In-depth Discussion on the Preparation of Inks for UV-curable Inkjet Technology

The most crucial consumable in inkjet printing is inkjet ink—a liquid ink that, under the influence of the electric field between the printhead of an inkjet printer and the substrate, can be jetted onto the substrate as required to form images and text.  It is a high-performance specialized ink that must meet strict requirements: it must be stable, non-toxic, and non-clogging to nozzles;  it should possess excellent moisture retention and jetting performance;  it must not corrode metal components such as printheads, nor be degraded by bacteria;  and it should be non-flammable and resistant to fading, among other properties.

The surface tension, viscosity, drying property, and color density of ink are critical factors in inkjet printing.  The chemical composition and performance of inkjet inks determine the quality of printed images, the characteristics of ink droplet ejection, and the reliability of the printing system.

When UV inks are used in inkjet printing, they must not only possess the general properties of UV printing inks but also specifically address three critical technical challenges: low viscosity, stability, and oxygen inhibition.

 ✿   low viscosity

Inkjet inks must have low viscosity.  Typically, the viscosity of thermal inkjet inks is 3–5 mPa·s at 25°C, while that of piezoelectric inkjet inks ranges from 3–30 mPa·s at the same temperature.  Therefore, it is quite challenging to formulate UV inkjet inks with a viscosity of <30 mPa·s.

To achieve this viscosity range, low-viscosity oligomers and reactive diluents must be used.  However, low-viscosity oligomers and reactive diluents usually have lower functionality—and this is unfavorable for improving the UV curing rate, as well as for the dispersion and stability of pigments.

 ✿  stability

The colorants used in UV inkjet inks are primarily pigments, which require nanoscale fineness.  These pigments must be dispersed in a low-viscosity polymer system without agglomeration, and no sedimentation should occur during storage.

To achieve this, it is necessary to select oligomers with good pigment wettability, pigments with excellent dispersibility, and dispersants with high stability.  After grinding and processing, an ink with a fineness of <1 μm, no sedimentation, and no agglomeration during storage is obtained.

 ✿  oxygen inhibition

When tiny ink droplets of UV inkjet ink are ejected from the printhead, they have an extremely large surface area.  This increased contact with air leads to more oxygen dissolving into the ink, and oxygen inhibition severely hinders the UV curing of the droplets.

Currently, the printhead movement speed of large-format inkjet printers is approximately 30 m/min—this requires the ink to have a fast UV curing rate and strong resistance to oxygen inhibition.  Therefore, in the formulation of UV inkjet inks, it is essential to address oxygen inhibition: high-activity photoinitiators, oligomers and reactive diluents with fast UV curing rates should be selected, and necessary anti-oxygen inhibition measures must be implemented.

To overcome the low viscosity challenge of UV inkjet inks, UV inkjet printer manufacturers install heating devices at the ink storage locations, which can heat the ink to approximately 50°C.  This provides favorable conditions for UV inkjet ink manufacturers to select oligomers and reactive diluents.

Meanwhile, UV inkjet inks are currently mainly used in advertising and promotional applications, where there are no specific requirements for the mechanical properties of the ink layer.  Therefore, in ink formulations, the content of oligomers is relatively low, and a higher proportion of reactive diluents can be used.

Currently, most UV inkjet printing equipment is based on UV-LED technology.  Therefore, UV inkjet inks must be improved upon the original UV inkjet ink formulations.  Beyond using oligomers and reactive diluents that offer fast UV curing rates and excellent resistance to oxygen inhibition, the key modification lies in optimizing photoinitiators.

Next, we will systematically analyze the main raw materials of UV-LED inkjet inks and their characteristics.

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