GB2489322A

GB2489322A - Radiation-curable ink

Info

Publication number: GB2489322A
Application number: GB201204960A
Authority: GB
Inventors: Kevin Burns
Original assignee: Sericol Ltd
Current assignee: Sericol Ltd
Priority date: 2011-03-23
Filing date: 2012-03-21
Publication date: 2012-09-26
Anticipated expiration: 2032-03-21
Also published as: GB201104853D0; GB201204960D0; GB2489322B

Abstract

A radiation-curable ink comprises of a compound having one ethylenically unsaturated group, such as (meth) acrylamide, (2(2-ethoxyethoxy ethyl acrylate), (2-phenoxy ethyl acrylate), cyclic trimethylolpropane formal acrylate, caprolactone acrylate, dicyclopentadienyl methacrylate or isobornyl acrylate; a compound having more than one ethylenically unsaturated group, such as ethylene glycol di(meth)acrylate, dicyclopentenyl di(meth)acrylate, triethylene glycol diacrylate, tetraethylene glycol di(meth)acrylate, pentaerythritol tetracrylate, trimethylolpropane trimethacrylate or tetramethylolmethane tetramethacrylate; an optionally substituted thiochroman-4-one; and a colourant; wherein all parts are by weight and the weight ratio of (a):(b) is >1.4:1. The inks are particularly suitable for ink jet printing flexible substrates. The inks can be cured rapidly using a low irradiation dose and provide prints having a low degree of post-cure colour shift.

Description

INKS

This invention relates to radiation-curable inks and to their use in ink jet printing.

Ink jet printing is a commonly used technique for printing substrates. The inks used vary depending on the properties required for the printed substrate.

Water-based, solvent-based and radiation-curable ink jet printing inks are commercially available.

Some radiation-curable inks suffer from the problem of slow cure, reducing the rate at which printed substrates can be produced. The dose of radiation used to cure the ink can be increased, but this also increases the printing cost because more power is then used by the radiation source. Furthermore, many radiation-curable inks suffer from the problem of colour shift after curing. In other words, the colour of the cured ink changes to an undesirable extent post-curing. There exists a need for inks which can be cured rapidly using a low irradiation dose and provide prints having a low degree of post-cure colour shift.

Furthermore, there is a need for ink compositions suitable for printing flexible substrates. Traditional inks can be brittle after curing and crack when the printed substrate is flexed in normal use. Ideally the flexibility of the ink after curing is achieved without adversely affecting the mechanical strength and robustness of the cured ink.

According to the present invention there is provided a radiation-curable ink comprising: (a) a compound having one ethylenically unsaturated group; (b) a compound having more than one ethylenically unsaturated group; (c) an optionally substituted thiochroman-4-one; and (d) a colorant; wherein all parts are by weight and the weight ratio of (a):(b) is >1.4:1.

In this specification (including its claims), the verb "comprise" and its conjugations is used in its non-limiting sense to mean that items following the word are included, but items not specifically mentioned are not excluded. In addition, reference to a feature by the indefinite article "a" or "an" does not exclude the possibility that more than one of the elements is present, unless the context clearly requires that there be one and only one of the elements. For example "having one" means having one and only one (not including two or more). The indefinite article "a" or "an" thus usually means "at least one".

The weight ratio of (a):(b) is preferably > 2:1, more preferably > 3:1. The ratio of (a):(b) is preferably c 100:1, more preferably < 50:1, especially < 20:1.

In a preferred embodiment, the ink comprises: to 99, more preferably 85 to 98.5, especially 90 to 98.2 parts in total of components (a) and (b); 0.1 to 10 parts, more preferably 0.25 to 7.5 parts, especially 0.5 to 5 parts of component (C); and 0.1 to 10 parts, more preferably 0.2 to 7.5 parts, especially 0.3 to 6 parts of component (d).

In one embodiment, the ink comprises 72 to 92 parts of (a) and 10 to 25 parts of (b).

The compounds having one or more than one ethylenically unsaturated group are optionally monomeric, oligomeric or polymeric, with monomeric compounds being preferred.

