# A Review on Bitumen Rejuvenation: Mechanisms, Materials, Methods and Perspectives
## 5 Perspectives  
### Improving Rejuvenors Characteristics  
As already discussed above, a rejuvenator is commonly made by simple oil waste for their low-cost and obvious environmental concerns. In fact, oil waste is the by-product of edible oils surely more produced in the world. Therefore, its re-utilization may provide a feasible method to minimize the amount of generated waste, providing a positive environmental impact. From a chemical point of view, the oil subjected to elevated temperature changes its chemical composition, producing molecules with high anti-oxidant properties, which are certainly advantageous, for example, in the exhausted oil utilization as rejuvenating of bitumen. However, the waste oils have some disadvantages, such as poor low temperature fluid properties, a propensity to oxidative degradation, a susceptibility to hydrolysis in acid media, which limits its application. For this reason, it is actually of greatest interest the proposal of chemical modifications of their structure to improve its physicochemical properties. In particular, if search should be narrowed only to vegetable oil waste (i.e., Waste Cooking Oils (WCO), then soybean oil, canola oil, coconut oil, castor oil, etc. ), esterification, [65] hydrogenation [66], epoxidation [67], acylation [24] are only some examples of adopted chemical transformations to enhance the performance of these oils. In fact, the major components of vegetable oil waste are triglycerides esters of glycerol with saturated and unsaturated long-chain fatty acids, therefore, the transesterification reaction may represent the most valid method to change their properties, and it progresses through hydrolysis and successive esterification of the hydrolyzed products as schematically shown in Scheme 1. This procedure paves the way to subsequent chemical modifications. An example is reported from [PERSON] et al. [68] that carried out the transesterification reaction by the treatment of exhausted oils with NaOH in CH\({}_{3}\)OH to obtain methyl esters of fatty acids. The latter were transformed in variously substituted triseter derivatives by the first epoxidation of unsaturated bounds present on of carbon backbone of R and a subsequent opening of oxirane ring to produce a final matrix with increased physicochemical properties, (i.e., better anti-wear ability, improved oxidation stability, etc.). However, it is evident that in proposing new synthetic methodologies of structural transformations, the chemists should take into account cheap and eco-friendly procedures. For this reason new perspectives on this typology of reaction may be, for example, the introduction of low-cost catalysts (i.e., Lanthanide salts, Fe salts, Zn salts, Cu salts, etc. ), which are known to be employed in many chemical manipulations [69; 70] in combination with green solvents, such as ionic liquid, Deep Eutectic Solvents (generally named DES) or water or Microwave irradiation [71; 72]. For example, in order to transform oil waste into more performing products, it is possible to suggest a synthesis process to open oxirane rings, formed on the unsaturated bounds of R chain, in mild conditions, introducing transformable functional groups, such as nitriles or nitro that may be subsequently oxidized in environmentally friendly solvents. Another suggested modification may be the introduction of acyl groups with alkyl or aromatic chains on the unsaturated scaffold of the carbon-chain [73; 74; 75]. The presence of alkyl or aromatic groups could lead to forming secondary interaction, such as Wan der Waals or \(\pi\)-\(\pi\) interactions that may confer to the modified oils a major branched substructure, varying the physicochemical characteristics, increasing the chemical stability of the substrates and obtaining their better performance. An alternative to the chemical catalysts could be the enzymatic catalysis, as proposed by [PERSON] and co-workers [76]. Candida rugosa lipase was used to hydrolyze oil wastes in a water solution with successive esterification by Amberlyst 15(H) resin catalyst.