The degradation products in polyurethane ester-based objects in museums: study of Iron adipate in miners’ shoes conserved at Deutsches Bergbau Museum

Abstract

This work deals with the degradation products of polyurethane (PUR)ester-based objects. The case studies are conserved at Deutsches Bergbau-Museum (German Mining Museum) in Bochum: the collection includes several pairs of miners’ shoes and their conservation is quite challenging, especially the sole made of polyurethane-ester polymers. Many degradation phenomena affect these particular artefacts, among them the presence of brown and glossy material on certain areas of the soles’ surface. The results gathered in this work suggest that these spots are mainly composed of iron adipates, as the resulting compounds of the interaction between iron and adipic acid. Iron from the metallic part of the sole structure reacts with adipic acid that is freed from the polymers network due to degradation processes; once it is formed, this liquid reaches the surface through cracks and fractures of the polyurethane sole. The synthesis of iron adipate compound was carried out with new and different procedures and the resulting products analysed with different techniques, such as X-ray diffraction (XRD) and Attenuated Total Reflection-Fourier Transform Infrared Spectroscopy (ATR-FTIR), Scanning Electron Microscopy (SEM) and Evolved Gas Analysis Gas Chromatography – Mass Spectrometry (EGA GC-MS) in order to characterize this degradation product and compare it with samples taken from miners’ shoes.