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UV system has sticking powerToday UV ink technology is in every part of printingapplications. Converters, because of the high pressure onquality request, commonly use UV inks on plastic or metalsubstrates. The main recurrent problem for offset printers isto find an ink which offers a good adhesion on suchsubstrates. This paper provides an overview of adhesionissues and presents an alternative UV technology thatresults in outstanding adhesion.Olivier Tonnoir.The poor adhesion of UV systems is a well known problem.This phenomenon is induced both by poor substrate wettingand, very importantly, by film shrinkage. The percentages offilm shrinkage for some monomers and for acrylatedBADGE illustrate the importance of the phenomenon onadhesion and the reasons for difficulties (Table 1).To ensure good adhesion on difficult substrates, it isessential to have the best possible cure response. In orderto achieve the best cure response, many actions on thesubstrate treatment, the lamp quality and improvement ofink chemical formula are required.

Treating the substrate improves surface energySynthetic substrates, such as polypropylene, have a verylow surface tension (less than 30 dynes/cm). Therefore, it isdifficult or even impossible to obtain adhesion. The solutionconsists of treating the plastic surface with a flame or withCorona equipment (electrical discharge), in order toincrease the surface energy by oxidation of the superficiallayer. The advantage of such treatments is that they can beapplied online (Figure 1 and Figure 2).

Doped lamps promote pre-gel formationMost UV printers use medium pressure mercury lamps tocure UV films. A bad deep cure is the main problemencountered with these lamps. As a consequence, thesurface cure results in local shrinkage, which pulls the filmaway from the substrate. This phenomenon results in a pooradhesion. Doped lamps can be used to first cure the coatingcore and then a mercury lamp is used to achieve a completecure of the coating surface. Doped lamps promote theformation of a pre-gel at the substrate-film interface, andthus allow adhesion.It is interesting to note that adhesion problems are lesscritical with porous substrates such as paper or cardboard,because the coating may be absorbed by the top layer andbecome part of the substrate when cured.Of course, printers have to invest in complementaryequipment to benefit from this technique and most of themare not committed to bear additional costs.

Photoinitiator choice influences polymerizationmechanismThe choice of photoinitiator is crucial. For UV formulators,constant effort is required to optimize film performance andfinal product cost.There are actually two different kinds of mechanismsinvolved in polymerization initiation. The first one is calledNorrish I, which initiates polymerization by molecularcleavage of a molecule. BDK (Benzyldimethylketal) is anexample of a molecule that undergoes a fragmentation of itsstructure.The second mechanism is known as Norrish II; it is basedon the excitation of a photoactivator that transfers theenergy received from UV lamp to the photoinitiator,generally a tertiary amine.The Norrish II mechanism favours a surface cure of the film.

The Norrish I mechanism results in a good surface cureresponse as well, but is more effective in the depth of thefilm. Other photoinitiators like phosphine oxides (such as thewell-known BAPO - Bis-acyl phosphine oxide) have beendeveloped to improve cure response in the film core.

A good choice of resin helps adhesionThe wetting ability of a resin depends on its surface tensionand this greatly influences the adhesion. The resinfunctionality is an important parameter too, itself dependanton molecular weight. The higher the molecular weight, thelower the double bond content and the lower the shrinkageeffect because of soft film properties.Regarding the double bond concentration, the use ofmonomers must be considered carefully because of themolecular size of those products and, thus, their high doublebond concentration level. To improve adhesion and takeadvantage of the solvent effect of monomers, the choicemust be oriented towards mono or difunctional monomers.Another important criterion is the reaction kinetics. It willaffect shrinkage effect: for example, epoxy resins have afaster reaction rate than polyester. This must be taken intoconsideration to formulate inks or varnishes dedicated tonon-porous substrates.In conclusion, the resin choice will have a dramatic effect onadhesion properties. Therefore, the choice has to combinethese parameters to give the right balance between the finalcured film properties to be achieved and the physicallimitations due to substrate characteristics. For a minimumimpact on adhesion, the monomer type should be chosen inorder to bring the maximum dilution efficiency.

Choice of additives can boost adhesionSome formulators use acidic compounds to improveadhesion on metallic substrates. UV resin manufacturerspropose the use of resins for those applications. Thelimitations are the substrate nature and the stability of theresins. Acid groups can promote gel formation of the resin.Particular attention must be paid to storage conditions.

