The film-forming mechanism of UV-curable water-based inks is very important for the preparation of water-based UV inks. Only by clarifying the entire film-forming process can the theoretical basis be found in the choice of resin, initiator, water, etc.
The drying mechanism of water-based UV photocuring systems is still controversial. It is generally believed that the drying mechanism of water-based UV inks is: after removing water through a drying device, under ultraviolet irradiation, the photoinitiator decomposes into free radicals or cations after absorbing radiation energy. It causes the polymer to undergo polymerization and cross-linking reaction, and solidifies into a three-dimensional network polymer in a short time. Radiation curing systems can be divided into free radical systems and cationic systems according to different types of photoinitiators. Since free radical systems are commonly used in radiation curing inks, it is recommended to use free radical systems in the development of waterborne UV inks.
The film-forming process of water-based UV ink is roughly divided into two stages:
1. Pre-volatile drying mechanism
Pre-drying is a process that must be carried out before photo-curing. Without pre-drying, the final result of photo-curing will be unsatisfactory. UV-curable water-based inks generally use appropriate heating methods, such as infrared baking or hot air drying, to shorten the initial water evaporation process. As the water gradually volatilizes, the dispersed polymer particles gradually move closer, but they can still move freely. Then as the moisture further volatilizes, the polymer particles reach close packing and become irreversible contact with each other. It is generally believed that the theoretical volume solid content is 74% at this time.
2. Mechanism of UV curing film formation
In the later period, the dispersed phase particles began to coagulate, and about 3% of water remained. At the same time, the curing group molecules containing functional groups undergo a curing reaction at the interface and inside of the dispersed phase particles under the trigger of the photoinitiator.
The curing process can generally be divided into four stages: â‘ interaction between light and photoinitiator, which may include absorption of light and interaction between photoinitiators; â‘¡ photoinitiator molecules rearrange to form free radicals Intermediate; â‘¢ The free radical interacts with the unsaturated group in the oligomer to initiate a chain or polymerization reaction; â‘£ The polymerization reaction continues and the liquid component is converted into a solid polymer.
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