By Cynthia Challener, CoatingsTech Contributing Writer

UV LED curing technology today offers numerous benefits in terms of sustainability, cost, and performance for certain applications—most notably graphics [primers and over print varnishes (OPVs)], wood fillers, fiber optics, electronic display coatings, and some non-graphic web coatings. The advantages being realized in these markets are generating interest from other sectors where UV LED curing technology has not reached the levels needed for commercialization. Interested end users are working closely with UV lamp/equipment manufacturers and coating formulators and their raw material (resin, additive, photoinitiator) suppliers to devise systems that will overcome current limitation and facilitate broader adoption of UV LED curing for industrial wood and other coatings, automotive/transportation, and many other applications.

Current State of the Market

Currently, UV LED curing is used most commonly for graphics applications, particularly for digital inkjet printing, screen printing, sheetfed offset printing, and flexo printing. These areas are growing rapidly, with the  fastest growth occurring in sheetfed offset printing, according to Scott Auger, global marketing manager with allnex. This is because a conventional press can be retrofitted quickly with little downtime or training while allowing flexibility to run or not run UV LED. “Sheetfed commercial printing is converting to UV LED for many reasons, but mainly for productivity, print quality improvements, and energy reduction; dry sheets of consistent quality are prepared instantly without powder, odor, or protective coatings and with brilliant, vibrant colors on any paper. In addition, spot, gloss, and strikethrough coatings are all possible with offset ‘on demand’ production,” he explains. Overall, Andrew Seecharan,market segment manager, Functional Packaging at BASF pegs the volume of UV LED inks and OPVs at approximately 5% of the market for energy curing inks and OPVs, or 3.3 kilotons, with the market growing at an estimated 4%.

UV LED curing is also used in optical fiber coating, electronics, and some wood coating and gel nail applications. “Today, most of the UV LED development for coatings has been in the graphics market (primarily OPVs and primers with some limited work in specialty), and fiber optics, wood filler coatings (not top coatings), and coatings for electronic assemblies,” says Jennifer Heathcote, global director of Business Development, Phoseon Technology. According to Dan Sweetwood, president and CEO of Allied PhotoChemical, when all applications are considered, the growth rate is estimated to be above 10% per year.

There are several applications for which UV LED curing has not yet been adopted. Few high-speed graphics applications have switched from conventional UV curing because the range of raw materials for formulating OPVs for high speed (> 700 ft/min) without high costs is limited, according to Seecharan. UV LED curing is also not used for thick-film, high-speed applications or applications where surface cure is a mandatory performance characteristic, according to Sweetwood. “Numerous industrial applications for UV coatings that require the lamps to be mounted at a distance (greater than 2 in.) or provide extreme functional or specialty photoinitiators used for these traditional UV coatings do not react in the longer UVA spectrum provided by UV LED,” she observes. Also, little work has been done with UV LED for industrial applications on parts with dramatic part profiles.

“UV LED is a new technology that is still evolving in supplier choice, capabilities, and cost.  Coating formulations need to be adjusted and tested for efficacy, especially in markets like automotive headlights that require long-term testing,” says Michela Fusco, global marketing manager at allnex. She notes that the largest installed base of UV LED lamps is in North America and Europe, and other regions have been waiting on the latest improvements to arrive before switching over.

Why UV LED Curing?

There are many good reasons to switch to UV LED curing if the technology can meet all of the requirements of an application. Like conventional UV curing with arc lamps or microwaves, LED curing allows the use of 100% solids formulations and provides fast curing with no thermal heating/ovens, increased production speeds, and a reduced footprint. Films are generated with excellent scratch and chemical resistance, and improved adhesion while waste and rejects are diminished. LEDs offer additional benefits, including a much longer lifetime over which marginal degradation of light output occurs, which is not the case for conventional lamps. LED lamps are instant on/off, reducing downtime. They consume 50–75% lower energy and provide for significantly reduced heat transfer to the substrate, allowing UV curing on heat-sensitive materials. Production speeds can be greater and floor space minimized further with LED curing. Because LED curing is based on solid-state technology, there are no moving parts and dramatically reduced maintenance requirements. The technology is also more sustainable, with no mercury-filled bulbs and no ozone that must be vented and managed. “Customers already using LED lamps have been able to validate the savings and, with lower investment costs, payback times are becoming more attractive, also in countries with lower energy costs,” observes Fusco. She adds that consistent output over time contributes to a more stable curing process improving quality delivered to market. Stable/consistent quality over a longer period also facilitates use in sensitive applications such as food packaging, where potential migration is a concern, according to Seecharan.

