UV-LED light curing in the field of wood coating

one. UV-LED light curing profile:

Radiation Curing is a process in which chemical formulations (coatings, inks, adhesives, etc.) are instantaneously converted from liquid to solid by means of energy irradiation. Since the energy used is often a UV-visible (ie UV light) source, it is also commonly referred to as UV curing or UV technology. UV-LED photocuring refers to the conversion of liquids such as inks, paints, paints, slurries, and adhesives into solids using light from LED sources. The LED curing technology is developed from the UV curing technology. The LED light source utilizes the photoelectric conversion principle, and the electrons and positive charges in the chip are converted into light energy by collision and collision during the moving process. LED has a low power consumption, long life, miniaturization, light weight, transient response, high output, no mercury, no ozone, etc., known as "a trump card to solve environmental problems." As an energy-saving and environmentally-friendly light source technology, the LED industry has become a national strategy of various countries and is widely used in the light curing industry. Table 1 compares the technical characteristics of LED curing and UV curing. It can be seen that compared with the conventional UV system, the LED curing system has the following advantages:

(1) Low energy consumption, only 1/4 of the power consumption of UV, greatly reducing energy consumption and CO2 emissions;

(2) The radiation amount is controllable, the calorific value is small, and the heat resistance of the mold or the like is poor or the printing heat of the thermal printing substrate is small;

(3) The long life, the LED light source is about 10 times longer than the life of the UV light source, the frequency of light source replacement is greatly reduced, and the equipment cost is reduced;

(4) It can be turned on and off instantaneously, without the preheating and cooling time necessary for UV curing, which improves the efficiency;

(5) Ozone-free production improves the working environment of workers and eliminates the need to install capture and incineration equipment to eliminate ozone hazards;

(6) The device is compact, simple to set up, and saves space.

Table 1 Comparison of technical characteristics of LED curing and UV curing

two. The development trend of wood coating technology

The haze, PM 2.5 and VOCs under the dome have seriously affected the lives and health of the people. In today's rapid economic development and rapid technological changes, environmental issues cannot be ignored, and environmental governance cannot be delayed. During the 12th Five-Year Plan period, the coatings industry developed rapidly and the total production value surged at a double-digit rate. According to the National Bureau of Statistics, total coating production increased from 9.66 million tons in 2010 to 17.18 million tons in 2015. At present, the market share of water-based coatings in Europe, America, Japan and other countries has reached 60%, while the proportion of China is obviously low. In addition to the widespread application in the construction industry, other areas are still dominated by solvent-based coatings. China is the world's largest paint production and consumption country, but the quality of coating products is low-end, serious homogenization, the domestic market is still immature, the international market is not competitive enough, overcapacity, and the industrial structure needs to be adjusted. According to the statistics of authoritative departments, the paint coating industry emits about 7 million tons of VOCs into the atmosphere every year. The paint coating industry is duty-bound to carry out VOCs emission reduction and prevention.

At present, the state has taken heavy measures to control environmental pollution. Relevant policies and laws such as “the most stringent environmental protection law”, “oil ban”, “paint consumption tax” and “oil to water” have been promulgated and implemented successively, and national environmental awareness has also been significantly improved. Transformation, furniture coating, such a key governance industry, urgently needs to be transformed, and taking the road of green energy conservation and environmental protection is an inevitable choice. Current environmentally friendly coating technologies include aqueous heat curing, 100% UV light curing, waterborne UV light curing, and waterborne LED light curing. China's water-based paints have been developed for decades, and they are always tepid. Due to their stricter requirements on technology and production processes, the cost is higher, and most domestic water-based paints do not meet the same coatings as traditional solvent-based paints. The effect of the installation, the painting process is affected by the construction environment. Water-based coatings must truly occupy the market. First of all, it is not a cost issue. It is urgent to upgrade and break through in the core technology research to ensure that the coating effect is comparable or surpasses that of oil paint. Even though some domestic key city governments have promoted the transformation of furniture coatings from solvent-based coatings to water-based thermosetting products by directly subsidizing enterprises, the relevant manufacturers still have no intention to adopt this technology. Because the wood furniture industry faces the following common technical problems:

1. It is very different from the highly regular plane parts such as wood flooring. The wood furniture lacquers vary in size and size, and often contain three-dimensional complex three-dimensional structures such as carved hollows. This status determines the wood furniture. Painting can only use spray-type coatings as a universal technical construction scheme, and only partially structured parts can be processed by UV curing. Because only a small part of the furniture painting involves flat coating, this explains why all the furniture companies that have launched the UV production line are under-utilized, but they are actually idle.

