Investigation of the Sharkskin melt instability using optical Fourier analysis

Abstract

An optical method allowing the characterization of melt flow instabilities typically occurring during an extrusion process of polymers and polymer compounds is presented. It is based on a camera‐acquired image of the extruded compound with a reference length scale. Application of image processing and transformation of the calibrated image to the frequency domain yields the magnitude spectrum of the instability. The effectiveness of the before mentioned approach is shown on Styrene‐butadiene rubber (SBR) compounds, covering a wide range of silica filler content, extruded through a Göttfert capillary rheometer. The results of the image‐based analysis are compared with the results from the sharkskin option, a series of highly sensitive pressure transducers installed inside the rheometer. A simplified version of the code used to produce the optical analysis results is included as supplementary material. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020, 137, 48806.

Mechano‐Optical Characterization of Extrusion Flow Instabilities in Styrene‐Butadiene Rubbers: Investigating the Influence of Molecular Properties and Die Geometry

“…Observing the extrudates obtained by the slit die at Figures 11a and 12a the proposed definitions [ 8,9,11 ] of sharkskin and stick‐slip met flow instabilities agree with the appearance of the extrudates. However, noticing the extrudates obtained by the round capillary die at Figures 11b and 12b, only the extrudate 7 s −1 for the SBR A can be categorized as surface instability and possibly designated as sharkskin.…”

“…Third, the Fourier transform (FT) analysis of the pressure signal from the piezoelectric transducers, can also be used to give the characteristic time periodicity. [ 8,9,11,23 ]…”

“…Attempting to define these regions, Gansen et al. [ 11 ] and Naue et al. [ 8,9 ] defined the surface and volume distortions referred to slit die (rectangular cross‐section area) geometry.…”

“…In the last 15 years, an online highly pressure‐sensitive in situ mechanical detection slit die has been developed to characterize the flow instabilities during extrusion. [ 10,11,13,14,22–28 ] This system was designed for coupling to both capillary rheometers and lab size extruders. [ 8,9 ] This slit die is equipped with three highly sensitive piezoelectric pressure transducers (Tr) along the die.…”

Role of molecular architecture and temperature on extrusion melt flow instabilities of two industrial LLDPE and LDPE polyethylenes investigated by capillary rheology, high‐pressure sensitivity slit die and optical analysis

“…Increasing extrusion throughput, usually, the smooth extrudate appearance has a transition to sharkskin first at low shear rates, then to stick–slip at medium shear rates, and last to gross melt fracture (GMF) at high shear rates. Definitions of the extrusion instabilities based on slit die geometry are discussed in the literature 8–11 . These definitions are based on visual observations and on the characteristic frequency (or time periodicity) of the time dependent pressure signal obtained by a custom high pressure sensitive slit die (HPSSD) 8,9,11–25 …”