UV LED (Ultraviolet Light Emitting Diode) is a new UV light source that emits light from semiconductors. It can directly convert electrical energy into light and radiation energy. Compared with traditional UV light sources such as mercury arc lamps and microwave electrodeless lamps, UV-LEDs have many advantages. This new generation of UV light source has been widely used in the fields of UV adhesives, UV inks, UV coatings, and 3D printing. It has played a role in promoting energy savings, environmental protection, and pollution reduction. The applications of UV LED mark a revolution in the light curing industry.
About UV LED
LED Lighting Principle
Materials for making LEDs include compounds of III-IV group elements, such as GaAs (gallium arsenide), GaP (gallium phosphide), GaAsP (gallium arsenide phosphide), and other semiconductors. The core part is a chip, which consists of a P-type semiconductor dominated by holes and an N-type semiconductor dominated by electrons. There is a transition layer between the two parts, which is called a PN junction (Figure 1).
Figure 1: Schematic diagram of LED structure
To emit light, electrons are injected from the N region into the P region under the forward voltage. When the electrons and holes recombine in the P region, they release excess energy in the form of photons, and then the LED lights up. The wavelength and color of light are related to the materials that form the PN junction. Different semiconductor materials emit light of different colors.
Since the 1960s, the red light LED (650 nm) was first developed by using gallium and arsenic phosphide GaAsP. Then people used elements such as indium (In) and nitrogen (N) in semiconductor materials to create green light (555 nm), yellow light (590 nm), and orange light (610 nm) LEDs. When they developed gallium indium nitride (GaInN) successfully, they got a blue-light LED. They mixed blue, red, and green light to produce white light LEDs, and created RGB LED which covers the entire visible light spectrum. In recent years, researchers have successively developed short-wavelength semiconductor materials for a UV LED light source (405, 395, 385, 375, 365 nm, etc.). These materials include aluminum nitride (AlN), gallium nitride (GaN), indium gallium nitride (InGaN), aluminum gallium nitride (AlGaN), and aluminum indium gallium nitride (AlInGaN).
At present, UV LEDs of 395, 385, 375, and 365 nm are the mainstream lights for light curing on the market. With the progress of LED semiconductor manufacturing technology, inventors are developing deep ultraviolet LEDs with a wavelength less than 350 nm or between 250-280 nm.
Performance comparison between UV-LED and mercury arc lamps
The spectrum of UV-LED is different from the continuous spectrum of traditional mercury arc lamps. Its spectral distribution concentrates in a narrow band of about 10-40 nm bandwidth, and emits no UVB (280-315 nm) or UVC (200-280 nm) (see Figure 2 and Figure 3).
Figure 2: Emission spectrum of a mercury arc lamp
Figure 3: Emission spectrum of UV-LED
As a semiconductor light source, UV LED has many advantages over traditional mercury arc lamps, microwave electrodeless lamps, and other light sources. First, it has a long lifespan, high efficiency, and good safety. Second, it needs low voltage and low operating costs. Third, it generates little heat, no mercury, and no ozone. Therefore, it is an energy-saving and environmentally friendly UV light source. It is in line with the green economic policy on pollution and consumption reduction.
The overall system energy efficiency of UV-LED has an unparalleled advantage when compared with that of other light sources. It is 4-5 times greater than that of medium- and high- pressure mercury lamps and can meet the requirements of high-power applications.
Features of UV-LED
Advantages of UV-LED
(1) Long lifespan, concentrated energy output, and high energy conversion efficiency
The common 395 nm and 385 nm UV-LED lamps can have a service life of more than 20,000 hours. The 365 nm UV-LED lamp can also have a service life of more than 10,000 hours. In contrast, the service life of mercury arc lamps is only 800-1000 hours, and that of microwave electrodeless lamps is 8000 hours. The main wave peak of a UV-LED lamp is narrow and single. More than 90% of the light output concentrates in the range of 10 nm near the main wave peak. So its energy conversion efficiency is high.
