Analytical approaches to identify potential migrants in polyester–polyurethane can coatings

Abstract

The safety of a polyester–polyurethane can coating has been assessed using a suite of complementary analytical methods to identify and estimate the concentrations of potential chemical migrants. The polyester was based on phthalic acids and aliphatic diols. The polyisocyanate cross-linking agent was 1-isocyanato-3-isocyanatomethyl-3,5,5-trimethyl cyclohexane homopolymer (IPDI) blocked with methylethylketone oxime (MEKO) to make a one-part formulation. The overall migrate, obtained using solvent extraction of cured films, comprised almost completely of 12 cyclic and one linear polyester oligomer up to molecular weight 800 and containing up to six monomer units. These 13 oligomers covered a total of 28 isomeric forms. Other minor components detected were plasticisers and surfactants as well as impurities present in the starting materials. There was no detectable residue of either the blocked isocyanate (<0.01 µg/dm²) used as the starting substance or the unblocked isocyanate (<0.02 µg/dm²). The level of extractable IPDI was used as an indicator of the completeness of cure in experimental coatings. These studies revealed that there was an influence of time, temperature and catalyst content. Polymerisation was also influenced by the additives used and by the ageing of the wet coating formulation over several months. These studies allow parameters to be specified to ensure that commercial production coatings receive a full cure giving low migration characteristics.

Keywords:

• can coatings
• migration
• analysis
• safety evaluation
• overall migrate

Acknowledgements

This work was carried out as a part of a Defra LINK project: FQS45 “New technologies and chemistries for food can coatings”. Funding by Defra and matching funds in kind from Valspar Corporation, Impress Group and H.J. Heinz are gratefully acknowledged. We also thank Agilent Technologies for the loan of the LC–TOF-MS instrumentation. The contents of this paper are the responsibility of the authors alone and should not be taken to represent the views of the supporting organisations.