Preferred ethylenically unsaturated groups are vinyl groups, (meth)acrylic groups, especially (meth)acrylate and (meth)acrylamide groups. Examples of ethylenically unsaturated groups include acrylamide (H2C=CHCONc) groups, methacrylamide (H2C=C(CH3)CONC) groups, acrylate (H2C=CHCO2) groups and methacrylate (H2C=C(CH3)C02-) groups.

As examples of compounds having one ethylenically unsaturated group there may be mentioned (meth)acrylamide, (meth)acryloylmorpholine, isobutoxymethyl(meth)acrylam ide, isobornyloxyethyl (meth)acrylate, isobornyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, ethyld iethylene glycol (meth)acrylate, t-octyl (meth)acrylamide, diacetone (meth)acrylamide, lauryl (meth)acrylate, d icyclopentad iene (meth)acrylate, d icyclopentenyloxyethyl (meth)acrylate, dicyclopentenyl (meth)acrylate, N5N-dimethyl(meth)acrylamide, tetrachlorophenyl (meth)acrylate, 2-tetrachlorophenoxyethyl (meth)acrylate, tetrahydrofu riuryl (meth)acrylate, tetrabromophenyl (meth)acrylate, 2-tetrabromophenoxyethyl (meth)acrylate, 2-trichlorophenoxyethyl (meth)acrylate, tribromophenyl (meth)acrylate, 2-tribromophenoxyethyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, N-vinylcaprolactam, N-vinylpyrrolidone, phenoxyethyl (meth)acrylate, butoxyethyl (meth)acrylate, pentachlorophenyl (meth)acrylate, bornyl (meth)acrylate and methyltriethylene diglycol (meth)acrylate and mixtures comprising two or more thereof.

Commercially available compounds having one ethylenically unsaturated group include: SR256 (2(2-ethoxyethoxy ethyl acrylate), SR339 (2-phenoxy ethyl acrylate), SR531 (cyclic trimethylolpropane formal acrylate), SR49SB (caprolactone acrylate), SR535 (dicyclopentadienyl methacrylate), SR 506D (isobornyl acrylate), 5R423 (isobornyl methacrylate), SR 313A, 313B and 313D (C12-C14 alkyl (meth)acrylates), all available from Sartomer Co. Inc. and Ciba Ageflex FM6 (n-hexyl (meth)acrylate available from Ciba Specialty Chemicals).

As examples of compounds having more than one ethylenically unsaturated group there may be mentioned ethylene glycol di(meth)acrylate, dicyclopentenyl di(meth)acrylate, triethylene glycol diacrylate, tetraethylene glycol d i(meth)acrylate, tricyclodecaned iyld imethylene d i(meth)acrylate, tris(2-hyd roxyet hyl)isocya nu rate d i(meth)acrylate, tris(2-hyd roxyethyl)isocya nu rate tri(meth)acrylate, ca prolactone-mod ified tris(2-hyd roxyethyl)isocyan u rate tri(meth)acrylate, trimethyloipropane tri(meth)acrylate, E0-modified trimethylolpropane tri(meth)acrylate, P0-modified trimethyloipropane tri(meth)acrylate, tripropylene glycol di(meth)acrylate, neopentyl glycol di(meth)acrylate, I,4-butanediol di(meth)acrylate, I,6-hexanediol di(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaeryth ritol tetra(meth)acrylate, polyester di(meth)acrylate, polyethylene glycol di(meth)acrylate, dipentaerythritol penta(meth)acrylate, d ipentaerythritol tetra(meth)acrylate, caprolactone-mod ified d ipentaerythritol hexa(meth)acrylate, caprolactone-mod ified d ipentaeryth ritol penta(meth)acrylate, d itrimethylolpropane tetra(meth)acrylate, E0-mod ified bisphenol A di(meth)acrylate, P0-modified bisphenol A di(meth)acrylate, E0-modified hydrogenated bisphenol A di(meth)acrylate, P0-modified hydrogenated bisphenol A di(meth)acrylate, E0-modified bisphenol F di(meth)acrylate and mixtures comprising two or more thereof.

