Effect of alternative crosslinking agents on the thermo-rheological properties of SBS-modified asphalts

Abstract

Current industrial practices rely on the modification of asphalt by thermoplastic elastomers, particularly styrene-butadiene-styrene, crosslinked with a small amount of sulfur. This technology allows the formation of a three-dimensional polymer network with a significant impact on thermo-rheological and engineering properties. However, gelation of the material and potential development of harmful sulfur emissions may occur. This work concentrates on the mechanism of modification and development of thermo-rheological properties of styrene-butadiene-styrene modified asphalt crosslinked by novel sulfur-based and sulfur-free crosslinkers. The properties of crosslinked modified asphalts were studied by Superpave binder specification tests and rheological tests conducted in the linear and non-linear viscoelastic region. The results were compared with currently used crosslinking technology employing elemental sulfur as well as with the technology without any crosslinking agent. The results from Superpave binder specification showed that the samples crosslinked with novel sulfur-based crosslinking agents were less sensitive to thermo-oxidative degradation than the sample crosslinked with elemental sulfur. This behavior was confirmed by the thermogravimetric analysis which also provided insights about decomposition and combustion behavior. In addition, modified asphalts crosslinked with sulfur and sulfur-based crosslinkers had the highest values of %Recovery_3.2kPa. The appearance of the “shoulder” on the master curves of G'(ω) and the local maximum followed by the local minimum on the master curves of tanδ(ω) suggested exceptional resistance of modified asphalts crosslinked by sulfur and sulfur-based crosslinkers to deformation. The development and strength of polymer network were evaluated by the presence of viscosity overshoots and stress overshoots in the steady-state viscosity and start-up of steady shear measurements, respectively. The effects of crosslinking technologies were also evaluated via mechanical glass transition temperature and modulated differential scanning calorimetry. Obtained results point to a considerable difference in the crosslinking mechanism which was also reflected by significant differences in morphology of prepared modified asphalts at ambient temperature. Although sulfur and sulfur-based crosslinking systems had superior performance, the non-sulfur-based crosslinking systems could play a significant role in the reduction of odor and sulfur emissions in densely populated areas.