(2) Low working temperature, no infrared heat radiation, and especially suitable for curing heat-sensitive materials
The temperature of the lamp body is below 100 ℃, and that of the lamp surface is about 60 ℃. While working, the temperature of the working surface only rises by about 5 ℃. It has no infrared heat radiation, so it will not cause thermal stress and thermal deformation of the workpiece. It is especially suitable for curing heat-sensitive materials. However, the surface temperature of the mercury arc lamp can reach as high as 600 ℃, and that of the working surface can reach about 80 ℃. It is not suitable for processing heat-sensitive materials.
(3) Instant light emission
The response time of UV-LED is at the microsecond level. You don’t need to warm it up. The number of openings and closings does not affect its service life and doesn’t require shutters. After turning on a mercury arc lamp, it takes 3-5 minutes to reach the complete spectral output. If you turn the lamp off, you cannot use it again immediately and must wait for 5 to 10 minutes for it to cool down. The number of times it opens and closes affects its service life.
(4) Low voltage, adjustable output power
UV-LED is a solid light-emitting device driven by DC low voltage, which is easier to control than mercury gas light. It is superior to mercury lamps in terms of luminous intensity, uniformity, and stability. The relationship between the output power of UV-LEDs and the driving current is stable. You can precisely adjust the UV output power by changing the current. The adjustment can correct for 1% or less. Mercury arc lamps require high voltage and rectifiers, and you cannot adjust the output power.
(5) No mercury pollution and no ozone production
Mercury is used in the production of mercury arc lamps, and the lamp tubes need to be recycled after they break down. It is prone to mercury pollution. Since 2013, a ban on mercury has reached a consensus in more than 140 countries and regions around the world. They signed the Minamata Convention to limit the use and emission of mercury in various industries. It aims to minimize the threat of mercury to the human body and the global environment. UV-LED does not use mercury, so the environmental protection advantage is very significant. At the same time, it doesn’t generate ozone. Therefore, it is imperative to replace the traditional mercury arc lamp ultraviolet light source with a UV-LED light source.
(6) Compact and versatile
The volume of a UV-LED is only 0.1 cm3, and it has a flexible configuration. As a point light source curing machine, it works under low DC voltage. The ultraviolet light emits with the help of the UV band optimized lens group. It is organized into a uniform circular spot, whose size is 3-20 mm. When using a rectangular irradiation head, it can form a rectangular irradiation area. When using line beam irradiation, it can form a slender linear beam with a wider irradiation area. You can also tilt the irradiation head by 45° or make the light emit from the side of it. In this way, UV LED greatly saves space and enables users to change it freely. It is suitable for narrow spaces.
UV LED curing machines
After permutation and combination, UV-LEDs become line and surface light sources for curing machines. The length of the line light source curing machine is 20-2000 mm. As for surface light source curing machines, you can customize the shape and size according to the light-emitting area. The irradiation uniformity of the surface light source is excellent. The variation in irradiation intensity between the edge and the center can reach 3%. UV-LED light sources can light up instantly, reaching 100% power ultraviolet output immediately. Its openings and closings won’t affect the service life. High energy, stable light output, and a good uniform irradiation effect improved production efficiency. It defeats mercury arc lamps because of these strengths.
At present, there are two ways to increase the irradiance of UV-LED curing light sources. The first is to add a lens with a converging effect in front of the single-tube UV-LED light. The second is to increase the quantity of it. The space array system design aims to realize the directional control and spatial accumulation of multi-tube UV-LED radiation energy. In such a manner, the irradiance of the light source can realize the UV curing of points, lines, and surfaces in various applications.
UV-LED first appeared in the form of a point light source. Later, by the space array combination, a UV-LED line light source and a surface light source emerged. Then people began using UV-LED light sources in various light-curing products. In addition, the arrangement of different arrays of UV-LEDs in one module is used to achieve outputs at different wavelengths. It can meet the needs of different types of photoinitiators in UV formulations. One module can contain five different wavelength UV-LEDs (365, 375, 385, 395, and 405 nm) to form a new light source wavelength region. Thus, it can be useful for different UV formula products.
