Application of New Technology of Curable UV Powder Coating

(Application of UV-Curable Powder Coating on Paperlike Substrates)

At present, environment-friendly powder coating has been applied in a wide range of fields. The development of this technology is based on the combination of a curable UV powder coating technology and an electromagnetic brush technology (EMB). The UV-curable coating can quickly form a 5-10mm thick uniform film and combine with the surface of the thermosetting material, which can greatly improve the performance of the surface of the print. The materials used for this coating may be wood, composites, plastics, and other materials.

In the printing industry, there are many reasons for the glazing of printing products, the most important of which is to make the appearance of the printed products more beautiful, and also to improve the chemical properties or firmness of the surface of the paper and rough materials by glazing. In the past, solvent-based glazing materials were often used, and fast-drying glazing solvents were used in faster printing processes.

Environmentally-friendly glazing technology requires that VOCs (volatile organic compounds) cannot be volatilized, so water-based glazing and UV-curing glazing are welcomed. Waterborne coatings in these two products still emit a small fraction of VOCs, and they do not achieve the desired gloss after coating, while low-viscosity liquid-curable UV coatings can meet printing requirements. When the metal is coated in industry, the powder coating resin develops rapidly due to environmental protection requirements. This solid coating material does not contain any soluble components, and these materials have a high recovery rate, and therefore, are more suitable for application in the coating process. Of course, there are still many restrictions on the universal application of such coatings. The main ones are:

Relatively high curing temperature (greater than 150°C);
· Coating speed is low;
Curing time is relatively long (more than 6 minutes);
The coating layer is relatively thick.

The above points have limited the widespread use of powdered coatings, making them more limited to metallic materials, while other heat-sensitive materials such as paper, wood, wood composites, and plastics are not suitable for this type of coating. Drying cure speeds limit the application of this coating to high speeds, such as the coating of metal coils.

But recently, two new types of technology have expanded the range of applications for powder coating, namely, UV high-speed, low-temperature curing; and the other, electromagnetic brush (EMB) technology enables powder coating to achieve rapid thin layer coating. This article will focus on these two technologies, talk about their specific situations and give examples.

First, UV curable powder coating technology UV curable powder coating technology is not just a name, at present, it has been applied in a variety of different materials. Two years ago, the use of UV-cured powder coating technology on metal materials, such as on electric motors and small trucks, was applied. In the past, some of the unit parts were to be assembled. For example, some sealing parts and temperature-sensitive parts were assembled after UV curing powder coating. After the curing temperature is lowered, the components can be assembled first, and then the entire coating can be performed. As a result, the assembly cost can be greatly reduced, and the frequency of assembly errors can be reduced to reduce the rejection rate. The current application on medium density fiberboard (MDF) is already available. Due to economic advantages, such as the reduction of material waste rejection rate, the timeliness of coating, and low inventory, UV-cured powder coating may replace the traditional melamine resin foil as the main raw material for coating in the future.

UV curable powder coating combines the advantages of traditional powder coating technology with liquid UV curing coating. Like the traditional powder coating, the new UV curable coating is also solid, because it contains almost no solvent, so it does not emit VOC, but also realizes the maximum recycling rate of the sprayed material. Using UV lamps instead of thermal curing, the curing temperature is reduced, from the original curing temperature is greater than 180 °C to about 100-120 °C, at the same time, the curing speed from the original need to 10 minutes to the current number Seconds. In addition, after applying the EMB technology, the thickness of the coating also becomes thinner.

A post-printing system with UV-curable powder coating equipment has been developed. The principle of powder coating is based on the unsaturation of polyester unsaturates with maleic acid or fumaric acid. Crosslinked with vinyl ester polyurethane. UV curable powder coating can be applied to a thin, uniform material similar to metal. UV powder coatings are simply composed of maleic acid esters, vinyl acid ethers, and some optoelectronic compounds. The UV-curable powder coating has a relatively low viscosity point at which the bookbinding system can operate. If the viscosity of the coating liquid is too large, the extrusion apparatus and the temperature control apparatus will be adjusted, and the UV-curable powder can be evenly applied to the material by using a conventional spray apparatus.

UV curable powder coating usually involves two steps: as shown in Figure 1.

After the powder is sprayed on the surface of the material, the powder is first melted under the action of infrared rays to form a film that is applied on the surface of the material, and then cured and dried by baking with a UV light source. These two processes ensure that the coating process is completed at a temperature that is not too high. There are several factors that affect the coating, ie the temperature of the UV light source is controlled within the range of 85-90°C. In addition, some pigments It also affects the performance of powdered coatings. The curing process also affects the effectiveness of the coating. Studies have shown that when the UV content is 300mj/cm2, the standard mercury lamp can well cure 100μm thick UV film, but the premise is that the UV content is high.

