Extrudate distortion

D.S. Kalika and M.M. Denn, Wall Slip and Extrudate Distortion in Linear Low-Density Polyethylene, J. Rheol., 31 815-834 (1987). 

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).]...

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. ... 

Although such numerical die design optimization techniques significantly improve the flow uniformity and reduce the level of internal stresses leading to extrudate distortion,... 

Fig. 1. Typical flow curve of commercial LPE. There are five characteristic flow regimes (i) Newtonian (ii) shear thinning (iii) sharkskin (iv) flow discontinuity or stick-slip transition in controlled stress, and oscillating flow in controlled rate (v) slip flow. There are three leading types of extrudate distortion (a) sharkskin like, (b) alternating bamboo like in the shaded region, and (c) spiral like on the slip branch. Industrial extrusion of polyethylenes is most concerned with flow instabilities occurring in regimes (iii) to (v) where the three kinds of extrudate distortion must be dealt with. The unit shows the approximate levels of stress where the sharkskin and flow discontinuity occur respectively. There is appreciable molecular weight and temperature dependence of the critical stress for the discontinuity. Other highly entangled melts such as 1,4 polybutadienes also exhibit most of the features illustrated herein...

Polymer melt rheologists constantly applied the notion of wall slip in describing a variety of polymer melt flow anomalies [ 10,11,14]. However, no explicit knowledge of a molecular mechanism for and microscopic origin of wall slip had been available. Therefore, for 40 years no explanation was given about why wall slip was pertinent and how it produced such melt flow anomalies as flow oscillations and sharkskin-like extrudate distortion. One objective of current research around the world is to explore the molecular nature of wall slip and establish a correlation between wall slip and some of the anomalous melt flow phenomena. 

Many polymers exhibit neither a measurable stick-slip transition nor flow oscillation. For example, commercial polystyrene (PS), polypropylene (PP), and low density polyethylene (LDPE) usually do not undergo a flow discontinuity transition nor oscillating flow. This does not mean that their extrudate would remain smooth. The often observed spiral-like extrudate distortion of PS, LDPE and PP, among other polymer melts, normally arises from a secondary (vortex) flow in the barrel due to a sharp die entry and is unrelated to interfacial slip. Section 11 discusses this type of extrudate distortion in some detail. Here we focus on the question of why polymers such as PS often do not exhibit interfacial flow instabilities and flow discontinuity. The answer is contained in the celebrated formula Eqs. (3) or (5). For a polymer to show an observable wall slip on a length scale of 1 mm requires a viscosity ratio q/q equal to 105 or larger. In other words, there should be a sufficient level of bulk chain entanglement at the critical stress for an interfacial breakdown (i.e., disentanglement transition between adsorbed and unbound chains). The above-mentioned commercial polymers do not meet this criterion. 

It was thought in the past that the only mechanism for wall slip would be polymer desorption, i.e., an adhesive breakdown [25, 53]. However, lack of a strong temperature dependence would be inconsistent with an activation process of chain desorption. Since the onset of the flow discontinuity (i.e., stick-slip) transition was found to occur at about the same stress over a range of experimental temperatures, it was concluded from the outset [9] that the phenomena could not possibly have an interfacial origin. Thus, the idea of regarding the flow discontinuity as interfacial did not receive sufficient and convincing theoretical and experimental support in the past, not only because the transition was often accompanied by severe extrudate distortion and hysteresis, but also because the molecular mechanism for such an interfacial transition involving wall slip was elusive. 

Extrudate distortion has been viewed as explicit evidence for melt flow instabilities or melt fracture. This is another calamitously misleading assertion in the massive literature of over 3000 papers on the subject. Because extrudate distortion was also observed even without any signature of slip, it was concluded by Blyler and Hart [32] that the slip process is not an essential part of the flow instability. This dilemma, that the flow anomalies including flow oscillation cannot be accounted for in terms of either a constitutive instability or interfacial slip mechanism, has persisted until very recently. Denn coined this plight the paradox [10b]. 

Besides eliminating the cause for extrudate distortion, it is important to remove the obscuring hysteresis around the transition so that the critical condition for the onset of spurt flow can be precisely determined. This can be accomplished by applying a discrete fixed pressure within each load (filling) of the bar-... 

Linear polyethylenes (PE) are one polymer that possess an important ingredient necessary for a display of interfacial stick-slip transition. In the past, the coincidence that PE is both the most widely used polymer and most prone to suffer from melt flow instabilities has challenged the PE industry. Today we still face the task of how to effectively remove instabilities that result in various types of extrudate distortions. 

