Characterization of Photo-Cross-Linked Polyethylene Pipes for Geothermal Energy Storage

This study investigates the morphology and thermo-mechanical properties of cross-linked polyethylene (PEX) pipes for potential use in high-temperature borehole thermal energy storage systems. Particular attention is given to a novel type of PEX pipe produced through photoinitiated cross-linking (PEX-e). Two formulations, PEX-e1 and PEX-e2, were analyzed and compared to peroxide-cross-linked polyethylene (PEX-a) and non-cross-linked bimodal polyethylene (PE100) pipes. The degree of cross-linking was evaluated via gel content, while cross-link density and molecular weight between cross-links were determined using dynamic mechanical analysis (DMA). Phase composition and molecular mobility were explored through ¹H static nuclear magnetic resonance (NMR), and the melting and crystallization behavior was assessed by differential scanning calorimetry (DSC). Oxidative stability and degradation were examined by using Fourier transform infrared (FTIR) spectroscopy, oxidation induction time (OIT) measurements, and thermogravimetric analysis (TGA). Both PEX-e formulations achieved satisfactory cross-linking degrees and exhibited remarkable OIT values. However, significant differences in cross-link distribution were noted, with PEX-e2 showing a less uniform dispersion of cross-links, which resulted in a lower storage modulus. FTIR analysis indicated that oxidation products were formed in PEX-e1 during cross-linking, highlighting the need for further optimization of the formulation and processing conditions.