UV LED is strictly UVA and eliminates any safety issues related to the invisible UVB and UVC wavelengths, ozone, and mercury that must be dealt with when using conventional UV curing technologies.

In the wood coatings sector, LED curing is attractive because it may be used in conjunction with any application techniques currently in use for UV, including roll coating, vacuum coating, and molding spray, according to Brian Leonard, business development manager for Flooring with Sherwin-Williams Industrial Wood Coatings. The primary advantages over UV in the wood coatings sector are no heat transfer to the substrate; roughly half the energy consumption, thus lowering manufacturing costs; stable UV output across the width of the conveyor; no ozone emissions, which means no venting is required; and a long bulb life (approximately 20,000 h vs 1,500 h for arc lamps and 2,500 h for microwave lamps). “There is no particular wood market segment growing faster with LED, but roller coating (i.e., flat panel finishing) is the primary application,” he notes. Even if UV LED is only used to cure the successive wood fillers with the final hard top coat cured with conventional UV, the board surface temperatures can still be significantly reduced resulting in less scrap and a greater range of viable board materials, according to Heathcote.

The same regulatory issues/concerns that apply to conventional UV chemistry also apply to UV LED. However, there are sustainability advantages to UV LED curing equipment, according to Heathcote. UV LED is strictly UVA and eliminates any safety issues related to the invisible UVB and UVC wavelengths, ozone, and mercury that must be dealt with when using conventional UV curing technologies. For the automotive/aerospace industry, for instance, AkzoNobel has developed UV LED coatings that are cured only with UVA because it is considered less harmful than UVB and UVC energy conventionally used with mercury lamps, according to Emmanuelle Provost, group leader—Development Lab in the company’s Europe Specialty Coatings business. Heathcote stresses that it is up to the manufacturer to work with suppliers to ensure that the UV LED equipment is matched to the formulation and that process control is implemented to ensure sufficient and repeatable cure.

Requirement for Matched Solutions

In fact, just as conventional UV curing systems are not “one-size-fits-all,” LED curing solutions, comprising the lamps, curing equipment, ink/coating/adhesive formulations, and material handling requirements, must be designed to meet the specific needs of each application (substrate width, profile, and sensitivities; line speed; distance from the UV source) and the desired performance and functionality of the cured formulation, according to Heathcote. Photopolymerization via UV curing depends on the wavelength (nm), irradiance (Watts/cm2), and energy density (Joules/cm2) required by the formulation. Distance from the surface, the machine speed, and the chemistry of the formulation have a direct impact on these requirements. “While UV LED curing is a proven technology, not all UV LED products on the market deliver the same cure results. As a result, manufacturers should work closely with suppliers to determine what works best for their needs,” Heathcote says.

Today, the most numerous installations for UV LED curing are processes such as inkjet, fiber and display coatings, and spot-cure adhesives and sealants; however, UV LED equipment and chemistry are quickly expanding into faster-speed processes such as narrow, mid, and wide web flexo, and in some cases have exceeded 1,000 fpm. Many are designed for heat-sensitive substrates for which conventional UV is inappropriate or applications that benefit from the monochromatic energy generated by UV LED bulbs. They also tend to be flat or of shapes that can easily be positioned close to the lamps and for which formulations have been developed to cure effectively at the relevant wavelengths (365–405 nm) of UV LED lamps. “The current successes and remaining potential of UV LED curing are driving further investment in many other application areas to promote and leverage recent advances in lamp technology, resins/photoinitiator chemistries, and formulations and enable wider use of UV LED curing,” Heathcote says. “The list of viable applications continues to grow, including those for coatings,” she adds.

