Embed Size (px)
Citation preview
Quelle/Publication:
Ausgabe/Issue:
Seite/Page:
European Coatings Journal
05/2004
44
UV ink-jet inks evolve to meet applications
Alexander Grant.Developing UV jet inks presents some challenges forfeatures such as stability, jetting and print quality. However,several innovative printers based on this technology are nowemerging on the market. These are helping to map thefuture of high-volume UV ink-jet printing.The usage of UV curable technology in a variety of coatingand ink applications is well known. The wide diversity offormulation materials, from varied functionality monomers tooligomers and photoinitiators and more recently hybridsystems, makes this chemistry highly adaptable to the everdemanding requirements in the world of printing [1]. Typicalprinting processes such as flexography, gravure or screenprinting utilise inks that have a range of viscosities andparticle sizes, which encompass a host of pigments fromcarbon blacks to metallics. However, whilst these printingtechniques are well established, they do not benefit from theflexibility offered by a digital printing process.Ink-jet printing has been around for many years, gainingstrong market position in small office home office (SOHO)and industrial marking segments, such as sell by dates onaluminium cans and barcodes on packaging items [2]. Theinks used therein are commonly water or solvent based andusually very low in viscosity (< 5 cPs at 25oC). This restrictsthe formulator somewhat in producing inks that can competewith other traditional printing processes, where viscositiescould be several orders of magnitude higher. Nevertheless,as print-head technology has evolved, most notably in thePiezo drop on demand (DOD) area, this has permittedusage of higher viscosity jet-inks and in so doing given agreater latitude to ink formulating. In particular, 100% UVjet-inks have pushed to the forefront within ink-jet productclasses. This is partly because UV materials tend to be wellsuited to print-head requirements, such as long open timesand highly reliable jetting but also offer resistance propertiesin-line with those expected from conventional printingtechniques. In turn, short to medium length print runs can beachieved rapidly and customised as desired.This paper discusses some of the challenges andconstraints in developing UV jet-inks, with some examplesfrom a jetting and stability perspective. This is followed bysome aspects relating to print quality. Finally, a mention isgiven to the emergence of several new innovative printers inthe market that utilise this technology type and help map theway forward for large volume UV ink-jet printing.
Formulation constraintsThe development of 100% UV jet-inks is constrained byseveral factors. This includes viscosity (commonly 8-22 cPsat jetting temperature), high stability (e.g. no settlement onstorage) and small particle size (generally < 1mm). Theformer often leads to the absence of oligomeric, resinous ormulti-functional species in the formulation whilst the lattercan put further restrictions on usage of certain pigmentclasses e.g. metallics and special effects pigments such asthermochromics and piezochromics.Demands are also placed on surface tension values; theseshould be sufficient to allow appropriate wetting onprint-head face-plate material and help meet print qualityspecifications through ink spread on the media. As thevolume of each drop ejected from a piezo DOD print-head istypically <100 ng, these drops are strongly affected by thechemistry of the ink's components. The sensitivity is such
that even additives at low level, such as surfactant, caninfluence how the drop breaks up, its length of tail (ligament)and its likelihood to form satellites (smaller drops that formfrom the break up of the initial drop). In turn, the latter canhave a major impact on print quality. To illustrate this, Figure1 shows various ligament lengths for three UV jet-inks thatvary only in their surfactant chemistry, ejected from aSpectra print-head.Furthermore, print-head face-plate material and internalcomponents need careful consideration in terms ofcompatibility with the jet-ink. Advancing and recedingdynamic contact angle profiles from a tensiometer, canserve as a useful indicator for differentiating two almostidentical UV jet-inks, prior to introducing them to aprint-head. According to the Wilhelmy method, dynamiccontact angle is calculated as a function of force, wettedlength of media and fluid surface tension, as shownschematically in Figure 2a. As an example, Figure 2b showsthe data collected for a print-head component dipped intotwo similar UV jet-ink formulations (inks A & B), highlightinga clear wetting difference between them.
Sensitivity and reactivityUV jet-inks have been shown to be more sensitive thangravure, flexo and screen UV ink to levels of oxygen in theatmosphere whilst curing [3]. In turn, this can affect curespeed and print engine design might necessarily includenitrogen blanketing. Hence, care is needed to maximise thereactivity and potency of photoinitiator and monomercombinations. This could be further complicated when verylow levels of odour and extractables are needed for certainmarket segments, such as food packaging, putting furtherrestrictions on photoinitiator and monomer selection.In order to avoid drying in the print-heads when machinesare not running for a period, it is necessary to usenon-volatile components in the ink formulation. This canexclude some highly reactive materials that could be ofbenefit in other printing processes.All these considerations need to be balanced further with theoverall fundamental requirements of sufficient resistanceand adhesion characteristics for the end user, in a mannerconsistent with alternative printing techniques. Furthermore,there are increasing demands being made on print qualityperformance in order to approach the standard ofconventional outputs.
