Unlike solvent, eco-solvent or water-based inks, UV-curable inks do not evaporate into the air. Instead these inks dry or “cure” instantly through a polymerization process. This process is initiated by exposing the inks to a certain spectrum and intensity of UV light.
UV inkjet inks are classified as 100% solids. The term “100% solids” may be confusing to many. You might wonder how an ink could be a liquid and be classified as 100% solids.
If you’re baffled by this as I was, here’s what they mean; 100% of what is printed on the media, stays on the media – kind of like the way Vegas works. By comparison, solvent inkjet inks are less than 20% solids. Solvent screen print inks are about 50% solids. Everything that isn’t a solid evaporates into thin air.
Here are the nuts and bolts of how UV-curable inks are formulated and how they cure. Two of the key ingredients in a UV curable ink formulations are monomers and oligomers. Of course, you must have heard of monomers. These are very small molecules that can combine, through a chemical process called polymerization, to form much larger molecules called polymers.
What are Monomers?
Monomers are classified as “reactive diluents”. So what does this fancy term mean? Reactive diluents are materials that ink manufacturers can use to replace organic solvents. In an ink formulation, the monomers perform the same purpose as a solvent, by reducing or thinning the viscosity of the ink.
But here’s the difference, and it’s an important one. Monomers don’t evaporate; solvents do. As the solvent in a conventional ink dries, it evaporates and these volatile organic chemicals or VOCs get into the air, which contributes to air pollution. Solvents in the air also pose a health risk to your employees, which are breathing this contaminated air.
What are Oligomers?
Now that we covered monomers, what are oligomers? Just like monomers, these molecules combine with monomers to form polymers. One difference between the two is that monomers are very small particles with a low molecular weight, whereas oligomers are larger molecules with a high molecular weight.
The function of an oligomer in the ink formulation is different, too. Whereas monomers are “reactive diluents” and perform the function of a thinner, oligomers are “reactive resins”. Just as a resin binds a paint or ink or vinyl film together, the larger reactive resins or oligomers of a UV curable ink form the backbone of the molecular chain to which the monomers bond in the polymerization process.
Of course, monomer and oligomers aren’t the only components that comprise a UV curable ink. The other components include photo initiators, pigments, and additives. Let’s start with the photo initiators. Their job is to get the ball rolling in the polymerization process.
What Polymerization Means
Here’s how they do it. When the ink is exposed to UV light, the photoinitiators absorb the UV energy and break down into reactive components that initiate the polymerization process. Polymerization is the chemical process in which the smaller monomers and oligomers bond together to form the much larger polymer molecules. As the smaller molecules polymerize, they entrap the pigments, which give the ink its color and the additives in the cured solid inks.
Not all photoinitiators are alike. In formulating UV curable inks, two types of photoinitiators are used. The most common or traditional type is called “free radical”. When the free radical photo initiator absorbs certain wavelengths of UV light, radicals are releases. If you were around in the turbulent era of the sixties, you know that radicals were agitators. The radicals in the ink are agitators, too. They start the chemical reaction known as “cross-linking”, which bonds one polymer chain to another. Cross-linking occurs almost instantaneously. You’ll note that I qualified my statement with the adverb “almost”. With some UV curable inks, there is a post cure period in which the inks continue to cure after the initial curing process.
UV curable inkjet inks are categorized as being either free radical or cationic. Most of the UV curable inkjet inks are the “free radical” type. These ink formulations cure only when exposed to certain wave lengths of UV light. Interrupt the light exposure and you interrupt the curing process.
There is however another type of photoinitiator. It’s called “cationic”. Cationic UV inks aren’t exactly the new kid on the block. These formulations have been around for years in the screen print and flexographic printing markets. Cationic UV inks cure differently than free radical inks. Once the curing process is initiated, it keeps going and going and going, just like the Energizer Bunny, until the ink is cured.
UV curing units can generate heat that can warp or damage some plastic films. Printers, that utilize curing lamps that emit very low curing heat, include the Mimaki UJV-160, a new UV curable inkjet printer using an LED light source. Not only do these low energy systems cut power consumption considerably, the low curing temperatures allow printing on heat sensitive plastics. Similar to cooking food in a microwave oven, the UV LED light source cures the ink from the inside out.
