Surface coating UV curing technology explained (2)

4. Current limitations of UV technology
Since the UV curing of coatings is still a technology that has not been around for a long time, there are still a series of problems to be solved. Therefore, the application of UV in coatings can continue to expand. The limitations of the technology are listed in Table 3. One of the most important issues is the economic factor, that is, the prices of basic raw materials used in the technology are relatively high. These raw materials include oligomers and monomers, which are much more expensive than the corresponding solvent-based coatings. However, we should keep in mind that this technology is a process that uses 100% solids, and it can be compensated for by saving solvents compared to its competitors solvent-based coatings. One of the major problems with UV curing, and the problem that is currently being gradually solved is the problem of the coloring system. The use of UV to penetrate pigment systems, especially white pigments, is difficult, however, recent research work using new photoinitiators and improvements in equipment design has been addressing this important issue. Since UV coatings are 100% solids, it is also difficult to apply them in some applications. In addition, it is difficult to reduce the viscosity of coatings to obtain good rheological properties. In order to overcome these particular difficulties, new technologies are being developed, especially in spray painting. UV is a linear curing technique that limits the types of substrates that can be coated and cured. In some applications, rapid solidification of the coating due to rapid solidification causes problems with the adhesion of the coating to the substrate; therefore, the coordination between the coating and the substrate is reduced during curing, and thus the adhesion is also reduced. Focus on. The toxicity of acrylic oligomers and monomers has until recently been a problem, mainly irritating to the skin. However, based on our experience, we have identified some of the monomers and oligomers associated with this problem and have basically controlled the problem. Finally, one of the major limiting factors in the promotion of this technology is the chemical problem of UV curing currently used. This is essentially a problem with the chemistry of acrylates as vinyl ethers are now more and more. Therefore, in order to extend the technology to a field that has not yet achieved satisfactory results, a wide range of chemical fields is needed.
Table 3 - General issues when replacing conventional systems with UV
1. Economy - High prices of basic materials
2. Difficulty of coloring system
3. Construction difficulties due to 100% solids
4. Linear curing problem
5. Problems of adhesion, matching of paint and substrate
6. Needs a wide range of chemistry
7. Toxicity (Irritation to skin)

5, application 

In addition to introducing paints in this section, inks are also introduced because, for many applications, the two aspects of research are linked. Many end-users use finishing paint to complete the entire process after printing. The main areas of UV applications in Australia are shown in Table 4. Coatings are used to etch the art industry with the largest and most diverse markets. The biggest application of UV coatings is the varnish and etching industry of packaging paper and packaging board materials after printing.
Table 4 - Main Areas of UV Applications in Australia
1. Print cover varnish
2. Lithographic ink
3. Screen printing ink
Wooden paint
5. Painting and Printing of Plastic Products
6. Current bank notes
7. Optical Fibers

However, there are many smaller market applications that can use this coating, and theoretically, any substrate may be UV-cured varnish coating. In terms of scale, inks constitute the second largest application area, especially in lithography and screen printing. In the latter field (ie screen printing), more than 50% of production lines in Australia have shifted to UV technology. Wood painting has also been actively adopted in the country. This is an interesting recovery phenomenon, especially in the coating of floors and doors, it is still the main application area of ​​the technology. In the second largest application area, plastic coatings are one of them. In this regard, the production of bank notes is of particular importance because it involves UV-curable coatings for polypropylene substrates. When the banknotes are paper, the initial output of the banknotes is 550 million. However, with the UV polymer system, the production of banknotes has been reduced to 200 million sheets due to the extension of the service life. In the world, especially in Australia, such banknotes have a major significance, so banknotes will be introduced separately in this article. Optical fiber is also one of the main areas in which this technology is used, and continues to be applied in more optical cable installations in China. 

The second common area of ​​application of UV technology is those areas that are not yet very established but have the greatest potential for development. This aspect has come to Table 5, where the hybrid printing system is the most promising, that is, the printing and painting of the system is carried out on a production line. One of the reasons for its development is the high utilization of equipment such as lithographic double coaters. The type of processing that can be accomplished with this type of equipment was known several years ago, but at the time such equipment could not be used. One of the main reasons for choosing this technology is economical, because it allows the most efficient use of UV processing.

Table 5 - UV applications with great potential for development
1. Joint implementation of printing and painting: traditional method, water method, UV method
2. Metal decoration and painting
3. Color paper coating
4. Lamination and bonding
5. Silicon mold release agent
6. Offset printing ink
7. Electronic products

In lithography, the mixing system that is used today is this: first, traditional printing, followed by a water-based sealant, and then a UV topcoat. A mixed system is also used for signage printing; a layer of water-based offset ink is followed by a UV top coat. The fields of application for the crucibles listed in Table 5 include a metal veneer overlay with a metal finish. Color paper coatings that improve the performance of recycled paper also particularly strongly require the use of UV technology. The UV processing of adhesives, especially adhesives for laminates, also has huge market capacity and potential in the long run. Silicon release coating is also a similar situation. Offset printing inks and electronic products are also important development areas. 

6. Fields with potential for development have been shown
UV technology has already been discussed in the application of those areas that have been established (in the table listed above). In addition to these areas, there are now many new areas of potential and technically very promising areas. Some of these areas have been bred in Broccoli and are summarized in Table 6. Including wide-format offset printing ink, this is another application after the successful application of narrow offset printing ink. Metallic coatings for steel pipelines have also adopted UV technology and have shown strong demand for future development. With the development of new and more effective photoinitiators intended to cure thick coatings, UV curing of coatings is under intensive research and has shown promising prospects for success. The use of water to reduce the viscosity of certain UV curing systems is also under investigation. The main problem with this work is that if excess water is used, it must be removed in one step before UV curing. If there is a small amount of water attached to the substrate, there will be undesirable pores. UV-curing coatings are promising for use in automotive coatings, especially interior parts, and are currently under investigation. In the field of wood painting, the application is being promoted from the floor to the furniture and the wood strips. Next is the UV curing powder coating. This is a potential area of ​​great significance; however, the chemical mechanisms associated with it need further study before their products meet the requirements of industrialization. Finally, the other two types of curing related to UV are actively being explored. They are visible light curing and laser curing, and they all have hopes to complement the trend of today's UV curing technology. 
Table 6 - Application of UV in areas where positive development is desired
Steel pipeline
2. Offset printing ink for wide paper
3. Development of photoinitiator for color paint
4. Water dilution UV curing system
5. Automotive coatings
6. Wood furniture coatings
7.UV curing powder
8. Wood trim strip coating
9. Visible light curing
10. Laser curing (to be continued)