Rheology of lubricated Polytetrafluoroethylene Compositions
0
0 0
Equipment and Operating Variables
0
ERNEST E. LEWIS AND CHARLES M.WINCHESTER1 Polychemicals Department, Research Division, Experimental Stafion, E. 1. du Ponf d e Nemours & Co., Inc., Wilmington, Del.
T
HE process for extruding Teflon tetrafluoroethylene resin in the form of a lubricated paste was developed to attain extrusion rates appreciably higher than those attainable by the more conventional high temperature processes. In a paper by Lontz, Jaffe, Robb, and Happoldt ( d ) , the methods of preparing the lubricated compositions and some simple techniques for fabricating tape and wire coverings were described. The development of paste extrusion has continued along several lines. I n the work described here the objective was to investigate more carefully the mechanisms and variables of the process in order to achieve improved performance and to permit the design of improved equipment. Therefore, instead of using a constant-rate wire coater, a simple rheometer was employed in which monofilaments could be extruded a t constant pressure. Equipment and Procedure The gas-piston rheometer (Figure 1) is a simple ram extruder with 0.75-inch cylinder, 10.75 inches long. The rheometer base is a steel tube 6.5 inches in outside diameter, 6 / 8 inch thick, with a hand hole for access to the extrudate and a base rjng welded t o the bottom. Bolted t o the top of the tube is the die holder with a recess for the die jacketed for heating or cooling. When the die is in position, the top face of the die is slightly higher than the surrounding surface, providing an intimate contact with the 1
Present address, Interchemioal Corp., Cincinnati, Ohio.
w
PISTON
bottom face of the cylinder when assembled. The cylinder is locked in position by means of an interrupted thread and is wrapped with a coil of copper tubing for temperature control. The first 0.75 inch of the piston is built to a close tolerance with the cylinder to minimize polymer extrusion around the piston. The remainder of the piston shaft is fluted to minimize frictional drag. Piston pressure is provided by the ram of a double-acting gas cylinder, 10 inches in diameter. Cylinder nitrogen supplies the necessary gas pressure v k the usual system of control valves and pressure gages. At the start of a run, the gas system is adjusted to giveJhe desired piston pressure and the ram put in the “up” position. The desired die and cylinder are assembled and the weighed polymer, usually 40 grams, is charged to the cylinder. The piston is inserted in the cylinder and allowed to come t o rest on the polymer, after which the polymer is preformed by the slow application of hand pressure, Then the orifice is plugged with the tapered end of a brass rod and pressure is applied with the ram of the gas cylinder. T o ensure that the system has reached the desired pressure, the orifice is plugged for 15 seconds after the gas system reaches the set pressure. The run is started and the beading is cut a t definite time increments. The length and caliper of each piece are measured and recorded. Unless otherwise state& all runs were made with the equipment a t 25’ C. When variables other than die dimensions were studied, a standard die was used which had a conical taper with an included angle of 120’ and an orifice 0.060 inch in diametkr and 0.457 inch long. Two special rheometers, one with 2.75-inch piston and the other with a 0.75-inch piston, were made t o determine the effect of piston diameter on the paste extrusion process. Both rheometers had 60’ t a ers and orifices 0.060 inch in diameter and 0.457 inch long; the End (Figure 2) of the large rheometer was later increased t o 1.0 inch. [The land of a cylindrical die is that portion of the die which has the same diameter as the orifice (see Figure 2).] Pressure was applied t o the piston of the large rheometer by means of a ao-ton, Watson-Stillman hydraulic press. The small rheometer. was substituted for the standard unit in the gas-piston press.
Polymer Used
TAPER TEMPERATURE
All the polymer used for these studies was polymerized as an aqueous dispersion, then coagulated by mechanical agitation. The coagulum was filtered, dried, and then blended with No, 45 white oil t o give a paste com osition of about 2 0 2 lubricant ,-CYLINDER r F L O W LINES by weight. Although the same polymerlubricant mixture was used for all runs in a given series of experiments, the same polymer was not used throughout all the work. Sources of Error
BASE
-B
i
‘ED
i Figure I. Ger Piston Rheometer
21
BEADING Figure 2.
