Polyester-based can coatings are used as replacements for epoxy-based coatings containing bisphenol A (BPA, CAS 80-05-7). During storage, polyester coatings can release non-intentionally added substances (NIAS), such as oligomers (FPF reported). Two studies published in 2018 also addressed the oligomers in polyester coatings.
An article by Emanuela Pietropaolo and colleagues from the Dipartimento di Scienze e Innovazione Tecnologica, Universita del Piemonte Orientale “Amedeo Avogadro,” Alessandria, Italy, published on October 4, 2018, in the peer-reviewed Journal of Chromatography A, presented a method for the conclusive identification and exact quantification of 16 oligomers (four linear and 12 cyclic) identified in two different types of polyester resins and final enamel can coatings. The method uses “ad hoc standards . . . synthesized by acylation reaction between alkyl diols and phthaloyl chlorides.” The different combinations of extraction conditions (at 60 °C in 95% or 50% ethanol for four hours or ten days) “showed no significant differences between the amounts of cyclic oligomers,” as most “extracts showed only a small amount of linear compounds and a prevalence of 2 + 2 cyclic oligomers.” The authors conclude that their study “shows the great importance of the synthesis of specific standards to allow exact quantification in food contact material migrates.”
The article by Malcolm Driffield and colleagues from the Packaging and MS Solutions team at the UK research agency Fera Science Ltd. published on February 26, 2018, in the peer-reviewed journal Food Additives & Contaminants: Part A, compared the concentrations of monomers and selected oligomers in foods with concentrations after migration or extraction. The scientists analyzed two different polyester coatings, both based upon 5-norbornene-endo-2,3-dicarboxylic anhydride (CAS 129-64-6), which were “commercially applied, packed with different food stuffs, processed and stored” for various time periods between eight and 43 months. The examined foods were selected “to represent a range of aqueous and acidic processed foods” and included beans, peas, corn, tomato products, soup, pumpkin, potatoes, and Jalapeno peppers. For extraction and migration experiments, “laboratory-prepared and cured panels” coated with either coatings were used. The laboratory procedure “mimicked as closely as possible industrial application and cure in order to minimize any potential differences between the coated panels and cans tested.” Extraction was done with acetonitrile for 24 hours at room temperature, and migration was carried out in water or ethanol at 121 °C for one hour followed by ten days at 40 °C.
The results showed that (1) “migration of selected monomers into the real foodstuffs analyzed, apart from two [(cream style corn and pumpkin)], did not exceed 50 µg/kg,” (2) “migration of oligomers into 50% aqueous ethanol . . . [was] higher than that into real foodstuffs,” (3) “migration levels determined in foodstuffs following long-term storage were significantly lower than those reported for simulants exposed to a similar coating following long-term storage”; this “question[s] the suitability of those conditions to represent realistic migration; (4) “there are no obvious trends in monomer or oligomer levels associated with different foodstuffs”; therefore, the authors recommend “to determine levels in the foodstuffs of interest.”
Pietropaolo, E., et al. (2018). “Synthesis, identification and quantification of oligomers from polyester coatings for metal packaging.” Journal of Chromatography A (published October 4, 2018).
Driffield, M., et al. (2018). “The determination of monomers and oligomers from polyester-based can coatings into foodstuffs over extended storage periods.” Food Additives & Contaminants: Part A 35:1200-1213.