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Curing of UV/LED/EB Litho Inks

Real Data on Migration Levels

by Don Duncan, Glenn Ribelin and Paul Robinson

Wikoff Color Corporation

SUBMITTED

Table I. Test Ink Descriptions


SUBMITTED

Table 2. Extraction/Migration Results

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In our younger days, we read a fair amount of Shakespeare. It seems that much of the furor and debate in the industry today about whether UV-LED curing is sufficient (or superior or even adequate) when compared UV lamps is “sound and fury, signifying nothing.” We’re just going to skip over the first part of the quote about being “a tale told by an idiot...”

As professional experimentalists, reliance upon data rather than upon hyperbole is an inbred characteristic. Therefore, a team of us within Wikoff Color decided to undertake some simple experiments to compare actual migration levels of UV lithographic inks cured by conventional UV lamps vs. UV LED arrays. There were doubts that UV-LED was up to the job, and it was time to find out. We expected that EB-cured lithographic inks would show lower migration levels than either type of UV curing, so we included EB litho inks as the standard.

All evaluations and comparisons of print made from varying ink formulas are troubled by the very large number of experimental variables, from variations in the purchased raw materials for the inks, to variations in ink manufacturing, to variations in the printing process (of which there are legion), to variations in the curing, to variations in the extraction test process on the print. And there are several other places where experimental error can creep in. We needed to keep it simple in order to have any chance of seeing trustworthy trends. We had no expectation that our results would be definitive or crystal clear – we were only hoping to see macroscopic trends and leave the fleshing out of the story to other researchers.

The Ink

So we picked a single printing process (lithography), a single ink color (process cyan), and as similar an ink formulation as possible for the three curing methods (UV lamp, UV-LED and EB). The three inks we created in our lab are listed in Table I.

The UV-1 and UV-2 inks were identical except for the photoinitiator selection. The EB ink was identical except with no photoinitiator, and re-scaled to 100%. Here is the basic ink formula for all three:

  • Internally manufactured Blue pigment dispersion
    • Phthalocyanine Blue (15:3)
    • Fatty acid-modified Epoxy Acrylate
    • GPTA
  • Hexafunctional Polyester Acrylate
  • EOTMPTA
  • GPTA
  • Inhibitor
  • Adhesion promoter
  • Clay
  • Photoinitiators (except for EB)

Here are the two different photoinitiator packages we used for the LED ink (UV-1) and the lamp ink (UV-2):

UV-1 (Optimized for 395 nm LED)

  • Michler’s Ethyl Ketone
  • Irgacure 819

UV-2 (Simple commercial UV lamp Ink)

  • ITX
  • EDAB (amine synergist)
  • Irgacure 907

The Substrate

After selecting the inks, we then had to select a substrate on which to print them. We decided on double polyboard, which is a paperboard with a polyethylene film laminated to both sides. This is a substantially impermeable substrate and is the standard packaging substrate for ice cream and much frozen and refrigerated food. This was 0.018 caliper ice cream carton stock obtained as a small roll from one of our customers, which we then cut down to the size for making laboratory prints.

Experimental Process

The prints were made with a Little Joe proofing press in our lab that was completely cleaned between ink samples. A new rubber blanket was installed before the first prints were pulled. The EB prints were made first, then the UV-1 and then the UV-2 prints.

We made five prints with each ink for each set of curing conditions on separate but practically identical substrate pieces cut from the roll of substrate. We then cured each set of prints under varying conditions and stacked the prints to allow face-to-back migration. Each stack of five prints was wrapped in aluminum foil.

We then let the stacks stand undisturbed in our climate-controlled lab for 1 week. After that time, we mailed the prints to the testing lab and requested that extraction testing be done on the back side of the three interior prints, thus giving extraction data in triplicate.

The Curing Conditions

EB – 3 Mrads

UV Lamp (Standard mercury)

  • 400 W/in, 500 fpm, 3 passes
  • 400 W/in, 500 fpm, 1 pass

UV-LED (395 nm)

  • 16W, 500 fpm, 3 passes
  • 16W, 300 fpm, 3 passes
  • 16W, 100 fpm, 3 passes
  • 16W, 300 fpm, 2 passes
  • 16W, 300 fpm, 1 pass

The UV lamp was a fresh 600 W/in lamp run at 400 W/in. The LED unit was a Phoseon 16W system. Both energy sources were mounted over a variable speed belt drive which was calibrated for speed. In the case where multiple passes were used, the print was passed under the curing unit at the noted speed and immediately placed back on the belt and passed under the curing unit again.

The EB unit is an ESI lab unit that is regularly calibrated and is used as part of our standard quality control processes.

The Extraction Test

We used Tom Hartman’s lab at the Center for Advanced Food Technology at Rutgers University. He has established himself as the industry standard in North America for migration testing for food packaging.

After consultation with Prof. Hartman, we agreed on solvent extraction testing in his custom stainless steel extraction cell (51 cm2), using 80 mL of 95% Ethanol for 24 hrs at 40°C, which is FDA Condition of Use E. After the extraction, the samples were spiked with deuterated anthracene as internal standard, concentrated to approx. 1 mL at RT, then analyzed by GCMS.

The unprinted substrate was tested in the same way, and those results used to normalize the data from other tests. Each printed (and unprinted) sample was tested in triplicate. The quantities of individual chemicals found in triplicate analyses were averaged and then normalized to blank stock. We then aggregated the individual chemicals into three classes of chemicals (photoinitiators per se, photoinitiator fragments and oils) and show the results in Table II.

Conclusions

“Fully cured” UV-LED inks (UV-1) and “Fully cured” UV lamp inks (UV-2) give similar migration levels, when considering all three types of chemical migrants.

“Undercuring” seems easier to achieve (really, harder to avoid) with lamps than with LED.

Residual photoinitiator and PI fragment levels were better for UV-LED than for lamps, but each type of ink uses different photoinitiators.

EB curing does, as expcected, give the lowest levels of extractable chemicals.

No monomers were detected in any sample, but only larger trifunctional monomers were used in the ink formulas.

The “Oils” were a mix of chemicals that seemed to have no traceable connection to the ink, the substrate or any cleaning materials used in our lab. These may be contamination from the extraction testing, or from some other unknown source.

The blank substrate gave large quantities of extractable chemicals, and the results varied quite a bit. The normalization process to subtract these from the test samples probably adds some noise and variability to the results.

Caveats

Only 400 W/in standard mercury lamps and 16 watt 395 nm LED units were tested. Nothing can be said about other power or other wavelength LED units.

This was designed to measure face-to-back migration and to avoid through-migration.

Despite running the data in triplicate, not everything lines up perfectly. Still, gross trends seem apparent.