1123
Extrusion Flow Patterns
Inmost runs a variable film of polymer is extruded around the rheometer piston, which results in a gradual increase in
1124
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 45, No. 5
h1ICROSCOPIC AND VISUAL OBSERVATIOSS.Microscopic examinations of split plugs and beading from paste extrusions show that three physical forms of the polymer are present. Throughout most of the cylinder where the polymer is not sheared, the original agglomerates or granules are seen in a compacted form. A t the end of the cylinder and in the first part of the die taper where the polymer is only slightly sheared, the paste is more homogeneous, indicating that the agglomerates are probably broken down into the ultimate particles. In the die taper immediately preceding the orifice where the polymer is sheared the most, and in the beading, long filaments or fibers can be seen even with the naked eye. For all sustained extrusions the entire cross section of the beading is fibrillated. Examination of the plugs and beading from low pressure extrusions shows that the fibrillation starts a t the periphery of the die and progresses inward with time. With the large-piston rheometer and frequently a t low rates of flow in the standard rheometer the beading curls as it emerges from the die, indicating a wide difference in strain between the periphery and core of the beading analogous to a stretched rubber hose filled with putty. In special cases where the polymer is difficult to shear, the long fibers are not present and the beading is weak and scaly in appearance. Electron micrographs show that the fibers, as seen by the naked eye, are actually ropes made of very h e fibers in which many of the original globular particles are interspersed. X-ray diffraction pictures indicate that the extent of crystalline orientation is barely detectable.
I
.05
Figure 3.
Electron Micrograph of Fiber from Unsintered Extruded Lubricated Polytetrafluoroethylene Scale. 0.5 inch
I
I
POLYMER PRESSURE
___I
3240 PSI.
.02
= 1 micron
the frictional drag of tbe piston and a corresponding decrease in the pressure available for extrusion. Another effect which tends to increase the extrusion rate as the size of the charge becomes smaller is the frictional drag of the polymer on the cylinder walls. This effect is most obvious with fairly rapid extrusions involving a large charge. Presumably variations in the pressuring system can also occur, but these are considered to be negligible. Another large factor is entrapped air, which seems to have a lubricating action particularly a t the start of a run. This effect is most evident with large charges, tendirig t o boost greatly the flow rate in the first part of an extrusion. In some cases the first sections of beading have large air bubbles. Apparently, as the piston applies pressure, the paste starts compacting a t the piston, and, as successive layers are compacted, the excess air is pushed toward the orifice. This problem is minimized by using a standard weight of charge and by preforming the charge in the rheometer with the slow application of hand pressure on the piston.
,
.o I 0' v) w
\
,005
z 5
E
,002
a
5
,001
LL J
,0005
.0002 .OOOl 0
1.0
2.0
5.0
EXTRUSION TIME, MINUTES
Figure 4.
Variation of Flow Rate with Time at Various Pressures
Rheological Studies Flow Mechanism. FLOW PATTERNS.If alternate layers of pigmented and unpigmented paste are charged to the rheometer and extruded, examination of the plug will show the flow pattern for the die system. The pattern for a die with a conical taper with an included angle of 120' is sketched in Figure 2. I n the cylinder the polymer moves in a dished front, then as the die is reached, the polymer in the center accelerates toward the orifice. I n the taper of the die this trend continues, while the polymer near the a d l s encounters increasing resistance and slows down markedly, Flow patterns for a given die seem to be the same regardless of the flow rate. However, a different pattern is obtained for each die design, as might be expected, and obAerved wall effects agree with the performance of each design as discussed belo-ir..
