Novel Sulphonium Salt Cationic Photoinitiators For Food Packaging Applications

UV curing systems for the graphic arts are dominated by acrylate chemistry, which accounts for as much as 95% of the total radiation curing market. Although there is a number of niche alternative curing technologies, by far the most important is cationic curing. This has been known for at least as long as free-radical acrylate chemistry, but its use faltered in its early years as a result of competing patent claims with the photoinitiators. Up until the mid-i 990s cationic curing technology struggled to compete with free-radical chemistry due to its higher cost, slow cure speed and limited range of raw-materials. However, its most important advantages of excellent resistance properties, adhesion to difficult substrates and inherently superior migration characteristics meant that it continued to survive in niche markets; Towards the late 1990s the increased use of narrow web flexo printing onto plastic substrates allowed significant market growth for cationic curing, only for the market to be badly damaged by the discovery of benzene as a by-product from the most commonly used triarylsulphonium salt photoinitiators in 1999.
In terms of their use in food packaging applications, cationic curing inks are desirable because, in theory at least, the post-cure reaction will continue until all the monomer is consumed, thus minimising migration. A further significant advantage is the superior mechanical properties of cationic inks and coatings following post cure, allowing them to survive processes such as steam sterilisation and retorting which are becoming increasingly important in the food packaging sector. These advantages are countered by the cost, slow cure speed, weak inks, high odour on cure and humidity inhibition to the extent that their use is often too much of a compromise, particularly when competing against an established technology like free-radical.