WO2022084027A1

WO2022084027A1 - Process for printing particles

Info

Publication number: WO2022084027A1
Application number: PCT/EP2021/077440
Authority: WO
Inventors: Laura GILSBACH; Maike TROMPA; Matthias DREIER; Rahul AGNIHOTRI
Original assignee: Actega Metal Print
Priority date: 2020-10-19
Filing date: 2021-10-05
Publication date: 2022-04-28
Also published as: EP4228902A1

Abstract

:The invention relates to a process for printing onto a substrate having an image-receiving surface, which comprises providing a donor surface, passing the donor surface through a coating station from which the donor surface exits coated with individual particles, and repeatedly performing the steps of: (i) providing a receptive layer, at least partially covering said image-receiving surface, (ii) treating the receptive layer by electromagnetic radiation in order to enhance its affinity to the particles greater than the affinity of the particles to the donor surface, (iii) contacting the receptive layer with the donor surface to cause particles to transfer from the donor surface to the treated receptive layer, thereby exposing regions of the donor surface from which particles are transferred to the receptive layer and thereby generating a plurality of individual particles adhered to the receptive layer; and (iv) returning the donor surface to the coating station to coat it again with individual particles in order to permit printing of a subsequent image on the substrate,characterized in that the receptive layer provided in step (i) contains or consists of a coating composition RC comprising10 - 95 wt.-% of a multifunctional acrylate MA with 2 – 16 acryloyl groups and additionally at least a sufficient number of further moieties selected from the group consisting of urethane functions, ester functions and ether functions, with the proviso that the sufficient number is such that in total at least 6 – 90 moiety-heteroatoms are provided,and with optionally a limited number of phenyl groups, where said number is limited by the proviso that there are at least 6 of the moiety-heteroatoms per one phenyl group.

Description

Title Process for printing particles

The present invention relates to a process for printing onto a substrate, an intermediate print product and a print product.

Foil imaging is deemed as to be one of the most practicable technologies for printing solid material on a substrate and might be used e.g. for metallization purposes. There are two different types of foil imaging, foil stamping on the one hand and foil laminating on the other hand. In a foil stamping process a metallic foil is transferred onto a surface through the transfer of heat and pressure. During the stamping process, a die is heated and applied to the foil and is then sandwiched between the die and the surface which should be stamped. The heated die pressing against the surface activates an adhesive within the foil, which fuses the foil to the substrate. Pressure and heat cause the relevant sections of the foil to become detached from the carrier material and become bonded with the printing surface. Foil laminating on the other hand differs from said foil stamping technology, as the latter requires a die to be heated. A typical foil laminating process consists of two steps: printing on the substrate and foiling itself. In the printing step a UV-curable laminating adhesive is printed onto the substrate in the areas which are to be foiled. In a second step a foil is brought into contact with the substrate as well as the uncured adhesive by a laminating roller. Subsequently the adhesive is cured through the foil using a conventional UV-lamp which bonds the foil to the substrate via the adhesive. By removing the foil reel from the substrate path the now bonded foil areas are removed from the carrier material and non-bonded foil is wound up on a reel.

However, the production of this foil waste is considered as a serious disadvantage of foil stamping and laminating: in practice a large amount of foil that is wasted during each stamp/laminating process, as any foil area that is not transferred to form the desired image on the substrate cannot be recovered for successive prints.

WO 2016/189515 proposes a printing process in which the waste and the costs for the foil is

SUBSTITUTE SHEET (RULE 26) reduced. Said process is a continuous method for printing particles (e.g. metal pigment flakes) and uses a printed trigger image before passing into an application unit, where a donor roll carries the particles from a reservoir to a receptive layer of the substrate to be printed. Only those particles that are in contact with the trigger image are used, the remainder return to the reservoir for future rotations. After transferring the particles to the receptive layer the donor surface is returned to coat it again with particles in order to allow a continuous printing process. Said process is especially used as an economical metallization technology which provides attractive metallic effects.

However, there still is a need to further improve the efficiency of the printing process and the quality of the received printing products.