Manufacturers, however, are often risk averse and tasked by management to prove a strong business case before switching to UV LED, Heathcote notes. “Unfortunately, as we have seen in many cases, the numerous benefits to the bottom line are not always fully understood before implementation.  As a result, many companies that should be switching to UV LED have not done so due to the ROI calculation.  For those who have switched to UV LED, the payback has almost always been quicker than originally projected,” she asserts. Typically, companies with existing conventional UV curing equipment with years of useful life remaining are often less likely to invest in new LED systems that cost additional money and will require requalification of the curing process. For new machine purchases, however, or in cases where the conventional equipment needs to be replaced, UV LED will often present the better total solution cost. In some cases, while the technology may be feasible at slower press speeds or narrower webs, it occasionally cannot be implemented economically at the very fast production speeds and wide webs required for many coatings applications today. In either case, the technology is advancing quickly and implementation costs continue to decline, according to Heathcote.

Some of these applications will require further development and customization of the technology before they will be suitable for practical implementation. “Specific lamp designs must be engineered for the various applications and markets that incorporate wavelength, irradiance, and energy density profiles specific to the needs of the application.  The beauty of LED is that the lamp can truly be designed to deliver the exact energy needs of the application.  This makes UV LED technology incredibly efficient compared to conventional UV curing,” asserts Heathcote.

 The Problem of Surface Cure

While UV LED curing provides excellent through cure, there are some issues with surface cure that both formulators and equipment/lamp suppliers are working to address. Because UV LED curing occurs at specific wavelengths rather than a broad range of wavelengths, the photoinitiators and resins developed for conventional UV curing do not often provide cured films with the same properties using UV LED systems. “The current technology consists of a blend of photoinitiators that can

pose migration, yellowing, and formulation issues,” says Auger. He adds that there are even fewer options for water-based UV coatings and inks.

As importantly, UV LED systems on the market deliver a wider range of energy to the coating (depending on the system), and eliminate shorter wavelength UV.  The longer wavelenth energy penetrates deeper into the material for good through cure, but tends to result in increased oxygen inhibition at the surface due to the production of fewer free radicals, which can negatively impact productivity. “Formulating functional and specialty chemistry with hard surface cure across the full range of protection

qualities achieved with conventional UV still requires development, particularly for industrial applications,” Heathcote agrees. “Many industrial coating companies simply haven’t invested as much research time as the ink and graphics coatings companies and continue to promote current radiation cure chemistry. They will only begin to expand their development activities when existing and potential customers (converters, manufacturers, and large brands) start demanding UV LED alternatives,” she observes. Sweetwood adds that there is a lack of commercially viable UVC output LED systems that can overcome the issues with surface cure.

This issue is one reason why UV LED curing for wood coatings is limited to primer or filler coatings and often not used for topcoats. “There are limited raw materials available that respond to the LED wavelengths (405 nm, 395 nm, 385 nm, 365 nm) for surface cure. There are very few products that can fully cure using LEDs without discoloration; mercury lamps are often used at the end of a finishing process to ensure a full cure,” Leonard says.

UV LED equipment and chemistry are quickly expanding into faster-speed processes such as narrow, mid, and wide web flexo, and in some cases have exceeded 1,000 fpm. Many are designed for heat-sensitive substrates for which conventional UV is inappropriate.

Possible solutions to the surface cure problem include isolating the coating from oxygen in some way, increasing the amount of energy reaching the surface, increasing the photoinitiator concentration, or changing the photoinitiator/resin chemistry. However, isolating oxygen from the coating surface is very challenging and increasing the amount of energy often increases the rate of polymerization and thus negatively impacts through cure. Higher photoinitiator concentration can have a similar negative effect. As a result, efforts have been focused on tailoring photoinitiator/resin packages.

Other Hurdles to Adoption

This need to tailor the chemistry for UV LED curing has led to a slower rate of new formulation development for many of the more challenging applications and curing conditions, particularly with respect to high-speed applications, according to Seecharan. As mentioned earlier, the installed base of conventional UV curing equipment is also slowing the introduction of UV LED systems. Seecharan also notes that more operator training and growth of the knowledge base are needed with respect to implementation and use of UV LED to speed up adoption. Finally, as with any new technology, costs remain relatively high vs established UV inks, OPVs, and other systems. The business case has yet to be developed in many applications.

