EB and UV-curable inks and coatings can be formulated to produce a high-gloss appearance with relative ease, but when it comes to matte and soft-touch variations, especially those striving for ultra-low gloss units, challenges arise. One way to circumvent those challenges is to incorporate the use of excimer.
What is matte?
Shiny, high-gloss surfaces appear smooth to the naked eye but also to the light that reflects off of them. When roughness or texture is introduced, light is scattered and results in a diffuse reflection the eye perceives as matte.
The Status Quo
Most low-gloss inks and coatings are formulated with matting agents – particles and waxes – to disrupt the surface and create roughness. These agents work well in traditional solvent-based systems. During the coating process, the solvent helps keep the viscosity low, and during the drying stage, the evaporation of the solvent makes it easy for the particles to stick out of the surface. Without an evaporative fraction, it is more difficult to get matting agents to the surface of EB and UV coatings, and viscosity control also can be an issue, as combating the increased viscosity from the agents requires changes to the active components of the formulation (i.e., monomers and oligomers).
Matting agents present other challenges as well. The agents can be difficult to keep dispersed in coatings, settling to the bottom of the bucket while it’s on the shelf. Even during production, with constant mixing, agents sometimes can collect in the doctor-blade chamber or not release properly from the cells of an anilox roll, which opposes consistency over the length of a run. Matting agents can influence coating properties, such as increasing staining or decreasing water resistance. While the coating itself may hold up just fine, the burnishing of matte particles can create noticeable areas of gloss when scuffed or scratched that give the appearance of early failure in these tests. And given the choice, anyone who’s cleaned up after a coating run will choose gloss over matte or soft-touch coatings because removing matting agents from a coater is tedious, to say the least.
An Alternative
Excimer lamps, via radio frequency or dielectric barrier discharge, produce a wavelength of light in the vacuum UV, typically 172 nm. 1,2 This short wavelength penetrates only nanometers into an ink or coating, polymerizing a very thin layer on the surface (Figure 1). Resting on uncured, wet coating, the shrinkage stresses that normally occur during polymerization wrinkle this thin layer, creating micro-folds and a matte appearance. Either UV or EB then is used to achieve through-cure of the coating. Imagine having just one high-gloss coating and being able to shift from gloss to matte with the flip of a switch!
Excimer and EB
The combination of excimer and UV generally is more common, but excimer and EB have been used together for gloss reduction in the décor paper industry for several years. While there seems to be much potential for this duo in industries such as packaging or coil coating, the difficulty in exploring excimer and EB for other applications is a lack of access. With the help of Hamamatsu, the author’s company was able to temporarily install an excimer lamp on its pilot line for a couple of months, allowing coating providers and end users the opportunity to investigate the technology. As newcomers to excimer, the shareable takeaways from these trials are fairly elementary, but hopefully they’ll provide a useful primer for those considering the technology.
At 172 nm, the light produced by an excimer lamp does not require a photoinitiator to initiate polymerization. Photons at this wavelength have 7.2 eV of energy, which is sufficient to break several common bonds (e.g., C−C, C−H and even C=C). This fact had previously been established 3 but obviously was not widely known as more than one coating supplier initially declined to participate because they thought utilizing excimer meant needing to add photoinitiators to their initiator-free EB coatings. None of the coatings or inks tested (>20) contained photoinitiators, and all responded to excimer. Maintaining initiator-free formulations is a crucial aspect of excimer adoption in applications where photoinitiators are problematic. 4
Similar to EB, excimer requires nitrogen inertization and creates ozone if not inerted. Oxygen in the air absorbs light at this wavelength, preventing it from reaching the coating surface. 3 Thus, improper inerting reduces or eliminates the matting effect of excimer and clearly can be seen by the increase in gloss level. One benefit of pairing EB and excimer is that they can make use of the same nitrogen supply!
