A Brief Talk on UV-LED Components and Curing Technology Application

A Brief Talk on UV-LED Components and Curing Technology Application

Jul 1, 2020

According to the "2019 Deep Ultraviolet LED Application Market Report-Sterilization, Purification and Water Treatment Market" released by LEDinside of TrendForce, the global UV LED market reached 299 million U.S. dollars in 2018, and it is expected that the market size will increase by 2023. Reached US$991 million, with a compound growth rate of 27% in 2018-2023. Looking ahead to the application market demand, in addition to the stable growth of the curing market, surface/air sterilization, static water sterilization, and flowing water sterilization will be the main growth drivers in the future.

Ⅰ、UV market analysis

At present, the UVA industrial curing market has the largest demand! The future UVC sterilization and purification market has the highest growth!

Industrial curing is currently the largest application field of UV LEDs. In 2018, it accounted for 41% of the total UV LED market. Products in the industrial curing field are mainly UV-A LEDs. It is estimated that the mercury lamp use prohibition regulations ("Minamata Convention") will be fermented in 2020. The active introduction of UV LED products by industrial curing equipment manufacturers in Europe, America and Japan will drive the demand of the market for modification and replacement. The market penetration rate of UV LED industrial curing modules will also reach 30-40% in 2020, driving the growth of the overall UV-A LED market. In the next 5 years, the market size of the industrial curing field will continue to maintain rapid growth, second only to the sterilization and purification market. In terms of UVA curing, Refond's purple light technology promotes the following two directions:

UV offset printing direction

Compared with ordinary UV printing, LED UV offset printing is more energy-saving and environmentally friendly, has a long life span, can reduce energy consumption by up to 70~80%, and has outstanding adaptability to printing materials.

UV wood coating industry

Compared with the traditional coating method, UV LED will not affect the moisture content of the substrate, and the construction process of UV LED fast curing is fully realized from the closed base, primer, color repair, and top coat, eliminating the need for traditional wood base , The long drying and curing time of the topcoat and the complicated process of low solid content and multiple spraying construction can realize full-line mechanization and assembly line operation, which greatly promotes the increase of factory productivity.

Ⅱ.Advantages of Refond UVA curing products

Introduction of UV-LED cooling system:

UV LED converts 15 to 25% of electrical energy into light, which is more efficient than traditional mercury lamps, but the remaining 75%-85% is still converted into heat. Therefore, the LED array must be kept cool.

For UV LED arrays, the cooling systems currently used are all air or water cooling.

Introduction of UV-LED secondary optical system:

Based on the final application requirements, the optical engineer must decide which shape, form and material should be used to best utilize the unique performance of the LED, and then also need to note that the LED is a floodlight form of light, unlike the mercury lamp mode The ultraviolet light is directed by the reflector to a specific point and has a certain focal length. We have simulated different secondary optical designs (see Table 4 below), and finally developed a patented secondary optics for offset printing (Figure 6) to solve the problem of insufficient energy of UV light source in offset printing.

Refond (patent number: CN208906633U)-schematic diagram of secondary optical design of offset printing light source

Ⅲ. Review of problems affecting the effect of UV curing (printing)

Review-the problem of oxygen inhibition

The mechanism of UV curing is a free radical reaction in most cases. Although cationic polymerization is also one of the reactions and is not affected by oxygen inhibition, because its application accounts for a small proportion, we are not much discussion here. UV curing is through the generation of free radicals under the irradiation of ultraviolet light by a photoinitiator, and then these free radicals initiate the polymerization of the oligomer and the acrylate double bond in the monomer to produce free radical chain growth, so as to achieve the formula product changes from liquid to solid. The oxygen present in the air easily combines with free radicals to form peroxy free radicals. The reactivity of peroxy radicals is extremely low, and no polymerization reaction occurs again, thereby preventing the progress of photocuring.

The diffusion rate of oxygen in low-viscosity liquids (such as water with a viscosity of 1mPa•s) is 10-5cm2/s, based on different viscosities in typical UV resins, the diffusion rate is 10-6-10-8cm2/s. Usually the UV curing time is between 0.5 and 5 seconds, so the depth that oxygen molecules can penetrate is 0.1-10 microns. This hypothesis can be confirmed by the experiment shown in Figure 9.

One of the solutions: looking for higher irradiation intensity and higher energy density

It is a very simple and feasible method to increase the radiation intensity or increase the energy density. It can easily increase the amount of free radicals and consume oxygen to overcome the effect of oxygen inhibition, thereby obtaining a non-sticky cured surface. The disadvantage of this method is that there will be excessive irradiation, which will bring other problems. The influence of irradiance on the double bond conversion rate is shown in Figure 3. It can be seen from Figure 10 that when the irradiance is increased from 15 to 90mW/cm2, the conversion rate is greatly improved.

The selection and use of photoinitiators in UV-LED curing has also attracted more and more attention. First, choose a photoinitiator whose absorption peak matches the emission spectrum of the light source. It can be seen from Table 1 that the emission spectrum of the UV-LED light source is between 360-405nm, with the highest intensity at 365nm, 375nm, 385nm, 395nm, and 405nm, which belong to the long-wave region, and long-wave photoinitiators should be used first. After further testing, several photoinitiators with the highest absorption rates were found at 365nm, 385nm, and 395nm wavelengths. In terms of efficiency, DETX and EMK are the best photoinitiators for UV-LED light sources. The specific test results are shown in Table 5, Table 6, and Table 7.

One of the solutions:

Look for photoinitiators with stronger absorption and more stable photoinitiators. In terms of efficiency, we have found two good photoinitiators to solve the problem of UV-LED curing.

Ⅳ. Refond offset printing curing UVA full spectrum system light source installation cases:

Ⅴ. Summary:Refond Optoelectronics’ purple light technology uses its LED packaging technology advantages, adopts multi-chip eutectic integration (low thermal resistance) and vacuum packaging and other patented technologies, and combines the strengths of secondary optical design to create ultra-high UV light power output Offset curing light source system! So as to replace the traditional mercury light source, it is applied to the light source transformation project in the offset sheet-fed printing machine. Because of its many advantages such as power saving and environmental protection, it has gradually gained the recognition and favor of the printing industry!