Photoinitiator mixtures and method for preparing same

Abstract

The present invention relates to specific oligomeric acylphosphine oxide photoinitiators and encompasses mixtures of these photoinitiators for use in radiation-curable compositions, particularly UV-curable inkjet inks. These compositions exhibit low odour and low migration properties, making them suitable for indoor decoration applications. Additionally, the invention includes inkjet printing methods involving the jetting of such UV- curable inkjet inks and a manufacturing method for a mixture of oligomeric acylphosphine oxide photoinitiators.

Description

DescriptionPHOTOINITIATOR MIXTURES AND METHOD FOR PREPARING SAMETechnical Field

[0001] The present invention relates generally to radiation curable technology.More particularly, to low odour and low migration photoinitiators based on oligo-condensation of acyl phosphine oxides for LED-based UV curing systems.Background Art

[0002] Radiation curable technology has undergone significant evolution in recent years, transitioning from mercury bulb-based exposure to LED exposure systems. This shift has primarily focused on the use of 385 nm and 395 nm LEDs as the predominant light sources for curing applications. The adoption of LED technology has brought about numerous advantages, including improved energy efficiency, longer operational lifetimes, and reduced environmental impact compared to traditional mercury-based systems.

[0003] However, this technological advancement has also introduced new challenges, particularly in the realm of photoinitiators. The transition to LED-based curing has severely limited the number of suitable photoinitiator classes available in the market. Currently, thioxanthones as type II initiators and acyl phosphine oxides as type I initiators represent the most important initiator classes for 385-395 nm LED exposure. This restriction in photoinitiator options has significant implications for various industries relying on radiation curable technology.

[0004] Thioxanthones, while effective in certain applications, have notable limitations. These photoinitiators are not well suited for use in varnishes, cyan inks and white inks due to their inherent tendency to cause yellowing upon exposure. This discoloration effect restricts their applicability in many high-quality printing and coating processes where colour accuracy and stability are crucial. As a result, acyl phosphine oxides have emerged asthe most important class of photoinitiators for 385-395 nm LED exposure systems.

[0005] Despite their widespread use, standard commercially available acyl phosphine oxides are not without their drawbacks. In recent years, there have been growing concerns regarding the toxicological profile of these compounds. The potential health and environmental risks associated with their use have prompted increased scrutiny and a search for safer alternatives. This situation has created a significant challenge for industries that rely heavily on radiation curable technology, as they must balance performance requirements with safety considerations.

[0006] Furthermore, standard acyl phosphine oxides generate medium volatile degradation products during the curing process. These byproducts often result in a persistent background odour, which can be particularly problematic in applications such as indoor decorations. The presence of these odours can negatively impact user experience and limit the use of radiation curable technology in sensitive environments. Additionally, the migration of standard acyl phosphine oxides poses challenges in food packaging and related applications, where strict regulations govern the use of potentially migratory substances.

[0007] The limitations of current photoinitiator technologies extend beyond toxicological and odour concerns. The performance characteristics of available photoinitiators may not fully meet the demands of advanced LED curing systems, particularly in terms of curing speed, depth of cure, and compatibility with various formulations. This gap in performance can lead to reduced efficiency in manufacturing processes and limitations in the types of products that can be effectively produced using LED curing technology.

[0008] To address these limitations, significant research efforts have focused on designing functionalized acyl phosphine oxides. To mitigate the volatility of degradation products, the acyl fragment of acyl phosphine oxides must be functionalized. Several approaches have been disclosed in prior art. WO 2014 / 129213 A (FUJIFILM) and WO 2014 / 051026 A (FUJIFILM) describe a method involving benzylic halogenation followed by further derivatizationto yield mesityl-functionalized acyl phosphine oxides. However, limited selectivity in halogenation and the multistep synthesis process are major drawbacks of this approach. WO 2017 / 086224 A (FUJIFILM) discloses a Friedel-Crafts approach to functionalize standard acyl phosphine photoinitiators, with a similar multistep Friedel-Crafts based approach also disclosed in EP 4393720 A (ARKEMA) . Friedel-Crafts reactions are known to generate significant waste and require special precautions due to the corrosive nature of typical Friedel-Crafts catalysts. A selective approach using nitration, avoiding highly corrosive chemistry but still requiring multistep synthesis, has been disclosed in WO 2019 / 243099 A (AGFA) and WO 2022 / 106100 A (AGFA) .

[0009] Further multistep synthesis approaches have been disclosed in US 2013328028 A (CH EIL INDUSTRIES) , JP 2019183051 A (KONICA MINOLTA) , WO 2019 / 071428 A (DIC) , CN 114507255 A (SOUTH CHINA UNIVERSITY OF TECHNOLOGY) , WO 2013 / 091521 A (SHENZHEN UV-CHEMTECH CO) and US 2007027229 A (IVOCLAR VIVADENT ) . Additional approaches have been published by YinPing et al. in the European Polymer Journal (2022) and Progress in Organic Coatings (2022).

[0010] All these approaches involve multistep synthesis, sometimes combined with corrosive chemistry, generating substantial waste and limiting their applicability to high-end applications such as dental products. The prior art approaches have limited economic viability for more general-purpose applications like indoor decoration and food packaging, and several have limited potential for low migration applications.

[0011] Therefore, there is a need to overcome the problems discussed above.The development of novel photoinitiator systems that address these challenges is crucial for the continued advancement of radiation curable technology. Such innovations should focus on enhancing safety profiles, reducing odour and migration issues, and improving overall performance in LED-based curing systems. Additionally, new photoinitiators should be compatible with a wide range of formulations and substrates to ensure broad applicability across various industries.Summary of invention

[0012] An approach has been developed that combines low odour and low migration properties with a cost-effective synthetic method based on the oligo-condensation of standard (commercial) acyl phosphine oxides.

[0013] Objects of the present invention are realized by oligomeric acyl phosphine oxide photoinitiators as defined in claim 1.

[0014] It is a further object of the present invention to provide a cost-effective synthetic process for the preparation of these oligomeric acyl phosphine oxides and mixtures thereof.

[0015] It is a further object of the present invention to provide radiation curable compositions and (inkjet) inks comprising at least one oligomeric acyl phosphine oxide according to the present invention.

[0016] These and other objects of the present invention will become apparent from the detailed description hereinafter.Description of embodimentsDefinitions

[0017] The term “alkyl group” means a functional group with all variants possible for each number of carbon atoms in the alkyl group, i.e. for one carbon atom: methyl, for two carbon atoms: ethyl, for three carbon atoms: n-propyl and isopropyl; for four carbon atoms: n-butyl, isobutyl and tertiary-butyl; for five carbon atoms: n-pentyl, 1 ,1-dimethyl-propyl, 2,2-dimethylpropyl and 2- methyl-butyl, etc.Unless otherwise specified, a substituted or unsubstituted alkyl group is preferably a Ci to Ce-alkyl group, more preferably a Ci to C4-alkyl group and most preferably a methyl group or ethyl group.

[0018] The term “alkoxy group” means a functional group with all variants possible for each number of carbon atoms in the alkoxy group, i.e. for one carbon atom: methoxy, for two carbon atoms: ethoxy, for three carbon atoms: n-propoxy and isopropoxy; for four carbon atoms: n-butoxyl, isobutoxy and tertiary-butoxy; etc.Unless otherwise specified, an alkoxy-group is preferably a Ci to Ce-alkoxy group, wherein a methoxy group and an ethoxy group are particularly preferred.

[0019] An alkenyl group is a functional group derived from alkenes and is characterized by the presence of a double bond. Unless otherwise specified, an alkenyl group is preferably a C2 to Ce-alkenyl group.

[0020] An alkynyl group is a functional group derived from alkynes and is characterized by the presence of a triple bond. Unless otherwise specified, an alkynyl group is preferably a C2 to Ce-alkynyl group.

[0021] The term “aryl group” means a monocyclic or polycyclic aromatic ring structure comprising only carbon atoms in the aromatic ring structure. Unless otherwise specified, an aryl group is preferably a phenyl group (CeHs-), or a naphthyl group.

[0022] The term “aralkyl group” means an aryl group attached to an alkyl chain.Unless otherwise specified, an aralkyl group is preferably an aryl group attached to a Ci to Ce-alkyl chain, more preferably a benzyl group (C6H5CH2-), where a phenyl group (CeHs-) is attached to a methylene group (-CH2-).

[0023] The term “alkaryl group” means a functional group having one or more alkyl groups attached to an aryl group. Essentially, it is the opposite of an aralkyl group.Unless otherwise specified, an alkaryl group is preferably a group having one, two, three or more alkyl groups attached to a phenyl group or a naphthyl group, more preferably a tolyl group (CH3C6H4-), where a methyl group (CH3-) is attached to a phenyl group.

[0024] The term “heteroaryl group” means a monocyclic or polycyclic aromatic ring comprising carbon atoms and one or more heteroatoms in the ring structure, preferably 1 to 4 heteroatoms independently selected from nitrogen, oxygen, selenium and sulphur.Preferably, a heteroaryl group is a monocyclic ring, and more preferably a heteroaryl group is a five- or six-membered ring substituted by one, two or three oxygen atoms, nitrogen atoms, sulphur atoms, selenium atoms or combinations thereof.Preferred examples of heteroaryl groups include, but are not limited to,pyridinyl, pyridazinyl, pyrimidyl, pyrazyl, quinolyl, triazinyl, pyrrolyl, pyrazolyl, imidazolyl, (1,2,3,)- and (1 ,2,4)-triazolyl, pyrazinyl, pyrimidinyl, tetrazolyl, furyl, thienyl, isoxazolyl, thiazolyl, isoxazolyl, and oxazolyl.

[0025] The term “alkylene group” means a bivalent functional group derived from an alkane by removing two hydrogen atoms from one or two carbon atoms, thereby creating two points of attachment.Unless otherwise specified, an alkylene group is preferably a Ci to C20- alkylene group, more preferably a Ci to Cio-alkylene group and most preferably a Ci to Ce-alkylene group.

[0026] The term “arylene group” means a bivalent functional group derived from an aromatic hydrocarbon (arene) by removing a hydrogen atom from two ring carbon atoms.Unless otherwise specified, an arylene group is preferably a phenylene group.

