Cleaner coatings for food cans
Cleaner coatings for metal cans to inhibit surface chemical migration into food are the result of a three-year study funded by Defra.
The project, called ‘New technologies and chemistries for food can coatings’, was managed by Julian Stocker from Heinz UK, based in Cheltenham. The goal was to study coating migration and how it can be minimised. ‘Present coatings are safe and comply with extensive international regulations,’ says Stocker. ‘But for food manufacturers this is not enough, their target must be zero contamination of food by the packaging.’
Paul Stenson, Chief Technology Officer at Valspar, an international coatings manufacturer that collaborated on the project, adds, ‘There has been continuing interest from consumer advocacy groups to understand the exact migration characteristics of coatings. This is also driven by the new REACH legislation, which affects the whole chemical industry in Europe’.
The researchers conducted kinetic tests on chemical migration from epoxy phenolic, epoxy anhydride, organosol and polyester polyurethane coatings. An approach using liquid chromatography time-of-flight mass spectrometry was developed to give more accurate information on the mass of individual molecules. This removed the need for authentic chemical standards for each possible minor impurity and reaction by-product.
Scientists found that the key parameters influencing the migration of chemicals into cans are – the level of wax used to lubricate the coating, the coating’s age, curing times, film thickness and food temperature. Each factor is dependent on the requirements of the food being packaged.
‘All coatings have to form a protective barrier. However, a coating which excels for one property may fail for another,’ explains Stocker. ‘All of the coatings were developed for reduced migration before undergoing extensive performance tests.’ Ideally, epoxy phenolic coatings should be cured for 10 minutes at 195-205ºC peak metal temperature. Five to eight grams per square metre is a good weight for obtaining the preferred film thickness.
While the composition of the epoxy-based coatings did not change, some technical advances were made to better understand the kinetics of their curing, says Stenson. ‘Progress was also made to further refine non-epoxy-based chemistries,’ he adds.
Valspar has commercially released faster-curing epoxy phenolic and polyester polyurethane coatings for three-piece food can interiors, and is continuing to develop on the solvent and water-based coatings created by the Defra project. Heinz has also adopted a new coating for some of its tins, though Stocker could not reveal details of its composition.
Leeds University, UK, and packaging specialist Impress, based in Deventer, The Netherlands, also worked on the research.