Self-curing acrylate resin for UV inks and coatings for indirect consumer product packaging applications

The discovery of contamination with Isopropyl Thioxanthone (ITX) in various Nestlé baby food brands in 2005 caused a huge stir in the European food packaging market. Millions of liters of product had to be withdrawn from store shelves. At that time, ITX was used as a photoinitiator in UV curable printing inks on the outer surface of liquid milk cartons. Despite the presence of aluminium as functional barrier, ITX was transferred to the inner side of the packaging through set-off in the reel. In contact with the food, ITX migrated from the inner side of the packaging into the food.

In 2009, the German authorities reported a migration of 4-Methyl Benzophenone (4-MBP) from cardboard boxes produced in the Netherlands containing muesli above the Specific Migration Limit (SML). In this case case, 4-MBP, present in an over print varnish (OPV) migrated through the cardboard and its’ polyethylene liner (which is not a functional barrier either), into the muesli. In this case, the muesli manufacturer (after consultation with the food authorities (who are the food authorities?) decided to take remaining muesli inventories off the market.

Both ITX and 4-MBP issues have put the use of UV curing for food packaging in a negative spotlight. Both standard photoinitiators were able to migrate into the food due to their very small molecular weight (about 200-250 Dalton). Since then, to improve the safety of UV curable inks and varnishes, so called polymeric photoinitiators have been developed. With polymeric photoinitiators, reacting the functionalized photoinitiator to a polymeric backbone increases its’ molecular weight and by doing so decreases its potential to migrate. The molecular weights of polymeric photoinitiators are typically 700 Dalton or higher.

Polymeric photoinitaitors can suffer some draw backs such as:

- Poor solubility in acrylates,

- Reduced ink flow leading to problems in ink transfer in ink ducts and ink rollers

- Low reactivity making it necessary to reduce printing speed.

To overcome above issues, Allnex has developing a “self-curing acrylate resin” which makes the addition of photoinitiators in printing inks or varnishes unnecessary.