Effect of Paraffin Microcapsule and Carbon Nanotube Content on the Thermal Behavior of Thermoplastic Polyurethane Nanocomposites with Thermal Energy Storage Capability

The development of multifunctional polymer composites capable of both heat conduction and latent heat storage is of great interest for advanced thermal management applications. In this work, thermoplastic polyurethane (TPU) nanocomposites containing microencapsulated paraffin-based phase change materials (PCMs) and multi-walled carbon nanotubes (MWCNTs) were systematically investigated. The microstructure, thermal stability, specific heat capacity, thermal diffusivity and conductivity of these composites were analyzed as a function of the PCM and MWCNTs content. SEM observations revealed the homogeneous dispersion of PCM microcapsules and the presence of localized MWCNT aggregates in PCM-rich domains. Thermal diffusivity measurements indicated a monotonic decrease with increasing temperature for all compositions, from 0.097 mm2·s−1 at 5 °C to 0.091 mm2·s−1 at 25 °C for neat TPU, and from 0.186 mm2·s−1 to 0.173 mm2·s−1 for TPU with 5 vol.% MWCNTs. Distinct non-linear behavior was observed around 25 °C, i.e., in correspondence to the paraffin melting, where the apparent diffusivity temporarily decreased due to latent heat absorption. The trend of the thermal conductivity (λ) was determined by the competing effects of PCM and MWCNTs: PCM addition reduced λ at 25 °C from 0.162 W·m−1·K−1 (neat TPU) to 0.128 W·m−1·K−1 at 30 vol.% PCM, whereas the incorporation of 5 vol.% of MWCNTs increased λ up to 0.309 W·m−1·K−1. In PCM-containing nanocomposites, MWCNT networks efficiently bridged the polymer–microcapsule interfaces, creating continuous conductive pathways that mitigated the insulating effect of the encapsulated paraffin and ensured stable heat transfer even across the solid–liquid transition. A one-dimensional transient heat-transfer model confirmed that increasing the matrix thermal conductivity accelerates the melting of the PCM, improving the dynamic thermal buffering capacity of these materials. Therefore, these results underlined the potential of TPU/MWCNT/PCM composites as versatile materials for applications requiring both rapid heat dissipation and effective thermal management.