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

• Published in: Polymers Vol. 14; no. 1; p. 44
• Main Authors: Kweon, Mu Sung, Embabi, Mahmoud, Shivokhin, Maksim E, Gupta, Anvit, Yan, Xuejia, Pehlert, George, Lee, Patrick C
• Format: Journal Article
• Language: English
• Published: Switzerland MDPI AG 23.12.2021; MDPI
• Subjects: batch foaming; Cellular structure; Chain branching; Crystallization; crystallization onset temperature; extensional hardening; Foaming; high-pressure differential scanning calorimetry; Impact strength; linear and long-chain branched polypropylene; Low temperature; Material properties; Melting points; Morphology; Parameter identification; Polymers; Polypropylene; Pressure drop; Process parameters; Resins; Rheology; Strain hardening; strain hardening ratio; Supercooling; Temperature; Viscosity; crystallization onset temperature; linear and long-chain branched polypropylene; batch foaming; strain hardening ratio; high-pressure differential scanning calorimetry; extensional hardening
• ISSN: 2073-4360; 2073-4360
• DOI: 10.3390/polym14010044

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.