THE EXTENSIONAL FLOW MIXER, EFM

There are several reports indicating that deformation and breakup of d ispersed drops is easier in extensional flow field than that in shear. This is particularly true for the systems containing the dispersed ph ase significantly more viscous than the matrix, e.g., blends in which the viscosity ratio lambda = eta(disp)/eta(matrix) greater than or equ al to 3.8. These reports led to development of an extensional flow mix er, EFM, a device in which multicomponent, multiphase system [e.g., po lymer alloys, blends, master-batches, filled systems] can be hydrodyna mically mixed by flowing through a series of convergent/divergent regi ons of increasing intensity. To be effective, EFM must be attached to a machine capable of melting and pressurizing the compound, preferably a single-screw extruder, SSE. In this paper, efficiency of two compou nding systems is compared, the first is made of a SSE and an EFM, whil e the second is a twin-screw extruder, TSE. To evaluate the efficiency three types of blends, all characterized by high viscosity ratio, lam bda greater than or equal to 3.8, were used: (i) high density polyethy lene dispersed in polystyrene, HDPE/PS, (ii) polypropylene impact-modi fied by addition of an ethylene-propylene elastomerm, EPR/PP, and (iii ) ultrahigh molecular weight polyethylene added to high density polyet hylene, UHMWPE/HDPE. System (i) was used to study the effect of compou nding on blend's morphology - the dispersion from the SSE + EFM compou nding unit was significantly finer than that from TSE. System (ii) was selected to examine usefulness of EFM for impact modification. The re sults demonstrated that SSE + EFM provided milder compounding conditio ns that less shear-degraded PP than TSE. The impact strength of specim ens prepared in SSE + EFM was superior to that of blends compounded in TSE. The system (iii) was studied to examine the relative merit of SS E + EFM over TSE to produce a finer dispersion of the UHMWPE domains t hat in turn would result in better dissolution of this ultrahigh molec ular weight fraction. Again in this case the SSE + EFM compounder outp erformed TSE - the dissolution of UHMWPE was significantly better with out parallel degradation of the resin.