Commercially available compounds having more than one ethylenically unsaturated group include: SR 295 (pentaerythritol tetracrylate); SR 350 (trimethylolpropane trimethacrylate); SR 351 (trirnethylolpropane triacrylate); SR 367 (tetramethylolmethane tetramethacrylate); SR 368 (tris(2-acryloxy ethyl) isocyanurate triacrylate); SR 399 (dipentaerythritol pentaacrylate); SR 444 (pentaerythritol triacrylate); SR 454 (ethoxylated (3) trimethylolpropane triacrylate); SR 833S (tricyclodecane dimethanol diacrylate), SR 833S (a diacrylate), SR 9041 (dipentaerythritol pentaacrylate ester) and CN964A85, available from Sartomer Co Inc. The ink optionally comprises a urethane acrylate oligomer. Where such a monomer contains one or more than one ethylenically unsaturated groups it is part of component (a) or (b) respectively. The urethane acrylate oligomer is preferably an aliphatic urethane oligomer Examples of commercially available aliphatic urethane oligomers include: CN 934 CN 934X50, CN 944B85, CN 945A60, CN 945B85, CN 953B70, CN 961 E75, CN 961 H81, CN 962, CN 963A80, CN 963B80, CN 963E75, CN 963E80, CN 963J85, CN 964, CN 964A85, CN 964B85, CN 964H90, CN 964E75, CN 965, CN 965A80, CN 966A80, CN 966B85, CN 966H90, CN 966180, CN 966J75, CN 966R60, CN 968, CN 982E75, CN 982P90, CN 983, CN 983B88, CN 984 and CN 985B88, all available from Sartomer, and mixtures comprising two or more thereof.

Examples of commercially available aromatic urethane oligomers include CN 970A60, CN 970E60, CN 970H75, CN 971 A80, CN 972, CN 973A80, CN 973H85, CN 973J75, CN 975, CN 977C70, CN 978, CN 980, CN 980M50, CN 981, CN 981 A75, CN 981 B88, ON 982A75 and CN 982B88, all from Sartomer, and mixtures comprising two or more thereof.

When a urethane acrylate oligomer is included in the ink the amount is typically 0.25 to 6wt%, especially 1 to 5wt%, relative to the total weight of components (a) to (d).

Optionally the ink further comprises an optionally substituted acyldiphenylphosphine oxide, preferably an optionally substituted benzoyl diphenylphosphine oxide.

The substituents which may be present on the optionally substituted acyldiphenylphosphine oxide are preferably selected from C14-alkyl, C1..4-alkoxy and halo (especially chloro), more preferably C14-alkyl, especially methyl.

As examples of optionally substituted acyldiphenylphosphine there may be mentioned isobutyryl-diphenylphosphine oxide, pivaloyl-diphenylphosphine oxide, 1-methyl-I -cyclohexanoyl-diphenylphosphine oxide, 2-ethylhexanoyl- diphenylphosphine oxide, p-toluyl-diphenylphosphine oxide, o-toluyld iphenylphosph me oxide, 3-pyridylcarbonyl-d iphenylphosph me oxide, acryloyldiphenylphosphine oxide, benzoyl-diphenylenephosphine oxide, 2,2-dimethyl-heptanoyl-diphenylphosphine oxide, terephthaloyl-bis-diphenylphosphine oxide, adi poyl-bis-d iphenylphosph me oxide, 2,4,6-trimethylbenzoyldiphenylphosphine oxide and mixtures comprising two or more thereof. 2,4,6-Trimethylbenzoyldiphenylphosphine oxide is particularly preferred.

Optionally substituted acyldiphenylphosphine oxides may be prepared by the general method described in US 4,324,744.

The amount of optionally substituted acyldiphenylphosphine oxide in the ink, when present, is preferably 3 to I4wt%, more preferably 3 to l3wt%, especially 3 to 1 Owt%, relative to the total weight of components (a) to (d).