Yet Rapid Progress Ensues

UV LED lamp/equipment manufacturers and formulators and their raw material suppliers have been working diligently to develop solutions that will enable wider adoption of UV LED curing across more challenging applications. Lamps are increasing in power (over 20 Watts/cm) and offering lower wavelengths (as low as 340 nm from average 395 nm), according to Auger. UV LED systems are increasingly becoming more efficient, better tuned to the application, and more economical, according to Heathcote.  They are also now available in both narrow (less than 30 in.), mid (30 to 60 in.), and wide (greater than 60 in.) widths. Higher energy, air-cooled UV LED systems from Phoseon Technology, she adds, eliminate the need for a liquid circulation chiller which consumes power, takes up space, exhausts its own heat during operation, and has the potential for plumbing leaks and condensation at coolant temperatures below the dew point. While liquid cooled UV LED curing systems have their place and will continue to be offered, Phoseon is quickly responding to the market’s demand for higher powered, air-cooled alternatives, according to Heathcote.

Inerting of LED systems and the use of dual curing (LED and conventional UV) are having an impact, as is increased training opportunities through co-supplier webinars, at trade conferences, and in industry committees, according to Bryant.

Most of the recent advances are due to coating formulators better understanding how UV LED devices actually behave and emit UV output, according to Heathcote. “UV LEDs are quite different from conventional arc lamps. Simply swapping the photoinitiators or increasing the amount of photoinitiator to achieve better absorption in the UVA wavelengths is often not enough. Coatings suppliers have learned a great deal by experimenting in the lab and optimizing their formulations for what the UV LED lamps deliver. The biggest achievements in the graphics industry in the last year have resulted in increasing the line speeds on press that OPVs and primers cure to as much as 900 fpm while achieving sufficient surface cure and no yellowing. Most formulators have figured out how to prevent yellowing.  The recent focus has been on pushing the limits of press speeds,” she explains. Phoseon also continues to work on short wavelength LED devices in the UVB and UVC ranges.  “Unfortunately,” Heathcote says, “these systems are still incredibly expensive, deliver only a few hundred mW/cm2, and have a very short useful life (less than 1,000 h). Short wavelength technology will continue to evolve, but the industry is several years away from practical commercial solutions.”

Raw material suppliers have introduced new photoinitiators and resins to try to address issues of cure speed, oxygen inhibition (surface cure), discoloration, and stability on press, according to Seecharan. For instance, BASF has introduced Laromer PO 9137 and Laromer PO 9139 in Europe with registrations in progress for a North American introduction, and continues development work to further improve print speed, surface cure, and reduced yellowing/discoloration of inks/OPVs. Allnex has introduced EBECRYL LED 03 booster and EBECRYL 5850 reactive resin to allow printing presses to run only UV LED lamps at top speeds. “Introducing an LED booster as an additive in existing formulations or adding quantities of more reactive resins is the right answer in most cases to achieve efficient LED curing,” Auger states.

The key for Leonard is developing LEDs that work with shorter wavelengths to help with surface cure and make LED a complete curing solution while eliminating the need for specially designed raw materials. “If LEDs could work in the range of 280–340 nm, they would offer a viable total curing solution,” he comments. He believes this area will be the focus in the next few years. “If the bulbs can advance, the coating technology will advance to where we will likely see faster growth,” Leonard adds. Sweetwood agrees that the development of UVC LEDs for UV curing is being pursued by all LED semiconductor manufacturers.

Several approaches are seen by Heathcote as having value for advancing UV LED curing in other markets. First is leveraging what has been achieved for formulations for the graphics industry (UV digital inkjet, screen, flexo, and offset) and applying it to chemistry in other markets. Second is designing the UV system to fit the needs of the chemistry and application. Third is further investigating how the chemistry reacts to UV LED light. “Conventional arc lamps are broad band and emit up to 3 Watts/cm2 peak irradiance. Microwave lamps are also broad band and emit up to 5 Watts/cm2. UV LED systems are commercially marketed at irradiances as high as 20 to 50 Watts/cm2. Understanding how the chemistry responds to irradiance values so significantly higher than conventional UV is important. A higher irradiance is not always better and is sometimes detrimental.  Energy density and spectral output were always what formulators focused on for conventional UV.  Energy density is as equally important as irradiance.  A high irradiance may deliver a high-energy density, but in many cases UV LED products on the market that are specified at a high irradiance often don’t provide sufficient energy density to cure at fast line speeds,” she observes.

