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Flow Enhancement in the Continuous Extrusion of Linear Low-Density Polyethylene† Jose´ Pe´ rez-Gonza´ lez Laboratorio de Reologı´a, Departamento de Fı´sica, Escuela Superior de Fı´sica y, Matema´ ticas, Instituto Polite´ cnico Nacional, C. P. 07300, Apartado Postal 75-076, Me´ xico D. F., Mexico
Morton M. Denn* The Benjamin Levich Institute for Physico-Chemical Hydrodynamics, City College of the City University of New York, New York, New York 10031
Enhanced throughput and the elimination of sharkskin were observed over an extended temperature range when linear low-density polyethylenes having the same molecular weight, polydispersity, and melt index, one with and one without commercial additives, were extruded through R-brass dies. The additives appear to play a significant role in the detailed flow behavior, however, most notably in an irreversible transition to a “normal” flow curve at stresses on the order of 0.3 MPa. There are indications of a slip-like phenomenon, including the occurrence of electrostatic charge on the extrudate. A high-throughput, low-extrusion-pressure processing window like that associated by Keller with a transition to a mesophase was observed for the polymer containing no additives, but not for the polymer containing additives. Introduction Polyethylene exhibits complex rheological behavior during extrusion, including the appearance of a succession of defects in the extrudates that are limiting factors in processing. These instabilities have recently been comprehensively reviewed.1 Two physicochemical issues emerge as relevant to the understanding of these phenomena in linear polyethylenes: polymer-die interactions and a possible transition to a structured phase at the die wall. The effect of capillary dies made of different metals on polyethylene extrusion was first explored by Benbow and Lamb.2 Ramamurthy3,4 reported distortion-free continuous extrusion of linear low-density polyethylene (LLDPE) through an R-brass film die, while observing distortions with the same polymer in the same extruder when an identical chrome-plated steel die was used; he attributed this result to enhanced adhesion between the polymer and the wall. Ramamurthy also reported that an induction time, dependent on the “cleanliness” of the die surface and the shear rate, was required to obtain extrudates without distortions. These results were independently reproduced only recently by Ghanta and co-workers5, who also reported the elimination of extrudate defects and flow instabilities in continuous extrusion of a LLDPE through an R-brass capillary under conditions where instabilities were observed for an identical stainless steel die. They interpreted their results as apparent slip in the brass die, however, rather than the enhanced adhesion proposed by Ramamurthy. † Dedicated to James Wei on the occasion of his 70th birthday. * Corresponding author. Phone: (212) 650-7444. Fax: (212) 650-6835. E-mail:
[email protected].
The possible shear-induced development of a structured phase in polyethylene melts close to a solid boundary was studied in a series of papers by Keller and co-workers, starting with Waddon and Keller6 and summarized in Kolnaar and Keller.7 They observed a “temperature window,” characterized by a large decrease in the extrusion pressure and distortion-free extrusion, in batch capillary rheometer experiments at temperatures just above those for which flow-induced crystallization is expected. Keller and co-workers attributed this behavior to the presence of a flow-induced mesophase, which was identified as the mobile hexagonal phase of polyethylene. The effect was dependent on the molecular weight and degree of branching, as well as on the materials of construction of the dies, and it was sensitive to startup conditions. The lower the molecular weight and the higher the degree of branching, the lower the temperatures at which the window was observed. Keller’s window effect might be the same phenomenon as the “stable island” reported by Pudjijanto and Denn,8 who observed a reduction in the pressure drop and the elimination of extrudate distortions in the oscillating flow regime of LLDPE in a narrow temperature and shear rate range above the quiescent crystallization temperature. It might also be related to the “easy flow” regime reported for a metallocene LLDPE by Pe´rez-Gonza´lez et al.,9 who found a minimum in the stress for the onset of spurt at a critical temperature and observed a significantly smaller shearthinning index and a delay in the onset of extrudate distortions relative to those observed for the same polymer at high temperatures. (It is interesting to note that Hussein and Williams10-12 recently reported evidence of a liquid-crystalline-like transition in highdensity polyethylene (HDPE) at temperatures above 200 °C.)