The ink optionally further comprises an optionally substituted thioxanthone.

The optional substituents which may be present on the optionally substituted thioxanthone (when present) are preferably selected from alkyl (e.g. C14-alkyl), alkoxy (e.g. C1..4-alkoxy) and halo (e.g. chloro) groups.

The preferred optionally substituted thioxanthone is of the Formula (1): (Ri)rn__1Q__ (R2)n Formula (1) wherein: each R1 and each R2 independently is a substituent other than hydrogen; m has a value of 0, 1, 2, 3 or 4; and n has a value of 0, 1 or 2.

Each R1 and each R2 independently is preferably C14-alkyl, C1..4-alkoxy or halo (especially chloro). In a particularly preferred embodiment, m has a value of 0, each R2 independently is C14-alkyl (especially isopropyl) and n has a value of 1 or2.

Optionally substituted thioxanthones may be prepared by general methods known in the art, for example by the cyclisation of 2-phenylthiobenzoic acid derivatives. The preparation can be performed in one step, e.g. by reacting a 2,2'-dithiodibenzoic acid compound with an aromatic compound in sulphuric acid.

Typical of the methods for preparing thioxanthones are those described in US 4,101,558 and US patent application Publication No. 2003229233.

As examples of optionally substituted thioxanthones there may be mentioned thioxanthone, 2-isopropylthioxanthone, 2-chlorothioxanthone, 2- dodecylthioxanthone, 2,4-diethylthioxanthone, 2,4-dimethylthioxanthone, 1- methoxycarbonylth ioxa nthone, 2-ethoxycarbonylth ioxa nthone, 3-(2- methoxyethoxyca rbonyl) th ioxanthone, 4-butoxycarbonylth ioxanthone, 3- butoxycarbonyl-7-methylth ioxanthone, I -cyano-3-chloroth ioxanthone, 1- ethoxycarbonyl-3-chloroth ioxanthone, 1 -ethoxycarbonyl-3-ethoxyth ioxanthone, 1- ethoxycarbonyl-3-aminoth ioxanthone, 1 -ethoxycarbonyl-3-phenylsudfurylth ioxanthone, 3,4-d i-[2-(2-methoxyethoxy) ethoxycarbonyl} thioxanthone, 1 -ethoxycarbonyl-3-(1 -methyl-I -morpholinoethyl) thioxanthone, 2-methyl-6-d imethoxymethylth ioxanthone, 2-methyl-6-(l, 1 -d imethoxybenzyl) th ioxanthone, 2-morpholinomethylth ioxanthone, 2-methyl-6-morpholinomethyl th ioxanthone, n-allylth ioxanthone-3,4-d icarboxim ide, n-octylth ioxanthone-3,4- dicarboximide, N-(I,1,3,3-tetramethylbutyl)thioxanthone-3,4-dicarboximide, 1- phenoxyth ioxanthone, 6-ethoxycarbonyl-2-methoxyth ioxanthone, 6-ethoxycarbonyl-2-methylth ioxanthone and th ioxanthone-2-polyethylene glycol ester and mixtures comprising two or more thereof.

As examples of commercially available optionally substituted thioxanthones there may be mentioned isopropyl thioxanthone (available as Speedcure TM ITX), 2,4-diethyl thioxanthone, 2-t butyl thioxanthone, 2-chlorothioxanthone, I-chloro-4-propoxyth ioxanthone (CPTX) and 2-propoxy th ioxanthone. Isopropyl th ioxanthone is particularly preferred.

The amount of optionally substituted thioxanthone in the ink, when present, is preferably 0.4 to 4.Swt%, more preferably 0.4 to 4wt%, especially 0.4 to 3wt%, relative to the total weight of components (a) to (d). The optional substituents which may be present on the optionally substituted thiochroman-4-one are preferably selected from alkyl (e.g. C14-alkyl, especially methyl), alkoxy (e.g. C14-alkoxy, especially methoxy) and halo (especially chloro) groups.