Wood Coatings

LED UV curing is attractive for industrial wood coating applications because it can be used for heat-sensitive wood substrates, including pine, fir, spruce, and mahogany. When resinous, oily woods get too hot, the resins or pitch can come to the surface and cause both discoloration and adhesion issues. There is great variation in wood composition, which makes it difficult to efficiently prepare these materials for coating. Lower-temperature UV LED curing is ideal for these woods and can potentially enable the use of lower-grade woods as well. For edge coating, UV LED makes it possible to use more company coating machines at higher speeds due to the consistent UV output. Roller coating UV LED lines are shorter and more efficient and benefit from reduced downtime and the ability to use less expensive stock. They also tend to have lower operating costs. Digital printing of the wood-grain look is also possible on many different materials. Even the texture of the grains in natural wood can be created for decorative and accent pieces.

Automotive/Transportation

As with the industrial wood sector, one of the biggest drivers for switching to UV LED curing in the transportation sector is the ability to achieve low-temperature curing. Lightweighting to boost fuel conservation is driving the use of plastics, composites, and lighter metallic alloys that can be sensitive to heat. Welding of these dissimilar materials is also not possible, leading to greater use of adhesives and sealants. UV LED curing can be ideal for this application as well, according to Heathcote.

Because UV LED curing is such a new technology in the automotive/aerospace sector, there is no clear estimate of the size of the market today, according to Provost. In the transportation sector, curing time is generally reduced by heating in ovens, although this approach is somewhat limited in the aerospace industry due to the large dimensions of many coated components. “UV LED curing is of growing interest as a means for reducing energy costs and shortening curing lines, but does represent an investment for most of our customers,” Provost says. Of particular interest is the use of inkjet for customization or fast application of complex drawings, he notes.

AkzoNobel has observed a transition from pure radical curing to hybrid systems. In response, the company is adapting its Autoclear UV system for car repair applications to meet the needs of aerospace customers for faster paint application of small pieces and airplane tails. Autoclear UV is a conventional polyurethane system that is UV catalyzed using a proprietary catalyst developed by AkzoNobel. The coating also cures without any UV light, which is important in the aerospace industry where shadow effects must be considered due to the shapes and dimensions of airplane parts. “We are now looking for customers willing to invest in UV LED curing systems. Several technologies have been proven but will not be implemented as long as customers don’t invest in this curing process,” Provost states. He adds that close collaboration with customers is important to the development of coatings that meet the requirements of their future LED curing lines.

Heathcote sees a number of potential commercial UV LED solutions being introduced to the transportation sector in the coming years, from screen-printed in-mold decorations for vehicle interiors to structural bonding of exterior components and light optically cured adhesives for electronic assemblies to printed appliqués and other markings. Longer-term, she expects to see UV LED curing of hard coats for in-mold decorating for vehicle interiors and coatings for mirrors, headlights, and the like. “Many of these applications are already achieved using conventional UV curing technology, and there is growing interest and exploration of UV LED solutions. Most of the development in this area is in fact driven by end users looking to find a way to meet their needs for lower-temperature curing using more sustainable technologies with lower operating costs,” Heathcote remarks.

Technology of the Future

UV LED curing technology has clearly been shown to offer many benefits, ranging from greater sustainability to greater productivity and quality. In many graphics markets, it is the preferred technology. In some cases, according to Heathcote, UV LED curing has been implemented where conventional UV curing could not. Even so, there remain many potential coatings applications that are not yet commercially feasible due to the need for advances in equipment and formulation chemistry designed to deliver functional cure properties at faster line speeds, greater distances, and for parts with complex profiles. All members of the UV LED value chain are tackling these issues head on and making rapid forward progress. As a result, adoption of UV LED curing will take place across a consistently expanding range of applications over the coming years. “UV LED curing technology is evolving rapidly in graphics, but all radcure applications even for consumer electronics and industrial plastics will move in this direction as the obstacles are removed,” states Auger.  Sweetwood, in fact, predicts that the development of LEDs will continue at a rapid pace, and this technology will become the curing equipment of choice within the next five to seven years.

CoatingsTech | Vol. 15, No. 5  | May 2018