Assessment of Long-Term Aged Polyurethane Foam in District Heating Pipes and the Applicability of FTIR Spectroscopy

Abstract

Polyurethane (PUR) foam used in district heating pipe systems plays a critical role in the performance and service life of the pipe assembly. Besides providing thermal insulation, the foam contributes to the structural integrity of the system by supporting the steel service pipe, maintaining the cohesion between the steel pipe and the outer casing, and limiting pipe displacement caused by thermal expansion. Over time, however, PUR foam undergoes ageing and degradation, which lead to deterioration of its thermal and mechanical properties. Fourier Transform Infrared Spectroscopy (FTIR) is commonly used to evaluate the chemical degradation of PUR by monitoring changes in molecular bonds. In this study, PUR foam samples extracted from field-aged district heating pipes at the end of their service life were investigated using FTIR and complementary analytical techniques. The outer surface of the foam exhibited strong darkening and FTIR measurements performed directly on this layer showed a significant reduction or disappearance of characteristic absorption bands. After removing approximately 1 mm of the surface layer, the typical PUR spectral features became detectable again. Additional investigations using X-ray diffraction (XRD) and chemical analysis were carried out to determine whether iron oxides or metallic ions originating from the steel service pipe contributed to this phenomenon; however, no evidence of such contamination was detected. Furthermore, nuclear magnetic resonance (NMR) analysis was conducted to examine potential hydrogen redistribution across the foam from the steel pipe side toward the casing. The results indicate that surface alterations in severely aged PUR foam may influence FTIR measurements and should be considered when interpreting spectroscopic analyses of field-aged insulation materials.