Influence of molecular weight on high- and low-expansion foam injection molding using linear polypropylene

Abstract

Long-chain branched (LCB) polypropylene (PP) exhibits strain hardening and increased melt strength not seen in typical linear PP. These benefits have seen LCB PP used broadly in foaming applications to produce high cell density foams due to decreased cell coalescence. However, these benefits may come at a significantly increased material cost. This work evaluates linear PP resins of varying molecular weight and polydispersity to determine their influence on foam expansion and cell nucleation behavior in a foam injection molding application. Resins with higher molecular weight and polydispersity reached higher expansion ratios owing to increased crystallization temperatures and improved extensional rheology. Low molecular weight was correlated to improved cell nucleation capability and higher cell density foams. This trend was attributed to decreased interfacial tension which decreases the energy barrier for cell nucleation. The results can inform future application-based resin design and selection by providing insights into the properties governing expansion and cell nucleation.