GB2517591A - Ink jet printing - Google Patents

Abstract

A printing process comprises applying a radiation-curable ink onto a substrate using an inkjet printer and curing the ink, the printer comprising a nozzle plate with a surface energy (SE) >25 dynes/cm and a piezoelectric actuator operating at a frequency (F) of ≥20 KHz. The ink comprises less than 5 wt.% organic solvent and has a polarity (P) satisfying Equation 1 and a surface tension (ST) (measured in dynes/cm at 25°C) satisfying Equation 2: wherein: D is the average droplet volume (picolitres) created by the piezoelectric actuator. A radiation-curable ink is also disclosed which comprises 40-80 pbw compound having one ethylenically-unsaturated group (preferably N-vinylcaprolactam with 2-phenoxy ethyl acetate), 1-10 pbw curable compound having two ethylenically-unsaturated groups and 1-20 pbw curable compound having three or more ethylenically-unsaturated groups (preferably urethane diacrylate oligomer combined with ethoxylated trimethylolpropane triacrylate and propoxylated neopentylglycol diacrylate), 0-4.9 pbw organic solvent, 0.1-15 pbw colorant, and 0.1-15 pbw photoinitiator.

Description

INK JET PRINTING

This invention relates to process for printing a substrate with a radiation-curable ink and to radiation-curable inks.

Ink jet printing is a commonly used technique for printing substrates.

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

Many ink jet printers contain an ink storage tank and a printhead comprising a piezoelectric actuator and a nozzle plate. The nozzle plate comprises numerous holes or nozzles. The piezoelectric actuator vibrates, creating pressure waves which eject droplets of the ink through the nozzles or holes and onto the substrate.

By controlling the position of the printhead relative to the substrate, and controlling which are nozzles are firing at any point in time, one can use the printer to form a desired pattern of ink on the substrate which corresponds to desired text, images etc. The printheads used in certain ink jet printers (e.g. in the wide format, graphics and industrial markets) typically fall into two categories, namely non-wetting' printheads and wetting printheads'. Generally the former comprise nozzle plates having a lower surface energy than the latter.

To achieve good print quality when using a wetting printhead', it is important that a uniform, thin film of ink is formed across the surface of the nozzle plate. If the printhead nozzle plate is incorrectly wetted, print quality defects may occur as a result of an irregular build-up of ink on the nozzle plate and from nozzle blockage, obstruction causing jet deviation, ink dripping, drop starvation or mis-firing. When using radiation-curable inks, the mis-firing and irregular wetting of the nozzle plate can result in cured or partially cured ink blocking or otherwise obstructing the nozzles. This can permanently damaging the printhead or require a greater level of printhead maintenance to keep the head functioning correctly.

After extensive research we have found that there is a relationship between the surface energy of the printhead nozzle plate, the frequency at which the piezoelectric actuator operates and the ink droplet volume and properties (notably the ink's polarity surface tension) which all impact on print performance. Further, we have found that by controlling the properties of the radiation-curable ink relative to the properties of the printhead components to satisfy Equations 1 and 2 below one may achieve good nozzle plate wetting and thereby obtain good quality prints.

According to a first aspect of the present invention there is provided a process for printing a substrate comprising the steps of applying droplets of a radiation-curable ink onto the substrate by means of an ink jet printer and curing the printed ink, wherein: (i) the ink jet printer comprises a nozzle plate having a surface energy (SE) of >25 in dynes/cm and a piezoelectric actuator operating at a frequency (F) of at least 20 KHz; and (ii) the radiation-curable ink comprises less than 5 wt% organic solvent and has the following features: (a) a polarity (P) which satisfies Equation 1: -20) t-I÷(ST/5) J Equation 1 wherein: ST is the surface tension of the ink in dynes/cm when measured at 25°C; and F is as hereinbefore defined; and D is the average volume of the droplets in pL created by the piezoelectric actuator; (b) a surface tension (ST) which satisfies Equation 2: ST < (SE-3) Equation 2 wherein ST and SE are as hereinbefore defined.

Thus with knowledge of the nozzle plate surface energy and the frequency at which the piezoelectric actuator operates, one may select the ink droplet volume and use an ink having the properties (polarity and surface tension) which satisfy Equations 1 and 2 and thereby achieve the benefits of the present invention.

