Extrudate instability

Slip at the wall is closely related to extrudate instabilities, but in normal flow situations within machines, in virtually all but exceptional cases, the no-slip condition is assumed for solving flow problems. 

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... 

As normally polymerized, PVF melts between IH5 and 210 °C and contains 12 18% inverted monomer units ft is normally considered a thermoplastic, but because of its instability above its melting point, it cannot be processed by conventional thermoplastic techniques Instead it is generally extruded into films in a solvent swollen (organosol) form and the solvent is subsequently evaporated and recovered Such films can be onented further to achieve specific mechanical properties PVF films are exceptionally weather and radiabon resistant considenng their modest fluonne content PVF is insoluble below 100 °C but, at higher temperatures, it dissolves in polar solvents like amides, ketones, tetramethylene sulfone, and tetramethylurea Resistance to acids and bases at room temperature IS good [1, 29 ... 

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... 

Screw and barrel wear can reduce the performance of the extruder by causing the specihc rate to decrease and the discharge temperature to increase. Screw wear is discussed in Sections 13.2 and 13.4.1. Although extremely rare, wear in the feed casing can lead to a rate reduction and process instabilities. This case study is presented in Section 12.7.7. 

In polymer processing practice, we need to ensure that the particulate gravitational mass flow rate of the hopper exceeds, over the complete operating range, the extruder open discharge rate (i.e., the rate without any die restriction). That is, hoppers must not be the production-rate limiting factor. Second, and more importantly, it is necessary for stable extrusion operations and extruded product quality that the flow be steady and free of instabilities of the particulate flow emerging from the hoppers. Finally, as we will see in Chapter 9, we need to know the pressure under the hopper in order to determine the pressure profile in a SSE. 

So far in this chapter we have looked into the viscous phenomena associated with the flow of polymer melts in capillaries. We now turn to the phenomena that are related to melt elasticity, namely (a) swelling of polymer melt extrudates (b) large pressure drops at the capillary entrance, compared to those encountered in the flow of Newtonian fluids and (c) capillary flow instabilities accompanied by extmdate defects, commonly referred to as melt fracture. ... 

Fig. 12.15 The ratio of entrance pressure drop to shear stress at the capillary wall versus Newtonian wall shear rate, T. . PP , PS O, LDPE +, HDPE , 2.5% polyisohutylene (PIB) in mineral oil x, 10% PIB in decalin A, NBS-OB oil. [Reprinted by permission from J. L. White, Critique on Flow Patterns in Polymer Fluids at the Entrance of a Die and Instabilities Leading to Extrudate Distortion, Appl. Polym. Symp., No. 20, 155 (1973).]...

Looking at the melt fracture of specific polymers, we see many similarities and a few differences. Polystyrene extrudates begin to spiral from smooth at t 105 N/m2, and at higher shear stresses, they are grossly distorted. Visual observations show a wine glass entrance pattern with vortices that are stable at low stress values and spiral into the capillary and subsequently break down, as t is increased. Clearly, melt fracture is an entrance instability phenomenon for this polymer. 

In the three polymers just named, two more observations are worth mentioning. Lirst, at the melt fracture onset, there is no discontinuity in the flow curve (t vs. y ). Second, as expected, because the entrance is the site of the instability, increasing L/Dq decreases the severity of extrudate distortions. 

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. ... 

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).]...

Upon exiting the die, the sheet extrudate will swell to a level determined by the polymer, the melt temperature, the die length-to-opening ratio, and the shear stress at the die walls. Additionally, flow instabilities will occur at values of the corrected shear stress at the wall, of the order of, but higher than 105 N/m2, as found by Vlachopoulos and Chan (58), who also concluded that, for PS, HDPE, and LDPE, the critical Sr in slits is 1.4 times higher than in tubes of circular cross section. Aside from these differences, the information presented in Section 12.1 and 12.2 applies to slit flow. 

Reactive extrusion has emerged from a scientific curiosity to an industrial process. Various types of extruders can be used, all with their specific advantages and disadvantages. Further development suffers from lack of kinetic and rheological data at high conversions and from uncertainties about heat transfer and reactor stability. Nonlinear effects in the process can give rise to instabilities that are of thermal, hydrodynamical or chemical origin. 

Hydrodynamic instability, where a small increase in die pressure leads to a larger local residence time, which in turn, through conversion, results in a larger viscosity. This viscosity increase will successively increase the die pressure even further. The positive feedback will be counterbalanced by the back-flow, because an increased viscosity also increases the pressure build-up ability of the extruder. An influence on the stability may be expected if the interaction parameters and the local viscosities are such that the positive feedback dominates. 

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]. 

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