Unveiling the Key Factors of Spatial and Chemical Structures to Pellet Strength of Tire Pyrolysis Carbon Black for Sustainable Rubber Recycling

Due to its considerable economic viability and environmental benefits in the recycling of waste tires, the applications of pyrolysis carbon black (CBp) from waste rubber pyrolysis are receiving increasing attention. However, the use of CBp is significantly hindered by its high pellet strength, which complicates its homogeneous dispersion within rubber matrices. In this work, the correlation between pellet strength and the structural and chemical properties of CBp was systematically investigated using synchrotron radiation X-ray nano-CT and XPS. The results indicate that the formation of compact structures, which confers high pellet strength, is attributed to a high oxygen (O) content and strong interfacial adhesion created by intertwined functional groups. Meanwhile, the presence of substantial undecomposed rubber residues, coupled with a low surface structure (the fractal dimension is 2.201 and lower than the other two), suggests that these residues coating the surface contribute significantly to the pellet strength. Although the surface chemistry of CBp is complex, functional groups such as C–OH and C═O facilitate hydrogen bonding (C–O•••H), and C–S forms cross-linked network formation, both of which enhance interfacial adhesion and thereby strengthen pellet strength. With the Weibull model, the minimum number of particles in high-strength pellets indicates that more residues and oxygen-containing groups adhere to small particles, forming big ones to constitute the pellet. The work aims to provide new insights into the CBp properties and develop high-performance materials for sustainable rubber recycling and green transformation of tire manufacturing.