Thus, it is an object of the present invention to provide an economical printing process which might be used as a metallization method. The corresponding print product should be of high quality.

The solution is a process for printing onto a substrate having an image-receiving surface, which comprises providing a donor surface, passing the donor surface through a coating station from which the donor surface exits coated with individual particles, and repeatedly performing the steps of:

(i) providing a receptive layer, at least partially covering said image-receiving surface,

(ii) treating the receptive layer by electromagnetic radiation in order to enhance its affinity to the particles greater than the affinity of the particles to the donor surface,

(iii) contacting the receptive layer with the donor surface to cause particles to transfer from the donor surface to the treated receptive layer, thereby exposing regions of the donor surface from which particles are transferred to the receptive layer and thereby generating a plurality of individual particles adhered to the receptive layer; and

SUBSTITUTE SHEET (RULE 26) (iv) returning the donor surface to the coating station to coat it again with individual particles in order to permit printing of a subsequent image on the substrate, characterized in that the receptive layer provided in step (i) contains or consists of a coating composition RC comprising

10 - 95 wt.-% of a multifunctional acrylate MA with 2 - 16 acryloyl groups and additionally at least a sufficient number of further moieties selected from the group consisting of urethane functions, ester functions and ether functions, with the proviso that the sufficient number is such that in total at least 6 - 90 moiety-heteroatoms are provided, and with optionally a limited number of phenyl groups, where said number is limited by the proviso that there are at least 6 of the moiety-heteroatoms per one phenyl group.

The acryloyl groups (sometimes “only” defined as H2C=CH-C(=O)-) in the relevant (invention) definition of the multifunctional acrylate MA are defined as to be (“the ester-type”): H2C=CH-C(=O)-O- (“acrylic acid ester function”). The ester segment contained in said acyryloyl group does not belong to the defined ..further moieties".

Additional explanation concerning the claim definition: “at least a sufficient number of further moieties selected from the group consisting of urethane functions, ester functions and ether functions, with the proviso that the sufficient number is such that in total at least 6 - 30 moiety-heteroatoms are provided”.

Said „Moiety-heteroatoms“ (generally O. possibly N) are only provided by the heteroatoms of the cited functions (urethane, ester and ether): The individual contribution of a single urethane group is 3 heteroatoms, of a single ester group is 2 heteroatoms and of a single ether group is 1 heteroatom. For example an O atom of a

SUBSTITUTE SHEET (RULE 26) possible additional hydroxyl group is not considered as to a „moiety-heteroatom”. With ..sufficient number" it is meant that there are enough of these (cited) “further moieties” in order to obtain (in addition) the requested number of moiety-heteroatoms.

The donor surface is typically made from a polymer that can be tailored in regard to its surface polarity and mechanical properties. Typically, the donor surface is made from an elastomer, for example a silicone-based material. One example is made by combining three silicone- based materials: a vinyl-terminated polydimethylsiloxane (Polymer VS 5000, Evonik) in an amount of about 58.45% by weight of the total composition (wt.%), a vinyl functional polydimethylsiloxane containing both terminal and pendant vinyl groups (Polymer RV 5000, Evonik), in an amount of about 11 wt.% and a branched structure vinyl functional polydimethylsiloxane (VQM 906, Evonik) in an amount of about 22.5 wt.%. To the mixture were added: a platinum catalyst (Catalyst 512, Evonik) in an amount of about 0.05 wt.%, a inhibitor (Inhibitor 600, Evonik) in an amount of 2.0 wt.%, and finally a reactive cross-linker such as a methyl-hydrosiloxane-dimethylsiloxane copolymer (Crosslinker 101 , Evonik) in an amount of 6.0 wt.%. This composition can be thermally cured to produce a hydrophobic elastomeric donor surface.

