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

The extrusion flow instabilities of two commercial styrene-butadiene rubbers are investigated as they vary in isomer content (1,4-cis, 1,4-trans, and 1,2 conformation) of the butadiene monomer and the molecular architecture (linear, branched). The investigated samples have similar multimodal molecular weight distribution. Two geometries of extrusion dies, slit and round capillary, are compared in terms of the type and the spatial characteristics of the flow instabilities. The latter are quantified using three methods: a highly pressure sensitive slit die, online and offline optical analysis. The highly pressure-sensitive slit die has three piezoelectric pressure transducers (Delta t approximate to 10(-3) s and Delta p approximate to 10(-5) bar) placed along the die length. The characteristic frequency (f(Char.)) of the flow instabilities follows a power law behavior as a function of shear rate to a 0.5 power for both materials, f(Char.) proportional to (gamma) over dot(app)(0.5). A qualitative model is used to predict the spatial characteristic wavelength (lambda) of the flow instabilities from round capillary to slit dies and vice versa. Slip velocities (V-s) are used to quantify the slippage at slit and round capillary dies as well.