The physical–chemical behaviour of amino cross-linkers and the effect of their chemistry on selected epoxy can coatings’ hydrolysis to melamine and to formaldehyde into aqueous food simulants

Abstract In this study, the suitability of four chemically different melamino cross-linkers for use in formulating epoxy coatings was investigated on the basis of the composition and of the tendency of cured coatings to hydrolyse to melamine and to formaldehyde when they were retorted in aqueous food simulants. The four cross-linkers were characterised for their composition identity, flow behaviour, thermal stability and for the presence of residual species. The different cross-linkers were used individually to cross-link selected epoxy coatings. The effects of the cross-linker chemistry, the curing conditions and the kinetics of the hydrolysis and subsequent migration processes, leading to melamine and formaldehyde were investigated following thermal treatments that were designed to represent the conditions of food sterilisation. The results show that each cross-linker type is different in its rheological characteristics, its solids content, its thermal behaviour and its physical properties. The chemistry of each cross-linker plays a major role in the manner in which the epoxy coatings undergo hydrolysis to release melamine and formaldehyde. The greatest migration of melamine (from an unpigmented epoxy anhydride coating, cured with the hexamethoxymethyl melamine cross-linker) into the 10% (v/v) aqueous ethanol food stimulant, after retorting at 131 °C, for 1 h was 525 μg/6 dm 2 . The greatest migration of formaldehyde into the simulant was also from this coating at 11 μg/6 dm 2 , when retorted at 131 °C for 1 h. The curing conditions affected the extent of the cross-linker hydrolysis. The influence of varying the curing time and the curing temperature was used to control the hydrolysis of the cross-linked, epoxy-based coatings. A decrease in the extent of cross-linker hydrolysis by 50–80% was achieved in all cases as the temperature of the curing was increased, in stages, from 160 °C to 200 °C.