Problems with UV-LED
(1) High cost: At present, the price of UV-LED point light sources is equal to or even slightly lower than that of mercury lamps. But for surface light sources using a large number of UV-LED array combinations, the cost remains high. This restricts the promotion and application of LED surface light sources. The reason for the high price of UV-LED is not only the high manufacturing cost but also the result of the technology monopoly of upstream manufacturers. Due to the patent protection, the price remains high. However, with the development of semiconductor manufacturing technology and domestic UV-LED packaging technology, the price of UV-LED will gradually decline. This will surely prompt the wide application of it.
(2) The wavelength of UV-LED is single, which doesn’t completely match with existing photoinitiators. At present, the commonly used UV-LED light sources are 405, 395, 385, 375, and 365 nm long-band ultraviolet light. Since the ultraviolet light emitted by UV-LED has a narrow peak, more than 90% of the light output is concentrated near the main peak. Almost all the energy lies in the UVA band, causing a lack in the UVC and UVB bands. It does not match the absorption spectrum of commonly used photoinitiators. It seriously affects the initiation efficiency of photoinitiators. Although the deep curing effect of UV-LED is excellent, the surface drying effect is not good.
(3) The power of a single UV-LED is still not large enough. The ability to overcome oxygen inhibition is poor, which affects surface curing. This is a big problem in the practical application of UV-LED light sources.
(4) The poor heat dissipation of UV-LED will accelerate the speed of light decay, and it is irreversible, which affects the service life. Therefore, we need to pay attention to the heat dissipation of it. Usually, water cooling or air cooling is used to cool it down. At the same time, UV-LEDs require an ambient temperature which must be lower than 35 °C, and a junction temperature which cannot be above 120 °C. In industrial use, sometimes the UV-LED curing machine cannot work because the ambient temperature is too high.
(5) UV-LED has only been used in the field of photocuring for around 10 years. Photoinitiators suitable for it are still being gradually developed. At the same time, there are only a few UV inks, UV coatings, and UV adhesives that are compatible with the characteristics of UV-LED. The development of these materials is urgently needed.
Applications of UV-LED
People first used UV-LED in the curing of UV adhesives. Due to its small size, excellent performance, and convenience of use, the UV-LED point light source curing machine is suitable for many industries. It emerges in the medical and health industries, optoelectronics, microelectronics, and information industries, as well as optical instrumentation, glass products, handicrafts, and jewelry industries.
Application in the printing industry
At the Drupa 2008 International Printing Exhibition held in Düsseldorf, Germany, in May 2008, three Japanese companies launched UV-LED printing systems, using the UV-LED developed by Toyo Ink. The live demonstration of the ink has opened the door for the application of it. At present, it has been applied in offset printing, flexo printing, screen printing, inkjet printing, and other printing fields. You can also find its use in the solder mask exposure process of producing printed circuit boards (PCBs).
Application in Optical Rapid Prototyping
At present, there are two categories of ultraviolet light sources used in photocuring rapid prototyping equipment. One is the high-end rapid prototyping equipment, mostly using ultraviolet lasers. The other is low-end rapid prototyping equipment using ultraviolet lamps.
Lasers have the advantages of high brightness, great directivity, monochromaticity, and coherence, making them an ideal light source for material processing. The common ultraviolet laser sources in rapid prototyping equipment include helium cadmium (He-Cd) laser (325 nm), argon ion (Ar+) laser (351-364 nm), N2 laser (337 nm), diode-pumped Nd:YOV4 three-frequency doubled laser (355 nm), etc. However, the price and maintenance costs of the laser system (including the laser, cooler, power supply, and external optical path) are expensive. This results in high manufacturing and use costs for rapid prototyping equipment. It limits the promotion of UV-curing rapid prototyping technology to a certain extent. On the other hand, UV lamps occupy the low-end market of UV curing rapid prototyping equipment with their price advantage. Although the cost of UV lamps is low, their service life is short, and the beam quality is poor. Besides, there is some pollution in the environment.
