Fundamental Foaming Mechanisms Governing Volume Expansion of Extruded PP Foams

摘要

This paper describes the fundamental foaming mechanisms that govern the volume expansion behavior of the extruded polypropylene foams. A careful analysis of extended experimental results indicates that the final volume expansion ratio of the extruded polypropylene foams blown with butane is governed by either loss of blowing agent or crystallization of polymer matrix. A CCD camera was installed at the die exit to carefully monitor the shape of the extruded polypropylene foam. The CCD images were analyzed to illustrate both these mechanisms of gas loss and crystallization during foaming at various temperatures, and it was observed that the maximum expansion ratio was achieved when the governing mechanism was changed from one to the other. In general, the gas loss mode was dominant at high temperatures whereas the crystallization mode was dominant at low temperatures. When the gas loss mode was dominant, the volume expansion ratio was increased by decreasing the temperature because of the reduced amount of gas lost. By contrast, when the crystallization mode was dominant, the expansion ratio was increased by increasing the temperature because of the delayed solidification of polymer. The processing window variation with the butane concentration, the change of the temperature ranges for the two governing modes, and the sensitivity of melt temperature variations to the volume expansion ratio are discussed in detail based on the obtained experimental results for both branched and linear polypropylene materials.