ANALYTICAL APPROACHES TO IDENTIFY POTENTIAL MIGRANTS IN POLYESTER-POLYURETHANE CAN COATINGS
Résumé
Abstract The safety of a polyester-polyurethane can coating has been assessed by 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 twelve cyclic and one linear polyester oligomers up to molecular weight 800 and containing up to six monomer units. These 13 oligomers were in 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 (< 0.01 µg/dm2) of the blocked isocyanate used as the starting substance and no detectable (< 0.02 µg/dm2) residue of the unblocked isocyanate either. The level of extractable IPDI was used as an indicator of the completeness of cure in experimental coatings. These studies revealed the 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 allowed parameters to be specified to ensure that commercial production coatings receive a full cure giving low migration characteristics.
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