To study the effect of the crosslink density of EPDM on the material properties of TPV, the ENB content was varied as presented in Table 4. The crosslink density was measured by swelling crumb TPV samples according to the ASTM D6815. It was observed that the crosslink density increased as ENB content increased in the TPV. Furthermore, as a result, a higher ENB content in the TPV lowered compression set due to enhanced elastic resilience of the EPDM crosslinks. ENB #3 (27.55 %) exhibited an improvement of approximately 12.4 % in the compression set compared to ENB #1 (31.45 %).
Figure 6 presents the temperature-dependent tanδ curves for TPV samples with varied ENB content. The Tg of EPDM increased as the crosslink density increased (Tg of ENB #1, ENB #2, and ENB #3 were -41.2 °C, -39.0 °C, and -38.0 °C, respectively.). This can be attributed to the fact that more thermal energy is required for the glass transition of the EPDM polymer chins because of reduced chain mobility by higher crosslink density. As shown in the scanning electron microscopy (SEM) images in Figure 7, the average particle diameter observed was 0.85–2.10 μm. According to the related literatures (Katbab et al., 2000; Lim et al., 2017; Xu et al., 2018), the EPDM particle diameter of TPV is in the range of 0.5–5 μm. Therefore, the particle observed in this study can be considered as EPDM particles. 
The average diameter of the EPDM particles decreased as the EPDM crosslink density increased. ENB #3 exhibited an average diameter of 0.85 μm which was 59.5% smaller compared to ENB #1 (2.1 μm). For the same EPDM content, the specific surface area of an EPDM particle increases as the average particle diameter decreases, implying an increase in the interfacial area between EPDM and PP. Therefore, when the crosslinking density of EPDM is increased during the dynamic vulcanization reaction, the EPDM particles are broken down to smaller particles increasing EPDM/PP interfacial slip energy. This increased energy dissipation resulted in higher tanδ values above Tg of EPDM as shown in Figure 7. ENB #3 exhibited the largest tanδ value (0.11) at 24 °C, which is the standard temperature condition for vehicle vibration test.