Structural and chemical analysis of bitumen using time-of-flight secondary ion mass spectrometry (TOF-SIMS)

- TOF-SIMS has been used successfully to characterize bitumen surfaces.
- Bitumen with natural wax shows phase separation and wax-enrichment on air-cooled surfaces.
- Wax-free bitumen shows homogeneous chemical and more aromatic structures on surfaces.
- A wax-free bitumen to which isolated natural wax is added also shows phase separation.
- Fracture surfaces of wax-containing bitumen reveal a different type of structure.

The chemical composition and structures of bitumen surfaces are characterised using time-of-flight secondary ion mass spectrometry (TOF-SIMS). The effect of wax is considered by comparing a wax-free bitumen with a bitumen that contains natural wax and a wax-free bitumen to which a small amount of wax has been added. The results demonstrate that TOF-SIMS is a powerful method for the chemical characterisation of surface structures and phase segregation phenomena in bitumen. It is evident that the structures formed on the bitumen surface are closely related to the wax content and that these structures, as well as the surface in general, are enriched in wax-related compounds (aliphatic hydrocarbons with a high degree of saturation). For the wax-free bitumen, the surface is characterised by a homogeneous distribution without chemical variations or phase structures and by a stronger signal intensity from aromatic compounds. When adding wax to the wax-free bitumen, extensive wax segregation occurs, but differently from the natural waxy bitumen, no bee structures are observed. Furthermore, fracture surfaces of all the wax-containing samples reveal circular structures, which are distinctly different from those observed on the original surfaces. The obtained chemical knowledge on bitumen surfaces and phase structures is of fundamental importance to understand performance differences of this type of materials.