Mechanical properties of polymer composites are well known to be affected strongly by internal defects such as voids (Takagi and Asano, 2008). Consequently, the density and void content usually serve as good indicators for composite strength (Borja et al., 2006; Takagi and Asano, 2008).
The measured density values were compared to the calculated ones on the basis of the following relationship, based on the rule of mixtures: