With the rapidly increasing use of UV-curable inkjet inks, more end users are finding new uses for their flatbed and roll-to-roll printers — many of which were never anticipated by the ink designers. This has challenged chemists to improve ink performance to adhere to more substrates, provide better durability, provide more flexibility and perform well at ever-increasing resolution and printer speeds. Since ink droplets fly through the air and land on the substrate, a printer can apply ink to anything. It’s when the ink lands that the trouble begins.

FREE RADICAL INK — PROS AND CONS
Today, flatbed inkjet printer users are almost certainly using UV-curable inks based on free radical UV chemistry. Suffice it to say, this technology works pretty well on most commonly used substrates. As with all chemistries and all inks, there are trade-offs. Free radical chemistry is very sensitive to oxygen. Oxygen quenches the curing reaction initiated by UV light, so free-radical ink curing must be fast enough that oxygen doesn’t have time to penetrate the ink. This is usually accomplished by using intense UV light on the ink droplets immediately after they are ejected. Since some pigments — particularly white, black and yellow — absorb UV radiation, it is more difficult to completely cure these inks at the substrate/ink interface, where there is the least UV light. This frequently requires greater UV intensity, more photo initiator (to allow for oxygen quenching) or a slower printing speed. More photo initiator adds to cost and can cause an objectionable odor. More intense UV light causes more heat on the substrate, and some substrates can’t tolerate the heat. Since speed is one of the most important attributes of an inkjet printer, the option of giving up speed otherwise available from the printheads is hard to swallow.

Additionally, substrates like glass provide an un-friendly surface to free radical ink chemistry. A primer is usually required to get adequate adhesion. In addition, the inks often shrink when cured, by as much as 20 percent. This can cause adhesion problems and cracking on some materials. Once the UV light is off, the reaction stops and is quenched by oxygen. It’s hard to get complete reaction if the UV exposure is not sufficient during the printing process, since little curing occurs afterwards.

So free radical chemistry works well, but it has drawbacks and trade-offs, and there is clearly room for continued improvement. How about the promise of better inks using cationic UV chemistry?

CATIONIC INK — A BETTER SOLUTION?
A lot of research work is underway, particularly by the scientists at Sun Chemical, to develop cationic UV inkjet inks that offer a good alternative to free radical UV chemistry. Why cationic? For one thing, it can be fully cured using heat after the initial UV exposure. This offers the printer designer the opportunity to use UV to fix the inks to the substrate in the printer, then complete the cure later. Cationic chemistry is not sensitive to oxygen, so oxygen quenching is not a problem. It requires less UV light to affect the initiation of curing, although it does cure more slowly than free radical. This may enable the use of laser diodes for UV exposure, reducing the heat and ozone emitted by conventional UV lamps. On the down side, cationic inks are very sensitive to moisture and to acids and bases. To get good results using cationic inks, it may be necessary to heat the substrate prior to printing to avoid the negative effects of moisture. Residual acidic or alkaline materials quench the reaction, so substrate choices must be carefully made. These inks have been shown to shrink less than free radical inks (only about 5- 10 percent), which helps improve adhesion. As a result, they generally adhere better to unprimed glass and other difficult substrates.

Cationic inks are purported to provide overall better adhesion, higher gloss, better opacity, better flexibility, better hardness, better chemical resistance and lower odor than free radical inks. So why aren’t we using them? One reason is that the development of the total ink package is behind that of free radical, which has carved out the position as leader in inkjet. The ingredients are more costly, so the ink pricing will likely be somewhat higher. Moisture and acid/base sensitivity are a significant problem for general purpose printing. For special applications, where the properties of cationic are necessary or desirable and free radical cannot provide them, cationic will be the choice.

UV INKS OF THE FUTURE
Both chemistries have their place. Like most things in life, each has pros and cons. For now, I believe free radical will dominate in the general purpose flatbed printer marketplace. When the advantages of cationic make a real difference, it will be the chemistry of choice. The good news is that we have multiple chemistries available, and as new specialty printing systems are developed and applied to more printing and decorating applications, these two chemistry tool kits will be available to allow UV inkjet to meet the performance challenges they present.