An investigation on the melamine self-condensation in polyester/melamine organic coating

Melamine is a common crosslinker widely used in the coil coating industry to crosslink hydroxyl functional polyesters. Theoretically, melamine can also self-condense with the consequence regions with higher crosslink density build up in the coating. Although the melamine self-condensation has been studied, most of works were based on melamine segregation on the coating surface; the existence of melamine self-condensation has only been modelled rather than observed. Utilising confocal Raman microscopy and nanoindentation, the regions with higher melamine concentration were clearly observed and characterised for the first time. It is primarily found from this work that the regions with higher melamine concentration show unique optical features. Those regions are also harder than the normal coating regions at a temperature above Tg; this suggests the cross-linking density in melamine-enriched region is very high, as expected. Moreover, the pigment concentration in the melamine-enriched regions is much lower (or even zero). A possible mechanism was also proposed in this paper.