The process according the present invention provides a sufficient transfer of the particles from the donor surface to the receptive layer. The curing properties of the receptive layer (of the coating composition RC) are such (universal) that an efficient curing is even possible at different printing speeds and receptive layer thicknesses. The hardening of the receptive layer is on the one hand fast enough in order to provide the appropriate acceptor properties (sufficient stickiness to the particles even in a fast printing process) but on the other hand not too intensive so that the flexibility of the hardened layer is remained (brittleness avoided). In other words: The cohesion of the cured layer should be appropriate (sufficient mechanic solidity/ strength) so that the coating does not deposit on the printing pressure roller and does not show mechanical brittleness (to hard). Additionally, the cured layer provides universal and efficient acceptor properties (adoption from the donor surface) for the particles. According to the present invention a sufficient (particle) covering of the receptive layer is achieved: By "sufficient" covering, it is meant that the coat of particles on the relevant substrate regions will be especially devoid of defects perceptible to the naked eye so that the intended visual effect is achieved. Generally, the received print product is optically

SUBSTITUTE SHEET (RULE 26) attractive and has a good quality. The gloss of a metallized substrate might be deemed as to be a corresponding quality feature.

Due to a preferred embodiment of the printing process according to the present invention, in step (ii) the electromagnetic radiation is performed by UV radiation and the coating composition RC comprises a sufficient amount of suitable UV polymerisation initiator.

For the UV radiation corresponding lamps using wavelengths of e.g. 200 - 350 nm might be provided.

The UV polymerization initiator might be selected from the group consisting of so called Type I photoinitiators such as hydroxyacetophenones, alkylaminoacetophenones, benzil ketals, dialkoxyacetophenones, benzoin ethers, phosphine oxides, acyloximino esters, so called Type II photoinitiators such as benzophenone, substituted benzophenones, thioxanthones, anthraquinones, benzoylformate esters, camphorquinone, as well as polymers with before mentioned radical forming groups attached to them.

One possibility of applying the receptive layer is according to an analogues technology element: in step (i) the receptive layer is applied to selected regions by indirect printing which is performed by offset printing, screen printing, flexographic printing and/or gravure printing.

Alternatively the receptive layer is applied according to a digital technology embodiment: in step (i) the receptive layer is applied to the substrate surface by direct printing, especially by direct jetting. In the latter case the coating composition RC should have an appropriate viscosity (low enough).

Normally, the receptive layer provided in step (i) has a thickness between 0.5 pm and 500 pm, where the thickness is determined via gravimetry.

SUBSTITUTE SHEET (RULE 26) The particles are preferably selected to adhere to the donor surface more strongly than they do to one another. This results in the applied layer being substantially a thin layer (preferably being a monolayer) of individual particles.

Typically, the printed particles contain or consist of metal.

Due to a special embodiment the particles contain or consist of flaky metallic pigments (often formed like platelets). Such flaky metallic pigments typically have an average thickness (h50) value in the range of 20 - 200 nm (which is determined with a scanning electron microscope according to the corresponding method as described in WO 2004/087816).

However, (even if not preferred) also non-metallic particles might be used. Examples of such „non-metallic“ particles: glass and ceramic (metal oxides), respectively including polymeric or inorganic coating of the particles.

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 an embodiment of the present invention the coating composition RC comprises 20 - 90 wt.-% of the multifunctional acrylate MA.

Typically at least a partial quantity of the multifunctional acrylate MA contains no phenyl group, 2 - 10 acryloyl groups and additionally at least a sufficient number of further moieties selected from the group consisting of urethane functions, ester functions and ether functions, with the proviso that the sufficient number is such that in total at least 11 - 20 moiety-heteroatoms are provided, where said partial quantity is preferably at least 30 wt.-%. The total absence (or sometimes also a “very” low number) of phenyl groups generally improves the quality (e.g. often by improving particle adoption properties and/or typically by avoiding mechanical brittleness).

SUBSTITUTE SHEET (RULE 26) In one embodiment of the invention at least a partial quantity of the multifunctional acrylate MA has a molecular weight of at least 500, where said partial quantity is at least 20 wt.-%.

