Inkjet printing method

Abstract

An inkjet printing method comprising the steps of: a) spraying a varnish on a substrate; and b) UV curing the varnish with a UV LED source having a spectral emission in the range of 360-420 nm after a curing delay of at least 2 seconds; wherein the varnish has a static surface tension of 28 mN / m or more; wherein the varnish contains more than 12% by weight, based on the total weight of the varnish, of an acylphosphine oxide photoinitiator; and wherein the curing delay is a period of time between dropping of the varnish liquid on the substrate and initiation of UV curing.

Description

Technical Field

[0001] The present invention relates to an inkjet printing method for varnishing substrates. Background Art

[0002] Varnishes are commonly used in the inkjet process to provide a glossy surface finish to the substrate that bears the image. Gloss is achieved by eliminating any micro-roughness and making the surface as flat as possible.

[0003] In order to produce a high-gloss surface, so-called free-flowing UV varnishes are used, which are self-leveling between the moment of application to the substrate and the moment of UV curing. Individual varnish droplets flow into each other to form a continuous varnish layer. For example, US2006198964 (HEIDELBERGER DRUCKMASCHINEN) discloses such a method for producing a smooth and high-gloss surface using a fast-flowing UV varnish.

[0004] The formation and spreading of the droplets are largely controlled by the surface tension of the varnish, which is usually reduced with surfactants. Varnishes typically have a very low static surface tension of about 20 to 22 mN / m to enable fast flow.

[0005] Different compositions for inkjet varnishes have been proposed in, for example, EP 2786878 A (HP SCITEX), US2015126638 A (MGIDIGITAL TECHNOLOGY), EP 3356479 A (MGIDIGITAL TECHNOLOGY), EP 2682273 A (AGFA) and EP 3070130 A (AGFA).

[0006] Different inkjet printing methods for improving the gloss of varnished substrates have also been proposed, for example, in US2019217649 (GREAT COMPUTER CORP), EP 2810783A (HP SCITEX) and EP2909039 A (OCE).

[0007] However, none of these approaches address the gloss artefacts that occur when two print swaths are "stitched" together. A print swath is a band of print produced by one pass of the print head. Splicing is the joining of the print swaths to produce a continuous layer. Larger print heads can produce wider swaths, so splicing is not necessary. However, such print heads become too expensive above a certain width of about 10 to 30 cm, and the printed substrates typically have a width of more than 1 meter. When splicing with a single-pass inkjet printer, the print heads are positioned so that there is a small overlap in the print swaths produced by the print heads. When splicing with a multi-pass inkjet printer, a small portion of the print swath produced in the first pass of the print head is superimposed by the print swath produced in the second pass of the print head. In both cases, the overlap typically produces gloss artifacts such as due to the so-called Marangoni effect. Figure 1 Small protrusions (1) and depressions (2) are shown. Although these protrusions and depressions provide a relief of only a few μm in height, they greatly affect the uniformity of the gloss.

[0008] There therefore still exists a need for an improved inkjet printing method for varnishing substrates, wherein gloss artifacts caused by splicing are reduced or absent. Summary of the invention

[0009] In order to overcome the above problems, a preferred embodiment of the present invention provides an inkjet printing method as set forth in claim 1 .

[0010] It has surprisingly been found that by controlling certain aspects of the varnish composition and applying a cure delay of a few seconds, gloss artifacts caused by the splicing of printed strips can be reduced or even eliminated. The varnishes in the present invention have a high static surface tension, which is contrary to conventional practice in the art for achieving fast flow of varnishes. UV curing is also controlled by using a UV LED source with a spectral emission in the range of 360-420 nm and by having a minimum amount of acylphosphine oxide photoinitiator in the varnish.

[0011] Other objects of the present invention will become apparent from the following description. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 A cross-sectional view of a substrate is shown having a second printed strip (4) partially overlapping a first printed strip (3) and exhibiting elevations (1) and depressions (2) in the overlapping region.

[0013] Figure 2A printing module (10) is shown, which contains a print head for color inks (11) and a print head for varnish (12) and UV LED modules A to C (15, 16, 17). In a multi-pass printer, the printing module (10) moves in a scanning direction (13) while the substrate is transported in a direction (14). DETAILED DESCRIPTION

[0014] Inkjet printing method

[0015] The inkjet printing method according to the present invention comprises the following steps: a) spraying a varnish on a substrate; and b) after a curing delay of at least 2 seconds, UV curing the varnish with a UV LED source having a spectral emission in the range of 360-420 nm; wherein the static surface tension of the varnish is above 28 mN / m; wherein the varnish contains more than 12 wt % of acylphosphine oxide photoinitiator based on the total weight of the varnish; and wherein the curing delay is the time period between the varnish droplet landing on the substrate and the start of UV curing.

[0016] A preferred inkjet printing process uses a varnish containing a mixture of multifunctional acrylates and methacrylates, wherein the multifunctional acrylate is preferably a difunctional acrylate.

[0017] In a preferred embodiment of the inkjet printing method, the varnish is heated at 45°C and 1,000 s -1 The viscosity at a shear rate of less than 13.0 mPa.s, more preferably in the range of 7.0 to 12.5 mPa.s, most preferably in the range of 7.2 to 9.8 mPa.s.

