Confocal Raman Microscopy Study of the Melamine Distribution in Polyester-Melamine Coil Coating

摘要

Hexa-Methoxy-Methyl-Melamine (HMMM), a common cross-linker used in combination with polyester resins to prepare thermosetting coatings can self-condense during curing and in theory build up regions with high cross-link density (melamine enrichment). This distribution may affect such properties as the 'local' glass-transition temperature and structural heterogeneity and thereby modify formability, stain resistance and weatherability. The self-condensation regions have not previously been observed in detail so their distribution in the film is unclear. Confocal Raman Microscopy (CRM) has been used to characterise the melamine distribution in three polyester-melamine paint systems: one clear and two pigmented coil coatings. Both the surface mapping and depth profiling features of CRM were used to perform a 3-D analysis. The depth profiling of the clear coating was performed in a non-destructive way, but due to the opaque nature of the pigments, cross-sections of the pigmented coatings were prepared in order to facilitate the analysis of the melamine distribution through the film thickness. Melamine enriched zones were found on the surfaces of all three samples. They appeared to be approximately spherical with diameters of around 5 m. It was also observed from the cross sections that the melamine enriched zones appeared as 'particles' distributed randomly through the coating. The Raman spectra collected at these regions show significantly higher triazine ring band intensity. All of these findings confirm the hypotheses constructed over the past 20 to 30 years.