Preferably the nozzle plate has a surface energy >30 dynes/cm, more preferably >35 dynes/cm. The surface energy is that when measured at 20°C.

Printheads comprising nozzle plates having the desired surface energy are commercially available from FUJIFILM Dimatix.

In a preferred embodiment, the nozzle plate is constructed from silicon or a silicon compound or carries a coating made from silicon or a silicon compound.

Such nozzle plates are particularly robust and resistant to caustic or aggressive inks. Printheads comprising nozzle plates having the desired surface energy and constructed from silicon or a silicon compound are available from FUJIFILM Dimatix under the trade name 0-Class" printhead.

The average volume of the droplets (D) in pL (picolitres) for a particular frequency and print mode is usually specified by the manufacturer of the printhead comprising the nozzle plate and may in many cases be selected by the user from a number of options, using software provided with the ink jet printer. For example, the Q-class printhead from FUJIFILM Dimatix is available as 0256/10 and Q256/30 models. With the 0256/10 model one may select drop sizes of lOpL, 2OpL or 3OpL. With the 0256/30 model one may select drop sizes of 2SpL, SOpL or BOpL. The volume of the droplets being created by the piezoelectric actuator is the volume before any droplet merging which may take place at the nozzle plate.

In greyscale printing it is possible for several droplets (e.g. three droplets, being for example 10 pL, 9 pL and 11 pL) to merge at the nozzle plate to create a single droplet (e.g. a single 30 pL droplet) and in this case D has the "pre-merging" value of 10 (not the post-merging" value of 30).

The actual dropsize D for a given printhead may change a little from that selected depending on the frequency F. For greyscale printing, it is possible to apply multiple pulses of electricity (typically 2, 3 or 4 pulses of electricity) to the piezoelectric actuator per Hz and thereby increase the effective frequency (F) of the piezoelectric actuator. For example, if three pulses of electricity are applied per Hz to a 20 KHz piezoelectric actuator, the effective frequency is 60 KHz and the value of F to be used in Equation 1 is 60. Similarly, if two pulses of electricity are applied per Hz to a 12 KHz piezoelectric actuator, the effective frequency is 24 KHz and the value of F to be used in Equation 1 is 24.

The radiation-curable ink preferably comprises: a) a compound having one ethylenically unsaturated group; b) a compound having more than one ethylenically unsaturated group; c) less than 5wt% organic solvent; and d) a colorant.

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".

One may ensure that the ink has a high polarity by including polar components within the ink and avoiding non-polar (or hydrophobic) components or keeping the content of such non-polar (or hydrophobic) components low. For example, polar components such as propylene carbonate, glycerol, phenoxyethyl (meth)acrylate and trimethyloipropane tri(meth)acrylate will generally increase the polarity of the ink. Non-polar or hydrophobic components such as isobornyl (meth)acrylate, octyl (meth)acrylate and decyl (meth)acrylate will generally decrease the polarity of the ink.

One may ensure that the radiation-curable ink has a low surface tension by including within it components which have a low surface tension and/or one or more surfactants. Components having low surface tension include lower alcohols (e.g. methanol, ethanol and isopropanol) and toluene, but account must be taken of the requirement for the ink to contain less than 5wt% organic solvent.

Numerous surfactants are commercially available and include ionic surfactantas and non-ionic surfactants (e.g. SurfynolTM surfactants).

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 (i.e. only 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, ethyldiethylene glycol (meth)acrylate, t-octyl (meth)acrylamide, diacetone (meth)acrylam ide, lauryl (meth)acrylate, dicyclopentadiene (meth)acrylate, dicyclopentenyloxyethyl (meth)acrylate, dicyclopentenyl (meth)acrylate, N5N- dimethyl(meth)acrylam ide, tetrachlorophenyl (meth)acrylate, 2-tetrachlorophenoxyethyl (meth)acrylate, tetrahydrofurfuryl (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), SR339c (2-phenoxy ethyl acrylate), SR395 (isodecyl acrylate), SR4B9D (tridecyl acrylate), SR531 (cyclic trimethylolpropane formal acrylate), SR495B (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).