Particle sizeWhen using typical piezo DOD print-heads it is vital toensure ink particle size is <1mm and stability is such thatthis does not change upon storage, in order to ensurenozzles remain blockage-free and jetting is consistentlyreliable over time. Particle size analysis techniques, such aslaser scattering from an ink diluted in solvent, gives anindication of average particle size. Over a period of time thiscan be re-measured to ensure that stability is satisfactory.However, this does not always correlate with settlementcharacteristics, which are important in the case of a jet-inkstored in a print-head. The "Turbiscan" (manufactured byFormulaction, France) offers a useful technique for rapidquantification of ink settlement characteristics, such asdegree of sedimentation, kinetics and evolution of particlesize.The apparatus employs a near infrared light source that
Vincentz Network +++ Schiffgraben 43 +++ D-30175 Hannover +++ Tel.:+49(511)9910-000
Quelle/Publication:
Ausgabe/Issue:
Seite/Page:
European Coatings Journal
05/2004
44
scans the height of a stationary sample, taking readingsevery 40 mm. Detectors are positioned to recordtransmission (clear and slightly turbid fluids) and backscatter(opaque fluids), in the case of most ink testing. Due to thevery sensitive nature of these measurements, stabilitytrends can be determined in a much faster manner thantraditional storage tests. For example, Figure 3 shows thebackscattering profile over several days for a solvent-basedjet-ink versus a UV jet-ink, employing the same pigmenttype. The supernatant formation rate in the former appearsmuch faster and the speed of sedimentation is alsodifferentiated. As a result of this testing it could beconcluded that this particular pigment is more likely to givelong term storage stability in UV jet-ink formulations thansolvent-based counterparts.Formulating stable low viscosity white UV jet-inks can bedifficult due to the high density of typical TiO2 and ZnSpigments. Settling and supernatant formation are usuallyunavoidable but the speed at which this occurs is verysensitive to the components present in the formulation. Asan illustrative example, a series of white UV jet-inks wereprepared with identical pigment, dispersant, monomer andphotoinitiator chemistries. Using dispersant level as avariant, measurements were made to follow changes inbackscattering over time. This was compared with averageparticle size data collected from a standard laser technique,with a solvent diluted sample. Irrespective of dispersantlevel, particle sizes for the three inks were identical with thelatter approach (Table 1).The traces of the three inks show some slight differences inthe rate of backscatter in the supernatant region of thesamples. However, looking at this area in some detail, it ispossible to determine the rate of settlement (Table 1).However, despite there being little difference in these inksfrom initial laser scattering particle size measurements, itwould appear that over a period of time the ink withdispersant level "C" is most likely to offer superior stability.
Print qualityAs ink-jet technology aims to gain ground in traditionalprinting markets, a key requirement is a high, if notmatching, standard of print quality. In order to achieve this,knowledge of ink and media parameters and how theyinteract with each other is essential.The IMAGE-IN project is a multi-partner collaborationfunded by the EU, bringing together partners from a diversityof backgrounds to focus on the challenge of improvingink-media understanding with a key output being how printquality can improve in an ink-jet process, utilising UVtechnology [4].Figure 4 helps highlight the challenge. Examples of twopackaging lids, printed on the left hand side with flexo UVink and on the right hand side with UV jet-ink, show someinteresting contrasts.As print quality is essentially a measure of how ink wets onmedia, the general dynamic wetting characteristics of an inkcan be useful. Figure 5 shows a dynamic surface tensionplot for several common UV monomers, measured using thebubble pressure method. In the field of ink-jet printing twospecific areas of the plot provide useful information. Theseare how the ink behaves at very low surface age (highbubble frequency) and at the time of high surface age (lowbubble frequency). These are related to jetting frequencyand print to cure times, respectively. The monomers chosenfor these measurements are all acrylic based but wet in acompletely different manner, suggesting some are perhapsmore suited to high frequency jetting than others and arelikely to provide a range of wetting differences on the printmedia.
Commercial printers, applications and hybridtechnologiesThe presence of UV ink-jet technology in the printing marketis growing rapidly as many new commercial graphicsprinters emerge. This includes machines that are capable ofprinting on a wide variety of packaging and display media atincreasingly faster speeds."The. Factory" from Dotrix is a leading example of a printengine that is capable of multi-media printing that fullyutilises UV jet-ink technology. This is especially appealing toshort and medium size print runs where a fast turn around ofcustomised packaging is required. The single pass ink-jetcolour engine (SPICE unit) from "The. Factory" is also usedin a label printing application. In this system, a SPICE unit isintegrated into a Mark Andy press, serving as a usefulexample of where traditional technology (web press) meetsdigital.In terms of rigid display graphics printing, the Durst "Rho"and Nur "Tempo" machines offer high resolution at fastoutput, covering a wide range of media, including acrylicsand polyesters. Outputs from these machines mighttraditionally have been made with screen printing processes.Other typical UV ink-jet applications embrace credit cardprinting, posters, golf balls, circuit boards, CDs, mobilephones, wood decor and textiles, all of which are dependenton high image quality, short to medium print run lengths andcan require a good degree of customisation.Incorporation of UV formulae to other ink-jet technologysegments can also be beneficial. For example 100%hot-melt jet-inks are useful for high print quality ontoabsorbent media [5]. However, resistance properties, whichtend to be relatively weak, can be improved upon markedlyby combining with UV free-radical chemistry, whilst retainingprint quality. Similarly, water-based formulations can beadapted to combine with UV free-radical or cationicchemistry thereby offering possible advantages over each ofthe single technologies, in a similar manner to conventionalwater-based UV inks [6].