Preferably the thiochroman-4-one has a C1..4-alkyl group (especially methyl) and a halo group (especially chloro).

The optionally substituted thiochroman-4-one is preferably of Formula (2): (R3)p(R4)q Formula (2) wherein: each R3 and each R4 independently is a substituent other than hydrogen; and p has a value of 0, 1, 2, 3 or 4; and q has avalue of 0, 1 or2.

Preferably each R3 independently is halo, R4 is C14-alkyl, p has a value of 1 or 2 and q has a value of 1.

Thiochroman-4-ones may be prepared by methods known in the art, for example as described in "Thiochroman-4-ones: synthesis and reactions" by S. Bondock; M.A. Metwally, Journal of Sulfur Chemistry, 1741-6000, volume 29, Issue 6, 2008, pages 623-653. S.W. Schneller also provides synthetic methods in the article entitled "Thiochromanones and Related Compounds", Advances in Heterocyclic Chemistry, volume 18, 1975, pages 59-97.

The colorant is preferably an oil-soluble dye or, more preferably, a pigment.

The pigment which can be used as colorant is not particularly limited, for example it can be an organic or inorganic pigment or a mixture thereof. Numerous commercially available pigments are listed in the Colour Index International.

Examples of red or magenta pigments include C.I. Pigment Red 3, 5, 19, 22, 31, 38, 43, 48:1, 48:2, 48:3, 48:4, 48:5, 49:1, 53:1, 57:1, 57:2, 58:4, 63:1, 81, 81:1,81:2,81:3,81:4,88,104,108,112,122,123,144,146,149,166,168,169, 170, 177, 178, 179, 184, 185, 208, 216, 226 and 257; C.l. Pigment Violet 3, 19, 23, 29, 30, 37, 50 and 88; and C.I. Pigment Orange 13, 16, 20 and 36.

Examples of blue or cyan pigments include CI. Pigment Blue 1, 15, 15:1, 15:2, 15:3, 15:4, 15:6, 16, 17-1,22,27,28,29,36 and 60.

Examples of green pigments include C.l. Pigment Green 7, 26, 36 and 50.

Examples of yellow pigments include C.I. Pigment Yellow 1, 3, 12, 13, 14, 17, 34, 35, 37, 55, 74, 81, 83, 93, 94, 95, 97, 108, 109, 110, 137, 138, 139, 153, 154, 155, 157, 166, 167, 168, 180, 185 and 193.

Examples of black pigments include carbon blacks and C.I. Pigment Black 7, 28 and 26.

Examples of white pigments include titanium dioxide and CI. Pigment White 6, 18 and 21.

The oil-soluble dye is preferably substantially insoluble in water (e.g. a water-solubility of below lwt% at 25°C) and soluble in the remaining components of the ink (solubility above lwt% at 25°C).

Examples of preferred oil-soluble dyes include: the C.I. solvent dyes mentioned below: C.I. Solvent: Black 3, 7, 27, 29 and 34; Yellow 14, 16, 19, 29, 30, 56, 82, 93 and 162; Red 1,3, 8, 18, 24, 27, 43, 49, 51, 72, 73, 109, 122, 132 and 218; Violet 3; Blue 2, 11, 25, 35, 38, 67 and 70; Green 3 and 7; and Orange 2; and the C.l disperse dyes mentioned below: CI. Disperse: Yellow 5, 42, 54, 64, 79, 82, 83, 93, 99, 100, 119, 122, 124, 126, 160, 184:1, 186, 198, 199, 201, 204, 224 and 237; Orange 13, 29, 31:1, 33, 49, 54, 55, 66, 73, 118, 119 and 163; Red 54, 60, 72, 73, 86, 88, 91, 92, 93, 11.1, 126, 127, 134,135,143,145,152,153,154,159,164,167:1,177,181,204,206, 207, 221, 239, 240, 258, 277, 278, 283, 311, 323, 343, 348, 356 and 362; Violet 33; Blue 56, 60, 73, 87, 113, 128, 143, 148, 154, 158, 165, 165:1, 165:2, 176, 183, 185, 197, 198, 201, 214, 224, 225, 257, 266, 267, 287, 354, 358, 365 and 368; and Green 6:1 and 9.