10.1021/ie0007771 CCC: $20.00 © 2001 American Chemical Society Published on Web 07/03/2001
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Table 1. Physical Properties of the Polymers polymer
density (kg/m3)
melt index (dg/min)
Mw
Mw/Mn
peak melting temp. (°C)
crystallinity (%)
C4-LLDPE C6-LLDPE
918 928
1.0 1.0
114 000 110 220
3.8 3.8
122.8 127.1
12 14
We report here on the flow curves and the onset of extrusion instabilities during the continuous extrusion of two similar linear low-density polyethylene resins, one a commercial polymer containing additives, over a wide temperature range using identical R-brass and stainless steel dies. The focus is on behavior that might be indicative of an effect of polymer additives, die materials of construction, and possible stress-induced phase transitions. Experimental Section We used two LLDPEs produced by Union Carbide, denoted GRSN-7047 NT 7 and HS-7028, respectively. GRSN-7047 is a copolymer with butene that is provided in granules; according to the manufacturer, it contains only a storage stabilizer. This butene-containing polymer is denoted hereafter as C4-LLDPE. HS7028 is a copolymer with hexene, provided in pellets, and contains a range of additives typical of commercial polymers, including primary and secondary antioxidants and antistatic and antiblock agents. This hexene-containing polymer is denoted hereafter as C6LLDPE. The properties of the two polymers are shown in Table 1. Further characterization can be found in Pudjijanto.13 C6-LLDPE has a larger high-temperature fraction in temperature-rise elution fractionation (TREF), indicating somewhat more high-density (linear) chains, which is consistent with the higher density of the resin; the low-temperature (very low density) fractions are similar. Continuous extrusion was carried out in a 3/4-in., 20:1 variable-speed single-screw extruder (Wayne Machine and Die Co.). Flow rates were determined by catching and measuring extrudates using a digital balance and a stopwatch; all points represent averages of multiple
TREF (%) e50 °C g90 °C 15.6 16.9
28.9 41.6
measurements in which the flow rates differed by less than 2%. The pressure was measured at the end of the extruder barrel with a Dynisco model TPH32A-10M-6/ 18 pressure transducer; 0.2 MPa variations were typically observed in the pressure transducer under steady flow conditions, corresponding to stress uncertainties that are less than the size of the data points reported subsequently. Five capillary dies with a length-to-diameter (L/D) ratio of 20 and an entry angle of 180° were used. Two capillaries with D ) 1.01 mm, one fabricated from 304stainless steel and one from CDA-464 naval brass, were used previously by Ghanta et al.5 Three capillaries with D ) 1.01, 1.19, and 1.59 mm were fabricated from CDA360 brass. The experiments were carried out in the temperature ranges from 135 to 200 °C for C4-LLDPE and from 145 to 200 °C for C6-LLDPE, with typical variations of (2 °C in the temperature close to the die. All runs were started by extruding Union Carbide Unipurge and then changing to extrusion of the desired polymer under a continuous 2.3 L/min flow of dry nitrogen to the hopper at a position just above the screw (see Ghanta et al.5). Unipurge is a polyolefin with silicate particles that provide abrasive cleaning to the extruder and die. Different times for die conditioning were investigated; the standard conditions were then set to no less than 3 h of polymer flow before starting measurements, with 2 h at 200 °C, followed by 1 h at the desired experimental temperature. Polymer leaving the die was transparent at all temperatures in the absence of surface distortions, indicating that melting was complete. Video images were obtained with a Cohu highperformance CCD camera. The observed behavior was unchanged over runs of as long as 5 h.
Figure 1. Flow curves for C4-LLDPE obtained with the 464-brass die at different temperatures.