Alternative resin technologyTo improve adhesion on difficult substrates, new resinsbased on chemically-modified epoxy acrylate are beingdeveloped. The modification consists in grafting adiisocyanate compound onto the lateral hydroxyl group thatresults from the reaction of acrylic acid on the epoxyfunctional group. Only one of the two functional groups isused to bind the two molecules. The other isocyanatefunction remains untouched. Thus, an isocyanateprepolymer is built (Equation 1), which can then be curedunder UV light.The purpose of the chemical modification is to create areactive centre in the molecule that is able to react with thesubstrate. The hydroxyl group available on the pre-treatedsurface reacts with isocyanate and ensures chemicalbonding of the resin.As soon as the film is cured under UV light, it becomes apart of the substrate, linked by a strong molecular bondingwithin the ink film and between the film and substrate. As aresult, adhesion is noticeably improved by the use of this insitu chemistry (Equation 2).Equation 2 illustrates the chemical bonding that results fromthe reaction between the hydroxyl groups from thepre-treated substrate and the NCO groups from the modifiedresin.Polyurethane derivatives are obtained by the reaction of

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isocyanate functions on hydroxyl groups. This reaction maystart at a low temperature (from 35°C) without catalyst. Butto make the reaction faster in an industrial process, it isusually catalysed by a tin derivative, in a very smallproportion (10 to 100ppm).

Reaction between resin and substrateIn this alternative technology, the starting resin alreadycontains catalyst from the chemical modification of theepoxy. Moreover, the curing process of the acrylate resinunder UV light induces release of heat (IR radiationgenerated by lamps and the polymer formation).Both the presence of catalyst and the released heat help toaccelerate the reaction between NCO and OH and thereforepromote bonding between substrate and resin. If hydroxylgroups are still available after UV curing, the reaction will goon off-line till the complete consumption of remaining OHgroups has occurred.Therefore, adhesion can be improved with time whileresidual acrylates and urethanes continue to react.

Dual Cure System used as an additiveAn example of the alternative resin technology is theapplication of a red ink onto a PE laminated substrate(Figure 3). The additive has just been mixed in the ink at alevel of 5% and 10%. For comparison, 5% and 10% ofbinder resin were added in the blank test. The taperesistance test was done immediately after curing and oneday later.An improvement in adhesion is obvious with a 5% additionof the additive and this clearly competes with resin. Ingreater proportions, resin levels differ. This shows that theDual Cure System can be used as an additive.Note that in that case, the substrate was not Corona treatedin order to increase differences. There are probably residualhydroxyl groups at the surface from "natural" oxidation,offering the possibility of reaction with isocyanate.

New technique has advantagesOf course, such a technology for adhesion improvement isof interest for printers, because of the wide applications andsubstrates that it could be used for. But, for ink formulators,it could also be the solution to their continuous problem ofhow to achieve a good printability without altering adhesion.Formulators usually use specific raw materials developed tohelp them in difficult applications. Chlorinated polyesterresins and monomers are often used. One example isHDDA but this is not always allowed because of its irritatingproperties. The development field is very narrow so it isdifficult to be creative in terms of formulation. The Dual CureSystem (DCS) makes ink formulators free to choose theirresin type to adapt the final properties such as gloss,chemical resistance and scratch resistance rather thanfocusing on adhesion because that is already achieved byusing the isocyanate prepolymer as an additive.Almost all resins from the most common families can beused and a good adhesion compromise can be obtained incombination with the DCS product.

Considering safety concernsThe main drawback comes from the technology itselfbecause it results in isocyanate content of the product. Thepasty additive is quite an isocyanate-rich product but it is notlabelled as toxic. The labelling only provides a Xn warning,which indicates a harmful hazard. Because of the hazard, itmust be manipulated avoiding direct contact with skin oreyes.

Chemical compatibility brings manufacturing

challengesAnother problem to be managed is the chemicalcompatibility of the product. Because of its isocyanatecontent, the additive cannot be mixed with compoundscontaining hydroxyl functions, such as epoxy acrylate resinsor photoinitiators such as2-hydroxy-2-methyl-1-phenyl-propane-1-one ("Darocur1173").As a matter of fact, all products likely to contain a group thatreacts with isocyanate must be avoided. For example,pigments that are based on chemically-modified rosin couldreact and cause an increase in the ink's viscosity. It istherefore necessary to perform a stability test on the finalproducts, even if the formulation resin exhibits the requestedchemical compatibility.Of course, if there is no possible compatibility between theadditive and the ink, the solution might be to use a primerformulation containing the additive. In that way, thehydroxyl-free product, may give the requested adhesion.