When the ink is to be cured by irradiation, one will usually select a colorant which will not absorb all of the curing radiation.

Typically the colorant is dispersed with the other ingredients of the ink using a dispersing device, for example, a ball mill, a sand mill, an attritor, a roll mill, an agitator, a Henschel mixer, a colloid mill, an ultrasonic homogenizer, a pearl mill, a wet type jet mill, a paint shaker or the like.

It is also possible to include a dispersant in the ink, especially when the colorant comprises a pigment. Although the type of dispersant is not particularly limited, it is preferred to use a high-molecular weight dispersant. Examples of the high-molecular weight dispersant include the SolsperseTM hyperdispersants. It is also possible to use a synergist with the dispersant. In the present invention, the dispersant (when included) is preferably added in an amount of from 1 to 50 parts by weight per 100 parts by weight of colorant.

The colorant may be a single component or a combination of two or more components (e.g. 2 or more pigments).

When the colorant comprises a pigment it preferably has an average particle size below 0.5 pm, more preferably below 0.3 pm. This is for storage stability advantages, and also because larger particles could block the fine nozzles of an ink jet printer if the ink is used in such a printer.

When calculating the number of parts of component (d), the weight of any additional ingredients (e.g. dispersant, diluent, liquid vehicle etc.) are not included.

Preferably the total number of parts by weight of components (a) to (d) adds up to 100. When the total number of parts by weight of components (a) to (d) adds up to 100 this does not rule out the presence of further components, it merely further defines the total amount of components (a) to (d).

If desired further ingredients may be included in the ink, e.g. an antioxidant, a UV stabiliser, a surfactant etc. When the ink contains an antioxidant, the preferred antioxidant is a sterically hindered tertiary or secondary amine. Examples of such amines include N, N-d iphenylam me, N-n itroso-d iphenylam in n itrosod iethylan il in phenylenediamine, an N,N'-di(Ci..4) alkyl-p-phenylenediamine such as N,N'-di-isobutyl-p-phenylenediamine, or N,N'-di-isopropyl-p-phenylenediamine, lrganoxTM 5057 (produced by Firma Ciba Spezialitaetenchemie), N-phenyl-p- phenylened iam me, N, N'-d iphenyl-p-phenylened iamine, N-isopropyl-N-phenyl-p-phenylenediamine, N,N'-di-s-butyl-p-phenylenediamine (KerobitTM BPD produced by BASF Aktiengesellschaft), N-phenyl-N'-isopropyl-p-phenylened iam me (VulkanoxTM 4010 produced by Bayer A C), N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine, N-phenyl-2-naphthylamine, iminodibenzyl, diphenylbenzidine, N-phenyltetraaniline, acridone, 3-bydroxydiphenylamine, 4-hydroxydiphenylamine and mixtures comprising two or more thereof.

When an antioxidant is included in the ink the amount is typically 0.01 to 0.25wt%, relative to the total weight of components (a) to (d).

UV stabilizers may be included in the ink to reduce or prevent premature polymerization during the manufacture and storage of the ink. Examples of suitable stabilisers include AdditolTM S100, SilO and 5120 from Cytec, FlorstabTM UV1, UV5, UV8, UV11 and UV12 from Kromachem; and TinuvinTM 328, 384, 1130, 400, 123, 292 and 5151 from Ciba.

When the ink contains a UV stabiliser, the amount present is selected so as not to unduly interfere with the process for radiation curing the ink in normal use.

Typically 0.1 to lwt% of the UV stabiliser (e.g. an aluminium salt) is used, relative to the total weight of components (a) to (d).

When a surfactant is included in the ink the amount is typically 0.1 to 2.Swt% surfactant, relative to the total weight of components (a) to (d).

Preferably the ink has a viscosity of from 15 to 30 cP, more preferably 18 to 27 cP, when measured at 25°C.