[0027] The term “heteroarylene group” means a bivalent functional group derived from a heteroarene by removing a hydrogen atom from two ring atoms. Heteroarenes are aromatic compounds where one or more carbon atoms in a ring are replaced by heteroatoms such as nitrogen, oxygen, or sulphur. Preferred examples of heteroarylene groups include, but are not limited to, pyridinylene, pyridazinylene, pyrimidylene, pyrazylene, quinolylene, triazinylene, pyrrolylene, pyrazolylene, imidazolylene, (1,2,3,)- and (1,2,4)-triazolylene, pyrazinylene, pyrimidinylene, tetrazolylene, furylene, thienylene, isoxazolylene, thiazolylene, isoxazolylene, and oxazolylene.

[0028] The term “cycloalkylene group” means a bivalent group derived from a cycloalkane by removing two hydrogen atoms from one or two carbon atoms in the ring.Unless otherwise specified, a cycloalkylene group is preferably a cyclohexylene group.

[0029] The term “substituted”, in e.g. a substituted alkyl group, means that the group may be substituted by other atoms than the atoms normally present in such a group, for an alkyl group: carbon and hydrogen. For example, a substituted alkyl group may thus include a halogen atom or a thiol group,while an unsubstituted alkyl group contains only carbon and hydrogen atoms. The latter applies also for an alkenyl group, an alkynyl group, an aralkyl group, an alkaryl group, an aryl group, a heteroaryl group, an alkoxy group and an aryloxy group.

[0030] Unless otherwise specified, a substituted functional group of the functional groups listed above is preferably substituted by one or more constituents selected from the group consisting of an ester group, an amide group, an ether group, a thioether group, a ketone group, an aldehyde group, a sulfoxide group, a sulfone group, a sulfonate ester group, a sulphonamide group, a free radical polymerizable ethylenically unsaturated group, -Cl, - Br, -I, -OH, -SH, -CN and -NO2. The free radical polymerizable ethylenically unsaturated group is preferably selected from the group consisting of an acrylate group, a methacrylate group, an acrylamide group, a methacrylamide group, a styrene group, a maleate group, a fumarate group, an itaconate group, a vinyl ether group, a vinyl ester group, an allyl ether group and an allyl ester group.Oligomeric Acylphosphine Oxide Photoinitiators

[0031] An oligomeric acylphosphine oxide photoinitiator of the invention has a chemical structure according to Formula (I):Formula (I), wherein,R1 to Re are independently selected from an alkyl group and an alkoxy group;R7 to R9 are independently selected from the group consisting of a hydrogen, an alkyl group, an alkenyl group, an alkynyl group, an aralkyl group, an alkaryl group, an aryl group, a heteroaryl group, an alkoxy group and an aryloxy group;R10 to R12 are independently selected from the group consisting of an alkyl group, an alkenyl group, an alkynyl group, an aralkyl group, an alkaryl group, an aryl group, a heteroaryl group, an alkoxy group and an aryloxy group;R13 to R15 are independently selected from the group consisting of an alkyl group, an alkenyl group, an alkynyl group, an aralkyl group, an alkaryl group, an aryl group and an heteroaryl group;n and m are integers describing the functionality;n is greater than or equal to 0;m is greater than or equal to 1 ; andn+m is greater than or equal to 3.

[0032] When the sum of n and m is less than 3, the oligomeric acylphosphine oxide photoinitiator tends to contain residual amounts of unreacted monofunctional acylphosphine oxide. This can lead to toxicological concerns when used in radiation curable compositions, as well as potential issues with odor and migration.

[0033] In a preferred embodiment, the oligomeric acylphosphine oxide initiator fulfils one, two, three, or all of the following conditions (a) to (d):a) Ri to Re are independently selected from an Ci to C4-alkyl group and an Ci to C4-alkoxy group;b) R? to Rg are independently selected from the group consisting of a hydrogen, an alkyl group and an alkoxy group;c) Rio to R12 are independently selected from the group consisting of an alkyl group, an alkoxy group and an aryl group; andd) R13 to R15 are independently selected from the group consisting of an alkyl group and an aryl group.

[0034] In another preferred embodiment, the oligomeric acylphosphine oxide initiator fulfils one, two, three, or all of the following conditions e) to h): e) Ri to Re are independently selected from an methyl group and a methoxy group, with a methyl group being the most preferred;f) R? to Rg are independently selected from the group consisting of a hydrogen, a methyl group and a methoxy group, with a methyl group being the most preferred;g) R10 to R12 are independently selected from the group consisting of an alkyl group, an alkoxy group and an aryl group, with an alkoxy group being particularly preferred; andh) R13 to R15 are independently selected from the group consisting of an alkyl group and an aryl group, with an aryl group being the most preferred.

[0035] In a further preferred embodiment, the above oligomeric acylphosphine oxide photoinitiator according is a compound where n is equal to 0.

[0036] In a further preferred embodiment, the above oligomeric acylphosphine oxide photoinitiator is a compound where Ri to Re are identical; R? to Rg are identical; Rio to R12 are identical; and R13 to R15 are identical.

[0037] R1 to Re are preferably methyl groups, as they ensure hydrolytic stability of the acylphosphine oxide group without introducing excessive steric bulk that could hinder oligomer formation. During the oligomerization of acylphosphine oxides, steric hindrance plays a significant role. When also R?to R9 are not hydrogen, then these subsitituents R1 to R9 can interfere with the formation of methylene bridges between initiator units. Methyl groups are small enough to allow condensation to proceed under mild conditions, while still contributing to the overall stability of the molecule.

[0038] R10 to R12 are preferably alkoxy groups, such as methoxy or ethoxy, as these enhance solubility. Acylphosphine oxides containing such alkoxy groups, and used as starting materials in the synthesis of the aforementioned oligomeric acylphosphine oxide photoinitiator, are typically liquid monoacylphosphine oxide photoinitiators, which further facilitates the synthesis.

[0039] Preferred examples include at least one of the oligomeric acylphosphine oxide photoinitiators or their mixtures, as listed in Table 1 below, though they are not limited to these.Table 1

[0040] Another preferred embodiment is a mixture of acylphosphine oxide photoinitiators including one or more oligomeric acylphosphine oxide photoinitiators as described above. Such a mixture may include an acylphosphine oxide photoinitiator wherein the integer n is equal to 0 and m is equal to 1.

[0041] In a particularly preferred embodiment, the mixture includes a photoinitiator selected from the group consisting of:

[0042] In a preferred embodiment, the mixture of acyl phosphine oxide oligomers consists of or includes substantially, i.e. more than 90 wt% based on the mixture, oligomeric acyl phosphine oxides wherein n+m represents a range from 1 to 10, more preferably from 1 to 6 and most preferably from 1 to 4.

[0043] The average molecular weight of the oligomeric acyl phosphine oxide photoinitiator according to the present invention is preferably between 750 and 2500, more preferably between 800 and 2000 and most preferably between 850 and 1500, calculated as a weight average of the dimer,trimer, tetramer, pentamer and hexamer in the mixture, where the composition of the mixture is determined by the LIPLC method as described in the examples. The concentration of the different oligomers is determined as area% in UV.Manufacturing Methods

[0044] A manufacturing method of a mixture of oligomeric acylphosphine oxide photoinitiators as described above may be performed the synthesis step:wherein Ri to R15 are defined as above and n and m represent an integer as defined above; andwherein the formaldehyde source is preferably a poly(oxymethylene).

[0045] In the above manufacturing method, a mixture of acyl phosphine oxides can be condensed to a mixture of acyl phosphine oxide oligomers.However, it is not excluded that R1, R3 and R5 are identical, R2, R4 and Re are identical, R7 to R9 are identical, R10 to R12 are identical, and R13 to R14 are identical.

[0046] In principle, any formaldehyde source can be used but poly(oxymethylene) is particularly preferred.

[0047] In a preferred embodiment, no cosolvent is used in the acidic medium.

[0048] In a further preferred embodiment, the acid used has an acidity constant of 1 or lower, measured in water at 25°C. Sulfonic acids and sulfuric acid are particularly preferred acids, methane sulfonic acid and sulfuric acid being the most preferred.

[0049] In principle, any acyl phosphine oxide, capable of reaction with a formaldehyde source in acidic conditions can be used. Ethyl(2,4,6- trimethylbenzoyl)phenyl-phosphinate (TPO-L) and 2,4,6- (trimethylbenzoyl)diphenylphosphorus oxide (TPO) are particularly preferred starting materials for oligomerization.

[0050] In a further embodiment of the present invention, said acyl phosphine oxides can be copolymerized with other aromatic monomers not having an acyl phosphine oxide photoinitiating moiety.

[0051] In a preferred embodiment, the mixture of acyl phosphine oxide oligomers consists of or includes substantially, i.e. more than 90 wt% based on the mixture, oligomeric acyl phosphine oxides wherein n+m represents a range from 1 to 10, more preferably from 1 to 6 and most preferably from 1 to 4.

[0052] A single oligomeric acyl phosphine oxide photoinitiator may be isolated from a mixture using chromatographic purification methods. These methods are well-known to those skilled in the art. However, a greater formulation flexibility is one of the advantages of working with a mixture.Radiation Curable Compositions and (Inkjet) Inks

[0053] Another aspect of the invention is a radiation curable composition including a polymerizable compound and an oligomeric acylphosphine oxide photoinitiator, or a mixture thereof, as described above.

[0054] The amount of the oligomeric acylphosphine oxide photoinitiator, or a mixture thereof, depends on the application for which it is used. Usually, the oligomeric acylphosphine oxide photoinitiator, or a mixture thereof, is used in an amount of 1 to 20 wt%, preferably 5 to 15 wt% based on the total weight of the radiation curable composition.

[0055] The radiation curable composition may be a colourless composition such as a colourless UV curable ink or inkjet ink, but preferably it contains a colorant, more preferably a colour pigment. Colourless UV curable (inkjet) inks may be used, for example, as a protective varnish or as a primer to improve adhesion of a printed image to a substrate

[0056] In addition to a colorant, the radiation curable composition may include other ingredients as desired, such as surfactants, dispersants, dispersion synergists, stabilizers, UV absorbers and the like.

[0057] A particularly preferred embodiment is a UV curable inkjet ink including a radiation curable composition as described above.

[0058] For having a good ejecting ability, the viscosity of the UV curable inkjet ink at the jetting temperature is preferably smaller than 30.0 mPa.s, more preferably smaller than 20.0 mPa.s, most preferably between 5.0 and 16.0 mPa.s at a shear rate of 1000 s1and at a jetting temperature between 30 and 70°C, preferably at a jetting temperature of 45°C.

[0059] The surface tension of the UV curable inkjet ink is preferably in the range of 20 mN / m to 35 mN / m at 25°C, more preferably in the range of about 22 mN / m to about 30 mN / m at 25°C. In these ranges, good ink spreading is obtained on a wide range of substrates.

