Melt fracture sharkskin

For uniform and stable extrusion it is important to check periodically the drive system, the take-up device, and other equipment, and compare it to its original performance. If variations are excessive, all kinds of problems will develop in the extruded product. An elaborate process-control system can help, but it is best to improve stability in all facets of the extrusion line. Some examples of instabilities and problem areas include 1) non-uniform plastics flow in the hopper 2) troublesome bridging, with excessive barrel heat that melts the solidified plastic in the hopper and feed section and stops the plastic flow 3) variations in barrel heat, screw heat, screw speed, the screw power drive, die heat, die head pressure, and the take-up device 4) insufficient melting or mixing capacity 5) insufficient pressure-generating capacity 6) wear or damage of the screw or barrel 7) melt fracture/sharkskin (see Chapter 2), and so on. 

The addition of specific fluoropolymers to polyolefins allows for improved processability, including elimination of melt fracture (sharkskin), reduced die build-up, lowered processing viscosity, reduced die pressure and abihty to increase extrusion rates [278-282]. The fluoropolymers are generally added at levels of 100-1000 ppm. Vinylidene fluoride-hexafluoro-propylene fluorocarbon elastomers are often mentioned in the patent examples, such as the commercial systems carrying the tradename Viton duPont. 

There are basically five types of melt fracture sharkskin, ripple, bamboo, wavy, and severe. These types of melt fracture are shown in Figures 7.8, 7.9, and 7.10. Sharkskin is shown in Figure 7.8 for a LLDPE. At the lowest apparent shear rate the extrudate is smooth but at ya = 112 s, the... 

The narrow molecular weight distribution means that the melts are more Newtonian (see Section 8.2.5) and therefore have a higher melt viscosity at high shear rates than a more pseudoplastic material of similar molecular dimensions. In turn this may require more powerful extruders. They are also more subject to melt irregularities such as sharkskin and melt fracture. This is one of the factors that has led to current interest in metallocene-polymerised polypropylenes with a bimodal molecular weight distribution. 

The site of the sharkskin distortion is again the die exit, and so is the screw thread pattern. The site of, and the mechanism for the gross extrudate distortion are problems that have no clear answers. The work of White and Ballenger, Oyanagi, den Otter, and Bergem clearly demonstrates that some instability in the entrance flow patterns is involved in HDPE melt fracture. Clear evidence for this can be found in Fig. 12.18. Slip at the capillary wall, to quote den Otter, does not appear to be essential for the instability region, although it may occasionally accompany it. ... 

With the discovery and development of metallocene-based LLDPEs with narrow MWD and high molecular weight, there has been a flurry of investigations with these polymers, because they exhibit sharkskin melt fracture at quite low and industrially limiting production rates. The objective of such studies is to increase the rate of production with... 

Fig. 12.19 Cold postextrusion micrographs as a function of the flow rate. The processing conditions were T = 177°C and no PPA. Each image is actually a composite of two micrographs in which the side and top are focused. The relative errors in throughputs are 0.05 Q = (a) 1.0, (b) 2.2, (c) 3.8, (d) 6.3, and (e) 11 g/min. The width of each image corresponds to 3 mm. [Reprinted by permission from K. B. Migler, Extensional Deformation, Cohesive Failure, and Boundary Conditions during Sharkskin Melt Fracture, J. Rheol., 46, 383 4-00 (2002).]...

Fig. 12.21 Sketch of the kinetics of the sharkskin instability, side view. [Reprinted by permission from K. B. Migler, Extensional Deformation, Cohesive Failure, and Boundary Conditions during Sharkskin Melt Fracture, J. Rheol., 46, 383 400 (2002).]...

Recently, Muliawan et al. (52), who have been studying melt fracture, and in particular sharkskin extrudate instabilities over the last decade, have presented interesting experimental results relating the extensional stress-Hencky strain behavior of polymer melts to their sharkskin (exit) and gross (capillary entrance) melt fracture behavior. For the purposes of this discussion, results obtained with two Nova Chemicals... 

Corporation film-grade Ziegler-Natta linear low density polyethylenes will be presented. They are Resin E, Nova FP-015-A, MFI = 0.55, p = 0.9175 g/cc, and Resin C, Nova PF-120-F, MFI = 1.00, p = 0.9170 g/cc. Their capillary-flow behavior in terms of apparent shear stress vs. apparent shear rate are shown on Fig. 12.24. The melt fracture onset is also noted in Figure 12.24 and the data presented in the table below, indicate that resin E undergoes both sharkskin and gross melt fracture at lower apparent shear rates and stresses. 

