First, the printing and curing characteristics of light-curing water-based ink
Light-curing water-based inks use water as a diluent, and a combination of dehumidification and UV radiation curing is used to achieve the printing process. Since this kind of ink uses water as the solvent, the printing process is: before the ink is cured, the wet ink is dried by removing water to obtain a dry film that can be touched, and then UV light irradiation is performed to achieve cross-link curing of the UV ink. See the light site cross-linking, no light is not cross-linked, washed out after the film can be obtained in a variety of patterns, which is difficult to achieve the general water-based ink. Light-curable water-based inks can be used in screen printing inks, gravure and flexo printing inks. However, light-curable water-based ink technology has been largely blank in the country so far, and it should have great development prospects.
Light-curing water-based inks require lower ink film thickness because of their lower viscosity, and are mostly in the micrometer range. If the film is too thick, the light curing of the lower ink film may be incomplete due to the light shielding effect of the upper film. The wet film drying methods include a direct heating and drying method, a high-speed air-drying method, and an infrared radiation method. Among them, the air-drying method is particularly suitable for an absorbent substrate such as paper or wood. For absorbent printing materials, there is no need for special drying. A small amount of moisture can be directly absorbed by the substrate.
In water-based UV-curing inks, because the amount of photoinitiator itself is small, and there is a certain degree of water vapor volatility, the drying temperature should be strictly controlled and the drying time can not be too long, otherwise it will easily cause the loss of photoinitiator.
The ink film formation process is very important for the final properties of the cured film, and the film formation process of the water-dispersible link material ink is more difficult to control than the water-soluble link material ink. The film formation process of a water-dispersible ink is actually a coalescence process between individual particles (the emulsion particles or hydrosol particles) in the system. When most of the water evaporates, these particles will be deformed, penetrate each other, and aggregate into numerous Small pieces. The most desirable coalescence occurs between polymer molecules, which results in a very uniform film. The water-removal drying process of the photo-curing water-dispersible ink is also advantageous for film formation due to the increase in temperature.
Second, the curing process of light-curing water-based ink
The curing of water-curable water-based inks generally uses medium-pressure mercury lamps, and the degree of curing is related to the amount of radiation received by the ink film. This depends on the effective light-absorbing ability and photoinitiation efficiency of the photoinitiators; on the other hand, it also relates to the use of mercury lamps. Line power (w/cm), irradiation time, and irradiation distance are related.
Light-curable aqueous inks have been cross-linked dry films after UV curing and generally do not require further post-curing. However, since light-curing water-based inks often contain carboxyl groups in order to obtain water-soluble or water-dispersible systems, these carboxyl groups remain in the dry film coating after photocuring, which has a negative effect on the coating properties (eg, ink film resistance to water Poor performance, etc.) Therefore, post-cure cross-linking agents have been added in many applications to further cure the ink film during or after photocuring and consume hydrophilic carboxyl groups.
In addition, since light is the key to film formation, the water removal operation of the light-curable aqueous ink does not substantially affect the performance of the final cured film. Therefore, after the photocuring technology is introduced into the water-based ink technology, the cross-linking curing reaction may not occur in the process of ingredients, storage, painting, and drying, and cross-link curing is performed only under light, so that the performance of the final cured film is easily controlled. The product quality is stable, which obviously reduces the construction difficulty of the ink.
Third, the advantages and disadvantages of light-cured water-based ink
Light-curing water-based inks use water as a diluent, and use UV radiation curing technology, which greatly contributes to the comprehensive performance of the two. Compared to traditional oil-soluble UV light-curable inks, light-curable water-based inks have the following advantages:
(1) It is not necessary to use reactive diluents to adjust the viscosity, which can solve the problems of VOC and toxicity and irritation;
(2) The viscosity and rheology of the ink can be controlled with water or a thickener; (3) A very thin coating can be obtained;
(4) Equipment, containers, etc. are easy to clean;
(5) It can be touched before light curing, and it can be stacked and repaired to ensure the smoothness of the cured film, simplifying the dust-proof operation;
(6) Greatly reduce fire hazards.
Of course, light-curable aqueous inks also have their disadvantages. Unlike traditional oil-soluble light-curable inks, most light-curable water-based inks require drying and water removal from the wet film prior to the photocuring process, resulting in increased energy consumption. In general, a drying device needs to be added on the basis of the existing photocuring production line, which will increase the space occupied by the device. At the same time, the production time will be prolonged due to the addition of the drying step, and the production efficiency will be reduced. In addition, the high surface tension of water (72mN/m) also brings a series of problems, such as poor wettability to the substrate (especially low surface energy) and pigments, easy to cause uneven dispersion, etc. Printing failures can be solved by adding co-solvents or surfactants. At the same time, the freezing point of water is higher than that of common organic solvents. In the course of transportation and storage, it is necessary to add antifreeze agents. The aqueous system is prone to bacteria, and antifungal agents need to be added to complicate the formulation.
Fourth, the classification of light-curable water-based ink
According to the composition and the water dispersion method, the light-curable aqueous ink can be divided into the following categories.
1. Light-curing water-based inks - emulsification type. By the addition of surfactants (emulsifiers), supplemented with high shear forces, it is possible to emulsify traditional photocurable resins into water-dispersed systems (oil-in-water systems). The emulsion has a relatively high solid content and can be directly utilized with ready-made ink raw materials, and the production process is relatively simple. The choice of emulsifier will directly affect the dispersion properties of the resin and the ink stability, rheological properties. The emulsifier consists of a hydrophilic group and a lipophilic group. The latter is generally a long alkane chain. After being miscible with the resin droplets, the hydrophilic group is located in the water, so that the resin droplets are dispersed and stabilized. There are many interactions between the droplets, such as electrostatic interaction, thermal diffusion, gravity, and space repulsion of emulsifier molecules. Under these forces, the droplets of the system can maintain a certain distance and remain stable for a certain period of time. The inside does not aggregate, and therefore, the ink resin component can be stably dispersed into a lotion. The acidity and alkalinity of the system will change the state of the ionic groups and affect the stability of the emulsion droplets, so this type of external emulsion ink is very sensitive to changes in the pH value. However, the added emulsifier will remain in the coating after curing, which will reduce the moisture resistance and affect the quality of the dry film.
2. Light-curable aqueous ink - mixed with a water-dispersible resin solution. The photo-curable hydrophilic polymer is mixed with a physically drying water-dispersible resin (usually an acrylic resin), and the photo-curable aqueous polymer component is dispersed in the aqueous phase through a non-photo-curing water-dispersible acrylic resin. However, since the content of the photo-curable component is low in the entire system, the cross-linking density of the resulting cured film is not high, and thus the chemical resistance is lower than that of the conventional photo-curing system.
3. Light-curing water-based inks - ion-based self-emulsification type. The ionic group is introduced into the resin skeleton, and then ions in the molecular chain are neutralized with a counter ion. The resin thus obtained has excellent self-emulsification properties. Due to its excellent wettability, very small particles can be formed in water, so that it has a long shelf life and very good shear stability.