A New High Sensitivity System to Detect Instabilities During the Extrusion of Polymer Melts

Abstract

This publication is concerned with the development of a novel high sensitivity instability detection die for a laboratory size extruder. The high sensitivity system consists of piezoelectric transducers placed along the die length to monitor local pressure fluctuations, having temporal and pressure resolution of up to Dt % 10 À3 s and Dp % 10 À5 bar, at a nominal pressure of 500 bar. The system is tested on short chain branched polyethylenes, namely on the detection and characterization of the sharkskin instability. It is shown that the determined instabilities are related to characteristic peaks in the corresponding in situ pressure Fourier-transform spectra and that other peaks are caused by different, but defined, sources. The in situ results obtained are validated through optical image analysis of the instabilities.

Investigation of the Sharkskin melt instability using optical Fourier analysis

“…Hence, only the signal at the frequency with the highest magnitude (in the frequency domain) needs to be considered. For the Fourier transform employed on the time dependent pressure data please refer to the following publications, where the method is described in detail …”

“…The compounds are extruded at a temperature of 100 ° C at shear rates ranging between as the sharkskin instability appears in this range. The measurements are carried out using the capillary rheometers Rheograph 25 and 50 from Göttfert with the sharkskin option …”

“…The optical method proposed in this article is not designed to replace the sharkskin procedure . That method remains the best approach for understanding instabilities in highly controlled laboratory experiments where the sharkskin option is available.…”

“…The method suggested in this article uses a Fourier analysis of imaging data instead to characterize melt flow instability and could be used in the context of a large‐scale manufacturing process independently of the shape of the dies. For polyethylene samples, Naue used a similar technique . Focusing on different image enhancement techniques, the image analysis can be significantly improved, leading to characteristic frequencies in the same order of magnitude with characteristic frequencies of the piezoelectric pressure transducer measurements for SBR compounds with varying silica content.…”

“…9,14 The optical method proposed in this article is not designed to replace the sharkskin procedure. 5,[15][16][17] That method remains the best approach for understanding instabilities in highly controlled laboratory experiments where the sharkskin option is available. In fact, the sharkskin option is used as a benchmark against which the quality of the optical method is assessed.…”