In view of the above requirement for polarity (P) to satisfy the stated formula, it is preferred for the ink not to contain large amounts of non-polar, ethylenically unsaturated compounds (e.g. SR 506D, SR423, SR395 and SR498D). However such non-polar, ethylenically unsaturated compounds may still be included in the ink and may in fact confer desirable properties on the ink, provided that the ink satisfies the requirements of feature (ii) defined above.

Preferably component a) comprises an N-vinyl amide or N-acryloyl amine (e.g. N-vinyl caprolactam, N-vinyl piperidine or N-acryloyl morpholine) and a monofunctional methacrylate (e.g. 2-phenoxy ethyl acrylate).

Preferably the ink comprises 1 to 90 parts, more preferably 40 to 80 pads, especially 50 to 75 parts by weight of compounds having one ethylenically unsaturated group.

Compounds having more than one ethylenically unsaturated group include compounds having two ethylenically unsaturated groups, compounds having three or more ethylenically unsaturated groups and mixtures comprising two or more thereof.

As examples of compounds having more than one ethylenically unsaturated group there may be mentioned ethylene glycol di(meth)acrylate, 1,10-decanediol diacrylate, dicyclopentenyl di(meth)acrylate, dipropylene glycol di(meth)acrylate, 3-methyl pentanediol diacrylate, divinyl ethers (e.g. DVE-3), triethylene glycol diacrylate, tetraethylene glycol di(meth)acrylate, tricyclodecanediyldimethylene di(meth)acrylate, tris(2-hydroxyethyisocyanurate di(meth)acrylate, tris(2- hydroxyethyisocyanurate tri(meth)acrylate, caprolactone-modified tris(2-hydroxyethyisocyanurate tri(meth)acrylate, trimethylolpropane tri(meth)acrylate, E0-modified trimethylolpropane tri(meth)acrylate, P0-modified trimethylolpropane tri(meth)acrylate, tripropylene glycol di(meth)acrylate, neopentyl glycol di(meth)acrylate, 1 4-butanediol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, polyester di(meth)acrylate, polyethylene glycol di(meth)acrylate, dipentaerythritol penta(meth)acrylate, dipentaerythritol tetra(meth)acrylate, caprolactone-modified dipentaerythritol hexa(meth)acrylate, caprolactone-modified dipentaerythritol penta(meth)acrylate, ditrimethylolpropane tetra(meth)acrylate, ethoxylated trimethylolpropane tri(meth)acrylate, propoxylated trimethylolpropane tri(meth)acrylate E0-modified 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 (trimethylolpropane 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) and SR 9041 (dipentaerythritol pentaacrylate ester).

Preferably the ink comprises 0 or 1 to 70 parts, more preferably 1 to 20 parts, especially 5 to 11 parts by weight of compounds having two (i.e. only two) ethylenically unsaturated groups.

Preferably the ink comprises 1 to 20 parts, especially 3 to 9 parts by weight of compounds having three or more ethylenically unsaturated groups.

The ink optionally comprises one or more oligomers having more than one ethylenically unsaturated group, e.g. urethanes and/or polyesters having more than one ethylenically unsaturated group.

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 acrylate oligomer Examples of commercially available aliphatic urethane acrylate oligomers include: CN 934 CN 934X50, ON 944B85, ON 945A60, CN 945B85, CN 953B70, CN 961 E75, ON 961 H81, ON 962, ON 963A80, ON 963B80, CN 963E75, ON 963E80, ON 963J85, ON 964, ON 964A85, ON 964B85, ON 964H90, ON 964E75, ON 965, ON 965A80, ON 966A80, ON 966B85, ON 966H90, ON 966180, ON 966J75, ON 966R60, ON 968, ON 982E75, ON 982P90, ON 983, ON 983B88, ON 984 and ON 985B88, all available from Sartomer, and mixtures comprising two or more thereof.

Examples of commercially available aromatic urethane acrylate oligomers include ON 970A60, ON 970E60, ON 970H75, ON 971 A80, ON 972, ON 973A80, ON 973H85, ON 973J75, ON 975, ON 977070, ON 978, ON 980, ON 980M50, ON 981, ON 981 A75, ON 981 B88, ON 982A75 and ON 982B88, all from Sartomer, and mixtures comprising two or more thereof.

