Curing UV inks
Jack Knight is director international technical service for INX International Ink Co. In his latest article, he discusses the UV process and answers questions from CanTech International readers
After a great turn out at Asia CanTech in Ho Chi Minh City, Vietnam, I spent the next three weeks travelling South East Asia. Not only did I see more customers than ever before during this period, I was also able to get to the questions that the metal decorators and can makers are asking answered in this article. Everyone I talked to said that their business had grown 20 to 30 per cent over the past two years and they predict that this will continue for the next 10 years.
The overwhelming questions were on the UV process. So let’s get to the science (no magic) as to how this power source turns a liquid to a solid. For help explaining the science I turned to Mark Gordon, energy curable chemist with INX. Thanks Mark.
James Zani from Cometa Can in Jakarta Indonesia asked the question ‘What spectrum is best for cure of INX UV Inks?’ The answer is, well, there isn’t one best answer and understanding what the spectrums are, which bulbs produce them and for which types of UV inks is important.
The UV spectrum is part of the electromagnetic radiation band between visible light (400NM) and X-rays (100NM). That spectrum is generally divided into three parts:
UVA 315 – 400NM (often called “long”)
UVB 280 – 315NM
UVC 100 – 280NM (often called “short”)
The significance of these three bands is mostly in relation to the UV source, the lamp. Most people use medium pressure mercury vapour (or H) bulbs, which has a spectral output that covers all three bands. The primary peaks of a mercury bulb are at 265NM (C band), 315NM (B band) and 365 NM (A Band). Still, the majority of its output is in B and C. Many photoinitiators will make use of these peaks and that is why mercury bulbs generally work quite well.
The type and amount of pigment in an ink has a huge impact on the cure properties because pigments also absorb UV wavelength and effectively compete with the photoinitiators. Dark colours and opaque whites are particularly affected by this. To overcome this, the formulator has to choose photoinitiators that make use of the longer UV wavelengths, UVA and UVB.
Another way to address this is to boost the amount of UVA and UVB available from the lamps. UV lamps can be modified with the addition of metal salts (halide lamps). A mercury bulb that has iron halide added (also called a D bulb) has much more UVA and UVB energy, but very little UVC. These are utilised to cure dark colours or thicker films, particularly if the ink has been modified to exploit these lamps.
For whites, the TiO2 absorbs all the UVC and UVB bands, and a good part of the UVA. An additive lamp where Gallium halide is added to the mercury bulb (called a V bulb) is best suited for this situation. This type of lamp only puts out energy in the longest part of the A spectrum and a little in the B. Only a couple of photoinitiators are efficient with this type of bulb. This is almost exclusively used for curing white inks only.
There isn’t really a best spectrum. The entire UV spectrum as a whole is used and it is up to the printer and ink supplier to figure out how best to utilise it. There are many more things to consider when using UV inks and the information given here is only a start.
In terms of what bulb to use, almost everyone uses standard medium-pressure mercury lamps. Consequently, most inks and coatings are formulated with that in mind. But with more people looking at additive lamps for the reasons stated above, it’s now possible for ink suppliers to provide products to address some of the cure issues that plague many printers. Matching ink chemistry to the type of lamp is a critical step that printers should not forget.
Another question I often hear from decorators who are making the transition from UV to conventional is why are the UV inks not as glossy before and after applying overprint varnish. The answer comes from the differences in the curing/drying process. Conventional inks dry by baking them for 10 minutes creating a hard shell over the pigments using the same process as the overprint varnish. UV inks are 100 per cent solids.
When they are cured in a fraction of a second, they bind the pigments in a continuous semi-solid film without the ability to create that same hard-shell that baking provides. This leaves the ink porous so that when the overprint varnish is applied it dives into the ink film leaving less varnish on the surface to reflect light giving the finished product with less gloss than the conventional process. The final results are very close for gloss but with some systems you can see a difference. ❑