A major phenomenon noted in the extrusion is the swelling of the paste as it emerges from the orifice. This swelling is evidence of recovery from elastic deformation of the polymer in the die. The magnitude of the observed swelling depends on the
amount of elastic deformation of the polymer, the length of time the polymer is confined in the orifice, the extrusion temperature, and the fluidity of the paste. ELEC~O MICROSCOPE N STUDIES. As shown in Figure 3, an electron micrograph of the extruded polymer (made by C. E. Wiloughby, Chemical Department, E. I. du Pont de Nemours & Co., Inc., Wilmington, Del.), the beading is composed of polymer particles interspersed throughout a network of polymer strings. As a few of these strings are seen in powders coagulated by stirring, but are not present in the original dispersion, it a p
INDUSTRIAL AND ENGINEERING CHEMISTRY
May 1953
pears that the strings are formed whenever the polymer is sheared. Hence, the extrusion involves forcing a mixture of globular particles and tangled strings of polytetrafluoroethylene into the orifice. During this process additional strings are formed, and the entire mass is transformed into a rope or yarn comprised of aligned strings and interspersed original particles. During sintering the strings and particles lose their identity and the polymer becomes homogeneous. Analysis of Data. The data from a typical series of extrusions with the same die and paste at different pressures are plotted as log flow rate against log time in Figure 4. At high pressures the flow rate is essentially.constant; at low pressures the flow rate rapidly decreases with time. At intermediate pressures a high initial flow rate rapidly drops to a minimum, then increases somewhat and tends to level off. A logical explanation of the behavior described by these curves can be made by assuming that the ease of extrusion a t a given pressure-i. e., flow rate-is influenced by the previous shear history or amount of previous deformation of the polymer, and that the amount of shear strain which the polymer undergoes in a given die depends on the flow rate. As reported above, the physical appearance of the polymer is changed from compacted agglomerates to long fibers in what may be called the transition zone4.e.; the die taper and the part of the cylinder immediately before the die. The material as charged to the transition zone has no previous shear history. When the extrusion is started, this material is easy t o extrude and an abnormally high rate of flow occurs. Each successive layer of polymer originating farther and farther away from the orifice is subjected to more and more shear as it passes through the transition zone and enters the hole. Therefore, the flow rate rapidly decreases as the paste initially in the transition zone is extruded I
0.02
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VOLUME EXTRUDED, CU. IN.
,
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0.1
I
0.2
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0.5
I
1.0
I
2.0
5.0
Change in Flow Rate with Volume Extruded at Constant Pressure
Therefore, the performance of a given composition in a given die is described by a curve of flow rate versus applied pressure. A family of performance curves is used to describe the effect of one variable such as approach design, land length, and temperature. These curves have an apparent yield value or pressure which should be interpreted as the minimum pressure required to sustain a continuous extrusion for that composition in that die. As described above, some extrusion is actually obtained a t lower pressures for a limited time.
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Figure 6.
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In rheology the flow of non-Newtonian materials is described by the general shape of their consistency curves or shear diagrams, which are plots of the rate of shear versus the shearing stress (1). The extrusion of a lubricated polytetrafluoroethylene paste, however, involves an irreversible physical change, making it a special case. Furthermore, as most of the shearing takes place in the taper rather than in the hole of the die, the shear behavior of a given composition cannot be described by a single curve which is independent of the die dimensions without resorting to empirical equations with parameters of no rheological significance ( 4 ) .
1 '
I
0.01 \
1125
'
If flow rate is plotted against volume extruded, the minimum of the curves for a given die occurs at the same volume regardless of pressure, polymer batch, or lubricant content. Figures 5 and 6 show five extrusions a t the same pressure with the same die, each extrusion with a different batch of semiworks polymer blended to the same lubricant composition. The total volume of material extruded at the minimum flow rate is roughly the volume of the transition zone. These flow fluctuations are masked in rapid extrusions and, therefore, essentially constant-rate extrusion occurs (Figure 4). I n some extrusions at low pressures the flow rate continuously decreases. In these cases all the material in the transition zone is not extruded.
Equipment Design Variables Taper Studies. Figure 7 compares the performance of a single batch of polytetrafluoroethylene paste in each of five dies with the same orifice diameter and land length but with different ta9er designs: No. 1, no taper (180'); No. 2, 120" conical taper; No. 3, 60' conical taper; No. 4, 120' conical taper followed by a rounded orifice entrance; and No. 5 , a streamlined taper as shown. The measured angle is the included angle of the cone. As shown by the performance curve, the resistance t o flow of the 180" die is very great. The best conical taper among those studied is 60",but the improvement of the streamlined die over the 60' taper is significant. The die with the 120" taper and the rounded throat was designed on the assumption that a large part of the resistance offered by the 120' conical die was due to the abrupt change from the taper to the orifice. Apparently the resistance a t this point is an important factor only at the lowest rates of flow, and in most of the range of flow rates the over-all resistance of the taper is more significant.