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Figure 2. Development of extrudate defects at 150 °C for C4-LLDPE in the 464-brass die. Wall stress ) (a) 0.124, (b) 0.157, (c) 0.175, (d) 0.202, (e) 0.225, and (f) 0.234 MPa.
Figure 3. Extrudate of C4-LLDPE at 150 °C in the 464-brass die showing the gel-like film; wall stress ) 0.191 MPa.
Results C4-LLDPE. The initial experiments with C4-LLDPE were carried out in the 464-brass and stainless steel dies at temperatures ranging from 135 to 200 °C, followed by experiments with the three 360-brass dies at 150 and 200 °C. Flow curves are presented without corrections to the data. 464-Brass Die. The flow curves obtained with the 464brass die at the different temperatures are shown in Figure 1. There are no regions of oscillating flow, consistent with the measurements of Ghanta et al.5 at 200 °C. (Spurt flow was observed with a shorter die conditioning time.) There is a curious inversion in the temperature range from 150 to 200 °C in which the flow rate decreases at fixed stress with increasing temperature. The flow curves exhibit local minima in stress with respect to shear rate at temperatures between 150 and 170 °C, followed by a rapid increase in stress; similarly, at low shear rates, the stress passes through a minimum with respect to temperature at about 150 °C. This behavior seems analogous to the “easy flow” described by Pe´rez-Gonza´lez et al.9 The polymer exhibited the small-amplitude, high-frequency surface distortion known as “sharkskin” at the lowest shear rates attainable with the extruder at all temperatures from 150 to 200 °C, but the defect disappeared as the shear rate was increased, until the polymer appeared almost free of distortions (see Figure 2), with very little extrudate swell. The disappearance of sharkskin was localized and then spread to cover the entire surface.
Figure 4. Appearance of sharkskin after detachment of the gellike film for C4-LLDPE at 150 °C in the 464-brass die.
The extrudates at temperatures from 150 to 200 °C were electrostatically charged at all shear rates above that at which the sharkskin began to disappear; the presence of the charge was easily detected by placing a steel knife blade near the extrudate, which caused the filament to deflect and establish contact with the knife. Finally, the gross surface distortion usually known as “melt fracture” began at flow rates beyond the region of “easy flow” at each temperature, and the pressure increased. The transient prior to reaching the minimum in the flow curve at 150 °C was sometimes longer than 30 min.
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Figure 5. Extrudate of C4-LLDPE with a neck-like distortion at 150 °C.
A gel-like film surrounded the extrudate at shear rates at and beyond the minimum in the flow curve; some images of the extrudate with the film are shown in Figure 3. The extrudate from the bulk showed a single Differential Scanning Calorimeter crystallization peak at 108 °C at a heating rate of 0.5 °C/min, whereas the film showed a second peak at 113 °C. (A second melting peak was also observed at higher temperature in the film, but this peak was harder to resolve.) The film detached periodically from the die lip, accompanied by an increase of around 1 MPa in the pressure drop and the appearance of sharkskin over a portion of the extrudate perimeter (see Figure 4). The pressure then decreased, and the defects on the extrudate disappeared. This process repeated periodically; the period was around 5 min at the flow curve minimum, decreasing with increasing shear rate until it could no longer be distinguished. The extrudate exhibited a larger-scale distortion beyond this oscillatory region, as shown in Figure 5, where there appears to have been formation of a neck. The polymer gives the appearance of having been rejoined at the neck after a loss of continuity, perhaps at the die entrance. All of the phenomena described in this section were reversible with respect to changes in the shear rate.
