Curing mechanism of cationic UV ink
The so-called cationic UV ink curing means that under the irradiation of ultraviolet light, the initiator produces an acidic active center, which then triggers a chain reaction of monomers and resins. Taking oxetane as an example, the approximate reaction process is shown in the figure.
In order to improve the reaction degree of the ink and minimize the residual amount of monomer or resin, the free radical curing function is usually introduced into the ink formula. The free radical curing focus is on drying the middle and bottom of the ink layer, and the cationic focus is on drying the surface of the ink layer. The two are combined. , will significantly improve the performance of the ink. In addition, the introduction of thermal curing is also one of the commonly used methods. For example, acid anhydride is very easy to cure with epoxy. The general principle is shown in the figure.
Main characteristics of cationic UV ink
1)Low migration characteristics
Cationic UV ink has a very high conversion rate, and the film-forming material after curing has good low migration characteristics.
There are often two or more functional groups on the monomer or resin molecules in cationic UV ink. They have dual curing functions. When exposed to UV or UV-LED light, they will initiate polymerization and produce a chain reaction. In a very short time (about 0.02 seconds), the conversion rate of the functional group will reach more than 80%; the remaining unreacted functional groups will continue to react, which is often called post-curing. After 48 hours, the conversion rate of the functional group will be greater than 99%.
This dual curing is fundamentally different from the radiation curing of conventional free radical inks. Conventional free radical inks have a conversion rate of about 80% for functional groups under ultraviolet light irradiation. When the ultraviolet light irradiation stops, the reaction stops immediately, and the unreacted monomers, functional groups and resins remain in the film-forming material of the ink. These residues will not only reduce the wear resistance and folding resistance of the ink, but also make it easy to stick. It is more likely to overflow and penetrate from the film-forming material of the ink and migrate to the packaging product, causing product contamination.
2)Low VOC properties
Cationic UV ink can significantly reduce the VOC of ink products.
We know that printing films mainly use traditional solvent-based gravure inks, which have a solvent content of up to about 60%. About 70% of solvents are added during the printing process. In this way, every ton of ink used will bring about 1.3 tons of VOC emissions. China consumes 350,000 tons of solvent-based gravure inks each year, and the VOC emissions brought will exceed 450,000 tons/year. Although printing companies have added VOC recovery and treatment devices under the pressure of environmental supervision in recent years, there are still a lot of VOC emissions due to management, production capacity and regional issues. Even if the VOC treatment device is normal, people who have been to solvent-based gravure workshops are deeply touched by the choking smell of solvents. The evaporation of solvents is everywhere, leaving front-line workers nowhere to hide.
If water-based inks are to be used for printing film products, adding alcohol is also essential, and the amount added is huge, sometimes even reaching more than 30% of the ink used.
Traditional free radical UV inks are still difficult to use for printing flexible packaging products due to problems such as adhesion, odor and migration.
3)High adhesion properties
Solvent-based inks and water-based inks are both volatile drying inks. As the solvent and water evaporate, the resin in the ink will fully wet the film, forming good van der Waals forces and intermolecular forces, which will produce good adhesion. Solvent-based inks have better adhesion due to their small molecular weight and high wetting ability.
The monomers and some resins used in conventional free radical UV curing inks will shrink by 3-10% during the curing process, causing a large displacement between the ink and the substrate, resulting in a decrease in the adhesion of the ink. Although a small amount of thermoplastic resins, such as thermoplastic acrylic resins, polyurethane resins, and chlorinated polypropylene resins, can be added to its formula to reduce the shrinkage deformation of the ink to less than 3%, effectively improving the adhesion of the ink, these thermoplastic resins, because they do not participate in UV curing, cause the physical properties of the film-forming material, such as wear resistance, chemical resistance, scratch resistance, and anti-rebound resistance, to be seriously reduced.
Cationic UV inks have good adhesion to film products. The shrinkage deformation of the monomers and resins used is usually less than 3% during curing, and some monomers are even slightly expanded. A good formula will make the shrinkage stress of the ink close to 0 during curing, and produce good adhesion with the film. Since the shrinkage deformation of the ink is very small, the printed product is not easy to deform, so the surface is smoother and less prone to deformation problems such as curling, which is more conducive to subsequent processing.
Of course, the reason why cationic UV ink has good adhesion to common films such as BOPP, PE, PET, PVC, PS, PC, PA, etc., in addition to the small shrinkage of cationic UV ink during curing, its unique structure and high polarity are also one of the main factors.
4)Curing speed and influencing factors
Cationic monomers have good adhesion to thin films due to their chemical properties, so less or no resin can be added to cationic UV inks, which means that the number of cationic functional groups per unit volume will be relatively large. In addition, the chain reaction during cationic curing is not sensitive to oxygen and there is no disadvantage of oxygen inhibition. In this way, high-speed curing cationic UV inks can be designed.
The curing speed of cationic UV inks is mainly restricted by four factors:
1 Photoinitiator addition ratio
2 UV radiation intensity
3 Ink temperature and ambient temperature
4 Ambient humidity
Application fields of cationic UV ink
1
Composite flexible packaging
2
Industrial heavy packaging
3
Bottle label products
4
In-film label
5
Protective film
6
Digital printing field