Investigation The Extrusion Foaming Process Of Polyproylene By Using Supercritical CO₂ As Foaming Agent

Polypropylene(PP) is a kind of abroad applied universal plastics for its advantages such as lower cost and easier recycle. And PP foams are becoming investigative hotspot in the thermoplastic foam industry, because of their unique advantages and performances such as thermostability, good buffer performance, easy degradation. So they can be applied in many fields like packing service, automotive industry and architecture. Until now, few countries except for Japan and Germany have controlled the key technologies of industrial production. However, PP extrusion foaming with supercritical CO₂ is considered as a new research direction, not only domestic institution but also some developed countries are still in research and development.Cell nucleation theory and main effects of extrusion foaming were systematically analyzed in this paper, then a series of test and measurement were taken on the basis of exist theory. The main effects of PP extrusion foaming were discussed. And the main reason of PP melt strength increase was generalized as the change of PP molecular chain. The PP macromolecular chain tangling between PP molecular chain was reinforced after branched structure was established, finally the melt elasticity and initial temperature of crystallization were increased. PP extrusion foaming experiment was done after the experiment setup was improved. The continuous PP extrusion foaming was well proceeded by optimizing process conditions. At last, the expansion ratio for the better PP foams was around 20, the cell size for the smaller cells was around 50μm and the cell density was about 10 cells/cm .Effects of process parameters on expansion ratio and cell structure were analyzed and investigated in detail, including the melt temperature, die temperature, die pressure and CO₂ content. And the suitable range of these parameters was also confirmed. As for the experiment system, the melt temperature should be controlled from 125℃to 140℃. Within this temperature range, the cell size reduced, the cell density and the expansion ratio increased by lowering the melt temperature. The die temperature also effected expansion ratio, so there was an optimum range in die temperature. The maximum expansion ratio was 23 when the die temperature was 150℃. The die pressure should be controlled from 7 MPa to 9 MPa . Cell size reduced , cell density and expansion ratio increased by increasing the die pressure. It is advantage for increasing the expansion ratio and cell density, and reducing cell size by increasing the CO₂ content. But the CO, content should not exceed the upper limit of CO₂ solubility on PP melt (<10wt% in this paper), or PP foams may exist big bubbles for the incomplete solution of excessive CO₂.In addition, some technical problems and engagement were taken as for the defects of PP foams, such as cell coalescence, foam contraction and gas loss.