[0060] While a single UV curable inkjet ink can be used in certain applications, it is preferable to use a UV curable inkjet ink set containing multiple differently coloured inkjet inks in accordance with the invention.

[0061] For printing multi-colour images, the UV-curable inkjet ink is preferably part of a UV curable inkjet ink set containing at least three, but most preferably at least four, UV curable inkjet inks in accordance with the invention. Such an inkjet ink set is preferably a UV curable CMYK or CRYK inkjet ink set, and may further include a UV curable white inkjet ink to enhance colour vibrancy. This inkjet ink set can also be extended with additional inks such as violet, green, red, blue, and / or orange to further expand the colour gamut of the image.

[0062] The UV-curable inkjet ink set can be enhanced by combining full-density inkjet inks with light-density inkjet inks. Full-density inkjet inks match the colour of their light-density counterparts but contain more of the same colorant. Full and light-density inks are sometimes referred to as dark and light colour inks, respectively. The combination of such dark and light colour inks, such as black and grey inks, improves image quality by reducing graininess.

[0063] The UV curable inkjet inks described in the invention have a reduced odour, making them highly suitable for indoor decoration applications. Low odour is essential when manufacturing indoor decorative articles for rooms and vehicles, such as furniture, wallpaper, doors, natural leather articles, textile fabrics and decorative panels such as flooring laminate panels.

[0064] For reproducing wood patterns, a CRYK ink set is preferred over a CMYK ink set, particularly for achieving a large colour gamut and good metamerism. In a preferred embodiment of such a UV curable inkjet ink set, the ink set includes: a cyan UV curable inkjet ink containing a betacopper phthalocyanine pigment; a red UV curable inkjet ink containing a pigment selected from the group consisting of C.L Pigment Red 57 / 1, C.L Pigment Red 122, C.L Pigment Red 144, C.L Pigment Red 170, C.L Pigment Red 175, C.L Pigment Red 176, C.L Pigment Red 187, C.L Pigment Red 188, C.L Pigment Red 202, C.L Pigment Red 207, C.L Pigment Red 242, C.L Pigment Red 254, C.L Pigment Red 272 and mixed crystals thereof; a yellow UV curable inkjet ink containing a pigment selected from C.L Pigment Yellow 74 C.L Pigment Yellow 83, C.L Pigment Yellow 97, C.L Pigment Yellow 110, C.L Pigment Yellow 120, C.L Pigment Yellow 139, C.L Pigment Yellow 138, C.L Pigment Yellow 150, C.L Pigment Yellow 151, C.L Pigment Yellow 154, C.L Pigment Yellow 155, C.L Pigment Yellow 175, C.L Pigment Yellow 180, C.L Pigment Yellow 181 , C.L Pigment Yellow 194, C.L Pigment Yellow 213, C.L Pigment Yellow 214 and mixed crystals thereof; and a black UV curable inkjet ink containing a carbon black pigment; preferably complemented by a white UV curable inkjet ink and / or a colourless UV curable inkjet ink. For an improvement in graininess, the ink set may contain full- and light-density UV curable inkjet inks.

[0065] When more vibrant colours are desired, the red UV curable inkjet ink is replaced by or supplemented with a magenta UV curable inkjet ink. This magenta ink preferably contains a pigment selected from the group consisting of C.L Pigment Violet 19 and its mixed crystals.

[0066] The above-described oligomeric acyl phosphine oxide photoinitiator is preferably present in an amount between 1 and 30 wt%, more preferably 2and 25 wt%, most preferably between 5 and 20 wt%, with the wt% based on the total weight of the UV curable inkjet ink.

[0067] The UV curable composition inkjet ink preferably contains 10 to 95 wt% of free radical polymerizable compounds, with the wt% based on the total weight of the UV curable inkjet ink.

[0068] The UV curable inkjet ink preferably includes polymerizable compound selected from the group consisting of isobornyl acrylate, vinyl methyl oxazolidinone, 2-(2-vinyloxyethoxy)ethyl acrylate, 4-tert-butylcyclohexyl acrylate, tricyclodecane dimethanol diacrylate, dipropylene glycol diacrylate, isodecyl acrylate, 3,3,5-trimethylcyclohexyl acrylate, lauryl acrylate, 4-hydroxybutyl acrylate, (3-ethyloxetane-3-yl)methyl acrylate, (2- methyl-2-ethyl-1 ,3-dioxolane-4-yl)methyl acrylate, benzyl acrylate, 1 ,6- hexanediol diacrylate, dodecanedioic acid diacrylate, tri(propylene glycol) diacrylate, and 2-(2-butoxyethoxy)ethyl acrylate. These polymerizable compounds provide advantages in reducing odour or toxicological issues.Polymerizable Compounds

[0069] There is no limitation on the type of polymerizable compounds used in the radiation curable compositions and UV curable (inkjet) inks of the invention as long as it is a compound that can be made to undergo a polymerization reaction by means of an initiating species generated from the oligomeric acyl phosphine oxide photoinitiator by the application of UV radiation. The polymerizable compound is preferably a free radical polymerizable compound.

[0070] The free radical polymerizable compound is preferably an ethylenically unsaturated compound. The free radical polymerizable chemistry may be a (meth)acrylate based polymerizable chemistry, but may for instance also be a thiol-ene and / or thiol-yne polymerizable chemistry. Suitable polymerizable monomers may be any monomer found in the Polymer Handbook Vol 1 + 2, 4th edition, edited by J. BRANDRUP et al., Wiley- Interscience, 1999.

[0071] In this invention, preferably a mixture of free radical polymerizable monomers and oligomers is utilized. These monomers and oligomers can have varying degrees of functionality, including mono-, di-, tri-, and higherfunctionality monomers. Such a mixture allows for fine-tuning the viscosity of the UV curable composition based on the properties of the oligomeric acyl phosphine oxide photoinitiator, which is especially relevant for UV curable inkjet inks.

[0072] A monofunctional monomer is typically used to enhance the flexibility of a cured layer, while a polyfunctional monomer is employed to improve its scratch resistance.

[0073] A monofunctional monomer contains a single free radical polymerizable group preferably selected from the group consisting of an acrylate, a methacrylate, an acrylamide, a methacrylamide, a styrene group, a maleate, a fumarate, an itaconate, a vinyl ether, a vinyl ester, an allyl ether and an allyl ester.

[0074] A polyfunctional polymerizable compound contains two, three or more free radical polymerizable groups preferably independently selected from the group consisting of an acrylate, a methacrylate, an acrylamide, a methacrylamide, a styrene group, a maleate, a fumarate, an itaconate, a vinyl ether, a vinyl ester, an allyl ether and an allyl ester.

[0075] In the present specification, both or either one of 'acrylate' and 'methacrylate' are sometimes referred to as '(meth)acrylate', and both or either one of 'acrylic' and 'methacrylic' are sometimes referred to as '(meth)acrylic'.

[0076] Examples of the (meth)acrylate used as the free radical polymerizable monomer include a monofunctional (meth)acrylate, a difunctional (meth)acrylate, a trifunctional (meth)acrylate, a tetrafunctional (meth)acrylate, a pentafunctional (meth)acrylate, and a hexafunctional (meth)acrylate.

[0077] In a preferred embodiment, the monofunctional monomers are selected from hexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, tert-octyl (meth)acrylate, isoamyl (meth)acrylate, decyl (meth)acrylate, isodecyl (meth)acrylate, stearyl (meth)acrylate, isostearyl (meth)acrylate, cyclohexyl (meth)acrylate, 4-n-butylcyclohexyl (meth)acrylate, bornyl (meth)acrylate, isobornyl (meth)acrylate, benzyl (meth)acrylate, 2- ethylhexyldiglycol (meth)acrylate, butoxyethyl (meth)acrylate, 2-chloroethyl(meth)acrylate, 4-bromobutyl (meth)acrylate, cyanoethyl (meth)acrylate, benzyl (meth)acrylate, butoxymethyl (meth)acrylate, 3-methoxybutyl (meth)acrylate, an alkoxymethyl (meth)acrylate, an alkoxyethyl (meth)acrylate, 2-(2-methoxyethoxy)ethyl (meth)acrylate, 2-(2-butoxyethoxy)ethyl (meth)acrylate, 2,2,2-trifluoroethyl (meth)acrylate, 1 H ,1 H,2H,2H-perfluorodecyl (meth)acrylate, 4-butylphenyl (meth)acrylate, phenyl (meth)acrylate, 2,4,5-trimethylphenyl (meth)acrylate, 4-chlorophenyl (meth)acrylate, phenoxymethyl (meth)acrylate, phenoxyethyl (meth)acrylate, glycidyl (meth)acrylate, glycidyloxybutyl (meth)acrylate, glycidyloxyethyl (meth)acrylate, glycidyloxypropyl (meth)acrylate, tetrahydrofurfuryl (meth)acrylate, a hydroxyalkyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 3-hydroxybutyl (meth)acrylate, dimethylaminoethyl (meth)acrylate, diethylaminoethyl (meth)acrylate, dimethylaminopropyl (meth)acrylate, diethylaminopropyl (meth)acrylate, trimethoxysilylpropyl (meth)acrylate, trimethylsilylpropyl (meth)acrylate, polyethylene oxide monomethyl ether (meth)acrylate, oligoethylene oxide monomethyl ether (meth)acrylate, polyethylene oxide (meth)acrylate, oligoethylene oxide (meth)acrylate, an oligoethylene oxide monoalkyl ether (meth)acrylate, a polyethylene oxide monoalkyl ether (meth)acrylate, dipropylene glycol (meth)acrylate, a polypropylene oxide monoalkyl ether (meth)acrylate, an oligopropylene oxide monoalkyl ether (meth)acrylate, 2-(meth)acryloyloxyethylsuccinic acid, 2-(meth)acryloyloxyhexahydrophthalic acid, 2-(meth)acryloyloxyethyl-2-hydroxypropyl phthalate, butoxydiethylene glycol (meth)acrylate, trifluoroethyl (meth)acrylate, perfluorooctylethyl (meth)acrylate, 2-hydroxy-3-phenoxypropyl (meth)acrylate, ethylene oxide(EO)-modified phenol (meth)acrylate, EO-modified cresol (meth)acrylate, EO-modified nonylphenol (meth)acrylate, propylene oxide(PO)-modified nonylphenol (meth)acrylate, EO-modified 2-ethylhexyl (meth)acrylate, dicyclopentenyl (meth)acrylate, dicyclopentenyloxyethyl (meth)acrylate, dicyclopentanyl (meth)acrylate, (3-ethyl-3-oxetanylmethyl) (meth)acrylate, and phenylethylene glycol(meth)acrylate; allyl derivatives such as allyl glycidyl ether; styrenics such as styrene, 4-methylstyrene, 4-hydroxystyrene, 4-acetostyrene, and styrenesulfonic acid; (meth)acrylonitrile; (meth)acrylamides (including N- mono and N,N-disubstituted) such as N-benzyl (meth)acrylamide; maleimides such as N-phenyl maleimide; vinyl derivatives such as vinylcaprolactam, vinylpyrrolidone, vinylimidazole, vinylnapthalene, and vinyl halides; vinylethers such as vinylmethyl ether; vinylesters of carboxylic acids such as vinylacetate, vinylbutyrate, and vinyl benzoate.

