Effects of pressure drop rate and CO₂ 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.

Tuning High and Low Temperature Foaming Behavior of Linear and Long-Chain Branched Polypropylene via Partial and Complete Melting

“…Therefore, it can be concluded that foaming induced at temperatures at least 8 °C above T c,onset may lead to sensible expansion of the molten polymer–gas mixture. When the foaming temperature is further above T c,onset , optimal foaming behavior—characterized by high volume expansion and uniform cell density—is achieved, which often emerges as a “mountain-shaped” profile in continuous foaming processes as the die temperature approaches the crystallization temperature of the polymer–gas mixture [ 39 ].…”

Improved Cell Morphology and Surface Roughness in High-Temperature Foam Injection Molding Using a Long-Chain Branched Polypropylene

“…Additionally, differences in foaming behaviors amongst resins between UC and AV CBA can be attributed to increased gas yield in AV CBA. As noted earlier, higher amounts of dissolved gas allow for increased cell density and smaller cell sizes [ 14 ]. The higher amounts of dissolved CO 2 also lower T c of PP [ 13 , 25 ] which decreases sensitivity to packing time amongst the resins.…”

“…Evidence of suppressed cell nucleation is reflected in the decrease in cell density with increasing packing time in all resins ( Figure 5 a,b). Further, the higher cell density, smaller cell sizes, and decreased sensitivity to packing time at higher CBA wt% can be partly attributed to decreased T c of PP due to CO 2 plasticization [ 13 , 14 , 25 ], which is further evidence that foaming under these test conditions is dominated by crystallization. From these trends, it can be postulated that the injected melt subjected to a longer packing time has been excessively cooled to a point that a larger portion of the polymer has solidified and lost more of its capability to foam upon mold opening.…”

“…A significant amount of research on improving PP foamability is focused on low melt flow rate (MFR) and low molecular weight distribution (MWD) polymers that are primarily used for extrusion foaming to manufacture foamed products with simple geometries, such as foam rods and sheets [ 13 , 14 ]. To create complex three-dimensional shapes with high expansion and good dimensional stability, mold-opening foam injection molding (MO-FIM) is used, which is a continuous foaming technique very similar to traditional injection molding but with the addition of a blowing agent and a foaming stage.…”

Morphological, thermal and mechanical properties of polypropylene foams via rotational molding

“…So far, some studies have been conducted on PP foams manufactured by injection molding, [13][14][15][16] compression molding [17][18][19] and extrusion molding. 20,21 For example, Ahmadi and Hornsby reported how the structure of PP foams was influenced by the injection molding processing conditions. 13 The effect of injection speed, shot weight, mold temperature and melt temperature were investigated.…”