Application of Environmental Protection, Energy-saving and Efficient Photocuring Coatings (I)

Since Bayer developed the first generation of UV-cured wood coatings in 1968, light-curable coatings have developed rapidly in the world. In 2004, China's photocurable coatings (excluding inks) were 23,000 tons, an increase of 64% over 2003. Compared with Western developed countries, China's UV-cured coating products still occupy only a small part of the market. With the development of communications, fiber optics, computers, printing, packaging, home appliances, building materials, and environmental protection legislation, China's radiation-cured coating product market will continue to maintain rapid growth over the coming period. The important new developments of environmentally friendly, energy-efficient and light-curable coatings are as follows.

Raw material modification:

â—†Initiator development and research
The initiators with high initiation efficiency, deep-curing, and residues that do not affect the performance of the product have achieved important results. In particular, the polymerized initiators exhibit excellent properties. It not only overcomes the odor of small-molecule initiator residues and Toxicity, negative impact on the aging properties of the cured film, but also due to the special nature of the polymer chain can greatly increase the efficiency of the initiation.

A noteworthy research result in the initiator is that the substituted maleimide compound has a good photo-initiating property, and it may become a highly efficient new type of industrial photoinitiator.

Low-viscosity oligomers
Ordinary oligomers have high viscosity. In order to meet the coating requirements, a large amount of monomer is added as a diluent to reduce the viscosity. The current development of spray-type photocurable coatings requires adding more reactive diluent to achieve the required spraying. Viscosity, but in addition to the problems of volatile, odor, and toxicity, the monomer can also increase the shrinkage of the coating during the curing process, resulting in performance problems such as poor adhesion. Therefore, research and development of low viscosity oligomers With monomer attention. The acrylated hyperbranched polyester oligomers have the characteristics of low viscosity and fast curing, and have a good application prospect.

The reactive diluent is modified to remove its irritation, for example, the development of ethoxylated or propoxylated monomers is the direction of monomer development.

Curing/heat curing

Light-curing powder coatings and water-based photocurable coatings
Photocuring resins require the addition of more low molecular weight reactive diluents. The addition of thinner can cause the following problems: Some thinners are irritating and harmful to the human body; thinners can penetrate into the pores of porous substrates such as wood, cement, and paper, and they are not easily exposed to light and cannot be cured, so they slowly move from Diffusion in the pores causes odor in the object to be coated for a long period of time. The thinner influences the properties of the film. The density of the double bond in the free radical curing system is greater than that of the oligomer, and the shrinkage during curing is large. Therefore, it is very significant to reduce the amount of diluent or no diluent in the photocuring system. Waterborne photocurable and photocurable powder coatings are two types of photocurable coatings with little or no diluent.

â—†Light curing powder coating:

Light-curable powder coatings combine the advantages of powder coatings and photo-curing coatings to make up for the shortcomings of general powder coatings.

Photocured powders are relatively stable at the melting temperature and are not prone to thermal reactions. Therefore, they do not agglomerate due to thermal reaction during extrusion mixing, and are stable when placed. After the photocurable powder coating is sprayed, it can be melted and leveled with infrared heating, and then cured by ultraviolet light irradiation. Since the melting temperature is low, it can be used for heat-sensitive substrates. Since the leveling process and the curing process are separately performed, the surface leveling can be controlled to achieve the desired decorative effect.

Light-cured powder coatings are free radicals and cations. The corresponding resins require certain polymerizable groups and at the same time require higher glass transition temperatures, making the preparation somewhat difficult. Cationic light-curing powder coatings generally use bisphenol epoxy as a base material and an initiator. Free radical photocurable coatings generally use acrylated epoxy or polyester. The use of unsaturated resins and vinyl etherified polyurethane to formulate powder coatings has certain advantages. Although these two resins have double bonds, they cannot be or cannot be polymerized alone. Only after uniform mixing can the polymerization be initiated by free radical initiators. Light-curing powder coatings can be applied by different methods.
(to be continued)