INDUSTRIAL AND ENGINEERING CHEMISTRY
1126 10.0
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Vol. 45, No. 5
noted, as might be expected with a greater residence time of the polymer in the die. The dies were not, however, in the exact order of increasing land length and above 7 inches per second the trend noted at lower flow rates apparently was reversed. The finish of the hole is probably important and may influence the amount of additional shear. The holes of all the dies used in this series had prominent drill marks. Effect of Piston Diameter. An extrusion pressure of over 10,000 pounds per square inch is necessary to sustain an extrusion rate of 2 inches per minute with the 2.75-inch rheometer. With the 0.75-inch rheometer the same extrusion rate is obtained with an extrusion pressure of only 2500 pounds per square inch. This large difference in extrusion pressure is undoubtedly due to the much greater shear to which the polymer is subjected at the periphery of the larger cylinder in moving through the greater radial distance to the orifice. Evidence of this greater shear is seen in the extrudate. The beading from the large rheometer is strong and severely curled with small fiber ends or loops
4.0-
LL
B I 3
0 0 .
2.0 -
2000
3000
4000
APPLIED POLYMER PRESSURE,
Figure 7.
J
5000
PSI
Effect of Taper Design on Performance
The caliper of the beading extruded a t different flow rates for these dies is shown graphically in Figure 8. For each die as the flow rate increases, and therefore the residence time of the polymer in the die decreases, the observed swelling of the extruded beading is greater. As the same paste was used and all dies have the same land length, comparison of the calipers of the extrudate at a given flow rate gives an indication of the relative amount of elastic deformation of the polymer caused by each die at that flow rate. With the exception of the modified 120" die, the dies fall in the same order as in the performance curves, Figure 7-i.e., a t a given flow rate the streamlined die produces the least deformation, followed by the 60°, modified 120°, and 120 O dies. I n general, then, the greater the resistance of the die, the more the polymer is deformed in achieving a given flow rate. The swelling data, however, indicate that the polymer undergoes less deformation in the modified 120" die than in the straight 120' die. Land Length Studies. Imperfections such as pimples, splits, and curling can be reduced or eliminated by using a die whose land or orifice length is at least 0.5 inch. Apparently uneven strains are introduced during the extrusion process, and the relaxation that occurs beyond the orifice accounts for these imperfections. Hence, by using a land of sufficient length, the residence time of the beading in the land is long enough to ensure that elastic recovery is essentially completed while the beading is still confined. Although increasing the land length markedly improves the quality of the extrudate, it also has its drawbacks. The rheometer performance curves for a series of dies with different land lengths, but with the same orifice diameter and taper, are shown in Figure 9. Obviously, the longer the land, the greater the pressure needed to obtain a given flow rate and the greater the pressure change needed to obtain a given change in flow rate. The data are replotted in Figure 10, illustrating that the higher the flow rate desired the larger is the increase in pressure required to overcome a given increase in land length. I n view of the increased resistance to flow offered by the longer dies, especially at the higher flow rates, it seems likely that the land is responsibIe for additional shear of the polymer. Evidence of this additional shear was seen in the observed swelling of the extruded beading. At a given flow rate below about 7 inches per second, a general trend toward lower calipers with longer lands was
I 0
POLYMER
Figure 8.
10 20 FLOW RATE, INCMES /SECOND
30
Effect of Taper Design and Flow Rate on Caliper of Extruded Beading
APPLIED
Figure 9.
POLYMER PRESSURE, PSI.
Effect of Land Length of Dies
INDUSTRIAL AND ENGINEERING CHEMISTRY
May 1953
1127
-on the surface. When drawn, it has little elongation and the ends a t a break are fibrillated. When the land length is increased from 0.46 to 1.0 inch, the curls and fibered ends are eliminated, but the beading is still strong, has little elongation, and has fibered ends a t a break. Early semiworks development of wire-coating extruders indicated that piston diameters over 3 inches were too large to obtain satisfactory operation and/or good quality insulation. I n the light of the rheometer results, those extruders either had insufficient available pressure or the extrudate was poor because of its highly strained condition. If the latter is the caae, a longer land on the die would help.