We have presented a thorough description and discussion about the molecular origin of the second kind (b) - bamboo like extrudate distortion - in the preceding Sect. 8. The present section is devoted to a specific illustration of the molecular origin of the type (a) distortion, i.e., sharkskin, which occurs in a range of stress/rate below the oscillatory flow or stick-slip transition, as indicated in Fig. 1. The next section will provide a brief discussion of the origin of the type (c), often spiral-like distortion. The macroscopic nature of the type (c) distortion was first discussed at least over 20 years ago [75]. Note that when the type (c) spiral distortion occurs on very fine length scales on the extrudate it can be and has sometimes been mistaken as sharkskin. 

Constitutive Instabilities, Extrudate Distortions and Melt Fracture 11.1... 

Gross extrudate distortions have constantly been linked to melt fracture. This alleged connection between a gross extrudate distortion and melt fracture re-... 

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. ... 

A true melt fracture by definition must involve some kind of bulk failure in melt flow. Either massive chain disentanglement or chain scission or both must occur in the bulk. Such a real cohesive breakdown away from the surface may also produce extrudate distortion. In general, other forms of irregular extrudate distortion do occur at high stresses. Only direct flow visualization may reveal the origin of such extrudate distortions. 

We have surveyed the most recent progress and presented a new molecular level understanding of melt flow instabilities and wall slip. This article can at best be regarded as a partial review because it advocates the molecular pictures emerging from our own work over the past few years [27-29,57,62,69]. Several results from many previous and current workers have been discussed to help illustrate, formulate and verify our own viewpoints. In our opinion, the emerging explicit molecular mechanisms have for the first time provided a unified and satisfactory understanding of the two major classes of interfacial melt flow instability phenomena (a) sharkskin-like extrudate distortion and (b) stick-slip (flow discontinuity) transition and oscillating flow. 

Extrudate distortion cannot generally be taken as evidence for melt fracture. For example, a commonly observed spiral-like distortion originates from vortex... 

Table 2. Molecular and macroscopic explanations of three dominant types of extrudate distortion in capillary flow of linear polyethylenes ...

Continuum Depiction Local oscillation of the melt-wall boundary condition in the exit region causes peturbations on the exit stress and die swell Oscillation of the overall stress due to unstable boundary condition produces cycles of melt compression and decompression in the barrel and fluctuations in the extrude swell The extrudate distortion arises from formation of secondary flow (vortices) in the barrel due to the strong converging flow near the die entry... 

In conclusion, it may be useful to highlight our current explanations for the first three types of extrudate distortion (sharkskin, oscillation and spirals) in polymer extrusion in a tabulated manner. In Table 2, using the example of linear polyethylenes, we not only list their molecular origins but their description from... 

In the existing literature, extrudate distortion has also been given the term melt fracture where the phrase takes its literal meaning of cohesive bulk failure. It will become clear that this practice of associating extrudate distortion with melt fracture is somewhat misleading... 

Kalika D.S. and Denn M. M., "Wall slip and extrudate distortion in linear low-density polyethylene," J. of Rheol. 31, 815-834 (1987). 

Major advantage can be taken of the fact that many aspects of extrudate distortions reveal the same physics of entangled chains, when changing from one polymer species to another. This physics depends little on the polymers chemical family, and on the temperature at which they melt [30, 31]. Better choices of experimental conditions may then result. Comparisons between results obtained for different chemical families of pol5uners must be made, and general laws and classifications appear. 

Figure 12a Extrudate distortion of PDMS LG3 at the exit of a capillary 2mm diameter and 20 mm long. Slip under stress controlled conditions. AP = 26 10 Pa...

Figure 12c Extrudate distortion for LLDPE at the exit of a capillary 0.5mm diameter and 20 mm long, during the second oscillating regime. The polymer flows downwards. The largest instantaneous flow rate is observed for iii). Photograph ii) shows the transition from small (photograph i)) to large instantaneous flow rate.

Melt fracture for highly filled composite materials has a more complex character compared to that of neat plastics. And this is, of course, due to the effect of the filler. For example, using a cone die (the entrance diameter 2.5 in., the exit diameter 0.300 in., the length of the die 12 in.) it was shown for the neat HDPE that there was no visual signs of extrudate distortion for any of the flow rate tested (5.5-177 g/min), unlike that for filled plastics. Similarly, the 10%-filled (ground rice hulls) HDPE did not exhibit any observable extrudate distortion. However, the 60% -filled composite... 

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