INVESTIGATION OF DEGRADATION IN BIO-BASED AND PETROLEUM BASED RESINS USED IN POLYESTER MELAMINE COIL COATINGS USING SPECTROSCOPY AND CHEMICAL FORCE MICROSCOPY

Abstract

The degradation of polyester coatings of three different formulations on steel substrates were critically analysed using photo-acoustic fourier transform infrared spectroscopy (PA-FTIR), atomic force microscopy (AFM), scanning electron microscopy (SEM), focussed ion beam (FIB) and chemical force microscopy (CFM). NS01, a coating with standard polyester resin, NS02, a 100% bio-sourced resin and NS03, a partially bio-sourced resin were each cross-linked with hexamethoxymethyl melamine and oven cured to reach a peak metal temperature of 232˚C. The cured coatings were subjected to accelerated weathering using the QUVA machine and natural weathering in Florida. The extent of photo-oxidation, using the characteristic OH band in FTIR 3600-3200cm⁻¹ was found to increase with respect to time and the gloss retention decreased. NS01 showed an expected trend of gradual increase whereas NS02 completely disintegrated by 1000 hours and NS03 out-performed NS01 until 1200 hours. The sudden degradation in NS03 is caused by the instability of the aliphatic chain in the resin backbone which is hindered after 1200 hours of exposure. The addition of Titanium dioxide, TiO₂ stabilises the coatings improving the durability of NS01 and NS03. A chemical force microscopy method was adopted to evaluate the chemistry on the substrate surface and depth profile after milling into the sample using a focussed ion beam. This technique has helped further the understanding of how chemical changes upon degradation manifest into physical changes ultimately causing failure which has not been investigated before.