UV Curing Flexo Inks Aper - UV Curing EB Curing

By David HelsbyThe Flexographic ProcessFlexography (Flexo) is a process inwhich the printing image stands up inrelief. A liquid ink (generic term forlow viscosity inks) is applied, wherebysolvent- or water-based inks or UVcuring can be used. UV-cured systemshave seen rapid growth in certainsegments of the flexo world, especiallyin the area of mid-size presses andnarrow web printing for labels. Theflexo process has several distinctivefeatures that must be considered whenformulating suitable UV curing inks:Press ConfigurationsFigure AFlexo processThere are three basic configurationtypes for flexo printing presses: stack common-impression in-lineThe first two types more commonly use solvent and water -basedsystems, but also UV-curing flexo inksare being used. In contrast, in-linepress configurations predominantlyuse UV-curing flexo inks.Printing UnitThe inking system meters out afine and controlled film of liquid inkand deposits it upon the surface of theprinting plate. It is essential to use lowviscosity inks to ensure they transferrapidly and uniformly, allowing for fastprinting speeds on non-absorbentmaterials such as films and foils. UVcuring flexo inks are usually in therange of 1000 mPas in viscosity.Although some users believe it wouldbe better to have lower viscosities, itis more important to have a balancedseries of inks with similar viscosities.Cure or running speeds haveincreased as press manufacturershave improved their design: 200 m/min are now possible.AniloxInk is applied to the printing platesurface by means of a screened(anilox) roller. The result is a simpleink feed system, especially whencompared with Litho presses and theircomplex water/ink balance. However,the process has its own uniquerequirements in terms of physicalcharacteristics, rheology and transferproperties. This is especially true forthe UV inks now in use. The aniloxroller is a crucial factor in achievinggood quality flexo printing and thereis still much to be learned when itcomes to applying UV-curing inks.Ceramic rollers and chrome-platedrollers can carry very differentamounts of ink when such factors ascell volume, geometry and screen sizeare considered. These factors are evenmore critical to the success of suitableJULY/AUGUST 2008 RADTECH REPORT 37Technical PaperUV Curing Flexo Inks

Technical PaperFigure BAnilox cellstaken in order to avoid plate damagefrom the UV ink chemistry. The mainareas of concern are with the selectionof the low molecular weight contentof the ink such as monomers andsome additives. As with the anilox,flexoplates should be selected forspecific UV curing inks.New PressesUV curing inks due to differing surfaceenergies and physical parameters,especially in comparison to conventional solvent-based inks. Thoroughtesting has been done in order toarrive at the optimum anilox and platecombinations for UV ink applications,and many manufacturers of presses,anilox and plates have specificrecommendations for UV inks.Doctor BladeThe doctor blade scrapes theexcess ink from the surface of theanilox as the ink is transferred to theflexoplate. In high-quality printing,reverse angle doctoring (using nylon,polyester or ultrahigh molecularweight polyethylene) has replacedsteel and thus cuts down aniloxwear significantly.On a number of UV-curing presses,a double doctor blade forms an inkchamber—creating a totally enclosedinking system—and makes thetransfer of UV-curing flexo inks muchmore efficient.FlexoplatesThese plates consist of lightsensitive photopolymer, which—whenexposed to UV radiation through aphotographic negative—polymerizeand become resistant to the washingout medium. In some aspects, thechemistry of the plates is similar tothe UV inks and care should thus be38 RADTECH REPORT JULY/AUGUST 2008The units of modern flexographicprinting presses are now engineered tovery tight tolerances.The ability to manufacture to thesestandards, in combination with thedevelopment of higher print quality UVcuring inks, has contributed to thegrowth in flexographic printing, and itsuse for higher quality products thanwas previously possible.Typical UV-Flexo ApplicationsOriginally, UV-curing flexo inks wereintroduced for the printing of labels andhave since become the dominanttechnology in that field. In the 90s, anumber of wide-web machines( 100 cm) were converted to UV curingbut the real growth came later in theFigure CFlexoplatemid-size printing machines (60 to100 cm). UV flexo printing inks andvarnishes are highly popular and stillgaining market share. One good reasonis the absence of solvents, whichreduces emissions and which wasprobably one of the main reasons forintroducing UV curing technology in thefirst place. Moreover, these productsalso offer superior print quality with nosolvent replenishment and rapidreadiness for use, provide good adhesionon flexible and many other substrates,offer high chemical and productresistance and allow for high runningspeeds. UV-curing flexo inks are nowprinting on nearly all ranges of substrates used for finished products suchas yogurt tops and cups; sugar, soup andspice sachets; flexible packaging; milkand juice cartons; pet food packagingand even cigarette packs.Materials for FlexographicFormulationsThe materials for UV-curableflexographic formulations includemonomers, oligomers, in-can stabilizersand photoinitiators.

