On the Post-cure Case (UV Inks)

Industry experts weigh in on the who, what, when, where, and why of the post-cure process.

by Jessica Millward, Associate Editor

THE APPARENT "chemistry mystery" occurring after a UV ink is cured significantly impacts the appearance of a finished printed product. Supplier sleuthing has led to these conclusions:

WHAT is (and what isn't) post-cure?

Though used throughout the industry, the term "post-cure," in many cases, is technically a misnomer. Rob Stendahl, corporate technical support supervisor for Akzo Nobel, clarifies: "Post cure only takes place with cationic UV-curable inks. The more common type of UV inks is cured through a UV-induced free radical polymerization process and has no post cure at all. What many people refer to as post cure is simply the relaxation of the high level of stresses built up into the ink film during the materials' incredibly fast cure process."

Sun Chemical Energy Curable Inks Market Manager Tony Bean concurs, explaining, "…[F]or free radical polymerization, the free radicals are consumed very quickly or quenched once they are generated." Once the ink lacks exposure to UV light, the free radicals deplete and curing ends.

WHEN is the cured ink "done"?

At Flint Ink, they refer to the "relaxation" period of typical UV inks as "post change," and estimate it as lasting approximately one day. Technical Manager, Radiation Cure, Rod Balmer asserts, "The amount of post-change will decrease with time at an exponential rate. Most of the change is likely to occur within the first few hours. At 24 hours after exposure we would expect to see little or no change."

Though post-cure or post-change can't be measured precisely, printers should focus on the printed results—and if they meet end-user standards by the end of the run. Kurt Hudson, UV products general manager at Water Ink Technologies, believes printers ought to concentrate on whether the ink is cured well enough at the end of the line to either pass all requirements of the end-use environment, or if it will pass a test predicting its eventual conformance to those requirements.

WHO determines successful UV ink curing?

Long before ink meets lamp, the effectiveness of a UV cure is set on course by the ink formulator. First, the change in finished substrate properties after curing is heavily influenced by the additives contained in the ink. Bean maintains, "Slip additives such as waxes and silicones, flow agents, and other materials can slowly or quickly come to the surface of the film and impact the performance of the ink."

Formulators also handle the tricky issue of pigment concentration. Pigment level plays a major role in the success of the finished product, as the pigment either absorbs or reflects the photons emitted by UV lamps. Akzo Nobel's Stendahl states, "Formulators must select photoinitiators that will absorb light at wavelengths not absorbed by the specific pigments." He further notes that with too great a pigment concentration, light simply cannot get through the ink film to the ink/substrate interface. Neither cure nor adhesion takes place.

WHY is ink film thickness such an issue?

The performance of UV inks is impacted, in numerous respects, by the ink film thickness of the ink laid down or amount of ink laid down. Balmer says while gloss can be enhanced by greater ink film thickness, proper depth cure is not so easy with a heavy film weight. The negative impact of a thick ink film is especially noticeable in darker colors, where the pigment absorbs UV light, competing with the photoinitiators, and resulting in under-cure. He concludes, "This in turn may lead to adhesion and rub resistance issues. Furthermore, if a UV lacquer is subsequently applied on top of such a film, this may result in scratch-off of the lacquer."

WHERE can printers avoid creating excessive ink thickness?

Accomplishing thorough cure throughout the ink film isn't simple when a particularly thick layer has been created. Zeller+Gmelin Senior Chemist Dale Smith attests excessive pressure on the print roller can force ink to the edge of the print area, resulting in build-ups.

Also, Smith relates, numerous tiny holes of non-coverage sometimes appear with rough or porous substrates. Because the press operator may perceive the problem as being a color issue rather than one of coverage, he might apply more ink, resulting in an overly thick film layer.

Finally, pinholing can also occur through poor flame treatment and/or contamination of the ink. As with porous substrates, these pinholes can be microscopic and mistakenly diagnosed as a color deficiency.