Slip-stick melt fracture

Finally, we shall mention the slip-stick melt fracture phenomenon — a phenomenon often observed in polymer extrusion — which is much related to the relative positions of the /Ltg(t, E) and pcit) processes. The phenomenon is a well-known problem in the polymer processing industry because it limits the output of polymer through an extruder. In a capillary flow, the decline of the viscosity observed as the shear rate (or flow rate) increases from the Newtonian region is much related to the damping factor... 

It has been proposed that polymers such as HDPE, which is considered a linear polymer, as well as other linear polymers such as PP and PS have a similar mechanism for fracture. In fact, the molecular weight dependence of the critical shear stress (tor) for fracture was found to be similar for linear polymers (Middleman, 1977). A statistical fit of the data for fracture gave the following relation for the critical shear stress for the onset of slip-stick melt fracture, Zcr, for linear polymers ... 

During extrusion of polymer melts with high throughputs, the elastic melt properties can also lead to elastic instabilities which can result in surface distortions of the extrudate. One example are wavy distortions also described as sharkskin. Depending on the polymer, this can also lead to helical extrudate structures (stick-slip effect) or to very irregular extrudate structures (melt fracture) at even higher throughput rates [10]. 

De Gennes (2007) has constructed a model for slippage planes in a sheared melt, based on a balance between reptation bridging and shear debonding (slip stick model). He proposed that slippage occurs on solid walls either at the container surface or on dust particles floating in the melt. There is critical stress for slippage, approximately equal to Ys of the plateau modulus, which means that melt fracture is expected at moderate stresses. 

An important element in melt fracture is also wall slip phenomenon [5, 49]. It is related to the so-called sharkskin, or sharkskin melt fracture, which is also called surface melt fracture. It is a low amplitude surface distortion of extruded polymer. Sharkskin is generally observed in case of linear polymers with narrow MWD, below the oscillating stick-slip transition. Sometimes (but not always), there is a change... 

It is recognized that HDPE with a high molecular weight typically does not show sharkskin and enters directly into the spurt stage when shear rate increases above a critical value. On the contrary, the sharkskin for narrow MWD LLDPE is a common phenomenon when the shear stress exceeds a critical value of 0.14 MPa, and at a further increase of the shear rate, the hot melt enters the spurt stage of melt fracture. It is noticeable that LDPE does not exhibit slip-stick even at high shear rates. 

Sharkskin, when the surface of the extrudate becomes visibly opaque, occurs at a wall shear stress level that is typically of the order of 0.1 MPa. At higher wall shear stress, typically of the order of 0.3 MPa, the flow becomes unsteady and the extrudate alternates between sharkskin and smooth segments (stick-slip, spurt flow, or cyclic melt fracture) [52, 53]. 

Melt fracture and distortion. The phenomena occur in materials at higher stresses and can be an indication of slip/stick in the die. The extrudate can display a wide variety of distortions from simple sharkskin (a very rough surface on the strand) to twists and kinks in the strand (gross melt distortion). These distortions usually occur at the exit of the die and do not affect the viscosity curve as long as the material does not slip. 

A number of other mechanisms [53-65] have been suggested for melt fracture. Based on a stick-slip mechanism, it is purported [53] that, above a critical shear stress, die polymer experiences intermittent slipping due to a lack of adhesion between itself and die wall, in order to relieve the excessive deformation energy adsorbed during the flow. The stick-slip mechanism has attracted a lot of attention [53-63], both theoretically and experimentally. The other school of drought [64,65] is based on thermodynamic argument, according to which, melt fracture can initiate anywhere in the flow field when reduction in the fluid entropy due to molecular orientation reaches a critical value beyond which the second law of thermodynamics is violated and flow instability is induced [64]. 

