Mixing in Extensional Flow Field

Abstract

Mixing is the most important operation in polymer processing. Uniformity of the molecular weight, degree of entanglement, temperature, and composition, is the prerequisite for good performance. Mixing of molten polymers is always within the laminar (as opposed to turbulent) flow regime. This paper presents an overview of the effect of flow type on mixing. The distributive mixing is best modeled considering either laminar or chaotic mixing. Here the system is ”passive“, i.e.the rheological properties are identical and the interface is ”invisible“. Furthermore, the effects of flow and morphology are ”decoupled“. By contrast, the dispersive mixing is best modeled by means of microrheology, complemented by coalescence. Both models – the laminar mixing and the microrheology – show the unambiguous superiority of the extensional flows for dispersive and distributive mixing, in particular, when the viscosity ratio of two principal components is large. However this superiority is balanced by practical difficulties in generating a flow that may ascertain a high level of extension.