma-Aldrich), 2.5g of Dynasylan^® VTMO (vinyltrimethoxy-silane, CAS No. 2768-02-7, Evonik), 2.5g of propyl silicate (CAS No. 682-01-9, Colcoat), 5g of AKT855 (titanium diisopropoxide bis(2,4- pentanedionate, CAS No. 17927-72-9, Gelest^®) and 2.5g of SIP6831.2 (platinum - divinyl- tetramethyldisiloxane complex in xylene, CAS No. 68478-92-2, Gelest^®). The priming substance was applied with a soaked tissue on the corona treated PET sheet. Excess was gently wiped out and the priming layer was allowed to dry for up to 30 minutes at RT, following which the curable PDMS composition was applied. The fluid was cured for two hours at l00-l20°C in a ventilated oven so as to form a donor surface suitable for embellishment particles.

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 features, 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. At least one of A and B is intended to mean either A or B, and may mean, in some embodiments, A and B.

Positional or motional terms such as“upper”,“lower”,“right”,“left”,“bottom”,“below”, “lowered”,“low”,“top”,“above”,“elevated”,“high”,“vertical”,“horizontal”,“front”,“back”, “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”, “approximately” 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 terms“about” and“approximately” precede a numerical value, it is intended to indicate +/- 15%, or +/-l0%, or even only +/- 5%, and in some instances the precise value. Furthermore, unless otherwise stated, the terms (e.g., numbers) used in this disclosure, even without such adjectives, should be construed as having tolerances which may depart from the precise meaning of the relevant term but would enable the invention or the relevant portion thereof to operate and function as described, and as understood by a person skilled in the art.

To the extent necessary to understand or complete the disclosure of the present disclosure, all publications, patents, and patent applications mentioned herein, including in particular the applications of the Applicant, are expressly incorporated by reference in their entirety by reference as is fully set forth herein.

While, for the sake of illustration, 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 based upon Applicant’s disclosure herein. The present disclosure is to be understood as not limited by the specific embodiments described herein. It is intended to embrace all such alternatives, modifications and variations and to be bound only by the spirit and scope of the disclosure and any change which come within their meaning and range of equivalency.

Certain marks referenced herein may be common law or registered trademarks of third parties. Use of these marks is by way of example and shall not be construed as descriptive or limit the scope of this disclosure to material associated only with such marks.

Claims

1. A system for thermal transfer printing onto a printing substrate, which comprises:

a) a movable transfer member having an imaging surface on a front side;

c) an imaging station at which energy is applied to one or more selected regions of the particle coating on the imaging surface to heat thermoplastic particles within each selected region of the particle coating and render the thermoplastic particles within each selected region tacky; and

d) a transfer station at which the imaging surface and a surface of the substrate are pressed against one another to cause the thermoplastic particles that have been rendered tacky to transfer to the surface of the substrate, thereby forming a receptive image on the surface of the substrate,

characterized by:

e) an embellishment station disposed downstream of the transfer station to apply embellishment particles onto the surface of the substrate, and

f) a heating station disposed downstream of the transfer station for heating at least a part of the receptive image prior to, and/or during, application of the embellishment particles, in order to ensure tackiness of the at least part of the receptive image and cause the embellishment particles to adhere only to the at least part of the receptive image.

2. A system as claimed in claim 1, wherein the polymer of the thermoplastic polymer has such an open-time that the energy applied in step c) is insufficient to maintain the thermoplastic particles forming the receptive image tacky until the embellishment particles are applied thereto and wherein the heating station comprises at least one of:

(i) a section disposed upstream of the embellishment station, and

(ii) a section disposed within the embellishment station.

3. A system as claimed in claim 1 or claim 2, wherein in the embellishment station a gas stream carrying the embellishment particles is blown onto the surface of the substrate bearing the receptive image by means of a nozzle that is surrounded by a cowling connected to a suction source.

4. A system as claimed in claim 1 or claim 2, wherein the embellishment station comprises another transfer member, serving as a donor surface, another coating station for coating the donor surface with the embellishment particles and another transfer station at which the donor surface, or a portion of the donor surface, is brought into contact with the surface of the substrate bearing the receptive image.

5. A system as claimed in claim 4, wherein the other coating station applies the embellishment particles to the donor surface as a monolayer particle coating.

