## 4 Discussion
### Influence of Base Rubber Type and Non-Dissipative Behavior of the Elastomer Compound  
In Figures 23 and 24, the tensile load-displacement response curves of samples with elastomer layers made of different types of compound are reported for direct comparison.  
Figure 21: Tensile load-displacement response curves for samples 14a and 14b, without bridge layers and 18a and 18b, with bridge layers.  
Figure 22: Tensile load-displacement response curves of sample 16b, without bridge layers, and samples 17a and 17b, with bridge layers.  
These figures are followed by Table 7, where the features of the compared samples are reported together with the experimental results.  
With regard to the influence of the rubber used to produce the elastomer, sample 12, which was built using a synthetic rubber-based compound was compared to sample 4, which was built using a natural rubber-based compound. As it can be seen in Table 7 and the curves in Figure 23, sample 12 shows greater strength and much greater ultimate elongation compared to sample 4. This happens even if sample 12 undergoes mixed failure, while sample 4 shows cohesive failure.  
It can be concluded that synthetic rubber provides the isolator with greater ductility, i.e., dissipative capacity in comparison with natural rubber.  
With regard to the influence of the non-dissipative behavior of the elastomer compound, sample 11, which was built using a non-dissipative compound was compared to sample 12, which is the control specimen in this case. As it can be seen from Table 7 and the curves in Figure 24, sample 11 has slightly greater strength, but much less ultimate elongation in comparison to sample 12. Hence, as was expected, the non-dissipative compound gives the isolator lower ductility.  
All samples, apart from samples 5 and 6, show satisfactory tensile behavior according to UNI EN 15129, i.e., they have a strength that is greater than 2 G.  
An increase in vulcanization time does not improve the adhesion between elastomer and steel layers, on the contrary, for samples 5 and 6, it reduced their capacity in terms of strength.  
The use of corundum in the sandblasting process does not improve or worsen the adhesion between elastomer and steel layers.  
The absence of an external covering layer does not improve the adhesion between the layers, but instead, it worsens it in terms of the deformation capacity and failure type.  
Using elastomer layers without an oily component. improves the adhesive behavior of elastomeric isolators, because they are able to avoid adhesive failure. Moreover, it increases the isolator ductility, i.e., its dissipative capacity.  
The use of refreshed elastomer and the use of bridge layers does not negatively influence the adhesion between steel and elastomer, whereas they may improve the elongation capacity, i.e., the ductility, and consequently, the dissipative capacity of the isolator.  
Synthetic rubber provides the isolator with greater ductility, i.e., dissipative capacity, in comparison with natural rubber, and greater strength.  
A non-dissipative compound gives the isolator lower ductility without affecting its strength.  
Conceptualization, M.P.; methodology, M.P.; software, F.P.; investigation, G.F. and M.P.; resources, G.R.; data curation F.P.; writing--original draft preparation, F.P., G.F. and M.P.; writing--review and editing, M.P.; visualization, F.P.; supervision, G.R.; project administration, G.R. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
ILPEA Industries S.p.A is gratefully acknowledged for providing the samples. The authors also acknowledge engineer [PERSON] for collaborating in the arrangement of the test setup.  
The authors declare no conflict of interest.