Isocyanate content requires careful handlingStill considering the isocyanate content, printers must becareful to manipulate the ink or primer. Isocyanate content ismuch lower in the ready-made product but is still reactiveand must be handled with care (gloves and protectiveclothes). However, the 2K products used by printers inflexography also contain isocyanate. The new inks have tobe handled with the same level of care.

End user applicationsAfter printing and curing under UV light, all non-reactiveproducts such as photoinitiators, stabilisers and additivesare trapped in the film. Experience related to low extractableresidual products in food packaging applications has shownthat non-reactive derivatives can migrate from the film coreand pollute some sensitive applications. For this reason theDCS cannot be used for food packaging.However, from a chemical point of view, residualisocyanates trapped in the film and unreacted isocyanatecompounds on the coating surface continue to be active. Asair moisture is a reactant for isocyanate functions, thefollowing chemical reaction water-isocyanate may occur(Equation 3).

Minimizing the risksLittle is known about the kinetics of such reactions becausethey depend on conditions such as temperature andconcentration.However, the migration of residual molecules is naturallylimited. About 90% of the available acrylic groups reactunder UV radiation, therefore only a 10% unreacted portionremains active. In order to limit the migration of residuals, anOPV can be applied on the ink to close the film.Finally, to ensure maximum safety regarding isocyanaterelease, it is possible to imagine an alternative solutionconsisting of the use of a modified DCS primer to promoteadhesion on the film. In this way, inks and OPV can bestandard ones. The primer is then trapped between the twophysical barriers of the substrate on one side and the ink onthe other side. An OPV top layer will guarantee an additionalsafety level against migration.This discussion shows that solutions can be found to ensuremaximum safety to the final end user customer. Bothchemical and technical considerations could be a responseto the legitimate questions the end user may have.

DSC usefull for a large range of substratesAs discussed above, the potential use of DCS in inks andprimers is very large. As it is used as an additive, it involves

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a minimum impact on the macroscopic properties of thebasic formulation. Converters (printers) can use it directly,just by adding and mixing the additive in the ink series,according to use recommendations and after stabilityvalidation with pre-tests are provided by UV formulators.A large range of substrates can also be used with thistechnology. Moreover, it has been shown that Coronatreatment is not necessary in all situations. This techniquecan be an economic alternative to heavy technical(expensive) solutions to adhesion problem. Of course, trialsmust be done and discussed with printers to ensure therequired adhesion level before the systems are used forapplications.Future developments to the technique will focus onimproving the viscosity of the additive to reduce its impacton the final properties of formulations. The main problems ofpolyurethane come from viscosity: the urethanization ofpolyols leads to a dramatic increase in viscosity. Furtherdevelopments of the DCS technique will be focused on rawmaterials selection.

References[1] G. McAuliffe, Polymers Paint Colour Journal, (1995) 1, p.24[2] A. Askienazy, R. Zwanenburg, E. Pezron, Journal ofRadiation Curing, 20, (1993) 3, p. 6[3] J-C. Sirost, P. Cole, Ink & Print, 11, (1993) 3, p. 14

Result at a glance- To improve adhesion on difficult substrates, new resinsbased on chemically modified-epoxy acrylate are beingdeveloped.- The modification results in an isocyanate prepolymer,which can be cured under UV light.- As soon as the film is cured under UV light, it becomes apart of the substrate, linked by a strong molecular bondingwithin the ink film and between the film and substrate.- For ink formulators, the technique could address thecontinuous problem of how to achieve a good printabilitywithout altering adhesion.- A large range of substrates can be used with thistechnology.- Corona treatment is not necessary in all situations.- This technique can be an economic alternative to heavytechnical (expensive) solutions to adhesion problem.

The author:-> Olivier Tonnoir studied chemistry at the University ofGrenoble, France. In 1983, he joined SICPA, global printingink and varnish manufacturer, as a R&D technician workingon UV inks and varnishes.After a number of differentassignments, he is now working as a project manager at theSICPA R&D Centre of Excellence dedicated to UV inks andvarnishes, which is located in Annemasse, France. Hedrives projects focused on organic and polymer synthesis aswell as UV technology.

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Figure 1: Substrate layer before treatment.

Figure 2: Chemical modification after Corona treatment of the substrate.

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Figure 3: Tape resistance of a red ink on PE laminated substrate.

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Equation 1: Reaction of an isocyanate on the epoxy secondary hydroxyl group.

Equation 2: Reaction of hanging isocyanate groups with layer hydroxyl groups.

Equation 3: Isocyanate reaction with water.

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