Elastic flow instabilities and shish-kebab formation during gel-spinning of ultra-high molecular weight polyethylene

Abstract

The flow behaviour during gel-spinning of semi-dilute solutions of ultra-high molecular weight polyethylene in paraffin oil was investigated in relation to fibre strength and morphology. Shish-kebab morphologies observed in the as-spun filaments originate from a solidification of long flow units already developed in the polymer solution during spinning. These flow units consist of alternating bundles of elongated molecules and highly entangled clusters of unoriented molecules. Extrudate stretching leads to a demolition of these long flow units by elastic flow instabilities. As a result, the tensile properties after hot-drawing of the fibres are strongly reduced. The inclusion of 1 wt% aluminium stearate in the spinning solution preserves the long bundle-like flow units even at high take-up speeds during extrusion. This additive suppresses the adsorption of the polyethylene on the die-wall and avoids the generation of elastic turbulences. Filaments with a strength between 2.5 and 3.0 G Pa can be obtained from these doped solutions after hot-drawing, in spite of high winding speeds up to 300 m min−1 in the spinning process.