Stainless Steel Die. The flow curves obtained with the stainless steel die are shown in Figure 6. These data did not show the anomalous temperature dependence seen in Figure 1, and horizontal time-temperature superposition was applied prior to the spurt regime with an Arrhenius function having an activation energy of 15.9 kcal/mol. The shear stresses in this set of experiments were consistently higher than those obtained with the brass die at the same temperatures and shear rates, and the extrudates in the temperature range between 150 and 200 °C exhibited sharkskin followed by pressure oscillations and melt fracture. In addition, oscillatory (“spurt”) flow was observed at temperatures from 150 to 200 °C, with a critical stress that decreased with temperature until it was suppressed at temperatures below 145 °C (compare refs 5, 7, and 9). Deep minima in the shear stress were observed at temperatures of 140 and 145 °C, and a gel-like layer was observed near the minima, as with the brass die. Electrostatic charging of the extrudates was not observed with oscillatory flow. Wall Slip. Flow curves at 200 °C for the three 360brass dies with the same L/D ratio are shown in Figure 7, together with the flow curve for the stainless steel capillary. There is a strong diameter dependence of the flow curves in the brass dies, but the curves for the two smallest diameters nearly overlap. The extrusion pressures with 360-brass dies were lower than those with the 464-brass, but the overall flow behavior and the development of extrudate distortions were similar; in particular, the plateau indicative of the oscillatory regime between the lower and upper branches of the flow curve that is prominent in the steel curve is absent for the 360-brass dies. C6-LLDPE. The C6-LLDPE resin is a copolymer with hexene and contains additives. Flow curves for the 464brass die at temperatures from 145 to 200 °C are shown in Figure 8. There are a number of striking features when compared to the flow curves for the similar C4LLDPE resin in Figure 1. First, a discontinuity in the flow curve was observed at all temperatures at a stress
Figure 6. Time-temperature superimposed flow curves for C4-LLDPE obtained with the stainless steel die.
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Figure 7. Flow curves for C4-LLDPE at 200 °C obtained with three 360-brass dies of different diameters and the stainless steel die.
Figure 8. Flow curves for C6-LLDPE obtained with the 464-brass die at different temperatures.
on the order of 0.3 MPa. Second, a stress minimum with increasing shear rate was not observed at any temperature. Third, although the flow curves from 150 to 180 °C were insensitive to temperature, they exhibited higher stresses at comparable shear rates than the C4LLDPE resin. The development of extrudate distortions was essentially the same as in C4-LLDPE prior to the stress jump: Sharkskin was observed at low shear rates, but it disappeared with increasing shear rate. Sharkskin appeared again following the jump, followed by spurt flow. The flow curves for both polymers in the 464-brass capillary at 200 °C are compared in Figure 9. The two flow curves at this temperature are very similar at stresses below 0.3 MPa, consistent with the similarities in molecular weight and polydispersity of the two
polymers. C6-LLDPE exhibits a sudden jump to a higher stress at a critical stress of about 0.3 MPa, however. After the jump, the flow corresponds to that obtained without using Unipurge and nitrogen during startup. The jump is irreversible, and the flow curve remains on a different branch as the extrusion pressure is decreased. Figure 10 shows the two branches of the flow curve at 180 °C: The filled circles are the flow curve obtained without Unipurge and nitrogen, while the inverted triangles represent the flow curve with Unipurge and nitrogen obtained by increasing from low pressures. The open triangles are the flow curve obtained with Unipurge and nitrogen, but starting at a high pressure. The squares are from the same run as the inverted triangles, but with decreasing extrusion pressure after the jump.
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Figure 9. Flow curves for C4-LLDPE and C6-LLDPE obtained with the 464-brass die at 200 °C.
Figure 10. Flow curves for C6-LLDPE obtained with the 464-brass die under different startup conditions at 180 °C.