Preferably the ink has a surface tension of from 20 to 40 mN/m, more preferably 25 to 35 mN/m, when measured at 25°C.

According to a second aspect of the present invention there is provided a process for printing a substrate comprising applying thereto an ink according to any one of the preceding claims and curing the ink. The ink is applied to the substrate by means of an ink jet printer. The process is particularly useful where the substrate is a flexible substrate.

Preferably the curing is performed using ultra violet light, especially using a light emitting diode which emits ultraviolet light (UV.-LED).

Printed substrates carrying a cured ink form a further feature of the present invention. Preferred substrates are metal, plastic, ceramic, glass, wood and flexible substrates such as papers, plastics sheets, balloons, textiles and apparel.

The invention in further illustrated by the following examples in which all parts and percentages are by weight unless otherwise stated.

Examples I and 2 and Comparative Example 1 The inks described in Table I were prepared by mixing the indicated number of parts by weight of the specified ingredients in a Silverson mixer for 30 minutes at a speed of 4000 rpm.

Table 1

Ingredient Example 1 Example 2 Comparative

Example I

((c) missing) Component (a) N-vinyl caprolactam 14.81 25.18 15.17 Phenoxy ethyl acrylate 35.14 30.77 35.57 Compound Isobornyl acrylate 15.23 15.23 having one ethylenically unsaturated group ________________________ ____________ ___________ _____________ Component (b) Ethoxylated 11.27 11.42 11.27 pentaeryth ritol Compound tetraacrylate ___________ __________ _____________ having more CN964A85 2.31 2.34 2.31 than one ethylenically unsaturated group ________________________ ____________ ___________ _____________ Component (c) 2-Methyl-6-chloro-thio-1.58 8.80 0 chroman-4-one __________ Colorant (d) 8.53 of PDI 8.64 of 8.53 dispersion _______________________ ___________ PD2 ____________ Other UV12 0.32 0.32 0.32 components (N, N'-d i-sec.-butyl-p-0.09 0.09 phenylene diamine) ___________ __________ _____________ BYK-307 0.03 0.03 0.03 TegoradTM 2100 0.5 0.50 0.5 LucirinTM TPO 9.4 6.00 9.4 SpeedcureTM ITX 0.79 __________ 1.58 lrgacureTM 819 ___________ 4.00 ____________ ______________ lrgacureTM 907 ___________ 2.00 ____________ Total 100 100 ___________

NotestoTablel:

1) The colorant PD1 is a 3Owt% dispersion of Cl. Pigment Blue 15:4 in a liquid vehicle comprising a dispersant. Therefore the amount of colorant present was 30% of the amount indicated in Table 1.

2) The colorant PD2 is 29.98wt% dispersion of C.l. Pigment Blue 15 in a liquid vehicle comprising propoxylated neopentyl glycol diacrylate and polymer amide dispersant. Therefore the amount of colorant present was 29.98% of the amount indicated in Table 1.

3) UV12 was a 3Owt% suspension of an aluminium tris (N-hydroxy-N-nitroso phenylaminato-O-O' salt in phenoxy ethyl acrylate. Therefore 0.32 parts of UV12 consisted of 0.096 parts of an aluminium tris (N-hydroxy-N-nitroso phenylaminato- 0-0' salt and 0.224 parts of phenoxy ethyl acrylate having one acrylate group.

4) BYK-307 is a polyether modified polydimethylsiloxane available from BYK USA Inc..

5) TegoradTM 2100 is a silicone acrylate wetting agent available from Evonik Tego Chemie GmbH.

6) LucirinTM TPO is 2,4,6-trimethylbenzoyldiphenylphosphine oxide.

7) SpeedcureTM ITX is isopropyl thioxanthone.

8) lrgacureTM 819 and lrgacureTM 907 are photoinitiators.

9) CN964A85 is an aliphatic polyester based urethane diacrylate oligomer blended with 15% SR306, tripropylene glycol diacrylate, obtained from Sartomer.

