High‐density polyethylene melt elasticity—some anomalous observations on the effects of molecular structure

Abstract

synopsisThe die swell behavior of polymeric melts is a manifestation of melt elasticity of these materials and is of considerable commercial as well as fundamental importance. Hence, knowledge of the effect of such molecular variables as molecular weight (MW) and molecular weight distribution (MWD) on melt elasticity is important from both commercial and basic rheological points of view. The effect of these variables on melt elasticity of broad-distribution polymers in the shear rate region of commercial interest is not unambiguously known, with most published theory and experiment being applicable to the low-shear behavior of narrow-distribution polymers and blends thereof. There is indication that die swell increases with increasing MW and broadening MWD. However, the current investigation of carefully characterized broad-distribution HDPE materials prepared specifically to examine the effects of various molecular variables on melt elasticity does not support this contention and, in fact, provides consistent evidence for the opposite result, i.e., decreasing die swell with increasing MW or broadening distribution. The various samples studied were prepared by fractionation removal or addition of component molecular species or by polymerization designed to provide systematic variation of molecular parameters. Overall MWD's of the samples were characterized, and die swell behavior was determined at 2OOOC over a wide shear rate region in a high L/D capillary both with and without annealing of extrudates. The results are presented showing effects of specific moIecular variables.

A comprehensive experimental study of the rheological behaviour of HDPE

“…those with frozen-in stresses. The data in figure 1 also show that B increases with both the average molecular weight (Mw for instance) and the molecular weight distribution (fl = Mw/Mn), in agreement with the results of Rogers [10] and Romanini [11] but in conflict with the data of Mendelson and Finger [12]. A quantitative dependence on both quantities is found through their product [11] In order to derive values of the first normal-stress difference from die-swell measurements it was neces- figure 3.…”

“…A quantitative dependence on both quantities is found through their product [11] In order to derive values of the first normal-stress difference from die-swell measurements it was neces- figure 3. It is seen that the swelling ratio after annealing also increases with r, Mw and fl, as found by other researchers [7,11,12]. The following evaluation of normal stresses has been restricted to the L / D ratio of 40 for reasons described in the literature [4], Among the several available theoretical analyses, Tanner's [4] has been chosen both because it can in some sense be considered the more rigorous [2,13] and because it has already been successfully employed to produce results that agree favourably with independently measurements [13].…”

The effect of molar mass distribution on extrudate swell of linear polymers

“…linear versus branched polymers), and the sparse variation in the polymer characteristics per study. For example, early work by Rogers [19] mentioned an increase of extrudate swell with M w , seemingly in contradiction with conclusions of Mendelson and Finger [20], who reported the opposite. Koopmans [25] showed that this was due to a different range of polymers (sparse variation), and united the two by showing the existence of a maximum extrudate swell at intermediate M w .…”

“…The effect of average molar mass, typically expressed via the weight-average molar mass M w , and of polydispersity has been studied among others by Graessley et al [18], Rogers [19], Mendelson and Finger [20], Racin and Bogue [21], Rokudai [22], Dealy and co-workers [23,24], Koopmans [17,25], and Wang and co-workers [26,27]. This approach has generated various valuable data sets, each focusing on different aspects affecting extrudate swell.…”

Die Swell of Complex Polymeric Systems

“…In these early studies [16][17][18], a wide range of experiments on polymer melt swelling were performed. Some workers have reported that die swell varies with temperature [19], pressure during processing [20], molecular weight and its distribution [21][22][23], as well as molecular structure [24][25][26][27] and compounding ingredients [28]. Studies of die swell as a function of length to diameter ratio of die (L/D), entry speed and shape of the die have been reported by Han [29], Cotten [30], Lenk [31] and Vinogradov and Malkin [32].…”