Investigation of melt flow instabilities in SBR: influence of MWD and microstructure at in situ pressure fluctuations as detected by capillary rheology

During the extrusion process of molten polymers, flow instabilities occur which manifest themselves as distortions on the surface and volume of the extrudates1. These phenomena include sharkskin or surface melt fracture, slip-stick or periodic melt fracture and gross melt fracture or large amplitude periodic and/or non-periodic chaotic anomalies2. In this study the pattern formation of the melt flow instabilities of pure SBR samples which are produced by solution (anionic) or emulsion polymerization are investigated. Those SBR have different bimodal and broad molecular weight distributions (MWD) and microstructure. This study use a high sensitive in situ mechanical pressure instability detection system3,4 in polymer processing equipment in combination with offline and online optical analysis. Thus, the characteristic frequency pattern of instabilities as a function of the apparent shear rate is correlated with the influence of the MWD and the microstructure of the material. INTRODUCTION The characteristic frequency of the melt fracture and the wall slip behavior of solutions of polymerized bimodal SBR resins, and emulsion polymerized SBR with broad MWD were investigated and compared in this study. Linear oscillatory rheological measurements have been done to obtain the linear rheological parameters as reptation and entanglement time, plateau modulus, zero shear viscosity and the molecular entanglement weight. By comparing the flow curve observed by capillary rheology measurements with the linear viscoelastic (LVE) data, it was observed that the apparent slip increased with content of low molecular weight (MW) component.5 It was empirically observed that the onset stress of sharkskin melt fracture was proportional to the plateau modulus. For polyethylene, it was proposed that the distinct separation of the two modes of the bimodal resin, especially the high content of small chains could cause the significant wall slip, and unusual melt fracture behaviors.5 Based on literature review for the bimodal metallocene PE6, first reported that the addition of low MW component drastically increased the melt flow index. Munoz-Escalona et al.7 found that the bimodal metallocene PE in the capillary flow showed more shear thinning behavior than the unimodal PEs of similar MW. Hence, lower viscosities in extrusion flow could be obtained with the bimodal resins. In this report upon to those previous studies, we discuss similar outcome for SBR materials and we correlate them with the characteristic frequency pattern of instabilities. EXPERIMENTAL The experimental setup is based on a Göttfert Rheotester 2000 capillary rheometer with a slit die 0.2 x 3 x 30 mm3. The die is equipped with three high sensitivity piezoelectric pressure transducers, see Fig. 1, and has an aspect ratio of W/H = 15, where W and H are Investigation of melt flow instabilities in SBR: influence of MWD and microstructure at in situ pressure fluctuations as detected by capillary rheology Christos K. Georgantopoulos1, Ingo F. C. Naue1, Andrea Causa2, Luciano Garro2, and Manfred Wilhelm1 1Karlsruhe Institute of Technology (KIT), Institute for Chemical Technology and Polymer Chemistry, D-76131 Karlsruhe, Germany 2Pirelli Tyre SpA, Viale Piero e Alberto Pirelli 25, 20126 Milan, Italy ANNUAL TRANSACTIONS OF THE NORDIC RHEOLOGY SOCIETY, VOL. 27, 2019