[0018] The curing delay is at least 2 seconds, preferably at least 3 seconds, most preferably at least 4 seconds.

[0019] The substrate preferably has a width of more than 30 cm, preferably more than 50 cm.

[0020] Inkjet varnish

[0021] Inkjet varnishes curable by UV radiation contain a polymerizable composition and one or more photoinitiators. The polymerizable composition typically contains monomers and oligomers. Photoinitiators are chemical compounds that initiate polymerization of monomers and oligomers by forming free radicals when exposed to actinic radiation.

[0022] Two types of photoinitiators can be distinguished for use in inkjet varnishes. Norrish type I initiators are initiators which break after excitation to immediately generate initiating free radicals. Norrish type II initiators are photoinitiators which are activated by actinic radiation and form free radicals by hydrogen abstraction from a second compound, which becomes the actual initiating free radical. This second compound is referred to as a polymerization synergist or coinitiator. Both type I and type II photoinitiators can be used in the present invention. Acylphosphine oxide photoinitiators are Norrish type I photoinitiators.

[0023] The fast flow of UV inkjet varnishes is usually achieved by low viscosity and by using a large amount of surfactants. Surfactants are used because of their ability to reduce surface tension. The surface tension determines whether the coating will wet and spread on the solid substrate or retract from the solid substrate. A large amount of surfactants may cause foaming problems during the varnishing process and during use in the inkjet printer. Foaming causes nozzle failure due to the presence of bubbles in the inkjet print head.

[0024] For high curing speeds, most inkjet inks and fluids are based on acrylate monomers and oligomers because they are more reactive than methacrylate monomers and oligomers.

[0025] It is also well known that multifunctional (meth)acrylate monomers cure faster than monofunctional (meth)acrylate monomers. The notation (meth)acrylate monomers represents both methacrylate monomers and acrylate monomers.

[0026] By making a specific mixture of monomers, it is possible to easily maximize the gloss of the splice. A preferred inkjet varnish contains a polymerizable composition having: a) 10 to 30 wt%, preferably 10 to 20 wt% of a monofunctional methacrylate; b) 40 to 70 wt%, preferably 45 to 65 wt% of a multifunctional acrylate; and c) 0 to 15 wt% of a monofunctional acrylate, wherein the weight percentages are based on the total weight of the varnish. In such a mixture, the monofunctional methacrylate slows down the UV curing speed, while the multifunctional acrylate accelerates the UV curing speed.

[0027] Acylphosphine oxide photoinitiator

[0028] Acylphosphine oxide photoinitiators are believed to be the most effective photoinitiators for curing UV curable compositions with UVLED sources having a spectral emission in the range of 360-420 nm.

[0029] One or more acylphosphine oxide photoinitiators may be used. The acylphosphine oxide photoinitiator may be a monoacylphosphine oxide photoinitiator or a bisacylphosphine oxide photoinitiator, or a combination of both.

[0030] Preferred acylphosphine oxide photoinitiators include trimethylbenzoyldiphenylphosphine oxide, ethoxyphenyl(2,4,6-trimethylbenzoyl)phosphine oxide, and bis(2,4,6-(2,4,6-trimethylbenzoyl)phenylphosphine oxide.

[0031] Acylphosphine oxide photoinitiators are also commercially available, such as Omnirad from IGM. TM TPO, TPO-L and 819.

[0032] A particularly preferred acylphosphine oxide photoinitiator is ethoxyphenyl (2,4,6-trimethylbenzoyl) phosphine oxide, as it is liquid at ambient temperature. High concentrations of non-liquid photoinitiators, such as trimethylbenzoyl diphenyl phosphine oxide and bis (2,4,6-(2,4,6-trimethylbenzoyl) phenyl phosphine oxide, may cause solubility problems to result in solid particles in the cured varnish, which negatively affects gloss.

[0033] To improve the splice gloss, the varnish contains more than 12% by weight, preferably more than 13% by weight, most preferably more than 14% by weight, of acylphosphine oxide photoinitiator, based on the total weight of the varnish.

[0034] In a preferred embodiment, the acylphosphine oxide photoinitiator is present in an amount of greater than 40 mmol acylphosphine oxide photoinitiator per 100 g varnish. At lower concentrations of acylphosphine oxide photoinitiator, insufficient improvement of splice-induced gloss artifacts is generally observed.

[0035] Gloss is usually only one aspect of the luxury aspect of packaging applications and interior decoration. Avoiding unwanted odors is another aspect of it. Upon UV exposure, acylphosphine oxide photoinitiators split into two free radical components: an acyl component and a phosphine oxide component. The phosphine oxide component is usually almost completely integrated into the polymeric network of the UV-curable polymerizable composition in the varnish. However, this is not the case with the acyl component, which may form aldehydes such as mesityleneformaldehyde after hydrogen abstraction, which causes unwanted odors. Therefore, in a preferred embodiment, the acylphosphine oxide photoinitiator is an acylphosphine oxide initiator in which the acyl group is selected from a benzoyl group substituted by a urea group, an oxamide group or a polymerizable group; a 2,6-dimethylbenzoyl group substituted by a urea group, an oxamide group or a polymerizable group at the 3-position; a 2,6-dimethoxybenzoyl group substituted by a urea group, an oxamide group or a polymerizable group at the 3-position; a 2,4,6-trimethylbenzoyl group substituted by a urea group, an oxamide group or a polymerizable group at the 3-position; and a 2,4,6-trimethoxybenzoyl group substituted by a urea group, an oxamide group or a polymerizable group at the 3-position. The polymerizable group is preferably a (meth)acrylate group, more preferably an acrylate group. Preferred acylphosphine oxides are disclosed in EP 3810710 A (AGFA) and WO 2014 / 051026 (FUJIFILM).