Fig. 12.24 Flow curves of LLDPE resins E and C, indicating the onset of sharkskin and gross melt fracture for each resin. T — 170°C, capillary D = 1 mm, L/D = 16, with entrance angle 2a = 180°. [Reprinted by permission from E. G. Muliawan, S. G. Hatzikiriakos, and M. Sentmanat, Melt Fracture of Linear Polyethylene, hit. Polym. Process., 20, 60 (2005).]...

Fig. 12.27 Surface morphological features of mLLDPE (ExxonMobil Exceed 350D60) extrudates obtained at 160 °C with a tungsten carbide die D — 0.767 and L — 25.5 mm just above and in the sharkskin melt fracture flow-rate region. [Reprinted by permission from C. G. Gogos, B. Qian, D. B. Todd, and T. R. Veariel, Melt Flow Instability Studies of Metallocene Catalyzed LLDPE in Pelletizing Dies, SPE ANTEC Tech. Papers, 48, 112-116 (2002).]...

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]. 

Sharkskin has been studied less widely than melt fracture but in fact may be a more common problem in actual operations. The distortion takes the form of transverse ridges (as distinct from the helical shapes in melt fracture) and is thought to be a consequence of the melt tearing as it emerges from the die. An explanation, briefly, is that melt close to the inside wall of the die is moving very slowly (not at all in the case of the layer immediately in contact with the wall) and as melt emerges and moves from the face at constant speed the outer layers may be stretched and tear. 

Polymer chains anchored on solid surfaces play a key role on the flow behavior of polymer melts. An important practical example is that of constant speed extrusion processes where various flow instabilities (called sharkskin , periodic deformation or melt fracture) have been observed to develop above given shear stress thresholds. The origin of these anomalies has long remained poorly understood [123-138]. It is now well admitted that these anomalies are related to the appearance of flow with slip at the wall. It is reasonable to think that the onset of wall slip is related to the strength of the interactions between the solid surface and the melt, and thus should be sensitive to the presence of polymer chains attached to the surface. 

It is simplest to think of sharkskin as a result of a quasi-periodic perturbation on the overall extrudate swell. This small amplitude fluctuation of extrudate swell arises from the oscillation of the boundary condition at the exit wall that produces an oscillation of the local stress level as the interfacial chains suffer a conformational instability. The local boundary condition oscillates between noslip and slip, resulting in the fluctuation of the stress level at the die exit. To determine whether some sort of melt fracture occurs, we need to know not only the... 

The term melt fracture has been applied from the outset [9,13] to refer to various types of visible extrudate distortion. The origin of sharkskin (often called surface melt fracture ) has been shown in Sect. 10 to be related to a local interfacial instability in the die exit region. The alternating quasi-periodic, sometimes bamboo-like, extrudate distortion associated with the flow oscillation is a result of oscillation in extrudate swell under controlled piston speed due to unstable boundary condition, as discussed in Sect. 8. A third type, spiral like, distortion is associated with an entry flow instability. The latter two kinds have often been referred to as gross melt fracture. It is clearly misleading and inaccurate to call these three major types of extrudate distortion melt fracture since they do not arise from a true melt fracture or bulk failure. Unfortunately, for historical reasons, this terminology will stay with us and be used interchangeably with the phase extrudate distortion. ... 

J.-M. Piau, N. El Kissi and B. Tremblay, Influence of upstream instabilities and wall slip on melt fracture and sharkskin phenomena during silicones extension through orifice dies, J. Non-Newtonian Fluid Mech., 34 (1990) 145-180. 

During the extrusion of polymers different defects and flow instabilities occur at very low Reynolds numbers. The commonly known ones are sharkskin, melt fracture, slip at the wall and cork flow. These defects are of commercial importance, since they often limit the production rate in polymer processing. Many researchers have been interested in the subject, and thorough reviews on flow stability and melt fracture have been written in the last 30 years [1-4]. More recently, two review papers deahng with viscoelastic fluid mechanics and flow stability, were published by Denn [5] and Larson [6]. However, although much work has been done in the field of extrusion distortions, controversy still exists regarding the site of initiation and physical mechanisms of the instabilities. 

Extrudate flow at the die outlet accelerates suddenly and the sharkskin cracks disappear immediately (Fig. 12a). This may be attributed to the fact that slip considerably reduces the stretch stresses at the die outlet and the fluid no longer cracks. Thus, as has been shown in numerous works [18, 24, 25] the extrudate is then opaque and irregular and has practicedly no fiirther swelling (Figure 12a). The appearance of the extrudate is the result of slip effects superimposed on melt fracture effects. 

Figure 13 Sharkskin and melt fracture for the flow of HDPE through an orifice die 1mm diameter (Stainless steel die, T = 185 C).

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