The cell morphologies of neat PP, PP/FKM(0.5), PP/FKM(1.0), and PP/FKM(2.0) foams prepared at 152 °C and 20 MPa are shown in Figure 2
. The cell size declined as the loading of FKM increased to 1.0 wt %. Cracked and consolidated cells appeared, and the cell continuity was poor in the neat PP foam. The PP/FKM foams exhibited a different cellular structure with different FKM contents. The cell size distributions of different foams are shown in Figure 2
. PP/FKM foams possessed narrower cell distributions than those of the neat PP foam. In particular, PP/FKM(1.0) and PP/FKM(2.0) foams exhibited much more uniform cell size distribution. Moreover, increased porosities were obtained as the loading of FKM increasing, which indicated a much better foaming ability of PP. These results implied that the enhanced diffusion rate and increased solubility of CO2
were obtained in the presence of FKM, resulting in a large porosity, which was also found in the previous studies on fluorinated ethylene propylene copolymer (FEP) foaming by scCO2
]. Furthermore, the independent solid-state FKM phase in the PP matrix may significantly increase the heterogeneous nucleation site in the foaming process.
The average cell diameter and cell density of the cellular structure of the neat PP and PP/FKM(0.5), PP/FKM(1.0), and PP/FKM(2.0) foams are summarized in Figure 3
a. The cell size of PP foams decreased from 65 to 23 μm as the loading of FKM increased from 0 to 1.0 wt % and the cell density increased significantly compared to neat PP foam, by more than 16 times. The foam density and expansion ratio of foamed samples are shown in Figure 3
b. The foam density declined as the content of FKM increased from 0 to 2.0 wt %, in agreement with the results in Figure 2
. The existence of FKM led to a higher diffusion rate and solubility of CO2
in the melt PP matrix, and enough CO2
could support cell growth for a long time [35
]. According to “Heterogeneous Nucleation Theory”, the formed nanoscale solid FKM phase in the PP matrix can act as the nucleating agent; it is vividly shown in Figure 4
. It is known that foaming is a rapid process for cells growing in a few seconds, which depends on the thermophysical and rheological properties of PP/CO2
mixtures, and this process is related to the change of temperature, pressure, and local stress, etc. In the multiple phase system, the existence of the FKM phase induces a mass of CO2
aggregation, which is similar to an “island”. During the process of release pressure, it was easy to cause the change of local stress around the “island”, and induce a large number of nucleation sites, which greatly increased the nucleation rate. Furthermore, a large number of cell sites, caused by heterogeneous nucleation, competed for the limited CO2
, which restricted the cell growth [37
]. Additionally, the suitable foaming conditions of FKM were about 230 °C and 30 MPa [35
]. A small amount of CO2
might dissolve into the FKM phase in the saturation process, so it might also enhance the foaming ability of PP/FKM samples.
The mechanical properties of the resultant PP foam are the important evaluation parameters for potential industrial applications. The tensile stress-strain and compressive stress-strain curves of neat PP and PP/FKM foams are shown in Figure 5
. The PP/FKM foams possess excellent stress and strain compared to neat PP foam. The tensile stress increased to more than 15 MPa and the tensile strain of PP/FKM foam reached 110%. The compressive strength results showed that PP/FKM exhibited higher stress than neat PP foam. The obtained outstanding mechanical properties were ascribed to the well-defined cellular structure and high continuous polygonal cell morphology of PP/FKM(1.0) foam [10
]. All these clear results signified the key role of FKM in preparing fine PP foam with excellent mechanical properties, which indicated promising engineering applications.