Introduction

LED curing solutions are utilized in the coatings industry because they offer highly reliable UV curing due to being a solid-state device. They provide deep through-cure and allow low surface temperatures that enable less expensive raw inputs. 

UV curing technology with light-emitting diodes (LEDs) is considerably more eco-friendly and resource-saving compared to the traditional technologies. The disposal of heavy metal lamps made of gallium and mercury is no longer required. Working conditions for the line operators are improved as there is no ozone formation and an exhaust system is thus not necessary, which could lead to higher noise levels. The lower energy consumption of LED light sources contributes to the protection of resources. Figure 1 illustrates the LED curing process.

Characteristics of UV LED Light Sources

Traditional UV lamps produce UV energy by generating an electric arc inside an ionized gas (typically mercury) chamber to excite atoms, which then decay, emitting photons. The photons emitted cover a broad range of the electromagnetic spectrum including some infrared and even visible light. Only about 20% is typically useful for UV curing.

A UV LED generates UV energy in an entirely different way. As an electric current (electrons) moves through a semiconductor device called a diode, it emits energy in the form of photons. The specific materials in the diode determine the photon wavelengths and, in the case of UV LEDs, the output is typically in a very narrow band +/-10 nm. Figure 2 compares the output of a 395 nm LED light source with a typical traditional UV lamp. It is important to note the difference in intensity and wavelength of the output; both are key to understanding a UV curing process.

Formulating Chemistries

For efficient and effective UV curing of a coating, the formulator seeks to overlap the UV light source output with the spectral absorption of the photoinitiator (PI). The amount of PI in a typical UV formulation is usually very small, less than 5%. PIs typically absorb across a range of wavelengths, not a narrow band. Many existing UV formulations developed for curing with a typical UV lamp use a broad spectrum PI. While there is often some absorption within the LED output range, it is clear that much of the PI absorption range is wasted. A more efficient cure is possible with a formulation designed specifically for LED curing using a PI with more concentrated absorption in the UV-A range.

New LED Innovation

Last year, Sherwin-Williams announced a new LED innovation aimed at manufacturers within the wood finishing industry – Full Curing LED. This breakthrough technology makes it possible to implement LED throughout the entire curing line, allowing for even more energy savings and quality improvements than before.  

“We´ve been working toward a solution like this since we first introduced LED curing in 2012. However, there was always a problem with unwanted yellowing effects due to difficulties with the topcoat. For a while, it seemed that a combination of LED and UV lamps was the only way to go, but we pushed forward and the R&D division finally came up with the right formula to make it work. It’s now possible to use LED lamps in every step of the curing process, eliminating the need for conventional UV lamps altogether. This will allow for a much more effective production”, says Lars Sandqvist, Sr. Technical Application Manager at Sherwin-Williams.
   
Due to the low curing temperatures of LED, it´s possible to coat materials such as pine and other resinous woods with significantly lower reject rates. The LED lamps also last thousands of hours, which decreases the overall maintenance cost compared to using conventional UV lamps and, without a warm up cycle, there is less down time. Additionally, LED lamps do not produce high temperatures or harmful ozone, which reduces the environmental impact. Several companies worldwide have already implemented LEDs in their curing lines and there are a variety of LED solutions available today, with interest booming.

“Our participation at the LIGNA exhibition last year really reinforced our belief that the market is ready; LED was everywhere. It´s only logical that we take this last step to facilitate a 100% LED curing line. Since we conducted this as an internal project without any client involvement, we now have to test Full Curing LED together with our machine suppliers and present it to our current LED customers. We´re very excited about this”, says Lars Sandqvist.

LED Technology Benefits

Less Heat
With no IR heat, LED curing technology significantly reduces work-piece surface temperature. With the ability to cure heat sensitive substrates, LED technology can add new capability and higher yields to coating applications. 

More Uptime
There is no warm up time required and less downtime due to the instant on/off of LEDs.  LED offers higher consistent yields and reduced waste compared to traditional UV curing. There are no reflectors to clean or shutters to replace. With no moving parts, the maintenance is minimal.

Greater Stability
Because of the uniformity and long-term consistency of LED lights, designers can develop safer, more stable, and more reliable processes. LEDs last more than 20,000 hours of operation, with only a minimal drop-off in power over the life of the curing unit.

More Compact Designs
Floor space is valuable in all types of businesses. LED light sources can be used to build more compact coatings lines, with shorter web paths that minimize waste.

Better Cure
Users report that LED light sources produce better cures and better adhesion on a wide range of materials. The longer wavelength output—such as the UV-A range seen from UV LEDs—penetrates through thick and pigmented systems producing through-cure of the material that ensures surface adhesion and the ability to cure pigmented wood coatings.

Energy Consumption 
Users can save up to 70% or more on energy bills.  Energy cost savings can be substantial, particularly if energy costs continue to rise.

Safer Workplace
LEDs contribute to workplace safety because they don’t generate dangerous UV-C radiation, excessive heat, or noise. The UV-A wavelength light is not harmful to the eyes.

Wood Coatings

One of the original drivers for developing LED wood coatings was to extend the use of UV curing to heat sensitive wood substrates such as pine, fir, spruce, mahogany, and other resinous, oily woods. Traditional UV lamps can cause problems on heat-sensitive wood materials. When a resinous wood such  as pine gets too hot, the resins or pitch come to the surface, or “bleed,” causing problems with coating adhesion and discoloration. For example, if the surface of a pine board exceeds 50 °C, the scrap rate increases significantly. Properly drying resinous wood is an important factor for successful UV coating. The inconsistencies of wood products from pallet to pallet and even board-to-board, make it desirable to find a lower-temperature UV-curing solution that will enable consistent finishing and coating results, and possibly enable the use of less costly lower-grade woods.

Edge Coating
Edge coating lines utilize LEDs to ensure consistent, high-quality results. Machines can be made more compact due to small form factor; speed can be increased due to consistent UV output; and the diffuse nature of LED light can be used to more effectively cure shaped surfaces which previously required multiple mercury lamps at various angles.

Roller Coating
LED is a perfect match for roller coating lines, both for gelling and full-cure stations. The benefits include better factory floor utilization due to shorter, more efficient lines; increased uptime with no degradation in UV output; less expensive input stock due to lack of infrared heat; and reduced operating cost by more efficient electricity use and no need for costly air ducting systems.

Digital Printing
While technically not a wood application, creating a wood-grain look on diverse materials is made possible with LED and ink-jet printing. By using a combination of pinning and full-cure light sources, realistic ‘look’ and more importantly ‘feel’ is enabled by creating texture to mimic the grains of natural wood. This is especially valuable in decorative and accent applications.

Automotive Applications

UV curing has been used for years in several areas of the automotive production process. UV provides near-instant curing of functional and decorative coatings. And the lightweight materials used in today’s automobiles benefit from the low temperatures enabled by LED technology.

Industrial Manufacturing

Countless manufacturing processes make use of the benefits of LED curing. Applications range from manual curing of the adhesives used during sub-component assembly, to high-speed curing of coatings and colorings of the finished products.

Conclusion

LED-based curing is becoming more widely accepted for industrial coating applications.  The coatings industry continues to challenge and drive the material formulation suppliers with LED wavelength optimized materials.  At the same time, LED curing technology has become more efficient in delivering higher energy to the media, thus driving environmentally clean, energy efficient and compact size units with increased throughput and process flexibility.