Preferably component b) comprises a urethane di-acrylate oligomer.

When a urethane acrylate oligomer is included in the ink the amount is typically 0.25 to 10 parts, especially 1 to 6 parts by weight.

The radiation-curable ink preferably comprises 0 to 4.9 parts, more preferably 0.1 to 4.5 parts, especially 0.5 to 4.5 parts of organic solvent.

In one embodiment all of the organic solvents present in the ink have a boiling point of 140 to 250C.

The organic solvent preferably has a higher polarity and optionally a lower surface tension than all of the radiation-curable components of the ink, e.g. components a) and b) (including bi) and b2)).

The most preferred organic solvents are selected from glycol ethers, cyclic lactones, organic carbonates, dibasic esters, bio-solvents and mixtures comprising two or more thereof Preferred glycol ethers include diethylene glycol diethyl ether, DowanolTM DPM, ethoxy propanol, propoxy propanol and butoxy propanol.

Preferred organic carbonates include propylene carbonate.

Preferred cyclic lactones include y-butyrolactone, y-valerolactone and 5-va lero I actone.

The preferred organic solvent comprises an organic carbonate (especially propylene carbonate) and/or a glycol ether (especially diethylene glycol diethyl ether)..

Organic solvents comprising y-butyrolactone and one or more glycol ethers and/or propylene carbonate are also preferred.

Dibasic esters typically comprise a di(C14 alkyl) ester of a saturated aliphatic dicarboxylic acid having 3 to 8 carbon atoms, e.g. of the following formula: R 0' ORL wherein: A represents (CH2)15; and R1 and R2 are the same or different and represent C14 -alkyl, preferably methyl or ethyl, and most preferably methyl.

Organic solvents do not cure when the radiation-curable ink is irradiated during the process of the present invention, i.e. they are inert. Therefore organic solvents are free from ethylenically unsaturated groups.

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 Cl. 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; Cl. Pigment Violet 3, 19, 23, 29, 30, 37, 50 and 88; and Cl. Pigment Orange 13, 16, 20 and 36.

Examples of blue or cyan pigments include Cl. 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 Cl. Pigment Green 7, 26, 36 and 50.

Examples of yellow pigments include Cl. 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 19 Examples of black pigments include carbon blacks and Cl. Pigment Black 7, 28 and 26.

Examples of white pigments include titanium dioxide and Cl. Pigment White 6, lBand2l.

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 Cl. Solvent dyes mentioned below: Cl. 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: Cl. 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.

Typically the colorant is dispersed with the other ingredients of the ink using a dispersing device, for example, a ball mill, bead mill, sand mill, attritor, roll mill, agitator, Henschel mixer, colloid mill, ultrasonic homogenizer, pearl mill, wet-type jet mill, 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, especially below 0.2 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. stabiliser, dispersant, diluent, liquid vehicle etc.) are not included.

The radiation-curable ink preferably comprises 0.1 to 15 parts, more preferably 0.5 to 10 parts, especially 0.75 to 8 parts of colorant.

Preferably the total number of parts by weight of components a) to d) (and e) when present) 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).

When UV light is used to cure the printed ink, the ink preferably contains one or more photoinitiators (component e)). Whilst any commercially available photoinitiator can be used which matches the radiation, those with a low tendency for yellowing are preferred. Examples of suitable photoinitators include alpha- hydroxyalkylphenones, such as 2-hydroxy-2-methyl-1 -phenyl propan-1 -one, 2- hydroxy-2-methyl-1 -(4-tert-butyl-) phenylpropan-1 -one, 2-hydroxy-[4 -(2- hydroxypropoxy)phenyl]-2-methylpropan-1 -one, 2-hydroxy-1 -[4-(2-hydroxyethoxy)phenyl]-2-methyl propan-1 -one, 1 -hydroxycyclohexylphenylketone and oligo[2-hydroxy-2-methyl-1 -{4-( 1 -methylvinyphenyl}propanone], alpha-aminoalkylphenones, alpha-sulfonylalkylphenones and acylphosphine oxides such as 2,4,6-trimethylbenzoyl-diphenylphosphine oxide (e.g. LucirinTM TPO), ethyl- 2,4,6-trimethylbenzoylphenylphosphinate and bis(2,4,6-trimethylbenzoy- phenylphosphine oxide (e.g. LucirinTM BAPO), benzophenone, 1- hydroxycyclohexyl phenyl ketone, benzil dimethylketal, bis(2,6-dimethylbenzoy- 2,4,4-trimethylpentylphosphine oxide and mixtures comprising two or more thereof. Such photoinitiators are known and are commercially available, for example under the trade names lrgacureTM, DarocurlM and LucerinTM (from BASE). An especially preferred photoinitiator is lrgacureTM 2959.

