- Rapid curing: UV inks can be cured at a relatively fast rate, and in the case of UV/LED inks, that curing can be achieved without generating a lot of heat, which isn’t the case with traditional mercury lamps.
- The ink doesn’t dry in the print head: With water based and solvent based inks there is always the potential risk of the ink drying prematurely. This is primarily because; these formulations dry by evaporation. So if any ink is left in the print head, it can cause clogging and eventually damage the print nozzles. With UV technology, the ink remains “open” until it’s irradiated by UV light, at which point curing takes place.
- Adhesion and toughness: UV inks can be formulated to adhere to a wide range of substrates and once properly cured can form very tough, resilient films making them highly suitable for outdoor displays or banners or packaging that demands a lot of wear and tear.
- Excellent color values: UV inks are typically very transparent, which produces a very clean color gamut. This is very advantageous when printing any kind of graphics, especially with four color process.
Though UV inkjet printing serves many benefits, but formulating its ink is quite challenging. Let's see how...
What Limits the Formulation?
There are a number of challenges associated with formulating UV or UV / LED inkjet inks
that are somewhat unique to this chemistry. But, as anyone who has attended any recent printing exhibitions can attest - This technology is virtually ubiquitous and is growing at a rapid pace
For the purpose of this article we’ll group UV and UV / LED together as the only key difference is the type of curing lamps and wavelength of light used to cure the inks
The figure below also shows that the LED curing range is very narrow compared to conventional UV curing.
Curing Range Comparison between UV- and LED UV- Lamp
Components of UV Ink
A typical UV ink formulation
consists of the following components:
- Other additives
Let's understand the significance of each component along with the challenge associated with it in detail...
Oligomers & Monomers
|I like to think of the Oligomer as being the “binder” or “resin” of the formulation, and the monomers as the diluents. In reality, both the oligomers and reactive monomers provide most of the ink’s physical properties, as these determine how the ink will cure, adhere, and perform on the substrate.
Oligomers tend to be fairly high viscosity liquids, while monomers tend to have much lower viscosities. Multifunctional acrylate monomers are used to provide lower ink viscosity, but they also can be used to modify the ink film properties, such as hardness, adhesion and chemical resistance.
Monofunctional acrylate monomers can be used to reduce the amount of cross-linking in the cured ink, which translates to reduced shrinkage for improved adhesion, flexibility and impact resistance.
So, What's Tricky?
The challenge comes when trying to formulate an ink
that has a high enough amount of oligomer - to impart good physical properties - while maintaining a low enough viscosity to successfully jet. Typically UV inks will jet at a viscosity range of between 6-12 cps
at jetting temperature. Since, most UV print heads have on-board heaters, which can significantly reduce the jetting viscosities by raising the jetting temperature (range of 40°-70° C).
Curing of UV inks is initiated by the choice of photoinitiator
, and this happens by either free radical curing or cationic curing. Free radical polymerization is currently the most popular curing method, which takes the ink through a series of steps in the curing process. Those steps being:
Radical formation → Initiation → Propagation → Termination
The amount of photoinitator in a UV ink or coating formulation can range from approximately 0.5% to 15%, and the photoinitiator composition can vary widely depending on the particular requirements needed. For example: Very thin coatings or very thick coatings. Whether the ink is clear or opaque also makes a difference, and typically mixtures of photoinitiators often are used to ensure both surface and through-cure of an ink.
Types of Photoinitiators
The two main types of free radical photoinitiators, are classified as either Type I or Type II. Type I photoinitiators are those that upon irradiation generate two free radicals.
- One of these free radicals is reactive and this initiates polymerization. The non-reactive free radical contributes to migrating species and extractables, which can be a problem in food packaging applications and those packages that are sensitive to odor. α-Hydroxyketone and 2-Hydroxy-2-methyl-1- phenyl-1-propanone are examples of type I photoinitiators.
- Type II photoinitiators contain benzophenone. They undergo a reaction where the excited state of the photoinitiator interacts with a co-initiator to generate free radicals. Benzophenone, which has a distinctive odor, is excluded from many food packaging applications. Thus, acrylated tertiary amine compounds are used instead.
This can be an issue with the curing of low viscosity acrylate systems. Oxygen diffuses more easily into low viscosity films, where it strongly inhibits free radical polymerization by scavenging, initiating or propagating radicals or by quenching the photoinitiator in its excited state
. This is often seen on the surface of the curing ink film, which often results in a tacky surface
even if the bulk of the ink film is properly cured. Some of the ways by which we solve this problem include:
- Curing under a nitrogen (or another inert gas like carbon dioxide) blanket. This method is very effective in suppressing oxygen inhibition, but it can be expensive and can prove very impractical, especially in the case of scanning-head UV ink jet platforms.
- Using photoinitiators that are less susceptible to oxygen inhibition or increasing the photoinitator concentration.
- Increase of UV light intensity (irradiance) is also useful, as it creates an overflow of free radicals that will compensate for the radicals consumed and neutralized by oxygen.
When formulating UV inks you can choose to use commercially available pigment dispersions
, either in solid / powder form or in liquid form. These are a good choice if you don’t have the capability to disperse your own pigments
, which require specialized micro media mills to achieve fine enough dispersions (50-200 nm). Pre-dispersed colorants can be incorporated into your ink formulation using a simple mixer. If you’re dispersing your own pigments, you’ll have to blend the pigment, a dispersant and usually a monomer. Put them through a pre-mix stage to ensure that all of the components are combined, and then run the mixture through a media mill for 24-36 hours depending on the output of the mill you’re using. Typical grinding medium are 0.3 mm sized zirconium beads.
Micro Media Mill
The types of additives that are used in UV inks include: