An investigation has been performed of the cell nucleation and initial growth behaviors in the foam processing of polypropylene (PP) in both the linear and branched forms. These materials were foamed in extrusion with the two blowing agents, CO, and isopentane. The cell density generally increased with an increased content of the blowing agent, for both CO, and isopentane. The effect of processing pressure on the cell density was distinct when CO, was used, whereas no pressure effect was observed in the foam processing with isopentane. The cell morphologies for the two PPs were found to be significantly different. A slightly lower nuclei density was observed in the branched PP foams than in the linear PP foams. However, the phenomenon of cell coalescence was observed much less in the branched PP foams. Most cells in the branched PP foams were closed, whereas in the linear PP foams they were connected to each other. The experimental results indicated that the branched structure played an important role in determining the cell morphologies through its effects on the melt strength and/or melt elasticity.

It is shown that Axisymmetric Drop Shape Analysis (ADSA) is well‐suited for the study of polymer melt surface tensions. The technique is not restricted to equilibrium surface (interfacial) tensions; it is also suitable for measuring the time dependence (or kinetics) of surface tension of polymer melts. Results for three polymers, polypropylene, polyethylene, and polystyrene, at temperatures above 170°C are reported. Contrary to the well‐known decrease of surface tension in low molecular weight surfactant solutions as a result of equilibration, an increase in the melt surface tension is observed under isothermal conditions.

An experimental investigation was conducted to research the cell nucleation behavior in the extrusion foam processing of polypropylene (PP) using hydrocerol and isopentane. While the hydrocerol and isopentane are considered to function a s the nucleating agent which determines the cell-population density and a s the blowing agent to control the volume expansion ratio respectively, both agents affected the cell-population density. In addition, synergistic effects of these agents on the cell density were observed. In foam processing with hydrocerol, a higher cell-population density was noted at lower processing pressures and at higher polymer flow rates. This phenomenon is of interest since the cell density, in general, increases a s the processing pressure increases when only a physical blowing agent is used in the foam processing. The experimental results indicate that the nucleation in the foam processing with hydrocerol is governed by a heterogeneous nucleation mechanism; also, the quality of the mixing of the polymer and the agents, as well a s the amount of gas lost during the plastication of the pellets in the barrel, exerts a strong influence on the resulting cell density.

The effect of talc on the cell morphology of polypropylene (PP) foams has been investigated. A branched high-melt-strength PP was used in this study to reduce the bubble coalescence phenomenon during the foam processing in extrusion with isopentane or CO2 as a blowing agent. When isopentane was processed with PP, the bubble nucleation in the extruded foams was dominated by the talc concentration, and the isopentane content did not strongly affect the cell-population density. However, when CO2 was used as the blowing agent, both the talc concentration and the CO2 concentrations played an important role in determining the cell-population density. Although previous studies on the foam processing with CO2 alone indicated that the cell-population density was a strong function of the processing pressure and the pressure-drop rate, neither of these processing parameters significantly influenced the cell morphology of the extruded PP foams when talc was employed as a nucleating agent.