Evolution of Morphological and Nanomechanical Properties of Bitumen Thin Films as a Result of Compositional Changes Due to Ultraviolet Radiation

This paper examines morphological and nanomechanical changes as compositional evolution occurs because of ultraviolet (UV) radiation in thin films of bituminous adhesives. Such an insight is important for developing strategies to enhance sustainable applications of such adhesives for coating and composite manufacturing. However, the shallow penetration depth of UV light in bitumen complicates efforts to isolate its effects. To address this question, this study examined the effects of UV exposure on spin-coated films of bitumen that are 600 nm thick. Bitumen thin films were UV-aged in an accelerated weathering machine and were characterized by scanning electron microscopy, atomic force microscopy, Fourier transform infrared spectroscopy, energy-dispersive X-ray spectroscopy, UV-visible absorption spectroscopy, and nanoindentation. As UV radiation continued from 0 to 400 h, the hydrocarbon content of thin films gradually decreased, leaving behind sulfates and carbonyls and reducing bitumen solubility in organic solvents. This in turn led the bitumen thin films to become thinner and chemically and morphologically more heterogeneous. Nanoindentation showed a dramatic increase in stiffness and loss of viscoelasticity after 20 h of UV exposure. Stiffness continued to increase up to 50 h and nearly reached a plateau. The study results provide insights into the effects of UV radiation on structure-property changes in bitumen thin films, explaining how UV exposure causes rapid embrittlement and failure in bituminous adhesives, coatings, and composites. This in turn enables the design of sustainable bituminous composites with enhanced resistance to UV aging.