Mixing in Extensional Flow Field

Abstract

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. Results of simulation and model experiments on drop deformabilty in converging and diverging (c-d) flow were used to develop a simple and inexpensive mixer capable of generating a high level of extension that lead to enhanced mixing capability. In consequence, an extensional flow mixer (EFM) was designed. In this device, a liquid mixture is repeatedly exposed to extensional flow fields and semi-quiescent zones. The c-d channels are of progressively increasing intensity. To reduce the pressure drop, and to prevent blockage, slit restrictions are used. The EFM (as well as its newer, dynamic version – DEFM) is fully adjustable, general-purpose mixers. Several its versions have been used for: polymer blending, incorporation of elastomers into resins, and dispersion of high viscosity polymers or ”gel particles“. For most mixing applications a singe-screw extruder equipped with either EFM or DEFM performed at least as well as a co-rotating, inter-meshing twin-screw extruder, with high efficiency screw configuration.