[0078] In a preferred embodiment, the polyfunctional acrylates are selected from difunctional (meth)acrylates including 1 ,6-hexanediol di(meth)acrylate, 1 ,10-decanediol di(meth)acrylate, neopentyl glycol di(meth)acrylate, 2,4-dimethyl-1 ,5-pentanediol di(meth)acrylate, butylethylpropanediol di(meth)acrylate, ethoxylated cyclohexanemethanol di(meth)acrylate, polyethylene glycol di(meth)acrylate, oligoethylene glycol di(meth)acrylate, ethylene glycol di(meth)acrylate, 2-ethyl-2- butylbutanediol di(meth)acrylate, neopentyl glycol hydroxypivalate di(meth)acrylate, EO-modified bisphenol A di(meth)acrylate, bisphenol F polyethoxydi(meth)acrylate, polypropylene glycol di(meth)acrylate, oligopropylene glycol di(meth)acrylate, 1,4-butanediol di(meth)acrylate, 2- ethyl-2-butylpropanediol di(meth)acrylate, 1 ,9-nonane di(meth)acrylate, propoxylated ethoxylated bisphenol A di(meth)acrylate, tricyclodecane di(meth)acrylate, dipropylene glycol di(meth)acrylate, tripropylene glycol di(meth)acrylate, and PO-modified neopentyl glycol di(meth)acrylate; from trifunctional (meth)acrylates including trimethylolpropane tri(meth)acrylate, trimethylolethane tri(meth)acrylate, an alkylene oxidemodified tri(meth)acrylate of trimethylolpropane, pentaerythritol tri(meth)acrylate, dipentaerythritol tri(meth)acrylate, trimethylolpropane tris((meth)acryloyloxypropyl) ether, an isocyanuric acid alkylene oxidemodified tri(meth)acrylate, propionic acid dipentaerythritol tri(meth)acrylate, tris((meth)acryloyloxyethyl) isocyanurate, hydroxypivalaldehyde-modified dimethylolpropane tri(meth)acrylate, sorbitol tri(meth)acrylate, propoxylated trimethylolpropane tri(meth)acrylate, and ethoxylated glycerol triacrylate;from tetrafunctional (meth)acrylates including pentaerythritol tetra(meth)acrylate, sorbitol tetra(meth)acrylate, ditrimethylolpropane tetra(meth)acrylate, dipentaerythritol propionate tetra(meth)acrylate, and ethoxylated pentaerythritol tetra(meth)acrylate;from pentafunctional (meth)acrylates including sorbitol penta(meth)acrylate and dipentaerythritol penta(meth)acrylate; and from hexafunctional (meth)acrylates including dipentaerythritol hexa(meth)acrylate, sorbitol hexa(meth)acrylate, an alkylene oxidemodified hexa(meth)acrylate of phosphazene, and c-caprolactone- modified dipentaerythritol hexa(meth)acrylate.

[0079] A particularly preferred polyfunctional acrylate is a polymerizable compound having two different polymerizable groups, such as a vinylether group and an acrylate group. Preferred vinylether (meth)acrylates are those disclosed in US 6310115 (AGFA) and in columns 3 and 4 of US 67679890 B (NIPPON SHOKUBAI).A particularly preferred compound is 2- (2’-vinyloxyethoxy)ethyl acrylate (VEEA). VEEA is especially to achieve a high curing degree.

[0080] A preferred UV curable inkjet ink for indoor decoration applications comprises the above-described acyl phosphine oxide photoinitiator and a mixture of free radical polymerizable monomers comprising of: a) 7 - 70 wt%, preferably 15 - 60 wt% and most preferably 25 - 50 wt% of 2- (2’- vinyloxyethoxy)ethyl acrylate b) 1 - 65 wt% , preferably 20 - 60 wt% of one or more polymerizable compounds selected from the group consisting of monofunctional acrylates and difunctional acrylates; and c) 0 - 55 wt% of one or more polymerizable compounds selected from the group consisting of trifunctional acrylates, tetrafunctional acrylates, pentafunctional acrylates and hexafunctional acrylates,wherein all weight percentages of a, b and c are based upon the total weight of the polymerizable composition of the UV curable inkjet ink. In general, the cured products of these UV curable inkjet inks exhibit a more pleasant odour.

[0081] Acrylates of the aforementioned monofunctional or polyfunctional monomers are preferred over their methacrylate counterparts due to their generally higher curing speed.

[0082] In a preferred embodiment, the radiation curable composition or UV curable (inkjet) ink includes an N-vinyllactam, such as N-vinylcaprolactam. Another particularly preferred monomer is vinyl methyl oxazolidinone, available as VMOX from BASF. These monomers are favored because they provide excellent ink curability and adhesion of the cured film to a recording medium.

[0083] In a preferred embodiment, the radiation curable composition or UV curable (inkjet) ink contains contain at least one of N-vinyl caprolactam, vinyl methyl oxazolidinone and (2’-vinyloxyethoxy)ethyl acrylate. The mixture of free radical polymerizable monomers may also include a (meth)acrylamide compound selected from the group consisting of (meth)acrylamide, N-methyl (meth)acrylamide, N-ethyl (meth)acrylamide, N-propyl (meth)acrylamide, N-n-butyl (meth)acrylamide, N-t-butyl (meth)acrylamide, N-butoxymethyl (meth)acrylamide, N-isopropyl (meth)acrylamide, N-methylol (meth)acrylamide, N,N-dimethyl (meth)acrylamide, N,N-diethyl (meth)acrylamide, and (meth)acryloyl morpholine.

[0084] The mixture of free radical polymerizable monomers in the radiation curable composition or UV curable (inkjet) ink may be supplemented by polymerizable oligomers and / or polymerizable polymers. In the present disclosure, the oligomer has preferably a weight-average molecular weight of 500 or more and 20,000 or less, and the polymer has preferably a weight-average molecular weight of more than 20,000.

[0085] Preferred free radical polymerizable oligomers and polymers are polyurethanes, polyesters, polyethers, polycarbonates, poly-carbamates, polyureas and straight-chain oligomers having the following polymerizable groups: acrylate, methacrylate, vinyl, acrylamide, methacrylamide, vinyl carbonate, vinyl ether, vinylester- vinyl carbamate groups, as well as their corresponding alkene and alkyne compounds.

[0086] A urethane (meth)acrylate oligomer is particularly preferred for free radical polymerization due to its excellent abrasion resistance, flexibility, and chemical resistance in the resulting cured product.

[0087] The urethane (meth)acrylate oligomer may be a compound having one or more urethane bonds and having one or more (meth)acrylate groups.

[0088] It is preferable to use a bifunctional urethane (meth)acrylate oligomer having a weight-average molecular weight between 3,000 and 20,000 to ensure optimal abrasion resistance, flexibility, and chemical resistance in the cured product.

[0089] As the bifunctional urethane (meth)acrylate oligomer, a commercially available product may be used. Examples of the commercially available product include CN963B80 and CN9001 (manufactured by ARKEMA and SHIKO UV-3200B, SHIKO UV-3300B, SHIKO UV-3310B, and SHIKO UV- 6630B (all manufactured by The Nippon Synthetic Chemical Industry Co., Ltd.).

[0090] The content of the urethane (meth)acrylate oligomer in the radiation curable composition or UV curable (inkjet) ink according to the present disclosure is preferably 1 % by mass or more and 30% by mass or less, more preferably 3% by mass or more and 20% by mass or less, and particularly preferably 5% by mass or more and 10% by mass or less with respect to the total mass of the composition from the viewpoint of the abrasion resistance, flexibility, and chemical resistance of the obtained cured product.

[0091] Another preferred alternative free radical curing chemistry is the so-called thiol-ene and thiol-yne chemistry. In such a chemistry, a combination of at least one polyfunctional thiol monomer and at least one polyfunctional polymerizable monomer is used. The polyfunctional polymerizable monomer is preferably a polyfunctional monomer having a plurality of polymerizable groups selected from a group consisting of a vinyl group, an acrylamide group, a methacrylamide group, a vinyl carbonate group, a vinyl ether group, a vinyl ester group, a vinyl carbamate group, an allyl ether groups, an allyl ester group and an alkyne group. Particularlypreferred are polymerizable compounds including allyl ether groups, vinyl carbonate groups and alkyne groups.

[0092] Synthesis of such monomers is disclosed in the relevant literature, for example in HURD, Charles D.. Vinylation and the Formation of Acylals. Journal Am. Chem.Soc. 1956, vol.78, no.1, p.104_106. ; LOBELL, M., et al. Synthesis of hydroxycarboxylic acid vinyl esters. MP Synthesis. 1994, vol.4, p.375-377. ; LEE, T. Y., et al. Synthesis, Initiation, and Polymerization of Photoinitiating Monomer. Macromolecules. 2005, vol.38, no.18, p.7529-7531. ; ATTA, A.M., etal. New vinyl ester resins based on rosin for coating applications. React. Funct. Po / ym.. 2006, vol.66, p.1596- 1608. ; WO 01 / 00634 A (WRIGHT CHEM CORP) ; and ROHR, Markus, et al. Solvent-free ruthenium-catalysed vinylcarbamate synthesis from phenylacetylene and diethylamine in ‘supercritical’ carbon dioxide. Green Chemistry. 2001, vol.3, p.123-125.Other Photoinitiators and Co-initiators

[0093] The oligomeric acyl phosphine oxide photoinitiator can be used as a single photoinitiator in the radiation curable composition or UV curable (inkjet) ink. Alternatively, it may be combined with other photoinitiators and / or a- co-initiators to form a photoinitiating system in the radiation curable composition or UV curable (inkjet) ink.

