The mystery of the mechanism of sharkskin: Case closed?
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Abstract
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| We utilize in-situ optical measurements in the exit region of a transparent sapphire capillary tube to determine the mechanism of sharkskin during extrusion. Upstream of the die exit, the polymer, a linear low density polyethylene (LLDPE), sticks at the wall-acceleration occurs in the last 25 microns from the exit. Downstream of the exit, high-speed video-microscopy reveals the existence of cohesive failure; the material splits into a surface layer and a core region. The material from the surface layer cyclically accumulates near the exit wall and then peels off via adhesive detachment. We also utilize polymer process additives which modify the surface and allows the polyethylene to slip, delaying the onset of sharkskin to higher flow rates. Comparing the modified and unmodified surfaces, we quantitatively assess the numerous suggested causes for sharkskin. We propose that the extensional deformation rate is the controlling parameter for the onset of sharkskin. |
