Unraveling structural evolution of crumb rubber derived from end-of-life tires in supercritical fluid environments

Abstract

The crumb rubber (CR) derived from end-of-life tires (ELTs) is widely used in paving asphalt modification, while pretreatments are generally necessary to improve the CR-asphalt compatibility. This study explores the structural changes in CR under various pretreatment scenarios, with an emphasis on mechanisms in supercritical carbon dioxide (ScCO₂) reaction environments. Two supercritical pretreatments were designed, including supercritical de-crosslinking (SCD) and supercritical swelling (SCS). Two other pretreatments were also considered for comparison: high-pressure de-crosslinking (HPD) and unpretreated (UPT). The results show that SCS causes slight random scission of rubber crosslinking structure, while SCD achieves uniform and thorough de-crosslinking of CR. However, high temperatures also unavoidably cause some structural damage during both supercritical pretreatments. Quantitatively, sol fraction increased from 3.8% (UPT) to 10.6% (HPD), 5.2% (SCS) and 21.1% (SCD), while the measured crosslink densities show a corresponding reduction after pretreatments from 13.1 mol/cm³ (UPT) to 4.5 mol/cm³ (SCD). Thermal analysis reveals that SCS induces minor “sol” content increase, while HPD and SCD greatly increase “sol,” with SCD further improving thermal stability of CR. Microstructural observations show distinct morphology changes, ranging from increased porosity with SCS to complete structural disruption under SCD. The supercritical pretreatment processes involve ScCO₂-induced swelling, enabling efficient and uniform de-crosslinking, accompanied by filler release under high temperatures. These findings provide insight into the mechanisms underlying CR pretreatment in ScCO₂ environments and demonstrate their practical implications such as improved CR-asphalt compatibility and environmental benefits of using ScCO₂ processes.