## 3 Results and Discussion
### Rejuvenating Effect vs. Fluxing Effect: Action Mechanism of Recycling Agents  
Generally speaking, this study shows how differently the two recycling agents function. The techniques applied in this study are able to provide indications on the effects of these recycling agents on aged bitumen, thereby enabling the characterization of their mechanisms of action. A real rejuvenator is capable of restoring the chemical compounds that were lost during aging, and also of breaking down the oxides formed with the hydrocarbons and sulfides that make up bitumen's composition [30; 31; 32; 33; 34]. The R1 recycling agent, which the results suggest is a real rejuvenator, is made up of a blend of compounds composed of amino groups, as can be seen in Table 2. It is very plausible that the amino derivatives interact with the part of the bitumen that was oxidized (the asphaltenes), thereby reconstructing their chemical structure and reversing the asphaltene aggregation phenomenon that brings about stiffness of aged bitumen. Asphaltenes are known to consist of condensed aromatic rings and heterocyclic compounds that contain nitrogen and sulfur, and oxidative aging depletes these compounds and also triggers the formation of oxides such as sulfoxides, which have been identified in aged bitumen [17]. The reconstruction of the chemical structure of aged bitumen by the R1 recycling agent could be as a result of the fact that amine groups, in general, consist of five-membered heterocyclic rings that contain nitrogen, and these compounds could either replace the depleted compounds in aged bitumen or supplement the existing ones, while also breaking down the dative bonds of the sulfoxide groups that are characteristically present in high quantities in aged bitumen. The R2 recycling agent, on the other hand, is a mix of plant esters. Given that the findings of this study suggest that R2 is not capable of interacting chemically with aged bitumen, but rather establishes a physical interaction by augmenting the maltenic fraction, it can be said that it is a mere fluxing agent. The R2 recycling agent is composed of plant esters; these also contain heterocyclic rings, but are predominantly composed of carboxylic acids, heterocyclic rings, and alcohol. Esters are also well known to function as excellent lubricants, and, in this case, it can be said that the action of R2 on oxidized bitumen is the dilution of oxidized asphaltenes and not real rejuvenation via the regeneration of heterocyclic or aromatic rings. This mechanism of action of the R2 recycling agent is not sufficient for the real rejuvenation of aged bitumen, and this could explain why R1 made up of amino derivatives is a better candidate at restoring the structural properties of aged bitumen compared to R2, which is made up of plant esters. Asphaltene aggregation is also characteristic of aged bitumen, and this is brought about by the interaction between the polyaromatic cores of the asphaltene molecules. Van de Waal forces between the aliphatic side chains of the asphaltene molecules also contribute to this aggregation and clustering [35; 36]. Since the AFM images show the de-clustering of the bee structures brought about by R1 in aged bitumen, it is possible that the tendency of alkyl groups to surround N atoms in amino group molecules increases the steric interference between the asphaltene molecules, thus destabilizing the asphaltene clusters in aged bitumen [22]. The esters that constitute the R2 recycling agent are less likely to influence steric interference, and more likely to promote van der Waal forces between asphaltene molecules that sustain asphaltene aggregation [35]. These properties indicate why R1 seems to be an ideal rejuvenator and R2 seems to be a softener of aged bitumen, as shown by the results of this study.