Discussion Die Conditioning. The conditioning of the brass die surfaces required up to 3 h of continuous extrusion, after which there was visual evidence of a change in the surface; the 464-brass die entry faces had a reddish coloration after extrusion, and the 360-brass a color between gray and green. This is consistent with Ramamurthy’s3,4 observation that an induction period is required before extrudate distortions disappear and explains why the effect, which has obvious processing significance, has never been seen in capillary rheometers, which operate in a batch mode. (Compare Ghanta et al.5) Electrostatic Charge. The observation of an electrostatic charge on the C4-LLDPE resin extrudate
coincident with the onset of the disappearance of sharkskin in the brass dies is suggestive of slip at the die wall, as first noted by Vinogradov and co-workers;14 see also Pe´rez-Gonza´lez.15 It is tempting to identify the mechanism with adhesive failure at the polymer/brass interface, as one usually associates electrostatic charge with friction between dissimilar materials, but this is simplistic reasoning, and slip at an interior surface cannot be ruled out by this observation. Perhaps the most striking feature is the fact that the charge appears when sharkskin begins to disappear, pointing to a slip mechanism and stress reduction prior to the die lip as the cause of the disappearance of the sharkskin. The absence of electrostatic charge on extrudates of C6LLDPE, despite the onset of apparent slip, is undoubt-
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edly a consequence of the presence of the antistatic agent in the commercial polymer. Stress Minima. The stress minima with increasing shear rate observed with the C4-LLDPE polymer in both the steel and brass dies (but in different regimes), together with the formation of the gel-like layer, suggest a flow-induced phase transition. The new state is unlikely to be to the stable orthorhombic phase of polyethylene, as that phase would be expected to form at the die entrance and lead to die blocking (which probably accounts for the sharp upturn in the flow curves at the lowest temperatures). The minima are reminiscent of Keller’s temperature window, and, following Keller,7 they might reflect a transition to the mobile hexagonal phase or some other mesophase over the outer portion of the extrudate. There is at least one important difference from Keller’s observations: The continuous extrusion experiments revealed a transient associated with periodic detachment of the gel-like layer for as long as 10 min following quasi-steady operation; this transient would not be observable on the time scale of the batch capillary rheometry employed by Keller and co-workers. Wall Slip. The insensitivity of the flow curve to the capillary diameter for C4-LLDPE in the two smallest brass dies is very unusual. This leads to a highly nonlinear Mooney plot (apparent shear rate versus reciprocal diameter at constant wall stress), indicating that the simple concept of a slip velocity as a unique function of shear stress is not applicable. Additives. It is unlikely that the large differences in behavior between the C4-LLDPE and C6-LLDPE resins are solely a consequence of the difference in the length of the short-chain branches or the different fractions of linear polymer, and the presence of a range of additives in the commercial C6-LLDPE resin is undoubtedly a major factor. The most striking difference is the irreversible discontinuity in the ascending flow curve for the C6-LLDPE resin. Superficially, there seems to be a transition on the ascending curve from slip to noslip, followed by massive slip associated with the spurt transition; the effect of massive slip then seems to be to change the wall conditions in a way that enhances adhesion on the descending curve. Operation without Unipurge and nitrogen seems to have the same effect on the wall conditions. What is perplexing about this scenario is that the additives undoubtedly include an antioxidant, while the perceived effect of the Unipurge is to remove the oxide layer from the die, and the purpose of the nitrogen blanket over the feed is to keep oxygen and moisture away from the polymer. Hence, the antioxidants among the additives would be expected to function in the same way as the Unipurge and the nitrogen, not the opposite. In any event, the presence of the additives in the commercial polymer at high stress levels apparently changes the wall conditions (compare ref 4), possibly by enhancing the adsorption of chains. The absence of the flow-curve minimum and the gellike layer in the C6-LLDPE resin could be a consequence of the small difference in chain structure and composition or a wall phenomenon associated with the presence of the additives.
