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Thioxanthones have been used as photoinitiators (P1) for curing pigmented coatings
since the mid 1970's (1). Of the various commercialized thioxanthone photoinitiators, ITX
has emerged as the most widely used based on its performance and cost. ITX is an
off-white to yellow solid with a broad melting point (49-72°C) due to the existence of both
2- and 4-isomers. ITX has an ultraviolet (UV) absorption spectrum which contains a peak
maximum at 385 nm and an extension coefficient of about 6500 I.mol-1,cm-1. It is this
absorption band that makes ITX and other thioxanthone photoinitiators efficient in curing
pigmented coatings since most pigments efficiently absorb light of wavelength less than 370
nm.
Although ITX has several attributes that make it an attractive P1 for curing
pigmented systems, it also has some major limitations. ITX is considered to impart a yellow
color to white coatings. Because of this it has found applications for use in darker shades
or in coatings where poor color is of little importance (2). Although ITX has better
solubility than other commercial thioxanthones, it is much less soluble in most resin systems
than benzophenone, ketals and acetophenone derivatives (3).
It is clear that if the color and solubility of ITX were improved, its use in pigmented
coatings, including whites, would be further broadened. Last year at RadTech North
America and again at the PRA conference in Harrogate we announced the results of
research directed at improvements in the ITX process which resulted in "White ITX".
Thioxanthone research has continued in our laboratories and has resulted in the synthesis of
a liquid thioxanthone (LTX) with excellent solubility, initiator activity equal to and in some
cases superior to ITX, and with reduced yellowing tendencies similar to "White ITX".
