Flow through a convergence. Part 1: Critical conditions for unstable flow

Abstract

The critical conditions leading to fracture in elongation and different types of flow instabilities were examined in uniaxial elongation and in a capillary rheometer equipped with dies having different entry profiles. Either ductile or brittle fracture may be observed, ductile being related to necking of material. The critical stress approach was used to predict fracture in elongation. All linear polymers studied in this work exhibited ductile fracture in elongation, but the transition to brittle fracture is discussed in relation to existing experiments with other materials. In a ductile h c t u r e regime, critical stress and work both increase with an increasing rate of deformation, whereas in a brittle regime the critical values remain constant. The converging flow studies indicated that two types of flow instability that have been previously related to each other, namely, pressure oscillations and volume distortions, are of different origins. The critical flow rate for pressure oscillations is independent of entry profile, and the origin for this type of instability lies along the wall of the capillary. On the other hand, the critical flow rate for volume distortions increased with a decreasing entry angle, indicating that volume distortions are not a consequence of pressure oscillation, nor are their origin at the cap& lary wall. Numerical simulations were used to determine the stress profies within the flow, and it was shown that the onset of volume distortions is directly related to the magnitude of elongational stress and work, and may therefore be considered to be caused by fracture in elongation. In dies with 90" entry profile, volume distortions were observed simultaneously with pressure oscillations, making it difficult to distinguish between the two phenomena. a maximum of four different types of flow instability 61 1 Towfiornmmsponduurshouldbeaddressed. a 1 2 t z h / HDPE and LLDPE Flow Rate, Qlet PP and LDPE r Flow Rate, Q&

Molecular interpretation of the “stick–slip” defect of linear polymers

“…To determine the plateau modulus of HDPE, we used the coefficients of the relaxation time spectra derived from rheological characterizations of the literature. We found G 0 N = 649, 000 Pa at 190 • C with the data of Arda and Mackley [50] and G 0 N = 677, 000 Pa at 200 • C with those of Miller et al [51].…”

Flow property at capillary extrusion for ethylene–tetrafluoroethylene copolymer

“…M a n u s c r i p t Kotera and Yamaguchi,5 Since the average molecular weight between entanglement couplings M e is inversely proportional to G N 0 , a polymer having high M e tends to show shark-skin failure at low shear stress [9]. Furthermore, it has been clarified that the narrow distribution of molecular weight also leads to the flow instability at low shear stress, which was theoretically derived using the concept of Deborah number [9].…”

“…Meller et al summarized that the shark-skin failure occurs beyond the critical shear stress at the die exit and the wavy melt fracture takes place beyond the critical elongational stress at the die entry [5]. Therefore, the rheological properties, especially strain-hardening behavior in elongational viscosity, affect the type of flow instability.…”

Flow instability for binary blends of linear polyethylene and long-chain branched polyethylene

“…Since it is generally accepted that a high viscous polymer melt slips on the wall, the slippage can be the origin of surface instability [35,43,44]. Gross melt fracture is attributed to the flow instability at a die entrance, and associated with long time relaxation mechanism [38,[45][46][47][48][49][50][51]. Yamaguchi et al found that gross melt fracture of LDPE can be avoided by applying intense shear history, which weakens the relaxation mechanism associated with long-chain branches, by shear modification [51].…”

“…Consequently, the longest relaxation time of the blend with a linear polyethylene having high molecular weight becomes longer than that of the LDPE [31]. The mechanisms of the flow instability at capillary extrusion have been investigated for a long time [25,26,[30][31][32][33][34][35][36][37][38]. It has been recognized that the flow instability can be classified into two types; one is rough surface, which is called as shark-skin failure, and the other is volumetric gross melt fracture.…”