Moisture-Driven Morphology Changes in the Thermal and Dielectric Properties of TPU-Based Syntactic Foams

Syntactic foams are a promising candidate for applications in marine, oil and gas industries in underwater cables and pipelines due to their excellent insulation properties. The effective transmission of electrical energy through cables requires insulation materials with a low loss factor and low dielectric constant. Similarly, in transporting fluid through pipelines, thermal insulation is crucial. However, both applications are susceptible to potential environmental degradation from moisture exposure, which can significantly impact the material's properties. This study addresses the knowledge gap by examining the implications of prolonged moisture exposure on thermoplastic polyurethane elastomer (TPU) and TPU-derived syntactic foam via various multi-scale material characterization methods. This research investigates a flexible syntactic foam composed of TPU and glass microballoons (GMBs) fabricated through selective laser sintering. The study specifically examines the effects of moisture exposure over periods of 90 and 160 days, in conjunction with varying GMB volume fractions of 0%, 20%, and 40%. It aims to elucidate the resulting microphase morphological changes, their underlying mechanisms, and the subsequent impact on thermal transport and dielectric properties, all in comparison to unaged samples of the same material. Our findings reveal that increasing the volume fraction of GMB in TPU-based syntactic foam reduces its thermal conductivity and specific heat capacity. However, moisture exposure did not significantly affect the foam's thermal conductivity. Additionally, we found that the dielectric constant of the syntactic foams decreases with increasing volume fraction of GMB and decreasing frequency of the applied field, which is due to limited molecular orientation in response to the field. Finally, moisture exposure affects the dielectric loss factor of TPU-based syntactic foams with GMBs, possibly due to the distribution morphology of hard and soft segments in TPU.