Curing and Drying Mechanism and Influencing Factors of Waterborne UV Ink

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.

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 waterborne materials refers to the photoinitiator, which absorbs the radiant energy of ultraviolet light, splits into free radicals under the irradiation of ultraviolet light, initiates polymerization of the prepolymer, crosslinks the graft, and cures 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.

Factors affecting the curing and drying of aqueous UV inks

There are many factors that affect the curing and drying of water-based UV inks. This article only discusses the main influencing factors. These factors are as follows:

1. Effect of pre-drying of aqueous system on photocuring

The curing condition before curing has a great influence on the curing speed. When it is not dry or incomplete, the curing speed is slow, and with the prolonging of the exposure time, the gelation rate does not increase significantly. This is because, although water has a certain effect on inhibiting the inhibition of oxygen, this only allows the ink film surface to rapidly solidify, and it only reaches the surface dryness, and it cannot reach the solid state. Because the system contains a large amount of water, when the system is cured at a certain temperature, the surface of the ink film rapidly solidifies as the moisture on the surface of the ink film rapidly evaporates, the water in the film layer is difficult to escape, and a large amount of water remains in the ink film. The ink film is prevented from further curing and the curing speed is reduced. In addition, the ambient temperature at the time of UV irradiation has a great influence on the curing of the UV ink. The higher the temperature, the better the curability. Therefore, if the preheating is performed, the curability of the ink will be enhanced and the adhesion will be better.

2. Effect of Waterborne UV Curing Resin on Photocuring

Water-based UV-curable resins undergo radical photo-curing. This requires that the resin molecules must have unsaturated groups. Under ultraviolet irradiation, the unsaturated groups in the molecules crosslink with each other, and the liquid coating becomes a solid coating. Floor. The method of introducing an acryloyl group, a methacryloyl group, a vinyl ether, or an allyl group is generally adopted so that the synthetic resin has an unsaturated group, so that it can be cured under suitable conditions, and the acrylate is often due to its high reactivity and often used. For radical-type UV curing systems, as the content of double bonds in the molecule increases, the cross-linking speed of the coating film increases and the curing speed increases. Also, resins of different structures have different effects on the curing speed, and the reactivity of various functional groups generally increases in the following order: vinyl ether <allyl <methacryloyl <acryloyl. Therefore, the introduction of acryloyl and methacryloyl groups is generally the main, so that the resin has a faster curing speed.