Effect of Ethylene Comonomer Content on the Foam Processing Window of Long-Chain Branched Polypropylene

Abstract

The non-optimized foam processing window (FPW) can create processing challenges and limit the possible materials with which a polymer of interest can be processed in emerging foaming technologies. To address this drawback, we aim to investigate the effect of ethylene comonomer content on the foaming behavior of polypropylene (PP), particularly the FPW. Two PP resins containing 0.75 and 1.5 wt % of ethylene comonomers were studied, while using a PP homopolymer as a control. High-pressure differential scanning calorimetry confirmed the decrease in the onset crystallization (T c-onset ) and end melting (T m-end ) temperatures with increasing ethylene comonomer content as a result of the defects the ethylene units induce in the crystalline structure of PP. Rheological measurements suggest that the addition of ethylene comonomer content resulted in negligible changes in the extensional and shear rheological responses of the PP-based resins. In addition, the foam expansion results and the observed morphologies reveal no significant change in the resins' foamability, characterized by the shape of their expansion profiles with respect to the foaming temperature. Interestingly, the FPW was found to be the sole affected parameter, as 1.5 wt % of ethylene comonomers resulted in a 10 °C shift in the FPW toward lower temperatures, compared to the PP homopolymer. The resins with 0 and 1.5 wt % of ethylene comonomer content showed similar mechanical responses under compression tests for their optimally expanded samples. These findings reveal a potentially effective approach for tuning the FPW of processed PP resins without compromising their foaming and mechanical performances. Consequently, foam processing techniques are further enhanced, resulting in a wider span of potential applications.

In Situ Visualization of Extensional Flow-Induced Crystallization in Polypropylene under High-Pressure CO2

“…These values were verified using high-pressure DSC, and the results are shown in Figure S4. This negative linear dependence between T c‑onset and CO 2 pressure is expected and has been reported in previous literature with similar pressure ranges. ,, This depression in T c‑onset can be attributed to the plasticizing effect of the CO 2 in the PP matrix. With CO 2 injected at increasing pressure, the gas molecules infiltrate the PP matrix, causing a significant increase in the free volume of the melt and the mobility of the PP chains, in addition to a direct decrease in T m o of the resins, as illustrated by Takada et al Therefore, as the polymer reaches a certain temperature ( T ) below T m o during the constant cooling phase, a lower T m o would directly result in a reduced extent of supercooling (i.e., T m o – T ).…”

“…Supporting batch foaming results have been reported in our previous work using fast-scanning calorimetry. 34 Similar to strain hardening, extensional flow-induced crystallization was shown in this section to be more pronounced in long-chain-branched polymers over linear counterparts. It is, therefore, believed that extensional flowinduced crystallization synergistically aids, along with strain hardening, the high-temperature expansion tendencies often exclusively encountered using long-chain-branched polymers.…”

“…This negative linear dependence between T c-onset and CO 2 pressure is expected and has been reported in previous literature with similar pressure ranges. 16,17,34 This depression in T c-onset can be attributed to the plasticizing effect of the CO 2 in the PP matrix.…”

“…32 However, since foaming is an inherently nonisothermal process, extensional rheology measurements using conventional rheometers (typically conducted isothermally at elevated temperatures in the absence of gas) are unlikely to solely represent the dynamic extensional behavior experienced by a polymer melt during cell growth. In fact, in our previous foaming studies, we have shown, using wide-angle X-ray scattering 33 and fast-scanning calorimetry, 34 that extensional flow-induced crystallization during the rapid cell growth phase significantly affects the foaming behavior but is seldom studied in the foaming community.…”

Foaming behaviors and mechanical properties investigation of high‐strength polyethylene terephthalate/polycarbonate bead foam

“…12 The excellent properties of polymer foam arouse people's interest in foam research. 13,14 Due to the high crystallinity and melting point of PET material, the resulting PET foam had high thermal stability and excellent mechanical strength after foaming. 15 Because of their ultra-high rigidity and thermal stability, PET foams have been widely used as structural core materials, 15 such as wind turbine blade inner cores, building materials, 16 and composite sandwich panels.…”