Effects of pressure drop rate and CO 2 content on the foaming behavior of newly developed high-melt-strength polypropylene in continuous extrusion

Abstract

We report systematic studies on the foamability of our novel high-melt-strength long-chain branched polypropylene under supercritical CO₂. Continuous foaming experiments were conducted using a tandem extrusion system and a set of filamentary dies with similar pressure drops but different pressure drop rates. The foam expansion was controlled by varying the temperature at the die exit. Under identical CO₂ loadings, the expansion ratio plotted as a function of die temperature exhibited similar shapes across multiple pressure drop rates. However, the shape of the curve varied across different amounts of CO₂, under which the highest achievable expansion ratio occurred at a lower die temperature with increasing CO₂ content. The cell density displayed strong dependence on both the pressure drop rate and the amount of dissolved CO₂. The effect of the latter became more apparent at lower pressure drop rates. The average cell size decreased with increasing CO₂ loading but generally showed weak dependence on pressure drop rate except at the highest value.