SHARKSKIN AND CRACKING OF POLYMER MELT EXTRUDATES

During the extrusion of polymers the extrudate leaving the die is smoo th and transparent when the flow rate is low enough, and may be very s wollen. As the flow regime progressively increases, and irrespective o f the polymer used in this study, scratches, i.e. small amplitude loca l cracks, appear on the surface of the extrudate, situated in longitud inal bands that become increasingly wider and more numerous, gradually invading the entire surface of the extrudate. The extruded rod thus l oses its transparency and becomes increasingly matt and opaque. With s lightly entangled polymers, this appearance remains as long as the flo w regime is stable. With moderately to highly entangled polymers, the scratches may evolve. Indeed stresses with these polymers may reach su fficiently high levels to produce cracks around the surface of the flu id as it leaves the die. These cracks penetrate deeply into the extrud ed rod just where it leaves the die. They close downstream of the outf low section owing to the relaxation of the polymer and the extrudate t hen has the characteristic appearance of sharkskin. By using highly en tangled fluids, i.e. those with very long characteristic times, it was possible to observe the formation of these cracks in detail. A birefr ingence experiment was performed in order to determine the stress fiel d in the outflow section. In the case of flow with sharkskin, birefrin gence patterns show that the number of fringes varies in time for a gi ven regime. This pulsation in the number of fringes is identical to th e period of crack formation. Lastly, it must be underlined that it is possible to significantly delay or eliminate the appearance of sharksk in, by considering the polymer flow through fluorinated dies. Characte rized by their particularly low surface energy, such dies cause polyme r 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 c onsequently it does not crack. (C) 1997 Elsevier Science B.V.