[0094] The other photoinitiator(s) may be a Norrish type I initiator and / or a Norrish type II initiator. A Norrish Type I initiator is a photoinitiator which cleaves after excitation, yielding the initiating radical immediately. A Norrish type II- initiator is a photoinitiator which is activated by actinic radiation and forms free radicals by hydrogen abstraction from a second compound that becomes the actual initiating free radical. This second compound is called a polymerization synergist or co-initiator.

[0095] Suitable Norrish type I and II photo-initiators are disclosed in CRIVELLO, J.V., et al. VOLUME III: Photoinitiators for Free Radical Cationic & Anionic Photopolymerisation. 2nd edition. Edited by BRADLEY, G.. London, UK: John Wiley and Sons Ltd, 1998. p.287-294.

[0096] The oligomeric acyl phosphine oxide photoinitiator may be combined with a photoinitiator selected from the group consisting of another acylphosphine oxide initiator a thioxanthone initiator, an a- hydroxyalkylphenone initiator and a carbazole initiator. Such combinations can improve curing speed further.

[0097] In order to increase the photosensitivity further, a co-initiator may be added, which preferably is an amine synergist.

[0098] Suitable examples of amine synergists can be categorized in three groups:1) tertiary aliphatic amines such as methyldiethanolamine, dimethylethanolamine, triethanolamine, triethylamine and N- methylmorpholine;(2) aromatic amines such as amylparadimethylaminobenzoate, 2-n- butoxyethyl-4-(dimethylamino) benzoate, 2-(dimethylamino)ethylbenzoate, ethyl-4-(dimethylamino)benzoate, and 2-ethylhexyl-4- (dimethylamino)benzoate; and(3) (meth)acrylated amines such as dialkylamino alkyl(meth)acrylates (e.g., diethylaminoethylacrylate) or N-morpholinoalkyl-(meth)acrylates (e.g. , N-morpholinoethyl-acrylate).

[0099] The total amount of photoinitiator used is preferably an amount of 1 to 30 wt%, more preferably 2 to 20 wt%, and most preferably 5 to 15 wt%, based on the total weight of the radiation curable composition or UV curable (inkjet) ink. Preferably, the oligomeric acyl phosphine oxide photoinitiator of the invention constitutes 60% to 100% of the total photoinitiator.

[0100] The total amount of coinitiator used is preferably an amount of 1 to 20 wt%, more preferably 2 to 15 wt%, and most preferably 5 to 10 wt%, based on the total weight of the radiation curable composition or UV curable (inkjet) ink.Colorants

[0101] The radiation curable composition may contain a colorant. The colorant may be a dye, a pigment or a combination thereof. Organic and / or inorganic pigments may be used. The colorant is preferably a pigment or a polymeric dye, most preferably an organic colour pigment. Organic colour pigments generally allow to obtain a much broader colour gamut than inorganic colour pigments. However, for the colours white and black,preferably inorganic pigments such as titanium dioxide respectively carbon black are used.

[0102] The pigments may be black, white, cyan, magenta, yellow, red, orange, violet, blue, green, brown, mixtures thereof, and the like. This colour pigment may be chosen from those disclosed by HERBST, Willy, et al. Industrial Organic Pigments, Production, Properties, Applications. 3rd edition. Wiley - VCH , 2004. ISBN 3527305769.

[0103] A preferred pigment for a UV curable cyan (inkjet) ink according to the invention is a beta-copper phthalocyanine pigment, with C.L Pigment Blue 15:3 or 15:4 being particularly preferred for minimal light fading of the cured product.

[0104] The pigments mentioned above for the red and yellow inks in CRYK ink set may also be used for (inkjet) inks in a different ink set.

[0105] The pigment in a magenta UV curable (inkjet) ink is preferably C.L Pigment Violet 19 or a mixed crystal thereof.

[0106] In a black UV curable (inkjet) ink according to the invention, the pigment is preferably a carbon black pigment. Suitable black pigments include carbon blacks such as Pigment Black 7 (e.g. Carbon Black MA8TMfrom MITSUBISHI CHEMICAL), Regal™ 400R, Mogul™ L, Elftex™ 320 from CABOT Co., or Carbon Black FW18, Special Black 250, Special Black 350, Special Black 550, Printex™ 25, Printex™ 35, Printex™ 55, Printex™ 90, Printex™ 150T from EVONIK. In a preferred embodiment, the carbon black pigment used is a pigment having less than 0.15% of toluene- extractable fraction using the method as described in section III, paragraph 5 of the Resolution AP(89) 1 dated 13 September 1989 published by the Council of Europe.

[0107] It is also possible to include mixtures of pigments in the UV curable (inkjet) ink. For example, in some inkjet ink application a neutral black inkjet ink is preferred and can be obtained, for instance, by mixing a black pigment and a cyan and / or magenta pigment into the ink. Also pigments may be combined to enlarge the colour gamut of an ink set.

[0108] An inkjet ink set may also include one or more spot colours. Silver and gold are often desired colours for making a product more attractive by giving it an exclusive appearance.

[0109] Suitable pigments include mixed crystals of the above preferred pigments.Mixed crystals are also referred to as solid solutions. For example, under certain conditions different quinacridones mix with each other to form solid solutions, which are quite different from both physical mixtures of the compounds and from the compounds themselves. In a solid solution, the molecules of the components enter into the same crystal lattice, usually, but not always, that of one of the components. The x-ray diffraction pattern of the resulting crystalline solid is characteristic of that solid and can be clearly differentiated from the pattern of a physical mixture of the same components in the same proportion. In such physical mixtures, the x-ray pattern of each of the components can be distinguished, and the disappearance of many of these lines is one of the criteria of the formation of solid solutions. A commercially available example is Cinquasia™ Magenta L 4540 from SUN CHEMICAL.

[0110] Pigment particles in inkjet ink should be sufficiently small to permit free flow of the ink through the inkjet-printing device, especially at the ejecting nozzles. It is also desirable to use small particles for maximum colour strength and to slow down sedimentation. The numeric average pigment particle size is preferably between 0.050 and 1 pm, more preferably between 0.070 and 0.300 pm and particularly preferably between 0.080 and 0.200 pm.

[0111] The numeric average pigment particle size of pigment particles is best determined with a Brookhaven Instruments Particle Sizer BI90plus based upon the principle of dynamic light scattering. The ink is then diluted, for example, with ethyl acetate to a pigment concentration of 0.002 wt%. The measurement settings of the BI90plus are: 5 runs at 23°C, angle of 90°, wavelength of 635 nm and graphics = correction function.

[0112] In the case of a UV curable white (inkjet) ink according to the invention, preferably a pigment with a refractive index greater than 1.60, preferably greater than 2.00, more preferably greater than 2.50 and most preferablygreater than 2.60 is used. The white pigments may be employed singly or in combination.

[0113] In a coloured inkjet ink the pigment is preferably present in an amount of 0.1 to 13.0 wt%. A dark colour inkjet ink preferably contains 1.5 to 13.0 wt%, more preferably 1.8 to 6.0 wt% of colour pigment based on the total weight of the inkjet ink, while a light colour inkjet ink preferably contains 0.1 to 1.3 wt%, more preferably 0.3 to 1.2 wt% of colour pigment based on the total weight of the inkjet ink.

[0114] A white inkjet ink preferably contains more than 13.0 wt%, more preferably 15.0 to 25.0 wt% of a white pigment based on the total weight of the inkjet ink.Dispersants

[0115] The UV curable (inkjet) ink preferably contains a dispersant for further improving pigment dispersion properties. For obtaining high printing reliability, the dispersant is preferably a polymeric dispersant. Such dispersant improves the reliability of an inkjet printing process due to a generally smaller sedimentation speed, especially when the dispersant contains secondary or tertiary amine groups.

[0116] Typical polymeric dispersants are copolymers of two monomers but may contain three, four, five or even more monomers. The properties of polymeric dispersants depend on both the nature of the monomers and their distribution in the polymer. Copolymeric dispersants preferably have the following polymer compositions:• statistically polymerized monomers (e.g. monomers A and B polymerized into ABBAABAB);• alternating polymerized monomers (e.g. monomers A and B polymerized into ABABABAB);• gradient (tapered) polymerized monomers (e.g. monomers A and B polymerized into AAABAABBABBB);• block copolymers (e.g. monomers A and B polymerized into AAAAABBBBBB) wherein the block length of each of the blocks (2, 3, 4, 5 or even more) is important for the dispersion capability of the polymeric dispersant;• graft copolymers (graft copolymers consist of a polymeric backbone with polymeric side chains attached to the backbone); and • mixed forms of these polymers, e.g. blocky gradient copolymers.

[0117] The polymeric dispersant has preferably a number average molecular weight Mn between 500 and 30000, more preferably between 1500 and 10000.

[0118] The polymeric dispersant has preferably a weight average molecular weight Mw smaller than 100,000, more preferably smaller than 50,000 and most preferably smaller than 30,000.

[0119] The polymeric dispersant has preferably a polydispersity PD smaller than 2, more preferably smaller than 1.75 and most preferably smaller than 1.5.

[0120] Commercial examples of polymeric dispersants are the following:DISPERBYK™ dispersants available from BYK CHEMIE GMBH;SOLSPERSE™ dispersants available from LUBRIZOL; TEGO™ DISPERS™ dispersants from EVONIK; EDAPLAN™ dispersants from MUNZING CHEMIE; ETHACRYL™ dispersants from LYONDELL;GANEX™ dispersants from ISP; DISPEX™ and EFKA™ dispersants from BASF; and DISPONER™ dispersants from DEUCHEM.

[0121] The dispersants may be used alone or in combination of two or more kinds thereof.

[0122] The polymeric dispersant is preferably used in an amount of 10 to 200 wt%, more preferably 20 to 100 wt%, most preferably 50 to 90 wt% based on the weight of the pigment.Dispersion Synergists

[0123] The UV curable inkjet ink may include a dispersion synergist to further improve the dispersion stability by a polymeric dispersant and thus also the printing reliability as less pigment can sediment in the nozzle of a print head upon stand-by of an inkjet device.

[0124] A dispersion synergist usually consists of an anionic part and a cationic part. The anionic part of the dispersion synergist exhibiting a certain molecular similarity with the colour pigment and the cationic part of the dispersion synergist consists of one or more protons and / or cations to compensate the charge of the anionic part of the dispersion synergist.

