Failure mechanism of zinc adipate as a β-nucleating agent for polypropylene in the presence of calcium stearate

Abstract

Although zinc dicarboxylic acids are highly effective β-nucleating agents (β-NAs), they lose their β-nucleating ability in isotactic polypropylene (iPP) upon the addition of calcium stearate (CaStr2). In this study, we investigated the failure mechanism of these β-NAs in terms of the chemical and physical interactions of zinc adipate (ZnAA) with CaStr2. No chemical reaction occurs between CaStr2 and ZnAA; rather, it is the physical interactions between this β-NA and CaStr2 that are responsible for the loss of β-nucleating ability. Based on the experimental findings and molecular dynamics (MD) simulations, we suggest a blocking effect of CaStr2 on nucleation as a possible failure mechanism. MD simulations confirmed that the aggregation of CaStr2 on the β-NA surface, observed experimentally using polarized optical microscopy and scanning electron microscopy, resulted from the binding energy for the interaction between ZnAA and CaStr2 being higher than that for iPP/CaStr2. Because the active nucleation sites on the surface of the β-NA were covered, lattice matching between ZnAA and the iPP chains was hindered. Therefore, the addition of CaStr2 weakens the β-nucleating ability of ZnAA, leading to inhibition of the epitaxial crystallization process, thereby decreasing the degree of β-crystal formation and, consequently, lowering the toughness of the resulting iPP.