| Melt-processible fluoropolymers, like Fluorinated Ethylene Propylene (FEP) and Polyvinylidene Fluoride (PVDF) exhibit excellent properties such as excellent chemical resistance, high continuous use temperatures, outstanding ageing and weathering stability, which make them appropriate for very special applications. Combined with these inherent properties are the attributes derived by foaming the material such as impact absorption and reduced mass of polymer per unit volume to yield products with markedly different with those foams currently available. However, the foaming processes of fluoropolymers are more complex compared with other polymers used for foaming, such as polypropylene (PP) or polystyrene (PS).Foaming processes of FEP with supercritical carbon dioxide (scCO2) as the blowing agent has been investigated. It is found that with increasing foaming temperature, the foamed FEPs showed an apparent increase in cell size. While cell size decreased with increasing saturation pressure. An abnormal phenomenon, that expansion ratio decreased with the increasing saturation pressure, has been found for the pure FEP foams. It has been explained by the relation between foaming and crystallization processes. In FEP, crystallization is faster than cell growth The melt viscosity of FEP is enhanced by the newly formed crystals and become detrimental for the increasing expansion ratio. This abnormal phenomenon may be typical for semi-crystalline polymers with very fast crystallization rate. In this work, the pure FEP foams obtained at 24 MPa and 250℃ have the highest cell density of 2.2×106 cells/cm3, average cell size of 87 μm and an expansion ration of 4.3. By adding the nucleating agent, the optimal foaming temperature is increased to 265℃. The average cell size of composite FEP foams can be reduced to 19μm while the cell density is increased to 2.0×108 cells/cm3. The pure FEP foams obtained at 12 MPa and 250℃ have the highest expansion ration of 5.4, and cell density of 2.2×106 cells/cm3, average cell size of 87 μm However, the highest expansion ration of composites was obtained at 24 MPa and 250 ℃, with a value of 5.3.This thesis also investigated the foaming properties of PVDF using scCO2 at various temperatures (150～163℃) and pressures (12～24 MPa). With increasing foaming temperature, the expansion ratio of foamed FEPs showed an apparent increase first and then decrease. While expansion ratio increased with increasing saturation pressure. It is found that the addition of nucleating agent had little influence on foaming behavior. While the mixing process had a notable effect on the properties, but the mechanism is not clear. It is believed that the mixing process may influence the crystallization behavior. At 12 MPa and 163℃, PVDF could foam with the highest expansion ratio,29 for foamed P0,37 for foamed P1 and 35 for foamed P4, respectively. The highest cell density ofPO was obtained at 12 MPa and 163℃ and the value was 5.6×108 cell/cm3, while the highest cell density of P1 and P4 was obtained at 24 MPa and 155℃ and the value was 1.7×109 cell/cm3,4.0×109 cell/cm3, respectively.The solubility of CO2 in PVDF was measured at temperatures from 353.15 K to 383.15 K and pressure up to 10 MP. The results demonstrated that the solubility of CO2 in PVDF increased with the elevated pressure and decreased with temperature. The experimental results also were correlated by Sanchez-Lacombe equation of state (SL EOS) with temperature-dependent binary interaction parameter k12. The results showed that SL EOS was a suitable model to predict the solubility of CO2 in PVDF. All of experiment proved that the crystal forms were keep unchangeable, but the crystal morphology became more perfect and crystallinity merely increased. |
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