[0036] Other photoinitiators and co-initiators

[0037] In addition to the acylphosphine oxides, other photoinitiators may also be used. However, many photoinitiators do not absorb in the range of 360-420 nm and are therefore preferably not incorporated into the varnish. Suitable other photoinitiators are disclosed in CRIVELLO, JV et al., Volume III: Photoinitiators for Free Radical Cationic, 2nd edition, BRADLEY, G. ed., London, UK: John Wiley and Sons Ltd, 1998, pp. 287-294.

[0038] Preferred other photoinitiators include thioxanthone, carbazole and α-hydroxy ketone. Specific examples of other photoinitiators may include, but are not limited to, the following compounds or combinations thereof: 1-hydroxycyclohexyl phenyl ketone, isopropyl thioxanthone and diethyl thioxanthone.

[0039] A disadvantage of other photoinitiators is that they often produce colored byproducts after UV curing. Especially thioxanthone-type photoinitiators can show strong photoinduced yellowing. It is desirable that the inkjet varnish is colorless after curing, otherwise it will cause color shifts in the image printed on the substrate. It is therefore preferred that the inkjet varnish contains less than 2% by weight, preferably less than 1% by weight of thioxanthone photoinitiator, based on the total weight of the varnish. Most preferably, the varnish contains no thioxanthone photoinitiator.

[0040] Most preferably, the varnish contains only acylphosphine oxides, since the latter are known to bleach after UV curing, thereby producing a colorless varnish.

[0041] Photoinitiators can also be combined with co-initiators. The most effective co-initiators are amine synergists. To further increase photosensitivity, UV inkjet varnishes may contain one or more amine synergists. Suitable examples of co-initiators can be divided into three categories:

[0042] (1) tertiary aliphatic amines, such as methyldiethanolamine, dimethylethanolamine, triethanolamine, triethylamine and N-methylmorpholine;

[0043] (2) aromatic amines, such as amyl p-dimethylaminobenzoate, 2-n-butoxyethyl-4-(dimethylamino)benzoate, 2-(dimethylamino)ethyl benzoate, ethyl-4-(dimethylamino)benzoate, and 2-ethylhexyl-4-(dimethylamino)benzoate; and

[0044] (3) (Meth)acrylated amines, such as dialkylaminoalkyl (meth)acrylates (eg diethylaminoethyl acrylate) or N-morpholinoalkyl-(meth)acrylates (eg N-morpholinoethyl-acrylate).

[0045] Preferred coinitiators are aminobenzoates.

[0046] In a preferred embodiment, the other photoinitiator is selected from the group consisting of non-polymeric multifunctional photoinitiators, oligomeric or polymeric photoinitiators and polymerizable photoinitiators. Such diffusion hindered photoinitiators exhibit much lower mobility in the cured layer of the UV curable varnish than low molecular weight monofunctional photoinitiators, such as benzophenone. The inclusion of diffusion hindered photoinitiators as well as diffusion hindered co-initiators not only has safety advantages for operators of inkjet printers or for interior decoration applications, but is also environmentally friendly because these compounds will not be leached from outdoor billboards by, for example, acid rain.

[0047] The total amount of amine synergist and photoinitiator is preferably no more than 35 wt%, more preferably no more than 30 or 28 wt%, based on the total weight of the varnish. Higher amounts generally result in insufficient improvement in splice gloss because solubility problems may occur.

[0048] Monofunctional methacrylate

[0049] Monofunctional methacrylates are polymerizable compounds containing only one polymerizable group, more particularly a methacrylate group.The inkjet varnish preferably contains 10 to 30% by weight, more preferably 10 to 20% by weight, of monofunctional methacrylate.

[0050] The monofunctional methacrylate may be one or more monomers or one or more oligomers, or a combination thereof.

[0051] Suitable monofunctional methacrylates include methacrylic acid, alkyl methacrylates (linear, branched and cycloalkyl), such as methyl methacrylate, n-butyl methacrylate, t-butyl methacrylate, cyclohexyl methacrylate, 2-ethylhexyl methacrylate and isotridecyl methacrylate; aryl methacrylates, such as benzyl methacrylate and phenyl methacrylate; and hydroxyalkyl methacrylates, such as hydroxyethyl methacrylate and hydroxypropyl methacrylate.

[0052] Preferred monofunctional methacrylates are tetrahydrofurfuryl methacrylate, alkyl methacrylate (preferably C methacrylate), 12 To C 15 alkyl esters), ethoxyethoxyethyl methacrylate, 2-phenoxyethyl methacrylate, ethoxylated-4-phenyl methacrylate, ureido methacrylate, behenyl methacrylate, and 3,3,5-trimethylcyclohexanol methoxypolyethylene glycol (350) methacrylate.

