Implication of wax on hindering self-healing processes in bitumen

This paper examines the molecular mechanism of self-healing and hindering effect of wax on self-healing in bituminous adhesives. To do so, computational analysis was combined with microscopy to show the effect of molecular binding and surface morphology on self-healing processes in bituminous adhesives. Specifically, here, we demonstrated how bitumen wax content can hinder self-healing of microcracks in bitumen by promoting formation of characteristic features (bees) on the surface of cracks. Surfaces of a fresh crack in bitumen are free of “bees”, but wax molecules migrate to the bitumen-air interface forming “bee” structures quickly. This can be attributed to “bees” being measurably less adhesive than other bitumen components. This was supported by our computational analysis showing interactions between wax and bitumen components (<22 kcal/mol) are much weaker than interactions among bitumen components themselves (>50 kcal/mol). In addition, the “bees” introduce nanoscale surface roughness that could prevent conformal contact and keep a large proportion of the surfaces separated. This was also evidenced in our density functional analysis showing electrostatic interactions decrease notably with increase of distance between wax crystals. Accordingly, presence of wax can hinder self-healing of bitumen by altering surface morphology and preventing conformal contact between two sides of a microcrack.