Ink composition and curing judgment

Ink curing is an extremely important link in the entire printing process and plays a decisive role in the quality of the product.

Compared with traditional inks such as solvent-based inks, water-based inks, and oxidized conjunctival inks, UV inks have the advantages of environmental protection, good printing effects, high production efficiency, and low cost of use.

The basic composition of UV ink

UV inks are divided according to printing methods, which can be divided into UV embossing inks, UV offset printing inks, UV flexo printing inks and UV screen printing inks, which are mainly composed of the following parts.

1.

It is equivalent to the resin in ordinary ink, which determines the printing performance of UV ink and various test performances, such as friction performance and solvent resistance.

2.

It is mainly to adjust the viscosity of UV ink and improve the fluidity of UV ink, which is equivalent to the diluent in ordinary ink.

3.

The main function is to absorb the energy of ultraviolet light to form free radicals, and to stimulate the cross-linking reaction of monomers and prepolymers to form a stable polymer with a network structure.

4.

Mainly determine the color of UV ink, varnish does not need to add pigments.

5.
Additives

Including leveling agents, surfactants, defoamers and other additives to improve the performance of UV inks.

The basic principle of UV ink curing

UV ink is cured by the action of ultraviolet rays.
Under the irradiation of ultraviolet rays, the photopolymerization initiator in UV ink absorbs photons of a certain wavelength and then turns into free molecules, which become free radicals.
Then through the transfer of energy between molecules, the polymerizable prepolymer and photosensitive monomer become In the excited state, a charge transfer complex is produced, and the complexes are continuously cross-linked and polymerized to solidify into a film.

The curing process of UV ink is a photochemical reaction process, that is, under the action of ultraviolet energy, the prepolymer is cured into a film in a very short time.
In addition to curing the surface of the UV ink, ultraviolet rays can penetrate deep into the liquid UV ink and stimulate the further curing of the deep ink film.

In traditional inks, oil-based inks are dried under the action of oxidation, while solvent-based or water-based inks are mainly dried by the evaporation of water or solvent, and part of the ink can penetrate the paper.
Therefore, compared with traditional inks, the polymerization and curing of UV inks are more thorough, without any evaporation or solvent contaminants, and the ink film is 100% cured.

The basic method to determine its curing

Combining the characteristics of UV ink itself and drawing on traditional ink drying judgment methods, the following methods can be used to judge the curing of UV ink.

1.
Check the adhesion of printing ink after stacking or winding

If the traditional oxidized conjunctival ink is not completely dried, the printed products will stick to each other after they are piled up or wound.
Similarly, printed products printed with UV inks will also have this problem if they are not cured thoroughly.
So this method can be used to judge the curing effect of the ink.

2.
Thumb pressing method

This method is the most convenient and fastest test method.
The basic process is to use your thumb to firmly press the cured surface of the printed product, and then squeeze it back and forth several times.
If the pressed place becomes rough or worn, and the surface of the thumb If there are ink marks, it means that the ink is not cured sufficiently.

3.
Nail test method

This method is similar to the thumb pressing method.
The basic basis is that the UV ink is cured from the surface and then gradually enters the interior.
If the ink is not fully cured, scratches will appear after nail scratching, and the fully cured UV ink will not be scratched, so as to determine whether the ink has been cured.

4.
Tape peeling test

Since UV inks are currently widely used on film materials or non-absorbent substrates (such as aluminized paper), the tape test method is applied to test the ink bonding performance.
Taking into account that if the UV ink is not fully cured, it will cause the ink and the printing material to become weaker, then after the tape test, the ink will be peeled off the surface of the printed product by the tape.
The amount of ink peeling is related to the type of tape, peeling speed, and ink curing conditions.

5.
Chemical test methods

Many end users regard the resistance of printing inks to chemical solvents (alcohol) as an important aspect of evaluating the printing performance of their products.
The fully cured UV ink has good chemical solvent resistance, while the uncured UV ink has poor chemical solvent resistance.
Insufficient curing of UV inks will have a rigid surface, but in fact the inside of the ink layer is still not cured and is soft, so it has poor resistance to chemical solvents.
In addition, acetone is also very suitable for testing the degree of curing of UV inks.
The method is to wipe the coating with a cloth soaked in acetone.
If the curing is not good, the coating will be dissolved.

6.
Other methods

The German Printing Association (FOGRA) recommends some test devices to check the anti-peeling and anti-blocking ability of UV inks, which can be tested according to the methods provided by the German industrial standards DIN16524 and DIN16525.