By Jim Hingst
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 photo initiators 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.
Vinyl Application Videos
Squeegee Technique. Nothing is more basic in vinyl application than the squeegee. But some squeegees work better than others. And there are right ways and wrong ways to use this simple tool. This video clip reviews squeegee selection, squeegee care and squeegee technique. Click to Link
Wet Applications. Dry applications are typically recommended for most vinyl applications. For those exceptions to the rule, this instructional video explains the right way to perform a wet application. Click to Link
Application of Window Graphics. Installing vinyl graphics on glass can be challenging, because the adhesive aggressively grabs onto this high energy surface. Repositioning graphics on window is generally difficult, if not impossible. This instructional video explains how to dry apply window graphics right the first time. It also describes the necessary steps required for surface preparation. Click to Link
Transferring Frosted Window Graphics Films. Because frosted window films are highly textured, transferring cut vinyl graphics can be problematic. This video provides direction in selection of the right application tape and how to apply these films quickly, easily and without problems. Click to Link
3-Step Surface Prep. This instructional video describes how to properly clean the surface of a vehicle before applying pressure sensitive vinyl graphics to a trailer or doing a full wrap of a car or van. This three step surface prep involves detergent washing, solvent cleaning and a final wipe down with IPA. Click to Link
Vinyl Application Over Rivets. Applying vinyl graphics on vehicle surfaces with rivets is challenging for most sign makers. Vinyl failures to these surfaces are all too common. This video demonstrates some tips and procedures, used by professional decal installers, which will make these demanding applications easier and more trouble-free. Click to Link
Vinyl Application to Corrugations. Learn how to apply vinyl graphics to corrugations in a relaxed state. Forcing films into the valleys of corrugations puts too much mechanical stress on the film. Sooner or later, the vinyl will lift or tent in valleys. Click to Link
Applying Air Egress Vinyls. Vinyl films with air egress release liners have almost eliminated bubbles and wrinkles from applied graphics. Air egress release liners with their highly textured surface structure are problematic. Not much will stick to these liners other than the vinyl film. At RTape our best premask solution for air egress liners is a special Conform® tape called 4761RLA. Click to Link
About Jim Hingst: After fourteen years as Business Development Manager at RTape, Jim Hingst retired. He was involved in many facets of the company’s business, including marketing, sales, product development and technical service.
Hingst began his career 42 years ago in the graphic arts field creating and producing advertising and promotional materials for a large test equipment manufacturer. Working for offset printers, large format screen printers, vinyl film manufacturers, and application tape companies, his experience included estimating, production planning, purchasing and production art, as well as sales and marketing. In his capacity as a salesman, Hingst was recognized with numerous sales achievement awards.
Drawing on his experience in production and as graphics installation subcontractor, Hingst provided the industry with practical advice, publishing more than 150 articles for publications, such as Signs Canada, SignCraft, Signs of the Times, Screen Printing, Sign and Digital Graphics and Sign Builder Illustrated. He also posted more than 325 stories on his blog (hingstssignpost.blogspot.com). In 2007 Hingst’s book, Vinyl Sign Techniques, was published. Vinyl Sign Techniques is available at sign supply distributors and at Amazon.
© 2016 Jim Hingst
Nice blog
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