Water-soluble Photoinitiators: Present and Future
While photopolymerization of water-based formulations is a well-established technology, especially for coating applications, and water-soluble photoinitiators have been known for a long time, they did not find widespread use in industrial applications. Preferentially liquid commercial oil-soluble photoinitiators, possibly in combination with a dispersant aid, can in fact successfully be used in most water-borne UV-curable coatings and inks. Research on new developments aimed at water-soluble photoinitiators was thus mostly restricted to academia. However, in recent years new technologies, such as UV-LED inkjet printing for packaging applications, or the increasingly important additive manufacturing processes in tissue engineering and other biomedical applications, which are based on photolithography of aqueous formulations, created a new need for fully water-soluble photoinitiators that in addition meet requirements such as absorbance in the near-UV–visible, low migration or favorable cytotoxic properties. These requirements, which also created a significant market need, have stimulated significant activity in this field in recent years, not only in academia, but also in industrial, application-focused research. Besides optimization of known concepts aimed at providing water-soluble photoinitiators, several novel approaches have thus been proposed and evaluated, and the introduction of water-soluble photoinitiators as commercial products is close to realization. This chapter gives an overview of recent developments with new water-soluble photoinitiators, but also includes some novel applications of known compounds in aqueous formulations.