Effect of Temperature The curves of flow rate versus applied pressure for 7", 25", and 50" C. are plotted in Figure 11. At 7" the flow rate a t a given pressure is faster than a t 25 and a much lower pressure is required to start and sustain extrueion. At pressures below 3300 pounds per square inch the flow rates a t 50" are about the same aa those a t 25O, but above that pressure the extrusion at 50" is faster. These results indicate that heating the die would assist the extrusion. The monofilaments extruded a t each temperature have different characteristics. The beading made a t 7" C. is rough, scaly, and rather weak. The polymer has been sheared only slightly, forming short fibers. Hence, a relatively small increase in polymer surface area occurs and there is less resistance t o flow. The beading extruded a t both 25 and 50' C. is smooth and fairly strong, and can be hand drawn 2 to 3 X before breaking. The beading from the 50" C. extrusion is stronger, particularly that extruded at the slower flow rates. At rapid flow rates the beading made a t both 7 " and 50 ' C. does not swell as markedly as the beading extruded a t 25" C. This indicates that at 7" C. less elastic deformation has occurred, while a t 50" C. either less elastic deformation has occurred or there has been more relaxation in the land of the die. The polymer has a different crystalline form a t each of these temperatures (3). Below 19" C. the crystals are triclinic and the molecules are most densely packed. Between 19" and 29O
O
12,000
10,000
-m
I-----.
/ 7.0 IN./SEC.
n W
K 2
2 8000a n a
1000
2000
3000
4000
5000
APPLIED POLYMER PRESSURE, PSI
Figure
11.
Effect of Temperature
32" C. the crystals are hexagonal. As the temperature is raised above 29" C., an increasing amount of disorder occurs along the long axis of the crystal. It can be postulated that the triclinic form is more resistant t o deformation, so that lese shearing occurs. With temperatures above 29 O C. the increasing disorder in the crystal permits a decrease in the elastic deformation and an increase in the plastic deformation of the polymer.
Summary The mechanism of lubricated paste extrusion of polytetrafluoroethylene appears to be a combination of permanent and elastic deformation in the region just before the orifice of the die. The amount of total deformation of the polymer in a given paste depends on the die design, the polymer temperature, and in some cases the flow rate. As a result of permanent or plastic deformation, the spherical polymer particles formed during polymerization are transformed into long fibers. Relaxation or recovery from elastic deformation tends t o occur both in the land and after extrusion. The amount of relaxation after extrusion, aR evidenced by the d e gree of swelling of the beading, depends on the amount of elastic deformation of the polymer, the fluidity of the paste, and the residence time of the polymer in the land, which in turn is a function of land length and flow rate.
W
f
2 3
3.0 IN. / SEC.
literature Cited
6000-
i2 a
4000
2000
1.0 IK / SEC.
-
.M4 .228
Figure I O .
Qreen, H., "Industrial Rheology and Rheological Structures," New York, John Wiley & Sons, 1949. (2) Lontz, J. F., Jraffe, J. A,, Robb, L. E., and Happoldt, W. B., IND. ENG.CHEM.,44,1805 (1952). (3) Pierce, R. H. H., Jr., Bryant, W. M. D., and Whitney, J. F., "An X-Ray Study of the Polymorphs of Teflon Tetrafluoroethylene Resin," Division of Polymer Chemistry, 121st Meeting, AMERICAN CHEMICAL SOCIETY, Buffalo, N. Y. (4) Reiner, M.,"Deformation and Flow," London, H. K. Lewis & Co., Ltd., 1949. (1)
.457
Effect
.914 LAND LENGTH, INCHES
1.821
of Land Length and Pressur6 on Flow Rate
R~CEIVED for review October 2, 1952. ACCEPTEDFebruary 24, 1953. Presented before the Division of Paint, Varnish, and Plastios Chemistry at the 122nd Meeting of the AMERICANCHEMICAL SOCIETY, Atlantic City, N. J.
