UV Ink Innovation in Screen Printing Applications (I)

UV ink composition and characteristics

In order to understand the latest technological advancement of UV inks and the improvement of the specific use performance of UV inks, it is helpful to grasp the basic knowledge of ink chemistry. UV ink formulations are mainly monomers and oligomers. The monomer is a low molecular weight reactive diluent that produces a homogeneous solution that imparts surface characteristics to the ink. There are three types of monomers: low-activity monofunctional monomers, moderately active bifunctional monomers, and trifunctional monomers. The trifunctional monomer cures fastest and produces the hardest surface. The monomer is 100% solid and does not release volatile organic compounds like solvents. Once the monomers are cured, they become part of the polymer matrix of the ink film.

Monomers impart the surface properties of the ink, and the oligomers determine the final properties of the cured ink layer, including flexibility, weatherability, and chemical resistance. The oligomers have a high molecular weight and form the main chemical entity of the UV ink. It has several kinds, there are epoxy acrylate, polyester, polyurethane and other kinds. When cured, they are linked differently at the molecular level. Acrylic type is most commonly used in screen printing inks because of its versatility.

Photoinitiators are important components for the initiation and completion of UV curing. They absorb the UV energy that the light source accumulates on the surface of the print. The UV energy causes the photoinitiator to split into active materials. These reactive materials then initiate the chemical reaction, ie, the polymerization reaction. Polymerization converts liquid inks into solid ink layers. There are two commonly used photoinitiators for screen printing inks: free radical photoinitiators and cationic photoinitiators.

UV light does not have enough energy to allow the active molecules in the ink layer to interact to generate free radicals. Therefore, a free radical photoinitiator should be added to the formulation. When photoinitiator molecules are exposed to certain wavelengths of UV light, they absorb light to generate free radicals, initiate cross-linking reactions, and as a result, undergo transient polymerization. Radical photoinitiators are most commonly used for more than 90% of UV screen printing inks.

UV inks formulated with cationic photoinitiators generally contain monomers and oligomers that are different from those formulated with free radical photoinitiators, but the true difference between these inks lies in the way photoinitiators react. Cationic photoinitiators are generally aryl tetravalent sulfide salts that, when exposed to UV light, generate an acid catalyst that initiates the polymerization reaction. Unlike the chemistry of free radicals, the cation reaction will continue after removing the UV light. Cationic photoinitiators account for only 5% to 8% of screen printing UV inks.

The physical state of the ink after curing is greatly affected by the light source structure. The final performance of the cured ink layer depends on the following four main factors:

1, UV radiation, expressed in mW/cm2

2, spectral output, UV light wavelength unit is nm

3, UV energy, expressed in mJ/cm2

4, infrared energy or heat

The latest UV inks have better hiding power and the thick ink layer can also be cured. It is no longer important to understand what UV exposure is and how it acts on the ink. There are many interactions between UV screen printing inks and UV lamps, all of which affect the performance of the finished ink and determine the optimal configuration of the curing device.

The two key factors in the curing process are the amount of radiation and energy that printers often measure but often misunderstand. The amount of radiation is the radiant power reaching the surface of the ink. The peak value of the radiation is the highest power value. It occurs at the focus of the UV light. If the reflector of the curing device does not collect light well, the amount of radiation is lower than the optimum value. It is the decrease of the curing speed or the insufficient curing of the ink layer.

UV energy is the product of two independent variables, radiation dose and time, and 1 watt per second equals 1 Joule of radiant energy. Radiation energy is generally mentioned when there is no indication of the amount of radiation, and the combination of the two will produce the best curing conditions. When curing pigments with high content, opaque inks, or thick ink layers, higher amounts of radiation have a greater effect on curing than energy.

Together with photoinitiator suppliers, ink formulators have developed inks that match the UV output of medium-pressure mercury vapor lamps. These lamps are used in most curing equipment for screen printing. Photoinitiators designed for pigment inks generally absorb wavelengths of 330 to 400 nm. For ink formulators, two different properties of the photoinitiator absorption curve must be taken into account.

1. Which wavelength of light is absorbed?

2. What is the intensity of absorption?

It is important to understand that all components of UV inks absorb UV energy but absorb different wavelengths. This concept is illustrated in Figure 1 and compares the absorption of pigmentless UV inks with pigment inks.

Figure 1 The UV component of the ink composition varies greatly. The above table illustrates this. Pigments and resins are affected by shorter UV wavelengths, and photoinitiators absorb more strongly for longer periods (to be continued)