# Using Plastic Waste in a Circular Economy Approach to Improve the Properties of Bituminous Binders
## 3 Results and Discussion
### Synthesis of the Principal Inferences and Findings  
In general terms, from the assessment of the works and tests carried out, it was possible to frame the following inferences: the HDPE waxes can be obtained in a relatively simple reaction system, with a very high yield, and one of the remaining products is a valuable fuel that can be used on-site to provide the energy required for the process (thus avoiding the consumption of energy generated by other sources); they change the viscosity of the binders, favouring a reduction in the handling temperature of binders/mastics/bituminous mixtures (when finished with functional additives); and contribute to the chemical recycling of plastics, as well as to the fulfilment of some of the goals defined in the U.N. 2030 Agenda.  
Other findings can be mentioned, namely that the test results strongly depended on the analysed samples, their representativeness, dimensions, heterogeneities, and measurement fidelity (test precision--repeatability and reproducibility). It was also possible to confirm that the advanced characterization of binders (e.g., with comprehensive rheological tests) is more suitable to help find compositions that present added value in more differentiating parameters. Traditional and standardized assessments are usually not enough to distinguish complex binders.  
In addition, some trends that could be of relevance for future developments in these sustainable production processes were noticeable. For example, as commented by [PERSON] in a recent review [39], the future for plastic waste can go through co-processing technologies, such as co-pyrolysis, including its blending with biomass. In the words of those authors, such technologies have _\"tremendous environmental and economic advantages\"_. Other researchers also pointed to similar co-processing technology [64] (pyrolysis of the mixture of HDPE with wood) as a promising method or referred to the usefulness of using renewable and low-cost catalysts, such as those derived from lignocellulosic waste [65].  
Further directions may also be taken in future research, namely evaluating the influence of mixing these waxes with nanoclays [30; 66], aged bitumen from reclaimed asphalt pavement (RAP) [67; 68], or polymeric capsules containing rejuvenators to promote self-healing [69]. Likewise, it will be helpful to study the long-term performance of different bituminous mixtures with a binder such as the PMB + wax presented in this study and evaluate their recyclability potential [70], including the advanced characterization after an accelerated ageing procedure [71; 72]. This evaluation can be performed using, among other test methods, the MSCR protocol following, for example, the European standard EN 16659--adopting two stress levels (0.1 kPa \(\wedge\) 3.2 kPa) at two temperatures (chosen between 40 \({}^{\circ}\)C and 90 \({}^{\circ}\)C).  
\begin{table}
\begin{tabular}{c c c c c c} \hline
**Parameters** & **PMB 1** & **PMB + Wax 1.2** & **PMB 3** & **PMB 4** & **PMB 5** & **PMB 6** \\
**(SHRP Specifications)** & **PMB 6** & **PMB 7** & **PMB 8** & **+ RPPW 4** & **+ FTW 4** \\ \hline G*/sin \(\lx@sectionsign\), \# 60 \({}^{\circ}\)C (kPa) & 6.09 & 46.23 & 1.10 & 1.11 & 8.90 \\ Upper limit temperature (\({}^{\circ}\)C) & 80.3 & 110.8 & 61.0 & 61.2 & \textgreater{}100 \\ \hline \end{tabular} 1  
\end{table}
Table 6: Effects of three different waxes on the rutting resistance parameters of two PMB.