2. In the spraying technical scheme, if the manufacturer chooses the water-based one-component or two-component heat-curing coating paint, the inherent technical weakness of the thermal curing itself is insufficient, which inevitably causes the hardness of the paint film to be low, with a pencil The hardness is generally measured in HB or below, and the water-based heat-curing paint is a slow-reacting curing, that is, it must wait for a long time after painting, and the paint film can reach a moderate degree of sanding, thereby entering the next construction process. This means that the film is dried either in a drying and dust-cleaning operation room with controlled conditions and high investment costs, or has to “see the sky” because the bad weather humidity and air dust content directly determine the film. Curing efficiency and performance; Today, many furniture and wood industry practitioners have generally accepted from the mentality that water-based heat curing coating technology is impossible to get a practice plan;

3. In the spraying technical scheme, if the manufacturer chooses UV curing, that is, 100% solid content UV curing, it is necessary to use a low viscosity acrylate type monomer and resin composite system formulation. Such a system is in the spraying operation. Since the acrylate monomers themselves are also highly volatile VOCs, not only are the unpleasant smells strong, the air pollution is large, and due to the skin or physiological irritancy of the acrylate compounds, the manufacturers are forced to use only the robots in the highly protected space. Construction is carried out. For large-scale construction of shaped objects, it is completely unacceptable for the manipulator to be able to perceive the three-dimensional shape of the object in a timely and efficient manner;

4. In the spraying technical scheme, if the manufacturer chooses water-based UV curing, due to the characteristics of the UV light source, on the one hand, it still cannot cope with the problem of insufficient curing of the irregular curved substrate. On the other hand, the UV light source is often medium-high pressure mercury. The lamp needs more than half an hour to warm up during work and the short-wavelength ultraviolet radiation in the work inevitably stimulates the oxygen in the air to generate a large amount of harmful ozone (O3) species. Therefore, the entire production line has to be shielded from the exhaust, resulting in "environmental protection." The result of energy saving, at the same time, it is more important to pay attention to the fact that the UV light source has a high starting voltage. In the high humidity and high dust environment of furniture factories, it is easy to cause voltage breakdown, which is very labor-intensive furniture enterprises. It is easy to cause major safety accidents such as mass casualties. In addition, UV light sources generally have a low life expectancy and usually have a service life of only about 800 hours. Therefore, the environmental treatment of a large number of mercury-containing scrap lamps is also a real headache. All of the above reasons explain why water-based UV coatings have never been able to achieve technology in furniture manufacturers.

LED surface light source is cured by water-based paint, and is still essentially a photo-curing curing technology. Therefore, the light cross-linking density is high, the paint film is dense, and the hardness of the paint film of 2H-3H or above can be easily achieved, and The high gloss and high fullness characteristics result in excellent wood protection and aesthetics, which is an unmatched advantage of waterborne thermal curing technology. Water-based LED light curing is first of all low cost, followed by energy saving and environmental protection. It is the ultimate environmentalist of wood furniture coating. This technology not only overcomes the shortcomings of traditional water-based heat-curing coatings, but also gives users the much-needed Construction freedom and effect characteristics, combined with low cost, energy saving and environmental protection advantages, have been promoted and applied in Shandong Wanjiayuan, Shandong Huari, Nantong Taisen, Shanghai Aurui and other furniture leading enterprises. The large-scale demonstration and promotion of this innovative technology will truly solve the pollution problem of the traditional furniture industry from the source, and more importantly, improve the production efficiency and economic benefits of China's furniture industry while saving energy and reducing emissions.

three. The key to waterborne UV-LED light curing coating technology:

Traditional mercury lamps are continuous spectra, emitting ultraviolet light from 200nm to 400nm. Currently, LEDs emit ultraviolet light only in several narrow bands such as 365nm, 385nm, 395nm, and 405nm, and the spectral width does not exceed 40nm. The key to the application of LED curing technology in the field of wood coating is to develop coatings. In order to adapt to the spectral characteristics of LED curing devices, the technical development of LED coatings is very demanding. The emission wavelength of the light source and the light transmission window and photoinitiator of the pigment. The absorption spectrum should be matched: on the one hand, the absorption peak of the photoinitiator is preferably in the "light transmission window" of the pigment; on the other hand, the photoinitiator needs to have stronger absorption ability and higher reaction efficiency, while absorbing The peak should match the emission wavelength.

