UV LED Inks: The Future of Narrow-web Printing or Just a Passing Fad?

By Niklas Olsson

Flint Group launched UV LED curable inks for narrow-web combination printing at Labelexpo Americas 2012. Since that time there have been many questions as to whether this technology is the wave of the future, or just a passing fad.

Flint Group has studied the benefits thoroughly during three years of development work and has documented the benefits. In close co-operation with development partners and beta test customers, UV LED technology has proven itself to be a viable option for narrow-web printing today, and with great opportunities to develop further into the future as a major cure method for high-quality narrow-web labels and product decorations. As this technology is new, let us look at some of the fundamentals behind the technology, as well as briefly describe the main benefits identified.

First, the basics

UV LED is an alternative curing technique to traditional mercury curing, the primary method used today to cure UV inks in the narrow-web industry. Used already in many industrial and practical applications, such as visible lighting, water purification, and nail curing, UV LED technology has now crossed over into printing applications. This curing method is now used regularly in UV inkjet, large-format UV screen, and UV sheetfed applications. And, as we will discuss in this article, beginning its growth in narrow-web label and package printing.

UV LED is an abbreviation for UltraViolet Light Emitting Diode. UV curing with an LED lamp occurs exactly the same way as a standard mercury lamp. The ink, coating, or adhesive, when exposed to the UV lamp, causes a polymerization reaction to occur. The differences? This curing method offers significant advantages over mercury curing (as we'll discuss later).

A free radical curing UV ink consists, typically, of a range of raw materials, where each ingredient gives certain properties to the finished ink (see Fig. 1).

Any UV curing ink will only cure when it is exposed to UV light. In this case, the oligomers and monomers react with the photoinitiators and a crosslink reaction begins. If there is not enough light, only surface cure will be achieved, which can result in poor adhesion.

The choice of pigment will affect the choice of photoinitiators, as they will compete to absorb the UV light.

The difference with UV LED

The main difference between a standard mercury lamp and an LED lamp is the wavelengths emitted from the lamps that cause curing to take place.

A standard lamp has a broad spectra of wavelengths that spread from UV-A through UV-B and UV-C and include visible and infrared. This means that an ink or coating formulation will have a mixture of photoinitiators to allow the product to achieve optimum cure.

An LED lamp has a very narrow wavelength, with a peak around 395 nm, although LED lamps are available with alternative wavelength diodes including 375 nm, 385 nm, 405 nm, and 410 nm (±20 nm). Therefore the ink and coating technology must be reformulated so that the reactivity and cure results are similar, but with the high-intensity LED output.

Formulation for LED curing

When formulating inks for UV LED curing, the main difference is related to the photoinitiator selection. In free radical inks a mixture of photoinitiators are used to match the wide spread wave lengths of mercury lamps. In UV LED a focused peak at 395 nm requires very careful selection of exactly the right photoinitiators to match the high energy peak.

When selecting a UV light source, it is not only important to consider the operating parameters of the UV source, but also the formulation of the material, which in many respects is even more critical to a successful curing "system." In terms of UV cure rate, the general assumption is that increased peak intensity is the key factor in increasing cure speed/cure rate. If a 4 W/cm2 UV LED light source is good, then 16 W/cm2 should be four times better! Of course, in practical terms the reality is much more complicated; cure rate is a function of not only the amount of UV energy, but also how well matched that UV energy is to the spectral response of the photoinitiator used in the formulation.

But it doesn't stop there; the photoinitiator is just one small component of the material, which also contains oligomers, pigments, and monomers. In fact, the photoinitiator makes up only a small percentage of the UV system (typically 5 to 10 percent). The photoinitiator or photoinitiator blend plays the pivotal role in determining the cure rate, but oligimer and monomer selection are also factors.

UV LED ink technology needs to consider the wavelength specifics and should be developed using raw materials that work in conjunction with a range of UV LED light sources. In Fig. 2, you see a wavelength comparison, which shows that traditional UV inks will not cure adequately with UV LED lamps. The peak is very specific and inks should be developed with this in mind.

Lamp and press suppliers

Many lamp and press manufacturers are working together and can provide UV LED lamps and UV LED presses. However, not all lamps are the same. Lamps can differ in the wavelength output, as well as the peak irradiance output among other parameters. Your ink, press, or lamp supplier can help you select the proper lamp specifications for your application needs. And, most press suppliers will offer new presses or retrofit an existing press.

LED lamp advantages

There are many advantages to using LED lamps. These can be split into economical and environmental advantages.

Economic/productivity/expanded capability advantages:

Press uptime—Less equipment faults, presses run longer and without lamp faults; higher certainty of cure with inks when using this process;
Faster cure/higher productivity—Better UV curing allows for faster speeds and more combination printing;
Energy efficient—UV LED lamps require about 50 percent less energy;
Long lifetime and low maintenance—UV LED lamps last in the range of 20,000 to 50,000 hours of run time as compared to a mercury lamp running of about 2,000 hours total time;
Solid state/instant on/off—There is no waiting for lamps to heat up or cool down saving valuable press time and substrate;
Expands the capability of the press by running heat sensitive/thin films on a press without heat management. An LED lamp does not have infrared and thus does not create heat as mercury lamps do;
UV LED lamps are very compact and flexible and can easily be adapted to any press platform;
UV LED systems are smaller and don't require large blowers to move air and thus take up less space in the production area.

Environmental and safety advantages:

Removes toxic mercury from the process and thus no need to deal with hazard waste disposal of lamps;
Removes ozone generation since there is no UV-C light output from lamp (ozone is a respiratory hazard and pollutant);
Lamps are not hot to touch and thus are safer;
Systems are much quieter, resulting in less noise pollution;
Safer cleaning and maintenance (very little required) and thus less risk of injury;
The bright blue light emitted from the UV LED lamps is not recommended to look at, but unlike standard mercury light it will not cause permanent eye damage.

Deeper curing with LED

Also important to note is that UV LED light, with its high peak irradiance and UV-A and visible light emission, provides a deeper penetrating light source. This provides advantages to cure thicker films (such as screen printing) and darker more opaque colors (such as blacks and opaque whites). This will enable printers that are doing combination printing to print faster speeds with more assuredness of cure and adhesion.

Return on investment

While the up-front installation costs for UV LED technology may be higher than traditional UV curing technologies, beta testing—at both development and converter facilities—demonstrates that UV LED technology delivers value not found in traditional mercury curing.

The data shows that converters will see a return on their investment—and in most cases payback will be realized in less than one year. In addition to the advantages mentioned above, specially designed UV LED inks:

When formulated correctly, will be suitable for most applications you do in conventional UV today.
Offer high color strength with excellent cure (curing more opaque colors better).
Are available in a wide range, including UV flexo, UV screen, flexo varnishes, adhesives, metallic inks.

The future

UV LED technologies are a strategic and significant progression into the future of printing. This is a new generation of technology, which provides increased productivity with less energy consumption while reducing a printer's carbon footprint.

Narrow web printers are entrepreneurs who are always looking for the next, and the best, technologies to move them and the industry ahead. UV LED technologies will enable these savvy printers to move into new applications and experience better operational performance. UV LED will have a significant impact on our industry.

About the author—Niklas Olsson is the global brand manager, narrow web for Flint Group – Packaging and Narrow Web. pP