[0053] Particularly preferred monofunctional methacrylates are those having a relatively high glass transition temperature T g , preferably above 25°C T g , more preferably a T above 40°C g Monofunctional methacrylate. Used to determine T g The method is the DSC method in ISO 11357-2:1999.

[0054] The most preferred monofunctional methacrylates include isobornyl methacrylate.

[0055] Monofunctional acrylate

[0056] Monofunctional acrylates are polymerizable compounds containing only one polymerizable group, more particularly an acrylate group. The inkjet varnish preferably comprises 0 to 15 wt %, more preferably 0 to 5 wt %, most preferably 0 wt % of monofunctional acrylate, the wt % being based on the total weight of the polymerizable composition.

[0057] The monofunctional acrylate may be one or more monomers or one or more oligomers, or a combination thereof.

[0058] Suitable monofunctional acrylates include isoamyl acrylate, octadecyl acrylate, lauryl acrylate, octyl acrylate, decyl acrylate, isoamylstyl acrylate, isooctadecyl acrylate, 2-ethylhexyl-diglycol acrylate, 2-hydroxybutyl acrylate, 2-acryloyloxyethylhexahydrophthalic acid, butoxyethyl acrylate, ethoxydiethylene glycol acrylate, methoxydiethylene glycol acrylate, methoxypolyethylene glycol acrylate, methoxypropylene glycol acrylate, phenoxyethyl acrylate, tetrahydrofurfuryl acrylate, isobornyl acrylate, 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, 2-hydroxy-3-phenoxypropyl acrylate, 2-acryloyloxyethylsuccinic acid, 2-acryloyloxyethylphthalic acid, 2-acryloyloxyethyl-2-hydroxyethyl-phthalic acid, lactone-modified flexible acrylates, and tert-butylcyclohexyl acrylate.

[0059] Other suitable monofunctional acrylates include caprolactone acrylate, cyclic trimethylolpropane formal acrylate, ethoxyethoxyethyl methacrylate, ethoxylated nonylphenol acrylate, isodecyl acrylate, isooctyl acrylate, octyldecyl acrylate, alkoxylated phenol acrylates, and tridecyl acrylate.

[0060] Preferred monofunctional acrylates are tetrahydrofurfuryl methacrylate, alkyl acrylate (preferably C 12 To C 15 Alkyl esters), ethoxyethoxyethyl acrylate, 2-phenoxyethyl acrylate, ethoxylated-4-phenyl acrylate and 3,3,5-trimethylcyclohexanol methoxypolyethylene glycol (350) acrylate.

[0061] Particularly preferred monofunctional acrylates are those having a relatively high glass transition temperature T g , preferably above 25°C T g , more preferably a T above 40°C g Monofunctional acrylate. Used to determine T g The method is the DSC method in ISO 11357-2:1999.

[0062] The most preferred monofunctional acrylate is isobornyl acrylate.

[0063] Multifunctional acrylate

[0064] Multifunctional acrylate means the presence of two, three or more polymerizable acrylate groups. The UV curable varnish preferably comprises 40 to 70 wt%, more preferably 45 to 60 wt% of multifunctional acrylate, the wt% being based on the total weight of the polymerizable composition.

[0065] In a preferred embodiment, the multifunctional acrylate is a difunctional acrylate containing two polymerizable groups, ie, two acrylate groups.

[0066] The multifunctional acrylate may be one or more monomers or one or more oligomers, or a combination thereof.

[0067] Suitable multifunctional acrylates include triethylene glycol diacrylate, tetraethylene glycol diacrylate, polyethylene glycol diacrylate, dipropylene glycol diacrylate, tripropylene glycol diacrylate, polypropylene glycol diacrylate, 1,4-butanediol diacrylate, 1,6-hexanediol diacrylate, 1,9-nonanediol diacrylate, neopentyl glycol diacrylate, dimethylol-tricyclodecane diacrylate, bisphenol A EO (ethylene oxide) adduct diacrylate, bisphenol A PO (propylene oxide) adduct diacrylate, hydroxypivalate neopentyl glycol diacrylate, propoxylated neopentyl glycol diacrylate, alkoxylated dimethylol tricyclodecane diacrylate and polytetramethylene glycol diacrylate, trimethylolpropane triacrylate, EO-modified trimethylolpropane triacrylate, tri(propylene glycol) triacrylate, caprolactone-modified trimethylolpropane triacrylate, pentaerythritol triacrylate, pentaerythritol tetraacrylate, pentaerythritol ethoxy tetraacrylate, dipentaerythritol hexaacrylate, ditrimethylolpropane tetraacrylate, glycerol propoxy triacrylate and caprolactam-modified dipentaerythritol hexaacrylate.

[0068] Other suitable difunctional acrylates include alkoxylated cyclohexanone dimethanol diacrylate, alkoxylated hexanediol diacrylate, dioxanediol diacrylate, dioxanediol diacrylate, cyclohexanone dimethanol diacrylate, diethylene glycol diacrylate, and neopentyl glycol diacrylate.

[0069] Other higher functional acrylates include propoxylated glycerol triacrylate and propoxylated trimethylolpropane triacrylate, di-trimethylolpropane tetraacrylate, dipentaerythritol pentaacrylate, ethoxylated pentaerythritol tetraacrylate, methoxylated glycol acrylates and acrylate esters.

[0070] Preferred multifunctional acrylates include dipropylene glycol diacrylate, tripropylene glycol diacrylate, 1,6-hexanediol diacrylate, cyclohexanone dimethanol diacrylate, polyethylene glycol 200 diacrylate, 3-methyl 1,5-pentanediol diacrylate, pentaerythritol tetraacrylate, trimethylolpropane triacrylate, and dipentaerythritol pentaacrylate.

[0071] Polymerization inhibitors

[0072] The UV curable varnish may contain a polymerization inhibitor. Suitable polymerization inhibitors include phenolic antioxidants, hindered amine light stabilizers, phosphor antioxidants, hydroquinone monomethyl ether (commonly used in (meth)acrylate monomers), and hydroquinone, tert-butylcatechol, and pyrogallol may also be used.

[0073] Suitable commercial inhibitors are, for example, Sumilizer manufactured by Sumitomo Chemical Co. Ltd. TM GA-80, Sumilizer TM GM and Sumilizer TM GS; Genorad from Rahn AG TM 16. Genorad TM 18 and Genorad TM 20; Irgastab from Ciba Specialty Chemicals TM UV10 and Irgastab TM UV22, Tinuvin TM 460 and CGS20; Floorstab from Kromachem Ltd TM UV series (UV-1, UV-2, UV-5 and UV-8), Additol from Cytec Surface Specialties TM S Series (S100, S110, S120 and S130).

[0074] A preferred polymerization inhibitor is Irgastab from BASF TM UV10.

[0075] Since excessive addition of these polymerization inhibitors reduces the sensitivity of the ink to curing, it is preferred to determine the amount capable of preventing polymerization before blending. The amount of the polymerization inhibitor is preferably less than 2% by weight based on the total weight of the UV curable varnish.

[0076] Surfactants

[0077] Surfactants can be used in UV curable inkjet varnishes to control the surface tension, which is preferably above 28 mN / m in the present invention, more preferably even above 29, 30 mN / m. The surface tension depends not only on the amount and type of surfactant, but also on the polymerizable compounds and other additives in the ink composition.

[0078] The surfactant may be anionic, cationic, nonionic or zwitterionic and is typically added in a total amount of less than 2 wt%, preferably less than 1 wt%, based on the total weight of the UV curable inkjet varnish.

[0079] Suitable surfactants include fluorinated surfactants, fatty acid salts, ester salts of higher alcohols, alkylbenzene sulfonate salts of higher alcohols, sulfosuccinate salts and phosphate ester salts (e.g., sodium dodecylbenzenesulfonate and sodium dioctylsulfosuccinate), ethylene oxide adducts of higher alcohols, ethylene oxide adducts of alkylphenols, ethylene oxide adducts of polyol fatty acid esters, and acetylene glycol and its ethylene oxide adducts (e.g., polyoxyethylene nonylphenyl ether, and SURFYNOL available from AIR PRODUCTS & CHEMICALS INC. TM 104, 104H, 440, 465 and TG).

[0080] Preferred surfactants include fluorinated surfactants (such as fluorinated hydrocarbons) and silicone surfactants. Silicones are typically siloxanes and can be alkoxylated, polyether modified, polyester modified, polyether modified hydroxy functional, amine modified, epoxy modified, and other modifications or combinations thereof. Preferred siloxanes are polymeric, such as polydimethylsiloxane.

[0081] Fluorinated or organosilicon compounds used as surfactants may be crosslinkable surfactants. Suitable copolymerizable compounds having a surface-active effect include, for example, polyacrylate copolymers, organosilicon-modified acrylates, organosilicon-modified methacrylates, acrylated siloxanes, polyether-modified acrylic-modified siloxanes, fluorinated acrylates and fluorinated methacrylates. These acrylates may be mono-, di-, tri- or higher functional (meth)acrylates.

[0082] Silicone surfactants are preferred in the UV curable varnish of the present invention, especially reactive silicone surfactants that are polymerizable together with the polymerizable compound during the curing step.

[0083] Examples of useful commercial silicone surfactants are those supplied by BYK CHEMIE GMBH (including Byk TM-302, 307, 310, 331, 333, 341, 345, 346, 347, 348, UV3500, UV3510 and UV3530), those supplied by TEGO CHEMIESERVICE (including Tego Rad TM 2100, 2200N, 2250, 2300, 2500, 2600 and 2700), Ebecryl from CYTEC INDUSTRIES BV TM 1360 polysiloxane hexaacrylate and Efka from EFKA CHEMICALS BV TM -3000 series (including Efka TM -3232 and Efka TM -3883).

[0084] Substrate and printed products

[0085] In the inkjet printing method of the present invention, the varnish is sprayed onto a substrate, whereby a varnish layer is formed from droplets of the varnish. The substrate preferably comprises a printed image, more preferably an inkjet printed color image.