The application of UV powder coating to MDF has better fluidity, stickiness and firmness.

Second, the electromagnetic brush technology The newly developed electromagnetic brush technology (EMB) comes from the application of copiers and laser printers. Its basic electromagnetic brush system is shown in Fig. 2.

The working process of the electromagnetic brush is the process of spraying the powder on the material. The powder particles will enter the automatic dispensing container under the action of the electromagnetic brush. The powder particles will be combined with the charged particles and the particles will be sprayed under the action of the magnetic. To the surface of the material. The thickness of the powder sprayed onto the surface of the material depends on the strength of the electricity. In this process, only when the powder particles overcome the gravitational force between the electrons, the surface of the material can be absorbed as shown in Figure 3, so that the powder particles can be controlled by electricity.

In order to test the process of EMB technology applied to smooth materials in powder coating, EMB test equipment has been developed. The schematic diagram of the equipment is shown in Figure 4. The powder particles enter the fixed container, the coating starts from the right side, and the coated material adheres to the surface of the roller. After the material passes the electromagnetic brush, the charged part of the material surface is attracted by the attraction force, and the powder particles are attracted to the surface of the material through the roller and the magnetic brush. This coating is very good for coating non-magnetic materials such as paper or cardboard. For magnetic materials, such as webs, it is necessary to prevent magnetic interference with particles. During the test, different materials can be coated with a thickness of 5-100 microns at a line speed of more than 130 meters per minute.

Third, coating on paper-like materials uses EMB technology, which can be applied on different materials that are similar to paper. In order to achieve this goal, when coating UV-curable powders, MDF materials are generally selected. After grinding classification, the powder size is from 5-20 microns, the average size is 11 microns, the powder and the load The material is mixed and sent to the EMB device.

IV. OPV on Cardboard and Printing Cardboard refers to paperboard with a thickness of about 0.3mm, which is often used in packaging and decoration. Therefore, it also needs a higher gloss. On the cardboard, an experiment was carried out to coat the curable powder with an offset printer at a printing speed of about 30 m/min. During coating, infrared rays were used to melt the powder and apply it. The UV light source cures and the resulting coating thickness is approximately 7-10 microns. The coating effect is very good, with high brightness and scratch resistance.

In the second experiment, full-color printing was performed using a Xeikon DCP-1 digital printer. The same curable powder coating was performed under the same conditions and conditions, and the coating thickness was also 7-10 μm. The effect after coating is also very good. This experiment shows that powder coating can also be used for some coating processes, such as coating the cover of magazines.

The use of EMB technology for UV-curable powder coating, and then IR IR and UV heating and curing process can be applied to a variety of printing methods, such as offset, gravure or flexo, can be performed offline, at the same time It can also be done online. The applicable materials can be paper jams or book covers used in packaging. This process can also be covered. Good chemical properties and coating effects make it possible to coat UV-curable powders in future industries.

Fifth, the application of wallpaper Advanced wallpaper can have multiple coatings to improve its durability. The UV curable powder coating can replace the traditional varnish and is coated with rollers using a UV curable coating liquid containing 16% solids. Its main advantage is that it is less volatile after coating with UV-curable powders because traditional paints contain more volatile solvents. Therefore, a UV-curable powder coating was applied on wallpaper using an EMB device approximately 17 inches wide at a speed of approximately 1 meter/second. After wall coating, the applicator roll can be cut to a 6" x 20" size and the coating can be melted under IR light and rapidly cured under a UV light source. The thickness of the coating is approximately 5-7 microns.

Sixth, partial printing and glazing In addition to the overall coating, UV curable powder coating can also be applied to the local coating or printing. In this process, UV-curable toners are not the same as traditional thermoplastic materials. To test this printing process, we used a small, traditional manuscript, using a blue UV-curable toner. During the printing process, the toner was melted by a heated roller to complete the graphic transfer process. Curing the toner, a paper turnover device causes the paper to be flipped from the printing unit as shown in Fig. 5. In order to ensure that the heat is not lost during the reversal, an additional IR unit is mounted on the turning device, after which the toner on the print is UV-cured. The test product passed the paper test, abrasion test and bend test, indicating that it has very good performance. Compared to traditional printing, UV-curable toner molecules have greater cohesion.

7. Conclusion Because of the application of MDF and hot melt materials, UV-curable powder coating has been applied in new areas. It can be successfully applied in the overall coating and local coating. The material can be cardboard, wallpaper and Digital printing application materials, etc., because of the relatively low temperature of curing, make the paper deformation and fading and other defects are well compensated. UV curable coatings can already be applied to thinner or pinhole materials such as paper, and UV curable coatings can also be applied to printing on suitable equipment.