Due to a special embodiment at least 80 wt.-% of the multifunctional acrylate MA contains maximal 6 acryloyl groups and additionally at least 4 moieties selected from the group consisting of urethane functions and ester functions.

Normally, the coating composition RC contains at least 60 wt.-% radical polymerizable components and preferably maximal 10 wt.-% non-radical-polymerizable solvents. Components which are normally not contained in the coating composition are thermal radical initiators and mineral fillers. However, polymeric (organic) resins might be contained. Preferably the receptive layer is selected so that it does not interfere with the desired printing effect (e.g., clear, transparent, and/or colorless).

The invention also concerns an intermediate print product manufactured by a printing process as described above.

Furthermore, the invention concerns a print product on the basis of said intermediate print product additionally comprising an overcoat layer covering the printed particles. The overcoat layer improves the stability of the print product.

Below the invention is described in more detail by providing examples.

The adoption of pigments by the receptive layer in the print process should be experimentally shown. Relevant “indicators” are especially the metallic gloss and the optical density (microscopical view) of the received (intermediate) print product.

Gloss measurement:

The gloss of the metallized surface of printed samples was measured using a glossmeter (device: micro-TRI-gloss manufactured by BYK-Gardner GmbH, D-82538 Geretsried, Germany). Since the measured surfaces are highly reflective, the measurement was performed using a 20° angle setting. For each sample five

SUBSTITUTE SHEET (RULE 26) measurements in different areas were performed and the values were arithmetically averaged.

measurement:

The optical density provides an indication of the amount of transferred metallic pigments. To determine the optical density a black/white transmission densitometer (device: 341 C manufactured by X-Rite Inc., Grand Rapids Ml 49512, USA) was used. To calibrate the pure substrate was first measured and the value set to zero. For each sample three measurements in different areas were performed and the values were arithmetically averaged.

Coverage measurement:

To determine the area covered by aluminum platelets after the metallization process, microscope images were made using a laser-scanning microscope VK-X 1100 (manufactured by Keyence Corporation, Osaka 533-8555, Japan). After generating a composite image at 150 x magnification, overlaying optical and laser images, it is possible to distinguish areas covered by pigment from free areas. Using the MultiFileAnalyzer Software (by Keyence Corporation, Osaka 533-8555, Japan) the free area can be calculated by relating the covered are in pm² to the entire image are in pm² resulting in a percentage value of an area covered by metallic pigments.

Print trials:

Metallized samples were prepared by applying the receptive coatings Example 1 and Wessco 3501 (comparative example - not according to the invention, manufactured by ACTEGA Schmid Rhyner AG, 8134 Adliswil, Switzerland) using a flexographic print station on a Digicon Series 3 finishing unit (manufactured by AB Graphics International Ltd., Bridlington YO15 3QY, United Kingdom) at different speeds on a gloss white polyethylene laminate substrate (RI-837/85 PE GLOSS WHITE manufactured by Ritrama S.p.A., 20867 Caponago, Italy). For the application an anilox roller from Sandon with 200 lines per inch (manufactured by Sandon Global, Cheshire WA7 1 SR, United Kingdom) and a flexographic printing plate nyloflex® Seal F (manufactured by Flint Group, D-70469 Stuttgart, Germany) were used. After application the trigger image was immediately cured inline using a conventional mercury-based UV-lamp

SUBSTITUTE SHEET (RULE 26) (GEW E2C, 120 W/cm manufactured by GEW Ltd., Crawley RH10 9QR, United Kingdom) at 100% power setting. Following the curing of the receptive coating the web was fed through an EcoLeaf metallization unit (manufactured by ACTEGA Metal Print GmbH, D-31275 Lehrte, Germany). The pigments used in the metallization process were EcoLeaf P110 aluminum platelets with a median thickness of around 40 nm (distributed by ACTEGA Metal Print GmbH, D-31275 Lehrte, Germany).

After application of the pigments the metallized samples were characterized by gloss (20° angle setting), optical density and measurement of covered area using a confocal laser scanning microscope.

