Abstract
Polymer additives are widely used to improve the performance of road bitumens including their resistance to hardening during oxidation, although their oxidative inhibitor effect has not been well documented. This study aims to investigate the effect of laboratory-simulated ageing on the microstructure, and rheological properties of Polymer Modified Bitumens (PMBs) prepared with a Styrene-Butadiene-Styrene (SBS) copolymer and sulphur as a cross-linker. The laboratory ageing was conducted through the Rolling Thin Film Oven (RTFO) and the Pressure Aging Vessel (PAV) procedures. The unaged and aged binders were characterised using the Multiple Stress Creep and Recovery (MSCR) test, Gel Permeation Chromatography (GPC) and fluorescence microscopy. The MSCR test showed that the cross-linked polymer had a higher capability of mitigating the effect of bitumen hardening on the rheological response, while for the PMBs without cross-linking, the polymer effects decreased significantly after RTFOT ageing. The GPC results showed that the aromatics, resins and asphaltenes fractions of the unmodified bitumens shifted towards heavier molecules after one cycle of PAV ageing but showed limited variation with further ageing. Similar behaviour was observed for the uncross-linked polymer binders, while for the cross-linked polymer binder, the behaviour depended on the polymer concentration.
Highlights
The modification of road bitumens with polymeric additives was developed through intense investigations which started decades ago and are still on-going (Airey, 2002, 2003; Goodrich, 1988; Lesueur, 2009; Lo Presti, 2013; Polacco, Filippi, Merusi, & Stastna, 2015; Yildrim, 2007; Zhu, Birgisson, & Kringos, 2014)
In the Rolling Thin Film Oven (RTFO), such an increase might depend on the volatilisation of the lighter fractions, while during the first cycle of Pressure Aging Vessel (PAV) it depends on the PAV-oxidative hardening
In the RTFO, the increase in the RH value is slightly dependent on the polymer concentration, due to the simultaneous effect of the ‘Volatilization of the light fractions’ and ‘Polymer degradation’
Summary
The modification of road bitumens with polymeric additives was developed through intense investigations which started decades ago and are still on-going (Airey, 2002, 2003; Goodrich, 1988; Lesueur, 2009; Lo Presti, 2013; Polacco, Filippi, Merusi, & Stastna, 2015; Yildrim, 2007; Zhu, Birgisson, & Kringos, 2014). Sulfoxides form at the start of bitumen ageing but only for a limited time, while the formation of ketones, carboxylic acid and anhydride occurs with time along with the oxygen uptake (Liu et al, 1998) The presence of such strong interactive forces contributes to increasing the content of heavy and polar groups of molecules known as asphaltenes. Mechanical characterisation has been contemplated to quantify bitumen oxidation The majority of these works aim to assess the role of the polymers in the oxidative mechanism through the comparison of physical parameters (i.e., penetration, softening point, viscosity, complex modulus and phase angle) of unmodified and polymer modified bitumens under the same level of ageing.
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