Flexo packagingMonomersMonomers are typically used inflexographic formulations at 30-60weight percent of the formulation.Choosing the right monomers iscritical. A wrong type of monomer canlead to plate swell and create registration issues. Monomers provide lowerviscosity and can influence rheology,cure rates and adhesion.Letdown OligomersLetdown Oligomers provideenhanced physical properties, uniqueend properties, cure speed andadhesion.to be optimized until the final formula-In-can StabilizersIn-can stabilizers are an importantpart of UV-curing flexo inks. Theysignificantly improve shelf life and inkstability during the grinding process.Our stabilizer product range increasesthe stability of UV-curable flexo inkformulations and has already been inuse for many years.The test inks were formulated byPhotoinitiatorsA full range of well-establishedphotoinitiators, including polymericphotoinitiators, for radically curingUV systems are commercially available.Grinding VehiclesGrinding vehicles are essential inthe development of flexographic inks.These vehicles efficiently break downpigment agglomerates and allow forefficient stabilization. Using thesevehicles will increase flow, stability andcolor development of the ink.AdditivesThere are many additives used tomanufacture UV-curing flexo inks.The huge range of products on themarket makes it difficult to give even arough overview. Successful startingpoint formulations, however, oftencontain quite complex additivecombinations which customers cansort through given advice fromformulators and suppliers. These needtion is meeting the specific requirements.UV-Curing Ink Formulationspreparing an optimal mill base and thenlet down to the final finished inks byadding monomers, oligomers, photoinitiators and additives. This methodallows for optimum grinding and millingof the pigments—eliminating the risk ofreactive photoinitiators and poorlygrinding oligomers and additives—thatare required in the final ink.The production of concentratedbases also allows for a number ofdifferent ink series to be producedby simply altering the let downformulation. The end formulationslisted in this report have been designedas a general purpose series, i.e. asstarting point formulations.Mill BasesThe mill base is the first and mostcrucial step in the process of making aUV flexo ink from a platform manufacturing system. The mill base containspigment, oligomer, monomer anda dispersant.Table 1Starting point formulationsYellowMagentaCyanBlackWhitePY 13PR 57:1PB 15:3PB 7PW Specialblack 250Titan 5.0035.0035.0050.00Grinding Oligomer60.0060.0064.0060.0049.00BYK .00100.00100.00100.00100.00PigmentSupplier CodeTotalsJULY/AUGUST 2008 RADTECH REPORT 39Technical PaperFigure D

Technical PaperTable 2Properties of finished Flexo InksSPF 524SPF 526SPF 530SPF 528SPF 532Mill base 50.5Modified Epoxy Acrylate9.59.513.510.010.0Modifed Urethane Acrylate5.05.05.05.0Reactive diluent (4 functional)9.0Reactive diluent (3 functional)15.015.011.018.57.0Reactive diluent (2 functional)21.419.419.515.012.9Uvitex OB0.1WAX OR SLIP ADDITIVE1.01.01.01.01.0Photoinitiator 11.91.92.42.52.0Photoinitiator 23.0Photoinitiator 30.80.80.91.0Photoinitiator 41.91.92.42.51.0Photoinitiator 51.5Photoinitiator 63.03.03.84.0Total100100100100Reactivity in m/min at 1 pass240W/cm mpHg lamp100approx. 100* toned with cyan and magenta mill basesThe pigment selection is the firstand some say most important step inthe manufacturing of UV curing inks.The pigments used here should onlyserve as a starting point, and westrongly recommend that thoroughtests be done to arrive at the optimumpigments for each individualproduct requirement. The mill baseformulations shown here can be used asbenchmark formulations to evaluatealternative pigments.The dispersant used in the mill basehas been chosen to compliment thegrinding oligomer and provides pigmentwetting and improved grinding properties. The dispersant additive is notrequired in the Blue and White.The mill bases were produced on aBühler SDY- 200 mill, with one loosepass at 15 bar, followed by up to 3passes at 20 bar, achieving a finenessof less than 5 micron with no peppers.Viscosity and Flow PropertiesThe viscosity and flow of a mill baseor a finished ink can be determined byTable 3Viscosity of inks measured on the Mettler RM260SPF 524Viscosity [Pa*s @ 25 C]40 RADTECH REPORT JULY/AUGUST 20081.01SPF 5261.03SPF 530SPF 5281.030.9SPF 5320.7

The flow plate method is a simplistic and comparative method. Depending on the product, 5 to 10 g are placedon a glass plate, allowed to settle for30 seconds and then the plate ispositioned upright at 80 [TBC1].Depending on the products beingcompared (i.e. finished ink or millbase),the flow length is measured in mm after1 or 3 minutes. The flow, viscosity andrheology curves of the bases have beencompared to bases made from ourstandard epoxy acrylate, reduced withtrimethylol propane tri(ethylene oxide)triacrylate.Flow plate resultsFlexo Inkssophisticated measurement on avariety of rheology testers, quicklychecked by a cone and plate viscometer or simply and visually comparedby the flow plate method. In thisparticular study we have used:Figure FRheology Curves an AR 550 Rheomat from TAInstruments to determine the rheologycurves and viscosity of the bases. a RM 260 Rheomat from Mettler for aQC viscosity check of the inks. a flow plate method to observe theflow characteristics.The mill bases are let down withmonomers, oligomers, photoinitiatorsand additives to produce the finishedink. By varying the letdowns, manydifferent inks can be made from thesame mill base. The letdown oligomersand monomers have been selected togive the finished ink properties. Thephotoinitiators are shown in breakback quantities but are added to theletdown as an eutectic mix. A breakback of the final ink formulation canalso be produced as an in-situ formulation run on a pearl mill such as BühlerK8 but special care is required withregard to the correct cooling of the milland/or the eutectic photoinitiators mixleft out of the grinding stage and addedin the final potting stages.Please note that the viscositiesachieved are part of an ink series withbalanced properties. Lower viscositiescan be achieved with some colors butare often counter productive when runon a press configuration if not inbalance with the rest of the colors.Glass Plate FlowMeasurementsThe results shown in the flow andrheology measurements are anindication of how the inks will performon the press. Different presses willJULY/AUGUST 2008 RADTECH REPORT 41Technical PaperFigure E