N. El Kissi

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.

The different capillary flow regimes of entangled polydimethylsiloxane polymers: macroscopic slip at the wall, hysteresis and cork flow

Abstract Polymers, melted at room temperature, have been extruded through capillary dies using either a controlled pressure system or an automatic capillary rheometer in which the rate of flow is controlled. When the polymer is highly entangled, macroscopic slip is observed at the wall. However, flow curves differ: for controlled pressure conditions, slip appears simultaneously with a sudden increase in the rate of flow, an dflow curves exhibit a hysteretic regime. For controlled flow conditions, slip is accompanied by oscillations of the rate of flow and pressure head around a mean value, as a result of polymer compressibility. The succession and evolution of the different flow defects are clearly identified. As the flow regime increases, scratches appear first. Then, beyond a critical value of wall shear stress in the exit region, cracks are formed just at the exit of the capillary die. These cracks are accompanied by the formation of “rings”, which are more easily observed as the molecular weight increases. At high flow rates, when macroscopic slip appears at the wall, the aspect of the extrudate depends on the system used. For controlled pressure conditions, the polymer is ejected in the form of an opaque, irregular jet, where swelling is quasi-non-existent. For controlled flow conditions, cork flow is observed. At higher flow regimes the extrudate becomes chaotic. The existence of a master curve was also shown for variations in average velocity vs. wall stress, during flow with macroscopic slip at the wall. It is important to note that this curve represents the sliding friction properties of the polymer under consideration.

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.

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.

Velocity field for polymer melts extrusion using particle image velocimetry: Stable and unstable flow regimes

Abstract Extrusion defects and flow instabilities are an important limitation in most polymer processing operations. Observation of the fluid kinematics to deduce the dynamical response of the polymer can be very useful to characterise these instabilities and understand the mechanisms involved in their triggering and enhancement. To do so, the flow of two linear polydimethylsiloxanes (PDMS) through an axisymmetric sudden contraction has been studied using pressure drop measurements as well as particles image velocimetry (PIV). This optical method for measuring flow velocities provides quantitative whole-field information even when flow conditions are non-stationary. It was shown that, for the two polymer melts studied, the flow is symmetric under stable conditions whereas non-symmetric and non-stationary flows are obtained under unstable conditions. These instabilities arise in the upstream part of the contraction. The extruded rod is then excited by these instabilities which generates the phenomenon of melt fracture at the die exit. In addition, the flow is found to be purely elongationnal on the axis during stable and unstable flow regimes. Under the experimental conditions investigated here, the resulting stretch rate on the centreline varies as x−3, x being the distance above the contraction plane. Moreover flow through contractions becomes unstable first around the centreline and close to the orifice plane. The instabilities then develop in the radial direction and invade progressively the entire upstream flow which completely looses its properties of symmetry: the flow is expected to have a spiralling motion and its two-dimensional (2D) projection looks like a knitting system. The frequency of the instabilities remains approximately the same while their amplitude increases with flow regimes. On the flow axis, instantaneous fluctuations of the velocity can reach ±30% of a mean value characteristic of a mean flow. These fluctuations are more important on each side of the axis where they can reach ±105% of a mean value.

The reduction of viscous extrusion stresses and extrudate swell computation using slippery exit surfaces

Wall slip has been pointed out as a way to reduce cracks and swelling on polymer extrudates. To investigate this phenomenon, the capillary extrusion of a purely viscous generalized Newtonian fluid is computed with the finite element code POLYFLOW with a realistic slip law. This fits with the friction curve data for a polydimethylsiloxane (PDMS) in a steel die. It is based on molecular dynamics theory and contains a critical stress below which there is qualitative adhesion at the wall. The existence of a slip boundary condition is shown to modify the morphology of the velocity field which tends toward a plug flow. It thus largely reduces the fully developed stress level and the exit stress concentration. Localized slip at the die exit appears even for fully developed flow under shear rate values which are lower than the critical shear rate corresponding to the occurrence of slip at the wall in a Poiseuille flow. This is explained by exit stress concentration. The effect of slip length is then examined: it is found that exit stress and extrudate swell can be largely reduced by using a very short slip length only. This result shows the interest of optimising slippery surfaces in the design of extrusion processes.

Numerical simulation of the transition from adhesion to slip with friction in generalized Newtonian Poiseuille flow

Abstract The axisymmetric Poiseuille flow of a purely viscous generalized Newtonian fluid under rate of flow controlled conditions is studied with a change in the boundary conditions at a transition point from an adhesive to a slip condition with friction at the wall. The friction law used originates from an experimental study by (J.M. Piau and N. El Kissi, J. Non-Newtonian Fluid Mech. 54 (1994) 121–142) using a capillary made of steel and a silicone fluid, and is based also on a molecular dynamics theory by (Yu. B. Chernyak, A.I. Leonov, Wear, 108 (1986) 105–138). It gives a non-linear multivalued dependance of the wall shear stress to the velocity at the wall. Moreover, wall shear stress values may become smaller than values obtained when adhesion prevails in the capillary. The shear stress must over-step some limiting stress level to trigger the wall slip. After checking slip boundary condition implementation for the case of Poiseuille flow with slip along the entire wall, the convergence and the validity of the computation was studied. Important morphologic changes of the flow field and the stress field appear around the transition point from adhesion to slip boundary condition. Slip at the wall allows the principal stress difference to be drastically reduced, except in the vicinity of the transition point where this difference is maximum. A peak in shear stress located upstream of the transition, and a peak in elongational stress located downstream of the transition, are observed at the wall. Fully developed near plug-like flows are obtained within about 1D only downstream of the transition point. It is concluded that the effect of slip on extrudates distorsion should appear clearly even when the exit slippery zone is reduced to 1D.

Aging and yielding in a sheared AOT/iso-octane/water lyotropic lamellar phase

We define a creep-flow-based measurement procedure to allow reliable and reproducible results on aging and yielding materials to be obtained. Investigation of the effects of different parameter such as the pre-shear time, the recovery time and the applied stress magnitude on the viscoelastic properties of a lyotropic liquid crystal phase is reported. Cryo-TEM observations indicate the formation of multiconnected bilayers at rest. Shearing the investigated material shows a propensity to acquire all the macroscopic properties of “soft jammed systems”. These properties are then interpreted in terms of shear-induced structural rearrangement on the basis of cryofracture observation obtained at different times after the preshear imposed.

Extruded Proton Exchange Membranes Based on Sulfonated Polyaromatic Polymers for Fuel Cell Application

A global approach of membranes elaboration in environmentally friendly conditions and at a moderate cost, for applications in the fuel cells industry is proposed here. New elaboration methods of polysulfones proton exchange membranes have been developed. Extrusion has been used to produce homogeneous films for PEMFC. Different routes are reported: chemical modification of extruded membranes, and for the first time, extrusion of acid and alkaline forms of modified polysulfone. An electrochemical study shows that extruded films exhibited a promising proton‐conductivity.