[0125] The dispersion synergist is preferably added in a smaller amount than the polymeric dispersant(s). The ratio of polymeric dispersant / dispersion synergist depends upon the pigment and should be determined experimentally. Typically, the ratio wt% polymeric dispersant / wt% dispersion synergist is selected between 2:1 to 100:1, preferably between 2:1 and 20:1.

[0126] Suitable dispersion synergists that are commercially available include Solsperse™ 5000 and Solsperse™ 22000 from LUBRIZOL. Suitable dispersion synergists for a diketopyrrolo-pyrrole pigment, a quinacridone pigment or a mixed crystal thereof include those disclosed in EP 1790698 A (AGFA GRAPHICS) , EP 1790696 A (AGFA GRAPHICS) , WO 2007 / 060255 (AGFA GRAPHICS) and EP 1790695 A (AGFA GRAPHICS) .

[0127] In dispersing C.L Pigment Blue 15:3, the use of a sulfonatedCu-phthalocyanine dispersion synergist, e.g. Solsperse™ 5000 from LUBRIZOL is preferred.Stabilizers

[0128] The UV curable (inkjet) ink may also contain a stabilizer, often referred to as a polymerization inhibitor. Due to the fact that an ink contains the polymerization inhibitor, a polymerization reaction before curing, such as during storage or transport, can be prevented. It can also improve the printing reliability. For example, a UV LED curable inkjet ink in a print head of an inkjet device is usually kept at a higher temperature such as 45 to 55 °C.

[0129] Suitable polymerization inhibitors include phenol type antioxidants, hindered amine light stabilizers, phosphor type antioxidants, benzoquinone, hydroquinone and derivatives, such as hydroquinone monomethyl ether commonly used in (meth)acrylate monomers.

[0130] Examples of the phenolic polymerization inhibitor include, but are not limited to the following substances, p-methoxy phenol, cresol, t-butyl catechol, di-t-butyl-p-cresol, hydroquinone monomethylether, a-naphthol, 3,5-di-t-butyl-4-hydroxytoluene, 2,6-di-t-butyl-4-methylphenol, 2,2'-methylene-bis(4-methyl-6-t-butylphenol), 2,2'-methylene-bis(4-ethyl-6- butylphenol), and 4,4'-thio-bis(3-methyl-6-t-butylphenol) and pyrogallol.

[0131] Suitable commercial inhibitors are, for example, Sumilizer™ GA-80, Sumilizer™ GM and Sumilizer™ GS produced by Sumitomo Chemical Co. Ltd.; Genorad™ 16, Genorad™ 18 and Genorad™ 20 from Rahn AG; Irgastab™ LIV10 and Irgastab™ LIV22, Tinuvin™ 460 and CGS20 from Ciba Specialty Chemicals; Floorstab™ UV range (UV-1, UV-2, UV-5 and UV-8) from Kromachem Ltd, Additol™ S range (S100, S110, S120 and S130) from Cytec Surface Specialties.

[0132] A preferred polymerization inhibitor is Irgastab™ UV10 from BASF. Other examples of polymerization inhibitor include TEMPO, TEMPOL, and Al cupferron.

[0133] The polymerization inhibitors may be used alone or in combination of two or more kinds thereof. In a preferred embodiment, the polymerization inhibitor is a mixture of different types of polymerization inhibitors.

[0134] Preferred polymerization inhibitors are mixtures of an oxyl free radicalbased polymerization inhibitor, a phenol-based polymerization inhibitor, and an amine-based polymerization inhibitor.

[0135] Suitable examples are given in EP 2851402 A (FUJIFILM) . The polymerization inhibitor is preferably present in an amount of 0.1 to 5 wt% based on the total weight of the free radical curable (inkjet) ink. Below 0.1 wt%, the undesired polymerization is insufficiently inhibited and above 5 wt% the curing speed is heavily reduced.Surfactants

[0136] The UV curable inkjet ink may contain a surfactant. The surfactant can be anionic, cationic, non-ionic, or zwitter-ionic. The surfactant is preferably present in an amount of 0.1 to 3 wt% based on the total weight of the UV curable inkjet ink. At higher concentrations than 3 wt%, the adhesion may deteriorate rapidly, while usually insufficient spreading of the ink is observed at concentration lower than 0.1 wt%.

[0137] The total quantity of surfactant is preferably less than 3 wt% based on the total weight of the ink and more preferably less than 1.5 wt% based on thetotal weight of the UV curable inkjet ink to prevent foaming of the ink in its container. Such foaming has a negative impact on the printing reliability.

[0138] Preferred surfactants are selected from fluoro surfactants (such as fluorinated hydrocarbons) and silicone surfactants. The silicone surfactants are preferably siloxanes and can be alkoxylated, polyester modified, polyether modified, polyether modified hydroxy functional, amine modified, epoxy modified and other modifications or combinations thereof. Preferred siloxanes are polymeric, for example polydimethylsiloxanes.

[0139] Preferred commercial silicone surfactants include BYKTM333 and BYKTMLIV3510 from BYK Chemie and Tegoglide™ 410 from EVONIK.

[0140] In a preferred embodiment, the surfactant is a polymerizable compound.

[0141] Preferred polymerizable silicone surfactants include a (meth)acrylated silicone surfactant. Most preferably the (meth)acrylated silicone surfactant is an acrylated silicone surfactant, because acrylates are more reactive than methacrylates.

[0142] In a preferred embodiment, the (meth)acrylated silicone surfactant is a polyether modified (meth)acrylated polydimethylsiloxane or a polyester modified (meth)acrylated polydimethylsiloxane.

[0143] Preferred commercially available (meth)acrylated silicone surfactants include: Ebecryl™ 350 , a silicone diacrylate from Cytec; the polyether modified acrylated polydimethylsiloxane BYK™ LIV3500, BYK™ LIV3510 and BYK™ LIV3530, the polyester modified acrylated polydimethylsiloxane BYK™ LIV3570, all manufactured by BYK Chemie; Tego™ Rad 2100, Tego™ Rad 2200N, Tego™ Rad 2250N, Tego™ Rad 2300, Tego™ Rad 2500, Tego™ Rad 2600, Tego™ Rad 2700, and Tego™ RC711 all manufactured by EVONIK. Another preferred silicone is Silwet™ L7500 from OSI SPECIALITIES BENELUX NV; Silaplane™ FM7711 , Silaplane™ FM7721 , Silaplane™ FM7731 , Silaplane™ FM0711 , Silaplane™ FM0721 , Silaplane™ FM0725, Silaplane™ TM0701 , Silaplane™ TM0701T all manufactured by CHISSO Corporation; and DMS-R05, DMS-R11, DMS- R18, DMS-R22, DMS-R31, DMS-U21 , DBE-U22, SIB1400, RMS-044, RMS-033, RMS-083, UMS-182, UMS-992, UCS-052, RTT-1011 and UTT- 1012 all manufactured by GELEST Inc..

[0144] Particularly preferred surfactants for the UV curable inkjet ink are Silmer™ surfactants from SILTECH CORPORATION, such as Silmer™ ACR Di- 1508.Preparation of UV Curable Inks and Inkjet Inks

[0145] The preparation of UV curable (inkjet) inks is well-known to the skilled person.

[0146] The average particle size and distribution of a colour pigment is an important feature for inkjet inks. The inkjet ink may be prepared by precipitating or milling the pigment in the dispersion medium in the presence of a dispersant.

[0147] Mixing apparatuses may include a pressure kneader, an open kneader, a planetary mixer, a dissolver, and a Dalton Universal Mixer. Suitable milling and dispersion apparatuses are a ball mill, a pearl mill, a colloid mill, a high-speed disperser, double rollers, a bead mill, a paint conditioner, and triple rollers. The dispersions may also be prepared using ultrasonic energy or using a microfluidizer.

[0148] Different types of materials may be used as milling media, such as glasses, ceramics, metals, and plastics. In a preferred embodiment, the grinding media can comprise particles, preferably substantially spherical in shape, e.g. beads consisting essentially of a polymeric resin or yttrium stabilized zirconium oxide beads.

[0149] In the process of mixing, milling and dispersion, each process is performed with cooling to prevent build-up of heat and as much as possible under light conditions in which actinic radiation has been substantially excluded.

[0150] The (inkjet) ink may contain more than one pigment, and may be prepared using separate dispersions for each pigment, or alternatively several pigments may be mixed and co-milled in preparing the dispersion.

[0151] The dispersion process can be carried out in a continuous, batch or semibatch mode.

[0152] The preferred amounts and ratios of the ingredients of the mill grind will vary depending upon the specific materials and the intended applications. The contents of the milling mixture comprise the mill grind and the milling media. The mill grind comprises pigment, polymeric dispersant and a liquidcarrier. For inkjet inks, the pigment is usually present in the mill grind at 5 to 50 wt%, excluding the milling media. The weight ratio of pigment over polymeric dispersant is preferably 20:1 to 1 :2, more preferably 2:1 to 1 :1.

[0153] The optimal milling time can vary and depends upon the pigment, mechanical means and residence conditions selected, the initial and desired final particle size, etc. In the present invention pigment dispersions with an average particle size of less than 100 nm may be prepared.

[0154] After milling is completed, the milling media is separated from the milled particulate product (in either a dry or liquid dispersion form) using conventional separation techniques, such as by filtration, sieving through a mesh screen, and the like. Often the sieve is built into the mill, such as for a bead mill. The milled pigment concentrate is preferably separated from the milling media by filtration.

[0155] In general, it is desirable to make the Inkjet inks in the form of a concentrated pigment dispersion, which is subsequently diluted to the appropriate concentration for use in the inkjet printing system. This technique permits preparation of a greater quantity of pigmented ink from the equipment. By dilution, the inkjet ink is adjusted to the desired viscosity, surface tension, colour, hue, saturation density, and print area coverage for a particular application.Inkjet Printing Methods

[0156] Another aspect of the invention is an inkjet printing method comprising the steps of: a) jetting a UV curable inkjet ink as described above on a substrate; and b) UV curing the UV curable inkjet ink with UV LEDs having an emission wavelength of 360 nm or larger.

[0157] The UV curing is preferably performed by UV LEDs having an emission wavelength larger than 360 nm, preferably larger than 370 nm and most preferably between 390 and 400 nm.

[0158] The UV curable inkjet ink is jetted by one or more print heads ejecting small droplets in a controlled manner through nozzles onto a substrate moving relative to the print head(s).

