Surface coating UV curing technology Detailed (1)

1. Introduction

As a new processing technology, UV has been commercialized in Australia for more than 25 years, and the market continues to look good. The technology is steadily growing at a rate of nearly 10% per year, especially in surface coatings and related industries. This article has two purposes. First, it introduces the technology to members who are likely to become new end users, including a preliminary introduction to the basic principles of the technology. The second is to introduce to current end-users the latest trends in the technology, especially in Australia. Because this progress is linked to the development of similar technologies overseas. In this second part, a brief introduction of the ongoing basic research related to the technology in Australia will be presented.

Australia has applied UV curing technology to many important applications in society, especially safety facilities. For example, Australia's polymer banknotes (bank currency) are the only currency in the world and are the result of research using UV coatings and UV curing technology. The process has been put into production for eight years and has been very successful in anti-counterfeiting. It is also a technology for exporting to earn foreign exchange, because the circulation currency of many countries now uses the technology of Australian Reserve Bank (Australian Reserve Bank) to produce.

2. Light curing technology

UV curing is a technique for obtaining a coating film that is also crosslinkable by rapid polymerization of a monomer/oligomer mixture. This rapid polymerization of UV systems is achieved with photoinitiators and high performance lamps. The resin system used in UV curing technology (Table 1) involves a basic oligomer, which is essentially a low molecular weight (about 2500) prepolymer, commonly used in urethane acrylate, epoxy Acrylates, polyester acrylates, or polyether acrylates, though they do not contain acrylate oligomers such as vinyl ethers, are also being colored. The viscosity of the oligomers is relatively high. In order to facilitate the application and increase the speed of cross-link curing, it is necessary to add a monomer as a reactive diluent to adjust the rheology of the resin. The structure of the reactive diluent has an important influence on the properties of the final coating film such as flowability, slip properties, wettability, swelling properties, shrinkage, adhesion and internal migration of the coating film. Reactive diluents can be monofunctional or polyfunctional, with the latter being preferred because it can increase the degree of crosslinking during cure. The performance of reactive diluents requires, dilution capacity, solubility, odor, ability to reduce the viscosity of the medium, volatility, functionality, surface tension, shrinkage during polymerization, and the glass transition temperature (Tg) of the homopolymer. The impact of the entire curing speed and toxicity. The monomers used should be monomers that are skin irritant and have a Draize value of no more than 3 levels. The typical monomer used as a reactive diluent is the tripropylene glycol dipropylene ether ester (TPGDA) variety (1).

Tripropylene glycol sulfonium (1) diacryloyl ester (TPGDA)
Table 1 - Typical Formula Compositions for UV-curable Coatings
Oligomeric 40-50
Monomer 40-50
Photoinitiators/Photosensitizers 1-10
Surfactant, Additive 0.5-1.0
Pigments (if necessary, their amount is controlled to obtain satisfactory coverage) 

The rapid polymerization reaction in the UV curing chemistry is effected by radical reactions that are actually generated under suitable photoinitiators and/or photosensitizers and high-performance light conditions. Photoinitiators that can generate free radicals and produce cationic intermediates can be used. However, in the modern industry, the former type (ie, a photoinitiator capable of generating radicals) is often used. The photo-generated radicals can be obtained by two methods. The first method is to generate a free radical pair (Equation) by the cleavage of a photoexcited molecule, which is a process inside the molecule. The sample in Equation 1 is Ciba Geigy's Irgacure 651 and the chemical name is 2,2-dimethoxy-2-phenylacetophenone. 

The second method of generating free radicals is the hydrogen abstraction reaction that occurs between the excited photosensitizer and the hydrogen donor molecule. A typical system for this method is a mixture of amines and benzophenones, of which triethylamine and benzophenone are the most economical ones. The amine benzophenone system is very economical and most commonly used, and its side effects such as yellowing are not a problem. The disadvantage is that the photoinitiator used in this method is in excess, which is required to achieve rapid curing, and photoinitiators that have not reacted after curing are left in the cured film. 

In the initiation technology of UV systems, the use of cationic photoinitiators has recently been studied. Triaryl sulfides are typical examples of such photoinitiators, as shown in equations 2, 3, and 4. In this cation system, the activation process is similar to the radical polymerization process, namely: light absorption, sensitization, initiation, transfer, and finally chain termination. One advantage of the cationic system is that the reaction is not inhibited by oxygen, whereas traditional free radical initiators are inhibited by oxygen. However, cationic initiators are sensitive to nucleophilic impurities such as water, which can neutralize cations and cause chain growth to end. Cationic initiators are relatively expensive in terms of price and are therefore not commonly used at present.

3. Vigorously promote UV technology

At present, the use of new technologies in the paint industry, especially in the art of etching, is increasing. High-quality etch art requirements and sound environmental protection laws and regulations have led to the widespread use of technologies such as UV in industry. The main influence that UV technology has already shown in the paint industry has been the favorable ecological environment. Therefore, it is very important for the world today that the environmental situation has caused concern.

The general advantages of using UV technology are given in Table 2 if compared to conventional solvent-based systems. From the perspective of benefiting the environment, the non-volatile nature of UV coatings is the key. In UV systems, end users realize that monomers can be used instead of solvents to reduce the viscosity of the paint. They do not volatilize the working environment and cause a greenhouse effect. The advantages of the UV curing technology listed in Table 2 are 25 years of experience since the use of the technology. In addition to being beneficial to environmental protection, UV materials are also fast-drying, have good adhesion to various substrates (chemically), and are easy to control in terms of processing degree. The construction equipment does not need to be prepared for the second day after being stopped overnight. . Another advantage of this technology is that there is less waste. The paint can be stored for a long time under suitable conditions away from heat and UV light, and can be reused. Compared with other countries in the world, the influence of solvents and related greenhouse effect is particularly important for Australia, so the use of UV technology in environmental protection has become one of its most important performance. Finally, other advantages of the use of UV technology include the ability to produce products that cannot be obtained with other technologies; because of the compactness of their equipment, the factory space can be saved; and the versatility and adaptability of the processing methods make it possible to have It is possible to quickly change the color variety. 

Table 2 - Where UV technology outperforms conventional systems
1. No solvent - Greenhouse effect
2 is conducive to environmental protection
3. Work is pollution-free
4. Rapid production
5. Room temperature curing
6. The product has unique properties
7. Save energy
8. Easy to clean
9. Save space
10. High gloss
11. Less waste
12. Method diversity, adaptability, quick product replacement

(to be continued)

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