Figure 9 represents the relation between the hardness of NRLF and ESP loading. Hardness of NRLF filled with ESP increases with increasing filler loading. The value of hardness for the controlled NRLF sample is 48, which is
in the range of R33 Medium. Hardness values for 2.5, 5.0, 7.5, and 10.0 phr of NRLF filled with ESP increase from 74.6, 83.0, and 85.2 to 98.2. The hardness values of the NRLF filled with ESP are in the range of R34 Firm, except for NRLF-10 phr of ESP, where the filled foam was in the hardness range of R35-X Firm. There are two reasons for this: the nature of filler and demobilizing effect of filler on the polymer chain. The ESP filler is harder than the polymer matrix; thus, when the filler content increases, the hardness increases. Moreover, good
dispersion of ESP filler in the NRLF matrix causes the
polymer chain to be demobilized on the filler surface,
thus increasing the stiffness and hardness of the NRLF
filled with ESP.