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The Mysteries and Myths of UV Curing

(December 2006) posted on Wed Dec 27, 2006

Discover how UV inks differ from other formulations and what it takes to print and cure them successfully.

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By Bea Purcell

For a chemist who is told to formulate an ink, the first raw material to select is the resin. Resins give the ink its major characteristics, such as adhesion and flexibility. Solvents (for conventional ink) and monomers (for UV ink) are selected to dissolve or "cut" the resins and pigments in the formula to a workable viscosity. Solvents and monomers are also selected to complement the resin in achieving the desired performance characteristics of cured or dry ink film. Finally, the additives contained in these inks are substances such as pigments (for color), flow agents, thickening powders, retarders for conventional inks, and catalysts for both conventional and UV formulations. In UV inks, these catalysts are photoinitiators.

UV inks employ photoinitiators ("photo" referring to light or radiation, and "initiate" meaning to start) to trigger the reaction that allows monomers and resin to bond and create the cured ink film. The photoinitiators absorb UV energy at certain wavelengths, creating "free radicals." The free radicals connect with the molecules of the resins and monomers, and they, in turn, cross-link with each other, forming chains of molecules we recognize as the "cured" ink film. Chemists call this cross-linking reaction polymerization.

UV inks are considered "100% solids" because almost everything in them is used up in the polymerization process. In conventional inks, solvents are evaporated and the cured ink film comprises only what remains.

One of the major advantages of UV over conventional inks is that no volatile organic compounds (VOCs) are released into the air during the curing process. When drying conventional inks, however, VOCs are evaporated with the solvents. Additionally, because UV curing relies on polymerization rather than evaporation, UV inks can be cured much more quickly and in less space than solvent-based inks. Finally, the lack of solvent in UV inks allows them to be used with higher mesh counts and support finer detail and higher print resolutions.

What does a UV-curing unit do? The function of the curing unit is to deliver the UV energy that sets off the photoiniators and starts the polymerization process. However, before we explore just how this energy is delivered, it might be beneficial to review the nature of electromagnetic energy.


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