[0159] A preferred print head for the inkjet printing system is a piezoelectric head.Piezoelectric inkjet printing is based on the movement of a piezoelectricceramic transducer when a voltage is applied thereto. The application of a voltage changes the shape of the piezoelectric ceramic transducer in the print head creating a void, which is then filled with inkjet ink. When the voltage is again removed, the ceramic expands to its original shape, ejecting a drop of ink from the print head. Piezoelectric print heads have proven to be the most reliable print heads in industrial printing.

[0160] A preferred piezoelectric print head is a so-called push mode type piezoelectric print head, which has a rather large piezo-element capable of ejecting also high viscous inkjet ink droplets. Such a print head is available from RICOH as the GEN5s print head.

[0161] A more preferred piezoelectric print head is a so-called through-flow piezoelectric drop-on-demand print head. Such a print head is, for instance available from TOSHIBA TEC as the CF1ou print head or from FUJIFILM DIMATIX as Samba™ G3L and G5L printheads. Through-flow print heads are preferred because they enhance the reliability of inkjet printing due to the ink circulation within the print head.

[0162] The inkjet print head preferably scans back and forth in a transversal direction across the moving ink-receiver surface. The inkjet print head may not print on the way back, but bi-directional printing is preferred for obtaining a high areal throughput. For maximizing high areal throughput, another printing method may be used that is known as a “single pass printing process”, which can be performed by using page wide inkjet print heads or multiple staggered inkjet print heads that cover the entire width of the ink-receiver surface. In a single pass printing process the inkjet print heads usually remain stationary and the ink-receiver surface is transported under the inkjet print heads.

[0163] However, the inkjet printing of the UV curable inkjet inks is more preferably performed in a multi-pass printing mode. Multi-pass printing is a very suitable technique to reduce banding in ink-jet printing or mask nozzle failure which may occur in single pass inkjet printings. Dots of ink, when still in liquid form, tend to run together due to surface tension. This is referred to as coalescence. To print a high-quality image, it is important to print individual round dots. But to achieve full saturated colours, the dotsmust overlap to completely cover the substrate. By only printing a portion of the image data so as to avoid simultaneously printing adjacent dots during each printing cycle, coalescence may be largely avoided.Additionally, by avoiding all horizontal adjacencies, the transverse speed of the printing mechanism can be increased up to two times the rated print speed of the print head. In a preferred embodiment, the number of passes used is to 2 to 6 passes, more preferably no more than 4 passes.

[0164] An advantage of using a multi-pass printing mode is that the UV curable inkjet inks are cured in consecutive passes, rather than in a single pass which would require a curing device with a high UV output. The print head lifetime is also larger for multi pass printing. While in single pass printing one side shooter is sufficient to replace the whole print head, in multi pass printing side shooters and even failings can be tolerated. Also the cost of a multi-pass printer is usually much lower, especially for wide format substrates.

[0165] For facilitating curing, the inkjet printer may include one or more oxygen depletion units. The oxygen depletion units place a blanket of nitrogen or other relatively inert gas (e.g. CO2), with adjustable position and adjustable inert gas concentration, in order to reduce the oxygen concentration in the curing environment. Residual oxygen levels are usually maintained as low as 200 ppm, but are generally in the range of 200 ppm to 1200 ppm.Inkjet Printed Articles

[0166] Another aspect of the invention is a cured product formed by UV curing the UV curable composition or (inkjet) ink in accordance with the invention.

[0167] The cured product is preferably a printed article comprising a cured composition derived from the radiation curable composition as described above.

[0168] There are no specific limitations on the nature of the printed articles, which can be either 3D-printed or printed on a substrate.

[0169] In a preferred embodiment, the printed article is selected from printed packaging, printed leather articles, and printed interior decoration articles.

[0170] More preferably, the printed article is produced using inkjet printing technology, which facilitates customization and personalization.

[0171] Printed packaging includes articles made from corrugated cardboard or folded carton.

[0172] Preferred printed leather articles are those made from natural leather.

[0173] Printed interior decoration articles include laminate products such as luxury vinyl tiles. However, the UV curable (inkjet) inks may also be directly printed on an article such as furniture or doors.

[0174] There are no limitations on the substrates for printing UV-curable (inkjet) inks; they can be plastic, metal, wood, or cellulose-based.EXAMPLESMaterials

[0175] All solvents and reagents were supplied by standard fine chemical suppliers such as TCI Europe, unless otherwise specified.

[0176] Ethyl (2,4,6-trimethylbenzoyl)phenylphosphinate (TPO-L) was supplied by IGM as Omnirad™ TPO-L

[0177] (2,4,6-Trimethylbenzoyl)diphenylphosphine oxide (TPO) was supplied by IGM as Omnirad™ TPO.

[0178] Di-p-tolyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO R) was supplied by King Brother Chem Co. Ltd. as CHEMACURE™ TPO R.

[0179] Silwet™ L7500 is a silicone based wetting agent supplied by Momentive Performance Materials GmbH.

[0180] Genomer™ 2253 is an acrylated amine oligomer supplied by Rahn.

[0181] DPGDA is dipropylene glycol diacrylate, a difunctional acrylate supplied by Sartomer as Sartomer™ SR508E.Methods1. Molecular Mass Determination Using TLC-MS

[0182] The molecular mass of the different components in the mixture was determined using TLC-MS, following this procedure:• TLC Run: Conducted under the conditions described in the synthetic examples.• Analysis: The TLC was analyzed using a CAMAG™ TLC-MS interface coupled to an AmaZon™ SL mass spectrometer (Bruker Daltonics) via an Agilent™ 1100 HPLC pump.• Blank Spectrum: A blank spectrum was obtained by eluting a spot on the TLC plate where no compounds were present with a 0.01 molar solution of ammonium acetate in methanol.• Sample Spectrum: A spectrum of the compound to be analyzed was obtained by eluting the spot of the compound with a 0.01 molar solution of ammonium acetate in methanol.• Spectrum Subtraction: The blank spectrum was subtracted from the sample spectrum, yielding the spectrum of the compound to be analyzed.2. Degree of Oligomerization Analysis Using UPLC-MS

[0183] The degree of oligomerisation was analyzed using a Waters UPLC / MS(SQD) system under the following experimental conditions. • Column: Waters Acquity™ BEH C18 (50 x 2.1 mm)• Sample Injection Volume: 2 pL• Flow Rate: 0.4 mL / min• Temperature: 40°C• Gradient Elution Scheme according to Table 2 using:Mobile Phase A: Water / Acetonitrile (90:10)Mobile Phase B: Water / Acetonitrile (5:95)

[0184] Table 23. Curing Efficiency

[0185] The surface cure was evaluated by wiping the surface five times with a Q- tip and the surface damage was evaluated on a scale from 0 to 5, where 0 means no visual damage, 1-2 means slight visual damage, 3-4 meansclear visual damage and 5 means hardly any surface cure and almost complete removal of the layer.4. Odour Test

[0186] After curing, the samples were stored in the dark for one hour with a cover on the cured layer. Subsequently, the covers were removed, and the odour of each sample was evaluated on a scale from 0 to 5, where 0 indicates no noticeable odour and 5 indicates a very pronounced odour. EXAMPLE 1

[0187] This example illustrates the synthesis of APOFORM-1 type of initiators. Synthesis

[0188]

[0189] 79.1 g (0.25 mol) ethyl (2,4,6-trimethylbenzoyl)phenylphosphinate (TPO-L) and 3.75 g (0.125 mol) paraformaldehyde were mixed together. 55 mL methane sulfonic acid was added dropwise over 20 minutes while keeping the temperature below 30°C. The reaction was allowed to continue for one hour, allowing the reaction temperature to decrease to room temperature. An additional 75 mL methane sulfonic acid was added over 30 minutes. During the addition, the temperature rose to 30°C. The reaction was allowed to continue over night at room temperature. The reaction mixture was slowly added to a mixture of 400 mL ice / water and 200 mL ethyl acetate, while keeping the temperature at 0°C. The ethyl acetate fraction was isolated, washed with 200 mL water, 100 mL of a 5w% NaHCOs solution in water and 200 mL of a 0.5 M NaCI solution. The ethyl acetate fraction was dried over MgSCU and evaporated under reduced pressure.76.2 g of APOFORM-1 (y : 94.7%) was isolated. (TLC analysis on aMERCK TLC Silica gel 6OF254 plate, eluent methylene chloride / methanol 95 / 5, main components : Rf = 0.49 for the compound where m=1 ; Rf = 0.36 for the compound where m=2. Only a minor amount of the compound with m=3 could be detected at Rf = 0.29, in combination with a minor amount of residual TPO-L at Rf = 0.67). The structure of the different compounds was confirmed using TLC MS.EXAMPLE 2

[0190] This example illustrates the synthesis of APOFORM-2 type of initiators. Synthesis

[0191] m ranging from 1 to 3

[0192] A suspension of 1.5 g (0.05 mol) paraformaldehyde in 8.6 mL methane sulfonic acid was prepared. 34.84 g (0.1 mol) (2,4,6- trimethylbenzoyl)diphenylphosphine oxide was dissolved in 40 mL methane sulfonic acid and added dropwise to the paraformaldehyde suspension while maintaining the temperature below 30°C. The reaction was allowed to continue for 16 hours at room temperature. 50 mL ethyl acetate was added to the reaction mixture and the mixture was added to a mixture of 400 mL water and 200 mL ethyl acetate at 0°C. The organic fraction was isolated, extracted first with a solution of 3 g NaHCOs in 250 mL water, followed by extraction with 200 mL of a 0.5 M NaCI solution. The organic fraction was isolated, dried over MgSCM and evaporated under reduced pressure. 25.9 g (y : 72%) of APOFORM-2 was isolated (TLC analysis on a MERCK TLC Silica gel 6OF254 plate, eluent methylene chloride / methanol 95 / 5, main components : Rf = 0.71 for the compound where m=1 ; Rf = 0.55 for the compound where m=2. Only a minor amountof the compound with m=3 could be detected at Rf = 0.4, in combination with a minor amount of residual TPO at Rf = 0.88).EXAMPLES

[0193] This example illustrates how to control the molecular weight of the photoinitiators according to the present invention, exemplified by the synthesis of APOFORM-1 type of initiators with variable molecular weight.Synthesis

[0194]

[0195] 31.6 g (0.1 mol) ethyl (2,4,6-trimethylbenzoyl)phenylphosphinate and 2.1 g (0.07 mol) (APOFORM-1 A) respectively 2.4 g (0.08 mol) (APOFORM-1 B) paraformaldehyde were mixed together. 25.9 mL methane sulfonic acid was added dropwise while maintaining the temperature below 30°C. The reaction was allowed to continue for 16 hours at room temperature. 50 mL ethyl acetate was added to the reaction mixture and the mixture was added to a mixture of 400 mL water and 200 mL ethyl acetate at 0°C. The organic fraction was isolated, extracted first with a solution of 3 g NaHCOs in 250 mL water, followed by extraction with 200 mL of a 0.5 M NaCI solution. The organic fraction was isolated, dried over MgSCM and evaporated under reduced pressure. A sample of each oligomer was analyzed using UPLC-MS according to the method described above. The distribution for each sample, determined by UPLC-MS, is summarized in Table 3.Table 3>

[0196] It becomes apparent from Table 3 that the degree of oligomerization can be controlled by controlling the molar ratio between ethyl (2,4,6- trimethylbenzoyl) phenylphosphinate and paraformaldehyde, allowing to tune the properties towards a target application.

