Surface Morphology and Chemical Mapping of UV-Aged Thin Films of Bitumen

This study decouples effects of ultraviolet (UV) exposure on the surface and bulk properties of bituminous composites. UV exposure accelerates oxidative aging of bitumen, but its effects are concentrated mostly at the surface of the bitumen. Here, we examine morphology and chemical composition of the surfaces of non-aged thin films of bitumen and UV-aged thin films of bitumen using combined atomic force microscopy (AFM) and photothermal infrared (AFM-IR) spectroscopy; we further compare the findings with those of bulk properties measured by Fourier transform infrared spectroscopy. The study results showed that the surfaces of non-aged and 20 h UV-aged bitumen appeared depleted in phenyl and CH3 groups compared to the bulk. This in turn indicates that the surfaces lacked asphaltenes and aromatics. It also shows that the saturated aliphatics found on the surface were almost all nonbranched linear chains. The ratio of CH3/CH2 between the surface and bulk was completely reversed as the aging progressed; the non-aged and 20 h UV-aged films showed a lower CH3/CH2 ratio at the surface relative to the bulk, but the 50 h UV-aged film showed a higher ratio. The latter observation was attributed to the chain scission and start of decomposition reactions breaking down the large compounds into smaller ones. In addition, sulfur hydroxyls present in the bulk were entirely absent from the surface. Furthermore, the chemical composition of the bee like structures on the bitumen surface could not be easily distinguished from the bulk composition by AFM-IR, but they appeared to be depleted in alkenes and hydroxyls compared to the areas around them. The study outcomes show variational effects of UV exposure on surface and bulk properties of bituminous composites. Such insights help formulators and manufacturers design functionally graded bitumen composites with enhanced resistance to UV aging at the surface.