The oscillating shear phenomenon in high density polyethylenes

Metzger, A. P.; Hamilton, C. W.

doi:10.1002/pen.760040209

Cited by 3 publications

(3 citation statements)

References 4 publications

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“…There are two "exceptions". (a) Sometimes introducing additives, 33 using Teflon capillary die, 34,35 or modifying the die wall surface 35 to reduce the surface energy has been shown to minimize the spurt flow instability for polyethylene. (b) For other less entangled polymers such as polystyrene (PS) and poly(dimethylsiloxane) (PDMS) of comparable molecular weight, the spurt flow characteristics are nowhere to be found under controlled speed, and no flow discontinuity transition is observed under controlled stress either.…”

Section: Discussionmentioning

confidence: 99%

Superfluid-Like Stick−Slip Transition in Capillary Flow of Linear Polyethylene Melts. 1. General Features

1996

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This paper explores rheological characteristics of and molecular mechanism for a superfluid-like stick−slip transition occurring under controlled pressure in capillary flow of a series of highly entangled linear polyethylene (PE) melts and establishes its connection with the spurt flow phenomenon. The transition is signified by a large discontinuity in the flow rate at a critical stress, resulting in a double value within the flow curve. The magnitude of the transition can be quantified in terms of an extrapolation length, b. In particular, the superfluid-like flow transition occurs throughout a range of temperatures from T = 180 to 260 °C as long as a critical stress, σc, is exceeded. It is found that σc increases linearly with T, and b c at the transition remains around 1.7 mm at all the temperatures for the PE (MH20) of weight-average molecular weight M w = 316 600. Thus the observed remarkably large interfacial slip is believed to be due to complete disentanglement of the adsorbed chains from free chains at the melt/wall interface at and beyond the transition. The amount of wall slip, as described by b, diminishes quickly with decreasing M w, in qualitative agreement with a simple scaling relation for noninteracting interfaces. The flow transition depends on the surface condition of the die wall and occurs at a considerably lower critical stress when the wall is treated by depositing a fluorocarbon elastomer to weaken the PE adsorption. Application of both controlled-pressure and controlled-piston speed conditions demonstrates that spurt flow instability originates from indeterminacy of the hydrodynamic boundary condition at the PE/die wall interfaces.

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Section: Discussionmentioning

confidence: 99%

Superfluid-Like Stick−Slip Transition in Capillary Flow of Linear Polyethylene Melts. 1. General Features

1996

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“…During the extrusion phase, when the reservoir of the rheometer is emptying, it is observed that the oscillation amplitude remains substantially constant, but that the period decreases, which reveals the key role played by the compressibility of the material (Metzger and Hamilton, 1964;Lupton and Regester, 1965;Weill, 1980b;Kalika and Denn, 1987;Hatzikiriakos and Dealy, 1992b;Durand, 1993;Sato and Toda, 2001;Baldi et al, 2013). Figure 17 shows that the decrease of the period is linear and that it affects both the pressure increase Rheol., 40, 83 -394 (1996).…”

Section: Description Of the Observed Phenomenamentioning

confidence: 91%

“…Figure 19 shows that its magnitude, both in pressure and flow rate (or apparent shear rate) variations, is a direct function of the L/D ratio of the capillary (Metzger and Hamilton, 1964;Ui et al, 1964;Den Otter, 1970;Vinogradov et al, 1972a;Lim and Schowalter, 1989;El Kissi and Piau, 1990;Durand et al, 1996). In particular, for an orifice die (L/ D & 0), there is no longer any pressure oscillation.…”

Section: Influence Of Parametersmentioning

confidence: 95%

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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