The Science of Solvent, UV, and LED Inkjet

The deciding factor is end-use application. Solvent and latex inkjet tends to be a superior choice for roll-to-roll printing on flexible media, for such products as banners and car wraps, because the ink is more pliable and better able to go over rivets and seams. (Also see “Latex Printing: Hope or Hype?” www.myprintresource.com/article/10828989.) Agfa solvent-based inks feature low profiles, lending them even to fabrics and sheer materials. UV-cured inkjet, on the other hand, is ideally suited for rigid media, including plastic, PVC, Styrene, board, and even granite. (Other UV applications include second-surface imaging and reverse printing inside polycarbonate glass.)

Inkjet 101

Each inkjet process has its reputation. Solvent-based inks are known to be more cost-effective, while UV-curable inks cost more per liter but are more sustainable. Industry analyst Henry Freedman compared the two technologies. “Significant differences between solvent and UV inks exist in both sustainability and economics,” said Freedman, who also edits the quarterly Tech Watch newsletter. “A UV system must be very complete in exposing all the ink chemistry so [as] not to release uncompleted drying chemicals into the environment. Solvent impact is more fully defined, being a more mature chemistry.”

Steve Emery, senior director of EFI’s ink business, added, “Solvent—up to 80 percent—is inherent in solvent ink. It is the carrier, in essence carrying the pigment dispersion [the color]. Solvent softens or swells the media so that the ink can penetrate into the top layer,” Emery explained. “The media and the ink almost become one, then the solvent evaporates off.” Agfa’s Salomon noted that so-called eco-solvent ink contains diluted solvent.

There is a chemical reaction within the UV curing process, too, of course. “The ultraviolet light is a catalyst,” Salomon went on to explain. “When UV rays hit the ink, it transforms from a liquid to a solid.”

Emery of EFI said, “At some point UV [inks] will crack. There is a deformation of ink, and it won’t come back together.” At the same time, UV inks are becoming more flexible. “Their elongation is getting better,” Emery noted, “so we’re beginning to see UV displace solvent on some car-wrap applications, but not on banners so much.” He added that EFI, along with media partner 3M, will be launching more flexible, fade-resistant UV ink sets later in 2013.

For UV-cured inkjet technology, monomers and oligomers are used to carry the pigment, Emery continued, which is dried (cured instantly) by a mercury arc lamp light source. According to Wikipedia, “an oligomer is a molecule that consists of a few monomer units, in contrast to a polymer that … consists of a nearly unlimited number of monomers.” A polymer is a chemical compound or mixture of compounds consisting of repeating structural units created through a process of polymerization (a.k.a. plasticization).

In physical chemistry, photoinitiated polymerization is a chain reaction of monomer to polymer initiated by a photogenerated radical or ion (source: McGraw-Hill Dictionary of Scientific & Technical Terms). In an UV inkjet printing context, ultraviolet light triggers photoinitiators that typically react to specific wavelengths within the 200- to 400-nanometer range. In the case of more heat-efficient, light-emitting diode (LED) curing, Emery noted, photons from the LED do the drying. “These photons interact with photoinitiators, which are a component of the ink set,” he pointed out.

Agfa’s Salomon is quick to counter with his own scientific equation, supporting the belief that inkjet is more about the media than the ink: “Ink represents only one or two percent of the mass of a finished graphic,” he summed up.

Economic Trade-Offs and ROI

In terms of upfront costs, eco-solvent inkjet printers range in price from about $20,000 to $25,000, Salomon said, while UV-curable device price points start at around $70,000 and can ramp up to $1 million, depending on the speed and sophistication of the output device. For Agfa’s high-speed :Anapurna flatbed models, prices range from $70,000 to $180,000, he added. Salomon encourages print firm owners to think in terms of productivity, not sticker price. “It’s all about how the work is getting done today, and technology drives consolidation. One M-Press can do the work of 100 devices from 10 years ago,” he explained, commenting on the commoditization of wide-format print. “At $10,000 a month, an entry-level machine could have a three- to four-year payback.”