The effect of chain architecture on “sharkskin” of metallocene polyethylenes

Fernández, Mercedes; Vega, Juan Francisco; Santamarı́a, Anton; Muñoz‐Escalona, A.; Lafuente, Pilar

doi:10.1002/1521-3927(20000901)21:14<973::aid-marc973>3.0.co;2-7

Cited by 27 publications

(17 citation statements)

References 28 publications

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“…In strictly linear polymers of moderate polydispersity index, i.e., mPEs, η o is directly proportional to τ o , which is an average relaxation time [125,135,142,144,146]. This conclusion is easily attainable from the linear viscoelastic model assuming a single exponential relaxation modulus G(t) in the terminal zone, and is considered to be consistent with the behaviour predicted by reptation theory for linear, long chain, branchless, monodisperse polymers [92,93].…”

Section: Relaxation Time and Steady-state Compliancementioning

confidence: 99%

Effect of long chain branching on linear-viscoelastic melt properties of polyolefins

et al. 2002

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Abstract:The aim of this review is to provide evidence that rheological testing is a potent tool for characterising polymers in the melt. An effort has been made in order to gather results in conventional and model polyolefins, and correlating them with phenomena occurring at the molecular level. We have focused our interest on long chain branching (LCB). In the case of materials containing long side-chain branches, strong effects on viscosity, elastic character and activation energy of flow are general features. Literature results mostly indicate that the effect of polydispersity on these parameters could be very similar to that expected due to the presence of LCB -notwithstanding that the effects of LCB seem to be stronger than those due to polydispersity for a given molecular weight. Different relaxation processes appear as a consequence of the presence of LCB: slower terminal relaxation behaviour than of linear counterparts, and faster additional branch relaxation at higher frequencies, clearly distinguishable from polydispersity effects. To measure the amount of LCB from limited viscoelastic data and molecular properties seems to be a suitable instrument to explain the rheological features of the different polymers, but it fails when the results are compared with measured values of LCB density in model polymers. The actual framework leads us to say that the number of branches is less important than the topology itself. Therefore, the position and architecture of the branches along the polymer main chain are the main factors that control the rheology of the material.

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Section: Relaxation Time and Steady-state Compliancementioning

confidence: 99%

Effect of long chain branching on linear-viscoelastic melt properties of polyolefins

et al. 2002

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“…At higher shear rates, regular surface instabilities can appear called a ‘sharkskin’, which is characterized by both small amplitude and high frequency distortions 3. Metallocene short‐chain branching polyethylenes,4 ethylene/propylene copolymers,5 polymers with high molecular weights and low polydispersities,6 and syndiotactic poly(propylenes),7, 8 are examples of materials that develop this instability. Upon increasing the shear rate, some polymers show spurt instability (also named stick‐slip, and is different to the sharkskin phenomenon) characterized by large pressure oscillations where the extrudate alternates between rough and smooth sections.…”

Section: Introductionmentioning

confidence: 99%

In situ Pressure Fluctuations of Polymer Melt Flow Instabilities: Experimental Evidence about their Origin and Dynamics

Palza

Naue

Wilhelm

2009

Macromol. Rapid Commun.

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Despite the practical importance of polymer melt instabilities, there is still a lack of experiments able to characterize in situ the origin and behavior of these phenomena. In this context, a new set-up consisting of high sensitive pressure transducers located inside a slit-die and an advanced mathematical framework to process in situ measurements of polymer melt instabilities, are developed and applied. Our results show for the first time that pressure oscillations can actually be detected inside the die under sharkskin conditions. This originates from a factor of 10(3) and 10(2) improvement in terms of time and pressure resolution. Furthermore, new evidence towards the propagation of the slip phenomena along the die in spurt instabilities are found.

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“…For a fixed polydispersity, the amplitude (Den Otter, 1971;Cogswell, 1977;Beaufils, 1989;Kurtz, 1994) and the frequency of the defect increase with the molecular weight. However, the critical shear stress for the onset of sharkskin is assumed to be independent of molecular weight (Ramamurthy, 1988;Kurtz, 1992;Vega et al, 1999;Fernandez et al, 2000).…”

Section: Influence Of Parametersmentioning

confidence: 91%

“…A series of stick/slip or entanglement/disentanglement at the die wall, just before the exit, as it can be observed in the oscillating zone, which is presented in details in section 4 (Kurtz, 1992;Molenaar and Koopmans, 1994;Wang and Drda, 1997a;Shore et al, 1997;Barone et al, 1998;Molenaar et al, 1998;Koopmans and Molenaar, 1998;Deeprasertkul et al, 1998;Fernandez et al, 2000;Nithi-Uthai and Manas-Zloczower, 2003). This periodic change of the boundary conditions would cause regular variations of extrudate swelling at the die exit, thereby leading to the sharkskin defect.…”

Section: Mechanisms and Interpretationsmentioning

confidence: 99%

Extrusion Defects and Flow Instabilities of Molten Polymers

Vergnes

2015

International Polymer Processing

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International audienceWhen certain critical conditions are exceeded, the flow of a polymer melt becomes unstable, giving rise to particular phenomena that are the subject of this review. These instabilities are a vital industrial problem, as they are the key to productivity of extrusion processes. Indeed, their onset leads to unacceptable products and is thus the upper limit of the processing conditions. They are also a fascinating scientific problem because, although studied and discussed for more than half a century, they are still not fully understood and their mechanisms are still sometimes controversial. In the present literature review, we present an overview of the studies carried out for fifty years, as well as our own opinion on the underlying mechanisms.3-2

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The effect of chain architecture on “sharkskin” of metallocene polyethylenes

Cited by 27 publications

References 28 publications

Effect of long chain branching on linear-viscoelastic melt properties of polyolefins

Effect of long chain branching on linear-viscoelastic melt properties of polyolefins

In situ Pressure Fluctuations of Polymer Melt Flow Instabilities: Experimental Evidence about their Origin and Dynamics

Extrusion Defects and Flow Instabilities of Molten Polymers

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