1 development history
Since 1990, several publicly patented technologies related to UV-curable powder coatings have been reported in Europe. Pass one of 1995.
Some of them have been approved, and the notable features of this kind of patented technology are:
(1) No solvent, no VOC emission between coatings;
(2) The application range of coatings is expanded, and powder coating of heat-sensitive materials is realized;
(3) shortening the curing time of the coating and saving energy.
The first companies to promote the commercialization of this new environmental protection technology are:
UV curable powder coatings-Herbert, Sweden, now DuPont powder coatings company;
UV TRIAB powder coating processing equipment-Swedish company;
Uv curing coatings and product management-STILEXO company, uk.
The surface coating of medium density fiberboard is the most mature application of ultraviolet curing powder coating technology, which has more advantages than liquid coating in environmental protection.
1. 1 Application of UV curable powder coatings in various countries
STILEXO Company in Britain is equipped with the UV-IR dual curing device of TRIAB Company, and the powder coating of DuPont Company is used to coat the medium density fiberboard. This powder sprayed MDF is mainly used to make furniture, audio and office supplies.
1998, the British Baode Electronics Company completed the debugging of the motor photocuring powder coating production line and put it into production one year later. Many wiring materials inside the motor are easy to deform when heated, so it is an effective way to solve this problem by using ultraviolet curing powder coating.
During the period of 1999, Voleo company carried out technical transformation on the coating of the second primer slurry of the automobile itself and the radiator of the automobile, and both of them were coated with ultraviolet curing powder coating, which achieved good results. For automobile radiators lined with rubber materials, it is very beneficial to use UV curing powder coatings to avoid thermal damage of rubber materials.
1.2 Extension of application field
Theoretically, the application fields of UV-curable powder coatings are basically the same as those of UV-curable liquid coatings. Judging from the market situation in 2000, UV-curable powder coatings on plastic surfaces are very promising. Polyflor Company of Britain sprayed PVC floor with UV-curable powder coating. This powder sprayed PVC floor has the following properties:
Long-term sun does not fade;
The dosage of floor cleaner is reduced by 40%;
The surface hardness is remarkably improved.
2 coating characteristics of UV-curable powder coatings
The coating formed by crosslinking reaction initiated by ultraviolet radiation has the following physical properties, chemical properties and economic properties.
2. 1 physical properties
Surface hardness is improved (pencil hardness can reach 4h);
The wear strength is improved (1㎏ 200 times under load);
The tensile strength of the coating film is improved;
Bubble reduction.
2.2 chemical properties
Resistant to organic solvents (insoluble in methyl ethyl ketone);
Pollution resistance.
2.3 Economic performance
Less powder drops when painting;
Small environmental impact (no VOC release);
Low energy consumption for coating and curing;
Curing equipment occupies less space;
Short curing time;
You can draw a three-dimensional picture.
Resin components mainly include the following types:
Acrylic acid or methacrylic acid system;
Epoxy resin system;
Polyester or polyester epoxy mixed system;
Polyurethane system;
Vinyl ether system.
In the future, we look forward to the emergence of high-performance polymer systems, such as silicone and organic fluorine.
The curing speed of UV-curable powder coatings is greatly influenced by pigment components. For example, when titanium dioxide is used to prepare white ultraviolet curing powder,
The ultraviolet absorption wavelength of anatase titanium dioxide and rutile titanium dioxide is different, which will obviously affect the curing process of the coating. Figure 1 is the ultraviolet absorption spectrum of anatase and rutile titanium dioxide.
Fig. 1 UV absorption spectra of anatase and rutile titanium dioxide.
Photoinitiator and curing accelerator are important guarantees for curing of photocurable powder. BAPO (diacyl phosphine hydroxide), AHK (2-
Hydroxyketone is a newly developed photoinitiator and curing accelerator, and its application effect is very good. Fig. 2 shows the UV absorption spectra and chemical structures of AHK and BAPO.
The heating and melting process (infrared radiation) conditions of UV curing powder coatings are as follows:
Pipeline speed: 0.5 ~ 2m/min；;
Pipeline length:195cm;
Radiation intensity: 32kW/㎡;
Total length of heating furnace: 2m.
A typical infrared heating device is 2m long and equipped with five infrared lamps, according to the needs of different substrates (such as medium density fiberboard and iron plate).
The surface heating temperature is different.
The UV curing process conditions of UV curing powder are as follows:
Total length of curing furnace:155cm;
The pipeline speed is1~ 5m/min;
Power: 80 ~160 w/cm;
Ultraviolet lamp wavelength: 320 ~ 420 nm.
In order to make ultraviolet light pass through the coating with the thickness of 60 ~ 100μ m and realize the complete curing of the coating, an ultraviolet lamp with long wavelength should be used, and gallium and mercury should be sealed in the lamp tube during manufacture. Pulsed ultraviolet radiation has a wide wavelength range, average radiation intensity and high transmittance.
Technical progress of UV-curable powder coatings
The traditional medium-pressure mercury lamp and metal halide lamp used in the ultraviolet curing process of powder coatings have the following problems:
The coating with a thickness of 50 ~ 100μ m has poor light transmittance;
UV curable powder coatings filled with high pigments are not cured sufficiently;
UV powder coatings with high content of black or titanium dioxide and yellow pigments cannot be cured in the short-wave ultraviolet range;
For three-dimensional coated substrates, UV curing of coating film is difficult.
Large-scale coated workpiece needs to use high-power curing device when coating UV powder coating, which is difficult to realize in practical operation. The newly developed pulsed ultraviolet curing technology is an effective means to solve the above problems. Using xenon lamp, the UV powder coating containing white pigment can be completely cured within 30s, while the transparent UV powder coating can be cured within 15s. Table 6 shows the performance comparison of the new pulsed ultraviolet radiation device.
The new pulsed ultraviolet radiation device has the following characteristics:
The radiation wavelength is in the range of 300 ~ 1000 nm, the radiation intensity distribution is uniform, and the photoinitiator selection range is wide;
Short-wave (about 300nm) ultraviolet light is suitable for curing transparent powder coatings;
Long wave (about 400nm) ultraviolet light is suitable for curing color UV powder coatings.
The power of the pulsed lamp tube is as high as 1× 105 W, and it can reach the specified energy output in a short time (65438+ 2 parts per million), so the radiation energy can penetrate the thick film and completely cure it. Figure 4-7 is the radiation spectrum of pulsed ultraviolet lamp and ordinary ultraviolet lamp.