A spectroscopic study of the degradation of polyurethane coil coatings.
The degradation of polyurethane (PU) coil coatings were studied with step scan phase modulation photo-acoustic (SS-PM-PA) FTIR, confocal Raman mapping (CRM) and scanning electron microscopy (SEM). PU coatings were oven cured for 30 seconds to reach a peak metal temperature of 232°C. The cured coatings were exposed in a QUV A accelerated ageing test with exposure time intervals of 1200 hours and 4098 hours. Isophorone diisocyanate (IPDI) cross-linker gave lower cross-linking density and degradation rate to the PU coating compared to hexamethylene diisocyanate (HDI). Cyclic trimer (CT) isocyanate cross-linker gave higher durability compared to biuret (BI). A primary amide and urea entity rich top-film was formed at the surface of degraded PU coatings, with characteristic IR bands at 1640 cm-1 and 1560 cm-1. The decomposition of allophanate in exposed HDI-CT cross-linked PU coating was indicated. The degradation of BI core produced additional urea linkage compared to allophanate. ɛ-caprolactam (Capro) blocked isocyanate gave lower cross-linking density and higher degradation rate compared to methyl ethyl ketoxime (MEKO), and 3,5 dimethyl pyrazole (DMP). The addition of melamine and HALS (less than 5%) improved the durability of PU coatings. The melamine linkage was more sensitive to the degradation compared to the urethane linkage. The higher NCO/OH resulted in more rapid degradation product build-up at the surface of the PU coating in the meantime deterred the decomposition of amide II type linkage. A FTIR peak fitting method was developed for generating degradation index plots, based on the knowledge of degradation chemistry of the PU coatings described above. The degradation rate correlation of the PU coatings exposed in the QUV A test and natural exposure sites including Liverpool, UK (LIV), Vereeniging, South Africa (SA) and Kuala Lumpur, Malaysia (KL) are demonstrated by using degradation index plot methods. The harshness of the natural exposure sites gives the order of KL > SA > LIV.
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