Nowadays, the UV-LED material growth technology and the preparation process keep developing. The luminous efficiency of commercial UV-LED has increased by an order of magnitude almost every ten years. The successful development of high-power UV-LED has promoted its application. Compared with traditional light sources such as lasers and mercury lamps, UV-LEDs have many advantages. These inherent characteristics determine its high cost-performance ratio, and the light-curing rapid prototyping industry makes full use of them.
The new light-curing forming system (LED-SLA) developed by Xi’an Jiaotong University uses high-power UV-LED as the light source. The ultraviolet light is converged onto the liquid surface of the photosensitive resin through the focusing mirror. The mechanical x-y table drives the focusing mirror to scan on the resin liquid surface to cure it (see Figure 4). Through comparative experiments on the energy consumption of three curing light sources, UV-LED, laser, and high-pressure mercury lamp, we can find that the light curing energy consumption of UV-LED is only 0.86% that of laser and 0.1% that of mercury lamp. It proves that LED-SLA has outstanding energy-saving advantages. However, SLA adopts the point-scanning method, which has a slow curing speed and low efficiency.
Now experts apply new methods such as LCD imaging technology and digital imaging technology (DLP). These help change the cross-sectional graphics into a dynamic mask. Then they expose and cure the entire layer of photosensitive resin. The view generator transforms the original point-scanning light-curing forming into surface-exposure light-curing forming, and one exposure cures a whole layer. This greatly speeds up the forming procedure. The surface forming SLA method not only has a fast forming speed but also shortens the curing time. The efficiency increases, yet the operating cost is lower.
Moreover, the light source transforms from an upper-mounted type to a lower-mounted type. It makes the equipment more compact and more convenient to use. In particular, the amount of photosensitive resin in the curing pool is greatly reduced. The manufactured model part will not swell and affect the accuracy when soaked in the resin liquid for a long time.
Figure 4: Schematic diagram of the LED-SLA system principle
① Computer ② Programmable power supply ③ X-Y workbench ④ LED light source
⑤ Focusing lens ⑥ Component ⑦ Resin level
Application in the coating industry
The painting industry is also an area where UV-LED is entering. At present, it has been applied to paper glazing and wood coating. Shenzhen Youwei Chemical Technology Co., Ltd. took the lead in developing water-based UV-LED coatings for wood decoration. It is not only helpful in the coating of furniture, wooden doors, cabinets, etc., but also in high-end fields such as interior decoration, containers, subways, and high-speed rail cars.
Application in the nail art industry
With the improvement of living standards, wearing nail armor has become a fashion for women to pursue beauty and enjoy life. The new technology of “phototherapy manicure” is to apply UV nail polish and cure it under ultraviolet light to form a protective coating. The traditional “phototherapy manicure” uses conventional mercury arc lamp irradiation to cure UV nail polish. The operation time is long, usually more than 6 minutes, and the mercury arc lamp emits light with obvious infrared heat. There is a clear burning sensation during manicures, and it may also damage the nail bed. Now people use UV-LED light source curing to provide safer, time-saving, and convenient UV-LED “phototherapy nails” .
Summary and outlook
The rapid development of UV-LED manufacturing and the advantages of it have attracted great attention from people who engage in the light curing industry. Everyone has given a very high evaluation to this technology. With the continuous promotion and application of UV-LED technology, it will become predominant in the light curing industry. As the luminous power of UV-LED increases and the cost decreases, experts are continuously developing new photoinitiators suitable for it. Its application will be more extensive in the future. A UV-LED light curing system will gradually replace mercury arc lamps in the photocuring of various UV coatings, UV inks, and UV adhesives. It has great market prospects and will become one of the most influential semiconductor products.
