by Paul Mills, contributing writer
One of the growing areas in UV coatings and printing is the growth of UV LED curing – a technology that offers great advantages. PostPress reached out to a long time consultant and expert in the field, Paul Mills, senior editor of the UV LED Curing Community website, to answer a few questions on the technology.
Q. What are some of the main benefits LED curing offers over more conventional UV curing?
Some of the advantages of LEDs (light emitting diodes) come from the “D” part, or diode aspects of emitting light. Semiconductors have replaced older technologies in so many areas. Old radios with vacuum tubes that burned out were replaced by transistor radios. TV sets with cathode ray tubes that were bulky, needed time to warm up and often had shrinking images as they deteriorated have been replaced by high-def LED screens. Even the incandescent light bulb, with its fragile filaments, has been replaced by robust LED lights.
For UV curing, LEDs are now replacing quartz tubes that rely on small drops of mercury to generate UV light. Mercury technology requires high voltages, generates large amounts of heat and the mercury emissions can be dangerous to worker health and safety if not properly handled. Mercury lamps, particularly the popular arc lamp variety, degrade every time the lamp is turned on and off. It takes a good deal of engineering to manufacture an arc lamp with very uniform output, and they require routine maintenance.
By contrast, LEDs are highly reliable, producing consistent output over extremely long time periods. They use less energy, produce less heat and the longer wavelength LEDs are relatively benign light sources. And, because they are solid-state light sources, they can be turned on and off nearly instantaneously with no degradation.
Q. There are a lot of decorative coatings out there today, such as raised profile UVs, textured UVs and glitter UV. Is LED the best choice for drying these types of specialty coatings?
It’s not so much the LED itself that makes it any more, or less, suitable for curing a specialty coating. From the coating’s point of view, the light source is a black box that emits photons, and molecules are pretty egalitarian when it comes to their photons. However, there are aspects of LED light sources that make it easier to control the photons. For instance, where mercury emits a broad spectrum, with multiple peaks of light spanning from 200 to over 400 nanometers, LEDs are like lasers in that they emit a strong peak of light at very specific wavelengths. This potentially allows chemists to perform some tricks when curing with LEDs. Also, since LEDs are tiny and an array can be put in spaces as small as those between inkjet heads, LEDs can “freeze” inks, keeping the ink dots from growing in size, or a process called “pinning”.
But it would be dishonest to say that every ink and press manufacturer is taking advantage of these capabilities. In fact, only a handful of leading suppliers are really tapped into the inherent advantages of LED. Many suppliers are just trying to substitute LEDs for arc lamps and treating the LED as a semiconductor version of a mercury light source. In that case, traditional light sources might even perform better, since the existing chemistry has been developed around mercury-based technology.
As competition for LED technology ramps up, firms that want to create more value with curing technology will embrace the unique aspects of LED to cure coatings with unique colors, textures and special effects.
Q. Is there a difference between installing a new LED system and converting a mercury-based system to LEDs?
If space aliens visited earth and had no prior knowledge of UV lamps, then adopting to LED would seem like a simple decision. But old habits die hard. An economist might call it switching cost, a physicist might call it inertia and a psychologist a habit. Changing paradigms is hard. For example, consider the tools and techniques used to measure UV lamp performance. With mercury lamps, broadband radiometers were the proper tool, since the mercury spectra are so broad. But a broadband radiometer can’t be used to measure a 395 nm LED and result in accurate measurements. That led to a lot of confusion about the claims of various manufacturers. Companies have now developed new devices tailored to measure these new light sources. So, for UV neophytes, the LED path may actually be easier than to those who have a long history with conventional UV curing who will have to make more adjustments.
Q. Is LED the answer to all curing/drying applications for coatings? When is more conventional UV curing still a better or, as good as, choice?
Someday most, if not all, applications will move toward LED curing technologies. But not yet.
Until now, there has been some rationale for sticking with arc lamps. LED sources were relatively expensive, particularly for larger curing footprints. Bu that gap has narrowed considerably, and in some cases LED can be less costly. LED capability has been correlated with output power, or irradiance. Years ago LED sources were 50 to 100 milliwatts. Today, 20 watt units are commonplace. These higher power units overcome many of the hurdles regarding cure speed, depth of cure and distance from the lamp to the substrate.
Finally, as alluded to above, advances in chemistry, which have been frustratingly slow, are beginning to emerge that will enable more LED applications. So, if arc lamps were to go away today, there would still be users with applications that would be stuck without an LED solution. In another five years there will likely be few curing applications that won’t be able to convert to LED, and the advantages of LED will make mercury lamps an obsolete technology down the road.
However, today the least desirable candidates for LED, and, thus, most well suited applications for conventional lamps, include: 1.) larger cure surfaces that are expensive to cure with LED arrays, 2.) coatings that require hard, scratch- and mar-resistant surfaces and 3.) surfaces with contours and profiles that present greater variability in distance from the part to the light source.
Q. What’s next in the future of LED curing?
The developments in the near future will be in two areas. First, LEDs are pushing the envelope to shorter and shorter wavelengths. These efforts are being driven more by germicidal/disinfection/food safety applications, but there will be clear spillover effects that will enable mixed-diode arrays to cure more coatings with harder surface properties than ever. One result of these developments will be the solid-state analog of the mercury arc lamp. This will mean losing some of the advantages of early LED sources, such as safer emission spectra. However, this is most likely inevitable.
The second area of development is in more LED specific chemistry that takes advantage of the novel characteristics of LEDs, such as special effects coatings that can be produced by high-speed strobing of the light source, or coatings that take advantage of the narrow wavelength spectra of LED sources. For example, coatings with light stabilizers that combat the deleterious effects of sunlight can be cured with LED sources whose emissions are not absorbed by the light stabilizer itself.
Paul Mills is the senior editor of the UV LED Community website. The site is a free, educational resource for UV LED chemistry (inks, coatings and adhesives), light sources, LED printing presses, and other UV LED related supplies and applications. The UV LED Community is operated by RadTech North America. Learn more at www.uvledcommunity.org