Characterisation, release and migration of phenolic compounds from resoles used in polyester-phenol coatings intended for food contact materials

Resoles are multifarious pre-polymeric resins produced by the condensation of basic chemicals phenols, formaldehyde and optionally aliphatic alcohols like butanol. They are widely used as cross-linkers to form resistant internal coatings on metal surfaces of cans, containers or closures. Although the application of resoles is common in food contact, usually little is known about their exact composition, the toxicological hazards of their individual constituents and the migration of phenolic compounds, e.g., of the potentially endocrine-disrupting chemical bisphenol F. Our study fills major gaps of knowledge in risk assessment, using the example of a two-layer polyester-phenol coating system, which is based on three different resoles and is commercially used for closures of infant food glass jars. Various analytical approaches, namely size-exclusion chromatography, nuclear magnetic resonance spectroscopy, liquid chromatography coupled to mass spectrometry, fluorescence and diode array detection as well as gas chromatography-mass spectrometry were evaluated to quantitatively characterise resoles. Additionally, derivatisation with dansyl chloride as well as Folin-Ciocalteu colorimetric assay was adapted first times to determine the total phenol content from technical resoles. Individual mono- and bisphenols were determined in resoles up to about 120 mg/g, while the concentration of bisphenol F isomers was below 10 mg/g. Migration from the coating system was determined after sterilisation (121°C, 1 h, 20% ethanol). Results were ~2 µg/dm2 for identified individual mono-phenols like 2-hydroxybenzyl alcohol and up to ~120 µg/dm2 for total phenolic compounds, representing approximately 7% of the overall migration. The migration of bisphenol F isomers was negligible below 0.3 µg/dm2. Potential exposure to migrating phenols was assessed based on the threshold of toxicological concern concept to be significantly below for individual phenols and in the same order of magnitude for total phenols compared to the respective thresholds calculated for infants.