Preferably the ink comprises 0.1 to 15 parts by weight, more preferably 0.5 to 12 parts by weight of photoinitiator(s).

Optionally the ink further comprises an anti-oxidant. When the ink contains an antioxidant, the preferred antioxidant is a sterically hindered tertiary or secondary amine. Examples of such amines include N,N-diphenylamine, N-nitroso-diphenylamine, nitrosodiethylaniline, p-phenylenediamine, an N,N'-di(Ci.4) alkyl-p-phenylenediamine such as N,N'-di-isobutyl-p-phenylenediamine, or N,N'-di-isopropyl-p-phenylenediam in lrganoxTM 5057 (produced by Firma Ciba Spezialitaetenchemie), N-phenyl-p-phenylenediamine, N, N'-diphenyl-p- phenylenediam i, N-isopropyl-N-phenyl-p-phenylenediam me, N, N'-di-s-butyl-p- phenylenediamine (KerobitTM BPD produced by BASE Aktiengesellschaft), N-phenyl-N'-isopropyl-p-phenylenediam me (VulkanoxTM 4010 produced by Bayer A N-(1,3-dimethylbuty-N'-phenyl-p-phenylenediamine, N-phenyl-2-naphthylamine, iminodibenzyl, N,N'-diphenylbenzidine, N-phenyltetraaniline, acridone, 3-hydroxydiphenylamine, 4-hydroxydiphenylamine, hydroquinone monomethyl ether, 4-hydroxy-2,2,6,6-tetramethylpiperidin-1 -oxyl and mixtures comprising two or more thereof.

When an antioxidant is included in the ink the amount is typically 0.01 to 0.25 parts by weight.

In one embodiment the radiation-curable ink is free from surfactants.

Curing rates may be increased by including an amine synergist in the ink.

Suitable amine synergists include, for example, free alkyl amines, e.g. triethylamine or triethanol amine; aromatic amines, e.g. 2-ethylhexyl-4-dimethylam inobenzoate, ethyl-4-dimethylaminobenzoate and also polymeric amines as polyallylamine and its derivatives. Curable amine synergists such as ethylenically unsaturated amines (e.g. acrylated amines) are preferable since their use provide inks having less odour due to their ability to be incorporated into the cured ink. When used, the amount of amine synergists is preferably from 0.1 to 1 Owt.% based on the total weight of the ink, more preferably from 0.3 to 3wt%.

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 5100, 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 e).

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 e).

Preferably the ink has a viscosity of from 5 to 40 cP, more preferably 15 to cP, especially 18 to 27 cP, when measured at 25°C.

Preferably the ink has a surface tension of 20 to 50 mN/m, more preferably 28 to 40 mN/m, when measured at 25°C.

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

Preferred substrates are metal, plastic, ceramic, glass, wood and flexible substrates such as papers, plastics sheets, balloons, textiles and apparel.

In a particularly preferred embodiment, the radiation-curable ink comprises: a) 40 to 80 parts of a compound having one ethylenically unsaturated group; bi) 1 to 20 parts of a curable compound having two ethylenically unsaturated groups; b2) 1 to 20 pads of a curable compound having three or more ethylenically unsaturated groups; c) 0 to 4.9 parts of organic solvent (preferably having a higher polarity and/or a lower surface tension than components a), bl) and b2)); d) 0.1 to 15 parts of colorant; and e) 0.1 to 15 parts of photoinitiator; wherein all parts are by weight.

