Surface-modified graphite based polymer nanocomposites for high-temperature geothermal applications

This research aims to develop novel economical and thermally-stable polymer nanocomposites to seal geothermal wells by dispersing surface-modified graphite into an inexpensive raw polymer. Using a mixture of nitric acid and sulfuric acid, a nano-scale graphite filler, namely small-size lamellar graphite (SFG15), was surface-modified, which introduced considerable oxygen and carboxylic (-COOH) groups to graphite surfaces, as verified by X-ray photoelectron spectroscopy (XPS) analysis. Polymer nanocomposites were fabricated by dispersing different concentrations of modified graphite SFG15, including 1.5, 3, 6, and 9 wt%, into the ethylene propylene diene monomer (EPDM) matrix. First, characterization techniques, including wide-angle X-ray scattering (WAXS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM), were employed to examine modified graphite dispersion in the nanocomposites. Then, dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA) were used to quantitatively study the key mechanical and thermal properties of the nanocomposites. It was found that introducing modified SFG15 to EPDM enhances its resistance to deformation at high temperatures significantly. Notably, 9 wt% of modified SFG15 showed superior performances in improving the storage modulus of EPDM by 191.17%, enhancing loss modulus by as high as 136.08%, and decreasing tan delta by 18.93%. After introducing modified graphite, the energy needed for melting EPDM was increased markedly, which was accompanied by improvement in its heat capacity. Compared with pure EPDM, the onset temperature of degradation for developed nanocomposites could be increased by over 30 degrees C, and the temperature of 50% degradation was raised by up to 20 degrees C. Possessing the improved mechanical and thermal performances, the nanocomposites developed could be a primary choice for materials used to seal geothermal wells.Highlights A technique is developed to reinforce low-cost polymers for high-temperature use. Reinforcement of EPDM with high concentrations of modified graphite is investigated. Modified graphite increases the high-temperature storage modulus of EPDM by 191.17%. Degradation temperatures of EPDM are enhanced by modified graphite considerably. Low-cost polymer composites are essential seals for application in deep geothermal wells.