Feature Article: April 1, 2000

Understanding Mechanisms of Mixing:
The Batch Intensive Mixer as a Process Simulator

Chris E. Scott
Department of Materials Science and Engineering
Massachusetts Institute of Technology

Melt compounding of components is the most common method for the preparation of polymer/polymer blends. The performance of such materials is critically dependent on the morphology or microstructure which is produced during the compounding process. Unfortunately, in most cases little is known about the details of the mechanisms which are responsible for the transformation of the morphology from the pellet size particles which are fed to the compounding equipment to the target morphology, which is often that of submicron domains of one polymer in a matrix of another polymer. Better understanding of these mechanisms is necessary in order to optimize existing compounding operations as well as to design new processing operations and equipment.

Research into mechanisms of mixing is challenging due to the complex flow fields, high temperature gradients, multiphase flow, and non-Newtonian rheologies encountered during polymer processing. In addition, commercial practice on large extruders operating at high speeds makes the material being processed relatively inaccessible to the investigator. Fortunately, laboratory batch intensive mixers can often be used to simulate more complex processing operations. These mixers can be quickly stopped to extract and quench samples. The mixing torque provides an indication of the changes in the sample rheology. In addition, the use of a transparent window on the mixer can provide information on the sample morphology, albeit on a limited size scale.

One mechanism of mixing for which our understanding has developed rapidly over the last few years with the aid of model experiments in batch intensive mixers is the mechanism of phase inversion during polymer/polymer blending. The key aspect of this mechanism is the morphological transformation which is illustrated schematically in Figure 1. A phase inversion is said to occur when a morphology consisting of polymer A (green) dispersed in polymer B (red) is transformed to a morphology consisting of polymer B dispersed in polymer A.

Phase inversion in low molecular weight systems (such as oil/water mixtures) has been known for many years. However, Dr. Shih at DuPont discovered only recently that this transformation could occur in the melting regime during the blending of two immiscible polymers. In one particular experiment, Dr. Shih was surprised to observe that compounding of a blend of 80% polyarylate with 20% rubber on a corotating twin screw extruder produced a coarse powdery mixture which consisted of "a large amount of unfused polyarylate particles dispersed in the rubber matrix" [Shih, 1991]. This contrasted dramatically with the desired morphology, which was a toughened engineering thermoplastic consisting of rubber particles dispersed in a polyarylate matrix! In order to simulate the extrusion process under controlled conditions, Shih used a batch intensive mixer with a programmed temperature rise to replicate the thermal history the material experienced in the twin screw extruder. This led him to discover that in this blend the rubber initially becomes the continuous phase, despite the fact that it comprises only 20% by weight of the blend, and that a subsequent phase inversion is necessary in order to produce the morphology desired for a toughened thermoplastic. Shih also used a transparent window on the batch mixer in order to observe changes in the flow characteristics of the mixture and correlate these with the mixing torque [Shih, 1992, 1995].

Research in the Polymer Processing Laboratory at MIT has focused on exploring the fundamentals of the phase inversion mechanism [Scott and Joung, 1996; Ratnagiri and Scott, 1996]. Using the batch intensive mixer as a process simulator, we have investigated the effects of various material and process parameters on i) whether or not phase inversion occurs and ii) if it does occur, the point at which it occurs.

In order to determine whether or not phase inversion occurs, the blend morphology must be sampled and characterized at various points during the blending process to determine whether or not the transformation illustrated in Figure 1 actually occurs. The capability to stop a batch mixer at any time in the process; quickly remove the sample; and quench it into liquid nitrogen to freeze the morphology has been critical to this effort.

In some systems, the mixing torque on the batch intensive mixer may be used to identify the phase inversion point. One system where this approach has been successful is illustrated in Figure 2. The blue circles represent the torque trace for a typical injection molding grade polystyrene in the batch intensive mixer. As the cold pellets are added to the mixer, the mixing torque rises dramatically. It then falls to a steady-state value as the polystyrene is softened and heated. The red curve gives the torque trace for the same polystyrene with 7.8% wt. of a low viscosity polyethylene wax. The processing behavior is quite different in this case. The small peak at 30 seconds is associated with the melting of the polyethylene wax. From 1 to 6 minutes of mixing time, the blend morphology consists of polystyrene dispersed in a matrix of polyethylene. The torque transition at 7 minutes is associated with the phase inversion which takes place to transform the morphology into that of polyethylene dispersed in polystyrene. We have successfully modeled the energy fluxes in the melting regime of this process and correlated the viscosity rise due to the phase inversion with theoretical predictions of blend rheology [Scott and Joung, 1996, 1999].

In conclusion, the laboratory batch intensive mixer is a useful process simulator for polymer mixing operations. Recent advances in the understanding of the phase inversion mechanism during polymer/polymer blending are just one example of where it has been successfully applied.

Ratnagiri, R.; Scott, C.E., SPE ANTEC, 42, p. 59 (1996).
Ratnagiri, R.; Scott, C.E., Polym. Engr. Sci., 39, 1823 (1999).
Scott, C.E., Joung, S.K., Polym. Engr. Sci., 36, 1666 (1996).
Shih, C.-K., SPE ANTEC, 37, p. 99 and p. 1715 (1991).
Shih, C.-K., Adv. Polym. Tech., 11, 223 (1992).
Shih, C.-K., Polym. Engr. Sci., 35, 1688 (1995).

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