Sharkskin and cracking of polymer melt extrudates

• Published in: Journal of non-Newtonian fluid mechanics 1997-02, Vol.68 (2), p.271-290
• Main Authors: El Kissi, N, Piau, J.-M, Toussaint, F
• Format: Article
• Language: English
• Subjects: Cracks; Extrusion; Polymer; Scratches; Sharkskin
• ISSN: 0377-0257; 1873-2631
• DOI: 10.1016/S0377-0257(96)01507-8

During the extrusion of polymers the extrudate leaving the die is smooth and transparent when the flow rate is low enough, and may be very swollen. As the flow regime progressively increases, and irrespective of the polymer used in this study, scratches, i.e. small amplitude local cracks, appear on the surface of the extrudate, situated in longitudinal bands that become increasingly wider and more numerous, gradually invading the entire surface of the extrudate. The extruded rod thus loses its transparency and becomes increasingly matt and opaque. With slightly entangled polymers, this appearance remains as long as the flow regime is stable. With moderately to highly entangled polymers, the scratches may evolve. Indeed stresses with these polymers may reach sufficiently high levels to produce cracks around the surface of the fluid as it leaves the die. These cracks penetrate deeply into the extruded rod just where it leaves the die. They close downstream of the outflow section owing to the relaxation of the polymer and the extrudate then has the characteristic appearance of sharkskin. By using highly entangled fluids, i.e. those with very long characteristic times, it was possible to observe the formation of these cracks in detail. A brirefringence experiment was performed in order to determine the stress field in the outflow section. In the case of flow with sharkskin, birefringence patterns show that the number of fringes varies in time for a given regime. This pulsation in the number of fringes is identical to the period of crack formation. Lastly, it must be underlined that it is possible to significantly delay or eliminate the appearance of sharkskin, by considering the polymer flow through fluorinated dies. Characterized by their particularly low surface energy, such dies cause polymer to slip at the wall even in low flow regimes. Thus, the fluid can be extruded under lower exit stresses without wetting the die exit and consequently it does not crack.