## 4 Conclusions  
In this work, the influence of the crosslinking density on the elastocaloric properties of NR has been investigated for the first time. Samples with varying degrees of crosslinking--2.9, 4.0 and 5.2 mol\(\cdot 10^{-4}\)/cm\({}^{3}\)--were produced by melt compounding and hot-pressing, and thermo-mechanically characterized. The assessment of the elastocaloric effect revealed that both the imposed deformation and the crosslinking density played a crucial role in the elastocaloric properties of NR. In particular, reducing the crosslinking degree and increasing the deformation led to enhancing the capability of the material to extract thermal energy from the environment. The NR sample characterized by a crosslinking density of 2.9 mol\(\cdot 10^{-4}\)/cm\({}^{3}\) (the least crosslinked sample) was found to be capable of extracting 16.4 J/cm\({}^{3}\) of thermal energy in one single refrigeration cycle when stretched at 500%. The evaluation of the Coefficient of Performance of the material (COP\({}_{\text{mat}}\)) confirmed the least crosslinked sample to be the best formulation among the ones prepared, exhibiting a COP\({}_{\text{mat}}\) ranging from 1.2 to 2.4.  
The investigation of the Mullins effect of the prepared sample highlighted the need to pre-stretch the rubber before its application, to ensure more reliable and consistent results, particularly in scenarios requiring significant deformations. It was found that the extent of strain played a pivotal role in the Mullins effect, whereas the impact of crosslinking density remained not entirely elucidated.  
In conclusion, the results underscore the significance of crosslinking density as one of the primary factors to be considered in enhancing the refrigeration potential of NR. It could be interesting to explore even lower crosslinking density and investigate if the elastocaloric properties could be even more improved. It is important to emphasize that the cooling performance of NR is far from that of conventional air conditioners. To improve the cooling power of natural rubber, a comprehensive research campaign is needed to investigate all material properties that could positively impact its elastocaloric performance. With the aim of enhancing the cooling power, future works will be focused on trying to reduce as much as possible the time of the refrigeration cycle; for example, working under forced convection conditions.  
Conceptualization, A.D., A.P. and L.F.; methodology, M.B.; software, M.B.; validation, L.F., A.D. and A.P.; formal analysis, M.B.; investigation, M.B.; resources, M.B.; data curation, M.B.; writing--original draft preparation, M.B.; writing--review and editing, L.F., G.F., A.P. and A.D.; visualization, A.D. and L.F.; supervision, L.F. and A.D.; project administration, A.D. and L.F. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
**Institutional Review Board Statement:** Not applicable.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Caloric Material** & \(\Delta\varepsilon\) **(mm/mm)** & \(\Delta\)**T\({}_{\text{cool}}\)/\(\Delta\sigma\)** (\({}^{\circ}\)C/MPa)** & \(\Delta\)**T/\(\Delta\varepsilon\)**(\({}^{\circ}\)C)** \\ \hline NR\_1.5S & 5 & 3.2 & 2.1 \\ Shape memory alloys (SMAs) & 0.03–0.05 & 0.013–0.045 & 300 \\ \hline \hline \end{tabular}
\end{table}
Table 8: Comparison of the elastocaloric properties in NR_1.5S and shape memory alloys [9].