Coating materials containingnone or only low amounts of solventsare gaining more and more signifi-cance. Naturally this also applies toUV-radiation curing systems.

Advantages of UV inks are obvious:Originally environmental policy was tothe fore – UV inks are nearly free ofsolvents and thus low pollutant. In themeantime, however, users have alsobeen convinced by the economic bene-fits of these systems. Layers applied aredry immediately after the curing processand further processing can take placestraight away. Thus production is quickerand therefore more cost efficient.

Radiation curing systems consist ofreactive resins (ol igomers or pre-polymers) and reactive thinners, alsocalled monomers. Due to chemicalreaction the ”thinner” is tightly linkedinto the polymer matrix thus contributingto the formation of the ink film. Powerdensity of the UV radiation by itself isnot sufficient to achieve polymerisation.For that reason photoinitiators or photosensitizers are mixed into inks andvarnishes.

There are two different UV-systems,with basically quite different reactionmechanisms:

Radical curingUV inks or varnishesCationic curingUV inks or varnishes

Currently radical curing ink types ownthe highest market share. Cationic curinginks are rather an exception, especiallyin screen printing applications.

The different mechanisms and theiradvantages and disadvantages are asfollows:

RADICAL CURING INKS

Unsaturated resins with high reactivitygroups, which lead to a cross-linkagereaction by free radicals are used forthese types. Generally these are acrylicresins or monomers with terminal acrylicacid groups (acrylic esters). In additionto acrylates other compounds withreactive double l inkages, such asunsaturated polyester resins and vinylmonomers like styrene can also becross-linked in this manner.

If subjected to UV-radiation the photo-initiators spontaneously decompose intofree radicals initiating a (chain) reactionfor the polymerisation. These radicalsreact with the double bonds of the resinsor monomers resulting in further growing(macro) radicals. Again further resin ormonomer molecules attach to theseradicals. A quickly growing chain isproduced, which is finally stopped by achain-breaking reaction. Thus a three-dimensionally cross-linked, insoluble andrigid structure of macromolecules isbeing formed. All this happens withinseconds or split seconds so thatimmediately after curing a highly resistantfilm accrues.

Oxygen in the air may interfere with thisreaction. The radicals required forpolymerisation tend to react with oxygenon the surface thus significantly reducingefficiency. Experts call this oxygeninhibition. Therefore some UV-driers runwith inert gas as this way belt speedcan be significantly increased or contentof (expensive) photoinitiator in the inkcan be considerably reduced. Inert gas

equipment is rarely used for screenapplications, however some - especiallythe automotive industry - are using themfor structured varnish applications.

Volume contraction takes place duringpolymerisation. This effect, also calledshrinkage, does have negative effects;adhesion on difficult substrates is moredifficult to achieve, on the other handtensions in the substrate may cause thesurface to bend; especially thin materialslike labels will then show the so-called”edge curl” effect. Typically shrinkageduring curing is about 5-15%, dependingon formulation.

Usually not fully reacted, radical curingUV ink types are more or less irritant tothe skin and also do have a sensitisingeffect. Therefore handling UV inks requiressuitable precautionary measures. Curedinks, on the other hand, do not presentany risks.

CATIONIC CURING INKS

Cationic curing inks have quite a differentreaction mechanism. The resin base ofthese types is a compound containingoxirane groups. The initiator used is amasked Lewis acid or Broensted acid,which is activated by exposure. Thefollowing procedure then correspondsto that of conventional acid curingsystems.

UV-RADIATIONCURING TECHNOLOGYUV-RADIATIONCURING TECHNOLOGYEdwin TafelmeierResearch & Development

O

O

X

HO

O

O

O

O

OO

O

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O

O X

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Usually the resin component used is acycloaliphatic epoxy resin with a highreactivity. The proton contained in theacid induces opening of the epoxidering initiating polymerisation with acontinuous chain growth. Shrinkage withthis type of polymerisation is much lower,usually about 3 - 5 %; thus there is amuch better adhesion on many sub-strates.

Contrary to polymerisation initiated byradicals here the breakdown productsof the photoinitiator are long-lasting;thus reaction continues after exposure.First curing mainly takes place at thesurface, and then slowly continues. Theacid can diffuse through the still liquidlayer thus curing parts, which had notyet been directly exposed. A curingprocess still continuing in the dark istaking place. This process e.g. can beof advantage when printing highlyopaque ink types, as the UV-light doesnot penetrate al l the way to thesubstrate, which on the other hand willcause insufficient curing with radicalcuring ink types.

The combination of epoxides withpolyoles allows large-scale adjustmentof film properties from rigid to flexibledepending on individual requirements.

Processing of cationic curing UV inksdoes not require special measurements,provided the following properties aretaken into consideration. As usual whenhandling any ink type, all machinery hasto be clean, as even minor traces ofalkaline materials may impair or evenstop curing completely. E.g. con-tamination with alkalines may be due toamines, often contained in radical UVcuring inks. Some pigments and fillersalso have alkaline properties, so thatcationic UV-overprint varnishes do notcure on printing inks containing alkalinepigments. In addition auxiliary agentsand addit ives may have alkal inecharacter; if added to inks these willbecome unusable. Therefore pre-testsare always necessary.

One of the distinct advantages of ioniccuring is that there is no sensitivityagainst oxygen. However high humiditymay have unfavourable effects on curing.Use of an IR drier before the UV drierwill be quite advantageous. On one handthe relative humidity is reduced, on theother hand even a slightly increasedtemperature will significantly improvespeed of reaction during UV-exposure.Also heat will increase post-curing afterdrying.

Generally cationic UV inks have a lowerrisk potential than radical curing inks.

Odour of the liquid ink and of cured inklayer is very low. These properties arefavourable aspects for users.

All advantages in short

● films show a very goodflexibility

● adhesion on plastics, metals and glass is very good

● nearly free of odour

● excellent abrasion resistance

● very high gloss and good hardness

● good chemical resistance

● good barrier effect

Despite these certainly excellent pro-perties there are the following limitations:

● Like with all epoxide systems, outdoorresistance is still limited, even thoughsignificant improvements could beachieved using certain additives.

● Raw material prices for cationic inksand varnishes are much higher thanthose of common radical systems.

At this point we would like to point outthe aff inity to conventional epoxyvarnishes, either processed as ovencuring system or 2-component system.Processing of these conventionalsystems is more time consuming andcostly than that of UV-curing systems,thus making up for cost differences ofmaterials. As a whole the UV technologycould even be cheaper.

Even though the amounts of cationicUV varnishes and inks used are quitelow the Research and DevelopmentDepartment of Coates is closely lookingat these ink types. Especially for thoseapplications, where use of conventionalUV inks will reach their capacity limit,we expect a chance to master newtechnical requirements. In addition highprices for photoinitiators at present, aredecreasing slowly, but surely.

If this tendency continues, cationicUV inks will surely gain significancein the future. Coates Screen InksGmbH is already prepared to facethese new requirements.

+49 911 64 22-242

+49 911 64 22-283

edwin.tafelmeier@coates.comi

Edwin Tafelmeier