Product
As the public's attention to environmental protection and sustainable development increases, green and environmentally friendly technologies are receiving unprecedented attention. Since December 1, 2020, the limit standards for the content of volatile organic compounds (VOCs) in chemical products such as cleaning agents, coatings, adhesives and inks have come into effect.
Photocuring technology, as a green, environmentally friendly and efficient coating curing technology, uses ultraviolet light to achieve curing and has experienced rapid development in the past few decades. Its basic principle is that under ultraviolet irradiation, the photoinitiator absorbs light energy of a specific wavelength to produce free radicals or cations, thereby initiating polymerization and cross-linking reactions of monomers and oligomers, quickly forming a high-molecular polymer with a network structure, and completing the curing process.
At present, among UV curing light sources, the use of traditional medium-pressure mercury lamps (known as high-pressure mercury lamps in China) is more common, although today it still occupies more than half of the market share. The main emission wavelength of the medium-pressure mercury lamp is 365nm, and there is also intensive emission output near 254nm and 208-331nm. However, mercury lamps have several defects:
1) Short service life, usually only 1000 to 2000 hours;
2) Light intensity will gradually decay, affecting the stability of the curing effect;
3) High energy consumption, high heat generation; long startup time, need to preheat;
4) Large size, difficult to install in space-constrained occasions.
In addition, a fatal weakness of mercury lamps is that they must use mercury, a substance that is harmful to the environment.
This problem faced by curing equipment did not fundamentally bring revolutionary changes to the light curing industry until the emergence of UV LED technology, and gradually showed a clear trend of replacing traditional technologies.
According to wavelength, ultraviolet light is usually divided into three bands: UVC (100-280nm), UVB (280-315nm) and UVA (315-400nm), as shown in Figure 1. The ultraviolet light emitted by traditional mercury lamps covers these three bands, and through doping (such as iron, gallium, etc.), the intensity of the spectrum in different bands or specific wavelengths can be adjusted to meet different curing application requirements.
The emergence of UV LED technology has brought new development opportunities for radiation curing technology. UV LED light sources are solid light sources and do not involve harmful substances such as mercury. UV LED light sources have the advantages of no preheating, instant use, no mercury pollution, relatively small equipment size, energy saving, long service life (up to tens of thousands of hours), no ozone generation and no air pollution to the environment, and the emitted light does not contain infrared parts, which is suitable for heat-sensitive substrates.
These advantages have accelerated the application and promotion of UV LED in the current environment that emphasizes energy saving, environmental protection and VOC control, so it has been rapidly developed and popularized in the past 10 years. UV LED curing technology has replaced traditional mercury lamp curing in many application fields such as adhesives and inks and has become the mainstream.
The most common UV light-curing products are UV coatings, UV inks and UV adhesives. Their curing speed is extremely fast, usually between a few seconds and tens of seconds; they are suitable for a variety of substrates, such as paper, wood, plastic, metal, leather, stone, glass, ceramics, etc., and are particularly suitable for heat-sensitive materials (such as paper, plastic or electronic components).
UV curing light source parameters include output wavelength distribution, irradiation intensity, irradiation area, etc. Since most of the standards related to photocuring are based on the traditional mercury lamp light source system, there are relatively few standards for photocuring technology based on UV LED light sources.
At present, the curing market is the main application field of UV LED, and the acceptance of each market segment continues to increase. The market penetration rate of UV LED in the field of radiation curing is about 10%-20%; among them, surface curing occupies a dominant position in the UV LED curing market, accounting for about 90%, and point-shaped UV LED irradiation curing is mainly used for UV bonding, while linear UV LED irradiation curing is less used due to the low irradiation energy density of the light source.
In the field of ink curing, the market mainly uses UV LEDs with wavelengths of 365 nm, 385 nm, and 395 nm for printing curing. UV LED light sources have shown varying degrees of penetration in the fields of exposure curing, inkjet printing, screen printing curing, offset printing curing, etc. In printed circuit exposure and inkjet curing, UV LED has basically replaced mercury lamps and has been widely used. In the fields of offset printing and screen printing, the replacement rate of UV curing light sources is less than 20%.
In the field of coating curing, the promotion of UV LED light sources is relatively slow, and the market penetration rate varies depending on the application field. In the optical fiber coating market, the penetration rate of UV LED curing market exceeds 20%; wood curing application is still in its infancy.
In the field of adhesive curing, UV LED light sources have basically achieved complete replacement in the field of electronic components and medical consumables curing; in the curing application of wood, automobiles, etc., it is in its infancy.
In the field of consumer curing, represented by nail curing, the market mainly uses UVA LED with a wavelength of 395 nm, which has basically achieved full industrial chain replacement.
Overall, the existing UV LED wavelengths are poorly matched with the absorption wavelengths of mainstream photoinitiators, and the supply of ultraviolet light source products in the range of 300 nm-330 nm is insufficient. As supporting UV LED formula products, they account for a relatively low proportion in the market of ultraviolet light curing formula products, and have great room for development in the future. Although UV LED curing applications are relatively mature, their large-scale industrial development is still in its infancy. On the other hand, research on adaptive materials for UV formulas is also actively underway, including long-wave absorption high-efficiency photoinitiators, long-wave high-reactivity photoinitiators, and high-efficiency antioxidant inhibitor additive materials that adapt to UVA LEDs.