A study on the depth distribution of melamine in polyester-melamine clear coats

[Display omitted] •The melamine distribution in polyester/melamine coatings has been studied.•Depth resolved quantification of the melamine crosslinker was achieved.•Free standing films behave differently than coatings applied on galvanized steel.•The highest melamine concentration is expected in the surface near region of a coating.•A new qualitative model on the melamine distribution is proposed. The depth distribution of melamine in polyester-based coatings was investigated by XPS (X-ray photoelectron spectroscopy) as well as Raman analysis on cryo-ultra-low-angle microtome (cryo-ULAM) prepared coatings on hot dip galvanized (HDG) steel panels. Additionally, free standing films were analyzed on the air/coating and the coating/substrate interfaces: for polyester/melamine ratios ≤20wt.% the melamine concentration was equal on both sides of free films, which is in accordance with literature. This finding could not be confirmed for the coatings applied on HDG steel panels that revealed a higher amount of melamine in the surface-near region. In addition various types of non-linear melamine concentration gradients have been detected over depth by XPS as well as Raman spectroscopy. This behavior was confirmed for all types of coatings irrespective of binder chemistry and curing temperatures in the range of 220–260°C. Whereas previous studies have mainly focused on higher melamine concentration where gradient properties are more pronounced, this work tries to fills the gap towards lower melamine contents, which are of greater relevance for industrial coatings. According to these results, a qualitative model on the melamine distribution has been established, which explains the observed non-linear gradients on the basis of transport phenomena and crosslinking kinetics within the curing process. This model is also in agreement with experimental findings from previous works.