Migration of melamine from thermally cured, amino cross-linked can coatings into an aqueous ethanol food simulant: aspects of hydrolysis, relative reactivity and migration

Aspects of melamine migration from epoxy-based coatings into a food simulant were studied. Four commercial amino-based cross linkers were incorporated into an epoxy anhydride coating system and into an epoxy phenolic coating system. The epoxy-based coatings were formulated, applied, cured and tested for migration by retorting in contact with the food simulant, 10% ethanol, at 131°C. The commercial melamino-based cross linkers used and the model coatings that were prepared using these cross linkers contained very low or non-detectable levels of free melamine. However, during retorting, the migration of melamine from the coatings increased as the retorting time was increased. This migration process is not the more classical diffusional process but rather the result of chemical attack (hydrolysis) of the coating. For these model can coatings, a substantial fraction of the melamino cross linker was hydrolysed although, curiously, the essential functional properties of the coating are retained. In all cases, for these model systems the migration of melamine was rather low because the cross linkers are used commercially in only small amounts - typically 1-2% of the dry film weight of the coatings. For the standard retorting conditions of 1 h, migration of melamine was up to 0.4 mg kg(-1), depending on the cross linker used. The cross linker that contained the methylol functionality (-CH₂OH group) gave rise to less melamine than did the alkylated cross linkers (methylated and butylated, -OCHv and -O(CH₂)₃CH₃, respectively). This observation could prove useful in formulating coatings with even lower melamine release characteristics.