Photoinitiators photolyze to produce products with various structures, some of which become darker in color due to the increased degree of conjugation or the influence of chromogenic substituents. The absorption wavelength of the colored product is not necessarily linearly correlated with the overall yellowing, but is also related to the width and intensity of the absorption peak and the interference of impurities. The common Irgacure369 and Irgacure907 photoinitiators have serious yellowing, which is related to their special substitution. They are not suitable for use in varnishes, white or light-colored systems, but are widely used in inks. The substituted benzoyl radicals produced by the photolysis of these two photoinitiators have amino or alkylthio groups in the para position, and the strong conjugation of the N and S atoms aggravates the light absorption effect of the product. Generally speaking, for cleavage photoinitiators, if the aromatic ring is damaged and there is a substituted amino group, the yellowing is more serious.
Our company's cationic photoinitiator Uyracure-180 is triarylsulfonium hexafluoroantimonate. Because the sulfur atom can be conjugated with three aromatic rings, the positive charge is dispersed, and it can be cracked after light excitation to produce polymerization active species. It has stronger reactivity than phosphates and does not yellow. The addition amount only needs 40-60% of triarylsulfonium hexafluorophosphate. It has higher crosslinking hardness, no migration, no odor, temperature resistance, and good deep curing. The absorption wavelength is 290nm, which has both high performance and economy. It is used in cationic curing transparent coatings, adhesives, LED packaging, inks and other fields.
