J.-M. Piau

Influence of upstream instabilities and wall slip on melt fracture and sharkskin phenomena during silicones extrusion through orifice dies

Abstract The flow of four different linear or branched silicones through thin-walled orifice dies has been visualized. The observation of the extrudate downstream of the orifice has allowed a thorough examination of the appearance and evolution of the various flow regimes. Thus it has been possible to detect very clearly a surface defect appearing in the form of scratches at the free surface, and whose appearance threshold has been thoroughly measured. Depending on the rheology of the fluids considered, these defects may evolve into more severe forms commonly designated by “loss of gloss” or “sharkskin”. Moreover, for all the silicones studied, the well-known melt fracture phenomenon has been observed. This defect first appears in the form of a regular helix, and then evolves into a chaotic regime as flow pressure increases. In other respects, the visualization of the upstream flow shows that this remains perfectly stable as long as the rupture regime has not been reached. Thus, it seems that the surface defect, even in its more severely evolved form, namely the sharkskin defect, is an exit phenomenon related to the relaxation of stretch strains at the orifice outlet. On the other hand, instabilities are generated in the upstream region for the flow regime at which melt fracture appears downstream. Consequently, it seems that this phenomenon should be attributed, at least in part, to the unstable phenomena developing in the elongational flow field upstream of the orifice die. This assumption is supported by the fact that experiments performed with various wall material have shown that wall slip was, in any case, unlikely or minor, so that it could not be responsible for the observed phenomena.

Sharkskin and cracking of polymer melt extrudates

Abstract 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.

Measurement and modelling of friction in polymer melts during macroscopic slip at the wall

Abstract This article examines the flow of two polydimethylsiloxanes, a polybutadiene and a polyethylene, in axisymmetrical capillaries over the entire range of possible flow rates. Measuring and plotting flow curves has shown that macroscopic slip at the wall occurs with these highly entangled polymers as soon as a sufficient level of stress is reached. For each capillary and each flow rate considered, the entry pressure losses at the die inlet was also estimated. A simple method is proposed for determining the polymers friction curve and it is shown that this curve is scarcely dependent on the dimensions of the dies used. The results obtained for each polymer enables stress variations at the wall to be represented as a function of slip velocity and their general shape to be deduced. The variations are distinctly non-linear, as they introduce a threshold, a maximum and two minimum levels of stress. Finally, on the basis of the experimental measurements, a procedure is proposed for modelling polymer slip at the wall during steady flow. Taking into account the existence of a static friction stress, it can be used to represent the general shape of stress variations at the wall as a function of slip velocity. Combined with the compressibility of the fluid, the law used gives reasonable access to variations in pressure as a function of time during extrusion regimes corresponding to cork flow.

Adhesion of linear low density polyethylene for flow regimes with sharkskin

This study examines the surface defects known as sharkskin and the physical mechanisms that cause its appearance. Authors of previous papers who studied different polyethylenes attribute the occurrence of sharkskin, either to the initiation of slip at the fluid–wall interface or to the existence of local tensile stresses at the die exit. To test the slip hypothesis, the present authors studied the flow of a linear low density polyethylene (LLDPE) through capillaries of different geometries. The results obtained show clearly that experimental methods for determining slip velocities do not give conclusive evidence of the existence of slip at the wall for the flow of the LLDPE used, under conditions that would normally give rise to sharkskin. The observations obtained using silicone fluids of different molecular weights and an LLDPE suggest that this phenomenon results from the cracking of the fluid at the die exit, due to the high tensile stresses in that region. Such an explanation is supported by flow bir...

Effect of surface properties on polymer melt slip and extrusion defects

Abstract The purpose of this study is to analyse the flow of a linear polydimethyl-siloxane of high molecular weight on plane surfaces that may interact differently with the silicone fluid used. Stainless steel surfaces and slightly rough silicon surfaces were examined, including silica surfaces where monolayers of fluorine chains were grafted on by silanation reactions. The interest and principle of these reactions are briefly recalled. The quality of the surface treatment was checked by wetting experiments. For each “fluid-wall material” pair, flow rate measurements were performed under a given pressure. In addition, the extrudate was photographed under various flow regimes. Combined analysis of the flow curves and observations at the die outlet helped to determine the effect of roughness and of the wall material for the specific flow conditions considered.

Low Reynolds number flow visualization of linear and branched silicones upstream of orifice dies

Abstract The results obtained from a series of experiments dealing with the flow of four polydimethylsiloxanes (PDMS) having different molecular characteristics through an orifice die are reported. A particular orifice die has been chosen for its convenience in view of numerical simulation. The use of adequate methods of visualization has permitted us to obtain the structure of both stable and unstable recirculating vortices developing upstream of the orifice, and to measure the velocity profiles on the axis by analysing the photonegatives. The whole set of experimental results, including rheometrical data and the measurement of the total pressure loss, permits the evaluation of non-dimensional parameters typical of the flow. The effect of shear-thinning, elongation, and elasticity have been quantified and discussed by using current results available in the literature. Finally, this study provides a fundamental experimental basis, which may be quite useful for the validation of numerical models using appropriate constitutive laws relative to mixed flows of viscoelastic materials with axisymetrical boundaries.

Shear rheometry of polydimethylsiloxanes. Master curves and testing of Gleissle and Yamamoto relations

AbstractThis study concerns the shear rheometry of a series of nine silicone fluids, all linear or branched polydimethylsiloxanes (PDMS), with varying mass distributions.The use of a rotative rheometer enabled characterization of these products in a dynamic regime for about five decades of pulsation ω. In addition, because of the use of rotative rheometers together with a capillary rheometer with controlled piston speed, the behavior of these products could be determined for about six orders of magnitude of shear rate

Easier flow of viscoplastic materials with ultrasonic longitudinal wall motion

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