The inks described above form a second aspect of the present invention.

Preferably the piezoelectric actuator operates at a frequency of at least 25 KHz, more preferably at least 30 KHz, especially at least 40 KHz. Preferably the piezoelectric actuator operates at a frequency of less than 100 KHz. In some ink jet printers it is possible to adjust the frequency (F) at which the piezoelectric actuator operates using software provided with the ink jet printer. Often the pre-set operational print modes of a printing machine will adjust frequency automatically depending on parameters such as number of print passes used to build the image, or number of heads selected.

In one embodiment the frequency at which droplets of the ink contact with the substrate is the same as the frequency at which the piezoelectric actuator operates. Thus one drop of ink is ejected from the printed for each vibration of the piezoelectric actuator. In another embodiment, the frequency at which droplets of the ink contact with the substrate is the lower than the frequency of the piezoelectric actuator. For example, droplets may merge before they contact with the substrate, e.g. in flight or at the nozzle plate.

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

The surface energy (SE) of the nozzle plate was measured using DataPhysics OCA 20 equipment employing three liquids: ethylene glycol (5pl), diiodomethane (2pl) and water (lOpI). Using this equipment, the static contact angle at 25°C was measure three times for each liquid, allowing a drop of each liquid to fall onto the nozzle plate surface. The resultant values for the static contact angles were inputted to the SE analysis too, and the surface energies were determined using the Owens, Wendt, Rabel & Kaelble (OWRK) method.

The surface tension (ST) of the inks at 25°C was determined in using a KSV Sigma tensiometer fitted with a platinum DuNouy ring. The tensiometer was set in Surface Tension Ring Mode, density measurement was obtained using method 2 provided with the equipment. The ST of each ink was measured 3 times and the average figure was used.

The interlacial tension (lET) of the inks was determined in using a KSV Sigma tensiometer fitted with a platinum DuNouy ring and weight accessory, usinga 2-phase sample and a temperature of 25°C. The low density phase was FC-72 periluoro compound and the ink was the high density phase. The Interfacial Tension Ring Mode was selected. The density difference between the two phases was inputted into hardware and method 2 selected. The FC-72 perfluoro compound had a density of 1.65 glcm3. The tensiometer was operated in push mode and the IFT was measured 3 times using Huh-Mason correction and the average figure was calculated.

The average volume of the droplets in picolitres (pL) at various frequencies is often provided along with technical literature accompanying the ink jet printer. However where such information is not provided, one may determine the average volume of the droplets by ejecting a known number of droplets (e.g. a total of 128 million droplets) from all nozzles into a pre-weighed receptacle, weighing the receptacle containing the droplets, calculating the average mass per droplet and dividing this mass by the density of the ink. In the Examples we used binary mode and hence there was no merging of droplets at the nozzle plate.

However had there been droplet merging (e.g. three into one), we would have needed to have taken account of this merging when calculating the average volume of pre-merged droplets (e.g. by dividing by 3).

The polarity of the inks was calculated from their ST and IFT using the software provided with the DataPhysics OCA 20 equipment mentioned above.

Using this software, the high density phase from method 4 (perfluorohexane) was selected. The ST and IFT of the ink were inputted and the polarity was calculated by the software using the WORK method according to the Owens-Wendt method.

An Apollo II ink evaluation rig was obtained from FUJIFILM Dimatix which comprised a QS256/10 printhead.

The following raw materials were used in the Examples: SR339c is 2-phenoxy ethyl acrylate, obtained as SartomerTM SR339c.

NVC is N-vinyl caprolactam.

SR454 is ethoxylated (3) trimethylolpropane triacrylate, obtained as SartomerTM 5R454.

UV12 is a 3Owt% suspension of an aluminium tris (N-hydroxy-N-nitroso phenylaminato-O-O' salt in phenoxy ethyl acrylate, obtained as Florstab UV12.

SR9003 is propoxylated neopentylglycol diacrylate, obtained as SartomerTM SR9003.

CN964 is an aliphatic polyester based urethane diacrylate oligomer, obtained from Sartomer as CraynorTM CN964.

