XPS and ToF-SIMS study of freeze-dried and thermally cured melamine-formaldehyde resins of different molar ratios

Abstract

The surface chemical characterization of melamine–formaldehyde (MF) resins by x-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) is examined in this study. Melamine–formaldehyde resins with different molar ratios of formaldehyde to melamine are synthesized and thermally cured. From XPS measurements, quantitative information is obtained and atomic chemical concentrations show the effect of the molar ratio for the freeze-dried resins. However, the thermally cured resins display a rather similar surface elemental composition. Moreover, because the binding energy values of the main N–C–N and N–C–O groups are too close, XPS does not help to identify changes in chemical structure after curing. The main ToF-SIMS negative and positive mass fragments of the MF resins are identified. Principal component analysis (PCA) is shown to be useful to determine and explain the main differences between all of the ToF-SIMS spectra. It allows us to distinguish the effect of the bulk chemical composition on the respective surface compositions of not only the uncured but also the cured MF resins. Moreover, extended interpretation leads to the identification of peaks characteristic of methylol groups and methylene ether bridges, but their absolute quantification is not straightforward. However, this result indicates that surface analysis helps to characterize the poorly defined chemical structure of MF resins. This is important for understanding the influence of surface chemistry on macroscopic surface properties such as chemical durability under exterior exposure. Copyright © 2000 John Wiley & Sons, Ltd.