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Thursday, 19 September 2019
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Raw materials & technologies, Raw materials, Coatings binders

Studying melamino crosslinkers for use in epoxy can coatings

Monday, 23 March 2015

Researchers studied the behaviour of amino crosslinkers and their effect on selected epoxy can coatings’ hydrolysis to melamine and formaldehyde into aqueous food simulants.

The crosslinker chemistry controls the extent of release of melamine and formaldehyde. Source: Pierre Brillot/Fotolia

The crosslinker chemistry controls the extent of release of melamine and formaldehyde. Source: Pierre Brillot/Fotolia

They investigated the suitability of four chemically different melamino crosslinkers for use in formulating epoxy coatings 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 crosslinkers were characterised for their composition identity, flow behaviour, thermal stability and for the presence of residual species. The different crosslinkers were used individually to crosslink selected epoxy coatings. The effects of the crosslinker 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.

Curing conditions affect extent of hydrolysis process

The results show that each crosslinker type is different in its rheological characteristics, its solids content, its thermal behaviour and its physical properties. The chemistry of each crosslinker 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 crosslinker) into the 10% (v/v) aqueous ethanol food stimulant, after retorting at 131 °C, for 1 h was 525 μg/6 dm2. The greatest migration of formaldehyde into the simulant was also from this coating at 11 μg/6 dm2, when retorted at 131 °C for 1 h. The curing conditions affected the extent of the crosslinker hydrolysis. The influence of varying the curing time and the curing temperature was used to control the hydrolysis of the crosslinked, epoxy-based coatings. A decrease in the extent of crosslinker 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.

The study is published in: Progress in Organic Coatings, Volume 78, January 2015, Pages 325-333.

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