additives, is extruded through R-brass dies over an extended temperature range. The additives appear to play a significant role in the detailed flow behavior, however, most notably in an irreversible transition to a “normal” flow curve at stresses on the order of 0.3 MPa. This irreversibility would seem to limit the use of brass dies as a practical commercial solution for the elimination of extrudate defects at high throughputs. There are indications of a slip-like phenomenon, including the occurrence of electrostatic charge on the extrudate, but the detailed mechanism, including the role of the additives, cannot be determined from these macroscopic experiments. A high-throughput, low-extrusionpressure processing window observed at low temperatures by a number of authors and associated by Keller with the appearance of a mesophase is observed for the polymer containing no additives, but not for the polymer containing additives. The small difference in backbone structure between the two polymers studied here precludes definitive identification of the reasons for the differences in flow behavior; the additives are the likely cause, but the mechanism by which they might act is elusive. Acknowledgment The experimental work was performed at the Lawrence Berkeley National Laboratory, with partial support from the Director, Office of Energy Research, Office of Basic Energy Science, Materials Science Division of the U.S. Department of Energy under Contract DE-AC07376SF00098. J. P.-G. acknowledges COFFA-IPN and CONACYT for permission and support to stay in Berkeley. Literature Cited (1) Denn, M. M. Extrusion Instabilities and Wall Slip. Annu. Rev. Fluid Mech. 2000, 33, 265. (2) Benbow, J. J.; Lamb, P. New Aspects of Melt Fracture. SPE Trans. 1963, 3, 7. (3) Ramamurthy, A. V. Wall Slip in Viscous Fluids and Influence of Materials of Construction. J. Rheol. 1986, 30, 337. (4) Ramamurthy, A. V. LLDPE Rheology and Blown Film Fabrication. Adv. Polym. Technol. 1986, 6, 489. (5) Ghanta, V. G.; Riise, B. L.; Denn, M. M. Disappearance of Capillary Instabilities in Brass Capillary Dies. J. Rheol. 1999, 43, 435. (6) Waddon, A. J.; Keller, A. A Temperature Window of Extrudability and Reduced Flow Resistance in High-MolecularWeight Polyethylene: Interpretation in Terms of Flow-Induced Mobile Hexagonal Phase. J. Polym. Sci. 1990, B30, 1063. (7) Kolnaar, J. W. H.; Keller, A. A Singularity in the Melt Flow of Polyethylene with Wider Implications for Polymer Melt Flow Rheology. J. Non-Newtonian Fluid Mech. 1997, 69, 71. (8) Pudjijanto, S.; Denn, M. M. A Stable “Island” in the SlipStick Region of Linear Low-Density Polyethylene. J. Rheol. 1994, 38, 1735. (9) Pe´rez-Gonza´lez, J.; de Vargas, L.; Pavlinek, V.; Hausnerova, B.; Saha, P. Temperature-Dependent Instabilities in the Capillary Flow of a Metallocene Linear Low-Density Polyethylene Melt. J. Rheol. 2000, 44, 441.
Conclusions
(10) Hussein, I. A.; Williams, M. C. Rheological Evidence for High-Temperature Phase Transitions in Melts of High-Density Polyethylene. Macromol. Rapid Commun. 1998, 19, 323.
The primary conclusion of this work is that enhanced throughput and the elimination of sharkskin are observed when LLDPE, both with and without commercial
(11) Hussein, I. A.; Williams, M. C. Anomalous Nonlinearities in Steady Shear of Polyethylene Melts. J. Non-Newtonian Fluid Mech. 1999, 86, 105.
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(12) Hussein, I. A.; Williams, M. C. DSC Evidence for Microstructure and Phase Transitions in Polyethylene Melts at High Temperatures. Macromolecules 2000, 33, 520. (13) Pudjijanto, S. Melt Fracture Phenomena in Linear LowDensity Polyethylene. M. S. Thesis, University of California, Berkeley, CA, 1994. (14) Vinogradov, G. V.; Malkin, A. Y.; Yanovskii, Y. G.; Borisenkova, E. K.; Yarlikov, B. V.; Berezhnaya, G. V. Viscoelastic Properties and Flow of Narrow Distribution Polybutadienes and Polyisoprenes. J. Polym. Sci., Part A-2 1972, 10, 1061.
(15) Pe´rez-Gonza´lez, J. Exploration of the Slip Phenomenon in the Capillary Flow of Linear Low-Density Polyethylene via Electrical Measurements. J. Rheol. 2001, 45, 845.
Received for review August 29, 2000 Revised manuscript received May 21, 2001 Accepted May 24, 2001 IE0007771
