Effect of Sulfur on Wood Tar Biopitch as a Sustainable Replacement for Coal Tar Pitch Binders

Coal tar pitch (CTP) is a residue formed from the distillation of coal tar and is widely used as a carbonizable and graphitizable binder for many industrial applications. However, CTP is fossil-derived and has recently been classified as a "sunset" status material under REACH due to its toxicity, which makes finding a sustainable alternative vital. In this work, bio-oil was synthesized from the pyrolysis of fresh eucalyptus sawdust, from which wood tar biopitch (WTB) was subsequently produced by a second distillation process. Chemical characterization revealed the presence of higher amounts of aromatic compounds and PAHs in the industrially used CTP relative to the WTB. Sulfur is widely used as a graphitization promoter for CTP but has not yet been used for biopitch alternatives. Hence, graphite/WTB and graphite/CTP composites were fabricated with varying amounts of sulfur and were subsequently carbonized and graphitized at 850 and 2500 °C, respectively. The use of WTB as a binder led to less porous composites after carbonization/graphitization with higher levels of shrinkage than those based on CTP, whereas the carbon yield was very similar for both systems. The incorporation of sulfur was found to promote more compact structures with higher levels of graphitization, leading to improved electrical and mechanical properties, particularly for the composites based on CTP due to the higher levels of graphitization achieved relative to the WTB. The electrical and mechanical performance found for the WTB-based composites, combined with the much lower toxicity, evidences the promise of WTB as a sustainable alternative to traditional CTP binders.