Capillary flow instability of ethylene polymer melts

Abstract

The capillary flow instability resulting in extrudate distortion has been studied for ethylene polymer melts using a molecular structure approach. It is found that the instability initiates at a critical value of elastic strain energy independent of (average) molecular weight for linear polyethylene. Once the flow breaks down, a slip interface within the melt is formed near the capillary wall, causing an abrupt increase in volumetric throughput. The velocity gradient within the melt remains continuous through the instability, however. Low molecular weight species present in the molecular weight distribution of linear polyethylene tend to suppress slip. Blends of linear and branched polyethylene exhibit instability behavior characteristic of both components throughout the entire range of composition. Results are discussed in terms of specific molecular mechanisms.

Extrusion Defects and Flow Instabilities of Molten Polymers

“…The fact that the flow rate jumps occur at different pressures implies the existence of a hysteresis loop between the two stable branches, which is followed periodically during the oscillations. This particular behavior of the linear polymers has been first reported by Bagley et al (1958) and then confirmed by many authors (Sabia and Mullier, 1962;Tordella, 1963;Lupton and Regester, 1965;Myerholtz, 1967;Blyler and Hart, 1970). Figure 16 shows a typical example of simultaneous oscillations of pressure and exit flow rate during the oscillating cycle.…”

Slip at Fluid-Solid Interface

“…These include direct relation between slip and viscosity, 41,99,120 molecular weight, 11,53,59,71,81,119 temperature, 54,79 pressure, 65,80 particle concentration, 13,101 or particle size; 81 and the reciprocal relation between slip and pressure, 37,46,56,59,60,97,98 fluid polarity, 28 density, 98 viscosity, 79,80 concentration, 68 or temperature. 23 However, some of Downloaded by [Fresno Pacific University] at 05: 28 27 December 2014 these correlations seem to be system-dependent, as it is obvious from the contradiction in some of these reported relationships, and hence any generalization requires solid experimental support.…”

“…37,46,48,86 In some cases, slip at wall is more likely to occur when the fluid is not given sufficient contact aging time to adhere to the solid. 66,119 Wall slip may also be influenced by the type of deformation and fluid motion. Oscillatory motion, for example, could be more susceptible to slip 127,145 due possibly to an increased importance of inertial, elastic, and time-dependent factors.…”

“…20,25,28,46,50,51,[79][80][81]98,124,125 In the case of multi-component fluids, such as emulsions, slip can depend on the concentration, particle size (absolute and relative to the size of flow duct), particle shape, molecular weight, and salinity. [11][12][13]15,21,49,51,53,59,68,101,117,119,[123][124][125][126][127][128][129][130] The onset and velocity of slip can also be affected by the presence, type, and quantity of impurities, such as dissolved gases or traces of minerals, in the fluid sample. 22,37,46,86,131 Another factor is the physical and chemical properties of the surface.…”

“…47,85,89,118 Positive wall slip contributes to the total flow throughput, and hence the total flow in slippery systems has contributions from slip as well as from bulk deformation. 78,101,[119][120][121] This may be quantified by the slip fractional flow rate which is the ratio of the volumetric flow rate of slip contribution to the total flow rate. 55,56,75,84,111,120 In some systems, such as microfluidic and viscoplastic, and over certain deformation rate regimes the contribution of wall slip to the total flow rate could be substantial.…”