To evaluate this value, the apparent density of
a non-cellular structure unreinforced phenolic material was
determined according to previous work [5,6]. The formulation of
this material was carried out in the same way than for the studied
foams apart from in this case the blowing agent was not added to
the formulation mixture. This material showed an apparent density
of 580 kg/m3, only a 25% higher than the cell wall density for the
unreinforced foam determined from the parameters shown in
Table 2. Therefore, the assumption of the foam cells are spheres is
acceptable for the studied system.