Photo-induced ageing processes in bitumen

Abstract

Asphalt roads and waterproofing membranes on roofs are exposed to sunlight, which induces irreversible changes in those materials. The underlying chemical processes initially affect the surface but later on impact the bulk chemistry and the mechanical properties. Bitumen, the binding and sealing material, is particularly susceptible to such influences. However, the molecular processes that initiate material deterioration are still poorly explored. Here, we show that ultraviolet and visible light cause the formation of a unique set of oxidation products and alter aromatic structures on the bituminous surface. Qualitative and quantitative information is obtained using infrared, fluorescence and NMR spectroscopy. Bulk ageing studies have shown a shift of molecules from the aromatics to the resins and asphaltenes fractions, potentially influencing colloidal stability. The evaluation of surface processes was facilitated in ageing studies with thin sub-µm specimens. We find that gaseous oxygen is essential for molecular changes to occur. The formation of carboxylic acids, esters and aliphatic alcohols, coupled with the degradation and restructuring of aromatic systems, demonstrates the unique nature of light ageing. This is due to a nuanced interplay between oxidation, degradation, condensation and aromatisation reactions. The ageing rates depend on the origin of the binder and increase as the wavelength of the light source decreases. Ageing follows an asymptotic growth with time. All results considered, this study enabled us to decipher critical molecular processes during photochemical ageing. In this regard, we hope to develop more effective strategies for mitigating and preventing the adverse impact of solar radiation on bituminous infrastructure in the future.