[0086] If the substrate comprises an inkjet-printed color image, the image contains a color layer formed by droplets of color ink. The droplet size of the droplets forming the clearcoat layer and the droplet size of the droplets forming the color layer are therefore preferably substantially equal, and / or the layer thickness of the clearcoat layer is greater than the layer thickness of the color layer, more preferably at least 1.2 times thicker. This leads to a more uniformly applied clearcoat layer, and thus a higher gloss.

[0087] In a preferred embodiment, the image on the substrate is at least partially printed using one or more UV-curable pigmented color inkjet inks jetted by the same inkjet printing device. The time period between the droplets of the one or more UV-curable pigmented color inkjet inks landing on the substrate and the start of UV curing is preferably shorter than 1 second. The latter generally improves image quality.

[0088] In a preferred embodiment, the varnish is applied imagewise over the color image. This is not only cost-effective, but also allows a certain relief to be created, for example on luxury goods packaging or interior decoration articles.

[0089] There is no practical restriction on the type of substrate on which the UV curable varnish of the present invention is inkjet printed. The substrate may have a ceramic, metal, glass, wood, paper or polymer surface for printing. The substrate may also be primed with white ink, for example.

[0090] The substrate may be porous, such as fabric, paper and cardboard substrates, or substantially non-absorbent, such as a plastic substrate having a polyethylene terephthalate surface.

[0091] Preferred substrates include surfaces of polyethylene, polypropylene, polycarbonate, polyvinyl chloride, polyesters such as polyethylene terephthalate (PET), polyethylene naphthalate (PEN) and polylactide (PLA), and polyimides.

[0092] The substrate may also be a paper substrate, such as plain paper or a resin coated paper, for example polyethylene or polypropylene coated paper. There is no practical limitation on the type of paper, which includes newsprint, magazine paper, office paper, wallpaper, and higher grammages, commonly referred to as paperboard, such as white lined chipboard, corrugated board, and packaging board.

[0093] The substrate may be transparent, translucent or opaque. Preferred opaque substrates include so-called synthetic papers, such as Synaps from Agfa-Gevaert. TM Grade, which is 1.10g / cm 3 or greater density opaque polyethylene terephthalate sheet.

[0094] There is no limitation on the shape of the substrate. It can be a flat sheet, such as paper or a polymer film, or it can be a three-dimensional object, such as a plastic coffee cup. The three-dimensional object can also be a container, such as a bottle or jerry-can for containing, for example, oil, shampoo, insecticide, pesticide, solvent, paint thinner or other types of liquids.

[0095] In a preferred embodiment of the inkjet printing method, the printed article is selected from point-of-purchase displays, billboards, vehicle graphics, backlit signage, display panels, stage graphics, architectural notices, billboards, and the like.

[0096] Inkjet printing device

[0097] The UV curable varnish is sprayed by one or more print heads that eject small droplets in a controlled manner through nozzles onto a substrate that is moving relative to the print heads.

[0098] In a preferred embodiment, the inkjet printing device used to jet the one or more UV curable color inkjet inks onto the substrate is also used to jet the UV curable varnish.

[0099] The preferred print head for the inkjet printing system is a piezoelectric head. Piezoelectric inkjet printing is based on the movement of a piezoelectric ceramic transducer when a voltage is applied to it. The application of voltage changes the shape of the piezoelectric ceramic transducer in the print head to create a void, which is then filled with inkjet ink or liquid. When the voltage is removed again, the ceramic expands to its original shape to eject ink drops from the print head. However, the inkjet printing method according to the present invention is not limited to piezoelectric inkjet printing. Other inkjet print heads can be used and include various types, such as continuous types as well as thermal, electrostatic and acoustic drop-on-demand inkjet types.

[0100] The inkjet print head typically scans back and forth across the moving ink-receiving surface in a transverse direction. The inkjet print head may not print on the return pass, but bidirectional printing is preferred to achieve high area throughput. Another preferred printing method is by a "single pass printing process", which can be performed using multiple staggered inkjet print heads covering the entire width of the ink-receiving surface. In a single pass printing process, the inkjet print head typically remains stationary and the ink-receiving surface is conveyed under the inkjet print head.

[0101] In a particularly preferred embodiment, the inkjet printing of the UV curable varnish is carried out in a multi-pass printing mode. Failed nozzles in a single-pass printing process produce line defects in the color image or gloss artifacts when printing varnishes.

[0102] UV curing device

[0103] The UV-curable varnishes according to the invention are cured by ultraviolet radiation.

[0104] The UV curing device may be arranged in combination with a printing module containing one or more print heads (12) for ejecting UV curable varnishes and may be operated therewith so that the curable composition is exposed to curing radiation soon after ejection. The printing module may contain one or more print heads (11) for ejecting UV curable colored inks.

[0105] In such an arrangement, it may be difficult to provide a sufficiently small radiation source connected to and operating with the print head. Therefore, a static fixed UV radiation source may be used, which is connected to the print module by means of a flexible radiation conducting device, such as a fiber optic bundle or an internally reflecting hose. Alternatively, the UV radiation may be supplied to the print module from a fixed source by an arrangement of mirrors.

