Shoemaker’s nightmare – Deterioration of shoe soles and tests for the conservation of degraded closed-cell polyester urethane museum objects

Elena Gómez Sánchez, Janine Köppen, Susanne Brunner, Giulia Mazzon, Corentin Chaussat, Cédric Bouallag

Deutsches Bergbau-Museum Bochum, Germany; Hochschule für Technik und Wirtschaft Berlin, Germany, Deutsches Museum München, Germany

This paper is part of an ongoing project that started in 2015. It deals with the degradation and conservation of polyurethane ester, based on a collection of twenty original miners’ shoes from the German Mining Museum in Bochum (Deutsches Bergbau-Museum Bochum, DBM). The shoes were produced between 1988 and 1992 and used by the workers in the coal mines, where they were exposed to environmental influences such as water, heat, salts and mechanical stress. Today, the elastic closed-cell polyurethane ester outsoles are in an advanced state of deterioration, as they show great losses, brittleness, deformation and white blooming on the dark surface. Therefore the shoes could no longer be on display and conservation treatments were necessary. In this context information on the ageing of polyurethane ester was gathered through artificial ageing and investigation of the natural degradation phenomena. Based on the results of chemical (FTIR and py-GCMS) and mechanical tests (contact angle, Shore hardness, compression set, abrasion and water vapour permeability) approaches for their conservation were developed. The work presents both consolidation tests and first tests to evaluate the possibility of cold storage for the material.

The naturally aged closed cell polyurethane ester was documented and classified regarding the phenomena visible on the surface of the shoe soles. The FTIR analysis of these degradation phenomena revealed that, although they differ in appearance and shape, they mainly consist of adipic acid. After analysing the shoe soles’ material, a similar test material by BASF was artificially aged. For this purpose test specimens were placed for eight weeks in a climate chamber at 70 °C and 98 % RH. The resulting material was brittle and partially broken, lacking cohesion.

Regarding the consolidation of broken and detached pieces a test series with artificially aged samples was carried out. First the samples were cut and re-adhered by using four different adhesives: Lascaux 303 HV, Lascaux 498 HV, Plextol B 500 and the PU-Dispersion PU 52. The bonding strength of the different adhesives was then detected through standardized tensile tests. Promising results were achieved both with Plextol B 500 and Lascaux 303 HV, the former having the highest bonding strength and the latter showing excellent flexibility.
First attempts were made on investigating the cold storage of naturally and artificially aged cellular elastic polyurethane ester. Samples were deep frozen at -20 °C for 28 days and the surface morphology studied by means of scanning electron microscopy. Although the tested samples had little cohesion originally, no further cracks appeared after freezing. Instead, an unidentified substance appeared in the form of little drops on the outer surface at regular intervals. Furthermore standardized methods were used for characterising the mechanical properties of the samples. Hardness was tested using Shore A and 00, elasticity using compression set, and abrasion using a grinding wheel. Regarding the mechanical properties, no major changes could be detected before and after freezing.