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.