Propoxy Substituted Thioxanthones As Photoinitiators And Sensitisers For The Uv Curing Of Pigmented Systems

Introduction

The ultra violet curing of pigmented surface coatings
is now well established in the litho and screen printing
sectors of the graphic arts. In these systems the pigments
themselves cause considerable constraint by absorbing uv
irradiation and competing with the photoinitiators for the
available energy. The darker pigments, blues and blacks are
much difficult to cure than the lighter reds and
yellows because of. this pigment absorption effect. (See
Figure 1). Photoinitiators are therefore required which can
absorb uv energy in a part of the spectrum where the
pigments have a absorption and this "absorption
window" is typically 300-400 nm. A further consideration is
the wavelength and relative output of the uv source, which,
for a typical medium pressure mercury lamp consists of
narrow line outputs at 254, 313, 366 and 404 nm. The
photoinitiator must have some absorption at one of these
lines in the "window" in order to absorb uv energy and
produce free radicals. Isopropyithioxanthone which has an
absorption maximum at 383 nm is widely used in pigmented
systems and has become an industrial standard sensitiser.

Recent work on propoxy substituted thioxanthones has
endeavoured to move the long wave absorption peak of the
initiator closer to the 404 nm output. This should further
reduce the absorption effect and at the same time
pick up longer wavelength energy which has greater
penetrative power and to better depth cure of
highly pigmented coatings.