It is obvious that in rubber processing there will be slip, because the surface of processing equipment in contact with gum rubbers and compounds is usually shiny. This is in contrast to plastic processing equipment, in which the surfaces become coated with a thin layer of the degraded material. With plastics this fact indicates the velocity of the melt at the metal interface is zero, i.e., laminar shear flow. A study of slip is multi-faceted, because there are many types of slip, a steady slip, slip with a lubricated layer, slip-stick, slip involving a fracture at the interface or ruhhing like a dynamic friction measurement. 

Slip/stick occurs when the shear rate at the die wall exceeds the adhesive force of the melt to the surface. When this occurs, the melt jerks forward as a plug, relieving the pressure behind it and allowing the oriented chain segments to recoil somewhat. Once the pressure is relieved the rate of movement of the polymer slows and it re-adheres to the die wall. Shear flow resumes until once again the shear rate exceeds the critical value [91]. The effect is also known as spurting due to the erratic polymer output associated with it. During slip/stick flow the pressure within the die fluctuates and the polymer output is unsteady, both of which may vary periodically or erratically. The effects of stick/slip are closely related to those of melt fracture. 

The results of melt fracture and slip/stick are most commonly observed during extrusion, the effects being manifest as a nonuniform extrudate. The nonuniformity may take the form of periodic fluctuations of the cross-sectional area (sometimes referred to as bamboo ), helices, rough, highly erratic extrudate profiles, and, in extreme cases, fragmentation of the extrudate. Some of the manifestations of slip/stick and melt fracture are illustrated schematically in Figure 59. 

Fig. 2 shows the flow curve for the neat exact 5361 at loot). The instability problem of the metallocene based polymers with narrow molecular distribution is well known. Fig. 3 shows the photographs of the extrudate samples with varying Dechlorane concentration collected during the capillary rheometers measurement. It is interesting to see that while the severe instabilities such as slip-stick and gross-melt fracture were observed at the shear rate from 177.8 s to 3162.2 s for the neat Exact resin, the severe instability appeared at the shear rate between 177.8 s and 562.2 s but disappeared at the shear rate above 1000.1 s for Exact/10% Dechlorane suspension. The shark-skin like instabilities were observed above the Dechlorane concentration of 20% and the shear rate at which the instability started to appear was decreased as the Dechlorane concentration was increased. Since the viscosity of the Dechlorane-filled systems was higher than that of the neat resin at all rates of shear, the instabilities are expected to develop the melt fracture at even lower shear rates. The shear viscosity vs. shear rate relationships measured with plate-plate rheometers and capillary rheometers are shown in Fig. 4. In this figure it is seen that both sets of data are reasonably matched. It is observed that at low shear rate range the viscosity increment due to the increase in the filler concentration is more pronounced than that at high shear rate. Both plate-plate and capillary measurements were carried out with constant shear rate (CSR) mode. While the capillary rheometer could accurately follow the preset shear rate values the plate-plate rheometer couldn t keep up with the preset shear rate values. Above two observations are due to the yield stress developed at low shear rate. At low shear rate particle-particle interaction dominates the flow phenomena and the yield stress was observed. At high shear rate hydrodynamic effect dominates the flow phenomena.

While the severe melt instabilities such as slip-stick and gross-melt fracture were observed for the neat Exact resin, the mild sharkskin-like instability was observed for the suspensions with Dechlorane concentration above 20%. The shear rate at which the instability started to appear was decreased as the Dechlorane concentration was increased. The thixotropic behavior of Exact 5361/60% Dechlorane suspension was observed using sequential step rate. 

When the pressure is increased much above 0.23 GPa, stick-slip extrusion occurs. At an extrusion pressure of 0.23 GPa, the extrusion stops when the viscosity increases sufficiently that the extrusion force cannot overcome the resistance to extrusion. During slip in stick-slip extrusion, a spiral fractured extrudate is produced that is similar to spiral products found in shear fracture of polymeric melts during extrusion (12). The flow profile in the solid state extruded rods is a deep shear parabola (10, 13, 24) (Figure 2) which suggests that stick-slip may arise from shear fracture of the extrudate. 

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