[0197] The structures of the different oligomers are given below.

[0198] Dimer:

[0199] Trimer:

[0200] Tetramer:

[0201] Pentamer:EXAMPLE 4

[0202] This example illustrates the synthesis of APOFORM-3 type of initiators.Synthesis

[0204] 25.9 mL methane sulfonic acid is slowly added at room temperature over 50 minutes to a mixture of 15.8 g (50 mmol) ethyl (2,4,6- trimethylbenzoyl)phenylphosphinate, 17.4 g (50 mmol) (2,4,6-trimethylbenzoyl)diphenylphosphine oxide and 1.95 g (65 mmol, calculated on formaldehyde) paraformaldehyde. The reaction was allowed to continue for 16 hours at room temperature, followed by two hours at 30°C. The reaction mixture was allowed to cool down to room temperature and diluted with 50 mL ethyl acetate. The mixture was added to 400 mL water at 0°C and extracted with 200 mL ethyl acetate. The aqueous phase was removed and the organic phase was extracted with 250 mL water, which was neutralized with 3 g NaHCOs and 10 mL 1N NaOH. The organic phase was isolated, extracted with 250 mL of a 0.5 M NaCI-solution, 350 mL of a 0.5 M NaCI-solution containing an additional 5 g NaHCOs and 250 mL of a 0.5 M NaCI solution. The organic phase was isolated, dried over MgSO4 and evaporated under reduced pressure. 29.9 g of APOFORM-3 was isolated (y : 88%). APOFORM-3 was characterized using TLC MS (TLC on a MERCK TLC Silica gel 6OF254 plate, eluent methylene chloride / methanol 95 / 5). The Rf and the structure of the main components are summarized in Table 4. Small amounts of residual TPO and TPO-L were detectable.

[0205] Table 4

[0206] This example illustrates the synthesis of APOFORM-9 type of initiators.Synthesis

[0207]

[0208] A solution of 37.6 g (0.1 mol) di-p-tolyl(2,4,6-trimethylbenzoyl)phosphine oxide in 63.7 g methane sulfonic acid was added to a suspension of 1.8 g (0.06 mol, calculated on formaldehyde) paraformaldehyde in 15.3 g methane sulfonic acid at room temperature over 90 minutes. The reaction was allowed to continue for 16 hours at room temperature. 50 mL ethyl acetate was added to the reaction mixture and the mixture was added to 400 mL water and 200 mL ethyl acetate at 0°C. The organic phase was isolated and extracted with a solution of 0.8 g NaHCOs in 200 mL water. The pH of the aqueous phase was checked and proved to be 7. If the pH would still be acidic, a second extraction is needed. The organic phase was isolated, dried over MgSCU and evaporated under reduced pressure.35 g (y : 89%) of APOFORM-9 was isolated. TLC-MS analysis of APOFORM-9 (TLC on a MERCK TLC Silica gel 6OF254 plate, eluent methylene chloride / methanol 95 / 5) proved that APOFORM-9 was a mixture with a different degree of oligomerization (Rf of the dimer : 0.54, Rf of the trimer : 0.39, Rf of the tetramer : 0.32).EXAMPLE 6

[0209] This example illustrates the curing efficiency and odor performance of oligomeric acyl phosphine oxides according to the present invention. It compares these results with those obtained using reference photoinitiators TPO, TPO-L, and their mixture.Preparation of Inkjet Inks

[0210] Comparative examples COMP-1 to COMP-3 and inventive examples INV- 1 to INV-6 were prepared as outlined in Table 5 and Table 6, respectively.The weight percentages (wt%) are calculated based on the total weight of the UV LED curable formulations.

[0211] Table 5

[0212] Table 6Results and Evaluation

[0213] Each of the comparative examples (COMP-1 to COMP-3) and inventive examples (INV-1 to INV-6) was coated on a PET175 substrate using a bar coater with a 10 pm wired bar. The samples were then cured using anAktiprint™ mini duo LED curing station at full power, with a linear transport speed of 10 m / min. The curing efficiency was assessed after 2 passes under the LED curing station.

[0214] A second set of samples were coated and covered with a PET175 substrate to retain potential volatiles created during curing. The samples were cured using two passes under an Aktiprint™ mini duo LED curing station at full power, with a linear transport speed of 10 m / min. After curing, the samples were stored in the dark for one hour. Subsequently, the covers were removed, and the odour of each sample was evaluated.

[0215] The results of the curing efficiency and the odour test are presented in Table 7.Table 7

[0216] Table 7 demonstrates that a wide variety of oligomeric acyl phosphine oxide photoinitiators, as per the invention, exhibit good surface cure performance comparable to standard commercial acyl phosphine oxides, but without any noticeable odour.

Claims

ClaimsClaim 1. An oligomeric acylphosphine oxide photoinitiator according toFormula (I), wherein,Ri to Re are independently selected from an alkyl group and an alkoxy group; R? to R9 are independently selected from the group consisting of a hydrogen, an alkyl group, an alkenyl group, an alkynyl group, an aralkyl group, an alkaryl group, an aryl group, a heteroaryl group, an alkoxy group and an aryloxy group;R10 to R12 are independently selected from the group consisting of an alkyl group, an alkenyl group, an alkynyl group, an aralkyl group, an alkaryl group, an aryl group, a heteroaryl group, an alkoxy group and an aryloxy group;R13 to R15 are independently selected from the group consisting of an alkyl group, an alkenyl group, an alkynyl group, an aralkyl group, an alkaryl group, an aryl group and an heteroaryl group;n and m are integers describing the functionality;n is greater than or equal to 0;m is greater than or equal to 1 ; andn+m is greater than or equal to 3.Claim 2. The oligomeric acylphosphine oxide photoinitiator according to claim 1 , wherein 1, 2, 3, or all of the conditions a) to d) are fulfilled wherein the conditions are:a) R1 to Re are independently selected from an Ci to C4-alkyl group and an Ci to C4-alkoxy group;b) R7 to R9 are independently selected from the group consisting of a hydrogen, an alkyl group and an alkoxy group;c) R10 to R12 are independently selected from the group consisting of an alkyl group, an alkoxy group and an aryl group; andd) R13 to R15 are independently selected from the group consisting of an alkyl group and an aryl group.Claim 3. The oligomeric acylphosphine oxide photoinitiator according to claim 1 , wherein 1, 2, 3, or all of the conditions e) to h) are fulfilled wherein the conditions are:e) Ri to Re are independently selected from an methyl group and a methoxy group;f) R? to Rg are independently selected from the group consisting of a hydrogen, a methyl group and a methoxy group;g) Rio to R12 are independently selected from the group consisting of an alkyl group, an alkoxy group and an aryl group; andh) R13 to R15 are independently selected from the group consisting of an alkyl group and an aryl group.Claim 4. The oligomeric acylphosphine oxide photoinitiator according to any one of claims 1 to 3, wherein n is equal to 0.Claim 5. The oligomeric acylphosphine oxide photoinitiator according to any one of claims 1 to 4, wherein Ri to Re are identical; R? to Rg are identical;Rio to R12 are identical; and R13 to R15 are identical.Claim 6. A mixture of acylphosphine oxide photoinitiators including one or more oligomeric acylphosphine oxide photoinitiators according to any one of claims 1 to 5.Claim 7. The mixture of acylphosphine oxide photoinitiators according to claim 6, wherein the mixture includes an oligomeric acylphosphine oxide photoinitiator selected from the group consisting of:Claim 8. A mixture of acylphosphine oxide photoinitiators according to claim 6 including more than 90 wt% of acyl phosphine oxides according to Formula (I) wherein n+m represents a range from 1 to 10; and wherein the wt% is based on the mixture of acylphosphine oxide photoinitiators.Claim 9. A radiation curable composition including a polymerizable compound and an oligomeric acylphosphine oxide photoinitiator according to any one of claims 1 to 5 or a mixture of acylphosphine oxide photoinitiators according to any one of claims 6 to 8.Claim 10. The radiation curable composition according to claim 8 or 9 including a colorant, preferably a colour pigment.Claim 11. A UV curable inkjet ink including the radiation curable composition of any one of claims 8 to 10.Claim 12. The UV curable inkjet ink of claim 11 , wherein the polymerizable compound includes one or more compounds selected from the group consisting of isobornyl acrylate, vinyl methyl oxazolidinone, 2-(2- vinyloxyethoxyjethyl acrylate, 4-tert-butylcyclohexyl acrylate, tricyclodecane dimethanol diacrylate, dipropylene glycol diacrylate, isodecyl acrylate, 3,3,5- trimethylcyclohexyl acrylate, lauryl acrylate, 4-hydroxybutyl acrylate, (3- ethyloxetane-3-yl)methyl acrylate, (2-methyl-2-ethyl-1 ,3-dioxolane-4-yl)methyl acrylate, benzyl acrylate, 1 ,6-hexanediol diacrylate, dodecanedioic acid diacrylate, tri(propylene glycol) diacrylate, 2-(2-butoxyethoxy)ethyl acrylate, and cyclohexylmethanol tetrafluoroacrylate.Claim 13. An inkjet printing method comprising the steps of:a) jetting a UV curable inkjet ink according to claim 11 or 12 on a substrate; andb) UV curing the UV curable inkjet ink with UV LEDs having an emission wavelength of 360 nm or larger.Claim 14. An inkjet printed article comprising a cured composition derived from the UV curable inkjet ink of claim 11 or 12.Claim 15. A manufacturing method of a mixture of oligomeric acylphosphine oxide photoinitiators according to claim 7 by the synthesis step:wherein the formaldehyde source is preferably a poly(oxymethylene).

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