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1 Curing and Drying Mechanism of Waterborne UV Ink
Water-based UV inks are primarily formulated from prepolymers (water-based photocurable resins), photoinitiators, pigments, amines, water, cosolvents, and other additives. The drying curing combines two kinds of drying modes of UV light curing and water-based ink pervaporation. Specifically, there are two main drying methods: pre-volatilization drying and ultraviolet light curing of the aqueous system.
1. Pre-evaporation drying mechanism
Pre-drying is a necessary step before photocuring. Without pre-drying, the final result of photocuring is not ideal. In the manufacture of water-based UV inks, water-based photocurable resins can be dissolved in water by adding a base or acid to make them carboxylate salts, where the reaction to form salts by adding ammonia water can be shown as:
R-COOH+NH→R-COO+NH (water-soluble)
In turn, the reactions that occur during the pre-drying process are:
R-COO+NH→R-COOH (water insoluble)+NH↑
2. Photocuring mechanism
The curing of UV-cured water-based materials means that under the irradiation of ultraviolet light, the photoinitiator absorbs the radiant energy of the ultraviolet light and then splits into free radicals, causing polymerization of the prepolymer, cross-linking of the grafts, and curing in a short time. Into a three-dimensional network of high-molecular polymer, to obtain a hardened film, the essence is to achieve chemical drying through the formation of chemical bonds. The curing process can generally be divided into four stages: 1 interaction between light and photoinitiator, which may include the absorption of light and the interaction between photoinitiators; 2 rearrangement of photoinitiator molecules, the formation of free radicals Intermediates; 3 Free radicals react with unsaturated groups in the oligomers to initiate chain or polymerization reactions; 4 Polymerization continues, and liquid components are converted to solid polymers.
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Prepolymers in general UV inks are generally very viscous and need to be diluted with a reactive diluent. The currently used diluents, acrylates, have varying degrees of skin irritation and toxicity, and many reactive diluents in the UV. In the process of light irradiation, there is still a problem that the reaction is incomplete, and the residual monomers are permeable, which may easily bring about hidden dangers of health and safety, and affect the long-term stability of the cured film. Although water-based inks have easily adjustable low viscosity and very low organic volatiles (VOC), their drying time is generally longer, most of them need to be heated, the heat resistance of the substrate is also limited, and the drying and curing cross-linking may be simultaneous. There are other deficiencies. Water-based UV inks use water and ethanol as diluents, combining the characteristics of water-based inks and UV inks. It is a new research direction in the field of UV inks.