Polymer Testing 84 (2020) 106390 Available online 1 February 2020 0142-9418/© 2020 Elsevier Ltd. All rights reserved. Test Method Capillary extrusion of polypropylene/high-density polyethylene immiscible blends as studied by rheo-particle image velocimetry Benjamín M. Marín-Santib� a~ nez a , Jos� e P� erez-Gonz� alez b, * , Guillermo G� omez-Herrera b , Francisco Rodríguez-Gonz� alez c a Escuela Superior de Ingeniería Química e Industrias Extractivas, Instituto Polit� ecnico Nacional, U. P. Adolfo L� opez Mateos, C. P. 07738, Ciudad de M� exico, Mexico b Laboratorio de Reología y Física de la Materia Blanda, Escuela Superior de Física y Matem� aticas, Instituto Polit� ecnico Nacional, U. P. Adolfo L� opez Mateos, C. P. 07738, Ciudad de M� exico, Mexico c Departamento de Biotecnología, Centro de Desarrollo de Productos Bi� oticos, Instituto Polit� ecnico Nacional, Col. San Isidro, C.P. 62731, Yautepec, Morelos, Mexico A R T I C L E INFO Keywords: Polymer blends Capillary rheometry Rheo-particle image velocimetry High-density polyethylene Polypropylene ABSTRACT The capillary extrusion of polypropylene (PP) and high-density polyethylene (HDPE) immiscible blends was studied in this work by rheo-particle image velocimetry (Rheo-PIV). The PP/HDPE blends were prepared by single screw extrusion and extruded through a transparent capillary die at a temperature of 200 � C and con- centrations of 80/20, 60/40, 40/60 and 20/80 wt%, respectively. PIV measurements described accurately the fow behavior of PP/HDPE blends and revealed continuous velocity profles in the die, without macroscopic phase separation, for all the blends in the resolution range of the PIV technique. The fow behavior of all the blends was shear-thinning (power-law) type and their viscosities laid in between the values corresponding to the neat polymers and increased in an exponential way along with the concentration of the highest viscosity component in the blend (HDPE). Also, it was found that the extruded blends acquired a stratifed morphology and that HDPE mitigates extrudate distortions in PP, meanwhile PP eliminates slip and fow instabilities in HDPE by migrating to the region of highest shear stresses in the die. Migration of PP to the capillary wall was corroborated by Raman spectroscopy measurements on the periphery of solid extrudates. Finally, via calculations of the density of the molten blends under fow using the velocity profles in the die, we show that the homo- polymers are compatible in the molten state and follow a simple inverse relation for their density, and an exponential one for their viscosity. 1. Introduction A trend in the polymer industry has been the development of new materials based on blends of two or more homopolymers, which allows the production of materials with properties different from their pre- cursors. Polymer blends may be overall classifed according to the miscibility of the involved homopolymers as miscible, immiscible or partially miscible. If the involved homopolymers are miscible a single- phase blend is obtained, otherwise a two or more phases blend is ex- pected. In practice, however, most of polymers are thermodynamically immiscible, then the properties of immiscible polymer blends may be optimized by controlling the morphology of the blend, which is in its time related to the rheological behavior of the neat polymers [1]. In general, rheological properties of molten polymer blends depend on the composition, physical-chemical properties of the components as well as on the morphology and interactions between phases [2]. Also, the dis- tribution of phases and the properties of the blend in the solid state depend on the processing conditions [3]. One of the most used processes to prepare polymer blends is continuous (single or twin) screw extrusion, which allows melting of the polymers and creation of intimal contact between the different phases. Since the processing of the sample is carried out in the molten state, knowledge of the rheological properties of the homopolymers and the blend is relevant to evaluate the quality of the product. Most of published work about the rheological behavior and pro- cessing of polymer blends consists of rheometrical studies, visualization and numerical simulations (see Ref. [3] for a review) with scarce reference to the fow kinematics in the extrusion die, wherein the fnal product is shaped. The knowledge of the fow kinematics of polymer melts is relevant for basic rheology as well as for polymer processing, * Corresponding author. E-mail address: jpg@esfm.ipn.mx (J. P� erez-Gonz� alez). Contents lists available at ScienceDirect Polymer Testing journal homepage: http://www.elsevier.com/locate/polytest https://doi.org/10.1016/j.polymertesting.2020.106390 Received 19 December 2019; Received in revised form 28 January 2020; Accepted 31 January 2020