Cyan PD is a cyan dispersion comprising 30%wt% of Cl. Pigment Blue 15:4 (i.e. a colorant) and the remainder (70%) consisted of n-propylene glycol diacrylate (59wt%), UV12 (lwt%) and a dispersant (lOwt%).

TPO is 2,4,6-trimethylbenzoyldiphenylphosphine oxide, a photoinitiator obtained from BASF under the name LucirinTM TPo.

lrg 184 is 1-hydroxy-cyclohexyl-phenyl-ketone, a photoinitiator obtained from Ciba under the name lrgacureTM 184.

JPC is propylene carbonate obtained from Agfa chemicals.

IBOA is isobornyl acrylate, obtained from Sartomer as SartomerTM SR506.

Red 355 is Cl. Solvent Red 119, a dye obtained from BASF as Neozapan Red 335.

CN964A85F is a urethane acrylate resin comprising lSwt% of n-propylene glycol diacrylate, obtained from Sartomer as CraynorTM C N 964A85 F.

Examples

Inks were prepared having the formulations described in Table 1 below.

The ST and polarity of the inks, determined as described above, are also shown in

Table 1:

Table 1

Ingredient Ink 1 Ink 2 Ink 3 Ink 4 Component a) SR339c 0 38.47 38.47 0 NVC 24.59 24.59 24.59 0 1 ethylenically unsaturated IBOA 38.47 0 0 84.0 group _____________ ____________ ___________ ____________ ____________ Component b) SR454 7.49 7.49 7.49 0 SR9003 4.0 4.0 1.0 0 >1 ethylenically CN964 0 4.00 4.00 0 unsaturated CN964A85F 4.0 0 0 15.0 group _____________ ____________ ___________ ____________ ____________ Component c) JPC 0 0 3 0 organic solvent Component d) Cyan PD 8.65 8.65 8.65 0 colorant Red 355 0 0 0 1.0 Other UV12 0.3 0.3 0.3 0 components TPO 9.5 9.5 9.5 0 ___________ lrg 184 3.0 3.0 3.0 0 Total number of parts 100 100 100 100 ST (dyne/cm) 32.0 36.5 34.8 30.8 Polarity 6.59 9.30 11.90 3.64 Notes: 1) The colorant Cyan PD is a 30 wt% dispersion of Cl. Pigment Blue 15:4 in a liquid vehicle comprising a dispersant. Thus for the inks containing Cyan PD, the amount of colorant present was 30% of the amount indicated in Table 1.

2) All amounts in Table 1 are in parts by weight.

Printing The inks described in Table 1 were printed onto paper and self-adhesive vinyl using the Apollo II rig and cured using UV light by passing the printed substrate through a LJV belt drier. This rig comprised a printhead comprising a nozzle plate having surface energy (measured as described above) of 48 dynes/cm and a variable frequency piezoelectric actuator, operating at the frequency 20KHz or 40KHz, as indicated in Table 2 and Table 3 respectively. All inks satisfied Equation 2.

Table 2 -Printing at 20KHz Comparative Example 1 Example 2 Comparative Example 1 ____________ ___________ Example 2 Ink 1 lnk2 lnk3 lnk4 ST (dyne/cm) 32.0 36.5 34.8 30.8 Polarity 6.59 9.30 11.90 3.64 Value arising from the 7.6 8.5 8.2 7.5 abovementioned Equation (i.e. >P) (i.e. <P) (i.e. <P) (i.e. >P) 1 ________________________________ ______________________________ ___________________________ ______________________________ Average droplet volume in 11 11 11 pL ______________ _____________ ____________ _____________ Predicted print quality at Poor Good Good Poor F2OKHz ______________ _____________ ____________ _____________ Table 3 -Printing at 40KHZ Comparative Comparative Example 3 Comparative Example 3 Example 4 ___________ ExampleS Ink 1 lnk2 lnk3 lnk4 ST (dyne/cm) 32.0 36.5 34.8 30.8 Polarity 6.59 9.30 11.90 3.64 Value arising from the 9.4 10.3 9.96 9.16 abovementioned Equation (i.e. >F') (i.e. >P) (i.e. <P) (i.e. >P) Average droplet volume 10 10 10 10 in pL _____________ ____________ ___________ ____________ Predicted print quality at Poor Poor Good Poor F=4OKHz _____________ ____________ __________ ____________ Note: the wetting of the nozzle plate with ink in the Apollo II rig (determined visually) was used as an indication of print quality that would arise had a commercial ink jet printer been used. "Predicted print quality" is therefore inferred from the visual examination of the nozzle plate flooding.

