Surface instability detection in highly-filled biocomposites from inline imaging during extrusion

The processing of wood fiber biocomposites, and in particular, the extrusion, is accompanied by multiple challenges, among which agglomeration, entanglement, slip, or surface instabilities being the most common ones. In the current work, we focus on the dynamics of surface instabilities during the single screw extrusion of highly filled wood fiber biocomposites. The biocomposites are polypropylene based with up to 40 wt% wood fiber content of custom compositions based on the commercial-grade by Stora Enso. To detect and quantify the dynamics of surface instabilities, inline image analysis was applied using an optical visualization system positioned at the die exit. Therefrom, space-time diagrams were constructed, and after that, via 2D-Fourier transform analysis, the spatio-tempoal spectral dynamics of the surface instabilities were determined as a function of the die (apparent) shear rate. The spectral dynamics show that melt instabilities of 40 wt% for example, detected via their characteristic (temporal) frequency and (spatial) wavenumber, dissipate with increasing the shear rate, such that at shear rates above ca. 90 1/s, no characteristic frequency and wavenumber can be distinguished i.e. instabilities can no longer be observed on the surface of the extrudates.