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

Abstract

While existing foam studies have identified processing parameters, such as high-pressure drop rate, and engineering measures, such as high melt strength, as key factors for improving foamability, there is a conspicuous absence of studies that directly relate foamability to material properties obtained from fundamental characterization. To bridge this gap, this work presents batch foaming studies on one linear and two long-chain branched polypropylene (PP) resins to investigate how foamability is affected by partial melting (Method 1) and complete melting followed by undercooling (Method 2). At temperatures above the melting point, similar expansion was obtained using both foaming procedures within each resin, while the PP with the highest strain hardening ratio (13) exhibited the highest expansion ratio (45 ± 3). At low temperatures, the foamability of all resins was dramatically improved using Method 2 compared to Method 1, due to access to lower foaming temperatures (<150 °C) near the crystallization onset. Furthermore, Method 2 resulted in a more uniform cellular structure over a wider temperature range (120–170 °C compared to 155–175 °C). Overall, strong extensional hardening and low onset of crystallization were shown to give rise to foamability at high and low temperatures, respectively, suggesting that both characteristics can be appropriately used to tune the foamability of PP in industrial foaming applications.

Foaming Performance of Linear Polypropylene Ionomers

“…To address this issue, Kweon et al conducted a detailed study to elucidate the governing factors in PP foamability from a more fundamental perspective. The authors demonstrated that the strain hardening behavior, quantified by the extensional SHR, is the deciding factor in PP foamability at temperatures above the end melting point . This conclusion is in good agreement with the results presented in this section considering the physical reinforcement effect ion clusters have on the polymer chain network under extension.…”

“…These results suggest that while some degree of strain hardening is necessary for foam expansion, a minimum level of ionic content is necessary to achieve high volume expansion and a wide foaming temperature window that are typically observed in commercial long-chain branched PP. In fact, iPP-ion2 and iPP-LCB showed almost identical expansion behavior over the temperature range 155–175 °C, despite linear PP being generally perceived as more challenging to produce highly expanded foams than long-chain branched PP . This observation further emphasizes the significance of sufficient ionic content in an inherently linear PP to allow it to exhibit a volume expansion behavior similar to the benchmark sample.…”

“…Then this quantity was multiplied by the average volume expansion ratio to obtain the cell density with respect to unfoamed polymer volume. 45 2.6. Computed Tomography (CT) Scanning.…”

“…Figure shows a schematic illustration of the molecular structure and the ion clusters network in the ionomers. In addition, a commercially available long-chain branched grade of HMS-PP homopolymer (“iPP-LCB”) was included in the batch foaming tests to serve as a frame of reference, given its good foamability reported in our previous study (resin A in ref ) at processing conditions similar to the ones used in the work presented here. As an indicator of the molecular structures of these resins, the branching index ( g ′ vis ) was used to measure the degree of branching.…”

“…Cell density measurements were also calculated from the acquired two-dimensional (2D) SEM images by obtaining the cell count per unit area and applying stereological methods for conversion on a per unit volume basis. Then this quantity was multiplied by the average volume expansion ratio to obtain the cell density with respect to unfoamed polymer volume …”