ConclusionsDemands on the UV jet-ink formulator are numerous. Thisincludes developing inks that print reliably, are fullycompatible with print-head internal and external componentsand offer resistance properties that can come close to thoseof traditional UV inks. Further requirements are made toensure the jet-inks offer excellent long-term stability andattain a print quality standard expected from conventionalprinting techniques. Despite the constraints present informulating UV jet-inks, such as particle size, viscosity andstability, many examples are given where this chemistry canapproach and indeed offer advantages over non-digitalprinting techniques. This is reflected in the emergence ofseveral new machines on the market that utilise thistechnology in a variety of applications such as packagingand flat bed printing for point of purchase displays. As thetechnology further evolves and new materials emerge, it islikely that 100% UV free-radical will remain the technologyof choice but other systems, such as cationic andwater-based UV, will gain some share.
References[1] Prepolymers & Reactive Diluents, Chemistry &Technology of UV&EB formulations for coatings, inks &paints (VOL II). Wiley. ISBN 0471978914.[2] D. Bucknall, Proc. NIP 19, International conference ondigital printing technologies, (2003), 552.[3] N. Caiger, Proc. NIP 15, International conference ondigital printing technologies, (1999), 116.
Vincentz Network +++ Schiffgraben 43 +++ D-30175 Hannover +++ Tel.:+49(511)9910-000
Quelle/Publication:
Ausgabe/Issue:
Seite/Page:
European Coatings Journal
05/2004
44
[4] Funded by the European Community through aFramework 5 grant (contract number:GRD1-CT-2002-00663). Partners: Sunjet (ink),AGFA-Gevaert N.V. (media), Dotrix N.V. (manufacture ofprinting technology), Teich AG (industrial printer), Universityof Oxford, University Joseph Fourier, Ardeje SARL(visualisation specialists).[5] WO patent No. 9,954,416.[6] US patent No. 6,232,361.
The author:> Alexander Grant, SunJet, received a PhD from theUniversity of Cambridge in 1995 in the synthesis of liquidcrystalline polymers for coating applications. He wasemployed as a Polymer Scientist with National Starch(Vinamul division) in the UK, synthesising emulsionpolymers for the paint and coatings industries, and thenjoined SunChemical's R&D centre in Carlstadt, NJ, USA, asa Scientist working on ink-jet formulations forcomputer-to-plate and packaging applications. He iscurrently Project Leader at SunJet, the ink-jet division ofSunChemical, at their headquarters in Bath, UK.
Vincentz Network +++ Schiffgraben 43 +++ D-30175 Hannover +++ Tel.:+49(511)9910-000
Quelle/Publication:
Ausgabe/Issue:
Seite/Page:
European Coatings Journal
05/2004
44
Figure 1: Drop profiles & volumes of three different surfactant modified cyan UVjet-inks jetted from a Spectra Print-Head..
Vincentz Network +++ Schiffgraben 43 +++ D-30175 Hannover +++ Tel.:+49(511)9910-000
Quelle/Publication:
Ausgabe/Issue:
Seite/Page:
European Coatings Journal
05/2004
44
Figure 2: a) Dynamic contact angle measurements by a Wilhelmy method..
Figure 2: b) Dynamic contact angle measurements for two UV jet-inks on print-headface-plate..
Vincentz Network +++ Schiffgraben 43 +++ D-30175 Hannover +++ Tel.:+49(511)9910-000
Quelle/Publication:
Ausgabe/Issue:
Seite/Page:
European Coatings Journal
05/2004
44
Figure 3: "Turbiscan" data for yellow solvent based (top) and UV (bottom) jet-inks.
Vincentz Network +++ Schiffgraben 43 +++ D-30175 Hannover +++ Tel.:+49(511)9910-000
Quelle/Publication:
Ausgabe/Issue:
Seite/Page:
European Coatings Journal
05/2004
44
Figure 4: Examples of packaging lids printed with UV flexo (left) and UV ink-jet (right)inks.
Vincentz Network +++ Schiffgraben 43 +++ D-30175 Hannover +++ Tel.:+49(511)9910-000
Quelle/Publication:
Ausgabe/Issue:
Seite/Page:
European Coatings Journal
05/2004
44
Figure 5: Dynamic surface tension vs surface age for four acrylic monomers: THFA -tetrahydrofurfuryl acrylate, IBOA - isobornyl acrylate, TDA - tetradecyl acrylate, NPGDA
- propoxylated neopentyl glycol diacrylate.
Vincentz Network +++ Schiffgraben 43 +++ D-30175 Hannover +++ Tel.:+49(511)9910-000
Quelle/Publication:
Ausgabe/Issue:
Seite/Page:
European Coatings Journal
05/2004
44
.
Vincentz Network +++ Schiffgraben 43 +++ D-30175 Hannover +++ Tel.:+49(511)9910-000