6. A system as claimed in any one of claim 1 to claim 5, wherein the heating station renders tacky a first part of the receptive image and the embellishment particles applied by the embellishment station adhere only to the first part of the receptive image, the system further comprising: (I) another embellishment station disposed downstream of the embellishment station to apply other embellishment particles different from the embellishment particles applied on the first part of the receptive image onto the surface of the substrate; and (II) another heating station disposed downstream of the embellishment station for heating a second part of the receptive image prior to, and/or during, application of the other embellishment particles at the other embellishment station, in order to ensure tackiness of the second part of the receptive image and cause the other embellishment particles to adhere only to the second part of the receptive image.

7. A system as claimed in any one of claim 1 to claim 6, further comprising at least one finishing station located downstream of the embellishment station.

8. A system as claimed in claim 7, wherein the at least one finishing station is selected from the group comprising a fusing station, a curing station, a levelling station, a burnishing station, a patterning station, a texturing station and an over-coating station.

9. A system as claimed in any one of claims 1 to claim 8, wherein the energy is applied by way of convection or thermal conduction.

10. A system as claimed in claim 9, wherein the energy is applied by way of a thermal print head in contact with a rear side of the movable transfer member.

11. A system as claimed in any one of claim 1 to claim 8, wherein the applied energy includes electromagnetic (EM) radiation generated by a laser beam emitting element or an array of individually addressable laser beam emitting elements.

12. A system as claimed in claim 11, wherein the laser beam emitting element or array of individually addressable laser beam emitting elements is a vertical-cavity surface- emitting laser (VCSEL) element or an array of VCSEL elements.

13. A system as claimed in claim 11 or claim 12, wherein the EM radiation is directed towards the thermoplastic particles via an optical system consisting of at least one optical lens.

14. A system as claimed in claim 13, wherein the optical system has a magnification of +1 or of -1.

15. A system as claimed in claim 13 or claim 14, wherein the at least one optical lens is a gradient index (GRIN) rod.

16. A system as claimed in any one of claim 1 to claim 15, wherein the heating station is operative to heat the receptive image, or a part thereof, by thermal conduction.

17. A system as claimed in claim 16, wherein the heating station is operative to heat the receptive image by coming into physical contact with the substrate.

18. A system as claimed in any one of claim 1 to claim 15, wherein the heating station is operative to heat the receptive image, or a part thereof, without coming into direct physical contact therewith.

19. A system as claimed in claim 18, wherein the heating station is operative to heat the receptive image, or a part thereof, by exposure to electromagnetic radiation.

20. A system as claimed in claim 18, wherein the heating station is operative to heat the receptive image, or a part thereof, by convection.

21. A method of thermal transfer printing onto a printing substrate, which comprises applying to a surface of a substrate a receptive image by:

a) providing a movable transfer member having an imaging surface;

b) applying to at least a segment of the imaging surface a monolayer coating of thermoplastic particles that are formed of, or coated with, a thermoplastic polymer; c) heating thermoplastic particles within one or more selected regions of the monolayer coating to render the thermoplastic particles within each selected region of the monolayer coating tacky; and

d) pressing the imaging surface and the surface of the substrate against one another to cause the thermoplastic particles that have been rendered tacky to transfer to the surface of the substrate, thereby forming a receptive image on the surface of the substrate,

characterized by:

e) applying embellishment particles onto the surface of the substrate, and f) heating the receptive image prior to, and/or during, application of the embellishment particles, in order to ensure tackiness of at least a part of the receptive image and cause the embellishment particles to adhere only to the at least part of the receptive image heated to tackiness.

22. A method as claimed in claim 21, wherein:

(i) the heating of the at least part of the receptive image includes heating by way of convection, electromagnetic radiation and/or thermal conduction; and wherein

(ii) [A] the applying of the embellishment particles includes a gas stream carrying the embellishment particles being blown onto the surface of the substrate by means of a nozzle that is surrounded by a cowling connected to a suction source; or [B] the applying of the embellishment particles includes coating another transfer member serving as a donor surface with the embellishment particles and bringing the donor surface, or a portion of the donor surface, into contact with the surface of the substrate bearing the receptive image.

23. A method as claimed in claim 22, which further comprises applying one or more treatments to the embellishment particles after the applying of the embellishment particles to the at least part of the receptive layer.

24. A substrate bearing printed image applied by thermal transfer, wherein the printed image comprises a first layer formed of thermoplastic first particles in contact with, and adhering to, the substrate and a second layer overlying the first layer formed of second particles, adhering to and/or at least partly embedded within the first layer, wherein the second particles, and/or the second layer, differ from the first particles, and/or the first layer, physically and/or chemically.

25. A substrate as claimed in claim 24, wherein the printed image was (a) printed in a thermal transfer printing system as claimed in any one of claim 1 to claim 20; and/or prepared according to a method as claimed in any one of claim 21 to claim 23.