Coalescence in thermoplastic olefin (TPO) blends under shear flow

Abstract

In this article, the capability of the Lee–Park (LP) model (Lee and Park, J Rheol 38:1405–1425, 1994) in predicting the extent of drop coalescence under transient shear has been evaluated. Thermoplastic olefin blends of polypropylene (PP) and three types of metallocene catalyzed ethylene copolymers (EC) with different melt viscosities were investigated. The interfacial tension between the PP and the ECs was determined by means of linear viscoelastic measurements using a simplified version of the Palierne (Rheol Acta 29:204–214, 1990) model as well as the Choi and Schowalter (Phys Fluids 18:420–427, 1975) equation. Flow-induced coalescence was investigated by shearing the samples at a very low shear rate of 0.01 s − 1. The size evolution and orientation of the dispersed droplets under shear were correlated with the transient rheological data. To account for the non-affine deformation, an additional slip parameter (Lacroix et al., J Non-Newton Fluid Mech 86:37–59, 1999) was introduced into the LP model. The modified model (LPL model) was found to predict well the morphological state of all blends in conjunction with the rheological data, whereas in most of the cases, the LP model significantly underestimated the interfacial area (Q). Coalescence was favored by a decrease of the viscosity of the dispersed phase. Smaller viscosity droplets increased the interfacial mobility and, hence, reduced the drainage time promoting the coalescence of two approaching droplets.