UV ink curing process (below)

Beer-Lambert's law states that the energy of UV light is reduced when passing through any material. When light energy passes through the ink layer, it is not absorbed and reflected by the uppermost ink layer, but passes through the material and reaches the bottom layer. We can use the following formula to describe the reduction of light energy.

In the formula, Io denotes the light energy in the range of wavelength λ, Ia denotes the absorbed light energy, Aλ denotes the absorbance in the wavelength λ range, and d denotes the ink film thickness.

According to the above formula, the relative light energy absorbed by the uppermost layer (1% thickness) and the lowest layer (1% thickness) of the ink film can be seen, and the light energy absorbed in the two regions is completely different, as shown in FIG. 1 .

When examining the spectral absorbance of photoinitiators, pigments, and prepolymers, we can clearly see that short-wavelength UV light energy (200-300 nm) is completely absorbed when it reaches the surface of the ink layer and cannot reach the film at all. The bottom of the floor. In general, the thickness of the ink film layer is certain, so the adhesion strength of the ink to the printing material is the first consideration. Even the photoinitiator absorbs all the light energy in its sensitive spectral range, so that the light energy in this part of the wavelength range cannot be absorbed by deeper photoinitiators, as shown in FIG. 2 and FIG. 3 . It can be seen that the photoinitiator is very suitable for a very thin transparent coating, not suitable for a certain thickness of ink, for the ink, should choose a long range of photoinitiators.

Most UV curing light sources contain two wavelengths of UV light energy (or three wavelengths of light energy if infrared energy is added). The short wavelength UV light energy is suitable for the curing of the ink film surface, and the long wavelength UV light energy is suitable for the deep curing of the ink film. This is mainly because the short-wavelength UV light energy is completely absorbed by the ink film surface and cannot reach the deep layer of the ink film. If the short wavelength UV light energy radiation is insufficient, the ink surface will be sticky; if the long wavelength UV light energy radiation is insufficient, it will affect the ink on the substrate surface of the auxiliary force. The thickness of the ink film layer benefits from the proper ratio of long- and short-wavelength light energy.

The ordinary mercury lamp radiates light energy of two wavelengths, but it radiates light energy of short wavelengths very strong, so it is more suitable for solidification of a thin coating of the ink layer. Materials with high absorbance, such as various additives and screen printing inks, contain photoinitiators with a long wavelength range, and therefore require long wavelengths of light energy for curing. Adding some other components to the mercury lamp allows the light source to radiate a large amount of UV light energy with a long wavelength, and of course radiate light energy with a short wavelength, so that the ink surface is well cured, as shown in FIG.

Adding a certain amount of metal halide to the UV lamp can improve the spectral range of the lamp. The amount of metal halide added is calculated accurately.

Effect of the optical factors of the curing equipment on the curing process

In addition to the ink itself, there are many factors that can affect the curing effect of the ink. These factors are the optical and physical properties of the curing equipment, and UV exposure equipment is the main factor.

1, UV light intensity

Indicates how much UV light energy is obtained per unit area in the specified wavelength range. The UV light intensity indicates the photon flux and the unit of measurement is W/cm2 or mW/cm2. The light intensity differs depending on the output power of the light source and its distance from the surface of the irradiated object. Directly below the light source, the light intensity is the strongest. Light intensity is a manifestation of the combined performance of power supply power, light source radiation, reflectance, focus, lamp size, and geometry. The intensity of light measured over a range of wavelengths is called the effective light intensity. The light intensity obtained on the surface is strong, and the UV light energy received by the ink and the coating is strong. The depth of the curing is mainly affected by the light intensity of the radiation, but has little to do with the length of the exposure time. The light intensity plays an important role in the solidification of the ink having a high light absorbency (good hiding property).

2, spectral distribution

It describes the relative radiant energy of the wavelength emitted by the lamp or the wavelength distribution of the energy of the light reaching the surface of the ink. In order to describe the distribution of UV energy, the distribution of light energy is usually based on the energy of the irradiated light in the wavelength range of 10 nm. This makes it easy to compare the spectral distribution of each different light source. In general, manufacturers of electric light sources will provide spectral profiles for each type of electric light source.

3, the effective energy of UV light source

It refers to the intensity of radiation reaching a unit area in a given wavelength range. It represents the total number of photons that reach the surface of the object. The energy of light is inversely proportional to the speed of photons and is proportional to the number of exposure light sources (the number of curing channels or the number of light sources arranged). The effective energy of the UV light source is a time accumulation of the radiant energy on the surface of the printing material, because the printing material is dried and solidified by a light source or another light source, and the effective energy is usually represented by J/cm2 or mJ/cm2.

4, infrared radiation intensity

The infrared radiation intensity indicates the infrared energy emitted by the quartz body in the UV light source. Its measurement unit is the same as the unit of the light intensity, and the surface temperature it generates may be beneficial and may cause troubles.

Most UV-curable inks have "optical thickness". The surface of the ink absorbs a lot of light energy. According to the different pigments in the ink and the thickness of the ink layer, the absorption rate of light energy in a certain wavelength range will also be different. The spectral absorbance of the ink during curing is a very important parameter. The absorption rate of the UV light source will affect the cured thickness of the ink, and the infrared absorption rate affects the observed temperature.

In order to achieve the best effect of the UV ink curing, the UV ink curing process and manner should be selected according to the UV ink printing product, the UV ink photochemical characteristics and the UV curing light source characteristics. Pay special attention to the optical characteristics of the ink and the matching of the ink and the curing light source so that the UV curing equipment can have a very relaxed operating environment.

The optical properties of the electric light sources and their interaction with the optical properties of the UV inks make the curing process perfect.

Source: 21st Century Fine Chemicals Network