Melamino migrants as breakdown products of applied amino resin cross-linked can coatings.

This thesis consists of seven chapters. These are: (1) General introduction; (2) Preparation of reagents, characterisation of raw materials and calibration of the curing oven; (3) Formulation procedures, coating applications and curing procedures; (4) Analyses of liquid coatings, analyses of cured coatings and migration studies; (5) Epoxy-based coatings— study of the migration of melamine, the migration of benzoguanamine and the migration of formaldehyde; (6) Poly(ester)-based coatings— study of the release and migration of melamine, of benzoguanamine and of formaldehyde; (7) General discussion/summary.
In the work that was carried out, an investigation of the phenomena of migration of melamine, of benzoguanamine and of formaldehyde, primarily during the retorting of the coatings, was undertaken. Five coating systems were investigated; these were unpigmented epoxy-anhydride coatings, TiO2 pigmented-epoxy anhydride coatings, epoxy-phenolic coatings, poly(ester)-urea coatings and poly(ester)-acrylic coatings. The coatings were applied through controlled repeatable procedures that were designed to represent industrial practices. The coatings were cured under controlled conditions relating to the temperature of curing and to the time over which the curing was performed. The curing oven was repeatedly calibrated to ensure that it was in a consistent and accurate working condition. This involved the need for a peak metal temperature (PMT), of 195 oC, maintained for 12 minutes and a PMT of 200 oC, held for 10 minutes, to be consistently achieved.
The raw material samples that comprised the samples and the coatings themselves were characterised/analysed for their compositional integrity. Several analytical techniques were used including NMR spectroscopy, FT-IR spectroscopy, mass spectrometry, scanning electron microscopy, UV-visible spectroscopy, fluorescence spectroscopy, liquid chromatography (UV detection) and gas chromatography (MS detection). Relevant application-based techniques that were used include rheometry, particle size analysis, surface wetting studies and hardness testing, each used as appropriate.
For the analyses of migrant species (melamine, benzoguanamine and formaldehyde) from the coatings, aqueous solutions of specified agents were used as food simulants (a simplified model food). These were used as food mimics, according to established European Commission standards. Elevated temperature treatments of cured coatings that were contained in the food simulants, in sealed containers, were carried out according to standard methods, to represent the effects of pressure cooking and the consequences of sterilisation of the canned foodstuffs during manufacture.
The effects of the cross-linker chemistry, the amount of cross-linker in the coatings, the curing conditions, the coating application procedures and the kinetics of the release processes were investigated as was the stability of the migrant species in retorted food stimulants and their potential for further reaction, after release.
Melamine, benzoguanamine and formaldehyde were not released from the coatings, at room temperature, after the cured coatings had come into contact with food simulants or with selected organic solvents, after a 24 hour exposure period. However, the migrant compounds were released from cured coatings that were retorted in the aqueous food simulants. The release was shown to be via the hydrolysis of the cross-linker components of the coatings. Such release was influenced by the 131 oC retorting temperature and the aqueous conditions. In all cases, the amounts of melamine, benzoguanamine and formaldehyde that were released and subsequently migrated into the food simulants were well within current limits, set by the European regulatory organisations.
The results show that increasing the amounts of the cross-linker compounds in the total coating formulations, up to 2% for epoxy coatings and up to 10% for poly(ester) coatings, does not always affect the hydrolysis of the cross-linking agents, although it leads to an increase in melamine release. Excluding the cross-linkers from formulation does not significantly affect thermal properties and the hardness of the coatings. As a consequence of this exclusion, melamine and benzoguanamine release and their subsequent migration should not occur. The results show that a substantial proportion (up to 90%, in unpigmented epoxy coatings) of the cross-linker suffers hydrolysis to give melamine and yet the coatings still clearly perform their function when used commercially.
The influence of varying the curing time and the curing temperature were factors that affected the amount of melamine that was released from the various coatings. This influence was more severe with the selected epoxy coatings than with selected poly(ester) coatings. For the epoxy coatings, in certain instances, increasing the curing temperature, in stages, from 160 oC to 240 oC, reduced the extent of cross-linker hydrolysis by up to 60 %. The presence of the TiO2 pigment particles in the epoxy anhydride coatings assisted in lowering the extent of melamine generation that would otherwise have occurred via hydrolysis. Also, the three grades of the TiO2 pigments gave similar contributions with respect to the reduced melamine release from the epoxy coatings.
The results also indicate that melamine and formaldehyde are released during the curing of the coatings in the oven. Using various pigmented coating formulations that were allowed to stand under stated laboratory conditions, for specified time periods, resulted in wet ageing occurring. Such wet ageing significantly affected the amount of melamine that was released under hydrolytic conditions, leading to 20% more melamine being released after storage for 30 weeks.