Water-based LED light-curing wood furniture green coating technology, on the one hand from the material technology point of view, there are two key components of the compound water-based light-curing coating:

A. Waterborne urethane acrylate dispersion, generally having a solid content of 30-60%, is a photocurable main component, having a olefinic bond (C=C) density in the main chain, and a carboxylic acid (tertiary amine). Neutralization salinity, polyurethane bond (NHCO) hydrophilic group distribution pattern, end group functionality, and glass-transition temperature have direct effects on film formation properties;

B. Waterborne LED photoinitiator, generally added in 3-5%, but is the core of the key raw materials, due to the low emission power of LED light source (usually 2-8 watts / cm 2 power density) and narrow wavelength range, which directly determines A photoinitiator product suitable for the coating of furniture wood must have two key properties at the same time, that is, the initiator itself has sufficient water-solubility in pure water to be compatible with water-based resins and other functional additives. It is compounded into a highly stable coating product. Secondly, the UV-Vis absorption maximum wavelength of the initiator itself must be precisely aligned with the emission wavelength of the LED source, thereby maximizing the use of photon energy and in a highly sensitive manner. An active radical species that induces chemical polymerization is produced.

On the other hand, from the compounding technology, the key is to fully overcome the long-term technical problems in the industry of formulation compatibility and storage stability of water-based photocuring terminal coatings. Consider the chemical synthesis, solid content, molecular weight, acid-base neutralization and glass transition temperature control technology of waterborne urethane acrylate type resin dispersion, flexible-rigid polymer segment coupling, hydrophilic-hydrophobic segment coupling and resin Film formation effect, compatibility of aqueous resins with aqueous photoinitiators, formulation auxiliaries, aqueous pigments or dye dispersions. It is also necessary to take into account the water solubility of the aqueous LED photoinitiator, the initiator-resin compatibility, the initiator-agent compatibility, and the light absorption properties of the initiator in different wood furniture coating formulations. By studying the compatibility of a series of aqueous resin dispersions and aqueous photoinitiators in the presence of filming aids and pigments/dyes, photo-curing structure-function relationship, accelerated aging and storage stability of formulations, formulation Problems such as spray stability, ultimately determine the optimal design of the design and continuously optimize and improve the improvement.

In addition, the LED light source is the key to the green coating technology of LED light-cured wood furniture. It must not only achieve effective curing, but also be easy to operate. The size and shape can be customized according to the assembly line. LED as a point source for the lighting industry has long been in a fierce competition market pattern, but the LED surface light source system is very new because of the demand for dense chip cooling. Only in the past five years, the market is budding and the technology threshold is high. In view of the small size and dexterity of LED, the LED surface light source system can be processed into any shape according to customer requirements. It is especially suitable for the light curing operation of any irregular type of irregular parts. Figure 1 can be matched with the reciprocating spray machine as shown in Figure 2. It can also be used. The hanging wire is shown in Figure 3. The ground wire is shown in Figure 4. Compared with UV lamp energy saving and power saving, there are huge advantages (see Table 2):

four. in conclusion

Since the new technology market for water-based LED light-curing wood furniture painting, users can truly experience the following outstanding technological benefits:

1. Due to the realization of the source of VOCs emission control, the original factory can be rectified on site to meet the government's environmental protection requirements, without the need to relocate, resulting in large-scale impairment or waste of fixed assets;

2. Water-based LED curing paint uses water as a green medium to completely eliminate the environmentally-friendly and artificially healthy solvent release, which greatly reduces the investment in key monitoring facilities of government security supervision departments such as traditional spray booths, and ensures the factory is stable daily. Safe operation;

3. The performance of LED light-curing paint film completely surpasses the water-based one-component or two-component heat-curable paint film, and is completely comparable to the solvent-based paint coating effect, so that the end user's confidence in the use of water-based LED light-curing paint is significantly enhanced;

4. LED light curing is suitable for the coating of various shaped objects, greatly increasing the production efficiency of the factory, and the LED curing of the film is completed instantaneously (in the form of a strong contrast, the traditional solvent-based paint film can take a day to several days to dry fully). The traditional 3-day workload can be completed in just 2 hours after the introduction of the technology, which greatly reduces the labor cost of the enterprise and greatly improves the profitability of the enterprise;

5. LED light curing uses long-wavelength light source. The longer the wavelength, the better the penetrating power, so it is very beneficial to achieve one-time rapid curing of thick film paint film (20-40 microns), while the traditional solvent type is to achieve specific film thickness. It must be repeatedly cured by repeated application (only a curing film thickness of about 10 microns can be cured in a single pass), which further enhances production efficiency;

6. Water-based LED furniture paint is colorless, odorless and has no human health risks. First-line workers like it, and the labor cost is greatly reduced compared with the solvent-based paint production line;

Based on the above factors and practical assessment, the introduction of water-based LED coating technology can save downstream furniture coating manufacturers about 30-50% of operating costs, is a typical example of mining value through technological innovation.