Metallized samples were prepared by applying the receptive coatings Example 2 and Flint Tactile Varnish UV D0-1200-408 N (comparative example - not according to the present invention, manufactured by Flint Group, D-70469 Stuttgart, Germany) using a rotary screen printing station on an Omet XFlex X6 (manufactured by Omet srl, 23900 Lecco, Italy) at 20 m/min on a coated paper laminate substrate (ADESTOR High Gloss Ws 80 manufactured by Lecta, 08019 Barcelona, Spain). For the application a RotaMesh® 75 rotary screen with an open area of 32% and mesh opening of 192 p (manufactured by SPGPrints B.V., 5831 Boxmeer, The Netherlands) was used. After application the trigger image was immediately cured inline using a conventional mercury-based UV-lamp (GEW E2C, 120 W/cm manufactured by GEW Ltd., Crawley RH10 9QR, United Kingdom) at 100% power setting. Following the curing of the receptive coating the web was fed through an EcoLeaf metallization unit (manufactured by ACTEGA Metal Print GmbH, D-31275 Lehrte, Germany). The pigments used in the metallization process were EcoLeaf P110 aluminum platelets with a median thickness of around 40 nm (distributed by ACTEGA Metal Print GmbH, D-31275 Lehrte, Germany).

After application of the pigments the metallized samples were characterized by gloss (20° angle setting) and measurement of the covered area using a confocal laser scanning microscope. A measurement of the optical density was not feasible due to the low transparency of the substrate laminate.

SUBSTITUTE SHEET (RULE 26) Metallized samples were prepared by jetting the receptive coating Example 3 using a MH5421 F piezoelectric inkjet print-head (manufactured by Ricoh Company, Ltd., Tokyo 143-8555, Japan) mounted on an Omet XFlex X6 (manufactured by Omet srl, 23900 Lecco, Italy) at 50 m/min on a gloss white polyethylene laminate substrate (RI-837/85 PE GLOSS WHITE manufactured by Ritrama S.p.A., 20867 Caponago, Italy). For the application the receptive coating was jetted with a triple pulse resulting in a drop volume of around 21 pL. After application the trigger image was immediately cured inline using a conventional mercury-based UV-lamp (GEW E2C, 120 W/cm manufactured by GEW Ltd., Crawley RH10 9QR, United Kingdom) at 80% power setting. Following the curing of the receptive coating the web was fed through an EcoLeaf metallization unit (manufactured by ACTEGA Metal Print GmbH, D-31275 Lehrte, Germany). The pigments used in the metallization process were EcoLeaf P110 aluminum platelets with a median thickness of around 40 nm (distributed by ACTEGA Metal Print GmbH, D-31275 Lehrte, Germany).

Examples 1 , 2 and 3 were presented in order to provide an improved process. In these examples the used multifunctional acrylates MA are Ebecryl 230 (difunctional which contains around 42 “moiety-heteroatoms” stemming from urethan and ether moieties), Ebecryl 5129 (hexafunctional which contains 6 “moiety-heteroatoms” stemming from urethan moieties), Ebecryl 8409 (difunctional which contains 14 “moiety-heteroatoms” stemming from urethane and ester moieties) and CN2505 (tetrafunctional which contains 10 “moiety-heteroatoms” stemming from ester and ether moieties). Each of said mentioned multifunctional acrylates MA do not contain a phenyl group. Further multifunctional acrylates MA which can be employed and have shown good results are e.g. SR344 (polyethyleneglycol (400) diacrylate), SR610 (polyethyleneglycol (600) diacrylate), SR499 (ethoxylated (6) trimethylolpropane triacrylate), SR502 (ethoxylated (9) trimethylolpropane triacrylate), SR9035 (ethoxylated (15) trimethylolpropane triacrylate), SR415 (ethoxylated (20) trimethylolpropane triacrylate), Miramer M2040 (polypropyleneglycol (400) diacrylate) and Miramer M2300 (ethoxylated (30) bisphenol A diacrylate).