Perhaps the biggest unknown as excimer newbies was whether or not inks and coatings had to be specially formulated to react to the excimer light. The answer? No… but also, sort of. All the inks and coatings reacted to the excimer, including the few that were pigmented – and, by luck or small-enough sample size, most produced excellent visual results when exposed to excimer, but not all reacted with a uniform appearance or wrinkle pattern that was pleasing to the eye. In addition to formulated coatings, several individual monomers/oligomers were exposed to the excimer, and there were one or two that did not react. When an excimer-unreactive pre-polymer was mixed with one that was reactive, the result was a reactive formulation. That all formulations do not react in the same way to excimer has advantages and disadvantages; more variety and control of the look and feel of the final product is possible, but also formulating for excimer appearance may conflict with the functional needs of the coating. As with most solutions, this may mean excimer isn’t right for every occasion.
The drastic reduction in gloss achieved by excimer is impressive to witness, much like experiencing the instantaneous cure of UV/EB coatings for the first time – magic! Figure 2 shows examples of high-gloss coatings matted using excimer; the lamp was set to one side of the web for a side-by-side comparison of the OPVs with and without excimer. Measured at 60˚, gloss values dropped from ~90 to 10-20 gloss units for several coatings. One drawback, however, is that a gradient of gloss levels doesn’t seem achievable by modulating the excimer effective irradiance alone; the effect tended to be all or nothing.
One of the excimer-matted gloss coatings in Figure 2 was compared to the same formulation matted with traditional matting agents; a couple of quick, bench-side tests revealed the former had approximately twice the MEK resistance and triple the scratch resistance as the latter. While not conclusive, these tests highlight some of the benefits of reducing or eliminating traditional matting agents.
A limitation of using an excimer with a PI-free EB-curable ink or coating is that the material cannot be pre-gelled with UV. Pre-gelling is a partial cure that sometimes precedes excimer exposure. It is used to prevent non-uniformities from forming and also can influence the final feel. Since it is accomplished by UV or UV LED, conventional pre-gelling would require the use of a photoinitiator. Fun fact: Low doses of EB (~2-5 kGy) also can be used to provide a PI-free pre-gel (Figure 3). Admittedly though, the expense of incorporating a second EB just for pre-gelling is a bit impractical; reformulating to avoid pre-gelling is a more likely solution. The majority of coatings trialed did not require a pre-gel step.
As a final bit of fun, a matte spot embellishment was attempted. Spot embellishment 5 typically is a matte flood coat with a gloss inkjet embellishment, since matte particles and jetting ink do not play well together. Here, a gloss flood coat was EB-cured, and then a clear, gloss inkjet ink was layered on top in stripes (much easier to do than spots when applying with a drawdown bar instead of an inkjet head!). The ink layer was exposed to the excimer, then the beam and – voilà! – matte spot embellishment (Figure 4).
In summary, excimer is a wonderful tool to add to the gloss-control toolbox. It integrates well with EB, it has its benefits and challenges, but, overall, excimer creates possibilities that would not be achievable with matting agents alone.
References
- Hamamatsu, Excimer Lamp Light Source: Flat Excimer, 2021. https://www.hamamatsu.com/content/dam/hamamatsu-photonics/sites/documents/99_SALES_LIBRARY/etd/FlatExcimer_TLSZ1027E.pdf
- Heathcote, J., Mattifying Surfaces with Excimer UV Curing, 2021. https://www.gewuv.com/mattifying-surfaces-with-excimer-uv-curing/
- Scherzer, T., Prager, L., Knolle, W., Naumov, S., Photoinitiator-free Photopolymerization of Arylates Using Short-Wavelength Excimer UV Radiation, RadTech 2006. https://www.radtech.org/proceedings/2006/papers/065.pdf
- Schissel, S., Photoinitiators Reevaluated: PI-free EB is About More than Just Migration, UV+EB Technology, (3) 2023.
- Schissel, S., Embellishing with EB, UV+EB Technology, (4) 2021.
Sage Schissel, Ph.D.
Applications Specialist
PCT Ebeam and Integration LLC
[email protected]
