UV Curing Mechanism and Application of Waterborne UV Ink

Release time:

2020-10-31 00:00

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The main base material of waterborne UV ink is waterborne photocurable resin, which also includes other auxiliary materials such as photoinitiators, pigments, water, cosolvents, and other additives. It is a new type of environmentally friendly ink that has attracted research interest among scientists in recent years.

Waterborne UV inks have been initially used in packaging and printing of food, medicine, tobacco, alcohol, and various daily necessities used by people. The key component of waterborne UV ink is waterborne UV curable resin, which acts as a connecting material in the ink, provides adhesion to the substrate material, and enables uniform wetting of the pigment, enabling the ink to have excellent physical and chemical properties, such as chemical resistance, wear resistance, and other characteristics. Photoinitiator is a key component of waterborne UV ink film formation, which is related to the film formation speed and quality of the ink film. This article focuses on the drying and curing mechanism of waterborne UV ink and its application in industry.

Drying and Curing Mechanism of Waterborne UV Ink

The drying and curing of water-based UV inks include both the light curing of traditional UV inks and the volatilization physical film forming drying of conventional water-based inks. The two specific drying methods are: heating pre volatilization drying at the front stage and UV radiation light curing drying at the rear stage.

(1) Pre volatilization drying mechanism

Pre drying is similar to conventional water-based drying, which is a drying process before UV curing. Pre drying must be thorough and sufficient to obtain a good ink film surface effect. The synthesis of waterborne UV curable resins, one of the key components of waterborne UV inks, is usually achieved by adding acids or bases to turn them into carboxylates with good hydrophilicity. For example, adding ammonia can increase their hydrophilicity. The specific reaction formula is: R-COOH+NH → R-COO -+NH+(water solubility) Conversely, The reaction formula in the pre drying process of water-based UV ink is: R-COO -+NH+→ R-COOH (water insoluble)+NH The pre drying process of water-based UV ink can usually be quickly dried by infrared, hot air, or microwave. After the volatilization and drying of water, amines, and various co solvents are complete, the next step of UV curing can be carried out to obtain good surface effects.

(2) Mechanism of UV curable film formation

Waterborne UV inks undergo UV curing machines. The photoinitiator in the formulation absorbs radiation energy and splits to produce corresponding free radicals and cations, initiating chain initiation, and initiating polymerization and crosslinking reactions of the prepolymers of waterborne UV curable resins. After polymerization, the macromolecules continue to undergo three-dimensional cross-linking reaction. When the free radical reaction completely loses its activity, the chain terminates, forming a three-dimensional network of polymer, and obtaining a hardened film. The specific reaction of photocuring polymerization includes the following three stages: ① Chain initiation: UV radiation penetrates the photoinitiator, causing excitation rearrangement and the generation of free radicals. Growth of the PI → PI. PI.+RCH=CH → PI ～ CH C.HR ② chain: Excited rearranged free radicals react with unsaturated bonds in aqueous UV prepolymer molecules, causing chain polymerization in the prepolymer. Termination of the chain from PI to CH C. HR+nCH=CHR → R (CH CHR) nCH C. HR (P) ③: After the free radical loses its activity, the reaction system undergoes low molecular crosslinking and solidification into a three-dimensional network of macromolecules, which are converted from a liquid state to a solid state. R+R→R-R P .+ In the three consecutive stages of UV curing, the speed of photoinitiation is the slowest, so the speed of photoinitiation is crucial to the overall UV curing reaction speed. How to properly select photoinitiators is crucial to controlling the UV curing speed.

Application of 02 water-based UV ink

(1) Application Fields of Waterborne UV Ink

Waterborne UV curable inks have the characteristics of rapid drying in production lines, so they can also be used on non paper substrates, such as PVC, PET, or synthetic paper. Compared to traditional oil ink based UV inks, even when printed on paper, their gloss, width, and friction resistance are also superior and different. Waterborne UV inks are currently used in various high-end ink fields such as photocurable polymer printing plates, screen printing, gravure and plate printing, wood and metal inks. Multicolor overprinting is now a unique process for many high-quality printing processes, and a thinner dry film coating can reflect a rough ink film effect on the substrate. Therefore, the low solid content characteristics of water-based UV resin itself give it greater advantages. Waterborne UV ink has good application value in the packaging and printing industries of food and medicine, wood finishing, tobacco and alcohol, and other daily necessities, especially in the printing of molded plywood and molded furniture wood.

(2) Application Prospect of Waterborne UV Ink

Aqueous UV ink not only inherits the advantages of traditional inks, but also has a high environmental value. Compared to traditional oily UV curable inks, it has significant improvements in various aspects and has many obvious advantages: ① Environmental safety, using water as a diluent to solve the problem without adding monomer diluents, which is conducive to protecting the environment and personal safety Good overprinting performance. Due to its low solid content of water-based UV ink, it solves the problem that the ink layer of ordinary UV ink is too thick, which affects overprinting Easy to control viscosity and rheology. Using water and thickeners to control the viscosity and rheology of ink can provide good rheological properties for high-precision printing. For example, high demand screen printing can be widely used in various high-precision and high-quality printing At the same time, because printing is mostly a common thin coating, water-based UV inks have unique advantages over the oxygen polymerization inhibition existing in traditional UV inks, and water-based UV can minimize the oxygen polymerization inhibition effect.

However, water-based UV ink still has some shortcomings that need to be improved Waterborne UV inks require more energy consumption when dried. Although water-based UV inks are environmentally friendly, their energy consumption is higher than that of traditional inks, because prior to the later stage of UV curing drying, the water-based system must be pre dried, often using infrared hot air or microwave with higher energy consumption. In addition, manufacturers need to invest in additional drying equipment and increase the occupancy of equipment in the factory, resulting in an increase in the average production time of products and a decrease in production efficiency The price is much more expensive than traditional ink, and the increase in production costs puts manufacturers under greater competitive pressure The printing process of water-based UV ink is complex and variable, relatively difficult to control, and its application on the substrate is poor in breadth, which is a certain gap from traditional ink; Due to its complex process, water-based UV inks require more production time to apply UV primer to the substrate or to properly surface treat the substrate, resulting in a significant decrease in yield and an invisible increase in production costs. Due to the increasing human demand for environmental protection, UV curing technology has developed rapidly in European and American countries that are leading in environmental protection and technology, and various technical patents and products are increasingly developing, such as Germany's Vinova. At the same time, the raw material technology of water-based UV ink is becoming increasingly mature, and water-based UV ink has become a key research object for scientific workers and technical personnel in the ink field. With the increasing demand for environmental protection, water-based UV ink will become a key research object of environmentally friendly ink, widely used in various printing industries.

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