Tables 2 and 3 show that when the printing process was performed such that the inks had a polarity satisfying the abovementioned equation, the predicted print quality was better than when this equation was not satisfied.

Claims (16)

1. CLAIMS1. A process for printing a substrate comprising the steps of applying a radiation-curable ink onto the substrate by means of an ink jet printer and curing the printed ink, wherein: (ii) the ink jet printer comprises a nozzle plate having a surface energy (SE) of >25 in dynes/cm and a piezoelectric actuator operating at a frequency (F) of at least 20 KHz; and (ii) the radiation-curable ink comprises less than 5 wt% organic solvent and has the following features: (a) a polarity (F) which satisfies Equation 1: ((F *0) +i÷(ST/5) ] Equation 1 wherein: ST is the surface tension of the ink in dynes/cm when measured at 25°C; F is as hereinbefore defined; and D is the average volume of the droplets in pL created by the piezoelectric actuator; and (b) a surface tension (SI) which satisfies Equation 2: ST < (SE-3) Equation 2 wherein ST and SE are as hereinbefore defined.
2. 2. A process according to claim 1 wherein the piezoelectric actuator operates at a frequency of at least 30 KHz.
3. 3. A process according to any one of the preceding claims wherein the piezoelectric actuator operates at a frequency of at least 40 KHz.
4. 4. A process according to any one of the preceding claims wherein the nozzle plate has a surface energy of >35 dynes/cm.
5. 5. A process according to any one of the preceding claims wherein the radiation-curable ink comprises: a) a compound having one ethylenically unsaturated group; b) a compound having more than one ethylenically unsaturated group; c) less than 5wt% organic solvent; and d) a colorant.
6. 6. A process according to claim 5 wherin component c) has a lower surface tension than components a) and b).
7. 7. A process according to claim 5 or 6 wherein the ink further comprises a photoinitiator.
8. 8. A process according to any one of the claims 5 to 7 wherein the ink comprises: a) 40 to 80 parts of a compound having one ethylenically unsaturated group; bi) 1 to 20 parts of a curable compound having two ethylenically unsaturated groups; b2) 1 to 20 parts of a curable compound having three or more ethylenically unsaturated groups; c) 0 to 4.9 parts of organic solvent; d) 0.1 to 15 parts of colorant; and e) 0.1 to 15 parts of photoinitiator; wherein all parts are by weight.
9. 9. A process according to any one of claims 5 to 8 wherein component a) comprises N-vinyl caprolactam and 2-phenoxy ethyl acrylate.
10. 10. A process according to any one of claims S to 9 wherein component b) comprises a urethane di-acrylate oligomer
11. 11. A process according to any one of the preceding claims wherein the frequency at which droplets of the ink contact with the substrate is the same as the frequency of the piezoelectric actuator.
12. 12. A process according to any one of claims 1 to 10 wherein droplets the frequency at which droplets of the ink contact with the substrate is the lower than the frequency of the piezoelectric actuator.
13. 13. A radiation-curable ink comprising: a) 40 to 80 parts of a compound having one ethylenically unsaturated group; bi) 1 to 10 parts of a curable compound having two ethylenically unsaturated groups; b2) 1 to 20 parts of a curable compound having three or more ethylenically unsaturated groups; c) 0 to 4.9 parts of organic solvent; d) 0.1 to 15 parts of colorant; and e) 0.1 to 15 parts of photoinitiator; wherein all parts are by weight.
14. 14. An ink according to claim 13 wherein component a) comprises N-vinyl caprolactam and 2-phenoxyethyl acrylate.
15. 15. An ink according to claim 13 or 14 wherein component b) comprises a urethane di-acrylate oligomer.
16. 16. An ink according to any one of claims 13 to 15 wherein the organic solvent, when present, has a higher polarity and optionally a lower surface tension than components a), bi) and b2).