[0106] In the present invention, the UV source is preferably a UV LED module. Due to its compactness, the UV LED can be easily incorporated into the scanning printing module. The preferred UV source is a UV source that exhibits a relatively long wavelength UV contribution with a dominant wavelength in the range of 360-420nm. Such a light source is preferred due to the consequent reduced light scattering to bring about more efficient internal curing.

[0107] To promote curing, the inkjet printer typically includes one or more oxygen depletion units. The oxygen depletion unit arranges a blanket of nitrogen or other relatively inert gas (e.g., CO2) at an adjustable position and adjustable inert gas concentration to reduce the oxygen concentration in the curing environment. Residual oxygen levels are typically kept as low as 200 ppm, but are typically in the range of 200 ppm to 1200 ppm.

[0108] Example

[0109] Measurement method

[0110] 1. Viscosity

[0111] Using Rotovisco from HAAKE TM RV1 viscometer at 45°C and 1,000 s -1 The viscosity of the UV curable composition was measured at a shear rate of 1.5 %.

[0112] 2. Surface tension

[0113] Use from GmbH, Germany The tensiometer K9 measures the static surface tension of UV curable inks after 60 seconds at 25°C.

[0114] 3. Splicing

[0115] The joints were evaluated by eye according to the criteria in Table 1. The appearance evaluation was carried out under direct lighting, i.e. not diffuse lighting, as this causes diffuse reflections and reduces the gloss impression. Direct lighting was carried out using 2 Philips MASTER TL-D Super 80 36W-840 cool white lamps. Observation of the surface was carried out at an angle of 45°, with the eye focusing on the surface itself, rather than on the reflected image of the light source. By focusing on the surface itself, information is obtained on flow / leveling defects (here the appearance of the joint).

[0116] Table 1

[0117] score standard C The splicing is clearly visible, poor uniform gloss appearance B Reduced visibility of splices and improved uniform gloss appearance A No visible splicing, uniform glossy appearance

[0118] Material

[0119] IBOMA is available as a Photomer TM 2012 Isobornyl methacrylate from IGM Resins

[0120] HDDA is available as a Sartomer TMSR238 1,6-Hexanediol diacrylate from SARTOMER

[0121] TPO is provided by IGM as Omnirad TM Trimethylbenzoyldiphenylphosphine oxide (MW=348) supplied by TPO

[0122] TPO-L is available as TPO-L Acylphosphine oxide photoinitiator obtained from IGM RESINS (MW=316)

[0123] CN3755 is available as a Sartomer TM CN3755 is a diacrylated amine synergist from SARTOMER

[0124] CN963B80 is available as Sartomer TM CN963B80 Urethane acrylate oligomer with two acrylate groups from ARKEMA (MW=1900)

[0125] Surf-1 is available as BYK TM UV 3510 Polyether modified polydimethylsiloxane surfactant obtained from BYK CHEMIE GMBH

[0126] Surf-2 is available as BYK TM 3566 Polyacrylate copolymer surfactant available from BYK CHEMIE GMBH

[0127] Surf-3 is available as a Silwet TM L7604 siloxane polyalkylene oxide copolymer surfactant from Momentive Performance Materials

[0128] Surf-4 is a siloxane polyalkylene oxide copolymer surfactant available from Momentive Performance Materials as Indusil 391

[0129] Surf-5 is available as Silwax TM B116 Liquid alkylated silicone surfactant from SILTECH CORPORATION

[0130] Surf-6 is available as a Synperonic TM 1310 Ethylene oxide / propylene oxide copolymer based on ethylenediamine for use as a low HLB surfactant from Croda Industrial Chemicals

[0131] UV10 is 4-hydroxy-2,2,6,6-tetramethylpiperidinyloxy sebacate available from Shanghai FINC Chemical Technology Co., Ltd.

[0132] Dibond substrate is available as 2 mm thick plate from ANTALIS having a polyethylene core sandwiched between two aluminum layers laminated thereto

[0133] Example 1

[0134] This example shows how to reduce or eliminate gloss non-uniformity caused by splicing in inkjet varnishing of a substrate.

[0135] Preparation of varnish composition

[0136] Varnishes COMP-1 to COMP-4 and INV-1 to INV-6 were all prepared in the same manner by mixing the components according to Tables 2 and 3. In order to keep the varnish compositions comparable, the surface tension was adjusted by using different surfactants at the same concentration. The weight percentage (wt%) of each component is based on the total weight of the varnish. The viscosity and static surface tension of each varnish composition were measured and are also listed in Tables 2 and 3 in mPa.s and mN / m, respectively.

