Table 2.

The equations and features of the morphological characteristics [56,57,58].

	Main Equation	Feature	Equation
Number average cell size	$D_n=\frac{{{\displaystyle \sum }}^{ }\left(n_i D_i\right)}{{{\displaystyle \sum }}^{ }{n}_i}$	Number average diameter of cells	(2)
Weight average cell size	$D_w=\frac{{{\displaystyle \sum }}^{ }\left(n_i D_i^2\right)}{{{\displaystyle \sum }}^{ }(n_i D_i)}$	Weight average diameter of cells	(3)
Polydispersity index	$PDI=\frac{D_w}{D_n}$	Cell size distribution	(4)
Cell density	${\rho }_{cell}=\frac{{\rho }_{bulk}}{{\rho }_{foam}} {\left(\frac{{{\displaystyle \sum }}^{ }{n}_i}{A}\right)}^{\frac{3}{2}}$	Population and number of cells inside the matrix	(5)

n_i is the number of cells with a diameter D_i, A is the surface analyzed of the foam, ρ_bulk is the density of the unfoamed matrix and ρ_foam is the foam density. For hardness (Shore A), a PTC Instrument (ASTM D2240, model 307 L, PTC Instruments, Los Angeles, California, USA) was used. Tensile testing (ASTM D412) was performed at room temperature on an Instron universal testing machine (USA) model 5565 with a 500 N load cell at a crosshead speed of 10 in/min. For the compression tests (ASTM D575), a dynamic mechanical analyzer (DMA) RSA3 TA Instrument (New Castle, DE, USA) was used with cylindrical samples (2.5 cm in diameter and 3.5 mm thick) and compressed at a rate of 0.01 mm/s. All the properties reported represent an average of a minimum of three samples.