DISPERSIVE MIXING IN IMMISCIBLE POLYMER BLENDS

Dispersive mixing of immiscible polymer blends as well as polymer syst ems containing solids is achieved in compounding equipment at two stag es of the system's processing experience: first, while one or more of the polymer components are melting, and second, after all polymer comp onents have melted. That is, the first mode of dispersive mixing occur s during the melting mechanism of ''dissipative mix melting'' (Ref. 1) , while the second is melt-melt mixing. During the compounding of a gi ven blend system, there are a number of processing parameters that can be changed in order to improve mixing. These range from machine opera ting variables to the addition of processing aids. If such processing changes fail to produce the desired morphology, the most common change to consider is the screw geometry. This, in practice involves a trial and error procedure, or the use of an existing database built from pr ior experience. The role which the thermomechanical and rheological pr operties of the blend component play in dissipative mix melting and me lt-melt mixing has not yet been well understood. The reason for this i s that although most blend systems have components which are strongly non-Newtonian and strongly viscoelastic, the thinking and rules of thu mb for mixing such materials has been heavily influenced by the analys is of G. I. Taylor (Ref. 2), who in 1932 addressed the phenomenon of t he dispersion of a single Newtonian droplet by a Newtonian matrix flow ing in laminar shear flow. This paper addresses the strong role that t he rheology of blend components, under processing conditions, play in laminar dispersive mixing of polymer blends. From a practical point of view, if the dispersion mechanisms and rates of dispersion depend on the component rheology, then such knowledge can lead us to the selecti on of advantageous mixing element designs and processing conditions. T he experimental results were obtained in dispersive mixing carried out in devices developed in the Polymer Mixing Study (Ref. 3). Such model devices include the Couette Flow Intensive Mixer (CIM) (Ref. 4), wher e a constant shear stress is applied on the blend components and the T win Screw Mixing Element Evaluator (TSMEE) (Ref. 5), where the mixing flows are those encountered in actual mixing/compounding operations. T he TSMEE will be described in the body of this paper together with its on- and off-line morphology determination capabilities and its in-lin e rheology sensor. The low-density polyethylene (LDPE) and polystyrene (PS) polymers studied were selected because they cover a wide spectru m of rheological properties.