[0137] Table 2

[0138] weight%: COMP-1 COMP-2 COMP-3 COMP-4 HDDA 51.10 56.40 59.48 55.40 IBOMA 16.05 16.75 17.67 17.75 CN3755 17.00 11.00 7.00 12.00 TPO 15.00 15.00 15.00 TPO-L 12.00 CN963B80 2.00 UV10 0.50 0.50 0.50 0.50 Surf-1 0.35 0.35 0.35 Surf-2 0.35 Total = 100.00 100.00 100.00 100.00 Viscosity 11.6 9.0 7.4 8.3 Surface tension 22.2 23.8 24.0 29.5

[0139] Table 3

[0140]

[0141]

[0142] Results and Evaluation

[0143] Varnishes COMP-1 to COMP-4 and INV-1 to INV-6 were prepared by using CMYKLcLk inkset from AGFA 1250 and Figure 2 The printing module (10) shown in FIG. 1 is a multi-pass printing press from AGFA. H3300HS printed on Dibond substrate. Test pattern bidirectionally printed by print head (11) and UV cured by UV LED module (15) at the same time, including unprinted area and area with text and multicolor graphics, also including 40cm x 40cm dark green square. Varnish compositions COMP-1 to COMP-4 and INV-1 to INV-6 were sprayed using print head (12) and with UV LED module (16) turned off. UV curing of the varnish was performed using UV LED module (17) after a delay of about 5 seconds.

[0144] The effect of splicing on gloss was evaluated for each printed sample and is shown in Table 4, which also shows the static surface tension of the varnish and the amount of acylphosphine oxide initiator in the varnish.

[0145] Table 4

[0146] Printing samples Static surface tension (mN / m) wt% APO Splicing score COMP-1 22.2 15 C COMP-2 23.8 15 C COMP-3 24.0 15 C COMP-4 29.5 12 C INV-1 30.8 15 B INV-2 30.1 15 B INV-3 30.0 15 A INV-4 34.4 15 B INV-5 31.6 15 A INV-6 31.7 15 A

[0147] It should be clear from Table 4 that only the samples of the present invention are able to reduce gloss artifacts because they meet the required static surface tension and minimum amount of acylphosphine oxide initiator in the clearcoat.

[0148] Reference numerals list

[0149] Table 5 shows Figure 1 and 2 The reference numerals used in

[0150] Table 5

[0151] 1 Raised 2 Depression 3 First printed strip 4 Second printed strip 10 Printing module 11 Print heads for color inks 12 Print heads for varnishes 13 Scan direction 14 Substrate conveying direction 15 UV LED Modules 16 UV LED Modules 17 UV LED Modules

Claims

1. An inkjet printing method comprising the steps of: a) spraying varnish on the substrate; and b) UV curing the varnish with a UV LED source having a spectral emission in the range of 360-420 nm after a curing delay of at least 2 seconds; The static surface tension of the varnish is above 28 mN / m; wherein the varnish contains greater than 12 wt % of an acylphosphine oxide photoinitiator based on the total weight of the varnish; and The curing delay is the time period between when the varnish drop is dropped onto the substrate and when UV curing begins.

2. The inkjet printing method according to claim 1, wherein the varnish contains a mixture of multifunctional acrylates and methacrylates.

3. The inkjet printing method according to claim 2, wherein the multifunctional acrylate is a difunctional acrylate.

4. The inkjet printing method according to any one of claims 1 to 3, wherein the varnish contains a polymerizable composition having: a) 10 to 30% by weight of monofunctional methacrylate; b) 40 to 70 wt % of a multifunctional acrylate; and c) 0 to 15% by weight of monofunctional acrylate, the weight percentages being based on the total weight of the clearcoat.

5. The inkjet printing method according to any one of claims 1 to 4, wherein the monofunctional methacrylate has a Tg greater than 25°C. 6 . The inkjet printing method according to claim 5 , wherein the monofunctional methacrylate comprises isobornyl methacrylate.

7. The inkjet printing method according to any one of claims 1 to 6, wherein the polymerizable composition does not contain monofunctional acrylate.

8. The inkjet printing method according to any one of claims 1 to 7, wherein the varnish is heated to 45°C and at 1,000 s -1 The viscosity is less than 13.0 mPa.s at a shear rate of .

9. The inkjet printing method according to any one of claims 1 to 8, wherein the acylphosphine oxide photoinitiator is an acylphosphine oxide initiator in which the acyl group is selected from the group consisting of a benzoyl group substituted with a urea group, an oxamide group or a polymerizable group; a 2,6-dimethylbenzoyl group substituted at the 3-position with a urea group, an oxamide group or a polymerizable group; a 2,6-dimethoxybenzoyl group substituted at the 3-position with a urea group, an oxamide group or a polymerizable group; a 2,4,6-trimethylbenzoyl group substituted at the 3-position with a urea group, an oxamide group or a polymerizable group; and a 2,4,6-trimethoxybenzoyl group substituted at the 3-position with a urea group, an oxamide group or a polymerizable group.

10. The inkjet printing method according to any one of claims 1 to 9, wherein the substrate comprises a printed image.

11. The inkjet printing method according to claim 10, wherein the printed image is an inkjet-printed color image formed by droplets of color ink.

12. The inkjet printing method according to claim 11, wherein a droplet size of the droplets forming the varnish layer and a droplet size of the droplets forming the color image are substantially equal.

13. The inkjet printing method according to claim 11 or 12, wherein the layer thickness of the varnish layer is greater than the layer thickness of the color image.

14. The inkjet printing method according to any one of claims 1 to 13, wherein the static surface tension of the varnish is at least 30 mN / m.

15. A varnished substrate produced by a process according to any one of claims 1 to 14, suitable for use as packaging or interior decoration.

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