Stiffness-Impact Energy Relationship

On the one hand, it is possible to increase the toughness of polymers by incorporating rubber or polyurethane (HD-TPU) particles. Compliant particles, if well dispersed, may influence the deformation behavior towards a higher energy absorption by activating localized cavitational craze-like and shear yielding mechanisms. Their lower stiffness, however, has a disadvantageous effect on the composite s modulus. 

on the other hand, stiff fillers are used instead, the modulus of the polymer can be enhanced, e.g., by adding only silicon carbide or calcium silicate 

---