# FTIR Characterization of Asphalt SARA Fractions in Response to Rubber Modification
###### Abstract  
Asphalt-rubber binders (A-RBs) have a long and deep history of use; however, little is known regarding the interrelated chemical behaviors and miscibility of rubber with the asphalt fractions [saturates, aromatics, resins, and asphaltenes (SARA)]. This study comprehensively attempted to address this knowledge deficiency by employing Fourier transform infrared spectroscopy (FTIR) to investigate the chemical evolution of A-RBs. A-RB interacted at 190 \({}^{\circ}\)C and 3000 min\({}^{-1}\) for 8 h was deemed to have the optimal rheological performance. FTIR of the liquid fractions of A-RB 190-3000 showed a prominent chemical shift in the SARA fractions, with new peaks that showed rubber polybutadiene (PB) and polystyrene migration into asphaltenes. Meanwhile, decreases in peaks with C-H aromatic bending and S=O stretching for the A-RB 190-3000 saturates showed that the rubber absorbed low-molecular-weight maltenes during swelling. Peaks associated with C=C aromatic appeared in saturates and aromatics, respectively, emphasizing that unsaturated components migrated from the rubber into the asphalt. Thermal analysis showed that rubber dissolution for this sample reached 82%. While a PB peak existed in asphaltenes of A-RB 220-3000, its intensity was diminished by depolymerization, thus compromising the integrity of the migrated rubber structure and generating less rheological enhancement. This study concludes that FTIR characterization of SARA fractions offers valuable insights into the interactions between asphalt and rubber, and that regulated processing conditions are essential for enhancing binder performance. F 27 June 202514 July 20251  
Scrap tire disposal in landfills causes severe problems, including environmental hazards and waste of valuable materials. Most scrap tires are repurposed for fuel, for construction applications, or as ground rubber [7]. Tire rubber is a complex crosslinked material composed of valuable components, including natural rubber (polyisoprene) with synthetic rubbers, styrene-butadiene, and butadiene rubber, that are crosslinked with sulfur and reinforced with carbon black. Valuable additives such as aromatics and antioxidants are also incorporated to increase flexibility, ease of processing, and resistance to aging or oxidation [8; 9]. Ground tire, crumb rubber, was first introduced for asphalt surface treatments in the mid-1960s and later adopted in hot mix asphalt applications during the 1970s [10]. When the asphalt binder is mixed with rubber, a combination of swelling, partial dissolution, and release of rubber constituents into the asphalt matrix occurs [11]. The complex interactions in asphalt-rubber binders (A-RBs) can be monitored effectively using Fourier transform infrared spectroscopy (FTIR). FTIR is a crucial tool for detecting chemical changes arising from additive inclusion into asphalt through changes in functional groups [12]. Carbonyl (C=O) at a 1700 cm\({}^{-1}\) wavenumber and sulfoxide (S=O) at 1036 cm\({}^{-1}\) were lessened for A-RBs compared to neat binders, indicating improved aging resistance [13]. The increase in aromatics (C=C) at 1600 cm\({}^{-1}\) in A-RBs was more pronounced in unaged samples; however, both short- and long-term aging reduced this increase due to the breaking of C=C bonds during aging [14]. Furthermore, increasing rubber content led to higher C=C intensity due to the degradation of rubber chains, releasing additional unsaturated components into the binder's matrix. Surface treatment of rubber with zinc stearate caused a higher intensity of the S=O peak at 1030 cm\({}^{-1}\)[15], influencing the high-temperature performance of binders. Earlier studies chemically examined the liquid phase of A-RBs, without the undissolved rubber, and established evidence for rubber polymeric component release into the asphalt matrix.