Hal Hinderliter

UV inks and coatings
November 27, 2007

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Question: We're having some occasional trouble with UV inks and coatings (see below).
Can you offer some advice?

Paulette Ott
Print Production Coordinator, PCA

Today’s answer comes from one of the foremost expert on UV inks and coatings, Don P. Duncan, Ph.D., Director of Research for Wikoff Color Corporation and co-chair of Radtech's Graphic Arts Focus Group:

Problem A
If we print a black solid using hybrid UV ink and then apply a UV coating over it inline, we are seeing cracking and/or stress whitening when bending a 180 degree score on the box.

Answer(s) for Problem A
This is usually one of three issues.  The most common cause is the paperboard drying out.  Sometimes, especially during the winter in colder climates, the humidity can drop very low.  If the paperboard is allowed to dry out so that its moisture content is below the design specifications, it will get brittle and crack upon scoring. 

A second reason that this might occur is if the ink or coating are cured beyond their design specifications.  If the UV-curable products see more UV energy, they can crosslink to a higher degree than the formulator planned.  This will cause them to become brittle and can result in score cracking, revealing fibers.  When this is occurring, the coating will usually show poor fingernail scratch resistance, due to its brittleness.

The third reason is if the improper coating is chosen for the job.  Some coatings are designed to cure to a very high crosslink density, and thus are quite brittle by design.  This also can result in score cracking.

Problem B
We sometimes create a "strike-through" effect by printing a black solid of UV ink then overprinting a matte varnish in selected areas, then run a gloss UV coating over top it all. However, when we test the adhesion with a tape pull, the areas where the varnish was applied are removed. (Some ink remains, but all the varnish and most of the ink pulls off; the substrate is undamaged as there are no board fibers on the tape.)

Answer for Problem B
Since all three layers (the UV ink, the matte varnish and the UV coating) are either completely or mostly lifted by the tape, this implies that the interlayer adhesion of the three products is very good.  That is, the ink is sticking to the varnish and the varnish is sticking to the coating.  The weak link appears to be at the board/ink interface.  While it is possible that something from the varnish is migrating through the cured UV ink and weakening the ink/board adhesion, this seems unlikely.  Since UV inks typically have very good adhesion to paperboard, but since you are seeing no fibers on the tape pulls, this suggests that there is something on the surface of the board that is slightly weakening the board/ink bond.  

Editor's note: For help in determining your UV print quality, consider the Thumb Twist Test Form from PIA/GATF. This test form includes several elements that allow for the evaluation of curing, image quality, and gloss characteristics. Simply print the form on a sheetfed UV/hybrid printing press, pull a sheet, place your thumb on any number of squares varying in coverage, and twist. If ink or coating appears on your thumb, the sheet is not cured. The form is supplied on CD-ROM in PDF format.

Posted by Hal Hinderliter on November 27, 2007 | Comments (1)