SUBSTITUTE SHEET (RULE 26) Table 1: Examples of receptive coatings

SUBSTITUTE SHEET (RULE 26)

Table 2: Results of metallization with receptive coatings

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Claims

Claims:

1 . Process for printing onto a substrate having an image-receiving surface, which comprises providing a donor surface, passing the donor surface through a coating station from which the donor surface exits coated with individual particles, and repeatedly performing the steps of:

(iv) returning the donor surface to the coating station to coat it again with individual particles in order to permit printing of a subsequent image on the substrate, characterized in that the receptive layer provided in step (i) contains or consists of a coating composition RC comprising

10 - 95 wt.-% of a multifunctional acrylate MA with 2 - 16 acryloyl groups and additionally at least a sufficient number of

SUBSTITUTE SHEET (RULE 26) further moieties selected from the group consisting of urethane functions, ester functions and ether functions, with the proviso that the sufficient number is such that in total at least 6 - 90 moiety-heteroatoms are provided, and with optionally a limited number of phenyl groups, where said number is limited by the proviso that there are at least 6 of the moiety-heteroatoms per one phenyl group. Printing process according to claim 1 , wherein in step (ii) the electromagnetic radiation is performed by UV radiation and the coating composition RC comprises a sufficient amount of suitable UV polymerisation initiator. Printing process according to claim 1 or 2, wherein step (i) comprises applying a receptive layer to selected regions by indirect printing which is performed by offset printing, screen printing, flexographic printing and/or gravure printing. Printing process according to claim 1 or 2, wherein in step (i) the receptive layer is applied to the substrate surface by direct printing, including by direct jetting. Printing process according to one of the preceding claims, wherein the receptive layer provided in step (i) has a thickness between 0.5 pm and 500 pm, where the thickness is determined via gravimetry. Printing process according to one of the preceding claims, wherein the particles contain or consist of metal. Printing process according to one of the preceding claims, wherein the particles contain or consist of flaky metallic pigments. Printing process according to claim 7, where the flaky metallic pigments have an average thickness (h50) value in the range of 20 - 200 nm which is

SUBSTITUTE SHEET (RULE 26) 15 determined with a scanning electron microscope. Printing process according to one of the preceding claims, wherein the coating composition RC comprises 20 - 90 wt.-% of the multifunctional acrylate MA. Printing process according to one of the preceding claims, wherein at least a partial quantity of the multifunctional acrylate MA contains no phenyl group, 2 - 10 acryloyl groups and additionally at least a sufficient number of further moieties selected from the group consisting of urethane functions, ester functions and ether functions, with the proviso that the sufficient number is such that in total at least 11 - 20 moiety-heteroatoms are provided, where said partial quantity is preferably at least 30 wt.-%. Printing process according to one of the preceding claims, wherein at least a partial quantity of the multifunctional acrylate MA has a molecular weight of at least 500, where said partial quantity is at least 20 wt.-%. Printing process according to one of the preceding claims, wherein at least 80 wt.-% of the multifunctional acrylate MA contains maximal 6 acryloyl groups and additionally at least 4 moieties selected from the group consisting of urethane functions and ester functions. Printing process according to one of claims 2 - 12, wherein the UV polymerization initiator is selected from the group consisting of so called Type I photoinitiators such as hydroxyacetophenones, alkylaminoacetophenones, benzil ketals, dialkoxyacetophenones, benzoin ethers, phosphine oxides, acyloximino esters, so called Type II photoinitiators such as benzophenone, substituted benzophenones, thioxanthones, anthraquinones, benzoylformate esters, camphorquinone, as well as polymers with before mentioned radical forming groups attached to them.

SUBSTITUTE SHEET (RULE 26) 16 Printing process according to one of the preceding claims, wherein the coating composition RC contains at least 60 wt.-% radical polymerizable components and preferably maximal 10 wt.-% non-radical-polymerizable solvents. An intermediate print product manufactured by a printing process according to of one of the claims 1 - 14. A print product on the basis of an intermediate print product according to claim 15 further comprising an overcoat layer covering the printed particles.

SUBSTITUTE SHEET (RULE 26)