Results The inks described in Table 1 above were coated onto a substrate (Mactac Imagin JT5929P gloss white self-adhesive PVC) using a 12 micron wire-wound bar. The coated substrates were then cured by passing them under a UV-LED irradiation source (a Baldwin 8W 395nm LED lamp) at a speed of 60 metres/minute enough times to produce a tack-free film. The L*a*b* values of the coated substrates immediately after curing and again 24 hours later were measured using a GretagMacbeth SpectroEye spectrophotometer. The colour shift, expressed as the LiE value, was calculated according to the equation: LiE = SQRT [(L*2 -L*1)2 + (a*2 -a*1)2 + (b*2 -b*1)2] wherein: SQRT means square root; L*1, a*1 and b*1 respectively mean the L*, a* and b* values immediately after curing; and L*2, a*2 and b*2 respectively mean the L*, a* and b* values 24 hours after curing.

The results are shown in Table 2.

Table 2

Example 1 Comparative Example 1 ((c) Example 2 _______ ____________________ missing) __________________ _______ L*a*b* values immediately after curing _________________ L*1 49.20 48.39 45.8 a*1 -24.61 -25.88 -19.67 b*1 -51.67 -50.17 -54.45 _______ L*a*b* values 24 hrs after curing __________________ L*2 49.63 49.19 46.6 a*2 -23.90 -24.09 -21.06 b*2 -52.94 -52.59 -54.46 Colour 1.52 3.12 1.6 shift

IXE ______________ ____________________ ____________

Table 2 shows that the Examples demonstrated less colour shift than the

Comparative Example.

In addition, Example 2 required an LED-UV dose of c200mJ/cm2 to produce a tack-free 12 micron film.

Claims

CLAIMS1. A radiation-curable ink comprising: (a) a compound having one ethylenically unsaturated group; (b) a compound having more than one ethylenically unsaturated group; (c) an optionally substituted thiochroman-4-one; and (d) a colorant; wherein all parts are by weight and the weight ratio of (a):(b) is >1.4:1.
2. An ink according to claim I wherein the weight ratio of (a):(b) is >3:1.
3. An ink according to any one of the preceding claims wherein the weight ratio of(a):(b) is <100:1.
4. An ink according to any one of the preceding claims which comprises: to 99 parts in total of components (a) and (b); 0.1 to 10 parts of component (c); and 0.1 to 10 parts of component (d); wherein all parts are by weight.
5. An ink according to any one of the preceding claims which comprises: to 98.2 parts in total of components (a) and (b); 0.5 to 5 parts of component (c); and 0.3 to 6 parts of component (d); wherein all parts are by weight.
6. An ink according to any one of the preceding claims which further comprises an optionally substituted acyldiphenylphosphine oxide.
7. An ink according to any one of the preceding claims which further comprises an optionally substituted thioxanthone.
8. An ink according to any one of the preceding claims wherein the optionally substituted thiochroman-4-one is of Formula (2): (R3)p(R4)q Formula (2) wherein: each R3 and each R4 independently is a substituent other than hydrogen; and p has a value of 0, 1, 2, 3 or 4; and q has a value of 0, 1 or 2.
9. An ink according to claim 8 wherein each R3 independently is halo, R4 is Ci 4-alkyl, p has a value of I or 2 and q has a value of 1.
10. An ink according to any one of the preceding claims which further comprises a urethane acrylate oligomer.
11. An ink according to any one of the preceding claims having a viscosity of from 15 to 30 cP when measured at 2500.
12. An ink according to any one of the preceding claims having a surface tension of from 20 to 40 mN/rn when measured at 2500.
13. A process for printing a substrate comprising applying thereto a ink according to any one of the preceding claims and curing the ink.
14. The process of claim 13 wherein the ink is applied to the substrate by means of an ink jet printer.
15. The process of claim 13 or 14 wherein the curing is performed using a light emitting diode which emits ultraviolet light.
16. A printed substrate carrying a cured ink according to any one of claims 1 to 12.