Melt Property and Extrusion Foaming Processability of Polypropylene Blended with Polytetrafluoroethylene

Abstract

Polypropylene (PP) has a wide range of application because of its high stiffness and high heat resistance. PP is, however, not suitable for extrusion foaming process, because the cells of PP have low resistance to rupture during expansion.
In order to give good processability to PP under extrusion foaming, PP is blended with polytetrafluoroethylene (PTFE). PTFE turns into many thin fibers during the mixing process, and these fibers may increase the resistance to rupture. Various samples of the blends were prepared by using three PP's with different viscosity values (melt index=0.4, 7.0 and 20) and various amounts of PTFE. As the indicators of the processability, the melt tension and the crystallization temperature of the blends were measured. The melt tension of the blends increased with PTFE content. The crystallization temperature also increased with PTFE content, especially when the PP with high viscosity was used.
The extrusion foaming process of these blends was studied by using a tandem extruder. It was found that the blends of low viscosity PP can be successfully made into foamed products. The expansion ratio of the products increased with PTFE content. However, the size of the cells in the foamed products had a wide distribution, and the volume fraction of the closed cells decreased with PTFE content. These problems of wide cell size distribution and low content of closed cell may be caused by the inhomogeneous distribution of PTFE fibers in the PP matrix.