Photoinitiator Migration Limits in Food Contact Print Finishes
Photoinitiator migration limits demand complete photopolymer conversion or functional containment to keep extractable residues below 10 parts per billion.
Type II Photoinitiators
Bimolecular Initiator
Molecules that require a co-initiator or hydrogen donor to generate free radicals upon exposure to ultraviolet light are widely used in UV-curable coatings for paperboard. These substances, known as type ii photoinitiators, absorb light energy and form an excited state but cannot fragment on their own. They must react with an amine synergist to start the polymerization process.
This bimolecular mechanism is common in overprint varnishes.
Reaction Kinetics
The rate of radical generation depends on the concentration of both the photoinitiator and the hydrogen-donating species in the wet coating. Because this is a bimolecular reaction, it is slightly slower than the cleavage of unimolecular initiators. However, these compounds are highly effective at curing the surface of the ink film, which is often inhibited by oxygen.
This surface cure provides excellent scratch and scuff resistance.
Industrial Application
Managing the migration of these initiators and their amine co-initiators is a major focus for converters producing food-grade folding cartons. If the amine synergist is not fully consumed, it can remain mobile and migrate through the paperboard, causing unpleasant odor and migration issues. To prevent this, ink formulators use polymeric amines that are too large to migrate.
Testing the finished prints with migration assays ensures that the cured varnish meets regulatory limits. This dry film testing is necessary to confirm that the package will not affect the taste or smell of the food. It ensures a safe barrier for the consumer.