## 3 Results and Discussion  
### Mechanical Behavior  
Two important parameters are obtained from DSR tests on bitumen: the complex modulus, \(G^{\bullet}\), and the phase angle, \(\delta\). These parameters can be used to characterize both viscous and elastic behavior of the binder [40]. The dependence of these quantities on the temperature gives rise to the so-called time cures. As the temperature increases or frequency decreases, bitumen begins to lose the majority of its elastic behavior (the storage modulus, \(G^{\prime}=G^{\bullet}\cos\delta\), decreases) and starts to behave as a viscous fluid  
Figure 2: Flowchart of the Electron Paramagnetic Resonance (EPR) test protocol. The preliminary power saturation experiments are necessary to determine the power of the incident microwave beam to be used in the following steps.  
(the loss modulus, \(G^{\prime\prime}=G^{*}\sin\delta\), increases). Thus, the limiting temperature in correspondence of which tan \(\delta\rightarrow\infty\) identifies the viscoelastic-sol transition temperature \(T_{\text{TR}}\) above which viscous behavior is highly predominant over elastic mechanical contribution. Figure 3 shows the time cure curves for virgin bitumen and bitumens modified with antioxidant additives and compared to analogous measurements performed after the aging process (RTFOT). Through data interpolation the asymptotic value of tan \(\delta\) intercepts the temperature axis and identifies \(T_{\text{TR}}\). Actually, in Figure 3 the temperature difference \(\Delta T=T_{\text{TR}}\) (aged) \(-\)\(T_{\text{TR}}\) (unaged) can be read in each graph while the histogram of Figure 4 illustrates a direct comparison of \(\Delta T\) for all the investigated samples.  
Upon addition of 2% \(w/w\) of anti-oxidants a slight increase of \(T_{\text{TR}}\) is observed for unaged samples, 69.4 \({}^{\circ}\)C (VC), 70.6 \({}^{\circ}\)C (LCS) and 71.2 \({}^{\circ}\)C (VC) compared to virgin bitumen 68.1 \({}^{\circ}\)C. The shift \(\Delta T\) (\(\pm\)0.2 \({}^{\circ}\)C as estimated error) of \(T_{\text{TR}}\) recorded upon heating treatment respect to the same unaged sample is an indication of an increase in hardening consistency of bitumen, which in turn can be correlated to the oxidation degree occurred in the material. Aging significantly changes both the chemical and physical properties of asphalts, which results in lower elasticity and higher stiffness. In absence of additive a shift of \(T_{\text{TR}}\) towards higher temperatures has been observed with \(\Delta T_{\text{CTRL}}\) = 6.5 \({}^{\circ}\)C. Then, the following sequence of \(\Delta T\) values has been detected for bitumen supplemented with inhibitors of free radicals: \(\Delta T_{\text{LCS}}\) = 4.5 \({}^{\circ}\)C, \(\Delta T_{\text{RH}}\) = 5.8 \({}^{\circ}\)C and \(\Delta T_{\text{VC}}\) = 7.2 \({}^{\circ}\)C. Those data clearly indicate a superior performance manifested by LCS in retarding oxidative hardening compared to the other tested antioxidants. Hitherto, the use of raw mixtures of natural phospholipids has been confirmed to be a powerful method to increase both the adhesion properties of bitumen [34] and its mechanical  
Figure 3: Time cure curves for virgin bitumen and bitumens modified with 2% \(w/w\) of LCS, RH and VC antioxidant additives, respectively. The increments of viscoelastic-sol transition temperature between aged (red symbols) and unaged (blue symbols) samples are indicated on the top axis of each graph.  
resistance to the action of thermal shocks [41]. Here, the discover of anti-aging properties of LCS adds another advantage of using these green compounds as multi-functional additives to enhance the bitumen performances. The presence of polyunsaturated fatty components in phospholipids providing carbon sites susceptible of oxidation may be responsible for the observed antioxidant activity [42] as it will be also confirmed by the analysis of EPR spectra.